%0 Journal Article
%T Determining the hydronium pK[Formula: see text] at platinum surfaces and the effect on pH-dependent hydrogen evolution reaction kinetics.
%A Zhong G
%A Cheng T
%A Shah AH
%A Wan C
%A Huang Z
%A Wang S
%A Leng T
%A Huang Y
%A Goddard WA
%A Duan X
%J Proc Natl Acad Sci U S A
%V 119
%N 39
%D 09 2022 27
%M 36122216
%F 12.779
%R 10.1073/pnas.2208187119
%X Electrocatalytic hydrogen evolution reaction (HER) is critical for green hydrogen generation and exhibits distinct pH-dependent kinetics that have been elusive to understand. A molecular-level understanding of the electrochemical interfaces is essential for developing more efficient electrochemical processes. Here we exploit an exclusively surface-specific electrical transport spectroscopy (ETS) approach to probe the Pt-surface water protonation status and experimentally determine the surface hydronium pKa [Formula: see text] 4.3. Quantum mechanics (QM) and reactive dynamics using a reactive force field (ReaxFF) molecular dynamics (RMD) calculations confirm the enrichment of hydroniums (H3O[Formula: see text]) near Pt surface and predict a surface hydronium pKa of 2.5 to 4.4, corroborating the experimental results. Importantly, the observed Pt-surface hydronium pKa correlates well with the pH-dependent HER kinetics, with the protonated surface state at lower pH favoring fast Tafel kinetics with a Tafel slope of 30 mV per decade and the deprotonated surface state at higher pH following Volmer-step limited kinetics with a much higher Tafel slope of 120 mV per decade, offering a robust and precise interpretation of the pH-dependent HER kinetics. These insights may help design improved electrocatalysts for renewable energy conversion.