Two major manifestations of proton membrane failure

The electrolyte of PEMFC is perfluorosulfonic acid proton exchange membrane (PFSA membrane), which mainly plays the role of blocking hydrogen and air, blocking electrons and conducting protons in fuel cell cells/electrolyzers. As the operation time increases, the structure of the PFSA membrane will change, and various functions will inevitably be affected, mainly in the following two aspects.

1. Gas cross-penetration

After working for a long time, the thickness of the PFSA membrane will be reduced, and defects such as cracks and perforations will appear. The ability to block gas will be greatly weakened, and hydrogen and air will cross-penetrate through the PFSA membrane, and the open circuit voltage (OCV) of the battery will decrease.

The mechanism of PFSA membrane defects can be mainly divided into two aspects: attenuation caused by mechanical damage and membrane degradation caused by electrochemistry.

The PFSA membrane will repeatedly expand and shrink under wet and hot cycles, resulting in stress cycles. Over time, cracks will occur;

The generation of cracks will promote the leakage and cross-penetration of reaction gases, aggravating electrochemical degradation;

The defects caused by electrochemical degradation will become stress concentration areas, which in turn accelerate mechanical damage.

Two major manifestations of proton membrane failure

SEM image of damaged PFSA membrane structure

The generation of micropores or microcracks does not cause PFSA membrane failure immediately, but expands to a certain size under the action of cyclic mechanical stress. This process accounts for 50% of the life of PFSA membrane. Therefore, the fatigue resistance of PFSA membrane is very important.

OCV is an important indicator to measure the life of PFSA membrane

Serious gas cross-penetration inside PFSA membrane will lead to a decrease in OCV. There are two opinions on this.

One view is that in the open circuit state, hydrogen penetrates into the cathode, and under the electrocatalytic action of the cathode, a mixed potential is generated to form a local half-cell, thereby lowering the OCV of the battery;

Another view is that after hydrogen penetrates into the cathode, it is adsorbed on the surface of the Pt catalyst, destroying the Pt-O interface, reducing the Pt-O mixed potential and thus reducing the OCV.

The reduction in OCV may also be caused by electronic short circuit.

We can distinguish between electronic short circuit and gas penetration by changing the gas partial pressure.

When the gas partial pressure increases, the gas pressure on both sides of the membrane is greater, the gas transmembrane permeation will intensify, and the OCV will decrease significantly; the same is true in reverse.

If the OCV decrease is related to the gas pressure, it means that it is related to gas cross-permeation, which is caused by the weakening of the gas barrier performance of the membrane; if the OCV decrease is not related to the gas pressure, it means that it is caused by electronic short circuit.

2. Decrease in proton conductivity

When the PFSA membrane is contaminated, the protons on the sulfonate group are replaced by other metal cations, which in turn leads to an increase in the material equivalent EW value of the membrane, a decrease in the number of protons that can be released, and a decrease in the amount of water that can be accommodated. The two together cause the proton conductivity of the proton membrane to decrease.

The EW value represents the mass of the membrane required for 1 mol of hydrogen ions. The smaller the EW value, the greater the proton density in the membrane and the better the conductivity of the membrane.

In addition, when the proton membrane is attacked by free radicals, the sulfonate group decomposes, or a polycondensation reaction occurs under a wet heat cycle to form anhydride, these will cause the proton membrane to lose protons, increase its EW value, and ultimately cause a decrease in conductivity.