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Hydrogen (H2) in the fuel electrode and oxygen (O2) in the air electrode pass through the electrolyte membrane, although in minute amounts. (Cross leakage) When the membrane thickness reduces as a result of the degradation of the electrolyte membrane, the amount of cross leakage increases, which is one cause of the reduction in voltage.
The fuel electrode uses not only platinum (Pt) but also an alloy catalyst consisting of ruthenium and platinum that is resistant to CO poisoning. However, this ruthenium dissolves in the course of the operation of the fuel cell, with the CO tolerance consequently degraded. The catalyst with degraded CO tolerance allows more and more CO to be adsorbed, which inhibits the activity of the catalyst and results in the voltage drop.
Nano-sized (※2) platinum (Pt) particles used in the catalyst grow gradually, reducing the effective electrode area, which is one cause of the voltage drop.
In this phenomenon, as carbon present in the catalyst layer corrodes, it becomes more and more hydrophile, with generated water being accumulated in the catalyst layer to lower the diffusivity of the air.
This is generally called “flooding.”