Proton Exchange Membrane Type

Proton Exchange Membrane Type – Fuel Cell Catalyst Coatings – Cheersonic

Proton exchange membranes can be divided into homogeneous membranes and composite membranes.

1) Homogeneous membrane

Homogeneous membranes can be divided into five different types according to the main chain composition and functional groups of the materials: perfluorosulfonic acid membranes, partially fluorinated sulfonic acid membranes, non-fluorinated sulfonic acid membranes, polybenzimidazole (PBI)/ H3PO4 membrane and alkaline ion membrane.

Perfluorosulfonic acid membranes, represented by Nafion, are the most commonly used PEMs and are used as benchmarks to characterize the performance of proton exchange membranes due to their excellent chemical and electrochemical stability and excellent proton conductivity. It has a unique structure, including a hydrophobic main skeleton of tetrafluoroethylene and a side chain with a hydrophilic terminal sulfonic acid group. The former makes it have certain physical strength and excellent chemical stability, while the latter makes it watery Has ideal proton conductivity. The presence of water in the membrane affects the PEM ion channel formation, size, and connectivity, which in turn determines the proton conductivity of the PEM.

Partially fluorinated sulfonic acid membranes mainly include radiation-grafted membranes and blended membranes based on commercial fluoropolymers. Currently, radiation-grafted membranes used in PEMFC are usually prepared using a two-step method: firstly, styrene or α,β,β-trifluorostyrene is grafted onto a fluorine-containing inert polymer membrane, which The inert polymer membrane is usually polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), copolymerized tetrafluoroethylene and hexafluoropropylene (FEP) or cross-linked ethylene and tetrafluoroethylene (ETFE), etc., and then sulfonated. chemical grafting. The research of blending modification method mainly focuses on the selection of blended membrane materials and the preparation of blended membranes. Generally, polysulfone (PS), polyacrylonitrile (PAN), polyvinylidene fluoride, cellulose acetate and other high-performance and high-performance materials are selected. Molecular materials are used as blended membrane materials. In recent years, research on medical polymer materials such as silk fibroin, chitosan and chitin has been carried out. Polymer blending or doping has also been used as a valuable technique to improve the mechanical, thermal, surface and proton conductivity properties of fluoropolymers. However, so far, there are no practical reports on partially fluorinated sulfonic acid membranes prepared by this technique.

Non-fluorine proton membranes are an important branch of PEM. These PEM materials include polyaryl ether, polyimide, polyetherimide, styrene and its derivatives. Poly(arylene ether)-based membranes are one of the most promising alternative PEM materials due to their processability, excellent thermochemical stability, good mechanical properties, and low cost. The sulfonic acid group is the proton exchange site of the polyarylether-based membrane, and it is easier to be introduced into the aromatic ring than the carboxylic acid and phosphonic acid. Polyimide-based sulfonic acid membrane, especially sulfonated six-membered ring (naphthalene) polyimide, due to its excellent chemical and thermal stability, high mechanical strength, good film-forming ability and low fuel Gas (or liquid) permeability and is considered an ideal candidate for PEM.

2) Composite film

Due to the limitation of the strength of the perfluorosulfonic acid resin itself and the preparation process, the homogeneous membrane has low mechanical strength, severe swelling, and thick thickness. It is currently difficult to obtain a practical homogeneous membrane with a thickness of less than 25 μm. In order to further reduce the film thickness, improve its own strength and reduce swelling, Gore Corporation of the United States has developed a polytetrafluoroethylene (ePTFE) reinforced composite PEM. This composite PEM fills PSFA into the micropores of PTFE, and on the premise of ensuring the mechanical properties of the membrane, the thickness of the membrane is further reduced to 10-20 μm or even lower, and the corresponding proton conductivity is greatly improved. At present, most of the PEMs for vehicle fuel cells have been changed to use composite membranes.

The composite proton exchange membrane is a membrane with a composite structure formed by injecting perfluorosulfonic acid resin into a reinforcing matrix material with a porous structure (such as PTFE, PVDF, etc.). The perfluorosulfonic acid resin is filled into the micropores of the porous reinforcing matrix material, which can not only block the proton conduction channel, maintain the proton conduction performance of the membrane, but also improve the mechanical strength and dimensional stability of the membrane. Microscopic reinforcement of perfluorosulfonic acid resin by PTFE membrane with micropores is the most mainstream preparation method for composite membranes. This enhancement process did not change the chemical properties of the perfluorosulfonic acid resin, but the thickness of the membrane could be greatly reduced to 10–20 μm, while its proton conductivity was improved (60 S/cm). By comparing the 20μm thick, 1100EW Gore-select film with the 175μm thick, 1100EW Nafon117 film, under the same water content, the tensile strength of the former is twice that of the latter, and the shrinkage rate after water loss is higher than that of the latter. 1/4 of the former, and the former is also significantly higher than the latter in terms of battery performance. However, the hydrogen permeability of the former is 4 times that of the latter due to the reduced film thickness.

3) Proton exchange membrane additives

The properties of proton exchange membrane homogeneous membrane and composite membrane can be improved by adding additives such as inorganic small molecules (such as SiO2, CeO2, etc.) or metal nanoparticles (such as Pt). The addition of inorganic small molecules such as SiO2 to the proton exchange membrane mainly modifies the membrane material by self-humidification, so that the fuel cell can maintain good water retention and moisture absorption performance under low humidity conditions, and at the same time, it can promote the reverse diffusion of the product water of the cathode to the membrane and anode. . The addition of inorganic small molecules such as CeO2 is mainly used as a free radical quencher, thereby eliminating the free radicals generated from the catalytic layer and improving the durability of the membrane. The addition of Pt nanoparticles to the cathode side of proton exchange membranes has received considerable attention in recent years and has already started mass production at major membrane manufacturers. The addition of Pt nanoparticles on the cathode side can simultaneously act as a self-humidifying additive and a free radical quencher for the membrane material, greatly improving its durability, and its performance has been practically verified.

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