Microenvironment Engineering of Mesoporous Metals for Ammonia Electrosynthesis from Nitrate: Advances, Mechanisms, and Prospects

ConspectusElectrocatalytic nitrate reduction to ammonia (NO3--to-NH3) offers a promising pathway to convert NO3- wastewater to high-value-added NH3 under ambient conditions with renewable electricity. The design of robust electrocatalysts that facilitate the completion of complex hydrodeoxygenation steps is a major challenge for efficient NH3 electrosynthesis from NO3-. Mesoporous metals, as a fantastic class of mesoscopic functional materials, not only retain the ability to control electrocatalytic nitrate reduction reactions (eNO3RR) at the atomic/molecular level but also induce new physicochemical properties with the nanoconfined mesoporous microenvironment. In this Account, we outline our recent progress in engineering the surface microenvironment of mesoporous metals to enhance the electrocatalytic NO3--to-NH3 performance.We start by introducing nanoconfinement effects to validate the mechanism of mesoporous metals on key intermediates in eNO3RR. Unlike conventional nonporous counterparts, nanoscale pores and channels of mesoporous metals present strong nanoconfinement of nitrogen-containing intermediates and active hydrogen (*H) radicals, which promotes the deeper electroreduction of NO3- by multistep hydrodeoxygenation routes and results in remarkable NH3 selectivity. To resolve the key challenge of longer mesopores that severely limit mass transfer efficiency, hierarchical mesoporous metals, including hollow mesoporous nanotubes and mesoporous nanocavities, are designed that effectively realize the synergistic promotion of NH3 yield rate and selectivity. Next, an enzyme-like tandem electrocatalyst with separated metal active sites is developed to alleviate the kinetic barrier of eNO3RR, which thus achieves NH3 electrosynthesis at ultralow overpotentials. Through coupling with anode oxygen reactions with mesoporous metals as a bifunctional electrocatalyst, eNO3RR is further promoted and delivers better performance for NH3 electrosynthesis in a more sustainable manner. Finally, we present the limitations and challenges in designing functional mesoporous metal electrocatalysts and propose prospects for further development of eNO3RR technologies. We hope that this Account will open an alternative in designing efficient mesoporous metals with optimized surface microenvironments for selective electrocatalysis and beyond.