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A novel single electrode supported direct methanol fuel cell
Electrochemistry Communications 11 (2009) 1530–1534
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
A novel single electrode supported direct methanol fuel cell Alfred Lam, David P. Wilkinson *,1, Jiujun Zhang 1 Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC, Canada V6T 1Z3 Institute for Fuel Cell Innovation, National Research Council, 4250 Wesbrook Mall, Vancouver, BC, Canada V6T 1W5
a r t i c l e
i n f o
Article history: Received 29 April 2009 Received in revised form 20 May 2009 Accepted 21 May 2009 Available online 29 May 2009 Keywords: Single electrode supported Membraneless fuel cell Direct methanol fuel cell (DMFC) Air breathing fuel cell Passive fuel cell Membrane electrode assembly
a b s t r a c t In this paper a single electrode supported direct methanol fuel cell (DMFC) is fabricated and tested. The novel architecture combines the elimination of the polymer electrolyte membrane (PEM) and the integration of the anode and cathode into one component. The thin film fabrication involves a sequential deposition of an anode catalyst layer, a cellulose acetate electronic insulating layer and a cathode catalyst layer onto a single carbon fibre paper substrate. The single electrode supported DMFC has a total thickness of 3.88 102 cm and showed a 104% improvement in volumetric specific power density over a two electrode DMFC configuration under passive conditions at ambient temperature and pressure (1 atm, 25 °C). Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Direct liquid fuel cells such as the direct methanol fuel cell (DMFC) offer the advantage of extended and continuous operation through the replacement of a fuel cartridge. Additionally, liquid fuels such as methanol have a high energy density (4820 Wh L1 [1]) and can be easily handled, stored and transported with existing infrastructure. Given the size constraints for many portable applications, the volume of a fuel cell is an important consideration. A conventional DMFC membrane electrode assembly (MEA) architecture consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. To simplify this design, the removal, replacement or integration of the electrode assembly components has been studied by various research groups. Previous work by the authors and others has shown that membraneless designs are possible [2–8]. In mixed reactant strip cells [9–13] and monolithic fuel cells [14–16] the anode and cathode have been integrated together onto the same planar side of a substrate in a side by side arrangement. In this preliminary study a single electrode supported DMFC is fabricated by a combination of PEM removal (membraneless) and electrode integration. This design is related to our previous study [2] where two electrodes are separated by a spacer in a membraneless configuration. In this configuration a gap must be maintained between the electrodes to prevent an electrical short circuit. An * Corresponding author. Tel.: +1 604 822 4888; fax: +1 604 822 6003. E-mail address: [email protected] (D.P. Wilkinson). 1 ISE Member. 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.05.049
alternative method to prevent short circuiting is to coat one of the electrode surfaces with a thin electrically non-conducting material so that the two separate electrodes are in physical but not electrical contact with each other (Fig. 1a). This configuration is an example of a two electrode system. In a single electrode supported architecture, as shown in Fig. 1b, an anode catalyst layer, an electrically insulating film (entire area) and a cathode catalyst layer are sequentially deposited onto a single carbon fibre substrate. In the research here, a cellulose acetate (CA) polymer was used as the electrically insulating film for the configurations shown in Fig. 1a and b. CA by itself does not conduct protons, however its hydrophilic properties allow for a liquid electrolyte (0.5 M H2SO4) to be soaked into its structure. This provides the ionic connection between the anode and cathode. In the operation of the fuel cell, a fuel electrolyte (5 M CH3OH and 0.5 M H2SO4) is supplied to the anode, and the cathode is open to the air. In the novel architecture, the thin film deposition allows for the fabrication of an electrode assembly that is a fraction of the thickness for a conventional two electrode architecture. Sequential ceramic deposition on a metal supported substrate has been investigated for high temperature (>400 °C) solid oxide fuel cells [17,18] but this approach has not been done for low temperature liquid fuel cells. Mixed reactant strip cells and monolithic fuel cells are other examples of PEM based single substrate fuel cells. In a strip cell, described by Barton et al [9], anode and cathode catalyst are deposited on the same planar side of Nafion Ò117 in a side by side arrangement and a mixed reactant stream is fed over the surface. A monolithic fuel cell described by Meyers et al. [14] has a similar arrangement but the fuel and oxidant are fed with separate
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A. Lam et al. / Electrochemistry Communications 11 (2009) 1530–1534
Fig. 1a. Schematic of a two electrode DMFC with a cellulose acetate (CA) film over the entire surface.
(39.8 wt% acetyl content; Mn 30,000) purchased from Sigma Aldrich was dissolved in acetone to 5 wt% and was deposited over the entire anode surface to thickness of 2.00 103 cm. For the single electrode supported DMFC a single Etek-TGPH060 carbon fibre paper with 20% wet proofing was used as a base substrate. The first anode layer, had a loading of 4.00 mg cm2 carbon supported (Vulcan XC-72) 40 wt% Pt–Ru (1:1 atomic ratio, or a/o) catalyst with a NafionÒ loading of 30 wt%. The electrically insulating CA film was loaded from a 5 wt% CA/acetone solution to a thickness of 2.07 103 cm over the entire active area and the final cathode layer had a loading of 1.36 mg cm2 carbon supported (Vulcan XC-72) 20 wt% Pt catalyst with a NafionÒ loading of 30 wt%. In the fabrication of a single electrode structure, short circuiting can become an issue if the cathode catalyst ink penetrates the insulating layer during the spray deposition process. Isopropyl alcohol was used as a solvent to make the catalyst ink. CA was chosen because it is soluble in acetone but not in isopropyl alcohol thus it is a barrier to penetration of the catalyst ink during fabrication. The electrode assembly was incorporated into a holder with perforated graphitic foil and Pt current collectors. A 2.0 cm2 active area single chamber glass cell at ambient temperature and pressure (25 °C, 1 atm) and an aqueous 5 M methanol/0.5 M H2SO4 anolyte was used to examine the electrode assembly performance. The polarization curves were developed with a Solartron 1420E Multistat operated in galvanostatic mode and the specific electrode potentials were monitored with a double junction saturated calomel electrode (SCE) located in the anodic chamber. The fuel cell resistance as a function of the electrode assembly configuration was recorded at an operating frequency of 1000 Hz using a Solartron 1260 FRA. 3. Results and discussion The size reduction of a fuel cell into a compact design is a key factor for the integration into portable electronic devices and other applications. This can be accomplished through the elimination
Fig. 1b. Schematic of a single electrode supported DMFC.
Carbon Fibre Paper
Cellulose Acetate
streams in adjacent channels on the same planar side. The sequential layered approach shown in this paper overcomes several disadvantages associated with both of these configurations. For instance, limitations with the area specific power density (W cm2) due to a 50% share a single substrate surface [14] and ohmic losses resulting from in-plane current collection are avoided by utilizing the entire substrate area and through plane current collection. Mixed reactant strip cells also have the added disadvantage of poor mass specific catalyst activity of the selective electrocatalysts used when compared with platinum based catalysts [9]. 2. Experimental An ink spray deposition method with an AccuSpray spray gun was used to fabricate the electrodes for a two electrode DMFC with cellulose acetate film (Fig. 1a). Etek-TGPH-060 carbon fibre paper with 20% wet proofing was used as a base substrate for the respective electrodes. The anode had a loading of 4.00 mg cm2 carbon supported (Vulcan XC-72) 40 wt% Pt–Ru (1:1 atomic ratio, or a/o) catalyst with a NafionÒ loading of 30 wt% and the cathode had a loading of 1.34 mg cm2 carbon supported (Vulcan XC-72) 20 wt% Pt catalyst with a NafionÒ loading of 30 wt% and a 1.10 mg cm2 Cabot carbon sublayer with 20 wt% PTFE. Powdered CA
Anode Catalyst Layer
Cathode Catalyst Layer
Fig. 2. SEM of a single electrode supported DMFC at a 210x magnification.
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A. Lam et al. / Electrochemistry Communications 11 (2009) 1530–1534
and/or integration of components and a reduction of certain geometric parameters (i.e., thickness and area). Previously, we have shown that it is possible to eliminate the membrane in direct liquid fuel cells by using a 3D anode structure in conjunction with a conductive fuel electrolyte. In a fuel cell a significant contribution to the overall voltage losses is attributed to ohmic overpotentials. The overall ohmic loss, gohmic (V), shown in Eq. (1), is the sum of the resistance of each component (n) in the electrode assembly multiplied by the current density, I (Acm2). Each individual resistance is a function of component thickness, ln (cm), resistivity, qn (ohmcm) and area, A (cm2).
gohmic ¼ I Roverall ¼ I
X n
Rn ¼ I
X ln q n
n
A
ð1Þ
With respect to the overall resistance, Roverall (Ohm), the electrolyte plays a significant role. A reduction in gap separation would reduce the overall resistance. In theory, a zero gap separation would eliminate the electrolyte component in Eq. (1). However, in practice there exist limitations to achieving a zero gap separation. Imperfections or highly rough surfaces will result in short circuiting of certain parts of the electrode when the electrodes are brought close together. In order to reduce the gap separation further a thin electrically non-conductive coating can be applied to the surface of the electrode. This prevents short circuiting and enables the two electrodes to be in physical but not electrical contact with each other. For this study, CA was chosen as the coating material for its ease of application onto an electrode surface and its hydrophilic and electrically
Fig. 3a. Polarization and power density curve on an area basis for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC at ambient temperature and pressure. The anode layer has a loading of 4.00 mg cm2 carbon supported (Vulcan XC-72) 40 wt% Pt–Ru and the cathode layer has a loading 1.36 mg cm2 carbon supported (Vulcan XC-72) 20 wt% Pt catalyst.
Fig. 3b. Individual reference potential of the anode and cathode for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC.
A. Lam et al. / Electrochemistry Communications 11 (2009) 1530–1534
1533
Fig. 4. Polarization and power density curve on a volumetric basis for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC at ambient temperature and pressure. The anode layer has a loading of 4.00 mg cm2 carbon supported (Vulcan XC-72) 40 wt% Pt–Ru and the cathode layer has a loading 1.36 mg cm2 carbon supported (Vulcan XC-72) Pt catalyst.
insulating properties. Although the proof of concept was carried out with CA, the single electrode supported DMFC is not limited by the use of this polymer. A CA layer was deposited onto the anode surface to a thickness of 2.00 103 cm for both the two electrode and single electrode supported configuration as shown in Fig. 1a and b. A distinct separation between the cathode catalyst layer and the anode catalyst layer formed by the CA layer is shown in Fig. 2 for an SEM of a single electrode supported DMFC. The CA film (entire area) represents only a small fraction of the electrode thickness and has provided an effective coating to prevent short circuiting between the anode and cathode catalyst layers. The resistance of the electrode assembly at 1000 Hz was 0.373 Ohm and 0.537 Ohm for the two electrode DMFC with a CA film (entire area) and single electrode supported architecture, respectively. One would expect a lower resistance for the thinner electrode assembly, however a higher interfacial resistance resulted when the cathode diffusion layer was removed and the current was collected directly from catalyst layer. This was confirmed by testing the resistance of the single electrode supported DMFC with an Etek TGPH-060 carbon fibre paper placed on the cathode surface. The resistance was 0.297 Ohm in this case. Fig. 3a shows, a comparison in performance of a two electrode and a single electrode supported DMFC with a similar CA film thickness of 2.00 103 cm. The power density on an area basis between the single and two electrode configuration is comparable at of 3.54 mW cm2 versus 3.02 mW cm2, respectively. To examine the individual contribution of each electrode to the overall cell voltage, a plot of the reference potentials after IR correction is shown in Fig. 3b. This plot reveals that difference in performance is primarily associated with the anode electrode. The true benefit in the integration onto a single substrate is not fully realized until the performance is normalized on a volume basis. The area specific performance in Fig. 3a for the two electrode and single electrode supported architecture was divided by the respective electrode assembly thickness of 6.69 102 cm and 3.88 102 cm. Fig. 4 shows that the single electrode supported DMFC significantly outperforms the two electrode architecture when the volume of the electrode assembly was considered. The maximum volumetric power density improved from 45.2 mW cm3 to 92.2 mW cm3.
The move toward a structure where the PEM and cathode diffusion layer is removed and all the layers are supported on a single substrate significantly reduces the electrode assembly thickness, volume and weight and overall material cost. 4. Conclusions A proof of concept single electrode supported DMFC with a total thickness of 3.88 102 cm has been successfully demonstrated. The thin film fabrication involved a sequential deposition of an anode catalyst layer, a cellulose acetate (CA) electronic insulating layer and a cathode catalyst layer onto a single carbon fibre paper substrate. The single electrode supported DMFC has significantly reduced the cost (membraneless and only one diffusion support) and manufacturing. The simple fabrication and compact nature of this electrode architecture shows promise for implementation into portable electronic devices and other applications where size is important. The maximum area specific power density was 3.54 mW cm2 and based on the electrode assembly volume, the volume specific power density was 92.2 mW cm3 under passive conditions at ambient temperature and pressure (25 °C, 1 atm). This is comparable to the 2010 Department of Energy (DOE) power density target of 100 mW cm3 for consumer electronics [19]. Further performance improvements are expected especially with respect to the material properties (e.g., optimization of the ionic conductivity in the porous insulating film). References [1] W. Qian, D.P. Wilkinson, J. Shen, H. Wang, J.J. Zhang, Journal of Power Sources 154 (2006) 202. [2] A. Lam, D.P. Wilkinson, J.J. Zhang, Electrochimica Acta 53 (2008) 6890–6898. [3] J.L. Cohen, D.A. Westly, A. Westly, H.D. Abruna, Journal of Power Sources 139 (2005) 96. [4] E.R. Choban, J.S. Spendelow, L. Gancs, A. Wieckowski, P.J.A. Kenis, Electrochimica Acta 50 (2005) 5390. [5] E.R. Choban, L.J. Markoski, A. Wieckowski, P.J.A. Kenis, Journal of Power Sources 128 (2004) 54. [6] R. Ferrigno, A.D. Stroock, T.D. Clark, M. Mayer, G.M. Whitesides, J. Am. Chem. Soc. Commun. 24 (2002) 12930. [7] F. Chen, M.H. Chang, M.K. Lin, Electrochimica Acta 52 (2007) 2506.
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[8] R.S. Jayashree, L. Gancs, E.R. Choban, A. Primak, D. Natarajan, L.J. Maroski, P.J.A. Kenis, J. Am. Chem. Soc. Commun. 127 (2005) 16758. [9] S.C. Barton, T. Patterson, E. Wang, T.F. Fuller, A.C. West, Journal of Power Sources 96 (2001) 329. [10] G.A. Louis, J.M. Lee, D.L. Maricie, J.C. Trocciola, US Patent No. 4, 248 (1981) 941. [11] T. Hibino, K. Ushiki, T. Sato, Y. Kuwahara, Solid State Ionics 81 (1995) 1. [12] T. Hibino, H. Tsunekawa, S. Tanimoto, M. Sano, Journal of the Electrochemical Society 147 (2000) 1338. [13] T. Hibino, A. Hashimoto, M. Suzuki, M. Yano, S-I Yoshida, M. Sano, Journal of the Electrochemical Society 149 (2002) A195.
[14] J.P. Meyers, H.L. Maynard, Journal of Power Sources 109 (2002) 76. [15] S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, T. Osaka, Electrochemistry Communications 6 (2004) 562. [16] Z. Xiao, C. Feng, P.C.H. Chan, I.-M. Hsing, Sensor and Actuators B 132 (2008) 576. [17] S. Hui, D. Yan, Z. Wang, S. Yick, C.D. Petit, W. Qu, A. Tuck, R. Maric, D. Ghosh, Journal of Power Sources 167 (2007) 336. [18] Z. Wang, J.O. Berghaus, S. Yick, C.D. Petit, W. Qu, R. Hui, R. Maric, D. Ghosh, Journal of Power Sources 176 (2008) 90. [19] V. Lightner, Small Fuel Cells 2005, Washington DC, USA, April 27–29, 2005.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
A novel single electrode supported direct methanol fuel cell Alfred Lam, David P. Wilkinson *,1, Jiujun Zhang 1 Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC, Canada V6T 1Z3 Institute for Fuel Cell Innovation, National Research Council, 4250 Wesbrook Mall, Vancouver, BC, Canada V6T 1W5
a r t i c l e
i n f o
Article history: Received 29 April 2009 Received in revised form 20 May 2009 Accepted 21 May 2009 Available online 29 May 2009 Keywords: Single electrode supported Membraneless fuel cell Direct methanol fuel cell (DMFC) Air breathing fuel cell Passive fuel cell Membrane electrode assembly
a b s t r a c t In this paper a single electrode supported direct methanol fuel cell (DMFC) is fabricated and tested. The novel architecture combines the elimination of the polymer electrolyte membrane (PEM) and the integration of the anode and cathode into one component. The thin film fabrication involves a sequential deposition of an anode catalyst layer, a cellulose acetate electronic insulating layer and a cathode catalyst layer onto a single carbon fibre paper substrate. The single electrode supported DMFC has a total thickness of 3.88 102 cm and showed a 104% improvement in volumetric specific power density over a two electrode DMFC configuration under passive conditions at ambient temperature and pressure (1 atm, 25 °C). Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Direct liquid fuel cells such as the direct methanol fuel cell (DMFC) offer the advantage of extended and continuous operation through the replacement of a fuel cartridge. Additionally, liquid fuels such as methanol have a high energy density (4820 Wh L1 [1]) and can be easily handled, stored and transported with existing infrastructure. Given the size constraints for many portable applications, the volume of a fuel cell is an important consideration. A conventional DMFC membrane electrode assembly (MEA) architecture consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. To simplify this design, the removal, replacement or integration of the electrode assembly components has been studied by various research groups. Previous work by the authors and others has shown that membraneless designs are possible [2–8]. In mixed reactant strip cells [9–13] and monolithic fuel cells [14–16] the anode and cathode have been integrated together onto the same planar side of a substrate in a side by side arrangement. In this preliminary study a single electrode supported DMFC is fabricated by a combination of PEM removal (membraneless) and electrode integration. This design is related to our previous study [2] where two electrodes are separated by a spacer in a membraneless configuration. In this configuration a gap must be maintained between the electrodes to prevent an electrical short circuit. An * Corresponding author. Tel.: +1 604 822 4888; fax: +1 604 822 6003. E-mail address: [email protected] (D.P. Wilkinson). 1 ISE Member. 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.05.049
alternative method to prevent short circuiting is to coat one of the electrode surfaces with a thin electrically non-conducting material so that the two separate electrodes are in physical but not electrical contact with each other (Fig. 1a). This configuration is an example of a two electrode system. In a single electrode supported architecture, as shown in Fig. 1b, an anode catalyst layer, an electrically insulating film (entire area) and a cathode catalyst layer are sequentially deposited onto a single carbon fibre substrate. In the research here, a cellulose acetate (CA) polymer was used as the electrically insulating film for the configurations shown in Fig. 1a and b. CA by itself does not conduct protons, however its hydrophilic properties allow for a liquid electrolyte (0.5 M H2SO4) to be soaked into its structure. This provides the ionic connection between the anode and cathode. In the operation of the fuel cell, a fuel electrolyte (5 M CH3OH and 0.5 M H2SO4) is supplied to the anode, and the cathode is open to the air. In the novel architecture, the thin film deposition allows for the fabrication of an electrode assembly that is a fraction of the thickness for a conventional two electrode architecture. Sequential ceramic deposition on a metal supported substrate has been investigated for high temperature (>400 °C) solid oxide fuel cells [17,18] but this approach has not been done for low temperature liquid fuel cells. Mixed reactant strip cells and monolithic fuel cells are other examples of PEM based single substrate fuel cells. In a strip cell, described by Barton et al [9], anode and cathode catalyst are deposited on the same planar side of Nafion Ò117 in a side by side arrangement and a mixed reactant stream is fed over the surface. A monolithic fuel cell described by Meyers et al. [14] has a similar arrangement but the fuel and oxidant are fed with separate
1531
A. Lam et al. / Electrochemistry Communications 11 (2009) 1530–1534
Fig. 1a. Schematic of a two electrode DMFC with a cellulose acetate (CA) film over the entire surface.
(39.8 wt% acetyl content; Mn 30,000) purchased from Sigma Aldrich was dissolved in acetone to 5 wt% and was deposited over the entire anode surface to thickness of 2.00 103 cm. For the single electrode supported DMFC a single Etek-TGPH060 carbon fibre paper with 20% wet proofing was used as a base substrate. The first anode layer, had a loading of 4.00 mg cm2 carbon supported (Vulcan XC-72) 40 wt% Pt–Ru (1:1 atomic ratio, or a/o) catalyst with a NafionÒ loading of 30 wt%. The electrically insulating CA film was loaded from a 5 wt% CA/acetone solution to a thickness of 2.07 103 cm over the entire active area and the final cathode layer had a loading of 1.36 mg cm2 carbon supported (Vulcan XC-72) 20 wt% Pt catalyst with a NafionÒ loading of 30 wt%. In the fabrication of a single electrode structure, short circuiting can become an issue if the cathode catalyst ink penetrates the insulating layer during the spray deposition process. Isopropyl alcohol was used as a solvent to make the catalyst ink. CA was chosen because it is soluble in acetone but not in isopropyl alcohol thus it is a barrier to penetration of the catalyst ink during fabrication. The electrode assembly was incorporated into a holder with perforated graphitic foil and Pt current collectors. A 2.0 cm2 active area single chamber glass cell at ambient temperature and pressure (25 °C, 1 atm) and an aqueous 5 M methanol/0.5 M H2SO4 anolyte was used to examine the electrode assembly performance. The polarization curves were developed with a Solartron 1420E Multistat operated in galvanostatic mode and the specific electrode potentials were monitored with a double junction saturated calomel electrode (SCE) located in the anodic chamber. The fuel cell resistance as a function of the electrode assembly configuration was recorded at an operating frequency of 1000 Hz using a Solartron 1260 FRA. 3. Results and discussion The size reduction of a fuel cell into a compact design is a key factor for the integration into portable electronic devices and other applications. This can be accomplished through the elimination
Fig. 1b. Schematic of a single electrode supported DMFC.
Carbon Fibre Paper
Cellulose Acetate
streams in adjacent channels on the same planar side. The sequential layered approach shown in this paper overcomes several disadvantages associated with both of these configurations. For instance, limitations with the area specific power density (W cm2) due to a 50% share a single substrate surface [14] and ohmic losses resulting from in-plane current collection are avoided by utilizing the entire substrate area and through plane current collection. Mixed reactant strip cells also have the added disadvantage of poor mass specific catalyst activity of the selective electrocatalysts used when compared with platinum based catalysts [9]. 2. Experimental An ink spray deposition method with an AccuSpray spray gun was used to fabricate the electrodes for a two electrode DMFC with cellulose acetate film (Fig. 1a). Etek-TGPH-060 carbon fibre paper with 20% wet proofing was used as a base substrate for the respective electrodes. The anode had a loading of 4.00 mg cm2 carbon supported (Vulcan XC-72) 40 wt% Pt–Ru (1:1 atomic ratio, or a/o) catalyst with a NafionÒ loading of 30 wt% and the cathode had a loading of 1.34 mg cm2 carbon supported (Vulcan XC-72) 20 wt% Pt catalyst with a NafionÒ loading of 30 wt% and a 1.10 mg cm2 Cabot carbon sublayer with 20 wt% PTFE. Powdered CA
Anode Catalyst Layer
Cathode Catalyst Layer
Fig. 2. SEM of a single electrode supported DMFC at a 210x magnification.
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A. Lam et al. / Electrochemistry Communications 11 (2009) 1530–1534
and/or integration of components and a reduction of certain geometric parameters (i.e., thickness and area). Previously, we have shown that it is possible to eliminate the membrane in direct liquid fuel cells by using a 3D anode structure in conjunction with a conductive fuel electrolyte. In a fuel cell a significant contribution to the overall voltage losses is attributed to ohmic overpotentials. The overall ohmic loss, gohmic (V), shown in Eq. (1), is the sum of the resistance of each component (n) in the electrode assembly multiplied by the current density, I (Acm2). Each individual resistance is a function of component thickness, ln (cm), resistivity, qn (ohmcm) and area, A (cm2).
gohmic ¼ I Roverall ¼ I
X n
Rn ¼ I
X ln q n
n
A
ð1Þ
With respect to the overall resistance, Roverall (Ohm), the electrolyte plays a significant role. A reduction in gap separation would reduce the overall resistance. In theory, a zero gap separation would eliminate the electrolyte component in Eq. (1). However, in practice there exist limitations to achieving a zero gap separation. Imperfections or highly rough surfaces will result in short circuiting of certain parts of the electrode when the electrodes are brought close together. In order to reduce the gap separation further a thin electrically non-conductive coating can be applied to the surface of the electrode. This prevents short circuiting and enables the two electrodes to be in physical but not electrical contact with each other. For this study, CA was chosen as the coating material for its ease of application onto an electrode surface and its hydrophilic and electrically
Fig. 3a. Polarization and power density curve on an area basis for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC at ambient temperature and pressure. The anode layer has a loading of 4.00 mg cm2 carbon supported (Vulcan XC-72) 40 wt% Pt–Ru and the cathode layer has a loading 1.36 mg cm2 carbon supported (Vulcan XC-72) 20 wt% Pt catalyst.
Fig. 3b. Individual reference potential of the anode and cathode for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC.
A. Lam et al. / Electrochemistry Communications 11 (2009) 1530–1534
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Fig. 4. Polarization and power density curve on a volumetric basis for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC at ambient temperature and pressure. The anode layer has a loading of 4.00 mg cm2 carbon supported (Vulcan XC-72) 40 wt% Pt–Ru and the cathode layer has a loading 1.36 mg cm2 carbon supported (Vulcan XC-72) Pt catalyst.
insulating properties. Although the proof of concept was carried out with CA, the single electrode supported DMFC is not limited by the use of this polymer. A CA layer was deposited onto the anode surface to a thickness of 2.00 103 cm for both the two electrode and single electrode supported configuration as shown in Fig. 1a and b. A distinct separation between the cathode catalyst layer and the anode catalyst layer formed by the CA layer is shown in Fig. 2 for an SEM of a single electrode supported DMFC. The CA film (entire area) represents only a small fraction of the electrode thickness and has provided an effective coating to prevent short circuiting between the anode and cathode catalyst layers. The resistance of the electrode assembly at 1000 Hz was 0.373 Ohm and 0.537 Ohm for the two electrode DMFC with a CA film (entire area) and single electrode supported architecture, respectively. One would expect a lower resistance for the thinner electrode assembly, however a higher interfacial resistance resulted when the cathode diffusion layer was removed and the current was collected directly from catalyst layer. This was confirmed by testing the resistance of the single electrode supported DMFC with an Etek TGPH-060 carbon fibre paper placed on the cathode surface. The resistance was 0.297 Ohm in this case. Fig. 3a shows, a comparison in performance of a two electrode and a single electrode supported DMFC with a similar CA film thickness of 2.00 103 cm. The power density on an area basis between the single and two electrode configuration is comparable at of 3.54 mW cm2 versus 3.02 mW cm2, respectively. To examine the individual contribution of each electrode to the overall cell voltage, a plot of the reference potentials after IR correction is shown in Fig. 3b. This plot reveals that difference in performance is primarily associated with the anode electrode. The true benefit in the integration onto a single substrate is not fully realized until the performance is normalized on a volume basis. The area specific performance in Fig. 3a for the two electrode and single electrode supported architecture was divided by the respective electrode assembly thickness of 6.69 102 cm and 3.88 102 cm. Fig. 4 shows that the single electrode supported DMFC significantly outperforms the two electrode architecture when the volume of the electrode assembly was considered. The maximum volumetric power density improved from 45.2 mW cm3 to 92.2 mW cm3.
The move toward a structure where the PEM and cathode diffusion layer is removed and all the layers are supported on a single substrate significantly reduces the electrode assembly thickness, volume and weight and overall material cost. 4. Conclusions A proof of concept single electrode supported DMFC with a total thickness of 3.88 102 cm has been successfully demonstrated. The thin film fabrication involved a sequential deposition of an anode catalyst layer, a cellulose acetate (CA) electronic insulating layer and a cathode catalyst layer onto a single carbon fibre paper substrate. The single electrode supported DMFC has significantly reduced the cost (membraneless and only one diffusion support) and manufacturing. The simple fabrication and compact nature of this electrode architecture shows promise for implementation into portable electronic devices and other applications where size is important. The maximum area specific power density was 3.54 mW cm2 and based on the electrode assembly volume, the volume specific power density was 92.2 mW cm3 under passive conditions at ambient temperature and pressure (25 °C, 1 atm). This is comparable to the 2010 Department of Energy (DOE) power density target of 100 mW cm3 for consumer electronics [19]. Further performance improvements are expected especially with respect to the material properties (e.g., optimization of the ionic conductivity in the porous insulating film). References [1] W. Qian, D.P. Wilkinson, J. Shen, H. Wang, J.J. Zhang, Journal of Power Sources 154 (2006) 202. [2] A. Lam, D.P. Wilkinson, J.J. Zhang, Electrochimica Acta 53 (2008) 6890–6898. [3] J.L. Cohen, D.A. Westly, A. Westly, H.D. Abruna, Journal of Power Sources 139 (2005) 96. [4] E.R. Choban, J.S. Spendelow, L. Gancs, A. Wieckowski, P.J.A. Kenis, Electrochimica Acta 50 (2005) 5390. [5] E.R. Choban, L.J. Markoski, A. Wieckowski, P.J.A. Kenis, Journal of Power Sources 128 (2004) 54. [6] R. Ferrigno, A.D. Stroock, T.D. Clark, M. Mayer, G.M. Whitesides, J. Am. Chem. Soc. Commun. 24 (2002) 12930. [7] F. Chen, M.H. Chang, M.K. Lin, Electrochimica Acta 52 (2007) 2506.
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