Structure dependent electrochemical performance of PbO2 thin-film electrode

Accepted Manuscript Structure dependent electrochemical performance of PbO2 thin-film electrode

Fang Fu, Weihua Yang, Chunyu Ke PII:

S0254-0584(18)30734-X

DOI:

10.1016/j.matchemphys.2018.08.069

Reference:

MAC 20912

To appear in:

Materials Chemistry and Physics

Received Date:

06 January 2018

Accepted Date:

23 August 2018

Please cite this article as: Fang Fu, Weihua Yang, Chunyu Ke, Structure dependent electrochemical performance of PbO2 thin-film electrode, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.08.069

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ACCEPTED MANUSCRIPT

Structure dependent electrochemical performance of PbO2 thin-film electrode Fang Fu, Weihua Yang*, Chunyu Ke

College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, China E-mail address: [email protected]

Abstract: Various structured PbO2 electrodes were prepared by surfactant-assisted anodic electrodeposition method. PbO2-PEG thin-film electrode is composed of small particles and shows a smooth surface. PbO2-PVA electrode displays a unique structure in which the PbO2 particles are embed in the PVA network structure. PbO2AOT electrode consists of bull-horn like particles. The morphologies and structures of PbO2 electrodes are highly influenced by the surfactant. PEG and PVA can decrease the particle size of PbO2 particle and AOT can dramatically alter the morphology of PbO2 particle. PbO2-PEG, PbO2-PVA, and PbO2-AOT electrodes exhibit excellent electrocatalytic activity, stability, corrosion resistance, and long service life compared with bare PbO2 electrode. This work demonstrates the relationship between the structure and electrochemical performances of PbO2 electrodes, and provides a feasible approach to fabricate stable structured electrode.

Keywords: lead dioxide; surfactant; electrodeposition; electrocatalytic.

1. Introduction PbO2 electrode has been extensively used for applications like lead acid batteries, electrochemical synthesis, ozone generation, and oxidation of organic pollutants in wastewater due to its high oxygen overpotential, electrical conductivity, and low cost [1-6]. However, pure PbO2 electrode exhibits short service life and low electro1

ACCEPTED MANUSCRIPT catalysis activity, particularly when they are employed at high potential [7]. To improve the electro-catalysis activity and stability of PbO2 electrode, some solid solution interlayers were introduced between Ti substrate and PbO2 coating [8,9]. In addition, some additives, such as F- [10], Bi3+ [11], Fe3+ [12], Pr3+ [13], TiO2 [14], ZrO2 [15], PTFE [16], and CeO2 [17] were incorporated into the PbO2 film. The research results suggest that the physical and chemical properties of PbO2 electrodes, such as morphology, crystal structure, and heterogeneous rate constant for anodic oxygen transfer reaction, are greatly improved by doping anions, cations, and metal oxides. However, the use of surfactants to enhance the structure and electrochemical performance of PbO2 electrode has rarely been reported. The surfactant is an amphiphilic molecule with a polar head at one end and a long hydrophobic tail at the other, which can spontaneously adsorb on the interface of two phases with different polarities. This unique characteristic is useful to lower the surface energy, improve the surface wetting, and restrict the particle aggregation [18]. Thus, the surfactants can be used as templates to prepare mesoporous structures [19], enhance electron transfer between the protein and the electrode [20], control the shape and dimension of grain [21], as well as to control the nucleation, growth, and morphology of aggregates [22]. Hence, the modification of PbO2 electrode with a surfactant would be an effective method to change the structure, morphology and electrochemical performance of this electrode [23-25]. In this work, different structured PbO2 electrodes were prepared by surfactantassisted anodic electrodeposition method. Among various structured electrodes, PbO2PEG electrode exhibits the best performance owing to its unique structure, lowest charge transfer resistance, and longest lifetime.

2. Experimental Titanium plates with a geometric area of 10 × 10 mm were selected as substrate material. Prior to coating, the plates were polished using emery paper, degreased with 40% NaOH solution, cleaned with 20% H2SO4 solution and etched in boiling 15% oxalic acid solution for 2 h. Subsequently, a Sb-SnO2 oxide interlayer, used to 2

ACCEPTED MANUSCRIPT strengthen the adherence and stability between the PbO2 film and titanium substrate, was obtained by thermal oxidation decomposition [9]. Modified PbO2 films were electrodeposited on the above Sb-SnO2 interlayer with a electrolyte solution composed of 0.1 M HNO3, 0.5 M Pb(NO3)2, 0.04 M NaF, and different kinds of surfactants (PEG, PVA, AOT). The optimal concentration of PEG, PVA, and AOT were 6, 0.4, and 0.6 g L-1, respectively. Electrodeposition was performed under constant current density for 1 h (60 mA cm-2 for 20 min, then 40 mA cm-2 for 40 min) at 333 K. The PbO2 electrodes prepared with surfactants in reaction solution are labeled with PbO2-PEG, PbO2-PVA, and PbO2-AOT. Bare PbO2 film was prepared without surfactant in reaction solution. The thickness and weight of different PbO2 films are basically the same. The morphology of PbO2 electrodes was examined using scanning electron microscopy (SEM) with a Hitachi S-3500 model. The structure and crystalline phases of the samples were examined by X-ray diffraction (XRD) using a Bruker D8 diffractometer with Cu Kα radiation at 20 kV and 40 mA, the scanning rate was 5° min-1. Electrochemical measurements were carried out in a classical three-electrode glass cell, which was composed of a Pt sheet as counter electrode and a saturated Hg/Hg2Cl2 electrode (SCE) as reference electrode. The measurement system consisted of a model 2273 potentiostat (Ametek, USA) and a microcomputer. The electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 120 kHz to 5 mHz at an alternating current (AC) signal amplitude of 5 mV. The EIS data was fitted using Zview software. Linear potential sweep (LPS) tests were conducted from 0 V to certain potentials until the current densities of PbO2 electrodes increased dramatically and reached to a relative high value. The sweep rate is 10 mV·s-1. IR corrections were made using the Rs obtained in the EIS study. The electrooxidation of phenol (50 mg L-1 in 0.5 M Na2SO4) was repeated five times on each sample in an electrochemical reactor having only one compartment and a volume of 50 mL at a current density of 60 mA cm-2 at 298 K. PbO2 electrode was used as anode while the stainless steel as cathode. During the tests, the treated 3

ACCEPTED MANUSCRIPT samples were collected from the reactor at certain intervals and then monitored with the aid of the spectrophotometer at constant wavelength (λ = 510 nm; Unico UV-2800 from Shanghai). The removal rate η can be calculated with Eq. (1) η ＝ (co-c)/co×100%

(1)

where co is the initial concentration of phenol and c is the concentration of phenol at certain time t. Electrode stability was investigated using accelerated life testing with a current density of 1.0 A cm−2 in 1.0 M H2SO4 solution at room temperature. During the accelerated life testing, electrolytic cell voltage was measured periodically, and the test was considered finished when the anode potential steeply increased above 10 V [8].

3. Results and Discussion

Fig. 1 SEM images of PbO2 electrodes (a. PbO2, b. PbO2-PEG, c. PbO2-PVA, d. PbO2-AOT).

Fig. 1 shows SEM images taken for PbO2, PbO2-PEG, PbO2-PVA, and PbO2AOT electrodes. It is found that there is an obvious difference in the morphology and structure between the modified and bare PbO2 electrodes. Fig. 1a displays the 4

ACCEPTED MANUSCRIPT morphology of bare PbO2 electrode. It can be seen that the bare PbO2 electrode is composed of irregular large particles and has a rough surface. The PbO2-PEG electrode shows a well-defined structure composed of smaller and uniform particles (Fig. 1b). The PbO2-PVA electrode displays a special structure that small PbO2 particles embed in PVA cross-linked network (Fig. 1c). This network structure is beneficial to restrict the growth of PbO2 and prevent the diffusion of electrolyte to electrode matrix. The PbO2-AOT electrode exhibits a very unique morphology formed by bull-horn like PbO2 particles (Fig. 1d). The above results suggest that the morphology and structure of PbO2 electrodes are greatly influenced by the surfactant. The surfactant, as a shape modifier, can selectively bind to the surfaces, which results

111

in slowing down the growth rate of PbO2 particles while decreasing their size [26].

Intensity

PbO2-AOT

PbO2-PVA

20

30

40

50

321

301

200

211

200

101

110

PbO2-PEG

60

70

PbO2

80

90

2  

Fig. 2 XRD patterns of PbO2 electrodes.

Fig. 2 illustrates the XRD patterns of PbO2 electrodes. All of the peaks can be unequivocally assigned to α-PbO2 and β-PbO2. In the XRD patterns of PbO2-PEG and PbO2-PVA electrodes, it can be observed that the peak intensity of β-PbO2 decreases and the peaks corresponding to α-PbO2 disappear. The reduction of peak intensity suggests that the smaller particles were formed in PbO2-PEG and PbO2-PVA electrodes [27-28], which is in good agreement with SEM results. In addition, it should be noted that the preferred crystal plane for bare PbO2 electrode is β(110), 5

ACCEPTED MANUSCRIPT whereas for PbO2-PEG and PbO2-PVA electrodes is β(101). The adsorption of PEG and PVA molecules on the electrode surface can alter the orientation of PbO2 crystalline planes during growing. XRD pattern of PbO2-AOT electrode exhibits a mixed phase composed of -PbO2 and β-PbO2. Thus, XRD results reveal that the surfactant has a high impact on crystal structure of PbO2. The Nyquist plots of PbO2 electrodes in 0.5 M H2SO4 solution are displayed in Fig. 3. All Nyquist diagrams of PbO2 electrodes show a regular semicircle for the entire frequency range. The equivalent circuit (EC) and simulation parameters of EIS spectra were evaluated using Zview software. The inset in Fig. 3 represents the equivalent circuit and highlights the EC data of EIS spectra, where Q is a constant phase element that generally describes the non-ideal behavior of the capacitance [29], Rt and Rs correspond to the charge transfer resistance and solution resistance, respectively.

-Z" / ohm cm2

200 175

PbO2

150

PbO2-PEG(6)

125

PbO2-AOT(0.4)

PbO2-PVA(0.6)

100 75 50 25 0

0

25

50

75

100

125

150

175

200

Z' / ohm cm2

Fig. 3 EIS of different PbO2 electrodes.

The simulation parameters of EIS spectra are listed in Table 1. From Fig. 3 and Table 1, it can be observed that the charge transfer resistance of PbO2-PEG, PbO2PVA, and PbO2-AOT electrodes was decreased. The PbO2-PEG electrode shows the smallest charge transfer resistance (Rt = 13.95 Ω cm2), which is associated to the welldefined structure of PbO2-PEG electrode. The smooth structure creates the conditions 6

ACCEPTED MANUSCRIPT for a fast electron transfer. The Rt values of PbO2-AOT and PbO2-PVA electrodes are 39.68 and 47.04 Ω cm2, respectively. Rt and electrochemical exchange current density i0 can be presented as Eq. (2) [30]: Rt = RT/nFi0

(2)

where n is the number of exchanged electrons and F is the Faraday constant. It can be speculated from Eq. (2) that Rt is inversely proportional to i0, thus, the exchange current densities and electrochemical reaction rates for modified electrodes would be improved.

Table 1 Simulated results of EIS of PbO2 electrodes modified with different surfactants Electrode

Rs (Ω cm2)

Q (Ω-1·cm-2·sn)

n

Rt (Ω cm2)

PbO2

1.40

9.18×10-5

0.65

181.20

PbO2-PEG

1.60

4.16×10-5

0.83

13.95

PbO2-PVA

0.84

1.25×10-5

0.75

47.04

PbO2-AOT

1.40

1.93×10-5

0.87

39.68

Fig. 4 Polarization curves of different PbO2 electrodes.

Oxygen evolution is a competitive reaction encountered during the electrochemical oxidation of organic pollutants. Generally, higher overpotential of 7

ACCEPTED MANUSCRIPT oxygen evolution for an electrode helps to improve the electrocatalytic activity [31]. Thus, linear potential sweep studies were conducted to investigate the effect of surfactants (PEG, AOT, and PVA) on the oxygen evolution of PbO2 electrodes. Fig. 4 illustrates the linear polarization curves of PbO2 electrodes. It can be seen that the oxygen evolution overpotential for the modified PbO2 electrodes has been significantly enhanced. PbO2-PEG exhibits the highest overpotential for oxygen evolution, followed by PbO2-AOT and PbO2-PVA. The overpotential of PbO2-PEG, PbO2-AOT, and PbO2-PVA electrodes are 2.15, 1.94, and 1.83 V, respectively. It is worth mentioning that all these values are much higher than that of bare PbO2 electrode i.e., 1.69 V. The above results indicate that the oxygen evolution on PbO2PEG, PbO2-AOT, and PbO2-PVA electrodes is shifted to a higher overpotential.

Fig. 5 Electrochemical degradation of phenol for five successive reactions with different PbO2 electrodes

The stability of electrode is an important factor that defines its practical application. To investigate the electrocatalytic stability of PbO2 electrodes, five successive experiments of phenol degradation were performed on each electrode. As shown in Fig. 5 and Table 2, the degradation curves and degradation ratio of phenol on surfactant modified PbO2 electrodes were comparable. It can be observed that 8

ACCEPTED MANUSCRIPT PbO2-PEG, PbO2-PVA, and PbO2-AOT electrodes have higher electrocatalytic stability and activity in comparison with the bare PbO2 electrode. PbO2-PVA electrode exhibits the highest degradation ratio of phenol (96.1%), followed by PbO2PEG (93.8%) and PbO2-AOT (90.4%). The different degradation ratios of phenol on modified electrodes demonstrate that the surfactant has great influence on the structure and electrochemical performance of PbO2 films.

Table 2 Phenol removal ratio for five successive reactions with different PbO2 electrodes.

12 PbO2 PbO2-PEG

10

PbO2-PVA

Potential / V

PbO2-AOT

8 6 4 2

0

20

40

60

80

100

Time / h

Fig. 6 Accelerated life tests of different PbO2 electrodes.

Accelerated life test was also carried out to characterize the stability of PbO2 electrodes (Fig. 6). It can be observed that there is a relatively long stable period before inactivation of the electrodes. However, during the stable stage, the slight 9

ACCEPTED MANUSCRIPT fluctuations and rising of the voltage may result from the dissolution of poor crystalline grains at the electrode surface. Finally, the potential increased rapidly to a value higher than 10 V. It can be clearly seen in Fig. 6 that the lifetimes of PbO2-PEG, PbO2-AOT, and PbO2-PVA electrodes are 102, 101, and 93 h, respectively, values far are much higher than that for the bare PbO2 electrode. The factors affecting the service life of PbO2 electrode are complicated. The main contributors are the detachment, consumption, and deactivation [32]. During the accelerated life testing, the film detachment was noticed on the surface of bare electrode, but no detachment was found on the surface of other electrodes. The explanation could be related to the surfactant that reduces the inner-stress of PbO2 electrode. Another factor influencing the service life of an electrode is the surface morphology. The smooth and dense morphology of PbO2-PEG and PbO2-PVA can effectively prevent the electrolyte from diffusing into the Ti substrate, thus the service life of these electrodes were greatly improved. Moreover, the good stability of PbO2AOT may be associated with its unique morphology and compact structure.

4. Conclusion In summary, the relationship between the morphology, structure and electrochemical performances of PbO2 electrodes are investigated and discussed herein. Different structured PbO2 electrodes were successfully prepared by surfactantassisted electrodeposition method. It was found that the surfactant play an important role in controlling the morphologies and structures of PbO2 electrodes. It was shown that the electrochemical performance of PbO2 electrodes strongly depends on their morphology and structure. PbO2-PEG electrode shows excellent electrocatalytic activity, stability, and service life, which is superior to other structures studied. The outstanding performance of PbO2-PEG electrode is related to its large surface area and compact structure, which are favorable for fast electron transfer and electrochemical reaction. This work offers fundamental insights into the structureproperty relationships of PbO2 electrodes, which will facilitate a control of the electrode structure. 10

ACCEPTED MANUSCRIPT Acknowledgements This work was supported by National Natural Science Foundation of China (Approval No. 21473063, 21103055).

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ACCEPTED MANUSCRIPT Highlights 

Various structured PbO2 thin-film electrodes are prepared.



Electrochemical performance of PbO2 depends on its structure and morphology.



PbO2-PEG electrode exhibits the best performance.



The performance of PbO2-PEG electrode arises from its smooth and dense

structure.

1