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

Journal Article · Mon Sep 19 00:00:00 EDT 2022 · Proceedings of the National Academy of Sciences of the United States of America

DOI:https://doi.org/10.1073/pnas.2208187119· OSTI ID:1888062

Zhong, Guangyan ^[1]; Cheng, Tao ^[2]; Shah, Aamir Hassan ^[1]; Wan, Chengzhang ^[1]; Huang, Zhihong ^[3]; Wang, Sibo ^[1]; Leng, Tianle ^[4]; Huang, Yu ^[5]; Goddard, III, William A. ^[6]; Duan, Xiangfeng ^[7]

---

1. Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095
2. Institute of Functional Nano &, Soft Materials, Jiangsu Key Laboratory for Carbon-Based Functional Materials &, Devices, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, People’s Republic of China, Materials and Process Simulation Center, California Institute of Technology, Pasadena, CA 91125
3. Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095
4. Materials and Process Simulation Center, California Institute of Technology, Pasadena, CA 91125
5. Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, California NanoSystems Institute, University of California, Los Angeles, CA 90095
6. Materials and Process Simulation Center, California Institute of Technology, Pasadena, CA 91125, Liquid Sunlight Alliance, California Institute of Technology, Pasadena, CA 91125
7. Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, California NanoSystems Institute, University of California, Los Angeles, CA 90095

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 pK _a $=$ 4.3. Quantum mechanics (QM) and reactive dynamics using a reactive force field (ReaxFF) molecular dynamics (RMD) calculations confirm the enrichment of hydroniums (H ₃ O ${}^{+*}$ ) near Pt surface and predict a surface hydronium pK _a of 2.5 to 4.4, corroborating the experimental results. Importantly, the observed Pt-surface hydronium pK _a 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.

• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0021266
• OSTI ID: 1888062
• Journal Information: Proceedings of the National Academy of Sciences of the United States of America, Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Issue: 39 Vol. 119; ISSN 0027-8424
• Publisher: Proceedings of the National Academy of Sciences
• Country of Publication: United States
• Language: English