Direct Hydrogen PEFC Systems

Direct hydrogen PEFC systems require extensive thermal and water management to ensure that the PEFC stack operates under the desired design conditions (Figure 3-10). Key components are... 

The choice between a direct hydrogen and a reformate-based system depend on the application. For light duty vehicles, most experts now prefer direct hydrogen systems (hence the focus of the U.S. DOE program), while for stationary applications natural gas reformer-based PEFC systems are favored. 

Chapter 6 is devoted entirely to PEFC systems, including hydrogen- and direct alcohol-based apphcations, issues, and degradation concerns. The specific devotion to PEFCs is based on my personal expertise and the fact the PEFC is the most broadly studied system and most likely to have future ubiquitous application in various applications. From a student perspective, the automotive application tends to draw students into the class, so that the PEFC tends to be the system of greatest student interest. Additionally, multiphase management for PEFCs is especially complex compared to other systems where only single phase flow is present in the reactant and product mixture. Due to its importance in stability, performance, and durability, special attention is taken to detail the water balance and flooding in PEFCs. 

A particular version of the PEFC is the direct methanol fuel cell (DMFC). As the name implies, an aqueous solution of methanol is used as fuel instead of the hydrogen-rich gas, eliminating the need for reformers and shift reactors. The major challenge for the DMFC is the crossover of methanol from the anode compartment into the cathode compartment through the membrane that poisons the electrodes by CO. Consequently, the cell potentials and hence the system efficiencies are still low. Nevertheless, the DMFC offers the prospect of replacing batteries in consumer electronics and has attracted the interest of this industry. 

An alternative to the use of H2 as fuel is methanol, which is a liquid fuel and easy to handle. This can be directly transformed to electrical current in a DMFC (direct methanol fuel cell). The DMFC allows a simple system design. However, presently achieved performance data of DMFC is not satisfactory and material costs are too high. As another alternative, methanol or hydrocarbons (e.g. natural gas, biogas) can be transformed to hydrogen on board the electric vehicle by a reformation reaction. This allows use of the H2-PEFC cell, which has a higher level of development. The reformate feed gas may contain up to 2.5% carbon monoxide (CO) by volume, which can be reduced to about 50ppm CO using a selective oxidizer (Wilkinson et al. [1997]). 

Throughout this textbook, the PEFC has been emphasized because it is a likely candidate for power replacement for portable, auxiliary, stationary, and automotive power systems. As shown in Figure 6.1, the PEFC class of fuel cells includes the hydrogen, direct methanol, direct alcohol, and other fuel cell systems utilizing a solid polymer electrolyte. While the higher temperature molten carbonate and solid oxide fuel cell systems are well-suited for steady power systems, only the low-temperature PEFC offers the rapid startup and lower operating temperatures (20-90°C) required for transient operation of portable, reserve, and automotive power applications. 

The effectiveness of EIS can be greatly enhanced with the use of a reference electrode, which has a stable potential at the time of measurement [3]. A suitable reference electrode allows discernment of the different electrode losses from the overall cell response, resulting in a more appropriate equivalent circuit. Ideally, the collective responses of the anode and cathode will add to the full cell resistance. Because the use of a stable reference electrode in many fuel cell systems is difficult, one common way to examine fuel cell behavior is the use of a dynamic hydrogen electrode (DHE). In this case, one of the electrodes is used as the DHE, with hydrogen flow at this location. It is assumed that the losses associated with the DHE are minor, and all polarizations measured can be attributed to the other electrode. This approach can be dubious and is not appropriate when there are phenomena at the DHE that can affect losses, such as anode dryout in a PEFC. Note that the DHE does not have to be the actual anode in the fuel cell but can be used at either electrode to examine the polarization of the opposing electrode. For example, a DHE can be used at the cathode of a DMFC to examine the polarization behavior of the anode in the DMFC. In this case, of course, the reaction does not galvanically proceed in the desired direction, and external power from a galvanostal/potentiostat system must be applied to drive the reaction in the desired direction. 

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