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

Abstract

Ammonia (NH<inf>3</inf>) is a carbon-free fuel and essential for producing chemicals like fertilizers. The conversion of toxic nitrite (NO<inf>2</inf>⁻) ions from contaminated water to NH<inf>3</inf> 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-NO<inf>2</inf>⁻ battery meanwhile working as an electrocatalytic system that can simultaneously degrade NO<inf>2</inf>⁻, produce NH<inf>3</inf> and generate electricity. Nanoparticle-assembled carbon-doped cobalt oxide (C/Co<inf>3</inf>O<inf>4</inf>) hollow nanotubes are found to be highly efficient for the NO<inf>2</inf>⁻ reduction reaction (NO<inf>2</inf>⁻RR), which shows a faradaic efficiency (FE) of nearly 100% for NH<inf>3</inf> 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⁻ + H<inf>2</inf>O → *NH + OH⁻), facilitating the NO<inf>2</inf>⁻RR process. The assembled Zn-NO<inf>2</inf>⁻ battery exhibits a power density of 6.03 mW cm⁻² and a highest NH<inf>3</inf> FE of 95.1%. This work not only initiates the Zn-NO<inf>2</inf>⁻ battery system for sustainable energy supply and NH<inf>3</inf> electrosynthesis but also offers an attractive electrocatalytic approach to realize the degradation of the NO<inf>2</inf>⁻ pollutant in wastewater.