Mo-doped spinel NiCo2O4 as a bifunctional electrocatalyst for highly efficient nitrate reduction to ammonia and oxygen evolution reaction

Abstract

Developing a highly efficient single catalyst capable of driving both multi-electron reductive and oxidative reactions remains a major challenge for integrated energy-environmental systems. One promising technique involves the electrocatalytic nitrate reduction reaction (EnitRR), which is recognised as a thermodynamically favourable alternative to direct nitrogen reduction and enables the simultaneous remediation of nitrate pollutants. This work developed a cation-engineered spinel oxide, Ni_0.9Mo_0.1Co₂O₄, that achieves the highest catalytic activity for both the EnitRR and OER. The optimised catalyst achieves an NH₃ yield of 4.20 mg h⁻¹ cm⁻² with ∼95% FE at −0.6 V vs. RHE. The accelerating multielectron kinetics are evident from the reduced charge-transfer resistance observed from operando potential-dependent EIS. The dynamic catalytic reconstruction, indicated by surface oxidation, is corroborated by post-reaction XPS, which shows an increase in the Ni³⁺ fraction from 41.75 to 75.72% and the emergence of N-containing surface species. The alkaline OER activity is accompanied by a low potential of 239 mV at 10 mA cm⁻² and a Tafel slope of 99.74 mV dec⁻¹ in 1 M KOH. Electronic structural analysis reveals that moderate Mo incorporation modulates Ni active centres, optimizes intermediate adsorption energy, and promotes defect-assisted nitrate activation. Furthermore, DFT calculations confirmed that Mo-induced d-band modulation and increased projected density of states (PDOS) near the Fermi level enhance electron transport and intermediate adsorption. Upon building a Zn–NO₃⁻ battery, the catalyst yielded high NH₃ production of up to 4.58 mg h⁻¹ cm⁻² at 30 mA cm⁻² with simultaneous electricity generation, achieving a peak power current density of 21.62 mW cm⁻² with 30 mA cm⁻². Cation engineering of spinel oxide is thereby established as an effective strategy for coupling environmental remediation with sustainable energy generation and green NH₃ synthesis.