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Carbon supported gold-iridium composite (AuIr/C) was synthesized by a facile one-step process and was investigated as the bifunctional catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The physical properties of the AuIr/C composite were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Although the Au and Ir in the AuIr/C did not form alloy, it is clear that the introduction of Ir decreases the average Au particle size to 4.2 nm compared to that in the Au/C (10.1 nm). By systematical analysis on chemical state of metal surface via XPS and the electrochemical results, it was found that the Au surface for the Au/C can be activated by potential cycling from 0.12 V to 1.72 V, resulting in the increased surface roughness of Au, thus improving the ORR activity. By the same potential cycling, the Ir surface of the Ir/C was irreversibly oxidized, leading to degraded ORR activity but uninfluenced OER activity. For the AuIr/C, Ir protects Au against being oxidized due to the lower electronegativity of Ir. Combining the advantages of Au and Ir in catalyzing ORR and OER, the AuIr/C catalyst displays an enhanced catalytic activity to the ORR and a comparable OER activity. In the 50-cycle accelerated aging test for the ORR and OER, the AuIr/C displayed a satisfied stability, suggesting that the AuIr/C catalyst is a potential bifunctional catalyst for the oxygen electrode.

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.

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.