Heterogenization of Cobalt–Phosphotungstate with Triazolium-Polymer through Ion Pairing for Facile Nitrate Adsorption and Electroreduction to Ammonia

Cobalt–phosphotungstate [PW₁₁O₃₉Co(H₂O)]⁵⁻ (Co–POM) is embedded in the microporous triazolium polymer (TOP) matrix to form composites that enhance the selectivity of electrocatalytic nitrate reduction (eNO₃RR). Surface and bulk analyses, including differential pulse voltammetry, corroborate the incorporation of Co–POM through anion metathesis into the protonated form of TOP. Real-time monitoring of the contact angle revealed a high surface energy of the composite 3 with ≈66 wt% Co–POM, implicating fast electrolyte adsorption. At −0.1 V versus. RHE, composite 3 exhibited eNO₃RR activity with 98 ± 1% Faradaic efficiency of ammonia in Britton–Robinson buffer (pH 3.3). ¹⁵N-labeling study confirms the source of ammonia as nitrate [NO₃]⁻. The rotating disc electrode study reveals the involvement of two and six electrons, unveiling the formation of key intermediates such as nitrite [NO₂]⁻ and hydroxylamine (NH₂OH). In situ infrared spectroscopy supported the formation of intermediates, including [NO₂]⁻ and (NH₂OH), providing insight into the reaction mechanism. The kinetic isotope effect value from the D₂O labeling study varies from 1.5 to 4.7, revealing the participation of the proton-coupled electron transfer process in the rate-limiting step. The ex situ Raman spectroscopy confirmed the adsorption of nitrate ions onto the catalyst's surface, validating the role of Co–POM in nitrate reduction.