The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 233

oxide electrolyzer. Solid oxides utilize gas phase reactants, whereas carbonates utilize
molten phase reactants. Molten carbonate contains 2

10 − 2 mol reducible tetrava-
lent carbon/cm 3 . The density of reducible tetravalent carbon sites in the gas phase
is considerably lower. Air contains 0.03% CO 2 , equivalent to only 1

×

10 − 8 mol
of tetravalent carbon/cm 3 , and flue gas (typically) contains 10-15% CO 2 , equiva-
lent to 2

×

10 − 5 mol reducible C(IV)/cm 3 . Carbonate's higher concentration of active,
reducible tetravalent carbon sites, logarithmically decreases the electrolysis potential,
and can facilitate charge transfer at low electrolysis potentials. (ii) Molten carbonates
can directly absorb atmospheric CO 2 , whereas solid oxides require an energy con-
suming pre-concentration process. (iii) Molten carbonates electrolyses are compatible
with both solid and gas phase products. (iv) Molten processes have an intrinsic ther-
mal buffer not found in gas phase systems. Sunlight intensity varies over a 24-hour
cycle, and more frequently with variations in cloud cover. This disruption to other
solar energy conversion processes is not necessary in molten salt processes. For exam-
ple as discussed in Section 8.4.3, the thermal buffer capacity of molten salts has been
effective for solar to electric power towers to operate 24/7. These towers concentrate
solar thermal energy to heat molten salts, which circulate and via heat exchange boil
water to drive conventional mechanical turbines.

×

8.3 DEMONSTRATED STEP PROCESSES

8.3.1 STEP hydrogen
STEP occurs at both higher electrolysis and higher solar conversion efficiencies than
conventional room temperature photovoltaic (PV) generation of hydrogen. Experimen-
tally, we demonstrated a sharp decrease in the water splitting potential in an unusual
molten sodium hydroxide medium, Figure 8.3.1, and as shown in Figure 8.3.2, three
series connected Si CPVs efficiently driving two series molten hydroxide water-splitting
cells at 500 ◦ C to generate hydrogen (Licht et al., 2003; Licht, 2005).
Recently we have considered the economic viability of solar hydrogen fuel pro-
duction. That study provided evidence that the STEP system is an economically viable
solution for the production of hydrogen (Licht et al., 2003; Licht, 2005).

8.3.2 STEP carbon capture
In this process carbon dioxide is captured directly, without the need to pre-concentrate
dilute CO 2 , using a high temperature electrolysis cell powered by sunlight in a single
step. Solar thermal energy decreases the energy required for the endothermic conver-
sion of carbon dioxide and kinetically facilitates electrochemical reduction, while solar
visible generates electronic charge to drive the electrolysis. CO 2 can be captured as solid
carbon and stored, or used as carbon monoxide to feed chemical or synthetic fuel pro-
duction. Thermodynamic calculations are used to determine, and then demonstrate, a
specific low energy, molten carbonate salt pathway for carbon capture.
Prior investigations of the electrochemistry of carbonates in molten salts tended to
focus on reactions of interest to fuel cells, (Sunmacher, 2007) rather than the (reverse)
electrolysis reactions of relevance to the STEP reduction of carbon dioxide, typically in