English
Proton exchange membrane fuel cell (PEMFC) can be said to be a "reverse" device for electrolyzing water. Electrolyzed water is the use of external power to electrolyze water to produce hydrogen and oxygen; The fuel cell is a process of electrochemical reaction between hydrogen and oxygen to produce water and electricity at the same time. PEMFC has hydrogen electrode and oxygen electrode, in which proton exchange membrane is used as electrolyte.
Therefore, proton exchange membrane (PEM) is one of the core basic materials of proton exchange membrane fuel cell (PEMFC), and its performance determines the performance and service life of the battery. In order to achieve the efficient and stable operation of hydrogen fuel cell, PEM is required to have high proton conductivity, good thermal and chemical stability, high mechanical strength and durability.
The preparation process of proton exchange membrane directly affects the performance of the membrane. At present, there are mainly two kinds of membrane preparation processes: melt forming method and solution forming method.
1. melting film forming method
Melt forming method, also known as melt extrusion method, is the earliest method used to prepare PFSA proton exchange membrane. The preparation process is to melt the resin and form a film by extrusion tape casting or calendering, and obtain the final product after transformation treatment. DuPont took the lead in commercial production of melt extrusion method. Solvay's aquivion series products also adopt a similar process, using short side chain perfluorinated sulfonic acid (PFSA) as raw material.
The film prepared by this method has uniform thickness, good performance and high production efficiency. It is suitable for mass production of thick film, and the production process does not need to use solvent, which is environmentally friendly.
The disadvantage is that, on the one hand, due to the process characteristics, the melt extrusion method can not be used to produce the film, which can not effectively solve the cost problem of PFSA proton membrane. On the other hand, the membrane made by extrusion needs to be hydrolyzed and transformed to obtain the final product, which is difficult to maintain the flatness of the membrane in this process. In view of the above problems can not be fundamentally solved, the research and application of melting method in the field of proton exchange membrane show a downward trend.
2. solution film forming method
Solution film forming method is the mainstream method used in scientific research and commercial products at present. It is generally prepared as follows: the polymer and modifier are dissolved in the solvent, then cast or cast, and finally the film is formed after drying and solvent removal. The solution film-forming method is suitable for most resin systems, which is easy to realize hybrid modification and microstructure design, and can also be used to prepare ultra-thin films, so it has attracted much attention.
The solution film forming method can be further divided into solution casting method, solution casting method and sol-gel method according to the difference of the later stage process.
2.1 solution casting method
The solution casting method is to directly cast the polymer solution in a flat mold, and form a film after volatilizing the solvent at a certain temperature. This method is simple and feasible, and is mainly used for laboratory basic research and formulation and process optimization in the early stage of commercialization.
2.2 solution casting method
The solution casting method is an extension of the solution casting method, which can be used for mass continuous production. Therefore, at present, the solution casting method is mostly used for commercial products (mainly PFSA proton exchange membrane).
The solution tape casting method can realize continuous production through roll to roll process, mainly including resin dissolution and transformation, solution tape casting, drying film forming and other processes. Compared with the melt extrusion method, its process is longer, the process is more complex, and the solvent needs to be recycled, but the advantage is that the product performance is better and the film thickness is thinner.
The main production companies are: Gore select film series, DuPont second/third generation Nafion film, Asahi acflex film, Asahi nitro flemion film, Dongyue Group, etc.
2.3 sol gel method
The sol-gel method is usually used to prepare organic-inorganic composite membranes, and the sol-gel process is used to achieve the uniform dispersion of inorganic fillers in the polymer matrix.
The brief preparation process is as follows:
The prepared polymer homogeneous membrane was swelled and immersed in a small molecular solvent containing alkoxides (Si, Ti, Zr, etc.), and the inorganic oxides were in situ doped into the membrane through the sol-gel process to obtain the composite membrane. The performance of the organic-inorganic composite membrane prepared in this way is generally better than that of the direct solution mixed membrane. The hydrogen fuel cell made of this film can still work stably at 130 ° high temperature, but it can not realize the continuous production of the film in large quantities.
