Environmental Engineering Reference
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Figure 8.3.1 VH 2 O, measured in aq.saturated or molten NaOH, at 1 atm. Steam is injected in the
molten electrolyte. O 2 anode is 0.6 cm 2 Pt foil. IR and polarization losses are minimized
by sandwiching 5mm from each side of the anode, oversized Pt gauze cathode. Inset:At
25 C, 3 electrode values comparing Ni and Pt working electrodes and with a Pt gauze
counterelectrode at 5mV/s. Modified with permission from Licht 2011.
alkali carbonate mixtures. Such mixtures substantially lower the melting point com-
pared to the pure salts, and would provide the thermodynamic maximum voltage for
fuel cells. However, the electrolysis process is maximized in the opposite temperature
domain of fuel cells, that is at elevated temperatures which decrease the energy of
electrolysis, as schematically delineated in Scheme 8.2.1. These conditions provide a
new opportunity for effective CO 2 capture.
CO 2 electrolysis splitting potentials are calculated from the thermodynamic free
energy components of the reactants and products (Licht, 2009; Licht et al., 2010a;
Chase, 1998) as E
4 or 2 for the respective conver-
sion of CO 2 to the solid carbon or carbon monoxide products. As calculated using
the available thermochemical enthalpy and entropy of the starting components, and
as summarized in the left side of Figure 8.3.3, molten Li 2 CO 3 ,viaaLi 2 O inter-
mediate, provides a preferred, low energy route compared to Na 2 CO 3 or K 2 CO 3
(via Na 2 OorK 2 O), for the conversion of CO 2 . High temperature is advantageous
as it decreases the free energy energy necessary to drive the STEP enodthermic pro-
cess. The carbonates, Li 2 CO 3 ,Na 2 CO 3 and K 2 CO 3 , have respective melting points
of 723 C, 851 C and 891 C. Molten Li 2 CO 3 not only requires lower thermody-
namic electrolysis energy, but in addition has higher conductivity (6 S cm 1 ) than
that of Na 2 CO 3 (3Scm 1 )orK 2 CO 3 (2Scm 1 ) near the melting point (Zhang and
=− G(reaction)/nF, where n
=
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