The fabrication and performance of a poly ...

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33 (2008) 2059 – 2063

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Technical Communication

The fabrication and performance of a poly(dimethylsiloxane) (PDMS)-based microreformer for application to electronics Ji Won Haa,, Jae Hyuck Jangb, Jae Hyoung Gilb, Sung-Han Kimb a

Department of Chemistry, Rice University, 6100 Main Street, Houston, TX 77005, USA Micro-Fuel Cell Team, Electro-Material and Device Laboratory, Central R&D Institute, Samsung Electro-Mechanics, Maetan 3-Dong, Yeoungtong-Gu, Suwon 442-838, South Korea

b

art i cle info

ab st rac t

Article history:

A miniaturized poly(dimethylsiloxane) (PDMS)-based methanol steam reformer having a

Received 22 August 2007

serpentine microchannel for application in a proton exchange membrane fuel cell (PEMFC)

Received in revised form

has been developed. The fabricated PDMS microreformer consists of four layers, and a

31 January 2008

commercial thin-flexible heater for reforming reaction is embedded in the PDMS layers.

Accepted 16 February 2008

The volume of a PDMS microreformer is about 10 cm3 . The commercial Cu=ZnO=Al2 O3

Available online 2 April 2008

reforming catalyst was used and the Cu=ZnO=Al2 O3 reforming catalyst particles of mean

Keywords: Fuel cell Reformer Hydrogen energy Poly(dimethylsiloxane) (PDMS)

1.

diameter 50270 mm was packed into the microchannels by fluidized method. In this study, the miniaturized PDMS microreformer was operated successfully in the operating temperatures of 1802240 C and 30–40% molar methanol conversion was achieved in the temperature range for the feed rate of 10 and 50 ml min

1

.

& 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Introduction

Fuel cells (FCs) have emerged strongly as an alternative power source owing to their high-energy efficiency and eco-friendly nature [1,2]. Among the wide variety of areas in which FCs can be employed, one of the promising fields of applications is their use as portable power sources making them prominent alternatives to lithium-ion batteries. In the context of portable and palm power sources, FCs have the potential of providing energy storage densities several times higher than those possible using current state-of-the-art lithium-ion batteries [3]. Two types of small FC systems have been proposed: direct methanol fuel cell (DMFC) systems and proton exchange membrane fuel cell (PEMFC) systems. DMFC systems have the

reserved.

advantage of room temperature operation but it offers only relatively low-power density due to methanol crossover through the polymer electrolyte membrane and the lowreaction rate of methanol oxidation over the anode electrocatalyst. In contrast, PEMFC systems generate electrical energy from concentrated hydrogen produced from steam reforming of methanol. This can meet the demands of advanced portable electronic devices because of their ability to deliver higher energy per volume and weight, and can be integrated as onboard power sources in the sub-Watt range to operate small sensors and actuators. Moreover, at slightly higher output powers, they can be used in other applications to power portable electronic devices like cell phones (1 W), laptop computers (20 W), etc [4].

Corresponding author. Tel.: +1 713 3483391; fax: +1 713 3485155.

E-mail address: [email protected] (J.W. Ha). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.02.031

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Fabrication and miniaturization of efficient microreformers for producing hydrogen from methanol are critical part for developing PEMFCs; thus, there has also been considerable research activity to develop a compact, efficient and miniaturized reformer in recent years [5–10]. The fabrication route for microreformer for PEMFC based on the micromachining technologies on silicon wafers was used by many research groups [11–16]. Kothare et al. developed a radial flow packed-bed microreactor based on the silicon micromachining [17]. Kundu et al. reported a silicon-based microreformer fabricated for a reformed-hydrogen FC for application in a cell phone [18]. However, the silicon micromachining process is very difficult, highly expensive and time-consuming. Moreover, silicon wafers would be easily broken by the external impact during the making process or experiments because they are very fragile. Kothare et al. reported a glass-based microreactor which was more simple in design, easy to fabricate, and economical than silicon-based microreactor [19]. However, this glass-based microreactor also has limitations in application for the development of cellular phone in companies. That is because the volume is relatively large, it is difficult to apply for massproduction, it has less reproducibility, it is very fragile, it is not easy to bond between glass wafers, etc. Therefore, extensive research efforts are necessary in order to develop novel and cheap FC materials which simplify manufacturing process and allow for mass-production in companies at a low cost. In this paper, for the first time, an elastomeric poly(dimethylsiloxane) (PDMS) was employed as a substrate for developing microreformers in order to overcome the above drawbacks. A PDMS offers many advantages over conventional substrate materials such as silicon [20] and glass [19]. It is less expensive and fragile and PDMS microreformers are fabricated faster with more reproducibility, because their fabrication mostly employs replication techniques, such as casting, molding and embossing. PDMS can be both reversibly and irreversibly bonded to other PDMS, glass and silicon oxide surfaces. In addition, multiple (30 or more) PDMS microchips can be produced rapidly from a single master at very low cost, so that PDMS is suitable for mass-production in companies. Furthermore, it is easy to interface the external world with a PDMS microreformer through silicon tubing and to bond two PDMS plates by corona discharges. Therefore, PDMS has numerous advantages over conventional materials such as silicon and glass. In the present work, a miniaturized methanol reformer with commercial Cu=ZnO=Al2 O3 catalyst-based microreformer for a small PEMFC is designed and fabricated using PDMS. The miniaturized microreformer consists of four layers, and a commercial thin-flexible heater for supplying heat is embedded in the PDMS layers. The microchannels are packed with commercial Cu=ZnO=Al2 O3 reforming catalyst particles of mean diameter 50270 mm. Then, the methanol conversion at the relatively low-operating temperatures of 1802220 C was investigated.

2.

Experimental section

2.1.

Chemicals and materials

A PDMS used in this work was obtained from Dow Corning (Midland, MI, USA) under the product name sylgard 184.

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Methanol used as a fuel was purchased from Sigma (St. Louis, MO, USA). Thin-flexible heaters were purchased from OMEGA Technologies Company (Stanford, CT, USA). Commercial reforming catalyst pellets used in this paper were obtained from Su¨d-Chemie. The catalyst particles of desired size range are obtained by grinding and sieving. The gas product was analyzed by micro-GC equipment (Agilent Technologies, Paolo Alto, CA, USA).

2.2.

Fabrication of PDMS microreformers

A schematic diagram of a PDMS microreformer with a serpentine microchannel is given in Fig. 1. A PDMS microreformer consists of four layers as shown in Fig. 1: an upper PDMS plate with a microchannel network, an embedded PDMS plate, a thin-flexible heater, and a lower PDMS plate. A 2 mm-thick PDMS plate with a serpentine microchannel was fabricated by a photolithography replica molding technique. This technique is well-described elsewhere [21,22]. A serpentine microchannel was 1000 mm in both width and depth. The irreversible bonding between the PDMS plates was performed by using corona discharge generated by the Tesla coil (BD-10A; Electro Technic Products, Inc., IL, USA). The bonding between PDMS plates and a thin-flexible heater was achieved by spreading surfaces of the heater with PDMS solutions and by curing at 75 C for 1 h in the oven. After bonding all plates, catalyst particles with mean diameter of 50270 mm were introduced into a microchannel by carrying out fluidized packing of catalyst particles. The catalyst powder was placed on the inlet port. Then, packing was achieved by applying air pressure using a 5-cc syringe. In this experiment, in order to trap catalyst particles, glass cotton was employed as a filter located in both inlet and outlet ports. Fig. 2(A) and (B) show photographic images of a PDMS microreformer: before catalyst packing (A) and after catalyst packing (B), respectively. The volume of the PDMS microreformer is about 10 cm3 . Fig. 2(C) and (D) are photographic images of a PDMS microreformer in side view (C) and a commercial thin-flexible heater (D).

2.3.

Experimental setup

The test system for methanol reforming using the PDMS microreformer is shown in Fig. 3. It consists of a liquid pump for feeding the liquid methanol–water mixture (1:1, molar H2 O : CH3 OH) at desired flow rates and a gas analyzer which provides an on-line analysis of the composition of the exit gases from the outlet port. A bubble flow meter was used for measuring flow rates of the exit gases. In order to remove unreacted methanol and water vapor, a cold trap has been inserted between outlet of the microreformer and the inlet of the gas analyzer. The microreformer was heated by applying direct current from DC power supply to the embedded thinflexible heater instead of a hotplate, and the temperature of the microreformer was measured by a thermocouple as shown in Fig. 1. In this experiment, the catalyst activation was performed prior to use. Thus, the fresh catalyst particles have been reduced by introducing a methanol/water feed at a temperature gradually increased from 150 up to 175 C over a period of about 10 h of operation. Due to the transparence of


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2061

Fig. 1 – A schematic diagram of PDMS-based microreformer with a serpentine channel.

3 cm

3.5 cm

Fig. 2 – Photographic images of (A) a PDMS microreformer before filling catalyst particles, (B) a PDMS microreformer after filling catalyst particles, (C) a PDMS microreformer in cross sectional view and (D) a commercial thin-flexible heater.

2.4.

Methanol reforming with a PDMS microreformer

Methanol reforming refers to the chemical reaction between methanol ðCH3 OHÞ and water ðH2 OÞ vapor for the production of hydrogen gas. This reaction is typically occurred in the presence of metal oxide catalyst at operating temperatures of 200–320 1C. The chemical reactions taking place during the steam reforming process are expressed as follows:

Fig. 3 – Schematic of experimental setup.

PDMS, the catalyst activation can be observed visually via a change in color of the catalyst powder from black to yellowish brown. After the catalyst activation phase, the operating temperature of the microreformer was gradually increased up to about 180–240 1C and the methanol conversion was investigated.

CH3 OH þ H2 O 2 CO2 þ 3H2 ,

(1)

CH3 OH 2 CO þ 2H2 ,

(2)

CO þ H2 O 2 CO2 þ 3H2 .

(3)

Reaction (1) is the main reforming reaction which gives the stoichiometric conversion of methanol to hydrogen. It can be regarded as the overall effect of reactions of methanol decomposition (2) and the water–gas shift reaction (3). In this study, for the first time, the feasibility of PDMS-based

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microreformer for PEMFC applications and the effect of an operating temperature ranging from 180–240 1C on molar methanol conversion were investigated at the feed rate of 10 1 and 50 ml min .

3.

Results and discussion

PDMS is one of popular substrate materials for lab-on-a-chip. However, there have not been practical applications of PDMS to microreformers to which the high temperature exceeding 250 1C is needed. In this work, for the first time, native PDMS was successfully employed as a substrate material for developing microreformer for PEMFC, and the methanol conversion on PDMS-based microreformer at the relatively low temperatures of 180–240 1C, was investigated. The thermal resistance of native PDMS prior to experiment was studied. A PDMS piece with 30 mm (horizontal) 30 mm (vertical) 5 mm (height) was placed on a hotplate. It is observed that PDMS can endure up to 250 1C without any damage and deformation. The flow rates that can be used for a PDMS microreactor which does not have a vaporization region were investigated. The relatively low-feed rates of 10 1 and 50 ml min were chosen for this test, and there was no breakage of a PDMS microreactor due to the sudden expansion of the liquid feed. To investigate the relationship between applied power to the heater and temperature, applying voltage was gradually increased and an internal temperature was measured. Fig. 4 shows the result of heater performance test. In this experiment, a desired temperature was adjusted based on the result shown in Fig. 4. Fig. 5 shows the trend of methanol conversion by varying the feed rates at operating temperatures from 180–240 1C. At a 1 fixed feed rate of methanol and water mixture of 10 ml min , the molar methanol conversion was about 9% at 180 1C and 35% at 220 C. At a fixed feed rate of methanol and water mixture of 50 ml min1 , the methanol conversion was about 12% at 180 C and 30% at 220 C. As shown in Fig. 5, when the methanol feed rate was increased, the methanol conversion decreased. The reduction in the methanol conversion seemed to be originated from the retention time of reactants with catalyst. It is obvious that the conversion will increase if the operation temperature is raised, but high-operating

300

(11, 280)

Temperature (°C)

250 (7.5, 220)

200 (5, 160)

150

Conversion (%)

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50 45 40 35 30 25 20 15 10 5 0

10 µl min-1

170

180

190

50 µl min

200 210 220 Temperature (°C)

230

240

250

Fig. 5 – Methanol conversion vs. feeding rates of 10 and 1 50 ll min at the operating temperature increased from 180 1 1 to 240 1C ðE10 ll min , ’50 ll min ).

temperature will accelerate the deactivation of the catalyst. In this study, it is not possible to raise the temperature beyond 240 C due to unstability of native-PDMS materials. Therefore, the catalyst performance test was carried out at the maximum temperature of 240 C, though the methanol conversion achieved at this temperature is about 37%. In this work, it is certainly observed that PDMS can be used for developing microreformers for PEMFC. Furthermore, it is expected that the molar methanol conversion will be improved by optimizing channel networks, feed rates, amounts of catalysts and the size of a catalyst particle at the same temperature ranges in the future works.

4.

Conclusions

A miniaturized methanol steam reformer based on poly(diemthylsiloxane) (PDMS) with a serpentine microchannel for application in a microproton exchange membrane fuel cell (PEMFC) is presented in this paper. PDMS provides a lot of advantages over conventional silicon substrate. It is less expensive and fragile. PDMS microreformers are fabricated faster with more reproducibility. The fabricated PDMS microreformer consists of four layers, and a commercial thinflexible heater for reforming reaction is embedded in the PDMS layers. The volume of a PDMS microreformer is about 10 cm3 . The catalyst used was commercial Cu=ZnO=Al2 O3 reforming catalyst. The Cu=ZnO=Al2 O3 reforming catalyst particles of mean diameter 50270 mm was packed into the microchannels by fluidized method. In this study, the miniaturized PDMS microreformer was operated successfully in the operating temperatures of 180 C2240 C and 30–40% molar methanol conversion was achieved in the temperature range for the feed rate of 10 and 50 ml min1 .

100 50

Acknowledgments

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1

2

3

4

5 6 7 Power (Watt)

8

9

10

11

12

Fig. 4 – Heater performance: temperature vs. applied power.

One of the authors (Ji Won Ha) is very glad to express his thanks and respect to his teacher, Professor Jong Hoon Hahn in the Chemistry Department of Pohang University of Science


and Technology, through this paper. The authors are grateful to the SAMSUNG Electro-Mechanics, Co., Ltd. and the Ministry of Commerce, Industry and Energy of the Korean Government for the financial support of this work. R E F E R E N C E S

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