The role of cations of the electrolyte for the pseudocapacitive behavior of metal oxide electrodes, MnO 2 and RuO 2

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1 Electrochimica Acta 50 (2004) The role of cations of the electrolyte for the pseudocapacitive behavior of metal oxide electrodes, MnO 2 and RuO 2 Sun Wen a, Joon-Woo Lee b, In-Hyeong Yeo b, Jongman Park c, Sun-il Mho a, a Department of Molecular Science and Technology, Ajou University, Suwon , Korea b Department of Chemistry, Dongguk University, Seoul , Korea c Department of Chemistry, Konkuk University, Seoul , Korea Received 2 June 2003; received in revised form 15 December 2003; accepted 24 February 2004 Abstract The energy storage process for amorphous hydrated manganese dioxide (MnO 2 ) is suggested as fast faradaic reactions occurring at the solid electrode surface with the reduction from Mn 4+ to Mn 3+. In order to understand the role of cations of the electrolyte for the MnO 2 electrode as a pseudocapacitor in aqueous KCl solution, we monitored the change of the capacitance by varying the concentration of the KCl electrolyte, the cation of the electrolyte, the ph of the solution, and the solvent. The charge storage for the metal oxide electrode such as MnO 2 is concluded to involve a fast redox reaction through both potassium ion exchange, MnO 2 + K + + e MnO 2 (OK) and proton exchange, MnO 2 + H + + e MnO 2 (OH) dependent upon the availability of cations in the electrolyte. The contribution of proton to the pseudocapacitive process is not negligible in aqueous solution Elsevier Ltd. All rights reserved. Keywords: Amorphous MnO 2 ; Metal oxide electrode pseudocapacitors 1. Introduction Electrochemical capacitors are of interest for the auxiliary energy storage in hybrid electrical devices powered by a rechargeable battery [1,2]. Energy storage mechanisms for the electrochemical capacitors include the separation of charges at the interfaces between a solid electrode and an electrolyte (double-layer capacitance), and/or fast faradaic reactions (underpotential deposition, intercalation, or redox process) occurring at or near a solid electrode surface at the appropriate potential. Redox process often takes place in conductive polymers or metal oxides [1 11]. Metal oxide electrode such as RuO 2 in an aqueous acidic electrolyte is known to be oxidized or reduced reversibly through a Corresponding author. Tel.: ; fax: address: mho@ajou.ac.kr (S.-i. Mho). mechanism involving proton exchange, RuO 2 + H + + e RuO 2 (OH),0 2 [12 14]. A large capacitance (350 F/g) and fast discharge are assumed only to be associated with the chemisorption and desorption of protons on an oxide surface in an aqueous electrolyte [4]. Most metal oxides are unfortunately unstable in strong acidic media. Amorphous hydrated manganese dioxide (MnO 2 ), which is unstable in acid solution, can also be an excellent pseudo-capacitive material with the high capacitance (200 F/g) and a relatively wide potential window in neutral electrolytes [9 11]. An aqueous KCl solution is widely used as a supporting electrolyte in the field of electrochemistry, as it has a relatively high ion conductivity; the K + ion has a smaller hydration sphere than other alkaline ions, as listed in Table 1 [15,16]. An optimal faradaic pseudocapacitor supposed to be charged by chemisorption of a working cation of the electrolyte, K +, at a reduced complex at the surface of the electrode [9]. In /$ see front matter 2004 Elsevier Ltd. All rights reserved. doi: /j.electacta

2 850 S. Wen et al. / Electrochimica Acta 50 (2004) Table 1 Ionic radii, ionic conductivity, and mobilities at infinite diluted aqueous solution and CH 3 CN solution [15,16] Size (Å) Conductivity (10 4 m 2 S/mol) Mobility (10 8 m 2 s/v) H + K + Na + Li + In crystals In aqueous solution In H 2 O In CH 3 CN In H 2 O In CH 3 CN this work, the role of cations of the electrolyte solution for the pseudocapacitive behavior of metal oxide electrodes is pursued. 2. Experimental The amorphous hydrated manganese dioxide (MnO 2 ) electrode material was prepared by mixing the KMnO 4 with Mn(CH 3 COO) 2 aqueous solutions at room temperature [9,17]. Because the electric conductivity of MnO 2 is lower than that of the metallic RuO 2 material, the carbon black was mixed with MnO 2 to increase the conductivity of the electrode. Mixtures of MnO 2 (70 wt.%), carbon black (25 wt.%), and EC (ethyl cellulose; 5 wt.%) were coated in the form of thin films on the current collectors such as Ti, Au, or ITO by a screen-printing method. Apparent geometric area of the MnO 2 electrode is ca cm 2. The thickness of the film is estimated to be about 20 m. The RuO 2 film electrode (area of ca cm 2, 50 m thickness) was formed on the titanium substrate by the thermal decomposition of a RuCl 3 solution at 380 C [18]. A three-electrode electrochemical cell was used for the cyclic voltammetric measurements. The reference electrode used in the experiments was an Ag/AgCl (sat d KCl) electrode. All potentials reported here are referred to the Ag/AgCl (sat d KCl) reference electrode. A Pt electrode (surface area of ca cm 2 ) was used as the counter electrode. The RuO 2 film electrode prepared by the thermal decomposition method or the MnO 2 screen-printed film electrode was used as a working electrode. The electrolytes were prepared using the deionized distilled water or distilled CH 3 CN. Cyclic voltammetry was performed by a potentiostat/galvanostat (EG&G PAR273A or Zahner IM6). The capacitance (C) is the ratio of the charge (q) on the electrode of a capacitor to the potential difference (v) between the electrodes; C = q/v. The capacitances were calculated from the cyclic voltammograms; the capacitance (C) is equal to the value of the current (amperes; coulombs/s) divided by the scan rate (Volt/s) at an arbitrary voltage. The current density and capacitance density were used for the figures, because the current response of the electrode was proportional to the electrode surface area. 3. Results and discussion 3.1. ph or pk dependency of the capacitance The use of ruthenium oxide, RuO 2, as an electrode material for the electrochemical capacitors in acidic solutions is known to base upon the rapid and reversible protonation/deprotonation at the surface sites of the electrode. In order to monitor the effect of the cations on the pseudocapacitive behavior of the RuO 2 electrodes, the capacitances of RuO 2 electrodes were measured both in H 2 SO 4 and in KCl electrolyte solutions. The K + ion is known to have a relatively high mobility in aqueous medium [15]. The cyclic voltammograms (CVs) of the RuO 2 electrode in aqueous H 2 SO 4 and KCl electrolyte solutions are measured at potentials extended to its oxidation and reduction limits. Shown in Fig. 1 are the representative CVs of RuO 2 electrode in both H 2 SO 4 and KCl electrolyte solutions. The CV curves of the RuO 2 electrode show an approximately constant current response to the applied voltage in the range of 0.2 to 1.2 V. The potential ranges are about the same in the two electrolyte solutions, and the capacitive current of the RuO 2 electrode in KCl solution is somewhat smaller than that in H 2 SO 4 solution. Considering the charging/discharging reaction of the ruthenium oxide electrodes in the acid solution, RuO 2 + H + + e RuO 2 (OH),0 2, we can write the rate law for the charging/discharging step as an elementary reaction; the charging rate = dq/dt = k 1 [H + ](q: charge, t: time, k 1 : the charging rate constant in the acid solution, [H + ]: the concentration of proton). The charging rate (dq/dt) should be equal to the proton adsorption rate (d[h + ]/dt) for the charge neutrality of the charging reaction. Hence, the charging/discharging rate can be written; dq/dt = d[h + ]/dt = k 1 [H + ]. The differential equation d[h + ]/dt = k 1 [H + ] has the solution; ln[h + ]=k 1 t. This equation is an integrated rate law, the integrated form of the rate law. For the charging/discharging processes of a capacitor, the rate of charging of q with time at an arbitrary voltage (v) is given by dq/dt = v dc/dt. Hence, the capacitance of the charging/discharging reaction involving the proton exchange is expected to show the logarithmic dependence on the proton concentration, i.e. ph ( log[h + ]). The capacitances of the RuO 2 electrode depend upon the concentrations of the electrolyte (H 2 SO 4 or KCl), and the calculated values of the capacitance from the currents of the CVs are plotted as functions of ph or pk ( log[k + ]) as shown in Fig. 2. Ca-

3 S. Wen et al. / Electrochimica Acta 50 (2004) Fig. 1. Cyclic voltammograms of RuO 2 electrode taken at 10 mv/s scan rate in aqueous (a) H 2 SO 4 (ph = 2.04) electrolyte and (b) KCl (pk = 2.00) electrolyte solution. pacitances show the linear dependences upon the ph or pk. The linear dependences of the capacitance of the RuO 2 in the acid electrolyte upon the ph and that in the neutral KCl electrolyte upon the pk imply that the charging processes are the first-order reactions with respect to the proton concentration and the potassium concentration, respectively. The slopes of the plot of capacitances versus the ph and pk are 8.2 and 3.3, respectively, in the linear range of ph or pk. At the high values of pk or ph (at ca. 5.0) of the electrolyte (at the extremely low concentrations of the cations), the capacitances of the RuO 2 electrode are very small in both electrolyte solutions with the values below 1 mf/cm 2. The capacitances in the acid electrolyte solution are larger than those in the KCl solution, for example, the capacitance of RuO 2 in acid electrolyte of ph = 1.0 is twice of that in neutral electrolyte of pk = 1.0. The larger value of the slope in the acid electrolyte solution than in the KCl solution is observed. The slope represents the rate of charge/discharge. The charging/discharging rate can be affected by the size of the cation, the size of the hydration sphere of the cation in the electrolyte, the mobility of the cation, and the rate of adsorption/desorption at the surface sites. The ionic size in Fig. 2. Linear regression fit of (a) capacitances against the ph of H 2 SO 4 electrolyte and (b) capacitances against the pk of KCl electrolyte solutions for RuO 2 electrode.

4 852 S. Wen et al. / Electrochimica Acta 50 (2004) the crystal lattices of potassium ion is far larger than proton, however, that in aqueous solution is smaller than proton because of the extreme hydration of proton in water. The rate of adsorption/desorption at the surface sites may as well be dependent upon the ion size and the dehydration/hydration rate. The ionic mobilities for proton and potassium ion in aqueous solution at infinite dilution are very different from each other as shown in Table 1 [15]. The large mobility of proton compared to that of potassium ion could be an important factor to the larger capacitance and the larger slope of dependence on the concentration. However, it is hard to conclude which is the most important factor determining the rate of charging/discharging the electrochemical capacitor without further experiments. The pseudocapacitive electrode material, MnO 2 is not stable in a strong acidic solution, and potassium chloride aqueous electrolyte is used in a faradaic pseudocapacitor. The pseudocapacitive behavior of the MnO 2 electrodes is examined in the KCl electrolyte aqueous solution, as a function of the concentration of K + ion. The CVs for the MnO 2 electrode between 0.0 and +1.0 V for the concentration range of 0.10 to 100 mm aqueous KCl solutions are shown in Fig. 3(A). The capacitances of the MnO 2 electrodes increase with the increase of the KCl concentration up to 100 mm. The capacitance shows a linear dependence upon the pk values up to pk of one, with the slope of 6.5 (shown in Fig. 3(B)). At the higher concentrations than 100 mm of electrolyte, the capacitance of the MnO 2 electrode prepared in the labora- Fig. 3. (A) Cyclic voltammograms of MnO 2 electrode taken at 10 mv/s scan rate in: (a) 100; (b) 10.0; (c) 1.0; and (d) 0.10 mm KCl electrolyte and (B) the linear regression fit of the capacitances against the pk of KCl electrolyte solution.

5 S. Wen et al. / Electrochimica Acta 50 (2004) Fig. 4. Cyclic voltammograms of MnO 2 electrode taken at 10 mv/s scan rate in aqueous (a) 10.0 mm KClO 4, (b) 10.0 mm NaClO 4, and (c) 10.0 mm LiClO 4 electrolyte solution. tory increases no more and stays about the same value of ca. 20 ± 3 mf/cm 2. The capacitances for MnO 2 do not increase higher than 20 ± 3 mf/cm 2 even with higher concentration of K + ion than 100 mm, which may imply the adsorption sites of the electrode surfaces being saturated by K + ion. The instability of the MnO 2 electrode in the acid solution limits to measure the capacitance at higher proton concentrations than ph of 3.0. For the experiment of controlling the K + ion concentration, the ph of the electrolyte was kept at ca. 6.0 ([H + ]=10 6 M), and the proton concentration was far lower than that of potassium ion in the solution. At the K + ion concentration of 0.10 mm, the capacitance is very small and negligible. However, at the K + ion concentration of 0.10 mm with adjusting the solution ph of 3.0, the capacitance is increased to 6.6 ± 0.7 mf/cm 2. The capacitance depends also on the proton concentration. Hence, the capacitance depends on the concentration of the available cation in the electrolyte solution Cation species and solvent dependency In order to study the effect of the cations on the pseudocapacitive behavior of the MnO 2 electrode in aqueous KCl solution, we monitored the variations of the capacitance of MnO 2 electrodes dependent on changing the electrolyte species (K +, Na +, and Li + ) and the solvents (protic and aprotic). Ionic sizes in the lattices and in aqueous solutions, ionic conductivity, and the mobilities of H +,Li +,Na +, and K + in aqueous and aprotic solution are listed in Table 1 [15,16]. The conductivities and mobilities of K + ion are far smaller than those of proton, but the largest among the alkaline ions in the Table. The CVs for MnO 2 between 0.0 and +0.8 V measured in 10.0 mm KClO 4, NaClO 4, and LiClO 4 electrolyte solutions are shown in Fig. 4. The capacitance of the MnO 2 electrode determined from the CV curves in aqueous solutions with 10.0 mm concentration of alkaline ion electrolyte is 13 ± 2 mf/cm 2. The capacitances of the MnO 2 electrode do not change by changing the cations from K + to Na + or Li + ion. The differences in the conductivity, mobility, and size among the alkaline ions are not able to vary the capacitance of MnO 2. The concentration of the alkaline ions is the only important factor for determining the capacitance of the electrodes. In order to eliminate the contribution from the proton to the charging/discharging processes for the MnO 2 electrode in the neutral electrolyte solutions, the capacitances are measured in an aprotic solvent. The CVs for the MnO 2 electrode in 10.0 mm NaClO 4 and LiClO 4 electrolytes dissolved in H 2 OorCH 3 CN between 0.8 and +1.0 V are shown in Fig. 5. Aqueous electrolyte solution has high conductivity, which renders high current flux limit. On the other hand, organic electrolyte solutions show very wide electrochemical windows with stability, which results in high cell voltages. The capacitances in aprotic solvents were decreased to the smaller value (7.4 ± 0.8 mf/cm 2 ) than that in aqueous solutions (ca. 13 ± 2 mf/cm 2 ) with the same electrolyte concentration. Even at the high ph (at ca. 6.0) of the electrolyte solution, where the proton concentration was far lower than that of potassium ion in the solution (pk = 2.0), the contribution of the proton to the charge storage for the pseudocapacitor is large. Therefore, the charging/discharging process for the MnO 2 electrode in aqueous electrolyte solution is speculated to involve a fast redox reaction through the proton exchange MnO 2 + H + + e MnO 2 (OH) if the proton is available in the electrolyte solutions, as well as through K + ion exchange, MnO 2 + K + + e MnO 2 (OK).

6 854 S. Wen et al. / Electrochimica Acta 50 (2004) Fig. 5. Cyclic voltammograms of MnO 2 electrode taken at 10 mv/s scan rate in: (a) 10.0 mm NaClO 4 dissolved in H 2 O; (b) 10.0 mm LiClO 4 dissolved in H 2 O; (c) 10.0 mm NaClO 4 dissolved in CH 3 CN; and (d) 10.0 mm LiClO 4 dissolved in CH 3 CN. 4. Conclusions The capacitances of RuO 2 electrodes were measured both in H 2 SO 4 and in KCl electrolyte solutions in order to study the role of the cations of the electrolyte for the pseudocapacitive behavior of the metal oxide electrodes. The capacitances in the acid solution are larger than those in the KCl solution. Capacitances show the linear dependences upon the ph and pk, which imply the charging processes being the first-order reactions with respect to the proton concentration and the potassium concentration, respectively. The charging/discharging reaction mechanism of the ruthenium oxide electrodes involves the proton exchange in the acid solution, and the potassium ion exchange in KCl solution. Observed is the larger slope for the plot of the capacitances versus the ph in the acid electrolyte solution than that versus pk in the KCl solution. The slope represents the rate of charge/discharge. The charging/discharging rate can be affected by the size of the cation, the size of the hydration sphere of the cation in the electrolyte, the mobility of the cation, and the rate of adsorption/desorption at the surface sites. The capacitances of the MnO 2 electrodes were monitored as functions of the concentration of the KCl electrolyte, the cation speices of the electrolyte, the ph of the solution, and the solvent. The capacitance of MnO 2 electrodes prepared in this laboratory increases with the KCl electrolyte concentration at lower concentrations than 100 mm and shows a linear dependence on the pk. The capacitance is saturated at a constant value of ca. 20 ± 3 mf/cm 2 at higher electrolyte concentration than 100 mm. The capacitance of the MnO 2 electrode shows little variation with changing the alkaline ions of the electrolyte such as KClO 4, NaClO 4, and LiClO 4 aqueous solutions (ca. 13 ± 2 mf/cm 2 in 10.0 mm electrolyte solution). The capacitances in aprotic solvents were drastically decreased to the smaller values than that in aqueous neutral solutions. Hence, the contribution of the proton to the pseudocapacitive process is not negligible. The charge storage mechanism of MnO 2 electrode is concluded to involve a fast redox reaction through both potassium ion exchange, MnO 2 + K + + e MnO 2 (OK) and proton exchange, MnO 2 + H + + e MnO 2 (OH). The metal oxide supercapacitors are charged by chemisorption of cation of the electrolyte, proton or the alkaline ion depending on the availability. Acknowledgement The authors wish to acknowledge the support from the Korea Science and Engineering Foundation (R ) and from the Korea Research Fund (BK21) for this study. References [1] B.E. Conway, J. Electrochem. Soc. 138 (1991) [2] B.E. Conway, Electrochemical Supercapacitors, Kluwer-Plenum, New York, [3] S. Sarangapani, B.V. Tilak, C.P. Chen, J. Electrochem. Soc. 143 (1996) [4] J.P. Zheng, P.J. Cygan, T.R. Jow, J. Electrochem. Soc. 142 (1995) [5] V. Srinivasan, J.W. Weidner, J. Electrochem. Soc. 147 (2000) 880. [6] T. Kudo, Y. Ikeda, T. Watanabe, M. Hibino, M. Miyayama, H. Abe, K. Kajita, Solid State Ionics (2002) 833. [7] Y.U. Jeong, A. Manthiram, Electrochem. Solid-State Lett. 3 (2000) 205.

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