Galvanic displacement on electrodeposited tangled Zn nanowire sacrificial template for preparing porous and hollow Ni electrodes in ionic liquid

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Galvanic displacement on electrodeposited tangled Zn nanowire sacrificial template for preparing porous and hollow Ni electrodes in ionic liquid Yi-Ting Chen a, Chien-Hung Li a, Po-Yu Chen a,b,c,⁎ a b c

Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan, ROC Department of Medical Research, Kaohsiung Medical University Hospital, Kaohsiung 807, Taiwan, ROC Department of Chemistry, National Sun Yat-sen University, Kaohsiung 804, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 2 September 2019 Received in revised form 25 October 2019 Accepted 1 November 2019 Available online xxxx Keywords: Electrodeposition Galvanic displacement Ionic liquid Porous nickel Urea oxidation

a b s t r a c t Tangled-Zn nanowire-composed porous structures were prepared via electrodeposition at particular applied potentials in an amide-type ionic liquid 1-butyl-1-methylpyrrolidinium bis((trifluoromethyl)sulfonyl) amide (IL [BMP][TFSA]) containing ZnCl2. The porous Zn was used as a sacrificial template for preparing a porous nickel duplicate via galvanic displacement in [BMP][TFSA] containing [NiCl4]2−. The solid tangled Zn-nanowires were converted to hollow NiZn-nanotubes in which a typical galvanic displacement reaction occurred through the alloying/dealloying mechanism, and the Kirkendall effect might also took place to form the bi-walled tubes at some spots. In general, the atomic content of Ni exchanged from Zn increased with the increment of reaction time, concentration of [NiCl4]2−, and temperature. The obtained NiZn could be almost completely dealloyed as verified by the XPS analyses to form a porous Ni electrode that displayed high stability and activity to electrooxidation of urea in alkaline solutions. This study indicated that [BMP][TFSA] played a crucial role on successfully cloning porous Ni from its Zn counterpart. © 2018 Elsevier B.V. All rights reserved.

1. Introduction The electrochemical oxidation of urea is currently very attractive to the electrochemical community because it can replace the high overpotential-needed oxygen evolution reaction (OER) in electrochemical water splitting so that the hydrogen production can be conducted in urea-containing waste drains such as urine using a lower cell voltage [1–3]. The production of hydrogen fuel and the decomposition of urea into low environment-impact products can be achieved simultaneously. Efficient electrocatalysts for urea oxidation reaction (UOR) are thus needed, and noble metal-based catalysts should be avoided because of the economic efficiency. Although many non-noble metals show activity [4], nickel and its compounds have been widely studied and recognized as exceptional active electrocatalysts towards UOR [5–11]. To increase the activity and stability of the Ni-based electrodes is important in order to make the production of hydrogen from urea waste drains feasible. An effective approach to increasing the activity of an electrode is to enlarge its surface ⁎ Corresponding author at: Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan, ROC. E-mail address: [email protected] (P.-Y. Chen).

area as high as possible because a larger surface area contains more active sites for the demanded reactions. An ionic liquid-Ni(II)-graphite composite electrode has ever been prepared for UOR and showed a high activity because the active site is in the molecular scale [12]. However, that electrode was not sufficiently robust because of the poorly mechanical strength of the ionic liquid-graphite paste. Hence, electrodeposition should be a better approach for preparing the robust Ni catalysts. In addition, nanostructures with hollow interior configurations produce high surface areas and are exceptionally suitable to electrochemical reactions. Importantly, hollow nanostructures can easily be synthesized by galvanic displacement reaction, one type of electroless depositions, and an efficient way of the formation of hollow nanostructures. Via the alloying-dealloying mechanism, which leads to high surface areas and abundant low-coordination atoms advantageous to catalytic reactions [13–18]. However, galvanic displacement is usually conducted in aqueous solutions, and suitable additives are needed in order to have a controllable displacement. Ionic liquids (ILs) have been recognized as good electrolytes for electrodeposition because of their many unique advantages such as wide electrochemical window, tailored physicochemical properties, and so on so forth [19,20]. Compared with aqueous electrolytes, the wider electrochemical windows of ILs give an opportunity for preparing a reactive

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template, which can be replaced by non-precious and relatively active metals such as nickel. In contrast to aqueous solutions, ILs including deep eutectic solvents (DESs) are rarely used as the media for galvanic displacement although a review article has been published [21]. Some active metals, such as aluminum [22] and III-V semiconductors [23], can be made via galvanic displacement in ILs. A very interesting study indicated that nickel nanostructure can be made via galvanic displacement on a copper-based template in DES [24]. In our previous study [25], it has been found that tangled Zn-nanowires could be electrodeposited from [BMP][TFSA] containing ZnCl2 at particular potentials. This finding encouraged us to use this special structure as a sacrificial template for preparing porous and hollow Ni electrode, which cannot be obtained directly by electrodeposition. The relatively high viscosity of ILs seemed to provide a unique environment beneficial to retain the template structure during the galvanic displacement because the same result cannot be achieved by using aqueous solution or choline chloride-ethylene glycol (ChCl-EG) DES. It was also found that the cations of ILs might be crucial to successfully retain the template structure. However, the preparation and characterizations of the porous and hollow Ni electrodes were focused here. The effects of cations will be presented elsewhere. To the best of our knowledge, no similar study has been reported. This study is able to provide tremendous insights to the galvanic displacement in ILs.

2. Experimental 2.1. Chemicals and apparatus The hydrophobic amide-type IL [BMP][TFSA] was synthesized by following the procedures reported previously [26]. The IL was stored in a glove box after drying it under vacuum at 120 °C for at least one day using a diffusion pump. Anhydrous ZnCl2 (99.995%) was purchased from Sigma-Aldrich. Anhydrous NiCl2 (99.99%), Zn foil (99.98%. Thickness: 0.25 mm), Ni wire (99.995%. Diameter: 0.5 mmø), Pt wire (99.95%. 0.5 mmø), and 304 stainless steel foil (SS foil. 99.98%. Thickness: 0.2 mm) were obtained from Alfa Aesar. Urea (CH4N2O. 99%) and NaOH (96%) were provided by J.T.Baker and Showa, respectively. All aqueous solutions were prepared by using the doubly-deionized water produced from the Millipore Smart Simplicity System (Resistance N 18 MΩ cm−1). All electrochemical experiments were conducted in a conventional one-compartment three-electrode electrochemical cell using a CHI 660E (CH Instruments, Inc.) electrochemical analyzer. Inside the PLHE-10-404 glove box (Innovative Technology), which was filled with highly pure N2 continuously circulated through a gas purifier to maintain the contents of O2 and H2O below 1 ppm, a Ag/AgClIL reference electrode was used [27], and a piece of Zn foil parallel to the working electrode surface was used as the counter electrode for voltammetric study and electrodeposition of Zn. A piece of Ni wire was used as the counter electrode for voltammetric study of Ni. The working electrodes were glassy carbon disc electrode (GCE. 3 mmø) and detachable stainless steel disc electrode (SSE. 3 mmø). Outside of the glove box, a Ag/ AgCl (saturated in NaCl) and a Pt-spiral were used as the reference and counter electrodes, respectively, in alkaline aqueous electrolytes. The electrodeposits were characterized with field-emission scanning electron microscope (FE-SEM. JEOL 6330 TF, ZEISS Supra 55, or Hitachi-SU8010) equipped with energy dispersive X-ray spectrometer (EDX), high-resolution scanning transmission electron microscope (HR-STEM. JEOL JEM-2100F CS STEM) coupled with EDX, grazing incident X-ray diffractometer (XRD. Bruker D8 Discover), and X-ray photon electron spectrometer (XPS. PHI 5000 VersaProbe) in order to have the surface observation, internal structure analysis, semi-quantitative elemental analysis, crystal structure analysis, and surface composition analysis, respectively.

2.2. Electrodeposition of tangled Zn nanowire sacrificial templates and galvanic displacement Electrodeposition of Zn was conducted at 80 °C in quiescent [BMP] [TFSA] containing 0.2 M ZnCl2 by applying a potential of −0.67 V (vs. Ag/AgClIL) at SSE. The electrodeposition was terminated once a charge density of 2.1C cm−2 was achieved (SSE\Zn). The SSE\Zn was immersed into pure and dry acetone to remove residual IL and then dried inside the glove box. Afterwards, the SSE\Zn was immersed in [BMP][TFSA] containing various concentrations of NiCl2 with four equivalents of [BMP][Cl] at 50 °C and 80 °C, respectively, for various periods to have the galvanic displacement occur. Because Zn could not be completely displaced by Ni, the obtained electrodes were denoted as SSE\NiZn(50 or 80 °C, replacement time) hereafter. 2.3. Electrolysis and voltammetric study of urea The obtained SSE\NiZn electrodes were activated by scanning the potential between 0.1 and 0.6 V for 20 cycles under a scan rate of 50 mV s−1 in 1 M NaOH. The XPS depth analysis indicated that Zn could be almost removed during the activation procedure to leave a morphologyunchanged SSE\Ni electrode (to prevent from confusing, the denotation of SSE\NiZn was still used after activation), which was used for the electrochemical study of urea. The electrode showed a high background current in pristine electrolyte after being used for the urea electrolysis. A regeneration procedure could reduce the background current by electrolyzing in neat 1 M NaOH at +0.5 V until the observation of a steady and small background current. 3. Results and discussion 3.1. Electrodeposition of tangled Zn nanowire sacrificial templates The voltammetric study and electrodeposition of Zn have been previously reported in detail [25]. The previous data indicated that tangled Zn nanowires could only be obtained within a highly narrow range of electrodepositing potential (ca. −0.6 to −0.7 V vs. Ag/AgClIL). In this study, it has been verified that the formation of tangled Zn nanowires was only feasible using a quiescent solution, indicating the orientation-dependent growth of electrodeposits due to an uneven distribution of electroactive species (such as ZnCl− 3 ) on the electrode surface [25,28]. Without stirring, the electrodeposition rate was quiet slow, which can be resolved by raising the temperature such as 80 °C. The tangled Zn nanowire sacrificial template was thus prepared by electrodeposition at −0.67 V and 80 °C from a 0.2 M ZnCl2 solution. The relevant SEM image of the template is shown in Fig. 1, and the inset indicated that the diameter of the tangled nanowires was near 100 nm. This unique structure could be successfully obtained at various temperatures while an exact potential was applied. Particularly, the temperature of 80 °C was used because of the considerations of an acceptable electrodepositing rate and the convenience of temperature control. 3.2. Galvanic displacement—the effects of air, chloride ion, temperature, and Ni(II) concentration The tangled Zn nanowire sacrificial templates were prepared by electrodeposition at SSE from [BMP][TFSA] with 0.2 M ZnCl2 at 80 °C (denoted as SSE\Zn). Afterwards, the galvanic displacement was conducted by immersing SSE\Zn in [BMP][TFSA] containing NiCl2 and four equivalents of [BMP][Cl] because NiCl2 is insoluble in [BMP][TFSA] and Ni(II) could be kept in the form of [NiCl4]2− [12]. Zn was gradually displaced by Ni via the following simplified reaction, Zn þ NiðIIÞ→ZnðIIÞ þ Ni

ð1Þ

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concentration, respectively. Temperature probably exhibited a significant effect on reaction rate because a high Ni at.% was obtained in a shorter period of reaction time at 80 °C (Fig. 4b). These results can be explained by Eq. (1) because the reaction goes right more with a longer reaction time, and a higher temperature and Ni(II) concentration. In addition, the Ni at.% seemed to reach a ceiling that was controlled by the Ni(II) concentration (Fig. 4b), implying that Zn could not be completely displaced. Namely, NiZn rather than Ni was obtained, and the highest displaceable amount of Zn was determined by the Ni(II) concentration. An equilibrium of NiZn alloy formation might be involved. It must be emphasized that [BMP][TFSA] played a crucial role in retaining the template morphology during galvanic displacement because the same results could not be achieved in water or ChCl-EG DES (data not shown). The relatively high viscosity of [BMP][TFSA] might induce a moderate displacement rate to make structure unchanged possible.

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3.3. Characterization of SSE\NiZn electrodes prepared via galvanic displacement Fig. 1. SEM images of SSE\Zn electrodeposited from quiescent [BMP][TFSA] containing 0.2 M ZnCl2 at 80 °C. Inset shows the magnified Zn nanowires.

to form SSE\NiZn because Zn could not be completely replaced (discussed later). It was found that the atomic contents (at.%) of Ni (determined with EDX) in NiZn deposits were significantly divergent among the replicate experiments if the SSE\Zn templates are exposed to air prior to conducting the galvanic displacement. Analogous atomic contents of Ni could be obtained as long as the galvanic displacement was directly conducted inside the glove box after SSE\Zn had been obtained. This behavior indicated that the Zn nanowires were easily oxidized by air, and the oxidized Zn could not be displaced by Ni. However, Zn nanowires were oxidized in different degree each time exposed to air, leading to the very divergent Ni contents. To avoid this problem, the sacrificial Zn templates were kept from exposing to air and directly used for the subsequent galvanic displacement inside the glove box. In addition to the effect resulting from air oxidation, the concentration of Cl− in the displacement solutions also showed the effect on galvanic displacement because Cl− is a stronger ligand than [TFSA] anion, which might affect the reduction potentials of the Ni(II) and Zn(II) species via the coordination reaction between Cl− and Ni2+/Zn2+. Interestingly, the experimental results revealed no significant dependence of Ni at.% on the concentration of Cl−. A Ni at.% of 58.21 ± 2.41 was obtained from a galvanic displacement conducted in [BMP][TFSA] containing 0.03 M NiCl2 and four equivalents of [BMP][Cl] at 50 °C. It was 53.42 ± 1.77 (at.%) when 16 equivalents of [BMP][Cl] was used. This behavior was not surprised because the redox couple cNi/aNi corresponding to Ni (II) + 2e− ⇌ Ni did not significantly change with the equivalents of Cl− (Fig. 2; [29]) except that the redox couple cCl2/aCl− associated with the redox of free Cl− ions (Cl2 + 2e− ⇌ 2Cl−) became more apparent when more Cl− ions were introduced. Therefore, the lowest four equivalents of Cl− was used to prepare the Ni(II) solutions for the following galvanic displacement experiments. According to the reaction shown in Eq. (1), the concentration of Ni (II) and reaction temperature inherently exhibit crucial effects on the galvanic displacement. Fig. 3 shows the SEM images of SSE\NiZn prepared from 0.05 M Ni(II) solution at 50 °C (Fig. 3a–c) and 80 °C (Fig. 3d–f), respectively. The insets show the corresponding zoom-out images. As can be seen, the structure of the Zn sacrificial templates could be maintained well after the galvanic displacement regardless of the temperature and Ni(II) concentrations (other samples prepared from 0.01 and 0.03 M Ni(II) are shown in Fig. S1 in Supplementary data). However, the Ni contents in these SSE\NiZn were very divergent. The results are collected in Fig. 4 that shows the Ni at.% (determined with EDX) increased with the increment of reaction time and Ni(II)

According to Fig. 4, the SSE\NiZn electrodes prepared from the 0.05 M Ni(II) solution at 50 °C were selected for the following study because their Ni at.% distributed in a wider range; it would be easier to study the dependence of the electrode performance on Ni contents. Another SSE\NiZn prepared from the same Ni(II) solution but at 80 °C and the displacement time of 30 min was selected as a comparison. Opencircuit potential (EOCP) was recorded during the galvanic displacement for the abovementioned four electrodes, and the results are shown in Fig. 5a. The behavior is similar to that reported previously [30]. In general, for the electrodes SSE\NiZn(50 °C, 60 min) and SSE\NiZn(80 °C, 30 min) that showed a plateau after a sufficient reaction time, their EOCP initially went in negative direction, reached a valley for an incubation time, and then continuously climbed to reach the plateau. It is reasonable that EOCP changed in positive direction during the displacement because Ni showed a more positive EOCP than Zn in [BMP][TFSA]. The incubation time as well as the climbing rate apparently depended on the temperature; namely, a shorter incubation time and a steeper climbing rate were observed at a higher temperature, which might imply a faster displacement at a higher temperature. Therefore, the EOCP - t curves can be used to monitor the galvanic displacement and to determine whether the reaction is accomplished. However, more experiments will be needed in the future to interpret meaning for shape of the curves; a very complicated change of concentration of various species on the electrode surface might be involved. The SSE\NiZn prepared

Fig. 2. CVs recorded at GCE in [BMP][TFSA] containing 0.05 M NiCl2 with the indicated equivalents of [BMP][Cl] at 50 °C. Scan rate: 50 mV s−1.

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Fig. 3. SEM images of SSE\NiZn prepared via galvanic displacement from [BMP][TFSA] containing 0.05 M NiCl2/0.2 M [BMP][Cl] at (a–c) 50 °C and (d–f) 80 °C, respectively. Insets show the relevant zoom out images and the displacement time is indicated in each micrograph.

Fig. 4. Dependence of atomic Ni content (in SSE\NiZn) on displacement time. Galvanic displacement was conducted at (a) 50 °C and (b) 80 °C, respectively, from [BMP][TFSA] containing 0.01, 0.03, and 0.05 M NiCl2 with four equivalents of [BMP][Cl].

Fig. 5. (a) Change of EOCP on time of galvanic displacement conducted in [BMP][TFSA] containing 0.05 M NiCl2/0.2 M [BMP][Cl]. Temperature and time used for the displacement are indicated. (b) XRD patterns of the four SSE\NiZn electrodes as indicated in (a), SSE\Zn, and SSE. The database information for Zn and Ni is provided.

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from the four major stages as indicated by the four curves (black, red, blue, and green) in Fig. 5a were further studied. First, their XRD patterns as shown in Fig. 5b were measured in which the diffraction signals of SSE, SSE\Zn, and SSE\NiZn(80 °C, 30 min) were also shown as comparisons. It was found that the signal intensity of Zn decreased with increasing the displacement time, indicating that more Zn was displaced by Ni via prolonging the reaction time. Unfortunately, the signal of the major facet (101) of Zn was embedded in the major signal of SSE although the mode of grazing incident X-ray was used. No Ni signal was observed because the crystal size and the thickness of Ni were too small and thin, respectively, which will be discussed in more detail in the next paragraph. Fig. 6 shows the TEM images of the deposits obtained from the four stages as indicated in Fig. 5a. Tangled nanowires were observed within every sample (Fig. 6a-d and a′-d′) but hollow structures (marked by circles) were only observed in the 30-min and 60-min (displacement time) samples; longer the displacement time more the hollow structures. The HR-STEM images (Fig. 6a″-d″) indicated that the tangled Zn-nanowires were single crystals and displaced by Ni from the exterior (Fig. 6a″). With the displacement lasting, Zn was gradually displaced by Ni (Fig. 6b″-d″) and the interior Zn might diffuse towards the surface so that forming the hollow structures. The exterior Ni was composed of extremely tiny crystals with random orientations, resulting in the polycrystalline characteristics of the selected area electron diffraction (SAED) patterns (see Fig. S2 in Supplementary data). It needs to emphasize that NiO rather than Ni was observed for the samples in Fig. 6b″ and c″ because Ni was oxidized once it was exposed to ambient air. Ni metal was found in the interior deposits (Fig. 6d″) because the exterior NiO might form a barrier against the air. The abovementioned results indicated that the sacrificial Zn template was displaced by Ni via the alloying/dealloying mechanism, typically involved in galvanic

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displacement [13]. In the initial stage, Zn atoms were displaced by Ni atoms to form a NiZn alloy (the alloying step), and this step is crucial. Otherwise, the morphology of the template cannot be retained [13,31]. Zn in the alloy was displaced by Ni in the dealloying step if galvanic displacement is being continued. However, Zn could not be completely displaced using the current experimental conditions even at 80 °C but it could be thoroughly removed via anodization, which will be discussed later. Fig. 7 shows the dark-field HR-STEM images and the relevant EDX line-scan analysis of a selected segment of a nanowire. Apparently, the solid Zn-nanowires were gradually converted to be hollow NiZnnanotubes from a reaction time of 5 min to 60 min. For the 30-min reaction time samples (Fig. 7b and c), the initial development of hollow structure (Fig. 7b) and the complete formation of hollow tube (Fig. 7c) were observed. A double-walled structure as indicated by the white arrow was also observed (Fig. 7c). The double-walled structure was more apparent for the tube shown in Fig. 7d; the formation of which structure should result from the Kirkendall effect (i.e. unbalanced interdiffusion between two materials) [13,32,33]. The EDX line-scan analysis further verified the formation of hollow structures because the distribution of Zn atoms was uniform for the 5-min sample (Fig. 7a) and then concentrated on the wall once the tubular structure was observed (Fig. 7c and d). The distribution of Ni atoms always concentrated peripherally on the wire (Fig. 7b–d), indicating that Zn was displaced since exterior layer, and the internal Zn atoms diffused towards the surface to be continuously displaced so that a hollow tube was finally formed. The EDX line-scan analysis also verified that NiZn instead of Ni was obtained. The porous and tubular hollow structure is able to produce a huge surface area and make mass transport more efficient. Both are very important to electrochemical reactions.

Fig. 6. TEM and HR-STEM images of the deposits obtained from the four SSE\NiZn electrodes as indicated in Fig. 5(a). The same series of samples are denoted by ′ and ″. Except for those in (d″), the facet distance is represented in black background for Ni but no background for Zn.

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Fig. 7. Dark-field HR-STEM images and EDX line-scan analysis of nanowires obtained from the SSE\NiZn electrodes prepared via galvanic displacement from [BMP][TFSA] with 0.05 M NiCl2/0.2 M [BMP][Cl] at 50 °C. The displacement time is indicated.

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As mentioned above, SSE\NiZn instead of SSE\Ni was obtained using the current experimental conditions. However, Zn could be thoroughly removed via anodization in 1 M NaOH. Fig. 8a and b show the highresolution XPS of Zn 2p and Ni 2p electrons for the SSE\NiZn before and after the anodization; SSE\Zn was also shown as a comparison. Fig. 8a shows that metallic Zn signals (Zn 2p3/2 1022.2 eV, Zn 2p1/2 1045.2 eV) were observed for SSE\Zn and SSE\NiZn prior to the anodization (i.e. SSE\NiZn(50 °C, 60 min)), indicating that Zn atoms existed on the surface of the deposits in both electrodes. After anodization, no Zn signal could be observed for the SSE\NiZn electrodes regardless of 20 or 20.5 cycles of potential scan; the difference between 20 and 20.5 cycles was the final potential stopped at 0.1 or 0.6 V. Fig. 8b indicates that metallic Ni (Ni 2p3/2 853.0 eV, Ni 2p1/2 870.2 eV) was observed after the galvanic displacement (solid curve; SSE\NiZn(50 °C, 60 min)). “*” belonging to the solid curve represents the satellite peaks of metallic Ni (Ni 2 p3/2,sat 856.2 eV, Ni 2p1/2,sat 873.8 eV), which also included the oxidized Ni signals (peak fitting can refer Fig. S3a in Supplementary data). After anodization, the intensity of metallic Ni peak significantly decreased (labeled by dashed straight lines), but two new pairs of peaks appeared at the higher BE region, which belong to the oxidized Ni and its satellite (Ni 2p3/2 855.8 eV with Ni 2p1/2

873.8 eV, Ni 2p3/2,sat 861.4 eV with Ni 2p1/2,sat 879.8 eV). The oxidized 363 Ni are possibly corresponding to the mixture of Ni(II/III) species. The de- 364 tail of XPS peaks deconvolution and identifications are presented in Supplementary data (Fig. S3b and Table S1). In addition, it is interesting and important to know whether Zn was completely removed because XPS can only detect the surface atoms. XPS depth analysis was conducted to obtain the answer, and the results are shown in Fig. 8c and d. Before anodization, both Zn and Ni distributed uniformly from the surface to the interior (Fig. 8c). After anodization, Zn was almost thoroughly removed and the oxygen at.% increased because Ni was converted to its oxides and/or hydroxides (Fig. 8d). These experimental data pointed out that anodization in alkaline solution could efficiently remove Zn from SSE\NiZn. Meanwhile, Ni was converted to its oxides and/or hydroxides that exhibit electrocatalytic activity towards urea oxidation. Anodization, therefore, was an efficient approach of electrode dealloying and activation in this study. 3.4. UOR at porous and hollow Ni nanowire-electrode The abovementioned Ni electrodes own high surface area, and porous and hollow structures that should be advantageous to

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Fig. 8. High-resolution XPS spectra for the selected regions: (a) Zn2p and (b) Ni2p for the indicated electrodes. Asterisks indicate the satellite peaks. The depth analyses of SSE\NiZn(50 °C, 60 min) prepared from [BMP][TFSA] with 0.05 M NiCl2/0.2 M [BMP][Cl] (c) before and (d) after anodization are shown.

electrocatalytic reactions because abundant active sites were exposed out. Fig. 9a shows the cyclic voltammograms (CVs) recorded at SSE in 1 M NaOH with and without urea. SSE shows unobvious oxidation current of urea until the concentration of 0.33 M (typical urea concentration in human urine) was introduced. However, SSE\NiZn (50 °C, 60 min) showed apparent oxidation current even when only 0.01 M urea was introduced (Fig. 9b). Other SSE\NiZn electrodes shown in Fig. 5b except for SSE\Zn (unstable in anodic scan) were also used as the working electrodes for the same experiments indicated in Fig. 9. A similar voltammetric behavior was observed except that the current response depended on the electrode used. The oxidation peak currents (j pa ) recorded at the four SSE\NiZn in 0 and 0.02 M urea, respectively, were collected in Table 1. In 0.33 M solution, some electrodes did not show apparent oxidation peak. The oxidation current at +0.5 V was collected in the same table instead. Apparently, jpa,0M depended on the displacement time (in other words, the Ni at.% in the electrode) when the same displacement temperature was employed. In general, jpa,0.02M and j0.5V,0.33M were positively correlated to jpa,0M. Namely, the higher the jpa,0M was recorded, the higher the jpa,0.02M and j0.5V,0.33M were observed, indicating that the reaction current of urea was controlled by the size of the oxidation wave Ni2+ → Ni3+ + e−, which equaled to the number of Ni2+ active sites. This behavior is reasonable because urea is electrocatalytically oxidized by following an EC mechanism as shown below [3]: − 6NiðOHÞ2ðsÞ þ 6OH − ðaqÞ ⇌6NiOOH ðsÞ þ 6H 2 OðlÞ þ 6e

ð2Þ

6NiOOH ðsÞ þ COðNH2 Þ2ðaqÞ þ H 2 OðlÞ ⟶6NiðOHÞ2ðsÞ þ N 2ðgÞ þ CO2ðgÞ ð3Þ Net reaction: − COðNH2 Þ2ðaqÞ þ 6OH − ðaqÞ ⟶N 2ðgÞ þ 5H 2 OðlÞ þ CO2ðgÞ þ 6e

ð4Þ

In short, Zn in SSE\NiZn shows a negligible contribution to the electrocatalytic UOR because Zn was almost completely removed during the electrode activation, and the electrode with a larger oxidation wave of Ni2+ → Ni3+ + e− shows a higher response to UOR. It needs to emphasize that the number of Ni2+ active site and the electroactive surface area of the abovementioned electrodes may be estimated by the size of the redox waves of Ni3+/Ni2+ (Fig. 9b) [34] and the double layer capacitance [35,36], respectively. However, a qualitative analysis was focused here, and a more detailed investigation should be conducted in the near future because to compare the electrode performance in a quantitative way is important to an electrocatalytic reaction. A conclusion can be made, instead; the condition of galvanic displacement that can produce the highest number of Ni2+ active site is able to reveal the highest activity of UOR. Although the CVs showed that the obtained SSE\NiZn electrodes exhibited electrocatalytic activity to UOR, to use these electrodes for bulk electrolysis is also important in order to estimate their stability in electrochemical decomposition of urea. Fig. 10a shows the current-time (jt) curve recorded at the indicated four electrodes in 1 M NaOH with 0, 0.02, and 0.33 M urea, respectively. The j-t curve of SSE was provided as a comparison. SSE showed a negligible current regardless of the

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Fig. 9. CVs recorded in 1 M NaOH with the indicated concentration of urea under the scan rate of 50 mV s−1 at (a) SSE and (b) SSE\NiZn(50 °C, 60 min) prepared from [BMP][TFSA] with 0.05 M NiCl2/0.2 M [BMP][Cl].

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urea concentration as to the four SSE\NiZn in 0 M urea solution. Apparent oxidation currents were observed for the four SSE\NiZn in 0.02 M urea, and their performance was differentiated when a high concentration of urea (0.33 M) was used. The fluctuation of current resulted from the abundant gas bubble formation on the electrode surface although the solution was vigorously stirred. According to Eq. (4), the gas bubble might be N2 gas. In our previous report, CO2 formation (another product in Eq. (4)) was detected in the form of carbonate although a different electrode was used [12]. The best electrode in Fig. 10a, i.e. the SSE \NiZn(50 °C, 60 min), was used for galvanostatic electrolysis under an applied current of 10 mA cm−2, and the chronopotentiograms are shown in Fig. 10b. As can be seen, the electrode potential stabilized approximately at 0.48 V (vs. Ag/AgCl) for three replicate experiments, indicating the high stability of the electrode. This experiment indicates that the obtained electrodes may exhibit the potential to be used for

t1:1 t1:2 t1:3

Table 1 Comparison of oxidation current among four selected electrodes in 1 M NaOH with 0, 0.02, and 0.33 M urea.

t1:4

(mAcm−2)

t1:5 t1:6 t1:7 t1:8

jpa,0M jpa,0.02M j0.5V,0.33M

SSE\NiZn (50 °C,15 min)

(50 °C,30 min)

(50 °C,60 min)

(80 °C,30 min)

13.67 26.27 56.74

17.80 24.83 51.49

26.98 34.21 86.30

20.64 31.16 84.67

Fig. 10. (a) Current-time (j-t) curves recorded at the indicated electrodes during the bulk electrolysis in 1 M NaOH with and without urea. Applied potential: +0.5 V. (b) Chronopotentiograms (E-t curves) recorded at SSE\NiZn(50 °C, 60 min) in 1 M NaOH with 0.33 M urea. Applied current: 10 mA cm−2. Three replicate results are shown.

urea analysis and hydrogen production from urea-containing waste 449 drains. 450

4. Conclusions

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Tangled-Zn nanowire-composed porous structure was easily prepared by potentiostatic electrodeposition in [BMP][TFSA] containing ZnCl2, which might result from the orientation-dependent growth of electrodeposits due to an uneven distribution of electroactive species on electrode surface in a highly viscous liquid. This structure was successfully used as a sacrificial template to be converted to structureunchanged NiZn via galvanic displacement in [BMP][TFSA] containing [NiCl4]2−. However, the solid nanowires were transformed to hollow tubes. The relatively high viscosity of [BMP][TFSA] might cause a moderate displacement rate so that template structure could be retained well during the conversion. The same results could not be achieved if aqueous solution or ChCl-EG DES was used as the displacement medium. Anodization in alkaline solutions was an efficient way for NiZn dealloying to form the hollow nanotube-composed porous Ni electrode, which showed an extremely high activity towards UOR because of the exposure of abundant Ni2+ sites. The electrode showed a high stability without morphology change in the periodic electrolysis and regeneration. Further investigations into the applications are currently underway in our laboratory.

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Y.-T. Chen et al. / Journal of Molecular Liquids xxx (xxxx) xxx

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Declaration of competing interest

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Dr. Po-Yu Chen, the corresponding author of the paper entitled “Galvanic displacement on electrodeposited tangled Zn nanowire sacrificial template for preparing porous and hollow Ni electrodes in ionic liquid”, is on behalf of all co-authors to declare that there is no interest of conflict.

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Acknowledgements

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This work was supported by the Ministry of Science and Technology (MOST) of ROC (Taiwan) (Grant MOST107-2113-M-037-009 and MOST108-2113-M-037-002). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.112050.

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Please cite this article as: Y.-T. Chen, C.-H. Li and P.-Y. Chen, Galvanic displacement on electrodeposited tangled Zn nanowire sacrificial template for preparing por..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112050