Cite this: RSC Advances, 2013, 3,2428

Carbon nanofiber–RuO2–poly(benzimidazole) ternaryhybrids for improved supercapacitor performance3

Received 5th November 2012,Accepted 3rd December 2012

DOI: 10.1039/c2ra22776b

www.rsc.org/advances

Beena K Balan,a Harshal D Chaudhari,b Ulhas K Kharulb and Sreekumar Kurungot*a

Carbon nanofiber–RuO2–poly(benzimidazole) ternary hybrid electrode material which integrates dual wall

decoration and interfacial area tuning for supercapacitor applications has been devised based on a simple

approach. This is achieved by decorating RuO2 nanoparticles of size ca. 2–3 nm along the inner and outer

walls of a hollow carbon nanofiber (CNF) support (F-20RuO2). In the next step, a proton conducting

polymer, phosphoric acid doped polybenzimidazole (PBI-BuI), interface is created along the inner and

outer surfaces of this material. A 103% increase in the specific capacitance is obtained for RuO2–PBI hybrid

material as compared to that of F-20RuO2 at the optimum level of the polymer wrapping. Apart from the

high specific capacitance, the RuO2–PBI hybrid materials exhibit enhanced rate capability and excellent

electrochemical stability of 98% retention in the capacitance. Such a remarkably high activity can be

primarily attributed to the efficient dispersion of active sites achieved by properly utilizing inner and outer

surfaces of CNF. Apart from this, the facile routes for ion transport created as a result of PBI incorporation

coupled with excellent interfacial contact between the RuO2 and the electrolyte resulting in the improved

utilization of the active material also contribute to the improved activity. In addition to this, the synergistic

effects of pseudocapacitive contribution from both the PBI-BuI and RuO2 also contribute to the redefined

performance characteristics.

Introduction

Electrochemical means for energy storage plays a key role inefficient and versatile use of energy as it can open up manypossibilities in the perspective of exploitation of renewableenergy.1 Considering the immense development of super-capacitors (SCs) in the field of energy storage devices,development of SCs has become one of the most targetedresearch areas to fulfil the future requirements of energystorage.2,3 New approaches which focus on the challenges andapplications on the rational design of advanced materials arecrucial for this and a notable improvement in performance hasachieved so far. However, in line with the search for novelelectrode materials, modifications in the existing state-of-artmaterials in new directions will be a logical way to meet thereal time requirements for several specific applications.Hydrous ruthenium oxide, the state-of-art supercapacitormaterial, is an ideal candidate for the charge storageapplications with its theoretical capacitance of the order of2000 F g21.4,5 Apart from this, RuO2 offers many promisingproperties such as high proton and electron conductivity, high

chemical stability etc., which can be tailored by adoptinginnovative preparation and integration strategies.6–8 So far,various approaches adopted for the property modulation ofRuO2 include exploration of composites with differentcarbonaceous materials, investigation of various syntheticprotocols for increasing and controlling morphology, porosity,and surface area etc.9–13

In a pseudocapacitive material like RuO2, the chargestorage processes are primarily surface reactions which aredominated by coupled electron–proton transfer process. Thisinvolves a reversible change in the valency of RuO2 by theexchange of protons with the solution.14,15 Hence theefficiency of the performance of a SC is determined by theavailability of the interspace between the electrode andelectrolyte where the energy-providing or storage processestake place. This makes the interfacial area engineeringimperative.16,17 Moreover, the presence of any species whichcan facilitate the exchange of electrons and protons/ionsbetween the electrode and electrolyte can also contribute tothe improved charge storage capability. Recently, conductingpolymers such as polyaniline, polypyrrole, poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate) etc. have been inten-sively explored in this field.13,18 Apart from this, for practicalapplications where high energy and power density are needed,high mass loading of the active material is necessary. In suchcases, the active material becomes densely packed and a verythin outer layer of the active materials is involved in the

aPhysical and Materials Chemistry Division, National Chemical Laboratory, Pune,

411008, India. E-mail: k.sreekumar@ncl.res.in; Tel: 02025902566bPolymer Science and Engineering Division, National Chemical Laboratory, Pune,

411008, India

3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra22776b

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charge–discharge process. This will result in low specificcapacitances.13 An ideal situation exists if the electrodematerial can be incorporated on both the inner and outersurfaces of one dimensional, hollow carbon nanomorpholo-gies like nanotubes and nanofibers. Such dual wall decorationwill result in the reduced thickness of the active material. Inaddition to this, it can also provide intriguing confinementeffect along with unique electronic and structural features.19,20

So far, all these promises have been explored individually andhave resulted into an accountable improvement in the area.However, it is worth mentioning that a method based on alogical combination of all the aforementioned approaches willresult into a significant improvement in the charge storagecapability.

In this context, we report here a facile move towards theperformance improvement of RuO2 and the key innovation inthe present strategy is the integration of interfacial area tuningwith dual wall decoration. Briefly, dual wall (inner and outer)decoration of hydrous RuO2 nanoparticles in the range of 2–3nm on a functionalized hollow carbon nanofiber (F-CNF) wasachieved by a modified polyol process (F-20RuO2). Properbalancing of the surface tension and polarity of the reactionmedium coupled with the benefits of the capillary effectinduced by the morphology of CNF mobilizes the desired dualwall decoration of the particles.21 Subsequently, interfacialarea tuning and a concomitant enhancement in the utilizationof the active material were achieved by incorporating a protonconducting polymer, phosphoric acid doped polybenzimida-zole (PBI-BuI), interface along the inner and outer surfaces ofF-20RuO2.22 Various steps involved in the design of F-20RuO2–PBI-BuI hybrid materials are systematically shown inScheme 1. Through this simple PBI-BuI incorporation,potentially conductive SC electrodes could be easily fabricated

and the amount of the PBI-BuI in the hybrid was fine tuned byvarying the polymer/carbon (P/C) ratio at 0.25, 0.5 and 1, whichare respectively designated as RP-0.25, RP-0.5 and RP-1 in thelater sections. Such a rational approach retains the electricalconductivity and structural integrity of the CNF whilemaintaining the continuous proton conducting network bythe polymer incorporation. These modifications will result inslight changes in the electrical conductivity of the system.Apart from this, the conducting path can also enhance thediffusion of electrolyte into the bulk of the electrode materialresulting in the utilization of the underlying active materialswhich remain as a part of the dead volume otherwise.Consequently, there is at least 50% increase in the specificcapacitance after the conductive wrapping and the greatestimprovement obtained at the optimum level of the polymerwrapping is 103%.

Experimental section

Materials

CNF was procured from Pyrograf Products, Inc, USA.Ruthenium chloride (RuCl3), 3,39-diaminobenzidine (DAB)and 5-tert-butyl isophthalic acid were purchased fromAldrich Chemicals. Polyphosphoric acid (PPA) was purchasedfrom Alfa Aesar. Ethylene glycol (EG), N,N-dimethyl acetamide(DMAc) and sulfuric acid (H2SO4) were procured from RankemChemicals. All the chemicals were used as received withoutany further purification. A poly(tetrafluoroethylene) (PTFE)filter paper (pore size, 0.45 mm; Rankem) was used for thefiltration. A 200 mesh carbon coated Cu grid (Ted Pella, Inc.)was used for the transmission electron microscopy (TEM)observations.

Functionalization of carbon support

For the functionalization of the carbon support, 1 g of thesupport material was dispersed well in 200 mL of 30% H2O2.This was then refluxed at a temperature of 60 uC for 5 h.Subsequently, the mixture was filtered, washed with DI waterand dried.

Preparation of F-20RuO2

20 wt% RuO2 loaded carbon nanofiber material (F-20RuO2)was prepared by a modified polyol process. Briefly, the hollowcarbon nanofiber was first activated by an H2O2 treatment tointroduce oxygen containing functional groups (FCNF). Therequired amount of the RuO2 precursor, RuCl3?H2O wasdissolved in a 3 : 2 mixture of ethylene glycol : water andwas added dropwise to the FCNF slurry in the same solvent.This was then kept for aging for 10–12 h at room temperature.The solvent composition and aging time are two criticalparameters in achieving the RuO2 decoration along the innerand outer surfaces of the substrate. The pH of this slurry wasthen adjusted to neutral by the addition of 1 M NaOH solution.This was then refluxed at 140 uC. Finally, the slurry was filteredand washed several times with DI water until neutral pH wasmaintained and the wet cake thus obtained was dried in anoven at 100 uC for 24 h.

Scheme 1 Various steps involved in the realization of the F-20RuO2–PBI-BuIhybrid material possessing significantly enhanced interfacial area and welldefined pathways for reactant transport. The highlighted portion is the cross-sectional view of a tube depicting the mode of integration and the pathways forion transport.

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Preparation of polybenzimidazole (PBI-BuI)

Polybenzimidazole was synthesized by solution polycondensa-tion method using polyphosphoric acid (PPA) as the solvent. Ina typical procedure, a three-necked flask equipped with amechanical stirrer, N2 inlet and CaCl2 drying tube was chargedwith 500 g of PPA, 10 g (0.0467 mol) of 3,39-diaminobenzidine(DAB) and the temperature was elevated to 140 uC. Aftercomplete dissolution of DAB, 10.37 g (0.0467 mol) of 5-tert-butylisophthalic acid was added; temperature was slowlyraised to 170 uC and maintained for 5 h under constant flowof N2. The temperature was further raised to 200 uC andmaintained for 14 h. After the completion of the reaction, thetemperature was lowered and the highly viscous reactionmixture was poured onto the stirred water. The precipitatedpolymer was crushed and thoroughly washed with water tillneutral to pH. The polymer was then kept overnight in 10%aqueous NaHCO3, washed with water until neutral pH andsoaked in methanol for 8 h to extract the water. Dried polymer(100 uC, 3 days) was further purified by dissolving in DMAc toget a 3 wt% solution, removing undissolved material, if any, bycentrifugation at 3000 rpm for 3 h and reprecipitation ontostirred water. The polymer was kept in methanol for 8 h,filtered, dried at 60 uC for 24 h and then in vacuum oven at 100uC for a week. The inherent viscosity of the polymer was 0.48dL g21 using a 0.2 g dL21 polymer solution in DMAc.

PBI incorporation in F-20RuO2

For the polymer incorporation, PBI-BuI solution in DMAc wasselected and 5 mg of F-20RuO2 hybrid was added to therequired volume of the 0.25 wt% polymer solution. To initiatethe PBI-BuI entry into the inner cavity and to get a uniformdispersion, the mixture was initially sonicated using a bathtype sonicator for 10 min. In the next step, the mixture waskept stirring for 10 h at room temperature to facilitate PBI-BuIentry into the tubular region and homogeneous coverage alongthe inner and outer surfaces. The same procedure wasrepeated for the incorporation of PBI-BuI at various P/C ratiosalso. Phosphoric acid doping was achieved by dipping theelectrodes in o-phosphoric acid for 24 h and subsequentlydried in a vacuum oven at 100 uC for 48 h. The CNF andF-20RuO2 at the P/C ratios of 0.25, 0.5 and 1.0 are respectivelydenoted as CP-0.25 and RP-0.25, CP-0.5 and RP-0.5 and CP-1and RP-1.

Characterization

The TEM images were taken by a TECNAI-T 30 modelinstrument operated at an accelerating voltage of 300 kV.Samples for TEM imaging were prepared by placing a drop ofthe catalyst sample in isopropanol onto a carbon-coated Cugrid (3 nm thick, deposited on a commercial copper grid forelectron microscope), dried in air and loaded into the electronmicroscopic chamber. X-Ray Diffraction (XRD) was conductedusing a Philips X9pert pro powder X-ray diffractometer (Cu-Karadiation, Ni filter). Thermogravimetric Analysis (TGA) wasperformed on a SDT Q600 TG-DTA analyzer. XPS measure-ments were carried out on a VG MicroTech ESCA 3000instrument at a pressure of .1 6 1029 Torr (pass energy of50 eV, electron takeoff angle 60u) using monochromatic Mg-Ka

(source, hn = 1253.6 eV). The overall resolution of theinstrument was y1 eV.

Electrochemical studies

All cyclic voltammetry (CV) and electrochemical impedancespectroscopy (EIS) analysis were performed on an AutolabPGSTAT30 (Eco Chemie) instrument. For the F-20RuO2, RP-0.25, RP-0.5 and RP-1 electrodes a two electrode configurationwas used however for the pristine CNFs and PBI-BuIincorporated CNFs; a three electrode configuration was used.The galvanostatic charge–discharge measurements were doneusing a Solatron SI1287 electrochemical interface withCarware software at different current densities in a twoelectrode configuration. The electrodes for the capacitancemeasurements were prepared by coating the respectivematerials on a carbon paper having an electrode area of 1 61 cm2 followed by drying at 120 uC in a vacuum oven. For thepreparation of the slurry, 90% of the active material and 10%Nafion binder were used in case of F-20RuO2 material and allthe hybrid materials were used as such without any externalbinder. The material loading on the electrode was kept as 1 mgcm22. An aqueous solution of 0.5 M H2SO4 was used as theelectrolyte for all the electrochemical analysis. The capacitanceof all the materials was calculated from cyclic voltammetry byusing the equation C = Q/V, were ‘‘Q’’ is the cathodic charge,‘‘V’’ is the discharge voltage. The capacitance (C) of thematerials was calculated from the discharge line using theformula C = I*t/DV, where I is the discharge current, t is thedischarge time and DV is the corresponding voltage drop. Themass specific capacitance was obtained by dividing C by themass of the electrodes.

Results and discussion

The structure and morphology of the F-20RuO2, RP-0.25, RP-0.5 and RP-1 materials are evaluated using XRD, TGA, XPS andTEM analysis. Fig. 1(a) shows the comparison of the XRDprofile obtained for CNF, F-20RuO2, RP-0.25, RP-0.5 and RP-1.For CNF, an intense peak at 2h = 26.4u is obtained. This can beattributed to the (002) plane diffractions from the graphiticcarbon of the CNF. The other low intense peaks obtained athigher 2h values correspond to the (100), (101), and (004)planes of CNF.23 After the RuO2 decoration, two additionalbroad peaks appeared between 2h = 25–40u and 50–65u. Thesebroad peaks are characteristic of amorphous RuO2 formed onthe CNF support due to the lower synthesis temperature of 140uC.24 The XRD profiles of F-20RuO2 after PBI-BuI incorporation(RP-0.25, RP-0.5 and RP-1) are also presented in Fig. 1(a). TheXRD profiles indicate that the characteristic peaks are retainedeven after the PBI-BuI incorporation. Further, to give addi-tional evidence for the effective decoration of RuO2 nanopar-ticles on CNF, XRD analysis of F-20RuO2, RP-0.25, RP-0.5 andRP-1, thermal annealed at a temperature of 500 uC for 5 h isalso conducted and is shown in Fig. 1(b). Sharp diffractionpeaks centered at around 2h values 28.1u, 35u, 55u and 67u areobserved for all the samples. These peaks can be ascribable tothe (110), (101), (211), and (310) reflections of the rutile type

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anhydrous RuO2 structure, which are consistent with theearlier reports.12 Moreover, the peak position and the FWHMof characteristic reflections remain the same in the polymerincorporated samples also. Hence from the XRD profile, it canbe deduced that the size, dispersion and crystallinity of thenanoparticles are retained after the polymer incorporationalso.

Further, to study the composition as well as the thermalstability, TG/DTG analysis of F-20RuO2, RP-0.25, RP-0.5 andRP-1 materials is carried out in air from RT–900 uC and theresults are given in Fig. 2(a) and (b). TG analysis of the pureCNF is also carried out under the same conditions and theresulting TGA profile is also included in Fig. 2(a). As can beseen from the Fig. 2(a), CNF shows a single step weight loss.However, the F-20RuO2 material shows a multistep weight lossand the residue content obtained after the weight loss is18.4%. From the DTG curve given in Fig. 2(b), a clear three stepweight loss is observed for F-20RuO2. The weight loss centeredat 150 uC and 250 uC can be attributed to the loss ofchemisorbed and crystalline water respectively.25 The thirdstep weight loss observed from 400 uC to 820 uC is due to theoxidation of the CNF support in the material. The slowerweight loss observed further confirms the amorphous nature

of the RuO2 in this material.12 Interestingly, the RP-0.25, RP-0.5 and RP-1 materials also display a three step weight loss.The weight loss centered at 250 uC in RP-0.25, RP-0.5 and RP-1can be attributed to the loss of crystalline water. The newweight loss observed in the temperature range of 350–600 uCcorresponds to the thermal degradation of the PBI-BuI and thethird weight loss observed from 600 uC is ascribable to the CNFoxidation in the RP-0.25, RP-0.5 and RP-1 materials. Therelative PBI-BuI contents calculated for the RP-0.25, RP-0.5 andRP-1 materials from the TGA curves presented in Fig. 2(a) are20.0, 31.1 and 48.2% respectively. Finally, the RuO2 loadingsobtained from the residue content for RP-0.25, RP-0.5 and RP-1 materials are 13.8, 11.2 and 8.5% respectively. These valuesare close to the calculated PBI-BuI content and RuO2 loadingin the respective samples, considering the total contributionfrom the CNF, PBI-BuI and RuO2.

As TEM has the potential to be a powerful technique forunderstanding the behaviour of materials in nanospace, thistechnique is used to confirm the nanoparticle decorationfollowed by the PBI-BuI incorporation. Accordingly, Fig. S1(a)and (b), ESI,3 which correspond to the TEM images of pristineCNF at different magnifications, clearly depict the peculiarmorphological characteristics like the open tips with largeenough inner diameter, the duplex structure of the systemwhich reveals the edges of the slanting graphene planes in the

Fig. 1 Powder XRD patterns obtained for CNF, F-20RuO2, RP-0.25, RP-0.5 andRP-1 (a) before and (b) after annealing.

Fig. 2 Comparison of the (a) TGA and (b) DTG curves of CNF, F-20RuO2, RP-0.25,RP-0.5 and RP-1 performed in air atmosphere from RT–900 uC at a heating rateof 10 uC min21.

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inner cavity and the smooth finish of the outer wall resultedfrom the stacking of the parallel graphene planes.26,27 TheF-20RuO2 (XPS are given in ESI, Fig. S2, ESI3), where thenanoparticle decoration is expected in the inner cavity as wellas on the outer walls, is also explored using TEM analysis andthe respective images at various magnifications are shown inFig. 3(a and b). The image clearly displays the excellentdispersion of RuO2 nanoparticles of size ca. 2–3 nm on theouter walls along with their confinement in the inner cavity.Even though it is difficult to differentiate the particledistribution along the inner and outer walls due to thedominance in the contrast by the particles lying on the outerwall, a separate experiment using pristine CNF unambiguouslyconfirmed RuO2 decoration along the inner wall (Fig. S3, ESI3).The logic of doing this control experiment was based on thefact that unless the pristine CNF is subjected to chemicalfunctionalization, the outer wall will remain inert and, undersuch a situation, the particle dispersion can occur exclusivelyalong the inner wall which is inherently active. The imageshown in Fig. S3, ESI3 clearly shows this and supplement ourconclusion that in Fig. 3(a and b), the RuO2 decoration occursalong the inner and outer walls of FCNF. Achieving such afinite distribution of small-scale nanoparticles is extremelyimportant as a significant improvement in the pseudocapaci-

tive effect occurs when electroactive materials approachnanoscale dimensions.28,29

In order to validate the extent of conductive wrappingeffected as a result of the PBI-BuI incorporation, the hybridmaterial obtained after the polymer incorporation was alsofollowed using TEM imaging. Accordingly, Fig. 3(c and d) showthe TEM images of RP-0.5 at different magnifications. Thestriking feature that can be visualized on comparison ofFig. 3(a) and (c) is the hazy nature of the image after the PBI-BuI incorporation. In TEM, presence of a polymer on the outersurface of carbon nanotube (CNT) may result in such hazyimages as one will be out of focus while trying to focus theother. Hence, it can be inferred that a thin wrapping of PBI-BuIis formed over the RuO2 nanoparticles, thus creating anexcellent proton conducting interfacial region upon phospho-ric acid doping. It is well known that PBI-BuI can act as athermally and mechanically stable containment for phospho-ric acid and subsequently can trigger proton mobilizationthrough the matrix.30 Apart from the haziness of the image,the inset of Fig. 3(d) shows a clear picture of the skin layer ofPBI-BuI formed on the RuO2 nanoparticles where the electronconducting medium (CNF) with the surface active groups(RuO2) meets the proton conducting path (PBI-BuI). Further, tofurnish an additional confirmation of the presence of PBI-BuIin the F-20RuO2, we have carried out TEM analysis at higher P/C contents. At the ratio of 0.5, the PBI-BuI content is justsufficient to form a thin and continuous coating on RuO2

nanoparticles. Of interest, while altering the P/C ratio from 0.5to 1, this layer expanded to almost 5–6 nm above the RuO2

layer (Fig. S4, ESI3). This is further manifested in the imageshown in the inset of Fig. S4, ESI.3 All these observationsclearly confirm the effective wrapping of PBI-BuI on the outersurface of F-20RuO2 material.

On the other hand, due to the complexity involved inimaging the three phases (i.e. CNF, RuO2 and PBI-BuI)simultaneously, a restriction in obtaining direct evidence forthe presence of PBI-BuI in the inner cavity of the hybrid arises.Hence, to provide evidence for the presence of the PBI-BuI inthe inner cavity, we carried out a logical move by diluting someof the experimental conditions wherein we have incorporatedPBI-BuI in pristine CNF. Accordingly, the TEM images of CP-1and CP-0.5 shown in Fig. 4(a) and (b), respectively, illustratethe formation of a uniform wrapping of the polymer ofthickness ca. 10 nm on the entire length of the CNF. Sinceboth CNF and PBI-BuI are carbon based materials, the contrastwill be narrow and so distinguishing the contrast between PBI-BuI and CNFs, especially in the inner cavity at low magnifica-tion will be difficult (Fig. S5, ESI3). However, the highmagnification image of the CP-0.5 given in Fig. 4(b) showsthe discontinuous filling of the polymer in the inner cavity ofthe CNFs. The presence of such vacant space provides awindow of opportunity to visualize the PBI-BuI layer formedalong the curved inner wall surfaces lying far from the voidsurface. The difference in the path length of the focusingbeam by few nanometers, which is a function of the diameterof CNF, help to demarcate the wrapping of PBI-BuI along the

Fig. 3 TEM images of (a and b) F-20RuO2 showing the fine distribution of NPs inthe inner cavity and outer wall and (c and d) are the representative images ofRP-0.5 which clearly indicate the formation of a thin layer of PBI-BuI on theouter wall of CNF. The insets in (d) show a clear picture of the skin layer of PBI-BuI formed on the RuO2 nanoparticles.

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curved surface from undesirable bulk filling in the innercavity, which is absent in the present case as evident from theTEM images. These observations unambiguously confirm theconfinement of PBI-BuI layers both along the inner and outerwalls of FCNF, leaving a continuous open path for thediffusion of the species along the inner wall. In addition tothis, the lower thickness of PBI-BuI observed in the case of CP-0.5 compared to that of CP-1 helps to provide a distinct benefitin terms of favouring the formation of a FCNF, RuO2 and PBI-BuI interlayer with placid interfacial contact. This can lead to areduction in the contact resistance between the electrodematerial and the electrolyte for the charge transport, which is acrucial parameter for attaining better capacitive property inmaterials.

After confirming the incorporation of PBI-BuI in F-20RuO2,the advantages of these hybrid materials as active super-capacitor electrodes were explored using CV, EIS and galvano-static charge–discharge measurements in a two-electrodeconfiguration in 0.5 M H2SO4. The CV analysis of F-20RuO2,RP-0.25, RP-0.5 and RP-1 was carried out in a potential rangeof 20.8–0.5 V at various scan rates. Fig. 5(a) shows thecomparison of the CV response obtained at a scan rate of 20mV s21. The increase in the current leading to a concomitantenhance in the area under the CV curve is a measure of theimprovement in the total capacitance of the material. Thus,the specific capacitances obtained for RP-0.25, RP-0.5 and RP-1are ca. 948, 1262 and 1060 F g21 respectively, which are 1.5, 2and 1.7 times higher as compared to 620 F g21 obtained forF-20RuO2. This result clearly demonstrates that the chargestorage capability of RuO2–PBI-BuI hybrid systems outper-forms that of F-20RuO2 in terms of higher capacitance. Theexcellent charge storage performances of F-20RuO2, RP-0.25,RP-0.5 and RP-1 are also evident from the CV response atvarious scan rates in the range of 5–300 mV s21 as shown inFig. S6(a–d), ESI3 respectively. The cyclic voltamograms of allthe materials exhibit almost rectangular shape; apart fromthis, the capacitance current of all the electrodes increaseswith increasing the scan rate which indicates better capaci-tance properties of these materials.31 Moreover, the specificcapacitance obtained for FCNF at the same loading is 4 F g21

only (Fig. S7, ESI3). Thus, almost the entire capacitanceobtained can be ascribed to the better utilization of theRuO2 material itself. This capacitance value obtained issignificantly higher than the capacitance values obtained forother RuO2 based systems at similar loading. For e.g. Popovet al. reported a specific capacitance value of 854 F g21 at 20wt% RuO2 loading.24 A specific capacitance of 522 F g21 (RuO2

alone) was reported for RuO2–MWCNT nanocomposites byMitani et al.9 For a graphene based composite of RuO2, Wuet al. obtained a specific capacitance of 250 F g21at 15 wt%loading.32

CV analysis of PBI-BuI incorporated pristine CNF was alsocarried out and the results are presented in Fig. S8, ESI.3 TheCV profile indicates that PBI-BuI participates in the faradaicprocess and contributes to the improvement of the specificcapacitance. There is a clear improvement in the specificcapacitance after the polymer incorporation and the highestimprovement (nearly 4 times) is obtained in case of the CP-1system. However, for the CNF–RuO2–PBI hybrids, the highestimprovement is obtained for the RP-0.5 system. This dis-crepancy observed can be attributed to the decreased RuO2

surface area accessed by the electrolyte at high polymer layerthickness. However, the results demonstrate a combined

Fig. 5 Comparison of (a) the cyclic voltammograms at the scan rate of 20 mVs21 and (b) impedance curves obtained for F-20RuO2, RP-0.25, RP-0.5 and RP-1electrodes in 0.5 M aqueous H2SO4.

Fig. 4 TEM images of (a) CP-1 and (b) CP-0.5 highlighting the PBI-BuI layerthickness on the outer wall and the portion of the PBI-BuI layer along the innerwall region of CNF.

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pseudocapacitive contribution from the active material andthe PBI-BuI.

The improved specific capacitance obtained by the con-ductive wrapping is further confirmed by the EIS and theNyquist plots obtained for all the four electrode materials areshown in Fig. 5(b). A sharp increase of the imaginary part ofEIS at lower frequency is due to capacitive behavior of theelectrode whereas the semicircular loop at higher frequenciesis ascribed to charge-transfer resistance of the electrode.33 Theequivalent series resistances (ESR) obtained for the F-20RuO2

electrode was y4.25 V. Once the polymer wrapping wasintroduced in the system, the ERS value decreases and thatobtained for RP-0.25 and RP-0.5 electrodes were 3.9 V and 3.4V, respectively. This is a clear indication of the improvementin the conductivity of the hybrid electrodes. However, on afurther increase in the polymer content, the ESR value remainsconstant but the plot shows a deviation from the idealbehaviour with a semicircular loop at the high frequencyregion.34 The observed trend in the ESR can be attributed to the following facts. The phosphoric acid doped PBI-BuI, used

in the present study for the interfacial area tuning, is a wellestablished candidate in the literature as a proton conductingmaterial.35 Hence it can improve the proton/ion diffusion intothe electrode material to participate in the redox reactionsalong with the interfacial area tuning. Though a sufficientamount of PBI-BuI is good for improving the conductivity, alarger amount of the same may result in diffusional limitationby creating long transport paths for ions. Hence, a properbalance between the PBI-BuI content and the conductivity iscrucial to have high capacitance performance.

Charge–discharge profiles of all the materials were alsotaken in a stable window of 0–1 V and a comparison of thecharge–discharge profiles obtained at the current density of 1A g21 is shown in Fig. 6(a). A linear voltage-time profile andthe highly symmetric charge–discharge characteristics areindicative of good capacitive behavior.36 As can be manifestedfrom Fig. 6(a), F-20RuO2, RP-0.25 and RP-0.5 displayed idealbehaviour with a substantially prolonged charging anddischarging time in RP-0.25 and RP-0.5 systems as comparedto the F-20RuO2 system. However, the RP-1 electrode shows anasymmetric behaviour with charging time significantly higherthan the discharging time. This result is consistent with thedistorted CV curve and the high ESR obtained from the EISanalysis. The specific capacitance calculated from the charge–discharge profile for F-20RuO2, RP-0.25 and RP-0.5 electrodesare respectively 605, 929 and 1232 F g21, which are comparableto those calculated from the CV analysis. In addition to this,the variation of the specific capacitance with the currentdensity, presented in Fig. 6(b), confirms that the rate capabilityof the RuO2-PBI hybrid electrodes is also significantlyimproved. The retentions in the capacitance obtained for theF-20RuO2, RP-0.25 and RP-0.5 electrodes are 73, 78 and 79%respectively as the current density increases from 1 to 5 A g21.Thus, the addition of PBI-BuI in the F-20RuO2 system resultedin an improvement in the specific capacitance as well as in therate capability and the RP-0.5 electrode exhibits the greatestimprovement of 103% in the specific capacitance along with

Fig. 6 (a) Comparison of the galvanostatic charge–discharge profiles obtainedat a current density of 1 A g21 and (b) the variation of the specific capacitancewith current density from 1 A g21 to 5 A g21 for F-20RuO2, RP-0.25, RP-0.5 andRP-1 electrodes: electrolyte 0.5 M H2SO4, potential window 0–1 V.

Fig. 7 Comparison of the cycling stability of the F-20RuO2 and RP-0.5 electrodesat a current density of 1 A g21.

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better rate capability. Such a remarkably high activity can beattributed to the efficient dispersion of active sites achieved byproperly utilizing the inner and outer surfaces of CNFresulting in the improved utilization of the active material.Apart from this, the facile routes for ion transport created as aresult of PBI incorporation coupled with excellent interfacialcontact between the RuO2 and the electrolyte also result in theimproved activity. In addition to this, the synergistic effects ofpseudocapacitive contribution from both the PBI-BuI andRuO2 also play a major role in redefining the performancecharacteristics.

Cycling stability is another important parameter whichdetermines the supercapacitor performance. Hence, thecycling performances of the F-20RuO2 and RP-0.5 electrodeswere also evaluated by performing the charge–discharge testsat a constant current density of 1 A g21 for 1500 cycles.Accordingly, Fig. 7 compares the cycling performance obtainedfor the F-20RuO2 and RP-0.5 electrodes in the potentialwindow of 0–1 V in 0.5 M H2SO4. As can be evident fromFig. 7, 98% of the initial capacitance is retained for the RP-0.5electrode, whereas the F-20RuO2 shows capacitance retentionof 92% only. The improved cycling stability obtained for theRP-0.5 electrode clearly signifies the importance of interfacialarea tuning in improving the supercapacitor performance.This is achieved by providing more active sites for the contactbetween electrode material and the electrolyte along with itscapability to stabilize the materials during the cycling.

Conclusions

Here, we demonstrate an innovative strategy for the perfor-mance improvement of SC electrodes based on the combina-tion of dual wall decoration of the active material and polymerwrapping. Such rationally designed electrode material exhib-ited almost 103% increase in the specific capacitance after theconductive wrapping. The improvement in the performanceobtained can be attributed to the continuous conducting pathcreated as a result of incorporation of PBI-BuI which canprovide excellent interfacial contact between the RuO2 and theelectrolyte resulting in the increased utilization of the activematerial. Apart from this, the dual wall decoration and thesynergistic effects of the pseudocapacitive contribution fromthe RuO2 and the CNF also render the composite material withhigh specific capacitance and good rate capability. We believethat such a simple strategy can offer great promise in theperformance improvement of SC electrode materials especiallyin the case of devising solid-state supercapacitors where ioninteraction with the active sites can only be facilitated bymeans of liquid-free conducting interfaces.

Acknowledgements

Special thanks go to Dr S. Pal, Director, NCL, Pune, for hiscontinuous encouragement. BKB acknowledges CSIR for the

research fellowship. KS is thankful to DST for the projectfunding (GAP 296126).

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