pubs.acs.org/Langmuir © 2009 American Chemical Society
Electrophoretic Deposition of Manganese Dioxide-Multiwalled Carbon Nanotube Composites for Electrochemical Supercapacitors Yaohui Wang and Igor Zhitomirsky* Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7 Received March 17, 2009. Revised Manuscript Received April 26, 2009 The cathodic electrophoretic deposition (EPD) method has been developed for the deposition of composite manganese dioxide-multiwalled carbon nanotube (MWCNT) films. Dopamine (DA) was shown to be an effective charging additive, which provides stabilization of manganese dioxide nanoparticles and MWCNTs in the suspensions. The influence of DA concentration on the deposition efficiency has been studied. EPD has been utilized for the fabrication of porous nanostructured films for application in electrochemical supercapacitors (ES). Obtained films were studied using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), cyclic voltammetry (CV), and impedance spectroscopy. CV data for the films tested in the 0.5 M Na2SO4 solutions showed capacitive behavior in the voltage window of 0-0.9 V. The highest specific capacitance (SC) of ∼650 F g-1 was obtained at a scan rate of 2 mV s-1. The SC decreased with an increasing scan rate in the range of 2-100 mV s-1. The deposition mechanism, kinetics of deposition, and charge storage properties of the films are discussed.
Introduction Cathodic electrophoretic deposition (EPD) is an important technique for the surface modification of metals and deposition of nanostructured films for electrochemical, electronic, catalytic, and biomedical applications.1-4 There is growing interest in the application of EPD for the fabrication of photonic crystals, protective coatings, and piezoelectric, ferroelectric, and magnetic films.5-9 It is known that EPD is a very useful technique for the deposition of polycrystalline films with controlled crystalline texture, patterned films, multilayer, and functionally graded composites.9-12 EPD is a simple and low-cost technique that can be used for the fabrication of uniform coatings on substrates with complex shapes.13 Similar to other colloidal techniques, EPD is based on the use of stable suspensions, containing well-dispersed colloidal particles. EPD is achieved via motion of charged particles in suspensions under an applied electric field, followed by particle coagulation and deposition at the electrode surface. Bath compositions for EPD usually include various additives, which *To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (905) 528-9295. Phone: (905) 525-9140. (1) Affoune, A. M.; Prasad, B. L. V.; Sato, H.; Enoki, T. Langmuir 2001, 17, 547–551. (2) Cao, G. J. Phys. Chem. B 2004, 108, 19921–19931. (3) Holgado, M.; Garcia-Santamaria, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Miguez, H.; Serna, C. J.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lopez, C. Langmuir 1999, 15, 4701–4704. (4) Boccaccini, A. R.; Kaya, C.; Chawla, K. K. Composites-Part A: Applied Science and Manufacturing 2001, 32, 997–1006. (5) Kira, A.; Umeyama, T.; Matano, Y.; Yoshida, K.; Isoda, S.; Isosomppi, M.; Tkachenkod, N. V.; Lemmetyinen, H.; Imahori, H. Langmuir 2006, 22, 5497–5503. (6) Kim, S.-K.; Lee, H.; Tanaka, H.; Weiss, P. S. Langmuir 2008, 24, 12936– 12942. (7) Neirinck, B.; Fransaer, J.; Van der Biest, O.; Vleugels, J. J. Eur. Ceram. Soc. 2009, 29, 833–836. (8) Van Der Biest, O.; Joos, E.; Vleugels, J.; Baufeld, B. J. Mater. Sci. 2006, 41, 8086–8092. (9) Boccaccini, A. R.; Zhitomirsky, I. Curr. Opin. Solid State Mater. Sci. 2002, 6, 251–260. (10) Besra, L.; Liu, M. Prog. Mater. Sci. 2007, 52, 1–61. (11) Corni, I.; Ryan, M. P.; Boccaccini, A. R. J. Eur. Ceram. Soc. 2008, 28, 1353–1367. (12) Kaya, C. Ceram. Int. 2008, 34, 1843–1847. (13) Sun, F.; Sask, K. N.; Brash, J. L.; Zhitomirsky, I. Colloids Surf., B 2008, 67, 132–139.
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provide effective stabilization and charging of inorganic particles in the suspensions.14-16 The electrostatic repulsion of inorganic particles is important for good dispersion and high electrophoretic mobility of the particles in the bulk of the suspension. However, the repulsion must be reduced at the electrode surface, where particles coagulate to form a deposit. It was shown that the high electric charge of the particles or polymer macromolecules does not necessarily allow formation of deposits by EPD.16 Recent studies highlighted the importance of pH change at the electrode surfaces and demonstrated the possibility of EPD using pH-dependent additives.16 Several investigations were focused on polymers with amine groups.17-19 In this approach, protonated polymers provided effective charging of the ceramic particles for cathodic EPD and precipitated as electrically neutral polymers in the high-pH region at the cathode surface. A new wave of interest in the development of processing additives for EPD is related to the application of this method in nanotechnology. EPD of nanoparticles with high surface areas requires the use of efficient additives. Such additives must be adsorbed on the particle surface and provide effective charging and dispersion of nanoparticles in suspensions and particle coagulation at the electrode surface. Dopamine (DA) is an attractive charging additive for application in EPD. As a member of the catecholamine family,20-25 (14) Ferrari, B.; Gonzalez, S.; Moreno, R.; Baudin, C. J. Eur. Ceram. Soc. 2006, 26, 27–36. (15) Ferrari, B.; Moreno, R. J. Eur. Ceram. Soc. 1997, 17, 549–556. (16) Zhitomirsky, I. Adv. Colloid Interface Sci. 2002, 97, 277–315. (17) Zhitomirsky, I. J. Mater. Sci. 2006, 41, 8186–8195. (18) Pang, X.; Zhitomirsky, I. Surf. Coat. Technol. 2008, 202, 3815–3821. (19) Grandfield, K.; Zhitomirsky, I. Mater. Charact. 2008, 59, 61–67. (20) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1995, 11, 4193– 4195. (21) Huang, W.; Jiang, P.; Wei, C.; Zhuang, D.; Shi, J. J. Mater. Res. 2008, 23, 1946–1952. (22) Hyon, M. S.; Ye, P. D.; Ivanisevic, A. Langmuir 2007, 23, 9472–9480. (23) Niederberger, M.; Garnweitner, G.; Krumeich, F.; Nesper, R.; Colfen, H.; Antonietti, M. Chem. Mater. 2004, 16, 1202–1208. (24) Rajh, T.; Chen, L. X.; Lukas, K.; Liu, T.; Thurnauer, M. C.; Tiede, D. M. J. Phys. Chem. B 2002, 106, 10543–10552. (25) Xu, C.; Xu, K.; Gu, H.; Zheng, R.; Liu, H.; Zhang, X.; Guo, Z.; Xu, B. J. Am. Chem. Soc. 2004, 126, 9938–9939.
Published on Web 05/18/2009
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DA is a strong surface complexant, which bonds strongly to oxide surfaces. Dopamine and other aromatic compounds are known to interact strongly with sidewalls of CNTs through π-π stacking.26-28 Such π-π interactions enabled dispersion of CNTs in solutions containing aromatic surfactants and polymers. The protonated DA’s terminal amino group can provide a pH-dependent charge and good dispersion of oxide nanoparticles and CNT for cathodic EPD. The results presented below indicate that DA can be used for cathodic EPD of manganese dioxide nanoparticles and manganese dioxide-multiwalled carbon nanotubes (MWCNTs). The goal of our investigation was the application of EPD for the fabrication of manganese dioxide-MWCNT composite films for electrodes of electrochemical supercapacitors (ES). We present experimental data on EPD, microstructure, and capacitive behavior of the composite films.
Experimental Procedures Materials and Film Deposition. Dopamine hydrochloride and KMnO4 were purchased from Aldrich Co. Manganese dioxide nanoparticles with an average particle size of 30 nm and an oxidation state of Mn of 3.6 were prepared by the reduction of aqueous KMnO4 solutions using the method described in a previous investigation.29 MWCNTs were supplied by Arkema Co. Electrophoretic deposits were obtained from suspensions of manganese dioxide and MWCNTs at constant voltages of 10-50 V on stainless steel foils. The distance between the stainless steel substrates and platinum counterelectrodes was 15 mm. The deposition time was varied in the range of 1-10 min. Characterization. TGA and DTA investigations were conducted in air at a heating rate of 5 °C min-1 using a thermoanalyzer (Netzsch STA-409). FTIR studies were performed using a Bio-Rad FTS-40 instrument. The DA adsorption on manganese dioxide was monitored using a quartz crystal microbalance (QCM 922, Princeton Applied Research) controlled by a computer. The mass Δm of adsorbed DA was calculated using Sauerbrey’s equation: 2F0 2 -ΔF ¼ pffiffiffiffiffiffiffiffiffiffi Δm ð1Þ A F q μq where ΔF is the frequency decrease of the QCM, F0 is the parent frequency of QCM (9 MHz), A is the area of the gold electrode (0.2 cm2), Fq is the density of the quartz, and μq is the shear modulus of quartz. The microstructures of the deposited films were investigated using a JEOL JSM-7000F scanning electron microscope. Capacitive behavior of the deposited films was studied using a potentiostat (PARSTAT 2273, Princeton Applied Research) controlled by a computer using PowerSuite electrochemical software. Electrochemical studies were performed using a standard threeelectrode cell containing a 0.5 M Na2SO4 aqueous solution, degassed with purified nitrogen gas. The surface area of the working electrodes was 1 cm2. The counter electrode was a platinum gauze, and the reference electrode was a standard calomel electrode (SCE). CV studies were performed within a potential range of 0-0.9 V versus the SCE at scan rates of 2-100 mV s-1. The SC was calculated using half the integrated area of the CV curve to obtain the charge (Q) and subsequently dividing the charge by the mass of the film (m) and the width of the potential window (ΔV): C ¼ Q=ðmΔVÞ
ð2Þ
(26) Lin, D.; Xing, B. Environ. Sci. Technol. 2008, 42, 7254–7259. (27) Woods, L. M.; Badescu, S. C.; Reinecke, T. L. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 155415-9. (28) Star, A.; Han, T.-R.; Gabriel, J.-C. P.; Bradley, K.; Gruner, G. Nano Lett. 2003, 3, 1421–1423. (29) Cheong, M.; Zhitomirsky, I. Surf. Eng. 2009, (in press).
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Figure 1. Film mass vs DA concentration in the 4 g L-1 manganese dioxide suspension at the deposition voltage of 20 V and a deposition time of (a) 1 or (b) 2 min.
Impedance spectroscopy investigations were performed in the frequency range of 0.1 Hz to 100 kHz at a voltage of 5 mV. Simulations of the impedance behavior were performed on the basis of the equivalent-circuit models using ZsimpWin version 3.10.
Results and Discussion The suspensions of manganese dioxide in ethanol were unstable and showed relatively fast sedimentation when ultrasonic treatment was interrupted. The cathodic deposits obtained from such suspensions were highly agglomerated and nonuniform. In contrast, relatively uniform deposits were obtained from well-dispersed and stable suspensions of manganese dioxide containing DA as a dispersant. Figure 1 shows typical dependences of the deposition yield versus DA concentration. The deposition rate increased with an increase in DA concentration (Figure 1). The mass of the deposited materials increased with an increase in deposition time, indicating the formation of films with different thicknesses (Figure 2). It is known that colloidal particles in suspensions exhibit a charge, which can be modified by the use of additives.16 The formation of cathodic deposits indicated that manganese dioxide particles were positively charged in ethanol suspensions. It is in this regard that alcohols behave as proton donors.30 Pure alcohols can ionize in the following way: RCH2 OH þ RCH2 OH f RCH2 O - þ RCH2 OH2 þ
ð3Þ
Damodaran and Moudgil30 have proposed a mechanism of particle charging in which the adsorbed alcohol ionized into a protonated alcohol and an alkoxide ion, followed by the dissociation of the protonated alcohol. The dissociated alcohol and alkoxide ion desorbed into the solution, leaving a proton on the particle surface. This resulted in the formation of positively charged particles in the suspensions. It is suggested that the addition of dopamine hydrochloride to the suspensions resulted in the adsorption of the protonated DA on the manganese dioxide surface. The adsorbed protonated DA (Figure 3) improved particle stability and increased particle charge. The deprotonation of the amino groups of DA in the high-pH region at the electrode surface16,31 promoted deposit formation. As a result, the increase in the deposition yield was observed with an increase in DA concentration (Figure 1). The adsorption of DA has been investigated using FTIR and QCM methods. Figure 4 shows FTIR spectra for as-prepared (30) Damodaran, R.; Moudgil, B. M. Colloids Surf., A 1993, 80, 191–195. (31) De, D.; Nicholson, P. S. J. Am. Ceram. Soc. 1999, 82, 3031–3036.
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Figure 2. Film mass vs deposition time for (a) 4 and (b) 9 g L
manganese dioxide suspensions containing 15 mg L-1 dopamine at a deposition voltage of 20 V.
Figure 4. FTIR spectra for (a) as-prepared manganese dioxide, (b) manganese dioxide deposited from the 4 g L-1 manganese dioxide suspension, containing 26 mg L-1 DA, and (c) as-received dopamine hydrochloride.
Figure 5. QCM data for the mass of adsorbed DA on the 1 μg manganese dioxide film (film area of 0.2 cm2) from the 100 mg L-1 DA solution vs time. Figure 3. Adsorption of protonated DA on manganese dioxide.
manganese dioxide, a deposit obtained from the manganese dioxide suspension containing DA and as-received dopamine hydrochloride. The FTIR spectrum of as-prepared manganese dioxide exhibited a very broad peak centered at 3400 cm-1 associated with the stretching vibration of OH groups of adsorbed water molecules.32,33 The bands at 1625, 1538, and 1412 cm-1 represented the vibrations related to interactions of Mn with OH and other surface groups.32,34,35 The broad peak below 750 cm-1 can be attributed to the vibrations of the Mn-O bonds.32-36 After the surface modification with DA, a new band at 1088 cm-1 appeared, which can be attributed to the aryl-oxygen stretching vibrations.20,37 However, the bending vibrations of OH groups of DA21,24 at 1365 cm-1 were not observed in the FTIR spectrum of manganese dioxide modified with DA. This is in good agreement with the proposed mechanism of adsorption of DA on oxide particles, which involves chelation of surface metal ions.21,38 QCM studies of DA adsorption were performed using a manganese dioxide thin film with a mass of 1 μg on a gold electrode of a quartz crystal. The film was deposited by EPD from a manganese dioxide suspension without DA. Figure 5 shows the (32) Yang, R.; Wang, Z.; Dai, L.; Chen, L. Mater. Chem. Phys. 2005, 93, 149– 153. (33) Wang, H.-e.; Qian, D.; Lu, Z.; Li, Y.; Cheng, R.; Li, Y. J. Phys. Chem. Solids 2007, 68, 1422–1427. (34) Ananth, M. V.; Pethkar, S.; Dakshinamurthi, K. J. Power Sources 1998, 75, 278–282. (35) Ni, J.; Lu, W.; Zhang, L.; Yue, B.; Shang, X.; Lv, Y. J. Phys. Chem. C 2009, 113, 54–60. (36) Eren, E.; Afsin, B.; Onal, Y. J. Hazard. Mater. 2009, 161, 677–685. (37) Wang, G.-L.; Xu, J.-J.; Chen, H.-Y. Biosens. Bioelectron. 2009, (in press). (38) Shultz, M. D.; Reveles, J. U.; Khanna, S. N.; Carpenter, E. E. J. Am. Chem. Soc. 2007, 129, 2482–2487.
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mass gain of the electrode after the injection of 100 mg L-1 DA into the ethanol solution. The increase in the electrode mass indicated adsorption of DA on manganese dioxide in agreement with FTIR data. DA has been also used for the stabilization of MWCNT suspensions. As pointed out above, the stabilization mechanism, discussed in the literature, involved π-π interactions of DA and MWCNTs. EPD from the suspensions containing MWCNTs and DA resulted in the deposition of MWCNTs on a cathode. Moreover, codeposition of manganese dioxide and MWCNTs was achieved using DA as a common charging and stabilizing additive. Obtained deposits were studied using TGA and DTA (Figure 6). The deposit prepared from the suspension without MWCNTs showed mass loss, which can be mainly attributed to dehydration. A small mass gain at ∼460 °C and a corresponding small exotherm in the DTA data could be attributed to oxidation of nonstoichiometric manganese dioxide. The total mass loss at 900 °C was found to be 15 mass %. The deposit, prepared from the suspension containing MWCNTs, showed an additional step in the mass loss in the range of 450-500 °C and an exotherm in the corresponding DTA data at ∼450 °C. The total mass loss at 900 °C was found to be 33 mass %. The additional mass loss can be attributed to burning out of MWCNTs and indicated the formation of composite manganese dioxide-MWCNT deposits. SEM studies of the films prepared from the manganese dioxide suspensions containing DA showed that film thickness can be varied in the range of 0-5 μm; however, cracking was observed when the film thickness exceeded ∼2 μm. The film cracking can be attributed to drying shrinkage. It is suggested that the deposition of thicker films requires the use of binders,16 which can prevent Langmuir 2009, 25(17), 9684–9689
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Figure 6. TGA (a and b) and DTA (c and d) data for deposits obtained from (a and c) the 4 g L-1 manganese dioxide suspension containing 8 mg L-1 DA and (b and d) the 9 g L-1 manganese dioxide suspension containing 15 mg L-1 DA and 0.6 g L-1 MWCNTs, with a deposition voltage of 20 V.
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Figure 8. CVs at a scan rate of 5 mV s-1 for the films prepared from (a) the 4 g L-1 manganese dioxide suspension containing 8 mg L-1 DA and (b) the 9 g L-1 manganese dioxide suspension containing 15 mg L-1 DA and 0.6 g L-1 MWCNTs, with a deposition voltage of 20 V and a film mass of 0.17 mg cm-2.
Figure 9. SC vs scan rate for films prepared from (a) the 4 g L-1
Figure 7. SEM image of the film prepared from the 9 g L-1
manganese dioxide suspension containing 15 mg L-1 DA and 0.6 g L-1 MWCNTs, with a deposition voltage of 20 V.
film cracking. The films with similar thicknesses prepared from manganese dioxide suspensions containing MWCNTs and DA showed reduced the level of cracking and increased porosity. SEM investigations (Figure 7) of such films indicated that MWCNTs with a high aspect ratio can provide microstructure reinforcement and reduce the level of cracking. It is also suggested that the increased porosity of the films containing MWCNTs can result in a reduced level of cracking due to the crack-tip blunting mechanism.39 SEM studies also showed that the incorporation of MWCNTs into the manganese dioxide film resulted in the reduced agglomeration of the manganese dioxide particles (Figure 7). The incorporation of MWCNTs into manganese dioxide films and microstructure changes resulted in improved electrochemical performance of the composite films. Figure 8 shows CV data for the films. The CVs presented in Figure 8 indicated capacitive behavior with a larger area of the CV for the film containing MWCNTs compared to the film of the same mass without MWCNTs. The difference can be attributed to the higher SC of the film containing MWCNTs. The SCs calculated from the CV data at different scan rates are shown in Figure 9. The manganese dioxide film showed a SC of 340 F g-1 at a scan rate of 2 mV s-1. The SC decreased with an increase in scan rate in the range of 2-100 mV s-1 (Figure 9). The manganese dioxide film containing (39) Deng, Z.-Y.; She, J.; Inagaki, Y.; Yang, J.-F.; Ohji, T.; Tanaka, Y. J. Eur. Ceram. Soc. 2004, 24, 2055–2059.
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manganese dioxide suspension containing 8 mg L-1 DA and (b) the 9 g L-1 manganese dioxide suspension containing 15 mg L-1 DA and 0.6 g L-1 MWCNTs, with a deposition voltage of 20 V and a film mass of 0.17 mg cm-2.
MWCNTs exhibited a higher SC in the same scan rate range with the highest SC being 650 F g-1 at 2 mV s-1. The charging mechanism of manganese dioxide is described by the following reaction:40,41 MnO2 þ A þ þ e - T MnO2 A
ð4Þ
where A+ is Li+, Na+, K+, or H+. Equation 4 indicates that high ionic and electronic conductivity of the active material is necessary for utilization of a high SC. A complicating factor in the application of manganese dioxide for ES is the low electronic and ionic conductivity of this material. Thin manganese dioxide films (∼1 μg cm-2) exhibited ideal capacitive behavior42 in a voltage window of 0-0.9 V and showed a SC of ∼700 F g-1. Previous investigations43 showed that the capacitance decreased from 400 to 177 F g-1 when the film mass increased from 50 to 200 μg cm-2. Obtained SC values are much lower than the theoretical SC44 of 1370 F g-1. The development of ES requires the use of porous active materials with a small particle size and a high surface area. The small (40) Athouel, L.; Moser, F.; Dugas, R.; Crosnier, O.; Belanger, D.; Brousse, T. J. Phys. Chem. C 2008, 112, 7270–7277. (41) Khomenko, V.; Raymundo-Pi~nero, E.; Beguin, F. J. Power Sources 2006, 153, 183–190. (42) Pang, S.-C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444–450. (43) Nagarajan, N.; Cheong, M.; Zhitomirsky, I. Mater. Chem. Phys. 2007, 103, 47–53. (44) Devaraj, S.; Munichandraiah, N. Electrochem. Solid-State Lett. 2005, 8, A373–A377.
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Figure 10. Nyquist plot of the complex impedance Z* = Z0 - iZ00
for the film, prepared from the 9 g L-1 manganese dioxide suspension containing 15 mg L-1 DA and 0.6 g L-1 MWCNTs, with a deposition voltage of 20 V, a film mass of 0.17 mg cm-2, and an electrode area of 1 cm2.
size and high surface area of manganese dioxide nanoparticles and film porosity are important for the access of the ion (A+) to the active material. The incorporation of MWCNTs into the manganese dioxide matrix45,46 can improve the electronic conductivity of the composite materials. Porous microstructure and reduced agglomeration of particles were beneficial for the capacitive behavior of the composite manganese dioxide-MWCNT films prepared by EPD using DA as a dispersant and charging agent. Figure 10 shows complex impedance data of the composite manganese dioxide-MWCNT film. The equivalent circuit of ES was discussed in several investigations47-50 and included RC transmission line,47 describing the porous electrode, and Warburg impedance W,49,51 representing the diffusion resistance of electrolyte inside the pores. Cn elements represent double-layer capacitance and pseudocapacitance, whereas Rn elements represent electrolyte resistance in pores, Faradaic resistance, and equivalent series resistance of the electrodes. Conway and Pell47 described the impedance of the porous electrode using a five-element (n = 5) circuit. A constant phase element (CPE) Q, rather than a pure capacitance C, was used in another investigation.52 The CPE element describes a “leaking” capacitor with microscopic roughness of the surface and capacitance dispersion of interfacial origin.52 Solution resistance RS is usually combined in series with the RC transmission line.53 The equivalent circuits should allow an optimum representation of the measured spectra with a minimum set of model parameters. Good agreement of simulated and measured voltage data (Figure 10) was found for the equivalent circuit containing a transmission line with three RC (or RQ) elements. The results of our investigation showed that cathodic EPD can be used for the fabrication of porous films of nanostructured manganese dioxide for application in ES. Recent studies54 highlighted the advantages of cathodic electrodeposition methods for (45) Ma, S.-B.; Nam, K.-W.; Yoon, W.-S.; Yang, X.-Q.; Ahn, K.-Y.; Oh, K.-H.; Kim, K.-B. Electrochem. Commun. 2007, 9, 2807–2811. (46) Ma, S.-B.; Nam, K.-W.; Yoon, W.-S.; Yang, X.-Q.; Ahn, K.-Y.; Oh, K.-H.; Kim, K.-B. J. Power Sources 2008, 178, 483–489. (47) Conway, B. E.; Pell, W. G. J. Power Sources 2002, 105, 169–81. (48) Huai, Y.; Hu, X.; Lin, Z.; Deng, Z.; Suo, J. Mater. Chem. Phys. 2009, 113, 962–966. (49) Ko¨tz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483–2498. (50) Rafik, F.; Gualous, H.; Gallay, R.; Crausaz, A.; Berthon, A. J. Power Sources 2007, 165, 928–934. (51) Sawai, K.; Ohzuku, T. J. Electrochem. Soc. 1997, 144, 988–995. (52) Pell, W. G.; Zolfaghari, A.; Conway, B. E. J. Electroanal. Chem. 2002, 532, 13–23. (53) Zhang, G.-Q.; Zhao, Y.-Q.; Tao, F.; Li, H.-L. J. Power Sources 2006, 161, 723–729. (54) Liu, D.; Zhang, Q.; Xiao, P.; Garcia, B. B.; Guo, Q.; Champion, R.; Cao, G. Chem. Mater. 2008, 20, 1376–1380.
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the fabrication of manganese dioxide for energy storage devices. The cathodic electrosynthesis from solutions of manganese acetate54 enabled the formation of manganese dioxide nanowall structures. The intercalation of Li ions in such porous structure resulted in high capacitance and high energy density. In our investigation, nanoparticles of manganese dioxide were used for EPD from colloidal suspensions instead of Mn2+ ions utilized for the film deposition in the cathodic electrosynthesis method.54 EPD offers the advantage of a higher deposition rate and the possibility of the deposition of doped films with a controlled composition.55 It is known that the fabrication of stable suspensions of manganese dioxide nanoparticles with a particle concentration of >1 mM presents difficulties.56 The dilute suspensions with a concentration of
