The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 221

Chase, 1998). In 1976 Wentworth and Chen wrote about “simple thermal decomposi-
tion reactions for storage of solar energy,'' with the limitation that the products of the
reaction must be separated to prevent back reaction (and without any electrochemical
component), (Wentworth, 1976) and as early as 1980 it was noted that thermal energy
could decrease the necessary energy for the generation of H 2 by electrolysis (Bockris,
1980). However, the process combines elements of solid state physics, insolation and
electrochemical theory, complicating rigorous theoretical support of the process. Our
photo-electrochemical thermal water-splitting model for solar/H 2 by this process, was
the first derivation of bandgap restricted, thermal enhanced, high solar water-splitting
efficiencies. The model, predicting solar energy conversion efficiencies that exceed
those of conventional photovoltaics was initially derived for AM(Air Mass)1.5, terres-
trial insolation, and later expanded to include sunlight above the atmosphere (AM0
insolation) (Licht, 2002; Licht, 2003). The experimental accomplishment followed,
and established that the water-splitting potential can be specifically tuned to match
efficient photo-absorbers, (Licht et al., 2003; Licht, 2005) eliminating the challenge
of tuning (varying) the semiconductor bandgap, and which can lead to over 30%
solar to chemical energy conversion efficiencies. Our early process was specific to H 2
and did not incorporate the additional temperature enhancement of excess super-band
gap energy and concentration enhancement of excess reactant to further decrease the
electrolysis potential, in our contemporary STEP process.
8.2 SOLAR THERMAL ELECTROCHEMICAL PRODUCTION
OF ENERGETIC MOLECULES:AN OVERVIEW
8.2.1 STEP theoretical background
A single, small band gap junction, such as in a silicon PV, cannot generate the mini-
mum photopotential required to drive many room temperature electrolysis reactions,
as shown in the left of Scheme 8.2.1. The advancement of such studies had focused
on tuning semiconductor bandgaps (Zou et al., 2001) to provide a better match to
the electrochemical potential (specifically, the water-splitting potential), or by utilizing
more complex, multiple bandgap structures using multiple photon excitation (Licht
et al., 2000; Licht 2001). Either of these structures are not capable of excitation beyond
the bandedge and cannot make use of longer wavelength sunlight. Photovoltaics are
limited to super-bandgap sunlight, h ν> E g , precluding use of long wavelength radia-
tion, h ν< E g . STEP instead directs this IR sunlight to heat electrochemical reactions,
and uses visible sunlight to generate electronic charge to drive these electrolyses.
Rather than tuning the bandgap to provide a better energetic match to the elec-
trolysis potential, the STEP process instead tunes the redox potential to match the
bandgap. The right side of Scheme 8.2.1 presents the energy diagram of a STEP process.
The high temperature pathway decreases the free energy requirements for processes
whose electrolysis potential decreases with increasing temperature. STEP uses solar
energy to drive, otherwise energetically forbidden, pathways of charge transfer. The
process combines elements of solid state physics, insolation (solar illumination) and
high temperature electrochemical energy conversion. Kinetics improve, and endother-
mic thermodynamic potentials decrease, with increasing temperature. The result is a