Carbon dioxide electrolysis and carbon deposition in alkaline-earth-carbonate-included molten salts electrolyzer

Article abstract of DOI:10.1039/C8NJ02965B

The electrochemical reduction of CO₂ in molten carbonates provides a comprehensive solution to end the detrimental global climate change, and convert and store conventional electricity in a stable chemical mode. In this work, we provide experimental validation of carbon deposition in CaCO₃-, SrCO₃- and BaCO₃-dissolved electrolytes. Carbon products aggregate on the cathodic surface and are then collected and characterized by electron dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, and X-ray diffraction (XRD) analysis. The results demonstrate that the alkaline earth carbonate additives sustain continuous CO₂ electrolysis and carbon electro-deposition. However, the micromorphology and microstructure of the carbon deposits are found to be significantly changed mainly because of the interface modification induced by the alkaline earth carbonate additives. In addition, a high yield of carbon nanotubes is observed in the cathodic carbon products by optimizing the electrolytic conditions. Compared to pure Li₂CO₃, alkaline earth carbonate additives provide carbon nanotubes with a thicker diameter and more prominent hollow structure.

Full text of DOI:10.1039/C8NJ02965B

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Carbon dioxide electrolysis and carbon deposition
in alkaline-earth-carbonate-included molten
salts electrolyzer
Cite this:DOI: 10.1039/c8nj02965b
a
a
b
a
a
Zhida Li,
Jinlian Li,
Yanyan Yua,
Wei Li, * Guanzhong Wang,
aLi Peng,
a
a
Di Gu,
Dandan Yuan
and Hongjun Wu
*
The electrochemical reduction of CO
the detrimental global climate change, and convert and store conventional electricity in a stable
chemical mode. In this work, we provide experimental validation of carbon deposition in CaCO -,
SrCO - and BaCO -dissolved electrolytes. Carbon products aggregate on the cathodic surface and are
then collected and characterized by electron dispersive spectroscopy (EDS), thermogravimetric analysis
TGA), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, and
X-ray diffraction (XRD) analysis. The results demonstrate that the alkaline earth carbonate additives
sustain continuous CO electrolysis and carbon electro-deposition. However, the micromorphology and
2
in molten carbonates provides a comprehensive solution to end
3
3
3
(
2
Received 14th June 2018,
Accepted 13th August 2018
microstructure of the carbon deposits are found to be significantly changed mainly because of the
interface modification induced by the alkaline earth carbonate additives. In addition, a high yield of
carbon nanotubes is observed in the cathodic carbon products by optimizing the electrolytic conditions.
DOI: 10.1039/c8nj02965b
Compared to pure Li
2 3
CO , alkaline earth carbonate additives provide carbon nanotubes with a thicker
rsc.li/njc
diameter and more prominent hollow structure.
4
,5
back to the mid-1960s. Recently, this technology has received
renewed interest largely due to the high necessity to reduce
Introduction
2
The worldwide awareness of the link between the global climate atmospheric CO levels and avoid detrimental climate change.
change and the ever-increasing carbon dioxide (CO ) accumulation The molten carbonate electrolyte that is suitable to deposit
2
2À
in the atmosphere has driven extensive investigations in tackling carbon products from CO should be able to dissolve O (which
2
the increase in CO emissions. Nevertheless, fossil fuels are still is an intermediate product of carbon electro-deposition) and
2
being widely utilized to meet the energy demand of human society, in turn help to dissolve atmospheric CO
2
and convert it into
, Na CO , K CO , and
particularly their mixture are the most used electrolysis media
is due to their properties of wide electrochemical window, rapid
2
À 6,7
and therefore, the rise in CO
coming decades. The strategy of CO
has already entered the view of scientific community, where CO
expected to be stored in an appropriate geological stratification reaction kinetics, less corrosion ability towards anode, etc.
safely. However, in addition to being a kind of greenhouse gas, Ijije et al. established an electrolyzer with Li CO –K CO binary
2
emissions will continue in the CO
3
.
Alkali carbonates of Li
2
CO
3
2
3
2
3
1,2
2
capture and storage (CCS)
2
6
,8,9
2
3
2
3
CO2 is also a significant carbon source abundant in nature. melts (mole ratio: 62 : 38) under CO2 or mixed N2 and CO2
Varying carbon dioxide capture and conversion (CCC) approaches atmospheres at 3.0–5.0 V and 540–700 8C, in which amorphous
such as chemical conversion, photochemical conversion and and various carbon nanostructures derived from dissolved CO
biological conversion have been proposed in order to create value were deposited onto a mild steel cathode. More recently, our
2
1
0
3
out of wasted CO
Electrochemical reduction of carbonate ions (CO
deposit solid carbon is a proven method that can be dated and carbon nanotubes were synthesized in this molten salt
2
exhaust and close the present carbon cycle.
group studied a Li
2 3 2 3 2 3
CO –Na CO –K CO ternary eutectic (61 : 22 : 17
2
À
3
) to by wt) and interestingly, unique porous honeycomb-like carbon
11
environment, arousing a further interest in this domain. Further-
more, carbon electro-deposition is also observed to proceed in
chloride/carbonate or fluoride/carbonate mixtures as long as Li is
a
b
Provincial Key Laboratory of Oil & Gas Chemical Technology, College of
Chemistry & Chemical Engineering, Northeast Petroleum University, Daqing
+
present. For example, a molten salts-electrolyzer of CaCl
LiCl–KCl successfully achieved the electrochemical reduction of
CO to carbon products, where the chlorides acted as the fluxing
2 3
–CaCO –
1
63318, China. E-mail: hjwu@nepu.edu.cn; Tel: +86-0459-6503331
College of Petroleum Engineering, Northeast Petroleum University, Daqing,
63318, China. E-mail: cyyping@sina.com; Tel: +86-0459-6504498
1
2
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6
agent to lower the melting point of this electrolyte. In addition, carbon surface. Subsequent to ultrasonic treatment, filtration,
tetravalent CO reduction into zerovalent solid carbon was fulfilled washing with ultrapure water and desiccation in an air drying oven,
in LiF–NaF–Na CO eutectic and in this electrolyte, CO
2
2À
was the purified carbon powders were collected and characterized by
2
3
3
confirmed to be regenerated from the combination between CO
energy dispersive spectroscopy (EDS, X-Max, the British Oxford
Instruments Co., Ltd), scanning electron microscope (SEM, SIGMA,
2
2À 12
and O
CO
.
2
capture and conversion based on the molten salt–electro- Zeiss company), thermal gravimetric analyzer (TGA, SDT-Q600, TA
lysis was mostly performed in alkali carbonates previously, but Instruments), Brunauer–Emmett–Teller analysis instrument (BET,
relevant thermodynamic calculations show that carbon electro- Tristar-3020, Micromeritics Instrument Corporation) and X-ray
6
deposition in alkaline earth carbonates is also possible. As a diffractometer (XRD, D/MAX 2200, Rigaku).
supporting example, BaCO
3
had been employed to stabilize the
2 3
High temperature electrolysis at over 700 8C in Li CO and
molten carbonate electrolyzer due to the lower Lewis basicity of BaO moderate amount (such as 10% by wt) of CaCO -, SrCO -, and
3
3
than Li O, Na O and K O, and the nickel loss was significantly BaCO -dissolved Li CO was subsequently performed. The
2
2
2
3
2
3
1
3,14
2
reduced during the electrolysis process.
carbon nanotubes via molten carbonate electrolysis, dissolving a galvanized iron wire disk as the cathode and a 10 cm nickel wire
certain amount of BaCO additive in molten Li CO generated a disk as the anode. In this case, pre-electrolysis with low current
high yield of valued carbon nanotubes as in the pure Li
However, up to now, few systematic studies of carbon production 20 mA cm for 5 min) was performed in order to control the
in alkaline-earth-carbonate-included molten salts have been growth of nickel nanoparticles on the cathodic surface. It
performed. Herein, a Li CO –Na CO –K CO –MCO (M = Ca, should be noted that the described current densities are in
When synthesizing shape and area of the electrodes were changed by using a 5 cm
2
3
2
3
11
À2
À2
2
CO
3
case.
density (6 mA cm for 15 min, 10 mA cm for 15 min and
À2
2
3
2
3
2
3
3
Sr, Ba) electrolyzer was fabricated to convert CO into energetic terms of the cathode active area. The corresponding carbon
2
carbon deposits, and the amount of MCO is regulated from 0% products were subjected to the same purification procedures as
3
to 30% in order to investigate the effect of alkaline earth the experiments performed at 600 8C and then, they were
carbonate fraction on the carbon generation process. Additionally, characterized by transmission electron microscopy (TEM,
electrolysis in Li
2
CO
3
, Li
2
CO
3
–CaCO
3
, Li
2
CO
3
–SrCO
3
and Li
2
CO
3
–
JEM-2100, JEOL).
BaCO at over 700 1C was also performed, and carbon nanotubes
3
become the preferable carbon products instead of other products.
Results and discussion
Carbon products aggregate on the cathode surface when electro-
lyzing the carbonate eutectic, and the digital photos of the
Experimental
Battery-grade anhydrous Li
Company of Shanghai, China. Na
BaCO were of analytical purity and supplied by BA Chemical BaCO
Reagent Factory of Tianjin, China. Analytically pure SrCO was evidently illustrated, the Li
2
CO
3
was provided by the Lithium galvanized iron plate cathode subsequent to electrolysis in
CO , K CO , CaCO and Li CO –Na CO –K CO , 30% CaCO -, 30% SrCO - and 30%
-included electrolyte are displayed in Fig. 1a. As is
CO –Na CO –K CO –CaCO system
2
3
2
3
3
2
3
2
3
2
3
3
3
3
3
3
2
3
2
3
2
3
3
purchased from the FC Chemical Reagent Factory of Tianjin, deposits a much thicker carbon product than the other electro-
2
China. A 4 cm nickel plate and a galvanized iron plate were lyte systems. This matches well with the higher voltage plots as
employed as the anode and cathode, respectively. A corundum 30% CaCO is dissolved in Li CO –Na CO –K CO induced by
3
2
3
2
3
2
3
crucible of 50 mL volume served as the electrolysis container. the poor conductivity of molten CaCO
Initially, 15 g Li CO , 15 g Na CO and 15 g K CO were mixed
and melted in a well-type crucible furnace. Then, 0% (0 g), 10% of the purified carbon products. As shown in Fig. 1b–e, C, O, Ni,
4.5 g), 20% (9 g) and 30% (13.5 g) alkaline earth carbonate was Au and Cl are present in the EDS spectrum. Au derives from the
dissolved in the Li CO –Na CO –K CO ternary eutectic. CaCO coating operation for SEM-EDS analysis; Cl is attributed to the
SrCO and BaCO melt at 1339 8C, 1700 8C and 1740 8C, respec- acid leaching with hydrochloric acid; and the presence of Ni is
3
(Fig. 2a).
2
3
2
3
2
3
EDS analysis is employed to reveal the elemental composition
(
2
3
2
3
2
3
3
,
3
3
tively; therefore, further increasing their fraction would result in caused by nickel oxide dissolution in the molten carbonates.
low fluidity. After heating to the presupposed temperature of Oxygen was previously investigated by Fourier transformed infra-
6
00 8C and stabilizing for 30 min, a constant current density of red (FTIR) spectroscopy, and the absorption peak located at
À2
À1
250 mA cm was applied and the electrolysis was performed 1110 cm induced by C–C–O stretching vibration indicated the
1
7,18
for 60 min. In addition to conventional electricity, the energy presence of oxygen-containing functional groups.
This is
consumption by melting the carbonate electrolyte and reducing common for active carbons and is believed to be favorable for
1
9,20
the carbonate ions (CO
2
) could be supplied by a sustainable capacitance behavior.
Content of O, Ni, Au and Cl in each
solar source, which has been stated in the STEP (solar thermal sample is very low, while C comprises an overwhelming mole
1
5,16
electrochemical production) process.
Then, the galvanized fraction of over 90%. Carbon products formed in the above four
iron plate covered by a thick layer of carbon products was pulled systems are further studied by TGA to reveal the thermal
out from the carbonate melts and cooled at room temperature. oxidation temperature, as presented in Fig. 1f. The initial minor
The carbon deposits were washed with sufficient hydrochloric mass loss before 350 8C is because of the release of volatile
acid (6 M) to remove the frozen electrolyte attached on the matter, such as the residual moisture in the carbon products.
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Li
2
CO
3
–Na
2
CO
3
–K
2
CO
3
–SrCO
3
2 3 2 3 2 3
and Li CO –Na CO –K CO –
BaCO
3
electrolytes. This high cell voltage, in turn, could be used
to explain the observation of thicker carbon deposits (Fig. 1a).
The observed carbon deposition could be attributed to the
7
,21
direct splitting of molten carbonates (eqn (1)).
mediate metal oxides act as efficient absorbent for atmospheric
CO and tend to reform the carbonate electrolyte (eqn (2)). For
example, the combination between Li O and CO greenhouse
gas has already been proven via a thermogravimetric method.
The inter-
2
2
2
2
2
As presented in the indicated literature, a visible mass loss was
presented when heating Li CO up to 950 8C under an open
2
3
atmosphere due to thermal decomposition (eqn (3)), but no
significant mass loss was observed when CO was injected or
2
excess Li
case, the reverse reaction of excess CO
form Li CO occurred and contributed to the decrease in mass
loss (eqn (4)). We inferred that this is also the case for the
alkaline earth carbonate CaCO (Fig. 2b). Before 600 8C, no
2
O was added under the same temperature. In the latter
2
and dissolved Li O to
2
2
3
3
visible mass loss is observed, but then an exponential increase
is present after heating to over 700 8C. This mass loss is caused
by the decomposition of CaCO into CaO and CO (eqn (5)).
3
2
When heated under CO
lower than 5%, which could be explained by the classical
physical chemistry theory that CaCO decomposition proceeds
only when the equilibrium partial pressure of CO is higher
than that of the atmosphere. Compared to CaCO , SrCO and
BaCO3 exhibit enhanced thermal endurance, and their mass
loss is lower than 5%, under either air atmosphere or CO2
atmosphere. The polarization ability of the central cation could
2 3
atmosphere, the mass loss of CaCO is
3
2
Fig. 1 (a) Images of the galvanized iron plate cathode subsequent to
molten carbonate electrolysis; (b–e) EDS spectrum of the electro-deposited
carbon products; (f) thermogravimetric analysis of the carbon products.
3
3
explain the strengthened thermal stability of SrCO
3 3
and BaCO
compared to CaCO . For alkaline earth carbonates, carbonate
3
ion provides a large radius; therefore, the stronger the polariza-
tion of the central cations, the greater would be the value of Z/R
(
Z and R represent cationic charge number and cationic radius,
2À
respectively) and more possible it is to take O to form metal
oxides and release CO into the atmosphere. In this manner, the
2
2
+
stronger polarization of Ca makes CaCO3 decompose at a
relatively lower temperature than SrCO or BaCO
Fig. 2 (a) Cell voltage plots recorded during the electrolysis process;
3
3
.
(b) thermogravimetric analysis of CaCO
3 3 3
, SrCO and BaCO under an
open air or CO atmosphere.
2
MCO (l) - MO(l) + C(s) + O (g)
(1)
(2)
3
2
MO(l) + CO (g) - MCO (l)
2
3
Afterwards, a sharp mass loss occurs at 350–400 8C agreeing well
with the thermal oxidation temperature of amorphous carbon
in atmosphere but lower than that of carbon nanotubes and
carbon spheres reported by our group. After heating to 450 8C,
nearly nothing is left, suggesting that the carbon products are
mainly carbon plus some oxygen-containing functional groups,
and this is consistent with the EDS spectrum.
(
In eqn (1) and (2), M represents Li , Na , K , Ca, Sr and Ba.)
2 2 2
Li CO (l) - Li O(l) + CO (g)
(3)
(4)
(5)
2
3
2
2
1
7
Li O(l) + CO (g) - Li CO (l)
2
2
2
3
3 2
CaCO (s) - CaO(s) + CO (g)
Fig. 2a depicts the cell voltage recorded by the electrochemical
Typical SEM image of the carbon products prepared by
workstation during the galvanostatic electrolysis process. The cell molten Li CO –Na CO –K CO electrolysis is shown in Fig. 3.
2
3
2
3
2
3
voltage of Li CO –Na CO –K CO electrolysis is lower than 2.0 V, Porous, honeycomb-like carbon structure with a diameter of
2
3
2
3
2
3
while the inclusion of alkaline earth carbonates provides a visible 300 to 500 nm is presented, and its BET surface area is
2
À1
increase in the recorded cell voltage due to the poor conductivity quantified as 193 m g , as listed in Table 1. The formation
of molten CaCO , SrCO and BaCO . In particular, the inclusion of honeycomb-like nanostructured carbon materials might be
of 30% CaCO in the Li CO –Na CO –K CO presents a cell caused by rapid carbon deposition as well as the co-deposition
voltage of over 3.0 V, which is higher than that obtained using of alkali metals. Porous carbon nanostructures with large BET
3
3
3
3
2
3
2
3
2
3
1
8
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formed, but the carbon flakes observed in carbonate eutectic
dissolving 10% and 20% CaCO (Fig. 4a–d) evolve into broken
3
carbon pieces instead. Agreeing well with SEM data, the BET
surface area of the carbon products is found to increase to
2
À1
2
64 m g when 30% CaCO is included, which is higher than
3
that obtained using Li
0%) and Li CO –Na CO
CaO deriving from the decomposition of CaCO
2
CO
–K
3
–Na
CO
2
CO
systems. Unlike alkali oxides,
is nearly
3 2 3 3
–K CO –CaCO (10% and
2
2
3
2
3
2
3
3
1
4
insoluble in molten carbonate electrolyte. Moreover, the inter-
mediate CaO is denser than molten carbonate and tends to
deposit on the cathode or the bottom of the cell, of which the
deposition on the iron plate cathode provides an opportunity to
substantially modify the interface between the iron cathode and
molten carbonate electrolyte. Carbon deposition occurs in this
unusual high CaO environment, and the observed morphological
Fig. 3 Micromorphology of the carbon products electro-deposited in the
2 3 2 3 2 3
Li CO –Na CO –K CO system at 600 8C.
3
evolution along with elevated CaCO fraction is presumably
induced in this manner. We have demonstrated in a previous
report the morphological evolution between carbon nanotubes
and quasi carbon spheres at 750 8C, where the quasi carbon
Table 1 BET surface areas of the carbon products deposited in various
electrolyte compositions
spheres are synthesized in CaCO -included carbonate electrolyte,
3
2
À1
Electrolyte
BET surface area (m g
)
while the exclusion of CaCO from the electrolyte provides higher
3
1
7
Li–Na–K
193
82
85
264
53
62
76
44
76
92
carbon nanotubes fraction in the cathodic carbon products.
It is therefore concluded that CaCO additive significantly
Li–Na–K–Ca (10%, wt)
Li–Na–K–Ca (20%, wt)
Li–Na–K–Ca (30%, wt)
Li–Na–K–Sr (10%, wt)
Li–Na–K–Sr (20%, wt)
Li–Na–K–Sr (30%, wt)
Li–Na–K–Ba (10%, wt)
Li–Na–K–Ba (20%, wt)
Li–Na–K–Ba (30%, wt)
3
influences the micromorphology and microstructure of the
cathodic carbon deposits because of the nearly insoluble CaO
deriving from CaCO decomposition.
3
Fig. 5 shows the representative SEM images of the carbon
products synthesized in SrCO -included molten carbonate electro-
3
lyte. The carbon products shown in Fig. 5a and b were synthesized
in 10% SrCO
in Fig. 5c and d were synthesized in 20% SrCO
3
additive dissolved carbonate eutectic; those shown
additive dissolved
3
surface area demonstrate brilliant application potential in the
23
2
adsorption, separation, and even catalytic conversion of CO .
Two approaches to increase the BET surface area are (i) decreasing
the electrolytic temperature and (ii) increasing the applied cell
voltage or current density. As a supporting example, the BET
surface area of the carbon products subsequent to molten
À2
Li
2
CO
3
–Na
2
CO
3
–K
2
CO
3
electrolysis with 100 mA cm current
2
À1
2
À1
density increases from 194 m g at 550 8C to 440 m g at
1
3
4
6
50 8C. In addition, under the same electrolysis temperature of
00 8C, Li CO –Na CO binary system electrolysis generates
2
3
2
3
2
À1
carbon products with a BET surface area of 39.9 m
g
at
À2
2
À1
5
0 mA cm , which increases to 80.3 m g when the current
À2 24
density is enlarged to 200 mA cm
.
Thus, the Li CO –Na CO
2 3 2 3
binary system is expected to synthesize carbon materials with
tunable BET surface area via the regulation of electrolytic
parameters of temperature, current density, cell voltage, etc.
Fig. 4 depicts the carbon products acquired from CaCO
molten carbonate electrolysis. When 10% CaCO is dissolved in
Li CO –Na CO –K CO eutectic, a mixture of flake-like carbon and
3
-included
3
2
3
2
3
2
3
irregular carbon particles is presented in the cathodically generated
carbon products, as evident in Fig. 4a and b. Upon further increasing
the CaCO
the carbon deposits comprise carbon flakes and quasi carbon
sphere clusters. Interestingly, when 30% CaCO is dissolved in
Li CO –Na CO –K CO melts, irregular carbon particles are still included carbonate melts electrolysis at 600 8C.
3
mass fraction to 20%, as illustrated by Fig. 4c and d,
3
3
Fig. 4 Micromorphology of the carbon products subsequent to CaCO -
2
3
2
3
2
3
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Fig. 5 Micromorphology of the carbon products deriving from SrCO
included carbonate melts electrolysis at 600 8C.
3
-
Fig. 6 Micromorphology of the carbon products deriving from BaCO
included carbonate melts electrolysis at 600 8C.
3
-
carbonate eutectic; and those shown in Fig. 5e and f were but a small amount of carbon nanotubes were observed, as
synthesized in 30% SrCO additive dissolved carbonate eutectic. marked with an arrow (Fig. 6f). The decomposition free energy
A common feature of the cathodically deposited carbon products of BaCO into BaO and CO is negative but higher than that of
3
3
2
2
6
is the presence of carbon flakes and sphere-like carbons as SrCO3 Li CO , Na CO and K CO , indicating that BaCO degradation
2
3
2
3
2
3
3
was dissolved in the Li
due to the absence of porous carbon structure and the presence mediate BaO originating from the BaCO
of compact flake-like and sphere-like structure, the BET surface considerable advantage in dissolving into molten carbonates
CO –Na CO –K CO
2 3 2 3 2 3
melts. In addition, would proceed in a more facile manner. Moreover, the inter-
3
splitting shows a
2
À1
14
areas of the carbon products are lower than 100 m
g
,
compared to alkali oxides. The dissolved BaO tends to react
of the with atmospheric CO to form BaCO . Thus, it is thermodynamically
electro- concluded that the BaCO additive demonstrates an even greater
lysis (Table 1). However, it is found that with the increase in the CO2 absorption behavior than the Li CO –Na CO –K CO
3
2
À1
signifying a visible decrease compared to 193 m
products obtained from molten Li CO –Na CO –K
g
CO
2
3
2
3
2
3
2
3
3
2
3
2
3
2
SrCO mass fraction, the BET surface area tends to elevate. This matrix. The enhanced CO dissolution promotes the carbonate
3
2
trend indicates an interesting phenomenon where the SrCO
3
regeneration process and facilitates compact carbon generation,
additive favors compact carbon products deposition within the resulting in a reduced BET surface area.
Li CO –Na CO –K CO system, but more SrCO provides an Carbon deposits formed in carbonate eutectic of Li
environment for the generation of loosened carbon structure Na CO –K CO , Li CO –Na CO –K CO –CaCO (30%), Li
with larger BET surface area. The BET surface area significantly Na CO –K CO –SrCO (30%) and Li CO –Na CO –K CO
2
3
2
3
2
3
3
2
2
CO
CO
–BaCO
3
–
–
2
3
2
3
2
3
2
3
2
3
3
3
2
3
2
3
3
2
3
2
3
2
3
3
depends on the particle size and in general, smaller particle size (30%) are further studied by nitrogen adsorption–desorption auto-
corresponds to carbon products with higher BET surface area. matic analysis. Nitrogen adsorption–desorption isotherm and pore
In this manner, it is expected that carbon products with smaller size distribution are presented in Fig. 7a and b, respectively.
particle size are acquired on increasing the SrCO
A certain amount of BaCO was previously utilized as a products of Li
functional additive to stabilize the molten carbonate electro- larger porosity than that of Li
lyzer due to the lower Lewis basicity (the ability to donate a pair those of 30% SrCO and BaCO
3
fraction.
Matching well with the BET surface area listed in Table 1, carbon
CO –Na CO –K CO –CaCO (30%) electrolysis show
CO –Na CO –K CO system, while
additives, on the contrary, exhibit
3
2
3
2
3
2
3
3
2
3
2
3
2
3
3
3
1
3,14,25
of electrons) of BaO than Li
2
O, Na
2
O and K
2
O.
Herein, decreased porosity.
X-ray diffraction patterns of the carbon products of CO
the SEM images of the carbon products electro-deposited in
2
electrolyte with varying BaCO fractions are displayed in Fig. 6. dissolved in molten carbonates were recorded in order to develop a
3
Amorphous carbon particles and carbon flakes were formed more profound understanding of the crystal structure, as presented
when 10% and 20% BaCO
Na CO –K CO composite, as evident in Fig. 6a–d. When 30% of essentially parallel layer planes composed of several condensed
BaCO was dissolved in Li
products still presented amorphous microstructure (Fig. 6e), are randomly oriented rather than in an ordered arrangement, and
3
were included in molten Li
2
CO
3
–
in Fig. 7c–f. Both amorphous carbon and graphitic carbon consist
2
3
2
3
27,28
3
2
CO
3
–Na
2
CO
3
–K
2
CO
3
system, carbon benzene rings.
However, layer planes in amorphous carbons
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of the carbon products to some extent. As further demonstrated
herein, BaCO
3 3
and SrCO additives generate compact carbon
structures with lower BET surface areas than Li CO –Na CO –
2
3
2
3
K CO melts. For CaCO , low CaCO fraction deposits carbon
2
3
3
3
products with compact microstructure, while sufficient amount
would preferably lead to loosened carbon structure production
with larger BET surface area than that obtained in the Li
Na CO –K CO system. It was previously measured that the
solubility of CaO in molten carbonates was 30 to 100 times less
2 3
CO –
2
3
2
3
3
0
3 2
than that of CaCO and 50 times less than that of Li O.
Hence, the interesting influence of CaCO3 on the carbon
production is believed to be mainly caused by the poor solubility
of CaO in molten carbonates.
Carbon nanotubes, instead of amorphous carbons, were
deposited by optimizing the electrolytic conditions. A nickel
2
wire disk with an area of 10 cm and a galvanized iron wire disk
2
with an area of 5 cm were employed as the anode and cathode,
respectively. A 50 mL-corundum crucible served as the electro-
lytic container in this study rather than the previously reported
1
1,29
nickel crucible.
nickel crucible acted as the anode simultaneously, and no
additional nickel anode was employed. Li CO melts at 723 8C
and to ensure good fluidity, pure Li CO electrolysis was also
carried out at over 730 8C. Herein, we performed the pure Li CO
electrolysis at 730 8C, 750 8C and 770 8C, as depicted in Fig. 8a–c,
respectively. Evidently, the pure Li CO electrolysis generates a
In the latter case, the inner wall of the
2
3
2
3
Fig. 7 Characterization of the carbon products. (a) Nitrogen adsorption–
desorption isotherm. (b) Pore size distribution. (c–f) X-ray diffraction
2
3
patterns of the carbon materials electro-deposited in Li
CO , Li CO –Na CO –K CO –CaCO (30%), Li CO –Na
SrCO (30%) and Li CO –Na CO –K CO –BaCO (30%), respectively.
2
CO
3
–Na
–K
2
CO
CO
3
–
2
3
K
2
3
2
3
2
3
2
3
3
2
3
2
CO
3
2
3
-
high yield of carbon nanotubes with an external diameter of
3
2
3
2
3
2
3
3
B100 nm and an inner diameter of merely tens of nanometers.
The length of the carbon nanotubes subsequent to pure Li CO3
2
this defect leads to majority of the differences between amorphous electrolysis varies from tens of microns to hundreds of microns.
carbon and graphitic carbon. Herein, a wide and weak carbon peak No visible change in the diameter of the synthetic carbon
located at 268 is observed for the above carbon products, signifying nanotubes is observed along with the increase in electrolytic
that the CO
are mostly amorphous. In contrast, on electrolyzing pure Li
23 8C, the cathodically generated carbon conformation is observed BaCO -included molten Li CO were performed at 750 8C, as
2
-derived carbon products in the above four electrolytes temperature.
2
CO at Preparation of carbon nanotubes in CaCO
3
3 3
-, SrCO - and
7
3
2
3
to produce carbon nanotubes in high yield and presents a presented in Fig. 8d–f, respectively. Compared to that of pure
sharp and enhanced carbon peak at 268 assigned to the well- Li CO electrolysis, alkaline earth carbonate additives provide
2
3
ordered hexagonal graphite (002) diffraction planes within the thicker carbon nanotube structure. The increase in the diameter
2
9
carbon nanotubes.
of the carbon nanotube structure is presumably because of the
A visible difference in the carbon structure and BET surface interface modification induced by the alkaline earth carbonate
area is presented on adding diverse alkaline earth carbonate additives. An additional observation from the TEM images is
with varying additive amounts. When alkaline earth carbonate that the carbon nanotubes of pure Li
2 3
CO present a very small
was dissolved in the Li CO –Na CO –K CO melts, the fluidity inner void, while a more prominent hollow structure is observed
2
3
2
3
2
3
and conductivity of the molten salt system significantly changed. when alkaline earth carbonate is added into molten Li CO . In
2
3
The former leads to an interface modification between the fact, the carbon nanotubes subsequent to pure Li CO electro-
2
3
carbonate electrolyte and cathode. Carbon electro-deposition in lysis were once considered as solid carbon nanofibers until TEM
2
9,31
the electrolyzer occurs on the cathodic surface, which is in reality characterization was employed.
The selective synthesis of
significantly dependent on the interfacial interaction. Therefore, carbon nanotubes and carbon nanofibers could be achieved
1
3
any minor change in the interface between the cathode and through isotopic labeling, where CO
2
favors a straightforward
1
2
the electrolyte would provide the cathodically formed carbon synthesis of carbon nanofibers and naturally abundant CO
products with different microstructures and BET surface areas. facilitates hollow carbon nanotubes production.
Moreover, the visible distinction of the cell voltages recorded The formation mechanism of carbon nanotubes in molten
2
3
1
during the galvanostatic electrolysis in varying electrolytes, carbonate environment is attributed to the nucleating effect of
which is induced by the changing conductivity as alkaline earth nickel nanoparticles deriving from the reduction and redeposition
carbonate additives are included, provides different carbon of dissolved nickel oxide. To deposit high quality uniform carbon
deposition rates. This would also impact the microstructure nanotubes, an initial low current step is required in order to
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Paper
Table 2 Comparison of the external diameter of the carbon nanotubes in
the literature
Container
Duration/min External diameter/nm Ref.
Corundum crucible r120
100
200–300
2500
This work
29
11
Nickel crucible
Nickel crucible
r120
1200
2
anode and provided larger active area than the 10 cm nickel
wire disk. The increase in anode area may facilitate nickel
release and form larger transitional metal nucleation sites,
providing the carbon nanotubes with larger diameter. As further
reported in ref. 11, extended electrolysis duration generates
proportionally thicker carbon nanotubes due to the continuous
carbon deposition, which in turn signifies the excellent regeneration
of the carbonates electrolyte.
Conclusions
2 3
Fig. 8 Carbon nanotubes prepared in pure Li CO
2
at (a) 730 8C, (b) 750 8C In this study, the conversion of CO into carbon fuel or even
and (c) 770 8C. Carbon nanotubes prepared in 750 8C alkaline-earth- valuable carbon nanotubes was achieved via electrolysis in
carbonate-included electrolyte (d) Li
f) Li CO –BaCO
2
CO
3
–CaCO
3
, (e) Li
2
CO
3
–SrCO
3
and
alkaline-earth-carbonate-included carbonate eutectic. The reduction
of tetravalent carbonate ions to zerovalent solid carbon occurs on
the cathode surface and significantly depends on the interfacial
(
2
3
3
.
interaction. Li
the alkaline earth carbonate generates a carbon product with a
porous, honeycomb-like structure. The addition of CaCO , SrCO
and BaCO leads to a significant change in the carbon morphology,
from a porous, honeycomb-like carbon to a mixture of carbon
flakes, sphere-like carbons and irregular carbon particles. The
morphological evolution after the addition of diverse alkaline
earth carbonates by varying amounts is mainly due to the inter-
face modification between the cathode and the carbonates
2 3 2 3 2 3
CO –Na CO –K CO electrolysis without dissolving
À2
deoxidize of nickel oxide. In this case, before using 200 mA cm
to obtain a rapid carbon nanotube deposition, current densities
3
3
À2
À2
À2
of 6 mA cm , 10 mA cm and 20 mA cm were initially
applied to the electrolyzer for 15 min, 15 min and 5 min,
respectively, to grow nickel nucleation sites on the cathodic
surface controllably. During this process, cell voltage increased
from B0.8 V to B1.05 V, which was sufficient to reduce nickel
oxide to form nickel nanoparticles but insufficient for carbon
deposition. Zinc metal is another essential factor for carbon
nanotubes growth, but acts in a different manner from nickel
anode. In the absence of zinc but in the presence of nickel, CO2
electrolysis produces amorphous carbon graphite and uncontrolled
nanofiber structures with circular or rectangular duct-like cross
sections. In contrast, when nickel rather than galvanized iron is
used as the cathode and ZnO (1% by wt) is added directly into the
electrolyte, the nickel crucible electrolysis still nucleates high-yield
3
electrolyte. Moreover, SrCO
BET surface area than the carbon products of Li
CO , while sufficient CaCO endows the carbon products with
a larger BET surface area than Li
3
and BaCO
3
additives provide a lower
2
CO –Na CO
3
2
3
–
K
2
3
3
2
2
CO
3 2 3 2 3
–Na CO –K CO (264 m
À1
2
À1
g
vs. 193 m g ), which could be attributed to the formation of
the nearly insoluble intermediate CaO. The XRD patterns signify
that the synthesized carbon products are of low crystallinity due
to the random order of the layer planes. Moreover, the carbon
11
and high-quality carbon nanotubes.
product of CO
SrCO and Li
2
dissolved in over 700 8C Li
2 3 3 2 3
CO –CaCO , Li CO –
The diameter of a nanotube significantly affects its mechanical
and electronic properties and thus can influence its performance
Herein, it is verified that the
electrolysis using alkaline earth carbonate additives provides
thicker carbon nanotubes production compared to the pure
3
2
CO –BaCO is seen to produce valuable carbon
3
3
nanotubes in high yield. This suggests that carbon nanotubes
synthesis could be fulfilled in a more economic electrolyte rather
than with rare and expensive pure Li CO . Summarily, the alkaline-
3
2–35
in a variety of applications.
2
3
earth-carbonate-included molten carbonates electrolyzer could
Li
2
CO
3
electrolysis. Additionally, we compare the carbon nano-
CO electrolyte presented in this work
as listed in Table 2. It is demonstrated
sustain continuous CO electrolysis and carbon electro-deposition,
2
tubes formed in the Li
and previous reports,
2
3
providing a new alternative electrolyte component to consume
1
1,29
unwanted CO
products.
2
greenhouse gas and generate reusable carbon
that besides the inclusion of the alkaline earth carbonate, the
electrolytic container and electrolysis duration would also impact
the carbon nanotubes deposition. The nickel crucible electrolysis
was observed to deposit carbon nanotubes at a high yield, but the
Conflicts of interest
29
carbon nanotubes exhibit wider diameter of 200–300 nm. In
ref. 29, the inner wall of the nickel container was utilized as the There are no conflicts of interest to declare.
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Acknowledgements
16 S. Licht, B. Wang, S. Ghosh, H. Ayub, D. Jiang and J. Ganley,
J. Phys. Chem. Lett., 2010, 1, 2363–2368.
This study was supported by the National Natural Science
Foundation of China (21476046, 21306022 and 21808030), the
Science Fund for Distinguished Young Scholars of the Heilongjiang
Province of China (JC2017002), the Natural Science Foundation of
Heilongjiang Province (E2018014), the Postdoctoral Launch
Fund of the Heilongjiang Province of China (LBH-Q17033) and
Northeast Petroleum University (KYCXTD201703, 2017PYQZL-07
and JYCX_CX03_2018).
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