Carbon fiber paper

Attwood PA, McNicol BD, Short RT. 1980. Electrocatalytic oxidation of methanol in acid electrolyte—Preparation and characterization of noble-metal electrocatalysts supported on pretreated carbon-fiber papers. J Appl Electrochem 10 213-222. 

The MPL is normally formed with carbon black and hydrophobic particles (PTFE). The diffusion layer is usually made out of carbon fiber paper (CFP) or carbon cloth (CC) and is a vital component of the MEA and fuel cell because it provides the following functions and properties ... 

In 2007, the same consulting company published another report in which the cost of the DLs had increased slightly to 6% of the overall cost of the stack, compared to the 5% previously estimated [2]. One issue with these analyses and predictions was that they were based on carbon cloth as the diffusion layer, but this material does not reflect what most of the fuel cell companies use (i.e., carbon fiber paper) [3]. 

In another report, James and Kalinoski [4] performed an estimation of the costs for a direct hydrogen fuel cell system used in automotive applications. The assumed system consisted of an 80 kW system with four fuel cell stacks, each with 93 active cells this represents around 400 MEAs (i.e., 800 DLs) per system. The study was performed assuming that the DL material used for both the anode and cathode sides would be carbon fiber paper with an MPL. In fact, the cost estimate was based on SGL Carbon prices for its DLs with an approximate CEP value of around US 12 m for 500,000 systems per year. Based on this report, the overall value of the DLs (with MPL) is around US 42.98 per kilowatt (for current technology and 1,000 systems per year) and 3.27 per kilowatt (for 2015 technology and 500,000 systems per year). Figure 4.2 shows the cost component distribution for this 80 kW fuel cell system. In conclusion, the diffusion layer materials used for fuel cells not only have to comply with all the technical requirements that different fuel cell systems require, but also have to be cost effective. 

As discussed previously, a number of different materials have been considered as potential candidates to be used as diffusion layers in PEMFCs and direct liquid fuel cells (DLFCs). The two materials used the most so far in fuel cell research and products are carbon fiber papers and carbon cloths, also known as carbon woven fabrics. Both materials are made from carbon fibers. Although these materials have been quite popular for fuel cells, they have a number of drawbacks—particularly with respect to their design and model complexity—that have led to the study of other possible materials. The following sections discuss in detail the main materials that have been used as diffusion layers, providing an insight into how these materials are fabricated and how they affect fuel cell performance. 

Table 4.1 shows some of the parameters and properties of the most common carbon fiber paper materials being produced commercially for use in PEM and DLFCs. Figure 4.3 shows an SEM (scanning electron microscope) picture of a carbon fiber paper without any coating. In the following subsection we will briefly discuss the fabrication process of carbon fibers and carbon fiber papers. 

List of Commercially Available Carbon Fiber Papers as Diffusion Layers in Fuel Cells... 

Scanning electron microscope picture of typical carbon fiber paper sheets used in fuel cells (a) Toray TGPH-060 CFP with no PTFE (reference bar indicates 500 pm) (b) close-up view of the TGPH-060 CFP with no PTFE (reference bar indicates 100 pm) (c) Toray TGPlT-060 CFP with 20% PTFE (reference bar indicates 500 pm) (d) close-up view of the TGPH-060 CFP with 20% PTFE (reference bar indicates 100 pm). 

Schematic of the typical and general steps in carbon fiber paper fabrication with wet-laid fibers.

For more information and details regarding the fabrication processes of PAN fibers and carbon fiber papers, please refer to Kinoshita [6], Decrecente, Layden, and Pike [8], and Mathias et al. [9]. 

Of the most common DL materials, carbon fiber papers are widely known for being mechanically weak because their microstructure is destroyed when excessive compression forces are applied to them (i.e., when compressing a fuel cell). This destruction of the materials affects the porosity, which has a... 

In PEMFCs, Ralph et al. [86] tested a Ballard Mark V single cell with two different DLs a carbon cloth (Zoltek PWB-3) and a carbon fiber paper (Toray TGP-090) all the other operating conditions stayed the same for bofh cases. It was observed that the carbon cloth demonstrated a distinct advantage over the CFP at high current densities (>600 mA/cm ), while at low current densities both DLs performed similarly. If was claimed fhaf this was because the CC material enhanced mass transport properties and improved the water management within the cell due to its porosity and hydrophobicity. 

It is also important to take into account that one of the main issues with the carbon fiber paper or cloth used as the DL is the uncontrolled variation in porosity (and other localized properties) of these manufactured conventional diffusion layers that is, the porosity characteristics between carbon papers are not repeatable [57]. These materials are difficult to improve because only average pore sizes and volume densities can be measured and much of the development has been based on empirical parameters. Thus, extensive work has focused on optimizing the MPL in order to reduce the differences within carbon paper fiber and cloth diffusion layers. 

Another important point regarding the fabrication process of MPLs is that, typically, when carbon fiber paper is used as the DL, the MPL is coated on only one surface of the CFP. However, when a carbon cloth DL is used, it is normally coated on both sides with MPLs. Section 4.3.S.4 will discuss these DLs with multiple microporous layers in more detail. 

Ofher fypes of carbon blacks, such as Kefjenblack EC-600JD and Denka, have also been used in MPLs for carbon fiber paper DLs [139,162]. Compared to Vulcan XC-72R, bofh carbon black powders performed better due to their lower resistance and their excellent water transport capabilities, which limit water flooding af higher currenf densifies [162]. 

Kannan, Menghal, and Barsukov [165] and Kannan and Munukutla [166] used a new form of parfially ordered graphitized nanocarbon black, called Pureblack carbon, as parf of fhe MPL for a carbon fiber paper DL. In addition, the nanocarbon black was mixed with nanofibrous carbon (Showa Denko) in order to improve mechanical strength. It was demonstrated that this composite MPL with Pureblack and the nanofibrous carbon performed better than an MPL with Vulcan CX-72R under fully humidified and ambienf pressure conditions, especially at higher current densities. 

Another important point regarding the fabrication process of MPLs is the fact that, typically, when carbon fiber paper is used as the DL, the MPL is coated just on one surface of the CLP. However, when a carbon cloth is used, a homogeneous water suspension of carbon powder and PTFE is filtered under vacuum onto both faces of the carbon cloth material to form the MPLs [153,158,161,171], followed by drying and sintering as mentioned earlier. Antolini et al. [161] were able to demonstrate that carbon cloth with double MPLs, for both the anode and the cathode sides, showed better performance than when a CFP was used as the cathode DL with one MPL. At low current densities, the difference between the two DLs was not as obvious, but it became more evident at higher current densities because the limiting current densities for each case are quite different ( 1.6 A cm for CFP vs. 2.7 A cm for CC) (see Figure 4.20 for more details). 

Wang et al. [131] used carbon composite MPLs (AB and Black Pearls 2000), similar to those presented in Reference 164, and coated them on both sides of a carbon fiber paper (similar to that for carbon cloths see Sechon 4.3.3.1). In this... 

Polarization citrve for PEMFCs with two different cathode diffusion layers carbon fiber paper with one MPL and carbon fiber cloth with two MPLs. Operating conditions ceU temperature of 85°C, O2/H2 dewpoint temperatures of 90/100°C gas pressures of 2 atm. CFP DL was a TGP-H-090 with 20 wt% PTFE in the MPL. CCs were PWB-3 from Stackpole cathode CC had 15 wt% PTFE in the MPL near the CL and 30 wt% PTFE in the MPL near the flow field. The anode CC had 15 wt% PTFE in both MPLs carbon loading on the MPL was not specified. The catalyst Pt loading was 0.4 mg cm and the Nation loading was 1.1 mg cm for all catalyst layers the membrane was a Nation 115. (Modified from E. Antolini et al. Journal of Power Sources 163 (2006) 357-363. With permission from Elsevier.)... 

Schmitz et al. [184] tested various carbon fiber papers with different thicknesses as cathode DLs in PEM fuel cells. It was observed that the cell resistance dropped when the thickness of the DL increased thus, thicker materials are desired in order to improve the electrical conductivity. It was also mentioned that the optimal thickness for the DL is usually between the thinnest and the thickest materials because the two extremes give the lowest performance. In fact, in thin DLs, the water produced can fill pores within the material, resulting in flooding and the blockage of available flow paths for the oxygen. Similarly, Lin and Nguyen [108] concluded that thinner DLs (without MPLs) were more prone to liquid water accumulation than thicker ones. 

In DMFCs, Xu et al. [119] tested various carbon fiber papers with different thicknesses (TGP-H-030, TGP-H-060, TGP-H-090, and TGP-H-120) as anode... 

The determination of the capillary pressure of a diffusion layer is critical, not only to have a better understanding of the mass transport mechanisms inside DLs but also to improve their design. In addition, the accuracy of mafhemafical models can be increased with the use of experimental data obtained through reliable techniques. Both Gostick et al. [196] and Kumbur et al. [199] described and used the MSP method in detail to determine the capillary pressures of differenf carbon fiber paper and carbon cloth DLs as a function of the nonwetting phase saturation. Please refer to the previous subsection and these publications for more information regarding how the capillary pressures were determined. 

Through-plane permeability is usually one of the most common parameters given by manufacturers for carbon fiber papers and carbon cloths, even though it is often not specified as through-plane permeability. It is important to note that commercial instruments, such as permeameters and Gurley method instruments, are used in the fuel cell industry to measure this permeability [197,218]. 

These tests were performed on materials with the same characteristics but with different thicknesses thus, the intrinsic thermal conductivity could be resolved at different temperatures and compression pressures. Through these tests, the thermal conductivity of TGP-H carbon fiber papers was measured and achieved the same value as that reported by the manufacturer. In addition, it was observed that the thermal conductivity of the CFPs decreased from 1.80 + 0.27 W m i K i (af 26°C) to 1.24 + 0.19 W m-i K i (at 73°C). This result was suggested to be due to the presence of carbonized thermosetting resin on the CFPs. The thermal conductivity of fhe resin, which is a thermosetting polymer and acts as a binder, decreases with increasing temperature. For carbon cloth (without any resin), no significant changes in thermal conductivity were noted when the temperature was increased. 

The compressive behavior of a DL is a very important mechanical property. Therefore, to study the mechanical properties of various diffusion materials (carbon cloths, carbon fiber papers, and carbon felts), Escribano et al. [251] used a compression cell. The sample diffusion materials were placed between the two plates of the cell, and the thickness and deflection of each sample were measured as a function of the compression pressure. These researchers... 

Soler, Hontanon, and Daza [268] tested two different FF designs with a number of carbon fiber paper and carbon cloth DLs in order to determine the best combination. They measured the pressure drop of the flow field in a nonactive fuel cell with each DL material with oxygen, air, and nitrogen. The researchers... 

Another example of neution imaging is the one presented by Yoshizawa et al. [273], who compared the performance of carbon cloth and carbon fiber paper with a parallel FF design. The CC had a better performance than the CFP at high current densities, but the CFP showed less water content over the whole active area. Thus, it was concluded that the CC diffusion layer was less influenced by the accumulation of water because the transport of oxygen toward the catalyst zones was sufficient while still keeping the membrane humidified. 

C. H. Liu, T. H. Ko, and Y. K. Liao. Effect of carbon black concentration in carbon fiber paper on the performance of low-temperature proton exchange membrane fuel cells. Journal of Power Sources 178 (2008) 80-85. 

X. Zhang and Z. Shen. Carbon fiber paper for fuel cell electrode. Fuel 81 (2002) 2199-2201. 

C. Ji, G. J. Fleming, M. Fleming, and M. Mathias. Acrylic fiber bonded carbon fiber paper as gas diffusion media for fuel cell. US Patent 2007/0218346 (2007). 

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