B368
Journal of The Electrochemical Society, 159 (4) B368-B370 (2012)
0013-4651/2012/159(4)/B368/3/$28.00 © The Electrochemical Society
Effect of Mg/Al Ratio on Hydroxide Ion Conductivity for Mg–Al
Layered Double Hydroxide and Application to Direct Ethanol
Fuel Cells
Kiyoharu Tadanaga, z Yoshihiro Furukawa, Akitoshi Hayashi, ∗ and Masahiro Tatsumisago
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Naka-ku, Sakai,
Osaka 599-8531, Japan
Mg–Al layered double hydroxides (LDHs) intercalated with carbonate anions were prepared by the co-precipitation process with
Mg/Al = 2, 3 and 4, and the effect of Mg/Al ratio on the ionic conductivity was examined. It is shown that the Mg/Al ratio influences
ionic conductivities of Mg–Al CO3 2− LDH. The conductivity of Mg-Al CO3 2− LDH with Mg/Al = 2 and 3 exhibited almost the
same ionic conductivity of about 1 × 10−3 S cm−1 at room temperature under 80% relative humidity, while that of LDH with
Mg/Al = 4 was lower by one order of magnitude. Thermal analyses showed that Mg-Al CO3 2− LDH with Mg/Al = 2 or 3 keeps
interlayer water molecules at higher temperatures compared with Mg/Al = 4, suggesting that the interlayer water molecules are
strongly held in the interlayer. The strongly held interlayer water molecules are assumed to contribute the high ionic conductivity
in Mg-Al CO3 2− LDH with Mg/Al = 2 and 3. The alkaline type direct ethanol fuel cells using Mg/Al CO3 2− LDHs as hydroxide
conducting electrolyte were confirmed to operate, and larger power was obtained in Mg-Al CO3 2− LDH with smaller Mg/Al ratio.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.007204jes] All rights reserved.
Manuscript submitted March 31, 2011; revised manuscript received November 22, 2011. Published January 18, 2012.
Layered double hydroxides (LDHs) are anionic clays consisting of
positively charged metal hydroxide layers with anions located in the
interlayer for charge compensation of the cationic layers. The chemical formula for LDHs is [MII 1−x MIII x (OH)2 ]x+ (An− )x/n · mH2 O, where
MII is a divalent metal cation, MIII is a trivalent cation, and An− is an
anion. Because of their potential application as anion exchangers, catalysts, electrochemical sensors and bio-active materials, LDHs have
received increasing attention in recent years.1–4
The layered structure in LDHs incorporates anions and water
molecules in the interlayer, and thus, they can be interesting materials
for use as solid electrolytes. Nevertheless, few reports have described
investigations of ionic conductivities of LDHs.5, 6 In fuel cells, LDHs
have been used only as fillers for proton conductive membranes,7, 8 or
effects of the addition of LDHs to the catalyst layer of alkaline-type
direct alcohol fuel cells have been reported.9, 10 Very recently, we examined the ionic conductivity for Mg-Al layered double hydroxides
(LDHs) intercalated with several inorganic anions11 reporting that
they have high ionic conductivity under 80% relative humidity. We
also clarified that Mg-Al LDH intercalated with CO3 2− is a hydroxide
ion conductor under the humidified condition by water vapor concentration cell measurements, and showed that Mg-Al CO3 2− LDH
functions as an electrolyte for alkaline type direct ethanol fuel cells
(DEFCs).12
In LDHs, trivalent cations replace divalent cation, and the replacement of divalent cations with trivalent cations generates a positive
charge on the hydroxide layers of LDH. Thus, the degree of replacement affects the amount of interlayer anions, and may also affect the
ionic conductivity.
In the present study, effects of Mg/Al ratio on the ionic conductivity
of the Mg–Al CO3 2− LDHs were examined. Performance of DEFCs
using the Mg–Al CO3 2− LDHs as electrolyte was also examined.
Experimental
Using the co-precipitation method with controlled pH, Mg–
Al CO3 2− LDHs were prepared.13 A mixed solution containing
Mg(NO3 )2 · 6H2 O and Al(NO3 )3 · 9H2 O with Mg2+ /Al3+ = 2, 3 and
4 was added dropwise into 0.3 M Na2 CO3 solution with stirring at
80◦ C. The pH of the mixture was adjusted to 10 by the addition of
2 M NaOH solution and the reaction mixture was aged at 80◦ C for
17 h. The resulting white precipitates were filtrated, washed with
distilled water, and dried at 80◦ C. The precipitates consisted of ag∗ Electrochemical Society Active Member.
z
E-mail: [email protected]
gregated particles with a size of a few micrometer, having a primary
particle diameter of from several tens to a few hundreds nanometers.
X-ray diffraction (XRD) measurements (CuKα) were performed
to identify crystalline phases using an XRD diffractometer (Shimazu XRD 6000). Differential thermal analysis and thermogravimetry
(DTA-TG) were carried out using a thermal analyzer (Rigaku Termoplus 8120), with a hearting rate of 10◦ C min−1 in N2 .
Electrical conductivities of the pellets obtained by cold pressing
under a pressure of 360 MPa were measured: the diameters and thickness of the pellets were 10 mm and about 0.5 mm, respectively, and
the estimated relative density of the pellet was more than 90%. Gold
was evaporated on both sides of the pellets as the electrodes. The conductivity of the pellets was determined from impedance data obtained
using an impedance analyzer (Solartron 1260; Solartron Analytical)
at frequencies of 1 Hz – 8 × 106 Hz.
Performance of DEFCs was examined as described previously.12
A passive-type direct ethanol fuel cell was fabricated using pelletized
Mg-Al CO3 2− LDH powder as electrolyte (thickness of the pellet was
about 0.3 mm). The anode and cathode electrodes with non-Pt catalysts
(HypermecTM ; ACTA S.p.A.) were used, where Ni-Co-based alloy on
carbon powder is used for the anode and Fe-Co-based alloy on carbon
powder is used for the cathode.14 The cathode and anode backing
layers were nickel form and carbon cloth, respectively. The pelletized
Mg-Al CO3 2− LDH was sandwiched with the two electrodes and the
gold-plated current collectors that were attached with cell fixtures. An
aqueous solution of ethanol and potassium hydroxide (10wt %EtOH10wt %KOH) was used as fuel. The volume of the anode compartment was ca. 25 cm3 , and the cathode was exposed to air. The active
area of the cell was about 0.4 cm2 . Polarization performances were
measured using a potentiostat and galvanostat (Autolab, PGSTAT30)
at R.T.
Results and Discussion
Figure 1 shows the XRD patterns of LDH with Mg/Al = 4, 3 and
2. The basal spacing calculated using those peaks is 0.804, 0.780, and
0.750 nm for Mg/Al = 4, 3, and 2, respectively. The basal spacing
decreases with a decrease in Mg/Al ratio from 4 to 2. The basal spacing
should be affected by the interaction between inorganic layers. As
mentioned above, in LDH, the replacement of divalent cations with
trivalent cations generates a positive charge on the hydroxide layers of
LDH. Decrease in the Mg/Al ratio means the increase in the amount of
substitution of Mg2+ with Al3+ and also the increase in charge of layer.
Therefore, the interaction between hydroxide layers is considered to
become strong and the basal spacing between layers decreased.
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Journal of The Electrochemical Society, 159 (4) B368-B370 (2012)
B369
Au / LDH / Au
(0 0 3)
(0 0 6)
10
Mg/Al = 4
-2
Mg/Al = 3
Conductivity / S cm-1
Intensity / arb.unit
d = 0.804nm
(0 0 3)
(0 0 6)
Mg/Al = 3
d = 0.780nm
10
-3
10
-4
10
-5
(0 0 3)
(0 0 6)
10
Mg/Al = 2
Mg/Al = 4
2.8
2.9
Mg/Al = 2
d = 0.750nm
20
30
40
2 /deg. (CuK )
80 %R.H.
3
3.1
3.2
1000/T / K -1
3.3
Figure 3. Temperature dependence of the conductivities for Mg-Al CO3 2−
LDH with Mg/Al = 2, 3 and 4.
50
Figure 1. XRD patterns of Mg-Al CO3 2− LDH with Mg/Al = 2, 3 and 4.
The DTA-TG curves of Mg-Al CO3 2− LDH with Mg/Al = 4, 3
and 2 are presented in Fig. 2. The weight loss at temperatures between
room temperature and 250◦ C can be assigned to the loss of interlayer
water in LDH, and the weight loss at higher temperature is assigned
to the elimination of H2 O from OH groups in the inorganic layer and
CO2 from CO3 2− ions.15 The endothermic peak in the DTA curve at
around 200◦ C is shifted to higher temperature side with decreasing
Mg/Al ratio. Since the charge density of a hydroxide layer becomes
large as Mg/Al becomes small, the interaction between the hydroxide
layer and interlayer water molecules should become strong. Thus,
Mg-Al CO3 2− LDH with Mg/Al = 2, or 3 keeps the interlayer water
molecules at higher temperatures compared with Mg/Al = 4.
Temperature dependence of the conductivities for Mg-Al CO3 2−
LDH with Mg/Al = 4, 3 and 2, at 80% of the relatives humidity is
shown in Fig. 3. The ionic conductivity of Mg-Al CO3 2− LDH with
Mg/Al = 2 and 3 is about 10−3 S cm−1 , while that of Mg-Al CO3 2−
LDH with Mg/Al = 4 is lower than 10−4 S cm−1 , at room temperature. Moreover, the activation energy of ionic conduction decreases
with a decrease in Mg/Al ratio. As shown in Fig. 2, interlayer water
molecules are strongly held in Mg-Al CO3 2− LDH with smaller Mg/Al
ratio. Because the interlayer molecules can contribute the retention of
adsorbed water under humidified conditions, the interlayer molecules
are assumed to play an important role in ionic conduction in Mg-Al
CO3 2− LDH. Thus, the water molecules strongly held in the layer
must contribute the high ionic conductivity and low activation energy
in Mg-Al LDH with Mg/Al = 2, and rather low ionic conductivity of
LDH with Mg/Al = 4.
Although Mg-Al LDHs are known as an anion exchanger, Mg-Al
LDH intercalated with CO3 2− is very stable against anion exchange,
and thus the ion conducting species is not CO3 2− . As reported previously, Mg-Al CO3 2− LDH with Mg/Al = 3 is confirmed to be a
hydroxide ion conductor under the humidified condition. The water
vapor concentration cell measurements for Mg-Al CO3 2− LDH Mg/Al
= 2 and 4 revealed that these LDHs are also hydroxide ion conductor.
Figure 4 shows the cell voltage and power density versus current
density for a passive-type DEFC with Mg-Al CO3 2− LDH with Mg/Al
= 2, 3 and 4, as an electrolyte and non-platinum catalysts, at room
temperature. The cells work as a DEFC; the larger current density
is observed at smaller Mg/Al ratio. The maximum power density of
about 48 mW cm−2 is obtained for the cell with Mg/Al = 2, which is
comparable to that of an active-type DEFC using those non-platinum
Mg/Al=2
Mg/Al=3
1
Mg/Al=4
50
0.8
40
0.6
30
0.4
20
-20
0.2
10
-40
0
Mg/Al = 2
20
0
100
200
300
400
500
Figure 2. DTA-TG curves of Mg-Al CO3 2− LDH with Mg/Al = 2, 3 and 4.
Cell voltage / V
40
0
50
100 150 200 250
Current density / mA cm-2
Power density / mW cm -2
Mg/Al = 3
Mg/Al = 4
0
300
Figure 4. Performances of the DEFCs using Mg-Al CO3 2− LDH with Mg/Al
= 2, 3 and 4 as the electrolyte at room temperature. An aqueous solution of
10wt %EtOH-10wt %KOH was used as fuel.
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B370
Journal of The Electrochemical Society, 159 (4) B368-B370 (2012)
ences ionic conductivities of Mg–Al CO3 2− LDH. The conductivity of
Mg-Al CO3 2− LDH with Mg/Al = 2 and 3 exhibited almost the same
ionic conductivity of about 1 × 10−3 S cm−1 at room temperature
under 80% relative humidity, while that of LDH with Mg/Al = 4 was
lower by one order of magnitude. Thermal analyses showed that MgAl CO3 2− LDH with Mg/Al = 2 or 3 keeps interlayer water molecules
at higher temperatures compared with Mg/Al = 4, suggesting that the
interlayer water molecules are strongly held in the interlayer. The
strongly held interlayer water molecules are assumed to contribute
the high ionic conductivity in Mg-Al CO3 2− LDH with Mg/Al = 2
and 3. The alkaline type DEFCs using Mg/Al CO3 2− LDHs as the
electrolyte were confirmed to operate, and larger power was obtained
in Mg-Al CO3 2− LDH with smaller Mg/Al ratio.
-6
Mg/Al=2
Mg/Al=3
Mg/Al=4
Z'' / Ω
-4
-2
0
0
1
2
3
Z' / Ω
4
5
Acknowledgments
6
Figure 5. Nyquist plots of the DEFCs using Mg-Al CO3 2− LDH with Mg/Al
= 2, 3 and 4, as the electrolyte at room temperature.
This work was partly supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology of Japan.
References
14
catalysts and an anion-exchange membrane or a passive DEFC using
Pd/MWCNT anode catalyst and the non-platinum cathode catalyst
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Nyquist plots of the DEFCs shown in Fig. 4, under the cell operation, are shown in Fig. 5. In the Z vs. Z plot, a few resistance
components were observed. The resistance observed in the highest frequency region is attributed to the resistance of the electrolyte. Thus,
the Nyquist plots show that the resistance of the electrolyte decreased
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= 2 and 3 is almost the same as shown in Fig. 1. However, in the
DEFC cell, KOH in the fuel slightly increased the ionic conductivity
of the solid electrolyte, and the degree of the increase in conductivity
should be higher in Mg/Al = 2. The superior performances at smaller
Mg/Al ratio must be caused by the increase in ionic conductivity with
a decrease in Mg/Al ratio.
Conclusions
Effects of Mg/Al ratio on the electrochemical properties of Mg-Al
CO3 2− LDH were examined. It is shown that the Mg/Al ratio influ-
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