(59b) Degradation Mechanisms in Fuel Cells and Batteries: Materials Challenges and System Mitigation

A key barrier to the widespread introduction of electric vehicles, hybrid electric vehicles, and fuel cell vehicles is cost. Related to their cost is the durability of these electrochemical energy storage and conversion devices. Through an understanding of the mechanisms of failure it is possible to 1) guide the development of new materials that are lower cost or more durable, 2) devise system architectures and controls to mitigate these failure mechanisms. Generally, these are transient phenomena and the challenges involve the coupling of transport, kinetics, and thermodynamics.

Two examples related to fuel cells are discussed in detail. The first is the oxidation of carbon associated with the starting and stopping of a PEM FC [1]. In the absence of a suitable materials solution, the system design is altered to reduce the loss of carbon to an acceptable level. A second example shows how a simple change to the power sharing algorithm for a fuel cell hybrid vehicle can dramatically reduce the dissolution of platinum. Only a small reduction in system efficiency accompanies this control strategy. More important, a framework is established to allow these types of trade-offs to be explored. [2]

A third example is the loss of cyclable lithium in a lithium-ion battery. Common electrolytes used in lithium ion cells are not stable in contact with lithium. Because a relatively stable film called the solid electrolyte interphase (SEI) is formed, the reaction of lithium with the electrolyte is limited. Nonetheless, some lithium becomes unavailable for shuttling between the insertion electrodes, and further the reaction continues albeit more slowly over the life of the battery. It is impractical to add excess lithium to these cells. At the same time the SEI represents a polarization of the cell that reduces its efficiency and limits power density.

 References

1.       N. Takeuchi, T. F. Fuller, J. Electrochem. Soc., 157, B135-B140 (2010).

2.       R. Chandrasekaran, W. Bi, T. F. Fuller, J. Power Sources, 182, 546-557 (2008).