The hydrogen evolution reaction (HER) in alkaline medium is of great importance for the economic production of sustainable hydrogen. Nevertheless, the origin of the sluggish reaction kinetics observed in alkaline compared to acid medium, remains a point of debate [1-5]. In order to facilitate this debate, we have undertaken a systematic determination of the HER kinetics on Pt and Ni within a broad range of temperatures and pressures in both alkaline and acidic environment. At present, the study of HER at high temperatures and pressures (HTP) is limited by the interim stability and lifetime of the available reference electrodes (RE) [6, 7]. Thus, a novel Palladium hydride (PdH) RE design has been developed that enables its utilization at HTP. A constant electrochemically driven supply of H₂ on the PdH surface ensures preservation of the mixed (α+β)-phase in the PdH layer and thereby stable potential at HTP conditions for prolonged periods. Furthermore, the potential of the PdH RE has been calibrated as a function of temperature and pressure with respect to the reversible hydrogen electrode (RHE). The potential of the PdH electrode in the mixed (α+β)-phase is ~55 mV vs. the RHE at 25 ºC, 1 bar, and observed to be independent of temperature and pressure. When the PdH is overcharged with H₂ to assume the β-phase only, its potential drops to ~0 mV vs. the RHE. Using the PdH RE, the kinetics of Pt and Ni for the HER are estimated with varying operating pressure, temperature and electrolyte concentration.

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