A\nMetal–Organic Framework Derived Porous Cobalt Manganese Oxide\nBifunctional Electrocatalyst for Hybrid Na–Air/Seawater Batteries

Abstract

Spinel-structured\ntransition metal oxides are promising non-precious-metal electrocatalysts\nfor oxygen electrocatalysis in rechargeable metal–air batteries.\nWe applied porous cobalt manganese oxide (CMO) nanocubes as the cathode\nelectrocatalyst in rechargeable seawater batteries, which are a hybrid-type\nNa–air battery with an open-structured cathode and a seawater\ncatholyte. The porous CMO nanocubes were synthesized by the pyrolysis\nof a Prussian blue analogue, Mn₃[Co­(CN)₆]₂·<i>n</i>H₂O, during air-annealing,\nwhich generated numerous pores between the final spinel-type CMO nanoparticles.\nThe porous CMO electrocatalyst improved the redox reactions, such\nas the oxygen evolution/reduction reactions, at the cathode in the\nseawater batteries. The battery that used CMO displayed a voltage\ngap of ∼0.53 V, relatively small compared to that of the batteries\nemploying commercial Pt/C (∼0.64 V) and Ir/C (∼0.73\nV) nanoparticles and without any catalyst (∼1.05 V) at the\ninitial cycle. This improved performance was due to the large surface\narea (catalytically active sites) and the high oxidation states of\nthe randomly distributed Co and Mn cations in the CMO. Using a hard\ncarbon anode, the Na-metal-free seawater battery exhibited a good\ncycle performance with an average discharge voltage of ∼2.7\nV and a discharge capacity of ∼190 mAh g^–1_{hard carbon} during 100 cycles (energy efficiencies\nof 74–79%).