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

Abstract

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∙mgcat.−1 with Faradaic efficiency (FE) of 89.32%, whereas that for Ni6enMe was 3.46 mg∙h−1∙mgcat.−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.

Keywords

• polyoxometalates
• metal-added
• electrocatalyst
• nitrate reduction