Dual Co Sites in n─n Type Heterojunction Enable Selective Electrochemical Co-Valorization of HMF and CO₂

Electrocatalytic oxidation of biomass-derived hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) and electrocatalytic reduction of CO₂ into CO are two highly investigated areas. Efficient electrocatalytic system design that combines CO₂ valorization with biomass upgrading offers a viable solution to produce high-value chemicals and renewable energy at the same time. Here, we demonstrate an interfacial-engineered CoS/Co─N─C n─n type heterojunction featuring unique dual Co sites and strong built-in electric field (BEF) effects, which enables efficient electrochemical coupling of 5-hydroxymethylfurfural oxidation reaction (HMFOR) and CO₂ reduction reaction (CO₂RR). The optimized catalyst achieves exceptional performance metrics, i.e., a record-low onset potential of 1.12 V (versus RHE), with 99% selectivity and 98.2% faradaic efficiency (FE) for 2,5-furandicarboxylic acid (FDCA) in HMFOR, coupled with 98.6% CO₂─to─CO selectivity and the FE average was retained 98.4% in CO₂RR, which outperform the previously reported state-of-the-art electrocatalysts. Moreover, the integrated HMFOR//CO₂RR system demonstrates impressive stability over 50 h continuous operation. Through systematic experimental examination and theoretical calculations, we reveal that the BEF boosts the formation of the unique dual Co coordination environments (Co─N₄ electron-deficient and Co─S electron-rich configurations) through modulation of charge transport dynamics, facilitating HMF activation through *OH intermediate stabilization while promoting multi-electron CO₂ reduction via charge accumulation. This work establishes a blueprint for developing multi-functional catalytic architectures that address the thermodynamic and kinetic challenges in coupled electrochemical systems, advancing the frontier of sustainable electrosynthesis technologies.