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

Solar-drivenreduction of CO₂ to solar fuels as an alternative to H2 via water splitting is an intriguing proposition. We modelthe solar-to-fuel (STF) efficiencies using realistic parameters basedon recently reported CO2 reduction catalysts with a highperformance tandem photoabsorber structure. CO and formate, whichare 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 efficiencytoward carbon products is mainly due to electrolyte resistance, notoverpotential. Using a cell design where electrolyte resistance isminimized makes formate the preferred product from an efficiency standpoint(reaching 22.7% STF efficiency). On the other hand, going beyond a2 electron reduction reaction, the more highly reduced products seemunviable with presently available electrocatalysts due to excessiveoverpotentials and poor selectivity. This work considers breakingup the multielectron reduction pathway into individually optimized,separate two-electron steps as a way forward.