The energy grade of CO2 is low from a thermodynamic
perspective, accounting for why any transformation
to hydrocarbon requires energy infusion.
The energy source should be provided without producing
more CO2, such as solar energy. Plants use
solar energy to perform natural photosynthesis, but
the energy transformation is low at the cost of supporting
their lives. Even under the optimal artificial
conditions, energy efficiency can only reach about
10% in macroalga under full sunlight [1]. Solar energy
is the Earth’s ultimate power supply. All energy
forms except geothermal or nuclear, such as fossil
fuel, bio-material, hydropower and wind are various
kinds of transformation from sun. Consequently, the
photoreduction of CO2 is particularly interesting,
and achieving a high efficiency for this reaction is
highly desired. The ultimate goal is to demonstrate
that artificial photosynthesis may be implemented via
the photoreduction of CO2 to produce hydrocarbons,
such as methanol or methane. That is, solar energy is
transformed and stored as chemical energy. Moreover,
methanol is the most promising photo-reduced product
of carbon dioxide because it can be transformed
into other useful chemicals using conventional chemical
technologies, or easily transported and used as
fuel-like renewable energy.