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Studies of structural and morphological properties of cuprate conductive ceramics after electrochemical treatment in alkaline electrolyte
Materials Chemistry and Physics 239 (2020) 121934
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Studies of structural and morphological properties of cuprate conductive ceramics after electrochemical treatment in alkaline electrolyte Angelina Stoyanova-Ivanova a, *, Peter Lilov a, Alexander Vasev a, Antonia Stoyanova b, Galia Ivanova b, Daniela Karashanova c, Valdek Mikli d a
Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784, Sofia, Bulgaria Institute of Electrochemistry and Energy Systems “Academician Evgeni Budevski”, Bulgarian Academy of Sciences, Acad. Georgi Bonchev Str., Bl. 10, 1113, Sofia, Bulgaria c Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Acad. Georgy Bonchev St., Bl. 109, 1113, Sofia, Bulgaria d Insitute of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate Str., 19086, Tallinn, Estonia b
H I G H L I G H T S
� BSCCO � BSCCO � BSCCO � BSCCO
ceramics display redox activity in an environment similar to the NiZn battery. ceramics are reduced during the charging process. reduction products improve the negative active mass conductivity . reduction products decrease the solubility of ZnO.
A R T I C L E I N F O
A B S T R A C T
Keywords: B(Pb)SCCO conductive ceramics Physicochemical and electrochemical properties Ni–Zn rechargeable batteries
This study is focused on detail research of the structural and morphological changes in cuprate ceramics Bi1.7Pb0.3Sr2CaCu2Ox (B(Pb)SCCO 2212) and Bi1.7Pb0.3Sr2CuOx (B(Pb)SCCO 2201) during the electrochemical treatment. Their electrochemical reduction behavior is investigated by Cyclic Voltammetry (CV) and Chro nopotentiometry (CP) in 7 M KOH. The used ceramic powders are physicochemically characterized before and after the electrochemical test. It has been found that ceramics are reduced during the charging process. The reduction products are different oxides and hydroxides as well as Cu and Bi metals. Their presence probably improve the overall conductivity of the electrode and the contact between the particles of the negative active mass. On the other hand, the Ca- and Sr-products in the case of B(Pb)SCCO 2212 modification decrease the solubility of ZnO in the electrolyte and suppress the shape change of the electrode.
1. Introduction This study is focused on the electrochemical redox behavior of Bi1.7Pb0.3Sr2CaCu2Oy (B(Pb)SCCO 2212) and Bi1.7Pb0.3Sr2CuOy (B(Pb) SCCO 2201) cuprate ceramics, which are known to be superconductors at and above liquid nitrogen temperatures [1]. There is insufficient in formation about the electrochemical reactivity of these ceramics [2], which can be relevant to room temperature applications of these ma terials which do not involve superconductivity. One such application is in electrocatalysts for oxygen reduction and hydrogen evolution reactions (ORR and HER) studied by Lim et al. [3] who found BSCCO 2212 to be electrochemically reactive in aqueous
media, but did not discuss the reduction products in detail. Another potential application based on the chemical properties of BSCCO-system ceramics is as a stabilizing additive in zinc anodes for rechargeable Ni–Zn batteries. It is well known that this green technology suffers from a variety of issues that are well documented in literature [4–6] and has found relatively few large-scale applications [4,7–9]. This technological limit is related to the shape change of zinc electrode with increasing charge/discharge cycle count, zinc electrode passivation, and growth of zinc metal dendrites leading to separator punctures and short-circuiting of the battery [10,11]. There are different approaches to overcoming these limitations, such as an optimizing the battery design, improvement of the Ni(OH)2 electrode, use of gel polymer electrolytes
* Corresponding author. E-mail address: [email protected] (A. Stoyanova-Ivanova). https://doi.org/10.1016/j.matchemphys.2019.121934 Received 23 May 2019; Received in revised form 30 July 2019; Accepted 1 August 2019 Available online 2 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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and electrolyte additives [12,13], a special 2-layer separators [14,15], modification of the charging process by optimization of charge mode with protection against recharging and hydrogen release [16,17] and etc. An already proven approach to minimize the above mentioned problems is by adding different oxides to the active mass of the negative electrode mass. The inclusion of B(Pb)SCCO in the Zn electrode is an effective solution. This may be related to the known beneficial effects of various simpler oxide additives, such as Ca(OH)2 [18–20], Bi2O3 [21], PbO [22], TiO2 [23], CuO [24] and others [25] in reducing shape change, dendrite growth and suppressing hydrogen evolution. Transi tion metal oxide additives to Ni–Zn batteries (Bi2O3, CuO, PbO) have been shown to undergo reduction during charging, prior to the main zinc deposition reaction [18–24]. Literature suggests formation of copper oxides and even metallic copper at high cathodic polarization [2,26]. In our previous studies of using YBCO, two modifications of B(Pb) SCCO or other conducting cuprate ceramics as additives to the zinc electrode mass of Ni–Zn alkaline batteries was studied [27–30]. The chemical stability of B(Pb)SCCO and YBCO materials to the highly alkaline batteries electrolytes (7 MKOH) has been confirmed by struc tural and surface morphology analysis as well as magnetic measure ments of ceramic samples after a prolonged exposure to the alkaline electrolyte [28,29]. The electrochemical tests have shown that the nickel-zinc cells with ceramics in the zinc electrode mass exhibit good performance stability. For example the discharge capacity of the cell with a zinc electrode containing carbon as conducive addition is lower (up to 30%) than the capacity of the cell with a zinc electrode with YBa2Cu3O7-x conductive ceramics [29]. The improvement of the electrochemical properties of Zn-electrode in the Ni–Zn battery cell by adding two types of B(Pb)SrCaCuO con ducting ceramics B(Pb)SCCO 2201 and B(Pb)SCCO 2212 is confirmed. The presence of both types of ceramics in the zinc electrode mass in creases the discharge capacity of the battery. The obtained results are explained again by an increase in the conductivity and homogeneity of the electrode mass in the presence of the used additives, on the one hand, and the formation of a highly conducting network between the zinc oxide particles, on the other [27]. The impedance studies have shown that the positive effect of the Bi1.7Pb0.3Sr2CuOy additives on the performance of the zinc electrode is not due to its intrinsic conductivity, but more probably owing to the reduction products of the ceramic that could be produced during charging the electrode [31]. Based on the above, it is of interest to examine in more detail the structural and morphological changes that occur in ceramics after their electrochemical treatment in alkaline solutions. This is an innovative study that provides more clarity about the mechanism of the ongoing processes. The determination of the reduction products of BSCCO ce ramics in an environment similar to the Ni–Zn battery may prove also useful in explaining how the improvement in Zn-electrode performance is achieved.
copper clad board with silver conductive paste to ensure good electrical conductivity. The board is then insulated with epoxy, leaving only one face of the B(Pb)SCCO tablets with 75 mm2 of surface area exposed to the electrolyte. 2.2. Experimental methods The investigated ceramics are physicochemically and electrochemi cally characterized. The physicochemical analyses are performed by the X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Xray Spectroscopy (EDX) before and after electrochemical tests (Chro nopotentiometry (CP) and Cyclic Voltammetry (CV)). The powder X-ray diffraction patterns are collected in the range from 10 to 80� 2θ with a constant step of 0.02� 2θ angle on Bruker D8 Advance diffractometer with Cu Kα radiation and Lynx Eye detector. Phase identification was performed with the Diffract plus EVA using ICDDPDF2 Database. The powder diffraction patterns are evaluated with the Topas-4.2 software package using the fundamental parameters peak shape description including appropriate corrections for the instrumental broadening and diffractometer geometry. The surface morphology of B(Pb)SCCO 2212 and 2201 ceramics after CP is compared to the one of freshly synthesized tablets by means of digitized Philips 515 scanning electron microscope. The data for the surface elemental composition of the samples were determined by Energy dispersive spectrometer Oxford Instruments INCA Energy system with a Zeiss EVO MA-15 scanning electron microscope with a LaB6 cathode. The qualitative and quantitative analyses are carried out at an accelerating voltage of 20 kV. The elemental mapping of the B(Pb)SCCO 2212 and 2201 surfaces before and after the chro nopotentiometric examination is appended as supplementary information. CP is performed using a computer controlled potentiostat/galvano stat - Bio-logic SP-200 to control the cell current and record data. The prepared B(Pb)SCCO (2201 and 2212) electrodes are used as working electrodes at a constant cathodic current of 10 mA for 30 min in a 7 M KOH electrolyte at 25 � C, the measured potential is referenced to a saturated calomel electrode (SCE). A polished platinum plate is used as a counter-electrode (CE). The open circuit potential is measured before applying cathodic current by Bio-logic SP-200 and potential value in electrochemical system reached constant value. To observe the initial electrochemical degradation stage, destruction of the ceramics’ structure and intermediate reaction products, after the CP testing the ceramic electrodes were removed from the electrolyte, rinsed with distilled water and dried with a hot air gun. The drying step is needed to prevent any further unwanted degradation of the ceramics as they are sensitive to water [2]. CV is carried out using the same cell and electrode configuration as in the CP experiment. Three scans are done on each sample in the potential window of 1.75 V to þ0.85 V vs. SCE, starting in the positive direction from the open circuit potential at a rate of 5 mVs 1. The goal of this experiment is to determine the potential windows in which the indi vidual redox-active metal components (Cu and Bi) undergo reduction.
2. Materials and methods 2.1. Materials Through solid state reaction ceramics of nominal composition: B(Pb) SCCO 2212 and B(Pb)SCCO 2201 are obtained. Samples of Bi–Sr–Ca–Cu–O system are produced by two-stage conventional solidstate synthesis from high-purity (99,99%) oxides and carbonates Bi2O3, PbO, CuO, SrCO3 and CaCO3. After thorough mixing and grinding and initial heat treatment at 780 � C for 24 h in air the powder obtained is ground and pressed into (5–6 MPa) round tablets 9,8 mm in diameter and 1.8 mm in thickness. The tablets are then sintered again: B(Pb)SCCO 2201 at 830 � C for 24 h, B(Pb)SCCO 2212 at 830 � C for 48 h in an air atmosphere. The working electrodes are prepared using both ceramics - B(Pb) SCCO 2212 and B(Pb)SCCO 2201. A ceramic tablet is glued to a piece of
3. Results and discussion In the work of Ivanova et al. [27] the same ceramic modifications (B (Pb)SCCO-2201 and B(Pb)SCCO-2212) are used an additives to the Zn-electode in Ni/Zn battery cell. The obtained results are shown in Fig. 1 with permission from the authors. It can be seen that their pres ence results in higher and more stable specific discharge capacity in comparison to the zinc electrode with classic carbon additive (the ca pacity of cells with B(Pb)SCCO additives decreased only by about 15–20% after 500 cycles). In Fig. 2 are compared the X-ray diffraction patterns of B(Pb)SCCO 2212 and B(Pb)SCCO 2201 ceramics [24] with those on the same 2
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Materials Chemistry and Physics 239 (2020) 121934
analysis. At potentials more negative than 1.22 V the subsequent reduction of some of these oxides may happen immediately after the initial breakdown, leading to a large increase in current. 3.2. SEM SEM results for the surface morphology of freshly synthesized ceramic materials B(Pb)SCCO 2212 and 2201 and electrochemically treated ceramics are presented in Fig. 4. It is well seen from the mi crographs (Fig. 4a and b) that two as-synthesized samples exhibit the typical for B(Pb)SCCO structure of the main matrix, consisting of large, flat terraces, laying in the a-b plane, predominantly. Some of them are located perpendicularly of this plane, according to the synthesis method and they are visualized as rods on the micrographs. In the same time, spheroid and significantly smaller in size (about tenths of a micrometer), compared to the terraces (few microns), grains are found on the terraces’ surface. They tend to form aggregates and are abundant in the surface morphology of B(Pb)SCCO 2212 (Fig. 4a) and rare in this one of B(Pb) SCCO 2201 (Fig. 4b). After CP testing the morphologies on B(Pb)SCCO 2212 and 2201 ceramics (Fig. 4c and d) are changed, compared to the pristine materials. The width and the height of the matrix terraces are significantly increased and abrupt edges and large facets are formed. The grainy microstructure is still present in both sample morphology, as well, but the free from specks surface of the terraces in B(Pb)SCCO 2212 is increased. This is probably due to partial decomposition of the ceramic structure in the electrolyte (Fig. 4c), while small grains of new phase, formed during the electrochemical treatment, have completely conquered the terraces in B(Pb)SCCO 2201 (Fig. 4d).
Fig. 1. Dependence of the specific discharge capacity of the zinc electrode with carbon, B(Pb)SCCO 2201 and B(Pb)SCCO 2212 ceramic additives on the cycle number, at current load density C/5 (13 mA cm 2). Graph reproduced with permission from of Ivanova et al. [27].
ceramic samples after their chronopotentiometry study. The results shown that after electrochemical treatment, the preservation of main phases with reduced intensity and appearance of Bi2O3 were observed. 3.1. CP The CP curves of B(Pb)SCCO 2212 and 2201 at a constant cathodic current of 10 mA (current density 13.3 mA cm 2) are presented in Fig. 3. The starting open circuit potentials are þ0.15 V for B(Pb)SCCO 2201 and þ 0.087 V for B(Pb)SCCO 2212. The potential of B(Pb)SCCO 2201 changes abruptly when current is applied, immediately falling to 0.9 V and settling to a plateau at 1.15 V. The potential of B(Pb)SCCO 2212 falls slower with a mild s-curve at 0.6 to 0.7 V, a plateau at 0.95 V is then reached and this potential remains relatively constant until the end of the experiment. The total charge passed in the experiments is 5 mAh, which is more than double the charge passed in the cathodic part of the first cycles of the CV experiments: 2.15 mAh for B(Pb)SCCO 2212 and 0.58 mAh for B(Pb)SCCO 2201. Therefore we speculate that an initial breakdown reaction may occur prior to reduction reactions occurring at potentials more negative than the plateau potentials above. This breakdown appears unhindered by its reaction products because the electrode potential remains relatively constant. This process may pro ceed until the complex structure of B(Pb)SCCO is completely destroyed, leading to the simpler oxides consisting of one or two metals (CuO, Bi2O3, Bi2CuO4, Ca(OH)2, Sr(OH)2), which were later detected by EDX
3.3. EDX Table 1 presents the atomic composition of untreated B(Pb)SCCO 2212 ceramic in multiple regions marked in the SEM image in Fig. 5a (magnification: 2000�) and in Table 2 - of untreated B(Pb)SCCO 2201 ceramic in the regions marked in the SEM image in Fig. 5b (magnifi cation: 5000�), respectively. The results confirm that the elemental composition of the freshly synthesized ceramics corresponds to the expected stoichiometric ratios for B(Pb)SCCO 2212 (Table 1: Spectra 1,2,3 and 7) and 2201 (Table 2: Spectrum 4). Inclusions of CuO are visible in both types of untreated ceramics (Table 1: Spectrum 6, Table 2: Spectrum 2), as well as sec ondary phases: CaSrCu3O5 (Table 1 Spectrum 4, 5) in B(Pb)SCCO 2212 and Sr2Cu3O5 (Table 2 Spectrum 1) in B(Pb)SCCO 2201. Table 3 presents the atomic composition of electrochemically treated B(Pb)SCCO 2212 in multiple regions marked on Fig. 6 and Table 4 - of treated B(Pb)SCCO 2201 in the regions marked on Fig. 6, respectively. Areas with high predominant copper content can be seen in the
Fig. 2. XRD patterns of synthesized untreated/treated B(Pb)SCCO 2212 (a) and untreated/treated B(Pb)SCCO 2201 (b). 3
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Materials Chemistry and Physics 239 (2020) 121934
Fig. 3. Potential vs. time curves of the chronopotentiometry experiments.
Fig. 4. SEM images of untreated B(Pb)SCCO 2212 (a) and 2201 (b) and of electrochemically treated B(Pb)SCCO 2212 (c) and 2201 (d). Table 1 Elemental composition of points of untreated B(Pb)SCCO 2212 (regions marked on the SEM image in Fig. 5a). Concentration: At. % Spectrum Element: Bismuth Lead Copper Strontium Calcium Oxygen
0.91 � 0.3 7.15 � 0.3 6.94 � 0.8 4.79 � 0.2 74.98 � 4.0
5.23 � 1.3 1.04 � 0.3 7.71 � 0.3 6.70 � 0.8 4.60 � 0.2 74.35 � 3.9
1
2
3
4
5
6
5.60 � 1.3 0.86 � 0.3 8.49 � 0.4 6.95 � 0.8 5.82 � 0.2 72.67 � 3.9
5.20 � 1.3 – 30.64 � 1.3 8.64 � 0.9 10.54 � 0.3 49.18 � 2.4
0.27 � 0.1 – 27.04 � 1.1 9.18 � 0.9 9.59 � 0.3 53.61 � 2.6
0.58 � 0.2 – 47.03 � 2.0 – – 52.97 � 2.5
– 0.90 � 0.3 6.62 � 0.3 5.04 � 0.6 5.29 � 0.2 77.09 � 4.2
5.07 � 1.3
4
7
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Materials Chemistry and Physics 239 (2020) 121934
Fig. 5. SEM images of regions examined by EDX in untreated B(Pb)SCCO 2212 (a) and B(Pb)SCCO 2201 (b).
another indication that Ca and Sr containing phases separate from the B (Pb)SCCO structure, forming simpler compounds. Spectra 4 and 8 in Table 3 show high bismuth content. Sr, Ca and Cu are also present in the spectra in varying amounts with no apparent relationships between them. Therefore, we conclude that they are likely present as separate phases such as Bi2O3, CuO and Ca- and Sr-hydroxides or carbonates. Several regions appear different from their immediate surroundings in Fig. 7. The spectra in points 3 and 7 appear similar in the SEM image and their elemental composition (Table 4) is nearly identical with a Sr: Pb ratio of about 1:1. Points 5 and 8 also have nearly the same composition along with the highest Sr and Pb concentration of all spectra in Table 4. Regions with Bi:Cu atomic ratios close to 2:1 were observed in both B(Pb)SCCO ceramics after the chronopotentiometry treatment (Table 3: Spectrum 1 and 2. Table 4: Spectrum 2,3 and 7). The region of Spectrum 1, Table 3 appears homogenous in the SEM image and is likely a single compound. This leads us to conclude that Bi2CuO4 is also present as a decomposition product in addition to CuO and Bi2O3. It should be also noted that the atomic ratios in Spectrum 5 in Table 3 and Spectrum 4 in Table 4 are very close to those of the pristine ce ramics, indicating that some of it remains unchanged.
Table 2 Elemental composition of untreated B(Pb)SCCO 2201 (regions marked on the SEM image in Fig. 5b). Concentration: At. % Spectrum
1
2
3
4
Element: Bismuth Lead Copper Strontium Oxygen
1.44 � 0.2 – 29.11 � 1.0 18.23 � 1.4 51.21 � 2.0
– – 49.92 � 2.0 – 50.08 � 2.1
5.00 � 1.1 11.38 � 1.9 6.30 � 0.21 27.61 � 1.7 49.70 � 1.3
11.61 � 2.3 1.82 � 0.9 6.17 � 0.2 12.58 � 1.1 67.82 � 2.3
electrochemically treated ceramics (Fig. 6: point 5; Table 3: Spectrum 5) as well as a distinct CuO grain in Fig. 6: point 6 and Table 4: Spectrum 6. Strontium is the major component in Spectrum 1 in Table 4. Sepa ration of alkaline earth element phases from the B(Pb)SCCO structure is a known decomposition process of cuprate ceramics [2] and this sepa ration is clearly visible in the corresponding SEM image (Fig. 7b). Different regions with high Ca, Sr and Cu content are visible in the supplementary EDX element maps (See Supplementary Fig. 4). This is
Table 3 Elemental composition of electrochemically treated B(Pb)SCCO 2212 (regions marked on the SEM image in Fig. 6). Concentration: At. % Spectrum
1
2
3
4
5
6
7
8
9
Element: Bismuth Lead Copper Strontium Calcium Potassium Oxygen
13.25 � 2.1 – 7.21 � 0.3 0.37 � 0.2 – 1.72 � 0.2 77.45 � 3.3
16.11 � 2.1 – 6.75 � 0.2 – – – 77.14 � 3.0
1.46 � 0.4 – 12.76 � 0.7 6.50 � 0.8 7.89 � 0.3 2.07 � 0.2 69.32 � 3.8
14.81 � 1.7 – 2.76 � 0.1 6.24 � 0.6 3.11 � 0.1 – 73.08 � 3.1
2.04 � 0.4 – 27.12 � 1.2 2.56 � 0.4 4.14 � 0.2 1.01 � 0.2 63.14 � 3.2
6.54 � 1.1 – 6.48 � 0.3 6.07 � 0.7 4.04 � 0.1 – 76.86 � 4.1
10.67 � 1.6 – 0.94 � 0.1 2.97 � 0.4 4.42 � 0.1 – 81.01 � 4.0
18.83 � 2.1 1.70 � 0.3 10.98 � 0.3 6.49 � 1.1 2.98 � 0.1 – 59.03 � 2.0
4.32 � 1.2 0.50 � 0.2 4.01 � 0.2 4.68 � 0.9 3.87 � 0.2 0.26 � 0.1 82.34 � 4.8
Fig. 6. SEM images of regions examined by EDX in electrochemically treated B(Pb)SCCO 2212. Magnification: 10000� - Fig. 6a. 2000x - Fig. 6b. 5
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Materials Chemistry and Physics 239 (2020) 121934
Table 4 Elemental composition of electrochemically treated B(Pb)SCCO 2201 (regions marked on the SEM image in Fig. 7). Concentration: At. % Spectrum
1
2
3
4
5
6
7
8
Element: Bismuth Lead Copper Strontium Potassium Oxygen
3.44 � 0.9 0.87 � 0.4 1.11 � 0.1 20.51 � 1.8 1.13 � 0.1 72.12 � 3.0
11.88 � 1.9 0.67 � 0.5 5.72 � 0.2 4.14 � 0.4 – 77.59 � 3.0
13.08 � 2.0 2.81 � 1.0 5.46 � 0.2 2.56 � 0.4 – 76.09 � 2.9
11.15 � 2.1 1.87 � 0.8 6.49 � 0.2 12.66 � 1.0 – 67.83 � 2.6
2.59 � 0.6 6.90 � 1.2 5.58 � 0.2 19.02 � 1.1
– – 46.47 � 1.9 0.77 � 0.1 – 52.76 � 2.4
15.77 � 2.2 2.96 � 1.1 5.98 � 0.2 2.27 � 0.4 – 73.03 � 2.6
2.79 � 0.6 7.25 � 1.4 5.08 � 0.2 20.49 � 1.3 – 64.40 � 2.0
65.91 � 2.0
Fig. 7. SEM images of regions examined by EDX in electrochemically treated B(Pb)SCCO 2201. Magnification: 10000� - Fig. 7а and 3000x - Fig. 7b.
3.4. CV
displays two slightly more distinct peaks: Ipc 1b and 2b (Fig. 8b). Copper and bismuth are the only metals in the composition of the ceramics that can be reduced under the experimental conditions. Therefore we sup pose that the observed current peaks are associated with their reduction to the corresponding metals in an order that agrees with the electro chemical reduction series. But due to the presence of other inactive alkaline-earth elements in the ceramics’ composition reduction occurs at more negative potentials than the reduction potentials of the simple oxides CuO and Bi2O3. For comparison, the Bi and Cu reduction po tential for simple oxides are respectively 0.067 V and 0.277 V vs. SCE [32]. Peaks associated with further reaction of the products of the initial cathodic scan appear on the second and third cycles (Fig. 8). Three broad oxidation peak regions can be distinguished. One indicates the
Cyclic Voltammetry is carried out to observe reactions occurring at more negative potentials than those reached in the chronopotentiometry experiment. Such reactions are relevant because the reduction of ZnO to Zn metal (the main reduction reaction in a Ni–Zn battery) occurs at 1.5 V vs. SCE. EDX results show that after CP testing, the reaction products consist of various oxides including CuO and Bi2O3 which may also be reduced to metals. During the first cycle CV reveals peaks related to the redox behavior of the ceramics themselves. No distinct oxidation peaks appear on the first scan until oxygen evolution begins. B(Pb)SCCO 2212 displays a single broadened reduction peak (or possibly two overlapping peaks) at very negative potentials Ipc 1a and 2a (Fig. 8a), while B(Pb)SCCO 2201
Fig. 8. CV curves of B(Pb)SCCO 2212 (a) and B(Pb)SCCO 2201 (b) in 7 M KOH solution. scan rate: 5 mV s 1. 6
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reoxidation of Bi metal to Bi2O3 (Ipa 1a). Under alkaline conditions copper metal is first oxidized to Cu2O (Ipa 2a). At more positive poten tials Cu2O and any remaining copper metal are oxidized to CuO (Ipa 3a). These two simultaneous reactions explain why peak Ipa 3a (CuO) has a greater maximum value and charge passed associated with it. The region between 0 V and þ0.5 V corresponds to the oxidation of copper to sol uble copper (II) species [26]. The potentials (Epa 1.2 and 3) at which these peaks occur are essentially the same for both B(Pb)SCCO 2212 and 2201 and are in close agreement with the theoretical equilibrium po tentials for the respective reactions and with those obtained by a similar study on the YBCO superconductor, which also examined the electro chemistry of copper and it’s oxides in alkaline solution [26]. This leads us to believe that the reduction reactions in cycle 1 lead to the same products: Cu and Bi metals, for both B(Pb)SCCO 2212 and 2201. Scan cycles 2 and 3 also reveal that the initial reduction process irreversibly destroys the ceramics. Three reduction peaks are present at less negative potentials than the initial reduction peaks of cycle 1. They represent the reduction of Bi2O3 to Bi0 (Ipc 20 a. 20 b). CuO to Cu2O (Ipc 30 a. 30 b) and simultaneous reduction of CuO and Cu2O to Cu0 (Ipc 20 a. 20 b). When used as an additive in rechargeable Zn electrodes the BSCCO ceramics will likely undergo reduction during the charging process. This is because the reduction peak potentials Epc 1, 2a and Epc 1b, 2b are more positive than the potential at which the main ZnO reduction re action takes place ( 1.5 V vs. SCE). This may lead to formation of metal particles, which improve the overall conductivity of the electrode and contact between the particles of active ZnO material. The calcium and strontium present in the ceramic are irreducible under our conditions and as a result of the destruction of the ceramic during reduction they form Sr(OH)2 and also Ca(OH)2 in the case of B(Pb)SCCO 2212. These products can decrease the solubility of ZnO in the electrolyte by for mation of insoluble CaZn(OH)4 [7,20] and SrZn(OH) and reduce the shape change of the electrode. The decrease the solubility of ZnO leads to increasing the utilization of the active material and a constant growth of Zn deposition in the electrode volume during the charge discharge process [20].
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(1975). US Patent 3876470. [12] S.J. Banik, R. Akolkar, Suppressing dendrite growth during zinc electrodeposition by PEG-200 additive, J. Electrochem. Soc. 11 (2013) D519–D523, https://doi.org/ 10.1149/2.040311jes. [13] J.W. Diggle, A. Damjanovic, The electrocrystallization of zinc dendrites in HighPurity, and inhibitor doped, alkaline zincate solutions, J. Electrochem. Soc. 1 (1970), https://doi.org/10.1149/1.2407441. [14] P. Arora, Z. Zhang, Battery separators, Chem. Rev. 10 (2004) 4419–4462, https:// doi.org/10.1021/cr020738u. [15] P. Kritzer, J.A. Cook, Nonwovens as separators for alkaline batteries, J. Electrochem. Soc. 5 (2007) A481, https://doi.org/10.1149/1.2711064. [16] K. Appelt, K. Jurewicz, Die l€ osliche zinkelektrode als anode alkalischer akkumulatoren, J. Power Sources 5 (3) (1980) 235–244, https://doi.org/10.1016/ 0378- 7753(80)80002-4. [17] L. Binder, K. Kordesch, Corrosion of zinc electrode mixtures in alkaline media, J. Electroanal. Chem. 180 (1984) 495–510, https://doi.org/10.1016/0368- 1874 (84)83603-5. [18] Y.F. Yuan, J.P. Tu, H.M. Wu, Y. Li, D.Q. Shi, X.B. Zhao, Effect of ZnO nanomaterials associated with Ca(OH)2 as anode material for Ni–Zn batteries, J. Power Sources 159 (2006) 357–360. https://doi.org/10.1016/j.jpowsour.2006.04.010. [19] C.-C. Yang, W.-C. Chien, C.L. Wang, C.-Y. Wu, Study the effect of conductive fillers on a secondary Zn electrode based on ball-milled ZnO and Ca(OH)2 mixture powders, J. Power Sources 172 (2007) 435–445. https://doi.org/10.1016/j. jpowsour.2007.07.053. [20] V. Caldeira, R. Rouget, F. Fourgeot, J. Thield, F. Lacoste, L. Dubau, M. Chatenetabe, Controlling the shape change and dendritic growth in Zn negative electrodes for application in Zn/Ni batteries, J. Power Sources 350 (2017) 109–116. [21] Y.F. Yuan, L.Q. Yu, H.M. Wu, Electrochemical performances of Bi based compound film-coated ZnO as anodic materials of Ni–Zn secondary batteries, Electrochim. Acta 56 (2011) 4378–4383. https://doi.org/10.1016/j.electacta.2011.01.006. [22] R. Shivkumar, G.P. Kalaignan, T. Vasudevan, Studies with porous zinc electrodes with additives for secondary alkaline batteries, J. Power Sources 75 (1998) 90–100. https://doi.org/10.1016/S1003-6326(14)63500-7. [23] S.-H. Lee, C.-W. Yi, K. Kim, Characteristics and electrochemical performance of the TiO2-coated ZnO anode for Ni-Zn secondary batteries, J. Phys. Chem. C115 (2011) 2572–2577, https://doi.org/10.1021/jp110308b. [24] Yong-Seok Lee, Young-Jin Kim, Kwang-SunRyu, The effects of CuO additives as the dendrite suppression and anti-corrosion of the Zn anode in Zn-air batteries, J. Ind. Eng. Chem. 78 (2019) 295–302. [25] C. Cui, M. Li, X. Zhou, X. Zhang, Synthesis of ZnO/carbon nanotube composites for enhanced electrochemicalperformance of Ni-Zn secondary batteries, Mater. Res. Bull. 112 (2019) 261–268. [26] M.T. San Jose, A.M. Epinosa, M.L. Tascon, M.D. Vazquez, P. Sanchez Batanbro, Electrochemical behaviour of copper oxides at a carbon paste electrode. Application to the study of the superconductor YBaCuO, Electrochim. Acta 36 (1991) 1209. https://doi.org/10.1016/0013-4686(91)85111-J. [27] G. Ivanova, et al., Improvement of the electrochemical properties of Ni-Zn rechargeable batteries by adding B(Pb)SrCaCuO conducting ceramics, Bulgar. Chem. Commun. 51–1 (2019) 66–72. [28] L. Stoyanov, S. Terzieva, A. Stoyanova, A. Stoyanova-Ivanova, M. Mladenov, D. Kovacheva, R. Raicheff, J. Progr. Res. Chem. 2 (2015) 83–91. http://www.scite cresearch.com/journals/index.php/jprc/article/view/455. [29] A.K. Stoyanova-Ivanova, S.D. Terzieva, G.D. Ivanova, M.A. Mladenov, D. G. Kovacheva, R.G. Raicheff, S.I. Georgieva, B.S. Blagoev, A.J. Zaleski, V. Mikli,
4. Conclusion For the first time the structural and morphological changes in B(Pb) SCCO ceramics after their electrochemical treatment in environment similar to the Ni–Zn battery are studied (7 M KOH, 25 � C). To elucidate the electrode changes during electrochemical tests, the ex-situ XRD and SEM/EDS experiments are also conducted. After the CV experiment various reduction products are found, such as CuO, Bi2O3, Bi2CuO4, Ca (OH)2 and Sr(OH)2. The CV-curves indicate that at more negative po tentials Cu and Bi compounds are further reduced to Cu and Bi metals. This may lead to formation of metal particles, which improve the overall conductivity of the electrode and the contact between the particles of active ZnO material. On the other hand, the Ca- and Sr-products in the case of B(Pb)SCCO 2212 modification decrease the solubility of ZnO in the electrolyte and suppress the shape change of the electrode. Therefore B(Pb)SCCO 2212 may have additional effect on Zn electrode perfor mance, compared to B(Pb)SCCO 2201, due to calcium content. Here, however, the higher cost and longer time for its synthesis must be considered. Acknowledgments This work was part of a bilateral project between the Bulgarian Academy of Sciences and Estonian Academy of Science, Tallinn Uni versity of Technology (Estonian projects TAR16016 and IUT-T4). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.121934. 7
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The use of high-temperature superconducting cuprate as a dopant to the negative electrode in Ni-Zn batteries, Bulg. Chem. Commun. 47 (2015) 41–48. https://www .researchgate.net/publication/278155751. [30] M. Mladenov, R. Raicheff, L. Stoyanov, A. Stoyanova-Ivanova, S. Terzieva, D. Kovacheva, Electrode mass for zinc electrode of alkaline rechargeable batteries”. BG Patent Reg. No 66730/28.8.2018 (in Bulgarian).
[31] A. Stoyanova-Ivanova, A. Vasev, P. Lilov, V. Petrova, Y. Marinov, A. Stoyanova, G. Ivanova, V. Mikli, Conductive ceramic based on the Bi-Sr-Ca-Cu-O HTSC system as an additive to the zinc electrode mass in the rechargeable Ni-Zn Batteries–Electrochemical Impedance Study, Comptes rendus de l’Academie bulgare des Sciences 72 (2019) 174–181. [32] A.J. Bard, R. Parsons, J. Jordan, International Union of Pure and Applied Chemistry, Standard Potentials in Aqueous Solution, M. Dekker, New York, 1985.
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Studies of structural and morphological properties of cuprate conductive ceramics after electrochemical treatment in alkaline electrolyte Angelina Stoyanova-Ivanova a, *, Peter Lilov a, Alexander Vasev a, Antonia Stoyanova b, Galia Ivanova b, Daniela Karashanova c, Valdek Mikli d a
Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784, Sofia, Bulgaria Institute of Electrochemistry and Energy Systems “Academician Evgeni Budevski”, Bulgarian Academy of Sciences, Acad. Georgi Bonchev Str., Bl. 10, 1113, Sofia, Bulgaria c Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Acad. Georgy Bonchev St., Bl. 109, 1113, Sofia, Bulgaria d Insitute of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate Str., 19086, Tallinn, Estonia b
H I G H L I G H T S
� BSCCO � BSCCO � BSCCO � BSCCO
ceramics display redox activity in an environment similar to the NiZn battery. ceramics are reduced during the charging process. reduction products improve the negative active mass conductivity . reduction products decrease the solubility of ZnO.
A R T I C L E I N F O
A B S T R A C T
Keywords: B(Pb)SCCO conductive ceramics Physicochemical and electrochemical properties Ni–Zn rechargeable batteries
This study is focused on detail research of the structural and morphological changes in cuprate ceramics Bi1.7Pb0.3Sr2CaCu2Ox (B(Pb)SCCO 2212) and Bi1.7Pb0.3Sr2CuOx (B(Pb)SCCO 2201) during the electrochemical treatment. Their electrochemical reduction behavior is investigated by Cyclic Voltammetry (CV) and Chro nopotentiometry (CP) in 7 M KOH. The used ceramic powders are physicochemically characterized before and after the electrochemical test. It has been found that ceramics are reduced during the charging process. The reduction products are different oxides and hydroxides as well as Cu and Bi metals. Their presence probably improve the overall conductivity of the electrode and the contact between the particles of the negative active mass. On the other hand, the Ca- and Sr-products in the case of B(Pb)SCCO 2212 modification decrease the solubility of ZnO in the electrolyte and suppress the shape change of the electrode.
1. Introduction This study is focused on the electrochemical redox behavior of Bi1.7Pb0.3Sr2CaCu2Oy (B(Pb)SCCO 2212) and Bi1.7Pb0.3Sr2CuOy (B(Pb) SCCO 2201) cuprate ceramics, which are known to be superconductors at and above liquid nitrogen temperatures [1]. There is insufficient in formation about the electrochemical reactivity of these ceramics [2], which can be relevant to room temperature applications of these ma terials which do not involve superconductivity. One such application is in electrocatalysts for oxygen reduction and hydrogen evolution reactions (ORR and HER) studied by Lim et al. [3] who found BSCCO 2212 to be electrochemically reactive in aqueous
media, but did not discuss the reduction products in detail. Another potential application based on the chemical properties of BSCCO-system ceramics is as a stabilizing additive in zinc anodes for rechargeable Ni–Zn batteries. It is well known that this green technology suffers from a variety of issues that are well documented in literature [4–6] and has found relatively few large-scale applications [4,7–9]. This technological limit is related to the shape change of zinc electrode with increasing charge/discharge cycle count, zinc electrode passivation, and growth of zinc metal dendrites leading to separator punctures and short-circuiting of the battery [10,11]. There are different approaches to overcoming these limitations, such as an optimizing the battery design, improvement of the Ni(OH)2 electrode, use of gel polymer electrolytes
* Corresponding author. E-mail address: [email protected] (A. Stoyanova-Ivanova). https://doi.org/10.1016/j.matchemphys.2019.121934 Received 23 May 2019; Received in revised form 30 July 2019; Accepted 1 August 2019 Available online 2 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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and electrolyte additives [12,13], a special 2-layer separators [14,15], modification of the charging process by optimization of charge mode with protection against recharging and hydrogen release [16,17] and etc. An already proven approach to minimize the above mentioned problems is by adding different oxides to the active mass of the negative electrode mass. The inclusion of B(Pb)SCCO in the Zn electrode is an effective solution. This may be related to the known beneficial effects of various simpler oxide additives, such as Ca(OH)2 [18–20], Bi2O3 [21], PbO [22], TiO2 [23], CuO [24] and others [25] in reducing shape change, dendrite growth and suppressing hydrogen evolution. Transi tion metal oxide additives to Ni–Zn batteries (Bi2O3, CuO, PbO) have been shown to undergo reduction during charging, prior to the main zinc deposition reaction [18–24]. Literature suggests formation of copper oxides and even metallic copper at high cathodic polarization [2,26]. In our previous studies of using YBCO, two modifications of B(Pb) SCCO or other conducting cuprate ceramics as additives to the zinc electrode mass of Ni–Zn alkaline batteries was studied [27–30]. The chemical stability of B(Pb)SCCO and YBCO materials to the highly alkaline batteries electrolytes (7 MKOH) has been confirmed by struc tural and surface morphology analysis as well as magnetic measure ments of ceramic samples after a prolonged exposure to the alkaline electrolyte [28,29]. The electrochemical tests have shown that the nickel-zinc cells with ceramics in the zinc electrode mass exhibit good performance stability. For example the discharge capacity of the cell with a zinc electrode containing carbon as conducive addition is lower (up to 30%) than the capacity of the cell with a zinc electrode with YBa2Cu3O7-x conductive ceramics [29]. The improvement of the electrochemical properties of Zn-electrode in the Ni–Zn battery cell by adding two types of B(Pb)SrCaCuO con ducting ceramics B(Pb)SCCO 2201 and B(Pb)SCCO 2212 is confirmed. The presence of both types of ceramics in the zinc electrode mass in creases the discharge capacity of the battery. The obtained results are explained again by an increase in the conductivity and homogeneity of the electrode mass in the presence of the used additives, on the one hand, and the formation of a highly conducting network between the zinc oxide particles, on the other [27]. The impedance studies have shown that the positive effect of the Bi1.7Pb0.3Sr2CuOy additives on the performance of the zinc electrode is not due to its intrinsic conductivity, but more probably owing to the reduction products of the ceramic that could be produced during charging the electrode [31]. Based on the above, it is of interest to examine in more detail the structural and morphological changes that occur in ceramics after their electrochemical treatment in alkaline solutions. This is an innovative study that provides more clarity about the mechanism of the ongoing processes. The determination of the reduction products of BSCCO ce ramics in an environment similar to the Ni–Zn battery may prove also useful in explaining how the improvement in Zn-electrode performance is achieved.
copper clad board with silver conductive paste to ensure good electrical conductivity. The board is then insulated with epoxy, leaving only one face of the B(Pb)SCCO tablets with 75 mm2 of surface area exposed to the electrolyte. 2.2. Experimental methods The investigated ceramics are physicochemically and electrochemi cally characterized. The physicochemical analyses are performed by the X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Xray Spectroscopy (EDX) before and after electrochemical tests (Chro nopotentiometry (CP) and Cyclic Voltammetry (CV)). The powder X-ray diffraction patterns are collected in the range from 10 to 80� 2θ with a constant step of 0.02� 2θ angle on Bruker D8 Advance diffractometer with Cu Kα radiation and Lynx Eye detector. Phase identification was performed with the Diffract plus EVA using ICDDPDF2 Database. The powder diffraction patterns are evaluated with the Topas-4.2 software package using the fundamental parameters peak shape description including appropriate corrections for the instrumental broadening and diffractometer geometry. The surface morphology of B(Pb)SCCO 2212 and 2201 ceramics after CP is compared to the one of freshly synthesized tablets by means of digitized Philips 515 scanning electron microscope. The data for the surface elemental composition of the samples were determined by Energy dispersive spectrometer Oxford Instruments INCA Energy system with a Zeiss EVO MA-15 scanning electron microscope with a LaB6 cathode. The qualitative and quantitative analyses are carried out at an accelerating voltage of 20 kV. The elemental mapping of the B(Pb)SCCO 2212 and 2201 surfaces before and after the chro nopotentiometric examination is appended as supplementary information. CP is performed using a computer controlled potentiostat/galvano stat - Bio-logic SP-200 to control the cell current and record data. The prepared B(Pb)SCCO (2201 and 2212) electrodes are used as working electrodes at a constant cathodic current of 10 mA for 30 min in a 7 M KOH electrolyte at 25 � C, the measured potential is referenced to a saturated calomel electrode (SCE). A polished platinum plate is used as a counter-electrode (CE). The open circuit potential is measured before applying cathodic current by Bio-logic SP-200 and potential value in electrochemical system reached constant value. To observe the initial electrochemical degradation stage, destruction of the ceramics’ structure and intermediate reaction products, after the CP testing the ceramic electrodes were removed from the electrolyte, rinsed with distilled water and dried with a hot air gun. The drying step is needed to prevent any further unwanted degradation of the ceramics as they are sensitive to water [2]. CV is carried out using the same cell and electrode configuration as in the CP experiment. Three scans are done on each sample in the potential window of 1.75 V to þ0.85 V vs. SCE, starting in the positive direction from the open circuit potential at a rate of 5 mVs 1. The goal of this experiment is to determine the potential windows in which the indi vidual redox-active metal components (Cu and Bi) undergo reduction.
2. Materials and methods 2.1. Materials Through solid state reaction ceramics of nominal composition: B(Pb) SCCO 2212 and B(Pb)SCCO 2201 are obtained. Samples of Bi–Sr–Ca–Cu–O system are produced by two-stage conventional solidstate synthesis from high-purity (99,99%) oxides and carbonates Bi2O3, PbO, CuO, SrCO3 and CaCO3. After thorough mixing and grinding and initial heat treatment at 780 � C for 24 h in air the powder obtained is ground and pressed into (5–6 MPa) round tablets 9,8 mm in diameter and 1.8 mm in thickness. The tablets are then sintered again: B(Pb)SCCO 2201 at 830 � C for 24 h, B(Pb)SCCO 2212 at 830 � C for 48 h in an air atmosphere. The working electrodes are prepared using both ceramics - B(Pb) SCCO 2212 and B(Pb)SCCO 2201. A ceramic tablet is glued to a piece of
3. Results and discussion In the work of Ivanova et al. [27] the same ceramic modifications (B (Pb)SCCO-2201 and B(Pb)SCCO-2212) are used an additives to the Zn-electode in Ni/Zn battery cell. The obtained results are shown in Fig. 1 with permission from the authors. It can be seen that their pres ence results in higher and more stable specific discharge capacity in comparison to the zinc electrode with classic carbon additive (the ca pacity of cells with B(Pb)SCCO additives decreased only by about 15–20% after 500 cycles). In Fig. 2 are compared the X-ray diffraction patterns of B(Pb)SCCO 2212 and B(Pb)SCCO 2201 ceramics [24] with those on the same 2
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analysis. At potentials more negative than 1.22 V the subsequent reduction of some of these oxides may happen immediately after the initial breakdown, leading to a large increase in current. 3.2. SEM SEM results for the surface morphology of freshly synthesized ceramic materials B(Pb)SCCO 2212 and 2201 and electrochemically treated ceramics are presented in Fig. 4. It is well seen from the mi crographs (Fig. 4a and b) that two as-synthesized samples exhibit the typical for B(Pb)SCCO structure of the main matrix, consisting of large, flat terraces, laying in the a-b plane, predominantly. Some of them are located perpendicularly of this plane, according to the synthesis method and they are visualized as rods on the micrographs. In the same time, spheroid and significantly smaller in size (about tenths of a micrometer), compared to the terraces (few microns), grains are found on the terraces’ surface. They tend to form aggregates and are abundant in the surface morphology of B(Pb)SCCO 2212 (Fig. 4a) and rare in this one of B(Pb) SCCO 2201 (Fig. 4b). After CP testing the morphologies on B(Pb)SCCO 2212 and 2201 ceramics (Fig. 4c and d) are changed, compared to the pristine materials. The width and the height of the matrix terraces are significantly increased and abrupt edges and large facets are formed. The grainy microstructure is still present in both sample morphology, as well, but the free from specks surface of the terraces in B(Pb)SCCO 2212 is increased. This is probably due to partial decomposition of the ceramic structure in the electrolyte (Fig. 4c), while small grains of new phase, formed during the electrochemical treatment, have completely conquered the terraces in B(Pb)SCCO 2201 (Fig. 4d).
Fig. 1. Dependence of the specific discharge capacity of the zinc electrode with carbon, B(Pb)SCCO 2201 and B(Pb)SCCO 2212 ceramic additives on the cycle number, at current load density C/5 (13 mA cm 2). Graph reproduced with permission from of Ivanova et al. [27].
ceramic samples after their chronopotentiometry study. The results shown that after electrochemical treatment, the preservation of main phases with reduced intensity and appearance of Bi2O3 were observed. 3.1. CP The CP curves of B(Pb)SCCO 2212 and 2201 at a constant cathodic current of 10 mA (current density 13.3 mA cm 2) are presented in Fig. 3. The starting open circuit potentials are þ0.15 V for B(Pb)SCCO 2201 and þ 0.087 V for B(Pb)SCCO 2212. The potential of B(Pb)SCCO 2201 changes abruptly when current is applied, immediately falling to 0.9 V and settling to a plateau at 1.15 V. The potential of B(Pb)SCCO 2212 falls slower with a mild s-curve at 0.6 to 0.7 V, a plateau at 0.95 V is then reached and this potential remains relatively constant until the end of the experiment. The total charge passed in the experiments is 5 mAh, which is more than double the charge passed in the cathodic part of the first cycles of the CV experiments: 2.15 mAh for B(Pb)SCCO 2212 and 0.58 mAh for B(Pb)SCCO 2201. Therefore we speculate that an initial breakdown reaction may occur prior to reduction reactions occurring at potentials more negative than the plateau potentials above. This breakdown appears unhindered by its reaction products because the electrode potential remains relatively constant. This process may pro ceed until the complex structure of B(Pb)SCCO is completely destroyed, leading to the simpler oxides consisting of one or two metals (CuO, Bi2O3, Bi2CuO4, Ca(OH)2, Sr(OH)2), which were later detected by EDX
3.3. EDX Table 1 presents the atomic composition of untreated B(Pb)SCCO 2212 ceramic in multiple regions marked in the SEM image in Fig. 5a (magnification: 2000�) and in Table 2 - of untreated B(Pb)SCCO 2201 ceramic in the regions marked in the SEM image in Fig. 5b (magnifi cation: 5000�), respectively. The results confirm that the elemental composition of the freshly synthesized ceramics corresponds to the expected stoichiometric ratios for B(Pb)SCCO 2212 (Table 1: Spectra 1,2,3 and 7) and 2201 (Table 2: Spectrum 4). Inclusions of CuO are visible in both types of untreated ceramics (Table 1: Spectrum 6, Table 2: Spectrum 2), as well as sec ondary phases: CaSrCu3O5 (Table 1 Spectrum 4, 5) in B(Pb)SCCO 2212 and Sr2Cu3O5 (Table 2 Spectrum 1) in B(Pb)SCCO 2201. Table 3 presents the atomic composition of electrochemically treated B(Pb)SCCO 2212 in multiple regions marked on Fig. 6 and Table 4 - of treated B(Pb)SCCO 2201 in the regions marked on Fig. 6, respectively. Areas with high predominant copper content can be seen in the
Fig. 2. XRD patterns of synthesized untreated/treated B(Pb)SCCO 2212 (a) and untreated/treated B(Pb)SCCO 2201 (b). 3
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Materials Chemistry and Physics 239 (2020) 121934
Fig. 3. Potential vs. time curves of the chronopotentiometry experiments.
Fig. 4. SEM images of untreated B(Pb)SCCO 2212 (a) and 2201 (b) and of electrochemically treated B(Pb)SCCO 2212 (c) and 2201 (d). Table 1 Elemental composition of points of untreated B(Pb)SCCO 2212 (regions marked on the SEM image in Fig. 5a). Concentration: At. % Spectrum Element: Bismuth Lead Copper Strontium Calcium Oxygen
0.91 � 0.3 7.15 � 0.3 6.94 � 0.8 4.79 � 0.2 74.98 � 4.0
5.23 � 1.3 1.04 � 0.3 7.71 � 0.3 6.70 � 0.8 4.60 � 0.2 74.35 � 3.9
1
2
3
4
5
6
5.60 � 1.3 0.86 � 0.3 8.49 � 0.4 6.95 � 0.8 5.82 � 0.2 72.67 � 3.9
5.20 � 1.3 – 30.64 � 1.3 8.64 � 0.9 10.54 � 0.3 49.18 � 2.4
0.27 � 0.1 – 27.04 � 1.1 9.18 � 0.9 9.59 � 0.3 53.61 � 2.6
0.58 � 0.2 – 47.03 � 2.0 – – 52.97 � 2.5
– 0.90 � 0.3 6.62 � 0.3 5.04 � 0.6 5.29 � 0.2 77.09 � 4.2
5.07 � 1.3
4
7
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Materials Chemistry and Physics 239 (2020) 121934
Fig. 5. SEM images of regions examined by EDX in untreated B(Pb)SCCO 2212 (a) and B(Pb)SCCO 2201 (b).
another indication that Ca and Sr containing phases separate from the B (Pb)SCCO structure, forming simpler compounds. Spectra 4 and 8 in Table 3 show high bismuth content. Sr, Ca and Cu are also present in the spectra in varying amounts with no apparent relationships between them. Therefore, we conclude that they are likely present as separate phases such as Bi2O3, CuO and Ca- and Sr-hydroxides or carbonates. Several regions appear different from their immediate surroundings in Fig. 7. The spectra in points 3 and 7 appear similar in the SEM image and their elemental composition (Table 4) is nearly identical with a Sr: Pb ratio of about 1:1. Points 5 and 8 also have nearly the same composition along with the highest Sr and Pb concentration of all spectra in Table 4. Regions with Bi:Cu atomic ratios close to 2:1 were observed in both B(Pb)SCCO ceramics after the chronopotentiometry treatment (Table 3: Spectrum 1 and 2. Table 4: Spectrum 2,3 and 7). The region of Spectrum 1, Table 3 appears homogenous in the SEM image and is likely a single compound. This leads us to conclude that Bi2CuO4 is also present as a decomposition product in addition to CuO and Bi2O3. It should be also noted that the atomic ratios in Spectrum 5 in Table 3 and Spectrum 4 in Table 4 are very close to those of the pristine ce ramics, indicating that some of it remains unchanged.
Table 2 Elemental composition of untreated B(Pb)SCCO 2201 (regions marked on the SEM image in Fig. 5b). Concentration: At. % Spectrum
1
2
3
4
Element: Bismuth Lead Copper Strontium Oxygen
1.44 � 0.2 – 29.11 � 1.0 18.23 � 1.4 51.21 � 2.0
– – 49.92 � 2.0 – 50.08 � 2.1
5.00 � 1.1 11.38 � 1.9 6.30 � 0.21 27.61 � 1.7 49.70 � 1.3
11.61 � 2.3 1.82 � 0.9 6.17 � 0.2 12.58 � 1.1 67.82 � 2.3
electrochemically treated ceramics (Fig. 6: point 5; Table 3: Spectrum 5) as well as a distinct CuO grain in Fig. 6: point 6 and Table 4: Spectrum 6. Strontium is the major component in Spectrum 1 in Table 4. Sepa ration of alkaline earth element phases from the B(Pb)SCCO structure is a known decomposition process of cuprate ceramics [2] and this sepa ration is clearly visible in the corresponding SEM image (Fig. 7b). Different regions with high Ca, Sr and Cu content are visible in the supplementary EDX element maps (See Supplementary Fig. 4). This is
Table 3 Elemental composition of electrochemically treated B(Pb)SCCO 2212 (regions marked on the SEM image in Fig. 6). Concentration: At. % Spectrum
1
2
3
4
5
6
7
8
9
Element: Bismuth Lead Copper Strontium Calcium Potassium Oxygen
13.25 � 2.1 – 7.21 � 0.3 0.37 � 0.2 – 1.72 � 0.2 77.45 � 3.3
16.11 � 2.1 – 6.75 � 0.2 – – – 77.14 � 3.0
1.46 � 0.4 – 12.76 � 0.7 6.50 � 0.8 7.89 � 0.3 2.07 � 0.2 69.32 � 3.8
14.81 � 1.7 – 2.76 � 0.1 6.24 � 0.6 3.11 � 0.1 – 73.08 � 3.1
2.04 � 0.4 – 27.12 � 1.2 2.56 � 0.4 4.14 � 0.2 1.01 � 0.2 63.14 � 3.2
6.54 � 1.1 – 6.48 � 0.3 6.07 � 0.7 4.04 � 0.1 – 76.86 � 4.1
10.67 � 1.6 – 0.94 � 0.1 2.97 � 0.4 4.42 � 0.1 – 81.01 � 4.0
18.83 � 2.1 1.70 � 0.3 10.98 � 0.3 6.49 � 1.1 2.98 � 0.1 – 59.03 � 2.0
4.32 � 1.2 0.50 � 0.2 4.01 � 0.2 4.68 � 0.9 3.87 � 0.2 0.26 � 0.1 82.34 � 4.8
Fig. 6. SEM images of regions examined by EDX in electrochemically treated B(Pb)SCCO 2212. Magnification: 10000� - Fig. 6a. 2000x - Fig. 6b. 5
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Materials Chemistry and Physics 239 (2020) 121934
Table 4 Elemental composition of electrochemically treated B(Pb)SCCO 2201 (regions marked on the SEM image in Fig. 7). Concentration: At. % Spectrum
1
2
3
4
5
6
7
8
Element: Bismuth Lead Copper Strontium Potassium Oxygen
3.44 � 0.9 0.87 � 0.4 1.11 � 0.1 20.51 � 1.8 1.13 � 0.1 72.12 � 3.0
11.88 � 1.9 0.67 � 0.5 5.72 � 0.2 4.14 � 0.4 – 77.59 � 3.0
13.08 � 2.0 2.81 � 1.0 5.46 � 0.2 2.56 � 0.4 – 76.09 � 2.9
11.15 � 2.1 1.87 � 0.8 6.49 � 0.2 12.66 � 1.0 – 67.83 � 2.6
2.59 � 0.6 6.90 � 1.2 5.58 � 0.2 19.02 � 1.1
– – 46.47 � 1.9 0.77 � 0.1 – 52.76 � 2.4
15.77 � 2.2 2.96 � 1.1 5.98 � 0.2 2.27 � 0.4 – 73.03 � 2.6
2.79 � 0.6 7.25 � 1.4 5.08 � 0.2 20.49 � 1.3 – 64.40 � 2.0
65.91 � 2.0
Fig. 7. SEM images of regions examined by EDX in electrochemically treated B(Pb)SCCO 2201. Magnification: 10000� - Fig. 7а and 3000x - Fig. 7b.
3.4. CV
displays two slightly more distinct peaks: Ipc 1b and 2b (Fig. 8b). Copper and bismuth are the only metals in the composition of the ceramics that can be reduced under the experimental conditions. Therefore we sup pose that the observed current peaks are associated with their reduction to the corresponding metals in an order that agrees with the electro chemical reduction series. But due to the presence of other inactive alkaline-earth elements in the ceramics’ composition reduction occurs at more negative potentials than the reduction potentials of the simple oxides CuO and Bi2O3. For comparison, the Bi and Cu reduction po tential for simple oxides are respectively 0.067 V and 0.277 V vs. SCE [32]. Peaks associated with further reaction of the products of the initial cathodic scan appear on the second and third cycles (Fig. 8). Three broad oxidation peak regions can be distinguished. One indicates the
Cyclic Voltammetry is carried out to observe reactions occurring at more negative potentials than those reached in the chronopotentiometry experiment. Such reactions are relevant because the reduction of ZnO to Zn metal (the main reduction reaction in a Ni–Zn battery) occurs at 1.5 V vs. SCE. EDX results show that after CP testing, the reaction products consist of various oxides including CuO and Bi2O3 which may also be reduced to metals. During the first cycle CV reveals peaks related to the redox behavior of the ceramics themselves. No distinct oxidation peaks appear on the first scan until oxygen evolution begins. B(Pb)SCCO 2212 displays a single broadened reduction peak (or possibly two overlapping peaks) at very negative potentials Ipc 1a and 2a (Fig. 8a), while B(Pb)SCCO 2201
Fig. 8. CV curves of B(Pb)SCCO 2212 (a) and B(Pb)SCCO 2201 (b) in 7 M KOH solution. scan rate: 5 mV s 1. 6
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reoxidation of Bi metal to Bi2O3 (Ipa 1a). Under alkaline conditions copper metal is first oxidized to Cu2O (Ipa 2a). At more positive poten tials Cu2O and any remaining copper metal are oxidized to CuO (Ipa 3a). These two simultaneous reactions explain why peak Ipa 3a (CuO) has a greater maximum value and charge passed associated with it. The region between 0 V and þ0.5 V corresponds to the oxidation of copper to sol uble copper (II) species [26]. The potentials (Epa 1.2 and 3) at which these peaks occur are essentially the same for both B(Pb)SCCO 2212 and 2201 and are in close agreement with the theoretical equilibrium po tentials for the respective reactions and with those obtained by a similar study on the YBCO superconductor, which also examined the electro chemistry of copper and it’s oxides in alkaline solution [26]. This leads us to believe that the reduction reactions in cycle 1 lead to the same products: Cu and Bi metals, for both B(Pb)SCCO 2212 and 2201. Scan cycles 2 and 3 also reveal that the initial reduction process irreversibly destroys the ceramics. Three reduction peaks are present at less negative potentials than the initial reduction peaks of cycle 1. They represent the reduction of Bi2O3 to Bi0 (Ipc 20 a. 20 b). CuO to Cu2O (Ipc 30 a. 30 b) and simultaneous reduction of CuO and Cu2O to Cu0 (Ipc 20 a. 20 b). When used as an additive in rechargeable Zn electrodes the BSCCO ceramics will likely undergo reduction during the charging process. This is because the reduction peak potentials Epc 1, 2a and Epc 1b, 2b are more positive than the potential at which the main ZnO reduction re action takes place ( 1.5 V vs. SCE). This may lead to formation of metal particles, which improve the overall conductivity of the electrode and contact between the particles of active ZnO material. The calcium and strontium present in the ceramic are irreducible under our conditions and as a result of the destruction of the ceramic during reduction they form Sr(OH)2 and also Ca(OH)2 in the case of B(Pb)SCCO 2212. These products can decrease the solubility of ZnO in the electrolyte by for mation of insoluble CaZn(OH)4 [7,20] and SrZn(OH) and reduce the shape change of the electrode. The decrease the solubility of ZnO leads to increasing the utilization of the active material and a constant growth of Zn deposition in the electrode volume during the charge discharge process [20].
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4. Conclusion For the first time the structural and morphological changes in B(Pb) SCCO ceramics after their electrochemical treatment in environment similar to the Ni–Zn battery are studied (7 M KOH, 25 � C). To elucidate the electrode changes during electrochemical tests, the ex-situ XRD and SEM/EDS experiments are also conducted. After the CV experiment various reduction products are found, such as CuO, Bi2O3, Bi2CuO4, Ca (OH)2 and Sr(OH)2. The CV-curves indicate that at more negative po tentials Cu and Bi compounds are further reduced to Cu and Bi metals. This may lead to formation of metal particles, which improve the overall conductivity of the electrode and the contact between the particles of active ZnO material. On the other hand, the Ca- and Sr-products in the case of B(Pb)SCCO 2212 modification decrease the solubility of ZnO in the electrolyte and suppress the shape change of the electrode. Therefore B(Pb)SCCO 2212 may have additional effect on Zn electrode perfor mance, compared to B(Pb)SCCO 2201, due to calcium content. Here, however, the higher cost and longer time for its synthesis must be considered. Acknowledgments This work was part of a bilateral project between the Bulgarian Academy of Sciences and Estonian Academy of Science, Tallinn Uni versity of Technology (Estonian projects TAR16016 and IUT-T4). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.121934. 7
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