Unlocking d-orbital tunability in nickel-based hydroxide by a high-entropy strategy for boosting electrocatalytic 5-hydroxymethylfurfural oxidation

Abstract

The electrocatalytic 5-hydroxymethylfurfural oxidation reaction (HMFOR) offers a sustainable and green route for upgrading biomass-derived chemicals and as a substitute for the energy-intensive oxygen evolution reaction. Nickel-based hydroxides are famous HMFOR electrocatalysts, whose activities are closely correlated with their electronic structures near the Fermi level. Recent advances identify lattice distortion as a crucial factor affecting the electronic structures of nickel-based hydroxides, thereby governing the electrocatalytic performance. Herein, we report the modulated d-orbital electronic state of Ni active sites in NiCoMnCrCu hydroxide via a high-entropy strategy, which induces lattice distortion that disrupts the NiO₆ octahedron symmetry. The distortion moves up the d_x²−y² orbital of the Ni²⁺ state closer to the Fermi level to promote the deprotonation step for generating reconstructed active sites. Meanwhile, it broadens the d_x²−y² orbital of the Ni³⁺ state, which facilitates the electron transfer between the reactants and the catalyst in the spontaneous chemical reaction. Hence, the high-entropy hydroxide displays superior HMFOR performance with a low onset of 1.334 V vs. the RHE, while selectively converting HMF into 2,5-furandicarboxylic acid (FDCA) with a high faradaic efficiency (above 90%) and fast production rate (2178.3 μmol cm⁻² h⁻¹). When employed in a flow cell for coupled biomass upgrading and hydrogen production, the system enables steady co-production of value-added FDCA and energy-saving hydrogen. The present study provides insights into modulation of the electronic states of active sites via a high-entropy strategy.