The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 245

Figure 8.3.8 Photograph lower left: coiled platinum before (left), and after (right), MgCl 2 electrolysis
forming Mg metal on the cathode (shown) and evolving chlorine gas on the anode. Main
figure: cathode size restriced cyclic voltammetry of Pt electrodes in molten MgCl 2 . Inset:
The measured full cell potential during constant current electrolysis at 750 ◦ C in molten
MgCl 2 . Lower right:Thermodynamic and measured electrolysis potentials in molten MgCl 2
as a function of temperature. Electrolysis potentials are calculated from the thermody-
namic free energies components of the reactants and products as E =− G(reaction)/2F.
Measured electrolysis potentials are stable values on Pt at 0.250A/cm 2 cathode (Licht
et al., 2011a). Lower right:A schematic representation of a separate (i) solar thermal and
(ii) photovoltaic field to drive both water purification, hydrogen generation, and the
endothermic electrolysis of the separated salts to useful products. Modified with
permission from Licht et al. 2011a.
the solid and liquid (Mg mp 649 ◦ C). The liquid magnesium is less dense than the
electrolyte, floats upwards, and eventually needs to be separated and removed to
prevent an inter-electrode short, or to prevent a reaction with chlorine that is evolved
at the anode. In a scaled-up cell configuration (not shown in Figure 8.3.8, a larger
Ni cathode (200 cm 2 cylindrical nickel sheet (McMaster 9707K35) was employed,
sandwiched between two coupled cylindrical Ni sheet anodes (total 200 cm 2 , of area
across from the cathode) in a 250 ml alumina (Adavalue) crucible, and sustains multi-
amp large ampere currents. The potential at constant current is initially stable, but this
cell configuration leads to electrical shorts, unless liquid magnesium is removed.