A Zn-nitrite battery as an energy-output electrocatalytic system for high-efficiency ammonia synthesis using carbon-doped cobalt oxide nanotubes

Ammonia (NH₃) is a carbon-free fuel and essential for producing chemicals like fertilizers. The conversion of toxic nitrite (NO₂⁻) ions from contaminated water to NH₃ in an electrocatalytic system is highly attractive both environmentally and sustainably and the system can potentially be changed from energy input to energy output. In this work, we develop a Zn–NO₂⁻ battery meanwhile working as an electrocatalytic system that can simultaneously degrade NO₂⁻, produce NH₃ and generate electricity. Nanoparticle-assembled carbon-doped cobalt oxide (C/Co₃O₄) hollow nanotubes are found to be highly efficient for the NO₂⁻ reduction reaction (NO₂⁻RR), which shows a faradaic efficiency (FE) of nearly 100% for NH₃ production in a wide potential window from −0.1 V to −0.6 V versus a reversible hydrogen electrode. The interstitial C dopant can induce a local electric field to greatly fascinate charge transfer, which lowers the energy barrier of the rate-determining step (*N + e⁻ + H₂O → *NH + OH⁻), facilitating the NO₂⁻RR process. The assembled Zn–NO₂⁻ battery exhibits a power density of 6.03 mW cm⁻² and a highest NH₃ FE of 95.1%. This work not only initiates the Zn–NO₂⁻ battery system for sustainable energy supply and NH₃ electrosynthesis but also offers an attractive electrocatalytic approach to realize the degradation of the NO₂⁻ pollutant in wastewater.