Proton Flux Engineering via Built‐in Electric Fields in N‐doped CuO@Co ₃ O ₄ @Ni(OH) ₂ Heterostructure for Rechargeable Zn‐NO ₃ ⁻ /5‐Hydroxymethylfurfural Multielectron Transfer Systems

Electrocatalytic coupling of nitrate reduction (NO₃RR) to ammonia with 5-hydroxymethylfurfural (HMF) oxidation to 2,5-furandicarboxylic acid (FDCA) enables simultaneous wastewater remediation and biomass valorization. However, developing efficient bifunctional electrocatalysts for these multiproton-coupled electron transfer reactions remains challenging as conventional single-active-site catalysts inherently suffer from linear scaling relationships between intermediates and adsorption energies, particularly sluggish proton transfer. To address this, we engineered a triphasic N-doped CuO@Co₃O₄@Ni(OH)₂ heterostructure with a gradient built-in electric field (BIEF), which synergistically enhances interfacial charge polarization and accelerates proton transport through dynamic coupling effects in both reactions: sufficient *H supply for NO₃RR and fast Ni(OH)₂/NiOOH redox cycling during HMF oxidation (HMFOR), thus achieving unprecedented bifunctional performance: at - 0.4 V versus RHE, Faradaic efficiency (FE) for NH₃ reaches 96.49% with a yield rate of 45.36 mg h^-1 cm^-2; under 1.53 V versus RHE, the FDCA FE achieves 95.23% with a yield of 95.24%. The bifunctional design reduces energy consumption by 31.39% at 10 mA cm^-2 in a NO₃RR||HMFOR flow electrolyzer compared to traditional electrolytic water splitting. A rechargeable Zn-NO₃ ^-/HMF battery shows 70-280 mV lower charging potential with exceptional cycling stability (>450 cycles). This work provides a new design paradigm for bifunctional electrocatalysts in sustainable energy conversion and waste valorization.