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

Ammonia (NH 3 ) is attracted as a potential carbon free energy carrier and as important feedstock for most of the fertilizers, chemicals, pharmaceutical related products. NH 3 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 CO 2 emissions into the atmosphere. Therefore, there is an urgent need to develop an alternative and sustainable route for NH 3 production under ambient conditions. Recently, electrocatalytic N 2 reduction to NH 3 production has attracted as a potential approach, but achieving high NH 3 yield and Faradaic efficiency, and avoiding competitive hydrogen-evolution reaction (HER) are still challenging. Nitrate/nitrite (NO 3 − /NO 2 − ) is the widely reported contaminant for eutrophication and carcinogens, which can be utilized as a nitrogen resource for electrocatalytic NO 3 − /NO 2 − reduction to NH 3 (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. This paper comprehensively reviews electrocatalytic NRA performance of the metal nanomaterials, bio-inspired metalloenzymes and bioelectrochemical systems. An insight into the rate-determining step and reaction mechanism of NRA process is summarized. The challenges and perspectives are put forward for the future design and application of advanced electrocatalytic NRA systems under low-temperature conditions. • This review summarizes the recent advances in electrocatalytic NRA. • The strategies for NH 3 yield and Faradic efficiency improvement are discussed. • Key factors and fundamental mechanisms of NRA are described. • Challenges and future pathways of NRA are summarized.