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Dynamic fuel cell gas humidification system

R. Kuhn a,*, Ph. Kruger a,e, S. Kleinau a, M. Dawson b, J. Geyer a, M. Roscher a, I. Manke d,Ch. Hartnig a,c

aZentrum fur Sonnenenergie- und Wasserstoff-Forschung, 89081 Ulm, GermanybUniversity of Salford, Salford, M5 4WT, UKcChemetall GmbH, Trakehnerstr. 3, 60487 Frankfurt, GermanydHelmholtz Centre Berlin for Materials and Energy (HZB), 14109 Berlin, GermanyeCONSULECTRA Unternehmensberatung GmbH, Weidestraße 122 a, 22083 Hamburg, Germany

a r t i c l e i n f o

Article history:

Received 4 October 2011

Received in revised form

23 January 2012

Accepted 29 January 2012

Available online 14 March 2012

Keywords:

PEM fuel cell

Humidification

Neutron imaging

EIS

* Corresponding author. Tel.: þ49 0 731 95 30E-mail addresses: Robert.Kuhn@zsw-bw.d

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.01.143

a b s t r a c t

Water management is one of the crucial factors regarding the performance and durability

of low temperature PEM fuel cells. Amongst other factors, the water balance in an oper-

ating fuel cell can be influenced by the humidification of the reaction gases. For transient

response investigations of the fuel cell behavior under fast humidification changes

a system is needed which is able to humidify the supplied gases in a highly dynamic and

reproducible way. Exact knowledge of the water content of the supplied gases is of utmost

importance to study humidification effects. In this contribution, a dynamic fuel cell

humidification system is presented. Reliability of the concept is proven by using three

different methods: straightforward dew point measurements, electrochemical impedance

spectroscopy (EIS) and in situ neutron radiography. The test setup is able to provide dew

point temperatures with a tolerance range of 1e3 K leading to a highly reproducible fuel

cell performance and water content of the complete cell.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction the current distribution proves directly the influence of excess

The performance of a fuel cell strongly depends on the water

management. In recent years there has been much focus on

the relationship between water management and the result-

ing cell performance using a wide variety of different tech-

niques [1e18]. In general, a direct correlation between water

content and performance is difficult to achieve with non-

destructive methods. In situ neutron and synchrotron radi-

ography are two comparatively recent methods which allow

quantification of the water distribution in the cell with high

spatial resolution [8e13,19e33]. Along with direct imaging,

combined methods allow the detection of correlated effects

on the local performance: locally resolved measurements of

822.e, kuhn.robert@arcor.de2012, Hydrogen Energy P

water or of drying on transport processes and in consequence

on the kinetics on a limited region of the electrochemically

active area [34e36]. Electrochemical impedance spectroscopy

has also been combined with imaging techniques to monitor

the cell response on the water content [37].

In general, thewater balance in a PEM fuel cell is influenced

by several transport processes. Diffusion, permeation and

capillary effects can lead to a water exchange between anode

and cathode. In the membrane, the electro-osmotic drag

(e-drag) is an additional transport process which is related to

the protonic current leading to a net transport of water from

the anode to the cathode side (Fig. 1). These effects are

strongly influenced by the chosen operation conditions, and

(R. Kuhn).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Investigated water transport effects in a PEM-FC.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 7 7 0 2e7 7 0 9 7703

the materials used for the gas diffusion layers (GDL) and the

membrane, as well as the cell geometries (flow field design)

have a great impact.

In an operating fuel cell, the contributions of the afore-

mentioned transport mechanisms are difficult to separate

from one another. This is further complicated by external

humidification of the reactant gases. Depending on the

humidification method used, crucial effects can be observed

when changing the humidification conditions in a fuel cell.

For example, if a humidification jar is used for the gas

humidification, the mass flow will increase compared to non

humidified gas. Effects like these must be known and taken

into account by the interpretation of the cell water manage-

ment and adjustment of the humidifying element.

From an application point of view, any humidifier can be

omitted as the fuel cell system would be self-sustaining and

the required amount of water is taken from the product water.

The development of such a system setup requires a very

accurate and validated humidification system for detailed

investigations of the water management and a deep insight in

the processes combined with an excellent estimation of the

time scale where these processes take place. State-of-the-art

humidification systems are:

i) Gasegas humidifier.

The dry gas flow is bypassed through a wet porous or semi

permeable media. The membrane based humidifier consists

of two gas channels (dry and wet), separated by a vapor water

permeable membrane [38e41]. For this system the wet gas

flow from the anodic or cathodic exhaust gas loop can be used

for the humidification. The geometry of the humidifying

system is a key factor for the efficiency. One example for an

effective design is the shell and tube gasegas concept [42]. In

general, the control on the exact humidification level and the

heat control of the inlet streams is limited, the big advantage

however is the possibility to scale these systems to suit the

requirements of applications with varying power outputs [38].

For larger, mainly stationary systems, an enthalpy wheel can

be employed to recover heat and water vapor from the air

streams [43]. This class of humidifiers utilizes a hygroscopic

corematerial that rotates between a dry andwet gas flow. This

rotation can carry hot moisture from the fuel cell exhaust

stream to the inlet stream leading to a humidified inlet flow

[39,42]. The control of such a system is also limited, however,

both systems can utilize the fuel cell exhaust steam which

brings them close to a self-humidifying system. Nevertheless,

the missing accuracy and controlability turns them insuffi-

cient for investigations of the water management of fuel cells.

ii) Humidification using direct water injection (steam,

liquid).

The water is directly injected in the fuel cell or in the gas

stream. Liquid water injection can result in pulsing effects

caused by evaporation and/or flooding of the fuel cell [44]. On

the other hand, steam injection requires a suitable setup to

evaporate the liquid phase which might also induce gas flow

pulses. For a stable equilibrium in the fuel cell water-

household and detailed studies these humidification

methods are in general not advisable.

iii) Humidification by means of a humidification jar (satu-

ration bubbler).

In this setup the inlet gas flow (anodic and/or cathodic gas

stream) is passed by a water filled jar with a controlled water

temperature. The design of the optimal humidification jar is

such that the dewpoint of the outlet gas flow is identical to the

temperature of the water that in turn can be controlled in

a very accurate way (see section 2). Compared to the above

mentioned steam humidifiers this method is based on

a continuous processes which avoids any pulsing or flooding

effects. The dew point of the inlet gas stream can be easily

controlled and adapted by changing the water temperature.

Themain disadvantages of this system are the requirement in

terms of volume and weight. In addition, changes of the dew

point, which is a change of the water temperature in the jar,

require a considerable time which prevents this systems from

a commercial step; however, for highly accurate and constant

humidification levels which are needed for fundamental

studies of fuel cell systems, this class of humidifiers repre-

sents the best choice.

Here we present a fuel cell humidification system which

was developedmainly for the purpose of investigating the cell

water management. In section 2, the system setup is

explained; calculations of the achieved dew point are given in

section 3 and the complete system is qualified based on

different measurement methods in section 4.

Fig. 3 e Piping and points of temperature measuring of the

humidification system.

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2. Description of the humidification system

The humidification system implemented is based on a water

bubbling device and can support fuel cells and stacks with an

active area of up to 500 cm2 at a current density of up to 1 A/

cm2. The remote-controlling of the mass flow controllers and

the electronic load is realized with Labview.

The core of the humidification system is a humidification

jar. The gas flowing through the jar is saturated according to

the dewpoint of liquidwater (Fig. 2). To avoid condensation on

the top of the jar and to ensure that the gas has the correct

dew point, the top of the jar is heated up; thereby, the heating

temperature of the lid is generally set to 10 K above the dew

point temperature of the water bucket. The resident time of

the gas in the humidification water must be long enough to

reach full water saturation in the gas. The dimensions of the

jar are thus chosen to ensure sufficient contact time (even for

high gas flows) and pathway with sufficient tortuosity is

guaranteed by stainless steel plates. The water temperature

inside the humidification jar is determined by means of

a thermocouple (type K) that controls the water heating. The

same type of thermocouple is used for temperature control in

the gas phase inside the vessel. In an ideal case the temper-

ature of the water and the gas temperature on the top are

identical, ensuring an accurate humidification of the gas flow.

In addition, themass of the water and the jar are large enough

to avoid oscillations caused by controlling interruptions.

For dynamic changes of the humidification, the systemhas

been constructed in a way that avoids long-lasting changes of

the water temperature inside the jar. The humidified gas

stream is therefore mixed with a dry gas stream to allow

adjustment to any desired dew point between room temper-

ature and the temperature of the humidification bucket

(Fig. 3). Adjustable mass flow controllers are used on the dry

and wet gas inlets. Condensation in the pipes is avoided by

keeping the temperature of the heating ribbons to 10 K above

the maximum desired dew point.

Fig. 2 e Schematic sketch of the humidification jar.

Additional valves positioned directly after the humidifiers

on the dry gas side allow precisely defined jumps in the range

between 0 and 100%humidificationwith respect to the chosen

dew point of the humidification jar.

The following crucial points have been fulfilled in the

presented setup to ensure a proper humidification:

i) Each component in between the water vessel and the

fuel cell is at the same (or higher) temperature as the

required dew point temperature to avoid condensation.

ii) If the gas flow rate increases, the pressure drop in the

fuel cell and in the piping is increased as well; the

increased pressure drop in turn leads to a higher pres-

sure in the humidification jar. As the amount of water

which can be taken up by the gas is a function of pres-

sure and temperature, the achieved dew point in the

humidification jar is related to the pressure in the jar;

the dew point temperature of the humidified gas will

therefore decrease.

iii) The mass of the flow through the humidification jar

increases by the amount of absorbed water. As a conse-

quence of air humidification the volume flows, as well as

the velocities in the fuel cell flow field, increase. The

resident time of the gas in the cell decreases, the

convective entry in the GDL increases and the cell

pressure drops.

The calculation of the humidification of the feed gases is

based on the assumption that the gases obey an ideal

behavior.

3. Calculation of the system dew point

For the calculation of the correct dew point temperature at the

cell inlet various temperatures and pressures have to be

taken into account (Figs. 4e6). The pressure values of the

gases before and after passing the cell are known; it is

assumed that the pressure before the cell is equal to the

humidification pressure. In general, it is easier to perform the

calculations based on mass flow rates instead of volume flow

rates as a proper balancing of the engaged masses is always

Fig. 4 e Dew point measurement for different gas flows at

35 �C and 1 bar; the entire gas stream is lead through the

humidification jar. (20 A y 200 mA/cm2, 35 A y 350 mA/

cm2, 50 A y 500 mA/cm2).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 7 7 0 2e7 7 0 9 7705

guaranteed. The MFC volume flow set point refers to the gas

properties at normal conditions of 25 �C and 1 bar. The inlet

mass flow rates can be calculated assuming the ideal gas law

is obeyed. Thus, for the mass flow before entering the cell

(mb_Cell) the following mass balance can be assumed:

mb Cell ¼ ðmMFC wet þmHumidÞ þmMFC dry:

where

mMFC_wet ¼ dry mass flow to the humidification vessel.

mMFC_dry ¼ dry mass flow direct to the cell.

mHumid is the additional mass flow from the humidifier

(commonly called: ‘makesmMFC_wet wet’) which is transported

in the gas flow and can be calculated from the two previous

masses; therefore, the saturation pressure for the gas flow can

be estimated based on the Antoine-Equation:

log�psat

� ¼ �A� �

B=�T Humid þ C

���;

where A ¼ 4.65430, B ¼ 1435.264 and C ¼ �64.848.

The water partial pressure of the humidified gas flow

equals psat. Together with the balance of partial pressures

Fig. 5 e Dew point measurement at a 35 �C at different

pressures; 100% of the flow passes the humidification jar.

(the sum of the partial pressures equals the overall pressure)

and the equation pi ¼ yi$pges the mole fraction ( yi) as well as

the mass water amount of the gas flow can be defined.

The amount of product water (per time unit) in the cell,

mp_water, is calculated using Faraday’s law. The totalmass flow

which has to be transported out of the cell is:

ma cell ¼ mb cell þmp water

At the outlet of the cell the dew point is calculated taking

the product water into account and critical operation points

regarding condensation in the cell can be avoided.

4. Verification of the humidification system

4.1. Dew point validation using a dew point mirror

The dew point of the gas stream was controlled by means of

a dew point mirror that can be heated (or cooled) with Peltier

modules. The dew point temperature is determined based on

the reflection of an incident light beamwhich is changed once

water vapor condenses on the surface of the mirror.

In Fig. 4 the determination of the dew points at different

gas flows are explained. The gas flow refers to different

current densities as indicated in the right upper corner. It can

be easily seen that the influence of the gas flow rates on the

dew point is very small, justifying the chosen dimensions of

the humidifier. In both cases deviations as low as 1e2 K were

found. The measured dew points are slightly above the

calculated temperatures which demonstrate that condensa-

tion inside the cell could be avoided.

The pressure of the dry gases is another key parameter

which has to be considered in more detail. Particularly for

pressurized operation of fuel cells as employed in automotive

applications, the dependence of the dew point on the actual

pressure has to be controlled and adjusted (if necessary). The

results from our measurements are summarized and

explained in more detail in Fig. 5. Different humidification

pressures resulting in dew points between 25 �C and 35 �C, and100%mixing ratio are shown. The observed deviation between

calculated and measured values is again in the range of 1e2 K

demonstrating that the employed concept of dew point

calculations is valid.

Fig. 6 e Dew point measurement at different dew points for

1 bar; 60% fully humidified gas, 40% dry gas.

Fig. 7 e Flow field geometry as used for in situ

measurements.

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For dynamic changes of the humidification level the

previously presented mixing between dry and fully humidi-

fied gases has been studied in more detail. A given gas flow

(equivalent to 500 mA/cm2 at l ¼ 5) was fully humidified at

different temperatures and afterward mixed at a fixed ratio of

60%. Based on the calculations mentioned above, the

following dew points are expected:

The following dew points are expected and compiled in

Table 1; in Fig. 6 the resulting values are displayed. Similar to

the measurements presented above the observed dew points

are in excellent agreement with the predicted ones with

deviations of less than 3 K.

These measurements of the dew points prove the suit-

ability of the chosen setup and dimensions based on variation

of mass flow rates, temperatures and pressures. The concept

of mixing dry and fully humidified gases which serves as

a basis for dynamic humidification strategies has been

verified.

4.2. Verification of dew point measurements by meansof combined neutron radiography and electrochemicalimpedance spectroscopy

4.2.1. Fuel cell setup and operation conditionsIn Fig. 7 the geometry of the employed threefold serpentine

flow field with an active area of 100 cm2 is shown; the cell is

operated in cross flow mode. A Gore MEA 5761 and

a symmetric GDL setup with SGL Sigracet 10BB gas diffusion

layer (GDL) were used.

In order to prove the humidification concept, the cell has

been operated in cross flow mode with pure hydrogen and

humidified air. The cell operating temperature of 60 �C was

achieved by an external thermostat.

The current densitywas set constant to 200mA/cm2with an

anode utilization of 60% and a cathode utilization of 20%. The

cathodic humidification jar was heated to 35 �C, the anodic gas

stream was not humidified. The air temperature at the inlet

was set to 70 �C; the hydrogen temperature was set to 25 �C.For the EIS measurements a ZAHNER IM6 with an elec-

tronic load EL300 was employed. The frequency spectrumwas

measured ranging from 100 mHz to 10 kHz with an amplitude

of 1 A.

4.2.2. Neutron radiography test setupThe neutron experiments were performed at the BER II

research reactor at the Helmholtz Centre Berlin for Materials

and Energy [45,46]. The detection system was based on

a (ZnS(Ag)-6Li) scintillator with an area of 20 � 20 cm2 and

a 2048 � 2048 pixel CCD camera (Andor DW436N-BV). Fig. 8

shows a schematic drawing of the used setup.

Table 1 e Calculated dew points for different humidifiertemperatures at 60% humidification ratio.

Water temperature [�C] Dew point [�C]

35 26.5

50 41

70 61.5

4.2.3. ResultsThe first step toward an experimentally reliable setup was

achieved by demonstrating the reproducibility of the obtained

images. This was achieved by comparing the respective water

content at different humidification levels in images taken on

two different days.

In Figs. 9 and 10 neutron radiographies at 12 and 35%

relative humidity of the cathodic and a dry anodic gas stream

and a current density of 200 mA/cm2 are displayed. A steady

state condition in the water distribution after changing the

humidity from 12% up to 35% is typically reached after one

hour. The images have been normalized in the standard way

with respect to an empty cell with no additional filters applied.

The performance of the cell was constant: The cell voltage

at 12% relative humidity was identical on day 1 and 2

Fig. 8 e Experimental setup for neutron imaging.

Fig. 9 e Neutron radiographies on day 1 at a current

density of 200 mA/cm2. 12% r.h. at the cathode inlet (left)

and 35% r.h. at the cathode inlet (right).

Fig. 11 e EIS spectra of the overall cell under different

cathode humidification.

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amounting to 725 mV. At 35% relative humidity the resulting

difference was in the range of 1e2 mV at a cell voltage of

738 mV.

A large difference between 12% and 35% relative humidity

at the cathode can be detected with maxima of liquid water

content toward the outlet of the flow fields. The mean thick-

ness averaged over the cell area of 100 cm2 is about 80 � 5 mm

and 65 � 5 mm for 35% and 12% humidity, respectively. The

water thickness can be calculated via the mean transmission

signal and the attenuation coefficient of liquid water [47e50].

However, no significant difference between the images

taken on day 1 and 2 can be seen. The good matching of the

two sets of images taken at different relative humidification

levels and at different experimental times proves the good

reproducibility of the chosen setup. In addition, the humidi-

fication of the gases does not change between levels and days

and can be considered well-defined.

Simultaneous to the imaging, electrochemical impedance

spectra (EIS) at 12% and 35% relative humidity were recorded

as displayed in Fig. 11. Both the EIS measurements and the

neutron radiographies were repeated several times at steady

state operating conditions. The Nyquist plots taken at the two

different humidification levels are in good agreement, under-

lining the general reliability of the chosen experimental setup.

For these spectra, the measurements were repeated five

times with simultaneous neutron imaging. Changing from

one humidification level to the next results in shifts of the

Fig. 10 e Neutron radiographies on day 2 at a current

density of 200 mA/cm2. 12% r.h. at the cathode inlet (left)

and (b) 35% r.h. at the cathode inlet (right).

spectra to higher values on the real scale. This is generally

attributed to lower water content in the membrane due to the

reduced humidification level. The equilibration time for

reaching a stable response of the EIS measurements is 30min.

This gives a good indication that processes which are

depending on the water level in the cell can be observed in

a frequency range from 100 mHz to 10 kHz reach an equilib-

rium under these conditions. A more detailed discussion

based on combined neutron imaging and EISmeasurements is

in progress.

5. Conclusion

An experimental setup for an accurate and reliable humidifi-

cation system was presented; main targets were an exact

adjustment of the dew point as well as highly reproducible

humidification levels. The relative humidity of the gas stream

has been determined in three different ways using straight-

forward dew point measurements, electrochemical imped-

ance spectroscopy (EIS) and direct neutron imaging in order to

conclude on the resulting humidification of the operating fuel

cell.

Dew points have been set with reasonable variations in

the range of 1e3 K as verified by mirror measurements. The

combined EIS and neutron radiography indicates that the

water content in the cell and the performance of the cell are

highly reproducible. Also the pressure changes in the system

and the dependency of the amount of steam in the feed

gas has been proven and verified by measurements and

calculations.

Acknowledgment

The authors would like to thank Joachim Scholta, Ludwig

Jorissen, Laszlo Kuppers, Matthias Messerschmidt and Tobias

Arlt for fruitful discussions.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 7 7 0 2e7 7 0 97708

The research activities were partially funded by the

German Federal Ministry for Education and Science (BMBF)

under grant numbers 03SF0324A and 03SF0324F (RuNPEM).

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