The selectivity in general is strongly related to the type and amount of alkali cation in the electrolyte. Computational studies have demonstrated that a decrease in the hydration radii of alkali metal cations induces an increase in the electric field, which stabilizes dipolar reaction intermediates. This increase in electric field enhances the activity and selectivity towards C2+ products. These studies show the advantages of using Cs + as an alkali metal cation in the electrolyte. In addition to the alkali metal cation type, concentration also plays an essential role for the selectivity, since it results in a larger surface charge density. At 50 mA cm -2 , both electrolyte concentrations exhibit similar local Cs however their calculated C2/C1 ratio (0.12 vs. 0.33) are quite different. This variation indicates the influence of other factors, such as different potentials and H2O content in terms of selectivity changes. In addition, the Faradaic efficiency of H2 at 0.01 M CsHCO3 electrolyte concentrations for all current density is approximately 5% smaller than that of 0.1 M CsHCO3. This observation suggests that a higher Cs + content leads to a higher HER efficiency, consistent with the literature. A study by El-Nagar et al. showed a similar trend where HER reaches a minimum at 0.02 KHCO3 and then gradually increases with higher electrolyte concentrations. Adding to the complexity and already existing hypotheses of how cations affect the performance, this study reveals that the presence of Cs + is often accompanied by H2O and can contribute to a higher HER.