Synthesis of the Co-N-C pre-catalyst. 0.5 mmol Co(OAc)2•4H2O (125 mg, Ac = acetate) was dissolved in 40 mL ethanol. An ethanol solution (10 mL) of 1,10-phenanthroline (Phen, 1.0 mmol, 180 mg) was added dropwise and the reaction mixture was sonicated for 30 min. 90 mg of Phen and 4 g of Mg(OH)2 were then added and the reaction mixture was further sonicated for 30 min.