1 Introduction
According to the BP World Energy Statistics Yearbook released in 2019, with the accelerated growth of global energy demand, global carbon emissions increased by 2% in 2018, which is higher than the historical average of 1.4%
[1]. The growth of carbon emissions and the increasingly serious environmental problems such as acid rain and haze have promoted the development and utilization of clean energy. Fuel cells (FCs) have the characteristics of simple reaction products, green and safety. Proton exchange membrane fuel cells (PEMFCs) are regarded as the most commercially valuable fuel cells due to their high energy conversion, high power density and fast cold start, and have received extensive attention
[2]. Proton exchange membrane fuel cells (PEMFCs) can be divided into traditional low temperature PEMFCs (LT-PEMFC) and high temperature PEMFCs (HT-PEMFC) according to their working temperature. The working temperature of the former is generally below 80 ℃, while that of the latter is between 100 ℃ and 200 ℃. Compared with low temperature, the operation of PEMFCs at high temperature has the advantages of improving the tolerance of catalyst to CO, accelerating the kinetics of electrode reaction, simplifying the management of water and heat, and reducing costs
[3⇓~5].
Proton exchange membranes (PEMs) are the core components of PEMFCs. In order to ensure the efficient operation of PEMFCs, PEMs need to have high Proton conductivity, low fuel permeability, good insulation, and good chemical stability, mechanical stability, structural and morphological stability.
At present, the most mature and successful commercialized PEMs are Nafion membranes jointly developed by General Electric Company and DuPont Company in the 1960s. As a representative of perfluorosulfonic acid (PFSA) membranes, Nafion has almost no shortcomings except high fuel permeability, and its comprehensive performance is very outstanding
[6]. However, the Nafion membrane is only suitable for LT-PEMFCs operating at a temperature not higher than 80 ℃. On the one hand, the membrane has poor high temperature moisture retention ability and will lose proton carrier water; On the other hand, the glass transition temperature of the film is low, and the film is easy to be chemically degraded at high temperature. In addition, Nafion is very expensive to produce and is an extremely expensive material
[7].
In order to prepare proton exchange membranes suitable for HT-PEMFCs, people have modified perfluorosulfonic acid PEMs, such as introducing hydrophilic inorganic metal oxides to solve the problem of water loss at high temperature, or introducing heteropoly acids to improve the proton conductivity at high temperature, but the working temperature of proton exchange membranes can not be increased to more than 120 ℃
[8][9]. Other sulfonic acid-based PEMs, such as sulfonated poly (aryl ether ketone), sulfonated poly (aryl ether sulfone), sulfonated polyimide, and sulfonated polyphenylene oxide, are also only suitable for LT-PEMFCs. The essential reason is that the sulfonic acid group can not be ionized in the dehydrated state and lacks a proton conductor.Even the Polybenzimidazole (PBI) material, which is regarded as the most promising material, needs a relative humidity higher than 50% to obtain a relatively high proton conductivity after being doped with sulfonic acid, which can not meet the needs of high temperature and anhydrous
[9].
Inorganic phosphoric acid doped polymer membrane materials are one of the most ideal candidates for high temperature proton exchange membranes. Although polymer substrates such as PBI can be doped with a variety of acids, most of the current research focuses on doping with phosphoric acid. Phosphoric acid has moderate acidity, good thermal stability and low vapor pressure. Most importantly, phosphoric acid can be self-ionized at high temperature in the absence of water, and its ionization degree is only slightly lower than that of sulfonic acid
[10]. Therefore, phosphoric acid can be used as a non-volatile proton carrier to completely replace water and realize proton conduction under high temperature, low humidity and even anhydrous conditions. As the substrate of phosphate-based high-temperature proton exchange membranes, the choice of polymers is relatively diversified. In addition to PBI, polyaryletherketone, polyarylethersulfone and polyphenylene ether containing basic groups have also been reported
[11⇓⇓~14]. The performance of polymer substrate has a very important impact on the mechanical strength, heat resistance and aging resistance of the film.
At present, the research on phosphoric acid doped high temperature proton exchange membrane has achieved rich results. In order to improve the comprehensive performance of the membrane, a large number of studies on the modification of the substrate polymer and the membrane itself have emerged. In addition, the selection range of substrate polymers is constantly expanding, and MOFs and COFs are also emerging as crystalline proton conductors in this field. In this paper, the mechanism of proton transport in PEMs and the current problems and challenges of phosphoric acid-based high-temperature PEMs are reviewed, and the modification of phosphoric acid doped high-temperature proton exchange membranes, including PBI and non-PBI, is reviewed comprehensively.The applications of new porous materials such as MOFs and COFs in the field of PEMs are summarized from the introduction of porous PBI. This kind of materials has a wide variety and strong designability. Although the relevant research is still in the initial stage, it has shown the potential for HT-PEMFCs. Finally, the advantages and disadvantages of various modification methods were briefly summarized and evaluated, and the application prospects of MOFs and COFs in this field were pointed out, aiming to help future research.
2 Proton conduction mechanism of proton exchange membrane
The mechanism of proton conduction is generally divided into two kinds, namely, Vehicular mechanism and Hopping mechanism/Grotthuss mechanism
[15].
According to the transport mechanism, protons combine with proton carriers (usually water molecules) and move directionally from anode to cathode through the microstructure of the membrane to complete proton conduction
[17]. According to the hopping mechanism, the proton transfer is based on the hydrogen bond network in the membrane. The proton jumps from a carrier to an adjacent proton carrier through the stretching vibration of the hydrogen bond, and the original proton carrier recovers its ability to accept protons, thus forming a directional and continuous proton transfer
[18]. The hopping mechanism and the transport mechanism can coexist and influence each other, for example, both mechanisms exist in Nafion film
[19]. In general, when there is sufficient water and hydrogen bonding sites in the membrane, it is easy to form a continuous hydrogen bonding network, which mainly transfers protons by Grotthuss mechanism, and the transfer efficiency is high. On the contrary, it mainly follows the transport mechanism, needs to overcome higher energy barriers, and has lower proton conduction efficiency
[20]. The activation energy of proton conduction of Grotthuss mechanism is 14 ~ 40 kJ/mol, which can be used as the criterion of proton conduction mechanism
[21,22].
It is generally believed that the proton conduction in PBI/H
3PO
4 membranes mainly follows the Grotthuss mechanism
[23⇓⇓~26]. The imidazole ring contains a Lewis basic N atom (-N), which can interact with phosphoric acid molecules to produce "bonded acid", and the phosphoric acid molecules can also be connected with the polymer skeleton and other phosphoric acid molecules through hydrogen bond interaction.In practice, the doping amount of phosphoric acid is often very high, and there are a large number of "free acids" between the molecular chains, which form a hydrogen bonding network with the polymer skeleton
[4]. The number of free acids is the main factor affecting the volume swelling ratio, and the proton conductivity increases with the increase of the number of free acids
[13][27].
In the literature, the total number of phosphoric Acid molecules adsorbed per polybenzimidazole repeat unit is referred to as Acid doping level (ADL). For example, the ADL in fig. 1 is 4, blue indicates bonded acid, and red indicates free acid.
Ma of Case Western Reserve University in the United States studied the proton conduction mechanism of PA-PBI membrane, and proposed that the order of proton transfer rate between different molecules is Case Western Reserve
[26]; The upper limit of the proton conductivity is given by the conductivity of the liquid H
3PO
4. Fig. 2 shows the main proton conduction paths in a concise manner
[4].
3 Problems and Challenges of Phosphoric Acid Based High Temperature Proton Exchange Membrane
Phosphoric acid doped high-temperature proton exchange membrane is essentially an acid-base composite membrane, which is made of polymers containing basic groups (such as imine, amide, imidazole groups, etc.) As the substrate to provide phosphoric acid adsorption sites and endow the material with processability. Phosphoric acid molecule is a proton carrier, and its content directly determines the proton conductivity. At the same time, phosphoric acid is equivalent to plasticizer, which can increase the distance between polymer chains and weaken the intermolecular force, thus enhancing the flexibility of the film, improving and improving the brittleness and processability of the film. However, high acid absorption rate also means large size swelling and mechanical strength reduction, and excessive phosphoric acid content can even lead to membrane deformation and breakage, which is an important and difficult point in the study of phosphoric acid membrane.
Another important problem is the loss of phosphoric acid. Through experimental research, Lee et al. believed that the loss of phosphoric acid was mainly caused by water vapor and liquid water on the cathode side. During the operation of PEMFCs, a large amount of water was produced and attached to the cathode.To a certain extent, it will absorb the phosphoric acid molecules in the membrane. Under the long-term high temperature operation of the battery, the free phosphoric acid will continue to diffuse outward and lose, which not only reduces the proton conductivity, but also corrodes other components of the battery
[28].
4 Modification of Phosphoric Acid Based High Temperature Proton Exchange Membrane
In order to prepare phosphoric acid-based proton exchange membranes with both proton conductivity and mechanical properties, researchers have carried out a variety of modifications on the substrate polymer. Since PBI proton exchange membranes are still regarded as the most potential candidate membrane materials for HT-PEMFCs, and the related research is the most in-depth and extensive, the following will be divided into two parts: PBI and non-PBI. The former part classifies and summarizes the modification methods of phosphate-based high-temperature proton exchange membranes, and the latter part introduces other types of high-temperature proton exchange membranes.
4.1 PBI high temperature proton exchange membrane
Since polybenzimidazole was first synthesized in 1961, poly [5 '-bibenzimidazole] (commonly known as m-PBI) has been the most widely studied and applied, and is also the earliest and most successful PBI commercialized.It has excellent heat resistance, fire resistance, high mechanical properties, as well as excellent chemical resistance and harsh environmental resistance, and is widely used in industry, military supplies, civil and other aspects
[29].
Due to the rigidity of the molecular chain and the strong hydrogen bonding interaction between the imidazole rings, m-PBI has poor solubility, and generally only m-PBI with low molecular weight (M
W=23~40 kDa) can be applied to proton exchange membranes
[30]. At room temperature, the ADL of m-PBI membrane can reach 5 ~ 6 when the membrane is immersed in 75 wt% phosphoric acid until the mass does not change. By increasing the temperature or acid concentration, the ADL can be increased, but not more than 10. At the same time, low molecular weight PBI has the problem of poor mechanical properties after adsorption of phosphoric acid
[31].
He Ronghuan et al found that increasing the molecular weight of m-PBI could significantly improve the mechanical strength of the membrane, but had little effect on the ADL and proton conductivity of the membrane
[32]. The target conductivity of PEMs for fuel cell vehicles given by the US Department of Energy (DOE) is the proton conductivity of 100 mS·cm
-1(120℃),m-PBI membrane, but there is not enough room for improvement to meet this standard
[33]. In view of the problems of m-PBI, researchers have proposed a variety of modification methods (m-PBI is often referred to as PBI in the literature, and the examples in this paper are referred to as m-PBI if not specified), which will be described below.
4.1.1 Main chain modified polybenzimidazole proton exchange membrane material
The modification of PBI backbone is based on the design of molecular structure, and some specific groups are introduced into the polymer backbone. In practical work, multiple groups are often introduced at the same time, which is introduced in two parts below.
(1) introduction of flexible group, bulky side groups, and non-coplanar structures
The introduction of flexible groups such as ether bond (— O —), carbonyl group (— CO —), sulfone group (—SO
2—), hexafluoroisopropyl group (—C(CF
3)
2), large side groups or non-coplanar structures into polymers can reduce the rigidity and symmetry of polymer chains, prevent the close packing of polymer chains and improve the solubility
[34⇓~36][37][38,39]. Liu et al. synthesized polybenzimidazole Ph-PBI and Me-PBI with asymmetric side groups and ether bonds (as shown in Figure 3). Although their molecular weights are as high as 104 kDa and 118 kDa, respectively, they can be completely dissolved in organic solvents such as DMSO, DMF, DMAc and NMP at a concentration of 10% at room temperature
[40]. After phosphoric acid doping, the proton conductivity of the membranes reaches 0.217 and 0.165 S·cm
-1 at 200 ℃, and the tensile strength is 5.5 and 6.1 MPa, respectively.
The novel polybenzimidazole (PBI) was synthesized by introducing the pyridine ring and diazafluorene structure into the main chain of polybenzimidazole, which was easily soluble in polar organic solvents at room temperature. The proton conductivity of the PBI film dried at 180 ℃ was up to 0.072 S·cm
-1, and the tensile strength was above 40 MPa, showing excellent mechanical properties
[41]. Jian et al. Introduced ether linkage and non-coplanar phthalazinone containing basic N atoms into the backbone of polybenzimidazole (as shown in Figure 4). The polymer has good solubility, the ADL of the membrane reaches 15.2, the proton conductivity at 160 ℃ reaches 0.13 S·cm
-1, and the tensile strength is 8.5 MPa
[42].
(2) introduce other functional group
By increasing the basic groups in the main chain of PBI, the affinity of the matrix to acid molecules can be improved, thus improving the ADL and proton conductivity of PBI membranes
[43]. The introduction of other functional groups such as sulfonic acid groups and hydroxyl groups can also improve the proton exchange performance and durability of the membrane
[44⇓~46].
The introduction of sulfonic acid groups into PBI can combine the common advantages of high-temperature membranes and low-temperature membranes. Phosphoric acid-doped sulfonated polybenzimidazoles (SPBIs) have been reported to have higher proton conductivity than the corresponding non-sulfonated polybenzimidazoles
[43,47]. The preparation methods of sulfonated polybenzimidazole membranes can be divided into two kinds: the synthesis of polymers with sulfonated monomers (sulfonation first) and the sulfonation of polymer membranes (sulfonation later). First sulfonation can design the target structure of the polymer, and can accurately control the degree of sulfonation, but most of the sulfonated monomers are not commercially available. Second sulfonation is very simple, but the reactivity of the polymer is very low. For example, m-PBI and poly (2,5-benzimidazole) (ABPBI) need high reaction temperature (450-475 ℃) to sulfonate, which will lead to cross-linked and brittle films with low degree of sulfonation
[48]. Poly [5 '-biphenylimidazole] (OPBI) has higher reactivity due to the electron-donating effect of the ether bond, and can be sulfonated with concentrated sulfuric acid or oleum at 80 ℃. Fang et al. Prepared sulfonated polybenzimidazole SOPBI by post-sulfonation of OPBI, which has good solubility in DMSO, excellent thermal properties, good film-forming properties and mechanical properties, but they did not study the doping behavior of phosphoric acid and the proton conductivity of SOPBI doped with phosphoric acid
[48].
Quartarone et al. prepared a series of SPBIs membranes with different degrees of sulfonation using a pre-sulfonation method (shown in fig. 5)
[44]. Under the same experimental conditions, SPBIs membranes with lower sulfonation degree have higher ADL and proton conductivity, which is also found in Zhang's work. This is due to the intramolecular interaction between the basic site of the imidazole ring and the sulfonic acid group, which can no longer contribute to the adsorption and dissociation of phosphoric acid
[49]. However, at the same ADL, the higher sulfonated SPBIs membranes have higher proton conductivity, and all SPBIs membranes have much higher proton conductivity than the unsulfonated PBI membranes, which indicates that the sulfonic acid group contributes to the proton conduction
[50]. The maximum power density of the H
2/ air fuel cell assembled by the SPBIs membrane is about 320 mW·cm
-2 at 150 deg C.
It has also been reported that hydroxyl groups are introduced into the structure of polybenzimidazole, and the hydroxyl groups can form a bridging structure with phosphoric acid molecules to fix phosphoric acid and prevent the loss of phosphoric acid, so polybenzimidazole containing hydroxyl groups shows high proton conductivity and durability
[41,45,46].
Benicewicz et al. synthesized 2OHPBI containing two hydroxyl groups (shown in fig. 6), and the phosphoric acid doped membrane of 2OHPBI exhibited the highest proton conductivity among PBI derivatives, up to a 0.35 S·cm
-1,ADL of 25.4 at 160 ° C
[45]. For comparison, p-PBI without hydroxyl groups had an ADL of 19.3 and a proton conductivity of 0.27 S·cm
-1 under the same conditions. This shows that the great improvement of proton conductivity is not only related to the doping level of phosphoric acid, but also to the structure.
Li et al. synthesized polybenzimidazole OHPyPBI containing pyridine and hydroxyl (as shown in Figure 7). When the ADL of the membrane is 8.6, the proton conductivity at 180 ℃ reaches 0.102 S·cm
-1, and the peak power density of the assembled H
2/O
2 fuel cell reaches 570 mW·cm
-2, both of which are higher than that of m-PBI. However, the tensile strength of the membrane is slightly lower than that of m-PBI with similar ADL, and the tensile strength of OHPyPBI/8.6PA at room temperature is only 4.6 MPa
[46].
4.1.2 Doped polybenzimidazole proton exchange membrane material
The doped composite membrane can be prepared by doping inorganic substances, proton conductors and other additives into the polymer, thereby improving the thermal stability, conductivity, mechanical properties and the like of the membrane. Generally, the introduction of inorganic phase can reduce the plasticizing effect of phosphoric acid on the membrane, thereby increasing the mechanical strength of the membrane, but for organic-inorganic composite membranes, the dispersion and compatibility of the inorganic phase in the polymer membrane must be considered, and inorganic nanoparticles may also aggregate at high temperatures. Based on the above problems, inorganic fillers are usually designed to be nano-scale or micro-scale, and various organic groups are introduced to the surface for chemical modification. Organic small molecules are more compatible as dopants of PBI, but there are fewer examples because of their poor stability. According to the type of dopant, it can be divided into the following four categories:
[51](1) dope inert hygroscopic oxides
Hydroxyl groups on the surface of water-absorbing oxides, such as SiO
2, TiO
2 and zeolites, can improve the water retention and acid retention of membranes
[52⇓⇓~55][56]. Lobato et al. prepared a micron level TiO
2 modified PBI membrane. Among a series of composite membranes with different TiO
2 contents (0%, 2%, 4%, 8% and 16%), the 2 wt%TiO
2 doped membrane showed the best phosphoric acid doping level and proton conductivity level, and reached the 0.043 S·cm
-1 at 150 ℃ under anhydrous conditions, which was higher than the pure m-PBI membrane
[54]. The tensile stress at break increases with the increase of the content of TiO
2.
Han et al improved the above work
[53]. Sulfonated titanium dioxide nanoparticles (s-TiO
2) were synthesized by introducing phenyl and sulfonic acid groups on the surface of TiO
2 nanoparticles, and were used to modify m-PBI. Similarly, the composite membrane with 2 wt%s-TiO
2 obtained the highest phosphoric acid doping level (ADL = 12.1), the proton conductivity at 150 ℃ was 0.096 S·cm
-1, and the peak power density was 621 mW/cm
2, which was 30% higher than that of the pure PBI membrane. The comparison shows that the modification effect of s-TiO
2 is better than that of TiO
2.
One way to increase the proton conductivity and thermal performance of PEMs is to dope solid inorganic proton-conducting materials, such as heteropolyacid (HPA), zirconium phosphate (ZrP), CsHPA, etc., which can increase acidic sites and provide additional pathways for proton transport
[57⇓~59].
Heteropoly acid (HPA) is a kind of solid acid with high conductivity and acidity, and its proton conduction mainly follows the Grotthuss mechanism
[60]. Scott et al prepared PWA/PBI and SiWA/PBI composite membranes doped with phosphoric acid, and the proton conductivity of the composite membrane containing 40% SiWA reached 0.1774 S·cm
-1 at 150 ℃
[57]. They also synthesized four kinds of heteropolyacid cesium salt nanoparticles: CsPOMo, CsPOW, CsSiOMo and CsSiOW, and compounded them with PBI to prepare proton exchange membranes
[59]. Phosphoric acid doped composite membranes have higher proton conductivity and cell performance than pure PA/PBI membranes, and the proton conductivity increases with the increase of CsHPA content in the composite membrane. The 30 wt% CsPOMo/PBI film exhibits a maximum conductivity of 0.12 S·cm
-1 at 150 ℃ in the absence of water, and the maximum power density of the H
2/O
2 fuel cell at the same temperature is about 0.6 W·cm
-2, but the mechanical strength is slightly lower.
Zirconium hydrogen phosphate (ZrP) can also improve the proton conductivity of the composite membrane. Xie et al compounded ZrP with SPBIs, and the proton conductivity of the SPBI-10-Z3 composite membrane was 0.08 S·cm
-1 at 180 ℃, which was 20% higher than that of the original PA/SPBIs membrane
[58].
(3) Doped carbon-based material
Doped carbon-based materials mainly include carbon nanotubes and graphene oxide
[61⇓⇓⇓~65]. Carbon nanotubes (CNTs) are often used as reinforcing materials for high performance polymers. Moreno et al. Synthesized a composite membrane of multi-walled carbon nanotubes and PBI. The proton conductivity and fuel cell power density of the PBI/CNT composite membrane were slightly higher than those of the pure PBI membrane. The tensile strength increased by 32% and the Young's modulus increased by 147%
[61]. Pillai et al. synthesized phosphonic acid modified multi-walled carbon nanotubes (P-MWNTS) and doped them into PBI, which improved the proton conductivity and mechanical properties at the same time. They proposed that the phosphonic acid groups on the side wall of P-MWNTS participated in the formation of proton conduction network (as shown in Figure 8), thus improving the conduction efficiency
[62].
Graphene oxide (GO) has a unique 2D structure and contains abundant hydrophilic oxygen-containing groups, such as carboxyl, hydroxyl and epoxy groups, which can provide additional proton transport pathways through hydrogen bonding
[63]. A typical graphene oxide structure model is shown in fig. 9. By modifying various organic groups on the surface of GO, the dispersibility of GO in organic solvents can be improved, thus improving the compatibility of GO with polymer substrates, which is essential for the gas permeability, dimensional stability and mechanical integrity of composite membranes
[64].
The proton conductivity of the composite membrane of sulfonated graphene oxide and PBI synthesized by Scott et al. Is almost twice that of the original GO/PBI
[63]. Abouzari-Lotf et al. Used phosphonic acid modified graphene oxide as the filler of Py-PBI, which improved the ADL and proton conductivity of the membrane. The proton conductivity of 1.5 wt% PGO/Py-PBI membrane reached 0.076 S·cm
-1 at 140 ℃, which was about four times that of the original Py-PBI membrane, and the tensile strength of the membrane was also improved
[64].
He et al. Applied triazole grafted graphene oxide MGO to PBI, which not only improved the tensile strength of the membrane, but also promoted proton transport
[65]. Phosphoric acid doped PBI/MGO membranes exhibit 0.135 S·cm
-1 proton conductivity at 180 ℃ in the absence of water.
(4) Doping organic matter
In fact, inorganic fillers modified by organic functional groups can also improve the compatibility with matrix materials, and some other works directly use small organic molecules such as imidazole and ionic liquids as dopants, which have better compatibility
[66⇓~68]. Hsu et al. Used organic proton conductor imidazole as the filler of F6PBI (PBI containing hexafluoroisopropyl), which improved the proton conductivity but reduced the mechanical properties, and the maximum proton conductivity was only 10
-3S·cm
-1 order of magnitude. Later, they used ionic liquid 1-hexyl-3-methylimidazolium triflate (HMI-Tf) to dope F6PBI, and the composite membrane showed 0.016 S·cm
-1 proton conductivity at 250 ℃ in the absence of water, with a slight decrease in mechanical properties
[68].
4.1.3 Cross-linked or grafted polybenzimidazole proton exchange membrane material
The existence of cross-linked network can limit the volume between polymer molecules, and also limit the movement of polymer molecules, and reduce the dimensional change of polymer membranes.Reasonable structural design can improve the dimensional stability and mechanical stability of the membrane without reducing the proton conductivity. According to the different types of intermolecular interactions, the common types of crosslinking can be divided into covalent crosslinking and ionic crosslinking.
Covalent crosslinking is the formation of covalent chemical bonds between polymer chains. In some literatures, the crosslinking of imidazole NH with nucleophilic groups (such as halides, epoxides) by S
N2 reaction is called covalent crosslinking, which accounts for the majority of the literature
[69]. In addition, there are other crosslinking agents and crosslinking methods, and NH may not be used as a crosslinking point.
He et al prepared two kinds of PBI crosslinked membranes using small molecules 1,3,5-tribromomethylbenzene (B3Br) and 1,3,5-tris (bromomethyl) -2,4,6-triethylbenzene (Be3Br) as crosslinkers
[69]. The proton conductivity of Be3Br-7.5%/11.6 PA membrane was 0.095 S·cm
-1 at 180 ℃ without humidification, and the tensile strength was 8. 5 MPa at room temperature. In general, the cross-linked structure slightly improves the phosphoric acid doping rate and proton conductivity of m-PBI, while the mechanical strength is significantly improved.
Liu et al. Synthesized 2,2 '-bis (chloromethyl) -5,5' -dibenzimidazole (2BIM-2Cl) as a crosslinking agent, which can not only react with the polymer backbone, but also "self-react", thus forming a variety of linear, branched and self-crosslinking structures (as shown in Figure 10)
[70][71]. The crosslinking agent provides additional imidazole groups and a larger free volume, thereby increasing the ADL and proton conductivity. The proton conductivity of the crosslinked membrane can reach up to 0.253 S·cm
-1 at 200 ℃.
Chen et al. Used bromomethylated polyxylene (Br-HPP) as a hyperbranched macromolecular crosslinking agent (its structure is shown in Figure 11), and then immersed the crosslinked membrane in triethylamine to quaternize the unreacted bromomethyl and methylene groups, thus introducing abundant quaternary ammonium ions
[72]. The phosphoric acid doping rate of the cross-linked membranes with different degrees of cross-linking is lower than that of the pure OPBI membranes, but the proton conductivity is higher, which can be attributed to the stronger acid-base interaction between the strong basic quaternary ammonium ions and phosphoric acid, which is conducive to the ionization of phosphoric acid and the formation of a stable hydrogen bond network. The swelling ratio of the crosslinked film is greatly reduced,
The mechanical strength and the proton conductivity are simultaneously improved. Phosphoric acid doped QOPBI-15 membrane (15 wt% crosslinker) has a proton conductivity of 0.049 S·cm-1 at 160 ℃ in the absence of water. The most outstanding mechanical property is the tensile stress up to 27 MPa.
Wang et al. Synthesized branched F6PBI, and then used poly-bisphenol a benzoxazine p (BA-a) as a crosslinking agent to obtain crosslinked polybenzimidazole
[73]. Crosslinker p (BA-a) itself contains N atoms, and the long chain structure can increase the free volume, so the adsorption capacity of PA increases with the increase of crosslinker content. The mechanical properties of C15F6-R2-6 membrane were also improved with the increase of proton conductivity. The proton conductivity of C15F6-R2-6 membrane was 0.073 S·cm
-1 and the tensile stress was 17 MPa at 160 ℃ in the absence of water. The peak power density of hydrogen-oxygen fuel cell assembled with C15F6R2-6 reached 690 mW·cm
-2 at 160 ℃.
Instead of taking NH as the crosslinking point, the methylene group between N and O of BA-a molecule attacked the electron-rich site of F6PBI and electrophilic substitution occurred. Crosslinking sites are indicated in fig. 12.
"Ionic crosslinking" is a concept first proposed by Kerres et al., which occurs when basic polymers (such as PBI) are blended with acidic polymers (usually polymers with —SO
3H groups). It is essentially a "crosslinking" based on Lewis acid-base reaction, forming hydrogen bonds or electrostatic interactions between polymer chains
[74]. In general, ionic crosslinked membranes are less stable and have a higher swelling ratio than covalently crosslinked membranes, resulting in poor mechanical properties
[31].
Other studies have shown that the thermal curing of PBI can lead to the transition from ionic crosslinking to covalent crosslinking. When the ionic crosslinked membrane is heated to a specific temperature, a Friedel-Crafts sulfonylation reaction of a sulfonic acid group and an electron-rich benzene ring occurs, and a stable C—SO
2—Cstructure is obtained
[75]. For example, Henkensmeier et al. blended the basic polymer PBI-OO with the polymer SES0005 with sulfonic acid group to obtain a uniform ionic cross-linked membrane. The membrane was thermally treated at 350 ℃ in an inert atmosphere, and the sulfonic acid group reacted with the electron-rich benzene ring of PBI-OO via Friedel-Crafts reaction to obtain a sulfonyl structure. The molecular chain structure before and after thermal curing is shown in Figure 13
[75].
After thermal curing, the phosphoric acid adsorption of the membrane decreased, but the proton conductivity and the performance of the assembled battery increased, especially the durability of the battery operation improved greatly, and the tensile strength of the membrane did not change much, which was above 10 MPa. The proton conductivity of the covalently cross-linked membrane c-BM 1 (5% SES0005, 95% PBI-OO) was 0.260 S·cm-1 at 160 ° C, twice as high as that of the same type of covalently cross-linked membrane with different proportions. The reason is not clear. They speculated that the low concentration of acidic polymer may lead to the formation of a microstructure conducive to the adsorption of phosphoric acid.
It is worth noting that the ether structure of PBI-OO makes the benzene ring more electron-rich, which is conducive to covalent crosslinking, while the stability of PBI-OO itself is poor, even completely dissolved in PA at 150 ℃. If m-PBI can be blended with SES0005 and then heat-cured, the stability of the film will be better, but the feasibility of this approach has not been verified.
The introduction of side chains containing basic groups (such as quaternary ammonium salt, imidazole, pyridine, pyrrolidone, etc.) On the polymer backbone through chemical reaction not only provides new basic sites, but also destroys the close packing of molecular chains and increases the free volume, so the doping level of phosphoric acid can be directly improved. For example, Liu et al. Grafted benzimidazole onto aromatic polybenzimidazole by N-substitution reaction. Under the condition of 200 ℃ without humidification, the highest proton conductivity reached 0.212 S·cm
-1,160℃, and the peak energy density of hydrogen-oxygen fuel cell was 443 mW·cm
-2. The comprehensive performance was good, but the mechanical strength decreased compared with that before grafting
[76].
In order to solve the problem of phosphoric acid loss, one strategy is to chemically immobilize phosphoric acid molecules on the polymer molecular chain, for example, by grafting. Yu et al. Synthesized polybenzimidazole PBI-EPA grafted with ethylene phosphoric acid
[47]. The grafted membrane itself has very poor mechanical strength and will dissolve in concentrated phosphoric acid at 70 ℃, so it is mixed with PBI to prepare proton exchange membrane. The blend membrane with PBI/PBIEPA mass ratio of 8 ∶ 2 has the highest PA content of 350 wt%, and the proton conductivity is 0.068 S·cm
-1. Although this grafting method fixes the phosphoric acid molecule, it sacrifices the hydrogen of imidazole, which is not conducive to proton transfer in essence. The highest proton conductivity of the grafted membrane reached 0.035 S·cm
-1 at 120 ℃ and 100% RH, and only 0.008 S·cm
-1 at 20% RH, which was not suitable for high temperature proton exchange membrane
[77].
4.1.4 Blend type polybenzimidazole proton exchange membrane material
Blending is a simple method to improve the dimensional stability, mechanical stability and thermal stability of proton exchange membranes. Basic PBI is most commonly blended with acidic polymers, such as sulfonated polyether ether ketone (SPEEK), sulfonated polyether sulfone, sulfonated poly (aryl ether sulfone), and the like
[78⇓~80]. There are also examples where PBI is blended with a polymer that does not contain sulfonic acid groups.
(1) blende PBI with sulfonated polymer
Sulfonated polymers usually have good solubility in organic solvents, good mechanical properties and low price, which can make up for the lack of solubility of PBI. The sulfonated poly (arylene ether) and PBI polymers can be ionically crosslinked, which is helpful to improve the proton conductivity. Bjerrum et al. prepared a blend membrane of sulfonated polysulfone SPSF and PBI
[78]. The proton conductivity of the mixed membranes with PBI mass fraction of 75% or 20% is higher than that of the pure PBI membrane, and the former is higher. The higher the sulfonation degree of SPSF is, the higher the proton conductivity is, which proves that the sulfonic acid group contributes to the proton conductivity. When the relative content of SPSF is low or the sulfonation degree of SPSF is low, the compatibility of PBI and SPSF becomes poor, and phase separation occurs, which is also a problem that needs special attention.
Zheng et al synthesized SPEEK/ABPBI (mass ratio 1:5) blend membranes with SPEEK of 16% and 32% sulfonation degrees
[79]. After phosphoric acid doping, the proton conductivity of (SPEEK16%)/ABPBI membrane is 0.109 S·cm
-1 at 100 ℃, which is 6 times higher than that of pure ABPBI membrane at the same condition. However, the proton conductivity of the mixed membrane begins to decrease greatly when the temperature rises to 100 ℃, which indicates that the contribution of sulfonic acid groups to proton conduction is largely dependent on water. However, the proton conductivity of the blend membrane was always higher than that of the pure PBI membrane even under completely dry conditions, and the blend membrane had better thermal stability than the pure ABPBI membrane.
(2) Blending of PBI with Other Polymers
The presence of the proton acceptor (-N) in the imidazole ring makes PBI very easy to absorb water and prevents the effective contact between the phosphate molecule and the PBI skeleton. Jana et al. Found that blending PVDF and PBI in a certain proportion can improve the doping level of phosphoric acid, because PVDF, as a hydrophobic polymer, can inhibit the water absorption capacity of the material, thus loading more acid
[81].
In order to solve the problem of phosphoric acid loss, it has also been studied to blend polymers containing phosphonic acid groups with PBI. Uch as ABPBI-PVPA hybrid membranes prepared by Bozkurt et al
[82]. In order to achieve a good mixing effect, ABPBI was dissolved in TFA, then water was added dropwise until the solution became uniform, and then mixed with PVPA/water solution. The molar ratio of ABPBI/PVPA repeating units in a series of blend membranes was controlled from 1: 0.5 to 1:4. Unfortunately, the proton conductivity of the membrane is very low, up to the order of 10
-3S·cm
-1.
4.2 Non-PBI high-temperature proton exchange membrane
The modification methods of PBI proton exchange membranes summarized above are also applicable to non-PBI proton exchange membranes. Therefore, this part only takes the polymer substrate itself as the standard for classification and introduction.
In addition to PBI, phosphoric acid-doped alkaline poly (arylene ether) s also account for a large proportion of high-temperature proton exchange membranes. Other polymers, such as polyimides, polyvinylpyrrolidone, polyphenylquinoxaline and polypyrrole with basic N heterocycles in the backbone, have the potential to be used as phosphate-based proton exchange membranes, but the related research is still very few.
4.2.1 Polyarylether polymer
Poly aryl ether (PAE) is a kind of engineering plastics with excellent properties, including polyether ether ketone, polyether ether ketone ketone, polyether sulfone, polyphenylene ether and so on. The main chain of PAE has a rigid p-phenylene support and a flexible ether structure, so it has good mechanical properties, thermal stability and a certain degree of flexibility. The sulfonation of PAE to prepare low-cost proton exchange membranes has been a hot spot in the field of traditional PEMs
[83]. The ability to dope phosphoric acid can be obtained by introducing basic groups such as quaternary amines and imidazoles into the PAE structure
[11⇓⇓~14].
Zhao et al. grafted 1,2,4-triazole onto the backbone of poly (aryl ether ketone) by nucleophilic substitution reaction
[13]. The proton conductivity of the grafted membrane with DS = 1.95 (DS stands for the degree of bromination, which is equivalent to the degree of grafting) at 190.d egree. C. is 0.053 S·cm
-1. He et al. Synthesized quaternized poly (arylene ether sulfone), and hydrolyzed siloxane EMPS under acidic conditions to obtain a membrane with Si — O — Si crosslinked network structure
[12]. The quaternary ammonium ion is beneficial to the adsorption of phosphoric acid, and the existence of the crosslinking structure improves the dimensional stability and the tensile strength of the membrane. The ADL of QPAES-10EPMS-90TEA is 12. 1, the highest proton conductivity is 0.074 S·cm
-1 at 180 ℃ without humidification, and the tensile strength is 9. 16 MPa at room temperature.
Lee et al. introduced triazole ring to the side chain of PPO through alkynyl-azide cycloaddition click reaction, and formed a partially crosslinked structure to obtain XTPPO crosslinked film. Fig. 14 is a schematic diagram of the molecular chain structure
[14]. The membrane with lower degree of crosslinking has higher proton conductivity, and the membrane with higher degree of crosslinking has higher phosphoric acid retention rate, although the acid absorption rate and battery performance are slightly reduced, because the tight crosslinking structure reduces the entry of water molecules. The highest proton conductivity of the crosslinked membrane was 0.064 S·cm
-1 at 180 ℃ without humidification.
4.2.2 Polyimide
Polyimide (PI) has excellent mechanical strength, outstanding thermal stability and chemical stability. Sulfonated polyimides (SPIs) are widely used as PEMs, but PI itself can not be doped with phosphoric acid. Kawakami et al. Blended PI with PBI and doped with phosphoric acid, the proton conductivity can reach 0.5 S·cm
-1 at 120 ℃ and 45% RH
[84]. Kowsari et al. Modified graphene oxide with an ionic liquid containing imidazole groups and used it in SPI to prepare a composite membrane
[85]. After doping with phosphoric acid, the highest proton conductivity of the composite membrane can reach 0.077 S·cm
-1 at 160 ℃ without humidification.
4.2.3 Polyvinylpyrrolidone/polyphenylquinoxaline
Poly vinyl pyrrolidone (PVP) has a nitrogen heterocyclic ring in its molecular chain, which is beneficial to phosphoric acid doping and proton conduction.However, due to its water solubility, it cannot be used as a proton exchange membrane by itself, and other polymers need to be blended to improve its mechanical properties, such as blending with PES and PVDF
[86].
He et al. prepared a series of composite films using polyvinylpyrrolidone-polyethersulfone (PVP-PES) blends as substrates and carbon quantum dots (CDs) as fillers
[87]. The proton conductivity of the original PVP-PES membrane was 0.029 S·cm
-1 at 180 ℃, and the maximum proton conductivity of the composite membrane was up to 0.086 S·cm
-1 at the same conditions, while the swelling ratio was basically unchanged, and the tensile stress at break remained at the same high level (> 10 MPa).
Lu et al. Used carbon nitride (CN) nanosheets with 2D structure to dope PVP-PES blends
[88]. CN can adsorb phosphoric acid, improve mechanical properties and reduce size swelling. When the doping amount reached 0. 5 wt%, the adsorption amount of phosphoric acid and the proton conductivity reached the highest value. The maximum proton conductivity of the P/CN-0.5/PA membrane is 0.104 S·cm
-1 at 160 ℃, and the peak power density is 512 mW·cm
-2.
Poly phenyl quinoxaline (PPQ) has excellent thermal stability and mechanical properties, and contains basic N atoms in the main chain, which has the potential to be used in phosphate-based high-temperature proton exchange membranes. The PPQ synthesized by Hsu et al. Will dissolve in the doping process of high concentration phosphoric acid, resulting in a serious decline in dimensional stability and mechanical properties. Therefore, they synthesized a cross-linked ABPPQ membrane, which greatly improved the mechanical properties, could be assembled into membrane electrodes, and was stable in fuel cell operating conditions
[89]. The highest proton conductivity of the crosslinked membrane was 0.025 S·cm
-1 at 160 ℃.
5 Application of Novel Porous Materials in High Temperature Proton Exchange Membrane
5.1 Porous PBI proton exchange membrane material
Changing the microstructure of the membrane can also change the physical properties, such as the amount of acid doping and proton conductivity of the membrane. Mecerreyes et al. Found that the porous structure can improve the phosphoric acid doping level of PBI and reduce acid loss
[90]. In general, a porogen such as dibutyl phthalate, polyvinyl alcohol, glucose, dimethyl phthalate, diethyl phthalate, dibutyl phthalate, triphenyl phosphate, lithium chloride, or the like can be used to form pores in the film
[91,92][93][94][90][95]. However, the porous membrane with high acid doping level will lead to the decline of mechanical properties, fuel penetration and other problems, which can be reduced to some extent by doping polymer and inorganic materials. The mechanical strength can also be improved by crosslinking, sandwich structure and other methods
[96,97][98].
Mehdipourghazi et al. Obtained porous membranes by mixing pore-forming agents LiCl and lignin with PBI
[99]. LiCl leads to the formation of micropores. Lignin is an anionic polymer, which can increase the number and size of pores by micro-phase separation with PBI, thus adsorbing more phosphoric acid. The highest proton conductivity of the porous membrane is 0.152 S·cm
-1 at 160 ℃. Xu et al compounded PBI with sulfonated graphene oxide (RGO) with high ion exchange capacity, and coated both sides of porous PBI to form a sandwich structure
[100]. The introduction of RGO did not affect the doping level of phosphoric acid (possibly due to the formation of hydrogen bonds between surface oxygen-containing groups and phosphoric acid), and the doping level of phosphoric acid increased with the increase of the mass fraction of PPBI in the porous layer. The sandwich membrane PBI-RGO/PPBI-80/PBI-RGO adsorbs 500 wt% phosphoric acid and has an ADL of 20.4, a proton conductivity of 0.113 S·cm
-1 at 170 ℃ without humidification and a tensile strength of 22.7 MPa.
5.2 Metal organic framework materials (MOFs)
Metal organic frameworks (MOFs), also known as Porous coordination polymers (PCPs), are one-dimensional, two-dimensional or three-dimensional networks constructed by self-assembly of Metal ions or clusters and organic ligands through coordination. There are many kinds of MOFs, which have high specific surface area, tailorable framework and pore structure, and high stability. In the past decade, the number of papers on MOFs has increased exponentially, including gas storage and separation, optical materials, magnetic materials, catalysis, proton conduction and so on
[101][102][103][104][105][106]. At present, a large number of MOFs have been confirmed to have the potential to conduct ions. There are two strategies to prepare proton-conducting MOFs. One is to modify acidic functional groups such as sulfonic acid group, carboxylic acid group and phosphonic acid group on the backbone of MOFs
[107~111]. The other is to insert non-volatile guest molecules, such as imidazole, triazole, histamine, acid molecules and
[112~114], into the channels to provide additional proton conduction paths. Proton conducting Mixed-matrix membranes (MMMs) can be prepared by blending MOFs with flexible polymer substrates, but few MOFs have been used as fillers for MMMs so far.
5.2.1 Proton conducting MOFs
Most of the reported proton conducting MOFs are still only suitable for low temperature operation (< 100 ℃), and the conductivity of these MOFs has reached a high level. For example, the layered Mg compound PCMOF10 with phosphonate as ligand reported by Shimizu et al. Has a proton conductivity of 0.035 S·cm
-1 at 70 ° C and 95% RH
[115]. The proton conductivity of Uio-66(SO
3H)
2 crystals with sulfonic acid groups synthesized by Hong et al. Can reach 0.084 S·cm
-1 at 80 ℃ and 90% RH
[116].
At present, there are few MOFs suitable for high-temperature operation. In 2011, Kitagawa et Al. Synthesized MOFs supporting histamine and imidazole molecules, named imidazole @ Al-MOF and histamine @ Al-MOF, respectively. The proton conductivity of the latter reached the order of 10
-3S·cm
-1 at 150 ℃ in the absence of water
[113].
The pore volume of MIL-101 is 1.9 cm
3·g
-1, so it has a high loading capacity, and the Cr (Ⅲ) carboxylate is one of the few porous coordination compounds resistant to strong acids
[117][106]. Ponomareva et al. filled inorganic acid H
2SO
4 and H
3PO
4 into the pores of MIL-101 to obtain H
2SO
4@MIL-101 and H
3PO
4@MIL-101.At 150 ℃ and relative humidity of only 0.13%, the proton conductivity of the two materials can reach 1.0×10
-2S·cm
-1 and 3.0×10
-3S·cm
-1, respectively
[106]. This result confirms that the stronger the acidity of the guest molecule, the better the proton conductivity of MOFs.
[118]. The defect sites provide space for uncoordinated H
3PO
4, H
2PO
4- and H
2O to move in the framework, which promotes the migration of protons in the framework and helps to improve the proton conductivity. The proton conductivity of the crystal with Zn ∶ P ∶ C ∶ N = 1 ∶ 1.6 ∶ 4.1 ∶ 5.8 can reach the 4.6×10
-3S·cm
-1 at 150 ℃. The advantages of the non-porous structure are demonstrated by the fact that the proton carrier encapsulated inside the crystal can be maintained up to the decomposition temperature of the complex, and fuel cell tests have confirmed that there is no fuel permeation problem.
5.2.2 MOFs composite membrane
As solid proton conducting materials, the proton conductivity of MOFs mostly needs to be tested by pressing. In order to meet the needs of film formation, it is a relatively simple method to compound with polymer matrix. As fillers, MOFs are mostly compounded with Nafion or SPEEK. For example, Jiang et al. compounded MIL-101 impregnated with phytic acid with Nafion, and the proton conductivity of 12 wt% MIL @ phytic/Nafion composite membrane can reach 0.228 S·cm
-1 at 100 ℃ and 100% RH, showing higher performance than Nafion membrane
[119]. Zang et al. Synthesized MIL-101, sulfonated MIL-101 and MIL-101 loaded with H
2SO
4, H
3PO
4 and CF
3SO
3H, and used them to compound with chitosan.The composite membrane loaded with the H
2SO
4, the H
3PO
4 and the CF
3SO
3H is heated at 80 deg C,The highest proton conductivities at 35% RH were 9.5×10
-2S·cm
-1, 8.3×10
-2S·cm
-1 and 9.4×10
-2S·cm
-1, respectively.Close to the level of complete hydration of Nafion
[120].
MOFs have good compatibility with PBI, especially ZIFs containing imidazole ester structure. In the past five years, MOFs/PBI composite membranes have been used for gas separation and
[121~123], but few studies have been carried out on MOFs/PBI composite proton exchange membranes. In 2018, Escorihuela et al. Used zeolite imidazole ester framework materials ZIF-8, ZIF-67 and binary mixtures of ZIF-8 and ZIF-67 in m-PBI by solution method to obtain a series of phosphoric acid doped composite membranes based on PBI
[101]. The proton conductivity of PBI @ ZIF-8 and PBI @ ZIF-67 composite membranes containing 5 wt% ZIFs at 200 ℃ in the absence of water was 3.1×10
-3S·cm
-1 and the proton conductivity of 4.1×10
-2S·cm
-1,PBI@ZIF-mix was 9.2×10
-2S·cm
-1, respectively, indicating the synergistic effect of ZIFs.
5.3 Covalent organic frameworks (COFs)
Covalent organic frameworks (COFs) are a class of porous materials linked by Covalent bonds, which have potential applications in gas collection and storage, catalysis, and fuel cells due to their porosity, high surface area, and abundant functional groups
[124][125][126].
The ordered pores of COFs are conducive to the movement of protons and can be loaded with proton carriers, so a large number of studies have embedded proton carriers such as
[127~129] phosphate, polyoxometalates, phytic acid, acetic acid and so on into the pores of COFs
[130][131][126]. Another strategy is to modify the framework of COFs directly with proton-conducting groups
[132]. Compared with inorganic fillers, COFs have better compatibility with polymers, so they can also be used to prepare hybrid matrix films. Banerjee et al. synthesized a self-supporting, highly crystalline flexible COF mediated by amino-p-toluenesulfonic acid (PTSA·H
2O) salt. The slow crystallization process allows the proton carrier PTSA·H
2O to be impregnated in situ in the ordered nanopore of COFs
[133]. The sulfonic acid-loaded COFs membrane had a proton conductivity of 0.078 S·cm
-1 (80.degree. C., 95% RH). Wu et al. Synthesized a covalent organic framework Z-COF modified by zwitterions. The framework of Z-COF contains both quaternary ammonium ions and sulfonic acid groups (as shown in Figure 16). The composite membrane obtained by blending with Nafion has the highest proton conductivity of 0.22 S·cm
-1(80℃,100%RH), which is 57.1% higher than that of Nafion
[132].
As far as the current research status is concerned, both proton-conducting COFs and hybrid membranes with other flexible polymers, although some of them have reached high proton conductivity, can not be used for HT-PEMFCs at low temperature and high humidity. Kharul et al. Synthesized flexible blend membranes of COFs and PBI and used them for gas separation. Because of the good compatibility between COFs and polymers, there is a large development space for proton exchange membranes
[134].
6 Conclusion and prospect
20 Since the concept of HT-PEMFCs was proposed in the 1990s, abundant research progress has been made in high-temperature proton exchange membranes. Inorganic phosphoric acid has good thermal stability, low vapor pressure, and self-ionization under high temperature and anhydrous conditions.Therefore, high temperature proton exchange membranes based on phosphoric acid have high proton conductivity, good chemical stability and thermal stability under high operating temperature, low relative humidity and even anhydrous conditions, and become the most suitable membrane materials for HT-PEMFCs.
In order to improve the comprehensive performance of phosphoric acid membrane, a variety of modification methods have been used in high temperature PEMs based on polybenzimidazole, polyarylether and other polymers. The advantages and disadvantages of various modification methods can be briefly summarized as follows: (1) Crosslinking and organic-inorganic composites are the research hotspots because of the high selectivity of crosslinking agents and inorganic fillers, which can often improve the proton conductivity and mechanical properties of phosphoric acid membranes at the same time; (2) The combination of multiple polymers is the simplest modification method and has wide applicability. When PBI is blended with sulfonated polymers, in addition to compatibility, the higher the proportion of the latter, the greater the dependence of proton conductivity on humidity; (3) The method of grafting basic groups on the polymer molecular chain can only improve the doping rate of phosphoric acid, but not the mechanical properties; (4) Compared with inorganic phosphoric acid, organic phosphonic acid groups are directly linked to the main chain or side chain of the polymer by C-P bonds, which overcomes the shortcomings of strong corrosiveness and easy loss of inorganic phosphoric acid, but the proton conductivity of this kind of proton exchange membrane is generally low, because the number of free phosphoric acid is necessary to improve the proton conductivity
[135][26].
MOFs and COFs have high specific surface area, good chemical stability, thermal stability, wide variety and strong designability, which can be used as crystalline proton conductive materials. However, due to the insoluble and infusible characteristics, it is still difficult to prepare self-supporting membranes with sufficient mechanical properties, and there are problems such as proton carrier loss and fuel penetration
[117]. Composites with flexible polymers seem to easily meet the needs of proton exchange membranes for fuel cells.
The theoretical and experimental research of high temperature fuel cells is still developing rapidly, and the research ideas of phosphoric acid doped proton exchange membranes for high temperature fuel cells will continue to broaden. How to reduce the cost and prolong the working life will become an important issue for the commercial use of PEMs.
the National Key Research and Development Program(Japan-China Joint Research Program)(2017YFE0197900)
Link Project of the National Natural Science Foundation of China and Guangdong Province(U1601211)
and the National Key Research and Development Program(2018YFA0702002)
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