For a greenhouse gas emission free society, hydrogen from water electrolysis is fundamental. Despite decades of study, the mechanism of the hydrogen evolution reaction (HER) on the best available catalyst, Pt, remains controversial. At least in part, understanding is complicated by not being able to disentangle the involved timescales ranging from femtoseconds (interfacial charge transfer) to milliseconds (mass transport).I approach this problem by performing perturbation experiments with ultrashort laser pulses that drive the HER close to its reversible potential and induce charge transfer along the Pt-H bond in the underpotential deposition region [1]. I disentangle the involved timescales by performing both i) time averaging electrochemical measurements under femtosecond laser irradiation and ii) time resolved sum frequency generation spectroscopy (SFS) of Pt-H after laser excitation.Ultrafast charge transfer is found to be a function of interfacial structure (electrode and electrolyte) and trends on single crystals and a microelectrode correlate with HER activity and allow insight into the reactions rate determining step. SFS provides information on Pt-H as a function of potential, electrolyte composition and temporal evolution during and after ultrafast charge transfer. Implications for the HER are discussed.[1] ChemElectroChem 2019, 6, 2675-2682
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