Key Role of Cations in Stabilizing Hydrogen Radicals for CO₂-to-CO Conversion via a Reverse Water-Gas Shift Reaction

Electrochemically converting CO₂ into valuable chemicals and fuels in acidic media is argued as a promising energy- and carbon-efficient route. Although several key roles of alkali cations have been unveiled, the alkali cation trends for CO₂ reduction remain largely elusive. With decreasing cation size from Cs⁺ to Li⁺, here we show that the apparent proton diffusion coefficient in 3.0 M Li⁺ is tens-fold lower than in 3.0 M K⁺ and 3.0 M Cs⁺ acidic electrolytes. Although Li⁺ has the strongest inhibition ability for proton transport, it acts the worst for both the CO₂-to-CO conversion and partial current density on Au catalysts. Unexpectedly, K⁺ with a higher proton transport performs the best for CO₂-to-CO conversion. We thus revisit the roles of alkali cations and find that hydrated K⁺ can stabilize hydrogen radicals benefiting CO₂ conversion at the electrode interface while for Li⁺ this is not the case. This study proposes that cation-stabilized atomic hydrogen assists in activating CO₂ via a reverse water-gas shift route under electrochemical conditions.