PEM has the ability to selectively penetrate the steam in the catalytic layer into the PEM

The diffusion of oxygen into the catalyst layer and the blocking of part of the channels by the liquid droplets formed in the diffusion layer constitute a resistance to the vapor infiltration of the catalyst layer toward the cathode channel. Therefore, during normal operation, the vapor pressure of the catalyst layer is relatively high. In the case of high current density, the oxygen flow rate is large, and the amount of steam generated in the catalytic layer is also large. In addition, the vapor resistance in the catalytic layer to the diffusion layer also increases, so that the partial pressure of steam in the catalytic layer will become higher. . PEM has a selective permeability, and it is difficult for steam in the catalyst layer to enter the PEM, which creates the conditions for the formation of two-phase countercurrent flow in the PEM porous media.

In order to analyze the formation mechanism of the two-phase countercurrent flow in PEM, the porous structure was simplified to a parallel channel consisting of a solid framework and void channels. The cathodic electrochemical reaction exotherm is equivalent to heating the system at the boundary. Ohm generates heat for the internal heat source of the PEM. Under normal operating conditions, liquid water is in the PEM fluid channel. However, due to the fact that the current density is not evenly distributed across the cross-section of the PEM, the current density at some local points is too high, and the Joule heat generated is sufficient to cause the phase change of the local water to generate steam.

The bubble bubble B1 in the fluid channel immediately adjacent to the cathode catalyst layer. Due to the large vapor pressure of the cathode catalyst layer, the bubble is not immediately taken out of the PEM by the liquid water. Instead, the bubble B1 will gradually grow under the Joule heat flow. At this time, the steam flows along the C1 channel in the opposite direction to the liquid water flow, and the liquid water before the bubble moves along the channel wall to the cathode side under the action of the capillary force. The replenishment of the liquid water will continue the evaporation process. . In the PEM porous media, this liquid and steam flow in the opposite direction, referred to herein as two-phase countercurrent.