Architecting Cobalt-Centered 2D Sandwich-Type Polyoxometalate Frameworks for Selective Electrocatalytic Nitrate Reduction to Ammonia.

Developing efficient electrocatalysts for neutral-media nitrate reduction (NO3-RR) is crucial for simultaneous nitrate remediation and sustainable ammonia (NH3) synthesis, yet hindered by sluggish reaction kinetics. Herein, we designed and synthesized four novel two-dimensional (2D) cobalt-centered hourglass-shaped [P4Mo6VO31]12--based crystalline materials via the hydrothermal method: (H2bimb)2[Co3{Mo6O15(HPO4)3(PO4)}2]·7H2O (1), (H2bimb)2[Co3{Mo6O15(HPO4)3(PO4)}2]·16H2O (2), [{Co(Hbimb)2}2][{Co(H2O)2}Co{Mo6O15(HPO4)2(PO4)2}2]·10H2O (3), and (H2bimb)2[Co(Hbimb)2][{Co(H2O)2}2Co{Mo6O15(HPO4)2(PO4)2}2]·18H2O (4) (bimb = 1,3-bis(1H-imidazol-1-yl)benzene), featuring tunable hourglass-type [P4Mo6VO31]12- clusters. The experimental results showed that compounds 1-4 all exhibited better NO3-RR performance, among which compound 4 exhibited excellent stability and outstanding redox activity performance with a Faradaic efficiency of 89.65% at -0.7 V (versus reversible hydrogen electrode) and an ammonia synthesis rate of 8.97 mg h-1 mgcat-1. This work extends the application of e-NO3-RR catalysts based on polymetallic oxides and opens up innovative pathways for sustainable ammonia synthesis through molecular-level structure modulation.