The solar thermal electrochemical production of energetic molecules: Step - Solar Energy Sciences and Engineering Applications - page 239

capture of atmospheric CO 2 , without a CO 2 pre-concentration stage. This consists
of the absorption of atmospheric CO 2 (in molten Li 2 CO 3 containing Li 2 O, to form
Li 2 CO 3 ), combined with a facile rate of CO 2 splitting due to the high carbonate con-
centration, compared to the atmospheric concentration of CO 2 , and the continuity of
the steady-state of concentration Li 2 O, as Li 2 CO 3 is electrolyzed in Equation 8.3.2.

8.3.3 STEP iron
A fundamental change in the understanding of iron oxide thermochemistry can open a
facile, new CO 2 -free, route to iron production. Along with control of fire, iron produc-
tion is one of the founding technological pillars of civilization, but is a major source of
CO 2 emissions. In industry, iron is still produced by the carbothermal greenhouse gas
intensive reduction of iron oxide by carbon-coke, and a carbon dioxide free process
to form this staple is needed.
The earliest attempt at electrowinning iron (the formation of iron by electrolysis)
from carbonate appears to have been in 1944 in the unsuccessful attempt to electrode-
posit iron from a sodium carbonate, peroxide, metaborate mix at 450-500 ◦ C, which
deposited sodium and magnetite (iron oxide), rather than iron (Andrieux and Weiss,
1944; Haarberg et al., 2007). Other attempts (Haarberg et al., 2007) have focused on
iron electrodepostion from molten mixed halide electrolytes, which has not provided
a successful route to form iron, (Wang et al., 2008; Li et al., 2009), or aqueous iron
electrowinning (Yuan et al., 2009; Palmaer and Brinell, 1913; Eustis, 1922; Mostad
et al., 2008) that is hindered by the high thermodynamic potential (E ◦ = 1.28 V) and

diminished kinetics at low temperature.
We present a novel route to generate iron metal by the electrolysis of dissolved iron
oxide salts in molten carbonate electrolytes, unexpected due to the reported insolubility
of iron oxide in carbonates. We report high solubility of lithiated iron oxides, and facile
charge transfer that produces the staple iron at high rate and low electrolysis energy,
and can be driven by conventional electrical sources, but is also demonstrated with
STEP procesess that decrease or eliminate a major global source of greenhouse gas
emissions (Licht, 2009; Licht et al., 2010a; Licht and Wu, 2011).
As recently as 1999, the solubility of ferric oxide, Fe 2 O 3 , in 650 ◦ C molten carbon-
ate was reported as very low, a 10 − 4 . 4 mole fraction in lithium/potassium carbonate
mixtures, and was reported as invariant of the fraction of Li 2 CO 3 and K 2 CO 3 (Qingeng
et al., 1999). Low solubility, of interest to the optimization of molten carbonate fuel
cells, had likely discouraged research into the electrowinning of iron metal from ferric
oxide in molten lithium carbonate. Rather than the prior part per million reported sol-
ubility, we find higher Fe(III) solubilities, in the order of 50% in carbonates at 950 ◦ C.
The CV of a molten Fe 2 O 3 Li 2 CO 3 mixture presented in Figure 8.3.5, and exhibits a
reduction peak at

0.8 V, on Pt (gold curve); which is more pronounced at an iron
electrode (light gold curve). At constant current, iron is clearly deposited. The cooled
deposited product contains pure iron metal and trapped salt, and changes to rust color
with exposure to water (Figure 8.3.5 photograph). The net electrolysis is the redox
reaction of ferric oxide to iron metal and O 2 , Equation 8.2.14. The deposit is washed,
dried, and is observed to be reflective, grey metallic, responds to an external magnetic
field, and consists of dendritic iron crystals.

−