The rapid depletion of the fossil fuel reserves, the increase of green-house gas emissions and other environmental pollutions, bring up an urgent need for the development of renewable and clean energy sources. The ability to use sunlight to produce fuels such as H2 and electricity from abundant, non-toxic resources, and to decompose environmental pollutions with benign catalysts, would revolutionize our civilization. Figure 1 shows a proposed scheme of the renewable energy economy, where electricity or hydrogen is produced under solar irradiation, then stored and finally used in a electric or hydrogen fuel cell car. However, current technologies are still far away for large scale applications due to the limit of low efficiency, high cost and/or short lifetime. To achieve this, technological breakthroughs and revolutionary developments in materials are extremely needed
