Phosphoric acid-loaded covalent triazine framework for enhanced the proton conductivity of the proton exchange membrane

Electrochimica Acta xxx (xxxx) xxx

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Phosphoric acid-loaded covalent triazine framework for enhanced the proton conductivity of the proton exchange membrane Xiang Sun a, Jun-Hua Song c, Hong-qian Ren a, Xiao-yang Liu a, Xiong-wei Qu a, Yi Feng a, *, Zhong-Qing Jiang b, **, Hui-li Ding a, *** a Institute of Polymer Science and Engineering, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300130, People’s Republic of China b Key Laboratory of Optical Field Manipulation of Zhejiang Province, Department of Physics, Zhejiang Sci-Tech University, Hangzhou, 310018, PR China c School of Chemistry, Beihang University, Beijing, 100000, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 August 2019 Received in revised form 21 October 2019 Accepted 5 November 2019 Available online xxx

The development of proton exchange membranes (PEMs) with high loading and stable electrolytes is currently critical and challenging for applications in new energy related devices such as proton exchange membrane fuel cells (PEMFC). In this study, a novel porous organic skeleton (Covalent triazine framework, recorded as CTFp) is synthesized as a material for immobilized guest molecules via a simple nucleophilic substitution reaction. The phosphoric acid molecule (H3PO4) is extruded into the CTFp porous organic framework by vacuum assisted method (VAM). Since the molecular size of H3PO4 is smaller than the window size of the micropores in CTFp, a high loading of H3PO4 is achieved. The large amounts of basic groups distributed in CTFp can form a strong electrostatic interaction with H3PO4, which ensures the low dynamic leakage of H3PO4. PEMs with high proton conductivity are developed by embedding phosphoric acid-loaded CTFp (H3PO4@CTFp) in a SPEEK matrix. The acid-base pair formed between H3PO4@CTFp network and SPEEK optimizes the interfacial interaction and enhances the dispersion of H3PO4@CTFp in the composite membrane. H3PO4 stored in CTFp provides rich proton hopping sites for proton transport. The hydrogen bond network formed by self-dissociation of high concentration H3PO4 molecules constructs a proton transfer channel with low energy barrier for proton transfer, thereby significantly enhancing the proton conductivity of the membrane. The results show that the proton conductivity of the composite membrane at 80
C is 0.313 S/cm when the filler content is 15%. It is worth noting that the phosphoric acid leakage rate of H3PO4@CTFp is only 15.3% after the filler is immersed in water at 60
C for 30 days. Therefore, the SPEEK/H3PO4@CTFp composite membranes are promising to develop new PEMs with low acid loss and high proton conductivity. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Phosphoric acid Covalent triazine framework Vacuum assisted method Proton exchange membrane Sulfonated poly(ether ether ketone)

1. Introduction Polymer electrolyte membrane fuel cells (PEMFC) have the potential to replace fossil energy due to their high energy conversion efficiency, green emissions, low noise operation and low maintenance costs [1e3]. However, the core component PEM in the fuel cell exhibits lower proton conductivity under high temperature or low humidity conditions, limiting the large-scale implementation

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Y. Feng), [email protected]. cn (Z.-Q. Jiang), [email protected] (H.-l. Ding).

of PEMFC [4]. As the most widely used PEM, perfluoro sulfonic acid polymers such as Nafion and sulfonated aromatic polymers have been successfully prepared as PEMs, which provide sufficient proton conductivity (~0.1 S/cm) under fully hydrated conditions [5e7]. At high temperatures, however, water loss reduces proton support and disrupts the proton pathway within the membrane, and most PEMs undergo a sharp decay under these conditions [8]. Therefore, optimizing the proton transport carrier and improving the proton conduction of the membrane, especially at high temperatures and low RH, remains challenging. An effective way to solve the above problems is to use amphoteric sites with proton donor and proton acceptor characteristics as carriers for proton transfer in PEM, such as phosphoric acid (H3PO4, PA) [9], imidazole (Im) [10] and ionic liquids (IL) [11]. Scholars have

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discovered that each amphoteric -PO3H2 group has two proton donor sites and one proton acceptor site [12]. Compared to the common sulfonic acid groups (69.9 kJ mol1), the phosphate group (37.2 kJ mol1) has a lower energy penalty during proton transfer, especially at low RH, exhibiting the highest proton conductivity. -PO3H2 not only has the advantages of high load carrier concentration, high proton conductivity, low dependence on moisture, [13,14] but also low proton transfer energy barrier. When -PO3H2 is used as a proton donor, it can be automatically dissociated to form a high concentration of intrinsic proton defects; when it is used as a proton acceptor, it promotes excessive proton or insufficient high mobility [15]. High concentration liquid phosphoric acid also exhibits high proton conductivity due to structural diffusion [16]. Qin and his colleagues [17] loaded H3PO4 onto a three-dimensional (3D) polyacrylamide-graft-starch (PAAm-g-starch) hydrogel material as a high temperature PEM. The results showed that the proton conductivity was as high as 0.109 S/cm at 180
C in completely anhydrous state. Cai et al. [18] combine the cellulose nanofibers with phosphoric acid and embed them in sulfonated poly (ether sulfone) (SPES) matrix. As a composite membrane of cellulose nanofibers doped with 0.25 mol/L phosphoric acid, the maximum conductivity reached 0.154 S/cm (80
C, 100 RH). Dang et al. [19] synthesized a series of imidazole microcapsules (ImMCs) as a phosphate reservoir to achieve high phosphoric acid retention of PEM. Interestingly, 10 wt% of ImMCs provided 78 wt% phosphoric acid loading and a 75-fold increase in proton conductivity compared to the control membrane. Direct introduction of PA may be more convenient and efficient considering the limitations of available monomers and the relative difficulty of introducing phosphonic acid groups, however, the loss of PA in the membrane is an unavoidable disadvantage. At present, metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have shown great potential as new porous proton conductive materials [20e23]. Thanks to its rich structural adjustability, high specific surface area and functional pore channels provide and accommodate a variety of proton carriers [24,25]. Ye and his group [26] embedded the free imidazole molecule into the metal-organic framework (NENU-3, ([Cu12(BTC)8(H2O)12 [HPW12O40])$Guest). The results show that the material Im@(NENU-3) exhibits high proton conductivity of up to 1.82  102 S/cm at 90% RH and 70
C. Li and his colleagues [24] immersed phytic acid (plant hexaphosphonic acid) in MIL101 by vacuum assist and the obtained phytic @ MIL101 as a new filler into Nafion to prepare a mixed proton exchange membrane. The study found that the conductivity can reach 0.0608 S/cm and 7.63  104 S/cm at 57.4% RH and 10.5% RH (2.8 times and 11.0 times higher than the original membrane, respectively). Yin et al. [27] used vacuum assisted method to impregnate phosphate molecules into SNW-1 covalent organic framework (COF), in order to achieve high loading and low guest leaching rate of H3PO4 in SNW-1. When the filler content is 15%, the composite membrane reveals an excellent proton conductivity of 0.0604 S/cm at 51% relative humidity and 80
C. Therefore, the use of functional porous materials to support guest molecules to ensure high impregnation and low leaching of guest molecules has become an efficient coping strategy. In this study, we first synthesize a large number of triazine rings, imine groups and microporous-rich covalent triazine network framework (CTFp). Subsequently, phosphate molecules are loaded onto CTFp via vacuum-assisted method and then incorporated into a SPEEK matrix to prepare composite membrane. The strong electrostatic interaction between the large number of imine groups distributed at the end of the CTFp network and the sulfonic acid groups on the SPEEK imparts high dispersion of the filler in the membrane. Phosphoric acid molecules stored in CTFp can selfdissociate and form a hydrogen bond network structure,

enhancing the hopping mechanism of protons in the membrane. The phosphoric acid content of CTFp was characterized by EDS, XPS and ICP. The composite membrane was characterized by SEM, FTIR and TGA. The proton conductivity of the prepared composite membrane was studied, and the Grotthuss mechanism of H3PO4@CTFp enhancing proton transfer in the composite membrane was speculated. 2. Experimental 2.1. Materials and chemicals Anhydrous piperazine (PIP, 99%) and cyanuric chloride (CC, 99%) were supplied by Aladdin Reagent and used directly without purification. N,N-diisopropylethylamine (DIPEA, 99%) was supplied by Tianjin Hengshan Chemical Technology Co., Ltd and distilled under vacuum. Phosphoric acid (H3PO4, 85%), tetrahydrofuran (THF,  99.7%), dichloromethane (CH2Cl2, 99.7%) and N,N-dimethylformamide (DMF,  99.7%) were purchased from Tianjin Jindong Tianzheng Fine Chemical Reagent Factory, in which tetrahydrofuran is dried with 4A molecular sieve, and then purified by vacuum distillation. Poly (ether ether ketone) (Victrex® PEEK, grade 450G) was supplied by Suzhou Qijiangda Engineering Plastics Co., Ltd. De-ionized water was used throughout the experiment. 2.2. Synthesis of covalent triazine framework(CTFp) CTFp was synthesized according to previous researchers’ reports [28]. A dried three-necked flask fitted with a condenser was charged with cyanuric chloride (1.4753g, 8 mmol), and THF (10 mL). Anhydrous piperazine (1.059 g, 12.3 mmol) and excess N,N-diisopropylethylamine (17 mL) were dissolved in 20 mL THF. The mixture was slowly dropped into a three-necked flask under icewater bath at 0
Ce5
C. After the addition was completed, the reaction was kept at this temperature for 2 h. Thereafter, the reaction flask was warmed to 45
C for 2 h under N2 atmosphere. Finally, the reaction was heated to 80
C for 24 h. After reaction, the reactor was cooled down to room temperature and the precipitated powder was isolated via filtration. The obtained CTFp was washed with water (30 mL  3) and THF (30 mL  5). The obtained product was placed in a vacuum oven and dried at 80
C under vacuum (under a vacuum of about 2.67 KPa) for 12 h to obtain a white solid powder (yield: 92%). 2.3. Immersion of phosphoric acid into CTFp Phosphoric acid was impregnated into CTFp via VAM by the following steps. First, the CTFp was treated by CH2Cl2 for 24 h under reflux to replace the high boiling point solvent by lower one. Second, pure-CTFp was treated in completely sealed three-necked flask under vacuum at 120
C for 12 h to remove the trace of residual dichloromethane and trapped air in pores. Third, the threenecked flask was cooled to 80
C. H3PO4 aqueous solution (50 wt %, 50 mL) was added into the three-necked flask under vacuum for 12 h. Fourthly, vacuum was removed and the mixture was stirred for 12 h. Phosphoric acid could be pressed into the cavities of CTFp by the pressure difference between the cavities and atmosphere. Finally, the treated pure-CTFp was centrifuged from the mixture, washed by water until the pH of supernatant liquid was neutral, and dried at 60
C under vacuum until constant weight. a light yellow powder could be obtained, which was designed as H3PO4@CTFp. Fig. 1 shows the specific experimental method process.

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Fig. 1. Schematic illustration of phosphoric acid loaded CTFp.

2.4. Synthesis of sulfonated Poly(ether ether ketone)(SPEEK) PEEK was dried at 80
C for 24 h in a vacuum oven before utilization. Dried PEEK (14 g) was added slowly to concentrated sulfuric acid (125 mL) with vigorous stirring at room temperature until dissolved and then kept stirring at 50
C for 8 h. The resultant SPEEK was precipitated from the solution via pouring it into an icewater bath to stop the sulfonation. The precipitate SPEEK was washed by de-ionized water until the filtrate was neutral (pH of 7). The SPEEK was dried at room temperature for 48 h and then at 60
C for another 24 h in a vacuum oven. The SPEEK degree of sulfonation can be calculated by 1HNMR to be 67.8%. 2.5. Preparation of composite membranes The membranes were prepared by solution-casting method. The covalent triazine skeleton impregnated with phosphoric acid (H3PO4@CTFp) were dispersed in 10 mL of DMF under ultrasonication and stirred for 12 h. Afterwards, 0.80 g of the SPEEK was added to the above mixture and stirred vigorously at room temperature for 24 h. Then the above solution was cast onto a glass plate and dried at 80
C in an oven for 12 h, followed by annealing at 120
C for 6 h further. After cooling to room temperature, the hybrid membrane was peeled off from the glass plate in deionized water. The resultant membrane was nominated as SPEEK/H3PO4@CTFp-X, where X (X ¼ 5, 10, 15 or 20) was the weight ratio of the H3PO4@CTFp to the SPEEK). The pristine SPEEK membrane was also fabricated in a similar procedure, without incorporating any H3PO4@CTFp, and was nominated as SPEEK.

photoelectron spectroscopy (XPS, ESCALAB 250Xi). The total elemental content of the sample was measured using an inductively coupled plasma spectrometer (ICP, Optima 8300). The pore size and specific surface area of the sample were measured using a fully automated specific surface and porosity analyzer (BET, ASAP 2460). The thermal stability of the sample was tested by thermogravimetric analysis (TGA, SDT/Q600) under a nitrogen atmosphere at a temperature increase rate of 10
C/min from room temperature to 800
C. The tensile properties of the membrane were tested by a microcomputer controlled electronic universal testing machine (CMT6104). Powder X-ray diffraction (PXRD) data were collected by using a Bruker D8 Advance, having high intensity microfocus rotating anode as X-ray generator. The radiation used for diffraction was CuKa (a ¼ 1.54 Å) with a Ni filter. All the PXRD Data of CTFp were recorded in the range of 2q ¼ 5e60
. The scan speed and the step size were set to 1
min-1 and 0.02
respectively. 2.7. Dynamic leakage rate of phosphoric acid The dynamic leakage rate of H3PO4 was tested by the following method. Weigh m0g H3PO4@CTFp and dip it into Vml deionized water, place it at 60
C for 5 days, then centrifuge and take the upper suspension, and immerse the remaining H3PO4@CTFp in fresh Vml deionized water again. The above impregnation process was carried out for 5 days as a cycle for a total of 30 days. The content of phosphorus (P) in the supernatant was measured by ICP. The formula for calculating the dynamic leak rate is as follows:


Ln ¼

2.6. Characterizations The morphology of the CTFp particles was observed by a field emission scanning electron microscope (FESEM, Nova Nano SEM450). The energy dispersion spectroscopy (EDS) mapping was performed in a high angle annular dark field (HAADF) scanning mode to observe the distribution of phosphate molecules in the CTFp material while obtaining a cross-sectional image of the membrane. Fourier Transform Infrared Spectroscopy (FTIR) were recorded with a spectrometer (VECTOR 22), equipped with attenuated total reflection (ATR) for characterizing functional groups in the membrane structure. The structure of the sample was measured on a13C solid state nuclear magnetic spectrometer (Agilent-NMR-vnmrs600) at a MAS rate of 75.4 MHz and 10 kHz. The elements on the surface of the sample were measured by X-ray

Pn

i¼1 V

unMH

 3 PO4

Mp

m0 up

(1)

The un (g mL1) in the formula is the phosphorus content of the centrifuged supernatant after the nth cycle, MH3 PO4 is the relative molecular mass of the phosphate molecule and Mp is the relative atomic mass of the phosphorus element,up (g g1) is the initial P content of the sample measured by ICP. 2.8. Measuring the water uptake, swelling ratio and IEC of the membrane The water uptake of the membrane can be determined by the difference between the sample weight (Wdry) in a completely dry state and the sample weight (Wwet) after complete hydration at different temperatures. The specific steps are as follows: the dry membrane sample is cut into a square shape (about 2 cm  2 cm)

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and its mass (Wdry) and length (Ldry) are recorded. Then the sample is immersed in deionized water at different temperatures for 24 h. The wet weight (Wwet) and wet length (Lwet) of the membranes are quickly measured after rapidly removing the moisture on the surface of the membranes. Each sample is measured at least three times and the average value is calculated, and the water uptake and swelling degree of the membranes are calculated by the following formula:

 Water uptakeð%Þ ¼

Wwet  Wdry Wdry

 Swelling ratioð%Þ ¼

Lwet  Ldry Ldry

  100 %

(2)

  100 %

(3)

In this experimental work, the IEC value of the membrane was determined by a conventional acid-base titration method. The sample membrane was vacuum dried at 80
C for 24 h, then a certain amount of sample (Wdry, g) was accurately weighed and immersed in 2 M NaCl solution for at least 48 h. Using phenolphthalein as an indicator, the above solution was titrated to neutral using a 0.01 mol/L NaOH standard solution, and the volume of the above-mentioned NaOH solution was recorded. The three measurements were averaged as a result, and the IEC value of the membrane was calculated according to the following formula:

 0:01  1000  V  NaOH IEC mmol , g1 ¼ Wd

(4)

2.11. Single PEMFC performance The performance of the synthesized membranes were determined using a single-cell MEA setup. The electrocatalyst used in the anode and cathode was Pt/C (40 wt% Pt, Johnson Matthey) and Pt/C (60% Pt, Johnson Matthey), respectively. Briefly, commercial Pt/C was first soaked in deionized water under sonication for 60 min. The suspension was then mixed with a solution of Nafion (20 wt%) in isopropyl alcohol. Excess isopropyl alcohol was added to control the total solid content to 5 wt%. After sonicating further for 60 min at room temperature, the well-dispersed catalyst ink was sprayed on the gas diffusion layer (25 BC, SGL) surface using an electrostatic spraying equipment with a platform heating temperature of 50
C. The Pt loadings on both the anode and cathode were 1.0 mg cm2. The MEA was obtained by hot-pressing the membrane in between the anode and cathode and followed by pressing at 140
C under the pressure of 50 kg cm2 for 3 min. The obtained MEA, flanked by grids and gaskets, was then assembled into a single cell having an active area of 5 cm2 with stainless steel plates. The single-cell performance was investigated using a fuel cell station under ambient pressure. The cell temperature was set to 65
C and the gas humidifying temperatures were controlled at 50
C for the anode and cathode, which corresponded to 30% RH. During the tests, the H2 and O2 gas flow rates were fixed at 200 and 100 mL min1, respectively, with the back pressure at 0.1 MPa. In all the cases, polarization curves of the PEMFCs are measured after the cells have been activated at 50 mA cm2 and 100 mA cm2 for 12 h, respectively. Each membrane was tested for three times.

3. Results and discussion 3.1. Synthesis and characterization of CTFp

2.9. Oxidative stability of the membrane The oxidative stability of the membranes was evaluated according to a typical method. Uniform size membrane samples (1 cm  1 cm) were immersed in Fenton’s reagent (3% H2O2, 3 ppm FeSO4) at 60
C. The oxidative stability of the membrane samples was characterized by the residual weight ratio (Ra) after 1 h of soaking and the time (tb) consumed for the membrane to completely dissolve.

2.10. Proton conductivity of the membrane The proton conductivity of the membrane was measured using a four-electrode AC impedance method [29]. The electrochemical impedance of the membrane was measured by an electrochemical workstation (CHI 660D) at a vibration voltage of 10 mV with a frequency range of 1e105 Hz. All membrane samples were fully hydrated by immersing them in water prior to testing. The test samples were immersed in deionized water at different temperatures for testing. Each sample was tested at least three times and the proton conductivity of the membrane was calculated according to equation (5):

s¼

l AR

(5)

where s represents the proton conductivity (S/cm) of the membrane, l (cm) is the distance between the two electrodes (the distance between the two electrodes is fixed at 1.5 cm), and A is the cross-sectional area of the sample membrane, R (U) is the resistance tested in the horizontal direction of the membrane.

The triazine network framework is constructed based on a simple nucleophilic substitution reaction between cyanuric chloride and anhydrous piperazine. Successful synthesis of the chemical structure of the CTFp network is confirmed by 13C solid state NMR. As shown in Fig. 2(a), the NMR spectrum of CTFp shows a significant carbon atom resonance peak at 42 ppm and 163 ppm. Specifally, resonance peak at 42 ppm is attributed to the sp3 hybridized carbon atom on the piperazine ring. At the same time, the signal peak at 163 ppm can be attributed to the unsaturated carbon atoms on the triazine ring. The formation of the CTFp network structure can be further verified by FTIR spectroscopy (Fig. 2(b)). The characteristic peaks appearing at 1560 cm1 and 1490 cm1 are attributable to the stretching vibration peak of C]N in the triazine ring in the network structure. The characteristic peaks at 2920 cm1 and 1161 cm1 correspond to the stretching vibration peaks of CeH and CeN in the piperazine ring, respectively, and it was confirmed that the covalent triazine framework was prepared by a simple nucleophilic reaction from cyanuric chloride and piperazine. The stretching vibration peak of NeH (3430 cm1) in the map indicates that a large number of imine groups are distributed at the end of the CTFp network structure. The two-dimensional layered morphology of CTFp can be observed in the SEM image (shown in Fig. 3(a)). It is found by SEM image that the CTFp particles have a lamellar structure, which is consistent with the two-dimensional layered structure [28]. The SEM image shows that the CTFp particles have a diameter between 400 and 500 nm and a layer thickness of approximately 50 nm. The porous structure and specific surface area of CTFp can be characterized by N2 adsorption. As can be seen from Fig. 3(b), CTFp has a

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Fig. 2. (a)

5

13

C solid-state NMR spectrum of CTFp; (b) FTIR spectrum of CTFp.

Fig. 3. (a) SEM image of CTFp; (b) Nitrogen adsorption-desorption isotherm of CTFp at 77 K, internal illustration of a pore size distribution histogram of CTFp.

typical І-type reversible adsorption curve [30]. The adsorption process exhibits rapid gas absorption at low relative pressures, followed by a flat adsorption process, which is typical of microporous materials. The slow rise of the adsorption curve and the hysteresis of the desorption may be due to the unique swelling characteristics of the organic framework [31]. The Langmuir specific surface area of CTFp is 446.6 m2/g, and the pore volume is 0.3 cm3/g. The high specific surface area and pore volume of the microporous material means a high degree of crosslinking of the product. The pore size distribution of CTFp can be observed from the inset in Fig. 3(b), and the main pore size of CTFp is concentrated at ~1.7 nm. The appearance of larger sized pores in the microporous material may be due to the mutual stacking of the layered structures, which is consistent with the phenomenon shown in Fig. 3(a). Owing to that the CTFp is one type of synthesized piperazinesubstituted covalent triazine framework, there is no standard JCPDS card to compare with. The diffraction peaks agree with the optimised simulation result in the reported work [28], which illustrate a certain kind of crystallinity (As shown in Fig. 4). However, the low intensity and broad diffraction peaks also suggest the structural disorder in longer distance. 3.2. Phosphoric acid loading into CTFp (H3PO4@CTFp) Phosphoric acid acts as a good proton carrier by loading it into a three-dimensional network to increase the proton conductivity of the covalent triazine backbone. The molecular diameter of H3PO4 is 0.373 nm, which is smaller than the CTFp pore diameter (1.7 nm). A large number of phosphate molecules can be efficiently

Fig. 4. PXRD pattern of CTFp.

immobilized in the cavities of the CTFp network structure by vacuum assisted method (VAM) and its large specific surface area. The FTIR spectrum of H3PO4@CTFp is used to detect the successful loading of H3PO4 into CTFp, as can be clearly seen in Fig. 5(a), compared to the FTIR curve of CTFp. New characteristic absorption peaks appeared at 1149 cm1 and 876 cm1, which can be attributed to the stretching vibration peak of -P]O and symmetric PeO

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Fig. 5. (a) FTIR spectra of H3PO4@CTFp and CTFp; (b) Adsorption-desorption isotherm curve of H3PO4@CTFp at 77 K. Inner illustration: a histogram of the pore size distribution of H3PO4@CTFp; (c) TGA curves of H3PO4@CTFp and CTFp.

in the impregnated phosphate molecule, respectively [32,33]. Furthermore, the new peak appearing at 1672 cm1 may be related to the bending vibration of the bound water (HeOeH), indicating that the water molecules are bound by phosphoric acid [34]. The N2 adsorption-desorption curve of H3PO4@CTFp confirms the loading level of phosphoric acid. As shown in Fig. 5(b), comparing with CTFp with high specific surface area [Fig. 3(b)], the specific surface area and pore diameter of CTFp after phosphoric acid impregnation [inset in Fig. 5(b)] can be almost ignored. The phosphate molecule occupies the pore volume in the CTFp so that H3PO4@CTFp no longer maintain its porous properties. It is not difficult to explain that the VAM method can completely fill the phosphate molecules into the pores inside the CTFp. The TGA curve of H3PO4@CTFp not only characterizes its thermal stability but also demonstrates the presence of H3PO4. For pure CTFp, the skeletal structure begins to decompose at about 400
C. However, for H3PO4@CTFp, the mass loss occurs first at around 180
C, Which is due to the reversible self-condensation dehydration of phosphate [35]. Through the above characterization analysis, phosphoric acid has been successfully immobilized into the CTFp porous network framework. The P content of the H3PO4@CTFp surface layer and the oxidation state of P are detected by XPS. The XPS survey spectrum is shown in Fig. 6(a). After the phosphate is immobilized, the new band appears at a binding energy of about 133 eV due to the P2p component in the phosphate group [36]. The content of the P element in the surface layer of H3PO4@CTFp (4e10 nm depth) can be calculated by the peak area ratio, which is 2.38 wt%. The oxidation state of the P element is observed as shown in Fig. 6(b),

and the main peak is 133.17 eV, which corresponds to the phosphate state [37]. The shoulder is also observed at 132.22 eV, indicating the presence of pentavalent oxidation state and PeO bond of P [38]. The distribution of phosphoric acid in H3PO4@CTFp is observed by EDS-mapping, and Fig. 6(c) revealed the distribution of P elements in the EDS-mapping display results, in which the N element distribution and the O element distribution are compared. It is apparent that it can be observed that the P element can be uniformly distributed in the network structure due to the electrostatic interaction between the uniformly distributed basic sites in the triazine skeleton and the phosphate molecules. Excellent phosphoric acid distribution can build a good hydrogen bond network structure for proton transfer. In order to evaluate the specific phosphoric acid content in CTFp, the P content in H3PO4@CTFp is calculated from the EDS spectrum and the results are shown in the internal table of Fig. 6(d), and the result is 6.05 wt%. In addition, the total P content in H3PO4@CTFp is characterized by ICP, and the results are similar to those of EDS, which is 6.85 wt%. It is indicated that 21.65 wt% H3PO4 content is fixed in the whole CTFp network structure, and the content of H3PO4 stored in the internal pores is much larger than that of surface storage compared with the fixed phosphoric acid content of 7.52 wt% (calculated by XPS) in H3PO4@CTFp. The H3PO4 content stored in the internal pores is much larger than the H3PO4 content stored on the surface, which means that most of the H3PO4 can be embedded in the interior of the network structure by the VAM method.

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Fig. 6. (a) XPS measurement spectrum of H3PO4@CTFp surface after phosphate fixation; (b) P2p XPS spectrum of H3PO4@CTFp; (c) Elemental distribution of H3PO4@CTFp detected by EDS-mapping (oxygen distribution; phosphorus distribution; distribution of nitrogen); (d) calculation of the content of each element in H3PO4@CTFp.

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3.3. Leakage rate of phosphoric acid by H3PO4@CTFp For long time, the leakage of phosphoric acid has been a major obstacle in practical applications. The dynamic leakage rate of H3PO4 from H3PO4@CTFp is calculated to evaluate the phosphoric acid retention characteristics of H3PO4@CTFp in 60
C water. The leaching curve is shown in Fig. 7. A clear trend is that the leakage rate of phosphoric acid slowly increases over time. However, under the 30-day test, the leak rate is only 15.3%. In the previous report, an acid-loaded porous material (H3PO4 @ MIL-101) had an acid leakage rate of up to 95% after 20 min [39]. The lower leakage rate of H3PO4@CTFp can be attributed to the following two aspects: Firstly, most of the phosphate molecules are retained inside the network structure by the VAM method, and it is difficult to flow out from the inside; In addition, the acid-base electrostatic interaction between a large number of basic sites distributed in CTFp and phosphoric acid ensures that most guest molecules are confined to the pores of CTFp during use. 3.4. Characterizations of composite membranes Fig. 8(a) is a photograph of a SPEEK/H3PO4@CTFp-15 composite membrane, and it can be seen from a macroscopic degree that the nanofiller is uniformly dispersed in the membrane without significant phase separation. By observing the cross-sectional morphology of the membrane by using FESEM, as shown in Fig. 8(b)e(f), Fig. 8(b) shows that the cross-section of the SPEEK substrate is very dense and smooth without voids. After incorporation of H3PO4@CTFp, the nanoparticles are evenly dispersed in the SPEEK matrix. As the content of H3PO4@CTFp increases, the cross section of the composite membrane becomes rougher, and the scale structure morphology can be more clearly seen in the cross section. In addition, all composite membranes showed no significant defects and showed a void-free morphology. The uniform structure of the composite membrane can be attributed to two reasons. First, the imine group present in CTFp can generate an electrostatic interaction with the sulfonic acid group in the SPEEK matrix to enhance its dispersibility; Secondly, as an organic phase, the organotriazine skeleton has good compatibility with the polymer matrix. Furthermore, it can be observed in Fig. 8(g) that the incorporation of the purely CTFp organic framework into the SPEEK matrix also shows good compatibility. It can be said that in the case of phosphoric acid-unloaded, the CTFp framework can contribute

Fig. 7. Dynamic leakage rate of H3PO4 from CTFp @ H3PO4 in water at 60
C.

more basic groups to enhance its dispersibility. It should be noted that when the content of H3PO4@CTFp reaches 15% by weight or more, slight agglomeration occurs. To investigate the interaction between the H3PO4@CTFp filler and the SPEEK matrix, the FTIR spectra are recorded and shown in Fig. 9. All of the membranes exhibited typical characteristic absorption peaks at 1217 cm1, 1076 cm1 and 1018 cm1, which can be attributed to the symmetrical/asymmetric stretching vibration of O]S]O of the eSO3H group on the SPEEK backbone [40]. After blending H3PO4@CTFp with the SPEEK matrix, the composite membrane exhibited a new characteristic absorption peak at 1538 cm1, which is caused by the stretching vibration of C]N in the triazine skeleton [28,41]. It is worth noting that the characteristic bands at 984 cm1 and 902 cm1 represent the stretching vibration of free H3PO4 and the symmetric stretching of SeO-, respectively [41,55], meaning that the phosphate molecules are stored in the composite membrane. Since the -SO- … þHeHN- or -S-O- … þHN]C acid-base pairs are formed between the SPEEK and CTFp interfaces, the reason for the occurrence of -S-O- is explained. The results indicate that the eNHe, -PO3H2 groups in the CTFp after immobilization of phosphoric acid form electrostatic interaction with the eSO3H groups in the SPEEK matrix, resulting in a decrease in transmittance. 3.5. Mechanical properties and thermal stability of composite membranes The thermal stability of the SPEEK blank membrane and the SPEEK/H3PO4@CTFp composite membrane is evaluated by TGA, and the results are shown in Fig. 10. The TGA curve shows three typical mass loss stages for all membranes. Taking the SPEEK blank membrane as an example, the first stage mass loss occurring before 200
C is caused by evaporation of residual solvent and bound water. The second mass loss occurring in the interval of 305
Ce380
C is attributed to the thermal decomposition of the sulfonic acid groups in the ion cluster. The final mass loss occurs in the region of 480
Ce620
C due to decomposition of the polymer backbone [42e44]. The SPEEK/H3PO4@CTFp composite membrane shows a thermal decomposition process similar to that of the SPEEK blank membrane. The difference is that the decomposition temperature of the PA stored in the membrane occurs at about 200
C (for dehydration condensation), resulting in the initial decomposition temperature of the composite membranes are turned to a lower value. As the content of H3PO4@CTFp is further increased, the decomposition initiation temperature of the composite membrane can be as low as 305
Ce252
C. Surprisingly, the SPEEK/CTFp-15 composite membrane exhibited higher mass loss than the other samples in the first stage (before 200
C). As can be seen from the figure, the mass loss of SPEEK/CTFp-15 in the first stage can be as high as nearly 10%. As mentioned above, the loss of moisture occurs in the first stage. It can be speculated that the pure CTFp framework without loading phosphoric acid can store a large amount of water molecules in advance due to its own abundant pores. In addition, since the water molecules stored in the pores are difficult to overflow, the CTFp framework realizes the conversion of free water into bound water for storage in the membrane. Nonetheless, the thermal stability of all membranes is as high as 250
C, which is sufficient to meet the practical application of PEM in hydrogen fuel cells. The stress-strain curves of the SPEEK and SPEEK/H3PO4@CTFp composite membranes are plotted in Fig. 11, and the mechanical property test results are summarized in Table 1. The hydrophobic structure in the polymer chain imparts good mechanical properties to SPEEK with a tensile strength of 31.0 MPa and an elongation at break of 22.9%. In comparison, for composite membranes

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Fig. 8. (a)Picture of SPEEK/H3PO4@CTFp-15 and SEM image of the cross section of (b)SPEEK, (c) SPEEK/H3PO4@CTFp-5, (d) SPEEK/H3PO4@CTFp-10, (e) SPEEK/H3PO4@CTFp-15, (f) SPEEK/H3PO4@CTFp-20, (g) SPEEK/CTFp-15.

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X. Sun et al. / Electrochimica Acta xxx (xxxx) xxx Table 1 Mechanical properties of SPEEK blank membrane and SPEEK/H3PO4@CTFp-X composite membranes.

Fig. 9. FTIR spectrum of SPEEK membrane and SPEEK/H3PO4@CTFp-X (X ¼ 5,10,15,20) composite membranes.

Fig. 10. TGA curve of SPEEK membrane and SPEEK/H3PO4@CTFp-X (X ¼ 5,10,15,20) composite membrane.

Membrane

Tensile Strength (MPa)

Elongation at break (%)

Young’s modulus (MPa)

SPEEK SPEEK/H3PO4@CTFp-5 SPEEK/H3PO4@CTFp-10 SPEEK/H3PO4@CTFp-15 SPEEK/H3PO4@CTFp-20 SPEEK/CTFp-15

31.0 33.6 34.9 47.0 28.1 43.1

22.9 10.6 10.1 3.3 2.5 14.3

1689 1901 2169 2459 2297 1943

incorporating 5 wt% to 15 wt% of H3PO4@CTFp, the tensile strength can be increased to 33.6 MPae47.0 MPa, and the elongation at break is reduced to 10.6%e2.5%. This is due to the electrostatic attraction between the basic site distributed on the triazine skeleton and the SPEEK interface, which effectively inhibits the mobility of the chain. At the same time, the internal fixed phosphoric acid molecule forms stronger electrostatic force and hydrogen bond with SPEEK, which also provides another explanation for the excellent mechanical properties of the composite membrane [36]. With the further loading of H3PO4@CTFp (>15 wt %), the tensile strength and Young’s modulus began to decrease, and the tensile strength and Young’s modulus of SPEEK/ H3PO4@CTFp 20 composite membrane are measured to be ~28.1 MPa and 2297 MPa, respectively. This may be due to excessive concentration of H3PO4@CTFp in the SPEEK matrix leading to partial aggregation. Therefore, stress concentration occurs. The fact that the composite membrane exhibits an extremely low elongation at break is generally explained by the fact that the addition of H3PO4@CTFp causes voids at the interface [as shown in Fig. 8(e)], causing the composite membranes to break at a lower elongation. At the same amount of incorporation, the SPEEK/CTFp-15 composite membrane exhibited slightly lower mechanical properties than the SPEEK/H3PO4@CTFp-15 composite membrane. The SPEEK/ CTFp-15 composite membrane had a mechanical strength of 43.1 MPa and an elongation at break of 14.3%. This is because the CTFp framework without loading phosphoric acid does not have the electrostatic force and hydrogen bonding effects of the phosphoric acid molecules and SPEEK. Therefore, the strengthening of the mechanical properties of the SPEEK matrix by the CTFp framework is slightly weaker than that of the H3PO4@CTFp framework. All results indicate that the triazine skeleton loaded with phosphoric acid is effective in enhancing the mechanical properties of the membrane.

3.6. Oxidative stability of composite membranes

Fig. 11. Stress-strain curve of SPEEK membrane and (X ¼ 5,10,15,20) composite membrane at room temperature.

SPEEK/H3PO4@CTFp-X

Oxidative stability determines the lifetime and long-term performance of PEMFC. Generally, during the operation of PEMFC, the by-product H2O2 produced by incomplete reduction of oxygen will generate free oxidative radicals ($OH and $OOH) by decomposition [45]. These highly oxidized free radicals will attack the polymer backbone causing membrane degradation. The time elapsed (t) after all the membranes were soaked in the Fenton reagent and the residual weight membrane (R) were collected in Table 2. It can be seen that the original SPEEK membrane with high DS degree is completely dissolved after soaking for 8 h, and the residual weight is 90.2% after soaking for 1 h. As reported in the literature [46,47], as with most sulfonated PEMs, the oxidative stability of SPEEK membranes decreases with increasing sulfonation. Excessive water absorption of the SPEEK membrane promotes the diffusion of hydrated $OH and $OOH radicals, which accelerates the degradation

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Table 2 Ion exchange capacity and oxidative stability of SPEEK blank membrane and composite membranes. Sample

Water uptake (%)

SPEEK SPEEK/H3PO4@CTFp-5 SPEEK/H3PO4@CTFp-10 SPEEK/H3PO4@CTFp-15 SPEEK/H3PO4@CTFp-20 SPEEK/CTFp-15 a b

Swelling Ratio (%)

IEC (meq/g)

25
C

80
C

25
C

80
C

35.32 34.06 31.71 30.21 29.17 32.40

692.54 415.63 243.50 135.32 103.93 175.98

12.00 11.17 10.95 10.55 10.21 9.60

122.73 90.95 58.38 42.29 30.50 40.91

1.84 1.64 1.54 1.47 1.40 1.42

Oxidative stability Ra (%)

tb (h)

90.2 93.0 93.6 93.8 94.8 92.4

＜8 h ＞13 h ＞13 h ＞13 h ＞13 h ＞12 h

The residual weight percentage of membranes after immersion in Fenton reagent (3% H2O2, 3 ppm FeSO4) for 1 h at 60
C. The elapsed time when the membranes dissolved completely (t).

of PEM [48,49]. In addition, the oxidative stability of all composite membranes was improved after the addition of the CTFp framework and the H3PO4@CTFp framework. The t values of all the composite membranes of SPEEK/H3PO4@CTFp were >13 h under the same conditions, and the residual weight ranged from 93.0% to 94.8%, which became larger as the amount of H3PO4@CTFp was increased. This phenomenon can be attributed to the following facts: First, the ionic cross-linking network formed by the basic group in the CTFp framework and the acidic group on the SPEEK backbone can hinder the attack of free radicals and impart better antioxidant capacity to the composite membranes [50,51]. Second, the introduction of phosphoric acid can facilitate the formation of hydrogen bond networks. Therefore, as the amount of H3PO4@CTFp incorporated increases, more phosphoric acid is introduced into the membrane. The dense hydrogen bond network structure will enhance the oxidative stability of the composite membranes. Although the SPEEK/CTFp-15 composite membrane also showed good oxidative stability, for example, the complete dissolution time t > 12 h and the residual weight is 92.4%. However, compared to H3PO4@CTFp, the enhancement effect of the purely CTFp framework on oxidative stability is slightly weaker. This is attributed to the water absorption effect caused by the porous structure inside the pure CTFp framework is more susceptible to attack by free radicals. 3.7. Water uptake and swelling degree of composite membranes The water uptake value is an important performance parameter for PEM because the water molecules provide a sufficient number of carriers for proton conduction [52]. However, extreme water uptake will seriously affect the mechanical properties and dimensional stability of PEM [53]. The water uptake curves and water

uptake values of the SPEEK membrane and the composite membrane at different temperatures are depicted in Fig. 12 (a) and Table 2, respectively. It can be intuitively seen from the figure that the water uptake content of all the membranes show a strong dependence on temperature. Specifically, for the SPEEK blank membrane, a terrible water uptake value is exhibited, which is as high as 692.54% at 80
C. This is attributed to the high degree of sulfonation of the polymer chain imparting a greater number of hydrophilic sites to the membrane matrix [54]. With the increase of the incorporation of H3PO4@CTFp, the water uptake curve of the composite membrane showed a gradual decline, which made the water uptake of the composite membrane at a reasonable level. For example, when the nano filler is added in an amount of 20% by weight, the water uptake value of the SPEEK/H3PO4@CTFp-20 composite membrane is only 103.93%, which shows similar water uptake value as other PEM substrates [55]. The reasons are as follows: First, we have obtained the loading of phosphate molecules (~21.65 wt%) in previous studies. Compared to the CTFp after loading phosphoric acid, there is no excess internal pores [eg Fig. 5 (b)], from which it can be inferred that in addition to the phosphate molecule, a large amount of water molecules are enriched in H3PO4@CTFp. The addition of H3PO4@CTFp converts the free water into bound water and stores it in the composite membrane in advance. Therefore, the composite membranes show a lower free water uptake. Secondly, due to the strong electrostatic interaction between the H3PO4@CTFp and the SPEEK interface, the size of the ion transport channel is narrowed, and the water uptake space of the SPEEK matrix is limited. Furthermore, the interaction of the phosphate and sulfonic acid groups replaces the interaction between the sulfonic acid groups and the water molecules [56]. The difference is that the SPEEK/CTFp-15 composite membrane exhibits a higher water absorption capacity at the same addition amount of

Fig. 12. Water uptake(a) and swelling ratio(b) curves for SPEEK blank membrane and composite membranes at different temperatures.

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15% by weight. For example, at 80
C, the water absorption of the SPEEK/CTFp-15 composite membrane is as high as 175.98%, which is 40.66% higher than that of the SPEEK/H3PO4@CTFp-15 composite membrane. This is because the purely CTFp framework is rich in a large number of internal pores, which can be used as a place to store water molecules. At the same time, the CTFp framework having a high specific surface area can adsorb a large amount of water molecules by capillary action. Furthermore, since water molecules stored in the pores are difficult to overflow, the CTFp framework achieves the function of converting free water into bound water for storage in the membrane (as mentioned in Section 3.5.). The dimensional stability of the membrane in water is an important indicator to ensure the long-term efficient operation of PEM. High swelling degree is unfavorable for the formation of proton transfer pathway and seriously affects the service life of PEM [57]. From the curve of the swelling ratio of the obtained membrane with temperature [Fig. 12 (b)], it can be seen that the change in the swelling ratio of the membrane exhibits the same regularity as the change in the water uptake rate of the membrane. It is not difficult to explain that as a large amount of water molecules penetrate into the membrane matrix, the spacing and free volume between the polymer chains will inevitably increase. Table 2 gives the specific values of the swelling ratio of the obtained membrane. Compared to the SPEEK membrane (swelling ratio up to 122.73% at 80
C), the addition of the H3PO4@CTFp network inhibits excessive swelling of the SPEEK matrix by limiting the mobility of the polymer chains. For example, the SPEEK/H3PO4@CTFp-20 composite membrane has a swelling ratio of only 30.5% even in an aqueous environment of 80
C. This is primarily due to the formation of an ionic crosslinking network in the polymer matrix by the covalent triazine backbone, which limits excessive swelling of the SPEEK matrix. Compared with the SPEEK/H3PO4@CTFp-15 composite membrane, the dimensional stability of the SPEEK/CTFp-15 composite membrane can be maintained at a low level while maintaining high water absorption (For example, at 80
C, at a higher water uptake of 175.98%, the swelling ratio is only 40.91%). On the one hand, the space provided by the pores inside the CTFp framework for the absorbed water molecules does not itself cause swelling of the membrane. On the other hand, since the CTFp framework is not loaded with phosphoric acid, the CTFp framework possesses more basic groups for forming an ion-crosslinking network with acidic groups on the SPEEK framework to inhibit the swelling behavior of the membrane. The above results indicate that moderate water uptake and dimensional stability are the main advantages of the SPEEK/H3PO4@CTFp composite membrane for fuel cells. 3.8. Proton conductivity and ion exchange capacity (IEC) of composite membranes The IEC value of the membrane reflects the information of the proton conduction sites from the side. The IEC values for all membranes are summarized in Table 2. Obviously, with the increase of the doping amount of H3PO4@CTFp, the IEC value of the composite membrane shows a decreasing trend. The IEC value of the SPEEK blank membrane has been tested to be 1.84 meq/g, which is consistent with its degree of sulfonation (DS) [54]. In comparison, the IEC value of the SPEEK/H3PO4@CTFp-X composite membrane is between 1.64 and 1.40 meq/g. The ion exchange capacity is generally considered to be related to the amount of ion exchange groups available in the membrane. A brief description is made through the following two points: First, the acid-base electrostatic interaction between the basic site in the CTFp network structure and the acidic group (including the phosphate group and the sulfonic acid group) limits the dissociation of Hþ from the acidic

group. Secondly, despite the introduction of phosphate groups in the composite membrane, the addition of CTFp loaded with a certain proportion of phosphoric acid in the membrane, to some extent, results in dilution of the sulfonic acid groups per unit mass of SPEEK. In addition, the IEC value is consistent with the WU capacity, confirming that the latter parameter is also closely related to the phosphate group contained in the polyelectrolyte [58]. Proton conductivity is one of the most important factors for PEM energy conversion in hydrogen fuel cells. Fig. 13 (a) shows the proton conductivity of membranes containing different phosphoric acid loadings measured at 100% RH but different temperatures. A clear trend is shown in the graph, that the proton conductivity of all membranes increases linearly with increasing temperature, indicating that proton conduction is thermally activated. The resulting composite membrane exhibited higher proton conductivity than the original SPEEK membrane over all test temperature ranges. For example, as the filler content is in the range of 0 wt% to 15 wt%, the proton conductivity tends to increase, and the highest proton conductivity of the SPEEK/H3PO4@CTFp-15 composite membrane at 80
C can reach 0.313 S/cm. Compared to the original membrane of SPEEK, the proton conductivity is ~0.139 S/cm, and its proton conductivity is increased by nearly 2.5 times. However, only high levels of fillers do not provide more conductivity values, and when the filler content increases to 20 wt%, the proton conductivity decreases slightly. It is inferred that this is due to the “blocking effect” produced by the H3PO4@CTFp filler aggregates prior to the strengthening of the membrane in a suitable amount of filler, which hinders the proton transfer pathway within the membrane [59,60]. All results indicate that the phosphate-loaded covalent triazine backbone is effective in enhancing the proton conductivity of PEM. This can be further explained by the following facts: (f1) In vehicle mechanisms, water molecules are often used as the primary carrier of PEM. The phosphate molecule is a new proton carrier due to its unique amphoteric effect. In addition, the phosphate molecule provides an additional new proton conduction site in combination with the sulfonic acid for the proton transport channel in the nanohybrid membrane [61]; (f2) In the previous tests, the CTFp porous framework exhibited good retention of H3PO4 molecules, and the high concentration of H3PO4 stored internally provided a hydrogen bond network within the membrane to provide a continuous transport path for proton transfer. A new transmission path with low energy barriers can provide higher electrical conductivity to the composite membrane by enhancing proton hopping; (f3) A large number of basic sites distributed in CTFp can form an acid-base pair transfer site with the eSO3H group on the molecular chain (e.g. -SO- … þHeHN- or -SO- … þHN]C Acid-base pair), which also plays an important role in enhancing proton conduction. This is also observed in other PEMs (such as filling imidazole to construct acid-base pairs) [62,63]. The proton conductivity of SPEEK/H3PO4@CTFp-15 and SPEEK/CTFp-15 is measured and compared. For proton conductivity at 80
C, 100% RH, the SPEEK/H3PO4@CTFp-15 composite membrane shows a higher conductivity, which is approximately 56.5% higher than that of the SPEEK/CTFp-15 composite membrane (0.200 S/cm). It can be said that the increase in the proton conductivity of SPEEK/H3PO4@CTFp15 is mainly attributed to the presence of H3PO4 molecules. To analyze the relationship between proton conduction and time in low humidity conditions, the time-dependent proton conductivity of the membrane at 40
C and 20% RH is tested, as shown in Fig. 13 (b). The strong dependence of proton transfer on the water content in the membrane results in a tendency for all proton conductivity of the membrane to decrease over time. It can be inferred that the loss of free water stored in the membrane inhibits the dissociation of the sulfonic acid groups and causes the proton conducting nanochannels to contract and break [64]. In general, the

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Fig. 13. (a) Proton conductivity of the SPEEK blank membrane and the SPEEK/H3PO4@CTFp-15 composite membrane as a function of temperature at 100% RH; (b) Time-dependent proton conduction of the membrane at 40
C and 20% RH; (c) Arrhenius plots of SPEEK, SPEEK/CTFp-15 and SPEEK/H3PO4@CTFp-15; (d) Arrhenius plots of different doping amounts of SPEEK/H3PO4@CTFp-X composite membranes.

proton conductivity of the SPEEK blank membrane decreased from the original 2.32  102 S/cm to 6.88  104 S/cm (a reduction of 97.0%) in 30 min. The addition of H3PO4@CTFp can effectively inhibit the decrease of proton conductivity. When SPEEK is mixed with 5 wt%, 10 wt%, 15 wt%, and 20 wt% fillers, the proton conductivity decreases by 85.8% (from 5.63  102 S/cm to 8.01  103 S/cm), 84.8% (from 6  102 S/cm to 9.10  103 S/cm), 76.8% (from 6.13  102 S/cm to 1.42  102 S/cm) and 83.4% (from 5.82  102 S/cm to 9.69  103 S/cm), respectively. The proton conductivity reduction curve of the hybrid membrane appears smoother and more stable. These results are illustrated by the following explanations: (1) Since the water evaporates at a low humidity, the continuous water network for proton transfer existing inside the hydrated membrane is gradually destroyed, thus the proton conductivity of the original SPEEK membrane sharply drops at a low humidity [65,66]. The water molecules stored in the pores in the H3PO4@CTFp network structure can compensate for the lost water molecules to maintain a continuous water network structure, ensuring that the hybrid membranes have higher conductivity even at low humidity; (2) Generally speaking, water acts as a proton carrier for most PEMs, however, the reduction in the number of carriers under low humidity conditions will seriously affect the transport of protons. For hybrid membranes, the incorporation of phosphoric acid provide new carriers for proton transfer. In addition, the hydrogen bond network structure formed by the phosphate molecule has a lower proton transfer energy barrier and is not destroyed at low humidity [67], which effectively ensures proton transfer through the Grotthus mechanism. It can be observed that the incorporation of a pure CTFp framework is also

effective in mitigating the decay of proton conductivity of the membrane at low humidity. For example, the SPEEK/CTFp-15 composite membrane is reduced from the original 3.35  102 S/ cm to 4.83  103 S/cm (reduced by 85.6%) within 30 min. As previously mentioned (described in Section 3.5), this can be attributed to the fact that the bound water stored in the CTFp network framework can continue to maintain continuous water networks structure for proton transport. In general, phosphoric acid is more suitable as an electrolyte and proton carrier for PEMFC to maintain high proton conductivity of the membrane at low RH. Activation energy (Ea) is usually defined as the minimum energy required for proton conduction. In general, a low energy barrier proton transfer channel is expected to exhibit a lower Ea value. We calculated the activation energy (Ea) of the membrane by linear fitting of the Arrhenius plots and are shown in Fig. 13 and Table 3. As can be seen from Fig. 13(c), after the CTFp framework was incorporated, the Ea value of the SPEEK/CTFp-15 composite membrane was reduced from the Ea value (28.09 kJ mol1) of the original SPEEK membrane to 27.21 kJ mol1. For the SPEEK/ H3PO4@CTFp-15 composite membrane, a lower Ea value (24.19 kJ mol1) is shown. The reduced activation energy can be explained as follows: (a) The addition of the CTFp framework. Due to the hydrophilicity of the eNHe and eSO3H groups in the membrane, water molecules tend to form intermediate hydrogenbonded water clusters between acidic sites and basic sites [68]. Protons are easily transferred from a cluster of water molecules on the SPEEK polymer chain to CTFp, which will build a low energy barrier for proton transfer. (b) For H3PO4@CTFp, the addition of a phosphate group can further enhance the acidity of the SPEEK/

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Table 3 Proton conductivity and activation energy of SPEEK blank and composite membranes. Sample

Activation energy (KJ/mol)

SPEEK SPEEK/H3PO4@CTFp-5 SPEEK/H3PO4@CTFp-10 SPEEK/H3PO4@CTFp-15 SPEEK/H3PO4@CTFp-20 SPEEK/CTFp-15

28.09 23.45 23.69 24.19 25.54 27.21

Proton conductivity (S/cm) 25
C

80
C

0.0232 0.0563 0.0600 0.0613 0.0582 0.0343

0.139 0.262 0.280 0.313 0.281 0.200

H3PO4@CTFp-15 composite membrane to facilitate proton transport. In addition, due to the high hydrogen bonding energy of the phosphate group, the phosphoric acid has a strong interaction with water molecules and sulfonic acid groups, which helps to establish a dynamic hydrogen bond network through the Grotthuss mechanism and promote the proton conductivity [69,70]. Therefore, the H3PO4@CTFp framework can establish a lower energy transfer channel than the pure CTFp framework. Fig. 13(d) shows the Ea values for different additions of the SPEEK/H3PO4@CTFp composite membranes. It can be seen that the activation energy required for an increase in the amount of incorporation is slightly increased. Part of the reason can be summarized as: The channel in H3PO4@CTFp becomes narrow due to the occupancy of more H3PO4 þ molecules. Proton transport of larger species (e.g. H5Oþ 2 , H9O4 , etc.) contained in the channel requires more energy [71]. In order to explain the high proton conductivity exhibited by the composite membrane more vividly, the mechanism of proton transfer in the composite membrane is presumed, as specifically described in Fig. 14. The carrier mechanism and the Grotthuss mechanism are involved in the proton transfer in the prepared membrane [72]. in terms of the Grothuss mechanism, a large number of triazine rings and imino groups distributed in the CTFp network act as basic sites to form acid-base pairs with acidic sites on the SPEEK backbone, providing a low energy barrier pathway for proton transfer. Protons can easily jump between the filler and the matrix. Later, the high concentration of phosphate molecules remaining in the internal channels of H3PO4@CTFp can form a continuous hydrogen bond network structure, and promote the efficient conduction of protons by constructing a new proton transfer pathway. In addition, for carrier-type transfer, protons

Fig. 15. Single PEMFC performance of SPEEK blank membrane, SPEEK/CTFp-15 and SPEEK/H3PO4@CTFp-15 with hydrated H2/O2 at 30%, 65
C.

react with water to form hydronium ions, which then diffuse through the membrane through free-volume cavities in the membrane. Since the diameter of the water molecules (~0.4 nm) is smaller than the window diameter of the CTFp network, not only the H3PO4 molecules are stored in the pores, but also a large number of water molecules clusters are enriched inside. The ordered porous structure interconnects the clusters of water molecules to form a hydration transfer channel, and the protons along the CTFp network can help the ions to diffuse. It is not difficult to understand that the addition of the H3PO4@CTFp network has significantly enhanced the transmission of protons in the membrane.

3.9. Single cell performance tests of composite membranes In the above test, the SPEEK/H3PO4@CTFp-15 composite membrane exhibited relatively good proton conductivity, and in order to further verify its cell performance, it can be compared with SPEEK/ CTFp-15 and SPEEK membranes to prepare MEA. The single cell performance is tested with hydrous H2/O2 gases at 65
C, 30% RH. The polarization and power density curves are shown in Fig. 15 and

Fig. 14. It is speculated that H3PO4@CTFp plays a role in enhancing the proton transfer mechanism in the composite membrane.

Please cite this article as: X. Sun et al., Phosphoric acid-loaded covalent triazine framework for enhanced the proton conductivity of the proton exchange membrane, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135235

X. Sun et al. / Electrochimica Acta xxx (xxxx) xxx

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Table 4 Single cell performance parameters of SPEEK blank and composite membranes. Sample

SPEEK SPEEK/H3PO4@CTFp-15 SPEEK/CTFp-15

Maximum Power Density (W cm2)

Open Circuit Potential (V)

Maximum Current density (mA cm2)

65
C,30% RH

65
C,30% RH

65
C,30% RH

192.9 325.8 229.2

0.691 0.762 0.775

1206.1 1899.8 1386.5

Table 4. The open-cell voltage (OCV) of the cell made with SPEEK/ H3PO4@CTFp-15 composite membrane (0.762 V) and SPEEK/CTFp15 composite membrane (0.775 V) are higher than that of the cell with the SPEEK membrane (0.691 V). This means that the incorporation of the covalent triazine framework prolongs the path of hydrogen permeation and the composite membrane does not exhibit significant gas permeability. At the same time, under the condition of 30% RH, the maximum power density and maximum current density of the cell with SPEEK/H3PO4@CTFp-15 composite membrane are 325.8 mW cm2 and 1899.8 mA cm2, which are 68.9% and 57.5% higher than the cell with SPEEK blank membrane, respectively. In addition, the maximum power density and maximum current density of the SPEEK/CTFp-15 membrane are also 18.8% and 15% higher than the SPEEK blank membrane. The results show that the incorporation of the H3PO4@CTFp network framework can significantly improve the performance of the fuel cell. The high acid retention of CTFp is critical for PA-doped PEMs used in hydrogen fuel cells. 4. Conclusion In this work, we used the VAM method to immobilize phosphoric acid into the covalent triazine network structure, since the diameter of the phosphate molecule is smaller than the window diameter of the internal pores of the CTFp network and the electrostatic attraction of phosphoric acid and internal basic sites. The prepared H3PO4@CTFp exhibited a low guest molecular leak rate. Subsequently, the obtained H3PO4@CTFp was embedded as a novel filler into the SPEEK matrix to prepare a composite membrane. Due to the electrostatic interaction and hydrogen bonding of H3PO4@CTFp and SPEEK matrix, it shows good interfacial compatibility. The incorporation of the H3PO4@CTFp network enhances the hopping mechanism of protons in the membrane. The acidic-PO3H2 group in H3PO4@CTFp provides a rich proton transfer site through which the phosphate molecules form a hydrogen bond network structure. A new proton transfer channel is constructed to impart higher proton conductivity to the composite membrane. For example, the SPEEK/H3PO4@CTFp-15 composite membrane has a proton conductivity of up to 0.313 S/cm (at 80
C, 100% RH), which is nearly 2.5 times higher than the proton conductivity of the original SPEEK membrane. In addition, the mechanical properties of the membrane and the dimensional stability in water are improved. This study shows that the developed covalent triazine porous organic framework can be used as a new multifunctional material for immobilizing phosphoric acid, showing great potential in PEMFC. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Phosphoric acid-loaded

covalent triazine framework for enhanced the proton conductivity of the proton exchange membrane “. Acknowledgments The authors acknowledge the financial support of the College of Science and Technology Research Project of Hebei Provincial, China (ZD2018044). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135235. References [1] D.T.N. Minh, H.S. Dang, D. Kim, Proton exchange membranes based on sulfonated poly(arylene ether ketone) containing triazole group for enhanced proton conductivity, J. Membr. Sci. 496 (2015) 13e20. [2] S.J. Peighambardoust, S. Rowshanzamir, M. Amjadi, Review of the proton exchange membranes for fuel cell applications, Int. J. Hydrogen Energy 35 (2010) 9349e9384. [3] S. Bose, T. Kuila, T.X.L. Nguyen, N.H. Kim, K.T. Lau, J.H. Lee, Polymer membranes for high temperature proton exchange membrane fuel cell: recent advances and challenges, Prog. Polym. Sci. 36 (2011) 813e843. [4] H.W. Zhang, P.K. Shen, Advances in the high performance polymer electrolyte membranes for fuel cells, Chem. Soc. Rev. 41 (2012) 2382e2394. [5] M.M. Nasef, Radiation-Grafted membranes for polymer electrolyte fuel cells: current trends and future directions, Chem. Rev. 114 (2014) 12278e12329. [6] A.K. Mishra, S. Bose, N.H. Kim, J.H. Lee, Silicate-based polymer-nanocomposite membranes for polymer electrolyte membrane fuel cells, Prog. Polym. Sci. 37 (2012) 842e869. [7] H.W. Zhang, P.K. Shen, Recent development of polymer electrolyte membranes for fuel cells, Chem. Rev. 112 (2012) 2780e2832. [8] C.H. Park, S.Y. Lee, D.S. Hwang, D.W. Shin, D.H. Cho, K.H. Lee, T.W. Kim, T.W. Kim, M. Lee, D.S. Kim, C.M. Doherty, A.W. Thornton, A.J. Hill, M.D. Guiver, Y.M. Lee, Nanocrack-regulated self-humidifying membranes, Nature 532 (2016) 480e483. [9] S.I. Lee, K.H. Yoon, M. Song, H. Peng, K.A. Page, C.L. Soles, D.Y. Yoon, Structure and properties of polymer electrolyte membranes containing phosphonic acids for anhydrous fuel cells, Chem. Mater. 24 (2012) 115e122. [10] C.H. Park, C.H. Lee, M.D. Guiver, Y.M. Lee, Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs), Prog. Polym. Sci. 36 (2011) 1443e1498. [11] S. Subianto, M.K. Mistry, N.R. Choudhury, N.K. Dutta, R. Knout, Composite polymer electrolyte containing ionic liquid and functionalized polyhedral oligomeric silsesquioxanes for anhydrous PEM applications, ACS Appl. Mater. Interfaces 1 (2009) 1173e1182. [12] L. Vilciauskas, S.J. Paddison, K.D. Kreuer, Ab initio modeling of proton transfer in phosphoric acid clusters, J. Phys. Chem. A 113 (2009) 9193e9201. [13] G.W. He, L.L. Nie, X. Han, H. Dong, Y.F. Li, H. Wu, X.Y. He, J.B. Hu, Z.Y. Jiang, Constructing facile proton-conduction pathway within sulfonated poly(ether ether ketone) membrane by incorporating poly(phosphonic acid)/silica nanotubes, J. Power Sources 259 (2014) 203e212. [14] M. Schuster, T. Rager, A. Noda, K.D. Kreuer, J. Maier, About the choice of the protogenic group in PEM separator materials for intermediate temperature, low humidity operation: a critical comparison of sulfonic acid, phosphonic acid and imidazole functionalized model compounds, Fuel Cells 5 (2005) 355e365. [15] H. Steininger, M. Schuster, K.D. Kreuer, A. Kaltbeitzel, B. Bingoel, W.H. Meyer, S. Schauff, G. Brunklaus, J. Maier, H.W. Spiess, Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group: a progress report, Phys. Chem. Chem. Phys. 9 (2007) 1764e1773. gues, K.D. Kreuer, D. Rodriguez, Proton conductivity in [16] T. Dippel, J.C. Lasse fused phosphoric acid; A 1H/31P PFG-NMR and QNS study, Solid State Ion. 61 (1993) 41e46. [17] Q. Qin, Q.W. Tang, B.L. He, H.Y. Chen, S.S. Yuan, X. Wang, Enhanced proton conductivity from phosphoric acid-incorporated 3D polyacrylamide-graft-

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Please cite this article as: X. Sun et al., Phosphoric acid-loaded covalent triazine framework for enhanced the proton conductivity of the proton exchange membrane, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135235