Gregory Offer | Imperial College London (original) (raw)

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Papers by Gregory Offer

Journal of Power Sources, 2015

ABSTRACT One of the attractive applications for reversible Solid Oxide Cells (SOCs) is to convert... more ABSTRACT One of the attractive applications for reversible Solid Oxide Cells (SOCs) is to convert CO2 into CO via high temperature electrolysis, which is particularly important for biogas upgrading. To improve biogas utility, the CO2 component can be converted into fuel via electrolysis. A significant issue for SOC operation on biogas is carbon-induced catalyst deactivation. Nickel is widely used in SOC electrodes for reasons of cost and performance, but it has a low tolerance to carbon deposition. Two different modes of carbon formation on Ni-based electrodes are proposed in the present work based on ex-situ Raman measurements which are in agreement with previous studies. While copper is known to be resistant towards carbon formation, two significant issues have prevented its application in SOC electrodes – namely its relatively low melting temperature, inhibiting high temperature sintering, and low catalytic activity for hydrogen oxidation. In this study, the electrodes were prepared through a low temperature metal infiltration technique. Since the metal infiltration technique avoids high sintering temperatures, Cu–Ce0.9Gd0.1O2−δ (Cu-CGO) electrodes were fabricated and tested as an alternative to Ni-CGO electrodes. We demonstrate that the performance of Cu-CGO electrodes is equivalent to Ni-CGO electrodes, whilst carbon formation is fully suppressed when operated on biogas mixture.

Environmental Innovation and Societal Transitions, 2015

Journal of Energy Storage, 2015

ABSTRACT Lithium batteries suffer from a number of safety concerns which limits their use in some... more ABSTRACT Lithium batteries suffer from a number of safety concerns which limits their use in some applications. Nail penetration tests are used by the battery industry to compare the safety of different batteries during an incursion by a metallic object through a battery that physically violates the containment and locally damages the internal structure. For most lithium chemistries, such as mixed metal oxide or lithium iron phosphate cathodes, these tests result in rapid and dangerous failure. Lithium sulfur is an important next generation ultra-high energy density battery chemistry which is also inherently safer. Results are reported for nail penetration tests on 16 Ah lithium sulfur batteries showing how they heat up by less than 10 °C during a 10 min penetration and then cool down rapidly after removal of the nail. Results of a nail penetration test under load are also reported for the first time, showing how the battery was capable of continuing to provide 1.6 A (C/10) of current to an external load, with only a 1% drop in voltage. The results should be of interest for applications requiring ultra-high energy densities, improved safety, and continuous provision of power for a short period after damage, particularly military, aviation, portable electronics and automotive industries.

Energy Environ. Sci., 2015

ABSTRACT Global greenhouse gas (GHG) emission targets can only be met by significantly decarbonis... more ABSTRACT Global greenhouse gas (GHG) emission targets can only be met by significantly decarbonising road transport. The only long term way to do this is via the electrification of powertrains combined with the production of low carbon electricity or hydrogen. Current assumptions and models, such as the IEA BLUE Map, demonstrate that this is technically possible, but assume growth in demand for transport services will only double by 2035 and triple by 2050, largely driven by growth in developing economies. However, another transport revolution, automated vehicles, could drive growth in transport services significantly further, which without electrification will have a large negative impact on efforts to curb transport related emissions. In contrast, it is shown in this paper that automated vehicles could significantly improve the economics of electric vehicles, and therefore make the electrification of powertrains more likely, which could help reduce emissions. Despite this uncertainty, little work has been done on understanding how these factors will affect each other, particularly the timing and uptake of automated vehicles and their effect on future transport related GHG emissions and economics, yet the impact on transport policy, infrastructure and society will be profound and should be of interest to policy makers, the automotive and energy industries, and society as a whole.

Fuel Cells, 2013

ABSTRACT Raman spectroscopy is a powerful characterization tool for improving the understanding o... more ABSTRACT Raman spectroscopy is a powerful characterization tool for improving the understanding of solid oxide fuel cells (SOFCs), capable of providing direct, molecularly specific information regarding the physical and chemical processes occurring within functional SOFCs in real time. In this paper we give a summary of the technique itself and highlight ex situ and in situ studies that are particularly relevant for SOFCs. This is followed by a case study of carbon formation on SOFC Ni-based anodes exposed to carbon monoxide (CO) using both ex situ and in situ Raman spectroscopy combined with computational simulations. In situ measurements clearly show that carbon formation is significantly reduced for polarized SOFCs compared to those held at open circuit potential (OCP). Ex situ Raman mapping of the surfaces showed clear variations in the rate of carbon formation across the surface of polarized anodes. Computational simulations describing the geometry of the cell showed that this is due to variations in gas access. These results demonstrate the ability of Raman spectroscopy in combination with traditional characterization tools, to provide detailed understanding of critical processes occurring within functional SOFCs.

ECS Transactions, 2013

ABSTRACT Ni-based solid oxide fuel/electrolysis cell electrodes are often optimized for fuel cell... more ABSTRACT Ni-based solid oxide fuel/electrolysis cell electrodes are often optimized for fuel cell operation without consideration of electrolysis-specific conditions. In-situ Raman spectroscopy was employed to investigate changes that occur in the surface chemistry of such electrodes during operation in electrolysis mode. Carbon deposition on Ni-gadolinium-doped ceria electrodes after CO2 electrolysis was demonstrated. Spatially resolved Raman spectroscopy has been shown to provide valuable insight into electrode optimization. Ex-situ Raman spectroscopy was used in conjunction with high resolution microscopy in order to further characterize the changes occurring within the electrodes during operation.

ECS Transactions, 2013

ABSTRACT Carbon formation within nickel-based solid oxide fuel cell (SOFC) anodes exposed to carb... more ABSTRACT Carbon formation within nickel-based solid oxide fuel cell (SOFC) anodes exposed to carbonaceous fuels typically leads to reduced operational lifetimes and performance, and can eventually lead to catastrophic failure through cracking and delamination. In-situ Raman spectroscopy has been shown to be a powerful characterization tool for the investigation of the dynamics of physical processes occurring within operational SOFCs in real time. Here we investigate the dynamics of carbon formation on a variety of nickel-based SOFC anodes as a function of temperature, fuel and electrical loading using Raman spectroscopy. We show that the rate of carbon formation throughout the SOFC anode can be significantly reduced through a careful consideration of the SOFC anode material, design and operational conditions.

2013 World Electric Vehicle Symposium and Exhibition (EVS27), 2013

6th IET International Conference on Power Electronics, Machines and Drives (PEMD 2012), 2012

ABSTRACT In this paper systems integration issues, such as electrical and thermal design and mana... more ABSTRACT In this paper systems integration issues, such as electrical and thermal design and management of full battery packs — often containing hundreds of cells — are discussed. The design and construction of a 9 kWh battery pack for a motorsports application is used as an example. The pack contained 504 lithium cells arranged into 2 sidepods, each containing 3 modules, with each module in a 12P7S configuration. This paper focuses on describing problems related to cells being connected in parallel, known as massively parallel packs. We also demonstrate how a full vehicle test can be used to identify malfunctioning strings of cells for further investigation. It is shown that normal inter-cell contact resistances can cause currents to flow unevenly within the pack, leading to cells being unequally worked. This is supported by a Matlab/Simulink model of one battery module, including contact resistances, which was able to reproduce the results that were seen in experimental tests. Over time the unequal current flowing through cells can lead to significant differences in cells' state of charge and open circuit voltages, large currents flowing between cells even when the load is disconnected, and ultimately, some cells discharging and aging more quickly than others and jeopardising the energy storage capacity and lifetime of the entire pack.

ECS Transactions, 2013

ABSTRACT Understanding the chemical processes that occur at the anode triple phase boundary (TPB)... more ABSTRACT Understanding the chemical processes that occur at the anode triple phase boundary (TPB) between Ni, YSZ and fuel molecules is essential in determining solid oxide fuel cell (SOFC) anode performance. With a growing interest in vibrational spectroscopy for studying such processes, the surface and vibrational properties of the materials nickel (Ni) and yttria stabilized zirconia (YSZ) are investigated using first principles atomistic simulations based on density functional theory and empirical potential models. The initial findings and the methodology followed to date are presented.

Environmental Innovation and Societal Transitions, 2014

Journal of Power Sources, 2015

ABSTRACT Monitoring of lithium-ion batteries is of critical importance in electric vehicle applic... more ABSTRACT Monitoring of lithium-ion batteries is of critical importance in electric vehicle applications in order to manage the operational condition of the cells. Measurements on a vehicle often involve current, voltage and temperature which enable in-situ diagnostic techniques. This paper presents a novel diagnostic technique, termed differential thermal voltammetry, which is capable of monitoring the state of the battery using voltage and temperature measurements in galvanostatic operating modes. This tracks battery degradation through phase transitions, and the resulting entropic heat, occurring in the electrodes. Experiments to monitor battery degradation using the new technique are compared with a pseudo-2D cell model. Results show that the differential thermal voltammetry technique provides information comparable to that of slow rate cyclic voltammetry at shorter timescale and with load conditions easier to replicate in a vehicle.

2013 World Electric Vehicle Symposium and Exhibition (EVS27), 2013

... 372 7.2. 4 MEA for Anion Exchange Membrane Fuel Cells..... ... 1 Catalyst LibraryPreparation.... more ... 372 7.2. 4 MEA for Anion Exchange Membrane Fuel Cells..... ... 1 Catalyst LibraryPreparation..... ... 661 14.3 Synthesis and Characterization of Pt Catalysts Supported on Carbon Nanotubes, Carbon Nanofibers and Metal Oxide Nanowires... ...

Hybrid and Electric Vehicles Conference 2013 (HEVC 2013), 2013

ABSTRACT The concept of a fuel cell-supercapacitor hybrid system involves the direct coupling of ... more ABSTRACT The concept of a fuel cell-supercapacitor hybrid system involves the direct coupling of the two devices to achieve the same benefits of hybridisation but without the need for costly DC-DC converters. Using an experimentally validated steady state fuel cell model and a transmission line based supercapacitor model, it has been shown that the passive hybridisation allows for efficiency gains of approximately 16% compared to a pure fuel cell system. Under load, the supercapacitors meets the peak power requirement due to their lower impedance giving the FC time to ramp up. Under no load conditions, the fuel cell gradually charges the supercapacitors back to the steady state thermodynamic equilibrium potential. A fast fourier transform analysis of the respective loads under an automotive drive cycle showed that the supercapacitors act as a low pass filter, reducing the magnitude of load oscillations from the fuel cell. This therefore addresses two of the main modes of fuel cell degradation in automotive applications: rapid power cycling and no load idling.

ECS Transactions, 2013

ABSTRACT In this study a novel electrode structure for both fuel cell and electrolysis was made b... more ABSTRACT In this study a novel electrode structure for both fuel cell and electrolysis was made by impregnation of a porous Gd-doped Ceria scaffold with Ni nano-particles. Electrical conductivity was assessed by the Van der Pauw method. Electrochemical performance in dry hydrogen was evaluated using three electrode DC and AC measurements over a range of temperatures and gas concentrations. The microstructure of the electrode was studied using Scanning Electron Microscopy. Encouraging results have been obtained for the performance of the electrode and results from these studies are reported.

Journal of Power Sources, 2015

ABSTRACT One of the attractive applications for reversible Solid Oxide Cells (SOCs) is to convert... more ABSTRACT One of the attractive applications for reversible Solid Oxide Cells (SOCs) is to convert CO2 into CO via high temperature electrolysis, which is particularly important for biogas upgrading. To improve biogas utility, the CO2 component can be converted into fuel via electrolysis. A significant issue for SOC operation on biogas is carbon-induced catalyst deactivation. Nickel is widely used in SOC electrodes for reasons of cost and performance, but it has a low tolerance to carbon deposition. Two different modes of carbon formation on Ni-based electrodes are proposed in the present work based on ex-situ Raman measurements which are in agreement with previous studies. While copper is known to be resistant towards carbon formation, two significant issues have prevented its application in SOC electrodes – namely its relatively low melting temperature, inhibiting high temperature sintering, and low catalytic activity for hydrogen oxidation. In this study, the electrodes were prepared through a low temperature metal infiltration technique. Since the metal infiltration technique avoids high sintering temperatures, Cu–Ce0.9Gd0.1O2−δ (Cu-CGO) electrodes were fabricated and tested as an alternative to Ni-CGO electrodes. We demonstrate that the performance of Cu-CGO electrodes is equivalent to Ni-CGO electrodes, whilst carbon formation is fully suppressed when operated on biogas mixture.

Environmental Innovation and Societal Transitions, 2015

Journal of Energy Storage, 2015

ABSTRACT Lithium batteries suffer from a number of safety concerns which limits their use in some... more ABSTRACT Lithium batteries suffer from a number of safety concerns which limits their use in some applications. Nail penetration tests are used by the battery industry to compare the safety of different batteries during an incursion by a metallic object through a battery that physically violates the containment and locally damages the internal structure. For most lithium chemistries, such as mixed metal oxide or lithium iron phosphate cathodes, these tests result in rapid and dangerous failure. Lithium sulfur is an important next generation ultra-high energy density battery chemistry which is also inherently safer. Results are reported for nail penetration tests on 16 Ah lithium sulfur batteries showing how they heat up by less than 10 °C during a 10 min penetration and then cool down rapidly after removal of the nail. Results of a nail penetration test under load are also reported for the first time, showing how the battery was capable of continuing to provide 1.6 A (C/10) of current to an external load, with only a 1% drop in voltage. The results should be of interest for applications requiring ultra-high energy densities, improved safety, and continuous provision of power for a short period after damage, particularly military, aviation, portable electronics and automotive industries.

Energy Environ. Sci., 2015

ABSTRACT Global greenhouse gas (GHG) emission targets can only be met by significantly decarbonis... more ABSTRACT Global greenhouse gas (GHG) emission targets can only be met by significantly decarbonising road transport. The only long term way to do this is via the electrification of powertrains combined with the production of low carbon electricity or hydrogen. Current assumptions and models, such as the IEA BLUE Map, demonstrate that this is technically possible, but assume growth in demand for transport services will only double by 2035 and triple by 2050, largely driven by growth in developing economies. However, another transport revolution, automated vehicles, could drive growth in transport services significantly further, which without electrification will have a large negative impact on efforts to curb transport related emissions. In contrast, it is shown in this paper that automated vehicles could significantly improve the economics of electric vehicles, and therefore make the electrification of powertrains more likely, which could help reduce emissions. Despite this uncertainty, little work has been done on understanding how these factors will affect each other, particularly the timing and uptake of automated vehicles and their effect on future transport related GHG emissions and economics, yet the impact on transport policy, infrastructure and society will be profound and should be of interest to policy makers, the automotive and energy industries, and society as a whole.

Fuel Cells, 2013

ABSTRACT Raman spectroscopy is a powerful characterization tool for improving the understanding o... more ABSTRACT Raman spectroscopy is a powerful characterization tool for improving the understanding of solid oxide fuel cells (SOFCs), capable of providing direct, molecularly specific information regarding the physical and chemical processes occurring within functional SOFCs in real time. In this paper we give a summary of the technique itself and highlight ex situ and in situ studies that are particularly relevant for SOFCs. This is followed by a case study of carbon formation on SOFC Ni-based anodes exposed to carbon monoxide (CO) using both ex situ and in situ Raman spectroscopy combined with computational simulations. In situ measurements clearly show that carbon formation is significantly reduced for polarized SOFCs compared to those held at open circuit potential (OCP). Ex situ Raman mapping of the surfaces showed clear variations in the rate of carbon formation across the surface of polarized anodes. Computational simulations describing the geometry of the cell showed that this is due to variations in gas access. These results demonstrate the ability of Raman spectroscopy in combination with traditional characterization tools, to provide detailed understanding of critical processes occurring within functional SOFCs.

ECS Transactions, 2013

ABSTRACT Ni-based solid oxide fuel/electrolysis cell electrodes are often optimized for fuel cell... more ABSTRACT Ni-based solid oxide fuel/electrolysis cell electrodes are often optimized for fuel cell operation without consideration of electrolysis-specific conditions. In-situ Raman spectroscopy was employed to investigate changes that occur in the surface chemistry of such electrodes during operation in electrolysis mode. Carbon deposition on Ni-gadolinium-doped ceria electrodes after CO2 electrolysis was demonstrated. Spatially resolved Raman spectroscopy has been shown to provide valuable insight into electrode optimization. Ex-situ Raman spectroscopy was used in conjunction with high resolution microscopy in order to further characterize the changes occurring within the electrodes during operation.

ECS Transactions, 2013

ABSTRACT Carbon formation within nickel-based solid oxide fuel cell (SOFC) anodes exposed to carb... more ABSTRACT Carbon formation within nickel-based solid oxide fuel cell (SOFC) anodes exposed to carbonaceous fuels typically leads to reduced operational lifetimes and performance, and can eventually lead to catastrophic failure through cracking and delamination. In-situ Raman spectroscopy has been shown to be a powerful characterization tool for the investigation of the dynamics of physical processes occurring within operational SOFCs in real time. Here we investigate the dynamics of carbon formation on a variety of nickel-based SOFC anodes as a function of temperature, fuel and electrical loading using Raman spectroscopy. We show that the rate of carbon formation throughout the SOFC anode can be significantly reduced through a careful consideration of the SOFC anode material, design and operational conditions.

2013 World Electric Vehicle Symposium and Exhibition (EVS27), 2013

6th IET International Conference on Power Electronics, Machines and Drives (PEMD 2012), 2012

ABSTRACT In this paper systems integration issues, such as electrical and thermal design and mana... more ABSTRACT In this paper systems integration issues, such as electrical and thermal design and management of full battery packs — often containing hundreds of cells — are discussed. The design and construction of a 9 kWh battery pack for a motorsports application is used as an example. The pack contained 504 lithium cells arranged into 2 sidepods, each containing 3 modules, with each module in a 12P7S configuration. This paper focuses on describing problems related to cells being connected in parallel, known as massively parallel packs. We also demonstrate how a full vehicle test can be used to identify malfunctioning strings of cells for further investigation. It is shown that normal inter-cell contact resistances can cause currents to flow unevenly within the pack, leading to cells being unequally worked. This is supported by a Matlab/Simulink model of one battery module, including contact resistances, which was able to reproduce the results that were seen in experimental tests. Over time the unequal current flowing through cells can lead to significant differences in cells' state of charge and open circuit voltages, large currents flowing between cells even when the load is disconnected, and ultimately, some cells discharging and aging more quickly than others and jeopardising the energy storage capacity and lifetime of the entire pack.

ECS Transactions, 2013

ABSTRACT Understanding the chemical processes that occur at the anode triple phase boundary (TPB)... more ABSTRACT Understanding the chemical processes that occur at the anode triple phase boundary (TPB) between Ni, YSZ and fuel molecules is essential in determining solid oxide fuel cell (SOFC) anode performance. With a growing interest in vibrational spectroscopy for studying such processes, the surface and vibrational properties of the materials nickel (Ni) and yttria stabilized zirconia (YSZ) are investigated using first principles atomistic simulations based on density functional theory and empirical potential models. The initial findings and the methodology followed to date are presented.

Environmental Innovation and Societal Transitions, 2014

Journal of Power Sources, 2015

ABSTRACT Monitoring of lithium-ion batteries is of critical importance in electric vehicle applic... more ABSTRACT Monitoring of lithium-ion batteries is of critical importance in electric vehicle applications in order to manage the operational condition of the cells. Measurements on a vehicle often involve current, voltage and temperature which enable in-situ diagnostic techniques. This paper presents a novel diagnostic technique, termed differential thermal voltammetry, which is capable of monitoring the state of the battery using voltage and temperature measurements in galvanostatic operating modes. This tracks battery degradation through phase transitions, and the resulting entropic heat, occurring in the electrodes. Experiments to monitor battery degradation using the new technique are compared with a pseudo-2D cell model. Results show that the differential thermal voltammetry technique provides information comparable to that of slow rate cyclic voltammetry at shorter timescale and with load conditions easier to replicate in a vehicle.

2013 World Electric Vehicle Symposium and Exhibition (EVS27), 2013

... 372 7.2. 4 MEA for Anion Exchange Membrane Fuel Cells..... ... 1 Catalyst LibraryPreparation.... more ... 372 7.2. 4 MEA for Anion Exchange Membrane Fuel Cells..... ... 1 Catalyst LibraryPreparation..... ... 661 14.3 Synthesis and Characterization of Pt Catalysts Supported on Carbon Nanotubes, Carbon Nanofibers and Metal Oxide Nanowires... ...

Hybrid and Electric Vehicles Conference 2013 (HEVC 2013), 2013

ABSTRACT The concept of a fuel cell-supercapacitor hybrid system involves the direct coupling of ... more ABSTRACT The concept of a fuel cell-supercapacitor hybrid system involves the direct coupling of the two devices to achieve the same benefits of hybridisation but without the need for costly DC-DC converters. Using an experimentally validated steady state fuel cell model and a transmission line based supercapacitor model, it has been shown that the passive hybridisation allows for efficiency gains of approximately 16% compared to a pure fuel cell system. Under load, the supercapacitors meets the peak power requirement due to their lower impedance giving the FC time to ramp up. Under no load conditions, the fuel cell gradually charges the supercapacitors back to the steady state thermodynamic equilibrium potential. A fast fourier transform analysis of the respective loads under an automotive drive cycle showed that the supercapacitors act as a low pass filter, reducing the magnitude of load oscillations from the fuel cell. This therefore addresses two of the main modes of fuel cell degradation in automotive applications: rapid power cycling and no load idling.

ECS Transactions, 2013

ABSTRACT In this study a novel electrode structure for both fuel cell and electrolysis was made b... more ABSTRACT In this study a novel electrode structure for both fuel cell and electrolysis was made by impregnation of a porous Gd-doped Ceria scaffold with Ni nano-particles. Electrical conductivity was assessed by the Van der Pauw method. Electrochemical performance in dry hydrogen was evaluated using three electrode DC and AC measurements over a range of temperatures and gas concentrations. The microstructure of the electrode was studied using Scanning Electron Microscopy. Encouraging results have been obtained for the performance of the electrode and results from these studies are reported.

The present disclosure relates to the analysis of battery performance. In particular, but not exc... more The present disclosure relates to the analysis of battery performance. In particular, but not exclusively, the disclosure concerns the provision of battery managements systems that enable users of battery powered devices or systems to be provided with information regarding battery characteristics.

Physical chemistry chemical physics : PCCP, Jan 21, 2015

Understanding of the complex electrochemical, transport, and phase-change phenomena in Li-S cells... more Understanding of the complex electrochemical, transport, and phase-change phenomena in Li-S cells requires experimental characterization in tandem with mechanistic modeling. However, existing Li-S models currently contradict some key features of experimental findings, particularly the evolution of cell resistance during discharge. We demonstrate that, by introducing a concentration-dependent electrolyte conductivity, the correct trends in voltage drop due to electrolyte resistance and activation overpotentials are retrieved. In addition, we reveal the existence of an often overlooked potential drop mechanism in the low voltage-plateau which originates from the limited rate of Li2S precipitation.

Physical chemistry chemical physics : PCCP, Jan 7, 2016

Lithium-sulfur cells present an attractive alternative to Li-ion batteries due to their large ene... more Lithium-sulfur cells present an attractive alternative to Li-ion batteries due to their large energy density, safety, and possible low cost. Their successful commercialisation is dependent on improving their performance, but also on acquiring sufficient understanding of the underlying mechanisms to allow for the development of predictive models for operational cells. To address the latter, we present a zero dimensional model that predicts many of the features observed in the behaviour of a lithium-sulfur cell during charge and discharge. The model accounts for two electrochemical reactions via the Nernst formulation, power limitations through Butler-Volmer kinetics, and precipitation/dissolution of one species, including nucleation. It is shown that the flat shape of the low voltage plateau typical of the lithium-sulfur cell discharge is caused by precipitation. During charge, it is predicted that the dissolution can act as a bottleneck, because for large enough currents the amount ...

Journal of Energy Storage, 2016