Abstract: Industrial ammonia synthesis catalyzed by Fe- and Ru-based catalysts is an energy-consuming process. The development of low-temperature active catalyst has been pursued for a century. Herein, we report that barium hydride (BaH2) can synergize with Co, leading to a much better low-temperature activity, i.e., the BaH2-Co/carbon nanotube (CNT) catalyst exhibits ammonia synthesis activity right above 150 °C; at 300 °C, it is 2 orders of magnitude higher than that of the BaO-Co/CNTs and more than 2.5-times higher than Cs-promoted Ru/MgO. Kinetic analyses reveal that the dissociative adsorption of N2 on the Co-BaH2 catalyst may not be the rate-determining step, as evidenced by the much smaller reaction order of N2 (0.43) and the lower apparent activation energy (58 kJ mol-1) compared with those of the unpromoted and BaO-promoted Co-based catalysts. BaH2, with a negative hydride ion, may act as a strong reducing agent, removing activated N from the Co surface and forming a BaNH species. The hydrogenation of the BaNH species to NH3 and BaH2 can be facilely carried out at 150 °C. The relayed catalysis by Co and BaH2 sites creates an energy-favored pathway that allows ammonia synthesis under milder conditions.