High Temperature Proton Exchange Membrane

High Temperature Proton Exchange Membrane – Fuel Cell Coating – Cheersonic

The development of human society faces major problems such as environmental pollution and energy shortage. It is urgent to develop new energy technologies with low or no pollution. Fuel cell is an efficient and clean energy conversion technology that can directly and continuously convert the chemical energy of fuel into electrical energy. Compared with traditional internal combustion engines, fuel cells are not limited by the Carnot cycle, and have the advantages of high energy conversion efficiency, low pollution, fast startup, modular structure and low noise. Proton exchange membrane fuel cell (PEMFC, whose structure is shown in Figure 1) is an important type of fuel cell and has received great attention. As one of its key components, the proton exchange membrane plays the role of conducting protons, isolating electrons and reactants at the cathode and anode. Aiming at the problems of low catalytic activity, poor catalyst tolerance, and complex hydrothermal management of low-temperature PEMFC, increasing its working temperature to 100–200 °C is an effective solution. In this paper, the main types, preparation and modification methods and proton conduction mechanism of high temperature proton exchange membrane (HTPEM) are reviewed. It has great potential for application as a proton exchange membrane, and the preparation of composite PBI high temperature proton exchange membrane, doped proton conductor types and performance improvement methods are discussed.

1. Research progress of high temperature proton exchange membrane

1.1 Main types of HTPEM

According to the composition, HTPEM is divided into pure polymer membrane, organic-inorganic composite membrane and pure inorganic membrane. In pure polymers, hydrophilic segments and hydrophobic segments aggregate respectively to form a microphase separation structure. The interconnected hydrophilic regions form proton transport channels, and the hydrophobic regions play a mechanical support role; organic-inorganic composite membranes are generally formed in polymers. The organic-inorganic interface exists in the film prepared from the functional inorganic material doped with medium, and the large amount of doping of inorganic components may affect the mechanical properties and even the film-forming properties of the film.

From the aspect of preparing HTPEM polymer materials, it is divided into perfluoropolymer, partially fluorinated polymer proton exchange membrane material and non-fluorinated polymer proton exchange membrane material. The proton conductivity of perfluorosulfonic acid (PFSA) membranes decayed severely at high temperature and low relative humidity. The chemical stability and mechanical properties of partially fluorinated polymer membranes are generally poor and cannot meet the needs of practical use. The polymers used in fluorine-free high temperature proton exchange membranes mainly include sulfonated polyarylene ether sulfone (SPSF), sulfonated polyarylene (sulfide) ether (SPAE), sulfonated polyether ether ketone (SPEEK), sulfonated polyimide ( SPI) and polybenzimidazole (PBI), etc.

1.2 Preparation and modification of HTPEM

Monolayer HTPEM can be prepared by casting, hot pressing, spin coating, spray coating and electrospinning. The preparation method of the multilayer film includes layer-by-layer casting, electrostatic layer-by-layer self-assembly and layer-by-layer spraying.

The proton conductivity, mechanical properties, wet and dry deformation properties, and anti-oxidation properties of membranes can be improved by doping functional materials, ionic cross-linking, covalent cross-linking, surface modification, and multi-layer composites.

1.3 Proton conduction mechanism

The driving force for proton conduction is the synergistic effect of electric field force and concentration difference. According to the different humidity of the working environment, the mass transfer mechanism of the proton exchange membrane mainly includes the vehicle mechanism and the hopping mechanism (also known as the Grotthuss mechanism). The transport mechanism requires the hydrophilic group in the membrane to form a continuous hydrophilic channel through the membrane, and the proton carrier (usually water molecule) binds the proton, and is transported to the cathode side through this channel to participate in the oxygen reduction reaction. The typical polymer is PFSA. The hopping mechanism believes that the proton donor and acceptor existing in the proton exchange membrane form hydrogen bonds and connect with each other to form a hydrogen bond network, and the conduction of protons is completed through the continuous formation-breaking process of hydrogen bonds. A typical representative of the hopping mechanism is the proton conductor doped polybenzimidazole (PBI) composite membrane, which has good proton conductivity at low relative humidity.

2. PBI type high temperature proton exchange membrane

2.1 Preparation of PBI-like HTPEM

PBI is a special engineering plastic with high glass transition temperature, good thermal stability, good mechanical properties and low gas permeability, and has become the most concerned HTPEM material at present. PBI is generally an aromatic tetraamine (a compound containing two o-diaminophenyl groups or its hydrochloride) and a diacid (or diester, dinitrile, dialdehyde, diamide), through melt polycondensation and The solution polycondensation method is produced by condensation polymerization.

2.2 Types of proton conductors doped in composite PBI films

PBI itself has extremely low proton conductivity and needs to be doped with a proton conductor to be used as a proton exchange membrane. Phosphoric acid (PA)-doped PBI films can still maintain high proton conductivity at high temperature and low humidity, so they have attracted much attention. However, when working at medium or high RH, PA is easily lost from the membrane, resulting in performance degradation. To solve this problem, water-soluble inorganic solid-state proton conductors such as heteropolyacids, superproton conductors (solid acids that can undergo superproton phase transition), etc. can be anchored (or adsorbed) on the water-insoluble matrix material, and then doped with Hybrid to PBI to prepare HTPEM.

Recently, our group found that PWA-intercalated FeSPP can be prepared in situ by adding water-soluble phosphotungstic acid (PWA) to the layered sulfonated iron phenylphosphonate (FeSPP). The intercalation of PWA significantly improved the proton conductivity of the proton conductor. At the same time, a covalent bond was formed between PWA and FeSPP, which inhibited the loss of PWA at high RH. The composite membrane prepared by doping into PBI had higher proton conductivity and Thermal stability.

2.3 Performance improvement method of composite PBI film

To further improve the performance of PBI-like HTPEM, the methods that can be taken include:

(1) Change the PBI structure, such as changing its degree of polymerization and distribution, changing its monomer structure, block structure and block length, side chain structure and distribution, and end group structure.

(2) PBI composite membranes were prepared by doping inorganic materials to control the microstructure of the membranes and improve the membrane properties. Recently, PBI membranes were doped with fibers, and FeSPP was used as proton conductor to prepare PBI/FeSPP/POAF composite membranes doped with pre-oxidized polyacrylonitrile fibers (POAF) and oPBI doped with glass fibers. /FeSPP/GF composite membrane, the structures of the two composite membranes are shown in Figure 5. In these two composite membranes, a dense hydrogen bond network structure can be formed between the fibers, proton conductors and polymers, and the wet and dry deformation of the composite membranes is significantly reduced at lower fiber doping levels (3%-10%). , the mechanical properties, thermal stability and antioxidant properties were improved, while the proton conductivity was not significantly affected.

(3) Preparation of PBI cross-linked membrane. Covalent or ionic crosslinking (acid-base blending) of PBI can improve membrane performance.

3 Conclusion

There are still many challenges to develop PBI-like HTPEMs doped with proton conductors with superior performance at high temperature and low humidity. First, it is necessary to design and synthesize a new type of proton conductor, so that the PBI film has high proton conductivity, while avoiding the loss of proton conductor and the degradation of mechanical properties, increased dry-wet deformation, and phase separation caused by higher doping levels. Secondly, the N—H bond on the end group and imidazole group of the composite PBI film is relatively active and prone to oxidative degradation. Therefore, it is necessary to design new methods or additives to effectively reduce or even eliminate oxidative degradation while avoiding significant impact on other membrane properties. In addition, the study of the relationship between the microstructure and membrane performance of the composite PBI membrane, including the regulation of the microphase separation structure in the membrane, the size and distribution of the hydration region and the free volume, etc., is an effective way to construct a proton transport channel and comprehensively improve the membrane performance. performance plan. Finally, the development of new polymer electrolytes is also a direction worthy of continuous exploration and attempts.

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