Open circuit/idling is recognized a

Open circuit/idling is recognized as the most serious condition for chemical degradation of the membrane for two primary reasons: strong gas permeation and high cathode potential. Here, the so-called gas permeation mainly corresponds to that of oxygen. Greater fluorine emission and higher average •OH density [100] at low load current confirm the damaging effect of open-circuit/idling condition. The details are discussed as follows: (a) Stronger oxygen permeation Gas permeation reaches a maximum under OCV condition. Firstly, the absence of gas consumption at OCV elevates the partial pressure of the reactant in the electrode . Thus, the residual oxygen concentration increases as the load current decreases, and oxygen permeation increases in accordance with Fick‘s law. Secondly, the absence of water production at OCV reduces the water content of the membrane and the partial pressure of water in the cathode. Inadequate swelling of the membrane increases membrane porosity, and the low partial pressure of water creates space for oxygen in the cathode [135]. Gummalla et al. [135] and Kundu et al. [144] studied the effect of the relative humidity (RH) of the supplied gas on the chemical degradation of the membrane. Their results show that a lower RH results in greater fluorine release. Finally, because the electro-osmotic water drag can help hydrogen crossover [123], it can also impede the transportation of dissolved oxygen to the anode. Therefore, under OCV condition, the oxygen permeates to the anode without the hindrance from electro-osmotic drag. Note that the idling condition is very similar to the OCV condition.