Enhanced Glycerol Electrooxidation Capability of NiO by Suppressing the Accumulation of Ni4+ Sites

Glycerol electrooxidation (GOR), as a typical nucleophilic biomass oxidation reaction, provides a promising anodic alternative for coupling green hydrogen generation at the cathode. However, the challenges of identifying active sites and elucidating reaction mechanisms greatly limit the design of high-performance catalysts. Herein, we use NiO and Ni/NiO as model catalysts to investigate glycerol oxidation. Electrochemical measurements and operando spectroscopic studies uncovered that Ni²⁺/Ni³⁺ species are the true active sites of NiO for GOR at lower potentials. However, the Ni²⁺/Ni³⁺ species formed on the NiO surface were easily converted to Ni⁴⁺ species (NiO₂) at higher potentials, which not only contributed to the overoxidation of glycerol electrolysis products but also worked as the main active sites of the competitive oxygen evolution reaction (OER), resulting in the rapid decay of Faradic efficiencies (FEs) at high potentials. Interestingly, for Ni/NiO, only Ni³⁺ species were formed on the surface. Experimental and density functional theory (DFT) investigations indicated that due to the relatively lower average valence state of Ni in Ni/NiO and strong electronic interaction on the Ni/NiO interface, the surface reconstruction of Ni/NiO was effectively manipulated. Only Ni/NiO → NiOOH (Ni³⁺) transformation was observed, and the formation of Ni⁴⁺ species was greatly suppressed. As a result, Ni/NiO delivered superior GOR activity, and the FE did not drop apparently at high potentials. This work offers mechanistic insight into how to identify and maintain the true active sites of catalytic materials for value-added nucleophile electrooxidation reactions.