ECS Transactions [ECS 220th ECS Meeting - Boston, MA (October 9 - October 14, 2011)] - Fabrication of a Transparent Supercapacitor Electrode Consisting of Mn-Mo Oxide/CNT Nanocomposite

Fabrication of a Transparent Supercapacitor Electrode Consisting of Mn-Mo Oxide/CNT Nanocomposite

M. Nakayamaa, K. Okamuraa, L. Athouëlb, O. Crosnierb, and T. Brousseb

a Materials Chemistry, Graduate School of Science and Engineering

Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan b Laboratoire de Génie des Matériaux et Procédés Associés, Polytech Nantes

Université de Nantes, 44306 Nantes Cedex 3, France

We have fabricated nanocomposite electrode materials consisting of manganese-molybdenum mixed oxide and carbon nanotubes (CNTs) for supercapacitor application. The process involves electrophoretic deposition (EPD) of multi-walled CNTs onto an ITO electrode, followed by anodic deposition of Mn-Mo oxide from an aqueous MnSO4 solution with MoO4

2– anions. The composite electrodes with a variety of combinations of Mn-Mo oxide and CNTs were evaluated systematically with the aim of maximizing the capacitive properties of the oxide. Optimization led to a high specific capacitance of 408 F g–1 (2 mV s–1) with an area-normalized capacitance of 31 mF cm–2.

Introduction

Global warming due to the continued production of CO2 has made us realize the importance and urgency of development of more advanced and environmentally friendly energy storage devices. Supercapacitors, one of the most promising candidates for next generation power devices, possess higher power density and longer cycle life than secondary batteries and higher energy density compared to conventional electrical capacitors. Supercapacitors can be classified as electrical double-layer capacitors (EDLCs) and pseudocapacitors (or redox capacitors) according to their charge storage mechanisms. The latter can store energy through fast and reversible redox reactions of electroactive materials with multiple oxidation states, such as transition metal oxides or conductive polymers. Transition of MnIII/MnIV involving a single electron transfer is responsible for the pseudocapacitance of MnO2. As the following equation indicates, simultaneous injection and ejection of cations (C+) and electrons contribute to the charge/discharge processes.

MnO2 + C+ + e- MnOOC [1] where the theoretical specific capacitance of MnO2 is computed to 1370 F g–1 (1). In fact, however, only 1/5 or 1/6 of the theoretical value is delivered. Such low practical specific capacitance of MnO2 can be attributed to its intrinsic poor electrical conductivity and dense morphology. To avoid these problems, mainly three strategies have been developed

(2): i) fabrication of micro- and nanostructured MnO2, ii) doping of other transition metals, and iii) combination with nanocarbons such as carbon nanotubes (CNTs).

ECS Transactions, 41 (22) 53-64 (2012)10.1149/1.3693063 © The Electrochemical Society

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CNTs are beneficial as a current collector for electrodepositing transition metal oxides owing to their high mechanical strength, high chemical stability, and excellent conductivity. Modification of CNTs onto the surface of electrode can be achieved by direct chemical vapor deposition (CVD) or dip-coating and electrophoretic deposition (EPD) from CNT dispersion (3). EPD is more convenient compared to CVD in terms of short fabrication time, simple apparatus, and suitability for mass production. It involves the transfer of charged particles dispersed in a suitable solvent towards an electrode under an applied electric field, which results in the accumulation of the particles to form a thin film on the electrode.

A complementary combination of potentially high pseudocapacitance of MnO2 and high electrical conductivity of CNTs can open a new possibility for fabricating electrode materials for ideal supercapacitors. Over the years various nanocomposites consisting of MnO2 and CNT have been constructed with aiming the improvement in the electrochemical utilization of MnO2. Chemical co-precipitation of MnO2 and CNTs gives nanocomposites in a powder form, where a binder is necessary to hold the composite in a current collector. In the absence of any additives, the introduction of MnO2 into CNTs has been achieved by spontaneous reduction of MnO4

– (4) on the carbon surface and anodic oxidation of Mn2+ (5–10) using electrodes coated with CNTs by CVD (7–10),dip-coating (6), and EPD (4,5) techniques. To the best of our knowledge, however, only a few groups have optimized the composition of MnO2 and CNTs to maximize the pseudocapacitance of the electrodes (7). Although a large specific capacitance can be obtained by decreasing the thickness of the active material, the area- and volume-normalized capacitance values are more crucial in practical use.

In this study, CNT films were fabricated by an EPD technique from a dispersion of positively charged particles of MWCNTs, followed by the anodic deposition of Mn and Mo mixed oxide from a bath containing Mn2+ and MoO4

2–. Composition of the composite electrodes was tuned to draw out the best performance with respect to capacitance values, rate capability, and electrochemical stability.

Experimental Electrophoretic Deposition of CNTs

EPD of CNTs was made according to a method described in the literature (11). To obtain CNTs with acidic sites, 0.06 g CNTs were refluxed in 20 mL of a mixture of nitric and sulfuric acids (1:3 by volume) at 120 °C for 30 min. The acid-treated CNTs were washed thoroughly with water, separated in a centrifuge at 5000 rpm for 30 min, and then dried in an oven at 80 °C for 24 h. 0.001 g of the acidified CNTs were charged positive by being dispersed ultrasonically in 30 mL of isopropyl alcohol containing 0.001 g of Mg(NO3)2. For EPD, an indium tin oxide (ITO)-coated glass slide (R = 10 Ω cm) and a platinum foil served as the working and counter electrodes, respectively. The exposed geometric area of the ITO electrode was 2.0×0.9 cm2. Prior to each experiment, the electrode surface was ultrasonicated in acetone for 10 min. The two electrodes were kept parallel at a fixed gap of 1 cm. EPD was performed by applying a constant DC voltage of 80 V, while the deposition time was altered. Thus, the charged CNT particles were deposited onto the cathode. After EPD, the CNT-coated electrode was washed with copious amount of water to remove soluble species and then dried under vacuum in a desiccator. The weight difference between the ITO electrodes with and without the CNT-

ECS Transactions, 41 (22) 53-64 (2012)



coating was measured by a Mettler Toledo XS205DU microbalance with an accuracy of 0.01 mg.

Electrodeposition of Mn-Mo Mixed Oxide

Electrodeposition of Mn and Mo mixed oxide was carried out in a standard three-electrode cell using a Hokuto Denko HZ-5000 potentio/galvanostat. A platinum sheet served as the counter electrode, and a standard Ag/AgCl electrode (in saturated KCl) was used as the reference electrode. The deposition bath used was a mixed aqueous solution containing 2 mM MnSO4, 20 mM Na2MoO4, and 100 mM Na2SO4. Mn-Mo mixed oxide was prepared on CNT-coated and uncoated ITO electrodes by cycling the electrode potential repeatedly between 0 and +1.0 V. The conditions employed had been optimized to produce a Mn-Mo oxide film with the highest pseudocapacitance on Pt electrode (12). The resulting ITO-supported film was rinsed thoroughly with water, dried under vacuum, and then submitted to structural and electrochemical characterization.

Structural Characterization

X-ray diffraction (XRD) data were acquired at 1° min–1 on a Rigaku Ultima

diffractmeter with Cu Kα radiation (λ = 0.154051 nm). Field emission scanning electron microscopy (FE-SEM) was conducted with a Hitachi S-4700Y SEM operating at 10 kV. The contents of Mn and Mo in the film were measured by an inductively coupled plasma-atomic emission spectrometer (ICP-AES, SII SPS-3500) after dissolving the sample in HNO3/HCl solution. The emitting light wavelengths in the ICP-AES measurements were 257.6 and 202.0 nm in order to detect Mn and Mo, respectively. The estimated moles of Mn and Mo were converted to the masses of the corresponding oxides of Mn (MnO2) and Mo (MoO3) because Mo6+ is not involved in the electrode reaction. A Shimadzu UV2400PC spectrometer was used to record UV/vis absorption spectra of the films deposited on ITO substrate.

Electrochemical Characterization

CVs of the Mn-Mo oxide films deposited on CNT-coated and uncoated ITO electrodes

were recorded at various scan rates, where an aqueous solution of 0.5 M Na2SO4 was employed as the supporting electrolyte. Specific capacitance (in F g–1) was calculated by integrating the CV curve to obtain the voltammetric charge (in C), and subsequently dividing it by the mass of Mn-Mo oxide (in g) and the potential window (in V) (12). Since it is dangerous to determine the mass of very thin films by a microbalance, the mass values were estimated based on the Mn and Mo contents that were determined by the above ICP-AES measurements.




Results and Discussion Preparation and electrochemical properties of CNT films

Fig. 1a shows photographs of CNT films deposited electrophoretically on an ITO substrate at various EPD times. The film obtained at short EPD time appears transparent. The mass deposited is plotted in Fig. 1b as a function of EPD time. As expected, an increase in EPD time causes an increase in the mass of CNTs. The mass increase becomes slow after 120 s, probably because the dispersion state of CNTs in solution could not be maintained at longer time.

Figure 1. (a) Photographs of the CNT film electrodes deposited on an ITO electrode by EPD for indicated periods of time. (b) Plots of the masses of deposited CNTs as a function of EPD time.

Fig. 2a shows CVs of the CNT-coated electrode prepared with EPD time of 30 s at noted scan rates. The CV curves show an almost rectangular shape even at high scan rates, being an indication of an ideal EDLC behavior arising from charge separation at the electrode/electrolyte interface. The calculated specific capacitances are plotted in Fig. 2b as a function of scan rate. The capacitance at 200 mV s–1 is as large as 71 % of that at 10 mV s–1. This demonstrates a high rate capability, characteristic of double layer charge, reflecting small resistance of the electrode and fast diffusion of electrolyte in the film. Figure 2. (a) CVs of the CNT-coated electrodes made with EPD time of 30 s. The measurements were carried out at noted scan rates. (b) Plots of the specific capacitances estimated from the CVs in Fig. 2a as a function of scan rate.




Electrodeposition of Mn-Mo Oxide Film on CNT/ITO Electrode

Fig. 3 shows CVs of ITO electrodes coated without (a) and with (b) CNTs when the electrode potential was cycled repeatedly in a Na2SO4 solution containing MnSO4 and Na2MoO4. An anodic current appearing at +0.6 V on the first cycle in Fig. 3a is attributable to the oxidation of Mn2+ to form MnO2 (Mn2+ + 2H2O → MnO2 + 4H+ + 2e–). We previously revealed that the electron transfer from MoO4

2– does not participate in the potential region examined, while the protons generated during the formation of MnO2 react with MoO4

2– to produce polyoxomolybdate through a dehydrated condensation reaction (by protonation and dehydration) (12). The condensed product is coprecipitated, resulting in Mn and Mo mixed oxide. The appearance of anodic peaks around +0.8 V at initial cycles reflects a characteristic of diffusion-limited oxidation of Mn2+ ions. The capacitive current as a result of redox reactions of the deposited Mn-Mo oxide becomes larger in size with the number of cycles, gradually hiding the Mn2+ oxidation current.

The oxidation of Mn2+ ions can also be observed from +0.6 V at the first anodic scan on the CNT-coated electrode (Fig. 3b), where the double-layer charging current is involved. The capacitive current due to the deposited Mn-Mo oxide similarly increases with the cycle number. Accordingly, basically the same electrochemical reaction takes place. It should be noted that the peaks around +0.8 V at initial cycles in Fig. 3b are broader than those in Fig. 3a. This suggests that the high specific surface area of CNTs can enlarge the real electrode surface area, cancelling the diffusion-controlled effect (13). Clearly, the nucleation and growth of Mn-Mo oxide on the substrate is affected by the underlying CNTs. XRD analysis revealed that Mn-Mo oxide in the nanocomposite is x-ray amorphous (not shown).

Figure 3. CVs of ITO electrodes coated (a) without and (b) with CNTs when the electrode potential was cycled repeatedly in 0.1 M Na2SO4 electrolyte containing 2 mM MnSO4 and 20 mM Na2MoO4

The deposited masses of Mn-Mo oxide on the CNT-coated and uncoated ITO electrodes were measured on ICP-AES data as a function of the number of cycles for electrodeposition. As described in the experimental section, the estimated moles of Mn and Mo by ICP-AES measurements were converted to the masses of the corresponding oxides of Mn (MnO2) and Mo (MoO3), and the sum of the masses was defined as the mass of the Mn-Mo mixed oxide. We found no significant difference in the deposition masses of Mn-Mo oxide between the CNT-coated and uncoated electrodes. On the other hand, the molar ratio of Mo to Mn was 0.2, which agrees with our previous work (12).

Fig. 4 depicts FE-SEM images of the CNT-coated electrodes before (a) and after (b) electrodeposition of Mn-Mo oxide, along with that of the Mn-Mo oxide film deposited on




bare ITO substrate (c). As shown in image a, CNTs cover the whole surface of the electrode and none of the underlying ITO is exposed. The randomly oriented CNTs form a well-entangled and interconnected nanoporous structure. As shown in image c, the original Mn-Mo oxide presents particles that are 150 to 200 nm in diameter. Such a particlulate morphology cannot be seen in image b where the same morphology as that (image a) of the CNT film appears. A closer look reveals that the diameter of CNT fibrils is in the range from 15 to 35 nm before electrodeposition of Mn-Mo oxide, while that of the fibrils in the composite is in the range from 30 to 60 nm. This suggests that the Mn-Mo oxide forms a thin coating layer on the surface of individual fibrils of CNTs. That is, the composite has a CNT core/Mn-Mo oxide shell structure. This is quite different from the situation where a specific nanostructure of MnO2 is grown electrochemically on the CNT surfaces.

Figure 4. FE-SEM images of the CNT films (a) before and (b) after electrodeposition of Mn-Mo oxide, along with (c) that of Mn-Mo oxide deposited on a bare ITO substrate.

Figure 5. CVs of the CNT-coated electrodes modified (a) without and with (b) pure Mn and (c) mixed Mn-Mo oxides, along with (d) that of Mn-Mo oxide deposited on bare ITO electrode.

Figure 5 depicts CVs of the CNT-coated electrodes further modified without (a) and with pure Mn (b) and mixed Mn-Mo (c) oxides, along with that (d) of the Mn-Mo oxide deposited on bare ITO substrate. They were recorded in 0.5 M Na2SO4 electrolyte at 20 mV s–1 after the steady-state was achieved. The broken lines exhibit the responses with an anodic potential limit of +0.8 V. It is worth noting that the CV of the Mn-Mo oxide combined with CNTs (Mn-Mo/CNT) is much larger than a simple sum of the contributions from CNT and Mn-Mo electrodes. The Mn-Mo/CNT electrode provides a nearly symmetrical rectangular shape, which is a proof of rapid response when the scan




direction is altered. The difference in shape of the CVs between the electrodes with and without underlying CNTs is more evident with the anodic limit of +1.0 V than +0.8 V. It is obvious that the introduction of CNTs enhances the rate capability of the Mn-Mo oxide, resulting in the expansion of the operating potential region. Pure MnO2 deposited on the CNT-coated electrode provides a much smaller current, reflecting the effect of the introduction of Mo, as we previously reported (12).

Figure 6 depicts in situ absorption spectra of the Mn-Mo oxides deposited on bare (a) and CNT-coated (b) ITO electrodes recorded when the electrode potential was stepped between 0 and +1.0 V at 0.2 V intervals. All the spectra were acquired after a 2 min equilibration period. Broad absorptions lying in the wavelength region from 400 to 800 nm with no distinct peaks can be ascribed to d-d transition of the Mn4+ state (t2g

3eg0). The

observed absorbance change evidences transparency of both electrodes. The absorption due to Mn4+ ions in the film increases with an increase in the anodic potential. From another viewpoint, the color change occurring concurrently with the capacitive behavior can be regarded as an optical evidence that the capacitance of the oxide arises from the faradaic redox reactions Mn3+/Mn4+. Figure 6. In situ absorption spectra of the Mn-Mo oxides deposited on (a) bare and (b) CNT-coated ITO electrodes, taken from 0 to +1.0 V at 0.2 V intervals. Composite Electrodes with Different Masses of Deposited CNTs

It is crucial to adjust the composition in the composite electrodes to maximize the capacitive performance. First, CNTs were deposited on ITO substrate with different EPD times, followed by electrodeposition of Mn-Mo oxide with a fixed number (36) of potential cycles. In Fig. 7, the anodic charges of the resulting electrodes, extracted from their steady-state CVs, are plotted versus the EPD time for the deposition of CNTs. For comparison purpose, the charge of the electrode coated with CNTs alone is also displayed. As shown in Fig. 2, the double-layer charge arising from the CNT coating increases with increasing the mass of deposited CNTs. We should note that the composite electrode exhibits much larger charge than those (broken line) expected from the sum of the contributions of individual electrodes, which is obvious in the region up to 120 s. This finding can be attributed to an improved electrochemical utilization of the deposited Mn-Mo oxide by underlying CNTs. The deposited amount of CNTs linearly increases with EPD time up to 120 s (Fig. 1), while the same amount of Mn-Mo oxide is deposited on them. Since the mixed oxide forms a thin coating layer on the surface of CNT fibrils, as shown in the SEM image of Fig. 5b, it is reasonable to assume that the Mn-Mo oxide layer becomes thinner as the mass of CNTs increases within 120 s. Further increase of the underlying CNTs did not lead to a significant increase of the charge. In other words, the




EPD time of 120 s is necessary to achieve the best electrochemical performance of the Mn-Mo oxide made with 36 cycles.

Figure 7. Plots of the anodic charges extracted from the CVs of CNT-coated electrodes further modified () with and () without Mn-Mo oxide by 36 potential cycles, as a function of EPD time for the CNT coating. The CVs were recorded in an electrolyte of 0.5 M Na2SO4 at a scan rate of 20 mV/s.

The effect of the scan rate on the CV response of the composite electrodes with different CNT masses is shown in Fig. 8. Distortion from rectangularity becomes smaller by the underlying of CNTs up to 30 s, but further increase distorts again.

Figure 8. CVs of Mn-Mo oxides deposited with 36 cycles on CNT-coated electrodes with EPD times of (a) 0, (b) 30, (c), 120, and (d) 240 s. The measurements were carried out in an aqueous electrolyte of 0.5 M Na2SO4 at noted scan rates. Composite Electrodes with Different Masses of Deposited Mn/Mo Oxide

Mn-Mo oxide was electrodeposited by applying different potential cycles on the electrode that had been coated with the fixed amount of CNTs (EPD 120s). The resulting composite electrodes were subjected to similar CV measurements. As shown in Fig. 9, the




anodic charge of the composite electrode increases sharply up to 36 cycles and then becomes slow. The electrodeposition of Mn-Mo oxide by more than 36 cycles led to a decrease in the specific capacitance because the increase of the anodic charge is not as large as the increase of the mass of Mn-Mo oxide.

Figure 9. Plots of the anodic charges extracted from the CVs of Mn-Mo oxides deposited on () CNT-coated and () uncoated ITO electrodes, as a function of the number of cycles for electrodeposition of the oxide. EPD time for the CNT coating was always 120 s. The CV measurements were conducted in an aqueous electrolyte of 0.5 M Na2SO4 at a scan rate of 20 mV/s.

Figure 10. Dependence of scan rate on (a) specific and (b) area- or volume-normalized capacitances of the Mn-Mo/CNT composites prepared with noted cycle numbers for the electrodeposition of Mn-Mo oxide. The CVs were recorded in an electrolyte of 0.5 M Na2SO4.

We investigated the influence of scan rate on the CV responses of the composite electrodes with different masses of Mn-Mo oxide, and the results are shown with respect to the specific capacitance and area- and volume-normalized capacitances in Fig. 10. The volume-normalized capacitance was estimated based on the thickness of the deposited CNT film that was measured based on the cross-sectional SEM image (inset). Generally, the specific capacitance decreases with the increase of potential scan rate because the electrolyte ions only reach the outer surface of the electrode at high scan rates. As in the cases of other transition metal oxide films, the smaller the loading of active materials, the larger the specific capacitance at all scan rates. The film made with 5 CV cycles provides a highest specific capacitance value of 693 F/g at a scan rate of 2 mV/s, which is larger than the values reported for several MnO2/CNT electrodes.5-7,10 We should note in Fig.




10b that the volume- or area-normalized capacitance of three composite electrodes are settled down to one value when the scan rate is increased. This value is attributed to the charge associated with the outer surface of the electrodeposited Mn-Mo oxide. However, the electrode made with 5 CV cycles does not reach this value, meaning an insufficient performance at high scan rates although it exhibits the highest specific capacitance. Accordingly, among all the electrodes fabricated in this study, the composite electrode consisting of Mn-Mo oxide made with 36 CV cycles and CNTs with EPD time of 120 s is the best electrode with effective capacitive performance. This electrode shows a specific capacitance of 157 F/g at 200 mV/s, which is as high as 48 % of that at 10 mV/s.

The Mn-Mo/CNT electrode thus optimized was subjected to a long-term cycle stability test at a scan rate of 20 mV/s for 1000 cycles (Fig. 11). During the first 20 consecutive cycles, the specific capacitance decrease 14 % of the initial value; however, after 20 cycles the decrease of specific capacitance was less than 9 % during 1000 cycles. Figure 11. Plots of the specific capacitance of the optimized Mn-Mo/CNT composite electrode as a function of the number of cycles, based on the CV data in 0.5 M Na2SO4 at 20 mV/s. Performance of a Symmetric Capacitor Based on the Mn-Mo/CNT Composite

Fig. 12 compares the galvanostatic charge/discharge characteristics of symmetric capacitors based on Mn-Mo (a) and Mn-Mo/CNT (b) films in 0.5 M Na2SO4, where the composite electrode optimized in the above section was used. The oxide without CNTs (Fig. 12a) exhibits more pronounced initial IR drops during the discharge of the electrode at increased current densities. In the presence of underlying CNTs (Fig. 12b), we can observe linear discharge curves and good symmetry, typical of an ideal capacitor. Figure 12. Galvanostatic charge/discharge curves of symmetric capacitors based on (a) Mn-Mo and (b) Mn-Mo/CNT films coated on an ITO electrode.




Fig. 13 presents Ragone plots for the symmetric capacitors. The specific energy (E) and power (P) were calculated by following equation from the galvanostatic charge/discharge cycles.

E = 1/2CΔV2 [2] P = QΔV/2t [3]

Where C, Q, ΔV, and t are specific capacitance (F g–1), total charge delivered (C), the potential of discharge (V), and discharge time (s), respectively. The energy density decreases with increasing discharge current. The capacitor with Mn-Mo exhibits 2.3 Wh kg–1 at a power density of 100 W kg–1 and 0.5 Wh kg–1 at 2,000 W kg–1, which corresponds to a 20 % retention. On the other hand, the energy density with Mn-Mo/CNT can reach 5.5 Wh kg–1 at 100 W kg–1 and still remains 4.0 Wh kg–1 at 2,000 W kg–1, indicating the high retention of energy density. This can be ascribed to the excellent conductivity of underlying CNTs and the fast ionic diffusion. Figure 13. Ragone plots for symmetric capacitors based on Mn-Mo and Mn-Mo/CNT films.

Conclusions

The composite electrodes consisting of Mn-Mo oxide and CNTs were fabricated by EPD of CNTs and the subsequent anodic deposition of Mn-Mo oxide. Electrochemical properties of the electrodes with various combinations of Mn-Mo oxide and CNTs were investigated with the aim of maximizing the electrochemical utilization of the oxide. A high specific capacitance of 408 F/g at 2 mV/s was obtained with an area-normalized capacitance of 31 mF/cm (or a volume-normalized capacitance of 76 F/cm3).

Acknowledgement

The work described in this report was conducted in Materials Chemistry, Graduate School of Science and Engineering at Yamaguchi University. I gratefully acknowledge the support provided by the U. S. Army Research Office under contract no. FA5209-10-P-0050.




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ECS Transactions [ECS 220th ECS Meeting - Boston, MA (October 9 - October 14, 2011)] - Fabrication of a Transparent Supercapacitor Electrode Consisting of Mn-Mo Oxide/CNT Nanocomposite