- Email: [email protected]
Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC)
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 x x x ( 2 0 1 6 ) 1 e8
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC) V.B. Oliveira a,**, J.P. Pereira b, A.M.F.R. Pinto a,* a
CEFT, Departamento de Engenharia Quı´mica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal b TU Delft, Department of Biotechnology, Office 1.84, Julianalaan 67, 2628 BC Delft, The Netherlands
article info
abstract
Article history:
One of the most significant obstacles for passive direct methanol fuel cells is the methanol
Received 21 January 2016
crossover, since methanol diffuses through the membrane generating heat but no power.
Received in revised form
This can be reduced if the cell operates with low methanol concentrations. However, this
11 June 2016
significantly reduces the energy density of the system since a large amount of water is
Accepted 13 June 2016
presented on the anode side that will produce no power. Also, water crosses the membrane
Available online xxx
towards the cathode flooding it and reducing the cell performance. In the present work, different anode diffusion layers were tested in order to select optimal working conditions
Keywords:
at high methanol concentrations. The results showed that the performance increased with
Passive direct methanol fuel cells
an increase of carbon cloth thickness due mainly to a decrease of methanol crossover.
High methanol concentrations
Regarding the carbon paper, better performances were achieved for lower thicknesses and
Methanol crossover
with a microporous layer (MPL) mainly due to a decrease of the anode overpotential ach-
Anode overpotential
ieved by a facilitated access of reactants. In this work, the best performance was achieved
Anode diffusion layer
using carbon cloth for a methanol concentration of 3 M with a corresponding power density of 24.2 mW/cm2. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, the energy needs for portable electronic devices are rising exponentially due to the increasing amount of applications (games, internet service, instant messaging and cameras), especially on cell phones. The conventional batteries are becoming scarce for the increasing power demand and complexity of these devices and the period that these devices can operate as truly portable is limited by the amount
of energy that can be stored in the conventional batteries. To overcome that, special attention is being devoted to fuel cell systems and particularly, to direct methanol fuel cells (DMFC) are considered as a main power provider for portable applications [1e3] The DMFC emerge as a promising power solution since methanol is a liquid at room temperature, is easy to handle, storage and distribute, has limited toxicity, the highest energy to carbon ratio of any alcohol and has low cost since it can be generated from natural gas, coal or biomass. However, the major disadvantages of using a liquid fuel is the
* Corresponding author. Tel.: þ351 225081675; fax: þ351 225081449. ** Corresponding author. Tel.: þ351 225081655; fax: þ351 225081449. E-mail addresses: [email protected] (V.B. Oliveira), [email protected] (A.M.F.R. Pinto). http://dx.doi.org/10.1016/j.ijhydene.2016.06.116 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
2
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 x x x ( 2 0 1 6 ) 1 e8
slow anode kinetics arising from a multi-step oxidation reaction at the anode and fuel crossover from the anode to the cathode side. This not only lowers the fuel utilization on the anode side, but degrades the cathode performance and generates extra heat. In a DMFC, the fuel and oxidant can be supplied in two different ways: an active or a passive one. The active systems need moving parts to feed the oxidant and the fuel to the cell requiring power to operate and allow higher power outputs since can be operated at optimal conditions of temperature and pressure, being for that reason more suitable for large fuel cells. However, these systems have higher costs and lower system energy densities. In the passive systems, the fuel is supplied to the anode from a fuel reservoir built-in in the anode and the air to the cathode, normally by natural convection and diffusion. This lead to a simpler and smaller design, making these systems better suited for small applications. The refueling of the passive DMFC is fast and the fuel can last several months. However, this simple design causes lower system performances due to the difficulty of getting a continuous and homogeneous supply of reactants to the cell. This limits the reaction rates, reducing the cell voltage and severely affecting the efficiency of the system. It is commonly accepted that two achieve the power outputs needed for portable applications, more concentrated methanol solutions are preferred. However this will lead to higher levels of methanol crossover with a consequent loss of performance. Another issue of using high methanol concentrations or even neat-methanol is the water problem. Since low water or no water is presented on the anode side, the water required for the methanol oxidation reaction need to be transported from the cathode to the anode, by diffusion, through the membrane. In this situation the anode performance is dependent not only on the water presented on the anode side but also on the water transport rate from the cathode. Therefore, in order to a passive system achieve the desirable levels of energy density, the key challenges such as the low rate of methanol oxidation, the methanol crossover and the water management need to be overcome. In order to achieve that, different solutions, such as, improving the methanol transport resistances and the catalyst activity through structural modifications are being proposed [4e17]. The diffusion layers do not participate directly in the electrochemical reactions but have several important functions: i) to allow the access of reactants to the catalyst layer; ii) to provide the removal of the products from the catalyst layer; iii) to electrically connect the catalyst layer to the collector plate; iv) allow heat removal and v) to provide mechanical support to the MEA (membrane electrode assembly). According to these roles, these layers should be sufficiently porous to allow the flow of both reactants and products, electrically and thermally conductive, sufficiently rigid to support the MEA, but must have some flexibility to maintain good electrical contacts. The porous facing the catalyst layer should be relatively small since the catalyst layer is made of discrete small particles. These requirements are best met by carbon based materials such as carbon papers and carbon cloths. Two structural parameters of these two materials affect the fuel cell performance: i) porosity, which influences the species transport and ii) the surface properties, the
hydrophobicity and roughness, which control the droplet/ bubble attachment or coverage on the diffusion layer surface. As mentioned, the diffusion layers are usually made of carbon materials, such as carbon paper and carbon cloth and in order to regulate the reactant supply and the product removal, on these layers, and control the amount of methanol that reaches the membrane different structural modifications, such as polytetrafluoroethylene (PTFE) treatments, adding a barrier layer and/or increase the thickness of the diffusion layer have been proposed [4e12]. In this work, a detailed experimental study regarding the effects of anode diffusion layer material and thickness on the performance of a passive DMFC is described. In order to achieve the energy outputs needed for real passive DMFC applications and towards the introduction, as soon as possible, of this technology in the market, the main goal of this work is to methodically vary available commercial materials and evaluate their influence on the passive DMFC performance.
Materials and methods Fuel cell design The experimental fuel cell consists of two acrylic end plates (open on the cathode side and with a reservoir on the anode side), two isolating rubber plates, two gold plated copper connector plates (with 36 holes with a diameter of 6 mm), two diffusion layers (GDL), two catalyst layers and a membrane (Fig. 1). The membrane used was Nafion 117 (with a thickness of 0.0178 cm) and the catalyst used on the anode and cathode side was, respectively, 4 mg/cm2 PteRu black (with a molar of ratio 1:1) and 4 mg/cm2 Pt black. The three-layer MEAs (membrane electrode assembly) were supplied by Fuel Cells Etc. Carbon cloth (CC) with two different thicknesses (0.400 mm and 0.425 mm) and with a MPL (0.410 mm) were used as anode diffusion layer. Two different carbon paper (CP) thicknesses
Fig. 1 e “In-house” passive direct methanol fuel cell.
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
3
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 x x x ( 2 0 1 6 ) 1 e8
Table 1 e Properties of the materials used as anode diffusion layers. GDL
Type Thickness (mm) Basic weight (g/m2) Air permeabilitya (s) Through-plane resistanceb (mUcm2) Porosity MPL
CC CC_A CC_MPL CP_B CP CP_MPL_C CP_MPL
Cloth Cloth Cloth Paper Paper Paper Paper
a b
0.400 0.425 0.410 0.110 0.190 0.240 0.340
122 149 180 49 84 90 130
<10 <10 <8 <10 <10 <85 <170
<5 <5 <13 <9 <7 <15 <10
0.83 0.83 0.80 0.78 0.78 0.80 0.80
No No Yes No No Yes Yes
Gurley Method. Based on ASTM C611 Method.
(0.110 mm and 0.190 mm) 20 (þ/ 5) wt. % PTFE treated and carbon paper with MPL (0.240 mm and 0.340 mm) were, also tested as anode diffusion layer. Therefore, in this work seven different diffusion layers were tested on the anode side three carbon cloths and four carbon papers. In all the experiments the cathode diffusion layer was carbon cloth with a thickness of 0.400 mm (CC). All the materials are woven and were supplied by QuinTech. The corresponding properties are displayed in Table 1. The diffusion layers were pressed on the three-layer membranes (membrane with the catalyst deposited/sprayed on the membrane) when assembling the fuel cell as shown in Fig. 1. As already mentioned, commercially available materials were selected for the diffusion and catalyst layers and for the membrane.
Experimental conditions In the experiments, a passive DMFC with an open area of 10.2 cm2 and a MEA area of 25 cm2 was operated at ambient conditions (ambient pressure and temperature) and with different methanol solutions on the anode compartment (1 M, 3 M and 5 M). The fuel cell temperature was controlled by a digital temperature controller and was set near ambient conditions, 20 C. Different methanol concentrations and anode diffusion layer materials with different thicknesses were tested. The experimental tests were performed with a commercial test station (Zahner-Elektrik GmbH & Co. kG) and the polarization curves were performed galvanostatically. The computer constantly monitors both current and voltage and these parameters are used to estimate and track the power output.
Results and discussion All the results presented in this section were explained under the light of model predictions for methanol crossover rate (methanol flux through the membrane from the anode to the cathode side) and anode overpotential from a previous developed and validated semi-analytical and onedimensional model considering the effects of heat and mass transfer, along with the electrochemical reactions occurring in a passive DMFC. The model assumes that the fuel cell
operates under steady-state conditions, only the liquid phase was considered in the anode side and membrane and only the gas phase was considered in the cathode side, the mass transport in the diffusion layers and membrane was described using effective Fick models, local equilibrium at interfaces was represented by partition functions, the catalyst layers were assumed to be a macrohomogeneous porous electrode, so reactions in these layers are modeled as a homogeneous reaction, methanol and water transport through the membrane was assumed to be caused by the combined effect of the concentration gradient between the anode and the cathode and the electro-osmosis force. All the model details (development, assumptions, validation and outputs) can be found in Ref. [18].
Diffusion layer material In order to evaluate the effect of the anode diffusion layer material on the fuel cell voltage and power density, two different materials, carbon cloth (CC) and carbon paper (CP), were used (properties in Table 1). As already mentioned, the tests were performed for three different methanol concentrations 1 M, 3 M and 5 M. The influence of the anode diffusion layer material on the cell voltage is shown in Fig. 2 while the impact on power density is presented in Fig. 3. It should be mentioned that carbon cloth is more porous and less tortuous than carbon paper, though carbon paper has excellent electronic conductivity [10]. Also, as can be seen in Table 1, CP is thinner than CC. As can be seen in Figs. 2 and 3 better performances are achieved using CC as anode diffusion layer, mostly due to its higher porosity and thickness. As mentioned, CC has higher porosity than CP enhancing the methanol transport from the methanol reservoir to the reaction zone, the catalyst layer. This is disadvantageous to crossover (as can be seen by the methanol flux values presented in Fig. 4) but has a very positive effect on methanol oxidation rate on the catalyst layer, allowing a higher methanol concentration in this layer. Therefore, more methanol is oxidized, increasing the methanol oxidation rate and the power output due to a decrease of the anode overpotential (Fig. 4). The CP with its lower porosity limits the amount of methanol that reaches the catalyst layer decreasing not only the methanol crossover through the membrane but, also the methanol oxidation reaction rate due to a lack of methanol on this layer. This can be clearly seen on
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
4
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 x x x ( 2 0 1 6 ) 1 e8
the model predictions for the methanol crossover flux thought the membrane and anode overpotential presented in Fig. 4. On the anode reaction gaseous carbon dioxide is produced and if the carbon dioxide bubbles are not removed efficiently
0.60
CC_1M CC_3M
Cell voltage (V)
0.50
CC_5M CP_1M
0.40
CP_3M CP_5M
0.30 0.20 0.10 0.00 0
10
20
30
40
50
Current density (mA/cm2)
Fig. 2 e Effect of anode diffusion layer material on cell performance for different methanol feed concentrations.
Power density (mW/cm2)
3.5
CC_1M CC_3M
3.0
Carbon cloth
CC_5M
2.5
CP_1M CP_3M
2.0
CP_5M
1.5
1.0 0.5
0.0 0
10
20
30
40
50
Current density (mA/cm2)
Fig. 3 e Effect of anode diffusion layer material on power density for different methanol feed concentrations.
7.00E-08
0.50
CC_3M_MF CP_3M_MF
6.50E-08
0.45
CC_3M_AO CP_3M_AO
6.00E-08
0.40
5.50E-08
0.35
5.00E-08
0.30
4.50E-08
0.25
anode overpotential (V)
Methanol flux (mol/cm2.s)
from the catalyst surface, they cover the surface decreasing the effective active area for methanol oxidation. The texture of carbon paper with a highly tortuous structure enhances the interactions between the bubbles and the solid. Hence, the gas tends to remain attached to the surface leading to a blockage of the active sites lowering the methanol oxidation reaction rate and consequently the cell performance. This can be perceived on the model predictions for the anode overpotential for the two materials tested and shown in Fig. 4. Model predictions for the methanol flux (MF) through the membrane from the anode to the cathode side and the anode overpotential (AO) for a 3 M concentration are also presented in Fig. 4. This concentration was chosen since, among all the concentrations tested, this was the one which leads to a better performance (Figs. 2 and 3). As previously referred, higher crossover rates and lower anode overpotentials are achieved with CC as GDL due to a lower methanol mass transport resistance. From these predictions it can be concluded that the use of CC as anode G. GDL has a positive effect on the anode reaction rate and a negative effect on crossover. However, the polarization and the power density curves show that the positive effect on the oxidation rate is more relevant than the negative one on crossover.
0.20
4.00E-08
0
10
20
30
40
50
Current density (mA/cm2)
Fig. 4 e Model predictions for the effect of anode diffusion layer material on methanol flux through the membrane (MF) and anode overpotential (AO) for a methanol concentration of 3M.
As mentioned, different structural modifications have been performed in order to reduce the methanol crossover and the most common one is to add a microporous layer (MPL) to the GDL. In this case the GDL has a dual-layer structure instead of a single one. The first layer is the backing layer and serves as a substrate for the second layer and the one is a MPL, usually made of carbon black powder with a hydrophobic agent [17]. Each layer acts as a diffusion layer for fuel and products transport. In order to evaluate the effect of adding a MPL to the GDL, in this work, carbon cloth with a single structure (CC and CC_A) and with MPL (CC_MPL) was used as anode diffusion layer. Two different thicknesses of the GDL without MPL (CC and CC_A) were also tested to study the effect of the GDL thickness on the passive DMFC performance. The polarization and the power density curves, for these studies, are presented, respectively, in Figs. 5 and 6. As can be seen in the plots, for the single layer structure a higher thickness (CC_A) leads to a higher fuel cell performance due to better fuel distribution, which not only allows a higher methanol concentration on the catalyst layer increasing the methanol oxidation rate and decreasing the anode overpotential, but also decreases the amount of methanol that crosses the membrane towards the cathode side (Fig. 7). The effect of the layer thickness on the decrease of the methanol crossover is well put in evidence by the significantly higher OCV values when using the CC_A layer (Table 2). Regarding the GDL with MPL (CC_MPL) and without MPL (CC) it can be seen that better performances are achieved with the dual layer structure (Table 2), since the MPL increases the methanol utilization due to a more uniformly reactant distribution on the electrode surface decreasing the anode overpotential and the methanol flux through the membrane [11,17] (Fig. 7).
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
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 x x x ( 2 0 1 6 ) 1 e8
0.70
Cell voltage (V)
(Table 2). The model predictions are in accordance with the experimental data, since the use of carbon cloth with a higher thickness (CC_A) leads to a lower methanol flux and anode overpotential.
CC_1M CC_3M
0.60
CC_5M CC_A_1M
0.50
CC_A_3M CC_A_5M
0.40
CC_MPL_1M
CC_MPL_5M
0.20
0.00
0
20
40
60
80
100
120
140
Current density (mA/cm2)
Fig. 5 e Effect of carbon cloth properties on cell performance for different methanol feed concentrations.
25
Power density (mW/cm2)
Carbon paper
CC_MPL_3M
0.30
0.10
20 CC_1M CC_3M
15
CC_5M CC_A_1M
10
CC_A_3M
CC_A_5M CC_MPL_1M
5
CC_MPL_3M CC_MPL_5M
0 0
20
40
60
80
100
120
140
Current density (mA/cm2)
Fig. 6 e Effect of carbon cloth properties on power density for different methanol feed concentrations.
7.00E-08
0.45
6.00E-08
0.40
5.00E-08
0.35 4.00E-08 0.30
CC_3M_MF CC_A_3M_MF CC_MPL_3M_MF CC_3M_AO CC_A_3M_AO CC_MPL_3M_AO
3.00E-08 2.00E-08 1.00E-08
0
20
40
60
0.25
anode overpotential (V)
Methanol flux (mol/cm2.s)
5
0.20
80
100
120
Current density (mA/cm2)
Fig. 7 e Model predictions for the effect of carbon cloth properties on methanol flux through the membrane (MF) and anode overpotential (AO) for a methanol concentration of 3M.
Fig. 7 shows the model predictions for the effect of carbon cloth properties on methanol flux through the membrane and anode overpotential, for a methanol concentration of 3 M (concentration which shows the higher power outputs)
To analyze the effect of the carbon paper thickness on the passive DMFC performance two different structures, CP and a CP_B with a lower thickness (Table 1) were used and the results are presented in Figs. 8 and 9, for three different methanol concentration. As can be seen, a lower thickness leads to a higher fuel cell performance due to a lower methanol mass transport resistance in the diffusion layer. A higher thickness in combination with a lower porosity characteristic of the carbon paper limits the amount of methanol that reaches the catalyst layer not supplying the amount of methanol needed for the methanol oxidation reaction. Therefore the methanol oxidation rate tends to be lower and the anode overpotential increases. The use of carbon paper with a thinner thickness (CP_B) is advantageous since it allows a higher methanol concentration on the catalyst layer, increasing its oxidation rate and decreasing its transport through the membrane towards the cathode. These explanations are confirmed by the model predictions in Fig. 10 on the effect of carbon paper thickness on methanol crossover and anode overpotential for a methanol concentration of 3 M. As expected, after the analysis of the experimental results, lower anode overpotentials and methanol crossover fluxes and consequently higher performances are achieved using carbon paper with a lower thickness (CP_B) as GDL. A similar study regarding the effect the carbon paper thickness on a passive DMFC for a dual-layer structure (GDL with MPL) was also performed. Two dual layer structures with two different thicknesses, CP_MPL_C (0.240 mm) and CP_MPL (0.340 mm), were tested and the experimental data are presented in Figs. 11 and 12. Regarding the use of a single GDL structure (Figs. 8 and 9) and a dual one, better results were achieved for the GDL with MPL due to a better methanol distribution and products removal on the anode. Its higher thickness and porosity lead to a higher methanol oxidation rate and consequently a lower anode overpotential and methanol flux through the membrane, since more methanol is consumed on the anode reaction [17]. Analyzing the two dual-layer structures tested (Figs. 11 and 12) having the same porosity, higher performances were obtained for the layer with the lower thickness due to a lower resistance to methanol transport to the catalyst layer, enabling a higher oxidation rate and as result lower anode overpotentials and higher power densities. The results presented in Figs. 11 and 12, also, show that using CP with a dual-layer structure better performances are achieved not with a methanol concentration of 3 M (as for the other materials tested) but using a 5 M methanol concentration. This is mainly due to the intrinsic properties of these materials (Table 1) which enable better reactants distribution, higher oxidation rates and lower crossover fluxes through the membrane.
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
6
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 x x x ( 2 0 1 6 ) 1 e8
Methanol concentration (M)
OCV (V)
Maximum power density (mW/cm2)
1 3 5 1 3 5 1 3 5
0.450 0.442 0.408 0.600 0.525 0.470 0.643 0.547 0.564
1.56 3.30 1.19 20.7 24.2 18.6 7.10 8.72 9.40
CC
CC_A
CC_MPL
0.70
0.45
5.60E-08 CP_B_3M_MF
5.20E-08
0.40
CP_3M_MF CP_B_3M_AO
0.35
CP_3M_AO
4.80E-08
0.30
4.40E-08
0.25
4.00E-08
0.20 0
10
20
30
40
50
60
70
Current density (mA/cm2)
Fig. 10 e Model predictions for the effect of carbon paper thickness on methanol flux through the membrane (MF) and anode overpotential (AO) for a methanol concentration of 3M.
CP_B_1M CP_B_3M
0.60
CP_B_5M
0.50
0.70
CP_MPL_C_1M
CP_1M
0.40
CP_MPL_C_3M
0.60
CP_3M
CP_MPL_C_5M
CP_5M
Cell voltage (V)
Cell voltage (V)
0.50
anode overpotential (V)
GDL
Methanol flux (mol/cm2.s)
6.00E-08
Table 2 e Effect of carbon cloth properties on the open circuit voltage (OCV) and maximum power density different methanol feed concentrations.
0.30 0.20 0.10
0.50
CP_MPL_1M CP_MPL_3M
0.40
CP_MPL_5M
0.30 0.20
0.00
0
10
20
30
40
50
60
70
0.10
Current density (mA/cm2) 0.00 0
Fig. 8 e Effect of carbon paper thickness on cell performance for different methanol feed concentrations.
20
40
60
80
100
120
Current density (mA/cm2)
Fig. 11 e Effect of carbon paper properties on cell performance for different methanol feed concentrations.
6
18
16
5
Power density (mW/cm2)
Power density (mW/cm2)
7
CP_B_1M
4
CP_B_3M
3
CP_B_5M CP_1M
2
CP_3M
CP_5M
1
0 0
10
20
30
40
50
60
70
Current density (mA/cm2)
Fig. 9 e Effect of carbon paper thickness on cell power density for different methanol feed concentrations.
14 12 10 CP_MPL_C_1M
8
CP_MPL_C_3M
6
CP_MPL_C_5M
4
CP_MPL_1M
CP_MPL_3M
2
CP_MPL_5M
0 0
20
40
60
80
100
120
Current density (mA/cm2)
Fig. 12 e Effect of carbon paper properties on power density for different methanol feed concentrations. These considerations are well confirmed in Fig. 13 by the model predictions regarding the effect of carbon paper properties on methanol flux and anode overpotential for a methanol concentration of 5 M, the one that conducts to
better performances (Figs. 11 and 12). As shown in Fig. 13, lower anode overpotentials and methanol fluxes are achieved for the CP_MPL_C. It is important to report that the lowest predicted methanol crossover fluxes when using all
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
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 x x x ( 2 0 1 6 ) 1 e8
0.50 0.45
4.00E-08 0.40
3.00E-08
0.35 CP_MPL_C_5M_MF
0.30
CP_MPL_5M_MF
2.00E-08
anode overpotential (V)
Methanol flux (mol/cm2.s)
5.00E-08
0.25
CP_MPL_C_5M_AO CP_MPL_5M_AO
0.20
1.00E-08 0
20
40
60
80
100
Current density (mA/cm2)
Fig. 13 e Model predictions for the effect carbon paper properties on methanol flux through the membrane (MF) and anode overpotential (AO) for a methanol concentration of 5M.
the materials tested were obtained with carbon paper with MPL as GDL (Figs. 4, 7 and 10), which allowed to work with higher methanol concentrations (5 M) without a significant loss on performance. These results are very important to achieve the goals needed for a passive DMFC in real applications: higher methanol concentrations, lower methanol crossover rates and higher power outputs leading to improved autonomies.
Conclusions A great challenge on the development of a passive DMFC, is to use cost-effective materials and cells with high energy den-
sities and low methanol crossover rates. To achieve these goals, it is necessary to use high methanol concentrations and use different diffusion layer materials with different thicknesses. This work aims to find solutions to these problems and contribute to an advance on the DMFC knowledge for portable applications. The results show that better performances are achieved using carbon cloth as GDL due to a higher thickness and porosity than carbon paper. A higher thickness allied to a higher porosity significantly decreases the methanol crossover rate through the membrane and allows an increase on the methanol oxidation reaction and consequently a decrease on the anode overpotential.
7
An anode diffusion layer made of carbon paper with a duallayer structure and with a lower thickness lead to higher fuel cell performances than a single structure and higher thicknesses mainly due to an improved porosity enabling a higher methanol oxidation rate on the catalyst layer and a reduction of the anode overpotential. This dual structure also allowed to work with higher methanol concentrations (5 M) and lower methanol crossover rates. In this work the maximum power density, 24.2 mW/cm2, was obtained using a Nafion 117 membrane, 4 mg/cm2 of PteRu and 4 mg/cm2 of Pt, as respectively, anode and cathode catalyst layers, carbon cloth with a higher thickness as anode diffusion layer, carbon cloth as cathode diffusion layer and a methanol concentration of 3 M. As a result of this work, a tailored MEA build-up with the common available commercial materials was proposed to achieve low methanol crossover rates and higher power densities operating with high methanol concentrations. This work shows that changes in fuel cell structure are effective ways of controlling the methanol crossover and increase the fuel cell performance. Further improvements, in passive DMFC systems, will be achieved when more cost effective materials for the different layers become commercially available.
Acknowledgments V.B. Oliveira acknowledges the post-doctoral fellowship (SFRH/BDP/91993/2012) supported by the Portuguese ~o para a Cie ^ncia e Tecnologia” (FCT), POPH/QREN and “Fundac¸a European Social Fund (ESF). POCI (FEDER) also supported this work via CEFT.
references
[1] Kamarudin SK, Achmad F, Daud WRW. Overview on the application of direct methanol fuel cell (DMFC) for portable electronic devices. Int J Hydrogen Energy 2009;34:6902e16. [2] Sundmacher K. Fuel cell engineering: toward the design of efficient electrochemical power plants. Ind Eng Chem Res 2010;49:10159e82. ~ o DS, Oliveira VB, Rangel CM, Pinto AMFR. Review on [3] Falca micro-direct methanol fuel cells. Renew Sustain Energy Rev 2014;34:58e70. [4] Wu QX, Zhao TS, Chen R, Yang WW. Effects of anode microporous layers made of carbon powder and nanotubes
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
8
[5]
[6]
[7]
[8]
[9]
[10]
[11]
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 x x x ( 2 0 1 6 ) 1 e8
on water transport in direct methanol fuel cells. J Power Sources 2009;191:304e11. Yuan T, Zou Z, Chen M, Li Z, Xia B, Yang H. New anodic diffusive layer for passive micro-direct methanol fuel cell. J Power Sources 2009;192:423e8. Yuan W, Zhou B, Hu J, Deng J, Zhang Z, Tang Y. Passive direct methanol fuel cell using woven carbon fiber fabric as mass transfer control medium. Int J Hydrogen Energy 2015;40:2326e33. Zainoodin AM, Kamarudin SK, Masdar MS, Daud WRW, Mohamad AB, Sahari J. High power direct methanol fuel cell with a porous carbon nanofiber anode layer. Appl Energy 2014;113:946e54. Li J, Ye D-d, Zhu X, Liao Q, Ding Y-d, Tian X. Effect of wettability of anode microporous layer on performance and operation duration of passive air-breathing direct methanol fuel cells. J Appl Electrochem 2009;39:1771e8. Yuan T, Yang J, Wang Y, Ding H, Li X, Liu L, et al. Anodic diffusion layer with graphene-carbon nanotubes composite material for passive direct methanol fuel cell. Electrochim Acta 2014;147:265e70. ~ o DS, Rangel CM, Pinto AMFR. Water Oliveira VB, Falca management in a passive direct methanol fuel cell. Int J Energy Res 2013;37:991e1001. Yuan W, Tang Y, Yang X, Liu B, Wan Z. Structural diversity and orientation dependence of a liquid-fed passive airbreathing direct methanol fuel cell. Int J Hydrogen Energy 2012;37:9298e313.
[12] Park Y-C, Kim D-H, Lim S, Kim S-K, Peck D-H, Jung D-H. Design of a MEA with multi-layer electrodes for high concentration methanol DMFCs. Int J Hydrogen Energy 2012;37:4717e27. [13] Shrivastava NK, Thombre SB, Mallick RK. Effect of diffusion layer compression on passive DMFC performance. Electrochim Acta 2014;149:167e75. [14] Wu QX, Zhao TS, Yang WW. Effect of the cathode gas diffusion layer on the water transport behavior and the performance of passive direct methanol fuel cells operating with neat methanol. Int J Heat Mass Transf 2011;54:1132e43. [15] Cao J, Chen M, Chen J, Wang S, Zou Z, Li Z, et al. Double microporous layer cathode for membrane electrode assembly of passive direct methanol fuel cells. Int J Hydrogen Energy 2010;35:4622e9. [16] Cao J, Wang L, Song L, Xu J, Wang H, Chen Z, et al. Novel cathodal diffusion layer with mesoporous carbon for the passive direct methanol fuel cell. Electrochim Acta 2014;118:163e8. [17] Kamarudin SK, Hashim N. Materials, morphologies and structures of MEAs in DMFCs. Renew Sustain Energy Rev 2012;16:2494e515. [18] Oliveira VB, Rangel CM, Pinto AMFR. One-dimensional and non-isothermal model for a passive DMFC. J Power Sources 2011;196:8973e82.
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC) V.B. Oliveira a,**, J.P. Pereira b, A.M.F.R. Pinto a,* a
CEFT, Departamento de Engenharia Quı´mica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal b TU Delft, Department of Biotechnology, Office 1.84, Julianalaan 67, 2628 BC Delft, The Netherlands
article info
abstract
Article history:
One of the most significant obstacles for passive direct methanol fuel cells is the methanol
Received 21 January 2016
crossover, since methanol diffuses through the membrane generating heat but no power.
Received in revised form
This can be reduced if the cell operates with low methanol concentrations. However, this
11 June 2016
significantly reduces the energy density of the system since a large amount of water is
Accepted 13 June 2016
presented on the anode side that will produce no power. Also, water crosses the membrane
Available online xxx
towards the cathode flooding it and reducing the cell performance. In the present work, different anode diffusion layers were tested in order to select optimal working conditions
Keywords:
at high methanol concentrations. The results showed that the performance increased with
Passive direct methanol fuel cells
an increase of carbon cloth thickness due mainly to a decrease of methanol crossover.
High methanol concentrations
Regarding the carbon paper, better performances were achieved for lower thicknesses and
Methanol crossover
with a microporous layer (MPL) mainly due to a decrease of the anode overpotential ach-
Anode overpotential
ieved by a facilitated access of reactants. In this work, the best performance was achieved
Anode diffusion layer
using carbon cloth for a methanol concentration of 3 M with a corresponding power density of 24.2 mW/cm2. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, the energy needs for portable electronic devices are rising exponentially due to the increasing amount of applications (games, internet service, instant messaging and cameras), especially on cell phones. The conventional batteries are becoming scarce for the increasing power demand and complexity of these devices and the period that these devices can operate as truly portable is limited by the amount
of energy that can be stored in the conventional batteries. To overcome that, special attention is being devoted to fuel cell systems and particularly, to direct methanol fuel cells (DMFC) are considered as a main power provider for portable applications [1e3] The DMFC emerge as a promising power solution since methanol is a liquid at room temperature, is easy to handle, storage and distribute, has limited toxicity, the highest energy to carbon ratio of any alcohol and has low cost since it can be generated from natural gas, coal or biomass. However, the major disadvantages of using a liquid fuel is the
* Corresponding author. Tel.: þ351 225081675; fax: þ351 225081449. ** Corresponding author. Tel.: þ351 225081655; fax: þ351 225081449. E-mail addresses: [email protected] (V.B. Oliveira), [email protected] (A.M.F.R. Pinto). http://dx.doi.org/10.1016/j.ijhydene.2016.06.116 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
2
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 x x x ( 2 0 1 6 ) 1 e8
slow anode kinetics arising from a multi-step oxidation reaction at the anode and fuel crossover from the anode to the cathode side. This not only lowers the fuel utilization on the anode side, but degrades the cathode performance and generates extra heat. In a DMFC, the fuel and oxidant can be supplied in two different ways: an active or a passive one. The active systems need moving parts to feed the oxidant and the fuel to the cell requiring power to operate and allow higher power outputs since can be operated at optimal conditions of temperature and pressure, being for that reason more suitable for large fuel cells. However, these systems have higher costs and lower system energy densities. In the passive systems, the fuel is supplied to the anode from a fuel reservoir built-in in the anode and the air to the cathode, normally by natural convection and diffusion. This lead to a simpler and smaller design, making these systems better suited for small applications. The refueling of the passive DMFC is fast and the fuel can last several months. However, this simple design causes lower system performances due to the difficulty of getting a continuous and homogeneous supply of reactants to the cell. This limits the reaction rates, reducing the cell voltage and severely affecting the efficiency of the system. It is commonly accepted that two achieve the power outputs needed for portable applications, more concentrated methanol solutions are preferred. However this will lead to higher levels of methanol crossover with a consequent loss of performance. Another issue of using high methanol concentrations or even neat-methanol is the water problem. Since low water or no water is presented on the anode side, the water required for the methanol oxidation reaction need to be transported from the cathode to the anode, by diffusion, through the membrane. In this situation the anode performance is dependent not only on the water presented on the anode side but also on the water transport rate from the cathode. Therefore, in order to a passive system achieve the desirable levels of energy density, the key challenges such as the low rate of methanol oxidation, the methanol crossover and the water management need to be overcome. In order to achieve that, different solutions, such as, improving the methanol transport resistances and the catalyst activity through structural modifications are being proposed [4e17]. The diffusion layers do not participate directly in the electrochemical reactions but have several important functions: i) to allow the access of reactants to the catalyst layer; ii) to provide the removal of the products from the catalyst layer; iii) to electrically connect the catalyst layer to the collector plate; iv) allow heat removal and v) to provide mechanical support to the MEA (membrane electrode assembly). According to these roles, these layers should be sufficiently porous to allow the flow of both reactants and products, electrically and thermally conductive, sufficiently rigid to support the MEA, but must have some flexibility to maintain good electrical contacts. The porous facing the catalyst layer should be relatively small since the catalyst layer is made of discrete small particles. These requirements are best met by carbon based materials such as carbon papers and carbon cloths. Two structural parameters of these two materials affect the fuel cell performance: i) porosity, which influences the species transport and ii) the surface properties, the
hydrophobicity and roughness, which control the droplet/ bubble attachment or coverage on the diffusion layer surface. As mentioned, the diffusion layers are usually made of carbon materials, such as carbon paper and carbon cloth and in order to regulate the reactant supply and the product removal, on these layers, and control the amount of methanol that reaches the membrane different structural modifications, such as polytetrafluoroethylene (PTFE) treatments, adding a barrier layer and/or increase the thickness of the diffusion layer have been proposed [4e12]. In this work, a detailed experimental study regarding the effects of anode diffusion layer material and thickness on the performance of a passive DMFC is described. In order to achieve the energy outputs needed for real passive DMFC applications and towards the introduction, as soon as possible, of this technology in the market, the main goal of this work is to methodically vary available commercial materials and evaluate their influence on the passive DMFC performance.
Materials and methods Fuel cell design The experimental fuel cell consists of two acrylic end plates (open on the cathode side and with a reservoir on the anode side), two isolating rubber plates, two gold plated copper connector plates (with 36 holes with a diameter of 6 mm), two diffusion layers (GDL), two catalyst layers and a membrane (Fig. 1). The membrane used was Nafion 117 (with a thickness of 0.0178 cm) and the catalyst used on the anode and cathode side was, respectively, 4 mg/cm2 PteRu black (with a molar of ratio 1:1) and 4 mg/cm2 Pt black. The three-layer MEAs (membrane electrode assembly) were supplied by Fuel Cells Etc. Carbon cloth (CC) with two different thicknesses (0.400 mm and 0.425 mm) and with a MPL (0.410 mm) were used as anode diffusion layer. Two different carbon paper (CP) thicknesses
Fig. 1 e “In-house” passive direct methanol fuel cell.
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
3
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 x x x ( 2 0 1 6 ) 1 e8
Table 1 e Properties of the materials used as anode diffusion layers. GDL
Type Thickness (mm) Basic weight (g/m2) Air permeabilitya (s) Through-plane resistanceb (mUcm2) Porosity MPL
CC CC_A CC_MPL CP_B CP CP_MPL_C CP_MPL
Cloth Cloth Cloth Paper Paper Paper Paper
a b
0.400 0.425 0.410 0.110 0.190 0.240 0.340
122 149 180 49 84 90 130
<10 <10 <8 <10 <10 <85 <170
<5 <5 <13 <9 <7 <15 <10
0.83 0.83 0.80 0.78 0.78 0.80 0.80
No No Yes No No Yes Yes
Gurley Method. Based on ASTM C611 Method.
(0.110 mm and 0.190 mm) 20 (þ/ 5) wt. % PTFE treated and carbon paper with MPL (0.240 mm and 0.340 mm) were, also tested as anode diffusion layer. Therefore, in this work seven different diffusion layers were tested on the anode side three carbon cloths and four carbon papers. In all the experiments the cathode diffusion layer was carbon cloth with a thickness of 0.400 mm (CC). All the materials are woven and were supplied by QuinTech. The corresponding properties are displayed in Table 1. The diffusion layers were pressed on the three-layer membranes (membrane with the catalyst deposited/sprayed on the membrane) when assembling the fuel cell as shown in Fig. 1. As already mentioned, commercially available materials were selected for the diffusion and catalyst layers and for the membrane.
Experimental conditions In the experiments, a passive DMFC with an open area of 10.2 cm2 and a MEA area of 25 cm2 was operated at ambient conditions (ambient pressure and temperature) and with different methanol solutions on the anode compartment (1 M, 3 M and 5 M). The fuel cell temperature was controlled by a digital temperature controller and was set near ambient conditions, 20 C. Different methanol concentrations and anode diffusion layer materials with different thicknesses were tested. The experimental tests were performed with a commercial test station (Zahner-Elektrik GmbH & Co. kG) and the polarization curves were performed galvanostatically. The computer constantly monitors both current and voltage and these parameters are used to estimate and track the power output.
Results and discussion All the results presented in this section were explained under the light of model predictions for methanol crossover rate (methanol flux through the membrane from the anode to the cathode side) and anode overpotential from a previous developed and validated semi-analytical and onedimensional model considering the effects of heat and mass transfer, along with the electrochemical reactions occurring in a passive DMFC. The model assumes that the fuel cell
operates under steady-state conditions, only the liquid phase was considered in the anode side and membrane and only the gas phase was considered in the cathode side, the mass transport in the diffusion layers and membrane was described using effective Fick models, local equilibrium at interfaces was represented by partition functions, the catalyst layers were assumed to be a macrohomogeneous porous electrode, so reactions in these layers are modeled as a homogeneous reaction, methanol and water transport through the membrane was assumed to be caused by the combined effect of the concentration gradient between the anode and the cathode and the electro-osmosis force. All the model details (development, assumptions, validation and outputs) can be found in Ref. [18].
Diffusion layer material In order to evaluate the effect of the anode diffusion layer material on the fuel cell voltage and power density, two different materials, carbon cloth (CC) and carbon paper (CP), were used (properties in Table 1). As already mentioned, the tests were performed for three different methanol concentrations 1 M, 3 M and 5 M. The influence of the anode diffusion layer material on the cell voltage is shown in Fig. 2 while the impact on power density is presented in Fig. 3. It should be mentioned that carbon cloth is more porous and less tortuous than carbon paper, though carbon paper has excellent electronic conductivity [10]. Also, as can be seen in Table 1, CP is thinner than CC. As can be seen in Figs. 2 and 3 better performances are achieved using CC as anode diffusion layer, mostly due to its higher porosity and thickness. As mentioned, CC has higher porosity than CP enhancing the methanol transport from the methanol reservoir to the reaction zone, the catalyst layer. This is disadvantageous to crossover (as can be seen by the methanol flux values presented in Fig. 4) but has a very positive effect on methanol oxidation rate on the catalyst layer, allowing a higher methanol concentration in this layer. Therefore, more methanol is oxidized, increasing the methanol oxidation rate and the power output due to a decrease of the anode overpotential (Fig. 4). The CP with its lower porosity limits the amount of methanol that reaches the catalyst layer decreasing not only the methanol crossover through the membrane but, also the methanol oxidation reaction rate due to a lack of methanol on this layer. This can be clearly seen on
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
4
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 x x x ( 2 0 1 6 ) 1 e8
the model predictions for the methanol crossover flux thought the membrane and anode overpotential presented in Fig. 4. On the anode reaction gaseous carbon dioxide is produced and if the carbon dioxide bubbles are not removed efficiently
0.60
CC_1M CC_3M
Cell voltage (V)
0.50
CC_5M CP_1M
0.40
CP_3M CP_5M
0.30 0.20 0.10 0.00 0
10
20
30
40
50
Current density (mA/cm2)
Fig. 2 e Effect of anode diffusion layer material on cell performance for different methanol feed concentrations.
Power density (mW/cm2)
3.5
CC_1M CC_3M
3.0
Carbon cloth
CC_5M
2.5
CP_1M CP_3M
2.0
CP_5M
1.5
1.0 0.5
0.0 0
10
20
30
40
50
Current density (mA/cm2)
Fig. 3 e Effect of anode diffusion layer material on power density for different methanol feed concentrations.
7.00E-08
0.50
CC_3M_MF CP_3M_MF
6.50E-08
0.45
CC_3M_AO CP_3M_AO
6.00E-08
0.40
5.50E-08
0.35
5.00E-08
0.30
4.50E-08
0.25
anode overpotential (V)
Methanol flux (mol/cm2.s)
from the catalyst surface, they cover the surface decreasing the effective active area for methanol oxidation. The texture of carbon paper with a highly tortuous structure enhances the interactions between the bubbles and the solid. Hence, the gas tends to remain attached to the surface leading to a blockage of the active sites lowering the methanol oxidation reaction rate and consequently the cell performance. This can be perceived on the model predictions for the anode overpotential for the two materials tested and shown in Fig. 4. Model predictions for the methanol flux (MF) through the membrane from the anode to the cathode side and the anode overpotential (AO) for a 3 M concentration are also presented in Fig. 4. This concentration was chosen since, among all the concentrations tested, this was the one which leads to a better performance (Figs. 2 and 3). As previously referred, higher crossover rates and lower anode overpotentials are achieved with CC as GDL due to a lower methanol mass transport resistance. From these predictions it can be concluded that the use of CC as anode G. GDL has a positive effect on the anode reaction rate and a negative effect on crossover. However, the polarization and the power density curves show that the positive effect on the oxidation rate is more relevant than the negative one on crossover.
0.20
4.00E-08
0
10
20
30
40
50
Current density (mA/cm2)
Fig. 4 e Model predictions for the effect of anode diffusion layer material on methanol flux through the membrane (MF) and anode overpotential (AO) for a methanol concentration of 3M.
As mentioned, different structural modifications have been performed in order to reduce the methanol crossover and the most common one is to add a microporous layer (MPL) to the GDL. In this case the GDL has a dual-layer structure instead of a single one. The first layer is the backing layer and serves as a substrate for the second layer and the one is a MPL, usually made of carbon black powder with a hydrophobic agent [17]. Each layer acts as a diffusion layer for fuel and products transport. In order to evaluate the effect of adding a MPL to the GDL, in this work, carbon cloth with a single structure (CC and CC_A) and with MPL (CC_MPL) was used as anode diffusion layer. Two different thicknesses of the GDL without MPL (CC and CC_A) were also tested to study the effect of the GDL thickness on the passive DMFC performance. The polarization and the power density curves, for these studies, are presented, respectively, in Figs. 5 and 6. As can be seen in the plots, for the single layer structure a higher thickness (CC_A) leads to a higher fuel cell performance due to better fuel distribution, which not only allows a higher methanol concentration on the catalyst layer increasing the methanol oxidation rate and decreasing the anode overpotential, but also decreases the amount of methanol that crosses the membrane towards the cathode side (Fig. 7). The effect of the layer thickness on the decrease of the methanol crossover is well put in evidence by the significantly higher OCV values when using the CC_A layer (Table 2). Regarding the GDL with MPL (CC_MPL) and without MPL (CC) it can be seen that better performances are achieved with the dual layer structure (Table 2), since the MPL increases the methanol utilization due to a more uniformly reactant distribution on the electrode surface decreasing the anode overpotential and the methanol flux through the membrane [11,17] (Fig. 7).
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
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 x x x ( 2 0 1 6 ) 1 e8
0.70
Cell voltage (V)
(Table 2). The model predictions are in accordance with the experimental data, since the use of carbon cloth with a higher thickness (CC_A) leads to a lower methanol flux and anode overpotential.
CC_1M CC_3M
0.60
CC_5M CC_A_1M
0.50
CC_A_3M CC_A_5M
0.40
CC_MPL_1M
CC_MPL_5M
0.20
0.00
0
20
40
60
80
100
120
140
Current density (mA/cm2)
Fig. 5 e Effect of carbon cloth properties on cell performance for different methanol feed concentrations.
25
Power density (mW/cm2)
Carbon paper
CC_MPL_3M
0.30
0.10
20 CC_1M CC_3M
15
CC_5M CC_A_1M
10
CC_A_3M
CC_A_5M CC_MPL_1M
5
CC_MPL_3M CC_MPL_5M
0 0
20
40
60
80
100
120
140
Current density (mA/cm2)
Fig. 6 e Effect of carbon cloth properties on power density for different methanol feed concentrations.
7.00E-08
0.45
6.00E-08
0.40
5.00E-08
0.35 4.00E-08 0.30
CC_3M_MF CC_A_3M_MF CC_MPL_3M_MF CC_3M_AO CC_A_3M_AO CC_MPL_3M_AO
3.00E-08 2.00E-08 1.00E-08
0
20
40
60
0.25
anode overpotential (V)
Methanol flux (mol/cm2.s)
5
0.20
80
100
120
Current density (mA/cm2)
Fig. 7 e Model predictions for the effect of carbon cloth properties on methanol flux through the membrane (MF) and anode overpotential (AO) for a methanol concentration of 3M.
Fig. 7 shows the model predictions for the effect of carbon cloth properties on methanol flux through the membrane and anode overpotential, for a methanol concentration of 3 M (concentration which shows the higher power outputs)
To analyze the effect of the carbon paper thickness on the passive DMFC performance two different structures, CP and a CP_B with a lower thickness (Table 1) were used and the results are presented in Figs. 8 and 9, for three different methanol concentration. As can be seen, a lower thickness leads to a higher fuel cell performance due to a lower methanol mass transport resistance in the diffusion layer. A higher thickness in combination with a lower porosity characteristic of the carbon paper limits the amount of methanol that reaches the catalyst layer not supplying the amount of methanol needed for the methanol oxidation reaction. Therefore the methanol oxidation rate tends to be lower and the anode overpotential increases. The use of carbon paper with a thinner thickness (CP_B) is advantageous since it allows a higher methanol concentration on the catalyst layer, increasing its oxidation rate and decreasing its transport through the membrane towards the cathode. These explanations are confirmed by the model predictions in Fig. 10 on the effect of carbon paper thickness on methanol crossover and anode overpotential for a methanol concentration of 3 M. As expected, after the analysis of the experimental results, lower anode overpotentials and methanol crossover fluxes and consequently higher performances are achieved using carbon paper with a lower thickness (CP_B) as GDL. A similar study regarding the effect the carbon paper thickness on a passive DMFC for a dual-layer structure (GDL with MPL) was also performed. Two dual layer structures with two different thicknesses, CP_MPL_C (0.240 mm) and CP_MPL (0.340 mm), were tested and the experimental data are presented in Figs. 11 and 12. Regarding the use of a single GDL structure (Figs. 8 and 9) and a dual one, better results were achieved for the GDL with MPL due to a better methanol distribution and products removal on the anode. Its higher thickness and porosity lead to a higher methanol oxidation rate and consequently a lower anode overpotential and methanol flux through the membrane, since more methanol is consumed on the anode reaction [17]. Analyzing the two dual-layer structures tested (Figs. 11 and 12) having the same porosity, higher performances were obtained for the layer with the lower thickness due to a lower resistance to methanol transport to the catalyst layer, enabling a higher oxidation rate and as result lower anode overpotentials and higher power densities. The results presented in Figs. 11 and 12, also, show that using CP with a dual-layer structure better performances are achieved not with a methanol concentration of 3 M (as for the other materials tested) but using a 5 M methanol concentration. This is mainly due to the intrinsic properties of these materials (Table 1) which enable better reactants distribution, higher oxidation rates and lower crossover fluxes through the membrane.
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
6
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 x x x ( 2 0 1 6 ) 1 e8
Methanol concentration (M)
OCV (V)
Maximum power density (mW/cm2)
1 3 5 1 3 5 1 3 5
0.450 0.442 0.408 0.600 0.525 0.470 0.643 0.547 0.564
1.56 3.30 1.19 20.7 24.2 18.6 7.10 8.72 9.40
CC
CC_A
CC_MPL
0.70
0.45
5.60E-08 CP_B_3M_MF
5.20E-08
0.40
CP_3M_MF CP_B_3M_AO
0.35
CP_3M_AO
4.80E-08
0.30
4.40E-08
0.25
4.00E-08
0.20 0
10
20
30
40
50
60
70
Current density (mA/cm2)
Fig. 10 e Model predictions for the effect of carbon paper thickness on methanol flux through the membrane (MF) and anode overpotential (AO) for a methanol concentration of 3M.
CP_B_1M CP_B_3M
0.60
CP_B_5M
0.50
0.70
CP_MPL_C_1M
CP_1M
0.40
CP_MPL_C_3M
0.60
CP_3M
CP_MPL_C_5M
CP_5M
Cell voltage (V)
Cell voltage (V)
0.50
anode overpotential (V)
GDL
Methanol flux (mol/cm2.s)
6.00E-08
Table 2 e Effect of carbon cloth properties on the open circuit voltage (OCV) and maximum power density different methanol feed concentrations.
0.30 0.20 0.10
0.50
CP_MPL_1M CP_MPL_3M
0.40
CP_MPL_5M
0.30 0.20
0.00
0
10
20
30
40
50
60
70
0.10
Current density (mA/cm2) 0.00 0
Fig. 8 e Effect of carbon paper thickness on cell performance for different methanol feed concentrations.
20
40
60
80
100
120
Current density (mA/cm2)
Fig. 11 e Effect of carbon paper properties on cell performance for different methanol feed concentrations.
6
18
16
5
Power density (mW/cm2)
Power density (mW/cm2)
7
CP_B_1M
4
CP_B_3M
3
CP_B_5M CP_1M
2
CP_3M
CP_5M
1
0 0
10
20
30
40
50
60
70
Current density (mA/cm2)
Fig. 9 e Effect of carbon paper thickness on cell power density for different methanol feed concentrations.
14 12 10 CP_MPL_C_1M
8
CP_MPL_C_3M
6
CP_MPL_C_5M
4
CP_MPL_1M
CP_MPL_3M
2
CP_MPL_5M
0 0
20
40
60
80
100
120
Current density (mA/cm2)
Fig. 12 e Effect of carbon paper properties on power density for different methanol feed concentrations. These considerations are well confirmed in Fig. 13 by the model predictions regarding the effect of carbon paper properties on methanol flux and anode overpotential for a methanol concentration of 5 M, the one that conducts to
better performances (Figs. 11 and 12). As shown in Fig. 13, lower anode overpotentials and methanol fluxes are achieved for the CP_MPL_C. It is important to report that the lowest predicted methanol crossover fluxes when using all
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
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 x x x ( 2 0 1 6 ) 1 e8
0.50 0.45
4.00E-08 0.40
3.00E-08
0.35 CP_MPL_C_5M_MF
0.30
CP_MPL_5M_MF
2.00E-08
anode overpotential (V)
Methanol flux (mol/cm2.s)
5.00E-08
0.25
CP_MPL_C_5M_AO CP_MPL_5M_AO
0.20
1.00E-08 0
20
40
60
80
100
Current density (mA/cm2)
Fig. 13 e Model predictions for the effect carbon paper properties on methanol flux through the membrane (MF) and anode overpotential (AO) for a methanol concentration of 5M.
the materials tested were obtained with carbon paper with MPL as GDL (Figs. 4, 7 and 10), which allowed to work with higher methanol concentrations (5 M) without a significant loss on performance. These results are very important to achieve the goals needed for a passive DMFC in real applications: higher methanol concentrations, lower methanol crossover rates and higher power outputs leading to improved autonomies.
Conclusions A great challenge on the development of a passive DMFC, is to use cost-effective materials and cells with high energy den-
sities and low methanol crossover rates. To achieve these goals, it is necessary to use high methanol concentrations and use different diffusion layer materials with different thicknesses. This work aims to find solutions to these problems and contribute to an advance on the DMFC knowledge for portable applications. The results show that better performances are achieved using carbon cloth as GDL due to a higher thickness and porosity than carbon paper. A higher thickness allied to a higher porosity significantly decreases the methanol crossover rate through the membrane and allows an increase on the methanol oxidation reaction and consequently a decrease on the anode overpotential.
7
An anode diffusion layer made of carbon paper with a duallayer structure and with a lower thickness lead to higher fuel cell performances than a single structure and higher thicknesses mainly due to an improved porosity enabling a higher methanol oxidation rate on the catalyst layer and a reduction of the anode overpotential. This dual structure also allowed to work with higher methanol concentrations (5 M) and lower methanol crossover rates. In this work the maximum power density, 24.2 mW/cm2, was obtained using a Nafion 117 membrane, 4 mg/cm2 of PteRu and 4 mg/cm2 of Pt, as respectively, anode and cathode catalyst layers, carbon cloth with a higher thickness as anode diffusion layer, carbon cloth as cathode diffusion layer and a methanol concentration of 3 M. As a result of this work, a tailored MEA build-up with the common available commercial materials was proposed to achieve low methanol crossover rates and higher power densities operating with high methanol concentrations. This work shows that changes in fuel cell structure are effective ways of controlling the methanol crossover and increase the fuel cell performance. Further improvements, in passive DMFC systems, will be achieved when more cost effective materials for the different layers become commercially available.
Acknowledgments V.B. Oliveira acknowledges the post-doctoral fellowship (SFRH/BDP/91993/2012) supported by the Portuguese ~o para a Cie ^ncia e Tecnologia” (FCT), POPH/QREN and “Fundac¸a European Social Fund (ESF). POCI (FEDER) also supported this work via CEFT.
references
[1] Kamarudin SK, Achmad F, Daud WRW. Overview on the application of direct methanol fuel cell (DMFC) for portable electronic devices. Int J Hydrogen Energy 2009;34:6902e16. [2] Sundmacher K. Fuel cell engineering: toward the design of efficient electrochemical power plants. Ind Eng Chem Res 2010;49:10159e82. ~ o DS, Oliveira VB, Rangel CM, Pinto AMFR. Review on [3] Falca micro-direct methanol fuel cells. Renew Sustain Energy Rev 2014;34:58e70. [4] Wu QX, Zhao TS, Chen R, Yang WW. Effects of anode microporous layers made of carbon powder and nanotubes
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116
8
[5]
[6]
[7]
[8]
[9]
[10]
[11]
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 x x x ( 2 0 1 6 ) 1 e8
on water transport in direct methanol fuel cells. J Power Sources 2009;191:304e11. Yuan T, Zou Z, Chen M, Li Z, Xia B, Yang H. New anodic diffusive layer for passive micro-direct methanol fuel cell. J Power Sources 2009;192:423e8. Yuan W, Zhou B, Hu J, Deng J, Zhang Z, Tang Y. Passive direct methanol fuel cell using woven carbon fiber fabric as mass transfer control medium. Int J Hydrogen Energy 2015;40:2326e33. Zainoodin AM, Kamarudin SK, Masdar MS, Daud WRW, Mohamad AB, Sahari J. High power direct methanol fuel cell with a porous carbon nanofiber anode layer. Appl Energy 2014;113:946e54. Li J, Ye D-d, Zhu X, Liao Q, Ding Y-d, Tian X. Effect of wettability of anode microporous layer on performance and operation duration of passive air-breathing direct methanol fuel cells. J Appl Electrochem 2009;39:1771e8. Yuan T, Yang J, Wang Y, Ding H, Li X, Liu L, et al. Anodic diffusion layer with graphene-carbon nanotubes composite material for passive direct methanol fuel cell. Electrochim Acta 2014;147:265e70. ~ o DS, Rangel CM, Pinto AMFR. Water Oliveira VB, Falca management in a passive direct methanol fuel cell. Int J Energy Res 2013;37:991e1001. Yuan W, Tang Y, Yang X, Liu B, Wan Z. Structural diversity and orientation dependence of a liquid-fed passive airbreathing direct methanol fuel cell. Int J Hydrogen Energy 2012;37:9298e313.
[12] Park Y-C, Kim D-H, Lim S, Kim S-K, Peck D-H, Jung D-H. Design of a MEA with multi-layer electrodes for high concentration methanol DMFCs. Int J Hydrogen Energy 2012;37:4717e27. [13] Shrivastava NK, Thombre SB, Mallick RK. Effect of diffusion layer compression on passive DMFC performance. Electrochim Acta 2014;149:167e75. [14] Wu QX, Zhao TS, Yang WW. Effect of the cathode gas diffusion layer on the water transport behavior and the performance of passive direct methanol fuel cells operating with neat methanol. Int J Heat Mass Transf 2011;54:1132e43. [15] Cao J, Chen M, Chen J, Wang S, Zou Z, Li Z, et al. Double microporous layer cathode for membrane electrode assembly of passive direct methanol fuel cells. Int J Hydrogen Energy 2010;35:4622e9. [16] Cao J, Wang L, Song L, Xu J, Wang H, Chen Z, et al. Novel cathodal diffusion layer with mesoporous carbon for the passive direct methanol fuel cell. Electrochim Acta 2014;118:163e8. [17] Kamarudin SK, Hashim N. Materials, morphologies and structures of MEAs in DMFCs. Renew Sustain Energy Rev 2012;16:2494e515. [18] Oliveira VB, Rangel CM, Pinto AMFR. One-dimensional and non-isothermal model for a passive DMFC. J Power Sources 2011;196:8973e82.
Please cite this article in press as: Oliveira VB, et al., Effect of anode diffusion layer (GDL) on the performance of a passive direct methanol fuel cell (DMFC), International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.116