Large-Scale Synthesis of Silver Manganese Oxide Nanofibers and Their Oxygen Reduction Properties

High surface area (95 m2/g) silver manganese oxide octahedral molecular sieve (Ag-OMS-2) nanofibers with the cryptomelane structure were prepared at the multigram scale under ambient pressure at 100 °C. A variety of techniques were used to characterized these materials, such as powder X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, N2 sorption, and X-ray photoelectron spectroscopy. Electrochemical and oxygen reduction properties of these materials were studied by employing cyclic voltammetry and linear sweep voltammetry on rotating disk electrodes techniques. A significant enhancement of oxygen reduction reaction (ORR) activity was observed for Ag-OMS-2 electrocatalysts as compared to Vulcan XC-72 carbon and phosphorus-doped mesoporous carbon materials. The superior catalytic activity was also reflected by a 2-fold increase in ORR currents and a positive shifted half-wave potential. Moreover, a mass activity of 32.9 mA mg–1 was achieved, which is ten times higher than the electrode made with pure carbon. The well-crystallized Ag-OMS-2 nanofibers also showed an average high discharge specific capacity of 2741 mAh/g of carbon in Li–O2 batteries in an initial study, which is 1.8 times higher than AgMnO4 was used. Three possible reasons why the Ag-OMS-2 is a better ORR catalyst are also presented and discussed.