Highly Exposed Low-Valence Ni Sites of NiO(111) for Efficient Electrocatalytic Biomass Upgrading

The catalytic activity of transition metal oxides (TMOs) is significantly influenced by the exposure of different crystal facets due to the distinct arrangements of surface atoms. However, the detailed structural change of TMOs with a specific exposed facet and the corresponding structure–activity relationship remains ambiguous. In this work, we successfully fabricated the NiO catalysts with different facet exposures, including (111), (100), and (110). When applied for the structure-sensitive electrocatalytic 5-hydroxymethylfurfural oxidation reaction (HMFOR), the as-prepared NiO(111) exhibited a low onset potential of 1.23 V and achieved a current density of 10 mA cm^–2 at 1.39 V. Moreover, it realized over 99% HMF conversion and approximately 99% selectivity of FDCA. Detailed in situ experiments demonstrated that the electrooxidation of HMF on NiO requires the simultaneous fulfillment of hydroxyl adsorption and strong affinity for HMF. Further, characterizations and DFT calculations confirm that the NiO(111) with numerous exposed and continuous low-valence Ni sites can realize both hydroxyl adsorption with the lowest adsorption energy of −1.78 eV and sufficient affinity sites for HMF with d–π interactions. Additionally, the unique atomic arrangement of the NiO(111) with the most pronounced charge transfer demonstrated superior charge-transfer capability compared to that of the (100) and (110). This work provides insights into the fine structure and surface evolution process of NiO with different exposed facets and offers guidance for designing active Ni sites for efficient electrocatalytic biomass conversion.