Chemical Engineering Journal xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Conversion of carbon dioxide into formate using a continuous electrochemical
reduction process in a lead cathode
Manuel Alvarez-Guerra ⇑, Sheila Quintanilla, Angel Irabien
Departamento de Ingeniería Química y Química Inorgánica, ETSIIT, Universidad de Cantabria, Avda. Los Castros, s/n 39005 Santander, Spain
h i g h l i g h t s
" A continuous electro-reduction process to convert CO2 in formate was studied.
" Ambient conditions in filter-press cell with lead plate as cathode were used.
" The influence of electrolyte flow rate was more limited than that of current density.
" Increasing j up to a limit increases rate production but lowers Faradaic efficiency.
" This study can be useful as a reference for assessing future research efforts.
a r t i c l e
i n f o
Article history:
Available online xxxx
Keywords:
Carbon dioxide
Valorization
Electrochemical reduction
Formate
Continuous filter-press cell
a b s t r a c t
Among the different strategies for CO2 emissions reduction and climate change mitigation that are being
considered, valorisation of CO2 appears as a more interesting option than carbon storage, since it allows
the recycling of CO2 into added-value products. The purpose of this work is to study the influence of key
variables on the performance of an experimental system for continuous electro-reduction of CO2 to
formate in aqueous solutions under ambient conditions, using a filter-press electrochemical reactor with
a lead plate as cathode. A 22 factorial design of experiments at different levels of current density (‘‘j’’) and
electrolyte flow rate/electrode area ratio (‘‘Q/A ratio’’) was followed by additional tests at the mean level
of each variable. The obtained results confirmed general suggested trends in previous works, but they
also offered a detailed analysis of the influence of these variables. Regarding j, it should be emphasised
that a significant increase of the rate of formate production was observed at the expense of lowering
the Faradaic efficiency when increasing j up to a limit value of 10.5 mA cm2, where a Faradaic efficiency
of 57% was maintained. The influence of Q/A turned out to be more limited than that of j, since the results
revealed that increasing the catholyte flow to overcome mass transport limitations only had beneficial
effects for the lowest Q/A ratios (i.e. 60.76 mL min1 cm2). This study is a reference for the evaluation
of future improvements in the development of these continuous electro-reduction processes for CO2
valorisation, derived from further research to overcome current limitations.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Anthropogenic emissions of greenhouse gases to the atmosphere are a major cause of global climate change [1]. According
to NOAA, atmospheric concentration of carbon dioxide (CO2), which
is the most significant anthropogenic greenhouse gas, increases
each year by about 2 ppm, continuing the rise toward 400 ppm
and beyond [2], and as highlighted by the Intergovernmental Panel
on Climate Change (IPCC), about three-fourths of this increase in
atmospheric CO2 can be attributed to fossil fuels combustion [3].
⇑ Corresponding author. Tel.: +34 942 20 67 77; fax: +34 942 20 15 91.
E-mail address: [email protected] (M. Alvarez-Guerra).
In spite of the fact that the ultimate goal for many countries is to
phase out fossil fuels in power production and in the transport
sector, the change to renewable energy sources is slow and predictions indicate that the world energy demand will still depend on
fossil fuels for many years to come [4].
In this context, different strategies for reducing CO2 emissions
and mitigating climate change are being considered, such as energy efficiency improvements, the reduction of carbon intensity
of the economy, or the capture of CO2 from flue gases and its subsequent storage for long-term isolation from the atmosphere
(‘‘Carbon Capture and Storage’’, CCS) [1,5,6], which has received
significant attention to date. However, the development of innovative processes for the recycling of captured CO2 appears as an
attractive option, since they would allow the valorisation of CO2
1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2012.06.099
Please cite this article in press as: M. Alvarez-Guerra et al., Conversion of carbon dioxide into formate using a continuous electrochemical reduction process
in a lead cathode, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.099
2
M. Alvarez-Guerra et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
by converting it into useful and valuable products such as fuels or
other derived hydrocarbons.
The chemical recycling of CO2 to produce carbon neutral renewable fuels and materials is considered as a feasible and powerful
new approach that is entering the stage of gradual practical implementation [7,8]. As detailed in complete reviews on CO2 valorisation (e.g. [9–13]), different categories of CO2 transformations can
be distinguished, such as chemical, photochemical, electrochemical, biological or inorganic transformations. Over the past few
years, electrochemical valorisation of CO2 has gained important
attention. Various works have reviewed different efforts in the
study of CO2 electro-reduction and have discussed mechanistic
aspects and reaction pathways [14–21]. Conversion of CO2 by
means of electrochemical reduction requires supplying electrical
energy to establish a potential between two electrodes in order
to allow CO2 to be transformed into reduced forms. Different
authors have suggested that the electrochemical conversion of
CO2 may be an excellent way of storing intermittent renewable
energy, which can be used for providing the electricity required
for this process and, at the same time, making the beneficial reuse
of captured CO2 possible [8,27,31,39,42]. In this way, it has been
highlighted that if the electrochemical reduction of CO2 to liquid
chemical compounds could be made with high efficiencies, it could
become a sustainable approach in the future for the production of
liquid fuels, providing a high energy density means of storing
renewable electricity as chemical energy [8,17,39,42].
The yield and type of the CO2 reduced forms that can be obtained by means of electrochemical reduction depends on different
factors, emphasising the nature and form of the material used as
cathode, the medium where the reaction takes place or the conditions of pressure and temperature. In this way, the main products
obtained in aqueous media at ambient conditions strongly depend
on the cathode: Cu electrodes mainly yield mixtures of hydrocarbons (mostly of methane and ethylene) and alcohols; CO is the
main product at metal electrodes like Au, Ag or Zn; and other metals such as In, Sn, Hg or Pb are selective for the production of formic
acid/formate [14,15,18,22].
Among the range of useful chemicals into which CO2 can be
electrochemically converted, reduction of CO2 to formic acid/ formate appears to have the best chance for the practical development of technical and economically viable processes [23]. The
variety of traditional industrial uses of these chemicals include
silage preservation, additive in animal feeds, textile finishing and
as a chemical intermediate [24]. Particularly, there is a growing
demand for formic acid to be used in pharmaceutical synthesis
and in paper and pulp production [25], and it has also been pointed
out as one of the most promising candidate fuels for low-temperature fuel cells [26]. Moreover, formate has proven to be an effective, non-corrosive, environmentally friendly deicing agent, which
is used in airports principally in Europe, but in small amounts
because it is expensive [27]. In this sense, formate/formic acid is
relatively expensive as currently produced [27], and these manufacture processes mainly involve the oxidation of hydrocarbons
or thermo-chemical processes based on the carbonylation of
methanol or sodium hydroxide [24], all of them have negative
environmental impacts [28].
Accordingly, electrochemical valorisation of CO2 to formate has
had special attention in the literature over the last years. Although
some works have used divided H-type cells [29–31] or fixed-bed
reactors [32–34], recent research efforts have been mainly focussed
on studies with filter-press flow-by type cells [25,27,28,35–42],
using different operation conditions and cathodes of very different
nature, such as tinned-copper mesh [28,36] or lead-plated stainless
steel woven mesh [38], indium-impregnated lead wire [35], lead
plates [25], tin particles (shots and granules) [37] or metal catalysts
(like lead [40], tin [27,39,41] or indium [42]) electrodeposited on
different substrates. Several studies have experimentally investigated the electrochemical reduction of CO2 to formate in continuous operation [27,28,36–38], showing the great potential of and
growing interest in this type of approach, but at the same time,
highlighting the need to continue obtaining experimental evidence
to guide the efforts for developing and improving these processes.
The aim of this work is to study the influence of key variables
like the current density and electrolyte flow rate on the performance of an experimental system for continuous electro-reduction
of CO2 to formate in a filter-press electrochemical reactor using lead
cathodes. A 22 factorial design of experiments at different levels of
current density and electrolyte flow rate was followed by subsequent more detailed studies of the influence of each of these variables on the performance of this process. This type of study will
make it possible to describe the system behaviour and it can be useful as a reference for the evaluation of future improvements.
2. Methods
A filter-press or parallel-plate type electrochemical cell (Micro
Flow Cell, ElectroCell A/S) was used in this study. The cell consisted
of two separated anodic and cationic compartments that were
divided by a Nafion 117 cation-exchange membrane. The working
electrode (i.e. the cathode) was a lead plate (99.99% Pb), while a
Dimensionally Stable Anode (DSA/O2) plate was used as the counter-electrode (i.e. the anode). Each of the electrodes had a surface
area of 10 cm2. A leak-free Ag/AgCl 3.4 M KCl reference electrode
was assembled in a PTFE frame of the cell. This assembly made it
possible to place the reference electrode very close to the Pb cathode surface. A schematic representation of the cell configuration is
shown in Fig. 1.
Fig. 2 shows the general diagram of the experimental laboratory
system used to develop this work. Electrochemical experiments
were carried out in continuous mode with an operating time of
Fig. 1. Scheme of the filter-press electrochemical cell.
Please cite this article in press as: M. Alvarez-Guerra et al., Conversion of carbon dioxide into formate using a continuous electrochemical reduction process
in a lead cathode, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.099
M. Alvarez-Guerra et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
3
Fig. 2. Diagram of the experimental set-up.
90 min under ambient temperature and pressure conditions. Two
magnetically stirred glass tanks served as the reservoirs for the
catholyte and the anolyte. Each electrolyte was circulated through
its own compartment using peristaltic pumps (Watson Marlow
323S/D) with a flow rate that varied depending on the experiment.
Experiments were carried out at galvanostatic conditions (i.e. at
a certain constant current density) with a potentiostat/galvanostat
AutoLab PGSTAT 302 N (Metrohm, Inc.) that was controlled with a
computer using the General Purpose Electrochemical System
(GPES) software.
An aqueous solution 0.45 M KHCO3 + 0.5 M KCl was used as catholyte, while 1 M KOH was used as anolyte. The solutions were prepared with ultra-pure water (18.2 MX cm at 273 K, MilliQ
Millipore system). Before the start of each experiment, CO2 was bubbled into the catholyte solution until saturation, and then continuous CO2 bubbling (flow rate = 200 mL min1 STP) was maintained
throughout the experiment. Before the beginning of each test, the
Pb cathode was pre-treated with 11% wt HNO3 for 1 min and then
rinsed with plenty of ultra-pure water in an ultrasound bath.
Each sample was analysed in duplicate by Ion Chromatography
(Dionex ICS 1100) to quantify the concentration of produced formate. Samples were taken at different times (15, 30, 60 and
90 min) of operation and the average value of the concentrations
of these samples was obtained for each experiment. Taking into
account the intrinsic variability associated with this type of electrochemical processes, at least two experiments were always
performed for each of the points studied at the same current density and electrolyte flow rate. When the difference between the
concentrations of formate obtained in the two experiments for a
same point and their average concentration was greater than
15%, a third replicate was also carried out. In this way, maximum
standard deviations for the replicates of each point were around
15% of the average product concentration, and for most of the
points, lower than 10%.
The performance of the process was assessed focussing on two
aspects: the rate of formate production and the Faradaic current
efficiency for formate production. The rate of formate production
was expressed as the quantity of formate obtained per unit of cathode area and unit of time (i.e. mol m2 s1). The Faradaic efficiency
for a certain product, which is a widely used figure of merit to
assess the performance of an electrochemical process, is the yield
based on the electrical charge passed during electrolysis [43]; it
can therefore be defined as the percentage of the total charge supplied that is used in forming that product.
3. Results and discussion
3.1. Factorial design of experiments at different current densities and
flow/electrode area ratio
A 22 factorial design of experiments was carried out in order to
explore the effects of current density and electrolyte flow rate on
the performance of the experimental laboratory-scale electroreduction process to convert CO2 into formate. The variable
‘‘flow/area’’ ratio (Q/A), defined as the feed flow rate used for the
catholyte divided by the working electrode area (10 cm2), was
studied in the range between 0.57 and 2.3 mL min1 cm2, while
the variable current density (j) was investigated in the range 2.5–
22 mA cm2. Three levels (low (), medium (0) and high (+)) within these ranges were defined for each variable: 0.57, 1.44 and
2.3 mL min1 cm2 for Q/A, and 2.5, 12.25 and 22 mA cm2 for j.
Table 1 summarises the results obtained in the different experiments. The statistical analysis of these factorial experiments was
performed with MinitabÒ 15 (Minitab Inc.) and MATLABÒ 7.10
(MathWorks, Inc.) in a sequential way: first only the high and
low levels of the 2 factors (j and Q/A) were analysed (i.e. 22 factorial
design); then the centre point (0, 0) was added; and finally, the 32
combinations of the 2 factors at the 3 levels were considered. In all
these analyses both the factors (j and Q/A) and the responses (rate
of formate production and Faradaic efficiency) were normalised in
the range [1, 1] in order to eliminate the influence of the absolute
values.
Please cite this article in press as: M. Alvarez-Guerra et al., Conversion of carbon dioxide into formate using a continuous electrochemical reduction process
in a lead cathode, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.099
4
M. Alvarez-Guerra et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Table 1
Results of the factorial design of experiments: levels of the variables (current density and flow/area ratio) and responses (rate of formate production and Faradaic efficiency).
Point
a
b
Measured formate
concentration (mg L1)
Current
density, j
X1a
Flow/area
ratio, Q/A
X2b
Rate 104
(mol m2 s1)
r
g
Faradaic
efficiency (%)
Normalised rate
[1, +1] ()
r
Normalised Faradaic efficiency
[1, +1] ()
g
1
49.1
37.8
+
+
+
+
4.18
3.22
36.7
28.2
1.00
0.43
0.62
0.85
2
37.3
38.6
0.79
0.81
60.8
62.9
1.00
0.98
0.05
0.11
3
124.0
131.4
+
+
2.62
2.77
23.0
24.3
0.08
0.17
1.00
0.96
4
14.4
12.1
+
+
1.23
1.03
94.7
79.6
0.74
0.86
1.00
0.58
5
60.1
66.2
0
0
0
0
3.21
3.53
50.5
55.6
0.42
0.62
0.23
0.09
6
65.5
71.2
+
+
0
0
3.49
3.80
30.6
33.3
0.59
0.77
0.79
0.71
7
20.6
19.2
0
0
1.10
1.02
84.8
79.0
0.82
0.86
0.72
0.56
8
37.3
40.7
0
0
+
+
3.18
3.47
50.1
54.6
0.41
0.58
0.24
0.12
9
129.2
130.8
0
0
2.73
2.76
43.0
43.5
0.14
0.16
0.44
0.43
Levels for current density (mA cm2): 22 (+), 2.5 (), 12.25 (0).
Levels for flow/area ratio (mL min1 cm2): 2.3 (+), 0.57 (), 1.44 (0).
Table 2
is the normalised Faradaic efficiency.
Statistical analysis of the 22 factorial experiments. r represents the normalised rate of formate production and g
Response
r ()
g ()
*
Current density (j), X1
Flow/area ratio (Q/A), X2
Regression model: response = b0 + b1X1 + b2X2
Main effect
Standard error
P*
Main effect
Standard error
P*
Coefficients (with 95% confidence bounds)
b0
b1
b2
1.316
1.295
0.079
0.075
0.000
0.000
0.393
0.476
0.079
0.075
0.056
0.025
0.238 ± 0.204
0.211 ± 0.193
0.658 ± 0.204
0.647 ± 0.193
0.196 ± 0.204
0.238 ± 0.193
R2
0.9375
0.9443
Significant (a = 0.05) if P < 0.05.
Table 3
is the normalised Faradaic efficiency.
Statistical analysis of the 22 + centre point (0, 0) factorial experiments. r represents the normalised rate of formate production and g
Response
r ()
g ()
*
Current density (j), X1
Flow/area ratio (Q/A), X2
Curvature
Main effect
Standard error
P*
Main effect
Standard error
P*
Centre point
P*
1.316
1.295
0.069
0.057
0.000
0.000
0.393
0.476
0.069
0.057
0.036
0.009
0.757
0.048
0.004
0.719
Significant (a = 0.05) if P < 0.05.
The results of the statistical analyses considering the low and
high levels of the factors are shown in Table 2. It is noteworthy that
current density had a strong significant negative main effect on
formate Faradaic efficiency, which can be interpreted as that the
effect of raising j from the low (2.5 mA cm2) to the high level considered (22 mA cm2) was to lower the normalised Faradaic efficiency in 1.29 within the coded scale [1, +1]. The significant and
positive main effect of current density on the rate of production
means that increasing j from its low to its high level resulted in
an increase of 1.31 in the value of rate normalised in the range
[1, +1]. Regarding the main effects of Q/A ratio, they were positive
on both the Faradaic efficiency and rate, but Table 2 reveals that
these effects were of minor magnitude compared to those corresponding to current density.
The results of a factorial design can also be expressed in terms
of a linear regression model [44], as shown in Table 2. The b’s are
parameters of the regression model, and X1 and X2 are variables
that represent the factors (current density and Q/A ratio, respectively). The highest values of b parameters correspond to those that
multiply ‘‘current density’’ (Table 2), indicating the severe influence of j on both rate and Faradaic efficiency, as already mentioned. The more limited influence of Q/A on both responses is
also revealed by the lower values of the corresponding regression
parameters that were obtained (Table 2).
The results of the 22 factorial experiments were then studied
adding the centre point (0, 0) in order to test for curvature. The
values of ‘‘centre point’’ reported in Table 3 measure the difference
between the average of the centre-point response and the average
of the factorial points [44]. This value for Faradaic efficiency was
small and statistically insignificant, which indicates that the
is
importance of curvature for modelling the response function g
very limited. However, as shown in Table 3, a large and significant
curvature effect was obtained for rate, suggesting the existence of a
non-linear behaviour.
Please cite this article in press as: M. Alvarez-Guerra et al., Conversion of carbon dioxide into formate using a continuous electrochemical reduction process
in a lead cathode, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.099
5
M. Alvarez-Guerra et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Table 4
Regression models considering all the factorial experiments, i.e. including the 3 levels of both factors X1 (current density) and X2 (flow/area ratio). r is the normalised rate of
is the normalised Faradaic efficiency.
formate production and g
Response
r ()
g ()
Coefficients ± 95% confidence bounds for different terms
b0
X1 (b1)
X2 (b2)
X1X2 (b12)
X1X1 (b11)
X2X2 (b22)
0.504 ± 0.170
0.156 ± 0.146
0.692 ± 0.091
0.664 ± 0.080
0.188 ± 0.091
0.201 ± 0.080
0.100 ± 0.111
0.115 ± 0.098
0.573 ± 0.158
0.101 ± 0.138
0.172 ± 0.158
0.155 ± 0.138
Final regression models that only included coefficients of terms significant with 95% confidence:
r ()
r ¼ b0 þ b1 X 1 þ b2 X 2 þ b11 X 21
b0
b1
0.389 ± 0.157
g ()
0.692 ± 0.111
b2
b11
R2
0.188 ± 0.111
0.573 ± 0.193
0.9431
g ¼ b1 X 1 þ b2 X 2 þ b12 X 1 X 2 þ b22 X 22
b1
b2
b12
b22
R2
0.664 ± 0.088
0.201 ± 0.088
0.115 ± 0.107
0.243 ± 0.088
0.9548
Fig. 3. Relationship between the rate of formate production and Faradaic efficiency for formate at different flow/area ratios (Q/A) and current densities (j).
Therefore, regression models were finally calculated including
all the factorial experiments (i.e. considering the 3 levels of both
j and Q/A) (Table 4). The models were successively fitted in such
a way that those terms whose b parameters were not statistically
significant with 95% confidence were removed in the following
modelling equation, until obtaining models that only contained
parameters with 95% confidence bounds that did not include the
zero value. The results are summarised in Table 4. In the case of
Faradaic efficiency, the low values of the other b parameters compared to that of b1, which represents the linear effect of current
density, confirmed the predominant negative influence of this
term. It has been possible to neglect the influence of b0 in Faradaic
after including the influences of combined effects.
efficiency g
Finally, regarding the rate response function, and apart from reiterating the positive influence of b1, it should be especially emphasised that the curvature detected can be modelled by including
the high, negative and significant coefficient b11 that represents
quadratic effects associated with the current density.
3.2. Influence of the variables: current density and flow/electrode area
ratio
A detailed study of the influence of current density and catholyte
flow/electrode area ratio on the performance of the process was
carried out. Therefore, additional experiments were performed at
different current densities (j = 4.5, 6.5, 8.5, 10.5 and 14 mA cm2)
using a constant flow/area ratio of Q/A = 1.44 mL min1 cm2 (i.e.
the intermediate value of the range of Q/A considered in
the factorial study). Similarly, additional experiments were also
carried out at more different flow/area ratios (Q/A = 0.76, 1.00,
1.90 and 2.10 mL min1 cm2) using a constant current density of
12.25 mA cm2 (i.e. the intermediate value of the range of j considered in the factorial study). Fig. 3 summarises the results of rate of
formate formation and Faradaic efficiency at all the different Q/A
and j studied.
Regarding the influence of current density, it can clearly be seen
in Fig. 3 that, depending on the 3 levels of j initially considered, two
levels of formate rate production were observed. One rate level
corresponds to j = 2.5 mA cm2, where the rate was around values
of 104 mol m2 s1, and the other rate level was observed for the
medium and high levels of j. It is noteworthy that increasing j from
2.5 mA cm2 to the medium value j = 12.25 mA cm2 caused the
rate to increase more than 3 times, whatever the level of Q/A ratio
was studied. However, further increase of j from 12.25 to
22 mA cm2 did not result in higher rate of formate production,
since the values of rate achieved at j = 22 mA cm2 were very similar to those observed at j = 12.25 mA cm2, especially at the low Q/
A level.
Please cite this article in press as: M. Alvarez-Guerra et al., Conversion of carbon dioxide into formate using a continuous electrochemical reduction process
in a lead cathode, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.099
6
M. Alvarez-Guerra et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Focussing on all the results obtained at Q/A = 1.44 mL min1
cm , Fig. 3 shows that the rate of formate production grew, almost
in a lineal way, when j was progressively increased from 2.5 up to
10.5 mA cm2. However, working at current densities higher than
10.5 mA cm2, this proportional increase of rate is not longer observed, but the rate of product formation became stable around a
value of 3.4 104 ± 0.25 104 mol m2 s1. According to these
results, it can be concluded that values close to 10.5 mA cm2 can
be considered as limit current densities in the experimental system
studied for the electrochemical conversion of CO2 into formate, because it was found that working at higher j did not improve the rate
of formation of this product. The excess of electric charge supplied
above this limit was then used in competitive reactions like H2 formation instead of in the production of more amount of the desired
product, which resulted in similar rates of formate production but
with lower and lower Faradaic efficiencies as j is increased.
Fig. 3 also illustrates that, for each certain level of Q/A, an increase of current density resulted in noticeably lower Faradaic efficiencies. The analysis of this figure focussing on the evolution of the
points at different j and Q/A = 1.44 mL min1 cm2 (highlighted
with green colours in the graph) allows confirming the tendency
that working with higher values of current density would result
in lowering the Faradaic efficiency. Other previous works on the
electrochemical conversion of CO2 to formate also reported this
same trend of lower formate Faradaic efficiencies when increasing
the current density [25,28,38]. However, according to the results
obtained in our study, different stages in the evolution shown in
Fig. 3 could be distinguished. When j was gradually increased from
2.5 up to 8.5 mA cm2, the rate of formation was proportionally
increasing but the Faradaic efficiency was falling pronouncedly
(from 81.9% at j = 2.5 mA cm2 to 56.2% at j = 8.5 mA cm2). Experiments at j = 10.5 mA cm2 still yielded higher rate than at
j = 8.5 mA cm2, keeping similar efficiency of 57%. However, from
this value of j = 10.5 mA cm2, increasing j did not result in significative better rates of formation, but on the contrary, resulted in
remarkable fall of the Faradaic efficiency. Therefore, the interpretation of these results suggest that, in our electrochemical system for
CO2 conversion into formate, working with constant Q/A =
1.44 ml min1 cm2 and increasingly higher j up to 10.5 mA cm2,
the greater supply of charge (i.e. of electrons for the electro-reduction) allowed obtaining more moles of formate per m2 of electrode
and per second, but with a worse use of this electric charge supplied
in terms of Faradaic efficiency. It can also be interpreted that the
electric charge supplied exceeding this limit of 10.5 mA cm2
would not be used to obtain more formate, but it would be employed in other competitive secondary reactions like the formation
of H2 due to water hydrolysis. Therefore, working at j higher than
10.5 mA cm2 would only result in worse Faradaic efficiencies
observed.
The results shown in Fig. 3 also allow analysing the influence of
the catholyte flow/electrode area ratio. Fig. 3 reveals that when
experiments were carried out at a same current density (i.e. supplying the same amount of charge to the cathode per unit of electrode area), increasing Q/A from the low to the medium level
initially considered made it possible to obtain a greater rate of formate production (i.e. to form more amount of formate per m2 of
electrode and second) and, especially, to achieve much higher Faradaic efficiency, which meant that the percentage of this supplied
charge that was actually used for the desired product was much
bigger. The reason for this behaviour may be attributed to the fact
that increasing Q/A from 0.57 to 1.44 mL min1 cm2 meant that a
higher volume of catholyte was made available to the cathode per
unit of time and of electrode area, which involves improving the
supply of mass for the electro-reduction and hence reducing process limitations due to mass transport. However, it is also important to emphasise that continuing increasing this mass supply
2
(i.e. increasing Q/A from 1.44 to 2.3 mL min1 cm2) did not have
this beneficial effect on the performance of the process: the rate
of formate production is almost the same and only a slight increase
in the Faradaic efficiency is observed (from 81.9 to 86.8%) at the
low level of j.
Moreover, if we focus on the results at constant j = 12.25 mA
cm2, it can be noticed that increasing Q/A from 0.57 to
0.76 mL min1 cm2 allowed increasing not only the rate of formate
obtained in about 30%, but also the Faradaic efficiency in approximately 13 points in % (Fig. 3). Nevertheless, experiments performed
at higher Q/A led to very similar results in both rate and Faradaic
efficiency (about 3.4 104 mol m2 s1 and 54%, respectively),
as clearly reveals the accumulation of points around these values
in Fig. 3 for j = 12.25 mA cm2 and Q/A P 0.76 mL min1 cm2.
Therefore, these results indicate that in the studied electrochemical
system increasing the catholyte flow to overcome mass transport
limitations only had beneficial effects for the lowest flow/area
ratios; at Q/A higher than 0.76 mL min1 cm2, increasing the catholyte flow rate did not result in better performance of the process,
which could suggest that in such conditions the electrochemical
reaction of formate formation may be limited by other aspects, such
as the adsorption in the surface of the working electrode. In any
case, further research would be needed to ascertain the mechanisms that control the conversion of CO2 into formate in this electrochemical continuous process.
4. Conclusions
This work presents new experimental results on the performance of a continuous electro-reduction process for the valorisation of CO2 into formate in aqueous solutions under ambient
conditions, using a filter-press type cell with lead plate as cathode.
The concentrations of formate measured in the different experiments ranged from 12 to 131 mg L1. The results obtained confirmed general trends suggested in previous works, but they also
offered a systematic and detailed analysis of the influence of key
variables like the current density and electrolyte flow rate on the
performance of this type of continuous system. The influence of
current density was found to be especially noteworthy, emphasising the significant increase of the rate of formate production at
the expense of lowering the Faradaic efficiency observed when
increasing j up to a limit value of 10.5 mA cm2. Further increase
of j did not improve the rate of formate production and only
resulted in falls in the % of Faradaic efficiency, which could be
attributed to the occurrence of competitive reactions like H2 formation. Results also indicated the existence of mass transport limitations, which had been identified in the literature as one of the
challenges to be overcome in these types of electrochemical processes [15,21,23,38]. However, working at catholyte flow rates
higher than a third of the maximum value of the range studied
did not improve the performance, so this may suggest that in such
conditions the reaction to produce formate could be limited by
other aspects, like adsorption and desorption equilibria in the lead
cathode surface. In any case, further research is needed to ascertain
the detailed mechanisms that regulate the reduction of CO2 to formate in this continuous electrochemical process.
The results of this study can be useful as a reference for the
assessment of future efforts in the development of these continuous electro-reduction processes for CO2 valorisation, helping as
well to guide further research in order to overcome their current
limitations. In this sense, there may be potential for improvement
through research focussed on new electrocatalytic materials that
could be used as cathode to allow working at higher current densities without loss of Faradaic efficiency, i.e. minimising undesired
competitive reactions. We also believe that acting on the nature of
Please cite this article in press as: M. Alvarez-Guerra et al., Conversion of carbon dioxide into formate using a continuous electrochemical reduction process
in a lead cathode, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.099
M. Alvarez-Guerra et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
the solvent used as reaction medium may also lead to improvements in the performance of the process. Particularly, the use of
ionic liquids as reaction solvent for the electro-reduction, due to
their excellent electrochemical properties and the possibility of
designing them ‘‘task-specifically’’ with a high solubility of CO2,
will deserve special attention in future investigations.
Acknowledgements
This work was conducted under the framework of the Spanish
Ministry of Science and Technology Project CTM2006-00317 and
of the Spanish Ministry of Science and Innovation Project
ENE2010-14828. We are grateful to the undergraduate student
Laura Piñero for her help with some experiments.
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in a lead cathode, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.099