Sustainable production of formic acid by electrolytic reduction of gaseous carbon dioxide

Article abstract of DOI:10.1039/c4ta03893b

A tin (Sn) nanostructure has been applied to a gas diffusion electrode for the direct electro-reduction of carbon dioxide (CO₂) in a zero-gap electrolytic cell. A Sn catalyst layer was evenly applied to a carbon substrate by a controlled spraying technique and the efficient catalytic conversion of gas-phase CO₂ to formic acid (HCOOH) demonstrated. We observed that the overall mean faradaic efficiency towards HCOOH remained above 5.0% over the entire reduction time. In addition, due to its compact configuration and surroundings at near ambient conditions the approach described is promising in both modularity and scalability. Sustainable energy sources such as solar, wind, or geothermal electricity could be used as a power source to minimize the large-scale operating cost.

Full text of DOI:10.1039/c4ta03893b

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Journal Name
RSCPublishing
ARTICLE
Sustainable production of HCOOH via an electrolytic
1
2
reduction of gas-phase CO
2
Cite this: DOI: 10.1039/x0xx00000x
a
b
b
bc
Seunghwa Lee, HyungKuk Ju, Revocatus Machunda, Sunghyun Uhm, Jae
b
d
*ab
Kwang Lee, Hye Jin Lee, and Jaeyoung Lee
Received 00th January 2012,
Accepted 00th January 2012
A tin (Sn) nanostructure onto gas diffusion electrode is applied for the direct
electroreduction of carbon dioxide (CO ) in a zeroꢀgap electrolytic cell. Sn catalyst layer is
evenly placed onto carbon substrates by a controlled spraying procedure and its efficient
2
DOI: 10.1039/x0xx00000x
www.rsc.org/
catalytic conversion of gasꢀphased CO to formic acid (HCOOH) is demonstrated. We observe
2
that the overall mean faradaic efficiency towards HCOOH stays over 5.0% during the entire
reduction time. Furthermore, note that the approach employed in this study is promising for
modularity and scalability due to its compact configuration and surroundings under near
ambient conditions. Sustainable energy sources such as solar, wind, and geothermal electricity
could be used as a continuous power source in order to decrease the overall cost for CO2
utilization.
.
Introduction
Technologies to curtail greenhouse gas emissions especially
1,2
CO have been carried out through a variety of methods.
2
In the 21st century our world is still searching for strategies Currently, carbon capture and storage (CCS) has received
to overcome the challenges associated with climate change, considerable attentions as one of the technologies to handle
1
ꢀ3
dependency on fossil fuels and limited natural resources. Solar large quantities of CO emissions. Sequestration has been the
2
energy is trapped in the atmosphere as heat by several gases major storage option for CO2 gas from power plants, but
including typically carbon dioxide (CO ), methane (CH ) and several shortcomings remain, including environmental and
2
4
chlorofluorocarbon (CFCs). The soꢀcalled greenhouse effect is safety concerns about the risk of leakage as well as
3
now widely accepted as a significant environmental threat. technological limitations.
Supplies of fossil fuels and natural gases are dwindling and this
Considerable efforts have been made on heterogeneous
has propelled a search for potential renewable and alternative catalysis for the conversion of CO2 gas into valueꢀadded
energy supply.
chemicals as an alternative longꢀterm solution for the CO
2
2,4
mitigation. The utilisation of CO gas is limited by the fact
2
that it requires significant activation energy for transformation
Electrochemical Reaction and Technology Laboratory, School of into organic chemicals thus a superior catalyst must be used to
Environmental Science and Engineering, Gwangju Institute of Science overcome the high activation energy barrier. Hence, the
a
and Technology (GIST), Gwangju, 500ꢀ712, South Korea. Eꢀmail: reduction process for selecting products necessitates the use of
5
jaeyoung@gist.ac.kr
specific metal catalysts. Azuma et al. summarized the types of
b
Ertl Center for Electrochemistry and Catalysis, Research Institute for catalysts within the periodic table showing product distributions
Solar and Sustainable Energies, GIST, Gwangju, 500ꢀ712, South Korea.
Advanced Materials and Processing Center, Institute for Advanced types of catalysts.6,7
in specific condition and Hori et al. divided them into four
c
Engineering (IAE), Gyeonggiꢀdo, 449ꢀ863, South Korea.
Effective conversion will thus require the use of good
d
Department of Chemistry and GreenꢀNano Materials Research Center, catalysts and systems which can convert large volumes using
Kyungpook National University, Daegu, 702ꢀ701, South Korea.
Electronic Supplementary Information (ESI) available: [details of any categorized into sp metals (Zn, Cd, Hg, In, Tl, Pb, Ti and Sn)
supplementary information available should be included here]. See for HCOOH and
metals (Pd, Pt, Cu, Ag and Au) for CO
low cost reactants available. In general, electrocatalysts are
†
d
DOI: 10.1039/b000000x/
according to the electronic configuration related to their
electrocatalytic behavior. Among these metals, we investigated
8
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Sn electrocatalyst for the selective production of HCOOH since
Sn is inexpensive and relatively less toxic compared to other
However, both studies required high temperatures and
metals. A new approach using zeroꢀgap cell with GDE was first pressures. Our previous work performed under mild ambient
introduced for the effective electrocatalytic reduction of gasꢀ conditions showed that the electroꢀdeposition of Sn did not
phase CO , while alleviating massꢀtransfer limitations caused provide a sufficient loading and a homogeneous coating on the
2
1
4
by the low solubility of CO in liquid phase systems.
entire electrode surface. As a result, a low and unstable
2
The direct production of small organic molecules from CO2 product yield was obtained. In this work, we describe a novel
is still far from feasibility and the large scale production is even method to homogeneously coat a thin layer of Sn nanopowders
more challenging. Of the small organic molecules, HCOOH on gas diffusion electrode (Figure 1) by a controlled spraying
produced from the reduction process have higher value from method and demonstrate the better performance than that of the
the energy required for their production, because of its previous work in terms of activity and longꢀterm stability.
significant uses in various areas such as fuels in fuel cells,
fertilizers, pharmaceuticals in addition to feedstocks for various
chemical industries, than other organic products such as Results and discussion
9
methanol (CH OH), ethylene (C H ) and methane (CH ). The
3
2
4
4
Identification of gas-phase CO electroreduction
main mechanism of the electroreduction of CO taking place at
the cathode is given by the following equations.
2
2
1
0
A fundamental and immediate question arises as to whether
the carbon converted to HCOOH originated from CO gas or
2
ꢀ
ꢁꢂꢃꢄꢅꢆ → ꢁꢂꢃꢄꢇꢈꢆ
(1)
(2)
(3)
(4)
(5)
not, since the corrosion of the carbon electrode is known to
ꢀ
ꢀꢁꢂꢃꢄꢇꢈꢆ ꢉ ꢊꢋ →ꢀ∙ ꢁꢂꢃꢋ
occur under the high overvoltage applied. Hence, our first
15
ꢄꢇꢈꢆ
investigation was focused on verifying the conversion of gasꢀ
ꢁꢂꢃꢋ
ꢉ ꢌ →ꢀ∙ ꢌꢁꢂꢂ
ꢍ
∙
ꢄꢇꢈꢆ
ꢄꢇꢈꢆ
phase CO into the liquid form of HCOOH.
2
∙
∙
ꢌꢁꢂꢂꢄꢇꢈꢆ ꢉ ꢊꢋ →ꢀ∙ ꢌꢁꢂꢂꢋ
ꢄꢇꢎꢆ
ꢌꢁꢂꢂꢋ
ꢉ ꢌ → ꢀꢌꢁꢂꢂꢌ
ꢍ
ꢄ
ꢇꢎ
ꢆ
The overall reaction of the formation of HCOOH is:
ꢍ
ꢋ
ꢀ
ꢀꢁꢂꢃꢄꢅꢆ ꢉ 2ꢌ ꢉ 2ꢊ → ꢌꢁꢂꢂꢌ
(6)
The catalytic activity of gaseous CO2 into the intended
product, HCOOH, could be reported by both the product yield
1
1
and faradaic efficiency. Therefore, improving the efficiency
of this conversion will result in the scaleꢀup of the system and
stabilization of CO2 gas emissions. Preti et al. reported a
faradaic efficiency of ca. 83% to produce HCOOH from CO2
under the high pressure of 180 bar and Yan et al. showed a
1
2,13
yield of 87% of HCOOH over about 300°C.
Figure 2. UVꢀspectroscopy analysis of the produced HCOOH
to compare experiments performed under different conditions :
(
a) commercial HCOOH for standard reference, (b)
electroreduction of gaseous CO , (c) electroreduction of inert
2
gas Ar and (d) baseline of deionized water.
Several liquid samples generated under various conditions
were analyzed to reveal the carbon source of produced HCOOH
by UVꢀspectroscopy in the wavelength region of 190ꢀ240 nm
(
Figure 2). Deionized water was used as a reference and the
maximum absorbance peak at 207 nm from commercial
HCOOH solution was matched with each resultant graph of
samples. In the absence of CO2 gas supplied, no peaks
1
corresponding to the HCOOH were observed. In addition, Hꢀ
NMR analysis was performed for the identification of the
carbon source more meticulously.
Figure 1. Schematic diagram of a zero gap electrolytic cell
configuration for the electroreduction of CO2 supplied directly
from gas phase.
1
6,17
In order to trace the carbon source of HCOOH product,
1
3
CO gas was supplied and converted into the liquid phase
2
2
| J. Name., 2012, 00, 1-3
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ART93ICLE
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product by the electrochemical reduction process. Carbon
After confirming the origin of carbon converted to
isotopes having mass number of 13 u are naturally occurring at HCOOH, the following step was to determine the optimal
1
3
as small proportion as about 2%. By intended formation of Cꢀ operating conditions of an electrolytic CO reduction system.
2
labeled HCOOH, the intensities of two types of HCOOH peaks Knowing how much Sn catalyst is activated at a given potential
1
in HꢀNMR spectra were reversed compared to that of the is important in the optimization of the system operating
1
2
commercial CꢀHCOOH peaks. As shown in Figure 3, it was conditions. To examine the Sn catalyst’s activity for the
1
3
observed that Cꢀlabeled HCOOH peaks increased sharply and electroreduction of CO , LSV curves were obtained in a zeroꢀ
2
this result allowed the identification of the carbon source of gap cell applying both Ar and CO . As shown in Figure 4, the
2
HCOOH, which was generated from the directly supplied CO2 voltammogram under CO2 supplied condition was recorded
gas.
more negative than that of using the same catalyst under Ar
supplied at the cathodic sweep rate of 20 mV sec .
ꢀ
1
Figure 4. Linear sweep voltammograms using Sn in (a) Ar and
(
b) CO gas supplied respectively. The reduction peak at ꢀ0.7 V
2
was attributed to the electroreduction of CO and another peak
2
appeared at the less negative potential was associated with the
ꢀ
1
hydrogen adsorption. Scan rate was 20 mV sec .
We believe that the higher negative current value observed
at the cathode voltage around ꢀ0.7 V (vs. RHE) was in
accordance with CO reduction.
2
1
Figure 3. A series of HꢀNMR spectra for HCOOH generated
1
3
by electrochemical reduction with (a) isotopic CO gas, (b)
2
1
2
12
CO gas and (c) commercial CꢀHCOOH as a reference.
2
Figure 5. Faradaic efficiencies depending on the various
voltages : (a) ꢀ0.5 V, (b) ꢀ0.7 V, (c) ꢀ0.9 V, (d) ꢀ1.2 V, (e) ꢀ1.4
V, (f) ꢀ1.6 V, (g) ꢀ1.8 V and (h) ꢀ2.0 V.
LSV and Optimization of operating conditions
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Based on the observation, the optimal voltage was investigated Morphological investigations of electrode surface
through experiments while keeping the same conditions of
temperature, and gas flow rate (Figure 5).
Used electrodes were further analyzed by examining any
visual changes in Sn electrocatalyst morphology since the
To suppress the hydrogen production process, the operating
temperature was set at room temperature and the gas flow rate
of both the anode and cathode were kept at the minimum
reactivity varied during CO electroreduction. The fabricated
2
electrode was characterized using FEꢀSEM to reveal the
compactness and the uniformity of surface morphology before
and after the electrolytic process. Uniformly nanostructured Sn
particles were observed on GDE before experiments but the
amount of Sn particles decreased after the electrocatalytic
reduction of CO2 ( Figure 7). As illustrated in Figure 7b,
1
8,19
required to prevent mass transfer limitations (Figure S1).
Long-term test for gas-phase CO elecroreduction
2
Under the optimal conditions based on the above particle loss was observed indicating that the decomposition of
experimental results, the electrochemical reduction of gasꢀ Sn particles could lead to degradation in long term
phased CO was carried out at ꢀ0.7 V in 10 h of operation.
2
Liquid phase product was collected and analyzed at every 20
min. The variation in currents and faradaic efficiencies in terms
of the electrolysis time was evaluated and the measurement
outcome is summarized in Figure 6. Sn electrode exhibited the
ꢀ
2
stable current density of 5 mA cm . The initial faradaic
efficiency towards HCOOH, an average of 12.5%, decreased to
about 7% after 3 h running and maintained this level for further
few hours until the end of the experiment. The accumulated
amount of HCOOH was calculated and the production rate was
slightly declined during the electrolytic process (inset of Figure
6
).
performance.
Figure 7. SEM images showing the topꢀview and crossꢀsection
of Sn nanostructured cathode with GDE. (a) As prepared and
(
b) after the electroreduction of CO . Particle loss occurred
2
during CO electrolysis..
2
Table 1 (Supplementary information) shows the weight of three
samples of Sn electrode used in CO2 electroreduction for 1 h, 2
h and 5 h respectively. From the measurement, it is expected
that Sn catalyst layer could remain and catalyse the CO2
electroreduction for over 10 h because total weight of Sn
catalysts placed on the carbon substrate was 31.5 mg. During
CO2 electrolysis of 1 h, decrease of the amount of Sn
nanoparticles was almost 2.3 mg and with respect to the
electrolytic process time, the particle loss in weight was
diminishing gradually similar to the trend of Faradaic efficiency
toward HCOOH. Hence, considering result of the longꢀterm
electrolysis as seen in Figure 6, the relative weak adhesion
between Sn nanoparticles and carbon electrode surface seemed
to be functioned as a degradation factor.
Figure 6. Electrocatalytic activity for the electroreduction of
CO on Sn was evaluated by analyzing the production rate of
2
HCOOH vs. time and the current density vs. time at the cathode
potential of ꢀ0.7 V. Inset image shows the accumulated amount
of HCOOH generated from CO electrolysis stably for 10 h.
2
From the products analysis, the remainder of faradaic
current for CO reduction was allotted for the formation of
2
hydrogen (H ). Decreasing CO2 reduction performance is
2
In addition, changes in crystallinity during the
electrocataysis were examined by XRD. As shown in Figure 8,
the electroreduction of CO2 did not cause any significant
changes in crystallinity.
attributed to several reasons such as the loss of the sprayed
catalyst particles on the GDE, the competitive hydrogen
evolution reaction (HER) and the deactivation of the catalyst
1
3ꢀ15,20
due to the exposure to concentrated CO gas.
2
4
| J. Name., 2012, 00, 1-3
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A
3
R
8
T
93IC
B
LE
If RDS is the formation of CO radical anion (Equation 2), the
2
ꢀ
1
Tafel slope is theoretically 118 mV dec . We obtained the
Tafel slope of 148 mV dec on Sn electrocatalyst.
ꢀ
1
Figure 8. XRD patterns of (a) asꢀprepared Sn electrode and
after electrochemical CO reduction in a zeroꢀgap cell for (b) 3
2
h and (c) 10 h, respectively
Figure 9. Tafel plot of partial current densities for HCOOH by
electroreduction of CO . Theoretical Tafel slope of (a) 118 mV
2
The intensities of Sn peaks decreased due to the loss of
catalyst particles from the electrode surface. The increment in
the carbon substrate peak also confirms that the catalyst was
peeled off from the support material. Consequently, the damage
of catalyst configuration on GDE was one of the main
ꢀ1
dec is compared with the Sn electrocatalyst slope of (b) 148
ꢀ1
mV dec indicating the formation of CO radical anion as RDS.
2
Tafel slopes of CO reduction for determining the reaction
2
mechanism have previously been studied by many research
1
3,14,20
contributors to the degradation of the CO electrolysis.
22ꢀ26
2
groups.
It was reported that the Tafel slope similar to our
Moreover, the CO2 reduction interface was significantly
results was consistent with CO reduction mechanism through
2
2
1
acidified because of sufficient protons from the anode part. It
the transfer of one electron to CO adsorbed on the electrode
2
was demonstrated that H was the dominant species from the
2
surface as an initial RDS.
product analysis in the cathode part. This undesirable reaction
pathway could be responsible for the decrease in HCOOH
production. In other words, the rate determining step (RDS) is
not the transportation of proton and the mass transfer limitation
In summary, the longꢀterm electrocatalytic stability of Sn
nanoparticles for gaseous CO reduction was extended in a zero
2
gap electrolytic cell. Further investigations on enhancing the
efficient CO2 conversion into HCOOH are underway in
conjunction with optimizing electrolyzer’s conditions such as
reactor design, stacking technology and modifying electrode
configuration relative to the activation of RDS.
problem of CO was also compensated by directly supplying
2
CO gas to the catalyst onto GDE in a zero gap cell system.
2
Investigation of RDS by Tafel slope
With these recognitions, Tafel plot was extracted by the Conclusions
same process of CO reduction to investigate the mechanistic
2
A direct zeroꢀgap electrolytic cell was successfully used for
pathway regarding RDS. Liquid products were sampled every
0 min and partial current densities for HCOOH production
the electrochemical reduction of gasꢀphase CO . We fabricated
2
2
a gas diffusion electrode with placed Sn nanostructure on
carbon for the stable production of HCOOH. A Tafel plot
showed that the reduction mechanism is dominated by the
formation of CO2 radical anion adsorbed on the electrode
surface. The direct supply of gasꢀphase CO2 could aid the
development of advanced industrial processes. Sustainable
energy sources such as solar, wind, and geothermal electricity
could be used as a continuous power source in order to decrease
were calculated from faradaic efficiencies. Electrochemical
performances for Tafel plot were measured within the window
overvoltage ranging from 0.4 to 0.8 V. Tafel law for multi steps
reaction is given by:
ꢑ
ꢑ
η ꢏ 2.3 ꢐꢒ ꢕ ꢖꢗꢘꢙ ꢛ 2.3 ꢐꢒ ꢔꢕ ꢖꢗꢘꢙꢀꢀꢀꢀꢀꢀꢀ(7)
ꢚ
ꢓ
ꢔ
ꢓ
ꢀ
2
where i is the exchange current density expressed in mA cm , the overall cost for CO utilization.
o
2
ꢀ
2
i is the electrode current density expressed in mA cm , α is the
transfer coefficient with dimensionless unit, is the
η
Acknowledgements
ꢀ
1
overvoltage expressed in V, f is the RT/F expressed in V . The
graph of η vs. log i is known as Tafel plot as shown in Figure 9.
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This work is supported by the Core Technology addition, the carbon source for the formation of HCOOH was
Development Program of the Research Institute for Solar and
Sustainable Energies (RISE), Gwangju Institute of Science and
Technology.
identified through 1Hꢀnuclear magnetic resonance (NMR)
(
Varian Inovaꢀ600 MHz, Varian) spectroscopy analysis at
Korea Basic Science Institute (KBSI, Gwangju Center, Korea).
1
3
13
CO2 gas (99 atom% C, Sigma Aldrich) was utilized to
Experimental Section
Fabrication method of MEA
reveal the reactive origin of carbon. The morphology and phase
of the crystallites of Sn nanopowders on the electrode were
examined by field scanning electron microscope (FEꢀSEM, Sꢀ
4
700, Hitachi) and Xꢀray diffraction (XRD, Miniflex II,
The fabrication of membrane electrode assembly (MEA)
was simplified and effectively carried out by loading Sn
nanopowder (SigmaꢀAldrich, > 99%, an average particle size of
Rigacku), respectively.
Notes and references
ꢀ
2
1
00 nm, 3.5 mg cm ) as a cathode catalyst onto the carbon
diffusion paper with 30wt.% of catalyst of Nafion solution
1
.
M. E. BootꢀHandford, J. C. Abanades, E. J. Anthony, M. J. Blunt, S.
Brandani, N. M. Dowell, J. R. Fernandez, M.ꢀC. Ferrari, R. Gross, J.
P. Hallett, R. S. Haszeldine, P. Heptonstall, A. Lyngfelt, Z. Makuch,
E. Mangano, R. T. J. Porter, M. Pourkashanian, G. T. Rochelle, N.
1
3,26
(
SigmaꢀAldrich, 10%) using
a
spray method.
This
hydrophobic cathode electrode could enhance the humidity of
CO2 gas supplying networks compared to the hydrogen
evolution reaction (HER) in the liquid and more favorable for
liquid products discharge from the GDE. Pt/C (Johnson
Shah, J. G. Yao, and P. S. Fennell, Energy Environ. Sci., 2014,
30.
7,
1
ꢀ
2
Matthey, 40%, 0.3 mg cm ) anode was prepared in the same
manner as that of cathode. The electrodes were placed on both
2
.
N. M. Dowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G.
Jackson, C. S. Adjiman, C. K. Williams, N. Shah, and P. Fennell,
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Energy Environ. Sci., 2010, 3, 1645.
2
(
Nafion 115, Dupont). The assembly of 9 cm active area was
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4
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.
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,
,
2
013,
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Experimental set-up for gas-phase CO electroreduction
2
1
M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe, T. Sakata, J.
Electrochem. Soc., 1990, 137, 1772.
The electroreduction of CO was carried out in a zero gap
2
cell operated by electrochemical workstation (Figure S2). A
potentiostat (PGSTATꢀ302N, Autolab) was used to apply a
cathodic voltage of ꢀ0.7 V (vs. RHE). The MEA was activated
Y. Hori, in Modern Aspects of Electrochemisty, ed. C. G. Vayeanas,
R. White and M. E. GamboaꢀAldeco, Springer, New York, 2008,
No. 42. pp. 141ꢀ153.
ꢀ
1
ꢀ1
with H (20 ml min ) at the anode and CO (40 ml min ) with
2
2
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1
.
.
M. Jitaru, D. A. Lowy, M. Toma, B. C. Toma, L. Oniciu, J. Appl.
Electrochem., 1997, 27, 875.
1
00% relative humidity at the cathode. Linear sweep
ꢀ
1
voltammetry (LSV) was performed at a scan rate of 20 mV s
Det Norske Veritas (DNV), Electrochemical Conversion of CO
Opportunities and Challenge, Norway, 2011, pp.7ꢀ8
2
–
for testing electrocatalytic activity and a Tafel plot of CO2
reduction was extracted to obtain information about the reaction
mechanism.
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Int. Ed. Engl., 2011, 123, 12759.
To quantify the concentration of produced HCOOH, UVꢀ
spectroscopy (UVꢀ1800, Shimadzu) was employed for
analyzing liquid phase samples (Figure S3). High performance
liquid chromatography (HPLC) (Alliance 2690, Waters) with
Shodex RSpak KCꢀG and KCꢀ811 column was also utilized to
evaluate the liquid phase product. 3 mM perchloric acid
1
3. P. Yan, F. Jin, J. Cao, B. Wu and G. Zhang, AIP Conf. Proc., 2010,
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Sridhar, J. Appl. Electrchem., 2012, 159, F353.
(
HClO ) was selected as a mobile phase with a flow rate of 1
4
1
1
7. I. Berregi, G. D. Campo, R. Caracena, J. I. Miranda, Talanta, 2007,
ꢀ
1
ml min at 25°C. HPLC analysis showed that the HCOOH
peak at the retention time of 9.19 min was the sole liquid
72
, 1049.
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013, 54, 4463.
,
product from electrochemical CO reduction (Figure S4). On
2
2
the other hand, the gasꢀphased reaction product was detected by
gas chromatography (GC) (Agilent 7890A, Agilent
Technologies) equipped with thermal conductivity detector
1
2
2
2
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(
TCD). Carboxen 1006 PLOT column (Superico) was used
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Zakharov and A. A. Zinchenko, Water, 2013, , 129.
ꢀ
1
with the carrier gas of N flowed at 1.5 ml min (Figrue S3). In
2
4
6
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ART93ICLE
DOI: 10.1039/C4TA038 B
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