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Highly conductive anion exchange membranes based on one-step benzylation modification of poly(ether ether ketone)
Journal of Membrane Science 574 (2019) 205–211
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Highly conductive anion exchange membranes based on one-step benzylation modification of poly(ether ether ketone) Zhenghui Zhanga,
⁎,1
, Xinle Xiaob,1, Xin Yana, Xian Liangb, Liang Wub,
T
⁎
a
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Wolong Street 1638, Nanyang, Henan 473061, PR China CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Centre of Chemistry for Energy Materials, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Anion exchange membrane Benzylation Fuel cell Poly(ether ether ketone) Side chain type
Side-chain-type quaternized aromatic polyelectrolytes named QBz-PEEK-x have been synthesized via one-step benzylation modification of commercial poly(ether ether ketone) (PEEK). By controlling the molar ratio between the benzylation reagent and PEEK, the substitution degree x could be tuned in a wide range (51.1%~ 91.6%). A series of five anion exchange membranes (AEMs) with high mechanical strength were facilely prepared via solution-casting of QBz-PEEK-x. Water uptake, swelling ratio and ion conductivity of these AEMs were investigated in detail. Of particular interest is that, at 60 °C, the AEM of QBz-PEEK-76.0% exhibited a moderate water uptake of 56%, a low swelling ratio of 15%, and an exceptionally high OH– conductivity of 155 mS cm−1 which was comparable to the top values ever reported. Moreover, a promising peak power density of 391 mW cm−2 was achieved when assembling QBz-PEEK-76.0% AEM in a H2/O2 single cell operated at 70 °C. This unprecedented “benzylation” method opens a new door to novel high performance side-chain-type AEMs.
1. Introduction With the increasing interest in the applications of anion-exchange membranes (AEMs) in electrochemical energy conversion and storage systems such as fuel cells and water electrolyzers, AEMs with high conductivity and good stability under alkaline/oxidative operational conditions are in need [1,2]. In comparison with traditional AEMs base on polyvinyls, AEMs based on aromatic polymers such as poly(phenylene) [3,4], poly(phenylene oxide) (PPO) [5–8], poly(ether sufone)s (PESs) [9–16], and poly(ether ketone)s (PEKs) [17–22] have been considered as more potential candidates due to their excellent thermal/ chemical stability, good film-forming ability and high mechanical strength. In particular, commercial PPO [5–8] and polysulfone (e.g. Udel® P3500) [9–12] have been extensively investigated due to their easy functionalization for AEMs. Comparatively, commercial PEEK (e.g. Victrex®)-an outstanding high performance engineering plastic is far less investigated as a raw material for AEMs, probably because of the poor solubility of PEEK in common solvents. Yan et al. [17,23] (Scheme 1a) succeeded in the chloromethylation of PEEK with chloromethyl octyl ether in concentrated sulfuric acid or its cosolvents with methanesulfonic acid at low temperature. However, this route could only lead
to main-chain-type AEMs, that is, the cationic head groups are tethered with the backbones via just a short −CH2− spacer. Bromination of PEKs containing benzylic methyl groups is an indirect route capable of preparing side-chain-type AEMs (Scheme 1b). Nonetheless, it required specific monomers that were not easily accessible [19]. To be noted, side-chain-type AEMs, in comparison with main-chain-type analogues, were generally reported to exhibit better comprehensive properties such as higher conductivity, lower swelling, and higher alkaline stability [16,19]. Therefore, a method of preparing side-chain-type AEMs based on PEEK is very appealing yet challenging. This work proposes an unprecedented method of tethering sidechain-type benzyl quaternary ammonium groups to the main-chains of aromatic polymers via benzylation modification. Although FriedelCrafts benzylation of arenes is classic in text books and has been revived recently [24–26], the introduction of functional ammonium groups to polymers directly via benzylation has never been reported to the best of our knowledge. Herein, PEEK, as an exemplary polymeric substrate, was modified with 4-chloromethylbenzyl trimethylammonium chloride (CMBzTMA) as a benzylation reagent via one-step benzylation reaction in triflic acid (TFSA). Thus-obtained PEEKs functionalized with different contents of side-chain-type quaternary ammonium groups were prepared into AEMs via solution casting. Key properties of these AEMs
⁎
Corresponding authors. E-mail addresses: [email protected] (Z. Zhang), [email protected] (L. Wu). 1 Zhenghui Zhang and Xinle Xiao contributed equally to this work. https://doi.org/10.1016/j.memsci.2018.12.080 Received 22 October 2018; Received in revised form 12 December 2018; Accepted 29 December 2018 Available online 30 December 2018 0376-7388/ © 2018 Published by Elsevier B.V.
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homogeneous solution, and then the solution was cast onto a piece of flat glass placed on a heated surface to evaporate the solvent at 70 °C for 24 h in a well-vented fume hood. The resultant membrane was detached off the glass plate automatically upon immersion into water, then sequentially subjected to immersion in 1.0 mol L−1 K2CO3 at 60 °C for 48 h during which the K2CO3 solution was refreshed about every 8 h, immersion in 1.0 mol L−1 HCl for 1 h, washing thoroughly with water to finally get QBz-PEEK-x membrane in Clˉ type. 2.5. Characterization 1
H NMR spectra were recorded on an Avance 400 spectrometer (Bruker, Germany) using D2O or DMSO-d6 (tetramethylsilane as internal reference) as the solvent. The mechanical property of membranes was measured on an auto tensile tester (PARAM® XLW, Labthink Company, China) under ambient condition at a crosshead speed of 30 mm min−1, and tensile strength (TS) and elongation at break (Eb) values were recorded. Thermogravimetric analysis (TGA) was conducted on a TGA Q500 (T.A.) under nitrogen at a heating rate of 10 °C min−1. Small angel X-ray scattering (SAXS) was conducted on a Saxesess mc2 (Anton Paar) using AEMs (Clˉ type) dried at 100 °C for 12 h prior to tests. The characteristic separation distance (d) is calculated by the Bragg’s law d = 2π/q, where q is the scattering vector.
Scheme 1. Routes to PEEK-based AEMs reported in the literature [17,19].
including water uptake, swelling, conductivity, alkaline stability and single cell performance were investigated to evaluate their potential applications in fuel cells.
2.6. Measurements 2.6.1. Ion conductivity OHˉ conductivity was measured following the previous method in our group [28]. ⅰ) Pretreatment of membrane samples: The membrane sample (1 cm × 4 cm) was converted to hydroxide form by soaking in 1 mol L−1 NaOH for 12 h, then thoroughly rinsed with deionized water; ⅱ) Testing of membrane impedance: the impedance of each membrane sample was measured by four-point probe alternating current (AC) impedance technique using an autolab PGSTAT 30 (Eco Chemie, Netherland) over the frequency range from 1 MHz to 100 Hz in the galvanostatic mode with an AC current amplitude of 0.1 mA. The hydroxide conductivity measurements in the longitudinal membrane direction under fully hydrated conditions were carried out with a Teflon cell which had two inner potential-sensing electrodes (platinum wire, 1 cm apart) and two outer current carrying electrodes (stainless steel, flat, 2 cm apart) immersed in water, and the membrane resistance was obtained from the Nyquist plot; ⅲ) Calculation of hydroxide conductivity: the hydroxide conductivity (σ) was calculated according to the following equation:
2. Experimental 2.1. Materials 1,4-bis(chloromethyl)benzene, triflic acid (TFSA), and methane sulfonic acid (MSA), and hexafluoroisopropanol (HFIP) were purchased from Energy Chemical Co. Ltd. (China). Dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidinone (NMP), tetrahydrofuran (THF), chloroform and other solvents were supplied by Shanghai Sinopham Chemical Reagent Co. Ltd. (China). Poly(ether ether ketone) (PEEK, Victrex®) was commercially supplied, and dried at 120 °C for 24 h before use. All other reagents and solvents were used as received. 2.2. Synthesis of CMBzTMA The benzylation reagent CMBzTMA was synthesized following a procedure adapted from the literature [27]. 1,4-Bis(chloromethyl)benzene (22 g) was dissolved in chloroform (150 mL), and then trimethylamine gas was introduced into the solution at a rate of about 2 mL/min for 10 h at room temperature. The precipitate was filtered, washed with THF, dried in vacuum to give CMBzTMA (25 g, 86% yield) as white powder. 1H NMR (400 MHz, D2O) δ 7.49 (d, 2H), 7.45 (d, 2H), 4.62 (s, 2H), 4.38 (s, 2H), 2.98 (s, 9H).
σ=
D R ×T ×W
(1)
where R was the obtained membrane resistance, D was the distance between potential sensing electrodes (here 1 cm), and T and W were the membrane’s thickness and width, respectively. Clˉ conductivity was measured in the same way, except using membranes in Clˉ type.
2.3. Benzylation modification of PEEK A representative procedure was as follows. CMBzTMA (1.6245 g, 6.937 mmol) was added to a homogeneous solution of PEEK (2.000 g) in TFSA (20 mL). The mixture was stirred at 60 °C for 24 h, then cooled to room temperature and poured into water slowly. Thus-obtained thread-like precipitate was rinsed with water using a Soxhlet apparatus for at least 24 h, and dried at 100 °C for 12 h to obtain white QBz-PEEKx. Benzylation modification of PEEK with various adding ratios of CMBzTMA to PEEK was conducted in a similar way.
2.6.2. Ion exchange capacity (IEC) IEC of AEMs was titrated as follows. First, the membranes in Clˉ type were dried to a constant mass and weighed (denoted as Wdry). After that, the membranes were immersed in 0.5 mol L−1 Na2SO4 aqueous solution for 24 h. Finally, the Cl- ions released from the membrane were titrated using an aqueous solution of AgNO3 (0.01 mol L−1) with K2CrO4 as the indicator. The IEC values were calculated from the amount of AgNO3 consumed (VAgNO3) and the weight of dry membrane samples (Wdry) using the following formula:
2.4. Preparation of membranes and ion-exchange treatment
IEC(mmol/g) = QBz-PEEK-x was firstly dissolved in DMAc to form a 5 wt% 206
V(AgNO3) × C(AgNO3) Wdry
(2)
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2.6.3. Water uptake (WU) and swelling ratio (SR) Regular rectangular pieces of membrane samples in OHˉ type were dried to a constant mass, and then the weight (Wdry) and length (Ldry) of the dry membranes were measured, respectively. Then, the membranes were immersed in deionized water for 24 h at a certain temperature. Subsequently, the membranes were quickly wiped with tissue paper, and the weight (Wwet) and length (Lwet) of the wet membranes were quickly measured, respectively. Water uptake (WU) and swelling ratio (SR) were calculated as follows:
WU(%) =
SR(%) =
(Wwet-Wdry) Wdry
(L wet-L dry) L dry
× 100%
× 100%
Table 1 Screening of acidic solvents for benzylation modification of PEEK. Entry
1 2 3 4 5 6 7 8 9 10
(3)
Solvent
MSA MSA MSA/HFIPd MSA/TFSAe TFSA TFSA TFSA TFSA TFSA TFSA
(4)
Ratioa
1.0 1.0 1.0 1.0 1.0 1.1 1.2 1.3 1.6 0.8
Conditionb
60 °C, 24 h 100 °C, 24 h 60 °C, 24 h 80 °C, 24 h 60 °C, 24 h 60 °C, 24 h 60 °C, 24 h 60 °C, 24 h 60 °C, 24 h 60 °C, 24 h
Solubilityc DMSO
DMAc
– – – – + + + + + Δ
– – – – + + + + + Δ
a
Adding molar ratio of CMBzTMA to the hydroquinone unit in PEEK. The initial polymer concentration was fixed at 1 g PEEK per 10 mL solvent. c – soluble, + insoluble, Δ swollen to gel (judged by observing the state of 0.01 g polymer in 5 mL solvent at 80 °C for 2 h). d 7/3 vol ratio. e 1/1 vol ratio. b
2.6.4. Single cell performance The H2/O2 single cell using an AEM of QBz-PEEK-76.0% was performed at 70 °C. Firstly, a catalyst ink was formed by ultrasonically mixing Pt/C (60%w/w in metal content) with a benchmarked QPPO ionomer solution in our group [28]. The resulting ink contained 20%w/ w of ionomer and 80%w/w of catalyst. Secondly, the catalyst ink was sprayed onto a piece of carbon paper (Toray TGP-H-060) to form gas diffusion electrode (GDE). The metal loading in both anode and cathode was controlled to be 0.5 mg/cm−2, and the electrodes were converted to OHˉ type via ion-exchanging prior to use. Thirdly, a piece of AEM of QBz-PEEK-76.0% (OHˉ type) was sandwiched between two GDEs to form a membrane electrode assembly (MEA) without hot pressing. Lastly, the MEA with an effective area of 12.25 cm2 was tested using an 890E Multi Range fuel cell test station (Scribner Associates, USA) in a galvanic mode. H2 and O2 were humidified at 70 °C (100% RH) and fed with a flow rate of 1000 cm3 min−1 without backpressure on both sides. The cell voltage at each current density was recorded after the power output was stabilized.
As shown in Table 1, different acidic solvents/cosolvents were screened under various reaction conditions. The solubility of the resultant products in DMSO and DMAc was used as a simple judgment of the extent of modification. MSA alone or its cosolvent with HFIP or TFSA (Entry 1–4) did not seem to be effective enough to obtain products soluble in DMSO or NMP, although MSA is favorable for its low cost and safe handling. It turned out that only TFSA (Entry 5) with the highest acidity was effective in catalyzing the benzylation modification under a relatively mild condition (60 °C, 24 h). According to a recent report on the benzylation reaction between small molecules, electronwithdrawing groups such as –F, –CF3 and –NO2 would dramatically deactivate benzyl alcohols for the benzylation reaction [26], thus requiring more acidic catalysts. Similarly, the benzyl chloride group of CMBzTMA should be deactivated by the electron-withdrawing ammonium group at the para-position. Therefore, highly acidic TFSA was required for the benzylation reaction of CMBzTMA with PEEK. Therefore, the condition for benzylation modification was set at 60 °C for 24 h in TFSA, and the molar ratio of CMBzTMA to PEEK was varied to tune the degree of substitution (i.e., x). As listed in Table 1 (Entry 5–10), six ratios ranging from 1.60 to 0.80 were tested. All thusobtained QBz-PEEK-x expect the one prepared at a molar ratio of 0.8 (Entry 10) became soluble in common solvents such as DMSO and DMAc, while original PEEK could not be dissolved in either DMSO or DMAc. The improved solubility of the resultant QBz-PEEK-x not only indicated successful modification, but also facilitated their characterization and application. The chemical structures of QBz-PEEK-x prepared at different molar ratios (Entry 5–9, Table 1) were characterized by 1H NMR spectrometry. As shown in Fig. 1, resonance signals at around 3.98 ppm (peak b), 4.41 ppm (peak c), and 2.93 ppm (peak d) could be reasonably assigned to Ar–CH2–N, Ar–CH2–Ar, and (CH3)3–N, respectively, which confirmed the introduction of side-chain ammonium groups via benzylation as expected. The integral ratios between these peaks were also in line with the expected structure of the side chain. Furthermore, according to the integral ratio of peak d to peak a (7.79 ppm, Ar–H orth to the carbonyl group), the substitution degree (i.e., x) at different molar ratios could be calculated from the following equation (Eq. 5), where Sa and Sd were the integral area of peak a and peak d, respectively.
3. Results and discussion 3.1. Synthesis of QBz-PEEK-x As illustrated in Scheme 2, firstly, CMBzTMA containing both one benzylchloride group and one benzyl ammonium group was synthesized via selective amination of 1,4-bis(chloromethyl)benzene. Then, CMBzTMA was used as a benzylation reagent to react with PEEK in order to introduce side-chain-type ammonium groups via one-step benzylation modification. Since PEEK is known to be soluble only in strong acids such as MSA and TFSA, we expected that these acids might be used as both solvent and catalyst for the benzylation reaction of PEEK.
x = Scheme 2. (a) Synthesis of CMBzTMA, and (b) modification of PEEK with CMBzTMA.
Sd /9 Sa /4
(5)
As listed in Table 2, x increased with the increase of molar ratio, and reached as high as 91.6% when the molar ratio was 1.6, suggesting that 207
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Fig. 1. 1H NMR spectra of QBz-PEEK-x.
solution casting. Thanks to the tough nature of raw PEEK, all these AEMs were endowed with high tensile strength above 35 MPa (TS, Table 2). It was also observed that TS of these AEMs decreased with the increase of x. A popular explanation for this trend in the literature was the plasticization effect of absorbed water which increased with x as well as IECth (will be described later). Nevertheless, for PEEK-based AEMs, the semi-crystalline nature of PEEK might also play an important role here. The original regular alignment of PEEK main-chains which contributes critically to TS would be disturbed by the introduction of side-chains, therefore the higher the degree of substitution was, the lower TS was. The thermal properties of QBz-PEEK-x were also analyzed via TGA. Representative TGA curves of QBz-PEEK-76.0% in different types of counter-ions are displayed in Fig. 2. Aside from the initial minor loss of absorbed water, QBz-PEEK-76.0% in OH– type started to decompose at 124 °C presumably due to OH–-mediated degradation of ammonium groups; Cl– type otherwise stayed stable until 193 °C; CF3SO3– type exhibited the highest thermal stability of up to 295 °C. Apparently, the counter-ions play an important role in the thermal stability of AEMs. In addition, when both in CF3SO3– type, QBz-PEEK-91.6%, in comparison with QBz-PEEK-76.0%, exhibited the same degradation temperature of ammonium groups but more weight loss, which was in consistence with the contents of ammonium groups.
Table 2 The effects of reaction ratios on the properties of QBz-PEEK-x and corresponding AEMs. Ratioa
xb
IECthc [mmol g−1]
TSd
Ebd
1.0 1.1 1.2 1.3 1.6
51.1% 66.2% 70.2% 76.0% 91.6%
1.31 1.58 1.64 1.73 1.95
65 63 50 45 35
31% 30% 35% 38% 48%
a
Molar ratio of CMBzTMA to the hydroquinone unit in PEEK. Degree of substitution calculated from 1H NMR. c Theoretic ion exchange capacity (Cl– type) calculated from the substitution degree. d Tensile strength and elongation at break tested under ambient condition. b
the substitution degree could be tuned in a wide range.
3.2. Preparation and properties of AEMs 3.2.1. Mechanical and thermal properties Since the aforementioned QBz-PEEK-x at molar ratios of 1.6, 1.3, 1.2, 1.1, and 1.0 were soluble in DMAc, transparent and homogeneous AEMs with theoretical ion exchange capacity (IECth, Cl– type) ranging from 1.31 to 1.95 mmol g−1 (Table 2) could be facilely prepared via 208
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similar trend to WU (Fig. 4b). It is worth mentioning that QBz-PEEK91.6% exhibited abnormally lower SR than QBz-PEEK-76.0% at lower temperature until 60 °C. The reason is not known for sure at the moment; presumably the ion-exchange treatment of these AEMs at 60 °C (in 1.0 mol L−1 K2CO3 at 60 °C for 48 h to replace CF3SO3– totally) interfered in the following measurement of SR. When comparing QBz-PEEK-x with other PEEK-based AEMs, e.g. main-chain-type QAPEEKOH [17] (Scheme 1a) and side-chain-type QMPAEK [19] (Scheme 1b), it was found that (1) at high substitution degree or IECth, QBz-PEEK-x exhibited much better resistance against swelling than QAPEEKOH (Table 3). For example, QAPEEKOH started to swell excessively (SR ~ 30%, 60 °C) when x just reached 70% (IECth = 2.00 mmol g−1) while QBz-PEEK-91.6% (x = 91.6%, IECth = 1.95 mmol g−1) remained a relatively low SR of 20% at 60 °C; (2) at medium IECth around 1.75 mmol g−1, these three kinds of AEMs (QBzPEEK-76.0%, QAPEEKOH-60%, and QMPAEK-75, Table 3) exhibited a similar SR (~ 15%) but very different WU at 60 °C. Among them, QBzPEEK-76.0% exhibited a moderate WU of 56%. It indicated that for QBz-PEEK-76.0%, the ammonium groups were in a suitable hydrated state (the number of absorbed water molecules per ammonium group i.e., λ ~ 18) to transport OH–.
Fig. 2. TGA curves of QBz-PEEK-x in different types of counter-ions.
3.2.4. Ion conductivity Both Cl– and OH– ion conductivity of QBz-PEEK-x AEMs in a temperature region of 30–80 °C are plotted in Fig. 5. As expected, Cl– conductivity of all QBz-PEEK-x AEMs increased with temperature. Cl– conductivity also increased with x until x was too high. When x reached 91.6%, the corresponding QBz-PEEK-91.6% otherwise exhibited lower Cl– conductivity than QBz-PEEK-76.0% in the whole temperature region. This should be ascribed to the dilution of ammonium groups by excessive water uptake when x reached too high. This phenomenon was more apparent in the cases of high x and high temperature (Fig. 5b). For example, OH– conductivity of QBz-PEEK-91.6% was even lower than that of QBz-PEEK-70.2%, and OH– conductivity of QBz-PEEK-91.6% and QBz-PEEK-76.0% dropped slightly when temperature increased from 70 °C to 80 °C. Therefore, an optimal content of ionic groups for a certain AEM series is very important. Notably, QBz-PEEK-76.0% exhibited a very high OH– conductivity of 155 mS cm−1 at 60 °C, which was 6–15 times that of other PEEK-based AEMs with similar IECth, e.g. main-chain-type QAPEEKOH-60% and side-chain-type QMPAEK-75 (Table 3). As a matter of fact, this value was in the top rank of OH– conductivity ever reported. For example, Coates et al. [29] reported a high OH– conductivity of about 135 mS cm−1 at 60 °C for an optimized crosslinked AEM prepared via ring-opening metathesis polymerization. Jannasch et al. [30] reported an even higher OH– conductivity of about 175 mS cm−1 at 60 °C for PPO-based AEMs with flexible tetra-piperidinium side chains. The main reason for such a high conductivity of QBz-PEEK-76.0% should be the existence of orderly hydrophilic domains/channels in the hydrophobic and crystalline matrix as discussed in Section 3.2.2.
Fig. 3. SAXS of QBz-PEEK-x (Cl– type).
3.2.2. Morphology The micromorphology of AEMs is an important factor to influence the ionic conductivity. SAXS analysis of the AEMs of QBz-PEEK-x (Fig. 3) showed that with the increase of x, peak b (the scattering vector q ~ 13.2 nm−1) which should arise from crystallization decreased, indicative of the decrease of the degree of crystallization with x. This suggested the introduction of side-chains impeded the crystallization of PEEK main-chains. In the low q region, when x increased from 70.2% to 76.0%, a distinct peak (peak a) appeared at q ~ 1.9 nm−1, suggesting the formation of a long range order in the membrane. The corresponding Bragg spacing was about 3.3 nm, which should be ascribed to the orderly clustering of the side-chain ammonium groups due to hydrophilic/hydrophobic phase separation [30]. However, when x further increased from 76.0% to 91.6%, no scattering peak could be detected any more in the low q region. Probably, excessive substitution of hydrophilic side-chains in QBz-PEEK-91.6% broke the hydrophilic/hydrophobic balance, thus impeding the formation of orderly hydrophilic domains/channels in the hydrophobic matrix as in QBz-PEEK-76.0%.
3.2.5. Alkaline stability In order to assess the alkaline stability of QBz-PEEK-x, QBz-PEEK76.0% (OH– type) was aged in 1 mol L−1 NaOH at 60 °C for a certain period. Since QBz-PEEK-76.0% after aging could no longer be dissolved in DMSO presumably due to crosslinking, 1H NMR spectrometry was not suitable for detailed characterization of the chemical change of QBz-PEEK-76.0%. Therefore, OH– conductivity and IEC measured by titration (i.e., IECti) were recorded at different intervals, and their remaining ratios with time are plotted in Fig. 6. It turned out that OH– conductivity and IECti dropped quickly in the first 4 days but almost leveled after about 6 days. After 8 days, the membrane remained tough and foldable, suggesting no cleavage of the main-chains that often happened to main-chain-type quaternized poly(aryl ether)s [31,32]. However, only 53% of IEC and 24% of conductivity were retained after 8 days, respectively, which is not so satisfying for potential long-term
3.2.3. WU and SR Water uptake (WU) of QBz-PEEK-x AEMs in OH– type at different temperature was depicted in Fig. 4a. In general, WU increased with temperature as well as x (i.e., IECth). For QBz-PEEK-x with relatively lower x (e.g., QBz-PEEK-51.1%, −66.2%, and −70.2%), the effect of temperature and x on WU was very faint; when x reached 76%, the effect became apparent; only when x reached 91.6%, the effect became prominent, especially at high temperature. Swelling ratio (SR), a more direct parameter akin to the dimensional change of AEMs, exhibited a 209
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Fig. 4. (a) WU and (b) SR of QBz-PEEK-x at different temperature. Table 3 Comparison of key properties of different PEEK-based AEMs reported in this work and the literature [17,19]. AEMs
IECtha [mmol g−1]
WUb
λb,c
SRb
Conductivityb [mS cm−1]
Ref.
QBz-PEEK-91.6% QAPEEKOH-70% QBz-PEEK-76.0% QAPEEKOH-60% QMPAEK-75
1.95 2.00 1.73 1.76 1.75
96% 150% 56% 32% 94%
30 42 18 10 30
20% 30% 15% 16% 15%
59 16 155 10 24
This work [17] This work [17] [19]
a b c
Theoretical ion exchange capacity calculated from the substitution degree. Tested at 60 °C. Absorbed water molecules per ammonium group, λ = (WU/18)/( IECth/1000).
Fig. 6. Remaining ratio of OH– conductivity and IECti of QBz-PEEK-76.0% after being immersed in 1 mol L−1 NaOH at 60 °C for different time.
usage in alkaline fuel cells. Nonetheless, a big improvement could still be seen when compared with main-chain-type QAPEEKOH. For example, OH– conductivity of QBz-PEEK-76.0% dropped 17% after 2 days while it was reported that OH– conductivity of QAPEEKOH-70% dropped as high as 53% after 2 days’ immersion in 1 mol L−1 KOH at 60 °C [18]. Further effort on improving alkaline stability of AEMs prepared via this novel “benzylation” method is underway. 3.2.6. Single cell performance Among QBz-PEEK-x AEMs, QBz-PEEK-76.0% AEM possessed the highest conductivity while maintaining low swelling. Therefore, to further assess the applicability of QBz-PEEK-x AEMs in alkaline fuel cells, the AEM of QBz-PEEK-76.0% was chosen to be tested in a H2/O2 single cell. The cell performance was recorded under an operation condition that both H2 and O2 were fed with a flow rate of 1000 cm3
Fig. 5. (a) Cl– and (b) OH– conductivity of QBz-PEEK-x at different temperature. 210
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Contreras, Y. Yan, Quaternary phosphonium-based polymers as hydroxide exchange membranes, ChemSusChem 3 (2010) 555–558. [12] A.D. Mohanty, Y.B. Lee, L. Zhu, M.A. Hickner, C. Bae, Anion exchange fuel cell membranes prepared from C–H borylation and Suzuki coupling reactions, Macromolecules 47 (2014) 1973–1980. [13] J. Wang, S. Li, S. Zhang, Novel hydroxide-conducting polyelectrolyte composed of an poly (arylene ether sulfone) containing pendant quaternary guanidinium groups for alkaline fuel cell applications, Macromolecules 43 (2010) 3890–3896. [14] J. Wang, Z. Zhao, F. Gong, S. Li, S. Zhang, Synthesis of soluble poly(arylene ether sulfone) ionomers with pendant quaternary ammonium groups for anion exchange membranes, Macromolecules 42 (2009) 8711–8717. [15] J. Yan, M.A. Hickner, Anion exchange membranes by bromination of benzylmethylcontaining poly(sulfone)s, Macromolecules 43 (2010) 2349–2356. [16] X. Li, G. Nie, J. Tao, W. Wu, L. Wang, S. Liao, Assessing the influence of side-chain and main-chain aromatic benzyltrimethyl ammonium on anion exchange membranes, ACS Appl. Mater. Interfaces 6 (2014) 7585–7595. [17] X. Yan, G. He, S. Gu, X. Wu, L. Du, H. Zhang, Quaternized poly(ether ether ketone) hydroxide exchange membranes for fuel cells, J. Membr. Sci. 375 (2011) 204–211. [18] X. Yan, S. Gu, G. He, X. Wu, W. Zheng, X. Ruan, Quaternary phosphonium-functionalized poly(ether ether ketone) as highly conductive and alkali-stable hydroxide exchange membrane for fuel cells, J. Membr. Sci. 466 (2014) 220–228. [19] K. Shen, J. Pang, S. Feng, Y. Wang, Z. Jiang, Synthesis and properties of a novel poly(aryl ether ketone)s with quaternary ammonium pendant groups for anion exchange membranes, J. Membr. Sci. 440 (2013) 20–28. [20] Z. Liu, X. Li, K. Shen, P. Feng, Y. Zhang, X. Xu, W. Hu, Z. Jiang, B. Liu, M.D. Guiver, Naphthalene-based poly(arylene ether ketone) anion exchange membranes, J. Mater. Chem. A 1 (2013) 6481–6488. [21] Z. Zhang, L. Wu, J. Varcoe, C. Li, A.L. Ong, S. Poynton, T. Xu, Aromatic polyelectrolytes via polyacylation of pre-quaternized monomers for alkaline fuel cells, J. Mater. Chem. A 1 (2013) 2595–2601. [22] H. Zarrin, J. Wu, M. Fowler, Z. Chen, High durable PEK-based anion exchange membrane for elevated temperature alkaline fuel cells, J. Membr. Sci. 394–395 (2012) 193–201. [23] X. Yan, X. Wu, G. He, S. Gu, X. Gong, J. Benziger, A methanesulfonic acid/sulfuric acid‐based route for easily‐controllable chloromethylation of poly(ether ether ketone), J. Appl. Polym. Sci. 132 (2015). [24] K. Mertins, I. Iovel, J. Kischel, A. Zapf, M. Beller, Transition-metal-catalyzed benzylation of arenes and heteroarenes, Angew. Chem. Int. Ed. 44 (2010) 238–242. [25] M. Rueping, B.J. Nachtsheim, A review of new developments in the Friedel–Crafts alkylation – From green chemistry to asymmetric catalysis, Beilstein J. Org. Chem. 6 (2010). [26] V. Vukovic´, E. Richmond, E. Wolf, J. Moran, Catalytic Friedel-Crafts reactions of highly electronically deactivated benzylic alcohols, Angew. Chem. Int. Ed. 56 (2017) 3131–3135. [27] R. Bartsch, W. Zhao, Z. Zhang, Facile synthesis of (ω-bromoalkyl)trimethylammonium bromides, Synth. Commun. 29 (1999) 2393–2398. [28] Y. Zhu, Y. He, X. Ge, X. Liang, M.A. Shehzad, M. Hu, Y. Liu, L. Wu, T. Xu, A benzyltetramethylimidazolium-based membrane with exceptional alkaline stability in fuel cells: role of its structure in alkaline stability, J. Mater. Chem. A 6 (2018) 527–534. [29] N.J. Robertson, H.A. Kostalik, T.J. Clark, P.F. Mutolo, Hc.D. Abruña, G.W. Coates, Tunable high performance cross-linked alkaline anion exchange membranes for fuel cell applications, J. Am. Chem. Soc. 132 (2010) 3400–3404. [30] H.S. Dang, P. Jannasch, High-performing hydroxide exchange membranes with flexible tetra-piperidinium side chains linked by alkyl spacers, ACS Appl. Energy Mater. 1 (2018) 2222–2231. [31] C. Fujimoto, D.S. Kim, M. Hibbs, D. 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Fig. 7. The polarization and power density curve of a single cell using the AEM of QBz-PEEK-76.0% (Both H2 and O2 were humidified at 70 °C (100% RH) and fed with a flow rate of 1000 cm3 min−1, the metal loading on both the anode and cathode was 0.5 mg cm−2).
min−1 and humidified at 70 °C (100% RH). As shown in Fig. 7, a peak power density as high as 391 mW cm−2 was achieved at a current density of 731 mA cm−2, indicative of a promising application of QBzPEEK-x AEMs in alkaline fuel cells. 4. Conclusions In summary, side-chain-type quaternized polyelectrolytes based on commercial PEEK have been synthesized via one-step benzylation modification. After a screening of several acidic solvents, TFSA showed the most effective catalysis for benzylation when using CMBzTMA as a benzylation reagent. By tuning the molar ratio between CMBzTMA and PEEK, a series of polyelectrolytes with a wide range of substitution degree (51.1%~91.6%) were obtained in TFSA. These polyelectrolytes were readily soluble in DMSO and DMAc, and their solutions could be facilely cast into membranes. Key properties of thus-obtained AEMs such as mechanical properties, WU, SR and ion conductivity were investigated in detail. In particular, AEM of QBz-PEEK-76.0% with an IEC of 1.73 mmol g−1 showed the best comprehensive properties. For example, QBz-PEEK-76.0% exhibited an exceptionally high OH– conductivity of 155 mS cm−1 at 60 °C while maintaining a low swelling ratio of 15%. An aging test of QBz-PEEK-76.0% in 1 mol L−1 NaOH at 60 °C suggested a big improvement on alkaline stability when compared with main-chain-type QAPEEKOH, although it is not sufficient for longterm usage in alkaline fuel cells at relatively high temperature. Moreover, when the AEM of QBz-PEEK-76.0% was assembled in a H2/ O2 single cell operated at 70 °C, a promising peak power density as high as 391 mW cm−2 could be achieved. It is envisioned that this novel “benzylation” method is versatile to other analogous polymers and benzylation reagents. Improving the alkaline stability by designing new benzylation reagents will be one emphasis of this “benzylation” method in the future. Acknowledgments We thank for financial support from the National Natural Science Foundation of China (No. 21304083) and the Special Foundation for Young Teachers of Nanyang Normal University (No. ZX2015011). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2018.12.080.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Highly conductive anion exchange membranes based on one-step benzylation modification of poly(ether ether ketone) Zhenghui Zhanga,
⁎,1
, Xinle Xiaob,1, Xin Yana, Xian Liangb, Liang Wub,
T
⁎
a
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Wolong Street 1638, Nanyang, Henan 473061, PR China CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Centre of Chemistry for Energy Materials, School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, PR China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Anion exchange membrane Benzylation Fuel cell Poly(ether ether ketone) Side chain type
Side-chain-type quaternized aromatic polyelectrolytes named QBz-PEEK-x have been synthesized via one-step benzylation modification of commercial poly(ether ether ketone) (PEEK). By controlling the molar ratio between the benzylation reagent and PEEK, the substitution degree x could be tuned in a wide range (51.1%~ 91.6%). A series of five anion exchange membranes (AEMs) with high mechanical strength were facilely prepared via solution-casting of QBz-PEEK-x. Water uptake, swelling ratio and ion conductivity of these AEMs were investigated in detail. Of particular interest is that, at 60 °C, the AEM of QBz-PEEK-76.0% exhibited a moderate water uptake of 56%, a low swelling ratio of 15%, and an exceptionally high OH– conductivity of 155 mS cm−1 which was comparable to the top values ever reported. Moreover, a promising peak power density of 391 mW cm−2 was achieved when assembling QBz-PEEK-76.0% AEM in a H2/O2 single cell operated at 70 °C. This unprecedented “benzylation” method opens a new door to novel high performance side-chain-type AEMs.
1. Introduction With the increasing interest in the applications of anion-exchange membranes (AEMs) in electrochemical energy conversion and storage systems such as fuel cells and water electrolyzers, AEMs with high conductivity and good stability under alkaline/oxidative operational conditions are in need [1,2]. In comparison with traditional AEMs base on polyvinyls, AEMs based on aromatic polymers such as poly(phenylene) [3,4], poly(phenylene oxide) (PPO) [5–8], poly(ether sufone)s (PESs) [9–16], and poly(ether ketone)s (PEKs) [17–22] have been considered as more potential candidates due to their excellent thermal/ chemical stability, good film-forming ability and high mechanical strength. In particular, commercial PPO [5–8] and polysulfone (e.g. Udel® P3500) [9–12] have been extensively investigated due to their easy functionalization for AEMs. Comparatively, commercial PEEK (e.g. Victrex®)-an outstanding high performance engineering plastic is far less investigated as a raw material for AEMs, probably because of the poor solubility of PEEK in common solvents. Yan et al. [17,23] (Scheme 1a) succeeded in the chloromethylation of PEEK with chloromethyl octyl ether in concentrated sulfuric acid or its cosolvents with methanesulfonic acid at low temperature. However, this route could only lead
to main-chain-type AEMs, that is, the cationic head groups are tethered with the backbones via just a short −CH2− spacer. Bromination of PEKs containing benzylic methyl groups is an indirect route capable of preparing side-chain-type AEMs (Scheme 1b). Nonetheless, it required specific monomers that were not easily accessible [19]. To be noted, side-chain-type AEMs, in comparison with main-chain-type analogues, were generally reported to exhibit better comprehensive properties such as higher conductivity, lower swelling, and higher alkaline stability [16,19]. Therefore, a method of preparing side-chain-type AEMs based on PEEK is very appealing yet challenging. This work proposes an unprecedented method of tethering sidechain-type benzyl quaternary ammonium groups to the main-chains of aromatic polymers via benzylation modification. Although FriedelCrafts benzylation of arenes is classic in text books and has been revived recently [24–26], the introduction of functional ammonium groups to polymers directly via benzylation has never been reported to the best of our knowledge. Herein, PEEK, as an exemplary polymeric substrate, was modified with 4-chloromethylbenzyl trimethylammonium chloride (CMBzTMA) as a benzylation reagent via one-step benzylation reaction in triflic acid (TFSA). Thus-obtained PEEKs functionalized with different contents of side-chain-type quaternary ammonium groups were prepared into AEMs via solution casting. Key properties of these AEMs
⁎
Corresponding authors. E-mail addresses: [email protected] (Z. Zhang), [email protected] (L. Wu). 1 Zhenghui Zhang and Xinle Xiao contributed equally to this work. https://doi.org/10.1016/j.memsci.2018.12.080 Received 22 October 2018; Received in revised form 12 December 2018; Accepted 29 December 2018 Available online 30 December 2018 0376-7388/ © 2018 Published by Elsevier B.V.
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homogeneous solution, and then the solution was cast onto a piece of flat glass placed on a heated surface to evaporate the solvent at 70 °C for 24 h in a well-vented fume hood. The resultant membrane was detached off the glass plate automatically upon immersion into water, then sequentially subjected to immersion in 1.0 mol L−1 K2CO3 at 60 °C for 48 h during which the K2CO3 solution was refreshed about every 8 h, immersion in 1.0 mol L−1 HCl for 1 h, washing thoroughly with water to finally get QBz-PEEK-x membrane in Clˉ type. 2.5. Characterization 1
H NMR spectra were recorded on an Avance 400 spectrometer (Bruker, Germany) using D2O or DMSO-d6 (tetramethylsilane as internal reference) as the solvent. The mechanical property of membranes was measured on an auto tensile tester (PARAM® XLW, Labthink Company, China) under ambient condition at a crosshead speed of 30 mm min−1, and tensile strength (TS) and elongation at break (Eb) values were recorded. Thermogravimetric analysis (TGA) was conducted on a TGA Q500 (T.A.) under nitrogen at a heating rate of 10 °C min−1. Small angel X-ray scattering (SAXS) was conducted on a Saxesess mc2 (Anton Paar) using AEMs (Clˉ type) dried at 100 °C for 12 h prior to tests. The characteristic separation distance (d) is calculated by the Bragg’s law d = 2π/q, where q is the scattering vector.
Scheme 1. Routes to PEEK-based AEMs reported in the literature [17,19].
including water uptake, swelling, conductivity, alkaline stability and single cell performance were investigated to evaluate their potential applications in fuel cells.
2.6. Measurements 2.6.1. Ion conductivity OHˉ conductivity was measured following the previous method in our group [28]. ⅰ) Pretreatment of membrane samples: The membrane sample (1 cm × 4 cm) was converted to hydroxide form by soaking in 1 mol L−1 NaOH for 12 h, then thoroughly rinsed with deionized water; ⅱ) Testing of membrane impedance: the impedance of each membrane sample was measured by four-point probe alternating current (AC) impedance technique using an autolab PGSTAT 30 (Eco Chemie, Netherland) over the frequency range from 1 MHz to 100 Hz in the galvanostatic mode with an AC current amplitude of 0.1 mA. The hydroxide conductivity measurements in the longitudinal membrane direction under fully hydrated conditions were carried out with a Teflon cell which had two inner potential-sensing electrodes (platinum wire, 1 cm apart) and two outer current carrying electrodes (stainless steel, flat, 2 cm apart) immersed in water, and the membrane resistance was obtained from the Nyquist plot; ⅲ) Calculation of hydroxide conductivity: the hydroxide conductivity (σ) was calculated according to the following equation:
2. Experimental 2.1. Materials 1,4-bis(chloromethyl)benzene, triflic acid (TFSA), and methane sulfonic acid (MSA), and hexafluoroisopropanol (HFIP) were purchased from Energy Chemical Co. Ltd. (China). Dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidinone (NMP), tetrahydrofuran (THF), chloroform and other solvents were supplied by Shanghai Sinopham Chemical Reagent Co. Ltd. (China). Poly(ether ether ketone) (PEEK, Victrex®) was commercially supplied, and dried at 120 °C for 24 h before use. All other reagents and solvents were used as received. 2.2. Synthesis of CMBzTMA The benzylation reagent CMBzTMA was synthesized following a procedure adapted from the literature [27]. 1,4-Bis(chloromethyl)benzene (22 g) was dissolved in chloroform (150 mL), and then trimethylamine gas was introduced into the solution at a rate of about 2 mL/min for 10 h at room temperature. The precipitate was filtered, washed with THF, dried in vacuum to give CMBzTMA (25 g, 86% yield) as white powder. 1H NMR (400 MHz, D2O) δ 7.49 (d, 2H), 7.45 (d, 2H), 4.62 (s, 2H), 4.38 (s, 2H), 2.98 (s, 9H).
σ=
D R ×T ×W
(1)
where R was the obtained membrane resistance, D was the distance between potential sensing electrodes (here 1 cm), and T and W were the membrane’s thickness and width, respectively. Clˉ conductivity was measured in the same way, except using membranes in Clˉ type.
2.3. Benzylation modification of PEEK A representative procedure was as follows. CMBzTMA (1.6245 g, 6.937 mmol) was added to a homogeneous solution of PEEK (2.000 g) in TFSA (20 mL). The mixture was stirred at 60 °C for 24 h, then cooled to room temperature and poured into water slowly. Thus-obtained thread-like precipitate was rinsed with water using a Soxhlet apparatus for at least 24 h, and dried at 100 °C for 12 h to obtain white QBz-PEEKx. Benzylation modification of PEEK with various adding ratios of CMBzTMA to PEEK was conducted in a similar way.
2.6.2. Ion exchange capacity (IEC) IEC of AEMs was titrated as follows. First, the membranes in Clˉ type were dried to a constant mass and weighed (denoted as Wdry). After that, the membranes were immersed in 0.5 mol L−1 Na2SO4 aqueous solution for 24 h. Finally, the Cl- ions released from the membrane were titrated using an aqueous solution of AgNO3 (0.01 mol L−1) with K2CrO4 as the indicator. The IEC values were calculated from the amount of AgNO3 consumed (VAgNO3) and the weight of dry membrane samples (Wdry) using the following formula:
2.4. Preparation of membranes and ion-exchange treatment
IEC(mmol/g) = QBz-PEEK-x was firstly dissolved in DMAc to form a 5 wt% 206
V(AgNO3) × C(AgNO3) Wdry
(2)
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2.6.3. Water uptake (WU) and swelling ratio (SR) Regular rectangular pieces of membrane samples in OHˉ type were dried to a constant mass, and then the weight (Wdry) and length (Ldry) of the dry membranes were measured, respectively. Then, the membranes were immersed in deionized water for 24 h at a certain temperature. Subsequently, the membranes were quickly wiped with tissue paper, and the weight (Wwet) and length (Lwet) of the wet membranes were quickly measured, respectively. Water uptake (WU) and swelling ratio (SR) were calculated as follows:
WU(%) =
SR(%) =
(Wwet-Wdry) Wdry
(L wet-L dry) L dry
× 100%
× 100%
Table 1 Screening of acidic solvents for benzylation modification of PEEK. Entry
1 2 3 4 5 6 7 8 9 10
(3)
Solvent
MSA MSA MSA/HFIPd MSA/TFSAe TFSA TFSA TFSA TFSA TFSA TFSA
(4)
Ratioa
1.0 1.0 1.0 1.0 1.0 1.1 1.2 1.3 1.6 0.8
Conditionb
60 °C, 24 h 100 °C, 24 h 60 °C, 24 h 80 °C, 24 h 60 °C, 24 h 60 °C, 24 h 60 °C, 24 h 60 °C, 24 h 60 °C, 24 h 60 °C, 24 h
Solubilityc DMSO
DMAc
– – – – + + + + + Δ
– – – – + + + + + Δ
a
Adding molar ratio of CMBzTMA to the hydroquinone unit in PEEK. The initial polymer concentration was fixed at 1 g PEEK per 10 mL solvent. c – soluble, + insoluble, Δ swollen to gel (judged by observing the state of 0.01 g polymer in 5 mL solvent at 80 °C for 2 h). d 7/3 vol ratio. e 1/1 vol ratio. b
2.6.4. Single cell performance The H2/O2 single cell using an AEM of QBz-PEEK-76.0% was performed at 70 °C. Firstly, a catalyst ink was formed by ultrasonically mixing Pt/C (60%w/w in metal content) with a benchmarked QPPO ionomer solution in our group [28]. The resulting ink contained 20%w/ w of ionomer and 80%w/w of catalyst. Secondly, the catalyst ink was sprayed onto a piece of carbon paper (Toray TGP-H-060) to form gas diffusion electrode (GDE). The metal loading in both anode and cathode was controlled to be 0.5 mg/cm−2, and the electrodes were converted to OHˉ type via ion-exchanging prior to use. Thirdly, a piece of AEM of QBz-PEEK-76.0% (OHˉ type) was sandwiched between two GDEs to form a membrane electrode assembly (MEA) without hot pressing. Lastly, the MEA with an effective area of 12.25 cm2 was tested using an 890E Multi Range fuel cell test station (Scribner Associates, USA) in a galvanic mode. H2 and O2 were humidified at 70 °C (100% RH) and fed with a flow rate of 1000 cm3 min−1 without backpressure on both sides. The cell voltage at each current density was recorded after the power output was stabilized.
As shown in Table 1, different acidic solvents/cosolvents were screened under various reaction conditions. The solubility of the resultant products in DMSO and DMAc was used as a simple judgment of the extent of modification. MSA alone or its cosolvent with HFIP or TFSA (Entry 1–4) did not seem to be effective enough to obtain products soluble in DMSO or NMP, although MSA is favorable for its low cost and safe handling. It turned out that only TFSA (Entry 5) with the highest acidity was effective in catalyzing the benzylation modification under a relatively mild condition (60 °C, 24 h). According to a recent report on the benzylation reaction between small molecules, electronwithdrawing groups such as –F, –CF3 and –NO2 would dramatically deactivate benzyl alcohols for the benzylation reaction [26], thus requiring more acidic catalysts. Similarly, the benzyl chloride group of CMBzTMA should be deactivated by the electron-withdrawing ammonium group at the para-position. Therefore, highly acidic TFSA was required for the benzylation reaction of CMBzTMA with PEEK. Therefore, the condition for benzylation modification was set at 60 °C for 24 h in TFSA, and the molar ratio of CMBzTMA to PEEK was varied to tune the degree of substitution (i.e., x). As listed in Table 1 (Entry 5–10), six ratios ranging from 1.60 to 0.80 were tested. All thusobtained QBz-PEEK-x expect the one prepared at a molar ratio of 0.8 (Entry 10) became soluble in common solvents such as DMSO and DMAc, while original PEEK could not be dissolved in either DMSO or DMAc. The improved solubility of the resultant QBz-PEEK-x not only indicated successful modification, but also facilitated their characterization and application. The chemical structures of QBz-PEEK-x prepared at different molar ratios (Entry 5–9, Table 1) were characterized by 1H NMR spectrometry. As shown in Fig. 1, resonance signals at around 3.98 ppm (peak b), 4.41 ppm (peak c), and 2.93 ppm (peak d) could be reasonably assigned to Ar–CH2–N, Ar–CH2–Ar, and (CH3)3–N, respectively, which confirmed the introduction of side-chain ammonium groups via benzylation as expected. The integral ratios between these peaks were also in line with the expected structure of the side chain. Furthermore, according to the integral ratio of peak d to peak a (7.79 ppm, Ar–H orth to the carbonyl group), the substitution degree (i.e., x) at different molar ratios could be calculated from the following equation (Eq. 5), where Sa and Sd were the integral area of peak a and peak d, respectively.
3. Results and discussion 3.1. Synthesis of QBz-PEEK-x As illustrated in Scheme 2, firstly, CMBzTMA containing both one benzylchloride group and one benzyl ammonium group was synthesized via selective amination of 1,4-bis(chloromethyl)benzene. Then, CMBzTMA was used as a benzylation reagent to react with PEEK in order to introduce side-chain-type ammonium groups via one-step benzylation modification. Since PEEK is known to be soluble only in strong acids such as MSA and TFSA, we expected that these acids might be used as both solvent and catalyst for the benzylation reaction of PEEK.
x = Scheme 2. (a) Synthesis of CMBzTMA, and (b) modification of PEEK with CMBzTMA.
Sd /9 Sa /4
(5)
As listed in Table 2, x increased with the increase of molar ratio, and reached as high as 91.6% when the molar ratio was 1.6, suggesting that 207
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Fig. 1. 1H NMR spectra of QBz-PEEK-x.
solution casting. Thanks to the tough nature of raw PEEK, all these AEMs were endowed with high tensile strength above 35 MPa (TS, Table 2). It was also observed that TS of these AEMs decreased with the increase of x. A popular explanation for this trend in the literature was the plasticization effect of absorbed water which increased with x as well as IECth (will be described later). Nevertheless, for PEEK-based AEMs, the semi-crystalline nature of PEEK might also play an important role here. The original regular alignment of PEEK main-chains which contributes critically to TS would be disturbed by the introduction of side-chains, therefore the higher the degree of substitution was, the lower TS was. The thermal properties of QBz-PEEK-x were also analyzed via TGA. Representative TGA curves of QBz-PEEK-76.0% in different types of counter-ions are displayed in Fig. 2. Aside from the initial minor loss of absorbed water, QBz-PEEK-76.0% in OH– type started to decompose at 124 °C presumably due to OH–-mediated degradation of ammonium groups; Cl– type otherwise stayed stable until 193 °C; CF3SO3– type exhibited the highest thermal stability of up to 295 °C. Apparently, the counter-ions play an important role in the thermal stability of AEMs. In addition, when both in CF3SO3– type, QBz-PEEK-91.6%, in comparison with QBz-PEEK-76.0%, exhibited the same degradation temperature of ammonium groups but more weight loss, which was in consistence with the contents of ammonium groups.
Table 2 The effects of reaction ratios on the properties of QBz-PEEK-x and corresponding AEMs. Ratioa
xb
IECthc [mmol g−1]
TSd
Ebd
1.0 1.1 1.2 1.3 1.6
51.1% 66.2% 70.2% 76.0% 91.6%
1.31 1.58 1.64 1.73 1.95
65 63 50 45 35
31% 30% 35% 38% 48%
a
Molar ratio of CMBzTMA to the hydroquinone unit in PEEK. Degree of substitution calculated from 1H NMR. c Theoretic ion exchange capacity (Cl– type) calculated from the substitution degree. d Tensile strength and elongation at break tested under ambient condition. b
the substitution degree could be tuned in a wide range.
3.2. Preparation and properties of AEMs 3.2.1. Mechanical and thermal properties Since the aforementioned QBz-PEEK-x at molar ratios of 1.6, 1.3, 1.2, 1.1, and 1.0 were soluble in DMAc, transparent and homogeneous AEMs with theoretical ion exchange capacity (IECth, Cl– type) ranging from 1.31 to 1.95 mmol g−1 (Table 2) could be facilely prepared via 208
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similar trend to WU (Fig. 4b). It is worth mentioning that QBz-PEEK91.6% exhibited abnormally lower SR than QBz-PEEK-76.0% at lower temperature until 60 °C. The reason is not known for sure at the moment; presumably the ion-exchange treatment of these AEMs at 60 °C (in 1.0 mol L−1 K2CO3 at 60 °C for 48 h to replace CF3SO3– totally) interfered in the following measurement of SR. When comparing QBz-PEEK-x with other PEEK-based AEMs, e.g. main-chain-type QAPEEKOH [17] (Scheme 1a) and side-chain-type QMPAEK [19] (Scheme 1b), it was found that (1) at high substitution degree or IECth, QBz-PEEK-x exhibited much better resistance against swelling than QAPEEKOH (Table 3). For example, QAPEEKOH started to swell excessively (SR ~ 30%, 60 °C) when x just reached 70% (IECth = 2.00 mmol g−1) while QBz-PEEK-91.6% (x = 91.6%, IECth = 1.95 mmol g−1) remained a relatively low SR of 20% at 60 °C; (2) at medium IECth around 1.75 mmol g−1, these three kinds of AEMs (QBzPEEK-76.0%, QAPEEKOH-60%, and QMPAEK-75, Table 3) exhibited a similar SR (~ 15%) but very different WU at 60 °C. Among them, QBzPEEK-76.0% exhibited a moderate WU of 56%. It indicated that for QBz-PEEK-76.0%, the ammonium groups were in a suitable hydrated state (the number of absorbed water molecules per ammonium group i.e., λ ~ 18) to transport OH–.
Fig. 2. TGA curves of QBz-PEEK-x in different types of counter-ions.
3.2.4. Ion conductivity Both Cl– and OH– ion conductivity of QBz-PEEK-x AEMs in a temperature region of 30–80 °C are plotted in Fig. 5. As expected, Cl– conductivity of all QBz-PEEK-x AEMs increased with temperature. Cl– conductivity also increased with x until x was too high. When x reached 91.6%, the corresponding QBz-PEEK-91.6% otherwise exhibited lower Cl– conductivity than QBz-PEEK-76.0% in the whole temperature region. This should be ascribed to the dilution of ammonium groups by excessive water uptake when x reached too high. This phenomenon was more apparent in the cases of high x and high temperature (Fig. 5b). For example, OH– conductivity of QBz-PEEK-91.6% was even lower than that of QBz-PEEK-70.2%, and OH– conductivity of QBz-PEEK-91.6% and QBz-PEEK-76.0% dropped slightly when temperature increased from 70 °C to 80 °C. Therefore, an optimal content of ionic groups for a certain AEM series is very important. Notably, QBz-PEEK-76.0% exhibited a very high OH– conductivity of 155 mS cm−1 at 60 °C, which was 6–15 times that of other PEEK-based AEMs with similar IECth, e.g. main-chain-type QAPEEKOH-60% and side-chain-type QMPAEK-75 (Table 3). As a matter of fact, this value was in the top rank of OH– conductivity ever reported. For example, Coates et al. [29] reported a high OH– conductivity of about 135 mS cm−1 at 60 °C for an optimized crosslinked AEM prepared via ring-opening metathesis polymerization. Jannasch et al. [30] reported an even higher OH– conductivity of about 175 mS cm−1 at 60 °C for PPO-based AEMs with flexible tetra-piperidinium side chains. The main reason for such a high conductivity of QBz-PEEK-76.0% should be the existence of orderly hydrophilic domains/channels in the hydrophobic and crystalline matrix as discussed in Section 3.2.2.
Fig. 3. SAXS of QBz-PEEK-x (Cl– type).
3.2.2. Morphology The micromorphology of AEMs is an important factor to influence the ionic conductivity. SAXS analysis of the AEMs of QBz-PEEK-x (Fig. 3) showed that with the increase of x, peak b (the scattering vector q ~ 13.2 nm−1) which should arise from crystallization decreased, indicative of the decrease of the degree of crystallization with x. This suggested the introduction of side-chains impeded the crystallization of PEEK main-chains. In the low q region, when x increased from 70.2% to 76.0%, a distinct peak (peak a) appeared at q ~ 1.9 nm−1, suggesting the formation of a long range order in the membrane. The corresponding Bragg spacing was about 3.3 nm, which should be ascribed to the orderly clustering of the side-chain ammonium groups due to hydrophilic/hydrophobic phase separation [30]. However, when x further increased from 76.0% to 91.6%, no scattering peak could be detected any more in the low q region. Probably, excessive substitution of hydrophilic side-chains in QBz-PEEK-91.6% broke the hydrophilic/hydrophobic balance, thus impeding the formation of orderly hydrophilic domains/channels in the hydrophobic matrix as in QBz-PEEK-76.0%.
3.2.5. Alkaline stability In order to assess the alkaline stability of QBz-PEEK-x, QBz-PEEK76.0% (OH– type) was aged in 1 mol L−1 NaOH at 60 °C for a certain period. Since QBz-PEEK-76.0% after aging could no longer be dissolved in DMSO presumably due to crosslinking, 1H NMR spectrometry was not suitable for detailed characterization of the chemical change of QBz-PEEK-76.0%. Therefore, OH– conductivity and IEC measured by titration (i.e., IECti) were recorded at different intervals, and their remaining ratios with time are plotted in Fig. 6. It turned out that OH– conductivity and IECti dropped quickly in the first 4 days but almost leveled after about 6 days. After 8 days, the membrane remained tough and foldable, suggesting no cleavage of the main-chains that often happened to main-chain-type quaternized poly(aryl ether)s [31,32]. However, only 53% of IEC and 24% of conductivity were retained after 8 days, respectively, which is not so satisfying for potential long-term
3.2.3. WU and SR Water uptake (WU) of QBz-PEEK-x AEMs in OH– type at different temperature was depicted in Fig. 4a. In general, WU increased with temperature as well as x (i.e., IECth). For QBz-PEEK-x with relatively lower x (e.g., QBz-PEEK-51.1%, −66.2%, and −70.2%), the effect of temperature and x on WU was very faint; when x reached 76%, the effect became apparent; only when x reached 91.6%, the effect became prominent, especially at high temperature. Swelling ratio (SR), a more direct parameter akin to the dimensional change of AEMs, exhibited a 209
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Fig. 4. (a) WU and (b) SR of QBz-PEEK-x at different temperature. Table 3 Comparison of key properties of different PEEK-based AEMs reported in this work and the literature [17,19]. AEMs
IECtha [mmol g−1]
WUb
λb,c
SRb
Conductivityb [mS cm−1]
Ref.
QBz-PEEK-91.6% QAPEEKOH-70% QBz-PEEK-76.0% QAPEEKOH-60% QMPAEK-75
1.95 2.00 1.73 1.76 1.75
96% 150% 56% 32% 94%
30 42 18 10 30
20% 30% 15% 16% 15%
59 16 155 10 24
This work [17] This work [17] [19]
a b c
Theoretical ion exchange capacity calculated from the substitution degree. Tested at 60 °C. Absorbed water molecules per ammonium group, λ = (WU/18)/( IECth/1000).
Fig. 6. Remaining ratio of OH– conductivity and IECti of QBz-PEEK-76.0% after being immersed in 1 mol L−1 NaOH at 60 °C for different time.
usage in alkaline fuel cells. Nonetheless, a big improvement could still be seen when compared with main-chain-type QAPEEKOH. For example, OH– conductivity of QBz-PEEK-76.0% dropped 17% after 2 days while it was reported that OH– conductivity of QAPEEKOH-70% dropped as high as 53% after 2 days’ immersion in 1 mol L−1 KOH at 60 °C [18]. Further effort on improving alkaline stability of AEMs prepared via this novel “benzylation” method is underway. 3.2.6. Single cell performance Among QBz-PEEK-x AEMs, QBz-PEEK-76.0% AEM possessed the highest conductivity while maintaining low swelling. Therefore, to further assess the applicability of QBz-PEEK-x AEMs in alkaline fuel cells, the AEM of QBz-PEEK-76.0% was chosen to be tested in a H2/O2 single cell. The cell performance was recorded under an operation condition that both H2 and O2 were fed with a flow rate of 1000 cm3
Fig. 5. (a) Cl– and (b) OH– conductivity of QBz-PEEK-x at different temperature. 210
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Fig. 7. The polarization and power density curve of a single cell using the AEM of QBz-PEEK-76.0% (Both H2 and O2 were humidified at 70 °C (100% RH) and fed with a flow rate of 1000 cm3 min−1, the metal loading on both the anode and cathode was 0.5 mg cm−2).
min−1 and humidified at 70 °C (100% RH). As shown in Fig. 7, a peak power density as high as 391 mW cm−2 was achieved at a current density of 731 mA cm−2, indicative of a promising application of QBzPEEK-x AEMs in alkaline fuel cells. 4. Conclusions In summary, side-chain-type quaternized polyelectrolytes based on commercial PEEK have been synthesized via one-step benzylation modification. After a screening of several acidic solvents, TFSA showed the most effective catalysis for benzylation when using CMBzTMA as a benzylation reagent. By tuning the molar ratio between CMBzTMA and PEEK, a series of polyelectrolytes with a wide range of substitution degree (51.1%~91.6%) were obtained in TFSA. These polyelectrolytes were readily soluble in DMSO and DMAc, and their solutions could be facilely cast into membranes. Key properties of thus-obtained AEMs such as mechanical properties, WU, SR and ion conductivity were investigated in detail. In particular, AEM of QBz-PEEK-76.0% with an IEC of 1.73 mmol g−1 showed the best comprehensive properties. For example, QBz-PEEK-76.0% exhibited an exceptionally high OH– conductivity of 155 mS cm−1 at 60 °C while maintaining a low swelling ratio of 15%. An aging test of QBz-PEEK-76.0% in 1 mol L−1 NaOH at 60 °C suggested a big improvement on alkaline stability when compared with main-chain-type QAPEEKOH, although it is not sufficient for longterm usage in alkaline fuel cells at relatively high temperature. Moreover, when the AEM of QBz-PEEK-76.0% was assembled in a H2/ O2 single cell operated at 70 °C, a promising peak power density as high as 391 mW cm−2 could be achieved. It is envisioned that this novel “benzylation” method is versatile to other analogous polymers and benzylation reagents. Improving the alkaline stability by designing new benzylation reagents will be one emphasis of this “benzylation” method in the future. Acknowledgments We thank for financial support from the National Natural Science Foundation of China (No. 21304083) and the Special Foundation for Young Teachers of Nanyang Normal University (No. ZX2015011). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2018.12.080.
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