High Durable PEK-Based Anion Exchange Membrane for Elevated Temperature Alkaline Fuel Cells

Journal of Membrane Science 394–395 (2012) 193–201

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Journal of Membrane Science

jo urnal homepage: www.elsevier.com/locate/memsci

High durable PEK-based anion exchange membrane for elevated temperature alkaline fuel cells

∗

Hadis Zarrin, Jason Wu, Michael Fowler, Zhongwei Chen

Dept. of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute for Sustainable Energy, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

a r t i c l e i n f o a b s t r a c t

Article history: Highly durable quaternary ammonium polyetherketone hydroxide (QAPEK-OH) membranes were devel-

Received 26 September 2011

oped for alkaline fuel cells operating at elevated temperatures via chloromethylation, quaternization and

Received in revised form

alkalinization method. The chemical reaction for chloromethylated polymer (CMPEK) was conﬁrmed by

22 December 2011 1

nuclear magnetic resonance ( H NMR) and the QAPEK-OH membranes were characterized by Fourier

Accepted 26 December 2011

transform infrared spectroscopy (FT-IR). The thermal properties of the membranes were determined by

Available online 31 December 2011

thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The ion-exchange capac-

ity (IEC), water uptake, mechanical properties and anion conductivity of the QAPEK-OH membranes were

Keywords:

measured to evaluate their applicability in alkaline fuel cells at elevated temperature. The anionic con-

Anion exchange membrane

−4 −2 −1

ductivity of the QAPEK-OH membrane varied from 8 × 10 to 1.1 × 10 S cm over the temperature

Alkaline fuel cell

◦

High durability range 20–100 C. The QAPEK-OH membrane showed excellent alkaline stability in 1 M KOH solution at

◦

High temperature 100 C. The excellent temperature durability of QAPEK-OHs makes them promising membrane materials

Ion conductivity for alkaline fuel cells.

1. Introduction within the membrane and the mass transfer, reducing the ohmic

and mass transfer overpotentials, respectively. Moreover, the elec-

Anion exchange membrane fuel cells (AEMFCs) are attracting trochemical kinetics of both the fuel oxidation reaction at the anode

considerable interest relative to proton exchange membrane fuel and the ORR at the cathode increase, resulting in minimized ther-

cells (PEMFCs) in recent years, primarily due to the distinct advan- modynamic voltage losses and higher overall cell voltages. Thus, in

tages when operating in an alkaline environment such as cathode order to successfully employ AEMFCs for applications such as vehi-

reaction kinetics and resistance polarization [1,2]. The inherently cle propulsion, high temperature stable AEMs must be produced

faster and much easier kinetics of the oxygen reduction reaction [2,7–11]. Accordingly, one of the primary challenges with AEMs

(ORR) in alkaline environment than in acid medium leads to the is the membrane physico-chemical stability, especially at elevated

use of non-noble and non-precious metal electrocatalysts such as temperatures.

silver (Ag) and nickel (Ni), making the AEMFC a potentially low Unlike polymer electrolyte membranes where highly chemi-

cost technology compared to PEMFCs, which employ platinum (Pt) cally stable perﬂuorinated membranes such as perﬂuorosulfonic

TM ®

catalysts [3,4]. Moreover, due to the less corrosive nature of an acid (DuPont Naﬁon ), control the market, commercially avail-

alkaline medium, higher durability is expected for AEMFCs. How- able AEMs are usually based on quaternary ammonium (QA)

ever, AEMFCs need to overcome strenuous technical and economic cross-linked polystyrene which albeit having been applied for

challenges which are speciﬁcally associated with fabrication of a water processing, cannot be used for AFCs due to low chemical

cost effective anion exchange membrane (AEM) possessing (i) high and thermal stability, as well as weak mechanical property. Dis-

anionic conductivity and (ii) exemplary physico-chemical and tem- tinctively, the polystyrene backbone is too rigid to produce ﬂexible

perature stability [1,2,5]. Due to the poor thermal, chemical and thin ﬁlms, and the QA functional group attached on polystyrene

◦

physical stability of current AEMs, the typical operating temper- tends to degrade at 40–60 C [5]. Furthermore, the blended ami-

◦

ature of AEMFCs is limited to 50 or 60 C [6]. However, AEMFC nated cross-linked polystyrene with other polymers limits the

performance can be increased by operation at elevated tempera- anion conductivity and may also reduce the chemical stability

tures which can be explained as follows. Increasing the operating of the membrane [7]. So far, many types of polymers, such as

temperature enhances both the charge (i.e., hydroxide) conduction polybenzimidazole (PBI) [12], polyphenylene oxide (PPO) [13],

cardo polyetherketone (PEK) [14], polyethersulfone (PES) [15], and

radiation-grafted FEET [16] and FEFP [17] have been used to pro-

∗ duce AEMs for application in alkaline fuel cells. Yet, the main

Corresponding author. Tel.: +1 519 888 4567x38664; fax: +1 519 746 4979.

E-mail address: [email protected] (Z. Chen). problems stayed with many known AEMs, such as the low chemical

194 H. Zarrin et al. / Journal of Membrane Science 394–395 (2012) 193–201

stability in concentrated alkaline environments, due to the decay of generate the hydrophobic oligomer block, DHBP (2.5 mmol,

the anion exchange groups (e.g., QA functional groups) in concen- 0.5356 g) and K2CO3 (2.75 mmol, 0.3795 g) were added to the solu-

trated alkali solution; or the weak thermal stability which limits tion with 12 mL of toluene and 6 mL of NMP, connected to the

them to be used for elevated temperature alkaline fuel cells [18]. Dean-Stark trap under nitrogen ﬂow. The temperature was kept

◦

In this paper, in order to accomplish the aforementioned objec- at 150 C for 4 h until the water was removed by toluene from

tives, a modiﬁed quaternary ammonium polyetherketone (QAPEK) the system. The reaction was continued under nitrogen ﬂow at

◦

based block copolymer electrolyte has been developed through 175–180 C for another 2 h until gaining a viscous copolymer. The

◦

the chloromethylation and quaternization method, demonstrating mixture was diluted with 12 mL of NMP at 100 C and added drop-

the requirements for AEMs with improved thermal, chemical and wise into a large excess of methanol with vigorously mechanical

mechanical stability for AEMFCs at elevated temperatures. Among stirring. The precipitated PEK block copolymer was washed and

different polymeric structures for AEMs, block copolymers possess puriﬁed with double deionized (DDI) water and methanol three

◦

higher water uptake and ion conductivity which can be assigned times. After being dried at 80 C under vacuum, the white PEK

to their sequential hydrophilic/hydrophobic structure with ionic copolymer ﬁbers were dissolved in 12 mL of NMP and then, re-

groups in the hydrophilic blocks, resulting in a helpful hydration precipitated in methanol, washed with DDI water and methanol

◦

effect [13,14]. Since one of the requirements and a key challenge for three times, and ﬁnally dried vacuum at 80 C in the oven.

AEMs is the satisfactory chemical stability, fully aromatic polymers

are expected to have notably high chemical resistance, leading to

2.3. Chloromethylation of PEK and membrane casting

the desirable durability for AEMFCs. Speciﬁcally, polyetherketones

(PEKs) and their derivatives are the focus of many investigations,

According to Fig. 2, the Friedel-Crafts chloromethylation reac-

and the synthesis of these materials has been widely reported.

tion of the PEK occurs at the activated aryl ring between

This family of aromatic copolymers is attractive for use in AEMs

the isopropylidene and ether bond by adding chloromethyl

because of their well-known chemical and hydrolytic stability

methylether (CMME) and a Lewis acid catalyst (e.g., ZnCl2). The

under harsh conditions [19–21]. The reason for that behavior is

product will be a halomethylated polymer. A 50 mL round-bottom

a microphase separation between the chemically different and

ﬂask was charged with PEK block copolymer (0.8 g) and 20 mL of

incompatible blocks which are attached to each other by covalent

1,1,2,2-tetrachloroethane (TCE) as the solvent. After stirring the

bonds, and which does not occur in random copolymers or in blends

mixture under nitrogen at room temperature (until PEK was com-

(macrophase separation) of the same composition [22]. Moreover,

pletely dissolved), a solution of CMME (29.28 mmol, 2.22 mL) and

AEMs containing QA functional groups as the cation exhibit higher

anhydrous ZnCl2 (0.0998 g, 0.732 mmol) was added dropwise to the

thermal and chemical stability than other quaternary cations such ◦

mixture and stirred at 35 C for 48 h. The mixture was poured slowly

as quaternary phosphonium (QP) or tertiary sulfonium (TS) groups

into a large excess of methanol, and washed with methanol three

[23–26]. This is mainly due to the higher basicity of QA (pKa = 31.9)

times. Then, the dried chloromethylated PEK (CMPEK) copolymer

than TS (pKa = 17.8) and QP (pKa = 17.4) compounds, respectively

was dissolved in NMP and ﬁltered via a fritted disk funnel. The ﬁl-

[27,28]. For the obtained QAPEK copolymer, the water uptake, ion ◦

trate was cast on a ﬂat glass plate and heated at 80 C for 1 day,

exchange capacity (IEC), ion conductivity, mechanical properties,

resulted in a thin 35–40 ␮m thick, transparent and tough mem-

thermal stability and chemical durability were investigated.

brane.

2. Experimental methods

2.4. Quaternization and alkalinization of CMPEK membrane

2.1. Materials

The solid-state alkaline electrolytes are typically prepared by

amination of halomethylated polymers with tertiary amines such

4,4 -isopropylidenediphenol (Bisphenol A/BPA), 4,4 -

as triethylamine (TEA). Fig. 3 clearly indicates the conversion from

dihydroxylbenzophenone (DHBP) and 4,4 -diﬂuorobenzophenone

a chloromethyl group into a quaternary ammonium (QA) group by

(DFBP) were purchased from TCI America and used as received.

immersing the CMPEK membrane in a 35 wt% TEA aqueous solution

Chloromethyl methyl ether (CMME), anhydrous zinc chlo-

at room temperature for 48 h. The last step corresponds to the ﬁnal

ride (ZnCl2), triethylamine (TEA), N-methylpyrrolidone (NMP) −

product, i.e., QAPEK-OH. To replace the Cl anion in the QAPEK with

and 1,1,2,2-tetrachloroethane (TCE) were purchased from −

OH , the membrane was placed in 1 M sodium hydroxide (KOH)

Sigma–Aldrich and used as received. All other reagents such

aqueous solution at room temperature for 48 h, and then, washed

as toluene and potassium hydroxide (KOH) were obtained from

with DDI water three times. The obtained QAPEK-OH membrane

other commercial sources and used as received.

was stored in a closed vessel ﬁlled with DDI water before being

used for any performance test.

2.2. Synthesis of PEK block copolymer via polycondensation

2.5. Measurements and characterization tests

The PEK block copolymer was synthesized through aromatic

nucleophilic substitution polycondensation reaction of activated

Nuclear magnetic resonance ( H NMR) spectra were measured

aromatic halide (DFBP) with BPA and DHBP in the presence of

at 500 MHz on a Bruker Analytik GmbH spectrometer. FT-IR spec-

a weak base, potassium carbonate, and a dipolar aprotic solvent,

tra were recorded on 35–40 ␮m thick membrane samples using a

NMP (Fig. 1). In a 50 mL 3-neck round-bottom ﬂask Bisphenol A

Avatar 320 FT-IR spectrometer. Thermogravimetric analysis (TGA)

(7.5 mmol, 1.7122 g) and K2CO3 (8.25 mmol, 1.6585 g) were mixed

was performed under nitrogen with a TGA Q500 V20.10 instrument

in 6 mL of NMP and 6 mL toluene, equipped with a Dean-Stark

◦

in the temperature range from 50 to 900 C at the heating rate of

receiver under nitrogen ﬂow. The reaction temperature was main-

◦ ◦ −1

10 C min . Differential scanning calorimetry (DSC) analysis was

tained at 150 C for 4 h until all the produced water was extracted

performed under nitrogen through the heating–cooling–heating

and collected in the Dean-Stark trap via toluene. After cooling

◦

cycle from 0 to 400 C with a DSC Q2000 V24.4 Build 116 instru-

the solution to the room temperature, DFBP (10 mmol, 2.182 g)

ment. Tensile measurements were carried out with an UMT-CETR

was added to the ﬂask and then, the temperature was increased

◦ −1

mechanical testing instrument at a speed of 0.05 mm min .

to 165 C for 2 h to obtain the hydrophilic oligomer. In order to

H. Zarrin et al. / Journal of Membrane Science 394–395 (2012) 193–201 195

Fig. 1. Synthesis of PEK block copolymer backbone by polycondensation.

2.5.1. Water uptake (WU) and ion exchange capacity (IEC) weight was constant, after which each sample was immersed in

◦

The membrane was vacuum-dried at 80 C for 10 h until con- DDI water overnight. Then, they were soaked in 0.01 M hydrochlo-

−

stant weight for the dried membrane was obtained. They were ric acid (HCl) for 48 h to exchange Cl ions with hydroxides. Back

then immersed in DDI water at given temperature for 4 h. After titration was then accomplished with 0.01 M NaOH (aq.) standard-

this time, the membranes were taken out, wiped with tissue paper, ized solution with phenolphthalein as an indicator. The IEC values

and quickly weighed on a microbalance. The water uptake of mem- were calculated using Eq. (2)

branes was calculated according to Eq. (1):

M V − M V

1 1 HCl 2 2 NaOH

W IEC = − W (2)

WU wet dry W

(%) = × 100 (1) dry W dry

−1

where IEC is the ion exchange capacity (mequiv. g ), M1 (molar)

where WU (%), Wwet and Wdry are the water uptake by weight

and V1 (mL) are the concentration and volume of HCl solution,

percentage, the weight of wet membrane and the weight of dry

respectively, before the titration; M2 (molar) and V2 (mL) are the

membrane, respectively.

concentration and volume of NaOH solution, respectively, used in

The IEC of membrane samples was determined by using the

the titration; Wdry (g) is the mass of dry sample.

back titration method. At ﬁrst, the samples were dried until the

Fig. 2. Chloromethylation process of PEK block copolymer through Friedel-Crafts reaction.

196 H. Zarrin et al. / Journal of Membrane Science 394–395 (2012) 193–201

Fig. 3. Quaternization/amination of CMPEK using TEA aqueous solution followed by alkalinization of the obtained QAPEK.

2.5.2. Anion conductivity 2.5.3. Alkaline stability

The anion conductivities of all QAPEK-OH membrane samples To measure the alkaline stability, the QAPEK-OH samples were

◦

were obtained by a four-electrode AC impedance spectroscopy [29] kept in a stirred aqueous solution of 1 M KOH at 60 and 100 C. Then,

with a CHI760D potentiostat control model. The impedance was after different speciﬁc time, each sample was rinsed and stored in

measured in the frequency range between 1 MHz and 0.1 Hz with DDI water before re-testing the anion conductivity. Furthermore,

perturbation voltage amplitude of 5 mV. The membranes and the the FT-IR and tensile strength tests were re-tested for samples after

electrodes were set in a Teﬂon cell, and the distance between the the stability test.

reference electrodes was 1 cm. The cell was placed in a thermo-

controlled chamber in DDI water for measurement. Conductivity

measurements under fully hydrated conditions were carried out 3. Results and discussion

with the cell immersed in DDI water. All samples were equilibrated

in water for at least 24 h prior to the conductivity measurement. As shown in Figs. 2 and 3, the AEM was prepared through

At a given temperature, the samples were equilibrated for at least three stages of chloromethylation, quaternization, and alkalin-

30 min before any measurements. Repeated measurements (four ization chemical reactions. Among them, chloromethylation and

times) were then taken at that given temperature with 10 min quaternization are two chief reactions which demonstrate the

interval until no more change in conductivity was observed. The anion conductivity [6]. The chloromethylation of polymer is a

anion conductivities () of all samples were then determined along valuable procedure that was identiﬁed about four decades ago.

the longitudinal direction, using Eq. (3) [30–32]. Because of the high reactivity of the tethered chloromethyl group,

chloromethylated polymer is the starting point for the synthesis

of “polymer-bound reagents” for the ion-exchange polymer elec-

= (3)

AR trolytes [33].

The H NMR spectra of the PEK and CMPEK block copolymers

where , L, R, and A denote the ionic conductivity, sample length

were well assigned to the supposed chemical structure (Fig. 4). In

(or distance between the reference electrodes in the membrane), 1

the H NMR spectrum of CMPEK (denoted as h in Fig. 4), a peak

the resistance of the membrane, and the cross-sectional area of the

assignable to the methylene protons of the chloromethyl groups

membrane, respectively.

on the polymer main chain was observed at 4.6 ppm, which was

H. Zarrin et al. / Journal of Membrane Science 394–395 (2012) 193–201 197

Fig. 4. H NMR spectra of PEK and CMPEK in deuterated chloroform (CDCl3).

absent in the H NMR spectrum of the bare PEK. The signals with of symmetric and asymmetric methylene ( CH2 ) groups, respec-

chemical shifts between 6.8 and 7.9 ppm were assigned to the aro- tively [34–40].

matic protons of PEK and CMPEK copolymers. The thermal stability of PEK, CMPEK and hydrated QAPEK-OH

In order to demonstrate whether the quaternary ammonium membrane were investigated by TGA curves, shown in Fig. 6. The

◦

(QA) groups are attached on the polymer chain, the elemental pure PEK copolymer started the degradation process around 461 C.

◦

composition of QAPEK-OH membrane was investigated through For CMPEK the weight loss (14%) observed at 320 C was attributed

the FT-IR spectroscopy (Fig. 5). The absorption bands at 3290 and to the chloromethylene groups. In QAPEK-OH membrane, as a result

−1

2359 cm were characteristic of QA groups [34,35]. The absorption of the strong hydrophilicity of the quaternary ammonium groups

−1

bands at 2873 and 2968 cm arise from the stretching vibrations and attracting water from the atmosphere, a slight weight loss

◦

(5.4%) was observed between 150 and 180 C, corresponding to the

Fig. 5. FT-IR spectrum of QAPEK-OH membrane. Fig. 6. TGA curves for PEK, CMPEK and QAPEK-OH copolymers in N2.

198 H. Zarrin et al. / Journal of Membrane Science 394–395 (2012) 193–201

Fig. 9. FT-IR spectrum of QAPEK-OH membranes treated in 1 M KOH solution at

Fig. 7. DSC curves for PEK, CMPEK and QAPEK-OH copolymers in N2.

◦

100 C after 24 and 72 h.

evaporation of absorbed water. This behavior has been commonly

and facilitate hydroxide transport. However, excessively high levels

found in other polymeric ion-exchange membranes. The weight

◦ of water uptake can result in membrane fragility and dimen-

loss (9%) observed for QAPEK-OH started at 200 C was related to

◦ sional change, which lead to the loss of mechanical properties [41].

the degradation of QA groups and the one at 320 C can be assigned

Basically, the amount of water uptake of QAPEK-OH membrane

to chloromethylene groups (18%), satisfying alkaline fuel cell appli-

is strongly dependent upon the amount of quaternary ammo-

cations at high temperatures.

nium hydroxide groups [42]. The deep effect of water uptake

The DSC analysis was performed to identify their glass transi-

on the anion conductivity as well as the mechanical property of

tion temperatures (Tg) of PEK, CMPEK and QAPEK-OH polymers

◦ AEMs is remarkable. Basically, when the membrane absorbs higher

(Fig. 7). For unmodiﬁed PEK copolymer, Tg was about 142 C. It can

amount of water, the number of available anion exchange sites

be seen that the Tg for CMPEK and QAPEK-OH has been shifted to

◦ increases, resulting in an increment in the anion conductivity.

higher temperature of 155 and 167 C, respectively. The increase of

Although the higher water uptake leads to the higher anion con-

Tg for the CMPEK and QAPEK-OH, implies that they are thermo-

ductivity, excessive uptake can also cause swelling and decrease

mechanically more stable than unmodiﬁed PEK membrane. The

the mechanical strength of the membrane [43–46]. The measured

membrane thus becomes more elastic in nature and can with- −1

water uptake and IEC were 16.67% (Eq. (1)) and 0.41 mequiv. g

stand higher temperatures. Thus, the results obtained from TGA

(Eq. (2)), respectively. To reach a high anion-conductive membrane,

and DSC shows the improved potential of the PEK based AEMs

16.67% water uptake was not enough for the developed QAPEK-OH

for high temperature operations of alkaline fuel cells applica-

membrane. The low water uptake value of the block QAPEK-OH

tions.

membrane can be related to (i) low ion exchange capacity and

The water uptake of aminated polymers is known to have a

(ii) the developed hydrophilic/hydrophobic phase-separated mor-

profound effect on the anion conductivity and mechanical proper-

phology [43,45].

ties of AEMs. Water molecules dissociate the alkaline functionality

Fig. 8. Anion conductivities of fully hydrated QAPEK-OH and commercial FAA mem- Fig. 10. Stress–strain curves of the QAPEK-OH membranes under hydrated condi-

◦ ◦

branes at different temperatures. tion in ambient atmosphere (25 C) before and after alkaline stability test at 100 C.

H. Zarrin et al. / Journal of Membrane Science 394–395 (2012) 193–201 199

Fig. 12. Anion conductivity after alkaline stability test of QAPEK-OH and commercial

◦

Fig. 11. Anion conductivity after alkaline stability test of QAPEK-OH and commercial

FAA in 1 M KOH at 100 C.

◦

FAA in 1 M KOH at 60 C.

The anion conductivity of the QAPEK-OH membrane was com- AEM materials used in fuel cells. As expected, an increase in tem-

pared to the commercial fumapem FAA from FuMA-Tech GmbH perature resulted in an increase in hydroxide conductivity based on

2 ◦

␮

(35–40 m thickness, 0.59 cm at 20 C), after the hydration in the simpliﬁed diffusion mechanism and thermal motion of hydrox-

DDI water for 24 h at the room temperature. FAA is a hydrocar- ides within the membrane [37]. This is attributed to the increase

bon membrane-polymer of aminated polyarylene (polysulfone) of free volume in the membrane at elevated temperatures which

chloride, hydroxide and carbonate salts anion exchange mem- expands the required channels for ion conduction [14]. For the

brane, used for alkaline fuel cells without the need of liquid FAA membrane, the enhancement of the anion conductivity with

electrolyte. In Fig. 8, the anion conductivity values of the mem- the increase of the temperature was in a gradual fashion, whereas

branes were shown and compared versus the temperature. In for the QAPEK-OH membrane the temperature had much higher

−2 −1

general, an anion conductivity above 10 S cm is required for effect on the improvement of the anion conductivity. The possible

◦

Fig. 13. Photographs of the QAPEK-OH and commercial FAA membranes before and after being treated with 1 M KOH solution at 100 C.

200 H. Zarrin et al. / Journal of Membrane Science 394–395 (2012) 193–201

explanation is that at elevated temperature the hydrophilic Acknowledgements

domains of QAPEK-OH membrane would easily become continu-

ous, leading to the expansion of overall hydrophilic domains [43]. This work was ﬁnancially supported by the Natural Sciences and

This phenomenon results in the enhancement of anion conduction Engineering Research Council of Canada (NSERC), and the Univer-

through the membrane by increasing the temperature. Moreover, sity of Waterloo.

◦

as the temperature increased to 100 C, the anion conductivity in

FAA membrane started to decrease. This can be attributed to the References

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◦

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