Abstract
There has been huge interest among chemical scientists in the electrochemical reduction of nitrate (NO(3)(-)) to ammonia (NH(4)(+)) due to the useful application of NH(4)(+) 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 Co(3)O(4)/graphene nanoribbon (GNR) electrocatalysts with excellent properties for the effective reduction of NO(3)(-) to NH(4)(+), where NH(4)(+) yield rate of 42.11 mg h(-1) mg(cat)(-1), FE of 98.7%, NO(3)(-) conversion efficiency of 14.71%, and NH(4)(+) selectivity of 100% were obtained, with the application of only 37.5 μg cm(-2) of the catalysts (for the best catalyst ─Co(3)O(4)(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 R(ct) values, along with the presence of several planes in Co(3)O(4) entangled with GNR and the occurrence of a kind of "(Co(3)(Co(CN)(6))(2)(H(2)O)(12))(1.333) complex" structure on the catalyst surface, in addition to the effective migration of NO(3)(-) 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, NO(3)(-), NO(2)(-), NO, and NH(2)OH, and the final product NH(4)(+), which are compatible with the proposed NO(3)(-) electroreduction mechanism. The Density Functional Theory (DFT) calculations helped confirm that the Co(3)O(4)(Cowt %55)GNR catalyst exhibited a better performance in terms of nitrate electroreduction in comparison with Co(3)O(4)(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).