The MCH theory predicts when the thermodynamic driving force is sufficiently large, the current density would level off at a value, i.e., 𝑗 '%( (Eq. 2B) , the magnitude of which contains contributions from both the electronic and vibrational couplings. Fitted 𝑗 '%( exhibits a cation-dependence in the sequence of Cs > K > Na ≈ TMA > Li > TEA > TPA (Fig. and). In Eq. 2B, 𝜌 -depends only on the electrode material and could thus be considered as cation-independent, and 𝛽 is also not affected by the electrolyte. With the impact of mass transport eliminated with the RDE method, [𝐶𝑂 # ] close to the electrode is expected to be the same as that in the bulk. With the identification of the proton donor in CO2RR, [𝐻 # 𝑂] refers to the concentration of water molecules in the cations' first hydration shell. Considering that cations are electrostatically drawn to electrode at large cathodic overpotentials, the cation concentration at the interface is expected to be substantially higher than that in the bulk electrolyte and dependent largely on the electrode potential. Thus, the variations of 𝑗 '%( with cations could be attributed primarily to 𝑉 *. # . Adopting the vibronically nonadiabatic PCET rate theory developed by Hammes-Schiffer and co-workers , 𝑉 *. factorizes into the electronic coupling and the overlap of the proton vibrational wavefunctions between the reactant and the product, as in Eq. 4: