High-Efficiency Electrochemical Nitrate Reduction to Ammonia on a Co₃O₄ Nanoarray Catalyst with Cobalt Vacancies

Electrocatalytic nitrate reduction reaction (NO₃RR) affords a bifunctional character in the carbon-free ammonia synthesis and remission of nitrate pollution in water. Here, we fabricated the Co₃O₄ nanosheet array with cobalt vacancies on carbon cloth (v_Co-Co₃O₄/CC) by in situ etching aluminum-doped Co₃O₄/CC, which exhibits an excellent Faradaic efficiency of 97.2% and a large NH₃ yield as high as 517.5 μmol h^-1 cm^-2, better than the pristine Co₃O₄/CC. Theoretical calculative results imply that the cobalt vacancies can tune the local electronic environment around Co sites of Co₃O₄, increasing the charge and reducing the electron cloud density of Co sites, which is thus conducive to adsorption of NO₃^- on Co sites for greatly enhanced nitrate reduction. Furthermore, the v_Co-Co₃O₄ (311) facet presents excellent NO₃RR activity with a low energy barrier of about 0.63 eV on a potential-determining step, which is much smaller than pristine Co₃O₄ (1.3 eV).