Relating Cu Phase Transitions to Selectivity Recovery During Pulsed CO2 Reduction on Gas Diffusion Electrodes

Abstract Pulsed electrochemical CO2 reduction on Cu catalysts represents a promising strategy to mitigate catalyst degradation and sustain high selectivity toward multicarbon (C2+) products. However, developing a detailed mechanistic understanding of the oxidative pulses remains challenging due to the inability to resolve the Cu oxidation state and the local electrolyte pH in the vicinity of the catalyst, especially inside a porous gas diffusion electrode. In this study, we investigate the impact of different potential pulse profiles on the recovery of CO and C2+ product selectivity, the latter representing C–C coupling. The potential-pH trajectories of the pulses are mapped onto calculated potential-pH Cu phase diagrams for CO2-saturated aqueous KCl and KHCO3 electrolytes to identify the thermodynamically accessible Cu phases during pulses. We show that abrupt pulses at a local alkaline pH lead to simultaneous C2+ and CO recovery. In contrast, quasi-adiabatic pulses, where the local pH remains equilibrated with the bulk pH, recover CO selectivity independently of C2+ products. Furthermore, in acidic KCl electrolytes, where no Cu oxide is thermodynamically accessible during a quasi-adiabatic pulse, the recovery of C2+ is attributed to Cu dissolution and redeposition. Importantly, this finding demonstrates that the formation of a Cu oxide phase, even temporarily during the pulse, is not required for C–C coupling. In contrast, in KHCO3 electrolyte, the formation of a Cu oxide phase remains thermodynamically accessible during pulses. Moreover, we show that the oxidative charge of the pulse recovering CO selectivity scales with prior reduction time, hypothesized to be a removal of surface poisoning species. Our findings enable rational design of pulse protocols based on electrolyte-, potential-, and pH-resolved Cu phase diagrams and provide a framework that can be extended to other catalysts and electrolytes.