Construction of NiCo-metaphosphate/hydroxide with synergistic heterostructure for sustainable biomass electro-oxidation

Coupling electrooxidation of 2,5-bis(hydroxymethyl)furan (BHMFOR) with electroreduction of 5-hydroxymethylfurfural (HMFRR) might be a feasible approach towards the sustainable synthesis of value-added chemicals. Rational design of efficient and durable catalysts is crucial to this novel strategy. Herein, we demonstrate that the NiCo phosphide (NiCoP) nanosheet could be anodically reconstructed into a hydroxide-modified metaphosphate nanosheet (i.e., NiCoPi/O_i). The NiCoPi/O_i exhibits excellent BHMFOR performance with a low onset potential of 1.23 V_RHE to yield 2,5-furandicarboxylic acid (FDCA) with nearly quantitative faradaic efficiency (FE), high selectivity (99%) and robust stability (for 10 cycles). A series of in situ spectroscopic analyses reveal dynamic cycles of NiOOH/Ni(OH)₂ on the catalyst surface under the BHMFOR conditions, verifying an indirect oxidation pathway. As demonstrated by density functional theory (DFT) calculations, benefiting from the synergistic effect of heterostructures, the NiCoPi/Ni(OH)₂ can enhance the adsorption of BHMF, hence promoting the BHMFOR through the indirect pathway. Furthermore, an electrode pair of NiCoP//NiCoPi/O_i is employed for the electrolysis of HMFRR/BHMFOR with graded electrolytes, fulfilling the concurrent synthesis of BHMF and FDCA with a combined FE of 134%. Our work thus provides new insights into catalyst design for biomass upgrading as well as a sustainable strategy for an overall conversion from HMF to FDCA.