Imperial College London

DrGregoryOffer

Faculty of EngineeringDepartment of Mechanical Engineering

Reader in Mechanical Engineering
 
 
 
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Contact

 

+44 (0)20 7594 7072gregory.offer Website

 
 
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Location

 

720City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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84 results found

Hales A, Diaz LB, Marzook MW, Zhao Y, Patel Y, Offer Get al., 2019, The cell cooling coefficient: A standard to define heatrejection from lithium-ion batteries, Journal of The Electrochemical Society, Vol: 166, Pages: A2383-A2395, ISSN: 0013-4651

Lithium-ion battery development is conventionally driven by energy and power density targets, yet the performance of a lithium-ion battery pack is often restricted by its heat rejection capabilities. It is therefore common to observe elevated cell temperatures and large internal thermal gradients which, given that impedance is a function of temperature, induce large current inhomogeneities and accelerate cell-level degradation. Battery thermal performance must be better quantified to resolve this limitation, but anisotropic thermal conductivity and uneven internal heat generation rates render conventional heat rejection measures, such as the Biot number, unsuitable. The Cell Cooling Coefficient (CCC) is introduced as a new metric which quantifies the rate of heat rejection. The CCC (units W.K−1) is constant for a given cell and thermal management method and is therefore ideal for comparing the thermal performance of different cell designs and form factors. By enhancing knowledge of pack-wide heat rejection, uptake of the CCC will also reduce the risk of thermal runaway. The CCC is presented as an essential tool to inform the cell down-selection process in the initial design phases, based solely on their thermal bottlenecks. This simple methodology has the potential to revolutionise the lithium-ion battery industry.

Journal article

Campbell I, Gopalakrishnan K, Marinescu M, Torchio M, Offer G, Raimondo Det al., 2019, Optimising lithium-ion cell design for plug-in hybrid and battery electric vehicles, Journal of Energy Storage, Vol: 22, Pages: 228-238, ISSN: 2352-152X

Increased driving range and enhanced fast charging capabilities are two immediate goals of transport electrification. However, these are of competing nature, leading to increased energy and power demand respectively from the on-board battery pack. By fine-tuning the number of layers versus active electrode material of a lithium ion pouch cell, tailored designs targeting either of these goals can be obtained. Achieving this trade-off through iterative empirical testing of layer choices is expensive and often produces sub-optimal designs. This paper presents a model-based methodology for determining the optimal number of layers, maximising usable energy whilst satisfying specific acceleration and fast charging targets. The proposed methodology accounts for the critical need to avoid lithium plating during fast charging and searches for the optimal layer configuration considering a range of thermal conditions. A numerical implementation of a cell model using a hybrid finite volume-spectral scheme is presented, wherein the model equations are suitably reformulated to directly accept power inputs, facilitating rapid and accurate searching of the layer design space. Electrode materials exhibiting high solid phase diffusion rates are highlighted as being equally as important for extended range as the development of new materials with higher inherent capacity. The proposed methodology is demonstrated for the common module design of a battery pack in a plug-in hybrid vehicle, thereby illustrating how the cost of derivative vehicle models can be reduced. To facilitate model based layer optimisation, the open-source toolbox, BOLD (Battery Optimal Layer Design) is provided.

Journal article

Wild M, Offer G, 2019, Lithium-Sulfur Batteries, Publisher: Wiley, ISBN: 9781119297864

The book contains a review of battery design and: Discusses electrochemistry of Li-S batteries and the analytical techniques used to study Li-S batteries Offers information on the application of Li-S batteries for commercial use Distills ...

Book

Campbell ID, Marzook M, Marinescu M, Offer GJet al., 2019, How Observable Is Lithium Plating? Differential Voltage Analysis to Identify and Quantify Lithium Plating Following Fast Charging of Cold Lithium-Ion Batteries, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, Vol: 166, Pages: A725-A739, ISSN: 0013-4651

Journal article

Hua X, Zhang T, Offer GJ, Marinescu Met al., 2019, Towards online tracking of the shuttle effect in lithium sulfur batteries using differential thermal voltammetry, Journal of Energy Storage, Vol: 21, Pages: 765-772

© 2019 Lithium sulfur (Li-S) batteries are an important next generation high energy density battery technology. However, the phenomenon known as the polysulfide shuttle causes accelerated degradation, reduced Coulombic efficiency and increased heat generation, particularly towards the end of charge. The real-time detection of the onset of shuttle during charge would improve the safety and increase cycle life of Li-S batteries in real applications. In this study, we demonstrate that the Differential Thermal Voltammetry (DTV) technique can be used for tracking shuttle during Li-S charging. By combining voltage and temperature measurements, DTV is shown to be sensitive to the magnitude of shuttle. We demonstrate significant differences in the DTV curves for Li-S cells charged at different currents and temperatures. Quantitative interpretations of the experimental DTV curves are performed through a thermally-coupled zero-dimensional Li-S model. The DTV technique, together with the model, is a promising tool for real-time detection of shuttle in applications, to inform control algorithms for deciding the end of charging, thus preventing excessive degradation and charge inefficiency.

Journal article

Zhao Y, Spingler FB, Patel Y, Offer GJ, Jossen Aet al., 2019, Localized swelling inhomogeneity detection in lithium ion cells using multi-dimensional laser scanning, Journal of The Electrochemical Society, Vol: 166, Pages: A27-A34, ISSN: 1945-7111

The safety, performance and lifetime of lithium-ion cells are critical for the acceptance of electric vehicles (EVs) but the detection of cell quality issues non-destructively is difficult. In this work, we demonstrate the use of a multi-dimensional laser scanning method to detect local inhomogeneities. Commercially available cells with Nickel Cobalt Manganese (NMC) cathode are cycled at various charge and discharge rates, while 2D battery displacement measurements are taken using the laser scanning system. Significant local swelling points are found on the cell during the discharge phase, the magnitude of swelling can be up to 2% of the cell thickness. The results show that the swelling can be aggravated by a combination of slow charge rate and fast discharge rate. Disassembly of the cells shows that the swelling points are matched with the location of ‘adhesive-like’ material found on the electrode surfaces. Scanning Electron Microscope (SEM) images show that the material is potentially blocking the electrodes and separators at these locations. We therefore present laser-scanning displacement as a valuable tool for defect/inhomogeneity detection.

Journal article

Zhao Y, Patel Y, Zhang T, Offer GJet al., 2018, Modeling the effects of thermal gradients induced by tab and surface cooling on lithium ion cell performance, Journal of The Electrochemical Society, Vol: 165, Pages: A3169-A3178, ISSN: 1945-7111

Lithium ion batteries are increasingly important in large scale applications where thermal management is critical for safety and lifetime. Yet, the effect of different thermal boundary conditions on the performance and lifetime is still not fully understood. In this work, a two-dimensional electro-thermal model is developed to simulate cell performance and internal states under complex thermal boundary conditions. Attention was paid to model, not only the electrode stack but also the non-core components (e.g. tab weld points) and thermal boundaries, but also the experiments required to parameterize the thermal model, and the reversible heat generation. The model is comprehensively validated and the performance of tab and surface cooling strategies was evaluated across a wide range of operating conditions. Surface cooling was shown to keep the cell at a lower average temperature, but with a large thermal gradient for high C rates. Tab cooling provided much smaller thermal gradients but higher average temperatures caused by lower heat removing ability. The thermal resistance between the current collectors and tabs was found to be the most significant heat transfer bottleneck and efforts to improve this could have significant positive impacts on the performance of li-ion batteries considering the other advantages of tab cooling.

Journal article

Zhang X-F, Zhao Y, Liu H-Y, Zhang T, Liu W-M, Chen M, Patel Y, Offer GJ, Yan Yet al., 2018, Degradation of thin-film lithium batteries characterised by improved potentiometric measurement of entropy change, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 20, Pages: 11378-11385, ISSN: 1463-9076

Journal article

Merla Y, Wu B, Yufit V, Martinez-Botas RF, Offer GJet al., 2018, An easy-to-parameterise physics-informed battery model and its application towards lithium-ion battery cell design, diagnosis, and degradation, Journal of Power Sources, Vol: 384, Pages: 66-79, ISSN: 0378-7753

Accurate diagnosis of lithium ion battery state-of-health (SOH) is of significant value for many applications, to improve performance, extend life and increase safety. However, in-situ or in-operando diagnosis of SOH often requires robust models. There are many models available however these often require expensive-to-measure ex-situ parameters and/or contain unmeasurable parameters that were fitted/assumed. In this work, we have developed a new empirically parameterised physics-informed equivalent circuit model. Its modular construction and low-cost parametrisation requirements allow end users to parameterise cells quickly and easily. The model is accurate to 19.6 mV for dynamic loads without any global fitting/optimisation, only that of the individual elements. The consequences of various degradation mechanisms are simulated, and the impact of a degraded cell on pack performance is explored, validated by comparison with experiment. Results show that an aged cell in a parallel pack does not have a noticeable effect on the available capacity of other cells in the pack. The model shows that cells perform better when electrodes are more porous towards the separator and have a uniform particle size distribution, validated by comparison with published data. The model is provided with this publication for readers to use.

Journal article

Mazur CM, Offer G, Contestabile MSM, Brandon Net al., 2018, Comparing the effects of vehicle automation, policy making and changed user preferences on the uptake of electric cars and emissions from transport, Sustainability, Vol: 10, ISSN: 2071-1050

Switching energy demand for transport from liquid fuels to electricity is the most promising way to significantly improve air quality and reduce transport emissions. Previous studies have shown this is possible, that by 2035 the economics of alternative powertrain and energy vectors will have converged. However, they don’t address if the transition is likely or plausible. Using the UK as a case study, we present a systems dynamics model based study informed by transition theory and explore the effects of technology progress, policy making, user preferences and; for the first time, automated vehicles on this transition. We are not trying to predict the future, but to highlight what is necessary in order for different scenarios to become more or less likely. Worryingly we show that current policies with the expected technology progress and expectations of vehicle buyers are insufficient to reach global targets. Faster technology progress, strong financial incentives or a change in vehicle buyer expectations are crucial, but still insufficient. In contrast the biggest switch to alternatively fuelled vehicles could be achieved by the introduction of automated vehicles. The implications will affect policy makers, automotive manufactures, technology developers and broader society.

Journal article

Few SPM, Schmidt O, Offer GJ, Brandon N, Nelson J, Gambhir Aet al., 2018, Prospective improvements in cost and cycle life of off-grid lithium-ion battery packs: An analysis informed by expert elicitations, Energy Policy, Vol: 114, Pages: 578-590, ISSN: 0301-4215

This paper presents probabilistic estimates of the 2020 and 2030 cost and cycle life of lithium-ion battery (LiB) packs for off-grid stationary electricity storage made by leading battery experts from academia and industry, and insights on the role of public research and development (R&D) funding and other drivers in determining these. By 2020, experts expect developments to arise chiefly through engineering, manufacturing and incremental chemistry changes, and expect additional R&D funding to have little impact on cost. By 2030, experts indicate that more fundamental chemistry changes are possible, particularly under higher R&D funding scenarios, but are not inevitable. Experts suggest that significant improvements in cycle life (eg. doubling or greater) are more achievable than in cost, particularly by 2020, and that R&D could play a greater role in driving these. Experts expressed some concern, but had relatively little knowledge, of the environmental impact of LiBs. Analysis is conducted of the implications of prospective LiB improvements for the competitiveness of solar photovoltaic + LiB systems for off-grid electrification.

Journal article

Marinescu M, O'Neill L, Zhang T, Walus S, Wilson T, Offer Get al., 2017, Irreversible vs Reversible Capacity Fade of Lithium-Sulfur Batteries during Cycling: The Effects of Precipitation and Shuttle, Journal of The Electrochemical Society, Vol: 165, Pages: A6107-A6118, ISSN: 1945-7111

Lithium-sulfur batteries could deliver significantly higher gravimetric energy density and lower cost than Li-ion batteries. Their mass adoption, however, depends on many factors, not least on attaining a predictive understanding of the mechanisms that determine their performance under realistic operational conditions, such as partial charge/discharge cycles. This work addresses a lack of such understanding by studying experimentally and theoretically the response to partial cycling. A lithium-sulfur model is used to analyze the mechanisms dictating the experimentally observed response to partial cycling. The zero-dimensional electrochemical model tracks the time evolution of sulfur species, accounting for two electrochemical reactions, one precipitation/dissolution reaction with nucleation, and shuttle, allowing direct access to the true cell state of charge. The experimentally observed voltage drift is predicted by the model as a result of the interplay between shuttle and the dissolution bottleneck. Other features are shown to be caused by capacity fade. We propose a model of irreversible sulfur loss associated with shuttle, such as caused by reactions on the anode. We find a reversible and an irreversible contribution to the observed capacity fade, and verify experimentally that the reversible component, caused by the dissolution bottleneck, can be recovered through slow charging. This model can be the basis for cycling parameters optimization, or for identifying degradation mechanisms relevant in applications. The model code is released as Supplementary material B.

Journal article

Ardani MI, Patel Y, Siddiq A, Offer GJ, Martinez-Botas RFet al., 2017, Combined experimental and numerical evaluation of the differences between convective and conductive thermal control on the performance of a lithium ion cell, Energy, Vol: 144, Pages: 81-97, ISSN: 0360-5442

Testing of lithium ion batteries is necessary in order to understand their performance, to parameterise and furthermore validate models to predict their behaviour. Tests of this nature are normally conducted in thermal/climate chambers which use forced air convection to distribute heat. However, as they control air temperature, and cannot easily adapt to the changing rate of heat generated within a cell, it is very difficult to maintain constant cell temperatures. This paper describes a novel conductive thermal management system which maintains cell temperature reliably whilst also minimising thermal gradients. We show the thermal gradient effect towards cell performance is pronounced below operating temperature of 25 °C at 2-C discharge under forced air convection. The predicted internal cell temperature can be up to 4 °C hotter than the surface temperature at 5 °C ambient condition and eventually causes layers to be discharge at different current rates. The new conductive method reduces external temperature deviations of the cell to within 1.5 °C, providing much more reliable data for parameterising a thermally discretised model. This method demonstrates the errors in estimating physiochemical paramet ers; notably diffusion coefficients, can be up to four times smaller as compared to parameterisation based on convective test data.

Journal article

Hunt I, Zhang T, Patel Y, Marinescu M, Purkayastha R, Kovacik P, Walus S, Swiatek A, Offer GJet al., 2017, The effect of current inhomogeneity on the performance and degradation of Li-S batteries, Journal of the Electrochemical Society, Vol: 165, Pages: A6073-A6080, ISSN: 0013-4651

The effect of thermal gradients on the performance and cycle life of Li-S batteries is studied using bespoke single-layer Li-S cells, with isothermal boundary conditions maintained by Peltier elements. A temperature difference is shown to cause significant current imbalance between parallel connected single-layer cells, causing the hotter cell to provide more charge and discharge capacities during cycling. During charge, significant shuttle is induced in the hotter Li-S cell, causing accelerated degradation of it. A bespoke multi-tab cell in which the inner layers are electrically connected to different tabs versus the outer layers, is used to demonstrate that noticeable current inhomogeneity occurs during the operation of practical multilayer Li-S pouch cells, which is expected to affect their performance and degradation. The observed thermal and current inhomogeneity should have a direct consequence on battery pack and thermal management system design for real world Li-S battery packs.

Journal article

Shibagaki T, Merla Y, Offer GJ, 2017, Tracking degradation in lithium iron phosphate batteries using differential thermal voltammetry, Journal of Power Sources, Vol: 374, Pages: 188-195, ISSN: 0378-7753

Diagnosing the state-of-health of lithium ion batteries in-operando is becoming increasingly important for multiple applications. We report the application of differential thermal voltammetry (DTV) to lithium iron phosphate (LFP) cells for the first time, and demonstrate that the technique is capable of diagnosing degradation in a similar way to incremental capacity analysis (ICA). DTV has the advantage of not requiring current and works for multiple cells in parallel, and is less sensitive to temperature introducing errors. Cells were aged by holding at 100% SOC or cycling at 1C charge, 6D discharge, both at an elevated temperature of 45 °C under forced air convection. Cells were periodically characterised, measuring capacity fade, resistance increase (power fade), and DTV fingerprints. The DTV results for both cells correlated well with both capacity and power, suggesting they could be used to diagnose SOH in-operando for both charge and discharge. The DTV peak-to-peak capacity correlated well with total capacity fade for the cycled cell, suggesting that it should be possible to estimate SOC and SOH from DTV for incomplete cycles within the voltage hysteresis region of an LFP cell.

Journal article

Cleaver T, Kovacik P, Marinescu M, Zhang T, Offer Get al., 2017, Perspective—commercializing lithium sulfur batteries: Are we doing the right research?, Journal of The Electrochemical Society, Vol: 165, Pages: A6029-A6033, ISSN: 0013-4651

A picture of the challenges faced by the lithium-sulfur technology and the activities pursued by the research community to solve them is synthesized based on 1992 scientific articles. It is shown that, against its own advice of adopting a balanced approach to development, the community has instead focused work on the cathode. To help direct future work, key areas of neglected research are highlighted, including cell operation studies, modelling, anode, electrolyte and production methods, as well as development goals for real world target applications such as high altitude unmanned aerial vehicles.

Journal article

Zhao Y, Patel Y, Hunt IA, Kareh KM, Holland AA, Korte C, Dear JP, Yue Y, Offer GJet al., 2017, Preventing lithium ion battery failure during high temperatures by externally applied compression, Journal of Energy Storage, Vol: 13, Pages: 296-303

© 2017 Elsevier Ltd Lithium-ion cells can unintentionally be exposed to temperatures outside manufacturers recommended limits without triggering a full thermal runaway event. The question addressed in this paper is: Are these cells still safe to use? In this study, externally applied compression has been employed to prevent lithium ion battery failure during such events. Commercially available cells with Nickel Cobalt Manganese (NCM) cathodes were exposed to temperatures at 80 °C, 90 °C and 100 °C for 10 h, and electrochemically characterised before and after heating. The electrode stack structures were also examined using x-ray computed tomography (CT), and post-mortems were conducted to examine the electrode stack structure and surface changes. The results show that compression reduces capacity loss by −0.07%, 4.95% and 13.10% respectively, measured immediately after the thermal testing. The uncompressed cells at 80 °C showed no swelling, whilst 90 °C and 100 °C showed significant swelling. The X-ray CT showed that the uncompressed cell at 100 °C suffered de-lamination at multiple locations after test, and precipitations were found on the electrode surface. The post-mortem results indicates the compressed cell at 100 °C was kept tightly packed, and the electrode surface was uniform. The conclusion is that externally applied compression reduces delamination due to gas generation during high temperature excursions.

Journal article

Wu B, Offer G, 2017, Environmental impact of hybrid and electric vehicles, Environmental Impacts of Road Vehicles : Past, Present and Future, Editors: Harrison, Hester, Publisher: Royal Society of Chemistry

Hybrid and electric vehicles play a critical role in reducing global greenhouse gas emissions, with transport estimated to contribute to 14% of the 49 GtCO2eq produced annually. Analysis of only the conversion efficiency of powertrain technologies can be misleading, with pure battery electric and hybrid vehicles reporting average efficiencies of 92% and 35% in comparison with 21% for internal combustion engine vehicles. A fairer comparison would be to consider the well-to-wheel efficiency, which reduces the numbers to 21–67%, 25% and 12%, respectively. The large variation in well-to-wheel efficiency of pure battery electric vehicles highlights the importance of renewable energy generation in order to achieve true environmental benefits. When calculating the energy return on investment of the various technologies based on the current energy generation mix, hybrid vehicles show the greatest environmental benefits, although this would change if electricity was made with high amounts of renewables. In an extreme scenario with heavy coal generation, the CO2eq return on investment can actually be negative for pure electric vehicles, highlighting the importance of renewable energy generation further. The energy impact of production is generally small (∼6% of lifetime energy) and, similarly, recycling is of a comparable magnitude, but it is less well studied.

Book chapter

Zhang T, Marinescu M, Walus S, Kovacik P, Offer GJet al., 2017, What Limits the Rate Capability of Li-S Batteries during Discharge: Charge Transfer or Mass Transfer?, Journal of the Electrochemical Society, Vol: 165, Pages: A6001-A6004, ISSN: 0013-4651

Li-S batteries exhibit poor rate capability under lean electrolyte conditions required for achieving high practical energy densities. In this contribution, we argue that the rate capability of commercially-viable Li-S batteries is mainly limited by mass transfer rather than charge transfer during discharge. We first present experimental evidence showing that the charge-transfer resistance of Li-S batteries and hence the cathode surface covered by Li2S are proportional to the state-of-charge (SoC) and not to the current, directly contradicting previous theories. We further demonstrate that the observed Li-S behaviors for different discharge rates are qualitatively captured by a zero-dimensional Li-S model with transport-limited reaction currents. This is the first Li-S model to also reproduce the characteristic overshoot in voltage at the beginning of charge, suggesting its cause is the increase in charge transfer resistance brought by Li2S precipitation.

Journal article

Walus S, Offer GJ, Hunt I, Patel Y, Stockley T, Williams J, Purkayastha Ret al., 2017, Volumetric expansion of Lithium-Sulfur cell during operation – Fundamental insight into applicable characteristics, Energy Storage Materials, Vol: 10, Pages: 233-245, ISSN: 2405-8297

During the operation of a Lithium-Sulfur (Li-S) cell, structural changes take place within both positive and negative electrodes. During discharge, the sulfur cathode expands as solid products (mainly Li2S or Li2S/Li2S2) are precipitated on its surface, whereas metallic Li anode contracts due to Li oxidation/stripping. The opposite processes occur during charge, where Li anode tends to expand due to lithium plating and solid precipitates from the cathode side are removed, causing its thickness to decrease. Most research literature describe these processes as they occur within single electrode cell constructions. Since a large format Li-S pouch cell is composed of multiple layers of electrodes stacked together, and antagonistic effects (i.e. expansion and shrinkage) occur simultaneously during both charge and discharge, it is important to investigate the volumetric changes of a complete cell. Herein, we report for the first time the thickness variation of a Li-S pouch cell prototype. In these studies we used a laser gauge for monitoring the cell thickness variation under operation. The effects of different voltage windows as well as discharge regimes are explored. It was found that the thickness evolution of a complete pouch cell is mostly governed by Li anodes volume changes, which mask the response of the sulfur cathodes. Interesting findings on cell swelling when cycled at slow currents and full voltage windows are presented. A correlation between capacity retention and cell thickness variation is demonstrated, which could be potentially incorporated into Battery Management System (BMS) design for Li-S batteries.

Journal article

Gopalakrishnan K, Zhang T, Offer GJ, 2017, A fast, memory-efficient discrete-time realization algorithm for reduced-order li-ion battery models, Journal of Electrochemical Energy Conversion and Storage, Vol: 14, ISSN: 2381-6872

Research into reduced-order models (ROM) for Lithium-ion batteries is motivated by the need for a real-time embedded model possessing the accuracy of physics-based models, while retaining computational simplicity comparable to equivalent-circuit models. The discrete-time realization algorithm (DRA) proposed by Lee et al. (2012, "One-Dimensional Physics-Based Reduced-Order Model of Lithium-Ion Dynamics," J. Power Sources, 220, pp. 430-448) can be used to obtain a physics-based ROM in standard state-space form, the time-domain simulation of which yields the evolution of all the electrochemical variables of the standard pseudo-2D porous-electrode battery model. An unresolved issue with this approach is the high computation requirement associated with the DRA, which needs to be repeated across multiple SoC and temperatures. In this paper, we analyze the computational bottleneck in the existing DRA and propose an improved scheme. Our analysis of the existing DRA reveals that singular value decomposition (SVD) of the large Block-Hankel matrix formed by the system's Markov parameters is a key inefficient step. A streamlined DRA approach that bypasses the redundant Block-Hankel matrix formation is presented as a drop-in replacement. Comparisons with existing DRA scheme highlight the significant reduction in computation time and memory usage brought about by the new method. Improved modeling accuracy afforded by our proposed scheme when deployed in a resource-constrained computing environment is also demonstrated.

Journal article

Zhang X-F, Zhao Y, Patel Y, Zhang T, Liu W-M, Chen M, Offer GJ, Yan Yet al., 2017, Potentiometric measurement of entropy change for lithium batteries, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 19, Pages: 9833-9842, ISSN: 1463-9076

Journal article

Parkes MA, Tompsett DA, d'Avezac M, Offer GJ, Brandon NP, Harrison NMet al., 2016, The atomistic structure of yttria stabilised zirconia at 6.7 mol%: an ab initio study., Physical Chemistry Chemical Physics, Vol: 18, Pages: 31277-31285, ISSN: 1463-9084

Yttria stabilized zirconia (YSZ) is an important oxide ion conductor used in solid oxide fuel cells, oxygen sensing devices, and for oxygen separation. Doping pure zirconia (ZrO2) with yttria (Y2O3) stabilizes the cubic structure against phonon induced distortions and this facilitates high oxide ion conductivity. The local atomic structure of the dopant is, however, not fully understood. X-ray and neutron diffraction experiments have established that, for dopant concentrations below 40 mol% Y2O3, no long range order is established. A variety of local structures have been suggested on the basis of theoretical and computational models of dopant energetics. These studies have been restricted by the difficulty of establishing force field models with predictive accuracy or exploring the large space of dopant configurations with first principles theory. In the current study a comprehensive search for all symmetry independent configurations (2857 candidates) is performed for 6.7 mol% YSZ modelled in a 2 × 2 × 2 periodic supercell using gradient corrected density functional theory. The lowest energy dopant structures are found to have oxygen vacancy pairs preferentially aligned along the ⟨210⟩ crystallographic direction in contrast to previous results which have suggested that orientation along the ⟨111⟩ orientation is favourable. Analysis of the defect structures suggests that the Y(3+)-Ovac interatomic separation is an important parameter for determining the relative configurational energies. Current force field models are found to be poor predictors of the lowest energy structures. It is suggested that the energies from a simple point charge model evaluated at unrelaxed geometries is actually a better descriptor of the energy ordering of dopant structures. Using these observations a pragmatic procedure for identifying low energy structures in more complicated material models is suggested. Calculation of the oxygen vacancy migration activat

Journal article

Gupta G, Wu B, Mylius S, Offer GJet al., 2016, A systematic study on the use of short circuiting for the improvement of proton exchange membrane fuel cell performance, International Journal of Hydrogen Energy, Vol: 42, Pages: 4320-4327, ISSN: 1879-3487

Proton exchange membrane fuel cells suffer from reversible performance loss during operation caused by the oxidation of the Pt catalyst which in turn reduces the electrochemically active surface area. Many fuel cell manufacturers recommend using short circuiting during the operation of the fuel cell to improve the performance of the cells over time. However, there is lack of understanding on how it improves the performance as wellas on how to optimise the short circuiting strategy for different fuel cell systems. We present a simple procedure to develop an optimised short circuiting strategy by maximising the cumulative average power density gain and minimising the time required to recover the energy loss during short circuiting. We obtained average voltage improvement from 10 to 12% at different current densities for a commercial H-100 system and our short circuiting strategy showed ~2% voltage improvement in comparison to a commercial strategy. We also demonstrated that the minimum short circuiting time is a function of double layer capacitance by the use of electrochemical impedance spectroscopy.

Journal article

Zhang T, Marinescu M, Walus S, Offer GJet al., 2016, Modelling transport-limited discharge capacity of lithium-sulfur cells, Electrochimica Acta, Vol: 219, Pages: 502-508, ISSN: 0013-4686

Lithium-sulfur (Li-S) battery could bring a step-change in battery technology with a potential specific energy density of 500 - 600 Wh/kg. A key challenge for further improving the specific energy-density of Li-S cells is to understand the mechanisms behind reduced sulfur utilisation at low electrolyte loadings and high discharge currents. While several Li-S models have been developed to explore the discharge mechanisms of Li-S cells, they so far fail to capture the discharge profiles at high currents. In this study, we propose that the slow ionic transport in concentrated electrolyte is limiting the rate capability of Li-S cells. This transport-limitation mechanism is demonstrated through a one-dimensional Li-S model which qualitatively captures the discharge capacities of a sulfolane-based Li-S cell at different currents. Furthermore, our model predicts that a discharged Li-S cell is able regain some capacity with a short period of relaxation. This capacity recovery phenomenon is validated experimentally for different discharge currents and relaxation durations. The transport-limited discharge behavior of Li-S cells highlights the importance of optimizing the electrolyte loading and electrolyte transport property in Li-S cells.

Journal article

Merla Y, Wu B, Yufit V, Brandon NP, Martinez-Botas R, Offer Get al., 2016, Extending battery life: A low-cost practical diagnostic technique for lithium-ion batteries, Journal of Power Sources, Vol: 331, Pages: 224-231, ISSN: 1873-2755

Modern applications of lithium-ion batteries such as smartphones, hybrid & electric vehiclesand grid scale electricity storage demand long lifetime and high performance which typicallymakes them the limiting factor in a system. Understanding the state-of-health during operationis important in order to optimise for long term durability and performance. However, thisrequires accurate in-operando diagnostic techniques that are cost effective and practical. Wepresent a novel diagnosis method based upon differential thermal voltammetry demonstratedon a battery pack made from commercial lithium-ion cells where one cell was deliberately agedprior to experiment. The cells were in parallel whilst being thermally managed with forced airconvection. We show for the first time, a diagnosis method capable of quantitativelydetermining the state-of-health of four cells simultaneously by only using temperature and 2voltage readings for both charge and discharge. Measurements are achieved using low-costthermocouples and a single voltage measurement at a frequency of 1Hz, demonstrating thefeasibility of implementing this approach on real world battery management systems. Thetechnique could be particularly useful under charge when constant current or constant power iscommon, this therefore should be of significant interest to all lithium-ion battery users.

Journal article

Kroupa M, Offer GJ, Kosek J, 2016, Modelling of Supercapacitors: Factors Influencing Performance, Journal of the Electrochemical Society, Vol: 163, Pages: A2475-A2487, ISSN: 0013-4651

The utilizable capacitance of Electrochemical Double Layer Capacitors (EDLCs) is a function of the frequency at which they are operated and this is strongly dependent on the construction and physical parameters of the device. We simulate the dynamic behavior of an EDLC using a spatially resolved model based on the porous electrode theory. The model of Verbrugge and Liu (J. Electrochem. Soc. 152, D79 (2005)) was extended with a dimension describing the transport into the carbon particle pores. Our results show a large influence of the electrode thickness (Le), separator thickness (Ls) and electrolyte conductivity (κ) on the performance of EDLCs. In agreement with experimental data, the time constant was an increasing function of Le and Ls and a decreasing function of κ. The main limitation was found to be on the scale of the whole cell, while transport into the particles became a limiting factor only if the particle size was unrealistically large. The results were generalized into a simplified relation allowing for a quick evaluation of performance for the design of new devices. This work provides an insight into the performance limitation of EDLCs and identifies the critical parameters to consider for both systems engineers and material scientists.

Journal article

McCarthy N, Chen R, Offer GJ, Thring Ret al., 2016, PTFE mapping in gas diffusion media for PEMFCs using fluorescence microscopy, International Journal of Hydrogen Energy, Vol: 41, Pages: 17631-17643, ISSN: 1879-3487

Differentiating between the various polytetrafluoroethylene based structures inside polymer electrolyte membrane fuel cells with a degree of certainty is necessary to optimize manufacturing processes and to investigate possible degradation mechanisms. We have developed a novel method using fluorescence microscopy for distinguishing the origin and location of PTFE and/or Nafion® in Membrane Electrode assemblies and the gas diffusion media from different sources and stages of processing. Fluorescent material was successfully diffused into the PTFE based structures in the GDM by addition to the ‘ink’ precursor for both the microporous layer and the catalyst layer; this made it possible to map separately both layers in a way that has not been reported before. It was found that hot pressing of membrane coated structures resulted in physical dispersion of those layers away from the membrane into the GDM itself. This fluorescence technique should be of interest to membrane electrode assembly manufacturers and fuel cell developers and could be used to track the degradation of different PTFE structures independently in the future.

Journal article

Propp K, Marinescu M, Auger DJ, O'Neill L, Fotouhi A, Somasundaram K, Offer GJ, Minton G, Longo S, Wild M, Knap Vet al., 2016, Multi-temperature state-dependent equivalent circuit discharge model for lithium-sulfur batteries, Journal of Power Sources, Vol: 328, Pages: 289-299, ISSN: 1873-2755

Lithium-sulfur (Li-S) batteries are described extensively in the literature, but existing computational models aimed at scientific understanding are too complex for use in applications such as battery management. Computationally simple models are vital for exploitation. This paper proposes a non-linear state-of-charge dependent Li-S equivalent circuit network (ECN) model for a Li-S cell under discharge. Li-S batteries are fundamentally different to Li-ion batteries, and require chemistry-specific models. A new Li-S model is obtained using a ‘behavioural’ interpretation of the ECN model; as Li-S exhibits a ‘steep’ open-circuit voltage (OCV) profile at high states-of-charge, identification methods are designed to take into account OCV changes during current pulses. The prediction-error minimization technique is used. The model is parameterized from laboratory experiments using a mixed-size current pulse profile at four temperatures from 10 °C to 50 °C, giving linearized ECN parameters for a range of states-of-charge, currents and temperatures. These are used to create a nonlinear polynomial-based battery model suitable for use in a battery management system. When the model is used to predict the behaviour of a validation data set representing an automotive NEDC driving cycle, the terminal voltage predictions are judged accurate with a root mean square error of 32 mV.

Journal article

Wu B, Parkes MP, de Benedetti L, Marquis AJ, Offer GJ, Brandon NPet al., 2016, Real-time monitoring of proton exchange membrane fuel cell stack failure, Journal of Applied Electrochemistry, Vol: 46, Pages: 1157-1162, ISSN: 1572-8838

Uneven pressure drops in a 75-cell 9.5-kWe protonexchange membrane fuel cell stack with a U-shaped flowconfiguration have been shown to cause localised flooding.Condensed water then leads to localised cell heating, resultingin reduced membrane durability. Upon purging of the anodemanifold, the resulting mechanical strain on the membranecan lead to the formation of a pin-hole/membrane crack and arapid decrease in open circuit voltage due to gas crossover.This failure has the potential to cascade to neighbouring cellsdue to the bipolar plate coupling and the current densityheterogeneities arising from the pin-hole/membrane crack.Reintroduction of hydrogen after failure results in cell voltageloss propagating from the pin-hole/membrane crack locationdue to reactant crossover from the anode to the cathode, giventhat the anode pressure is higher than the cathode pressure.Through these observations, it is recommended that purging isavoided when the onset of flooding is observed to preventirreparable damage to the stack.

Journal article

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