ABSTRACT
Energy and environment have become the top two concerns and challenges of mankind nowadays, and these two issues are closely correlated to each other. As estimated, over 80% of energy sources
of the human society are obtained from burning of fossil fuels such as coal, oil, and natural gas [1-3]. However, carbon dioxide and other toxic gases released from fossil fuel burning are causing many potential environmental problems such as the greenhouse effect. In addition, fossil fuel deposits on the earth are not unlimited. As a result, new technologies making use of alternative renewable and clean energy sources have become major research objectives and challenges of scientists in the twenty-fi rst century.
