Electrochemistry Communications 13 (2011) 1521–1524
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Production of iron and oxygen in molten K2CO3–Na2CO3 by electrochemically
splitting Fe2O3 using a cost affordable inert anode
Huayi Yin a, Diyong Tang a, Hua Zhu a, Yu Zhang b, Dihua Wang a, c,⁎
a
b
c
School of Resources and Environmental Science, Wuhan University, Wuhan 430072, PR China
Department of Radiochemistry, China Institute of Atomic Energy, Beijing, 102413, PR China
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China
a r t i c l e
i n f o
Article history:
Received 26 August 2011
Received in revised form 10 October 2011
Accepted 10 October 2011
Available online 14 October 2011
Keywords:
Molten salt electrochemistry
Iron
Molten carbonate
Inert anode
Green process
Oxygen production
a b s t r a c t
Iron oxide was electrochemically split into iron and oxygen gas in molten Na2CO3–K2CO3 at 750 °C using a
solid iron oxide cathode and a Ni10Cu11Fe alloy inert anode. Fe2O3 was electrochemically reduced to Fe on
the cathode, releasing oxygen anions into the electrolyte and which were oxidized on the anode to generate
O2. The cathodic current efficiency was as high as 95% and the energy consumption for producing 1 kg iron
was 2.87 kWh, only half of the current industrial energy consumption of blast-furnace steel production.
Due to the cost-affordable inert anode and the high energy efficiency, the method demonstrated in this
work shows promise as a practical “green” iron production process.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The modern iron and steel industry annually produces more than
1 × 10 9 tonnes of steel and releases more than 2 × 10 9 tonnes of CO2
accounting for 3%–4% of the total world greenhouse gas emissions
[1]. Production of iron utilizing carbon-free processes would eliminate this major greenhouse gas emission source. Along with the development of renewable electricity, electrolytic production of iron in
high temperature molten salts has drawn increasing attention in recent years [2–7]. Reported methods for electrolytic production of
iron in molten salts can be divided into three techniques: (1) electrodeposition of solid iron from molten salt such as molten lithium carbonate [5]; (2) electrowinning of liquid iron from high temperature
molten oxide [6]; (3) electro-deoxidation of solid iron oxide in molten calcium chloride or molten alkaline hydroxide [3,4,7]. Solid iron
produced by electrodeposition in molten salts is dendritic and difficult to separate the product from the molten salt [5]. Production of
molten iron and oxygen using an iridium inert anode was recently realized by electrochemical decomposition of molten oxide above
liquidus-temperature of iron [6], but a cost-affordable inert anode is
still absent. Although the electrochemical reduction of solid iron
oxide to iron in molten calcium chloride was demonstrated as a
high energy efficiency process [7], an inert anode in the halogen molten salts is also very challenging [8–12]. Efforts to replace molten
CaCl2 with molten hydroxide were performed [3,4], but it was theoretically limited by the narrow electrochemical potential window
and ambiguous mass transfer kinetics of oxygen anions in the melt.
An ideal supporting molten electrolyte for production of iron by
electroreduction of solid iron oxide should meet the requirements
of: (1) low solubility of iron oxide; (2) wide potential window;
(3) facile mass transfer of oxygen ions; and (4) compatibility with
an inert anode. Molten Na2CO3–K2CO3 has a wide electrochemical
potential window [13–15], good oxygen anion conductivity and a
weak ability to dissolve the transition metal oxides [5] so that it
might be an ideal molten electrolyte for iron production. In this
paper, we aimed to develop a more practical “green” iron production
process in molten Na2CO3–K2CO3 by electrochemically splitting solid
iron oxide and test the stability of a low-cost material (nickel alloy)
as an inert anode.
2. Experimental
2.1. Electrolyte and electrodes
⁎ Corresponding author at: School of Resources and Environmental Science, Wuhan
University, Wuhan 430072, PR China. Tel./fax: + 86 27 68775799.
E-mail address: [email protected] (D.H. Wang).
1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2011.10.009
About 500 g anhydrous Na2CO3–K2CO3 (Na2CO3:K2CO3 =
59:41mol%) was put into an alumina crucible which was located in a
closed-end sealed steel reactor. After drying at 150 °C for 12 h, the
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H. Yin et al. / Electrochemistry Communications 13 (2011) 1521–1524
temperature was slowly raised up to 750 °C in argon to melt the carbonate. Porous Fe2O3 pellets (20 mm in diameter) were prepared by diepressing of Fe2O3 powder and sintering in air at 750 °C for 2 h. All chemicals (AR grade) were purchased from Shanghai Chemical Regent Company. The pellets were attached to an iron wire to form an assembled
cathode. Ni10Cu11Fe alloy anode was fabricated by casting the mixed
nickel, copper and iron powders in an alumina tube (ID: 20 mm) at
1600 °C in argon atmosphere.
2.2. Cyclic voltammetry measurements
Cyclic voltammetry (CV) measurements were conducted using an
iron wire electrode (d = 1 mm), an Fe2O3 coated iron wire electrode
(FCE) and a piece of Ni10Cu11Fe alloy electrode (d = 5 mm), respectively. The FCE was manufactured by repeatedly dipping an iron wire
into an Fe2O3 powder/ethanol suspension (3 g Fe2O3 in 5 mL ethanol
under sonication). A silver wire (d = 2 mm) and the Ni10Cu11Fe rod
(d = 5 mm) were served as the pseudo-reference electrode (RE) and
counter electrode (CE), respectively. The potential of RE was regularly
checked by the CV measurements using an iron wire electrode and
it was found that its stability was acceptable (~20 mV shift in an
hour) during the CV measurements. The potential value as shown in
the presented CV curves was calibrated accordingly. The CV measurements were conducted on a CHI1140 electrochemical workstation
(Shanghai Chenhua Instrument Co. Ltd., China).
bare alloy. While the current becomes very low in the potential range of
−0.15 to 0.65 V starting from the 3rd cycle, this result demonstrates
that the alloy surface is covered by a protective oxide layer after the
first two polarization cycles. The anodic current increases with
polarizing the potential more positive than 0.65 V, but it does not destroy the protective layer for the passivation state is retained very
well in the potential range of −0.15 to 0.65 V after further anodic polarization. Therefore, we can ascribe the increasing anodic current at potentials more positive than 0.65 V to the oxygen evolution rather than
to the trans-passivation of the electrode. The results suggest that an
electronically conductive stable oxide layer can form on the alloy surface in the melt under the anodic polarization condition. Therefore,
the Ni10Cu11Fe alloy could act as an inert anode in the melt.
Using an Fe2O3 pellet cathode and a Ni10Cu11Fe rod anode, constant cell voltage electrolysis was performed. The Fe2O3 pellet precursor turns to gray metallic sponge with good conductance and a
reflective metallic shimmer as shown in Fig. 2a after electrolysis for
11 h under constant cell voltage of 1.9 V. The adhered salt on the
cathode surface was very easily removed by washing in water. XRD
(Fig. 2b) and EDX (inset of Fig. 2f) analysis confirmed that the product
was pure iron and there were no detectable impurities. The sample
was further analyzed by infrared carbon and sulfur analyzer and the
carbon content was found to be as low as 0.035%. The products
obtained under constant cell voltage of 1.5 V for 5 h were mainly
NaFe2O3 and a small amount of iron. NaFe2O3 as a product under
lower cell voltage suggests that the peak c1 in Fig. 1a might be related
2.3. Electrolysis and product characterization
Constant voltage electrolysis was performed using a solid Fe2O3
pellet (~1.1 g) cathode and the Ni10Cu11Fe alloy anode (d= 20 mm)
controlled by a DC power system (Shenzhen Neware Electronic Ltd.,
China), the current–time curve was recorded at the same time. During
electrolysis, the outlet gas from the anode was monitored by a gas chromatography instrument (GC9800, Shanghai Kechuang Chromatograph
Instruments Co. Ltd., China). The obtained products were characterized
by X-ray diffraction spectroscopy (XRD, Shimadzu X-ray 6000 with Cu
Kα1 radiation at λ = 1.5405 Å), scanning electron microscopy (SEM,
FEI Sirion field emission), energy-dispersive X-ray spectroscopy (EDX,
EDAX GENESIS 7000) and the infrared carbon and sulfur analyzer (Horiba-EMIR-8200, Japan).
3. Results and discussion
The electrochemical potential window of molten K2CO3–Na2CO3 at
the cathodic end is limited by the deposition of alkaline metals or carbon.
The cyclic voltammograms (CVs) recorded from iron electrode and FCE
were shown in Fig. 1a. The decomposition of the electrolyte takes place
when the applied potential is shifted to −1.5 V as evidence of the continuous increasing of the reduction current. For FCE, there are two more reduction peaks (c1 and c2) prior to the decomposition of the electrolyte,
which could be ascribed to the reduction of Fe2O3 coating. Two reduction
peaks indicated that the reduction of Fe2O3 might involve at least two
steps. The peak c2 should be related to the formation of iron metal,
while c1 may be due to the formation of possible intermediate products
(Fe3O4, FeO or NaFe2O3). It will be further analyzed based on the XRD
tests in the following text.
A satisfactory inert anode is crucial for a green electro-metallurgical
process in molten salts. Based on the previous investigations in literatures of metallic electrodes in molten cryolite (by others, especially de
Nora et al.) [10,11], molten carbonate fuel cell [16] and molten calcium
chloride [8], nickel alloys were selected as targeted inert anodes in this
work. It was expected that a stable, electronically-conductive oxide
layer could form on the alloy surface in molten K2CO3–Na2CO3. Fig. 1b
shows CVs of nickel alloy (Ni10Cu11Fe) electrode in the melt at
750 °C under a relatively low potential scan rate (5 mV/s). Large current
presented in the first and second cycle is related to the oxidation of the
Fig. 1. CVs recorded from (a) iron electrode (solid line) and FCE (dot line) (scan rate:
100 mV/s) and (b) Ni10Cu11Fe electrode in molten K2CO3–Na2CO3 at 750 °C (scan
rate: 5 mV/s).
H. Yin et al. / Electrochemistry Communications 13 (2011) 1521–1524
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Fig. 2. (a) Digital photos of the porous Fe2O3 before (left) and after (right) constant cell voltage electrolysis (1.9 V, 11 h). (b) XRD patterns of the cathode products obtained at the
indicated conditions. (c–f) SEM images of Fe2O3 (c) and the products electrolyzed under constant cell voltage of 1.5 V for 5 h (d), 1.9 V for 5 h (e) and 1.9 V for 11 h (f), the inset is
the EDX spectrum of the corresponding product.
to the electro-intercalation of Na + into Fe2O3. Thus, the reduction
Fe2O3 might contain two steps:
þ
Fe2 O3 þ Na þ e − ¼ NaFe2 O3
þ
NaFe2 O3 þ 5e − ¼ Na þ 2Fe þ 3O
ð1Þ
2−
ð2Þ
The SEM morphology of Fe2O3 powder and the products obtained
at different electrolysis conditions were shown in Fig. 2c–f. The size of
the original Fe2O3 particle was around 500 nm. The particle size of
product obtained under constant cell voltage of 1.5 V for 5 h was
1–4 μm with flake morphology, much bigger than that of Fe2O3. Combining with the XRD analysis (Fig. 2b), the growth of the particles was
due to the formation of NaFe2O3 through electro-intercalation of Na +.
The morphology of the iron product obtained under 1.9 V for 5 h is
soybean-like in a particle size of 10–20 μm. When the electrolysis
time was prolonged to 11 h, the product was sponge-like iron. The
mass of the product was ~ 0.76 g from electrolysis of a piece of
Fe2O3 pellet (~1.1 g) cathode. The energy consumption was calculated from the cell voltage (1.9 V) and consumed charge which was integrated from the current–time plot (Fig. 3a). The direct energy
consumption was 2.87 kWh/(kg-Fe) with a current efficiency of
~ 95%. Fig. 3b presents the gas chromatograms of outlet gas before
and during electrolysis. The oxygen content of the outlet gas from
the reactor remarkably increased during electrolysis, demonstrating
oxygen gas generated on the anode. Furthermore, the Ni10Cu11Fe
alloy was minimally consumed after serving as anode for more than
150 h in the melt. A gray coating formed on its surface after electrolysis (inset in Fig. 3a), and its mass and size remained almost
unchanged, indicating Ni10Cu11Fe alloy is a satisfactory inert anode
in the melt. All of the results demonstrated that Fe2O3 can be electrochemically split into iron and oxygen in molten carbonates using the
nickel alloy anode, providing a green process for producing iron and
oxygen from iron oxide in molten carbonate.
4. Conclusions
Direct electrochemical production of iron and oxygen from solid
Fe2O3 in Na2CO3–K2CO3 eutectic at 750 °C was realized using a costaffordable inert anode. The Fe2O3 can be electrochemically reduced
to low carbon iron in two steps. This work offers a green process for
production of iron and oxygen from iron oxide in molten carbonate
with high current efficiency and low energy consumption. Furthermore, this process can be also applied to production of other metals
such as Ni, Co, Mn and stainless steel without greenhouse gas
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H. Yin et al. / Electrochemistry Communications 13 (2011) 1521–1524
Acknowledgments
The authors thank the NSFC (grant nos. 20873093, 51071112),
MOE of China (NCET-08-0416) and the Fundamental Research
Funds for Central Universities of China for financial support. The authors also extend their thanks to Dr. James Yurko for his kind help
on the English polishing.
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Fig. 3. (a) Current–time plot recorded during constant voltage electrolysis of Fe2O3 at
constant cell voltage of 1.9 V (inset photos are Ni10Fe11Cu anode before and after electrolysis for 150 h). (b) Gas chromatograms recorded from the outlet gas before and
during electrolysis on the anode side.
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