Biomass-Derived Fe–Ni Alginate Hydrogel Electrodes for Sustainable Nitrate Upcycling to Ammonia

Abstract The electrocatalytic nitrate reduction reaction (NO3–RR) provides a sustainable and carbon-efficient strategy for ammonia (NH3) production, a vital industrial feedstock. However, the development of efficient electrocatalysts remains a significant bottleneck hindering its practical advancement. In alkaline media, effective electrocatalysts should not only activate nitrate but also promote water dissociation to supply proton equivalents for ammonia formation. Herein, we report the innovative application of sodium alginate (SA) hydrogels as a monolithic electrode for NO3–RR. The Fe3+ and Ni2+ ions coordinate with the carboxyl and hydroxyl groups in the sodium alginate to form a stable 3D porous network structure (FeNi/SA). In this structure, Ni sites promote water dissociation, whereas Fe sites facilitate nitrate reduction; the cooperation between these dual active sites accelerates ammonia formation. Consequently, the FeNi/SA hydrogel exhibits exceptional electrocatalytic performance, requiring potentials of only 0.0579 and –0.34 V (vs RHE) to achieve current densities of –10 and –100 mA cm–2, respectively. Notably, it delivers a high ammonia yield rate of 7.745 mg h–1 cm–2 with an impressive Faradaic efficiency of 93.79%. Experiment and density functional theory calculations results indicate that Ni sites promote the generation of *H and modulate the electronic structure of Fe, lowering the energy barrier for the critical *NO to *NOH transition and accelerating NH3 formation. Furthermore, integrating the NO3–RR process with an air-stripping system enables the efficient recovery of NH4Cl, offering a viable pathway for converting nitrogenous waste into high-value products. This work highlights the potential of hydrogel-based materials as a new frontier for robust nitrate-to-ammonia conversion.