Proceedings of MATSUS23 & Sustainable Technology Forum València (STECH23) (MATSUS23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.217
Publication date: 22nd December 2022
Electron transport layers (ETLs) have been used in photovoltaics (PV) cells particularly for silicon PV for efficient charge collection resulting in significantly improving its power conversion efficiency. However, when employing ETLs for Si photoelectrochemical cells particularly for CO2 reduction (CO2R) there are additional constraints to consider. First the ETLs must be stable to the reduction reaction (for example CO2R) under operation. Second, the ETLs when interfaced with a catalyst layer needs to be selective to the reduction reaction of interest (CO2R) and inert to other reactions (like hydrogen evolution reaction (HER)). Design and exploration of new ETLs for CO2R photocathodes must take into account these considerations and in this work, we show that TaOx satisfies all of the above criteria. TaOx films were synthesized by both pulsed laser deposition (PLD) and RF sputtering. In both cases, careful control of the oxygen partial pressure during growth was required to produce ETLs with acceptable electron conductivity. p-Si/TaOx photocathodes were interfaced with ca. 10 nm of a CO2R catalyst: Cu or Ag. Under front illumination with simulated AM 1.5G in CO2-saturated bicarbonate buffer, we observed, for both metals, faradaic efficiencies for CO2R products of ~50% and ~ 30% for PLD TaOx and RF sputtered TaOx, respectively, at photocurrent densities up to 8 mA cm-2. p-Si/TiO2/Cu photocathodes were also evaluated but produced mostly H2 (> 97%) due to reduction of the TiO2 to Ti metal under CO2R conditions. In contrast, a dual ETL photocathode (p-Si/TiO2/TaOx/Cu) was selective for CO2R, which suggests a strategy for separately optimizing selective charge collection and the stability of the ETL/water interface. The RF sputtered TaOx ETL based Si photocathode was also found to be stable for CO2R for about 240 mins, with metal crossover from the counter electrode being a limiting factor. Our techno-economic analysis shows that the reported system, if scaled, could allow for an economically viable production of feedstocks for chemical synthesis under the adoption of specific CO2 credit schemes, thus become a significant component to carbon-neutral manufacturing. Further evidence of efficient metal oxide catalyst support for CO2R is unravelled by ambient pressure x-ray photoelectron spectroscopy.
This material is based on work performed by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266.
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