J Solid State Electrochem DOI 10.1007/s10008-016-3276-6

ORIGINAL PAPER

Electrodeposition of Al, Zn, and Pt on silver-coated textile fibres from ionic liquids Yasin Cengiz Celik 1 & Giridhar Pulletikurthi 1 & Frank Endres 1

Received: 26 December 2015 / Revised: 28 May 2016 / Accepted: 1 June 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract In the present paper, we report on the electrodeposition of aluminium, zinc and platinum on silver-coated textile fibres from ionic liquids. For electrodeposition of Al, the 60:40 mol% mixture of AlCl3/1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) and 1.7 M AlCl 3 in 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSA) were employed. It was observed that microcrystalline aluminium was electrodeposited on the textile fibres in 60:40 mol% AlCl3/[EMIm]Cl. The deposited Al layers either on single fibres or on textile assemblies are well adherent and uniform. An adherent, homogeneous and nanocrystalline Al layer was obtained on the silver-coated textile samples from 1.7 M AlCl3/[Py1,4]TFSA at 75 °C. The obtained Al layers from 60:40 mol% AlCl3/[EMIm]Cl on the textile fibres exhibit a good corrosion resistance in an aqueous iodide/iodine electrolyte. Furthermore, we obtain Al microtubes from the investigated ionic liquids after dissolving the textile fibres. In addition, zinc electrodeposition was carried out on the textile samples from 60:40 mol% ZnCl2/ [EMIm]Cl at 80 °C. The electrodeposition of platinum on the textiles was done from 50 mM PtCl2 in 1-butyl-1methylpyrrolidinium dicyanamide ([Py1,4]DCA).

Keywords Electrodeposition . Textile fibres . Al . Zinc . Pt . Ionic liquids

Introduction The use of solar energy in daily life has triggered a growing research interest for the past few decades. The use of an inexhaustible, portable, and versatile energy source may considerably improve modern life, e.g. by integrating a wearable solar energy source to portable devices that consume electrical energy such as health monitoring systems, bio-sensors, smart phones, tablet computers, and GPS. For this reason, research on fibre-based solar cells has increased rapidly in the last decades [1–9]. In comparison to other type of solar cells, dye-sensitised solar cells (DSSCs) seem reasonably appropriate for the production of photovoltaic fibres as DSSCs are relatively easy to prepare, cheap, flexible, and non-toxic. DSSC is a photoelectrochemical device that converts solar energy into electricity. After the pioneering work of Grätzel and O’Regan on DSSC in 1991 [10], numerous studies were focussed on this new generation photovoltaic cells. At present, the power conversion efficiency of DSSCs is around 13 % [11]. Scheme 1a shows the principle of a DSSC. When a photon hits a ruthenium (Ru)-based dye that is adsorbed on a semiconducting film (TiO2), the excited dye ejects an electron to the conduction band of the semiconductor [13]. The oxidised dye molecule is regenerated by the following reaction: 3I − → I 3 − þ 2e−

* Frank Endres [email protected] 1

Clausthal University of Technology, Clausthal-Zellerfeld, Germany

ð1Þ

The electrons ejected from the excited dye molecules move through the semiconductor and reach the conductive substrate. The flow of electrons across the electrodes generates the current. At the platinum-coated cathode

J Solid State Electrochem Scheme 1 a Schematic diagram and operation principle of a dyesensitised solar cell, where S ground state of the sensitiser, S+ excited state of the sensitiser, S* oxidised sensitiser, CB conduction band. Reproduced from [12]. b Working of a textile DSSC (according to TITK e.V., Germany)

catalysts, the following reaction takes place and regenerates the electrolyte: I 3 − þ 2e− → 3I −

metalised textile fibres using ionic liquids. As a side result, we present aluminium microtubes made from ionic liquids.

ð2Þ

In order to realise textile-based solar cells, silver-coated textile fibres (as electrodes) are needed to be protected from corrosion by the iodide/iodine electrolyte. The coating of silver covered textile with, e.g., valve metals is only achievable with ionic liquids, as electroplating of Al on textile fibres is quite difficult from other electrolytes. The electrodeposition of Al, e.g., is not possible from aqueous electrolytes because of the reactive character of Al. The electrodeposition of Al is feasible from molten salts; however, this method is not suitable to electrodeposit Al on textile fibres due to high operating temperatures where the textile fibres would not withstand. For this purpose, we have employed ionic liquids. The electrodeposition of aluminium has been extensively studied in ionic liquids composed of organic halides and aluminium halides [14–23]. Furthermore, several studies reported the electrodeposition of Al from non-chloroaluminate ionic liquids, [Py1, 4]TFSA [24, 25] and [Py1,4]TfO [26]. Scheme 1b shows the working principle of a textile DSSC. The anode is composed of silver-coated textile fibre, covered with a layer of a valve metal. ZnO and the dye are attached to the valve metal layer and in contact with the gel electrolyte containing iodide/iodine. The role of the valve metal layer is to protect the silver layer against iodine in the gel electrolyte; otherwise, Ag can rapidly react with the electrolyte. The same strategy can also be applied to the counter electrode, where the silver-coated textile is electrocoated with Pt to prevent the corrosion of Ag against the electrolyte. As one way to such DSSCs, we investigated the electroplating of aluminium, zinc and platinum on silver

Experimental The ionic liquids used in this study were 1-ethyl-3methylimidazolium chloride ([EMIm]Cl), 1-butyl-1methylpyrrolidinium dicyanamide ([Py1,4]DCA), 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py 1 , 4 ]TFSA) and 1-ethyl-3-methylimid azolium bis(trifluoromethylsulfonyl)amide ([EMIm]TFSA). These liquids were purchased from IOLITEC, Germany. The purities of [EMIm]Cl, [Py1,4]TFSA, [EMIm]TFSA and [Py1,4]DCA given by the supplier are 98, 99.5, 99 and 98 %, respectively. [Py1,4]TFSA, [Py1,4]DCA and [EMIm]TFSA were further dried under vacuum at 100 °C for 2 days to remove the water content as well as possible. The other ionic liquid was dried at 60 °C for 2 days. Anhydrous AlCl3 grains were purchased from FLUKA (99 %) and used without any further purification. Zinc chloride (Fluka, 99 %) was used as a Zn2+ source, and platinum dichloride (PtCl2, Sigma-Aldrich, 99.9 %) was used as a source for Pt2+ ions. Iodine, tetrapropylammonium iodide, ethylene carbonate and acetonitrile were purchased from Sigma-Aldrich with the purity of 99.8, 98, 98 and 99.8 %, respectively, and were used to prepare the iodide/ iodine electrolyte. For Al electrodeposition experiments, two Al plates were used as counter electrodes and an Al wire was used as reference. The purity of Al wires is 99.999 % (purchased from Alfa). The cell consists of three electrodes: a working electrode (WE), a counter electrode (CE) and a reference electrode (RE). A silver-coated textile fibre was used as a WE, which

J Solid State Electrochem

Fig. 1 Three different types of the Shieldex® textile fibres used for electrodeposition. From left to right: a single textile fibre, b comb-like textile fibre and c textile assemblies

was inserted in a Teflon frame. Two different types of textile fibres were employed, namely, Shieldex® (22 dtex, 100 and 240 μm) and Elitex® (22 dtex and 240 μm), where 22 dtex has a fibre diameter of 50 μm, and the other two have a diameter of 100 and 240 μm. These textile fibres consist of polyamide and mantled with a silver layer. Shieldex® fibre has around 15 wt% of silver, and Elitex® has a silver content of 50 wt% (source: TITV e.V., Greiz, Germany). Shieldex® textile fibres were procured from Statex Productions & Vertriebs GmbH, Bremen, Germany, and Elitex® textile fibres were procured from TITV e.V.

Fig. 2 Cyclic voltammograms of [EMIm]Cl/AlCl3 (40/60 mol%) on a silver-coated textile fibre(s) at RT: a Shieldex® 22 dtex, b comb-like Shieldex® 22 dtex textile fibres, c Shieldex® 22 dtex textile fibre assemblies, d Elitex® 22 dtex textile fibre assemblies. Scan rate 10 mV/s

The electrolyte used for the electrodeposition of microcrystalline aluminium is prepared by mixing AlCl3 and [EMIm]Cl in 60:40 mol%. AlCl3 was added in portions to the [EMIm]Cl to prevent the thermal decomposition of the liquid, as the reaction is highly exothermic. AlCl3 and [Py1,4]TFSA form a uniform solution at a concentration of 1.5 M AlCl3, but electrodeposition of Al is not possible at this concentration. A biphasic mixture was made with an initial concentration of 1.6 M AlCl3 in [Py1,4]TFSA. At this concentration, reducible AlCl3 species are present in the upper phase of the biphasic mixture. Owing to the viscosity of this upper phase, experiments were carried out at higher temperatures for Al deposition. For the Zn electrodeposition experiments, two Zn plates and a Zn wire (purchased from Alfa, 99.99 %) were used as counter and reference electrodes, respectively. For the electrodeposition of zinc, a Lewis acidic [EMIm]Cl/ZnCl2 ionic liquid was prepared. Similar to [EMIm]Cl/AlCl3, [EMIm]Cl/ ZnCl 2 is prepared in a molar ratio of 2:3 by mixing [EMIm]Cl with ZnCl2. For Pt electrodeposition, Pt wires (purchased from Alfa, 99.99 %) were used as the quasi-reference and counter electrodes. For electrodeposition of Pt, PtCl2 with a concentration of 50 mM was dissolved in [Py1,4]DCA. The electrochemical measurements were carried out using a PARSTAT 2263 potentiostat/galvanostat controlled by PowerCV and PowerStep software. For the characterisation of the coatings, high-resolution scanning electron microscopy (SEM, Carl Zeiss DSM 982 Gemini) and energy-dispersive X-ray spectroscopy (EDX) were performed.

J Solid State Electrochem Fig. 3 SEM images and EDX of Al deposits on a silver-coated single Shieldex® 22 dtex, b comb-like Shieldex® 22 dtex textile fibres, c Elitex® 22 dtex textile fibre assemblies, d Shieldex® 22 dtex textile fibre assemblies. e Optical microscope image of silver-coated Shieldex® 22 dtex textile fibre assemblies before Al deposition and f optical microscope image of Al deposit on silver-coated Shieldex® 22 dtex textile fibre assemblies after Al deposition at −0.3 V for 45 min

Figure 1 shows the silver-coated textile substrates. Three different types of the textile fibre samples were used in this study: (a) one single textile fibre (0.3 cm × 1 cm open window), (b) comb-like textile fibre with ten different single textile fibres (1 cm × 1 cm open window) and

(c) textile assemblies (1 cm × 4 cm open window). The fibres are fixed in a laminating film and are attached to a bundle of silver-coated textiles, connected to a Cu plate for electrical connection.

Results and discussion Al deposition on silver-coated textile fibres in [EMIm] Cl/AlCl3

Fig. 4 CVof Al deposition on silver-coated comb-like Elitex® 22 dtex in [Py1,4]TFSA/AlCl3 (1.7 M) at 75 °C. Scan rate 10 mV/s

Figure 2a represents the CV of [EMIm]Cl/AlCl 3 (40/ 60 mol%) on a silver-coated single textile fibre at room temperature. An increase in cathodic current is seen at −0.025 V which corresponds to the reduction of Al2Cl7− to Al. On switching the potential at −0.2 V, an oxidation process begins at approximately 0.06 V which is correlated with the dissolution of the deposited Al. The process is reversible, which is typical for Al electrodeposition from these liquids. The Coulombic efficiency was calculated from the ratio of charge passed for stripping and deposition processes, and it was found to be 91 % for Shieldex® 22 dtex. Aluminium

J Solid State Electrochem Fig. 5 SEM images of two different samples after Al deposition on comb-like Elitex® 22 dtex in [Py1,4]TFSA/AlCl3 (1.7 M) at 75 °C. The Ag layer is uniformly coated with nanocrystalline Al after deposition at −1.2 V for 10 min

deposition experiments were performed at various electrode potentials. It was found that at −0.08 V, for 10 min in [EMIm]Cl/AlCl3, a uniform and adherent aluminium layer with a thickness of around 2.8 μm could be obtained. Consequently, the deposition of Al on such fibres is similar to the deposition on flat samples. For examining whether a bunch of textile fibres can be coated with Al or not, the experiments were also conducted on the comb-like structure which contains ten textile fibres parallel to each other. Figure 2b shows the CV of [EMIm]Cl/AlCl3 on comb-like textiles. In the CV, an increase in reduction current is seen below −0.025 V which corresponds to the reduction of Al2Cl7− to Al and the oxidation process commences at around −0.03 V. On comparing the CVs of the single textile fibre and comb-like structure in Fig. 2a, b, it can be seen that on the comb-like samples, the current is ∼10 times to that of a single textile due to increase in the surface area of the comb-like sample. The shoulder at about 0.05 V in the cathodic current regime can be referred either to the formation of Al-Ag alloy or an under potential deposition (UPD) of Al on Ag-coated textile fibres. Lee and Wheeler reported the alloy formation and UPD of Al on polycrystalline gold and Au(111) electrodes from chloroaluminate liquids with benzene as an additive [27]. The Coulombic efficiency was found to be 97 % for comb-like Shieldex® 22 dtex textile fibres. Similar to a single textile fibre, an adherent, uniform aluminium layer was deposited on silver-coated comb-like textile fibres at −0.08 V in pure [EMIm]Cl/AlCl3. It was observed

that a deposition time of 15 min at −0.08 V was best to obtain an adherent and uniform Al layer with a thickness of around 3 μm on the comb-like electrodes. Further experiments were conducted on the textile assemblies. Figure 2c shows the CV of [EMIm]Cl/AlCl3 (40/ 60 mol%) on silver-coated textile assemblies Shieldex® 22 dtex. Typical Al deposition and stripping peaks of the textile assemblies are seen wherein the Al deposition begins at approximately −0.06 V, the oxidation starts at around −0.02 V. Al deposition on textile assemblies was carried out at −0.3 V for around 45 min. Figure 2d shows the CV of [EMIm]Cl/ AlCl3 (40/60 mol%) on a silver-coated Elitex® 22 dtex textile assembly. This CV is similar to the one shown in Fig. 2c. A reduction peak is seen at approximately −0.01 V which could be due to an UPD process of Al on silver. The beginning of the bulk Al deposition is around −0.05 V, and in the anodic scan, the Al stripping process starts around −0.05 V. The Coulombic efficiencies were found to be 87 and 93 % for Shieldex® 22 dtex textile fibre assemblies and Elitex® 22 dtex textile fibre assemblies, respectively. In summary, all the CVs show similar electrochemical behaviour having a single reduction process for the Al deposition and a single oxidation process for the dissolution of Al. On all the employed textile samples, a small reduction peak was observed prior to the Al deposition process which is—presumably—due to UPD of Al on silver or an alloying of Al and Ag; this reduction process was observed at approximately −0.05 V in all cases. To obtain Al deposits, a constant potential electrolysis was carried out on the textile fibres. Figure 3a shows the SEM

Fig. 6 SEM images and their corresponding EDX spectra of Al-coated Elitex® 240 μm which were left in iodide/iodine electrolyte for 3 months. Some damage in the textile appears which could be due to non-uniform

coating of Al/silver layer on the textile due to defects in the silver layer. a Pure Al layer is still intact with the sample. b The polyamide textile fibre

J Solid State Electrochem Fig. 7 SEM image of Al-coated single fibre (Shieldex® 240 μm) which was left in iodide/iodine electrolyte for 3 months. Al coating itself looks intact

micrograph of the aluminium-coated textile, and its corresponding EDX analysis is depicted as an inset of Fig. 3a. Figure 3b shows the SEM image of one fibre from a comblike sample after the aluminium deposition. The Al coating was further confirmed by EDX spectra, which is shown also in Fig. 3b. The EDX spectrum shows peaks of Al and O with negligible amounts of C and Cl. The presence of oxygen indicates the formation of an oxide layer on the Al coating due to surface oxidation. The elements C and Cl are from the ionic liquid residue on the sample. The EDX analysis shows that a high purity of the aluminium coating was obtained on silvercoated textiles. Figure 3c illustrates the Al-coated Elitex® 22 dtex textile assemblies. It is evident that the Al deposition is uniform and adheres well to the textile fibres. However, there are few particles seen where the textile fibres are intertwined and these particles are found to be Al(OH)xCly, which are only found at the parts where the textile fibres are intertwined. This happens when the samples were not cleaned well enough; thus, the residues react immediately with moisture as soon as taken out of the glove-box. The following reaction describes the first step of the reaction: Al2 Cl7 − þ H2 O→AlðOHÞCl2 þ HCl þ AlCl4 −

Fig. 8 SEM images of Al-coated a Shieldex® 22 dtex without any other further treatment after the Al deposition, b Elitex® 22 dtex after staying 12 h in [Py1,4]TFSA/ AlCl3 (1.8 M) (in lower phase) at 90 °C

ð3Þ

The SEM image shown in Fig. 3c reveals the importance of a cleaning process, especially removal of the electrolyte to obtain a pure Al coating. To improve the cleaning process, the samples were immersed in [EMIm]TFSA at 80 °C for 3 min under stirring. It was found that the neat ionic liquid could easily remove the remainders of [EMIm]Cl/AlCl3. Figure 3d illustrates such a purified Al layer on the silvercoated textile assemblies. The high-resolution image shows that no impurities are present in the intertwined sections of the textile unlike the impurities present in Fig. 3c. After washing in [EMIm]TFSA at 80 °C, the sample has a very pure layer of Al. The EDX in Fig. 3d shows only the presence of Al with negligible amounts of oxygen and carbon. Figure 3e presents an optical microscope image of the silver-coated Shieldex® 22 dtex textile fibre assemblies. After the Al deposition at −0.3 V for 45 min at RT, the sample was coated with an adherent and homogeneous Al layer as it can be seen in Fig. 3f. Electrodeposition of Al in [Py1,4]TFSA/AlCl3 Nanocrystalline Al exhibits some advantages such as high surface area, hardness and corrosion resistance [28]. Thus, we also tried to deposit Al from [Py1,4]TFSA/AlCl3 (1.7 M)

J Solid State Electrochem

Fig. 9 SEM images of Al-coated a–e Shieldex® 22 dtex after staying 17 h in [EMIm]Cl/AlCl3 at 90 °C and f Shieldex® 22 dtex stayed 17 h in [EMIm]Cl/AlCl3 at 110 °C

[24] from which it is obtained as a nanomaterial. The CV in Fig. 4 shows that Al deposition starts at around −0.8 V and dissolution is found at around 0.1 V. The diagram reveals that the process is not fully reversible as the anodic peak is much smaller than the cathodic one. A small reduction shoulder was observed before the Al reduction. A similar electrochemical behaviour was found at 75 °C in [Py1,4]TFSA/AlCl3 (1.6 M) on gold-coated glass in [24]. The textile fibres were cleaned after deposition like described above for [EMIm]Cl/AlCl3. Figure 5 shows the SEM images of an Al-coated sample which was coated at −1.2 V for 10 min. The Al layer with around 3 µm thickness is uniformly coated with nanocrystalline Al.

Fig. 10 Cyclic voltammogram of [EMIm]Cl/ZnCl2 (40/60 mol%) on a silver-coated Shieldex® 22 dtex at 80 °C. Scan rate 10 mV/s. CVexhibits typical Zn deposition and stripping behaviour

Aluminium corrosion test in an iodide/iodine electrolyte An iodide/iodine electrolyte was prepared to test whether the Al layer can prevent the corrosion of silver against the iodide/ iodine containing gel electrolyte. The electrolyte was prepared by adding 0.1 M iodine and 1 M tetrapropylammonium iodide in a mixture of ethylene carbonate and acetonitrile with a volume ratio of 4:1 (the composition of electrolyte is obtained from the project partner, Derck Schlettwein, University of Giessen, Germany). The Al-coated textile fibres with a thickness of ∼5 μm were exposed to this electrolyte. The Al-coated samples (from [EMIm]Cl/AlCl3, 40/60 mol%) were placed in a closed container containing approximately 20 mL of the iodide/iodine electrolyte in the glove box for 3 months. After 3 months, the samples were washed with isopropanol, water and dried at 60 °C for 10 min. Figure 6 shows that the Al layer on Elitex® 240-μm single fibre was not subject to corrosion in the presence of the iodide/ iodine electrolyte for 3 months, indicating that the deposited Al layer could protect the silver coating on the textile. In Fig. 6, it can be seen that some defects arise there; a closed Al layer does not form, due to imperfections in the silver layer underneath. The EDX spectrum in Fig. 6 reveals that the Al layer is not corroded and still remains on the textile sample. The elemental analysis of part 2 shows that the defect sites on the textile only consist of polyamide fibres. In Fig. 7, it can be seen that the Al layer on a single fibre (Shieldex® 240 μm) was also not damaged in the iodide/ iodine electrolyte after 3 months. The holes appearing on the Al coating result from defects in the silver layer underneath. The aluminium corrosion test in an iodide/iodine electrolyte reveals that the iodide/iodine containing electrolyte is not corrosive against the electrodeposited Al layer.

J Solid State Electrochem

Fig. 11 a SEM images and EDX of Zn deposit on a silver-coated Shieldex® 22 dtex in [EMIm]Cl/ZnCl2 (40/60 mol%) at −0.05 V for 10 min at 80 °C, b at higher magnification; c SEM images and EDX of

Zn deposit on a silver-coated Elitex® 22 dtex in [EMIm]Cl/ZnCl2 (40/ 60 mol%) at −0.05 V for 10 min at 70 °C, d at higher magnification

Synthesis of Al microtubes

depicts another part of the same sample, and as it can be seen, there is no more fibre under the lifted Al layer. The sample in Fig. 9f was placed in [EMIm]Cl/AlCl3 at 110 °C (ca. 17 h), and the dissolved textile fibre has left an Al microtube. It is evident that the Al layers in Fig. 9 look porous which might have assisted the diffusion of ionic liquid through the porous structure, thereby dissolving the polyamide.

We observed that polymer fibres dissolve very rapidly in [EMIm]Cl/AlCl3 (40:60 mol%) and in the upper phase of the biphasic mixture of [Py1,4]TFSA/AlCl3 (1.8 M) at above 90 °C. Consequently, we deposited Al on a single fibre and removed the polymer fibre subsequently at 90 °C. Figure 8a presents the cross-sectional areas of Al-coated Shieldex® 22 dtex. After staying for 12 h in [Py1,4]TFSA/ AlCl3 (1.8 M) (in lower phase of biphasic mixture [Py1, 4]TFSA/AlCl3 (1.8 M) at 90 °C, the polyamide textile fibre has been dissolved and, as can be seen in Fig. 8b, an Al mantle is left. Figure 9 shows a series of SEM images after dissolution in [EMIm]Cl/AlCl3. Figure 9b–d illustrates different parts of the sample shown in Fig. 9a at higher magnification. After staying 17 h at 90 °C, no more polymer was detected. Figure 9e Fig. 12 a SEM images of Zn on silver-coated Elitex® 22 dtex textile assemblies, after Zn deposition in [EMIm]Cl/ZnCl2 (40/60 mol%) at −0.055 V for 30 min at 80 °C, b at higher magnification

Electrodeposition of Zn from [EMIm]Cl/ZnCl2 Zinc is a possible alternative to Al to prevent the corrosion of Ag; thus, we have also attempted to deposit Zn films on the silver-coated textile fibres. Figure 10 presents the CV of [EMIm]Cl/ZnCl2 (40/60 mol%) on silver-coated single textile Shieldex® 22 dtex at 80 °C. An increase in cathodic current is seen at approximately −0.02 V which corresponds to the reduction of Zn(II) species (ZnCl3−, Zn2Cl5− and Zn3Cl7−) to Zn

J Solid State Electrochem

Figure 11c, d shows that the Zn layer has a thickness of around 3 μm on Elitex® 22 dtex. Figure 12 shows that textile assemblies can also be coated uniformly with Zn. After 30 min of deposition at −0.055 V, Elitex® 22 dtex was completely covered with Zn at 80 °C. Similar to single textile fibres, this sample was also washed with isopropanol and deionised water outside of the glove-box to remove the remaining ionic liquid. The washed samples were dried in an oven at 60 °C for 10 min. We can conclude that silver-coated textile fibres can—in principle—also be coated with an adherent and uniform layer of Zn.

Electrodeposition of Pt in [Py1,4]DCA/PtCl2

Fig. 13 Cyclic voltammogram of 50 mM PtCl2 in [Py1,4]DCA on a silver-coated comb-like Shieldex® 100 μm at 100 °C. Scan rate 10 mV/s

[29]. Electrodeposition of zinc from Lewis acidic ZnCl2 and EMImCl was reported by I.W. Sun et al. [29–31]. An oxidation process begins at approximately −0.07 Vand is correlated with the dissolution of the deposited Zn. The process is completely reversible which is usual for Zn electrodeposition from these ionic liquids, similar to Al deposition in acidic [EMIm]Cl/AlCl3. In contrast to AlCl3, ZnCl2 is less sensitive to moisture and the Zn-coated textile samples were therefore only washed with isopropanol and deionised water outside of the glove-box to wash off the remaining ionic liquid. The washed samples were dried in an oven at 60 °C for 10 min. Figure 11a, b shows the SEM micrograph and EDX of the zinc-coated textile fibre (Shieldex®) after electrodeposition at −0.05 V for 10 min at 80 °C. The silver film is coated with an adherent and uniform zinc layer with a thickness of around 5 μm. The EDX shows peaks of Zn and O with negligible amounts of C. The presence of oxygen indicates the formation of a ZnO layer on the Zn coating due to the surface oxidation. Carbon results from the ionic liquid residue on the sample.

Fig. 14 SEM images and EDX of two different samples after Pt deposition on comb-like Shieldex® 100 μm at −1.8 V for 1 h at 100 °C in [Py1,4]DCA with 50 mM PtCl2

A platinum layer is supposed to be the inert counter electrode material for the textile-based DSSCs. Although electrodeposition of Pt is possible from aqueous solutions, due to the hydrogen evolution, it is often not feasible. Furthermore, the electrochemistry of platinum in ionic liquids has not been widely studied. For the first time, He et al. [32] reported some results on the electrodeposition of platinum in 1-butyl-3methylimidazolium tetrafluoroborate ([BMIm]BF4) and from 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF6) at a glassy carbon electrode using H2PtCl6· (H2O)6 as a source for Pt. It was reported that a uniform Pt deposit was obtained on glassy carbon in both ionic liquids containing 18 mM H2PtCl6·(H2O)6. The electrodeposition of platinum in ionic liquids was also reported by Huang et al. [33]. In this study, 1-butyl-1-methylpyrrolidinium dicyanamide ([Py1,4]DCA) and PtCl2 were employed. Like other metal halide salts, PtCl2 dissolves easily in this ionic liquid due to the strong coordinating ability of the DCA− anion [33]. In 50 mM PtCl2/[Py1,4]DCA, a reduction process (Pt2+ to Pt) was observed in the cathodic regime of the CV, but a good adhesive Pt coating could not be obtained at 40 °C on a tungsten wire [33]. As the polyamide textile easily dissolves in acidic solutions, H2PtCl6·(H2O)6 was not used as the precursor. The electrodeposition of platinum was carried out in [Py1,4]DCA with

J Solid State Electrochem

50 mM PtCl 2 at 100 °C on a silver-coated comb-like Shieldex® 100 μm. In this case, we did not observe dissolution of textile fibres. Figure 13 shows the cyclic voltammogram of 50 mM PtCl2 in [Py1,4]DCA. In the forward scan of the CV, a reduction process of Pt2+ to Pt was observed at approximately −1.25 V. No oxidation peak was observed in the backward scan. Such an electrochemical behaviour was also reported by Huang et al. [33]. Figure 14 shows the SEM image of the silver-coated textile fibre after the Pt electrodeposition experiment. A uniformly coated textile can be observed in the SEM image. The EDX analysis reveals that textile fibres were uniformly coated with a thin layer of Pt.

5. 6. 7. 8. 9. 10. 11.

12. 13.

Conclusions

14.

In this paper, we have shown that the electrochemical deposition of metals such as Al and Zn on Ag-coated textile-based electrodes in ionic liquids is feasible. These metal-coated textile fibres are interesting for DSSCs. Microcrystalline aluminium can be deposited in 60:40 mol% AlCl3/[EMIm]Cl at RT either on single fibres or on textile assemblies. The electrodeposited Al layer shows a good resistance in iodide/iodine electrolyte which is necessary to protect the silver electrode against corrosion by the iodide/iodine containing electrolyte of DSSC. An adherent and homogeneous nanocrystalline Al could also be deposited on textile fibres in 1.7 M AlCl3/[Py1, 4]TFSA at 75 °C. Electrodeposition of Zn is also possible in 60:40 mol% ZnCl2/[EMIm]Cl at above 75 °C. As a counter electrode in the DSSC, Pt deposition has also been investigated and a very thin Pt layer can be deposited on textile fibres in [Py1,4]DCA with 50 mM PtCl2 at 100 °C.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

References 28. 1. 2. 3. 4.

Zou D, Wang D, Chu Z, Lv Z, Fan X (2010) Coord Chem Rev 254: 1169–1178 Chen T, Qiu L, Yang Z, Peng H (2013) Chem Soc Rev 42:5031– 5041 Pan S, Yang Z, Li H, Qiu L, Sun H, Peng H (2013) J Am Chem Soc 135:10622–10625 Liu D, Zhao M, Li Y, Bian Z, Zhang L, Shang Y, Xia X, Zhang S, Yun D, Liu Z, Cao A, Huang C (2012) ACS Nano 6:11027–11034

29. 30. 31. 32. 33.

Weintraub B, Wei Y, Wang ZL (2009) Angew Chem 121: 9143–9147 Yang Z, Sun H, Chen T, Qiu L, Luo Y, Peng H (2013) Angew Chem 125:7693–7696 Toivola M, Ferenets M, Lund P, Harlin A (2009) Thin Solid Films 517:2799–2802 Bedeloglu AC, Demir A, Bozkurt Y, Sariciftci NS (2010) Text Res J 80:1065–1074 Neudeck A, Zimmermann Y, Mohring U (2015) J Solid State Electrochem 19:3–12 O’Regan B, Grätzel M (1991) Nature 353:737–740 Mathew S, Yella A, Gao P, Humphry-Baker R, Curchod BFE, Ashari-Astani N, Tavernelli I, Rothlisberger U, Nazeeruddin MK, Grätzel M (2014) Nat Chem 6:242–247 Hsu CY, Chen YC, Lin RYY, Ho KC, Lin JT (2012) Phys Chem Chem Phys 14:14099–14109 Nazeeruddin MK, Baranoff E, Grätzel M (2011) Sol Energy 85: 1172–1178 Giridhar P, El Abedin SZ, Endres F (2012) Electrochim Acta 70: 210–214 Tsuda T, Hussey CL, Stafford GR, Bonevich JE (2003) J Electrochem Soc 150:C234–C243 Hurley FH, Wier TP Jr (1951) J Electrochem Soc 98:207–212 Keyes DB, Swann S Jr, Klabunde W, Schicktanz ST (1928) Ind Eng Chem 20:1068–1069 Jiang T, Chollier Brym MJ, Dubé G, Lasia A, Brisard GM (2006) Surf Coat Technol 201:1–9 Lipsztajn M, Osteryoung RA (1983) J Electrochem Soc 130:1968– 1969 Carlin RT, Crawford W, Bersch M (1992) J Electrochem Soc 139: 2720–2727 Lai PK, Skyllas-Kazacos M (1987) Electrochim Acta 32:1443– 1449 Yang CC (1994) Mater Chem Phys 37:355–361 Moffat TP, Stafford GR, Hall DE (1993) J Electrochem Soc 140: 2779–2786 El Abedin SZ, Moustafa EM, Hempelmann R, Natter H, Endres F (2005) Electrochem Commun 7:1111–1116 El Abedin SZ, Moustafa EM, Hempelmann R, Natter H, Endres F (2006) Chem Phys Chem 7:1535–1543 Giridhar P, El Abedin SZ, Endres F (2012) J Solid State Electrochem 16:3487–3497 Lee JJ, Mo Y, Scherson DA, Miller B, Wheeler KA (2001) J Electrochem Soc 148:C799–C802 Endres F, Bukowski M, Hempelmann R, Natter H (2003) Angew Chem Int Ed 42:3428–3430 Hsiu SI, Huang JF, Sun IW, Yuan CH, Shiea J (2002) Electrochim Acta 47:4367–4372 Chen PY, Sun IW (2001) Electrochim Acta 46:1169–1177 Lin YF, Sun IW (1999) Electrochim Acta 44:2771–2777 He P, Liu H, Li Z, Li J (2005) J Electrochem Soc 152:E146–E153 Huang HY, Su CJ, Kao CL, Chen PY (2010) J Electroanal Chem 650:1–9