Novel quaternary ammonium functional addition-type norbornene copolymer as hydroxide-conductive and durable anion exchange membrane for direct methanol fuel cells

Article abstract of DOI:10.1039/c5ra09393g

Novel quaternary ammonium functional addition-type norbornene copolymers (Q_CnP(BN/PhBN), n = 1, 6, 10, 12) with different alkyl side chain length comb-shaped structures or different contents of 2-(4-phenyl-butoxymethy-lene)-5-norbornene (PhBN) (22-77%) are synthesized via copolymerization of functionalized norbornenes, and their corresponding hydroxide-conductive anion exchange membranes (AEMs) with effective hydrophilic-hydrophobic separation are prepared and confirmed by TEM or SEM. The achieved AEMs show high ion exchange capacity (1.83 mmol g^-1), as well as low methanol permeability (1.97-20.4 × 10^-7 cm² s^-1), which are lower than that of Nafion. The ionic conductivity increases with the operation temperature increasing and is observed up to 4.14 × 10^-3 S cm^-1. The AEMs exhibit excellent dimensional stability with a swelling degree in plane between 0.9-3.3% and good chemical stability under 6 M NaOH solution even after a month. Membrane electrode assembly (MEA) is fabricated by using the alkalized Q_C12P(BN/PhBN)-77 as the AEM and tested in an alkaline direct methanol fuel cell. The open circuit voltage (OVC) of 0.54 V and the maximum current density of 66 mW cm^-2 are achieved at 80 °C, respectively.

Full text of DOI:10.1039/c5ra09393g

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PAPER
Novel quaternary ammonium functional addition-
type norbornene copolymer as hydroxide-
conductive and durable anion exchange membrane
for direct methanol fuel cells
Cite this: RSC Adv., 2015, 5, 63215
ab
Jingyin Liu,ab Hongyu Zhu,a Yan Zhenga and Defu Chenc
*
Xiaohui He,
Novel quaternary ammonium functional addition-type norbornene copolymers (QCnP(BN/PhBN), n ¼ 1, 6,
10, 12) with different alkyl side chain length comb-shaped structures or different contents of 2-(4-phenyl-
butoxymethy-lene)-5-norbornene (PhBN) (22–77%) are synthesized via copolymerization of functionalized
norbornenes, and their corresponding hydroxide-conductive anion exchange membranes (AEMs) with
effective hydrophilic–hydrophobic separation are prepared and confirmed by TEM or SEM. The achieved
AEMs show high ion exchange capacity (1.83 mmol gꢀ1), as well as low methanol permeability (1.97–20.4
ꢁ 10ꢀ7 cm2
s
ꢀ1), which are lower than that of Nafion®. The ionic conductivity increases with the
operation temperature increasing and is observed up to 4.14 ꢁ 10ꢀ3 S cmꢀ1. The AEMs exhibit excellent
dimensional stability with a swelling degree in plane between 0.9–3.3% and good chemical stability
under 6 M NaOH solution even after a month. Membrane electrode assembly (MEA) is fabricated by
using the alkalized QC12P(BN/PhBN)-77 as the AEM and tested in an alkaline direct methanol fuel cell.
The open circuit voltage (OVC) of 0.54 V and the maximum current density of 66 mW cmꢀ2 are
achieved at 80 ꢂC, respectively.
Received 19th May 2015
Accepted 16th July 2015
DOI: 10.1039/c5ra09393g
www.rsc.org/advances
reaction kinetics,3–6 (ii) AEMDMFC can operate with platinum
free catalyst and greatly reduce the cost of the device,7–9 (iii) new
1 Introduction
routes are available for addressing water management, and (iv)
the electro osmotic drag of water from the air cathode to the fuel
anode will lower fuel crossover.10 Despite these benets, the
AEMDMFC suffers from the low diffusion coefficient of
hydroxide ions. A membrane with high ionic conductivity may
be obtained by the formation of nanochannels that facilitate ion
transport or improve its ion exchange capacity.
There are various polymeric materials that can be used in
AEMs, such as polyolen,11–13 poly(ether ketone),14 poly-
sulfone15,16 and so on. The AEMs can be obtained by further
chloromethylation, quaternization and alkalization. And the
ammonium groups, phosphonium groups and sulfonium
groups are always chosen for the solid polymer electrolyte.17–20
Among the different species of exchange groups, ammonium
groups are thought to have a higher thermal and chemical
stability compared to other groups.
Fuel cells are efficient energy conversion devices that can
directly convert chemical energy to electricity and have been
attracting attention as a clean energy technology.1,2 They are
mainly divided into proton exchange membrane direct meth-
anol fuel cells (PEMDMFC) and anion exchange membrane
direct methanol fuel cells (AEMDMFC). The most widespread
fuel cell technologies are based on the proton-exchange
membrane fuel cell (PEMFC). Naon, produced by DuPont, is
the preferred polymer electrolyte membrane in the PEMFC,
which is a perfiuorosulfonated copolymer that exhibits high
ionic conductivity and dimensional stability. Nevertheless,
PEMFCs must use platinum as the catalyst or other precious
metal-based catalysts, and have limited lifetime for easy
degradation. Meanwhile, PEMFCs have the slow rate of oxygen
reduction under acidic conditions.
As an alternative to PEMDMFC, AEMDMFC have received
great interest in recent years due to the following advantages:
(i) a high pH value will provide enhancement in the electrode
Addition-type polynorbornene (PNB) is a kind of suitable
polymer material for DMFC with high thermal stability, good
mechanical properties, excellent dielectric properties, low
methanol permeability, as well as good chemical stability.21–23
aSchool of Materials Science and Engineering, Nanchang University, 999 Xuefu Avenue,
Nanchang 330031, China. E-mail: hexiaohui@ncu.edu.cn
We have applied PNB to PEMFC in our previous studies.24–28
A
bJiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University,
high ionic conductivity can be improved by increasing the
amount of functional groups in the membrane. An effective
strategy that we herein present is to introduce the functional
999 Xuefu Avenue, Nanchang 330031, China
cSchool of Civil Engineering and Architecture, Nanchang University, 999 Xuefu Avenue,
Nanchang 330031, China
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group onto the polymer structure, and to avoid attacking the dropwise to the solution over a 15 min period. The reaction is
b-hydrogen of the ammonium from the hydroxyl ions that running at room temperature for 8 hours, and then quenched
leading to the degradation or loss of the mechanical properties with a 10% sodium bicarbonate solution. The organic solution
of the membrane. Meanwhile, we design the molecular struc- is passed through a plug of Celite, rinsed with water, and dried
ture with nanophase separation which can form nanochannels over magnesium sulfate. The solution is ltered and dried by
between a hydrophobic matrix and hydrophilic side groups on using rotary evaporation to yield the crude product. The crude
the basis of Naon's high conductivity with the similar product is further eluted over silica gel with petroleum ether,
structure.
and the rst of two UV active spots that seen by TLC are
In this study, we choose PNB as an organic polymer matrix to collected, and dried to obtain the product as a light yellow
get high thermal stability, good dimensional stability and high liquid (5.4 g, 77% yield). 1H NMR (CDCl3, d, ppm): 7.26–7.15
ionic conductivity. Initial series of anion-conductive and (m, 5H, C6H5–), 2.6 (t, 2H, C6H5–CH2–), 3.37 (t, 2H, –CH2-Br),
durable quaternary ammonium functional addition-type nor- 1.87–1.72 (m, 4H, –CH2–CH2–).
bornene copolymer (QCnP(BN/PhBN), n ¼ 1, 6, 10, 12) with
different alkyl side chains length comb-shaped structures or BN synthesis, and the product is a yellow liquid. 1H NMR
different contents of 2-(4-phenyl-butoxymethy-lene)-5- (CDCl3, d, ppm): 7.26–7.15 (m, 5H, C6H5), 6.12–6.05 (m, 2H,
The PhBN is synthesized by using the same route as that of
norbornene (PhBN) (22–77%) are innovatively designed and –CH]CH–), 3.4 (t, 2H, norbornene-CH2–O–), 3.27 (t, 2H,
prepared by using nickel catalyst in a vinyl-addition polymeri- O–CH2–C3H6–), 2.6 (t, 2H, C6H5–CH2–), 2.76–0.84 (m, 6H, nor-
zation fashion. An important consequence is that the intro- bornene-6H), 1.68–1.1 (m, 4H, –CH2–CH2–).
duction of different long alkyl side chains pendant to the
2.2.3 Synthesis of P(BN/PhBN). Copolymerization of BN
quaternary ammonium cation and the formation of comb- and PhBN (molar ratio ¼ 9 : 1 or 7 : 3 or 5 : 5) is carried out by
shaped structures while choosing PNB as polymer main using a homogeneous catalyst that combination of bis(b-keto-
chain, which can effectively build hydrophilic–hydrophobic naphthylamino) nickel(II) and B(C6F5)3 in a round-bottom ask
separation, and thus enhance the ionic conductivities of these with a magnetic stirring bar and sufficiently purged with
materials by forming the nanochannels. The obtained AEMs nitrogen. The appropriate B(C6F5)3 solid, anhydrous toluene,
performances application in direct methanol fuel cell can be BN, PhBN, and nickel(II) catalyst are added to the reactor
controllable by tuning the content of PhBN or adding different sequentially. The copolymerization is conducted at 60 ꢂC for
long alkyl side chains pendant to the nitrogen-centered cation. 24 h and is terminated by adding HCl/EtOH (v/v ¼ 1/9). The
resulting P(BN/PhBN) copolymers {weight-average molecular
weight (Mw) ¼ 2.72 ꢁ 105, 1.94 ꢁ 105, 1.33 ꢁ 105 g molꢀ1
,
2 Experimental
2.1 Materials
respectively. Molecular weight distribution [MWD z 2]} are
separated off, washed with fresh methanol, and dried in vacuo
at 40 ꢂC until the weight remains constant. The total volume of
the liquid phase is kept to be 10 mL.
Toluene is reuxed and distilled from sodium and benzophe-
none under dry nitrogen, trimethylamine aqueous solution (30
wt%), tin(IV) chloride, chloromethyl methyl ether (CMME),
phosphorus tribromide are obtained from Sigma-Aldrich Co.
Sodium hydroxide, trichloromethane, and 5-norbornene-2-
methanol are purchased from Puyang Huicheng chemical Co.,
Ltd, tetrahydrofuran (THF) is purchased from Tianjin Damao
Chemical Reagent Factory of China, and chloroform-d is
obtained from Aldrich Chemical. All chemicals are used as
received, unless otherwise be specied. The catalyst, bis(b-ke-
tonaphthylamino) nickel(II), is synthesized according to the
method reported in our previous article.29
2.3 Chloromethylation, quaternization, and membrane
casting
Fresh trichloromethane (10 mL) is added to P(BN/PhBN) (0.2 g).
Chloromethyl methyl ether (CMME) (0.15 mol) and SnCl4
(0.01 mol) are then injected into the solution (Caution: CMME
is carcinogenic and potentially harmful to human health). The
reaction is allowed to continue for 5 h at 55 ꢂC. Then the
methanol is poured into the reaction mixture to get the crude
chloromethylated polymer, named CP(BN/PhBN), and it is
recovered by washing the precipitate several times with
methanol.
2.2 Addition polymerization
2.2.1 Synthesis of 2-butoxymethylene norbornene (BN).
The obtained chloromethylated copolymers CP(BN/PhBN)
The monomer BN is synthesized by using a procedure reported (0.20 g) are dissolved in THF (3 mL) and further quaternized
by Liu et al.24
by adding N,N-dimethylhexylamine (0.1 mol) to the copolymer
2.2.2 Synthesis of 2-butoxymethylene norbornene (PhBN). at room temperature for 24 h. The solution is ltered with a
4-Pheny-1-bromide-butane (M2) is synthesized by using a 450 nm PTFE membrane lter and then cast onto a at glass
procedure reported by Hayward et al. with minor modica- plate. The cast solution is dried at room temperature for 24 h to
tions.30 4-Phenylbutanol (M1) (5.0 g, 3.33 ꢁ 10ꢀ2 mol) in dry obtain a ductile membrane, named QCnP(BN/PhBN). The chlo-
methylene chloride (100 mL) is added to a 250 mL ask, and ride ions in the lm are exchanged for hydroxide ions by
sealed under a nitrogen atmosphere. The ask is placed in an soaking in 0.1 M NaOH for about 48 h. Aer washing several
ice water bath, and the mixture is stirred for 20 min. Phos- times with water, and the QCnP(BN/PhBN) membranes in
phorus tribromide (3.16 mL, 3.33 ꢁ 10ꢀ2 mol) is added hydroxide form are achieved.
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For the measurement of water uptake, the membranes are
immersed in deionized water for 48 h under different temper-
ature conditions, and the wet membranes are weighted aer the
surface being wiped with tissue paper. These wet membranes
are dried at a set temperature of 60 ꢂC until constant dry weight
is obtained. The uptake of water (hw) is calculated by using the
following equation:
2.4 Characterization
2.4.1 1H NMR spectra. The chemical structures of the
synthesized monomers and polymers are conrmed by
analyzing the 1H NMR and recorded on A Bruker AvanceIII 600
spectrometer. Chloroform-d is used as the NMR solvent. The
ion exchange capacity (IEC) and degree of chloromethylation
1
(DC) are determined from the respective H NMR spectra.
mwet ꢀ mdry
2.4.2 GPC. The gel permeation chromatography (GPC) is
conducted with a Breeze Waters system equipped with a Rheo-
dyne injector, a 1515 Isocratic pump, and a Waters 2414 differ-
ential refractometer by using polystyrenes as the standard and
tetrahydrofuran (THF) as the eluent at a ow rate of 1.0 mL minꢀ1
and 40 ꢂC through a Styragel column set, and the Mw is ranging
from 102 to 106.
hwð%Þ ¼
ꢁ 100%
(3)
mdry
where mdry and mwet are the weight of dry and wet membrane
samples, respectively. The uptake of water is the average of the
measurement by three times.
2.4.6 Ion-exchange capacity (IEC). The ion-exchange
capacity is dened as the ratio between the number of
exchangeable ionic groups and the weight of the dry membrane.
The IECs of membranes are determined by the back titration
method. Briey, the dried OHꢀ form membranes are immersed
in 50 mL 0.01 M HCl aqueous solutions for 24 h, followed by
back titration of 0.01 M NaOH solution as the indicator. The 30
mL 0.01 M HCl solution is used as the blank sample for the
contrast experiment. The measured IEC (mmol gꢀ1) of the
membrane is calculated as follows:
2.4.3 Thermal analysis and mechanical properties.
Thermal degradation and stability of the membranes are
investigated by using a thermo-gravimetric analyzer (TGA)
(Perkin-Elmer instrument TGA 7ꢀ) 1under a nitrogen atmosphere
at a heating rate of 20 ꢂC min from room temperature to
800 ꢂC. The mechanical properties are measured on a CMT8502
Machine model GD203A (ShenZhen Sans Testing Machine,
China) at a speed of 5 mm minꢀ1
.
2.4.4 Contact angle measurements and microscopic char-
acterizations. Contact angle measurements are performed on
surfaces of every prepared membranes with drops of water by
using Powereach JC2000A Interface Tension/Contact Angle
Measure Equipments to study the hydrophilicity. Samples are
spin coated onto the quartz glass substrates at a spin speed of
2000 rpm for 40 s and then thermally annealed at 60 ꢂC for 5 min.
The surface morphology of the dried membranes are studied
by a JEOL JEM-2100F transmission electron microscope (TEM).
Samples for transmission electron microscopy (TEM) are prepared
as follows:31 membranes are stained by soaking in 1 mol Lꢀ1
sodium tungstate solution overnight, then washed several times
with water and dried under vacuum at room temperature. The
samples are dissolved in THF and coated on copper grids.
The surface and cross section morphology of the membranes
are investigated by scanning electron microscope (SEM) and
Environmental Scanning Electron Microscope (ESEM, FEI
Quanta 200). All samples are soaked in the liquid nitrogen and
fractured, followed by the sputtering of a thin layer of gold.
2.4.5 Dimensional stability and uptake of water. The
swelling ratio of the OHꢀ form membranes in plane (SRp) and in
thickness (SRt) direction are measured by the following
equations:
CHCIVHCl ꢀ CNaOHVNaOH
IEC ¼
(4)
M
where VNaOH and VHCl is the consumed volume (mL) of the NaOH
solution and HCl solution, respectively. CNaOH and CHCl are the
concentration of NaOH solution and HCl solution (mol Lꢀ1), M
is the mass of dry membrane.
2.4.7 Membrane conductivity measurements. The ionic
conductivity (d) of the membrane is evaluated at different
temperatures (30–90 ꢂC) by three-electrode electrochemical
impedance spectra method, using a CHI660 electrochemical
workstation (CH Instruments). The sample is sandwiched
between two PTFE gaskets in a PTFE diffusion cell that
composed of two symmetrical chambers. The cell is lled with
the electrolyte composed of 1 M NaOH solution.
The two platinum wires are used as the working electrode
and counter electrode, as well as a Ag/AgCl electrode function-
alized as the reference electrode that introduced into the elec-
trolyte solution. The impedance spectra are recorded with the
help of ZPlot/ZView soware under an ac perturbation signal of
10 mV over the frequency range of 1–105 Hz. The electric
resistance of the system (without membrane divided) is
measured as R1, and the electric resistance of the system (with
membrane divided) is measured as R2. The difference value
(R2 ꢀ R1) is the testing value of the anion exchange membrane
resistance (R). The ionic conductivity (d) of the membrane is
calculated from the following equation:
lwet ꢀ ldry
SRp ¼
(1)
(2)
ldry
twet ꢀ tdry
SRt ¼
tdry
d ¼ l/(RA)
(5)
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
where lwet
¼
awetbwet and ldry
¼
adrybdry are the average
where d, l, R and A are the ionic conductivity, thickness of
membranes, the resistance of the membrane, and the cross-
sectional area of the membrane, respectively.
length of wet and dry membrane samples, respectively, in
which, a and b are the lengths and widths of wet and dry
membrane samples, respectively. twet and tdry are the thickness
of wet and dry membranes, respectively.
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corresponding to a/b, the peak at 2.6 ppm can be assigned to the
hydrogen corresponding to c (Fig. 1a).
2.4.8 Methanol permeability measurements. An organic
glass diffusion cell is used to obtain the methanol permeability
of the membrane. The diffusion cell is composed of two
membrane sample. One chamber of the cell (V1) is lled with
5 M (C1) methanol solution in distilled water. The other
chamber (V2) is lled with water. A sample is clamped between
the two chambers. Methanol permeates across the membrane
by the concentration difference between the two chambers,
which can prompt methanol into distilled water. Then we
collect the solution from distilled water at every same time, the
concentration of it is dened C2. The methanol concentration in
the receiving chamber as a function of time is given by the
equation:
The chloromethyl group is introduced to P(BN/PhBN)
(Fig. 1b), and chloromethylated protons appear at 4.5 ppm.
The contents of PhBN in copolymers can be carefully controlled
by adjusting the monomer feed ratios (BN/PhBN ¼ 9/1, 7/3, 5/5).
And it can be calculated to be (a) 22%, (b) 40%, and (c) 77% by
the following equation.
A
B
2x
¼
4x þ 4y
A is the integral area of peak a/b, B is the c, x was set as the
contents of PhBN, y is set as the contents of BN, and x + y ¼ 1.
At the same time, the degree of chloromethylation can be
1
calculated by the same method with the area ratio of H NMR
C2(t) ¼ [ADKC1(t1 ꢀ t0)/(V1l)]
(6)
peaks k and m, which are 87%, 71%, and 64% and shown in
Table 4.
where A (cm2) is the membrane area, l (cm) is the membrane
thickness, D is the methanol diffusivity, and K is the partition
coefficient between the membrane and the adjacent solution.
The product DK means the membrane permeability (P):
The P(BN/PhBN) copolymers are obtained by using Ni(II)/
B(C6F5)3 catalytic system and characterized by GPC (as shown in
Fig. 2). The Mw data of the obtained copolymers are all up to 105,
and the WMD data are all relative narrow (WMD ¼ 1.75–2.49).
The relative narrow MWD and appearing as a single modal in
the GPC chromatogram indicate that copolymerization occurs
at the single active site, and the products are the true copoly-
mers instead of blends of homopolymers. The polymerization
results are presented in Table 1.
P ¼ (C2(t)V1l)/[AC1(t1 ꢀ t0)]
(7)
C2 is measured several times during the permeation experiment
and the methanol permeability is obtained from the slope of the
straight line. The methanol concentration is measured by using
a gas chromatography of Agilent 7890A GC system equipped
with a FID detector.
2.4.9 Single-cell performance tests. The catalyst solution is
prepared by mixing 40% Pt/C (Hesen) with a solution of 5 wt%
Naon (DuPont) and isopropanol, and then sonicated for 5 h to
get a homogeneous solution. The catalyst solution is sprayed
onto the carbon paper to obtain a catalyst layer with a Pt loading
of 0.5 mg cmꢀ2 for both anode and cathode.
3.2 Membrane appearance and surface hydrophilicity
All the alkaline membranes with a thickness around 200 nm are
homogeneous, exible, and yellow. The membranes and their
corresponding solution before alkalization are presented in
Fig. 3.
Values of measured contact angles are shown in Fig. 4. It is
assumed that the polynorbornene main chain showed strong
hydrophobicity which restricted the hydrophilicity of the poly-
mers. The graed side chain could effectively improve the
hydrophilicity of the polynorbornene main chain, which could
be illuminated by the relatively small contact angles of all the
MEA (membrane electrode assembly) is fabricated by hot-
pressing the alkalized QC12P(BN/PhBN)-77 AEM along with the
anode and cathode catalyst layer for 5 min under pressure of
6 MPa at 60 ꢂC. The MEA is evaluated in a single fuel cell with an
active area of 4 cm2 in methanol/air. Methanol and air are
supplied to enter the anode and cathode channels at a ow rate
of 3 mL minꢀ1 and 300 mL minꢀ1, respectively. Polarization
curves are obtained by using a commercial fuel cell evaluation
membranes.
QC12P(BN/PhBN)-77 membrane has the best
hydrophilicity, indicating that the smaller contact angle can be
observed in membranes with longer side chain or lager contents
of hydrophilicity PhBN segment.
ꢂ
system (Arbin BT 2000) at 80 C.
3.3 Microscopic characterizations
Furthermore, the microscopic characterizations of the alkaline
membranes are studied by TEM and SEM. All of the alkaline
membranes show no cracks and holes on the membrane
surface indicate the dense nature of the membrane (Fig. 5a and
3 Results and discussion
3.1 Synthesis and characterization of comb-shaped
copolymers
A series of copolymers P(BN/PhBN), chloromethylated copoly- c). The even-distributed nanosized spot like structure on the
mers CP(BN/PhBN), and quaternized copolymers QCnP(BN/ membrane surface indicate that the micro phase separation
PhBN) are synthesized as shown in Scheme 1.
(Fig. 5b) is formed between the hydrophobic main chain and
The structures of P(BN/PhBN) and CP(BN/PhBN) are char- the hydrophilic side chain structure of polymer. And the white
acterized by 1H NMR spectroscopy (Fig. 1), it proves that the particle is called the ion domain which is benecial to the
polymers are vinyl-addition type by the resonance absence of formation of ion transport channel32,33 and can improve the
the proton hydrogen connected to the double bond at 6.12– ionic conductivity. The micro-morphology stemmed from the
6.05 ppm. The peak at 3.4 ppm can be assigned to the hydrogen comb-shaped copolymer is further conrmed by TEM
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Scheme 1 Synthesis of comb-shaped P(BN/PhBN), CP(BN/PhBN), and QCnP(BN/PhBN) copolymers.
technique. The TEM images of alkalized QC6P(BN/PhBN)-22 P(BN/NB) main chains while the dark regions are corresponded
provide a direct evidence of biphasic morphology for the poly- to the hydrophilic PhBN side chains.
mer membranes and exhibit a clear hydrophilic–hydrophobic
microphase separation with wormlike and interconnect hydro-
philic network nanochannels (Fig. 5d). This unique micro-
3.4 Thermal stability and mechanical properties
morphology supports sufficient hydroxide conducting through Fig. 6 shows the TGA data for the comb-shaped copolymers
uniformly distributed well-connected ion pathways. The light QCnP(BN/PhBN) (in OHꢀ form). From the diagram we can know,
regions are corresponded to the hydrophobic domains of the its decomposition can be divided into four parts in the TGA
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Secondly, the elimination of quaternary ammonium groups
occurs at 160 ꢂC, and the loss percentage increases with the
gra ratio increasing. Thirdly, the mass loss commences at
ꢂ
280–420 C that is accredited to the decomposition of the side
chains of copolymers.31 Fourthly, the weight loss at around
430 ꢂC is related to the degradation of the polymer backbone.26
Based on these observations, we can conclude that the thermal
stability of this membrane is able to satisfy the requirement of
application in direct methanol fuel cell.
The tensile curves of the copolymer lms with different
PhBN content and different alkyl side chains are shown in
Fig. 7, and the mechanical properties are presented in Table 2.
The lm of copolymer that contains 22.0% PhBN shows the best
mechanical properties, and the tensile strength is about
6.1–7.9 MPa and the elastic modulus is 300.1–488.5 MPa, the
toughness and strength are enough to meet the application
requirements in fuel cells.
3.5 Water uptake and dimensional swelling
For fuel cell application, water uptake (WU) is of particular
importance in evaluating the performance of the AEMs. Excess
water adsorption will lead to deterioration in the thermal,
mechanical, dimensional, chemical stabilities and ionic
concentration. Table 3 shows the water uptake data of the
copolymer membranes evaluated at 25 ꢂC and 60 ꢂC. Generally,
higher IEC values can give higher water uptake because both
properties are strongly related to the amount of quaternary
Fig. 1 1H NMR spectra of copolymers (in CDCl3): (a) P(BN/PhBN);
(b) chloromethylation of P(BN/PhBN).
ammonium group (Tables
3 and 4). QC12P(BN/PhBN)-77
membranes with maximum chloromethyl graing ratio
exhibits 16.3% of water uptake at 25 ꢂC, which is lower than
Naon 117.
The dimensional stability of these membranes, which is one
of the crucial properties of AEMs for their application in fuel
cells, is evaluated by the water swelling ratio. A low in-plane
swelling is particularly expected to inhibit the deamination of
catalyst layer caused by dimensional mismatch. Herein, the in-
plane swelling ratio is low (the highest value is 4.21% at 60 ꢂC),
suggesting that the obtained membranes can meet the
requirements for dimensional stability in fuel cells application.
Fig. 2 The GPC curves of BN/PhBN copolymers with PhBN molar
ratios: (a) 22%, (b) 40%, (c) 77% obtained by bis(b-ketonaphthylamino)
Ni(II)/B(C6F5)3 system.
3.6 IEC, ionic conductivity and methanol permeability
curve for QP(BN/PhBN). Firstly, a slight loss between 50 and
150 C corresponding to the evaporation of absorbed water is
observed due to strong hydrophilicity of the quaternary
ammonium groups attracting water from the atmosphere.
The ion-exchange capacity (IEC) data of AEMs are calculated to
be from 0.91 to 1.81 mol gꢀ1 by the integral ratios, which is in
good agreement with the calculated values from the copolymer
compositions and the degree of chloromethylation. And these
ꢂ
Table 1 The results of vinyl-addition type copolymerizations with different monomer feed ratiosa
Sample
BN/PhBN (mol molꢀ1
)
Mol% of PhBN in copolymers
Activity (gpolymer (molNi h)ꢀ1
)
Yield (%)
Mw (ꢁ105)
PDI
P(BN/PhBN)-22
P(BN/PhBN)-40
P(BN/PhBN)-77
9/1
7/3
5/5
22
40
77
1.2 ꢁ 104
1.0 ꢁ 104
0.6 ꢁ 104
63.7
47.7
25.6
2.72
1.94
1.33
2.49
2.29
1.75
Conditions: (1) n[Cat.] is 5.0 ꢁ 10ꢀ6 mol; (2) co-catalyst is B(C6F5)3; (3) B/Cat./monomer is 20/1/2000 (molar ratio); (4) polymerization temperature
a
sets as 60 ꢂC; (5) solvent: toluene; (6) Vp ¼ 10 mL; (7) polymerization time: 24 h.
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Fig. 3 The photographs of QC12P(BN/PhBN) with different degree of chloromethylation: (1) QC12P(BN/PhBN)-22; (2) QC12P(BN/PhBN)-40;
(3) QC12P(BN/PhBN)-77.
IEC values calculated from 1H NMR are similar to the values content increasing in the membrane matrix. The QC12P(BN/
determined from titration, further suggesting that quaterniza- PhBN)-77 show the highest ionic conductivity of 4.14 ꢁ 10ꢀ3
S
tion of the chloromethyl groups occurs successfully, and has no cmꢀ1, lower than Naon117 (9.56 ꢁ 10ꢀ2 S cmꢀ1),35 which may
side effects (Table 4). QCnP(BN/PhBN) with maximum graing be due to the lower water uptake. More quaternary ammonium
ratio has IEC of 1.81 ꢁ 10ꢀ3 mol gꢀ1, which is relatively higher groups that facilitate transporting hydroxide and moderate
than that of Naon 117 might be due to higher quaternary water retention ability resulting from the comb-shaped struc-
ammonium group.
ture may be two of the reasons for the observed variations in
It is well known that the IEC values directly depend on the ionic conductivity values. Combined with the moisture content
content of quaternary ammonium group incorporated into the data (Table 3) of this lm, we discover that with the growth of
polymer and thus they are indicative of the actual ion exchange the quaternary ammonium side chain, the water absorption of
sites available for ionic conduction.34 Ionic conductivity data for this polymer membranes is enhanced. It is noteworthy that
different composite membranes are also presented in Table 4. ionic conductivity also increases. The above results indicate that
It can be seen that conductivity values increase with the PhBN the comb-polymer can build micro-phase separation, which can
Fig. 4 Water contact angle images of alkalized (a) QC1P(BN/PhBN)-22; (b) QC6P(BN/PhBN)-22; (c) QC10P(BN/PhBN)-22; (d) QC12P(BN/PhBN)-
22; (e) QC12P(BN/PhBN)-40; (f) QC12P(BN/PhBN)-77 membranes.
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Fig. 5 The SEM and TEM images of alkalized QC6P(BN/PhBN)-22 membrane: (a) SEM surface (30 mm); (b) SEM surface (1 mm); (c) SEM cross-
section (50 mm); (d) TEM image of the copolymer staining with tungstate ions.
efficiently adjust the transmission channel of OHꢀ, this theory
has been conrmed in the study of proton exchange
membrane.36
The
temperature-dependent
conductivity
measurements from 30 to 80 ꢂC are conducted and shown in
Fig. 7 The tensile curves of alkalized QCnP(BN/PhBN) with different
quaternary ammonium groups.
Fig. 6 TGA thermograms of the alkalized QCnP(BN/PhBN) membranes
recorded using a 20 ꢂC minꢀ1 heating rate under nitrogen atmosphere.
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Table 2 The key mechanical properties of alkalized QCnP(BN/PhBN)
membranes
methanol to penetrate into the membranes. The methanol
permeability of the membranes increased with the hydrophilic
side chain, because of the hydrophilicity contributes to
increasing water uptake and methanol permeability and thus
causing broadening of the ion-conducting channels to some
extent.
Tensile strength Elongation at Elastic modulus
Sample
at break (MPa) break (%)
(MPa)
Q
Q
C1P(BN/PhBN)-22 7.2
C6P(BN/PhBN)-22 6.4
4.5
4.2
32.7
23.7
4.8
450.2
488.5
393.1
300.1
206
QC10P(BN/PhBN)-22 7.9
QC12P(BN/PhBN)-22 5.1
Q
Q
3.7 Chemical stability
C12P(BN/PhBN)-40 3.2
C12P(BN/PhBN)-77 3.3
Besides the excellent thermal stability, the long-term tolerance
of QCnP(BN/PhBN) membrane is also investigated in NaOH
solutions (6 mol Lꢀ1). The hydroxide conductivities of QC1P(BN/
8.6
159.2
PhBN)-22,
Q
C12P(BN/PhBN)-22
and
QC12P(BN/PhBN)-77
Fig. 8. We observe the increase of conductivity with the opera-
tion temperature increasing. Delightfully, in contrast to the
QP(BN/PhBN), ionic conductivity increased with increasing of
the quaternary ammonium group into the polymer matrix and
the test temperatures.
The relevant data of methanol permeability for all QCnP(BN/
PhBN) membranes are determined and presented in Table 4.
Methanol permeability coefficients for these QCnP(BN/PhBN)-22
membranes (1.99–2.01 ꢁ 10ꢀ7 cm2 sꢀ1) are lower than one
order of magnitude in comparison with Naon117 (1.31 ꢁ 10ꢀ6
cm2 sꢀ1).37 It's known that the permeation of liquid molecules
across polymer membrane happened via the diffusion mecha-
nism, and the permeability of penetrate (methanol is one of the
case) is the product of its solubility and diffusivity. On account
of the narrow free void volume, the rigid structure of the
addition-type polynorbornene main chain hindered the
membranes are plotted as a function of time in Fig. 9. In alka-
line environments, the degradation of quaternary ammonium
groups is usually derived from the strong nucleophilicity of the
OHꢀ anions, which leading to displacement (SN2) and Hofmann
elimination reactions. Aer an initial period of decline behavior
for 5 days, the conductivity values of the QC1P(BN/PhBN)-22
membrane is remained constant over longer immersion
period. However, the conductivity values of the QC12P(BN/
PhBN)-22 and QC12P(BN/PhBN)-77 decrease little as time goes
on because of the b-H on the ammonium leading to Hofmann
elimination reactions. These results demonstrate that the
chemical stability of the comb-shaped membranes is excellent.
3.8 DMFC single cell performance
The alkalized QC12P(BN/PhBN)-77 is chosen as AEM for testing
the single cell performance owing to the highest hydroxide ion
Table 3 Thickness, water uptake, in-plane swelling and in-thickness swelling of QCnP(BN/PhBN) AEMs
In-
Water uptake (%)
In-plane swelling (%)
thickness swelling (%)
Thickness
(nm)
Sample
C1P(BN/PhBN)-22
QC6P(BN/PhBN)-22
QC10P(BN/PhBN)-22
25 ꢂC
60 ꢂ
C
25 ꢂ
C
60 ꢂC
25 ꢂ
C
60 ꢂ
C
Q
80
90
125
109
132
145
1.8
2.5
4.9
5.8
8.5
3.4
4.2
8.8
9.7
16.1
23.1
1.02
1.25
1.78
2.01
2.75
3.3
1.18
1.32
2.03
2.88
3.59
4.21
0.63
0.86
1.0
0.9
1.5
0.77
0.93
1.41
1.43
1.95
5.2
Q
C12P(BN/PhBN)-22
QC12P(BN/PhBN)-40
C12P(BN/PhBN)-77
Q
16.3
4.4
Table 4 DC, IEC, anion conductivity, methanol permeability of the QCnP(BN/PhBN) AEM
IEC(ꢁ10ꢀ3 mol gꢀ1
)
Sample
DC (%)
Calculateda
Titratedb
Conductivity (80 ꢂC) (10ꢀ3 S cmꢀ1
)
Methanol permeabilityc (10ꢀ7 cm2 sꢀ1
)
QC1P(BN/PhBN)-22
87
87
87
87
71
64
0.91
0.91
0.91
0.91
1.18
1.81
0.85
0.90
0.89
0.93
1.14
1.83
1.32
1.63
2.05
2.23
3.53
4.14
2.01
1.99
2.01
2.00
4.01
20.40
Q
C6P(BN/PhBN)-22
QC10P(BN/PhBN)-22
C12P(BN/PhBN)-22
QC12P(BN/PhBN)-40
Q
Q
C12P(BN/PhBN)-77
a
b
Calculated by the 1H NMR spectrum Fig. 1b. Calculated by the titrated method. c All data were tested at room temperature.
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excellent dimensional stability with swelling degree in plane
between 0.9–3.3%, as well as high ion exchange capacity up to
1.83 mmol gꢀ1. The thermal stability and mechanical properties
of the membranes are also excellent. When the membranes are
soaked into NaOH solution (6 M), their ionic conductivity
hardly decayed aer a week. Meanwhile, the conductivity
increase with the operation temperature or IEC, and up to
4.14 ꢁ 10ꢀ3 S cmꢀ1 at 80 ꢂC. Moreover, methanol permeability
of these membranes is in the range of 1.97–20.4 ꢁ 10ꢀ7 cm2 sꢀ1
,
which is lower than that of Naon®. The alkalized QC12P(BN/
PhBN)-77 AEM Performance of methanol/air fuel cell show a
maximum open circuit voltage (OVC) of 0.54 V and current
Fig. 8 Ionic conductivity of alkalized QCnP(BN/PhBN) membrane as a density of 66 mW cmꢀ2 at 80 ꢂC, respectively. Further work will
function of temperature.
focus on enhancing its ion conductivity and cell performance to
satisfy the AEMs requirement as potential application in DMFC.
Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (51463014 and 21164006).
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