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Application of polysulphone based anion exchange membrane electrolyte for improved electricity generation in microbial fuel cell
Accepted Manuscript Application of Polysulphone based Anion Exchange Membrane Electrolyte for Improved Electricity Generation in Microbial Fuel Cell
Elangovan Mahendiravarman, Dharmalingam Sangeetha PII:
S0254-0584(17)30547-3
DOI:
10.1016/j.matchemphys.2017.07.038
Reference:
MAC 19845
To appear in:
Materials Chemistry and Physics
Received Date:
13 June 2016
Revised Date:
23 November 2016
Accepted Date:
11 July 2017
Please cite this article as: Elangovan Mahendiravarman, Dharmalingam Sangeetha, Application of Polysulphone based Anion Exchange Membrane Electrolyte for Improved Electricity Generation in Microbial Fuel Cell, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys. 2017.07.038
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Application of Polysulphone based Anion Exchange Membrane Electrolyte for Improved Electricity Generation in Microbial Fuel Cell Elangovan Mahendiravarman and Dharmalingam Sangeetha* Department of Mechanical Engineering, Anna University, Chennai-600 025, India *Corresponding author Email: [email protected] Tel No: +91-44-22357763, Fax No: +91-44-22357744 Abstract The present study has been designed on the synthesis and utilization of quaternized polysulphone (QPSU) both as a membrane and a catalyst binder for microbial fuel cells (MFCs). The effect of surface roughness on biofilm growth and the impact of oxygen and specific substrate crossover of the anode and cathode for both membranes was studied. Besides, the effect of specific cathode binders of sulphonated poly (sulphone) (SPSU), Quaternized poly sulphone (QPSU) and Poly tetra fluro ethylene (PTFE) on the overall cell performance was measured using their electrochemical performance studies such as linear sweep voltammetry (LSV), cyclic voltammetry (CV) etc. The performance of synthesized membrane was compared to a commercially obtained anion exchange membrane AMI-7001. Based on the results obtained, QPSU can be considered not only as a suitable membrane alternative to replace commercially available AEM but also as an effective binder material for MFC applications.
Keywords: Anion exchange membrane, Polysulphone, Surface roughness, Ionic binders, Biofilm.
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1. Introduction Microbial fuel cells (MFCs) is an emerging frontline technology in the field of recovering energy from wastewater[1,2]. A Single-chamber, air cathode MFC is more advantageous than a two-chamber MFC in design, construction, performance and passive aeration for oxygen reduction [3]. A Membrane Electrode Assembly (MEA) type MFC delivers more power due to low internal resistance and reduced distance between the two electrodes. As membranes prevent electrodes from touching, short circuit is avoided. Nafion and PTFE are the commonly used catalyst binders that are used to bind platinum over a carbon electrode. In the case of MEA, though Nafion acts as a good sandwich between the membrane and the electrodes, its high cost impedes the commercialization [4]. Although both PTFE and Nafion can bind catalyst particles so as to form a catalyst layer, they cannot conduct hydroxide ions. Hence, these two polymer materials are not a good choice as the binder for preparing MEAs for alkaline membrane fuel cells [5]. In general, Proton Exchange Membranes (PEMs) and Cation Exchange Membranes (CEMs) are used to separate the anode and cathode in MFCs. However, some researchers have found that while using PEMs or CEMs, transport of cations other than protons leads to a reduced pH in the anode chamber which impairs the microorganism activity and at the same time an increased pH in the cathode chamber reduces the cathode potential [6–8]. J-cloth, a highly porous, biodegradable material, is one of the most successfully tried out separators in MFCs. However, this material biodegraded over time [9-11]. Glass fiber mats and nylon filters, which are permeable, economical, non-biodegradable coarse-pore materials are also used. These materials generally deliver more power density than ion exchange membranes. For example, nylon filters with pore sizes from 0.2 µm to 160 µm produce power density ranging from 443 to 908 mW m−2, Zhang et al. [12]. But an increased power density leading to decreased coulombic efficiency (CE) shows an inverse correlation
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between the two parameters. Actually it happens due to oxygen cross over from the aerobic cathode into the anaerobic anode chamber. Eliminating the membrane is one simple solution for this problem, but the coulombic efficiency (CE) may be much lower than that of MFCs with a membrane [13–15]. Another solution is to use Anion Exchange Membranes (AEMs) which are capable of inhibiting the cation transfer [16]. A dual-chamber MFC with an AEM has reached a power density of 610 mW m−2 compared to MFCs with a Nafion membrane (514 mWm−2) and a CEM (480 mWm−2) [17]. The increased power density, perhaps might be a result of the improved activity of microorganisms in the anode chamber due to more slowly decreasing pH [18]. In the AEM configuration the concentration gradients of the different phosphate buffer species of HPO42-, PO43- in anode and cathode are equivalent in concentration [19]. This is an indication for the shuttling of charges through buffer species and then transported through the membrane. This reveals that AEM facing the solution catalysts side produces a higher power density and a lower cathode resistance compared to MFCs with CEM. Recently, there has been a surge in alkaline fuel cell research activities and interest. Many researchers are switching their research outlook from an exclusive focus on AEM and AEMFCs. Progress is being made, research funding for AEMFCs is seeing an upward trend, and more papers on the subject are being published [20, 21, 22, 23]. Commercial anion exchange membranes are generally reinforced with poly (vinyl chloride) for enhanced mechanical stability [24, 25]. It is a normal practice to attach PVC with the ionomeric component of AEM. The physiochemical properties of membrane viz, ion exchange capability and permeability change over long period due to dehydrochlorination of poly (vinyl chloride). The membranes, used once, turn black in colour with a significant loss of rigidity and increased water uptake resulting in enlarged thickness. The polymer itself was damaged and peeled off from the PVC cloth [25]. Such a chemical degradation makes the
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membranes weak and unsuitable for high and long term pH applications. This is common for many commercially available AEMs, such as Tokuyama’s AMX and AM1 membranes that change colour from light yellow to black within an hour of exposure to 1 M NaOH [26]. Hitherto, only a few AEMs have been synthesized and tested successfully as anion exchange membranes for MFCs due to the technical glitches mentioned above [22, 27, 28]. The persistent interest of the membrane scientists for polysulphone is due to its excellent features such as solubility in a wide range of aprotic polar solvents, good film forming nature, high thermal, chemical and mechanical resistance on the entire pH range, sustainability in oxidative medium, moderate reactivity in aromatic electrophilic substitution reactions [29, 30]. In the present study, the authors have reported the synthesis and characterization of QPSU and its performance was compared to the commercial AMI-7001 in a single chambered air cathode MFC. The effect of surface roughness on biofilm growth, influence of oxygen and specific substrate crossover on the anode and cathode were also carried out. Besides, the impact of fabricated cathode binders of sulphonated polysulphone, Quaternized polysulphone and PTFE on the overall cell performance was estimated using their electrochemical studies such as linear sweep voltammetry (LSV), cyclic voltammetry. 2. Experimental 2.1. Materials Polysulphone (PSu, Mw–35,000), PTFE solution, platinum, iso propyl alcohol, sodium acetate, sodium dihydrogen phosphate, disodium phosphate, ammonium chloride, potassium chloride, magnesium sulphate, calcium c hloride, minerals, vitamins (Sigma Aldrich) were obtained commercially and used as received without further purification. Other
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like
chloroform,
methanol,
triethylamine
(TEA),
5
dimethylformamide,
paraformaldehyde, chlorotrimethylsilane and stannic chloride were purchased from Sisco Research Laboratory Pvt. Ltd, Mumbai, India. Vulcan XC-72 (20% of Platinum in carbon support) and activated carbon were purchased from Arora-Mathey and E-Merck Pvt. Ltd and the carbon cloth was obtained from Cobat carbon Inc. Commercially available AMI-7001 membrane was purchased from Membrane International, Inc., USA. 2.2. Preparation of AEM The polysulphone (PSU) based AEM was prepared as described elsewhere [31] The polysulphone based anion exchange membrane was prepared via three steps: (1) chloromethylation, (2) quaternization and (3) alkalization. The illustration diagram for the preparation of the QPSU is shown in Fig. 1.
Figure 1.Schematic diagram of QPSU synthesis
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2.2.1. Chloromethylation of polysulphone In chloromethylation, 5 g of polysulphone was dissolved in 75 mL of chloroform at 70°C. After dissolving the polymer, a mixture of paraformaldehyde (3.4 g) and chlorotrimethylsilane (14.7) mL was added drop wise with continuous stirring and then catalytic amount of stannous chloride was added into a polymer mixture containing flask at 70°C and maintained for 18 h. Then the polymer was terminated into methanol to remove excess catalyst and paraformaldehyde. Before drying, several times it was washed with deionized water and methanol. 2.2.2. Quaternization of polysulphone The chloromethylated polysulphone was weighed and poured into a round bottomed flask and dissolved required quantity of N, N dimethyl formamide used as the solvent. For quaternization, a required quantity of triethylamine was drop wise added. Then the mixture was allowed at 70°C for 20 h with continuous stirring. Later, the membrane was obtained using solution casting method and dried at 60°C for 24 h under vacuum oven. 2.2.3. Alkalization of polysulphone The quaternized membrane was immersed in a 1.0 M aqueous potassium hydroxide solution at 30°C for 24 h. The same process was repeated thrice for complete ion exchange. Before using, the alkalized membrane was washed with de-ionized water several times and soaked in de-ionized water prior to use. The static and kinetic properties of the membranes were determined and the obtained values are tabulated (Table 1).
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Table 1 Membrane properties Specifications
QPSU
AMI-7001
1.8 × 10-2
1.72 × 10-2
Ion exchange capacity (meq/g)
1.02
1.3
Water uptake (%)
41 ±6
17
Thickness (µm)
30 ±4
450
KO (cm/s)
2.1 × 10-5
0.12 × 10-5
KA (cm2s)
5.1 x 10-8
4.8 x 10-8
Cost $/m2
~150
80
Conductivity (mS/ cm)
2.3. Characterization of membranes 2.3.1. Structural characterization of membranes The structural conformation of PSU, CMPSU and QPSU was identified with an FTIR Alpha Bruker spectrometer in transmittance mode and 1H NMR Bruker Advance 500 MHz multinuclear FT-NMR spectrometer. 2.3.2. Topological characterization of membranes The prepared membranes surface morphology was viewed using Hitachi S-3400N model scanning electron microscope. The surface roughness of the membranes were absorbed using a non-contact profilo meter (Taylor Hobson hardness tester) and the respective average roughness (Ra) values were calculated using Taly Map Platinum 5.1.1.5374 software.
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2.3.3 Electrochemical measurements 2.3.3.1. Cyclic voltammetry In this study, electrochemical behaviour of the respective polymer catalyst binders were studied with cyclic voltammetry (CV) using a potentiostat (PC 4/750), Bio logic, France). CV was performed from -0.4 mV to +0.5 mV at a scan rate of 10 mV/s. The data were logged with a personal computer connected to the potentiostat. The carbon electrode was used as a working electrode loaded with 4.0 mg/cm2 of Platinum. The counter electrode was a platinum wire, and an Ag/AgCl saturated KCl; +197 ± 2 mV, ALS, RE-1B Japan) was used as the reference electrode. All three electrodes were inserted into a test-vial (15-mL) to avoid any contact between the electrodes. 2.3.3.2. Linear Sweep Voltammetry The catalyst binders were analysed using LSV to determine their electrochemical behaviour. The LSV was performed at 10 mV/s on the cathodes at 30°C. The test-vial (15mL) was filled with 13 mL of 200 mM phosphate buffer solution (PBS) (18.304 g L 21 Na2HPO4, 9.808 g L 21 NaH2PO4, 0.13 g L 21 KCl, 0.31 g L 21 NH4Cl, pH = 7) and equipped with a 1 cm2 platinum square counter electrode and an Ag/AgCl saturated KCl; +197 ± 2 mV, ALS, RE-1B Japan). 2.3.4. Physiochemical characterization 2.3.4.1. Conductivity, Ion exchange capacity, water uptake and thickness of membranes A custom-made conductivity cell (0.62 cm2) was equipped with a piece of fully hydrated membrane and A.C. impedance was measured in a galvanostatic mode (Bio logic VSP, France) over a frequency range of 1 MHz to 100 Hz under 10 mV oscillation potential. The ion exchange capacity (IEC) of the membrane was estimated following the procedure
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described elsewhere [27]. The alkalized polymer membrane was then equilibrated with 50 mL of 0.01M HCl aqueous solution for 24 h, followed by back titration method [32]. The IEC of the anion-exchange membrane was calculated based on the formula: IEC = (Mo −Mt)/Wd
(1)
Where Mo is the moles of HCl added originally and Mt is the moles of HCl or equivalent to the moles of potassium hydroxide consumed during back titration and Wd is the weight of the dried membrane. The membrane sample was soaked in de-ionized water for 24 h at 30°C and then the surface water was carefully wiped off with paper. The water uptake was calculated based on the following formula: Water uptake = (Ww −Wd)/Wd ×100%
(2)
Where Ww is the weight of wet membrane and Wd is the weight of dried membrane. AEM was obtained by casting the preweighed, QPSU ionomer dissolved in DMF solution onto a glass plate while keeping the thickness of the membrane in the range of 25-30 µm. 2.3.4.2. Oxygen and Substrate crossover of membranes A two chambered bottle MFC was employed as described earlier [33] to find out the oxygen mass transfer and substrate (sodium acetate) crossover for both the QPSU and AMI 7001 membranes using uninoculated bottle-MFC reactors and nutrient medium. Initially, nitrogen gas was purged through the system to remove oxygen until the DO level (Extech 407510A, Taiwan) remains constant. The mass transfer coefficient of oxygen through the membrane, Ko (cm s-1), was calculated as follows [34]: KO = V ⁄ At ln [(CO-C) ⁄ CO]
(3)
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Where, V is the total volume of the liquid in anode chamber (cm3), A is the crosssectional area of the membrane (cm2), CO is the initial saturated oxygen concentration in anode chamber (mol cm-3), and C is the measured oxygen concentration in cathode chamber (mol cm-3) at time t (s). The diffusion coefficient Do (cm2s-1) was calculated using the formula: DO = KOLt
(4)
Where, Lt is the thickness of the membrane. The contents of both chambers were stirred with a magnetic stir bar during the oxygen transfer experiments. Substrate crossover was also checked in a dual chambered MFC, in which the anode chamber was filled with 50 mM of sodium acetate and the cathode chamber with the same amount of distilled water. The acetate concentration in the chamber was measured with a gas chromatograph (KONIK HRGC - 4000B series) equipped with a flame ionization detector and an EC-1000 capillary column (Alltech, Deerfield, IL, USA). The samples were taken out from the cathode chamber for every 30 min. The tests for acetate transport were conducted with stirring. The C2 is the acetate in the anode chamber at time t. The mass transfer coefficient for acetate was determined using the formula [40] KA= -V ⁄ 2At ln [(CO-2C2) ⁄ CO]
(5)
2.3.5. MFC construction and operation The fabricated MFC consisted of an acrylic cylindrical chamber 4 of cm length and 3 cm diameter (empty bed volume of 28 mL), anode surface area per volume of 12.5 m2/m3 separated by a QPSU anion exchange membrane (a projected area of 12.5 cm2) [31]. The carbon cloth used as anode electrode was made wet proof with 30% PTFE and subsequently coated with carbon bilayer 3 g/cm2 of Vulcan XC 72. The preparation of the carbon bilayer
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slurry includes mixing of 70 wt% Vulcan XC- 72 and 30 wt% PTFE binder solutions with the appropriate amount of double distilled water and isopropyl alcohol, which resulted in a black mixture. The black mixture was first ultra sonicated for 30 min and then coated (brush coating) on carbon cloth and used as both anode and cathode. Finally the cloth was dried in a vacuum oven at 100°C for 2 h and kept in a muffle furnace at 350°C for 30 min. After the preparation of the carbon bilayer, cathode alone was fabricated with carbon supported platinum black of 0.5 mg/cm2 by a similar procedure, where double distilled water and isopropyl alcohol were used with respect to the wt. % of platinum on carbon catalyst [37]. Two MFCs, one with synthesized QPSU membrane and the other with commercially available AEM (AMI-7001) were used. Both MFCs were inoculated with the solution from an anode chamber of three years operated MFC reactor, which was originally inoculated with the domestic wastewater collected from Anna University sewage treatment plant (Chennai, India). The anode was filled with a solution containing specific organic substrate acetate, which was prepared on the basis of COD, at a concentration of 1 g COD/L, in a phosphate buffered nutrient medium (PBM) containing: NH4Cl 0.31 g/L, NaH2PO4.H2O 4.97 g/L, Na2HPO4.H2O 2.75 g/L, KCl 0.13 g/L and minerals and vitamins (each 12.5 mL). The chamber was refilled every time, with the voltage going down below 20 mV at the end of one cycle of operation, until the system reaches the steady state condition. All tests were conducted in a 30°C temperature controlled room. 2.3.6. MFC measurements and calculations The potential was measured using an Ag/AgCl saturated KCl; +197 ± 2 mV, ALS, RE-1B Japan) reference electrode. The voltage, V, between the anode and cathode and also a cathode with respect to the reference electrode were measured using a digital multimeter (Model 702, Metravi, India). The external circuit was completed with a fixed load
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(resistance) of 1000 Ω. A set of variable resistors (1 M Ω-100 Ω) were used to determine the power generation as a function of load. Current (I) and power (P =IV) were calculated as previously described [38]. The Columbic Efficiency (CE) was calculated using the following equation. CE = Cp/CT ×100 %
(6)
Where, Cp is the total Coulombs calculated by integrating the current over time. CT is the theoretical amount of Coulombs that can be produced from acetate and is calculated as; CT = FbSv/ M
(7)
Where, F is Faraday’s constant (96485 C/mol of electrons), b is the number of mol of electrons produced per mol of substrate (acetate) (b=8), S (g/L) the substrate (acetate) concentration, v (mL) the liquid volume and M the molecular weight of the substrate (acetate) (M = 82.3 g) [39]. 3. Results and discussion 3.1. Fourier transform infra-red spectroscopy The functional group of these polymers were investigated by FT-IR. The FT-IR spectra of the PSU, CMPSU and QPSU are shown in Fig. 2. In the spectrum of PSU, the characteristic peak at 3083 cm−1 was due to the aromatic stretch. The sharp peaks at 1594, 1509 and 1590 cm−1 were due to the vibration of the aromatic hydrocarbons. The peak at 1246 cm−1 was assigned to the asymmetric vibration of the ether linkage. The characteristic bands at 1208 and 1020 cm−1 were from the symmetric and the asymmetric stretching vibration of the S=O bond. In the spectrum of CMPSU, the increase in the intensity of the peak at 559 cm−1 might be due to C–Cl stretch. This showed that CMPSU successfully underwent a Friedel-Crafts reaction. The characteristic peak of N-CH2 at 1478 cm−1 and the
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sharp bands at 1395, 1354 and 1314 cm−1 indicate the presence of quaternary ammonium group. The broad and strong absorption band around 3352 cm-1 was assigned to the stretching vibration of O-H.
` Figure 2. FT-IR spectra of (A) PSU, (B) CMPSU and (C) QPSU 3.2. Proton NMR spectroscopy 1H
NMR spectra of PSU, CMPSU and QPSU are shown in Fig. 3. Before
chloromethylation, the pristine PSU showed chemical shifts of (a) 7.27–7.8 (multi Hs on phenyl groups) and (b) 1.73 (CH3). After chloromethylation, the characteristic peak of −CH2Cl corresponding to the newly formed chloromethyl group could be seen at 4.6 ppm, which clearly confirms the CMPSU has been successfully synthesized [26]. While in QPSU appearance of new chemical shift at 2.72 ppm (CH3 linked to N) and disappearance of 4.6 ppm confirmed the formation of the quaternized polysulphone.
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Figure 3. Proton NMR spectra of (A) PSU, (B) CMPSU and (C) QPSU 3.3. Scanning electron microscope Surface morphology of chloromethylated polysulphone (CMPSU) clearly indicates the presence of scattered pores with <1µm. However, these pores were found to be absent in the images of quarternized polysulphone (QPSU) samples that are observed in figure 4-C & 4-D. The uniform and solid dense morphology of QPSU clearly suggests that the disappearance of the membrane voids would be due to the wide distribution of the grafted larger domain of quaternary ammonium groups [40].
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Figure 4. SEM images of (A-B) CMPSU, (C-D) QPSU 3.4. Effect of catalyst binder on oxygen reduction reaction QPSU, SPSU and PTFE ionomers were used as binders for membrane electrode assembly (MEA) preparation, which also acted as a catalyst binder for microbial fuel cell. Fig. 5 (A) shows the analysed curves of CVs of carbon electrodes coated with respective binder containing Pt catalyst. In a potential field, electrons were found to move in and out of the active sites, resulting in redox current signals [41]. The voltammograms in oxygenated PBS solution were conducted at neutral pH and exhibited different current responses. The electrode with QPSU binder showed highest current density of 0.7 J/A cm-2 compared to SPSU (0.42 J/A cm-2) and PTFE (0.45 J/A cm-2). The broad peaks seen in the CV of QPSU might be due to oxygen absorption by the QPSU but the same was predominantly absent in the CV of SPSU and PTFE. The LSV technique is based on the fact that an electrode with different polymer binders produces different current densities according to the nature of the polymer. The observation that, electrodes with SPSU based binder had a lower current density than the
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electrodes with QPSU and PTFE polymer binders, highlights the fact that the structure of the polymer backbone is very important in determining the electrochemical performance of the cathodes (Fig. 5 (B)). By far, sulfonated or quaternary systems are considered as the ideal backbone. The presence of more hydrophilic sites in the QPSU might have allowed more exposure of the reactive sites for oxygen reduction compared to SPSU. In the case of PTFE, albeit it is hydrophobic with a fluorinated backbone, its porous nature might be the reason for more oxygen intake than SPSU for ORR reaction.
Figure 5. Electrochemical analysis of catalyst binders of QPSU, SPSU and PTFE (A) Cyclic voltammetry (B) linear sweep voltammetry 3.5. Power generation during MFC operation synthesized with QPSU and AMI-7001 Two identical single chambered air cathode microbial fuel cells (SCMFCs) with MEA electrodes containing QPSU and AMI-7001were used for this study. Initially, both the anode chambers were inoculated with wastewater from Anna University sewage treatment plant and the system was maintained aerobically for substantial bacterial growth. Later, it was confined anaerobically for culture enrichment. The difference in cell voltage with substrate was measured periodically. The sterile medium voltage was found to be less than 80 mV which
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might be due to biological and chemical factors influencing the cell potential between the two electrodes. When the inoculated anode chambers were subjected anaerobically for electricity generation, both the systems immediately generated an initial circuit voltage in the range of 235 mV on the addition of substrate (sodium acetate). But a gradual decrease in the voltage was noticed in a period of 5 days operation. With substantial addition of substrate after 7 days, the cell potential again rose to a modest 328 mV. A rather high cell voltage of 700 mV was recorded on day 30 after adequate substrate enhancement. This observation indicates that the quantum of circuit voltage as well as the performance of MFC with time is mainly due to the consumption of nutrients from the medium by microbes. The cell voltage remained almost constant (740±20 mV) during the span up to day 62 and thereafter gradual decrease was observed in spite of substrate addition. This clearly discloses the fact that there is a limitation on substrate addition, over period of time. It seems that adding substrates indefinitely has no effect on the microbial metabolism and therefore on the output voltage. On reaching the steady state, the cell voltage, power and current density were measured as a function of external circuit resistance. The circuit was completed with a load of resistance ranging from 1M Ω to 100 Ω and the measurements were recorded for 30 min interval. After the inclusion of resistance, a surprising trend in cell potential was observed, viz, the cell potential increased with external resistance. The reason for this remarkable observation is that at lower resistances oxidation of substrates takes place and at higher resistances microbes release electrons to the anode and finally through the external circuit. The MFC with QPSU generated a cell voltage of 753 mV with a maximum power density of 810 mW/m2, which was higher than that of MFC with AMI-7001 (731 mV and 575 mW/m2). (Fig. 6). CE measured for the two different membranes increased with current density [Equ. (4) and (5)]. The QPSU (69 ± 6%) membrane achieved the highest CE when compared to AMI-7001 (62 ± 3%).
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The ionic conductivity of a membrane is directly influenced by the dimension of the functional groups and absorbed water molecules [42]. An increase in the hydrophilic fraction of the membrane is associated with an expansion of the isolated hydrophilic domain which becomes interconnected upon hydration. This leads to the formation of channels for ion transportation. This is in good agreement with the better performance of the hydrophilic QPSU membrane than the AMI-7001 in the present study.
Figure 6. Power generation during MFC operation for synthesized QPSU and AMI7001 A close analysis on the use of both catalyst binder and ionomer in MEA preparation and their impact on the performance of MFCs with fabricated cathode binders of SPSU, QPSU and PTFE reveals that the cathode with QPSU binder has better performance due to the increased hydrophilicity with the combination of more ionic groups compared to the cathodes with SPSU and PTFE binders. The higher hydrophilicity of QPSU binder might be the reason behind an improved transport of phosphate ions (H2PO42) from the main proton carrier solution of PBS at neutral pH to the reaction sites. Computer simulations also show
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that phosphate ions do not react at the catalytic interface, but rather protons are directly responsible for oxygen reduction [43]. Therefore, it is expected that H2PO42 ions diffuse through the binder polymer to the interface of the reaction zone in the cathode and release protons to replace those consumed by the oxygen reduction reaction (ORR) [44]. The PTFE, a non-ionic, hydrophobic binder with a fluorinated backbone, showed a reasonable higher performance in electrochemical tests as its porous nature might have allowed more oxygen than the SPSU for ORR reaction. In the cathode with sulphonated poly(sulphone) binder with a range of sulfonate content (IEC = 2.31 meq g-1), the occurrence of sulfonate moieties may be the cause behind increased
hydrophilicity, but the higher concentrations of
negatively charged sulfonate groups decreased the electrochemical performance of the cathodes. Because, the ionic concentration gradient at the catalyst interface caused by the presence of sulfonate groups in the binders decreased the cathode performance. The maximum power density obtained for the MFC with a QPSU cathode was 954 mW m-2, whereas the same for MFCs with SPSU and PTFE cathodes were 773 and 805 mW m-2 respectively (Fig. 7 (B)). The increase in the maximum power density might be due to improvement in cathode potentials (Fig.7 (A)), since all the cells showed similar anode potentials. The coulombic efficiency (CE) of the MFC with the QPSU (61%) based cathode was lower than SPSU ( 66%) and PTFE (63%) binders. This trend is seen in few other studies that shows an inter correlation between the cell performance and CE. It seems that an increase in oxygen transfer to the cathode results in higher power density, but lower CE and vice-versa. [45].
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Figure 7. (A) Effect of anode and cathode potential of QPSU, SPSU and PTFE (B) Polarization curves of QPSU, SPSU and PTFE 3.6. Influence of oxygen diffusion on anode performance During MFC operation, Oxygen diffusion from the aerobic cathode to the anaerobic anode chamber is a very significant issue. The voltage loss through a rise in redox potential is due to substrate consumption, which is available for electricity generation or loss by aerobic bacteria [46]. In the experiment, the dissolved oxygen (DO) concentration of AMI 7001 was found to increase from 0.16 to 0.94 mg/L within10 h (Fig.8 A). When compared with QPSU membrane was found to increase from 0.17 to 0.3 mg/L only with the same duration, due to oxygen transfer property of the corresponding membranes. The oxygen mass transfer coefficient (KO) and the oxygen diffusion coefficient (DO) for the AMI 7001 were found to be 0.12 × 10-5 cm/s and 2.4 × 10-7 cm2/s respectively. The MFC with QPSU showed KO = 2.1 × 10-5 cm/s and DO= 4.8× 10-7 cm2/s, which was lower than that of AMI 7001. The increased oxygen cross over contributed to the high substrate loss causing in the low power generation in AMI 7001 [47]. Significantly, QPSU produced higher power density with low substrate loss. Being an avid electron acceptor, oxygen enters into the anode to accept electrons, hence
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decreases the CE of the MFC [48]. It was reported that the CE of membrane-less MFC was less than 20% when compared to MFC with membrane [49]. (A)
(B)
Figure 8. (A) Oxygen crossover, (B) Acetate diffusion through QPSU and AMI-7001 membranes 3.7. Influence of substrate crossover on cathode performance The substrates in wastewater may crossover through the membranes from the anode chamber to the cathode chamber during MFC operation, which is opposite to the trend of oxygen diffusion. The acetate mass transfer coefficient (KA) values of QPSU and AMI-7001 membranes were found to be 5.1 x 10-8 cm/s and 4.8 x 10-8 cm/s, respectively (Fig. 8 B). The increased KA value of QPSU membrane may be attributed to the higher value of ion exchange capacity compared to AEM-7001 membrane. The movement of catholyte with low buffer strength and electrons from anode to cathode always increased the crossover and inherently maintains the charge neutrality between the anode and cathode chamber [50]. AEMs, by the virtue of their anion transfer property, enables carbon based metabolites such as acetates, butyrates and propionates to diffuse across solid membranes at a slower rate [50]. On entering the cathode chamber, substrates get oxidized on the cathode surface, leading to internal short circuit inside the MFC. This ultimately ends up in reduced CE adding extra electrons for the ORR at the cathode [51,52]. Furthermore, the substrate
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crossover promotes the formation of a biofilm by the growth of aerobic bacteria on the cathode [51]. One of the ways to prohibit such undesirable effects is to change, the catholyte solution continuously in the cathode chamber. Besides, the MnCo2O4 based cathode catalyst reduces oxygen to OH- ion through an efficient four electron reduction path. The vantage point is that the OH- ions are transported to anode chamber via anion exchange membrane and combined with H+ to form water. It works like an alkaline fuel cell in which OH- ions are moved from cathode to anode. Hence, the mass transfer from anode to cathode is ruled out [27]. 3.8. Influence of surface roughness on biofilm growth Biofouling depends on the surface topology of the membrane, and usually a rough surface is more vulnerable to the biofilm formation compared to the smooth surface [53]. Fig. 9 (A-B) and (C-D) shows that commercially available AMI-7001 membrane having more surface roughness of 0.82 ±5 µm than prepared QPSU anion exchange membranes of 0.29 ±3 µm. The increased surface roughness represents the peaks and valleys of the AMI7001 membrane surface, which formally results in an augmentation of the area available for microbial settlement. These observations are consistent with previous reports claiming that bacterial adhesion is favoured by surface roughness close to the size of the microbial cells [54]. It was proved that higher surface roughness of AMI 7001 favours membrane biofouling, which deteriorates MFC performance.
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Figure 9. Image of surface roughness of (A-B) QPSU and (C-D) AMI-7001 4. Conclusion The present study deals with the development of an efficient anion exchange membrane and its application in SCMFC. The prepared QPSU membrane improved the performance in terms of their conductivity, water uptake, oxygen and substrate crossover, surface roughness and its effect on both the anode and the cathode functionalities. The impact of specific ionic binders on the overall performance of MFC cathodes along with changes in the cathode materials like fabricated cathode binders of SPSU, QPSU and PTFE were found. Their electrochemical properties and MFC performance in single chamber air-cathode MFCs were measured. The higher hydrophilicity of QPSU binder might have facilitated the improved transport of phosphate ions from the main proton carrier solution of PBS to the reaction sites and subsequently enhanced the overall performance. Acknowledgement The authors would like to thank Department of Science and Technology-Alternate Fuel Technology for funding the project vide the sanction letter No. DST/TSG/AF/2010/09 and
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Department of Mechanical Engineering, Anna University, Chennai, India for providing necessary laboratory facilities. References
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ACCEPTED MANUSCRIPT Highlights Quaternized polysulphone (QPSU) was found as a suitable membrane in MFC QPSU exhibited lower surface roughness QPSU showed good electrical cell performance over the AMI-7001 membrane QPSU not only a suitable membrane but also as an effective binder material for MFC
Elangovan Mahendiravarman, Dharmalingam Sangeetha PII:
S0254-0584(17)30547-3
DOI:
10.1016/j.matchemphys.2017.07.038
Reference:
MAC 19845
To appear in:
Materials Chemistry and Physics
Received Date:
13 June 2016
Revised Date:
23 November 2016
Accepted Date:
11 July 2017
Please cite this article as: Elangovan Mahendiravarman, Dharmalingam Sangeetha, Application of Polysulphone based Anion Exchange Membrane Electrolyte for Improved Electricity Generation in Microbial Fuel Cell, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys. 2017.07.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Application of Polysulphone based Anion Exchange Membrane Electrolyte for Improved Electricity Generation in Microbial Fuel Cell Elangovan Mahendiravarman and Dharmalingam Sangeetha* Department of Mechanical Engineering, Anna University, Chennai-600 025, India *Corresponding author Email: [email protected] Tel No: +91-44-22357763, Fax No: +91-44-22357744 Abstract The present study has been designed on the synthesis and utilization of quaternized polysulphone (QPSU) both as a membrane and a catalyst binder for microbial fuel cells (MFCs). The effect of surface roughness on biofilm growth and the impact of oxygen and specific substrate crossover of the anode and cathode for both membranes was studied. Besides, the effect of specific cathode binders of sulphonated poly (sulphone) (SPSU), Quaternized poly sulphone (QPSU) and Poly tetra fluro ethylene (PTFE) on the overall cell performance was measured using their electrochemical performance studies such as linear sweep voltammetry (LSV), cyclic voltammetry (CV) etc. The performance of synthesized membrane was compared to a commercially obtained anion exchange membrane AMI-7001. Based on the results obtained, QPSU can be considered not only as a suitable membrane alternative to replace commercially available AEM but also as an effective binder material for MFC applications.
Keywords: Anion exchange membrane, Polysulphone, Surface roughness, Ionic binders, Biofilm.
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1. Introduction Microbial fuel cells (MFCs) is an emerging frontline technology in the field of recovering energy from wastewater[1,2]. A Single-chamber, air cathode MFC is more advantageous than a two-chamber MFC in design, construction, performance and passive aeration for oxygen reduction [3]. A Membrane Electrode Assembly (MEA) type MFC delivers more power due to low internal resistance and reduced distance between the two electrodes. As membranes prevent electrodes from touching, short circuit is avoided. Nafion and PTFE are the commonly used catalyst binders that are used to bind platinum over a carbon electrode. In the case of MEA, though Nafion acts as a good sandwich between the membrane and the electrodes, its high cost impedes the commercialization [4]. Although both PTFE and Nafion can bind catalyst particles so as to form a catalyst layer, they cannot conduct hydroxide ions. Hence, these two polymer materials are not a good choice as the binder for preparing MEAs for alkaline membrane fuel cells [5]. In general, Proton Exchange Membranes (PEMs) and Cation Exchange Membranes (CEMs) are used to separate the anode and cathode in MFCs. However, some researchers have found that while using PEMs or CEMs, transport of cations other than protons leads to a reduced pH in the anode chamber which impairs the microorganism activity and at the same time an increased pH in the cathode chamber reduces the cathode potential [6–8]. J-cloth, a highly porous, biodegradable material, is one of the most successfully tried out separators in MFCs. However, this material biodegraded over time [9-11]. Glass fiber mats and nylon filters, which are permeable, economical, non-biodegradable coarse-pore materials are also used. These materials generally deliver more power density than ion exchange membranes. For example, nylon filters with pore sizes from 0.2 µm to 160 µm produce power density ranging from 443 to 908 mW m−2, Zhang et al. [12]. But an increased power density leading to decreased coulombic efficiency (CE) shows an inverse correlation
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between the two parameters. Actually it happens due to oxygen cross over from the aerobic cathode into the anaerobic anode chamber. Eliminating the membrane is one simple solution for this problem, but the coulombic efficiency (CE) may be much lower than that of MFCs with a membrane [13–15]. Another solution is to use Anion Exchange Membranes (AEMs) which are capable of inhibiting the cation transfer [16]. A dual-chamber MFC with an AEM has reached a power density of 610 mW m−2 compared to MFCs with a Nafion membrane (514 mWm−2) and a CEM (480 mWm−2) [17]. The increased power density, perhaps might be a result of the improved activity of microorganisms in the anode chamber due to more slowly decreasing pH [18]. In the AEM configuration the concentration gradients of the different phosphate buffer species of HPO42-, PO43- in anode and cathode are equivalent in concentration [19]. This is an indication for the shuttling of charges through buffer species and then transported through the membrane. This reveals that AEM facing the solution catalysts side produces a higher power density and a lower cathode resistance compared to MFCs with CEM. Recently, there has been a surge in alkaline fuel cell research activities and interest. Many researchers are switching their research outlook from an exclusive focus on AEM and AEMFCs. Progress is being made, research funding for AEMFCs is seeing an upward trend, and more papers on the subject are being published [20, 21, 22, 23]. Commercial anion exchange membranes are generally reinforced with poly (vinyl chloride) for enhanced mechanical stability [24, 25]. It is a normal practice to attach PVC with the ionomeric component of AEM. The physiochemical properties of membrane viz, ion exchange capability and permeability change over long period due to dehydrochlorination of poly (vinyl chloride). The membranes, used once, turn black in colour with a significant loss of rigidity and increased water uptake resulting in enlarged thickness. The polymer itself was damaged and peeled off from the PVC cloth [25]. Such a chemical degradation makes the
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membranes weak and unsuitable for high and long term pH applications. This is common for many commercially available AEMs, such as Tokuyama’s AMX and AM1 membranes that change colour from light yellow to black within an hour of exposure to 1 M NaOH [26]. Hitherto, only a few AEMs have been synthesized and tested successfully as anion exchange membranes for MFCs due to the technical glitches mentioned above [22, 27, 28]. The persistent interest of the membrane scientists for polysulphone is due to its excellent features such as solubility in a wide range of aprotic polar solvents, good film forming nature, high thermal, chemical and mechanical resistance on the entire pH range, sustainability in oxidative medium, moderate reactivity in aromatic electrophilic substitution reactions [29, 30]. In the present study, the authors have reported the synthesis and characterization of QPSU and its performance was compared to the commercial AMI-7001 in a single chambered air cathode MFC. The effect of surface roughness on biofilm growth, influence of oxygen and specific substrate crossover on the anode and cathode were also carried out. Besides, the impact of fabricated cathode binders of sulphonated polysulphone, Quaternized polysulphone and PTFE on the overall cell performance was estimated using their electrochemical studies such as linear sweep voltammetry (LSV), cyclic voltammetry. 2. Experimental 2.1. Materials Polysulphone (PSu, Mw–35,000), PTFE solution, platinum, iso propyl alcohol, sodium acetate, sodium dihydrogen phosphate, disodium phosphate, ammonium chloride, potassium chloride, magnesium sulphate, calcium c hloride, minerals, vitamins (Sigma Aldrich) were obtained commercially and used as received without further purification. Other
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like
chloroform,
methanol,
triethylamine
(TEA),
5
dimethylformamide,
paraformaldehyde, chlorotrimethylsilane and stannic chloride were purchased from Sisco Research Laboratory Pvt. Ltd, Mumbai, India. Vulcan XC-72 (20% of Platinum in carbon support) and activated carbon were purchased from Arora-Mathey and E-Merck Pvt. Ltd and the carbon cloth was obtained from Cobat carbon Inc. Commercially available AMI-7001 membrane was purchased from Membrane International, Inc., USA. 2.2. Preparation of AEM The polysulphone (PSU) based AEM was prepared as described elsewhere [31] The polysulphone based anion exchange membrane was prepared via three steps: (1) chloromethylation, (2) quaternization and (3) alkalization. The illustration diagram for the preparation of the QPSU is shown in Fig. 1.
Figure 1.Schematic diagram of QPSU synthesis
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2.2.1. Chloromethylation of polysulphone In chloromethylation, 5 g of polysulphone was dissolved in 75 mL of chloroform at 70°C. After dissolving the polymer, a mixture of paraformaldehyde (3.4 g) and chlorotrimethylsilane (14.7) mL was added drop wise with continuous stirring and then catalytic amount of stannous chloride was added into a polymer mixture containing flask at 70°C and maintained for 18 h. Then the polymer was terminated into methanol to remove excess catalyst and paraformaldehyde. Before drying, several times it was washed with deionized water and methanol. 2.2.2. Quaternization of polysulphone The chloromethylated polysulphone was weighed and poured into a round bottomed flask and dissolved required quantity of N, N dimethyl formamide used as the solvent. For quaternization, a required quantity of triethylamine was drop wise added. Then the mixture was allowed at 70°C for 20 h with continuous stirring. Later, the membrane was obtained using solution casting method and dried at 60°C for 24 h under vacuum oven. 2.2.3. Alkalization of polysulphone The quaternized membrane was immersed in a 1.0 M aqueous potassium hydroxide solution at 30°C for 24 h. The same process was repeated thrice for complete ion exchange. Before using, the alkalized membrane was washed with de-ionized water several times and soaked in de-ionized water prior to use. The static and kinetic properties of the membranes were determined and the obtained values are tabulated (Table 1).
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Table 1 Membrane properties Specifications
QPSU
AMI-7001
1.8 × 10-2
1.72 × 10-2
Ion exchange capacity (meq/g)
1.02
1.3
Water uptake (%)
41 ±6
17
Thickness (µm)
30 ±4
450
KO (cm/s)
2.1 × 10-5
0.12 × 10-5
KA (cm2s)
5.1 x 10-8
4.8 x 10-8
Cost $/m2
~150
80
Conductivity (mS/ cm)
2.3. Characterization of membranes 2.3.1. Structural characterization of membranes The structural conformation of PSU, CMPSU and QPSU was identified with an FTIR Alpha Bruker spectrometer in transmittance mode and 1H NMR Bruker Advance 500 MHz multinuclear FT-NMR spectrometer. 2.3.2. Topological characterization of membranes The prepared membranes surface morphology was viewed using Hitachi S-3400N model scanning electron microscope. The surface roughness of the membranes were absorbed using a non-contact profilo meter (Taylor Hobson hardness tester) and the respective average roughness (Ra) values were calculated using Taly Map Platinum 5.1.1.5374 software.
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2.3.3 Electrochemical measurements 2.3.3.1. Cyclic voltammetry In this study, electrochemical behaviour of the respective polymer catalyst binders were studied with cyclic voltammetry (CV) using a potentiostat (PC 4/750), Bio logic, France). CV was performed from -0.4 mV to +0.5 mV at a scan rate of 10 mV/s. The data were logged with a personal computer connected to the potentiostat. The carbon electrode was used as a working electrode loaded with 4.0 mg/cm2 of Platinum. The counter electrode was a platinum wire, and an Ag/AgCl saturated KCl; +197 ± 2 mV, ALS, RE-1B Japan) was used as the reference electrode. All three electrodes were inserted into a test-vial (15-mL) to avoid any contact between the electrodes. 2.3.3.2. Linear Sweep Voltammetry The catalyst binders were analysed using LSV to determine their electrochemical behaviour. The LSV was performed at 10 mV/s on the cathodes at 30°C. The test-vial (15mL) was filled with 13 mL of 200 mM phosphate buffer solution (PBS) (18.304 g L 21 Na2HPO4, 9.808 g L 21 NaH2PO4, 0.13 g L 21 KCl, 0.31 g L 21 NH4Cl, pH = 7) and equipped with a 1 cm2 platinum square counter electrode and an Ag/AgCl saturated KCl; +197 ± 2 mV, ALS, RE-1B Japan). 2.3.4. Physiochemical characterization 2.3.4.1. Conductivity, Ion exchange capacity, water uptake and thickness of membranes A custom-made conductivity cell (0.62 cm2) was equipped with a piece of fully hydrated membrane and A.C. impedance was measured in a galvanostatic mode (Bio logic VSP, France) over a frequency range of 1 MHz to 100 Hz under 10 mV oscillation potential. The ion exchange capacity (IEC) of the membrane was estimated following the procedure
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described elsewhere [27]. The alkalized polymer membrane was then equilibrated with 50 mL of 0.01M HCl aqueous solution for 24 h, followed by back titration method [32]. The IEC of the anion-exchange membrane was calculated based on the formula: IEC = (Mo −Mt)/Wd
(1)
Where Mo is the moles of HCl added originally and Mt is the moles of HCl or equivalent to the moles of potassium hydroxide consumed during back titration and Wd is the weight of the dried membrane. The membrane sample was soaked in de-ionized water for 24 h at 30°C and then the surface water was carefully wiped off with paper. The water uptake was calculated based on the following formula: Water uptake = (Ww −Wd)/Wd ×100%
(2)
Where Ww is the weight of wet membrane and Wd is the weight of dried membrane. AEM was obtained by casting the preweighed, QPSU ionomer dissolved in DMF solution onto a glass plate while keeping the thickness of the membrane in the range of 25-30 µm. 2.3.4.2. Oxygen and Substrate crossover of membranes A two chambered bottle MFC was employed as described earlier [33] to find out the oxygen mass transfer and substrate (sodium acetate) crossover for both the QPSU and AMI 7001 membranes using uninoculated bottle-MFC reactors and nutrient medium. Initially, nitrogen gas was purged through the system to remove oxygen until the DO level (Extech 407510A, Taiwan) remains constant. The mass transfer coefficient of oxygen through the membrane, Ko (cm s-1), was calculated as follows [34]: KO = V ⁄ At ln [(CO-C) ⁄ CO]
(3)
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Where, V is the total volume of the liquid in anode chamber (cm3), A is the crosssectional area of the membrane (cm2), CO is the initial saturated oxygen concentration in anode chamber (mol cm-3), and C is the measured oxygen concentration in cathode chamber (mol cm-3) at time t (s). The diffusion coefficient Do (cm2s-1) was calculated using the formula: DO = KOLt
(4)
Where, Lt is the thickness of the membrane. The contents of both chambers were stirred with a magnetic stir bar during the oxygen transfer experiments. Substrate crossover was also checked in a dual chambered MFC, in which the anode chamber was filled with 50 mM of sodium acetate and the cathode chamber with the same amount of distilled water. The acetate concentration in the chamber was measured with a gas chromatograph (KONIK HRGC - 4000B series) equipped with a flame ionization detector and an EC-1000 capillary column (Alltech, Deerfield, IL, USA). The samples were taken out from the cathode chamber for every 30 min. The tests for acetate transport were conducted with stirring. The C2 is the acetate in the anode chamber at time t. The mass transfer coefficient for acetate was determined using the formula [40] KA= -V ⁄ 2At ln [(CO-2C2) ⁄ CO]
(5)
2.3.5. MFC construction and operation The fabricated MFC consisted of an acrylic cylindrical chamber 4 of cm length and 3 cm diameter (empty bed volume of 28 mL), anode surface area per volume of 12.5 m2/m3 separated by a QPSU anion exchange membrane (a projected area of 12.5 cm2) [31]. The carbon cloth used as anode electrode was made wet proof with 30% PTFE and subsequently coated with carbon bilayer 3 g/cm2 of Vulcan XC 72. The preparation of the carbon bilayer
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slurry includes mixing of 70 wt% Vulcan XC- 72 and 30 wt% PTFE binder solutions with the appropriate amount of double distilled water and isopropyl alcohol, which resulted in a black mixture. The black mixture was first ultra sonicated for 30 min and then coated (brush coating) on carbon cloth and used as both anode and cathode. Finally the cloth was dried in a vacuum oven at 100°C for 2 h and kept in a muffle furnace at 350°C for 30 min. After the preparation of the carbon bilayer, cathode alone was fabricated with carbon supported platinum black of 0.5 mg/cm2 by a similar procedure, where double distilled water and isopropyl alcohol were used with respect to the wt. % of platinum on carbon catalyst [37]. Two MFCs, one with synthesized QPSU membrane and the other with commercially available AEM (AMI-7001) were used. Both MFCs were inoculated with the solution from an anode chamber of three years operated MFC reactor, which was originally inoculated with the domestic wastewater collected from Anna University sewage treatment plant (Chennai, India). The anode was filled with a solution containing specific organic substrate acetate, which was prepared on the basis of COD, at a concentration of 1 g COD/L, in a phosphate buffered nutrient medium (PBM) containing: NH4Cl 0.31 g/L, NaH2PO4.H2O 4.97 g/L, Na2HPO4.H2O 2.75 g/L, KCl 0.13 g/L and minerals and vitamins (each 12.5 mL). The chamber was refilled every time, with the voltage going down below 20 mV at the end of one cycle of operation, until the system reaches the steady state condition. All tests were conducted in a 30°C temperature controlled room. 2.3.6. MFC measurements and calculations The potential was measured using an Ag/AgCl saturated KCl; +197 ± 2 mV, ALS, RE-1B Japan) reference electrode. The voltage, V, between the anode and cathode and also a cathode with respect to the reference electrode were measured using a digital multimeter (Model 702, Metravi, India). The external circuit was completed with a fixed load
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(resistance) of 1000 Ω. A set of variable resistors (1 M Ω-100 Ω) were used to determine the power generation as a function of load. Current (I) and power (P =IV) were calculated as previously described [38]. The Columbic Efficiency (CE) was calculated using the following equation. CE = Cp/CT ×100 %
(6)
Where, Cp is the total Coulombs calculated by integrating the current over time. CT is the theoretical amount of Coulombs that can be produced from acetate and is calculated as; CT = FbSv/ M
(7)
Where, F is Faraday’s constant (96485 C/mol of electrons), b is the number of mol of electrons produced per mol of substrate (acetate) (b=8), S (g/L) the substrate (acetate) concentration, v (mL) the liquid volume and M the molecular weight of the substrate (acetate) (M = 82.3 g) [39]. 3. Results and discussion 3.1. Fourier transform infra-red spectroscopy The functional group of these polymers were investigated by FT-IR. The FT-IR spectra of the PSU, CMPSU and QPSU are shown in Fig. 2. In the spectrum of PSU, the characteristic peak at 3083 cm−1 was due to the aromatic stretch. The sharp peaks at 1594, 1509 and 1590 cm−1 were due to the vibration of the aromatic hydrocarbons. The peak at 1246 cm−1 was assigned to the asymmetric vibration of the ether linkage. The characteristic bands at 1208 and 1020 cm−1 were from the symmetric and the asymmetric stretching vibration of the S=O bond. In the spectrum of CMPSU, the increase in the intensity of the peak at 559 cm−1 might be due to C–Cl stretch. This showed that CMPSU successfully underwent a Friedel-Crafts reaction. The characteristic peak of N-CH2 at 1478 cm−1 and the
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sharp bands at 1395, 1354 and 1314 cm−1 indicate the presence of quaternary ammonium group. The broad and strong absorption band around 3352 cm-1 was assigned to the stretching vibration of O-H.
` Figure 2. FT-IR spectra of (A) PSU, (B) CMPSU and (C) QPSU 3.2. Proton NMR spectroscopy 1H
NMR spectra of PSU, CMPSU and QPSU are shown in Fig. 3. Before
chloromethylation, the pristine PSU showed chemical shifts of (a) 7.27–7.8 (multi Hs on phenyl groups) and (b) 1.73 (CH3). After chloromethylation, the characteristic peak of −CH2Cl corresponding to the newly formed chloromethyl group could be seen at 4.6 ppm, which clearly confirms the CMPSU has been successfully synthesized [26]. While in QPSU appearance of new chemical shift at 2.72 ppm (CH3 linked to N) and disappearance of 4.6 ppm confirmed the formation of the quaternized polysulphone.
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Figure 3. Proton NMR spectra of (A) PSU, (B) CMPSU and (C) QPSU 3.3. Scanning electron microscope Surface morphology of chloromethylated polysulphone (CMPSU) clearly indicates the presence of scattered pores with <1µm. However, these pores were found to be absent in the images of quarternized polysulphone (QPSU) samples that are observed in figure 4-C & 4-D. The uniform and solid dense morphology of QPSU clearly suggests that the disappearance of the membrane voids would be due to the wide distribution of the grafted larger domain of quaternary ammonium groups [40].
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Figure 4. SEM images of (A-B) CMPSU, (C-D) QPSU 3.4. Effect of catalyst binder on oxygen reduction reaction QPSU, SPSU and PTFE ionomers were used as binders for membrane electrode assembly (MEA) preparation, which also acted as a catalyst binder for microbial fuel cell. Fig. 5 (A) shows the analysed curves of CVs of carbon electrodes coated with respective binder containing Pt catalyst. In a potential field, electrons were found to move in and out of the active sites, resulting in redox current signals [41]. The voltammograms in oxygenated PBS solution were conducted at neutral pH and exhibited different current responses. The electrode with QPSU binder showed highest current density of 0.7 J/A cm-2 compared to SPSU (0.42 J/A cm-2) and PTFE (0.45 J/A cm-2). The broad peaks seen in the CV of QPSU might be due to oxygen absorption by the QPSU but the same was predominantly absent in the CV of SPSU and PTFE. The LSV technique is based on the fact that an electrode with different polymer binders produces different current densities according to the nature of the polymer. The observation that, electrodes with SPSU based binder had a lower current density than the
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electrodes with QPSU and PTFE polymer binders, highlights the fact that the structure of the polymer backbone is very important in determining the electrochemical performance of the cathodes (Fig. 5 (B)). By far, sulfonated or quaternary systems are considered as the ideal backbone. The presence of more hydrophilic sites in the QPSU might have allowed more exposure of the reactive sites for oxygen reduction compared to SPSU. In the case of PTFE, albeit it is hydrophobic with a fluorinated backbone, its porous nature might be the reason for more oxygen intake than SPSU for ORR reaction.
Figure 5. Electrochemical analysis of catalyst binders of QPSU, SPSU and PTFE (A) Cyclic voltammetry (B) linear sweep voltammetry 3.5. Power generation during MFC operation synthesized with QPSU and AMI-7001 Two identical single chambered air cathode microbial fuel cells (SCMFCs) with MEA electrodes containing QPSU and AMI-7001were used for this study. Initially, both the anode chambers were inoculated with wastewater from Anna University sewage treatment plant and the system was maintained aerobically for substantial bacterial growth. Later, it was confined anaerobically for culture enrichment. The difference in cell voltage with substrate was measured periodically. The sterile medium voltage was found to be less than 80 mV which
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might be due to biological and chemical factors influencing the cell potential between the two electrodes. When the inoculated anode chambers were subjected anaerobically for electricity generation, both the systems immediately generated an initial circuit voltage in the range of 235 mV on the addition of substrate (sodium acetate). But a gradual decrease in the voltage was noticed in a period of 5 days operation. With substantial addition of substrate after 7 days, the cell potential again rose to a modest 328 mV. A rather high cell voltage of 700 mV was recorded on day 30 after adequate substrate enhancement. This observation indicates that the quantum of circuit voltage as well as the performance of MFC with time is mainly due to the consumption of nutrients from the medium by microbes. The cell voltage remained almost constant (740±20 mV) during the span up to day 62 and thereafter gradual decrease was observed in spite of substrate addition. This clearly discloses the fact that there is a limitation on substrate addition, over period of time. It seems that adding substrates indefinitely has no effect on the microbial metabolism and therefore on the output voltage. On reaching the steady state, the cell voltage, power and current density were measured as a function of external circuit resistance. The circuit was completed with a load of resistance ranging from 1M Ω to 100 Ω and the measurements were recorded for 30 min interval. After the inclusion of resistance, a surprising trend in cell potential was observed, viz, the cell potential increased with external resistance. The reason for this remarkable observation is that at lower resistances oxidation of substrates takes place and at higher resistances microbes release electrons to the anode and finally through the external circuit. The MFC with QPSU generated a cell voltage of 753 mV with a maximum power density of 810 mW/m2, which was higher than that of MFC with AMI-7001 (731 mV and 575 mW/m2). (Fig. 6). CE measured for the two different membranes increased with current density [Equ. (4) and (5)]. The QPSU (69 ± 6%) membrane achieved the highest CE when compared to AMI-7001 (62 ± 3%).
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The ionic conductivity of a membrane is directly influenced by the dimension of the functional groups and absorbed water molecules [42]. An increase in the hydrophilic fraction of the membrane is associated with an expansion of the isolated hydrophilic domain which becomes interconnected upon hydration. This leads to the formation of channels for ion transportation. This is in good agreement with the better performance of the hydrophilic QPSU membrane than the AMI-7001 in the present study.
Figure 6. Power generation during MFC operation for synthesized QPSU and AMI7001 A close analysis on the use of both catalyst binder and ionomer in MEA preparation and their impact on the performance of MFCs with fabricated cathode binders of SPSU, QPSU and PTFE reveals that the cathode with QPSU binder has better performance due to the increased hydrophilicity with the combination of more ionic groups compared to the cathodes with SPSU and PTFE binders. The higher hydrophilicity of QPSU binder might be the reason behind an improved transport of phosphate ions (H2PO42) from the main proton carrier solution of PBS at neutral pH to the reaction sites. Computer simulations also show
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that phosphate ions do not react at the catalytic interface, but rather protons are directly responsible for oxygen reduction [43]. Therefore, it is expected that H2PO42 ions diffuse through the binder polymer to the interface of the reaction zone in the cathode and release protons to replace those consumed by the oxygen reduction reaction (ORR) [44]. The PTFE, a non-ionic, hydrophobic binder with a fluorinated backbone, showed a reasonable higher performance in electrochemical tests as its porous nature might have allowed more oxygen than the SPSU for ORR reaction. In the cathode with sulphonated poly(sulphone) binder with a range of sulfonate content (IEC = 2.31 meq g-1), the occurrence of sulfonate moieties may be the cause behind increased
hydrophilicity, but the higher concentrations of
negatively charged sulfonate groups decreased the electrochemical performance of the cathodes. Because, the ionic concentration gradient at the catalyst interface caused by the presence of sulfonate groups in the binders decreased the cathode performance. The maximum power density obtained for the MFC with a QPSU cathode was 954 mW m-2, whereas the same for MFCs with SPSU and PTFE cathodes were 773 and 805 mW m-2 respectively (Fig. 7 (B)). The increase in the maximum power density might be due to improvement in cathode potentials (Fig.7 (A)), since all the cells showed similar anode potentials. The coulombic efficiency (CE) of the MFC with the QPSU (61%) based cathode was lower than SPSU ( 66%) and PTFE (63%) binders. This trend is seen in few other studies that shows an inter correlation between the cell performance and CE. It seems that an increase in oxygen transfer to the cathode results in higher power density, but lower CE and vice-versa. [45].
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Figure 7. (A) Effect of anode and cathode potential of QPSU, SPSU and PTFE (B) Polarization curves of QPSU, SPSU and PTFE 3.6. Influence of oxygen diffusion on anode performance During MFC operation, Oxygen diffusion from the aerobic cathode to the anaerobic anode chamber is a very significant issue. The voltage loss through a rise in redox potential is due to substrate consumption, which is available for electricity generation or loss by aerobic bacteria [46]. In the experiment, the dissolved oxygen (DO) concentration of AMI 7001 was found to increase from 0.16 to 0.94 mg/L within10 h (Fig.8 A). When compared with QPSU membrane was found to increase from 0.17 to 0.3 mg/L only with the same duration, due to oxygen transfer property of the corresponding membranes. The oxygen mass transfer coefficient (KO) and the oxygen diffusion coefficient (DO) for the AMI 7001 were found to be 0.12 × 10-5 cm/s and 2.4 × 10-7 cm2/s respectively. The MFC with QPSU showed KO = 2.1 × 10-5 cm/s and DO= 4.8× 10-7 cm2/s, which was lower than that of AMI 7001. The increased oxygen cross over contributed to the high substrate loss causing in the low power generation in AMI 7001 [47]. Significantly, QPSU produced higher power density with low substrate loss. Being an avid electron acceptor, oxygen enters into the anode to accept electrons, hence
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decreases the CE of the MFC [48]. It was reported that the CE of membrane-less MFC was less than 20% when compared to MFC with membrane [49]. (A)
(B)
Figure 8. (A) Oxygen crossover, (B) Acetate diffusion through QPSU and AMI-7001 membranes 3.7. Influence of substrate crossover on cathode performance The substrates in wastewater may crossover through the membranes from the anode chamber to the cathode chamber during MFC operation, which is opposite to the trend of oxygen diffusion. The acetate mass transfer coefficient (KA) values of QPSU and AMI-7001 membranes were found to be 5.1 x 10-8 cm/s and 4.8 x 10-8 cm/s, respectively (Fig. 8 B). The increased KA value of QPSU membrane may be attributed to the higher value of ion exchange capacity compared to AEM-7001 membrane. The movement of catholyte with low buffer strength and electrons from anode to cathode always increased the crossover and inherently maintains the charge neutrality between the anode and cathode chamber [50]. AEMs, by the virtue of their anion transfer property, enables carbon based metabolites such as acetates, butyrates and propionates to diffuse across solid membranes at a slower rate [50]. On entering the cathode chamber, substrates get oxidized on the cathode surface, leading to internal short circuit inside the MFC. This ultimately ends up in reduced CE adding extra electrons for the ORR at the cathode [51,52]. Furthermore, the substrate
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crossover promotes the formation of a biofilm by the growth of aerobic bacteria on the cathode [51]. One of the ways to prohibit such undesirable effects is to change, the catholyte solution continuously in the cathode chamber. Besides, the MnCo2O4 based cathode catalyst reduces oxygen to OH- ion through an efficient four electron reduction path. The vantage point is that the OH- ions are transported to anode chamber via anion exchange membrane and combined with H+ to form water. It works like an alkaline fuel cell in which OH- ions are moved from cathode to anode. Hence, the mass transfer from anode to cathode is ruled out [27]. 3.8. Influence of surface roughness on biofilm growth Biofouling depends on the surface topology of the membrane, and usually a rough surface is more vulnerable to the biofilm formation compared to the smooth surface [53]. Fig. 9 (A-B) and (C-D) shows that commercially available AMI-7001 membrane having more surface roughness of 0.82 ±5 µm than prepared QPSU anion exchange membranes of 0.29 ±3 µm. The increased surface roughness represents the peaks and valleys of the AMI7001 membrane surface, which formally results in an augmentation of the area available for microbial settlement. These observations are consistent with previous reports claiming that bacterial adhesion is favoured by surface roughness close to the size of the microbial cells [54]. It was proved that higher surface roughness of AMI 7001 favours membrane biofouling, which deteriorates MFC performance.
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Figure 9. Image of surface roughness of (A-B) QPSU and (C-D) AMI-7001 4. Conclusion The present study deals with the development of an efficient anion exchange membrane and its application in SCMFC. The prepared QPSU membrane improved the performance in terms of their conductivity, water uptake, oxygen and substrate crossover, surface roughness and its effect on both the anode and the cathode functionalities. The impact of specific ionic binders on the overall performance of MFC cathodes along with changes in the cathode materials like fabricated cathode binders of SPSU, QPSU and PTFE were found. Their electrochemical properties and MFC performance in single chamber air-cathode MFCs were measured. The higher hydrophilicity of QPSU binder might have facilitated the improved transport of phosphate ions from the main proton carrier solution of PBS to the reaction sites and subsequently enhanced the overall performance. Acknowledgement The authors would like to thank Department of Science and Technology-Alternate Fuel Technology for funding the project vide the sanction letter No. DST/TSG/AF/2010/09 and
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Department of Mechanical Engineering, Anna University, Chennai, India for providing necessary laboratory facilities. References
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ACCEPTED MANUSCRIPT Highlights Quaternized polysulphone (QPSU) was found as a suitable membrane in MFC QPSU exhibited lower surface roughness QPSU showed good electrical cell performance over the AMI-7001 membrane QPSU not only a suitable membrane but also as an effective binder material for MFC