1. Introduction
The photo(electro)chemical splitting water into hydrogen and oxygen has attracted tremendous attention in recent years because of its great potential in solar energy conversion and storage applications [1]. In respect to hydrogen evolution reaction, a two-electron-transfer process, oxygen evolution reaction is more complex, which involves fourelectron-transfer process. Currently, owing to the slow four-electron transfer rate and the high activation energy barrier for O\\O bond formation, the oxygen evolution is the main obstacle limiting the efficiency of overall water splitting [2]. Searching environmentally-friendly and efficient oxygen evolution catalysts (OECs) is an urgent and challenging task among the solar water splitting community. In nature, a Mn4CaO5 cluster as the oxygen evolving complex in photosystem II (PS II), has been identified as the catalytic site for the fourelectron involved water oxidation [3]. Inspired by the natural photosynthesis process in PS II, synthetic manganese-containing compounds are thought to be very promising candidates as functional water oxidation catalysts to mimic the role of Mn4CaO5 cluster in water oxidation [4]. Various nanostructured solid manganese oxides (MnOx) including MnO2, Mn2O3, Mn3O4, MnOx, MnO, Mn3(PO4)2·3H2O, CaMn2O4·xH2O and amorphous CaxMnOy have been synthesized and tested as OECs in either electrochemical or photochemical systems [5–12].
