Low-temperature PEMFC

The reforming process (as applied to a hydrocarbon or alcohol) yields a product stream that consists predominantly of hydrogen, carbon monoxide, carbon dioxide, water, unconverted feedstock, and trace by-products. This product stream mixture, called reformate, is unsuitable for direct use in low-temperature PEMFC and AFC, and some trace by-products (notably organosulfur compounds) will poison both high-temperature fuel cells and low-temperature fuel cells. A membrane for separating and purifying hydrogen from reformate must also be chemically compatible with the compounds in the reformate stream. 

For systems based on low-temperature PEMFCs, the optimal metal ammine would be a material that has a suitable ammonia release profile below 80 °C. This would take advantage of heat integration similar to the way in which waste heat is used to release hydrogen from a metal hydride canister. The metal ammine complex should release the ammonia without causing any additional system losses than that of the ammonia cracker (a theoretical efficiency of 86% and typically above 70% in real systems Thomas and Parks, 2006). 

The CO content in the hydrogen-rich gas at the outlet of shift reactor is sufficient to operate a high temperature PEMFC. For the operation of a low temperature PEM fuel cell the carbon monoxide content has to reduce further. The content will be reduced in the gas precision cleaning stage to a level that low-temperature PEMFC can tolerate. The gas precision cleaning stage is downstream of the shift reactor and upstream of the PEMFC anode located (see Fig. 6.2). 

DAFCs have attracted much attention due to their potential as a clean power source for many applications. For example, stationary systems providers have now demonstrated, all over the world, the ability of low temperature PEMFCs to answer... 

A PEMFC operating at 80°C produces a large amount of low value heat that must be damped in order to maintain the operating temperature. In this respect the low temperature PEMFC is technologically difficult to be integrated in the combined heat and power... 

Perfluorosulfonic add (PFSA) membranes continue to be the industry standard for low-temperature PEMFCs due to their excellent proton conductivity, mechanical and chemical stability that is difficult to surpass. The Nafion membrane produced by DuPont has been the most studied (Mauritz and Moore, 2004 Grot, 2008). Nafion membranes are coded according to the polymer equivalent weight (EW) (first two digits), the membrane thickness (in mil, 1/1000 inch, corresponding to 25 pm) - third, or third and fourth digits) thus Nafion N117 is polymer EW 1100,7 mil thickness. In parallel with these developments, advances have been made on related perfluorinated ionomers that differ from the Nafion -type polymer... 

Fig. 23. Fuel processor configuration with a low temperature PEMFC fuel cell (courtesy ECN).

In low-temperature PEMFCs, those that operate in the range of 65-80°C, the peroxide pathway is problematic because if the peroxide is not quickly decomposed into water, it can form radicals that chemically attack the membrane, causing premature failure. 

The ORR is a major source of efficiency loss in a PEMFC, but it is not the only source of the so-called overpotential. In general, there are also resistive and mass transport losses. The resistive losses are associated with the finite conductivities of the electrolyte and the electrodes and the contact resistance losses at the plate and GDL interfaces, whereas mass transport losses are associated with the lack of adequate fuel or air reaching the reaction sites within the electrode. For low-temperature PEMFC systems that operate below the boiling point of water, mass transport losses are primarily due to the buildup of liquid water in the electrode or gas diffusion layer (referred to as flooding ). Mass transport losses are also associated with the tortuous pathways that exist in the porous electrodes and the GDL. Reducing all of these sources of potential losses is important for achieving the highest possible efficiency. 

Since the discovery of carbon nanotubes in the early 1990s [273] there has been emerging interest in their applicability as catalyst supports for low-temperature PEMFCs. Recently, Lee et al. reviewed the area of Pt electrocatalyst preparation techniques using carbon nanotubes and nanofibers as supports [274]. Here, the emphasis will be on the impact of novel nanostructured carbon supports (ordered mesoporous materials, nanotubes, and nanofibers) on the electrocatalytic activity with respect to direct fuel cell anodes. 

The durability of the catalysts and their supports has been investigated in real or simulated low-temperature PEMFC condition [18, 39, 45]. It has been found that the electrochemical active surface area of the electrodes will be decreased during PEMFC running [45, 46]. The decrease in the electrochemical active surface area contributes to the main performance degradation of PEMFC [47]. Pt or Pt-alloy... 

For a low-temperature PEMFC, the most common problems for reliability and durability are degradation of catalysts and catalyst support oxidation. A number of papers have been published in this area, the details of which will be given in Section 18.2.2. It is expected that at high temperatures, these problems will be more pronounced. Recently, however, high-temperature PEMFC catalysts have begun to attract researchers attention, and several papers have been published. Liu et al. [24, 25] developed a Pt4Zr02/C catalyst and found that it was more durable than Pt/C in PEMFCs operated at 150 °C. Other research activities have mainly focused on the development of oxidation-resistant supports (Section 18.2.3). 

Research on the degradation of catalysts has mainly focused on low-temperature PEMFCs (< 90 °C) [28-42]. For high-temperature operation, studies on eatalyst degradation have been in the areas of phosphoric acid fuel cells (PAFCs) and PBI-based MEAs [41-43]. Since the catalysts used in PAFCs are the same as those in PEMFCs, the degradation mechanisms should be applicable to high-temperature PEMFCs. Normally, catalyst degradation includes two parts Pt catalyst degradation and carbon support oxidation. 

The catalyst layer structures and components in HT-PEMFCs should be different from diose used in low-temperature PEMFCs. For example, water management in an HT-PEMFC is not a problem and thus the required hydrophobicity of the catalyst layer might not be a factor for HT-MEAs. Obviously, Nafion, the commonly used ionomer for low-temperature MEAs, is not suitable for HT applications. Unfortunately, die design and evaluation of HT catalyst layers have not yet attracted attention. This is most likely due to the lack of suitable materials for HT-MEA fabrication. For example, although a significant number of publications look at high-temperature membrane and ionomer development, these ionomers have seldom been used in a catalyst layer to replace Nafion, possibly due to the technical difficulties of doing so. 

At this point we would like to leave the field of low temperature PEMFCs for an excursion to solid oxide fuel cells (SOFCs) and consider an example of an LCA for SOFC modules (with each module combining many stacks within a steel pressrrre vessel) that are rrrass-produced for central power production. It can be assumed that the length of the stack flow field can be related to not orrly the mass of stack materials (e.g. the amount of stainless steel or cerarrrics used) but also to the stack efficiency, which subsequently dictates in part the nurrrber of modules needed to achieve the desired power. Including corrsideration of the balance of plant, the length of the flow field also dictates the amonrrt of fuel available for combustion off the stacks (also related to the cherrristry of balance-of-plant emissions) and therefore the energy that might also be obtained by the overall... 

The type of the catalyst employed in the fuel cell depends on the type of fuel, the solid electrolyte used, and the operating temperature. Here we will consider the recent trends in catalysis for the two major types of the fuel cells, including low-temperature proton exchange membrane fuel cells and high-temperature SOFCs. Depending on the fuel used, low-temperature PEMFCs fall into two major categories hydrogen and direct methanol fuel cells (DMFCs). 

In Equation 5.156, the values of A and B, current density Xq, internal specific resistance r, and limiting current density/l depend on the type and design of the fuel cell. In Table 5.4, representative values of these parameters are listed for low-temperature (PEMFC) and high-temperature fuel cells (SOFC). For SOFC, the current density is large compared to the PEFC. The activation loss for the SOFC should be calculated from the full BV equation. In Figure 5.29, the cell voltages for PEMFC and SOFC are plotted using these parameters. 

Typical Parameters for Low-Temperature PEMFC and High-Temperature SOFC... 

BASF Fuel Cells (formerly PEMEAS or Celanese Ventures) produces polybenzimidazole (PBI)-based high-temperature membrane and electrode assemblies sold under the brand name Celtec . These MEAs operate at temperatures between 120 °C and 180 °C. One of the distinct advantages of high-temperature PEMFCs is exhibited in their high tolerance toward fuel gas impurities, such as CO (up to 3%), H2S (up to 10 ppm), NH3, or methanol (up to several percent), compared to low-temperature PEMFCs. Additionally, waste heat can be effectively used and, therefore, the overall system efficiency is increased. 

Table 3 summarizes the most important degradation modes observed in high-temperature MEAs operating up to 200 °C. However, it must be noted that, except for acid loss modes, which are unique to liquid acid based fuel cells, aU other degradation modes are also observed in low-temperature PEMFCs. 

PEMFCs employ a thin PEM for electrolytes. Currently, there are five main types of PEMFCs, differing from each other by their electrolytes (i.e ion and OH" ion), input fuels (i.e., H2 gas fuel and methanol/water fuel), and operating temperatures (1) low-temperature PEMFCs (LT-PEMFCs H2/O2 [or air] fuel,... 

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