2010 MEMS Hur Membrane Less Fuel Cell

8/6/2019 2010 MEMS Hur Membrane Less Fuel Cell

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MEMBRANELSS MICRO FUEL CELL CHIP

ENABLED BY SELF-PUMPING OF FUEL-OXIDANT MIXTURE

Janet I. Hur1, Dennis D. Meng2 and Chang-Jin CJ Kim11University of California, Los Angeles (UCLA), California, U.S.A.2 Michigan Technological University, Houghton, Michigan, U.S.A.

ABSTRACT

Despite the significant research in micro fuel cells,miniaturizing a whole fuel cell system to below a

few millimeters did not appear feasible until veryrecently, because ancillary parts (e.g. external pump,gas separator) are needed to complete a self-standing system. Building on the recently

discovered self-pumping fuel cell free of theancillary parts, here we design and test new self-

pumping fuel cell architecture free of the membraneelectrode assembly (MEA). Benefiting from theselectivity of catalysts on each electrode under slowflow rates, we find that the device can be run in amixed stream instead of the two streams ofunmixed laminar flows of fuel and oxidant, greatly

simplifying the design. Proof-of-concept deviceverifies the mechanism of embedded fuel pumpingand CO2 degassing, while producing 8 mW/cm

2 ofpeak power density.

INTRODUCTION

While their regular-scale counterparts are wellaccepted for many applications, microscale fuel

cells are yet to be realized mainly because a self-standing whole system would require all the

ancillary parts to be miniaturized and packaged intoa small volume (e.g., below 1 cm3). To address thisso-called packaging penalty, a bubble-pumpingmechanism has recently been developed [1],

eliminating the external pump and gas separator ina full fuel cell loop [2] (Figure 1 (a)). In order to

further improve such a small and simple fuel cellsystem, membrane electrode assembly (MEA) hasnow been identified as a main challenge.

On the other hand, in an effort to eliminate theMEA from the fuel cell design, fuel and oxidantwere pumped across a microchannel in laminarfashion with little mixing [4]. In this membraneless

fuel cell, proton is diffused across the laminarinterface, and the streams of fuel and oxidant flowon top of their own respective electrodes (i.e.,anode and cathode) without significant mixing(Figure 1 (b) [5]). However, both fuel and oxidantstreams require certain pumping rates in order to

maintain the laminar interface and keep the fuel

from crossing over to cathode. This could be aproblem in our device, since the CO2 bubble actionsin the self-pumping mechanism [1] may disrupt the

laminar interface and result in a slow feed rate.

Most recently, it has been shown that the fuel and

oxidant can flow mixed if a proper catalyst (notplatinum) is used so that each electrode selectively

oxidizes and reduces the fuel and oxidantindividually [6]. In most of the case, platinum is

reactive catalyst for both fuel oxidizing and oxygenreducing. If a cathode is comprised of platinum

catalyst, the electrode will face both oxidation andreduction resulting in mixed potential. Therefore, itis preferred to have non-platinum-based catalyst inmixed fuel-oxidant stream. While implementing themechanism, we further found that mixed fuel workseven with platinum-based catalyst on cathode when

the flow rate is sufficiently low, which is the casefor bubble pumping.

This new discovery, along with the self-regulatingnature of our self-feeding mechanism, has allowedus to design a dramatically simple yet efficient fuel

cell system.

(a) (b)

Vt

Vgleft Vg

right

Vsleft

Vsright

channel necks

bubble generator

hydrophilic hydrophobic

porousmembrane

1

2

3

Figure 1 Proposed self-pumping fuel cell is based on (a) bubble pumping mechanismreported by Meng and Kim [1]; 1: directionalbubble growth; 2: built-in bubble displacement;3: symmetric bubble collapse. (b) Laminar flow

based fuel cell designed by Jayashree et al. [5].

978-1-4244-5763-2/10/$26.00 2010 IEEE 168


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MECHANISM

Our self-feeding mechanism is based on the byproduct CO2 generated by the electrochemical

reaction [2]. Figure 2 describes the mechanism ofbubble pumping to self-feed the fuel. In the reactionchannel, small bubbles nucleate and merge to growinto a bigger bubble. As the bubble vents out of the

channel eventually, fresh fuel is fed into thechannel, starting the next pump cycle. Since the

bubble generation rate is directly related to thereaction rate, the pumping rate can be regulated bythe external load (i.e., current output). The resultingself-regulating phenomenon delivers the fuel on

demand, eliminating the need for active feedbackbetween the load and fuel delivery.

FABRICATION

A proof-of-concept device was built by placing aCNC-milled 1.5 mm-thick PMMA sheet between agraphite anode and an air-breathing cathode (Figure3). Sidewalls of PMMA channel were polished

down to achieve optically smooth surface for visual

verification of bubble pumping. The microchannel

in the PMMA sheet contained a check-valvestructure to guide bubble growth toward one

direction. A diverging channel shape along thedownstream to the outlet also guides the bubbleduring the reaction.

A fuel inlet, waste outlet and vent hole were drilledin the graphite anode, which was loaded with 10

mg/cm2

Pd and 1.5 mg/cm2Nafion

(10 wt%). The

pre-catalyzed gas-diffusion cathode wasadditionally loaded with 2 mg/cm

2of Pt and 0.1

mg/cm2 of Nafion[7]

After the rubber cement was applied between layers

to serve as a gasket, fuel cell assembly was clampedwith paper binder clip to avoid the leaking and bulging during the experiment. Finally,polypropylene tubings were connected to inlets and

outlets.

EXPERIMENT

The fuel cartridge is quite simple as it provides fuel(HCOOH) in oxidant medium (1M H2SO4) as asingle stream (vs. two laminar streams of fuel andoxidant). While fuel was supplied through the inleton the anode, oxygen was supplied from ambient

air through the air-breathing cathode.

Although platinum catalyst on the cathode readilyoxidizes the fuel in a mixed stream, we havediscovered that under a low fuel flow rate the fuelcell shows an encouraging potential differencebetween anode and cathode. In our tests, the initial

drop of the open circuit potential (OCP) returned

Figure 3 A schematic view of the proof-of-concept fuel cell device tested in this paper.

Graphite anode (shown transparent) has threedrilled holes for fuel inlet, waste outlet and vent.PMMA sheet is machined to form amicrochannel with check valve for directional bubble growth. Carbon cloth is used for gas-

diffusion cathode.

Anode

Cathode

Check valve Anode

Cathode

Check valve

(a) CO2 bubbles nucleate on anode catalyst

when electrical load is applied.

CO2Mixed fuel

Outlet

Inlet

CO2Mixed fuel

Outlet

Inlet

(b) Bubbles accumulate and grow towardventing membrane (red) while it is blocked bycheck valve on the inlet site.

Porous membranePorous membrane

(c) When the bubble reaches the hydrophobicventing membrane, bubble is captured and startsto vent out.

(d) While the bubble shrinks fresh fuel is pulledin starting the next pumping cycle.

Figure 2 Cross-sectional view of device toshow the schematic of bubble growth inside the

reaction chamber.

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over time, recovering normal fuel cell behavior. Inthe same way, when constant potential was applied,

initial drop of current density was seen and gainingback the normal current density over time (Figure4). We speculate this unexpected but wonderfully

encouraging result was because a protection layerformed on the cathode, isolating the fuel in themixed stream from the cathode surface.

Verification of pumping

Figure 5 shows video stills during actual fuel celloperation. As expected, small bubbles nucleated onanode quickly merged to a bigger bubble and grew

toward the outlet (to the right), whereas the otherside (to the left) was blocked by the check valve.While the bubble expanded, the current outputgradually dropped due to the decreased effectivereaction area. As the bubble was released throughthe vent membrane, fresh fuel is fed into the

channel, again starting the next pump cycle withhigh power. This self-pumping cycle wasrepeatable until the fuel ran out and recorded for a

few cycles (Figure 6).

Fuel cell performance

Although fuel cell performance was expected to bepoor due to the dead volume inside the channel, ourdevice showed an 80 mA/cm2 current density

(Figure 7), which is comparable to othermembraneless cells that employ a complicated

architecture including an external pump. Weobserved fuel utilization up to 94 % at feed rates as

low as 2.7 L/min. However, the feed rate andutilization fluctuated over time, prompting us to

Figure 6 Current density measured over time,verifying bubble pumping is sustainable. Smalloscillations are caused by small bubbles nearthe outlet spontaneously nucleated and vented.

As the bubbles grow and block the channel,current output decreases. When the bubbles vent

out and pull fuel into the channel, the current

output increases rapidly.

Figure 4 Current output recorded over timeunder 0.2 V load. Performance rapidlydecreases when mixed fuel is forced into thechannel but recovers as the self-pumping takes

over.

Figure 5 A cycle of actual bubble pumping captured from video, visually confirming the self-pumping

mechanism driven by the byproduct CO2.

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develop countermeasures.

Finally, we demonstrated a stacked system of twofuel cell chips (Figure 8) each self-sufficient to

showcase the scalable manufacturing of thereported architecture.

CONCLUSIONS

In an endeavor to develop a self-standing energysource with high energy density, we have shown aself-feeding micro fuel cell chip that does not usemembranes nor an external liquid pump and gas

separator. The proof-of-concept chips weresuccessfully fabricated and stacked to power ahandheld fan. Next step on this work is to furtherminiaturize the chip into microscale and build ascalable stacked system.

ACKNOWLEDGEMENTS

This work has been supported by the National

Science Funding (NSF) Grant No. 0824269. Janet I.Hur was supported by NSF IGERT: Materials

Creation Training Program (MCTP) DGE-0654431 and the California NanoSystems Institute.

REFERENCES

[1] D. D. Meng and C.-J. Kim, Micropumping of

liquid by directional growth and selective ventingof gas Bubbles,Lab on a Chip, 8 (2008), pp. 958-968.[2] D. D. Meng, and C.-J. Kim, An active micro-direct methanol fuel cell with self-circulation offuel and built-in removal of CO2 bubbles, Journalof Power Sources, 194 (2009), pp. 445-450.[3] D. D. Meng, and C.-J. Kim, Embedded self-circulation of liquid fuel for a micro directmethanol fuel cell, Proceedings of the 20th IEEE International Conference on Micro ElectroMechanical Systems, Kobe, Japan, 2007, pp. 85-88.

[4] E. Kjeang, N. Djilali, and D. Sinton,Microfluidic fuel cells: A review Journal ofPower Sources, 186 (2009), pp. 353-369.[5] E. R. Choban, L. J. Markoski, A. Wieckowski,and P. J. A. Kenis, Microfluidic fuel cell based onlaminar flow, Journal of Power Sources, 128

(2004) pp. 54-60.[6] D. T. Whipple, R. S. Jayashree, D. Egas, N.

Alonso-Vante, and P. J. A. Kenis, Rutheniumcluster-like chalcogenide as a methanol tolerantcathode catalyst in air-breathing laminar flow fuelcells, Electrochemica Acta, 54 (2009), pp. 4384-

4388.[7] R. S. Jayashree, L. Gancs, E. R. Choban, A.

Primak, D. Natarajan, L. J. Markoski, P. J. A. Kenis,Air-breathing laminar flow-based microfluidic fuelcell, Journal of the American Chemical Society,127 (2005), pp. 16758-16759.

Figure 8 Two fuel cell chips stacked and

connected in series are powering a handheldfan.

0 20 40 60 80

0.0

0.2

0.4

0.6

0.8Voltage

Power density

Current density (mA/cm2)

Voltage(V)

0

2

4

6

8

10 P

owerdensity(mW/cm2)

Figure 7 Performance of a fuel-cell chip using2M formic acid (fuel) in 1M sulfuric acid

(oxidant). It shows power density up to 7.2mW/cm

2and OCP of 0.61 V.

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2010 MEMS Hur Membrane Less Fuel Cell