行政院國家科學委員會專題研究計畫 成果報告
子計畫一：質子交換膜燃料電池流道設計：流道板及擴散層
熱流設計(實驗研究)
計畫類別： 整合型計畫 計畫編號： NSC91-2218-E-110-009-執行期間： 91 年 08 月 01 日至 92 年 07 月 31 日 執行單位： 國立中山大學機械與機電工程學系(所) 計畫主持人： 謝曉星 計畫參與人員： 黃青峰、林志益、田棨薰 報告類型： 精簡報告 處理方式： 本計畫涉及專利或其他智慧財產權，2 年後可公開查詢中
華
民
國 92 年 9 月 10 日
行政院國家科學委員會補助專題研究計畫
成 果 報
告 □期
中進度
報
告
微型燃料電池元件之設計與製作－子計畫一：質子交
換膜燃料電池流道設計：流道板及催化層熱流設計(實
驗研究)
計畫類別：□ 個別型計畫
整合型計畫
計畫編號：NSC 91－2218－E－110－009
執行期間： 91 年 08 月 01 日至 92 年 07 月 31 日
計畫主持人：謝曉星 教授
共同主持人：
計畫參與人員：黃青峰、林志益、田棨薰
成果報告類型(依經費核定清單規定繳交)： 精簡報告 □完整報
告
本成果報告包括以下應繳交之附件：
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□國際合作研究計畫國外研究報告書一份
處理方式：除產學合作研究計畫、提升產業技術及人才培育研究
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執行單位：國立中山大學機械與機電工程學系
中 華 民 國 92 年 07 月
31 日
Abstr act：
A novel design and microfabrication were developed for a micro PEM (Proton Exchange Membrane) fuel cell with a cross section of 5cm2 and thickness (for a single cell) of about 800μm. A new design and fabrication processes were developed for both flow field plate and MEA parts. Air flows were completed in hydrogen fuel cells with low input pressure and low velocity. Performance tests of polarization curves and power density distribution were conducted and discussed.
Introduction：
Proton exchange membrane fuel cell (PEMFC) has many advantages compared with other types of fuel cell as portable power for mobility. The application of fuel cells to portable power is motivated by numerous occasions such as 3C products [1,2] and challenged by several factors. For instance, high power density and high energy-to-weight ratio [3,4]. To make PEMFC a commercial reality, much development work has been done and focused on the performance improvement of polymer electrode, electro catalysts, and electro materials [5].
Miniaturizing fuel cells for portable application, as mentioned before like 3C products, however, is not simply a matter of reducing the corresponding components dimensions. Rather, new design and manufacturing processes must be examined and developed simultaneously.
As one may know, traditional/conventional macro-sized fuel cell components are limited by characteristic fabrication constraints. They are, just name a few, the machining of flow passage is constrained by the brittleness of graphite, and molding is limited in deep narrow channels. All of these traditionally were done by cutting, drilling, molding, grinding, etc. and it will become less economically while dealing with a rather small PEMFC. Instead, processes such as fabrication methodologies used in the convectional semi-conductor industry may be more appropriate in batch, as well as continuous production to manufacture a microfuel cell to reach powerfuel, highly efficient and low cost goal.
Regarding a wide array of portable electronic telecommunication and computing devices, including cellular telephones, game devices, and portable computers. It is expected that these machines will become more diverse, more numerous in the near future. As a result, several papers have recently reported their results to make the fuel cell product miliaturized by using sputter-deposited technology onto MEAs, [4,6] and fuel cell electrodes [7]. In addition to the sputter-deposited method used in those aspects, flow field plates design has also been attracted a lot of attention. For instance, Buchi and Sclerer [8] presented experimental results for in-situ resistance measurements of Nation 117 membranes in PEMFCs with different flow field design. It was found without forced gas convection, the fuel cell performance could be higher. Based on the foregoing discussion, a novel design and microfabrication processes adapted from bulk micromaching processes were developed for a micro PEMFC with a cross section of 5cm2 and thickness of a single cell about 800μm as shown in Fig. 1 and the objectives of this paper are three fold：
1. A new design for MEA, a flow field plate, and current collector. 2. A new fabrication process for MEA, a flow field plate, and current
collector.
4.Instead of the conventional hot pressing assembly if possible, thin film deposited and layer growth technology with surface mount technology was employed.
During the preparation of this paper, we have just and surprisingly found two papers [4,9] which are similar to the present novel design concept and fabrication processes for MEA and flow field plates, and they are worth mentioning. In fact, Muller et.’s paper was just published.
Design Salient Features and Key Factor s：
The characteristic feature of the new microfuel cell configuration is an integrated novel design and it consists PMMA flow field plate with narrow and deep channel made by excimer microsystem technology, platinum (Pt) sputtering deposited on MEA (Nafion 117), and an ultra thin copper layer again sputtering deposited on PMMA flow field plate used as a current collector. The relevant geometric design and fabrication parameters are listed in Table 1. The advantages of this integrated fabrication process are：A few nm (~55nm) sputtered Pt film corresponding to a catalyst loading level of 0.15 mg/cm2 can be reached. The flow field plate using excimer laser fabrication process may have a larger effective flow passage even up to 20% increase in contact area as compared to conventional cells. Two key issues should be considered for catalyst used in this new design. Namely, it should be resulted in Pt loading reduced as low as 0.15 mg/cm2 and the polarization curve (VI curve) characteristics, i.e., power rating can still reach up to one third (~25mW/cm2 at 0.65V) of conventional fuel cell.
Experimental
Before experimental tests, a prototype microfuel cell should be prepared. MEA Preparation
To achieve the cell as small as possible, the thin fuel platinum deposited MEAs was used in this study. It was made through processes using an ultra-thin sputtered platinum films deposited directly onto Nafion 117 (183 μ m) membranes (ElectroChem, Inc.) via a PFG 300RF Sputter manufactured by a German company under the working condition of 300W and 40 seconds agron at 3x10-5 torr with a sputter deposition rate of about 2 nm/s, and nominal Pt film thickness of 20～60 nm. Detailed operating condition is listed in Table 2. Before sputtering process going, smooth Nafion was obtained from the supplier, and cutted with an area of 5 cm2 and it was then washed by DI water for 5 mins. The electrolyte membranes were preheated to 75~85 ° C with H2SO4 and H2O2 (3:1 a/o) to remove organic and impurities,
respectively, and washed it again with DI for another 5 mins. Finally, the membranes were dried by N2 gas.
Excimer Laser Lithography for PMMA Flow Structure
Besides the above-stated MEAs preparation, microfabrication of flow structure like flow-field plate using techniques adopted from an excimer laser bulk micromachining process. PMMA materials were chosen for prototyping complex geometric structures such as serpentine channel patterns used for flow path. Electrically conductive regions acted as electrode for current collector were patterned on the surface by sputtering copper of a thickness about 200 nm. Due to Cu metal is not very stable, silver (Au) may be one of the appropriate choices as an alternate for future applications. The channels/or slots of the flow structure were patterned (mask 2mm x 2mm) by dry etching a 2.25cm2 square PMMA substrate with thickness of 250ìm using an ArF excimer laser (wavelength 193 nm, 100 mJ max power) with an etch rate of 10ìm /min. The same dimension flow structure was made for anode and cathode. The channels were about 400ìm wide, and 200ìm deep with a rib spacing of 50ìm as shown in Fig. 2 for both anode and cathode. The ratio of total flow volume of cathode to anode is 1.
Following the above-mentioned fabrication steps, Fig. 3 summarizes and highlights salient steps in the fabrication sequence for a prototype microfuel cell construction. Fig. 4 shows a photograph of the fuel test fixture, which includes excimer laser lithography PMMA flow fields with serpentine channels, providing a 5cm2 single cell geometry. The sandwiched layered (flow field plate + MEAs) structure was finally pressed onto 2 mm PMMA gasket (housing) as shown in Fig. 5. The entire assembly was bonded using surface mount technology /or hot pressing whenever applicable.
Pure compressed hydrogen and ambient air were used as reactant gases and their flow rates were controlled with a stoichiometry ratio of hydrogen and oxygen of 6. The feed flow rates of the anode were kept at 10 cc/min. The hydrogen gas was delivered at an inlet pressure of about 4 atm. Current density versus cell voltage curves of a single cell were taken using a Gamry PC 4/750 potentiostat meter interfaced to a PC at constant electric current and recorded after the system reached steady state (about 10 mins). The cell was operated at 60℃ and 1 atm.
Results and Discussion：
Microscopy Investigation of MEAs
Both SEM (JSM-6330TF, JEOL) and AFM (EX 139710, TOPMETRIX) images examination for sputtered platinum deposited MEAs were performed prior to bonding. It is recognized that the full cell catalyst effectiveness can be increased by sputter deposition [6]. A typical MEAs contains a three phase matrix of electrolyte and carbon supported catalyst and functioned to allow effective gas and water diffusion, proton as well as electron transport to and from the catalyst sites. The top view SEM images in Fig. 6 were analyzed to determine the surface coverage of the sputter deposited film. There are two factors that limit the fuel cell performance [6] which are the rate of oxygen reduction and oxygen diffusional resistance. The morphological characteristics of the catalyst layer were measured. SEM produced the real images of surface structure of MEAs. It was hoped that application of sputter deposited Pt loadings would enhance fuel cell performance by increasing the rate of oxygen reduction and simultaneously decreasing the oxygen diffusional resistance. Fig. 6 zoomed out as perspective of platinum films of thichness 55 nm with an area of 1.5 × 1.2ìm 2. Pt did not form continuous films on the Nafion 117 substrate. However, the platinum islands form a continuous net work on the surface. It can also be seen that the crack sites are widespread and envelops the entire surface. The cracks are about 25nm wide and space on average of 150nm apart. The catalyst activity (or the number of active catalytic sites) was reduced due to a reduction of the gas access with increasing the crack sites and depth of the crack unless a legitimate three phase zone was obtained [4]. This can be further observed from Fig. 7 of AFM images.
Figures 7 illustrated the top view with an area of 200nm×200nm (Fig. 7a) and 3-D view (Fig. 7b) for topography. As one can see, from top view, the results seem very similar to those of SEM results, as shown in Fig. 6. However, based on the 3-D view of Fig. 7(b), one can clearly observe the depth of the cracks and the gas access path as well as the morphology of the legitimate three phase (solid-platinum, liquid-MEA, and gas- H2/or air) zone. In general, significant surface texturing is noted.
This is also found in O’Hayre [4] for Pt sputter lording MEAs with a thickness of 30nm. This surface texturing plays a role to disrupt the Pt film to become discontinuity. The Pt film cracks would occur due to the increased constraint effects between two different materials (Pt metal and Nafion substrate). It is hoped that this behavior would result in an occurrence of three-phase zone and also keep its amount high. However, based on Figs. 7, there seems quite a few three phase zones found. Polarization Characteristics
Polarization characteristics (VI curves) measurements of MEA with new flow field plates at the ambient condition of this type micro fuel cell were performed. In this present design, power density of 25mW/cm2 at 0.65V is reported when the fuel is applied with nonhumidified hydrogen at about 6 atm and air at nearly 1atm,
respectively. The results were shown in Fig. 8. As expected, higher potential is achieved at lower temperature within the low current density range. This is because the non-humidified fuel would cause the efficient path of the dry fuel gas like H2 to
the catalyst layer. Furthermore, because of using air oxidant, nitrogen gas would occupy a substantial portions of reaction area, the effective reaction area is relatively reduced as compared to that of pure oxygen was applied. As a result, the oxygen reduction becomes the rate-limiting step of the overall reaction. In fact, the present maximum power output can reach to 25 mW/cm2 at Pt sputter loading thickness of about 55 nm, which is a little bit higher than the results reported by O’Hayre [4]. Also shown in Fig. 8 is the power density of the present single cell. It is found that the maximum power density can reach 31 mW/cm2 at 0.4V. Higher current density (≧ 200mA/cm2) would be reached by optimization of the thickness of catalytic layer and deposition methods of Pt on Nafion. Further study would include this aspect.
Conclusion：
For the first time, a combined novel design and micro fabrication for PMMA flow field plates and platinum loading MEAs were developed for a micro PEMFC with a cross section of 5cm2. The cell only contains two flow field plates and the Pt sputtered-loading MEAs. Microscopes examination of the Pt sputter loading was made and results were discussed. The test loop was setup and the performance tests were conducted for polarization curves and power density distribution. For the present single cell, a reliable and stable power output of 25 mW/cm2 at 0.65V was obtained at an ambient temperature of 25℃.
Acknowledgements：
This work was performed in University Microsystem Laboratory, Center for Nanoscience and Technology, National Sun Yat-Sen University. The effort is sponsored by the National Science Council (NSC), Taiwan, ROC, Grant numbers NSC 91-2218-E-110-009 and 91-2218-E-110-010.
References
：
1. Meyer s J . P. ; Maynar d H. L. (2002) Design Considerations for Miniaturized PEM Fuel Cells. J. of Power Sources 109 : 76-88.
2. Chang H ; Kim J R ; Cho J H ; Kim H K ; Chai K H (2002) Materials and Processes for Small Fuel Cells. Solid State Ionics 148 : 601-606.
3. Choi K H ; Peck D H ; Kim C E ; Shin D R ; Lee T H (2000) Water
Transport in Polymer Membranes for PEMFC. J. of Power Sources 86 : 197-201. 4. Hayre R O’ ; Lee S J ; Cha S W ; Pr inz F B (2002) A Sharp Peak in the
Performance of Sputtered Platinum Fuel Cells at Ultra-Low Platinum Loading. J. of Power Sources 109 : 483-493.
5. Lee S J ; A Chang-Cien ; Cha S W ; O’Hayre R Y ; Par k I ; Saito Y ; Pr inz F B (2002) Design and Fabrication of a Micro Fuel Cell Array with ‘‘flip-flop’’ Interconnection. J. of Power Sources 112 : 410-418.
6. Hang A T ; White R E ; Weidner J W ; Huang W ; Shi S ; Stoner To ; Rona N (2002) Increasing Proton Exchange Membrane Fuel Cell Catalyst Effectiveness Through Sputter Deposition J. of Electrochemical Society 149 : A280-A287. 7. Cho S Y ; Lee W M (1999) Performance of Proton Exchange Membrane Fuel
Cell Electrodes Prepared by Direct Deposition of Ultrathin Platinum on the Membrane Surfaces. J. of Electrochemical Society 146 : 4055-4060.
117 Membranes in Polymer Electrolyte Fuel Cells. J. of Electro chemistry 404 : 37-43.
9. Muller M ; Muller C ; Grombale F ; Woffee M ; Menz W (2003) Micro-structured Flow Field for Small Fuel Cells. Microsystem Technology 9 : 159-163.
Table 1 Geometric parameters
material dimensions
Channel width
(μm) Channel depth (μm) Flow field plate Polymethylmethacrylate
(PMMA) 22.5mm× 22.5mm×250 μm (L×W× H) 400 200 Catalyst layer Pt (99.99%) 22.5mm×22.5mm×55nm(L×W×H)
MEA Nafion 117 (ElectroChem) 22.5mm×22.5mm×183μm(L×W×H) Table 2 Operating condition for sputtering platinum film on Nafion 117
sputtered conditions parameter
mode RF
target Pt (99.99%)ψ2”x3mm t
atmosphere Ar
ion gauge 3×10-5 torr
operating vacuum 1×10-5torr
RF power 80 W
triggered power 20 W
temperature of the cooling water 16℃
mass flow control 12.5 SCCM
thicker platinum film 0.15mg/cm2 (55nm thickness)
substrate temperature 40℃~60℃
Fig. 1 Detailed Structure for Single Cell PEMFC 50 (rib) unit:μm 400 (channel) 22500 22500 20000
Fig.2 The dimensions of flow field plate Platinum deposited with MEMS process
1. Sputtering thin Pt film on Nafion 117
Pt (55nm)
Nafion (183μm)
2. Mask design
Flow field pattern
2mm
2mm
4.Excimer Laser Micromachining : Mask Dragging
Mask
PMMA
V
3. Excimer Laser Micromachining : Mask Projection
Mask
PMMA
Cu (1μm)
5. Sputtering thin Cu film on flow field plate
6. Bonding
H2 Air
MEA
flow field plate Fig.3 Fabrication processes of micro fuel cell cell
(a) MEA with thin Pt film (b) Flow field plate with thin Cu film
Fig.4 The photograph of MEA and flow field plate
1cm
1cm
Fig.5 The photograph of micro fuel cell
55nm Pt film (1.5x1.2μm2)
Fig.6 SEM image showing the morphological evolution of thin platinum film sputtered on Nafion 117
(a) Top view (200nm×200nm) (b) 3-D view (200nm×200nm)
Fig.7 AFM images showing the morphological evolution of thin platinum films
0 50 100 150 200 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 po w er de ns ity ( m W /c m 2 )
Fig.8 The polarization curve for the present single cell
ce ll v o lta g e (V )
current density (mA/cm2)
0 5 10 15 20 25 30 35
