Hydronium ions inhibit CO2 reduction on coinage metals

The carbon-efficient electrochemical reduction of CO2 (CO2RR) requires acidic electrolytes to mitigate irreversible electrolyte carbonation. However, CO2RR only takes place in acidic electrolytes upon addition of spectator cations and the mechanistic basis for this requirement remains unclear. Herein, we present scanning voltammetry experiments conducted in bulk acidic electrolytes in conjunction with dynamic electrochemical mass spectrometry (DEMS) analysis of CO2 consumption and product formation. By quantifying both faradaic CO2 consumption via electroreduction and non-faradaic CO2 consumption via electrolyte carbonation in real-time, we deconvolute the direct role of the added electrolyte cations from their indirect role in fostering a transient local pH swing. We find that across coinage metals and electrolyte compositions, CO2RR commences only upon substantial depletion of hydronium and coincides with non-faradaic CO2 uptake via carbonation. Doping Ag with Pt to promote hydronium consumption via efficient H2 evolution serves to lower the overpotential for CO2RR onset in scanning voltammetry experiments by ~1 V. These findings together indicate that interfacial alkalinization is a pre-requisite for, rather than merely a consequence of CO2RR, and that the primary role of the supporting electrolyte cations is to establish a non-equilibrium interfacial ionic environment depleted in hydronium ions. The findings imply a putative mechanistic model in which hydronium ions serve to inhibit CO2 activation at coinage metal surfaces. This work highlights the value of pre-steady state real-time analysis of both faradaic and non-faradaic interfacial reactions and establishes a self-consistent mechanistic picture for CO2RR in bulk acidic electrolyte.