Composition-Dependent Microcube NiCoO2 Binder-Free Electrode with Enhanced Diffusion for Efficient Electrochemical Nitrate Reduction to Ammonia.

Electrochemical nitrate reduction reaction (NO3RR) has recently emerged as an environmentally benign route for sustainable NH3 synthesis under mild conditions. However, the complex 8-electron transfer pathway and various intermediates often lead to sluggish kinetics and limited selectivity. Therefore, the mass transport and conductivity of the electrode play a critical role in the NO3RR process. Herein, NiCoO2 microcubes with different sizes were directly grown on nickel foam (NiCoO2/NF) by regulating the Ni:Co precursor ratio. Increasing cube size induces the formation of three dimensional architecture that facilitates effective electrolyte diffusion within the electrode. Enhanced mass transport was experimentally confirmed by the increased diffusion coefficient determined from electrochemical analysis. Meanwhile, the optimized Ni:Co ratio also modulates the electronic structure of NiCoO2, contributing to improved catalytic activity. Benefiting from these structural and electronic features, the NiCoO2/NF catalyst exhibits outstanding NO3RR activity, achieving an NH3 yield rate of 4133.94 µg h-1 cm-2 and a Faradaic efficiency of 94.56% at -0.1 V vs. RHE. Notably, a remarkable energy efficiency of 39.10% was obtained even at 0 V vs. RHE. This work demonstrates that microcube NiCoO2/NF can serve as highly efficient and durable binder-free electrocatalysts for selective and energy-efficient nitrate reduction toward sustainable NH3 synthesis.