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PES nano composite membranes for microbial fuel cell
European Polymer Journal 99 (2018) 222–229
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Synthesis, characterization and performance evaluation of Fe3O4/PES nano composite membranes for microbial fuel cell
T
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Luca Di Palmaa, Irene Bavassoa, , Fabrizio Sarasinia, Jacopo Tirillòa, Debora Pugliab, Franco Dominicib, Luigi Torreb a b
Department of Chemical Engineering, Materials Environment & UdR INSTM, Sapienza-Università di Roma, Roma, Italy Department of Civil and Environmental Engineering & UdR INSTM, University of Perugia, Terni, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Microbial fuelcell Polymer nanocomposites Magnetite Proton exchange membrane Polyethersulfone
In this study, nanocomposites based on polyethersulfone (PES) with different amounts of Fe3O4 nanoparticles have been synthesized, to be used as proton exchange membranes in a microbial fuel cell (MFC). Such new lowcost separators were fabricated by melt blending and tested in an MFC system. The membranes have been characterized in terms of their mechanical and thermal properties and the results compared to those of commercially available ones (Nafion 117 and CMI 7000). The efficiency of the newly synthesized membranes was assessed in H-type MFC system. Synthetic wastewater using sodium acetate as carbon source was prepared. Total Organic Carbon (TOC) reduction, pH and Open Circuit Voltage (OCV) were daily monitored. Linear Sweep Voltammetry (LSV) was used to optimize the amount of nanoparticles in terms of maximum current and power. The maximum power (9.59 ± 1.18 mW m−2) and current density (38.38 ± 4.73 mA m−2) generation were obtained by using a composite with 20 wt% of nanoparticles. Results of mechanical characterization pointed out that increasing nanoparticles content can compromise the mechanical properties of membranes leading to a significant brittle behavior, while the tensile strength was found to be suitable for durable MFC operations.
1. Introduction Microbial Fuel Cell (MFC) is a bioelectrochemical device able to produce electricity from waste or organic substrates by using microorganisms as biocatalysts [1]. Various configurations of MFCs have been developed, including tubular, double chamber (H-type) and single chamber [2]. The most studied architecture is the H-type configuration that includes an anodic and cathodic compartment both containing an electrode. In the anodic chamber occurs the oxidation reaction while in the cathodic compartment a reduction reaction is promoted by means of oxygen supply [3]. The wastewater is fed in the anodic compartment and to ensure neutral pH condition, a buffer phosphate is used, because the acidic condition in anolyte affects the oxidation activity of bacteria [4]. In this chamber, during oxidation reaction step, the electrons are generated and their transfer from the anode to the cathode is regulated by an external resistance [5]. In cathodic compartment, a buffer solution is generally used along with, if required, another electro acceptor such as ferricyanide [6]. Anode and cathode are separated by a membrane (Proton Exchange Membrane-PEM) that allows proton transfer from the anode to the cathode. Recently, power density and removal
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efficiency (Coulombic efficiency, %) enhancements have been achieved in a single chamber configuration, where the cathode is directly exposed to the air or protected by a diffusion layer to reduce water losses and the internal resistance [7]. Membranes can be simply coupled to the cathode, or the cathode itself is covered by a PEM layer. The membrane is one of the most important and crucial components of the MFCs. It must support protons transfer preventing the transfer of other undesired materials (oxygen and substrates) [8]. From the electrical point of view, the membrane contributes to increase the internal resistance of the system with lower power output due to the low ionic exchange capacity [9]. But, on the other hand, a porous membrane can compromise the efficiency of the system by reducing the Coulombic efficiency and the power density because of oxygen and substrate losses [10]. Different materials are used as PEM. The most common PEM is Nafion (112 and 117) but some problems are associated with its use: oxygen leakage, substrate losses, cation transport and high cost [11]. Nafion performances are strongly correlated to membrane morphology: composite membranes have been proposed and tested to achieve better performances. Bajestani and Mousavi [12] characterized Nafion/TiO2 membranes (1 wt% of TiO2) obtained by solution casting using different
Corresponding author. E-mail address: [email protected] (I. Bavasso).
https://doi.org/10.1016/j.eurpolymj.2017.12.037 Received 11 October 2017; Received in revised form 4 December 2017; Accepted 28 December 2017 Available online 29 December 2017 0014-3057/ © 2017 Elsevier Ltd. All rights reserved.
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(50 rpm screw speed, 3 min mixing time, and temperature profile: 330–340–350 °C), were produced in force control mode with a filmature die in control of temperature fixed at 355 °C. The film of neat PES was produced by setting a counterforce of 2100 N, a speed of the drawoff roller of 300 mm/min and a torque of the take-up roller equal to 35 N mm; to optimize the parameters for the sheets of composites, the counterforce was increased to 2500 N and to 3200 N for formulations with 5 and 20 wt% of magnetite nanoparticles, respectively.
solvents (DMF, DMAc, NMP) and found that DMF was the best solvent, thus leading to the best morphology, the highest porosity, and the maximum proton conductivity. Nowadays much attention has been paid to polymer matrix membranes as replacement of Nafion-based membranes aiming at reducing their cost [13]. The combination with nanoparticles is currently investigated to improve separation performances and increase thermal and mechanical properties of the resulting nanocomposite membranes [14]. Fe3O4 magnetic nanoparticles are commonly used in different fields such as medical [15], bio-analytical and sensor applications [16], water remediation [17] and catalysis [18] due to their magnetic and conductive behavior. Furthermore, such inorganic particles are ecofriendly and can be synthesized by simple and cheap methods [19]. Several authors have synthesized novel membranes based on sulfonated thermoplastic polymers and nanoparticles, such as SiO2 [11] and, especially, Fe3O4 [19,20]. In all these studies, the composite membranes were found to outperform standard Nafion 117 in terms of power density under the same operating conditions. Another common aspect to all these studies is the manufacturing route of the composite membranes, namely solvent casting. Solution processing is the most common method for fabricating polymer nanocomposites mainly because it is both amenable to small sample sizes and effective [21]. The main disadvantage of this process lies in its inherent incompatibility with industrial standards with limited possibilities of scaling up, which is still hindering the commercial applications of MFCs. In an attempt to overcome these issues, in this study we report on the fabrication of nanocomposite membranes by melt blending. This technique consists of blending nanoparticles with the polymer matrix in the molten state, thus in principle leading to a minimization of capital costs because this process is compatible with existing compounding devices such as extruders and mixers. At the same time this process has a much more environmentally friendly character as no solvents are required. In this study Fe3O4/PES nanocomposite membranes were fabricated by melt extrusion and their performances were evaluated in an H-type MFC. The newly developed membranes were characterized in terms of mechanical, thermal, chemical and electrochemical properties and compared to standard commercial membranes, namely Nafion 117 and CMI-7000. In MFC tests, sodium acetate as sole electron donor was used: total organic carbon (TOC) removal over time was measured, and the system was characterized by linear sweep voltammetry (LSV) and open circuit voltage (OCV) measurements.
2.3. Commercial membranes Nafion 117 (Dupont Co., USA) and Ultrex CMI-7000 (gel polystyrene cross linked with divinylbenzene) were used as traditional membranes to benchmark the performances of the newly synthesized membranes. Both membranes were pretreated before their use. Nafion was pretreated by boiling in 0.5 M sulfuric acid (prepared in distilled water) for 1 h three times and then washed with distilled water [19]. CMI-7000 was boiled in a solution of NaCl (5%) for 24 h at 40 °C. The surface area of all membranes used in this work was 8.75 cm2. 2.4. PEM characterization 2.4.1. Water uptake The water uptake (Wut) of the composite membranes was calculated by the difference between the wet (Ww) and dry (Wd) weight at room temperature. For the wet weight, all samples were immersed in deionized water for 24 h. The dry weight was measured after vacuum drying at 100 °C for 12 h. The Wut was calculated using Eq. (1): (1)
Wut (%) = ([Ww−Wd]/ Wd ) × 100
2.4.2. Ion exchange capacity Membrane Ion Exchange Capacity (IEC) was determined by titration method [23]. The samples were soaked for 24 h in 50 mL of sodium chloride solution (1 M) in which H+ of the membranes were replaced by Na+. The protons released were neutralized using sodium hydroxide solution (0.05 N) with phenolphthalein as indicator. The IEC (meq g−1) was calculated using Eq. (2):
IEC = VNaOH (mL) ∗Normality of the titrant (NaOH ) / weight of the dry membrane
(2)
2. Materials and methods 2.4.3. Oxygen permeability The oxygen transfer coefficient (Ko) was determined in an inoculated H-type MFC. Each chamber was fed with distilled water: anodic chamber was supplied by the oxygen sensor (SEVENGO PROMettler Toledo) while the cathodic chamber was continuously aerated. Before starting the analysis, anodic solution was bubbled with nitrogen flow in order to remove the dissolved oxygen in water. Ko (cm s−1) was calculated using Eq. (3):
2.1. Synthesis of Fe3O4 nanoparticles The magnetite nanoparticles were synthesized by co-precipitation, which is an easy and convenient way to synthesize nanoparticles (metal oxides and ferrites) from aqueous salt solutions by the addition of a base under inert atmosphere at room temperature or at elevated temperature. The details of the procedure are fully reported elsewhere [22]. In a typical synthesis, Fe3O4 magnetic nanoparticles were prepared on the basis of co-precipitation of Fe2+ and Fe3+ with a molar ratio of 3:2. In this study, polyethylene glycol was used as a stabilizing agent. Ammonium hydroxide (NH4OH) was added as precipitating agent under a nitrogen atmosphere.
(3)
K o = −V / At ln[(C0−C )/ C0] 2
where V (mL) is the liquid volume in the anodic chamber, A (cm ) is the cross-sectional area of the membrane, t (s) is the time and C0 and C (mg L−1) are the concentration of saturated oxygen in cathodic chamber and the dissolved oxygen concentration at time t, respectively.
2.2. Synthesis of magnetite nanoparticle–PES nanocomposite membranes using melt-blending technique
2.4.4. Membrane resistance measurement-test cell In order to evaluate the electrochemical property of all membranes, an H-Type MFC system was used to carry out the experiment. The membranes were placed between the two cylindrical compartments and hermetically fixed by a clamp. Both chambers were equipped with a Pt electrode and filled with 0.05 M sodium sulphate solution. Galvanostatic tests were conducted using current intensity (I) values in the range 0–100 µA. Potential (E) values were recorded and the internal
Ultrason® E3010 from BASF, an injection moulding and extrusion PES grade, was used as matrix. Two composite formulations with magnetite content of 5 and 20 wt % in sheet form were manufactured using a microextruder DSM Xplore 15 CC Micro Compounder coupled with a DSM Film Device. The molten composite samples, obtained with optimized compounding parameters 223
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Fig. 1. (a) Representative TEM and (b) FESEM micrographs showing nanosized magnetite particles.
2.4.8. Surface roughness The surface roughness of the membranes was investigated using a contact profilometer (Talyscan 150 by Taylor Hobson) and the respective surface roughness (Ra) values were measured using TalyMap software. 2.5. MFC set-up and operation H-type MFC was used to perform all tests. Anode and cathode chambers, in pyrex glass with a volume of 250 mL, were filled with buffered (pH = 7) synthetic solution [24]using sodium acetate (2 g L−1) as unique electron donor and buffer phosphate [25],respectively. Carbon paper electrodes were placed in both chambers and they were connected by a titanium wire closed by a resistor (180 Ω). A reference electrode (Ag/AgCl Crison 5240) was placed in the anodic compartment while an air diffuser was activated in cathodic chamber to enhance reduction reactions.
Fig. 2. Typical stress–strain curves from tensile tests for PES-based nanocomposites and commercial membranes.
2.5.1. Analytical measurements pH (GLP21 Crison) and Open Circuit Voltage (VSP Bio-Logic Potentiostat) were daily monitored and, once reached a stable potential value, Linear Sweep Voltammetry (LSV) was performed in order to evaluate the maximum value of power and current. At the end of each test, the Total Organic Carbon of the solutions used was measured (TOC-L Analyzer- Shimadzu). The Coulombic Efficiency (CE) was calculated using Eq. (5):
resistance (R) was obtained by using Ohm’s law:
R = E /I
(4)
2.4.5. Mechanical characterization of membranes Type 1BA samples (l0 = 30 mm) in accordance with UNI EN ISO 527-2 were used for tensile tests, which were carried out in displacement control using a crosshead speed of 10 mm/min on a Zwick/Roell Z010. The results reported in the work are the average of at least five tests for each material formulation. The measurements were performed at room temperature. The specimens for the mechanical characterization were cut from the films (thickness = 200–350 μm).
CE (%) = M
∫0
t
Idt / FbVanΔCOD
(5)
where M is the molar of the oxygen, I is the current over the time (t), F is the Faraday’s constant, b is the number of electrons exchanged per mole of oxygen (b = 4) and COD identify the Chemical Oxygen Demand of the compounds present in the wastewater [26]. In case of sodium acetate it was observed (data not reported) that the COD/TOC ratio is almost constant and equal to 2.8 ± 0.73. For this reason Eq. (5) was adapted to use TOC values [46].
2.4.6. Morphological characterization The morphology of the synthesized nanoparticles was investigated by field emission scanning electron microscopy (Zeiss, Auriga) and transmission electron microscopy (JEM 2011, Jeol). All specimens were sputter coated with chromium prior to examination.
3. Results and discussion 3.1. Morphology of nanoparticles
2.4.7. Thermal characterization of membranes Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) tests on membranes were performed using a SETSYS Evolution system by Setaram from 25 °C to 800 °C with a heating rate of 10 °C/min under nitrogen flow.
The morphology of the as-synthesized magnetite nanoparticles was investigated by FESEM and TEM, whose representative micrographs are shown in Fig. 1. The powder consisted of pseudo-spherical nanoparticles with a size range of 10–20 nm. From FESEM observations the 224
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Fig. 3. (a) Tensile strength, (b) Young’s modulus and (c) strain at failure for PES-based nanocomposites and commercial membranes.
Fig. 4. (a) Weight loss and (b) DTG curves with temperature of the different membranes.
relatively low temperature). In the present case, the synthesized nanoparticles have been subjected to an XRD analysis [22]. Phase identification of magnetite and maghemite by conventional X-ray diffraction method is not simple because both have the same spinel structure and their lattice parameters are almost identical. However, some authors reported that in the XRD pattern associated with the maghemite phase there are two additional peaks located at angles lower than 30° [29–31]. These intensities can be used to differentiate the magnetite phase. These peaks were not detected in the synthesized nanoparticles and, if some oxidation occurred, it was not significant, also due to the
nanoparticles appear as aggregates in densely packed secondary particles, as found in other studies [27]. These nanoparticles have been already characterized in terms of thermal stability and crystalline phase in a previous publication [22] and were found to exhibit a high thermal stability with a weight loss over the temperature range from 120 °C to 800 °C of about 2% (therefore perfectly compatible with the processing temperatures of PES) and the typical cubic structure of magnetite [28], with no additional crystalline phases identified. It is known that magnetite can be oxidized to maghemite, a process that occurs faster in nanoparticles than in the bulk even in mild oxidizing conditions (at 225
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Fig. 5. Surface roughness images of (a) Nafion 117, (b) CMI-7000, (c) PES0, (d) PES5 and (e) PES20.
presence of the stabilizing agent that provides more stability [32].
Table 1 Water uptake, Ion Exchange Capacity and Oxygen mass transfer of commercial and nanocomposite membranes. Sample
Wut [%]
IEC [meq g−1]
Ko [cm s−1]
3.2. Mechanical and thermal properties of PEMs
Nafion 117 CMI-7000 PES0 PES5 PES20
10.90 ± 1.23 27.51 ± 1.72 1.11 ± 0.27 1.18 ± 0.43 1.59 ± 0.30
0.92 0.07 0.03 0.03 0.01
1.97x 10−4 4.25x 10−4 8.04x 10−4 9.08x 10−4 1.03 x 10−3
The tensile behavior of PES-based composites is shown in Fig. 2compared to that of commercially available membranes, while Fig. 3summarizes the relevant mechanical properties. Significant differences can be noted among the membranes. Nafion exhibited the highest ductility while the PES-based nanocomposites showed a macroscopic brittle behavior, which was worsened by the addition of increasing amount of nanoparticles. This behavior has been frequently observed in composites reinforced with particles [21,33,34] and can be ascribed to the dispersion state of the
± ± ± ± ±
0.08 0.10 0.16 0.13 0.02
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PES with increasing nanoparticle content was detected, with onset degradation temperature that shifted from 565.84 °C to 556.91 °C for neat PES and PES20, respectively. All nanocomposite membranes were found to be much more thermally stable compared to the commercial membranes and different degradation behaviors were pointed out. Nanocomposite membranes were characterized by one degradation step ascribed to chain random scission and carbonization to release SO2from the sulfone group and phenol from the ether group at the temperature of the maximum thermogravimetric loss [36,37]. On the contrary, commercial membranes exhibited a more complex degradation behavior characterized by several weight loss steps. The first one, observed at around 100–140 °C and common to all the tested membranes, is related to the desorption of water bonded to the sulfonic groups, which was almost negligible in PES-based membranes [38], as confirmed by water uptake measurements. The Nafion 117 membrane showed 3 decomposition stages: the first stage (290–400 °C) may be associated with a desulfonation process, while the second stage (400–460 °C) may be related to side-chain decomposition and the third stage (460–550 °C) to PTFE backbone decomposition [39]. Also for CMI-7000 multiple stages of thermal degradation were observed, related to the decomposition of the perfluoroether side chains and to the degradation of the main chain at T > 550 °C [40]. Fig. 6. (a) OCV trends in commercial and nanocomposite membranes using sodium acetate (2 g L-1): (♦) neat PES, (□) PES5, (▴) PES20, (○) CMI-7000 and (■) Nafion 117; (b) Total Organic Carbon removal% (grey) and Columbic Efficiency% (black) after 15 days of treatment.
3.3. Surface roughness Surface roughness images are reported in Fig. 5 for all the membranes investigated in the present study. The average values of surface roughness (Ra), for Nafion 117, CMI-7000 and PES-based membranes were found to be equal to 0.016, 0.216, 0.017, 0.129 and 1.215 μm. It is clear that the roughness of PES-based membranes increased significantly with increasing magnetite particle content, as observed in other studies [19,20], with the lowest values showed by neat PES and Nafion 117. Surface roughness is a very important property for membranes because it directly influences the fouling tendency, which is reported to increase with roughness, thus retarding the performance of the membrane [19]. At the same time, the formation of a thin biofilm on the surface can reduce the oxygen crossover from the cathodic chamber to the anodic one that increases the anodic aerobic environment, thus resulting in a better efficiency of the MFC [20].
nanoparticles inside the polymer matrix. In the case when relatively large agglomerates would remain in the matrix, a propagating crack could meet a local stress concentration and then easily induce the onset of final failure. Obviously, embrittlement effects are more likely to occur at higher filler contents, where more agglomerates can be found. It is suggested that in the present study the shear forces acting in the molten state were not able to completely disentangle the clustered magnetite nanoparticles, especially at 20 wt%. The tensile strength of nanocomposites decreased, compared to the neat polymer, with increasing nanoparticle content due to imperfect dispersion and poor load transfer, which was not optimized in the present study. In fact, system requirements for mechanical reinforcement include large aspect ratio (rule of mixtures, Halpin-Tsai etc.) and a good dispersion to allow for efficient load transfer to the nanofillers and for a more uniform stress distribution and minimization of stress concentration centers. A decrease in nanofiller aspect ratio is to expected with increasing agglomeration. Despite these negative effects with increasing filler amount, all the synthesized nanocomposite membranes exhibited higher tensile strength compared to the commercial membranes, in addition to a significantly higher Young’s modulus due to the presence of nanoparticles that hindered the macromolecular mobility. Therefore it can be concluded that the nanocomposite membranes have enough strength to be used in MFC, which is particularly useful if long durations are needed, as deformation due to the formation of thick biofilms on the membrane surface can affect its efficiency [35]. With regard to the thermal stability of the different formulations, TGA curves and derivative thermograms (DTG) under nitrogen atmosphere are reported in Fig. 4. A slight decrease in thermal stability of
3.4. Ion exchange capacity, oxygen crossover and water uptake In Table 1 are summarized the values of water uptake, IEC and oxygen transfer coefficient for all membranes investigated in this work. Water uptake is another crucial characteristic of a membrane because it enhances the proton conductivity of the membrane [41]. Water uptake of PES-based membranes gradually increased with increasing content of Fe3O4 nanoparticles, even if it was lower than that exhibited by commercial membranes, thus confirming the results from TGA analysis. A possible way to improve water uptake is to apply a suitable pretreatment that can allow for up to 20% increase [8]. Pretreatment in general is not suitable for enhancing IEC [42], which in turn can be positively affected by sulfonation treatments, as confirmed by other studies [41,43]. In the present study, IEC values for nanocomposite membranes were lower than those of commercial membranes due to the lack of
Table 2 Electrochemical performances of MFC using commercial and nanocomposite membranes. Sample
Pmax [mW m−2]
Imax [mA m−2]
OCVmax [mV]
Membrane resistance [kΩ]
Nafion 117 CMI-7000 PES0 PES5 PES20
17.68 ± 0.61 12.58 ± 1.22 0.08 ± 0.01 1.66 ± 0.21 9.59 ± 1.18
69.27 ± 2.38 49.30 ± 4.78 1.12 ± 0.04 6.65 ± 0.86 38.38 ± 4.73
612.50 580.00 139.50 485.00 552.50
5.79 ± 0.19 6.13 ± 0.28 72.93 ± 2.02 46.28 ± 2.91 8.88 ± 0.11
227
± ± ± ± ±
12.50 32.00 2.50 15.00 29.50
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nanoparticles) without pretreatment, which showed results close to those obtained with commercial membranes: the OCV values of 485.00 mV, 552.50 mV, 580.00 mV and 612.50 mV respectively for PES5 (with 5 wt% of Fe3O4 nanoparticles), PES20 (with 20 wt% of Fe3O4 nanoparticles), CMI-7000 and Nafion 117 were obtained. A TOC removal of 75% was measured in all tests after 15 d, and a CE % = 11.36% using the PES20 membrane was calculated with a power and current density of about 9.59 ± 1.18 mW m−2and 38.38 ± 4.73 mA m−2, respectively. In addition, the nanocomposite membranes exhibited better thermal stability and mechanical properties compared to commercial membranes, thus suggesting their use for extended periods. However, a further increase in nanoparticle content beyond 20 wt% could compromise the operation of the MFC, since the anaerobic condition in the anodic chamber could not be ensured due to the increase of the oxygen transfer coefficient. The highest Ko (1.03 × 10−3 cm s−1) was in fact observed for PES20. Therefore, PES20 could be considered a good compromise between electrochemical and chemical performances. The PES-Fe3O4 system offers several possibilities to be improved in terms of water uptake and ionic exchange capacity by tailoring the degree of sulfonation of the polymer matrix and the surface functionality of the magnetite nanoparticles.
sulfonic acid groups in magnetite nanoparticles. This suggests the need to tailor the surface functionality of magnetite nanoparticles [44] in order to increase water uptake and IEC. Agglomeration of nanoparticles can be the cause of the decrease of IEC value at the highest nanoparticle content [11]. Amarked difference in oxygen transfer coefficient was observed among the membranes. Upon increasing the magnetite content, the PES-based membranes showed higher oxygen mass transfer, likely due to enhanced void space in membranes, generated due to the inorganic (magnetite) and organic (polymer) inter-space [45]. With regard to the potential application of these membranes in MFC systems, an even higher magnetite concentration can negatively affect the electrochemical performances: anaerobic condition in the anodic chamber cannot be possible if the membrane allows for the transfer of the oxygen that acts as the electron acceptor during the degradation of the organic matter [25]. 3.5. MFC performance OCV was daily recorded and the trends obtained in all tests were the same: OCV gradually increased and after 6 days a stable potential was recorded in all tests (Fig. 6a). In table 2 the electrochemical parameters measured in all tests are summarized. As expected, the commercial membranes showed the best results and neat PES (PES0) cannot be compared with the other membranes: the absence of the nanoparticle affects the electrochemical performances of the MFC and an OCV equal to 139.50 mV was recorded. The nanocomposite membranes exhibited better performances with increasing magnetite content. In particular, with PES20 membrane, the maximum OCV reached a value similar to that observed with commercial membranes. Pmax and Imax values were lower than those obtained with commercial membranes but improvements were observed on the electrochemical performances of the cell with increasing Fe3O4 content up to 20 wt%. This can be explained considering membrane resistances. While commercial membranes were characterized by resistances in the range of 5.79–6.13 kΩ in the adopted conditions, PES0 exhibited a value of 72.93 kΩ. A resistance reduction of 36% was observed by using PES5 and a further reduction of 80% was reached passing from 5 wt% to 20 wt% of Fe3O4. This suggests that the performances of the membrane in terms of internal resistance can be improved by an extra addition of nanoparticles and 20 wt% of Fe3O4 can be considered sufficient to avoid a membrane resistance comparable with that of the commercial ones used in this work. In Fig. 6b the treatment efficiency in terms of mineralization of the organic matter using the H-Type MFC system with different membranes is reported. A TOC removal of about 75% after 15 days of treatment was observed in all tests. As expected, MFC system with commercial membranes exhibited the highest values of CE%, however nanocomposite membranes with 20 wt% of Fe3O4 showed promising results with a CE% of 11.36%. Further studies devoted to the evaluation of a possible pretreatment effect are required, so as to improve electrochemical performances of the MFC without increasing the content of nanoparticles of the membrane or even trying to reduce it.
Acknowledgements The authors gratefully thank Dr. Mamadou Traore from LSMP-CNRS UPR 3407 Institut Galilée-Université Paris 13 for performing TEM analysis. References [1] A. Sivasankaran, D. Sangeetha, Influence of sulfonated SiO2 in sulfonated polyether ether ketone nanocomposite membrane in microbial fuel cell, Fuel 159 (2015) 689–696. [2] Z. Du, H. Li, T. Gu, A state of the art review on microbial fuel cells: a promising technology for wastewater treatment and bioenergy, Biotechnol. Adv. 25 (2007) 464–482. [3] H. Wang, J. Park, Z.J. Ren, Practical energy harvesting for microbial fuel cells: a review, Environ. Sci. Technol. 49 (2015) 3267–3277. [4] G.S. Jadhav, M.M. Ghangrekar, Performance of microbial fuel cell subjected to variation in pH, temperature, external load and substrate concentration, Bioresour. Technol. 100 (2009) 717–723. [5] A. Modestra, Microbial fuel cell: critical factors regulating bio-catalyzed electrochemical process and recent advancements, Renew. Sustain. Energy Rev. 40 (2015) 779–797. [6] B. Min, B.E. Logan, Continuous electricity generation from domestic wastewater and organic substrates in a flat plate microbial fuel cell, Environ. Sci. Technol. 38 (2004) 5809–5814. [7] S. Cheng, H. Liu, B.E. Logan, Increased performance of single-chamber microbial fuel cells using an improved cathode structure, Electrochem. Commun. 8 (2006) 489–494. [8] M. Ghasemi, S. Shahgaldi, M. Ismail, Z. Yaakob, W.R.W. Daud, New generation of carbon nanocomposite proton exchange membranes in microbial fuel cell systems, Chem. Eng. J. 184 (2012) 82–89. [9] Y. Fan, E. Sharbrough, H. Liu, Quantification of the internal resistance distribution of microbial fuel cells, Environ. Sci. Technol. 42 (2008) 8101–8107. [10] J. Xing, W. Ramli, W. Daud, M. Ghasemi, K. Ben, M. Ismail, Xing Ion exchange membranes as separators in microbial fuel cells for bioenergy conversion: a comprehensive review, Renew. Sustain. Energy Rev. 28 (2013) (2013) 575–587. [11] A. Sivasankaran, D. Sangeetha, Y.-H. Ahn, Nanocomposite membranes based on sulfonated polystyrene ethylene butylene polystyrene (SSEBS) and sulfonated SiO2 for microbial fuel cell application, Chem. Eng. J. 289 (2016) 442–451. [12] M. Bazrgar, S.A. Mousavi, Effect of casting solvent on the characteristics of Nafion / TiO2 nanocomposite membranes for microbial fuel cell application, Int. J. Hydrogen Energy 41 (2015) 476–482. [13] M. Rikukawa, K. Sanui, Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers, Prog. Polym. Sci. 25 (2000) 1463–1502. [14] G.L. Jadav, P.S. Singh, Synthesis of novel silica-polyamide nanocomposite membrane with enhanced properties, J. Memb. Sci. 328 (2009) 257–267. [15] R.A. Revia, M. Zhang, Magnetite nanoparticles for cancer diagnosis, treatment, and treatment monitoring: recent advances, Mater. Today (Kidlington) 19 (2016) 157–168. [16] C. Fenzl, T. Hirsch, A.J. Baeumner, Nanomaterials as versatile tools for signal amplification in (bio)analytical applications, TrAC – Trends Anal. Chem. 79 (2016) 306–316.
4. Conclusions Fe3O4/PES nanocomposite membranes were fabricated by melt extrusion and their performances as proton exchange membranes were evaluated in an H-type microbial fuel cell. Different amounts of nanoparticles (5 wt% and 20 wt%) were tested, aiming at improving the performances of MFC such as power, current density and OCV. The results of this study indicate that nanocomposite PES-Fe3O4 membranes are promising materials for MFC application. The increasing Fe3O4 nanoparticles content resulted in an improvement of electrochemical performances especially for PES20 (with 20 wt% of 228
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Synthesis, characterization and performance evaluation of Fe3O4/PES nano composite membranes for microbial fuel cell
T
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Luca Di Palmaa, Irene Bavassoa, , Fabrizio Sarasinia, Jacopo Tirillòa, Debora Pugliab, Franco Dominicib, Luigi Torreb a b
Department of Chemical Engineering, Materials Environment & UdR INSTM, Sapienza-Università di Roma, Roma, Italy Department of Civil and Environmental Engineering & UdR INSTM, University of Perugia, Terni, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords: Microbial fuelcell Polymer nanocomposites Magnetite Proton exchange membrane Polyethersulfone
In this study, nanocomposites based on polyethersulfone (PES) with different amounts of Fe3O4 nanoparticles have been synthesized, to be used as proton exchange membranes in a microbial fuel cell (MFC). Such new lowcost separators were fabricated by melt blending and tested in an MFC system. The membranes have been characterized in terms of their mechanical and thermal properties and the results compared to those of commercially available ones (Nafion 117 and CMI 7000). The efficiency of the newly synthesized membranes was assessed in H-type MFC system. Synthetic wastewater using sodium acetate as carbon source was prepared. Total Organic Carbon (TOC) reduction, pH and Open Circuit Voltage (OCV) were daily monitored. Linear Sweep Voltammetry (LSV) was used to optimize the amount of nanoparticles in terms of maximum current and power. The maximum power (9.59 ± 1.18 mW m−2) and current density (38.38 ± 4.73 mA m−2) generation were obtained by using a composite with 20 wt% of nanoparticles. Results of mechanical characterization pointed out that increasing nanoparticles content can compromise the mechanical properties of membranes leading to a significant brittle behavior, while the tensile strength was found to be suitable for durable MFC operations.
1. Introduction Microbial Fuel Cell (MFC) is a bioelectrochemical device able to produce electricity from waste or organic substrates by using microorganisms as biocatalysts [1]. Various configurations of MFCs have been developed, including tubular, double chamber (H-type) and single chamber [2]. The most studied architecture is the H-type configuration that includes an anodic and cathodic compartment both containing an electrode. In the anodic chamber occurs the oxidation reaction while in the cathodic compartment a reduction reaction is promoted by means of oxygen supply [3]. The wastewater is fed in the anodic compartment and to ensure neutral pH condition, a buffer phosphate is used, because the acidic condition in anolyte affects the oxidation activity of bacteria [4]. In this chamber, during oxidation reaction step, the electrons are generated and their transfer from the anode to the cathode is regulated by an external resistance [5]. In cathodic compartment, a buffer solution is generally used along with, if required, another electro acceptor such as ferricyanide [6]. Anode and cathode are separated by a membrane (Proton Exchange Membrane-PEM) that allows proton transfer from the anode to the cathode. Recently, power density and removal
⁎
efficiency (Coulombic efficiency, %) enhancements have been achieved in a single chamber configuration, where the cathode is directly exposed to the air or protected by a diffusion layer to reduce water losses and the internal resistance [7]. Membranes can be simply coupled to the cathode, or the cathode itself is covered by a PEM layer. The membrane is one of the most important and crucial components of the MFCs. It must support protons transfer preventing the transfer of other undesired materials (oxygen and substrates) [8]. From the electrical point of view, the membrane contributes to increase the internal resistance of the system with lower power output due to the low ionic exchange capacity [9]. But, on the other hand, a porous membrane can compromise the efficiency of the system by reducing the Coulombic efficiency and the power density because of oxygen and substrate losses [10]. Different materials are used as PEM. The most common PEM is Nafion (112 and 117) but some problems are associated with its use: oxygen leakage, substrate losses, cation transport and high cost [11]. Nafion performances are strongly correlated to membrane morphology: composite membranes have been proposed and tested to achieve better performances. Bajestani and Mousavi [12] characterized Nafion/TiO2 membranes (1 wt% of TiO2) obtained by solution casting using different
Corresponding author. E-mail address: [email protected] (I. Bavasso).
https://doi.org/10.1016/j.eurpolymj.2017.12.037 Received 11 October 2017; Received in revised form 4 December 2017; Accepted 28 December 2017 Available online 29 December 2017 0014-3057/ © 2017 Elsevier Ltd. All rights reserved.
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(50 rpm screw speed, 3 min mixing time, and temperature profile: 330–340–350 °C), were produced in force control mode with a filmature die in control of temperature fixed at 355 °C. The film of neat PES was produced by setting a counterforce of 2100 N, a speed of the drawoff roller of 300 mm/min and a torque of the take-up roller equal to 35 N mm; to optimize the parameters for the sheets of composites, the counterforce was increased to 2500 N and to 3200 N for formulations with 5 and 20 wt% of magnetite nanoparticles, respectively.
solvents (DMF, DMAc, NMP) and found that DMF was the best solvent, thus leading to the best morphology, the highest porosity, and the maximum proton conductivity. Nowadays much attention has been paid to polymer matrix membranes as replacement of Nafion-based membranes aiming at reducing their cost [13]. The combination with nanoparticles is currently investigated to improve separation performances and increase thermal and mechanical properties of the resulting nanocomposite membranes [14]. Fe3O4 magnetic nanoparticles are commonly used in different fields such as medical [15], bio-analytical and sensor applications [16], water remediation [17] and catalysis [18] due to their magnetic and conductive behavior. Furthermore, such inorganic particles are ecofriendly and can be synthesized by simple and cheap methods [19]. Several authors have synthesized novel membranes based on sulfonated thermoplastic polymers and nanoparticles, such as SiO2 [11] and, especially, Fe3O4 [19,20]. In all these studies, the composite membranes were found to outperform standard Nafion 117 in terms of power density under the same operating conditions. Another common aspect to all these studies is the manufacturing route of the composite membranes, namely solvent casting. Solution processing is the most common method for fabricating polymer nanocomposites mainly because it is both amenable to small sample sizes and effective [21]. The main disadvantage of this process lies in its inherent incompatibility with industrial standards with limited possibilities of scaling up, which is still hindering the commercial applications of MFCs. In an attempt to overcome these issues, in this study we report on the fabrication of nanocomposite membranes by melt blending. This technique consists of blending nanoparticles with the polymer matrix in the molten state, thus in principle leading to a minimization of capital costs because this process is compatible with existing compounding devices such as extruders and mixers. At the same time this process has a much more environmentally friendly character as no solvents are required. In this study Fe3O4/PES nanocomposite membranes were fabricated by melt extrusion and their performances were evaluated in an H-type MFC. The newly developed membranes were characterized in terms of mechanical, thermal, chemical and electrochemical properties and compared to standard commercial membranes, namely Nafion 117 and CMI-7000. In MFC tests, sodium acetate as sole electron donor was used: total organic carbon (TOC) removal over time was measured, and the system was characterized by linear sweep voltammetry (LSV) and open circuit voltage (OCV) measurements.
2.3. Commercial membranes Nafion 117 (Dupont Co., USA) and Ultrex CMI-7000 (gel polystyrene cross linked with divinylbenzene) were used as traditional membranes to benchmark the performances of the newly synthesized membranes. Both membranes were pretreated before their use. Nafion was pretreated by boiling in 0.5 M sulfuric acid (prepared in distilled water) for 1 h three times and then washed with distilled water [19]. CMI-7000 was boiled in a solution of NaCl (5%) for 24 h at 40 °C. The surface area of all membranes used in this work was 8.75 cm2. 2.4. PEM characterization 2.4.1. Water uptake The water uptake (Wut) of the composite membranes was calculated by the difference between the wet (Ww) and dry (Wd) weight at room temperature. For the wet weight, all samples were immersed in deionized water for 24 h. The dry weight was measured after vacuum drying at 100 °C for 12 h. The Wut was calculated using Eq. (1): (1)
Wut (%) = ([Ww−Wd]/ Wd ) × 100
2.4.2. Ion exchange capacity Membrane Ion Exchange Capacity (IEC) was determined by titration method [23]. The samples were soaked for 24 h in 50 mL of sodium chloride solution (1 M) in which H+ of the membranes were replaced by Na+. The protons released were neutralized using sodium hydroxide solution (0.05 N) with phenolphthalein as indicator. The IEC (meq g−1) was calculated using Eq. (2):
IEC = VNaOH (mL) ∗Normality of the titrant (NaOH ) / weight of the dry membrane
(2)
2. Materials and methods 2.4.3. Oxygen permeability The oxygen transfer coefficient (Ko) was determined in an inoculated H-type MFC. Each chamber was fed with distilled water: anodic chamber was supplied by the oxygen sensor (SEVENGO PROMettler Toledo) while the cathodic chamber was continuously aerated. Before starting the analysis, anodic solution was bubbled with nitrogen flow in order to remove the dissolved oxygen in water. Ko (cm s−1) was calculated using Eq. (3):
2.1. Synthesis of Fe3O4 nanoparticles The magnetite nanoparticles were synthesized by co-precipitation, which is an easy and convenient way to synthesize nanoparticles (metal oxides and ferrites) from aqueous salt solutions by the addition of a base under inert atmosphere at room temperature or at elevated temperature. The details of the procedure are fully reported elsewhere [22]. In a typical synthesis, Fe3O4 magnetic nanoparticles were prepared on the basis of co-precipitation of Fe2+ and Fe3+ with a molar ratio of 3:2. In this study, polyethylene glycol was used as a stabilizing agent. Ammonium hydroxide (NH4OH) was added as precipitating agent under a nitrogen atmosphere.
(3)
K o = −V / At ln[(C0−C )/ C0] 2
where V (mL) is the liquid volume in the anodic chamber, A (cm ) is the cross-sectional area of the membrane, t (s) is the time and C0 and C (mg L−1) are the concentration of saturated oxygen in cathodic chamber and the dissolved oxygen concentration at time t, respectively.
2.2. Synthesis of magnetite nanoparticle–PES nanocomposite membranes using melt-blending technique
2.4.4. Membrane resistance measurement-test cell In order to evaluate the electrochemical property of all membranes, an H-Type MFC system was used to carry out the experiment. The membranes were placed between the two cylindrical compartments and hermetically fixed by a clamp. Both chambers were equipped with a Pt electrode and filled with 0.05 M sodium sulphate solution. Galvanostatic tests were conducted using current intensity (I) values in the range 0–100 µA. Potential (E) values were recorded and the internal
Ultrason® E3010 from BASF, an injection moulding and extrusion PES grade, was used as matrix. Two composite formulations with magnetite content of 5 and 20 wt % in sheet form were manufactured using a microextruder DSM Xplore 15 CC Micro Compounder coupled with a DSM Film Device. The molten composite samples, obtained with optimized compounding parameters 223
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Fig. 1. (a) Representative TEM and (b) FESEM micrographs showing nanosized magnetite particles.
2.4.8. Surface roughness The surface roughness of the membranes was investigated using a contact profilometer (Talyscan 150 by Taylor Hobson) and the respective surface roughness (Ra) values were measured using TalyMap software. 2.5. MFC set-up and operation H-type MFC was used to perform all tests. Anode and cathode chambers, in pyrex glass with a volume of 250 mL, were filled with buffered (pH = 7) synthetic solution [24]using sodium acetate (2 g L−1) as unique electron donor and buffer phosphate [25],respectively. Carbon paper electrodes were placed in both chambers and they were connected by a titanium wire closed by a resistor (180 Ω). A reference electrode (Ag/AgCl Crison 5240) was placed in the anodic compartment while an air diffuser was activated in cathodic chamber to enhance reduction reactions.
Fig. 2. Typical stress–strain curves from tensile tests for PES-based nanocomposites and commercial membranes.
2.5.1. Analytical measurements pH (GLP21 Crison) and Open Circuit Voltage (VSP Bio-Logic Potentiostat) were daily monitored and, once reached a stable potential value, Linear Sweep Voltammetry (LSV) was performed in order to evaluate the maximum value of power and current. At the end of each test, the Total Organic Carbon of the solutions used was measured (TOC-L Analyzer- Shimadzu). The Coulombic Efficiency (CE) was calculated using Eq. (5):
resistance (R) was obtained by using Ohm’s law:
R = E /I
(4)
2.4.5. Mechanical characterization of membranes Type 1BA samples (l0 = 30 mm) in accordance with UNI EN ISO 527-2 were used for tensile tests, which were carried out in displacement control using a crosshead speed of 10 mm/min on a Zwick/Roell Z010. The results reported in the work are the average of at least five tests for each material formulation. The measurements were performed at room temperature. The specimens for the mechanical characterization were cut from the films (thickness = 200–350 μm).
CE (%) = M
∫0
t
Idt / FbVanΔCOD
(5)
where M is the molar of the oxygen, I is the current over the time (t), F is the Faraday’s constant, b is the number of electrons exchanged per mole of oxygen (b = 4) and COD identify the Chemical Oxygen Demand of the compounds present in the wastewater [26]. In case of sodium acetate it was observed (data not reported) that the COD/TOC ratio is almost constant and equal to 2.8 ± 0.73. For this reason Eq. (5) was adapted to use TOC values [46].
2.4.6. Morphological characterization The morphology of the synthesized nanoparticles was investigated by field emission scanning electron microscopy (Zeiss, Auriga) and transmission electron microscopy (JEM 2011, Jeol). All specimens were sputter coated with chromium prior to examination.
3. Results and discussion 3.1. Morphology of nanoparticles
2.4.7. Thermal characterization of membranes Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) tests on membranes were performed using a SETSYS Evolution system by Setaram from 25 °C to 800 °C with a heating rate of 10 °C/min under nitrogen flow.
The morphology of the as-synthesized magnetite nanoparticles was investigated by FESEM and TEM, whose representative micrographs are shown in Fig. 1. The powder consisted of pseudo-spherical nanoparticles with a size range of 10–20 nm. From FESEM observations the 224
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Fig. 3. (a) Tensile strength, (b) Young’s modulus and (c) strain at failure for PES-based nanocomposites and commercial membranes.
Fig. 4. (a) Weight loss and (b) DTG curves with temperature of the different membranes.
relatively low temperature). In the present case, the synthesized nanoparticles have been subjected to an XRD analysis [22]. Phase identification of magnetite and maghemite by conventional X-ray diffraction method is not simple because both have the same spinel structure and their lattice parameters are almost identical. However, some authors reported that in the XRD pattern associated with the maghemite phase there are two additional peaks located at angles lower than 30° [29–31]. These intensities can be used to differentiate the magnetite phase. These peaks were not detected in the synthesized nanoparticles and, if some oxidation occurred, it was not significant, also due to the
nanoparticles appear as aggregates in densely packed secondary particles, as found in other studies [27]. These nanoparticles have been already characterized in terms of thermal stability and crystalline phase in a previous publication [22] and were found to exhibit a high thermal stability with a weight loss over the temperature range from 120 °C to 800 °C of about 2% (therefore perfectly compatible with the processing temperatures of PES) and the typical cubic structure of magnetite [28], with no additional crystalline phases identified. It is known that magnetite can be oxidized to maghemite, a process that occurs faster in nanoparticles than in the bulk even in mild oxidizing conditions (at 225
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Fig. 5. Surface roughness images of (a) Nafion 117, (b) CMI-7000, (c) PES0, (d) PES5 and (e) PES20.
presence of the stabilizing agent that provides more stability [32].
Table 1 Water uptake, Ion Exchange Capacity and Oxygen mass transfer of commercial and nanocomposite membranes. Sample
Wut [%]
IEC [meq g−1]
Ko [cm s−1]
3.2. Mechanical and thermal properties of PEMs
Nafion 117 CMI-7000 PES0 PES5 PES20
10.90 ± 1.23 27.51 ± 1.72 1.11 ± 0.27 1.18 ± 0.43 1.59 ± 0.30
0.92 0.07 0.03 0.03 0.01
1.97x 10−4 4.25x 10−4 8.04x 10−4 9.08x 10−4 1.03 x 10−3
The tensile behavior of PES-based composites is shown in Fig. 2compared to that of commercially available membranes, while Fig. 3summarizes the relevant mechanical properties. Significant differences can be noted among the membranes. Nafion exhibited the highest ductility while the PES-based nanocomposites showed a macroscopic brittle behavior, which was worsened by the addition of increasing amount of nanoparticles. This behavior has been frequently observed in composites reinforced with particles [21,33,34] and can be ascribed to the dispersion state of the
± ± ± ± ±
0.08 0.10 0.16 0.13 0.02
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PES with increasing nanoparticle content was detected, with onset degradation temperature that shifted from 565.84 °C to 556.91 °C for neat PES and PES20, respectively. All nanocomposite membranes were found to be much more thermally stable compared to the commercial membranes and different degradation behaviors were pointed out. Nanocomposite membranes were characterized by one degradation step ascribed to chain random scission and carbonization to release SO2from the sulfone group and phenol from the ether group at the temperature of the maximum thermogravimetric loss [36,37]. On the contrary, commercial membranes exhibited a more complex degradation behavior characterized by several weight loss steps. The first one, observed at around 100–140 °C and common to all the tested membranes, is related to the desorption of water bonded to the sulfonic groups, which was almost negligible in PES-based membranes [38], as confirmed by water uptake measurements. The Nafion 117 membrane showed 3 decomposition stages: the first stage (290–400 °C) may be associated with a desulfonation process, while the second stage (400–460 °C) may be related to side-chain decomposition and the third stage (460–550 °C) to PTFE backbone decomposition [39]. Also for CMI-7000 multiple stages of thermal degradation were observed, related to the decomposition of the perfluoroether side chains and to the degradation of the main chain at T > 550 °C [40]. Fig. 6. (a) OCV trends in commercial and nanocomposite membranes using sodium acetate (2 g L-1): (♦) neat PES, (□) PES5, (▴) PES20, (○) CMI-7000 and (■) Nafion 117; (b) Total Organic Carbon removal% (grey) and Columbic Efficiency% (black) after 15 days of treatment.
3.3. Surface roughness Surface roughness images are reported in Fig. 5 for all the membranes investigated in the present study. The average values of surface roughness (Ra), for Nafion 117, CMI-7000 and PES-based membranes were found to be equal to 0.016, 0.216, 0.017, 0.129 and 1.215 μm. It is clear that the roughness of PES-based membranes increased significantly with increasing magnetite particle content, as observed in other studies [19,20], with the lowest values showed by neat PES and Nafion 117. Surface roughness is a very important property for membranes because it directly influences the fouling tendency, which is reported to increase with roughness, thus retarding the performance of the membrane [19]. At the same time, the formation of a thin biofilm on the surface can reduce the oxygen crossover from the cathodic chamber to the anodic one that increases the anodic aerobic environment, thus resulting in a better efficiency of the MFC [20].
nanoparticles inside the polymer matrix. In the case when relatively large agglomerates would remain in the matrix, a propagating crack could meet a local stress concentration and then easily induce the onset of final failure. Obviously, embrittlement effects are more likely to occur at higher filler contents, where more agglomerates can be found. It is suggested that in the present study the shear forces acting in the molten state were not able to completely disentangle the clustered magnetite nanoparticles, especially at 20 wt%. The tensile strength of nanocomposites decreased, compared to the neat polymer, with increasing nanoparticle content due to imperfect dispersion and poor load transfer, which was not optimized in the present study. In fact, system requirements for mechanical reinforcement include large aspect ratio (rule of mixtures, Halpin-Tsai etc.) and a good dispersion to allow for efficient load transfer to the nanofillers and for a more uniform stress distribution and minimization of stress concentration centers. A decrease in nanofiller aspect ratio is to expected with increasing agglomeration. Despite these negative effects with increasing filler amount, all the synthesized nanocomposite membranes exhibited higher tensile strength compared to the commercial membranes, in addition to a significantly higher Young’s modulus due to the presence of nanoparticles that hindered the macromolecular mobility. Therefore it can be concluded that the nanocomposite membranes have enough strength to be used in MFC, which is particularly useful if long durations are needed, as deformation due to the formation of thick biofilms on the membrane surface can affect its efficiency [35]. With regard to the thermal stability of the different formulations, TGA curves and derivative thermograms (DTG) under nitrogen atmosphere are reported in Fig. 4. A slight decrease in thermal stability of
3.4. Ion exchange capacity, oxygen crossover and water uptake In Table 1 are summarized the values of water uptake, IEC and oxygen transfer coefficient for all membranes investigated in this work. Water uptake is another crucial characteristic of a membrane because it enhances the proton conductivity of the membrane [41]. Water uptake of PES-based membranes gradually increased with increasing content of Fe3O4 nanoparticles, even if it was lower than that exhibited by commercial membranes, thus confirming the results from TGA analysis. A possible way to improve water uptake is to apply a suitable pretreatment that can allow for up to 20% increase [8]. Pretreatment in general is not suitable for enhancing IEC [42], which in turn can be positively affected by sulfonation treatments, as confirmed by other studies [41,43]. In the present study, IEC values for nanocomposite membranes were lower than those of commercial membranes due to the lack of
Table 2 Electrochemical performances of MFC using commercial and nanocomposite membranes. Sample
Pmax [mW m−2]
Imax [mA m−2]
OCVmax [mV]
Membrane resistance [kΩ]
Nafion 117 CMI-7000 PES0 PES5 PES20
17.68 ± 0.61 12.58 ± 1.22 0.08 ± 0.01 1.66 ± 0.21 9.59 ± 1.18
69.27 ± 2.38 49.30 ± 4.78 1.12 ± 0.04 6.65 ± 0.86 38.38 ± 4.73
612.50 580.00 139.50 485.00 552.50
5.79 ± 0.19 6.13 ± 0.28 72.93 ± 2.02 46.28 ± 2.91 8.88 ± 0.11
227
± ± ± ± ±
12.50 32.00 2.50 15.00 29.50
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nanoparticles) without pretreatment, which showed results close to those obtained with commercial membranes: the OCV values of 485.00 mV, 552.50 mV, 580.00 mV and 612.50 mV respectively for PES5 (with 5 wt% of Fe3O4 nanoparticles), PES20 (with 20 wt% of Fe3O4 nanoparticles), CMI-7000 and Nafion 117 were obtained. A TOC removal of 75% was measured in all tests after 15 d, and a CE % = 11.36% using the PES20 membrane was calculated with a power and current density of about 9.59 ± 1.18 mW m−2and 38.38 ± 4.73 mA m−2, respectively. In addition, the nanocomposite membranes exhibited better thermal stability and mechanical properties compared to commercial membranes, thus suggesting their use for extended periods. However, a further increase in nanoparticle content beyond 20 wt% could compromise the operation of the MFC, since the anaerobic condition in the anodic chamber could not be ensured due to the increase of the oxygen transfer coefficient. The highest Ko (1.03 × 10−3 cm s−1) was in fact observed for PES20. Therefore, PES20 could be considered a good compromise between electrochemical and chemical performances. The PES-Fe3O4 system offers several possibilities to be improved in terms of water uptake and ionic exchange capacity by tailoring the degree of sulfonation of the polymer matrix and the surface functionality of the magnetite nanoparticles.
sulfonic acid groups in magnetite nanoparticles. This suggests the need to tailor the surface functionality of magnetite nanoparticles [44] in order to increase water uptake and IEC. Agglomeration of nanoparticles can be the cause of the decrease of IEC value at the highest nanoparticle content [11]. Amarked difference in oxygen transfer coefficient was observed among the membranes. Upon increasing the magnetite content, the PES-based membranes showed higher oxygen mass transfer, likely due to enhanced void space in membranes, generated due to the inorganic (magnetite) and organic (polymer) inter-space [45]. With regard to the potential application of these membranes in MFC systems, an even higher magnetite concentration can negatively affect the electrochemical performances: anaerobic condition in the anodic chamber cannot be possible if the membrane allows for the transfer of the oxygen that acts as the electron acceptor during the degradation of the organic matter [25]. 3.5. MFC performance OCV was daily recorded and the trends obtained in all tests were the same: OCV gradually increased and after 6 days a stable potential was recorded in all tests (Fig. 6a). In table 2 the electrochemical parameters measured in all tests are summarized. As expected, the commercial membranes showed the best results and neat PES (PES0) cannot be compared with the other membranes: the absence of the nanoparticle affects the electrochemical performances of the MFC and an OCV equal to 139.50 mV was recorded. The nanocomposite membranes exhibited better performances with increasing magnetite content. In particular, with PES20 membrane, the maximum OCV reached a value similar to that observed with commercial membranes. Pmax and Imax values were lower than those obtained with commercial membranes but improvements were observed on the electrochemical performances of the cell with increasing Fe3O4 content up to 20 wt%. This can be explained considering membrane resistances. While commercial membranes were characterized by resistances in the range of 5.79–6.13 kΩ in the adopted conditions, PES0 exhibited a value of 72.93 kΩ. A resistance reduction of 36% was observed by using PES5 and a further reduction of 80% was reached passing from 5 wt% to 20 wt% of Fe3O4. This suggests that the performances of the membrane in terms of internal resistance can be improved by an extra addition of nanoparticles and 20 wt% of Fe3O4 can be considered sufficient to avoid a membrane resistance comparable with that of the commercial ones used in this work. In Fig. 6b the treatment efficiency in terms of mineralization of the organic matter using the H-Type MFC system with different membranes is reported. A TOC removal of about 75% after 15 days of treatment was observed in all tests. As expected, MFC system with commercial membranes exhibited the highest values of CE%, however nanocomposite membranes with 20 wt% of Fe3O4 showed promising results with a CE% of 11.36%. Further studies devoted to the evaluation of a possible pretreatment effect are required, so as to improve electrochemical performances of the MFC without increasing the content of nanoparticles of the membrane or even trying to reduce it.
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4. Conclusions Fe3O4/PES nanocomposite membranes were fabricated by melt extrusion and their performances as proton exchange membranes were evaluated in an H-type microbial fuel cell. Different amounts of nanoparticles (5 wt% and 20 wt%) were tested, aiming at improving the performances of MFC such as power, current density and OCV. The results of this study indicate that nanocomposite PES-Fe3O4 membranes are promising materials for MFC application. The increasing Fe3O4 nanoparticles content resulted in an improvement of electrochemical performances especially for PES20 (with 20 wt% of 228
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