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New modification procedure of zinc powder in neodymium nitrate solution for improving the electrochemical properties of alkaline zinc electrodes
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 70 (2009) 45–54
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New modification procedure of zinc powder in neodymium nitrate solution for improving the electrochemical properties of alkaline zinc electrodes Liqun Zhu, Hui Zhang , Weiping Li, Huicong Liu Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e in fo
abstract
Article history: Received 3 February 2008 Received in revised form 21 August 2008 Accepted 4 September 2008
Neodymium conversion films are directly deposited on the surface of zinc powder by means of ultrasonic impregnation to prepare the modified electrode material in order to obtain high-performance zinc electrodes applied in alkaline medium. Scanning electron microscopy, X-ray diffraction and other characterization techniques are used to analyze the formation and distribution of neodymium conversion coatings imposing different methods and process parameters. Simultaneously, the electrochemical properties of corresponding zinc electrodes are also studied through potentiodynamic polarization and cyclic voltammetry. Results demonstrate that the distribution features of neodymium conversion layers are changed by adjusting ultrasonic time and irradiation power, which contributes to different enhancement extents for the electrochemical performance of zinc electrodes. Especially, the neodymium conversion coatings generated by ultrasonic impregnation at an ultrasonic power of 550 W for an irradiation time of 10 min play a very efficient role in obtaining fine corrosion resistance and persistent cycle behavior of zinc electrode. Besides, ultrasonic impregnation is verified to have a great advantage, as compared with simple impregnation, because the neodymium conversion layers formed under the action of ultrasonic agitation and cavitation phenomenon can obviously improve the electrochemical performance of zinc electrodes. & 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds A. Surfaces C. Electron microscopy C. X-ray diffraction D. Electrochemical properties
1. Introduction Zinc powder, as an active material of zinc electrode, has many merits such as low cost, electrochemical reversibility, high specific energy and low equilibrium potential [1–3], which make it suitable for wide applications in alkaline battery systems [4,5]. However, the electrochemical performance of zinc electrode is usually limited by corrosion [6–8] and dendrite growth [9–11] in alkaline electrolyte, and these issues lead to high capacity loss and short cycle life of the zinc electrode. In the last few years, many attempts have been made to develop modified zinc electrodes for reducing the problems. For example, Zhu et al. [12] evaluated that lanthanum and neodymium hydroxides coated on zinc electrodes through electrolysis could suppress dendrite growth of zinc. Vastalarani and co-workers [13] reported that the reversibility and corrosion protection of zinc electrode in alkaline medium were improved by electrostatic deposition of a conducting polymer onto the electrode. Nevertheless, not much attention has been paid to the modification of zinc powder itself and to the influence of modified zinc powder on the properties of corresponding zinc electrode.
Corresponding author. Tel.: +86 1082317113; fax: +86 1082317133.
E-mail address: [email protected] (H. Zhang). 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.09.005
In addition, it has been found that many metals immersed in rare earth salt by a simple impregnation process [14] can increase corrosion protection [15,16]. Hinton et al. [17,18], who made the first effort on the field, suggested that many local chemical cells emerged on the heterogeneous surface of the immersed metal due to the existence of grain boundaries, exposed inclusion, intercrystalline compound, native oxide layer and other sub-microscopic defects. Thus, the electrochemical reactions of immersed metal dissolution and oxygen reduction were proposed to preferentially occur at those defect sites. They also confirmed that the reactions of oxygen reduction generated an alkaline environment, which resulted in the precipitation of rare earth conversion films on the surface of the immersed metal [19]. In the present paper, ultrasonic impregnation technique has been utilized for the modification of zinc powder to form neodymium conversion layers onto it, taking into account many merits of ultrasound that are associated with ultrasonic agitation and cavitation phenomenon [20,21]. Among them, cavitation phenomenon has been proved to be the predominant effect as reported elsewhere [22]. Apart from the remarkable enhancement of mass transfer rate and intense activation of electrochemical reactions, the cavitation phenomenon has a significant effect on the dissolution of oxygen, which contributes to a good alkaline solution in the liquid–solid (Nd(NO3)3–Zn) system. In this respect, we aim to investigate the merits of ultrasonic impregnation on the
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distribution and effect of neodymium conversion coatings in comparison with the simple impregnation process, as well as the influence of neodymium conversion films generated with different ultrasonic time and power on the electrochemical behavior of zinc electrodes consisting of such modified zinc powder.
2. Experimental All chemicals were of analytical grade, and doubly distilled water was used for preparing all solutions. The modification of zinc powder was carried out at room temperature by immersing bare powder into 0.045 mol L1 Nd(NO3)3 solution that was placed in an ultrasonic apparatus with a constant ultrasonic frequency of 40 kHz. The neodymium conversion layers were respectively formed on zinc powder with different operating times (5, 10, 15 and 20 min) or various output powers (220, 330, 440, 550, 660 W) of ultrasonic irradiation. Simultaneously, we expounded the modified case of the simple impregnation process without the application of ultrasound for comparison only. The morphology of bare or modified zinc powder was observed using a JEOL JSM-5800 scanning electron microscopy (SEM). The point composition on the surface of modified zinc powder was examined through energy dispersion spectrometry (EDS) with a GENESIS 60S instrument. Auger electron spectroscopy (AES) was employed to further analyze the surface composition of modified zinc powder on a PHI 700 instrument. The AES spectra were acquired at a primary electron beam energy of 5 keV and a primary electron beam current of 10 nA. X-ray diffraction (XRD) measurements were performed on a Bruker’s D8 diffractometer with Cu Ka radiation (l ¼ 1.5406 A˚). y/2y diffractograms were obtained in the scan range of 20–1001 with a step of 0.031. Later, zinc electrode that had an apparent area of 1 cm2 was prepared from a mixture of 1.0 g bare or modified zinc powder with a few drops of ethanol, electrolyte and PTFE. Here nickel foam was used as the current collector to incorporate the above mixture and shape a porous electrode. All electrochemical measurements were performed with a CHI 660a type of electrochemical system connected to a microcomputer running dedicated software. The electrochemical experiments were conducted by means of a typical three-electrode cell where a Ni plate was the counter electrode, Hg/HgO was used as the reference electrode and a zinc electrode acted as the working electrode. The potentials were measured versus a Hg/HgO reference electrode, and the
currents were obtained to be equivalent to the current densities owing to the 1 cm2 apparent area of zinc electrodes. The electrolyte used throughout this work was 6 M KOH solution saturated with ZnO. Then the zinc electrodes were polarized from 1.7 to 1.0 V at a constant scan rate of 0.5 mV s1 by the potentiodynamic polarization technique. The voltammetric characteristics of the zinc electrodes with bare or modified zinc powder were also investigated in the potential range of 0.6 to 1.8 V and the scan rate was always 10 mV s1. Moreover, all electrochemical measurements were repeated three times to ensure that the data were reproducible. The electrochemical curves that had medium current values for each sample of modified zinc powder were, respectively, selected from the three reproducible data to reveal in the figures.
3. Result and discussion 3.1. Characteristic analysis The surface micrographs of bare zinc powder and the powder modified in 0.045 mol L1 Nd(NO3)3 solution by simple impregnation for a treatment time of 10 min are exhibited in Fig. 1. Based on the clean and uneven surface of bare zinc powder, simple impregnation process leads to the formation of a small quantity and non-uniform distribution of white deposits on the surface of the modified zinc powder. The composition of white deposits can be obtained by the EDS method, and the result certifies the presence of Nd and O elements in the modified zinc powder. This fact implies that Nd2O3 and/or Nd(OH)3 are generated in the white deposits growing on the surface of modified zinc powder. However, no Nd signal can be observed at the dark regions on the modified zinc powder from EDS spectra. It appears that Nd-rich particles are distributed on the modified zinc powder in the form of white deposits. In order to accurately confirm the surface composition of modified zinc powder, Fig. 2 illustrates the AES spectra for two typical dark regions on the zinc powder modified by simple impregnation. The results indicate that the dark regions on the modified zinc powder also contain Nd and O elements without exception. It is therefore reasonable to conclude that thin neodymium conversion layers cover the surface of modified zinc powder, and the neodymium conversion coatings consist of basal
Fig. 1. SEM photographs obtained from (a) bare zinc powder, and (b) the zinc powder modified by the simple impregnation for a treatment time of 10 min.
ARTICLE IN PRESS L. Zhu et al. / Journal of Physics and Chemistry of Solids 70 (2009) 45–54
Intensity (a.u.)
membrane and Nd-rich particles. Simultaneously, AES measurements would generally give the C peak due to the adventitious surface cabon [23]. As can also be noted from the above SEM images, it is necessary to find a better solution, that is the
Nd
Nd C
Zn
Zn
Zn
Zn O
200
400 600 800 Kinetic energy (eV)
1000
Fig. 2. AES spectra acquired from two typical dark regions on the surface of the zinc powder modified by means of the simple impregnation.
47
ultrasonic impregnation method, to promote the formation and improve the distribution state of neodymium conversion films containing Nd-rich particles. Fig. 3 presents the SEM images of the zinc powder modified by ultrasonic impregnation at an ultrasonic power of 550 W with different treatment times. It can be clearly seen that the effect of ultrasonic time on the formation and distribution of Nd-rich particles growing in neodymium conversion coatings is remarkable. As expected, Nd-rich particles on the surface of modified zinc powder multiply and the particulate size of Nd-rich particles increases as ultrasonic time expends. It seems that Nd-rich particles gradually grow up through the extension of irradiation time. Moreover, the accumulation phenomenon of Nd-rich particles appears on the surface of modified zinc powder for an ultrasonic time of 15 or 20 min. All these suggest that an ultrasonic time of 10 min provides the surface of modified zinc powder with uniform distribution of neodymium conversion layers containing a small size of Nd-rich particles. Differences are observed regarding the morphology of modified zinc powder for a treatment time of 10 min with variable ultrasonic power as revealed in Fig. 4(a–e). With ultrasonic power of 220 and 330 W, Nd-rich particles are asymmetrically distributed on the surface of zinc powder. And it is obvious that the modification of zinc powder with an ultrasonic power of 440 or 550 W leads to the direct formation and uniform distribution of
Fig. 3. SEM images of the zinc powder modified by the ultrasonic impregnation with different ultrasonic times: ultrasonic impregnation for (a) 5 min; (b) 10 min; (c) 15 min; (d) 20 min.
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L. Zhu et al. / Journal of Physics and Chemistry of Solids 70 (2009) 45–54
Fig. 4. Microgram changes of the zinc powder modified by the ultrasonic impregnation at different ultrasonic powers: ultrasonic impregnation at (a) 220 W; (b) 330 W; (c) 440 W; (d) 550 W; (e) 660 W.
considerable Nd-rich particles owing to the favorable roles of high-intensity ultrasound. Especially, an ultrasonic power of 550 W creates the most uniform distribution of neodymium conversion films with a small size and great amounts of Nd-rich particles growing on the modified zinc powder. However, the quantity and size of Nd-rich particles on the zinc powder modified
with an ultrasonic power of 660 W decrease possibly due to the drastic agitation of ultrasound. In fact, the formation of neodymium conversion coatings during sonochemical modification can also be confirmed through the pH value variation of the Nd(NO3)3 solution before and after bare zinc powder is modified at an ultrasonic power of 550 W for
ARTICLE IN PRESS L. Zhu et al. / Journal of Physics and Chemistry of Solids 70 (2009) 45–54
Nd
3þ
2Nd
þ 3OH ! NdðOHÞ3 #
3þ
(1)
þ 6OH ! Nd2 O3 3H2 O #
(2)
3000
2.5 1
2.0
2
1.5 1.0 3
4
5
0.5 1 2 3 4 5
0.0
-1.6
-1.5 -1.4 -1.3 -1.2 Potential (V vs.HgO/Hg)
Bare 5 min 10 min 15 min 20 min
-1.1
-1.0
Fig. 6. Potentiodynamic polarization curves of zinc electrodes in 6 M KOH saturated with ZnO solution for bare zinc powder or the powder modified with different ultrasonic times. Scan rate: 0.5 mV s1.
2500 2000
Fig. 6 shows the potentiodynamic polarization curves of zinc electrodes for bare zinc powder and the powder modified with different ultrasonic time at the same irradiation power of 550 W. The corrosion potential (Ecorr) and the corrosion current density (icorr) of zinc electrodes are also obtained from the extrapolation of anodic and cathodic Tafel lines as listed in Table 1. Simultaneously, the protective efficiency (P) in Table 1 is determined as P ¼ ð1 icorr =i0corr Þ 100% [24], where i0corr and icorr denote the corrosion current density of zinc electrodes with bare and modified zinc powder, respectively. With the zinc powder modified using different ultrasonic times, the Ecorr values of several zinc electrodes have slightly moved towards a negative direction in comparison with bare zinc powder. However, there is significant reduction in the icorr values of zinc electrodes, indicating that all kinds of neodymium conversion layers formed with different ultrasonic time enhance the property of the corrosion resistance of zinc electrodes in alkaline electrolyte. The improved property of zinc electrode is related to the time parameter of ultrasonic irradiation. With the spread of ultrasonic time, the icorr value initially decreases and then increases, and correspondingly the maximum P value is detected when the zinc powder is modified for an ultrasonic time of 10 min. This is indicative of the best corrosion protection of zinc electrode containing the zinc powder modified with an ultrasonic time of 10 min as compared with other times, which is possibly explained by the uniform distribution of neodymium conversion layers
-0.5 -1.7
Zn, (112) Zn, (200) Zn, (201)
Intensity (a. u.)
3500
Zn, (004)
4000
3.2. Effect of ultrasonic time
Bare zinc powder without any treatment Zn, (103) Zn, (110)
4500
Zn, (102)
Zn, (002) Zn, (100) Zn, (101)
Fig. 5 shows the X-ray diffraction patterns of bare zinc powder and the powder modified in 0.045 mol L1 Nd(NO3)3 solution by ultrasonic impregnation at an ultrasonic power of 550 W for an irradiation time of 10 min. In addition, the XRD diffractogram for bare zinc powder treated in distilled water by ultrasound with the same ultrasonic condition is also measured for comparison only. All the peaks in the patterns of three kinds of zinc powder correspond to the hexagonal phase of zinc, and no extra lines other than the hexagonal phase of zinc are observed. Therefore, it is noted that the content of Nd(OH)3 and/or Nd2O3 in modified zinc powder is too small to be determined by XRD. However, it is interesting to notice from the XRD pattern of the zinc powder modified by ultrasonic impregnation that the formed neodymium conversion layers have a great impact on the lattice parameters of zinc. The lattice parameters of bare zinc are a ¼ 2.6612 A˚ and c ¼ 4.9430 A˚ and those of bare zinc treated only by ultrasound are a ¼ 2.6617 A˚ and c ¼ 4.9390 A˚, whereas those of modified zinc coating with neodymium conversion films are a ¼ 2.6584 A˚ and c ¼ 4.9312 A˚. The obvious decrease in the lattice parameters from the zinc powder modified by ultrasonic impregnation is possibly attributed to the presence of Nd-rich particles growing in the neodymium conversion coatings on some defect sites of zinc powder. Defect sites on zinc powder are responsible for the corrosion and dendrite formation of zinc, and the neodymium
conversion layers formed on zinc powder can therefore likely improve the electrochemical performance of zinc electrodes.
log (i, mA cm-2)
an ultrasonic time of 10 min. The pH value of the original solution is found to be 5.1 by the exact pH paper, whereas the value of the immersion solution ascends to 8.5 after the zinc powder is treated by ultrasonic impregnation. The fact denotes that the ultrasonic impregnation process provides a good alkaline environment as a result of the strongly activated oxygen reduction reactions. And the OH can be rapidly generated and supplied into liquid medium owing to the continual dissolution of oxygen promoted by cavitation bubbles’ growth and collapse. Moreover, the Nd3+ concentration in the remaining solution after modification is determined by means of spectrophotometry, which decreases to 0.019 mol L1 from the original 0.045 mol L1. All these results contribute to the formation of neodymium conversion films on the modified zinc powder. As a result, the general reactions for the deposition of neodymium conversion layers are likely to be written as
49
Modified zinc powder with ultrasonic impregnation
1500
Table 1 Corrosion studies of zinc electrodes for bare zinc powder and the powder modified with various ultrasonic times
1000 Bare zinc powder in water with ultrasound
500 0 20
40
60
80
100
2-Theta (degree) Fig. 5. X-ray diffractograms of bare zinc powder and the powder modified in 0.045 mol L1 Nd(NO3)3 solution by the ultrasonic impregnation at an ultrasonic power of 550 W for a treatment time of 10 min. The XRD pattern of bare zinc powder treated in distilled water by ultrasound is also tested for comparison only.
Ultrasonic time (min)
Ecorr (70.002 V)
icorr (70.381 mA cm2)
P (%)
Bare 5 10 15 20
1.353 1.361 1.365 1.360 1.360
67.5 8.678 6.167 8.082 9.816
– 87.1 90.9 88.0 85.4
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containing a small size of Nd-rich particles. In the case of an ultrasonic time longer than 10 min, the slightly decreased protection against corrosion of zinc electrode is possibly ascribed to the fact that the accumulation phenomenon of Nd-rich particles occurs on the surface of modified zinc powder. Hence, it is found that the neodymium conversion films formed for an ultrasonic time of 10 min are most effective in enhancing the corrosion inhibition capability of zinc electrodes. The 20th cycle curves of zinc electrodes containing bare zinc powder or the powder modified at an output power of 550 W with different time parameters are shown in Fig. 7. During the potentiodynamic scan starting from 600 mV, an irreversible anodic current peak appears around 1000 mV due possibly to the re-establishment of zinc oxidation products as reported elsewhere [25,26]. Later, a cathodic peak (C) emerges in the potential range of 1612 to 1784 mV during the negative-going scan. This is related to the reduction of the accumulated Zn(OH)2/ ZnO that remain on the electrode surface and are not dissolved in the electrolyte. However, an anodic peak (A) is attributed to the oxidation of Zn into Zn(OH)2/ZnO forming in 956 to 1053 mV during the positive-going scan. The anodic and cathodic peak potentials (EA and EC), and the anodic and cathodic peak currents (iA and iC) obtained from the cycle curves are shown in Table 2. At the same time, the difference between the anode and cathodic peak potentials (DEP ¼ EAEC), which is a measure of reversibility of the electrode reaction, is also listed in Table 2. After the zinc powder is modified with various operating times, the EA values of all zinc electrodes containing modified zinc powder have moved towards a negative direction, while the EC values have exhibited a positive shift. It can be concluded that the DEP values for all samples of modified zinc powder decrease as
A
200
Current (mA)
100 2
0
3
-100 1 2 3 4 5
4
-200
1
5 C
Bare 5 min 10 min 15 min 20 min
-300 -1800
-1600
-1200 -1400 -1000 Potential (mV vs.Hg/HgO)
-800
compared with bare zinc powder. These facts indicate that neodymium conversion layers formed on zinc powder elevate the reaction reversibility of zinc electrodes. In addition, both the iA and iC values for all samples of modified zinc powder obviously diminish, especially the iC values reveal a relatively greater drop. The decrease in the peak currents may be linked to the increased corrosion protection of zinc electrodes as confirmed by the above potentiodynamic polarization measurements. All these imply that the formation of neodymium conversion coatings on zinc powder significantly reduces the cathodic reaction rate, which favors the uniform charge and the excellent cycle stability of zinc electrodes. In particular, the neodymium conversion layers formed with an ultrasonic time of 10 min enhance the reaction reversibility of zinc electrode to the greatest extent as a result of the smallest DEP value that is 559 mV at the 20th cycle in comparison with other time parameters. Moreover, the minimum iA and iC values have a great advantage in the cycle stability of zinc electrodes. All these facts demonstrate that the neodymium conversion coatings generated on zinc powder for an ultrasonic time of 10 min provide the zinc electrode with the best reversibility and the highest stability. Consequently, the results obtained from the above electrochemical measurements manifest that the ultrasonic time of 10 min is considered to be an optimum time parameter for great availability of neodymium conversion layers in improving the electrochemical performance of zinc electrode.
3.3. Influence of ultrasonic power Similarly, the typical potentiodynamic polarization curves of zinc electrodes are presented in Fig. 8 for comparing the corrosion properties among bare zinc powder and the powder modified for the same ultrasonic time of 10 min with variable irradiation power, and the corresponding results are listed in Table 3. Compared with bare zinc powder, the Ecorr values of several zinc electrodes composed of modified zinc powder with different ultrasonic powers have a slightly negative shift, whereas the icorr values have exhibited a remarkable decrease. This illustrates that the variety of neodymium conversion films obtained with different ultrasonic powers are obviously helpful in providing corrosion protection and slowing down the corrosion rate of zinc electrodes. Moreover, large variations of icorr value and P-value are observed among several samples of zinc powder modified with different ultrasonic powers. It suggests that ultrasonic power plays a momentous role in the available properties of the formed neodymium conversion layers.
-600
2.5
Fig. 7. Cyclic voltammograms of zinc electrodes at the 20th cycle for bare zinc powder and the powder modified with different ultrasonic times. Scan rate: 10 mV s1. C: cathodic peak; A: anodic peak.
Table 2 Parameters derived from the 20th cycle behavior by cyclic voltammetry for zinc electrodes using bare zinc powder or the powder modified with various ultrasonic times Ultrasonic time (min)
EA (73 mV)
EC (72 mV)
iA (72 mA)
iC (71 mA)
DEP
Bare 5 10 15 20
956 990 1053 1038 1021
1784 1658 1612 1647 1678
235 151 145 167 154
282 154 131 161 164
828 668 559 609 657
1
2.0 log (i, mA cm-2)
50
3
4
2
1.5 1.0 6 5
1 2 3 4 5 6
0.5 0.0
(mV)
-0.5 -1.7
-1.6
-1.5 -1.4 -1.3 -1.2 Potential (V vs.HgO/Hg)
Bare 220 W 330 W 440 W 550 W 660 W
-1.1
-1.0
Fig. 8. Corrosion measurements of zinc electrodes at a scan rate of 0.5 mV s1 for bare or modified zinc powder using different ultrasonic powers.
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Table 3 Data elicited from potentiodynamic polarization curves of zinc electrodes with bare or modified zinc powder at different ultrasonic powers Ultrasonic power (W)
Ecorr (70.003 V)
icorr (70.52 mA cm2)
P (%)
Bare 220 330 440 550 660
1.353 1.365 1.362 1.363 1.365 1.362
67.5 34.67 20.97 15.86 6.167 10.62
– 48.6 68.9 76.5 90.9 84.3
51
Table 4 Numerical values obtained from the cycle behavior of zinc electrodes containing bare zinc powder or the powder modified with variable ultrasonic power Ultrasonic power (W)
EA (72 mV) EC (73 mV) iA (73 mA) iC (73 mA)
Bare 220 330 440 550 660
956 1044 1047 1069 1053 1057
1784 1694 1677 1656 1612 1612
235 201 219 157 145 153
282 251 250 180 131 138
DEP (mV)
828 650 630 587 559 555
A
Current (mA)
200 100 0
5
4
6
1 2 3 4 5 6
-100 3 2
-200 1 C
Bare 220 W 330 W 440 W 550 W 660 W
-300 -1800
-1600
-1400
-1200
-1000
-800
-600
Potential (mV vs.Hg/HgO) Fig. 9. The 20th cyclic voltammograms of zinc electrodes in 6 M KOH saturated with ZnO solution at a sweep rate of 10 mV s1 for bare zinc powder and the powder modified with different ultrasonic powers. C: cathodic peak; A: anodic peak.
In the case of the zinc powder modified at an ultrasonic power of 550 W, the icorr value of zinc electrode that is only 6.167 mA cm2 has been exhibited to be the least one and the P-value that attains 90.9% is the largest one in comparison with other power parameters. That is to say, the neodymium conversion films generated with an ultrasonic power of 550 W have been proved to be particularly effective in inhibiting the corrosion of zinc electrode. Nevertheless, when the ultrasonic power increases to 660 W, the P-value decreases. It may be the reason that the incremental agitation intensity decreases the combining force between the modified zinc powder and the neodymium conversion layers. Therefore, it is clear from this figure and table that an ultrasonic power of 550 W is a superior parameter for the favorable behavior of neodymium conversion coatings in the corrosion protection of zinc electrode once it is compared with other power parameters. Fig. 9 and Table 4 give some contrasts of the 20th cyclic voltammograms among bare zinc powder and the powder modified with different ultrasonic powers for the same treatment time of 10 min. Regarding bare zinc powder, the DEP value of zinc electrode at the 20th cycle is by far greater, as well as the iA and iC values are also much larger compared with all samples of zinc powder modified using various power parameters. Furthermore, the zinc electrode made up of bare zinc powder becomes bulgy after the 20th cycle is completed, which is potentially explained in terms of the dendrite growth of zinc. On the basis of these data and analyses, it is found that bare zinc powder cannot be well used as the electrode material due to inferior reversibility and stability. Among these samples of zinc powder modified by adjusting the ultrasonic power, the iA, iC and DEP values gradually decrease
as the ultrasonic power increases up to 550 W. Then, the iA and iC values slightly increase at an output power of 660 W in contrast to an ultrasonic power of 550 W. Therefore, the minimum values in the iA, iC and a smaller DEP value correlate to an ultrasonic power of 550 W, implying that the optimal power during the modification is detected to be 550 W for important impacts of neodymium conversion films on the cycle performance of zinc electrode in a beneficial way. Moreover, the surface of the zinc electrode containing the zinc powder modified with an ultrasonic power of 550 W is smooth and even after cycle scan terminates. Indicates that the neodymium conversion films containing Nd-rich particles could effectively impede the dendrite growth of zinc. Dendrite formation can also be confirmed by the SEM images of zinc electrodes after the 20th cycle tests as exhibited in Fig. 10. For zinc electrode containing bare zinc powder, it can be clearly seen that some dendrite sediments are formed on the zinc electrode after the cycles, which leads to low stability of the electrode during the cycles. Nevertheless, no dendrite formations are found on the zinc electrode using modified zinc powder at an ultrasonic power of 550 W, indicating great dependence of outstanding cycle stability on the generated neodymium conversion layers. Combined with the above characterization analysis, it can be concluded that the surface morphology of modified zinc powder and the distribution state of neodymium conversion coatings are responsible for the performance enhancement of zinc electrodes.
3.4. Comparison between the two modified methods As a part of our investigation, Fig. 11 compares the polarization behavior and the corrosion inhibition capability of zinc electrodes with the zinc powder modified, respectively, by means of simple impregnation and ultrasonic impregnation at 550 W for the same treatment time of 10 min. As expected, corrosion current of zinc electrode with the powder modified by simple impregnation, which is obtained from the relevant Tafel lines, is by far larger than that by ultrasonic impregnation. Thus an obvious evidence for the availability of ultrasound is also given with the corresponding protective efficiency gained from the equation P ¼ ð1 icorr =i0corr Þ 100% in Fig. 11. In the case of the ultrasonic impregnation process, the protective efficiency of zinc electrode achieves a very high value, that is 90.9%, showing that the application of ultrasound can crucially induce chemical modification of zinc powder and thus importantly enhance the protection property of zinc electrode against corrosion. All these facts are also benefited from many advantages of ultrasound, including the sharp activation of oxygen reduction reaction, the speedy mass transport and the high intensity of cavitation attack during the modification process. The results indicate that the modification methods have an essential role in the nature and properties of neodymium conversion layers
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Fig. 10. SEM micrographs of zinc electrodes after the 20th cycle measurements for giving a contrast of (a) bare zinc powder and (b) the zinc powder modified by ultrasonic impregnation at 550 W.
Fig. 11. Effect of modification methods on the property of corrosion resistance of zinc electrodes in 6 M KOH saturated with ZnO solution.
generated on zinc powder, and the much greater improvement over the electrochemical properties of zinc electrodes is accomplished by modifying the electrode material using the ultrasonic impregnation method. Also, cyclic voltammograms of zinc electrodes composed of the zinc powder modified by simple impregnation or by ultrasonic impregnation at 550 W are compared for the same treatment time of 10 min as shown in Fig. 12, and Table 5 gives the cycle parameters obtained from the contrastive cycle curves. Compared with the simple impregnation process at the same scan number, the DEP value of zinc electrode containing the zinc powder modified by ultrasonic impregnation decreases, suggesting a marked enhancement in the reaction reversibility of the electrode. Moreover, the zinc electrode with the powder modified by ultrasonic impregnation has smaller iA and iC values. It is therefore inferred that Nd-rich particles presenting in the neodymium conversion films are mainly seeded on the defect sites of zinc powder by the ultrasonic impregnation process and thus they can more effectively improve the cycle stability of zinc electrode. On repetitive scanning, the iA and iC values for the simple impregnation process speedily increase, especially the iC value is remarkably changed to be larger than the iA one at the 20th cycle.
As far as the zinc powder modified by ultrasonic impregnation is concerned, the shape of cycle curves keeps almost invariable, and the iA and iC values rise within a narrower range as the scan number increases. These facts would be favorable to uniformly charge and discharge the electrodes, to prevent the dendrite growth of zinc and thus to increase the cycle stability of the electrodes. In addition, previous studies [12,27,28] have suggested that the area of cathodic peak corresponds to the charge due to the reduction of Zn(OH)2/ZnO, which are undissolved in the electrolyte. That is to say, the area of cathodic peak is attributed to the amount of Zn(OH)2/ZnO retained in the zinc electrode. This provides a justification for high cycle performance and low capacity loss of zinc electrode. The greater area of cathodic peak for the zinc electrode containing the zinc powder modified by ultrasonic impregnation is noticeable. This is a clear indication that the neodymium conversion coatings formed by ultrasonic impregnation significantly impede the dissolution of the zinc oxidation products in the electrolyte, and thus favor the capacity maintenance of zinc electrodes. All these results demonstrate that the enhanced cycle performance of zinc electrode is also relative to the modification methods, and the zinc electrode using the zinc powder coated with neodymium conversion coatings by ultrasonic impregnation is provided with better reversibility and higher stability. Simultaneously, the ultrasonic impregnation process is also confirmed to be an effective method for the beneficial modification of zinc powder and the available formation of neodymium conversion layers, which is ascribed to the agitation and cavitation of the imposed ultrasound.
4. Conclusions The present paper is devoted to the direct precipitation of neodymium conversion films on zinc powder by means of simple impregnation or ultrasonic impregnation for extensive application in alkaline zinc electrode systems. Characterization techniques confirm that neodymium conversion layers containing Nd-rich particles emerge on the surface of modified zinc powder, as well as the distribution state of neodymium conversion coatings depends on different methods and variable modification parameters. Simultaneously, it is clarified that the availability of neodymium conversion layers in providing corrosion protection
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200
53
200
A
A
100 Current (mA)
Current (mA)
100
0
1st
-100
10th
1st 10th 20th
0
20th
-100
20th
-200
1st 10th 20th
1st 10th
C
C
-1800 -1600 -1400 -1200 -1000 -800 Potential (mV vs.Hg/HgO)
-600
-1800 -1600 -1400 -1200 -1000 -800 Potential (mV vs.Hg/HgO)
-600
Fig. 12. Influence of modification techniques on the 1st, 10th and 20th cycle voltammograms of zinc electrodes in 6 M KOH saturated with ZnO solution. (a) The zinc powder modified by the simple impregnation for 10 min; (b) the zinc powder modified by ultrasonic impregnation at ultrasonic power of 550 W for 10 min. C: cathodic peak; A: anodic peak.
Table 5 Comparison of cycle parameters of zinc electrodes in 6 M KOH saturated with ZnO solution between the modification methods Modified process
Cycle number
Simple impregnation
1 10 20
Ultrasonic impregnation
1 10 20
EA (74 mV)
EC (77 mV)
iA (73 mA)
iC (74 mA)
DEP (mV)
999 992 950
1548 1639 1715
142 148 175
101 147 199
549 647 765
1044 1053 1053
1571 1617 1612
124 128 145
95.8 128 131
527 564 559
and enhancing cycle performance of zinc electrodes is also affected by different methods and process parameters. Moreover, great improvements over corrosion protection and cycle performance of zinc electrodes are achieved from the uniform distribution of neodymium conversion layers formed by using ultrasonic impregnation at an operating power of 550 W for a treatment time of 10 min. In the non-conventional modification process, the protective efficiency reaches 90.9% and the difference between the anode and cathodic peak potentials only reaches 559 mV at the 20th cycle, indicating the remarkably improved electrochemical performance of zinc electrodes due possibly to the generation of Nd-rich particles growing in neodymium conversion films on some defects sites of zinc powder.
Acknowledgements Funding of this research by grant from the Innovation Foundation of BUAA for Ph.D. graduates is gratefully acknowledged. References [1] Dwaine Coates, Elio Ferreira, Allen Charkey, An improved nickel/zinc battery for ventricular assist systems, J. Power Sources 65 (1997) 109–115. [2] M.-L. Doche, J.-Y. Hihn, F. Touyeras, J.P. Lorimer, T.J. Mason, M. Plattes, Electrochemical behavior of zinc in 20 kHz sonicated NaOH electrolytes, Ultrason. Sonochem. 8 (2001) 291–298. [3] X. Gregory Zhang, Fibrous zinc anodes for high power batteries, J. Power Sources 163 (2006) 591–597.
[4] G.Q. Zhang, X.G. Zhang, A novel alkaline Zn/MnO2 cell with alkaline solid polymer electrolyte, Solid State Ionics 160 (2003) 155–159. [5] Y.F. Yuan, J.P. Tu, H.M. Wu, Y.Z. Yang, D.Q. Shi, X.B. Zhao, Electrochemical performance and morphology evolution of nanosized ZnO as anode material of Ni–Zn batteries, Electrochim. Acta 51 (2006) 3632–3636. [6] G. Juhel, B. Beden, C. Lamy, J.M. Leger, R. Vignaud, Effect of the surfactant ‘‘forafac’’ on hydrogen evolution on a zinc electrode, Electrochim. Acta 35 (1990) 479–481. [7] M. Yano, S. Fujitani, K. Nishio, Y. Akai, M. Kurimura, Effect of additives in zinc alloy powder on suppressing hydrogen evolution, J. Power Sources 74 (1998) 129–134. [8] Amar Prasad Yadav, Atsushi Nishikata, Tooru Tsuru, Oxygen reduction mechanism on corroded zinc, J. Electroanal. Chem. 585 (2005) 142–149. [9] C. Cachet, Z. Chami, R. Wiart, Electrode kinetics connected to deposit growth for zinc electrodeposition: influence of surfactants, Electrochim. Acta 32 (1987) 465–474. [10] J.M. Wang, L. Zhang, J.Q. Zhang, Effects of bismuth ion and tetrabutylammonium bromide on the dendritic growth of zinc in alkaline zincate solutions, J. Power Sources 102 (2001) 139–143. [11] Chang Woo Lee, K. Sathiyanarayanan, Seung Wook Eom, Mun Soo Yun, New alloys to improve the electrochemical behavior of zinc anodes of zinc/air battery, J. Power Sources 160 (2006) 1436–1441. [12] Ji-ling Zhu, Yun-hong Zhou, Hanxi Yang, Effect of lanthanum and neodymium hydroxides on secondary alkaline zinc electrode, J. Power Sources 69 (1997) 169–173. [13] J. Vatsalarani, S. Geetha, D.C. Trivedi, P.C. Warrier, Stabilization of zinc electrodes with a conducing polymer, J. Power Sources 158 (2006) 1484–1489. [14] Ali Eftekhari, Electrochemical behavior and electrocatalytic activity of a zinc hexacyanoferrate film directly modified electrode, J. Electroanal. Chem. 537 (2002) 59–66. [15] M.G.S. Ferreira, R.G. Duarte, M.F. Montemor, A.M.P. Simo˜es, Silanes and rare earth salts as chromate replacers for pre-treatments on galvanized steel, Electrochim. Acta 49 (2004) 2927–2935. [16] M.F. Montemor, A.M. Simo˜es, M.G.S. Ferreira, Composition and behavior of cerium films on galvanized steel, Prog. Org. Coat. 43 (2001) 274–281. [17] B.R.W. Hinton, R.P.M. Procter, The effect of stain-rate and cathodic potential on the tensile ductility of X-65 pipeline steel, Corros. Sci. 23 (1983) 101–123.
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[18] B.R.W. Hinton, L. Wilson, The corrosion inhibition of zinc with cerous chloride, Corros. Sci. 29 (1989) 967–975. [19] B.R.W. Hinton, Corrosion inhibition with rare earth metal salts, J. Alloys Compd. 180 (1992) 15–25. [20] Sandra M. Martins-Franchetti, Roberto N. Domingos, Rejane C. Trombini, Modifications on PVC films by ultrasound, Ultrason. Sonochem. 9 (2002) 139–141. [21] R. Walker, Ultrasound improves electrolytic recovery of metals, Ultrason. Sonochem. 4 (1997) 39–43. [22] M.-L. Doche, J.-Y. Hihn, A. Mandroyan, R. Viennet, F. Touyeras, Influence of ultrasound power and frequency upon corrosion kinetics of zinc in saline media, Ultrason. Sonochem. 10 (2003) 357–362. [23] O. Lunder, J.C. Walmsley, P. Mack, K. Nisancioglu, Formation and characterisation of a chromate conversion coating on AA6060 aluminium, Corros. Sci. 47 (2005) 1604–1624.
[24] Kunitsugu Aramaki, A self-healing protective film prepared on zinc by treatment in a Ce(NO3)3 solution and modification with Ce(NO3)3, Corros. Sci. 47 (2005) 1285–1298. [25] R. Shivkumar, G. Paruthimal Kalaignan, T. Vasudevan, J. Power Sources 75 (1998) 90–100. [26] R. Shivkumar, G. Paruthimal Kalaignan, T. Vasudevan, Effect of additives on zinc electrodes in alkaline battery systems, J. Power Sources 55 (1995) 53–62. [27] M. Ghaemi, R. Amrollahi, F. Ataherian, M.Z. Kassaee, New advances on bipolar rechargeable alkaline manganese dioxide-zinc batteries, J. Power Sources 117 (2003) 233–241. [28] Robab Khayat Ghavami, Zahra Rafiei, Performance improvements of alkaline batteries by studying the effects of different kinds of surfactant and different derivatives of benzene on the electrochemical properties of electrolytic zinc, J. Power Sources 162 (2006) 893–899.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
New modification procedure of zinc powder in neodymium nitrate solution for improving the electrochemical properties of alkaline zinc electrodes Liqun Zhu, Hui Zhang , Weiping Li, Huicong Liu Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e in fo
abstract
Article history: Received 3 February 2008 Received in revised form 21 August 2008 Accepted 4 September 2008
Neodymium conversion films are directly deposited on the surface of zinc powder by means of ultrasonic impregnation to prepare the modified electrode material in order to obtain high-performance zinc electrodes applied in alkaline medium. Scanning electron microscopy, X-ray diffraction and other characterization techniques are used to analyze the formation and distribution of neodymium conversion coatings imposing different methods and process parameters. Simultaneously, the electrochemical properties of corresponding zinc electrodes are also studied through potentiodynamic polarization and cyclic voltammetry. Results demonstrate that the distribution features of neodymium conversion layers are changed by adjusting ultrasonic time and irradiation power, which contributes to different enhancement extents for the electrochemical performance of zinc electrodes. Especially, the neodymium conversion coatings generated by ultrasonic impregnation at an ultrasonic power of 550 W for an irradiation time of 10 min play a very efficient role in obtaining fine corrosion resistance and persistent cycle behavior of zinc electrode. Besides, ultrasonic impregnation is verified to have a great advantage, as compared with simple impregnation, because the neodymium conversion layers formed under the action of ultrasonic agitation and cavitation phenomenon can obviously improve the electrochemical performance of zinc electrodes. & 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds A. Surfaces C. Electron microscopy C. X-ray diffraction D. Electrochemical properties
1. Introduction Zinc powder, as an active material of zinc electrode, has many merits such as low cost, electrochemical reversibility, high specific energy and low equilibrium potential [1–3], which make it suitable for wide applications in alkaline battery systems [4,5]. However, the electrochemical performance of zinc electrode is usually limited by corrosion [6–8] and dendrite growth [9–11] in alkaline electrolyte, and these issues lead to high capacity loss and short cycle life of the zinc electrode. In the last few years, many attempts have been made to develop modified zinc electrodes for reducing the problems. For example, Zhu et al. [12] evaluated that lanthanum and neodymium hydroxides coated on zinc electrodes through electrolysis could suppress dendrite growth of zinc. Vastalarani and co-workers [13] reported that the reversibility and corrosion protection of zinc electrode in alkaline medium were improved by electrostatic deposition of a conducting polymer onto the electrode. Nevertheless, not much attention has been paid to the modification of zinc powder itself and to the influence of modified zinc powder on the properties of corresponding zinc electrode.
Corresponding author. Tel.: +86 1082317113; fax: +86 1082317133.
E-mail address: [email protected] (H. Zhang). 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.09.005
In addition, it has been found that many metals immersed in rare earth salt by a simple impregnation process [14] can increase corrosion protection [15,16]. Hinton et al. [17,18], who made the first effort on the field, suggested that many local chemical cells emerged on the heterogeneous surface of the immersed metal due to the existence of grain boundaries, exposed inclusion, intercrystalline compound, native oxide layer and other sub-microscopic defects. Thus, the electrochemical reactions of immersed metal dissolution and oxygen reduction were proposed to preferentially occur at those defect sites. They also confirmed that the reactions of oxygen reduction generated an alkaline environment, which resulted in the precipitation of rare earth conversion films on the surface of the immersed metal [19]. In the present paper, ultrasonic impregnation technique has been utilized for the modification of zinc powder to form neodymium conversion layers onto it, taking into account many merits of ultrasound that are associated with ultrasonic agitation and cavitation phenomenon [20,21]. Among them, cavitation phenomenon has been proved to be the predominant effect as reported elsewhere [22]. Apart from the remarkable enhancement of mass transfer rate and intense activation of electrochemical reactions, the cavitation phenomenon has a significant effect on the dissolution of oxygen, which contributes to a good alkaline solution in the liquid–solid (Nd(NO3)3–Zn) system. In this respect, we aim to investigate the merits of ultrasonic impregnation on the
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distribution and effect of neodymium conversion coatings in comparison with the simple impregnation process, as well as the influence of neodymium conversion films generated with different ultrasonic time and power on the electrochemical behavior of zinc electrodes consisting of such modified zinc powder.
2. Experimental All chemicals were of analytical grade, and doubly distilled water was used for preparing all solutions. The modification of zinc powder was carried out at room temperature by immersing bare powder into 0.045 mol L1 Nd(NO3)3 solution that was placed in an ultrasonic apparatus with a constant ultrasonic frequency of 40 kHz. The neodymium conversion layers were respectively formed on zinc powder with different operating times (5, 10, 15 and 20 min) or various output powers (220, 330, 440, 550, 660 W) of ultrasonic irradiation. Simultaneously, we expounded the modified case of the simple impregnation process without the application of ultrasound for comparison only. The morphology of bare or modified zinc powder was observed using a JEOL JSM-5800 scanning electron microscopy (SEM). The point composition on the surface of modified zinc powder was examined through energy dispersion spectrometry (EDS) with a GENESIS 60S instrument. Auger electron spectroscopy (AES) was employed to further analyze the surface composition of modified zinc powder on a PHI 700 instrument. The AES spectra were acquired at a primary electron beam energy of 5 keV and a primary electron beam current of 10 nA. X-ray diffraction (XRD) measurements were performed on a Bruker’s D8 diffractometer with Cu Ka radiation (l ¼ 1.5406 A˚). y/2y diffractograms were obtained in the scan range of 20–1001 with a step of 0.031. Later, zinc electrode that had an apparent area of 1 cm2 was prepared from a mixture of 1.0 g bare or modified zinc powder with a few drops of ethanol, electrolyte and PTFE. Here nickel foam was used as the current collector to incorporate the above mixture and shape a porous electrode. All electrochemical measurements were performed with a CHI 660a type of electrochemical system connected to a microcomputer running dedicated software. The electrochemical experiments were conducted by means of a typical three-electrode cell where a Ni plate was the counter electrode, Hg/HgO was used as the reference electrode and a zinc electrode acted as the working electrode. The potentials were measured versus a Hg/HgO reference electrode, and the
currents were obtained to be equivalent to the current densities owing to the 1 cm2 apparent area of zinc electrodes. The electrolyte used throughout this work was 6 M KOH solution saturated with ZnO. Then the zinc electrodes were polarized from 1.7 to 1.0 V at a constant scan rate of 0.5 mV s1 by the potentiodynamic polarization technique. The voltammetric characteristics of the zinc electrodes with bare or modified zinc powder were also investigated in the potential range of 0.6 to 1.8 V and the scan rate was always 10 mV s1. Moreover, all electrochemical measurements were repeated three times to ensure that the data were reproducible. The electrochemical curves that had medium current values for each sample of modified zinc powder were, respectively, selected from the three reproducible data to reveal in the figures.
3. Result and discussion 3.1. Characteristic analysis The surface micrographs of bare zinc powder and the powder modified in 0.045 mol L1 Nd(NO3)3 solution by simple impregnation for a treatment time of 10 min are exhibited in Fig. 1. Based on the clean and uneven surface of bare zinc powder, simple impregnation process leads to the formation of a small quantity and non-uniform distribution of white deposits on the surface of the modified zinc powder. The composition of white deposits can be obtained by the EDS method, and the result certifies the presence of Nd and O elements in the modified zinc powder. This fact implies that Nd2O3 and/or Nd(OH)3 are generated in the white deposits growing on the surface of modified zinc powder. However, no Nd signal can be observed at the dark regions on the modified zinc powder from EDS spectra. It appears that Nd-rich particles are distributed on the modified zinc powder in the form of white deposits. In order to accurately confirm the surface composition of modified zinc powder, Fig. 2 illustrates the AES spectra for two typical dark regions on the zinc powder modified by simple impregnation. The results indicate that the dark regions on the modified zinc powder also contain Nd and O elements without exception. It is therefore reasonable to conclude that thin neodymium conversion layers cover the surface of modified zinc powder, and the neodymium conversion coatings consist of basal
Fig. 1. SEM photographs obtained from (a) bare zinc powder, and (b) the zinc powder modified by the simple impregnation for a treatment time of 10 min.
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Intensity (a.u.)
membrane and Nd-rich particles. Simultaneously, AES measurements would generally give the C peak due to the adventitious surface cabon [23]. As can also be noted from the above SEM images, it is necessary to find a better solution, that is the
Nd
Nd C
Zn
Zn
Zn
Zn O
200
400 600 800 Kinetic energy (eV)
1000
Fig. 2. AES spectra acquired from two typical dark regions on the surface of the zinc powder modified by means of the simple impregnation.
47
ultrasonic impregnation method, to promote the formation and improve the distribution state of neodymium conversion films containing Nd-rich particles. Fig. 3 presents the SEM images of the zinc powder modified by ultrasonic impregnation at an ultrasonic power of 550 W with different treatment times. It can be clearly seen that the effect of ultrasonic time on the formation and distribution of Nd-rich particles growing in neodymium conversion coatings is remarkable. As expected, Nd-rich particles on the surface of modified zinc powder multiply and the particulate size of Nd-rich particles increases as ultrasonic time expends. It seems that Nd-rich particles gradually grow up through the extension of irradiation time. Moreover, the accumulation phenomenon of Nd-rich particles appears on the surface of modified zinc powder for an ultrasonic time of 15 or 20 min. All these suggest that an ultrasonic time of 10 min provides the surface of modified zinc powder with uniform distribution of neodymium conversion layers containing a small size of Nd-rich particles. Differences are observed regarding the morphology of modified zinc powder for a treatment time of 10 min with variable ultrasonic power as revealed in Fig. 4(a–e). With ultrasonic power of 220 and 330 W, Nd-rich particles are asymmetrically distributed on the surface of zinc powder. And it is obvious that the modification of zinc powder with an ultrasonic power of 440 or 550 W leads to the direct formation and uniform distribution of
Fig. 3. SEM images of the zinc powder modified by the ultrasonic impregnation with different ultrasonic times: ultrasonic impregnation for (a) 5 min; (b) 10 min; (c) 15 min; (d) 20 min.
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Fig. 4. Microgram changes of the zinc powder modified by the ultrasonic impregnation at different ultrasonic powers: ultrasonic impregnation at (a) 220 W; (b) 330 W; (c) 440 W; (d) 550 W; (e) 660 W.
considerable Nd-rich particles owing to the favorable roles of high-intensity ultrasound. Especially, an ultrasonic power of 550 W creates the most uniform distribution of neodymium conversion films with a small size and great amounts of Nd-rich particles growing on the modified zinc powder. However, the quantity and size of Nd-rich particles on the zinc powder modified
with an ultrasonic power of 660 W decrease possibly due to the drastic agitation of ultrasound. In fact, the formation of neodymium conversion coatings during sonochemical modification can also be confirmed through the pH value variation of the Nd(NO3)3 solution before and after bare zinc powder is modified at an ultrasonic power of 550 W for
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Nd
3þ
2Nd
þ 3OH ! NdðOHÞ3 #
3þ
(1)
þ 6OH ! Nd2 O3 3H2 O #
(2)
3000
2.5 1
2.0
2
1.5 1.0 3
4
5
0.5 1 2 3 4 5
0.0
-1.6
-1.5 -1.4 -1.3 -1.2 Potential (V vs.HgO/Hg)
Bare 5 min 10 min 15 min 20 min
-1.1
-1.0
Fig. 6. Potentiodynamic polarization curves of zinc electrodes in 6 M KOH saturated with ZnO solution for bare zinc powder or the powder modified with different ultrasonic times. Scan rate: 0.5 mV s1.
2500 2000
Fig. 6 shows the potentiodynamic polarization curves of zinc electrodes for bare zinc powder and the powder modified with different ultrasonic time at the same irradiation power of 550 W. The corrosion potential (Ecorr) and the corrosion current density (icorr) of zinc electrodes are also obtained from the extrapolation of anodic and cathodic Tafel lines as listed in Table 1. Simultaneously, the protective efficiency (P) in Table 1 is determined as P ¼ ð1 icorr =i0corr Þ 100% [24], where i0corr and icorr denote the corrosion current density of zinc electrodes with bare and modified zinc powder, respectively. With the zinc powder modified using different ultrasonic times, the Ecorr values of several zinc electrodes have slightly moved towards a negative direction in comparison with bare zinc powder. However, there is significant reduction in the icorr values of zinc electrodes, indicating that all kinds of neodymium conversion layers formed with different ultrasonic time enhance the property of the corrosion resistance of zinc electrodes in alkaline electrolyte. The improved property of zinc electrode is related to the time parameter of ultrasonic irradiation. With the spread of ultrasonic time, the icorr value initially decreases and then increases, and correspondingly the maximum P value is detected when the zinc powder is modified for an ultrasonic time of 10 min. This is indicative of the best corrosion protection of zinc electrode containing the zinc powder modified with an ultrasonic time of 10 min as compared with other times, which is possibly explained by the uniform distribution of neodymium conversion layers
-0.5 -1.7
Zn, (112) Zn, (200) Zn, (201)
Intensity (a. u.)
3500
Zn, (004)
4000
3.2. Effect of ultrasonic time
Bare zinc powder without any treatment Zn, (103) Zn, (110)
4500
Zn, (102)
Zn, (002) Zn, (100) Zn, (101)
Fig. 5 shows the X-ray diffraction patterns of bare zinc powder and the powder modified in 0.045 mol L1 Nd(NO3)3 solution by ultrasonic impregnation at an ultrasonic power of 550 W for an irradiation time of 10 min. In addition, the XRD diffractogram for bare zinc powder treated in distilled water by ultrasound with the same ultrasonic condition is also measured for comparison only. All the peaks in the patterns of three kinds of zinc powder correspond to the hexagonal phase of zinc, and no extra lines other than the hexagonal phase of zinc are observed. Therefore, it is noted that the content of Nd(OH)3 and/or Nd2O3 in modified zinc powder is too small to be determined by XRD. However, it is interesting to notice from the XRD pattern of the zinc powder modified by ultrasonic impregnation that the formed neodymium conversion layers have a great impact on the lattice parameters of zinc. The lattice parameters of bare zinc are a ¼ 2.6612 A˚ and c ¼ 4.9430 A˚ and those of bare zinc treated only by ultrasound are a ¼ 2.6617 A˚ and c ¼ 4.9390 A˚, whereas those of modified zinc coating with neodymium conversion films are a ¼ 2.6584 A˚ and c ¼ 4.9312 A˚. The obvious decrease in the lattice parameters from the zinc powder modified by ultrasonic impregnation is possibly attributed to the presence of Nd-rich particles growing in the neodymium conversion coatings on some defect sites of zinc powder. Defect sites on zinc powder are responsible for the corrosion and dendrite formation of zinc, and the neodymium
conversion layers formed on zinc powder can therefore likely improve the electrochemical performance of zinc electrodes.
log (i, mA cm-2)
an ultrasonic time of 10 min. The pH value of the original solution is found to be 5.1 by the exact pH paper, whereas the value of the immersion solution ascends to 8.5 after the zinc powder is treated by ultrasonic impregnation. The fact denotes that the ultrasonic impregnation process provides a good alkaline environment as a result of the strongly activated oxygen reduction reactions. And the OH can be rapidly generated and supplied into liquid medium owing to the continual dissolution of oxygen promoted by cavitation bubbles’ growth and collapse. Moreover, the Nd3+ concentration in the remaining solution after modification is determined by means of spectrophotometry, which decreases to 0.019 mol L1 from the original 0.045 mol L1. All these results contribute to the formation of neodymium conversion films on the modified zinc powder. As a result, the general reactions for the deposition of neodymium conversion layers are likely to be written as
49
Modified zinc powder with ultrasonic impregnation
1500
Table 1 Corrosion studies of zinc electrodes for bare zinc powder and the powder modified with various ultrasonic times
1000 Bare zinc powder in water with ultrasound
500 0 20
40
60
80
100
2-Theta (degree) Fig. 5. X-ray diffractograms of bare zinc powder and the powder modified in 0.045 mol L1 Nd(NO3)3 solution by the ultrasonic impregnation at an ultrasonic power of 550 W for a treatment time of 10 min. The XRD pattern of bare zinc powder treated in distilled water by ultrasound is also tested for comparison only.
Ultrasonic time (min)
Ecorr (70.002 V)
icorr (70.381 mA cm2)
P (%)
Bare 5 10 15 20
1.353 1.361 1.365 1.360 1.360
67.5 8.678 6.167 8.082 9.816
– 87.1 90.9 88.0 85.4
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containing a small size of Nd-rich particles. In the case of an ultrasonic time longer than 10 min, the slightly decreased protection against corrosion of zinc electrode is possibly ascribed to the fact that the accumulation phenomenon of Nd-rich particles occurs on the surface of modified zinc powder. Hence, it is found that the neodymium conversion films formed for an ultrasonic time of 10 min are most effective in enhancing the corrosion inhibition capability of zinc electrodes. The 20th cycle curves of zinc electrodes containing bare zinc powder or the powder modified at an output power of 550 W with different time parameters are shown in Fig. 7. During the potentiodynamic scan starting from 600 mV, an irreversible anodic current peak appears around 1000 mV due possibly to the re-establishment of zinc oxidation products as reported elsewhere [25,26]. Later, a cathodic peak (C) emerges in the potential range of 1612 to 1784 mV during the negative-going scan. This is related to the reduction of the accumulated Zn(OH)2/ ZnO that remain on the electrode surface and are not dissolved in the electrolyte. However, an anodic peak (A) is attributed to the oxidation of Zn into Zn(OH)2/ZnO forming in 956 to 1053 mV during the positive-going scan. The anodic and cathodic peak potentials (EA and EC), and the anodic and cathodic peak currents (iA and iC) obtained from the cycle curves are shown in Table 2. At the same time, the difference between the anode and cathodic peak potentials (DEP ¼ EAEC), which is a measure of reversibility of the electrode reaction, is also listed in Table 2. After the zinc powder is modified with various operating times, the EA values of all zinc electrodes containing modified zinc powder have moved towards a negative direction, while the EC values have exhibited a positive shift. It can be concluded that the DEP values for all samples of modified zinc powder decrease as
A
200
Current (mA)
100 2
0
3
-100 1 2 3 4 5
4
-200
1
5 C
Bare 5 min 10 min 15 min 20 min
-300 -1800
-1600
-1200 -1400 -1000 Potential (mV vs.Hg/HgO)
-800
compared with bare zinc powder. These facts indicate that neodymium conversion layers formed on zinc powder elevate the reaction reversibility of zinc electrodes. In addition, both the iA and iC values for all samples of modified zinc powder obviously diminish, especially the iC values reveal a relatively greater drop. The decrease in the peak currents may be linked to the increased corrosion protection of zinc electrodes as confirmed by the above potentiodynamic polarization measurements. All these imply that the formation of neodymium conversion coatings on zinc powder significantly reduces the cathodic reaction rate, which favors the uniform charge and the excellent cycle stability of zinc electrodes. In particular, the neodymium conversion layers formed with an ultrasonic time of 10 min enhance the reaction reversibility of zinc electrode to the greatest extent as a result of the smallest DEP value that is 559 mV at the 20th cycle in comparison with other time parameters. Moreover, the minimum iA and iC values have a great advantage in the cycle stability of zinc electrodes. All these facts demonstrate that the neodymium conversion coatings generated on zinc powder for an ultrasonic time of 10 min provide the zinc electrode with the best reversibility and the highest stability. Consequently, the results obtained from the above electrochemical measurements manifest that the ultrasonic time of 10 min is considered to be an optimum time parameter for great availability of neodymium conversion layers in improving the electrochemical performance of zinc electrode.
3.3. Influence of ultrasonic power Similarly, the typical potentiodynamic polarization curves of zinc electrodes are presented in Fig. 8 for comparing the corrosion properties among bare zinc powder and the powder modified for the same ultrasonic time of 10 min with variable irradiation power, and the corresponding results are listed in Table 3. Compared with bare zinc powder, the Ecorr values of several zinc electrodes composed of modified zinc powder with different ultrasonic powers have a slightly negative shift, whereas the icorr values have exhibited a remarkable decrease. This illustrates that the variety of neodymium conversion films obtained with different ultrasonic powers are obviously helpful in providing corrosion protection and slowing down the corrosion rate of zinc electrodes. Moreover, large variations of icorr value and P-value are observed among several samples of zinc powder modified with different ultrasonic powers. It suggests that ultrasonic power plays a momentous role in the available properties of the formed neodymium conversion layers.
-600
2.5
Fig. 7. Cyclic voltammograms of zinc electrodes at the 20th cycle for bare zinc powder and the powder modified with different ultrasonic times. Scan rate: 10 mV s1. C: cathodic peak; A: anodic peak.
Table 2 Parameters derived from the 20th cycle behavior by cyclic voltammetry for zinc electrodes using bare zinc powder or the powder modified with various ultrasonic times Ultrasonic time (min)
EA (73 mV)
EC (72 mV)
iA (72 mA)
iC (71 mA)
DEP
Bare 5 10 15 20
956 990 1053 1038 1021
1784 1658 1612 1647 1678
235 151 145 167 154
282 154 131 161 164
828 668 559 609 657
1
2.0 log (i, mA cm-2)
50
3
4
2
1.5 1.0 6 5
1 2 3 4 5 6
0.5 0.0
(mV)
-0.5 -1.7
-1.6
-1.5 -1.4 -1.3 -1.2 Potential (V vs.HgO/Hg)
Bare 220 W 330 W 440 W 550 W 660 W
-1.1
-1.0
Fig. 8. Corrosion measurements of zinc electrodes at a scan rate of 0.5 mV s1 for bare or modified zinc powder using different ultrasonic powers.
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Table 3 Data elicited from potentiodynamic polarization curves of zinc electrodes with bare or modified zinc powder at different ultrasonic powers Ultrasonic power (W)
Ecorr (70.003 V)
icorr (70.52 mA cm2)
P (%)
Bare 220 330 440 550 660
1.353 1.365 1.362 1.363 1.365 1.362
67.5 34.67 20.97 15.86 6.167 10.62
– 48.6 68.9 76.5 90.9 84.3
51
Table 4 Numerical values obtained from the cycle behavior of zinc electrodes containing bare zinc powder or the powder modified with variable ultrasonic power Ultrasonic power (W)
EA (72 mV) EC (73 mV) iA (73 mA) iC (73 mA)
Bare 220 330 440 550 660
956 1044 1047 1069 1053 1057
1784 1694 1677 1656 1612 1612
235 201 219 157 145 153
282 251 250 180 131 138
DEP (mV)
828 650 630 587 559 555
A
Current (mA)
200 100 0
5
4
6
1 2 3 4 5 6
-100 3 2
-200 1 C
Bare 220 W 330 W 440 W 550 W 660 W
-300 -1800
-1600
-1400
-1200
-1000
-800
-600
Potential (mV vs.Hg/HgO) Fig. 9. The 20th cyclic voltammograms of zinc electrodes in 6 M KOH saturated with ZnO solution at a sweep rate of 10 mV s1 for bare zinc powder and the powder modified with different ultrasonic powers. C: cathodic peak; A: anodic peak.
In the case of the zinc powder modified at an ultrasonic power of 550 W, the icorr value of zinc electrode that is only 6.167 mA cm2 has been exhibited to be the least one and the P-value that attains 90.9% is the largest one in comparison with other power parameters. That is to say, the neodymium conversion films generated with an ultrasonic power of 550 W have been proved to be particularly effective in inhibiting the corrosion of zinc electrode. Nevertheless, when the ultrasonic power increases to 660 W, the P-value decreases. It may be the reason that the incremental agitation intensity decreases the combining force between the modified zinc powder and the neodymium conversion layers. Therefore, it is clear from this figure and table that an ultrasonic power of 550 W is a superior parameter for the favorable behavior of neodymium conversion coatings in the corrosion protection of zinc electrode once it is compared with other power parameters. Fig. 9 and Table 4 give some contrasts of the 20th cyclic voltammograms among bare zinc powder and the powder modified with different ultrasonic powers for the same treatment time of 10 min. Regarding bare zinc powder, the DEP value of zinc electrode at the 20th cycle is by far greater, as well as the iA and iC values are also much larger compared with all samples of zinc powder modified using various power parameters. Furthermore, the zinc electrode made up of bare zinc powder becomes bulgy after the 20th cycle is completed, which is potentially explained in terms of the dendrite growth of zinc. On the basis of these data and analyses, it is found that bare zinc powder cannot be well used as the electrode material due to inferior reversibility and stability. Among these samples of zinc powder modified by adjusting the ultrasonic power, the iA, iC and DEP values gradually decrease
as the ultrasonic power increases up to 550 W. Then, the iA and iC values slightly increase at an output power of 660 W in contrast to an ultrasonic power of 550 W. Therefore, the minimum values in the iA, iC and a smaller DEP value correlate to an ultrasonic power of 550 W, implying that the optimal power during the modification is detected to be 550 W for important impacts of neodymium conversion films on the cycle performance of zinc electrode in a beneficial way. Moreover, the surface of the zinc electrode containing the zinc powder modified with an ultrasonic power of 550 W is smooth and even after cycle scan terminates. Indicates that the neodymium conversion films containing Nd-rich particles could effectively impede the dendrite growth of zinc. Dendrite formation can also be confirmed by the SEM images of zinc electrodes after the 20th cycle tests as exhibited in Fig. 10. For zinc electrode containing bare zinc powder, it can be clearly seen that some dendrite sediments are formed on the zinc electrode after the cycles, which leads to low stability of the electrode during the cycles. Nevertheless, no dendrite formations are found on the zinc electrode using modified zinc powder at an ultrasonic power of 550 W, indicating great dependence of outstanding cycle stability on the generated neodymium conversion layers. Combined with the above characterization analysis, it can be concluded that the surface morphology of modified zinc powder and the distribution state of neodymium conversion coatings are responsible for the performance enhancement of zinc electrodes.
3.4. Comparison between the two modified methods As a part of our investigation, Fig. 11 compares the polarization behavior and the corrosion inhibition capability of zinc electrodes with the zinc powder modified, respectively, by means of simple impregnation and ultrasonic impregnation at 550 W for the same treatment time of 10 min. As expected, corrosion current of zinc electrode with the powder modified by simple impregnation, which is obtained from the relevant Tafel lines, is by far larger than that by ultrasonic impregnation. Thus an obvious evidence for the availability of ultrasound is also given with the corresponding protective efficiency gained from the equation P ¼ ð1 icorr =i0corr Þ 100% in Fig. 11. In the case of the ultrasonic impregnation process, the protective efficiency of zinc electrode achieves a very high value, that is 90.9%, showing that the application of ultrasound can crucially induce chemical modification of zinc powder and thus importantly enhance the protection property of zinc electrode against corrosion. All these facts are also benefited from many advantages of ultrasound, including the sharp activation of oxygen reduction reaction, the speedy mass transport and the high intensity of cavitation attack during the modification process. The results indicate that the modification methods have an essential role in the nature and properties of neodymium conversion layers
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Fig. 10. SEM micrographs of zinc electrodes after the 20th cycle measurements for giving a contrast of (a) bare zinc powder and (b) the zinc powder modified by ultrasonic impregnation at 550 W.
Fig. 11. Effect of modification methods on the property of corrosion resistance of zinc electrodes in 6 M KOH saturated with ZnO solution.
generated on zinc powder, and the much greater improvement over the electrochemical properties of zinc electrodes is accomplished by modifying the electrode material using the ultrasonic impregnation method. Also, cyclic voltammograms of zinc electrodes composed of the zinc powder modified by simple impregnation or by ultrasonic impregnation at 550 W are compared for the same treatment time of 10 min as shown in Fig. 12, and Table 5 gives the cycle parameters obtained from the contrastive cycle curves. Compared with the simple impregnation process at the same scan number, the DEP value of zinc electrode containing the zinc powder modified by ultrasonic impregnation decreases, suggesting a marked enhancement in the reaction reversibility of the electrode. Moreover, the zinc electrode with the powder modified by ultrasonic impregnation has smaller iA and iC values. It is therefore inferred that Nd-rich particles presenting in the neodymium conversion films are mainly seeded on the defect sites of zinc powder by the ultrasonic impregnation process and thus they can more effectively improve the cycle stability of zinc electrode. On repetitive scanning, the iA and iC values for the simple impregnation process speedily increase, especially the iC value is remarkably changed to be larger than the iA one at the 20th cycle.
As far as the zinc powder modified by ultrasonic impregnation is concerned, the shape of cycle curves keeps almost invariable, and the iA and iC values rise within a narrower range as the scan number increases. These facts would be favorable to uniformly charge and discharge the electrodes, to prevent the dendrite growth of zinc and thus to increase the cycle stability of the electrodes. In addition, previous studies [12,27,28] have suggested that the area of cathodic peak corresponds to the charge due to the reduction of Zn(OH)2/ZnO, which are undissolved in the electrolyte. That is to say, the area of cathodic peak is attributed to the amount of Zn(OH)2/ZnO retained in the zinc electrode. This provides a justification for high cycle performance and low capacity loss of zinc electrode. The greater area of cathodic peak for the zinc electrode containing the zinc powder modified by ultrasonic impregnation is noticeable. This is a clear indication that the neodymium conversion coatings formed by ultrasonic impregnation significantly impede the dissolution of the zinc oxidation products in the electrolyte, and thus favor the capacity maintenance of zinc electrodes. All these results demonstrate that the enhanced cycle performance of zinc electrode is also relative to the modification methods, and the zinc electrode using the zinc powder coated with neodymium conversion coatings by ultrasonic impregnation is provided with better reversibility and higher stability. Simultaneously, the ultrasonic impregnation process is also confirmed to be an effective method for the beneficial modification of zinc powder and the available formation of neodymium conversion layers, which is ascribed to the agitation and cavitation of the imposed ultrasound.
4. Conclusions The present paper is devoted to the direct precipitation of neodymium conversion films on zinc powder by means of simple impregnation or ultrasonic impregnation for extensive application in alkaline zinc electrode systems. Characterization techniques confirm that neodymium conversion layers containing Nd-rich particles emerge on the surface of modified zinc powder, as well as the distribution state of neodymium conversion coatings depends on different methods and variable modification parameters. Simultaneously, it is clarified that the availability of neodymium conversion layers in providing corrosion protection
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200
53
200
A
A
100 Current (mA)
Current (mA)
100
0
1st
-100
10th
1st 10th 20th
0
20th
-100
20th
-200
1st 10th 20th
1st 10th
C
C
-1800 -1600 -1400 -1200 -1000 -800 Potential (mV vs.Hg/HgO)
-600
-1800 -1600 -1400 -1200 -1000 -800 Potential (mV vs.Hg/HgO)
-600
Fig. 12. Influence of modification techniques on the 1st, 10th and 20th cycle voltammograms of zinc electrodes in 6 M KOH saturated with ZnO solution. (a) The zinc powder modified by the simple impregnation for 10 min; (b) the zinc powder modified by ultrasonic impregnation at ultrasonic power of 550 W for 10 min. C: cathodic peak; A: anodic peak.
Table 5 Comparison of cycle parameters of zinc electrodes in 6 M KOH saturated with ZnO solution between the modification methods Modified process
Cycle number
Simple impregnation
1 10 20
Ultrasonic impregnation
1 10 20
EA (74 mV)
EC (77 mV)
iA (73 mA)
iC (74 mA)
DEP (mV)
999 992 950
1548 1639 1715
142 148 175
101 147 199
549 647 765
1044 1053 1053
1571 1617 1612
124 128 145
95.8 128 131
527 564 559
and enhancing cycle performance of zinc electrodes is also affected by different methods and process parameters. Moreover, great improvements over corrosion protection and cycle performance of zinc electrodes are achieved from the uniform distribution of neodymium conversion layers formed by using ultrasonic impregnation at an operating power of 550 W for a treatment time of 10 min. In the non-conventional modification process, the protective efficiency reaches 90.9% and the difference between the anode and cathodic peak potentials only reaches 559 mV at the 20th cycle, indicating the remarkably improved electrochemical performance of zinc electrodes due possibly to the generation of Nd-rich particles growing in neodymium conversion films on some defects sites of zinc powder.
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