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Electrochimica Acta 56 (2011) 8122– 8128

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

arbon/PbO2 asymmetric electrochemical capacitor based on methanesulfoniccid electrolyte

hilippe Perreta, Zohreh Khania,c, Thierry Brousseb, Daniel Bélangerc, Daniel Guaya,∗

INRS-Énergie, Matériaux et Télécommunication, 1650 Boulevard Lionel-Boulet, C. P. 1020, Varennes, Québec, J3X 1S2, CanadaLaboratoire de Génie des Matériaux et Procédés Associés EA 2664, Polytech Nantes, Université de Nantes, BP50609, 44 306 Nantes Cedex 3, FranceDépartement de Chimie, Université du Québec à Montréal, Case Postale 8888, succursale Centre-Ville, Montréal, Québec, H3C 3P8, Canada

r t i c l e i n f o

rticle history:eceived 26 April 2011eceived in revised form 21 May 2011ccepted 26 May 2011vailable online 13 July 2011

a b s t r a c t

A new hybrid electrochemical capacitor based on an activated carbon negative electrode, lead dioxide thinfilm and nanowire array positive electrode with an electrolyte made of a lead salt dissolved in methane-sulfonic acid was investigated. It is shown that the maximum energy density and specific capacity of theC/PbO2 nanowire system increase during the first 50 cycles before reaching their maximum values, whichare 29 Wh kg−1 and 34 F g−1, respectively, at a current density of 10 mA cm−2 and a depth of discharge

eywords:anowirebO2

lectrodepositionlectrochemical capacitoremplate synthesis

(positive active electrode material) of 3.8%, that corresponds to a 22C rate. This is 7–8 times higher thanthe corresponding maximum values reached with a C/PbO2 thin film cell operated in the same conditions.After an initial activation period, the performances of the C/PbO2 nanowire system stay constant and donot show any sign of degradation during more than 5000 cycles. For comparison, the C/PbO2 thin filmsystem exhibits a 50% decrease of its performances in similar conditions.

© 2011 Elsevier Ltd. All rights reserved.

. Introduction

Electrochemical capacitors are energy storage devices exhibit-ng high power density (>5 kW/kg) and impressive cyclability>5 × 105 cycles) [1–4]. However, they suffer from a limited energyensity (≈5 kWh/kg), which is the major focus of most of nowadaysesearch efforts. Increasing the maximum energy of electrochem-cal capacitor can be achieved by improving the cell capacitancer widening the cell voltage window, according to the followingquation:

max = 12

Ccell(Vmax)2 (1)

here the maximum energy density, Emax, is related to the cellapacitance Ccell and to the square of the maximum cell volt-ge, Vmax. Aqueous-based electrochemical capacitors have severaldvantages compared to organic-based devices. However, the celloltage of symmetrical electrochemical capacitors is restricted to1 V owing to the limited stability of the aqueous electrolyte [5,6].his limitation can be overcome by replacing a carbon capacitive

lectrode with a faradic electrode. This was suggested by sev-ral authors who have used Ni(OH)2 [7–9], PbO2 [10,11] or MnO212–14] as positive electrode material together with an activated

∗ Corresponding author.E-mail addresses: guay@emt.inrs.ca, daniel.guay@emt.inrs.ca (D. Guay).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.05.125

carbon negative electrode. On these materials, the overpotentialfor the oxygen evolution reaction is large, which results in anincrease of the maximum cell voltage and hence an increase of theenergy density of the related hybrid device. A detailed calculationof the benefit of such a strategy was reported by Zheng [9]. Forexample, the maximum energy density of a hybrid carbon/Ni(OH)2electrochemical capacitor is a factor of 6 larger than an organic-based symmetrical carbon electrochemical capacitor [9]. Briefly,this arises as a consequence of the positive faradic electrode hav-ing a high overpotential for oxygen evolution reaction and a highercapacity than a carbon electrode. Moreover, it charges and dis-charges at constant potential, allowing the negative carbon-basedelectrode to be used in a larger potential window, thus increasingits capacity.

Among the different options available, the activatedcarbon–PbO2 hybrid electrochemical capacitor seems to beone of the best choices due to its high voltage (≈2 V in aqueouselectrolyte) and the low cost of carbon, PbO2 and sulfuric acid.Moreover recycling PbO2 and sulfuric acid is now well masteredand financially self-sufficient. Despite the many advantages of thisaqueous based hybrid device, Pell and Conway [10] have pointedout several limitations which can be summarized as follow:

(i) the limited cyclability of the PbO2 positive electrode whichrequires the use of an excess electrode material to compensatethe fading of the capacity upon cycling;

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P. Perret et al. / Electrochim

ii) the limited power capability of the PbO2 positive electrodewhich limits the power density of the hybrid cell.

The first limitation is related to the sulfation of the PbO2 elec-rode upon repeated cycling which results in a poor cycle life ofhe overall device. This phenomenon is inevitable in sulfuric acid.ecently, a new flow battery system based on the PbO2/Pb redoxouple cycled in a new electrolyte composed of methanesulfoniccid and lead methanesulfonate has been described [15–21]. Athe cathode, this completely changes the redox processes from aolid/solid couple in sulfuric acid (Eq. (2)) to a solid/solvated ionsn methanesulfonic based electrolyte (Eq. (3)).

bO2(S) + HSO−4(aq) + 3H+ + 2e− ↔ PbSO4(S) + 2H2O (2)

bO2(S) + 4H+ + 2e− ↔ Pb2+(aq) + 2H2O (3)

So when the system is discharged, PbO2 is reduced into Pb2+(aq),

hich means that the electrode is dissolved into the electrolyte.pon charging, Pb2+

(aq) is oxidized and PbO2 is electroplated on theurface of the electrode. To achieve this, methanesulfonic acid isandatory since Pb+2 cations are highly soluble in that electrolyte

15]. In that case, sulfation is not a limitation anymore and a bet-er cyclability can be expected from this system. There are otherlectrolytes that could be used such as perchloric acid, hexafluo-osilicic acid and tetrafluoroboric acid, but these acids are plaguedith safety hazards not found with methanesulfonic acid.

The second limitation of the positive electrode could be over-ome by increasing the electrochemically active surface area ofhe PbO2 positive electrode. For example, PbO2 nanowires haveeen recently prepared using an electrodeposition method through

porous alumina membrane, leading to a noticeable increase ofhe effective surface area [22–26]. The same approach that consistsf using a nanostructured oxide electrode material to increase thelectrochemically active surface area has also been used elsewheren the study of RuO2 [27–29], NiO [30,31] and MnO2 [32–34].

In this paper, we propose to couple both approaches (electrolytehange and PbO2 nanostructuration) in order to check their influ-nce on the cycling ability and the power capability of a C/PbO2ybrid electrochemical capacitor device.

. Experimental

Lead dioxide (PbO2) was electrochemically deposited from 0.1 M Pb(NO3)2/0.1 M HNO3 solution at a current density of.5 mA/cm2 (galvanostatic mode). The pH of the solution wasdjusted to 5.0 by adding small amounts of sodium hydroxide.hese conditions were chosen based on the results of a previoustudy showing that both thin films and PbO2 nanowires can berepared in these conditions [23]. The current efficiency for theeposition of PbO2 is ∼96%. All chemicals were reagent grade andsed without further purification. All solutions were prepared withe-ionized water (18 M� cm). All deposits were performed at roomemperature (20–23 ◦C).

PbO2 was deposited in the form of films and nanowires ontoitanium substrates (1 cm2). They were first mechanically polishedStruers, P#240 silicon paper) and then sonicated in methanol dur-ng 5 min to remove any silicon particles that could have beenncluded in the substrate. This last operation was repeated twice.he substrates were then etched in hot (90 ◦C) oxalic acid 10 wt.%olution during 1 h. Following that, the Ti substrates were thor-ughly rinsed with de-ionized water and dried under a constant

ow of nitrogen gas.

Thin films of PbO2 were prepared by direct electrodeposition oni substrate and scanning electron microscopy (SEM) micrographsf the as-prepared film are shown in Fig. 1A and B at two different

cta 56 (2011) 8122– 8128 8123

magnifications. As-deposited PbO2 thin films have a rough surfaceand the deposit is dense with no apparent porosity.

The experimental setup for the preparation of PbO2 nanowireswas first used by Taberna et al. for the preparation of Fe3O4 based Cunanowires electrode for lithium-ion battery applications [35]. Theworking electrode (WE) is made of three parts, namely a Ti sub-strate, an anodic aluminium oxide (AAO) membrane and a porousglass plate. Commercially available AAO membranes (Anodisc 25,Whatman International Ltd.) were used as porous templates. In thisstudy, AAO membranes with 200 nm pore diameter were used. Theporous glass plate (3 cm diameter glass plate, porosity between 145and 174 �m, Ace Glass Inc.) allows for the pressure applied by theclamps to be distributed evenly at the surface of the AAO mem-brane. The open structure of the porous glass plate ensures thatdiffusion of dissolved Pb2+ species to the AAO membrane is nothindered. Finally, two clamps are holding together the differentparts that are making the working electrode. The use of clampsminimizes the gap between the Ti substrate and the AAO mem-brane. The AAO membrane was humidified with electrolyte beforebeing inserted in the experimental setup to minimize air bubblesin the membrane. The counter electrode (CE) was a stainless steelplate and a saturated calomel electrode (SCE) was used as refer-ence electrode. A Luggin capillary was employed to minimize theiR drop. During the course of the experiment, the Ti substrate ispositively polarized and PbO2 deposition occurs at its surface. Fol-lowing deposition, the AAO membrane was dissolved in 1 M NaOHsolution at 80 ◦C during 10 min. All samples were rinsed with waterafter plating. Scanning electron microscopy (SEM) micrographs ofas-deposited PbO2 nanowires are shown in Fig. 1C and D. Denseand well ordered PbO2 nanowires are observed at the surface of theelectrode. The nanowires have a length of ca. 15 �m and a diam-eter of ca. 200 nm, which corresponds to the pore diameter of thealumina membrane.

All attempts made to measure the BET surface area of ourdeposits failed. A rough estimate has indicated that the amountof material should be increased by at least a factor 10 to reach thedetection of N2-based BET equipment. Instead, we have relied on asimple calculation based on geometrical considerations to estimatethe surface area of PbO2 nanowires.

We could not find any evidence that excessive growth of TiO2is occurring during the deposition of PbO2. As far as we can tellfrom impedance spectroscopy measurements (data not shown), themain contribution to the uncompensated resistance is due to theelectrolyte. This is believed to arise as a consequence of the forma-tion of a dense and non-porous PbO2 film at the surface of the Tielectrode that prevents excessive growth of resistive TiO2. As wewill see later on, PbO2 deposits with thicknesses in the 20–30 �mrange could be grown with any difficulty.

The carbon composite electrodes were prepared by mixing90 wt.% of the active material (carbon powder from Cabot Corp.,surface area 1500 m2/g) with 10 wt.% of polytetrafluoroethylene(PTFE-60 wt.% suspension, 6 wt.% surfactant) in ethanol. Then, thispaste was dried, cold-rolled and pressed at 5 MPa during 3 min ontoa stainless steel grid (40 mesh). It was then used without any furthertreatment.

Hybrid electrochemical capacitors were tested in an electro-chemical cell, using 0.1 M CH3SO3H + 0.1 M Pb(NO3)2 + 4 M NaNO3as an electrolyte. An excess of sodium nitrate was added to lowerthe resistivity of the electrolyte (�electrolyte = 4 � cm) and increase(by ∼10%) the specific capacitance of the carbon electrode. Also, inpresence of 4 M NaNO3, the charge and discharge processes on thecarbon electrode are more reversible, leading to a cyclic voltammo-

gram with a more rectangular shape than observed otherwise.

The cell voltage was controlled using a potentiostat–galvanostat(Solartron SI 1287). A saturated calomel electrode (SCE) referencewas used to follow the potential of the PbO2 electrode. The potential

8124 P. Perret et al. / Electrochimica Acta 56 (2011) 8122– 8128

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ing was ca. −0.1 V, so that the maximum cell voltage reached uponcharging was ca. 1.7 V. The lower cutoff voltage during dischargewas set at 0.7 V.

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ig. 1. Scanning electron microscopy micrographs of PbO2 deposited on Ti substrat0C in both cases.

f the activated carbon electrode was then obtained by subtractinghe PbO2 electrode potential from the total cell voltage. The systemas charged and discharged at 10 mA cm−2 with 0.7 and 1.7 V as

utoff cell voltage.

. Results and discussion

Fig. 2 shows the cyclic voltammograms (CVs) of an activatedarbon and a PbO2 thin film electrode in 0.1 M CH3SO3H + 0.1 Mb(NO3)2 + 4 M NaNO3. The carbon electrode was cycled from −0.5o + 0.7 V vs SCE. As expected, the CV has an almost rectangularymmetrical shape. The specific capacitance of that electrode is07 F g−1 of active carbon material at 10 mV s−1. The specific capac-

tance of a symmetrical carbon/carbon electrochemical capacitorade with these electrodes would be 1/4 of the single electrode

apacitance (≈27 F g−1). The use of this electrode in a symmetricalarbon/carbon electrochemical capacitor would yield a maximumheoretical voltage output of ca 1.0 V in this electrolyte [6], leadingo a specific energy of 3.75 Wh kg−1 based on Eq. (1).

The CV of PbO2 exhibits two well-defined oxidation and reduc-ion peaks at 1.55 and 1.10 V, respectively. The potential differenceetween the oxidation and reduction peaks points to the poorinetics of the PbO2/Pb2+ couple. The same observation was madelsewhere and this irreversibility has been identified as the majorource of loss in energy efficiency during cycling of a lead/lead

ioxide methanesulfonic acid flow battery [19].

Based on these CVs, the maximum charging cell voltage of aell-balanced hybrid electrochemical capacitor made of a nega-

ive activated carbon electrode and a positive PbO2 electrode would

and B) thin film and (C and D) array of nanowires. The electrodeposited charge was

be ca. 2.05 V, almost 1 V larger than a symmetrical carbon/carboncapacitor. However, in the system under investigation, the chargeimbalance between both electrodes was such that the minimumelectrode potential reached by the carbon electrode during charg-

Potential vsSCE / V

Fig. 2. Cyclic voltammograms of an activated carbon electrode and an array of PbO2

nanowires in 0.1 M CH3SO3H + 0.1 M Pb(NO3)2 + 4 M NaNO3 at 10 mV s−1.

P. Perret et al. / Electrochimica Acta 56 (2011) 8122– 8128 8125

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ig. 3. Scanning electron microscopy micrographs of PbO2 nanowire after differeanowire, (C) in the discharged state during the first cycle, (D–F) in the charged sta

In this study, we focused on the effect of the morphologynd accessible surface area of PbO2 on the electrochemical per-ormances of the positive electrode by comparing two hybridlectrochemical capacitors with the same negative carbon elec-rode but different positive PbO2 electrodes, viz. PbO2 thin filmnd PbO2 nanowires. In a first series of measurements, the effectf cycling on the morphology of the PbO2 positive electrode wasssessed for the two types of deposit. So, a series of scanning elec-ron microscopy micrographs of PbO2 nanowire electrodes cycledn methanesulfonic acid are shown in Fig. 3. For these measure-

ents, five different samples were used and each was cycled forifferent periods of time. The micrograph of as-deposited PbO2anowire array is shown in Fig. 3A and B (sideview and topview,espectively). The SEM micrograph of Fig. 3C was taken after therst discharge. The diameter of the NWs seen in Fig. 3C (dischargedtate) are smaller than in the as-deposited state (Fig. 3C), indicatinghat a significant amount of PbO2 has dissolved in the electrolyte.his is consistent with the fact that Pb+2 species are soluble inethasulfonic acid (see Eq. (3)). Each nanowire has a very rough

urface compared to the much smoother surface of as-depositedbO2. This roughness extends to the entire length of each nanowire.his indicates that the reduction reaction is not limited to thetmost outer portion of the nanowires. This is strong evidence that

ling period in methanesulfonic acid: (A and B) side and top view of as-depositedr 5, 30 and 100 complete cycles.

the electrolyte has access to the entire length of the nanowire. After5 charge/discharge cycles, the top view micrograph in Fig. 3D showsthat the deposit is still composed of individual nanowire. The meandiameter of the NW is larger than in the discharged state (Fig. 3C),indicating that a significant amount of PbO2 has been re-depositedcompared to the discharge state. Again this is consistent with themechanism proposed earlier for the charging and discharging ofthe positive electrode material in methanesulfonic acid (Eq. (3)).However, the NW has a prismatic shape in contrast to the cylin-drical shape observed in the as-deposited state. This indicates thatpreferential growth of the NWs is occurring in a radial direction,which would be the preferred growth axis if soluble Pb+2 ions arebeing trapped in the structure of the PbO2 nanowire array. Individ-ual nanowires are still discernable after 30 charge/discharge cycles(see Fig. 3E). After 100 charge/discharge cycles, the initial nanowirestructure is no longer distinguishable, although the film has a veryporous structure.

Low magnification SEM micrographs of a PbO2 thin film andPbO2 nanowire array after 100 charge/discharge cycles are dis-

played in Fig. 4. It is clear from a comparison of both micrographsthat the latter has a more open and porous structure than theformer one. So, although the initial structure of the nanowirearray is not retained after 100 charge/discharge cycle, the struc-

8126 P. Perret et al. / Electrochimica Acta 56 (2011) 8122– 8128

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For the C/PbO2 nanowires electrochemical capacitor, there is anincrease of the Ccell and Emax (activation period) during the first50 cycles. This increase of the Ccell and Emax values is quite signif-icant and amounts to more than 40% of their initial values, which

Fig. 4. Scanning electron microscopy micrographs of PbO2 (A) thin film and (

ure of the deposit is distinctively different and shows a markedependency on the structure of the as-deposited film. Dissolu-ion and recrystallization of PbO2 during the charge and dischargerocesses of the electrode are not unilaterally determining theorous structure of the deposit and, in that respect, the micro-raphs of Fig. 4 clearly point to the crucial role the structure ofhe as-deposited film is playing on the determination of the cycledtructure. After 100 charge/discharge cycles, the XRD structuref the PbO2 nanowire array has evolved from a mixture of �-bO2 and �-PbO2 to a deposit made exclusively of �-PbO2 (dataot shown), which is consistent with data from the literature36].

Fig. 5 shows the variation of the specific capacitance and max-mum specific energy of both types of hybrid electrochemicalapacitors with the cycle number. The mass of PbO2 was constantor both electrodes (47 mg) and corresponds to an electrodepositedharge of 40C (the current efficiency for the deposition of PbO2s close to 100% (29)). In both cases, the negative electrode was

n activated carbon electrode (41.8 mg, 107 F g−1). The chargend discharge cycles were performed at a constant current of

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ig. 5. Specific capacitance (A and C) and maximum specific energy (B and D) of/PbO2 nanowires (A and B) and C/PbO2 thin film (C and D) hybrid electrochemicalapacitor measured at constant charge and discharge rate (10 mA cm−2) in a 0.1 MH3SO3H + 0.1 M Pb(NO3)2 + 4 M NaNO3. The mass of PbO2 on the positive electrodeas the same in both cases and corresponds to an electrodeposited charge of 40C.

owire array in the charged state after 100 complete charge/discharge cycles.

±10 mA cm−2, which corresponds to ±0.21 A g−1 of PbO2. The spe-cific capacitance, CSPE was calculated using the following equation:

CSPE = i

mt × ∂V∂t

(4)

where i, mt and ∂V/∂t are the discharge current (i = 10 mA cm−2 andthe surface area is 1 cm2), the total mass of the active material inboth electrodes (which is 47.0 + 41.8 = 88.8 mg in all cases) and therate of variation of the cell voltage during the discharge cycle (seeFig. 5), respectively. In both cases, the maximum energy densitywas calculated from Eq. (1), with Ccell = CSPE and Vmax = 1.3 V (seeFig. 6).

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Fig. 6. Potential-time evolution of (A and C) the positive and negative electrodeand (B and D) the cell potential in (A and B) a C/PbO2 thin film and (C and D) aC/PbO2 nanowires hybrid electrochemical capacitor measured at constant chargeand discharge rate (10 mA cm−2) in a 0.1 M CH3SO3H + 0.1 M Pb(NO3)2 + 4 M NaNO3.In the case of PbO2 thin film, the curves corresponding to cycle (black square) 1,(red circle) 10, (green up triangle) 350 and (blue down triangle) 4050 are shown.In the case of PbO2 nanowires electrode, the curves corresponding to cycle (blacksquare) 1, (red circle) 10, (green up triangle) 350 and (blue down triangle) 5050 areshown. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of the article.)

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P. Perret et al. / Electrochim

re ∼24 F g1 and ∼20 Wh kg−1, respectively. The reason underly-ng this phenomenon is unknown for the moment but it maye related to a change (increase) of the wettability at the PbO2anowire/electrolyte interface and an extension of the surface con-act area between the PbO2 nanowire and the electrolyte. Thisspect needs further investigation.

After 50 cycles, the Ccell and Emax values are ∼34 F g1 and29 Wh kg−1, respectively. This is close than an 8-fold increase

ompared to the maximum energy density of a symmetrical car-on/carbon electrochemical capacitor made with such carbonlectrodes. Such improvement of the energy density was pre-icted by Zheng [9] for hybrid devices. Briefly, this is due to theigh discharge potential of the faradic electrode, which increaseshe maximum cell voltage, and its flat discharge plateau whichnhances the cell capacitance. As we will see later on, there is anxcess of carbon material at the negative electrode that could beecreased in an optimized version of this hybrid devices. This would

ead to a further increase of the maximum energy density.The Ccell and Emax values stay constant until the end of the

easurements that were stopped after 5000 cycles. This system isighly reversible and there is no fading of the specific capacitancend maximum energy density over 5000 charge and dischargeycles. In the case of the C/PbO2 thin film electrochemical capaci-or, the initial Ccell and Emax values are ∼4.9 F g−1 and ∼4.1 Wh kg−1.nlike the previous case, at the beginning of the cycling, there iso activation period where the specific capacitance (and the spe-ific energy) of the system is increased. Indeed, both Ccell and Emax

alues decreased slowly with cycling, and after 4000 cycles, thepecific capacitance of the C/PbO2 thin film system has decreasedo ∼50% of its original value. The maximum Ccell and Emax values ofhe C/PbO2 nanowires array capacitor is a factor of 7 higher thanhe maximum value reached by the C/PbO2 thin film system. Thetructure of the PbO2 positive electrode has a dramatic effect onhe specific capacitance and maximum specific energy attainablen these systems.

The evolution of the cell voltage and of both electrode potentialsuring the discharge cycle are shown in Fig. 6. As seen in Fig. 6A,he potential of the positive electrode of the C/PbO2 thin film sys-em varies abruptly from 1.60 to 1.15 V at the very beginning of theischarge cycle. This latter value corresponds to the peak potentialf the reduction peak observed in the CV of that electrode recordedn the same electrolyte. This abrupt variation is due to the slowinetics of the Pb(II)/PbO2 redox interconversion in methanesul-onic acid. Further discharge brings the potential of the positivelectrode to 0.80 V. During that period, the potential of the nega-ive electrode varies only slightly from −0.15 to 0.10 V. Obviously,n that case, the C/PbO2 thin film system is limited by its positivelectrode

In the case of the C/PbO2 nanowires system, the potential ofhe positive electrode decreases from 1.60 to 1.20 V, similarly tohat was observed previously for the PbO2 thin film based system.owever, in contrast to that latter system, the potential of the pos-

tive electrode stays constant at ca. 1.10 V later during discharge.ver that period, the potential of the negative electrode increases

teadily from its initial value of ca −0.10 V to ca 0.40 V. In theseonditions, the lower cutoff voltage (0.70 V) is reached and the mea-urement is stopped. Obviously, the hybrid C/PbO2 nanowires arrayapacitor is limited by the negative electrode. At the end of the dis-harge period, the depth of discharge of the positive electrode isnly ca. 3.8%.

As shown elsewhere, oxidation and reduction of PbO2 is notestricted to the material that is closer to the bulk of the elec-rolyte but occur as well in those sections of the nanowires that

re closer to the substrate [24]. This assertion is supported by aomparison of the SEM micrographs of Fig. 3A and C, which showshat, upon reduction in methanesulfonic acid, the structure of the

cta 56 (2011) 8122– 8128 8127

PbO2 nanowires is modified over a significant portion of its length.Assuming that the electrolyte can access the entire length of thePbO2 nanowires, it can be shown that the ratio between the accessi-ble surface area of PbO2 nanowire, SPbO2 nanowire, and PbO2 thin film,SPbO2 thin film, is close to 160, assuming that 15 �m long nanowirewith a diameter of 200 nm are arranged in a hexagonal patternat the surface of the substrate with a center to center distanceof 250 nm. This value is consistent with that found elsewhere for∼40–50 �m long PbO2 nanowires grown in similar conditions [26].All measurements were performed at a discharge rate of 22C and,accordingly, it is not surprising that the performance of the C/PbO2nanowire hybrid electrochemical capacitor is superior to that of theC/PbO2 thin film system on account of the much larger accessiblesurface area.

It remains to understand why the specific capacitance and max-imum specific energy of the C/PbO2 nanowire system is not fadinglike this is the case for the C/PbO2 thin film electrochemical capac-itor. It is hypothesized that, during discharge, a large fraction ofsoluble Pb+2 species are trapped within the structure of volumedefined by the void between the nanowires and are available to bere-deposited in the same section of the nanowire. Again, assum-ing the same tri-dimensional arrangement of the nanowires and adepth of discharge of ca. 3.6%, the concentration of Pb+2 in the voidspace between the nanowires is 0.7 M, which is 7 times higher thanthe concentration of Pb+2 in the bulk of the electrolyte. This trap-ping effect within the structure of the deposit might explain thegood cyclability of the C/PbO2 nanowires electrochemical capaci-tor. It is also hypothesized that the length of the nanowire structurewill be an important parameter to determine the cyclability of thesystem and this aspect is currently under investigation.

4. Conclusion

A new hybrid electrochemical capacitor has been studied usingactivated carbon electrode, lead dioxide and an electrolyte com-posed of 0.1 M methanesulfonic acid and 0.1 M lead nitrate. In thecase of the C/PbO2 nanowire electrochemical capacitor (depth ofdischarge 3.6% and 22C rate of discharge), the specific capacitanceis 34 F g−1 and the maximum energy density is 29 Wh kg−1. Thesevalues are seven times larger than those measured on a C/PbO2 thinfilm electrochemical capacitor. The mean cell voltage during thedischarge cycle of the C/PbO2 nanowires electrochemical capacitoris 1.0 V. Up to 5000 charge/discharge cycles were performed at 22Crate without any sign of performance degradation. For comparison,Yu et al. have tested a carbon/PbO2 asymmetric electrochemicalcapacitor in sulfuric acid electrolyte and showed that the specificcapacitance retains 83% of its maximum value after 3000 cyclesat 4C rate [11]. This difference of behavior under cycling is mostprobably due to the fact that PbO2 does not undergo sulfation inmethanesulfonic acid. Considering its performances and its cyclingbehavior, this system might constitute an interesting alternativefor a new hybrid electrochemical capacitor, and further optimiza-tion, including (but not limited to) the mass ratio of AC to PbO2,will be performed in a forthcoming study.

Acknowledgements

The authors would like to thank the Natural Sciences and Engi-neering Research Council of Canada (NSERC), the Fonds Québécoisde la Recherche sur la Nature et les Technologies (FQRNT) and Axion

des Affaires Etrangères” (MAE) and “Ministère des Relations Inter-nationales du Québec” (MRI) are also gratefully acknowledged forsupporting part of this work within the framework of project 61-

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128 P. Perret et al. / Electrochim

10. One of the authors, P. Perret, thanks MAE and MRI for financialupport.

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