The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 219

The solar thermal electrochemical
production of energetic molecules: Step
Stuart Licht
Department of Chemistry, GeorgeWashington University,Washington, DC, USA
8.1 INTRODUCTION
Anthropogenic release of carbon dioxide and atmospheric carbon dioxide have reached
record levels. One path towards CO 2 reduction is to utilize renewable energy to pro-
duce electricity. Another, less explored, path is to utilize renewable energy to directly
produce societal staples such as metals, bleach, fuels, including carbonaceous fuels.
Whereas solar-driven water splitting to generate hydrogen fuels has been extensively
studied (Vayssieres 2009; Rajeshwar et al., 2008), there have been few studies of solar
driven carbon dioxide splitting. “ CO 2 is a highly stable, noncombustible molecule,
and its thermodynamic stability makes its activation energy demanding and challeng-
ing (Ohla et al., 2009).'' In search of a solution for climate change associated with
increasing levels of atmospheric CO 2 , the field of carbon dioxide splitting (solar or
otherwise), while young, is growing rapidly, and as with water splitting, includes the
study of photoelectrochemical, biomimetic, electrolytic, and thermal pathways of car-
bon dioxide splitting (Graves et al., 2011; Barber 2009). Recently we introduced a
global process for the Solar Thermal Electrochemical Production (STEP) of energetic
molecules, including CO 2 splitting (Licht 2009; Licht et al., 2010; Licht 2011) as well
as the solar production of metals, fuels, bleach and other staples (Licht 2009, Licht
et al., 2010a; Licht et al., 2010b; Licht and Wang 2010; Licht 2011; Licht et al.,
2011b; Licht, and Wu. 2011; Licht et al., 2011a).
The direct thermal splitting of CO 2 requires excessive temperatures to drive any
significant dissociation. As a result, lower temperature thermochemical processes using
coupled reactions have recently been studied (Stamatiou et al., 2010; Venstrom and
Davidson 2011; Chueh and Haile, 2010; Miller et al., 2008). The coupling of mul-
tiple reactions steps decreases the system efficiency. To date, such challenges, and
the associated efficiency losses, have been an impediment to the implementation of the
related, extensively studied field of thermochemical splitting of water (Rajeshwar et al.,
2008). Photoelectrochemistry probes the energetics of illuminated semiconductors in
an electrolyte, and provides an alternative path to solar fuel formation. Photoelectro-
chemical solar cells (PECs) can convert solar energy to electricity, (Licht, 1987; Licht
and Peramunage, 1990; Oregan and Gratzel, 1991; Licht, 1998; Licht, 2002) and
with inclusion of an electrochemical storage couple, have the capability for internal
energy storage, to provide a level output despite variations in sunlight (Licht et al.,
1987; Licht et al., 1999). Solar to photoelectrochemical energy can also be stored