The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 244

8.3.4 STEP chlorine and magnesium production
(chloride electrolysis)
The predominant salts in seawater (global average 3.5 0.4% dissolved salt by mass)

are NaCl (0.5 M) and MgCl 2 (0.05 M). The electrolysis potential for the industrial
chlor-alkali reaction exhibits little variation with temperature, and hence the conven-
tional generation of chlorine by electrolysis, Equation 8.2.11, would not benefit from
the inclusion of solar heating (Licht, 2009). However, when confined to anhydrous
chloride splitting, as exemplified in the lower portion of Figure 8.2.1, the calculated
potential for the anhydrous electrolysis of chloride salts is endothermic for the elec-
trolyses, which generate a chlorine and metal product. The application of excess heat,
as through the STEP process, decreases the energy of electrolysis and can improve the
kinetics of charge tranfer for the Equation 8.2.12 range of chloride splitting processes.
The thermodynamic electrolysis potential for the conversion of NaCl to sodium and
chlorine decreases, from 3.24 V at the 801 ◦ C melting point, to 2.99 V at 1027 ◦ C (Licht,
2009). Experimentally, at 850 ◦ C in molten NaCl, we observe the expected, sustained
generation of yellow-green chlorine gas at a platinum anode and of liquid sodium (mp
98 ◦ C) at the cathode. Electrolysis of a second chloride salt, MgCl 2 , is also of particular
interest. The magnesium, as well as the chlorine, electrolysis products are significant
global commodities. Magnesium metal, the third most commonly used metal, is gen-
erally produced by the reduction of calcium magnesium carbonates by ferrosilicons
at high temperature, (Li and Xie, 2005) which releases substantial levels of carbon
dioxide contributing to the anthropogenic greenhouse effect. However, traditionally,
magnesium has also been produced by the electrolysis of magnesium chloride, using
steel cathodes and graphite anodes, and alternative materials have been invesitgated
(Demirci and Karakaya, 2008).
Of significance here to the STEP process is the highly endothermic nature of anhy-
drous chloride electrolysis, such as for MgCl 2 electrolysis, in which solar heat will also
decrease the energy (voltage) needed for the electrolysis. The rest potential for electrol-
ysis of magnesium chloride decreases from 3.1 V, at room temperature, to 2.5 V at the
714 ◦ C melting point. As seen in Figure 8.3.8, the calculated thermodynamic poten-
tial for the electrolysis of magnesium chloride continues to decrease with increasing
temperature, to

±

2.3 V at 1000 ◦ C. The 3.1 V energy stored in the magnesium and
chlorine room temperature products, when formed at 2.3 V, provide an energy sav-
ings of 35%, if sufficient heat applied to the process can sustain this lower formation
potential. Figure 8.3.8 also includes the experimental decrease in the MgCl 2 electroly-
sis potential with increasing temperature in the lower right portion. In the top portion
of the figure, the concurrent shift in the cyclic voltammogram is evident, decreasing
the potential peak of magnesium formation, with increasing temperature from 750 ◦ C
to 950 ◦ C. Sustained electrolysis and generation of chlorine at the anode and magne-
sium at the cathode (Figure 8.3.8, photo inset) is evident at platinum electrodes. The
measured potential during constant current electrolysis at 750 ◦ C in molten MgCl 2 at
the electrodes is included in the figure.
In the magnesium chloride electrolysis cell, nickel electrodes yield similar results
to platinum, and can readily be used to form larger electrodes. The nickel anode
sustains extended chlorine evolution without evident deterioration; the nickel cathode
may slowly alloy with deposited magnesium. The magnesium product forms both as

∼