Determining the hydronium pKα at platinum surfaces and the effect on pH-dependent hydrogen evolution reaction kinetics

Electrocatalytic hydrogen evolution reaction (HER) is critical for green hydrogen generation and exhibits 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 = 4.3. A molecular dynamics (MD) and reactive dynamics using a reactive force field (ReaxFF) quantum mechanics (QM) calculations confirm the enrichment of hydroniums (H3O+) near Pt surfaces 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 a 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.

Authors: Guangyan Zhong, Tao Cheng, Aamir Hassan Shah, Chengzhang Wan, Zhihong Huang, Sibo Wang, Tianle Leng, Yu Huang, William A. Goddard III, Xiangfeng Duan

Journal: Proc. Natl. Acad. Sci. U.S.A., 2022, 119, e2208187119

Full Text

Please email to tcheng@suda.edu.cn if you need the full text.