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Conjugated Microporous Polymers with Dense Sulfonic Acid Groups as Efficient Proton Conductors Si-Jie Yang, Xuesong Ding, and Bao-Hang Han Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00926 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Conjugated Microporous Polymers with Dense Sulfonic Acid Groups as Efficient Proton Conductors

Si-Jie Yang,†,‡ Xuesong Ding,*,† Bao-Hang Han*,†,‡

† CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China

1 ACS Paragon Plus Environment

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ABSTRACT: Proton exchange membrane fuel cells (PEMFC), emerging as green and sustainable energy sources, have attracted extensive attention in recent decades. Porous organic polymers (POPs), which feature in high surface area values, tunable pore sizes, excellent thermal and chemical stabilities, and the flexibility to incorporate specific functional groups, have recently displayed their striking images as potential electrolytes for fuel cells. In this work, BO-CMP-1 and BO-CMP-2 that possess rich π-structure and permanent porosity and have high thermal and chemical stability were synthesized through Suzuki–Miyaura coupling reaction. Owing to their rigid structures and abundant electrophilic substitution positions, these two novel porous polymers were covalently decorated with dense sulfonic acid groups by post-sulfonation, as denoted by SBO-CMP-1 and SBO-CMP-2.

The proton conductivity of SBO-CMPs is

systematically studied to evaluate their performance as proton-conductive materials. It was found that their performance is highly humidity- and temperature-dependent and they show relatively high proton conductivity. For SBO-CMP-1 and SBO-CMP-2, their proton conductivity is 1.29 × 10–2 and 5.21 × 10–3 S cm–1, respectively at 70 °C and 100% relative humidity (RH). Low activation energy values of 0.32 eV for SBO-CMP-1 and 0.40 eV for SBO-CMP-2 suggest the Grotthuss mechanism for proton conduction.

KEYWORDS: Proton Conduction; Proton Exchange Membrane; Porous Organic Polymers; Post-Sulfonation


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INTRODUCTION In recent decades, people are more concerning about energy problems because of the shortage of natural resources like fossil fuels or the environmental pollution caused by the consumption of these energies. The fuel cells, as a new kind of energy transformation device, have been considered as a green and sustainable energy source, in which proton exchange membrane fuel cells (PEMFC) are widely studied and used in a broad range of applications.1 Fuel cells’ performance is to large extent determined by the properties of the proton exchange membranes. The electrolyte membranes, commonly should have, high efficiency of proton mobility but low electronic conductivity, and excellent chemical and physical stability under fuel cell operating condition to ensure long lifetime.2 Various inorganic or composite materials have been explored to be employed as the electrolyte membrane candidates.3,4 Among those materials, metal–organic frameworks (MOFs) have been mostly studied in recent years due to its rationally designable structure and post-functionalizable property. Examples demonstrated that by incorporating acid–base groups or impregnating proton carriers (imidazole and triazole) into their frameworks, MOFs present competitive proton transfer ability to the commercialized Nafion.5,6,7 As possessing the similar ordered structure of MOFs, covalent organic frameworks (COFs), have also been explored to be proton-conducting materials recently.8,9,10,11,12,13,14 However, most MOFs and COFs suffer from instability in harsh conditions, thus impeding the post-synthetic modulation to incorporate the frameworks with strong acid groups directly or hindering the


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long-term use under moisture conditions. Recently, some research groups have put their attention to transfer the proton conduction concept to more robust porous organic polymers (POPs),15 which composed entirely of light elements and covalent bonds. Up to now, several studies have achieved great start-up on POPs-based proton electrolyte by impregnating proton donor within the skeletons of POPs or directly decorating acidic groups to realize high proton conduction.16,17,18,19,20 Conjugated microporous polymers (CMPs), as a category of POPs featuring conjugated structure, have been widely studied.21,22,23,24,25 CMPs can perform as promising platforms for gas sorption and separation,26,27,28 encapsulation of solvents or any other organic chemicals,29,30 catalysis,31,32,33 photo-electronics,34,35 and chemosensing.36,37 Because of their conjugated structure and aromatic property, CMPs exhibit long-term stability under harsh conditions, like high humidity, high temperature, acid or alkali environment, which are superior to most MOFs or COFs-based materials. Meanwhile, CMPs possess rich channels and pores inside their networks. Inspired by the previous works, we herein adopt the proton conduction concept to CMPs. We employed two biolefin substituted benzoquinone-based CMPs (BO-CMPs), which were reported by our group recently,38 as proton conductor’s precursors. Due to the abundant aromatic rings in their skeletons, these two polymers are incorporated with dense sulfonic acid groups by post-sulfonation. This simple sulfonation process endows the obtained polymers with high water affinity and they are chemical and physical stable as well. The intrinsic proton conductivity of these sulfonated CMPs operated in


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various humidity and temperature working conditions were systematically measured to evaluate their performance as proton-conductive materials.

EXPERIMENTAL SECTION Materials 1,4-Phenylenediboronic acid and 4,4'-biphenyldiboronic acid were purchased from Macklin. Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) and chlorosulfonic acid were purchased from Energy Chemical. Potassium carbonate (K2CO3), 1,4-dioxane, tetrahydrofuran (THF), dichloromethane (DCM), and other commonly used organic solvents were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. The monomer 9,10-bis(dibromomethylene)-9,10-dihydroanthracene (BBMA) was synthesized according to the reported procedure39 and the BO-CMPs were prepared according to the method we reported before.38 Preparation of SBO-CMPs BO-CMP-1 (200 mg) was suspended in 12 mL of DCM, then the mixture was stirred for 1 h before 2.5 mL chlorosulfonic acid was added dropwise with reaction bottles placed in an ice water bath. This suspension was stirred continuously for 4 days at room temperature. Then, the mixture was poured into 1 L of ice water, stirring for 6 h. The precipitate was collected by filtration and washed with water and methanol. Finally, 266 mg of brownish black powder was obtained after dried under vacuum. The SBO-CMP-2 was prepared following the same procedure as that of SBO-CMP-1. BO-CMP-2 (200


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mg) was suspended in 12 mL of DCM, then treated with 2.5 mL chlorosulfonic acid, and finally obtained as a dark green solid (220 mg). Proton Conductivity Measurements To investigate their proton conductivity, the materials were finely ground, and then pressed in the form of films of 0.05–0.13 cm thickness and 13 mm of diameter under the pressure of 12 MPa. The obtained pellets were equipped with two home-made Pt blocking electrodes with two platinum wires connecting the outside. Before measurement, the hand-made cell was placed in a humidity control chamber made by our own for at least 24 hours. Several RH of 43%, 53%, 75%, 80%, 97%, and 100% were realized by using different saturated aqueous salt solutions of K2CO3, Mg(NO3)2·6H2O, NaCl, KCl, K2SO4, and pure water. The Nyquist plots were collected from Autolab-302 electrical chemical workstation with frequency response analyzer (FRA) (Metrohm AG, Switzerland), applying AC voltage amplitude of 10 mV and frequency range from 1 MHz to 10 Hz. The obtained Nyquist plots were fitted using Z-View software and the resistance values were extrapolated from equivalent circuit simulation, while the proton conductivity values were obtained with the following equation.

where L and A stands for the thickness and areas of the pellets respectively and R is the resistance of the pellets. The activation energy (Ea) for the materials was estimated from the following equation.


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where σ is the proton conductivity, σ0 is the preexponential factor, kB is the Boltzmann constant, and T is the temperature. To acquire the conductivity at diverse temperature, we equilibrated all the sample pellets at 100% RH for over 24 h, and then transferred to the oven with target temperature and equilibrated for at least 2 h before measurements.

RESULTS AND DISCUSSION BO-CMPs were prepared through a [4 + 2] polycondensation process depicted in Scheme 1. 1,4-Phenylenediboronic acid (M1) and 4,4'-biphenyldiboronic acid (M2) were

reacted

with

tetrabromobisolefin

monomer

9,10-bis(dibromomethylene)-9,10-dihydroanthracene (BBMA), respectively, to obtain BO-CMP-1 and BO-CMP-2 through one-step Suzuki–Miyaura reaction. The as-prepared

materials are

insoluble in

common

organic

solvents

such as

N,N-dimethylformamide (DMF), dichloromethane (DCM), tetrahydrofuran (THF), and remain stable even exposed to acid and alkali solutions. The thermal gravimetric analysis (TGA) reveals that BO-CMPs have no obvious weight loss before 500 °C. All the results suggest the polymers are cross-linked and have high chemical and thermal stability. The BO-CMPs were characterized by Fourier transform infrared spectroscopy (FT-IR),

13

C solid-state NMR spectroscopy, and elemental analysis (shown in Figures

1a, S1, S2 and Table S1, Supporting Information). The N2 sorption isotherms at 77 K of BO-CMP-1 and BO-CMP-2 exhibit a combination of type I and type IV curves


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according to IUPAC classification,40 indicating the presence of permanent micropores and mesopores in the polymers. The Brunauer–Emmett–Teller (BET) specific surface area value of BO-CMP-2 with longer repeating phenyl units is 1030 m2 g–1, while the value is 440 m2 g–1 for BO-CMP-1 with shorter repeating units.

Scheme 1. The preparation process of (a) BO-CMPs and (b) SBO-CMPs. The sulfonation process was accomplished through electrophilic substitution by chlorosulfonic acid at room temperature. This transformation process lasted for 4 days and accompanied by an apparent color change from red to dark brown for BO-CMP-1 and yellow to dark green for BO-CMP-2. The chemical structures of sulfonated


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polymers were characterized by FT-IR spectroscopy measurement (Figures 1a and S1, Supporting Information). In addition to the stretching bands at 2910–3050 and 1620 cm–1 originated from the aromatic C–H and aromatic C–C vibration modes, respectively, new strong peaks appear at 1180 and 1376 cm–1 for SBO-CMP-1 and 1174 and 1375 cm–1 for SBO-CMP-2 after sulfonation, which are assigned to S=O=S symmetric and asymmetric stretching modes,41 demonstrating the successful introduction of sulfonic acid groups. The detailed information about sulfur existed within the skeletons was given by elemental analysis, X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) spectra (Figures 1b, S3, S4 and S5, Table S1, Supporting Information). The signals at 169.6 and 234.0 eV are assigned to be S 2p and S 2s, showing the consistent result with FT-IR as well as EDX data. The narrow scan spectrum in the S2p region of SBO-CMPs were measured to probe the oxidation state of sulfur (Figure S4, Supporting Information). The asymmetrical S2p was split into two peaks at 168.6 and 169.1 eV corresponding to the binding energies of the S2p3/2 and S2p1/2 state in –SO3H, respectively. The EDX-mapping reveals that sulfur element is evenly dispersed within these polymers. The content of accessible sulfonic acid groups in SBO-CMPs were further estimated by acid-base titration, which showed 27.6 wt% and 23.9 wt% –SO3H of SBO-CMP-1 and SBO-CMP-2, respectively. The TGA curves (Figure S6, Supporting Information) for SBO-CMP-1 and SBO-CMP-2 exhibit loss in thermal stability when compared with the parent polymers, which is derived from the introduction of active sulfonic acid groups.


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Figure 1 (a) FT-IR spectra of monomer (blue), BO-CMP-1 (black), and SBO-CMP 1 (red). (b) XPS spectrum of SBO-CMP-1. The 13C NMR spectra of the sulfonated polymers don’t change greatly, except for a slight shift of the signals to low field (Figure S2, Supporting Information), which may due to the electron-withdrawing effects of sulfonic acid groups. The powder X-ray diffraction patterns represent broad peaks that verify the polymers have no long-distance order before and after sulfonation (Figure S7, Supporting Information). SEM images reveal that BO-CMP-1 and BO-CMP-2’s morphological difference is negligible and the morphology remains after sulfonation (Figure S8, Supporting Information). Porosities and Water Capture Properties. To evaluate the porosity of sulfonated materials, nitrogen sorption isotherm measurements at 77 K were performed on these samples. The nitrogen sorption curves are shown in Figure 2a and the detailed data about porous properties are listed in Table S2 (Supporting Information). It reveals that after sulfonation, the BET specific surface area values of both the materials reduce significantly, to 40 and 250 m2 g–1 for


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SBO-CMP-1 and SBO-CMP-2, respectively. That means the pores inside SBO-CMP-1 are almost filled with the sulfonic acid groups, while SBO-CMP-2 remain part of its micropores. This difference may result from the variety of pore volumes of these two parent polymers for BO-CMP-2 (pore volume of 0.72 cm3 g–1), more than twice as much as that of BO-CMP-1. Consistent results are also found in the pore size distribution of BO-CMPs and SBO-CMPs calculated by the nonlocal density functional theory (NLDFT) method (Figure 2b). It shows that pores with pore size below 2 nm are predominant in BO-CMPs. After sulfonation, SBO-CMP-1’s porosity almost disappears while SBO-CMP-2 still has pores with size below 2 nm. For a humidity mediated proton transport system, in order to realize efficient proton conduction, a material should have good ability to incorporate as much water as possible so that an extended hydrogen-bonded network can be established. The water vapor sorption isotherms of BO-CMPs and SBO-CMPs were measured at 298 K (Figures 2c and 2d). BO-CMP-1 and BO-CMP-2 exhibit nearly trace uptake at the low relative pressure, and the uptake only begins at high relative pressure owing to the affinity between the adsorbed water molecules and the vapor. In contrast, high water uptakes are achieved for SBO-CMPs and the total adsorbed water can reach 42 and 31 wt% of their own weight at P/P0 = 0.95 for SBO-CMP-1 and SBO-CMP-2, respectively, indicating good affinity between water molecules and the SBO-CMPs’ frameworks. Moreover, the water uptake of SBO-CMP-1 is much higher than that of SBO-CMP-2 although it has smaller BET specific surface area and pore volume values, which may


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attribute to its larger content of sulfonic acid groups.

Figure 2 (a) Nitrogen sorption isotherms of BO-CMPs and SBO-CMPs. (b) Pore size distribution profiles for polymers calculated using the NLDFT method. (c) Water vapor sorption isotherms of BO-CMP-1 and SBO-CMP-1 at 298 K. (d) Water vapor sorption isotherms of BO-CMP-2 and SBO-CMP-2 at 298 K. Proton Conductive Properties. The high capacity of water and the presence of strong Brønsted acid groups within the pores indicate the possible utility of SBO-CMPs as water-mediated proton conducting materials. In order to characterize the proton-conducting performance, alternating-current (AC) impedance method was used to measure the pelletized samples. We firstly investigated the conductivity dependence on temperature by testing the materials from 30 to 70 °C under 100% relative humidity (RH). The Nyquist plots are


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shown in Figures 3a and S10 (Supporting Information). The impedance of these materials decreases as the temperature growing, and both BO-CMP-1 and BO-CMP-2 exhibit low proton conductivity. With rising temperature, the conductivity grows slightly from 2.88 × 10–7 and 5.15 × 10–7 S cm–1 at 30 °C to 2.72 × 10–6 and 4.68 × 10–6 S cm–1 at 70 °C, respectively. It is noted that after sulfonation, SBO-CMP-1 that is endowed with pendant sulfonic acid groups show proton conductivity of 4.15 × 10–3 S cm–1 at 30 °C, which increases to 4.64 × 10–3 when the temperature turns to 50 °C. Upon further rise in temperature to 70 °C, the conductivity increases greatly and reaches to 1.29 × 10–2 S cm–1, which is about four orders of magnitudes greater than that of BO-CMP-1 under the same conditions. For SBO-CMP-2, its conductivity rises from 1.16 × 10–3 S cm–1 at 30 °C steadily to 5.21 × 10–3 S cm–1 at 70 °C, which is three orders higher than that of the parent BO-CMP-2. Although the conductivity of SBO-CMP-1 is one order of magnitude less than the commercialized Nafion® 112 (0.14 S cm–1, 65 °C, 100%),42 Nafion® 211 (0.15 S cm–1, 80 °C, water),43 and Nafion® 117 (0.15 S cm–1),44 it is comparable to or overmatch many well performed polymers with sulfonic acid groups, such as SPFEK (5.37 × 10–3 S cm–1, 80 °C, water), SPAE (1.02 × 10–2 S cm–1, 80 °C, water), and Nafion® 115 (2.03 × 10–2 S cm–1, 80 °C, water).45 The proton conductivity results of other materials were also listed in Table S3 (Supporting Information) for comparison.


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Figure 3 (a) The Nyquist plots of SBO-CMP-1 at various temperature under 100% RH. (b) The proton conductivity of SBO-CMP-1 and BO-CMP-1 measured under 100% RH and at different temperature. To get an insight into the proton transport mechanism, we calculated the activation energy Ea of the parent and sulfonated materials from Arrhenius plots collected at different temperatures (Figures 4a and 4b). There are two principle mechanisms relating to proton transport process, namely, Grotthuss and vehicular mechanisms.4,7,46 Usually, Grotthuss mechanism possesses an activation energy less than 0.4 eV, where protons migrate within a water established hydrogen-bonded network. In contrast, vehicular mechanism that involves proton transportation process via proton carriers (H2O, NH3) refers to a higher activation energy usually larger than 0.4 eV. Here, the activation energy calculated for BO-CMP-1 and BO-CMP-2 are 0.48 and 0.56 eV, respectively, indicating a “vehicle” proton transport mechanism. This result is easy to understand because BO-CMP-1 and BO-CMP-2 are pure hydrocarbon organic frameworks without any functional groups, thus the proton migrates as OH3+ bonded to the water “vehicle”. The activation energy of SBO-CMP-1 and SBO-CMP-2


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are 0.32 and 0.40 eV, suggesting Grottuss proton transport mechanism. Therefore, the protons are transported by a hooping pathway through extended hydrogen-bonding networks, which largely facilitates the proton transfer efficiency compared with the parent polymers’ migration way.47,48

Figure 4 (a) Arrhenius plots for BO-CMP-1 and SBO-CMP-1. (b) Arrhenius plots for BO-CMP-2 and SBO-CMP-2. We then performed the impedance measurement under different RH conditions at 25 °C (Figure S11, Supporting Information). Several RH 43%, 53%, 75%, 80%, 97%, and 100% were realized by using home-made chamber containing different saturated aqueous salt solutions of K2CO3, Mg(NO3)2·6H2O, NaCl, KCl, K2SO4, and pure water. Before measurement, the home-made cell was placed in the humidity control chamber for at least 24 hours. At the low RH of 43%, the conductivity of SBO-CMP-1 is calculated to be 2.35 × 10–7 S cm–1, and as the RH growing to 100%, the conductivity increases to 4.23 × 10–3 S cm–1 (Figure 5a). As for SBO-CMP-2, the conductivity increases from 1.91 × 10–7 S cm–1 at 43% RH to 1.57 × 10–3 S cm–1 at 100% RH (Figure 5b). This phenomenon was also observed in many water-mediated proton


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conductors.16,49,50 The gradual increase in the low RH range and large increase in the range of high RH confirm that the proton conductivity of these materials is strongly dependent on water uptake. The swelling in the thickness was observed after measurement, the pellet swells by 20% and 15% in thickness for SBO-CMP-1 and SBO-CMP-2, respectively, which is common for proton conducting membranes after hydration due to the huge water uptake.

Figure 5 The proton conductivity of (a) SBO-CMP-1 and (b) SBO-CMP-2 measured under different RH at 25 °C. It is noted that the proton conductivity of SBO-CMP-1 is 2.5 times as high as that of SBO-CMP-2, and the fact is that SBO-CMP-1 has less pore volume but higher water uptakes than SBO-CMP-2. Considering the higher sulfonic acid content within SBO-CMP-1 network, we assume that compared with the existence of permanent channels for water molecule or proton transportation, the formation of infinite hydrogen bonding networks established by water and sulfonic acid interaction can provide a more efficient pathway for proton transportation.


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CONCLUSIONS In summary, we have prepared two BO-CMPs that could be efficiently sulfonated through post-modulation process due to its abundant π-structure and high chemical stability derived from rigid backbones. The proton conductivity of SBO-CMP-1 and SBO-CMP-2 are significantly enhanced as compared with their parent materials, and their proton transporting performances are highly humidity- and temperature-dependent. SBO-CMP-1 achieves high conductivity of 1.29 × 10–2 S cm–1 at 70 °C and 100% RH, which is comparable with many MOFs-based and POPs-based proton conducting materials, indicating the CMPs are potentially applicable as membrane materials in PEMFCs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org/. Details of experimental conditions for preparing BO-CMPs and SBO-CMPs, corresponding porosity data, SEM images, PXRD patterns, TGA curves, solid-state 13

C/CP-MAS NMR spectra, XPS and EDX spectra, and the corresponding data of

proton conductivity.


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AUTHOR INFORMATION Corresponding Authors *Phone: +86 10 8254 5708. Email: [email protected] *Phone: +86 10 8254 5576. Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The financial support of the National Science Foundation of China (Grants No. 21674026 and 21474027), and the Ministry of Science and Technology of China (Grant No. 2014CB932200) is acknowledged.

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For Table of Contents Only

SBO-CMPs are employed as proton conducting materials via post-sulfonation, and SBO-CMP-1 achieves a remarkable conductivity of 1.29 × 10–2 S cm–1.


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