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Synthesis and characterization of crosslinked poly(vinyl alcohol)/layered double hydroxide composite polymermembranes for alkaline direct ethanol fuel cells

L. Zeng, T.S. Zhao*, Y.S. Li

Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

Hong Kong SAR, China

a r t i c l e i n f o

Article history:

Received 2 August 2012

Received in revised form

13 September 2012

Accepted 13 September 2012

Available online 12 October 2012

Keywords:

Fuel cell

Direct ethanol fuel cell

Poly (vinyl alcohol)

Layered double hydroxide

Ionic conductivity

Anion exchange membrane

* Corresponding author. Tel.: þ86 852 2358 8E-mail address: [email protected] (T.S. Zh

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.09.0

a b s t r a c t

The low ionic conductivity and low thermal stability of conventional quaternary ammo-

nium group functionalized anion-exchange membranes (AEM) are two key parameters that

limit the performance of AEM direct ethanol fuel cells (AEM DEFCs). The present work is to

address these issues by synthesizing crosslinked poly (vinyl alcohol)/layered double

hydroxide (PVA/LDH) hybrid membranes with solution casting method. The experimental

results indicate that incorporating 20 wt.% LDH into the PVA resulted in not only a higher

ionic conductivity, but also a lower ethanol permeability. The performance test of the DEFC

using the PVA/LDH hybrid membrane shows that the fuel cell can yield a power density of

82 mW cm�2 at 80 �C, which is much higher than that of the AEM DEFC employing the

quaternary ammonium group functionalized membrane. A constant current discharge test

shows that the PVA/20LDH membrane can be operated stably at relatively high

temperatures.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction Asan important component of AEMDEFCs, anion exchange

Anion exchange membrane direct ethanol fuel cells (AEM

DEFCs) that utilize alkaline anion exchange membranes

(AAEMs) as the solid electrolyte have recently been attracted

increasing attention [1e3]. The most significant feature of

DEFCs is that ethanol is not only environmentally friendly, but

also has a higher energy density, less toxic and widely avail-

able than other possible alcohol fuels. In particular, the

change in the electrolyte membrane from conventionally acid

to base will dramatically boost the kinetics of both the ethanol

oxidation and oxygen reduction reactions, even with non-

precious metal catalysts on both the anode and cathode of

the fuel cell [4,5].

647.ao).2012, Hydrogen Energy P89

membraneshavebeenextensively investigated in recent years

[6,7]. The existing commercial AEMsconsisting of hydrocarbon

main chains andquaternary ammoniumfunctional groups are

capable of selectively conducting hydroxide anions, but they

have a low conductivity. Moreover, the functional groups in

conventional AEMs will undergo a fast degradation when the

operating temperature becomes higher than 60 �C [5,8,9]. Poly

(vinyl alcohol) (PVA), therefore, has been selected to form

hybrid membranes for alkaline direct alcohol fuel cells.

Studies of PVA-based hybrid membranes have shown that the

solid matrix of PVA could hold alkaline electrolyte while

inorganic additives could improve the ionic conductivity and

thermal stability of membranes. Yang [10] prepared a PVA/

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 4 2 5e1 8 4 3 218426

TiO2 composite membrane by directly blending PVA with

nanoscale titania; the compositemembrane exhibited an ionic

conductivity of 12 mS cm�1 with 20 wt.% titania at 30 �C, butthe power density was as low as 7.54 mW cm�2 at 60 �C for

alkaline direct methanol fuel cells (DMFCs). Xiong et al. [11]

studied quaternization PVA and mixed with tetraethox-

ylsilanes (TEOS) to form organic-inorganic hybrid AEMs; the

membranes showed a low methanol permeability and a high

ratio of conductivity to permeability with the silica content of

5 wt.%. Wang et al. [12] prepared hybrid anion exchange

membranes based on PVA and 3-(trimethylammonium)

propyl-functionalized silica (TMAPS), which takes the role of

OH� transportation and disorders the crystallinity of PVA. In

addition, fumed silica [13], alumina [14], hydroxyapatite [15]

and bentonite [16] were blended with PVA to form solid poly-

mer electrolyte membranes.

A problem with the above-mentioned composite

membranes is that inorganic particles are unstable in

concentrated base media, resulting in fast degradation of the

membranes during the long-term operation, thereby limiting

their applications to fuel cells. Layered double hydroxides

(LDHs) as a kind of anionic clays that widely used as anion

exchangers [17], are stable in alkaline media. Recently, Tada-

naga et al. [18,19] observed that MgeAl CO2�3 LDH showed the

same electromotive force as anion exchangemembranes; their

studies showed that MgeAl CO2�3 LDH was able to conduct

hydroxide ions at an ionic conductivity of 1.1 � 10�2 S cm�1 at

80 �Cunder80RH%.The featuresof stableproperties inalkaline

media and a high OH� conductivity make the MgeAl LDH

suitable for alkaline exchangemembranes. However, this kind

of inorganic membranes may suffer from bad flexibility,

limiting their applications in fuel cells. An addition of organic

polymer into matrix of MgeAl LDH to form an intercalation

hybrid membrane not only can enhance the mechanical

property but also can increase its ionic conductivity.

In this work, we prepared hybrid membranes with MgeAl

LDH and organic PVA by a solution cast method. MgeAl LDH

with different loadingswas added into thediluted PVAsolution

and cast to formcompositemembranes. The thermal property,

crystallinity, ionic conductivity and ethanol permeability of the

obtained composite membranes were characterized. An AEM

DEFCemploying thecompositemembranewas also tested. The

results showed that the PVA/LDH composite membrane

exhibited better performance than the commercial A301

membrane did, especially at high temperatures.

2. Experimental

2.1. Synthesis of layered double hydroxides

Layered double hydroxides were synthesized through co-

precipitation method [18,19]. 0.6 M Mg (NO3)2 and 1 M Al

(NO3)3 with Mg2þ/Al3þ ¼ 3 were mixed by a magnetic stirrer at

80 �C. The mixture was added dropwise into 0.3 M Na2CO3

aqueous solution. Subsequently, pH of the mixture was

adjusted to 10 by adding 2 M NaOH solution. The precipitates

were collected by filtration after themixture was aged at 80 �Covernight. The precipitates were then washed with deionized

(DI) water several times and dried at 80 �C.

2.2. Preparation of PVA/xLDH composite membranes

The alkaline PVA/xLDH composite membranes were prepared

by solution castingmethod. The predeterminedweight of LDH

was ultrasonically dispersed into ethanol for 6 h, and 5 wt.%

PVA polymer solution was then slowly added. The mixture

was continuously stirred for another 6 h at 40 �C. 5 wt.%

glutaraldehyde (GA) as the crosslinker was dropwise added

into the mixture, and several drops of 1 M NaOH (Here the

base was used as catalyst instead of acid HCl to avoid disso-

lution of LDH)was then added as the catalyst. After stirring for

2 h, the mixture was degassed in a vacuum for 15 min. The

viscous solution was cast onto a clean glass plate using

a micrometer adjustable film applicator. The composite

membranes were peeled from the glass plate after drying in

a vacuum oven at 60 �C for 24 h. The composite membranes

are 60 mm thick, and represented by PVA/xLDH, where x

stands for the content of LDH. A composite membrane with

thickness of 44 mm was also prepared for comparison with

commercial A301membrane (The thickness is 44 mm; the ionic

exchange capacity and ionic conductivity is 1.6 mmol g�1 and

25 mS cm�1, respectively). All the membranes were immersed

into 4 M KOH solution for 1 day prior to testing.

2.3. Crystal structure, morphology and thermalanalyses

The crystal structures of the LDH and PVA/xLDH composite

membranes were analyzed with a Philips high resolution

X-ray diffraction system (model PW1825) using a CuKa source

operating at 40 keV. Surface morphologies of the composite

membranes were determined by scanning electron micros-

copy (JEOL-6300F). SEM-EDX mapping was operated at 15 kV.

The thermal property was measured using a PerkineElmer

Pyris 7 TGA system. The data were collected by heating the

samples from 25 to 600 �C, under N2 atmosphere with a heat-

ing rate of 10 �C min�1.

2.4. Measurements of ethanol permeability throughmembranes

The ethanol permeability through amembranewasmeasured

with a homemade apparatus. The membrane was tightly

sandwiched between two chambers with the same volume

(40ml). Chamber Awas filledwith 1M ethanol while Chamber

B was filled with DI water. The concentration of ethanol in

Chamber B due to permeation was measured by gas chro-

matography (6890 GC Instrument, Agilent Technologies). Each

chamber contained a magnetic stirrer for mixing during the

measurement. The ethanol concentration in Chamber B can

be determined by [14,20]:

CBðtÞ ¼ AVDKL

CAðt� t0Þ (1)

where C is the concentration of ethanol, A and L are the area

and thickness of the membrane, respectively, D and K stand

for the ethanol diffusivity and partition coefficient, V is the

volume of chamber, and a t0¼ L2/D is the time lag. The ethanol

permeability P¼DK is determined by the slope of the variation

in ethanol concentration with time.

10 20 30 40 50 60 70 80

(006)

Inte

nsity

(a. u

.)

2θ(°)

(003)

Fig. 1 e XRD patterns of the MgeAl layered double

hydroxides. The inset shows the TEM image of MgeAl

LDHs.

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2.5. Ionic conductivity measurements

The ionic conductivity of the membrane was measured

with a four electrode conductivity clamp with an AC

impedance method [20]. A sample membrane with

40 mm � 10 mm � 0.06 mm (long � wide � thick) was put in

the conductivity clamp. The clamp was then placed into

a humidity chamber (HP-2000, DAIHAN Labtech Co. LTD) with

a desired temperature and humidity. AC impedance

measurements were carried out using potentiostat (EG&G

Princeton, model 273A) coupled with frequency response

detector (Model 1025) after the sampleswere equilibratedwith

the experimental temperature at least 30 min. The spectra

were recorded at the frequency range from 100 kHz to 1 Hz

with the wave amplitude of 5 mV. During the measurement,

the relative humidity was kept at 95%. The influence of

humidity on the ionic conductivity also tested with the range

of relative humidity from 20% to 95%.

The membrane resistance (RU) can be obtained by calcu-

lating the intercept of the high frequency region. The ionic

conductivity s can be obtained from:

s ¼ Ld�W � RU

(2)

where L represents the distance of the potential electrodes, W

the width of potential electrodes and d the membrane

thickness.

2.6. MEA fabrication and fuel cell testing

A Pd/C catalyst (10 wt.% metal loading, Aldrich) and a non-

platinum HYPERMECTM catalyst (Acta) was employed as the

anode and cathode catalyst, respectively. The catalyst loading

at the anode and cathode was both 1 mg cm�2. The prepara-

tion of the anode and cathode can be found elsewhere [21,22].

The fuel cell testing was carried out with an electric load

system (BT2000, Arbin Instrument, Inc.). The MEA was sand-

wiched between two fixture plates (made from stainless steel),

one side of which was machined to form a single serpentine

flow field (1.0mm inwidth, 0.5mm in depth, and 1.0mm in rib

width). At the anode, an aqueous ethanol solutionmixed with

KOHwas supplied by a peristaltic pump at a constant flow rate

of 1 ml min�1. Pure oxygen was supplied to the cathode with

a flow rate of 100 standard cubic centimeters per minute

(sccm). The operating temperature was maintained by an

electrical heating rod andmeasured by a thermocouple placed

nearby the anode and cathode current collectors (i.e. the

fixture plates). The polarization curves were not recorded

until 24 h after assembling the MEA.

3. Results and discussion

3.1. Mg-Al LDHs characterization

Fig. 1 shows the XRD patterns of the synthesized MgeAl LDHs

with Mg2þ/Al3þ ¼ 3. The diffraction pattern of the sample is

consistent with the standard diffraction pattern (ICSD PDF#

54-1030). We also synthesized MgeAl LDHs by varying the

ratio between Mg and Al from 2:1 to 3:1 and 4:1; the XRD

characterizations showed the synthesized MgeAl LDHs with

the different ratios had the similar diffraction patterns, indi-

cating the same structures. For this reason, the ratio was fixed

to 3:1 for all the samples reported in this work.

The X-ray pattern shown in Fig. 1 shows that there exist

two strong peaks, which are marked with (003) and (006),

respectively. The basal spacing of (003) which calculates from

the c-value of the subcell containing one interlayer spaces is

0.776 nm, which is consistent with the result reported else-

where [23]. The TEM image of typical MgeAl LDHs samples is

also presented in Fig. 1, showing the MgeAl LDHs sample are

relatively uniform and thin hexagonal platelets with an

average lateral size of 100 nm. Obviously, the as-prepared

MgeAl LDHs has the high quality in terms of size and

uniformity. The uniform and well-dispersed MgeAl LDHs

particles are beneficial for the preparation of composite

membranes.

3.2. Morphology and crystalline structure of PVA/LDHmembranes

The morphologies of the composite membrane are presented

in Fig. 2. Fig. 2AeB shows the surface and cross-section

appearance of PVA/20LDH, respectively. It can be seen that

the composite membrane has a uniform morphology without

aggregates or chunks, which can be attributed to the well

dispersion process. However, with a high content of LDHs

(Fig. 2C), the PVA/30LDH membrane shows some aggregation

phase located on the cross-section, making the composite

membrane more brittle than pure PVA, which may lower

down the mechanical property of the composite membrane.

The selected area of PVA/20LDHs for EDXmapping also shows

in Fig. 2(B). The EDX mapping analysis is shown in Fig. 2DeF,

from which it can be observed oxygen, aluminum and

magnesium are uniformly distributed on the cross-section,

indicating that the homogeneous phase forms.

Fig. 2 e SEM images (A) surface view and (B) cross section view of PVA/20LDH membrane (red rectangle: selected area for

SEM-EDX mapping); (C) cross section view of PVA/30LDH membrane; (D)-(F) SEM-EDX mapping of O, Al and Mg. (For

interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

10 20 30 40 50

Inte

nsity

(a.u

.)

2θ(°)

PVA PVA/10LDH PVA/20LDH PVA/30LDH

Fig. 3 e XRD patterns of the PVA and PVA/xLDH composite

membranes (black dash line stands for 2q [ 19.5�).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 4 2 5e1 8 4 3 218428

The crystallinity of the compositemembranes is confirmed

by XRD presented in Fig. 3. A broad peak exhibits at 2q ¼ 19.5�

belongs to the characteristic peak of PVA. After adding LDHs

into the PVA matrix, the intensity of this peak decreases with

increasing the LDH content, reducing the crystallinity. As

a kind of semi-crystalline membrane, the crystallinity of PVA

is determined by the hydroxyl groups in the side-chain. After

being intercalated into the layer structure of LDHs, the side-

chain of PVA was restricted by the layer structures, resulting

in the reduced crystallinity [11,24]. Also, due to the strong

interfacial adhesion coming from the hydrogen bonding

between the interlayer structures of LDHs, PVA matrix was

intercalated into the layer structure, instead of exfoliating all

of the layer structure of LDHs. The intercalation process

enables the strong interaction between PVA polymer chain

and the layered double hydroxides.

3.3. Thermal property

The TGA thermograms of the PVA/xLDH composite

membranes, along with those of the neat PVA (crosslinked)

PVA PVA/10LDH PVA/20LDH PVA/30LDH0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Perm

eabi

lity(

10-7cm

2 s-1)

Membrane

Fig. 5 e Ethanol permeability of the PVA and PVA/xLDH

composite membrane.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 4 2 5e1 8 4 3 2 18429

and neat LDH, are compared in Fig. 4. There are two stages for

LDH decomposition. The first weight loss stage is due to the

loss of bound water, with the temperature ranging from 25 �Cto 161.5 �C (17wt.%). The secondweight loss stage is due to the

dehydroxylation of the layer structure, with the temperature

ranging from 161.5 �C to about 500 �C (23 wt.%). The TGA

curves of the composite membranes show three stages of

weight loss, which are ascribed to the evaporation of absorbed

water, the degradation of PVA polymer and LDH, the cleavage

backbone of PVA polymer, respectively. In the first stage, it can

be seen the weight loss of the composite membranes is lower

than that of neat PVA, which means that the addition of LDH

increases the absorption capability of water molecules.

Meanwhile, the secondweight loss stage of LDH overlaps with

the secondweight loss stage of PVA comparedwith TGA curve

of neat PVA, which leads to the degradation of the composite

membrane at lower temperature compared with PVA (i.e.

229 �C for composite membrane vs 295 �C for PVA) at the

second stage. Nevertheless, it is found that the weight loss of

the hybrid membrane is lower than that of neat LDH and the

degradation temperature of hybrid membranes moves

forward to the high temperature (229 �C for composite

membrane vs 161 �C for LDH).

3.4. Ethanol permeability

Fig. 5 shows the ethanol permeability through the composite

membrane. It can be seen that the ethanol permeability has

a dramatic reduction after adding the LDHs into the

membrane. The ethanol permeability through the PVA/10LDH

composite membrane is 1.767 � 10�7 cm2 s�1, which is

much lower than that of the neat PVA membrane

(4.545 � 10�7 cm2 s�1). The PVA polymer was intercalated into

the layer structure of LDHs, resulting in a tortuous path for

ethanol transportation. Therefore, the ethanol permeabilities

of the composite membranes are lower than that of PVA

membrane. It also can be seen that the permeabilities of the

PVA/xLDH composite membranes decrease with increasing

the LDH content.

It should be mentioned that the ethanol permeability was

measured with a diffusion cell without applying current. The

0 100 200 300 400 500 6000

10

20

30

40

50

60

70

80

90

100

Wei

ght l

oss(

%)

Temperature( )

PVA PVA/10LDH PVA/20LDH PVA/30LDH LDH

Fig. 4 e TGA curves of PVA and PVA/xLDH composite

membranes.

ethanol permeability through the composite membranes

installed in the DEFC should be even lower than the values

mentioned above, as themigration of hydroxide ions from the

cathode to the anode helps reduce the rate of ethanol

crossover.

3.5. Ionic conductivity

Fig. 6 shows the ionic conductivity of the PVA/xLDH composite

polymer membranes. The temperature ranged from 30 �C to

90 �Cwith the relative humidity of 95%. The ionic conductivity

is significantly increased after the addition of LDHs, from

6.2 mS cm�1 of neat PVA to 17.0 mS cm�1, 26.6 mS cm�1,

27.8 mS cm�1, corresponding to PVA/10LDH, PVA/20LDH and

PVA/30LDH at 30 �C, respectively. The reasonwhy the addition

of LDHs into PVA improves of ionic conductivity can be

explained as follows. First, the MgeAl CO2�3 LDH was testified

to be a hydroxide conductor by a concentration cell as

mentioned in the introduction section. Meanwhile, alkaline

doped PVA was also demonstrated to conduct the OH�.Therefore there will be exist synergic effect between PVA and

LDHs due to the strong interfacial adhesion proved by XRD

and TGA. Secondly, the crystallinity of PVA polymer was

inhibited by the LDHs and the amorphous phase is increased

as discussed above. The improvement of amorphous domain

created more free volumes, which are favorable for the

increase in the ionic conductivity.

Due to the hydrophilic property of LDH, an increase of LDH

in the composite membrane will bond more free water

molecules, which effectively impacts on the ionic conduc-

tivity. Therefore, the ionic conductivity of PVA/20LDH ismuch

higher than that of PVA/10LDH. However, a further increase in

the LDH content has a less influence on the ionic conductivity

comparing PVA/30LDH with PVA/20LDH. This can be

explained as follows. It commonly hypothesizes that the

transport of hydroxide ions follows the Grotthuss mechanism

that the ion transportation depends on the chain of water. The

free water will affect the anion transportation while another

kind of water, bound water, has less influence on the ionic

30 40 50 60 70 80 90

0.01

0.02

0.03

0.04

0.05 PVA PVA-10LDH PVA-20LDH PVA-30LDH

Ioni

c co

nduc

tivity

(S c

m-1)

Temperature( )

A

2.7 2.8 2.9 3 3.1 3.2 3.3

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5B

ln(io

nic

cond

uctiv

ity, S

cm

-1)

1000/T(K-1)

PVA PVA-10LDH PVA-20LDH PVA-30LDH

Fig. 6 e (A) Ionic conductivity of composite membranes

(Relative humidity [ 95%); (B) Arrhenius plots (Neat PVA

membrane also showed for comparison).

100 90 80 70 60 50 40 30 20 10

0.004

0.008

0.012

0.016

0.020

0.024

0.028

0.032

0.036

Ioin

c co

nduc

tivity

(S c

m)

Relative humidity(%)

PVA-20LDH PVA

20

40

60

80

100

σ/σ

(%)

Fig. 7 e Effect of relative humidity on the ionic conductivity

(sRH/s95% means the percentage of the ionic conductivity

under different humidity compared with the ionic

conductivity under RH [ 95%).

0 50 100 150 200 250 300 350 4000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Cel

l vol

tage

(V)

Current density(mA cm-2)

44μm 60μm

0

10

20

30

40

50

60

70

Po

wer

den

sity

(mW

cm

-2)

Fig. 8 e Polarization and power density curves of AEM

DEFC employing PVA/20LDH composite polymer

membrane with different thickness. Closed symbols

represent cell voltage, open symbols represent power

density (anode: 3 M ethanol D1 M KOH, 1 mL minL1;

cathode: dry oxygen, 100 sccm; temperature: 60 �C).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 4 2 5e1 8 4 3 218430

mobility. With an increase in the LDH content, the formation

of LDH aggregate was demonstrated by the SEM morphology

and XRD results as mentioned above. The LDH aggregates will

improve the content of bound water due to the strong inter-

action between the interlayer structures, which will has less

influence on the anion transport.

The ionic conductivity of the polymer exchangemembrane

is influenced by the humidity, as membranes need water

molecules to distribute functional group and transport

hydrated ions. In a proton exchange membrane, the proton

conductivity is influenced by humidity, especially at high

temperatures. Here, we also show that humidity is one of the

parameters that affect the hydroxide ion conductivity. The

influence of the relative humidity on ionic conductivity is

presented in Fig. 7. It can be seen that ionic conductivity is

significantly affected by the relative humidity. When the

relative humidity decreases from 95% to 20%, the ionic

conductivity of PVA reduces from 8.21 mS cm�1 to

3.09 mS cm�1, a reduction of 63%. In the case of the PVA/

20LDH composite membrane, the ionic conductivity declines

from 33.92mS cm�1 to 20.16 mS cm�1, a reduction of 41%. The

reason can be explained as follows. Humidity influences the

solvation of charged ions, whichwill impact on themobility of

ions. With the strong interfacial adhesion between LDH and

PVA, the absorbed water molecules are held in the interlayer

structures of LDH, which plays the role of solvated hydroxide

ions, resulting in the less reduction of ionic conductivity

under lower humidified condition.

3.6. Cell performance of AEM DEFCs

Fig. 8 shows the polarization and power density curves of MEA

employing the PVA/20LDHs membranes with different thick-

nesses. The power density is increased from 34.04 mW cm�2

to 61.30 mW cm�2 when the thickness of hybrid membrane

reduces from 60 mm to 44 mm. The improved performance

when reducing the membrane thickness can be attributed to

facilitate back-transport of water from the anode to the

0 10 20 30 40 50 60 70 800.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-0.5

0.0

0.5

1.0

0.264Ω

0.251Ω

-Zimag(Ω)

Zreal(Ω)

0.5Hrs 80Hrs

Cel

l vol

tage

(V)

Time(h)

Fig. 10 e Transient voltage of the AEM DEFC employing

PVA/20LDH at a constant current density of 50 mA cmL2.

Inset shows AC impedance spectra of MEA before and after

constant current discharge (Frequency: 100 kHze0.1 Hz,

temperature: 80 �C, anode: 3 M ethanol D1 M KOH,

1 mL minL1; cathode: dry oxygen, 100 sccm; membrane

thickness: 60 mm).

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cathode [25], resulting in the improvement of the cathode

performance. Also, the ohmic loss ascribed to membrane is

reduced when decreasing the thickness of themembrane [26].

Although the ethanol crossover will also be increased with

a reduction in the membrane thickness, the open cell voltage

is slightly reduced (i.e. 0.756 V for 60 mmand 0.754 V for 44 mm),

indicating the lower ethanol crossover. This should be resul-

ted from the well obstruction of ethanol crossover as pre-

sented in Fig. 5.

The cell performances with the PVA/20LDH and commer-

cial A301 membranes are presented in Fig. 9. The cell perfor-

mances were measured with the same electrodes, membrane

thickness (i.e. 44 mm), and operating conditions. It is seen the

maximum power densities of the MEA employing A301 and

PVA/20LDH at 60 �C are 31.97 mW cm�2 and 61.31 mW cm�2,

respectively. When the temperature increased to 80 �C, themaximum power densities of MEA employing A301 and PVA/

20LDH are increased to 37.11 mW cm�2 and 81.92 mW cm�2.

Clearly, the cell performance of MEA employing PVA/20LDH is

obviously higher than that of A301, especially at 80 �C,which is

attributed to the high ionic conductivity as shown in Fig. 6. The

large difference of power density at 80 �C is due to the different

ionic transportmechanism. A301membraneusing quaternary

ammonium functional group to conduct the OH� is readily

degraded at high temperatures (>60 �C), while the PVA/xLDHs

composite membranes using coordinate sites, local structural

relaxation and segmental motion to transfer the OH� can

afford the high temperature with high ionic conductivity.

The stability and durability of long-term operation is

another concern for AEM DEFCs. The voltage degradation

curves of AEM DEFCs employing PVA/20LDH at constant

current of 50 mA cm�2 at 80 �C for more than 80 h are pre-

sented in Fig. 10. The cell voltage steady decreases from initial

0.481 V (at 0.5 h) to 0.405 V (at 80 h) with the decay rate of

83.5 mVh�1. Also, the AC impedance spectra ofMEA before and

after constant current discharge are measured and shown in

Fig. 10. The ohmic resistance of the membrane can be

0 50 100 150 200 250 300 350 400 4500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Cel

l vol

tage

(V)

Current density(mA cm-2)

PVA/20LDH 60°C PVA/20LDH 80°C A301 60°C A301 80°C

0

10

20

30

40

50

60

70

80

90

Pow

er d

ensi

ty(m

W c

m-2)

Fig. 9 e Polarization and power density curves of AEM

DEFC employing PVA/20LDH composite polymer

membrane and commercial A301 at different temperature.

Closed symbols represent cell voltage, open symbols

represent power density. (anode: 3 M ethanol D1 M KOH,

1 mL minL1; cathode: dry oxygen, 100 sccm).

obtained through calculating the intercept of the high

frequency impedance loop; the membrane resistance

increases from 0.251 U to 0.264 U. It should be mentioned that

the thickness of membrane is 60 mm and thereby the ionic

conductivity of membrane reduces from 23.9 mS cm�1 to

22.7 mS cm�1, with the degradation of 4.9%. This result

demonstrated the PVA/xLDH composite membrane can be

used at high temperatures. In the impedance spectra, the

second semicircle with a larger time constant belongs to the

anode activation process due to the sluggish kinetics of

ethanol oxidation reaction. It can be seen that the diameter of

second semicircle becomes larger after long-term operation,

which is ascribed to the degradation of the anode. The results

of impedance spectra are consistent with the result reported

elsewhere [8]. It was reported the performance degradation of

AEM DEFCs was mainly attributed to agglomeration and

growth of anode catalyst particles, which caused the reduc-

tion of the electroactive surface area. This is the main reason

lead to the reduction of the cell voltage.

In summary, the MEA employing the PVA/LDH membrane

exhibits a peak power density of 61.31 mW cm�2,

81.92mW cm�2 at 60 �C and 80 �C, respectively. The long-term

operation at 80 �C indicated that the PVA/LDH membrane

couldmaintain the ionic conductivity with lower degradation.

4. Conclusions

Poly (vinyl alcohol)-layered double hydroxide composite

polymermembraneswas successfully developed by a solution

cast method. PVA polymer was uniformly intercalated into

the layered structures of LDH to form a uniform membrane.

Due to the strong interfacial interaction between PVA and

LDH, ionic conductivity of the composite membranes was

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 4 2 5e1 8 4 3 218432

enhanced, and the ethanol permeability of the membranes

was reduced. The cell performance with the PVA/20LDH

membrane was measured and peak power densities of

61.31 mW cm�2 and 81.92 mW cm�2 at 60 �C and 80 �C were

achieved, which are higher than that using the commercial

A301 membrane. A more than 80 h discharge process at 80 �Cemploying the PVA/20LDH membrane demonstrated that the

ionic conductivity of the membrane was only slightly

decreased, from 23.9 mS cm�1 to 22.7 mS cm�1. The PVA/LDH

composite membrane has shown a potential application in

AEM DEFCs, especially at high operating temperatures.

Acknowledgments

The work described in this paper was fully supported by

a grant from the Research Grants Council of the Hong Kong

Special Administrative Region, China (Project No. HKUST9/

CRF/11G).

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