Periodic Reporting for period 4 - HybridSolarFuels (Efficient Photoelectrochemical Transformation of CO2 to Useful Fuels on Nanostructured Hybrid Electrodes) Reporting period: 2021-07-01 to 2022-03-31 Summary of the context and overall objectives of the project Given that CO2 is a greenhouse gas, using the energy of sunlight to convert CO2 to transportation fuels (such as methanol) and basic chemicals (such as syngas) represents a value-added approach to the simultaneous generation of alternative fuels and environmental remediation of carbon emissions. Photoelectrochemistry has been proven to be a useful avenue for solar water splitting. CO2 reduction, however, is multi-electron in nature (e.g. 6 e- to methanol) with considerable kinetic barriers to electron transfer. It therefore requires the use of carefully designed electrode surfaces to accelerate e- transfer rates to levels that make practical sense. In addition, novel flow-cell configurations have to be designed to overcome mass transport limitations of this reaction. We design and assemble nanostructured hybrid materials to be simultaneously applied as both adsorber and cathode material to photoelectrochemically convert CO2 to valuable products. The three main goals of this project are to (i) gain fundamental understanding of morphological-, size-, and surface functional group effects on the PEC (PEC) behavior at the nanoscale (ii) design and synthesize new functional hybrid materials for PEC CO2 reduction, (iii) develop flow reactors for PEC CO2 reduction. We have demonstrated how to assemble complex photoelectrode architectures with components that have precisely defined functionality and complementarity. We have shown how such rationally designed hybrid nanostructures outperform their single component counterparts. Based on our custom-developed (photo)electrochemical cells we were able to overcome mass transport limitations and achieve high current density operation. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far The project focuses on the photoelectrochemical (PEC) conversion of carbon dioxide to useful products. We vigorously studied the effect of size, morphology, and surface functional groups of the photoelectrodes at the nanoscale. As a first step, we designed model systems to deconvolute the effect of the three main processes (light absorption, charge carrier transport and charge transfer), which dictate the solar-to-fuel conversion efficiency in a PEC cell. We studied bimetallic oxides, metal halide, lead halide perovskites in this vein. As the next step, we assembled hybrid photoelectrodes, where different components are responsible for the different processes. For example, nanocarbon-containing photoelectrodes outperformed their bare SC counterpart, due to enhanced charge carrier transport. We have developed and adapted different in situ electrochemical methods, to better understand the light-induced processes both inside the SC photoelectrodes, as well as at the SC/electrolyte interface. Finally, we designed, prepared and studied PEC flow cells to achieve unprecedentedly high CO2 conversion efficiencies). For example, a concentrated photovoltaic cell has been integrated into a custom-made heat managed photo-electrochemical device. With solar concentrations, the solar-to-CO conversion efficiency reaches 17% while maintaining a current density of 150 mA cm-2 in the electrolyzer and a CO selectivity above 90%, representing an overall 19% solar-to-fuel conversion efficiency. We have published over 31 high impact papers so far, in internationally leading journals. In addition, the group members gave dozens of presentations on leading international conferences, to disseminate the results. As for exploitation, an ERC proof of concept grant on this topic was awarded and successfully completed. We were also able to strengthen our ties with the relevant industrial actors, and new technology development orientated projects have span out from the ERC project. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) We have developed and employed an in situ ultrafast spectroelectrochemistry method to understand the charge carrier dynamics of perovskite photoelectrodes under electrochemical control. This allows to better understand chemical events occurring upon irradiation. Elaborating on this tool, we can rationally design new efficient and stable photoelectrodes for CO2 conversion. We have designed, fabricated and validated novel electrochemical and photoelectrochemical cell architectures. These cells operate with direct CO2 gas feed, and generate different gas products. We have gained record high performance in electrochemical CO2-to-CO conversion (1 A cm−2), and direct paired photoelectrochemical glycerol oxidation and CO2 reduction (120 mA cm-2 at 10 sun).In collaboration with EPFL, we integrated a concentrated photovoltaic cell into a custom-made heat managed photo-electrochemical device. With solar concentrations up to 450 suns, the solar-to-CO conversion efficiency reaches 17% while maintaining a current density of 150 mA cm-2 in the electrolyzer and a CO selectivity above 90%, representing an overall 19% solar-to-fuel conversion efficiency.