Investigation of bubble effect in microfluidic fuel cells by a simplified microfluidic reactor

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Investigation of bubble effect in micro

ﬂuidic fuel cells by a simpliﬁed

micro

ﬂuidic reactor

Jin-Cherng Shyu

a,*

_{, Chung-Sheng Wei}

b

_{, Ching-Jiun Lee}

c

_{, Chi-Chuan Wang}

d

a_{Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan} b_{Nanya Technology Corporation, Taoyuan 33859, Taiwan}

c_{Institute of Applied Mechanics, National Taiwan University, Taipei 10617, Taiwan}

d_{Department of Mechanical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan}

a r t i c l e i n f o

Article history:

Received 16 November 2009 Accepted 23 April 2010 Available online 4 May 2010 Keywords:

Microﬂuidic Solubility

Gas/liquid two-phaseﬂow Fuel cell

a b s t r a c t

This study experimentally examines the influence of two-phase flow on the fluid flow in membraneless microfluidic fuel cells. The gas production rate from such fuel cell is firstly estimated via corresponding electrochemical equations and stoichiometry from the published measured currentevoltage curves in the literature to identify the existence of gas bubble. It is observed that O2bubble is likely to be generated in Hasegawa’s experiment when the current density exceeds 30 mA cm2_{and 3 mA cm}2_{for volumetric}_flow rates of 100mL min1and 10mL min1, respectively. Besides, CO2bubble is also likely to be presented in the Jayashree’s experiment at a current density above 110 mA cm2_{at their operating volumetric liquid}_flow rate, 0.3 mL min1. Secondly, a 1000-mm-width and 50-mm-depth platinum-deposited microfluidic reactor is fabricated and tested to estimate the gas bubble effect on the mixing in the similar microchannel at different volumetricflow rates. Analysis of the mixing along with the flow visualization confirm that the membraneless fuel cell should be free from any bubble, since the mixing index of the two inlet streams with bubble generation is almostfive times higher than that without any bubble at the downstream.

1. Introduction

In recent years, much attention has been focused on fuel cells because of their attractive advantages, such as higher efficiency of energy conversion and no carbon dioxide emission, over the conventional combustion of gasoline and other fossil fuels. Among all kinds of current fuel cells being studied, polymer electrolyte membrane (PEM) fuel cell employing perfluorosulfonic acid poly-mer membranes, such as NafionÒ, yields the best performance for stationary and transportation applications. As a consequence, researches associated with the PEM fuel cells development had been well elaborated.

Despite the cited promising features, the PEM has certain intrinsic problems, such as tearing, deterioration, and inefﬁcient prevention of fuel crossover as stated by Savadogo [1]. Besides, decreasing the membrane thickness results in higher ionic conductivity but is susceptible to the fuel crossover. Moreover, in order to avoid the dehydration of the PEM for a better performance, PEM fuel cell systems like that proposed by Nguyen and White[2],

Ferng et al.[3], and Williams et al.[4]as well as Wang et al.[5]are rather complex for incorporating both an external humidiﬁcation system and a heat exchanger, leading to some concerns in well managing a PEMFC system.

With the advance of microfabrication technology, it has become possible to fabricate a novel microfluidic fuel cell operated without a PEM by transporting the aqueous fuel, oxidant and/or electrolyte streams in a single microchannel through different inlets under laminarflow condition. In recent years, the so-called microfluidic membraneless fuel cell has been proposed and tested by several research groups such as Ferrigno et al.[6], Choban et al. [7e9], Cohen et al.[10,11], Jayashree et al.[12], Hasegawa et al.[13], and Sun et al.[14]. Various designs of some published researches per-taining to membraneless microfluidic fuel cell are summarized in

Table 1andFig. 1. With this smart design, both fuel crossover and water management could be avoided due to the nature of laminar

liquidﬂow. Furthermore, adjustments of ﬂow rates and channel

dimensions allow more precise control of the electrochemical processes that are taken place at the catalyst-covered electrodes. Such microﬂuidic fuel cells are considered as a feasible micropower sources for miniature and portable devices. These portable devices include not only cell phones and laptop computers, but also clinical and diagnostic tests, microanalytical systems for ﬁeld tests, and global positioning systems[7].

* Corresponding author. Tel.: þ886 7 3814526x5343; fax: þ886 7 3831373. E-mail address:[email protected](J.-C. Shyu).

Contents lists available atScienceDirect

Applied Thermal Engineering

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o ca t e / a p t h e r m e n g

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Table 1

Summary of various design parameters for some published membraneless microﬂuidic fuel cell researches.

Authors Aqueous reactant Catalyst/position Catalyst deposition Channel size (mm) Flow rate (mL/min)

Fuel Oxidant Anode Cathode

Choban et al.[7] HCOOH KMnO4 Pt-black Pt-black Electrodeposition W H ¼ 0.5 0.5 0.3e0.8

O2/H2SO4 W H ¼ 1.0 1.0

Opposite sidewalls L¼ 30 mm

Cohen et al.[11] H2/KOH O2/H2SO4 Pt Pt E-beam evaporation W H ¼ 1.0 0.25

H2/H2SO4 O2/KOH W H ¼ 1.0 0.38 0. 5e2.0

Opposite sidewalls L¼ 50 mm

Choban et al.[9] Pt/Ru Apply the catalyst suspension on the electrode and dry it 0.15e0.4

CH3OH O2/H2SO4 Ptþ Ru Pt-black W H ¼ 0.75 1.0

CH3OH/H2SO4 Pt-black L¼ 29 mm

Opposite sidewalls

Hasegawa et al.[13] H2O2/NaOH H2O2/H2SO4 Pt Pt Argon ion sputter W H ¼ 1.0 0.05 1.44

The same surface

Cohen et al.[10] HCOOH/H2SO4 O2/H2SO4 Pt Pt E-beam evaporation W H ¼ 1.0 0.25 0.5

W H ¼ 1.0 0.38

Opposite sidewalls L¼ 50 mm 0.5

Choban et al.[8] CH3OH/H2SO4 O2/H2SO4 Pt/Ru Pt-black Dry the catalyst suspension applied on the electrode W H ¼ 0.75 1.0 0.3

CH3OH/KOH O2/KOH L¼ 29 mm Opposite sidewalls Fig. 1 . Two typical designs of published membraneless micro fl uidic fuel cell with two inlets: (a) anode cataly st and cathode cataly st are on the opposite sidew alls, (b) both anode cataly st and cathode cataly st are on the same surface. Fig. 2. Schematic illustration of the present Y -shaped micro fl uidic react or design, whose cataly st is used to catalyze the H 2_O 2 decomposition to imitate the bubble gener ation at one electrode of the membraneless micro fl uidic fuel cell. J.-C. Shyu et al. / Applied Thermal Engineer ing 30 (20 10) 1863 e 1871 18 6 4

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2H2O2/2H2Oþ O2 (7)

Photos that represent hydrogen peroxide and water with injection of red dye in the Y-shaped microﬂuidic reactor at 0.1 M subjected to various volumetric ﬂow rates are shown in Fig. 6, where (a)e(c) is 1

m

L min1; (d)e(f) is 10

m

L min1; (g)e(i) is 100

m

L min1; and (j)e(l) is 1000

m

L min1. As is clearly shown in theﬁgure, bubble is generated when the volumetric ﬂow rate is less or equal to 100

m

L min1. A calculation of the corresponding gas superficial velocity and liquid superficial velocity suggests the two-phase flow patterns fall with the slug/bubbly region of the

corresponding microchannel based on the_{ﬂow pattern proposed}

by Kawaji and Chung[20]. Note that in microchannel, the size of the detached bubble is comparable or much larger than the micro-channels, this is especially applicable when the system pressure is low or moderate like present case. In this regard, the bubble/slug being generated can easilyﬁll up the channel, signiﬁcantly altering the interface of the two streams, or even mixing up the two streams completely as shown inFig. 7(a). In either case, it would doom the performance of fuel cell due to less effective electrode area or crossover phenomenon. Actually, compared to the case without any addition of H2O2, denoted as 0 M, the mixing index estimated by Eq. (4)of the two inlet streams due to the effect of bubble generation in

the microchannel is almost ﬁvefold as compared to that at the

downstream shown inFig. 7(b) with a lower volumetricﬂow rate, 0.001 mL min1.

In addition, the periodic generation and detachment of the bubble/slug_{flow pattern result in intermittent flow field along the} microchannel, which eventually lead to an unstable fuel cell oper-ation. This can be made clear from the progress offlow pattern (e.g.

Fig. 6(a)e(c), (d)e(f) or (g)e(i)) where the generated small bubbles agglomerated with each other to become larger slug/plug, yet the detachment of larger slug give way to liquid refilling. The cyclic intermittent process caused by the slugflow inevitably offset the performance of required laminarflow. In the meantime, the two-phaseflow pattern is no longer in existence when the volumetric flow rate is raised to 1000

m

L min1. This again agrees with the foregoing discussion that no bubble will be observed provided that it is within the solubility threshold. Analogous observation prevails for a concentration of 1.0 M as seen in Fig. 8. However, higher concentration results in more violent reaction. Hence, a number of bubbles being generated and agglomeration of these bubbles becomes more pronounced, leading to an even worse performance of the fuel cell at this situation. In addition, theﬂow pattern shown in bothFig. 6(j)e(l) andFig. 8(j)e(l) at a volumetric ﬂow rate of 1000

m

L min1indicated that the mixing performance between two streams would be close to that without any addition of H2O2, as denoted as 0 M inFig. 7(b), since there is almost no bubble presence in the microchannel.

Similar analysis was also made for two different kinds of mem-braneless microﬂuidic fuel cells that also produce carbon dioxide

[7,12]. It is found in Fig. 9 that carbon dioxide produced in the electrochemical reaction can be completely dissolved into the liquid

stream in the Choban_{’s experiment whereas the carbon dioxide}

produced in the Jayashree’s experiment is normally below the

threshold value of solubility at their operating volumetric liquidflow rate, 0.3 mL min1. It is only at a current density above 110 mA cm2, as shown inFig. 9, two-phaseflow may be likely to occur. Such critical value is almost identical to the limiting value observed by Jayashree et al.[12]. Beyond this limiting current density, the authors suggested that crossover of formic acid to the cathodic stream would happen. In view of the foregoing discussions andflow visualization, it is therefore concluded that the influence of the gas/liquid

two-phaseﬂow on the performance of such membraneless microﬂuidic

fuel cell may be quite crucial, and it is recommended that checking

the threshold solubility with an operatedﬂow rate is needed prior to conducting numerical simulation or experimental study for such membraneless microﬂuidic fuel cell.

4. Conclusions and suggestions

This study presents an analysis and aflow visualization exper-iment to examine the influence of two-phase flow on the fluid flow in membraneless microfluidic fuel cells. Prior the flow visualization experiment, an analysis is performed based on the comparison of the solubility of the gaseous product with the gas generation rate obtained by stoichiometry (Eqs.(5) and (6)) via their corresponding chemical equations.

The calculated results show that oxygen bubble is likely to be generated in Hasegawa’s experiment[13]when the current density exceeds 30 mA cm2and 3 mA cm2for volumetric ﬂow rates of 100

m

L min1and 10

m

L min1, respectively. Besides, it is found that the CO2 bubble is also likely to be presented in the Jayashree’s experiment [12] at current density above 110 mA cm2 at their operating volumetric liquidflow rate, 0.3 mL min1. According to those calculation results, it is demonstrated that larger bubble in such microfluidic fuel cell would be easier to be formed either at the condition of higher current production or at a lower volumetric flow rate of the reactant.

For further clariﬁcation of this observation, a ﬂow visualization

using a 1000-

m

m-width and 50-

m

m-depth platinum-deposited

Y-shaped microchannel is also conducted under laminar ﬂow

condition. Based on theflow visualization results, it is found that the presence of bubbles/slug is strongly connected with threshold solubility. When the operation is above the limit, two-phase_flow prevails and the generated bubble/slugs can easilyfill up the whole

microchannel, thereby altering the ﬂow ﬁeld completely. Such

bubble generation in the microchannel causes the mixing enhancement of the two inlet streams. Compared to the case without any H2O2 addition, the mixing index of the two inlet streams due to the effect of bubble generation in the microchannel

is almostﬁvefold at the downstream when operated at a lower

volumetricﬂow rate,1

m

L min1. By contrast, there are no detectable bubbles/slugs when theﬂow is being operated below the solubility limit. Hence, as theﬂow rate increases, 1000

m

L min1, the mixing enhancement in the present study is insignificant due to no visible bubble formed in the microchannel. It is therefore concluded that the influence of the gas/liquid two-phase flow on the performance of such membraneless microfluidic fuel cell may be quite crucial.

A microfluidic membraneless fuel cells with high-aspect-ratio microchannel had been fabricated for observation of the bubble formation during the electric generation process. Measurements of the relevant polarization curve along withflow field visualization and related in-depth discussion of bubble effect on the perfor-mance of such microfluidic membraneless fuel cell is now being carried out. The gas bubble is indeed generated during the electric generation process. Both polarization measurement andflow fluid observation results will be presented in the near future.

Acknowledgement

Theﬁnancial support provided to this study from the National Science Council of Taiwan under the Contract No. NSC 98-2221-E-151-056 is gratefully acknowledged.

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