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Porous current collectors for passive direct methanol fuel cells
Electrochimica Acta 52 (2007) 4317–4324
Porous current collectors for passive direct methanol fuel cells R. Chen, T.S. Zhao ∗ Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Received 4 October 2006; received in revised form 5 December 2006; accepted 5 December 2006 Available online 17 December 2006
Abstract A passive direct methanol fuel cell (DMFC) with its cathode current collector made of porous metal foam was investigated experimentally. The measured polarization curves, constant-current discharging behavior and EIS spectra showed that the passive DMFC having the porous current collector yielded much higher and much more stable performance than did the cell having the conventional perforated-plate current collector with high methanol concentration operation. It was demonstrated that the improved performance for the porous current collector was attributed to: (i) the enhanced oxygen transport on the cathode as a result of a larger specific transport area, (ii) the increased operating temperature as a result of the lower effective thermal conductivity of the porous structure, and (iii) the faster water removal as a result of the capillary action in the porous structure, The experimental results also revealed that the porous current collector with a smaller pore size yielded higher performance as a result of the lower cell resistance. © 2006 Elsevier Ltd. All rights reserved. Keywords: Fuel cell; Passive DMFC; Metal foam; Effective thermal conductivity; Cell performance; Oxygen transport
1. Introduction The high energy density of liquid methanol as well as simple and compact power systems make the direct methanol fuel cells (DMFC) a promising power source for portable electronic devices. Over the past decade, extensive efforts [1–8] have been made to the study of the DMFC with the fuel fed by a liquid pump and oxidant fed by a gas compressor. In order to make the DMFC more competitive with conventional battery technologies, however, it is essential to eliminate some auxiliary devices such as liquid pumps and gas fans/blowers so that the overall DMFC system becomes much simpler and much more compact. For this reason, the concept of a passive DMFC without external pumps and other ancillary devices has been proposed and studied [9–18]. A typical passive-feed DMFC consists of a fuel reservoir, an anode current collector, a membrane electrode assembly (MEA), and a cathode current collector. Methanol is introduced to the active layer primarily by diffusion without any external means of liquid transport and oxygen is taken passively from the ambient air without any means of air movement. This type of passive DMFCs not only offers the advantage of simple
∗
Corresponding author. Tel.: +852 2358 8647; fax: +852 2358 1543. E-mail address: [email protected] (T.S. Zhao).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.12.015
and compact systems but also makes it possible to eliminate the parasitic power losses for powering ancillary devices required in active DMFCs. Because of these advantages, the passive DMFC has received much more attention. Chen and Yang [10] investigated the effect of operating conditions on the power density of an air-breathing DMFC. Liu et al. [11] studied sintered stainless steel fiber felt as the gas diffusion layer in an air-breathing DMFC. The effect of methanol concentration was also studied in this work. Kim et al. [12] fabricated and tested a single cell and monopolar DMFC stack operating under passive and airbreathing conditions. Shimizu et al. [13] reported their activities regarding the research and development of DMFCs that operated passively at room temperature. Park et al. [15] reported the optimal methanol solution appeared at 4.0 M in passive DMFCs. Kho et al. [17] investigated the variation in the open circuit voltage (OCV) and the cell temperature with time as a consequence of the methanol crossover. The previous studies have indicated that the passive DMFC usually has to be operated with higher methanol concentration, as the methanol transport from a built-in fuel reservoir to the anode catalyst layer in this type of passive fuel cell relies on diffusion. Since there is plenty of room up to pure methanol, methanol transport on the anode is actually not a problem. Rather, higher methanol concentration offers the advantage of increasing the energy density of the fuel cell, provided that the
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rate of methanol crossover can effectively be reduced. In contrast, the problem of mass transport of oxygen and water on the cathode is more challenging, as this type of fuel cell operates on the air-breathing mode and there is no external means to enhance oxygen transport and water removal. As a result, the passive fuel cell frequently operates under the oxygen-starving and water-flooding conditions. In addition, it was recently found that passive DMFCs operate at near room temperature and it is important to increase the cell operating temperature so that electrochemical kinetics of both methanol oxidation and oxygen reduction reactions can be improved [18,19]. In summary, on the constraint without any external means of air movement, it is critical to design and optimize the architecture of the passive DMFC to ensure higher oxygen transfer and water removal rates as well as a higher cell operating temperature. The cathode current collector is one of the key components of the passive DMFC, which not only collects the electric current but also offers the passages for oxygen and water transport. Furthermore, since in the passive DMFC, the heat generated within the cell is predominately lost from the cathode current collector, it is essential to employ a material with low effective thermal conductivity as the cathode current collector such that the heat loss can be lowered and the cell can be operated at a higher temperature to achieve better performance. Therefore, in the passive DMFC, the cathode current collector must have high electric conductivity, good mechanical strength, more uniform transport area and low effective thermal conductivity. In this work, a passive DMFC with its cathode current collector made of porous metal foam was investigated experimentally. We show that this type of porous current collector can not only provide a higher oxygen transfer rate and a more effective water removal rate, but also can render a higher cell operating temperature.
As a consequence, the cell having this type of porous current collector yielded significantly higher performance and much more stable operation than did the cell having the conventional perforated-plate current collector. 2. Experimental 2.1. Membrane and electrode assembly (MEA) A pretreated Nafion 115 membrane with a thickness of 125 m was employed in this work. The pretreatment procedures included boiling the membrane in 5 vol.% H2 O2 , washing in DI water, boiling in 0.5 M H2 SO4 and washing in DI water for 1 h in turn. The pretreated membranes were kept in the DI water prior to the fabrication of MEAs. Single-side ELAT electrodes from ETEK were used in both anode and cathode, where carbon cloth (E-TEK, Type A) were used as the backing support layer with 30 wt% PTFE wet-proofing treatment. The catalyst loading on the anode side was 4.0 mg/cm2 with PtRu black (1:1 a/o), while the catalyst loading on the cathode side was 2.0 mg/cm2 using 40 wt% Pt on Vulcan XC-72. Furthermore, 0.8 mg/cm2 dry Nafion® ionomer was coated onto the surface of each electrode. Finally, MEA with an active area of 4.0 cm2 were fabricated by hot pressing at 135 ◦ C and 4 MPa for 3.0 min. More detailed information about the MEA fabrication can be found elsewhere [20]. 2.2. Single cell fixture As shown in Fig. 1, the MEA mentioned above was sandwiched between an anode and a cathode current collector. The entire cell setup was then held together between an anode and a
Fig. 1. Schematic of the passive DMFC.
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cathode fixture, both of which were made of transparent organic glass. A 5.0 mL methanol solution reservoir was built in the anode fixture. Methanol was transferred into the catalyst layer from the built-in reservoir, while oxygen, from the surrounding air, was transferred into the cathode catalyst layer through the opening of the cathode fixture. The cell temperature was measured by a thermocouple (Type T), which was installed on the outer surface of the anode gas diffusion layer. The anode current collector was made of a perforated 316L stainless steel plate of 1.5 mm in thickness. Two different types of cathode current
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collectors, as shown in Fig. 2, were fabricated and tested. One was the conventional perforated-plate current collector made of a 316L stainless steel plate, while the other was the porous current collector made of a Ni–Cr alloy metal foam plate of 1.0 mm in thickness. The Ni–Cr alloy metal foam supplied by the RECEMAT® International offers over 95% porosity and the estimated average pore diameter of 0.4 mm. A plurality of 2.6 mm-circular holes was drilled in both the current collectors, serving as the passages of fuel and oxidant, which resulted in an open ratio of 47.8%. A 200 nm platinum layer was sputtered onto the surface of the perforated-plate and porous current collectors to reduce the contact resistance with the electrodes. 2.3. Electrochemical instrumentation and test conditions An Arbin BT2000 electrical load interfaced to a computer was employed to control the condition of discharging and record the voltage–current curves. For each discharging current point along the I–V curve, a 60 s waiting time was used to obtain the stable voltage. The temperature of the cell was measured by the Arbin BT2000 built-in function. All the experiments of the passive DMFCs were performed at room temperatures of 21.8–22.6 ◦ C and the relative humidity of 63–75%. Prior to the performance test, the MEA was installed in an active cell fixture and activated at 70 ◦ C about 24 h. During the activation period, 1.0 M methanol was fed at 1.0 mL min−1 , while oxygen was supplied under atmospheric pressure at a flow rate of 50 mL min−1 . 3. Results and discussion Fig. 3 compares the cell performance of the porous and perforated-plate current collectors with 2.0 M methanol concentration operation. The polarization curves at low current densities do not show too much difference between both the current collectors. With increasing current density, the voltage with the porous current collector became lower than that with the perforated-plate current collector. However, at high current
Fig. 2. Schematic of the current collector. (a) Perforated-plate current collector and (b) Porous current collector.
Fig. 3. Comparison in cell performance between different cathode current collectors. Anode: 2.0 and 4.0 M methanol; cathode: room temperature: 22.1 ◦ C, relative humidity: 75%.
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densities, the perforated-plate current collector tended to show a rapid decline in voltage, implying that the corresponding limiting current density of the perforated-plate current collector would be lower than that of the porous current collector. The performance behavior shown in Fig. 3 for lower methanol concentration can be explained as follows. The cathode current collector affects the performance of the passive DMFC in two major aspects: oxygen transport and water removal (flooding) on the cathode. In addition, different types of current collectors may cause different internal electrical resistances due to the contact between the gas diffusion layer and the current collector. Since the rate of methanol crossover is lower with lower methanol concentration operation, the demand of the oxygen on the cathode is smaller. Under such a circumstance, both the porous and perforated-plate current collectors may provide sufficient oxygen at low current densities. Hence, at low current densities, both the current collectors did not make difference in cell performance. As current density increases, the ohmic loss becomes more important in the voltage losses. We measured the internal cell resistance of the passive DMFC with both current collectors and found the cell with the porous current collector (630 m cm2 ) was almost twice larger than the cell with the perforated-plate current collector (324 m cm2 ). The increased internal cell resistance was attributed to the increased contact resistance between the porous current collector and the gas diffusion layer, as the porous plate has an extremely high (over 95%) porosity. As a result, the higher internal cell resistance of the cell with the porous current collector led to lower cell voltages in moderate current densities. As current density further increases, the demand of oxygen supply increases. In the cell with the perforated-plate current collector, oxygen is transported to the catalyst layer through the drilled holes in the current collector. In the cell with the porous current collector, however, oxygen can be supplied to the catalyst layer not only through the drilled holes but also through other pores of the porous plate. Hence, the use of the porous current collector offers a much lower overall mass transfer resistance of oxygen, leading to higher cell performance. Moreover, the reduced water flooding resulting from the capillary force in the porous plate can also enhance the oxygen transport to improve the cell performance. In addition, the cell operating temperature was measured by installing a thermocouple at the outer surface of the anodic diffusion layer. The transient operating temperature of the cell operated with 2.0 M methanol solution under the OCV condition is shown in Fig. 4. It is found that the passive DMFC with the porous current collector showed a slightly higher cell operating temperature than did the perforated-plate current collector. This is in part because the high porosity of the porous plate causes a lower effective thermal conductivity compared with the stainless steel plate, which can reduce the heat loss on the cathode to increase the cell operating temperature, and in part because the increased oxygen transport can completely oxidize the permeated methanol and produce more heat, resulting in the increased cell operating temperature. The higher temperature led to the improved electrochemical kinetics of methanol oxidation and oxygen reduction reaction, thereby a better performance at high current densities. However, since the temperature difference between both the cur-
Fig. 4. Variation in cell operating temperature. Anode: 2.0 and 4.0 M methanol; cathode: room temperature: 22.1 ◦ C, relative humidity: 75%.
rent collectors at the low methanol concentration operation is rather small. Therefore, at high current densities, the improved cell performance of passive DMFC with the porous current collector is mainly attributed to the enhanced oxygen transfer rate as a result of the larger specific transport area and the reduced water flooding as a result of the capillary force in the porous plate. We now present the cell performance of the passive DMFC with 4.0 M methanol concentration operation in Fig. 3. Compared with the cell performance at the lower methanol concentration of 2.0 M, it can be seen from Fig. 3 that the cell performance, including the limiting current density and the peak power density, increased from 23.35 to 28.43 mW/cm2 when methanol concentration was increased from 2.0 to 4.0 M. The increased performance as a result of higher methanol concentration can be mainly attributed to the increased methanol permeation rate, which increases the operating temperature and thus improves the electrochemical kinetics of both methanol oxidation and oxygen reduction reactions [18]. It is interesting to notice from Fig. 3 that at low current densities the porous current collector yielded higher voltages, including open circuit voltage (OCV), than did the perforated-plate current collector, and at moderate current densities, the cell voltages for both the current collectors were almost the same. At high current densities, the perforated-plate current collector exhibited a rapid drop in voltage, meaning that the limiting current density was almost reached. However, the cell with the porous current collector was still away from reaching the limiting current density. This behavior confirms that the overall mass transfer resistance of the porous current collector is lower than that of the perforatedplate current collector. In addition, the measured cell operating temperature under the OCV condition shown in Fig. 4 indicates that the porous current collector showed a higher cell operating temperature, which can be attributed to the following factors. First, the higher oxygen transfer rate using the porous current collector yields a higher rate of the exothermic reaction between
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Fig. 5. Nyquist plots of passive DMFC impedance spectra. Cell voltage: 0.30 V; anode: 4.0 M methanol; cathode: room temperature: 22.4 ◦ C, relative humidity: 75%.
the permeated methanol and oxygen on the cathode. Second, the lower effective thermal conductivity of the porous current collector reduces the heat loss to the ambient. As a result, the use of the porous plate as the cathode current collector can lead to a higher cell operating temperature. The higher operating temperature enhances the electrochemical kinetics of methanol oxidation and oxygen reduction, resulting in a higher OCV and higher voltages at low current densities. At moderate current densities, the improved electrochemical kinetics as a result of the increased operating temperature and the increased oxygen transfer rate compensated the higher internal cell resistance of the cell with the porous current collector. As a result, the voltages at moderate current densities became almost the same with both the current collectors. In summary, the enhanced oxygen transfer rate and the lower effective thermal conductivity of the porous current collector are two major reasons that yielded better cell performance of the passive DMFC with higher methanol concentration operation. As mentioned above, the improved performance of the passive DMFC with the porous current collector can be mainly attributed to the enhanced oxygen transfer rate on the cathode. In order to further prove this point, the electrochemical impedance spectra (EIS) for the passive DMFC with the porous and perforated-plated current collectors were measured at 0.3 V with 4.0 M methanol solution. The impedance spectra are compared in Fig. 5. It is seen from this figure that the cell with the porous current collector showed a larger impedance spectra than did the perforated-plate current collector at the high frequency, whereas the porous current collector exhibited a smaller impedance spectra at the low frequency. The arc at high frequency is related to the internal cell resistance [21]. The increased contact resistance as a result of an extremely high (over 95%) porosity resulted in the increased internal cell resistance for the cell with the porous current collector. Hence, the porous current collector showed larger impedance spectra at the high frequency. At the low frequency, the arc was mainly
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Fig. 6. Transient discharging voltage at a constant-current density (80 mA cm−2 ) with a start from the cell to be fueled with methanol solutions between different cathode current collectors. Anode: 4.0 M methanol; cathode: room temperature: 22.6 ◦ C, relative humidity: 71%.
attributed to the mass transfer resistance. The smaller impedance spectra at the low frequency for the porous current collector implied that the overall mass transfer resistance was smaller than the perforated-plate current collector, meaning that the oxygen transfer rate is enhanced. The above EIS spectra confirmed that the improved performance of the porous current collector is attributed primarily to the enhanced oxygen transfer rate as a result of the larger specific transport area. To investigate the operation stability of the passive DMFC with the porous current collector, we also tested the long-term operation of the passive DMFC with both cathode current collectors. The transient discharging cell voltage of the passive DMFC at a constant-current density (80 mA cm−2 ) with a start from the cell to be fueled with 5.0 mL methanol solution at 4.0 M is shown in Fig. 6. It is seen that both the perforated-plate current collector and porous current collectors exhibited an increase in voltage from the beginning to about 15 min as a result of the increased operating temperature, as evidenced from the simultaneously measured temperature shown in Fig. 7. It is interesting to notice that in this initial period the perforated-plate current collector yielded higher voltages than did the porous current collector. This can be attributed to the fact that the cell with the perforated-plate current collector has a lower internal cell resistance. However, after 15 min, the voltage of the cell with perforated-plate current collect began to drop sharply, whereas the voltage of the cell with the porous current collector continued to increase. During the tests, it was also observed that a large amount of liquid water was built-up in the perforated-plate current collector, leading to a rapid decrease in the oxygen transfer rate, thereby a lower cell operating temperature and lower cell voltage. However, this was not the case for the cell with the porous current collector. It is significant to notice that the porous current collector yielded substantially higher voltages than did the perforated-plate current collector. The improved performance can be mainly attributed to the enhanced oxygen
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Fig. 7. Comparison in cell operating temperatures at constant-discharging condition between different cathode current collectors. Anode: 4.0 M methanol; cathode: room temperature: 22.6 ◦ C, relative humidity: 71%.
Fig. 8. Transient discharging voltage at a constant-current density (80 mA cm−2 ) with the active methanol feed between different cathode current collectors. Temperature: 30 ◦ C; anode: 4.0 M methanol, 1.0 mL min−1 ; cathode: room temperature: 21.8 ◦ C, relative humidity: 63%.
transport in the porous plate as the result of the larger specific mass transfer area and the faster water removal rate due to the capillary action in such a porous structure. It is also noticed from Fig. 6 that the perforated-plate current collector exhibited significant fluctuations during the discharge period, mainly because of the periodic buildup and removal of liquid water in this type of current collector. In contrast, the cell with the porous current collector discharged much more stably. It should be mentioned that the methanol consumption due to the electrochemical reaction and methanol crossover caused a decrease in methanol concentration in the fuel reservoir. Therefore, the discharging behaviors for both the current collectors shown in Fig. 6 were actually caused by the combined effect of the time-dependent methanol concentration, oxygen transport and cell operating temperature. In order to further demonstrate the improved performance using the porous current collector was due to the enhanced mass transport of oxygen instead of other factors, we tested the same DMFC with methanol solution supplied by a pump while keeping the cathode at the air-breathing operation mode and maintaining a fixed cell operating temperature. To this end, the anode current collector for the passive operation mode was replaced by a current collector made of a 316L stainless steel plate with a thickness of 1.0 mm, in which a single serpentine flow field, consisting of a flow channel with a cross-sectional area of 1.0 mm × 1.0 mm, was formed. Similarly, to reduce the contact resistance between the anode current collector and electrode, a 200 nm platinum layer was sputtered onto the surface of the anode current collector. The methanol solution with 4.0 M was supplied with the flow rate controlled by a digital HPLC micro-pump (Series III). To ensure that all the experiments were performed with the same mass transfer rate of methanol and with the same heat removal rate from the cell by the methanol stream, methanol was supplied at a flow rate of 1.0 mL min−1 . The cell operating temperature was controlled at 30 ◦ C by a temperature controller. The transient
discharging voltage at a constant-current density of 80 mA cm−2 between the cell with the porous current collector and the cell with the perforated-pate current collector under the condition of the cathode air-breathing operation but the active methanol feed is compared in Fig. 8. It is seen that although the perforated-plate current collector yielded higher voltage in the initial period, but the porous current collector exhibited substantially higher voltages during the remaining discharge period. This proves that the improved performance using the porous current collector is because of the enhanced oxygen transport as a result of the larger specific mass transfer area and the faster water removal rate due to the capillary action in such a porous structure. In addition, the comparison between Figs. 6 and 8 indicates that the active methanol feed operation yielded much more stable cell performance than did the passive operation. This fact indicates that the time-dependent methanol concentration is one of the reasons for instability in the passive DMFC. Moreover, it is also clear from Figs. 6 and 8 that the passive methanol feed operation yielded much better cell performance than did the active operation. This is because the temperature in the active methanol feed mode was maintained at 30 ◦ C, whereas the operating temperature of the passive methanol feed mode was higher. This fact implies that the temperature effect is a key factor affecting the performance of the passive DMFC. In this work, we also tested the passive DMFC using the porous current collector with and without drilled holes by the long-term operation with a start from the cell to be fueled with 5.0 mL methanol solution at 4.0 M. The cell performance is compared in Fig. 9. It is seen that the passive DMFC using the porous current collector without holes showed almost the same performance as the cell with drilled holes. This reveals that the porous structure can provide the sufficient oxygen transfer rate in the passive DMFC as a result of a larger specific transport area and the faster water removal rate due to the capillary action in such a porous structure. This fact also indicates that there is no need
R. Chen, T.S. Zhao / Electrochimica Acta 52 (2007) 4317–4324
Fig. 9. Transient discharging voltage at a constant-current density (80 mA cm−2 ) with a start from the cell to be fueled with 5.0 mL methanol solution for the porous current collector with/without holes. Anode: 4.0 M methanol; cathode: room temperature: 22.6 ◦ C, relative humidity: 71%.
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tally. The experimental results showed that although the passive DMFC with the porous current collector has a higher internal cell resistance, the cell performance of the passive DMFC having the porous current collector still yielded much better performance than did the perforated-plate current collector under the high methanol concentration operation and high current densities. It was demonstrated that the improved performance for the porous current collector was attributed to the enhanced oxygen transport on the cathode as a result of a larger specific transport area, the increased operating temperature as a result of the lower effective thermal conductivity of the porous structure, and the more effective water removal as a result of the capillary action in such a porous structure. The EIS results further confirmed that the porous current collector exhibited a lower overall mass transfer resistance on the cathode as a result of a larger specific transport area and thus enhanced the oxygen transport. The constant-current discharging tests showed that the passive DMFC with the porous current collector yielded better and much more stable performance than did the cell with the conventional perforated-plate current collector. It was also shown that the significant improvement on the operation stability was attributed to the fact that the capillary action in the porous current collector can reduce the liquid water buildup on the cathode. In addition, it is found that the porous current collector with the pore size of 0.4 mm showed the highest performance because of the lowest cell resistance. Moreover, the use of the porous plate as the cathode current collector can greatly reduce the weight of the overall DMFC system. Acknowledgements
Fig. 10. Effect of the pore size of the porous current collector on the performance of passive DMFC. Anode: 4.0 M methanol; cathode: room temperature: 22.4 ◦ C, relative humidity: 75%.
to fabricate holes in the porous plate. Moreover, the effect of the pore size of the porous plate on the cell performance was investigated and the results are shown in Fig. 10. It can be seen from this figure that the porous current collector with the pore size of 0.4 mm yielded the highest performance. During the testing, we also measured the cell resistance and found that the cell resistance was increased from 560 to 1216 m cm2 as the pore size increased from 0.4 to 0.9 mm. Larger pores led to the poor mechanical support on the gas diffusion layer and thus the poor contact, resulting in the higher contact resistance. As a consequence, the porous current collector with the pore size of 0.4 mm showed the highest performance. 4. Concluding remarks A passive DMFC with its cathode current collector made of porous metal foam was fabricated and investigated experimen-
The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 622305). The authors also thank RECEMAT® International for supplying the metal foam samples in this work. References [1] C.K. Dyer, J. Power Sources 106 (2002) 31. [2] Z. Qi, A. Kaufman, J. Power Sources 110 (2002) 177. [3] X. Ren, T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, J. Electrochem. Soc. 147 (2000) 466. [4] Q. Ye, T.S. Zhao, H. Yang, J. Prabhuram, Electrochem. Solid-State Lett. 8 (2005) A52. [5] J. Prabhuram, T.S. Zhao, C.W. Wong, J.W. Guo, J. Power Sources 134 (2004) 1. [6] K. Scott, W.K. Taama, P. Argyropoulos, J. Power Sources 79 (1999) 43. [7] R.Z. Jiang, D. Chu, J. Electrochem. Soc. 151 (2004) A69. [8] Q. Ye, T.S. Zhao, Electrochem. Solid-State Lett. 8 (2005) A211. [9] R. Hockaday, C. Navas, Proceedings of the Conference on Portable Fuel Cells, Switzerland, June 21–24, Lucerne, 1999, p. 45. [10] C.Y. Chen, P. Yang, J. Power Sources 123 (2003) 37. [11] J.G. Liu, G.G. Sun, F.L. Zhao, G.X. Wang, G. Zhao, L.K. Chen, B.L. Yi, Q. Xin, J. Power Sources 133 (2004) 175. [12] D. Kim, E.A. Cho, S.A. Hong, I.H. Oh, H.Y. Ha, J. Power Sources 130 (2004) 172. [13] T. Shimizu, T. Momma, M. Mohamedi, et al., J. Power Sources 137 (2004) 277.
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[14] G.G. Park, T.H. Yang, Y.G. Yoon, W.Y. Lee, C.S. Kim, Int. J. Hydrogen Energy 28 (2003) 645. [15] J. Han, E.S. Park, J. Power Sources 112 (2002) 477. [16] B.K. Kho, I.H. Oh, S.A. Hong, H.Y. Ha, Electrochim. Acta 50 (2004) 781. [17] B.K. Kho, B. Bae, M.A. Scibioh, J. Lee, H.Y. Ha, J. Power Sources 142 (2005) 50.
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Porous current collectors for passive direct methanol fuel cells R. Chen, T.S. Zhao ∗ Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Received 4 October 2006; received in revised form 5 December 2006; accepted 5 December 2006 Available online 17 December 2006
Abstract A passive direct methanol fuel cell (DMFC) with its cathode current collector made of porous metal foam was investigated experimentally. The measured polarization curves, constant-current discharging behavior and EIS spectra showed that the passive DMFC having the porous current collector yielded much higher and much more stable performance than did the cell having the conventional perforated-plate current collector with high methanol concentration operation. It was demonstrated that the improved performance for the porous current collector was attributed to: (i) the enhanced oxygen transport on the cathode as a result of a larger specific transport area, (ii) the increased operating temperature as a result of the lower effective thermal conductivity of the porous structure, and (iii) the faster water removal as a result of the capillary action in the porous structure, The experimental results also revealed that the porous current collector with a smaller pore size yielded higher performance as a result of the lower cell resistance. © 2006 Elsevier Ltd. All rights reserved. Keywords: Fuel cell; Passive DMFC; Metal foam; Effective thermal conductivity; Cell performance; Oxygen transport
1. Introduction The high energy density of liquid methanol as well as simple and compact power systems make the direct methanol fuel cells (DMFC) a promising power source for portable electronic devices. Over the past decade, extensive efforts [1–8] have been made to the study of the DMFC with the fuel fed by a liquid pump and oxidant fed by a gas compressor. In order to make the DMFC more competitive with conventional battery technologies, however, it is essential to eliminate some auxiliary devices such as liquid pumps and gas fans/blowers so that the overall DMFC system becomes much simpler and much more compact. For this reason, the concept of a passive DMFC without external pumps and other ancillary devices has been proposed and studied [9–18]. A typical passive-feed DMFC consists of a fuel reservoir, an anode current collector, a membrane electrode assembly (MEA), and a cathode current collector. Methanol is introduced to the active layer primarily by diffusion without any external means of liquid transport and oxygen is taken passively from the ambient air without any means of air movement. This type of passive DMFCs not only offers the advantage of simple
∗
Corresponding author. Tel.: +852 2358 8647; fax: +852 2358 1543. E-mail address: [email protected] (T.S. Zhao).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.12.015
and compact systems but also makes it possible to eliminate the parasitic power losses for powering ancillary devices required in active DMFCs. Because of these advantages, the passive DMFC has received much more attention. Chen and Yang [10] investigated the effect of operating conditions on the power density of an air-breathing DMFC. Liu et al. [11] studied sintered stainless steel fiber felt as the gas diffusion layer in an air-breathing DMFC. The effect of methanol concentration was also studied in this work. Kim et al. [12] fabricated and tested a single cell and monopolar DMFC stack operating under passive and airbreathing conditions. Shimizu et al. [13] reported their activities regarding the research and development of DMFCs that operated passively at room temperature. Park et al. [15] reported the optimal methanol solution appeared at 4.0 M in passive DMFCs. Kho et al. [17] investigated the variation in the open circuit voltage (OCV) and the cell temperature with time as a consequence of the methanol crossover. The previous studies have indicated that the passive DMFC usually has to be operated with higher methanol concentration, as the methanol transport from a built-in fuel reservoir to the anode catalyst layer in this type of passive fuel cell relies on diffusion. Since there is plenty of room up to pure methanol, methanol transport on the anode is actually not a problem. Rather, higher methanol concentration offers the advantage of increasing the energy density of the fuel cell, provided that the
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rate of methanol crossover can effectively be reduced. In contrast, the problem of mass transport of oxygen and water on the cathode is more challenging, as this type of fuel cell operates on the air-breathing mode and there is no external means to enhance oxygen transport and water removal. As a result, the passive fuel cell frequently operates under the oxygen-starving and water-flooding conditions. In addition, it was recently found that passive DMFCs operate at near room temperature and it is important to increase the cell operating temperature so that electrochemical kinetics of both methanol oxidation and oxygen reduction reactions can be improved [18,19]. In summary, on the constraint without any external means of air movement, it is critical to design and optimize the architecture of the passive DMFC to ensure higher oxygen transfer and water removal rates as well as a higher cell operating temperature. The cathode current collector is one of the key components of the passive DMFC, which not only collects the electric current but also offers the passages for oxygen and water transport. Furthermore, since in the passive DMFC, the heat generated within the cell is predominately lost from the cathode current collector, it is essential to employ a material with low effective thermal conductivity as the cathode current collector such that the heat loss can be lowered and the cell can be operated at a higher temperature to achieve better performance. Therefore, in the passive DMFC, the cathode current collector must have high electric conductivity, good mechanical strength, more uniform transport area and low effective thermal conductivity. In this work, a passive DMFC with its cathode current collector made of porous metal foam was investigated experimentally. We show that this type of porous current collector can not only provide a higher oxygen transfer rate and a more effective water removal rate, but also can render a higher cell operating temperature.
As a consequence, the cell having this type of porous current collector yielded significantly higher performance and much more stable operation than did the cell having the conventional perforated-plate current collector. 2. Experimental 2.1. Membrane and electrode assembly (MEA) A pretreated Nafion 115 membrane with a thickness of 125 m was employed in this work. The pretreatment procedures included boiling the membrane in 5 vol.% H2 O2 , washing in DI water, boiling in 0.5 M H2 SO4 and washing in DI water for 1 h in turn. The pretreated membranes were kept in the DI water prior to the fabrication of MEAs. Single-side ELAT electrodes from ETEK were used in both anode and cathode, where carbon cloth (E-TEK, Type A) were used as the backing support layer with 30 wt% PTFE wet-proofing treatment. The catalyst loading on the anode side was 4.0 mg/cm2 with PtRu black (1:1 a/o), while the catalyst loading on the cathode side was 2.0 mg/cm2 using 40 wt% Pt on Vulcan XC-72. Furthermore, 0.8 mg/cm2 dry Nafion® ionomer was coated onto the surface of each electrode. Finally, MEA with an active area of 4.0 cm2 were fabricated by hot pressing at 135 ◦ C and 4 MPa for 3.0 min. More detailed information about the MEA fabrication can be found elsewhere [20]. 2.2. Single cell fixture As shown in Fig. 1, the MEA mentioned above was sandwiched between an anode and a cathode current collector. The entire cell setup was then held together between an anode and a
Fig. 1. Schematic of the passive DMFC.
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cathode fixture, both of which were made of transparent organic glass. A 5.0 mL methanol solution reservoir was built in the anode fixture. Methanol was transferred into the catalyst layer from the built-in reservoir, while oxygen, from the surrounding air, was transferred into the cathode catalyst layer through the opening of the cathode fixture. The cell temperature was measured by a thermocouple (Type T), which was installed on the outer surface of the anode gas diffusion layer. The anode current collector was made of a perforated 316L stainless steel plate of 1.5 mm in thickness. Two different types of cathode current
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collectors, as shown in Fig. 2, were fabricated and tested. One was the conventional perforated-plate current collector made of a 316L stainless steel plate, while the other was the porous current collector made of a Ni–Cr alloy metal foam plate of 1.0 mm in thickness. The Ni–Cr alloy metal foam supplied by the RECEMAT® International offers over 95% porosity and the estimated average pore diameter of 0.4 mm. A plurality of 2.6 mm-circular holes was drilled in both the current collectors, serving as the passages of fuel and oxidant, which resulted in an open ratio of 47.8%. A 200 nm platinum layer was sputtered onto the surface of the perforated-plate and porous current collectors to reduce the contact resistance with the electrodes. 2.3. Electrochemical instrumentation and test conditions An Arbin BT2000 electrical load interfaced to a computer was employed to control the condition of discharging and record the voltage–current curves. For each discharging current point along the I–V curve, a 60 s waiting time was used to obtain the stable voltage. The temperature of the cell was measured by the Arbin BT2000 built-in function. All the experiments of the passive DMFCs were performed at room temperatures of 21.8–22.6 ◦ C and the relative humidity of 63–75%. Prior to the performance test, the MEA was installed in an active cell fixture and activated at 70 ◦ C about 24 h. During the activation period, 1.0 M methanol was fed at 1.0 mL min−1 , while oxygen was supplied under atmospheric pressure at a flow rate of 50 mL min−1 . 3. Results and discussion Fig. 3 compares the cell performance of the porous and perforated-plate current collectors with 2.0 M methanol concentration operation. The polarization curves at low current densities do not show too much difference between both the current collectors. With increasing current density, the voltage with the porous current collector became lower than that with the perforated-plate current collector. However, at high current
Fig. 2. Schematic of the current collector. (a) Perforated-plate current collector and (b) Porous current collector.
Fig. 3. Comparison in cell performance between different cathode current collectors. Anode: 2.0 and 4.0 M methanol; cathode: room temperature: 22.1 ◦ C, relative humidity: 75%.
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densities, the perforated-plate current collector tended to show a rapid decline in voltage, implying that the corresponding limiting current density of the perforated-plate current collector would be lower than that of the porous current collector. The performance behavior shown in Fig. 3 for lower methanol concentration can be explained as follows. The cathode current collector affects the performance of the passive DMFC in two major aspects: oxygen transport and water removal (flooding) on the cathode. In addition, different types of current collectors may cause different internal electrical resistances due to the contact between the gas diffusion layer and the current collector. Since the rate of methanol crossover is lower with lower methanol concentration operation, the demand of the oxygen on the cathode is smaller. Under such a circumstance, both the porous and perforated-plate current collectors may provide sufficient oxygen at low current densities. Hence, at low current densities, both the current collectors did not make difference in cell performance. As current density increases, the ohmic loss becomes more important in the voltage losses. We measured the internal cell resistance of the passive DMFC with both current collectors and found the cell with the porous current collector (630 m cm2 ) was almost twice larger than the cell with the perforated-plate current collector (324 m cm2 ). The increased internal cell resistance was attributed to the increased contact resistance between the porous current collector and the gas diffusion layer, as the porous plate has an extremely high (over 95%) porosity. As a result, the higher internal cell resistance of the cell with the porous current collector led to lower cell voltages in moderate current densities. As current density further increases, the demand of oxygen supply increases. In the cell with the perforated-plate current collector, oxygen is transported to the catalyst layer through the drilled holes in the current collector. In the cell with the porous current collector, however, oxygen can be supplied to the catalyst layer not only through the drilled holes but also through other pores of the porous plate. Hence, the use of the porous current collector offers a much lower overall mass transfer resistance of oxygen, leading to higher cell performance. Moreover, the reduced water flooding resulting from the capillary force in the porous plate can also enhance the oxygen transport to improve the cell performance. In addition, the cell operating temperature was measured by installing a thermocouple at the outer surface of the anodic diffusion layer. The transient operating temperature of the cell operated with 2.0 M methanol solution under the OCV condition is shown in Fig. 4. It is found that the passive DMFC with the porous current collector showed a slightly higher cell operating temperature than did the perforated-plate current collector. This is in part because the high porosity of the porous plate causes a lower effective thermal conductivity compared with the stainless steel plate, which can reduce the heat loss on the cathode to increase the cell operating temperature, and in part because the increased oxygen transport can completely oxidize the permeated methanol and produce more heat, resulting in the increased cell operating temperature. The higher temperature led to the improved electrochemical kinetics of methanol oxidation and oxygen reduction reaction, thereby a better performance at high current densities. However, since the temperature difference between both the cur-
Fig. 4. Variation in cell operating temperature. Anode: 2.0 and 4.0 M methanol; cathode: room temperature: 22.1 ◦ C, relative humidity: 75%.
rent collectors at the low methanol concentration operation is rather small. Therefore, at high current densities, the improved cell performance of passive DMFC with the porous current collector is mainly attributed to the enhanced oxygen transfer rate as a result of the larger specific transport area and the reduced water flooding as a result of the capillary force in the porous plate. We now present the cell performance of the passive DMFC with 4.0 M methanol concentration operation in Fig. 3. Compared with the cell performance at the lower methanol concentration of 2.0 M, it can be seen from Fig. 3 that the cell performance, including the limiting current density and the peak power density, increased from 23.35 to 28.43 mW/cm2 when methanol concentration was increased from 2.0 to 4.0 M. The increased performance as a result of higher methanol concentration can be mainly attributed to the increased methanol permeation rate, which increases the operating temperature and thus improves the electrochemical kinetics of both methanol oxidation and oxygen reduction reactions [18]. It is interesting to notice from Fig. 3 that at low current densities the porous current collector yielded higher voltages, including open circuit voltage (OCV), than did the perforated-plate current collector, and at moderate current densities, the cell voltages for both the current collectors were almost the same. At high current densities, the perforated-plate current collector exhibited a rapid drop in voltage, meaning that the limiting current density was almost reached. However, the cell with the porous current collector was still away from reaching the limiting current density. This behavior confirms that the overall mass transfer resistance of the porous current collector is lower than that of the perforatedplate current collector. In addition, the measured cell operating temperature under the OCV condition shown in Fig. 4 indicates that the porous current collector showed a higher cell operating temperature, which can be attributed to the following factors. First, the higher oxygen transfer rate using the porous current collector yields a higher rate of the exothermic reaction between
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Fig. 5. Nyquist plots of passive DMFC impedance spectra. Cell voltage: 0.30 V; anode: 4.0 M methanol; cathode: room temperature: 22.4 ◦ C, relative humidity: 75%.
the permeated methanol and oxygen on the cathode. Second, the lower effective thermal conductivity of the porous current collector reduces the heat loss to the ambient. As a result, the use of the porous plate as the cathode current collector can lead to a higher cell operating temperature. The higher operating temperature enhances the electrochemical kinetics of methanol oxidation and oxygen reduction, resulting in a higher OCV and higher voltages at low current densities. At moderate current densities, the improved electrochemical kinetics as a result of the increased operating temperature and the increased oxygen transfer rate compensated the higher internal cell resistance of the cell with the porous current collector. As a result, the voltages at moderate current densities became almost the same with both the current collectors. In summary, the enhanced oxygen transfer rate and the lower effective thermal conductivity of the porous current collector are two major reasons that yielded better cell performance of the passive DMFC with higher methanol concentration operation. As mentioned above, the improved performance of the passive DMFC with the porous current collector can be mainly attributed to the enhanced oxygen transfer rate on the cathode. In order to further prove this point, the electrochemical impedance spectra (EIS) for the passive DMFC with the porous and perforated-plated current collectors were measured at 0.3 V with 4.0 M methanol solution. The impedance spectra are compared in Fig. 5. It is seen from this figure that the cell with the porous current collector showed a larger impedance spectra than did the perforated-plate current collector at the high frequency, whereas the porous current collector exhibited a smaller impedance spectra at the low frequency. The arc at high frequency is related to the internal cell resistance [21]. The increased contact resistance as a result of an extremely high (over 95%) porosity resulted in the increased internal cell resistance for the cell with the porous current collector. Hence, the porous current collector showed larger impedance spectra at the high frequency. At the low frequency, the arc was mainly
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Fig. 6. Transient discharging voltage at a constant-current density (80 mA cm−2 ) with a start from the cell to be fueled with methanol solutions between different cathode current collectors. Anode: 4.0 M methanol; cathode: room temperature: 22.6 ◦ C, relative humidity: 71%.
attributed to the mass transfer resistance. The smaller impedance spectra at the low frequency for the porous current collector implied that the overall mass transfer resistance was smaller than the perforated-plate current collector, meaning that the oxygen transfer rate is enhanced. The above EIS spectra confirmed that the improved performance of the porous current collector is attributed primarily to the enhanced oxygen transfer rate as a result of the larger specific transport area. To investigate the operation stability of the passive DMFC with the porous current collector, we also tested the long-term operation of the passive DMFC with both cathode current collectors. The transient discharging cell voltage of the passive DMFC at a constant-current density (80 mA cm−2 ) with a start from the cell to be fueled with 5.0 mL methanol solution at 4.0 M is shown in Fig. 6. It is seen that both the perforated-plate current collector and porous current collectors exhibited an increase in voltage from the beginning to about 15 min as a result of the increased operating temperature, as evidenced from the simultaneously measured temperature shown in Fig. 7. It is interesting to notice that in this initial period the perforated-plate current collector yielded higher voltages than did the porous current collector. This can be attributed to the fact that the cell with the perforated-plate current collector has a lower internal cell resistance. However, after 15 min, the voltage of the cell with perforated-plate current collect began to drop sharply, whereas the voltage of the cell with the porous current collector continued to increase. During the tests, it was also observed that a large amount of liquid water was built-up in the perforated-plate current collector, leading to a rapid decrease in the oxygen transfer rate, thereby a lower cell operating temperature and lower cell voltage. However, this was not the case for the cell with the porous current collector. It is significant to notice that the porous current collector yielded substantially higher voltages than did the perforated-plate current collector. The improved performance can be mainly attributed to the enhanced oxygen
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Fig. 7. Comparison in cell operating temperatures at constant-discharging condition between different cathode current collectors. Anode: 4.0 M methanol; cathode: room temperature: 22.6 ◦ C, relative humidity: 71%.
Fig. 8. Transient discharging voltage at a constant-current density (80 mA cm−2 ) with the active methanol feed between different cathode current collectors. Temperature: 30 ◦ C; anode: 4.0 M methanol, 1.0 mL min−1 ; cathode: room temperature: 21.8 ◦ C, relative humidity: 63%.
transport in the porous plate as the result of the larger specific mass transfer area and the faster water removal rate due to the capillary action in such a porous structure. It is also noticed from Fig. 6 that the perforated-plate current collector exhibited significant fluctuations during the discharge period, mainly because of the periodic buildup and removal of liquid water in this type of current collector. In contrast, the cell with the porous current collector discharged much more stably. It should be mentioned that the methanol consumption due to the electrochemical reaction and methanol crossover caused a decrease in methanol concentration in the fuel reservoir. Therefore, the discharging behaviors for both the current collectors shown in Fig. 6 were actually caused by the combined effect of the time-dependent methanol concentration, oxygen transport and cell operating temperature. In order to further demonstrate the improved performance using the porous current collector was due to the enhanced mass transport of oxygen instead of other factors, we tested the same DMFC with methanol solution supplied by a pump while keeping the cathode at the air-breathing operation mode and maintaining a fixed cell operating temperature. To this end, the anode current collector for the passive operation mode was replaced by a current collector made of a 316L stainless steel plate with a thickness of 1.0 mm, in which a single serpentine flow field, consisting of a flow channel with a cross-sectional area of 1.0 mm × 1.0 mm, was formed. Similarly, to reduce the contact resistance between the anode current collector and electrode, a 200 nm platinum layer was sputtered onto the surface of the anode current collector. The methanol solution with 4.0 M was supplied with the flow rate controlled by a digital HPLC micro-pump (Series III). To ensure that all the experiments were performed with the same mass transfer rate of methanol and with the same heat removal rate from the cell by the methanol stream, methanol was supplied at a flow rate of 1.0 mL min−1 . The cell operating temperature was controlled at 30 ◦ C by a temperature controller. The transient
discharging voltage at a constant-current density of 80 mA cm−2 between the cell with the porous current collector and the cell with the perforated-pate current collector under the condition of the cathode air-breathing operation but the active methanol feed is compared in Fig. 8. It is seen that although the perforated-plate current collector yielded higher voltage in the initial period, but the porous current collector exhibited substantially higher voltages during the remaining discharge period. This proves that the improved performance using the porous current collector is because of the enhanced oxygen transport as a result of the larger specific mass transfer area and the faster water removal rate due to the capillary action in such a porous structure. In addition, the comparison between Figs. 6 and 8 indicates that the active methanol feed operation yielded much more stable cell performance than did the passive operation. This fact indicates that the time-dependent methanol concentration is one of the reasons for instability in the passive DMFC. Moreover, it is also clear from Figs. 6 and 8 that the passive methanol feed operation yielded much better cell performance than did the active operation. This is because the temperature in the active methanol feed mode was maintained at 30 ◦ C, whereas the operating temperature of the passive methanol feed mode was higher. This fact implies that the temperature effect is a key factor affecting the performance of the passive DMFC. In this work, we also tested the passive DMFC using the porous current collector with and without drilled holes by the long-term operation with a start from the cell to be fueled with 5.0 mL methanol solution at 4.0 M. The cell performance is compared in Fig. 9. It is seen that the passive DMFC using the porous current collector without holes showed almost the same performance as the cell with drilled holes. This reveals that the porous structure can provide the sufficient oxygen transfer rate in the passive DMFC as a result of a larger specific transport area and the faster water removal rate due to the capillary action in such a porous structure. This fact also indicates that there is no need
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Fig. 9. Transient discharging voltage at a constant-current density (80 mA cm−2 ) with a start from the cell to be fueled with 5.0 mL methanol solution for the porous current collector with/without holes. Anode: 4.0 M methanol; cathode: room temperature: 22.6 ◦ C, relative humidity: 71%.
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tally. The experimental results showed that although the passive DMFC with the porous current collector has a higher internal cell resistance, the cell performance of the passive DMFC having the porous current collector still yielded much better performance than did the perforated-plate current collector under the high methanol concentration operation and high current densities. It was demonstrated that the improved performance for the porous current collector was attributed to the enhanced oxygen transport on the cathode as a result of a larger specific transport area, the increased operating temperature as a result of the lower effective thermal conductivity of the porous structure, and the more effective water removal as a result of the capillary action in such a porous structure. The EIS results further confirmed that the porous current collector exhibited a lower overall mass transfer resistance on the cathode as a result of a larger specific transport area and thus enhanced the oxygen transport. The constant-current discharging tests showed that the passive DMFC with the porous current collector yielded better and much more stable performance than did the cell with the conventional perforated-plate current collector. It was also shown that the significant improvement on the operation stability was attributed to the fact that the capillary action in the porous current collector can reduce the liquid water buildup on the cathode. In addition, it is found that the porous current collector with the pore size of 0.4 mm showed the highest performance because of the lowest cell resistance. Moreover, the use of the porous plate as the cathode current collector can greatly reduce the weight of the overall DMFC system. Acknowledgements
Fig. 10. Effect of the pore size of the porous current collector on the performance of passive DMFC. Anode: 4.0 M methanol; cathode: room temperature: 22.4 ◦ C, relative humidity: 75%.
to fabricate holes in the porous plate. Moreover, the effect of the pore size of the porous plate on the cell performance was investigated and the results are shown in Fig. 10. It can be seen from this figure that the porous current collector with the pore size of 0.4 mm yielded the highest performance. During the testing, we also measured the cell resistance and found that the cell resistance was increased from 560 to 1216 m cm2 as the pore size increased from 0.4 to 0.9 mm. Larger pores led to the poor mechanical support on the gas diffusion layer and thus the poor contact, resulting in the higher contact resistance. As a consequence, the porous current collector with the pore size of 0.4 mm showed the highest performance. 4. Concluding remarks A passive DMFC with its cathode current collector made of porous metal foam was fabricated and investigated experimen-
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