Improved performance of PEM fuel cell using carbon paper electrode prepared with CNT coated carbon fibers

Improved performance of PEM fuel cell using carbon paper electrode prepared with CNT coated carbon fibers

Electrochimica Acta 54 (2009) 7476–7482 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elec...

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Electrochimica Acta 54 (2009) 7476–7482

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Improved performance of PEM fuel cell using carbon paper electrode prepared with CNT coated carbon fibers Priyanka H. Maheshwari, R.B. Mathur ∗ Carbon Technology Unit, Division of Engineering Materials, National Physical Laboratory, New Delhi 110012, India

a r t i c l e

i n f o

Article history: Received 1 July 2009 Received in revised form 30 July 2009 Accepted 30 July 2009 Available online 7 August 2009 Keywords: Carbon fiber Fuel cell Power density Polarization Porosity

a b s t r a c t Porous conducting carbon paper has been identified as the most suitable material to be used as a backing material for the fuel cell electrode. The surface of carbon fiber, the major constituent of the carbon paper was modified by: (1) removing the functional groups by heat cleaning process and (2) coating the non-functionalized carbon fiber with multi-walled carbon nanotubes (MWCNTs). This has a marked influence on the fiber–matrix interactions during later stages of processing of carbon paper that helped in controlling its various characteristic properties. Using the carbon paper formed with CNT coated carbon fiber as electrode, the maximum power density achieved from a unit fuel cell was found to be 783 mW/cm2 as compared to 630 mW/cm2 when the paper was formed with normal fiber. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Fuel cells have gained attention in the last few years not only due to depleting energy resources but also due to growing concern about urban air pollution and consequent environmental problems. These are electrochemical cells that use hydrogen and oxygen/air as the fuel and oxidant respectively, generating energy and water as the only by-products. Because of their capability of directly converting chemical energy into electrical energy, they have high efficiency [1,2]. Among all the fuel cells PEM fuel cell is much preferred because of its high power density, favorable stability of cell voltage during lifetime, absence of corrosive liquid electrolyte, favorable efficiency and low operating temperature. The electrode substrate also known as gas diffusion layer provides mechanical support for the electrocatalysts. It allows diffusion of gaseous reactants from the bulk flow streams to the reaction site within the catalyst layer and removal of the product water from the reaction site to the bulk flow streams. It transfers heat and electrons through the cells and maintains a uniform contact pressure between the catalyst layer and the matrix containing the electrolyte. Because of its critical role in fuel cell performance, much effort has been guided towards optimizing its properties [3–10]. Porous conducting carbon paper has been identified as the

∗ Corresponding author. Tel.: +91 11 45608426; fax: +91 11 45609310. E-mail address: [email protected] (R.B. Mathur). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.07.085

most promising material as a GDL, not only because of its high conductivity that allows easy flow of electrons but also because of its fine porosity that allows uniform distribution of the reactant gases over its surface. The high mechanical strength of the carbon paper provides mechanical integrity to the MEA. In their previous studies the authors have reported the process of making porous conducting carbon paper by first making carbon fiber preform by paper making technology followed by its subsequent impregnation with resin, compression molding and carbonization [9]. Because of the strong fiber matrix interaction during the curing and carbonization cycles (with the commercially available fibers), large volume shrinkage occurred along the thickness of the sample which not only resulted in the decrease in the porosity of the samples but also in a non-uniform pore size distribution (PSD). In the present study we report two different approaches of controlling the fiber–matrix interactions (and hence the porosity and pore size distribution) by modifying the surface of carbon fibers. In the first case by using surface cleaned carbon fibers along with the neat fibers (as received commercial fibers) in varying proportions, while in the second case the surface of the carbon fiber has been further modified by MWCNTs coating. References have been cited whereby CNTs have been effectively used as a catalyst support for better utilization of the later, which in turn leads to increased performance [11–15] and durability [16] of fuel cell. We report here significant improvement in the physical and electrical properties of the carbon paper (produced by this new technique), which in turn contributes to the enhanced performance of the fuel cell.

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Fig. 1. Schematic of the experimental set up for the growth of MWCNT on carbon fiber surface by CVD technique.

2. Experimental 2.1. Preparation of carbon paper PAN based T 300 carbon fiber chopped in 10 mm length was used as the starting material. The commercially available fiber is normally functionalized to make bonds with the resin and surface coated with thin layer of resin (0.1%) for better handling and wetting with any resin system. Different steps were taken at the initial stage to modify the fiber surface so as to influence the properties of the resulting carbon paper. Thus carbon fiber performs were prepared [9] with - As received carbon fiber (commercially available), type A. - Desized and heat treated carbon fiber, type B. - Type B fiber coated with CNT, type C. The type B carbon fiber was produced by heating the carbon fiber “type A” at 750 ◦ C for an hour in an inert atmosphere. This fiber was then coated with nanotubes in a continuous manner using chemical vapor deposition (CVD) process. Ferrocene was used as the source of Fe catalyst whereas toluene was used as hydrocarbon source. The type B carbon fiber (with all sizing and functional groups removed) tow was passed through 12 wt% ferrocene solution and squeezed through a hole of diameter 1 mm, so that it is uniformly coated with ferrocene, prior to its entry into the CVD reactor (Fig. 1). The liquid feed solution (Toluene) is injected into the reactor at the rate of 7 ml/h through the capillary tube and carried to the reaction zone by the steady flow of argon. The flow rate of argon was maintained at 2 l/min, and the furnace temperature at 750 ◦ C. At the exit end the fiber was collected onto a spool which in turn was connected to a motor that pulls the fiber tow. The pulling speed was adjusted to control residence time of the fiber in the effective reaction zone which was nearly 5 min. The amount of CNT growth was thus controlled. The SEM picture of the type C fibers clearly shows a uniform coating of MWCNTs on the fiber surface (Fig. 2). The amount of MWCNTs grown on the fiber surface was determined through thermal gravimetric analysis of the samples as discussed later in the text. In the first set of experiments carbon fiber performs were prepared [9] using different vol. ratios of type A (as received) and type B (heat treated) carbon fibers, whereas in the second set of experiments the carbon fiber performs were prepared using different vol. ratios of type A and type C (CNT coated) carbon fiber. Fig. 2 (inset) shows the SEM of the carbon fiber perform processed with type C carbon fiber, indicating a strong anchoring of CNTs even after the processing. The carbon fiber performs thus prepared were impregnated with phenolic resin (obtained from ‘IVP India Ltd.’) such that the ratio of reinforcement and the resin is 1:1 by volume. The impregnated performs were then molded into sheets by compression molding technique. Following molding, a post-cure is performed at 150 ◦ C for 2 h in air to ensure full curing and cross-linking of

the binder material before carbonization. The samples so obtained were known as green samples. These were further heated to 2200 ◦ C in an inert atmosphere with a heating rate of 900 ◦ C/h [9,10]. In the following text the carbon paper samples formed with type A, type B and type C carbon fibers are referred to as a, b, and c respectively. The samples formed with different proportions of type A and type B fiber (3:1, 1:1, 1:3) are termed as b1, b2, b3 and those formed with different proportions of type A and type C fiber (3:1, 1:1, 1:3) are termed as c1, c2, and c3 respectively. 2.2. Characterization of the carbon paper The electrical resistivity of the paper was measured using the four probe technique. Kiethley 224 programmable current source was used for providing current. The voltage drop was measured by Keithley 197 A auto-ranging microvolt DMM. The Flexural strength and modulus of the carbon paper samples were measured on the INSTRON machine model—4411 according to ASTM: D 1184-69. The stress strain curves along with the flexural strength of the samples were directly recorded from the software provided with the machine. Fractured surfaces of the individual samples were observed under the SEM, model Leo S440. The porous network was determined using mercury porosimetry analyzer (model: Poremaster (33/60), P/N 05060) obtained from Quantachrome Instruments, USA. In this method, mercury (Hg) with its very high surface tension (4.80 × 10−5 J cm−2 ) is forced into the pores of the sample. The amount of Hg uptake as a function of pressure allows one to calculate the total porosity as well as the pore size distribution.

Fig. 2. SEM image of the surface of type C carbon fibers. Inset showing SEM image of the carbon fiber preforms prepared with type C carbon fiber, indicating a strong anchoring of MWCNTs with the carbon fiber surface.

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Fig. 3. (a) TGA and (b) DTG curve of MWCNT coated carbon fiber.

2.3. Formation of membrane electrode assembly and fuel cell performance

Fig. 4. Variation in the flexural strength and flexural modulus of the carbon paper with different proportions of type A and type B carbon fibers.

The carbon paper samples of size 25 cm2 were teflonised and Membrane Electrode Assembly (MEA) was prepared [10] with 0.4 mg/cm2 Pt loading and Nafion membrane-1135 as the electrolyte. The fuel cell was evaluated [10] while operating it at 70 ◦ C temperature and 100% humidified conditions. The fuel (hydrogen) and oxidant (oxygen) streams were applied to the anode and cathode at atmospheric pressures respectively and the gas stoichiometry for both the gases is kept constant at 1.5 slpm.

As shown in Fig. 4 the weak fiber–matrix bonding is responsible for decrease in the flexural modulus of the paper with increasing amount of type B fiber and reaches as low as 6.2 GPa for sample ‘b’ as compared to 15 GPa for sample ‘a’. The lower modulus or stiffness results in improved flexibility in the paper. The fracture behavior (Fig. 6) of the carbon paper also shows more plastic behavior with increasing amount of type B fiber (b 3 and b).

3. Results and discussion 3.1. Amount of CNT growth on carbon fibers TGA analysis was used to determine the amount of CNT growth on the surface of type C carbon fiber. The TGA run was carried out in air from RT to 1000 ◦ C at the rate of 10 ◦ C/min and kept isothermal at 1000 ◦ C for 30 min, to ensure complete oxidation or expunge of all carbon material. As shown in Fig. 3, a weight loss of nearly 97.4% was observed (curve a), indicating nearly 2.6% of ash as leftover which could be the catalyst (Fe or its oxide). The two DTG peaks in curve b correspond to weight loss due to MWCNTs (570 ◦ C) and carbon fibers (750 ◦ C). Since the amount of catalyst in pure as grown MWCNTs is found to be 13–15% [17], the remainder 2.6% of catalyst in the present case would therefore correspond to 18–20% MWCNTs coated on carbon fibers. 3.2. Characteristics of the carbon paper Fig. 4 shows the variation in the mechanical properties of the carbon paper with different ratios of type A and type B carbon fibers. As shown in the figure the flexural strength of the carbon paper decreases with increasing amount of type B fiber (from an average of 58 MPa to nearly 40 MPa). This is expected, since the process known as heat cleaning removes the functional groups from the carbon fiber surface, which in turn weakens the fiber matrix interactions, thus leading to decrease in the flexural strength. This is also evident from the SEM micrographs (Fig. 5a) of the fractured surface of the carbon paper ‘b’, which indicates significant fibers pull outs, suggesting a weak fiber–matrix bonding. On the other hand the SEM of the fractured surface of carbon paper ‘a’, (Fig. 5b) shows a completely brittle fracture due to strong fiber–matrix interactions [18] as evident by the columnar type of matrix microstructure at the fiber–matrix interface. This is responsible for higher strength and brittle fracture of the composite paper.

Fig. 5. (a) SEM image of the fractured surface of the carbon paper formed with type B fiber. (b) SEM image of the fractured surface of the carbon paper formed with type A fiber, showing strong fiber matrix bonding.

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Fig. 6. Stress–strain curves of different carbon paper samples, during mechanical testing.

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Fig. 8. SEM image of the fractured surface of the green sample (cg) formed with type C fiber showing fiber pull outs. SEM image in the inset clearly showing MWCNTs on the fiber surface.

Fig. 7 shows the variation in the mechanical properties of the carbon paper produced with different ratios of type A and type C fibers. As shown in the figure the strength of the carbon paper increases with increasing amount of type C fiber which is quite unexpected. The reason being, non-functionalized CNTs have always been found to have poor wetting with the polymer matrix which was supposed to persist during the carbonization as well. In order to understand the unusual behavior it was thought interesting to study the strength of the sample at the green (polymer) stage also for both type A and type C fiber based papers. An average on ten samples show that the flexural strength of the green sample formed with type A fiber (ag) is 268.74 ± 37.1 MPa. The strength for the green sample prepared with type C fiber (cg) on the other hand was found to be 164.1 ± 27.8 MPa (∼40% less). The results were confirmed by the SEM micrographs of the fractured surface of the green sample ‘cg’, that show lot of fiber pull outs indicating a very weak CNT coated fiber–polymer interaction (Fig. 8). The SEM picture (inset) clearly shows presence of MWCNTs on the fiber surface at this stage, which is not clearly visible after carbonization (Fig. 10). There is a marked difference in the flexural modulus of the green samples ‘ag’ and ‘cg’ i.e. 15.1 ± 3.2 GPa and 10.8 ± 2.7 GPa respectively. This is also reflected in the fracture behavior of the two samples as shown in Fig. 9, while the behavior for sample ‘ag’ is primarily elastic that of ‘cg’ is more or less plastic.

The completely opposite behaviour in the flexural strength of the two samples after carbonization suggests the formation of strong carbon–carbon bond between the MWCNT and carbon matrix as compared to carbon fibers and carbon matrix. This could be due to the large surface area of the MWCNTs, and totally different interactions of the two phases at the nanoscale. This results in high strength and brittle fracture of the carbon–carbon composite (Fig. 10). However as shown in Fig. 7, the flexural modulus of the carbon paper maintains the same trend as in the green samples showing decrease in the value with increasing amount of type C fiber. As a result of this peculiar behavior the carbon paper sample ‘c’ shows high degree of flexibility while maintaining highest strength. This is also evident from the fracture behavior of the samples (Fig. 6), showing large strain to failure of these samples which is as high as 0.68% as compared to 0.37% for sample ‘a’, an improvement by a factor of two. Thus the incorporation of type C fiber helps improving the strength as well as the flexibility in the composite paper. The result is quite significant with regard to the durability of the carbon paper samples during the entire operation of the fuel cell. The nature of interactions of the matrix with the fiber have a marked influence on the volume shrinkage or the thickness of the

Fig. 7. Variation in the flexural strength and flexural modulus of the carbon paper with different proportions of type A and type C carbon fibers.

Fig. 9. Stress–strain curves of the green samples formed with type A and type C fiber.

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Table 1 Effect on the pore size and its distribution for the carbon paper samples prepared with different proportions of type A and type B fibers. S. no.

a b1 b2 b3 b

% volume in intervals Up to 10 ␮m

10–20 ␮m

20–30 ␮m

30–40 ␮m

40–50 ␮m

50–60 ␮m

60–70 ␮m

70–1000 ␮m

Up to 50 ␮m

>50 ␮m

8.3 7.7 6.7 4.7 5.6

2.7 4.0 2.8 2.6 3.2

5.5 7.1 5.4 5.5 8.7

27.1 31.8 33.0 33.6 41.7

26.3 26.1 31.8 35.2 26.4

9.7 8.8 8.9 7.5 4.5

2.9 3.3 1.9 2.1 1.7

17.5 11.2 9.5 8.8 8.2

69.9 76.7 79.7 81.6 85.6

30.1 23.3 20.3 18.4 14.4

Fig. 10. SEM image of the fractured surface of the carbon paper formed with type C fiber indicating a brittle fracture.

paper during processing from green stage to carbonized stage and is exemplified in Fig. 11. As shown in the figure, the thickness of the carbon paper shows an increasing trend with increasing amount of type B fibers. With the total weight of the constituent material remaining the same, increase in the thickness of the paper therefore results in decrease in its density (Fig. 11). This in turn affects the total porosity as well as shown in Fig. 12. The pore size distribution of the carbon paper samples with increasing amount of type B fiber is shown in Table 1. The improvement in the pore size distribution (PSD) is clearly visible, i.e. there is an increase in the number of pores with smaller diameter. As already mentioned, the fiber–matrix interaction and hence the shrinkage in the samples is reduced considerably by using type B fiber, thus leading to a more desirable PSD. In case of carbon paper samples prepared with combination of type A and type C fibers, the trend is reversed. It was found that the

Fig. 11. Variation in the density and thickness of the carbon paper with different proportions of type A and type B carbon fibers.

Fig. 12. Variation in the porosity of the carbon paper with different proportions of type A and type B and type A and type C carbon fibers.

thickness of the carbon paper decreases with increasing amount of type C fibers (Fig. 13). For sample ‘c’ (100% type C fiber) the interactions are so strong that the thickness of the carbon paper is reduced to 0.24 mm as compared to 0.30 mm for sample ‘a’. As expected the porosity of the samples also show a decreasing trend with increasing amount of type C fiber (Fig. 12). A close look at all the curves shown in Figs. 12 and 13 it is concluded that the changes in the properties are moderate up to 75% of type C fiber, after which the changes are drastic (for 100% type C fiber). The pore size distribution of the carbon paper samples with increasing amount of type C fiber is shown in Table 2. As shown the PSD shifts toward pores with smaller diameter as the as the amount of type C fiber increases from 0 to 75% (samples a, c1–c3). However for sample c the trend is reversed probably due to stronger interactions.

Fig. 13. Variation in the density and thickness of the carbon paper with different proportions of type A and type C carbon fibers.

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Table 2 Effect on the pore size and its distribution for the carbon paper samples prepared with different proportions of type A and type C fibers. S. no.

a c1 c2 c3 c

% volume in intervals Up to 10 ␮m

10–20 ␮m

20–30 ␮m

30–40 ␮m

40–50 ␮m

50–60 ␮m

60–70 ␮m

70–1000 ␮m

Up to 50 ␮m

>50 ␮m

8.3 3.7 6.9 1. 4.7

2.7 2.4 3.7 3.3 6.2

5.5 6.7 12.8 13.9 22.8

27.1 36.1 31.3 50.0 30.4

26.3 30.4 24.4 16.0 12.8

9.7 7.7 8.2 3.3 6.6

2.9 2.9 1.9 1.9 3.8

17.5 10.1 10.5 10.3 12.2

69.9 79.3 79.2 84.3 77.2

30.1 20.7 20.7 15.6 22.7

The in-plane electrical resistivity of the carbon paper shows a slight decrease with increasing amount of type B fiber (Fig. 14) whereas it shows a marked reduction with increasing amount of type C fiber and becomes almost half for sample ‘c’ as compared to sample ‘a’. This is because of the fine coating of highly conducting CNTs on the carbon fiber surface, since the electrical conductivity of MWCNT is an order of magnitude higher than the PAN based carbon fiber. The CNTs dispersed in between the fiber network might also help in increasing the conductivity of the composite by facilitating the percolation of current. 3.3. The current–voltage (I–V) performance The polarization curve is the most important characteristic of the fuel cell, in a way that it can be used for diagnosis of the various factors affecting the fuel cell performance. Fig. 15 shows the comparative performance of the unit fuel cells assembled using different carbon paper samples as discussed in Section 2.1. From the curve it is clear that the open circuit potential of the fuel assemblies is around 0.98–1.05 V as compared to the theoretical fuel cell potential of 1.23 V due to generation of internal (stray) currents. When the electrical circuit is closed the potential further drops as a function of current density due to certain unavoidable losses. As soon as the reaction starts the activation over potential comes into picture. Since same amount of Pt loading is maintained for different carbon paper samples, it does not affect much on their comparative performance. As the reaction proceeds, it is greatly influenced by the ohmic polarization, i.e. the voltage loss due to resistance offered by the various cell components (carbon paper, in this particular case) and later due the concentration polarization. The concentration losses in the fuel cell occurs when the reactant is rapidly consumed at the electrode due to the electrochemical reaction so that a concentration gradient is established, and is greatly affected not only by the porosity [10] and the pore size distribu-

Fig. 15. Comparative performance of the PEM fuel cell using different carbon paper samples.

tion of the electrode substrate but also on the partial pressure of the reactant gas. As shown in Fig. 15 the maximum power density of the fuel cell obtained with carbon paper formed with type A fiber (i.e. a) is nearly 630 mW/cm2 . The performance of the fuel cell increases as the amount of heat treated fiber is increased in the paper (samples b2 and b). Power density as high as 755 mW/cm2 have been achieved for carbon paper (b) prepared with type B fiber, thus indicating an improvement of nearly 20%. The papers with combinations of the two fiber types show intermediate values of power densities. Since the different carbon paper samples (formed with the combination of type A and type B carbon fibers) show very little variation in their resistivity values (Fig. 14), the variation in the performance curves can mainly be attributed to improved porosity and pore size distribution (Table 1). The increase in the porosity of the samples and a uniform PSD greatly helps the reactant gases to reach the reaction sites. Non-uniformity in the PSD may result in the reactant gases taking the path of least resistance (larger pores) to flow, leaving the remaining part of the electrode flooded and electrochemically inactive. The size of the pore is another important constraint. Decreasing the pore size increases the total meniscus area through which the diffusion occurs thus leading to an effective utilization of the reactant gases. Similarly the performance of the carbon paper with increasing amount of type C fiber (c2 and c3) also increases accordingly (see Table 2). Power density as high as 783 mW/cm2 have been achieved for carbon paper prepared with 75% type C fiber. This is a further improvement of ∼4% over carbon paper sample b, may be attributed to the higher values of electrical conductivity of the paper (Fig. 14), that helps in reducing the ohmic losses and results in higher value of power density obtained. 4. Conclusions

Fig. 14. Variation in the electrical resistivity of the carbon paper with different proportions of type A and type B and type A and type C carbon fibers.

A novel technique, of coating controlled amount of MWCNTs onto the carbon fiber surface on a continuous scale has been

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established. The performance of the unit fuel cell ensured 24% higher power density when the electrodes were formed by using CNTs coated fibers as compared to the electrode formed with as received (commercially available) fiber. The improved performance can be attributed to the high electrical conductivity and uniform pore size distribution of the carbon paper. A favorable PSD helps a great deal in minimizing the diffusion polarization in the fuel cell thus increasing its performance even at high current densities. A narrow pore size distribution with pore size of 30–50 ␮m is ideal for good fuel cell performance. Use of CNT coated fiber also assisted in maintaining high strength along with introducing flexibility in the paper. This greatly ensures durability of paper especially during the fuel cell assembly and its life cycle performance. Acknowledgements The authors are grateful to the scientists of Central Electrochemical Research Institute, Chennai for evaluation of the fuel cell performance and also to HEG, Bhopal, for carrying out the pore size distribution analysis on the carbon paper samples. Authors also acknowledge the help from Mr. R.K. Seth for TGA analysis and Dr. Vinay Gupta for SEM micrographs. The studies were carried out under the CSIR, NMITLI program on ‘The development of fuel cells based on hydrogen’.

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