Homometallic dual-site Co/Ni-polyoxometalates with water-assisted proton channel for boosting nitrate-to-ammonia electrocatalysis: decoupling active hydrogen generation from nitrate reduction.

Electrocatalytic nitrate reduction to ammonia (e-NO3RR) is hindered by competitive adsorption of active hydrogen (*H) and nitrate intermediates on conventional single/heterometallic dual sites. Herein, we construct two novel sandwich-type Co2-GeMo10 and Ni-GeMo10 polyoxometalates (POMs) electrocatalysts with homometallic dual active sites and in-situ formed water-assisted proton channels. Different from previously reported heterometallic tandem catalysts, the single Co or Ni metal species realizes functional differentiation via distinct coordination environments, achieving spatial decoupling of *H generation and nitrate reduction. The intrinsic hydrogen-bond network constructs ordered one-dimensional proton transport pathways, which greatly lowers proton migration barriers and suppresses the competing hydrogen evolution reaction (HER). Benefiting from the above merits, Co2-GeMo10 delivers a superior NH3 yield rate of 22.30 mg h-1 mgcat-1 and a Faradaic efficiency of 86.13%, which is ∼40% higher than our former heterometallic POMs catalysts. Combining in-situ FTIR and DFT calculations, we unambiguously reveal the site-specific catalytic mechanism: ligand-coordinated sites are responsible for water dissociation and *H production, while POM-bonded sites drive the stepwise reduction of nitrate. This work proposes a novel homometallic site engineering strategy, and provides new insights for designing advanced e-NO₃RR electrocatalysts.