The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are crucial half-reactions of green electrochemical energy storage and conversion technologies, such as electrochemical water-splitting devices and regenerative fuel cells. Researchers always committed to synthesizing earth-abundant-element-based nanomaterials as high-efficiency electrocatalysts for realizing their industrial applications. In this review, we briefly elaborate on the underlying mechanisms of OER and ORR during the electrochemical process. Then, we systematically sum up the recent research progress in representative metal-free carbon (C)-based electrocatalysts; metal-nitrogen-C electrocatalysts; and nonprecious-metal OER/ORR electrocatalysts, including transition-metal oxides, phosphides, nitrides/oxynitrides, chalcogenides, and carbides. Among these, some representative bifunctional electrocatalysts for the OER/ORR are mentioned. In particular, we discuss the effects of physicochemical properties-morphology, phases, crystallinity, composition, defects, heteroatom doping, and strain engineering-on the comprehensive performance of the abovementioned electrocatalysts, with the aim of establishing the nanostructure-function relationships of the electrocatalysts. In addition, the development directions of OER and ORR electrocatalysts are determined and highlighted. The generic approach in this review expands the frontiers of and provides inspiration for developing high-efficiency OER/ORR electrocatalysts.
In order to understand the changes in the ecological environment of Miyun Reservoir after the South-to-North Water Transfer Project, our study investigated the changes in the composition and structure of two indicator species in the Miyun Reservoir and applied the rotifer biological assessment method to analyse the changes of water ecological environment. Our results showed that a total of 10 species of the genera Trichocerca and Brachionus were identified during the investigation with an average annual abundance of 542.124 ind./L and 102.68 ind./L, respectively. Among them, T. pusilla and T. cylindrica were the dominant species of the genus Trichocerca, and the majority of Brachionus species were B. diversicornis, B. angularis, B. caudatus. Variance results showed that there were significant differences in the abundance of these two rotifer pupations at different inter-annual and spatial scales during the survey. The biological evaluation indicated that the water quality has gradually changed from eutrophication to oligotrophication after the South-to-North Water Transfer Project, and the ecological environment was maintained well. Our results will provide necessary ecological parameters for water environment protection and ecological safety management of Miyun Reservoir.
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