Corrosion of carbon support during start-up and shut-down of proton-exchange-membrane fuel cells is well-known and widely reported in the literature. Today, this is an accepted degradation mechanism. However, the journey from severe performance loss observed in a fuel-cell vehicle to our present understanding was difficult. In fact, measurements of cell potential and current did not help elucidate the mechanism at all. It was only through mathematical modeling that the corrosion process was clarified. No new physics was needed, just the careful assembly of existing knowledge of kinetics and transport for a start-up transient.
Capacity fade in lithium-ion batteries is also a well-known issue. Generally, the formation and slow growth of the solid electrolyte interphase (SEI) results in irreversible consumption of lithium. This loss of cyclable lithium translates into a reduced capacity of the battery with age. In addition to capacity fade, power fade is critical for many applications. A prime example is the battery for a charge-sustaining hybrid-electric vehicle. Here, the energy storage system is used for recovering energy and power assist, not sustained propulsion of the vehicle. Therefore, power capability is at least if not more important than capacity. The power capability is characterized by “cell resistance.” As the SEI thickness grows, the ohmic resistance of the cell increases. However, the magnitude of the increase cannot explain the power fade. Typically, for a pulse of current of ten or so seconds is used in practice to measure “cell resistance.” For transients of this length, this quantity is not simply the ohmic resistance of the cell--concentration polarizations are significant. This behavior is explored with well-established mathematical models of lithium-ion cells. Again, a mathematical model that uses existing physics is able to shed light on the nature of the power fade.