Proceedings of MATSUS Fall 2024 Conference (MATSUSFall24)
DOI: https://doi.org/10.29363/nanoge.matsusfall.2024.256
Publication date: 28th August 2024
Life cycle assessment (LCA) studies clearly indicate that a minimum 10% solar to hydrogen (STH) conversion efficiency and a 10-year lifetime are required for a positive energy return on energy invested (ERoRI) for photoelectrochemical (PEC) water splitting [1-3]. Analysis of published reports reveals that many lab-scale and larger demonstrations satisfy the STH criterion, but no demonstration comes within an order of magnitude of the lifetime requirement. A drastic improvement in the demonstrated/projected lifetime of PEC water splitting schemes is urgently needed if this approach is to compete with hydrogen generation via renewables (wind/solar) coupled to electrolyzers. Although the LCA and technoeconomic analysis (TEA) of PEC CO2 reduction (CO2R) are less numerous, examination of the related electrocatalytic literature points to minimum lifetimes of five years or more [4].
Two experimental systems which address the PEC CO2R stability challenge will be presented.
Many promising light absorbers are not stable under the conditions of PEC CO2R reduction. Use of a transparent electron transport layer (ETL) can address this challenge, but the ETL must have good electronic transport, be stable under CO2R conditions, and, ideally, be catalytically inert for the competing hydrogen evolution reaction (HER). Oxygen-deficient TaOx satisfies these criteria, and p-Si/TaOx/Cu photocathodes have CO2R faradaic efficiencies (FE) >50% at photocurrent densities up to 8 mA cm-2 under 1 sun illumination with multi-hour stability [5]. Dissolution of the Cu co-catalyst appears to be the dominant degradation mechanism, suggesting that redeposition of the catalyst could extend the lifetime.
Are there any materials which have intrinsic activity/stability (that is, without the aid of electron transport/protection layers and/or co-catalysts) as photocathodes for CO2R? This question is especially pertinent if operation in aqueous media is considered. Reports of photocatalytic CO2R using metal sulfides are suggest a starting point for materials discovery [6]. 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 FEs of 28-32% and 14%, respectively. Extensive structural characterization (Raman, ambient pressure XPS, XAS) shows that Cu (In,Ga)S2 photocathodes are stable for at least a few hours. Interestingly, as would be predicted by considerations of equilibrium (Pourbaix) stability, Se-alloyed photocathodes corrode rapidly. Additionally, Cu(In,Ga)S2 films with lower bandgaps also appear to be unstable. These 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 PEC CO2R stability of CIGS-based photocathodes..
Technoeconomic analyis was supported HydroGEN Advanced Water Splitting Materials Consortium under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office. ETL and CIGS work was supported 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.