Effective Nitrate Electroconversion to Ammonia Using an Entangled Co₃O₄/Graphene Nanoribbon Catalyst

There has been huge interest among chemical scientists in the electrochemical reduction of nitrate (NO₃^-) to ammonia (NH₄⁺) due to the useful application of NH₄⁺ in nitrogen fertilizers and fuel. To conduct such a complex reduction reaction, which involves eight electrons and eight protons, one needs to develop high-performance (and stable) electrocatalysts that favor the formation of reaction intermediates that are selective toward ammonia production. In the present study, we developed and applied Co₃O₄/graphene nanoribbon (GNR) electrocatalysts with excellent properties for the effective reduction of NO₃^- to NH₄⁺, where NH₄⁺ yield rate of 42.11 mg h^-1 mg_cat^-1, FE of 98.7%, NO₃^- conversion efficiency of 14.71%, and NH₄⁺ selectivity of 100% were obtained, with the application of only 37.5 μg cm^-2 of the catalysts (for the best catalyst ─Co₃O₄(Cowt %55)GNR, only 20.6 μg cm^-2 of Co was applied), confirmed by loadings ranging from 19-150 μg cm^-2. The highly satisfactory results obtained from the application of the proposed catalysts were favored by high average values of electrochemically active surface area (ECSA) and low <i>R</i>_ct values, along with the presence of several planes in Co₃O₄ entangled with GNR and the occurrence of a kind of "(Co₃(Co(CN)₆)₂(H₂O)₁₂)_1.333 complex" structure on the catalyst surface, in addition to the effective migration of NO₃^- from the cell cathodic branch to the anodic branch, which was confirmed by the experiment conducted using a H-cell separated by a Nafion 117 membrane. The in situ FTIR and Raman spectroscopy results helped identify the adsorbed intermediates, namely, NO₃^-, NO₂^-, NO, and NH₂OH, and the final product NH₄⁺, which are compatible with the proposed NO₃^- electroreduction mechanism. The Density Functional Theory (DFT) calculations helped confirm that the Co₃O₄(Cowt %55)GNR catalyst exhibited a better performance in terms of nitrate electroreduction in comparison with Co₃O₄(Cowt %75), considering the intermediates identified by the in situ FTIR and Raman spectroscopy results and the rate-determining step (RDS) observed for the transition of *NO to *NHO (0.43 eV).