Effect of calcination temperature on hydrophobicity of microporous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading

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 6 ) 1 e1 2

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Effect of calcination temperature on hydrophobicity of microporous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading € ¨ rk, Ays‚e Bayrakc¸eken Yurtcan* Ays‚enur Oztu Chemical Engineering Department, Atatu¨rk University, 25240 Erzurum, Turkey

article info

abstract

Article history:

Water management is the most critical issue for the reliable PEM fuel cell operation.

Received 18 September 2016

Microporous layer (MPL) is an important component of gas diffusion media and its hy-

Received in revised form

drophobicity affects the water removal capability of the cell. Hydrophobic property

4 November 2016

changes with different parameters and in this study, the effect of calcination temperature

Accepted 5 November 2016

on the hydrophobicity of the MPLs was investigated. MPLs were prepared with two

Available online xxx

different molecular weights of polydimethylsiloxane (PDMS) polymer and they were calcined at different temperatures. The results show that calcination temperature has

Keywords:

considerable effect on the hydrophobic characteristic of the MPL. The fuel cell MPL pre-

Water management

pared with lower molecular weight of PDMS and calcined at 300
C gives the best perfor-

Microporous layer

mance even though its hydrophilic character.

PDMS

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Hydrophobicity PEM fuel cell

Introduction PEM fuel cells are important candidates to meet the increasing energy demand of the world because of obtaining high-power density from them without giving any harm to the environment. PEM fuel cells still have some important topics that must be improved to extend their utilization in daily life applications. Cell performance and durability are mostly affected from liquid water amount in the cell and therefore, water management has been widely studied so far achieving the maximum energy from the cell and making suitable it for using at any application. Water flooding is leading undesirable phenomena that is related with water management in PEM

fuel cell [1]. Common strategies have been applied to alleviate water flooding in PEM fuel cell such as inclusion of hydrophobic materials to the gas diffusion layer [2e6], addition of microporous layer (MPL) between the catalyst layer and gas diffusion layer [7e12], redesign of cell components with different configurations [13e15] and flow-field region [16e18], alteration of cell operation conditions; for instance temperature and pressure of cell [19e21], reactant gas flow rates [22,23], reactant gas humidity levels [20e22,24,25]. Gas diffusion medium is dimerous structure that contains macroporous carbon paper or fiber substrate and microporous layer (MPL) located over it. MPL facilitates the removal of liquid water from the interface of catalyst layer and gas

* Corresponding author. Fax: þ90 442 231 45 44. € ¨ rk), [email protected] (A. Bayrakc¸eken Yurtcan). E-mail addresses: [email protected] (A. Oztu http://dx.doi.org/10.1016/j.ijhydene.2016.11.041 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. € ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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diffusion layer towards flow field and moderates the severe effect of water flooding on the cell performance by virtue of smaller porosity. Hydrophobic materials are preferable in the MPL content according as gas diffusion layer (GDL) to provide easily repelling liquid water from the MPL surface [26]. Especially at the cathode side, MPL prohibits the accumulation of liquid water so it helps to obtain lower resistance against oxygen gas transport. This case reflects positively on cell performance [27]. The researchers commonly have positive impression about the addition of MPL both for obtaining higher power density and more durability from the cell [28e30]. Carbon black and hydrophobic agent are two main materials of the MPL ink [26]. Fluoropolymer derivatives have been mostly used as hydrophobic agents in the GDL or MPL ink but within the scope of this study, polydimethylsiloxane (PDMS) polymer was preferred to bring innovation to MPL structure. Polydimethylsiloxane (PDMS) polymer has some principal advantages such as quite intrinsic hydrophobicity, chemical inertness, thermal and mechanical strength, flexibility, optical transparency [31]. These are forceful properties to introduce PDMS in the content of MPL ink as a novel hydrophobic material. In recent years, PDMS polymer has become prominent at certain studies that are related with the fabrication of superhydrophobic surfaces. Yong et al. [32] demonstrated the effective way of 3D pattern-structured superhydrophobic surfaces fabrication prepared with PDMS polymer by using femtosecond laser etching method. Bao et al. [33] developed a novel method to fabricate superhydrophobic surfaces by using various metal oxide nanoparticles such as ZnO, Al2O3, Fe3O4 and coated them on various substrates followed by treatment with PDMS polymer. They said that the wettabilities of various substrates changed from hydrophilicity to superhydrophobicity by the combination of improved surface roughness with metal oxide nanoparticles and low surface energy with PDMS polymer. At another study, Cholewinski et al. [34] developed a robust bilayer superhydrophobic coating that contains PDMS-functionalized silica particles on top and an epoxy bonding layer at the base by using facile dip-coating process. They found that this bilayer coating maintains its mechanical strength even if it exposes the external stress. Shah et al. [35] fabricated micro PEM fuel cells by using silicon and PDMS base substrates. PDMS base substrate contains micro-channels for gas transfer and membrane electrode assembly locates vertically onto this substrate. Pt and Pd catalysts were separately used in the catalyst layer and performance curves were obtained at different operating conditions. Authors indicate these PDMS based micro PEM fuel cells are promising and perfectible for laboratory scale applications even though their performance results are lower than standard PEM fuel cells at equivalent Pt loading. PDMS polymer is also introduced to membrane inner structures. As an example of this, composite membrane was constructed with proton conductive sulfonated poly(ether ether ketone) (S-PEEK) and non-conductive PDMS polymer [36]. Suspensions of S-PEEK particles in the polymer matrix are adjustable by applying electric field to these particles and thus composite

membrane structure can be controlled. Additionally, applied electric field stimulates chaining of the S-PEEK particles and this case improves membrane properties such as conductivity, water uptake ability and durability. It was also stated that S-PEEK/PDMS membranes may provide more durability against electrode degradation due to their anisotropic swelling behavior in water so they can be conceivable in fuel cell applications with further development. Surface hydrophobicity is usually defined as the measure of water contact angle and it must be higher than 150
C to characterize any superhydrophobic solid surface. Water droplet takes a spherical form on hydrophobic surface and it easily slides while the surface is tilted in some degree. Lower surface energy and roughness are determinant factors for surface wettability [37]. Hydrophobicity is affected from many parameters and calcination temperature is also one of them. Calcination temperature has been taken into consideration so far to modify the properties of materials such as sorption capacity, surface activity, pore volume and diameter, moisture content and hydrophilic-hydrophobic character. Calcination procedure must be performed under the melting point of material with regard to temperature [38]. Li et al. [39] prepared the superhydrophobic coating that contains polydimethylsiloxane (PDMS) and hydrophobic nanosilica (SiO2) particles and they investigated the calcination temperature effect on the microstructure, component, transmittance and hydrophobicity of these coatings. They found that the increment at calcination temperature led the replacement of hydrophobic Si-CH3 groups with hydrophilic Si-OH groups and this case changed the surface wettability from hydrophobicity to hydrophilicity. Liu et al. [40] synthesized wormhole-like mesoporous nanocrystalline TiO2 and they investigated the effect of calcination temperature on physical parameters, photo-induced hydrophilicity and photocatalytic activity of these samples. It was indicated that the water contact angles of the calcined mesoporous nanocrystalline TiO2 thin films decreased with the increase of calcination temperature. Yang et al. [41] prepared organicinorganic hybrid Pd/SiO2 material by using solegel method and calcined at different temperatures such as 200, 350, 500, 600 and 750
C in air atmosphere to understand the effect of calcination temperature on the phase change, particle size distribution and hydrophobicity of this hybrid material. It was stated that maximum water contact angle was obtained at the film calcined at 350
C and from this point on, the values started to decrease as the temperature increased because of decomposition of some Si-CH3 groups. In the light of these informations, it can be said that calcination temperature is also conceivable parameter to change the surface hydrophobicity of the MPL in PEM fuel cell. The hydrophobicity of gas diffusion media is important for the balance of liquid water in PEM fuel cell. In this study, microporous layers were formed onto the gas diffusion backing layers by using two different molecular weights of PDMS polymer and these layers were calcined at different temperatures based on the idea of modification of MPL surface hydrophobicity thus it was aimed to contribute water management issue in a different way.

€ ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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Experimental methods Chemical materials The bare gas diffusion layer (Freudenberg, X0090 IX92 CX316) as carbon paper was obtained from the company. In general procedure of MPL fabrication, carbon black is used for providing electrical conductivity and hydrophobic polymer is used for gaining hydrophobic property to the structure [42]. Vulcan XC72R (Cabot) was used as carbon black and two kinds of PDMS polymer that have different molecular weights were used as hydrophobic agent in the MPL. Lower molecular weight PDMS (236.53 g/mole, Aldrich) is called as PDMS-1 and higher molecular weight PDMS (117,000 g/mole, Alfa Aesar) is called as PDMS-2 through this study. 1,2 propanediol (99,5 wt.%, Aldrich) was used as solvent in the MPL ink. Afterwards, catalyst layer was made on each prepared MPL to complete the electrode of the fuel cell. Pt/C catalyst (Pt/C, 66,7 wt.% Pt, Tanaka), Nafion solution (15 wt.%, Ion Power), 1,2 propanediol (99,5 wt.%, Aldrich) and pure water were used for preparation of the catalyst ink. In MEA structure, Nafion-212 polymeric membrane was used to maintain proton conductivity during the operation of fuel cell.

Preparation of microporous layers All of MPLs were prepared with the same procedure by applying MPL ink on bare gas diffusion layer via spraying method. Polymer loading was kept low because of quite superhydrophobic characteristic of PDMS and it was chosen as 1 wt.% for both PDMS-1 and PDMS-2 in MPL inks. Carbon black loading amount was adjusted as 1 mg per unit GDL area (2,1 cm  2,1 cm) to provide suitable MPL thickness and also electrical conductivity. 1,2 propanediol was used as solvent. Materials of MPL ink were put into a bottle based on the calculated amounts and then solution was stirred by homogenizer. Afterwards, solution was poured into the reservoir of spraying gun and ink application started. The temperature of the vacuum plate was set to 60
C during the spraying procedure to remove the solvent from the surface and to provide exact loading amount for the MPL materials. After spraying MPL inks on bare GDLs, these layers firstly were held in drying oven at 80
C through half an hour and then they were calcined at five different temperatures such as 100
C, 200
C, 300
C, 400
C, and 500
C in muffule furnace for one hour. The properties of the MPLs are shown as a summary in Fig. 1.

Catalyst layer and membrane electrode assembly (MEA) preparation Preparation of the catalyst layer was conducted subsequently preparation of the MPL. Commercial Pt/C catalyst, Nafion solution, 1,2 propanediol and distilled water were used for catalyst ink. This solution was stirred until the homogeneity was obtained and then application of ink onto the GDL was performed via spraying method as well as application of the MPL ink. Pt amount was adjusted a lower value such as 0.1 mg Pt/cm2 after drying the both electrodes over hot plate at 60
C

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in order to evaporate the solvent. Anode and cathode electrodes were prepared simultaneously according to the same procedure. These are combined with together as the polymer electrolyte membrane was placed between them to construct membrane electrode assembly (MEA) structure. Nafion-212 membrane was used as polymeric membrane for all cells. Hot-press was used to combine these three important parts of cell consisting from anode electrode-membrane-cathode electrode, respectively. The temperature was set 130
C and compression pressure was set to 400 psi. No further heat treatment was applied to the electrodes. Afterwards from the waiting for 4 min, hot-press was released and MEA structure was obtained.

Characterization Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Thermal Gravimetric Analysis (TGA) and contact angle measurements were conducted to analyze prepared MPL surfaces. Perkin Elmer Spectrum One FTIR spectrometer was used to characterize special chemical bonds of MPL surfaces and understand any change at chemical bonds by means of calcination process. Spectrum of samples were taken between 4000 cm1 and 400 cm1 wavelengths. The surface and intersection images of MPLs were taken by using JEOL JSM-7001FTTLS LV model SEM device. Thus the informations about morphology and thickness of MPLs were obtained. Netzsch STA 409 PC Luxx Thermal Analyzer was used for each MPL surface to determine their thermal stability and measure the weight loss of materials with the effect of temperature. In air atmosphere, MPL surfaces were exposed to thermal stress in the temperature range of 25e1000
C with the heating rate of 10
C/min. Additionally, water contact angles were measured to see hydrophobicity degree of each MPL surface. Contact angle is defined as the tangent angle between water droplet and solid surface depending on the surface baseline and the higher value of contact angle means hydrophobic surface. Primarily the images were taken by digital camera while the water droplet was on the MPL surface and the angles were measured through these images by using AutoCAD 2013 program.

Electrochemical characterization Fuel cell performance tests were conducted by Henatech™ (600 W) single fuel cell test station. Polarization curves were obtained to evaluate the electrochemical performance of each cell. Generally cell potential is plotted versus current density in a typical polarization curve and obtaining higher current density from the cell means that it gives intended performance in the positive way. Test station contains connections that provide the entrance and exit of reactant gases, silicon gasket for sealing, graphite bipolar plates, gold coated metal plates that work as current collectors, screws for compression and external heated reservoirs that are present for humidification of reactant gases before they arrive inside to cell. The reactant gases were humidified at different temperatures through the performance measurements to investigate the moderating power of MPL surfaces on the detrimental effect of liquid water amount. Humidification temperatures were set to

€ ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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Fig. 1 e Summary of MPL properties of PEM fuel cells.

50, 60, 65, 70
C at both electrodes and these are respectively corresponding to 40%, 64%, 80%, 100% relative humidity levels. Cell temperature was set to 70
C and the gas lines were also heated with heating wires to prevent condensation of humidified gases before access to cell. Firstly system was purged with nitrogen for a while and then reactant gases were enabled. Hydrogen gas was used as fuel and oxygen gas was used as oxidant and they were sent to test station system at equal flow rate as 0.25 slpm. Starting from the open circuit potential (OCV) value, cell potential decreased with the 0.05 V step until 0.1 V and corresponding currents at each potential were recorded after the occurence of stability. Following the first measurement, cell was waited through 30 min at 0.1 V and then second one was started to record current values. Same procedure was applied until the current values at each potential almost remain unchanged rephrasing this case as steady-state.

Results and discussion Fourier Transform Infrared Spectroscopy (FTIR) Fig. 2a shows the FTIR spectra of MPL surfaces with PDMS-1 and Fig. 2b shows the FTIR spectra of MPL surfaces with PDMS-2 calcined at different temperatures. Characteristic absorptions at 1268 cm1 for C-H deformation of Si-CH3 and at 805 cm1 for C-Si-C asymmetric stretching vibrations of PDMS polymers are seen. The doublet that is observed more clearly for PDMS-2 between 1100 and 1000 cm1 [43]. The peaks at 2964 cm1 and at 3054 cm1 represent, respectively, asymmetric CH3 stretching vibration and aromatic CeH stretching vibration [44]. The peak at around 3750 cm1 agrees with eOH group of silanol moeties [45]. There is no significant difference between the spectra of MPL surfaces prepared with PDMS-1 and PDMS-2. The peaks in Fig. 2b are seen more sharply because of dominant PDMS characteristic of PDMS-2 by means of higher molecular weight. Several peaks become indistinct together with increasing calcination temperature

because of degradation of PDMS polymer on the MPL surface but yet it still observed no clear difference between the calcined surfaces. This case is attributed to fact that the spectral features of materials such as vinyl and silane groups can be the same before and after curing and maybe it occured in here for calcination likewise curing [44].

Scanning Electron Microscopy (SEM) The SEM images of MPL surfaces prepared with both PDMS polymers are shown in Fig. 3a. These images represent the MPL surfaces without coating of catalyst layer for each one. Common perception when looking at these images is inclusion of PDMS to MPL ink provides surface smoothness. Increasing of calcination temperature generates gradual disentangle of MPL surfaces for PDMS-1 and PDMS-2. As it is understood from the images, PDMS-1 shows less thermal durability according to the PDMS-2. The bottom GDL surface starts to appear when the MPL surface with PDMS-1 calcined at 400
C in comparison with equivalent position of PDMS-2. This case arises from considerable molecular weight difference between both PDMS polymers. Chain length affects the degradation of polymer and higher molecular weight carried PDMS-2 one step further according to the PDMS-1 in terms of thermal durability. The last images of Fig. 3a belong to MPL surfaces calcined at 500
C for both PDMS polymers and it is seen that MPL surfaces completely disintegrated by means of temperature and spaghetti shape of base GDL surfaces came into sight. Fig. 3b shows SEM images that belong to intersections of MPLs. Both of anode and cathode electrode MPLs were pressed with each other as the polymeric membrane was placed between them and this triple structure was cut into half to scan for intersection images. MPL layer thicknesses can be determined from these images and it is seen that increasing of calcination temperature leads to decrease in thickness. MPL layers waste away as the temperature increasing to 500
C. Keeping the calcination temperature at optimum level provides more integrative MPL structure.

€ ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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Fig. 2 e FTIR spectra of calcined MPLs with a) PDMS-1 b) PDMS-2.

Thermal gravimetric analysis (TGA) Thermal gravimetric analysis was made to determine the weight percents of materials that stayed with MPL surface after the calcination process. Thus, it was aimed to have an opinion about the thermal durability of the MPL surfaces. Additionally, Vulcan XC72R, PDMS-1 and PDMS-2 were also exposed to thermal gravimetric analysis in order to see their own thermal durability characteristics. Fig. 4 shows TGA results of these materials. Vulcan XC72R resists to thermal stress until 600
C and its degradation begins after exceeding

this temperature. PDMS-1 has the lowest thermal durability among them and it is over completely at 200
C. According to the PDMS-1, PDMS-2 further tolerates the deformation effect of high temperature and it deteriorates beyond 400
C. The difference between the thermal durabilities of two PDMS polymers can be attributed to the different molecular weights and polymer chain lengths. Fig. 5a and b show the thermal gravimetric analysis of MPL surfaces prepared with PDMS-1 and PDMS-2, respectively. Decomposition of the MPL surfaces starts to decay earlier so long as the calcination temperature decreases according to

€ ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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Fig. 3 e (a) SEM images of MPL surfaces at 100 mm (b)SEM images of intersections of MPLs at 100 mm.

the both graphics. It was interpreted as calcination process at 500
C leads the degradation of both PDMS polymers almost completely and the residual carbon black provides slightly more thermal durability than the other calcined surfaces. Jovanovic et al. noted a conclusion about the better oxidative thermal stability of polysiloxanes because of many dimethylsiloxy groups on the main chain of these type of polymers [46]. The maximum degradation rates of MPL surfaces

Fig. 3 e (continued).

€ ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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prepared with PDMS-1 and PDMS-2 start with exceeding 400
C and those are over completely about to 800
C [47]. It can be seen that MPL surfaces have good thermal stability in air atmosphere.

Contact angle measurements Contact angle measurements were made to investigate how the PDMS treatment affects the MPL surface hydrophobicity. GDL or MPL generally are exposed to hydrophobic material treatment to repel water easily from these surfaces and thus alleviating the negative effect of excess liquid water amount on the fuel cell performance and durability. Water management requires the balance on the liquid water amount providing membrane humidification and avoidance of flooding at the same time so proper hydrophobic treatment of GDL, MPL or catalyst layer works for this purpose. Choun et al. prepared the cathode catalyst layer with PDMS to enhance hydrogen cell durability and they measured the contact angles of cathode electrode surfaces with and without PDMS. They indicated that the hydrophobicity of PDMS treated electrode surface improves substantially by means of PDMS polymer according to the other untreated electrode [48]. Therefore, it is expected to obtain high contact angle values from the prepared MPL surfaces with PDMS-1 and PDMS-2 based on this study. Fig. 6 shows the images of water droplets on the MPL surfaces in sequence of increasing calcination temperature and Table 1 also gives average contact angle values separately from these images. The results clearly reveal that calcination temperature has significant effect on the surface hydrophobicity. Contact angle values sharply decrease and surface property changes from hydrophobicity to hydrophilicity with the high calcination temperature for both PDMS polymers. MPL surfaces prepared with PDMS-1 preserve the hydrophobicity until the calcination at 300
C while the hydrophobicity of the MPL surfaces prepared with PDMS-2 retains up to 500
C. This case confirms the less thermal stability of PDMS-1 polymer in accordance with TGA analysis itself. The remarkable difference between the hydrophobic properties of surfaces demonstrates that calcination temperature must be considered to modify surface wetting characteristics.

PEM fuel cell performance tests Water plays a critical role because it has reciprocal effect on the fuel cell performance and durability. Polymeric membrane must be well hydrated to maintain proton conductivity, on the other hand excess liquid water leads to mass transfer limitations and flooding usually occurs. From this point of view there must be balance on the liquid water amount so in this sense reasonable gas humidification gains importance for better and reliable fuel cell performance [49]. Sluggish oxygen reduction reaction (ORR) leads sharp voltage drop at the beginning of polarization curve and this region is called as activation polarization region. At the medium of polarization curve, the resistances against to flow of ions and electrons become crucial and they merge under the name of ohmic resistance that causes the drop of cell voltage

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almost linearly with the increasing current density. Lastly at the concentration polarization region of curve, mass transfer limitations manifest while high current density values are drawing from the cell. In this area, transfer of reactant gases can be interrupted because of blockage of gas diffusion medium pores with excess liquid water and the reactions can not proceed effectively at the electrodes. In this case, cell potential drops substantially because of reactants starvation at the active regions of catalyst layer [50]. Following figures represent performance curves of cells that contain MPLs prepared with PDMS polymers and calcined at different temperatures except for 500
C because of complete degradation of MPL surface at this temperature. Fig. 7 belongs to cells with PDMS-1/MPL and calcined at 100, 200, 300, 400
C, respectively. In the same way, Fig. 8 shows the performance results of cells with PDMS-2/MPL and calcined at 100, 200, 300, 400
C. Polarization curves were performed under different relative humidity levels of gases such as 40%, 64%, 80% and 100% in order to understand liquid water tolerance of MPL surfaces. When it is examined the curves of cells with PDMS-1/MPL, lower level of gas humidity pulls down the cell performance according to the other temperatures in all cases. The cells calcined at 100
C and 400
C give their worst performances at 40% RH. Proton conductivity strongly depends on wellhumidified membrane. Insufficient humidification and the poor amount of produced water by cell reaction retard water back diffusion from gas diffusion layer to polymeric membrane. At low current density region (activation polarization), membrane dehydration is one of the reasons of cell voltage loss and this situation can also be valid in here so cell voltages of these cells decreased sharply at the beginning of polarization curve. On the other hand, the cells calcined at 200
C and 300
C show enhanced performance at 40% RH according to others. This case confirms the back-diffusion ability of their MPLs and it can be interpreted as ohmic resistance is getting better for these two cells due to better membrane humidification [51]. Pore size of MPL is an important factor for optimum water permeability. As the calcination temperature increases, volume of the macro pores in the prepared MPLs increases while the volume of the micro pores decreases [52]. The enhancing performance at 40% RH up to cell calcined at 300
C may depend on enlarged macro pore volume and facilitation of water passage through the membrane. Voltage drops follow nearly linear trend for all cells with PDMS-1/MPL at intermediate current density region but when we compared with four cells between each other, the voltage of cell calcined at 400
C drops quicker than those of others. The reason of this behavior is most probably related with MPL thickness. As it can be seen from the SEM intersection images of MPLs, the cell calcined at 400
C has the thinner MPL thickness and some deformations are present. Therefore this cell has the lowest water retention capability. Water saturation decreases at the interface of catalyst layer and MPL, thus it leads occurring higher ohmic resistance at the electrode. Membrane dehydration occurs with this case and the performance decline of this cell at 40% RH level can be the confirmation about it. MPL thickness is also another important parameter for providing

€ ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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(b)

Fig. 5 e Thermal gravimetric analysis of MPLs with a) PDMS-1 b) PDMS-2.

effective water flux distribution through the layer and optimum value for thickness must be determined [51,53]. Generally moderate level of humidification works well for proper water management and the results obtained at 64% RH and 80% RH validate this case. All of the cells with PDMS-1/ MPL show the better performance at these humidity levels. The highest level of gas humidity surpasses in terms of the performance at first but it starts to degrade at high current density region because of over liquid water that results from both high humidity level and accelerating rate of produced water by reaction. It can be deduced from the results that high humidity level have adverse effect on the cell performance because non-uniform distribution of local current density occurs in both cases [49]. Weng et al. developed inhomogeneous MPL that has gradient hydrophobicity and they investigated the performance and durability of cells that contain these MPLs under various relative humidity conditions. They indicated that when the cell operated at high humidity

Fig. 4 e TGA analysis of MPL ink materials (a) Vulcan XC72R (b) PDMS-1 (c) PDMS-2. € ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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Table 1 e Contact angles of MPL surfaces. MPL PDMS-1/100
C PDMS-2/100
C PDMS-1/200
C PDMS-2/200
C PDMS-1/300
C PDMS-2/300
C PDMS-1/400
C PDMS-2/400
C PDMS-1/500
C PDMS-2/500
C

Fig. 6 e Water droplet images on the MPL surfaces. condition there must be effective water removal from inside of the cell to prevent blockage of passageways of gases, otherwise sharp decrease in cell performance will occur because of excess water [54]. Liquid water amount affects charge transfer resistance of the electrode so the voltage losses related with mass transfer limitations come through. Generally this kind of decrease in cell performance is seen under at the 100% RH condition for all fuel cells with PDMS-1/ MPL. The cell with PDMS-1/MPL and calcined at 300
C is the best one in terms of the tolerance against high humidity level

Contact angle 142
139
135
138
65
135
49
133
14
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condition and the highest current density values were obtained during the operation of this cell. There is an instant change at slope of cell voltage at 64% RH about 1200 mA/cm2 in the polarization curve of the cell with PDMS-1/MPL calcined at 300
C. This may be arise from the well hydration of membrane with the increasing amount of produced water at high current density region in the cell [55]. It is needed to remember that calcination temperature changed the MPL surface morphology and so surface wettability. Thus, one of the reasons of difference between the cell performance results may originate from the changing hydrophobic property via calcination temperature. MPL with PDMS1 and calcined at 300
C shows the hydrophilic character with low value of contact angle. Because the best performance result belongs to this cell, it can be said that pore size of MPL is probably determinant factor at enhancing cell performance and it provides convenient cell operation media. Impregnation of water and transport of gases are related with hydrophobicity and porosity of diffusion medium [56]. Generally the fuel cells with PDMS-2/MPL show the worse performance according to the PDMS-1. Polymer chains aggregate in high molecular weight of PDMS polymer and it can affect the rheological behavior of MPL ink solution. It was interpreted as MPL surface has the tighter structure with high molecular weight of PDMS and this dense pattern may complicate the gas and water transport so the cell performances degrade [57]. Almost all cells of PDMS-2/MPL give their best values at 64% RH level as in the same with PDMS-1 but only the cell calcined at 300
C gives its best at 40% RH at high current density region surprisingly. This can be explained as the following. PDMS-1 and PDMS-2 have different molecular weights so they differ from each other with regard to chainlength, cross-linking density and the number of free and pendant chains. Higher chains mobility of PDMS-2 provides better adsorption to surface and creates more contact points on it thus the surface wettability enhances [58]. From this point PDMS-2 is more effective at wetting the surface so low humidity level of reactant gases can suffice the operate cell with enhancing performance. It may be exceeded the water content limitations at other humidity levels for this polymer. Higher voltage losses at the activation polarization region originate from lower value of exchange current densities and the MPL porosity degree is generally responsible from this case [59]. PDMS-2 may create viscous medium in the MPL and this probably decreases the porosity of the structure. Additionally severe effect of water flooding can be seen from the

€ ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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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 6 ) 1 e1 2

Fig. 7 e PEM fuel cell performance curve of cell that includes MPL with PDMS-1 at calcined a) 100
C b) 200
C c) 300
C d) 400
C.

Fig. 8 e PEM fuel cell performance curve of cell that includes MPL with PDMS-2 at calcined a) 100
C b) 200
C c) 300
C d) 400
C. € ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041

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 6 ) 1 e1 2

states of polarization curves at high current density region for all cells with PDMS-2/MPL. Especially at 100% RH, cells fail in terms of the tolerance against high water content according to the cells with PDMS-1/MPL. The results explicitly show the severe effect of calcination temperature on the MPL hydrophobicity hence water management capability of MPL and finally cell performance. The most favorable calcination temperature is 300
C for both PDMS polymers. Relative humidity of gases is also important for water management as the cell operation parameter. It was understood that PDMS-1 is more suitable than PDMS-2 with regard to obtaining better cell performance. Finally it can be suggested that both of calcination temperature and molecular weight of polymer are likely effective on pore size of MPL structure.

Conclusions In the scope of this study, microporous layers were prepared with two kinds of hydrophobic PDMS polymer and the effect of calcination temperature on the surface hydrophobicity was investigated. PDMS polymer gains excellent hydrophobic property to MPL surface though its quite low amount. Two different molecular weights of PDMS were compared with each other and the best performance result was obtained with the cell that contains MPL prepared with PDMS-1 and calcined at 300
C. This result proves that the optimum properties have to be provided in MPL in order to enhance the water management and get better performance from the fuel cell. The effect of calcination temperature on the surface wettability is apparent from the water droplet images so this parameter must be taken into consideration.

Acknowledgements The authors are gratefully acknowledged the financial support by Turkish Scientific and Technological Research Council [Grant Number: 113M205]; and Atatu¨rk University [Grant Number: 2015/134]. The authors are also acknowledge the Freundenberg company for providing gas diffusion layer.

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€ ¨ rk A, Bayrakc¸eken Yurtcan A, Effect of calcination temperature on hydrophobicity of microPlease cite this article in press as: Oztu porous layers prepared with two different molecular weights of PDMS polymer on PEM fuel cell performance with low Pt loading, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.041