Nanowires made of “SnO 2 on CuO” coupled with a GaInP/GaInAs/Ge photovoltaic cell show an interesting solar-to-CO conversion efficiency of 13.4% [9].
Using perennial energy for coprocessing CO 2 and water is of paramount importance
for our future and for an extended C-recycling [10, 11]. Engineering new photomaterials such as mixed oxides, coupling a n-, and a p-type semiconductor is a way
to avoid charge recombination and address the selectivity issue. Photocatalysts
selectivity toward one of the reduction products of CO 2 is a key issue to solve. In
fact, the energy necessary for product separation is a key factor in determining the
overall energetics of a process.
Working in water, the ideal would be to produce a chemical which is not
water-soluble, so that it can escape the condensed phase. Looking at Fig. 10.7, CH 4
and CO leave the water phase together with excess CO 2 , while formic acid is
water-soluble and remains in water. If only methane or only CO in the gas phase
were produced even if coupled to formic acid, the product separation would be
easier; viceversa, the separation of methane and CO eventually both present in the
gas phase with CO 2 , requires more energy for separation. Spending energy means
today emit CO 2 , as the use of PV is not yet at such economic level to drive the
separation of industrial processes relevant to very cheap materials such as CO and
CH 4 . Therefore, CuI presents quite interesting photo properties but should be
improved in order to cut one of the gaseous products.
10.5 PV-Driven Processes
The utilization of PV-electricity in CO 2 reduction can occur either in a direct or an
indirect way. In the latter, PV-H2 is produced which is then used for CO 2 reduction,
as already discussed in Chap. 9. In the former, CO 2 is directly used in electrochemical processes and converted into a variety of compounds. Moreover, the
direct reduction can be performed in a variety of routes as described in the following paragraphs.
10.5.1 Electrochemical Reduction of CO 2
In an electrochemical device, electricity drives two processes occurring, respectively, at the cathode (reduction) and the anode (oxidation). Usually, the electrodes
are immersed in a liquid which may even take part in the process. Working in
water, it is worth to recall the low solubility of CO 2 and the fact that it can originate
a variety of species, according to the pH. Hydrogen carbonate, HCO 3
− is most
abundant and can be reduced instead of CO 2 . The low solubility of CO 2 in water
increases the energy necessary for the reduction, which often occurs at less negative
potential than the thermodynamic ones. However, the pH of the solution must be
kept under control.
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10 Solar Chemistry and CO 2 Conversion
Using perennial energy for coprocessing CO 2 and water is of paramount importance
for our future and for an extended C-recycling [10, 11]. Engineering new photomaterials such as mixed oxides, coupling a n-, and a p-type semiconductor is a way
to avoid charge recombination and address the selectivity issue. Photocatalysts
selectivity toward one of the reduction products of CO 2 is a key issue to solve. In
fact, the energy necessary for product separation is a key factor in determining the
overall energetics of a process.
Working in water, the ideal would be to produce a chemical which is not
water-soluble, so that it can escape the condensed phase. Looking at Fig. 10.7, CH 4
and CO leave the water phase together with excess CO 2 , while formic acid is
water-soluble and remains in water. If only methane or only CO in the gas phase
were produced even if coupled to formic acid, the product separation would be
easier; viceversa, the separation of methane and CO eventually both present in the
gas phase with CO 2 , requires more energy for separation. Spending energy means
today emit CO 2 , as the use of PV is not yet at such economic level to drive the
separation of industrial processes relevant to very cheap materials such as CO and
CH 4 . Therefore, CuI presents quite interesting photo properties but should be
improved in order to cut one of the gaseous products.
10.5 PV-Driven Processes
The utilization of PV-electricity in CO 2 reduction can occur either in a direct or an
indirect way. In the latter, PV-H2 is produced which is then used for CO 2 reduction,
as already discussed in Chap. 9. In the former, CO 2 is directly used in electrochemical processes and converted into a variety of compounds. Moreover, the
direct reduction can be performed in a variety of routes as described in the following paragraphs.
10.5.1 Electrochemical Reduction of CO 2
In an electrochemical device, electricity drives two processes occurring, respectively, at the cathode (reduction) and the anode (oxidation). Usually, the electrodes
are immersed in a liquid which may even take part in the process. Working in
water, it is worth to recall the low solubility of CO 2 and the fact that it can originate
a variety of species, according to the pH. Hydrogen carbonate, HCO 3
− is most
abundant and can be reduced instead of CO 2 . The low solubility of CO 2 in water
increases the energy necessary for the reduction, which often occurs at less negative
potential than the thermodynamic ones. However, the pH of the solution must be
kept under control.
186
10 Solar Chemistry and CO 2 Conversion