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

Abstract

Solar-driven reduction of CO 2 to solar fuels as an alternative to H 2 via water splitting is an intriguing proposition. We model the solar-to-fuel (STF) efficiencies using realistic parameters based on recently reported CO 2 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 H 2 (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 twoelectron steps as a way forward.

Sequential catalysis controls selectivity in electrochemical CO2 reduction on Cu

“…32 Furthermore, recent calculations by Vesborg and Seger have shown that breaking down the multistep CO2R process into two steps, first from CO2 to CO on one catalyst and subsequently from CO to further reduced products such as ethanol on another catalyst, might be a strategy to overcome fundamental efficiency limitations. 33 In EC CO2R, there have been numerous recent studies including bimetallic alloys 34,35 and phase separated nanoparticles. [36][37][38][39][40][41] Of interest here are those for which a CO generating metal is used in tandem with Cu for which a sequential catalysis mechanism is proposed.…”

Beyond Water Splitting: Efficiencies of Photo‐Electrochemical Devices Producing Hydrogen and Valuable Oxidation Products

“…When water is oxidized this difference is 1.23 V, whereas bromide ions are oxidized at 1.09 V. As noted by Vesborg and Seger, PEC reactions with operating potentials near 2 V are optimally suited to match the efficiency of two-photon tandem PEC devices. [35] Given that the BrER reaction operates at 1.48 V, whereas both the OER and ClER operate at 1.73 V, it is clear that deviation from the optimal tandem device voltage significantly affects solar-tohydrogen fuel production and the BrER efficiency is mitigated by the misalignment of the reaction operating potential with the optimal tandem device potential.…”

“…In tandem devices, the efficiency of the bottom cell is hampered by low energy photon absorption of the electrolyte. [35] www.advsustainsys.com…”

“…[26,27] PEC systems for water splitting [17,[28][29][30][31][32][33] have been extensively modeled, and to less extent PEC systems for CO 2 reduction have been as well. [34,35] To the best of our knowledge, in-depth modeling of halide oxidation, or oxidation of water to H 2 O 2 , has yet to be published. In this perspective, we therefore model single and dual absorber devices where hydrogen (as a fuel) and valuable oxidation products are simultaneously produced.…”

Design principles for efficient photoelectrodes in solar rechargeable redox flow cell applications

“…large overpotential or poor selectivity) and cost of electrification of the stored energy (e.g. fuel cells) impede the wide implementation of these PEC fuel systems 5,6 . Another approach that has received attention for solar energy storage is integration with secondary batteries 6,7 .…”