Environmental Engineering Reference
In-Depth Information
externally in chemical form, when it is used to drive the formation of energetically
rich chemicals. Photochemical, and photoelectrochemical, splitting of carbon dioxide
(Yan et al., 2011; Zhou et al., 2011; Richardson, Holland, Carpenter 2011; Barton
et al., 2008; Kaneco et al., 2009; Pan and Chen, 2007) have demonstrated selective
production of specific fuel products. Such systems function at low current density and
efficiencies of
1 percent, and as with photoelectrochemical water splitting face sta-
bility and bandgap challenges related to effective operation with visible light (Licht,
2002; Murphy, 2008; Currao, 2007).
The electrically driven (nonsolar) electrolysis of dissolved carbon dioxide is under
investigation at or near room temperature in aqueous, non-aqueous and PEM media
(Narayanan, et al., 2011; Delacourt and Newman 2010; Dufek et al., 2011; Gangeri
et al., 2009; Innocent et al., 2008; Wang et al., 2009; Chu et al., 2008; Yano et al.,
2007; Hori et al., 2005; Ogura et al., 2004). These are constrained by the thermo-
dynamic and kinetic challenges associated with ambient temperature, endothermic
processes, of a high electrolysis potential, large overpotential, low rate and low elec-
trolysis efficiency. High temperature, solid oxide electrolysis of carbon dioxide dates
back to suggestions from 1960 to use such cells to renew air for a space habitat,
(Martin, 1965; Chandler et al., 1966; Erstfield, 1979; Stancati et al., 1981; Richter,
1981; Mizusaki et al., 1992; Tao et al., 2004; Green et al., 2008) and the sustainable
rate of the solid oxide reduction of carbon dioxide is improving rapidly (Meyers et al.,
2011; Kim-Lohsoontorn et al., 2011; Ebbesen et al., 2010; Jensen et al., 2010; Fu
et al., 2010; Stoots et al., 2010; Fu et al., 2010). Molten carbonate, rather solid oxide,
fuel cells running in the reverse mode had also been studied to renew air in 2002 (Lueck
et al., 2002). In a manner analogous to our 2002 high temperature solar water splitting
studies and described below (Licht, 2002; Licht, 2003; Licht et al., 2003; Licht, 2005),
we showed in 2009 that molten carbonate cells are particularly effective for the solar
driven electrolysis of carbon dioxide, (Licht, 2009; Licht, Wang et al., 2010a; Licht
et al., 2011a; Licht 2011) and also CO
2
-free iron metal production (Licht and Wang,
2010; Licht et al., 2011b; Licht, 2011).
Light-driven water splitting was originally demonstrated with TiO
2
(a semincon-
ductor with a bandgap, E
g
>
3.0 eV) (Fujishima and Honda, 1972). However, only a
small fraction of sunlight has sufficient energy to drive TiO
2
photoexcitation. Studies
had sought to tune (lower) the semiconductor bandgap to provide a better match to the
electrolysis potential (Zou et al., 2001). In 2000, we used external multiple bandgap
PV
s (photovoltaics) to generate H
2
by splitting water at 18% solar energy conversion
efficiency (Licht et al., 2000; Licht, 2001). However, that room temperature process
does not take advantage of additional, available thermal energy.
An alternative to tuning a seminconductor bandgap to provide a better match to
the solar spectrum, is an approach to tune (lower) the electrolysis potential (Licht
et al., 2003; Licht, 2005). In 2002, we introduced a photo-electrochemical ther-
mal water splitting theory, (Licht, 2002) which was verified by experiment in 2003,
for H
2
generation at over 30% solar energy conversion efficiency, and providing the
first experimental demonstration that a semiconductor, such as Si (E
g
=
∼
1.1 eV), with
bandgap lower than the standard water-splitting potential (E
◦
H
2
O
(25
◦
C)
1.23 V), can
directly drive hydrogen formation (Licht et al., 2003; Licht, 2005). With increasing
temperature, the quantitative decrease in the electrochemical potential to split water to
hydrogen and oxygen had been well known by the 1950s (deBethune and Licht, 1959;
=
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