Periodic Reporting for period 2 - EXISTAR (EXtending Interface Science To Atmospheric-pressure Reactions)
Periodo di rendicontazione: 2023-01-01 al 2024-06-30
The performance of materials used for electrochemical energy storage (batteries), or the catalytic conversion of waste products into industrially useful chemicals, is typically defined by reactions occurring at their interface with a liquid or gas environment. As the reaction conditions change so these interfaces change, with often dramatic effects on their structure, chemistry and performance. Understanding the evolution of these interfaces is critical to developing the improved materials needed for a more sustainable economy. However, it is extremely challenging to extract information from these interfaces during operation, due to interference from the bulk phases either side, which scatter most interface-sensitive probes.
The ambition of this project is to pioneer enclosed environmental reaction cells to extend the operation of a range of interface-sensitive characterisation techniques to liquid and high-pressure gas environments, such that the chemical and structural evolution of material interfaces can be resolved under realistic operating conditions. The motivation is to be able to directly explore and hence understand the key processes occurring at material interfaces that underpin sustainable technologies. This will transform our understanding of interfacial processes which will be key to the design of future materials for sustainable energy applications.
The operando characterisation techniques developed in this project will be made widely available to the research community thanks to the relatively low cost of such enclosed cells and their portability across existing characterisation equipment already available at many research institutions. Although battery electrode and catalyst interfaces are the primary focus of this project, these will serve as exemplar cases, with the methodology readily transferable to many other research fields, including materials synthesis, electrocatalysis, and environmental science.
These new capabilities will enable both accelerated screening of new catalyst and electrode materials and a rational approach to their optimisation based on understanding the fundamental origins of their performance. By observing the interfacial behaviour of transition metal oxides across different applications (batteries and catalysis), key insights are expected that will inform the design and discovery of new materials, cycling protocols and reaction conditions, beyond the current trial-and-error approach. For example, operando studies of electrolyte decomposition in Li-ion batteries will aid us in the selection of additives and charging protocols.
The overarching aim of this project is to develop these new characterisation capabilities and demonstrate their importance through the study of materials interfaces relevant to future sustainable technologies. The key objectives are to:
(a) Develop enclosed-cell approaches to extend the operation of a range of complementary interface-sensitive characterisation techniques to liquid and high-pressure gas (up to 10 bar) environments, such that the chemical and structural evolution of representative material interfaces can be resolved under realistic operating conditions.
(b) Develop new deposition pathways for integrating battery electrode and catalyst materials of controlled chemical composition and morphology with the windows used in the enclosed reaction cells
(c) Establish understanding of structure-property relationships for transition metal oxide interfaces used as lithium-ion battery cathodes, and heterogeneous catalysts.
(d) Connect the behaviours observed in lithium-ion battery cathodes with catalysts used for chemical feedstock and liquid fuel synthesis, to identify common trends that are generally applicable to TMO interfaces.
(e) Inform the development of materials solutions to problems such as battery capacity fade and poor catalyst selectivity, and perform operando measurements to validate these solutions.
The ambition of this project is to pioneer enclosed environmental reaction cells to extend the operation of a range of interface-sensitive characterisation techniques to liquid and high-pressure gas environments, such that the chemical and structural evolution of material interfaces can be resolved under realistic operating conditions. The motivation is to be able to directly explore and hence understand the key processes occurring at material interfaces that underpin sustainable technologies. This will transform our understanding of interfacial processes which will be key to the design of future materials for sustainable energy applications.
The operando characterisation techniques developed in this project will be made widely available to the research community thanks to the relatively low cost of such enclosed cells and their portability across existing characterisation equipment already available at many research institutions. Although battery electrode and catalyst interfaces are the primary focus of this project, these will serve as exemplar cases, with the methodology readily transferable to many other research fields, including materials synthesis, electrocatalysis, and environmental science.
These new capabilities will enable both accelerated screening of new catalyst and electrode materials and a rational approach to their optimisation based on understanding the fundamental origins of their performance. By observing the interfacial behaviour of transition metal oxides across different applications (batteries and catalysis), key insights are expected that will inform the design and discovery of new materials, cycling protocols and reaction conditions, beyond the current trial-and-error approach. For example, operando studies of electrolyte decomposition in Li-ion batteries will aid us in the selection of additives and charging protocols.
The overarching aim of this project is to develop these new characterisation capabilities and demonstrate their importance through the study of materials interfaces relevant to future sustainable technologies. The key objectives are to:
(a) Develop enclosed-cell approaches to extend the operation of a range of complementary interface-sensitive characterisation techniques to liquid and high-pressure gas (up to 10 bar) environments, such that the chemical and structural evolution of representative material interfaces can be resolved under realistic operating conditions.
(b) Develop new deposition pathways for integrating battery electrode and catalyst materials of controlled chemical composition and morphology with the windows used in the enclosed reaction cells
(c) Establish understanding of structure-property relationships for transition metal oxide interfaces used as lithium-ion battery cathodes, and heterogeneous catalysts.
(d) Connect the behaviours observed in lithium-ion battery cathodes with catalysts used for chemical feedstock and liquid fuel synthesis, to identify common trends that are generally applicable to TMO interfaces.
(e) Inform the development of materials solutions to problems such as battery capacity fade and poor catalyst selectivity, and perform operando measurements to validate these solutions.
An enclosed atmospheric pressure catalysis cell has been developed for performing X-ray absorption and photoelectron spectroscopy (XAS/XPS), that can accommodate X-ray and electron transparent windows. This allows heating to temperatures in excess of 500 °C whilst feeding gases at pressures in excess of 4 bar. This has been used for proof of concept XAS measurements at synchrotron beamlines, and then gone on to be used for studying the oxidation station of copper catalysts used for the conversion of carbon dioxide to methanol. The potential to perform XPS at elevated pressures has also been demonstrated using this cell, and a number of reactions catalysed by metallic nanoparticles have been studied.
An enclosed electrochemical cell for Li-ion battery studies also been developed that can also incorporate X-ray and electron transparent windows, and can be assembled and sealed in an Argon glovebox, so that the moisture-sensitive battery components can be contained within it. This cell has been used for an operando XAS study of the solid electrolyte interphase that forms on silicon electrodes. A specialist technique involving a modulated X-ray beam was used so that the photocurrent related to the XAS measurement could be separated from the faradaic currents associated with lithiating and delithiating the silicon.
The reliable fabrication of clean, suspended-graphene windows that are electron transparent has been developed. This uses a gold support film that can be etched away before performing XPS measurements. With this approach we have been able to demonstrate the operation of a liquid flow cell in a laboratory based XPS instrument, and observe the gradual removal of the gold support by exposure to the etchant. The membranes that can be produced with this process are now being using in combination with the atmospheric pressure catalysis cell and the electrochemical cell to enable operando XPS studies.
Work has also been carried out to develop suitable model catalyst and electrode materials for the XAS and XPS studies. Thin-film anode and cathode materials for Li-ion batteries have been produced and their electrochemical behaviour benchmarked against commercial battery materials to ensure they are representative. Catalyst nanoparticles of controlled composition and size have been produced using a size-selected nanoparticle deposition system, and their catalytic performance for different reactions of interest has been tested. The model catalysts and electrodes are starting to now by used for the operando studies which are the current focus of the project.
An enclosed electrochemical cell for Li-ion battery studies also been developed that can also incorporate X-ray and electron transparent windows, and can be assembled and sealed in an Argon glovebox, so that the moisture-sensitive battery components can be contained within it. This cell has been used for an operando XAS study of the solid electrolyte interphase that forms on silicon electrodes. A specialist technique involving a modulated X-ray beam was used so that the photocurrent related to the XAS measurement could be separated from the faradaic currents associated with lithiating and delithiating the silicon.
The reliable fabrication of clean, suspended-graphene windows that are electron transparent has been developed. This uses a gold support film that can be etched away before performing XPS measurements. With this approach we have been able to demonstrate the operation of a liquid flow cell in a laboratory based XPS instrument, and observe the gradual removal of the gold support by exposure to the etchant. The membranes that can be produced with this process are now being using in combination with the atmospheric pressure catalysis cell and the electrochemical cell to enable operando XPS studies.
Work has also been carried out to develop suitable model catalyst and electrode materials for the XAS and XPS studies. Thin-film anode and cathode materials for Li-ion batteries have been produced and their electrochemical behaviour benchmarked against commercial battery materials to ensure they are representative. Catalyst nanoparticles of controlled composition and size have been produced using a size-selected nanoparticle deposition system, and their catalytic performance for different reactions of interest has been tested. The model catalysts and electrodes are starting to now by used for the operando studies which are the current focus of the project.
A new interface-sensitive technique has been introduced for tracking the decomposition reactions occurring at the electrode-electrolyte interface in Li-ion batteries associated with the formation of the solid electrolyte interphase (SEI). Total Electron Yield X-ray Absorption Spectroscopy under potentiostatic control, has shown that LiF is first formed at the anode, and then on cycling to lower potentials organic components form on top of this. It has further been shown that the additive flouroethylene carbonate (FEC) leads to an earlier onset of SEI formation i.e. at higher potentials, which can explain the improved cycling stability seen for silicon anodes. As the silicon is delithiated the large change in volume leads to damage to the SEI at higher potentials, and with FEC present this is rapidly healed such that less SEI needs to reform on the next cycle.
Furthermore, new understanding of the role of the reactant mixture during methanol synthesis has been developed. It is observed that CO helps to maintain metallic sites on the Cu catalyst, which are critical to the ongoing dissociation of hydrogen and CO2. Therefore the inclusions of CO contributes to a process that is more robust to variations in reaction conditions, particularly given that large kinetic barriers for certain processes (e.g. H2 activation) can lead to effective catalyst deactivation if the desired oxidation state is lost. Our results highlight the importance of studying these phenomena at pressures close to realistic industrial conditions, as this can alter both the equilibrium and reaction kinetics.
In the field of Li-ion battery cathode materials, by combining different core level spectroscopies in a systematic way it has been possible to properly distinguish bulk redox processes from interfacial degradation in LiNiO2 electrodes. This has shown that molecular oxygen formation is a surface degradation process rather than a bulk redox process, as some reports have recently suggested. This is important for designing mitigation strategies to extend battery life, as it highlights the critical role of reactions at the interface with the electrolyte rather than restructuring within the bulk of the cathode material.
Furthermore, new understanding of the role of the reactant mixture during methanol synthesis has been developed. It is observed that CO helps to maintain metallic sites on the Cu catalyst, which are critical to the ongoing dissociation of hydrogen and CO2. Therefore the inclusions of CO contributes to a process that is more robust to variations in reaction conditions, particularly given that large kinetic barriers for certain processes (e.g. H2 activation) can lead to effective catalyst deactivation if the desired oxidation state is lost. Our results highlight the importance of studying these phenomena at pressures close to realistic industrial conditions, as this can alter both the equilibrium and reaction kinetics.
In the field of Li-ion battery cathode materials, by combining different core level spectroscopies in a systematic way it has been possible to properly distinguish bulk redox processes from interfacial degradation in LiNiO2 electrodes. This has shown that molecular oxygen formation is a surface degradation process rather than a bulk redox process, as some reports have recently suggested. This is important for designing mitigation strategies to extend battery life, as it highlights the critical role of reactions at the interface with the electrolyte rather than restructuring within the bulk of the cathode material.