Heterostructured Bismuth Vanadate/Cobalt Phosphate Photoelectrodes Promote TEMPO-Mediated Oxidation of 5-Hydroxymethylfurfural

Motivated by replacing the kinetically unfavorable oxygen evolution reaction (OER) and producing value-added products in photoelectrochemical cells (PECs), we report that bismuth vanadate (BiVO4) photoelectrodes modified with a cobalt phosphate (CoPi) overlayer facilitate 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated selective oxidation of 5-hydroxymethylfurfural (HMF). CoPi layers with sufficient thickness were found to reduce the potential required for TEMPO oxidation by 0.5 V, as well as increase charge injection efficiency sevenfold compared to BiVO4 without CoPi. Furthermore, the undesired OER was completely suppressed when using the heterostructured photoanodes. Transient photocurrent measurements suggested that CoPi alleviates recombination losses resulting from the back reduction of oxidized TEMPO. The PECs with BiVO4/CoPi bilayer achieved 88 % yield to 2,5-furandicarboxylic acid (FDCA) from HMF oxidation under mild conditions, whereas <1 % FDCA was generated with BiVO4. These findings suggest that suppression of the back reduction process substantially improves the efficiency of the oxidation, giving a potential route to more efficient solar fuel/chemical production.