Electrocatalytic reduction of nitric oxide (NO) to ammonia (NH3) represents a potential solution for improving the disrupted nitrogen cycle balance. Unfortunately, designing efficient electrocatalysts for NO reduction reaction (NORR) remains a notable challenge, especially at low concentrations. Herein, a displacement-alloying strategy is reported to successfully induce the phase transition of Co nanoparticles supported on carbon nanosheets from face-centered cubic (fcc) to hexagonal close-packed (hcp) structure through Ru incorporation. The obtained RuCo alloy with hcp phase structure (hcp-RuCo) exhibits apparent NORR activity with a record-high Faraday efficiency of 99.2% and an NH3 yield of 77.76µg h-1 mgcat -1 at -0.1V versus reversible hydrogen electrode at a NO concentration of 1vol %, surpassing Co nanoparticles with fcc phase structure and most reported catalysts. Density functional theory calculations reveal that the excellent NORR activity of hcp-RuCo can be attributed to the optimized electronic structure of Co site and lowered energy barrier of the potential rate-determining step through phase transition. Furthermore, the assembled Zn-NO battery using hcp-RuCo as the cathode achieves a power density of 2.33mW cm-2 and an NH3 yield of 45.94µg h-1 mgcat -1. This work provides a promising research perspective for low-concentration NO conversion.