Synthesis and characterization of novel sulfonated polyimides from 1,4-bis(4-aminophenoxy)-naphthyl-2,7-disulfonic acid

PAPER www.rsc.org/ees | Energy & Environmental Science

Synthesis and characterization of novel sulfonated poly(arylene thioether)ionomers for vanadium redox flow battery applications†

Dongyang Chen, Shuanjin Wang,* Min Xiao and Yuezhong Meng*

Received 18th August 2009, Accepted 29th October 2009

First published as an Advance Article on the web 21st December 2009

DOI: 10.1039/b917117g

High-molecular-weight poly(arylene thioether ketone) (PTK) and poly(arylene thioether ketone

ketone) (PTKK) polymers were successfully synthesized by one-pot polymerization of N,N0-dimethy-S-

carbamate masked dithiols with activated dihalo compounds, followed by post-sulfonation using

chlorosulfonic acid as the sulfonation agent in dichloromethane solution to give the production of

sulfonated poly(arylene thioether ketone) (SPTK) and sulfonated poly(arylene thioether ketone

ketone) (SPTKK) with appropriate ion-exchange capacities. The chemical structures were confirmed

by 1H NMR, FT-IR and elemental analysis (EA). The thermal properties were fully investigated by

TGA-IR. The synthesized SPTK and SPTKK polymers are soluble in aprotic solvents such as

N,N0-dimethylacetamide (DMAc), N,N0-dimethylformamide and dimethyl sulfoxide, and can be cast

into membranes on a glass plate from their DMAc solution. The proton conductivities of these

membranes are comparable to Nafion117 membranes under the same conditions. Cell performance

tests showed that the vanadium redox flow batteries (VRBs) assembled with SPTK and SPTKK

membranes possessed higher Coulombic efficiencies than VRBs assembled with Nafion117 membranes

at the current density of 50 mA cm�2, because of their one-order-of magnitude lower VO2+

permeabilities. In conclusion, these ionomers could be promising candidates as proton-exchange

membranes for vanadium redox flow battery (VRB) applications.

Introduction

Vanadium redox flow batteries (VRBs) have attracted extensive

attention as candidates for stationary energy storage facilities

owing to their advantages such as long cycle life, flexible

assembly, fast response time, deep-discharge capability and facile

maintenance.1–3 One of the most significant challenges for flow

batteries is to block the cross contamination of the anolyte and

catholyte, which can be avoided by adopting the same element

electrolytes (vanadium in VRBs). The manufacture of VRB

power stacks and electrolytes is now sufficiently mature for

commercial exploitation. However, the performance of VRB

separators is still under development and receives a lot of effort

worldwide.4–9 Basic qualifications for VRB separators are both

low resistance in the electrolyte and high selectivity for protons.

Perfluorinated ionomers such as Nafion membranes (DuPont)

are the most commonly used proton conducting membranes for

their high proton conductivity and excellent chemical stability.

However, they suffer greatly from electrolyte permeation due to

their large hydrophilic channels which form when the membrane

is fully hydrated.9

Sulfonated aromatic polymers based proton exchange

membranes (PEMs) are of great interest for a variety of

The Key Laboratory of Low-carbon & Energy Conservation of GuangdongProvince, Institute of Optoelectronic and Functional Composite Materials,Sun Yat-Sen University, Guangzhou 510275, PR China. E-mail:[email protected]; [email protected]

† Electronic supplementary information (ESI) available: Synthesis of themonomers and polymers and 1H NMR spectra for PTK, PTKK, SPTKand SPTKK. See DOI: 10.1039/b917117g

622 | Energy Environ. Sci., 2010, 3, 622–628

applications, because of their high proton conductivity and good

processability, such as fuel cells,10–14 nanofiltration,15,16 actua-

tors17 and so on. PEMs being used as VRB separators have

emerged as a hot research topic for VRB researchers. Generally,

alternative sulfonated aromatic polymers include sulfonated

poly(arylene ether)s, poly(arylene ketone)s, poly(arylene

sulfone)s, poly(arylene imide)s and their copolymers. Neverthe-

less, poly(arylene thioether)s have received less attention.

Poly(arylene thioether)s are an important class of high-perfor-

mance polymers with excellent mechanical properties, chemical

stabilities, and good compatibility for inorganic fillers. There-

fore, it is expected that sulfonated poly (arylene thioether)s will

possess a good performance for PEM applications.

Conventionally, sulfonated aromatic polymers are made by

direct copolymerization with primary sulfonated monomers or

post-sulfonation of polymers. In direct polymerization, the

degree of sulfonation can be precisely controlled by adjusting the

ratio of sulfonated monomers to unsulfonated monomers,

however, the thermal stability of these polymers is limited by the

degradation of sulfonic acid groups at the ortho-positions of the

ether bond. This problem can be avoided by post-sulfonation of

aromatic polymers to attach the sulfonic acid groups to the

pendant phenyl rings, although the reaction is hard to isolate

from other side reactions.

We have reported the post-sulfonation of many aromatic

polymers,18,19 and demonstrated that PEMs with fluorenyl

groups exhibit high proton conductivity and chemical

stability.20,21 In this work, we report the synthesis of poly(arylene

thioether ketone)s by a one-pot polymerization method. The new

polymers were then post-sulfonated using chlorosulfonic acid as

This journal is ª The Royal Society of Chemistry 2010

a sulfonation agent to give highly proton conductive sulfonic

fluorenyl groups. The water uptake, proton conductivity,

thermal stability, oxidative stability and VO2+ permeability of the

as-made membranes from these new sulfonated polymers were

investigated in detail to evaluate them as promising proton

exchange membranes for vanadium redox flow battery applica-

tions.

Experimental

Membrane preparation

The sulfonated ionomers were dissolved in DMAc solution at

a high concentration and cast into membranes on a glass plate by

drying them at 60 �C for 10 h before eventually drying at 120 �C

under vacuum for 24 h. The yellowish membranes were peeled off

the glass plate, immersed in 1 M H2SO4 solution at 80 �C for 1 h,

washed with deionized water several times and then kept in

deionized water until use.

Characterization

Nuclear magnetic resonance (NMR) spectra were recorded at

400 MHz using a Bruker DRX NMR instrument and the

chemical shifts were listed in parts per million (ppm) downfield

from tetramethylsilane (TMS). Elemental analyses were per-

formed on a Vario EL elemental analyzer, ELEMENTAR Co.,

Germany. Gel permeation chromatography (GPC) was per-

formed on a Waters Breeze system equipped with a Waters

Styragel column, Waters 515 HPLC pump and Waters 2414

refractive index detector, chloroform as an elution solvent at

a flow rate of 1 mL min�1 and polystyrene as standards for

calibration. HPLC analysis was performed on SHIMADZU

HPLC instrument equipped with LC-20AT pump and SPD-20A

detector. FT-IR spectra were recorded on a PerkinElmer Spec-

trum 100 Fourier transform infrared spectrometer with

membrane samples. The inherent viscosity of the ionomers was

determined using an Ubbelohde viscometer in DMAc at 20 �C.

Melting points were taken on a SGW X-4 melting point appa-

ratus. Thermal stability was analyzed using a PerkinElmer Pyris

Diamond TG/DTA analyzer. The temperature was increased

from 50 �C to 500 �C at a heating rate of 10 �C min�1 under N2

atmosphere. The proton conductivity measurements were carried

out on a Solartron 1255 B frequency response analyzer coupled

with a Solartron 1287 electrochemical interface in the frequency

range of 1 Hz to 1 MHz. The glass-transition temperature (Tg)

was determined on a Seiko 220 DSC instrument at a heating rate

of 10 �C min�1 under nitrogen protection. The tensile properties

were determined by a SANS (Shenzhen, China) electromechan-

ical universal test machine (model CMT-4014).

Ionic exchange capacity (IEC)

The IEC of the membranes was determined by titration

according to the literature.22 The dried membrane was weighed

and immersed in a 1 M HCl solution at 80 �C for 5 h to protonize

the sulfonic acid groups. The membrane was then immersed in

deionized water to remove the excess acid absorbed in the

membrane. The membrane was then immersed in a 2 M solution

of NaCl (aq) for 24 h to replace the protons of the sulfonic acid


groups with sodium ions. The number of replaced protons was

titrated using 0.1 M NaOH (aq) solution, with phenolphthalein

as the indicator. The IEC was calculated using the following

equation:

IEC ¼ DVNaOH � CNaOH

Wd

�mol g�1

�(1)

Where DVNaOH is the consumed volume of NaOH solution,

CNaOH is the concentration of NaOH solution and Wd is the

weight of the sample. The equivalent weight (EW) and sulfona-

tion degree (SD) were also calculated, using the following

equations:

EW ¼ 1

IEC

�g mol-1

�(2)

SD ¼ FW� IEC

1� 80IECðsulfonic acid groups=repeated unitsÞ (3)

Where FW is the molar weight of the repeat units of the

synthesized ionomers.

Water uptake and swelling ratio

The dry membranes were weighed and immersed in deionized

water at different temperature for 24 h. The water uptake was

defined as the weight change to that of the dry membrane. The

swelling ratio was described as the thickness change rate of the

wet membrane. They can be calculated via the following equa-

tions:

W ð%Þ ¼ ðWw �WdÞWd

� 100% (4)

S ð%Þ ¼ d1 � d0

d0

� 100% (5)

Where Wd and Ww are the weights of the membranes before and

after water absorption, respectively; d0 and d1 are the thickness of

the membranes before and after water absorption, respectively.

Oxidative properties

Oxidative stabilities were tested in Fenton’s reagent (3 wt% H2O2

+ 2 ppm FeSO4) at 80 �C.23 The membranes with thickness

ranging from 120–160 mm were immersed in Fenton’s reagent

and conditioned at 80 �C using a water bath shaker. The values

for each sample were determined from the times of commence-

ment to break.

Proton conductivity

The proton conductivity (s) of the membranes in the transverse

direction was determined by measuring the impedance spec-

troscopy of a cell with the given membrane sample sandwiched

between two gold electrodes. The conductivity was calculated

from the impedance plot with a computer curve-fitting technique

according to the electrode area of the cell and the thickness of the

membrane, which was measured with a micrometer.

Energy Environ. Sci., 2010, 3, 622–628 | 623

s ðs cm�1Þ ¼ d

RS(6)

Where S and d are the face area and thickness of the membrane,

and R is derived from the low intersect of the high-frequency

semicircle on a complex impedance plane with the Re (Z0) axis.

VO2+ permeability

The permeation of VO2+ was measured by a method similar to

the literature.24 40 mL of 1 M VOSO4 in 2 M H2SO4 solution and

40 mL of 1 M MgSO4 in 2 M H2SO4 solution were used to fill two

reservoirs respectively. The membrane area exposed to the elec-

trolytes was 5.3 cm2. MgSO4 was used to equalize the osmotic

pressure. The two solutions were continuously magnetically

stirred and the concentration of VO2+ was measured using a UV–

visible spectrophotometer (Model: 752-P, Shanghai Xianke CO.

China) at regular time intervals (1 h) at room temperature.

The VO2+ permeability was defined as the VO2+ diffusion

coefficient (D) of the membrane calculated by Fick’s first law of

diffusion on the assumption of a pseudo-steady-state condition

inside the membrane, and is calculated using the following

equation:

J ¼ �DjC1 � C2j

d(7)

Where J is the flux of VO2+, C1 and C2 are the concentrations of

VO2+ in each reservoir respectively; d is thickness of the

membrane.

Scheme 2 Synthesis of PTK (3a), PTKK (3b), SPTK (4a) and SPTKK (4b).

Cell performance

Samples for discharge tests were sandwiched between two pieces

of carbon paper before being clamped in the graphite bipolar

with serpentine flow field. The anodic electrolyte was 40 mL of

1 M VO+2 in 2 M H2SO4 solution and the cathodic electrolyte was

40 mL of 1 M V2+ in 2 M H2SO4 solution. They were pumped

through the cell unceasingly during the test at room temperature.

The effective area of membrane was 4.86 cm2 and the limit

voltage was set to be 0.75 V. The Coulombic efficiency (CE) was

calculated using eqn (8):

Scheme 1 Synthesis of N,N0-dimethyl

Table 1 Properties of the polymers SPTK and SPTKK

Sample Yield (%) hinh (Dl/g)b IEC/mmol g�1 EW/g

SPTK 81 0.77 1.29 772SPTKK 75 0.78 1.91 523

a Measured at room temperature. b Where hinh is inherent viscosity


CE ¼ Cd

Ctc

� 100% (8)

Where Cd is the discharge capacity and Ctc is the theoretic charge

capacity (where we assume the charge efficiency is 100%) of the

cell.

Results and discussion

Synthesis and chemical structure analysis

Monomer 2 was synthesized by a Newman–Kwart rearrange-

ment reaction as depicted in Scheme 1 and copolymerized with

different dihalides to produce high-molecular-weight poly-

(thioether ketone)s. Because the polymerization only occurs at

temperatures higher than 240 �C and the difluorobenzophenone

is easy to sublime at about 80 �C, the benzophenone was used as

the solvent to keep the reactants dissolved before their subli-

mation for the polymerization. The molar amount of monomer 2

is slightly higher than that of dihalides because side-reactions

occurs at the polymerization temperature. The reaction was first

run at 200 �C for 3 h then at 240 �C for another 3 h. Increasing

the reaction time has no effect on increasing the molecular

weight, but the product color turned form yellowish to brown.

Poly(arylene thioether ketone) (PTK) and poly(arylene thioether

-S-carbamate masked monomer 2.

mol�1

Wateruptake (%)a

Swellingratio (%)a

Water molecules persulfonic acid group (l)a

11.9 5.8 5.119.3 7.6 5.6


Table 2 Sulfonation degree (SD) characterization of the polymers SPTK and SPTKK

Sample

Elemental analysis

S/C�1 (10�2)

SD

C H SCalculatedby EA

Calculatedby titration

SPTK 62.17 4.29 12.00 19.30 0.75 0.81SPTKK 56.88 4.65 12.80 22.50 1.80 1.82

Fig. 1 TGA and DTG curves of SPTK and SPTKK.

ketone ketone) (PTKK) were obtained with weight-average

molecular weights higher than 50 kDa.

Sulfonic acid groups can be easily introduced onto the PTK

and PTKK polymers by post-sulfonation using chlorosulfonic

acid as the sulfonation agent in dichloromethane solution. The

sulfonation degree can be readily controlled by varying the

reactant ratio, and the sulfonation position of the polymer chain

is found to be influenced by the concentration of chlorosulfonic

acid.15–18 Therefore, we adopted low-concentration chlor-

osulfonic acid solutions and the molar ratio of polymer repeating

unit to chlorosulfonic acid was 1 : 5 for the sake of suitable

proton conductivity as proton exchange membranes for vana-

dium redox flow battery (VRB) applications. The resulting

sulfonated poly(arylene thioether ketone) (SPTK) and

sulfonated poly(arylene thioether ketone ketone) (SPTKK)

polymers are soluble in polar organic solvents such as DMSO,

DMF and NMP (N-methylpyrrolidinone), and can be cast into

tough, flexible, and transparent membranes from their solutions.

The basic properties of SPTK and SPTKK are shown in Table 1.

The sulfonation degree (SD) was determined by both titration

and elemental analysis (EA) as shown in Table 2. Since the

sulfonated ionomers are insoluble in CH2Cl2, the longer

distances between each fluorenyl group are favorable for the

sulfonation reaction. As can be seen from Table 1, SPTKK has

a higher IEC and SD than SPTK as expected. On the other hand,

the EWs of SPTK and SPTKK are successfully controlled to be

adequate for satisfied proton conductivity.1H NMR was conducted to confirm the structure of the

synthesized polymers. The 1H NMR peaks of unsulfonated

polymers are well separated and easy to assign. We were con-

cerned more about the 1H NMR peaks of the target sulfonated

polymers. Because the chemical shift value reflects the electron

environments of protons, the relative value is generally the same

for the same substance in different solvents. Taking the protons

on other phenyl rings as references, it can be seen that the

chemical shift of the protons on the fluorenyl rings changed after

the sulfonation. The 1H NMR spectra of PTK (CDCl3) and

SPTK (DMSO-d6) are shown in Fig. S1.† The protons 1, 2, 7 and

8 can be simply assigned according to the integral area

and chemical shift value, and were chosen for reference. It can be

seen that the chemical shift value of protons 3, 4, 5 and 6 on

fluorenyl rings obviously changed for SPTK. The same circum-

stance happened for SPTKK as shown in Fig. S2.†

The FT-IR spectra also confirm the sulfonation of polymers

PTK and PTKK. Samples were cast into films at a thickness of

about 30 mm, dried under vacuum for 24 h and then exposed to

test immediately. As shown in Fig. S3,† the new absorptions at

1090 and 1032 cm�1 for SPTK and SPTKK in turn are observed.

They are attributed to the symmetric and asymmetric stretching

vibrations of S–O bond respectively, indicating the existence of


sulfonic acid groups in SPTK and SPTKK. Furthermore, the

strong adsorption band from 3300 to 3700 cm�1 for SPTK and

SPTKK which corresponds to the adsorption of hydroxyl groups

also demonstrate the successful introduction of hydrophilic

sulfonic acid groups.

Thermal properties

The thermal properties of the ionomers SPTK and SPTKK were

investigated by both DSC and TGA in a nitrogen atmosphere.

DSC was performed in the temperature range of 30–310 �C and

the second scan was initiated after quenching to the room

temperature from the first scan. No glass transition was found in

the second scan curves due to the inherent nature of ionomers.

The TGA and DTG curves of SPTK and SPTKK are shown in

Fig. 1. There are two typical weight-loss stages in both TGA

curves of SPTK and SPTKK, which are same as the other

sulfonated aromatic ionomers reported. The first weight-loss

stage ranging from about 300–400 �C is considered to represent

the decomposition of sulfonic acid groups, while the second

weight-loss step started from about 400 �C are accepted to

represent the degradation of the aromatic polymer main chain.

The 5% weight loss temperatures of SPTK and SPTKK are

around 350 �C, which validates the high thermal stability of both

samples.

In order to investigate the thermal degradation behavior of

SPTK and SPTKK, TGA-IR technology was applied and the

results are depicted in Fig. 2. TGA tests were carried out in

a nitrogen atmosphere in the temperature range from room

temperature to 600 �C at a heating rate of 10 �C min�1. The

evolved gases were led to FT-IR spectrometer directly through

a connected heated gas line. The operation conditions of the FT-

IR were as follows: frequency range: 4000–500 cm�1; resolution:


Table 3 Proton conductivity and VO2+ permeability of SPTK, SPTKKand Nafion 117 membranes at room temperature

MembraneProton conductivity/�10�2S cm�1

VO2+

permeability/m2 s�1

SPTK 1.05 1.2 � 10�13

SPTKK 1.36 3.1 � 10�13

Nafion 117 2.03 4.9 � 10�12

Fig. 2 TGA-IR spectra of the decompositions of (a) SPTK; (b) SPTKK.

4.0 cm�1; scan rate: 8 scans/s. It can be seen from Fig. 5 that the

decomposition products at about 300–400 �C are SO2 for both

SPTK and SPTKK, indicating the loss of sulfonic acid groups.

Thereafter, the degradation product of SPTK starting from

about 400 �C is SO2 while that of SPTKK is CO2. This implies

the main chain degradation of SPTK is the thioether bond while

that of SPTKK is the acyl group.

Water affinity and proton conductivity

The water uptake and swelling ratio measured at room

temperature are listed in Table 1. SPTKK shows better water

Fig. 3 The water uptake and l of SPTK and SPTKK as a function of

temperature.


affinity than SPTK owing to its higher IEC value. The

dependence of water uptake on temperature is depicted in

Fig. 3. It is apparent that the amounts as well as the rate of

water uptake increased with increasing temperature. The water

molecules per sulfonic acid group (l) of each membrane are

also calculated and depicted in Fig. 6. While the water uptake

of SPTKK is much larger than SPTK, the l values are similar

which reveals the similar hydration state of sulfonic acid groups

in these two membranes. The water absorbed plays an impor-

tant role in proton conduction. However, an excess of absorbed

water leads to exaggerated swelling so as to decrease the

selectivity and destroy the mechanical properties of the

membrane. Compared to Nafion 117 membranes, the synthe-

sized ionomers SPTK and SPTKK exhibit lower water uptake

and l values, but lower VO2+ permeabilities, which will be

discussed in following section.

The proton conductivity under full hydrated conditions was

measured and is listed in Table 3. The proton conductivity of

Nafion 117 membranes was also measured under the same

conditions for comparison. The proton conductivity of Nafion

117 membranes is 2.03 � 10�2 S cm�1, slightly higher than

1.05 � 10�2 S cm�1 for SPTK and 1.36 � 10�2 S cm�1 for

SPTKK, respectively. However, the proton conductivity of

SPTK and SPTKK is comparable to the most reported

sulfonated aromatic ionomers with the same IEC value.25,26

The dependence of proton conductivity on temperature is

shown in Fig. 4. It is obvious that the proton conductivity

increases with increasing temperature for both SPTK and

SPTKK. Therefore, the working conditions of the assembled

battery can be optimized by taking the temperature into

consideration.

Fig. 4 The proton conductivity of SPTK and SPTKK as a function of

temperature.


Fig. 6 Discharge characteristics of VRBs assembled with SPTK, SPTKK

and Nafion 117 membranes at the current density of 50 mA cm�2.

Table 4 Mechanical, oxidative and thermal properties of the polymersSPTK and SPTKK

SampleTensilestrength/MPa

Elongation atbreak (%)

Oxidativetimea/min Td�5%/�C

SPTK 23 32 89 352SPTKK 18 25 73 347

a Times were recorded from commencement to break of samples.

Mechanical properties and oxidative stability

The mechanical properties were determined at 20 �C and 50%

relative humidity. The samples were cut into dumbbell shape

with a size of 50mm � 2 mm. The cross-head speed was

controlled at a constant value of 2 mm min�1. Because of the

assembly necessary and the impact of electrolytes, the mechan-

ical properties of membranes are essential important for vana-

dium redox flow battery (VRB) applications. In respect that the

backbones of SPTK and SPTKK are rigid aromatic segments,

the tensile strength of them are relative high for the VRB

application as shown in Table 4.

The oxidative stability of the samples was investigated as

antioxidative stability in Fenton’s reagent (3 wt% H2O2 + 2 ppm

FeSO4) at 80 �C. Samples were dissolved in the reagent little-by-

little with flocculent products appearing. The reaction time was

recorded and listed in Table 4. The water uptake and swelling

ratio of SPTKK is higher than SPTK, enhancing the attack

opportunities of free radicals for SPTKK in the absorbed water.

Therefore, SPTK shows longer oxidative time as demonstrated in

Table 4. Both samples exhibit high oxidative stability, referring

to the literature.18, 22

VO2+ permeability and cell performance

Vanadium redox flow batteries (VRBs) employ V2+/V3+ and VO+2/

VO2+ redox couples in sulfuric acid solution as its anolyte and

catholyte. The permeation of these four species takes place

spontaneously and causes battery capacity loss which is consid-

ered as the main reason for self-discharge. Because of their

different ionic diameters, their permeabilities in the same

Fig. 5 The VO2+ permeation of SPTK, SPTKK and Nafion 117

membranes as a function of time.


membranes are distinctly varied and the self-discharge rate is

determined by the largest one. The electrolytes are prepared from

the electro-oxidation or electro-reduction of VOSO4 solution.

VO2+ permeability is always taken as a nominal property for VRB

separators with regard to their self-discharge characteristics.

The VO2+ permeations of SPTK, SPTKK and Nafion 117

membranes as a function of time are shown in Fig. 5. It can be

seen that the permeation increases linearly with time for all

membranes. The permeations of SPTK and SPTKK are

dramatically lower than Nafion 117 membranes at the same time

interval. The VO2+ permeabilities are calculated and listed in

Table 3. The VO2+ permeabilities of SPTK and SPTKK are one

order of magnitude lower than Nafion 117 membranes. In

conclusion, the VRBs assembled with SPTK and SPTKK are

expected to obtain higher Coulombic efficiencies than those

assembled with Nafion 117 membranes.

The discharge characteristics of VRBs assembled with SPTK,

SPTKK and Nafion 117 membraned at a current density of

50 mA cm�2 are shown in Fig. 6. It is clear that the discharge time

of VRB-SPTKK is the longest while the discharge voltage of

VRB-Nafion 117 is the highest. All curves declined slowly with

the discharge time ensuring a steady output of electricity. The

Coulombic efficiencies are calculated to be 79.6%, 81.8% and

75.1% for VRB-SPTK, VRB-SPTKK and VRB-Nafion 117

respectively. The Coulombic efficiency is influenced by the VO2+

permeability of the membrane and the discharge voltage is

determined by the proton conductivity. All of these results are in

good accordance with the above discussion, demonstrating the

promising properties of the synthesized ionomers SPTK and

SPTKK.

Conclusions

Novel sulfonated poly(arylene thioether) ionomers (SPTK and

SPTKK) containing fluorenyl groups can be readily synthesized

with appropriate ion exchange capacities for the vanadium redox

flow battery (VRB) applications. The ionomers are soluble in

aprotic solvents and can be cast into membranes from their N,N0-

dimethylacetamide (DMAc) solutions. The 5% weight loss

temperatures of the ionomers are around 350 �C, and the


degradation product of SPTK at 400–500 �C is SO2 while that of

SPTKK is CO2, which implies different degradation mechanisms

of the aromatic main chain of the ionomers. The proton

conductivities of SPTK and SPTKK are comparable to Nafion

117 membranes, however, their VO2+ permeabilities are much

lower than Nafion 117. The Coulombic efficiencies of VRBs

assembled with SPTK, SPTKK and Nafion 117 membranes are

79.6%, 81.8% and 75.1% respectively. In conclusion, these ion-

omers could be promising candidates as proton exchange

membranes for VRB applications.

Acknowledgements

The authors would like to thank the China High-Tech Devel-

opment 863 Program (Grant No.: 2007AA03Z217), Guangdong

Province Sci & Tech Bureau (Key Strategic Project Grant No.:

2003C105004, 2006A10704004, 2006B12401006), and Guangz-

hou Sci & Tech Bureau (2005U13D2031) for financial support of

this work.

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