Effective Nitrate\nElectroconversion to Ammonia Using\nan Entangled Co<sub>3</sub>O<sub>4</sub>/Graphene Nanoribbon Catalyst

There has been huge interest among chemical scientists\nin the\nelectrochemical reduction of nitrate (NO<sub>3</sub><sup>–</sup>) to ammonia (NH<sub>4</sub><sup>+</sup>) due to the useful application\nof NH<sub>4</sub><sup>+</sup> in nitrogen fertilizers and fuel. To\nconduct such a complex reduction reaction, which involves eight electrons\nand eight protons, one needs to develop high-performance (and stable)\nelectrocatalysts that favor the formation of reaction intermediates\nthat are selective toward ammonia production. In the present study,\nwe developed and applied Co<sub>3</sub>O<sub>4</sub>/graphene nanoribbon\n(GNR) electrocatalysts with excellent properties for the effective\nreduction of NO<sub>3</sub><sup>–</sup> to NH<sub>4</sub><sup>+</sup>, where NH<sub>4</sub><sup>+</sup> yield rate of 42.11 mg\nh<sup>–1</sup> mg<sub>cat</sub><sup>–1</sup>, FE of\n98.7%, NO<sub>3</sub><sup>–</sup> conversion efficiency of\n14.71%, and NH<sub>4</sub><sup>+</sup> selectivity of 100% were obtained,\nwith the application of only 37.5 μg cm<sup>–2</sup> of\nthe catalysts (for the best catalyst Co<sub>3</sub>O<sub>4</sub>(Cowt U)GNR, only 20.6 μg cm<sup>–2</sup> of Co was\napplied), confirmed by loadings ranging from 19–150 μg\ncm<sup>–2</sup>. The highly satisfactory results obtained from\nthe application of the proposed catalysts were favored by high average\nvalues of electrochemically active surface area (ECSA) and low <i>R</i><sub>ct</sub> values, along with the presence of several\nplanes in Co<sub>3</sub>O<sub>4</sub> entangled with GNR and the occurrence\nof a kind of “(Co<sub>3</sub>(Co(CN)<sub>6</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>12</sub>)<sub>1.333</sub> complex” structure\non the catalyst surface, in addition to the effective migration of\nNO<sub>3</sub><sup>–</sup> from the cell cathodic branch to\nthe anodic branch, which was confirmed by the experiment conducted\nusing a H-cell separated by a Nafion 117 membrane. The in situ FTIR\nand Raman spectroscopy results helped identify the adsorbed intermediates,\nnamely, NO<sub>3</sub><sup>–</sup>, NO<sub>2</sub><sup>–</sup>, NO, and NH<sub>2</sub>OH, and the final product NH<sub>4</sub><sup>+</sup>, which are compatible with the proposed NO<sub>3</sub><sup>–</sup> electroreduction mechanism. The Density Functional\nTheory (DFT) calculations helped confirm that the Co<sub>3</sub>O<sub>4</sub>(Cowt U)GNR catalyst exhibited a better performance in terms\nof nitrate electroreduction in comparison with Co<sub>3</sub>O<sub>4</sub>(Cowt u), considering the intermediates identified by the\nin situ FTIR and Raman spectroscopy results and the rate-determining\nstep (RDS) observed for the transition of *NO to *NHO (0.43 eV).