Water Balance in PEFC

A key element in PEFC performance is the water balance. There is a complex relationship between moisture content and performance in PEFCs. For high ionic conductivity, the polymer electrolyte membrane must have high moisture content [8], However, as discussed in Chapter 5, liquid accumulation can restrict reactant availability at the electrode. 

Diffusion media Material type Geometric parameters Microporous layer Hydrophobic additive Compression Heat transfer Phase change 

Flow channel interface Interfacial parameters Surface energy (coatings) Channel/land ratio Pressure drop Convection 

Other interactions Anode/cathode interaction Catalyst layer properties Coolant channels 

1 Overall Water Balance Fuel Cell Mass Balance 

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While there has been significant activity in this area in the patent literature, relatively little effort has been reported in peer-reviewed publications until quite recently. Several factors complicate an investigation into subfreezing phenomena. The first of these is that, other than mapping the nature and extent of phase changes as a function of temperature, freeze-related phenomena are inherently transient problems, as waste heat will raise the internal cell temperatures and stack exit temperatures well above ambient conditions. While there are many papers describing thermal and water balance in PEFC systems, many of these early papers describe... 

Perry, M.L. and Darling R.M. (2004) Optimizing PEFC stack design and operation for energy and water balance in transportation systems, Electrochem. Soc. Proc., PV2004-21, 634. 

Psychrometrics is the study of nonreacting moist air mixtures and is critical to understand the water balance in low-temperature PEFCs. Nonreacting moist mixtures can be evaluated exactly like other nonieacting gas mixtures, but since engineering with moist air mixtures is so common, additional parameters and special charts have been developed to aid in calculation and analysis, hi most fuel cells, water is produced as a product and must be removed from the fuel cell as part of the effluent mixture. In low-temperature PEFCs, the water balance is critical to maintain proper electrolyte conductivity while avoiding electrode flooding. 

While achieving an overall water balance in a PEFC will generally improve performance compared to a highly flooded or dry condition, the liquid water distribution in a PEFC is generally highly nonuniform, and small accumulations or areas of drying can result in substantially reduced performance and durability. 

Flooding Condition Adjustment When the water balance is accumulating liquid water mass, a periodic ejection of droplets can maintain the water balance in the fuel cell since the liquid droplets are so dense compared to gas phase ejection. An illustration of the process of water buildup and ejection from the DM is shown in Figure 6.19. In the steady state, the water from generation must be exactly balanced by that removed. Although water droplet ejection is a periodic process, a H2 PEFC can be operated in a net flooding condition and still achieve relatively stable performance with periodic ejection. If the liquid water accumulation restricts gas-phase flow to the catalyst surface, performance instability will occur, however, until a new equilibrium is achieved. 

We have just discussed the global water balance in a PEFC, but we have also mentioned that actual flooding loss is a localized phenomenon that can occur as a film resistance and pore filling in the catalyst layer, DM, and channels. To grasp the localized flooding phenomenon, it is also important to understand the macroscopic water transport processes which occur within the fuel cell media. 

Water Flux in Polymer Electrolyte Membranes Water flux in the solid electrolyte membrane of the PEFC must be understood to grasp the concept of a local water balance in the fuel cell. From Chapter 5, we know that the ionic conductivity of perfluorosulfonic acid-based solid polymer electrolytes is a strong function of water content. Within the electrolyte, there are four basic modes of transport, as schematically illustrated in Figure 6.21 ... 

Obviously, the CCL not only determines the rate of currenf conversion and the major portion of irreversible voltage losses in a PEFC, but also plays a key role for the water balance of the whole cell. Indeed, due to a benign porous structure with a large portion of pores in the nanometer range, the CCL emerges as favorite water exchanger for PEFCs. Once liquid wafer arrives in gas diffusion layers or flow fields, PEFCs are unable to handle if. 

Water management is one of the most critical and widely studied issues in PEFC. Water management is referred to as balancing membrane hydration with flooding avoidance. These are two conflicting needs to hydrate the polymer electrolyte and to avoid flooding in porous electrodes and GDL for reactant/ product transport. 

Distributions of water and reactants are of high interest for PEFCs as the membrane conductivity is strongly dependent on water content. The information of water distribution is instrumental for designing innovative water management schemes in a PEFC. A few authors have studied overall water balance by collection of the fuel cell effluent and condensation of the gas-phase water vapor. However, determination of the in situ distribution of water vapor is desirable at various locations within the anode and cathode gas channel flow paths. Mench et al. pioneered the use of a gas chromatograph for water distribution measurements. The technique can be used to directly map water distribution in the anode and cathode of an operating fuel cell with a time resolution of approximately 2 min and a spatial resolution limited only by the proximity of sample extraction ports located in gas channels. 

Based on system requirements discussed above, fuel cell APUs will consist of a fuel processor, a stack system and the balance of plant. Figure 1-11 lists the components required in SOFC and PEFC systems. The components needed in a PEFC system for APU applications are similar to those needed in residential power. The main issue for components of PEFC systems is to minimize or eliminate the use of external supplied water. For both PEFC and SOFC systems, start-up batteries (either existing or dedicated units) will be needed, since external electric power is not available. 

Multiphase flow in porous media is a very important topic for low-temperature PEFCs because water produced at the cathode flows through the porous catalyst layers and porous gas diffusion media. Any local blockage of normally open pores restricts reactant flow to the reaction sites, a phenomenon known as flooding. The water balance and flooding in a PEFC is described in detail in Chapter 6. Here, the basic fundamentals that describe two-phase flow in porous media are described to guide and understanding. 

Example 5.18 Determination of Coolant Flow Rate Required Consider a 100-plate, 10-kWe PEFC stack operating at 48% thermal efficiency, as determined from a stack voltage measurement. Each individual fuel cell is to be cooled by a flowing Uquid water coolant channel. In order to balance the water generated in the fuel cell and prevent flooding, it is... 

For DAFCs using a liquid feed, like the DMFC, the water balance and fuel crossover problem are more acute than the hydrogen fuel cell. Dilute Uquid solutions, thicker membranes, and capillary pressure management are used to control these two issues. As a result of the high methanol and water crossover in the DMFC, the open-circuit potential is very low, and performance is also low compared to the H2 PEFC. However, the use of diffusion barriers in the anode and capillary pressure management eliminates the need for highly dilute methanol solutions, and these systems may ultimately be more appropriate than their H2 PEFC counterparts for portable applications. 

For the PEFC, the water balance is highly coupled to the temperature distribution, as discussed. However, direct measurement of localized temperature is difficult, due to the two-phase nature of flow in the gas channels and the small through-plane dimensions of a typical electrolyte. Besides infrared measurement, the most conunonly applied technique is the direct embedding of a thermocouple or thermistor within the bipolar plate. This approach is acceptable for most fuel cell varieties. If all thermal transport parameters, such as specific heat, thermal conductivity, and contact resistance, are known, calculation of the temperature profiles within the fuel cell can be accomplished using embedded thermocouple data and analytical or computational heat transfer models. 

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. 

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