Trans. Nonferrous Met. Soc. China 23(2013) 1382-1389

Influence of nano-CeO₂ on coating structure and properties of electrodeposited Al/α-PbO₂/β-PbO₂

Zhen CHEN¹, Qiang YU¹, Deng-hui LIAO², Zhong-cheng GUO², Jian WU²

1. Faculty of Science, Kunming University of Science and Technology, Kunming 650093, China;

2. Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China

Received 6 April 2012; accepted 10 September 2012

Abstract: Al/α-PbO₂/β-PbO₂ composite electrodes doped with rare earth oxide (CeO₂) were prepared by anodic oxidation method investigate the influence of nano-CeO₂ dopants on the properties of Al/α-PbO₂/β-PbO₂-CeO₂ electrodes and the impact of α-PbO₂ as the intermediate layer. The results show that using α-PbO₂ as the intermediate layer will benefit the crystallization of β-PbO₂ and β-PbO₂ is more suitable as the surface layer than α-PbO₂. CeO₂ dopants change the crystallite size and crystal structure, enhance the catalytic activity, and even change the deposition mechanism of PbO₂. The doping of CeO₂ in the PbO₂ electrodes can enhance the electro-catalytic activity, which is helpful for oxygen evolution, and therefore reduce the cell voltage.

Key words: rare earth; CeO₂; composite electrode material; α-PbO₂; β-PbO₂; cell voltage; inert anode

1 Introduction

Studies on inert anode materials used in zinc-plating industry have mostly focused on conventional anode materials, such as lead-based alloys, dimensionally stable anode, Au, Pt and glassy carbon [1-3]. However, the disadvantages of conventional anode materials including high cell voltage, poisonousness, low mechanical strength, inefficient conductivity and impurity product caused by anode re-dissolve have always perplexed us [4,5]. Therefore, further researches have been undertaken to discover more efficient electrode materials and metal oxide-film electrodes, such as PbO₂, SnO₂ and RuO₂ [6-10].

Lead dioxide (PbO₂) electrode has been extensively used in electrochemical industry and regarded as an excellent metal oxide electrode because of its low price compared with noble metals, good chemical stability, and high catalytic activity for oxygen evolution. The application of PbO₂ electrode is largely dependent upon its structure, morphology, and phase composition. In order to further improve the electrochemical properties of PbO₂ toward various applications, incorporating some ions or particles such as rare earth into the film of lead oxide was investigated.

It is well known that PbO₂ has two different crystallographic forms: orthorhombic and tetragonal (α and β). α-PbO₂ is obtained from alkaline solution and β-PbO₂ from acid solution [11]. α-PbO₂ has a better contact between particles, and a more compact structure than β-PbO₂. Unfortunately, more compact structure leads to bad conductivity compared with β-PbO₂ [12]. Different electro-catalytic activities of α and β forms of PbO₂ were observed in other studies. It was also observed that the structure of crystallization of PbO₂ films influenced the electro-catalytic properties of the material [13-19].

A new type of PbO₂ anode was widely used in electrolysis [20]. This electrode is made up of four layers: a metal base, a conductive layer (protecting the substrate from passivation), α-PbO₂ as the intermediate layer, and β-PbO₂ as surface layer. Aluminum is relatively cheap and has a good conductivity. The electrode material by electrodepositing lead dioxide on Al substrate has huge market prospects. Rare earth oxides, as powerful oxidants, can easily catalyze the oxidation of organics. It was reported that [21-24] metal oxides have been successfully doped into PbO₂ electrodes by anodic codeposition. However, few researches on the doping of rare earth oxide into PbO₂ electrodes have been reported.

In this work, α-PbO₂ was chosen as intermediate layer, and β-PbO₂ as the surface layer. Ce(IV) was introduced as a doping agent into PbO₂ electrode for its superior properties [25] were successfully doped by the anodic codeposition method. It changes the crystallite size and enhances the catalytic activity in the oxidation of the material. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) were used to examine the changes in the coating.

2 Experimental

2.1 Preparation of coatings

After a series of pretreatments: sand blasting→ degreasing (40 g/L Na₃PO₄, 10 g/L Na₄SiO₄, 3 min)→ chemically etching (10 mL/L HF, 50 mL/L HNO₃, 90 s) → coating by a conductive layer, 1060# Al was used as an anode and machined to dimensions of 40 mm×10 mm×1 mm. Pure lead was used as a cathode. The electrodeposition of α-PbO₂ was conducted in an alkaline bath, while that of β-PbO₂ was conducted in an aqueous bath. The plating conditions are listed in Table 1 and Table 2, respectively. Two types of anodes were made: one was Al/conductive layer/β-PbO₂ (No. 1), and the other was Al/conductive layer/α-PbO₂/β-PbO₂ (No. 2). The difference between them was the intermediate layer α-PbO₂. CeO₂ was introduced as a doping agent into the aqueous β-PbO₂ electrolyte. By the anodic codeposition method, CeO₂ was successfully doped into No. 2 electrode.

2.2 Measuring instruments

Micrographs of the coating surface were obtained by scanning electron microscopy (ESEM, FEI, Quanta200). The structures of the films were analyzed by X-ray diffraction (XRD) with Co K_α radiation in a standard X-ray diffract meter (Rigaku D/max-1200X). The element contents were tested by energy-dispersive spectroscopy (EDAX-Phoenix). The over-potential was tested in 1.3 mol/L ZnSO₄+1 mol/L H₂SO₄ (pH=4.5) using polarization curves at a scanning rate of 50 mV/s. (CHI660D, Chenhua, China). During the measurement, a three-electrode system was used. The working electrode was the composite coating (1 cm×1 cm). The reference electrode was saturated calomel electrode (SCE) used directly in contact with the working solution. The counter electrode was platinum (1 cm×1 cm).The measurement was conducted at room temperature.

3 Results and discussion

3.1 Choice of intermediate layer

In this work, α-PbO₂ was obtained at a constant current density of 20 mA/cm² for 30 min in the solution containing 180 g/L NaOH saturated with PbO(s), and was used as the intermediate layer. A typical XRD pattern of α-PbO₂ deposited on Al/conductive layer is shown in Fig. 1. The surface morphology of α-PbO₂ coating is shown in Fig. 2. The XRD pattern indicates the presence of α-PbO₂. The characteristic peaks are α-PbO₂ (scrutinyite), and β-PbO₂ (plattnerite) is not observed. The grain of α-PbO₂ looks like rod or fiber. Numerous crystal edges and a few pores are found in such coating. However, these defects are proved to be helpful for the deposition of β-PbO₂ in further study.

Fig. 1 XRD pattern of α-PbO₂ (scrutinyite)

Table 1 Deposition condition for α-PbO₂ coatings

Table 2 Deposition condition for β-PbO₂ coatings

Fig. 2 SEM image of α-PbO₂ coating (scrutinyite)

Fig. 3 XRD patterns of β-PbO₂ coatings

The XRD patterns of two types of β-PbO₂ coatings (No. 1 and 2) shown in Fig. 3 all accord with the PDF#41—1492, which is plattnerite (β-PbO₂). No. 1 was prepared directly on the Al conductive layer, while No. 2 was prepared on the α-PbO₂ layer. The main crystal plane of No. 1 is (200), while that of No. 2 is (101). The main crystal plane of No. 2 is consistent with the results reported by ABACI et al [26]. Furthermore, No. 2 has more peaks: (002), (312), (202), (222), (312), and they are all consistent with the PDF#41—1492, showing that No. 2 is better crystallized. Apparently, α-PbO₂ as intermediate layer will improve the crystallization of β-PbO₂.

Table 3 Reactant and product of anodic process and  [27,28]

Table 4 Reaction of anodic process and  [27,28]

Fig. 4 CV curves of α-PbO₂and β-PbO₂ coatings

Table 3 shows the  values of several substances, which may participate as a reactant or product in producing PbO₂. Table 4 presents the of each reaction and demonstrates that thermodynamic reaction followed by ease of PbO>PbO₂>Pb₂O₃>Pb₃O₄. Figure 4 shows that the precipitation potential of α-PbO₂ is lower than that of β-PbO₂, which requires small electromotive force (E). So, according to the equation: ΔG=-nEF (ΔG>0), when the product is α-PbO₂, the ΔG is low, the energy barrier is smaller and therefore the thermodynamic stability is poor. Researchers also conducted a study on this issue and came to conclusions that the thermodynamic stability of β-PbO₂ is better than that of α-PbO₂ [29]. Above all, the thermodynamic reaction (precipitation potential) followed by ease of PbO>α-PbO₂>β-PbO₂>Pb₂O₃>Pb₃O₄. In this case, β-PbO₂ coating is more suitable as a surface anode materials.

Figure 5(a) presents the β-PbO₂ prepared directly on the conductive layer (β_D), while Fig. 5(b) presents β-PbO₂ prepared on α-PbO₂ layer (αβ). The morphology of the β_D shown in Fig. 5(a) looks like a nappe sheet, whereas that of αβ looks like a typical pyramid and the average grain size is 25 μm. It is obvious that coatings prepared on the α layer are more compact and uniform. Studies show that [30-32], α-PbO₂ intermediate layer can effectively increase the firmness of the lead dioxide coating and the substrate, and the ease of the distortion of electrodeposition, and make the surface layer β-PbO₂ distribute more uniformly. Therefore, new type of PbO₂ electrode is composed of α-PbO₂ intermediate layer and β-PbO₂ surface layer, so a better catalytic activity is obtained [33].

3.2 CeO₂ as doping agent

The compositional analysis of Al/α-PbO₂/β-PbO₂ electrodeposits was performed under different CeO₂ concentrations in bath using energy dispersive spectroscopy. EDS spectra of β-PbO₂-CeO₂ films deposited under different conditions are shown in Fig. 6. It is found from Fig. 6 that with increasing the CeO₂ concentration in bath from 10 to 50 g/L, Ce contents are 2.97%, 2.90%, 3.47%, 3.44% and 3.73%, respectively.

Fig. 5 SEM images of β-PbO₂ prepared directly on conductive layer (a) and on α-PbO₂ layer (b)

Fig. 6 EDS spectra of β-PbO₂-CeO₂ coatings with different concentrations of CeO₂

Obviously, particles will get more opportunity to be embed into Al/α-PbO₂/β-PbO₂ composite by adding more nucleation points on the anodic surface. More particles suspending in the electrolyte are good for CeO₂ to stay, absorb and form nucleation points. As a result, increasing the CeO₂ concentration will surely help the co-deposition. Rare earth oxides can easily catalyze the oxidation of organics and change the surface morphology.

Fig. 7 SEM images of β-PbO₂ with different concentrations of CeO₂

When ceria dioxide is used, as shown in Fig. 7, the surface morphology presents nodular shape. The average grain sizes decrease with the increase of Ce content. The average grain sizes are shown in Table 5. The present experimental results indicate that the crystallization process of PbO₂ can be affected by adulteration of CeO₂, thus changing surface microstructure of the coating. The average grain size of PbO₂ decreases with the adulteration of CeO₂ because the doped ceria dioxide provides a new center for PbO₂ to nucleate, which hinders the further growth of PbO₂. In other words, the nucleation rate is faster than the growing rate [34].

Table 5 Grain size of coatings

3.3 Over potential and catalytic activity for oxygen evolution

In zinc plating industry, anodic over potential, mainly caused by oxygen evolution reaction, will consume a large percentage of cell voltage. Therefore, a low oxygen evolution potential will save electricity and reduce the cell voltage a lot. Figure 8 gives the polarization curves of Al/α-PbO₂/β-PbO₂-CeO₂ and Al/β-PbO₂ electrodes in 1.3 mol/L ZnSO₄+1 mol/L H₂SO₄ (pH=4.5). Compared with Al/α-PbO₂/β-PbO₂, adulteration of CeO₂ can apparently lower the over-potential of PbO₂. According to the Tafel equation:

where β is the transfer coefficient and J⁰ is the exchange current density, dividing the intercept of linear by its slope, J⁰ can be obtained, and the fitting curves are shown in Fig. 9. The exchange current densities are listed in Table 6, confirming that Al/α-PbO₂/β-PbO₂-CeO₂ has a higher catalytic activity for oxygen evolution than Al/β-PbO₂. Moreover, adulteration of CeO₂ can greatly influence the exchange current. When CeO₂ concentra- tion in bath is 30 g/L, the exchange current of Al/α-PbO₂/β-PbO₂-CeO₂ electrode is the highest, indicating that the content of Ce can easily catalyze the oxidation of organics. On the other hand, the surface structure of PbO₂ has a strong influence on the catalytic activity for oxygen evolution [35-37]. The SEM images of Al/α-PbO₂/β-PbO₂-CeO₂ and Al/α-PbO₂/β-PbO₂ electrode show that the morphologies of the two types of electrodes are rather different. The surface of Al/α-PbO₂/ β-PbO₂ electrode presents many micro-cracks, and looks like a typical pyramid shape; the grains are more dispersive and large. Al/α-PbO₂/β-PbO₂-CeO₂ is quite compact, and the grain size decreases. Therefore, the active surface area of Al/α-PbO₂/β-PbO₂-CeO₂ electrode is larger than that of Al/β-PbO₂. Moreover, electronic conduction exists in the rare earth oxides [38]. This demonstrates that ceria dioxide, as the catalytic sites, could strengthen the interface electron transfer of the electrode.

Fig. 8 Polarization curves of PbO₂ electrodes prepared with different concentrations of CeO₂ in bath

Fig. 9 Linear fitting of polarization curves of PbO₂ electrodes prepared with different concentrations of CeO₂ in bath

Table 6 Kinetic parameters of oxygen evolution

3.4 AC impedance

Figure 10 shows the Nyquist plots of the coatings prepared under different CeO₂ concentrations. It can be seen that when CeO₂ concentrations are 10 and 50 g/L, the plots are not the same as the others. Apparently, CeO₂ as a doping agent changes the reaction mechanism. Coatings, which were prepared under CeO₂ concentrations of 10 and 50 g/L, are coarse or can easily fall off. Therefore, only the other three were computed and analyzed. When CeO₂ concentration is 20-40 g/L, polarization resistance (R_p) becomes large first and then gets small. When the fitting software (ZSimpWin) is used to fit and interpret the Nyquist plots, the obtained equivalent circuit is shown in Fig. 11 and the impedance fitting values are listed in Table 7. The equivalent circuit is composed of a solution resistance (R_s), two constant phase elements Q (CPE) which represent the coating capacitance and the double layer capacitance, a coating resistance and a charge transfer resistance (R_ct). Generally, CPE/Q may exist because of the rough surface or uneven distribution of the electric field [39]. Table 7 shows that the values of n₁ are all equal to 1, indicating that dispersion effect does not occur between the solution/coating layers. While values of n₂ are all not equal to 1, indicating that dispersion effect occurs under the coating layer, or maybe occurs in electric double layer. When the value of n₂ is closer to 1, the coating layer is smoother and dispersion effect is weaker. When CeO₂ concentration is 30 g/L, dispersion effect of the coating is weaker in that n₂ is maximum, which demonstrates a smoother surface. It is considered that in the low-frequency area, the corrosion resistance of the composite coatings can be reflected by the impedance. Such impedance corresponds to the the charge transfer resistance (R_ct). R_ct of the three coatings is not very different, only about 100 Ω/cm². As the same as the dispersion effect, the largest R_ct is tested on the coating when CeO₂ concentration is 30 g/L. This is probably because CeO₂ particles are firstly absorbed on the coating surface, by lowing the reaction area, formation of the oxidation and hydrated layer was hindered, and therefore R_ct increases. When CeO₂ particles are wrapped into the metal composite, the reaction area or active area is exposed again, oxidation and hydration occur, and then R_ct decreases. This is probable why coatings will have different mechanisms when prepared with different CeO₂ concentrations. Particles experience a complex process, including adsorption→desorption→inlay→ coating. So the composite deposition process is actually complicated.

Fig. 10 Nyquist plots of coatings with different CeO₂ concentrations

Fig. 11 Equivalent circuit of composite coatings in zinc plating solution

Table 7 Fitting parameters of equivalent circuit of electrode

4 Conclusions

1) α-PbO₂ as intermediate layer can improve the deposition of β-PbO₂, including structure and crystal plane. β-PbO₂ has better thermodynamic stability, and is more suitable as a surface layer.

2) Rare earth oxide dopants can decrease the grain size of PbO₂ and enlarge the active surface area and therefore change the content of Ce element in the coating.

3) The change of catalytic activity is mainly due to the specific role of the doped ceria dioxide and the different structures of electrodes.

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纳米CeO₂对电沉积Al/α-PbO₂/β-PbO₂镀层结构和性能的影响

陈 阵¹，余 强¹，廖登辉²，郭忠诚²，武 剑²

1. 昆明理工大学 理学院，昆明 650093；

2. 昆明理工大学 冶金与能源工程学院，昆明 650093

摘  要：采用阳极氧化法制备掺杂稀土氧化物(CeO₂)的 Al/α-PbO₂/β-PbO₂复合电极，考察CeO₂的掺杂以及α-PbO₂作为中间层对Al/α-PbO₂/β-PbO₂电极性能的影响。结果表明：α-PbO₂作为中间层有利于β-PbO₂的结晶，β-PbO₂比α-PbO₂更适合作电极表层；CeO₂的掺杂能够改变晶粒尺寸和晶粒结构，提高电极的催化活性，并改变PbO₂晶粒的沉积机理；掺杂CeO₂的PbO₂电极的电催化活性能得到有效提高，从而降低析氧电位和槽电压。

关键词：稀土；CeO₂；复合电极材料；α-PbO₂；β-PbO₂；槽电压；惰性阳极

(Edited by Xiang-qun LI)

Foundation item: Project (50964008) supported by the National Natural Science Foundation of China; Project (2010287) supported by Analysis and Testing Foundation of Kunming University of Science and Technology, China

Corresponding author: Qiang YU; Tel: +86-13708460246; E-mail: yuqiang0015@163.com

DOI: 10.1016/S1003-6326(13)62607-2