Materiales nanocarbonosos para su uso en pilas PEM de alta temperatura

Improving ofMicro Porous Layer basedonAdvanced CarbonMaterials for HighTemperature Proton ExchangeMembrane Fuel Cell ElectrodesH. Zamora1, P. Canizares1,M. A. Rodrigo1, J. Lobato1*1Chemical EngineeringDepartment, University of Castilla-LaMancha,Camilo JoseCelaAv.N12,Ciudad Real 13071, Spain.

Received September 09, 2014; accepted January 12, 2015; published online

Abstract

This work reports the study of four different carbon materialsfor their application as carbon material in microporous layersfor high temperature proton exchange membrane fuel cellselectrodes. The microporous layers were prepared with car-bon black (a commercial one, Vulcan XC72), two differentcarbon nanofibers, CNF, (Ribbon and Platelet structure) andcarbon nanospheres, all of them prepared in our lab. Themicroporous layers were characterized by XRD. The hydro-phobicity, electrical conductivity, and permeability to differ-ent gases were also evaluated. The stability is an importantissue to be overcome in the field of proton exchange mem-

brane fuel cells. Thus, accelerated thermal and electrochemi-cal degradation tests in phosphoric acid media were carriedout to evaluate the stability of the different advanced materi-als tested under the same conditions. From all the performedessays, the carbon nanospheres were the best nano-carbonmaterials because of the lower degradation degree shown bythe microporous layer prepared with them and the good con-ductivity and permeability achieved, whereas CNF with a Pla-telet structure showed a low electrochemical stability due totheir greater edge plane exposure which favors their corrosion.

Keywords: Carbon nanofibers, Carbon nanospheres, HighTemperature PEMFC, Microporous layer, PBI electrodes

1 Introduction

The inherent disadvantages of fossil fuels in terms of envir-onmental effects associated with the production, transporta-tion and use of energy have become increasingly relevant.Therefore, fuel cells as alternative power resources, havegained increased attention. Fuel cells are widely considered tobe efficient and non-polluting power sources offering muchhigher energy densities and energy efficiencies as compared toother conventional power sources. Of the different fuel celltypes, the polymer electrolyte membrane fuel cell (PEMFC)has received considerable attention over the past decade foruse in transportation, vehicles and portable devices, based onits low operating temperature, high efficiency and power den-sity, nearly zero emission, and silent working conditions [14].Over recent years, extensive research has been conducted onhigh temperature proton exchange membrane fuel cells(HT-PEMFCs) due to the numerous advantages offered by thistechnology [5]. Working at high temperatures, above 100 C,requires new membrane materials that differ from the tradi-tionally used Nafion or similar polymers. Among the differentpossibilities, polybenzimidazol (PBI)-based membranes are

particularly relevant, apparently the most appropriate for bothautomobile and stationary applications [5, 6].

Membrane-electrode assembly (MEA) is a key componentof PEMFCs, consisting of a polymer electrolyte membrane,catalyst layers for the anode and cathode, and gas diffusionlayers (GDLs). The GDL situated between the catalyst layerand bipolar plate is subdivided into the gas diffusion backing(GDB) and the micro-porous layer (MPL). The GDB, consistingof carbon fibers, collects the current and physically supportsthe platinum on carbon particles, the greatest catalyst used.On the other hand, the MPL, consisting of porous, nano-sizedcarbon powders, provides the proper pore structure for thediffusion of reactant gases and liquids, minimizing electriccontact resistance between the catalyst layer and bipolarplates, and managing the water balance during production,expulsion, supply and evaporation [1, 5, 6].

Extensive studies on the MPL have been conducted toinvestigate the effects of carbon powder type on PEM fuel cell

[*] Corresponding author, [email protected]

FUEL CELLS 00, 0000, No. 0, 19 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1

ORIGINALRESEARCHPAPER

DOI: 10.1002/fuce.201400139

Zamora et al.: Improving of Micro Porous Layer based on Advanced Carbon Materials

performance [79]. For example, Passalacqua et al. preparedthe MPLs using different types of carbon blacks in order todetermine the effect of porous carbon structure on fuel cellperformance. Performance improvement was attributed tohigher pore volume and smaller pore size of acetylene black,facilitating gas diffusion and reducing the amount of wateraccumulation inside the MPL [79]. These results were foundfor a system running at a low temperature (80 C). For hightemperature PEMFCs, only one study conducted by ourresearch group [5] is currently available, as recently reportedby Chandan et al. [6].

In studies and commercial applications, carbon blacks, suchas Vulcan XC-72, are typically used as supports for Pt andPt-alloy catalysts for fuel cells. MPL usually contains VulcanXC-72 as well. The high surface area (250 m2 g1 for VulcanXC-72), low cost and availability of carbon blacks serve toreduce overall fuel cell costs. However, carbon blacks havetheir share of potential problems including: 1) thermochemicalinstability; 2) corrosion under acidic conditions; 3) pore sizeand pore distribution which affect interaction between Nafionionomer and catalyst particles, and 4) organosulfur impuritiesof CB and deep micropores or recesses that trap catalyst nano-particles, reducing accessibility and catalyst activity [1012].Therefore, carbon nanostructures formed by rolled-graphenesheets are currently being examined in order to overcomethese limitations. Typically, nanostructures are: 1) carbonnanotubes (CNT), 2-D nanostructures that may be singlewalled or multi-walled; 2) carbon nanofibers (CNF), filamen-tous and with graphene sheets that form fibers, sometimeshaving a hollow interior. Based on the disposition angle of the

graphene axis, these fibers are classified as Platelet (CNFP),Fishbone and Parallel [13]. Finally, there are 3) carbon nano-spheres (CNS), formed by graphene sheets in the form of hol-low spheres. All of these nanostructured materials haverevealed improved electrochemical and thermochemical resis-tance. Furthermore, higher power densities, improved ECSAof the catalyst and greater catalyst efficiency have been foundwhen testing them as catalyst supports for PEMFCs [1316].

It should be noted that our system differs from the Nafion-based PEMFC, since it only has one phase on the cathode sideand the electrode surface environment is more aggressive dueto the high temperatures (100200 C) and the presence ofphosphoric acid.

Therefore, the aim of this study is to assess the behavior ofthree different advanced carbon materials (Carbon Nano-spheres, CNS, Ribbon Carbon Nanofibers, CNFR, and PlateletCarbon Nanofibers, CNFP, and Vulcan CX72 for comparativepurposes) in the microporous layer of electrodes for phospho-ric acid doped, PBI-based HT-PMFCs. Structural analysis anddurability tests have been performed on the MPLs utilizingdifferent materials and under operating conditions that arecomparable to those of a PBI-based HT-PEMFC.

2 Physicochemical Characterization of CarbonMaterials

2.1 XRD Analysis

XRD analyzes were performed to evaluate crystallite sizeand crystallinity. Figure 1 shows the XRD patterns of different

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Fig. 1 XRD of carbonaceous powders materials: A) CNFP; B) CNFR; C) CNS; D) Vulcan Carbon.


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carbon materials. It may be observed that they all have a char-acteristic graphite (0 0 2) peak of 2q = 25 [15, 17]. Table 1reveals the crystallite sizes (LC), and the average interlayerspacing (d002). LC and d002 were calculated with the expres-sions 1 and 2, respectively:

LC 0:89 lB cos q (1)

d002 l

2 sin q (2)

where LC is the crystal size (nm), l corresponds to the Karadiation of copper (l = 0.15418 nm), B is the peak width ofImax/2 and q is the angle corresponding to Imax, with the lattertwo expressed in radians. The average interlayer spacing, d002,is related to the graphitization index, with lower values forthis parameter indicating a higher graphitic structure.

The Ribbon carbon nanofibers (CNFR) had the greatestcrystal size while the Vulcan Carbon had the smallest. Never-theless, all of the materials presented values within the rangethat has typically been found in literature for these com-pounds [15, 16]. On the other hand, the parameter d002 wassimilar in all cases. The lowest value was found for the carbonnanospheres (CNS), indicating that these materials have amore graphitic structure.

2.2 MPL Characterization

2.2.1 XRD

As a starting point, the different MPLs were characterizedby XRD analyses in order to study the surface of the layer con-taining the different carbon materials. XRD patterns of theMPLs are shown in Figure 2. Thus, apparent crystallite sizeand apparent distance between planes d002a, were calculatedin the same manner as for the powders. The values obtainedfor the different MPLs are shown in Table 2.

Values obtained for LC and d002 differ from the valuesshown in Table 1. This is due to the addition of PTFE in themicroporous layer which causes agglomerations, as well asthe surface of the GDL (below the MPL) which was also ana-lyzed by the X-ray. It may be seen that the trend for crystallite

Table 1 Data obtained from the XRD analyses and BET surface per-formed on the different carbon powders.

Material LC (nm) d002 (nm) BET (m2 g1)

CNFR 5.4 0.343 130.4

Vulcan 2.3 0.348 250.0

CNFP 3.5 0.347 144.2

CNS 3.6 0.341 14.3

Fig. 2 XRD of the MPL prepared with each carbonaceous material.

Table 2 Data obtained from the XRD analyses for each MPL, initial, postthermal treatment (TT) and post CV (electrochemical degradation test).

Material LCa (nm) d002a (nm) % Increase

CNFR 21.451 0.335

Vulcan 18.481 0.335

CNFP 21.448 0.336

CNS 24.563 0.336

CNFR post TT 22.667 0.337 5.68

Vulcan post TT 20.876 0.336 12.96

CNFP post TT 24.201 0.336 12.82

CNS post TT 25.594 0.337 4.20

CNFR post CV 23.327 0.338 8.74

Vulcan post CV 23.330 0.338 26.24

CNFP post CV 24.789 0.338 15.58

CNS post CV 24.794 0.336 0.94


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size LC and interlayer spacing (d002) changes with respect tothe powder, due to the previously described effects. Therefore,these values are considered to be apparent and shall be usedfor comparative purposes when making stability assessmentsas described later in this work.

2.2.2 SEM

The surface appearance of the different electrodes was stud-ied using SEM micrographs. Figure 3 shows the SEM of theMPL with the different carbonaceous materials at two magni-fications.

Vulcan has a more uniform surface, since the powder issomewhat finer and is distributed more uniformly along thesurface. The nanofibers have a similar distribution, althoughthey are somewhat grainy in the case of CNFR, similar to thatobserved in carbon nanotubes, having a similar morphology[20]. The addition of Teflon and the depositing of the inks onthe distribution electrodes are different, and agglomeratesmay form, as shown in Figure 3, leading to changes in crystalsize. The filaments observed in the CNS and Vulcan fiber net-work correspond to the carbon support paper.

At higher magnifications (Figure 3b), it is possible to differ-entiate the different morphologies of the carbonaceous materi-als such as carbon fibers in the case of CNFR and CNFP[15, 21], and the spherical shape of the CNS, which is quitesimilar to that obtained by the TEM photographs [16]. Theimage obtained for the Vulcan is quite similar to thoseobtained by other authors [22].

Nevertheless, in all cases, the MPL surfaces were found tobe homogenous.

2.2.3 Hydrophobicity

The degree of hydrophobicity was measured as describedin the Experimental section. Table 3 shows the left and rightcontact angle of each sample. CNSs are found to be morehydrophobic, while the Ribbon nanofibers have lower contactangles and therefore, are less hydrophobic. The MPL signifi-cantly contributes to the increase in the electrodes hydropho-bicity [23], since the GDL is considerably less hydrophobicdue to its low angle contact [19]. The higher hydrophobicity ofMPL means better water management, leading to lower fuelcell degradation [23]. In all cases, the left and right angles havethe same value. Therefore, the surfaces are found to be quitehomogeneous and regular as seen by SEM.

2.2.4 Reactive Gas Permeability

The reactive gas permeability was assessed using Darcyslaw (Eq. (3)) [24]

Q K S DPm L (3)

where Q (m3s1) is the volumetric flow of the gas, K (m2) is thepermeability coefficient, S (m2) is the pass area, L (m) is thethickness of the electrode, m (Pa.s) is the viscosity of the gasand DP is the pressure difference.

Table 4 shows the permeability values of air, oxygen andhydrogen for the different MPLs prepared with the advancedcarbon materials. It may be seen that the permeability ofhydrogen is the highest in all cases, since it is the smallest ele-ment. With respect to the MPLs, those prepared with CNS andthe carbon Vulcan XC72 are more permeable to the different

Fig. 3 SEM photographs of each MPL magnified 100: A) CNFR; B)Vulcan Carbon; C) CNFP; D) CNS (1500); E and F) Higher magnifica-tion to CNFP (E) and CNFR (F).

Table 3 Contact angles for each MPL sample.

Sample Left Angle () Right Angle ()

GDL 39 40

CNFp 144.5 143

CNFr 118 119

CNS 142 142

Vulcan XC 72 133 132

Table 4 MPL permeability for different gases.

Sample Air permeability (m2) O2 permeability (m2) H2 permeability (m

2)

CNFp 3.95 1012 3.68 1012 4.01 1012

CNFr 6.52 1012 4.81 1012 6.28 1012

CNS 9.51 1011 5.27 1011 1.03 1010

VulcanXC 72

8.44 1011 9.35 1011 1.02 1010




gases than those prepared with both CNFs, ribbon and plate-let. This may be due to the more compact surfaces found inthe MPLs prepared with CNFs, hindering the flow of gases, asseen in Figure 3. Nevertheless, all values have been found tobe quite similar to those found in literature [25].

2.2.5 Electrical Conductivity

In addition to providing an appropriate structure toenhance gas diffusion of the reactants and products, the MPLmust also provide also good electrical conductivity, meaningthat electrodes must have low resistivity (high electrical con-ductivity). Thus, resistivity measurements were performed onthe MPL with the different carbon materials at three differenttemperatures. Furthermore, for comparative purposes, theelectrical conductivity of a commercial GDL coated with anMPL was also measured. Figure 4 shows the evolution of solidphase conductivity with temperature for each sample. First, itwas found that the conductivity in all cases is between 125and 200 S/cm, similar to the values found in literature [26].On the other hand, those values are much higher than the val-ues observed for the gas diffusion layers (GDLs) based on car-bonaceous materials which are approximately 7 S/cm [27].Therefore, it may be said that all prepared MPLs provide lessohmic resistance, possibly due to greater structural stabilityand improved interlayer contact.

The MPL having both CNFs shows the highest electricalconductivity at all temperatures tested. The electrical conduc-

tivity values for carbon nanofibers were reported to fall withina range between 0.5 and 30 S/cm. Therefore, the electrical con-ductivity of CNFs may be classified as being at an intermedi-ate interval between high conductive carbon blacks and highlygraphitic carbons. However, their apparent electrical conduc-tivity also depends on the packing fraction and porosity [28].Nevertheless, it should be considered that, in this study, elec-trical conductivities are found for the microporous layer, notfor powders, explaining the considerable differences in thevalues. Furthermore, the technique used to measure electricalconductivity plays also an important role and may justify thedifferent values.

The MPL with the CNS has conductivity values that aresimilar to those of the MPL with Vulcan and even to the com-mercial GDL at all temperatures, although slight differencesare seen at the highest temperature. Nevertheless, it may beconcluded that these advanced carbon materials are good can-didates to form a part of the electrode MPL for HT-PEMFCs,from an electrical conductivity perspective.

2.2.6 Thermal Degradation

Thermal degradation tests were conducted in phosphoric acidmedia as will be described later, in the Experimental section.After completing the thermal degradation assays, XRD ana-lyses were performed in order to examine changes in surfacestructure of the different MPLs. Table 2 shows the values thatwere obtained and Figure 5 shows, for example, XRD patternsbefore and after thermal treatment of the MPLs prepared withcommercial Vulcan and CNS. It is evident that the MPL withthe CNSs shows a lower increase (4.2%) in crystallite size(LCa), indicating a lower degradation process for this surface.On the other hand, MPLs prepared with CNFs have highercrystal size value increases, meaning that this nanoparticle suf-fers from greater thermal degradation in the phosphoric acidmedia. The highest value of degradation corresponds to Vul-can carbon, with an increase in apparent crystal size of12.95%. This corresponds to changes found in the XRD, with aconsiderable decrease in the carbon peak for Vulcan Carbonand a negligible decrease found in CNS. As seen in Table 2,CNFP had a greater increase (12.82%) than CNFR (5.89%), dueto agglomeration of the particle materials [23].

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2.2.7 Electrochemical Degradation

Electrochemical assays were performed in order to estab-lish the electrochemical stability of the MPLs prepared withdifferent advanced carbon materials. Cyclic Voltammetry (CV)is an important electrochemical tool that may reveal evidenceof surface oxidation in different carbon materials. It should benoted that the cyclic voltammetries (CVs) were carried out byimmersing the electrodes in a 2 M phosphoric acid solution,since the electrolyte used in the HT-PEMFC is phosphoric aciddoped PBI based membranes. Therefore, all MPL surfaceswere exposed to the phosphoric acid media, whereas in anactual fuel cell, an extra layer (catalyst layer) exists betweenthe MPL and the electrolyte. Thus, in the VC test, the degrada-tion process is accelerated. Figure 6 shows the cyclic voltam-mograms number 1 and 350 for all samples prepared in thisstudy, recorded during the potential cycling (0.4 V to 0.8 Vapprox. vs. Ag/AgCl electrode).

The initial cycle of the MPL prepared with the commercialcarbon black, Vulcan XC72 has high oxidation currents at highpeak potentials, approximately 0.8 V (1V vs. RHE) probablydue to impurities which are easily oxidized [29]. The high initialvoltammetric current peaks may also be attributed to oxida-tion which is caused by defects on the particle surfaces [30].The oxidation of carbon surface on carbon blacks in hot phos-phoric acid is well known at potentials nearing 0.6 V (vs. RHE)[31]. However, it should be noted that this observation wasmade for a Pt catalyst on Vulcan XC72. The formation of a sur-face oxide and the oxidation of carbon may be illustratedusing the following sequence of reactions [32]:

C H2O COsurf 2H 2e (4)

COsurf H2O CO2 2H 2e (5)

In the absence of Pt, as in our study, the further oxidationof COsurf to carbon dioxide has a very slow kinetic rate, poten-tially explaining the lack of CO2 bubbles on the surface of theelectrode during the CV test.

On the other hand, the voltammograms of the both carbonnanofibers are similar to one another and differ from thatobtained for the Vulcan XC72. In this case, the steep hydroqui-none-quinone (HQ-Q) peak at 0.4 V approx. (vs. AG/AgCl)observed in the anodic CV curve is indicative of surface oxida-tion. It is easy to see how surface charge density increases withthe number of cycles due to the surface reaction of the elec-trodes prepared with both CNFs. This change may be calcu-lated by subtracting the pseudocapacitance charge from thetotal charge in the HQ-Q region [29, 33] and integrating thearea under the peak. According to various authors, [3436] thecalculated charge is assumed to be a faradaic charge generatedfrom the one electron HQ-Q redox reaction:

> C O e H $ C OH (6)

Thus, the MPL with CNFR had an area of 0.805 mC in thefirst cycle and 0.892 mC in the 350th cycle, an increase of10.7%. Regarding the MPL with CNFP, the first cycle had areaof 1.277 mC, greater than the one for the CNFR, and in the350th cycle, the area increased by 16.5 % (1.488 mC). Therefore,it is clear that the carbon nanofibers, with a platelet structure,suffer from greater electrochemical degradation. The greater

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Fig. 6 Cyclic voltammogram curves, 1st and 350th cycle, for each sample recorded at potential cycling between approx. 0.4 and 0.8 V. (vs. Ag/AgCl) in 2 M H3PO4.




edge plane exposure and number of defected sites of theCNFP enhance its properties as catalyst support but it is alsofound that the corrosion of carbon materials begins at theseedge planes. This explains the poorer degree of electrochemi-cal stability for the MPL prepared with CNFP as compared tothe CNFR. The MPL prepared with CNSs does not show sig-nificant changes in voltammograms during the 350 cycles,indicating a high degree of electrochemical stability underthose operating conditions.

As was done for thermal degradation, all of the carbonac-eous electrodes were analyzed by XRD; the results of the sameare presented in Table 2. CNFR and CNFp show largeincreases in LCa crystallite size (8.74 and 15.57%, respectively).Also, the MPL prepared with the commercial carbon black,Vulcan XC72 had the largest increase in the parameter LCa,indicative of a large change on the surface tested. As expected,the surface of the MPL prepared with carbon nanospheres didnot change based on the parameters calculated from XRD-analyses.

In addition, all of these comments were confirmed by SEManalyses. Figure 7 shows images of the MPL surfaces after theCV tests in acid media. Large cracks may be seen on the sur-face of the MPL prepared with Vulcan XC72 and the MPL pre-pared with the CNFP, in accordance with data calculated fromthe XRD analyses. No appreciable changes were evident onthe surface of the CNS.

The high resistance of CNSs may be explained by their lowBET surface (seen in Table 1), and their spherical bodies, freeof edges or having lower edge plane exposure, leading to alower number of defected sites that may result in corrosion.This makes it hard for the acid media to attack the carbon sur-face. Another possible explanation may be the greater degreeof graphitization (high ratio between the LC and d002 parame-ters), meaning that CNSs are more stable than other materials[16]. In other words, materials with higher values of BET sur-face (Vulcan Carbon and CNFP) may suffer from more degra-dation than materials having lower BET values (CNFR, CNS)and they are not as appealing for the microporous layer asthey would be if they were used as catalyst support. There-fore, this property may directly influence the chemical resis-tance of the carbon materials, due to the larger area exposed tothe acid media. It should be noted that the low BET value ofthe MPL with CNS does not decrease the gas permeability ofthis layer, as previously stated. Furthermore, the excellent gra-phitic character of CNS [16] makes this material more resistantto the corrosion. This may not only explain the high resistanceto the electrochemical corrosion in acid media but also thehigh heat resistance of the MPLs prepared with CNSs.

3 Conclusions

In this work, carbon black (a commercial type, VulcanXC72), two different types of CNFs (Platelet, CNFP, and Rib-bon, CNFR, synthesized in our department) and carbon nano-spheres, CNSs, synthesized in our department, were evalu-

ated as carbon materials for MPLs in high temperature-PEMFC electrodes.

The following conclusions may be reached from the study:Both of the tested CNFs reveal the largest degree of conduc-

tivity behavior but they also revealed a high degradation pro-cess during thermal and electrochemical tests, therefore theiruse as carbon materials is not recommended for the micropor-ous layer in this kind of technology due to the numerousdefected sites found in their oxidation process structure. Onthe other hand, the carbon nanospheres, CNSs, are very prom-ising candidates to form the electrode MPL for PBI based HT-PEMFCs, since they have revealed a very good thermal resis-tant in phosphoric acid media and a very high electrochemicalresistance in acceleration electrochemical degradation tests.This was explained based on their high graphitization degreeand compacter structure.

Fig. 7 SEM photographs for each MPL (right) after the CV tests. For com-parison purposes, on the left side, the photographs of the MPL from Fig-ure 3 are shown. Magnification 100: A) CNFR, B) Vulcan Carbon; C)CNFP; and D) CNS.




4 Experimental Section

4.1 Materials

Vulcan Carbon XC-72 used in this work was provided byCABOT Corp. Carbon nanofibers Platelet and Ribbon, weresynthesized by ethylene and hydrogen into a fluidized bedreactor in our laboratory, based on the methods described inprevious works [15]. Carbon nanospheres were prepared frombenzene and hydrogen in a fixed-bed reactor at atmosphericpressure, in accordance with procedures used in previousworks [16].

4.2 Methods

In previous works performed in our laboratory, the Teflonand carbon content in the MPL was optimized for a PBI basedHT-PEMFC [5, 11]. First, inks with different types of carbonac-eous compounds were prepared with a carbon concentrationof 2 mg/cm2 and a content of 10% weight PTFE. These valueswere found to be optimum for PBI based HT-PEMFCs in pre-vious studies [5, 18, 19]. Isopropyl alcohol was used as the sol-vent to prepare the ink which it was air sprayed onto a com-mercial GDL (Toray Graphite paper TGPH-120, 0.35 mm)

4.2.1 XRD

XRD measurements were conducted on a Philips PW-1700diffractometer with a rotating anode, applying K.alpha corre-sponding to the transition from copper radiation for differentsamples. Sampling of the radius was carried out between 0and 90 with a scan speed of 0.1 2q/sg.

4.2.2 Hydrophobicity

To determine the degree of hydrophobicity, a 20 m liter dropwas deposited on a small sample of each electrode and afterstanding for 1 hour, zoom shooting was conducted for each ofthe samples and the contact angle was measured between thedroplet and the electrode surface. A higher contact anglemeans a greater electrode hydrophobicity [19].

Both right (D) and left (L) angles were measured. A com-parison of the angle values for the same sample offers infor-mation on the homogeneity of the surface, with a smaller dif-ference between right and left angle indicating a morehomogeneous surface.

4.2.3 Permeability

One of the features required of any gas diffusion layer isgas access to the catalytic layer. It is possible to assess theappropriateness of the carbonaceous layer is by determiningthe permeability, which may be calculated with Darcys law(Eq. (3)).

Permeability was evaluated using home-made equipmentthat was designed and created in-house. In this set up, flow isregulated by a flow controller, while the pressure drop is eval-uated with a water-filled U-tube pressure gauge. The pressure

drop was measured in a 1.5 cm diameter pipe, at flow rates of5, 4, 3, and 2 l/min of air, hydrogen and oxygen. Pressuredrop values were represented versus flow values, obtainingthe coefficient of permeability from the slope of the fit, accord-ing to Darcys Law.

4.2.4 Conductivity

Conductivity is a property that measures the ease withwhich a charge carrier migrates under the action of an electricfield. The basic device used to measure electrode conductivityconsists of two 8 x 2 cm2 plates having a thickness of 1 cm.The plate supporting the electrodes is made of stainless steeland has a screw drilling in order to seal the system. Above it isa thin plate that offers insulation for the compressible Teflonelectrical insulation, where the 5 1 cm2 electrode is placed.Two gold plates are situated at the ends and in the center, sep-arated by 1 cm, are two gold wires that measure the potentialdifference, based upon which the electrical conductivity ismeasured. Finally, a Teflon plate is situated on the top, con-taining openings that permit the electrode to reach equilibra-tion with the surrounding environment.

Resistivity of each electrode was calculated at 125, 150 and170C using an AUTOLAB PGSTAT 30 potentiostat / galvano-stat with a FRA (Frequency Response Analysis) working mod-ule. Electrical conductivity was measured at a frequency rangeof 10.000 to 100 Hz, with a potential difference of 0 V andamplitude of 0.01 V. The average of the points was calculatedwith y = 0 for each electrode resistivity, expressed in ohms.Conductivity was calculated using the following Eq. (7):

s 1RW

I cmS cm2 (7)

4.2.5 Thermal Resistance

Considering that these materials are used for electrodesthat will potentially operate at high temperatures (100200 C),high durability issues are of considerable importance whencommercializing this technology. Therefore, heat resistance ofthe prepared electrode was evaluated for each of the carbonac-eous materials. This test was performed at 185 C for 8 hours,after having impregnated the electrode with 85% phosphoricacid (PA) with a load of 13 mg PA/cm2. Acid load reaching theelectrodes under the conditions of operation of the fuel cellwas approximately 0.17 mg/cm2. A higher loading and ele-vated temperatures were used to ensure breakdown of the ele-ments in the time assay. After 8 hours, the samples wereweighed and underwent XRD analysis to assess changes inthe crystallite size and surface of the sample. No changes inweight were found.

4.2.6 Electrochemical Degradation

In order to evaluate the electrochemical corrosion, sampleswere subjected to cyclic voltammetries, performed in a reactorfilled with 2M H3PO4 operating at room temperature, with a




reference electrode Ag/AgCl having a potential of 0.210 V. Inorder to remove the Oxygen solved in the acid media, N2 gaswas bubbled in the solution during, at least, 20 min. AnAUTOLAB potentiostat/galvanostat was used. Maximumand minimum voltages were 0.790 and 0.41 V vs Ag/AgCl,respectively, and scanning was conducted at 20 mV/s. Underthese conditions, 350 cycles were performed on each sample inorder to assess the electrochemical degradation of the differentMPLs.

Acknowledgments

The authors wish to thank the European Commission asthis work was supported by the Seventh Framework Pro-gramme of the CISTEM project (FCH-JU Grant AgreementNumber 325262). The authors also wish to thank to Prof. J. L.Valverde (Head of the Catalysis Group of our Chemical Engi-neering Department) for the supply of CNFs and CNS.

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