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Electrochimica Acta 56 (2011) 6696–6701
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Fibrous MnO2 electrode electrodeposited on carbon fiber for a fuel cell/battery system Bokkyu Choi, Sunmook Lee, Chihiro Fushimi, Atsushi Tsutsumi ∗ Collaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan
a r t i c l e
i n f o
Article history: Received 9 February 2011 Received in revised form 16 May 2011 Accepted 17 May 2011 Available online 26 May 2011 Keywords: Electrodeposition Manganese dioxide Carbon fiber Fuel cell/battery system Cathode
a b s t r a c t A novel fibrous MnO2 electrode for a fuel cell/battery system is fabricated on carbon fiber by the electrodeposition method. The characteristics of the fibrous MnO2 electrode are examined by electrochemical impedance spectra, galvanostatic performance and cyclic voltammetry. The experimental results indicate that the fibrous MnO2 electrodes are superior to pasted electrodes because of the following: (i) better contact between MnO2 and the electrical conducting material; (ii) high charge-transfer rate because of a smaller diameter than conventional electrodeposited MnO2 particles (thus it is expected that the specific surface area would be higher); and (iii) a low overpotential. The morphology and the crystal structure of the fibrous MnO2 electrode are investigated by scanning electron microscopy and X-ray diffraction, respectively. The entire surface of the carbon fiber is found to be coated with ␥-MnO2 after 2 h of electrodeposition at 0.01 A dm−2 current density. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Fuel cells are a promising power generation system with high power generation efficiency (40–60%) [1] and high energy density (800–10,000 Wh kg−1 ) [2]. However, the fuel cell reactions are restricted to a solid (electrode)/liquid (electrolyte)/gas (cathodic and anodic reactants) three-phase boundary, which leads to a low power density (approximately 500 W kg−1 ) [2,3]. A large number of fuel cell stacks have been assembled to obtain sufficient power output for use in automobiles (50–75 kW) and with distributed power (200–300 kW) [4,5]. Moreover, expensive catalysts such as platinum are used in the electrodes of the fuel cells to increase the reaction rate. Recently, secondary batteries have also been attracting a lot of attention as electrical power storage systems in conjunction with efficient power generation systems [5,6]. The power density of secondary batteries is higher (∼10,000 W kg−1 ) [3] than that of fuel cells because their reactions occur over a large reaction area at the solid (electrode)/liquid (electrolyte) two-phase boundary. However, the energy density of secondary batteries is relatively low (20–200 Wh kg−1 ) [3] because their capacity is determined by the total amount of the electrode’s active materials [6]. In previous papers, a novel concept for fuel cell/battery (FCB) systems that function as fuel cells and as secondary batteries with high energy and power densities has been proposed [7–11].
∗ Corresponding author. Tel.: +81 3 5452 6727; fax: +81 3 5452 6728. E-mail address: [email protected] (A. Tsutsumi). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.057
Figs. 1 and 2 show the reaction mechanism and the cell configuration of the FCB system, respectively. This system is composed of an anode (metal hydride; MH) [8] and a cathode (manganese dioxide; MnO2 ) [7,11] in an alkaline electrolyte. The system can be recharged by supplying electric power and also by supplying gases such as hydrogen and oxygen to the anode and the cathode at ambient temperatures, respectively. In the FCB system, the fuel cell reaction is split into two reactions, which occur at the two-phase regions. One is a chemical gaseous charge reaction at the solid/gas two-phase boundary. The other is an electrochemical reaction at the solid/liquid two-phase boundary. In the anode of the FCB system, MH can be charged by supplied hydrogen gas at the solid/gas two-phase boundary and the charged MH can be electrochemically discharged at the solid/liquid two-phase boundary [8]. In the cathode of the FCB system, MnOOH, which is formed by the discharge of MnO2 , can be chemically recharged by gaseous oxygen at the solid/gas two-phase boundary (oxygen reduction reaction) and an electrochemical discharge reaction of MnO2 occurs at the solid/liquid two-phase boundary [11]. All the reactions in the FCB system proceed at the solid/liquid or solid/gas two-phase regions leading to large reaction areas. This improves the power generation efficiency as well as the power density compared with a conventional fuel cell, because of the suppression of the overvoltage [11]. However, a relatively large resistance overpotential still exists in the cathode because the MnO2 electrode is poorly constructed [12]. It is, therefore, essential for a FCB system that an advanced electrode structure is developed to obtain better performance. Arnott and Donne have reported that the electrochemical performance can depend significantly on the connectivity between
B. Choi et al. / Electrochimica Acta 56 (2011) 6696–6701
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Fig. 1. Schematic diagram of the reaction mechanism in the fuel cell/battery (FCB) system.
MnO2 and graphite [12]. The reasons are: (1) the discharge reaction of MnO2 is a homogenous process based essentially on concerted proton and electron insertion into the host ␥-MnO2 structure and (2) electrode impedance is dependent on the effective establishment of the MnO2 -graphite-electrolyte three-phase boundary with the proximity between proton (from the electrolyte) and electron (from graphite) insertion determining the impedance [12]. Hence, the practical working potential of the MnO2 electrode can come close to reaching its theoretical value if the connectivity between MnO2 and graphite is improved. In recent research reported by Sakai et al., carbon fibers were used as a current collector in a nickel-metal hydride secondary battery [13,14] and a lithium secondary battery [15,16]. They observed that the long cycle life and high rate capability were achieved because the fibrous electrodes have good connectivity between the active materials and the substrate, which leads to a reduction in the ohmic resistance. In the present paper, a novel fibrous MnO2 electrode for a FCB system was fabricated on carbon fiber by the electrodeposition method, and the characteristics and performance of the fibrous MnO2 electrode in the FCB system were investigated. 2. Experimental 2.1. Preparation of the electrodes Among the methods to synthesize MnO2 , electrodeposition synthesis is most popular and used widely to fabricate electrolytic manganese dioxide (EMD) for use as a cathode material in alkaline manganese dioxide cells [17–23]. EMDs are conventionally fabricated by the following procedure: (i) electrodeposition on an anode substrate in a sulfate electrolyte at around 92–99 ◦ C, (ii) mechanical removal of EMD deposits from the anode, (iii) washing with distilled water and dilute ammonia (or an alkaline solution), and (iv) finally grinding down. The obtained EMD particles are relatively large around 10 m. The cathode for an alkaline battery is then prepared by mixing the EMD powder, graphite (conducting material) and binder materials. In the present study, the electrode fabricating method was modified by directly electrodepositing MnO2 onto the carbon fibers.
Fig. 2. Schematic diagram of the cell configuration in the fuel cell/battery (FCB) system.
Fig. 3 shows a schematic diagram of the electrochemical plating cell. The MnO2 /carbon fiber electrode was prepared as follows: the electrolyte was composed of 0.66 M MnSO4 in 0.34 M H2 SO4 . Carbon fibers (HTA-12, TOHO TENAX Co., Ltd.) were used as anode materials for the electrodeposition of MnO2 . The end of carbon fibers was tied by a copper mesh and connected with a lead wire for an electrical contact. A copper mesh (∅ 0.20 mm × 80 mm × 20 mm, 50 mesh, Nilaco Corp.) and a Hg/Hg2 SO4 electrode were used as the cathode and the reference electrode, respectively. Before the electrodeposition, the anodes were immersed in 0.66 M MnSO4 containing 0.34 M H2 SO4 for 2 h at 85 ◦ C. The MnO2 was then electrodeposited galvanostatically onto the carbon fibers (total surface area: approximately 1 dm2 ) with an approximate current density of 0.01 A dm−2 for 1–4 h. The electrodeposited MnO2 electrodes were thoroughly washed with deionized water and 1 M KOH aqueous solution for neutralization and then dried in an oven at 80 ◦ C overnight. The amount of MnO2 was determined from the difference in the amount of carbon fibers by measuring it before and after the electrodeposition. For comparison, pasted MnO2 electrodes were also fabricated by mixing EMD (∼10 m in diameter, Tosoh Corp.) powder, carbon black (CB; #3050B, 50 nm in average diameter, Mitsubishi Chemical Corp.), and poly(tetrafluoroethylene) (PTFE; Aldrich) in a mass ratio of 10:3:1. These mixtures were then rolled on both sides of nickel foam, which was followed by baking the foam in an oven at 110 ◦ C for 30 min. The foam was then pressed at 6 MPa for 10 min. The total weight of the electrode was approximately 150 mg while the surface area and the thickness of the fabricated electrode were 3.14 cm2 and 0.6 mm, respectively. 2.2. Characterization The electrochemical characterization of the MnO2 /carbon fiber electrodes was undertaken in 6 M KOH aqueous solution. Before
Fig. 3. Schematic of the electrochemical plating cell.
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the electrochemical measurements, the electrodes were immersed in a 6 M KOH aqueous solution at room temperature under reduced pressure (−16.6 kPa) for approximately 12 h to completely wet the electrodes with the electrolyte solution. Electrochemical measurements such as cyclic voltammograms (CV) and galvanostatic performance were carried out with an Analytical 1480 Multistat (Solartron) at 25 ◦ C. A Ag/AgCl electrode and a nickel-foam of 30 mm in diameter were used as a reference and a counter electrode, respectively. CV measurements were carried out under the following conditions: 0.1 mV s−1 scan rate, 5 cycles, potential scan range from −0.5 to 0.4 V. The discharge/charge experiments were conducted at 0.2 C (1 C being the rate of current at which the cell or working electrode is fully charged in 1 h). 5 discharge/charge cycles were carried out. Electrochemical impedance spectra (EIS) were measured using a frequency response analyzer (1255B, Solartron) within a frequency range from 100 kHz to 10 mHz. Scanning electron microscope (SEM) images were recorded (JSM-7000F, JEOL, Ltd.) and the crystal structure of the electrodes was measured with an X-ray diffractometer (XRD miniflex, Rigaku Corp.) at a scan rate of 2◦ min−1 within 2 = 5 − 90◦ . 3. Results and discussion 3.1. MnO2 electrodeposition onto carbon fibers A SEM image of the carbon fibers that were used as a substrate and an electrical conducting material are shown in Fig. 4. The carbon fibers are approximately 6.5 m in diameter and the surface is very clean and smooth. Fig. 5(a)–(d) shows SEM images of the electrodes that were electrodeposited over 1, 2, 3, and 4 h, respectively. For the 1 h electrodeposition (Fig. 5(a)), agglomerates of MnO2 with a size of 2–4 m were observed on the surface of the carbon fibers. When the electrodeposition time was more than 2 h, the surface of
Fig. 4. SEM image of the raw carbon fibers.
the carbon fibers was coated with MnO2 . Longer electrodeposition times led to thicker fibrous electrode diameters of approximately 9, 11, and 14 m for the electrodes electrodeposited over 2, 3, and 4 h, respectively. These results indicate that MnO2 was deposited well on the surface of the carbon fibers and the current density and thickness of the fibrous electrodes can thus be controlled by adjusting the electrodeposition time. Fig. 6 shows the XRD pattern of the fibrous electrode, which was electrodeposited over 4 h. The peaks are present at 2 = 21.9◦ , 34.1◦ , 36.9◦ , 41.9◦ , 55.6◦ and are typical ␥-MnO2 peaks [24]. Thus, it is confirmed that ␥-MnO2 can be generated in the same manner as in a conventional method using a titanium or carbon substrate even if carbon fiber is used as an anode.
Fig. 5. SEM images for MnO2 electrodeposited on the carbon fibers at a current density of 0.01 A dm−2 over (a) 1, (b) 2, (c) 3, and (d) 4 h.
B. Choi et al. / Electrochimica Acta 56 (2011) 6696–6701
Table 1 Open circuit potential (OCP) and charge-transfer resistance (Rct ) of the MnO2 electrodes that were electrodeposited onto carbon fibers (CF) at 0.01 A dm−2 over 1–4 h.
Intensity / a.u.
γ-MnO2
OCP/V Rct /ohm
0
20
40
60
80
6699
100
2θ / degree Fig. 6. X-ray diffraction pattern of MnO2 electrodeposited on carbon fibers at a current density of 0.01 A dm−2 over 4 h.
3.2. Characterization of the fibrous MnO2 electrodes Electrochemical impedance measurements were performed in 6 M KOH aqueous to investigate the charge-transfer resistance of the fibrous MnO2 electrodes and their dependence on the thickness of the MnO2 layer that was electrodeposited on the carbon fibers. Nyquist plots of the electrodes after electrodeposition times of 1, 2, 3, and 4 h are shown in Fig. 7(a). According to the experimental observations, an equivalent electrical circuit is suggested, as shown in Fig. 7(b). Here, Rsol is the solution resistance from the reference electrode to the current collector for the MnO2 electrodes. Rct is the resistance to charge transfer at the electrode–electrolyte interface. Rl , L, Zw , and CPE are the inductance reactance, the inductance, the Warburg impedance, and the constant-phase element, respectively. A semicircle in the high-frequency region and a straight line in the low-frequency region are present in Fig. 7(a). Before the semicircle, an inductance loop is present at 100 kHz to around 50 kHz, which comes from the effects of the stray electric field generated by the cables [25]. In the present research, the semicircles in the
Fig. 7. (a) Nyquist plots of the MnO2 electrodes that were electrodeposited onto carbon fibers at a current density of 0.01 A dm−2 over () 1, () 2, (♦) 3, and () 4 h. (b) The corresponding equivalent circuit.
CFEMD (1 h)
CFEMD (2 h)
CFEMD (3 h)
CFEMD (4 h)
−0.116 1.203
−0.012 0.464
−0.020 0.769
−0.027 0.684
high-frequency region are evaluated for the charge-transfer resistance. By fitting the semicircles, the charge-transfer resistances of the electrodes that were electrodeposited on the carbon fibers over 1, 2, 3, and 4 h are estimated to be 1.203, 0.464, 0.769, and 0.684 , respectively. Table 1 lists the charge-transfer resistance (Rct ) of the MnO2 electrodes that were electrodeposited onto carbon fibers at 0.01 A dm−2 from 1 to 4 h. It has been reported that the Rct of the pasted EMD (Tosoh Corp.) electrode was 0.78 with a working electrode area of 1 cm2 [22]. Although the fibrous electrode that was electrodeposited for 1 h has a relatively high Rct because of the removal of agglomerates from the surface of the carbon fibers, other fibrous electrodes have a lower Rct than that of the conventional pasted electrode. In particular, the Rct of the fibrous electrode that was electrodeposited over 2 h is reduced by 40% compared with that of the pasted electrode. Additionally, the fibrous electrodes have a high open circuit potential (OCP), which corresponds to the EIS results, as listed in Table 1. These results indicate that direct electrodeposition onto the carbon fibers can lead to an improved connectivity between MnO2 and the electrical conduction material, and to a higher charge-transfer rate at the electrode–electrolyte interface. In addition, it is expected that the specific surface area
Fig. 8. (a) CV curves (5th cycle) of the pasted EMD electrode and the MnO2 electrodes that were electrodeposited on carbon fibers (CFEMD) over 1, 2, 3 and 4 h at 0.1 mV s−1 . (b) Normalized CV curves by the amount of active materials.
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of the thin carbon fiber electrode would increase because the surface area is proportional to the charge-transfer rate. However, the fibrous electrodes that were electrodeposited over 3 and 4 h have proximate values with the Rct of the pasted electrode because the fibrous electrodes have a relatively thick MnO2 layer which would interrupt proton and electron insertion into center of MnO2 layer. The difference in Rsol for all the curves is attributed to the variable distances between the electrodes and the references in this experiment. The cyclic voltammetry (CV) and the discharge/charge cycling performance of the fibrous MnO2 electrodes were also investigated. The electrodes were sandwiched between two nickel-foams to prevent the MnO2 agglomerates from exfoliating. Fig. 8 shows the 5th CV curves for each electrode. All the CV curves are typical of the current peaks of the electrodeposited MnO2 . The peak near −0.4 V can be assigned to the reduction reaction of MnO2 and the other two peaks at −0.2 and −0.15 V can be assigned to the oxidation reaction of Mn3+ because of the formation of pyrolusite and ramsdellite, respectively [26]. The current peaks of the fibrous electrodes are smaller than those of the pasted electrode (the amount of MnO2 : 107 mg) because the quantities of MnO2 electrodeposited on the carbon fibers over 1, 2, 3 and 4 h are approximately 8.3, 31.3, 28.2 and 68.4 mg, respectively, which is less than the amount of MnO2 in the pasted electrode. For a direct comparison among the curves, CV curves normalized by the amount of MnO2 are shown in Fig. 8(b). The reduction current peaks of the fibrous electrodes are located at a more positive potential than that of the pasted electrode because of the better connectivity between MnO2 and the
conductive materials in the fibrous electrodes. The potentials of the electrodes are located at a more positive potential when the fibrous electrodes are thinner because a thinner fibrous electrode has a higher charge-transfer rate, as shown in the EIS result. Fig. 9 shows the 3rd discharge curves of the pasted EMD electrode and the fibrous electrodes. In order to remove other influences on the discharge behavior such as oxygen gas which generates when electrodes are overcharged more than their capacity, discharge curves are normalized by setting 0.1 V to zero capacity point as shown in Fig. 9(b). In terms of the discharge performance, the working potentials of the fibrous electrodes that were electrodeposited at less than or equal to 2 h increases at up to 0.1 V. Additionally, the potentials of the fibrous MnO2 electrodes decrease slowly. On the other hand, the working potentials of the fibrous electrodes electrodeposited over 3 and 4 h are similar to that of the pasted electrode at 100–200 mAh g−1 in Fig. 9(a). However, they are smaller than that of the pasted electrode in Fig. 9(b). In addition, the potentials of the fibrous electrodes with a relatively thick MnO2 layer decrease rapidly around 200 mAh g−1 (160 mAh g−1 in Fig. 9(b)) and the amount of discharge reaches approximately 70% (52% in Fig. 9(b)) of the theoretical capacity. These indicate that proton (from the electrolyte) and electron (from the carbon fiber) insertion into MnO2 is interrupted when the diameter of the electrodeposited electrodes are as thick as the MnO2 bulk of the pasted electrode. From the CV and the charge/discharge results, it can be concluded that the working potential and the percentage of reactions of the active materials improves because the thin fibrous MnO2 electrodes reduce the resistance overpotential. 4. Conclusions A novel fibrous MnO2 electrode was fabricated on carbon fiber by the electrodeposition method and its characteristics as a cathode in a fuel cell/battery system were investigated. The morphology and the crystal structure of the fibrous electrodes were confirmed by SEM and XRD measurements. These results indicate that the entire surface of the carbon fiber was covered with MnO2 after 2 h of electrodeposition at a current density of 0.01 A dm−2 and the crystal structure of the MnO2 corresponded to ␥-MnO2 . Electrochemical impedance spectra, galvanostatic performance, and cyclic voltammetry show that the fibrous electrodes possess superior electrochemical characteristics compared to pasted electrodes and these include: excellent connectivity between MnO2 and the electrical conducting material, high charge-transfer rate because of a smaller diameter (thus it is expected that the specific surface area would increase), and a low overpotential. Furthermore, the electrodeposition method allows the thickness of the electrodeposited MnO2 to be controlled. Acknowledgements The authors would like to thank Dr. Tetsuo Sakai at National Institute of Advanced Industrial Science and Technology (AIST) for useful advice. Financial support in the form of a Grant-in-Aid for Scientific Research (B) (number 19360434) and a Grant-in-Aid for JSPS Fellows (number 22·0342) from the Japanese Society for the Promotion of Science (JSPS) is gratefully acknowledged. References
Fig. 9. (a) Discharge curves (3rd cycle) of the pasted EMD electrode and the MnO2 electrodes that were electrodeposited onto carbon fibers (CFEMD) over 1, 2, 3 and 4 h at a 0.2 C discharge rate. (b) Normalized discharge curves by setting 0.1 V to zero capacity point.
[1] A. Kirubakaran, S. Jain, R.K. Nema, Renewable and Sustainable Energy Reviews 13 (2009) 2430. [2] H. Chen, T.N. Cong, W. Yang, C. Tan, Y. Li, Y. Ding, Progress in Natural Science 19 (2009) 291. [3] S. Campanari, G. Manzolini, F.G. Iglesia, Journal of Power Sources 186 (2009) 464. [4] M. Winter, R.J. Brodd, Chemical Reviews 104 (2004) 4245.
B. Choi et al. / Electrochimica Acta 56 (2011) 6696–6701 [5] E.D. Geeter, M. Mangan, S. Spaepen, W. Stinissen, G. Vennekens, Journal of Power Sources 80 (1999) 207. [6] R.M. Dell, Solid State Ionics 134 (2000) 139. [7] S. Lee, B. Choi, N. Hamasuna, C. Fushimi, A. Tsutsumi, Journal of Power Sources 181 (2008) 177. [8] B. Choi, S. Lee, H. Kawai, C. Fushimi, A. Tsutsumi, International Journal of Hydrogen Energy 34 (2009) 2058. [9] B. Choi, S. Lee, C. Fushimi, A. Tsutsumi, Journal of Power Sources 194 (2009) 1150. [10] B. Choi, S. Lee, C. Fushimi, A. Tsutsumi, Journal of Chemical Engineering Japan 43 (2010) 224. [11] B. Choi, S. Lee, C. Fushimi, A. Tsutsumi, Electrochimica Acta 55 (2010) 8771. [12] J.B. Arnott, S.W. Donne, Journal of the Electrochemical Society 154 (8) (2007) A776. [13] T. Sakai, T. Iwaki, T. Mukai, T. Takasaki, K. Tsutsumi, K. Nishimura, Japanese Patent JP2010225354-A (Issued October 7, 2010). [14] T. Iwaki, T. Mukai, K. Nishimura, T. Sakai, T. Takasaki, K. Tsutsumi, WO Patent WO2010058574-A1 (Issued May 27, 2010).
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[15] T. Iwaki, T. Mukai, K. Nishimura, T. Sakai, T. Takasaki, K. Tsutsumi, Japanese Patent JP2010160912-A (Issued July 22, 2010). [16] T. Sakai, J. Yao, T. Mukai T, T. Takasaki, K. Tsutsumi, K. Nishimura, WO Patent WO2010089991-A1 (Issued August 12, 2010). [17] J.P. Rethinaraj, S. Visvanathan, Journal of Power Sources 42 (1993) 335. [18] E. Preisler, Journal of Applied Electrochemistry 19 (1989) 559. [19] K. Matsuki, T. Endo, H. Kamada, Electrochimica Acta 29 (1984) 983. [20] K. Matsuki, T. Endo, H. Kamada, Electrochimica Acta 30 (1985) 1329. [21] S. Chou, F. Cheng, J. Chen, Journal of Power Sources 162 (2006) 727. [22] M. Ghaemi, L. Khosravi-Fard, J. Neshati, Journal of Power Sources 141 (2005) 340. [23] M.E. Armacanqui, R.J. Ekern, Journal of Applied Electrochemistry 22 (1992) 593. [24] Y. Chabre, J. Parmetier, Progress in Solid State Chemistry 23 (1995) 1. [25] J.B. Arnott, G.J. Browning, S.W. Donne, Journal of the Electrochemical Society 153 (7) (2006) A1332. [26] S.W. Donne, G.A. Lawrance, D.A.J. Swinkels, Journal of the Electrochemical Society 144 (1997) 2954.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Fibrous MnO2 electrode electrodeposited on carbon fiber for a fuel cell/battery system Bokkyu Choi, Sunmook Lee, Chihiro Fushimi, Atsushi Tsutsumi ∗ Collaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan
a r t i c l e
i n f o
Article history: Received 9 February 2011 Received in revised form 16 May 2011 Accepted 17 May 2011 Available online 26 May 2011 Keywords: Electrodeposition Manganese dioxide Carbon fiber Fuel cell/battery system Cathode
a b s t r a c t A novel fibrous MnO2 electrode for a fuel cell/battery system is fabricated on carbon fiber by the electrodeposition method. The characteristics of the fibrous MnO2 electrode are examined by electrochemical impedance spectra, galvanostatic performance and cyclic voltammetry. The experimental results indicate that the fibrous MnO2 electrodes are superior to pasted electrodes because of the following: (i) better contact between MnO2 and the electrical conducting material; (ii) high charge-transfer rate because of a smaller diameter than conventional electrodeposited MnO2 particles (thus it is expected that the specific surface area would be higher); and (iii) a low overpotential. The morphology and the crystal structure of the fibrous MnO2 electrode are investigated by scanning electron microscopy and X-ray diffraction, respectively. The entire surface of the carbon fiber is found to be coated with ␥-MnO2 after 2 h of electrodeposition at 0.01 A dm−2 current density. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Fuel cells are a promising power generation system with high power generation efficiency (40–60%) [1] and high energy density (800–10,000 Wh kg−1 ) [2]. However, the fuel cell reactions are restricted to a solid (electrode)/liquid (electrolyte)/gas (cathodic and anodic reactants) three-phase boundary, which leads to a low power density (approximately 500 W kg−1 ) [2,3]. A large number of fuel cell stacks have been assembled to obtain sufficient power output for use in automobiles (50–75 kW) and with distributed power (200–300 kW) [4,5]. Moreover, expensive catalysts such as platinum are used in the electrodes of the fuel cells to increase the reaction rate. Recently, secondary batteries have also been attracting a lot of attention as electrical power storage systems in conjunction with efficient power generation systems [5,6]. The power density of secondary batteries is higher (∼10,000 W kg−1 ) [3] than that of fuel cells because their reactions occur over a large reaction area at the solid (electrode)/liquid (electrolyte) two-phase boundary. However, the energy density of secondary batteries is relatively low (20–200 Wh kg−1 ) [3] because their capacity is determined by the total amount of the electrode’s active materials [6]. In previous papers, a novel concept for fuel cell/battery (FCB) systems that function as fuel cells and as secondary batteries with high energy and power densities has been proposed [7–11].
∗ Corresponding author. Tel.: +81 3 5452 6727; fax: +81 3 5452 6728. E-mail address: [email protected] (A. Tsutsumi). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.057
Figs. 1 and 2 show the reaction mechanism and the cell configuration of the FCB system, respectively. This system is composed of an anode (metal hydride; MH) [8] and a cathode (manganese dioxide; MnO2 ) [7,11] in an alkaline electrolyte. The system can be recharged by supplying electric power and also by supplying gases such as hydrogen and oxygen to the anode and the cathode at ambient temperatures, respectively. In the FCB system, the fuel cell reaction is split into two reactions, which occur at the two-phase regions. One is a chemical gaseous charge reaction at the solid/gas two-phase boundary. The other is an electrochemical reaction at the solid/liquid two-phase boundary. In the anode of the FCB system, MH can be charged by supplied hydrogen gas at the solid/gas two-phase boundary and the charged MH can be electrochemically discharged at the solid/liquid two-phase boundary [8]. In the cathode of the FCB system, MnOOH, which is formed by the discharge of MnO2 , can be chemically recharged by gaseous oxygen at the solid/gas two-phase boundary (oxygen reduction reaction) and an electrochemical discharge reaction of MnO2 occurs at the solid/liquid two-phase boundary [11]. All the reactions in the FCB system proceed at the solid/liquid or solid/gas two-phase regions leading to large reaction areas. This improves the power generation efficiency as well as the power density compared with a conventional fuel cell, because of the suppression of the overvoltage [11]. However, a relatively large resistance overpotential still exists in the cathode because the MnO2 electrode is poorly constructed [12]. It is, therefore, essential for a FCB system that an advanced electrode structure is developed to obtain better performance. Arnott and Donne have reported that the electrochemical performance can depend significantly on the connectivity between
B. Choi et al. / Electrochimica Acta 56 (2011) 6696–6701
6697
Fig. 1. Schematic diagram of the reaction mechanism in the fuel cell/battery (FCB) system.
MnO2 and graphite [12]. The reasons are: (1) the discharge reaction of MnO2 is a homogenous process based essentially on concerted proton and electron insertion into the host ␥-MnO2 structure and (2) electrode impedance is dependent on the effective establishment of the MnO2 -graphite-electrolyte three-phase boundary with the proximity between proton (from the electrolyte) and electron (from graphite) insertion determining the impedance [12]. Hence, the practical working potential of the MnO2 electrode can come close to reaching its theoretical value if the connectivity between MnO2 and graphite is improved. In recent research reported by Sakai et al., carbon fibers were used as a current collector in a nickel-metal hydride secondary battery [13,14] and a lithium secondary battery [15,16]. They observed that the long cycle life and high rate capability were achieved because the fibrous electrodes have good connectivity between the active materials and the substrate, which leads to a reduction in the ohmic resistance. In the present paper, a novel fibrous MnO2 electrode for a FCB system was fabricated on carbon fiber by the electrodeposition method, and the characteristics and performance of the fibrous MnO2 electrode in the FCB system were investigated. 2. Experimental 2.1. Preparation of the electrodes Among the methods to synthesize MnO2 , electrodeposition synthesis is most popular and used widely to fabricate electrolytic manganese dioxide (EMD) for use as a cathode material in alkaline manganese dioxide cells [17–23]. EMDs are conventionally fabricated by the following procedure: (i) electrodeposition on an anode substrate in a sulfate electrolyte at around 92–99 ◦ C, (ii) mechanical removal of EMD deposits from the anode, (iii) washing with distilled water and dilute ammonia (or an alkaline solution), and (iv) finally grinding down. The obtained EMD particles are relatively large around 10 m. The cathode for an alkaline battery is then prepared by mixing the EMD powder, graphite (conducting material) and binder materials. In the present study, the electrode fabricating method was modified by directly electrodepositing MnO2 onto the carbon fibers.
Fig. 2. Schematic diagram of the cell configuration in the fuel cell/battery (FCB) system.
Fig. 3 shows a schematic diagram of the electrochemical plating cell. The MnO2 /carbon fiber electrode was prepared as follows: the electrolyte was composed of 0.66 M MnSO4 in 0.34 M H2 SO4 . Carbon fibers (HTA-12, TOHO TENAX Co., Ltd.) were used as anode materials for the electrodeposition of MnO2 . The end of carbon fibers was tied by a copper mesh and connected with a lead wire for an electrical contact. A copper mesh (∅ 0.20 mm × 80 mm × 20 mm, 50 mesh, Nilaco Corp.) and a Hg/Hg2 SO4 electrode were used as the cathode and the reference electrode, respectively. Before the electrodeposition, the anodes were immersed in 0.66 M MnSO4 containing 0.34 M H2 SO4 for 2 h at 85 ◦ C. The MnO2 was then electrodeposited galvanostatically onto the carbon fibers (total surface area: approximately 1 dm2 ) with an approximate current density of 0.01 A dm−2 for 1–4 h. The electrodeposited MnO2 electrodes were thoroughly washed with deionized water and 1 M KOH aqueous solution for neutralization and then dried in an oven at 80 ◦ C overnight. The amount of MnO2 was determined from the difference in the amount of carbon fibers by measuring it before and after the electrodeposition. For comparison, pasted MnO2 electrodes were also fabricated by mixing EMD (∼10 m in diameter, Tosoh Corp.) powder, carbon black (CB; #3050B, 50 nm in average diameter, Mitsubishi Chemical Corp.), and poly(tetrafluoroethylene) (PTFE; Aldrich) in a mass ratio of 10:3:1. These mixtures were then rolled on both sides of nickel foam, which was followed by baking the foam in an oven at 110 ◦ C for 30 min. The foam was then pressed at 6 MPa for 10 min. The total weight of the electrode was approximately 150 mg while the surface area and the thickness of the fabricated electrode were 3.14 cm2 and 0.6 mm, respectively. 2.2. Characterization The electrochemical characterization of the MnO2 /carbon fiber electrodes was undertaken in 6 M KOH aqueous solution. Before
Fig. 3. Schematic of the electrochemical plating cell.
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the electrochemical measurements, the electrodes were immersed in a 6 M KOH aqueous solution at room temperature under reduced pressure (−16.6 kPa) for approximately 12 h to completely wet the electrodes with the electrolyte solution. Electrochemical measurements such as cyclic voltammograms (CV) and galvanostatic performance were carried out with an Analytical 1480 Multistat (Solartron) at 25 ◦ C. A Ag/AgCl electrode and a nickel-foam of 30 mm in diameter were used as a reference and a counter electrode, respectively. CV measurements were carried out under the following conditions: 0.1 mV s−1 scan rate, 5 cycles, potential scan range from −0.5 to 0.4 V. The discharge/charge experiments were conducted at 0.2 C (1 C being the rate of current at which the cell or working electrode is fully charged in 1 h). 5 discharge/charge cycles were carried out. Electrochemical impedance spectra (EIS) were measured using a frequency response analyzer (1255B, Solartron) within a frequency range from 100 kHz to 10 mHz. Scanning electron microscope (SEM) images were recorded (JSM-7000F, JEOL, Ltd.) and the crystal structure of the electrodes was measured with an X-ray diffractometer (XRD miniflex, Rigaku Corp.) at a scan rate of 2◦ min−1 within 2 = 5 − 90◦ . 3. Results and discussion 3.1. MnO2 electrodeposition onto carbon fibers A SEM image of the carbon fibers that were used as a substrate and an electrical conducting material are shown in Fig. 4. The carbon fibers are approximately 6.5 m in diameter and the surface is very clean and smooth. Fig. 5(a)–(d) shows SEM images of the electrodes that were electrodeposited over 1, 2, 3, and 4 h, respectively. For the 1 h electrodeposition (Fig. 5(a)), agglomerates of MnO2 with a size of 2–4 m were observed on the surface of the carbon fibers. When the electrodeposition time was more than 2 h, the surface of
Fig. 4. SEM image of the raw carbon fibers.
the carbon fibers was coated with MnO2 . Longer electrodeposition times led to thicker fibrous electrode diameters of approximately 9, 11, and 14 m for the electrodes electrodeposited over 2, 3, and 4 h, respectively. These results indicate that MnO2 was deposited well on the surface of the carbon fibers and the current density and thickness of the fibrous electrodes can thus be controlled by adjusting the electrodeposition time. Fig. 6 shows the XRD pattern of the fibrous electrode, which was electrodeposited over 4 h. The peaks are present at 2 = 21.9◦ , 34.1◦ , 36.9◦ , 41.9◦ , 55.6◦ and are typical ␥-MnO2 peaks [24]. Thus, it is confirmed that ␥-MnO2 can be generated in the same manner as in a conventional method using a titanium or carbon substrate even if carbon fiber is used as an anode.
Fig. 5. SEM images for MnO2 electrodeposited on the carbon fibers at a current density of 0.01 A dm−2 over (a) 1, (b) 2, (c) 3, and (d) 4 h.
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Table 1 Open circuit potential (OCP) and charge-transfer resistance (Rct ) of the MnO2 electrodes that were electrodeposited onto carbon fibers (CF) at 0.01 A dm−2 over 1–4 h.
Intensity / a.u.
γ-MnO2
OCP/V Rct /ohm
0
20
40
60
80
6699
100
2θ / degree Fig. 6. X-ray diffraction pattern of MnO2 electrodeposited on carbon fibers at a current density of 0.01 A dm−2 over 4 h.
3.2. Characterization of the fibrous MnO2 electrodes Electrochemical impedance measurements were performed in 6 M KOH aqueous to investigate the charge-transfer resistance of the fibrous MnO2 electrodes and their dependence on the thickness of the MnO2 layer that was electrodeposited on the carbon fibers. Nyquist plots of the electrodes after electrodeposition times of 1, 2, 3, and 4 h are shown in Fig. 7(a). According to the experimental observations, an equivalent electrical circuit is suggested, as shown in Fig. 7(b). Here, Rsol is the solution resistance from the reference electrode to the current collector for the MnO2 electrodes. Rct is the resistance to charge transfer at the electrode–electrolyte interface. Rl , L, Zw , and CPE are the inductance reactance, the inductance, the Warburg impedance, and the constant-phase element, respectively. A semicircle in the high-frequency region and a straight line in the low-frequency region are present in Fig. 7(a). Before the semicircle, an inductance loop is present at 100 kHz to around 50 kHz, which comes from the effects of the stray electric field generated by the cables [25]. In the present research, the semicircles in the
Fig. 7. (a) Nyquist plots of the MnO2 electrodes that were electrodeposited onto carbon fibers at a current density of 0.01 A dm−2 over () 1, () 2, (♦) 3, and () 4 h. (b) The corresponding equivalent circuit.
CFEMD (1 h)
CFEMD (2 h)
CFEMD (3 h)
CFEMD (4 h)
−0.116 1.203
−0.012 0.464
−0.020 0.769
−0.027 0.684
high-frequency region are evaluated for the charge-transfer resistance. By fitting the semicircles, the charge-transfer resistances of the electrodes that were electrodeposited on the carbon fibers over 1, 2, 3, and 4 h are estimated to be 1.203, 0.464, 0.769, and 0.684 , respectively. Table 1 lists the charge-transfer resistance (Rct ) of the MnO2 electrodes that were electrodeposited onto carbon fibers at 0.01 A dm−2 from 1 to 4 h. It has been reported that the Rct of the pasted EMD (Tosoh Corp.) electrode was 0.78 with a working electrode area of 1 cm2 [22]. Although the fibrous electrode that was electrodeposited for 1 h has a relatively high Rct because of the removal of agglomerates from the surface of the carbon fibers, other fibrous electrodes have a lower Rct than that of the conventional pasted electrode. In particular, the Rct of the fibrous electrode that was electrodeposited over 2 h is reduced by 40% compared with that of the pasted electrode. Additionally, the fibrous electrodes have a high open circuit potential (OCP), which corresponds to the EIS results, as listed in Table 1. These results indicate that direct electrodeposition onto the carbon fibers can lead to an improved connectivity between MnO2 and the electrical conduction material, and to a higher charge-transfer rate at the electrode–electrolyte interface. In addition, it is expected that the specific surface area
Fig. 8. (a) CV curves (5th cycle) of the pasted EMD electrode and the MnO2 electrodes that were electrodeposited on carbon fibers (CFEMD) over 1, 2, 3 and 4 h at 0.1 mV s−1 . (b) Normalized CV curves by the amount of active materials.
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of the thin carbon fiber electrode would increase because the surface area is proportional to the charge-transfer rate. However, the fibrous electrodes that were electrodeposited over 3 and 4 h have proximate values with the Rct of the pasted electrode because the fibrous electrodes have a relatively thick MnO2 layer which would interrupt proton and electron insertion into center of MnO2 layer. The difference in Rsol for all the curves is attributed to the variable distances between the electrodes and the references in this experiment. The cyclic voltammetry (CV) and the discharge/charge cycling performance of the fibrous MnO2 electrodes were also investigated. The electrodes were sandwiched between two nickel-foams to prevent the MnO2 agglomerates from exfoliating. Fig. 8 shows the 5th CV curves for each electrode. All the CV curves are typical of the current peaks of the electrodeposited MnO2 . The peak near −0.4 V can be assigned to the reduction reaction of MnO2 and the other two peaks at −0.2 and −0.15 V can be assigned to the oxidation reaction of Mn3+ because of the formation of pyrolusite and ramsdellite, respectively [26]. The current peaks of the fibrous electrodes are smaller than those of the pasted electrode (the amount of MnO2 : 107 mg) because the quantities of MnO2 electrodeposited on the carbon fibers over 1, 2, 3 and 4 h are approximately 8.3, 31.3, 28.2 and 68.4 mg, respectively, which is less than the amount of MnO2 in the pasted electrode. For a direct comparison among the curves, CV curves normalized by the amount of MnO2 are shown in Fig. 8(b). The reduction current peaks of the fibrous electrodes are located at a more positive potential than that of the pasted electrode because of the better connectivity between MnO2 and the
conductive materials in the fibrous electrodes. The potentials of the electrodes are located at a more positive potential when the fibrous electrodes are thinner because a thinner fibrous electrode has a higher charge-transfer rate, as shown in the EIS result. Fig. 9 shows the 3rd discharge curves of the pasted EMD electrode and the fibrous electrodes. In order to remove other influences on the discharge behavior such as oxygen gas which generates when electrodes are overcharged more than their capacity, discharge curves are normalized by setting 0.1 V to zero capacity point as shown in Fig. 9(b). In terms of the discharge performance, the working potentials of the fibrous electrodes that were electrodeposited at less than or equal to 2 h increases at up to 0.1 V. Additionally, the potentials of the fibrous MnO2 electrodes decrease slowly. On the other hand, the working potentials of the fibrous electrodes electrodeposited over 3 and 4 h are similar to that of the pasted electrode at 100–200 mAh g−1 in Fig. 9(a). However, they are smaller than that of the pasted electrode in Fig. 9(b). In addition, the potentials of the fibrous electrodes with a relatively thick MnO2 layer decrease rapidly around 200 mAh g−1 (160 mAh g−1 in Fig. 9(b)) and the amount of discharge reaches approximately 70% (52% in Fig. 9(b)) of the theoretical capacity. These indicate that proton (from the electrolyte) and electron (from the carbon fiber) insertion into MnO2 is interrupted when the diameter of the electrodeposited electrodes are as thick as the MnO2 bulk of the pasted electrode. From the CV and the charge/discharge results, it can be concluded that the working potential and the percentage of reactions of the active materials improves because the thin fibrous MnO2 electrodes reduce the resistance overpotential. 4. Conclusions A novel fibrous MnO2 electrode was fabricated on carbon fiber by the electrodeposition method and its characteristics as a cathode in a fuel cell/battery system were investigated. The morphology and the crystal structure of the fibrous electrodes were confirmed by SEM and XRD measurements. These results indicate that the entire surface of the carbon fiber was covered with MnO2 after 2 h of electrodeposition at a current density of 0.01 A dm−2 and the crystal structure of the MnO2 corresponded to ␥-MnO2 . Electrochemical impedance spectra, galvanostatic performance, and cyclic voltammetry show that the fibrous electrodes possess superior electrochemical characteristics compared to pasted electrodes and these include: excellent connectivity between MnO2 and the electrical conducting material, high charge-transfer rate because of a smaller diameter (thus it is expected that the specific surface area would increase), and a low overpotential. Furthermore, the electrodeposition method allows the thickness of the electrodeposited MnO2 to be controlled. Acknowledgements The authors would like to thank Dr. Tetsuo Sakai at National Institute of Advanced Industrial Science and Technology (AIST) for useful advice. Financial support in the form of a Grant-in-Aid for Scientific Research (B) (number 19360434) and a Grant-in-Aid for JSPS Fellows (number 22·0342) from the Japanese Society for the Promotion of Science (JSPS) is gratefully acknowledged. References
Fig. 9. (a) Discharge curves (3rd cycle) of the pasted EMD electrode and the MnO2 electrodes that were electrodeposited onto carbon fibers (CFEMD) over 1, 2, 3 and 4 h at a 0.2 C discharge rate. (b) Normalized discharge curves by setting 0.1 V to zero capacity point.
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