Carbon paper PEMFC

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

Chen et al recently reported the fabrication and characterization of a high power self-breathing PEMFC with optimally designed wet KOH etched flow-fields and electrodes [49]. Ni/Cu/Au layers are used for current collecting, the 1.5 /im-thick in-between layer instead of a thick Au layer allowing to reduce the fabrication cost. The MEA is composed of a classical Nafion -1135 membrane with Pt-alloy sprayed carbon paper on both sides. The two silicon electrodes are sandwiched and pressed at RT with the MEA, and then sealed with epoxy. A base-chip formed by drilled Pyrex glass anodically bonded with KOH eched silicon acted both as H2 inlet/outlet manifolds and as a support for a... 

A well-qualified substrate layer should have excellent gas permeability, high electron conductivity, smooth surface, good mechanical strength, proper wettability, stable chemical and heat properties, as well as low cost. The most common SL materials used in PEMFCs are carbon-fiber-based products, such as non-woven carbon papers and woven carbon cloths. This is mainly due to their high porosity (> 70%) and good electrical conductivity. Besides these carbon materials, some metal substrates such as sintered porous titanium and stainless steel fiber felt have also been explored as SLs [12-14] because of their high mechanical strength, ductility, and low cost. 

It can be observed fliat PEMFC performance employing carbon cloth GDLs is different from that when using carbon paper GDLs. Ralph et al. [39] showed that at high current density wifli internal humidification in Ballard Mark V cells the carbon cloth offered a distinct advantage compare to carbon paper. They deemed the surface porosity and hydrophobieity of the carbon cloth substrate to be more favorable for the movement of liquid water, and thus both water management and... 

Carbon materials, such as carbon cloth and carbon paper, are also good substrates for the deposition of photocatalysts, due to their low resistivity and cost. They are also widely used in the proton exchange membrane fuel cell (PEMFC), and are also commercially available at www.fuelcellstore.com. Carbon cloth/paper is supplied with an untreated surface or reinforced with PTFE. Since the photocatal5ftic reactions at the anode side involve three phases liquid electrol)4e, solid anode photocatalyst, and the produced gaseous CO2, carbon materials with hydrophilic surfaces are preferred in the fabrication of photoanodes. Compared to the previous two substrates, carbon material can be used directly without any pretreatment. 

A PEMFC consists of two electrodes and a solid polymer membrane, which acts as an electrolyte. The proton exchange membrane is sandwiched between two layers of platinum porous electrodes that are coated with a thin layer of proton conductive material. This assembly is placed between two gas diffusion layers such as carbon cloth, carbon paper enabling transfer of electrical current and humidified gas. Some single cell assemblies can be mechanically compressed across electrically conductive separators to fabricate electrochemical stacks. 

Each of these electrodes have some advantages and disadvantages. The carbon paper electrode are more expensive than the carbon cloth electrodes. Efforts to reduce the cost of this component has been reported by Rajalakshmi et al. (2005). Evolution of the preparation method of PEMFC electrodes is described by Antolini (2004). 

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. 

Once the paper is cured, the carbonization and graphitization steps are performed under an inert environment. The exact temperatures for these two steps depend on the desired characteristics of the final products. In general, it is difficult to determine the best temperatures for these steps. In addition, the rate at which the sample is heated inside the furnace is an important parameter that determines the characteristics of the final product. For example, Mathur et al. [11,12] determined that a carbonization temperature of around 2,000°C (with a 900°C/hour heat rate) and a graphitization temperature of 2,500°C produced a CFP that performed the best in a PEMFC because it improved the hydrophobic properties of the material. 

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. 

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.)... 

In this chapter, after recalling the working principles and the different kinds of fuel cells, the discussion will be focused on low-temperature fuel cells (AFC, PEMFC, and DAFC), in which several kinds of carbon materials are used (catalyst support, gas-diffusion layer [GDL], bipolar plates [BP], etc.). Then some possible applications in different areas will be presented. Finally the materials used in fuel cells, particularly carbon materials, will be discussed according to the aimed applications. To read more details on the use of carbon in fuel cell technology, see the review paper on The role of carbon in fuel cell technology recently published by Dicks [6],... 

The GDL is located on the back of the CL in order to improve gas distribution and water management in the cell. This layer has to be porous to the reacting gases, must have good electronic conductivity, and has to be hydrophobic so that the liquid produced water does not saturate the electrode structure and reduce the permeability of gases. The GDL needs to be resilient and the material of choice for the PEMFC is usually carbon fiber, paper or cloth, with a typical thickness of 0.2-0.5mm [74,75], This macroporous support layer is coated with a thin layer of carbon black mixed with a dispersed hydrophobic polymer, such as P I LL, in order to make it hydrophobic. This latter compound can, however, reduce the electronic conductivity of the GDL, and limit the three-phase boundary access. 

The cost of Gas Diffusion Media (MPL + GDL) [13] is quoted to be aroimd 10- 15 per m, and is likely to be primarily determined by processing costs. Both carbon/graphite paper as woven stmctures are being used as substrate, which is made hydrophobic by, for example, a PTFE coating. The microporous layer mostly consists of carbon powder mixed with a PTFE emulsion, which is cured by a heat treatment. Clear directions for cost reduction have not been found. As the gas diffusion media play a critical role in the performance of the PEMFC, especially determining the maximum power output makes it worthwhile to focus on the GDM performance, rather than on the cost per m. ... 

The carbon powder corrosion in the MPL can also occur in the environment of an operating PEMFC. Porosimetry measurements indicate that carbon is lost from the MPL dining operation. " Porosimetry measurements have also shown that the GDL pore stmcture changes during lifetime tests. Large pore (30-60 pm diameter) volume has decreased, while small-pore volume increases. The loss in large-pore volume is probably due to irreversible compression due to cell compression. However, there are ordy a few hterature papers that examine mechanical degradation and review the effect of compression of the GDL on PEMFC performance. ... 

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