The electrochemical NRR to produce ammonia under ambient conditions represents an attractive prospect in nitrogen fixation which can greatly ease the energy crisis and environmental pollution. However, suffering from awful N₂ activation results in low ammonia yield rate and Faradaic efficiency in electrochemical NRR. In this work, we devote to construct FeNi heteronuclear sites in FeNi-PBA with accurate coordination structure to facilitate N₂ activation employing a side-on configuration. Control experiments have verified N₂ activation on FeNi heteronuclear sites was enhanced compared with that on FeFe sites. Meanwhile, the particle size of PBA was decreased to boost active sites. FeNi-PBA nanocrystals with ultrasmall size (10 ± 2.8 nm) through microwave-assisted synthesis achieve an ammonia yield rate of 17.7 mg_NH3 g_cat.⁻¹ h⁻¹ and Faradaic efficiency of 27.5% at a bias of −0.2 V (V vs. RHE) in 0.05 M H₂SO₄ solution. Faradaic efficiency achieved in this work well exceeds than that of most Fe based transition metal electrocatalysts.