Asymmetric defective site-triggered triple synergistic modulation in nanoconfined aerogel for superior electrochemical conversion of low-concentration nitrate into ammonia

Electrocatalytic conversion of low-concentration nitrate (NO₃⁻) into ammonia (NH₃) under ambient conditions is expected to provide an effective solution to the global nitrogen cycle imbalance. However, this process is hindered by slow reaction kinetics and the competing hydrogen evolution reaction (HER). Herein, we anchored oxygen vacancy-containing hollow Co₃O₄ nanoparticles on waste spirulina residue-derived reduced graphene oxide aerogel (Vo-HCo₃O₄@SRGA) for electrocatalytic low-concentration NO₃⁻ reduction. Finite element simulation demonstrates that the nanoconfined SRGA significantly increases the local concentration of NO₃⁻, thereby accelerating the reaction kinetics. Moreover, the Vo is able to disrupt the local structural symmetry of Co-O-Co sites. The asymmetric active site (Vo) can simultaneously enhance NO₃⁻ adsorption, promote water dissociation, and inhibit hydrogen evolution. Thanks to the triple synergistic modulation of Vo and the nanoconfined effect of SRGA, Vo-HCo₃O₄@SRGA exhibits unprecedented activity (NH₃-N yield rate: 1.53 mg h⁻¹ cm⁻²; NH₃-N Faraday efficiency: 96.5%) superior to most of the reported advanced electrocatalysts under low-concentration NO₃⁻ conditions. This work cleverly combines macroscopic modification with microscopic fine tuning of catalysts, which is expected to open up new opportunities in the direction of pollutant resourcing.