Figure 5.

Cation- and pH-dependent reaction entropy change of HER/HOR on Pt RDE, determined by (Appendix S2). Temperature-dependent CV of HER/HOR measured on Pt at pH 1 in H₂-saturated 0.1 M HClO₄ electrolyte (a) with 0.1 M LiClO₄. Temperature-dependent CV of HER/HOR measured on Pt at pH 13 of H₂-saturated (b) 0.1 M LiOH and (c) 0.1 M CsOH. Insets show the zoom-in of zero overpotential region. Temperature-dependent HER/HOR potential vs the potential of reference electrode (Hg/HgSO₄ for pH 1 and Hg/HgO for pH 13) at 293 K (d) at pH 1 and (e) at pH 13. (f) Cation dependence of the exchange current density j₀ and reorganization energy, as a function of reaction entropy, ΔS, of HER/HOR at pH 1 and pH 13. Note that the kinetics (j₀ and λ) of HER/HOR on Pt RDE at pH 1 was limited by mass transport (Figure S4) and cannot be separated from diffusion current density, thus the exchange current density at pH 1 shown in Figure S5c does not reflect the pure HER/HOR kinetics at pH 1. The thermodynamic HER/HOR entropy changes were found to increase in the sequence of Cs⁺ < Rb⁺ < K⁺ < Na⁺ < Li⁺, where adsorbed Cs⁺ and Rb⁺ could expel interfacial water molecules and weaken the hydrogen-bonding network of surrounding water molecules at OHL, which leads to an increase in molar entropy of interfacial water molecules S_H2O(l) and interfacial hydronium S_H3O⁺(aq), and a corresponding decrease in HER/HOR reaction entropy, ΔS = S_H2(g) – 2 × S_H3O⁺(aq) in acid (2H_(aq)⁺ + 2e^– ↔ H_2(g)) or ΔS = S_H2(g) + 2 × S_OH^–(aq)) – 2 × S_H2O(l) in alkaline electrolytes (2H₂O_(l) + 2e^– ↔ H_2(g) + 2OH_(aq)). Error bars were obtained from the standard deviation of 2–3 independent measurements. Full data points are available in Figure S10.