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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.

Three-dimensional nanoporous metals of highly porous structure and interconnected ligaments are attractive for electrochemical reactions. Herein, freestanding 3D nanoporous silver (np-Ag) is prepared by a facile electrochemical approach, i.e., first electro-oxidizing silver to silver halides followed by electro-reduction of the silver halides by controlling the potential applied to the electrode. The np-Ag displays 130 and 11.5 times enhancement in catalytic activity for the oxygen reduction reaction and the formaldehyde electro-oxidation reaction, respectively, relative to the flat polycrystalline silver, and even outperforms the commercial nano-Pt catalyst. Detailed experimental and theoretical studies discover that both the facilitated mass transportation in the 3D interconnected porous structures and the favored kinetics contribute to the superior electro-catalytic activity of np-Ag.

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.

The morphologically and compositionally controlled synthesis of coordination polymers and spinel oxides is highly desirable for realizing new advanced nanomaterial functionalities. Here we develop a novel and scalable strategy, containing a "copolymer-co-morphology" conception, to shape-controlled synthesis of various types of Prussian blue analogues (PBAs). Three series of PBAs MyFe1-y[Co(CN)6]0.67·nH2O (MyFe1-y-Co, M = Co, Mn and Zn) with well-controlled morphology have been successfully prepared through this strategy. Using MnyFe1-y-Co PBAs as the model, by increasing the relative content of Mn, flexible modulation of the morphology could be easily realized. In addition, a series of porous MnxFe1.8-xCo1.2O4 nano-dices with well-inherited morphologies and defined cation distribution could be obtained through a simple thermal treatment of the PBAs. All these results demonstrate the good universality of this novel strategy. When evaluated as an electrocatalyst, the octahedral-site Mn(III)/Mn(IV) content in MnxFe1.8-xCo1.2O4, mainly determined by sensitive (57)Fe Mössbauer in combination with X-ray photoelectron spectroscopic techniques, was discovered to be directly correlated with the oxygen reduction/evolution reaction (ORR/OER) activity.