Microporous layers based on poly(vinylidene fluoride) and sulfonated poly(vinylidene fluoride)

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Microporous layers based on poly(vinylidene fluoride) and sulfonated poly(vinylidene fluoride) Aldo Bottino, Gustavo Capannelli, Antonio Comite*, Camilla Costa, Ai Lien Ong
degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Dipartimento di Chimica e Chimica Industriale, Universita Italy

article info

abstract

Article history:

In this study, electrically conductive microporous layers (MPLs) were prepared through the

Received 15 March 2015

phase inversion technique by coagulation in a water bath of a mixture of either poly(-

Received in revised form

vinylidene fluoride) or sulfonated poly(vinylidene fluoride) and graphite and carbon black

18 August 2015

as electrically-conductive fillers (ECFs). The novel gas diffusion layers (GDL) were obtained

Accepted 26 August 2015

by casting the MPLs on a wet proof carbon cloth. Poly(vinylidene fluoride) (PVDF), due to its

Available online 26 September 2015

excellent chemical stability, thermal resistance and hydrophobic character was proposed in place of the more expensive and less processable PTFE. Moreover, sulfonated poly(-

Keywords:

vinylidene fluoride) (PVDFS, 1.9% sulfonation degree) it was investigated as binding agent

Microporous layer

in order to obtain MPLs with improved characteristics especially in view of the possibility of

PVDF

preparation of gas diffusion electrodes (GDEs) by the deposition of electro-active catalysts.

Sulfonated PVDF

MPLs morphology, water contact angle, electrical resistance, and through-plane air

Phase inversion

permeability were carefully examined. The PVDF and PVDFS MPLs were assembled with a

Air through plane permeability

catalysts coated Nafion membrane and the performance of the resulting membrane elec-

PEM fuel cell

trode assemblies were compared in a fuel cell fed with air and hydrogen. While electrical resistance of MPLs was slightly influenced by the different preparation conditions, the air permeability considerably increased by switching the solvent from N-methyl-2pyrrolidone (NMP) to dimethyl sulfoxide (DMSO) and by increasing the air exposure time of the composite film after casting as confirmed by the morphological analysis through scanning electron microscopy. As an indication of the MPLs performance, at the operating current density of 0.60 A cm
2, the single cell voltage of the proton exchange membrane fuel cell (PEMFC) was enhanced from about 0.43 to 0.5 V for PVDF based MPLs to about 0.60 V of PVDFS based MPL. The use of the phase inversion technique open several possibilities to tailor the MPL structure to specific fuel cell applications and moreover offers a more straightforward and scalable preparation method. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ39 0103538746; fax: þ39 0103538759. E-mail address: [email protected] (A. Comite). http://dx.doi.org/10.1016/j.ijhydene.2015.08.099 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

Introduction Though fuel cell systems have now reached a degree of technological maturity and appear destined to form the cornerstone of future energy technologies [1,2], the water flooding within gas diffusion electrodes still remains as a hot issue of concern, especially in the field of low temperature proton exchange membrane (PEM) fuel cell. The excessive water may dramatically decrease PEM fuel cell performance by hindering gas diffusion, as well as by covering the active sites of the electrocatalyst, forming a dead reaction zone [3e5]. Thus, the properties of gas diffusion layer (GDL) materials have a direct impact on fuel cell operation. The GDL is usually formed by a macroporous backing layer (carbon paper or carbon cloth), coated on one side or both sides with a microporous layer (MPL), composed by an electrically conductive material, typically carbon black, and a hydrophobic binding agent, typically polytetrafluoroethylene (PTFE). The role of MPL is to enhance the gas diffusion layer's performance by minimizing electronic contact resistance, improving gas transport and reducing the water flooding [6]. Chou et al. [7] found that the behavior of water drainage is controlled by the pore configuration instead of the hydrophobicity in GDL. Nam et al. [8] suggested that the effectiveness of the MPL might be further improved by designing them with large holes for water passage into the GDL, while leaving the microporosity dry for gas transport. Zhan et al. [9] concluded that gas diffusion increased with the increase of porosity as well as the porosity gradient along the microporous layer thickness. Tang et al. [10] reported that MPLs with graded-porosity have better PEM fuel cell performance than those consisting of conventional homogenous MPLs. Chu et al. [11] suggested that PEM fuel cell performance can be enhanced by enlarging macropore volume in diffusion layer while smaller porosity raises the mass transport resistance and further depresses the PEM fuel cell performance [12e14]. Recently, there has been a significant effort expended in pursuit of alternative materials for PEM fuel cell. The motive is primarily to obtain lower cost materials that perform better than the current state of the art [15]. More recently, few sulfonated polymeric materials have been proposed as an alternative material to Nafion by the aims of seeking cheaper material with enhancing proton conductivity and catalyst utilization. Sambandam and Ramani [15] used sulfonated poly(ether ketone) (SPEEK) as a cathode electrode binder while  le  ke  et al. [16] investigated a gas diffusion electrode conBe taining sulfonated polyether ionomers. Proton-conducting fuel cell composite membranes of chitosan and sulfonated polysulfone were studied by Smitha et al. [17]. The excellent physical, chemical and electrical properties of poly(vinylidene fluoride) (PVDF) make it an interesting material for applications in batteries and fuel cells as reviewed by Ameduri [18]. Numerous studies were devoted to the preparation of separators for Li-ion batteries where the

14691

electrolyte was made of a liquid electrolyte filling the pores of a macroporous PVDF membrane. PVDF/ionomers blend have been extensively studied for the preparation dense proton conducting polymeric membranes for application in either PEM fuel cells or Direct Methanol Fuel Cells (DMFC) to limit methanol crossover [19,20]. Because of its chemical resistance, thermal stability and hydrophobic character, poly(vinylidene fluoride) (PVDF) could be used instead of the more expensive and less processable PTFE in GDL manufacturing. For this last purpose, MPLs have been manufactured by introducing PVDF of functionalized PVDF as a binder in the ink composition replacing PTFE and adopting the spray technique to distribute the ink on an electron conducting substrate as well as a carbon cloth [21,22]. PVDF exhibited a comparable performance to PTFE as binder in gas diffusion electrodes composition [23]. Cabasso et al. [24] first developed GDL with a controlled porosity and pore size by coagulating a mixture of activated carbon and PVDF dissolved in an organic solvent in a non-solvent medium. A PVDF MPL with improved performance was then obtained by Hitomi [25]. The inversion phase technique consists in the coagulation of a polymer dissolved in an appropriate solvent in a non-solvent bath resulting in the formation of a porous structure and it is widely used in for the preparation of porous membranes. Most of the commercial PVDF porous membranes are fabricated through inversion phase method [26]. Recently, PVDF cathodes for microbial fuel cells were prepared by using the inversion phase technique [27], and the authors showed that the PVDF component was inexpensive and performed efficiently compared to a conventional PTFE based cathode. In the previous work [6], we have investigated the effect of preparative parameters on the characteristics of nonsupported PVDF MPL, prepared by a versatile and easy phase inversion technique. The performance of the prepared PVDFMPL coated GDL was also compared with a commercially available GDL. The obtained results demonstrated the potential of this new material and procedure in PEM fuel cell components fabrication. The importance of controlling morphology, hydrophobicity and electrical conductivity of the PVDF MPLs, for ensuring the desired performance of the GDL was also shown. Whereas, the present work extends the previous studies on the preparation of novel PVDF MPLs and, in addition, investigates the use of a sulfonated PVDF (PVDFS) as new binding agent in order to obtain MPL with improved characteristics especially in view of the possibility of preparation of gas diffusion electrodes (GDEs) by the deposition of electroactive catalysts. PVDF and PVDFS MPLs were prepared by phase inversion technique using carbon black and graphite as an electrically-conductive filler blend, and wet-proof carbon cloth as a macroporous backing layer. Morphology, surface contact angle, electrical resistance and through-plane air permeability of the obtained MPLs were carefully examined. A single PEM test cell was also assembled and the resulting membrane electrode assemblies (MEAs) performance induced by the different MPLs properties was studied in high humidification conditions.

14692

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

Experimental Preparation of PVDF and PVDFS microporous layers (MPLs) Both PVDF and PVDFS MPLs were prepared via phase inversion technique [28,29] by using a commercial PVDF (Foraflon 1000HD, Atochem) and a laboratory-made PVDFS as binding agents. The sulfonated poly(vinylidene fluoride) (PVDFS) was obtained by heterogeneous reaction of PVDF with fuming sulfuric acid (20% free SO3 basis, Sigma Aldrich) at 70  C for 30 min. The sulfonated polymer was washed and hydrolyzed with a solution of water and ethanol (30% vol.) to a boil and dried [30]. The average viscometric mass, Mw, of PVDFS was about 21 kDa. The determination of the sulfonation degree was carried out by decomposition of a PVDFS membrane € niger sample in an oxygen flask (commonly known as Scho flask) with an absorbing NaOH solution [31,32]. Sulfates in solution were measured by means of ion chromatography. Carbon black (Vulcan XC-72R, Cabot Inc., particle size ~50 nm) and graphite (Timrex HSAG 300, TIMCAL, D10 ¼ 1.2 mm; D50 ¼ 6.0 mm; D90 ¼ 32.1 mm) were used as electrically conductive filler. The electrically conductive fillers were dispersed in a PVDF or PVDFS solution, obtained by dissolving the polymer (10 wt %) in N-methyl-2-pyrrolidone (NMP, >99%, ACS reagent, SigmaeAldrich) or dimethyl sulfoxide (DMSO, >99, 9%, ACS reagent, SigmaeAldrich). The dispersion were mixed alternately by using ultrasonic bath and magnetic stirrer for at least 24 h, for obtaining homogeneous slurry that was successively cast by a knife as a thin film (final thickness of the MPL of about 50 mm) onto a commercial water-proof carbon cloth, (thickness about 350 mm, B1-A standard wet proofing plain carbon cloth, ex E-TEK Division). After a proper exposure period in a controlled environment (20  C and 60% relative humidity, r.h.), the cast slurry film was immersed into a coagulation bath (water), to form the MPL that was rinsed extensively (about 70 h) with deionized water in order to remove the solvent (both NMP and DMSO are very soluble in water) as much as possible before drying at room temperature. Table 1 lists the various MPLs prepared, along with some preparative details.

Scanning electron microscopy observation Scanning electron microscope (SEM, LEO Stereoscan 440) equipped with analytical EDS system (Oxford link with a Ge

detector with a resolution of 120 eV) was used to investigate the morphology of the MPLs. Dried MPL samples were fixed to a SEM spin stub with a conductive adhesive prior to be coated with a thin layer of gold (20 nm) by sputtering. Cross-sections were prepared by fracturing the samples into liquid nitrogen.

Contact angle measurement Liquid water contact angle at the surface of the MPL was measured by sessile-drop method, using a contact angle system (Erma G-1 contact angle meter) at 20  C. At least five spots of deionised water drops of about 4 ml were placed onto the surface of each MPL sample, using a GC syringe and the angles were measured with a goniometer.

Electrical resistance measurement The through-plane electrical resistance of MPLs was measured at 20  C by using a 2-point resistance measurement device (Keithley, DMM 2000), by blocking the sample between two copper cylindrical plates (surface area of 0.20 cm2). Prior to resistance measurements, the two copper cylindrical plates were pretreated with metal polish wadding followed by acetone cleaning to remove any surface contaminants. The samples have been clamped between the two copper plates by using a dynamometric screwdriver with a torque moment of 30 cNm. A current was passed through the cell and the voltage drop across the MPL sample was measured. The measured resistance values represent the overall resistance of the sample and the two contact resistances between samples and the copper plates.

Through-plane air permeability measurement Through-plane air permeability was determined at 20  C, by measuring the pressure drop through MPL corresponding to a given air flux. The through-plane permeability has been carried out with a very simple set-up, composed of two mass flow controllers (Brooks Mass Flow Controllers 5820E Series) with different full scale (100 ml/min and 500 ml/min, accuracy 1% of full scale value) and two differential pressure transducers (Testo 520, full scale 1000 mbar and 200 mbar respectively and accuracy 0.2% of full scale value) to measure the inlet relative pressure and the differential pressure between the two sides of the samples. The air flow rate was measured by a bubble soap flowmeter. The samples have been always assembled in

Table 1 e PVDF and PVDFS MPLs coated GDLs. MPL

Polymer solvent

NGVI

NMP

DGVI

DMSO

DGVII

DMSO

SGVDII

DMSO

Weight ratio 1 Polymer/ ECF 1 Graphite 0.5 Vulcan 1 Graphite 0.5 Vulcan 1 Graphite 0.5 Vulcan 1 Graphite 0.5 Vulcan

Binding agent

Exposure time (min)

Coagulation bath temperature Contact angle   ( C) (q )

PVDF

0.5

12

97

PVDF

0.5

12

129

PVDF

8

12

130

PVDFS

8

12

119

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

the sample cell with the MPL side at the inlet. The MPL sample area was 0.80 cm2.

Membrane-electrode assemblies and PEM fuel cell tests Commercial catalyst coated membranes (PAXITECH Co.), consisting of a Nafion® 212 membrane (DuPont) with a Pt loading of 0.5 mg , cm
2 for both anode an cathode side was used to investigate the single cell performance of the MPL coated GDLs. The membrane-electrode assembly (MEA) was constructed with the MPL sample only at the cathode and a wet-proof commercially available gas diffusion backing layer at the anode. The MEA was directly assembled in the fuel cell hardware by carefully pressing using a dynamometric tool without any thermal treatment (e.g., hot pressing). The performance of the single cell (active surface area of 5 cm2) was measured with hydrogen (H2) as fuel gas and air as oxidant at cell temperature of 65  C, without back pressure in the fuel cell test station (Fideris Innovative Solutions). The fuel cell test station was equipped with a bubble humidifier for both the fuel and the oxidant gases. The inlet relative humidity was 100% as measured by a hygrometric sensor (DeltaOhm). The external humidification temperatures of H2 and air were constantly kept at 75  C and 70  C, respectively, as suggested by Srinivasan [33]. These harsh conditions (very high humidification) were chosen in order to better highlight the different behavior of GDLs even in presence of water flooding. H2 flow rate of 0.35 L min
1 and air flow rate of 0.71 L min
1 were fed. Prior to the polarization curves recording, assembled cells were activated by setting polarization at ramp mode and operated continuously till a stable performance was obtained.

Results and discussion Morphology Scanning electron microscope (SEM) images of cross-section and surface (these latter taken at two different magnifications) of various PVDF MPL prepared by varying the polymer solvent and the exposure period of casting slurry before immersion in the water bath are reported in Fig. 1a1ec4. In Fig. 1a4ec4, SEM images of a PVDFS-MPL and in Fig. 1a5ec5, SEM images of a commercially available gas diffusion backing layer are also shown for comparison purposes. By using NMP as PVDF solvent, MPL (NGVI) with a crosssection structure (Fig. 1a1) composed of a microporous thin surface layer, supported by a thicker sub-layer with large cavities was formed. PVDF-MPLs cast from DMSO (DGVI, DGVII; Fig. 1a2ec3) present a very similar cross-section structure, but their surfaces appear quite different, being characterized by the presence of larger pores (1c2,1c3) and long crazes (1b2,1b3). This roughness profile is very similar to that of other types of MPLs, prepared from carbon black and PTFE as demonstrated in previous studies [14,34,35]. By comparing Fig. 1b2,c2 with Fig. 1b3,c3, it is apparent that the surface porosity strongly increases with increasing the exposure period of the casting slurry before immersion in the water bath. The comparison between the low magnification images

14693

of PVDF (DGVII, Fig. 1b3) and PVDFS (SDGVII, Fig. 1b4) MPLs does not indicate profound differences in the number and size of crazes, while careful inspection of high magnification images reported in Fig. 1c3 and 1c4 reveals the presence pore of larger size on the surface of the PVDFS-MPL. Fig. 1a5 illustrates the cross-section structure of a commercially available wetproofed carbon cloth (the gas diffusion backing) under higher magnification (10 mm), while Fig. 1b5ec5 show its surface morphology at two different magnifications. The surface roughness of a blank gas diffusion layer (Fig. 1b5,c5) is significantly smoothed by the presence of a MPL (Fig. 1b1eb4), regardless of the long-crazes formed on the MPLs surfaces. Fig. 1a6 shows The cross-section of a microporous layer of typical commercial gas diffusion layer and its surface is reported in Fig. 1a6,b6,c6. The thickness of the commercial MPL is about 30 mm and its surface seems smooth but large cracks are present. It is well known that the morphology of polymer films prepared by phase inversion technique depends on the exchange rate between solvent and coagulant during final immersion step [28]. By using a good PVDF solvent like NMP [29,36], rapid exchange between solvent and water causes an increase of the PVDF concentration at the film surface. This leads to the formation of a relatively dense polymer film (skin) that hinders further exchange between water and solvent. Therefore the precipitation rate of the polymer in the region beneath the skin is considerably reduced and consequently, a porous sub-layer with large pores and cavities is formed. By using a poor and highly hygroscopic PVDF solvent like DMSO [20], the casting slurry reaches an “incipient precipitation” state during exposure period in air. This determinates a lower exchange between water and solvent during immersion step with the consequent formation of a more porous skin layer (DGVI, Fig. 1b2,c2). By prolonging the exposure period, a further increase of the surface porosity is observed (DGVII, Fig. 1b3,c3). It is due to the higher amount of humidity absorbed from the air that favors the demixing phenomena of the casting slurry before water immersion. In addition, new large pores are created by the deeper penetration of the casting slurry into the carbon cloth that enhances its stretching action on the film, which is forming in the water bath [37,38]. By using a less hydrophobic binder such as PVDFS [30], humidity absorption during exposure period of the casting slurry is enhanced at the presence of sulfonic (
SO3H) groups, and alters the exchange rate between solvent and coagulant. As a consequence, a more porous MPL is obtained.

Contact angle Contact angle values of various MPLs are reported in the last column of Table 1. The water contact angle on the various MPLs was sensibly higher that the expected one (82 ) for a virgin PVDF film [39,40]. The MPL prepared by NMP-PVDF slurry (NGVI) presents a contact angle value (97 ) considerably lower than that of other MPLs (DGVI, DGVII), obtained by using DMSO as PVDF solvent. These differences can be explained on the basis of the different morphologies of the MPLs surface, reported in Fig. 1a1-c3. The MPLs are composed of an hydrophobic PVDF porous structure with a fine

14694

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

Fig. 1 e SEM images of cross-section (a) and skin surface (bec) of different PVDF and PVDFS MPL-based gas diffusion layers: (a1ec1) NGVI, (a2ec2) DGVI, (a3ec3) DGVII, (a4ec4) SDGVII, (a5ec5) Commercial gas diffusion backing layer, (a6ec6) commercial gas diffusion layer with a conventional MPL.

dispersion of particles fillers that interacts with the water drop. Therefore, the contact angle value depends on the ratio between the amount of PVDF and fillers on the surface. The MPL cast from the NMP-PVDF slurry (NGVI) exhibits a relatively smooth surface characterized by a relevant presence of evenly dispersed fillers particles that adsorb the water drop. It

leads to a strong decrease of the contact angle. On the contrary, the surface of the MPLs cast from DMSO-PVDF slurries (DGVI, DGVII) is dominated by large voids. Thus, the amount of fillers is too low to ensure a proper water adsorption. Consequently, a higher contact angle is measured. As expected, by utilizing PVDFS (SDGVII) as binder agent, the MPL

14695

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

present a relatively lower contact angle, due to the less hydrophobic character of the sulfonated polymer [30].

Electrical resistance and air permeability

Fig. 3 e Air flux against the pressure drop through the PVDF and PVDFS MPL-based gas diffusion layers.

compactness, which provide minimum contact resistance and continuous conductive pathway. On the contrary, gas permeability is predicted to increase with surface roughness and through porosity, which serve as mass transport paths during fuel cell operation. The counter-dependent of the electrical resistance and gas permeability of MPL on its thickness and other material parameters were experimentally demonstrated in the previous work [6]. Therefore, the lower through-plane electrical resistance and gas permeability of the MPL cast from NMP-PVDF slurry (NGVI) is reasonably connected to its relatively dense and smooth surface (Fig. 1b1,c1), which provides better electronic contact resistance but restrains the gas transport through the MPL. The results of the MPLs prepared from DMSO-PVDF (DGVI, DGVII) slurries also can be explained on the basis of the SEM images reported in Fig. 1b2ec3. As far as

0,12

0,12 Resistance Air Permeance -

0,11

Resistance (Ohm)

0,1

0,11 0,1

0,09

0,09

0,08

0,08

0,07

0,07

0,06

0,06

0,05

0,05

0,04

0,04

0,03

0,03

0,02

0,02

0,01

0,01

0

Fig. 2 e Electrical resistance of the DGVII sample at different torque moment values of the screwdriver used to clamp the test cell.

0 NGVI

DGVI

DGVII

SDGVII

Fig. 4 e Electrical resistance and air permeability of the PVDF and PVDFS MPL-based gas diffusion layers.

Air permeance, (x10 -1 mol/s·Pa·m2)

Fig. 2 shows the effect of the torque moment applied to the test cell on the electrical resistance of the sample DGVII. It can be noted, as expected, that the electrical resistance decreases by increasing the torque moment due to the reduction of the contact resistances and of the more compact structure of the sample. For a purpose of comparison, all the other electrical resistance measurement reported were obtained with a torque moment of 30 cN m which was the same applied also during the PEM fuel cell tests. Fig. 3 shows the behavior of the air flux through the MPLs as a function of the pressure drop. The SDGVII MPL exhibits the highest air flux while the NGVI MPL exhibits the lowest one. For all the samples the experimental data are well represented by a straight line. The slope of the regression line gives the air permeance or the air flow rate per unit of MPL surface area and unit of pressure drop across it. A good MPL should be very electrically conductive and with a good capacity to both distribute the reactants to the proton exchange membrane and remove the products and in particular water vapor. A good compromise between gas permeance and electrical resistance is of paramount importance for the good performance of the fuel cell. Electrical resistance and air permeance values of different MPLs are reported together in Fig. 4. As it can be seen, by replacing NMP with DMSO as PVDF solvent, the electrical resistance of the resulting MPL increases from ca. 0.05 U (NGVI) to ca.0.07 U (DGVI) while the gas permeability becomes considerably higher. An increase of the exposure period of the DMSO cast slurry allows obtaining a MPL (DGVII) with improved gas permeability and lower electrical resistance. The same trend is observed for the MPL (SDGVII) prepared by using PVDFS as binder. Electrical resistance is expected to be correlated to the surface smoothness and conductive bulk

14696

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

the MPL obtained from PVDFS (SDGVII) is concerned, the mild sulfonation degree of PVDFS by introducing the eSO3H group to the former PVDF, improved columbic interactions between conductive carbon fibers of GDL and enhanced compatibility with the neighborhood materials, such as carbon supported catalyst layer and carbon cloth. As a consequence, lower electrical resistance was achieved. Although, it appears that the low eSO3H content does not deeply affect the electrical resistance, while the gas permeation through MPL is strongly enhanced by the highly porous structure (Fig. 1b4,c4). In particular, by the presence of large pores size as according to HagenePoiseuille relationship for viscous flow, the permeability through pore depends on the 4th power of the pore radius.

Polarization data analysis Fig. 5 compares the polarization curves of different MPLs. It is apparent that the PEM fuel cell performance is remarkably affected by the increase of the MPL porosity. As previously demonstrated in the prior study [6], the presence of a PVDF MPL significantly enhances the performance of fuel cell, compared with a commercially available gas diffusion layer. Consequently in the present work, the effect is significant also with the increased MPL porosity, especially in the mass transport control region with the current density higher than 0.50 A cm
2. Furthermore, by using PVDFS as binding agent, a more encouraging PEM fuel cell performance is obtained. At the operating current density of 0.60 A cm
2, single cell voltage of PEM fuel cell is enhanced from about 0.43 V of PVDFMPL coated gas diffusion layer to 0.6 V of PVDFS-MPL coated gas diffusion layer. This positive impact not only limited to the mass transport control region but also extended to the activation and ohmic regions (Fig. 5). Therefore it appears reasonable to think that by operating at much lower humidifications (e.g., RH 30%) as commonly used by developers as well as by optimizing the hot pressing operation more encouraging results should be reached. The obtained result can be attributed to the key characteristic of different MPLs (i.e. morphology, porosity, contact angle, gas permeability, electrical resistance) highlighted in

the previous sections. The results showed that, by placing a MPL between the GDL and the catalyst layer creates a saturation jump across the interface. It alters the four former characteristics (morphology, porosity, contact angle and gas permeability) and affects the water droplet attachment or water coverage in MPL. Because, it changes the original surface roughness, overall porosity, water production and gas flow rate through the pores of whole GDL. Smooth surface such as those of the MPL cast from NMP-PVDF slurry makes water droplets detachment difficult, resulting in severe water coverage on the surface and increased mass transport loss [41]. Thus, the worst performance was obtained. Whereas, the increased porosity MPLs with an appropriate polymer solvent (DMSO) and with the presence of eSO3H group, better cell performance was observed. Despite of it counteract effect on the electrical contact resistance for the ohmic control region, the relatively rough textural surface of the increased porosity MPLs facilitates water droplet detachment for better performance in the mass transport control region, as demonstrated in Fig. 5. Since, the external contact angle measurements were more indicative of the MPL surface roughness than the capillary forces inside the GDL pores, which reflects the real measurement of water removal capacity [42]. The positive effect of the PVDFS on MPL porosity (Fig. 1a4ec4), electrical resistance and gas permeability (Figs. 3 and 4) that did not reduce substantially the surface water contact angle (Table 1) of MPL, contribute to improve the PEM fuel cell performance. With the role of the PVDFS, improvement in the performance of the PEM fuel cell is facilitated, not only by enlarging the threephase boundary in the catalyst layer, but also by providing ionic-conduction paths as well as by imparting negative charge to catalyst active site with concomitant of sulfur present in the carbon support as proposed [43]. The performances obtained are not very far from some performances in close conditions reported in literature with similar materials [44]. However it is also important to notice that overhumidified operating conditions adopted to perform the measurements allows to better highlight the different behavior of GDLs since favors high voltage losses especially in the GDL mass transfer region (Fig. 5) and consequently are far from the operating conditions (much lower humidifications, e.g., r.h. 30%) used by developers. Recognizing that along with the fact that the MEA have not been hot pressed, but directly assembled in the fuel cell hardware by using a dynamometric tool it appears reasonable to conclude that by optimizing testing conditions as well as the contact between the MPL and the catalysts layer on the protonic membrane more encouraging results for practical application should be reached.

Conclusion

Fig. 5 e Polarization curves of the PVDF and PVDFS MPLbased gas diffusion layers.

Novel electrically conductive PVDF and PVDFS MPLs for PEM fuel cell application were prepared by phase inversion technique and characterized by SEM observations, water contact angle, electrical resistance and gas permeability measurements. MPLs characteristics were controlled by varying the polymer solvent (NMP, DMSO) as well as the exposure period in (from 0.5 to 8 min) of the cast slurry before immersion in the

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

water bath. The best PEM fuel cell performance were obtained from highly porous and gas permeable PVDF and PVDFS MPLs (especially these latter), prepared from DMSO slurries immersed in water after exposure in air for 8 min. For instance, at the operating current density of 0.60 A cm
2, single cell voltage of PEMFC is enhanced from 0.43 V of PVDFMPL coated gas diffusion layer to 0.60 V of PVDFS-MPL coated gas diffusion layer. The use of the phase inversion techniques open several possibilities to tailor the MPL structure to specific fuel cell applications and moreover offers a more straightforward and scalable preparation method. Further studies are still ongoing to:  thoroughly study the performance of the MPL in both PEMFC and in Direct Methanol Fuel Cells (DMFC);  optimize MEA assembly by achieving a better contact between the GDL and the catalyst coated membrane; for example we have also developed a PVDF/Nafion blend proton conducting membrane which should ensure a better compatibility with the PVDF based MPLs;  introduce a proton conductor and a catalyst in the novel PVDF and PVDFS MPLs. Improvements are expected by the addition of graphene sheets and nanotubes. A more detailed study of the influence of the phase inversion technique parameters (e.g. couple solvent/non-solvent, bath temperature, air exposure time, etc.) will be carried out.

references

[1] Hoogers G. Fuel cell technology handbook. Boca Raton (USA): CRC Press; 2002. [2] Sørensen B. Hydrogen and fuel cells emerging technologies and applications. 2nd ed. Amsterdam (The Netherlands): Elsevier; 2012. [3] Park G, Sohn Y, Yim S, Yang T, Yoon Y, Lee W, et al. Adoption of nano-materials for the micro-layer in gas diffusion layers of PEMFCs. J Power Sources 2006;163:113. [4] Benziger J, Nehlsen J, Blackwell D, Brennan T, Itescu J. Water flow in the gas diffusion layer of PEM fuel cells. J Membr Sci 2005;261:98. [5] Chen J, Matsuura T, Hori M. Novel gas diffusion layer with water management function for PEMFC. J Power Sources 2004;131:155. [6] Ong AL, Bottino A, Capannelli G, Comite A. Effect of preparative on the characteristic of poly(vinylidene fluoride)based microporous layer for proton exchange membrane fuel cells. J Power Sources 2008;183:62. [7] Chou YI, Siao ZY, Chen YF, Sung LY, Yang WM, Wang CC. Water permeation analysis on gas diffusion layers of proton exchange membrane fuel cells for Teflon-coating annotation. J Power Sources 2010;195:536. [8] Nam JH, Lee KL, Hwang GS, Kim CJ, Kaviany M. Microporous layer for water morphology control in PEMFC. Int J Heat Mass Transf 2009;52:2779. [9] Zhan Z, Xiao J, Zhang Y, Pan M, Yuan R. Gas diffusion through differently structured gas diffusion layers of PEM fuel cells. Int J Hydrogen Energy 2007;32:4443. [10] Tang H, Wang S, Pan W, Yuan R. Porosity-graded microporous layers for polymer electrolyte membrane fuel cells. J Power Sources 2007;166:41.

14697

[11] Chu HS, Yeh C, Chen F. Effects of porosity change of gas diffuser on performance of proton exchange membrane fuel cell. J Power Sources 2003;123:1. [12] Chang MH, Chen F, Teng HS. Effects of two-phase transport in the cathode gas diffusion layer on the performance of a PEMFC. J Power Sources 2006;160:268. [13] Roshandel R, Farhanieh B, Saievar-Iranizad E. The effects of porosity distribution variation on PEM fuel cell performance. Renew Energy 2005;30:1557. [14] Chen J, Xu H, Zhang H, Yi B. Facilitating mass transport in gas diffusion layer of PEMFC by fabricating micro-porous layer with dry layer preparation. J Power Sources 2008;182:531. [15] Sambandam S, Ramani V. Effect of cathode binder IEC on kinetic and transport losses in all-SPEEL MEAs. Electrochim Acta 2008;53:6328.  le  ke  AB, Miyatake K, Uchida H, Watanabe M. Gas diffusion [16] Be electrodes containing sulfonated polyether ionomers for PEFCs. Electrochim Acta 2007;53:1972. [17] Smitha B, Devi DA, Sridhar S. Proton-conducting composite membranes of chitosan and sulfonated polysulfone for fuel cell application. Int J Hydrogen Energy 2008;33:4138. [18] Ameduri B. From vinylidene fluoride (VDF) to the applications of VDF-containing polymers and copolymers: recent developments and future trends. Chem Rev 2009;109:6632. [19] Jung H-Y, Park J-K. Long-term performance of DMFC based on the blend membrane of sulfonated poly(ether ether ketone) and poly(vinylidene fluoride). Int J Hydrogen Energy 2009;34:3915. [20] Lufrano F, Baglio V, Staiti P, Antonucci V, Arico' AS. Performance analysis of polymer electrolyte membranes for direct methanol fuel cells. J Power Sources 2013;243:519. [21] Lin H-L, Wu T-J, Lin Y-T, Wu H-C. Effect of polyvinylidene difluoride in the catalyst layer on high-temperature PEMFCs. Int J Hydrogen Energy 2015;40:9400. [22] Zhang F, Zhang H, Qu C, Ren J. Poly(vinylidene fluoride) based anion conductive ionomer as a catalyst binder for application in anion exchange membrane fuel cell. J Power Sources 2011;196:3099. [23] Su H, Pasupathi S, Bladergroen B, Linkov V, Pollet BG. Optimization of gas diffusion electrode for polybenzimidazole-based high temperature proton exchange membrane fuel cell: evaluation of polymer binders in catalyst layer. Int J Hydrogen Energy 2013;38:11370. [24] Cabasso. I, Yuan. Y., Xu. X, Gas diffusion electrodes based on poly(vinylidene fluoride) carbon blends, US Patent 5,783,325 (1998). [25] Hitomi. S, Electrode for furl cell and process for the preparation thereof, US Patent 2001/0,041,283 A1 (2001). [26] Liu F, Hashim NA, Liu Y, Moghareh Abed MR, Li K. Progress in the production and modification of PVDF membranes. J Membr Sci 2011;375:1. [27] Yang W, He W, Zhang F, Hickner MA, Logan BE. Single-step fabrication using a phase inversion method of poly(vinylidene fluoride) (PVDF) activated carbon air cathodes for microbial fuel cells. Env Sci Technol Lett 2014;1:416. [28] Strathmann H, Kock K. The formation mechanism of phase inversion membranes. Desalination 1977;21:241. [29] Bottino A, Camera-Roda G, Capannelli G, Munari S. The formation of microporous polyvinylidene difluoride membranes by phase separation. J Membr Sci 1991;57:1. [30] Munari. S, Vigo. F, Capannelli. G, Uliana. C, Bottino. A, Method for the preparation of asymmetrical membranes, U. S Patent 4,188,354 (1980). € niger W. Analytical procedures for the flask combustion [31] Scho method. In: Proceedings of the international symposium on microchemistry. Pergamon Press; 1958. p. 93.

14698

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 4 0 ( 2 0 1 5 ) 1 4 6 9 0 e1 4 6 9 8

[32] Fung YS. Sample dissolution for elemental analysis j oxygen flask combustion, reference module in chemistry, molecular sciences and chemical engineering - encyclopedia of analytical science. 2nd ed. 2005. p. 137. [33] Srinivasan S. Fuel cells: from fundamentals to applications. New York (USA): Springer Science; 2006. [34] Wang XL, Zhang HM, Zhang JL, Xu HF, Tian ZQ, Chen J, et al. Micro-porous layer with composite carbon back for PEM fuel cells. Electrochim Acta 2006;51:4909. [35] Han M, Chan SH, Jiang SP. Development of carbon-filled gas diffusion layer for polymer electrolyte fuel cells. J Power Sources 2006;159:1005. [36] Lin DJ, Chang HH, Chen TC, Lee YC, Cheng LP. Formation of porous poly(vinylidene fluoride) membranes with symmetric or asymmetric morphology by immersion precipitation in the water/TEP/PVDF system. Eur Polym J 2006;42:1581. [37] Bottino A, Capannelli G, Comite A. Preparation and characterization of novel porous PVDF-ZrO2 composite membranes. Desalination 2002;146:35. [38] Bottino A, Capannelli G, Comite A, Oliveri M. Development of novel membranes with controlled porosity from fluorinated polymer. Filtration 2004;4:130.

[39] Dalal EN. Calculation of solid surface tensions. Langmuir 1987;3:1009. [40] Wadekar MN, Patil YR, Ameduri B. Superior thermostability and hydrophobicity of poly(vinylidene fluoride-cofluoroalkyl 2-trifluoromethacrylate). Macromolecules 2014;47:13. [41] Wang Y, Wang CY, Chen KS. Elucidating differences between carbon paper and carbon cloth in polymer electrolyte fuel cells. Electrochim Acta 2007;52:3965. [42] Gurau V, Bluemle MJ, Castro ESD, Tsou YM, Mann JA, Zawodzinski TA. Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells: 1. wettability (internal contact angle to water and surface energy of GDL fibers). J Power Sources 2006;160:1156. [43] Selvarani G, Sahu AK, Choudhury NA, Sridhar P, Pitchumani S, Shukla AK. A phenyl-sulfonic acid anchored carbon-supported platinum catalyst for polymer electrolyte fuel cell electrodes. Electrochim Acta 2007;52:4871. [44] Escribano S, Blachot J-F, Etheve J, Morin A, Mosdale R. Characterization of PEMFCs gas diffusion layers properties. J Power Sources 2006;156:8.