Hydrogen Evolution Reaction on Transition Metals: Promoting Water Dissociation By Tuning the Surface Oxophilicity.

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Hydrogen evolution reaction (HER) has been at the core of the development of fundamentals of electrochemistry and electrocatalysis. In acidic electrolytes, it has been generally accepted that the differences in reaction rates/activity are closely related to the differences in hydrogen adsorption free energy on different materials. In recent years, the fundamental investigation of the HER has been enjoying a renewed vigor, but this time in alkaline electrolytes, where there is much less consensus on the underlying activity descriptors. While some groups insist on the single descriptor of hydrogen binding energy, our group has continuously pointed out to the bifunctional nature of the HER in alkaline environments, i.e. a good catalyst needs both a beneficial OHad energetics and beneficial Had energetics. In most cases, the OHad-metal energetics is by far the more unfavorable of the two and OHad adsorption therefore represents the rate determining step. In some cases (e.g. polycrystalline iridium), the OHad-metal energetics no longer represents a significant obstacle in the HER process; hence the catalyst's rate determining step becomes Had adsorption and the catalyst starts behaving as it would in acid – as a pseudo mono-functional catalyst. For most materials, however, the HER rates in alkaline environments are significantly lower than in acid. One such example is copper (Cu), where reported values for HER activity in alkaline are indeed lower but are interestingly scattered over a wide range of values. In a manuscript recently published [1], we were interested in the underlying cause of these discrepancies. We explore how the different history the polycrystalline copper surfaces experienced changes their electrochemical properties (e.g. the HER activity). By using detailed spectroscopic surface analysis, we were able to find different extents of oxidation on various copper surfaces and link them to different hydrophilicity of the surfaces. With the established trends we propose that the formation of Cu(I)-OHad‐‐‐OH2 activated complex is essential in lowering the energy barrier for water dissociation, which leads to higher activity for the HER in alkali environments. [1] Farinazzo Bergamo Dias Martins, P. et al. Hydrogen evolution reaction on copper: Promoting water dissociation by tuning the surface oxophilicity. Electrochem. commun. 100, 30-33 (2019).