The hydrogen evolution reaction (HER) is a fundamental and critical process that enables H₂ production through electrochemical water splitting. Conventional HER catalysts are based on metal-containing materials in which the metal atoms function as the catalytic centers. Recently, we demonstrated a metal-free organic molecular electrocatalyst (OMEC) for HER in acidic media with high activity and durability.^{1, 2} This OMEC is based on a small pyridine-based heterocycle. A density functional theory (DFT) study of the HER mechanism catalyzed by this molecule revealed that the thermodynamic limiting potential is related to the atomic hydrogen adsorption energy at the pyridyl-N site.^{1, 2} Further, our systematic study of the substituent effects on the proton-coupled electron transfer (PCET) potential at nitrogen-containing heterocycles provided guidance to the rational design of OMECs for HER.³

In this talk, we will present a study on the substituent effects on the activity of the organic molecular electrocatalyst mentioned above.^1,2 Several substituents with electron-withdrawing/donating character will be included in this study. The HER activity of these substituted molecules is determined experimentally through electrochemical measurement. Their HER onset potentials are further correlated with the H adsorption energy values at pyridyl-N sites predicted from DFT calculations. Through the combined experimental-computational approach, we demonstrate the structure-activity relationship within this particular type of OMECs, guiding future OMEC research.

Acknowledgments

Financial support for this research by Los Alamos National Laboratory through Laboratory Directed Research and Development (LDRD) Exploratory Research (ER) program is gratefully acknowledged.

References

1. X. Yin, L. Lin, H. T. Chung, U. Martinez, A.M. Baker, S. Maurya, P. Zelenay. Organic Molecular Catalyst for Hydrogen Evolution Reaction, Meeting Abstracts, The Electrochemical Society, MA2017-02, 1659 (2017).
2. X. Yin, L. Lin, U. Martinez, H. T. Chung, P. Zelenay. Bifunctional Organic Molecular Electrocatalyst for Hydrogen Evolution Reaction and Hydrogen Peroxide Production, Meeting Abstracts, The Electrochemical Society, MA2018-02, 1609 (2018).
3. X. Yin, E. F. Holby, P. Zelenay. Structure-Activity Data Mining for Hydrogen Evolution Reaction at Organic Molecular Electrocatalysts, Meeting Abstracts, The Electrochemical Society, MA2019-01, 1429 (2019).