The efficient synthesis of ammonia using carbon-footprint-free hydrogen under mild conditions is a grand challenge in chemistry today. To achieve this objective, novel concepts are needed for the activation process and catalyst. This article briefly reviews catalytic activation of N2 for ammonia synthesis under mild conditions. The features of the various activation methods reported so far are summarized, looking chronologically back at progress in heterogeneous catalysts since the use of iron oxide for the Haber-Bosch process, and finally the technical challenges to be overcome are described. Establishing low work functions for the support materials of the metal catalysts is one key to reducing the activation barrier to dissociate N2. Surfaces of electride materials that preserve the character of the bulk are shown to be useful for this purpose. The requirements of desired catalysts are high efficiency at low temperatures, Ru-free compositions, and chemical robustness in the ambient atmosphere.