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

Abstract

Photoelectrochemical (PEC) CO 2 reduction (CO 2 R) on semiconductors provides a promising route to convert CO 2 to fuels and chemicals. However, most semiconductors are not stable under CO 2 R conditions in aqueous media and require additional protection layers for long-term durability. To identify materials that would be stable and yield CO 2 R products in aqueous conditions, we investigated bare Cu(In,Ga)S 2 (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 CO 2 R. The bare CIGS photocathodes had faradaic yields of 14% for HCOO À and 30% for CO in 0.1 M KHCO 3 electrolyte without the use of any co-catalysts under 1 sun illumination at an applied bias of À0.4 V vs. RHE and operated stably for 80 min. Operando Raman spectroscopy under CO 2 R conditions showed that the dominant A 1 mode of CIGS was unaffected during operation. Post-mortem X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analysis suggests that the CO 2 R 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 CO 2 R products, particularly HCOO À. These results show that CIGS is a promising semiconductor material for performing direct semiconductor/ electrolyte reactions in aqueous media for the PEC CO 2 R. Broader context Artificial photosynthetic systems use sunlight to convert CO 2 to value added products. These are photoelectrochemical (PEC) devices that rely on semiconductor-electrolyte junctions. However, very few photocathode semiconductor materials are stable and yield CO 2 reduction (CO 2 R) products without any protection layers and/or co-catalysts in aqueous media. This severely limits the artificial photosynthesis community from investigating direct semiconductor-electrolyte reactions and exploiting the rich interface chemistry relevant to PEC CO 2 R. Herein, we show stable PEC CO 2 R operation (41 h)