Photoelectrochemical upgrading of biomass-derived compounds over hematite nanorods decorated with bimetallic zeolitic imidazolate frameworks

Replacing the kinetically sluggish oxygen evolution reaction (OER) with biomass oxidation reaction at photoanodes would provide a cost-effective and energy-efficient pathway to the simultaneous production of hydrogen at the cathode and value-added chemicals at the anode in a photoelectrochemical (PEC) configuration. In this work, titanium-doped hematite nanorods (Hem) decorated with CoNi bimetallic zeolitic imidazolate frameworks (ZIF) were developed via room temperature deposition to serve as the photoanode for 5-hydroxymethylfurfural (HMF) oxidation. By employing 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as the redox mediator in HMF-containing alkaline electrolyte, the obtained CoNi-ZIF/Hem photoanode exhibits excellent activity for PEC TEMPO-mediated HMF oxidation reaction (HMFOR), with photocurrent density reaching 1.09 mA cm−2 at a low bias of 1.1 V vs. reversible hydrogen electrode (RHE). Experimental results and theoretical calculations demonstrate that CoNi-ZIF would accelerate charge transfer and separation as well as strengthen TEMPO adsorption on the surface, which benefits PEC TEMPO-mediated HMFOR for 2,5-furandicarboxylic acid (FDCA) production. With high HMF conversion of ~99% and FDCA yield of ~98% achieved in a well-designed PEC flow-cell reactor within 2 h under 1 sun illumination, encouragingly, the as-prepared photoanode presents excellent PEC performances for TEMPO-mediated oxidation of various aldehyde-containing biomass-derived compounds. This successful attempt on the surface decoration of hematite photoanodes for HMFOR is believed to pave a new way to the rational design and assembly of economic and efficient PEC configuration for paired biomass valorization and hydrogen production.