Electron Transport Layers for CO2 Reduction: Conductivity, Selectivity and Stability

Electron transport layers (ETLs) used as components of photocathodes for light-driven CO₂ reduction (CO₂R) in aqueous media should have good electronic transport, be stable under CO₂R conditions, and, ideally, be catalytically inert for the competing hydrogen evolution reaction (HER). Here, using planar p-Si (100) as the absorbing material, we show that TaO_x satisfies all three of the above criteria. TaO_x films were synthesized by both pulsed laser deposition (PLD) and radio-frequency (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/TaO_x photocathodes were interfaced with ca. 10 nm of a CO₂R catalyst: Cu or Au. 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 TaO_x and RF sputtered TaO_x, respectively, at photocurrent densities up to 8 mA cm^-2. p-Si/TiO₂/Cu photocathodes were also evaluated but produced mostly H₂ (&gt;97%) due to reduction of the TiO₂ to Ti metal under CO₂R conditions. In contrast, a dual ETL photocathode (p-Si/TiO₂/TaO_x/Cu) was selective for CO₂R, which suggests a strategy for separately optimizing selective charge collection and the stability of the ETL/water interface. The maximum photovoltage obtained with p-Si/TaO_x/Cu devices was 300 mV which was increased to 430-460 mV by employing ion implantation to make pn⁺-SiTaO_x/Cu structures. Photocathodes with RF sputtered TaO_x ETLs are stable for CO₂R for at least 300 min. 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 becoming a significant component to carbon-neutral manufacturing.<br/>Further to understand the why PLD TaO_xelectron transport layer exhibits higher product selectivity over RF sputtered TaO_x, we investigated using ambient pressure x-ray photoelectron spectroscopy (APXPS) how the Cu catalyst oxidation state changes with different CO₂ partial pressures. There was evidence on favorable change in the oxidation state of Cu towards Cu²⁺ when exposed to higher partial CO₂ pressures with different TaO_xsupports.