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Electrochemical CO2 reduction to formate on indium catalysts prepared by electrodeposition in deep eutectic solvents
Journal Pre-proofs Electrochemical CO2 reduction to formate on indium catalysts prepared by electrodeposition in deep eutectic solvents Barbara Bohlen, Daniela Wastl, Johanna Radomski, Volker Sieber, Luciana Vieira PII: DOI: Reference:
S1388-2481(19)30260-7 https://doi.org/10.1016/j.elecom.2019.106597 ELECOM 106597
To appear in:
Electrochemistry Communications
Received Date: Revised Date: Accepted Date:
16 September 2019 30 October 2019 31 October 2019
Please cite this article as: B. Bohlen, D. Wastl, J. Radomski, V. Sieber, L. Vieira, Electrochemical CO2 reduction to formate on indium catalysts prepared by electrodeposition in deep eutectic solvents, Electrochemistry Communications (2019), doi: https://doi.org/10.1016/j.elecom.2019.106597
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Electrochemical CO2 reduction to formate on indium catalysts prepared by electrodeposition in deep eutectic solvents Barbara Bohlena, Daniela Wastla, Johanna Radomskia,b, Volker Siebera,b and Luciana Vieiraa* a Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB Bio-, Electro- and
Chemocatalysis BioCat, Straubing Branch, Schulgasse 11a, 94315 Straubing, Germany b Technical University of Munich, Campus Straubing for Biotechnology and Sustainability,
Schulgasse 16, 94315 Straubing, Germany, * [email protected]
Abstract The electrochemical conversion of CO2 to high-value molecules is an elegant alternative for combining CO2 utilization with renewable energy conversion and storage. Herein we report the preparation and characterization of indium catalysts for the electrochemical CO2 reduction to formate. Indium coatings were prepared by electrodeposition from a deep eutectic solvent (DES) comprising 1:2 molar choline chloride and ethylene glycol (12CE). The electrochemical behavior of indium chloride in this DES was investigated by cyclic voltammetry (CV) on copper, glassy carbon (GC) and platinum electrodes. The effect of InCl3 concentration, electrolyte temperature and deposition method on the phase and morphology of the coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Indium deposits on copper and carbon were deployed as catalysts for the CO2 electrolysis in aqueous media. Chemical analysis by HPLC, GC, and NMR revealed an optimum efficiency toward formate at -1.9 V vs. Ag/AgCl. Indium coatings prepared by potentiostatic deposition showed faradaic efficiencies (FE) up to 72.5 %. Gas diffusion electrodes (GDE) coated with indium led to formate concentrations up to 76 mM and formation rates of 0.183 mmol cm-2 h-1, which was considerably superior to indium coatings on planar electrodes.
Keywords Carbon dioxide utilization; indium electrodeposition; electrodeposition from deep eutectic solvents; electrochemical CO2 reduction; indium gas diffusion electrodes
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1. Introduction Indium is a metal with diverse technological applications, such as component for semiconductors [1], flat-panel displays, and in solar cells [2-4]. The electrodeposition of In from aqueous solutions takes place at rather negative potentials, leading to massive H2 evolution [5]. The efficiency of the process is therefore low, and the structure, morphology, and stability of the deposits can be affected by the gas evolution [3, 5]. Alternatives are the electrochemical deposition of In in organic electrolytes, ionic liquids or deep eutectic solvents (DESs) [2, 3, 6]. DESs are eutectic solutions of two or more components [7]. The application of DES as electrolytes for electrodeposition opens up to several advantages since they have a great capacity of dissolving metal salts compared to organic and aqueous solutions [7]. The electrochemical potential window is also considerably wider than aqueous solutions, enabling the electrodeposition of metals at more negative potentials, otherwise not possible due to hydrogen evolution [8-10]. Choline-chloride based DES has been already used for indium electrodeposition on Cu [6, 11], W [6] and Au [12] electrodes. Nevertheless, the deposition of In on different electrode materials and its use as catalysts is a rather less explored subject. Herein, we extend the electrode range to include Pt, glassy carbon (GC), and carbon paper. The electrochemical CO2 reduction (eCO2R) on indium electrodes is known to lead mainly to formate with considerably high faradaic efficiencies (FE up to 95 %) [13-16]. New architectures of In electrodes, including gas diffusion layer (GDL) as substrate, are reported to improve the FE and formation rates [17-21]. In this work, we report a study of indium electrodeposition from DESs on Cu, Pt and carbon. Indium deposits on Cu and carbon were used as electrocatalysts for the eCO2R to formate. This is the first report of catalyst preparation for CO2 reduction by electrodeposition in DESs.
2. Experimental 1:2 molar choline chloride and ethylene glycol (12CE) electrolytes were prepared from choline chloride (ChCl) and ethylene glycol (EG) as described previously [10]. InCl3 was added to 12CE to reach concentrations of 0.1 and 0.05 M at 80 ºC and stirred for 2 h. 2
The electrochemical characterization of the InCl3 solutions in 12CE was carried out from room temperature (RT) to 100 °C in a 3-electrode cell purged with argon. The temperature of the cell was controlled and maintained with a thermometer in a sand bath. Ag wires were used as quasi-reference electrode (RE), Ti-Ir meshes as counter electrode (CE) and Cu sheets, carbon paper, glassy carbon or Pt as working electrodes (WE). Electrodeposition of In on Cu sheets and carbon paper were carried out under stirring (500 rpm) to increase mass transport. The geometric area of the Cu electrodes was delimited using an insulant lacquer. A potentiostat/galvanostat Autolab PGSTAT128N controlled by the software NOVA 1.11 (Metrohm, Switzerland) was used for potential and current control. The crystal structures of the deposits were characterized by X-ray diffraction (XRD) recorded on a Rigaku MiniFlex 600 diffractometer, with Cu Kα radiation and a silicon strip detector D/teX Ultra. The surface morphology was analyzed with SEM using two different equipment: Hitachi S2700 (Tokyo) and Zeiss, DSM 940 A (Oberkochen, Germany). Electrochemical measurements in CO2 and Ar saturated solutions of 0.5 M KHCO3 were carried out in a 3-electrode cell using In coatings as WE, Ag/AgCl RE and Ti-Ir mesh CE. All potentials presented for the eCO2R are referred to Ag/AgCl. CO2 electrolysis with chemical analysis was performed in an H-Cell for planar electrodes and in a Gaskatel cell for GDEs. Nafion N117 was used as a separator and a GDL was used as support for the In coatings in the GDE assembly. A potentiostat/galvanostat Autolab PGSTAT204 connected to a 10 A current booster and controlled by the software NOVA 2.1 (Metrohm, Switzerland) was used as a current and potential source. During the CO2 reduction experiments, gas samples were collected with gas bags and changed every 30 min. The CO2 flow was controlled with a mass flow controller (CO2 020 sccm, Brooks Instrument, USA) and set constant to 10 mL min-1. Offline gas analysis was performed on a gas chromatograph equipped with a TCD detector (GC-TCD System GC-2010 series of Shimadzu). Liquid products were analyzed by nuclear magnetic resonance (1H-NMR, JEOL JNM-ECA 400 MHz) and highperformance liquid chromatography (HPLC, Shimadzu LC20A).
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3. Results and discussion 3.1. Electrodeposition of indium from 12CE Cyclic voltammetry of 0.1 M InCl3 solutions in 12CE was carried out on GC, Pt, and Cu electrodes. Figure 1a shows the cyclic voltammograms (CVs) of these three electrode materials in the DES in the absence and presence of InCl3 at room temperature. The appearance of cathodic and anodic waves in the solutions containing InCl3 can be observed for all electrode materials. On Cu electrodes, a well-defined reduction peak related to the reduction of indium chloride species to metallic In can be observed at -0.6 V. This single deposition peak indicates a single-step process for the reduction of indium species onto Cu surfaces [6]. This deposition potential is consistent to that reported in the literature for Cu electrodes in 0.05 M InCl3 in 12CE (-0.8 V) [11]. The positive shift in peak potential observed here is due to the higher indium concentration in solution. A comparison of these two indium salt concentrations is shown in the support information (SI, Fig. S1). Indium deposition on GC electrodes takes place at more negative potentials than Cu (-0.75 V). The CV on GC depicts a typical nucleation loop, indicating a larger overpotential to initiate the indium nucleation and growth, in accordance with previous reports of In deposition on such substrates [3, 6, 11]. The coulombic efficiency, calculated as the ratio of the total anodic over the total cathodic charge, is rather low (27 %), indicating incomplete dissolution and/or cathodic side-reactions. Yet, the visibly smaller oxidation peak, compared to the reduction peak, suggests incomplete dissolution of indium from GC. Consequently, the second and third CV cycle are thermodynamically more favorable and a smaller nucleation loop is observed (SI, Fig. S2) [3]. As for Pt electrodes, which have much lower overpotential for H2 evolution in this electrolyte compared to GC and Cu [22], no defined peak attributed to indium deposition was observed. Nevertheless, the current density in the electrolyte containing InCl3 is significantly higher compared to that in the absence of indium species. This increase in current density indicates that indium electrodeposition takes place along with side reactions - H2 evolution from the reduction of the hydroxyl groups from EG or ChCl, which masks the indium deposition peak on platinum. An
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anodic peak at -0.48 V, attributed to the dissolution of indium from the surface, confirms indium deposition on this substrate. Anodic peaks attributed to the dissolution of indium deposited on Cu and GC electrodes arise at -0.34 and -0.49 V, respectively. Both electrodes became visually pale gray after the 3 voltammetric scans, confirming incomplete stripping of In from the substrate. Compared to Cu, the anodic peak current for indium dissolution on both GC and Pt are significantly lower. Coulombic efficiency for indium electrodeposition on GC, Pt, and Cu substrates, were 27, 06 and 55 % respectively. Low efficiency for Pt confirms that the main reaction on this electrode is the electrodecomposition of the DES, besides residual water reduction, to generate hydrogen [22]. The effect of the temperature on the electrodeposition of indium from 0.1 M InCl3 in 12CE was systematically investigated for GC and Cu electrodes (Fig. 1b). Increasing the temperature causes a decrease of the electrolyte viscosity and improves the mobility of ionic species in solution [8, 12]. Consequently, the current density increases and the peak potential for indium deposition shifts positively [23, 24]. Starting the CVs at RT and gradually increasing up to 80 °C for GC (temperature limit of the electrode used) shifted the peak potential from -0.74 V at RT to -0.72, 0.66 and -0.53 V at 40, 60 and 80 °C, respectively. The associated cathodic peak current increased from -0.076 to -2.465 mA, from RT to 80 °C. On copper, the effect of temperature on the peak potential was not so pronounced as for GC, whereas the cathodic peak current increased considerably from -2.10 to -4.49 mA from RT to 100 °C. The CV parameters such as peak current, peak potential, and coulombic efficiency obtained for GC and Cu electrodes at different temperatures are listed in Table S1 (SI). At GC electrodes, coulombic efficiency slightly decreases with temperature, indicating side-reactions at a higher temperature, such as reduction of hydroxyl groups from EG and ChCl [10].
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Figure 1: CVs of 0.1 M InCl3 in 12CE at 10 mV s-1. The CVs started at open circuit potential (OCP) with a negative scan direction. a) Different electrode materials at RT. Gray lines show the background CVs of the DES without InCl3 on each electrode. OCP values were: +0.30, +0.22 and -0.32 V vs. Ag/Ag+ for GC, Pt and Cu electrodes; b) Temperature variation on GC and Cu electrodes.
The electrochemical behavior of InCl and InCl3 solutions in 12CE on W and Cu electrodes was reported by Barrado et.al [6]. The authors showed CVs of 0.07 M In(III) in 12CE at 70 °C on Cu, with a cathodic peak current of -1.5 mA cm-2 at -0.8 V vs. Ag/Ag+. This reported peak current is considerably lower and at a more positive potential than those observed in our studies for Cu at 60 °C (-3.52 mA cm-2 at -0.65 V), which can be a result of to their lower InCl3 concentration compared to the herein reported (0.1 M). The results observed for the increase in temperature on Cu electrodes are also consistent with the reports presented by Alcanfor et al. for In deposition on Cu from a 0.05 M InCl3 in 12CE [11]. The authors report an indium deposition peak at -0.74 V, with a peak current of -2 mA cm-2 at 80 °C. A higher current density (-3.54 mA cm-2) was achieved in this work due to the higher concentration of indium in solution.
3.2.Characterization of In deposits on Cu Indium
deposits
were
prepared
on
copper
foils
at
constant
current
(galvanostatically) or constant potential (potentiostatically) (Fig. 2). SEM images from coatings prepared at galvanostatic (In-G2) and potentiostatic (In-P2) conditions 6
reveal that In-G2 (Fig. 2b) has a rougher surface with larger grains, whereas the sample In-P2 (Fig 2a) has smaller grains more distanced from each other. Potential-controlled electrodeposition at -0.6 V showed a rather constant current (Fig. S3). The deposition time was controlled (300 s) to achieve -1 C of charge transfer. The galvanostatic deposition is usually a more reproducible method since the charge consumed can be controlled beforehand by the deposition time [25]. During the galvanostatic deposition at -3 mA cm-2, the potential was about -0.52 V and -2.4 C of charge was passed during 800 s. The parameters used for the preparation of several indium catalyst are presented in Table S2 in the SI. X-ray diffractograms of samples In-G2 and In-P2 (Fig. 2c) resemble typical reflexes of metallic In and Cu3In7 surface alloys. These patterns are consistent to previous reports for In deposits on Cu substrates [6, 11].
Figure 2: Characterization of the indium coatings electrodeposited from 0.1 M InCl3 in 12CE solutions at 100 °C. SEM images from coating deposited a) potenstiotatically at -0.6 V for 300 s (In-P2) and b) galvanostatically at -3 mA cm-2 for 800 s (In-G2); c) XRD pattern of InG2 and In-P2.
3.3.Electrochemical CO2 reduction on In coatings In-G2 and In-P2 were used as electrodes for multiple steps chronoamperometric (CA) experiments in CO2 and Ar saturated solutions of 0.5 M KHCO3. The activity towards CO2 reduction was pre-evaluated by the difference in current density in the 7
two solutions. It is important to remark that this current gap in Ar and CO2 saturated electrolytes cannot be completely correlated to CO2 reduction without chemical analysis. The pH becomes more acidic when saturating a KHCO3 solution with CO2, favoring H2 evolution [26]. This method was used as a rapid screening and comparison for different indium coatings in the presence of dissolved CO2, and thereby choosing the coatings with the highest current densities for further chemical analysis of products. The complete screening of several indium coatings is shown in the SI. Multiple-step CA of In-P2 and In-G2 on Cu are depicted in Figure 3a. In-G2 shows considerably higher current densities at all step potentials compared to In-P2. In-G2 reached a maximum current density of -27.8 mA cm-2 at -2.0 V, compared to 14.5 mA cm-2 for In-P2 at the same potential. Moreover, the current density gap for CO2/Ar saturated solutions is significantly larger for In-G2, which becomes even more pronounced at more negative potentials. For In-P2, this gap is rather small for all applied potentials. This difference in current density can be caused by the higher In loading and different roughness of In-G2. Thus, In-G2 was chosen as a catalyst for further experiments. Potential screening with chemical analysis of the products was carried out for InG2 by decreasing the potential stepwise from -1.4 to -2.0 V in intervals of 30 min. Liquid and gas samples were collected at the end of each step-potential. Chemical analysis of the products shows the presence of formate, H2, and CO (Fig. 3). These products are consistent with the classification of indium catalysts by Hori et al [13]. Decreasing the potential led to an increase in current density (Fig. 3b), reaching a maximum of -8 mA cm-2 at -2.0 V. FEs were higher at potentials more negative than 1.7 V, with the highest FE of 52.7 % at -1.9 V. The concentration of formate produced at each potential (not the cumulative) increased at more negative potentials, reaching a maximum of 4.97 mM at -2.0 V (Fig. 3c). Nonetheless, at such highly negative potential, the FE for H2 increases, decreasing the efficiency for formate.
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Figure 3: a) multiple step CA of In-G2 and In-P2 at the indicated potential for 2 min each in 0.5 M KHCO3 solutions saturated with CO2 or Ar. For the potential screening with In-G2: b) average current density achieved for 30 min at each step potential; c) concentration of formate obtained after each step potential (non-cumulative); d) faradaic efficiency for the products.
The product distribution observed in this work is in agreement with reports for different In-based catalysts [16, 20]. A similar systematic study of the potentialdependent eCO2R was carried out by Luo et al [27]. The group compared the catalytic activity of an In foil and a porous In catalyst. Both catalysts led to a similar product distribution with applied potential, yet the porous In catalysts showed the highest FE towards formate due to its higher surface area. The eCO2R is a three-phase reaction involving the CO2 gas, the solid catalyst and the liquid electrolyte. Thus, for typical planar electrodes in saturated CO2 solutions, the CO2 conversion is limited by the solubility of CO2 in the electrolyte, which is quite low in water. To overcome this limitation, it is possible to change the cell configuration to GDE cells. In this kind of electrode assembly, the gas diffuses through a membrane, thereby enabling a constant supply of CO2 to the catalyst/electrolyte interface [29]. Consequently higher current densities and higher product concentrations can be achieved [28]. GDEs with indium coatings were prepared by galvanostatic deposition at -2 mA cm-2 for 20 min at 80 ºC from 12CE on carbon paper. This indium deposit was assembled attached to gas diffusion membrane as a gas diffusion electrode (In-GDL). CO2 electrolysis on In-GDL, In-G2 and In-P2 was carried out at constant potential of -1.9 V for 2 h (Fig. 4). A peak of FE for formate was observed for all three electrodes for the first 30 min: 72.48, 60.29 and 48.90 % for In-P2, In-G2 and In-GDL, 9
respectively. Yet, we want to highlight that it is important to evaluate not only the FE, but the whole formation rate of products, especially on potential-controlled experiments. Experiments at constant potential with low current densities and lower concentrations of formate could lead to higher FE. Nevertheless, we aim to achieve high product concentrations at higher current densities. Comparing the two planar electrodes, the net concentration of the formate is higher for the In-G2 (19.17 mM) compared to In-P2 (12.7 mM). In-GDL increases the formate concentration in almost 4-fold: 76.39 mM after 2 h of electrolysis with formation rate of 0.183 mmol cm-2 h-1, while the planar catalyst showed rates of 0.0719 (In-G2) and 0.0476 mmol cm-2 h-1 (In-P2).
Figure 4: Faradaic efficiency for potentiostatic CO2 electrolysis at -1.9 V in 0.5 M KHCO3 saturated with CO2, for 2 h in with the following catalysts a) In-P2, b) In-G2 and c) In-GDL. d) formate concentration for each catalyst during the 2 h electrolysis.
SEM images of the In coatings on carbon paper show a coverage of indium particles on the carbon fibers (Fig. 5). EDX of the sample (Fig. S7) show reflexes of indium only, besides carbon from the substrate. Remarkably, the deposit is not lost after 2 h of potentiostatic experiment, indicating good adhesion of the indium coating on the carbon fibers for this period of time.
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Figure 5: SEM image of the catalysts In-GDL a) and b) before and c) and d) after the CO2 reduction experiments at -1.9 V for 2 h.
A comparative study of In/C-GDL and In foil as catalysts for the CO2 reduction showed that better performance was obtained for GDE compared to indium foils: higher current density and formation rates [20]. The In/C-GDL electrode was prepared by immobilizing indium and carbon particles on a GDL using Nafion as binder. FE up to 45 % for formate on In/C-GDL was reported, with higher efficiencies in the first 30 min of reaction. However, the formation rate was rather low: 0.0504 mmol cm-2 h-1 compared to that reported in this work (0.183 mmol cm2
h-1). The authors observed a decrease in specific surface area due to the binder used
in the electrode preparation. An advantage of the method herein described is that no binder is necessary for the metal immobilization by electrodeposition on the GDL. A literature comparison of formation rates and FE can be found in the SI.
4. Conclusions Electrodeposition of indium from 12CE on copper and carbon has been successfully applied as a method for catalyst preparation for electrochemical CO2 reduction. Gas diffusion electrodes containing electrodeposited indium catalysts were produced with good adhesion and successfully applied for the CO2 reduction to formate. To the best of our knowledge, this is the first application of electrodeposition from DESs for eCO2R catalysts preparation. The catalytic activity of the final coatings were affected by the loading and the roughness of indium. Indium coatings on copper sheets led to formate FE of 72.48 %, whereas the deposits assembled as GDE led to FE of 48.90 %. The net formate concentration (76.39 mM) and formation rate (0.183 mmol cm-2 h-1), 11
however, were significantly higher for In-GDLs. Catalyst preparation by electrodeposition from DES can be extended to other metals, increasing the range of possible products from eCO2R.
5. Acknowledgments This work was supported by the Project CELBICON in the EU Framework Program Horizon 2020 (GA Nr. 679050). The authors are indebted to Professor Cordt Zollfrank (TUM Campus Straubing) for allowing the use of the XRD and SEM, to Manuela Kaiser for assistance in the lab and to Leonardo Castaneda Losada for fruitful discussions.
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Indium was deposited from deep eutectic solvents on copper and carbonaceous substrates Indium coatings were used as catalysts for the electrochemical CO2 reduction The indium coatings were stable after two-hour CO2 electrolysis The main product of the electrochemical CO2 reduction was formate The use of gas diffusion electrodes improved the formation rate of formate
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S1388-2481(19)30260-7 https://doi.org/10.1016/j.elecom.2019.106597 ELECOM 106597
To appear in:
Electrochemistry Communications
Received Date: Revised Date: Accepted Date:
16 September 2019 30 October 2019 31 October 2019
Please cite this article as: B. Bohlen, D. Wastl, J. Radomski, V. Sieber, L. Vieira, Electrochemical CO2 reduction to formate on indium catalysts prepared by electrodeposition in deep eutectic solvents, Electrochemistry Communications (2019), doi: https://doi.org/10.1016/j.elecom.2019.106597
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Electrochemical CO2 reduction to formate on indium catalysts prepared by electrodeposition in deep eutectic solvents Barbara Bohlena, Daniela Wastla, Johanna Radomskia,b, Volker Siebera,b and Luciana Vieiraa* a Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB Bio-, Electro- and
Chemocatalysis BioCat, Straubing Branch, Schulgasse 11a, 94315 Straubing, Germany b Technical University of Munich, Campus Straubing for Biotechnology and Sustainability,
Schulgasse 16, 94315 Straubing, Germany, * [email protected]
Abstract The electrochemical conversion of CO2 to high-value molecules is an elegant alternative for combining CO2 utilization with renewable energy conversion and storage. Herein we report the preparation and characterization of indium catalysts for the electrochemical CO2 reduction to formate. Indium coatings were prepared by electrodeposition from a deep eutectic solvent (DES) comprising 1:2 molar choline chloride and ethylene glycol (12CE). The electrochemical behavior of indium chloride in this DES was investigated by cyclic voltammetry (CV) on copper, glassy carbon (GC) and platinum electrodes. The effect of InCl3 concentration, electrolyte temperature and deposition method on the phase and morphology of the coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Indium deposits on copper and carbon were deployed as catalysts for the CO2 electrolysis in aqueous media. Chemical analysis by HPLC, GC, and NMR revealed an optimum efficiency toward formate at -1.9 V vs. Ag/AgCl. Indium coatings prepared by potentiostatic deposition showed faradaic efficiencies (FE) up to 72.5 %. Gas diffusion electrodes (GDE) coated with indium led to formate concentrations up to 76 mM and formation rates of 0.183 mmol cm-2 h-1, which was considerably superior to indium coatings on planar electrodes.
Keywords Carbon dioxide utilization; indium electrodeposition; electrodeposition from deep eutectic solvents; electrochemical CO2 reduction; indium gas diffusion electrodes
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1. Introduction Indium is a metal with diverse technological applications, such as component for semiconductors [1], flat-panel displays, and in solar cells [2-4]. The electrodeposition of In from aqueous solutions takes place at rather negative potentials, leading to massive H2 evolution [5]. The efficiency of the process is therefore low, and the structure, morphology, and stability of the deposits can be affected by the gas evolution [3, 5]. Alternatives are the electrochemical deposition of In in organic electrolytes, ionic liquids or deep eutectic solvents (DESs) [2, 3, 6]. DESs are eutectic solutions of two or more components [7]. The application of DES as electrolytes for electrodeposition opens up to several advantages since they have a great capacity of dissolving metal salts compared to organic and aqueous solutions [7]. The electrochemical potential window is also considerably wider than aqueous solutions, enabling the electrodeposition of metals at more negative potentials, otherwise not possible due to hydrogen evolution [8-10]. Choline-chloride based DES has been already used for indium electrodeposition on Cu [6, 11], W [6] and Au [12] electrodes. Nevertheless, the deposition of In on different electrode materials and its use as catalysts is a rather less explored subject. Herein, we extend the electrode range to include Pt, glassy carbon (GC), and carbon paper. The electrochemical CO2 reduction (eCO2R) on indium electrodes is known to lead mainly to formate with considerably high faradaic efficiencies (FE up to 95 %) [13-16]. New architectures of In electrodes, including gas diffusion layer (GDL) as substrate, are reported to improve the FE and formation rates [17-21]. In this work, we report a study of indium electrodeposition from DESs on Cu, Pt and carbon. Indium deposits on Cu and carbon were used as electrocatalysts for the eCO2R to formate. This is the first report of catalyst preparation for CO2 reduction by electrodeposition in DESs.
2. Experimental 1:2 molar choline chloride and ethylene glycol (12CE) electrolytes were prepared from choline chloride (ChCl) and ethylene glycol (EG) as described previously [10]. InCl3 was added to 12CE to reach concentrations of 0.1 and 0.05 M at 80 ºC and stirred for 2 h. 2
The electrochemical characterization of the InCl3 solutions in 12CE was carried out from room temperature (RT) to 100 °C in a 3-electrode cell purged with argon. The temperature of the cell was controlled and maintained with a thermometer in a sand bath. Ag wires were used as quasi-reference electrode (RE), Ti-Ir meshes as counter electrode (CE) and Cu sheets, carbon paper, glassy carbon or Pt as working electrodes (WE). Electrodeposition of In on Cu sheets and carbon paper were carried out under stirring (500 rpm) to increase mass transport. The geometric area of the Cu electrodes was delimited using an insulant lacquer. A potentiostat/galvanostat Autolab PGSTAT128N controlled by the software NOVA 1.11 (Metrohm, Switzerland) was used for potential and current control. The crystal structures of the deposits were characterized by X-ray diffraction (XRD) recorded on a Rigaku MiniFlex 600 diffractometer, with Cu Kα radiation and a silicon strip detector D/teX Ultra. The surface morphology was analyzed with SEM using two different equipment: Hitachi S2700 (Tokyo) and Zeiss, DSM 940 A (Oberkochen, Germany). Electrochemical measurements in CO2 and Ar saturated solutions of 0.5 M KHCO3 were carried out in a 3-electrode cell using In coatings as WE, Ag/AgCl RE and Ti-Ir mesh CE. All potentials presented for the eCO2R are referred to Ag/AgCl. CO2 electrolysis with chemical analysis was performed in an H-Cell for planar electrodes and in a Gaskatel cell for GDEs. Nafion N117 was used as a separator and a GDL was used as support for the In coatings in the GDE assembly. A potentiostat/galvanostat Autolab PGSTAT204 connected to a 10 A current booster and controlled by the software NOVA 2.1 (Metrohm, Switzerland) was used as a current and potential source. During the CO2 reduction experiments, gas samples were collected with gas bags and changed every 30 min. The CO2 flow was controlled with a mass flow controller (CO2 020 sccm, Brooks Instrument, USA) and set constant to 10 mL min-1. Offline gas analysis was performed on a gas chromatograph equipped with a TCD detector (GC-TCD System GC-2010 series of Shimadzu). Liquid products were analyzed by nuclear magnetic resonance (1H-NMR, JEOL JNM-ECA 400 MHz) and highperformance liquid chromatography (HPLC, Shimadzu LC20A).
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3. Results and discussion 3.1. Electrodeposition of indium from 12CE Cyclic voltammetry of 0.1 M InCl3 solutions in 12CE was carried out on GC, Pt, and Cu electrodes. Figure 1a shows the cyclic voltammograms (CVs) of these three electrode materials in the DES in the absence and presence of InCl3 at room temperature. The appearance of cathodic and anodic waves in the solutions containing InCl3 can be observed for all electrode materials. On Cu electrodes, a well-defined reduction peak related to the reduction of indium chloride species to metallic In can be observed at -0.6 V. This single deposition peak indicates a single-step process for the reduction of indium species onto Cu surfaces [6]. This deposition potential is consistent to that reported in the literature for Cu electrodes in 0.05 M InCl3 in 12CE (-0.8 V) [11]. The positive shift in peak potential observed here is due to the higher indium concentration in solution. A comparison of these two indium salt concentrations is shown in the support information (SI, Fig. S1). Indium deposition on GC electrodes takes place at more negative potentials than Cu (-0.75 V). The CV on GC depicts a typical nucleation loop, indicating a larger overpotential to initiate the indium nucleation and growth, in accordance with previous reports of In deposition on such substrates [3, 6, 11]. The coulombic efficiency, calculated as the ratio of the total anodic over the total cathodic charge, is rather low (27 %), indicating incomplete dissolution and/or cathodic side-reactions. Yet, the visibly smaller oxidation peak, compared to the reduction peak, suggests incomplete dissolution of indium from GC. Consequently, the second and third CV cycle are thermodynamically more favorable and a smaller nucleation loop is observed (SI, Fig. S2) [3]. As for Pt electrodes, which have much lower overpotential for H2 evolution in this electrolyte compared to GC and Cu [22], no defined peak attributed to indium deposition was observed. Nevertheless, the current density in the electrolyte containing InCl3 is significantly higher compared to that in the absence of indium species. This increase in current density indicates that indium electrodeposition takes place along with side reactions - H2 evolution from the reduction of the hydroxyl groups from EG or ChCl, which masks the indium deposition peak on platinum. An
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anodic peak at -0.48 V, attributed to the dissolution of indium from the surface, confirms indium deposition on this substrate. Anodic peaks attributed to the dissolution of indium deposited on Cu and GC electrodes arise at -0.34 and -0.49 V, respectively. Both electrodes became visually pale gray after the 3 voltammetric scans, confirming incomplete stripping of In from the substrate. Compared to Cu, the anodic peak current for indium dissolution on both GC and Pt are significantly lower. Coulombic efficiency for indium electrodeposition on GC, Pt, and Cu substrates, were 27, 06 and 55 % respectively. Low efficiency for Pt confirms that the main reaction on this electrode is the electrodecomposition of the DES, besides residual water reduction, to generate hydrogen [22]. The effect of the temperature on the electrodeposition of indium from 0.1 M InCl3 in 12CE was systematically investigated for GC and Cu electrodes (Fig. 1b). Increasing the temperature causes a decrease of the electrolyte viscosity and improves the mobility of ionic species in solution [8, 12]. Consequently, the current density increases and the peak potential for indium deposition shifts positively [23, 24]. Starting the CVs at RT and gradually increasing up to 80 °C for GC (temperature limit of the electrode used) shifted the peak potential from -0.74 V at RT to -0.72, 0.66 and -0.53 V at 40, 60 and 80 °C, respectively. The associated cathodic peak current increased from -0.076 to -2.465 mA, from RT to 80 °C. On copper, the effect of temperature on the peak potential was not so pronounced as for GC, whereas the cathodic peak current increased considerably from -2.10 to -4.49 mA from RT to 100 °C. The CV parameters such as peak current, peak potential, and coulombic efficiency obtained for GC and Cu electrodes at different temperatures are listed in Table S1 (SI). At GC electrodes, coulombic efficiency slightly decreases with temperature, indicating side-reactions at a higher temperature, such as reduction of hydroxyl groups from EG and ChCl [10].
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Figure 1: CVs of 0.1 M InCl3 in 12CE at 10 mV s-1. The CVs started at open circuit potential (OCP) with a negative scan direction. a) Different electrode materials at RT. Gray lines show the background CVs of the DES without InCl3 on each electrode. OCP values were: +0.30, +0.22 and -0.32 V vs. Ag/Ag+ for GC, Pt and Cu electrodes; b) Temperature variation on GC and Cu electrodes.
The electrochemical behavior of InCl and InCl3 solutions in 12CE on W and Cu electrodes was reported by Barrado et.al [6]. The authors showed CVs of 0.07 M In(III) in 12CE at 70 °C on Cu, with a cathodic peak current of -1.5 mA cm-2 at -0.8 V vs. Ag/Ag+. This reported peak current is considerably lower and at a more positive potential than those observed in our studies for Cu at 60 °C (-3.52 mA cm-2 at -0.65 V), which can be a result of to their lower InCl3 concentration compared to the herein reported (0.1 M). The results observed for the increase in temperature on Cu electrodes are also consistent with the reports presented by Alcanfor et al. for In deposition on Cu from a 0.05 M InCl3 in 12CE [11]. The authors report an indium deposition peak at -0.74 V, with a peak current of -2 mA cm-2 at 80 °C. A higher current density (-3.54 mA cm-2) was achieved in this work due to the higher concentration of indium in solution.
3.2.Characterization of In deposits on Cu Indium
deposits
were
prepared
on
copper
foils
at
constant
current
(galvanostatically) or constant potential (potentiostatically) (Fig. 2). SEM images from coatings prepared at galvanostatic (In-G2) and potentiostatic (In-P2) conditions 6
reveal that In-G2 (Fig. 2b) has a rougher surface with larger grains, whereas the sample In-P2 (Fig 2a) has smaller grains more distanced from each other. Potential-controlled electrodeposition at -0.6 V showed a rather constant current (Fig. S3). The deposition time was controlled (300 s) to achieve -1 C of charge transfer. The galvanostatic deposition is usually a more reproducible method since the charge consumed can be controlled beforehand by the deposition time [25]. During the galvanostatic deposition at -3 mA cm-2, the potential was about -0.52 V and -2.4 C of charge was passed during 800 s. The parameters used for the preparation of several indium catalyst are presented in Table S2 in the SI. X-ray diffractograms of samples In-G2 and In-P2 (Fig. 2c) resemble typical reflexes of metallic In and Cu3In7 surface alloys. These patterns are consistent to previous reports for In deposits on Cu substrates [6, 11].
Figure 2: Characterization of the indium coatings electrodeposited from 0.1 M InCl3 in 12CE solutions at 100 °C. SEM images from coating deposited a) potenstiotatically at -0.6 V for 300 s (In-P2) and b) galvanostatically at -3 mA cm-2 for 800 s (In-G2); c) XRD pattern of InG2 and In-P2.
3.3.Electrochemical CO2 reduction on In coatings In-G2 and In-P2 were used as electrodes for multiple steps chronoamperometric (CA) experiments in CO2 and Ar saturated solutions of 0.5 M KHCO3. The activity towards CO2 reduction was pre-evaluated by the difference in current density in the 7
two solutions. It is important to remark that this current gap in Ar and CO2 saturated electrolytes cannot be completely correlated to CO2 reduction without chemical analysis. The pH becomes more acidic when saturating a KHCO3 solution with CO2, favoring H2 evolution [26]. This method was used as a rapid screening and comparison for different indium coatings in the presence of dissolved CO2, and thereby choosing the coatings with the highest current densities for further chemical analysis of products. The complete screening of several indium coatings is shown in the SI. Multiple-step CA of In-P2 and In-G2 on Cu are depicted in Figure 3a. In-G2 shows considerably higher current densities at all step potentials compared to In-P2. In-G2 reached a maximum current density of -27.8 mA cm-2 at -2.0 V, compared to 14.5 mA cm-2 for In-P2 at the same potential. Moreover, the current density gap for CO2/Ar saturated solutions is significantly larger for In-G2, which becomes even more pronounced at more negative potentials. For In-P2, this gap is rather small for all applied potentials. This difference in current density can be caused by the higher In loading and different roughness of In-G2. Thus, In-G2 was chosen as a catalyst for further experiments. Potential screening with chemical analysis of the products was carried out for InG2 by decreasing the potential stepwise from -1.4 to -2.0 V in intervals of 30 min. Liquid and gas samples were collected at the end of each step-potential. Chemical analysis of the products shows the presence of formate, H2, and CO (Fig. 3). These products are consistent with the classification of indium catalysts by Hori et al [13]. Decreasing the potential led to an increase in current density (Fig. 3b), reaching a maximum of -8 mA cm-2 at -2.0 V. FEs were higher at potentials more negative than 1.7 V, with the highest FE of 52.7 % at -1.9 V. The concentration of formate produced at each potential (not the cumulative) increased at more negative potentials, reaching a maximum of 4.97 mM at -2.0 V (Fig. 3c). Nonetheless, at such highly negative potential, the FE for H2 increases, decreasing the efficiency for formate.
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Figure 3: a) multiple step CA of In-G2 and In-P2 at the indicated potential for 2 min each in 0.5 M KHCO3 solutions saturated with CO2 or Ar. For the potential screening with In-G2: b) average current density achieved for 30 min at each step potential; c) concentration of formate obtained after each step potential (non-cumulative); d) faradaic efficiency for the products.
The product distribution observed in this work is in agreement with reports for different In-based catalysts [16, 20]. A similar systematic study of the potentialdependent eCO2R was carried out by Luo et al [27]. The group compared the catalytic activity of an In foil and a porous In catalyst. Both catalysts led to a similar product distribution with applied potential, yet the porous In catalysts showed the highest FE towards formate due to its higher surface area. The eCO2R is a three-phase reaction involving the CO2 gas, the solid catalyst and the liquid electrolyte. Thus, for typical planar electrodes in saturated CO2 solutions, the CO2 conversion is limited by the solubility of CO2 in the electrolyte, which is quite low in water. To overcome this limitation, it is possible to change the cell configuration to GDE cells. In this kind of electrode assembly, the gas diffuses through a membrane, thereby enabling a constant supply of CO2 to the catalyst/electrolyte interface [29]. Consequently higher current densities and higher product concentrations can be achieved [28]. GDEs with indium coatings were prepared by galvanostatic deposition at -2 mA cm-2 for 20 min at 80 ºC from 12CE on carbon paper. This indium deposit was assembled attached to gas diffusion membrane as a gas diffusion electrode (In-GDL). CO2 electrolysis on In-GDL, In-G2 and In-P2 was carried out at constant potential of -1.9 V for 2 h (Fig. 4). A peak of FE for formate was observed for all three electrodes for the first 30 min: 72.48, 60.29 and 48.90 % for In-P2, In-G2 and In-GDL, 9
respectively. Yet, we want to highlight that it is important to evaluate not only the FE, but the whole formation rate of products, especially on potential-controlled experiments. Experiments at constant potential with low current densities and lower concentrations of formate could lead to higher FE. Nevertheless, we aim to achieve high product concentrations at higher current densities. Comparing the two planar electrodes, the net concentration of the formate is higher for the In-G2 (19.17 mM) compared to In-P2 (12.7 mM). In-GDL increases the formate concentration in almost 4-fold: 76.39 mM after 2 h of electrolysis with formation rate of 0.183 mmol cm-2 h-1, while the planar catalyst showed rates of 0.0719 (In-G2) and 0.0476 mmol cm-2 h-1 (In-P2).
Figure 4: Faradaic efficiency for potentiostatic CO2 electrolysis at -1.9 V in 0.5 M KHCO3 saturated with CO2, for 2 h in with the following catalysts a) In-P2, b) In-G2 and c) In-GDL. d) formate concentration for each catalyst during the 2 h electrolysis.
SEM images of the In coatings on carbon paper show a coverage of indium particles on the carbon fibers (Fig. 5). EDX of the sample (Fig. S7) show reflexes of indium only, besides carbon from the substrate. Remarkably, the deposit is not lost after 2 h of potentiostatic experiment, indicating good adhesion of the indium coating on the carbon fibers for this period of time.
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Figure 5: SEM image of the catalysts In-GDL a) and b) before and c) and d) after the CO2 reduction experiments at -1.9 V for 2 h.
A comparative study of In/C-GDL and In foil as catalysts for the CO2 reduction showed that better performance was obtained for GDE compared to indium foils: higher current density and formation rates [20]. The In/C-GDL electrode was prepared by immobilizing indium and carbon particles on a GDL using Nafion as binder. FE up to 45 % for formate on In/C-GDL was reported, with higher efficiencies in the first 30 min of reaction. However, the formation rate was rather low: 0.0504 mmol cm-2 h-1 compared to that reported in this work (0.183 mmol cm2
h-1). The authors observed a decrease in specific surface area due to the binder used
in the electrode preparation. An advantage of the method herein described is that no binder is necessary for the metal immobilization by electrodeposition on the GDL. A literature comparison of formation rates and FE can be found in the SI.
4. Conclusions Electrodeposition of indium from 12CE on copper and carbon has been successfully applied as a method for catalyst preparation for electrochemical CO2 reduction. Gas diffusion electrodes containing electrodeposited indium catalysts were produced with good adhesion and successfully applied for the CO2 reduction to formate. To the best of our knowledge, this is the first application of electrodeposition from DESs for eCO2R catalysts preparation. The catalytic activity of the final coatings were affected by the loading and the roughness of indium. Indium coatings on copper sheets led to formate FE of 72.48 %, whereas the deposits assembled as GDE led to FE of 48.90 %. The net formate concentration (76.39 mM) and formation rate (0.183 mmol cm-2 h-1), 11
however, were significantly higher for In-GDLs. Catalyst preparation by electrodeposition from DES can be extended to other metals, increasing the range of possible products from eCO2R.
5. Acknowledgments This work was supported by the Project CELBICON in the EU Framework Program Horizon 2020 (GA Nr. 679050). The authors are indebted to Professor Cordt Zollfrank (TUM Campus Straubing) for allowing the use of the XRD and SEM, to Manuela Kaiser for assistance in the lab and to Leonardo Castaneda Losada for fruitful discussions.
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Indium was deposited from deep eutectic solvents on copper and carbonaceous substrates Indium coatings were used as catalysts for the electrochemical CO2 reduction The indium coatings were stable after two-hour CO2 electrolysis The main product of the electrochemical CO2 reduction was formate The use of gas diffusion electrodes improved the formation rate of formate
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