Water Activation as a Limiting Factor for High Energy Efficiency in CO2 Electroreduction

Abstract The sustainable synthesis of multicarbon (C2+) products via electrochemical carbon dioxide reduction (CO2R) is one of the key pillars for carbon valorization. However, practical deployment is often stalled by severe energy inefficiencies that are exacerbated under the neutral and acidic conditions used to minimize carbon losses. This paper consolidates recent experimental and theoretical advances to elucidate how cation-induced shielding and interfacial alkalization govern cell performance. We show that, at industrial current densities, severe local proton depletion triggers an extreme interfacial alkalization that shifts the effective thermodynamic baseline of the reaction, incurring an operational thermodynamic penalty of 0.83 V (for valuable products like ethylene or ethanol). While a Nernstian “discount” arising from bulk-to-interfacial pH gradients partially offsets this cost, our analysis identifies a persistent net thermodynamic voltage penalty (NTVP) of ∼0.6 V for modern zero-gap membrane electrode assembly (MEA) cells—which acts as a practical energy floor for direct CO2R. To bypass this penalty, we evaluate different emerging strategies, ranging from anodic compensation to proton-donating cations. We suggest that breaking this energy floor for multicarbon products will require a delicate synchronization of interfacial proton delivery and dimerization kinetics—a missing link that must first be demonstrated through high-rate acidic COR. Until these selective acidic pathways reach maturity, we conclude that the sequential CO2-to-CO and CO-to-C2+ cascade architecture remains the most robust industrial strategy to circumvent the operational energy floor and achieve high-efficiency multicarbon production.