The work performed for this project was carried out at the Lawrence Berkeley National Laboratory in California (US) and at the Department of Chemistry at the University of Cambridge (UK). The focus has been on investigating how interfaces in electrochemistry and heterogeneous catalysis behave under reaction conditions. X-ray spectroscopy and scanning probe microscopy techniques that have been specially adapted to operate at ambient pressures (tens of mbar) have been applied to study the behaviour of solid-gas interfaces. This has included fundamental studies of the pressure-dependent adsorption of carbon dioxide and methanol on low-index faces of Cu single-crystals which serve as models for the complex facetted nanoparticles typically used in carbon dioxide reduction and methanol oxidation reactions. X-ray photoelectron spectroscopy has been used to obtain a precise account of the chemical species adsorbed on the Cu surface, the pressures at which they dissociate, and the corresponding oxidation state of the Cu catalyst. Scanning tunnelling microscopy has been used to observe the structures adopted by the adsorbed species on the catalyst surface, and how the catalyst surface restructures at pressures of up to 20 mbar.
A new technique has been developed that allows photoelectron spectroscopy to be performed at atmospheric pressures in liquid or gas environments (see Figure). This is based on using graphene membranes to separate the gas/liquid from the vacuum conditions needed for electron detection, whilst the graphene is thin enough to allow the transmission of photoelectrons from the high-pressure side. Methods have been developed for producing stable graphene membranes that allow these measurements to be performed, and protocols for depositing electrode and catalyst materials onto these membranes have been established. Special liquid and gas reaction cells have been designed to accommodate these graphene membranes and successfully tested at various synchrotron facilities. Work has also been undertaken on applying X-ray absorption spectroscopy (XAS) to study solid-liquid interfaces. A three-electrode liquid flow cell that incorporates a metal-coated silicon nitride membrane (100nm thick) as both X-ray transparent window and working electrode, has been designed that allows measurement under electrochemical control. These operando X-ray spectroscopy techniques have been used to study electrolyte decomposition and resulting formation of a solid-electrolyte interphase (SEI) on high capacity silicon anodes for lithium-ion batteries. The effect of common additives such as fluoroethylene carbonate on the formation and structure of the SEI have been revealed, improving our understanding of how SEI structure can be altered to minimise capacity fade during repeated cycling. To complement the powerful chemical information these X-ray spectroscopy techniques can provide, scanning probe microscopy has been applied to study the structure of solid-liquid interfaces. Operando NMR has also been used to investigate the Lithiation mechanisms of the Si and SiO as high-capacity anode materials for Li-ion batteries.
