Graphene Supported Platinum Nanoparticles as

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Conductive Polymer/Graphene Supported Platinum Nanoparticles as Anode Catalysts for the Extended Power Generation of Microbial Fuel Cells Georgepeter Gnana kumar,*,† Christopher Joseph Kirubaharan,† Subramani Udhayakumar,† Chandrasekaran Karthikeyan,† and Kee Suk Nahm*,‡ †

Department of Physical Chemistry, Madurai Kamaraj University, Madurai-625-021, Tamil Nadu, India School of Chemical Engineering and Department of Hydrogen Fuel Cell Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: Platinum (Pt) nanoparticles anchored over reduced graphene oxide (rGO) and rGO/conductive polyaniline (PANI) composites were synthesized and exploited as anode catalysts in microbial fuel cells (MFC). PANI bridges rGO and Pt nanoparticles through the electrostatic interaction/π−π stacking force/hydrogen bonding and Pt−N bond, respectively, and increased the intrinsic stability of rGO/PANI/Pt composite. The electrocatalytic performances of rGO/PANI/Pt exhibited the better oxidation current and lower internal resistance over the prepared rGO/PANI and rGO/Pt composites as evidenced from the cyclic voltammetric and electrochemical impedance techniques, respectively. By the combined efforts of active support, high electrical conductivity, and number of catalytic active sites, the prepared rGO/PANI/Pt nanocomposite exhibited a maximum MFC power density of 2059 mW/m2 with the concrete life durability. Thus, the proposed approach has paved new dimensions in not only the preparation of rGO-supported conductive polymer nanocomposites but also its applications as effective anode catalysts in MFCs.

1. INTRODUCTION Energy efficiency has become a significant factor for the increasing global energy demand in which the significance of environmental balanced energy devices is vital for the green and sustainable future.1,2 Of particular interest are microbial fuel cells (MFC), which are capable of delivering inexpensive and environmentally friendly routes for the generation of sufficient energy.3−5As global energy demand continues to grow, the ways to improve the energy efficiency of MFCs are essential. The overall performances of MFCs are determined by the intimate contact of anode−biofilm−anolyte and are effectually determined by the electron transfer efficiency of electrodes, which urges the modification of electrodes with the conductive materials.3,6,7 Carbonaceous nanomaterials find its extensive applications in electrochemistry owing to its high surface area, prompt electrical conductivity, better mass transport of reactants, maximized availability for the anchoring of electrocatalysts, efficient collection and transfer of electrons, and so forth.8,9 Although the pronounced carbon materials such as carbon nanotubes (CNT)10 and carbon black11 find its applications in MFCs, it still lacks in the bacterial cellular toxicity and vulnerability toward the oxidation,10,11 respectively, which dragged the attention toward active carbon support graphene. Graphene is a two-dimensional single atom thick sheet of sp2 hybridized carbon atoms arranged in a honeycomb lattice that exhibits superior electrical, mechanical, thermal, and chemical properties that are comparable or even superior over the CNTs.12−14 Current large-scale production of graphene involves the chemical exfoliation of graphite in strong acids © 2014 American Chemical Society

and the resulting graphene oxide (GO) contains heavily oxygenated graphene sheets bearing epoxy and hydroxyl functional groups on their basal planes and carbonyl and carboxyl groups on the sheet edges.15 However, the electrical conductivity of GO sheets was reduced by the destructed conjugated sp2 hybridized network via the hydrophilic functional groups.15,16 The electrical conductivity of GO is increased by the restorage of disrupted π system/sp2 hybridized network and is achieved by the removal of aforementioned functional groups through the reduction of GO.17 Hence, few efforts have been devoted on the utilization of reduced GO (rGO) as anode catalyst in MFCs. Zhang et al. modified the stainless steel mesh anode with graphene nanosheets and the modified electrode exhibited a maximum MFC power density of 2668 mW/m2.18 The crumpled graphene-modified carbon cloth (CC) exhibited a maximum power density of 3.6 W/m3, which is 2-fold higher than that of activated carbon modified electrode.19 Liu et al. modified CC with graphene and the corresponding electrode exhibited a maximum power density of 52.5 mW/m2, which is 2.7-fold higher than that of bare CC.20 The enhancement in MFC power generation of graphene modified electrode is ascribed to the enhanced surface area, biocompatibility, high electron transfer efficiency, and so forth.17−19 Though rGO exhibits better electrical conductivity over GO, it still lacks in the active surface area and electrical Received: Revised: Accepted: Published: 16883

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through their structural, morphological, and electrochemical properties.

conductivity in comparison with graphene. In addition, the inevitable aggregation of rGO sheets exhibited unsatisfactory electrochemical performances.21 Hence, it is essential that rGO should be complexed with the conductive materials for the betterment of electrical conductivity and prevention of rGO sheets aggregation, which could be effectively satisfied by the complex formation with the conductive polymer/nanometal catalysts. Conductive polymers such as polypyrrole,22 polyaniline (PANI),23 poly(aniline-co-o-aminophenol),24 and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)25 were utilized as anode catalysts in MFCs, owing to its prompt electron conduction properties. Among the aforementioned conducting polymers, PANI has been considered as a unique polymer, owing to its low cost, easier synthesis protocols, high conductive properties, environmental stability, and biocompatibility. However, certain limitations of PANI such as swelling, shrinkage, poor stability, and sluggish electron transfer properties limited the durability performances of MFCs.26 The large van-der Waals force and hydrogen bonding exerted between −NH group of PANI and the left out oxygen moieties of rGO improved the electrical conductivity, catalytic stability, and mechanical properties of PANI. The complex formation of PANI with rGO could not only provides a solution for the negative impacts of PANI but also relieves the aggregation of rGO sheets through its synergistic combination with PANI. Yong et al. fabricated the PANI hybridized three-dimensional graphene foam and the corresponding anode exhibited a maximum MFC power density of 768 mW/m2, which is about four times higher than that of bare electrode.27 The faster electron transfer rate stimulated by the electrochemically reduced GO/PANI exhibited the maximum power density of 1390 mW/m2.28 Zhao et al. modified the graphene nanoribbons with PANI by using the electropolymerization technique and the corresponding anode catalyst exhibited a maximum power density of 856 mW/m2, which is about six times higher than that of bare electrode.29 Though the aforementioned efforts could bring forth the improvement in MFC power densities, durability, and maximum MFC performances crises of anode catalysts have not been resolved yet. On the other side, anchoring of metal nanocatalysts over rGO is the ultimate strategy in improving the dispersion of metal nanoparticles, which facilitates the electrochemically available surface area of electrodes and electrocatalytic active sites.30 Among the metal nanoparticles studied, Pt exhibits excellent catalytic activity and is still the most efficient catalyst for most of the electrochemical reactions.31 Although Pt nanoparticles were homogeneously anchored over the rGO sheets, it could be easily detached from the carbon support during the ultrasonic treatment involved in the slurry preparation and under the aqueous environment, owing to its low adhesion with the carbon support. Hence, it is essential to reinforce the strength of rGO supported Pt nanoparticles and the corresponding issue has become a big challenge. To tackle the aforementioned significant issues, the research effort on Pt nanoparticles anchored over PANI layered rGO has been reported. If a conductive polymer and metal nanoparticles could be effectively interfaced with rGO, superior properties could be capitalized, which may bring forth the efficient MFC performances. The objective of this work is to analyze the impact of a combination of active carbon support, conductive polymer, and metal nanoparticles over the MFC performances

2. EXPERIMENTAL METHODS 2.1. Materials. Aniline (Merck, 99.5%), ammonium persulfate (APS) (Sigma-Aldrich, >98%), chloroplatinic acid (H2PtCl6) (Sigma-Aldrich, >99%), ferricyanide (Sigma-Aldrich, >99%), graphite powder (Sigma-Aldrich, 99.99% trace metals), formic acid (Merck, 85%), phosphate-buffered saline (PBS) (Sigma-Aldrich, pH = 7.2), poly(tetrafluoroethylene) (PTFE) (Sigma-Aldrich, 60 wt % dispersed in water) were derived and used without any further purification. Carbon cloth electrodes derived from Electrosynthesis Co. Inc., Lancaster N.Y. GC-14 were pretreated by ethanol and water and dried in a vacuum oven at 60οC. 2.2. Preparation of rGO. GO was synthesized from natural graphite powder according to the modified Hummers method as described elsewhere.32 A volume of 0.5 M HCOOH was gradually added to the GO dispersion (1 mg/mL) and refluxed at 80 °C for 30 min. The obtained product was collected via centrifugation at 12 000 rpm and dried under vacuum at 100 °C for 24 h. 2.3. Synthesis of rGO/Pt Nanocomposite. A volume of 1.5 mM H2PtCl6 was gradually added to the GO dispersion (1 mg/mL) and magnetically stirred at 80 °C for 10 min. It was followed by the addition of 0.5 M HCOOH and the resultant mixture was refluxed at 80 °C for 30 min. The obtained product was dried under vacuum at 100 °C for 24 h. 2.4. Synthesis of rGO/PANI Nanocomposite. A 500 μL aliquot of aniline and 1 N HCl were added to the GO dispersion (1 mg/mL) and magnetically stirred. To the above mixture, appropriate amount of APS was gradually added and kept in an ice-cold condition. After a few minutes, the abrupt color change was observed from black to green and the reaction was allowed to be continued for 5 h with the addition of 0.5 M HCOOH. The resultant product was collected and repeatedly washed with ethanol and water mixture. 2.5. Synthesis of rGO/PANI/Pt Nanocomposite. A volume of 1.5 mM H2PtCl6 was gradually added to the GO/ PANI dispersion (1 mg/mL) and magnetically stirred at 80 °C for 10 min. It is proceeded by the addition of 0.5 M HCOOH and the reaction mixture was refluxed at 80 °C for 30 min. The obtained product was separated by using centrifugation and dried under vacuum at 100 °C for 24 h. 2.6. Modification of Anode. Appropriate amount of prepared nanocatalysts were mixed with 1 wt % PTFE solution and ultarsonicated for 30 min. Then the resultant mixture was sprayed onto the surface of carbon cloth (1 cm × 1.5 cm) via spray technique and dried at 100 °C for 12 h. 2.7. Microorganism Cultivation. Escherichia coli cells were cultivated according to the procedure described elsewhere.33 2.8. Characterizations. The prepared nanomaterials were characterized by using transmission electron microscopy (TEM) (Hitachi-H-7650), UV−vis spectroscopy (Agilent8453), X-ray powder diffractometer (XRD) (Rigaku D/max2500), FT-IR spectroscopy (PerkinElmer) and Raman spectroscopy (JY-HR800). The electrical conductivities of prepared nanostructures were measured by using Agilent multimeter, Cleveland, Ohio, U.S.A., through a four-probe method. 2.9. Electrochemical Characterizations. Electrochemical measurements were performed in a conventional one compartment cell with the three electrode system, consisting of bare/ modified carbon cloth, Ag/AgCl and Pt wire as a working, 16884

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Figure 1. TEM images of (a) rGO, (b) rGO/Pt, (c) rGO/PANI, (d) and (e) rGO/PANI/Pt and (f) FFT pattern of rGO/PANI/Pt (Inset, SAED pattern).

induced between the phenyl group and basal planes of aniline and GO, respectively. The hydrogen bonding interactions (O− H···N) are resulted from the hydroxyl groups of GO sheets and −N− of aniline. The adsorbed aniline was polymerized through in situ polymerization technique with the aid of APS and PANI layer was homogeneously surrounded on the edges of GO sheets. It has not only increased the thickness of GO sheets but also disrupted the transparent nature of GO sheets. After the addition of HCOOH with GO/PANI, GO was reduced into rGO, which would neither affect the morphology nor the thickness of rGO/PANI (Figure 1c) from that of GO/PANI composite. rGO has not exhibited any aggregation and was completely used as a substrate for PANI to produce hierarchical nanocomposites. Though the morphology and structure of anchored Pt nanoparticles in rGO/PANI/Pt (Figure 1d and e) is similar to rGO/Pt (Figure 1b), an increment in the thickness of rGO, disruption in the transparent nature and more number of Pt nanoparticles was observed for the rGO/PANI/Pt composite. A strong covalent nitride bond (Pt−N) induced between PANI and Pt through the free electron pair of N and the space orbital of Pt, respectively,34 leads to the strong adhesion and maximum number of Pt nanoparticles over the rGO/PANI matrix. The average particle size of uniformly anchored Pt nanoparticles over rGO/PANI matrix is found to be 5 nm. The ordered lattice fringes of Pt nanoparticles with the same orientation were observed from the FFT pattern of rGO/PANI/Pt and the lattice spacing distance was found to be 0.23 nm, corresponds to the face-centered cubic (fcc) structure of Pt (111) (Figure 1f and inset in 1f). For the confirmation of elements present in the prepared nanostructures and nanocomposites, EDAX measurement was carried out (Supporting Information Figure S2). The existence of C, O, and N elements in rGO/PANI composite represent the complex formation of PANI with rGO (Figure S2a in Supporting Information). For the rGO/Pt nanocomposite, C, O, and Pt elements were observed (Figure S2b, Supporting Information). The EDAX pattern of rGO/PANI/Pt composite exhibited the C, O, N, and Pt elements, ensuring the presence of PANI and Pt over the rGO sheets (Figure S2c Supporting Information). 3.2. UV−Vis Absorption Studies. The reduction of GO and formation of rGO composites were ensured from UV−vis

reference, and counter electrode, respectively. The three electrode system was placed in 1 M glucose and 300 μM HNQ mediated Escherichia coli solution under anaerobic conditions and the cyclic voltammetric studies were studied in CHI-650D analytical system. The electrochemical impedance spectroscopy (EIS) measurements of the studied electrodes were conducted in a frequency range of 300 kHz-0.001 kHz under an open circuit conditions with aid of CHI-650D instrument. 2.10. MFC Performances. The MFC performances of prepared catalysts were evaluated by using a dual chamber MFC system as described elsewhere.33 Polarization and power output of the constructed MFCs were monitored using a data acquisition system [(Agilent multimeter) Cleveland, Ohio, U.S.A.] by varying the external resistances in the range of 510 kΩ−120 Ω.

3. RESULTS AND DISCUSSION 3.1. Morphological Properties. The transparent and thin layer structure with some wrinkles and folds on the surface and edges were observed for rGO (Figure 1a). From Figure 1b it is clear that Pt nanoparticles were homogeneously anchored over the surface of rGO sheets with the narrow size distribution and the growth of Pt nanoparticles focuses on the edge of rGO layer. The presence of carboxyl, hydroxyl, and epoxide functional groups on the surface of GO layers act as binding/ adsorption sites16 for the anchoring of Pt4+ ions. It was followed by the chemical reduction of GO and Pt4+ ions into rGO and Pt nanoparticles, respectively, with the aid of HCOOH as a single reducing agent (Supporting Information Figure S1). The expanded interlayer distance of rGO and large and uniformly distributed active sites are also helpful in homogeneously anchoring the Pt nanoparticles over the rGO sheets and the average particle size of anchored Pt nanoparticles is found to be 5 nm. GO served as a template and provided active nucleation sites and high surface area for the effective adsorption of aniline over its surface16 and the interaction of GO with aniline occurs through electrostatic interaction/π−π stacking force/hydrogen bonding (Figure 2). The structural electrostatic interaction is observed between the electron withdrawing carboxyl, carbonyl, and epoxy functional groups of GO and electron donating groups (−NH−) of aniline. The π−π stacking forces are 16885

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Figure 2. Proposed scheme for the preparation of rGO/PANI/Pt nanocomposite.

absorption spectroscopy and the corresponding UV−vis absorption spectra are given in Figure 3. The GO dispersion exhibits SPR bands at 238 (π−π* transition of aromatic CC bonds) and 303 nm (n−π* transition of CO bonds). After the treatment with HCOOH, the SPR band observed at 238 nm of GO has been red-shifted to 267 nm and CO band was disappeared for rGO, ensuring the restorage of electronic conjugation/aromatic structure. A similar trend has been observed for the rGO/Pt composite as that of rGO and it is clear that Pt does not exhibit any significant SPR band in UV− vis region. In addition to the band observed at 267 nm, a polaron−π* transition of PANI was observed at 405 nm for the rGO/PANI composite, indicating the intercalation of PANI with the rGO matrix.39−42 The characteristic band of PANI found at 405 nm has been blue-shifted to 374 nm for rGO/ PANI/Pt, which indicates the closer contact of PANI with rGO, owing to the increased steric hindrance. 3.3. XRD Analysis. The crystalline structure and structural purity of prepared nanostructures were evaluated by XRD patterns. The native XRD peak of graphite powder observed at

Figure 3. UV−vis absorption spectra of (a) GO (b) rGO, (c) rGO/Pt, (d) rGO/PANI, and (e) rGO/PANI/Pt. 16886

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26° was disappeared and an intense peak observed at 10.27° ensured the formation of GO, which is assigned to the (001) plane (Figure 4a).43 An interlayer distance of ∼0·86 nm was

Figure 5. FT-IR spectra of (a) GO, (b) rGO, (c) rGO/Pt, (d) rGO/ PANI, and (e) rGO/PANI/Pt.

attributed to the CN stretching of secondary aromatic amine, benzenoid ring (CC stretching deformations), and quinonoid ring (NQN) vibrations, respectively.55−57 In addition, CH out of plane bending vibration and CH bonds in the benzene rings are found at 800 and 780−580 cm−1, respectively.58,59 The presence of benzenoid and quinonoid ring vibrations clearly ensured the strong grafting of PANI onto the surface of rGO. For the rGO/PANI/Pt composite, CN stretching, CC stretching deformations, and NQN quinonoid ring vibrations; the characteristic bands of PANI were red-shifted to 1302, 1503, and 1582 cm−1 (Figure 5e), respectively, indicating the complex formation of Pt with rGO/PANI. 3.5. Raman Analysis. D and G bands are the significant Raman spectrum features of GO and are found at 1344 and 1598 cm−1, respectively (Figure 6a). mode of k point photons

Figure 4. X-ray diffraction patterns of (a) GO (b) rGO, (c) rGO/Pt, (d) rGO/PANI, and (e) rGO/PANI/Pt.

observed for GO, which is larger than that of graphite powder and is attributed to the intercalation of oxide functional groups. The expansion of CO2 evolved into the interstices between graphene sheets resulted in the exfoliation process. The disappearance of (001) reflection plane ensured the complete reduction of GO and an appearance of a new broad reflection plane at 23.5° ensured the presence of stacked graphene layers of rGO (Figure 4b).44,45 rGO/Pt nanocomposite exhibited the characteristic diffraction patterns of Pt at 39.9, 46.3, 67.7, 81.8, and 86.02° (Figure 4c) and are assigned to the (111), (200), (220), (311), and (222) facets of the fcc structure, respectively.46−48 The average crystallite size of Pt extracted from the (220) facet centered at 67.7° using standard Scherrer equation is found to be 5 nm, which is consistent with the TEM observation. For the rGO/PANI composite, two peaks were observed at 19.5 and 25° (Figure 4d) and are attributed to the (100) and (110) facets of crystalline PANI, respectively.49,50 All of the characteristic diffraction patterns of rGO, PANI, and Pt were observed for the prepared rGO/PANI/Pt nanocomposite, ensuring the composite formation. The fcc structure of Pt nanoparticles has not been altered in the rGO/PANI/Pt nanocomposite as observed from the corresponding diffraction pattern (Figure 4e) and the observed average crystallite size of Pt is 5 nm, which is similar to the size of Pt in rGO/Pt composite. From the observed diffraction patterns, it is clear that HCOOH has served as a single reducing agent for the reduction of GO and Pt4+ ions, leading to the formation of rGO and Pt nanoparticles, respectively. 3.4. FT-IR Analysis. Figure 5a exhibits the FT-IR spectrum of GO and the −OH stretching vibration of GO was found from the broad band at 3470 cm−1. The FT-IR spectrum of GO illustrates CO stretching of carboxylic groups at 1728 cm−1, CC stretching vibration at 1634 cm−1, CO stretching vibrations of carboxyl and alkoxy groups at 1396 and 1096 cm−1, respectively (Figure 5a).51,52 After the reduction of GO with the aid of HCOOH, the characteristic CO and CO stretching vibrations of GO were completely disappeared, representing the removal of oxygen functionalities (Figure 5b).53,54 For the rGO/PANI composite (Figure 5d), the characteristic bands found at 1294, 1490, and 1568 cm−1 are

Figure 6. Raman spectra of (a) GO, (b) rGO, (c) rGO/Pt, (d) rGO/ PANI, and (e) rGO/PANI/Pt.

of A1g symmetry and first-order scattering of E2g The D and G bands are occurred from the k point photons of A1g symmetry and first-order scattering of E2g mode, respectively.43 The observed bands elucidated that the extensive oxidation destructed the sp2 character and induced the formation of defects in the graphene sheets. The ID/IG ratio of rGO is found to be 1.09 (Figure 6b), which is higher than that of GO (0.89) 16887

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(Figure 6a), ensuring the size decrement of sp2 domains and deoxygenation process.60−62 The increased ID/IG ratio of rGO/ Pt (1.20) reflected the increased disorder structure of rGO and other structural defects (Figure 6c). The D band of rGO/PANI composite has been shifted to lower frequencies and is attributed to the fact that the electron pairs of N atoms of PANI resonated with the adjacent benzene structures of rGO.63 In addition, the new bands observed at 822, 1187, and 1220 cm−1 for the rGO/PANI are attributed to the substituted benzene ring deformation, CH vibrations in quinoid/phenyl groups, and CN stretching vibrations of PANI, respectively (Figure 6d).64−66 Though the characteristic bands of PANI observed in rGO/PANI were also observed in rGO/PANI/Pt, a decrement in the intensities was notified (Figure 6e). The electrical conductivity observed for rGO is 22.3 S/cm, which is much higher than that of GO (10−5 S/cm), owing to the restored conjugated π system. The synergetic interaction exerted between rGO and Pt exhibited an increment in the electrical conductivity of 36 S/cm for the rGO/Pt composite. The electrical conductivity of rGO/Pt is inferior to the rGO/ PANI/Pt, owing to the insufficient electrical contact point from the randomly dispersed and few agglomerated Pt nanoparticles over the rGO surface. Meanwhile, a strong covalent bond induced between the PANI and Pt of rGO/PANI/Pt resulted in the homogeneous distribution and more number of Pt nanoparticles, leading to the enhanced electrical conductivity of 47 S/cm. 3.6. TGA Analysis. The TGA profile of prepared nanostructures is given in Figure 7. A lower weight loss of

oxygen labile functional groups during the reduction process.37,60 Though rGO/Pt composite exhibited a similar thermal profile as that of rGO, a minimal weight loss of 23% (Figure 7d) observed for the composite ensured the high thermal stability among the prepared nanostructures and is attributed to the high crystallinty of the corresponding composite given via Pt nanoparticles. rGO/PANI composite exhibited an inferior thermal stability over rGO (Figure 7e), owing to the decomposition of PANI in the rGO matrix. After 300 °C, the relatively higher weight loss of rGO/PANI was observed over rGO, which is attributed to the deprotonation of PANI through the loss of dopant HCl and polymeric degradation of skeleton PANI units.66,67 However, rGO/ PANI/Pt (Figure 7f) exhibited an improved thermal stability over rGO/PANI owing to its improved crystallinity. 3.7. Electrochemical Performances. The electrocatalytic activities of prepared nanostructures were evaluated using cyclic voltammetry and the cyclic voltammograms obtained under Escherichia coli, HNQ and glucose solution are given in Figure 8. The well-defined redox peaks were observed for all of the

Figure 8. Cyclic voltammograms of (a) bare CC, (b) rGO/CC, (c) rGO/PANI/CC, (d) rGO/Pt/CC, and (e) rGO/PANI/Pt/CC obtained under Escherichia coli, HNQ and glucose at a scan rate of 50 mV/s.

studied electrodes. Though bare CC exhibited significant redox peaks, lower peak currents were observed due to its limited conductivity. The electrocatalytic performances of CC were improved by rGO and rGO supported nanocomposites as observed from the increased peak currents. The cyclic voltammograms of rGO/PANI/CC and rGO/PANI/Pt/CC obtained under buffer solution is given in Supporting Information Figure S3. The two pairs of redox peaks were observed for the rGO/PANI/CC under buffer solution, which are the characteristic peaks of PANI and are ascribed to the leucoemeraldine/emaraldine and emaraldine/pernigraniline structural conversions. Although a similar trend has been observed for the rGO/PANI/Pt/CC, increased redox currents were observed, which is attributed to the high electrocataytic activity of the rGO/PANI/Pt composite. Under the Escherichia coli in buffer solution, rGO/PANI/CC and rGO/PANI/Pt/CC exhibited the increased redox currents over the buffer solution, which indicates that glucose can be oxidized in the presence of Escherichia coli. Among the studied electrodes, rGO/PANI/Pt/ CC exhibited maximum peak currents that are attributed to the

Figure 7. TGA patterns of (a) graphite, (b) GO, (c) rGO, (d) rGO/ Pt, (e) rGO/PANI, and (f) rGO/PANI/Pt.

4% observed for the graphite powder ensured its excellent thermal stability (Figure 7a). The oxidation of graphite powder induces the oxygen labile functional groups such as carboxyl, epoxy, hydroxyl, and so forth, over the surface of GO, which destructed the conjugated sp2 hybridized network and decreased the thermal stability of GO. The significant weight losses of GO were observed between 100, 200, and 203 °C and are attributed to the loss of physisorbed water molecules and pyrolysis of oxygen containing functional groups, respectively (Figure 7b).60 After the reduction of GO with HCOOH, rGO exhibited an improved thermal stability with a weight loss of 35% (Figure 7c), ensuring the removal of majority of the 16888

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centers improved the electrochemical performances of rGO/ PANI/Pt/CC. 3.8. MFC Performances. The electrocatalytic activities of synthesized nanostructures toward MFC performances were evaluated and given in Figure 10. Bare CC exhibited the power

improved electrical conductivity, number of conduction channels, and enhanced surface area of the rGO/PANI/Pt composite. The peak to peak separation (ΔEp) was also lowered for the rGO/PANI/Pt/CC in comparison with the studied electrodes, indicating the high electron transfer rate. The improved electrical conductivity of rGO/PANI/Pt composite increased the electron transfer efficiency and promoted the contact between the anolyte and exposed surface area of the electrode. It increased the inter facial contact between the electrode and microorganisms and thereby the oxidation current was increased. The maximum oxidation of glucose observed from the increased oxidation current ensured the electrogenic activity of Escherichia coli. Hence, it is clear that rGO/PANI/Pt composite harvested number of electrons through the direct electron transfer from the oxidation of glucose via Escherichia coli. The interfacial properties between electrode and anolyte were identified from EIS technique (Figure 9) and all of the

Figure 10. Fuel cell performances of MFC equipped with (■) bare CC, (●) rGO/CC, (▲) rGO/PANI/CC, (▼) rGO/Pt/CC, and (⧫) rGO/PANI/Pt/CC.

and current densities of 377 mW/m2 and 3336 mA/m2, respectively, which are the lowest among the studied electrodes due to the limited electrical conductivity of electrode. rGO/CC exhibited the improved MFC performances owing to its ability to shuttle, store, and transfer electrons. rGO has the potential surface area to adopt the conductive polymer in its surface that avoids the swelling and shrinkage of the conductive polymer. rGO provided number of active sites for the nucleation of PANI and also generates excellent electron transfer pathways. The large specific surface area and good biocompatibility of rGO/PANI composite increased the charge transfer efficiency and cell−material interaction, which promoted the MFC power and current densities to 1050 mW/m2 and 6200 mA/m2, respectively. An increase in the PANI mass content from 1 to 11% with GO (1%) gradually increased the thickness of PANI layer on rGO sheets, which increased the charge transfer between rGO and PANI. However, a further increment in the thickness of PANI layer over rGO by enhancing the PANI content beyond 11% resulted in the decreased charge transfer between rGO and PANI, resulting lower MFC performances. The smaller sized Pt nanoparticles uniformly anchored over the rGO sheets increased the electrochemically available surface area of the rGO/Pt composite. The Pt nanoparticles anchored over the rGO sheets served as spacers and prevented the restacking of rGO sheets, which construct the both faces of rGO sheets are accessible. The transfer of electrons between π orbitals of rGO and d orbitals of Pt atoms stimulated the high electrical conductivity and surface area of the rGO/Pt composite, which in turn the number of catalytic sites was exposed for the rGO/Pt/CC. By the efforts of above, the maximum power and current densities of 1258 mW/m2 and 6800 mA/m2, respectively, were observed for the rGO/Pt/CC. rGO/PANI/Pt/CC exhibited the maximum MFC power and current densities of 2059 mW/m2 and 9559 mA/m2 , respectively. A strong bond exerted between rGO and PANI through π−π stacking/electrostatic interactions/hydrogen

Figure 9. Impedance spectra of (■) bare CC, (●) rGO/CC, (▲) rGO/PANI/CC, (▼) rGO/Pt/CC, and (⧫) rGO/PANI/Pt/CC obtained under Escherichia coli, HNQ, and glucose.

obtained Nyquist plots include semicircles and straight lines at high and low frequencies, respectively. The semicircle and linear portions represent the electron transfer and diffusion limited processes, respectively. The interfacial charge transfer resistance (Rct) is identified from the diameter of a semicircle, representing the resistance of electrochemical reactions on the electrode and determines the electron transfer kinetics of redox probe at the electrode interface.68−70 The limited electrical conductivity is responsible for the high interfacial chargetransfer resistance of bare CC. The charge transfer resistance of CC was reduced with the modification of rGO, owing to its improved electrical conductivity. The effectively paneled Pt nanoparticles increased the catalytic active sites of rGO/Pt nanocomposite and PANI increased the electron conduction channels of rGO/PANI nanocomposite that reduced the charge transfer resistance of the corresponding electrodes. rGO/ PANI/Pt/CC exhibited the lowest charge transfer resistance among the studied electrodes and is attributed to the synergistic combination of number of conduction channels and active sites given via PANI and Pt, respectively. The combination of improved electrochemically active surface area and active 16889

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Pt composite conceived only 4 h for reaching the maximum OCV from the first discharge cycle to the second charge cycle after the addition of fresh inoculums which is much lesser than the studied electrodes. Hence, it is clear that rGO/PANI/Pt composite exhibited rapid start up for the charge process and lower degradation during the discharge process. The PANI bridges with Pt and rGO through the Pt−N bond and electrostatic interaction/π−π stacking force/hydrogen bonding, respectively, has extensively stabilized the composite, which not only prohibits the agglomeration of Pt nanoparticles but also the restacking of rGO sheets. The active rGO sheets provided a strong support for the anchoring of metal nanoparticles and avoided the agglomeration of nanoparticles even under the presence of an aqueous environment which collectively increased the durability of rGO/PANI/Pt/CC equipped MFC. The post morphological images of rGO/Pt (Supporting Information Figure S5a) and rGO/PANI/Pt (Supporting Information Figure S5b) observed after 411 and 510 h, respectively, of MFC operation are given in Figure S5. Few free zones in rGO and agglomeration of Pt nanoparticles were observed for the rGO/Pt nanocomposite, indicating the detachment of Pt nanoparticles from rGO matrix. Though a slight increment in the size of Pt nanoparticles was observed for rGO/PANI/Pt in comparison with the freshly prepared catalyst (Figure 1d and e), a similar morphology maintained in the system revealed the robustness of the prepared composite. By the synergistic combination of active carbon support, conducting polymer, and noble nanometal, the prepared rGO/PANI/Pt composite exhibited higher OCV at the initial stage of MFC operation itself and displayed excellent durability performances, owing to the strong interaction exerted among the three components. It has been reported that PANI modified carbon cloth exhibited a maximum MFC current density of 0.29 mA/cm2 under the Escherichia coli medium.71 Scott et al.11 and Li et al.24 reported the maximum MFC power densities of 26.5 and 27.4 mW/m2, respectively, for the PANI modified anodes under the wastewater medium. The obtained lower power output and limited durability performances of PANI are not influential enough to compete with the conventional energy systems. On the other side, electrochemically reduced GO (ErGO) exhibited a maximum MFC power density of 52.5 mW/m2 under the Pseudomonas aeruginosa medium20 and Hou et al. reported the maximum power density of 1003 mW/m2 for the ErGO/CC under the anerobic sludge medium.28 Although improved MFC power outputs were observed for the rGO over PANI due to its enhanced electrical conductivity, durability crisis still persists in the rGO based MFC systems. rGO sheets and PANI exhibited restacking and swelling behaviors under an aqueous environment, which deteriorated the surface area and other unique properties of corresponding nanostructures in the MFC system, and thereby, the corresponding fuel cell performances were lowered. The composite formation of rGO with PANI reported in this study effectively tackled the negative impacts of the aforementioned individual components through their large van-der Waals and hydrogen bonding interactions and the resultant rGO/PANI composite exhibited an improved MFC power density of 1050 mW/m2 and durability of 350 h, which is superior than the 3D graphene/ PANI foam27 (maximum power density of 768 mW/m2 under S. oneidensis medium) and graphene nanoribbons/PANI29 (maximum power density of 856 mW/m2 under S. oneidensis medium). Although Pt nanoparticles anchored rGO or PANI

bonding provided excellent conduction channels associated with an improved electrical conductivity. PANI bridges rGO with Pt and increased the number of electrocatalytic active sites and provided continuous electron conduction channels for the effective electron transfer. It is also clear that the smaller rGO sheets allowed the uniform dispersion of PANI and synergetic effect exerted among the active support, conductive polymer, and nanometal catalyst of rGO/PANI/Pt associated with the intimate contact and remarkable stabilization influenced the maximum MFC performances. The relationship between mass activity of Pt nanoparticles in rGO/PANI/Pt composite and MFC performances is given in Supporting Information Figure S4. As the Pt mass content increased from 0.2 to 1% with that of 5% GO/PANI, the corresponding MFC performances were gradually increased and reached a maximum power density of 2059 mW/m2 for the 1% Pt mass content with GO/PANI (5%), which is ascribed to the increased catalytic active sites, number of grain boundaries, and surface area of the composite. With the further increment of Pt mass content (1.2 and 1.4%), the MFC performances were significantly decreased and are due to the agglomeration of Pt nanoparticles and decreased surface area. In general, nanostructured materials exhibit agglomeration under aqueous environment which may degrade the long-term efficiencies of applied devices. To evaluate the electrocatalytic stabilities of studied catalysts, voltage−time profile of MFCs were studied evaluated at a fixed external resistance of 1000 Ω and are shown in Figure 11. The anolyte and catholyte were

Figure 11. Representative cell voltage−time profile of MFC equipped with (■) bare CC, (●) rGO/CC, (▲) rGO/PANI/CC, (▼) rGO/ Pt/CC, and (★) rGO/PANI/Pt/CC.

replaced with the new media when the MFC voltage was dropped below 0.05V. The unmodified CC exhibited faster degradation over the repetitive cycles and the degradation behavior was restricted by the prepared nanostructures. Though rGO/CC, rGO/PANI/CC, and rGO/Pt/CC exhibited prompt stabilities over the bare CC, life cycle performances of the said electrodes are inferior to the rGO/PANI/Pt composite. During the course of MFC operation, an OCV variation of 0.13 V was observed between 3 and 471 h for the rGO/PANI/Pt/ CC and conceived a maximum time of 140 h for the discharge process of first cycle. However, CC, rGO/CC, rGO/PANI/CC, and rGO/Pt/CC exhibited a rapid decrement in the discharge process as evidenced from Figure 11. In addition, rGO/PANI/ 16890

dx.doi.org/10.1021/ie502399y | Ind. Eng. Chem. Res. 2014, 53, 16883−16893

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Young Scientist Grant No. SR/FT/CS-113/2010(G). This work was supported by the Human Resources Development program (No. 20114030200060) of the Korea Institute of Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

have been reported for the electrochemical applications, the physical interaction exerted between rGO sheets or PANI matrix and Pt nanoparticles is not influential enough to maintain the surface area under the repetitive cycles of MFC environment, which faded the rGO or PANI/Pt composite in MFC applications. Hence, a covalent interaction of Pt−N has been generated between Pt and PANI in this report, which prohibited the agglomeration of Pt nanoparticles, swelling behavior of PANI and restacking of rGO sheets. The synergetic interactions exerted among the components of rGO/PANI/Pt facilitated the higher electrical conductivity and surface area of the composite, and thereby, the contact between electrode surface and electrolyte was facilitated. By the collective efforts of above, the prepared rGO/PANI/Pt composite exhibited a maximum MFC power density of 2059 mW/m2 and its robust characteristics increased the concrete life durability of MFC for more than 500h, which are far superior than the PANI/Pt black71 (maximum power output of 9 mW under Escherichia coli medium), PANI/Pt72 (maximum power density of 2.9 W/ m3 under H. anomala medium) and multiwalled carbon nanotubes/PANI/Pt73 (maximum power density of 613.5 mW/m2 under Escherichia coli medium) composites.

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4. CONCLUSION The rGO supported PANI/Pt nanocomposites were synthesized through the simple in situ polymerization and reduction techniques and the complex formation of prepared composite was identified from the structural characterizations. The inter facial properties between the electrode surface and anolyte was promoted by the increased electrical conductivity of rGO/ PANI/Pt composite. The lower charge transfer resistance observed for the rGO/PANI/Pt composite validated the above. By the combined efforts of high electrical conductivity and number of catalytic active sites, rGO/PANI/Pt composite harvested number of electrons and exhibited the maximum MFC performances. The active carbon support prohibited the agglomeration of nanoparticles and prevented the swelling of a conductive polymer under aqueous environment and thereby the durability performances of MFCs were facilitated for more than 500 h.

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ASSOCIATED CONTENT

S Supporting Information *

Proposed scheme for the preparation of rGO/Pt composite, EDAX patterns of prepared nanostructures, cyclic voltammograms of rGO/PANI/CC and rGO/PANI/Pt/CC obtained under PBS, MFC power density of rGO/PANI/Pt/CC as a function of Pt mass content and TEM images of prepared nanostructures after the post MFC operation are given in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 91-958-575-2997. *E-mail: [email protected]. Fax: +82-63-270-2306. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Department of Science and TechnologySERB, New Delhi, India Fast Track Project for 16891

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