- Email: [email protected]
Water-transport in outer micro-porous layers for direct methanol fuel cells
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Water-transport in outer micro-porous layers for direct methanol fuel cells Hun Suk Im a, Seongyop Lim b,c,*, Sang-Kyung Kim b, Dong-Hyun Peck b, Doohwan Jung b,c, Won Hi Hong d,** a
Korea Atomic Energy Research Institute, KAERI, Daejeon, Republic of Korea Fuel Cell Research Center, KIER, 71-2, Jangdong, Daejeon 305-343, Republic of Korea c Advanced Energy Technology, UST, Daejeon, Republic of Korea d Department of Chemical and Biomolecular Engineering, KAIST, Daejeon, 305-701, Republic of Korea b
article info
abstract
Article history:
The outer micro-porous layer (MPL) between the gas diffusion layer and channel of the
Received 28 February 2011
bipolar plate was studied for both sides of the electrodes in DMFC, with particular attention
Received in revised form
to the effects of the hydrophobicity of the MPL on mass transport as well as cell perfor-
14 April 2011
mance. Water-transport behavior from the electrodes to the channel was observed through
Accepted 23 April 2011
the transparent window of the single cell with membrane-electrode assemblies (MEAs)
Available online 12 June 2011
including three combinations of outer MPLs. The crossover amount of methanol as well as water through the membrane was measured, and the mass balance, based on the
Keywords:
measured flux, was established to understand the mass transport in MEAs. The design of
Direct methanol fuel cells
outer MPLs is discussed for the best cell performance.
The net water-transport coefficient
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Micro-porous layer
reserved.
Hydrophobicity Mass transport
1.
Introduction
During the past decade, there have been intensive efforts, concentrating on the research and development of the direct methanol fuel cells (DMFCs) technology [1]. For the best performance of DMFC, superior cell components such as catalysts, membrane, and gas diffusion layers (GDLs) are required, and also appropriate manufacturing techniques of membrane-electrode assembly (MEA) should be established for reducing the internal resistance and efficient mass transports of reactants and products [2].
The gas diffusion layer (GDL) typically consists of a backing layer (BL) and a micro-porous layer (MPL). Carbon papers, carbon cloths, or carbon felts are generally used as the substrate materials for backing layer. GDL can be treated by a selected binder such as polytetrafluoroethyelene (PTFE) to control its hydrophobicity. According to the circumstances, MPL is typically applied to the interface between BL and catalyst layer (CL), and its main role is to additionally control the properties of diffusion media such as hydrophobicity and characteristics of pores including pore size, porosity, and tortuosity. MPL also has other functions, for example leveling
* Corresponding author. Fuel Cell Research Center, KIER, 71-2, Jangdong, Daejeon 305-343, Republic of Korea. Tel.: þ82 42 860 3073; fax: þ82 42 860 3739. ** Corresponding author. Tel.: þ82 42 350 3959; fax: þ82 42 350 3910. E-mail addresses: [email protected] (S. Lim), [email protected] (W.H. Hong). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.159
4666
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
the uneven surface of BL for the coating of CL with ease as well as for the reduction of contact resistance in MEA. The GDL including BL and MPL governs the transport phenomena in MEA, and its careful design is necessary to obtain the best performance of MEA. Hydrophobic MPL is known to be effective for water removal at the cathode, forming a high hydraulic liquid pressure [3e9]. Liquid water-transport phenomena in the diffusion media including MPL has been investigated in the polymer electrolyte membrane fuel cell [10,11]. In the case of a passive DMFC, it has been reported that minimizing of the water loss by hydrophobic MPL at the cathode enabled waterneutral operation conditions [3] and also that the methanol crossover could be reduced [8]. In this study, the MPL at the interface between the channel and BL (‘outer’ MPL, below) was investigated in terms of its effect on mass transport and MEA performance. A carbon cloth was used as the BL, and MPL was applied to both sides of the BL surface. Hydrophobic MPL at the interface between the BL and CL (‘inner’ MPL, below) was fixed as the same composition and thickness, while the outer MPL of different hydrophobicity was applied to both the anode and cathode. Hence, each electrode consisted of four layers: the CL, inner MPL, BL and outer MPL. The outer MPL in this study was relatively very thin. Although its main role was intended to reduce the contact resistance with the channel, the outer MPL also can significantly affect the mass transport in MEA. Three kinds of MEAs by combination of hydrophilic outer MPL for the anode and hydrophilic or hydrophobic outer MPL for the cathode were prepared. Water discharge from GDL to channel was first visualized, and the mass flux such as water and methanol was measured to establish the mass balance in MEA. The effects of outer MPL properties on the cell performance as well as the mass transport were discussed based on the results.
2.
Experimental methods
2.1.
Features of MEAs and single cell test conditions
MEA was manufactured by controlling the hydrophobicity of MPL. MEAs were supplied by Hyupjin I&C (Republic of Korea). The size of the electrode was 4 cm2. Two types of MPL were applied to make GDL: one was relatively hydrophilic (Mphi), and the other hydrophobic (Mpho). The contact angle of hydrophobic MPL (Mphi) is 150 , and that of hydrophilic MPL (Mphi) is 110 . Both MPLs were in a hydrophobic range, but notations such as Mpho and Mphi are to create clear distinctions between MPLs. The GDL basically consisted of an inner MPL, BL and outer MPL, and the inner MPL was fixed as Mpho. Three kinds of MEAs were used in this study, and the layer structure of each MEA is described in Fig. 1. For the outer MPL, the first MEA was prepared with Mphi at the anode and Mphi at the cathode (AICI), another with Mphi at the anode and Mpho at the cathode (AICO), and the last one with Mpho at the anode and Mpho at the cathode (AOCO). Other layers in all MEAs except for the outer MPL were the same. A single cell was operated with 1.0 M methanol solution at 2 ml/min and air at 200 ml/min at 60 C. The channel configuration of the test cell is 1-serpentine (the channel width
1 mm and rib 0.7 mm) on both sides. Air and methanol solution is fed in the count-current flow.
2.2.
Visible unit cell and observation of liquid droplets
As shown in Fig. 2, the fuel cell unit was composed of a stainless still end plate and carbon graphite bipolar plate. The particular fuel cell unit with a transparent window at one side was manipulated to observe the liquid behaviors in the cathode channel. A transparent window is made by polycarbonate because of its thermal stability and mechanical hardness during operation. Several behaviors of water droplets depending on the hydrophobic property of MPL can be observed through the transparent window. The hydrophobic property of MPL shows different tendencies in the cathode channel. A three-electrode system is applied to the cathode electrode side to measure the exact behavior of cathode voltage. Graphite carbon was used as a reference electrode under hydrogen mood inside the Teflon tube. A reference electrode was located 3 cm from the cathode electrode to minimize resistance. Performance was measured by the Princeton Applied Research Model 264A under the same current conditions with a single cell test: 1 M methanol solution at 2 ml/min and air at 200 ml/min at 60 C. However, a cathodic potential has not yet measured so far.
2.3. Observation of water behaviors and measurement of pressure drop Movements of water droplets were recorded by a Samsung Camera in real time. At the same time, a pressure drop was also measured by a manometer, TESTO 521. These two methods were conducted for monitoring the effect of water accumulation on the change of the pressure drop. The pressure drop change of the GDL showed different behaviors according to the hydrophobic properties of the gas diffusion layer. All experiments were conducted with 1 M methanol solution at 2 ml/min and air at 200 sccm at 60 C.
2.4.
Calculation of net water-transport coefficient
Under the constant current mode (160 mA/cm2), a visible cell had been operated at 60 C for 2 h with air at 200 ml/min. Liquid from the outlet was gathered in a vial treated with ice. A chiller was used to condensate the vapor water as well as liquid water. Water movement is explained by three kinds of driving forces: 1) an electro-osmotic drag force, 2) a diffusion, and 3) a back convection. And the net water-transport is expressed at the given constant current density in the following. ic (1) JWC ¼ JEO þ JDiff þ JBC ¼ a F a¼
Jwc F i
NH2 o ¼ JORR þ JMOR þ Jwc A
(2) (3)
JWC is the water crossover flux through the membrane. a is the net water-transport coefficient. JEO is the molar flux of water
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
4667
Fig. 1 e A schematic diagram of (a) AICI, (b) AICO and (c) AOCO.
by electro-osmotic drag force. JDiff is the molar flux of water by diffusion of molecules. Jbc is the molar flux of water by back convection from the cathode into the anode. ic is the equivalent methanol crossover current density at the cathode side.
NH2 o is the molar flux of water measured by experimentally collecting water and is experimentally obtained as the result of oxygen reduction reaction (ORR), methanol oxidation reaction (MOR) in the cathode and crossovered water from anode to cathode. ‘A’ means the surface area of electrode. JORR is the molar flux of water due to the ORR (Oxygen Reduction Reaction) at the cathode catalyst layer. JMOR is the molar flux of water due to the MOR (Methanol Oxidation Reaction) at the cathode catalyst layer.
2.5.
Methanol crossover
In-situ methanol crossover was measured in the same structure of the constant current mode in the system referred from Gottesfeld et al. [12]. A methanol solution was fed to the anode. On the other hand, pure water as an inert atmosphere was fed to the cathode. At this state, oxidation current was loaded into not the anode, but the cathode. The oxidation of methanol happens at the cathode, and hydrogen is formed at the anode from proton transported from the cathode into the anode. This method was used to measure the steady state limiting current density determining the flux rate of permeating methanol by electro-osmotic drag for a given current density.
2.6.
Analysis of water-transport in the electrode
Analysis of water distribution inside the electrode was based on the two-phase and isothermal systems. Two-phase flow behaviors are strongly influenced by material property and operation conditions such as the hydrophobicity of the porous media, saturation, capillary pressure, temperature, and flow rate, and so on. The following equations are used for calculation of water saturation gradient in the porous electrode. The results are shown in Fig. 7.
2.6.1. Fig. 2 e Visible single cell; (a) transparent window to observe the behavior of water in the cathode channel and (b) 3-electrode unit cell system.
Leverett function
The most important relationship that must be established for an accurate prediction of the liquidegas transport in the diffusion media is the relationship between the capillary
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
pressure and the liquid saturation. The Leverett function from soil science has been applied to investigate the capillary transport behavior of the porous media in multiphase models. (4)
where k, f, and q are the permeability and porosity of the porous media and a contact angle, respectively, and J(s) means the Leverett function for scaling drainage capillary pressure curves:
(5)
At the cathode, liquid transport based on Darcy’s law is expressed as ul ¼
k kl ¼ kr ; kr ¼ s3 VPC where k m
(6)
Here, kl, kr, and k are respectively the liquid permeability, relative permeability, and absolute permeability of the porous media. The gradient of capillary pressure is induced as PC ¼ Pl Pg Pg zconst VPczVPl
(7)
Therefore, the capillary pressure gradient will be induced from combination of Eqn. (4) and Eqn. (5). 1=2 f dJ ds VPczVPl ¼ gcos q k ds dx
(8)
At the steady state, the water saturation distribution in the cathode catalyst layer is equal to the summation of the amount of water crossover going to the cathode diffusion media and the amount of water generated in the catalyst layer due to the ORR. Using the mass balance on the control volume, the continuity equation for the liquid water inside the cathode electrode is Jw$c þ
i MH2 O ¼ rl ul 2F w
(9)
2 O is the molecular weight of the water, and J Here, MH w·c is the w flux of water crossover from anode to cathode. Through rearranging and inserting the values in the Eqn. (6), the desired the local saturation values is obtained.
i kl 2O ¼ VPc Jw$c þ MH w m 2F
rmix ¼ ð1 a2 Þr1 þ a2 r2
Performance of MEA
Fig. 3 depicts the performances of three MEAs: AICI, AICO, and AOCO. The cell performance of MEAs was operated under the condition; 1 M methanol solution at 2 ml/min and air at 200 ml/min at 60 C. The cell performances were appeared to be similar except for the limiting current density (AOCO < AICI < AICO). AOCO had the strongest concentration polarization and the lowest limiting current density (around 300 mA/cm2). This result shows that MPL basically did not affect the activation overpotential but the mass transport limitation, as expected.
3.2.
Behaviors of water droplets in the cathode channel
The behavior of water discharging at the cathode was observed for three MEAs in the constant current operation at the current density of 120 mA/cm2, as shown in Fig. 4. The images were collected at the times of 5, 300, 600 and 900 s. For AICI with a hydrophilic outer MPL (Mphi) at the cathode (Fig. 4aed), a few water drops formed at the interface of GDL and the channel spread over the hydrophilic cathode area, and then a large drop or water band was formed over the surface. On the other hand, on the surface of hydrophobic cathodes (AICO and AOCO), a lot of water droplets just showed up (Fig. 4eei). Compared to the water band at the channel bottom in the case of AICI, water droplets in the cases of AICO and AOCO look more spherical, which means that the surface contacting them is hydrophobic. And after the growing of water droplets, they were united into a bigger droplet that blocked the cathode channel at the several points. The amount of AOCO water droplets appeared to be less than that of AICO. It is probably because the hydrophobic MPL (Mpho) in the anode blocked the waster permeation and the water flux from the anode to the cathode was reduced.
3.3.
Pressure drop at the cathode
Fig. 5 shows the pressure drop profiles at the cathode, when MEA was operated at various current densities of 120, 160,
(10) 0.8
On the other hand, at a steady state, the local saturation in the anode catalyst layer is equal to the net mixture flux including water crossover, the consumed methanol flux by MOR, the crossover methanol, and the carbon dioxide flux by MOR. Here the boundary values of saturation are determined with the two-phase (liquidegas) volume fraction. 1 a ¼ ð1 a1 Þ a2 ¼ ð1 c2 Þr2 u2 2 1þ c2 r1 u1
3.1.
(11)
100
0.7 80 0.6
2
JðsÞ ¼ 1:417ð1 sÞ 2:120ð1 sÞ2 þ1:263ð1 sÞ3 if q < 90 JðsÞ ¼ 1:417s 2:120s2 þ 1:263s3 if q > 90
Results and discussion
0.5
60
0.4 40
0.3 AICI voltage AICO voltage AOCO voltage AICI power density AICO power density AOCO power density
0.2 0.1
(12)
where xi is the kinetic mass concentration of species i and r is the density.
Power density (wm/cm )
1=2 f JðsÞ Pc ¼ gcos q k
3.
Voltage (V)
4668
20
0.0 0
40
80
120
160
200
240
280
320
0 360
Current density (mA/cm2)
Fig. 3 e IV curve of each MEA (AICI, AICO, and AOCO).
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
4669
Fig. 4 e Behaviors of water discharging and were taken in 5 s, 300 s, 600 s and 900 s. MEA #1 (aed) : AICI, MEA #2 (eeh) : AICO and MEA #3 (iel) : AOCO.
and 200 mA/cm2. During operation at the low current density of 120 mA/cm2, the cathode pressure drop profiles for all MEA samples showed relatively small fluctuation, and the pressure drop tended to slightly increase depending on the increase of the current density. The range of the pressure drop was 0.4e1.0, 0.2e0.3, and 0.4e0.9 kPa for AICI, AICO, and AOCO samples. In the case of AICO, the pressure drop appeared to reach to the steady state, and it did not increase so significantly according to the current density. Only at 200 mA/cm2, intermittent fluctuation was observed. On the other hand, in cases of AICI and AOCO, the profiles were not stable, especially at 160 and 200 mA/cm2. The fluctuation of profiles was the severest in the case of AICI, which means that the hydrophilic GDL at the cathode is not efficient to smooth water discharge. When compared to the results of cell performance (Fig. 3), AICO, which showed
the most stable pressure drop, exhibited the highest limiting current density among the samples. Hence, the hydrophobicity of MPL was confirmed to significantly affect the efficiency of water removal at the cathode and also the cell performance.
3.4. Net water-transport coefficient and methanol crossover The flux of water and methanol through MEA was measured at 160 and 240 mA/cm2, as described in the experimental section. Fig. 6a shows the methanol crossover for MEA samples. The methanol crossover was higher at the lower current density since the flow rate of methanol solution was fixed. AICI with a hydrophilic outer MPLs on both side of electrodes showed the highest methanol crossover. MEA
4670
a
1.4 1.2
2.0e-7
120 mA/cm2 160 mA/cm2 2 200 mA/cm
1.0
2
Pressure drop (kPa)
2.5e-7 160 mA/cm2 240 mA/cm2
J MOR (mol/cm s)
a
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
0.8 0.6
1.5e-7
1.0e-7
0.4 5.0e-8
0.2 0.0
0.0 0
200
400
600
800
1000
1200
1400
1600
AICI
1800
AOCO
MEAs
Time (s)
b
AICO
b
7e-6 2
160 mA/cm 2 240 mA/cm
1.40 6e-6
5e-6
0.80
2
120 mA/cm2 2 160 mA/cm 200 mA/cm2
1.00
J WC (mol/cm s)
Pressure drop(kPa)
1.20
4e-6
3e-6
0.60
2e-6
0.40
1e-6
0
0.20
AICI
0
200
400
600
800
1000
1200
1400
1600
1800
AICO
AOCO
MEAs
Time (s)
c
Fig. 6 e (a) Methanol oxidation flux and (b) water crossover from the anode to the cathode at a given current density (160 mA/cm2 and 240 mA/cm2 - AICI, AICO, and AOCO).
1.4
Pressure drop (kPa)
1.2 120 mA/cm2 160 mA/cm2 2 200 mA/cm
1.0 0.8 0.6 0.4 0.2 0.0 0
200
400
600
800
1000
1200
1400
1600
1800
Time (s)
Fig. 5 e Pressure drop change of (a) AICI, (b) AICO, and (c) AOCO.
samples with more hydrophobic outer MPLs resulted in a decrease of methanol crossover. The effect of methanol crossover on the cell performance was considered. For the current density 160 mA/cm2, the crossover current density, which was calculated from the methanol crossover amount (Fig. 6a), was 116, 98, and 70 mA/
cm2 for AICI, AICO, and AOCO, respectively. The methanol crossover in AICI was almost doubled with respect to that in AOCO. The cell potentials were estimated with the measured crossover current density (Fig. 6a), based on the Butler-Volmer equations (the relating parameters were empirically obtained). From the calculation, the cell potentials are 0.504 V at the current density 160 mA/cm2 with no methanol crossover assumed, and 0.475, 0.479, and 0.485 V at the same current density with the crossover current density of 116, 98, and 70 mA/cm2, respectively. Even if the crossover current (116 and 70 mA/cm2) is doubled, the potential drop is calculated around 10 mV (0.475 and 0.485 V). The effect of methanol crossover on the cell performance hence must be less critical, although the absolute values from calculation may have some error range. The performance difference in Fig. 3 resulted from a complex effect of many factors in construction of MEA including the methanol crossover. Water flux was also measured as shown in Fig. 6b, and it was increased according to the current density, which indicates that the water flux is caused by the electro-osmotic drag to a considerable extent. The dependency on the MPL properties appeared to follow a similar trend with the methanol crossover. From the water flux, the net water transfer
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
4671
except for outer MPLs, were the same. Such an effect on mass transport appears not to affect the initial cell performance such as the maximum power density, but a potential effect on the durability in the long-term operation can be expected.
4.
Conclusions
The cell performance and mass transport were examined for three MEA samples with different combinations of outer MPLs at the anode and cathode. A hydrophobic MPL, irrespective of the electrode sides, was found to block the crossover of methanol as well as water. The hydrophilic MPL at the cathode was confirmed to aggravate flooding at the GDL and channel, resulting in unstable water discharge, as shown in the observation of water droplets and measuring of the pressure drop profiles. Based on the results in this study, the combination of high liquid saturation at the anode and low flooding at the cathode was confirmed to result in the best performance of DMFC in the aspect of the limiting current density and stable mass transport. However, this study is limited to the experimental approach, and the optimum properties of MPL cannot be quantitatively suggested. Also, the porosity factor of the MPL and GDL should be considered together for discussion on this issue. Further analysis should be conducted together with demonstration.
Acknowledgement This work was supported by the New & Renewable Energy R&D program (2008NFC08P030000) under the Ministry of Knowledge Economy, Republic of Korea. Fig. 7 e Local saturation distribution (a) at 160 mA/cm2 (b) at 240 mA/cm2.
coefficient was calculated to be 2.67, 2.4 and 2.1 for AICI, AICO, and AOCO, respectively.
3.5.
Local saturation in the electrode
Based on the flux measurement, the saturation profiles for the anode and the cathode were drawn, as shown in Fig. 7. At the anode, the liquid fraction is dominant (high saturation), while the gaseous molecules fill the space in the cathode. The saturation became slightly higher according to the increase of the current density, which means that the flux of liquid fractions increased overall in MEA. The saturation profile was strongly governed by the hydrophobicity of diffusion media. The AICO sample, which showed the best performance in terms of the limiting current density and pressure drop profiles, is characterized by higher saturation at the anode and lower at the cathode. In this study, the outer MPLs were found to have a significant effect on the mass transport in MEA, although other components of MEA,
references
[1] Gottesfeld S. Development and demonstration of direct methanol fuel cells for consumer electronics applications. In: The Fuel Cell World-Proceeding, Lucerne, EFCF, pp. 35e41 [2] Barbir F. PEM fuel cells: theory and practice. pp. 73e74. Elsevier Academic Press; 2005. [3] Peled E, Blum A, Aharon A, Philosoph M, Lavi Y. Novel approach to recycling water and reducing water loss in DMFCs. Electrochem Solid-State Lett 2003;6:A268e71. [4] Blum A, Duvdevani T, Philosoph M, Rudoy N, Peled E. Waterneutral micro direct-methanol fuel cell for portable applications. J Power Sources 2003;117:22e5. [5] Ren XM, Henderson W, Gottesfeld S. Electroosmotic drag of water in ionomeric membranes d new measurements employing a direct methanol fuel cell. J Electrochem Soc 1997;144:L267. [6] Lu GQ, Liu FQ, Wang CY. Low crossover of methanol and water through thin membranes in direct methanol fuel cells. Electrochem Solid-State Lett 2005;8:A1. [7] Kim H, Oh J, Kim J, Chang H. Membrane electrode assembly for passive direct methanol fuel cells. J Power Sources 2006;162:497. [8] Liu FQ, Lu GQ, Y. C. Water and methanol crossover in direct methanol fuel cell-effect of anode diffusion media. J Electrochem Soc 2006;153:A543.
4672
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
[9] Krishnamurthy B, Deepalochania S. Effect of PTFE content on the performance of a direct methanol fuel cell. Int J Hydrogen Energy 2009;34:446. [10] Meng H. Multi-dimensional liquid water transport in the cathode of a PEM fuel cell with consideration of the microporous layer (MPL). Int J Hydrogen Energy 2009;34:5488.
[11] Ju H, Luo G, Wang CY. Probing liquid water saturation in diffusion media of polymer electrolyte fuel cells. J Electrochem Soc 2007;154:B218. [12] Ren X, Springer TE, Zawodzinski TA, Gottesfeld S. Methanol transport through nafion membranes electro-osmotic drag effects on potential step measurements. J Electrochem Soc 2000;147:466.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Water-transport in outer micro-porous layers for direct methanol fuel cells Hun Suk Im a, Seongyop Lim b,c,*, Sang-Kyung Kim b, Dong-Hyun Peck b, Doohwan Jung b,c, Won Hi Hong d,** a
Korea Atomic Energy Research Institute, KAERI, Daejeon, Republic of Korea Fuel Cell Research Center, KIER, 71-2, Jangdong, Daejeon 305-343, Republic of Korea c Advanced Energy Technology, UST, Daejeon, Republic of Korea d Department of Chemical and Biomolecular Engineering, KAIST, Daejeon, 305-701, Republic of Korea b
article info
abstract
Article history:
The outer micro-porous layer (MPL) between the gas diffusion layer and channel of the
Received 28 February 2011
bipolar plate was studied for both sides of the electrodes in DMFC, with particular attention
Received in revised form
to the effects of the hydrophobicity of the MPL on mass transport as well as cell perfor-
14 April 2011
mance. Water-transport behavior from the electrodes to the channel was observed through
Accepted 23 April 2011
the transparent window of the single cell with membrane-electrode assemblies (MEAs)
Available online 12 June 2011
including three combinations of outer MPLs. The crossover amount of methanol as well as water through the membrane was measured, and the mass balance, based on the
Keywords:
measured flux, was established to understand the mass transport in MEAs. The design of
Direct methanol fuel cells
outer MPLs is discussed for the best cell performance.
The net water-transport coefficient
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Micro-porous layer
reserved.
Hydrophobicity Mass transport
1.
Introduction
During the past decade, there have been intensive efforts, concentrating on the research and development of the direct methanol fuel cells (DMFCs) technology [1]. For the best performance of DMFC, superior cell components such as catalysts, membrane, and gas diffusion layers (GDLs) are required, and also appropriate manufacturing techniques of membrane-electrode assembly (MEA) should be established for reducing the internal resistance and efficient mass transports of reactants and products [2].
The gas diffusion layer (GDL) typically consists of a backing layer (BL) and a micro-porous layer (MPL). Carbon papers, carbon cloths, or carbon felts are generally used as the substrate materials for backing layer. GDL can be treated by a selected binder such as polytetrafluoroethyelene (PTFE) to control its hydrophobicity. According to the circumstances, MPL is typically applied to the interface between BL and catalyst layer (CL), and its main role is to additionally control the properties of diffusion media such as hydrophobicity and characteristics of pores including pore size, porosity, and tortuosity. MPL also has other functions, for example leveling
* Corresponding author. Fuel Cell Research Center, KIER, 71-2, Jangdong, Daejeon 305-343, Republic of Korea. Tel.: þ82 42 860 3073; fax: þ82 42 860 3739. ** Corresponding author. Tel.: þ82 42 350 3959; fax: þ82 42 350 3910. E-mail addresses: [email protected] (S. Lim), [email protected] (W.H. Hong). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.159
4666
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
the uneven surface of BL for the coating of CL with ease as well as for the reduction of contact resistance in MEA. The GDL including BL and MPL governs the transport phenomena in MEA, and its careful design is necessary to obtain the best performance of MEA. Hydrophobic MPL is known to be effective for water removal at the cathode, forming a high hydraulic liquid pressure [3e9]. Liquid water-transport phenomena in the diffusion media including MPL has been investigated in the polymer electrolyte membrane fuel cell [10,11]. In the case of a passive DMFC, it has been reported that minimizing of the water loss by hydrophobic MPL at the cathode enabled waterneutral operation conditions [3] and also that the methanol crossover could be reduced [8]. In this study, the MPL at the interface between the channel and BL (‘outer’ MPL, below) was investigated in terms of its effect on mass transport and MEA performance. A carbon cloth was used as the BL, and MPL was applied to both sides of the BL surface. Hydrophobic MPL at the interface between the BL and CL (‘inner’ MPL, below) was fixed as the same composition and thickness, while the outer MPL of different hydrophobicity was applied to both the anode and cathode. Hence, each electrode consisted of four layers: the CL, inner MPL, BL and outer MPL. The outer MPL in this study was relatively very thin. Although its main role was intended to reduce the contact resistance with the channel, the outer MPL also can significantly affect the mass transport in MEA. Three kinds of MEAs by combination of hydrophilic outer MPL for the anode and hydrophilic or hydrophobic outer MPL for the cathode were prepared. Water discharge from GDL to channel was first visualized, and the mass flux such as water and methanol was measured to establish the mass balance in MEA. The effects of outer MPL properties on the cell performance as well as the mass transport were discussed based on the results.
2.
Experimental methods
2.1.
Features of MEAs and single cell test conditions
MEA was manufactured by controlling the hydrophobicity of MPL. MEAs were supplied by Hyupjin I&C (Republic of Korea). The size of the electrode was 4 cm2. Two types of MPL were applied to make GDL: one was relatively hydrophilic (Mphi), and the other hydrophobic (Mpho). The contact angle of hydrophobic MPL (Mphi) is 150 , and that of hydrophilic MPL (Mphi) is 110 . Both MPLs were in a hydrophobic range, but notations such as Mpho and Mphi are to create clear distinctions between MPLs. The GDL basically consisted of an inner MPL, BL and outer MPL, and the inner MPL was fixed as Mpho. Three kinds of MEAs were used in this study, and the layer structure of each MEA is described in Fig. 1. For the outer MPL, the first MEA was prepared with Mphi at the anode and Mphi at the cathode (AICI), another with Mphi at the anode and Mpho at the cathode (AICO), and the last one with Mpho at the anode and Mpho at the cathode (AOCO). Other layers in all MEAs except for the outer MPL were the same. A single cell was operated with 1.0 M methanol solution at 2 ml/min and air at 200 ml/min at 60 C. The channel configuration of the test cell is 1-serpentine (the channel width
1 mm and rib 0.7 mm) on both sides. Air and methanol solution is fed in the count-current flow.
2.2.
Visible unit cell and observation of liquid droplets
As shown in Fig. 2, the fuel cell unit was composed of a stainless still end plate and carbon graphite bipolar plate. The particular fuel cell unit with a transparent window at one side was manipulated to observe the liquid behaviors in the cathode channel. A transparent window is made by polycarbonate because of its thermal stability and mechanical hardness during operation. Several behaviors of water droplets depending on the hydrophobic property of MPL can be observed through the transparent window. The hydrophobic property of MPL shows different tendencies in the cathode channel. A three-electrode system is applied to the cathode electrode side to measure the exact behavior of cathode voltage. Graphite carbon was used as a reference electrode under hydrogen mood inside the Teflon tube. A reference electrode was located 3 cm from the cathode electrode to minimize resistance. Performance was measured by the Princeton Applied Research Model 264A under the same current conditions with a single cell test: 1 M methanol solution at 2 ml/min and air at 200 ml/min at 60 C. However, a cathodic potential has not yet measured so far.
2.3. Observation of water behaviors and measurement of pressure drop Movements of water droplets were recorded by a Samsung Camera in real time. At the same time, a pressure drop was also measured by a manometer, TESTO 521. These two methods were conducted for monitoring the effect of water accumulation on the change of the pressure drop. The pressure drop change of the GDL showed different behaviors according to the hydrophobic properties of the gas diffusion layer. All experiments were conducted with 1 M methanol solution at 2 ml/min and air at 200 sccm at 60 C.
2.4.
Calculation of net water-transport coefficient
Under the constant current mode (160 mA/cm2), a visible cell had been operated at 60 C for 2 h with air at 200 ml/min. Liquid from the outlet was gathered in a vial treated with ice. A chiller was used to condensate the vapor water as well as liquid water. Water movement is explained by three kinds of driving forces: 1) an electro-osmotic drag force, 2) a diffusion, and 3) a back convection. And the net water-transport is expressed at the given constant current density in the following. ic (1) JWC ¼ JEO þ JDiff þ JBC ¼ a F a¼
Jwc F i
NH2 o ¼ JORR þ JMOR þ Jwc A
(2) (3)
JWC is the water crossover flux through the membrane. a is the net water-transport coefficient. JEO is the molar flux of water
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
4667
Fig. 1 e A schematic diagram of (a) AICI, (b) AICO and (c) AOCO.
by electro-osmotic drag force. JDiff is the molar flux of water by diffusion of molecules. Jbc is the molar flux of water by back convection from the cathode into the anode. ic is the equivalent methanol crossover current density at the cathode side.
NH2 o is the molar flux of water measured by experimentally collecting water and is experimentally obtained as the result of oxygen reduction reaction (ORR), methanol oxidation reaction (MOR) in the cathode and crossovered water from anode to cathode. ‘A’ means the surface area of electrode. JORR is the molar flux of water due to the ORR (Oxygen Reduction Reaction) at the cathode catalyst layer. JMOR is the molar flux of water due to the MOR (Methanol Oxidation Reaction) at the cathode catalyst layer.
2.5.
Methanol crossover
In-situ methanol crossover was measured in the same structure of the constant current mode in the system referred from Gottesfeld et al. [12]. A methanol solution was fed to the anode. On the other hand, pure water as an inert atmosphere was fed to the cathode. At this state, oxidation current was loaded into not the anode, but the cathode. The oxidation of methanol happens at the cathode, and hydrogen is formed at the anode from proton transported from the cathode into the anode. This method was used to measure the steady state limiting current density determining the flux rate of permeating methanol by electro-osmotic drag for a given current density.
2.6.
Analysis of water-transport in the electrode
Analysis of water distribution inside the electrode was based on the two-phase and isothermal systems. Two-phase flow behaviors are strongly influenced by material property and operation conditions such as the hydrophobicity of the porous media, saturation, capillary pressure, temperature, and flow rate, and so on. The following equations are used for calculation of water saturation gradient in the porous electrode. The results are shown in Fig. 7.
2.6.1. Fig. 2 e Visible single cell; (a) transparent window to observe the behavior of water in the cathode channel and (b) 3-electrode unit cell system.
Leverett function
The most important relationship that must be established for an accurate prediction of the liquidegas transport in the diffusion media is the relationship between the capillary
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
pressure and the liquid saturation. The Leverett function from soil science has been applied to investigate the capillary transport behavior of the porous media in multiphase models. (4)
where k, f, and q are the permeability and porosity of the porous media and a contact angle, respectively, and J(s) means the Leverett function for scaling drainage capillary pressure curves:
(5)
At the cathode, liquid transport based on Darcy’s law is expressed as ul ¼
k kl ¼ kr ; kr ¼ s3 VPC where k m
(6)
Here, kl, kr, and k are respectively the liquid permeability, relative permeability, and absolute permeability of the porous media. The gradient of capillary pressure is induced as PC ¼ Pl Pg Pg zconst VPczVPl
(7)
Therefore, the capillary pressure gradient will be induced from combination of Eqn. (4) and Eqn. (5). 1=2 f dJ ds VPczVPl ¼ gcos q k ds dx
(8)
At the steady state, the water saturation distribution in the cathode catalyst layer is equal to the summation of the amount of water crossover going to the cathode diffusion media and the amount of water generated in the catalyst layer due to the ORR. Using the mass balance on the control volume, the continuity equation for the liquid water inside the cathode electrode is Jw$c þ
i MH2 O ¼ rl ul 2F w
(9)
2 O is the molecular weight of the water, and J Here, MH w·c is the w flux of water crossover from anode to cathode. Through rearranging and inserting the values in the Eqn. (6), the desired the local saturation values is obtained.
i kl 2O ¼ VPc Jw$c þ MH w m 2F
rmix ¼ ð1 a2 Þr1 þ a2 r2
Performance of MEA
Fig. 3 depicts the performances of three MEAs: AICI, AICO, and AOCO. The cell performance of MEAs was operated under the condition; 1 M methanol solution at 2 ml/min and air at 200 ml/min at 60 C. The cell performances were appeared to be similar except for the limiting current density (AOCO < AICI < AICO). AOCO had the strongest concentration polarization and the lowest limiting current density (around 300 mA/cm2). This result shows that MPL basically did not affect the activation overpotential but the mass transport limitation, as expected.
3.2.
Behaviors of water droplets in the cathode channel
The behavior of water discharging at the cathode was observed for three MEAs in the constant current operation at the current density of 120 mA/cm2, as shown in Fig. 4. The images were collected at the times of 5, 300, 600 and 900 s. For AICI with a hydrophilic outer MPL (Mphi) at the cathode (Fig. 4aed), a few water drops formed at the interface of GDL and the channel spread over the hydrophilic cathode area, and then a large drop or water band was formed over the surface. On the other hand, on the surface of hydrophobic cathodes (AICO and AOCO), a lot of water droplets just showed up (Fig. 4eei). Compared to the water band at the channel bottom in the case of AICI, water droplets in the cases of AICO and AOCO look more spherical, which means that the surface contacting them is hydrophobic. And after the growing of water droplets, they were united into a bigger droplet that blocked the cathode channel at the several points. The amount of AOCO water droplets appeared to be less than that of AICO. It is probably because the hydrophobic MPL (Mpho) in the anode blocked the waster permeation and the water flux from the anode to the cathode was reduced.
3.3.
Pressure drop at the cathode
Fig. 5 shows the pressure drop profiles at the cathode, when MEA was operated at various current densities of 120, 160,
(10) 0.8
On the other hand, at a steady state, the local saturation in the anode catalyst layer is equal to the net mixture flux including water crossover, the consumed methanol flux by MOR, the crossover methanol, and the carbon dioxide flux by MOR. Here the boundary values of saturation are determined with the two-phase (liquidegas) volume fraction. 1 a ¼ ð1 a1 Þ a2 ¼ ð1 c2 Þr2 u2 2 1þ c2 r1 u1
3.1.
(11)
100
0.7 80 0.6
2
JðsÞ ¼ 1:417ð1 sÞ 2:120ð1 sÞ2 þ1:263ð1 sÞ3 if q < 90 JðsÞ ¼ 1:417s 2:120s2 þ 1:263s3 if q > 90
Results and discussion
0.5
60
0.4 40
0.3 AICI voltage AICO voltage AOCO voltage AICI power density AICO power density AOCO power density
0.2 0.1
(12)
where xi is the kinetic mass concentration of species i and r is the density.
Power density (wm/cm )
1=2 f JðsÞ Pc ¼ gcos q k
3.
Voltage (V)
4668
20
0.0 0
40
80
120
160
200
240
280
320
0 360
Current density (mA/cm2)
Fig. 3 e IV curve of each MEA (AICI, AICO, and AOCO).
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
4669
Fig. 4 e Behaviors of water discharging and were taken in 5 s, 300 s, 600 s and 900 s. MEA #1 (aed) : AICI, MEA #2 (eeh) : AICO and MEA #3 (iel) : AOCO.
and 200 mA/cm2. During operation at the low current density of 120 mA/cm2, the cathode pressure drop profiles for all MEA samples showed relatively small fluctuation, and the pressure drop tended to slightly increase depending on the increase of the current density. The range of the pressure drop was 0.4e1.0, 0.2e0.3, and 0.4e0.9 kPa for AICI, AICO, and AOCO samples. In the case of AICO, the pressure drop appeared to reach to the steady state, and it did not increase so significantly according to the current density. Only at 200 mA/cm2, intermittent fluctuation was observed. On the other hand, in cases of AICI and AOCO, the profiles were not stable, especially at 160 and 200 mA/cm2. The fluctuation of profiles was the severest in the case of AICI, which means that the hydrophilic GDL at the cathode is not efficient to smooth water discharge. When compared to the results of cell performance (Fig. 3), AICO, which showed
the most stable pressure drop, exhibited the highest limiting current density among the samples. Hence, the hydrophobicity of MPL was confirmed to significantly affect the efficiency of water removal at the cathode and also the cell performance.
3.4. Net water-transport coefficient and methanol crossover The flux of water and methanol through MEA was measured at 160 and 240 mA/cm2, as described in the experimental section. Fig. 6a shows the methanol crossover for MEA samples. The methanol crossover was higher at the lower current density since the flow rate of methanol solution was fixed. AICI with a hydrophilic outer MPLs on both side of electrodes showed the highest methanol crossover. MEA
4670
a
1.4 1.2
2.0e-7
120 mA/cm2 160 mA/cm2 2 200 mA/cm
1.0
2
Pressure drop (kPa)
2.5e-7 160 mA/cm2 240 mA/cm2
J MOR (mol/cm s)
a
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
0.8 0.6
1.5e-7
1.0e-7
0.4 5.0e-8
0.2 0.0
0.0 0
200
400
600
800
1000
1200
1400
1600
AICI
1800
AOCO
MEAs
Time (s)
b
AICO
b
7e-6 2
160 mA/cm 2 240 mA/cm
1.40 6e-6
5e-6
0.80
2
120 mA/cm2 2 160 mA/cm 200 mA/cm2
1.00
J WC (mol/cm s)
Pressure drop(kPa)
1.20
4e-6
3e-6
0.60
2e-6
0.40
1e-6
0
0.20
AICI
0
200
400
600
800
1000
1200
1400
1600
1800
AICO
AOCO
MEAs
Time (s)
c
Fig. 6 e (a) Methanol oxidation flux and (b) water crossover from the anode to the cathode at a given current density (160 mA/cm2 and 240 mA/cm2 - AICI, AICO, and AOCO).
1.4
Pressure drop (kPa)
1.2 120 mA/cm2 160 mA/cm2 2 200 mA/cm
1.0 0.8 0.6 0.4 0.2 0.0 0
200
400
600
800
1000
1200
1400
1600
1800
Time (s)
Fig. 5 e Pressure drop change of (a) AICI, (b) AICO, and (c) AOCO.
samples with more hydrophobic outer MPLs resulted in a decrease of methanol crossover. The effect of methanol crossover on the cell performance was considered. For the current density 160 mA/cm2, the crossover current density, which was calculated from the methanol crossover amount (Fig. 6a), was 116, 98, and 70 mA/
cm2 for AICI, AICO, and AOCO, respectively. The methanol crossover in AICI was almost doubled with respect to that in AOCO. The cell potentials were estimated with the measured crossover current density (Fig. 6a), based on the Butler-Volmer equations (the relating parameters were empirically obtained). From the calculation, the cell potentials are 0.504 V at the current density 160 mA/cm2 with no methanol crossover assumed, and 0.475, 0.479, and 0.485 V at the same current density with the crossover current density of 116, 98, and 70 mA/cm2, respectively. Even if the crossover current (116 and 70 mA/cm2) is doubled, the potential drop is calculated around 10 mV (0.475 and 0.485 V). The effect of methanol crossover on the cell performance hence must be less critical, although the absolute values from calculation may have some error range. The performance difference in Fig. 3 resulted from a complex effect of many factors in construction of MEA including the methanol crossover. Water flux was also measured as shown in Fig. 6b, and it was increased according to the current density, which indicates that the water flux is caused by the electro-osmotic drag to a considerable extent. The dependency on the MPL properties appeared to follow a similar trend with the methanol crossover. From the water flux, the net water transfer
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
4671
except for outer MPLs, were the same. Such an effect on mass transport appears not to affect the initial cell performance such as the maximum power density, but a potential effect on the durability in the long-term operation can be expected.
4.
Conclusions
The cell performance and mass transport were examined for three MEA samples with different combinations of outer MPLs at the anode and cathode. A hydrophobic MPL, irrespective of the electrode sides, was found to block the crossover of methanol as well as water. The hydrophilic MPL at the cathode was confirmed to aggravate flooding at the GDL and channel, resulting in unstable water discharge, as shown in the observation of water droplets and measuring of the pressure drop profiles. Based on the results in this study, the combination of high liquid saturation at the anode and low flooding at the cathode was confirmed to result in the best performance of DMFC in the aspect of the limiting current density and stable mass transport. However, this study is limited to the experimental approach, and the optimum properties of MPL cannot be quantitatively suggested. Also, the porosity factor of the MPL and GDL should be considered together for discussion on this issue. Further analysis should be conducted together with demonstration.
Acknowledgement This work was supported by the New & Renewable Energy R&D program (2008NFC08P030000) under the Ministry of Knowledge Economy, Republic of Korea. Fig. 7 e Local saturation distribution (a) at 160 mA/cm2 (b) at 240 mA/cm2.
coefficient was calculated to be 2.67, 2.4 and 2.1 for AICI, AICO, and AOCO, respectively.
3.5.
Local saturation in the electrode
Based on the flux measurement, the saturation profiles for the anode and the cathode were drawn, as shown in Fig. 7. At the anode, the liquid fraction is dominant (high saturation), while the gaseous molecules fill the space in the cathode. The saturation became slightly higher according to the increase of the current density, which means that the flux of liquid fractions increased overall in MEA. The saturation profile was strongly governed by the hydrophobicity of diffusion media. The AICO sample, which showed the best performance in terms of the limiting current density and pressure drop profiles, is characterized by higher saturation at the anode and lower at the cathode. In this study, the outer MPLs were found to have a significant effect on the mass transport in MEA, although other components of MEA,
references
[1] Gottesfeld S. Development and demonstration of direct methanol fuel cells for consumer electronics applications. In: The Fuel Cell World-Proceeding, Lucerne, EFCF, pp. 35e41 [2] Barbir F. PEM fuel cells: theory and practice. pp. 73e74. Elsevier Academic Press; 2005. [3] Peled E, Blum A, Aharon A, Philosoph M, Lavi Y. Novel approach to recycling water and reducing water loss in DMFCs. Electrochem Solid-State Lett 2003;6:A268e71. [4] Blum A, Duvdevani T, Philosoph M, Rudoy N, Peled E. Waterneutral micro direct-methanol fuel cell for portable applications. J Power Sources 2003;117:22e5. [5] Ren XM, Henderson W, Gottesfeld S. Electroosmotic drag of water in ionomeric membranes d new measurements employing a direct methanol fuel cell. J Electrochem Soc 1997;144:L267. [6] Lu GQ, Liu FQ, Wang CY. Low crossover of methanol and water through thin membranes in direct methanol fuel cells. Electrochem Solid-State Lett 2005;8:A1. [7] Kim H, Oh J, Kim J, Chang H. Membrane electrode assembly for passive direct methanol fuel cells. J Power Sources 2006;162:497. [8] Liu FQ, Lu GQ, Y. C. Water and methanol crossover in direct methanol fuel cell-effect of anode diffusion media. J Electrochem Soc 2006;153:A543.
4672
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 e n e r g y 3 7 ( 2 0 1 2 ) 4 6 6 5 e4 6 7 2
[9] Krishnamurthy B, Deepalochania S. Effect of PTFE content on the performance of a direct methanol fuel cell. Int J Hydrogen Energy 2009;34:446. [10] Meng H. Multi-dimensional liquid water transport in the cathode of a PEM fuel cell with consideration of the microporous layer (MPL). Int J Hydrogen Energy 2009;34:5488.
[11] Ju H, Luo G, Wang CY. Probing liquid water saturation in diffusion media of polymer electrolyte fuel cells. J Electrochem Soc 2007;154:B218. [12] Ren X, Springer TE, Zawodzinski TA, Gottesfeld S. Methanol transport through nafion membranes electro-osmotic drag effects on potential step measurements. J Electrochem Soc 2000;147:466.