Qiong Cai

Dr Qiong Cai


Reader in Chemical and Process Engineering
PhD, FEA, FRSC
+44 (0)1483 686561
12 BC 02

Academic and research departments

Department of Chemical and Process Engineering.

Biography

University roles and responsibilities

  • Academic Integrity Officer (2013-2016, 2020 - date)
  • Director of Postgraduate Research (2016 - 2017)
  • University HPC Cluster Stakeholder Group
  • University Ethics Committee (Deputy Chair for Faculty of Engineering and Physical Sciences))

    Affiliations and memberships

    Fellow
    Higher Education Academy
    Member
    The Peer Review College of Engineering and Physical Sciences Research Council (EPSRC)
    Member
    The Scientific Board of the Energy Storage SUPERGEN Hub
    Member
    The Scientific Board of the H2FC SUPERGEN Hub
    Member
    International Society of Electrochemistry
    Member
    American Chemical Society

    Research

    Research interests

    Research projects

    Research collaborations

    Indicators of esteem

    • Keynote and Invited Talks at National and International Conferences

      Keynote: Multiscale modelling for battery electrode materials design, 3rd International Conference on Energy Storage Materials (ICEnSM), Shenzhen, China, 29 November – 01 December 2019.

      Keynote: Lattice-Boltzmann model for liquid water transport and oxygen diffusion in cathode of polymer electrolyte membrane fuel cell with electrochemical reaction, The VI Symposium on Hydrogen, Fuel Cells, and Advanced Batteries, Porto, Portugal, 19-23 June 2017.

      Invited talk: Understanding the role of porous structure in the performance of redox flow batteries, The Annual Meeting of UK Redox Flow Battery Network, Lancaster University, Lancaster, UK, 12 September 2019.

      Invited talk: Lattice Boltzmann model study on redox flow battery, Imperial-MIT Workshop on Redox Flow Batteries, Imperial College London, UK, 07 December 2017.

      Invited talk: Towards multi-scale materials design for energy storage and conversion, UK-China Workshop on Efficient Energy Utilisation, Nanjing, China, 10-13 August 2017.

      Invited talk: Design of nanoporous carbon materials for sodium ion batteries, The International Conference on Advanced Energy Materials and Advanced Nanomaterials, University of Surrey, 14 September 2016.

    • Invited Seminars at Universities and Institutions

      1. Multiscale modelling aided materials design for energy storage and conversion, University of Edinburgh, Edinburgh, UK, 24 July 2019.

      2. Multiscale materials design for energy conversion and storage, University of Newcastle, Newcastle, UK, 15 May 2019.

      3. Multiscale materials modelling for energy conversion and storage, UCL, London, UK, 10 September 2018.

      4. Multiscale modelling led materials design and formulation engineering, Beijing Institute of Technology, Beijing, China, 04 July 2018.

      5. Multiscale modelling led materials design and formulation engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China, 02 July 2018.

      6. Towards multiscale materials design for energy conversion and storage, Lancaster University, Lancaster, UK, 06 February 2018.

      7. Towards multiscale materials design for energy conversion and storage, Guangdong University of Technology, Guangzhou, China, 30 August 2017.

      8. Towards multiscale materials design for energy conversion and storage, Southern University of Science and Technology, Shenzhen, China, 28 August 2017.

      9. Towards multiscale materials design for energy conversion and storage, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, China, 25 August 2017.

      10. Towards multiscale materials design for energy conversion and storage, Institute of Physics, Chinese Academy of Sciences, Beijing, China, 20 August 2017.

      11. Modelling solid oxide fuel cells: linking molecular mechanisms with system design, SusHGEN Spring School on Fuel Cells and Hydrogen Technology, Newcastle University, Newcastle Upon Tyne, UK, 22 March 2012.

      12. Hydrogen production using solid oxide electrolysers: from materials structure to system design, The Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, China, 04 January 2012.

      13. Hydrogen production through steam electrolysis: designing the solid oxide electrode materials and electrolysis stack, at EMPA (Swiss Federal Laboratories for Materials Testing and Research), Switzerland, 6th August 2009.

    • Conference Chair

      Session Chair at the 22nd Topical Meeting “Materials Engineering and Process, Optimization at Electrified Solid/Liquid Interfaces” of the International Society of Electrochemistry, Waseda University, Tokyo, Japan, 15-18 April 2018.

      Session Chair at the UK STFC Battery Network Meeting, Abingdon, UK, 30-31 May 2017.

      Session Chair at International Conference on Advanced Energy Materials and Advanced Nanomaterials, University of Surrey, Guildford, 12-14 September 2016.

      Session Chair at 12th EUROPEAN SOFC & SOE FORUM, Luzern, Switzerland, 5-8th July 2016.

      Session Chair at HYSYDAYS 2009 - 3rd World Congress of Young Scientists on Hydrogen Energy Systems, Turin, Italy, 07-09 October 2009.

    • Regular Referee for International Journals

      ACS Applied Materials and Interfaces; Energy Storage Materials;

      Journal of Materials Chemistry A; Journal of Power Sources;

      Applied Energy; Journal of Energy Chemistry;

      ChemElectroChem; Electrochimica Acta;

      International Journal of Hydrogen Energy; Fuel Cells;

      Langmuir; Journal of Chemical Information and Modelling;

      Chemical Engineering Science; Scientific Reports...

    Supervision

    Postgraduate research supervision

    My teaching

    My publications

    Highlights

    For a complete list of my publications please visit my Google Scholar webpage or my ResearchGate webpage.

    Publications in Progress

    Invited Book Chapters

    E. Olsson, Q. Cai, Computational Studies on Na-Ion Electrode Materials, in Sodium-Ion Batteries: Materials, Characterization, and Technology, Wiley, to be published in June 2022.

    A. Karatrantos, E. Olsson, Q. Cai, Computational Modelling of Sodium-Ion Battery Electrolytes, in Handbook of Sodium-Ion Batteries: Materials and Characterization, Jenny Stanford Publishing, to be published in May 2022

    Journal Articles under Peer-Review / In Press

    Sailin Liu, Ruizhi Zhang, Jianfeng Mao, Yunlong Zhao, Qiong Cai*, Zaiping Guo*, From room temperature to harsh temperature applications: Fundamentals and perspectives on electrolytes in zinc metal batteries, Science Advances, 2022, 8, eabn5097.

    William Bell, Qiong Cai, A techno-economic analysis of a polymer electrolyte membrane water electrolyser supplying specific hydrogen applications, International Journal of Energy Technology, under review

    D. Zhang, W. Liu, C. Wu, Q. Cai*, Performance modelling and design of 3D electrode for Li-ion batteries, Energy Technology, under review

    Emilia Olsson, Jiale Yu, Haiyan Zhang, Huiming Cheng, Qiong Cai*, Atomic-scale design of anode materials for alkali metal (Li/Na/K)-ion batteries: progress and perspectives, Advanced Energy Materials, under review. 

    Juyan Zhanga, Lan Zhang, Jiashen Mengd, Bohua Wen, Kashem M. Muttaqi, Md. Rabiul Islam, Yunlong Zhao, Qiong Cai,* Suojiang Zhang,* High-performance rechargeable aluminum-ion batteries enabled by conversion-type FeF3 cathode and carbon nanotube-modified separator, Advanced Energy Materials, under review

    Emilia Olsson, Jonathon Cottom, Hande Alptekin, Heather Au, Maria Crespo-Ribadeneyra, Maria-Magdalena Titirici, and Qiong Cai*, Investigating the role of surface roughness and defects on EC breakdown, as a precursor to SEI formation in hard carbon sodium-ion battery anodes, Small, under review

    Yameng Fan, Gemeng Liang, Zhijie Wang, Anita M. D'Angelo, Bernt Johannessen, Lars Thomsen, Bruce Cowie, Jingxi Li, Fangli Zhang, Yunlong Zhao, Wei Kong Pang*, Qiong Cai*, and Zaiping Guo*, Strengthening Oxygen Lattice to Achieve Better-Performance Cobalt-free Li-Rich Layered Oxide Cathodes, Nature Communications, under review

    Xuting Li, Zhanghua Fu, Jing Wang, Xinhong Zhao, Yukun Zhang, Weizhe Liu, Qiong Cai * and Cheng Hu, Dilithium Phthalocyanine as Electrolyte Additive for the Regulation of Ion Solvation and Transport towards Dendrite-free Li Metal Anodes, Journal of Materials Chemistry A, under review

    Xuhui Yao, Johann Perera, Emilia Olsson, Manman Wang, Jianan Wang, Qiong Cai, David E. J. Armstrong, Nianhua Peng,* Roger Webb, Yunlong Zhao*, Xenon ion implantation induced compressive stress for preventing dendrite penetration in solid-state electrolytes, Small, 2022, 10.1002/smll.202108124

    Gemeng Liang, Emilia Olsson, Jinshuo Zou, Zhibin Wu, Jingxi Li, Cheng-Zhang Lu, Anita M. D'Angelo, Bernt Johannessen, Lars Thomsen, Bruce Cowie, Vanessa K. Peterson, Qiong Cai, Wei Kong Pang*, and Zaiping Guo* Introducing 4s-2p orbital hybridization to stabilize spinel oxide cathodes for lithium-ion batteries, Angewandte Chemie, 2022, 10.1002/anie.202201969

    Publications

    Tengfei He, Teng Zhang, Zhirong Wang, Qiong Cai (2022)A comprehensive numerical study on electrochemical-thermal models of a cylindrical lithium-ion battery during discharge process, In: Applied Energy313118797 Elsevier

    Modelling the electrochemical and thermal behaviours of cylindrical lithium-ion batteries (LIBs) is complicated by their multi-unit jellyroll structure. To evaluate the accuracy of cylindrical LIB models, eight electrochemical-thermal models (ECT) with different levels of fidelity and dimensionality (from one-dimensional (1D) to three-dimensional (3D) electrochemical and thermal models) are established for a Li[Ni8Co1Mn1]O2/graphite 18,650 type cylindrical LIB. The effect of different levels of model simplification on the predicted LIB thermal and electrochemical characteristics are compared under different discharge and cooling rates. Non-uniformity indexes are also introduced to compare the differences between the eight models for predicting electrochemical reactions and heat generation non-uniformity. The accuracy and computation time of different models are compared, and the applicable scope of different models is discussed comprehensively. Furthermore, the non-uniformity mechanism inside the battery are also analysed. The present work can be used to help other researchers select appropriate electrochemical thermal models under different applicable conditions and study the battery thermal management system.

    Tianhao Yu, Qiong Cai, Guoping Lian, Y Bai, X Zhang, Xiangping Zhang, Lei Liu, Suojiang Zhang (2021)Mechanisms behind high CO2/CH4 selectivity using ZIF-8 metal organic frameworks with encapsulated ionic liquids: A computational study, In: Chemical Engineering Journal419129638 Elsevier

    CO2/CH4 separation using ionic liquids (ILs) encapsulated metal-organic frameworks (MOFs), especially ZIF-8, has shown promise as a new technique for separating CO2 from CH4. However, the mechanisms behind the high CO2/CH4 selectivity of the method remain indistinct. Here we report the progress of understanding the mechanisms from examining the ZIF-8 aperture configuration variation using DFT and MD simulations. The results indicate that the pristine aperture configuration exhibits the best separation performance, and the addition of ILs prevents the apertures from large swing (i.e. configuration variation). Subsequently, the effect of IL viscosity on the ZIF-8 configuration variation was investigated. MD simulations also show that the pristine aperture configuration is more stabilized by ILs with large viscosity (0-87Cp). Further increase of IL viscosity above 87Cp did not result in noticeable changes in the aperture stability.

    Yameng Fan, Wenchao Zhang, Yunlong Zhao, Zaiping Guo, Qiong Cai (2021)Fundamental understanding and practical challenges of lithium-rich oxidecathode materials: Layered and disordered-rocksalt structure, In: Energy Storage Materials Elsevier

    Lithium-rich oxides have been considered as one of the most promising cathode materials for lithium-ion batteries due to the competitively high specific capacity contributed by both anionic and cationic redox reactions. Recent years have witnessed the successful achievements of lithium-rich oxides with layered structure due to their high capacity. More recently, another new class of lithium-rich oxides with disordered-rocksalt structure has been demonstrated to possess high capacity and high energy density with moderate structural changes during cycling. A comprehensive comparison and greater fundamental understanding of these two classes of lithium-rich oxides have not been achieved, however, which has greatly hindered the further application of these lithium-rich materials. Herein, an overview of the lithium-rich oxide cathode materials, including the conventional layered and the emerging disordered structure, is provided. Their structural characteristics, mechanisms of anionic redox reactions, and lithium diffusion pathways are systematically summarized. The key challenges to practical application and related strategies to enhance the electrochemical performance of the lithium-rich oxide cathodes are also discussed. At the end, the possible directions for the future development of the lithium-rich oxide cathodes are outlined.

    Jiale Yu, H. Zhang, Emilia Olsson, Tianhao Yu, Z Liu, Shangshang Zhang, X Huang, Wenwu Li, Qiong Cai (2021)A novel amorphous P4SSe2 compound as an advanced anode for sodium-ion batteries in ether-based electrolytes, In: Journal of Materials Chemistry A: materials for energy and sustainability9pp. 12029-12040 Royal Society of Chemistry

    Anode materials with superior electrochemical performance are urgently needed for the development of sodium (Na)-ion batteries (NIBs). Ternary phosphides can deliver better electrochemical performance than binary counterparts due to the synergistic effect created by their multinary components. However, ternary phosphides can suffer from severe volume expansion during cycling due to their high crystallinity, leading to poor cycle performance. Here we, for the first time, synthesize the amorphous ternary phosphorus chalcogenide P4SSe2 compound, via a facile and scalable high energy ball milling method. This amorphous P4SSe2 compound shows great tolerance to volume expansion and fast kinetics, owing to its stepwise reaction and intermediate electrochemical products with superior conductivity. When applied as an anode for NIBs, the as-synthesized P4SSe2 delivers a reversible specific capacity of 1038 mA h g−1 with an initial coulombic efficiency up to 87%. Additionally, it shows more durable cycle life in the diglyme electrolyte than that in the EC:DEC electrolyte, which can be attributed to favorable molecular energy levels and stronger solvation energy, as confirmed by density functional theory calculations and X-ray photoelectron spectroscopy depth profile experiments, thus leading to the formation of an inorganic inner layer in solid electrolyte interphase films and preventing electrode material degradation from continuous side reactions with the electrolyte. Broadly, this study may excite more research interest in the synthesis of amorphous ternary phosphides and their application in the energy storage field.

    Naiguang Wang, Yixiang Huang, Jingjing Liu, Xusheng Yang, Weipeng Xie, Qiong Cai, Songyuan Zheng, Zhicong Shi (2021)AZ31 magnesium alloy with ultrafine grains as the anode for Mg-air battery, In: Electrochimica Acta Elsevier

    Fabricating the magnesium alloy with fine grains, low dislocation density, and weak grain orientation is of crucial importance to enhance its anode performance for primary aqueous battery. However, this structure mode can hardly be realized for bulk magnesium alloy via the conventional approaches such as plastic working. Herein, we construct an AZ31 magnesium alloy with ultrafine grains (667.28 ± 291.35 nm) by using the spark plasma sintering of the alloy powder that has been treated via high-energy ball milling. This alloy exhibits weak grain orientation and its dislocation density is not increased compared to the precursor alloy. Benefiting from the unique microstructure, the modified AZ31 displays significantly more active behaviour with enhanced capacity during the discharge of Mg-air battery, as compared with the precursor AZ31 that has the grain size of 472.89 ± 154.31 μm. Furthermore, the impact of ultrafine grains on the discharge behaviour is also analysed based on microstructure characterization and electrochemical response.

    Emilia Olsson, Jonathon Cottom, Qiong Cai (2021)Defects in Hard Carbon: Where Are They Located and How Does the Location Affect Alkaline Metal Storage?, In: Small17(18)2007652 Wiley

    Hard carbon anodes have shown significant promise for next‐generation battery technologies. These nanoporous carbon materials are highly complex and vary in structure depending on synthesis method, precursors, and pyrolysis temperature. Structurally, hard carbons are shown to consist of disordered planar and curved motifs, which have a dramatic impact on anode performance. Here, the impact of position on defect formation energy is explored through density functional theory simulations, employing a mixed planar bulk and curved surface model. At defect sites close to the surface, a dramatic decrease (≥50%) in defect formation energy is observed for all defects except the nitrogen substitutional defect. These results confirm the experimentally observed enhanced defect concentration at surfaces. Previous studies have shown that defects have a marked impact on metal storage. This work explores the interplay between position and defect type for lithium, sodium, and potassium adsorption. Regardless of defect location, it is found that the energetic contributions to the metal adsorption energies are principally dictated by the defect type and carbon interlayer distance.

    Thomas Keal, Alin-Marin Elena, Karen Stoneham, MR Probert, Clotilde Cucinotta, Andrew Logsdail, Andrea Zen, Phil Hasnip, Ian Bush, M Watkins, Dario Alfe, Chris-Kriton Skylaris, Basile Curchod, QIONG CAI, Scott Woodley (2022)Materials and Molecular Modelling at the Exascale, In: Computing in science & engineering24(1)pp. 36-45 IEEE

    Progression of computational resources towards exascale computing makes possible simulations of unprecedented accuracy and complexity in the fields of materials and molecular modelling (MMM), allowing high fidelity in silico experiments on complex materials of real technological interest. However, this presents demanding challenges for the software used, especially the exploitation of the huge degree of parallelism available on exascale hardware, and the associated problems of developing effective workflows and data management on such platforms. As part of the UKs ExCALIBUR exascale computing initiative, the UK-led MMM Design and Development Working Group has worked with the broad MMM community to identify a set of high priority application case studies which will drive future exascale software developments. We present an overview of these case studies, categorized by the methodological challenges which will be required to realize them on exascale platforms, and discuss the exascale requirements, software challenges and impact of each application area.

    XUHUI YAO, Emilia Olsson, J Zhao, W Feng, Wen Luo, SL Tan, Meng Huang, YUNLONG ZHAO, Jianyu Huang, QIONG CAI, Liqiang Mai (2022)Voltage plateau variation in a bismuth-potassium battery, In: Journal of materials chemistry. A, Materials for energy and sustainability10(6)pp. 2917-2923 Royal Society of Chemistry

    Voltage plateau is an eigenvalue associated with the electrochemical reaction and hence a feasible tool for real-time detection of the state of charge in batteries. The variation of voltage plateau generally relates to the degradation of batteries, indicating the changes in the structure and/or the composition of electrode materials. In this work, we focus on a voltage plateau variation in a bismuth-potassium cell, investigating the electrode evolutions by two in situ methodologies. The phase transition reveals an intermediate phase in the reaction process, featuring a very limited but constant mass fraction. An irreversible structural collapse that follows leads to the exponential magnifying of the intermediate phase, taking responsibility for the distinct diffraction peaks and new voltage plateaus in the following cycles. This reaction mechanism builds the bridge between voltage plateau variation and electrode evolution, highlighting a combined effect of intermediate phase and structural collapse.

    Sean Grabe, Joseph Paul Baboo, Stephen Tennison, Teng Zhang, Constantina Lekakou, Eleftherios I Andritsos, QIONG CAI, Stephen Downes, Stephen Hinder, John F. Watts (2022)Sulfur infiltration and allotrope formation in porous cathode hosts for lithium-sulfur batteries, In: AIChE journale17638 Wiley Periodicals LLC

    We investigate sulfur infiltration and formation of lower order allotropes in heated porous hosts during fabrication of lithium-sulfur (Li-S) battery cathodes. Sulfur existence in cathode ultramicropores has been an important question for Li-S batteries, as ultramicropores reduce the polysulfides " shuttle effect " but also delay sulfur dissolution and Li + ion diffusion in the trapped solid sulfur. A novel continuum-level model is presented including heat transfer and sulfur infiltration, either from the top of a porous host or from the porous host particle surface, and taking into account the pore size distribution. A novel decay factor in modeling sulfur infiltration incorporates the pore wall repulsion energy and allotrope formation energy (predicted by density functional theory [DFT] simulations). Simulations are performed for a microporous carbon fabric host and an activated carbon powder host with bimodal micropore and macropore size distribution , with Raman and X-ray photoemission spectroscopy (XPS) spectroscopy confirming the predicted existence of linear S 6 and S 4 in ultramicropores. K E Y W O R D S lithium-sulfur batteries, melt and vapor infiltration, pore size distribution in cathode, sulfur allotropes 1 | INTRODUCTION Lithium-sulfur (Li-S) batteries have been in the research focus in the last decade because of their high theoretical energy density, 2510 Wh kg À1 , abundance and low toxicity of sulfur. 1 Intensive effort has been devoted on the cathode, where sulfur is incorporated in a porous conductive host with the aim for the host to not only enhance electronic conductivity but also allow for cathode expansion on the insertion of Li + ions and formation of polysulfides Li 2 S x during discharge. 2 Additionally, effective cathode hosts offer means to trap the sulfur and polysulfides to limit polysulfide shuttling between cathode and anode, which would cause reduction in capacity and coulombic efficiency. 3,4 Suitable microstructural architectures, including small pores, pores with necks and hollow particles, 5,6 and hosts functionalized with chemical groups with high adsorption energy to sulfur and sulfides 7 can provide such good traps. Additional specifications for host porosity and pore size distribution (PSD) target a sulfur mass greater than 5 mg cm À2 , making up at least 70% of the cathode weight, to realize the high theoretical energy density of Li-S batteries. 8 There is a vast range of porous host material candidates for Li-S battery cathodes: porous carbons, 5,6,9 activated carbon (AC) fabrics or fibers, 10,11 graphene and graphene oxide, 12–15 and

    NICOLA PIASENTIN, GUOPING LIAN, QIONG CAI (2021)Evaluation of Constrained and Restrained Molecular Dynamics Simulation Methods for Predicting Skin Lipid Permeability, In: ACS omega6(51)pp. 35363-35374 American Chemical Society

    Recently, molecular dynamics (MD) simulations have been utilized to investigate the barrier properties of human skin stratum corneum (SC) lipid bilayers. Different MD methods and force fields have been utilized, with predicted permeabilities varying by few orders of magnitude. In this work, we compare constrained MD simulations with restrained MD simulations to obtain the potential of the mean force and the diffusion coefficient profile for the case of a water molecule permeating across an SC lipid bilayer. Corresponding permeabilities of the simulated lipid bilayer are calculated via the inhomogeneous solubility diffusion model. Results show that both methods perform similarly, but restrained MD simulations have proven to be the more robust approach for predicting the potential of the mean force profile. Critical to both methods are the sampling of the whole trans-bilayer axis and the following symmetrization process. Re-analysis of the previously reported free energy profiles showed that some of the discrepancies in the reported permeability values is due to misquotation of units, while some are due to the inaccurately obtained potential of the mean force. By using the existing microscopic geometrical models via the intercellular lipid pathway, the permeation through the whole SC is predicted from the MD simulation results, and the predicted barrier properties have been compared to experimental data from the literature with good agreement.

    Jiale Yu, H. Zhang, Y Lin, Junyao Shen, Y Xie, X Huang, QIONG CAI, H-H Huang (2022)Amorphous phosphorus chalcogenide as an anode material for lithium-ion batteries with high capacity and long cycle life, In: Journal of energy chemistry68pp. 658-668 Elsevier B.V

    A novel amorphous phosphorus chalcogenide (a-P4SSe2) compound as anode material delivers excellent electrochemical performance for lithium-ion batteries, owing to its isotropic nature, synergetic effect and unique reaction mechanism. [Display omitted] The ever-increasing demands for modern energy storage applications drive the search for novel anode materials of lithium (Li)-ion batteries (LIBs) with high storage capacity and long cycle life, to outperform the conventional LIBs anode materials. Hence, we report amorphous ternary phosphorus chalcogenide (a-P4SSe2) as an anode material with high performance for LIBs. Synthesized via the mechanochemistry method, the a-P4SSe2 compound is endowed with amorphous feature and offers excellent cycling stability (over 1500 mA h g−1 capacity after 425 cycles at 0.3 A g−1), owing to the advantages of isotropic nature and synergistic effect of multielement forming Li-ion conductors during battery operation. Furthermore, as confirmed by ex situ X-ray diffraction (XRD) and transmission electron microscope (TEM), the a-P4SSe2 anode material has a reversible and multistage Li-storage mechanism, which is extremely beneficial to long cycle life for batteries. Moreover, the autogenous intermediate electrochemical products with fast ionic conductivity can facilitate Li-ion diffusion effectively. Thus, the a-P4SSe2 electrode delivers excellent rate capability (730 mA h g−1 capacity at 3 A g−1). Through in situ electrochemical impedance spectra (EIS) measurements, it can be revealed that the resistances of charge transfer (Rct) and solid electrolyte interphase (RSEI) decrease along with the formation of Li-ion conductors whilst the ohmic resistance (RΩ) remains unchanged during the whole electrochemical process, thus resulting in rapid reaction kinetics and stable electrode to obtain excellent rate performance and cycling ability for LIBs. Moreover, the formation mechanism and electrochemical superiority of the a-P4SSe2 phase, and its expansion to P4S3−xSex (x = 0, 1, 2, 3) family can prove its significance for LIBs.

    NING MAO, TENG ZHANG, Z Wang, QIONG CAI (2022)A systematic investigation of internal physical and chemical changes of lithium-ion batteries during overcharge, In: Journal of power sources518230767 Elsevier B.V

    The overcharge of lithium-ion batteries (LIBs) can not only cause irreversible battery degradation and failure but also trigger detrimental thermal runaway. This paper presents a systematic investigation of the electrical and thermal behaviors of LIBs during overcharge up to thermal runaway, and reveals the underlying physical, structural, and chemical changes at each electrode at different stages of overcharge using microscopic and spectroscopy characterizations. The overcharge process of LIBs with Li(Ni0.6Mn0.2Co0.2)O2 cathodes can be divided into four stages. Stage I involves the decomposition of LiMnO2 and LiNiO2 to MnO and NiO, respectively, accompanied by a slight collapse of the cathode. During stage II, the cathode forms a relatively stable hexagonal phase H3 while Li plating occurs at the anode surface. Moreover, NiO and Ni(OH)2 decompose into metallic Ni and release a large amount of heat. During stage III, MnO2 is formed on the cathode, which is irreversibly damaged, and unstable substance LiH is formed on the anode, which accelerates thermal runaway onset. During stage IV, the battery ruptures at approximately 174% state of charge (SOC), followed by thermal runaway in just 20 s. During overcharge, the crystallinity of the cathode decreases with the increase of SOC. Upon charging LIBs to different SOCs, the side reactions produce different substances. The red mark represents the new substances that may be produced in each stage, which participate in the next reaction, and generate abundant heat and gas, eventually leading to thermal runaway. [Display omitted] •A comprehensive and systematic investigation of LIB overcharge behavior is presented.•The decomposition mechanism of the NMC311 cathode during overcharge is speculated.•The evolution of the electrode crystal structure during LIB overcharge is examined.•A four-stage degradation and failure process during LIB overcharge is revealed.•The findings help to understand the thermal runaway behavior during LIB overcharge.

    TIANHAO YU, QIONG CAI, GUOPING LIAN, Lei Liu (2022)Molecular dynamics studies on separation of CO2/CH4 by the ionic liquids encapsulated ZIF-8, In: Journal of membrane science644120117 Elsevier B.V

    The utilisation of ZIF-8 membrane with encapsulated ionic liquids (IL@ZIF-8 composite membrane) has been demonstrated as one of the most efficient approaches for CO2/CH4 separation. However, compared to pure ZIF-8 membrane, the mechanisms behind the CO2/CH4 selectivity enhancement caused by the encapsulation of ILs still remain indistinct. Here, molecular dynamics (MD) simulations have been performed to investigate the separation and diffusion of gas molecules in the IL@ZIF-8. The simulated results show that the selectivity enhancement is because the adsorbed CO2/CH4 ratio has been increased when ILs are encapsulated into ZIF-8. Detailed analysis indicates that the diffusion of CO2/CH4 molecules follows three different pathways of the hexagonal microstructure of IL@ZIF-8 composite, of which the route to the nearest composite cells (route I) has the highest possibility, and gives the smallest mean diffusion displacement. Lastly, the effect of CO2/CH4 ratio in the initial gas mixture has been explored, and we found that CO2/CH4 selectivity is related to the concentration of CO2, where three regions with different slopes have been classified. The diffusion mechanisms of CO2/CH4 in IL@ZIF-8 composite were investigated by MD simulations, including diffusion routes and effects of CO2 concentration. [Display omitted] •The diffusion mechanisms of CO2/CH4 molecules in IL@ZIF-8 composite were fully investigated by MD simulations.•A route with minimum length for gas molecules diffusion has been identified based the mean diffusion displacement analysis.•MD simulation reveals a general relationship between the CO2/CH4 selectivity and CO2 concentration.

    Naiguang Wang, J Liang, Jingjing Liu, QIONG CAI, JUERONG LI, Jinpeng Chen, TAO HUANG, Zhicong Shi (2022)CoFe nanoparticles dispersed in Co/Fe-N-C support with meso- and macroporous structures as the high-performance catalyst boosting the oxygen reduction reaction for Al/Mg-air batteries, In: Journal of power sources517230707 Elsevier B.V

    Constructing the M-N-C (M = Fe, Co, etc.) with meso- and macroporous structures is an effective strategy for designing the high-performance catalysts towards oxygen reduction reaction (ORR). However, these structures can hardly be achieved via the pyrolysis of classic metal-organic frameworks. Herein, we prepare the CoFe nanoparticles with surface oxides dispersed in Co/Fe-N-C support via carbonizing melamine and ethylenediaminetetraacetic acid chelated with metal ions. This hybrid exhibits high proportions of meso- and macropores along with excellent ORR catalytic activity, as evidenced by the half-wave potentials of 0.91 and 0.61 V (vs. RHE) in 0.1 M KOH and 3.5 wt% NaCl, respectively. The Al/Mg-air batteries with this catalyst exhibit superior performance to those employing Pt/C, primarily derived from the enlarged pore diameters and the exposed ORR active sites substantially enhancing the air-electrode performance. Our study provides an avenue to broaden the pore sizes of M-N-C for boosting its ORR catalytic activity. •The hybrid catalyst composed of CoFe nanoparticles and Co/Fe-N-C support is prepared.•This hybrid has high ratios of meso- and macropores with exposed ORR active sites.•Excellent ORR catalytic activity is achieved due to the unique structure of hybrid.•The Al/Mg-air batteries with hybrid exhibit better performance than those with Pt/C.

    Hande Alptekin, Heather Au, Emilia Olsson, Jonathon Cottom, Anders C. S. Jensen, Thomas F Headen, QIONG CAI, Alan J. Drew, Maria Crespo-Ribadeneyra, Maria‐Magdalena Titirici (2021)Elucidation of the Solid Electrolyte Interphase Formation Mechanism in Micro‐Mesoporous Hard‐Carbon Anodes, In: Advanced materials interfaces9(8)2101267 Wiley
    UTSAB GUHAROY, TOMAS RAMIREZ REINA, JIAN LIU, Qiao Sun, SAI GU, QIONG CAI (2021)A theoretical overview on the prevention of coking in dry reforming of methane using non-precious transition metal catalysts, In: Journal of CO2 utilization53101728 Elsevier Ltd

    •Non-precious catalysts for production of syngas from CO2 dry reforming of methane.•Extensive review of Ni-based bimetallic and transition metal phosphides.•Fundamental mechanisms of anti-coking and stability of catalysts in DRM reactions.•Recommendation of future research directions in non-precious catalysts for DRM. It is worthwhile to invest in the development of CO2 reforming of methane, as it presents a promising alternative for transforming two global warming gases into a very versatile product such as syngas. A syngas rich feed gas presents extensive prospects for existing downstream industrial processes for producing valuable fuels and chemicals. The commercialization of the DRM process greatly depends upon the development of low cost, non-precious transition metal-based catalysts, to provide a desirable balance between catalytic activity and stability. In this review, the progress in the advancements of non-precious catalytic materials have been discussed from a theoretical point of view. A theoretical perspective gives an opportunity to gain fundamental information at the atomic level, such as the interaction of reaction intermediates with particular crystal facets (typically active sites in the reaction), combined with electronic structure insights, directly influencing the kinetic behaviour of the catalyst system. Theoretical insights into the DRM reaction mechanisms on non-precious Ni-based bimetallic and transition metal phosphide catalysts are extensively discussed, together with the mitigation mechanisms to avoid carbon deposition and catalyst deactivation under DRM reaction conditions. Prospects of future development of DRM are also provided, highlighting the importance of computational chemistry studies in the development of the next-generation advanced DRM catalysts.

    ELEFTHERIOS ANDRITSOS, CONSTANTINA LEKAKOU, QIONG CAI (2021)Single-Atom Catalysts as Promising Cathode Materials for Lithium–Sulfur Batteries, In: Journal of physical chemistry. C125(33)pp. 18108-18118 American Chemical Society
    ARGYRIOS KARATRANTOS, Md Sharif Khan, C Yan, Reiner Dieden, Koki Urita, T. Ohba, QIONG CAI (2021)Ion Transport in Organic Electrolyte Solutions for Lithium-ion Batteries and Beyond, In: Journal of Energy and Power Technology3(3)pp. 1-1

    The performance of metal-ion batteries at low temperatures and their fast charge/discharge rates are determined mainly by the electrolyte (ion) transport. Accurate transport properties must be evaluated for designing and/or optimization of lithium-ion and other metal-ion batteries. In this review, we report and discuss experimental and atomistic computational studies on ion transport, in particular, ion diffusion/dynamics, transference number, and ionic conductivity. Although a large number of studies focusing on lithium-ion transport in organic liquids have been performed, only a few experimental studies have been conducted in the organic liquid electrolyte phase for other alkali metals that are used in batteries (such as sodium, potassium, magnesium, etc.). Atomistic computer simulations can play a primary role and predict ion transport in organic liquids. However, to date, atomistic force fields and models have not been explored and developed exhaustively to simulate such organic liquids in quantitative agreement to experimental measurements.

    Yiyan Wang, ZHONG LI, P Zhang, Yuan Pan, Ying Zhang, QIONG CAI, S. Ravi P. Silva, JIAN LIU, G.X. Zhang, X Sun, Z Yan (2021)Flexible carbon nanofiber film with diatomic Fe-Co sites for efficient oxygen reduction and evolution reactions in wearable zinc-air batteries, In: Nano energy87106147 Elsevier Ltd

    Carbon nanofiber (CNF) papers have been widely used in many renewable energy systems, and the development of its catalytic function is of great significance and a major challenge. In this work, we pioneer a time- and cost-efficient strategy for the preparation of large-area flexible CNF films with uniformly distributed diatomic FeN3-CoN3 sites (Fe1Co1-CNF). Due to the excellent compatibility and similar functionality of the pre-designed ZnFeCo-NC precursors (ZnFeCo-pre) with the electrospun polymer polyacrylonitrile (PAN), the mixture of ZnFeCo-pre and PAN can be co-electrospun and subject to a standard CNF fabrication process. The resulting Fe1Co1-CNF exhibits excellent bifunctional catalytic performance for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), attributing to the abundant dual catalytic FeN3-CoN3 sites which are mutually beneficial for attaining optimal electronic properties for the adsorption/desorption of reaction intermediates. The assembled liquid-electrolyte ZAB provides a high specific power of 201.7 mW cm−2 and excellent cycling stability. More importantly, due to the good mechanical strength and flexibility of Fe1Co1-CNF, portable ZAB with exceptional shape deformability and stability can be demonstrated, in which Fe1Co1-CNF utility as an integrated free-standing membrane electrode. These findings provide a facile strategy for manufacturing flexible multi-functional catalytic electrodes with high production. [Display omitted] •Large-area self-standing flexible CNF film with diatomic Fe-Co sites was developed.•The diatomic Fe-Co sites render optimized adsorption of O-containing intermediates.•The Fe1Co1-CNF exhibits superior bifunctional ORR/OER performance.•The Fe1Co1-CNF shows great potential in liquid/flexible Zn-air battery.

    TIANHAO YU, Emilia Olsson, GUOPING LIAN, Lei Liu, Feng Huo, Xiangping Zhang, QIONG CAI (2021)Prediction of the Liquid–Liquid Extraction Properties of Imidazolium-Based Ionic Liquids for the Extraction of Aromatics from Aliphatics, In: Journal of chemical information and modeling61(7)pp. 3376-3385 American Chemical Society

    Liquid–liquid extraction (LLE) is an important technique to separate aromatics from aliphatics since these compounds have very similar boiling points and cannot be separated by distillation. Ionic liquids (ILs) are considered as potential extractants to extract aromatics from aliphatics. In this paper, molecular dynamics (MD) simulations were used to predict the extraction property (i.e., capacity and selectivity) of ILs for the LLE of aromatics from aliphatics. The extraction properties of seven different ILs including [C2mim]­[Tf2N], [C2mim]­[TFO], [C2mim]­[SCN], [C2mim]­[DCA], [C2mim]­[TCM], [C4mim]­[Tf2N], and [C8mim]­[Tf2N] were investigated. Results show that ILs with shorter alkyl chain cations and [Tf2N]− anion exhibit better extraction efficiency than other ILs, which is in agreement with previously reported experimental data on the extraction of toluene from aliphatics and further validated the reliability of the proposed model. The binding energies between ILs and organic molecules were calculated by the density functional theory, which help explain the different extraction behaviors of different ILs. The symmetry-adapted perturbation theory analysis was performed to further understand the interaction mechanisms between ILs and organics. Our study shows that the [Tf2N]− anion also has the best extraction capability for heavier aromatics (o-xylene, m-xylene, and p-xylene) from common aliphatics (heptane and octane). The MD modeling approach can be a low-cost in silico tool for the high-throughput fast screening of ILs for the LLE of aromatics from aliphatics.

    Emilia Olsson, Jonathon Cottom, Heather Au, Maria-Magdalena Titirici, Qiong Cai (2021)Investigating the effect of edge and basal plane surface functionalisation of carbonaceous anodes for alkali metal (Li/Na/K) ion batteries, In: Carbon177pp. 226-243 Elsevier

    Alkali metal ion batteries are instrumental in the widespread implementation of electric vehicles, portable electronics, and grid energy storage. From experimental characterisation of hard carbons, these carbon anodes were shown to contain a variety of functional groups. Through density functional theory simulations, the effect of functional groups (O, OH, NH2, and COOH) on edges and basal plane surfaces of carbonaceous materials on the adsorption of lithium, sodium, and potassium are investigated. These simulations show that the functionalisation of H-terminated edges and curved surfaces rather than basal planes is more energetically favourable and thus more likely to be present. Comparison of experimental FTIR and computational vibrational frequency analysis confirmed the occurrence of the investigated functional groups (O, OH, NH2, and COOH) in the synthesised hard carbon materials. Metal adsorption on the functionalised models showed that adsorption energies were stronger on the functionalised basal plane in comparison to the functionalised edge sites and contribute to the metal ion immobilization and consequent irreversible capacity loss. The metal adsorption on the curved surface was further improved by the addition of functional groups, benefitting the initial lithiation/sodiation/potassiation of the carbon anode. Hence, the morphology of the functionalised carbon systems plays an important role in the charge/discharge performance of carbonaceous anodes.

    James B Robinson, Kai Xi, Vasant Kumar, Andrea C Ferrari, Heather Au, Maria-Magdalena Titirici, Andres Parra-Puerto, Anthony R Kucernak, Samuel D S Fitch, Nuria Garcia-Araez, Zachary L Brown, Mauro Pasta, Liam Furness, Alexander J Kibler, Darren A Walsh, Lee R Johnson, Conrad Holc, Graham N Newton, Neil R Champness, Foivos Markoulidis, Carol Crean, Robert C T Slade, Eleftherios I Andritsos, Qiong Cai, Shumaila Babar, Teng Zhang, Constantina Lekakou, Nivedita Kulkarni, Alexander J E Rettie, Rhodri Jervis, Michael Cornish, Monica Marinescu, Gregory Offer, Zhuangnan Li, Liam Bird, Clare P Grey, Manish Chhowalla, Daniele Di Lecce, Rhodri E Owen, Thomas S Miller, Dan J L Brett, Sebastien Liatard, David Ainsworth, Paul R Shearing (2022)2021 roadmap on lithium sulfur batteries, In: JPhys Energy3(3)031501 IOP Publishing

    Batteries that extend performance beyond the intrinsic limits of Li-ion batteries are among the most important developments required to continue the revolution promised by electrochemical devices. Of these next-generation batteries, lithium sulfur (Li–S) chemistry is among the most commercially mature, with cells offering a substantial increase in gravimetric energy density, reduced costs and improved safety prospects. However, there remain outstanding issues to advance the commercial prospects of the technology and benefit from the economies of scale felt by Li-ion cells, including improving both the rate performance and longevity of cells. To address these challenges, the Faraday Institution, the UK's independent institute for electrochemical energy storage science and technology, launched the Lithium Sulfur Technology Accelerator (LiSTAR) programme in October 2019. This Roadmap, authored by researchers and partners of the LiSTAR programme, is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the LiSTAR consortium. In compiling this Roadmap we hope to aid the development of the wider Li–S research community, providing a guide for academia, industry, government and funding agencies in this important and rapidly developing research space.

    Qirui Tian, Weiqing Zhou, Qiong Cai, Guanghui Ma, Guoping Lian (2021)Concepts, processing, and recent developments in encapsulating essential oils, In: Chinese Journal of Chemical Engineering30pp. 255-271 Elsevier

    By the virtue of their olfactory, physicochemical, and biological characteristics, essential oils (EOs) have drawn wide attention as additives in daily chemicals like perfume or personal care products. Nevertheless, they are physicochemically unstable and susceptible to degradation or loss. Microencapsulation offers a feasible strategy to stabilize and prolong release of EO. This review summarizes the recognized benefits and functional properties of various preparation and characterization methods, wherein innovative fabrication strategies and their formation mechanisms are especially emphasized. Progress in combining detecting/measuring technologies with kinetic modelling are discussed, to give an integral approach of controlling the dynamic release of encapsulated EOs. Moreover, new development trends of EOs capsules are also highlighted.

    Naiguang Wang, J Liang, Jingjing Liu, Yixiang Huang, H Shao, QIONG CAI, M Hu, Zhicong Shi (2021)AS61 Magnesium Alloy with Nano-Scale Mg2Sn Phase as a Novel Anode for Primary Aqueous Magnesium Battery, In: Journal of the Electrochemical Society168(10) IOP Publishing
    Hassan Alzahrani, Christophe Antoine, Koichi Aoki, Lane Baker, Sebastien Balme, Cameron Bentley, Gourav Bhattacharya, Paul W. Bohn, Qiong Cai, Chan Cao, Daniel Commandeur, Richard M. Crooks, Martin Edwards, Andrew Ewing, Kaiyu Fu, Alina Galeyeva, Rui Gao, Thom Hersbach, Robert Hillman, Yong-Xu Hu, Lei Jiang, Frederic Kanoufi, Christine Kranz, Shaochuang Liu, Tobias Löffler, Yitao Long, Julie MacPherson, Kim McKelvey, Shelley Minteer, Michael Mirkin, Andrew Mount, Richard Nichols, Wojciech Nogala, Denis Öhl, Kaipei Qiu, Hang Ren, Jennifer Rudd, Wolfgang Schuhmann, Zuzanna Siwy, Zhongqun Tian, Patrick Unwin, Yixian Wang, Patrick Wilde, Yanfang Wu, Zhugen Yang, Yilun Ying (2018)Processes at nanoelectrodes: general discussion, In: Faraday Discussions210pp. 235-265 Royal Society of Chemistry
    Anders C. S. Jensen, Emilia Olsson, Heather Au, Hande Alptekin, Zhengqiang Yang, Stephen Cottrell, Koji Yokoyama, Qiong Cai, Maria-Magdalena Titirici, Alan J. Drew (2019)Local mobility in electrochemically inactive sodium in hard carbon anodes after the first cycle., In: Journal of Materials Chemistry A8(2)pp. 743-749 Royal Society of Chemistry

    Sodium ion batteries are a promising alternative to current lithium ion battery technology, providing relatively high capacity and good cycling stability at low cost. Hard carbons are today the anodes of choice but they suffer from poor rate performance and low initial coulombic efficiency. To improve the understanding of the kinetics of sodium mobility in these materials, muon spin rotation spectroscopy and density functional theory calculations were used to probe the intrinsic diffusion of sodium in a characteristic hard carbon sample. This revealed that atomic diffusion between sites is comparable to that observed in transition metal oxide cathode materials in sodium ion batteries, suggesting that the poor rate performance is not limited by site–site jump diffusion rates. In addition, diffusion was observed in the sodium that is irreversibly stored during the first cycle, suggesting that some of these sodium atoms are not immobilised in the solid electrolyte interface (SEI) layer but are still blocked from long range diffusion, thereby rendering the sodium electrochemically inactive.

    Polymer electrolyte membrane (PEM) fuel cells have higher efficiency and energy density and are capable of rapidly adjusting to power demands. Effective water management is one of the key issues for increasing the efficiency of PEMFC. In the current study, a three-dimensional (3D) lattice Boltzmann model is developed to simulate the water transport and oxygen diffusion in the gas diffusion layer (GDL) of PEM fuel cells with electrochemical reaction on the catalyst layer taken into account. In this paper, we demonstrate that this model is able to predict the liquid and gas flow fields within the 3D GDL structure and how they change with time. With the two-phase flow and electrochemical reaction coupled in the model, concentration of oxygen through the GDL and current density distribution can also be predicted. The model is then used to investigate the effect of microporous layer on the cell performance in 2D to reduce the computational cost. The results clearly show that the liquid water content can be reduced with the existence of microporous layer and thus the current density can be increased.

    Mehdi Choolaei, QIONG CAI, BAHMAN AMINI HORRI (2021)Green synthesis and characterisation of nanocrystalline NiO-GDC powders with low activation energy for solid oxide fuel cells, In: Ceramics international47(23)pp. 32804-32816 Elsevier Ltd

    This work reports the preparation of nanocrystalline Ni-Gd0.1Ce0.9O1.95 (NiO-GDC) anode powders using a novel single-step co-precipitation synthesis method (carboxylate route) based on ammonium tartrate as a low-cost green precipitant. The thermogravimetric analysis (TGA) of the synthesised powder showed the complete calcination/crystallisation of the resultant precipitates to take place at 500 °C. The prepared NiO-GDC powder was coated on a GDC electrolyte disc and co-sintered at 1300 °C. A mixture of La0.6Sr0.4Co0.2Fe0.8O3−δ and GDC was used as the cathode material and subsequently coated onto the anode-electrolyte bilayer, resulting in the fabrication of a NiO-GDC|GDC|La0.6Sr0.4Co0.2Fe0.8O3−δ-GDC cell. The crystallite size of both NiO and CeO2 phases were estimated using the X-ray powder diffraction (XRD) profiles and were calculated to be ~14 nm. Applied H2 temperature-programmed reduction (H2-TPR) analysis indicated a synergetic effect among different anode composites' constituents, where an intense interaction between the dispersed NiO nanocrystalline particles and the GDC crystallite phase had weakened the metal-oxygen bonds in the synthesised anode composites, resulting in a strikingly high catalytic activity at temperatures as low as 300 °C. The electrochemical impedance spectroscopy (EIS) and the electrochemical performance of the fabricated cells were measured over a broad range of operating temperatures (500–750 °C) and H2/Ar-ratios of the anode fuel (e.g. 100%–15%). Quantitative analysis from the EIS data and the application of the distribution of relaxation times (DRT) method allowed for the estimation of the activation energies of the anodic high and intermediate frequency processes that were 0.45 eV and 0.76 eV, respectively. This is the first report of a NiO-GDC synthesis, where a considerable improvement in activation energy is observed at the low-temperature region. Such low activation energies were later associated with the adsorption/desorption process of water molecules at the surface of NiO-GDC composite, indicating a high activity towards hydrogen oxidation.

    Mohammadmehdi Choolaei, Qiong Cai, Robert C.T. Slade, Bahman Amini Horri (2018)Nanocrystalline gadolinium-doped ceria (GDC) for SOFCs by an environmentally-friendly single step method, In: Ceramics International44(11)pp. 13286-13292 Elsevier

    Nanocrystalline gadolinium-doped ceria (GDC) was synthesized by a single step, low cost and environmentally friendly method using ammonium tartrate as an inexpensive, green and novel precipitant. The precipitate obtained during the process was calcined at 400 and 600 °C and the effect on the final microstructural properties of the powders of differing process variables were studied. The synthesized GDC samples were analysed using a range of different techniques, including XRD, TG/DSC, FESEM, STEM, and FT-IR and Raman spectroscopies. The thermal (TG/DSC), XRD and Raman spectroscopic analyses confirm the formation of a single crystalline phase with a cubic (fluorite) unit cell and formed at a low calcination temperature (400 °C). XRD profiles permitted estimation of crystallite sizes as

    Q Cai, NP Brandon, CS Adjiman (2010)Modelling the dynamic response of a solid oxide steam electrolyser to transient inputs during renewable hydrogen production, In: Frontiers of Energy and Power Engineering in China4(2)pp. 211-222

    Hydrogen is regarded as a leading candidate for alternative future fuels. Solid oxide electrolyser cells (SOEC) may provide a cost-effective and green route to hydrogen production especially when coupled to a source of renewable electrical energy. Developing an understanding of the response of the SOEC stack to transient events that may occur during its operation with intermittent electricity input is essential before the realisation of this technology. In this paper, a one-dimensional (1D) dynamic model of a planar SOEC stack has been employed to study the dynamic behaviour of such an SOEC and the prospect for stack temperature control through variation of the air flow rate. Step changes in the average current density from 1.0 to 0.75, 0.5 and 0.2 A/cm have been imposed on the stacks, replicating the situation in which changes in the supply of input electrical energy are experienced, or the sudden switch-off of the stack. Such simulations have been performed both for open-loop and closed-loop cases. The stack temperature and cell voltage are decreased by step changes in the average current density. Without temperature control via variation of the air flow rate, a sudden fall of the temperature and the cell potential occurs during all the step changes in average current density. The temperature excursions between the initial and final steady states are observed to be reduced by the manipulation of the air flow rate. Provided that the change in the average current density does not result in a transition from exothermic to endothermic operation of the SOEC, the use of the air flow rate to maintain a constant steady-state temperature is found to be successful. © 2010 Higher Education Press and Springer-Verlag Berlin Heidelberg.

    Q Cai, NP Brandon, CS Adjiman (2011)Investigation of the active thickness of solid oxide fuel cell electrodes using a 3D microstructure model, In: Electrochimica Acta56(28)pp. 10809-10811

    A 3D microstructure model is used to investigate the effect of the thickness of the solid oxide fuel cell (SOFC) electrode on its performance. The 3D microstructure model, which is based on 3D Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three phase boundary (TPB) length. The electrode model can also be used to perform transport and electrochemical modelling such that the performance of the synthetic electrode can be predicted. The dependence of the active electrode thickness, i.e. the region of the anode, which is electrochemically active, on operating over-potential, electrode composition and particle size is observed. Operating the electrode at an over-potential of above 200 mV results in a decrease in the active thickness with increasing over-potential. Reducing the particle size dramatically enhances the percolating TPB density and thus the performance of the electrode at smaller thicknesses; a smaller active thickness is found with electrodes made of smaller particles. Distributions of local current generation throughout the electrode reveal the heterogeneity of the 3D microstructure at the electrode/electrolyte interface and the dominant current generation in the vicinity of this interface. The active electrode thickness predicted using the model ranges from 5 μm to 15 μm, which corresponds well to many experimental observations, supporting the use of our 3D microstructure model for the investigation of SOFC electrode related phenomena. © 2011 Elsevier Ltd. All rights reserved.

    Argyrios Karatrantos, Sharif Khan, Tomonori Ohba, Qiong Cai (2018)The effect of different organic solvents on sodium ion storage in carbon nanopores, In: Physical Chemistry Chemical Physics20(9)pp. 6307-6315 Royal Society of Chemistry

    In this study fully atomistic grand canonical Monte Carlo (GCMC) simulations have been employed to study the behaviour of electrolyte salt (NaPF6) and different non-aqueous (organic) solvents in carbon nanopores, to reveal the structure and storage mechanism. Organic solutions of Na+ and PF6 - ions at 1 M concentrations were considered, based on the conditions in operational sodium ion batteries and supercapacitors. Three organic solvents with different properties are selected: ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC). The effects of solvents, pore size and surface charge were quantified by calculating the radial distribution functions and ionic density profiles. It is shown that the organic solvent properties and nanopore confinement can affect the structure of the organic electrolyte solution. For the pore size range (1-5 nm) investigated in this paper, the surface charge used in this study, can alter the sodium ions but not the solvent structure inside the pore.

    E Lorente, JA Peña, J Herguido, Q Cai, NP Brandon (2009)Conceptual design and modelling of the Steam-Iron process and fuel cell integrated system, In: International Journal of Hydrogen Energy34(13)pp. 5554-5562

    The Steam-Iron process, based on the redox reaction of iron oxides (FeO + 4H ↔ 3Fe + 4HO), is an interesting alternative to other methods of storing and generating pure hydrogen. In order to evaluate the ability of the Steam-Iron process to supply hydrogen to a solid oxide fuel cell (SOFC), a mathematical model for the oxidation process in a fixed bed reactor has been developed and is used to estimate the behaviour of the reactor under various operating conditions (e.g. amount of iron, steam flow rate, temperature). As a result of the simulations, information is provided for the preliminary design of the reactor and the selection of optimal reaction conditions. Furthermore, we have shown that the Steam-Iron reactor can be successfully integrated with an SOFC, and two system options have been explored to determine the overall system efficiency. © 2009 International Association for Hydrogen Energy.

    Q Cai, A Buts, NA Seaton, MJ Biggs (2006)Characterization of the porosity of a microporous model carbon, In: Studies in Surface Science and Catalysis160pp. 257-264 Elsevier

    The intersecting capillaries model (ICM), combined with the Monte Carlo simulation approach, was applied to characterize a computer-generated microporous "model carbon" with known structure, in order to evaluate the realism of this characterization method. The "partial" PSDs for three species (CH, CF and SF) were obtained by comparing the Monte Carlo simulated isotherms in the slit pores of the ICM with the isotherms generated from the model carbon. There is good agreement between model carbon-generated isotherms and the isotherms predicted based on the overall PSDs (by combining the partial PSDs). The overall PSD agree well with the real PSD of the model carbon in their dominant pore size range. These results support the validity and the realism of this characterization method for the characterization of porous carbons. © 2007 Elsevier B.V. All rights reserved.

    MJ Biggs, A Buts, Q Cai, NA Seaton (2006)Absolute assessment of adsorption-based microporous solid characterisation methods, In: Studies in Surface Science and Catalysis160pp. 79-86 Elsevier
    Emilia Olsson, Qiong Cai, Jonathon Cottom, Rasmus Jakobsen, Alexander L. Shluger (2019)Structural, elastic, vibrational and electronic properties of amorphous Sm₂O₃ from Ab Initio calculations, In: Computational Materials Science169109119pp. 1-8 Elsevier

    Rare earth oxides have shown great promise in a variety of applications in their own right, and as the building blocks of complex oxides. A great deal of recent interest has been focused on Sm₂O₃, which has shown significant promise as a high-k dielectric and as a ReRAM dielectric. Experimentally, these thin films range from amorphous, through partially crystalline, to poly-crystalline, dependent upon the synthetic conditions. Each case presents a set of modelling challenges that need to be defined and overcome. In this work, the problem of modelling amorphous Sm₂O₃ is tackled, developing an atomistic picture of the effect of amorphization on Sm₂O₃ from a structural and electronic structure perspective.

    The pore size distribution (PSD) and the pore-network connectivity of a porous material determine its properties in applications such as gas storage, adsorptive separations, and catalysis. Methods for the characterization of the pore structure of porous carbons are widely used, but the relationship between the structural parameters measured and the real structure of the material is not yet clear. We have evaluated two widely used and powerful characterization methods based on adsorption measurements by applying the methods to a model carbon which captures the essential characteristics of real carbons but (unlike a real material) has a structure that is completely known. We used three species (CH, CF, and SF) as adsorptives and analyzed the results using an intersecting capillaries model (ICM) which was modeled using a combination of Monte Carlo simulation and percolation theory to obtain the PSD and the pore-network connectivity. There was broad agreement between the PSDs measured using the ICM and the geometric PSD of the model carbon, as well as some systematic differences which are interpreted in terms of the pore structure of the carbon. The measured PSD and connectivity are shown to be able to predict adsorption in the model carbon, supporting the use of the ICM to characterize real porous carbons. © 2007 American Chemical Society.

    Tanveer Hussain, Emilia Olsson, Khidhir Alhameedi, Qiong Cai, Amir Karton (2020)Functionalized Two-Dimensional Nanoporous Graphene as Efficient Global Anode Materials for Li‑, Na‑, K‑, Mg‑, and Ca-Ion Batteries, In: Journal of physical chemistry. C124(18)pp. 9734-9745 American Chemical Society

    Two-dimensional nanoporous graphene (NPG) with uniformly distributed nanopores has been synthesized recently and shown remarkable electronic, mechanical, thermal, and optical properties with potential applications in several fields. Here, we explore the potential application of NPG as an anode material for Li-, Na-, K-, Mg-, and Ca-ion batteries. We use density functional theory calculations to study structural properties, defect formation energies, metal binding energies, charge analysis, and electronic structures of NPG monolayers. Pristine NPG can bind effectively K+ cations but cannot sufficiently bind the other metal cations strongly, which is a prerequisite of an efficient anode material. However, upon substitution with oxygen-rich functional groups (e.g., O, OH, and COOH) and doping with heteroatoms (B, N, P, and S), the metal binding ability of NPG is significantly enhanced. Of the considered systems, the S-doped NPG (S-NPG) binds the metal cations most strongly with binding energies of −3.87 (Li), −3.28 (Na), −3.37 (K), −3.68 (Mg), and −4.97 (Ca) eV, followed by P-NPG, O-NPG, B-NPG, and N-NPG. Of the substituted NPG systems, O-substituted NPG exhibits the strongest metal binding with binding energies of −3.30 (Li), −2.62 (Na), −2.89 (K), −1.67 (Mg), and −3.40 eV (Ca). Bader charge analysis and Roby–Gould bond indices show that a significant amount of charge is transferred from the metal cations to the functionalized NPG monolayers. Electronic properties were studied by density of states plots, and all the systems were found to be metallic upon the introduction of metal cations. These results suggest that functionalized NPG could be used as a global anode material for Li-, Na-, K-, Mg-, and Ca-ion batteries.

    Emilia Olsson, Tanveer Hussain, Amir Karton, Qiong Cai (2020)The adsorption and migration behavior of divalent metals (Mg, Ca, and Zn) on pristine and defective graphene, In: Carbon (New York)163pp. 276-287 Elsevier Ltd

    The need for sustainable and large-scale energy supply has led to significant development of renewable energy and energy storage technologies. Divalent metal ion (Mg, Ca, and Zn) batteries are promising energy storage technologies for the sustainable energy future, but the need for suitable electrode materials have limited their commercial development. This paper investigates, at the atomic scale, the adsorption and migration of Mg, Ca, and Zn on pristine and defective graphene surfaces, to bring insight into the metal storage and mobility in graphene and carbon-based anodes for divalent metal ion batteries. Such atomistic studies can help address the challenges facing the development of novel divalent metal battery technologies, and to understand the storage differences between divalent and monovalent metal-ion batteries. The adsorption of Ca on the graphene-based system is shown to be more energetically favorable than the adsorption of both Mg and Zn, with Ca showing adsorption behavior similar to the monovalent ions (Li, Na, and K). This was further investigated in terms of metal migration on the graphene surface, with much higher migration energy barriers for Ca than for Mg and Zn on the graphene systems, leading to the trapping of Ca at defect sites to a larger extent.

    Jin Bai, Xiao Chen, Emilia Olsson, Huimin Wu, Shiquan Wang, Qiong Cai, Chuanqi Feng (2020)Synthesis of Bi.sub.2S.sub.3/carbon nanocomposites as anode materials for lithium-ion batteries, In: Journal of materials science & technology50pp. 92-102 Elsevier B.V

    Metal sulfides such as Bismuth sulfide (Bi.sub.2S.sub.3) hold immense potential to be promoted as anode materials for lithium-ion batteries (LIBs), owing to their high theoretical gravimetric and volumetric capacities. However, the poor electrical conductivity and volume expansion during cycling hinder the practical applications of Bi.sub.2S.sub.3. In this work, we used pyrrole and glucose as carbon source to design the surface carbon coating on the surface of Bi.sub.2S.sub.3 particles, to improve the structural stability of Bi.sub.2S.sub.3. Two composite materials were synthesized -- Bi.sub.2S.sub.3 coated with nitrogen doped carbon (Bi.sub.2S.sub.3@NC), and Bi.sub.2S.sub.3 coated with carbon (Bi.sub.2S.sub.3@C). When used as anode active materials, both Bi.sub.2S.sub.3@NC and Bi.sub.2S.sub.3@C showed improved performance compared to Bi.sub.2S.sub.3, which confirms surface carbon coating as an effective and scalable way for the modification of Bi.sub.2S.sub.3 material. The electrode based on Bi.sub.2S.sub.3@NC materials demonstrated higher performance than that of Bi.sub.2S.sub.3@C, with an initial discharge capacity of 1126.5 mA h/g, good cycling stability (500 mA h/g after 200 cycles at 200 mA/g) and excellent rate capability. Finally, Li storage and migration mechanisms in Bi.sub.2S.sub.3 are revealed using first principle density functional theory calculations.

    Heather Au, Hande Alptekin, Anders C. S Jensen, Emilia Olsson, Christopher A O’Keefe, Thomas Smith, Maria Crespo-Ribadeneyra, Thomas F Headen, Clare P Grey, Qiong Cai, Alan J Drew, Maria-Magdalena Titirici (2020)A revised mechanistic model for sodium insertion in hard carbons, In: Energy & environmental science13(10)pp. 3469-3479 Royal Society of Chemistry

    Hard carbons have shown considerable promise as anodes for emerging sodium-ion battery technologies. Current understanding of sodium-storage behaviour in hard carbons attributes capacity to filling of graphitic interlayers and pores, and adsorption at defects, although there is still considerable debate regarding the voltages at which these mechanisms occur. Here, ex situ23Na solid-state NMR and total scattering studies on a systematically tuned series of hard carbons revealed the formation of increasingly metallic sodium clusters in direct correlation to the growing pore size, occurring only in samples which exhibited a low voltage plateau. Combining experimental results with DFT calculations, we propose a revised mechanistic model in which sodium ions store first simultaneously and continuously at defects, within interlayers and on pore surfaces. Once these higher energy binding sites are filled, pore filling occurs during the plateau region, where the densely confined sodium takes on a greater degree of metallicity.

    Hassan Alzahrani, Cameron Bentley, Rob Burrows, Chan Cao, Qiong Cai, Chrys Chikere, Richard M. Crooks, Johan Dunevall, Martin Edwards, Andrew Ewing, Rui Gao, Robert Hillman, Mohi Kahram, Frederic Kanoufi, Christine Kranz, Jean-François Lemineur, Yitao Long, Kim McKelvey, Michael Mirkin, Stacy Moore, Wojciech Nogala, Hang Ren, Wolfgang Schuhmann, Patrick Unwin, Andrea Vezzoli, Henry White, Katherine Willets, Zhugen Yang, Yilun Ying (2018)Dynamics of nanointerfaces: general discussion, In: Faraday Discussions210pp. 451-479 Royal Society of Chemistry
    Naiguang Wang, Wenping Li, Jianwen Liang, Yixiang Huang, Qiong Cai, Mingchang Hu, Yuanye Chen, Zhicong Shi (2020)δ-MnO2 nanowires supported on carbon black with oxygen-containing functional groups for enhanced electrocatalytic oxygen reduction reaction, In: Journal of alloys and compounds846156396 Elsevier B.V

    The δ-MnO2 nanowires are fabricated and chemically bonded with the carbon black that has appropriate amounts of oxygen containing functional groups. These short δ-MnO2 nanowires are (006) crystal plane-dominated and the hybrid (δ-MnO2 nanowires/carbon black) exhibits enhanced electrocatalytic activity towards oxygen reduction reaction (ORR). The half-wave potential (0.82 V (vs. RHE)) and limiting-current density (5.47 mA cm−2) of the hybrid in alkaline medium are close to those of 20 wt% Pt/C, respectively. The hybrid is used as cathodic catalyst in a Zn-air battery cell, which displays a peak power density of 138.0 mW cm−2, comparable to that using Pt/C catalyst (142.8 mW cm−2). This excellent catalytic performance is attributed to the unique microstructure of the hybrid that accelerates the kinetics of ORR. Furthermore, the ORR catalytic mechanism is also systematically analysed based on the microstructural characterization and electrochemical response.

    Tingting Zhang, Emilia Olsson, Mohammadmehdi Choolaei, Vlad Stolojan, Chuanqi Feng, Huimin Wu, Shiquan Wang, Qiong Cai (2020)Synthesis and Electrochemical Properties of Bi2MoO6/Carbon Anode for Lithium-Ion Battery Application, In: Materials13(5)1132 MDPI

    High capacity electrode materials are the key for high energy density Li-ion batteries (LIB) to meet the requirement of the increased driving range of electric vehicles. Here we report the synthesis of a novel anode material, Bi2MoO6/palm-carbon composite, via a simple hydrothermal method. The composite shows higher reversible capacity and better cycling performance, compared to pure Bi2MoO6. In 0–3 V, a potential window of 100 mA/g current density, the LIB cells based on Bi2MoO6/palm-carbon composite show retention reversible capacity of 664 mAh·g−1 after 200 cycles. Electrochemical testing and ab initio density functional theory calculations are used to study the fundamental mechanism of Li ion incorporation into the materials. These studies confirm that Li ions incorporate into Bi2MoO6 via insertion to the interstitial sites in the MoO6-layer, and the presence of palm-carbon improves the electronic conductivity, and thus enhanced the performance of the composite materials.

    © 2015 Elsevier Ltd. All rights reserved.The effective conductivity of thick-film solid oxide fuel cell (SOFC) electrodes plays a key role in their performance. It determines the ability of the electrode to transport charge to/from reaction sites to the current collector and electrolyte. In this paper, the validity of the recently proposed 3D resistor network model for the prediction of effective conductivity, the ResNet model, is investigated by comparison to experimental data. The 3D microstructures of Ni/10ScSZ anodes are reconstructed using tomography through the focused ion beam and scanning electron microscopy (FIB-SEM) technique. This is used as geometric input to the ResNet model to predict the effective conductivities, which are then compared against the experimentally measured values on the same electrodes. Good agreement is observed, supporting the validity of the ResNet model for predicting the effective conductivity of SOFC electrodes. The ResNet model is then combined with the volume-of-fluid (VOF) method to integrate the description of the local conductivity (electronic and ionic) in the prediction of electrochemical performance. The results show that the electrochemical performance is in particular sensitive to the ionic conductivity of the electrode microstructure, highlighting the importance of an accurate description of the local ionic conductivity.

    C Hu, C Kirk, Qiong Cai, C Cuadrado- Collados, J Silvestre-Albero, F Rodríguez- Reinoso,, MJ Biggs (2017)A High-Volumetric-Capacity Cathode Based on Interconnected Close-Packed N-Doped Porous Carbon Nanospheres for Long-Life Lithium–Sulfur Batteries, In: Advanced Energy Materials7(22)1701082 Wiley

    This study reports a Li–S battery cathode of high volumetric capacity enabled by novel micro- and mesostructuring. The cathode is based on monodisperse highly porous carbon nanospheres derived from a facile template- and surfactant-free method. At the mesoscale, the nanospheres structure into interconnected close-packed clusters of a few microns in extent, thus facilitating the fabrication of dense crack-free high areal sulfur loading (5 mg cm−2) cathodes with high electrical conductivity and low cathode impedance. A combination of the nitrogen doping (5 wt%), high porosity (2.3 cm3 g−1), and surface area (2900 m2 g−1) at the microscale enables high sulfur immobilization and utilization. The cathode delivers among the best reported volumetric capacity to date, above typical Li-ion areal capacity at 0.2 C over 200 cycles and low capacity fading of 0.1% per cycle at 0.5 C over 500 cycles. The compact cathode structure also ensures a low electrolyte requirement (6 µL mg−1), which aids a low overall cell weight, and further, among the best gravimetric capacities published to date as well.

    Yaxiang Lu, Lianqin Wang, Kathrin Preuß, Mo Qiao, Maria-Magdalena Titirici, John Varcoe, Qiong Cai (2017)Halloysite-derived nitrogen doped carbon electrocatalysts for anion exchange membrane fuel cells, In: Journal of Power Sources372pp. 82-90 Elsevier

    Developing the low-cost, highly active carbonaceous materials for oxygen reduction reaction (ORR) catalysts has been a high-priority research direction for durable fuel cells. In this paper, two novel N-doped carbonaceous materials with flaky and rod-like morphology using the natural halloysite as template are obtained from urea nitrogen source as well as glucose (denoted as GU) and furfural (denoted as FU) carbon precursors, respectively, which can be directly applied as metal-free electrocatalysts for ORR in alkaline electrolyte. Importantly, compared with a benchmark Pt/C (20wt%) catalyst, the as-prepared carbon catalysts demonstrate higher retention in diffusion limiting current density (after 3000 cycles) and enhanced methanol tolerances with only 50-60mV negative shift in half-wave potentials. In addition, electrocatalytic activity, durability and methanol tolerant capability of the two N-doped carbon catalysts are systematically evaluated, and the underneath reasons of the outperformance of rod-like catalysts over the flaky are revealed. At last, the produced carbonaceous catalysts are also used as cathodes in the single cell H2/O2 anion exchange membrane fuel cell (AEMFC), in which the rod-like FU delivers a peak power density as high as 703 mW cm−2 (vs. 1106 mW cm−2 with a Pt/C benchmark cathode catalyst).

    Q Cai, Z-H Huang, F Kang, J-B Yang (2004)Preparation of activated carbon microspheres from phenolic-resin by supercritical water activation, In: Carbon42(4)pp. 775-783

    Supercritical water (SCW) has been employed as an efficient activating agent for th preparation of activated carbon microspheres (P-ACS) with developed mesopores from phenolic-resin. Several processing factors that influenced the activation reaction, including activation temperature, activation duration, supercritical pressure and water flow rate were investigated. Increasing activation temperature and duration lead to larger porosity and higher specific surface area as demonstrated in the samples. Supercritical pressure change has little effect on the activation; however, there are indications that a slight increase in mesoporosity can be obtained when the pressure was raised to 36 MPa or higher. Higher water flow rate slightly enhanced the development of microporosity but had little effect on the mesoporosity. Compared with the traditional steam activation, SCW activation can produce P-ACS with more mesoporosity and higher mechanical strength. © 2004 Elsevier Ltd. All rights reserved.

    Emilia Olsson, Jonathon Cottom, Heather Au, Zhenyu Guo, Anders C. S Jensen, Hande Alptekin, Alan J Drew, Maria‐Magdalena Titirici, Qiong Cai (2020)Elucidating the Effect of Planar Graphitic Layers and Cylindrical Pores on the Storage and Diffusion of Li, Na, and K in Carbon Materials, In: Advanced functional materials30(17)1908209 Wiley

    Hard carbons are among the most promising materials for alkali-ion metal anodes. These materials have a highly complex structure and understanding the metal storage and migration within these structures is of utmost importance for the development of next-generation battery technologies. The effect of different carbon structural motifs on Li, Na, and K storage and diffusion are probed using density functional theory based on experimental characterizations of hard carbon samples. Two carbon structural models—the planar graphitic layer model and the cylindrical pore model—are constructed guided by small-angle X-ray scattering and transmission electron microscopy characterization. The planar graphitic layers with interlayer distance 6.5 Å, when the graphitic layer separation becomes so wide that there is negligible interaction between the two graphitic layers. The cylindrical pore model, reflecting the curved morphology, does not increase metal storage, but significantly lowers the metal migration barriers. Hence, the curved carbon morphologies are shown to have great importance for battery cycling. These findings provide an atomic-scale picture of the metal storage and diffusion in these materials.

    Juyan Zhang, Xuhui Yao, Ravi K Misra, Qiong Cai, Yunlong Zhao (2020)Progress in electrolytes for beyond-lithium-ion batteries, In: Journal of materials science & technology44pp. 237-257 Elsevier Ltd

    The constant increase in global energy demand and stricter environmental standards are calling for advanced energy storage technologies that can store electricity from intermittent renewable sources such as wind, solar, and tidal power, to allow the broader implementation of the renewables. The grid-oriented sodium-ion batteries, potassium ion batteries and multivalent ion batteries are cheaper and more sustainable alternatives to Li-ion, although they are still in the early stages of development. Additional optimisation of these battery systems is required, to improve the energy and power density, and to solve the safety issues caused by dendrites growth in anodes. Electrolyte, one of the most critical components in these batteries, could significantly influence the electrochemical performances and operations of batteries. In this review, the definitions and influences of three critical components (salts, solvents, and additives) in electrolytes are discussed. The significant advantages, challenges, recent progress and future optimisation directions of various electrolytes for monovalent and multivalent ions batteries (i.e. organic, ionic liquid and aqueous liquid electrolytes, polymer and inorganic solid electrolytes) are summarised to guide the practical application for grid-oriented batteries.

    Argyrios Karatrantos, Qiong Cai (2016)Design of Nanoporous Carbons as Anode Materials for Sodium (Na) Ion Batteries, In: ECS Transactions72(36)pp. 11-14 Electrochemical Society

    Molecular dynamics simulations have been employed to study the structural properties of non-aqueous (organic) electrolyte solutions confined within carbon nanopores. The effects of pore size and surface charge density were quantified by calculating ionic density profiles and concentration within the pores. Graphene slit pores of widths 0.72-10 nm were considered. The graphene surfaces were charged with densities ranging from 0 (neutral pores), -0.8e/nm2 , -1.2e/nm2 , -2e/nm2. As the surface charge density increases, more Na+ ions enter the pores. When the graphene surface is highly charged the Na+ ions are adsorbed due to counterion condensation effect.

    A two dimensional, along the channel, non-isothermal, two-phase flow, anode partial flooding model was developed to investigate the effects of relative humidity, stoichiometric flow ratio and channel length, as well as their interactive influence, on the performance of a PEM (proton exchange membrane) fuel cell. Liquid water formation and transport at the anode due to the condensation of supersaturated anode gas initiated by hydrogen consumption was considered. The model considered the heat source/ sink in terms of electrochemical reaction, Joule heating and latent heat due to water phase-transfer. The non-uniform temperature distributions inside the MEA (membrane electrode assembly) and channels at various stoichiometric flow ratios were demonstrated. The Peclet number was used to evaluate the contributions of advection and diffusion on liquid water and heat transport. Results indicated that higher anode relative humidity is required to the improved cell performance. As the decrease in the anode relative humidity and increase in channel length, the optimal cathode relative humidity was increased. The initial increase in stoichiometric flow ratio improved the limiting current densities. However, the further increases led to limited contributions. The Peclet number indicated that the liquid water transport through the electrode was mainly determined by the capillary diffusion mechanism.

    The penetration of intermittent renewable energies requires the development of energy storage technologies. High temperature electrolysis using solid oxide electrolyser cells (SOECs) as a potential energy storage technology, provides the prospect of a cost-effective and energy efficient route to clean hydrogen production. The development of optimal control strategies when SOEC systems are coupled with intermittent renewable energies is discussed. Hydrogen production is examined in relation to energy consumption. Control strategies considered include maximizing hydrogen production, minimizing SOEC energy consumption and minimizing compressor energy consumption. Optimal control trajectories of the operating variables over a given period of time show feasible control for the chosen situations. Temperature control of the SOEC stack is ensured via constraints on the overall temperature difference across the cell and the local temperature gradient within the SOEC stack, to link materials properties with system performance; these constraints are successfully managed. The relative merits of the optimal control strategies are analyzed.

    PR Shearing, Q Cai, R Clague, NP Brandon, C Adjiman, AJ Marquis, J Gelb, R Bradley, PJ Withers (2011)Microstructural characterisation of SOFC electrodes: Observations and simulations, In: ECS Transactions35(2 PART)pp. 1367-1377

    SOFC electrodes are typically porous composite materials bringing ionic, electronic and pore phases into intimate contact. These electrodes must fulfill a broad range of criteria from diffusion and electrocatalysis to mechanical support and redox tolerance. Historically design and optimisation have been largely empirical and characterisation of electrode microstructures at sub-micron length scales has been restricted to two-dimensional electron microscopy. In recent years, the development and application of focused ion beam and X-ray nano tomography tools has enabled characterisation of electrode microstructures in three dimensions providing unprecedented access to a wealth of microstructural information (see e. g [1,2]). As well as improving our understanding of existing electrode geometries, these tools have also been successfully applied to evaluate design and manufacturing strategies. With improved availability and functionality of high-resolution tomography tools, we can start to explore the effects of processing and operation on microstructure and performance. Using the unique benefits of non-destructive synchrotron X-ray nano-CT, we have explored microstructural evolution processes in-situ, using so-called "4D tomography", facilitating an improved understanding of electrode aging and durability. These tomography platforms are however most powerful when used in conjunction with relevant simulation tools [3,4]. Here we present the results of finite element simulations, exploring coupled electrochemistry and transport and stress in composite SOFC electrodes, utilising experimentally derived microstructural frameworks. ©The Electrochemical Society.

    Emilia Olsson, Guoliang Chai, Martin Dove, Qiong Cai (2019)Adsorption and migration of alkali metals (Li, Na, and K) on pristine and defective graphene surfaces, In: Nanoscale11(12)pp. 5274-5284 Royal Society of Chemistry

    In this paper, a computational study of Li, Na, and K adsorption and migration on pristine and defective graphene surfaces is conducted to gain insight into the metal storage and mobility in carbon-based anodes for alkali metal batteries. Atomic level studies of the metal adsorption and migration on the graphene surface can help address the challenges faced in the development of novel alkali metal battery technologies, as these systems act as convenient proxies of the crystalline carbon surface in carbon-based materials including graphite, hard carbons and graphene. The adsorption of Li and K ions on the pristine graphene surface is shown to be more energetically favourable than Na adsorption. A collection of defects expected to be found in carbonaceous materials are investigated in terms of metal storage and mobility, with N- and O-containing defects found to be the dominant defects on these carbon surfaces. Metal adsorption and migration at the defect sites show that defect sites tend to act as metal trapping sites, and metal diffusion around the defects is hindered when compared to the pristine surface. We identify a defect where two C sites are substituted with O and one C site with N as the dominant surface defect, and find that this defect is detrimental to metal migration and hence the battery cycling performance.

    William Perren, Arkadiusz Wojtasik, Qiong Cai (2018)Removal of Microbeads from Wastewater Using Electrocoagulation, In: ACS Omega3(3)pp. 3357-3364 American Chemical Society

    The need for better microplastic removal from wastewater streams is clear, to prevent potential harm the microplastic may cause to the marine life. This paper aims to investigate the efficacy of electrocoagulation (EC), a well-known and established process, in the unexplored context of microplastic removal from wastewater streams. This premise was investigated using artificial wastewater containing polyethylene microbeads of different concentrations. The wastewater was then tested in a 1 L stirred-tank batch reactor. The effects of the wastewater characteristics (initial pH, NaCl concentration, and current density) on removal efficiency were studied. Microbead removal efficiencies in excess of 90% were observed in all experiments, thus suggesting that EC is an effective method of removing microplastic contaminants from wastewater streams. Electrocoagulation was found to be effective with removal efficiencies in excess of 90%, over pH values ranging from 3 to 10. The optimum removal efficiency of 99.24% was found at a pH of 7.5. An economic evaluation of the reactor operating costs revealed that the optimum NaCl concentration in the reactor is between 0 and 2 g/L, mainly due to the reduced energy requirements linked to higher water conductivity. In regard to the current density, the specific mass removal rate (kg/kWh) was the highest for the lowest tested current density of 11 A/m2, indicating that low current density is more energy efficient for microbead removal.

    Rhazaoui Rhazaoui, Qiong Cai, Paul Shearing, Claire Adjiman, NP Brandon (2011)Solid Oxide Fuel Cell Electrode 3D Microstructure and Performance Modeling, In: ECS Transactions35(1)pp. 1097-1105 Electrochemical Society

    A strong correlation exists between the performance of Solid Oxide Fuel Cells (SOFCs) and their electrode microstructures, requiring an improved understanding of this relationship if more effective application-specific SOFC electrodes are to be designed. A model has been developed capable of generating a random 3D electrode microstructure and predicting its performance by analyzing structure properties such as porosity, percolation of the various phases and the length and distribution of triple phase boundaries. A Monte Carlo process is used initially to randomly position spherical particles of the three different phases, in a packed bed. Next, the pore former particles are removed. The remaining particles are then expanded uniformly to represent the sintering process, resulting in a network of particles of ionic and electronic phases overlapping each other, creating a distinctive, examinable electrode. This paper presents the impact of a range of technologically important parameters such as particle size and sintering expansion coefficient on electrode performance.

    PR Shearing, Q Cai, JI Golbert, V Yufit, CS Adjiman, NP Brandon (2010)Microstructural analysis of a solid oxide fuel cell anode using focused ion beam techniques coupled with electrochemical simulation, In: Journal of Power Sources195(15)pp. 4804-4810
    Mattia Turchi, Guoping Lian, Qiong Cai, Ian Wood, Jeremy Rabone, Massimo Noro (2017)Multi-scale modelling of solute partition equilibria of micelle-water and microemulsion-water systems using molecular dynamics and COSMOtherm, In: Computer Aided Chemical Engineering40pp. 2773-2778 Elsevier

    Complex formulations such as emulsions are widely used for enhancing the solubility and delivery of functional ingredients. Many experiments have been reported to evaluate how functional chemical compounds partition between phases of complex structures of micelles and emulsions. A great challenge is to predict these thermodynamic properties of wide chemicals. Here we explore a multi-scale approach for in-silico prediction of the partition coefficient in two steps: At first a molecular dynamic simulation (MD’s) is performed to determine the micelle and emulsion structure of the simulated system. In the second stage the predicted micellar and emulsion structure file is processed in COSMOtherm to determine the Gibbs free energy profile and so the partition coefficient of the whole structure of the aggregate. We report initial progress in predicting the micelle-water partition of a wide chemical space in a model SDS micelle system. The predicted partition coefficient is then compared to published experimental data in order to evaluate the accuracy and reliability of the methodology. Further work will be carried for real-world emulsion systems to achieve a good agreement between calculated and experimental data.

    Zhaoxia Yang, Qiong Cai, Chuanqi Feng, Huimin Wu, He Mei (2019)Synthesis and electrocatalytic activity of Ni0.85Se/MoS2 for hydrogen evolution reaction, In: INTERNATIONAL JOURNAL OF HYDROGEN ENERGY44(48)pp. 26109-26117 ELSEVIER

    Recently, the replacement of expensive platinum-based catalytic materials with nonprecious metal materials to electrolyze water for hydrogen separation has attracted much attention. In this work, Ni0.85Se, MoS2 and their composite Ni0.85Se/MoS2 with different mole ratios are prepared successfully, as electrocatalysts to catalyze the hydrogen evolution reaction (HER) in water splitting. The result shows that MoS2/Ni0.85Se with a molar ratio of Mo/Ni = 30 (denoted as M30) has the best catalytic performance towards HER, with the lowest overpotential of 118 mV at 10 mA cm -2, smallest Tafel slope of 49 mV dec -1 among all the synthesized materials. Long-term electrochemical testing shows that M30 has good stability for HER over at least 30 h. These results maybe due to the large electrochemical active surface area and high conductivity. This work shows that transition metal selenides and sulfides can form effective electrocatalyst for HER.

    Utsab Guharoy, Tomas Ramirez Reina, Sai Gu, Qiong Cai (2019)Mechanistic Insights into Selective CO2 Conversion via RWGS on Transition Metal Phosphides: A DFT Study, In: Journal of Physical Chemistry C123(37)pp. 22918-22931 American Chemical Society

    Selective conversion of CO2 to CO via the reverse water gas shift (RWGS) reaction is an attractive CO2 conversion process, which may be integrated with many industrial catalytic processes such as Fischer−Tropsch synthesis to generate added value products. The development of active and cost friendly catalysts is of paramount importance. Among the available catalyst materials, transition metal phosphides (TMPs) such as MoP and Ni2P have remained unexplored in the context of the RWGS reaction. In the present work, we have employed density functional theory (DFT) to first investigate the stability and geometries of selected RWGS intermediates on the MoP (0001) surface, in comparison to the Ni2P (0001) surface. Higher adsorption energies and Bader charges are observed on MoP (0001), indicating better stability of intermediates on the MoP (0001) surface. Furthermore, mechanistic investigation using potential energy surface (PES) profiles showcased that both MoP and Ni2P were active toward RWGS reaction with the direct path (CO2* → CO* + O*) favorable on MoP (0001), whereas the COOH-mediated path (CO2* + H* → COOH*) favors Ni2P (0001) for product (CO and H2O) gas generation. Additionally, PES profiles of initial steps to CO activation revealed that direct CO decomposition to C* and O* is favored only on MoP (0001), while H-assisted CO activation is more favorable on Ni2P (0001) but could also occur on MoP (0001). Furthermore, our DFT calculations also ascertained the possibility of methane formation on Ni2P (0001) during the RWGS process, while MoP (0001) remained more selective toward CO generation.

    Mattia Turchi, Qiong Cai, Guoping Lian (2019)In Silico Prediction of the Thermodynamic Equilibrium of Solute Partition in Multiphase Complex Fluids: A Case Study of Oil–Water Microemulsion, In: Langmuir35(33)pp. 10855-10865 American Chemical Society

    Multiphase complex fluids such as micelles, microemulsions, and dispersions are ubiquitous in product formulations of foods, pharmaceuticals, cosmetics, and fine chemicals. Quantifying how active solutes partition in the microstructure of such multiphase fluids is necessary for designing formulations that can optimally deliver the benefits of functional actives. In this paper, we at first predict the structure of a heptane/butanol/sodium dodecyl sulfate droplet in water that self-assembled to form a microemulsion through the molecular dynamics (MD) simulation and subsequently investigate the thermodynamic equilibrium of solute partitioning using COSMOmic. To our knowledge, this is the first time that the MD/COSMOmic approach is used for predicting solute partitioning in a microemulsion. The predicted partition coefficients are compared to experimental values derived from retention measurements of the same microemulsion. We show that the experimental data of droplet–water partition coefficients (Kdroplet/w) can be reliably predicted by the method that combines MD simulations with COSMOmic.

    Utsab Guharoy, Estelle Le Saché, Qiong Cai, Tomas Ramirez Reina, Sai Gu (2018)Understanding the role of Ni-Sn interaction to design highly effective CO2 conversion catalysts for dry reforming of methane, In: Journal of CO2 Utilization27pp. 1-10 Elsevier

    CO2 reforming of methane is an effective route for carbon dioxide recycling to valuable syngas. However conventional catalysts based on Ni fail to overcome the stability requisites in terms of resistance to coking and sintering. In this scenario, the use of Sn as promoter of Ni leads to more powerful bimetallic catalysts with enhanced stability which could result in a viable implementation of the reforming technology at commercial scale. This paper uses a combined computational (DFT) and experimental approach, to address the fundamental aspects of mitigation of coke formation on the catalyst’s surface during dry reforming of methane (DRM). The DFT calculation provides fundamental insights into the DRM mechanism over the mono and bimetallic periodic model surfaces. Such information is then used to guide the design of real powder catalysts. The behaviour of the real catalysts mirrors the trends predicted by DFT. Overall the bimetallic catalysts are superior to the monometallic one in terms of long-term stability and carbon tolerance. In particular, low Sn concentration on Ni surface effectively mitigate carbon formation without compromising the CO2 conversion and the syngas production thus leading to excellent DRM catalysts. The bimetallic systems also presents higher selectivity towards syngas as reflected by both DFT and experimental data. However, Sn loading has to be carefully optimized since a relatively high amount of Sn can severely deter the catalytic performance.

    Meshaal F Alebrahim, I.A. Khattab, Qiong Cai, Mohammed Sanduk (2017)Practical study on the electrochemical simultaneous removal of copper and zinc from simulated binary-metallic industrial wastewater using a packed-bed cathode, In: Egyptian Journal of Petroleum26(2)pp. 225-234 Elsevier

    In this work, electrochemical-simultaneous removal of copper and zinc from simulated binary-metallic industrial wastewater containing different ratios of copper to zinc was studied using a packed-bed continuous-recirculation flow electrolytic reactor. The total nominal initial concentration of both metals, circulating rate of flow and nominal initial pH were held constant. Parameters affecting the removal percent and current efficiency of removal, such as applied current and time of electrolysis were investigated. Results revealed that increased current intensity accelerated the removal of metals and diminish current efficiency. It was also observed that selective removal of both metals is possible when the applied current was of small intensity. Moreover, the factors that led to loss of faradaic efficiency were discussed.

    Tomas Ramirez Reina, Utsab Guharoy, Emilia Olsson, Sai Gu, Qiong Cai (2019)Theoretical insights of Ni2P (0001) surface towards its potential applicability in CO2 conversion via dry reforming of methane, In: ACS Catalysis9(4)pp. 3487-3497 American Chemical Society

    This study reports the potential application of Ni2P as highly effective catalyst for chemical CO2 recycling via dry reforming of methane (DRM). Our DFT calculations reveal that the Ni2P (0001) surface is active towards adsorption of the DRM species, with the Ni hollow site being the most energetically stable site and Ni-P and P contributes as co-adsorption sites in DRM reaction steps. Free energy analysis at 1000 K found CH-O to be the main pathway for CO formation. The competition of DRM and reverse water gas shift (RWGS) is also evidenced with the latter becoming important at relatively low reforming temperatures. Very interestingly oxygen seems to play a key role in making this surface resistant towards coking. From microkinetic analysis we have found greater oxygen surface coverage than that of carbon at high temperatures which may help to oxidize carbon deposits in continuous runs. The tolerance of Ni2P towards carbon deposition was further corroborated by DFT and micro kinetic analysis. Along with the higher probability of C oxidation we identify poor capacity of carbon diffusion on the Ni2P (0001) surface with very limited availability of C nucleation sites. Overall, this study opens new avenues for research in metal-phosphide catalysis and expands the application of these materials to CO2 conversion reactions.

    K Rhazaoui, Q Cai, CS Adjiman, NP Brandon (2014)Towards the 3D modeling of the effective conductivity of solid oxide fuel cellelectrodes – II. Computationalparameters, In: Chemical Engineering Science116pp. 781-792

    The effective conductivity of a thick-film solid oxide fuel cell (SOFC) electrode is an important characteristic used to link the microstructure of the electrode to its performance. A 3D resistor network model, the ResNet model, developed to determine the effective conductivity of a given SOFC electrode microstructure was introduced in earlier work (Rhazaoui et al., Chem. Eng. Sci. 99, 161-170, 2013). The approach is based on the discretization of each structure into voxels (small cubic elements discretizing the microstructure). In this paper, synthetic structures of increasing complexity are analyzed before an optimum discretization resolution per particle diameter is determined. The notion of Volume Elements (VEs), based on the Volume-Of-Fluid method, is then introduced in the model to allow larger structures to be modelled and is used to analyze synthetic structures as well as an experimental Ni/10ScSZ electrode. The behaviour of the model output is examined with respect to increasing aggregation resolutions for several synthetic microstructures of varying compositions, with the aid of extracted skeletonized paths of charge-conducting pathways. A ratio of VE size to voxel size of 5 is shown to be appropriate. The first comparison of calculated and measured effective conductivities is presented for the Ni/10ScSZ electrode considered. The computed effective conductivities are found to be consistent with observations made on the microstructure itself and skeletonized network paths, and support the findings of earlier work with respect to the minimum sample size required to characterize the entire anode from which it is extracted.

    Q Cai, NP Brandon, CS Adjiman (2011)Modelling the 3D microstructure and performance of solid oxide fuel cell electrodes: Computational parameters, In: Electrochimica Acta56(16)pp. 5804-5814

    In this paper, the computational parameters for a 3D model for solid oxide fuel cell (SOFC) electrodes developed to link the microstructure of the electrode to its performance are investigated. The 3D microstructure model, which is based on Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials. Once formed, the synthetic electrodes are discretized into voxels (small cubes) of equal sizes from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three-phase boundary (TPB) length. Transport phenomena and electrochemical reactions taking place within the electrode are modelled so that the performance of the synthetic electrode can be predicted. The degree of microstructure discretization required to obtain reliable microstructural analysis is found to be related to the particle sizes used for generating the structure; the particle diameter should be at least 20-40 times greater than the edge length of a voxel. The structure should also contain at least 25 discrete volumes which are called volume-of-fluid (VOF) units for the purpose of transport and electrochemical modelling. To adequately represent the electrode microstructure, the characterized volume of the electrode should be equivalent to a cube having a minimum length of 7.5 times the particle diameter. Using the modelling approach, the impacts of microstructural parameters on the electrochemical performance of the electrodes are illustrated on synthetic electrodes. © 2011 Elsevier Ltd. All rights reserved.

    L Xing, Q Cai, X Liu, C Liu, K Scott, Y Yan (2016)Anode partial flooding modelling of proton exchange membrane fuel cells: Optimisation of electrode properties and channel geometries, In: Chemical Engineering Science146pp. 88-88 Elsevier

    A two-dimensional, along-the-channel, two-phase flow, non-isothermal model is developed which represents a low temperature proton exchange membrane (PEM) fuel cell. The model describes the liquid water profiles and heat distributions inside the membrane electrode assembly (MEA) and gas flow channels as well as effectiveness factors of the catalyst layers. All the major transport and electrochemical processes are taken into account except for reactant species crossover through the membrane. The catalyst layers are treated as spherical agglomerates with inter-void spaces, which are in turn covered by ionomer and liquid water films. Liquid water formation and transport at the anode is included while water phase-transfer between vapour, dissolved water and liquid water associated with membrane/ionomer water uptake, desorption and condensation/evaporation are considered. The model is validated by experimental data and used to numerically study the effects of electrode properties (contact angel, porosity, thickness and platinum loading) and channel geometries (length and depth) on liquid water profiles and cell performance. Results reveal low liquid water saturation with large contact angle, low electrode porosity and platinum loading, and short and deep channel. An optimal channel length of 1 cm was found to maximise the current densities at low cell voltages. A novel channel design featured with multi-outlets and inlets along the channel was proposed to mitigate the effect of water flooding and improve the cell performance.

    Q Cai, A Buts, NA Seaton, MJ Biggs (2008)A pore network model for diffusion in nanoporous carbons: Validation by molecular dynamics simulation, In: Chemical Engineering Science63(13)pp. 3319-3327

    A hybrid molecular dynamics simulation/pore network model (MD/PNM) approach is developed for predicting diffusion in nanoporous carbons. This approach is computationally fast, and related to the structure of the real material. The PNM takes into account both the geometrical (a distribution of pore sizes) and topological (the pore network connectivity) characteristics of nanoporous carbons, which are obtained by analysing adsorption data. The effective diffusion coefficient is calculated by taking the transport diffusion coefficients in single slit-shaped model pores from MD simulation and then computing the effective value over the PNM. The reliability of this approach is evaluated by comparing the results of the PNM analysis with a more rigorous, but much slower, simulation applied to a realistic model material, the virtual porous carbon (VPC). We obtain good agreement between the diffusion coefficients for the PNM and the VPC, indicating the reliability of the hybrid MD/PNM method and it can be used in industry for materials design. © 2008 Elsevier Ltd. All rights reserved.

    Hassan Alzahrani, Christophe Antoine, Lane Baker, Sebastien Balme, Gourav Bhattacharya, Paul W. Bohn, Qiong Cai, Chrys Chikere, Richard M. Crooks, Naren Das, Martin Edwards, Cyril Ehi-Eromosele, Niklas Ermann, Lei Jiang, Frederic Kanoufi, Christine Kranz, Yitao Long, Julie MacPherson, Kim McKelvey, Michael Mirkin, Richard Nichols, Wojciech Nogala, Juan Pelta, Hang Ren, Jennifer Rudd, Wolfgang Schuhmann, Zuzanna Siwy, Zhongqun Tian, Patrick Unwin, Liping Wen, Henry White, Katherine Willets, Yanfang Wu, Yilun Ying (2018)Processes at nanopores and bio-nanointerfaces: general discussion, In: Faraday Discussions210pp. 145-171 Royal Society of Chemistry
    Duo Zhang, Qiong Cai, Oluwadamilola O. Taiwo, Vladimir Yufit, Nigel P Brandon, Sai Gu (2018)The effect of wetting area in carbon paper electrode on the performance of vanadium redox flow batteries: A three-dimensional lattice Boltzmann study, In: Electrochimica Acta283pp. 1806-1819 Elsevier

    The vanadium redox flow battery (VRFB) has emerged as a promising technology for large-scale storage of intermittent power generated from renewable energy sources due to its advantages such as scalability, high energy efficiency and low cost. In the current study, a three-dimensional(3D) Lattice Boltzmann model is developed to simulate the transport mechanisms of electrolyte flow, species and charge in the vanadium redox flow battery at the micro pore scale. An electrochemical model using the Butler-Volmer equation is used to provide species and charge coupling at the surface of active electrode. The detailed structure of the carbon paper electrode is obtained using X-ray Computed Tomography(CT). The new model developed in the paper is able to predict the local concentration of different species, over-potential and current density profiles under charge/discharge conditions. The simulated capillary pressure as a function of electrolyte volume fraction for electrolyte wetting process in carbon paper electrode is presented. Different wet surface area of carbon paper electrode correspond to different electrolyte volume fraction in pore space of electrode. The model is then used to investigate the effect of wetting area in carbon paper electrode on the performance of vanadium redox flow battery. It is found that the electrochemical performance of positive half cell is reduced with air bubbles trapped inside the electrode.

    Qiong Cai, NP Brandon, AWV Haw, CS Adjiman (2012)Hydrogen production through steam electrolysis: A model-based study, In: Computer Aided Chemical Engineering30pp. 257-261 Elsevier

    Hydrogen production using solid oxide electrolyser cells (SOECs) has attracted increasing research attention as it may provide a cost-effective and green route to hydrogen generation especially when coupled to a source of renewable or nuclear energy. Developing control strategies for the SOEC stack to respond to changes or disturbances that may occur during its operation is necessary to support the development and demonstration of this technology. A one-dimensional (1D) dynamic model of a planar SOEC stack developed at Imperial College has been employed to study optimal control strategies. In this paper, some preliminary results are reported for two control strategies during a change of operating regime - maximizing hydrogen production and minimizing electrical energy consumption. The results offer optimal control policies for the chosen situations and provide a good starting point for identifying the optimal control strategy in practical operation. © 2012 Elsevier B.V.

    Q Cai, NP Brandon, E Luna-Ortiz, CS Adjiman (2010)The effects of operating conditions on the performance of a solid oxide steam electrolyser: A model-based study, In: Fuel Cells10(6)pp. 1114-1128

    To support the development of hydrogen production by high temperature electrolysis using solid oxide electrolysis cells (SOECs), the effects of operating conditions on the performance of the SOECs were investigated using a one-dimensional model of a cathode-supported planar SOEC stack. Among all the operating parameters, temperature is the most influential factor on the performance of an SOEC, in terms of both cell voltage and operation mode (i.e. endothermic, thermoneutral and exothermic). Current density is another influential factor, in terms of both cell voltage and operation mode. For the conditions used in this study it is recommended that the SOEC be operated at 1,073 K and with an average current density of 10,000 A m , as this results in the stack operating at almost constant temperature along the cell length. Both the steam molar fraction at the inlet and the steam utilisation factor have little influence on the cell voltage of the SOEC but their influence on the temperature distribution cannot be neglected. Changes in the operating parameters of the SOEC can result in a transition between endothermic and exothermic operation modes, calling for careful temperature control. The introduction of air into the anode stream appears to be a promising approach to ensure small temperature variations along the cell. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

    K Rhazaoui, Q Cai, NP Brandon, CS Adjiman (2013)Towards the 3D modeling of the effective conductivity of solid oxide fuel cell electrodes: I. Model development, In: Chemical Engineering Science99pp. 161-170

    The effective conductivity of a thick-film solid oxide fuel cell (SOFC) electrode is an important characteristic used to link the microstructure of the electrode to its performance. A 3D resistor network model that has been developed to determine the effective conductivity of a given SOFC electrode microstructure, the Resistor Network or ResNet model, is introduced in this paper. The model requires the discretization of a 3D microstructure into voxels, based on which a mixed resistor network is drawn. A potential difference is then applied to this network and yields the corresponding currents, allowing the equivalent resistance and hence conductivity of the entire structure to be determined. An overview of the ResNet modeling methodology is presented. The approach is general and can be applied to structures of arbitrary complexity, for which appropriate discretization resolutions are required. The validity of the model is tested by applying it to a set of model structures and comparing calculated effective conductivity values against analytical results. © 2013 Elsevier Ltd.

    Q Cai, Z-H Huang, F-Y Kang (2005)Comparative study of phenolic resin-based activated carbons by means of supercritical water activation and steam activation, In: Xinxing Tan Cailiao/New Carbon Materials20(2)pp. 122-128

    A new activation method, supercritical water activation (650°C, 32 Pa), and a traditional method, steam activation (650°C), were used to prepare phenolic resin based activated carbons. Based on pore structure characterization of the samples by nitrogen adsorption and weight loss behavior of the starting materials by TG/ DSC analysis, the effects of the two different activation methods and the degree of carbonization of the starting materials on the evolution of the pore structure of phenolic resin-based activated carbons were obtained. Results show that: (1) supercritical water activation benefits the development of mesoporosity, while steam activation benefits the development of microporosity; (2) activated carbons with high specific surface area and mesoporosity were obtained at a low degree of burn-off from phenolic resin-based carbons carbonized to a low degree.

    Duo Zhang, Antonio Bertei, Farid Tariq, Nigel Brandon, Qiong Cai (2019)Progress in 3D electrode microstructure modelling for fuel cells and batteries: Transport and electrochemical performance, In: Progress in Energy1(1)012003pp. 1-35 IOP Publishing

    Electrode microstructure plays an important role in the performance of electrochemical energy devices including fuel cells and batteries. Building a clear understanding of how the performance is affected by the electrode microstructure is necessary to design the optimal electrode microstructure, to achieve better device performance. 3D microstructure modelling enables us to perform simulations directly on a 3D electrode microstructure and thus link structure with performance. This paper provides an extensive review on the current state of the art in 3D microstructure modelling of transport and electrochemical performance for four promising electrochemical energy technologies: solid oxide fuel cells (SOFCs), proton exchange membrane fuel cells (PEMFCs), redox flow batteries (RFBs) and lithium ion batteries (LIBs). Each technology has different electrode microstructures and processes, and thus presents different challenges. The most commonly used modelling methods including the finite element method (FEM) and the finite volume method (FVM) are reviewed, together with the developing lattice Boltzmann method (LBM), with the advantages and disadvantages of each method revealed. Whilst FEM and FVM have been extensively applied in simulating SOFC and LIB electrodes where the methods are capable of dealing with single phase (gas or liquid) transport, they face challenges in simulating the multiphase phenomenon present in PEMFC and some RFB electrodes. LBM is, on the other hand, well suited in simulating gas-liquid two phase flow and applications in PEMFCs and RFBs, as well as single-phase phenomenon in SOFCs and LIBs. The review also points to current challenges in 3D microstructure modelling, including the simulations of nanoscale gas transport and phase transition, moving interfaces associated with structural changes, accurate reactions kinetics, experimental validation, and how to make 3D microstructure modelling truly impactful through the design of better electrochemical devices.

    Q Cai, DJL Brett, NP Brandon, D Browning (2010)A sizing-design methodology for hybrid fuel cell power systems and its application to an unmanned underwater vehicle, In: Journal of Power Sources195(19)pp. 6559-6569

    Hybridizing a fuel cell with an energy storage unit (battery or supercapacitor) combines the advantages of each device to deliver a system with high efficiency, low emissions, and extended operation compared to a purely fuel cell or battery/supercapacitor system. However, the benefits of such a system can only be realised if the system is properly designed and sized, based on the technologies available and the application involved. In this work we present a sizing-design methodology for hybridisation of a fuel cell with a battery or supercapacitor for applications with a cyclic load profile with two discrete power levels. As an example of the method's application, the design process for selecting the energy storage technology, sizing it for the application, and determining the fuel load/range limitations, is given for an unmanned underwater vehicle (UUV). A system level mass and energy balance shows that hydrogen and oxygen storage systems dominate the mass and volume of the energy system and consequently dictate the size and maximum mission duration of a UUV. © 2010 Elsevier B.V. All rights reserved.

    M. S. Khan, A. V. Karatrantos, T. Ohba, Q. Cai (2019)The effect of different organic solvents and anion salts on sodium ion storage in cylindrical carbon nanopores, In: Physical Chemistry Chemical Physics Royal Society of Chemistry

    In this article fully atomistic Molecular Dynamics simulations were employed to study the behaviour of electrolyte salts (NaPF6, NaBF4, and NaTFSI) and different organic solvents (PC, EC, and EMC) in cylindrical carbon nanotubes, in order to reveal the storage mechanism. Organic solutions at 1 M concentrations were considered in bulk reservoir solutions, at the operational condition of sodium ion batteries. The effects of the solvents, nanotube diameter, and different anions (PF6 -, BF4-, and TFSI-) are quantified by calculating the number of ions inside the nanotubes, solvation number and radial distribution functions. The solvent, anion and cylindrical nanoconfinement can influence the organic electrolyte solution structure.

    Yash Boyjoo, Haodong Shi, Emilia Olsson, Qiong Cai, Zhong‐Shuai Wu, Jian Liu, Gao Qing (Max) Lu (2020)Molecular‐Level Design of Pyrrhotite Electrocatalyst Decorated Hierarchical Porous Carbon Spheres as Nanoreactors for Lithium–Sulfur Batteries, In: Advanced energy materials10(20)2000651 Wiley

    Lithium–sulfur batteries (LSBs) are a class of new‐generation rechargeable high‐energy‐density batteries. However, the persisting issue of lithium polysulfides (LiPs) dissolution and the shuttling effect that impedes the efficiency of LSBs are challenging to resolve. Herein a general synthesis of highly dispersed pyrrhotite Fe1−xS nanoparticles embedded in hierarchically porous nitrogen‐doped carbon spheres (Fe1−xS‐NC) is proposed. Fe1−xS‐NC has a high specific surface area (627 m2 g−1), large pore volume (0.41 cm3 g−1), and enhanced adsorption and electrocatalytic transition toward LiPs. Furthermore, in situ generated large mesoporous pores within carbon spheres can accommodate high sulfur loading of up to 75%, and sustain volume variations during charge/discharge cycles as well as improve ionic/mass transfer. The exceptional adsorption properties of Fe1−xS‐NC for LiPs are predicted theoretically and confirmed experimentally. Subsequently, the electrocatalytic activity of Fe1−xS‐NC is thoroughly verified. The results confirm Fe1−xS‐NC is a highly efficient nanoreactor for sulfur loading. Consequently, the Fe1−xS‐NC nanoreactor performs extremely well as a cathodic material for LSBs, exhibiting a high initial capacity of 1070 mAh g−1 with nearly no capacity loss after 200 cycles at 0.5 C. Furthermore, the resulting LSBs display remarkably enhanced rate capability and cyclability even at a high sulfur loading of 8.14 mg cm−2.

    Q Cai, MJ Biggs, NA Seaton (2008)Effect of pore wall model on prediction of diffusion coefficients for graphitic slit pores, In: Physical Chemistry Chemical Physics10(18)pp. 2519-2527

    The effect of the pore wall model on the self-diffusion coefficient and transport diffusivity predicted for methane in graphitic slit pores by equilibrium molecular dynamics (EMD) and non-equilibrium MD (NEMD) is investigated. Three pore wall models are compared-a structured wall and a smooth (specular) wall, both with a thermostat applied to the fluid to maintain the desired temperature, and a structured wall combined with the diffuse thermalizing scattering algorithm of MacElroy and Boyle (Chem. Eng. J., 1999, 74, 85). Pore sizes ranging between 7 and 35 Å and five pressures in the range of 1-40 bar are considered. The diffuse thermalizing wall yields incorrect self-diffusion coefficients and transport diffusivities for the graphitic slit pore model and should not be used. Surprisingly, the smooth specular wall gives self-diffusion coefficients inline with those obtained using the structured wall, indicating that this computationally much faster wall can be used for studying this phenomenon provided the fluid-wall interactions are somewhat weaker than the fluid-fluid interactions. The structured wall is required, however, if the transport diffusivity is of interest. © the Owner Societies.

    Duo Zhang, Antoni Forner-Cuenca, Oluwadamilola O. Taiwo, Vladimir Yufit, Fikile R. Brushett, Nigel P. Brandon, Sai Gu, Qiong Cai (2020)Understanding the role of the porous electrode microstructure in redox flow battery performance using an experimentally validated 3D pore-scale lattice Boltzmann model, In: Journal of Power Sources447227249 Elsevier

    The porous structure of the electrodes in redox flow batteries (RFBs) plays a critical role in their performance. We develop a framework for understanding the coupled transport and reaction processes in electrodes by combining lattice Boltzmann modelling (LBM) with experimental measurement of electrochemical performance and X-ray computed tomography (CT). 3D pore-scale LBM simulations of a non-aqueous RFB are conducted on the detailed 3D microstructure of three different electrodes (Freudenberg paper, SGL paper and carbon cloth) obtained using X-ray CT. The flow of electrolyte and species within the porous structure as well as electrochemical reactions at the interface between the carbon fibers of the electrode and the liquid electrolyte are solved by a lattice Boltzmann approach. The simulated electrochemical performances are compared against the experimental measurements with excellent agreement, indicating the validity of the LBM simulations for predicting the RFB performance. Electrodes featuring one single dominant peak (i.e., Freudenberg paper and carbon cloth) show better electrochemical performance than the electrode with multiple dominant peaks over a wide pore size distribution (i.e., SGL paper), whilst the presence of a small fraction of large pores is beneficial for pressure drop. This framework is useful to design electrodes with optimal microstructures for RFB applications.

    Argyrios Karatrantos, Qiong Cai (2016)Effect of pore size and surface charge on Na ion storage in carbon nanopores, In: Physical Chemistry Chemical Physics

    Na ion batteries (NIBs) are considered as a promising low cost and sustainable energy storage technology. To better design nanoporous carbons as anode materials for NIBs, molecular dynamics simulations have been employed to study the behavior of Na+ ions (as well as PF6- ions) confined within carbon nanopores, in the presence of non aqueous (organic) solvent. The effects of pore size and surface charge density were quantified by calculating ionic density profiles and concentration within the pores. Carbon slit pores of widths 0.72-10 nm were considered. The carbon surfaces were charged with densities ranging from 0 (neutral pores), -0.8e/nm2 , -1.2e/nm2 , -2e/nm2 . Organic solutions of Na+ and PF6− at 1M concentrations were considered at operating sodium ion batteries conditions. As the surface charge density increases, more Na+ ions enter the pores. In all pores, when the surface is highly charged the Na+ ions move toward the negatively charged graphene surfaces because of counterion condensation effects. In some instances our results reveal the formation of multiple layers of adsorbed Na+ inside the pores. Both nanopore width and surface charge alter the density profiles of ions and solvent inside the pores.

    Josh Bates, CONSTANTINA LEKAKOU, QIONG CAI, FOIVOS MARKOULIDIS, Robert Prof Slade, Gareth Hinds (2019)Modelling and Simulations of Energy Storage Devices

    In electrochemistry, numerical models are used to predict the activity of energy storage devices such as batteries and supercapacitors. Novel battery technologies, such as lithium-sulphur batteries, benefit from simulation studies in optimising their materials, and more specifically in this study, their porous cathodes. Porous carbon is typically used as the electrode in different supercapacitor configurations, as well as the cathode structural material in Li-S batteries. Previous models in the literature simulate the porous electrodes with a single uniform pore size. In this project a novel model has been devised, incorporating multiple pore sizes of the electrode material, determined from a pore size distribution.

    Mattia Turchi, Qiong Cai, Guoping Lian (2019)An evaluation of in-silico methods for predicting solute partition in multiphase complex fluids – A case study of octanol/water partition coefficient, In: Chemical Engineering Science197pp. 150-158 Elsevier

    Solute partition in multiphase fluids is an important thermodynamic phenomenon and performance attribute for a wide range of product formulations of foods, pharmaceuticals and cosmetics. Experimental evaluation of partition coefficients in complex product formulations is empirical, difficult and time consuming. In-silico methods such as fragment constant method and group contribution method require parameter fitting to the experimental data and are limited to relatively simple fluids. Recently, a method combining molecular dynamics (MD) and quantum chemical (QC) calculation of screening charge density function has been reported. The method does not only use fundamental properties of intermolecular force and charge density function, which does not require parameter fitting to the experimental data, but also applies to complex fluid structures such as micelles. In this work, the predictive accuracy of the combined method of MD and QC is evaluated. Using widely available octanol-water partition coefficients as a case study, the performance of the combined MD and COSMOmic for predicting octanol/water partition coefficients has been compared with those of the EPI Suite™ fragment constant method, UNIFAC group contribution method and COSMOtherm. The prediction of the combined MD/COSMOmic method is the closest to the best performing fragment constant method which was specifically designed for the octanol-water system. The combined MD/QC method proves to be the most promising and robust method applicable to a wide range of complex structures of multiphase fluid systems.