Enhancing the selectivity of Nafion membrane by incorporating a novel functional skeleton molecule to improve the performance of direct methanol fuel cells

Article abstract of DOI:10.1039/c9ta10215a

Conventional fillers have limitations on the modification of proton exchange membranes because of differences in the sizes and physicochemical properties of matrix molecules. Designing skeleton molecules is a vital way to address the limitations by enabli

Full text of DOI:10.1039/c9ta10215a

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
PAPER
Enhancing the selectivity of Nafion membrane by
incorporating a novel functional skeleton molecule
to improve the performance of direct methanol fuel
cells†
Cite this: J. Mater. Chem. A, 2020, 8,
96
1
a
b
a
a
a
a
Jialin Li,
Fanzhe Bu, Chunyu Ru, Hao Jiang, Yuting Duan, Yinan Sun,
a
a
a
ac
Xingtong Pu, Lichao Shang, Xifang Li and Chengji Zhao
*
Conventional fillers have limitations on the modification of proton exchange membranes because of
differences in the sizes and physicochemical properties of matrix molecules. Designing skeleton
molecules is a vital way to address the limitations by enabling precise distribution in the matrix and
inducing automatic nanoscale aggregation and separation of hydrophilic–hydrophobic phases. In this
work, a novel SDF-PAEK polymer was synthesized with a rigid hydrophobic backbone, short high-density
trifluoromethyl side chains and long flexible aliphatic pendant side chains as a skeleton molecule for the
Nafion membrane. Due to the unique molecular interaction selectivity during the membrane formation
process, SDF-PAEK automatically matches the Nafion molecular conformation by self-assembly.
Combining an improved solution formulation and membrane-casting method, the degree of
composition can therefore rise to 20%, which can effectively reduce the cost. Morphological studies
show that there is a certain degree of bicontinuous phase microcrystalline domains, and the proton
transport channels are highly concentrated. In contrast to the Nafion membrane, SDF-PAEK@Nafion-15%
Received 16th September 2019
Accepted 25th November 2019
4
ꢁ3
exhibits better performances with higher selectivity (9.73 ꢀ 10 S s cm ) and single-cell maximum
DOI: 10.1039/c9ta10215a
ꢁ2
ꢂ
power density (PDmax–139 mW cm , at 80 C), which demonstrates the feasibility of SDF-PAEK as
rsc.li/materials-a
a novel PEM skeleton molecule for fuel cell applications.
membranes created with only one type of molecule. In those
membranes, mass transport relies on discontinuous ion clus-
Introduction
Over the past several decades, scientists have realised that an ters separated from each other in hydrophobic domains, which
10,11
effective way to improve the single-cell performance of direct obviously hinders proton conductivity.
Therefore, an
methanol fuel cells (DMFCs) is to improve the proton conduc- improvement strategy is to introduce a variety of water-retaining
1
2–14
tivity of its key component, the proton exchange membrane particles, such as inorganic oxide particles,
heteropoly
to enhance the hydrophilic
philic groups were investigated, including fabricated macro- nature of the membranes. In this way, the number of hydration
15–17
18,19
(
PEM). Therefore, various membrane materials with hydro- acid,
and graphene oxides,
1–3
molecules containing hydrophilic side chains, cross-linked molecules in hydrophilic channels increases, which increases
4–6
7–9
membranes,
and porous organic protogenic polymers.
the proton conductivity of membranes signicantly. However,
However, it is challenging to form at interconnected hydro- due to the differences in sizes, gravity, and physicochemical
philic nanoscale conduction channels even in the porous properties between the particles and membrane matrix, the

llers tend to agglomerate during the membrane-casting
a
process (even in the solvent-drying process). The subsequent
discrete ion cluster phases growing around the particles also
Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun
1
b
30012, People's Republic of China. E-mail: jialinmury@163.com
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun generate a degree of separation away from the membrane. This
1
30022, People's Republic of China
20–22
can seriously affect the performance of DMFCs.
Incorporating macromolecule llers with
c
Key Laboratory of Advanced Batteries Physics and Technology (Ministry of Education),
a
particular
Jilin University, Changchun 130012, PR China
conductive ability into matrix molecules can reduce the ller
loss problem and form hydrophilic phases because of physical
crosslinking, and stable chemical and ionic bonds between the
1
†
Electronic supplementary information (ESI) available: H NMR spectrum of
DMNF; TGA curves of membranes; cross-sectional SEM images of different
membranes; photographs of composite membranes; Nyquist plots of single cell
3,23,24
impedance spectra; liquid uptake and swelling ratio of membranes; the ller and matrix molecules.
However, because the differ-
ꢂ
polarization and power density curves of membranes at 25 C. See DOI:
ences in molecular morphologies generate thermodynamic
1
0.1039/c9ta10215a
196 | J. Mater. Chem. A, 2020, 8, 196–206
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry A
immiscibility, the degree of mixing is usually low (<10 wt%), hydrochloric acid (Sinopharm Chemical Reagent Co. Ltd, 37%),
which leads to separation among the hydrophilic ion clusters 1,2-dichloroethane (Shanghai Chemical Reagent Co. Ltd). N,N-
on the dissimilar structures and makes them stay too far to Dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), and N-
form connected hydrophilic proton transport channels. More- methyl-2-pyrrolidone (NMP), used as solvents, were vacuum-
over, the performance improvements mostly depend on the distilled before use. Naon powder was obtained by evapo-
chemical regulation of chain structures whereas the physical rating the commercial solvent D2020 (20 wt%).
blending of polymers and ion nanoscale channels are rarely
3,7,25–27
chemically modied.
The skeleton molecule is one kind
Synthesis of the monomer
of organic composite component which is different from
conventional llers. Synthesized by matching the matrix mole-
cule structure and resembling a skeleton of the matrix, these
skeleton molecules enhance the mechanical properties of
composite membranes and show a distinctive predominance
for functional groups with specic characteristics. The matched
molecular structures, sizes, and exibilities can improve the
thermodynamic compatibility and selectivity of the molecular
interactions between the skeleton molecules and the matrix
molecules in the membrane-casting process. Additionally, while
enhancing some of the specic properties of the membrane
materials, the adjustable chemical affinity also allows the
skeleton molecule to simultaneously contribute a somewhat
better material performance.
In a 200 mL beaker, 3,5-di(triuoromethyl) aniline (5.7 g) and
concentrated hydrochloric acid (10 mL) were stirred until they
were dissolved in 100 mL water at 10 C. And then sodium
nitrite solution (1.8 g sodium nitrite in 10 mL water) was added
dropwise into the aniline solution. Aer stirring for 1 hour, the
solution underwent ltration and the ltrate was then slowly
added from a dropping funnel to a 1000 mL beaker containing
ꢂ
1
1
,4-benzoquinone (1.6 g) and sodium bicarbonate (10.5 g) at 5–
0 C. Aer a 4 hour stirring process, the product was thor-
ꢂ
oughly washed with deionized water and allowed to dry out
naturally. Aerward, the product (7.1 g), zinc powder (4.3 g),
deionized water (43 mL) and toluene (64 mL) were added into
a four-necked ask equipped with a mechanical stirrer, a ther-
mometer, a nitrogen inlet and a Dean–Stark apparatus with
a reux condenser. Aer reuxing for 6 hours at 80 C, the hot
mixture was then ltered. And the crude was dried and recrys-
tallized from methanol to give grey needles of DFBP.
At present, Naon, a state-of-the-art peruorosulfonic acid
molecule with a exible aliphatic carbon main chain structure,
ꢂ
ꢁ1
exhibits a high proton conductivity (z0.1 S cm ) but also high
1,28
solvent dependence and low molecular rigidity.
A small
quantity of water loss can cause the nanoscale proton transport
29–31
channels to shrink and collapse quickly.
Herein, this paper Synthesis of DF-PAEK
reports a novel type of proton-conducting polymer to be used as
the skeleton molecule containing a hydrophobic domain—
a rigid phenyl and naphthyl ring backbone and high-density of
short, rigid triuoromethyl side chains; and a hydrophilic
domain—long exible hydrophilic aliphatic side chains that are
similar to Naon. Aer incorporating the skeleton into the
Naon matrix at the molecular level, the methanol permeability
of the membranes can be reduced by over 50%. In contrast to
the conventional mixed ller-matrix, the degree of compatibility
between the two molecules is also greatly improved by adjusting
the solution formulation and optimizing the membrane-casting
method. Inside the membranes, skeleton molecules guide the
Naon molecule to automatically form hydrophilic–hydro-
phobic ionic nanophase separation. Hydrophilic ion clusters
aggregate to create proton transport channels with a certain
1
,5-Bis(4-uorobenzoyl)-2,6-dimethoxynaphthalene
(DMNF)
(4.56 g, 33
(12.97 g, 30 mmol), DFBP (7.56 g, 30 mmol), K CO
2
3
mmol), tetramethylene sulfone (50 mL) and toluene (10 mL)
were dissolved in a three-necked ask with continuous stirring
in a nitrogen atmosphere. The solution was then heated to
ꢂ
120 C for 2 hours until the water by-product generated in the
polymerization step was eliminated. While the toluene was
slowly drained, the temperature was gradually raised to 200 C,
and the reaction was sustained for 12 hours. Then, the viscous
product was washed with deionized water. The product was
ꢂ
ꢂ

nally dried at 60 C for 48 hours and ultimately named DF-
PAEK.
Synthesis of SDF-PAEK
connectivity. This unique nanophase separation–aggregation The graing of exible pendent sulfonated groups onto the
structure is formed by the entropy-driven self-assembly of polymer was performed according to a procedure reported
32,33
amphipathic sulfonated poly(arylene ether ketone) (SPAEK) previously.
DF-PAEK (5 g, 7.1 mmol) was dissolved in 125 mL
chains prearranged in the casting process.
rened dichloromethane, and then 1 M BBr –dichloromethane
3
solution (20 mL) was dropwise added into that ask. Aer the
reaction sustained for 24 hours, 500 mL aqueous ethanol
solution was added into the system for quenching. The
collected brick red product, named OH-DF-PAEK, was washed
Experimental section
Materials
2,6-Dimethoxynaphthalene, 4-uorobenzoyl chloride, 3,5- with deionized water. Then pure OH-DF-PAEK (2.49 g, 3.56
di(triuoromethyl) aniline, and 1,4-benzoquinone were mmol), NaH (0.568 g, 14.26 mmol) and 1,4-butanesultone (3.1
purchased from Sigma-Aldrich. Boron tribromide (BBr ), 1,4- mL, 30 mmol) were dissolved in 50 mL DMSO in a nitrogen
3
ꢂ
butanesultone and sodium hydride (NaH) (60%) were atmosphere with constant stirring at 85 C for 8 hours. Finally,
purchased from Aladdin Scientic Co. Ltd, Shanghai. Zinc the solution was poured into acetone to precipitate the product.
powder (Shanghai Chemical Reagent Co. Ltd, 95%); The solid SDF-PAEK was obtained aer acetone purication.
This journal is © The Royal Society of Chemistry 2020
J. Mater. Chem. A, 2020, 8, 196–206 | 197

View Article Online
Journal of Materials Chemistry A
Paper
ꢂ
test, samples were heated from 100 to 800 C at a speed of
Preparation of SDF-PAEK@Naon-x composite membranes
ꢂ
ꢁ1
10 C min in a nitrogen atmosphere in a Pt crucible. The
This paper employs a solution method different from the usual
method to cast membranes. As shown in Scheme 1, dry Naon
powder was dissolved in DMSO as the casting solution with
a concentration of 10 wt%. Aer being stirred for 72 hours, the
DF-PAEK powder was then directly dissolved in the solution.
Finally, DMAc was added into the DMSO solution in a volume
relevant data are shown in Fig. S2.†
Mechanical strength
Tensile strength, Young's modulus, and elongation at break of
the membranes were measured at room temperature on a tensile
ꢂ
ꢁ1
fraction of 5% at 30 C and stirred for 24 hours. The claried and
tester (SHIMADZU AG-I 1KN) at a speed of 2 mm min
.
transparent dispersion was poured onto an 8 cm ꢀ 8 cm hori-
ꢂ
zontal glass plate aer ltration and heated in an oven at 70 C
Liquid adsorption properties
for 36 hours to remove the solvent. Aerwards, the membranes
The weight and size (in the area and thickness) of the dry
membranes were measured before the test. Then the
membranes were soaked in a series of solutions (deionized
were peeled off and then immersed in 0.5 M H
2
SO
4
solution at
ꢂ
60 C for 24 hours, followed by thorough rinsing with deionized
ꢂ
water. All membranes were thoroughly dried in vacuum at 60 C.
ꢂ
water, 1 M methanol, 2 M methanol) at 25, 40, 60 and 80 C for
2
4 hours, respectively. The liquid uptake (LU) and swelling ratio
Measurements
(SR) of membranes were then obtained using the equations
The chemical structures of DMNF, DFBP, and the polymers given below:
1
were conrmed by H NMR spectroscopy on a Bruker Avance
W
wet ꢁ Wdry
510 spectrometer, which was conducted with DMSO-d
6
as the
LU ¼
SRarea
ꢀ 100%
ꢀ 100%
(1)
(2)
(3)
W
dry
solvent and tetramethylsilane (TMS) as the internal standard.
S
wet ꢁ Sdry
¼
Electron microscope observations
S
dry
The microstructure and morphology of the membranes were
studied by high-resolution transmission electron microscopy
T
wet ꢁ Tdry
SRthickness
¼
ꢀ 100%
T
dry
(HRTEM) recorded on a JEM-2100F. Before observation,
membranes were immersed in 1 M (CH COO) Pb aqueous where Wwet and Wdry are the weights of wet and dry membranes,
3
2
solution for 72 hours, then rinsed with deionized water and Swet and Twet are the area and thickness of wet membranes,
ultimately dried at room temperature. The stained samples respectively. Sdry and Tdry are the area and thickness of dry
were then embedded in epoxy resin and sectioned with membranes, respectively.
a microtome. Thick samples were placed on copper grids.
Ion exchange capacity (IEC)
Thermal stability
The IEC of membranes was determined by titration via the
36
Thermogravimetric analysis (TGA) was performed on a Pyris-1- method reported previously. Firstly, dry membranes were
TGA (PerkinElmer). Before the test, the samples were kept at equilibrated in 1 M sodium chloride solution for 72 h to release
ꢂ
1
20 C in vacuum for 24 hours to remove moisture. And in the the protons completely. Aerwards, the solution was titrated
Scheme 1 The preparation procedure of SDF-PAEK@Nafion-x composite membranes.
198 | J. Mater. Chem. A, 2020, 8, 196–206
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry A
with standard NaOH solution calibrated with potassium
hydrogen phthalate. The IEC titration was carried out three
times, and the average value was calculated using:
Results and discussion
The synthesis of NFBP, DF-PAEK, and SDF-PAEK-x
As shown in Scheme 2, DMNF was synthesized by the Friedel–
Cras acylation of 2,6-dimethoxynaphthalene with 4-uo-
robenzoyl chloride and catalysed by anhydrous ferric chloride
V
NaOH ꢀ CNaOH
IEC ¼
ꢀ 100%
(4)
W
dry
where VNaOH and CNaOH mean the volume (mL) and concen-
tration (mol L ) of NaOH solution, respectively.
37
according to the method previously reported. Through a three-
ꢁ
1
step synthetic process, the synthesis of DFBP was accomplished
accordingly. As shown in Scheme 3, SDF-PAEK was synthesized by
an aromatic nucleophilic-substitution–polycondensation con-
Degree of sulfonation (DS)
1
sisting of three steps. H NMR analysis was used to examine the
DS was calculated using
DS ¼ 2(AHm/AHa
)
(5)
1
where AHm is the integral area of H NMR spectra assigned to
2
the –OCH – groups connected with naphthalene rings, and AHa
is the integral area assigned to the phenyl protons adjacent to
carbonyl groups.
Proton conductivity
The proton conductivity (s) was measured on a Princeton
Applied Research Model 2273 potentiostat/galvanostat/FRA via
a four-electrode AC impedance method from 100 kHz to 1 Hz.
The equation is:
Scheme 2 The synthesis procedure of DMNF, DFBP.
s ¼ L/(RWT)
(6)
where L means the distance between electrodes; R means the
membrane resistance; W and T mean the width and thickness of
the samples, respectively.
Methanol crossover measurements
The methanol crossover was measured on a Princeton Applied
Research Model 2273 potentiostat/galvanostat/FRA via the
linear sweep voltammetry (LSV) method combined with
32
a DMFC. The value was calculated using:
L ꢀ CDmax
P ¼ 6
(7)
F ꢀ k ꢀ CMeOH
2
ꢁ1
where P means the methanol crossover value (cm s ), L is the
thickness of membranes (cm), CDmax means the maximum
ꢁ2
current of the LSV (mA cm ), F means Faraday constant (C
ꢁ
1
mol ), k is the drag correction factor ꢁ0.739 (2 M methanol);
MeOH is the concentration of methanol solution-2 M.
C
Single-cell performance evaluation
Aer mixing the catalyst and the Naon solution at room
temperature, it was evenly sprayed on the two sides of the
membranes. Aer drying, the catalyst coated membranes
(
CCMs) were obtained. Then the gas diffusion layers (GDLs)
ꢂ
were sandwiched on both sides of the CCMs. At 80 C, the anode
side was supplied with different concentrations of methanol
ꢁ
1
solution (1 M or 2 M) at a rate of 2 mL min , and prehumidied
ꢁ
1
oxygen was fed at a rate of 30 mL min on the cathode side.
ꢂ
The membrane electrode assembly (MEA) was activated at 60 C
for 2 hours before the formal test.
Scheme 3 The synthesis procedure of SDF-PAEK.
J. Mater. Chem. A, 2020, 8, 196–206 | 199
This journal is © The Royal Society of Chemistry 2020

View Article Online
Journal of Materials Chemistry A
Paper
product in each step, and the results (Fig. 1 and S1†) clearly in Fig. 1(c) and (d). In Fig. 1(d), the characteristic peak of the
illustrated changes in molecular structures during the synthetic phenolic hydroxyl groups substantially decreased, while the
process. The process from DF-PAEK to OH-DF-PAEK was a deme- characteristic peaks assigned to methylene groups on the exible
thylation reaction, the conversion of methoxyl groups to hydroxyl aliphatic chain emerged (4.02, 2.36, 1.55, and 1.46 ppm). The
1
groups. Comparing Fig. 1(b) with Fig. 1(c), the chemical shis of other H NMR peaks remained unchanged. This indicated that
the polymers in the two spectra were consistent. However, there SDF-PAEK-x was successfully prepared. Meanwhile, 1.92 as the
still existed a signicant difference, as the characteristic peak of corresponding value of x, namely DS, could be calculated
34,35
the methoxyl groups attached to the naphthyl groups at 3.76 ppm according to eqn (5).
in Fig. 1(b) completely disappeared in Fig. 1(c); at the same time,
the characteristic peak belonging to the phenolic hydroxyl groups Morphology study
was missing in Fig. 1(b) but appeared in Fig. 1(c) (9.86 ppm). This
Fig.
2 shows the typical nanoscale morphology of the
indicates that the methoxyl groups in DF-PAEK had been entirely
converted to phenolic hydroxyl groups by a demethylation reac-
tion. Finally, OH-DF-PAEK underwent a graing reaction in which
most of the hydroxyl groups were transformed into exible
pendant side chains with sodium sulfonate. This could be
membranes. In these TEM images, the black domains represent
ion clusters formed by an aggregation of the hydrophilic sulfonic
acid groups aer Pb ion staining. The ion clusters in composite
2+
membranes are generally uniform in size and evenly distributed,
whereas the ion clusters in recast Naon are in a high degree of
self-aggregation, unequal-sized, and mutually separated. The
long exible aliphatic side chains of SDF-PAEK, containing
hydrophilic groups far away from the main chains, show
matched size, exibility, hydrophilicity and chemical affinities to
the Naon side chains. Therefore, in the cast solution, this allows
the hydrophilic side chains of different molecules to selectively
interact, driving them to aggregate by subordinate-assembly and
form nanohydrophilic phase domains with the self-assembly.
Similarly, hydrophobic groups of different molecules form
hydrophobic domains that are separated from the hydrophilic
phase. These TEM images indicate that the microstructures are
efficiently regulated in the composite membranes.
1
observed by contrasting the H NMR spectrum of SDF-PAEK-1.92
The size and distribution of different ion clusters are related
to the composition amount of the SDF-PAEK skeleton molecules.
It can be observed from Fig. 2(a) and (b) that as the composition
amount of the skeleton molecule increases, the dispersed ion
1
Fig. 1 H NMR spectra of (a) DFBP, (b) DF-PAEK, (c) HO-DF-PAEK, and
(
d) SDF-PAEK.
Fig. 2 TEM images of SDF-PAEK@Nafion composite membranes: (a) SDF-PAF@Nafion-10%; (b, e and f) SDF-PAEK@Nafion-15%; (c) SDF-
PAEK@Nafion-20%; (d) recast Nafion.
200 | J. Mater. Chem. A, 2020, 8, 196–206
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry A
cluster domains tend to connect, and the scattered ion channels conductivity is a vital indicator of PEM properties, which
effectively connect. This structure is similar to that of Naon, but reects an essential relationship with single-cell performance.
the number of connected channels is larger than that of Naon. As shown in Fig. 3, SDF-PAEK-1.92 showed the highest values of
However, in Fig. 2(c), the separation degree of the separated ion conductivity, and the proton conductivity of composite
clusters signicantly increases, and the number of inter- membranes was much higher than that of recast Naon. In the
penetrating channels decreases. The above results indicate that SDF-PAEK membrane, a large number of hydrophilic sulfonic
to some extent, intermolecular thermodynamic incompatibility acid molecules at the end of side chains could gather together to
41
occurs due to different molecular structures; this intermolecular form the proton channels. When the amount of SDF-PAEK in
thermodynamic incompatibility affects the degree of connection the composite membranes increased from 10 wt% to 20 wt%,
among the ion clusters, and therefore, the Naon must form the proton conductivity of SDF-PAEK@Naon membranes
either a dispersed or a co-continuous morphology to maintain initially experienced an upward trend but then dropped aer-
38
the functionality of the membrane. The high magnication ward. The addition of skeleton molecules caused the sulfonic
observations of SDF-PAEK@Naon membranes are presented in acid groups to aggregate; the rigid phenyl and naphthyl rings
Fig. 2(e) (250 000x) and Fig. 2(f) (1 500 000x). The apparently with the high uorine content density parts containing the
ordered lattice structures, in black domains, which possess structure in repetitive units came together to form the overall
straight sides and regular ends, are well dispersed and have hydrophobic phase domain. As displayed in the TEM images,
39
a narrow distribution in the average size of 5 to 6 nm. These the proton transport channels induced by the hydrophilic–
results further illustrate that the unique skeleton molecule of hydrophobic phase separation were eventually distributed,
SDF-PAEK can facilitate phase separation between hydrophilic which signicantly promoted proton transport and signicantly
and hydrophobic aggregations to form nanochannels, simulta- enhanced proton conductivity.
40
neously promoting proton transport.
Nevertheless, intermolecular incompatibility existed
between the SDF-PAEK containing rigid groups and the exible
chain-based Naon molecules. The incompatibility increased as
the composition amount of SDF-PAEK increased, thus sepa-
Proton conductivity (s)
The proton conductivity (s) values measured at different rating some of the small scattered ion clusters from each other.
temperatures are displayed in Fig. 3 and Table 1. Proton An extension of the distance between two adjacent proton
transport groups would shorten the hydrogen bond lifetime in
42
the transport channels of the uorination system. Accord-
ingly, when the temperature increased, membranes with
a closer distance between the internal sulfonic acid groups
would possess a higher conductivity. The measured experi-
mental data conrmed the above statements—with 15 wt%
skeleton molecules, the material exhibited the highest proton
ꢁ1
ꢂ
conductivity of 0.197 S cm at 80 C.
Liquid uptake (LU) and swelling ratio (SR) of membranes
To study the effects of different methanol concentrations on LU
and SR, the membranes were immersed in 0 M (deionized
water), 1 M and 2 M methanol aqueous solutions at different
temperatures. As shown in Fig. 4, Tables S1–S3,† the trends of
LU and SR are similar. The LU and SR increased with increase in
temperature and methanol concentration, respectively. Aer
immersion in DI water, the LU and SR values of recast Naon
membrane appeared to be lower than those of composite
Fig. 3 Proton conductivity of the proton exchange membranes at
different temperatures.
Table 1 The thickness (mm), IEC, proton conductivity (s), maximum current density (CDmax), methanol permeability (P) and selectivity (F) of
membranes
ꢁ1
s (S cm
)
Thickness
(mm)
IEC
(mequiv g
CDmax
(mA cm
P
F
ꢁ1
ꢂ
ꢂ
ꢁ2
ꢁ6
2
ꢁ1
4
ꢁ3
Samples
)
25 C
80 C
)
(10 cm s
)
(10 S s cm )
SDF-PAEK@Naon-10%
SDF-PAEK@Naon-15%
SDF-PAEK@Naon-20%
Recast Naon
52.67
49.33
49.33
52.33
0.79
0.86
0.93
0.65
0.091 ꢃ 0.003
0.093 ꢃ 0.002
0.087 ꢃ 0.002
0.077 ꢃ 0.001
0.183 ꢃ 0.005
0.197 ꢃ 0.004
0.171 ꢃ 0.005
0.140 ꢃ 0.002
227.90
203.48
265.64
387.46
2.13
2.03
2.67
4.08
8.56
9.73
6.42
3.71
This journal is © The Royal Society of Chemistry 2020
J. Mater. Chem. A, 2020, 8, 196–206 | 201

View Article Online
Journal of Materials Chemistry A
Paper
Methanol permeability and selectivity
High methanol permeability stands for high methanol cross-
over which can degrade the fuel efficiency and single-cell
performance of DMFCs. Therefore, reducing fuel perme-
2
1,44
ability is a vital method to improve DMFC performance.
The results of the LSV of the methanol resistance of MEAs are
shown in Fig. 5 and Table 1. The 2 M methanol solution was
ꢂ
introduced into the anode side, and at 80 C, recast Naon
ꢁ
2
exhibited the largest CDmax of 387.46 mA cm . The above
result indicated that under these conditions, the methanol
resistance efficiency of the composite membranes was higher
than that of Naon. As the SDF-PAEK composition increased
from 10% to 15%, the CDmax of the composite membrane
ꢁ
2
ꢁ2
decreased from 227.90 mA cm to 203.48 mA cm , which
indicated that skeleton molecules exhibited some enhance-
ment effects on the methanol resistance of composite
4
5
membranes. However, with the addition of skeleton mole-
cules further increased from 15% to 20%, CD increased to
65.64 mA cm , due to the separation of molecules inducing
max
ꢁ
2
2
a larger void space inside the membrane and thus increasing
the methanol permeability.
It is also essential to maintain sufficient proton conductivity
while reducing methanol permeability. Combined with the
proton conductivity, the selectivity of the materials was calcu-
lated. The selectivity value of SDF-PAEK@Naon-15% (9.73 ꢀ
4
ꢁ3
1
0 S s cm ) was more than 1.5 times higher than that of recast
Fig. 4 LU of PEMs at different concentrations: DI water (a), 1 M (b) and
2
water (d), 1 M (e) and 2 M (f) methanol.
4
ꢁ3
M (c) methanol; SR (in area) of PEMs at different concentrations: DI Naon (3.71 ꢀ 10 S s cm ). The penetration of methanol is
also inseparable from proton transport channels. As shown in
Fig. 5(c), the distinctive aggregation–separation of hydrophilic–
hydrophobic phases and the characteristic functional groups of
the composite membranes play a vital role in the selection of
ꢂ
membranes at 80 C. This may be due to the incompatibility
between the Naon matrix and SDF-PAEK skeleton molecules. A
complete interpenetrating network structure could not be
formed, and there was certainly void space among the mole-
cules. At higher temperatures, the number of physically cross-
linked networks between the different aggregated phases was
reduced to a certain degree, so that more water molecules were
allowed to enter the membrane interior.
However, in methanol solution, the LU and SR of Naon
were higher than that of composite membranes at each
temperature, as Naon showed a higher affinity to methanol
43
than SDF-PAEK and absorbed more methanol molecules.
Moreover, skeleton molecules inhibited the degree of matrix
swelling; strong intermolecular forces and physical crosslinking
between macromolecules limited the mutual penetration
2
among Naon, H O, and methanol molecules. As the additional
amount of SDF-PAEK increased from 10% to 15%, all the LU
and SR values of composite membranes decreased. When this
amount reached 20%, the negative effects of incompatibility
were further highlighted: the degrees of molecular separation
and the void space among different phase molecules increased.
In summary, SDF-PAEK as a skeleton molecule of the
membrane can limit the swelling effect of Naon molecules in
methanol aqueous solutions, thus enhancing the dimensional
stability of the membrane for DMFCs which work in the envi-
ronment of methanol aqueous solutions.
Fig. 5 (a) The LSV curves of membranes for testing methanol cross-
over at 80 C, (b) methanol permeability (P) and selectivity (F) of
composite membranes (CM) and recast Nafion (RN); (c) schematic
diagram for the proton transport channel of the prepared membranes.
ꢂ
202 | J. Mater. Chem. A, 2020, 8, 196–206
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry A
ꢂ
Table 2 The mechanical properties of PEMs, OCV values and PDmax of DMFCs at different methanol concentrations (80 C)
PDmax
(mW cm
ꢁ2
OCV (V)
1 M
)
Tensile strength
(MPa)
Young's
modulus (MPa)
Elongation at
break (%)
Samples
2 M
1 M
2 M
SDF-PAEK@Naon-10%
SDF-PAEK@Naon-15%
SDF-PAEK@Naon-20%
Recast Naon
0.763
0.812
0.726
0.676
0.678
0.730
0.617
0.528
107
112
96
128
139
113
94
16.5 ꢃ 1.6
14.4 ꢃ 0.6
15.2 ꢃ 0.7
13.7 ꢃ 0.3
237 ꢃ 26
291 ꢃ 15
312 ꢃ 13
240 ꢃ 20
170 ꢃ 11
160 ꢃ 16
125 ꢃ 23
154 ꢃ 5
81
methanol and protons. In this way, the selectivity of the transport resistance, and CPE and CPE are the constant phase
1
2
composite membrane can be higher than that of recast Naon. elements connected in parallel with charge transfer and mass
transport resistances. From Fig. 6(a), (b) and S5,† it can be
observed that as the concentration of methanol solutions
increased, the radius of each impedance arc decreased, which
Mechanical properties
As shown in Table 2, the comprehensive mechanical perfor-
mances of composite membranes are better than those of recast
Naon. Tensile strength is the critical value of the transition
from uniform plastic deformation to locally concentrated
plastic deformation when membranes are stretched. In contrast
to recast Naon, SDF-PAEK containing rigid phenyl and naph-
thyl rings served as the reinforcement site; therefore, composite
membranes incorporating skeleton molecules could reach
reveals the improvement in the methanol oxidation reaction
and the restriction on the oxygen reduction reaction, since
a small amount of methanol that crossed over from the anode
side sharply restricted the transport of protons to the cathode
48
side. The ohmic resistance values of the different materials
were close, whereas the polarization resistance values of the fuel
cell fabricated with recast Naon were the highest of those
determined here. The addition of skeleton molecules caused
the impedance arcs of membranes to converge. However, for
composite membranes, as the amount of SDF-PAEK increased,
the polarization resistance presented a similar growing trend as
well, which might be related to the incompatibility between the
rigid molecules of SDF-PAEK and the exible aliphatic chain
molecules of Naon. It is worth noting that the charge-transfer
resistances of the three composite membranes increased aer
an initial drop as the amount of SDF-PAEK increased from 10%
to 15%. The composite membrane with a 15% composition
44,46
higher tensile strength.
However, there was some incom-
patibility between SDF-PAEK and Naon molecules. When the
addition amount of SDF-PAEK was relatively high, for example,
up to 20%, the negative effects of intermolecular incompati-
bility resulted in a reduction in material density and mechan-
47
ical strength.
Electrochemical behaviour
According to the DMFC equivalent circuit (Fig. 6(c)), different
amount requires the lowest resistance to charge transfer, which
single cells were investigated to obtain the Nyquist plots at
36,49
was consistent with its highest proton conductivity.
In the
ꢂ
0
.25 V and 80 C. The components of the equivalent circuit are
high-frequency region, small arcs with the centers below the
real axis appear in the Nyquist plots, suggesting that a disper-
sion effect may occur at the same time. The capacitance
frequency response characteristics of the solid electrode double
layers display inconsistency and deviation with the pure
capacitance to some extent. There was a certain degree of
nonuniformity between composite membranes with catalyst
layers, and it was lower than that between the Naon membrane
and catalyst layers.
as follows: R
U
is the high-frequency resistance, L is the pseudo
inductance, Rct is the charge transfer resistance, Rmt is the mass
Fig. 6 Nyquist plots of single cell impedance spectra for (a) 1 M
methanol solution, (b) 2 M methanol solution, and (c) the DMFC Fig. 7 The polarization and power density curves with different
ꢂ
equivalent circuit for the evaluation of the measured impedance methanol solution concentrations at 80 C: (a) CMeOH ¼ 1 M, and (b)
spectra.
C
MeOH ¼ 2 M.
This journal is © The Royal Society of Chemistry 2020
J. Mater. Chem. A, 2020, 8, 196–206 | 203

View Article Online
Journal of Materials Chemistry A
Paper
Table 3 DMFC performances for different kinds of composite membranes
a
ꢂ
ꢁ2
Samples
T ( C)
CMeOH (M)
OCV (V)
PDmax (mW cm
)
SDF-PAEK@Naon-15% [this work]
80
60
70
70
80
60
80
80
70
80
90
80
25
2
1
1
1
2
2
2
2
2
2
2
2
4
0.730
0.84
0.71
0.6
0.82
0.79
0.836
0.541
0.52
0.58
0.7
139
130
98
59
Naon-sulfonated silica
60
Naon-zeolite hybrid
Naon-mordenite hybrid
6
1
80
1
2
SPEEK/sSrZrO
3
(3 wt%)
110
20.7
111.53
103.0
84.5
28
120
47
35.3
6
2
SPVDF-co-HFP/SPA
p-BPAF@Naon-7.5 (ref. 24)
N/PVFP-BI-9.5–0.5 (ref. 63)
23
NP-composite
Naon-TiO hybrid
Naon-GO platelet hybrid
13
2
64
Naon/SNPAEK-7.5% (ref. 65)
C-PEEK-25 (ref. 66)
0.77
0.8
a
The operating temperature of DMFC mentioned in references.
The radius values of the arcs correspond to the ionic crossover can be reduced, which decreased the depolarization
58
impedance of PEMs. An increase in the impedance of any other losses and then improved the power density. The low fuel cell
parts of the cell causes the semi-circular arc to move toward the performance revealed the proton-transport problem in the high
36,50–52
low-frequency region.
The electrochemical impedance test current density region at a concentration of 1 M, whereas at
of DMFC shows a series of uncertainties, and related research a concentration of 2 M, the reduced concentration polarization
needs further exploration. In summary, the composite improved cell performance at high current density. Under the
membrane aer incorporating skeleton molecules in the Naon same conditions, the PDmax of the 15% composite membranes
membrane possessed lower proton transfer impedance than was higher than that of the 10% composite membranes.
recast Naon.
However, for the 20% composite membrane, the negative effect
Different concentrations of methanol solution were fed into of the incompatibility between the rigid backbone structure of
ꢂ
ꢂ
the anode sides at 25 C and 80 C to conduct DMFC polariza- SDF-PAEK molecules and the exible backbone of Naon
tion studies on different membranes. The corresponding OCV molecules was highlighted, resulting in a decrease in power
and power density data are shown in Fig. 7, S6† and Table 2. It is density. This was consistent with the test results of EIS.
easy to nd that with the increase of methanol concentration,
Furthermore, comparison of DMFC performance between
the OCV values of all MEAs were reduced, because of more fuel SDF-PAEK@Naon-15% and other polymer membranes re-
permeating through the membrane from the anode sides to the ported in previous literature was made, and the results are
cathode sides. And the fuel permeation led to an increase in summarized in Table 3. It can be seen that the composite
methanol crossover and inverse voltage, thus resulting in membrane SDF-PAEK@Naon-15% exhibits a comparable and
higher depolarization losses of the OCV. However, under the even better single-cell performance.
same conditions, composite membranes exhibited better
methanol resistance and reduced depolarization losses in
comparison with the values of recast Naon membranes, so the
Conclusions
OCV values of composite membranes were higher than that of
A series of SDF-PAEK@Naon-x membranes, as novel PEM
53–55
recast Naon.
The SDF-PAEK@Naon-15% membrane
materials suitable for DMFC application, were prepared by
incorporating a novel SDF-PAEK skeleton molecule into Naon
membranes. Compared to recast Naon, the cost of composite
membranes was reduced dramatically since the composition
can reach 20%. Moreover, skeleton molecules promoted the
automatic aggregation–separation of hydrophilic–hydrophobic
phases by self-assembly, thus enhancing the proton conduc-
tivity of composite membranes. The TEM analyses demon-
strated the existence and even distribution of ion cluster
microphase lattices and bicontinuous proton transport chan-
showed the highest OCV (0.812 V, 2 M), which was consistent
with the highest conductivity test results.
However, when the methanol concentration increased from
1
M to 2 M, the maximum power density (PDmax) values of all
membranes were improved. This indicated that the positive
impact of increasing fuel concentration was greater than the
negative impact of increased methanol crossover. Under the
condition of 2 M methanol solution, the PDmax of the SDF-
ꢁ
2
ꢂ
PAEK@Naon-15% membrane reached 139 mW cm at 80 C.
In the composite membrane, Naon molecules were restruc-
tured around skeleton molecules, where the ion clusters tended
toward aggregation. More proton transport channels inter-
connected (veried by HRTEM images) and thus exhibited
ꢂ
nels. At 80 C, the proton conductivity of SDF-PAEK@Naon-
ꢁ1
1
5% was 0.197 S cm , whereas the methanol permeability
value was reduced by half over that of the recast Naon. The
composite membrane thus exhibited a 2.6-fold increase in
selectivity. The addition of SDF-PAEK also enhanced the
mechanical strength and dimensional stability of composite
56,57
higher affinity toward hydration molecules.
At the same
time, due to the dense hydrophobic domains, the methanol
204 | J. Mater. Chem. A, 2020, 8, 196–206
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Journal of Materials Chemistry A
membranes. The SR value of the composite membrane was 38% 14 W. Zhengbang, H. Tang and P. Mu, J. Membr. Sci., 2011, 369,
of that of the recast Naon (in area, 2 M aqueous methanol 250–257.
solution), signicantly reducing the manufacturing difficulties 15 F. Meng, N. V. Aieta, S. F. Dec, J. L. Horan, D. Williamson,
of the MEA. The single-cell performance test showed that the
PDmax of PAEK@Naon-15% was 139 mW cm (at 80 C, 2 M
methanol aqueous solution), which was much higher than that
M. H. Frey, P. Pham, J. A. Turner, M. A. Yandrasits,
S. J. Hamrock and A. M. Herring, Electrochim. Acta, 2007,
53, 1372–1378.
ꢁ
2
ꢂ
of the recast Naon with a maximum power density of 94 mW 16 J. L. Malers, M.-A. Sweikart, J. L. Horan, J. A. Turner and
ꢁ
2
cm . All these results demonstrate that composite membranes
have the potential to be used as PEMs for DMFC applications, 17 J. R. Ferrell III, M.-C. Kuo, J. A. Turner and A. M. Herring,
and this provides opportunities to research and develop alter- Electrochim. Acta, 2008, 53, 4927–4933.
native PEMs by incorporating skeleton molecules into the 18 H. Beydaghi, M. Javanbakht, B. Ahmad, ab P. Salarizadeh,
A. M. Herring, J. Power Sources, 2007, 172, 83–88.
Naon matrix.
H. Ghafarian-Zahmatkesh, S. Kashe and E. Kowsari, RSC
Adv., 2015, 5, 74054–74064.
1
2
2
2
9 D. He, H. Tang, Z. Kou, M. Pan, X. Sun, J. Zhang and S. Mu,
Adv. Mater., 2017, 29, 1601741.
0 G. Rambabu, N. Nagaraju and S. D. Bhat, Chem. Eng. J., 2016,
Conflicts of interest
There are no conicts to declare.
306, 43–52.
1 Y. Zhang, W. Cai, F. Si, J. Ge, L. Liang, C. Liu and W. Xing,
Chem. Commun., 2012, 48, 2870.
2 A. K. Sahu, S. Meenakshi, S. D. Bhat, A. Shahid, P. Sridhar,
S. Pitchumani and A. K. Shukla, J. Electrochem. Soc., 2012,
Acknowledgements
We acknowledge the nancial support from the Natural Science
Foundation of China (No. 21875088).
159, F702–F710.
2
2
3 H. Lin, T. L. Yu, L. Huang, L. Chen, K. Shen and G. Jung, J.
Power Sources, 2005, 150, 11–19.
4 C. Ru, Y. Gu, Y. Duan, C. Zhao and H. Na, J. Membr. Sci.,
2019, 573, 439–447.
References
1
2
3
4
N. Li, C. Wang, S. Y. Lee, C. H. Park, Y. M. Lee and
M. D. Guiver, Angew. Chem., Int. Ed., 2011, 50, 9158–9161.
D. Guo, A. N. Lai, C. X. Lin, Q. G. Zhang, A. M. Zhu and
25 A. Guimet, L. Chikh, A. Morin and O. Fichet, J. Membr. Sci.,
2016, 514, 358–365.
Q. L. Liu, ACS Appl. Mater. Interfaces, 2016, 8, 25279–25288. 26 P. Prapainainar, Z. Du, P. Kongkachuichay, S. M. Holmes
C. Wang, N. Li, D. W. Shin, S. Y. Lee, N. R. Kang, Y. M. Lee
and M. D. Guiver, Macromolecules, 2011, 44, 7296–7306.
N. R. Kang, S. Y. Lee, D. W. Shin, D. S. Hwang, K. H. Lee,
and C. Prapainainar, Appl. Surf. Sci., 2017, 421, 24–41.
27 T. Y. Inan, H. Do ˘g an, E. E. Unveren and E. Eker, Int. J.
Hydrogen Energy, 2010, 35, 12038–12053.
D. H. Cho, J. H. Kim and Y. M. Lee, J. Power Sources, 2016, 28 C. H. Park, S. Y. Lee, D. S. Hwang, D. W. Shin, D. H. Cho,
3
07, 834–843.
K. H. Lee, T. Kim, T. Kim, M. Lee, D. Kim, C. M. Doherty,
A. W. Thornton, A. J. Hill, M. D. Guiver and Y. M. Lee,
Nature, 2016, 532, 480–483.
5
6
L. Lei, X. Zhu, J. Xu, H. Qian, Z. Zou and H. Yang, J. Power
Sources, 2017, 350, 41–48.
X. Wang, S. Wang, C. Liu, J. Li, F. Liu, X. Tian, H. Chen, 29 K. A. Mauritz and R. B. Moore, Chem. Rev., 2004, 104(10),
T. Mao, J. Xu and Z. Wang, Electrochim. Acta, 2018, 283,
91–698.
J. Zheng, Q. He, C. Liu, T. Yuan, S. Zhang and H. Yang, J.
Membr. Sci., 2015, 476, 571–579.
4535–4586.
6
30 K. S. Siegert, F. R. Lange, E. R. Sittner, H. Volker,
C. Schlockermann, T. Siegrist and M. Wuttig, Rep. Prog.
Phys., 2015, 78, 13001.
7
8
9
A. L. Mong, S. Yang and D. Kim, J. Membr. Sci., 2017, 543, 31 K. Kreuer and G. Portale, Adv. Funct. Mater., 2013, 23, 5390–
33–142.
5397.
M. Sadakiyo, H. Okawa, A. Shigematsu, M. Ohba, T. Yamada 32 S. Kundu, M. W. Fowler, L. C. Simon and S. Grot, J. Power
and H. Kitagawa, J. Am. Chem. Soc., 2012, 134, 5472–5475.
Sources, 2006, 157, 650–656.
0 M. Liu, L. Chen, S. Lewis, S. Y. Chong, M. A. Little, T. Hasell, 33 Z. Xu, Z. Qi and A. Kaufman, J. Power Sources, 2003, 115, 49–
I. M. Aldous, C. M. Brown, M. W. Smith, C. A. Morrison, 53.
L. J. Hardwick and A. I. Cooper, Nat. Commun., 2016, 7, 34 B. Wang, L. Hong, Y. Li, L. Zhao, C. Zhao and H. Na, ACS
1
1
1
2750.
Appl. Mater. Interfaces, 2017, 9, 32227–32236.
35 C. Ru, Y. Gu, Y. Duan, C. Zhao and H. Na, J. Membr. Sci.,
2019, 573, 439–447.
1
1
1 H. Xu, S. Tao and D. Jiang, Nat. Mater., 2016, 15, 722–726.
2 G. Gnana kumar and A. Manthiram, J. Mater. Chem. A, 2017,
5
, 20497–20504; C. de Bonis, D. Cozzi, B. Mecheri, 36 J. P. Diard, B. Le Gorrec, C. Montella, C. Poinsignon and
A. D'Epifanio, A. Rainer, D. De Porcellinis and S. Licoccia,
Electrochim. Acta, 2014, 147, 418–425.
3 C. de Bonis, D. Cozzi, B. Mecheri, A. D'Epifanio, A. Rainer,
G. Vitter, J. Power Sources, 1998, 74, 244–245.
37 K. Shao, J. Zhu, C. Zhao, X. Li, Z. Cui, Y. Zhang, H. Li, D. Xu,
G. Zhang, T. Fu, J. Wu and W. Xing, J. Polym. Sci., Part A:
Polym. Chem., 2009, 47, 5772–5783.
1
D. De Porcellinis and S. Licoccia, Electrochim. Acta, 2014,
147, 418–425.
This journal is © The Royal Society of Chemistry 2020
J. Mater. Chem. A, 2020, 8, 196–206 | 205

View Article Online
Journal of Materials Chemistry A
Paper
3
3
4
4
4
4
4
4
4
4
4
4
5
5
8 G. He, Z. Li, J. Zhao, S. Wang, H. Wu, M. D. Guiver and 52 S. Asghari, A. Mokmeli and M. Samavati, Int. J. Hydrogen
Z. Jiang, Adv. Mater., 2015, 27, 5280–5295. Energy, 2010, 35, 9283–9290.
9 D. A. Tomalia, A. M. Naylor and A. William, Angew. Chem., 53 F. A. Zakil, S. K. Kamarudin and S. Basri, Renewable
Int. Ed. Engl., 1990, 29, 138–175. Sustainable Energy Rev., 2016, 65, 841–852.
0 C. Wang, D. W. Shin, S. Y. Lee, N. R. Kang, G. P. Robertson, 54 B. Bae, T. Yoda, K. Miyatake, H. Uchida and M. Watanabe,
Y. M. Lee and M. D. Guiver, J. Mater. Chem., 2012, 22, 25093. Angew. Chem., Int. Ed., 2010, 49, 317–320.
1 T. D. Gierke, G. E. Munn and F. C. J. Wilson, J. Polym. Sci., 55 G. Gnana Kumar, J. Mater. Chem., 2011, 21, 17382.
Polym. Phys. Ed., 1981, 19, 1687–1704.
56 M. Ranjani, D. J. Yoo and G. Gnana Kumar, J. Membr. Sci.,
2018, 555, 497–506.
57 G. Gnana kumar and A. Manthiram, J. Mater. Chem. A, 2017,
5, 20497–20504.
2 Y. Song, J. Huo, N. Zhang, J. Bao, X. Zhang, X. Ruan and
G. He, J. Phys. Chem. C, 2018, 122, 1982–1989.
3 M. Gil, X. Ji, X. Li, H. Na, J. Eric Hampsey and Y. Lu, J. Membr.
Sci., 2004, 234, 75–81.
4 M. Han, G. Zhang, K. Shao, H. Li, Y. Zhang, M. Li, S. Wang
and H. Na, J. Mater. Chem., 2010, 20, 3246.
5 X. Ren, T. E. Springer, T. A. Zawodzinski and S. Gottesfeld, J.
Electrochem. Soc., 2000, 147(2), 466–474.
6 H. Lin, C. Zhao, Y. Jiang, W. Ma and H. Na, J. Power Sources,
58 G. Gnana Kumar, J. Shin, Y. Nho, I. S. Hwang, G. Fei,
A. R. Kim and K. S. Nahm, J. Membr. Sci., 2010, 350, 92–100.
59 J. Kim, S. Kim, K. Nam and D. Kim, J. Membr. Sci., 2012, 415–
416, 696–701.
60 Z. Chen, B. Holmberg, W. Li, X. Wang, W. Deng, R. Munoz
and Y. Yan, Chem. Mater., 2006, 18, 5669–5675.
61 S. Al-Batty, C. Dawson, S. P. Shanmukham, E. P. L. Roberts
and S. M. Holmes, J. Mater. Chem. A, 2016, 4, 10850–10857.
62 K. Dutta, S. Das and P. P. Kundu, J. Membr. Sci., 2015, 473,
94–101.
2
011, 196, 1744–1749.
7 S. Shi, G. Chen, Z. Wang and X. Chen, J. Power Sources, 2013,
38, 318–323.
8 S. M. Rezaei Niya and M. Hoorfar, J. Power Sources, 2013, 240,
81–293.
2
2
63 S. Wang and H. Lin, J. Power Sources, 2014, 257, 254–263.
9 S. M. Rezaei Niya, R. K. Phillips and M. Hoorfar, J. 64 I. Nicotera, C. Simari, L. Coppola, P. Zygouri, D. Gournis,
Electroanal. Chem., 2016, 775, 273–279.
0 J. T. Mueller and P. M. Urban, J. Power Sources, 1998, 75, 139–
S. Brutti, F. D. Minuto, A. S. Aric `o , D. Sebastian and
V. Baglio, J. Phys. Chem. C, 2014, 118, 24357–24368.
65 B. Wang, L. Hong, Y. Li, L. Zhao, C. Zhao and H. Na, ACS
Appl. Mater. Interfaces, 2017, 9, 32227–32236.
66 L. Lei, X. Zhu, J. Xu, H. Qian, Z. Zou and H. Yang, J. Power
Sources, 2017, 350, 41–48.
1
43.
1 S. Yang, C. Chen and W. Wang, J. Power Sources, 2010, 195,
319–2330.
2
206 | J. Mater. Chem. A, 2020, 8, 196–206
This journal is © The Royal Society of Chemistry 2020