Phase-stabilized core@shell NiFe2O4@CoFe2O4 nanocages for the integrated energy and electroreduction of nitrate-to-ammonia

Abstract

Electrochemical nitrate reduction reaction (ENR) provides an eco-friendly route to ammonia (NH3) synthesis, positioning it as a viable substitute to the traditional Haber-Bosch method. However, accomplishing high efficiency and selectivity is challenging because of competing hydrogen production and reaction instability. Herein, we present phase-stabilized NiFe2O4@CoFe2O4 (NFCO) core@shell nanocages synthesized via a controlled annealing process using Prussian blue analogs. These analogs are self-assembled from divalent Ni and Co species within a trivalent Fe-CN framework, which serves as a sacrificial template. These nanocages impressively enhanced the ENR to NH3 with a Faradaic efficiency of 95% at-0.4 V vs. RHE via a direct eight-electron N-end reduction pathway. The NiFe2O4 core facilitates rapid charge transfer, while the CoFe2O4 shell boosts NO3-adsorption and stabilizes reaction intermediates, effectively suppressing hydrogen evolution. Theoretical calculations and in situ Raman spectroelectrochemistry unveil ENR mechanisms and possible limiting steps on NFCO sites. Beyond the ENR, integrating NFCO into a Zn-NO3-battery enables simultaneous energy generation and stable NH3 production, demonstrating an open-circuit voltage of 1.4 V and a power density of 1.54 mW cm-2. This approach advances the design of competent, stable catalysts for large-scale, sustainable NH3 generation and NO3-removal, with promising environmental applications.