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Titanium substrate based micro-PEMFC operating under ambient conditions
Electrochemistry Communications 9 (2007) 511–516 www.elsevier.com/locate/elecom
Titanium substrate based micro-PEMFC operating under ambient conditions Nianfang Wan, Cheng Wang, Zongqiang Mao
*
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China Received 27 August 2006; received in revised form 9 October 2006; accepted 12 October 2006 Available online 14 November 2006
Abstract A novel design and fabrication technique of micro-PEMFC based on titanium substrate is described. The titanium substrate was fabricated with microflow channels using microfabrication techniques, followed by surface treating and coating with a microporous layer (MPL). It served as a combined functional component as gas diffusion layer, current collector and flow plate. A catalyst coated membrane (CCM), fabricated by directly coating catalyst layer onto Nafion 112 membrane, was integrated with the titanium substrates to assemble the micro-PEMFC. The titanium substrate based micro-PEMFC showed some unique characteristics and acceptable performance while operating under ambient conditions. 2006 Elsevier B.V. All rights reserved. Keywords: Proton exchange membrane fuel cells; Titanium substrate; Microfabrication; Self-breathing
1. Introduction Microproton exchange membrane fuel cells (PEMFCs) has become a hot research field and been paid great attentions in recent years. Microfuel cells can potentially meet the high requirements of portable electronics devices for new power sources due to the high theoretical energy density. However, some critical technical challenges have to be overcome to accelerate the commercialization. For instance, to further reduce the volume and weight, simplify components, reduce cost and be more suitable for mass production are all of significant importance in the development of micro-PEMFCs. Typically, the state-of-the-art fuel cells consist of many layered components including the electrolyte membrane, catalyst layer, gas diffusion layer (GDL) and bipolar plate. The GDL, which supports MEA, distributes gas, removes product water and provides electric conductivity, usually adopts carbon-fiber-based non-woven paper or woven *
Corresponding author. Tel.: +86 10 62780537; fax: +86 10 62771150. E-mail addresses: [email protected] (N. Wan), [email protected] (Z. Mao). 1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.10.025
cloth. However, carbon paper is mechanically weak. Excessive compression force can destroy the microstructure and reduce porosity of it, and thus lead to high mass transport overpotential [1]. The flow plate, most commonly using graphite, is intrinsically brittle, costly in flow field machining and hard to fabricate microflow channel. It takes the major volume and weight of a fuel cell stack. Therefore, for microfuel cells, many traditional materials such as carbon paper and graphite-based flow plate need to be substituted by more suitable materials. Recently, silicon-based MEMS (Micro Electromechanical Systems) technology has been introduced into microfuel cells [2–5]. It showed advantages compared with conventional fuel cell technology, however, many problems still exist such as the fragility of silicon and its high fabrication cost. In this study, we presented a novel design and fabrication process for micro-PEMFC using titanium substrate. The titanium substrate was fabricated with microflow channels by microfabrication technique. After some processes of treatment, it can serve as a combined functional component as gas diffusion layer and flow plate. A catalyst coated membrane (CCM), fabricated by directly coating catalyst layer onto Nafion 112 membrane, was integrated with the
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N. Wan et al. / Electrochemistry Communications 9 (2007) 511–516
titanium substrates to assemble the micro-PEMFC. Titanium has good corrosion resistance, high electrical conductivity, high mechanical strength, good thermal conductivity, relatively low specific gravity and low cost. Microfabrication technique can offer precision, accuracy and good repeatability for fabrication of microflow channels. To the best of our knowledge, although there are many publications concerning different metal sheets as flow plate for fuel cells, using titanium substrate fabricated by microfabrication technique has not been reported before. The unique physical properties and combined functions of the titanium substrate are expected to contribute some unique features to micro-PEMFC such as the high mechanical strength, compactness and lightweight. The performance of the microH2/air fuel cell based on this titanium substrate was measured under ambient conditions. To date, acceptable performance results have been reached. 2. Experimental The titanium substrate employed was 100 lm thick. Photochemical wet etching technique was adopted to fabricate microchannels on it. The major fabrication processes consist of patterning and etching. The etching solution was composed of HF and HNO3. Titanium is prone to form an electrical-insulating layer on its surface. Therefore, the porous titanium substrate was etched and coated with a Ru layer prior to use. After that, a microporous layer composed of 3.5 mg cm 2 Vulcan XC-72 carbon and 40 wt.% PTFE was coated onto the titanium substrate. The Vulcan XC-72 carbon was firstly dispersed in deionized water, followed by adding PTFE emulsion and mixing uniformly in ultrasonic bath. The ink was coated onto the substrate and dried at 100 C. It was repeated several times until the desired amount was achieved. Lastly the coated titanium substrate was sintered at 345 C for 30 min. A catalyst coated membrane (CCM) with geometric active area of 4 cm2 (2 cm · 2 cm) was fabricated by directly coating catalyst layer onto Nafion 112 membrane. The catalyst ink was prepared by dispersing Pt/C (47.8%, Tanaka), isopropanol, and Nafion dispersion (5%, DuPont). The membrane was put on a vacuum table and heated while the catalyst ink was spraying. Lastly, the coated membrane was dried in a vacuum oven at 130 C for 1 h. The amount of Pt loading was about 0.4 mg cm 2 at both anode and cathode. The integration process of a single micro-PEMFC was as follows: the CCM was sandwiched between the two treated titanium substrates, followed by hot-pressing at 9 MPa, 140 C for 2 min. The measurement of the micro-PEMFC was all under ambient conditions. For all of the tests, pure hydrogen and air at normal pressure without pre-heating and prehumidifying were used as fuel and oxidant, respectively. The H2/air self-breathing fuel cell was assembled in a hardware consisting of two plastic end plates [6]. The cathode side was open directly to the environment. Hydrogen was conducted into a void flow field with dead-ended stream.
For another test with a forced flow velocity of air, the testing hardware consisted of graphite plates and Au coated stainless steel end plates. The flow rate was adjusted by a mass flowmeter. All of the current–potential (I–V) characteristics were measured by an electronic load system. A JSM-6301F Scanning electron microscope was adopted to examine the microstructure of the titanium substrate and the morphology of CCM. 3. Results and discussion Fig. 1 shows the microstructure of flow channels. The width of the channels was 150 lm and the ribs were 80 lm wide. The ratio of opening area of the titanium substrate was about 65%. The geometric active area was 4 cm2 (2 cm · 2 cm). This configuration of flow field showed the best performance among several different designs. By using the microfabrication technique, other types of flow fields can be patterned and precisely fabricated to further improve the fuel cell performance. There have been numerous studies on different flow fields design and optimization including modeling and experiments for fuel cells with a forced reactant supply mode. However for passive microfuel cells, these studies seem much less. In the later work of designing more smart flow field for completely passive fuel cells, several factors should be considered: Firstly, a large portion of the electrode area exposed to its surroundings must be kept. Without active movement of the reactant, the mass transport to the electrode active zones is mainly by diffusion not convection. So, the ratio of opening area of the titanium substrate should be as large as possible to reduce the mass transport overpotential. Secondly, the ribs of the titanium substrate should be small in size and uniformly distributed to maintain more uniform reactant concentration. Large size of ribs may block the mass diffusion and lead to dead zones [7]. However, the mechanical strength of the titanium ribs may be weakened with the
Fig. 1. Scanning electron micrograph (SEM) image of microchannels of titanium substrate fabricated by photochemical etching technique.
N. Wan et al. / Electrochemistry Communications 9 (2007) 511–516
decreasing size. Lastly, other factors which may have effects on the performance also need to be focused. For instance, considering the function as current collector, the in-plane electrical resistance of the titanium and the electrode should be reduced. This requires the ribs be uniformly distributed and the channel size be shortened to make a short path of electron transfer. However, cathode flooding is likely to happen with the reducing size of channels [8] and optimization is needed. Fig. 2 gives the SEM images of the fabricated titanium substrate with a MPL. It can be seen that the cross-section of the ribs of the titanium (Fig. 2a) was nearly hexagon. The MPL was filled into the channels and onto the surface of the titanium substrate. Fig. 2b shows the morphology of the surface of MPL. It displays a microporous structure, which is essential to the mass transport. The adding of PTFE can increase the hydrophobicity of MPL, provide capillary wicking of water from the catalyst layer and facilitate the removal of product water.
513
The total thickness of the metal-based miniature fuel cell fabricated was about 280 lm. Although it was significantly thinner than a conventional single fuel cell, usually several millimeters, it showed high mechanical strength. The high compression force of fuel cell hardware with flow field could not easily deform it due to the high mechanical strength of the supporting titanium substrate. It is known that high compression force on fuel cell can reduce the interfacial resistance and ensure good sealing. This is significantly important to obtain high performance of fuel cell. The titanium ribs can sustain most of the compression force, which means the pressure shared on the MPL is greatly alleviated, so the MPL can keep the porosity even with high compression. As a consequence, the mass transport in MPL cannot be impaired even with excessive compression force. The titanium substrate fabricated had good electronic conductivity. The electrical conductivity of titanium is several orders of magnitude higher than carbon. Although the titanium substrate was porous, when coated
Fig. 2. SEM images of fabricated titanium substrate with MPL: (a) cross-section, (b) surface.
Fig. 3. SEM images of the morphology of CCM: (a) cross-section, (b) surface.
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with a MPL, its conductivity was still much higher than a typical carbon paper. This was confirmed by simply measuring the electrical resistance with a microohm meter. Fig. 3 shows the morphology of CCM. It can be seen from Fig. 3a that the catalyst layer was in intimate contact with the membrane. An intimate interface between the catalyst layer and the membrane contributes to good mass and charge transfer and low resistance. The thickness of the catalyst layer was nearly uniform. From Fig. 3b, we can see numerous catalyst agglomerates and micropores uniformly distributed in the catalyst layer. The performance of the titanium-based H2/air fuel cell with a fixed flow rate of air is showed in Fig. 4. Under ambient pressure and at 22 C, the maximum power density was above 220 mW cm 2. At 0.6 V, the current density was 250 mA cm 2. In the current density regime ranging from 50 to 650 mW cm 2, the slope of the I–V curve was nearly linear, exhibiting a typical ohmic polarization. At high current density regime even above 600 mA cm 2, no obvious limiting current density can be seen. This showed the titanium-based H2/air fuel cell did not suffer from the oxygen mass transport limitation during the process of measurement. Fig. 5 shows the performance under air self-breathing conditions. The fuel cell worked under completely passive conditions. The cathode was directly open to the surroundings and self-breath the air. Due to the good mechanical strength of titanium-based PEMFC, the flow field of the anode was void, without any supporting ridge. This enlarges the exposed area of the electrodes as well as simplifies the flow field design and fabrication processes of fuel cell. Current was collected by the edge of the titanium substrate. The maximum power density of fuel cell operating in this way was about 120 mW cm 2. Comparing to the flowing gas supply mode, the near half power loss can be primarily attributed to the slower oxygen transport of the
self-breathing cathode. A forced flow velocity of air can facilitate the access of oxygen to the reaction site. Especially, gas flow velocity will increase as the size of flow channels decreases. This further increases the oxygen delivery rate. In a passive mode without forced air flow velocity, oxygen diffuse to the active site driven by the concentration gradient and the delivery rate is much slower. Additionally, for a self-breathing cathode, the removal of liquid water to surroundings is mainly by evaporation. While for a forced flow one, liquid water can be removed not only by quicker convective evaporation but also by the shear force exerted by the high gas flow velocity [9]. As a result, more water will be accumulated at a self-breathing cathode and block the transport of reactant. At the anode, while operating at a self-breathing mode, the hydrogen supply is in a dead-ended stream. After the fuel cell works for a while, little amount of water may also be accumulated at the anode. So, the mass transport limitation may also exist at the anode. However, due to the higher diffusion velocity of hydrogen, it is less serious compared to the situation at the cathode. An intermittent disclosing the after-reaction products, separation and recycle of the fuel may be beneficial to the removal of water and to get high fuel utilization. The long-term performance of the microfuel cell was demonstrated in Fig. 6. The cell worked at an intermittent mode. Usually, it worked continuously during daytime, shut down at night and restarted up next morning. The cell voltage was recorded at 250 mA cm 2. It was seen that the cell voltage fluctuated slightly during the accumulated operation time of about 200 h, between 0.64 V and 0.58 V. Note that the fuel cell worked under ambient conditions. The temperature, air humidity and water content of the membrane were not constant, changing with the environment and working conditions. This led to the fluctuation of the performance. Nonetheless, there was no obvious degradation observed during the operation period.
Fig. 4. Performance of micro-H2/air fuel cell (flow of H2: 30 ml/min, flow of air: 80 ml min 1, 22 C and at normal pressure).
N. Wan et al. / Electrochemistry Communications 9 (2007) 511–516
515
Fig. 5. Performance of micro-self-breathing fuel cell (about 22 C and at normal pressure).
4. Conclusions
Fig. 6. The long-term performance of microfuel cell (flow of H2: 20 ml min 1, flow of air: 50 ml min 1, ambient temperature and normal pressure, at current density of 250 mA cm 2).
In this study, factors affecting the life durability should be specially focused on the corrosion problem of metal substrate and the contact resistance between the MEA and the titanium substrate. It has been showed that the contact resistance between the untreated titanium sinter and MEA is quite large and platinum coating on the titanium sinter can greatly reduce the contact resistance [10]. However, platinum is high cost. In this study, less cost Ru was adopted as a coating material. Although it showed acceptable performance and durability in the 200 h operation, the long-term corrosion resistance and contact resistance between the MEA and the titanium substrate need to be further tested. Furthermore, more economical, corrosion resistive and lower contact resistance surface-treatment techniques on titanium are left to be explored.
A micro-PEMFC adopting titanium substrates with a total thickness of 280 lm was successfully fabricated. By using microfabrication techniques, the titanium substrate was fabricated with smart microflow channels. The MPL coated titanium substrates was used as a multifunctional component as GDL, current collector and flow plate. The micro-PEMFC based on this titanium substrate showed high mechanical strength, good mass transport, and compactness and lightweight. The performance of the microPEMFC was evaluated both under completely self-breathing conditions and with a forced flow velocity of reactant under ambient pressure and at 22 C. For the self-breathing H2/air PEMFC, the maximum power density of about 120 mW cm 2 has been achieved so far. While under the latter conditions, the maximum power density has exceeded 220 mW cm 2 due to the faster mass transport. The microPEMFC showed no obvious drop in performance after 200 h operation. These results demonstrated that the titanium-based micro-PEMFC had the potential to be an efficient power source for portable applications. References [1] W.-K. Lee, C.-H. Ho, J.W. Van Zee, M. Murphy, J. Power Sources 84 (1999) 45. [2] S.C. Kelley, G.A. Deluga, W.H. Smyrl, Electrochem. Solid State Lett. 3 (9) (2000) 407. [3] Jeremy P. Meyers, Helen L. Maynard, J. Power Sources 109 (2002) 76. [4] Shinji Motokawa, Mohamed Mohamedi, Toshiyuki Momma, Shuichi Shoji, Tetsuya Osaka, Electrochem. Commun. 6 (2004) 562. [5] Jingrong Yu, Ping Cheng, Zhiqi Ma, Baolian Yi, Electrochim. Acta 48 (2003) 1537. [6] Nianfang Wan, Gang Wang, J. Power Sources 159 (2006) 951. [7] S.W. Cha, S.J. Lee, Y.I. Park, Y. Saito, F.B. Prinz, Proceedings of the 1st International Conference on Fuel Cell Science, Engineering and
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N. Wan et al. / Electrochemistry Communications 9 (2007) 511–516
Technology, American Society of Mechanical Engineers, Rochester, NY, 2003, p. 143. [8] Suk Won Cha, Ryan O’Hayre, Fritz B. Prinz, Solid State Ionics 175 (2004) 789.
[9] F.Y. Zhang, X.G. Yang, C.Y. Wang, J. Electrochem. Soc. 153 (2) (2006) A225. [10] T. Hottinen, M. Mikkola, T. Mennola, P. Lund, J. Power Sources 118 (2003) 183.
Titanium substrate based micro-PEMFC operating under ambient conditions Nianfang Wan, Cheng Wang, Zongqiang Mao
*
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China Received 27 August 2006; received in revised form 9 October 2006; accepted 12 October 2006 Available online 14 November 2006
Abstract A novel design and fabrication technique of micro-PEMFC based on titanium substrate is described. The titanium substrate was fabricated with microflow channels using microfabrication techniques, followed by surface treating and coating with a microporous layer (MPL). It served as a combined functional component as gas diffusion layer, current collector and flow plate. A catalyst coated membrane (CCM), fabricated by directly coating catalyst layer onto Nafion 112 membrane, was integrated with the titanium substrates to assemble the micro-PEMFC. The titanium substrate based micro-PEMFC showed some unique characteristics and acceptable performance while operating under ambient conditions. 2006 Elsevier B.V. All rights reserved. Keywords: Proton exchange membrane fuel cells; Titanium substrate; Microfabrication; Self-breathing
1. Introduction Microproton exchange membrane fuel cells (PEMFCs) has become a hot research field and been paid great attentions in recent years. Microfuel cells can potentially meet the high requirements of portable electronics devices for new power sources due to the high theoretical energy density. However, some critical technical challenges have to be overcome to accelerate the commercialization. For instance, to further reduce the volume and weight, simplify components, reduce cost and be more suitable for mass production are all of significant importance in the development of micro-PEMFCs. Typically, the state-of-the-art fuel cells consist of many layered components including the electrolyte membrane, catalyst layer, gas diffusion layer (GDL) and bipolar plate. The GDL, which supports MEA, distributes gas, removes product water and provides electric conductivity, usually adopts carbon-fiber-based non-woven paper or woven *
Corresponding author. Tel.: +86 10 62780537; fax: +86 10 62771150. E-mail addresses: [email protected] (N. Wan), [email protected] (Z. Mao). 1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.10.025
cloth. However, carbon paper is mechanically weak. Excessive compression force can destroy the microstructure and reduce porosity of it, and thus lead to high mass transport overpotential [1]. The flow plate, most commonly using graphite, is intrinsically brittle, costly in flow field machining and hard to fabricate microflow channel. It takes the major volume and weight of a fuel cell stack. Therefore, for microfuel cells, many traditional materials such as carbon paper and graphite-based flow plate need to be substituted by more suitable materials. Recently, silicon-based MEMS (Micro Electromechanical Systems) technology has been introduced into microfuel cells [2–5]. It showed advantages compared with conventional fuel cell technology, however, many problems still exist such as the fragility of silicon and its high fabrication cost. In this study, we presented a novel design and fabrication process for micro-PEMFC using titanium substrate. The titanium substrate was fabricated with microflow channels by microfabrication technique. After some processes of treatment, it can serve as a combined functional component as gas diffusion layer and flow plate. A catalyst coated membrane (CCM), fabricated by directly coating catalyst layer onto Nafion 112 membrane, was integrated with the
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N. Wan et al. / Electrochemistry Communications 9 (2007) 511–516
titanium substrates to assemble the micro-PEMFC. Titanium has good corrosion resistance, high electrical conductivity, high mechanical strength, good thermal conductivity, relatively low specific gravity and low cost. Microfabrication technique can offer precision, accuracy and good repeatability for fabrication of microflow channels. To the best of our knowledge, although there are many publications concerning different metal sheets as flow plate for fuel cells, using titanium substrate fabricated by microfabrication technique has not been reported before. The unique physical properties and combined functions of the titanium substrate are expected to contribute some unique features to micro-PEMFC such as the high mechanical strength, compactness and lightweight. The performance of the microH2/air fuel cell based on this titanium substrate was measured under ambient conditions. To date, acceptable performance results have been reached. 2. Experimental The titanium substrate employed was 100 lm thick. Photochemical wet etching technique was adopted to fabricate microchannels on it. The major fabrication processes consist of patterning and etching. The etching solution was composed of HF and HNO3. Titanium is prone to form an electrical-insulating layer on its surface. Therefore, the porous titanium substrate was etched and coated with a Ru layer prior to use. After that, a microporous layer composed of 3.5 mg cm 2 Vulcan XC-72 carbon and 40 wt.% PTFE was coated onto the titanium substrate. The Vulcan XC-72 carbon was firstly dispersed in deionized water, followed by adding PTFE emulsion and mixing uniformly in ultrasonic bath. The ink was coated onto the substrate and dried at 100 C. It was repeated several times until the desired amount was achieved. Lastly the coated titanium substrate was sintered at 345 C for 30 min. A catalyst coated membrane (CCM) with geometric active area of 4 cm2 (2 cm · 2 cm) was fabricated by directly coating catalyst layer onto Nafion 112 membrane. The catalyst ink was prepared by dispersing Pt/C (47.8%, Tanaka), isopropanol, and Nafion dispersion (5%, DuPont). The membrane was put on a vacuum table and heated while the catalyst ink was spraying. Lastly, the coated membrane was dried in a vacuum oven at 130 C for 1 h. The amount of Pt loading was about 0.4 mg cm 2 at both anode and cathode. The integration process of a single micro-PEMFC was as follows: the CCM was sandwiched between the two treated titanium substrates, followed by hot-pressing at 9 MPa, 140 C for 2 min. The measurement of the micro-PEMFC was all under ambient conditions. For all of the tests, pure hydrogen and air at normal pressure without pre-heating and prehumidifying were used as fuel and oxidant, respectively. The H2/air self-breathing fuel cell was assembled in a hardware consisting of two plastic end plates [6]. The cathode side was open directly to the environment. Hydrogen was conducted into a void flow field with dead-ended stream.
For another test with a forced flow velocity of air, the testing hardware consisted of graphite plates and Au coated stainless steel end plates. The flow rate was adjusted by a mass flowmeter. All of the current–potential (I–V) characteristics were measured by an electronic load system. A JSM-6301F Scanning electron microscope was adopted to examine the microstructure of the titanium substrate and the morphology of CCM. 3. Results and discussion Fig. 1 shows the microstructure of flow channels. The width of the channels was 150 lm and the ribs were 80 lm wide. The ratio of opening area of the titanium substrate was about 65%. The geometric active area was 4 cm2 (2 cm · 2 cm). This configuration of flow field showed the best performance among several different designs. By using the microfabrication technique, other types of flow fields can be patterned and precisely fabricated to further improve the fuel cell performance. There have been numerous studies on different flow fields design and optimization including modeling and experiments for fuel cells with a forced reactant supply mode. However for passive microfuel cells, these studies seem much less. In the later work of designing more smart flow field for completely passive fuel cells, several factors should be considered: Firstly, a large portion of the electrode area exposed to its surroundings must be kept. Without active movement of the reactant, the mass transport to the electrode active zones is mainly by diffusion not convection. So, the ratio of opening area of the titanium substrate should be as large as possible to reduce the mass transport overpotential. Secondly, the ribs of the titanium substrate should be small in size and uniformly distributed to maintain more uniform reactant concentration. Large size of ribs may block the mass diffusion and lead to dead zones [7]. However, the mechanical strength of the titanium ribs may be weakened with the
Fig. 1. Scanning electron micrograph (SEM) image of microchannels of titanium substrate fabricated by photochemical etching technique.
N. Wan et al. / Electrochemistry Communications 9 (2007) 511–516
decreasing size. Lastly, other factors which may have effects on the performance also need to be focused. For instance, considering the function as current collector, the in-plane electrical resistance of the titanium and the electrode should be reduced. This requires the ribs be uniformly distributed and the channel size be shortened to make a short path of electron transfer. However, cathode flooding is likely to happen with the reducing size of channels [8] and optimization is needed. Fig. 2 gives the SEM images of the fabricated titanium substrate with a MPL. It can be seen that the cross-section of the ribs of the titanium (Fig. 2a) was nearly hexagon. The MPL was filled into the channels and onto the surface of the titanium substrate. Fig. 2b shows the morphology of the surface of MPL. It displays a microporous structure, which is essential to the mass transport. The adding of PTFE can increase the hydrophobicity of MPL, provide capillary wicking of water from the catalyst layer and facilitate the removal of product water.
513
The total thickness of the metal-based miniature fuel cell fabricated was about 280 lm. Although it was significantly thinner than a conventional single fuel cell, usually several millimeters, it showed high mechanical strength. The high compression force of fuel cell hardware with flow field could not easily deform it due to the high mechanical strength of the supporting titanium substrate. It is known that high compression force on fuel cell can reduce the interfacial resistance and ensure good sealing. This is significantly important to obtain high performance of fuel cell. The titanium ribs can sustain most of the compression force, which means the pressure shared on the MPL is greatly alleviated, so the MPL can keep the porosity even with high compression. As a consequence, the mass transport in MPL cannot be impaired even with excessive compression force. The titanium substrate fabricated had good electronic conductivity. The electrical conductivity of titanium is several orders of magnitude higher than carbon. Although the titanium substrate was porous, when coated
Fig. 2. SEM images of fabricated titanium substrate with MPL: (a) cross-section, (b) surface.
Fig. 3. SEM images of the morphology of CCM: (a) cross-section, (b) surface.
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with a MPL, its conductivity was still much higher than a typical carbon paper. This was confirmed by simply measuring the electrical resistance with a microohm meter. Fig. 3 shows the morphology of CCM. It can be seen from Fig. 3a that the catalyst layer was in intimate contact with the membrane. An intimate interface between the catalyst layer and the membrane contributes to good mass and charge transfer and low resistance. The thickness of the catalyst layer was nearly uniform. From Fig. 3b, we can see numerous catalyst agglomerates and micropores uniformly distributed in the catalyst layer. The performance of the titanium-based H2/air fuel cell with a fixed flow rate of air is showed in Fig. 4. Under ambient pressure and at 22 C, the maximum power density was above 220 mW cm 2. At 0.6 V, the current density was 250 mA cm 2. In the current density regime ranging from 50 to 650 mW cm 2, the slope of the I–V curve was nearly linear, exhibiting a typical ohmic polarization. At high current density regime even above 600 mA cm 2, no obvious limiting current density can be seen. This showed the titanium-based H2/air fuel cell did not suffer from the oxygen mass transport limitation during the process of measurement. Fig. 5 shows the performance under air self-breathing conditions. The fuel cell worked under completely passive conditions. The cathode was directly open to the surroundings and self-breath the air. Due to the good mechanical strength of titanium-based PEMFC, the flow field of the anode was void, without any supporting ridge. This enlarges the exposed area of the electrodes as well as simplifies the flow field design and fabrication processes of fuel cell. Current was collected by the edge of the titanium substrate. The maximum power density of fuel cell operating in this way was about 120 mW cm 2. Comparing to the flowing gas supply mode, the near half power loss can be primarily attributed to the slower oxygen transport of the
self-breathing cathode. A forced flow velocity of air can facilitate the access of oxygen to the reaction site. Especially, gas flow velocity will increase as the size of flow channels decreases. This further increases the oxygen delivery rate. In a passive mode without forced air flow velocity, oxygen diffuse to the active site driven by the concentration gradient and the delivery rate is much slower. Additionally, for a self-breathing cathode, the removal of liquid water to surroundings is mainly by evaporation. While for a forced flow one, liquid water can be removed not only by quicker convective evaporation but also by the shear force exerted by the high gas flow velocity [9]. As a result, more water will be accumulated at a self-breathing cathode and block the transport of reactant. At the anode, while operating at a self-breathing mode, the hydrogen supply is in a dead-ended stream. After the fuel cell works for a while, little amount of water may also be accumulated at the anode. So, the mass transport limitation may also exist at the anode. However, due to the higher diffusion velocity of hydrogen, it is less serious compared to the situation at the cathode. An intermittent disclosing the after-reaction products, separation and recycle of the fuel may be beneficial to the removal of water and to get high fuel utilization. The long-term performance of the microfuel cell was demonstrated in Fig. 6. The cell worked at an intermittent mode. Usually, it worked continuously during daytime, shut down at night and restarted up next morning. The cell voltage was recorded at 250 mA cm 2. It was seen that the cell voltage fluctuated slightly during the accumulated operation time of about 200 h, between 0.64 V and 0.58 V. Note that the fuel cell worked under ambient conditions. The temperature, air humidity and water content of the membrane were not constant, changing with the environment and working conditions. This led to the fluctuation of the performance. Nonetheless, there was no obvious degradation observed during the operation period.
Fig. 4. Performance of micro-H2/air fuel cell (flow of H2: 30 ml/min, flow of air: 80 ml min 1, 22 C and at normal pressure).
N. Wan et al. / Electrochemistry Communications 9 (2007) 511–516
515
Fig. 5. Performance of micro-self-breathing fuel cell (about 22 C and at normal pressure).
4. Conclusions
Fig. 6. The long-term performance of microfuel cell (flow of H2: 20 ml min 1, flow of air: 50 ml min 1, ambient temperature and normal pressure, at current density of 250 mA cm 2).
In this study, factors affecting the life durability should be specially focused on the corrosion problem of metal substrate and the contact resistance between the MEA and the titanium substrate. It has been showed that the contact resistance between the untreated titanium sinter and MEA is quite large and platinum coating on the titanium sinter can greatly reduce the contact resistance [10]. However, platinum is high cost. In this study, less cost Ru was adopted as a coating material. Although it showed acceptable performance and durability in the 200 h operation, the long-term corrosion resistance and contact resistance between the MEA and the titanium substrate need to be further tested. Furthermore, more economical, corrosion resistive and lower contact resistance surface-treatment techniques on titanium are left to be explored.
A micro-PEMFC adopting titanium substrates with a total thickness of 280 lm was successfully fabricated. By using microfabrication techniques, the titanium substrate was fabricated with smart microflow channels. The MPL coated titanium substrates was used as a multifunctional component as GDL, current collector and flow plate. The micro-PEMFC based on this titanium substrate showed high mechanical strength, good mass transport, and compactness and lightweight. The performance of the microPEMFC was evaluated both under completely self-breathing conditions and with a forced flow velocity of reactant under ambient pressure and at 22 C. For the self-breathing H2/air PEMFC, the maximum power density of about 120 mW cm 2 has been achieved so far. While under the latter conditions, the maximum power density has exceeded 220 mW cm 2 due to the faster mass transport. The microPEMFC showed no obvious drop in performance after 200 h operation. These results demonstrated that the titanium-based micro-PEMFC had the potential to be an efficient power source for portable applications. References [1] W.-K. Lee, C.-H. Ho, J.W. Van Zee, M. Murphy, J. Power Sources 84 (1999) 45. [2] S.C. Kelley, G.A. Deluga, W.H. Smyrl, Electrochem. Solid State Lett. 3 (9) (2000) 407. [3] Jeremy P. Meyers, Helen L. Maynard, J. Power Sources 109 (2002) 76. [4] Shinji Motokawa, Mohamed Mohamedi, Toshiyuki Momma, Shuichi Shoji, Tetsuya Osaka, Electrochem. Commun. 6 (2004) 562. [5] Jingrong Yu, Ping Cheng, Zhiqi Ma, Baolian Yi, Electrochim. Acta 48 (2003) 1537. [6] Nianfang Wan, Gang Wang, J. Power Sources 159 (2006) 951. [7] S.W. Cha, S.J. Lee, Y.I. Park, Y. Saito, F.B. Prinz, Proceedings of the 1st International Conference on Fuel Cell Science, Engineering and
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N. Wan et al. / Electrochemistry Communications 9 (2007) 511–516
Technology, American Society of Mechanical Engineers, Rochester, NY, 2003, p. 143. [8] Suk Won Cha, Ryan O’Hayre, Fritz B. Prinz, Solid State Ionics 175 (2004) 789.
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