A molten calcium carbonate mediator for the electrochemical conversion and absorption of carbon dioxide

Article abstract of DOI:10.1039/d0gc02626c

High-temperature molten salts are an excellent electrolyte to bring about redox reactions at a rapid rate without using rationally designed nano-structured catalysts. However, the large-scale electrolyzer is constrained by the use of expensive and resource-deficient lithium salts. Using Earth-abundant CaCO3 releases the pressure of using strategic lithium resources, but the low solubility of CaO in molten carbonates disables the capability of capturing CO2. In addition, the separation of carbon from water-insoluble CaO and CaCO3 consumes a large amount of acids. To tackle these challenges, we report a CaCO3-containing molten carbonate electrolyzer to prevent the use of lithium salts, and a molten CaCl2 dissolver to separate carbon from CaO that is soluble in molten CaCl2 and can capture CO2 by carbonization. More importantly, we develop a salt-soluble-to-water-insoluble approach for producing ultrafine CaCO3 using molten salt as a soft template. Overall, this study opens a pathway to use cheap and Earth-abundant molten CaCO3 as a mediator to convert CO2 to oxygen at a cost-effective inert anode, with value-added carbon at the cathode, and ultrafine CaCO3 through a salt-to-solution process.

Full text of DOI:10.1039/d0gc02626c

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Paul T. Anastas et al.
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DOI: 10.1039/D0GC02626C
A molten calcium carbonate mediator for the
electrochemical conversion and absorption of carbon
dioxide
Xiang Chen1, Haijia Zhao1, Jiakang Qu1, Diyong Tang2, Zhuqing Zhao1, Hongwei Xie1, Dihua Wang2,
Huayi Yin*1,3
1. Key Laboratory for Ecological Metallurgy of Multimetallic Mineral of Ministry of Education, School of
Metallurgy, Northeastern University, Shenyang, P. R. China, 110819
2. School of Resource and Environmental Science, Wuhan University, Wuhan, P.R. China, 430072
3. Key Laboratory of Data Analytics and Optimization for Smart Industry, Ministry of Education, Northeastern
University, Shenyang, P. R. China,110819
Email: yinhy@smm.neu.edu.cn
Abstract: High-temperature molten salts are an excellent electrolyte to entail redox reactions
at a rapid rate without using rationally designed nano-structured catalysts. However, the large-
scale electrolyzer is constrained by expensive and resource-deficient lithium salts. Employing
earth-abundant CaCO3 releases the pressure of using strategic lithium resources, but the low
solubility of CaO in molten carbonates disables the capability of capturing CO2. In addition,
the separation of carbon from water-insoluble CaO and CaCO3 consumes a large amount of
acids. To tackle these challenges, we report a CaCO3-containing molten carbonate electrolyzer
to prevent the use of lithium salt, and a molten CaCl2 dissolver to separate carbon from CaO
that is soluble in molten CaCl2 and can capture CO2 by carbonization. More importantly, we
develop a salt-soluble-to-water-insoluble approach to producing ultrafine CaCO3 by using
molten salt as a soft template. Overall, this paper opens a pathway to use the cheap and earth-
abundant molten CaCO3 as a mediator to converting CO2 to oxygen at a cost-effective inert
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anode, value-added carbon at the cathode, and ultrafine CaCO3 thorough a salt-to-solution
process.
Keywords: CaCO3, molten carbonate, electrolysis, CO2 reduction, carbon materials
1. Introduction
Electrochemical valorization of CO2 is the key to closing the carbon cycle and achieving
global sustainability. Using renewable energy-powered electrolyzers to capture and reduce
carbon dioxide is becoming a central focus to balancing the carbon cycle and therefore tacking
the climate change1. With the ever-decreasing cost of renewable energies, almost reaching the
cost-level of fossil-generated power2, 3, electrochemically converting CO2 to value-added
chemicals and fuels gradually becomes commercially viable4-6. Most state-of-the-art
electrochemical technologies aim to convert gaseous CO2 to CO or organic chemicals in
aqueous solutions where water molecules provide the protons and electrons give energy to form
new chemical bonds4. However, the electrochemical conversion of CO2 in aqueous solutions
needs to rationally design catalysts, engineer the electrolyte/electrode interface, and employ gas
diffusion electrode to improve the product selectivity and increase the relatively low current
density caused by the low solubility of CO2 in aqueous solutions7, 8. In addition to atmospheric
CO2 (~2.5x1012 tones), a larger portion of CO2 exists in the carbonate rocks (~1014 tones) and
the hydrosphere (~1016 tones)9, 10. Thus, solid carbonate minerals are another primary CO2
carrier. The electrochemical conversion of carbonate minerals could be another avenue to
valorize CO2 by using feedstocks that are available and being easily concentrated, and the
dissociation of carbonate will generate basic oxide being capable of capturing CO2 through
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carbonization or being directly used as raw materials for cement production11.
In addition to aqueous solutions, high-temperature molten salts have been proven as
excellent electrolytes to convert CO2 to carbons or hydrocarbons if water vapor was supplied12-
18. Unlike the aqueous solution, high-temperature molten salts have a wider electrochemical
window than that of water to allow the conversion CO2 to carbon with a high product selectivity,
and the high operating temperature overcomes the kinetic barrier and thereby enables a rapid
reduction rate without tentatively designed catalysts. For example, the industrially deployed
Hall-Heroult process for producing liquid aluminum has been commercialized for ~100 years.
However, the solubility of CO2 in molten salts is still low19, 20, so the direct electrochemical
reduction of gaseous CO2 cannot be sustained at a high current density. Different from aqueous
solutions, the carbonate anions can be reduced to carbon in molten salt while releasing oxide
2-
into the molten salt to capture CO2 and then replenish CO3 ions. In this case, the rates of
generating oxide, the dissolution of oxide in molten salt, and the carbonation of oxide should
be balanced to avoid any polarization or side reactions in addition to producing carbon. To date,
most researches focus on lithium-containing molten carbonates due to their relatively low
melting point and acceptable oxide solubility (Tab. S1)12-18. However, the lithium resource is
limited because of the ever-increasing lithium-ion battery market21, 22. Thus, a lithium-free and
earth-abundant molten carbonate salt is required in terms of considering the large-scale
deployment and the cost requirement of raw materials.
Calcium carbonate, the primary constituent of limestone, is cheap, earth-abundant,
environmentally friendly, and easily accessible, which covers ~15% of the continental surface23,
24. This means that the limestone is a huge CO2 reservoir25. On the other hand, the limestone
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could act as an abundant medium performing the CO2 transformation. Licht et al. have proved
that CaCO3 can be reduced to C and CaO in Li2CO3-containing molten salt26. However, the
solubility of CaO is very low in ternary molten carbonate (Na2CO3-K2CO3-Li2CO3 eutectic),
o
and the solubilities of both Ca(OH)2 (0.173 g/100g-H2O, 20 C) and CaCO3 (0.0006 g/100g-
H2O, 20 oC) in water are small, so that the separation of the electrolytic products from molten
salt consumes a large amount of water and acids. Thus, a lithium-free molten carbonate
electrolyzer and an efficient separation method to separate CaO or CaCO3 from the electrolytic
carbon are urgently needed.
To circumvent the challenge of using lithium salts and the difficulty of product separation,
herein we employ a lithium-free molten carbonate, CaCO3-Na2CO3-K2CO3, to convert molten
CaCO3 to solid carbon and CaO at the cathode and oxygen at a low-cost Ni10Cu11Fe inert
anode. Subsequently, the CaO and carbon can be separated in molten CaCl2 because both CaO
and CaCO3 are highly soluble in molten CaCl227, 28. Since both CaO and CaCO3 are highly
soluble in molten CaCl2, the CaO can be either separated by water leaching or directly used for
absorbing CO2 by carbonization. Moreover, the ultrafine CaCO3 with a diameter ranging from
500 to 800 nm can be obtained by a soft template approach, in which CaCO3 is highly soluble
in molten CaCl2 while CaCO3 is insoluble in water but CaCl2 is highly soluble in water. Because
CaCO3 can be substituted by the limestone, we use the cheap limestone as a CO2 host to enable
the electrochemical conversion of CO2 to value-added carbon materials and oxygen. Finally,
the sodium-storage performances of the electrolytic carbon were evaluated as well.
2. Experimental
2.1 Electrochemistry of molten Na2CO3-K2CO3-CaCO3
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The electrochemistry of molten carbonates was studied by cyclic voltammetry (CV) using
a three-electrode setup contained in an alumina crucible that was filled with molten carbonates.
The alumina crucible (inner diameter: 80 mm, height: 100 mm) was housed in a one-end-closed
stainless steel (SS) test vessel protected by argon flow (99.99%, Shenyang Shuntai Special Gas
Co., Ltd.). First, 450 g of the mixture of Na2CO3 and K2CO3 (Na: K = 59: 41, mol%) was dried
under vacuum in the test vessel at 250 oC for 12 h. Then, the test vessel was filled with Ar and
then was continuously flowed with Ar in the following steps. After that, the temperature was
o
o
raised to 750 C at 5 C min-1 to melt the salt. Second, 50 g of CaCO3 (99.9%, Sinopharm
Chemical Reagent Co., Ltd) was added into the molten Na2CO3-K2CO3. After two hours, pre-
electrolysis was performed between a nickel sheet cathode (1×2 cm) and a home-made
Ni10Cu11Fe anode (diameter: 2 cm) under a constant cell voltage of 1.8 V for 2 h. Third, a
copper wire (diameter: 1 mm), a silver wire (diameter: 1 mm) and the same Ni10Cu11Fe rod
served as the working electrode, the pseudo-reference electrode, and the counter electrode,
respectively. The CV measurement was performed at an electrochemical workstation (CH
Instruments Inc., Shanghai, China, CHI1130a). All anhydrous CaCO3, Na2CO3, K2CO3, CaCl2,
and CaO were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. A
Ni10Cu11Fe anode (Ni: Cu: Fe = 79: 10 :11, weight ratio) was made by casting from liquid
alloys prepared from the mixture of metal powders.
Electrolysis was conducted in a two-electrode system contained in the same setup as that
was used for CV measurements. The Ni10Cu11Fe alloy rod and a nickel sheet served as the
anode and the cathode, respectively, which were immersed in a molten CaCO3-Na2CO3-K2CO3
o
electrolyte (Ca: Na: K = 23: 32: 45 mol%, eutectic at 665 C). During electrolysis, the two-
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electrode system was exposed in the CO2 atmosphere. The power of the electrolysis DpOroI:c10e.s10s3e9s/D0GC02626C
was supplied by a battery testing system (Shenzhen Neware Co., Ltd, Shenzhen, China), and
the operating temperatures of molten salt ranged from 710 to 850 oC. During the electrolysis,
the outlet gases produced during electrolysis was monitored by gas chromatograph (FULI,
9790H). After electrolysis, the cathodic product was taken out from the molten salt, cooled
down in the CO2 atmosphere, and then transferred to the molten salt dissolver.
2.2 CaO dissolution in molten CaCl2 and CO2 absorption
The electrolytic products were soaked in molten CaCl2 contained in an alumina crucible
that was housed in a SS test vessel filled with argon, and the operating temperature was kept at
o
800 C. Then, the soaked product was lifted from the molten salt, cooled down in the Ar
atmosphere, washed with water, and vacuum-dried at 80 oC for 10 h. After soaking, CO2 was
directly bubbled into the molten CaCl2 that had been used for soaking the electrolytic products.
The flow rate of CO2 was 40 mL min-1. After absorption, the melt was cooled and dissolved in
water to remove CaCl2, and the residue was dried for further characterization.
To qualitatively measure the CO2 absorption capability of molten CaCl2-CaO, an
absorption cell was constructed in an Al2O3 crucible filled with 100 g of CaCl2 containing 1 wt%
and 3 wt% CaO, respectively. The absorption process was conducted in a closed SS reactor at
o
800 C and CO2 gas was injected through the top of the sealed reactor with a flow rate of 40
mL min-1. The electrolyte was sampled to analyze the CaCO3 concentration after 10 min, 30
min, 1 h, 3 h, and 5 h, respectively. The absorptive amount of carbon dioxide is calculated based
on the amount of calcium carbonate in the molten salt (Equation 1).
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Carbonization reaction: CaO(l) + CO2(g) = CaCO3(l)
(1)
2.3 Characterization
The crystal structures of all products were characterized by X-ray diffraction (XRD,
Shimadzu X-ray 6000 with Cu K radiation at =1.5405 Å) and confocal Raman
microspectroscopy (HR800, 100-240V 50/60Hz 488nm 100mw). Their morphologies were
characterized by scanning electron microscopy (SEM, FEI Quanta FEG 250) equipped with
EDS detector. Besides, the specific surface area of the electrolytic carbon was measured by
Brunauer-Emmett-Teller analysis (BET, Quantachrome NOVA 1200e, Surface Area & Pore
Size Analyzer). The procedure of assembling half-type coin cells and measuring the sodium-
storage performance of electrolytic carbons was described in Supporting Information.
3. Results and discussions
3.1 Electrochemistry of CaCO3
CaCO3 is rich in the earth’s crust, not only serving as a low-cost feedstock for making
commodity chemicals but also a huge CO2 reservoir. High-temperature molten carbonates can
dissolve CaCO3 and provide a wide electrochemical window to allow the electrochemical
reduction of CaCO3 (Fig. 1a). After electrolysis, the solid product of the mixture of C and CaO
can be separated by dissolving CaO into molten CaCl2 because of the high solubility of CaO in
the molten CaCl227, 28 (Figs. 1b-c). Then, the as-received CaCl2-CaO melt is able to absorb CO2
through the carbonization reaction without the passivation effect caused by the formation of the
CaO@CaCO3 core-shell structure because both CaO and CaCO3 are highly soluble in the
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molten CaCl2 (Fig. 1d)27, 28. At last, CaCl2 and CaCO3 are recycled by a water-leaching process,
whereby the CaCO3 is insoluble in water while the CaCl2 is highly soluble (Fig. 1e). Therefore,
CaCO3 act as a mediator to enable the electrolysis of CO2 to produce C and O2.
Fig. 1. Schematic illustration of the molten calcium carbonate mediator for the
electrochemical reduction and absorption of carbon dioxide.
CaCO3 rather than Li2CO3 is chosen as the active carbonate species due to thermodynamic
considerations, resource abundance, and cost. As shown in Fig. 2a, lithium has the lowest
abundance while the highest price among Na-, K-, and Ca-carbonates, i. e, the price of Li2CO3
is 100 times higher than that of CaCO3 while the abundance of lithium is only 1/2500 of calcium.
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Thus, the electrochemical reduction of CaCO3 in the lithium-free carbonate is more practically
viable than in the commonly used Li-containing molten salts (Tab. S1). In molten Na2CO3-
K2CO3-CaCO3, CaCO3 is the electro-active species that can be reduced because it has been
proven that carbonate ions cannot be reduced to carbon in Na2CO3-K2CO320, 29, 30. In this regard,
the electrolysis only consumes CaCO3 (Fig. 2b) while Na2CO3-K2CO3 acts as the supporting
electrolyte. Like Li2CO3, CaCO3 can be reduced to carbon at a potential of 1.5V more positive
than the deposition of Ca (Fig. 2c), thermodynamically convincing CaCO3 as the electro-active
species. After adding CaCO3 into molten Na2CO3-K2CO3, a reduction peak (-1.4 V vs. Ag) prior
to the Na deposition (-1.8 V vs. Ag) is due to the reduction of CaCO3 (Fig. 2d). Thus, CaCO3
can be reduced in the lithium-free molten carbonates.
Fig. 2. (a) Profiles of the abundances of Li, Na, K, and Ca in the Earth’s crust and the price of
their corresponding carbonates, data derived from the reference31. (b) Flowsheet of a molten
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CaCO3 mediator for the electrochemical reduction and absorption of carbon dioxiDdOeI.: 1(0c.)1039/D0GC02626C
Standard potential profiles of Li2CO3 and CaCO3 as a function of temperature, all
thermodynamic data are obtained from HSC Chemistry 6.0 and the activity of all involved
species is at unit 1. (d) Cyclic voltammograms recorded from a Cu electrode in Na2CO3-
K2CO3-10 wt% CaCO3 melts at 750 oC under Ar atmosphere.
3.2 Electrolysis of molten CaCO3-Na2CO3-K2CO3
The electrolysis of molten CaCO3-Na2CO3-K2CO3 is to electrochemically splits CaCO3 to
carbon and CaO at the cathode and to oxygen at the Ni10Cu11Fe inert anode. According to the
32
phase diagram of CaCO3-Na2CO3-K2CO3 , ~20wt% of CaCO3 can be dissolved in the Na2CO3-
K2CO3 eutectic at ~665 oC, which ensures a rapid mass transfer rate during the electrolysis. The
electrolytic carbons obtained in molten CaCO3-Na2CO3-K2CO3 are amorphous at 710 and 750
oC, and the amorphous carbon starts to graphitize when increasing the temperature to 800 and
850 oC (Fig. 3a). However, the amorphous carbon cannot be fully converted to graphite at 850
oC, which is different from the electrochemical conversion of amorphous carbon to graphite in
molten CaCl2 that reported by Jin et al33. It has also been observed that the amorphous carbon
o
cannot be converted to graphite in molten NaCl-KCl-MgCl2 at 650 C34, 35. Thus, the
composition of molten salt, operating temperature, and electrolysis parameters determine the
graphitization of amorphous carbon. In addition, Raman spectra of electrolytic carbons also
suggest that a higher operating temperature leads to the higher degree of graphitization that is
reflected by the ID:IG ratios (ID: IG refers the intensity ratio of the D band and the G band) (Fig.
3b). In addition to operating temperatures, the degree of graphitization is also determined by
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the cell voltage, i.e., electrolytic carbons obtained at a lower cell voltage has a higherDdOeIg: 1r0e.1e03o9f/D0GC02626C
graphitization (Figs. S1a-b). The degree of graphitization could relate to the growth rate of
carbon and the interplay between the carbon and metals produced from side reactions caused
by relatively high cell voltages. In addition, electrolytic carbons have a variety of morphologies,
containing carbon sheets, particles, spheres, etc (Figs. 3c-f).
Fig. 3. (a-b) XRD patterns (a) and (b) Raman spectra of the electrolytic carbons obtained at
3.0 V in molten CaCO3-Na2CO3-K2CO3 at operating temperatures ranging from 710 to 850
oC. (c-f) SEM images of the electrolytic carbons obtained at 3.0 V at (c) 710, (d) 750, (e) 800
and (f) 850 oC, respectively.
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A cost–affordable and durable oxygen-evolution inert anode is crucial to enable the
reduction of CO2 rather than electro-refining carbon if a carbon electrode was used. The
Ni10Cu11Fe alloy has been proven to be a low-cost and long-lasting inert anode in molten
Na2CO3-K2CO314, 38, 39. Recently, Wang et al. revealed that the stability of the Ni-based inert
40
anodes was determined by the operating temperatures in molten Li2CO3-Na2CO3-K2CO3 . The
Ni-based anode is a stable oxygen-evolution anode in molten Li2CO3-Na2CO3-K2CO3 when the
operating temperature is higher than 650 oC. Surprisingly, the Ni10Cu11Fe is a stable oxygen-
evolution inert anode in molten CaCO3-Na2CO3-K2CO3. After working as an anode in molten
CaCO3-Na2CO3-K2CO3 for 100 h, the dimension of the anode does not change while a black
oxide scale forms at the surface of the anode (the inset of Fig. 4a). The black oxide scale
consists of NiO and NiFe2O4 (Fig. 4b), which agrees well with that obtained in molten Na2CO3-
38
K2CO3 . The stability of the anode is due to the good electrical conductivity and chemical
stability of spinal NiFe2O4 that has been proven to be an excellent inert anode material in the
molten salts41, 42. Thus, the inertness of the anode is due to the formation of an oxide scale which
enables oxygen evolution (Fig. 4a). From the cross-section SEM of the alloy (Fig. 4c), a dense
oxide scale with a thickness of ~100 μm formed on the surface of the alloy substrate. From the
results of linear scan analysis (inset of Figs. 4c, e), the concentration in the outermost layer has
the highest oxygen and iron concentration, indicating that the Fe- and oxide-rich region mainly
consists of NiFe2O4. Moreover, a thin copper-rich region was observed between the oxide scale
and the alloy substrate, which is beneficial to strengthen the connection of the oxide scale and
bulk metal39. According to the EDS analysis, the structure of the oxide scale is shown in Fig.
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4d. Thus, the electrochemical reactions take place at the cathode and the anode can be expressed
as equations 2-3.
Cathode reaction: CaCO3 + 4e- = C + CaO + 2O2-
Anode reaction: 2CaCO3 - 4e- = 2Ca2+ + 2CO2(g) + O2(g)
The overall reaction can be expressed as equation 4.
Overall reaction: 3CaCO3 = 3CaO + C + 2CO2(g) + O2(g)
(2)
(3)
(4)
Fig. 4. (a) Gas chromatograms of the outlet gas before and during electrolysis using the
Ni10Cu11Fe anode, the inset pictures are digital photos of Ni10Cu11Fe electrode before and
after electrolysis. (b) XRD pattern of the oxide scale from the Ni10Cu11Fe alloy. (c) Cross-
sectional SEM images and their corresponding linear concentration profiles of each element
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across the interface of the alloy/oxide scale interface. (d) Schematic of the structurDeOoI:f1t0h.1e039/D0GC02626C
interface of alloy/oxide scale. (e) EDS mapping images of the interface of the alloy/oxide
scale interface.
Current efficiency and energy consumption are measured by the collected carbon and
consumed electricity. The current efficiency is an indicator of the selectivity of the
electrochemical reactions. Among various conditions, the current efficiency ranges from 76.0%
to 98.9% (Fig. 5a), and the highest current efficiency of the electrolysis is obtained under 800
oC at 2.8V for 3 h (inset of Fig. 5a). The current efficiency decreases when the cell voltage is
higher than 3.0 V because a higher voltage may trigger side reactions such as co-deposition of
sodium. The energy consumption increases with increasing the electrolysis voltage, and the
minimum energy consumption for producing 1 kg of carbon ranges from 20.6 to 34.0 kW h of
electrical energy (Fig. 5b). So, the cost of electricity for producing 1 kg of electrolytic carbon
ranges 0.8 to 1.4 US$ assuming that the cost of electricity is US$0.04 kW h–13. Note that the
electricity
for
powering
the
furnace
is
not
considered.
Fig. 5. (a) Profiles of current efficiency as a function of cell voltage in molten CaCO3-Na2CO3-
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K2CO3 at 710, 750, 800, and 850 oC, respectively. (b) Profiles of energy consumptDioOnI: 1a0s.1a039/D0GC02626C
function of the cell voltage at 710, 750, 800, and 850 oC, respectively.
3.3 Absorption of the CO2 in molten CaCl2-CaO melt
As shown in Fig. S2, the electrolytic products obtained from molten CaCO3-Na2CO3-
K2CO3 consists of CaO, CaCO3 and a small quantity of (Na, K)2CO3 (the obvious characteristic
peaks of electrolytic carbons was not observed in the XRD analysis due to an amorphous
structure of electrolytic carbons) (Fig. S2f). Because CaO and CaCO3 are insoluble in water,
the electrolytic product wrapped in nickel foam was firstly soaked in molten CaCl2 to dissolve
CaO and CaCO3 (Fig. S2b). Unlike the conventional way of separating CaO and CaCO3 in
mineral acid solutions, the CaO and CaCO3 are soluble in molten CaCl2, which are then
separated from the electrolytic carbon. After the CaO and carbonate dissolved in molten CaCl2
(Fig. S2c, d), the electrolytic carbons automatically peeled off from the nickel sheet (Fig. S2e).
Since CaO is a well-known solid-absorbent for the capture of CO243, 44, the dissolved CaO has
a higher activity to combine with CO2 through a rapid gas-liquid-conversion process in the
molten salt without the deactivation effect caused by the CaCO3 layer45. The soluble CaO in
molten salts has a fast absorption capacity for CO2 and 99% of CaO can be converted to CaCO3
within 30 min in the molten CaCl2 containing 1 wt% or 3 wt% CaO (Fig. 6a). After absorption,
the quenched salt was dissolved in water and the insoluble product was CaCO3 (Fig. 6b), further
indicating that CaO had been completely converted to CaCO3. Thus, molten CaCl2 is a good
solvent to dissolve CaO, and the obtained CaCl2-CaO is capable of absorbing CO2, regenerating
CaCO3 and thereby making CaCO3 as a mediator. Therefore, the net reaction is
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electrochemically splitting CO2 to C and O2.
Fig. 6. (a) Profiles of CO2 absorption as a function of time in molten CaCl2 with 1 wt% and 3
wt% CaO, respectively. (b) XRD pattern of the insoluble solid product obtained by dissolving
CaCl2-1 wt% CaO in water. (c-d) SEM images of (c) commercial CaCO3 and (d) CaCO3
derived by dissolving CaCl2-CaCO3 in water.
The separation of CaCO3 from the solidified CaCl2-CaCO3 is accomplished by a water-
leaching process, which provides us a delicate approach to downsizing particles through a
chemical etching process in water. At high temperatures, molten CaCl2-CaCO3 is a single phase
because CaCO3 is highly soluble in molten CaCl2, while CaCl2 and CaCO3 exist in two different
phases in water. The particle size of CaCO3 feedstock is bigger than 20 µm (Fig. 6c), and the
particle size of the obtained CaCO3 is reduced to 500~800 nm (Fig. 6d). Moreover, the particle
size and morphology of CaCO3 vary with increasing the concentration of CaCO3 in the molten
CaCl2. The sphere-like CaCO3 was obtained from dissolving CaCl2-1 wt% CaO and the rod-
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and sheet-like CaCO3 was obtained from dissolving CaCl2-3 wt% and 5 wt% CaODiOnI: w10a.1t0e3r9,/D0GC02626C
respectively. Note that the dimensions of both CaCO3 rods and sheets were less than 5 µm (Fig.
S3), meaning that the template-removing process significantly reduces the particle size and
tailors the morphologies of CaCO3 by tuning the CaCO3 concentration in molten CaCl2. As we
know, the ultrafine calcium carbonate has been widely used as additives, medicine, food, etc46,
47. Compared with the currently deployed methods involving multiple steps, complex synthesis
parameters47, 48, the molten salt approach could open a radically different door to synthesizing
micro- and nano-CaCO3 in an efficient and green manner.
Fig. 7 illustrates the concept of tuning the particle size and morphology of CaCO3 through
a concept of salt-soluble-water-insoluble approach. First, CaCO3 dissolves in molten CaCl2,
forming a homogeneous liquid phase (Fig. 7a). In the subsequent quenching process, CaCl2
and CaCO3 begin to co-crystallize to form a solid phase where CaCO3 and CaCl2 are mixed at
an atomic scale (Fig. 7a). In this scenario, CaCO3 distributes in the CaCl2 matrix (Fig. 7b). In
other words, CaCl2 serves as the template that can be removed to tailor the structure and
morphologies of CaCO3. The elegance of this approach takes advantage of using different
solubilities of CaCO3 in molten CaCl2 and water, and the CaCl2 template can be etched by the
low-cost and environmentally friendly water. In addition to CaCO3, this approach can be used
to prepare various functional materials based on the different solubilities in different media.
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Fig. 7. (a) Schematic of the process to tailor the particle size of CaCO3 using salt-to-water
conversion process, and (b) the proposed mechanistic illustration of the salt-to-water
conversion process.
3.4 Sodium-storage performance of electrolytic carbons
Electrolytic carbons show good sodium-storage performances. The electrolytic carbon
obtained at 710 oC and 3.0 V is denoted as EC-710-3.0. A reduction peak at ~0.41 V (vs. Na+/Na)
is due to the formation of an solid electrolyte interface (SEI) film36, and the sharp cathodic peak
at 0.005 V (vs. Na+/Na) and the anodic peak at 0.2 V (vs. Na+/Na) are attributed to the reversible
insertion/extraction of Na+ from the graphited phase (Fig. S4a)37. The feature of a rectangular-
shape CVs indicates a capacitive behavior of the carbon electrode, and the capacitive
contributions account for 84% of the capacity (Fig. S5). The first discharge capacity of the EC-
710-3.0 is 236.0 mAh g-1 with a Coulombic efficiency of 34.3% in sodium-ion batteries
(denoted as SIBs) (Fig. S4b). The low Coulombic efficiency could be resulted from the large
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surface area (Fig. S6), which could be improved by a post-heat treatment. Moreover, the EC-
710-3.0 delivers 142.8 mAh g-1 at 1000 mA g-1 (Fig. S4c), and exhibits excellent cycling
stability, i.e., the discharge capacity remains 210 mAh g-1 at 100 mA g-1 after 500 cycles with
a capacity retention rate of 89.0% (Fig. S4d). The sodium-storage capacity of electrolytic
carbons decreases with increasing the temperature of electrolysis (Fig. S7a, Tab. S4), which
could relate to the particle size and specific surface area of electrolytic carbons (Fig. S6).
4. Conclusions
A molten Na2CO3-K2CO3-CaCO3 carbonate electrolyzer can transform CaCO3 into
carbon and CaO at the cathode and O2 at the Ni10Cu11Fe inert anode with a current efficiency
of over 90%. Instead of electrochemically reducing CO2, CaCO3 is a CO2 carrier being reduced
in the form of CO32- that has a high solubility in molten Na2CO3-K2CO3-CaCO3. The electrolytic
CaO and carbon can be separated by a molten CaCl2-soaking process, by which CaO dissolves
in molten CaCl2 forming CaCl2-CaO melts. The CaCl2-CaO can be either used to capture CO2
to form CaCl2-CaCO3 melts, or to be directly used for producing Ca(OH)2 that is a key material
for cement production. In addition, a concept of salt-soluble-water-insoluble approach has been
proven as an effective approach to tailoring the particle size and morphologies of CaCO3,
thereby opening a new pathway to synthesize CaCO3 with tunable particle sizes and structures.
Further, electrolytic carbons are potential sodium-storage anode candidates that deliver a
specific capacity of ~210 mAh g-1 at 100 mA g-1 after 500 cycles with a capacity retention of
89.0%. Considering the huge limestone deposit and the ever-decreasing price of renewable-
powered electricity, the electrolysis of molten CaCO3 offers new opportunities for the
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valorization of CO2, preparation of energy storage materials and oxygen, production of clean
cement, and fabricating of functional CaCO3.
Acknowledgments
We greatly thank the financial support from the Fundamental Research Funds for the
Central Universities (N2025035), Xingliao Project (XLYC1807042), and the 111
Project (B16009).
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140x90mm (150 x 150 DPI)