Unbiased photoelectrochemical tandem reaction for concurrent oxidations with ultra-high overall faradaic efficiency

Abstract

The selective oxidation of biomass-derived compounds remains a critical challenge in sustainable biorefining. With 5-hydroxymethylfurfural (HMF) employed as a model reactant, current solar-driven HMF conversions and efficiencies of single photoelectrode-based photoelectrochemical (PEC) systems are suboptimal. Here, we report an unbiased tandem PEC system that enables concurrent selective oxidation of HMF to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) at both photoelectrodes, achieving a groundbreaking faradaic efficiency (FE_HMFCA) of 153.4%. The photoanode, composed of NiOOH-modified BiVO₄ (BVO), regulates the generation of hydroxyl radicals (˙OH) to enhance oxidative reactivity, as demonstrated by ESR spectroscopy. In situ XAS reveals that Ru-doped CuBi₂O₄ (CBO) at the photocathode promotes the oxygen reduction reaction (ORR), generating superoxide (˙O₂⁻) that further participates in the selective oxidation of HMF. This synergistic “dual-core processor” mechanism enables the tandem PEC device to achieve a stable photocurrent of 0.68 mA cm⁻², with an HMFCA selectivity exceeding 75% for both photoelectrodes. Additionally, a self-powered PEC system driven by solar energy demonstrates industrial scalability, achieving 355.2 mmol per m² HMFCA yield. This work provides foundational insights into the design of highly selective and efficient PEC systems for biomass valorization, setting a new benchmark for carbon-neutral chemical production and industrial scalability.