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

Abstract

Electrochemical water splitting is a key process for producing sustainable and clean hydrogen. Typically, high valence state sites are favorable for oxidation evolution reaction, while low valence states are suitable for hydrogen evolution reaction (HER) due to efficiency and stability issues. However, we propose a high valence state of Co3+in Ni9.5Co0.5-S-FeOxhybrid as active center for efficient and stable HER, while structural analogues with relatively low chemical states show much worse performance. The Ni9.5Co0.5-S-FeOxcatalyst can drive alkaline HER with an ultra-low overpotential of 175mV for 1000mAcm-2at the industrial working temperature of 60oC, with an excellent stability over 300h. Moreover, this material can work as a favorable bi-functional catalyst for both OER and HER, with a low cell voltage being 1.730V to achieve 1000mAcm-2for overall water splitting over 300h. Synchrotron radiation based X-ray absorption spectroscopy (XAS) clearly identifies the high valence Co3+sites, while in-situ XAS during HER and theoretical calculations reveal the favorable electron capture capability at Co3+and suitable H adsorption/desorption energy around the high valence sites, which can accelerate the HER reaction. The understanding of high valence state to drive the reductive reaction may pave the way for the rational design of energy-related catalysts.