where 𝐶 1 represents a monovalent cation and 𝑛 is the number of water molecules in the first hydration shell of the cation. It is clear that the thermodynamics of Eq. 3A depends on the ability of 𝐶 1 (𝐻 # 𝑂) 2 to donate H + , i.e., 𝑝𝐾 % (𝐶 1 (𝐻 # 𝑂) 2 ), as shown in Eq. 3B (derivations included in Supplementary Note 3). The linear relation between 𝐸 * * and ∆𝐺 /0& can be derived based on the previous reports (Supplementary Note 3). The slope correlating 𝐸 * * and ∆𝐺 /0& in Fig. (-0.21) is generally in line with the calculated value based on the data from literature (~ -0.12) , supporting the reliability of our results. Thus, these results provide compelling evidence that the water molecules in the first hydration shell of cations serve as the proton source in CO2RR on Au, and the cations impact the equivalent standard potential of the RDS by tuning the acidity of these water molecules.