Gas Diffusion Layer from Multiwalled Carbon Nanotubes/Polyacrylonitrile Composite Fiber for Proton Exchange Membrane Fuel Cell

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1 Gas Diffusion Layer from Multiwalled Carbon Nanotubes/Polyacrylonitrile Composite Fiber for Proton Exchange Membrane Fuel Cell CHATWARIN POOCHAI, THIRAWUDH PONGPRAYOON* Department of Chemical Engineering, Center of Eco-Materials and Cleaner Technology King Mongkut s University of Technology North Bangkok 1518 Piboonsongkram Road, Bangsue, Bangkok THAILAND tpongprayoon@yahoo.com, thp@kmutnb.ac.th Abstract: - Gas diffusion layer (GDL) is an important part of electrode of proton exchange membrane fuel cells (PEMFC). In order to improve the electrical conductivity, carbon nanofiber paper (CNP) was prepared from multiwalled carbon nanotubes/polyacrylonitrile composite fiber. PtRu/C was used as anodic and chathodic catalyst for PEMFC testing. The performance of PEMFC was examined with the comparison between the prepared CNP and commercial carbon paper (CP) as gas diffusion layer (GDL) combined with Nafion membrane (N115) for making the membrane electrode assembly (MEA). The results showed that the maximum power densities of MEA using the prepared CNP and commercial CP were 385 and 334 mw/cm 2 corresponding to 800 and 700 ma/cm 2, respectively. Thus the PEMFC performance was improved approximately 13%. Key-Words: - carbon nanofiber paper, modified multiwalled carbon nanotubes, gas diffusion layer 1 Introduction In order to develop proton exchange membrane fuel cell (PEMFC), the electrode substrate playing an important role to affect fuel cell performance is required as the high quality electrode materials. In spite of high conductive carbon paper (CP) as commercial materials produced with high temperature pyrolysis for high degree of graphite, carbon fiber paper substrate has been considered as an ideal conductive electrode due to; 1) a thin, highly conductive, light weight matrix with small pores, 2) high flexibility, 3) mechanical and chemical stability, 4) higher durability, 5) versatility and 5) lifetime of the electrode [1-4]. However, the carbon fiber paper is easily broken, changes its structures, and closes of porosity [5]. That is not suitable to use as fuel cell electrode according to Ohmic resistance effect. In order to fabricate the high electrical conductive carbon nanofiber paper, carbon nanotubes (CNTs) is an interesting material for increasing the comprehensive performance of carbon fiber paper [6] due to high surface area and high electrical conductivity. That makes CNTs very attractive to be used as electrodes [7,8]. In this study, multiwalled carbon nanotubes (MWCNTs) were used with surface modification as the disperse phase inside polyacylonitrile (PAN) matrix to produce the carbon nanofiber paper (CNP). The prepared CNP with PtRu/C catalyst was used to fabricate gas diffusion layer (GDL) of electrode including both anode and cathode. Before mixing with PAN, MWCNTs were modified by coating with PAN nanofilm via admicellar polymerization technique. For testing the performance of PEMFC, gas diffusion layer (GDL) with using PtRu/C catalyst was combined used with Nafion membrane N115 to prepare the membrane electrode assembly (MEA). The morphology of the prepared CNP and PtRu/C catalyst was characterized by SEM and chemical compositions were characterized by EDS. The fuel cell performance was examined by polarization curve. 2 Experimental 2.1 Chemical and materials Carbon nanotubes powder was produced from Department of physic and material science, Chiang Mai University (Thailand). Polyacrylonitrile (Mw 150,000), ruthenium trichloride, isopropyl alcohol, and sodium borohydrate were purchased from Sigma Aldrich (USA). Hydrogen hexachloroplatinic (IV) acid solution was purchased from Fluka (USA). Carbon black (Vulcan XC-72), carbon paper (EC- TP1-060T) and Nafion Membrane N115 were ISBN:

2 purchased from Electrochem (USA). Sodium hydroxide was purchased from Merck (German). All chemical used are analytical reagent grade and used without further purification. 2.2 Fabrication of carbon nanofiber paper The electrospun CNP modified MWCNTs/PAN nanofiber sheet was prepared by stabilization and subsequently carbonization. Firstly, the stabilization of the nanofiber sheet was taken in the air atmosphere by heating at 230 C with heating rate of 7.5 C/min for 2 hours to arrange the ladder structure of PAN chains. Next, it was carbonized into the horizontal stainless steel chamber with vacuum condition in the furnace with heating rate of 10 C/min to 1100 C for 1 hour to obtain the CNP. The electrical conductivity of CNP was calculated by dividing the resistance of cm 2 area by its thickness. The electrical conductivity was calculated according to equation 1 and 2. ρ= RA / t (1) K = 1/ ρ (2) where ρ is bulk resistance (ohm), R is resistance (obtained from I-V curve), t is thickness, A is surface area, and K is conductivity (S.m -1 ). The resistance was obtained from the curve of the relationship between voltage and current using the linear sweep voltammetry method. 2.3 Synthesis of PtRu/C catalyst According to J. Zhang et al. (2004) [9], 10 % (wt) PtRu/C were prepared by adding 2.7 mmol H 2 PtCl 6 and 2.7 mmol RuCl 3 into 5 g of Vulcan XC-72R carbon black in 25 ml isopropyl alcohol. The solution was continuously stirred for 40 min to complete adsorption of Pt and Ru on the surface of carbon black particle. After that, ph of solution was adjusted from 8 to 10 with 0.1 M NaOH. Then, the mixture was heated at 80 C. Subsequently, 10% NaBH 4 as reducing agent was slowly added by drop wise and then the solution was continuously stirred for 2 hours. The suspension solution was cooled down to room temperature and the cake was washed with DI water through filter paper (No.5) and then dried at 70 C in an oven overnight. 2.4 Single fuel cell testing The fuel cell performance was tested by polarization curve. The single cell (1 1 cm 2 dimension of reaction area) composed of the prepared MEA and a pair of graphite bipolar plate with serpentine flow channels. Nitrogen gas was fed to the single cell in order to test gas leak and to clean the flow channels. After that, hydrogen gas and oxygen gas were purged to the anode and cathode at flow rate of 100 and 200 cm 3 /min, respectively, At cathode side, 70% moisture at 80 C was supplied. The potential of the single cell was measured during the current density was applied from 0 to 900 ma/cm 2. 3 Results and Discussion 3.1 D.C. electrical conductivity The electrical conductivity of differentially prepared CNPs was observed comparing to the commercial CP (EC-TP1-660T), as shown in Fig 1. Different CNPs consisted of CNP without MWCNTs, and CNPs with as-received MWCNTs or PAN-coated MWCNTs, various loading from 0.0 to 1.0% (wt) of each. As the benchmark, EC-TP1-660T had the highest conductivity of approximately 2.70 S/cm, whereas the CNP without MWCNTs or pure CNP was approximately 0.05 S/cm as the lowest. Both were used as reference. In the case of pure CNP, the low electrical conductivity appeared due to its structure containing partly sp 3 hybridizations of carbon-carbon and even substantial dangling bonds pacified with chemisorbed hydrogen. The disorder and sp 3 defects may generate an effective band gap between mobile filled valence band state and empty conduction band states [10]. The electrical conductivity depends on the level of the band gap. In order to reduce its band gap level, MWCNTs should be mixed into PAN to be the composites for a better electrical conductivity. Conductivity (S/cm) EC-TP1-060T 0% 0.2% 0.4% 0.6% 0.8% 1% Sample as-received MWCNTs/CNP PAN-coated MWCNTs/CNP Commercial CP CNP without MWCNTs Fig. 1 D.C. Electrical conductivity measurement of CNPs containing as-received MWCNTs or PANcoated MWCNTs, compared to commercial CP (EC-TP1-060T, surface area = 1.0 cm 2 and thickness = 0.19 mm) and pure CNP as the references ISBN:

3 The electrical conductivity of the CNTs with loading of both as-received MWCNTs and PANcoated MWCNTs was increased up to 0.63 S/cm and 1.30 S/cm, respectively, the highest value of each at loading of 0.6% (wt). When the loading was higher, their conductivity was slightly decreased. The decreasing in electrical conductivity of CNPs at high loading appeared due to low dispersion and distribution of MWCNTs particle inside the fiber that was suggested by J.S. Im et al.(2009) [11]. The CNP with optimum loading of the PAN-coated MWCNTs at 0.6% wt showed higher conductivity than that of the CNP loading the as-received MWCNTs because the PAN-coated MWCNTs dispersed better in the fiber due to the similar chemical structure between the coated PAN film on MWCNTs surface and the PAN matrix of the fiber. 3.2 Characterization of the CNP containing PAN-coated MWCNTs The morphology of CNP containing PAN-coated MWCNTs at 0.6% (wt) was examined by scanning electron microscope (SEM), JEOL model JSM- 5410LV, as shown in Fig 2. Fig 2A and 2C shows the smooth surface and complete fiber of the CNP. These indicated that the structure was not destroyed by stabilization and carbonization for producing the CNP. For the cross-section view as shown in Fig 2B, the image of the CNP showed porous morphology with thickness approximately 40 µm. Hence, the microstructure inside the CNP will improve gas permeability and water removal when it will be used 600X, (C) top view at 2,500X, and (D) size distribution of the CNP (obtained from C) as gas diffusion layer (GDL) of PEMFC. That is the advantage for this application. According to Fig 2C, the diameter of the CNP fibers was analyzed. It was found that the average diameter was 880 nm along the fibers with S.D of 118. The size distribution of the fibers composed in the CNP is displayed in Fig 2D. 3.3 Characterization of the synthesized PtRu/C catalyst The characterization of the synthesized PtRu/C catalyst was performed by scanning electron microscopy (SEM) cooperating combination with energy dispersive spectroscopy (EDS), Oxford model ISIS 300EDS as shown in Fig 3. The SEM image of the synthesized PtRu/C catalyst showed a high degree of agglomeration and the very roughness surface with high porosity of the catalyst. EDS pattern was also observed to analyze the element contents of the catalyst. The atomic ratio of Pt to Ru was approximately 1:1. Oxygen was found at 92.10% (by atomic weight) on the catalyst surface. That is very important for oxygen reduction for cathodic catalyst. It also showed the impurity peak of sodium (Na), which occurred due to sodium hydroxide for adjusting ph 8 of the solution, but it did not affect the activity on the hydrogen oxidation and oxygen reduction in fuel cell during testing. The EDS data were summarized in Table 1. Fig. 3 The SEM image (left) and EDS pattern (right) of PtRu particles on carbon black (Vulcan N112) Table 1 Percentage of the elements of PtRu/C catalyst observed from EDS Fig. 2 SEM images of the morphology of the CNP containing PAN-coated MWCNTs at 0.6% (wt); (A) top view at 100X, (B) cross-section view at Element %Content S.D Pt Ru ISBN:

4 O Fuel cell performance The fuel cell performance of the MEA fabricated from the prepared CNP and from the commercial CP was observed from their polarization curves as shown in Fig 4. From Fig 4A, the potential loss of the polarization curve of the MEA fabricated from the CNP was higher than that of the MEA fabricated from the commercial CP when the current density was applied until to 400 ma/cm 2. That is the cause of lower electrical conductivity of the CNP comparing to that of the commercial CP [12]. Later when the current density above 400 ma/cm 2 was applied, the reactants were rapidly reacted at the interfacial electrode corresponding to rapidly generate water as by-product. It was observed that the potential loss of the MEA with the CNP was lower than that of the MEA with the commercial CP. The reason is that the smaller pore size of the CNP produces the small water droplet that easily transfers to the outside of catalyst layer, whereas the larger pore size of the commercial CP produces the large water droplet that can form the liquid film of water blocking the active site of catalyst [13]. Therefore, the GDL of the MEA with the CNP represented better fuel cell performance than that with the commercial CP did at high current density applied due to better water management. But at low current density applied, the GDL of the MEA with the commercial CP represented the better fuel cell performance than that with the CNP due to higher electrical conductivity of the commercial CP. Fig 4B shows the power density of a single PEMFC. The maximum power densities of the fuel cell prepared by both of the CNP and the commercial CP as GDL were found at 385 and 334 mw/cm 2 at the applied current density of each at 800 and 700 ma/cm 2, respectively. Thus the performance of the fuel cell prepared from the prepared CNP was improved approximately 13% comparing to the performance of the fuel cell prepared from the commercial CP. 4 Conclusion The CNP prepared from the PAN nanofiber containing 0.6% (wt) PAN-coated MWCNTs was the optimum condition for the highest electrical conductivity of 1.3 S/cm, however it was lower than that of the commercial CP. But the power density maximum of its GDL was higher than that of the GDL prepared from the commercial CP, approximately 13%, observed at the maximum of each of 385 and 334 mw/cm 2, at 800 and 700 ma/cm 2 current density, respectively, with the reason of its good water management to protect tolerance water flooding inside the cell affecting on the active site of catalyst. Potential (V) Power Density (mw/cm 2 ) A B Prepared carbon nanofiber paper as GDL Com merical Carbon Paper (EC-TP1-060T) as GDL C u rre n t D e n sity (m A /c m 2 ) Prepared carbon nanofiber paper as DGDL Com m erical Carbon Paper (EC-TP1-060T ) as GDL C urrent Den sity (m A /cm 2 ) Fig. 4 Fuel cell performance observation of the MEA prepared from the prepared CPN and the commercial CP; (A) fuel cell polarization curve and (B) power density of a single fuel cell (1 1cm 2 ) as a function of current density Acknowledgements The authors gratefully acknowledge the financial support from the National Nanotechnology Center (NANOTEC) Thailand with the contact number p References: [1] Lemons, R.A., Fuel cells for transportation, Journal of Power Sources, Vol. 29, 1990, pp [2] Liu, C.H., Ko, T.H., Chang E.C., Lyu H.D., Liao, Y.K., Effect of carbon fiber paper made from carbon felt with different yard weights on the performance of low temperature proton exchange membrane fuel cells, Journal of Power Sources, Vol. 180, 2008, pp [3] Mathur, R.B., Maheshwari, P.H., Dhami, T.L., and Tandon, R.P., Characteristics of the carbon paper heat-treated to different temperatures and ISBN:

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