The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 231

needed, and the product of Equation 8.2.9, carbon monoxide is a significant indus-
trial gas with a myriad of uses, including the bulk manufacturing of hydrocarbon
fuels, acetic acid and aldehydes (and detergent precursors), and for use in industrial
nickel purification (Elschenbroich and Salzer, 1992). To alleviate challenges of fos-
sil fuel resource depletion, CO is an important syngas component and methanol is
formed through the reaction with H 2 . The ability to remove CO 2 from exhaust stacks
or atmospheric sources, also limits CO 2 emission. Based on our original analogous
experimental photo-thermal electrochemical water electrolysis design, (Licht et al.,
2003; Licht, 2005) the first CO 2 STEP process consists of solar driven and solar ther-
mal assisted CO 2 electrolysis. In particular, in a molten carbonate bath electrolysis cell,
fed by CO 2 .

CO 3

cathode: 2CO 2 (g) + 2e −→ (molten) + CO (g)

anode: CO 3 (molten) → CO 2 (g) + 1 / 2O 2 (g) + 2e −

cell: CO 2 (g) → CO (g) + 1 / 2O 2 (g) (8.2.10)

Molten alkali carbonate electrolyte fuel cells typically operate at 650 ◦ C. Li, Na or
K cation variation can affect charge mobility and operational temperatures. Sintered
nickel often serves as the anode, porous lithium doped nickel oxide often as the cathode,
while the electrolyte is suspended in a porous, insulating, chemically inert LiAlO 2
ceramic matrix (Sunmacher, 2007).
Solar thermal energy can be used to favor the formation of products for electrolyses
characterized by a negative isothermal temperature coefficient, but will not improve the
efficiency of heat neutral or exothermic reactions. An example of this restriction occurs
for the electrolysis reaction currently used by industry to generate chlorine. During
2008, the generation of chlorine gas (principally for use as bleach and in the chlor-
alkali industry) consumed approximately 1% of the world's electricity, (Pellegrino,
2000) prepared in accord with the industrial electrolytic process:

E ◦ (25 ◦ C)

2NaCl + 2H 2 O → Cl 2 + H 2 + 2NaOH; = 2 . 502 V (8.2.11)

In the lower left portion of Figure 8.2.1, the calculated electrolysis potential for this
industrial chlor-alkali reaction exhibits little variation with temperature, and hence
the conventional generation of chlorine by electrolysis would not benefit from the
inclusion of solar heating. This potential is relatively invariant, despite a number of
phase changes of the components (indicated on the figure and which include the melting
of NaOH or NaCl). However, as seen in the figure, the calculated potential for the
anhydrous electrolysis of chloride salts is endothermic, including the electrolyses to
generate not only chlorine, but also metallic lithium, sodium and magnesium, and can
be greatly improved through the STEP process:

MCl n → n / 2Cl 2 + M;

(8.2.12)

E ◦ MCl-split (25 ◦ C)

= 3 . 98V-M = Na, 4 . 24V-K, 3 . 98V-Li, 3 . 07V-Mg

The calculated decreases for the anhydrous chloride electrolysis potentials are in the
order of volts per 1000 ◦ C temperature change. For example, from 25 ◦ Cuptothe