Unveiling the role of cobalt doping in optimizing ammonia electrosynthesis on iron-cobalt oxyhydroxide hollow nanocages

3d transition metal catalysts are effective for the electrocatalytic nitrogen (N₂) reduction reaction (NRR) to produce ammonia (NH₃), but the role of active sites remains elusive. Herein, a series of iron-cobalt oxyhydroxide hollow nanocages (FeCoOOH HNCs) were constructed via controlled Co doping. The as-obtained FeCoOOH HNCs with an Fe/Co ratio of 1 : 1 exhibited a high faradaic efficiency of 14.7% and superior NH₃ formation rate of 16.8 µg h⁻¹ mg_cat⁻¹ at −0.3 V vs. RHE. In situ Raman spectra disclose the existence of intermediates and identify the reaction pathway. Density functional theory (DFT) calculations reveal that Co doping could lower the energy barrier of *N₂ → *NNH → *NNHH, induced by the preferential proton adsorption on Co sites to drive NH₃ electrosynthesis. Moreover, FeCoOOH HNCs with a suitable Fe/Co ratio could boost the *N₂ activation due to the bolstered polarization of adsorbed N₂, while increasing the energy barrier for the hydrogen evolution reaction. This work provides an intriguing strategy towards efficient NRR electrocatalysis by the elaborate design of two 3d transition metals.