In situ observation of photoelectrochemical water oxidation intermediates for selective biomass upgrading with simultaneous hydrogen production

Abstract

In a water-splitting reaction, the anodic biomass upgrading with the simultaneous cathodic hydrogen production is a promising sustainable energy development for future needs. In the electrocatalytic water oxidation process, the reactive intermediates generated in the oxygen evolution reaction (OER) can be used for the selective oxidation of organic molecules to value-added chemicals. Herein, we fabricated a multifunctional catalyst, composed of phosphate-functionalized few-layer phosphorene (FLP-P) and bismuth ferrite (BFO) nanosheets (referred to as FLP-P-BFO), for photoelectrochemical (PEC) water-splitting reactions with the advantages of an efficient charge separation and the hybrid excitations of electrons and photons. At the cathode, the PEC-assisted FLP-P-BFO-catalytic reaction exhibits excellent hydrogen evolution reaction (HER) activity of a low overpotential of 110 mV at 10 mA cm−2 and a small Tafel slope of 51 mV dec-1. At the anode, the high FLP-P-BFO-catalytic efficiency is attributed to the bifunctional mechanism, in which the functionalized phosphate groups help to stabilize the Fe-OOH intermediate, thus mitigating the energy demand in the OER process. The bifunctional mechanism was validated by both pH-dependent and isotope-labeling examinations. With the assistance of in situ Raman spectroscopy, the optimal electrochemical conditions for the maximal production of the Fe-OOH intermediate in OER were obtained for the biomass upgrading of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) with 100% conversion and selectivity, but without reducing the HER activity. The anodic biomass conversion, from HMF (a carbon–neutral feedstock) to the high value-added FDCA (a monomer of bio-plastics), with the simultaneous cathodic H2 production is a promising sustainable energy innovation. This novel strategy of employing PEC-assisted FLP-P-BFO-catalytic biomass upgrading to value-added products is extendable to combine the cathodic HER with many other anodic hydrocarbon oxidations for future energy-related applications.