Origin of photoelectrochemical CO2 reduction on bare Cu(In,Ga)S2 (CIGS) thin films in aqueous media without co-catalysts†

EES catalysis Pub Date : 2025-01-15 DOI:10.1039/D4EY00233D
Rajiv Ramanujam Prabhakar, Sudhanshu Shukla, Haoyi Li, R. Soyoung Kim, Wei Chen, Jérôme Beaudelot, Jan D’Haen, Daniely Reis Santos, Philippe M. Vereecken, Gian-Marco Rignanese, Ethan J. Crumlin, Junko Yano, Bart Vermang and Joel W. Ager
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Abstract

Photoelectrochemical (PEC) CO2 reduction (CO2R) on semiconductors provides a promising route to convert CO2 to fuels and chemicals. However, most semiconductors are not stable under CO2R conditions in aqueous media and require additional protection layers for long-term durability. To identify materials that would be stable and yield CO2R products in aqueous conditions, we investigated bare Cu(In,Ga)S2 (CIGS) thin films. We synthesized CIGS thin films by sulfurizing a sputtered Cu–In–Ga metal stack. The as-synthesized CIGS thin films are Cu-deficient and have a high enough bandgap (1.7 eV) suitable to perform CO2R. The bare CIGS photocathodes had faradaic yields of 14% for HCOO and 30% for CO in 0.1 M KHCO3 electrolyte without the use of any co-catalysts under 1 sun illumination at an applied bias of −0.4 V vs. RHE and operated stably for 80 min. Operando Raman spectroscopy under CO2R conditions showed that the dominant A1 mode of CIGS was unaffected during operation. Post-mortem X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analysis suggests that the CO2R stability could be related to self-protection caused by the in situ formation of oxides/hydroxides of Ga and In during operation. Density functional theory (DFT) calculations also reveal that Ga and In are the preferential sites for the adsorption of CO2R products, particularly HCOO. These results show that CIGS is a promising semiconductor material for performing direct semiconductor/electrolyte reactions in aqueous media for the PEC CO2R.

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