The dominant Haber–Bosch process to produce ammonia, arguably the most important chemical in support of global food supply, is both energy and carbon intensive, resulting in substantial environmental impacts. Electrocatalytic nitrogen reduction reaction (NRR) powered by renewable electricity provides a green synthetic route for ammonia, but still suffers from insufficient yield rate and Faradaic efficiency. Single-atom electrocatalysts (SACs) have the potential to transform this catalytic process; however, controllable synthesis of SACs with high loading of active sites remains a big challenge. Here we utilize bacterial cellulose with rich oxygen functional groups to anchor iron (Fe) and cobalt (Co), realizing high density, atomically dispersed, bimetallic Fe–Co active sites. For electrocatalytic NRR, our catalyst design delivers a remarkable ammonia yield rate of 579.2 ± 27.8 μg h−1 mgcat.−1 and an exceptional Faradaic efficiency of 79.0 ± 3.8%. The combined theoretical and experimental investigations reveal that the operando change in coordination configuration from [(O-C2)3Fe–Co(O-C2)3] to [(O-C2)3Fe–Co(O-C)C2] is the enabling chemistry. Our findings suggest a general approach to engineer SACs that can drive critical reactions of relevance for sustainability.