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).
Cai Q, Brandon NP, Adjiman CS (2011) Modelling the 3D microstructure and performance of solid oxide fuel cell electrodes: Computational parameters, Electrochimica Acta 56 (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.
Shearing PR, Cai Q, Clague R, Brandon NP, Adjiman C, Marquis AJ, Gelb J, Bradley R, Withers PJ (2011) Microstructural characterisation of SOFC electrodes: Observations and simulations, ECS Transactions 35 (2 PART 2) 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.
Rhazaoui K, Cai Q, Adjiman CS, Brandon NP (2014) Towards the 3D modeling of the effective conductivity of solid oxide fuel cellelectrodes ? II. Computationalparameters, Chemical Engineering Science 116 pp. 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.
Cai Q, Brandon NP, Luna-Ortiz E, Adjiman CS (2010) The effects of operating conditions on the performance of a solid oxide steam electrolyser: A model-based study, Fuel Cells 10 (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.
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.
Rhazaoui K, Cai Q, Brandon NP, Adjiman CS (2013) Towards the 3D modeling of the effective conductivity of solid oxide fuel cell electrodes: I. Model development, Chemical Engineering Science 99 pp. 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.
Cai Q, Buts A, Seaton NA, Biggs MJ (2008) A pore network model for diffusion in nanoporous carbons: Validation by molecular dynamics simulation, Chemical Engineering Science 63 (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.
Cai Q, Brett DJL, Brandon NP, Browning D (2010) A sizing-design methodology for hybrid fuel cell power systems and its application to an unmanned underwater vehicle, Journal of Power Sources 195 (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.
Cai Q, Adjiman CS, Brandon NP (2014) Optimal control strategies for hydrogen production when coupling solid oxide electrolysers with intermittent renewable energies, Journal of Power Sources 268 pp. 212-224
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.
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/cm2 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.
Cai Q, Huang Z-H, Kang F, Yang J-B (2004) Preparation of activated carbon microspheres from phenolic-resin by supercritical water activation, Carbon 42 (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.
Cai Q, Buts A, Biggs MJ, Seaton NA (2007) Evaluation of methods for determining the pore size distribution and pore-network connectivity of porous carbons, Langmuir 23 (16) pp. 8430-8440
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 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 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.
Cai Q, Huang Z-H, Kang F-Y (2005) Comparative study of phenolic resin-based activated carbons by means of supercritical water activation and steam activation, Xinxing Tan Cailiao/New Carbon Materials 20 (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.
Lorente E, Peña JA, Herguido J, Cai Q, Brandon NP (2009) Conceptual design and modelling of the Steam-Iron process and fuel cell integrated system, International Journal of Hydrogen Energy 34 (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.
Shearing PR, Cai Q, Golbert JI, Yufit V, Adjiman CS, Brandon NP (2010) Microstructural analysis of a solid oxide fuel cell anode using focused ion beam techniques coupled with electrochemical simulation, Journal of Power Sources 195 (15) pp. 4804-4810
Cai Q, Biggs MJ, Seaton NA (2008) Effect of pore wall model on prediction of diffusion coefficients for graphitic slit pores, Physical Chemistry Chemical Physics 10 (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.
Rhazaoui K, Cai Q, Kishimoto M, Tariq F, Somalu MR, Adjiman CS, Brandon NP (2015) Towards the 3D modelling of the effective conductivity of solid oxide fuel cell electrodes - Validation against experimental measurements and prediction of electrochemical performance, Electrochimica Acta 168 pp. 139-147
© 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.
Cai Q, Brandon NP, Adjiman CS (2011) Investigation of the active thickness of solid oxide fuel cell electrodes using a 3D microstructure model, Electrochimica Acta 56 (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 ¼m to 15 ¼m, 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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
Alzahrani Hassan, Antoine Christophe, Baker Lane, Balme Sebastien, Bhattacharya Gourav, Bohn Paul W., Cai Qiong, Chikere Chrys, Crooks Richard M., Das Naren, Edwards Martin, Ehi-Eromosele Cyril, Ermann Niklas, Jiang Lei, Kanoufi Frederic, Kranz Christine, Long Yitao, MacPherson Julie, McKelvey Kim, Mirkin Michael, Nichols Richard, Nogala Wojciech, Pelta Juan, Ren Hang, Rudd Jennifer, Schuhmann Wolfgang, Siwy Zuzanna, Tian Zhongqun, Unwin Patrick, Wen Liping, White Henry, Willets Katherine, Wu Yanfang, Ying Yilun (2018) Processes at nanopores and bio-nanointerfaces: general discussion, Faraday Discussions 210 pp. 145-171
Royal Society of Chemistry
Alzahrani Hassan, Bentley Cameron, Burrows Rob, Cao Chan, Cai Qiong, Chikere Chrys, Crooks Richard M., Dunevall Johan, Edwards Martin, Ewing Andrew, Gao Rui, Hillman Robert, Kahram Mohi, Kanoufi Frederic, Kranz Christine, Lemineur Jean-François, Long Yitao, McKelvey Kim, Mirkin Michael, Moore Stacy, Nogala Wojciech, Ren Hang, Schuhmann Wolfgang, Unwin Patrick, Vezzoli Andrea, White Henry, Willets Katherine, Yang Zhugen, Ying Yilun (2018) Dynamics of nanointerfaces: general discussion, Faraday Discussions 210 pp. 451-479
Royal Society of Chemistry
Alzahrani Hassan, Antoine Christophe, Aoki Koichi, Baker Lane, Balme Sebastien, Bentley Cameron, Bhattacharya Gourav, Bohn Paul W., Cai Qiong, Cao Chan, Commandeur Daniel, Crooks Richard M., Edwards Martin, Ewing Andrew, Fu Kaiyu, Galeyeva Alina, Gao Rui, Hersbach Thom, Hillman Robert, Hu Yong-Xu, Jiang Lei, Kanoufi Frederic, Kranz Christine, Liu Shaochuang, Löffler Tobias, Long Yitao, MacPherson Julie, McKelvey Kim, Minteer Shelley, Mirkin Michael, Mount Andrew, Nichols Richard, Nogala Wojciech, Öhl Denis, Qiu Kaipei, Ren Hang, Rudd Jennifer, Schuhmann Wolfgang, Siwy Zuzanna, Tian Zhongqun, Unwin Patrick, Wang Yixian, Wilde Patrick, Wu Yanfang, Yang Zhugen, Ying Yilun (2018) Processes at nanoelectrodes: general discussion, Faraday Discussions 210 pp. 235-265
Royal Society of Chemistry
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
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.
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.
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.