Rationally Designed Manganese-Based Metal-Organic Frameworks as Altruistic Metal Oxide Precursors for Noble Metal-Free Oxygen Reduction Reaction.

The sluggish oxygen reduction reaction (ORR) at the cathode is challenging and hinders the growth of hydrogen fuel cells. Concerning kinetic values, platinum is the best known catalyst for ORR; however, its less abundance, high cost, and corrosive nature warrant the development of low-cost catalysts. We report the hydrothermal synthesis of two novel Mn-based metal-organic frameworks (MOFs), [Mn(DOT)(HO)] () and [Mn(DOT)(BPY)(THF)] () (DOT = 2,5-dihydroxyterephthalate; BPY = 4,4'-bipyridine). contains dimeric Mn(II) centers where the two corner-shared MnO octahedra fuse to give rise to an infinite MnO cluster, whereas the two Mn(II) ions coordinate to DOT and BPY moieties to give rise to a pillared structure in and form a 3D → 3D homo-interpenetration MOF with a twofold interpenetrated net. The pyrolysis of as-synthesized Mn-MOFs at 600 °C under N produced exclusively porous α-MnO composites ( and ), with the BET surface area of 90.8 (for ) and 179.3 m g (for ). These mesoporous MOF-derived α-MnO composites were modified as cathode materials for the electrocatalytic reduction of oxygen. The onset potential for the oxygen reduction reaction was found to be 0.90 V for and 0.93 V for versus RHE in 0.1 M KOH solution, with the current density of 4.8 and 6.0 mA cm, respectively, at 1600 rpm. Based on the RDE/RRDE results, the electrocatalytic oxygen reduction occurs majorly via the four-electron process. The electrocatalyst is cheap, easy to use, retains 90% of its activity after 10 h of continuous use, and offers higher recyclability than Pt/C. The onset potential maximum current density and kinetic values ( = 11.68 mA cm and Tafel slope = 85.0 mV dec) obtained in this work are higher than the values reported for pure MnO.