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Electrochimica Acta 55 (2010) 6101–6108

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

edox poly[Ni(saldMp)] modified activated carbon electrode in electrochemicalupercapacitors

ei Gaoa, Jianling Lia,∗, Yakun Zhanga, Xindong Wanga, Feiyu Kangb

Department of Physical Chemistry, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, ChinaDepartment of Material Science and Engineering, Tsinghua University, Beijing 100083, China

r t i c l e i n f o

rticle history:eceived 5 February 2010eceived in revised form 21 May 2010ccepted 25 May 2010vailable online 31 May 2010

eywords:

a b s t r a c t

The complex (2,2-dimethyl-1,3-propanediaminebis(salicylideneaminato))–nickel(II), [Ni(saldMp)], wasoxidatively electropolymerized on activated carbon (AC) electrode in acetonitrile solution. Thepoly[Ni(saldMp)] presented an incomplete coated film on the surface of carbon particles of AC electrodeby field emission scanning electron microscopy. The electrochemical behaviors of poly[Ni(saldMp)] modi-fied activated carbon (PAC) electrode were evaluated in different potential ranges by cyclic voltammetry.Counterions and solvent swelling mainly occurred up to 0.6 V for PAC electrode by the comparison of

1/2

upercapacitoredox polymerctivated carbononaqueous systemolymerization

D C values calculated from chronoamperometry experiments. Both the Ohmic resistance and Faradayresistance of PAC electrode gradually approached to those of AC electrode when its potential was rangingfrom 1.2 V to 0.0 V. Galvanostatic charge/discharge experiments indicated that both the specific capaci-tance and energy density were effectively improved by the reversible redox reaction of poly[Ni(saldMp)]film under the high current density up to 10 mA cm−2 for AC electrode. The specific capacitance of PAC

g thedox p

electrode decreased durinThis study showed the re

. Introduction

Electrochemical double-layer supercapacitors (EDLCs), due tots’ outstanding power density and extremely long lifetime vs.ithium ion battery, are widely used as an auxiliary power sourcen electric vehicles and other electric devices [1]. However, therawback of low energy density for activated carbon (AC) elec-rode significantly limits EDLCs’ wide application. Incorporatingome conducting polymers on top of porous activated carbonaterials is one practical approach to compensate this shortcom-

ng. Conducting polymers (e.g. polyaniline [2–4], polypyrrole [5,6],olythiophene and its derivatives [7–9]) have been investigated asctive materials of supercapacitor, which have larger capacitancend higher energy density than activated carbon but faster declinef performance.

Most polymers which have been employed as the electroac-ive materials for supercapacitors are either p-doped or n-dopedeversibility in aqueous solutions and nonaqueous systems. When

-doped (n-doped) electrochemically, electrons are removed frominjected onto) the polymer backbone through the external circuit,hile anions (cations) from the electrolyte solution are incorpo-

ated into the polymer film to maintain overall charge neutrality

∗ Corresponding author. Tel.: +86 10 62332651; fax: +86 10 62332651.E-mail address: lijianling@ustb.edu.cn (J. Li).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.05.076

first 50 cycles but thereafter it remained constant for the next 200 cycles.olymer may be an attractive material in supercapacitors.

© 2010 Elsevier Ltd. All rights reserved.

[10–12]. This makes the polymer susceptible to the attack of reac-tive species in the electrolyte solution, which may reduce the cyclelife.

We noticed that one type of conducting polymer produced byelectrooxidization of Ni (salen)-type monomer (see Chart 1) has thepotential to be used as electroactive material of supercapacitors.Nickel(II) complexes with N2O2 Schiff base ligands derived fromsalicylaldehyde have long been used as homogeneous electrocata-lysts in the reduction of alkyl and aryl halides [13–15]. Oxidationof these nickel(II) complexes can lead to polymerization on theconducting electrode surfaces and generate electroactive films inmoderate/weak donor solvents [16–19]. The polymerization of Ni(salen)-type monomers has been reported by several groups. Thosestudies have focused on the electropolymerization process, poly-mer structure, and identification of the redox couples [20–25].

Ni(salen) species are characteristic of square planar or nearlyplanar in steric orientation [26–28]. In the process of the elec-trooxidization of Ni(salen)(II)-type monomers, the oxidized speciesreact with each other to form a stack structure [21], meanwhile,charge-compensating counterions in the electrolyte penetrate intothe polymer so as to provide electroneutrality (processes that like

p-doped and d-doped).

The polymer films produced by Ni[salen]-type monomers maybe combined with porous activated carbon materials to improvethe performance of EDLC capacitors (specific capacitance, energydensity, etc.). In this paper, we prepared the poly[Ni(saldMp)](3)-

6102 F. Gao et al. / Electrochimica Acta 55 (2010) 6101–6108

maccattm

2

2

Gpmp1JN(ttiN7

monomer-free solution of 1 M Et3MeNBF4 in acetonitrile.

Chart 1. Structure of some typical Ni[salen]-type monomers.

odified AC electrode by a two-step potential pulse procedurend reported the characterization of PAC electrode throughyclic voltammetry, electrochemical impedance spectroscopy andhronoamperometry. The morphology of PAC electrode was char-cterized via field emission scanning electron microscopy. Finally,he galvanostatic charge/discharge tests were conducted in ordero evaluate the stability of poly[Ni(saldMp)] as an electroactive

aterial in electrochemical supercapacitor.

. Experimental

.1. Materials

Acetonitrile (AN, >99.9%, A.R. grade) was purchased fromuangdong Xilong Chemical Co., Ltd. Tetrabutylammoniumerchlorate (TBAP, >99.9%, C.P. grade) and triethylmethylam-onium tetrafluoroborate (Et3MeNBF4, >99.9%, C.P. grade) were

urchased from ZhongShengHuaTeng Co., Ltd. 2,2-Dimethyl-,3-propanediamine (dMp, >97%, GC,T) was purchased from&Kchemica Co., Ltd. They were used as received. The complexi(saldMp) monomer (2,2-dimethyl-1,3-propanediaminebis

salicylideneaminato))–nickel(II) was synthesized followinghe procedure in the literature and recrystallized from ace-

onitrile [29]. Results from CHN elemental analysis weren agreement with the expected theoretical composition fori(C19H20N2O2) = 367.07 g mol−1. Anal. Calc.: C, 62.17; H, 5.49; N,.63. Found: C, 62.52; H, 5.56; N, 7.94%.

Scheme 1. The two-step potential pulse procedure.

Activated carbon was purchased from Kuraray Chemical Co.Ltd, with BET surface area around 1683 m2 g−1. The AC electrodewas prepared by mixing the activated carbon with polyvinylidenefluoride (PVDF) and commercial carbon black (80:10:10 wt%) in N-methyl-2-pyrrolidone (NMP) until homogeneous slurry. The slurrywas coated on a 20-�m-thick Al foil (current collector) with thecoating thickness of 100 �m.

2.2. Poly[Ni(saldMp)] film electropolymerization on graphite andAC electrode

Linear sweep potential method was used for the preparation ofpoly[Ni(saldMp)] on the graphite electrode: the sweep potentialranged from 0.0 V to 1.2 V, and the sweep rate was set to 10 mV s−1

for the total of 10 cycles. A two-step potential pulse step procedure(Scheme 1) was used for the preparation of poly[Ni(saldMp)]-modified AC electrode (PAC): 2 s pulse time at 0.8 V followed by30 s relaxation time at 0.0 V, for the total of 250 cycles. Thereis a limitation of diffusion rate for the monomer molecules andelectrolyte in the solution during the electrochemical polymer-ization occurs on AC electrode due to the large surface area andhighly porous structure of the activated carbon. Based on thisconsideration, the two-step potential pulse step procedure wasadopted to polymerize a thin polymer film on carbon grains ofAC electrode. During the short pulse step (2 s) of the procedure,the monomers nearby the surface of carbon grains are electrooxi-dized and deposit on the surface. During the subsequent relaxationstep (30 s) of the procedure, the monomers locating in the bulkof electrolyte solution diffuse into the inner of the AC electrodeso that the monomer molecules which were exhausted in pre-ceding pulse can be compensated and electrooxidized in the nextstep.

The electropolymerization was conducted in a closed three-electrode compartment cell. The working electrodes were polishedgraphite electrode (˚ = 1 cm), or 100-�m-thick activated carbonelectrode (1 cm × 1 cm), an AC electrode with 8 cm2 surface area asthe counter electrode and a capillary Ag|AgCl wire as the referenceelectrode (all potentials here are given vs. Ag|AgCl) [30,31]. Thepolymerization solution was made with acetonitrile, 0.1 mM com-plex Ni(saldMp) monomer, and 0.1 M TBAP. The poly[Ni(saldMp)]and PAC electrodes were then washed in acetonitrile in order toremove any soluble species from the film and were tested in a

Apparent surface coverage (� app, mol cm−2) was calculatedusing the equation � app = �Q/nFA, where n is the number of trans-ferred electrons, F is the Faraday’s constant, A is the area of ACelectrode in cm2, and �Q is the charge (area) difference under the

F. Gao et al. / Electrochimica Acta 55 (2010) 6101–6108 6103

Table 11H chemical shifts of the H2(saldMp) ligand and its Ni(saldMp) complex.

Materials 1H chemical shift

Ha Hb Hc Hd He Hf Hg –OH

H2(saldMp)a 1.08 3.49 6.89 7.25 7.26 6.97 8.34 13.55

ca

2

vZpoaAHCJ

tiD1tcaa2

tsa

3

3

mFFottslsa

3

PP0oBso

Ni(saldMp)b 0.88 3.31 6.49 7.12 7.16 6.65 7.41

a CDCl3 solution.b CD3CN solution.

yclic voltammetric anodic oxidation wave of the PAC electrodend AC electrode at 5 mV s−1.

.3. Procedure and equipment

The morphology of the electrooxidized films was observedia field emission scanning electron microscopy (FESEM), using aeiss SuprATM 55 microscope. Photomicrographs were taken onoly[Ni(saldMp)] modified AC electrodes. All the films were thor-ughly rinsed with acetonitrile prior to analysis. The elementalnalyses (C, H and N) were performed on a Carlo Erba Elementalnalyzer model Flash EA1112. The 1H NMR spectra of the ligand2(saldMp) and its complex Ni(saldMp) monomer were obtained inDCl3 and CD3CN solutions in a JEOL 600 MHz spectrometer model

NM-ECA 600.All electrochemical measurements were conducted in a closed

hree-electrode compartment cell and were studied using a VMP2nstrument coupled to a PC with EC-Lab software (version 9.30).uring the electrochemical measurements, the PAC electrode ofcm2 and AC electrode with larger area (8 cm2) were exposed to

he electrolyte solution containing 1 M Et3MeNBF4. Electrochemi-al Impedance Spectroscopy measurements were conducted overfrequency range of 100 kHz to 10 MHz using 10 mV sine-wave

mplitude. The electrode was polarized at the test potential for0 min to ensure that the electrode had reached equilibrium.

The galvanostatic charge/discharge tests were conducted withhe 1 cm2 PAC electrode and bare AC electrode, and placed in theame closed electrolyte cell. The Ag/AgCl reference electrode waslso used to monitor the potential of PAC electrode during cycling.

. Results and discussion

.1. Spectroscopic characterization

The 1H NMR spectrums of CD3CN solution of Ni(saldMp)onomer and CDCl3 solution of H2(saldMp) ligand are shown in

ig. 1. The spectrums are obviously sharp and well resolved. Inig. 1(a), the singlet peak at 13.55 ppm is assignable to the protonf hydroxyl in the H2(saldMp) ligand, but it disappears in the spec-rum of Fig. 1(b), which is indicative of the coordination betweenhe ligand and nickel(II) cation. The other proton assignments areummarized in Table 1. Comparing the spectrum of H2(saldMp)igand with that corresponding to its Ni(II) complex, the chemicalhift deviation of Hg is comparatively bigger than those of Ha, Hb

nd Hc–e,f, which is due to the influence of coordination effect.

.2. Cyclic voltammetry (CV)

The typical cyclic voltammograms of poly[Ni(saldMp)] andAC electrodes in 1 M Et3MeNBF4/AN are shown in Fig. 2.oly[Ni(saldMp)] electrode possesses a couple of redox peaks at

.770 V and 0.862 V in Fig. 2(a), which attributes to the transitionf poly[Ni(saldMp)] between oxidation state and reduction state.y comparison between the CVs of poly[Ni(saldMp)] and graphiteubstrate, one should be noted that portion of 0.0–0.6 V has nobvious change which indicates that the polymerization of com-

Fig. 1. 600 MHz 1H NMR spectrums of H2(saldMp) ligand CDCl3 solution andNi(saldMp) complex CD3CN solution.

plex Ni(saldMp) on graphite has little effect on the double-layerabsorption and desorption process on the surface of graphite sub-strate. Additionally, the portion of 0.6–1.2 V of poly[Ni(saldMp)]on graphite electrode may be viewed as the sum of double-layerabsorption/desorption current of graphite substrate and faradiccurrent of poly[Ni(saldMp)].

Dahm proposed that establishment of the poly[Ni(salen)] filmoccurs in two stages: the first step corresponds to formation of amolecular acceptor–donor assembly in which the monomer unitsare arranged to give a relative stable deposit, and the second stepinvolves irreversible oxidative coupling of the aromatic rings [21].In addition, considering the complexity of the surface property andporous microstructure of activated carbon, we choose 0.8 V as thepulse potential during electropolymerization of Ni(saldMp) so asto prevent the large scale of linkage of phenyls between adjacentNi(saldMp) monomer units, which will bring about an irreversiblestep to give fully polymerized film and thus constrain the diffu-sion actions of counterions and the supporting electrolyte. The CVsbehavior of PAC electrode and bare AC electrode at same scan rate(10 mV s−1) are compared in Fig. 2(b). Firstly, it is characterized bya set of very broad waves, which peak potential are centered at ca.

0.9 V/0.7 V and associated with oxidation/reduction state transitionprocess in poly[Ni(saldMp)] film. Secondly, the area of responsivecurrents of PAC electrode during the scan range 0–1.2 V coveredcompletely the area of AC electrode, which indicates that the formerhas larger charge storage ability than the latter. The sweep ranges of

6104 F. Gao et al. / Electrochimica Ac

F(s1

Ctowpooisoe

3

tcatccatsco

ig. 2. Cyclic voltammograms of (a) graphite and poly[Ni(saldMp)] electrodes, andb) AC and PAC electrodes under different potential ranges in 1 M Et3MeNBF4/ANolution. Scan rate: 5 mV s−1. Structure of the Ni(saldMp) monomer is given in Chart(3).

V for the PAC electrode were subsequently enlarged from 0–0.9 Vo 0–1.2 V, the shape of CV maintained unchanged except that thexidation and reduction waves became widen. The ratios of Qox/Qre

ere subsequently 0.980, 0.978, 0.969, and 0.952 under differentotential sweep range, which indicated that the irreversible linkagef phenyls between monomer modules within polymer film did notccurred so that the PAC electrode remained the identical chargenput/output ability. The behaviors shown in Fig. 2(b) also demon-trate that the PAC electrode can be employed under the potentialf 1.2 V, thus no need for changing the potential range of positivelectrode for EDLC supercapacitor.

.3. Field emission scanning electron microscopy (FESEM)

Photomicrographs were obtained for the polymer film grown onhe AC electrode (Fig. 3). The morphology of AC electrode (Fig. 3(a))onsists of small blocky-shaped particles (2–5 �m in length) whichre apart from intervals. Fig. 3(b) shows the photomicrograph ofhe bare surface of a carbon particle of the AC electrode and it isharacterized by plenty of pores. It seems that there is no obvioushange for the morphology of PAC electrode (Fig. 3(c)) whereas the

ppearance of particles turns to be rugged. In contrast to Fig. 3(b),he polymer film can be significantly observed which grew on theurface of the carbon particle (Fig. 3(d)). Based on the morphologyhange of the polymer film from right to left in Fig. 3(e), the tracef the polymer growth may be presumed that small polymer seg-

ta 55 (2010) 6101–6108

ments or sites appeared on the commence of electropolymerizationprocess, then these segments gradually grew and linked each otherto form a thin film, following the rounded humps appeared on thefilm.

3.4. Chronoamperometry (CA)

The potential-step chronoamperometry was employed to assesshow the poly[Ni(saldMp)] film influence the charge transportbehavior of AC electrode. The potentiostatic data of the redox poly-mer have generally been treated according to a diffusion modelin which the chronoamperometric curve of an semi-infinitely filmis described by Cottrell equation [33]: i = nFAD1/2c/t1/2�1/2, whereD represents diffusion coefficient for counterions and c representsthe concentration of electroactive species.

Fig. 4 depicts typical chronoamperometric responses of AC elec-trode and PAC electrode, i vs. t−1/2 representation. These responsecurves could be divided into three domains. In domain 1, when timegoes to zero, both the currents of AC and PAC electrodes approach tolimiting values, which are lower than ones expected theoretically.These were attributed to the resistance effects of the experimen-tal circuit (e.g. uncompensated solution resistance [23,34,35], thecontact resistance and the resistance of carbon [36]). The currentsof PAC electrode are lower than those of AC electrode in domain 1,which is because PAC has a higher electronic resistance due to thecoated surface, hence lower current. Comparing the gap of currentin domain 1 between AC and PAC electrode under the potential stepof 0 → 1.2 V and 1.2 → 0 V, it is clear that the currents gap betweenAC and PAC electrode is less at the potential step of 1.2 → 0 V. Itis indicative that the electronic resistance of poly[Ni(saldMp)] isgreater at low potential whereas less at high potential. This is con-sistent with the low electronic conductivity of polymer under lowpotential proposed by Dahm et al. [21].

The domain 2 corresponds to the diffusion-controlled regime,which can be explained using the Cottrell equation to yield chargetransport dynamics. Pause and Pickup assume that the Cottrellequation (semi-infinitive diffusion system) pertains to the centralportion of the i vs. t−1/2 plot, and the most reasonable slope to adoptfor data analysis is the tangent to the curve from the origin [37].In this way, the D1/2C values are extracted in Fig. 4 and shown inTable 2. Actually, a linear relationship is found at domain 2 as seenin Fig. 4; however, its slope does not go through the origin, whichis associated with true diffusivity in the outer surface of the carbonparticles. Marcelo Zuleta et al. considered that the steep slope ofdomain 2 may be due to surface irregularities and pores penetrat-ing the surface, making the characteristic length of the diffusionfield much smaller than that of the particles [36]. In contrast withthe linear sections of AC electrode and PAC electrode in domain 2,the steep slope get smaller for the PAC electrode and this may beinfluenced by the poly[Ni(saldMp)] film which increases the diffi-culty for ionic diffusion into pores locating the surface of carbonparticles.

In domain 3, the fall-off of the currents means the semi-infinitivediffusion condition gets invalid at later time. The carbon particlesand polymer film can not be taken as semi-infinite system, as theresult, a finite diffusion boundary condition must be invoked. Thatis to say, there is a changeover, from a semi-infinite diffusion condi-tion to a finite diffusion condition. This also means the changeoverbetween two conditions corresponds to the transition part betweendomain 2 and domain 3. It can be observed clearly from Fig. 4 thatthe transition part of PAC electrode occurs at later time than that of

AC electrode, because the diffusion layer is thicker for the former.Additionally, the change of transition part looks unclear under thepotential step 1.2V → 0.0 V since the polymer film under oxidationpotential is swollen by the injection of counterions and solution soas to be in favor of diffusion process.

F. Gao et al. / Electrochimica Acta 55 (2010) 6101–6108 6105

F 10 kV)e ctrode

sbssfiws

TV

ig. 3. Field emission scanning electron micrographs of (a) an AC electrode (5k×,lectrode (5k×, 10 kV), and (d and e) the surfaces of carbon particles on the PAC ele

Table 2 provides the D1/2C values of AC and PAC electrode undereveral potential steps. We observed the D1/2C value differenceetween AC and PAC electrode is smaller under higher potential

tep of 0.6 → 1.2 V and 1.2 → 0.6 V than that under other potentialteps. It can be explained in terms of solution effects on polymerlm: the polymer has a compact structure under low potentialhereas gets loose when applied higher potential, and ion diffu-

ion within the film is faster under high potential, the injection

able 2alues of DCA

1/2C, obtained from the linear part of the chronoamperometric response usin

Electrode � (106 mol cm−2) 107DCA1/2C (mol cm−2s−1/2)

0.0 → 0.6 V 0.6 → 0.0 V

AC – 4.90 4.62PAC 1.60 4.13 4.19

, (b) the surface of a carbon particle on the AC electrode (200k×, 10 kV), (c) a PAC(200k×, 10 kV).

of ions and swelling of film take place mainly up to 0.6 V duringpolymer film oxidation.

3.5. Electrochemical impedance spectroscopy (EIS)

EIS was used to evaluate the low-frequency capacitance as wellas the Ohmic resistance and Faraday resistance of AC and PAC elec-trode. Fig. 5 shows the complex impedance plots for AC and PAC

g the Cottrell equation.

0.6 → 1.2 V 1.2 → 0.6 V 0.0 → 1.2 V 1.2 → 0.0 V

5.33 5.21 10.4 9.925.00 4.92 8.39 8.83

6106 F. Gao et al. / Electrochimica Acta 55 (2010) 6101–6108

F�m

ebclfitpToc

sesroodpptm

FP

Table 3Electrochemical impedance spectroscopy parameters for AC and PAC electrodes.

Potential CLFa (mF cm−2) RH

b (� cm2) RCTc (� cm2) ı (mS/cm)

0.0d 542.62 3.49 2.13 2.870.0 528.35 4.18 3.94 2.390.4 683.61 4.23 3.45 2.360.7 706.19 4.05 2.90 2.471.0 751.13 3.79 2.47 2.641.2 771.58 3.74 2.22 2.67

a

ig. 4. i vs. t−1/2 plot for a 0.0 → 1.2 V and 1.2 → 0.0 V potential step,app = 1.60 nmol cm−2 film of poly[Ni(saldMp)] deposited on AC electrode inonomer-free electrolyte solution, 1 M TBAP in AN.

lectrode under different potentials (0.0, 0.4, 0.7, 1.0, and 1.2 V). Foroth electrodes, the high-frequency data form two flattened semi-ircular arcs and the low-frequency data lies on an almost verticaline. The former maybe attribute to the electrochemical reactions ofunctional groups which were generated during the thermal chem-cal processing in the formation of the activated carbon as well ashe redox reaction between oxidation and reduction states of theolymer film, the later corresponds to the behavior of a capacitor.he shapes of low-frequency of PAC electrodes are similar with thatf AC electrode, which means the modified AC electrode remainsapacitor behavior, as shown in Fig. 5.

The high-frequency intercepts, RH, and the length of flattenedemicircular arcs, Rct, of both electrodes are given as functions of thelectrode potential in Table 3. RH can be regarded as the equivalenteries resistance containing the solution resistance and electronicesistance of electrode. Rct reflects the total reaction resistancef functional groups of carbon and oxidation/reduction transitionf poly[Ni(saldMp)]. All the data of resistances of PAC electrodeemonstrate the uniformity that they get peak values under low

otentials and gradually reduce to the values of AC electrode whenotentials get close to 1.2 V. This is in line with the conclusion inhe analysis of above potential-step chronoamperometry measure-

ent that the ionic diffusion gets easier within the polymer film

ig. 5. Complex plane impedance plots for AC electrode at potential of 0.0 V, andAC electrodes at 0.0–1.2 V in 1 M Et3MeNBF4/AN solution.

Low-frequency capacitance (10 MHz).b High-frequency intercept (10 kHz).c The length of semicircular arcs.d The EIS data of AC electrode, others are data of PAC electrode.

when it tends to be oxidative state by reason of solution effect. Theconductivities of AC and PAC electrode were estimated from theEIS results. In Table 3, all the conductivities of PAC electrode aresmaller than that of AC electrode, which means that the combi-nation of poly[Ni(saldMp)] with AC particles reduced the electricconductibility. However, the conductivity of PAC electrode gradu-ally increases with applied potential and approaches to that of ACelectrode.

The capacitances were calculated from the imaginary compo-nents of the impedance, as functions of the frequency. Fig. 6 depictsthe variation of capacitance in the low-frequency (10 MHz to 10 Hz)with multiple electrode potential for various oxidation state ofpoly[Ni(saldMp)] film. All the capacitances of PAC electrodes arehigher than that of bare AC electrode except for the data underthe potential of 0.0 V. The phenomenon can be explained that thepoly[Ni(saldMp)] film grown on the surface of carbon particlescomes about reversible oxidation/reduction reaction to contributeredox pseudo-capacitance, which greatly increases the total capac-itance of AC electrode, but the reaction does not happen under lowpotential such as 0 V. Actually, when the polymer was depositedon the surface of carbon, it covered most of the outer surface of thecarbon to reduce the double electric layer capacitance of AC elec-trode. As a result, the capacitance of PAC electrode is lower thanthat of bare AC electrode under the potential of 0.0 V.

3.6. Galvanostatic charge/discharge cycling

Fig. 7(a) shows the typical charge/discharge behavior ofpoly[Ni(saldMp)], which has two linear dependences on time. Thesection of 0.6–1.2 V corresponds to the transition between oxida-tion state and reduction state of poly[Ni(saldMp)] which can restore

Fig. 6. The capacitances of AC and PAC electrode as functions of frequency underdifferent potentials in 1 M Et3MeNBF4/AN solution.

F. Gao et al. / Electrochimica Acta 55 (2010) 6101–6108 6107

Fg

atdotcwcicpNmDoTrc

aaaibAiisp1otfipoptpnc

hF

possessed good ionic diffusion ability from D C values, especially

ig. 7. The typical charge/discharge curves at current density of 2 mA cm−2: (a)raphite and poly[Ni(saldMp)] electrodes, and (b) AC and PAC electrodes.

nd release charge reversibly, the section of 0.0–0.6 V correspondso the double-layer absorption/desorption process which is accor-ant with graphite electrode. In conclusion, the poly[Ni(saldMp)]n graphite electrode not only preserves the EDLC capacitance ofhe surface of graphite substrate, but also introduces the pseudo-apacitance of poly[Ni(saldMp)] for reversible redox reaction. Itas expected that the characteristic of poly[Ni(saldMp)] modifi-

ation effect may be achieved on activated carbon material andmproved the performance. Fig. 7(b) shows the charge/dischargeurves of AC and PAC electrodes. The charge/discharge time wasrolonged effectively for PAC electrode by the polymerization ofi(saldMp) on activated carbon particles, however, the improve-ent extent was limited and did not come up to graphite substrate.uring the polymerization, lots of polymer segments appearedn the forward AC electrode and suspended in the electrolyte.his problem impaired the polymerization efficiency. The reasonesponsible for the problem is unclear so far. We are currentlyontinuing our research in solving this problem.

The constant charge/discharge measurement results for the ACnd PAC electrodes are shown in Fig. 8. It can be seen that thedditional redox pseudo-capacitance of poly[Ni(saldMp)] is notffected visibly by current density from 1 mA cm−2 to 12 mA cm−2

n Fig. 8(a). The specific capacitances of PAC electrode increasey 18.8–13.6% in the current density range, comparing those ofC electrode. Meanwhile, the energy densities increase accord-

ng to equation E = 1/2CV2 at the same current density range. Its meaningful that both the specific capacitance and energy den-ity are effectively improved by the reversible redox reaction ofoly[Ni(saldMp)] film under the large current density up to about2 mA cm−2 for AC electrode. Due to the large specific surface areaf AC electrode, the massive poly[Ni(saldMp)] can be grown onhe extensive area, it offers huge reversible pseudo-capacitance,urthermore, the polymer film possesses good ionic diffusion abil-ty under oxidative state which has been demonstrated in theotential-step chronoamperometry test, which makes the poresf carbon under the polymer film effective, and then retains largeortion of double electric layer capacitance of carbon. Therefore,he poly[Ni(saldMp)] film play a positive part in enhancing theerformance of AC electrode. The columbic efficiency in Fig. 8(b)early remains unchanged, which reflecting on the approximately

omplete reversibility of the redox reaction of poly[Ni(saldMp)].

The stabilities of AC electrode and PAC electrode upon cyclingave been evaluated and the decay of specific capacitance is given inig. 8(b). During the 250 cycles, the specific capacitance of AC elec-

Fig. 8. (a) The correlation between the specific capacitance and current density forAC and PAC electrode, and (b) variation of the discharge capacity with the numberof constant current cycles at about 2000 mA g−1. Electrolyte: 1 M Et3MeNBF4 in AN.

trode decayed from 605.3 mF cm−2 to 556.7 mF cm−2, a decreasefrom 715.9 mF cm−2 to 645.4 mF cm−2 for the PAC electrode wasobserved during the first 50 cycles then it remained nearly con-stant for the next 200 cycles. The specific capacitance of PACelectrode was still 9.5% higher than that of AC electrode at 250th.The columbic efficiencies kept the values near to 99% and 97% for ACand PAC electrode. The analysis of galvanostatic charge/dischargecycling indicated that the poly[Ni(saldMp)] could improve the spe-cific capacitance steadily for short cycle times, the stability forlonger cycle times will be investigated in the next study.

4. Conclusions

The poly[Ni(saldMp)] was electropolymerizated by the two-step potential pulse method on the activated carbon particles.From the cyclic voltammetry curves, the couple of oxidative andreductive current peaks were observed at ca. 0.9 V and 0.7 V. Thepoly[Ni(saldMp)] modified AC electrode had better charge stor-age ability than the AC electrode and could work at the potentialof 0–1.2 V. SEM micrographs show the poly[Ni(saldMp)] presentsa film on the surface of carbon particles, and the morphologyof PAC electrode had no obvious change except that the parti-cles looked to be rugged. Based on the data of CA and EIS, theelectronic resistance of poly[Ni(saldMp)] film was greater at lowpotential whereas less at high potential. The poly[Ni(saldMp)] film

1/2

under oxidation state. The capacitance of PAC electrode at low fre-quency gradually increased as a function of potential. The specificcapacitance and energy density of PAC electrode got enhanced atcurrent density range from 1 mA cm−2 to 12 mA cm−2, meanwhile

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[34] M.E.G. Lyons, in: M.E.G. Lyons (Ed.), Electroactive Polymer Electrochemistry.Part 1. Fundamentals, Plenum Press, New York, 1994 (Chapter 1).

108 F. Gao et al. / Electrochim

he columbic efficiency approached to 100%. The electropolymer-zation of poly[Ni(saldMp)] on AC electrode is an attractive methodo improve the performance of AC electrode for EDLC capacitor,specially for the large current condition. The next work is tomprove the electropolymerization condition and the stability to

aintain the initial performance.

cknowledgments

Financial supports of this work by Beijing Natural Science Foun-ation of China (No. 2093039) and Program for New Centuryxcellent Talents in University (NECT) are gratefully acknowledged.

eferences

[1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals andTechnological Applications, Kluwer Academic/Plenum, New York, 1999.

[2] F. Fusalba, D. Bélanger, J. Phys. Chem. B 103 (1999) 9044.[3] K.E. Ryu, S.K. Jeong, J. Joo, K.M. Kim, J. Phys. Chem. B 111 (2007) 731.[4] Y.Y. Cao, T.E. Mallouk, Chem. Mater. 20 (2008) 5260.[5] S.K. Mondal, K. Barai, N. Munichandraiah, Electrochim. Acta 52 (2007) 3258.[6] C.Q. Sun, J. Guo, K. Lian, J. Phys. Chem. C 112 (2008) 14838.[7] J.P. Ferraris, M.M. Eissa, I.D. Brotherston, D.C. Loveday, A.A. Moxey, J. Elec-

troanal. Chem. 459 (1998) 57.[8] C. Arbizzani, M.C. Gallazzi, M. Matragostino, M. Rossi, F. Soavi, Electrochem.

Commun. 3 (2001) 16.

[9] K.S. Ryu, Y.G. Lee, Y.S. Hong, Y.J. Park, X.L. Wu, K.M. Kim, M.G. Kang, N.G. Park,

S.H. Chang, Electrochim. Acta 50 (2004) 843.10] P. Daum, R.W. Murray, J. Phys. Chem. 85 (1981) 389.11] P. Daum, R.W. Murray, J. Electroanal. Chem. 103 (1979) 289.12] P. Daum, J.R. Lenhard, D. Rolison, R.W. Murray, J. Am. Chem. Soc. 102 (1980)

4649.

[[

[

ta 55 (2010) 6101–6108

13] C. Gosden, K.P. Healy, D. Pletcher, J. Chem. Soc., Dalton Trans. 8 (1978) 972.14] K.P. Healy, D. Pletcher, J. Organomet. Chem. 186 (1980) 401.15] C. Gosden, J.B. Kerr, D. Pletcher, R. Rosas, J. Electroanal. Chem. Interfacial Elec-

trochem. 117 (1981) 101.16] K.A. Goldsby, J. Coord. Chem. 19 (1988) 83.17] K.A. Goldsby, L.A. Hoferkamp, Chem. Mater. 1 (1989) 348.18] P. Audebert, P. Capdevielle, M. Maumy, New J. Chem. 16 (1992) 697.19] M. Maumy, P. Capdevielle, P.H. Aubert, P. Audebert, M. Roche, New J. Chem. 21

(1997) 621.20] F. Bedioui, E. Labbe, S. Gutierrez-Granados, J. Devynck, J. Electroanal. Chem. 301

(1991) 267.21] C.E. Dahm, D.G. Peters, J. Simonet, J. Electroanal. Chem. 410 (1996) 163.22] M. Vilas Boas, C. Freire, B. Castro, P.A. Christensen, A.R. Hillman, Inorg. Chem.

36 (1997) 4919.23] M. Vilas Boas, C. Freire, B. Castro, A.R. Hillman, J. Phys. Chem. B 102 (1998) 8533.24] I.C. Santos, M. Vilas-Boas, M.F.M. Piedade, C. Freire, M.T. Duarte, B. Castro, Poly-

hedron 19 (2000) 655.25] P. Audebert, P. Hapiot, P. Capdevielle, M. Maumy, J. Electroanal. Chem. 338

(1992) 269.26] G. Maki, J. Chem. Phys. 28 (1958) 651.27] H.L. Chen, Y.H. Pan, S. Groh, T.E. Hagan, D.P. Ridge, J. Am. Chem. Soc. 113 (1991)

2766.28] K. Yoshida, S. Isoda, Chem. Mater. 19 (2007) 6174.29] R.H. Holm, G.W. Everett, A.A. Chakravorty, Prog. Inorg. Chem. 7 (1966) 183.30] A. Petr, L. Dunsch, A. Neudeck, J. Electroanal. Chem. 412 (1996) 153.31] K. Ding, M. Zhao, Q. Wang, Rus. J. Electrochem. 43 (2007) 1082.33] A.J. Bard, L.R. Faulkner, Electrochemical Methods Fundamentals and Applica-

tions, John Wiley & Sons, New York, 1980, p. 142.

35] P.K. Ghosh, A.J. Bard, J. Electroanal. Chem. 169 (1984) 113.36] M. Zuleta, M. Bursell, P. Bjornbom, A. Lundblad, J. Electroanal. Chem. 549 (2003)

101.37] C.D. Pause, P.G. Pickup, J. Phys. Chem. 92 (1988) 7002.