The sealing body plays a supporting and sealing role in the fuel cell. The attenuation of the sealing body will lead to problems such as internal and external leakage of the stack, relaxation of assembly pressure and change of contact resistance, resulting in failures such as reduced performance of the stack, accelerated attenuation of service life, safety of hydrogen leakage and unqualified insulation. Next, we will discuss the attenuation of silicone rubber in the fuel cell environment in detail.

To sum up, the main requirements of fuel cell sealant are as follows:

1) Air tightness: air leakage will affect the performance of fuel cell;

2) Acid resistance: the pH value of sulfonic acid group in proton exchange membrane in fuel cell is 1-2, and the pH value of generated water in electrochemical reaction is 3-5 [8];

3) Moisture resistance: water will be generated in electrochemical reaction, which is a high humidity environment, and the sealing material shall have moisture resistance;

4) Temperature resistance: the operating temperature of fuel cell stack is generally between 60 ℃ and 90 ℃, and the cold start temperature is - 40 ℃;

5) Low material precipitation: the sealing material will gradually age and degrade during use, and the precipitates (such as metal ions, fillers, sealant degradation particles, etc.) will not cause problems such as proton exchange membrane aging, catalyst poisoning, micropore blockage of gas diffusion layer, etc;

6) Mechanical properties: the sealing material shall have vibration absorption and impact resistance, and its compression shall be able to compensate the thickness tolerance of membrane electrode, bipolar plate and other components in the manufacturing process, and will not lead to overpressure or insufficient pressure of membrane electrode components.

The main chain of silicone rubber is silicone oxygen bond and the branch chain is organic group. By changing the types of branched organic groups and fillers, the viscosity, hardness after curing, mechanical properties and curing mode of silicone rubber can be changed, so as to meet different application requirements. In terms of bond energy, the carbon carbon bond is 345 kJ / mol, while the silicon oxygen bond is 451 kJ / mol, so silica gel has better high temperature resistance; The amorphous molecular structure also makes it have good low-temperature performance. Its suitable temperature range is - 50 ℃ ~ 250 ℃, which fully meets the temperature requirements of PEMFC for sealing materials.

Some literatures believe that silica gel is not suitable for long-life stack in simulated solution (12) μ G / g H2SO4 and 1.8 μ In g / GHF), the chemical corrosion resistance of silicone rubber is significantly lower than that of fluorosilicone and EPDM, and the silicone rubber is obviously degraded in practice. However, some fuel cell manufacturers do use silicone rubber as sealing material, and its stack life has reached tens of thousands of hours. This contradiction is undoubtedly worthy of attention.