Intermolecular Acid–Base‐Pairs Containing Poly (p‐Terphenyl‐co‐Isatin Piperidinium) for High Temperature Proton Exchange Membrane Fuel Cells

How to optimize and regulate the distribution of phosphoric acid in matrix, and pursuing the improved electrochemical performance and service lifetime of high temperature proton exchange membrane (HT‐PEMs) fuel cell are significant challenges. Herein, bifunctional poly (p‐terphenyl‐co‐isatin piperidinium) copolymer with tethered phosphonic acid (t‐PA) and intrinsic tertiary amine base groups are firstly prepared and investigated as HT‐PEMs. The distinctive architecture of the copolymer provides a well‐designed platform for rapid proton transport. Protons not only transports through the hydrogen bond network formed by the adsorbed free phosphoric acid (f‐PA) anchored by the tertiary amine base groups, but also rely upon the proton channel constructed by the ionic cluster formed by the t‐PA aggregation. Thorough the design of the structure, the bifunctional copolymers with lower PA uptake level (<100%) display prominent proton conductivities and peak power densities (99 mS cm−1, 812 mW cm−2 at 160 °C), along with lower PA leaching and higher voltage stability, which is a top leading result in disclosed literature. The results demonstrate that the design of intermolecular acid–base‐pairs can improve the proton conductivity without sacrificing the intrinsic chemical stability or mechanical property of the thin membrane, realizing win‐win demands between the mechanical robustness and electrochemical properties of HT‐PEMs.


Introduction
The proton exchange membrane plays a crucial role in impeding fuel gas and conducting protons, which directly impacts the output performance and operational lifetime of high temperature proton exchange membrane fuel cells (HT-PEMFCs). [1,2]Generally, the phosphoric acid (PA) doped polybenzimidazole (PBI) membranes are utilized to prepare membrane electrode assembly (MEA) for HT-PEMFCs. [3,4]he state-of-the-art high temperature proton exchange membranes (HT-PEMs) primarily rely on the doped PA molecules for rapid proton transportation, in which a small quantity of PA molecules is anchored on the imidazole base groups via acid-base interactions, while the remainder free PA (f-PA) produces a continuous proton transporting channels in the network of the polymer. [5,6]It has been found that the majority of the published literature blindly increases PA uptake levels in membranes with regard to developing an extensive and integrated ion transport pathway. [7]Actually, this method can adversely affect the entire fuel cell system.First, only a small portion of the adsorbed PA molecules is bound to the polymer main chain by acidbase interaction, and the remainder f-PA molecules have a plasticizing effect on the polymer main chain, which significantly weakens the van der Waals force between the polymer chains, resulting in decline of dimensional stability and mechanical properties of membrane. [8,9]Secondly, the produced gaseous water increases the partial pressure of water in the reaction gas, thereby accelerating the leaking rate of f-PA from the membrane under the driving of current and the strong binding energy between water molecules and f-PA molecules. [10]Meanwhile, it is unavoidable that the reaction activity of catalytic layer will be deteriorated by the f-PA, which may even corrode the metal bi-plate of the fuel cell system. [11,12][15] The true point of developing high-performance proton exchange membrane is to achieve a compromise between several properties.Consequently, optimizing and regulating the distribution of PA in the membrane matrix, minimizing the loss of f-PA, boosting the mechanical properties of the PA doped How to optimize and regulate the distribution of phosphoric acid in matrix, and pursuing the improved electrochemical performance and service lifetime of high temperature proton exchange membrane (HT-PEMs) fuel cell are significant challenges.Herein, bifunctional poly (p-terphenyl-co-isatin piperidinium) copolymer with tethered phosphonic acid (t-PA) and intrinsic tertiary amine base groups are firstly prepared and investigated as HT-PEMs.The distinctive architecture of the copolymer provides a well-designed platform for rapid proton transport.Protons not only transports through the hydrogen bond network formed by the adsorbed free phosphoric acid (f-PA) anchored by the tertiary amine base groups, but also rely upon the proton channel constructed by the ionic cluster formed by the t-PA aggregation.Thorough the design of the structure, the bifunctional copolymers with lower PA uptake level (<100%) display prominent proton conductivities and peak power densities (99 mS cm À1 , 812 mW cm À2 at 160 °C), along with lower PA leaching and higher voltage stability, which is a top leading result in disclosed literature.The results demonstrate that the design of intermolecular acid-base-pairs can improve the proton conductivity without sacrificing the intrinsic chemical stability or mechanical property of the thin membrane, realizing win-win demands between the mechanical robustness and electrochemical properties of HT-PEMs.membranes, and pursuing the improved electrochemical performance and service lifetime of HT-PEMFCs are significant challenges to this field.
[18][19] Because the phosphonated polymers with covalently linked t-PA groups can produce protons by dissociation and conduct protons at anhydrous and high temperatures.Protons can conduct through the continuous ion-rich channels formed by t-PA groups at the same time. [20]Moreover, compared to f-PA molecules, the t-PA group has better thermal stability and higher resistance to degradation under high temperature.Ideally, one approach to addressing the PA leak problem is using the phosphonated polymers as intrinsic proton conductors. [21,22]It was discovered, however, that because the t-PA group of phosphonated polymers has a lower first-order dissociation constant than f-PA, and the ion mobility of t-PA group of the phosphonated polymers is also significantly lower.[25] For example, the polyethylene bearing covalently bounded t-PA groups can form hydrogen-bonding layered aggregates that facilitate the protons transporting, but the proton conductivity only reaches 0.1 mS cm À1 at 150 °C at anhydrous condition. [26]urthermore, the synthetic difficulties and sophisticated regulation of the phosphonated polymer nanostructure are still major challenges for practical application.
To simultaneously optimize the electrochemical properties and physical properties of HT-PEMs, we first synthesized a bifunctional copolymer containing both the covalent t-PA groups and intrinsic tertiary amine base groups, in which the t-PA groups can produce protons and the tertiary amine base groups can adsorb appropriate amount of the f-PA molecules.The designed bifunctional copolymer (bifunctional poly (p-terphenyl-co-isatin piperidinium) with intermolecular acid-basepairs, abbreviated as P/PTIP-x) has shown intriguing performance as HT-PEM.With a precise design of the molecular structure of the polymer, the amounts of the f-PA molecules and t-PA groups can be tailored accurately by varying the feed ratio of the monomers during the copolymer synthesis.Finally, the proton transport channel and wellconnected hydrogen-bonded networks in P/PTIP-x membranes can be constructed not only through the interaction between the intrinsic tertiary amine base groups and the f-PA molecules, but also by the aggregation of the t-PA groups that form a microscopic phase separation region.

Polymer Synthesis and Characterization
Bifunctional poly(p-terphenyl-co-isatin piperidinium) was synthesized via a three-step procedure (Figure 1).Initially, poly(p-terphenyl-coisatin piperidinium) (PTIP-x) copolymer was synthesized via the Friedel-Crafts reaction with p-terphenyl, isatin, and N-methyl-4piperidone, catalyzed by superacid catalysis. [27,28]The feed ratio of isatin and N-methyl-4-piperidone is manipulated to synthesize PTIP-x with different contents of isatin unit.Both the PTIP-x copolymers are highly soluble in most common solvents at room temperature with the assistance of trifluoroacetic acid, such as dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), dimethylacetamide (DMAC) and so on.The 1 H-NMR spectra of PTIP-x copolymers are presented in Figure S1, Supporting Information and confirm the chemical structure of PTIP-x.The chemical shift between 7.00-7.75ppm are ascribed to the arylene protons and the signals between 2.0 and 3.6 ppm are ascribed to the methylene and methyl protons of piperidinium ring. [29]The single peak at 10.86 ppm corresponds to the N-H protons of the isatin unit.Then, the N-H groups of PTIP-x can be converted quantitatively into the covalent t-PA groups by the phosphonated reaction with phosphorus oxychloride (POCl 3 ) using pyridine as catalyst, followed by hydrolysis in vast quantities of water.As shown in Table S1, Supporting Information, the intrinsic viscosities of P/ PTIP-20, P/PTIP-40, P/PTIP-60, P/PTIP-80 in DMSO are 1.437, 1.223, 0.917 and 0.887 dL g À1 in turns, indicating that they have high molecular weights for membrane fabrication.After phosphonation, the solubility of the P/PTIP-x decreases with the increase of t-PA content.Due to the strong intermolecular hydrogen bonding between the t-PA groups and base groups, small amounts of TFA have to be added to facilitate the dissolution of P/PTIP-x in DMSO by destroying the intermolecular hydrogen bonding.The chemical structures of P/PTIP-x are confirmed by 1 H-NMR and 31 P-NMR spectroscopy (Figures S2 and S3, Supporting Information).Compared with 31 P-NMR spectra of the pure PA (0.00 ppm), 31 P-NMR spectra of P/PTIPx shows one typical peak at À1.00 ppm, which indicates the successful tethering of PA groups onto the macromolecules of PTIP-x.The difference in the static water contact angle between the PTIP-20 and P/PTIP-20 dry membrane shows that the polymer becomes more hydrophilic after introducing t-PA groups (Figure S4, Supporting Information).In addition, the IR spectrum of P/PTIP-x reveals -P=O peak at 1254 cm À1 , -P-OH peak at about 970 cm À1 (Figure S5, Supporting Information).
As depicted in Figure 1, P/PTIP-x has both alky tertiary amine base groups and t-PA groups, in which the molar ratio of the tertiary amine base group to the t-PA group can be tailored by varying the feed ratio of the monomers (isatin to N-methyl-4-piperdone).The tertiary amine base groups in P/PTIP-x can function well with f-PA groups by acidbase interaction, and then form continuous and fast proton conducting network with the assistance of t-PA groups.The phosphoric acid capacity (PAC) of P/PTIP-20, P/PTIP-40, P/PTIP-60, and P/PTIP-80 are 1.149, 2.157, 3.049, and 3.843 mmol/g in turns, The PAC values increase with the increase of the t-PA group content and the decrease of the tertiary amines base group content.It is believed that PAC has a big influence on the PA uptake value and proton conductivity as discussed below. [30]Photographs of P/PTIP-x membranes were shown in Figure S6, Supporting Information.Furthermore, the P/PTIP-x membrane remains structurally integrity after the prolonged soaking in PA at high temperature.Here, we present for the first time an efficient structural design strategy for regulating the ratio between t-PA and f-PA levels in HT-PEMs.adsorb f-PA rapidly and achieve the saturated PA uptake state within 4 h.The saturated PA uptake levels and volume swelling rates of the X %PA@P/PTIP-x membranes are much lower than those of the PBI membranes (X%PA@P/PTIP-x, X% represents the PA uptake level of the P/PTIP-x membrane).The P/PTIP-20 with the highest ratio of base groups to t-PA groups has the highest PA uptake of 90% and volume swelling rate of 74%.Whereas, the volume swelling of 30%PA@P/ PTIP-80 is only 3%.That is, when the amount of t-PA group increases, the PA uptake level (adsorbed f-PA amount) and volume swelling of the P/PTIP-x membrane decrease gradually.Simultaneously, due to the strong hydrogen bonding force inside the P/PTIP-x structure, there is limited amount of internal free space for PA adsorption, therefore the volume swelling of the membrane diminishes progressively.The mechanical properties of the dry and PA doped P/PTIP-x membranes are shown in Figure S7, Supporting Information and Figure 2c.Owing to the presence of the flexible units in the PBI main chain and rigid repeating units in the P/PTIP-x chain, the mechanical properties of the P/PTIP-x membranes are different with the PBI membrane.Nonetheless, we focus primarily on the mechanical properties of the PA doped membranes, since these properties directly impact the overall lifespan of HT-PEMFCs. [31]Compared with the PA@P/PTIP-x membrane, 250%PA@PBI exhibits a low tensile strength (9 MPa) because of the plasticizing effect of f-PA molecules.In contrast, the PA@P/PTIP-x membrane with lower PA uptake (<100%) displays good mechanical strength, which are helpful to improve the cell performance of HT-PEMFCs.

PA Doping, Volume Swelling, Mechanical and Chemical Stability
The overall thermal and chemical stabilities of membrane are crucial to the endurance of HT-PEMs. [32]The chemical stability of membranes was evaluated by accelerating degradation in Fenton's reagent.Under realistic operating conditions of HT-PEMFCs, the backbone of the HT-PEMs will be attacked by peroxide radicals (ÁOH and ÁOOH), which results in membrane deterioration and poor mechanical strength. [33,34]e assessed the oxidative stability of the pristine membrane in Fenton's reagent, which is determined by the fragmentation time and degradation degree for the dry membranes.As shown in Figure 2d, compared to the P/PTIP-20 membrane with t-PA groups, PTIP-20 membrane is more susceptible to peroxide radical attack.More importantly, the P/ PTIP-x membranes become more endurance toward ÁOH and ÁOOH radicals as the amounts of t-PA groups in copolymer increase.Among the P/PTIP-x membranes, the appearance of P/PTIP-40 membrane remains integrality over 192 h in Fenton reagent.That suggests that, the stronger interaction between the t-PA group and intrinsic tertiary amine base groups of P/PTIP-40 structure shields the active groups and protects the backbone of copolymer from radical attack and chemical degradation.In conclusion, we believe that the P/PTIP-x membrane with low PA uptake and volume swelling has the potential to produce HT-PEMFCs with high mechanical stability and extended durability.

Morphology Observation
AFM images illustrate the adhesion force of the pure P/PTIP-x membranes, and the different colored regions represent different adhesion forces, which reflect the deferent attraction between the tip probe and the inserted ionic cluster. [35]The bright region corresponds to the aggregation of the covalent t-A group with strong attraction, whereas the black portion represents the presence of the tertiary amine base groups with weak attraction. [36,37]Notably, the brilliant region gradually increases as the amounts of t-PA groups increase, as does the average adhesion force of the P/PTIP-x polymer.As shown in the inserts of Figure 3a-d, the P/PTIP-20 membrane with the maximum base and minimum t-PA groups has the lowest average adhesion force (3 nN) while the P/PTIP-80 membrane with the minimum base and maximum groups has the highest average adhesion force (24 nN).According to the aforesaid results, there is an apparent phase separation structure in the P/PTIP-x polymer because of the presence of bifunctional groups, which will greatly influence the proton transporting behavior of the P/PTIP-x membranes.TEM results display a phase-separated structure similar to those of AFM mapping, and the black region also indicates the presence of the hydrophilic t-PA group (Figure 3e,f and Figure S8, Supporting Information).The aggregation of ion clusters could serve as proton transport channels for proton transmission.After saturated with 85 wt% PA solution at 100 °C, the PA doped P/PTIP-x (denoted as PA@P/PTIP-x) were obtained with different PA uptake level.Figure 3h-k depict the AFM adhesion images of the PA@P/PTIP-x membrane, and the bright yellow zone in AFM images reveals the distribution of adsorbed f-PA in the P/PTIP-x membrane.Due to the synchronous incorporation of t-PA groups and base groups, the adsorption and distribution of PA molecules in P/PTIP-x matrices differed from those of polymers containing only base groups.As shown in Figure 3g, the hydrophilic clusters formed by t-PA groups of P/PTIP-x polymer are interconnected by narrow ionic channels and form ion-conducting paths inside matrix.Thus, the protons not only pass across the hydrogen-bonded network generated by f-PA molecules but also transport through the micro channel formed by t-PA groups in PA@P/PTIP-x membrane. [38]Additionally, compared to P/PTIP-x dry membrane, the surface adhesion morphology of PA@P/PTIP-x is dramatically different.The average adhesion force of 90%PA@P/PTIP-20 membrane increases to 23 nN from 3 nN of the pristine undoped membrane.Because the lower content of base groups, the average adhesion force of 30%PA@P/PTIP-80 membrane is only 31 nN, which is slightly higher than 24 nN of the pristine undoped membrane.Notably, f-PA in PA@P/PTIP-x membrane significantly affects the phase region, and displays better microphase separation for the rapidly proton conduction (Figure S9, Supporting Information).

Proton Conductivity
As depicted in Figure 4a,b, PA doped P/PTIP-x membranes show high proton conductivities with low PA uptakes (<100%) over the Energy Environ.Mater.2024, 7, e12621 temperature range of 80-180 °C.Especially, the PA doped P/PTIP-x (PA@P/PTIP-20) membrane with lower PA uptake (only 90%) exhibits higher proton conductivity than the PA doped PBI membrane with higher PA uptake (250%).For example, the 250%PA@PBI membrane has a conductivity of only 45 mS cm À1 , while the 90%PA@P/ PTIP-20 membrane exhibits the highest conductivity of 99 mS cm À1 at 160 °C (Figure 4a).This is due to the fact that the P/PTIP-20 with t-PA groups can provide a continuous hydrogen bond network as well as a sufficient proton source for proton transfer even with lower PA uptake.Meanwhile, to highlight the role of the t-PA groups of the phosphonated polymer, the proton conductivities of the PA doped P/ PTIP-20 and PTIP-20 membranes are compared under the same PA uptake level.The proton conductivities of 90%PA@PTIP-20 is dramatically lower than those of P/PTIP-20 over the test temperature range.As depicted in Figure 4b, the proton conductivities of the PA@P/PTIP-x membranes with different PA uptakes are shown as a function of temperature.The activation energies of the PA@P/PTIP-x membranes vary significantly and are shown in Figure S10, Supporting Information.As more t-PA groups in P/PTIP-x, the amount of the adsorbed f-PA gradually decreases, resulting the dramatic decrease in proton conductivity.For instance, 30%PA@P/PTIP-80 has significantly lower proton conductivity (3 mS cm À1 ) than 90%PA@P/PTIP-20 (106 mS cm À1 ) at 180 °C due to the low PA uptake (Figure 4b).This suggests that, unlike the PBI membranes, the bifunctional P/PTIP-x copolymer effectively uses both t-PA groups and f-PA molecules to construct an efficient network for proton conduction and can deliver high proton conductivity with low PA uptake level.
Since PA leakage is one of the major problems for HT-PEMs at high temperature and anhydrous conditions, it is important to evaluate the PA retention behavior of the membrane.The accelerated test of the proton conductivity stability at specific temperatures and anhydrous conditions is used to monitor the PA leaching rate from the PA doped membrane. [15]igure 4c shows that the proton conductivities of 250%PA@PBI and 90%@PTIP-20 membrane decrease seriously in beginning 144 h, which can be attributed to the large amount of f-PA.In contrast, the P/PTIP-20 with bifunctional groups shows a higher proton conductivity stability under the same condition.It is noteworthy that 70%PA@P/PTIP-40 membrane holds the highest retention of proton conductivity, which is related to the intermolecular interaction resulting from the appropriate ratio between t-PA group and the tertiary amine base groups in P/PTIP-40 (Figure 4d).Although the P/PTIP-60 has higher t-PA groups content, the PA uptake of doped P/PTIP-60 membrane with 45% still declines seriously, indicating that the f-PA molecules migrate from membrane will cause serious damage to the hydrogen bond network, and increase the internal resistance and causing an eventually drop of proton conductivity.Thus, the molar ratio between the fixed t-PA groups and the adsorbed f-PA molecules is crucial for the proton conducting behavior of the PA doped P/PTIPx membranes.
The PA uptake level has a significant effect on the morphology of the membrane, thus regulating the formation of hydrogen bond network and proton transport channels, which ultimately affect the electrochemical performance of the membrane. [39,40]As shown in Figure 4e, the bright yellow area in the AFM image of the 30% PA@P/PTIP-20 membrane represents the f-PA molecule aggregation region.A more uniformly distribution of brilliant yellow areas can be seen in Figure 4f, which means that 60%PA@P/PTIP-20 has more distinct microphase separation for the rapidly proton conduction.With varying microphase separation of P/PTIP-20 membrane by tailoring the adsorbed f-PA amount, the proton conductivity of membrane can be improved considerably.The 30%PA@P/PTIP-20 membrane has a lower proton conductivity of only 20 mS cm À1 , while the 60%PA@P/PTIP-20 membrane gives a higher proton conductivity of 30 mS cm À1 at 180 °C (as shown in Figure 4g).This implies that the phase separation structure of the bifunctional copolymer is critical for obtaining high proton conductivity of the PA doped P/PTIP-x membranes with low PA uptake (<100%), which can fabricate an effective hydrogen bond network and proton transport channels for proton transport (Figure 4h).This design permits the purpose of improving electrochemical performance of HT-PEMs without sacrificing the mechanical property of the HT-PEMs with high PA uptake.Energy Environ.Mater.2024, 7, e12621

Fuel Cell Performance
Up to now, the thick HT-PEMs had to be used for HT-PEMFCs testing owing to the poor mechanical properties of acid-doped membranes with high PA uptake.The high thickness results in the high internal resistance and low energy density of the cell. [41]It is one of the big challenges to fabricate high-performance HT-PEMFC by using thinner and stable H-PEMs with lower charge transfer resistance. [42,43]The physical and chemical stability of the thin membrane are major concerns at high temperature and under anhydrous conditions. [44,45]Furthermore, it should be noted that the degradation of the cell voltage gradually increases, and the operational difficulty of MEA construction becomes more serious as the membrane thickness decreases. [46]In this work, due to the outstanding mechanical features and high proton conductivities of the PA@P/PTIP-x membranes, we evaluated the HT-PEMFC performance using the membrane thickness of only 25 AE 5 lm thickness (Figure 5a), while most literatures used >50 lm thickness membrane.Figure 5b,c and Figure S12, Supporting Information show the polarization curves of the MEAs with the PA@P/ PTIP-x membranes at 140 °C.The P/PTIP-x based MEAs display high open circuit voltages, which further indicate low fuel permeability and dense structure of the thin PA@P/PTIP-x membranes.Owing to the higher proton conductivity for 90%PA@P/PTIP-20, the peak power density (PPD) of the MEA with 90%PA@P/PTIP-20 reaches 572 mW cm À2 with small flow rate and without backpressure, which is higher than that of the MEA with 250%PA@PBI (Figure S11, Supporting Information).Compared with the MEAs 70%PA@P/PTIP-40 and 45%PA@P/PTIP-60, the MEA with 90%PA@P/PTIP-20 MEA exhibits the most superior performance (678 mW cm À2 for 90% PA@P/PTIP-20 vs. 594 mW cm À2 for 70%PA@P/PTIP-40 with higher gas flow rate and backpressure).At the higher operating temperature of 160 °C (Figure 5d,e), the PPD of the MEA with 90%PA@P/ PITP-20 increases to 812 mW cm À2 , which is 1.18 times higher than that of the MEA with 70%PA@P/PTIP-40, and the maximum current density even reaches 2050 mA cm À2 .Due to the presence of by the fixed t-PA groups in the P/PTIP-x, the bifunctional copolymer with low PA uptake could establish a stable hydrogen bond network and continuous proton transport channel for proton rapidly diffusion across the Energy Environ.Mater.2024, 7, e12621 matrix.][49][50][51][52][53][54][55] The electrochemical performance of the most HT-PEMs can only be improved by increasing the PA uptake level (>200%); however, the huge amount of PA molecules typically has detrimental effects on the durability of the entire fuel cell system.In contrast, the PA@P/PTIP-x membrane with the optimized PA uptake (<100%) exhibits high performance by the synergistic effect between t-PA groups and the adsorbed f-PA anchored by the tertiary amines base group.
The durability of HT-PEMFC is evaluated by monitoring the voltage stability of the cell.The MEAs with PA@P/PTIP-x were initially cycled at different current densities.As shown in Figure 5g, the cells with all PA@P/PTIP-x show good rate performance, and no evident voltage degradation is observed at different current densities.After cycling at high current density of 500 mA cm À2 , the voltage can recover to 0.70 V when the current density recovers to 100 mA cm À2 .We then monitored the voltage changes of the cell with PA@P/PTIP-x membranes for100 h test at 150 mA cm À2 with a low flow rate and without back-pressure at 140 °C (Figure 5h).It is noted that the voltage Energy Environ.Mater.2024, 7, e12621 degradation rate of the 90%@P/PTIP-20 membrane is 0.45 mV h À1 , retaining 93.5% of the original voltage.As shown in Figure S13, Supporting Information, the PPD retention of the cell with 90%PA@P/ PTIP-20 is 70% after 100 h.Comparatively, the cell with 70%PA@P/ PTIP-40 possesses a lower voltage degradation rate of 0.08 mV h À1 , and voltage decay rate of only 1.1% under the same evaluating conditions.Additionally, after 100 h, the PPD retention of the cell with 70% PA@P/PTIP-40 is 90%, as illustrated in Figure S14, Supporting Information showing that the lower PA uptake of PEMs can afford better cell performance with lower voltage degradation rate and higher PDD retention.In order to simulate the PA leaching rate of the PEMs in the cell, the PA retention behavior of the PA@P/PTIP-40 membrane was investigated by exposing the membrane in water vapor.As shown in Figure 5i, the 70%PA@P/PTIP-40 membrane shows higher PA retention than those of other membranes.We believe that the strong intermolecular hydrogen bond interactions give the 70%PA@P/PTIP-40 membrane higher PA retention and ensure the proton source over a long period of time.Furthermore, the higher mechanical properties of the P/PTIP-40 also guarantee the MEA higher durability.The outstanding performance of the PA@P/PTIP-x validates the superior stability and unique structural advantage of the bifunctional copolymer as HT-PEMs.

Conclusions
In summary, we report a novel bifunctional copolymer for high performance HT-PEMs with high power output and durability.By simultaneously regulating the proportion of the tertiary amines base groups with t-PA groups, the saturated PA uptake of the bifunctional poly(pterphenyl-co-isatin piperidinium) can be accurately controlled.Owing to the highly continuous ions conducting channels formed by combining the adsorbed f-PA molecules and the t-PA groups, the bifunctional copolymers with low PA uptake (<100%) exhibit high proton exchange conductivity, high peak power density and good durability.Therefore, the new design concept of the bifunctional copolymer can be a new avenue to develop excellent performance HT-PEMs, and then give valuable conception on the practical application of HT-PEMFCs.

Experimental Section
Synthesis of PTIP-x copolymer: Different PTIP-x copolymer (x represents the molar percentage of isatin in the total moles of isatin and piperidinium) was prepared based on previous literature. [27,28]Poly(p-terphenyl-co-isatin piperidinium) (PTIP-x) was synthesized with p-terphenyl, isatin and N-methyl-4-piperidone by the superacid catalyzed step growth polymerization.Briefly, a typical synthesis procedure of PTIP-80 is as follows.At room temperature, p-terphenyl (14.60 mmol, 336.21 mg), isatin (12.88 mmol, 189.50 mg) and N-methyl-4piperidone (3.22 mmol, 36.43 mg) were added into a three-neck flask with mechanical stirring.Then, dichloromethane (CH 2 Cl 2 , 14.00 mL) was added to the flask to dissolve the monomers until a clear solution was formed.After the solution was cooled to below 10 °C, TFA (1.00 mL) and TFSA (14.00 mL) were introduced slowly to the reactor with vigorous mechanical stirring.The reaction temperature was maintained for 4-10 h until an extremely viscous solution with a dark green color was produced.And then the reaction mixture was poured into methanol to precipitate the copolymer (PTIP-80).
Synthesis of P/PTIP-x copolymer: Bifunctional poly(p-terphenyl-co-isatin piperidinium) (P/PTIP-x) copolymer is prepared from PTIP-x copolymer by a phosphonated reaction. [56]This is firstly report on the synthesis of the bifunctional P/PTIP-x copolymer.Briefly, PTIP-80 (1.00 g) was completed dissolved in 10.00 mL NMP at 80 °C with vigorous stirring.Afterwards, as soon as the temperature of reaction flask was cooled to below 10 °C, excess POCl 3 and the purified pyridine (n POCl3 /n pyridine = 1/1 mmol mmol À1 ) in NMP were added dropwise into above solution, and then stirred continuously for 12 h at below 10 °C.Then, DI water was dripped into the aforesaid solution with stirring for 12 h and the temperature was maintained below 10 °C, during which a gradual hydrolysis reaction will proceed.The white precipitate was then dumped into vast quantities of water to completely convert -P-Cl groups to -P-OH, which was agitated for 24 h.Finally, the white fiber named P/PPIT-80 was filtered and washed with deionized water for several times and then dried under vacuum at 100 °C.
Preparation P/PTIP-x membranes: The P/PTIP-x membranes were fabricated by a solution casting method.A typically preparation of P/PTIP-80 membrane is described as follows: P/PTIP-80 copolymer (200 mg) was dissolved in DMSO (10 mL) with small amount of TFA (20 lL) to obtain a diluted and homogeneous cast-solution.As the P/PTIP-x copolymer has poor solubility in DMSO, it is necessary to use TFA to disrupt the intermolecular hydrogen bonds in P/PTIP-x copolymer to facilitate the dissolution.Subsequently, the solution was filtered through a sintered glass funnel and cast on a clean glass plate.After dried at 70 °C for 10 h and 100 °C for another 10 h, a transparent membrane was obtained and immersed in DI water to remove the residue solvent.Transparent, flexible and dry membranes with a thickness of 20 AE 5 lm were obtained after dried at 100 °C for 24 h.

Figure
Figure2a,b depicts the variation trend of the PA uptake and volume swelling of the P/PTIP-x membrane in 85 wt% PA solution at 100 °C.In comparison with PBI membrane, the P/PTIP-x membranes can

Figure 2 .
Figure 2. a) PA uptake and b) volume swelling ratio of PA doped P/PTIP-x membrane.c) Mechanical properties of PA-doped PBI and P/PTIP-x membranes.d) Oxidative stabilities of pristine membrane.

Figure 5 .
Figure 5. a) Schematic of HT-PEMFC based on P/PTIP-x membranes.Fuel cell performance of the MEA based on PA@P/PTIP-x membranes at different testing conditions: b) 0.08/0.16SLPM of A/C flow rate and 0 bar A/C back pressure at 140 °C, c) 0.20/0.40SLPM of A/C flow rate and 0.3 bar A/C back pressure at 140 °C, d) 0.08/0.16SLPM of A/C flow rate and 0 bar A/C back pressure at 160 °C, e) 0.20/0.40SLPM of A/C gas flow rate and non 0.3 bar A/C back pressure at 160 °C.f) Comparison of peak power density and PA uptake level of PA@P/PTIP-x membrane with other reported HT-PEMs.g) Rate performance test of the fuel cells.h) Durable test of the fuel cell at 140 °C under 0.08/0.16SLPM of A/C flow rate and 0 bar A/C back pressure.i) PA retention of different membranes.