Double Activation of Water Splitting by Strong Cation-Water Interaction

Abstract

Presented herein is the possibility of activating water molecules associatively by cations in electrolytes, as well as active sites of electrocatalysts. Cation-water interaction (CW), weakening the intramolecular O-H bonds of water molecules, significantly affected dissociation of water molecules in proton-deficient media, resulting in hydrogen evolution reaction (HER) based on water reduction reaction (WRR). Both the quantitative and qualitative hydration nature of cations (i.e., hydration strength and number) determined the strength of CW and therefore the WRR activity. The cationic dependency of CW on electrolytes was confirmed by bulk and surface-specific spectroscopic techniques. After the cation-water complexes based on strong CW (cation(water)(n)) were defined as the main reactants for WRR, the intermediate adsorbate on the catalyst surface was suggested to be water molecules doubly coordinated to cations as well as active sites (cation-water-catalyst). The double activation picture of the tricomponent intermediate was strongly supported by the surface-specific Raman spectra, confirming the polarization-induced weakening of the O-H bonds of water molecules near the Pt catalyst surface in addition to the CW-induced O-H weakening found in the electrolyte as well as on the catalyst surface. The smallest divalent Be2+ among a series of test cations, including the monovalent, divalent, and trivalent ones, showed the most remarkable WRR kinetic gain, the superiority of which was expected from its high charge density nature guaranteeing strong hydration strength and cationic acidity. The beryllium anomaly to eminently weaken the O-H bonds accelerated WRR at pH2 with 600 mV overpotential gain for 150 mA cm(-2) hydrogen production (c.f., 28 mA cm(-2) with Na+).