Performance Limits of Photoelectrochemical CO₂ Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products

Solar-driven reduction of CO2 to solar fuels as an alternative to H2 via water splitting is an intriguing proposition. We model the solar-to-fuel (STF) efficiencies using realistic parameters based on recently reported CO2 reduction catalysts with a high performance tandem photoabsorber structure. CO and formate, which are both two-electron reduction products, offer STF efficiencies (20.0% and 18.8%) competitively close to that of solar H2 (21.8%) despite markedly worse reduction catalysis. The slightly lower efficiency toward carbon products is mainly due to electrolyte resistance, not overpotential. Using a cell design where electrolyte resistance is minimized makes formate the preferred product from an efficiency standpoint (reaching 22.7% STF efficiency). On the other hand, going beyond a 2 electron reduction reaction, the more highly reduced products seem unviable with presently available electrocatalysts due to excessive overpotentials and poor selectivity. This work considers breaking up the multielectron reduction pathway into individually optimized, separate two-electron steps as a way forward.