In the context of the GMPNP simulations, the cations (i) differ in terms of their solvated size (a i in equation ( )) and the hydration number of each ion (w i in equation ( )) (the values of both parameters for each cation can be found in the SI). The trend of solvated sizes of the cations in an aqueous solution follows the order Cs + <K + <Na + <Li + which is the inverse of the trend for the neutral atoms due to difference in the hydration properties amongst the alkali metal cations . Figure in the SI shows an illustration of the effect of cations with different sizes on the concentration at the steric limit and the thickness of the EDL. This can be used to understand the difference in concentration profiles of the cations in the EDL as shown in Figure , with Cs + being the smallest solvated cation reaching the highest concentration at the OHP followed very closely by K + with a similar solvated size, followed by Na + and Li + in that order. The resulting trend in the electric field strength at the OHP is shown in Figure . Despite reaching a higher concentration at the OHP and therefore a higher charge density, the electric field strength for Cs + is found to be weaker as compared to K + . This is due to the fact that Cs + polarizes fewer water molecules compared to the other alkali metal cations and therefore the drop in relative permittivity of the solution at the OHP is smaller based on the equation (12) (see Figure ). Multiple experimental results have shown Cs + to benefit the activation of CO 2 relative to other cation types . The electric field strength close to the cathode surface in the presence of the cations has been regarded as an important factor leading to the observed experimental results owing to the interaction of the field with the adsorbed polar reactive species . However, based on our results, we believe that the observed improvement in the reduction of CO 2 in the presence of Cs + cannot be understood solely on the basis of electric field strength. The specific ion interactions of the cation with the surrounding water and the CO 2 , as well as with the charged metal surface, also need to be taken into account.