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Oxygen reduction reaction (ORR) is becoming increasingly important with the development of fuel cells and metal-air batteries. Manganese oxides have been one of the focuses of recent research for Pt-alternative ORR catalysts. However, the structure-activity relationships of manganese oxides have not been well studied or understood. In the present work, we report a new finding that there is a strong dependence of the ORR activity of Mn 3 O 4 on its morphology. By adopting different solvents in the wet-chemical synthesis, we are able to tailor the morphology of Mn 3 O 4 from nanoparticles (NP-L, 12.5 nm and NP-S, 5.95 nm) to nanorods (NR, exposure of Mn 3 O 4 (101)) and nanoflake (NF, exposure of Mn 3 O 4 (001)). Surprisingly, surface-specific activity of NF toward the ORR was found to be one order of magnitude higher than NP-L. The morphology-activity relationships of Mn 3 O 4 were further studied through a combination of electrochemical experiments and density functional theory (DFT) calculations. It was discovered that the formation of *OOH, concomitant with the first electron transfer, is the potential determining step, which is thermo-dynamically more facile on Mn 3 O 4 (001) than (101) plane. The underlying mechanism could be ascribed to the strong interaction between O 2 and Mn 3 O 4 (001) surface as indicated by the DFT calculations. The study enlarges our understanding of Mn 3 O 4 catalysis and provides clues for rational design of highly efficient transitional metal oxide electrocatalysts for the ORR.

A series of PdO/C catalysts were prepared by using the PdCl2 and Mn3O4/C as the precursors. The physical properties and the catalytic activity toward the oxygen reduction reaction (ORR) in alkaline media of the prepared PdO/C catalysts were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), as well as transmission electron microscopy (TEM) and the electrochemical techniques. The results show that, with the reaction time increasing, the main species PdO forms on the surface of carbon accompanied by a decreasing of the Mn content, implying a replacement of Mn3O4 with PdO on the carbon surface. The morphology of the irregular aggregates composed of PdO nanoparticles (about 2.3 nm) is similar to that of Mn3O4 particles. The PdO/C catalyst with the reaction time of 4 h and the Mn content of 0.03 wt.% (PdO/C-4 h) exhibits the highest activity toward the ORR, with the onset potential and the half-wave potential of the ORR shifting positively about 0.017 V and 0.020 V, respectively, compared to those on the commercial Pd/C (20 wt. %, E-TEK). The specific and mass current densities for he ORR obtained at 0.850 V (vs. RHE) on the PdO/C-4 h catalyst are about 2.64 mAcm(Pd) (2) and 1.11 mA mu g(Pd) (1), respectively, being about 2.67 and 2.18 times to those on the Pd/C. After 2000 potential cycles between 0.424 V and 1.124 V for accelerated aging tests, the better catalytic stability of the PdO/C-4 h catalyst toward the ORR over the Pd/C is observed, with the half-wave potential of the ORR shifting negatively only about 0.013 V, less than that on the latter one. All these findings demonstrate that the catalytic activity and stability of PdO/C-4 h catalyst for the ORR are superior to those of the commercial Pd/C, and the PdO/C-4 h catalyst would be a promising candidate for the non-platinum catalysts in alkaline media. (C) 2014 Elsevier Ltd. All rights reserved.

A 3D graphene oxide/carbon sphere supported silver composite (Ag/GO/C) was synthesized using graphene oxide as the reducing agent. The reducing process of Ag+ was monitored by the ultra violet-visible (UV-vis) absorption spectrometer and the physical properties of the Ag/GO/C composite were characterized by Fourier transform infrared spectrometer (FTIR), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the dispersive Ag nanoparticles are anchored uniformly on the surface of GO sheets with a mean size of about 6.9 nm. With introducing carbon black, the Ag nanoparticles aggregated slightly. Compared with its counterpart Ag/GO, the Ag/GO/C composite showed a significantly enhanced activity towards the oxygen reduction reaction in alkaline media. The enhancement can be ascribed to the 3D composite support, which not only improves the electrical conductivity, but also enforces the mass transport in the catalyst layer facilitating the reactants access to the active sites. Moreover, the Ag/GO/C composite exhibits good tolerance to alcohols, carbonates and tetramethylammonium hydroxide. This work is expected to open a new pathway to use GO as a reducing agent to synthesize electrocatalysts without surfactants. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.

Ag-MnOx/C composites were prepared using AgNO3 and KMnO4 as the precursors and Vulcan XC-72 as the support. The physical properties of the Ag-MnOx/C composites were investigated via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The activity and the stability of the series of Ag-MnOx/C composites toward the oxygen reduction reaction (ORR) in alkaline media were investigated through the electrochemical techniques. The results show that the main species MnO2 and Ag2O in the fresh sample convert into Mn3O4 and Ag(0), respectively, after the heat treatment in N2 at 300 °C (Ag-MnOx/C-300). The Ag-MnOx/C-300 sample shows the highest activity toward the ORR, with the half-wave potential of the ORR shifting negatively only 0.035 V compared to that on the commercial 40 wt. % Pt/C (JM). The electron transfer number during the ORR on the Ag-MnOx/C composite increases with the value close to four after the heat treatment at 300 °C, which is mainly attributed to the formation of Ag(0), rather than Mn3O4. The heat treatment brings about a better catalytic stability of the composite, and no obviously negative shift takes place for the half-wave potential of the ORR on the Ag-MnOx/C-300 composite after 1000 cycles accelerated aging test. The maximum power density of the zinc-air battery with the Ag-MnOx/C-300 air electrode reaches up to 130 mW cm−2, higher than those based on the Pd/C and Pt/C cathode catalysts, which shows that the Ag-MnOx/C-300 composite is a promising candidate as the catalyst for the air electrode.