Hollow Nanoreactors Modulate Mass Transfer and Dual‐Species Spillover to Boost Nitrate‐to‐Ammonia Reduction in Real Wastewater

Copper‐based materials are promising catalysts for electrocatalytic nitrate reduction to ammonia (NH 3 ), while their use in real wastewater with low nitrate concentrations is hindered by poor mass transfer and high energy barrier. To overcome these limitations, we design a hollow Cu/MoS 2 ‐550 nanoreactor, consisting of hollow MoS 2 support loaded with Cu single atoms and clusters. At the mesoscale, the hollow MoS 2 support features a heat‐exchanger‐fin‐like structure that accelerates mass transfer, thereby promoting local enrichment of NO 3 − . At the microscale, precise modulation of sulfur vacancy concentration in MoS 2 triggers dual‐species spillover, namely reverse hydrogen spillover and *NO spillover from Cu single atoms to Cu clusters, which lowers the energy barrier of deep hydrogenation step. As a result, the Cu/MoS 2 ‐550 nanoreactor achieves an NH 3 Faradaic efficiency (FE) of 98.14% and a yield rate of 27.46 mg h −1 mg cat −1 . Furthermore, when assembled into an Al‐NO 3 − battery operating in real wastewater containing only ∼ 0.76 mM NO 3 − , the battery runs stably for 120 h, delivers an NH 3 FE of 53.20%, and maintains a nitrate removal rate of 91.17%. This work provides cross‐scale modulation strategies to overcome mass‐transfer bottlenecks and energy barriers in multi‐electron transfer reactions, offering a potential pathway for environmental applications.