lable at ScienceDirect

Energy 88 (2015) 202e208

Contents lists avai

Energy

journal homepage: www.elsevier .com/locate/energy

A study of influence on nanocomposite membrane of sulfonatedTiO2 and sulfonated polystyrene-ethylene-butylene-polystyrenefor microbial fuel cell application

Sivasankaran Ayyaru, Sangeetha Dharmalingam*

Department of Mechanical Engineering, Anna University, Chennai 600 025, India

a r t i c l e i n f o

Article history:Received 14 August 2014Received in revised form26 March 2015Accepted 2 May 2015Available online 27 June 2015

Keywords:Microbial fuel cellNanocompositesMetal oxidesProton conductivitySulfonated TiO2

Proton exchange membrane

* Corresponding author. Tel.: þ91 44 2235 7763; faE-mail address: sangeetha@annauniv.edu (S. Dhar

http://dx.doi.org/10.1016/j.energy.2015.05.0150360-5442/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Microbial fuel cell (MFC) is a device that uses bacteria as a catalyst to oxidize various substrates forsimultaneous electricity generation and wastewater treatment. In the present work, (sulfonated TiO2

(S-TiO2)/polystyrene ethylene butylene polystyrene) SPSEBS nanocomposite membranes were preparedby solution casting. The IEC (ion exchange capacity), water uptake, proton conductivity and MFC per-formance of the composite membranes were explored. SPSEBS-S-TiO2 membrane (7.5%) exhibited thehighest IEC value, water uptake and proton conductivity capacity. The results revealed that the incor-poration of sulfonated TiO2 improved the proton conductivity of the SPSEBS membrane effectively andexhibited the highest peak power density of 1345 ± 17 mWm�2 for SPSEBS-S-TiO2 7.5%, when comparedto 695 ± 7 mWm�2 and 835 ± 8 mWm�2 obtained for SPSEBS and SPSEBS-TiO2 membranes respectivelyin a (single chambered microbial fuel cell) SCMFC. In comparison to previously reported work withNafion (300 ± 10 mWm�2) in MFCs, the composite membrane delivered more than 4-fold higher powerdensity. The oxygen mass transfer coefficient (KO) of nanocomposite membranes decreased withincorporation of the sulfonated TiO2 which in turn increased the (columbic efficiency) CE.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The problem of global climate change caused by greenhousegases and environmental pollution has forced researchers andmanufacturers to search for alternative sources of energy.

[1]. In order to reduce the impact on environment and tostimulate sustainability, new energy efficient and renewable en-ergy systems are being considered effectively [2]. Number ofresearch and development programmes has been focused in thisarea; especially on MFCs. Microbial Fuel cells are very promisingenergy conversion devices that produce electricity from variousorganic matters [3e5]. Single-chamber, airecathode MFCs have thegreatest potential for practical applications due to their simpledesign and the direct use of oxygen in air [6]. The main challengesfor improving MFC (Microbial fuel cell) performance are increasingpower, increasing the recovery of electrons from the substrate(Coulombic efficiency; CE), and reducing material costs [7] in a waythat allows for scalable designs.

x: þ91 44 22357744.malingam).

An ideal proton exchange membrane used in MFC shouldfacilitate the transfer of protons from anode to cathode while at thesame time it should inhibit the transfer of other materials such asthe fuel (substrate) and the electron acceptor (oxygen). Nafion isthe widely used (proton exchange membrane) PEM in MFCs,despite the existence of a number of problems associated with itsuch as high cost, oxygen leakage, substrate loss, cation transport(other than protons) [8e12]. The oxygen diffusion into the anodecompartment from cathode is a serious problem in an MFC, whichconsumes electrons in the anode compartment, thereby reducingthe columbic yield and increases the substrate loss either by aerobicrespiration by facultative bacteria, or inhibits the growth of obligateanaerobes [13]. To overcome these drawbacks we must choose analternate PEM.

Metal oxides have been extensively investigated as inorganicadditives in proton conductingmembranes used as ionic separatorsin polymer electrolyte membrane fuel cells (PEMFCs) [14,15]. In theresearch of higher temperature proton-exchange membrane withadequate performances at low (relative humidity) RH, variouscompounds, such as SiO2, TiO2 and ZrO2, have been added to aNafion matrix [16e20]. However, the excessive incorporation ofthese nonconductive inorganic compounds in PEMnormally results

S. Ayyaru, S. Dharmalingam / Energy 88 (2015) 202e208 203

in a decrease of proton conductivity. It was overcome by theaddition of metal oxides, in the form of nano or submicrometricparticles, which improved the water retention and the thermo-mechanical stability of the membranes [22].

In most cases the metal oxide composite membrane showedbetter performance at high temperatures only. But at low tem-perature the PEM with fully humidified metal oxide could notimprove the proton conductivity than the pristine membrane. Thishygroscopic property could not always lead to a desired perfor-mance improvement under fully humidified condition. Most re-searchers thought that the proton conductivity of the compositemembranes under fully humidified condition was remarkablyreduced due to incorporating with these non-proton-conductivehygroscopic metal oxides [18,21e25]. As far as MFC is consid-ered, it operates at ambient temperature (30 �C) under fully wetcondition. Here we need to increase the proton conductivity atroom temperature and wet condition. So the objective of increasedPEM conductivity can be achieved by adding proton conductiveacidic additives like sulfonated groups grafted metal oxide(SeTiO2particles).

Always the hydrated membrane form gives better performance,because the water molecules create a path for transport of protons.But in case of high temperature thewater molecules get evaporatedfrom the membrane. To retain the hydrated form of the membraneat high temperature the metal oxides have been used. Sulfonatedgroups of TiO2 not only create a path for proton transport but alsoact as vehicle due to its negative charge. Whereas both metal oxide(eOH groupmetal oxide) andwatermolecule can only create a pathfor proton transport but cannot act as vehicle due to their neutralcharge.

Other types of very interesting compounds, to be effectivelyused as membrane additives, consists of solid electrolytes such as(heteropolyacids) HPAs [26]. Their conductivity is of the same orderof magnitude as that of mineral acids, around 0.02e0.1 S cm�1 atroom temperature, but the high affinity for polar media makesthem easily soluble [27]. Similar to HPAs, the sulfated metal oxideshave become subjects of intensive studies, being more stable thanother solid super acids. In general, the incorporation of inorganicsolid acids in conventional Nafion-type membranes is of primaryinterest, having the dual function of improving water retention aswell as providing additional acidic sites [28]. However, almost all ofthese composite membranes reported in literatures were preparedwith Nafion for HT-PEMFC only. In case of MFC, Nafion has its owndrawbacks; hence we have selected SPSEBS as the base polymerdue its low cost and excellent properties [11,12]. In our previouswork with SPSEBS in MFC, it was found that SPSEBS procured 106%higher performance than commercial Nafion due its higher protonconductivity and lower oxygen permeability [11]. In this paper,SPSEBS-S-TiO2nanocomposite membranes were prepared for lowtemperature MFC. The crystal structures of the composite mem-branes were characterized by (X-ray powder diffraction) XRD.Proton conductivity was obtained from the electrochemicalimpedance spectroscopy and single chamber-microbial fuel cellperformance of composite membranes was evaluated.

2. Materials and methodology

2.1. Preparation of sulfonated TiO2

TiO2 nanoparticles (25 nm) were purchased from Adrich, USA.The sulfonated TiO2 was prepared using sulphuric acid according tothemethod reported byWang L et al. [29] Chang-Chun Ke et al. [30]Chiu-Hsun Lin et al. [31]. One gram of TiO2 nanoparticles was addedto 15 ml of 0.5 M sulphuric acid under ultrasonication for 1 h. Thefinal SeTiO2white powder was obtained by drying the solution at

100 �C for 24 h. Sulfonation of TiO2 was confirmed by FTIR andSEM-EDX in our previous work [32].

2.2. Preparation of nanocomposites

Sulfonated TiO2 with different content (0, 2.5, 5, 7.5,and 10 wt%)was added into the SPSEBS/THF (tetrahydrofuran) solution, thenstirred mechanically and then degassed by ultrasonication. 7.5 wt%of pure TiO2 with SPSEBS/THF matrix was prepared for comparisonpurpose. The prepared mixture was slowly poured onto a glass dishin an amount that would give a thickness of 180 ± 5 mm for theformed nanocomposite membrane. The glass dish was thenallowed to evaporate the solvent slowly. The membranes whichwere thus obtained were designated as SPSEBS, SPSEBS-S-TiO22.5%, SPSEBS-S-TiO2 5%, SPSEBS-S-TiO2 7.5%, and SPSEBS-S-TiO2 10%according to the weight percentage of the filler added.

2.3. Instrumental characterization

2.3.1. FT-IR spectrum analysisFT-IR spectrum of the nanocomposite membranes was recorded

from 500 to 4000 cm�1 using an Alpha Bruker FTIR spectrometerfor confirming the increase in eSO3H functionality with respect toincrease in filler (SeTiO2) concentration.

2.3.2. Morphology observationThe membrane surface of the various composite membranes

was observed by means of JEM-5600LV (scanning electron micro-scope) SEM. All specimens were dried and coated with a thin layerof gold before observation.

2.3.3. X-ray diffraction (XRD)The XRD spectrum of membrane samples were recorded using

“X” Pert Pro diffracto meter. The scanning angle was 1e80� withscanned at a rate of 1� min�1.

2.4. Water uptake

Water uptake properties of the nanocomposite membraneswere measured at room temperature. The (water uptake) Wut wascalculated from the (wet weight) Wwet and the (dry weight)Wdry of the membranes using the following equation:

Wut ¼ (Wwet e Wdry) / Wdry � 100% (1)

The dry weight (Wdry) of the membrane was obtained afterdrying the membranes in vacuum oven at 100 �C for 12 h, and thewet weight (Wwet) was obtained after removing the excess liquidwater from the soaked membrane surface through dipping indeionized water for 24 h.

2.5. The ion exchange capacity

The (ion exchange capacity) IEC of nanocomposite was deter-mined by the back-titration method.1.0 g of the sample was soakedin 20 ml of 0.5 M H2SO4 overnight to ensure that all the eSO3Hgroups were exchanged with Hþ ions. Then the Hþ-formed samplewas washed with distilled water till the pH value was 7. The com-posite membrane was immersed in saturated potassium chloridesolution overnight, to allow replacement of protons with Kþ ions.Protons released from the membrane were neutralized by 0.01 Nsodium carbonate solutions. Phenolphthalein was used as theindicator. The IEC was calculated using the following formula.

Fig. 1. FTIR spectra of various concentration of sulfonated composite membranes,unsulfonated membrane composite and base membrane.

S. Ayyaru, S. Dharmalingam / Energy 88 (2015) 202e208204

IEC ¼ Titre value ðin mlÞ � Normality of Na2CO3

weight of dry polymer membraneðin gÞ meq=g (2)

2.6. Membrane conductivity measurement

The conductivity of each composite membrane was measuredusing ac impedance spectroscopy. An applied voltage of 10 mV anda frequency range of 1 MHze100 Hz were employed using apotentiostat (BioLogic VSP, France) electrochemical system. Thesample was soaked in water at ambient temperature (30 �C) for24 h and then sealed between two electrodes with an area of0.62 cm2. The impedance measurement was then carried out. Theconductivity values were calculated using Eq. (3)

s ¼ L/RA (3)

Where s is the proton conductivity of themembrane (S/cm), L is themembrane thickness (cm), R is the membrane resistance (U) and Ais the area of the electrode (cm2) [29,31].

2.7. MFC studies

2.7.1. MFC construction and operationThe anode electrode used was 30wt.% wet proofed carbon cloth.

The cathodewas prepared using a carbon cloth having 0.5 mg cm�2

platinum coating as a catalyst. The MEA was prepared by sand-wiching the composite membrane between the anode and cathodeelectrodes as mentioned in our previous reports [11,21,32]. TheMEA was further assembled into a cylindrical single-chamberedMFC (28 mL) with the cathode-side of the MEA facing the air. Theanode chamber was filled with nutrient solution containing 1 g/Lglucose, 0.1 g/L yeast, 0.5 g/L NaCl, 0.28 g/L NH4Cl, 0.1 g/L, 0.68 g/LKH2PO4, 0.87 g/L K2HPO4, having a pH of 7 which was then inoc-ulated with wastewater that was collected from Anna Universitysewage treatment plant (Chennai, India).

2.7.2. Analytical measurements and calculationsCell voltagewas recorded using a digital multimeter (Model 702,

Metravi, India). Polarization curves were obtained by varying theapplied external resistance at a time interval of 10 min. Currentdensity and power density were determined by Ohm's law [9,11].

. Oxygen mass transfer coefficients for each composite mem-brane were determined by following the protocol of O LefebVreet al. (2009). The (dissolved oxygen) DO probe (Extech 407510A,Taiwan) was inserted in an uninoculated MFC fitted with thedesired composite membrane and filled with distilled water pre-viously sparged with nitrogen gas to remove DO. The mass transfercoefficient of oxygen in themembrane, KO (cm s�1) was determinedby monitoring the DO concentration over time and using Eq. (4):

KO ¼ �V/At ln [(COeC)/CO] (4)

Where, V is the liquid volume in the anode chamber, A is themembrane cross-sectional area, CO is the saturated oxygen con-centration in the cathode chamber, and C is the DO concentration inthe anode chamber at time t [9].

The Coulombic Efficiency (CE) was calculated using followingEq. 5

CE ¼ Cp/CT � 100% (5)

where, Cp is the total Coulombs calculated by integrating the cur-rent over time. CT is the theoretical amount of Coulombs that can beproduced from the amount of glucose which was degraded [9,11].

Internal resistance was characterized using (electrochemicalimpedance spectrometer) EIS methods. The impedance measure-ments were taken from 500 kHz to 1 MHz by applying a sine wave(10 mV rms) on top of the bias potentials with a potentiostat((BioLogic VSP, France). The three-electrode mode is used toanalyze an individual electrode. To know the anode internal resis-tance, anode and cathode electrodes were used as working andcounter electrodes, respectively. The reference electrode was anAg/AgCl (KCl-sat.) electrode [33]. All tests were done in triplicatesand the average values were calculated.

3. Results and discussion

3.1. FTIR spectrum

Fig. 1 exhibits the FT-IR spectra of the SPSEBS, SPSEBS-TiO2,SPSEBS-S-TiO22.5%, SPSEBS-S-TiO2 5%, SPSEBS-S-TiO2 7.5%, andSPSEBS-S-TiO2 10%. It was slightly difficult to differentiate thestructural changes of the sulfonated composite membranes andSPSEBS from Fig. 1. This was due to the presence ofeSO3H groups inthe side chains of SPSEBS, that made it difficult for the identificationof theeSO3Hattached to the surfaceof TiO2nano-particles. Thepeakat 3400 cm�1was the characteristic signal ofeOH.On increasing theSeTiO2 concentration hydroxyl peaks were widened, due to theeOH group of sulfonated TiO2. These results also revealed that thehygroscopic property increased while adding sulfonated TiO2.

3.2. Morphology observation

The surface morphology was analyzed from scanning electronmicroscope (Fig. 2). The inorganic particles appeared to have welldispersed in both low and high wt % composites. The filler particleswere distributed relatively in a uniform fashion. The filler particleswere observed to be clearly embedded in the polymer matrix,which established the connectivity in the composites. The SeTiO2particles were dispersed homogeneously with the interspaces filledwith SPSEBS up to 7.5%, while the 10% TiO2 composite membranesshowed formation of agglomeration on the surface of the polymermatrix. This implies that the prepared compositemembranes (up to7.5%) can be expected to perform consistently well in the MFCs.From the images, it was observed that the concentration of fillersgradually increased from 2c to 2f in the sulfonated composite

Fig. 2. SEM images of (a) SPSEBS, (b) SPSEBS-TiO2 (c) SPSEBS-S-TiO22.5%, (d) SPSEBS-S-TiO25%, (e) SPSEBS-S-TiO27.5% TiO2 and (f) SPSEBS-S-TiO210% composite membranes.

S. Ayyaru, S. Dharmalingam / Energy 88 (2015) 202e208 205

membranes. The unsulfonated composite membrane with 7.5% ofTiO2 represented by 2b also showed the presence of higher con-centration of filler (see Fig. 2).

3.3. X-ray diffraction (XRD)

X-ray diffraction (XRD) analysis is an effective technique to studythe morphological information, especially the crystallinity of thecomposite membranes. Fig. 3 shows the XRD patterns of SPSEBS,SPSEBS-TiO2 and SPSEBS-S-TiO2 composite membranes. From theXRD profiles, it can be seen that on increasing the concentration ofsulfonatede TiO2 from 2.5% to 7.5 % the crystallinity of the compositemembranesdecreased.This indicated that thepresenceof amorphousdomain in the sulfonated nanocomposite membranes which wasgreatly decreased the crystallinity. The addition of TiO2 fillers couldnot decrease the peak intensity of SPSEBS-TiO2 membrane whencompared with Sulfonated composites. SPSEBS-TiO2 membrane

clearly showed the reflectiondue to crystallineTiO2 on the surfacebutin the case of sulfonated composite membranes no clear reflectionswere found. This behaviour suggested that TiO2 particles present insulfonated composite membranes were amorphous nature.

The addition of SeTiO2 fillers resulted in changes in crystallineproperties, the intensity of which depended on the amount ofadded SeTiO2. The crystallinity of the sulfonated membranedecreased with the increase in the percentage of SeTiO2till 7.5%. Asthe SeTiO2concentration further ascended beyond 7.5%, due tohigher loading of SeTiO2, (10%) inhomogeneous composite mem-brane was formed.

3.4. Water uptake and ion exchange capacity

Water uptake and ion exchange capacityof themembraneplayanimportant role in conductivity.With the increase inwateruptake andIEC, proton conductivity increased due to increase in the mobility of

Fig. 3. XRD patterns of SPSEBS, unsulfonated SPSEBS-TiO2 composite membrane anddifferent concentration of sulfonated SPSEBS-S-TiO2 composite membranes.

Fig. 4. Nyquist plot for proton conductivity of different SPSEBS-S-TiO2 compositemembranes, non sulfonated SPSEBS-TiO2 composite membrane and SPSEBS.

S. Ayyaru, S. Dharmalingam / Energy 88 (2015) 202e208206

ions in the water phase. To confirm the hygroscopic property ofmembranes by incorporating SeTiO2 and TiO2 particles, the wateruptakesweremeasured. Table 1 lists the IEC andwater uptake valuesof the membranes at room temperature. The SPSEBS membraneswith sulfonic acid functionalized TiO2 showed higher water uptakeswhen compared to the SPSEBS membranes with non-functionalizedTiO2. The increased water uptake content of composite membranewas attributed to the incorporated SeTiO2particles in the compositemembrane that can supplymore acid sites to absorbwater comparedwith the SPSEBS-TiO2 and pristine SPSEBS membranes. Thisconfirmed thehydrophilic nature of the SeTiO2. However, increasingthe loading of SeTiO2 from 7.5 % to 10 % did not significantly changethe measured liquid water uptake.

The SPSEBS-S-TiO2 7.5% membrane showed the highest IECvalue of 3.35 meq g�1 compared with the pristine SPSEBS of1.89 meq g�1and the SPSEBS-TiO2 7.5% of 1.62 meq g�1. Nano-composite membranes containing 2.5e7.5% SeTiO2 showed higherIEC values than pristine as well as un-sulfonated TiO2 compositemembrane. The increased IEC values illustrated that more acid siteswere resulted from acidic SeTiO2 particles existed in SPSEBS-S-TiO2composite membranes. The improved acidic property wasexpected to enhance the membrane proton conductivity [18]. Onthe other hand, the SPSEBS-TiO2 membrane showed the lowest IECvalue due to the presence of non-proton-conductive TiO2 particles.

3.5. Proton conductivity

The results of impedance measurement for the determination ofproton conductivity are plotted in Fig. 4. The proton conductivitiesfor SPSEBS, SPSEBS-TiO2 7.5%, SPSEBS-S-TiO2 2.5%, SPSEBS-S-TiO2

Table 1Comparison of IEC values and water uptake of the SPSEBS-S-TiO2 composites,SPSEBS-TiO2 and SPSEBS membranes.

Membrane type IEC (mmol g�1) Water uptake (%)

SPSEBS 1.89 163 ± 3SPSEBS-TiO2 7.5% 1.62 170 ± 5SPSEBS-TiO2-SO3H 2.5% 2.25 185 ± 8SPSEBS-TiO2-SO3H 5% 3.02 200 ± 9SPSEBS-TiO2-SO3H 7.5 3.35. 220 ± 11SPSEBS-TiO2-SO3H 10% 3.02 218 ± 10

5%, SPSEBS-S-TiO2 7.5%, and SPSEBS-S-TiO2 10% were 1.52 � 10�2,1.08 � 10�2, 1.75 � 10�2, 2.51 � 10�2, 3.57 � 10�2 and2.72 � 10�2 S cm�1 respectively. The proton conductivity of all thesulfonated membranes increased distinctively with the increasingSeTiO2 concentration upto 7.5% and then decreased above 7.5%. Theconductivity of the composite membranes with sulfonated SeTiO2was superior to that of SPSEBS and SPSEBS-TiO2 membranes. Thus,the addition of SeTiO2effectively increased the proton conductivityof the membranes compared with SPSEBS-TiO2 membranes. Keet al. obtained similar results with sulfonated SiO2 in Nafion at wetcondition in the chemical fuel cell [27]. The lower proton conduc-tivity of SPSEBS-TiO2 membrane compared with SPSEBS under wetcondition was due to the non-proton conductive property of TiO2.

The reasons for proton conductivity of sulfonated compositemembranes were: (1) water uptake of the composite membraneincreased with the SeTiO2addition as shown in Table 1; (2) anadditional proton conduction pathway was created due to molec-ular water absorption by SeTiO2, part of the proton can be con-ducted by hopping mechanism between TiO2eSO2eOH and watermolecules [34e36]. However, higher SeTiO2 loading (10%) did notcontribute to any additional increase in proton conductivity. Theincreased Hþ diffusion of the SeTiO2 based SPSEBS membraneswhen compared to the pure SPSEBS membrane is the combinedresults of the enhanced water uptake as well as the acidity.

3.6. MFC performances

The enrichment and adaptation of the electrochemically activebacteria in the SCMFCs were performed in batch mode under afixed external load of 500 U and was carried out for a period ofapproximately 3weeks. All the MFCs inoculated with volume 10%V/V initially was slower and the power density output rose aftereach batch feed, then after reaching a peak a decline was observeddue to consumption of the fuel. All MFCs attained the peak poweraround 3rd or 4th batch (300 or 400 h), beyond which no furtherincrease in the power density was observed which suggested thatthe anode biofilm was enriched with electrochemically-activebacteria thus rendering it stable. Subsequently the polarizationcurves were obtained from all SCMFCs using various resistances(1 MUe50 U) at 20 min interval period which was required forstabilization of the voltage.

Fig. 5. Performance of SPSEBS-S-TiO2 composite membranes, SPSEB-TiO2 compositemembrane and SPSEBS membrane in single chamber MFC.

Table 2Comparison of oxygenmass transport, columbic efficiency and internal resistance ofSPSEBS-S-TiO2 composites, SPSEBS-TiO2 and SPSEBS membranes.

Membrane type Oxygen mass transport(KO) (cm/s)

Columbicefficiency (%)

Internalresistance (U)

SPSEBS 3.5 � 10�5 cm/s 75 ± 3 66 ± 4SPSEBS-TiO2 7.5% 0.8 � 10�5 cm/s 72 ± 2 60 ± 3SPSEBS-TiO2-SO3H 2.5% 0.88 � 10�5 cm/s 80 ± 4 55 ± 2SPSEBS-TiO2-SO3H 5% 0.7 � 10�5 cm/s 83 ± 3 45 ± 1SPSEBS-TiO2-SO3H 7.5% 0.64 � 10�5 cm/s 87 ± 4 35 ± 0.8SPSEBS-TiO2-SO3H 10% 0.60 � 10�5 cm/s 85 ± 5 37 ± 0.5

Table 3Comparison of sulfonated composite membranes with commercial Nafion 117membrane.

Membrane specifications SPSEBS-TiO2-SO3H (7.5%) Nafion® 117

Proton conductivity (S cm�1) 3.574 � 10�2 2 � 10�2

IEC (meq g�1) 3.35 0.982Water uptake (%) 220 22Thickness (mm) 180 175Ko (cm s�1) 0.64 � 10�5 1.6 � 10�4

Power density (mW m�2) 1345 ± 17 300 ± 7Cost (m�2) 200 1500

S. Ayyaru, S. Dharmalingam / Energy 88 (2015) 202e208 207

Fig. 5 shows the SCMFC (single chambered microbial fuel cell)performance of various membranes. The power density measuredwith the composite membranes with sulfonated inorganic fillers,SPSEBS-S-TiO2 2.5%, SPSEBS-S-TiO2 5%, SPSEBS-S-TiO2 7.5%, andSPSEBS-S-TiO2 10% were 975 ± 11, 1200 ± 15, 1345 ± 17 and1105 ± 13 mWm�2 respectively. The power density of SPSEBS-TiO27.5% and SPSEBS were 835 ± 8 and 695 ± 7 mWm�2 respectively. Itwas noticed that the performance of SPSEBS-S-TiO2 membraneswas superior to SPSEBS-TiO2 and SPSEBS membranes, because ofthe increased number of acid sites (eSO3H) in the polymer matrixthat effectively increased the proton conductivity and improved theperformance in MFC in wet condition. The performance wasobserved to increase with increasing SeTiO2 content up to 7.5%(where the highest performance was observed) and beyond which,the performance of compositemembranes decreased. This declinedperformance was caused by aggregation of SeTiO2 conductors inmembranes. Aggregates of such inorganic materials reduce themobility of Hþ ion and thus presumably have a negative effect onthe performance of composite systems. This result was wellconsistent with previous proton conductivity measurements.

Cheng Bi et al. and L. Wang et al. reported better performanceswith the sulfonated metal oxide based membranes than the pris-tine and non-sulfonated membrane at wet condition in PEM fuelcell [18,29]. Alessandra D'Epifanio et al. have prepared compositemembrane with Nafion and sulfonated ZrO2 and its performancewas evaluated in 30% (relative humidity) RH at 70 �C. The com-posite membrane produced 200% higher performance than apristine Nafion membrane [37].

Different types of composite membranes have been investigatedin MFC and their results were compared with commercial Nafion[38,39]. In comparison to our previously reported work with Nafion(300 ± 10 mW/m2) in MFCs under the same condition [10], thecomposite membrane (SPSEBS-S-TiO2 7.5%) delivered 4-fold higherpower density. This was mainly due the sulfonation of TiO2 whichincreased the IEC, proton conductivity and MFC performance. Thismay make this approach useful for constructing large scale MFC. Incontrast, SPSEBS-TiO2 composite membrane showed better per-formance than SPSEBS, due to the low oxygen permeability ofcomposites, which maintained the anaerobic condition andreduced the aerobic metabolism in anode compartment.

Table 2 lists the internal resistance, CE and oxygen mass trans-port of the different sulfonated composite membranes, non-

sulfonated composite membrane and pristine SPSEBS. The low in-ternal resistance of all the composite membranes under operatingconditions were attributed to the incorporation of hygroscopicSeTiO2 and TiO2 particles. Compared to TiO2 additive, the SeTiO2particles contributed more to decrease the internal resistance dueto its higher proton-conductive behaviour. The oxygen masstransfer coefficient of SPSEBS (3.5 � 10�5 cm s�1) was found to bemuch higher than other membranes. The SPSEBS-S-TiO2 compositeand SPSEBS-TiO2 membranes had lower oxygen mass transfer co-efficients due to incorporation of inorganic metal oxides. The ox-ygen mass transfer coefficient rates decreased with an increase inthe amount of SeTiO2 added into the SPSEBS matrix.

Table 3 shows the comparison of sulfonated composite mem-branes with commercial Nafion membrane. In comparison toNafion 117 basedMFC under the same condition [11], the SPSEBS-S-TiO2 7.5% membrane based MFC delivered more than 4-folderhigher power density. In addition, 600 ± 7 mW m�2 obtained withSPSEBS in our previous work [9] was also dramatically increased to1345 ± 17 mW m�2 by incorporation of sulfonated TiO2 (7.5%)which means a 124% higher power density with sulfonated com-posite membrane. It is noted that the present work remarkablyimproved the power generation. The organic compound removalwas found to be around 75 ± 10% for all the composite membranesstudied in the microbial fuel cell during the performance.

The demonstrated composite membrane is an environmentallysafe, efficient and a cheaper alternate for the Nafion. The testing ofthe prepared composite membranes showed excellent results andit would be a promising candidate for application in MFCs for thetreatment of waste water and simultaneous electricity generation.Lower oxygen crossover, increased CE and low sludge formation areadditional supports for wastewater treatment applications. Addi-tionally, the membranes are also non-fluorinated in nature andhence would prove to be cost efficient also.

4. Conclusion

In this paper, unique composite SPSEBS-S-TiO2 membranes withhigh water uptake and good proton conductivity that will be apotential candidate as PEM for MFC applications were prepared.The composite membranes were characterized by XRD, FT-IR, SEMtechniques and the physical and chemical properties were studied.

S. Ayyaru, S. Dharmalingam / Energy 88 (2015) 202e208208

Among the studied membranes, SPSEBS-S-TiO2 membrane (7.5%)exhibited the highest IEC value, proton conductivity capacity,lowest internal resistance and cell performance. The improvedperformance was due to the sulfonation effect of SeTiO2 particleswhich combined the proton conductivity with water absorbingproperties. The oxygen mass transfer coefficient (KO) of compositemembranes was also found to be lower than SPSEBS. This workdemonstrated that these sulfonated inorganic fillers in poly-electrolyte nanocomposite membranes are an effective approach toimprove the MFC performances. In comparison to our previouswork with SPSEBS, the sulfonated composite membrane produced124% higher power output in an SCMFC that may be attributed to (i)the higher proton conductivity (ii) lower oxygen permeability (iii)lower internal resistance of the sulfonated composite membranes.Thus suchmembranes have great potential to be used as a PEM for ahigh-power MFC. This work may also provide a new universalapproach for improving other MFCs. Our future studies will befocused on the antifouling properties of sulfonated membranes andthe use of different types of sulfonated metal oxides like ZrO2 andSiO2.

Acknowledgement

The authors thank, the Department of Science and Technology(DST) India (letter no. DST/TSG/AF/2010/09, dt.01-10-2010) andUGC-Meritorial Student Fellowship (2010e2013) Department ofChemistry, Anna University Chennai, India for their financial sup-port to carry out this work.

References

[1] Rosenberg E, Lind A, Espegren KA. The impact of future energy demand onrenewable energy production e case of Norway. Energy 2013;61:419e31.

[2] Puksec T, Mathiesen BV, Novosel T, Duic N. Assessing the impact of energysaving measures on the future energy demand and related GHG (greenhousegas) emission reduction of Croatia. Energy 2014;76:198e209.

[3] Logan BE, Hamelers B, Rozendal R. Microbial fuel cells: methodology andtechnology. Environ Sci Technol 2006;40:5181e92.

[4] Jiang D, Li B. Granular activated carbon single-chamber microbial fuel(GASCMFCs): a design suitable for large-scale wastewater treatment pro-cesses. Biochem Eng J 2009;47:31e7.

[5] Xie S, Liang P, Chen Y, Xia X, Huang X. Simultaneous carbon and nitrogenremoval using an oxic/anoxic-biocathode microbial fuel cells coupled system.Bioresour Technol 2011;102:348e53.

[6] Luo Y, Zhang F, Wei B, Liu G, Zhangc R, Logan BE. The use of cloth fabricdiffusion layers for scalable microbial fuel cells. Biochem Eng J 2013;73:49e52.

[7] Hidalgo D, Tommasi T, Cauda V, Porro S, Chiodoni A, Bejtka K, et al. Stream-lining of commercial Berl saddles: a new material to improve the performanceof microbial fuel cells. Energy 2014;71:615e23.

[8] Chae KJ, Choi M, Ajayi FF, Park W, Chang IS, Kim IS. Mass transport through aproton exchange membrane (Nafion) in microbial fuel cells. Energy Fuels2008;22:169e76.

[9] Ayyaru S, Dharmalingam S. Development of MFC using sulphonated polyetherether ketone (SPEEK) membrane for electricity generation from waste water.Bioresour Technol 2011;102:11167e71.

[10] Lefebvre O, Shen Y, Tan Z, Uzabiaga A, Chang IS, Ng HY. A comparison ofmembranes and enrichment strategies for microbial fuel cells. BioresourTechnol 2011;102:6291e4.

[11] Ayyaru S, Letchoumanane P, Dharmalingam S, Stanislaus AR. Performance ofsulfonated polystyrene-ethylene-butylene-polystyrene membrane in micro-bial fuel cell for bioelectricity production. J Power Sources 2012;217:204e8.

[12] Sangeetha D, Bhavani P. Preparation and characterization of proton exchangemembrane based on SPSEBS/PSU blends for fuel cell applications. Energy2011;36:3360e9.

[13] Kim H, Chang IS, Moon H. Microbial fuel cell-type biochemical oxygen de-mand sensors. Encycl Sensors 2006;10:1e12.

[14] Adjemian KT, Lee SJ, Srinivasan S, Benziger J, Bocarsly AB. Silicon oxide nafioncomposite membranes for proton-exchange membrane fuel cell operation at80e140 �C. J Electrochem Soc 2002;149:256e61.

[15] Jalani NH, Dunn K, Datta R. Synthesis and characterization of Nafion®-MO2(M ¼ Zr, Si, Ti) nanocomposite membranes for higher temperature PEM fuelcells. Electroch Acta 2005;51:553e60.

[16] Herring AM. Inorganicepolymer composite membranes for proton exchangemembrane fuel cells. Polym Rev 2006;46:245e96.

[17] Sahu AK, Selvarani G, Pitchumani S, Sridhar P, Shukla AK. A sol-gel modifiedalternative nafion-silica composite membrane for polymer electrolyte fuelcells. J Electrochem Soc 2007;154:123e32.

[18] Bi C, Zhang H, Zhang Y, Zhu X, Ma Y, Dai H, et al. Fabrication and investigationof SiO2 supported sulfated zirconia/Nafion® self-humidifying membrane forproton exchange membrane fuel cell applications. J Membr Sci 2008;325:742e8.

[19] Lakshminarayana G, Nogami M, Kityk IV. Synthesis and characterization ofanhydrous proton conducting inorganiceorganic composite membranes formedium temperature proton exchange membrane fuel cells (PEMFCs). Energy2010;35:5260e8.

[20] Savadogo O. Emerging membranes for electrochemical systems e part II. Hightemperature composite membranes for polymer electrolyte fuel cell (PEFC)applications. J Power Sources 2004;127:135e61.

[21] Nunes SP, Ruffmann B, Rikowski E, Vetter S, Richau K. Inorganic modificationof proton conductive polymer membranes for direct methanol fuel cells.J Membr Sci 2002;203:215e25.

[22] Silva VS, Ruffmann B, Silva H, Gallego YA, Mendes A, Madeira LM, et al. Protonelectrolyte membrane properties and direct methanol fuel cell performance:characterization of hybrid sulfonated poly (ether ether ketone)/zirconiumoxide membranes. J Power Sources 2005;140:34e40.

[23] Zhang G, Zhou Z. Organic/inorganic composite membranes for application inDMFC J. Membr Sci 2005;261:107e13.

[24] Arico AS, Baglio VD, Blasi A, Creti P, Antonucci PL, Antonucci V. Influence of theacidebase characteristics of inorganic fillers on the high temperature per-formance of composite membranes in direct methanol fuel cells. Solid StateIonics 2003;161:251e65.

[25] Baglio V, Arico AS, Di Blasi A, Antonucci V, Antonucci PL, Licoccia S, et al.NafioneTiO2 composite DMFC membranes: physico-chemical properties ofthe filler versus electrochemical performance. Electrochim Acta 2005;50:1241e6.

[26] Malhotra S, Datta R. Membrane-supported nonvolatile acidic electrolytesallow higher temperature operation of proton-exchange membrane fuel cells.J Electrochem Soc 1997;144:L23e6.

[27] Rao PM, Wolfson A, Kababya S, Vega S, Landau MV. Immobilization ofmolecular H3PW12O40 heteropolyacid catalyst in alumina-grafted silica-geland mesostructrued SBA-15 silica matrices. J Catal 2005;232:210e25.

[28] Thampan TM, Jalani NH, Choi P, Datta R. Systematic approach to design highertemperature composite PEMs. J Electrochem Soc 2005;152:A316e25.

[29] Wang L, Zhao D, Zhang HM, Xing DM, L Yi B. Water-retention effect ofcompositemembranes with different types of nanometer silicon dioxide.Electrochem Solid St 2008;11:201e4.

[30] Ke CC, Li XJ, Shen Q, Qu SG, Shao ZG, L Yi B. Investigation on sulfuric acidsulfonation of in-situ solegel derived Nafion/SiO2 composite membrane. Int JHydrogen Energy 2011;36:3606e13.

[31] Yang CC, Lin CT. Preparation of the PVA/TiO2 nanocomposite polymer mem-branes by a sol-gel process for alkaline DMFC. ECS Trans 2008;6:17e44.

[32] Ayyaru S, Dharmalingam S. Improved performance of microbial fuel cellsusing sulfonated polyether ether ketone (SPEEK) S-TiO2 nanocompositemembrane. RSC Adv 2013;3:25243e51.

[33] He Z, Mansfeld F. Exploring the use of electrochemical impedance spectros-copy (EIS) in microbial fuel cell studies. Energy Environ Sci 2009;2:215e9.

[34] Zhao D, Yi BL, Zhang HM, Yu HM. MnO2/SiO2eSO3H nanocomposite ashydrogen peroxide scavenger for durability improvement in proton exchangemembranes. J Membr Sci 2010;346:143e51.

[35] Junjie Y, Guangbin Z, Hongting P. Preparation and properties of Nafion®/hollow silica spheres composite membranes. J Membr Sci 2008;325:742e8.

[36] Nogami M, Nagao R, Wong C. Proton conduction in porous silica glasses withhigh water content. J Phys Chem B 1998;102:5772e5.

[37] D'Epifanio A, Navarra MA, Weise FC, Mecheri B, Farrington J, Licoccia S, et al.Composite Nafion/sulfonated zirconia membranes: effect of the filler surfaceproperties on proton transport characteristics. Chem Mater 2010;22:813e21.

[38] Rahimnejad M, Ghasemi M, Najafpour GD, Ismail M, Mohammad AW,Ghoreyshi AA, et al. 2011, Synthesis, characterization and application studiesof self-made Fe3O4/PES nanocomposite membranes in microbial fuel cell.Electrochim Acta 2011;85:700e6.

[39] Venkatesan PN, Dharmalingam S. Characterization and performance studyon chitosan-functionalized multi walled carbon nano tube as separator inmicrobial fuel cell. J Membr Sci 2013;435:92e8.