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Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry
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Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry D. Chery, V. Albin, A. Melendez-Ceballos, V. Lair*, M. Cassir Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France
article info
abstract
Article history:
The electrochemical transformation of CO2 into more valuable chemicals such as CO offers
Received 30 December 2015
great interest in the energetic field. In this paper, the feasibility of this valorisation process
Received in revised form
is investigated by cyclic voltammetry in different binary molten carbonates: Li2CO3e
22 April 2016
Na2CO3 (52:48 mol. %), Li2CO3eK2CO3 (62:38 mol. %) and Na2CO3eK2CO3 (56:44 mol. %) and
Accepted 9 June 2016
in a ternary eutectic: Li2CO3eNa2CO3eK2CO3 (43.5:31.5:25 mol. %). The eutectic nature, the
Available online xxx
oxoacidity, temperature and scanning conditions on the reduction and oxidation peaks is thoroughly analysed. The reduction peak related to CO2 reduction into CO is diffusion-
Keywords:
controlled and slow quasi-reversible; in specific experimental conditions, it is split into
CO2 valorisation
two peaks of similar intensity. Re-oxidation peaks are most probably due to dissolved and
CO
adsorbed CO. Based on our experimental investigation under varied conditions such as
Alkali molten carbonates
molten salt composition, oxoacidity, temperature of the system and thermodynamic pre-
Electrochemical reduction
dictions regarding carbon presence, two mechanistic schemes are proposed, involving
Cyclic voltammetry
gaseous, dissolved and adsorbed species.
Oxalates
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the last decade, the idea of valorising the greenhouse effect gas, carbon dioxide, by reducing it electrochemically has attracted many researchers. The main focus has been to produce useful chemicals and/or fuels, such as carbon, carbon monoxide, methane, ethylene, formate, and some alcohols (methanol, ethanol, and propanol). An interesting review has been presented by Jones et al. analysing the reduction process in aqueous media [1], but other room temperature media are cited as potentially interesting [2]. Progressively, since CO2 appears as a key molecule in the energetic field, high
temperature processes, requiring less expensive electrical energy (even if cheaper thermal energy increases), are emerging. Among the devices dedicated to high temperature transformation of carbon dioxide, mainly solid oxide electrolysers have been investigated, in particular in view of converting H2O and CO2 into syngas (H2 þ CO) by a co-electrolysis process [3]. Even though devices based on chloride-based molten salts (NaCleKCl, CaCl2eCaO) have been mentioned in the literature [4e6], molten carbonates are most probably the best melts for transforming CO2 into C or CO. Indeed, alkali molten carbonates enable the capture of CO2 molecule by its strong solubilisation and favour the electrolysis conditions; a review has been recently published describing the interest of
* Corresponding author. E-mail address: [email protected] (V. Lair). http://dx.doi.org/10.1016/j.ijhydene.2016.06.094 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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such media [7]. Molten carbonates are the well-known electrolytes of the molten carbonate fuel cell (MCFC) and are used also in other devices [8]. One of the main advantages of molten alkali carbonates is their capability in solubilising CO2 molecule. This solubility is favoured by CO2 chemical reactivity in carbonates, e.g. forming hypothetical species such as C2 O5 2 [9e11]. The electrolysis of carbon dioxide has been mainly focussed on the production of amorphous carbon, in particular nanopowders with high specific surface area suitable for lithium-ion batteries [6,12e16]. The conversion of CO2 into fuels such as CO has been evidenced by some authors [17e19]. Moreover, large-scale electrolysers for the production of carbon or carbon monoxide, with the energy provided by solar thermal energy, have been explored in the recent years [20e22]. All these studies are lacking of a systematic analysis of the electrolyte and of the electrolysis conditions. In previous studies, we have developed a systematic thermodynamic approach combined a first electrochemical experimental approach to compare the behaviour of different molten carbonates towards the CO2 reduction [18,19]. The reduction of CO2 into CO has been evidenced in Li2CO3eNa2CO3 and Li2CO3eK2CO3 eutectics under pure CO2 atmosphere, in the temperature range of 575e650 C; re-oxidation peaks have been detected but not fully analysed [18]. This work is dedicated to the optimisation of electrochemical transformation of CO2 into CO, by analysing thoroughly the reduction and re-oxidation of all the carbon species, varying a large number of experimental conditions (nature of the electrolyte, oxoacidity of the melt, temperature, previous conditioning of the electrodes and addition of species that might be part of the electrochemical reactions) and cycling voltammetry parameters. This approach that can be qualified of fundamental was undertaken in order to establish a complete mechanistic route related to CO2 reduction at a gold inert working electrode.
Materials and method Preparation of molten carbonates eutectics For binary eutectics, a mixture of carbonates was prepared by mixing up Li2CO3 and either K2CO3 or Na2CO3 at a molar ratio of 62:38 mol. % or 42:58 mol. % respectively, or Na2CO3 and K2CO3 at a molar ratio of 56:44 mol. %. For the ternary eutectic, a mixture of carbonates was prepared by mixing up Li2CO3, K2CO3 and Na2CO3 at a molar ratio of 43.5:31.5:25 mol. %. Then, under a pure CO2 atmosphere (100%, 1 bar), LieK, LieNa and LieNaeK carbonate eutectics were heated up to 650 C, whereas the binary eutectic NaeK was heated up to 750 C, for 12 h with a temperature rate of 3 C min1 in order to form the melt. Experiments were carried out at either 650 C, 600 C or 575 C to study the effect of temperature for the LieK, LieNaeK and LieNa mixtures. The experimental studies were done in an oxoacidic environment, under one bar of carbon dioxide and under less oxoacidic environments with argon (CO2-argon 20:80%), (CO2-argon 30:70%), (CO2-argon 50:50%) in volumic ratio. The carbonates Li2CO3, Na2CO3, K2CO3 were SigmaeAldrich reagents (purity > 98%). These powders were dried at 180 C for 12 h before mixing up them and were
introduced into an alumina crucible cell. The total weight of the mixture of carbonates was 70 g.
Electrochemical measurements The high-temperature electrochemical cell was a singlecompartment crucible of dimensions 70 50 mm2 contained in an alumina Al2O3 reactor of dimensions 250 60 mm2, hermetically sealed by a stainless steel cover with a Viton Oring. The whole electrochemical set-up was fully described in a previous paper [23]. Temperature was controlled and hold constant at the selected temperature by means of a calibrated chromel/alumel thermocouple. Cyclic voltammetry was carried out in a three-electrode set-up. The working electrode was a gold plate (2 cm2). The Au electrode surface was polished on SiC grinding papers of P2400, and P4000, with a rotor at 300 rpm rotational speed. The counter electrode was a 16 cm2 gold spiral wire. The reference electrode was a silver wire dipped into an anhydrous Ag2SO4 (101 mol kg1) eutectic melt in an alumina cylinder closed by a porous alumina membrane (porosity less than 1 mm). Measurements were performed using a PGSTAT 30 Autolab Ecochemie BV. The scanning rate was varied between 50 and 500 mV s1 at different temperatures (575, 600, 650 and 750 C).
Results and discussion According to the literature and our previous theoretical and electrochemical studies, the selective reduction of CO2 into CO is likely in LieK and LieNa molten carbonates in specific oxoacidic and low temperature conditions (650 C) [18,19]. However, the peaks obtained by cyclic voltammetry were not always well defined and easy to interpret. In particular, oxidation peaks were impossible to analyse (flat with low current density). This is a lack of objective elements to analyse thoroughly the redox mechanisms involved. Thus led us to increase the measurement temperature, for instance using NaeK melts. For other molten carbonates, we realised, when necessary, a pre-electrolysis treatment at a potential lower than the reduction potential of CO2 into CO for 360 s (<-1V vs. Agþ/Ag), which allows a better definition of the reduction and oxidation peaks. Binary eutectics, such as Li2CO3eNa2CO3 (52:48 mol. %), Li2CO3eK2CO3 (62:38 mol. %) and Na2CO3e K2CO3 (56:44 mol. %), and a ternary mixture, Li2CO3eNa2 CO3eK2CO3 (43.5:31.5:25 mol. %), were investigated.
Nature of the redox systems in LieNaeK and NaeK Fig. 1 shows cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 575 C under 100% CO2, P (CO2) ¼ 1 bar at a gold flag electrode after pre-electrolysis at 1.2 V vs. Agþ/Ag during 360 s for different scan rates. As already mentioned, the preelectrolysis treatment is needed at this temperature to activate the electrode. Thus, in order to better define the reduction process and detect the oxidation peaks. As previously described, one reduction peak, attributed to CO2 reduction is observed at about 1.1 V vs. Agþ/Ag (C1) and two oxidation peaks are observed at about 0.8 V vs. Agþ/Ag (A1) and 0.45 V vs. Agþ/Ag (A2, visible at only higher scan rates), respectively.
Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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4,8
A1
A2
3,2 1,6
j / mA cm
-2
-1.2 V (360s)
0,0 -1,6 -3,2 -1
-4,8
C2
-6,4 -8,0
50 mV s -1 100 mV s -1 200 mV s
Ei -1,6
-1,4
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
+
E / V vs. Ag / Ag
Fig. 1 e Cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 575 C under 100% CO2, P(CO2) ¼ 1 bar at a gold flag electrode (S ¼ 2 cm2) after pre-electrolysis at ¡1.2 V vs. Agþ/Ag during 360 s for three scan rates. Initial potential: Ei ¼ ¡1.50 V vs. Agþ/Ag. In all cases, the absolute value of the current density increases with increasing scan rate. The reduction process of CO2 is diffusion-controlled because C1 peak current density variation (jp) with scan rate (v) is nonlinear. This nonlinear behaviour indicates that the electroactive species is under a dissolved form. Moreover, neither the plot of jp as a function of √v nor the plot of peak potential (Ep) as a function of log v are linear. These non-linear behaviours tend to prove that the reaction process is quasi-rapid. Na2CO3eK2CO3 (56:44 mol. %) carbonate eutectic melts at a significantly higher temperature with respect to the three others (melting point of 710 C compared to 501 C, 488 C and 397 C for Li2CO3eNa2CO3 (52:48 mol. %), Li2CO3eK2CO3 (62:38 mol. %) and Li2CO3eNa2CO3eK2CO3 (43.5:31.5:25 mol. %), respectively). Fig. 2 shows the cyclic voltammograms
3
obtained in Na2CO3eK2CO3 at 750 C at a gold flag electrode (S ¼ 2 cm2) under CO2-Argon (30:70) for differents scan rates. At this higher temperature, peaks are better defined with no need of pre-electrolysis. Two reduction peaks (C1 and C2) and two oxidation peaks (A1 and A2) are observed whatever the scan rate. As in the case of the ternary eutectic, peak C2 at about 1.2 V vs. Agþ/Ag can be attributed to CO2 reduction and as jp is non-linear either with respect to √v or to v and jp is non-linear with respect to log v, the process is diffusioncontrolled and quasi-rapid. The oxidation peak (A2) is difficult to analyse because it is quite broad and of low current density. The second oxidation peak A1 varies linearly as a function of v (y ¼ 0.0084x þ 0.0006, with R2 ¼ 0.993), does not vary linearly with √v and its peak potential is constant whatever the value of log v. Moreover this peak has more or less a symmetrical shape and should be due to an adsorbed species, probably adsorbed CO (no trace of C at the gold electrode). Reduction peak C1, associated to oxidation peak A1 also varies linearly with v (y ¼ 0.0102x þ 0.0004, with R2 ¼ 0.997), but not with √v and very slightly with log v; it has also a quasisymmetrical shape and should be due to an adsorbed species. It might be a pre-adsorbed CO2 peak but it is difficult to conclude so far with respect to the nature of the adsorbed species; nevertheless, a mechanism will be suggested later in this paper. To conclude on the nature of the CO2 reduction process in the two studied eutectics and the two others previously analysed [18], a brief analysis follows. In all cases, one or two reduction peaks and one or two oxidation peaks are observed. The reduction of dissolved CO2 (A2) is quasi-rapid or slightly slow and diffusion-controlled. CO dissolved oxidation is likely (A2), but the peak is broad and more difficult to analyse. Some species are most probably adsorbed (peaks A1 and C1) but further analyses are required to fully understand the processes involved. Moreover, one can notice that all the peaks, especially oxidation ones are better defined at higher temperature.
Temperature effect 15
A1
10
5
j / mA cm
-2
A2 0
Ei -5
C1
-10
-15
C2 -1.4
-1.2
-1.0
-0.8
-0.6
-0.4
50 mV s 100 mV s 200 mV s 300 mV s 400 mV s 500 mV s
-0.2
0.0
+
E / V vs. Ag / Ag
Fig. 2 e Cyclic voltammograms in Na2CO3eK2CO3 at 750 C at a gold flag electrode (S ¼ 2 cm2) under CO2-Argon (30:70) for differents scan rates. Initial Potential: Ei ¼ 0.10 V vs. Agþ/Ag.
Temperature effect on cyclic voltammograms was analysed in Li2CO3eK2CO3 (62:38 mol%), as depicted in Fig. 3. As mentioned, peaks are better defined when a pre-electrolysis is imposed before the cyclic voltammetry (360 s electrolysis at 1.1 V vs. Agþ/Ag in this case), especially if the partial pressure of CO2 is less than 1 bar. Each curve presents a reduction peak (C2) at about 1.10 V vs. Agþ/Ag and two oxidation broad peaks (A1 and A2). When increasing the temperature, current density of the reduction peak increases and its potential is shifted towards more negative potentials as given in Table 1, where experimental potentials are compared to thermodynamic predictions developed in previous works [18,19]. Thermodynamic values are lower than experimental ones due to kinetics and to the difficulty in defining a comparable reference electrode in calculations, as discussed previously [19]. Nevertheless, the variation of potentials with temperature is consistent. The increase in the current density (absolute values) with the temperature indicates that the solubility of CO2 increases with the temperature. This surprising behaviour has already been described in molten salts and
Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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6
A1
4
A1 (c)
5 4
-1.1 V (360s)
(b)
3
A2
2 -2
0
j / mA cm
j / mA cm
-2
2
-2
(a)
A2
1
-1.2 V (360s)
0 -1 -2
-4
% CO -Ar
-3
-6
575 °C 600 °C 650 °C
C2
Ei -1.4
-1.2
-1.0
-0.8
-0.6
-0.4
Ei
-4 -5
-0.2
30 70 50-50 70-30
C2
-1.4
-1.2
-1.0
+
E / V vs. Ag / Ag
-0.8
-0.6
-0.4
-0.2
0.0
+
E / V vs. Ag / Ag
Fig. 3 e Influence of the temperature on cyclic voltammograms obtained in Li2CO3eK2CO3 at a gold flag electrode (S ¼ 2 cm2) under CO2-Argon (20:80) atmosphere. Scan rate of 100 mV s¡1. Pre-electrolysis at ¡1.1 V vs. Agþ/ Ag during 360 s electrolysis. Initial potential: Ei ¼ ¡1.35 V vs. Agþ/Ag. especially in molten carbonates [10,24]. This temperature effect is the same whatever the molten carbonate eutectic. Kinetics is enhanced and the higher is the temperature, the larger is the current density and peaks are better defined. Only one low intensity oxidation peak can be observed at 575 C. At higher temperatures, two oxidation peaks can be identified, probably corresponding to dissolved CO and adsorbed CO oxidations, respectively. The current densities (absolute values) increase with the temperature and potentials are shifted towards lower potentials, showing a favourable kinetic effect. The fact that A2 peak, corresponding to dissolved CO is only significantly observed at 650 C indicates that the solubility of CO increases with the temperature, as already observed for CO2 [10,24].
Fig. 4 e Cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 600 C at a gold flag electrode after a pre-electrolysis at ¡1.2 V vs. Agþ/Ag electrolysis during 360 s under different atmospheres. Initial potential: Ei ¼ ¡1.40 V vs. Agþ/Ag for (a) and Ei ¼ ¡1.35 V vs. Agþ/Ag for (b) and (c). n ¼ 100 mV s¡1.
(30:70), (50:50), (70:30) and (100:0), in volumic ratio. Only some major results are presented here. Fig. 4 presents cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 600 C, after a 360 s pre-electrolysis under different CO2 partial pressures. As previously described, a major reduction peak (C2) is observed as well as two oxidation ones (A1 and A2). It is worth noting that all these peaks increase with CO2 partial pressure, e.g. A2 peak, in general broad and poorly defined, increases and is better defined when increasing P (CO2). In brief, rising P (CO2) increases the amount of dissolved CO2, favouring its reduction and, consequently, the amount of CO dissolved. The same observation can be done for most of the alkali molten carbonates, except for LieNa melt (Fig. 5) for which an additional reduction peak is observed at a more negative potential than
Oxoacidity effect 2.5
A1
2.0
b a
1.5 1.0
-2
A2
-1.1 V (360s)
0.5
j / mA cm
As seen previously, C2 reduction peak attributed to CO2 reduction is dependent on the melt oxoacidity, which varies with the temperature, the nature and composition of the molten carbonates [18]. Thus, the influence of the partial pressure of CO2 was investigated in different eutectics, at different temperatures. The total pressure was fixed at 1 bar and P (CO2) was varied by mixing two gases (CO2 and Ar) in order to avoid parasitic reactions due to air or oxygen, for instance. Five compositions of (CO2:Ar) were studied: (20:80),
0.0 -0.5 -1.0 -1.5
C2
-2.0 -2.5
Table 1 e Peak potential values of the reduction peak (C2) in LieK vs. the temperature, from Fig. 3 and from Ref. [19]. T Peak potential observed Theoretical potential peak ( C) for P (CO2) ¼ 0.2 bar for P (CO2) ¼ 0.2 bar and (V vs. Agþ/Ag) P (CO) ¼ 0.1 bar (V vs. O2/Li2O) 575 600 650
1.06 1.08 1.11
0.61 0.62 0.64
-3.0
Ei
-3.5 -1.4
% CO -Ar
C3 -1.2
20-80 30-70
-1.0
-0.8
-0.6
-0.4
-0.2
+
E / V vs. Ag / Ag
Fig. 5 e Cyclic voltammograms in Li2CO3eNa2CO3 at 600 C at a gold flag electrode (S ¼ 2 cm2) after ¡1.1 V vs. Agþ/Ag during 360 s under different atmospheres. Initial potential: Ei ¼ ¡1.35 V vs. Agþ/Ag. n ¼ 100 mV s¡1.
Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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C2 in all the experimental conditions, with P (CO2) < 0.7 bar and a pre-electrolysis before the potential scan. It is worth noting that for higher pressure of CO2, only one reduction peak C2 is observed with or without pre-electrolysis. Fig. 5 presents cyclic voltammograms in Li2CO3eNa2CO3 at 600 C at a gold flag electrode (S ¼ 2 cm2) after 360 s preelectrolysis at 1.1 V vs. Agþ/Ag under different CO2 partial pressures. The second reduction peak (at about 1.2 V vs. Agþ/ Ag) seems to be more affected by the increase in CO2 partial pressure. Up to now, the reduction of CO2 was supposed to yield CO formation with a 2 electrons process [11]. The existence of these two peaks suggests that the reduction process could be a two-step process, with a non-stable intermediate. The height of the peak being similar, each step should involve the same electron number. Different groups have already suggested different intermediates in the CO2 reduction process such as C2 O5 2 , CO2 or C2 O4 2 [9e11].
Oxalates addition: suggested mechanisms In our experimental conditions, the reduction potential is not negative enough to obtain carbon formation onto the electrode, contrarily to other studies privileging this process [6,13e16]. Moreover, no trace of carbon was observed on the gold electrode. For this reason, a two-step reduction involving a non-stable intermediate is likely. Among the intermediates mentioned in the literature, oxalates (C2 O4 2 ) are the unique species stable enough in solution and easy to add in the molten mixture. Fig. 6 depicts oxalate addition effect in Li2CO3eNa2CO3eK2CO3 eutectic at 600 C under an oxoacidic environment. Curves (b) and (c) of Fig. 6 present the cyclic voltammograms obtained for different pre-electrolysis conditions, without oxalates addition. For both negative potential
A2
10
A1
(a)
(b) (c)
-2
j / mA cm
C2 Ei
Ei
(a) with oxalates (b) without oxalates (c) without oxalates P (CO ) = 1 bar; 100 mV s
C3 -20
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
2 CO2 þ 2 e ¼ 2 CO 2 Thus, CO2 follows:
(1a)
anion is chemically transformed into CO as
2 2 CO 2 /CO þ CO3
(1b)
The overall reaction is: 2 CO2 þ 2 e /CO þ CO2 3
(1c)
Then, regarding the two oxidation peaks, A1 corresponds to a two-electron process. Indeed, after oxalates addition, the oxidation peak A1 is enhanced and we suggest that the reoxidation of oxalates involves two electrons and produces CO2, according to: (1d)
It should be noted that A2 oxidation peak is supposed to be the reaction (1c) in the opposite way. It is worth noting that both CO2 and CO seem to be stabilised in the molten carbonates under the solvated species form as described in reactions (1e) and (1f). The oxalates (C2 O4 2 ) and dicarbonate ions (C2 O5 2 ) are, respectively, CO and CO2 solvated species, according to the following equilibria:
0
-10
domains, only one reduction peak is observed (C2) at about 0.95 V vs. Agþ/Ag. Curve (a) presents a cyclic voltammogram obtained after the addition of 10 mg of oxalates. The C2 current density reduction peak is bigger with respect to the other curves (b) and (c) (about 3 times). Moreover, a second reduction broad peak (C3) appears at about 1.2 V vs. Agþ/Ag, with a height comparable to C2. This indicates that the same numbers of electrons are involved. It is worth noting that the two anodic peaks (A2 and A1) are better defined with the addition of oxalates compared to all the experiments realised until now in different eutectics and experimental conditions. Furthermore, peak A2 is higher than peak A1, whereas the contrary has always been observed so far. According to the balance of our results, including in particular the effect of oxalate addition, and taking into account the number of reduction peaks observed, two hypothetic mechanisms are suggested depending on the number of reductive peaks observed (C2 or C2 and C3). If only one reduction peak (C2) is observed, the reduction process involves most probably 2 electrons as seen in reaction (1a). CO2 is most probably first adsorbed before its reduction into an intermediate ion CO2 which yields CO (1b) via a chemical reaction with the carbonates [9e11]. The two corresponding reactions and the overall reaction are listed below: First the reduction of CO2 into CO2 involves two electrons:
CO þ CO2 3 ) 2CO2 þ 2 e !
-1.1 V(360s) -1.2 V(360s)
5
0.0
+
E / V vs. Ag / Ag
Fig. 6 e Cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 600 C at a gold flag electrode (S ¼ 2 cm2) under 100% CO2 at 1 bar, (a) after addition of 10 mg of oxalates, initial potential Ei ¼ ¡0.90 V vs. Agþ/Ag, and without oxalates (b) after preelectrolysis at ¡1.1 V vs. Agþ/Ag (360 s), initial potential Ei ¼ ¡1.50 V vs. Agþ/Ag, (c) after pre-electrolysis at ¡1.2 V vs. Agþ/Ag (360 s), initial potential Ei ¼ ¡1.35 V vs. Agþ/Ag.
2 CO þ CO2 3 ¼ C2 O4
(1e)
2 CO2 þ CO2 3 ¼ C 2 O5
(1f)
Fig. 7 is a scheme of this hypothetical mechanism where only one peak C2 is observed. If two reduction peaks are observed (C2 and C3). Then a two-step reduction process is most likely. The first step is described by reaction (1a), but contrarily to the first suggested
Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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Nevertheless, identification of such species requires further analyses either in situ or ex situ such as high temperature NMR or Raman for solvated species and gas chromatography and/or thermogravimetric measurements for gases.
Conclusions
Fig. 7 e Hypothetical mechanism scheme of the CO2 electrochemical reduction in molten alkali at an inert gold flag electrode for one reduction peak (C2). mechanism, the intermediate species CO2 is reduced to form CO2 2 in reaction (1g) before reacting with carbonates to form CO, reaction (1h). These reactions are suggested when two reduction peaks are observed and are related to the presence of peak C3. 2 2 CO 2 þ 2 e ) 2 CO2 !
(1g)
CO2 2 anion then reacts with carbonates and is transformed into CO: 2 CO2 2 þ CO2 /CO þ CO3
(1h)
Concerning the oxidation processes, the reactions are similar to those described above. A complete scheme of the reduction process involving the two reduction peaks C2 and C3 can be proposed, as depicted in Fig. 8. All these reactions are suggested regarding our electrochemical results, thermodynamic considerations and intermediate species suggested in the literature [9e11].
The reduction of CO2 into CO is feasible and can be optimised in molten alkali carbonates according to experimental conditions. In most of the molten carbonate eutectics, a diffusioncontrolled reduction is observed and the process is mostly quasi-rapid at a gold working electrode. The reduction depends on the molten carbonates used, on the temperature and on the melt oxoacidity. Moreover, one or two reduction peaks were observed depending on the experimental conditions. Literature points out that different intermediates species can be formed in the reduction of CO2 into CO; thus, we have added oxalates into the carbonate melt to investigate more concretely the electrochemical process. Based on our electrochemical analysis, thermodynamic predictions and the literature, we have proposed two reduction mechanisms and some oxidation hypotheses. Thus, two mechanistic schemes, involving the species in all their forms, gaseous, dissolved and adsorbed, are presented. However, it is worth pursuing this work by analysing the outlet gases by gas chromatography and the intermediate species by in situ NMR or RAMAN spectroscopy. Measurement of CO2 and CO solubility can also be considered as a key parameter. Moreover, other electrochemical techniques should be used, as chronopotentiometry, in particular for detecting the presence of adsorbed species and for having a deeper insight on the CO2 reduction process.
Acknowledgements This work was partially supported by the French program PLANEX ANR-11-EQPX-0-01 and by the CNRS exploratory project “Emergence CO2” in 2014.
references
Fig. 8 e Hypothetical mechanism scheme of the CO2 electrochemical reduction in molten alkali at an inert gold flag electrode for two reduction peaks (C2 and C3).
[1] Jones JP, Surya Prakash GK, Olah GA. Electrochemical CO2 reduction: recent advances and current trends. Irs J Chem 2014;54:1451e66. [2] Rees NV, Compton RG. Electrochemical CO2 sequestration in ionic liquids; a perspective (mini review). Energy Environ Sci 2011;4:403e8. [3] Bidraw F, Kim G, Corre G, Irvine JTS, Vohs JM, Gorte RJ. Efficient reduction of CO2 in a solid oxide electrolyser. Electrochem Solid-State Lett 2008;11:B167e70. [4] Novoselova IA, Oliynyk NF, Voronina AB, Volkov SV. Electrolytic generation of nano-scale carbon phases with framework structures in molten salts on metal cathodes. Z Naturforsch A Phys Sci 2008;63:467e74. [5] Otake K, Kinoshita H, Kikuchi T, Suzuki RO. CO2 gas decomposition to carbon by electro-reduction in molten salts. Electrochim Act 2013;100:293e9.
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[6] Ijije HV, Lawrence RC, Chen GZ. Carbon electrodeposition in molten salts: electrode reactions and applications. RSC Adv 2014;4:35808e17. [7] Chery D, Lair V, Cassir M. Overview on CO2 valorization: challenge of molten carbonates. Front Energy Res 2015;3:43. [8] Cassir M, McPhail S, Moreno A. Strategies and new developments in the field of molten carbonates and hightemperature fuel cells in the carbon cycle. Int J Hydrogen Energy 2012;37:19345e50. [9] Claes P, Thirion B, Glibert J. Solubility of CO2 in the molten carbon-ate Na2CO3eK2CO3 (42 mol%) eutectic mixture at 800 C. Electrochim Acta 1996;41:141e6. [10] Claes P, Moyaux D, Peeter D. Solubility and solvation of carbon dioxide in the molten Li2CO3/Na2CO3/K2CO3 (43.5:31.5:25.0 mol-%) eutectic mixture at 973 K I. Experimental part. Eur J Inorg Chem 1999;4:583e8. [11] Peelen WH, Hemmes K, de Wit JH. CO2 reduction in molten 62/38 mole% Li/K carbonate mixture. Electrochim Acta 1997;43:763e9. [12] Groult H, Kaplan B, Komaba S, Kumagai N, Gupta V, Nakajima T. Simon B ? Lithium insertion into carbonaceous anode materials prepared by electrolysis of molten LieKeNa carbonates. J Electrochem Soc 2003;150:G67e75. [13] Le Van K, Groult H, Lantelme Dubois M, Avignant D, Tressaud A, Komaba S, et al. Electrochemical formation of carbon nano-powders with various porosities in molten alkali carbonates. Electrochim Acta 2009;54:4566e73. [14] Yin H, Mao X, Tang D, Xiao W, Xing L, Zhu H, et al. Capture and electrochemical conversion of CO2 to value-added carbon and oxygen by molten salt electrolysis. Energy & Environ Sci 2013;6:1538e45. [15] Tang D, Yin H, Mao X, Xiao W, Wang DH. Effects of applied voltage and temperature on the electrochemical production
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of carbon powders from CO2 in molten salt with an inert anode. Electrochim Acta 2013;114:567e73. Ijije H, Chen G. Electrochemical manufacturing of nanocarbons from carbon dioxide in molten alkali metal carbonate salts: roles of alkali metal cations. Adv Manuf 2015:1e10. Kaplan V, Wachtel E, Gartsman K, Feldman Y, Lubomirsky I. Conversion of CO2 to CO by electrolysis of molten lithium carbonate. J Electrochem Soc 2010;157:B552e6. Chery D, Albin V, Lair V, Cassir M. Thermodynamic and experimental approach of electrochemical reduction of CO2 in molten carbonates. Int J Hydrogen Energy 2014;39:12330e9. Chery D, Lair V, Cassir M. CO2 electrochemical reduction into CO or C in molten carbonates: a thermodynamic point of view. Electrochim Acta 2015;160:74e81. Licht S. STEP (Solar Thermal Electrochemical Photo) generation of energetic molecules: a solar chemical process to end anthropogenic global warming. J Phys Chem C 2009;113:16283e92. Licht S. Efficient solar-driven synthesis, carbon capture, and desalinization, STEP: solar thermal electrochemical production of fuels, metals, bleach. Adv Mater 2011;23:5592e612. Licht S, Cui B, Wang B. STEP carbon capture e the barium advantage. J CO2 Util 2013;2:58e63. Lair V, Albin V, Ringuede A, Cassir M. Theoretical predictions vs. experimental measurements of the electrical conductivity of molten Li2CO3-K2CO3 modified by additives. Int J Hydrogen Energy 2012;37:1957e64. Kanai Y, Fukunaga K, Terasaka K, Fujioka S. Mass transfer in moltensalt and suspended molten salt in bubble column. Chem Eng Sci 2013;100:153e9.
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Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry D. Chery, V. Albin, A. Melendez-Ceballos, V. Lair*, M. Cassir Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France
article info
abstract
Article history:
The electrochemical transformation of CO2 into more valuable chemicals such as CO offers
Received 30 December 2015
great interest in the energetic field. In this paper, the feasibility of this valorisation process
Received in revised form
is investigated by cyclic voltammetry in different binary molten carbonates: Li2CO3e
22 April 2016
Na2CO3 (52:48 mol. %), Li2CO3eK2CO3 (62:38 mol. %) and Na2CO3eK2CO3 (56:44 mol. %) and
Accepted 9 June 2016
in a ternary eutectic: Li2CO3eNa2CO3eK2CO3 (43.5:31.5:25 mol. %). The eutectic nature, the
Available online xxx
oxoacidity, temperature and scanning conditions on the reduction and oxidation peaks is thoroughly analysed. The reduction peak related to CO2 reduction into CO is diffusion-
Keywords:
controlled and slow quasi-reversible; in specific experimental conditions, it is split into
CO2 valorisation
two peaks of similar intensity. Re-oxidation peaks are most probably due to dissolved and
CO
adsorbed CO. Based on our experimental investigation under varied conditions such as
Alkali molten carbonates
molten salt composition, oxoacidity, temperature of the system and thermodynamic pre-
Electrochemical reduction
dictions regarding carbon presence, two mechanistic schemes are proposed, involving
Cyclic voltammetry
gaseous, dissolved and adsorbed species.
Oxalates
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the last decade, the idea of valorising the greenhouse effect gas, carbon dioxide, by reducing it electrochemically has attracted many researchers. The main focus has been to produce useful chemicals and/or fuels, such as carbon, carbon monoxide, methane, ethylene, formate, and some alcohols (methanol, ethanol, and propanol). An interesting review has been presented by Jones et al. analysing the reduction process in aqueous media [1], but other room temperature media are cited as potentially interesting [2]. Progressively, since CO2 appears as a key molecule in the energetic field, high
temperature processes, requiring less expensive electrical energy (even if cheaper thermal energy increases), are emerging. Among the devices dedicated to high temperature transformation of carbon dioxide, mainly solid oxide electrolysers have been investigated, in particular in view of converting H2O and CO2 into syngas (H2 þ CO) by a co-electrolysis process [3]. Even though devices based on chloride-based molten salts (NaCleKCl, CaCl2eCaO) have been mentioned in the literature [4e6], molten carbonates are most probably the best melts for transforming CO2 into C or CO. Indeed, alkali molten carbonates enable the capture of CO2 molecule by its strong solubilisation and favour the electrolysis conditions; a review has been recently published describing the interest of
* Corresponding author. E-mail address: [email protected] (V. Lair). http://dx.doi.org/10.1016/j.ijhydene.2016.06.094 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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such media [7]. Molten carbonates are the well-known electrolytes of the molten carbonate fuel cell (MCFC) and are used also in other devices [8]. One of the main advantages of molten alkali carbonates is their capability in solubilising CO2 molecule. This solubility is favoured by CO2 chemical reactivity in carbonates, e.g. forming hypothetical species such as C2 O5 2 [9e11]. The electrolysis of carbon dioxide has been mainly focussed on the production of amorphous carbon, in particular nanopowders with high specific surface area suitable for lithium-ion batteries [6,12e16]. The conversion of CO2 into fuels such as CO has been evidenced by some authors [17e19]. Moreover, large-scale electrolysers for the production of carbon or carbon monoxide, with the energy provided by solar thermal energy, have been explored in the recent years [20e22]. All these studies are lacking of a systematic analysis of the electrolyte and of the electrolysis conditions. In previous studies, we have developed a systematic thermodynamic approach combined a first electrochemical experimental approach to compare the behaviour of different molten carbonates towards the CO2 reduction [18,19]. The reduction of CO2 into CO has been evidenced in Li2CO3eNa2CO3 and Li2CO3eK2CO3 eutectics under pure CO2 atmosphere, in the temperature range of 575e650 C; re-oxidation peaks have been detected but not fully analysed [18]. This work is dedicated to the optimisation of electrochemical transformation of CO2 into CO, by analysing thoroughly the reduction and re-oxidation of all the carbon species, varying a large number of experimental conditions (nature of the electrolyte, oxoacidity of the melt, temperature, previous conditioning of the electrodes and addition of species that might be part of the electrochemical reactions) and cycling voltammetry parameters. This approach that can be qualified of fundamental was undertaken in order to establish a complete mechanistic route related to CO2 reduction at a gold inert working electrode.
Materials and method Preparation of molten carbonates eutectics For binary eutectics, a mixture of carbonates was prepared by mixing up Li2CO3 and either K2CO3 or Na2CO3 at a molar ratio of 62:38 mol. % or 42:58 mol. % respectively, or Na2CO3 and K2CO3 at a molar ratio of 56:44 mol. %. For the ternary eutectic, a mixture of carbonates was prepared by mixing up Li2CO3, K2CO3 and Na2CO3 at a molar ratio of 43.5:31.5:25 mol. %. Then, under a pure CO2 atmosphere (100%, 1 bar), LieK, LieNa and LieNaeK carbonate eutectics were heated up to 650 C, whereas the binary eutectic NaeK was heated up to 750 C, for 12 h with a temperature rate of 3 C min1 in order to form the melt. Experiments were carried out at either 650 C, 600 C or 575 C to study the effect of temperature for the LieK, LieNaeK and LieNa mixtures. The experimental studies were done in an oxoacidic environment, under one bar of carbon dioxide and under less oxoacidic environments with argon (CO2-argon 20:80%), (CO2-argon 30:70%), (CO2-argon 50:50%) in volumic ratio. The carbonates Li2CO3, Na2CO3, K2CO3 were SigmaeAldrich reagents (purity > 98%). These powders were dried at 180 C for 12 h before mixing up them and were
introduced into an alumina crucible cell. The total weight of the mixture of carbonates was 70 g.
Electrochemical measurements The high-temperature electrochemical cell was a singlecompartment crucible of dimensions 70 50 mm2 contained in an alumina Al2O3 reactor of dimensions 250 60 mm2, hermetically sealed by a stainless steel cover with a Viton Oring. The whole electrochemical set-up was fully described in a previous paper [23]. Temperature was controlled and hold constant at the selected temperature by means of a calibrated chromel/alumel thermocouple. Cyclic voltammetry was carried out in a three-electrode set-up. The working electrode was a gold plate (2 cm2). The Au electrode surface was polished on SiC grinding papers of P2400, and P4000, with a rotor at 300 rpm rotational speed. The counter electrode was a 16 cm2 gold spiral wire. The reference electrode was a silver wire dipped into an anhydrous Ag2SO4 (101 mol kg1) eutectic melt in an alumina cylinder closed by a porous alumina membrane (porosity less than 1 mm). Measurements were performed using a PGSTAT 30 Autolab Ecochemie BV. The scanning rate was varied between 50 and 500 mV s1 at different temperatures (575, 600, 650 and 750 C).
Results and discussion According to the literature and our previous theoretical and electrochemical studies, the selective reduction of CO2 into CO is likely in LieK and LieNa molten carbonates in specific oxoacidic and low temperature conditions (650 C) [18,19]. However, the peaks obtained by cyclic voltammetry were not always well defined and easy to interpret. In particular, oxidation peaks were impossible to analyse (flat with low current density). This is a lack of objective elements to analyse thoroughly the redox mechanisms involved. Thus led us to increase the measurement temperature, for instance using NaeK melts. For other molten carbonates, we realised, when necessary, a pre-electrolysis treatment at a potential lower than the reduction potential of CO2 into CO for 360 s (<-1V vs. Agþ/Ag), which allows a better definition of the reduction and oxidation peaks. Binary eutectics, such as Li2CO3eNa2CO3 (52:48 mol. %), Li2CO3eK2CO3 (62:38 mol. %) and Na2CO3e K2CO3 (56:44 mol. %), and a ternary mixture, Li2CO3eNa2 CO3eK2CO3 (43.5:31.5:25 mol. %), were investigated.
Nature of the redox systems in LieNaeK and NaeK Fig. 1 shows cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 575 C under 100% CO2, P (CO2) ¼ 1 bar at a gold flag electrode after pre-electrolysis at 1.2 V vs. Agþ/Ag during 360 s for different scan rates. As already mentioned, the preelectrolysis treatment is needed at this temperature to activate the electrode. Thus, in order to better define the reduction process and detect the oxidation peaks. As previously described, one reduction peak, attributed to CO2 reduction is observed at about 1.1 V vs. Agþ/Ag (C1) and two oxidation peaks are observed at about 0.8 V vs. Agþ/Ag (A1) and 0.45 V vs. Agþ/Ag (A2, visible at only higher scan rates), respectively.
Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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4,8
A1
A2
3,2 1,6
j / mA cm
-2
-1.2 V (360s)
0,0 -1,6 -3,2 -1
-4,8
C2
-6,4 -8,0
50 mV s -1 100 mV s -1 200 mV s
Ei -1,6
-1,4
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
+
E / V vs. Ag / Ag
Fig. 1 e Cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 575 C under 100% CO2, P(CO2) ¼ 1 bar at a gold flag electrode (S ¼ 2 cm2) after pre-electrolysis at ¡1.2 V vs. Agþ/Ag during 360 s for three scan rates. Initial potential: Ei ¼ ¡1.50 V vs. Agþ/Ag. In all cases, the absolute value of the current density increases with increasing scan rate. The reduction process of CO2 is diffusion-controlled because C1 peak current density variation (jp) with scan rate (v) is nonlinear. This nonlinear behaviour indicates that the electroactive species is under a dissolved form. Moreover, neither the plot of jp as a function of √v nor the plot of peak potential (Ep) as a function of log v are linear. These non-linear behaviours tend to prove that the reaction process is quasi-rapid. Na2CO3eK2CO3 (56:44 mol. %) carbonate eutectic melts at a significantly higher temperature with respect to the three others (melting point of 710 C compared to 501 C, 488 C and 397 C for Li2CO3eNa2CO3 (52:48 mol. %), Li2CO3eK2CO3 (62:38 mol. %) and Li2CO3eNa2CO3eK2CO3 (43.5:31.5:25 mol. %), respectively). Fig. 2 shows the cyclic voltammograms
3
obtained in Na2CO3eK2CO3 at 750 C at a gold flag electrode (S ¼ 2 cm2) under CO2-Argon (30:70) for differents scan rates. At this higher temperature, peaks are better defined with no need of pre-electrolysis. Two reduction peaks (C1 and C2) and two oxidation peaks (A1 and A2) are observed whatever the scan rate. As in the case of the ternary eutectic, peak C2 at about 1.2 V vs. Agþ/Ag can be attributed to CO2 reduction and as jp is non-linear either with respect to √v or to v and jp is non-linear with respect to log v, the process is diffusioncontrolled and quasi-rapid. The oxidation peak (A2) is difficult to analyse because it is quite broad and of low current density. The second oxidation peak A1 varies linearly as a function of v (y ¼ 0.0084x þ 0.0006, with R2 ¼ 0.993), does not vary linearly with √v and its peak potential is constant whatever the value of log v. Moreover this peak has more or less a symmetrical shape and should be due to an adsorbed species, probably adsorbed CO (no trace of C at the gold electrode). Reduction peak C1, associated to oxidation peak A1 also varies linearly with v (y ¼ 0.0102x þ 0.0004, with R2 ¼ 0.997), but not with √v and very slightly with log v; it has also a quasisymmetrical shape and should be due to an adsorbed species. It might be a pre-adsorbed CO2 peak but it is difficult to conclude so far with respect to the nature of the adsorbed species; nevertheless, a mechanism will be suggested later in this paper. To conclude on the nature of the CO2 reduction process in the two studied eutectics and the two others previously analysed [18], a brief analysis follows. In all cases, one or two reduction peaks and one or two oxidation peaks are observed. The reduction of dissolved CO2 (A2) is quasi-rapid or slightly slow and diffusion-controlled. CO dissolved oxidation is likely (A2), but the peak is broad and more difficult to analyse. Some species are most probably adsorbed (peaks A1 and C1) but further analyses are required to fully understand the processes involved. Moreover, one can notice that all the peaks, especially oxidation ones are better defined at higher temperature.
Temperature effect 15
A1
10
5
j / mA cm
-2
A2 0
Ei -5
C1
-10
-15
C2 -1.4
-1.2
-1.0
-0.8
-0.6
-0.4
50 mV s 100 mV s 200 mV s 300 mV s 400 mV s 500 mV s
-0.2
0.0
+
E / V vs. Ag / Ag
Fig. 2 e Cyclic voltammograms in Na2CO3eK2CO3 at 750 C at a gold flag electrode (S ¼ 2 cm2) under CO2-Argon (30:70) for differents scan rates. Initial Potential: Ei ¼ 0.10 V vs. Agþ/Ag.
Temperature effect on cyclic voltammograms was analysed in Li2CO3eK2CO3 (62:38 mol%), as depicted in Fig. 3. As mentioned, peaks are better defined when a pre-electrolysis is imposed before the cyclic voltammetry (360 s electrolysis at 1.1 V vs. Agþ/Ag in this case), especially if the partial pressure of CO2 is less than 1 bar. Each curve presents a reduction peak (C2) at about 1.10 V vs. Agþ/Ag and two oxidation broad peaks (A1 and A2). When increasing the temperature, current density of the reduction peak increases and its potential is shifted towards more negative potentials as given in Table 1, where experimental potentials are compared to thermodynamic predictions developed in previous works [18,19]. Thermodynamic values are lower than experimental ones due to kinetics and to the difficulty in defining a comparable reference electrode in calculations, as discussed previously [19]. Nevertheless, the variation of potentials with temperature is consistent. The increase in the current density (absolute values) with the temperature indicates that the solubility of CO2 increases with the temperature. This surprising behaviour has already been described in molten salts and
Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
4
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6
A1
4
A1 (c)
5 4
-1.1 V (360s)
(b)
3
A2
2 -2
0
j / mA cm
j / mA cm
-2
2
-2
(a)
A2
1
-1.2 V (360s)
0 -1 -2
-4
% CO -Ar
-3
-6
575 °C 600 °C 650 °C
C2
Ei -1.4
-1.2
-1.0
-0.8
-0.6
-0.4
Ei
-4 -5
-0.2
30 70 50-50 70-30
C2
-1.4
-1.2
-1.0
+
E / V vs. Ag / Ag
-0.8
-0.6
-0.4
-0.2
0.0
+
E / V vs. Ag / Ag
Fig. 3 e Influence of the temperature on cyclic voltammograms obtained in Li2CO3eK2CO3 at a gold flag electrode (S ¼ 2 cm2) under CO2-Argon (20:80) atmosphere. Scan rate of 100 mV s¡1. Pre-electrolysis at ¡1.1 V vs. Agþ/ Ag during 360 s electrolysis. Initial potential: Ei ¼ ¡1.35 V vs. Agþ/Ag. especially in molten carbonates [10,24]. This temperature effect is the same whatever the molten carbonate eutectic. Kinetics is enhanced and the higher is the temperature, the larger is the current density and peaks are better defined. Only one low intensity oxidation peak can be observed at 575 C. At higher temperatures, two oxidation peaks can be identified, probably corresponding to dissolved CO and adsorbed CO oxidations, respectively. The current densities (absolute values) increase with the temperature and potentials are shifted towards lower potentials, showing a favourable kinetic effect. The fact that A2 peak, corresponding to dissolved CO is only significantly observed at 650 C indicates that the solubility of CO increases with the temperature, as already observed for CO2 [10,24].
Fig. 4 e Cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 600 C at a gold flag electrode after a pre-electrolysis at ¡1.2 V vs. Agþ/Ag electrolysis during 360 s under different atmospheres. Initial potential: Ei ¼ ¡1.40 V vs. Agþ/Ag for (a) and Ei ¼ ¡1.35 V vs. Agþ/Ag for (b) and (c). n ¼ 100 mV s¡1.
(30:70), (50:50), (70:30) and (100:0), in volumic ratio. Only some major results are presented here. Fig. 4 presents cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 600 C, after a 360 s pre-electrolysis under different CO2 partial pressures. As previously described, a major reduction peak (C2) is observed as well as two oxidation ones (A1 and A2). It is worth noting that all these peaks increase with CO2 partial pressure, e.g. A2 peak, in general broad and poorly defined, increases and is better defined when increasing P (CO2). In brief, rising P (CO2) increases the amount of dissolved CO2, favouring its reduction and, consequently, the amount of CO dissolved. The same observation can be done for most of the alkali molten carbonates, except for LieNa melt (Fig. 5) for which an additional reduction peak is observed at a more negative potential than
Oxoacidity effect 2.5
A1
2.0
b a
1.5 1.0
-2
A2
-1.1 V (360s)
0.5
j / mA cm
As seen previously, C2 reduction peak attributed to CO2 reduction is dependent on the melt oxoacidity, which varies with the temperature, the nature and composition of the molten carbonates [18]. Thus, the influence of the partial pressure of CO2 was investigated in different eutectics, at different temperatures. The total pressure was fixed at 1 bar and P (CO2) was varied by mixing two gases (CO2 and Ar) in order to avoid parasitic reactions due to air or oxygen, for instance. Five compositions of (CO2:Ar) were studied: (20:80),
0.0 -0.5 -1.0 -1.5
C2
-2.0 -2.5
Table 1 e Peak potential values of the reduction peak (C2) in LieK vs. the temperature, from Fig. 3 and from Ref. [19]. T Peak potential observed Theoretical potential peak ( C) for P (CO2) ¼ 0.2 bar for P (CO2) ¼ 0.2 bar and (V vs. Agþ/Ag) P (CO) ¼ 0.1 bar (V vs. O2/Li2O) 575 600 650
1.06 1.08 1.11
0.61 0.62 0.64
-3.0
Ei
-3.5 -1.4
% CO -Ar
C3 -1.2
20-80 30-70
-1.0
-0.8
-0.6
-0.4
-0.2
+
E / V vs. Ag / Ag
Fig. 5 e Cyclic voltammograms in Li2CO3eNa2CO3 at 600 C at a gold flag electrode (S ¼ 2 cm2) after ¡1.1 V vs. Agþ/Ag during 360 s under different atmospheres. Initial potential: Ei ¼ ¡1.35 V vs. Agþ/Ag. n ¼ 100 mV s¡1.
Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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C2 in all the experimental conditions, with P (CO2) < 0.7 bar and a pre-electrolysis before the potential scan. It is worth noting that for higher pressure of CO2, only one reduction peak C2 is observed with or without pre-electrolysis. Fig. 5 presents cyclic voltammograms in Li2CO3eNa2CO3 at 600 C at a gold flag electrode (S ¼ 2 cm2) after 360 s preelectrolysis at 1.1 V vs. Agþ/Ag under different CO2 partial pressures. The second reduction peak (at about 1.2 V vs. Agþ/ Ag) seems to be more affected by the increase in CO2 partial pressure. Up to now, the reduction of CO2 was supposed to yield CO formation with a 2 electrons process [11]. The existence of these two peaks suggests that the reduction process could be a two-step process, with a non-stable intermediate. The height of the peak being similar, each step should involve the same electron number. Different groups have already suggested different intermediates in the CO2 reduction process such as C2 O5 2 , CO2 or C2 O4 2 [9e11].
Oxalates addition: suggested mechanisms In our experimental conditions, the reduction potential is not negative enough to obtain carbon formation onto the electrode, contrarily to other studies privileging this process [6,13e16]. Moreover, no trace of carbon was observed on the gold electrode. For this reason, a two-step reduction involving a non-stable intermediate is likely. Among the intermediates mentioned in the literature, oxalates (C2 O4 2 ) are the unique species stable enough in solution and easy to add in the molten mixture. Fig. 6 depicts oxalate addition effect in Li2CO3eNa2CO3eK2CO3 eutectic at 600 C under an oxoacidic environment. Curves (b) and (c) of Fig. 6 present the cyclic voltammograms obtained for different pre-electrolysis conditions, without oxalates addition. For both negative potential
A2
10
A1
(a)
(b) (c)
-2
j / mA cm
C2 Ei
Ei
(a) with oxalates (b) without oxalates (c) without oxalates P (CO ) = 1 bar; 100 mV s
C3 -20
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
2 CO2 þ 2 e ¼ 2 CO 2 Thus, CO2 follows:
(1a)
anion is chemically transformed into CO as
2 2 CO 2 /CO þ CO3
(1b)
The overall reaction is: 2 CO2 þ 2 e /CO þ CO2 3
(1c)
Then, regarding the two oxidation peaks, A1 corresponds to a two-electron process. Indeed, after oxalates addition, the oxidation peak A1 is enhanced and we suggest that the reoxidation of oxalates involves two electrons and produces CO2, according to: (1d)
It should be noted that A2 oxidation peak is supposed to be the reaction (1c) in the opposite way. It is worth noting that both CO2 and CO seem to be stabilised in the molten carbonates under the solvated species form as described in reactions (1e) and (1f). The oxalates (C2 O4 2 ) and dicarbonate ions (C2 O5 2 ) are, respectively, CO and CO2 solvated species, according to the following equilibria:
0
-10
domains, only one reduction peak is observed (C2) at about 0.95 V vs. Agþ/Ag. Curve (a) presents a cyclic voltammogram obtained after the addition of 10 mg of oxalates. The C2 current density reduction peak is bigger with respect to the other curves (b) and (c) (about 3 times). Moreover, a second reduction broad peak (C3) appears at about 1.2 V vs. Agþ/Ag, with a height comparable to C2. This indicates that the same numbers of electrons are involved. It is worth noting that the two anodic peaks (A2 and A1) are better defined with the addition of oxalates compared to all the experiments realised until now in different eutectics and experimental conditions. Furthermore, peak A2 is higher than peak A1, whereas the contrary has always been observed so far. According to the balance of our results, including in particular the effect of oxalate addition, and taking into account the number of reduction peaks observed, two hypothetic mechanisms are suggested depending on the number of reductive peaks observed (C2 or C2 and C3). If only one reduction peak (C2) is observed, the reduction process involves most probably 2 electrons as seen in reaction (1a). CO2 is most probably first adsorbed before its reduction into an intermediate ion CO2 which yields CO (1b) via a chemical reaction with the carbonates [9e11]. The two corresponding reactions and the overall reaction are listed below: First the reduction of CO2 into CO2 involves two electrons:
CO þ CO2 3 ) 2CO2 þ 2 e !
-1.1 V(360s) -1.2 V(360s)
5
0.0
+
E / V vs. Ag / Ag
Fig. 6 e Cyclic voltammograms in Li2CO3eNa2CO3eK2CO3 at 600 C at a gold flag electrode (S ¼ 2 cm2) under 100% CO2 at 1 bar, (a) after addition of 10 mg of oxalates, initial potential Ei ¼ ¡0.90 V vs. Agþ/Ag, and without oxalates (b) after preelectrolysis at ¡1.1 V vs. Agþ/Ag (360 s), initial potential Ei ¼ ¡1.50 V vs. Agþ/Ag, (c) after pre-electrolysis at ¡1.2 V vs. Agþ/Ag (360 s), initial potential Ei ¼ ¡1.35 V vs. Agþ/Ag.
2 CO þ CO2 3 ¼ C2 O4
(1e)
2 CO2 þ CO2 3 ¼ C 2 O5
(1f)
Fig. 7 is a scheme of this hypothetical mechanism where only one peak C2 is observed. If two reduction peaks are observed (C2 and C3). Then a two-step reduction process is most likely. The first step is described by reaction (1a), but contrarily to the first suggested
Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094
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Nevertheless, identification of such species requires further analyses either in situ or ex situ such as high temperature NMR or Raman for solvated species and gas chromatography and/or thermogravimetric measurements for gases.
Conclusions
Fig. 7 e Hypothetical mechanism scheme of the CO2 electrochemical reduction in molten alkali at an inert gold flag electrode for one reduction peak (C2). mechanism, the intermediate species CO2 is reduced to form CO2 2 in reaction (1g) before reacting with carbonates to form CO, reaction (1h). These reactions are suggested when two reduction peaks are observed and are related to the presence of peak C3. 2 2 CO 2 þ 2 e ) 2 CO2 !
(1g)
CO2 2 anion then reacts with carbonates and is transformed into CO: 2 CO2 2 þ CO2 /CO þ CO3
(1h)
Concerning the oxidation processes, the reactions are similar to those described above. A complete scheme of the reduction process involving the two reduction peaks C2 and C3 can be proposed, as depicted in Fig. 8. All these reactions are suggested regarding our electrochemical results, thermodynamic considerations and intermediate species suggested in the literature [9e11].
The reduction of CO2 into CO is feasible and can be optimised in molten alkali carbonates according to experimental conditions. In most of the molten carbonate eutectics, a diffusioncontrolled reduction is observed and the process is mostly quasi-rapid at a gold working electrode. The reduction depends on the molten carbonates used, on the temperature and on the melt oxoacidity. Moreover, one or two reduction peaks were observed depending on the experimental conditions. Literature points out that different intermediates species can be formed in the reduction of CO2 into CO; thus, we have added oxalates into the carbonate melt to investigate more concretely the electrochemical process. Based on our electrochemical analysis, thermodynamic predictions and the literature, we have proposed two reduction mechanisms and some oxidation hypotheses. Thus, two mechanistic schemes, involving the species in all their forms, gaseous, dissolved and adsorbed, are presented. However, it is worth pursuing this work by analysing the outlet gases by gas chromatography and the intermediate species by in situ NMR or RAMAN spectroscopy. Measurement of CO2 and CO solubility can also be considered as a key parameter. Moreover, other electrochemical techniques should be used, as chronopotentiometry, in particular for detecting the presence of adsorbed species and for having a deeper insight on the CO2 reduction process.
Acknowledgements This work was partially supported by the French program PLANEX ANR-11-EQPX-0-01 and by the CNRS exploratory project “Emergence CO2” in 2014.
references
Fig. 8 e Hypothetical mechanism scheme of the CO2 electrochemical reduction in molten alkali at an inert gold flag electrode for two reduction peaks (C2 and C3).
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Please cite this article in press as: Chery D, et al., Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.06.094