In PEMFC operation, water is formed as a reaction product at the positive (oxygen) electrode. It can be seen from reaction (3.2a) that water reaches the side of the membrane that is in contact with the positive electrode, also by being dragged along in the form of hydrated protons. As a result of reaction (3.1a), on the other hand, water leaves the side of the membrane that is in contact with the hydrogen electrode. In part, this unilateral membrane water transport is compensated by the backdiffusion of water due to the concentration gradient that develops, but it is not completely compensated. Asymmetry of the hydraulic pressure on both sides of the membrane may also contribute to membrane water transport. All this leads to excess water at the oxygen electrode during cell operation, while the water content of the membrane next to the hydrogen electrode decreases.
Both of these effects have negative consequences for fuel cell operation.
Excess water at the positive electrode may lead to flooding of the pores of the active layers through which oxygen reaches the catalyst. This will affect the performance of this electrode and may in the end lead to complete cessation of oxygen access to the active layer, and thus to a cessation of fuel cell operation.
Dehydration of the membrane next to the hydrogen electrode, on the other hand, raises the ohmic resistance and leads to a lower discharge voltage of the cell.
To avoid these situations, the hydrogen that is supplied is usually saturated with water vapor, and the oxygen is circulated. Passing next to the electrode, the oxygen becomes saturated with water vapor. It then reaches a chamber with a lower temperature, where the water vapor condenses, and dry oxygen is returned to the electrodes. Sometimes, the periodic release of excess oxygen saturated with water vapor is practiced. The liquid water that is produced may be used for other purposes, such as drinking water.
All these processes must be controlled quite carefully. If water withdrawal is too fast, there is a risk of water loss from the swollen membrane, which not only leads to a drastic rise in resistance but also to fragilization. Cracks may then develop across which the gases may mix, yielding an explosive mixture, with all the catastrophic consequences that ensue.
A large problem in PEMFC operation is a possible partial condensation of water vapor when temperature gradients are present in the fuel cell, and a dual- phase water system develops. The liquid water forming within the MEA or in the channels of the bipolar plates interferes with the access of reactant gases to 54 PROTON-EXCHANGE MEMBRANE FUEL CELLS
the catalytically active layer, thus causing additional polarization of the electrodes and a drop in the cell’s discharge voltage.
For this reason, selecting optimum operating parameters for the system of water withdrawal from operating fuel cells is a very important condition for stable operation of a PEMFC. Many studies on this topic have been carried out. The problem is complicated by the fact that the rate of water formation by reaction (3.2a) is directly proportional to the current. Most often, fuel cells operate under conditions of constantly changing discharge currents (dynamic operating mode). Yet the parameters of the systems for water withdrawal (such as the rate of oxygen supply and circulation) usually are conditioned to a particular value of the rate of water formation. As a result, under real-world operating conditions of the fuel cell, transient situations may arise where the electrodes are flooded temporarily or the membrane partially dries out.
At UTC Power, a peculiar proprietary method of water withdrawal and membrane moistening was developed. Porous plates are pressed against the gas compartments of the fuel cell. Along the back side of these plates, cooling water is circulated at a pressure somewhat lower than the gas pressure. Water produced in the reaction diffuses through the pores of these plates into the circulating flow. When there is a risk of the membrane drying out, water is driven through the plates in the opposite direction (Perry and Kotso, 2004).
Optimum water management continues to be a central concern of PEMFC battery R&D.
The problem of water management in PEMFCs was the topic of three recent review papers. A particular focus in a review by Li et al. (2008) is diagnosis and mitigation of water flooding. Two classes of strategy to mitigate flooding have been developed. The first is based on system design and engineering and is often accompanied by significant parasitic power loss. The second class is based on MEA design and engineering. Reviews by Schmittinger and Vahidi (2008) and Yousfi-Steiner et al. (2008) analyze the influence of poor water management on long-term performance and durability of PEMFCs.
3.3.2 Heat Management
In the high-power-density operation of these fuel cells, large amounts of heat are generated. It follows from what was said in Section 1.3.2 that at a discharge voltage of 0.75 V, the thermal energy generated is approximately as large as the electrical energy generated. At lower voltages, of 0.6 V, for instance, the thermal energy generated is half again as much as the electrical energy generated. Under these conditions, efficient heat management is possible only by cooling with a liquid heat-transfer medium (water or other). For efficient heat transfer, the temperature of the medium must be at least 101C lower than the battery’s operating temperature.
The heat-exchange cooling plates mentioned above are usually designed like the usual cell units with membrane, but without electrodes. The gases supplied pass along one side of the membrane, and the heat-transfer fluid passes along
3.3 SPECIAL FEATURES OF PEMFC OPERATION 55
the other side of the membrane. After leaving these plates, the heat-transfer fluid reaches the definitive heat exchanger, usually equipped with cooling ribs radiating excess heat to the surroundings. The hot heat-transfer fluid can also be used for heating purposes (heat cogeneration).
3.3.3 Partial Pressure of the Reactant Gases
A higher partial pressure of the reactant gases (hydrogen and oxygen) leads to higher rates of the corresponding electrode reactions and thus to a certain decrease in electrode polarization (i.e., an increase in discharge voltage). It must be remembered, however, that the energy effect is not very large, because energy is consumed for compressing the gases (Kazim, 2005).
An important problem for PEMFCs is that of replacing pure oxygen (requiring special equipment for storage and transport) by oxygen from the ambient air. The partial pressure of oxygen in air is low (about 0.2 bar). With a passive air supply (air-breathing electrodes), where the air is not precompressed or pumped, the performance drop is rather significant. In this case, difficulties also arise in the elimination of water accumulating at the cathode. These difficulties can be eliminated in part when using an active air supply by compressing the air to a few bar and circulating it. Air breathing is rarely used, not only in PEMFCs but also in other fuel cell plants of relatively high power. For low-power fuel cells intended for power supply to portable equipment, passive air supply is the only possibility for the attainment of sufficiently low weight and volume. The problems arising under these condi- tions are discussed in Chapter 14.
3.3.4 Influence of Ambient Temperature
The working temperature of a PEMFC battery (about 801C) is usually maintained by the heat liberated during discharge. It is controlled by special equipment regulating the work of the cooling system. The ambient temperature as a rule is lower than the battery’s working temperature. High and low ambient temperatures as a rule have no important influence on battery function. Where needed, the battery may be thermally insulated.
Major problems arise during startup of a cold battery, which for instance has been warehoused or had a long idle period. When the battery’s tempera- ture is higher than the freezing point of water, slow heating by drawing cur- rent through an external circuit of low or zero resistance is feasible. When its temperature is still lower, ice incrustations may have formed in the gas-diffusion and catalyst layers from condensed and frozen water, partly or completely blocking gas access to the catalyst. Freezing water increasing in volume may also destroy the structure of the electrode. Prior to shutting down a hot operating battery, therefore, one must use dry gas to remove all residues of water vapor (Ahluwalia and Wang, 2006; Guo and Qi, 2006;
56 PROTON-EXCHANGE MEMBRANE FUEL CELLS
Oszcipok et al., 2006). In this dried state, a battery will withstand tens of freeze–
thaw cycles without marked performance loss (Hou et al., 2006).