(Keynote) Spectroscopic and Theoretical Studies of HOR/HER Electrochemical Interfaces

Abstract

Hydrogen fuel cells and water splitting electrolyzers are clean electrochemical devices toward the sustainable utilization of H2 as an energy carrier to replace traditional fossil fuels, in which hydrogen oxidation and evolution (HOR/HER) are two key reactions. Regardless of the apparent simplicity of these two reactions that involve *H as the only surface intermediates, the reaction mechanisms are still under intensive debate.It has been well reported that the HOR/HER kinetics in alkaline media is about two orders of magnitude slower than that in acid environment on Pt-group metals, while the reasons are not fully revealed. In this work, power surface-enhanced infrared absorption spectroscopy (SEIRAS) is utilized to directly monitoring the structures of HOR/HER electrochemical interfaces in a wide pH range. Through the changes of vibrational wavenumbers of Pt-H and O-H stretching, we find both the weakened *H and H2O binding strength on Pt as the solution pH increases, which are important contributions to the lowered reaction kinetics. Through constructing density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulation models that contain various interfacial parameters, HER activation energy barriers as a function of pH are further elaborated.In addition, Ru adatoms have been found to significantly accelerate HER/HOR kinetics on Pt in alkaline media. However, the promotion mechanism is unclear. Through directly monitoring the vibrational features of Pt-H and Ru-H on Ru-modified Pt thin films, we find that the HER/HOR kinetics is proportional to the Ru coverage, which unambiguously demonstrates Ru overlayer as the true active sites, instead of an auxiliary to Pt through the oxophilicity or alloying effects as proposed previously. DFT calculations illustrate that the strain and electronic effects of the Pt substrate lead to the more suitable binding energies of *H and *OH on the Ru overlayer, which lower the activation barrier of the rate-determining Volmer step.The authors acknowledge the support from Hong Kong Research Grant Council (16308420, 16310419, C6011-20G, and HKUST PDFS2021-6S08), Hong Kong Branch of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (SMSEGL20SC01), and Hong Kong Innovation and Technology Commission (ITC-CNERC14EG03).