Ultrafine SmMn2O5-δ electrocatalysts with modest oxygen deficiency for highly-efficient pH-neutral magnesium-air batteries

Abstract Mg-air batteries are of high significance and promise for sustainable energy storage and conversion, due to remarkably hightheoretical specific energy density, eco-friendliness, and low cost.However, the commercialization of Mg-air batteries has been impeded by the low efficiency of non-noble-metal-based electrocatalysts for oxygen reduction reaction (ORR) in pH-neutral electrolyte. Herein, we effectively regulate the ORR activity in NaCl solution over mullite SmMn2O5-δ nanocrystals by engineering their oxygen deficiency over a wide range using both laser irradiation and low temperature annealing methods. The optimized catalyst SMO4.86 with moderate oxygen deficiency shows the highest ORR activity in pH-neutral electrolyte, approaching that of commercial Pt/C catalyst, and long-term stability. Density functional theory calculations reveal that the introduction of oxygen vacancies reduces the work function and leads to higher Fermi levels, thus resulting in stronger catalyst-intermediates interaction and easier electron transfer from the catalyst to the intermediates. With SMO4.86 as the air electrode, the home-made Mg-air battery delivers a power density as high as 101 mW cm-2 and a high specific capacity of 1770 mAh g-1 with excellent durability (~115 h), superior to the commercial Pt/C electrode. This work provides an effective strategy to substantially promote the pHneutral ORR kinetics in Mg-air batteries.