Photocathode Materials for Carbon Dioxide Reduction in Aqueous Media

The prospect of performing solar to chemical energy conversion using the using non-equilibrium electrons and holes produced by illumination of semiconducting materials has been of interest since the pioneering work of Fujishima and Honda on water splitting by TiO₂ [1]. There are many reports of materials, notably metal oxides, which function as photoanodes for water oxidation [2]. However, far fewer materials have intrinsic activity (that is, without use of electron transport/protection layers and/or co-catalysts) as photocathodes for hydrogen evolution and, especially for CO₂ reduction (CO₂R) [3]. Moreover, many studies of semiconductor photocathodes are performed in non-aqueous solvents because the materials corrode rapidly if water is used [4].<br/>Reports of photocatalytic CO2R using metal sulfides are suggest a starting point for materials discovery [5]. More specifically, the Cu(In,Ga)(S,Se)₂ (CIGS) alloy family is interesting due to the extensive study of its properties as photovoltaic materials and its wide bandgap tuning range. Indeed, co-catalyst free Cu(In,Ga)S₂ (CIGS) thin-film photocathodes (Eg ~ 1.8 eV) reduce CO₂ to CO and HCOO^- in aqueous media at faradaic efficiencies of 28-32% and 14%, respectively. Extensive structural characterization (Raman, ambient pressure XPS, XAS) shows that Cu (In,Ga)S₂ (CIGS) photocathodes are stable for at least a few hours. Interestingly, as would be predicted by considerations of equilibrium (Pourbaix) stability, Se-alloyed photocathodes are not stable and corrode rapidly. Additionally, Cu(In,Ga)S₂ films with lower bandgaps also appear to be unstable.<br/>These finding merit a revisiting of high-throughput computational searches for CO₂R photocathode materials, as Cu(In,Ga)S₂ was not previously identified as a candidate [6]. More generally, our findings suggest that the previously unexplored Cu-deficient surface composition and specific surface defects, especially deep anti-site defects, might be playing a key role in governing the unique photoelectrochemical behavior of CIGS. More generally, the stable performance of CIGS photocathodes for CO₂R in aqueous media provides an opportunity to study complex, light-driven, catalytic processes at a semiconductor-electrolyte interface.<br/><br/>This research 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. The work also received funding from the Marie Sklodowska-Curie Actions (MSCA) project CHALCON under grant agreement no. 101067667.<br/><br/>1. Fujishima, A.; Honda, K. <i>Nature</i> <b>1972</b>, <i>238</i>, 37.<br/>2. Osterloh, F. E. <i>Chem. Soc. Rev.</i> <b>2013</b>, <i>42</i>, 2294–2320.<br/>3. Heller, A. <i>Acc. Chem. Res.</i> <b>1981</b>, <i>14</i>, 154–162.<br/>4. Liu, Y.; Xia, M.; Ren, D.; Nussbaum, S.; Yum, J.-H.; Grätzel, M.; Guijarro, N.; Sivula, K. <i>ACS Energy Lett. </i><b>2023</b>, <i>8</i>, 1645–1651.<br/>5. Li, X.; Sun, Y.; Xu, J.; Shao, Y.; Wu, J.; Xu, X.; Pan, Y.; Ju, H.; Zhu, J.; Xie, Y. <i>Nat. Energy</i> <b>2019</b>, <i>4</i>, 690–699.<br/>6. Siron, M.; Andriuc, O.; Persson, K. A. <i>J. Phys. Chem. C</i> <b>2022</b>, <i>126</i>, 13224–13236.