Robust Cu(0) active sites stabilized by high surface pH gradient for ultra-low-potential HMF oxidation

Abstract

Electrochemically converting biomass-derived 5-hydroxymethylfurfural (HMF) into a valuable product, particularly 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), is a promising method for achieving a sustainable chemical industry. Cu-based catalysts can drive the HMF oxidation reaction an at ultra-low potential. However, the active potential window of Cu-based catalysts is limited by the oxidation of Cu⁰ when the potential reaches the redox potential of Cu⁰/Cu¹. Considering that the oxidation of Cu⁰ is dependent on the accessibility of OH⁻, we present a design principle to stabilize highly active Cu⁰ against oxidation to Cu–OH_ad by utilizing the large pH gradient formed during catalysis on high-roughness Cu electrodes. Open circuit potential decay transient tests (OCP) confirm that the diffusion time for OH⁻ increases threefold on the Cu nanowire surface, and the surface pH value is much lower during the catalysis. In situ Raman and IR spectra demonstrate that the inhibition of Cu–OH_ad could expand the active potential range of Cu⁰ by 200 mV. As a result, Cu nanowire (Cu NWs) catalysts with the highest roughness factor offer a wide potential range of 0.1–0.5 V vs. RHE for HMF oxidation with a high current density of ∼100 mA cm⁻². This strategy can further support the novel design of fuel cells in combination with the oxygen reduction reaction (ORR) and a stable output current of >40 mA cm⁻² can be achieved.