**2. Necessity**

A large amount of fossil fuels has been combusted since the eighteenth century, so that the atmospheric CO2 concentration increased gradually. According to the data of the National Oceanic and Atmospheric Administration (NOOA), the CO2 concentration has exceeded 400 ppm in May, 2013, and it reached 402 ppm in May 2014 [54]. It is believed that the atmospheric CO2 concentration will exceed 550 ppm at the end of this century [55,56]. The sudden increase of CO2 concentration in the atmosphere can be attributed to the over-use of the fossil fuels. Currently, more than 80% of the global energy supply origins from fossil fuels, which generates a large amount of CO2 in the atmosphere.

A hundred years ago, Arrhenius suggested that CO2 emissions from the burning of fossil raw materials would lead to an increase in global temperatures [57,58]. Today, CO2 has been widely accepted as the chief culprit causing global warming, climate upheaval, and many other environmental problems. Various environmental problems will become much sharper if there are no e ffective measures are taken to curb CO2 emissions [59]. When the atmospheric CO2 content rises to 450 ppm, the accompanying increase in global temperature will seriously aggravate the cessation of the hot salt circulation, and environmental problems, like melting of glaciers, will take place [60].

In the 21st century, in addition to serious environmental problems, the energy crisis is also a global issue a ffecting human society. In 2008, the total global energy consumption was 132,000 megawatts. According to the U.S. [61] Energy Information Administration, this number will continuously grow, and the total energy consumption in 2040 is expected to be twice of that in 2020. Although the over exploitation and use of fossil energy has caused global warming and energy crisis, we can still find some opportunities and challenges to debate these issues [62]. For example, while using a suitable method to convert CO2 into energy materials or valuable industrial raw materials is a promising solution to close the carbon loop, and can alleviate the dependence of human beings on fossil energy and solve environmental problems that are caused by CO2 emissions [63,64].
