Solid oxide electrochemical reactors and processes for carbon dioxide and water splitting

The increasing contributions of renewable energy sources into the electricity grid necessitates large-scale energy storage to balance supply and demand due to their inherent intermittency. Storing electrical energy in chemical bonds by electrolysis of CO2 and / or H2O is one option. The aims of this project were to develop and characterise (micro)-tubular solid oxide electrolysers for the reduction of CO2 and/or steam at temperatures of 700–800 °C. Micro-tubular hollow fibre reactors were fabricated by phase inversion. Ni(O) – yttria stabilised zirconia (YSZ) cermet electrodes (electrolysis cathode) and YSZ electrolyte (15-50 μm) were simultaneously co-extruded and sintered, followed by the application of a lanthanum strontium doped manganite (LSM) – YSZ|LSM electrode (electrolysis anode) onto the outer surface, which was subsequently sintered. At 800 °C, current densities of up to -1.0 A cm-2 were achieved at ca. 1.8 V for CO2 electrolysis with a silver wire and silver conductive paste cathodic lumen current collector. Replacing the silver wire with nickel and removing any paste additives resulted in a 50 % increase in current density. Electrode polarization for steam and co-electrolysis (H2O/CO2 co-feed) was 62-382 % lower compared to CO2 electrolysis, with the extent depending on the current collector design; the silver paste had a greater detrimental effect on the electrode performance of the SOE operating with CO2. Evidence supporting dual-step co-electrolysis with electro-generation of hydrogen preceding the heterogeneous chemical reaction of H2 with CO2 included electrochemical performance, adsorption modelling, diffusion considerations, and response to silver paste. However, isotopic studies to differentiate between (electro)chemical processes using labelled C18O2 and H216O were inconclusive due to oxygen-18 exchange occurring between C18O2 and H216O, within the alumina feed tube, despite the absence of a Ni-YSZ cathode acting as a catalyst. To further characterize the intrinsic CO2 reduction mechanism, the surface exchange kinetics of C18O2 on YSZ and oxide diffusion coefficients, without electrochemical polarization, were determined using secondary ion mass spectrometry. These results facilitated the analyses of SOE experiments using oxygen-18 tracers that compared the effect of applied current on oxide ion transport rates within the hollow fibre reactors. Techno-economical evaluation of intra-day energy storage using the micro-tubular reactors cyclically in electrolyser and fuel cell operational mode resulted in an electricity storage cost of £0.016 per kWh, considering capital and operating costs (assuming £0.1 per kWh electricity costs), which is lower than current pumped hydroelectric storage (£0.05 per kWh).Open Acces