Phosphoric acid doped triazole-containing cross-linked polymer electrolytes with enhanced stability for high-temperature proton exchange membrane fuel cells

Journal Pre-proof Phosphoric acid doped triazole-containing cross-linked polymer electrolytes with enhanced stability for high-temperature proton exchange membrane fuel cells Joseph Jang, Do-Hyung Kim, Min-Kyoon Ahn, Cheong-Min Min, Su-Bin Lee, Juyi Byun, Chanho Pak, Jae-Suk Lee PII:

S0376-7388(19)31736-3

DOI:

https://doi.org/10.1016/j.memsci.2019.117508

Reference:

MEMSCI 117508

To appear in:

Journal of Membrane Science

Received Date: 11 June 2019 Revised Date:

6 September 2019

Accepted Date: 23 September 2019

Please cite this article as: J. Jang, D.-H. Kim, M.-K. Ahn, C.-M. Min, S.-B. Lee, J. Byun, C. Pak, J.-S. Lee, Phosphoric acid doped triazole-containing cross-linked polymer electrolytes with enhanced stability for high-temperature proton exchange membrane fuel cells, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/j.memsci.2019.117508. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Phosphoric

acid

doped

triazole-containing

cross-linked

polymer

electrolytes with enhanced stability for high-temperature proton exchange membrane fuel cells

Joseph Jang a, Do-Hyung Kim b, Min-Kyoon Ahn a, Cheong-Min Min a, Su-Bin Lee a, Juyi-Byun a, Chanho Pak b,*, Jae-Suk Lee a,*

a

School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123

Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea b

Graduate Program of Energy Technology, School of Integrated Technology, Institute of Integrated Technology,

Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea

Keywords: Polymer electrolyte membranes, Cross-linked membranes, Triazole, Phosphoric acid, HTPEMFCs

*

Corresponding authors. E-mail: [email protected] (J.-S. Lee), [email protected] (C. Pak)

ABSTRACT Phosphoric acid (PA) leaching is one of the endurance issues for the use of PA-doped polymer electrolytes in high-temperature proton exchange membrane fuel cells (HTPEMFCs). A chemical cross-linking approach is presented that adequately addresses this issue. The cross-linked membrane (XTPPO) was synthesized from poly(2,6-dimethyl-1,4phenylene oxide) containing triazole groups (TPPO) on the side chains by in situ casting and click reaction with 1,7-octadiyne. XTPPO exhibited more than 90% of the membrane retained after gel fraction test. The cross-linked membrane showed high thermal stability with TD5% = 325 oC. Cross-linking also led to enhanced oxidative and mechanical stability. PA doping was controlled with the amount of triazole and the degree of cross-linking. The highest proton conductivity measured in the anhydrous condition was 64 mS/cm at 180 oC.

1. Introduction Proton exchange membrane fuel cells (PEMFCs) are electrochemical devices converting the chemical energy of hydrogen into electricity with high efficiency. Most of the PEMFCs currently being used are based on perfluorosulfonic acid (PFSA) membranes. For highperformance fuel cells, PFSA membranes should be kept hydrated in the operational temperature range that limits its use above 100 oC. The low operational temperature devices possess several challenges for commercialization, such as, slow electrode kinetics, low tolerance to carbon monoxide, complex water and heat management, and the need for noble metal catalysts; all of which escalate manufacturing cost [1].

The high-temperature PEMFCs (HT-PEMFCs) have recently been introduced as a solution for the above technical problems [2-5]. The usual operating temperature range of HT-PEMFCs is 100-180 oC. Since in such devices, water can’t be used, suitable protonconducting materials are used in place of water [6]. Phosphoric acid (PA) is a nonaqueous proton conducting material with the highest intrinsic proton conductivity [7]. Phosphonated polymers have been used for HT-PEMFCs. However, the proton conductivities were not promising because of the immobilized phosphonic acids with low mobility [8-12]. Wainright proposed PA-doped polybenzimidazole (PBI) as the polymer electrolyte membrane in HTPEMFCs [13]. PBI is an aromatic thermoplastic polymer containing benzimidazoles with high thermal stability and excellent chemical resistance. The N and NH sites are involved in hydrogen bonding to give high mechanical strength. Furthermore, being efficient proton acceptors and donors, these sites contribute to proton hopping [2,3,14-16]. Since this discovery, enormous research on the synthesis of modified PBIs has been carried out to improve the PEM performance [17-24]. Also, other N-heterocycles, as well as imidazoles, are currently under study for HT-PEMFCs [25-32]. Triazole is one of the N-heterocycles having three nitrogens in the ring and has recently attracted attention for high-performance HT-PEMs [33-41]. The proton conduction mechanism of triazole was investigated by ab initio molecular dynamics. It is found that triazole rings conduct protons by structural diffusion (Grötthuss mechanism), like imidazole and water [42-45]. Liu et al. performed cyclic voltammetry (CV) measurements to investigate the electrochemical stabilities of PEMs containing imidazole and triazole, respectively [46,47]. A large irreversible oxidation peak appeared in the case of imidazole and the oxidation products were absorbed on the platinum surface. However, no discernable redox peaks were observed for the triazole, which implied that the triazole is electrochemically stable under the fuel cell conditions. The proton conductivities were compared, and the

triazole showed much higher conductivity in both the PA-doped and undoped states. It might be due to the more amphoteric characteristic of triazole which may allow efficient acid-base interaction with phosphoric acid for proton hopping [48]. The pKa values of phosphoric acid, imidazole and 1,2,3-triazole are shown in Table S2. Due to PA-leaching, long term durability is a significant concern for commercialization of the HT-PEMFCs, and several attempts have been made to resolve this issue. It was reported that quaternary ammonium ion was effective in preventing PA leaching due to its strong ion pair interaction between ammonium and phosphate [49, 50]. Pu et al. and Quartarone et al. fabricated composite membranes with inorganic fillers containing basic groups on the surface to enhance the PA retention [51-53]. Oono et al. performed long term durability test of PBI and cross-linked PBI. It was proved that the chemical cross-linking reduced PA leaching efficiently. The PBI membranes dissolved during power generation, but the cross-linked PBI membranes retained their original thickness [54, 55]. Higher durability of the membrane was also able to be achieved by a polymer cross-linking approach in several other studies [2,20,56-60], which confirmed that cross-linking is essential for HT-PEMs since the mechanical strength decreases when the membrane is PA-doped [61-65]. In spite of several studies on reducing PA leaching, the durability issue of the HT-PEMFCs remains unresolved. In this study, we report a facile synthesis of cross-linked HT-PEMs containing triazole and phosphoric acid. Triazole was selected because of the efficient acid-base interaction with phosphoric acid for proton hopping. The precursor polymer was cross-linked in situ by alkyne-azide cycloaddition reaction. It is beneficial to introduce additional triazoles through the cross-linking since the proton hopping sites also increase. The cross-linking was quite effective in enhancing the chemical, oxidative, mechanical stabilities, and PA retention ability. The characterization details, proton conductivity, and cell performance results are presented.

2. Experimental 2.1. Materials Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Sabic with molecular weight of 70000 g/mol. N-bromosuccinimide (NBS), copper(I) bromide(CuBr), 85 wt% phosphoric acid solution, and N,N,N′,N′,N″-pentamethyl diethylene triamine (PMDETA) were purchased from Sigma-Aldrich. Chlorobenzene and N-methyl-2-pyrrolidone (NMP) were purchased from Alfa Aesar. 1-Hexyne and 1,7-octadiyne were purchased from Tokyo Chemical Industry. 2,2′-Azobis(2-methylpropionitrile) (AIBN) was purchased from Junsei Chemical. Sodium azide was purchased from Acros. All the reagents and chemicals were used as received without further purification.

2.2. Bromination of Poly(2,6-dimethyl-1,4-phenylene oxide) For bromination in the benzylic position, a typical procedure with 35% degree of bromination (DB) is as follows: PPO (4.00 g, 33.3 mmol), NBS (2.97 g, 17 mmol), AIBN (0.55 g, 3.33 mmol) were dissolved in chlorobenzene (75 mL). The reaction solution was stirred under reflux in a nitrogen atmosphere at 135 oC for 12 h. The reaction solution was precipitated in ethanol (300 mL) and washed several times. The yield was 4.66 g (95% w/w). In the case of DB 30 and DB 40, NBS 2.67 g (15 mmol) and NBS 3.26 g (18.3 mmol) were used, respectively.

2.3. Azidation of brominated poly(2,6-dimethyl-1,4-phenylene oxide)

The BPPO-35 (5.00 g, 11.9 mmol of bromine group, 35% degree of bromination) was dissolved in NMP (49 ml) and stirred at 60 oC until the polymer was completely dissolved. After that NaN3 (2.32 g, 35.6 mmol) was added into the solution. The reaction mixture was stirred at 60 oC for 12 h. The solution was precipitated in ethanol (250 mL) and washed several times. The azido PPO polymer (APPO) was filtered and dried in a vacuum oven at 80 o

C. The yield was 4.10 g (90% w/w).

2.4. Synthesis of triazole modified poly(2,6-dimethyl-1,4-phenylene oxide) The azide groups were modified to triazole groups by alkyne-azide cycloaddition click chemistry as reported by Li et al. [66]. Some azide groups were deliberately left unreacted to be used for cross-linking. The procedure for the synthesis of the triazole modified APPO (TPPO) is as follows. APPO-35 (3.4 g, 8.9 mmol of azide) and CuBr (0.20 g, 1.42 mmol) were dissolved in NMP (89 mL). After completely dissolving the contents, the reaction flask was degassed by bubbling argon gas for 1h. 1-Hexyne (0.65 mL, 5.7 mmol) and PMDETA (0.59 mL, 2.8 mmol) were then added into the solution. The reaction mixture was stirred under a nitrogen atmosphere at 50 oC for 16 h. The solution was precipitated in DI water and washed several times. The product was named as TPPO-35-15, where 35 refers to 35% degree of original bromination and 15 is the degree of cross-linkable azide groups. It was dried in a vacuum oven at 80 oC. The yield was 3.44 g (90% w/w).

2.5. Fabrication of cross-linked triazole modified poly(2,6-dimethyl-1,4-phenylene oxide) membranes

The cross-linked TPPO (XTPPO) membranes were prepared by the in situ casting and click reaction of TPPO with 1,7-octadiyne as a cross-linker. For the preparation of XTPPO35-15, TPPO-35-15 (0.77 g, 0.92 mmol of azide groups) is dissolved in NMP (10.7 mL). 1,7Octadiyne (89 uL, 0.69 mmol) was added into the solution and stirred at 80 oC for 20 min. The solution was filtered and mixed with CuBr (0.17 mmol, in NMP). After sonicating for 10 min, the solution was cast on a glass petri dish and heated to 70 oC, 100 oC, and 130 oC for 12 h, 1 h, and 1 h, respectively. The membrane was detached from the glass petri dish by immersing in DI water. The membrane was then immersed in 2 M HCl (aq) to remove CuBr [67] followed by immersing in boiling water to remove the acid [49].

2.6. Phosphoric acid doping and swelling ratio The XTPPO membrane was dried in a vacuum oven at 80 oC for 12 h. After measuring the weight and length, it was immersed in 85 wt% phosphoric acid solution at 120 oC for 15 h. The membrane was taken out and wiped with tissue wipes. After drying at 100 oC for 12 h, PA uptake and the length of each sample were measured immediately. The PA uptake ratio of the membrane (Wdoping) was calculated by the following equation:

% =

−

× 100

where WPA is the weight of PA-doped membrane and WD is the weight of the undoped dry membrane. The dimensional swelling ratio of the membrane was calculated by the following equation:

% =

−

× 100

−

% =

−

% =

! "#

% =

$

× 100

× 100

−$ × 100 $

where LPA, TPA, APA and VPA are the length, thickness, area and volume of the PA-doped membrane, respectively. LD, TD, AD and VD are the length, thickness, area and volume of the undoped dry membrane.

2.7. Phosphoric acid leaching test The phosphoric acid leaching problem is one of the major obstacles for commercialization of PEMFCs. It is reported that acid leaching is severe below 140 oC [49,68]. The acid leaching test was carried out by exposure of the PA-doped membranes to water vapor. The PA-doped membranes were kept in a thermo-hygrostat, Espec SH-241, with 80 oC and at 50% relative humidity condition for 1h [49]. The PA retention ratio of the membrane was calculated by the following equation:

PA retention % =

-

× 100

where WR is the retained PA uptake of the membrane after the leaching test and WA is the initial PA uptake of the membrane before the leaching test.

2.8. Structural characterization

The nuclear magnetic resonance (NMR) spectra of the polymer samples were measured by a JEOL JNM 400 WB FT-NMR spectrometer with deuterated chloroform as the solvent and tetramethylsilane as the standard. Fourier transform infrared spectroscopy (FT-IR) of membrane samples were collected between 4000 cm-1 and 400 cm-1 using attenuated total reflection method on Perkin Elmer Frontier FT-IR/NIR. The molecular weight and polydispersity of the polymer samples were estimated by size exclusion chromatographymultiangle laser light scattering (SEC-MALLS) equipped with a 515 HPLC pump (Waters), a set of four Styragel columns connected in series (HR 0.5, HR 1, HR 3, and HR 4 with hole sizes of 50, 100, 500, and 1000 Å, respectively, Waters), a miniDAWN TREOS light scattering detector (Wyatt Technology), and a Optilab T-rEX refractive index detector (Wyatt Technology). The SEC-MALLS operated in THF:triethyamine (98:2 v:v) with an elution rate of 1.0 mL/min at 40 °C.

2.9. Mechanical and thermal properties Mechanical properties of membrane samples were measured by Test One TO-101 at room temperature at a strain rate of 10 mm/min. The membrane samples were prepared with 40 mm length and 5 mm in width. For each membrane, at least four samples were measured, and the average value was calculated. Thermogravimetric analysis (TGA) was performed by a Perkin Elmer TGA 4000 at a heating rate of 10 oC/min from 50 to 700 oC under nitrogen flow. Differential scanning calorimetry (DSC) was measured by Perkin Elmer DSC 4000 at a heating rate of 10 oC/min from 50 to 210 oC under nitrogen flow.

2.10. Oxidative stability and gel fraction test

To test oxidative stability, the membrane samples were immersed into Fenton’s reagent (3% H2O2 solution containing 4 ppm Fe2+) at 80 oC. After soaking for 27 h the samples were removed from the solution and dried in a vacuum oven, and weighed, from which the oxidative stability was calculated. The effect of cross-linking was determined from the gel fraction test of the membranes by treating it in a suitable solvent. The membrane samples were immersed in NMP at 80 oC for 12 h. The sample was then removed from the solvent and dried. From the weight of the dried sample, the stability of the cross-linked membranes in the chosen solvent was calculated.

2.11. Proton conductivity In-plane proton conductivity for PA-doped membranes was measured with Scribner Associate Inc. MTS 740 and BT-710 sample clamp. Zahner-Electrik IM 6 potentiostat was used to measure the voltage-current response curve by using CV. All samples were cut into 1 cm × 4 cm size, and the average thickness of the samples was obtained by a micrometer. The measurement was performed with a slew rate of 10 mV/s between -0.1 and 0.1 V in the temperature range 100-180 oC and in anhydrous condition. The conductivity was calculated by the following formula:

Proton conductivity, σ =

5×

×

where L is the distance between reference electrodes, R is the membrane resistance, W and T are the width and thickness of the samples, respectively.

2.12 Membrane electrode assembly (MEA) and fuel cell test

MEAs were fabricated with Celtec® cathode gas diffusion electrode (GDE) for cathode from BASF and GDE for anode from Dongjin Semichem Co. Ltd.. Both electrodes have Pt loading of 1.0 mg/cm2. The GDEs and PA-doped XTPPO membranes were cut into a square shape with 10.24 cm2 and 16.0 cm2, respectively. The electrodes were PA-doped with 75 μmol/cm2 in the cathode and 42 μmol/cm2 in the anode by dropping 50% PA aqueous solution on the surfaces and drying in 80 oC oven for 3h. The sub-gasket with 50 μm thickness from CNL energy was put between the GDE and the membrane, which made the active electrode area 7.84 cm2. MEAs were prepared by hot pressing at 2 MPa at 140 oC for 2 min [39,40,69]. The fuel cell test of the MEAs was carried out in a fuel cell station (Scitech, Korea). The single cells were activated at 150 oC before performing the electrochemical tests in order to distribute the PA through the interfaces. Dry hydrogen and air were supplied at a constant flow of 300 mL/min and 1,000 mL/min, respectively. The current started from zero until the cell voltage reaches 0.4 V and this was repeated for 50 cycles. I-V polarization curves were obtained at 150 oC with dry hydrogen and dry air flow at a rate of 100 mL/min and 300 mL/min, respectively. The current held at each point for 120 s. Electrochemical impedance spectroscopy (EIS) was performed in the same condition using an electrochemical impedance analyzer (SP-150, Bio-logic SAS) at 0.6 V with 10 mV amplitude and a frequency range from 10 mHz to 100 kHz.

3. Results and discussion 3.1. Preparation of polymers and membranes

It has been reported by Chang et al. that the flexible movement of N-heterocycle is desirable for high proton conductivity than when rigidly held in the polymer backbone [40]. TPPO was synthesized to attach triazole rings on the side chains by polymer modifications of PPO in different steps, as shown in Scheme 1. First, bromination reaction occurred on the benzylic position of PPO. The 1H NMR spectrum of BPPO is shown in Fig. S1 (a). The signals at 4.33 ppm and 2.08 ppm are assigned to the methylene protons (Hf) and protons from methyl groups (Hb,d), respectively. The other peaks at 6.35-6.75 ppm (Ha, Hc, He) are from aromatic rings. The degree of bromination was calculated by comparing the integrated peak area of Hf and Hb,d. The bromine groups were substituted with the azide groups via nucleophilic substitution reaction. The transformation was 100%, as confirmed by 1H NMR, which is shown in Fig. S1 (b). The signal of methylene protons at 4.33 ppm disappeared entirely, and the new signal at 4.20 ppm appeared, which was assigned to the CH2N3 protons. Alkyne-azide cycloaddition click reaction was carried out to functionalize the APPO. The conversion of azide groups into triazole groups was confirmed in 1H NMR in Fig. S1 (c). The peak of methylene protons shifted from 4.20 ppm to 5.32 ppm. Also, new signals from triazoles and alkyl spacers appeared at 7.17 ppm (Hg), 2.60 ppm (Hh), 1.58 ppm (Hi), 1.32 ppm (Hj) and 0.89 ppm (Hk). The XTPPO membranes were prepared by in situ castings and click reaction of TPPO and 1,7-octadiyne as a cross-linker. The cross-linking reaction was very efficient, in agreement with other reports [70-72]. As shown in the FT-IR study (Fig. 1), the azide peak at 2100 cm-1 completely disappeared after cross-linking. This cross-linking strategy is beneficial because there is no need to sacrifice the nitrogen atoms through conventional alkylation cross-linking methods [73-75].

3.2. Gel fraction test Gel fraction test was carried out to ascertain that cross-linking was successful. As shown in Fig. 2, all the membrane samples maintained the structural integrity after immersing in NMP at 80 oC for 12 h. The weight of the samples was over 90% after solvent treatment. Because of the high degree of cross-linking over 10, there might be no big difference of gel fraction [76,77]. The non-cross-linked membranes (TPPO-30, TPPO-35 and TPPO-40) completely dissolved in NMP in 1 h.

3.3. Thermal properties Thermal stability of polymer electrolytes is one of the most critical features for long term durability of fuel cells, especially in high-temperature operation. The thermal stability of XTPPO membranes was investigated in both PA undoped and PA-doped states by TGA under the nitrogen atmosphere. As shown in Fig. 3(a), two weight loss steps were observed for undoped membrane samples. The first weight loss beginning at about 240 oC was due to degradation of triazole groups on the side chains [36]. Since alkyne-azide cycloaddition click reaction is not reversible, the weight loss is not from reverse click reaction [78-80]. The second step observed at around 345 oC was attributed to the degradation of the polymer main chain. The 5% weight degradation temperatures (TD5%) was observed at around 325 oC for each sample. In the case of PA-doped states (Fig. 3b), weight loss started earlier at about 140 o

C because of the dehydration of phosphoric acid. During the dehydration process, condensed

phosphoric acids (H4P2O7, H5P3O10) and water molecules were formed [37]. However, the degradation phenomena of the polymer membranes itself were observed in the temperature

ranges similar to the undoped membranes. The TGA result shows that the XTPPO membranes are thermally stable enough to be used for HT-PEMFC (120-180 oC). DSC curves are shown in Fig. S2. Since cross-linking limits the movement of the polymer, the glass transition temperature increased with cross-linking.

3.4 Oxidative stability To ensure long term operation of the PEMFCs, oxidative stability is another significant factor. It is because the incomplete reduction of oxygen generates HO• and HO2• radicals during the fuel cell operation. The free radicals attack the polymer membrane, which causes performance degradation. As observed in Fig. 4, the cross-linked membranes showed superior oxidative stabilities as compared to the non-cross-linked membranes. Also, a higher degree of cross-linking enhanced oxidative stability. It is because the higher degree of crosslinking makes the better network structure of the membrane to retain the integrity. Also the more densely cross-linked structure reduces the possibility of radical species to attack the polymer chain [55]. This result demonstrates that cross-linking is essential for excellent durability of polymer electrolyte membranes.

3.5 Phosphoric acid doping and dimensional swelling The PA-doped XTPPO membranes were obtained by immersing the dried membranes into 85 wt% phosphoric acid solution at 120 oC for 15h. As shown in Table 1, the PA uptake initially increased with the triazole content since relatively basic triazole groups could absorb the phosphoric acid via the acid-base interaction. In addition, the XTPPO membranes with a lower degree of cross-linking showed higher PA uptake because the less cross-linked

membrane is swollen more to have large voids for containing the PA. The FT-IR spectra for the PA-doped membranes are shown in Fig. 5. The peaks at 1115 cm-1 and 956 cm-1 were attributed to characteristic absorptions of H2PO4- and H1PO42-, respectively [81]. Besides, the intensity of the C=N peak at 1688 cm-1 was reduced even as a small amount of PA was doped. The results indicate that protons were transferred from PA to triazole by the acid-base interaction. The dimensional swelling showed the same trend as the PA uptake.

3.6 Mechanical properties An assessment of the mechanical strength of the polymer electrolyte membranes is essential to fabricate membrane electrode assembly. However, PA doping reduces the mechanical properties of membranes because of the plasticizing effect of PA. Table 2 displays the mechanical properties of the cross-linked membranes. For the undoped states, it was seen that the tensile strength increased and then decreased as the quantity of triazole increased. It might be explained with π-π stacking effect of triazole and alkyl spacer effect. The π-π stacking of aromatic molecules has been known to enhance the intermolecular interaction [82]. On the contrary, however, the alkyl side chains disturb the intermolecular interaction by the steric hindrance effect. It is suggested that the side chain effect was superior to π-π stacking effect for the DB 40 samples. Besides, a higher degree of crosslinking was shown to enhance mechanical strength. Cross-linked membranes showed higher tensile strength and elongation compared to non-cross-linked membranes. It might be due to the cross-linking effect on membrane toughness [83]. Elongation was shown to decrease continuously with increasing triazole quantity. It might be due to the side chains which interrupt bond rotation of polymer main chain. When the membranes were PA-doped, the tensile strengths were reduced as expected. However, all of the PA-doped cross-linked

membranes showed fairly good mechanical strength of over 10 MPa. Contrary to the undoped membranes, the elongation of PA-doped membranes increased with the quantity of triazole because the PA uptake increased as well.

3.7 Phosphoric acid retention ability To prove the effect of membrane cross-linking on the PA retention ability, leaching test was carried out in 80 oC and 50% relative humidity for 1h. As shown in Fig. 6, the membranes with a higher quantity of triazole show more severe PA leaching. It is attributed to the higher PA uptake, which allows plasticizing the polymer matrix to accommodate more water molecules. However, a higher degree of cross-linking suppressed PA leaching. It might be due to the densely cross-linked structure, which reduces the opportunity of water to penetrate the PA-doped membranes. As a result, the PA retention ability was enhanced as the degree of cross-linking increases. An additional PA leaching test was performed for XTPPO30-15 which showed the highest PA retention (Fig. S3). The membrane was kept in different humidity conditions and exposure times. PA retention seems to reach an equilibrium after 1h, which means no more PA leaching occurs with longer exposure times.

3.8 Proton conductivity Ohmic loss is inversely proportional to proton conductivity. So, proton conductivity is one of the critical factors for the performance of the PEMFCs. Proton conductivities of the XTPPO membranes were measured from 100 to 180 oC in anhydrous condition. As observed in Fig. 7, proton conductivities increased with increasing temperature as expected. Also, conductivities were proportional to the amount of triazole and PA. It is because more amount

of triazole and PA can facilitate continuous hydrogen bonding network in which protons are transported more efficiently. It is shown that the degree of cross-linking, to some extent, affected the proton conductivities. As explained in Section 3.5, a higher degree of crosslinking suppresses the PA uptake, which leads to lower conductivities. The PA-doped XTPPO-40-10 with the highest PA uptake also showed the highest proton conductivity (64 mS/cm) among the XTPPO membranes, which is superior to cross-linked PBI membranes [73,84,85]. Also, proton conductivities were measured with PA leached XTPPO membranes (Fig. S6). Though the membranes showed reduced proton conductivities because of the PA loss after leaching test, the conductivities were proportional to the amount of PA.

3.9 Single cell performance Membrane electrode assemblies were fabricated for HT-PEMFC test. In Table 3, the voltages at 0.2 A/cm2 from the polarization curve are listed, and less cross-linked membranes show higher cell performances. I-V polarization curves for each MEA are shown in Fig. 8. For the membranes with a lower degree of cross-linking, XTPPO-40-10 and XTPPO-35-10 show similar performance and XTPPO-30-10 shows lower performance. It seems not to follow the trend with the proton conductivities, but the I-V curves for less cross-linked membranes well match the EIS in Fig. S4. However, the membranes with a higher degree of cross-linking showed different behaviors. XTPPO-35-15 shows similar curves like less crosslinked membranes, but XTPPO-30-15 and XTPPO-40-15 show mass transfer loss in the higher current density region. Overall, the less cross-linked membranes showed better performances compared to more cross-linked membranes. It might be due to the different chain mobility of the membranes (Fig. S2) [48]; less cross-linked membranes could make more dynamic chain movement for proton conduction. In addition, it might be due to the PA

loss from the membrane during MEA activation, which blocks the active sites in the catalyst layer, causing mass transport resistance to increase [86,87]. When viewed from the new PA loss mechanism [50], rigid membranes could lose more PA, which might explain why the more cross-linked XTPPO membranes showed mass transfer loss. The fuel cell test shows the promising potential of XTPPO membranes for HT-PEMFCs. It will be able to enhance the cell performance with an improvement of cell design.

Conclusions The cross-linking approach in controlling phosphoric acid (PA) leaching and improving membrane stability for HT-PEMFCs was very effective. The degree of cross-linking in the membranes was controlled by varying the triazole functionality in the side chains. This approach was effective in retaining PA due to the dense network structure - a higher degree of cross-linking resulting in better prevention of PA leaching. Besides, oxidative and mechanical stabilities were enhanced significantly. All the cross-linked membranes showed excellent thermal stability suitable to be operable in the high-temperature range of 100 - 180 oC. As expected, the less cross-linked membranes showed higher proton conductivity and better performances due to higher PA uptake and more dynamic chain mobility. Though a high degree of cross-linking showed better membrane stability, the fuel-cell performance slightly decreased. Hence, the optimization of the degree of cross-linking is essential for HT-PEMFCs. Our cross-linking technique to form triazole rings in situ is a promising approach for the development of stable, high performance and robust HT-PEMFCs.

Scheme 1. Synthesis and fabrication of cross-linked triazole modified poly(2,6-dimethyl-1,4phenylene oxide) membranes (XTPPO).

Fig. 1. FT-IR of spectra of TPPO-30-15 (before cross-linking) and XTPPO-30-15 (after cross-linking). The azide peak completely disappeared.

Fig. 2. Gel fraction test of XTPPO membranes performed in NMP at 80 oC for 12 h. (a) The membrane samples in NMP at 80 oC after 12 h, and (b) the residual weight of the membranes after solvent treatment.

Fig. 3. TGA curves of (a) PA-undoped XTPPO membranes and (b) PA-doped XTPPO membranes.

Fig. 4. Oxidative stability test of XTPPO membranes performed at 80 oC in Fenton’s reagent (3% H2O2 containing 4 ppm Fe2+) for 27 h. (a) The XTPPO membrane samples after the Fenton test, and (b) The residual weight of the XTPPO membranes after the Fenton test.

Fig. 5. FT-IR spectra of XTPPO-30-15 and PA-doped XTPPO-30-15 with PA uptake 8, 23, 73, and 105%.

Fig. 6. Phosphoric acid leaching test of PA-doped XTPPO membranes performed at 80 oC and RH 50% for 1h. The XTPPO-30-15 shows the highest PA retention.

Fig. 7. Proton conductivities of PA-doped XTPPO membranes in anhydrous condition.

Fig. 8. I-V polarization curves of the MEA single cells at 150 oC under dry hydrogen and air.

Table 1 PA doping behavior of the XTPPO membranes. PA uptake (%)

PA doping

XTPPO-30-10

110 ± 8

XTPPO-35-10

Membrane

level

a

Dimensional swelling (%) Length

Thickness

Area

Volume

5.7 ± 0.3

13 ± 1

11.1 ± 1

27.7 ± 2

41.9 ± 4

150 ± 10

7.0 ± 0.4

16 ± 1

15.8 ± 2

34.6 ± 2

55.8 ± 6

XTPPO-40-10

211 ± 14

9.0 ± 0.4

25 ± 1

36.2 ± 4

56.3 ± 2

112.8 ± 9

XTPPO-30-15

106 ± 7

5.5 ± 0.2

10 ± 1

15.2 ± 2

21.0 ± 2

39.4 ± 5

XTPPO-35-15

142 ± 11

6.7 ± 0.3

13 ± 1

20.7 ± 3

27.7 ± 2

54.1 ± 6

XTPPO-40-15

175 ± 12

7.5 ± 0.3

18 ± 1

29.4 ± 3

39.2 ± 2

80.2 ± 7

a

Mol of PA/mol of triazole

Table 2 Mechanical properties of the XTPPO membranes.

Membrane

Young's modulus (MPa)

Tensile strength (MPa)

Elongation at break (%)

PA undoped

PA doped

PA undoped

PA doped

PA undoped

PA doped

a

746 ± 51

-

46 ± 4

-

5.0 ± 0.4

-

a

824 ± 72

-

59 ± 4

-

3.8 ± 0.3

-

a

725 ± 64

-

52 ± 5

-

3.2 ± 0.4

-

XTPPO-30-10

1077 ± 82

219 ± 18

85 ± 6

12.7 ± 2

10.3 ± 1.1

43.7 ± 4

XTPPO-35-10

1156 ± 69

119 ± 11

86 ± 5

13.7 ± 1

8.5 ± 0.7

54 ± 6

XTPPO-40-10

1044 ± 70

82 ± 8

73 ± 6

11.5 ± 2

7.9 ± 0.9

63.4 ±5

XTPPO-30-15

1107 ± 74

254 ± 21

88 ± 6

17 ± 2

9.8 ± 0.8

38.8 ± 4

XTPPO-35-15

1169 ± 76

188 ± 17

90 ± 5

17.1 ± 2

8.8 ± 0.7

70.9 ± 6

XTPPO-40-15

1103 ± 68

81 ± 7

79 ± 6

12.7 ± 3

8.2 ± 0.8

80.2 ± 7

M-5 [29]

-

-

-

7

-

67

44-Ref [39]

-

113.8 ± 14.7

14.7 ± 0.9

-

96.4 ± 13.9

PESB [84]

2800 ± 20

78 ± 25

17.5 ± 1.3

89 ± 11

263 ± 25

TPPO-30 TPPO-35 TPPO-40

a

102.2 ± 1.4

Non-cross-linked membranes swelled significantly in PA solution.

Table 3 Electrochemical properties of MEA single cells.

XTPPO-30-10

V at 0.2 A/cm2 (V) 0.53

Rohm (Ωcm2) 0.287

Rct (Ωcm2) 0.772

XTPPO-35-10

0.61

0.187

0.433

XTPPO-40-10

0.60

0.191

0.432

XTPPO-30-15

0.52

0.316

0.504

XTPPO-35-15

0.57

0.274

0.710

XTPPO-40-15

0.56

0.244

0.557

Membrane

Fig. S1. 1H NMR spectra of (a) BPPO-40, (b) APPO-40, and (c) TPPO-40-15.

Fig. S2. DSC curves of TPPO-30 (non-cross-linked), XTPPO-30-10 (cross-linked), and XTPPO-30-15 (cross-linked).

Fig. S3. PA leaching test of PA-doped XTPPO-30-15 performed at 80 oC with different relative humidity and exposure times. Four samples were used for each condition.

Fig. S4. EIS measurement of the MEA single cells at 0.6V and the equivalent circuit used to fit the EIS measurement in this work.

Fig. S5. SEC curves of BPPO-40, APPO-40 and TPPO-40-10.

Fig. S6. Proton conductivities of PA-doped XTPPO membranes after PA leaching test. (B): Before PA leaching test, (A): After PA leaching test.

Table S1. Flexibility and PA uptake of XTPPO membranes. XTPPO membranes Degree of bromination

30

35

40

45

Flexibility a

PA uptake (Wt%)

O

-b

8

O

114

10

O

110

13

O

108

15

O

106

17

X

-

5

O

-b

8

O

154

10

O

150

13

O

145

15

O

142

17

X

-

5

O

-b

8

O

220

10

O

211

13

O

183

15

O

175

17

X

-

10

O

-b

15

O

-b

17

X

-

Degree of cross-linking 5

a

O: flexible, X: brittle

b

PA uptake could not be determined as the sample swelled excessively.

Table S2. pKa values of phosphoric acid, imidazole and 1,2,3-triazole. pKa1 2.14 7.18 a 1.17 a

Phosphoric acid [88] Imidazole [47] 1,2,3-Triazole [47] a

pKa2 7.20 14.52 9.26

pKa3 12.37 -

Protonated form

Table S3. Molecular weight of BPPO-40, APPO-40 and TPPO-40-10. Polymer

Mn (kDa)

Mw (kDa)

Polydispersity (Mw/Mn)

BPPO-40 APPO-40 TPPO-40-10

88.3 81.7 111.0

104.2 100.9 172.8

1.180 1.234 1.556

Table S4.

Pt loading (mg/cm2)

Flow rate (mL/min)

Temperature

Cathode

Anode

Cathode

Anode

(℃)

Voltage at 0.2A/cm2 (V)

XTPPO-30-10

1.0

1.0

Air, 300

H2, 100

150

0.53

XTPPO-35-10

1.0

1.0

Air, 300

H2, 100

150

0.61

XTPPO-40-10

1.0

1.0

Air, 300

H2, 100

150

0.60

XTPPO-30-15

1.0

1.0

Air, 300

H2, 100

150

0.52

XTPPO-35-15

1.0

1.0

Air, 300

H2, 100

150

0.57

XTPPO-40-15

1.0

1.0

Air, 300

H2, 100

150

0.56

M-5 [29]

0.60

0.60

O2, 60

H2, 120

160

0.59

Radel-PY-IM [32]

0.67

0.67

Air, 300

H2, 100

160

0.54

48-Lo-C [39]

0.58

0.29

Air, 450

H2, 110

160

0.60

PESB [84]

0.6

0.4

O 2, -

H 2, -

160

0.70

Membrane

Comparison of single cell performances with other research.

Acknowledgment This work was supported by the National Strategic Project - Fine Particle of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT), the Ministry of Environment (ME), and the Ministry of Health and Welfare (MOHW) (2017M3D8A1091937).

This work was also supported by a “Nobel Research Project” grant

for Grubbs Center for Polymers and Catalysis funded by the GIST in 2019.

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Highlights -

Phosphoric acid doped triazole-containing polymer electrolytes for high-temperature operation.

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Cross-linking proceeds through alkyne-azide reaction to introduce additional triazoles.

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Oxidative and mechanical stability are remarkably enhanced with cross-linking.

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Phosphoric acid leaching was reduced by membrane cross-linking.

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The highest proton conductivity of XTPPO-40-10 was 64 mS/cm.