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.
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.
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.
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.
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.
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
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 are beneficial for metal storage, but do not have significant contribution to rapid metal diffusion. Fast diffusion is shown to take place in 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.
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.
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.
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.
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.
Multiphase complex fluids such as micelles, microemulsions and dispersions are ubiquitous in product formulations of food, 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 thesis, in-silico methods are employed for the prediction of solutes partition coefficient in complex microstructures. Recently, the method that combines molecular dynamics (MD) and the Conductor like Screening Model for Micellar systems (COSMOmic) has been reported for modelling complex fluid structures. In this research work, at first the MD/COSMOmic approach is tested for predicting the partition coefficients of neutral solutes in simple octanol/water system and compared with other in-silico methods including EPI-suite, COSMOtherm and UNIFAC. After validating the MD/COSMOmic for the octanol/water system, the approach is further used for predicting the partition coefficient of neutral solutes in sodium dodecyl sulfate (SDS)/heptane/butanol in water microemulsion. The MD/COSMOmic approach is also tested for predicting the partition coefficients of both neutral and charged solutes in mixed micelles of sodium laureth ether sulfate (SLES) and fatty acids. While the combined MD/COSMOmic shows good accuracy in predicting the partition coefficient of neutral solutes, it fails in predicting the partition coefficients of charged solutes. In order to obtain more accurate prediction for the charged solutes, the method that combines steered molecular dynamic (SMD) simulation and umbrella sampling (US) was employed using the latest polarizable force field (CHARMM-Drude). This approach shows to accurately predict the partition coefficients of the charged solutes.
It has become increasingly important to control carbon dioxide (CO¬2) emissions and at the same time generate fuel sources to meet the growing global energy consumption need. CO2 (dry) reforming of methane (DRM) is a viable process as it generates fuel (syngas) and utilises greenhouse (CH4 and CO2) gas at the same time. The success of this process relies on the development of suitable noble-metal free catalysts. First principle’s based computational methods, such as density functional theory (DFT), has become a powerful predictive tool for catalyst development in modern science. Therefore the main objective of this thesis work has been to investigate suitable catalysts using computational methods for gas–phase CO2 utilisation reactions. In this research work, DFT calculations provided us with the fundamental insights into the DRM mechanism over bimetallic Sn/ Ni (111) periodic model surfaces. This analysis showed that low Sn concentration on Ni surface effectively mitigates carbon formation without compromising the CO2 conversion and the syngas production, showcasing superior characteristics of the bimetallic catalyst towards carbon tolerance stability. Other heterogeneous catalysts such as Ni2P and MoP have also been studied in this thesis. Theoretical analysis of DRM reaction on the unexplored nickel phosphide Ni2P (0001) surface showcased suitable syngas production under DRM reaction temperatures with low carbon deposition formation on the surface. This was mainly attributed to a lower number of active sites available for carbon adsorption compared to oxygen on the Ni2P (0001) surface. DFT study on activation of CO2 and CO on MoP (0001) and Ni2P (0001) surfaces showcased selective CO production from CO2 to be possible on both the surfaces. Further, direct CO activation is favoured on the MoP (0001) surface. Surface bounded oxygen removal on Ni2P (0001) is reasonably favourable. Findings from this thesis work will be beneficial in developing more robust catalysts for gas phase CO2 utilisation reactions and could contribute to a better understanding of CO2 conversion processes, catalysts deactivation and thus helping to develop new families of powerful catalysts for a greener society
Die filling is a critical process step in many industries. The majority of powders or powder mixtures utilised in these industries are fine powders. Powder properties dictate the die filling rate. Die filling may cause segregation in a powder blend, which can, for example, have the detrimental effect of changing the concentration of an API in a medicinal tablet.
Using a wide range of powders, the effect of die size and die orifice shape, were investigated, as well as die orientation and aspect ratio of non-axis symmetrical dies. Passive and active die filling were compared, and the effect of varying shoe length during active die filling was also studied. Hopper flow and die filling were modelled using physical equations. The predictions of these equations were compared to the Beverloo Equations. Furthermore, size and density induced segregation of binary powder mixtures during die filling was studied.
The mass flow rate of the small die was lower than the large die even when normalised by die orifice surface area due to the trapped air dampening the flow rate more for orifices with small surface area. Higher critical filling speeds were achieved with all powders when using broad non-axis symmetrical dies in the parallel orientation. For die filling in a rotary die filling system, an empirical correlation for the prediction of the weight in the die at a given shoe speed and the critical filling speeds of any powder was developed. For hopper flow and passive die filling, physical equations were developed that were able to predict the mass flow rate of a wide range of powder more accurately than the Beverloo Equation. Finally, two segregation mechanisms were identified (sieving segregation and fluidization segregation) to occur during die filling and the speed of the shoe affected the balance between the two mechanisms.
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.
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.
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.
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.
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.
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.
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).
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
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.
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.
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
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.
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.
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.
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, 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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
© 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.
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.
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 < 20 nm, which was further confirmed by STEM and FESEM micrographs indicating the formation of quasi-spherical particles with uniform particle sizes in the range 10–30 nm. This study will aid understanding of effects of process variables on the properties of doped metal-oxide powders prepared using the carboxylate route
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.