Highly stable self-crosslinked anion conductive ionomers for fuel cell applications

Article abstract of DOI:10.1039/c4ra02507e

We report a simple method for the preparation of self-crosslinked anion conductive ionomers (SCL-ACIs) via polycondensation-Friedel-Crafts alkylation reactions without using any catalyst or crosslinking agent. The reported alkaline stable and methanol impervious SCL-ACIs with 2.45 mequiv. g ^-1 ion-exchange capacity (IEC) and 68 mS cm^-1 hydroxide ion conductivity were assessed as suitable candidates for alkaline direct methanol fuel cell (ADMFC) applications. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02507e

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Highly stable self-crosslinked anion conductive
ionomers for fuel cell applications†
Cite this: RSC Adv., 2014, 4, 19238
ab
Amaranadh Jastiab and Vinod K. Shahi
*
Received 21st March 2014
Accepted 10th April 2014
DOI: 10.1039/c4ra02507e
www.rsc.org/advances
1
We report a simple method for the preparation of self-crosslinked polymers by quantitative H NMR analysis.19 Therefore, devel-
anion conductive ionomers (SCL-ACIs) via polycondensation–Friedel– oping of highly conductive and chemical resistant ACIs in an
Crafts alkylation reactions without using any catalyst or crosslinking eco-friendly manner is a challenging issue.20,21
agent. The reported alkaline stable and methanol impervious SCL-ACIs
with 2.45 mequiv. gꢀ1 ion-exchange capacity (IEC) and 68 mS cmꢀ1 linked ionomers with stability and reasonable conductivity.22–25
Research interest was rendered for developing self-cross-
hydroxide ion conductivity were assessed as suitable candidates for Zhuang et al. reported self-crosslinked tertiary quaternary
alkaline direct methanol fuel cell (ADMFC) applications.
ammonium poly sulfone based alkaline polymer electrolyte.23 In
spite of good dimensional stability, these membranes are
sensitive for Hofmann elimination (due to presence of b-
hydrogen atoms on crosslinked quaternary ammonium groups)
and further deteriorates membrane conductivity. Further, Hui
Na et al. also reported self-crosslinked anion-exchange
membrane (AEM) by introducing bromine atoms onto benzylic
positions of propenyl-containing poly(ether sulfone), which
exhibits activity to undergo Friedel–Cras electrophilic substi-
tution C-alkylation reaction with aromatic benzene ring under
heat treatment.24,25 In this process, a ne temperature control is
necessary otherwise crosslinking of all benzyl bromide groups
may be possible and thus results low hydroxide conductivity
Fuel cells are green and efficient power sources for stationary,
mobile, and automotive applications.1,2 Recently, ADMFCs have
showed potential advantages over direct methanol fuel cells
(DMFCs), because the former avoids the use of noble metal
catalysts as well as catalyst poisoning, exhibits better reaction
kinetics (fuel oxidation and oxygen reduction) in alkaline media
and less fuel loss results in depletion of cell cost.3–5 ACIs based
on polystyrene containing quaternary ammonium hydroxide
are commercially available but they are unstable in alkaline
media due tꢀo membrane degradation happening by the nucle-
ophilic (OH ) attack on the quaternary ammonium group.6–8
Several other types of ACIs based on poly(phenylene oxide),
poly(ether-imide), or poly(arylene ether)s have been reported in
the literature.9–15
Poly(arylene ether)s based ACIs were typically prepared by
chloromethylation reaction using a carcinogenic chloromethyl
methyl ether (CMME) reagent.16–18 In this process, ne control
over degree and position of functionalization is also quite
difficult. Recently, Hickner et al. studied chemical stabilities of
quaternary-ammonium groups containing anion exchange
(13.4–18.8 mS cmꢀ1 at 20 C) due to less availability of benzyl
ꢁ
bromide groups for quaternization. For improved hydroxide
conductivity and stability, it is also interesting for architectural
tailoring of self-crosslinked ionomers with hydrophilic and
hydrophobic phase separation, in which hydrophilic block
enhances conductivity and stability by hydrophobic block via in
situ self-crosslinking reaction. The hydrophobic blocks were
prepared by nucleophilic substitution polycondensation of
bromo-substituted allyl bisphenol and 4,40-diuorophenyl
sulfone, while hydrophilic blocks by 2,20,3,30-tetrakis-
(chloromethyl)bisphenol (TCMBP) and 4,40-diuorophenyl
sulfone monomers.
aElectro-Membrane Processes Division, CSIR-Central Salt and Marine Chemicals
We prepared highly stable SCL-ACIs using bromo
substituted 2,20-diallyl bisphenol (ABP-Br) and TCMBP mono-
mers and in situ self-crosslinking was achieved during poly-
merization process. Since, polycondensation reaction has been
Research Institute (CSIR-CSMCRI), Council of Scientic
& Industrial Research
(CSIR), Gijubhai Badheka Marg, Bhavnagar-364 002, Gujarat, India. E-mail:
vkshahi@csmcri.org; vinodshahi1@yahoo.com; Fax: +91-0278-2566970
bAcademy of Scientic and Innovative Research, CSIR-Central Salt and Marine
Chemicals Research Institute (CSIR-CSMCRI), Council of Scientic & Industrial
Research (CSIR), Bhavnagar-364 002, Gujarat, India
ꢁ
carried out at below 200 C, bromo-substituted benzyl groups
easily undergo self-crosslinking reaction due to best leaving
nature of bromide nucleophile26 but not the chloromethyl
† Electronic supplementary information (ESI) available: Detailed experimental
procedures are included. See DOI: 10.1039/c4ra02507e
19238 | RSC Adv., 2014, 4, 19238–19241
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groups.27,28 The ACI membranes were obtained by dissolving
SCL-ACIs was synthesized by a nucleophilic substitution
alkaline polymer (quaternary ammonium hydroxide containing polycondensation of ABP-Br, 4,40-diuorophenyl sulfone,
ionomers) in NMP under thermal treatment may be due to TCMBP monomers in the presence of potassium carbonate in
presence of more hydrophilic moieties in the polymer matrix dry DMAc followed by amination with trimethylamine solution
generated by quaternization and alkalization of un-crosslinked (35%) for 12 h at room temperature. The detailed membrane
chloromethyl groups.
preparation procedure (Scheme 1) has been discussed in ESI.†
Reported SCL-ACIs contain two vicinal quaternary ammo- Since the reaction temperature was maintained around 140–170
nium groups and avoid degradations such as Hofmann elimi- ꢁC, conversion of terminal double bond into internal double
nation and Sommelet–Hauser rearrangement. Vicinal bond (isomerization) and crosslinking of vinyl bromides was
ammonium groups also resist nucleophilic (OHꢀ) attack; achieved simultaneously. In this case, the self-crosslinking (C-
further suppress the Stevens rearrangement as well as SN2 alkylation) was not achieved by chloromethyl groups due to the
substitution reaction due to steric hindrance. ABP-Br monomer presence of two vicinal –CH2Cl groups on aromatic rings. This
was prepared in two steps; synthesis of 2,20-diallyl bisphenol would not allow for the formation of stable carbocation which
(ABP) monomer and followed by treatment with N-bromo- can easily undergoes C-alkylation with aromatic ring of another
succinamide (NBS). The TCMBP was synthesized by in situ pre- molecule in the copolymer. On the other hand, alkenyl bromide
chloromethylation of bisphenol A (BPA). BPA monomer was groups presented in copolymer easily undergoes Friedel–Cras
treated with potassium carbonate to achieve electrophilic- C-alkylation due to the best leaving nature of bromide nucleo-
substitution reaction via formation of phenoxides, which acts as phile.24–26 The prepared self-crosslinked ionomers were desig-
an electron donating groups. Further, reaction of bisphenol nated as SCL ACI-X where X is the DCM (90%, 55%, and 35%)
phenoxides with formaldehyde and thionyl chloride (SOCl2) corresponding to the 2.45, 1.75 and 1.46 meq. gꢀ1 ion exchange
gives TCMBP monomer. The detailed procedure for synthesis of capacity (IEC) as estimated from 1H NMR spectra (Fig. S2, ESI†).
ABP, ABP-Br and TCMBP has been discussed in ESI.† Prepared
monomers were characterized by 1H NMR spectrum (Fig. S1(a)– ability for fuel cell applications such as water uptake (WU) and
(c) in ESI†). ion conductivity (s) were measured by previously reported
The important membrane properties to assess their suit-
Fig. S1(a)† ABP: d 1.77 (s, 6H, –C(CH3)2), 3.62 (d, 4H, –CH2), method and are summarized in Fig. 1(a).29 The SCL-ACI-90 (2.45
4.94–5.14 (dd, 4H, ]CH2), 5.45–5.90 (m, 2H, ]CH), 6.71–6.85 meq. gꢀ1 IEC) exhibited 46% water uptake 10.2% swelling ratio
(m, 2ArH ortho to –C(CH3)2), 7.73 (s, 2ArH ortho to –C(CH3)2), and 68 mS cmꢀ1 hydroxide conductivity. This clearly demon-
7.12–7.28 (m, 2ArH ortho to –OH). Fig. S1(b)† ABP-Br: d 1.89 (s, strated the dependency of water uptake and hydroxide
6H, –C(CH3)2), 5.42–5.33 (d, 2H, –CHBr), 5.1–5.27 (dd, 4H, ] conductivities of developed ACIs on IEC (Fig. 1(a)). Relatively
CH2), 5.9–6.1 (m, 2H, ]CH), 6.7–6.8 (m, 2ArH ortho to low WU values were attributed to the crosslinked hydrophobic
–C(CH3)2), 8.10 (s, 2ArH ortho to –C(CH3)2), 7.04–7.15 (m, segments, which inhibited excessive membrane swelling. While
2ArH ortho to –OH). Fig. S1(c)† TCMBP: d 4.61 (s, 4H, ortho to presence of more number of hydrophilic segments were
–C(CH3)2), 4.74 (s, 4H, ortho to –OH), 1.53 (s, 6H, –C(CH3)2), contributed for improved IEC/conductivity. The hydroxide
7.00 (d, 2ArH, ortho to –C(CH3)2), 7.06 (d, 2ArH, ortho to –OH). conductivity of prepared SCL-ACI-90% membrane exhibited
Because of strong activating –OH group by resonance effect, comparatively high conductivity than other crosslinked
electrophilic substitution occurred at both ortho positions, membranes reported in the literature (Table S2, ESI†).
and a single –CH2Cl peak along with aromatic singlet peak
Hydroxide ion conductivity for prepared SCL-ACIs was varied
were expected. But, in 1H NMR spectrum of TCMBP, two between 68–22 mS cmꢀ1 corresponding to 2.45–1.46 meq. gꢀ1
–CH2Cl peaks were observed along with a double doublet IEC values. High hydroxide conductivity for self-crosslinked
of two dissimilar aromatic protons (appeared to be merged). membranes was observed may be due to the formation of ion
These observations conrmed electrophilic substitution conducting channels developed by hydrophilic–hydrophobic
at both ortho-positions of –OH and –C(CH3)2 groups phase separation. Further, hydroxide ion conductivity for SCL-
(caused by the activating nature of –C(CH3)2 group by induc- ACIs followed Arrhenius-type temperature dependent behav-
tive effect).
iour (activation energy: 6.93–16.97 kJ molꢀ1) with respect to IEC
The degree of chloromethylation (DCM) for TCMBP mono- due to formation of inter-linked hydrophilic channels leads to
mer was estimated by 1H NMR spectra (Fig. S1(c) in ESI†) by the fast ion conduction (Fig. 1(b)). Mechanical properties of devel-
integral peaks ratio of ‘4’, and ‘5’ (assigned for –CH2Cl group) to oped SCL-ACIs were also measured (Table S4, ESI†).
those of peak ‘3’ (assigned for –C(CH3)2 group) by following
equation.
The alkaline stability of SCL-ACIs was investigated by alkali
treatment (6 M NaOH) at 60 ꢁC for 300 h and percentage loss in
IEC, hydroxide conductivity was measured (Table S3 in ESI†).
The developed SCL-ACIs did not exhibit any degradations such
as Sommelet–Hauser rearrangement (due to absent of ortho
hydrogen adjacent to quaternary ammonium groups), Hof-
mann elimination (owing to absent of b-hydrogen). Further, the
presence of two bulky vicinal quaternary ammonium groups
suppresses the effective attack of OHꢀ nucleophile due to steric
hindrance in spite of having more a-H atoms and attached
2AðH4 þ H5Þ
DCM ¼
AðH3Þ
where A(H4 + H5) is the sum of integral area for both H4 and H5
peaks (assigned for –CH2Cl group), and A(H3) is the area of the
H3 peak (assigned for –C(CH3)2 group) (Fig. S1(c)†). TCMBPs
with different DCMs (90%, 55% and 35%) were prepared under
varied conditions (Table S1, ESI†).
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Scheme 1 Reaction scheme for SCL-ACIs preparation.
Fig. 1 (a) Variation of hydroxide ion conductivity (s), and water uptake (%) with IEC (meq. gꢀ1) at 30 ꢁC: (circle) water uptake; (square)
conductivity; (b) temperature dependence of hydroxide ion conductivity for SCL-ACIs with different IECs.
hydroxide ions. So, the less deterioration in hydroxide performance assessment.30 In spite of more number of
conductivity was happened by both of Stevens rearrangement quaternary ammonium groups, reported SCL-ACIs showed
and nucleophilic substitution (OHꢀ). Only less than 10% dete- quite low methanol permeability (2.31–4.57 ꢂ 10ꢀ8 cm2 sꢀ1
)
rioration in conductivity and IEC was showed by SCL-ACIs in at 60 ꢁC in 2 M MeOH and proportional with respect to DCM
spite of having more quaternary ammonium groups aer 300 (Fig. 2). Self-crosslinked hydrophobic segments in the SCL-
hours under strong alkaline treatment conrmed their dura- ACIs may be responsible for methanol impervious nature.
bility for ADMFC application.
Further, about (16.88–18.59) ꢂ 105 S cmꢀ3 s selectivity values
ꢁ
Selectivity parameter (ratio of hydroxide conductivity and at 60 C for these SCL-ACIs were obtained due to their high
methanol permeability) plays a key role during ADMFCs conductivity showed by four quaternary ammonium
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Acknowledgements
CSIR-CSMCRI Communication no. 065/2014. Financial assis-
tance received from the Ministry of New and Renewable Energy
(MNRE), Government of India (project no. 102/79/2010-NT) is
gratefully acknowledged. Authors and thankful for Analytical
Science Division, CSIR-CSMCRI for instrumental support.
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