Condensation and reduction of CO2 gas by electrochemical systems
with molten salts and ionic liquids electrolyte
Hokkaido University

Toshihiro Shimada

Abstract
We constructed an electrochemical system that condenses dilute CO2 gas by using
molten salts and ionic liquids as electrites.

During fabrication and testing, it was

found that the wettability of the porous support ceramics with electrites is important toprevent the destructing gas leak.

In particular, we found that the wettability between

Si3N4 ceramics and the molten carbonate salt was extremely poor (the contact angle wasmore than 150o).

This result strongly suggests that Si3N4 is a potential candidate ofprotective coating of the electrochemical system using molten salts.

Finally wesucceeded to demostrate the condensation of dilute CO2 gas by the electrochemicalsystem.
1. Introduction
Reduction of CO2 gas exaust is necessary to prevent and reverse the grobal warming
process.

The ultimate means for that is to equip pumps that condense and collect CO2

gas in every chimney and chemically reduce the CO2 to make fuels and chemicals.
This type of technology consumes energies in the form of electricity, but it might become
economically feasible in the near future in view of Carbon Offset, not to mension about
the advance of solar energy conversion in the long term.

This research project aimed

at the assembling of electrochemical CO2 gas condensation system by using molten
carbonate salts and ionic liquids.

We also made experimental attempts of reducing

CO2 by electrochemical processes (Fig.1).
The mebrain which we would like to fabricate in this research is something like
"Maxwell's Daemon" reversing the entropy transport.

How to perform this action withminimum energy is the key issue in the recovery of CO2 from the exhaust gas, which
will become important in the near future in relation to the carbon offset.

We believethat the higher efficiency is obtained by thermomechanical way such as absorption anddesorption controlled by pressure/temperature difference but by electrochemistry which
is used in fuel cell.

Since it is difficult to oxidize or reduce CO2 electrochemically, thereport on the electrochemical pump has rarely be found[1,2].

We focus on the molten

carbonate salt-type fuel cell to make the CO2 pump, in which the reduction of O2 is
coupled with the transport of CO2. This kind of devices can be fabricated with a compact－－size and easily attached to the existing chimneys in various facilities.

Fig.1 Proposal of CO2 tansporting membrane
2. Experiment
A. Electrodes and electrolytes
We first used the carbonate molten salts as the electrolytes, since the possibility inthe fuel cell is established.
ratio,We chose the mixture of NaCO3: LiCO3 = 48: 52 (molar

eutectic point 514℃) because of its low melting point.

We used porous Ni as theelectrodes. We successfully immersed the molten salt into the porous Ni as shown in
the SEM images below (Fig.2).
after immesion

porous Ni
immesion
tomolten salt at650 oC in air

Fig.2 Porous electrode and immersion to molten salt
B. Electrolyte support
 It is necessary to use an porous ceramic electrolyte support that is resistant to the－－strongly corrosive molten carbonate salts.

Since little is known about the corrosion ofceramics to carbonate salts, we examined the properties of Si3N4 and Al2O3 ceramics.
Si3N4 is characteristic in its high mechanical strength and high resistance to thermalstress.

Al2O3 ceramics has thecharacteristics of high mechanical strength and highcorrosion reistance.
 We synthesized porous Si3N4 ceramics.

We mixed Si3N4 power, carbon fiber andadded Al2O3 and Y2O3 as the sintering reagent.

The mixture was thoroughly mixed

with a ball mill (one day with ethanol), follwed by sintered in a hotpress at 1850 oC
24MPa.

We used commetial Al2O3 porous ceramics.

SEM images and X-ray

diffraction pattern (XRD) are shown in Fig.3.

Al2O3porousceramic
ceramic
Al2O3 porous

Al2O3 porous
ceramic
Si3N4 porous ceramic
Si3N4 porous
ceramic

Fig.3C.

Characterization of electrolyte supporting porous ceramics

Assemble of electrochemical cell We fabricated an electrochemical cell as shown in Fig.4. Porous ceramics and porous
Ni electrodes were separately immersed in molten carbonate and they were put
together and glued from outside by alumina adhesive which has excellent gas sealing
properties (left figure).

The electrode-electrolyte complex was further glued to one endof alumina tube and then covered by alumina tube with larger diameter.

They were allheated in the electric furnace during the operation. In the outer tube (cathode tube)
mixed gas of Ar, CO2 and O2 was flown, while in the inner tube He gas was flown in aconstant rate and the gas flowing to outside was collected and analyzed by gas
chromatography.

－－CO2 concentaration 2-20 vol%immesion to molten
salts

electricfurnaceheat
treatment

Ni wireAl2O3tube
porous Ni(electrode)gas analysis byAlumina

1sccm flow

GCadhesiveFig. 4 Electrochemical CO2 pump system
D. Experiment using ionic liquids and attempt to reduce CO2 electrochemically
Molten carbonate salt electrochemical systems must be operated at relatively hightemperature of 650 oC and it might deteriorate the energy efficiency.

Therefore, we

attempted to use ionic liquids with carbonate species to lower the temperature.

We

mixed Ag2CO3 to various imidazorium chloride derivative ionic liquids to make the ionicliquid with carbonate species by precipitating AgCl.

We found a problem during thisexperiment that CO2 is emitted from the liquid associated with silver salt preciptation
and we wer not successful in the synthesis of new ionic liquids with CO32- component.
We attempted various other way but we have not obtained fully positive results untilnow.

We are planning to make efforts to fabricate a room temperature electrochemical

CO2 pump by using ionic liquids.
3. Results
A. Discorvery of extremely low wettabilitty between Si3N4 and molten carbonate salts.
 The figure below shows the GC analysis results of anode gas when Si3N4 was used as
the electolyte support. It is noticed that cathode gas (mixture of Ar,O2 and CO2) isleaked to the anode side.

On the other hand, there was no noticeable leak when Al2O3

ceramic was used as the support. Since it is impossible to condense CO2 with suchleaks in the system, we examined the cause of the leak in Si3N4 support.
 We mirror-polished the surface of Si3N4 ceramics in the experiment.

The samecarbonate salt powder as in the electrochemical system was placed on the surface andheated. As shown in Fig. 6 (a), the molten salt became spherical and the contact angle
was found to be more than 150o.

It is reasonably explained that the gas leak occurredbecause the molten salt could not enter into the small holes of Si3N4 due to this lowwettability.

On the other hand, the wettability of Al2O3 was perfect with the contact－－GC of Cathode gas (10% CO2)  GC of annode gas in the case of                Si3N4 ceramic support

Fig.5
angle almost 0o as in Fig. 6 (b).

GC of anode gas in the case of Al2O3 ceramic support

GC analysis

It leads to the good adhesion of the molten salt to theinside of porous Al2O3.
The present result about Si3N4 is negative in view of making the electrochemical CO2pumps, but this finding is extremely useful to develop a coating material forelectrochemical system with corrosion resistance using the carbonate salts.

We arecontinuing the experiments about wettability and corrosion resitance of Si3N4
fabricated by plasma-assisted chemical vapor deposition (plasma CVD) with variousmolten salts including various carbonates and phosphates.

(b) Al2O3(a) Si3N4

contact angle >150o

contact angle ~ 0oFig. 6 Contact angles of moletn carbonate salt (a)Si3N4,(b) Al2O3B. Performance of an electrochemical CO2 pump using porous Al2O3 ceramic support
 The results shown above indicate that electrochemical system without gas leak can beassembled by using Al2O3 ceramic electrolyte support.
the electrochemical system operated as a CO2 pump.

－－Fig. 7 shows the performance ofCO2 conc. in anode gas - voltage characteristics

ＣＯ２㻌

Current-Voltage characteristc

Fig.7 Performance of electrochemical CO2 pump
The current-voltage curve in the left figure shows that current at the same voltage isincreased when the concentration of CO2 in the cathode gas is increased.

It suggeststhat CO32- derived from CO2 gas carry the current. The CO2 concentration in the rightfigure shows that the CO2 concentration in the anode gas is proportinal only to thecurrent, without strong dependence on the cathode gas concentration.

These results

clearly indicate that the CO2 pump operation is realized by the present system. We
have achieved the first objective of the proposed research.
4. Summary and future outlook
Systems for CO2 condensation and recovery from the exhaust gas are actively studied
right now, but main stream of the research is on the chemical adsorption and release of
CO2.

This type of system is inevitably poorly efficient because it uses

thermomechanical work.

In the present research, we aimed at construction ofelectrochemical pump for the CO2 condensation from dilute gas by using carbonate
molten salts including ionic liquids.

This approach is promising in terms ofsize-reduction and efficiency improvements.

We attempted to find good catalysts for

the CO2 ionization.
As a result. we have succeeded in the assembling CO2 pump system using molten
carbonate salts.

As a byproduct, we found that the wettability of Si3N4 and carbonate

molten salts are extemely poor (contact angle > 150o).

The latter results opens a newtechnique for corrosion protection to the molten salt devices such as fuel cells using
Si3N4 coating.
The remaining works on this proposal are as follows.

Using low temperaturemolten salts such as ionic liquids to reduce the operation temperature and improve the－－power efficiency. Also we need to find a good catalyst to promote the reaction CO2 + 1/2
O2 + 2e- → CO32- at low temperature. We will continue these researches after theperiod of JFE 21 Century Foundation funding because we have found promisingfindings.
The present work was supported by JFE 21 Century Foundation.

In the end of thisreport, we would like to thank JFE 21 Century Foundation for supporting us about thisproject.

We have made our idea of electrochemical CO2 pump into reality and obtained

various hints for future research.

References
1. H. Kasai, et al. DENKI KAGAKU No. 6 (1998) 635-640
2. K. Sugiura, et al. J. Power Sources 118 (2003) 218-227
3. "Change in the surface terrace and step structure after electrochemical reduction ofgraphite", S. Kubota, T. Yonezawa, T. Nagahama and T.Shimada, Chem. Lett., inpress.
4. "Low wettability between Si3N4 with various molten salts - possible protective coatingfor fuel cells", Y. Matsuda, Y. Katsuyama, H. Kiyono, T. Shimada, in preparation.

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