Interfacial enrichment by covalent organic framework microsphere layer enables efficient electrocatalytic nitrate reduction at low concentrations

Low-concentration nitrate (NO 3 − ) contamination has become a critical global challenge in water management, posing risks to both ecosystems and human health. Electrocatalytic NO 3 − reduction reaction (NO 3 RR) offers a sustainable and technologically viable strategy for the removal of NO 3 − at low concentrations. However, the practical application is severely hampered by mass-transport limitations and competing hydrogen evolution. Herein, we introduce a physical interfacial engineering strategy by coating commercial Cu foam with a covalent organic framework (COF) microsphere layer to enrich NO 3 − at the electrode-electrolyte interface and thereby enhance the catalytic performance of NO 3 RR at low concentrations. Notably, hollow COF microspheres with enclosed cavities create a confined microenvironment that promotes local nitrate accumulation without altering the intrinsic electronic properties of the Cu substrate. This interfacial architecture imparts a pronounced substrate-enrichment effect, sustaining sufficient local nitrate concentrations in dilute electrolytes. As a result, the adsorption and conversion of NO 3 − are significantly enhanced, and the current plateau due to diffusion limit is effectively mitigated. The optimized composite electrode delivers a maximum Faradaic efficiency of 84.6% and a nitrate removal rate of 63.3% h −1 at 5 mM NO 3 − , substantially surpassing bare Cu foam and setting a benchmark for low-concentration NO 3 RR. Operando Raman spectroscopy and finite element simulations corroborate the enhanced interfacial nitrate enrichment and reaction pathway. This work establishes structural enrichment as a general paradigm for overcoming diffusion-limited conditions in dilute-substrate electrocatalysis.