Electrocatalytic nitrate/nitrite reduction to ammonia synthesis using metal nanocatalysts and bio-inspired metalloenzymes

Ammonia (NH3) is attracted as a potential carbon free energy carrier and as important feedstock for most of the fertilizers, chemicals, pharmaceutical related products. NH3 is industrially produced by conventional Haber-Bosch process under harsh experimental conditions (high temperature and high pressure), and this process requires high-energy consumption and produces large amount of CO2 emissions into the atmosphere. Therefore, there is an urgent need to develop an alternative and sustainable route for NH3 production under ambient conditions. Recently, electrocatalytic N2 reduction to NH3 production has attracted as a potential approach, but achieving high NH3 yield and Faradaic efficiency, and avoiding competitive hydrogen-evolution reaction (HER) are still challenging. Nitrate/nitrite (NO3-/NO2- ) is the widely reported contaminant for eutrophication and carcinogens, which can be utilized as a nitrogen resource for electrocatalytic NO3- /NO2- reduction to NH3 (NRA) via eight/six-electron transfer process. Unfortunately, electrocatalytic NRA using metal nanomaterials are rarely investigated. In this review, we discuss the electrocatalytic NRA performance containing reactivity, selectivity, Faradaic efficiency and cycling stability of metal nanocatalysts, bio-inspired metalloenzymes and bioelectrochemical system. After this overview, we investigate the key factors, rate-determining step and the reaction mechanism that controlling the NRA performance. Finally, we summarize the challenges and future pathways guiding the design of effective nanomaterials and reaction systems to promote the industrial application of electrocatalytic NRA.