(215z) Ionic Liquid Mediated Electrochemical Reduction of CO2 in a Microbial Electrolysis Cell

Ionic liquid,
1-ethyl-3-methylimidazolium tetrafluoroborate
(EMIM-BF4) has been proved to be a promising catalyst for CO₂
electrochemical reduction with a silver nanoparticle deposited electrode ADDIN EN.CITE
<EndNote><Cite><Author>Rosen</Author><Year>2011</Year><RecNum>30</RecNum><DisplayText>[1]</DisplayText><record><rec-number>30</rec-number><foreign-keys><key
app="EN"
db-id="95dffxdd1zszaqeve5aprzxndxp0zr52awzv">30</key></foreign-keys><ref-type
name="Journal
Article">17</ref-type><contributors><authors><author>Rosen,
Brian A.</author><author>Salehi-Khojin,
Amin</author><author>Thorson, Michael R.</author><author>Zhu,
Wei</author><author>Whipple, Devin
T.</author><author>Kenis, Paul J.
A.</author><author>Masel, Richard
I.</author></authors></contributors><titles><title>Ionic
Liquid-Mediated Selective Conversion of CO2 to CO at Low
Overpotentials</title><secondary-title>Science</secondary-title></titles><periodical><full-title>Science</full-title></periodical><pages>643-644</pages><volume>334</volume><number>6056</number><dates><year>2011</year><pub-dates><date>Nov
4</date></pub-dates></dates><isbn>0036-8075</isbn><accession-num>WOS:000296494700047</accession-num><urls><related-urls><url>&lt;Go
to
ISI&gt;://WOS:000296494700047</url><url>http://www.sciencemag.org/content/334/6056/643.full.pdf</url></related-u...1]. As Brian et.
al., reported, by applying 1.5 V cell voltage, efficient CO₂
reduction to CO can be achieved in a two-chamber electrolysis cell with
EMIM-BF4 as the electrolyte  ADDIN EN.CITE
<EndNote><Cite><Author>Rosen</Author><Year>2012</Year><RecNum>29</RecNum><DisplayText>[2]</DisplayText><record><rec-number>29</rec-number><foreign-keys><key
app="EN" db-id="95dffxdd1zszaqeve5aprzxndxp0zr52awzv">29</key></foreign-keys><ref-type
name="Journal Article">17</ref-type><contributors><authors><author>Rosen,
Brian A.</author><author>Haan, John
L.</author><author>Mukherjee,
Prabuddha</author><author>Braunschweig,
Bjoern</author><author>Zhu, Wei</author><author>Salehi-Khojin,
Amin</author><author>Dlott, Dana D.</author><author>Masel,
Richard
I.</author></authors></contributors><titles><title>In
Situ Spectroscopic Examination of a Low Overpotential Pathway for Carbon
Dioxide Conversion to Carbon
Monoxide</title><secondary-title>Journal of Physical Chemistry
C</secondary-title></titles><periodical><full-title>Journal
of Physical Chemistry
C</full-title></periodical><pages>15307-15312</pages><volume>116</volume><number>29</number><dates><year>2012</year><pub-dates><date>Jul
26</date></pub-dates></dates><isbn>1932-7447</isbn><accession-num>WOS:000306725200014</accession-num><urls><related-urls><url>&lt;Go
to
ISI&gt;://WOS:000306725200014</url><url>http://pubs.acs.org/doi/abs/10.1021/jp210542v</url></related-urls></urls...2]. Here, in this work, we tried to
evolve the electrolysis cell to a microbial electrolysis cell to decrease the
cell voltage employed for CO₂ reduction process. Microbial electrolysis
cell (MEC) is a type of bioelectrochemical system, in which oxidation reaction
is catalyzed by electrochemical active bacteria with the solid anode as the
electron acceptor in anode chamber and then the electrons pass through the out
circuit power supply to cathode. Finally, the electrons drive the reduction
reaction in cathode chamber. With the help of electrochemically active
bacteria, the active energy for anodic oxidation reaction can be significantly
reduced and thus results in a more negative anode potential. Because the
cathodic reduction reaction has a lower redox potential than anodic oxidation
reaction in MEC, more negative anode potential can decrease the cell potential that
needed for driving the cathodic reduction reaction. Based on this principle, a
typical electrochemically active bacteria shewanella oneidensis MR-1 was grown in the anode chamber using a
carbon cloth electrode. 100 ml 18 mol% EMIM-BF4 was
filled into the cathode chamber and the headspace is 20 ml. The cathode is
silver nanoparticle deposited graphite block electrode. After sparging CO₂ overnight, different levels of cell
voltage were applied and the gas composition in the headspace was analyzed by
GC. To evaluate the effect of different electrode potential on CO₂
reduction, the two chamber reactor was firstly operated in abiotic
three-electrode system for 12 hours. The counter electrode was a carbon cloth
electrode and a Ag/AgCl
reference electrode was placed in the anode chamber. The potential of the
working electrode (a silver nanoparticle deposited graphite block electrode)
was controlled at a range from 0 to -0.8 V (vs. Ag/AgCl ) by a potentiostat. As shown in figure 1, when the working
electrode potential was controlled at -0.4 V, the CO production rate was the
highest. After 12 hours operation, the accumulated CO concentration was around
0.0035%. Surprisingly, decreasing the potential to a more negative level did
not enhance the CO production but methane production was trigged. The methane
production rate reached the maximal level at -0.5 V. Further decreasing the working
electrode potential to -0.8 V decreases the methane production rate
dramatically. As an alternative reduction reaction in cathode chamber, hydrogen
evolution reaction became the predominant reaction at a potential more negative
than -0.7 V (Fig. 2). These results are different from Brian et.
al. reported. In their case, no methane production was
detected in the gas phase and hydrogen production could be inhibited. The
possible reason is our reactors were operated in batch mode for more than 12
hours for each test. During the long term operation, the pH, water content and
other conditions might be varied. For example, the pH decreased from 5.69 to
lower than 2.5 in 12 hours operation. These changes leads to methane and hydrogen production at
different potential levels. Besides operating the reactor in the three
electrode mode, the same experiments were conducted in a MEC mode. The anode
potential during all the MEC tests was around -0.3 V and all experiments were
operated for 24 hours. The cell voltage was varied from 0 to -0.5 V. In MEC
mode tests, CO, methane, and hydrogen production rates follow a similar trend
as that in three electrode mode. Especially for methane production, the final
concentration in the gas phase in headspace was almost 100 times higher than
that in the three electrode mode (Fig. 3). This is due to higher current
density in MEC mode than in abiotic three electrode system with the help of
electrochemically active bacteria catalyst. Also this high methane production
rate indicated that methane production can be an important CO₂
reduction pathway besides CO₂ to CO reduction mechanism. Based on
all these results, integrating ionic liquid mediated CO₂ electrochemical
reduction with MEC can be a promising technology for CO₂ utilization
and fixation.

Figure
1.
CO, methane and H₂ production in the three electrode system

Figure
2.
CO, methane and H₂ production in MEC

References

 ADDIN EN.REFLIST [1]    B.A.
Rosen, A. Salehi-Khojin, M.R. Thorson, W. Zhu, D.T. Whipple, P.J.A. Kenis, R.I.
Masel, Science 334 (2011) 643.

[2]    B.A. Rosen,
J.L. Haan, P. Mukherjee, B. Braunschweig, W. Zhu, A. Salehi-Khojin, D.D. Dlott,
R.I. Masel, Journal of Physical Chemistry C
116 (2012) 15307.