A Novel High Temperature Fuel Cell Proton Exchange Membrane with Nanoscale Phase Separation Structure Based on Crosslinked Polybenzimidazole with Poly(vinylbenzyl chloride)

A semi-aromatic polybenzimidazole (DPBI) is synthesized via polycondensation of decanedioic acid (DCDA) and 3,3-diaminobenzidine (DAB) in a mixed phosphorus pentoxide/methanesulfonic acid (PPMA) solvent. Ascribing to in-situ macromolecular crosslinker of ploly((vinylbenzyl chloride) (PVBC), a robust crosslinked DPBI membrane (DPBI-xPVBC, x refers to the weight percentage of PVBC in the membrane) can be obtained. Comprehensive properties of the DPBI and DPBI-xPVBC membranes are investigated, including chemical structure, antioxidant stability, mechanical strength, PA uptake and electrochemical performances. Compared with pristine DPBI membrane, the PA doped DPBI-xPVBC membranes exhibit excellent antioxidative stability, high proton conductivity and enhanced mechanical strength. The PA doped DPBI-10PVBC membrane shows a proton conductivity of 49 mS cm−1 at 160 °C without humidification. Particularly, it reveals an enhanced H2/O2 single cell performance with the maximum peak power density of 405 mW cm−2, which is 29% higher than that of pristine DPBI membrane (314 mW cm−2). In addition, the cell is very stable in 50 h, indicating the in-situ crosslinked DPBI with a macromolecular crosslinker of PVBC is an efficient way to improve the overall performance of HT-PEMs for high performance HT-PEMFCs.


Synthesis of Semi-Aromatic Polybenzimidazole (DPBI)
DPBI was synthesized by solution polycondensation reaction of DCDA and DAB with PPMA as condensating agent and solvent according to the following procedure. In a 100 mL three-neck round-bottomed flask was filled with 3.75 g P 2 O 5 and 37.5 g MeSO 3 H and equipped with a mechanical stirrer and a nitrogen inlet and outlet. The mixture was stirred at 80 • C for approximately 1 h, until a transparent solution was obtained. DCDA (1.0117 g, 5 mmol) was added and stirred at this temperature to dissolve it completely. Then, DAB (1.0716 g, 5 mmol) was added and stirred for 1 h, the reaction temperature was promoted to 120 • C and kept for 5 h. The reaction mixture was raised to 140 • C and continuously stirred for another 0.5 h. The final viscous solution was slowly poured into deionized water so as to make the polymer precipitated. The obtained fibrous polymers were crushed by a high-speed blender, washed with water and filtered. The polymer was stirred in NaHCO 3 solution (10 wt.%) to neutralize the residual acid in the polymer, and then filtered and washed with water and acetone, respectively, till the pH of the filtrate is neutral. Finally, it was dried in an oven at 120 • C for 24 h. The synthesis process is shown in Scheme 1. promoted to 120 °C and kept for 5 h. The reaction mixture was raised to 140 °C and continuously stirred for another 0.5 h. The final viscous solution was slowly poured into deionized water so as to make the polymer precipitated. The obtained fibrous polymers were crushed by a high-speed blender, washed with water and filtered. The polymer was stirred in NaHCO3 solution (10 wt.%) to neutralize the residual acid in the polymer, and then filtered and washed with water and acetone, respectively, till the pH of the filtrate is neutral. Finally, it was dried in an oven at 120 °C for 24 h. The synthesis process is shown in Scheme 1. Scheme 1. Preparation procedure of semi-aromatic DPBI based on dicarboxylic acid and crosslinked DPBI-xPVBC membranes.

Preparation of Crosslinked DPBI-xPVBC Membranes
As presented in Scheme 1, the DPBI-xPVBC membrane with crosslinked three-dimensional (3D) network was prepared by typical solution casting method and in situ crosslinking reaction. Both DPBI and PVBC powders were dissolved separately in m-cresol to form 5 wt% solutions under magnetic stirring at 140 °C and 80 °C, respectively. The two solutions were mixed in a given mass ratio and magnetically stirred for 1 h at room temperature. The produced mixture was directly poured onto a clean glass plate, glass rod was used to spread the solution evenly. Then the cross-linked membrane was obtained by a multi-step heating process (80 °C, 3 h; 120 °C, 3 h; 150 °C, 2 h; 190 °C, 2 h). The resultant membranes were named as DPBI-xPVBC, where x represents the weight percentage of PVBC in membrane. As a control membrane, the pristine DPBI membrane was also prepared by the same solution casting method. All the membranes were dried in an oven at 100 °C for 24 h to remove moisture before use. Scheme 1. Preparation procedure of semi-aromatic DPBI based on dicarboxylic acid and cross-linked DPBI-xPVBC membranes.

Preparation of Crosslinked DPBI-xPVBC Membranes
As presented in Scheme 1, the DPBI-xPVBC membrane with crosslinked three-dimensional (3D) network was prepared by typical solution casting method and in situ crosslinking reaction. Both DPBI and PVBC powders were dissolved separately in m-cresol to form 5 wt% solutions under magnetic stirring at 140 • C and 80 • C, respectively. The two solutions were mixed in a given mass ratio and magnetically stirred for 1 h at room temperature. The produced mixture was directly poured onto a clean glass plate, glass rod was used to spread the solution evenly. Then the cross-linked membrane was obtained by a multi-step heating process (80 • C, 3 h; 120 • C, 3 h; 150 • C, 2 h; 190 • C, 2 h). The resultant membranes were named as DPBI-xPVBC, where x represents the weight percentage of PVBC in membrane. As a control membrane, the pristine DPBI membrane was also prepared by the same solution casting method. All the membranes were dried in an oven at 100 • C for 24 h to remove moisture before use. using a Ubbelohde viscometer at 30 • C, and the concentration of polymer solution using m-cresol as the solvent was controlled at about 5 mg mL −1 . The surface and cross-section morphologies of the membranes were measured using a scanning electron microscopy (UHR FE-SEM, SU8010, Hitachi Ltd., Tokyo, Japan).

Gel Fraction Measurement
Gel fraction of the crosslinked DPBI-xPVBC membranes in the organic solvent were estimated through monitoring the weight loss by immersing the membrane in m-cresol at 80 • C for 12 h so as to obtain the crosslinking degree. The pristine DPBI membrane was also tested as a control. The gel fraction of DPBI-xPVBC membrane can be calculated by using the following equation: where W 1 and W 2 are the weights of dry membranes before and after testing, respectively.

PA Doping Content and Proton Conductivity
The PA doping content of all the membranes were obtained by immersing the membrane in 85 wt.% PA for a controlled time at room temperature. Before PA doping, the membrane samples were dried in an oven at 100 • C for 24 h, and their weight were recorded as W dry . After PA doping, the excess PA remained on the sample surface was wiped off with tissue paper, and its weight was denoted as W wet . To ensure the accuracy of the results, three samples were prepared for each membrane to get the average value. Finally, the PA doping content (ADC%) of membranes and volumetric swelling ratio is calculated in Equations (2) and (3) based on the weight and volume gains of membrane samples.
The proton conductivity was measured by a four-probe AC impedance method with an Autolab PGSTAT204 electrochemical workstation in the frequency range of 0.1 Hz to 105 Hz, and an oven was used to control the testing temperature (80 • C~180 • C). The proton conductivity (σ) was determined on the Equation (3): where L, R and A are the thickness, ohmic resistance and cross-sectional area of the membrane, respectively. The ohmic resistance was obtained from the Nyquist plots. In this work, all the proton conductivities were measured as proton transport in the direction of the membrane thickness.

Mechanical Property
The mechanical property of the PA doped membrane was measured by employing a mechanical strength machine (CM640, Shenzhen, China). The stress-strain curves were analyzed at a constant elongation speed of 5 mm min −1 at room temperature in air atmosphere, and the size of the membrane samples was 40-50 mm in length and 4-6 mm in width. All results obtained came from the average value of at least three samples.

Oxidative Stability
To reflect the durability of the membranes, the oxidative stability of the PA doped membrane was investigated by Fenton's test (3 wt.% H 2 O 2 solution with 4 ppm Fe 2+ ) at 80 • C for 48 h. The oxidative stability (OS, %) was estimated as the remaining weight of all the membranes after testing by Equation (4). In order to guarantee the accuracy of the data, three samples of each membrane were taken for the experiment to obtain the average value.
where W 4 and W 3 are the weights of membrane samples before and after soaking, respectively.

Fuel Cell Test
The membrane electrode assemblies (MEAs) were fabricated by a combination of gas diffusion electrodes (GDEs, Hensen Company, 1.0 mg cm −2 Pt/C) with Pt catalyst (0.4 mg cm −2 Pt loading) and the PA doped PEM without hot pressing. The active area and Pt loading of each electrode were 4 cm 2 and 0.4 mg cm −2 , respectively. Components of a single cell similar with sandwich structure include two metallic polar plates with single serpentine flow-fields, PET gaskets, MEA, which were stacked in sequence and fixed together by bolts. The fuel cell test was performed with a single cell testing system at 160 • C at a constant flow rate of 80 mL min −1 and 160 mL min −1 for H 2 and O 2 without humidification under ambient pressure, respectively. The polarization curve was recorded and controlled using a fuel cell testing system equipped with operation control software and an electronic load unit controller, which was purchased from Dalian Yuke Innovation Technology Co., Ltd., Dalian, China. For a short-term durability test, the single cell was loaded with a constant current density of 150 mA cm −2 at 140 • C for 50 h. Before this durability test, the cell was employed without further activation.

Structure Characterization of DPBI and DPBI-xPVBC
In order to identify the viscosity-average molecular weight of DPBI polymer, the flow time of solvent and solution was measured in sequence with an Ubbelodhe viscometer. The viscosity-average molecular weight of DPBI polymer is 2.88, showing a high molecular weight. The chemical structure of DPBI were determined by 1 H NMR spectra, as shown in Figure 1a. All characteristic proton signals of imidazole denoted as H 1 , biphenyl denoted as H 2 , H 3 , H 4 , and methylene denoted as H 5 can be observed. The peak of a chemical shift between 12 and 13 ppm is attributed to the proton characteristic of the imidazole ring. The peaks with chemical shift of 7-8 ppm are ascribed to the protons of the benzene. The peaks in the 1-3 ppm are assigned to the methylene hydrogen on the semi-aromatic chain. The 1 H NMR results confirm the successful synthesis of DPBI.
In addition, in terms of structural characterization, the FT-IR spectra of DPBI-xPVBC, DPBI and PVBC are compared in Figure 1b. The most interrelated absorption peaks are observed at 1269, 708 and 672 cm −1 for the chloromethyl group of PVBC, and the adsorption peak at 1269 cm −1 is also consistent with −C-Nof imidazole ring. The peaks at 1449 cm −1 and 800 cm −1 are concerned in the −C=Cand −C=Nin-plane stretching vibration of PVBC and DPBI, and the peaks at 3400~3100 cm −1 is ascribed to the tensile vibration of N-H. In addition, there are two obvious peaks at 2932 cm −1 , 2848 cm −1 , matching with the stretching vibration peak of the C-H backbone. The characteristic absorption peaks described above distinctly confirms the successful in situ crosslinking reaction between DPBI and PVBC during fabricating the DPBI-xPVBC membranes.
The valid crosslinking reaction is confirmed via the solubility test in m-cresol for DPBI-xPVBC. The DPBI-30PVBC membranes have high gel content after treatment in m-cresol for 12 h at 80 • C, as depicted in Figure 1c. The gel contents of the DPBI-10PVBC and DPBI-20PVBC membranes are 93% and 88%, implying that the crosslinking reaction are successfully occurred between the chloromethyl groups of PVBC and the =N-H groups of benzimidazole and the highly crosslinked structure of DPBI-xPVBC are confirmed. In addition, in terms of structural characterization, the FT-IR spectra of DPBI-xPVBC, DPBI and PVBC are compared in Figure 1b. The most interrelated absorption peaks are observed at 1269, 708 and 672 cm −1 for the chloromethyl group of PVBC, and the adsorption peak at 1269 cm −1 is also consistent with −C-N-of imidazole ring. The peaks at 1449 cm −1 and 800 cm −1 are concerned in the −C=C-and −C=N-in-plane stretching vibration of PVBC and DPBI, and the peaks at 3400~3100 cm −1 is ascribed to the tensile vibration of N-H. In addition, there are two obvious peaks at 2932 cm −1 , 2848 cm −1 , matching with the stretching vibration peak of the C-H backbone. The characteristic absorption peaks described above distinctly confirms the successful in situ crosslinking reaction between DPBI and PVBC during fabricating the DPBI-xPVBC membranes.
The valid crosslinking reaction is confirmed via the solubility test in m-cresol for DPBI-xPVBC. The DPBI-30PVBC membranes have high gel content after treatment in mcresol for 12 h at 80 °C, as depicted in Figure 1c. The gel contents of the DPBI-10PVBC and DPBI-20PVBC membranes are 93% and 88%, implying that the crosslinking reaction are successfully occurred between the chloromethyl groups of PVBC and the =N-H groups of benzimidazole and the highly crosslinked structure of DPBI-xPVBC are confirmed.
The oxidative stability is a pivotal factor affecting the performance of HT-PEMs. Due to the continuous provision of pure O2 at the cathode side, the PEM of HT-PEMFCs must have high oxidative stability. In addition, the radicals, including peroxyl (OOH) and hydroxyl (OH) can attack the polymer chains and result in the PEM degradation [30]. As depicted in Figure 1d, the pristine semi-aromatic DPBI membrane has the maximum weight loss of about 24 wt.% after Fenton's test for 48 h at 80 °C, suggesting the low oxidative stability of DPBI. While the DPBI-xPVBC membranes exhibit only a slight weight loss after test, and their remain weights are 92%, 92%, 90% for the DPBI-10PVBC, DPBI-20PVBC and DPBI-30PVBC membranes, respectively, manifesting an admissible antioxidant durability of the crosslinked DPBI-xPVBC membranes. It is believed that the ·OH and HOO· radicals attack the hydrogen atom of N-H and C-H bonds of PBI backbone, and The oxidative stability is a pivotal factor affecting the performance of HT-PEMs. Due to the continuous provision of pure O 2 at the cathode side, the PEM of HT-PEMFCs must have high oxidative stability. In addition, the radicals, including peroxyl (OOH) and hydroxyl (OH) can attack the polymer chains and result in the PEM degradation [30]. As depicted in Figure 1d, the pristine semi-aromatic DPBI membrane has the maximum weight loss of about 24 wt.% after Fenton's test for 48 h at 80 • C, suggesting the low oxidative stability of DPBI. While the DPBI-xPVBC membranes exhibit only a slight weight loss after test, and their remain weights are 92%, 92%, 90% for the DPBI-10PVBC, DPBI-20PVBC and DPBI-30PVBC membranes, respectively, manifesting an admissible antioxidant durability of the crosslinked DPBI-xPVBC membranes. It is believed that the ·OH and HOO· radicals attack the hydrogen atom of N-H and C-H bonds of PBI backbone, and lead to the degradation of HT-PEMs [31][32][33]. Within a certain range, both the DPBI-xPVBC membranes with crosslinked structure show high oxidative stability, whose residue weights are higher than the pristine DPBI during the Fenton's test, showing that the crosslinked 3D network is beneficial for improving the stability. Compared with DPBI-10PVBC and DPBI-20PVBC membranes, the DPBI-30PVBC shows slightly low residue weight after Fenton's test. The higher content of crosslinker PVBC gives higher gel content but lower oxidative stability to DPBI-30PVBC compared with DPBI-10PVBC and DPBI-20PVBC. Presumably, with increasing the PVBC content, the benzyl chloride cannot react completely with the =N-H groups of imidazole units in DPBI, and the residue benzyl chloride groups are unstable and can produce benzyl radicals easily, accelerating the degradation of the membrane. Considering about the crosslinking degrees, PA uptake ability and oxidative stability, the content of the macromolecular crosslinker should be controlled below 30% for using as HT-PEMs. Additionally, the oxidation stability test of PA doped membranes was also tested in order to be closer to the conditions of reliable to fuel cells. According to the test phenomenon, PA doped DPBI membrane started to break into pieces after immersing in Fenton reagent for nearly 2 h, while PA doped DPBI-10PVBC started to break into pieces after immersing in Fenton reagent for nearly 4 h. In comparison with the pristine DPBI membrane, the notable improvement in antioxidant performance of the crosslinked DPBI-xPVBC membrane could be put down to the reduction of N-H content in the crosslinked membrane, which enhanced the resistance to free radical attack.

Performance Analysis of the Crosslinked DPBI-xPVBC Membranes
The proton conductivity of HT-PEM is a crucial peculiarity for HT-PEMFC application, even though the PEM exhibits high stability and good conformation, its low proton conductivity will place restrictions on its cell performance. For the PA doped HT-PEMs, since proton transport is fulfilled by means of the dissociation of PA molecules, high ADC is a prerequisite to achieve high proton conductivity. However, due to the "plasticizing effect" of the doped PA, the superabundant ADC will bring about the deterioration of mechanical properties of the membranes. The PA adsorbed speed of the common PBI based membrane is low and it need long time to achieve an equilibrium adsorption, resulting in the difficulty for controlling the ADC. Compared with m-PBI, the semi-aromatic DPBI herein has softer aliphatic unit in the polymer chain, which make it be able to adsorb PA very quickly. Combining the macromolecular-structure design and post-crosslinking strategy, the crosslinked DPBI-xPVBC membranes are developed to solve the trade-off between the proton conductivity and mechanical strength of HT-PEMs.
Firstly, ADC values of DPI membrane are 123%, 165% and 237% for the different immersing time of 0.5 min, 1 min and 2 min, in turn. It is thus clear that the DPBI membrane possesses the characteristics of the high PA adsorption capacity and fast adsorption rate, achieving the effect of instantaneous adsorption of PA. So, it is easy to control the PA adsorption capacity by adjusting the temperature and time. Initially, multiple DPBI membranes with ADC at or around 100% were selected to assess the proton conductivity and mechanical property, and the results were summarized in Table 1.
The stress strain curves of the dry and PA doped DPBI with 110% ADC are shown in Figure 2. The dry DPBI membrane shows a high tensile strength of 61.0 MPa and an elongation at break of 11.6%. After doping with PA, the 110%PA@DPBI membrane has tensile strength of only 13.2 MPa and high elongation at break of 260%. Meanwhile, the 110%PA@DPBI membrane exhibits a low proton conductivity of 19 mS cm −1 at 160 • C. Because of the strong plasticizing effect of PA for the DPBI membrane, it is impossible to promote the proton conductivity by increasing the PA doping level.    The y%PA@DPBI-xPVBC membranes with different ADCs were obtained by adjusting the immersion time in PA (y% refers to the ADC value of the membrane), and their proton conductivities as a function of the temperature without humidification and their mechanical properties at ambient environment were also assessed, as revealed in Figure 3. As seen in Figure 3a, the in-plane proton conductivity of the entire PA doped DPBI xPVBC membranes increase with the increase of the temperature. Whereas a plateau emerges at 160 • C, resulting from the PA dehydration in the range of 160 • C to 180 • C [34,35]. For example, with increasing temperature from 80 • C to 160 • C, the proton conductivity of 103% PA@ DPBI-10PVBC varies from 11 mS cm −1 to 32 mS cm −1 . Here, a schematic diagram is presented in Scheme 2 to explain the proton transport mechanism under the anhydrous conditions. It reveals that there are hydrogen bonds between PA molecules and imidazole rings, generating effective and continuous proton transport channels in accordance with Grotthuss mechanism. For the crosslinked membranes with the similar ADC, the DPBI-10PVBC membrane has the highest proton conductivity at the same temperature, due to the decrease content of DPBI with the increase of the PVBC content. In comparison with DPBI-20PVBC and DPBI-30PVBC membranes, the DPBI-10PVBC membrane has more imidazole groups, which can facilitate PA molecules forming more hydrogen-bonding networks for proton conduction. At the same time, considering that the PA doped HT-PEMs require higher ADC to obtain high proton conductivity at the anhydrous condition, the 235%PA@DPBI-10PVBC membrane seems to be a good choice. As depicted in Figure 3a, the proton conductivity of the DPBI-10PVBC membrane increases from 32 mS cm −1 to 49 mS cm −1 when ADC increases from 103% to 235% at 160 • C.
The excellent mechanical performance of HT-PEMs is one of the key points for longterm stable operation of HT-PEMFCs. Xiao et al. reported that the polymer electrolyte membranes with a tensile strength greater than 1.5 MPa can be applied in fuel cell performance testing [36]. Therefore, we should adhere to the principle that the PA doped membrane occupy superior mechanical properties on the premise that the proton conductivity meets the requirements. Figure 3b presents the typical stress-strain curves of various PA doped crosslinked membranes, and the detail relevant values are also illustrated in Table 2. As shown in Figure 3b and Table 2, the 227%PA@DPBI-10PVBC membrane exhibits the tensile strength and elongation at break of 12.6 MPa and 96.1%, respectively, which is much better than that of the DPBI membrane with the equivalent ADC. The enhanced mechanical properties of the crosslinked membrane are resulted from the low swelling ratio after the crosslinked reaction. Besides, it is noticed that the swelling ratio increased with the decrease of the PVBC content. It is worth noting that the high crosslinking degree would and make the membrane stiffer and even brittle [37]. The mechanical properties of the as-prepared crosslinked membrane confirm this expected tendency. After comprehensive consideration, the PA doped DPBI-10PVBC membrane was selected for subsequent fuel cell performance and durability test. Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 13 Scheme 2. Schematic of proton transport of PA@ DPBI-xPVBC membrane. The excellent mechanical performance of HT-PEMs is one of the key points for longterm stable operation of HT-PEMFCs. Xiao et al. reported that the polymer electrolyte membranes with a tensile strength greater than 1.5 MPa can be applied in fuel cell performance testing [36]. Therefore, we should adhere to the principle that the PA doped membrane occupy superior mechanical properties on the premise that the proton conductivity meets the requirements. Figure 3b presents the typical stress-strain curves of various PA doped crosslinked membranes, and the detail relevant values are also illustrated in Table  2. As shown in Figure 3b and Table 2, the 227%PA@DPBI-10PVBC membrane exhibits the tensile strength and elongation at break of 12.6 MPa and 96.1%, respectively, which is much better than that of the DPBI membrane with the equivalent ADC. The enhanced mechanical properties of the crosslinked membrane are resulted from the low swelling ratio after the crosslinked reaction. Besides, it is noticed that the swelling ratio increased with the decrease of the PVBC content. It is worth noting that the high crosslinking degree would and make the membrane stiffer and even brittle [37]. The mechanical properties of the as-prepared crosslinked membrane confirm this expected tendency. After comprehensive consideration, the PA doped DPBI-10PVBC membrane was selected for subsequent fuel cell performance and durability test.  The excellent mechanical performance of HT-PEMs is one of the key points for longterm stable operation of HT-PEMFCs. Xiao et al. reported that the polymer electrolyte membranes with a tensile strength greater than 1.5 MPa can be applied in fuel cell performance testing [36]. Therefore, we should adhere to the principle that the PA doped membrane occupy superior mechanical properties on the premise that the proton conductivity meets the requirements. Figure 3b presents the typical stress-strain curves of various PA doped crosslinked membranes, and the detail relevant values are also illustrated in Table  2. As shown in Figure 3b and Table 2, the 227%PA@DPBI-10PVBC membrane exhibits the tensile strength and elongation at break of 12.6 MPa and 96.1%, respectively, which is much better than that of the DPBI membrane with the equivalent ADC. The enhanced mechanical properties of the crosslinked membrane are resulted from the low swelling ratio after the crosslinked reaction. Besides, it is noticed that the swelling ratio increased with the decrease of the PVBC content. It is worth noting that the high crosslinking degree would and make the membrane stiffer and even brittle [37]. The mechanical properties of the as-prepared crosslinked membrane confirm this expected tendency. After comprehensive consideration, the PA doped DPBI-10PVBC membrane was selected for subsequent fuel cell performance and durability test.   Before testing the cell performance, the surface and cross section morphologies of DPBI and DPBI-10PVBC membranes were characterized by FE-SEM. In Figure 4a-d, both membranes show a completely uniform and compact structure with no pores, which is critical for separating fuel and oxidant during the cell operation and benefits fuel cell performance. The polarization curve of a single cell with the 235%PA@DPBI-10PVBC membrane at 140 • C without humidification and backpressure is displayed in Figure 4e.
First, the open circuit voltage is up to 0.94 V, suggesting that the PEM has remarkable gas barrier properties. Secondly, the highest power density (405 mW cm −2 ), which is much higher than that of the pristine DPBI membrane (314 mW cm −2 ). Additionally, it is comparable or even better HT-PEMFC performance than that of some recent reports for m-PBI and some other analogues containing PBI, as listed in Table 3. It is mainly credited to the excellent proton conductivity and better mechanical strength. Furthermore, a short-term dynamic test of the 235% PA@DPBI-10PVBC was also monitored to assess the reliability of the single HT-PEMFC, and the cell voltage was plotted as a function of time. As revealed in Figure 4f, the voltage drops from~0.644 V to~0.605 V during the test period of 50 h, and the voltage attenuation rate is 0.78 mV h −1 . The improved performance of the single cell can be attributed to a result of the crucial contribution of its suitable proton conductivity and excellent mechanical property resulting from the crosslinking structure. The above results imply a potential application of the DPBI-xPVBC crosslinking membrane in HT-PEMFCs.
the open circuit voltage is up to 0.94 V, suggesting that the PEM has remarkable gas barrier properties. Secondly, the highest power density (405 mW cm −2 ), which is much higher than that of the pristine DPBI membrane (314 mW cm −2 ). Additionally, it is comparable or even better HT-PEMFC performance than that of some recent reports for m-PBI and some other analogues containing PBI, as listed in Table 3. It is mainly credited to the excellent proton conductivity and better mechanical strength. Furthermore, a short-term dynamic test of the 235% PA@DPBI-10PVBC was also monitored to assess the reliability of the single HT-PEMFC, and the cell voltage was plotted as a function of time. As revealed in Figure 4f, the voltage drops from ~0.644 V to ~0.605 V during the test period of 50 h, and the voltage attenuation rate is 0.78 mV h −1 . The improved performance of the single cell can be attributed to a result of the crucial contribution of its suitable proton conductivity and excellent mechanical property resulting from the crosslinking structure. The above results imply a potential application of the DPBI-xPVBC crosslinking membrane in HT-PEMFCs.

Conclusions
DPBI, a semi-aromatic PBI with enhanced comprehensive fuel cell performances was successfully synthesized. Furthermore, a series of crosslinked DPBI-xPVBC HT-PEMs can be then obtained with excellent antioxidation stability, high proton conductivity and improved mechanical strength using PVBC macromolecular crosslinker by a simple in situ crosslinking reaction. The crosslinked DPBI-10PVBC membrane with a gel fraction of 88% and an ADC of 235% displays the highest conductivity of approximately 49 mS cm −1 at 160 • C and the highest tensile strength of 12.6 MPa at room temperature. Meantime, the maximum power density of the HT-PEMFC with PA doped DPBI-10PVBC membrane can reach 405 mW cm −2 , with an average voltage attenuation of 0.78 mV h −1 during the short-term stability test of 50 h. In summary, the crosslinked DPBI-xPVBC PEM shows a promising application in HT-PEMFCs.