1 Copyright © 2009 by ASME

Proceedings of IMECE09 2009 ASME International Mechanical Engineering Congress and Exposition

November 13-19, 2009, Lake Buena Vista, Florida, USA

IMECE2009-12062

EFFECT OF VIBRATION ON THE LIQUID WATER TRANSPORT OF PEM FUEL CELLS

Luis Breziner, Peter Strahs, Parsaoran Hutapea Composites Laboratory, Department of Mechanical Engineering, Temple University

Philadelphia, PA, USA

ABSTRACT The objective of this research is to analyze the effect of

vibration on the liquid water transport across the gas diffusion

layer (GDL) for hydrogen PEM fuel cells. This study is of great

importance, since many fuel cells operate under a vibrating

environment (such as the case in automotive applications) and

this may influence the liquid water blockage across the GDL at

different current densities, affecting the overall fuel cell

performance. From our findings, no report about this effect

could be found in the literature.

The problem was developed in two main steps. First, an

analytical model was stated, using current models for water

transport in porous media. Then, a series of experiments were

carried, monitoring the performance of the fuel cell for different

parameters of oscillation.

For sinusoidal vibration at 10, 20 and 50Hz (2 g of

magnitude), a decrease in the fuel cell performance by 2.2%,

1.1% and 1.3% was recorded when compared to operation at no

vibration respectively. For 5 g of magnitude, the fuel cell

reported a drop of 5.8% at 50 Hz, whereas at 20 Hz the

performance increased by 1.3%.

Although more extensive experimentation is needed to

identify a relationship between magnitude and frequency of

vibration affecting the performance of the fuel cell as well as a

throughout examination of the liquid water formation in the

cathode, it can be shown that sinusoidal vibration, overall,

affects the performance of PEM fuel cells.

INTRODUCTION

Fuel cells are electrochemical devices that produce

electricity directly from a chemical reaction. These devices are

very efficient, quiet and produce no polluting emissions

compared to electricity generators that use fossil fuels. One of

the most common types is the proton exchange membrane fuel

cell (PEMFC), which uses hydrogen and oxygen (or air) to

produce electric power. The only byproducts of a PEMFC are

heat and water.

The operating principle of a fuel cell was discovered in the

mid-19th

century followed by small demonstrators, which were

built solely by scientific curiosity. It was until the early 1960s

that fuel cells made its first practical application in NASA’s

Gemini and Apollo Space Programs to produce electricity for

critical equipment inside the space shuttles [1].

The applications of PEM fuel cells have been demonstrated

as base for future vehicular power trains by the majority of car

manufacturers. Some of the advantages include zero emissions,

better efficiency and performance, and quieter operation when

comparable with gasoline engines of the same power output.

The effect of vibration on the performance of the fuel cells,

however, has not been clearly documented in the literature.

Since PEM fuel cells produce large amounts of water

(especially for the size needed in automotive applications), the

mechanical vibration might have an effect in the removal of

water, helping the stack to prevent (or worsen) internal water

flooding.

Probably the closest study relating the effects of vibration

in the water transport of PEM fuel cells was documented by

Palan et. al. [2] in which vibro-acoustic methods were proposed

in order to enhance the water removal from the active reaction

sites of the cell. Palan et. al. did not perform any experiments

using an actual fuel cell, but studied water droplets movement

over vibrating plates. The result and numerical model may be

applied to an actual fuel cell, but only limited to the case of a

water drop leaving the GDL (into the gas channel) and does not

apply for the water transport inside of the GDL.

Another approach is to consider the porous characteristic of

the GDL and study the effects of vibration in transport in porous

media. In the study by Xiao et. al. [3], experiments using glass

beads as the porous media and a hydrocarbon as fluid are

subject to vibration. It is also supported by a numerical study

based on a network model, where the movement of ganglia (in

this case, the hydrocarbon) is predicted using some standard

parameters such as capillary pressure, gravity and viscosity.

This remarkable study, unfortunately, cannot be used for this

project since the structure of the porous media changes with the

DRAFT

2 Copyright © 2009 by ASME

reordering of the glass beads. In our case the GDL is assumed

to be rigid (or at least, to have constant porosity).

There are more numerical models for the effect of vibration

in the characteristics of porous media. Charrier et. al. [4], and

Malashetty and Padmavathi [5] proposed a complete model for

analyzing the effects of buoyancy forces when a fluid-saturated

porous layer is heated from below and subject to mechanical

oscillations. From these models, it is shown how vibration

affects the convection and other properties of the fluid;

however, the boundary conditions given to the model are for a

closed system (no flow across the boundary). These models,

nevertheless, will serve as a guideline for further analysis.

The problem of the liquid water transport in PEM fuel cells

has been documented by Pasaogullari and Wang [6], where

there has been extensive analysis on the effect of water

saturation in the performance of the cell. Here, a proposed

function for the saturation ratio dependent on the distance from

the electrode (towards the flow channel) is presented. Still, it

cannot be used for predicting the effects of vibration since this

function is not explicitly dependent on time (or for the same

matter, frequency). In addition to that, it also neglects the effect

of gravity.

MODELING EQUATIONS Most of the modeling done in PEM fuel cells assumes that

the water produced and transported out the GDL is in the vapor

form. However, Pasaogullari and Wang [7] proposed a two-

phase modeling of the water transport across the MEA, which

account for the effects of both liquid water and vapor. Here, the

liquid water is transported by capillary action through the

gradient in capillary pressure.

For hydrophobic GDLs, the capillary pressure is negative

since the liquid water pressure is greater than the vapor. It is

more convenient to express the capillary pressure as a function

of the pore saturation:

s =

Vliq

Vpore

(1)

which is the percentage of the volume of liquid water occupied

in the pore. Using the Leverette function, the capillary pressure

is given by Pasaogullari and Wang [7]:

pc = σ cosθc

ε

K

1/2

(1.417s − 2.120s2 +1.2633) (2)

where σc, θc, ε and K are the surface tension of the water, the

contact angle (> 90° for hydrophobic materials), the porosity

and the permeability of the porous membrane.

A practical method to estimate the water saturation in the

GDL is by observing the formation of drops on the surface of

this membrane (facing the open oxygen channel). Through the

use of a clear window on the cathode side, some experiments

designed by Ous and Arcoumanis [8] reveal the drop size

growth, and thus, the rate of formation on the surface.

Obviously, this experiment does not predict the saturation inside

of the membrane, but indicates a much higher pore water

distribution directly under the drop, compared to the areas of

the GDL without water in the surface (Figure 1).

Figure 1. Water formation in the surface of the GDL during fuel cell

operation. (Adapted from [7])

EFFECTS OF VIBRATION IN POROUS MEDIA The effects of vibration on a porous layer are well

documented by Charrier et. al. [4] and Razi et. al. [9]. In their

study, the thermal stability of the Rayleigh-Bénard problem

(applied for porous media) is analyzed for a vertical vibration,

parallel to the temperature gradient since the layer is heated

from below. The modeling equations for this problem, i.e., the

conservation of mass and momentum, assuming constant

temperature across the GDL (because of its very small

thickness) are:

∇⋅ u = 0 (3)

( )2 ˆsin( )liq

cp g b tt K

µρρ ω ω

ε

∂+ = −∇ + +

∂

uu k

(4)

In this case, however, the capillary pressure is not

sufficiently defined by Equation 2 since the effects of time do

not affect the saturation. With the proper boundary and initial

conditions, Equations 3 and 4 can be solved if:

p

c= f (s) s = f (x,t) (5)

Thus said, a model for pc is necessary in order to find a

solution. This may be obtained either empirically or

numerically, using a model derived from percolation theory in

porous media. The reason why the saturation must additionally

dependent on time is to allow the frequency of vibration to have

an effect on its distribution.

Now, if a solution is found for this set of equations (a

model for the saturation dependent of time and space), the

overall effect of saturation in the performance of the cell, is

given by Pasaogullari and Wang [6] as:

i = (1− s)ai0

refC

reaction

O2

Cref

O2

expαF

RT∆V

a

(6)

Here, the factor (1-s) expresses the area reduction in the

catalysts, due to the liquid saturation at that edge of the GDL.

So, in order to quantify the variation in performance in a PEM

fuel cell due to vibration, the saturation must be known at the

electrode-GDL interface.

3 Copyright © 2009 by ASME

EXPERIMENTAL APPROACH In order to investigate the effect of vibration in the

performance of the fuel cell, the parameters of operation must

be carefully controlled. Before running the experiments, a

considerable amount of time and effort was dedicated in the

experimental set-up. All apparatus, beginning from the fuel cell

testing station, the vibration assembly and the fuel cell itself,

were designed and manufactured from scratch.

In order to address the challenge of embedding a clear

window in the cathode side of the PEM fuel cell, custom-made

gas flow plates and end plates were required. This work was a

complete in-house design and manufacturing process. A

commercially available MEA was used (ElectroChem Inc.,)

with a 50 cm2 of active area, platinum content of 0.5 mg/cm

2 for

both electrodes and a hydrophobic GDL made out of ELAT®.

Also, in order to prevent a drop in conductivity due to the

corrosion of the aluminum-made gas flow plates as well as a

possible contamination of the platinum catalyst in the MEA, a

conductive, non-corrosive Zirconium-based coating (ZN54 by

Eclat Industries Inc.) was applied to the plates. The assembled

fuel cell is shown in Figure 2.

Figure 2. Custom-made fuel cell stack with clear window

In order to control the oscillations while observing the

water formation inside the window, a special structure was

designed for this experiment. The heart of the device is a

mechanical shaker, as shown in Figure 3, enabling the vertical

oscillation for the upper assembly. This assembly was made out

of aluminum bars that support the fuel cell at one side and a

camera at the other. This structure is fully adjustable, helping

the window to be easily focused.

This final prototype showed very stable performance in

preliminary testing until a possible premature degradation of the

MEA may have caused the sudden decrease in performance.

This prototype was tried again with two more brand-new MEA’s

of the same supplier, but inexplicably, the fuel cell never

performed as it did before. The window gasket was then

removed and sealed again but showed no improvement. After a

thorough revision of the integrity of the cell without finding any

possible cause for this misfortune, the prototype for the custom-

made fuel cell was halted and the experiments were performed

using a commercial, 36 cm2 single cell PEM fuel cell.

Figure 3. Image of the shaker assembly during testing

The operating conditions for the fuel cell are described in

Table 1.

Table 1. Testing condition for the fuel cell

Parameter Value

Hydrogen flow rate (STP) 80 mL/min

Air flow rate (STP) 400 mL/min

Hydrogen Inlet Gauge Pressure (no load) 27.5 kPa

Air Outlet Gauge Pressure (no load) 31.0 kPa

Temperature of Reactants 20 – 25 °C

Hydrogen Humidity (Inlet) < 10%

Air Humidity (Inlet) > 95%

The test consists of constant current loads, spaced in

intervals of 5 minutes. The first step was a no load current (0

A), followed by 0.1 A, and then 0.5 A. From here, each step was

incremented by 0.5 A until the current reaches 4 A. Once the

test was finished, the load was disconnected and the fuel cell re-

conditioned again.

In addition to the parameters described before, each

polarization curve was subjected to a vibration cycle as

described in Table 1. For each magnitude and frequency of

vibration a minimum of 4 polarization curves were obtained.

To ensure proper magnitude and frequency reading from

the accelerometers, a low-pass Butterworth filter was

implemented (using Lab VIEW) with the cut-off frequencies

listed in Table 2.

RESULTS The polarization curves obtained for the fuel cell are shown

in Figures 4 through 6. The performance of the cell was

significantly reduced due to the low temperatures of the gases

coming into the cell. Also, due to the recommendations of the

manufacturer, the fuel cell was always operated above 0.6 Volts

to prevent a premature degradation of the MEA. Thus said, the

4 Copyright © 2009 by ASME

tests show that the fuel cell behaved reliably given these limited

operation conditions.

Table 2. Testing condition for the fuel cell

Magnitude (g) Frequency (Hz) Cut-off frequency (Hz)

- No Vibration -

2 10 20

2 20 50

2 50 70

- No Vibration -

5 20 50

5 50 70

- No Vibration -

3 20 50

3 50 70

Figure 4. Polarization curve at 2 g of amplitude (above). Detail of the chart at high current densities (below). Error bars omitted above for clarity.

It can be seen that vibration with a magnitude of 2 g tended

to decrease the performance of the cell for all the frequency

range tested. For 5 g of magnitude, the two frequencies of the

experiment showed different results. The higher frequency, 50

Hz, presented a large decay in the performance starting at the

medium current density range of the polarization curves (150

mA/cm2). On the other hand, the 20 Hz-frequency run improved

the performance of the cell starting at low current densities

(from 25 mA/cm2) and maintained as the current was increased.

In the case for 3 g, both frequencies show a decrease in

performance compared to the other two conditions.

Figure 5. Polarization curve at 3 g of amplitude (above). Detail of the chart at high current densities (below). Error bars omitted above for clarity.

A more detailed observation at the typical operation range

of a single fuel cell (0.6 to 0.7 V) for the tested conditions is

shown in the bottom charts of Figures 4 through 6. In these

charts, it can be seen that the performance at the lowest

magnitude tested (2 g) was reduced by 2.2%, 1.1% and 1.3%

for 10, 20 and 50 Hz of frequency respectively. Because the

lack of a transparent window in the cathode side of the fuel cell,

it was not possible to determine the effect of these frequencies

in the rate of formation of water droplets in the GDL surface -or

for that matter, the removal of them. It may be the case (at least

for vibration of 2 g of acceleration) that at low frequencies the

removal of liquid water from the GDL was impaired resulting in

a loss of performance, however, this cannot be supported based

on these results only.

The case at 3 g showed an overall, steady decrease in

performance throughout the current density range. At the point

of maximum current recorded, this amplitude showed a

reduction of 3.7% for 20 Hz and 3.0% for 50 Hz.

On the other hand, the case with the highest magnitude of

vibration (5 g) had two very different results. The data for 20

Hz, showed an increase in performance of 1.3%, whereas in the

50 Hz case the performance dropped by 5.8%. The case for 50

5 Copyright © 2009 by ASME

Hz may be explained using the same argument mentioned

before, that is, vibration affects negatively the removal of liquid

water out of the GDL. This hypothesis can be supported further,

due to the polarization curve that correlates with those of fuel

cells experiencing flooding of the GDL, as seen by the sharp,

progressive decay in performance at the highest current

densities. In the 20 Hz case, however, vibration may had

improved this water removal rate, since the performance was

constantly above the reference case and showed no signs of

flooding. Regardless of the cause, this difference was

statistically significant (up to 99%).

Figure 6. Polarization curve at 5 g of amplitude (above). Detail of the chart at high current densities (below). Error bars omitted above for clarity.

In order to distinguish between all the vibration cases,

Figure 7 compares the voltage output of the cell at the

maximum tested current density. From all the vibration

frequencies, the one in the 20 Hz range seem to have the better,

or at least the lesser impact on the power output of the cell.

However, as mentioned before, the vibration at 20 Hz might

have a particular beneficial effect in the liquid water removal

from the cathode while keeping the MEA properly humidified,

all this by keeping an adequate saturation level in the GDL that

enables a better flow of the reactants to the catalyst. These

results do not prove that vibration affect the liquid water

transport in the cathode’s GDL, but it definitely opens the door

for further investigation on this phenomena.

Figure 7. Comparison of the fuel cell voltage at 4 A (275 mA/cm

2)

for different vibration parameters.

Another interesting observation is that the acceleration of

vibration seems to have an effect in the deviation in

performance from the no-vibration operation. Referring to

Figures 4 through 6, the higher the acceleration, the more offset

from the no-vibration line (observe that for 2 g the data is closer

to the no-vibration performance when compared to the 3 or 5 g

case.) This may suggest that the water saturation is more

affected by the acceleration magnitude than by the frequency.

CONCLUSIONS Based on the results, it was found that both frequency and

acceleration of vibration affect the power characteristics of a

fuel cell (as observed in the polarization curves.) At the

maximum current density of the experiment, 275 mA/cm2, the

performance dropped for all frequencies at 2 g by 2.2%, 1.1%

and 1.3% for 10 Hz, 20 Hz and 50 Hz respectively. The case of

3 g showed a decrease for both frequencies of 3.7% and 3.0%

(20 and 50 Hz respectively). Last, the case of 5 g, an increase in

performance at 20 Hz by 1.3% was opposed to a significant

drop of 5.8% at 50 Hz.

Also, since the overall decay in performance for both

magnitudes of vibration start in mid-range current density (from

about 100 mA/cm2 to 150 mA/cm

2), it is conjectured that the

decay may be due to the transport of liquid water in the MEA.

As seen on Figures 4 through 6, there is not much difference in

the performance at low current densities (and consequently, a

low water production in the cathode). This probably is the

strongest indicator that the decay in performance may be due to

an alteration of the water transport in the cell and not to other

factors, such as reactant concentrations or diffusion

mechanisms.

Another area of interest was the proposal of the 1-D

analytical model for the liquid water transport in the cathode.

Unfortunately, based on the current literature, a relationship for

the saturation dependent of both time and space (so the

6 Copyright © 2009 by ASME

frequency and magnitude of vibration affect directly the

saturation level) could not be found.

ACKNOWLEDGMENTS The authors like to thank the Ben Franklin Nanotechnology

Institute and the Tempe University’s Office of Vice President

for Research, and Future Faculty Fellow program for their

financial support.

REFERENCES

[1] Barbir, F., 2005, PEM Fuel Cells: Theory and Practice,

Elsevier Academic Press, Burlington, MA.

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acoustical methods”, Journal of Power Sources, 161, 1, 1116-

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[3] Xiao, M., Reddi, L., Steinberg, S., 2006, “Effect of

Vibrations on Pore Fluid Distribution in Porous Media”,

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[4] Charrier, M. C., Razi, Y. P., Maliwan, K., Mojtabi, A.,

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