Hard-Sphere Mixture Thermodynamics Are Necessary to Capture Mixed Cation Effects in CO2-to-CO Electroreduction

The rate of the electrochemical reduction of CO2 (CO2RR) is sensitive to the identity of the alkali metal cation, with many mechanistic studies examining their interfacial effect in single cation electrolytes. However, cation effects in mixed cation electrolytes remain underexplored, despite their relevance in zero-gap, anion exchange membrane electrolyzers or in their deliberate use as bulk electrolytes. We herein explore the impact of size-sensitive, competitive interfacial assembly of cations in mixed cation systems and predict their resultant influence on the rate of CO2RR. The rate of Au-catalyzed CO2RR to CO was examined in Cs:X bicarbonate mixtures, where X is either Li or Na, with Cs mole fractions ranging from 1.0 to 0.0. Electrolysis experiments reveal that CO production rates increase with diminishing returns with increasing Cs mole fraction and applied potential, implying preferential accumulation of Cs cations over the cocation at the interface. Continuum scale modeling with finite-size effects demonstrates that Cs outcompetes Li and Na at the interface under polarization. This effect qualitatively agrees with experiment using the Boublík-Mansoori-Carnahan-Starling-Leland (BMCSL) model, which captures hard-sphere mixture thermodynamics. This work demonstrates the utility of mixed cation electrolytes as a platform to reduce the usage of Cs in CO2RR devices, as well as establishing the necessity of BMCSL to capture CO2RR effects in mixed cationic interfaces.