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Electrode-electrolyte interactions in choline chloride ethylene glycol based solvents and their effect on the electrodeposition of iron
Electrochimica Acta 312 (2019) 303e312
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Electrode-electrolyte interactions in choline chloride ethylene glycol based solvents and their effect on the electrodeposition of iron Jorge D. Gamarra*, Kristof Marcoen, Annick Hubin, Tom Hauffman Vrije Universiteit Brussel, Research Group of Electrochemical and Surface Engineering (SURF), Brussels, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 December 2018 Received in revised form 25 April 2019 Accepted 25 April 2019 Available online 5 May 2019
Although deep eutectic solvents (DES) and ethylene glycol (EG) electrolytes are gaining ground as media for the electrodeposition of metals, the influence of these electrolytes on the electrode and electrodeposits has not been elaborated before. In this work, we investigate how choline chloride: ethylene glycol based (ChCl:EG) electrolytes interact with the glassy carbon (GC) working surface at different potentials and, how these interactions change during the electrodeposition of iron from 1:2 and 1:4 ChCl:EG electrolytes. GC substrates are exposed to both electrolytes in absence of iron in order to study the pure electrolyte-substrate interactions. Linear sweep voltammetry was used to determine the potential range in which the different reactions occur, while time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to detect and identify the molecules adsorbed on the surface. In all systems, there is an accumulation of either choline or its derivatives on the electrode surface, with 1:4 ChCl:EG electrolytes showing different decomposition products from 1:2 ChCl:EG electrolytes. Choline accumulation and decomposition was also found on iron electrodeposits. The electrolyte composition has a major impact on the chemical speciation of iron, and on the deposit's adherence to the substrate. These are two crucial characteristics that define the efficiency of iron deposition from DES. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Iron deposition Time-of-flight secondary ion spectrometry Non aqueous solvents
1. Introduction Deep eutectic solvents (DES) are promising electrochemical media [1]. They offer a low-cost alternative to ionic liquids and a broader electrochemical potential window than aqueous electrolytes, opening the door to a range of applications in electrochemistry [2,3]. One of these applications is the electrodeposition of metallic nanoparticles and coatings [4,5], where they should reduce the problem of hydrogen embrittlement. The existence of DES relies on strong interactions between their components; typically, a quaternary ammonia salt such as choline chloride acting as a hydrogen bond acceptor(HBA), and an organic molecule like ethylene glycol as a hydrogen bond donor (HBD). These interactions allow the ions from the HBA and the HBD to move as a single entity [6], determining the electrochemical stability of DES and other ionic liquid analogues. There is a growing body of work on how these HBA-HBD clusters interact with surfaces under different conditions [7] but very little is known about the effect of the
* Corresponding author. E-mail address: [email protected] (J.D. Gamarra). https://doi.org/10.1016/j.electacta.2019.04.154 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
electrochemical decomposition on the electrode and the electrolyte. Previous studies of the electrochemical decomposition of DES were done via gas chromatography [8]. This approach can miss the effects and species that are formed and bound with the electrode surface, which is the focus of this work. The formation and adsorption of species on the surface can lead to inefficiencies in redox processes [9] or passivisation of the deposited nanoparticles [5]. Thus, it is essential to know which are the species that are being produced and adsorbed on the electrode and how the composition of the electrolyte affects them. The studies about the electrochemical decomposition by Haerens et al. [8]. the possible cathodic process of choline based DES is described. The decomposition is ed by the Hoffman elimination in choline chloride and this determines the reduction potential limit of the electrolyte. The main drive for this reaction is the formation of choline hydroxide. This is formed at the cathode where the local concentration of OH- is increased by the reduction of water. The hydroxide is unstable and it decomposes into trimethylamine and an enol. The electrochemical decomposition can also lead to the formation of the similar byproducts like trimethylamine as seen in Fig. 1, so gas chromatography is unable to determine the origin of the byproducts or any adsorbates and complexes that could be
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Fig. 1. Reaction pathways for the decomposition of choline chloride, Top: Hoffman elimination; Bottom: a possible route for electrochemical decomposition [8].
formed on the electrode surface. The studies on how the nature of DES affects the deposition process are focused on the speciation and transport properties of the deposited material, but the reactions between electrolyte and substrate are just as relevant. The electrochemical double layer in DES and other ionic liquids has been described as a system that tends to overcrowding [10]. In this system the ions accumulate in several layers of the same charge on the surface of the electrode, and the co-ions and hydrogen bond donors are left in the following layers [11,12]. In this work, we focus on the interactions between an ideal inert substrate such as glassy carbon (GC) and choline chloride ethylene glycol based solvents. We study the effect of different HBD ratios on the surface reactions and finally how these can affect the electrodeposition of iron. Iron has been deposited using DES based on different hydrogen bond donors like urea and ethylene glycol, with varying levels of success [9,13]. Deposition from DES helps to avoid the typical problems present in aqueous media such as hydroxide formation at the anode and Fe(II) to Fe(III) oxidation. Bock and Manh report low efficiency and a tendency to form oxides [13,14] from ChCl:Urea. Deposits were also done in ethylene glycol and glycerol based solvents by Miller et al. and Panzeri [9,15]. The high concentration of chloride creates a very aggressive environment for the substrate and deposit. To solve this the use of excess amounts of ethylene glycol, leading to a non eutectic mixture, have been proposed. These ethylene glycol solutions show comparable properties like conductivity and electrochemical window to DES, and they yield iron deposits under similar conditions with consistent efficiency above 80%. Still, the effect of the electrolyte on the substrate is not fully understood. The excess ethylene glycol is known to affect the speciation of iron in solution, but there is no clear understanding on how this affects the deposited material. In this study, to determine the adsorbed species on the electrode and the electrodeposited iron, time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used. ToF-SIMS offers high detection sensitivity, for single elements and molecular fragments up to 1 ppm, and mass imaging with sub-micron spatial resolution. By using the mass spectra collected on the different points of the surface, it is possible to build a 2D image of the chemical distribution. The innovative aspect brought by this technique is that by using the spectra and chemical maps from the choline:ethylene glycol solutions, we can provide a better understanding of the solvent-deposit interactions than the current state of the art. We analyze the relation between composition and potential and identify if any species are adsorbed and their effect on the deposition of iron. We focus on the molecular fingerprint of the electrode-electrolyte interactions in choline based media, instead of electrochemical reaction analysis to bring additional information
to existing literature. 2. Materials and methods Choline chloride (Sigma-Aldrich, > 98%) was mixed with ethylene glycol (Merck, 99.5%) in molar ratios 1:2 and 1:4. The mixtures were stirred and kept at 80C for 6 h and nitrogen gas is bubbled through the solution before the experiments. A Karl Fischer coulometer was used to control the water quantity before each experiment and an average value of (1.0 ± 0.1) weight % of water was obtained. 0.1 M Iron chloride (II) dihydrate (Merck, 99% was added for the electrodeposition experiments. Glassy carbon electrodes were SIGRADUR K (HTW Germany), 7 mm diameter disc mounted in resin polished with 0.05 mm alumina were used for analysis and deposition plates were 10 20 mm used as received after rinsing with DI water. Linear sweep voltammetry (LSV) and chonoamperometry (CA) measurements were performed in an electrochemical cell with a three-electrode system, consisting of a silver wire quasi reference electrode (Ag/AgCl QRE), a dimensionally stable counter electrode, made of titanium oxide covered with ruthenium indium titanium oxides. This is coupled to a glassy carbon working electrode. All the potentials mentioned throughout the manuscript refer to Ag/AgCl QRE constructed by inserting a chloridised silver wire into a fritted glass capillary filled with 1:2 M ratio choline chloride to ethylene glycol. LSV was done between OCP and 1.15 V vs Ag/AgCl QRE at 1 mV/s for all solutions and CA measurements were done at selected potentials for 600s in order to ensure steady state conditions. The samples were then rinsed repeatedly with water and ethanol and dried. The deposit characterization was done using a Jeol-7100 F field emission scanning electrom microscope at 15 keV and 10 mm working distance. This was done to determine the microstructure and the morphology of the deposit. ToF-SIMS measurements were performed with a TOF. SIMS 5 instrument from ION-TOF GmbH (Münster, Germany). Positive ion mass spectra were acquired using a 30 keV Bi3þ primary ion beam operated in the high current bunched mode for high mass resolution (approximately 8000 at 29 mm (29Siþ)). In this mode, a lateral resolution of 3 mm is achieved, with a surface sensitivity of 1e5 nm. The pulsed ion beam target current was approximately 0.60 pA. Large area images were acquired on glassy carbon substrates, exposed to DES under varying polarisation conditions, by rastering an area of 1.0 mm 2.5 mm, divided into 10 patches of 500 mm 500 mm. Each patch was analysed for 30 s, with a pixel density of 256 pixels/mm, to ensure that the static limit of 1 1013 ions cm2 analysis1 was not exceeded. The accuracy or deviation of a mass assignment (in ppm) is calculated by subtracting the theoretical mass from the experimental mass of a fragment and dividing this difference by the experimental mass.
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Deviations (either negative or positive) with an absolute value below 50 ppm are indicative of good assignments. In each spectrum or image, intensities are normalised to the total ion intensity, typically to reduce topographic and matrix effects. This allows for a relative quantification between samples with similar chemistries [16,17]. 3. Results and discussion 3.1. Electrochemical profile of 1:2 and 1:4 ChCl:EG electrolytes on GC The solutions were polarised from their equilibrium potential to 1.15 V (vs Ag/AgCl QRE), via linear sweep voltammetry (Fig. 2). This is done to determine the potentials at which relevant electrochemical processes happen. 1.15 V (vs Ag/AgCl QRE) was chosen as the end point based on reports by Miller for iron deposition [9] and the onset of water splitting after 1.2 V. The blank solution doesn't reach 1.0 103A =cm2 , one of the definitions used for electrolyte decomposition [18]. Based on the voltammogram, different polarisation potentials were chosen to study the interactions between electrolyte and the electrode surface. The first point chosen was the open circuit potential (EOC). Here electrode-electrolyte interactions have no external driver other than the affinity of the solvent for the electrode. This will provide the baseline for ToF-SIMS analysis to identify the products that are appearing at each of the potentials of interest. In the 1:4 solution, there is an increase of current as the potential goes from 0 to 0.60 V. It has been suggested that this is due to the adsorption and possible breakdown of free choline on the surface [19], since in the 1:4 mixtures, the excess ethylene glycol is forming more hydrogen bonds with other EG molecules rather than the choline ions [9,20,21]. So 0.60 V was chosen to check this hypothesis. After this, a third potential was chosen at 0.95 V because this is the reported deposition potential for iron in DES and ethylene glycol solutions [9]. Finally, at 1.15 V vs QRE, the electrolyte and/or the water in it are being electrochemically reduced so this is the final point analysed.
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3.2. Study of the potential dependent interactions between GC electrodes and ChCl:EG solutions 3.2.1. 1:2 Choline chloride: ethylene glycol GC electrodes were immersed in a 1:2 ChCl:EG mixture for 600s under varying polarisation potentials (i.e., open circuit potential (EðOCÞ ), 0.60 V, 0.95 V and 1.15 V). Fig. 3 shows an overlay of ToF-SIMS mass spectra acquired from large area (1 mm 2.5 mm) imaging on exposed GC substrates. Remarkably in all spectra (but especially at EðOCÞ ) is the presence of polydimethylsiloxane (PDMS), indicated by a peak found at a nominal mass of 73 m/z, which is attributed to SiC3 H þ 9 . PDMS is a common contaminant, known to be present in tape and markers, detected by ToF-SIMS with a very high sensitivity. Furthermore, high Naþ intensities are observed for GC substrates that have been exposed to more negative potentials. This may suggest that contaminant sodium ions present in the ChCl:EG mixture were dragged towards the electrode by the negative potential. Nevertheless, sodium and PDMS are common contaminants, do not overlap with any fragment of interest, and therefore are of no concern. Most important for this study are fragments that are indicative for the presence of a ChCl:EG solvent residue on the electrodes. Ethylene glycol residues would not be easily distinguishable from the carbonaceous contamination that is always present on any kind of sample, but choline, which contains a nitrogen, can be easily recognised. Fragments associated to a choline residue were found in the mass spectra. The most intense ones are listed in Table 1. A peak found at a nominal mass of 104 m/z was assigned to C5 H14 NOþ and represents the molecular fragment of choline. From the spectrum overlay in Fig. 3 it is clear that the choline residue is much stronger present on GC substrates that have been exposed to more negative potentials compared to the GC substrate at EðOCÞ . More information about the choline residues can be extracted by looking at the large area (1 mm 2.5 mm) ToF-SIMS images, specifically for fragments of interest. Fig. 4 shows the spatial distribution of C5 H14 NOþ and C5 H12 Nþ on GC substrates exposed to varying polarisation potentials. Both fragments represent the choline residue, their pattern is quite similar, but the C5 H14 NOþ signal is more intense. Choline residues are found on every sample,
Fig. 2. Linear sweep voltammetries (LSV) on and GC electrodes of ChCl:EG mixtures in 1:2 (EOC ¼ 0.55 V) and 1:4 ratios (EOC ¼ 0.34 V).
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Fig. 3. Overlay of ToF-SIMS mass spectra (0e109 m/z) obtained on glassy carbon substrates exposed to a 1:2 ChCl:EG mixture under varying polarisation potentials. Spectra were acquired from large area (1 mm 2.5 mm) imaging.
Table 1 Mass fragments associated to a choline residue on GC electrodes. Fragment
Mass (amu)
Deviation (ppm)
C5 H14 NOþ C3 H 8 N þ C3 H 9 N þ C5 H12 N þ
104.1097 58.0680 59.0741 86.1006
26.4 49.7 19.4 48.1
even on the GC that was immersed without applying any overpotential. At EðOCÞ and 0.60 V, choline deposits seem to concentrate in microdomains with diameters ranging from 20 mm to 300 mm. Choline adsorption significantly increases on GC substrates at 0.95 V and 1.15 V, and is no longer concentrated in specific microdomains. It appears that the amount of choline residue on GC electrodes increases as a function of negative overpotential. This fits descriptions by Hammond [7] of the nanostructure of the interface for DES as tightly packed Stern layers of cholinium. The accumulation of choline residue leads to a layer of reactive quaternary ammonium salts that makes up the surface at all times.
3.2.2. 1:4 Choline chloride: ethylene glycol Similar ToF-SIMS measurements were performed on GC substrates immersed in a 1:4 ChCl:EG mixture, in order to evaluate the effect of an increased ethylene glycol concentration on the choline residues. Fig. 5 shows an overlay of mass spectra obtained on GC exposed to a 1:2 (top) and a 1:4 (bottom) mixture of ChCl:EG at 0.60 V. In the 0e109 m/z mass range, the largest differences are found at nominal masses 86 m/z and 91 m/z, where peaks attributed to C5 H12 N þ and C7 H þ 7 , respectively, seem to dominate the spectrum obtained on GC exposed to a 1:4 ChCl:EG mixture. Peak areas normalised to total ion intensity are plotted at different potentials for C5 H14 NOþ, 13 CC4 H12 Nþ and 13 CC6 H þ 7 in Fig. 6, both for the 1:2 ChCl:EG series as for the 1:4 ChCl:EG series. þ þ þ 13 C isotopes of C5 H12 N and C7 H 7 were chosen because C5 H12 N
and C7 H þ 7 signals were saturated in the 1:4 ChCl:EG series. A strong increase in intensity of C5 H12 Nþ and C7 Hþ 7 is observed on each GC substrate that has been immersed in a 1:4 ChCl:EG mixture (Fig. 6). On the other hand, while C5 H14 NOþ intensities were increasing as a function of applied negative overpotential for the 1:2 ChCl:EG series (Fig. 6a), this behaviour is not observed in the 1:4 ChCl:EG series. The C5 H14 NOþ intensities measured on GC substrates exposed to 0.95 V and 1.15 V are remarkably low in the 1:4 ChCl:EG series compared to their analogues in the 1:2 ChCl:EG series. The strong increase in intensity of C5 H12 N þ , compared to the intensity of the molecular choline fragment C5 H14 NOþ , points out that the solvent deposits formed on GC in a 1:4 ChCl:EG mixture are chemically different from the choline residues found in the 1:2 ChCl:EG series. This becomes even more evident in the ToF-SIMS images (Fig. 7) that show the spatial distribution of C5 H14 NOþ and the C5 H12 N þ isotope on GC substrates exposed to varying polarisation potentials for the 1:4 ChCl:EG series. Remarkably, the spatial distribution of C5 H12 Nþ , a fragment that had been assigned characteristic for the choline residue in the 1:2 ChCl:EG series (Table 1), does not match with the C5 H14 NOþ pattern in the 1:4 ChCl:EG series. This means that two chemically different solvent deposits are formed on the GC substrates. The C5 H14 NOþ signal still represents the choline residue, while C5 H12 N þ is now mainly attributed to a deposit specifically formed in an ethylene glycol rich environment (1:4), hereafter referred to as the Ch:EG deposit. In general, the amount of adsorbed choline (represented by the C5 H14 NOþ signal in Fig. 7) is very low compared to the 1:2 ChCl:EG analogue and seems to be concentrated in smaller microdomains, which do not grow that much at high overpotentials (0.95 V and 1.15 V). It seems that the formation of Ch:EG deposits is preventing the surface to be fully covered by a choline layer. It is unclear why Ch:EG deposits (represented by the C5 H12 N þ isotope) are highly concentrated in specific domains at 0.60 V and 1.15 V, and more homogeneously distributed at EðOCÞ and 0.95 V.
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Fig. 4. Large area (1.5 mm 2.5 mm) ToF-SIMS images show the spatial distribution of C5 H14 NOþ (top row) and C5 H12 N þ (bottom row) on GC electrodes exposed to a 1:2 ChCl:EG mixture under varying polarisation potentials. Plotted intensities are normalised to total ion intensity.
Fig. 5. Overlay of ToF-SIMS mass spectra (0e109 m/z) obtained on glassy carbon substrates exposed to a 1:2 ChCl:EG mixture (top) and a 1:4 ChCl:EG mixture (bottom) under an applied potential of 0.60 V. Spectra were acquired from large area (1 mm 2.5 mm) imaging.
As Fig. 6 already showed that the C7 H þ 7 signal follows the same trend as the C5 H12 Nþ signal, also the C7 Hþ 7 images (not shown) give exactly the same pattern as the C5 H12 Nþ images. Therefore, also C7 H þ 7 can be assigned as a characteristic fragment for the Ch:EG deposit. Although the mass spectra in Fig. 5 only show the 0e109 m/z mass range, other fragments in a higher mass range were found to be characteristic for the Ch:EG deposit as well and are listed in Table 2. Fragments as C10 H14 Nþ and C11 H16 N þ , characteristic for
the Ch:EG deposit, are larger than the molecular choline fragment alone. This suggests that in excess of EG, a large molecular network is formed. Choline ions may have reacted with EG and/or other choline ions to form the Ch:EG deposit. 3.3. Effects of solvent on the deposition of iron Iron was electrodeposited from a 1:2 and a 1:4 ChCl:EG
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Fig. 6. ToF-SIMS C5 H14 NOþ (a), 13 CC4 H12 N þ (b) and 13 CC6 H þ 7 (c) peak areas normalised to total ion intensity are plotted for GC substrates exposed to 1:2 ChCl:EG mixtures and 1:4 ChCl:EG mixtures at varying polarisation potentials.
Fig. 7. Large area (1.5 mm 2.5 mm) ToF-SIMS images show the spatial distribution of C5 H14 NOþ (top row) and ChCl:EG mixture under varying polarisation potentials. Plotted intensities are normalised to total ion intensity.
Table 2 Mass fragments associated to the Ch:EG deposit formed on GC electrodes in a 1:4 ChCl:EG mixture. C5 H12 N þ and are saturated, therefore their deviations are slightly higher. Fragment Nþ
C5 H12 C7 H þ 7 C7 H 8 N þ C8 H10 N þ C9 H12 N þ C10 H14 N þ C11 H16 N þ
Mass (amu)
Deviation (ppm)
86.1031 91.0608
77.8 72.0
106.0662 120.0793 134.0944 148.1096 162.1239
10.6 11.9 15.2 17.0 23.8
13 CC4 H12 N
þ
(bottom row) on GC electrodes exposed to a 1:4
electrolyte. LSV measurements (Fig. 8) show how the amount of ethylene glycol can affect the reactivity of iron. It has been regularly assumed that there is no significant contribution from the electrolyte decomposition based on the 2 orders of magnitude difference between the currents achieved during iron deposition and the currents measured in blank solutions (green curves in Fig. 8). In 1:2 ChCl:EG mixtures, there is a single sharp increase in current around 0.95 V. The reduction of iron and solvent decomposition cannot be distinguished electrochemically at this point. Meanwhile in the 1:4 ChCl:EG electrolyte, the first electrochemical event starts at 0.65 V, with a small shoulder at slightly more positive potential. This process stabilises from 0.75 V, until a second reaction, likely
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Fig. 8. Linear Sweep Voltammetry for iron chloride (0.1 M) solutions of Choline chloride and ethylene glycol on GC. Blank solutions under same conditions present in green and gray. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
due to solvent decomposition, starts to dominate after 1.00 V. Scanning electron microscopy (SEM) images in Fig. 9 show the overall effect of the electrolyte and the applied overpotential on the structure of the deposits. In fact, large differences in appearance were already seen with the naked eye. Flaky deposits, full of cracks, are formed under exposure to the highest overpotential (1.15 V), both in the 1:2 ChCl:EG electrolyte (Fig. 9c) as in the 1:4 ChCl:EG electrolyte (Fig. 9d). Fewer flakes (not shown here in the SEM image, but later on in the ToF-SIMS image) are observed for the
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deposit formed at 0.95 V in the 1:2 ChCl:EG mixture. Fig. 9a shows that, besides flaky deposits, also small metal nuclei are formed under these conditions. Finally, the most uniform deposit is obtained at 0.95 V in the 1:4 ChCl:EG electrolyte; Fig. 9b shows a uniform film constituted of small aggregates. Large area ToF-SIMS imaging was performed to study the chemical composition of the different deposits. Fig. 10 provides a general overview; for each condition, a Feþ image (blue intensity scale) is given, next to an overlay image of Feþ (blue), the C5 H14 NOþ isotope (red) and C5 H12 Nþ (yellow). All images were set to the same scaling for ease of comparison. Fig. 10a shows that flaky iron deposits are formed on GC at 0.95 V in a 1:2 ChCl:EG electrolyte. Besides these flaky deposits, the presence of iron based nuclei, as observed in SEM (Fig. 9a), is confirmed. The overlay images obtained for the flaky iron deposits at 1.15 V (Fig. 10c and d) show a complementary pattern between Feþ on the one hand and electrolyte residue fragments (represented by 13 CC4 H14 NOþ and C5 H12 Nþ ) on the other hand. Remarkably, the 13 CC4 H14 NOþ pattern and the C5 H12 Nþ pattern coincide in the 1:4 ChCl:EG series, while they show different patterns in the 1:2 ChCl:EG series. Based on the blank studies in section 3.2, one would expect the opposite trend. This demonstrates that the process has become more complex in the presence of iron and that one should be careful to make a direct comparison with the blank studies. Furthermore, it should be noticed that the rinsing step, which is performed after removal of the sample from the electrolyte, is difficult to control on the flaky iron deposits. Due to poor adhesion to the substrates, flaky iron deposits are easily removed by rinsing. Although the Feþ patterns in Fig. 10 show us the distribution of the iron deposits, they do not give information about the speciation of iron in these deposits. Information about the speciation of iron was extracted from ToF-SIMS imaging in negative polarity. Fig. 11
Fig. 9. Scanning electron microscopy images of iron deposits on GC from (top row) 1:2 ChCl:EG mixtures at (a) 0.95 V and (c) 1.15 V and from (bottom row) 1:4 ChCl:EG mixtures at (b) 0.95 V and (d) 1.15 V.
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Fig. 10. ToF-SIMS Feþ , 13 CC4 H14 NOþ and C5 H12 N þ peak areas normalised to total ion intensity are plotted for GC substrates exposed to 1:2 ChCl:EG mixtures and 1:4 ChCl:EG mixtures at varying polarisation potentials.
shows the ToF-SIMS images of fragments identified as FeCl 2 and FeC2 H3 O 2 . FeCl2 is representative of quick formation of iron chloroxides. FeC2 H3 O 2 is representative for the iron-glycolate complex. FeC2 H3 O2 Cl 2 has been proposed by Miller et al. as the dominant charge carrier in 1:4 solutions [9] over FeCl4 , which dominates in 1:2. This glycolate can act as a capping agent, inhibiting further growth and aggregation, while driving the granular morphology seen in Fig. 9b. Mainly iron chlorides are found in deposits obtained from 1:2 ChCl:EG electrolytes (Fig. 11a and c), consistent with the higher relative concentration of chloride in this electrolyte. In contrast, the deposit surface formed at 0.95 V in a 1:4 ChCl:EG electrolyte seems to consist mainly of iron-glycolate complexes (Fig. 11b), whereas the presence of iron chlorides is largely reduced. Interestingly, at the highest overpotential (1.15 V), deposits mainly consist of iron chlorides again (Fig. 11d). This is probably due to the electrolyte breakdown. Fig. 12 gives a final overview of the relative distribution of the different iron containing species. Iron-glycolate complexes, which are desirable, are dominantly formed at 0.95 V in a 1:4 ChCl:EG electrolyte. All other conditions result into iron chloride layer on the surface.
4. Conclusions hlGC substrates were exposed to blank 1:2 and 1:4 ChCl:EG electrolytes under varying polarisation potentials and analysed afterwards with ToF-SIMS. Choline residues were found on GC substrates exposed to the 1:2 ChCl:EG electrolyte, with the amount of adsorbed choline strongly increasing as a function of negative potential, i.e., GC electrode surfaces were saturated with choline at 0.95 V and 1.15 V. A different type of residue was found in the 1:4 ChCl:EG series. In excess of EG, a large molecular network is formed, supposedly by reactions between ethylene glycol and choline. This ChCl:EG deposit prevents accumulation of choline residues on the GC electrodes at high negative overpotentials; no saturation of solvent residue is observed at 0.95 V and 1.15 V. In a second set of experiments, the electrodeposition of iron was evaluated. At 1.15 V, flaky deposits full of cracks were obtained, both from 1:2 as from 1:4 ChCl:EG electrolytes. These deposits showed poor adhesion to the substrate and were identified by ToFSIMS as iron chloride deposits. Iron chloride deposits were also found on GC exposed to 0.95 V in the 1:2 ChCl:EG electrolyte. The best quality iron deposit was obtained at 0.95 V in the 1:4 ChCl:EG
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Fig. 11. ToF-SIMS FeCl 2 and FeC2 H3 O2 peak areas normalised to total ion intensity are plotted for GC substrates exposed to 1:2 ChCl:EG mixtures and 1:4 ChCl:EG mixtures at varying polarisation potentials. All images were set to the same scale.
Acknowledgements J.D.G acknowledges financial support by the Colombian Administrative Department of Science, Technology and Innovation (Colciencias). SURF acknowledges the Hercules program under grant agreement ZW13 07 for the ToF-SIMS measurements. T.H. acknowledges financial support by Research Foundation Flanders (FWO). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.04.154.
Fig. 12. Relative presence of iron complexes for GC substrates exposed to 1:2 ChCl:EG mixtures and 1:4 ChCl:EG mixtures at varying polarisation potentials.
electrolyte. A uniform layer of iron glycolates was formed under these conditions. While this work has focused only on the interactions between ChCl:EG electrolytes and GC substrates, there exists a broad spectre of DES and non aqueous media, applied for metal electrodeposition on various substrates. A detailed study on the metal complexes and corrosion of metallic substrates as a function of water content will be described in a follow up publication.
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[6] M.H. Mamme, S.L.C. Moors, H.A. Terryn, J. Deconinck, J. Ustarroz, F. De Proft, Atomistic insights into the electrochemical double layer of choline chlorideurea deep eutectic solvent: a clustered interfacial structuring, J. Phys. Chem. Lett. 9 (2018) 6296e6306. [7] O.S. Hammond, H. LI, C. Westermann, F. Endres, A.Y. Al-Murshedi, A.P. Abbott, G. Warr, K.J. Edler, R. Atkin, Nanostructure of the deep eutectic solvent/platinum electrode interface as a function of potential and water content, Nanoscale Horizons (2018). [8] K. Haerens, E. Matthijs, K. Binnemans, B. Van der Bruggen, Electrochemical decomposition of choline chloride based ionic liquid analogues, Green Chem. 11 (2009) 1357. [9] M.A. Miller, J.S. Wainright, R.F. Savinell, Iron electrodeposition in ionic-liquid electrolytes for flow batteries, J. Electrochem. Soc. 164 (2017) A796eA803. [10] Z.A. Goodwin, A.A. Kornyshev, Underscreening, overscreening and doublelayer capacitance, Electrochem. Commun. 82 (2017) 129e133. [11] M.V. Fedorov, A. a. Kornyshev, Ionic liquids at electrified interfaces, Chem. Rev. (2014) 2978e3036. [12] R. Costa, C.M. Pereira, A. Fernando Silva, Structural ordering transitions in ionic liquids mixtures, Electrochem. Commun. 57 (2015) 10e13. € ck, S.-E. Wulf, Electrodeposition of iron films from an ionic liquid (ChCl/ [13] R. Bo urea/FeCl3 deep eutectic mixtures), Transactions of the IMF 87 (2009) 28e32. [14] T.L. Manh, E.M. Arce-Estrada, I. Mejía-Caballero, E. Rodríguez-Clemente, nchez, J. Aldana-Gonz W. Sa alez, L. Lartundo-Rojas, M. Romero-Romo, , Iron electrodeposition from Fe(II) ions dissolved in a M. Palomar-Pardave
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrode-electrolyte interactions in choline chloride ethylene glycol based solvents and their effect on the electrodeposition of iron Jorge D. Gamarra*, Kristof Marcoen, Annick Hubin, Tom Hauffman Vrije Universiteit Brussel, Research Group of Electrochemical and Surface Engineering (SURF), Brussels, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 December 2018 Received in revised form 25 April 2019 Accepted 25 April 2019 Available online 5 May 2019
Although deep eutectic solvents (DES) and ethylene glycol (EG) electrolytes are gaining ground as media for the electrodeposition of metals, the influence of these electrolytes on the electrode and electrodeposits has not been elaborated before. In this work, we investigate how choline chloride: ethylene glycol based (ChCl:EG) electrolytes interact with the glassy carbon (GC) working surface at different potentials and, how these interactions change during the electrodeposition of iron from 1:2 and 1:4 ChCl:EG electrolytes. GC substrates are exposed to both electrolytes in absence of iron in order to study the pure electrolyte-substrate interactions. Linear sweep voltammetry was used to determine the potential range in which the different reactions occur, while time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to detect and identify the molecules adsorbed on the surface. In all systems, there is an accumulation of either choline or its derivatives on the electrode surface, with 1:4 ChCl:EG electrolytes showing different decomposition products from 1:2 ChCl:EG electrolytes. Choline accumulation and decomposition was also found on iron electrodeposits. The electrolyte composition has a major impact on the chemical speciation of iron, and on the deposit's adherence to the substrate. These are two crucial characteristics that define the efficiency of iron deposition from DES. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Iron deposition Time-of-flight secondary ion spectrometry Non aqueous solvents
1. Introduction Deep eutectic solvents (DES) are promising electrochemical media [1]. They offer a low-cost alternative to ionic liquids and a broader electrochemical potential window than aqueous electrolytes, opening the door to a range of applications in electrochemistry [2,3]. One of these applications is the electrodeposition of metallic nanoparticles and coatings [4,5], where they should reduce the problem of hydrogen embrittlement. The existence of DES relies on strong interactions between their components; typically, a quaternary ammonia salt such as choline chloride acting as a hydrogen bond acceptor(HBA), and an organic molecule like ethylene glycol as a hydrogen bond donor (HBD). These interactions allow the ions from the HBA and the HBD to move as a single entity [6], determining the electrochemical stability of DES and other ionic liquid analogues. There is a growing body of work on how these HBA-HBD clusters interact with surfaces under different conditions [7] but very little is known about the effect of the
* Corresponding author. E-mail address: [email protected] (J.D. Gamarra). https://doi.org/10.1016/j.electacta.2019.04.154 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
electrochemical decomposition on the electrode and the electrolyte. Previous studies of the electrochemical decomposition of DES were done via gas chromatography [8]. This approach can miss the effects and species that are formed and bound with the electrode surface, which is the focus of this work. The formation and adsorption of species on the surface can lead to inefficiencies in redox processes [9] or passivisation of the deposited nanoparticles [5]. Thus, it is essential to know which are the species that are being produced and adsorbed on the electrode and how the composition of the electrolyte affects them. The studies about the electrochemical decomposition by Haerens et al. [8]. the possible cathodic process of choline based DES is described. The decomposition is ed by the Hoffman elimination in choline chloride and this determines the reduction potential limit of the electrolyte. The main drive for this reaction is the formation of choline hydroxide. This is formed at the cathode where the local concentration of OH- is increased by the reduction of water. The hydroxide is unstable and it decomposes into trimethylamine and an enol. The electrochemical decomposition can also lead to the formation of the similar byproducts like trimethylamine as seen in Fig. 1, so gas chromatography is unable to determine the origin of the byproducts or any adsorbates and complexes that could be
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Fig. 1. Reaction pathways for the decomposition of choline chloride, Top: Hoffman elimination; Bottom: a possible route for electrochemical decomposition [8].
formed on the electrode surface. The studies on how the nature of DES affects the deposition process are focused on the speciation and transport properties of the deposited material, but the reactions between electrolyte and substrate are just as relevant. The electrochemical double layer in DES and other ionic liquids has been described as a system that tends to overcrowding [10]. In this system the ions accumulate in several layers of the same charge on the surface of the electrode, and the co-ions and hydrogen bond donors are left in the following layers [11,12]. In this work, we focus on the interactions between an ideal inert substrate such as glassy carbon (GC) and choline chloride ethylene glycol based solvents. We study the effect of different HBD ratios on the surface reactions and finally how these can affect the electrodeposition of iron. Iron has been deposited using DES based on different hydrogen bond donors like urea and ethylene glycol, with varying levels of success [9,13]. Deposition from DES helps to avoid the typical problems present in aqueous media such as hydroxide formation at the anode and Fe(II) to Fe(III) oxidation. Bock and Manh report low efficiency and a tendency to form oxides [13,14] from ChCl:Urea. Deposits were also done in ethylene glycol and glycerol based solvents by Miller et al. and Panzeri [9,15]. The high concentration of chloride creates a very aggressive environment for the substrate and deposit. To solve this the use of excess amounts of ethylene glycol, leading to a non eutectic mixture, have been proposed. These ethylene glycol solutions show comparable properties like conductivity and electrochemical window to DES, and they yield iron deposits under similar conditions with consistent efficiency above 80%. Still, the effect of the electrolyte on the substrate is not fully understood. The excess ethylene glycol is known to affect the speciation of iron in solution, but there is no clear understanding on how this affects the deposited material. In this study, to determine the adsorbed species on the electrode and the electrodeposited iron, time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used. ToF-SIMS offers high detection sensitivity, for single elements and molecular fragments up to 1 ppm, and mass imaging with sub-micron spatial resolution. By using the mass spectra collected on the different points of the surface, it is possible to build a 2D image of the chemical distribution. The innovative aspect brought by this technique is that by using the spectra and chemical maps from the choline:ethylene glycol solutions, we can provide a better understanding of the solvent-deposit interactions than the current state of the art. We analyze the relation between composition and potential and identify if any species are adsorbed and their effect on the deposition of iron. We focus on the molecular fingerprint of the electrode-electrolyte interactions in choline based media, instead of electrochemical reaction analysis to bring additional information
to existing literature. 2. Materials and methods Choline chloride (Sigma-Aldrich, > 98%) was mixed with ethylene glycol (Merck, 99.5%) in molar ratios 1:2 and 1:4. The mixtures were stirred and kept at 80C for 6 h and nitrogen gas is bubbled through the solution before the experiments. A Karl Fischer coulometer was used to control the water quantity before each experiment and an average value of (1.0 ± 0.1) weight % of water was obtained. 0.1 M Iron chloride (II) dihydrate (Merck, 99% was added for the electrodeposition experiments. Glassy carbon electrodes were SIGRADUR K (HTW Germany), 7 mm diameter disc mounted in resin polished with 0.05 mm alumina were used for analysis and deposition plates were 10 20 mm used as received after rinsing with DI water. Linear sweep voltammetry (LSV) and chonoamperometry (CA) measurements were performed in an electrochemical cell with a three-electrode system, consisting of a silver wire quasi reference electrode (Ag/AgCl QRE), a dimensionally stable counter electrode, made of titanium oxide covered with ruthenium indium titanium oxides. This is coupled to a glassy carbon working electrode. All the potentials mentioned throughout the manuscript refer to Ag/AgCl QRE constructed by inserting a chloridised silver wire into a fritted glass capillary filled with 1:2 M ratio choline chloride to ethylene glycol. LSV was done between OCP and 1.15 V vs Ag/AgCl QRE at 1 mV/s for all solutions and CA measurements were done at selected potentials for 600s in order to ensure steady state conditions. The samples were then rinsed repeatedly with water and ethanol and dried. The deposit characterization was done using a Jeol-7100 F field emission scanning electrom microscope at 15 keV and 10 mm working distance. This was done to determine the microstructure and the morphology of the deposit. ToF-SIMS measurements were performed with a TOF. SIMS 5 instrument from ION-TOF GmbH (Münster, Germany). Positive ion mass spectra were acquired using a 30 keV Bi3þ primary ion beam operated in the high current bunched mode for high mass resolution (approximately 8000 at 29 mm (29Siþ)). In this mode, a lateral resolution of 3 mm is achieved, with a surface sensitivity of 1e5 nm. The pulsed ion beam target current was approximately 0.60 pA. Large area images were acquired on glassy carbon substrates, exposed to DES under varying polarisation conditions, by rastering an area of 1.0 mm 2.5 mm, divided into 10 patches of 500 mm 500 mm. Each patch was analysed for 30 s, with a pixel density of 256 pixels/mm, to ensure that the static limit of 1 1013 ions cm2 analysis1 was not exceeded. The accuracy or deviation of a mass assignment (in ppm) is calculated by subtracting the theoretical mass from the experimental mass of a fragment and dividing this difference by the experimental mass.
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Deviations (either negative or positive) with an absolute value below 50 ppm are indicative of good assignments. In each spectrum or image, intensities are normalised to the total ion intensity, typically to reduce topographic and matrix effects. This allows for a relative quantification between samples with similar chemistries [16,17]. 3. Results and discussion 3.1. Electrochemical profile of 1:2 and 1:4 ChCl:EG electrolytes on GC The solutions were polarised from their equilibrium potential to 1.15 V (vs Ag/AgCl QRE), via linear sweep voltammetry (Fig. 2). This is done to determine the potentials at which relevant electrochemical processes happen. 1.15 V (vs Ag/AgCl QRE) was chosen as the end point based on reports by Miller for iron deposition [9] and the onset of water splitting after 1.2 V. The blank solution doesn't reach 1.0 103A =cm2 , one of the definitions used for electrolyte decomposition [18]. Based on the voltammogram, different polarisation potentials were chosen to study the interactions between electrolyte and the electrode surface. The first point chosen was the open circuit potential (EOC). Here electrode-electrolyte interactions have no external driver other than the affinity of the solvent for the electrode. This will provide the baseline for ToF-SIMS analysis to identify the products that are appearing at each of the potentials of interest. In the 1:4 solution, there is an increase of current as the potential goes from 0 to 0.60 V. It has been suggested that this is due to the adsorption and possible breakdown of free choline on the surface [19], since in the 1:4 mixtures, the excess ethylene glycol is forming more hydrogen bonds with other EG molecules rather than the choline ions [9,20,21]. So 0.60 V was chosen to check this hypothesis. After this, a third potential was chosen at 0.95 V because this is the reported deposition potential for iron in DES and ethylene glycol solutions [9]. Finally, at 1.15 V vs QRE, the electrolyte and/or the water in it are being electrochemically reduced so this is the final point analysed.
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3.2. Study of the potential dependent interactions between GC electrodes and ChCl:EG solutions 3.2.1. 1:2 Choline chloride: ethylene glycol GC electrodes were immersed in a 1:2 ChCl:EG mixture for 600s under varying polarisation potentials (i.e., open circuit potential (EðOCÞ ), 0.60 V, 0.95 V and 1.15 V). Fig. 3 shows an overlay of ToF-SIMS mass spectra acquired from large area (1 mm 2.5 mm) imaging on exposed GC substrates. Remarkably in all spectra (but especially at EðOCÞ ) is the presence of polydimethylsiloxane (PDMS), indicated by a peak found at a nominal mass of 73 m/z, which is attributed to SiC3 H þ 9 . PDMS is a common contaminant, known to be present in tape and markers, detected by ToF-SIMS with a very high sensitivity. Furthermore, high Naþ intensities are observed for GC substrates that have been exposed to more negative potentials. This may suggest that contaminant sodium ions present in the ChCl:EG mixture were dragged towards the electrode by the negative potential. Nevertheless, sodium and PDMS are common contaminants, do not overlap with any fragment of interest, and therefore are of no concern. Most important for this study are fragments that are indicative for the presence of a ChCl:EG solvent residue on the electrodes. Ethylene glycol residues would not be easily distinguishable from the carbonaceous contamination that is always present on any kind of sample, but choline, which contains a nitrogen, can be easily recognised. Fragments associated to a choline residue were found in the mass spectra. The most intense ones are listed in Table 1. A peak found at a nominal mass of 104 m/z was assigned to C5 H14 NOþ and represents the molecular fragment of choline. From the spectrum overlay in Fig. 3 it is clear that the choline residue is much stronger present on GC substrates that have been exposed to more negative potentials compared to the GC substrate at EðOCÞ . More information about the choline residues can be extracted by looking at the large area (1 mm 2.5 mm) ToF-SIMS images, specifically for fragments of interest. Fig. 4 shows the spatial distribution of C5 H14 NOþ and C5 H12 Nþ on GC substrates exposed to varying polarisation potentials. Both fragments represent the choline residue, their pattern is quite similar, but the C5 H14 NOþ signal is more intense. Choline residues are found on every sample,
Fig. 2. Linear sweep voltammetries (LSV) on and GC electrodes of ChCl:EG mixtures in 1:2 (EOC ¼ 0.55 V) and 1:4 ratios (EOC ¼ 0.34 V).
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Fig. 3. Overlay of ToF-SIMS mass spectra (0e109 m/z) obtained on glassy carbon substrates exposed to a 1:2 ChCl:EG mixture under varying polarisation potentials. Spectra were acquired from large area (1 mm 2.5 mm) imaging.
Table 1 Mass fragments associated to a choline residue on GC electrodes. Fragment
Mass (amu)
Deviation (ppm)
C5 H14 NOþ C3 H 8 N þ C3 H 9 N þ C5 H12 N þ
104.1097 58.0680 59.0741 86.1006
26.4 49.7 19.4 48.1
even on the GC that was immersed without applying any overpotential. At EðOCÞ and 0.60 V, choline deposits seem to concentrate in microdomains with diameters ranging from 20 mm to 300 mm. Choline adsorption significantly increases on GC substrates at 0.95 V and 1.15 V, and is no longer concentrated in specific microdomains. It appears that the amount of choline residue on GC electrodes increases as a function of negative overpotential. This fits descriptions by Hammond [7] of the nanostructure of the interface for DES as tightly packed Stern layers of cholinium. The accumulation of choline residue leads to a layer of reactive quaternary ammonium salts that makes up the surface at all times.
3.2.2. 1:4 Choline chloride: ethylene glycol Similar ToF-SIMS measurements were performed on GC substrates immersed in a 1:4 ChCl:EG mixture, in order to evaluate the effect of an increased ethylene glycol concentration on the choline residues. Fig. 5 shows an overlay of mass spectra obtained on GC exposed to a 1:2 (top) and a 1:4 (bottom) mixture of ChCl:EG at 0.60 V. In the 0e109 m/z mass range, the largest differences are found at nominal masses 86 m/z and 91 m/z, where peaks attributed to C5 H12 N þ and C7 H þ 7 , respectively, seem to dominate the spectrum obtained on GC exposed to a 1:4 ChCl:EG mixture. Peak areas normalised to total ion intensity are plotted at different potentials for C5 H14 NOþ, 13 CC4 H12 Nþ and 13 CC6 H þ 7 in Fig. 6, both for the 1:2 ChCl:EG series as for the 1:4 ChCl:EG series. þ þ þ 13 C isotopes of C5 H12 N and C7 H 7 were chosen because C5 H12 N
and C7 H þ 7 signals were saturated in the 1:4 ChCl:EG series. A strong increase in intensity of C5 H12 Nþ and C7 Hþ 7 is observed on each GC substrate that has been immersed in a 1:4 ChCl:EG mixture (Fig. 6). On the other hand, while C5 H14 NOþ intensities were increasing as a function of applied negative overpotential for the 1:2 ChCl:EG series (Fig. 6a), this behaviour is not observed in the 1:4 ChCl:EG series. The C5 H14 NOþ intensities measured on GC substrates exposed to 0.95 V and 1.15 V are remarkably low in the 1:4 ChCl:EG series compared to their analogues in the 1:2 ChCl:EG series. The strong increase in intensity of C5 H12 N þ , compared to the intensity of the molecular choline fragment C5 H14 NOþ , points out that the solvent deposits formed on GC in a 1:4 ChCl:EG mixture are chemically different from the choline residues found in the 1:2 ChCl:EG series. This becomes even more evident in the ToF-SIMS images (Fig. 7) that show the spatial distribution of C5 H14 NOþ and the C5 H12 N þ isotope on GC substrates exposed to varying polarisation potentials for the 1:4 ChCl:EG series. Remarkably, the spatial distribution of C5 H12 Nþ , a fragment that had been assigned characteristic for the choline residue in the 1:2 ChCl:EG series (Table 1), does not match with the C5 H14 NOþ pattern in the 1:4 ChCl:EG series. This means that two chemically different solvent deposits are formed on the GC substrates. The C5 H14 NOþ signal still represents the choline residue, while C5 H12 N þ is now mainly attributed to a deposit specifically formed in an ethylene glycol rich environment (1:4), hereafter referred to as the Ch:EG deposit. In general, the amount of adsorbed choline (represented by the C5 H14 NOþ signal in Fig. 7) is very low compared to the 1:2 ChCl:EG analogue and seems to be concentrated in smaller microdomains, which do not grow that much at high overpotentials (0.95 V and 1.15 V). It seems that the formation of Ch:EG deposits is preventing the surface to be fully covered by a choline layer. It is unclear why Ch:EG deposits (represented by the C5 H12 N þ isotope) are highly concentrated in specific domains at 0.60 V and 1.15 V, and more homogeneously distributed at EðOCÞ and 0.95 V.
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Fig. 4. Large area (1.5 mm 2.5 mm) ToF-SIMS images show the spatial distribution of C5 H14 NOþ (top row) and C5 H12 N þ (bottom row) on GC electrodes exposed to a 1:2 ChCl:EG mixture under varying polarisation potentials. Plotted intensities are normalised to total ion intensity.
Fig. 5. Overlay of ToF-SIMS mass spectra (0e109 m/z) obtained on glassy carbon substrates exposed to a 1:2 ChCl:EG mixture (top) and a 1:4 ChCl:EG mixture (bottom) under an applied potential of 0.60 V. Spectra were acquired from large area (1 mm 2.5 mm) imaging.
As Fig. 6 already showed that the C7 H þ 7 signal follows the same trend as the C5 H12 Nþ signal, also the C7 Hþ 7 images (not shown) give exactly the same pattern as the C5 H12 Nþ images. Therefore, also C7 H þ 7 can be assigned as a characteristic fragment for the Ch:EG deposit. Although the mass spectra in Fig. 5 only show the 0e109 m/z mass range, other fragments in a higher mass range were found to be characteristic for the Ch:EG deposit as well and are listed in Table 2. Fragments as C10 H14 Nþ and C11 H16 N þ , characteristic for
the Ch:EG deposit, are larger than the molecular choline fragment alone. This suggests that in excess of EG, a large molecular network is formed. Choline ions may have reacted with EG and/or other choline ions to form the Ch:EG deposit. 3.3. Effects of solvent on the deposition of iron Iron was electrodeposited from a 1:2 and a 1:4 ChCl:EG
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Fig. 6. ToF-SIMS C5 H14 NOþ (a), 13 CC4 H12 N þ (b) and 13 CC6 H þ 7 (c) peak areas normalised to total ion intensity are plotted for GC substrates exposed to 1:2 ChCl:EG mixtures and 1:4 ChCl:EG mixtures at varying polarisation potentials.
Fig. 7. Large area (1.5 mm 2.5 mm) ToF-SIMS images show the spatial distribution of C5 H14 NOþ (top row) and ChCl:EG mixture under varying polarisation potentials. Plotted intensities are normalised to total ion intensity.
Table 2 Mass fragments associated to the Ch:EG deposit formed on GC electrodes in a 1:4 ChCl:EG mixture. C5 H12 N þ and are saturated, therefore their deviations are slightly higher. Fragment Nþ
C5 H12 C7 H þ 7 C7 H 8 N þ C8 H10 N þ C9 H12 N þ C10 H14 N þ C11 H16 N þ
Mass (amu)
Deviation (ppm)
86.1031 91.0608
77.8 72.0
106.0662 120.0793 134.0944 148.1096 162.1239
10.6 11.9 15.2 17.0 23.8
13 CC4 H12 N
þ
(bottom row) on GC electrodes exposed to a 1:4
electrolyte. LSV measurements (Fig. 8) show how the amount of ethylene glycol can affect the reactivity of iron. It has been regularly assumed that there is no significant contribution from the electrolyte decomposition based on the 2 orders of magnitude difference between the currents achieved during iron deposition and the currents measured in blank solutions (green curves in Fig. 8). In 1:2 ChCl:EG mixtures, there is a single sharp increase in current around 0.95 V. The reduction of iron and solvent decomposition cannot be distinguished electrochemically at this point. Meanwhile in the 1:4 ChCl:EG electrolyte, the first electrochemical event starts at 0.65 V, with a small shoulder at slightly more positive potential. This process stabilises from 0.75 V, until a second reaction, likely
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Fig. 8. Linear Sweep Voltammetry for iron chloride (0.1 M) solutions of Choline chloride and ethylene glycol on GC. Blank solutions under same conditions present in green and gray. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
due to solvent decomposition, starts to dominate after 1.00 V. Scanning electron microscopy (SEM) images in Fig. 9 show the overall effect of the electrolyte and the applied overpotential on the structure of the deposits. In fact, large differences in appearance were already seen with the naked eye. Flaky deposits, full of cracks, are formed under exposure to the highest overpotential (1.15 V), both in the 1:2 ChCl:EG electrolyte (Fig. 9c) as in the 1:4 ChCl:EG electrolyte (Fig. 9d). Fewer flakes (not shown here in the SEM image, but later on in the ToF-SIMS image) are observed for the
309
deposit formed at 0.95 V in the 1:2 ChCl:EG mixture. Fig. 9a shows that, besides flaky deposits, also small metal nuclei are formed under these conditions. Finally, the most uniform deposit is obtained at 0.95 V in the 1:4 ChCl:EG electrolyte; Fig. 9b shows a uniform film constituted of small aggregates. Large area ToF-SIMS imaging was performed to study the chemical composition of the different deposits. Fig. 10 provides a general overview; for each condition, a Feþ image (blue intensity scale) is given, next to an overlay image of Feþ (blue), the C5 H14 NOþ isotope (red) and C5 H12 Nþ (yellow). All images were set to the same scaling for ease of comparison. Fig. 10a shows that flaky iron deposits are formed on GC at 0.95 V in a 1:2 ChCl:EG electrolyte. Besides these flaky deposits, the presence of iron based nuclei, as observed in SEM (Fig. 9a), is confirmed. The overlay images obtained for the flaky iron deposits at 1.15 V (Fig. 10c and d) show a complementary pattern between Feþ on the one hand and electrolyte residue fragments (represented by 13 CC4 H14 NOþ and C5 H12 Nþ ) on the other hand. Remarkably, the 13 CC4 H14 NOþ pattern and the C5 H12 Nþ pattern coincide in the 1:4 ChCl:EG series, while they show different patterns in the 1:2 ChCl:EG series. Based on the blank studies in section 3.2, one would expect the opposite trend. This demonstrates that the process has become more complex in the presence of iron and that one should be careful to make a direct comparison with the blank studies. Furthermore, it should be noticed that the rinsing step, which is performed after removal of the sample from the electrolyte, is difficult to control on the flaky iron deposits. Due to poor adhesion to the substrates, flaky iron deposits are easily removed by rinsing. Although the Feþ patterns in Fig. 10 show us the distribution of the iron deposits, they do not give information about the speciation of iron in these deposits. Information about the speciation of iron was extracted from ToF-SIMS imaging in negative polarity. Fig. 11
Fig. 9. Scanning electron microscopy images of iron deposits on GC from (top row) 1:2 ChCl:EG mixtures at (a) 0.95 V and (c) 1.15 V and from (bottom row) 1:4 ChCl:EG mixtures at (b) 0.95 V and (d) 1.15 V.
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Fig. 10. ToF-SIMS Feþ , 13 CC4 H14 NOþ and C5 H12 N þ peak areas normalised to total ion intensity are plotted for GC substrates exposed to 1:2 ChCl:EG mixtures and 1:4 ChCl:EG mixtures at varying polarisation potentials.
shows the ToF-SIMS images of fragments identified as FeCl 2 and FeC2 H3 O 2 . FeCl2 is representative of quick formation of iron chloroxides. FeC2 H3 O 2 is representative for the iron-glycolate complex. FeC2 H3 O2 Cl 2 has been proposed by Miller et al. as the dominant charge carrier in 1:4 solutions [9] over FeCl4 , which dominates in 1:2. This glycolate can act as a capping agent, inhibiting further growth and aggregation, while driving the granular morphology seen in Fig. 9b. Mainly iron chlorides are found in deposits obtained from 1:2 ChCl:EG electrolytes (Fig. 11a and c), consistent with the higher relative concentration of chloride in this electrolyte. In contrast, the deposit surface formed at 0.95 V in a 1:4 ChCl:EG electrolyte seems to consist mainly of iron-glycolate complexes (Fig. 11b), whereas the presence of iron chlorides is largely reduced. Interestingly, at the highest overpotential (1.15 V), deposits mainly consist of iron chlorides again (Fig. 11d). This is probably due to the electrolyte breakdown. Fig. 12 gives a final overview of the relative distribution of the different iron containing species. Iron-glycolate complexes, which are desirable, are dominantly formed at 0.95 V in a 1:4 ChCl:EG electrolyte. All other conditions result into iron chloride layer on the surface.
4. Conclusions hlGC substrates were exposed to blank 1:2 and 1:4 ChCl:EG electrolytes under varying polarisation potentials and analysed afterwards with ToF-SIMS. Choline residues were found on GC substrates exposed to the 1:2 ChCl:EG electrolyte, with the amount of adsorbed choline strongly increasing as a function of negative potential, i.e., GC electrode surfaces were saturated with choline at 0.95 V and 1.15 V. A different type of residue was found in the 1:4 ChCl:EG series. In excess of EG, a large molecular network is formed, supposedly by reactions between ethylene glycol and choline. This ChCl:EG deposit prevents accumulation of choline residues on the GC electrodes at high negative overpotentials; no saturation of solvent residue is observed at 0.95 V and 1.15 V. In a second set of experiments, the electrodeposition of iron was evaluated. At 1.15 V, flaky deposits full of cracks were obtained, both from 1:2 as from 1:4 ChCl:EG electrolytes. These deposits showed poor adhesion to the substrate and were identified by ToFSIMS as iron chloride deposits. Iron chloride deposits were also found on GC exposed to 0.95 V in the 1:2 ChCl:EG electrolyte. The best quality iron deposit was obtained at 0.95 V in the 1:4 ChCl:EG
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Fig. 11. ToF-SIMS FeCl 2 and FeC2 H3 O2 peak areas normalised to total ion intensity are plotted for GC substrates exposed to 1:2 ChCl:EG mixtures and 1:4 ChCl:EG mixtures at varying polarisation potentials. All images were set to the same scale.
Acknowledgements J.D.G acknowledges financial support by the Colombian Administrative Department of Science, Technology and Innovation (Colciencias). SURF acknowledges the Hercules program under grant agreement ZW13 07 for the ToF-SIMS measurements. T.H. acknowledges financial support by Research Foundation Flanders (FWO). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.04.154.
Fig. 12. Relative presence of iron complexes for GC substrates exposed to 1:2 ChCl:EG mixtures and 1:4 ChCl:EG mixtures at varying polarisation potentials.
electrolyte. A uniform layer of iron glycolates was formed under these conditions. While this work has focused only on the interactions between ChCl:EG electrolytes and GC substrates, there exists a broad spectre of DES and non aqueous media, applied for metal electrodeposition on various substrates. A detailed study on the metal complexes and corrosion of metallic substrates as a function of water content will be described in a follow up publication.
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