Sulfonated graphene oxide and titanium dioxide coated with nanostructured polyaniline nanocomposites as an efficient cathode catalyst in microbial fuel cells

Journal Pre-proof Sulfonated graphene oxide and titanium dioxide coated with nanostructured polyaniline nanocomposites as an efficient cathode catalyst in microbial fuel cells

Farhan Papiya, Prasanta Pattanayak, Vikash Kumar, Suparna Das, Patit Paban Kundu PII:

S0928-4931(19)33162-5

DOI:

https://doi.org/10.1016/j.msec.2019.110498

Reference:

MSC 110498

To appear in:

Materials Science & Engineering C

Received date:

24 August 2019

Revised date:

18 November 2019

Accepted date:

26 November 2019

Please cite this article as: F. Papiya, P. Pattanayak, V. Kumar, et al., Sulfonated graphene oxide and titanium dioxide coated with nanostructured polyaniline nanocomposites as an efficient cathode catalyst in microbial fuel cells, Materials Science & Engineering C (2019), https://doi.org/10.1016/j.msec.2019.110498

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© 2019 Published by Elsevier.

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Sulfonated graphene oxide and titanium dioxide coated with nanostructured polyaniline nanocomposites as an efficient cathode catalyst in microbial fuel cells Farhan Papiyaa, Prasanta Pattanayaka, Vikash Kumarb, Suparna Dasc and Patit Paban Kunduad*

Advanced Polymer Laboratory, Department of Polymer Science & Technology, University of

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Department of Civil and Environmental Engineering, Indian Institute of Technology, Patna,

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Calcutta, 92, A. P. C. Road, Kolkata – 700 009, India.

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Department of Chemical Engineering, Indian Institute of Technology, Roorkee– 247667, India.

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Department of Chemistry, Jadavpur University, Kolkata – 700032, India.

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Corresponding author. E-mail address: [email protected], [email protected] (P. P. Kundu).

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Bihar, India

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Journal Pre-proof ABSTRACT: In this study, sulfonated graphene oxide (SGO) was synthesized as potential conducting matrix to improve the properties of catalyst for single chamber microbial fuel cells (SC-MFCs). Here, TiO2 and Polyaniline (PAni) nanoparticles were anchored over SGO and the resulting SGOTiO2-PAni nanocomposites were used as a potential cathode catalyst in MFCs. We have also

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examined the performance of SGO-TiO2-PAni compared to GO-TiO2-PAni and TiO2-PAni catalyst. The structural and morphological analyses were examined using a variety of

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characterization techniques. TiO2 nanoparticles bridged PAni and SGO through hydrogen

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bonding/electrostatic interaction and improved the thermal stability of SGO-TiO2-PAni catalyst.

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The electrochemical characterizations of these nanocatalysts suggest that the SGO-TiO2-PAni

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showed higher reduction current value (-0.46 mA), enhanced stability, and lower internal resistance (46.2 Ω) in comparison to GO-TiO2-PAni and TiO2-PAni towards oxygen reduction

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reactions (ORR). Consequently, MFC using SGO-TiO2-PAni demonstrated a maximum power density of 904.18 mWm-2 than that of GO-TiO2-PAni (734.12 mWm-2), TiO2-PAni (561.5 mWm) and Pt/C (483.5 mWm-2). The enhanced catalytic activity of SGO-TiO2-PAni catalyst was

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ascribed to the high electronic conductivity and long-term permanence of the nanocomposite. These superior electrochemical results suggested that the SGO-TiO2-PAni catalyst could be applied as a potential alternative to the commercial Pt/C cathode catalyst for the application of MFCs.

Keywords: Sulfonated graphene oxide; Nanocomposite; Microbial fuel cells; Power density; Cathode catalyst.

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Journal Pre-proof 1. Introduction Energy crisis has always been a big issue in the new century because of the exhaustion of natural resources, which has spiked interest in the exploration of environment friendly and renewable energy materials and sources. The increasing energy demands inspired us to look for a suitable energy resource. Scientists have keenly looked upon the idea of conversion and storage

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devices by efficient utilization of the available alternative energy sources [1, 2]. Fuel cells are one such device that has been widely studied to acquire energy for modern society. Among the

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various types of fuel cells studied (e.g. H2O2 fuel cell, direct alcohol fuel cell, direct formic acid

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fuel cell etc.,), microbial fuel cells (MFCs) have attracted considerable attention as it is a

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promising green technology that can utilize organic wastewater and simultaneously produce

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bioelectricity [2,3]. As a substrate, domestic or industrial wastewater has widely been used because of their easy availability, rich source of microorganisms that can be used as biocatalysts

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for oxidizing fuel and obtaining energy. The biological processes of these microorganisms also

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help to generate specific nutrients in wastewater, which thereby oxidizes the substrates to produce energy [4,5]. On the other hand, cathode catalysts play an important role in oxygen

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reduction reaction (ORR) that is generally thwarted by a slower reaction kinetics, which hampers the overall performance in MFCs. Until now, platinum (Pt) based catalysts are known as the most effective catalysts towards ORR, although the lowering of performance over time, expensiveness and lower viability hinders its profound application in the system [6-9]. Thus, to reduce the cost of the MFC and to make it commercial, more attention needs to be paid to develop low cost catalyst with high performance to replace Pt/C. Nowadays to develop a potential cheap cathode catalyst, various transition metal oxide based catalysts such as V2O5[10], Carbon supported nickel-phthalocyanine/MnOx[11], CeO2 3

Journal Pre-proof doped Pt/C [12], GO-Zn/Co [13], Spinel-type Cu/Co and Ni/Co- oxides [14], αFe2O3/polyaniline [15], Mo2C/MoO2 [16], Polyaniline/β-MnO2 [17] and δ-MnO2 [18] materials are used as cathode catalysts of MFCs. Among these metal oxides, titanium dioxide (TiO2), an important semiconductor material possesses better electrochemical activity, chemical stability, low toxicity, environmental friendly and abundant availability [19-23].

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Commercially available, low cost carbon-based materials such as carbon blacks, carbon fibre, carbon felt and carbon nanotubes (CNT) have been widely used for MFCs [24-31] due to

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their chemical stability, large surface area and high electrical conductivity. However, these

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materials have some disadvantages such as they demonstrated poor bacterial adhesion and low

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EET efficiency. The graphene oxide exhibits high thermal conductivity, excellent mobility of

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charge carriers, good mechanical stability and a large specific surface area (2630 m2g-1) [32-35]. Moreover, graphene oxide (GO) can easily be functionalized and it is biocompatible. GO sheet is

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composed of two-dimensional single layer sp2 hybridized carbon atoms and amphiphilic material

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with hydrophilic regions that contain carboxylic, hydroxyl and epoxy groups. Therefore, GO is a unique material that incorporates various role as functional materials such as polymers, metal

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nanoparticles and metal-organic frameworks (MOFs) in nanocomposite materials, energy storage, polymer composite materials, catalysis and biomedical applications [36-43]. It has been observed that the polymer nanocomposites based on graphene demonstrate superior thermal, electrical, mechanical, optical and electrochemical properties in comparison to conventional graphite-based composites or neat polymers [36-39]. In recent years, heterogeneous-conducting polymer composites have received worldwide attention. Among the conducting polymers such as polyaniline (PAni), polypyrrole (PPy), polythiophene (PTh), poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-methyl) thiophene (PMeT), polyaniline (PAni) is one of the most widely 4

Journal Pre-proof investigated polymers in practical applications because of the ease of synthesis, low cost, good environmental stability and comparatively good process ability [36-39, 44-48]. Graphene-PAni nanocomposite exhibits superior efficiency as it showed higher conductivity and specific capacitance of 10 Scm-1 and 531 Fg-1 respectively compared to the pure PAni, which exhibited conductivity and specific capacitance of 2 Scm-1 and 216 Fg-1 respectively [36-38, 49]. Also, recent studies have shown that the sulfonation of any organic material is known to increase

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proton transport, which assists in fast electrochemical reaction [39,49,50]. For this reason, in this

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study, to enhance the electrochemical performance, graphene oxide was sulfonated using

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diazotization reaction.

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In the present work, we reported the facile synthesis of polyaniline-coated sulfonated

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graphene oxide doped with TiO2 hybrid nanocomposites, synthesized through a simple in situ oxidative polymerization technique. These synthesized nanocomposites (SGO-TiO2-PAni, GO-

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TiO2-PAni and TiO2-PAni composites) to be employed as the cathode material in MFC

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application were then characterized by FT-IR, XRD, XPS, FE-SEM and EDX. It is observed that, SGO-TiO2-PAni showed the best performance in comparison to GO-TiO2-PAni and TiO2-

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PAni composites in terms of lower internal resistance and higher oxidation current value. From the CV analysis, the SGO-TiO2-PAni nanocomposites exhibited a reduction peak current of -0.46 mA at 0.412 V and excellent cyclic stability up to 50 cycles. Moreover, this catalyst also performed as a superior catalyst, compared to the well-known of Pt/C cathode. 2. EXPERIMENTAL SECTION 2.1. Materials

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Journal Pre-proof Natural flake of graphite powder, sulfuric acid and hydrochloric acid were purchased from Loba chemicals, India. Titanium isopropoxide Ti[OCH(CH3)2]4) was obtained from Sigma Aldrich. Sulfanilic acid, aniline (Ani), ammonium persulfate (APS), methanol, ethanol, sodium nitrate (analytically pure), potassium permanganate (KMnO4), hydrogen peroxide (analytical pure, 30% w/w), 2- isopropanol, sulfanilic acid and sodium hydroxide (NaOH) were obtained from Merck Millipore, India. Catalysts (Pt/C), Nafion-117 membranes and 5 wt% Nafion resin were bought

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from M/S Anabond Synergy Fuel Cell India. All experiments were performed using de-ionized

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(DI) water.

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2.2. Synthesis of graphene oxide (GO)

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The Synthesis of GO was done by modified Hummer’s method as reported earlier [35,46].

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Briefly, graphite powder (2 g) is mixed with 1 g of NaNO3 and 200 mL concentrated H2SO4. To avoid agglomeration, the suspension was ultra-sonicated for 30 min to obtain a homogeneous

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dispersion, which was then kept in an ice bath. KMnO4 (oxidizing agent) was added gradually to

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this mixture and stirring it continuously. Then, DI water (300 ml) was added to the stirred

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mixture and the temperature was maintained at 0°C throughout the reaction. After stirring the mixture for 1 h in the ice bath, the whole set up of the reaction was kept for overnight under continuous stirring condition at room temperature. 100 mL of 30% H2O2 solution was added to terminate the reaction. The obtained product was washed thoroughly with DI water to make pH of the filtrate to 7 and then washed with ethanol. Finally, the obtained brown colored product was dried out in a vacuum oven at 40°C. Lastly, the exfoliated GO sheets were obtained by 1 h ultra-sonication. 2.3. Synthesis of sulfonated graphene oxide (SGO)

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Journal Pre-proof SGO was synthesized by sulfonating GO using sulfanilic acid [49-52]. 2.5 g of sulfanilic acid and 1.0 g of NaNO2 were mixed into 50 mL aqueous solution of NaOH (2 wt%). Then the mixture was heated at 60 °C followed by cooled to room temperature. After that, this solution was mixed in 0.1 M HCl (100 mL) solution and stirred in an ice bath due to the formation of diazonium salt. 100 mL of DI water and isopropanol (1:1) solution containing GO (1 g) was mixed into the solution of diazonium salt and vigorously stirred for 4 h in an ice bath. Then the

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mixture was centrifuged, repeatedly washed with water. Finally, SGO was dried at 50 °C in an

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oven.

Fig. 1. A schematic to represent the synthesis of SGO

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Journal Pre-proof 2.4. Synthesis of Titanium dioxide (TiO2) Titanium dioxide (TiO2) nanoparticles were synthesized by sol-gel method [53-58]. Isopropanol and titanium isopropoxide were mixed into a round bottom flask and stirred for 10 min. Then 1 mol HCl was added drop-wise under stirring condition into the above mixture in the pH range of 1. The gel was prepared when both solutions were mixed together under vigorous stirring condition. After that, the gel was repeatedly washed with ethanol and dried in vacuum oven at

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100 °C for 3 h. Finally, the obtained yellowish TiO2 powder was calcined at the temperature

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ranging from 200 to 600 oC for 5 h.

2.5. Methodology for the synthesis of SGO-TiO2-PAni, GO-TiO2-PAni, TiO2-PAni

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nanocomposite

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SGO-TiO2-PAni hybrid was synthesized by a multi step process. PAni was synthesized by in-situ

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polymerization method, coated on the synthesized SGO-TiO2 nanocomposite. In the first step, SGO (0.5 g) and TiO2 (0.5 g) were uniformly dispersed in the solution of water and isopropanol

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by using ultrasonic treatment for 30 min. In the second step, SGO-TiO2 dispersion was placed in

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a round bottom flask, to which aniline (1.0 ml) was added, and the dispersion was stirred in an ice bath. Ammonium per sulfate (APS) dissolved in 1 M HCl solution was then added drop-wise to initiate the polymerization of aniline on the SGO-TiO2 solution. The reaction mixture rapidly turned to greenish black slurry. This greenish black precipitate was filtered after 4 h. The precipitate was then properly washed with DI water, acetone and methanol and finally dried in vacuum oven at 50 °C. Similarly, GO-TiO2-PAni was prepared by exactly following the above mentioned procedure in the presence of GO and TiO2-Pani was synthesized in absence of GO. 8

Journal Pre-proof 2.6. Fabrication of membrane electrode assemblies (MEAs) Pre-treatment of the carbon cloths and membranes were carried out by following the procedures of our earlier reports [35,39,46]. To obtain the fine catalyst ink, synthesized catalyst material was first stirred for 2h in the solution of DI water, 2-propanol (1:1 ratio) and 5 wt% Nafion solution and then ultrasonicated for half an hour until a fine ink slurry of catalyst was formed. Then, the catalyst ink was loaded on the surface of the carbon cloth. Loading of the catalyst on the

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electrode surface (diameter = 4 cm) was maintained to 3 mgcm-2. MEAs were fabricated by

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applying hot press procedure reported in our earlier publications [35, 39].

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2.7. Inoculum preparation and construction of single chambered-microbial fuel cell (SC-MFC)

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The method used for the inoculums preparation is followed from the previous methods [35, 59].

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MFC tests were performed in an autoclaved single chambered plexiglass cell (air-cathode, inner volume = 150 mL). After that the chambers were assembled with the prepared MEA, where the

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catalyst painted electrode (cathode) was kept at the air facing side for ORR. The chamber was

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filled with the microbe enriched inoculums, acting as an electron contributor. The different

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catalyst loaded SC-MFC reactors connected with digital multimeter were set up for comparing the performance of the catalysts. All the MFC tests were operated at room temperature. A schematic diagram of SC-MFC is shown in Figure 2.

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Fig. 2. Schematic diagram of single chambered MFC. 3. Characterization of the catalyst 3.1. Instrumentation The synthesized materials were characterized by FT-IR spectroscopy (Bruker;Model: Alpha E) FT-IR spectrophotometer). XRD spectroscopy of the samples were conducted using CuKα radiation (λ = 1.5406 A) operating at 30 mA, 40 kV and at an angle of 2θ (within the range of 10° to100°) at a fixed scan rate of 1° min-1 by using a Goniometer instrument. The samples were characterized by X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) 10

Journal Pre-proof by using mono chromatic α-radiation of 1253.6 eV [Omicron ESCA Probe, Taunusstein, Germany) and NETZSCH TG 209F1 Libra instrument respectively. Raman spectrum was analyzed by using a Renishaw 1000B Raman diffractometer with a laser excitation of 633 nm. Size and morphology of the synthesized materials were examined by field emission scanning electron microscopy (FE-SEM, HITACHI-S4800) and transmission electron microscopy (TEM,

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Tecnai G2 F20, FEI, USA) combined with energy dispersive spectroscopy (EDS) analysis.

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3.2. Electrochemical measurements

The electrochemical characterizations of the electrocatalysts were carried out at room

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temperature in a standard three-electrode electrochemical cell (Gamry Bob’s cell) associated

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with Gamry Potentiostat-600 instrument (Warminster, PA, USA). At first, 5 mg of synthesized

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materials were ultrasonically dispersed in a mixture of 6 mL of deionized water, 1 mL of isopropanol and 5 μL of 5% Nafion® solution for 30 min. Consequently, we obtained a well-

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dispersed catalyst ink slurry. 10 μL of the catalyst ink slurry was carefully loaded on a glassy

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carbon of the working electrode. For electrochemical experiments, catalyst ink coated glassy

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carbon electrode served as the working electrode, Pt wire as counter and Ag/ AgCl as a reference electrode. Neutral PBS solution (0.1 M) was used as electrolyte, where working electrode potentials were scanned in different modes. Cyclic voltammetric (CV) curves of the synthesized nanocomposites were swept between -0.15 V to +2.15 V at different scan rates (from 5 mVs-1 to 100 mVs-1). Similarly, linear sweep voltammetry (LSV) were conducted at a scan rate of 100 mVs-1 from -0.15 V to +2.15 V. Electrochemical impedance spectroscopy (EIS) was performed by employing a voltage of 5 mV at frequencies ranging from 1 Hz to 105 Hz.

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Journal Pre-proof Further, chronoamperometry (CA) measurements were conducted to analyze the activity of the catalysts under quasi-steady state conditions. The CA experiments were performed at room temperature at +0.35V for 17000 s in a nitrogen-saturated 0.1 M PBS electrolyte solution. The electrolyte was deoxygenated by purging the system with nitrogen before the experiment. 3.3. Analysis and calculation

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Current (I) and potential (V) of all cells were regularly monitored in 1day gap, by utilizing a

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multimeter (Keithley Instruments, Cleveland, OH, USA), and a potentiostat (G600; Gamry

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Instrument Inc., Warminster, PA, USA) linked with a PC. The MFCs ran for 30 days. After stabilization of the circuit, current (I) and potential (V) were measured. Power densities (P) were

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measured by using the following equation, P = IV/A

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Where, A represents the surface area of the projected electrode. The polarization curves obtained

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4.1. FTIR spectroscopy

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4. Result and Discussion:

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for all of the MFCs were plotted by applying the different external resistance.

FT-IR spectroscopy of GO, SGO, SGO-TiO2 and SGO–TiO2-PAni were presented in Fig. 3. FTIR of GO shows the characteristic absorption bands for the stretching vibration of hydroxyl groups (3150 cm-1), carbonyl groups C=O (1722 cm-1), carboxyl groups C–O (1553.2 cm-1), epoxy bond C–O (1225.3 cm-1) and alkoxy bond C–O (1053 cm-1) of graphene oxide [35,46, 57, 60]. The FT–IR spectra for both of GO and SGO show two similar peaks at 1727 cm-1 for C=O stretching and at 3151 cm-1 for the hydrogen bonded O-H groups, respectively. In addition, SGO exhibits two characteristic peaks at 1110 and 1243.5 cm-1 that could be ascribed to the symmetric

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Journal Pre-proof and asymmetric stretching of the –SO3H group, respectively. The C–C aromatic bond is verified by the peak at 1415 cm-1 [51]. The major peaks found in the FT–IR spectrum of SGO-TiO2 are at 3190, 1631, 1484, 1120, and 690 cm-1, where the first two peaks confirms the presence of SGO and the last peak appears due to Ti–O–Ti. The vibration peaks at 1110 cm−1 in SGO and 1120 cm-1 in SGO-TiO2 confirmed the presence of the –SO3H groups [51]. FT-IR spectra of PAniSGO-TiO2 nanocomposite show characteristic changes because of the interactions of PAni with

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SGO and TiO2. Furthermore, the C=O stretching vibration of -COOH group is shifted to 1631

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cm-1 due to the amide bond formation in the hybrid nanocomposite. All of the characteristic

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peaks of PAni appear in the ternary nanocomposite, confirming that PAni is decorated on the

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SGO surface through the amide linkage; the absorption bands at 1525 cm-1 (C=C) and 1225 cm-1 (epoxy C–O) indeed suggest the presence of SGO in the nanocomposites [39,49,51,57,61]. The

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phenomenon indicated that the PAni-SGO-TiO2 nanocomposite were well synthesized.

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Journal Pre-proof Fig. 3. FT–IR spectra of GO, SGO, SGO–TiO2 and TiO2–PAni–SGO. 4.2. XRD spectra analysis To analyze the crystallographic material of pure GO, SGO, PAni, SGO-PAni, TiO2 and SGOTiO2-PAni, the X-ray diffraction patterns were represented in supporting information Fig. S1. In the XRD spectra of GO, the sharp peak at the 2θ value of 10.5° represents (001) plane

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[35,46,60,61]. After the sulfonation of GO, the intense peak at 2θ = 10.3˚ disappears along with

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the appearance of the new broad peaks at 13o, 26o and 43o, suggesting that a certain amount of GO is reduced and exfoliated during the covalent grafting [62,63]. The pristine Pani shows a

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peak approximately at 2θ = 25°, which is the characteristic peak of PAni polymer [39,49,61].

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Furthermore, the diffraction peaks of SGO-PAni had broad peaks and appeared at

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2θ ≈ 12° and 25° exactly at the same position as PAni; however, these peaks

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were found to have comparatively lower intensity than PAni due to the good attachment with SGO [63]. XRD of TiO2 showed very tiny peaks at 25.5°, 38°, 48°, 54°, 63° and 75°

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indicating the presence of anatase TiO2 [54,56,58,64]. The respective characteristic peaks were

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also observed in SGO-TiO2-PAni nanocomposite. The above observations confirmed that the PAni was coated successfully on SGO-TiO2. 4.3. XPS analysis: Structural characterization of samples XPS measurements were used to study the surface elemental compositions and chemical states of SGO-TiO2-PAni nanocomposites. As presented in Fig. 4a, the full survey spectrum (0 to 1200 eV) of SGO-TiO2-PAni nanocomposites contained the characterization peaks of S2s, S2p, C1s, N1s, Ti 2p, O1s, signifying the presence of these elements on the SGO-TiO2-PAni nanocomposites. The peaks at 226.7, 161.3 and 162.7 eV in the Fig 4b are ascribed to the 14

Journal Pre-proof binding energies of S2s, S2p3/2 and S2p1/2 electrons, respectively [65-67]. Fig. 4c shows the C1s peaks appeared at 283.9, 284.6, 285.8, 286.3and 288.4 eV, representing the C=C, C-C/C-H, C-OH, C-O-C, and O-C=O stretching, respectively [35,53,57,60,61,62]. The peak at 283.9 eV is appeared due to the presence of C-S bond. This result clearly confirms the successful incorporation of the sulfonic acid groups on the surface of GO [65,66]. Additionally, the peaks of N1s in Fig. 4d is found at the binding energy of ~399.3eV, ascribed to the imine nitrogen atom

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[61,68]. Two wide peaks at the binding energies of 458.8 and 465.5 eV appeared in Fig. 4e due

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to Ti 2p3/2 and Ti 2p1/2 respectively; this confirms the characteristic presence of Ti4+ in TiO2

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[53,65,68]. The peak at 532.1eV in O1s region (Fig. 4f) is attributed to the Ti-O-C bond, which

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indicates the close interfacial contact between SGO layers and TiO2 NPs. This result has also been confirmed by the results of SEM and TEM images, which are discussed in the later part of

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this manuscript. We have found that the binding energy of O1s in SGO-TiO2-PAni is shifted

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from 531.30 to 531.79 eV, indicating strong interactions between the oxygen of TiO2 and the imine nitrogen [65,68]. So, these data prove the successful synthesis of SGO-TiO2-PAni ternary

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the ternary components.

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nanocomposites, as well as provide an indication towards the intense interfacial connection of

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Fig. 4. (A) Full survey scan and high-resolution spectra of XPS analysis of SGO-TiO2-PAni

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nanocatalyst (b) S 2p region; (c) C 1s region; (d) N 1s region, (e) Ti 2p region and (f) O 1s region.

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4.4. Morphology of nanocomposites 4.4.1. FE-SEM analysis

The morphology of the synthesized composite materials was characterized through FE-SEM. It is clear from Fig 5a that the GO displays a crumpled, very thin and layered structure. As exhibited in Fig. 5b, the SGO are randomly oriented, relatively crumpled, closely associated with each other, forming a disordered solid and this structure is consistent with those reported in the literatures [51,69,70]. Therefore, different pattern of GO and SGO images indicates successful

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Journal Pre-proof anchoring of sulfonic acid on graphene oxide sheet. As shown in Fig. 5c, PAni exhibits nanorodlike structure along with fewer spheres. This result indicates that the PAni first forms spheres, which are then joined to nano sized rods during the polymerization process [69-71]. When TiO2 embedded with SGO, the layer structure is still observed in Fig. 5e along with the increased thickness of the sample. Furthermore, this morphology in PAni-TiO2 suggests the strong coupling effect between PAni and TiO2 as reported in Fig.5f. The SEM image of GO-TiO2-PAni

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composite displays a lower number of nano rods of PAni along with the higher number of

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spheres as reported in Fig. 5h [51,71]. Moreover, as observed in the FESEM images of Fig. 5g

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and 5h, the TiO2 nanoparticles on the surface of SGO-TiO2-PAni (Fig.5h) are more

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homogeneously dispersed compared to the GO-TiO2-PAni nanocomposite (Fig.5g). As shown in Fig. 5i, EDAX spectrum confirmed the presence of carbon, oxygen, sulfur, nitrogen and titanium

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elements in SGO-TiO2-PAni nanocomposite (Fig. 5i).

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Journal Pre-proof Fig. 5. FE-SEM images of (a) GO, (b) SGO, (c) PAni, (d) TiO2, (e) SGO-TiO2, (f) TiO2-PAni, (g) GO-TiO2-PAni, (h) SGO-TiO2-PAni and (i) EDAX spectrum of SGO-TiO2-PAni nanocomposite. 4.4.2. TEM Analysis In Fig. 6, the TEM analysis was performed to further characterize the surface study and the

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internal morphology of GO, SGO, TiO2-SGO and SGO-TiO2-PAni. From the TEM, it is found

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that the GO sheets are more aligned than the those of GO–SO3H (SGO) which are randomly

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oriented and crumpled (Fig. 6a,6b). This observation is consistent with the SEM analysis. Therefore, we can conclude that the GO displays a multilayer structure. However, on the

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sulfonation of GO, it forms a disordered solid due to the closely association of crumpled

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structures of GO [72]. The dissimilarity in the structure of SGO and GO suggests that the sulfonic acid is well anchored to GO. In Figure 6c, the TEM images of TiO2-SGO

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nanocomposite clearly indicates that GO sheets are also well coordinated with the TiO2

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nanoparticles [53,68]. The lowly magnified TEM images in Fig. 6d-e, representing the structure

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of the SGO-TiO2-PAni ternary nanocomposites, is significantly different from that of TiO2-SGO. This is likely to be due to coating of the surface and interspaces of the TiO2 and SGO by PAni (53,61,62,68). The high-resolution TEM (HRTEM) images in Fig. 6f-h exhibit that the surface of TiO2-SGO is covered by the transparent PAni nanosheets. This image also suggests that the PAni and TiO2-SGO are in close interfacial contact, which is in accordance with the SEM results. Size distribution histogram analysis of synthesized nanocomposite also demonstrates the uniform dispersion of nanoparticles onto the supporting matrix (Fig. 6i). This experiment suggests that the mean size of TiO2 nanoparticles is 11 nm.

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Fig. 6. TEM images of (a) GO, (b) SGO, (c) SGO-TiO2, (d) GO-TiO2-PAni and (e,f,g,h) SGO-

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TiO2-PAni, (i) histogram image of TiO2-SGO-PAni.

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4.5. Raman spectroscopy

Raman spectra of all the studied materials, i.e. GO, SGO, GO-TiO2-PAni and SGO-TiO2-PAni are shown in supporting information Fig.S2. A G-band at 1580 cm-1 is observed due to the Raman-active vibration of E2g symmetry in graphite. Whereas, a D-band at 1350 cm-1 is attributed to the disordered layers of the graphene [35,52,60]. These results confirm the existence of the aromatic carbon sheets in the synthesized materials. The two distinct bands at ~1340 and ~1590 cm-1 are ascribed to the D-band and G-band of GO, respectively. The intensity ratio of D/G (ID/IG) indicates the disorder, structural defects and defect density of the synthesized materials. The intensity ratio, I(D)/I(G) for GO and SGO was found to be 0.9015 and 1.03, 19

Journal Pre-proof respectively, indicating surface functionalization of the former by sulfonic acid group and the presence of more defects in SGO [52,60]. However, the ID/IG value for SGO-TiO2-PAni nanocomposite is found to increase this value to 1.04, demonstrating the presence of the highest defects and disorders of the SGO carbon material in the nanocomposites compared to GO-TiO2PAni (ID/IG=0.9936) [67,71,73].

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4.6. Thermogravimetric analysis

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Fig. S3 shows the thermograms of GO, SGO, TiO2, PAni, PAni-TiO2, PAni-SGO and SGO-

-p

TiO2-PAni nanocomposite from 50 to 600 oC in air atmosphere. All the samples showed distinct steps of thermal degradation. The first weight loss between 50 to 150 ºC was mainly due to the

re

loss of water molecules and residual solvent within the materials [62]. Obviously, the

lP

thermogram of GO and PAni were reliable with the other literatures [61,68]. In case of SGO, the first step observed between 50 to 150 ºC was attributed to the loss of absorbed water and residual

na

solvent. The second step, at 250-350 ºC, is ascribed to the decomposition of the –SO3H groups.

ur

The last step appeared at 450-500 ºC is attributed to the oxidation of carbon. For the PAni-SGO

Jo

composite, the second step, at 250-350 ºC, was ascribed to the decomposition of the sulfonic acid and the dopant molecules adsorbed on the polyaniline, and to the degradation of oligomers [68]. Thermogram of TiO2 nanoparticles showed very slight weight loss over the entire temperature range, proving to be a thermal stable material. Furthermore, SGO-TiO2-PAni nanocomposite exhibited more resistance towards thermal degradation; this might be due to the strong interactions between SGO-TiO2 and PAni surface. The incorporated TiO2 restrained the decomposition of SGO-TiO2-PAni chain as confirmed by their higher onset degradation temperature.

20

Journal Pre-proof 4.7. Electrochemical performance: Electrochemical properties of nanocomposite 4.7.1. Cyclic Voltammetry (CV) analysis The performances of the TiO2-PAni nanocomposite as a catalyst for the ORR were investigated through different electrochemical techniques. As we know, the electrochemical activity of catalytic materials depends on their structures and surface composition. We have conducted the

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cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurement in phosphate buffer

ro

solution of the synthesized catalyst materials to evaluate their electrocatalytic activity [39, 60,74-

-p

76]. As shown in Fig. 7a, TiO2-PAni nanocomposite showed significant redox peaks, although lower peak currents were observed probably due to its limited conductivity. Furthermore, the

re

electrocatalytic performance of graphene oxide were improved by sulfonated GO and it was

lP

observed that the SGO supported TiO2- PAni nanocomposites exhibits increased peak currents. However, a similar trend was observed for the SGO-TiO2-PAni composite along with the higher

na

redox peak currents. At the potential of 0.412 V, SGO-TiO2-PAni nanocatalyst exhibited a peak

ur

current of -0.46 mA, which is 1.24 times higher compared to that of the catalyst with GO-TiO2-

Jo

PAni nanocomposite (-0.37 mA) and 1.5 times higher compared to that of the catalyst with TiO2PAni (-0.32 mA). It is reported that the increase in peak current is indicative of enhancement of electrochemical activity [77-81]. Noticeably, this order of significant peak current was: SGOTiO2-PAni (-0.46 mA) > Pt/C (-0.42 mA) GO-TiO2-PAni (-0.37 mA) > TiO2-PAni (-0.32 mA), indicating the highest electrochemical activity of SGO-TiO2-PAni. The maximum peak current of SGO-TiO2-PAni nanocomposite unambiguously shows that it has improved electrical conductivity, large catalyst surface area, and higher number of conduction channels and enhanced surface area. Again, the oxygen reduction peak potential of the electrode with SGOTiO2-PAni nanocomposite is observed at 0.412 V, which is more positive than that obtained with 21

Journal Pre-proof GO-TiO2-PAni catalyst (0.40 V) and Pt/C catalyst (0.403 V). Interestingly, the oxygen reduction current obtained for SGO-TiO2-PAni catalyst is found to be higher in comparison to the Pt/C catalyst. The higher peak current value and more positive peak potential value for the nanocomposite with SGO-TiO2-PAni catalyst confirm its potential use towards ORR. Additionally, Fig. 7b demonstrates the CV of SGO-TiO2-PAni nanocatalyst at different

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scan rates (5, 10, 15, 30, 50 and 100 mVs-1) and it is observed that the cathodic peak shifts toward left direction (less positive potentials) with the increasing scan rate while the anodic peak

ro

shifts toward right direction (more positive potentials) [39,82,83]. Moreover, it is also observed

-p

that, the peak current value increases linearly with the increasing scan rates from 5 mVs-1 to 100

re

mVs-1. The reduction current peak is found to be -0.24 mA at the scan rate of 5 mVs-1, which is

lP

then increased to -0.41 mA at the scan rate of 100 mVs-1. Furthermore, to test the stability of the SGO-TiO2-PAni nanocomposite, 50 cycles of CV

na

has been performed and presented in fig 7c. It is observed that the TiO2-SGO-PAni

ur

nanocomposite exhibited outstanding stability, confirming SGO-TiO2-PAni as a potential

Jo

candidate towards ORR in neutral media [39]. 4.7.2. Linear Sweep Voltammetry (LSV) analysis To further explore the activity of catalyst, LSV of all catalysts was carried out within the potential range from 0.02 V to 1.2V. As can be seen in Fig. 7d, the onset potentials of SGOTiO2-PAni represented a maximum reduction current value of -0.48 mA, which was the highest reduction current value in the non-platinum cathode catalyst. The TiO2-PAni, achieved the minimum reduction current value of -0.31mA, followed by GO-TiO2-PAni (-0.40 mA) to SGOTiO2-PAni (-0.45 mA). Furthermore, SGO-TiO2-PAni nanocomposites were significantly shifted 22

Journal Pre-proof to more positive onset potential as compared to other TiO2 based catalyst and Pt/C catalyst [35, 46,49,50,84-86]. These results were in accordance with the results of CV, elucidating that the SGO-TiO2-PAni catalyst exhibited the highest reduction peak current with the highest positive peak potential. Moreover, the –SO3H group of SGO interacted with the PAni-TiO2 nanoparticles by hydrogen bond formation, resulting better dispersion, distribution and utilization of the SGOTiO2-PAni nanocomposite catalyst. Therefore, an enhancement of the electrocatalytic activity is

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observed for the SGO-TiO2-PAni nanocomposite. The reduction peak current value of the

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nanocomposites can be arranged as follows, i.e. TiO2-PAni (-0.31 mA)< GO-TiO2-PAni (-0.40

-p

mA)
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0.425 V and 0.43 V, respectively. Therefore, it can be concluded that SGO-TiO2-PAni

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ur

na

lP

nanocomposite possessed superior electrocatalytic activity.

23

Journal Pre-proof Fig. 7. (a) Cyclic Voltammetry (CV) and (d) Linear Sweep Voltammetry (LSV) of synthesized nanocomposites materials (b) Different scan rate CV analysis and (c) 50 cycle CV analysis of SGO-TiO2-PAni nanocatalyst. 4.7.3. Chronoamperometric performance The stability of all the synthesized catalysts, TiO2-PAni, GO-TiO2-PAni and SGO-TiO2-PAni

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was performed by chronoamperometry in PBS at a fixed potential +0.35V (vs Ag/AgCl) for

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17000 sec (Fig. 8a). A rapid decay of current was observed initially for all the studied

-p

nanocomposites due to the charging current. The initial current of 0.042 mA (at zero second) for Pt/C (10 wt% Pt) catalyst was dropped to 0.008 mA after 17000 sec due to the dissolution of the

re

catalyst system in neutral PBS solution [35,39,60,82,87-88]. On the other hand, GO-TiO2-PAni

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and TiO2-PAni exhibited comparatively lower initial current of 0.021 mA and 0.01 mA along with the limiting current values of 0.005 mA and 0.003 mA respectively. The higher retention

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current obtained for these two catalysts were due to the presence of minimal structural defects in

ur

PAni and its longer conjugation path length. Furthermore, SGO-TiO2-PAni exhibited highest

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initial current (0.064 mA), highest limiting current (0.015 mA) and higher current retention after 17000 s in comparison to the other studied catalysts, indicating its superior stability. The presence of sulfonic acid groups in the SGO matrix was probably responsible in avoiding the agglomeration and dissolution of the nanoparticles in the solution. Therefore, uniformly dispersed TiO2-PAni nanoparticles on SGO support matrix assisted in achieving improved stability in SGO-TiO2-PAni catalyst material on repeated cycling. Consequently, the tested SGOTiO2-PAni nanocatalyst can be a potential cathode catalyst in MFC usage. 4.7.4. Electrochemical impedance spectroscopy (EIS)

24

Journal Pre-proof Table 1. Electrochemical impedance spectroscopy (EIS) parameters of different hybrid catalyst materials. Pt/C

TiO2-PAni

GO-TiO2-PAni

SGO-TiO2-PAni

Rs (Ω)

2.64

2.13

2.03

1.88

Rct (Ω)

78.58

54.3

53.1

46.2

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EIS parameters

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EIS study was carried out to evaluate the charge transport characteristics of synthesized

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electrocatalyst at electrode/electrolyte interface. Fig. 8b represents the nyquist plots of all electrocatalyst with distinct semicircles followed by a straight line in neutral PBS. The inset

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figure in Fig. 8b showed a Randles type equivalent circuit including solution resistance (Rs),

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charge-transfer resistance (Rct), a Warburg diffusion resistance (Zw), and a constant phase

na

element (CPE). The obtained EIS parameters (Rct and Rs) are presented in Table 1, connected with the characterization of ORR [35,46,82,86,89-91]. The Rct of the catalysts is represented by

ur

the diameter of the semicircle in the nyquist plot and it is directly related to the interfacial

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interaction between the reactant or electrolyte and catalyst. Furthermore, the EIS spectra of Pt/C, TiO2-PAni, GO-TiO2-PAni and SGO-TiO2-PAni have been plotted in Figure 8b to evaluate their activities. It seems that, the SGO-TiO2-PAni nanocomposite shows the Rct value of 46.2 Ω, which is significantly lower compared to GO-TiO2-PAni (53.1), TiO2-PAni (54.3) and Pt/C (78.58 Ω). Moreover, the Rct value obtained for the studied nanocomposites fall in the order of SGO-TiO2-PAni (46.2)
Journal Pre-proof nanocomposite. Furthermore, the smaller Rct of SGO-TiO2-PAni nanocomposite indicates the faster reaction rate kinetics along with a lower electron-transfer resistance compared to the other studied catalysts. This faster reaction rate kinetics is responsible for the increases in the ORR rate due to decrease in ORR overpotential in addition to the highest reduction current found from the SGO-TiO2-PAni electrocatalyst. Moreover, the superior electrochemical activity of the SGOTiO2-PAni is also due to the presence of sulfonic acid functional groups, which assist in

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facilitation of the electron transfer reaction mechanism. This result is attributed to the better

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connection between the TiO2-PAni nanoparticles and SGO. As a result, SGO-TiO2-PAni

-p

exhibited higher conductivity as well as lower Rct value. Therefore, the CV, LSV,

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chronoamperommetry and EIS results demonstrate that the SGO-TiO2-PAni nanocomposite has the highest electrocatalytic activity, highest stability and lowest resistance towards ORR, among

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ur

na

lP

all the synthesized catalysts.

Fig.8. (a) Chronoamperometric curve and (b) Nyquist plots of different catalyst systems in phosphate buffer solution (PBS).

26

Journal Pre-proof 4.8. Performance analysis of MFCs with different cathode catalysts Table 2: MFC Performance based on different catalyst system Potential

Maximum

Maximum

(mV)

Current Density

power density

(mA/m2)

(mW/m2) 561.5±20

569

1633.17

Pt/C

305

1788.2

GO-TiO2-PAni

636

2031.9

734.12±21

SGO-TiO2-PAni

719

904.18±25

ro

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TiO2-PAni

-p

Catalysts

lP

re

2107.64

483.5 ±16

na

To evaluate the performance of the synthesized cathode catalysts, the cell voltage and power density curves of SGO-TiO2-PAni, GO-TiO2-PAni and TiO2-PAni were examined at variable

ur

external resistance in three batch cycles test under the same operating conditions; also the Pt/C

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(10 wt%) was used for comparison (Fig. 9a and Table 2). These MFCs were monitored in batch wise for about 1 month with synthetic wastewater (i.e. synthesized mixed microbial culture) as biocatalysts at anode, to compare the performance of the prepared cathode catalyst in the system. As can be seen from Table 2 and Fig. 9a, the SGO-TiO2-PAni represented a maximum power density (MPD) of 904.18mWm-2 (at a cell potential of 473 mV), which was the maximum power output than that of GO-TiO2-PAni (734.12mWm-2) and TiO2-PAni (561.5mWm-2), and even higher than that of Pt/C cathode (483.5 mWm-2). The highest power performance of MFC-SGOTiO2-PAni could be attributed to the lower charge transfer resistance of 46.2Ω than the other MFCs, which suggested the SGO-TiO2-PAni was appropriate for applying in MFC cathode. 27

Journal Pre-proof Moreover, the polyaniline-coated sulfonated graphene oxide doped with TiO2 hybrid nanocatalysts showed an excellent electrochemical performance, and its power density was 1.23, 1.6 and 1.88 times higher than that of MFCs catalyzed by GO-TiO2-PAni, TiO2-PAni and Pt/C respectively under the same operating condition [35,39,46,74]. Additionally, at different resistances, the curves of individual anode and cathode electrode potentials versus current

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densities were showed in supporting information Fig. S4. Furthermore, as can be seen from Fig. 9b, the maximum open circuit voltage (OCV) of the MFC

ro

reached 0.719 V for SGO-TiO2-PAni, 0.636 V for GO-TiO2-PAni and 0.569V for TiO2-PAni

-p

after three batch cycles of operation. Distinctions in OCVs were ascribed to the different

re

nanocomposite materials used in the method. Moreover, the higher OCV value is related to a

lP

higher reaction rate. Noticeably, the OCV value of SGO-TiO2-PAni had a slowly decreasing trend, which indicating inferior stability of the SGO in comparison to the other catalysts. The

na

outstanding structure of SGO-TiO2-PAni nanocomposite synthesized by the method reported in

ur

this article plays a significant role towards the enhanced catalytic activity [92,93]. The smaller particle size and uniform dispersion of TiO2 on the SGO support matrix could be responsible for

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the higher surface area of the catalyst. Again, PAni plays an important role for the better interaction to the TiO2 and SGO. As a result, the SGO-TiO2-PAni nanocomposite achieved the highest conductivity and stability along with the lowest resistance. The highest conductivity of SGO-TiO2-PAni nanocomposite facilitates better electron transfer, resulting in significant improvement towards the cathode performance. These results demonstrate that SGO-TiO2-PAni composite can be a good alternative cathode catalyst compared to Pt in practical MFC applications.

28

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Journal Pre-proof

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Fig. 9. (a) Polarization curve and (b) Open circuit voltage (OCV) of different catalyst system.

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4.9: MFC performance comparison with other literature Here, the obtained results are compared with recently developed non-PGM catalysts used in

na

MFCs (Table 3). As referenced, Li et al showed low-cost biochar derived from corncob as

ur

cathode catalyst that reached a MPD of 458.85 mW m-3 in MFC application [74]. Notably, some works reported that rose flower-like nitrogen-doped NiCo2O4/carbon (N-NiCo2O4/C)

Jo

nanocomposite exhibited slightly lower power density than the Pt/C electrode [76]. In another instance, Mahalingam et al. demonstrated reduced graphene oxide-V2O5 nanocomposite as aircathode that generated 533 mW m-2 MPD [10]. Besides, Tian et al., 2018 has reported that nitrogen-doped activated carbon (AC) exhibited the 2-times higher power density than the Pt/C catalyst [26]. In addition, Yang et al. demonstrated that GO-Zn/Co nanocomposite as the cathode catalyst generated the power density of 773 mW m-2, which was slightly higher than the Pt/C catalyst (744 mW m-2) [13]. Some scholars studied the Polyaniline (PANI)/reduced graphene oxide (rGO) decorated on mesophase pitch-based carbon fiber brush (Pitch-CB) with a 29

Journal Pre-proof remarkable improvement in power density (862 mW m-2) compared to the PANI-Pitch-CB and Pitch-CB pure graphene [38]. Besides, some scholars also studied the double-loaded reduced graphene oxide decorated V2O5 with a remarkable improvement in output performance compared to the single-loaded and triple loaded reduced graphene oxide-V2O5 nanocomposite [41]. The MFCs using the nanoflower-shaped graphene oxide hybridized MgO (GO/MgO) nanocomposite prepared by Li et al. showed the MPD of 755.63 mW m-2, which was 86% of that

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estimated for the Pt/C MFCs (870.75 mW m-2) [33]. In addition, for spinel cobalt based oxides

ro

doped with copper and nickel as the cathode ORR catalyst, the catalyst reached MPD of 567.58

-p

mW/m2 (Ortiz-Martínez et al., 2016) [14]. Consequently, 3 wt% nanophase CeO2 homogeneously deposited on the Pt/C (3 wt% CeO2 doped Pt/C) was also reported as the

re

potential ORR catalyst at cathode; it showed enhanced MPD of 840 mW m-2 (Li et al., 2016)

lP

[12]. Moreover, in the current study, the non- precious metal catalysts (SGO-TiO2-PAni)

na

employed as cathode catalyst exhibited an enhance performance in MFCs. This is possibly due to high conductivity and stability along with the lower internal resistance, thereby enhancing the

ur

ORR kinetics. However, MFCs performance was difficult to compare directly with other

Jo

literatures. Significant differences were observed probably due to the different operational strategy process, the anode type, anodic bacterial respiration, cell configuration, anode performance, substrate types, microbes, ion intensity and concentration, etc., rather than only cathode catalyst. Therefore, for better comparison, similar MFCs reactors were assembled by using Pt/C as cathode catalysts under identical operation condition. Consequently, in evaluation, SGO-TiO2-PAni composite outperformed most of compounds studied in this work, producing 1.88 times higher power output compared to Pt-based cathodes. Considering the commercial prices of the developed cathode catalyst used in this work, the final cost of the SGO-TiO2-PAni

30

Journal Pre-proof composite came out as ~0.0497 $/cm2. This, in turn, promises a cheaper and better alternative over the widely used Pt/C, which costs quite higher as ~2.75 $/cm2 in general usage. On average, cost of SGO-TiO2-PAni is approximately 55.3 times lower than that of the commercial Pt/C. This, in effect, makes the SGO-TiO2-PAni a superior cathode catalyst in MFC with higher electrocatalytic activity and better viability.

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Table 3. Comparisons analysis of the SGO-TiO2-PAni cathode catalyst activity of MFCs

P Max (mW m2 )

Ref.

Fish market waste water

533 (512)

[10]

Sodium acetate

840 (617)

[12]

Carbon cloth

Bacterial solutions

773 (20 wt % Pt 744)

[13]

graphite granules along with a bar graphite

Industrial wastewater

567.58 (647.95)

[14]

Cathode electrode material

Anode electrode material

V2O5/rGO

SS wire mesh

SS wire mesh

3 wt% CeO2 doped Pt/C

Carbon cloth

GO-Zn/Co nanocomposites

Carbon cloth

Spinel-type Cu/Co and Ni/Co- oxides

Carbon cloth

Substrate

lP

re

-p

Catalyst

na

carbon brush

ur

Jo

ro

with other literature values.

M 500 δ-MnO2

Carbon cloth

Carbon cloth

Sodium acetate

213 (483)

[18]

Nitrogen-doped activated carbon

Carbon cloth

Carbon cloth

Sewage from treatment plant

1042 (482)

[26]

GO/MgO

Carbon cloth

Carbon felt

Sodium acetate

755.63 (870.75)

[33]

PANI/rGO

Carbon cloth

Mesophase pitch-based CFs

Wastewater treatment plant

862 (-)

[38]

31

Journal Pre-proof rGO-V2O5 with the 30 wt. % wetdouble-loaded proofed carbon nanocomposite cloth

graphite brush

Sodium acetate

1668 (2004)

[41]

Biochar CC-650

Carbon cloth

Carbon felt

Sodium acetate

458.85 mW m-3 (-)

[74]

N-NiCo2O4/C

Carbon cloth

Carbon felt

Sodium acetate

1249.86 (1322.71) mW m-3

[76]

NG/Co-N

Carbon cloth

Bacterial culture medium

713.6 (571.3) Pt/C catalyst (JM 40%)

[78]

SGO-TiO2-PAni

Carbon cloth

904.18 (483.48)

[This study]

Mixed bacterial culture

lP

re

-p

Carbon cloth

ro

of

graphite felts

5. CONCLUSIONS

na

( ): Values found in MFCs with Pt/C cathode as a control

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In summary, TiO2 embedded PAni grafted on sulfonated graphene oxide materials (SGO-TiO2-

Jo

PAni) have been synthesized by a simple chemical reduction technique, where, it has been used as catalyst for cathodic ORR in SC-MFC. Comparatively, other modified catalysts namely TiO2PAni, GO-TiO2-PAni and commercially used Pt/C has been individually tested as cathode catalyst in MFCs. The presence of TiO2 and PAni within SGO support matrix were confirmed by FTIR analysis, where strong interactions were affirmed with purity of indigenous composite materials. Morphological analysis revealed uniform dispersion of nanoparticles on SGO support matrix and the EDX result confirmed the purity of the materials. Electrochemically, SGO-TiO2PAni showed enhanced electrical conductivity over Pt/C and other designed catalysts, revealing

32

Journal Pre-proof increased ORR with minimal charge transfer resistance. A maximum power density of 904.18 mWm-2 was observed with SGO-TiO2-PAni, which was 1.88 times higher than that of Pt/C employed MFC. Potentiometric analysis e.g., CV, LSV, chronoamperometry and EIS also revealed good agreement with the experimental results of MFCs. In effect, easy fabrication, lower cost and capability for higher power generation of SGO-TiO2-PAni, makes it an apt

with improved efficiency and minimal cost effectiveness.

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ro

Acknowledgement

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cathode catalyst that can potentially be applied in future field scale bio-electrochemical systems

FP would like to thank the University Grant Commission (UGC), Govt. of India for Senior

re

Research Fellowship [Sr. No. 061310272, Ref. No. 23/06/2013 (i) EU-V]. PP is grateful to the

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University Grants Commission (UGC, India) for providing the Rajiv Gandhi National Fellowship with Disabilities [No. F./2014-15/RGNF-2014-15D-GEN-WES- 58763]. Authors are

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Conflicts of interest

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duly thankful to Dr. Praveen Singh Gehlot for the characterization of samples.

References

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The authors declare no competing financial interest.

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Author Statement It is hereby mentioned that the work described has not been published previously and it is not under consideration for publication elsewhere. All the authors have contributed in the

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experiments as well as in the preparation of the manuscript and all of them have given approval to the final version of the manuscript. The authors declare no competing financial interest.

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Submission also implies that, if accepted, it will not be published elsewhere in the same form, in

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English or in any other language, without the written consent of the Publisher.

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Declaration of competing interest

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Authors have no conflict of interest.

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Grphical Abstract

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Journal Pre-proof Highlights 

Sulfonated graphene oxide (SGO) was synthesized by sulfonating GO using sulfanilic acid.



The facile synthesis of SGO and titanium dioxide (TiO2) were coated with nanostructured polyaniline (PAni).



Ternary hybrid nanocomposite of SGO-TiO2-PAni exhibited superior ORR activity as

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SGO-TiO2-PAni showed maximum power density of 904.18 mWm-2.

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

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cathode catalyst in MFCs.

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