DOI: 10.1002/cplu.201300153

Redox-Mediated Synthesis of Functionalised Graphene:A Strategy towards 2D Multifunctional Electrocatalysts forEnergy Conversion ApplicationsSreekuttan M. Unni, Siddheshwar N. Bhange, Bihag Anothumakkool, andSreekumar Kurungot*[a]

Introduction

As nanoscience has emerged as a fast-developing field of re-search, the markets for various nanomaterials with specificstructural and property characteristics are expected to widensignificantly in the coming years. In the urge for developingsmaller, lighter and more efficient systems, surfaces of variousnanomaterials have been identified as potential platforms tointegrate the required multifunctional features. In this way, thematerial properties can be effectively tuned for specific appli-cations. Reports on the successful demonstration of multifunc-tional features for system-specific applications are available inthe literature.[1] Such multifunctional nanomaterials are expect-ed to bring breakthroughs in device fabrication owing to theirability to function as smart materials. Eventually, such develop-ments could lead to radical changes in redefining the designstrategies and performance characteristics of various systems.In this context, carbon nanostructures have recently been at-tracting great interest, since applications of many of these ma-terials span a variety of niche areas such as energy, electronicsand biology.[2] From its infancy, two-dimensional (2D) carbon,named graphene, conquered all the allotropes of carbon andreached acme because of its excellent and tuneable proper-ties.[3] It is widely anticipated that graphene and its derivatives

are going to play a major role in a wide range of applications,including emerging fields such as batteries, fuel cells, superca-pacitors and solar cells.

Tuning the physicochemical properties of graphene bysimple functionalisation has gained more significance in recentresearch in view of effectively broadening the adaptability ofthe materials for specific applications.[4] There are a handful ofreports on the basal plane and edge plane functionalisation ofgraphene and the interactions of basal plane or edge planewith guest atom or molecule, which lead to changes in thep–p conjugation profile of the sp2 carbon.[5] This inevitablychanges the electron density on carbon and then alters theproperties of the material, which in turn renders multifunction-al behaviours to the system. A few reports are available on theapplication of functionalised graphene for fuel-cell electrodes,dye-sensitised solar cells and supercapacitors.[6] Many of themdeal with graphene oxide (GO) as the starting material for thefunctionalisation, but as a major concern, many such processesfail to retain the sp2 nature of the carbon after functionalisa-tion. In this scenario, fine-tuning the properties of graphene byincorporating new and desired functional groups by retainingthe sp2 characteristics of the carbon framework is a challengingendeavour.

Chemical reduction of GO is widely studied and many reduc-ing agents, such as hydrazine, hydroquinone and sodium boro-hydride, are employed for graphene synthesis. These methodsare very effective to produce good-quality graphene sheetsfrom GO. However, further processability of the reduced gra-phene oxide (RGO) for surface modification is very difficult be-cause of its poor stability and dispersibility in solvents.[7] Apart

A simple, one-step synthetic route for developing a two-di-mensional multifunctional electrocatalyst is reported, by thefunctionalisation of graphene using oxidised ethylenedioxy-thiophene (O-EDOT). The mutually assisted redox reaction be-tween graphene oxide (GO) and EDOT facilitates the reductionof GO to graphene with a concomitant deposition of O-EDOTon the surface of the graphene. The oxidised surface of GOcatalyses the reaction without using an added reducing agent,so a controlled and uniform deposition of O-EDOT is ensuredon the surface of graphene, which essentially prevents the re-stacking of the layers. UV/Visible, IR, Raman and X-ray photo-

electron spectroscopy give valid evidence for the reductionand functionalisation of graphene sheets. The functionalgroups present on the surface of graphene are found to tunethe physical and chemical properties of graphene. Conse-quently, the functionalised material displays enhanced electro-catalytic activity for the reduction of oxygen to water and I3�

to I� relative to pristine graphene. These distinct property char-acteristics make the material a versatile cathode electrocatalystfor both alkaline anion-exchange membrane fuel cells and dye-sensitised solar cells.

[a] S. M. Unni, S. N. Bhange, B. Anothumakkool, Dr. S. KurungotPhysical and Materials Chemistry DivisionNational Chemical LaboratoryPashan Road, Pune 411008, Maharashtra (India)Fax: (+ 91) 20-2590-2636E-mail : k.sreekumar@ncl.res.in

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cplu.201300153.

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from the aforementioned reducing agents, certain monomersof conducting polymers have also been reported to functionas effective reducing agents for accomplishing the reductionof GO. For example, there are reports on synthesising RGO byusing aniline and pyrrole as the reducing agents.[8] Mutually as-sisted oxidation and reduction (i.e. , redox) reactions betweenGO and these types of monomers not only accomplish reduc-tion of GO but also, in many such cases, the oxidation prod-ucts of the monomers functionalise the surface and tune theproperties of graphene.[8, 9] Notably, the functionalised gra-phene derived by the redox reaction between aniline and RGOis reported to display efficient charge storage properties.[8a]

Among the different potential monomers, ethylenedioxy-thiophene (EDOT) has emerged as a potential candidate be-cause the polymer synthesised from EDOT, namely poly(3,4-ethylenedioxythiophene) (PEDOT), shows interesting propertycharacteristics that make it attractive as a catalyst for polymerelectrolyte membrane fuel cells (PEMFCs), electrode materialfor supercapacitors and batteries, counter electrode for dye-sensitised solar cells (DSSCs) and sensor applications.[10] Apartfrom pristine PEDOT, its composites with carbon nanomorphol-ogies, such as carbon nanotubes, fullerenes and graphene,also display enhanced performance in energy-relatedareas.[10e, 11] There are reports on the outstanding performanceof graphene–PEDOT composites as catalyst support materials,electrodes for supercapacitors, counter electrodes for DSSC ap-plications and so forth.[4, 12] Most of the composite synthesesdiscussed in these cases include reduction of GO followed bythe oxidation of EDOT on the surface of RGO by using oxidis-ing agents such as ammonium persulfite or FeCl3.[12a, 13] In mostof the graphene composites prepared by these routes, theactual properties of graphene are suppressed because of ex-tensive surface coverage of the composite materials. Therefore,a process that offers good control of the oligomer growth onthe surface of graphene is an important requirement to effec-tively bring the desired functionality on the surface.

In this scenario, we have tried to explore the possibility ofexecuting direct oxidation of EDOT mediated by the functionalgroups present on the surface of GO and to subsequently ach-ieve the reduction of GO to graphene. This study leads toa facile synthetic approach for devising a graphene-based 2Dmultifunctional electrocatalyst through a mutually assistedredox reaction between EDOT and GO, in which EDOT is oxi-dised (O-EDOT) concomitantly with the reduction of GO to gra-phene. Compared with the available reports of graphene–PEDOT composites, in the present case the oxidation of EDOThas been achieved without using any oxidising agent.[12a, c–e] In-stead, the oxidised surface of GO itself facilitates oxidation ofEDOT during the course of the reaction with a simultaneousreduction of GO to RGO. The presence of O-EDOT on graphenesheets further helps to reduce the restacking of the sheets. Inaddition, the anchored O-EDOT moiety on the graphene sur-face plays a significant role in activating the carbon atoms to-wards multiple functions. For example, the O-EDOT-graftedgraphene (hereinafter referred to as GEDOT, Scheme 1) hasshown remarkable activities for the electrochemical reductionof both oxygen and triiodide, which makes the material versa-

tile for both PEMFC and DSSC applications. To the best of ourknowledge, there are no such reports dealing with the utilisa-tion of EDOT as a monomer as well as a reducing agent to ac-complish in situ reduction of GO and to form a multifunctionalnanocomposite with promising capabilities to function as cath-odes for PEMFCs and DSSCs.

Results and Discussion

EDOT is reported as an agent for reducing many metal salts,such as auric acid and PdCl2.[14] Similarly, GO is also reportedfor oxidising sugar and para-phenylenediamine.[15] In the pres-ent case, during the reaction of GO and EDOT at 95 8C for12 hours, EDOT monomer starts to become oxidised by releas-ing electrons to form the oxidation products of EDOT. At thesame time, the electrons thus generated help to reduce GO tographene. Similar types of electron-transfer reactions are re-ported in the case of pyrrole as a reducing agent.[8b] TheO-EDOT, the oligomers of EDOT, can be attached on the sur-face of the reduced graphene by p–p interactions between O-EDOT and graphene. Thus, the overall process is a mutually as-sisted redox reaction between GO and EDOT, which brings thetwo essential requirements of the final functionalised material :the electrically conductive base of graphene and the uniformand controlled deposition of the electroactive layer of O-EDOTon the surface. The prepared material was characterised byusing TEM, UV/Vis, IR, Raman and X-ray photoelectron spec-troscopy (XPS) to understand the extent of reduction of GOand the surface modifications induced by the functionalisation.A detailed electrochemical evaluation was performed to unrav-el the interesting multifunctional electrocatalytic activities ofthe material.

High-resolution transmission electron spectroscopy (HRTEM)images indicate the formation of a thin deposit on the surfaceof graphene (GEDOT, Figure 1 b, c). This can be visualised bycarefully looking at the differences of the surfaces of the gra-phene prepared by NaBH4 reduction (RGO, Figure 1 a) andGEDOT. Compared with RGO, the GEDOT surface has some ap-parent roughness presumably owing to the presence ofO-EDOT. This was further confirmed by the selected-area elec-tron diffraction (SAED) pattern as shown in the inset of Fig-ure 1 d, which shows irregular spots instead of the clear hexag-onal spots expected from a smooth graphene surface (inset ofFigure 1 a). Scanning electron microscopy images of GEDOT atdifferent magnifications are given in Figure S1 in the Support-ing Information. Most importantly, the oxidation of EDOT to

Scheme 1. Schematic representation of the preparation of the O-EDOT-func-tionalised graphene.

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O-EDOT on the surface of GO also concomitantly mobilises thein situ reduction of GO to graphene, which is evidenced bya set of spectroscopic investigations as detailed below.

UV/Visible spectroscopic analysis (Figure 2 a) of GO, GEDOTand RGO in water shows absorption peaks at 227.6, 259.5 and264.0 nm, respectively. This redshift in the absorption peakafter the reduction indicates the restoration of the electroniccolligation in the graphene sheets.[16] The slight difference inthe absorption peak positions of GEDOT and RGO is probablycaused by the intervention of the grafted O-EDOT moieties onthe surface of GEDOT. Interestingly, no absorption correspond-ing to PEDOT is observed in the absorption spectra, whichgives clear evidence of the partial oxidation of EDOT ratherthan complete oxidative polymerisation on the graphene sur-face. The FTIR spectrum (Figure 2 b) of GO shows peaks corre-

sponding to hydrogen-bonded �OH groups (3357 cm�1), vibra-tions resulting from �C=O (1719 cm�1) and �C�C� (1612 cm�1)along with peaks at 1151, 1023 and 862 cm�1, which are relat-ed to the vibrations of �C�O of ether, alkoxide and epoxide,respectively. Interestingly, the broad peaks corresponding to�OH and �C=O vanish completely after the reduction anda new peak corresponding to the vibration of the �C=C�bond appears at 1516 cm�1. Appearance of this new peak au-thenticates the in situ reduction of graphene as EDOT is oxi-dised to O-EDOT. The peaks at 1052, 1207 and 968 cm�1 repre-sent the vibrations of �C�O�, whereas the peaks at 784 and648 cm�1 give evidence for the vibration of �C�S originatingfrom O-EDOT. NaBH4-reduced graphene, that is, RGO, alsoshows similar vibrations except for the vibrations from thethiophene rings.

Raman spectra of GO and GEDOT are given in Figure 2 c. GOdisplays two peaks at 1327.9 and 1608.3 cm�1 correspondingto the D and G band, respectively. The G band of GEDOT ap-pears at 1594.2 cm�1, thus indicating a recovery of the sp2

carbon atoms with some apparent defects. Down-shifting ofthe G band from 1585.0 cm�1 (corresponding to RGO) to1582.2 cm�1 indicates the charge transfer between O-GEDOTand the graphene layers in GEDOT. The broad band at1327.0 cm�1 and the extrusion at 1506.2 cm�1 account for thepresence of O-EDOT on the surface of graphene. The ratio ofD- and G-band intensities (ID/IG) is often used to understandthe level of chemical modification on the surface of graphene.The ID/IG ratios of GEDOT and GO are respectively 1.0 and 1.1,which reveal the reduction of GO. From the thermogravimetricanalysis results presented in Figure S2, the amount of the oxi-dation product is estimated to be approximately 20 wt %.

X-ray diffraction (XRD) analysis clearly indicates the reduc-tion of GO in the presence of EDOT (Figure 2 d). GO showsa diffraction peak at 2 q= 10.48 with an increase in the d spac-ing to 8.2 �. This is attributed to the introduction of oxygen-containing functional groups, mainly carboxylic acid, epoxideand hydroxyl groups, between the planes of graphene. The ab-sence of the GO peak (2 q= 9.988) and recurrence of the (002)diffraction (2 q= 25.168) after the reaction with EDOT indicatethe reduction of GO to GEDOT. Also, this plane is almost over-lapping with the corresponding diffraction plane of RGO,which unambiguously confirms that the extent of GO reduc-tion achieved in the presence of EDOT is comparable with thefeatures developed under the NaBH4 reduction process. Thus,the in situ reduction of GO to graphene mediated by EDOTand a controlled deposition of O-EDOT on graphene could beachieved very effectively during the process. The mutually as-sisted redox reaction between EDOT and GO is augmentedfrom the electrical conductivity measurements as well. Themeasured value of the electrical conductivity of GEDOT is20 S m�1, which is significantly higher than the reported valuesof graphene obtained by reducing GO using biomolecules.[17]

At the same time, the electrical conductivity measured in thepresent case is lower than that for graphene prepared by thethermal reduction of GO.[18] This deviation from the conductivi-ty in the case of GEDOT can be ascribed to the presence of un-doped O-EDOT as a thin layer on the surface of graphene.

Figure 1. HRTEM images of( a) RGO and (b–d) GEDOT under different magni-fications. The insets of (a) and (d) represent the selected-area electron dif-fraction (SAED) patterns of RGO and GEDOT, respectively.

Figure 2. a) UV/Vis spectra, (b) FTIR spectra, (c) Raman spectra and (d) X-raydiffraction patterns of GO, GEDOT and RGO. ID/IG = ratio of D- and G-band in-tensities.

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Further, the XPS analysis of GO detects the presence of65 wt % carbon and 35 wt % oxygen, which leads to a surfaceC/O ratio of 1.8. On the other hand, measurement of theC/O ratio after the reduction process gives an enhancement ofthe value to 3.04, corresponding to a surface composition of70 wt % carbon and 23.5 wt % oxygen. This enhancement inthe C/O ratio after the reduction process can be ascribed tothe removal of the surface oxygen moieties during the reduc-tion of GO to GEDOT. Also, 6.5 wt % sulfur has been detectedin the reduced graphene and this further augments the pres-ence of oxidative products of EDOT on the surface of gra-phene. The oxygen atoms present in O-EDOT also contributeto the total weight percentage of oxygen in GEDOT. Figure 3 a

shows the XPS spectra of sulfur, which could be deconvolutedinto four peaks corresponding to the binding energies of162.24, 163.70, 161.54 and 164.90 eV. The peaks at 163.70 and164.90 eV arise from the spin-split doublet (P3/2 and P1/2) ofneutral sulfur.[19] More interestingly, the peak corresponding tothe doped state of sulfur is absent in the higher bindingenergy region. The peaks at 161.54 and 162.24 eV arise fromneighbouring S–S interactions of different molecules. The2p orbital energy value of the sulfur atom in the monomericthiophene is 164.50 eV.[20] This structural motif found in GEDOThas 0.40 eV higher binding energy and this apparently indi-cates charge transfer of electrons from sulfur to the carbon ofgraphene. Thus, O-EDOT acts as an n-type dopant to causepartial electron transfer from O-EDOT to graphene. In accord-ance with this, the sp2 carbon peak in C 1s spectra (Figure 3 band Figure S3) is broadened and shifted to a lower bindingenergy (from 284.40 to 284.30 eV), thus giving valid evidenceon the interaction of O-EDOT with the graphene surface.

To validate the role of EDOT for the reduction of GO, we per-formed a controlled experiment by heating at reflux a solutionof GO for 12 hours at 95 8C without introducing the EDOT mo-nomer into the reaction system. This experiment does not leadto any apparent changes in the GO solution and the physicalnature of the reaction mixture after the reaction remained vir-tually identical to the state before the commencement of thereaction. UV/Vis and XRD analyses of the synthesised sampleclearly show that the resulting material is none other than GO(Figures S4 and S5). These observations clearly validate thatthe reduction of GO is mediated by the intervention of theEDOT moiety in the system.

The cyclic voltammograms for the oxygen reduction reaction(ORR) on GEDOT in nitrogen- and oxygen-saturated 0.1 m KOHare shown in Figure 4 a. Formation of the peak corresponding

to the ORR exclusively under the oxygen saturation conditionsvalidates the oxygen reduction activity of GEDOT. As can beseen from Figure 4 a, the onset potential corresponding tooxygen reduction appears at �0.052 V versus the Hg/HgO ref-erence electrode. Further, the kinetics of the ORR of all the cat-alysts was studied by using a rotating disk electrode (RDE)with a catalyst loading in the working electrode of0.225 mg cm�2 in 0.1 m KOH using a three-electrode cell assem-bly. A scan rate of 10 mV s�1 was maintained and linear sweepvoltammograms were recorded at electrode rotation speeds of900, 1200, 1600, 2000 and 2500 rpm. In the case of the experi-ments using a state-of-the-art Pt/C (40 wt %) catalyst, a Pt load-ing of 0.106 mg cm�2 was maintained. The Koutecky–Levich(K-L) equation [Eq. (1)] was applied to calculate the kinetic cur-rent density :

1j¼ 1

j1þ 1

jkþ 1

jfð1Þ

in which j is the measured current density, jk is the kinetic cur-rent density, jl is the diffusion (mass-transfer)-limited currentdensity and jf is the film diffusion current. In the present case,jf can be neglected as the amount of Nafion applied is signifi-cantly low and hence may not affect the limiting current densi-ty. Also, jk can be represented as [Eq. (2)]:

jk ¼ nFkCO2ð2Þ

Figure 3. Deconvoluted XPS spectra of (a) S 2p and (b) C 1s of GEDOT.BE = binding energy.

Figure 4. a) Cyclic voltammograms of GEDOT in N2- and O2-saturated 0.1 m

KOH at a scan rate of 50 mV s�1 at an electrode rotation speed of 900 rpm.b) Linear sweep voltammograms of GEDOT, RGO and RGO + PEDOT recordedat an electrode rotation speed of 2000 rpm and a scan rate of 10 mV s�1 inO2-saturated 0.1 m KOH solution. c, d) Linear sweep voltammograms ofc) GEDOT and d) Pt/C obtained before and after an ADT at an electrode rota-tion rate of 2000 rpm in O2-saturated 0.1 m KOH solution at a scan rate of10 mV s�1.

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in which n is the number of electrons, F is the Faraday con-stant (96485.5 C), k is the reaction rate constant and CO2

is theconcentration of the dissolved oxygen in the electrolyte solu-tion (1.2 � 10�6 mol cm�3). The representation corresponding tojl is as follows [Eq. (3)]:

jl ¼ 0:62nFCO2D2=3

O2n�1=6w1=2 ð3Þ

in which n is the number of electrons, F is the Faraday con-stant (96485.5 C), CO2

is the bulk O2 concentration (1.2 �10�6 mol cm�3), DO2

is the diffusion coefficient (1.9 �10�5 cm2 s�1) of O2 in the electrolyte, n is the kinematic viscosi-ty of the electrolyte (0.01009 cm2 s�1) and w is the rotation rateof the electrode in radians per second (2p rpm/60). A plot of1/j versus w�1/2, known as the K-L plot, varies linearly and they intercept gives the inverse of kinetic current jk, and the corre-sponding slope gives the number of electrons involved in theORR.

Comparative linear sweep voltammograms taken by the RDEmethod are shown in Figure 4 b. A significant positive shift inthe onset potential as well as the higher limiting current ofGEDOT relative to RGO reveal the enhanced activity of GEDOTfor the ORR. The onset potential for the ORR of GEDOT is�0.052 V versus Hg/HgO, which is 100 mV higher than that forRGO. Similarly, GEDOT shows a much improved limiting currentdensity value, which is two times higher than that for RGO.Even though the ORR activity of GEDOT is still less than thatfor Pt/C (Figure S6), the overpotential for ORR on the presentsystem is lower relative to the few reported cases of Pt-freeelectrocatalysts derived from graphene.[6a, 21] For example, Jeonet al. reported an overpotential of 0.20 V in their tridodecylme-thylammonium chloride-functionalised graphene[21b] and Niuet al. reported again an overpotential of 0.20 V for their nitro-gen-doped graphene synthesised by a wet-chemical methodwith respect to the commercial catalysts.[21a] These values arequite high compared with the overpotential of 0.13 V obtainedin the case of GEDOT. To further prove the effect of functionali-sation in GEDOT towards the ORR activity, we used a physicalmixture consisting of equal amounts of PEDOT and RGO andevaluated the ORR activity by following the same testing pro-tocol. From Figure 4 b, it is clear that the RGO/PEDOT mixture(RGO + PEDOT) shows significantly low onset potential and lim-iting current values relative to pure RGO and GEDOT. This fur-ther validates that GEDOT is an oligomer (O-EDOT)-functional-ised graphene and not PEDOT-coated graphene.

Quantification of the electrons transferred during the ORRcalculated from the K-L plots (j�1 versus w�1/2) as shown in Fig-ure S7c and d leads to values of two and three for grapheneand GEDOT, respectively. The electron-transfer number ofGEDOT and RGO at different potential values is given in Fig-ure S8. Dai et al. reported a three-electron transfer mechanismfor their poly(diallyldimethylammonium chloride)-functional-ised carbon nanotubes.[22] Therefore, the GEDOT follows dioxy-gen reduction pathways, which involve both two- and four-electron transfer. To further understand the amount of perox-ide formation during the ORR, we performed rotating ring discelectrode analysis. From the results obtained, graphene and

GEDOT produce approximately 60 and 40 % hydrogen perox-ide, respectively, at �0.8 V. This clearly proves nearly 20 % re-duction in H2O2 production after the surface modification ofgraphene using O-EDOT, which is attributed to the modifica-tion of the carbon environment of graphene by surface func-tional groups. Another important advantage worth mentioningis the tenfold increase in the value of jk of GEDOT, which is12.4 mA cm�2 at �0.36 V compared with just 1.2 mA cm�2 inthe case of RGO.

Further, from a practical perspective, the oxygen reductionsites of the materials once formed should be able to surviveunder strong electrochemical environments to enable the ma-terial to serve as an efficient electrocatalyst for PEMFCs, forwhich stringent durability criteria exist. To validate the stabilityof GEDOT, an accelerated durability test (ADT) was performedat a potential range of �0.28 to 0.47 V versus Hg/HgO for1000 cycles. Linear sweep voltammograms were recordedbefore and after the ADT. A 30 mV difference in the onset po-tential and a 0.6 mA cm�2 difference in the limiting currentdensity were observed after GEDOT was subjected to the ADT(Figure 4 c). This negative shift in the onset potential after theADT indicates some amount of removal of the oxidation prod-uct of EDOT during the forced oxidation of carbon. Removal ofO-EDOT from GEDOT reduces the charge distribution of carbonin the graphene matrix, which in turn reduces the onset poten-tial for oxygen reduction. Thus, the ADT clearly validates thepivotal role played by the surface O-EDOT moiety in bringingthe ORR activity to the graphene surface. An ADT of the state-of-the-art 40 wt % Pt/C catalyst also leads to a similar reductionin the ORR activity in terms of both the onset potential andthe limiting current density, as can be seen from Figure 4 d.However, the extent of performance degradation in this case isslightly lower than that of the GEDOT system, which is expect-ed from the significantly higher number of active sites presenton the commercial catalyst owing to its large Pt content onthe surface.

As another important tolerance test, the methanol crossovereffect on the ORR was monitored by current–time (i–t) chro-noamperometry (Figure 5 a) by introducing 3 m methanol after300 s at �0.05 V versus Hg/HgO at an electrode rotation rateof 1000 rpm. A sharp decrease in the ORR current was ob-served on Pt/C after the addition of methanol. However, theamperometric response for GEDOT remained almost un-changed even after the addition of methanol, which clearlyvalidates the fuel selectivity of GEDOT.

Along with the capability of GEDOT to catalyse oxygen re-duction as detailed above, the system is also found to facilitatereduction of I3

� to I� . The reaction is significantly important inthe case of DSSCs in which the counter electrodes, which areagain mainly based on Pt, catalyse the reduction of I3

� to I� .Comparative cyclic voltammograms of GEDOT, RGO, RGO +

PEDOT and Pt towards the reduction of I3� in a potential

window of �0.4 to 1 V versus ferrocene (external references) inacetonitrile are shown in Figure 5 b. Two pairs of peaks are ob-served for electrodes based on Pt, GEDOT and RGO + PEDOT.The redox couple at higher positive potential is attributed tothe oxidation and reduction of I2/I3

� and that at lower poten-

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tial corresponds to the oxidation and reduction of I�/I3� . In

cyclic voltammetry, we rely mostly on the peak potential Epp

between the I�/I3� redox couple to measure the activity of the

catalysts. In the present case, the DEpp of Pt is 179 mV and thatof GEDOT is 286 mV. The RGO + PEDOT mixture shows a DEpp

value of 418 mV. Interestingly, RGO does not show any peakfor I�/I3

� reduction whereas GEDOT displays the clear redoxcouples corresponding to the reactions.

Even though Pt retains some advantage over GEDOT interms of its lower DEpp, it should be noted that the activity dis-played by GEDOT is very promising in the context of develop-ing cost-competent counter electrodes for DSSC applications.In view of the low corrosion resistance and high cost of Pt, re-alisation of such viable alternatives for DSSC cathodes is imper-ative for redefining the commercial viability of photovoltaiccells.[23] Graphene and functionalised graphenes prepared bydifferent methods and their applications as counter electrodesfor DSSC have been recently reported by a few groups.[24] Insome cases, comparable reduction properties with Pt counterelectrodes have been reported. For example, Ho et al.[25] havedemonstrated a functionalised graphene-based counter elec-trode for DSSCs that displays a DEpp value of 450 mV, which isless than the value obtained in the case of GEDOT. In addition,the DEpp of GEDOT in the present case is better than the re-cently reported values obtained by Chou et al.[24a] (581 mV) fornitrogen-doped graphene and Jo et al.[26] (580 mV) for gra-phene nanosheets as the counter electrodes.

Device performance for DSSCs was analysed by using Pt,GEDOT, RGO and RGO + PEDOT mixture as cathode. From Fig-ure 5 c and Table 1, it can be seen that the Pt-based cathodeshows values of short-circuit current density (Jsc), open-circuitvoltage (Voc), fill factor (FF) and power conversion efficiency of10.00 mA cm�2, 0.77 V, 63.78 % and 5.02 %, respectively. In the

case of the GEDOT system, Jsc is 8.94 mA cm�2, Voc is 0.79 V, theFF is 53.03 % and the efficiency is 3.87 %. Even with its higherVoc, GEDOT has a lower Jsc and FF than Pt. However, there hasbeen a significant improvement in the performance of GEDOTrelative to RGO, which shows extremely low Jsc and FF valueswith an overall power conversion efficiency as low as 0.23 %.The major differences in the performance profiles of GEDOTand RGO as illustrated herein clearly authenticate the efficacyof the O-EDOT functionalisation of the graphene surface tobuild the required surface features for the conversion of I3

� toI� . In the case of the RGO + PEDOT mixture, Voc, Jsc, FF andpower conversion efficiency are comparatively lower thanthose of GEDOT, which further confirms the role of O-EDOTfunctionalisation of graphene for the triiodide reduction.

Electrochemical impedance spectroscopy was used to un-derstand the charge-transfer resistance of the different catalystmaterials (Figure 5 d). Nyquist plots were obtained from thesymmetric cells fabricated from the same materials. The Ny-quist plots show three distinct resistance regions for GEDOT,RGO + PEDOT and Pt. These correspond to the series resistance(Rs), which appears as the high-frequency intercept of the realaxis, charge-transfer resistance (Rct), indicated by the diameterof the first semicircle loop, and Nernst diffusion impedance(ZN) in the low-frequency side of the spectrum owing to theredox couple transport in the electrolyte. The calculated Rs, Rct

and ZN values of GEDOT are 30.7, 6.67 and 13.58 W, respective-ly. The corresponding values for Pt are 33.76, 26.86 and 0.94 W,respectively. Similarly, the RGO + PEDOT mixture shows almostthe same Rs (29.00 W) value as that of GEDOT, but its Rct

(15.25 W) and ZN (34.99 W) values are higher than those ofGEDOT. RGO shows the highest Rct, which is reflected in thevery much lower efficiency of the cell as well. Owing to thelower Rct of GEDOT relative to the other graphene samples, itperforms exceptionally well in the cell with a power conversionefficiency of 3.87 %. Interestingly, compared with Pt, GEDOTshows a significantly lower charge-transfer resistance and thisopens up a great prospect that the material can be utilised asa potential alternative to the Pt counter electrode in DSSCs byadopting an optimised fabrication protocol.

The catalytic activity of GEDOT for the reduction of I3� to I�

and even oxygen can be mainly attributed to the greateramounts of lattice defects and functional groups present onthe surface and also to the spin density changes as reportedby Zhang et al. in a theoretical study.[27] On analysing thesethree probable contributing factors, they claimed that the spindensity changes on carbon could be the dominating factor for

Figure 5. a) Current–time (i–t) chronoamperometric response for the ORR atGEDOT and Pt/C in an O2-saturated 0.1 m KOH solution at �0.05 V versusHg/HgO by adding 3 m methanol. b) Cyclic voltammograms ofI2/I3

� and I�/I3� reactions in acetonitrile solution containing 0.1 m LiClO4,

5 mm LiI and 0.5 mm I2 at a scan rate of 20 mV s�1. c) Current density–volt-age (j–V) curves of I3

�/I� DSSC using Pt, GEDOT, RGO and RGO + PEDOT ascounter electrodes. d) Nyquist plots for the I3

�/I� symmetrical cells based onPt, GEDOT, RGO and RGO + PEDOT. Inset: see text for details.

Table 1. Short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF)and power conversion efficiency for DSSCs with Pt, GEDOT, RGO andRGO + PEDOT as counter electrodes.

Counter electrode Jsc [mA cm�2] Voc [V] FF [%] Efficiency [%]

Pt 10.00 0.77 63.78 5.02GEDOT 8.94 0.79 53.03 3.87RGO 1.53 0.78 22.86 0.28RGO + PEDOT 8.13 0.73 47.94 2.91

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modulating the catalytic activity of the surface. Subsequently,this was validated experimentally by Yang et al. in recent stud-ies on sulfur doping on graphene, in which sulfur seemed toplay a major role in manipulating the surface electron densi-ty.[28] Similarly, in the present case, the O-EDOT moieties pres-ent on the surface of GEDOT are also expected to create spindensity changes on the carbon adjacent to the functionalgroups. The evidence for charge transfer between O-EDOT andgraphene sheets as obtained from the XPS analysis clearly sup-ports the modulations on the surface electron distribution,which may make the surface fertile towards reduction ofoxygen and triiodide. One important piece of evidence on therole of the surface moieties as a determining factor for theelectrocatalytic activities has been obtained from the previous-ly discussed ADT profile. The forced oxidation of carbon duringthe ADT is expected to concomitantly remove the O-EDOTmoieties from the graphene surface. The ORR performance hasbeen reduced in accordance with the exposure during theADT. In addition to this, RGO, which does not contain O-EDOTmoieties on the surface, has not shown appreciable activity to-wards the ORR. By correlating these two results, it can be con-cluded that it is the oxidative products of EDOT on the surfaceof the RGO that bring the required electrocatalytic activity tothe surface. However, a detailed investigation is required tounravel the exact nature of such surface interactions froma molecular point of view. A study in this direction by usingpowerful surface analysis methods is currently in progress.

Conclusion

A 2D multifunctional electrocatalyst is synthesised by the mu-tually assisted redox reaction between graphene oxide (GO)and ethylenedioxythiophene (EDOT). The oxidised surface ofGO catalyses the reaction without use of an added reducingagent, so a controlled and uniform deposition of oxidisedEDOT (O-EDOT) could be ensured on the surface of graphene,which essentially prevents the restacking of the layers. TheO-EDOT moieties present on the graphene surface are foundto tune the physical and chemical properties of graphene andconsequently the material shows enhanced activities towardsthe electrochemical reduction of oxygen and I3

� , which are thetwo well-known processes occurring in polymer electrolytemembrane fuel cells and dye-sensitised solar cells (DSSCs), re-spectively. Even though the oxygen reduction reaction (ORR)activity of GEDOT is less than that of the state-of-the-art Pt/Ccatalyst, the overpotential for the ORR on the present systemis much lower than that of the few reported cases of Pt-freeelectrocatalysts derived from graphene. Similarly, the activitydisplayed by O-EDOT-grafted graphene for the reduction of I3

�

to I� is also very promising in the context of developing cost-competent counter electrodes for DSSC applications. The elec-trochemical investigation gives valid evidence on the role ofthe surface O-EDOT moieties as a determining factor for bring-ing favourable electronic modifications on the surface toenable it to function as an efficient electrocatalyst for the re-duction reactions. The interactions between O-EDOT and re-duced graphene modulate the surface electron distribution

and make the surface fertile towards the reduction of oxygenand triiodide.

Experimental Section

Materials

Graphite powder (purity 99.8 %) was purchased from Alfa Aeser.EDOT was procured from Aldrich whereas H2SO4, H3PO4, KMnO4,ethanol and dimethylacetamide were purchased from RankemChemicals, India. All the chemicals were used without any furtherpurification.

Graphene oxide synthesis

GO was prepared by following a reported procedure.[29] Briefly,a 9:1 mixture of concentrated H2SO4/H3PO4 (360/40 mL) was addedto a mixture of graphite flakes (3.0 g) and KMnO4 (18.0 g) ina round-bottomed flask. The reaction mixture was then heated to50 8C for 12 h. The mixture was cooled to room temperature andthen poured onto ice-cold water (400 mL) containing 30 % H2O2

(3 mL). The resulting yellow solution was centrifuged (10 000 rpmfor 10 min) and the supernatant layer was decanted away. The re-maining solid material was then washed in succession with 200 mLeach of water, 30 % HCl and ethanol followed by washing withwater several times. Finally, the dispersion of GO in water was coa-gulated by using ether (200 mL), and the resulting suspension wasfiltered over a PTFE membrane having a pore size of 0.45 mm. Theresidue collected on the membrane was vacuum-dried overnight(12 h) at room temperature.

Functionalisation of graphene

The oxidised ethylenedioxythiophene (O-EDOT)-functionalised gra-phene was prepared by the self-oxidation/reduction of EDOT andGO (Scheme 1). In a typical synthesis, GO (50 mg) was well dis-persed in deionised water (150 mL) followed by the addition ofEDOT (1 mL). The resulting solution was heated at reflux for 12 hat 95 8C. After completion of the reaction, the resulting black solu-tion was filtered using 0.45 mm pore size PTFE filter paper andwashed successively with ethanol, water and dimethylacetamide.The black residue obtained after filtration was dried in a vacuumoven for 3 h at 70 8C. For the purpose of comparison, a batch ofgraphene was also prepared from GO with NaBH4 as the reducingagent. In a typical synthesis procedure, the solution containing GOwas heated at reflux for 3 h at 130 8C after the addition of NaBH4.Subsequently, the mixture was filtered and the residue waswashed with ethanol and water. Finally, the wet cake was driedunder vacuum at 70 8C for 3 h.

Acknowledgements

S.M.U. and S.K. acknowledge UGC and DST, New Delhi (SR/S1/PC-05/2011), respectively, for financial assistance. We also thankOnkar Game and Dr. S. B. Ogale for the DSSC measurements.Support from CSIR, New Delhi is acknowledged. We are alsograteful to Dr. S. Pal, Director, NCL, for his support and continu-ous encouragement.

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Keywords: electrocatalysts · fuel cells · graphene · redoxchemistry · solar cells

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Received: April 24, 2013

Revised: June 22, 2013

Published online on July 22, 2013

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