Characterization Of Ion Exchange Membranes For Polymer
Electrolyte Fuel Cells
Rishon R. Benjamin, Francis Richey, Holly L.S. Salerno, Isthiaque Ahmed, Yuesheng Ye, Yossef A. Elabd
Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA
Introduction
Experimental Continued
Since the late 1960’s, fuel cells and solid polymer electrolyte membranes have garnered
much interest. Due to their versatility, fuel cells and polymer electrolyte membranes can be used in a
variety of settings including automobiles, mobile devices such as laptops and cell phones, and
stationary power sources. In addition, the use of fuel cells in automobiles could reduce the emission
of harmful gases such as Carbon Monoxide thereby preserving the environment.
A fuel cell is a device that utilizes an oxidation-reduction reaction to produce electricity. In the
case of a proton exchange membrane fuel cell (PEMFC), hydrogen gas is oxidized to produce
hydronium ions and electrons. The hydronium ions traverse the proton exchange membrane (PEM)
to reach the cathode where it is used to reduce oxygen gas to water. On the other hand, in an
alkaline fuel cell (AFC) hydrogen gas reacts with hydroxide ions at the anode to produce water and
electrons. Water reacts at the cathode with oxygen gas to produce hydroxide ions which traverse
the anion exchange membrane (AEM) to the anode.
Currently, Nafion® is considered to be the state-of-the-art PEM because of its high cationic
conductivity, its resistance to chemical attack and its ability to perform at high temperatures.
However, the use of Nafion requires the use of Pt as a catalyst at the electrodes. This makes
PEMFC’s using Nafion financial liabilities
AFC‘s, on the other hand, can function with non-noble metallic catalysts at the electrodes.
However, the lack of mechanical stability of these membranes, especially at temperatures required
to run a fuel cell, has not yet resulted in a state-of-the-art AEM [1].
PEMFC
Room Temperature Conductivities
DSC
Based on the figure below, it can be seen that the current AEM’s and alternate forms of PEM’s
do not conduct as well as the PEM Nafion 212.
The AEM’s and PEM’s that were analyzed were made up of a three monomer sequence that
repeated themselves in the form ABCBA. One can see transitions representative of the properties of
the component monomers. It is hypothesized that the 2nd transition, is that of the ionic group and the
one that tends to dictate most of the properties of the polymers.
Figure 3: EIS conductivity cell and instrument
Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) was performed on various PEM’s and AEM’s to
determine the glass transition temperatures (Tg) of these membranes. This is the temperature at
which polymers assume a glassy texture thereby making them brittle.
To do this, approximately 5 mg of membrane samples were placed in aluminum pans. The
pans were then heated to 200 °C, cooled to -90 °C (using Helium gas) and then heated again to 200
Figure 7 : Ionic conductivities for PEM’s and AEM’s
PEM2 and PEM3 are two different types of PEM’s developed by Kraton polymers. It can also
be noted that casting these membranes using different solvents, Cyclohexane/ Heptane (CHH) and
Dimethyl Acetamide (DMAc), results in varying ionic conductivities.
°C. The heat flow into/out of the sample was then plotted as a function of temperature.
Fuel Cell Tests
Figure 1: The difference in the functioning principles of PEMFC and AFC
Objective and Experimental
Objective: Analyze the physical and mechanical properties of various AEM’s and
PEM’s.
Water Uptake
Tg(2)
88.17
81.92
93.57
88.47
Tg(3)
111.75
109.65
105.96
Tg(4)
114.64
Tg(5)
162.79
127.41
145.95
Figure 11: Glass transition temperatures
Fuel Cell Data
Membrane electrode assemblies (MEA) were prepared and tested in a fuel cell. To prepare
the MEA, two Teflon® fiberglass decals were painted with the catalyst ink solution. The catalyst ink was
made by mixing 20 wt % Pt/C mixture and 5 wt % Nafion® solution or 11 wt% PEM1-9200 solution. The
membrane was then placed between the decals and the setup was heat pressed at 266 degrees
Fahrenheit and 3000 psi for 30 seconds to transfer the ink directly on the membrane.
The fuel cell was operated at 25%, 50%, 75%, and 100% relative humidities. This was done
by changing the temperature at which the fuels were introduced into the system. The temperatures to be
used were determined based on the Antoine equation:
Figure 8: Ionic conductivity in relation to IEC
Figure 8 also indicates that the there is a correlation between IEC, a measure of how many
moles of ionic groups( e.g .sulfonic acid) are present per gram of polymer, and conductivity. A higher
IEC provides the ions with more binding sites thereby increasing conductivity.
EIS was used to generate Nyquist plots of the real and imaginary components of the
impedance (R) of the membranes. Impedance of the membranes were recorded by fitting a circle to the
plots generated and using the high intercept of the circle as R. Based on these measures and the
formula σ = L/(R*A), the ionic conductivities were calculated.
Figure 13: Fuel cell data for PEM1-9200
MEA’s consisting of Nafion 212 and PEM1-9200 were operated in the fuel cell at the same
conditions. Nafion® 212 proved to be the better membrane generating a maximum power density of
280 mW/cm^2 at 50% relative humidity compared to 140 mW/cm^2 generated by PEM1-9200 at 75%
relative humidity. Ideally the performance for both membranes should have improved as the relative
humidity increases for the same reasons outlined before, but mass transport loses could have
prevented this from happening.
The results of these fuel cell runs will be used to determine the effectiveness of AEM’s.
Successful MEA’s made from AEM’s will be able to generate a similar power density with a similar
loading of a non-noble catalyst.
P is the desired vapor pressure and A,B,C are substance-specific constants. The cell itself was
maintained at a constant temperature of 50°C
.
Figure 5: The fuel cell test station and an MEA
The loading of the catalyst on the decals were 0.4mg/cm^2, the flow rate at the anode was
0.4L/min, and the that at the cathode was 1L/min. An appropriate load was applied across the circuit to
drop the cell voltage by 0.01V for every data point until the voltage reached 0.1V.
Future Work
Figure 9: Nyquist plots
1) Determine effective non-noble catalysts that can be used in AFC’s
2) Create catalyst inks for them and develop MEA’s for AFC’s
Results and Discussion
Figure 2: Effect of water on membrane conductivity
However, if the membranes absorb too much water, they become soft and lose their
mechanical stability. Hence, water uptake studies were conducted on various membranes. To do
this, the membranes were cut into 3cm X 0.5cm strips, weighed, and immersed in de-ionized water
(DI) for 24 hours. The membranes were dabbed dry and re-weighed after removal and % water
uptake was calculated.
3) Optimize AFC performance based on temperature, humidity, catalyst loading, flow rate of fuels, and
heat press techniques.
Water Uptake
Water uptake studies indicate that AEM1-900, AEM5-9200, AEM6-9200, and PEM1-9200
absorb high amounts of water thereby compromising their mechanical integrities. The PEM Nafion®
212 and the remaining AEM’s absorb a relatively low amount of water thereby making them more
durable and mechanically stable.
PEM1-9200
Nafion 212
Acknowledgements
I would like to extend my sincere thanks to Drexel University’s Pennoni Honors College and the
STAR program for giving me an opportunity to engage in this ten-week internship.
I would like to thank my mentor Dr. Elabd for all his guidance and encouragement and for giving
me a chance to work in his lab.
Conductivity
Electrochemical Impedance Spectroscopy (EIS) was used to measure ionic conductivities of
the PEM’s and AEM’s . The 3cm X 0.5 cm hydrated strips of membranes were used and their
conductivities were determined at room temperature and as functions of varying relative humidities
(RH). The RH was varied from 25 % to 90% in increments of 25% at a constant temperature of
50°C. Both these procedures were done using the 4-point conductivity technique whereby an
alternating voltage (1e6 Hz to 1Hz) of 10 mV was applied across the membranes so as to record
impedance. A caliper and micrometer were used to measure the width and thickness of the
membranes. These values were used to determine the cross-sectional area (A) of the membrane.
The distance between the electrodes (L) measuring the voltage across the membranes was also
measured.
The reason knowing the ionic conductivities (σ ) of these membranes is of importance is
because ion transport lies at the heart of a fuel cell. When used in fuel cells and other
electrochemical devices, membranes that do not conduct ions as well usually result in low power
densities and current outputs.
Figure 12: Fuel cell data for PEM Nafion® 212
Conductivity v Relative Humidity
Water plays an important role in the effective functioning of polymer electrolyte membranes.
Solid polymer electrolytes usually contain regions that are concentrated with ions known as ion
clusters [2]. As the water content increases, the isolated ion clusters become interconnected to form
a uniform network as seen in figure 2 below:
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Tg (1)
-47.51
-55.67
-53.8
-54.21
-56.61
AFC
Figure 4: The DSC instrument
TEMPLATE DESIGN © 2008
Name
AEM5-9100
AEM5-9200
AEM6-9200
AEM7-9200
PEM1-MD9200
Lastly, I would like to thank all the graduate and undergraduate students in the lab; thank you for
all your guidance, advice, support, and cooperation.
References
Figure 10: Conductivity v Relative Humidity at 50 °C
The graph in figure 10 indicates that ionic conductivity increases as relative humidity
increases. This occurs because a higher relative humidity results in more water being absorbed by the
membranes. As a result of this there is an increase in the ordering of ionic domains and thereby an
increase in the number of points on the membrane where the cations/anions can fix onto.
Figure 6: Water uptake for PEM’s and AEM’s
[1] Varcoe, J. R. and Slade, R. C. T. (2005), Prospects for Alkaline Anion-Exchange Membranes in Low Temperature
Fuel Cells. Fuel Cells, 5: 187–200. doi: 10.1002/fuce.200400045
[2] DeLuca, N. W. and Elabd, Y. A. (2006), Polymer electrolyte membranes for the direct methanol fuel cell: A review.
Journal of Polymer Science Part B: Polymer Physics, 44: 2201–2225. doi: 10.1002/polb.20861
[3] Phosphoric Acid And PEM Fuel Cells. Photograph. Fuel Cell Basics. FCTec. Web. 10 Aug. 2011.
[4] Alkali Fuel Cells. Photograph. Fuel Cell Basics. FCTec. Web. 10 Aug. 2011.
[5] MEA. Photograph. Fuelcellstore.com. Fuel Cell Store. Web. 14 Aug. 2011.