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TiO2 nanocomposite membranes for microbial fuel cell application
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e7
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Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application Majid Bazrgar bajestani*, Seyyed Abbas Mousavi* Department of Chemical & Petroleum Engineering, Sharif University of Technology, Iran
article info
abstract
Article history:
Synthesis and characterization of Nafion/TiO2 membranes (TiO2 1 wt%) with different
Received 18 June 2015
solvents (DMF, DMAc, NMP) for proton exchange membrane operating at Microbial Fuel
Received in revised form
Cell (MFC) was investigated in this study. Nanocomposite membranes are studied due to
30 October 2015
their better physical properties and higher production voltage in comparison with Nafion
Accepted 1 November 2015
112 in MFC systems. Nafion/TiO2 nanocomposite membranes were prepared by solution
Available online xxx
casting Method. The structures of membranes were investigated by Scanning Electron Microscopy (SEM). In addition, water uptake, proton conductivity, and ion exchange ca-
Keywords:
pacity (IEC) of membranes were measured and compared with Nafion 112 in microbial fuel
Nanocomposite
cell. The nanocomposite membrane prepared with DMF solvent has the best morphology,
Morphology
the highest porosity, and the maximum proton conductivity. Subsequently, the mem-
Solvent
branes were tested in a real MFC system and the nanocomposite membrane prepared with
Microbial fuel cell
DMF solvent exhibited the maximum MFC voltage.
Proton conductivity
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A global concern in the modern world is depletion of natural energy sources. Fossil fuels pollute the air and are exhaustible. Wastewaters contain soluble and insoluble materials that cause substantial environment problems [1e4]. Fuel cell technology is a promising alternative to fossil fuels [5] that is environmental friendly and reduces CO2 emission. Microbial Fuel Cell (MFC) is a type of fuel cell, which converts bioconvertible substrates to electricity using biocatalysts. The main advantage of these microbial fuel cells (MFCs) is their use in treating wastewater and electricity generation [6]. The power production of MFCs is affected by several factors
including bacterial composition, electrodes, media, and membranes. Among these, the membrane of the MFC is the most important part of the system. The membrane should transfer protons from the anode to the cathode while prevents the transfer of other materials [7]. The review of the literature about different membranes in MFC signifies the suitability of Nafion as a membrane in MFCs. Nafion membranes have many applications due to their high chemical and electrochemical stability, reasonable mechanical strength (especially when reinforced), selective and high ionic conductivity, and their ability to provide electronic insulation [8]. It is reasonable to hypothesize that the membrane properties, which are clearly correlated to membrane morphology, is strongly influenced by the interaction of
* Corresponding authors. Tel.: þ98 21 6616 6427; fax: þ98 21 6602 2853. E-mail addresses: [email protected] (M. Bazrgar bajestani), [email protected] (S.A. Mousavi). http://dx.doi.org/10.1016/j.ijhydene.2015.11.036 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e7
polymer and solvent during casting, as well as by residual solvent in the as-cast membrane [9e14]. This study used a constant amount of 1 wt% of TiO2 that has the best characterization regarding nafion composite membranes [15]. The main difference between this work and previous works lies in synthesizing and characterizing Nafion/ TiO2 membranes (with the optimum weight percent of TiO2) with different solvents (DMF, DMAc, NMP) that the results were studied and compared with Nafion 112 and tested in microbial fuel cell performance.
Experimental Materials Nafion 1100 was purchased from DuPont as a 15 wt.% solution. TiO2 Degussa P-25 was supplied by Germany Degussa Company. Other chemicals were supplied by Merck and used as received.
Membrane preparation The nafion solution contains water and isopropyl alcohol as the major volatile constituents. The diluted original nafion solution was heated at T ¼ 50 C to evaporate solution alcohols. Then, nafion was precipitated by adding NaOH solution (1 M) and dried at T ¼ 50 C until obtaining a dry residue. Successively, it was dissolved in (10%, w/w, solution) and then was filtrated. An appropriate amount of TiO2 powder (1%, w/ w) was added and dispersed in an ultrasonic bath. The composite membranes were prepared by casting the solutions using a petri dish. The petri dish was kept inside an oven at 60 C for 4 h, and then the temperature was increased to 120 C for 3 h. The membranes were peeled from the petri dish after the solvent evaporated. These membranes were washed using deionized water and then followed by 12 h of drying in oven at 100 C. Each membrane was treated by boiling for 1 h in 5% H2O2, 0.5 M H2SO4, and deionized water to obtain the acid form of the membrane. Finally, they were stored in deionized water.
Morphology The morphology of the prepared membranes was studied with Scanning Electron Microscopy (SEM). The membranes were fractured in liquid nitrogen and then coated with gold to see the cross-section structures of the membranes. SEM was utilized to investigate the surface and cross morphologies, porosity of membrane, and distribution of TiO2 particles in it.
Conductivity The proton conductivity of membranes was measured by twoelectrode AC impedance method [16]. Membrane was placed between two electrodes and conductivity was measured in the potentiostatic mode. The spectra were recorded between 100 Hz and 100 kHz and maximum perturbation amplitude of 10 mV. All the samples were put in water for 12 h prior to test.
The proton conductivity (s) of the membranes was calculated from impedance data by the Eq. (1): s ¼ L=R S
(1)
where L and S are the thickness and area of the membrane, respectively, and R is the resistance of the membrane obtained from Nyquist plot [16e18].
Water uptake Water uptake was calculated considering the difference between the wet and dry weight of a composite membrane. To obtain the wet weight, the membrane sample was equilibrated with distilled water at room temperature overnight. After that, the wet membrane was removed, dried with tissue papers, and then weighed. The dry weight was also measured after vacuum drying of the membrane. The water uptake was calculated by the following equation: Water uptake % ¼ 100 ððwð2Þ wð1ÞÞ=wð1ÞÞ
(2)
where w(2) and w(1) are weight of wet and dry membrane, respectively [18,19].
Ion exchange capacity The ion exchange capacity of the membrane sample was determined by titration method [20,21]. The membrane was submerged in 0.1 N NaOH solution for 24 h with continuous stirring. To complete ion exchange, the membranes were submerged again in anOther 0.1 N NaOH solution for additional 24 h. In this process, the Hþ of the nafion were substituted by Naþ through immersion of the sample in the NaOH solution. The two solutions were mixed and a sample was taken from the mixture. This sample was titrated using 0.1 N HCl solution to substitute H ions present in the sample, along with phenolphthalein as indicator. The ion exchange capacity of the membrane, E (meq/g), was calculated from eq. (3) [20,21]. Eðmeq=gÞ ¼ ðVðNaOHÞ NðNaOHÞ VðHClÞ NðHClÞÞ=WðdÞ (3) where, V(NaOH) and V(HCl) are the volumes (ml) of NaOH and HCl sample solutions used in the titration, respectively, and N(NaOH) and N(HCl) are the normalities of NaOH and HCl solutions, respectively. W(d) is the weight (gr) of the dry membrane.
MFC configuration Two cylindrical and H-shaped chambers measuring 1.25 L in volume and separated by a membrane were constructed using Plexiglas. Oxygen was fed continuously by an air pump in the cathode. The cathode and anode surface areas were 50 cm2. The anode chamber contained 1 L of buffer of K2HPO4 and HCl (PH ¼ 7) and 250 ml of a mixed culture sludge (obtained from return sludge flow of sedimentation tank of DMF waste water). The sludge was fed with DMF and molasses. The cathode chamber contained buffer of K2HPO4 and HCl (PH ¼ 7).
Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e7
Electrodes were stainless steel wire mesh. The schematic representation of the resultant MFC is shown in Fig. 1. Open Circuit Voltage (OCV) differs from electrical potential between two terminals of microbial fuel cell when disconnected from any circuit. The voltage was measured using a digital multifunction volt meter.
Results and discussion Morphology Fig. 2 shows the surface SEM images of the Nafion/TiO2 (1%) nanocomposite membranes prepared by the solution casting method with different solvents. It indicates that the Nafion/ TiO2 membranes with 1 wt. % of TiO2 particles are homogenous. A low concentration of TiO2 particles decreases the particle aggregation. TiO2 nanoparticles in the Nafion polymeric matrix were homogeneously dispersed. In a polymeresolvent system, there are three different interactions, namely, polymerepolymer, solventesolvent, and polymeresolvent. In a good solvent, the polymer is unfolded and polymeresolvent interactions are stronger. The Strength of polymeresolvent interactions establishes the properties of casting solvent and the morphologies of the final membranes [22]. Strength of the polymeresolvent interactions can be estimated by “solubility parameter”, d, which is the square root of the cohesive energy density. This parameter represents the strength of attractive force between molecules. Polymeresolvent interactions in polymer solutions were determined based on the difference between solubility parameters of solvents and polymer. Hansen [23] considered the dispersive forces (dd), permanent dipoleedipole interaction (dp), and hydrogen bonding forces (dh) to calculated d as follow: d ¼d dþd pþd h 2
2
2
2
(4)
The difference in Hansen solubility parameters between polymer and solvent was obtained using the equation: D¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðdp; d ds; dÞ þ ðdp; p ds; pÞ2 þ ðdp; h ds; hÞ
(5)
where P and S portray the polymer and solvent, respectively, and d, p, and h represent dispersive, polar, and hydrogen bonding components of the Hansen solubility parameters,
3
respectively. In the present study, the Hansen solubility parameters of the three different solvents and Nafion obtained from literature [23,24]. The D values between solvents and Nafion are summarized in Table 1 The smaller difference between the solubility parameters of solvent and polymer indicates the stronger polymeresolvent interaction. The polymeresolvent interactions of the three solvents decrease approximately in the below order: DMAc > DMF > NMP. The polymeresolvent interaction between DMF and Nafion should be stronger than the polymeresolvent interaction between NMP and Nafion, due to existence of strong hydrogen bonding interaction between DMF and Nafion that may sometimes play a significant role in the formation of micro phaseseparated structures. Though, their D values are close to each other [25]. In the case of selected three casting solvents in this study, DMF and DMAc are good solvents for Nafion such that DMF membrane has a more homogeneous surface than DMAc and the surfaces of both of these membranes are in turn more homogeneous than NMP-Nafion membrane surface. The different morphologies observed in the membranes might be due to the variation in the solvent volatility and also the interactions of solvents and nafion. Among the solvents, DMF and DMAc are more volatile than NMP. The SEM cross section images in Fig. 3 show porosity of the prepared membranes. Pure nafion (wt.% of TiO2 ¼ 0) has a very dense structure, nafion composite membrane (1% TiO2) with DMF solvent is porous uniformly in all of its cross section, nafion composite membrane (1% TiO2) with DMAc solvent is less porous and has a dense depth, and nafion composite membrane (1% TiO2) with NMP solvent is the least porous composite membranes.
Water uptake Water uptake is related to the basic membrane properties. Water in the membrane affects the ionomer microstructure, cluster, channel size, and plasticizes and also modifies the mechanical properties [26]. Water uptake properties of the composite membranes and pure cast nafion membrane were measured with de-ionized water. Table 2 shows the percentage of water uptake for a pure nafion and composite nafion membranes with TiO2 loading of 1 wt%. The enhanced water uptake of Nafion/TiO2 composite membranes can be attributed to the hygroscopic nature of TiO2 [27]. A number of water molecules can be aligned by hydrogen bonding with the OH groups present on the surface of TiO2 particles [28]. It can be seen that the water uptake of Nafion/TiO2 with DMF solvent has the highest value. The difference between water uptake of composite membranes and different solvents ones is related to porosity of the membranes. Membranes with higher porosity exhibit higher water uptake: DMF composite membrane water uptake > DMAc composite membrane water uptake > NMP composite membrane water uptake > Pure nafion membrane water uptake.
Conductivity
Fig. 1 e Schematic diagram of cubic two chambers MFC.
Fig. 4 shows the Nyquist plots of Nafion/TiO2 (1%) membranes at fully hydrated condition and ambient temperature with
Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
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Fig. 2 e SEM surface images of the Nafion/TiO2 composite membrane prepared by solution casting method. (a) Nafion with DMF (TiO2 0%) (b) Nafion with DMF (TiO2 1%) (c) Nafion with NMP (TiO2 1%) (d) Nafion with DMAc (TiO2 1%).
different solvents. The proton conductivity of the composite membranes is shown in the Table 2. For comparison, the proton conductivity of pure nafion and nafion112 is also shown. It can be seen that the proton conductivity of Nafion/ TiO2 with DMF solvent has the highest value equal to 12.6 mS/ cm and impregnation of TiO2 slightly increases the
conductivity value. The proton migration in Nafion occurs in two ways, which are Grotthuss and vehicular mechanisms [30]. In Grotthuss mechanism, protons hop from the H3Oþ donor acid site to neighboring acceptor water molecule, while protons transfer by the hydronium ions in vehicular mechanism [31e33]. With an increasing in the porosity of
Table 1 e Hansen solubility parameters for selected solvents, and the difference in solubility parameters between solvent and Nafion. DMF DMAc NMP Nafion
ddðMPaÞ1=2
dpðMPaÞ1=2
dhðMPaÞ1=2
dðMPaÞ1=2
D
17.4 16.8 18.0 17.4
13.7 11.5 12.3 12.5
11.3 10.2 7.2 9.6
24.8 22.1 23.1 23.5
2.1 1.3 2.48 e
Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
Fig. 3 e SEM cross section images of the Nafion/TiO2 composite membrane prepared by solution casting method. (a) Nafion with DMF (TiO2 0%) (b) Nafion with NMP (TiO2 1%) (c) Nafion with DMAc (TiO2 1%) (d) Nafion with DMF (TiO2 1%).
Table 2 e Samples characterization. Sample Nafion112 Nafion/DMF Nafion/DMF Nafion/DMAc Nafion/NMP
TiO2 (%)
Thickness (mm)
Proton conductivity (mS/cm)
0 0 1 1 1
50 60 55 55 60
4.2 3.9 12.6 4.5 3.3
Water uptake (%) 30 51 45 43
IEC (meq/g) 0.9 0.85 1.32 1.08 0.77
Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
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Fig. 4 e The Nyquist plot for membranes in the frequency range from 100 kHz to 100 Hz.
Table 3 e MFC tests. Sample OCV at SS conditions (mV) TiO2 (%) content
Nafion112
Nafion/DMF
Nafion/DMF
Nafion/DMAc
Nafion/NMP
321 0
302 0
330 1
325 1
250 1
membranes, there is an upsurge in water uptake. Water is essential for good conduction in both mentioned mechanisms, thus, proton conducting is increased. Fig. 3 shows that proton conductivity of membrane has increased for 1% loading of TiO2. The increase in the ion exchange capacity and water content of the composite membrane, which facilitate the transfer of protons, is the reason for increase in proton conductivity.
Ion exchange capacity Ion Exchange Capacity (IEC) is the number of replaceable ions (Hþ) per unit mass of the dry membrane. These Hþ ions are relatively weakly attached to SO3 groups and can move from anode to the cathode based on the Grotthuss mechanism [28]. Table 2 shows that the IEC of the composite nafion membrane increases with 1% TiO2 loading and composite membrane with DMF solvent has the highest IEC. The high oxidizing potential of TiO2 leads to oxidation of water molecules, which results in the formation of TieOH groups on the particles surface [34]. Additional OH groups increase the ion exchange sites number in the membranes. Thus, number of replaceable protons increases and is measured by ion exchange capacity. The IEC differences between composite membrane values with different solvents ones are probably due to the power of interactions between nafion, solvents, and TiO2 and different morphologies of the membranes.
MFC performance Finally, the membranes were applied to a MFC system. Better values for the maximum Open Circuit Voltage (OCV) at steady state conditions of DMF-Nafion/TiO2 (1%) is expected according to the results of proton conduction measurements, which
suggested the highest proton conductivity for this membrane. It is understood that an increase in proton conductivity causes the OCV of MFC increases at steady state condition (Table 3).
Conclusion The effects of three different casting solvents on the morphologies and microbial fuel cell performances of nafion membranes were studied by means of a series of characterization methods. DMF was the best solvent for nafion/TiO2 composite. Membrane morphology can be controlled by changing the solvent. The morphology appears to be governed by solvent volatility combined with polymeresolventenano particle interactions. It seems to be correlated to the conductivity difference between the nafioneDMAc, nafioneNMP, and nafion-DMF membranes. By changing morphology, water uptake and subsequently conductivity was changed. MFC performance is dependent on the membrane resistance and conductivity. In general, the relative strength of interactions between polymer and solvent determines the properties of casting solvent, the morphologies, and membrane properties. Composite membranes with high porosity provide high water uptake and high conductivity and subsequently lead to better microbial fuel cell performance.
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Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
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Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application Majid Bazrgar bajestani*, Seyyed Abbas Mousavi* Department of Chemical & Petroleum Engineering, Sharif University of Technology, Iran
article info
abstract
Article history:
Synthesis and characterization of Nafion/TiO2 membranes (TiO2 1 wt%) with different
Received 18 June 2015
solvents (DMF, DMAc, NMP) for proton exchange membrane operating at Microbial Fuel
Received in revised form
Cell (MFC) was investigated in this study. Nanocomposite membranes are studied due to
30 October 2015
their better physical properties and higher production voltage in comparison with Nafion
Accepted 1 November 2015
112 in MFC systems. Nafion/TiO2 nanocomposite membranes were prepared by solution
Available online xxx
casting Method. The structures of membranes were investigated by Scanning Electron Microscopy (SEM). In addition, water uptake, proton conductivity, and ion exchange ca-
Keywords:
pacity (IEC) of membranes were measured and compared with Nafion 112 in microbial fuel
Nanocomposite
cell. The nanocomposite membrane prepared with DMF solvent has the best morphology,
Morphology
the highest porosity, and the maximum proton conductivity. Subsequently, the mem-
Solvent
branes were tested in a real MFC system and the nanocomposite membrane prepared with
Microbial fuel cell
DMF solvent exhibited the maximum MFC voltage.
Proton conductivity
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A global concern in the modern world is depletion of natural energy sources. Fossil fuels pollute the air and are exhaustible. Wastewaters contain soluble and insoluble materials that cause substantial environment problems [1e4]. Fuel cell technology is a promising alternative to fossil fuels [5] that is environmental friendly and reduces CO2 emission. Microbial Fuel Cell (MFC) is a type of fuel cell, which converts bioconvertible substrates to electricity using biocatalysts. The main advantage of these microbial fuel cells (MFCs) is their use in treating wastewater and electricity generation [6]. The power production of MFCs is affected by several factors
including bacterial composition, electrodes, media, and membranes. Among these, the membrane of the MFC is the most important part of the system. The membrane should transfer protons from the anode to the cathode while prevents the transfer of other materials [7]. The review of the literature about different membranes in MFC signifies the suitability of Nafion as a membrane in MFCs. Nafion membranes have many applications due to their high chemical and electrochemical stability, reasonable mechanical strength (especially when reinforced), selective and high ionic conductivity, and their ability to provide electronic insulation [8]. It is reasonable to hypothesize that the membrane properties, which are clearly correlated to membrane morphology, is strongly influenced by the interaction of
* Corresponding authors. Tel.: þ98 21 6616 6427; fax: þ98 21 6602 2853. E-mail addresses: [email protected] (M. Bazrgar bajestani), [email protected] (S.A. Mousavi). http://dx.doi.org/10.1016/j.ijhydene.2015.11.036 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
2
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polymer and solvent during casting, as well as by residual solvent in the as-cast membrane [9e14]. This study used a constant amount of 1 wt% of TiO2 that has the best characterization regarding nafion composite membranes [15]. The main difference between this work and previous works lies in synthesizing and characterizing Nafion/ TiO2 membranes (with the optimum weight percent of TiO2) with different solvents (DMF, DMAc, NMP) that the results were studied and compared with Nafion 112 and tested in microbial fuel cell performance.
Experimental Materials Nafion 1100 was purchased from DuPont as a 15 wt.% solution. TiO2 Degussa P-25 was supplied by Germany Degussa Company. Other chemicals were supplied by Merck and used as received.
Membrane preparation The nafion solution contains water and isopropyl alcohol as the major volatile constituents. The diluted original nafion solution was heated at T ¼ 50 C to evaporate solution alcohols. Then, nafion was precipitated by adding NaOH solution (1 M) and dried at T ¼ 50 C until obtaining a dry residue. Successively, it was dissolved in (10%, w/w, solution) and then was filtrated. An appropriate amount of TiO2 powder (1%, w/ w) was added and dispersed in an ultrasonic bath. The composite membranes were prepared by casting the solutions using a petri dish. The petri dish was kept inside an oven at 60 C for 4 h, and then the temperature was increased to 120 C for 3 h. The membranes were peeled from the petri dish after the solvent evaporated. These membranes were washed using deionized water and then followed by 12 h of drying in oven at 100 C. Each membrane was treated by boiling for 1 h in 5% H2O2, 0.5 M H2SO4, and deionized water to obtain the acid form of the membrane. Finally, they were stored in deionized water.
Morphology The morphology of the prepared membranes was studied with Scanning Electron Microscopy (SEM). The membranes were fractured in liquid nitrogen and then coated with gold to see the cross-section structures of the membranes. SEM was utilized to investigate the surface and cross morphologies, porosity of membrane, and distribution of TiO2 particles in it.
Conductivity The proton conductivity of membranes was measured by twoelectrode AC impedance method [16]. Membrane was placed between two electrodes and conductivity was measured in the potentiostatic mode. The spectra were recorded between 100 Hz and 100 kHz and maximum perturbation amplitude of 10 mV. All the samples were put in water for 12 h prior to test.
The proton conductivity (s) of the membranes was calculated from impedance data by the Eq. (1): s ¼ L=R S
(1)
where L and S are the thickness and area of the membrane, respectively, and R is the resistance of the membrane obtained from Nyquist plot [16e18].
Water uptake Water uptake was calculated considering the difference between the wet and dry weight of a composite membrane. To obtain the wet weight, the membrane sample was equilibrated with distilled water at room temperature overnight. After that, the wet membrane was removed, dried with tissue papers, and then weighed. The dry weight was also measured after vacuum drying of the membrane. The water uptake was calculated by the following equation: Water uptake % ¼ 100 ððwð2Þ wð1ÞÞ=wð1ÞÞ
(2)
where w(2) and w(1) are weight of wet and dry membrane, respectively [18,19].
Ion exchange capacity The ion exchange capacity of the membrane sample was determined by titration method [20,21]. The membrane was submerged in 0.1 N NaOH solution for 24 h with continuous stirring. To complete ion exchange, the membranes were submerged again in anOther 0.1 N NaOH solution for additional 24 h. In this process, the Hþ of the nafion were substituted by Naþ through immersion of the sample in the NaOH solution. The two solutions were mixed and a sample was taken from the mixture. This sample was titrated using 0.1 N HCl solution to substitute H ions present in the sample, along with phenolphthalein as indicator. The ion exchange capacity of the membrane, E (meq/g), was calculated from eq. (3) [20,21]. Eðmeq=gÞ ¼ ðVðNaOHÞ NðNaOHÞ VðHClÞ NðHClÞÞ=WðdÞ (3) where, V(NaOH) and V(HCl) are the volumes (ml) of NaOH and HCl sample solutions used in the titration, respectively, and N(NaOH) and N(HCl) are the normalities of NaOH and HCl solutions, respectively. W(d) is the weight (gr) of the dry membrane.
MFC configuration Two cylindrical and H-shaped chambers measuring 1.25 L in volume and separated by a membrane were constructed using Plexiglas. Oxygen was fed continuously by an air pump in the cathode. The cathode and anode surface areas were 50 cm2. The anode chamber contained 1 L of buffer of K2HPO4 and HCl (PH ¼ 7) and 250 ml of a mixed culture sludge (obtained from return sludge flow of sedimentation tank of DMF waste water). The sludge was fed with DMF and molasses. The cathode chamber contained buffer of K2HPO4 and HCl (PH ¼ 7).
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Electrodes were stainless steel wire mesh. The schematic representation of the resultant MFC is shown in Fig. 1. Open Circuit Voltage (OCV) differs from electrical potential between two terminals of microbial fuel cell when disconnected from any circuit. The voltage was measured using a digital multifunction volt meter.
Results and discussion Morphology Fig. 2 shows the surface SEM images of the Nafion/TiO2 (1%) nanocomposite membranes prepared by the solution casting method with different solvents. It indicates that the Nafion/ TiO2 membranes with 1 wt. % of TiO2 particles are homogenous. A low concentration of TiO2 particles decreases the particle aggregation. TiO2 nanoparticles in the Nafion polymeric matrix were homogeneously dispersed. In a polymeresolvent system, there are three different interactions, namely, polymerepolymer, solventesolvent, and polymeresolvent. In a good solvent, the polymer is unfolded and polymeresolvent interactions are stronger. The Strength of polymeresolvent interactions establishes the properties of casting solvent and the morphologies of the final membranes [22]. Strength of the polymeresolvent interactions can be estimated by “solubility parameter”, d, which is the square root of the cohesive energy density. This parameter represents the strength of attractive force between molecules. Polymeresolvent interactions in polymer solutions were determined based on the difference between solubility parameters of solvents and polymer. Hansen [23] considered the dispersive forces (dd), permanent dipoleedipole interaction (dp), and hydrogen bonding forces (dh) to calculated d as follow: d ¼d dþd pþd h 2
2
2
2
(4)
The difference in Hansen solubility parameters between polymer and solvent was obtained using the equation: D¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðdp; d ds; dÞ þ ðdp; p ds; pÞ2 þ ðdp; h ds; hÞ
(5)
where P and S portray the polymer and solvent, respectively, and d, p, and h represent dispersive, polar, and hydrogen bonding components of the Hansen solubility parameters,
3
respectively. In the present study, the Hansen solubility parameters of the three different solvents and Nafion obtained from literature [23,24]. The D values between solvents and Nafion are summarized in Table 1 The smaller difference between the solubility parameters of solvent and polymer indicates the stronger polymeresolvent interaction. The polymeresolvent interactions of the three solvents decrease approximately in the below order: DMAc > DMF > NMP. The polymeresolvent interaction between DMF and Nafion should be stronger than the polymeresolvent interaction between NMP and Nafion, due to existence of strong hydrogen bonding interaction between DMF and Nafion that may sometimes play a significant role in the formation of micro phaseseparated structures. Though, their D values are close to each other [25]. In the case of selected three casting solvents in this study, DMF and DMAc are good solvents for Nafion such that DMF membrane has a more homogeneous surface than DMAc and the surfaces of both of these membranes are in turn more homogeneous than NMP-Nafion membrane surface. The different morphologies observed in the membranes might be due to the variation in the solvent volatility and also the interactions of solvents and nafion. Among the solvents, DMF and DMAc are more volatile than NMP. The SEM cross section images in Fig. 3 show porosity of the prepared membranes. Pure nafion (wt.% of TiO2 ¼ 0) has a very dense structure, nafion composite membrane (1% TiO2) with DMF solvent is porous uniformly in all of its cross section, nafion composite membrane (1% TiO2) with DMAc solvent is less porous and has a dense depth, and nafion composite membrane (1% TiO2) with NMP solvent is the least porous composite membranes.
Water uptake Water uptake is related to the basic membrane properties. Water in the membrane affects the ionomer microstructure, cluster, channel size, and plasticizes and also modifies the mechanical properties [26]. Water uptake properties of the composite membranes and pure cast nafion membrane were measured with de-ionized water. Table 2 shows the percentage of water uptake for a pure nafion and composite nafion membranes with TiO2 loading of 1 wt%. The enhanced water uptake of Nafion/TiO2 composite membranes can be attributed to the hygroscopic nature of TiO2 [27]. A number of water molecules can be aligned by hydrogen bonding with the OH groups present on the surface of TiO2 particles [28]. It can be seen that the water uptake of Nafion/TiO2 with DMF solvent has the highest value. The difference between water uptake of composite membranes and different solvents ones is related to porosity of the membranes. Membranes with higher porosity exhibit higher water uptake: DMF composite membrane water uptake > DMAc composite membrane water uptake > NMP composite membrane water uptake > Pure nafion membrane water uptake.
Conductivity
Fig. 1 e Schematic diagram of cubic two chambers MFC.
Fig. 4 shows the Nyquist plots of Nafion/TiO2 (1%) membranes at fully hydrated condition and ambient temperature with
Please cite this article in press as: Bazrgar bajestani M, Mousavi SA, Effect of casting solvent on the characteristics of Nafion/TiO2 nanocomposite membranes for microbial fuel cell application, International Journal of Hydrogen Energy (2015), http://dx.doi.org/ 10.1016/j.ijhydene.2015.11.036
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Fig. 2 e SEM surface images of the Nafion/TiO2 composite membrane prepared by solution casting method. (a) Nafion with DMF (TiO2 0%) (b) Nafion with DMF (TiO2 1%) (c) Nafion with NMP (TiO2 1%) (d) Nafion with DMAc (TiO2 1%).
different solvents. The proton conductivity of the composite membranes is shown in the Table 2. For comparison, the proton conductivity of pure nafion and nafion112 is also shown. It can be seen that the proton conductivity of Nafion/ TiO2 with DMF solvent has the highest value equal to 12.6 mS/ cm and impregnation of TiO2 slightly increases the
conductivity value. The proton migration in Nafion occurs in two ways, which are Grotthuss and vehicular mechanisms [30]. In Grotthuss mechanism, protons hop from the H3Oþ donor acid site to neighboring acceptor water molecule, while protons transfer by the hydronium ions in vehicular mechanism [31e33]. With an increasing in the porosity of
Table 1 e Hansen solubility parameters for selected solvents, and the difference in solubility parameters between solvent and Nafion. DMF DMAc NMP Nafion
ddðMPaÞ1=2
dpðMPaÞ1=2
dhðMPaÞ1=2
dðMPaÞ1=2
D
17.4 16.8 18.0 17.4
13.7 11.5 12.3 12.5
11.3 10.2 7.2 9.6
24.8 22.1 23.1 23.5
2.1 1.3 2.48 e
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Fig. 3 e SEM cross section images of the Nafion/TiO2 composite membrane prepared by solution casting method. (a) Nafion with DMF (TiO2 0%) (b) Nafion with NMP (TiO2 1%) (c) Nafion with DMAc (TiO2 1%) (d) Nafion with DMF (TiO2 1%).
Table 2 e Samples characterization. Sample Nafion112 Nafion/DMF Nafion/DMF Nafion/DMAc Nafion/NMP
TiO2 (%)
Thickness (mm)
Proton conductivity (mS/cm)
0 0 1 1 1
50 60 55 55 60
4.2 3.9 12.6 4.5 3.3
Water uptake (%) 30 51 45 43
IEC (meq/g) 0.9 0.85 1.32 1.08 0.77
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Fig. 4 e The Nyquist plot for membranes in the frequency range from 100 kHz to 100 Hz.
Table 3 e MFC tests. Sample OCV at SS conditions (mV) TiO2 (%) content
Nafion112
Nafion/DMF
Nafion/DMF
Nafion/DMAc
Nafion/NMP
321 0
302 0
330 1
325 1
250 1
membranes, there is an upsurge in water uptake. Water is essential for good conduction in both mentioned mechanisms, thus, proton conducting is increased. Fig. 3 shows that proton conductivity of membrane has increased for 1% loading of TiO2. The increase in the ion exchange capacity and water content of the composite membrane, which facilitate the transfer of protons, is the reason for increase in proton conductivity.
Ion exchange capacity Ion Exchange Capacity (IEC) is the number of replaceable ions (Hþ) per unit mass of the dry membrane. These Hþ ions are relatively weakly attached to SO3 groups and can move from anode to the cathode based on the Grotthuss mechanism [28]. Table 2 shows that the IEC of the composite nafion membrane increases with 1% TiO2 loading and composite membrane with DMF solvent has the highest IEC. The high oxidizing potential of TiO2 leads to oxidation of water molecules, which results in the formation of TieOH groups on the particles surface [34]. Additional OH groups increase the ion exchange sites number in the membranes. Thus, number of replaceable protons increases and is measured by ion exchange capacity. The IEC differences between composite membrane values with different solvents ones are probably due to the power of interactions between nafion, solvents, and TiO2 and different morphologies of the membranes.
MFC performance Finally, the membranes were applied to a MFC system. Better values for the maximum Open Circuit Voltage (OCV) at steady state conditions of DMF-Nafion/TiO2 (1%) is expected according to the results of proton conduction measurements, which
suggested the highest proton conductivity for this membrane. It is understood that an increase in proton conductivity causes the OCV of MFC increases at steady state condition (Table 3).
Conclusion The effects of three different casting solvents on the morphologies and microbial fuel cell performances of nafion membranes were studied by means of a series of characterization methods. DMF was the best solvent for nafion/TiO2 composite. Membrane morphology can be controlled by changing the solvent. The morphology appears to be governed by solvent volatility combined with polymeresolventenano particle interactions. It seems to be correlated to the conductivity difference between the nafioneDMAc, nafioneNMP, and nafion-DMF membranes. By changing morphology, water uptake and subsequently conductivity was changed. MFC performance is dependent on the membrane resistance and conductivity. In general, the relative strength of interactions between polymer and solvent determines the properties of casting solvent, the morphologies, and membrane properties. Composite membranes with high porosity provide high water uptake and high conductivity and subsequently lead to better microbial fuel cell performance.
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