RuO₂ loaded into porous Ni as a synergistic catalyst for hydrogen production

Abstract

Electrolytic hydrogen by renewable electricity such as solar and wind power is considered as a sustainable energy storage approach. In this work, a porous nano/microarchitectured RuO₂/Ni composite catalyst has been elaborately designed via a facile and controllable route. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), linear scanning voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were used to scrutinize the catalysts and the electrochemical performance. The designed RuO₂/p-Ni catalyst significantly displays enhanced catalytic activity and long-term durability toward hydrogen production compared with a Pt catalyst. The excellent performance of the composite catalyst could be ascribed to the fact that RuO₂ can be well incorporated into the constructed porous Ni network with large specific surface area. The presence of RuO₂ and the Ni network in pairs on the surface of the composite catalyst may not only result in a synergistically enhanced catalytic effect between RuO₂ and the porous Ni network by hydrogen spillover, but also ensure that RuO₂ firmly binds with the porous Ni network, consequently ensuring the long-term durability of the catalyst during the whole reaction.

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Introduction

Concern about climate change and exhaustible sources of fossil fuels has led to an urgent search for alternative and renewable energy sources and carriers. Hydrogen energy is a clean, environmentally friendly resource and considered as one of the most promising candidates for replacing fossil fuels in the future.¹ Currently, most hydrogen is produced by steam reforming natural gas and gasification of coal and petroleum coke.² However, these traditional fossil fuels are not renewable energy sources and the problem of CO₂ release has still not been solved.³ Reversible hydrogen fuel cell technology provides an essential link between renewable energy sources such as solar and wind energy, and sustainable energy carriers. Electrochemically splitting water into hydrogen by renewable energy is a green process and plays an important role in energy systems because electrolytic hydrogen can not only work as an energy vector/carrier and as an energy storage medium, but also overcome the intermittent nature of solar energy and wind energy.^4–8 When abundant renewable energy is available, excessive energy may be stored in the form of hydrogen by water electrolysis, which then could be utilized in hydrogen fuel cells to generate electricity or as a fuel gas again. That is to say, the interconversion of water and hydrogen presents a significant, environmentally responsible, carbon-free alternative for hydrogen generation.

Although in the water splitting process the water oxidation reaction is more challenging, the cathode catalyst with high over-potential and cost can also not be ignored for the hydrogen evolution reaction (HER). The best known catalyst for the HER is the typical precious metal Pt, but the scarcity and high cost prohibit its wide-scale applications. Therefore, some research efforts have been devoted to reducing Pt consumption or to replacement of Pt with other materials.^9–13 One alternative is RuO₂, which displays an exchange current density comparable to Pt at only 1/20 the cost per unit mass.^14,15 Furthermore, it has greater tolerance for heavy metals and considerable stability during long-term electrolysis.^16–20 On the other hand, some studies have been dedicated to developing non-precious metal substitutes.^21–24 Among these candidates, porous Ni-based materials with large surface areas obtained through leaching active elements out of the precursor showed significantly enhanced catalytic activity for the HER. These electrode materials have large specific surface areas, while their intrinsic activities are still inferior when compared to precious metal catalysts. In addition, the over-potentials of porous Ni-based materials for the HER gradually increase as the reaction progresses because the active species dissolve at high potentials and hydrogen bubbles are trapped in the small holes (<10 nm).²⁵

Based on the above considerations, herein we develop a facile and controllable electrodeposited method to address both the catalytic performance and cost issues by constructing a porous nano/microarchitectured interconnected RuO₂/Ni composite material. The porous Ni network grown in situ on a Ni substrate using a hydrogen bubble template possesses high surface area, an open porous structure, and can strongly adhere to the Ni substrate. This makes it a promising candidate for constructing a novel HER catalyst with high activity and stability. After RuO₂ is deposited onto the porous Ni network, both the RuO₂ and the porous Ni network are simultaneously exposed to the reactive two-phase zone. Although the deposited amount of RuO₂ is very little, the constructed composite catalyst can significantly improve the catalytic performance for the HER by incorporating the merits of high intrinsic activity from RuO₂ with the exposure of more active sites provided by high surface area.

Experimental section

Fabrication of the RuO₂/p-Ni composite cathode

Home-made porous Ni, named as p-Ni, with an area of 50.0 mm × 10.0 mm, was used as a support for depositing RuO₂ in an aqueous solution containing 0.005 mol L⁻¹ RuCl₃. The electrodeposition was carried out in a three-electrode cell system at 298 K, with the above p-Ni substrate as the working electrode, a Pt foil counter electrode, and a saturated Ag/AgCl (3 mol L⁻¹ KCl) reference electrode. Finally, the prepared RuO₂/p-Ni composite material was heated at 573 K for 2 h in air.

Characterization techniques

The surface morphology and the microstructure of the samples were analyzed by field-emission scanning electron microscopy (FE-SEM, FEI Nova 400 FEG), X-ray diffraction (XRD-6000, Shimadzu) and X-ray photoelectron spectroscopy (XPS, PHI 550 ESCA/SAM), respectively. Electrochemical measurements were conducted in a three-electrode cell system. The electrode area of the working electrode was 1 cm². A Pt foil counter electrode in parallel orientation to the working electrode was used and a saturated calomel electrode (SCE) as the reference electrode. All potentials mentioned in this work were converted to values with reference to a reversible hydrogen electrode (RHE). The catalytic performance of the prepared catalysts toward the HER was systematically investigated in a 6 mol L⁻¹ NaOH electrolyte. Electrochemical impedance spectroscopy (EIS) measurements were conducted in the frequency range of 100 kHz to 0.01 Hz with a voltage excitation amplitude of 5 mV. Long-term stability experiments were carried out at a cathode current density of 500 mA cm⁻² over 100 h.

Results and discussion

Fig. 1 shows XRD patterns of the support and catalyst. It can be seen that, three diffraction peaks at 44.5°, 51.8° and 76.3° can be indexed to the diffraction from the (111), (200) and (220) planes of Ni metal, respectively (JCPDS card no. 40-0850). In addition to the diffraction peaks from Ni metal, for the RuO₂/p-Ni catalyst, the broad and weak peaks are observed at 28.72° and 35.11°, which can be assigned to the diffraction peaks of RuO₂ (JCPDS card no. 40-1290). This suggests that the deposited RuO₂ particles are relatively small on the porous Ni support. To further demonstrate the presence of RuO₂, RuO₂/p-Ni was analyzed by X-ray photoelectron spectroscopy (XPS). As presented in Fig. 2a, the atomic percentage of elemental Ru in RuO₂/p-Ni is estimated to be 5.68%. In addition, the O 1s spectra from Fig. 2b show the presence of O²⁻ at 529.7 eV, OH⁻ at 531.1 eV, and less intense water molecules at 532.2 eV which might have resulted from water adsorption from the atmosphere prior to XPS measurement.^26,27 XPS spectra of the Ru 3d_3/2,5/2 core level for the RuO₂/p-Ni catalyst are shown in Fig. 2c. The peaks are relatively broad, pointing to the existence of multivalent oxidation states. According to the binding energy of Ru(0) (280.0 eV), RuO₂ (280.7 eV) and RuO₂·xH₂O (281.4 eV),^28,29 the Ru 3d_5/2 peak around 282 eV can be attributed to the presentation of ruthenium hydroxides/oxides. From the results of the XRD and XPS analysis, it can be confirmed that RuO₂ was successfully deposited on the porous Ni support.

Fig. 1 XRD patterns of (a) p-Ni and (b) RuO₂/p-Ni.

Fig. 2 XPS core level spectra of RuO₂/p-Ni catalyst: (a) survey; (b) O 1s; (c) Ru 3d; (d) Ni 2p.

Typical FE-SEM images of p-Ni, RuO₂/p-Ni, and RuO₂ are presented in Fig. 3. As illustrated in Fig. 3a₁ and a₂, the porous nano/microarchitectured interconnected Ni network has been grown in situ on the Ni substrate by a facile and controllable process accompanying hydrogen production where hydrogen bubbles play a template role in the formation of the unique porous architecture.^30–36 The obtained p-Ni clearly exhibits an open porous structure with typical large pores of 5–10 μm in diameter. This unique structure remarkably increases the surface area of the Ni substrate and is beneficial for dispersing the deposited RuO₂. Fig. 3b₁ and b₂ show that RuO₂ is uniformly distributed and confined inside the porous Ni network. The fact that RuO₂ and the porous Ni network appear in pairs on the surface of the catalyst is likely to play a synergistic role in enhancing the HER activity of the RuO₂/p-Ni. Moreover, the porous structure of the porous Ni network is well maintained and no pore-blocking occurred upon the incorporation of RuO₂, although the pore diameter of the RuO₂/p-Ni is obviously reduced relative to that of p-Ni. The sufficient exposure of active sites to the two-phase zone from the unique RuO₂ composite catalyst can thus ensure the enhanced HER activity as discussed in the following sections. This is in contrast to RuO₂ deposited on the smooth Ni substrate, i.e., RuO₂, as depicted in Fig. 3c₁ and c₂, where the Ni substrate is well covered by the close-packed RuO₂ deposits. Therefore, only RuO₂ is exposed to the two-phase zone and is utilized during the HER; the catalytic role of the Ni substrate is probably weakened more or less. The lower HER activity appears to be rational for RuO₂. A detailed discussion follows.

Fig. 3 FE-SEM images of (a₁ and a₂) p-Ni, (b₁ and b₂) RuO₂/p-Ni, and (c₁ and c₂) RuO₂.

The catalytic activity of the HER on the prepared catalysts was evaluated by means of linear scan voltammetry (LSV) measurements in a 6 mol L⁻¹ NaOH solution. For the purpose of comparison, the catalytic activity of the Pt catalyst for the HER was also investigated under the same conditions. It can be seen from the results in Fig. 4 that the onset potentials of the RuO₂/p-Ni and Pt catalysts for the HER are more positive when compared to those of pure RuO₂, p-Ni and Ni. This indicates that the RuO₂/p-Ni and Pt catalysts have higher intrinsic activity than other catalysts. In addition, both the RuO₂/p-Ni and Pt catalysts have almost the same onset potentials and catalytic activity below the cathode current density of 40 mA cm⁻². However, the RuO₂/p-Ni catalyst manifests superior apparent catalytic activity for the HER than the Pt catalyst at large current densities (> 40 mA cm⁻²). In other words, RuO₂ incorporated into the porous nano/microarchitectured interconnected Ni network possesses much more exposed active sites to be utilized during the HER.

Fig. 4 LSV of Ni, p-Ni, RuO₂, Pt, and RuO₂/p-Ni catalysts in a 6 mol L⁻¹ NaOH electrolyte at a sweep rate of 5 mV s⁻¹.

To obtain further insight into the HER activities of the different catalysts, the kinetic parameters (Tafel slope and exchange current density i₀) calculated from the polarization curves test are summarized in Table 1. The Tafel slope plays an important function in estimating the mechanism from the rate-determining step (rds) of a multi-step reaction.^37,38 Generally, the accepted mechanism of hydrogen evolution in alkaline media involves initial proton discharge to form an adsorbed hydrogen on the metal surface, MH_ads (Volmer reaction, eqn (1)), followed by the electrochemical desorption of the hydrogen (Heyrovsky reaction, eqn (2)) and/or the chemical desorption by the recombination of the adsorbed hydrogen (Tafel reaction, eqn (3)).

H₂O + M + e⁻ → MH_ads + OH⁻ (1)

H₂O + MH_ads + e⁻ → H₂ + M + OH⁻ (2)

MH_ads + MH_ads → H₂ + 2M (3)

Table 1 Kinetic parameters for HER in a 6 mol L⁻¹ NaOH solution

Catalysts	Over-potential η100^a (mV)	Tafel slope (mV dec^−1)		Exchange current density i0 (A cm^−2)
		Low η	High η	Low η	High η
a η at a current density of 100 mA cm^−2.
Ni	593	132	184	6.64 × 10^−6	5.83 × 10^−5
p-Ni	401	83	148	2.83 × 10^−5	3.85 × 10^−4
RuO2	171	47	141	7.97 × 10^−4	2.73 × 10^−3
RuO2/p-Ni	118	43	121	1.89 × 10^−3	3.41 × 10^−2
Pt	135	41	126	2.24 × 10^−3	2.69 × 10^−2

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The theoretically expected Tafel slopes at 25 °C are 120 mV when eqn (1) is the rds, 40 mV when eqn (2) is the rds and 30 mV when eqn (3) is the rds. Consequently, the adsorption and desorption of H atoms on the surface of the catalyst are competitive processes. A good HER catalyst should have the ideal trade-off between the binding and release of H atoms. That is, binding that is neither too strong nor too weak would favour the overall HER. As indicated in Table 1, in the light of low Tafel slop, low over-potential and large exchange current density, none of Ni, p-Ni or RuO₂ is better than Pt, however, the constructed composite catalyst, RuO₂/p-Ni, is comparable to Pt. Furthermore, EIS results from Fig. 5 indicate that the polarization resistance of RuO₂/p-Ni obviously decreased compared with that of p-Ni. It possesses two depressed capacitive semicircles, which are characteristic behavior of porous and rough surfaces.³⁹ In general, the high frequency semicircle is related to the pores of the electrode material and is independent of the kinetics of the faradic process, and the low frequency semicircle is controlled by the charge transfer process. Consequently, the HER occurs on RuO₂/p-Ni under a mixture of charge transfer and diffusion control. Such enhanced activity also validated our preliminary prediction that there is a synergistic effect between RuO₂ and the porous Ni network. It is well known that RuO₂ is actually a remarkably good catalyst for the HER.^40–43 After RuO₂ particles are deposited onto the porous Ni network, both RuO₂ and the porous Ni network are simultaneously exposed to the reactive two-phase zone. The porous Ni network is employed as the secondary active site supplier and cooperates with the highly dispersed RuO₂ particles to enhance integral activity for the HER. From the fitted curve of the Ni 2p XPS spectra in Fig. 2d, it can be observed that the peaks at 853.0 eV with its satellite at 858.9 eV are characteristic of metallic Ni; the peaks at 854.7 with its satellite at 861.0 eV are assigned to Ni²⁺.^27,44 This suggests the presence of hydroxide/oxide on the surface of the catalyst, which is a byproduct of Ni in preparing the porous Ni by a cathodic electrodeposition method accompanying hydrogen generation. The formed hydroxide/oxide can result in a high activity for the electrochemical dissociation of water (eqn (1)), however, they are poor at converting the resulting H_ad intermediates to H₂.^11,45 The adsorbed H on the surface of the nickel species (eqn (1)) might diffuse to the nearby ruthenium surfaces by the spillover effect, followed by the electrochemical desorption of the hydrogen (eqn (2)).¹¹ The active sites occupied by the adsorbed H on the nickel species could be promptly recovered and continue to process eqn (1). Accordingly, the most important condition for the HER in alkaline medium is a synergy between the effectiveness of the catalyst to break water molecules and to efficiently form hydrogen that subsequently can be adsorbed and desorbed on the catalyst surface.

Fig. 5 Nyquist plots of p-Ni, RuO₂/p-Ni, and Pt catalysts obtained at 50 mV over-potential.

High stability is important for a catalyst, especially for its industrial applications. To probe the stability of the prepared catalysts for the catalytic HER, the long-term performance of the catalysts was examined at a cathode current density of 500 mA cm⁻² in a 6 mol L⁻¹ NaOH solution. The initial over-potentials (η₅₀₀) of the RuO₂/p-Ni, RuO₂ and Pt catalysts are 0.354 V, 0.461 V and 0.376 V, respectively. The RuO₂/p-Ni catalyst manifests a lower over-potential for the HER than the Pt catalyst at large current densities. After 100 h electrolysis, the RuO₂/p-Ni and Pt catalysts still displayed low over-potentials and durability unlike the pure RuO₂ catalyst. Fig. 6 shows the variation in potential versus time for typical RuO₂/p-Ni and RuO₂ catalysts. It is noticeable that the RuO₂/p-Ni catalyst possesses almost constant over-potential during the electrolysis. Furthermore, the surface morphology of RuO₂/p-Ni (Fig. 7a) exhibits no perceptible change after 100 h electrolysis. EDS mapping also validates the presence of Ru and Ni, which are uniformly distributed within the RuO₂/p-Ni catalyst. In sharp contrast, parts of the active deposits of the RuO₂ catalyst have obviously peeled off (Fig. 7b). There are several causes that are thought to be responsible for the improved catalytic performance of the RuO₂/p-Ni catalyst for the HER. First of all, although RuO₂ should not be thermodynamically stable during cathodic polarization, it has been found experimentally that it possesses a considerable stability, which has been ascribed to a sufficient electronic conductivity of RuO₂, preventing its reduction.^46–48 Secondly, the unique porous structure is beneficial for enlarging the surface area of the catalyst to provide more active sites. Thirdly, the porous Ni network grown in situ on the Ni substrate may enhance the interaction between RuO₂ and the porous Ni network. Accordingly, the stable architecture ensures that the RuO₂/p-Ni catalyst works well and works long-term during the whole electrolysis process.

Fig. 6 Potential variation as a function of electrolysis time at a cathode current density of 500 mA cm⁻² in a 6 mol L⁻¹ NaOH electrolyte.

Fig. 7 FE-SEM micrograph (a) of used RuO₂/p-Ni after 100 h electrolysis reaction along with EDS mapping (c and d); FE-SEM micrograph (b) of used RuO₂ after 100 h electrolysis reaction.

Conclusions

In summary, the novel porous nano/microarchitectured RuO₂/Ni composite catalyst as a promising industrial electrocatalyst for the HER has been successfully fabricated through a facile and controllable method. The unique characteristics of the composite material incorporate the merits of high intrinsic activity from RuO₂ and the exposure of more active sites provided by high surface area, which make it an ideal catalyst for the HER. The presence of RuO₂ and a porous Ni network in pairs on the surface of the catalyst results in a synergistic effect between RuO₂ and the Ni network which ensures enhanced catalytic in the HER. The stable porous architecture due to the enhanced interaction between RuO₂ and the porous Ni network ensures that the RuO₂/p-Ni catalyst works well during the whole reaction.

Acknowledgements

This research work was financially sponsored by National Basic Research Program of China (Grant no.: 2012CB720300), by National Natural Science Foundation of China (Grant no.: 51072239, and 21306232) and by the Fundamental Research Funds for the Central Universities (CDJZR12228802 and CDJXS11132229).

Notes and references

1. M. S. Dresselhaus and I. L. Thomas, Nature, 2001, 414, 332.
2. J. D. Holladay, J. Hu, D. L. King and Y. Wang, Catal. Today, 2009, 139, 244.
3. Hydrogen production road map: Technology pathways to the future, FreedomCAR & Fuel partnership, Hydrogen Production Technical Team, 2009.
4. K. Zeng and D. K. Zhang, Prog. Energy Combust. Sci., 2010, 36, 307.
5. N. Armaroli and V. Balzani, Angew. Chem., Int. Ed., 2006, 46, 52.
6. R. van de Krol, Y. Q. Liang and J. Schoonman, J. Mater. Chem., 2008, 18, 2311.
7. Y. Zhang, Z. Schnepp, J. Cao, S. Ouyang, Y. Li, J. Ye and S. Liu, Sci. Rep., 2013, 3, 2163.
8. W. F. Pickard, A. Q. Shen and N. J. Hansing, Renewable Sustainable Energy Rev., 2009, 13, 1934.
9. V. Bansal, A. P. O'Mullane and S. K. Bhargava, Electrochem. Commun., 2009, 11, 1639.
10. M. Trueba, S. P. Trasatti and S. Trasatti, Mater. Chem. Phys., 2006, 98, 165.
11. R. Subbaraman, D. Tripkovic, D. Strmcnik, K. C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic and N. M. Markovic, Science, 2011, 334, 1256.
12. J. B. Cheng, H. M. Zhang, H. P. Ma, H. X. Zhong and Y. Zou, Electrochim. Acta, 2010, 55, 1855.
13. A. Doner, F. Tezcan and G. Kardas, Int. J. Hydrogen Energy, 2013, 38, 3881.
14. J. Fournier, P. K. Wrona, A. Lasia, R. Lacasse, J. M. Lalancette and H. Menard, J. Electrochem. Soc., 1992, 139, 2372.
15. Platinum Today, Johnson Matthey, London, 2013.
16. E. R. Kotz and S. Stucki, J. Appl. Electrochem., 1987, 17, 1190.
17. C. Iwakura, M. Tanaka, S. Nakamatsu, H. Inoue, M. Matsuoka and N. Furukawa, Electrochim. Acta, 1995, 40, 977.
18. L. Chen, D. Guay, F. H. Pollak and F. Levy, J. Electroanal. Chem., 1997, 429, 185.
19. C. Chabanier, E. Irissou, D. Guay, J. F. Pelletier, F. Sutton and L. B. Lurio, Electrochem. Solid-State Lett., 2002, 5, E40.
20. D. Rochefort, P. Dabo, D. Guay and P. M. A. Sherwood, Electrochim. Acta, 2003, 48, 4245.
21. P. D. Tran, S. Y. Chiam, P. P. Boix, Y. Ren, S. S. Pramana, J. Fize, V. Artero and J. Barber, Energy Environ. Sci., 2013, 6, 2452.
22. H. Vrubel and X. Hu, Angew. Chem., Int. Ed., 2012, 51, 12703.
23. Z. B. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara and T. F. Jaramillo, Nano Lett., 2011, 11, 4168.
24. H. X. Dong, T. Lei, Y. H. He, N. P. Xu, B. Y. Huang and C. T. Liu, Int. J. Hydrogen Energy, 2011, 36, 12112.
25. C. A. Marozzi and A. C. Chialvo, Electrochim. Acta, 2001, 46, 861.
26. D. Rochefort, P. Dabo, D. Guay and P. M. A. Sherwood, Electrochim. Acta, 2003, 48, 4245.
27. N. Kitakatsu, V. Maurice, C. Hinnen and P. Marcus, Surf. Sci., 1998, 407, 36.
28. K. S. Kim and N. Winograd, J. Catal., 1974, 35, 66.
29. K. H. Kim, K. S. Kim, G. P. Kim and S. H. Baeck, Curr. Appl. Phys., 2012, 12, 36.
30. H. C. Shin and M. L. Liu, Chem. Mater., 2004, 16, 5460.
31. Y. Li, Y. Y. Song, C. Yang and X. H. Xia, Electrochem. Commun., 2007, 9, 981.
32. B. J. Plowman, A. P. O'Mullane, P. R. Selvakannan and S. K. Bhargava, Chem. Commun., 2010, 46, 9182.
33. S. Cherevko, N. Kulyk and C. H. Chung, Nanoscale, 2012, 4, 103.
34. S. Cherevko, X. Xing and C. H. Chung, Electrochem. Commun., 2010, 12, 467.
35. X. H. Xia, J. P. Tu, Y. Q. Zhang, Y. J. Mai, X. L. Wang, C. D. Gu and X. B. Zhao, J. Phys. Chem. C, 2011, 115, 22662.
36. L. Vazquez-Gomez, S. Cattarin, P. Guerriero and M. Musiani, Electrochim. Acta, 2008, 53, 8310.
37. J. O. M. Bockris and E. C. Potter, J. Electrochem. Soc., 1952, 99, 169.
38. N. Spataru, J. Appl. Electrochem., 1996, 26, 397.
39. B. Losiewicz, A. Budniok, E. Rowinski, E. Lagiewka and A. Lasia, Int. J. Hydrogen Energy, 2004, 29, 145.
40. E. R. Kotz and S. Stucki, J. Appl. Electrochem., 1987, 17, 1190.
41. N. Krstajic and S. Trasatti, J. Appl. Electrochem., 1998, 28, 1291.
42. L. Vazquez-Gomez, S. Cattarin, P. Guerriero and M. Musiani, Electrochim. Acta, 2007, 52, 8055.
43. L. Vazquez-Gomez, S. Cattarin, P. Guerriero and M. Musiani, J. Electroanal. Chem., 2009, 634, 42.
44. G. H. Yu, C. L. Chai, F. W. Zhu, J. M. Xiao and W. Y. Lai, Appl. Phys., 2001, 78, 1706.
45. N. Danilovic, R. Subbaraman, D. Strmcnik, K. C. Chang, A. P. Paulikas, V. R. Stamenkovic and N. M. Markovic, Angew. Chem., Int. Ed., 2012, 51, 12495.
46. L. Chen, D. Guay, F. H. Pollak and F. Levy, J. Electroanal. Chem., 1997, 429, 185.
47. C. Chabanier, E. Irissou, D. Guay, J. F. Pelletier, F. Sutton and L. B. Lurio, Electrochem. Solid-State Lett., 2002, 5, E40.
48. D. Rochefort, P. Dabo, D. Guay and P. M. A. Sherwood, Electrochim. Acta, 2003, 48, 4245.

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