Ternary composite of polyaniline graphene and TiO2 as a bifunctional

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Materials and Interfaces

Ternary composite of polyaniline graphene and TiO2 as a bifunctional catalyst to enhance the performance of both the bioanode and cathode of a microbial fuel cell Thi Hiep Han, Nazish Parveen, Jun Ho Shim, Anh Thi Nguyet Nguyen, Neelima Mahato, and Moo Hwan Cho Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05314 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Ternary composite of polyaniline graphene and TiO2 as a bifunctional catalyst to enhance the performance of both the bioanode and cathode of a microbial fuel cell Thi Hiep Han1,2,3, Nazish Parveen3,4, Jun Ho Shim5, Anh Thi Nguyet Nguyen5, Neelima Mahato3, Moo Hwan Cho3* 1

Department for Management of Science and Technology Development, Ton Duc Thang

University, Ho Chi Minh City, Vietnam 2

Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam

3

School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541,

Republic of Korea 4

Flexible Display and Printed Electronics Laboratory, Department of Chemical and

Biochemical Engineering, Dongguk University-Seoul, 04620, Seoul, South Korea 5

Department of Chemistry, Daegu University, Gyeongsan-si, Gyeongsangbuk-do, 38453,

Republic of Korea Thi Hiep Han ([email protected]) Nazish Parveen ([email protected]) Jun Ho Shim ([email protected]) Anh Thi Nguyet Nguyen ([email protected]) 1


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Neelima Mahato ([email protected]) *Corresponding author: Moo Hwan Cho Email: [email protected]; Tel.: +82-53-782-8419; Fax: +82-53-810-4631 Thi Hiep Han and Nazish Parveen contributed equally to this work.

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Abstract Microbial fuel cells (MFCs) are a potential sustainable energy resource by converting organic pollutants in wastewater to clean energy. The performance of MFCs is influenced directly by the electrode material. In this study, a ternary PANI-TiO2-GN nanocomposite was used successfully to improve the performance of both the cathode and anode MFC. The PANITiO2-GN catalyst exhibited better oxygen reduction reaction (ORR) activity in the cathode, particularly as a superior catalyst for improved extracellular electron transfer (EET) to the anode. This behavior was attributed to the good electronic conductivity, long-term stability, and durability of the composite. The immobilization of bacteria and catalyst matrix in the anode facilitated more EET to the anode, which further improved the performance of the MFCs. The application of PANI-TiO2-GN as a bifunctional catalyst in both the cathode and anode helped decrease the cost of MFCs, making it more practical.

Keywords: Microbial fuel cells; PANI-TiO2-GN; capacitive electrode; oxygen reduction reaction; immobilized anode.

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1. INTRODUCTION

Microbial fuel cells (MFCs) are electrochemical devices that use a biocatalyst to generate electricity from waste. The performance of MFCs depends on a range of factors, in which the electrode design plays a key role. Considerable efforts have been made to design new electrodes1 or synthesize catalysts to enhance the performance of electrodes for MFCs.2 In particular, the key objectives are to synthesize an efficient and cost-effective anode to promote extracellular electron transfer (EET) as well as a cathodic catalyst with high catalytic activity for the oxygen reduction reaction (ORR).3 In the case of the cathode, the slow kinetics of the ORR is a major factor limiting the performance of MFCs. Pt is a conventional catalyst used to accelerate the ORR kinetics because of its high ORR catalytic activity. On the other hand, Pt is rare and expensive, accounting for one half of the capital cost of laboratory scale MFCs.4 Moreover, its activity can be poisoned by some environment factors, such as sulfur compounds, methanol, etc. Therefore, many ORR catalysts, such as advanced Pt alloys, transition metal oxide and chalcogenide composites, core-shell nanoparticles, and carbon-based catalysts and their composite with non-noble metals, have been studied to alter the Pt electrode.5 ORR catalysts should possess excellent electrical conductivity and a large surface area to enhance the ORR kinetics by decreasing the cathodic over-potential.6 In the case of the anode, slow extracellular electron transfer (EET) to the anode is the major limiting factor of a MFC. Electron transfer to the anode follows various EET mechanisms, such as outer-membrane proteins, excreted mediators, and extracellular conductive nanowires.7 Modification of the anode is considered to be an easier approach to 4



improve the EET. The modified anode should meet some of the requirements, such as large surface area with the appropriate porosity, high electronic conductivity, excellent biocompatibility, high stability and durability, and low cost.8 Another effective approach to enhance the EET is the immobilization of bacteria embedded in the catalyst mixture on the anode.9-11 In the immobilization matrix, the cell is in close contact with the catalyst. Thus, the electron generated can be transferred easily to the neighboring catalyst, which acts as a bridge for electron transfer to the electrode. This method is particularly helpful in the case of a thick biofilm, where the electrons need to travel a long distance to reach the electrode. Polyaniline (PANI) is a highly conductive polymer that has been studied extensively in MFCs to increase the anodic conductivity and EET efficiency. The long polymer chain could enter the bacterial cell membrane and draw out the electrons via the metabolic pathway.12 The high surface area and surface roughness of PANI enhances the bio-compatibility-modified anode and the behavior of the inoculated bacterial culture.13 Moreover, the lower cycling stability of the material in the charge-discharge process limits its applications. The decoration of nanocomposites of PANI with metal oxides, such as Fe3O4, Co3O4, SnO2, and TiO2, can improve the capacity, cycling ability, and charge/discharge property.14 Among these metal oxides, TiO2 is cost-effective, environmentally friendly, biocompatible, and chemically stable. Although TiO2 has poor ionic and electronic conductivity, the TiO2-based structure can increase the structural stability and prolong the life cycle of a material. To enhance the electrode performance of MFCs, traditional electrodes are improved using graphene (GN) because of its outstanding conductivity and large specific surface area, which generates a huge number of active sites. A GN/carbon cloth anode was reported to enhance the energy conversion efficiency and power output 3 and 2.7 fold, respectively.15 5


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Thus far, several studies have examined the application of binary composites of PANI, GN, and TiO2 as an anode catalyst in MFCs. The integration of TiO2 with conducting polymers16 or GN17 can improve the electron transfer efficiency of TiO2, and enhance the MFC performance. The PANI hybridized 3D GN sheet anode exhibited a 4 times higher maximum power density than traditional carbon cloth by providing multiple highly conductive pathways.18 To the best of the authors’ knowledge, there are no reports of ternary PANI-TiO2-GN nanocomposites in MFCs. A recent study indicated that the integration of PANI-TiO2-GN results in a large surface area and porosity, excellent electronic conductivity, good biocompatibility, and high stability and durability.19 In particular, the coordination of GN and TiO2 enhances the electrode conductivity and stability considerably, and improves the super-capacitance of the ternary PANI-TiO2-GN nanocomposite.20 Consequently, the ternary nanocomposite has been estimated to be an excellent catalyst to increase the performance of MFCs. In this study, a ternary PANI-TiO2-GN nanocomposite was used as a bifunctional catalyst to enhance the performance of both the cathode and anode electrodes. The bifunctional activity of the PANI-TiO2-GN catalysts can help make the MFC process relatively simple and cost-effective.

2. MATERIAL AND METHODS 2.1. Chemicals & Materials. Graphite sheets, 10 × 1.5 × 0.5 cm3 in size, were supplied by KOMAX Industrial Co. Ltd, South Korea and used to synthesize GN. The aniline monomer and Nafion solution were purchased from Sigma-Aldrich, USA. Titanium isopropoxide (TIP) was obtained from Daejung Chemicals and Metals Co. Ltd, S. Korea. Carbon tetrachloride (CCl4), sodium perchlorate (NaClO4), sodium thiosulphate (Na2S2O3), 6



potassium persulphate (PPs, K2S2O8), ammonia solution (NH3 35 %), hydrochloric acid (HCl), sodium hydroxide (NaOH), ethyl alcohol (C2H6O), methanol (CH3OH) and acetone (C3H6O) were purchased from Duksan Co. Ltd, S. Korea. Plain carbon paper (without wet proof) and 10 wt. % Pt coated on carbon paper (0.5 mg/cm2) were provided by Fuel Cell Earth LLC., USA. The Pt/C powder catalyst (20% Pt) was purchased from Alfa Aesar, USA. De-ionized water supplied by Pure Roup 30 water purifier system was used in all experiments. All other chemicals used in this study were of analytical grade and used as received.

2.2. Catalysts Fabrication and Characterization. The PANI-TiO2-GN was synthesized using the method described elsewhere.19 Briefly, few-layered GN was obtained electrochemically from a graphite sheet. The oxidant solution was then added drop-wise to a beaker containing 10 mL of CCl4, 0.5 mL of TIP, 5 mg of GN, and 1 mL of aniline monomer in a stirring ice bath. As a result, TiO2 was generated simultaneously with aniline polymerization on the GN. The PANI-TiO2-GN composite was filtered, washed, neutralized, and finally doped with a 0.1 M HCl solution overnight and dried in an oven at 80 oC for 1 day. The external morphology of the ternary catalyst was examined by scanning electron microscopy (SEM, S-4100 Hitachi, Japan) and field emission transmission electron microscopy (FETEM from Tecnai G2 F20, FEI, USA). Phase analysis of the material was confirmed by X-ray diffraction (XRD, PANalytical, X’Pert-PRO MPD, The Netherlands).

2.3. Cathode Electrode Preparation. The catalyst-coated electrode was prepared by mixing the catalyst with a Nafion binder solution at a weight ratio of 9:1 in 5 mL of ethanol. The mixture was sonicated until the catalyst was well dispersed (usually for 60 min). The mixture was dropped by pipette on both sides of carbon paper with a size of 2.5 × 4.5 7


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cm2 and dehydrated at room temperature. The plain carbon paper was also used as the cathode electrode in a parallel experiment as the control.

2.4. Anode Electrode Preparation: Immobilization of bacteria on the Electrode. S. oneidensis MR1 was streaked on an agar plate from -80 oC glycerol stock and incubated at 37 oC overnight. An individual colony was inoculated into 25 mL of Luria broth (LB) medium in a 250-mL flask and cultivated overnight at 250 rpm at 37 oC. The S. oneidensis MR1 cells were collected by centrifugation at 8000 × g for 10 min. The cells were then washed with a 50 mM phosphate buffer solution (PBS, pH 7.0) and re-suspended in 1 mL of PBS for later use. The optical density (OD600) of S. oneidensis MR1 in this solution was 8.33. The bacteria-entrapped nanocomposite anode was prepared as recommended by Yuan et al. (2009).9 Briefly, a 0.15 g sample of the composite was dispersed homogeneously by sonication in 5 mL of de-ionized water for 2 h and stirred vigorously until a paste-like material formed. The S. oneidensis MR1 suspension was placed into a glass bottle containing the nanocomposite paste. After mixing by stirring, 0.25 mL of a 15 M sodium lactate solution and 0.5 mL of Nafion binder (14% w/w) were added to the mixture. The final mixture was stirred for at least 2 min. The mixture was then spread on both sides of carbon paper, dried on a clean bench at room temperature for 30 min, and used for the MFC anode. Two other anodes were prepared as controls using the same procedure. The first control was no PANITiO2-GN/Immobilized, S. o (in the absence of PANI-TiO2-GN catalyst). The remaining one was PANI-TiO2-GN/Non-immobilized S. o (0.5 mL of planktonic cell and 0.25 mL of 15 M sodium lactate were added to the anolyte).

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2.5. Microbial Fuel Cells (MFCs) Assembly and Operation. The conventional H-type MFCs were assembled, as described previously.1,21 A Nafion® 117 membrane (Dupont, USA) was inserted between the anode and cathode chamber. The anode chamber was filled with 10 mL of an overnight culture of S. oneidensis MR1 and 250 mL of LB medium. Before filling the anode chamber, LB medium was purged continuously with nitrogen gas for 10 min to scavenge any available dissolved oxygen, and then autoclaved at 121 oC for 16 min. The cathode chamber contained a 50 mM phosphate-buffered saline (PBS, pH 7) solution (NaH2PO4.2H2O, 3.32 g/L; Na2HPO4.12H2O, 10.36 g/L; KCl, 0.13 g/L; NH4Cl, 0.31 g/L;) and was purged continuously with air at 150 cc air/min. Prior to the set-up, the MFCs were sterilized by autoclaving at 121 °C for 30 min. Titanium was used to hang the electrodes and was connected to a resistance box with a fixed load of 1000 Ω. All MFCs were kept in a closed chamber with a controlled temperature of 30 oC. The experiments were performed in duplicate to assess the reproducibility of the data and a typical data set is presented.

2.6. Measurement and Analysis. The voltage output was recorded continuously using a digital multi-meter (Agilent 34405A, Agilent Technologies, Inc., USA). The power density generation was calculated using the equation: P = V2/(ARext)

(1)

where V is the recorded voltage; A is the anode surface area; and Rex is the external resistance. Once the MFCs reached the stationary phase, the open circuit voltage (OCV) was measured overnight. The polarization test was then performed by changing the Rex from 100 Ω to 6000 Ω, and stabilizing each resistor for 30 min. A polarization curve and power density 9


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curve were acquired by plotting the voltage and power density as a function of the current density, respectively. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were accomplished using a potentiostat (Versa STAT 3, Princeton Research, USA). Both EIS (0.01Hz to 100k Hz, amplitude 10mV) and CV (scan rate of 50 mV/s) were conducted in a three-electrode single chamber reactor. A KCl saturated Ag/AgCl and platinum plate were used as the reference and counter electrodes, respectively. The working electrode was the plain carbon paper/carbon paper-coated catalysts (in the case of the cathode) or anode after running in the MFCs (in the case of the anode). The electrolyte was a 50 mM PBS solution, pH 7 (in the case of the cathode) or LB media (in the case of the anode). Linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) was measured using a RDE-2 of BASi electrochemical analyzer (Bioanalytical Systems Inc., USA) according to the methodology reported elsewhere.22 First, the working electrode (glassy carbon, 0.071 cm2) was wet-polished on an Alpha A polishing cloth (Mark V Lab Inc., USA) using 0.05 μm aluminum slurries. The electrode was then rinsed in de-ionized water, and sonicated in de-ionized water for 3 min to eliminate the aluminum slurry residue. Finally, the electrode was drop-coated with the catalysts, dried at room temperature, and a layer of Nafion binder was deposited on top of the catalyst layer. All RDE experiments were performed in an oxygen-saturated PBS solution (50 mM, pH 7) at a scan rate of 10 mV/s, and a potential range of -1.0 - 0.5 V. The reference and counter electrode was a saturated calomel electrode (SCE) and Pt wire, respectively. The ORR currents were recorded at electrode rotating speeds ranging from 400 to 3600 rpm. 10



2.7. Pretreatment for SEM Observation. The morphology of PANI-TiO2-GN and S. oneidensis MR1 embedded in the matrix of the catalyst were analyzed by scanning electron microscopy (SEM). The pretreatment procedure of S. oneidensis MR1 followed the modified protocol reported elsewhere.23 Briefly, the bacteria-embedded catalyst anode was cut into 0.5 × 0.5 cm2, and fixed directly in a solution containing 2% formaldehyde and 2.5% glutaraldehyde 4 °C overnight. The samples were washed with sodium phosphate buffer (SPB, 0.2 M) before being fixed for 90 min with an OsO4 solution (3 mL of 2% OsO4, 3 mL de-ionized water, and 1.5 mL of 0.2 M SPB). After fixation, the samples were rinsed and dehydrated in a graded series of ethanol solutions (50%, 70%, 80%, 90%, 95%, and 100%), incubated in isoamyl acetate for 20 min, and dried with a critical-point dryer (HCP-2, Hitachi, Japan). Before being examined by SEM (S-4100 Hitachi, Japan), the samples were fixed to SEM stubs and coated with white gold for 200 seconds by ion sputtering (E-1030, Hitachi, Japan).

3. RESULTS AND DISCUSSION This study focused on the application of the ternary PANI-TiO2-GN composite in MFCs. Thus, the behavior of the ternary PANI-TiO2-GN composite in MFCs is discussed in detail. The contents related to the synthesis procedure and the characterization of PANI-TiO2-GN have been already explained with the help of various slandered spectroscopic and microscopic techniques in a previous study for a better understanding.19 Figures S1 and S2 in Supporting Information present the possible reaction mechanism of PANI-TiO2-GN synthesis and some relevant data (SEM, TEM, and XRD), respectively.

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3.1. Application as ORR Catalysts in MFC Cathode. 3.1.1. ORR Activity. Linear swipe voltammetry on a RDE was performed to measure the ORR activity of the as-synthesized catalysis. The results showed that PANI, PANI-TiO2, and PANI-TiO2-GN have a similar onset potential (0.038 V vs. SCE), while more limitinglike current levels of the ORR at the PANI-TiO2-GN were observed, even for the same potential range where no limiting current was found at the PANI and PANI-TiO2 (Figures 1a and b). A previous study reported increased catalyst activity towards the ORR with increasing PANI/TiO2 ratio. Further increases in PANI did not improve the catalytic activity any further.24 Therefore, the improvement of the ORR activity can be attributed mainly to the high surface area per unit volume of the PANI-TiO2-GN providing a larger numbers of active sites. Moreover, the N in the framework of PANI may also have acted as an active center for the ORR.25 Good linearity from Koutecky-Levich (K-L) plots at a potential of -0.6V shows the first order kinetics with respect to the reactant concentration. Based on the slope of the K-L plots, the number of electrons transferred (n) for the ORR activity was calculated as follows:22 1 𝑗

1

1

𝑘

𝑑

=𝑗 +𝑗 ,

2/3

𝑗𝑑 = 0.62𝑛𝐹𝐶𝑂2 𝐷𝑂2 𝑣 −1/6 𝑤 1/2

(2)

where j is the measured limiting current density; jk is the kinetic current density; jd is the diffusion-limited current density; F is the Faraday constant; 𝐶𝑂2 (1.2 × 10−6 mol/cm3) is the saturated concentration of oxygen; 𝐷𝑂2 (1.9 × 10−5 cm2/s) is the diffusion coefficient of oxygen; 𝑣 is the kinematic viscosity of the solution; and 𝑤 is the electrode rotation rate. Using the K-L equation, the number of electrons transferred for PANI, PANI-TiO2 and 12



ternary composite of PANI-TiO2-GN were estimated to be 2, 3, and 3.3, respectively. This suggests that oxygen is reduced via a two-electron transfer pathway for PANI but through a four-electron transfer pathway in the case of PANI-TiO2 and PANI-TiO2-GN in the PBS solution.

Figure 1. (a) LSVs of the catalysts in O2 saturated PBS electrolyte at 1600 rpm, (b) LSVs of PANI-TiO2-GN in O2 saturated PBS at different rotation rates. (c) K–L plots for different catalysts at -0.6 V vs. SCE. The charge transfer resistance of the electrodes and the diffusion interaction between the electrolyte and electrode interface were examined by EIS. The Nyquist plots with the different catalysts coated on carbon paper displayed a distinct semicircle arc in the high frequency area accompanied by a straight line in the low frequency area, as shown in Figure 13


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2. The diameter of the semicircle arc at high frequency depicts the charge transfer resistance (Rct) of the respective electrode.26 After decorating with catalysts, the diameter of the semicircle arc of the electrodes decreased significantly, indicating that the catalysts with high electronic conductivity facilitates charge transfer to the electrode. Figure 2c shows the corresponding electrical equivalent circuits, and compares the measured and simulated spectra using the fitting algorithms. The corresponding equivalent electrical circuit is provided in the Supporting Information (Figure S3). The impedance data corresponding to PANI, PANI-TiO2, and PANI-TiO2-GN can be fitted with an equivalent electrical circuit containing three resistors and two non-ideal capacitors or CPEs (Constant Phase Elements): Rs (solution resistance), and two additional resistors, i.e., R1 (or Rct) and R2, respectively, corresponding to charge transfer resistance and resistance towards the adsorption-desorption of electrolyte elements as well as reaction intermediates on the electrode surface.27 When the R1 and n values are small, enhanced anodic reactions or oxidation are observed and the surface imposes less resistance to the electrochemical processes. In contrast, when the R1 and n values are high, the surface is unaffected by oxidation and the reduction reaction at the cathode is suppressed.28 This can be explained further by the formation of an electrochemical capacitive double layer on the electrode surface comprising both electrolyte molecules/ions and the reaction intermediates. A high R1 value indicates more charge transfer resistance and fewer reaction intermediates. In this regard, PANI-TiO2-GN exhibits relatively lower R1 and R2 values corresponding to the resistance against charge transfer and adsorption desorption processes, suggesting faster reaction rates. The R1 values followed the order of PANI-TiO2-GN < PANI-TiO2 < PANI

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(1216 Ω < 3244 Ω < 3988 Ω), showing that PANI-TiO2-GN has the highest electrical conductivity.

Figure 2. (a) Nyquist plots of the plain carbon paper electrode (no catalyst) and modified by catalysts. (b) Enlargement of the high frequency area. (c) Curve fitting circuit of EIS diagram.

3.1.2. Performance of the Catalysts in the MFC Cathode. The real ORR activity of the as-synthesized catalysts was examined in H-type MFCs and assessed in terms of the power density peak (Figure 3a), polarization (Figure 3b), and stability (Figure 3c). As shown in Figure 3a, the MFC with PANI-TiO2-GN generated a more than 3 times higher maximum power density than that of the MFC with PANI-TiO2 (30.21 mW/m2 vs. 9.86 mW/m2), whereas the maximum power density of the MFC with the PANI electrode was quite low (approximately 2 mW/m2), which was similar to that of the MFC without the catalyst electrode. The polarization curve indicated that the cell voltage of the MFC with the 15


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PANI-TiO2-GN catalyst decreases rapidly in the activation region along with a slow decrease in the ohmic region. In contrast, the cell voltage of the MFC with PANI and PANI-TiO2 decreased much more rapidly than that of PANI-TiO2-GN. Furthermore, the MFC with the PANI-TiO2-GN catalyst showed a higher OCV than that with the PANI-TiO2 and PANI catalysts. These results show that PANI-TiO2-GN exhibits better ORR catalytic activity. The PANI-TiO2-GN catalyst may facilitate the flow of electrons and mass transfer of chemical species to the electrode surface, resulting in lower resistance and potential loss compared to the MFC with the other catalysts.29 This claim is in agreement with the internal resistance estimated using the power density peak method,30 which revealed a decrease in the internal resistance of the MFC system with PANI-TiO2-GN as an ORR catalyst (PANI-TiO2-GN 1607 Ω < PANI-TiO2 1834 Ω < PANI 3379 Ω). The stability of the catalysts for the ORR in a MFC was evaluated by plots of the power density vs. time after several cycles (Figure 3c). Each cycle was identified by a decrease in voltage and recovery of the voltage after changing to new LB media. In the first cycle, the power density of the MFC with PANI was slightly higher than that of PANI-TiO2. On the other hand, it decreased and could not be recovered after 1 cycle, indicating that the ORR activity of PANI was not as stable as PANI-TiO2. When TiO2 was incorporated with PANI, the stability of the PANI-TiO2 composite was enhanced but there was still an approximately 57% decrease in power density after 4 cycles. The introduction of GN to the PANI-TiO2 composite resulted in the highest activity with the longest stability compared to the other catalysts. The MFC with the ternary composite, PANI-TiO2-GN, as the ORR catalyst produced the highest power density of approximately 28 mW/m2 and only 17.8% power density loss after 4 cycles. The high stability of PANI-TiO2-GN was attributed to both the 16



interaction of GN in the composite and the stabilizing nature of TiO2, which hinders the decomposition of the PANI chains.19 Overall, the performance of the catalysts in the MFC cathode was observed in the following order: PANI-TiO2-GN > PANI-TiO2 > PANI, which is in accordance with the intrinsic ORR activity observed by the RDE measurements. Although the performance of PANI-TiO2-GN was poorer than that of Pt-C (maximum power density about 40 mW/m2, data not shown), PANI-TiO2-GN was found to be a good ORR catalyst because of its good performance, long-term stability, and durability.

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Figure 3. (a) Power density curve, (b) polarization curve, and (c) variation of the power density vs. time of the MFCs with the plain carbon paper electrode and carbon paper decorated with the as-synthesized catalysts as cathode electrodes. The power density curve and polarization curve was obtained at 130 h after initiation of the MFC.

3.2. Immobilization of the PANI-TiO2-GN Catalyst Enhanced Anode Performance. 3.2.1. Performance of MFC with Immobilized Anode. The effects of the PANITiO2-GN catalyst and immobilization of the cell on the electrode on the anode performance were investigated. Both MFCs with the PANI-TiO2-GN immobilized S. o and nonimmobilized S. o anodes were more effective for energy production than that of the plain carbon paper-immobilized S. o anode. As shown in the power curves (Figure 4a), the maximum power density of the MFC with the PANI-TiO2-GN immobilized S. o anode was highest at 79.3 mW/m2, which was 1.3 and 2.7 times higher than that of the PANI-TiO2-GN non-immobilized S. o and the plain carbon paper immobilized S. o anode, respectively. The high power output of the MFC with the non-immobilized cell in the start-up time was caused mostly by the planktonic cells, not by the biofilm at the steady state. Therefore, the power curves and polarization curves were measured at 230 h when the three systems produced a stable power density. These results were in good agreement with the power density output vs. time, as shown in Figure 4c. Moreover, the immobilized cell in the matrix of PANI-TiO2-GN helps decrease the internal resistance of the MFC system. The MFC with the anode-coated PANI-TiO2-GN non-

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immobilized S. o showed an internal resistance of 1116.3 Ω. This value was only 800 Ω when the cell was embedded in the catalyst. The immobilized system without the catalyst showed an internal resistance of 1677.4 Ω, which is approximately 2 times higher than that with the catalyst.

Figure. 4 (a) Power density curve, (b) polarization curve, and (c) variation of the power density vs. time of MFC with PANI-TiO2-GN immobilized S. oneidensis MR1 anode (P-TG/Immobilized S. o), carbon paper-coated PANI-TiO2-GN non-immobilized S. oneidensis MR1 (P-T-G/Non-immobilized S. o), and plain carbon paper-immobilized S. oneidensis MR1 (No P-T-G/Immobilized S. o). The power density and polarization curves were measured at 230 h after initiating the MFC. 19


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3.2.2. Electrochemical Activity of the Bioanodes. CV is one of the most powerful techniques for investigating the electrochemical activity of a catalyst. The obtained voltammograms provide insight into the redox nature of a catalyst and the electrochemical reactions occurring on the electrode surface.31 The CVs of the P-T-G/Non-immobilized S. o anode showed distinct redox peaks, whereas the redox peaks of the No P-T-G/Immobilized S. o were negligible (Figure 5a). Two pairs of redox peaks were detected for the P-T-G/Nonimmobilized S. o, which are the typical peaks of PANI and are caused by the transition between the leucoemeraldine and emeraldine states.32 The significant redox peaks with the high redox current of the P-T-G/Non-immobilized S. o anode indicate the high electrocatalytic activity of the P-T-G composite, which was attributed to its higher electrical and ionic conductivity.33 Furthermore, the large rectangular area observed from the CV curve indicates the large pseudocapacitance of the PANI-TiO2-GN composite. This might be due to the high porosity of this composite. The Nyquist plots in Figure 5b show that two anodes with PANI-TiO2-GN had a lower charge transfer resistance than the anode without the catalyst, which is represented by the smaller diameter of the arc at high frequency.34 The immobilization of the bacteria cell and PANI-TiO2-GN catalyst further decreases the charge transfer resistance, showing that the facile electron transfer pathway between the entrapped S. oneidensis MR1 and anode is caused by their tight communication. The enhanced performance of the anode electrode is the synergistic effect of all three components in the ternary nanocomposite. As a result, the ternary PANI-TiO2-GN nanocomposite will gain not only the excellent biocompatibility of PANI, but also the large 20



specific areas of GN and chemical stability of TiO2. In particular, the PANI nanofibers with a network structure behave as nanowires for enhanced extracellular electron transfer by the effective interaction with the redox active sites on the bacterial outer-membrane.7 The c-type cytochromes (c-Cyts) in the membrane act as a “molecular wire” and contribute significantly to the electron transfer process.35 On the other hand, the non-conductive peptide chains would cover the active centers of the outer-membrane c-Cyts, which inhibit the direct EET of the exoelectrogens to the anode electrode.36 Owing to their special nature, GN can form GN– biomolecules bio-composites, which offer an excellent chance to overcome the inhibition, and improve electron transfer.11 Moreover, because electron transfer from S. oneidensis MR1 to the electrode also rely on contact between the S. oneidensis MR1 cells and electrode,37 the immobilization of the cell and catalyst on the electrode provides direct and tight contact of the cell and catalysts on the electrode, facilitating electron transfer. Therefore, the highest power output was also achieved in the MFC with the P-T-G/Immobilized S. o anode.

Figure 5. (a) Cyclic voltammograms (CVs) and (b) electrochemical impedance spectra (EIS) of carbon paper-immobilized S. oneidensis MR1 and PANI-TiO2-GN catalyst (P-TG/Immobilized S. o), carbon paper-coated PANI-TiO2-GN catalyst (P-T-G/Non-immobilized 21


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S. o) and plain carbon paper-immobilized S. oneidensis MR1 (No P-T-G/Immobilized S. o) after operating in the MFC (LB media electrolyte, scan rate of 50 mV/s).

3.2.3 Morphology of the Bacteria in the Matrix of the PANI-TiO2-GN Catalyst. Figures 6 presents SEM images of S. oneidensis MR1 embedded in the matrix of the PANI-TiO2-GN catalyst. The bacteria cells had close interactions with the PANI-TiO2-GN nanoparticles and entrapped inside the pores of the catalyst layer, thereby facilitating electron transfer from bacterial cells to the anode (Figure 6a). At high magnification, the nanowires (pili) produced by S. oneidensis MR1, which are electrically conductive,38 were observed clearly in Figure 6b. Overall, the enhancement of the power production of the MFC with the immobilization of the microorganism-catalyst anode can be explained. The S. oneidensis MR1 is an exoelectrogenic bacterium that can transfer electrons extracellularly via the outer-membrane protein c-Cyts or pili. 39 The immobilized cell on the anode has some benefits compared to the planktonic cell system. The cells that interact closely with the electrode facilitate electron transfer; hence, they are expected to contribute significantly to the power output of MFCs.10 In the absence of a catalyst, the number of exoelectrogenic bacteria is limited by the restricted anode surface area. The matrix of exoelectrogenic bacteria and PANI-TiO2-GN catalyst immobilized on the anode will assist in this case because direct contact of the bacteria on the electrode and the exoelectrogenic bacteria entrapped inside the catalyst pores through the catalyst channel can transfer electrons to the anode. The highly conductive and nanoporous PANI-TiO2-GN can bridge the bacteria and anode to enable the EET process. The high capacitance of the catalyst also helps capture electrons and form a capacitive bridge between 22



the conductive biofilm and electrode, which provides an effective flow of electrons generated by the electroactive bacteria towards the electrode.13

Figure 6. Morphology of S. oneidensis MR1 embedded in the matrix of the PANI-TiO2-GN catalyst. The red arrows in Figure b indicate the nanowires (pili) of S. oneidensis MR1.

4. CONCLUSIONS A ternary PANI-TiO2-GN nanocomposite was used as a bifunctional catalyst to improve the performance of both the anode and cathode of MFCs. The nanoporous structure of the composite facilitates better electrode/electrolyte contact and better substrate diffusion to the electrode. Furthermore, the ternary PANI-TiO2-GN nanocomposite possesses a high specific area, excellent biocompatibility, good conductivity and chemical stability, making it an excellent ORR and EET catalyst. As an ORR catalyst, PANI-TiO2-GN shows the best catalytic activity with the highest power output and stability compared to PANI-TiO2 and PANI. In the anode, the immobilization of bacteria in the catalyst matrix improves the EET significantly, thereby enhancing the performance of the MFC. The PANI-TiO2-GN immobilized S. o anode MFC generated a high power density of 79.3 mW/m2, which was 1.3 23


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and 2.7 times higher than that of the PANI-TiO2-GN non-immobilized S. o and the plain carbon paper immobilized S. o anode, respectively.

ASSOCIATED CONTENT Supporting Information Synthesis mechanism and characterization of ternary PANI-TiO2-GN nanocomposite. Comparison of the measured and the simulated spectra using the equation corresponding to the equivalent electrical circuits. The bode plots for the real Z and phase show good fitting with the proposed circuit and show relevance to the elements included in the circuit.

AUTHOR INFORMATION Corresponding Author *

Tel.: +82-53-782-8419. Fax: +82-53-810-4631. E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This

study

was

supported

by

Priority

Research

Centers

Program (grant

no:

2014R1A6A1031189) through the National Research Foundation of Korea funded by the Ministry of Education in Korea.

24



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Ternary composite of polyaniline graphene and TiO2 as a bifunctional

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