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To establish an ultimate energy conversion system consisting of a water-splitting photocatalyst and a fuel cell, it is necessary to further increase the efficiencies of the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR). Recently, it was demonstrated that thiolate (SR)-protected gold clusters, Au n (SR) m , and their related alloy clusters can serve as model catalysts for these three reactions. However, as the previous data have been obtained under different experimental conditions, it is difficult to use them to gain a deep understanding of the means to attain higher activity in these reactions. Herein, we measured the HER, OER, and ORR activities of Au n (SR) m and alloy clusters containing different numbers of constituent atoms, ligand functional groups, and heteroatom species under identical experimental conditions. We obtained a comprehensive set of results that illustrates the effect of each parameter on the activities of the three reactions. Comparison of the series of results revealed that decreasing the number of constituent atoms in the cluster, decreasing the thickness of the ligand layer, and substituting Au with Pd improve the activities in all reactions. Taking the stability of the cluster into consideration, [Au 24 Pd(PET) 18 ] 0 (PET = 2-phenylethanethiolate) can be considered as a metal cluster with high potential as an HER, OER, and ORR catalyst. These findings are expected to provide clear design guidelines for the development of highly active HER, OER, and ORR catalysts using Au n (SR) m and related alloy clusters, which would allow realization of an ultimate energy conversion system. † Electronic supplementary information (ESI) available: Geometrical structure of each cluster, MALDI mass spectra, UV-vis spectra, schematic of the proposed energy conversion system, additional linear sweep voltammograms of the products. See

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Perfluorinated surfactants (PFSs) in groundwater were analyzed to reveal their distribution and sources. Sixteen groundwater and spring samples were collected from the Tokyo metropolitan area, and nine PFSs, including perfluorooctane-sulfonate (PFOS) and perfluorooctanoate (PFOA), were analyzed by liquid chromatography-tandem mass spectrometry. A column test using artificial street runoff was also performed to study their behavior. PFSs were detected in all groundwater samples, some at concentrations comparable to those in wastewater and street runoff, suggesting widespread contamination of groundwater by PFSs. In particular, PFOS -was more abundant in groundwater than in rivers, wastewater, and street runoff. This was attributed to its production from the degradation of its precursors, as supported by the column test. The occurrence of short-chain perfluorocarboxylates (PFCAs) in groundwater was also consistent with the results of the column test, showing that limited amounts of short-chain PFCAs were removed by soil, as the efficiency of removal increased with the chain length. We evaluated the contributions of PFCAs from wastewater and surface runoff to groundwater by using two indicators, the long/(short + long) ratio and the even(even + odd) ratio. Both ratios showed good agreement in their calculated contributions in heavily contaminated groundwater where breakthroughs likely occurred. Wastewater and surface runoff contributed to 54-86% and 16-46% of PFCAs, respectively, in groundwater.