Hexanuclear nickel-added silicotungstates as high-efficiency electrocatalysts for nitrate reduction to ammonia

Ammonia (NH₃) is widely used in a wide range of fields because of its high energy density, and NH₃ is simple to liquefy and transport. Nitrate is also a source of pollution of the environment and drinking water sources. Therefore, there is a pressing demand for the design and production of high-efficiency catalysts for the nitrate reduction reaction (NO₃RR). Herein, two nickel-added polyoxometalates (NiAPs), namely, [Ni(en)₂][Ni₆(μ₃-OH)₃(en)₃(H₂O)₆(B-α-SiW₉O₃₄)]₂·6H₂O (Ni₆en) and [Ni(enMe)₂(H₂O)₂][Ni₆(μ₃-OH)₃(H₂O)₆(enMe)₃(B-α-SiW₉O₃₄)]₂·8H₂O (Ni₆enMe) (en = ethylenediamine, enMe = 1,2-diaminopropane), were effectively synthesized under hydrothermal conditions that contained several electrons and were used as electrocatalytic nitrate reduction reaction (e-NO₃RR) catalysts. The structures of the compounds were characterized by using various instruments such as powder X-ray diffraction (PXRD) spectroscopy, infrared (IR) spectroscopy, thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) method, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). e-NO₃RR tests were performed using electrochemical workstation. Results show that Ni₆en and Ni₆enMe have high-efficient electrochemical catalytic nitrogen reduction to NH₃. The highest NH₃ yield rate for Ni₆en was 3.66 mg∙h⁻¹∙mg_cat.⁻¹ with Faradaic efficiency (FE) of 89.32%, whereas that for Ni₆enMe was 3.46 mg∙h⁻¹∙mg_cat.⁻¹ with FE of 86.75% at a low voltage (−0.5 V vs. reversible hydrogen electrode (RHE)). This finding creates a novel path for manufacturing highly effective NO₃RR electrocatalysts using metal-added polyoxometalate as the catalyst in ambient settings. Furthermore, the findings of this research provide practical advice for creating effective electrocatalytic catalysts.