Highlights O-functionalization at the edges of boron carbonitride induces charge polarization effect on B. 0.1 M HCl serves to preserve the catalyst active site from poisoning effect by electrolyte anions. Experimental and theoretical findings go hand-in-hand towards high yield of ammonia. Ammonia has been recognized as the future renewable energy fuel because of its wide-ranging applications in H 2 storage and transportation sector. In order to avoid the environmentally hazardous Haber–Bosch process, recently, the third-generation ambient ammonia synthesis has drawn phenomenal attention and thus tremendous efforts are devoted to developing efficient electrocatalysts that would circumvent the bottlenecks of the electrochemical nitrogen reduction reaction (NRR) like competitive hydrogen evolution reaction, poor selectivity of N 2 on catalyst surface. Herein, we report the synthesis of an oxygen-functionalized boron carbonitride matrix via a two-step pyrolysis technique. The conductive BNCO (1000) architecture, the compatibility of B-2 p z orbital with the N-2 p z orbital and the charging effect over B due to the C and O edge-atoms in a pentagon altogether facilitate N 2 adsorption on the B edge-active sites. The optimum electrolyte acidity with 0.1 M HCl and the lowered anion crowding effect aid the protonation steps of NRR via an associative alternating pathway, which gives a sufficiently high yield of ammonia (211.5 μg h −1 mg cat −1 ) on the optimized BNCO (1000) catalyst with a Faradaic efficiency of 34.7% at − 0.1 V vs RHE. This work thus offers a cost-effective electrode material and provides a contemporary idea about reinforcing the charging effect over the secured active sites for NRR by selectively choosing the electrolyte anions and functionalizing the active edges of the BNCO (1000) catalyst.