High Valence State Sites as Favorable Reductive Centers for High-Current-Density Water Splitting

Electrochemical water splitting is a promising approach for producing sustainable and clean hydrogen. Typically, high valence state sites are favorable for oxidation evolution reaction (OER), while low valence states can facilitate hydrogen evolution reaction (HER). However, here we proposed a high valence state of Co³⁺ in Ni_9.5 Co_0.5 -S-FeO_x hybrid as the favorable center for efficient and stable HER, while structural analogues with low chemical states showed much worse performance. As a result, the Ni_9.5 Co_0.5 -S-FeO_x catalyst could drive alkaline HER with an ultra-low overpotential of 22 mV for 10 mA cm^-2 , and 175 mV for 1000 mA cm^-2 at the industrial temperature of 60 °C, with an excellent stability over 300 h. Moreover, this material could work for both OER and HER, with a low cell voltage being 1.730 V to achieve 1000 mA cm^-2 for overall water splitting at 60 °C. X-ray absorption spectroscopy (XAS) clearly identified the high valence Co³⁺ sites, while in situ XAS during HER and theoretical calculations revealed the favorable electron capture at Co³⁺ and suitable H adsorption/desorption energy around Co³⁺ , which could accelerate the HER. The understanding of high valence states to drive reductive reactions may pave the way for the rational design of energy-related catalysts.