Figure 6.

Altering interfacial structure at the Pt surface in aqueous electrolytes through noncovalent interactions associated with the hydrated cations, water molecules, and negatively charged Pt surface. The potential-dependent OH stretching features of in situ SEIRAS spectra of HER/HOR measured from 0.9 V_RHE to −0.3 V_RHE in a H₂-saturated aqueous solution of 0.1 M of (a) LiOH, (b) KOH, and (c) CsOH, where the reference spectrum was taken at 1.1 V_RHE. Full spectra before and after subtracting by reference spectrum (at 1.1 V_RHE) are available in Figure S11. (d) Deconvolution of the OH stretching peak at 0 V_RHE into three components: 3570 cm^–1 (weakly H-bonded/isolated water), 3450 cm^–1 (asymmetric H-bonded water), and 3270 cm^–1 (symmetric H-bonded water). The potential dependence of the relative fractions of (e) isolated water, (f) asymmetric H-bonded water, and (g) symmetric H-bonded water. (h) Reorganization energy and exchange current density of HER/HOR measured on Pt RDE in 0.1 M hydroxide of Li⁺, K⁺, and Cs⁺ as a function of the relative fraction of isolated water. Error bars were obtained from the standard deviation of 2 independent measurements. (i) Scheme summarizing the proposed mechanisms of cation dependent kinetics of HER/HOR, where Li⁺ ions retain their solvation shell intact and promote strongly hydrogen-bonded interfacial water layer, enhance proton-coupled electron transfer rate of the Volmer or Heyrovsky step by facilitating interfacial water molecule reorganization. Cs⁺ ions partially desolvate and attach to the negatively charged Pt surface, promoting weakly H-bonded interfacial water molecules, leading to an increase of the barrier of proton transfer during the Volmer or Heyrovsky step in HER/HOR on the Pt surface.