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Chem. Mater. 2004, 16, 3184-3190
Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor Mathieu Toupin,† Thierry Brousse,*,†,‡ and Daniel Be´langer*,† De´ partement de Chimie, Universite´ du Que´ bec a` Montre´ al, Case Postale 8888, succursale Centre-Ville, Montre´ al, Que´ bec H3C 3P8, Canada, and Laboratoire de Ge´ nie des Mate´ riaux, Ecole Polytechnique de l’Universite´ de Nantes, La Chantrerie, rue Christian Pauc, BP50609, 44306 Nantes Cedex 3, France Received March 2, 2004. Revised Manuscript Received June 2, 2004
The charge storage mechanism in MnO2 electrode, used in aqueous electrolyte, was investigated by cyclic voltammetry and X-ray photoelectron spectroscopy. Thin MnO2 films deposited on a platinum substrate and thick MnO2 composite electrodes were used. First, the cyclic voltammetry data established that only a thin layer of MnO2 is involved in the redox process and electrochemically active. Second, the X-ray photoelectron spectroscopy data revealed that the manganese oxidation state was varying from III to IV for the reduced and oxidized forms of thin film electrodes, respectively, during the charge/discharge process. The X-ray photoelectron spectroscopy data also show that Na+ cations from the electrolyte were involved in the charge storage process of MnO2 thin film electrodes. However, the Na/ Mn ratio for the reduced electrode was much lower than what was anticipated for charge compensation dominated by Na+, thus suggesting the involvement of protons in the pseudofaradaic mechanism. An important finding of this work is that, unlike thin film electrodes, no change of the manganese oxidation state was detected for a thicker composite electrode because only a very thin layer is involved in the charge storage process.
Introduction Electrochemical supercapacitors are currently investigated in various academic and industrial laboratories because they can be used as complementary charge storage devices to conventional batteries in various applications that require peak power pulses.1,2 In these electrochemical supercapacitors, the energy being stored is either capacitive or pseudocapacitive in nature. The capacitive or nonfaradaic process is based on charge separation at the electrode/solution interface, whereas the pseudocapacitive process consists of faradaic redox reactions that occur within the active electrode materials. The most widely used active electrode materials are carbon,3,4 conducting polymers,5,6 and both noble7-9 and transition-metal oxides.10-29 The main motivation for the use of transition-metal oxides lies in their low cost compared to noble metal oxides such as ruthenium7,8 and iridium9 oxides. The initial studies were performed on nickel oxide10 and cobalt oxide11 but more recently iron12-15 and manganese oxides14,16-29 were investigated. The research efforts focused on compounds providing high cyclability and capacitance. On the other hand, the charge storage mechanism of MnO2 has not been investigated in detail. Until now, two mechanisms were proposed to explain * To whom correspondence should be addressed. E-mail:
[email protected] (T.B.) and belanger.daniel@ uqam.ca (D.B.). † Universite ´ du Que´bec a` Montre´al. ‡ Ecole Polytechnique de l’Universite ´ de Nantes. (1) Conway, B. E. Electrochemical Supercapacitors, Scientific Fundamentals and Technological applications; Kluwer Academic/Plenum Press: New York, 1999. (2) Burke, A. J. Power Sources 2000, 91, 37.
the MnO2 charge storage behavior. The first one implies the intercalation of protons (H+) or alkali metal cations (C+) such as Li+ in the bulk of the material upon reduction followed by deintercalation upon oxidation.17
MnO2 + H+ + e- S MnOOH
(1)
MnO2 + C+ + e- S MnOOC
(2)
or
The second mechanism is based on the surface adsorption of electrolyte cations (C+) on MnO216
(MnO2)surface + C+ + e- S (MnO2-C+)surface
(3)
where C+ ) Na+, K+, Li+. This mechanism was proposed following the observation of significant difference of the cyclic voltammogram and the capacitance of MnO2 in the presence of various metal alkali cations in the electrolyte.16 It should be noticed that both proposed (3) Frackowiak, E.; Be´guin, F. Carbon 2001, 39, 937. (4) Lin, C.; Popov, B. N.; Ploehn, H. J. J. Electrochem. Soc. 2002, 149, A167. (5) Villers, D.; Jobin, D.; Soucy, C.; Cossement, D.; Chahine, R.; Breau, L.; Be´langer, D. J. Electrochem. Soc. 2003, 150, A747. (6) Fusalba, F.; El Mehdi, N.; Breau, L.; Be´langer, D. Chem. Mater. 2000, 12, 2581. (7) Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6. (8) Soudan, P.; Gaudet, J.; Guay, D.; Be´langer, D.; Schulz, R. Chem. Mater. 2002, 14, 1210. (9) Conway, B. E.; Birss, V.; Wojtowicz, J. J. Power Sources 1997, 66, 1. (10) Nelson, P. A.; Owen, J. R. J. Electrochem. Soc. 2003, 150, A1313.
10.1021/cm049649j CCC: $27.50 © 2004 American Chemical Society Published on Web 07/16/2004
Charge Storage Mechanism of MnO2 Electrode
mechanisms involved a redox reaction between the III and IV oxidation states of Mn. The mechanism based on the solid-state diffusion of protons in the bulk of the material is similar to that proposed for RuO2.7 However, only a limited fraction of the MnO2 composite is electrochemically active, thus suggesting that the protonic diffusion in the bulk of the MnO2 compound might not be as fast as in the case of RuO2.21 Subsequently, the charge storage might only involved the surface atoms of the MnO2 crystallites or a very thin layer. Then it might be plausible to assume that ions from the electrolyte would participate in the charge compensation process. On the other hand, the reported capacitance ranging between 150 and 200 F/g for composite electrode cannot be only associated to the formation of the classical double layer.21 Hence, the nature of the charge storage mechanism must be pseudocapacitive. This work aimed at getting a better understanding of the charge storage mechanism in manganese dioxide electrodes when cycled in aqueous electrolyte. The electrodes were characterized by cyclic voltammetry and X-ray photoelectron spectroscopy in order to determine a change of the manganese valence upon charge/ discharge. Additionally, the experimental results were used to determine whether the charge storage process was limited to the surface of the oxide or if it occurred inside the bulk of the material. Experimental Section Preparation of the MnO2 Powder. The MnO2 powder was synthesized by coprecipitation.21 Briefly, KMnO4 and MnSO4‚H2O were mixed in a 2:3 molar ratio, leading to a dark brown precipitate. The amorphous nature of the as-synthesized MnO2 powder was confirmed by the X-ray diffraction (XRD) spectrum (Figure SI1), which shows broad peaks related to a poorly crystallized compound. From chemical analysis, the stoichiometry for the powder was determined to be K0.02MnO2H0.33, 0.53H2O. Thereafter, the compound will be named “MnO2” despite that it does not reflect the exact composition of the sample. (11) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1998, 145, 4097. (12) Wu, N.-L.; Wang, S.-Y.; Han, C.-Y.; Wu, D.-S.; Shiue, L.-R. J. Power Sources 2003, 113, 173. (13) Wu, N. L. Mater. Chem. Phys. 2002, 75, 6. (14) Brousse, T.; Be´langer, D. Electrochem. Solid-State Lett. 2003, 6, A244. (15) Brousse, T.; Delahaye, T.; Be´langer, D. In preparation. (16) Lee, H. Y.; Goodenough, J. B. J. Solid State Chem. 1999, 144, 220. See also a more detailed version of this study in the following: Lee, H. Y.; Manivannan, V.; Goodenough, J. B. C. R. Acad. Sci. Paris 1999, t. 2, Se´ rie II c, 565. (17) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444. (18) Lee, H. Y.; Kim, S. W.; Lee, H. Y. Electrochem. Solid-State Lett. 2001, 4, A19. (19) Hu, C. C.; Tsou, T. W. Electrochem. Comm. 2002, 4, 105. (20) Chin, S. F.; Pang, S. C.; Anderson, M. A. J. Electrochem. Soc. 2002, 149, A379. (21) Toupin, M.; Brousse, T.; Be´langer, D. Chem. Mater. 2002, 14, 3946. (22) Brousse, T.; Toupin, M.; Be´langer, D. J. Electrochem. Soc. 2004, 151, A614. (23) Jiang, J.; Kucernak, A. Electrochim. Acta 2002, 47, 2381. (24) Hu, C. C.; Tsou, T. W. Electrochim. Acta 2002, 47, 3523. (25) Jeong, Y. U.; Manthiram, A. J. Electrochem. Soc. 2002, 149, A1419. (26) Broughton, J. N.; Brett, M. J. Electrochem. Solid-State Lett. 2002, 5, A279. (27) Kim, H.; Popov, B. N. J. Electrochem. Soc. 2003, 150, D56. (28) Hu, C.-C.; Wang, C.-C. J. Electrochem. Soc. 2003, 150, A1079. (29) Chang, J.-K.; Tsai, W.-T. J. Electrochem. Soc. 2003, 150, A1333.
Chem. Mater., Vol. 16, No. 16, 2004 3185 Scanning electron micrographs revealed that the assynthesized R-MnO2 powder is made of spherical grains (Figure SI2). The length scale is systematically indicated as a white bar on the bottom left corner of the micrographs. A statistical analysis of the grain diameter performed over more than 100 particles yielded a Gaussian distribution centered at 420 nm with a standard deviation of 190 nm. Each grain seems to result from the agglomeration of smaller particles (Figure SI3). Using the geometric surface of spherical grains (420 nm diameter assuming a density of 4.8 g/cm3) the specific surface was estimated to a value close to 3 m2/g. The specific surface area determined from BET measurements (160 ( 3 m2/g) is larger than this value thus indicating that pores and voids exist inside the grains examined by scanning electron microscopy. Preparation of the Electrodes. To investigate the influence of both the thickness and the composition of the electrodes, thick film (≈100 µm) and thin film (
