Download A hydrophilic-hydrophobic dual-layer microporous layer

Journal of Power Sources 294 (2015) 232e238
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Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
A hydrophilic-hydrophobic dual-layer microporous layer enabling the
improved water management of direct methanol fuel cells operating
with neat methanol
X.H. Yan, T.S. Zhao*, G. Zhao, L. An, X.L. Zhou
Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR,
China
h i g h l i g h t s
A hydrophilic-hydrophobic dual-layer MPL for passive DMFCs is proposed.
The novel MPL enables a significant improvement of water management.
The use of this MPL to either electrode leads to a significant performance boost.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 February 2015
Received in revised form
15 May 2015
Accepted 8 June 2015
Available online xxx
Passive direct methanol fuel cells (DMFCs) operating with neat methanol can achieve the maximum
system energy density. However, the anodic methanol oxidation reaction requires reactant water, which
is completely supplied by water generated at the cathode, causing the system to experience a critical
issue known as water starvation. A solution to this problem involves increasing the water recovery flux to
meet the rate of water consumption of the anodic reaction, and increase the local water concentration as
high as possible at the anode catalyst layer (CL) to improve the anodic kinetics. In the present work, a
new microporous layer (MPL) consisting of a hydrophilic layer and a hydrophobic layer is proposed. The
purposes of these two layers are to, respectively, trap and retain water and to create capillary pressure to
prevent water loss. Our experiments have shown that the use of this novel MPL at the anode and cathode
can increase the rate of water recovery and water retention, resulting in an increase in the local water
concentration. As a result, the use of this dual-layer MPL to either electrode of a passive DMFC operating
with neat methanol leads to a significant performance boost.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Fuel cells
Direct methanol fuel cells
Neat methanol operation
Water starvation
1. Introduction
Passive DMFCs have garnered increasing attention as the next
generation power source for portable electronic devices [1e4],
owing to its unique features, such as compact structure, convenience in fuel storage, low operating temperature and high theoretical energy density. Conventional passive DMFCs operate with
diluted methanol (i.e., 2.0 M4.0 M) to minimize the hindrances
caused by methanol crossover, but the most striking feature of high
energy density output (~4800 Wh L1) is sacrificed as a result. Thus,
it is ideal to operate passive DMFCs with highly concentrated fuel,
* Corresponding author.
E-mail address: [email protected] (T.S. Zhao).
http://dx.doi.org/10.1016/j.jpowsour.2015.06.058
0378-7753/© 2015 Elsevier B.V. All rights reserved.
and retard the rate of methanol crossover to maintain high
performance.
Past efforts to achieve this objective have been extensively
made. The approaches attempted can be divided into three categories: developing new electrolyte membranes with low methanol
permeability [5,6]; modifying the anode diffusion layer to increase
the transport resistance of methanol [7,8]; and introducing a barrier layer to control the rate of methanol transport before it reaches
the electrode [9]. Among them, the vapor-feed supply mode is one
of the most effective approaches [10e14], which enables passive
DMFCs to operate with neat methanol. Under this setup, however,
water starvation is a major issue as water is one of the reactants of
the methanol oxidation reaction (MOR) at the anode, and relies
completely on the supply of production from the cathode. To
address this issue, a sufficiently large water recovery flux must be
X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238
ensured. Masdar et al. [15] adopted a hydrophobic air filter at the
cathode to retard water loss and thus enhance water recovery.
Zhang and Feng [16] treated the cathode backing layer by
increasing the PTFE content to 40 wt. %, to enhance the water backflow effects. Li and Faghri [17] attempted to improve the water
recovery by optimizing the cathode open ratio. Xu et al. [18] proposed various approaches to increase the rate of water back
transport, including thinning the membrane thickness, placing an
air filter, and adopting a water management layer at the cathode.
Previous efforts have indeed achieved varying degrees of success in water management, but have altered the original structure
of DMFCs, resulting in increased system complexity. In the present
work, we propose a new microporous layer (MPL) consisting of a
hydrophilic layer and a hydrophobic layer. Highly hydrophobic
material, polytetrafluoroethylene (PTFE), is used as the binder in
the hydrophobic layer, and cross-linked polyvinyl alcohol (PVA),
both hygroscopic and highly hydrophilic, is used in the hydrophilic
layer. Taking advantage of the disparate features of these two
binder materials, the hydrophilic layer traps and retains water,
while the hydrophobic layer creates capillary pressure to prevent
the water loss, resulting in an overall increase in the local water
concentration. The schematic description of the hydrophilichydrophobic dual-layer MPL is shown in Fig. 1.
A water gradient is created throughout the passive DMFC
membrane, due to the higher concentration of water generated at
the cathode in contrast with the anode, which produces no water.
This gradient drives the water to diffuse from the cathode toward
the anode. The corresponding flux can be expressed as:
Cc Ca
dm
Km rs cos q ε 1=2
Jc ¼ JðsÞ
mMH2 O dm K
(2)
where Km is the permeability of the membrane, r is water density, m
is viscosity of liquid water, MH2O is molecular weight of water, and
scosq(ε/K)1/2 J(s) is the capillary pressure created by the MPL [19].
Another contributor to the water crossover is electro-osmotic
drag by proton transport from the anode to the cathode, which
can be expressed as:
Jeo ¼ nd
i
F
(3)
where nd, i and F denote, respectively, the electro-osmotic drag
coefficient in the membrane, current density and Faraday's
constant.
Therefore, the net water recovery flux can be obtained from Eqs.
(1)e(3) as:
Jw ¼ Jd þ Jc Jeo ¼ Deff
Clc Cla Km rs cos q ε 1=2
i
JðsÞ nd
mMH2 O dm K
F
dm
(4)
Meanwhile, the rate of water consumption by the anodic MOR is
obtained as:
2. Theoretical
Jd ¼ Deff
233
(1)
where Deff is the effective diffusivity of water through the membrane, dm is the thickness of membrane, Cc and Ca represent the
water concentrations at the cathode and anode surfaces facing the
membrane, respectively.
The generation of water at the cathode creates a gradual pressure difference throughout the membrane, resulting in a convection flux directed from the cathode to the anode:
JMOR ¼
i
6F
(5)
From these equations, it is apparent that the basic requirement
for a complete 6-electron transfer reaction to be true is to ensure
Jw JMOR > 0. As shown in Eq. (4), a larger water back flux can be
achieved by upgrading the water concentration at the cathode CL
and increasing the capillary pressure. For this reason, a hydrophilichydrophobic dual-layer MPL is proposed, in which the hydrophilic
layer located between the CL and hydrophobic layer elevates the
water concentration at the cathode and enhances the effect of back
diffusion. In addition, the hydrophobic layer is used to maintain the
convection flux.
It is noteworthy to mention that Wu et al. [20] found that there
exists a mole ratio of water to methanol at the anode CL under neat
methanol operation. When the mole ratio is larger than the critical
value, the anode potential stabilizes to a value close to that of the
Fig. 1. Schematic description of the hydrophilic-hydrophobic dual-layer MPL.
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X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238
diluted-methanol operation and typically this value is larger than 2.
Therefore, it is inadequate to focus entirely on ensuring high water
recovery flux of Jw, but that water concentration at the anode CL
must also be monitored to improve the anodic kinetics. Since water
concentration at the anode CL is higher than that of the fuel
reservoir, further loss of water toward the fuel reservoir should be
observed. Based on this knowledge, the dual-layer MPL is also
proposed for the anode, in which the hydrophobic layer is used to
retard water loss and the hydrophilic layer is employed to trap and
retain the recovered water.
3. Experimental
3.1. Membrane electrode assembly
3.1.1. Standard MEA
A pretreated Nafion® 212 membrane of thickness 50 mm was
employed in this work. Toary-060 carbon papers with 15 wt.% PTFE
wet-proofing treatment worked as the backing layers. At the anode
and cathode, the conventional hydrophobic MPL was fabricated
using XC-72 carbon powder with a loading of 3.0 mg cm2 and
adopted PTFE as the binder, with mass ratio of 30 wt.%. The catalyst
layers of the anode and cathode consisted of PtRu black (Johnson
Matthey®) with a loading of 4.0 mg cm2 and carbon supported Pt
(60% Pt, Johnson Matthey®) with a loading of 2.0 mg cm2,
respectively; both the mass ratios of Nafion ionomer were 20 wt.%.
The catalyst layers were fabricated by a catalyst-coated membrane
(CCM) method to provide direct coating on the membrane,
ensuring the effect of different MPL structures to be investigated
under a constant condition. The active area of the MEA was 4.0 cm2.
3.2. Single cell and measurement instrumentation
The MEA was placed between an anode and a cathode perforated current collector. This setup was sandwiched by the anode
and the cathode fixture and a 5 mL fuel tank was constructed at the
anode. In contrast with conventional passive DMFC structure, a gas
gap with a thickness of 10 mm, a perforated plate with an open
ratio of 15% and a 28 mm pervaporation membrane were employed
at the anode to realize the neat methanol operation as shown in
Fig. 3.
An Arbin BT2000 electrical load was used to control the
discharge condition and record the data. Prior to the performance
test, the MEA was installed in an active cell fixture and activated for
12 h at 60 C. During the activation process, 1.0 M methanol was fed
at 1.0 mL min1, while dry air was supplied under the atmospheric
pressure at a flow rate of 100 mL min1, and discharge tests were
performed for several times until the performance reached a
plateau without a further change. To ensure that the DMFC was
running with neat methanol, the MEA was dried at ambient temperature for 12 h before the performance test to remove the water
that initially existed inside the membrane. During the performance
test, the voltage stabilized at about 60s for each discharge current
point.
To measure the water recovery flux, the cathode fixture was
changed to an active mode and a water trap filled with Drierite
(anhydrous CaSO4) was connected. The water was gathered at the
open circuit voltage for 8.0 h. The details about this test can be
found from Ref. [21].
The various MPLs were characterized by a contact angle measurement system, scanning electron microscope (JEOL-6300) and
energy dispersive X-ray spectrum.
4. Results and discussion
3.1.2. MEA with dual-layer MPL
To form the dual-layer MPL, an inner hydrophilic layer was
fabricated between the CL and the outer hydrophobic layer with
polyvinyl alcohol as the binder. The loading of the XC-72 carbon
powder changed from 0.3 mg cm2 to 1.5 mg cm2, which determined the thickness of the hydrophilic layer; the mass ratio of the
PVA binder changed from 5 wt. % to 20 wt. %. Due to the watersoluble nature of PVA, hydrogel must be formed through a crosslinking reaction, after the fabrication of the hydrophilic layer. This
was formed by immersing the GDL coated hydrophilic layer into
5 wt.% glutaraldehyde (GA) (which worked as the crosslinker), and
adding several drops of 5wt.% HCl to work as the catalyst as shown
in Fig. 2. The entire process lasted for 2 h. The MPL was then washed
with DI water.
Fig. 2. The cross-linking reaction between PVA and glutaraldehyde.
4.1. Effect of the dual-layer MPL at the cathode
Ensuring a sufficient water recovery flux to supply the water
required by the MOR is imperative. To increase the water back
Fig. 3. Schematic of a passive DMFC operating with neat methanol: 1-Fuel reservoir;
2-Pervaporation membrane; 3-Perforated plate; 4-Gas gap; 5-CO2 venting hole; 6Current collectors; 7-MEA.
X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238
transport rate, the dual-layer MPL was used at the cathode. Fig. 4
gives a cross-sectional view of the dual-layer MPL structure (hydrophilic layer: 0.3 mg cm2 carbon, 10 wt.% PVA). From the EDX
mappings, the hydrophilic part does not consist of F element,
indicating that a very thin hydrophilic layer was coated on the
conventional hydrophobic layer.
Fig. 5 shows the effect of the dual-layer MPL on the cell performance of the DMFC operating with neat methanol. It demonstrates regardless of changes in the carbon loading of the
hydrophilic layer, the MEA with dual-layer MPL always exhibits a
better performance than does the conventional MEA. For instance,
the power density of the MEA with hydrophilic layer of 0.5 mg cm2
carbon is 29.2 mW cm2, 29% larger than that of a conventional
MEA (22.6 mW cm2). The cross-linked PVA is a highly hygroscopic
and hydrophilic material, as a result the hydrophilic layer adopting
the PVA binder can help to trap and retain produced water of the
cathode ORR to enhance the water concentration at the cathode
catalyst layer, the higher water concentration increase the water
diffusion to the anode and further improve the kinetics of the
anodic MOR, thereby boosting the cell performance. The increase in
water recovery flux by the dual-layer MPL is proven in Fig. 6.
Improvement in water back transport can be further confirmed by
the limiting current density. In our previous work, we have proven
that the limiting current density of the passive DMFCs under neat
methanol operation is due to the lack of water for MOR [14]. Fig. 5
indicates that the current density increases significantly after the
application of dual-layer MPL, thereby removing this limitation.
These results suggest that the cathode dual-layer MPL has excellent
water trapping and retention abilities and is able to improve water
recovery for passive DMFCs operating with neat methanol.
The effect of carbon loading in the hydrophilic layer (the mass
ration of PVA is 10 wt.%) is shown in Fig. 5. It shows that there is a
slight improvement in cell performance as the carbon loading increases from 0.3 mg cm2 to 0.5 mg cm2. A further increase in the
carbon loading, however, leads to a performance decline. This can
be attributed to the fact that a larger carbon loading indicates a
thicker hydrophilic layer which exhibits better water retention
ability. Therefore, enhancing the water concentration at the cathode CL and increasing the water back transport helps to boost the
anodic MOR. It should be noted, however, that the hydrophilic layer
has a negative effect on oxygen transport to the cathode CL. When
water accumulates in the hydrophilic layer, oxygen transport
235
Fig. 5. Effect of the dual-layer MPL at the cathode with different carbon loadings on
cell performance (the PVA content is 10 wt.%).
Fig. 6. Effect of the dual-layer MPL at the cathode on the water recovery flux with
different carbon loading.
Fig. 4. Cross-sectional view of the hydrophilic-hydrophobic dual-layer MPL and the EDX mappings of fluorine and carbon.
236
X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238
through the water-rich layer will be hindered which will lower the
cathode ORR, since the order of magnitude for oxygen diffusivity in
air is 105, while the value in water is just 109 [22,23]. This phenomenon suggests a thin hydrophilic layer is preferred at the
cathode.
Fig. 7 shows the contact angles of various MPLs adopting
different binders. The contact angle of the conventional hydrophobic MPL is as high as 140.3 . For MPLs adopting the PVA binder,
the contact angles are smaller than 50 , meaning that the PVA can
introduce the highly hydrophilic property. An increase in PVA
content from 5 wt.% to 20 wt.% decreases the contact angle from 42
to 23 , thus improving the ability to trap water.
Fig. 8 exhibits the effect of the dual cathode MPL, with different
PVA contents, on the performance of passive DMFCs under neat
methanol operation. It is demonstrated that increasing the PVA
content from 5 wt.% to 15 wt.% improves cell performance up to a
point, but any further increase in PVA content results in a performance decrease. The discussion and reasoning behind this is
similar to that of the above analysis. As shown in Fig. 7, larger PVA
content introduces better water trapping and retention abilities,
which enhances water recovery. Though similarly, an accumulation
of too much water hinders oxygen transport and lowers ORR.
Nevertheless, even with PVA content above the higher boundary of
Fig. 8. Effect of the dual-layer MPL at the cathode with different PVA content on cell
performance (the carbon loading is 0.5 mg cm2).
20 wt.%, the peak power density can still reach 26 mW cm2, a
result that is 15% higher than the cell performance of a conventional
hydrophobic structure. An optimum PVA content within the
Fig. 7. Contact angles of various MPLs: (a) hydrophilic layer with 5 wt.% PVA; (b) hydrophilic layer with 10 wt.% PVA; (c) hydrophilic layer with 15 wt.% PVA; (d) hydrophilic layer
with 20 wt.% PVA; (e) conventional hydrophobic MPL with 30 wt.% PTFE.
X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238
237
boundaries of 5 wt.% and 15 wt.% should be found to maximize the
cell performance.
These results suggest that the hydrophilic-hydrophobic duallayer MPL at the cathode can help to trap and retain the produced
water at the cathode to enhance the water back transport. Moreover, since the water-rich layer shows the negative effects on the
oxygen transport, both the PVA content and the thickness of the
hydrophilic layer show significant influence on the cell
performance.
4.2. Effect of the dual-layer MPL at the anode
It is crucial to recognize that the MOR at the anode requires not
only a sufficiently large water recovery flux, but also a high local
water concentration at the anode CL. Hence it is useful to retain the
recovered water at the anode CL by the hydrophilic-hydrophobic
dual-layer MPL. The influence of the anode dual-layer MPL on the
water management and cell performance was also investigated.
The effect of the dual-layer MPL at the anode only on the cell
performance is shown in Fig. 9. It is apparent that the DMFC with
dual-layer MPL yields a much higher power density than does the
conventional DMFC, demonstrating the favorable effect of the duallayer MPL at the anode. For instance, when the carbon loading of
the hydrophilic layer is at 1.5 mg cm2, the peak power density is
measured to be 32.6 mW cm2, which is 44% higher than the yields
of conventional MEA. Meanwhile, it can be seen that performance
improves with an increase in carbon loading. This is due to the
thicker hydrophilic layer, which has a better ability to retain water,
elevating the water concentration at the anode CL and improving
the kinetics of anodic MOR. It should be noted, however, that
although the highest power density is achieved at the carbon
loading of 1.5 mg cm2, the voltage is lower at the high current
density region, indicating mass transport is hindered by the
thickness of the layer. Therefore the optimal carbon loading for the
hydrophilic layer is 1.0 mg cm2, where the peak power density is
still as high as 31.5 mW cm2. These results indicate that a relative
thick hydrophilic layer is preferred at the anode.
Fig. 10 shows the influence of the dual-layer MPL with varying
PVA content in hydrophilic part. When the PVA content is at the
lower end of 5 wt.%, water conservation at the anode CL is insufficient. However, when the PVA content is increased to 15 wt.%, the
cell performance begins to decline. It can be understood similarly to
the situation at the cathode, whereby excessive enhancements of
the hydrophilic property leads to an increase in accumulated water,
Fig. 9. Effect of the dual-layer MPL at the anode with different carbon loadings on cell
performance (the PVA content is 10 wt.%).
Fig. 10. Effect of the dual-layer MPL at the anode with different PVA content on cell
performance (the carbon loading is 1.0 mg cm2).
slowing down the removal of CO2 gas.
The long-term discharge behaviors of the DMFCs with the
optimized dual-layer MPL and the conventional one were tested at
40 mA cm2 and are shown in Fig. 11. It is evident that the use of
this dual-layer MPL to either electrode enabled a significant increase in the cell voltage. The gradual decrease in the cell voltage
with time is attributed to the decrease in the contact area between
the liquid methanol and the pervaporation membrane (the cell is
placed vertically) and the condensation of liquid water on the
surface of the pervaporation membrane, which leads to a decrease
in the methanol delivery rate. In addition, a sudden drop in the cell
voltage is seen when the contact area between the liquid methanol
and the pervaporation membrane becomes zero, meaning that
there is no more methanol supply. The fuel efficiency is obtained
from the equation:
h ¼ Dmrea;MeOH DmMeOH ;
(6)
where Dmrea,MeOH is the methanol consumed by the anode reaction
and DmMeOH is the total methanol supplied [24,25]. The fuel efficiency increases from 41.8% to 44.6% and 48.6% when the dual-layer
MPL is used at the cathode and the anode, respectively. As
Fig. 11. Long-term discharge behavior at the current density of 40 mA cm2.
238
X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238
discussed above, the dual-layer MPL at either side can enhance the
local water concentration at the anode CL, which leads to a lower
methanol crossover rate and hence a larger fuel efficiency.
5. Concluding remarks
In the present work, we have demonstrated that the
hydrophilic-hydrophobic dual-layer MPL is effective for trapping
and retaining water by taking the advantage of the disparate features of PVA and PTFE. The dual-layer MPL at the cathode can increase the cathode water concentration to facilitate the water back
diffusion which supplies the required reactant for anodic MOR.
Meanwhile, the dual layer MPL at the anode conserves water from
the cathode at the anode catalyst layer. The effects of both result in
a higher water concentration, improve the kinetics of the anodic
reaction and enhance the overall cell performance. The parameters
of the dual-layer MPL structure have been optimized and the
modification at the anode shows more significant improvement.
Acknowledgements
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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