Fuel cells materials failures

As stated, one of the fundamental problems encountered in the direct oxidation of hydrocarbon fuels in SOFCs is carbon deposition on the anode, which quickly deactivates the anode and degrades cell performance. The possible buildup of carbon can lead to failure of the fuel-cell operation. Applying excess steam or oxidant reagents to regenerate anode materials would incur significant cost to SOFC operation. The development of carbon tolerant anode materials was summarized very well in several previous reviews and are not repeated here [7-9], In this section, the focus will be on theoretical studies directed toward understanding the carbon deposition processes in the gas-surface interfacial reactions, which is critical to the... 

It is important to operate the fuel cell at different compression pressures in order to determine the correct compression pressure for a DL material. If the applied compression pressures are too high, the DLs may deform, both the porosity and permeability of the DL decrease, and the probability of failure modes increases significantly. On the other hand, if the pressures are too low, then gas leaks and serious contact resistance between the components of the cell may be present. Various studies have been presented in which the compression pressure of the fuel cell is varied in order to observe how the cell s performance is affected [25,183,252]. In general, there is an optimal compression pressure range in which the cell s performance is the highest however, this depends on the DL material and on the MPL thickness (see Figure 4.21). 

Based on our observations, we generalize the fuel cell membrane degradation and failure mechanisms as the schematics in Fig. 23. So far, the evidence has shown that defects formation and growth play an important role both in chemical and in mechanical degradation processes. Drawing an analogy with material corrosion behavior,... 

Solid oxide fuel cells (SOFCs) involve structures composed of multiple ceramic layers, or interleaved layers of ceramic membranes and metal interconnects. To eliminate premature mixing of fuel and air gases or leaking of these gases from interior regions of the structure, the interfaces of adjacent layers are sealed with a glass or ceramic seal. These seals must withstand the high-temperature environment of the SOFC over its lifetime. Therefore these materials must be thermally matched with the adjacent layers to minimize transient stress risers and eliminate the potential for consequent seal failure. 

High-temperature corrosion is a practical problem in most applications of metals and alloys at elevated temperatures in corrosive environments. In power plants, chemical and petrochemical process industries, for aircraft engines, heat treatment and other metallurgical processes, and new technologies such as waste-incineration plants, high-temperature fuel cells, and so on, the metallic materials must he carefully selected, to allow sufficient lifetime and avoid premature failure. Sometimes processes are not possible since the materials would not withstand the process conditions that must he adapted to the available materials. 

Weber and Newman [68] developed one-dimensional model to study the stresses development in the fuel cell. They showed that hygro-thermal stresses might be an important reason for membrane failure, and the mechanieal stresses might be particularly important in systems that are non-isothermal. However, their model is one-dimensional and does not include the effects of material property mismatch among PEM, GDL, and bipolar plates. 

It has to be kept in mind that usually the temperature influence is less important, since large temperature gradients inside the fuel cell are normally avoided, for several reasons. First, a large temperature gradient imposes thermal stress on the respective materials and is a source of accelerated degradation or failure. Second, the electrochemical reaction is sensitive to temperature. An increase in temperature leads to an increase in current density. This in turn would amplify a nonideal current distribution, leading to an increase in losses connected with cross-currents. 

Durability testing takes a long time in an operation environment, which is difficult as normally several thousand hours are necessary to obtain a meaningful conclusion. In the development of durability testing, some in situ and ex situ methods and techniques for material evaluation have been used, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), CV, EIS and so on. However, new electrochemical and/or physical techniques are desirable to gain a better understanding of durability failure modes and then improve fuel cell durability and reliability. 

Besides the bipolar plate, the gasket material is an important component of the fuel cell stack and tends to be heavily underestimated. It plays a key role in the mechanical properties of the stack, compensated tolerances and partially determines the mechanical compression of the MEA. Inappropriately selected gasket materials may cause failure of the MEA or fracture of the bipolar plates. Despite the fact that the gasket has only to seal the stack, it is a highly challenging issue due to the tolerances of the other components which have to be managed. And last but not the least, the gasket has to be cost attractive. 

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