Atomic-level reactive sites for electrocatalytic nitrogen reduction to ammonia under ambient conditions

•The functions of reactive sites in electrocatalytic NRR and the electrocatalytic NRR mechanism are introduced.•The typical strategies to construct and characterize atomic-level reactive sites are summarized.•The recent theoretical and experimental advances of single-atoms, dual-atoms, metal clusters, vacancies, and dopants in electrocatalytic NRR are discussed.•The rigorous protocols for electrocatalytic NRR are listed.•The challenges and perspectives of atomic-level reactive sites in electrocatalytic NRR are proposed.