Ammonia synthesis by the electrochemical technique is a promising solution to replace the energy-intensive Haber-Bosch process in industry, which inevitably requires high-performance catalysts. However, the development of catalysts is still limited by the inferior activation of the stable N&z.tbd;N bond. Herein, a robust N 2 -activation mode is proposed, which addresses the electron donation mechanism to both the N 2 antibonding orbitals of p y and p z , simultaneously. Following this strategy, the single cluster catalyst (SCC) of Fe 4 /GaS has realized remarkable nitrogen reduction reaction (NRR) performance with an ultra-low overpotential ( η ) of 0.08 V by density functional theory (DFT) calculations. N 2 is completely activated in a side-on adsorption configuration on the hollow site of Fe 4 /GaS, where both degenerate N 2 -π* orbitals are properly hybridized to the frontier orbitals of the Fe 4 cluster, as proven by fragment orbital analysis. This work proposes an efficient strategy for N 2 activation, and also provides a valid design guideline for further research. A robust N 2 activation mode is proposed via transferring electrons to the N 2 -π* orbitals, p y and p z .