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

Abstract

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.