Temperature effects on the kinetics of a PbO₂ electrosynthesis process in an alkaline bath

Abstract

An electrochemical investigation of temperature on the electrosynthesis of lead dioxide in alkaline solutions was performed using a rotating disk electrode (RDE). In the Pb(II)-containing alkaline solution, the reaction taking place at 0.6 V_SCE is under the mixed control of ionic transport and charge transfer. The Koutechy–Levich equation was used to calculate the kinetics of the PbO₂ electrodeposition process at 0.6 V_SCE. The results indicate that temperature has a positive influence on the diffusion of Pb(II). Moreover, temperature has a positive influence on the apparent heterogeneous rate constant of Pb(II) oxidation reactions within a temperature range of 25–40 °C. XRD and SEM results show that PbO₂ synthesized in the alkaline solution consists of pure α phase within a temperature range of 25–45 °C. The intensity of the (200) crystallographic plane shows the same variation in the calculated apparent heterogeneous rate constant k of the PbO₂ electrodeposition process. The deposits are composed of rounded nanocrystallites when the temperatures of solutions are lower than 40 °C, but the deposits synthesized at 40 °C and 45 °C exhibit rod-like crystallites.

---

1. Introduction

Lead dioxide (PbO₂) has attracted considerable attention owing to its well-proven advantages, including low electrical resistivity, low cost, ease of preparation, good chemical stability in acid media, high overpotential for oxygen evolution reactions (OER), and a relatively large surface area.^1–4 Thus, lead dioxide has already been used in waste water treatment,^5–7 ozone generation,^8,9 lead-acid batteries,^10–12 analytical sensors,¹³ and the electrowinning process.¹⁴

It is well known that PbO₂ has two phases: α phase and β phase. Electrodeposition is a traditional way to prepare these two phases of PbO₂.¹⁵ The conditions (mainly composition and temperature) of the synthesizing bath determine the phase of deposits.¹⁶ To the best of our knowledge, α-PbO₂ is less studied than β-PbO₂ due to its lower electrochemical activity and electrical conductivity.¹⁷ Despite the shortcomings mentioned above, α-PbO₂ is chemically more stable than β-PbO₂; consequently, it promotes longer life cycles of lead-acid batteries.¹⁸ The capacity and electrical conductivity of lead-acid batteries was determined by β-PbO₂ and α-PbO₂, respectively.¹⁹ In addition, it is beneficial to ozone production,²⁰ presents the same performance of β-PbO₂ as a pH sensor²¹ and is the first step in the electrochemical fabrication of metallic lead nanowires (NWs).²² In addition, as an interlayer of the electrode, the binding force of β-PbO₂ and its underlayer would be improved by α-PbO₂.²³ Pure α-PbO₂ can be deposited from an acetic acid lead bath²⁴ or a methanesulfonic acid lead bath²⁵ at the suitable conditions. However, a stress-free α-PbO₂ can be obtained by electrodeposition from an alkaline lead bath.^26,27

In the electrochemical field, cyclic voltammetry (CV) technology is a general and powerful technique to study reactions in aqueous solutions, surface deposition and adsorption.^28,29 It can be used to investigate the kinetics of charge transfer at the electrode/electrolyte interface when paired with a rotating disk electrode (RDE).^30–33 The current generated by an electrochemical process under a given potential gradient is determined by two parts: first is transport of reactant ions to the electrode, and the second is the kinetics of its conversion at the electrode surface. Rotating disk electrode can easily establish a steady-state condition. In this condition, the Koutecky–Levich equation can express the relative contribution of mass transport and kinetics (in terms of resistance to charge transfer) to the current generated in an RDE experiment.³⁴

In the present work, we studied and carried out the effects of temperature on electrodeposition of lead dioxide in alkaline solutions. In these conditions, the kinetics of the electrodeposition process was well scrutinized using the Koutechy–Levich equation. The effects of temperature on kinetic parameters D and k were determined. The phase compositions and surface microstructures of deposits synthesized on the platinum surface were determined by means of XRD and SEM, respectively. These research results are helpful to provide the theoretical basis and technical support for lead-acid batteries.

2. Experimentation

2.1. Cyclic voltammetry and rotating disk electrode

The cyclic voltammetry experiments were carried out using aqueous solutions of Pb(II) and NaOH at certain ratios. Alkaline solutions were prepared by dissolving litharge (PbO) in NaOH solution. The concentrations of Pb(II) and NaOH were controlled at 0.15 M and 3 M, respectively. Analytical grade reagents (AR) and twice-distilled water were used for all solutions. High purity nitrogen gas was purged into the solutions before applying the potential. The bath temperature changed from 25 °C to 45 °C. A three-electrode system was employed. The working electrode was a Pt RDE (Ametek) with an exposed Pt area of 0.196 cm² mounted to a rotator. A platinum electrode was the counter electrode and a saturated calomel electrode (SCE) was the reference one. The reference electrode and working electrode were linked by Luggin capillary filled with agar and potassium chloride. In addition, the distance between the capillary and the working electrode surface was about 4d (d = diameter of capillary). Cyclic voltammetry curves and anodic polarization curves were obtained using an electrochemical workstation (PARSTAT2273). All potentials were given against the SCE.

2.2. Apparent heterogeneous rate constant and diffusion coefficient

Values of the apparent heterogeneous rate constants k and diffusion coefficients D for Pb(II) oxidation were calculated from the linear plots of I⁻¹ versus ω^−1/2 according to the Koutecky–Levich equation:

I⁻¹ = (0.62nFAD^2/3ν^−1/6Cω^1/2)⁻¹ + (nFAkC)⁻¹ (1)

where ω is the angular velocity of Pt-RDE (rad s⁻¹), C is the bulk concentration of the reacting species (mol m⁻³), ν is the kinematic viscosity of the solution (m² s⁻¹), F is the Faraday constant (C mol⁻¹), A is the surface area of Pt-RDE (m²), and n is the effective number of electrons exchanged in the reaction. The rate constants k and diffusion coefficients D were measured at E = 0.6 V. Kinematic viscosity ν was calculated by the equation:

ν = μ/ρ (2)

where μ is the dynamic viscosity of the solution (Pa s), and ρ is the density of the solution (kg m⁻³). The dynamic viscosity μ was measured by Brookfield viscometer, and the density was the ratio of mass and volume, which were measured by normal method.

2.3. Characterization of surface and phase composition

The phase composition and surface microstructure characteristics of PbO₂ synthesized on Pt-RDE were measured by D/Max-2200 X-ray diffractometer (XRD) and Nova NanoSEM450 scanning electron microscope (SEM), respectively. The samples were prepared by galvanostatic polarization (10 mA cm⁻²) in the solutions at different temperatures. The solution was consisted of 0.15 M Pb(II) and 3 M NaOH, and the synthesis time was controlled at 1 h.

3. Results and discussion

3.1. Cyclic voltammetry study

Voltammograms measured in the solution of 3 M NaOH + 0.15 M Pb(II) and blank solution (3 M NaOH) at 30 °C are shown in Fig. 1. There is only one peak C₁ in the blank curve, which can be related to oxygen evolution. All peaks related to the solution containing Pb(II) are referred to with letters. Five main peaks (A to E) can be detected on the curve of the Pb(II)-containing solution. On the anodic branch of the curve, peak C corresponds to the oxygen evolution. Peak A seems to be the process of Pb(II) oxidation, and the product of the oxidation process is PbO₂ which is supported by the data of XRD shown in Fig. 6. In the potential range of 0.7–1.1 V, the current density is stable, and PbO₂ electrosynthesis occurred simultaneously with oxygen evolution in this range. Peak B is a mixed peak, which consists of oxygen evolution and Pb(II) → PbO₂. On the cathodic branch, peak D and E correspond most likely to the process of PbO₂ → Pb₃O₄ and PbO₂ → Pb(II), respectively. In the potential range of 0.3–1.0 V, current density is positive and stable at cathodic branch, indicating that the formation of PbO₂ is continuously occur through the negative scan. The current densities of peak A and E are not on the same order of magnitude, indicating that the reaction (Pb[II] → PbO₂) is an irreversible process. In the anodic scanning branch, the two curves doesn't coincide in the potential range of 0.4–1.3 V, which would be caused by continuous formation of PbO₂ in Pb(II)-containing solution.

Fig. 1 Cyclic voltammogram of a Pt electrode in 3 M NaOH + 0.15 M Pb(II) solution at 30 °C, in comparison with a blank solution (scan rate, 50 mV s⁻¹).

Fig. 2 shows the effect of varying the temperature as well as the potential scan range (Pb[II] and NaOH controlled at 0.15 M and 3 M). The current density of peak A increases with bath temperature. The potential of peak A was also affected by temperature. These measures shifted slightly to a more negative potential as the temperature increased. This could be explained by the influence of temperature on the faster diffusion of Pb(II) and the quicker electron exchange of oxidation reaction (Pb[II] → PbO₂). Analysis of Fig. 3 and 4 below confirmed this speculation. Peak B does not existed in all curves, due to the mixed peak it is. In addition, the current density of peak C increased with temperature, indicating that the oxygen evolution reaction was promoted by the increase of temperature. It is because the increase of temperature can promote the electron-transfer in oxygen evolution reaction.³⁵ As cathodic peaks, peaks D and E can reflect the formation weight of the PbO₂ generated in anodic scan.³⁶ The current densities of peak D and E both increased with temperature, and peak D disappeared as temperature increased to 40 °C. The current density and potential of peak E obtained at 40 °C and 45 °C are very close, which reflect the formation weights of the PbO₂ generated in anodic scan at these two temperatures are similar.

Fig. 2 Cyclic voltammogram of a Pt electrode in solution (0.15 M Pb[II] and 3 M NaOH) at different temperatures (scan rate, 50 mV s⁻¹).

Fig. 3 Anodic polarization curves of a Pt RDE in solution (0.15 M Pb[II] and 3 M NaOH) at different temperatures and rotating speeds, (a): 25 °C, (b): 30 °C, (c): 35 °C, (d): 40 °C, (e): 45 °C (scan rate, 50 mV s⁻¹).

Fig. 4 Koutechy–Levich plot obtained at diffusion plateau (0.6 V) of different bath temperatures.

3.2. Rotating disk electrode study

Fig. 3 shows the anodic polarization curves of a solution (0.15 M Pb[II] and 3 M NaOH) at different temperature and rotating speeds varying from 25 to 45 °C and 400 to 2000 rpm, respectively. The five graphs of Fig. 3 show almost identical features. There is no reaction in the potential range of 0–0.42 V; above 0.42 V the current starts to increase; the current reaches a stable region at approximately 0.55–0.85 V. Some curves for high rotation speeds are disordered in the potential range of 0.9–1.2 V. This is because the binding force of PbO₂ and the Pt substrate is not very strong, so the generated PbO₂ exfoliated from the substrate. The measured dependencies of current within the stable region are obvious at varying rotation speeds. This result is unambiguous evidence that the transport (mass transfer) of Pb(II) to the electrode surface affects the rate of the electrode process.

Fig. 4 shows the Koutechy–Levich plot obtained at the diffusion plateau (0.6 V) of different temperatures. The curves indicate that there is direct proportionality between I⁻¹ and ω^−1/2, and the intercept on I axes is not zero. A result can be drawn that a mixed control (ion transport and kinetics) was present in the reaction taking place at 0.6 V.^34,37 A curve has been drawn through the points obtained and the goodness of fit of each line is shown in Fig. 4. Diffusion coefficient D has been calculated from the value of the slope, using eqn (1) above. The values of the parameters used in the equation and the results of all calculations are shown in Table 1; all parameters are experimental. It has been assumed that the reaction taking place at 0.6 V is Pb(II) → Pb(IV) (hence n = 2). The apparent heterogeneous rate constant k has also been calculated from the value of the intercept of the fitted line. The diffusion coefficient D and apparent heterogeneous rate constant k have an obvious relationship with the temperature of the solution. The value of diffusion coefficient increased with bath temperature, which indicates temperature has a positive influence on the diffusion of Pb(II) at a temperature range of 25–45 °C. The current density change of peak A in Fig. 2 may also reflect the difficulty of Pb(II) diffusion in alkaline solutions. In addition, the value of apparent heterogeneous rate constant k increased at first, then decreased a little as bath temperature increased from 25 °C to 45 °C. Formula (3) is the main reaction of PbO₂ electrodeposition in alkaline solutions.^38–40 The apparent heterogeneous rate constant rise within the temperature range of 25–40 °C could be explained by thermodynamic analysis. The thermodynamic analysis shows that the standard electrode potential of formula (3) falls from 0.6392 V to 0.5644 V as the bath temperature rises from 25 °C to 100 °C.⁴⁰ Therefore, the increase in bath temperature is beneficial to promote the electrosynthesis of α-PbO₂. However, the further increasing of bath temperature (higher than 40 °C) does not increase the apparent heterogeneous rate constant k. In fact, the rate constant k calculated from the bath temperature of 45 °C is a little lower than that calculated from 40 °C. This is because the electrosynthesis of α-PbO₂ is an exothermic reaction, and the faster the reaction runs, the larger amount of heat releases. As a result, the further increasing of bath temperature will not improve the reaction rate of Pb(II) oxidation.⁴¹ The process of PbO₂ electrodeposition on the surface of RDE is shown in Fig. 5.

HPbO₂⁻ + OH⁻ − 2e⁻ → PbO₂ + H₂O (3)

Table 1 Verification of Koutecky–Levich equation

Temperature	Koutecky–Levich equation: I^−1 = (0.62nFAD^2/3ν^−1/6Cω^1/2)^−1 + (nFAkC)^−1
	nF (C mol^−1)	C (mol m^−3)	A (m^2)	ν (m^2 s^−1)	Intercept	Slope	k (m s^−1)	D (m^2 s^−1)
25 °C	1.93 × 10^5	150	1.96 × 10^−5	4.36 × 10^−6	6.23	474.91	2.83 × 10^−4	6.63 × 10^−10
30 °C	1.93 × 10^5	150	1.96 × 10^−5	4.08 × 10^−6	3.3	438.29	5.34 × 10^−4	7.36 × 10^−10
35 °C	1.93 × 10^5	150	1.96 × 10^−5	3.72 × 10^−6	2.17	421.26	8.14 × 10^−4	7.63 × 10^−10
40 °C	1.93 × 10^5	150	1.96 × 10^−5	3.27 × 10^−6	1.45	405.97	1.22 × 10^−3	7.81 × 10^−10
45 °C	1.93 × 10^5	150	1.96 × 10^−5	2.54 × 10^−6	1.5	354.25	1.18 × 10^−3	8.99 × 10^−10

---

Fig. 5 Schematic diagram of the PbO₂ electrodeposition process on RDE surface.

3.3. Phase compositions and surface microstructures

The XRD patterns of PbO₂ synthesized on Pt electrode surface in solution (0.15 M Pb[II] and 3 M NaOH) at different temperatures are shown in Fig. 6. XRD analysis confirms that PbO₂ synthesized in an alkaline solution consists of pure α phase matching the ICDD database card #72-2440.⁴² However, not all characteristic peaks are present and relative intensities are not in agreement with the ICDD card, which was also observed by R. Inguanta.⁴³ A preferential orientation of growth in the (200) crystallographic plane can easily be observed, and the α-PbO₂ deposit is polycrystalline. Compared to (200) crystallographic plane, the intensity of other crystallographic planes is much lower. The intensity of the (200) crystallographic plane initially increased along with the bath temperature (<40 °C), then decreased when bath temperature exceeded 40 °C, which shows the same variation of the calculated apparent heterogeneous rate constant k. This indicates that increases in the apparent heterogeneous rate constant k may have some relationships with the preferential orientation of growth in the (200) crystallographic. The average crystallite size (L_hkl) of synthesized α-PbO₂ was estimated from the full width at half maximum height (FWHM) of the peaks at 36.2°. The Debye–Scherrer equation, which is valid for nanosized crystallites, was employed in the estimation, the values obtained at investigated bath temperatures are reported in Table 2. It can be observed that grain size increased from 35.3 nm to 42.8 nm as bath temperature increased from 25 °C to 40 °C. In addition, the grain size decreased a little from 42.8 nm to 38.7 nm as bath temperature changed from 40 °C to 45 °C. This effect can be attributed to the faster growth of formed nuclei as apparent heterogeneous rate constant k increases.

Fig. 6 XRD patterns of PbO₂ synthesized on Pt electrode surface in solution (0.15 M Pb(II) and 3 M NaOH) at different temperatures.

Table 2 Average size of crystalline grains of PbO₂ synthesized at different bath temperatures

Bath temperature/°C	25	30	35	40	45
dg/nm	35.3	36.5	38.1	42.8	38.7

---

In order to assess the influence of bath temperature, the microstructure of deposited samples was also characterized by SEM. Fig. 7 shows the SEM images of PbO₂ synthesized on Pt electrode surface in solution (0.15 M Pb(II) and 3 M NaOH) at different temperatures. As can be seen, compact deposits were obtained with alkaline solutions, but the general morphological trends occurred when varying bath temperature. The deposits are all composed of rounded nanocrystallites when the deposition temperature was lower than 40 °C. As bath temperature is larger than 40 °C, its crystallite is longer and finer than any other deposits. This phenomenon can be due to the higher intensity of the (200) crystallographic plane of XRD patterns of deposit obtained in solution at 40 °C and 45 °C and the apparent heterogeneous rate constant k of 40 °C and 45 °C obtained in part 3.2. Higher apparent heterogeneous rate constants involve higher electrodeposition rates, and higher electrodeposition rates may lead to more orientated crystallization, subsequently yielding the rod-like crystallite. In addition, the crystallite size initially increased and subsequently decreased as bath temperature increased, which can be observed in Fig. 7, confirming the average size of crystallite calculated using the Debye–Scherrer equation (Table 2).

Fig. 7 SEM images of PbO₂ synthesized on Pt electrode surface in solution (0.15 M Pb(II) and 3 M NaOH) at different temperatures (a: 25 °C; b: 30 °C; c: 35 °C; d and d′: 40 °C; e: 45 °C).

4. Conclusions

In this study, the effects of bath temperature were investigated by cyclic voltammetry and rotating disk electrode techniques in order to study the electrodeposition process affected by bath temperature. In the Pb(II)-containing alkaline solution, the reaction taking place at 0.6 V is under the mixed control of ionic transport and charge transfer. The values for diffusion coefficient D and apparent heterogeneous rate constant k of the electrodeposition process at 0.6 V were calculated using the Koutechy–Levich equation. The value of the diffusion coefficient increased with bath temperature, which indicates bath temperature has a positive influence on the diffusion of Pb(II) within a temperature range of 25–45 °C. In addition, the value of the apparent heterogeneous rate constant k increased at first, then decreased a little as the bath temperature increased from 25 °C to 45 °C. The deposits were successfully synthesized on the Pt surface via anodic galvanostatic polarization in alkaline solution within a temperature range of 25–45 °C. XRD and SEM were employed to investigate the effects of bath temperature on phase composition and surface microstructures of deposits. The results confirmed that PbO₂ synthesized in an alkaline solution consists of pure α phase. The preferential orientation of growth along the (200) crystallographic plane can be observed on the XRD patterns of α-PbO₂ deposits. The intensity of the (200) crystallographic plane initially increased with the rise in temperature (<40 °C), then decreased when bath temperature exceeded 40 °C, which shows the same variation for the calculated apparent heterogeneous rate constant k. The average crystallite size of the α-PbO₂ deposits obtained from alkaline bath within a temperature range of 25–45 °C changed from 35.3 nm to 42.8 nm. The deposits are composed of rounded nanocrystallites when the temperatures of solutions were lower than 40 °C, but the deposits synthesized at 40 °C and 45 °C exhibited rod-like crystallite.

Acknowledgements

Authors gratefully acknowledge the financial supports of the Specialized Research Fund for the Doctoral Program of the Ministry of Education of China (Project No. 20125314110011); the Key Project of Yunnan Province Applied Basic Research Plan of China (Project No. 2014FA024); the National Natural Science Foundation of China (Project No. 51564029).

References

1. A. M. Couper, D. Pletcher and F. C. Walsh, Chem. Rev., 1990, 90, 837–865.
2. H. Yang, B. Chen, H. Liu, Z. Guo, Y. Zhang, X. Li and R. Xu, Int. J. Hydrogen Energy, 2014, 39, 3087–3099.
3. M. Panizza, I. Sirés and G. Cerisola, J. Appl. Electrochem., 2008, 38, 923–929.
4. L. Chang, Y. Zhou, X. Duan, W. Liu and D. Xu, J. Taiwan Inst. Chem. Eng., 2014, 45, 1338–1346.
5. D. C. Johnson, J. Feng and L. L. Houk, Electrochim. Acta, 2000, 46, 323–330.
6. H. An, Q. Li, D. Tao, H. Cui, X. Xu, L. Ding and J. Zhai, Appl. Surf. Sci., 2011, 258, 218–224.
7. J. Xing, D. Chen, W. Zhao, X. Zhao and W. Zhang, RSC Adv., 2015, 66, 53504–53513.
8. R. Amadelli, L. Armelao, A. B. Velichenko, N. V. Nikolenko, D. V. Girenko, S. V. Kovalyov and F. I. Danilov, Electrochim. Acta, 1999, 45, 713–720.
9. J. Wu, H. Xu and W. Yan, RSC Adv., 2015, 25, 19284–19293.
10. H. Y. Chen, L. Wu, C. Ren, Q. Z. Luo, Z. H. Xie, X. Jiang and Y. R. Luo, J. Power Sources, 2001, 95, 108–118.
11. A. Moncada, M. C. Mistretta, S. Randazzo, S. Piazza, C. Sunseri and R. Inguanta, J. Power Sources, 2014, 256, 72–79.
12. A. Moncada, S. Piazza, C. Sunseri and R. Inguanta, J. Power Sources, 2015, 275, 181–188.
13. D. Velayutham and M. Noel, J. Appl. Electrochem., 1993, 23, 922–926.
14. C. J. Yang and S. M. Park, Electrochim. Acta, 2013, 108, 86–94.
15. H. Bode, Lead-Acid Batteries, Wiley, New York, 1977.
16. D. Devilliers, T. Dinh, E. Mahé, V. Dauriac and N. Lequeux, Electroanal. Chem., 2004, 573, 227–239.
17. B. Chen, Z. Guo, H. Huang, X. Yang and Y. Cao, Acta Metall. Sin., 2009, 22, 373–382.
18. A. J. Salkind, A. G. Cannone and F. A. Trumbure, Hand-book of Batteries, McGraw-Hill, New York, 2002.
19. R. Fitas, L. Zerroual, N. Chelali and B. Djellouli, J. Power Sources, 2000, 85, 56–58.
20. K. Kinoshita, Electrochemical Oxygen Technology, Wiley, New York, 1992.
21. A. Eftekhari, Sens. Actuators, B, 2003, 88, 234–238.
22. R. Inguanta, E. Rinaldo, S. Piazza and C. Sunseri, Electrochem. Solid-State Lett., 2010, 13, K1–K4.
23. S. He, R. Xu, G. Hu and B. Chen, Electrochemistry, 2015, 83, 974–978.
24. R. Inguanta, S. Piazza and C. Sunseri, J. Electrochem. Soc., 2008, 155, K205–K210.
25. I. Sirés, C. T. J. Low, C. Ponce-de-León and F. C. Walsh, Electrochim. Acta, 2010, 55, 2163–2172.
26. M. Ueda, A. Watanabe and T. Kameyama, J. Appl. Electrochem., 1995, 25, 817–822.
27. D. Devilliers, M. T. Dinh Thi, E. Mahé and Q. Le Xuan, Electrochim. Acta, 2003, 48, 4301–4309.
28. K. Polat, M. L. Aksu and A. T. Pekel, J. Appl. Electrochem., 2002, 32, 217–223.
29. R. Wartena, J. Winnick and P. H. Pfromm, J. Appl. Electrochem., 2002, 32, 725–733.
30. J. Prentice, Electrochemical Engineering Principles, Prentice Hall, 1991.
31. J. M. Maciel and S. M. L. Agostinho, J. Appl. Electrochem., 2000, 30, 981–985.
32. A. Neville, T. Hodgkiess and A. P. Morizot, J. Appl. Electrochem., 1999, 29, 455–462.
33. A. J. Bard, and L. R. Faulkner, Electrochemical Methods, John Wiley & Sons, 2001.
34. F. Brusciotti and P. Duby, Electrochim. Acta, 2007, 52, 6644–6649.
35. M. H. Miles, G. Kissel, P. W. T. Lu and S. Srinivasan, J. Electrochem. Soc., 1976, 123, 332–336.
36. A. B. Velichenko, R. Amadelli, E. V. Gruzdeva, T. V. Luk’yanenko and F. I. Danilov, J. Power Sources, 2009, 191, 103–110.
37. T. D. Cabelka, D. S. Austin and D. C. Johnson, J. Electrochem. Soc., 1984, 131, 1595–1602.
38. G. Hu, R. D. Xu, S. W. He, B. M. Chen, H. T. Yang, B. H. Yu and Q. Liu, Trans. Nonferrous Met. Soc. China, 2015, 25, 2095–2102.
39. S. He, R. Xu, G. Hu and B. Chen, RSC Adv., 2016, 6, 3362–3371.
40. B. Chen, Z. Guo and X. Yang, China Nonferrous Metall., 2009, B2, 54–58.
41. P. Delahay, M. Pourbaix and P. Van Rysselberghe, J. Electrochem. Soc., 1951, 98, 57–64.
42. International Centre for Diffraction Data, Power Diffraction File (2007), Pennsylvania, USA, card number 72-2440 for α-PbO₂.
43. R. Inguanta, F. Vergottini, G. Ferrara, S. Piazza and C. Sunseri, Electrochim. Acta, 2010, 55, 8556–8562.

---