The hydrogen evolution and oxidation reactions (HER/HOR) are the most fundamental reactions in electrochemistry. They are also the electrochemical reactions taking place in practical devices such as H₂ fuel cells and electrolyzers for the conversion between hydrogen fuel and electricity. Despite their fundamental and practical importance, the HER/HOR kinetics has remained not explicitly clear even for the platinum (Pt) electrode that has been mostly studied. This is manifested by the lack of consensus on why the HER/HOR rate of Pt is about two orders of magnitude slower in alkaline (pH = 0) than in acid (pH = 13),¹ a long-standing puzzle in electrochemistry. Clear understanding of electrochemical reaction kinetics requires clear understanding of the charge transfer and chemical transformation at the electrode-electrolyte/membrane interface (EEI), which is however hampered by the lack of means to explore the complex electrochemical processes at the EEI in situ/operando.

Herein, by employing a set of N-methylimidazoles as the HER/HOR promotors and interface probes, we gave a reasonable explanation on how the HER/HOR of Pt proceeds within the EEI in aqueous solution.² A combination of in situ spectroscopic characterization on the EEI showed that the N-methylimidazoles promoted the HER/HOR of Pt in alkaline solution by reorientating interfacial water against the interfacial electric field. We accordingly proposed that the HER/HOR in acidic and alkaline solution proceeds via diffusion of proton and hydroxide, respectively, through the H-bond network of interfacial water by the Grotthuss mechanism (Figure 1). We further demonstrated that adding 1,2-dimethylimidazole into the alkaline solution fed into the cathode of an anion exchange membrane electrolyzer led to an 40% performance improvement. The diffusion roles of interfacial water unraveled here may apply to many other electrochemical processes in aqueous solution.

Acknowledgements

This work was supported by the Office of Naval Research (ONR) grant N000141712608. This research used beamline 7-BM (QAS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory Contract DE-SC0012704. Beamline operations were supported in part by the Synchrotron Catalysis Consortium Grant DE-SC0012335.

1. Durst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci. 7, 2255-2260 (2014).
2. Sun et al. Mechanistic insights into pH-dependent hydrogen electrocatalysis by flipping interfacial water. Under revision by Nat. Energy.