conductivity of ion exchange materials
TRANSCRIPT
The Pennsylvania State University
The Graduate School
Department of Energy and Mineral Engineering
CONDUCTIVITY OF ION EXCHANGE MATERIALS A Thesis in
Energy and Mineral Engineering
by
Tomoki Naya
2010 Tomoki Naya
Submitted in Partial Fulfillment of the Requirements
for the Degree of
Master of Science
May 2010
ii
The thesis of Tomoki Naya has been reviewed and approved* by the following:
Serguei N. Lvov
Professor of Energy and Mineral Engineering &
Material Science and Engineering
Thesis Co-Advisor
Sridhar Komarneni
Distinguished Professor of Clay Mineralogy
Thesis Co-Advisor
Derek Elsworth
Professor of Energy and Mineral Engineering
Jeffrey R.S. Brownson
Assistant Professor of Energy and Mineral Engineering
Yaw D. Yeboah
Professor of Energy and Mineral Engineering
Head of the Department of Energy and Mineral Engineering
* Signatures are on file in the Graduate School
iii
ABSTRACT
Proton exchange membrane fuel cells (PEMFCs) and alkaline fuel cells (AFCs) have
received much attention as two of the promising energy conversion systems which can be
applied in many areas. In order to improve performance of the cells, the ionic conductivity of the
ion exchange membranes (IEMs) have been studied by many research groups and a considerable
number of IEMs and materials have been synthesized to show high conductivities in wide ranges
of temperature and humidity.
The measurement of ionic conductivity is crucially important in order to evaluate the
performance of a newly synthesized material. Conventionally, the in-plane gas phase
measurement has been used for thin-film organic membranes and the through-plane gas phase
measurement has been used for inorganic-based membranes. However, the in-plane method
measures the conductivity in a different direction from the operating condition of PEMFCs and
AFCs. In order to overcome this problem, we applied the through-plane measurement for the
thin-film membrane and compared the results with those of the conventional in-plane
measurement. Similar approach was implemented for the liquid phase measurements.
As a result, we found that the ionic conductivities in the membrane measured in the
through-plane direction were significantly lower than those measured in the in-plane direction.
Thus, we concluded that the through-plane measurement has some challenges to be solved and
the through-plane measurement should be carried out more carefully than the in-plane
measurement.
iv
In addition to these studies, we have investigated newly synthesized OH- conductive
inorganic materials using x-ray diffraction (XRD), scanning electron microscopy (SEM), and the
ionic conductivity measurement in the through-plane direction. The materials showed high
conductivities comparable to other reported inorganic materials in the low temperature and low
humidity ranges. However, their conductivities are much lower than the conventionally used in
potassium chloride-based electrolyzers. Therefore, we need further improve the ionic
conductivity of the OH- conductive inorganic materials for the development of an efficient AFC.
v
TABLE OF CONTENTS
LIST OF FIGURES ..................................................................................................................... viii
LIST OF TABLES ......................................................................................................................... xi
NOMENCLATURE ..................................................................................................................... xii
ACKNOWLEDGEMENT ............................................................................................................ xv
Chapter 1 Introduction .................................................................................................................... 1
1.1 Fuel cells ............................................................................................................................. 1
1.1.1 PEM fuel cells ............................................................................................................... 2
1.1.2. Alkaline fuel cells ......................................................................................................... 4
1.2 Ion exchange membranes .................................................................................................... 5
1.2.1 Basis of ion exchange membranes ............................................................................... 5
1.2.2 Development of IEMs for PEMFCs and AFCs ........................................................... 5
1.3 Problem statement and methodology ................................................................................ 10
Chapter 2 Theory and experimental technique ............................................................................ 11
2.1 Ion conducting mechanisms .............................................................................................. 11
2.1.1 Ion exchange mechanism by thermodynamics .......................................................... 11
2.1.2 Ion exchange mechanism in Nafion membrane ......................................................... 12
2.2 Conductivity measurements .............................................................................................. 14
2.2.1 Conductivity cells ...................................................................................................... 14
2.2.2 Calculation for each measurement ............................................................................. 18
vi
2.3 Electrochemical impedance spectroscopy ........................................................................ 19
2.3.1 Modeling for equivalent circuit ................................................................................. 19
2.3.2 Basic theory of electrochemical impedance spectroscopy ......................................... 20
Chapter 3 Measurement for PEMs ............................................................................................... 23
3.1 In-plane measurement in gas phase .................................................................................. 23
3.1.1 Experimental setup..................................................................................................... 23
3.1.2 Results ........................................................................................................................ 25
3.2 Through-plane measurement in gas phase ........................................................................ 28
3.2.1 Experimental setup..................................................................................................... 28
3.2.2 Results ........................................................................................................................ 28
3.3 Through-plane measurement in liquid phase .................................................................... 33
3.3.1 Experimental setup..................................................................................................... 33
3.3.2 Results ........................................................................................................................ 34
3.3.4 PEM measurement summary ..................................................................................... 35
Chapter 4 Measurement for OH- conductive materials ................................................................ 36
4.1 Synthesis and characterizations of inorganic materials .................................................... 36
4.1.1 Background on layered double hydroxides and hydroxyapatite ................................ 36
4.1.2 Synthesis of sample materials .................................................................................... 39
4.1.3 Characterizations of sample materials ....................................................................... 41
4.2 Conductivity measurement ............................................................................................... 45
4.2.1 Sample preparations and experimental setup ............................................................. 45
vii
4.2.2 Results ........................................................................................................................ 46
Chapter 5 Conclusion .................................................................................................................. 50
BIBLIOGRAPHY ......................................................................................................................... 51
viii
LIST OF FIGURES
Figure 1-1: The concept of fuel cells……………………………………………………………...1
Figure 1-2: Typical polarization curve for a PEMFC…………………………………………….3
Figure 1-3: The structure of Nafion membrane…………………………………………………...6
Figure 2-1: The interface between the solution and the membrane……………………………...11
Figure 2-2: The structure model of Nafion membrane…………………………………………..13
Figure 2-3: The conductivity cell in liquid phase………………………………………………..14
Figure 2-4: The cross section of the cell…………………………………………………………15
Figure 2-5: The conductivity cell in gas phase…………………………………………………..15
Figure 2-6: The cross section and the width of the membrane…………………………………..16
Figure 2-7: The sample and electrodes of in-plane method……………………………………..16
Figure 2-8: The interface concept between the electrodes and the electrolyte…………………..19
Figure 2-9: The equivalent circuit………………………………………………………………..20
Figure 2-10: Typical impedance plots…………………………………………………………...21
Figure 3-1: Gas phase measurement setup………………………………………………………23
ix
Figure 3-2: Ohmic responses in the in-plane gas phase measurement…………………………..25
Figure 3-3: Conductivity in the in-plane gas phase measurement……………………………….26
Figure 3-4: The Nyquist plot in the through-plane gas phase measurement…………………….29
Figure 3-5: The equivalent circuit model for the measurement…………………………………30
Figure 3-6: Conductivity in the through-plane gas phase measurement………………………...30
Figure 3-7: Nyquist plot in liquid phase measurement of solution and membrane……………...34
Figure 4-1: Schematic representation of the layered double hydroxide structure……………….37
Figure 4-2: Hydroxyapatite structure projected on x, y plane…………………………………...38
Figure 4-3: Scanning electron micrograph showing the particle morphology and size of Chloride
containing Mg-Al LDH (JK-3)…………………………………………………………………..42
Figure 4-4: Scanning electron micrographs showing the particle morphology and size of Chloride
containing Ca-Al LDH (JK-4) at two different magnifications………………………………….43
Figure 4-5: X-ray diffraction patterns of (a) chloride containing Mg-Al LDH (JK-3) and (b)
chloride containing Ca-Al LDH (JK-4)………………………………………………………….44
Figure 4-6: X-ray diffraction pattern of hydroxide containing Mg:Al LDH (YN 252)…………44
Figure 4-7: X-ray diffraction pattern of hydroxyapatite (YN 148)……………………………...45
Figure 4-8: Nyquist plots of inorganic materials at 95 %RH……………………………………46
x
Figure 4-9: OH- Conductivities in inorganic materials…………………………………………48
xi
LIST OF TABLES
Table 1-1: The reported organic membrane examples and their ionic conductivities
for PEMFCs….……………………………………………………………………………………8
Table 1-2: The reported inorganic material examples and their ionic conductivities
for PEMFC………………………………………………………………………………………..8
Table 1-3: The reported membrane examples and their ionic conductivities for AFCs………….9
Table 3-1: The experimental procedure………………………………………………………….24
Table 3-2: Conductivity data in the in-plane gas phase measurement…………………………...27
Table 3-3: Conductivity data in the through-plane gas phase measurement....………………….32
xii
NOMENCLATURE
A Surface area of a membrane (cm2)
a Activity
a Activity in membranes
B Tafel slope (V)
C Capacitance (F)
Cdl Double layer capacitance (F)
d Diameter of a disk (cm)
E Potential difference (V)
Ediff Diffusion potential (V)
EDon Donnan potential (V)
f Frequency (Hz)
H Height of a membrane in the in-plane direction (cm)
k Reaction rate
L Length of a membrane in the in-plane direction (cm)
l Length of a membrane in the through-plane direction (cm)
I Current (A)
i Current density (A cm-2)
j Imaginary unit
k Boltzmann constant (1.3806504×10-23 J K-1)
M Molarity (mol L-1)
n Degree of phase difference
Q Total electrical charge passed in a circuit (C)
Qo Modified capacitance (F)
xiii
R Molar gas constant (8.314 J K-1 mol-1)
Rct Charge transfer impedance (Ω)
Rmem Resistance of a membrane (Ω)
Rsol Resistance of a solution (Ω)
Rtot Resistance through the all components (Ω)
T Thermodynamic temperature (K)
t Time (s)
V Applied voltage (V)
V0 Amplitude of AC voltage (V)
W Width of a membrane in the in-plane direction (cm)
Z Impedance (Ω)
Greek
σ Ionic conductivity (mS cm-1)
σ0 Pre-exponential factor
Abbreviations
AFC Alkaline Fuel Cell
PEMFC Proton Exchange Membrane Fuel Cell
PAFC Phosphoric Acid Fuel Cell
MCFC Molten Carbonate Fuel Cell
SOFC Solid Oxide Fuel Cell
FCV Fuel Cell Vehicle
IEM Ion Exchange Membrane
xiv
IEC Ion Exchange Capacity
S-PPQ Sulfonated Polyphenylquinoxaline
HPA Heteroplyacid
GPTS Glycidoxypropyltri-methoxy
SPS Sulfonated PhenyltriethoxySilane
BMImCl Butyl-Methyl-Imidazolium Chloride
PWA Phosphotungstic Acid
SPEEK Sulfonated Polyether
PAMPS AcrylAmido-Methil-Propane Sulfonic acid
VP VinylPyradine
DABCO Diazabicyclo-Octane
SEM Scanning Electron Microscopy
XRD X-ray Diffraction
RH Relative Humidity
PID Proportional Integral Derivative
V.P. Vapor Pressure
EIS Electrochemical Impedance Spectroscopy
CPE Constant Phase Element
DOE Department of Energy
LDH Layered Double Hydroxides
xv
ACKNOWLEDGEMENT
I finished this thesis with countless technical and intellectual supports. I would like to
show my gratitude and respect to the individuals who contributed to my work.
Thanks to my thesis advisor, Dr. Serguei N. Lvov for letting me join his laboratory and
supervise my lab works and paper writing. Even though my background in electrochemistry was
not enough and my work was very slow, he patiently gave me great advises that helped me learn
new field on my own. This experience will definitely help me with my future work in both
scientific and military work.
I am grateful to my thesis co-advisor, Dr. Sridhar Komarneni for giving me many
instructive advises and telling me how to prepare the samples, grammatical details in my papers,
and how to find a good reference in the material science field.
Thanks to my committee members, Dr. Derek Elsworth and Dr. Jeffrey R.S. Brownson.
They received my offer to be my committee members with good grace, and gave inspiring
instructions and questions on my thesis work. In tackling these questions, my skill as a
researcher was improved greatly.
Special thanks to Dr. Mark V. Fedkin and Dr. Elena Chalkova. They directly supported
my experiments in the lab, including the preparation for the samples and conductivity
measurements.
Thanks to my colleagues, Chunmei Wang, Justin R. Beck, Mark S. LaBarbera, Richard S.
Schatz, Matthew A. Brown. Delightful conversation with them always encouraged me to conduct
xvi
my experiment and improved my English skills. Special thanks are to Chunmei Wang who
supported my experimental activity and give me multiple suggestions on my work. Justin Beck
also helped me do proof-reading and many clues for my study and communication skills.
I also wish to thank all my friends who cheered my life in State College, PA. Although it
was sometimes too tough for me to manage to my significant amount of assignments, they
facilitated those works as catalysts.
Finally, thanks to my supervisors in Japan, Col. Inoue and Maj. Ueno. They got in touch
with me closely and dealt with my student life financially and mentally. Financial support for my
student life was provided by the Japanese Ministry of Defense.
1
Chapter 1 Introduction
1.1 Fuel cells
A fuel cell is an energy conversion system that consumes hydrogen and oxygen in order
to produce electricity. Other fuels can also be consumed for some kinds of fuel cells. The cell has
the half-cell chemical reactions occur on the electrode surfaces. The reduction reaction occurs at
the cathode and the oxidation reaction proceeds at the anode. A simple schematic of the concept
for different kinds of fuel cells is shown in Figure 1-1.
Figure 1-1: The concept of fuel cells[1,2].
AFC, PEMFC, PAFC, MCFC, and SOFC stand for alkaline fuel cell, proton exchange membrane
(polymer electrolyte membrane) fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell,
and solid oxide fuel cell, respectively. The key factors that decide the reaction on the electrode
2
surfaces are electrodes, electrolytes, and the operating conditions. The half-cell chemical
reactions vary depending on these factors. Since the fuel cell does not work as a heat engine, it
avoids the efficiency limit of the Carnot cycle and shows higher efficiency [3].
PEMFCs and AFCs operate at relatively lower temperature and can potentially use a
solid state ion exchange membrane as the electrolyte, which would help these fuel cells to
increase their compactness, mechanical strength, and the range of applications.
1.1.1 PEM fuel cells
PEMFCs have a wide range of applications, such as the power source for fuel cell
vehicles (FCVs), stationary power plants, and energy storage [1]. The half-cell chemical
reactions in hydrogen PEMFCs are
Cathode: 12
O2 + 2H+ + 2e− → H2O
Anode: H2 → 2H+ + 2e−
Considerable studies have been carried out in order to improve the performance of PEMFCs.
One of the effective ways to examine the performance of a fuel cell is to observe the potential
difference and the current. The plot of this potential difference and current is called “polarization
curve” [4]. The curve typically shows a shape such as in Figure 1-2 below.
3
Figure 1-2: Typical polarization curve for a PEMFC [4]
This curve has three well defined regions, and these are called, “activation polarization losses”,
“ohmic losses”, and “concentration polarization losses,” respectively. The activation polarization
losses are related to the rate of reactions on the electrode surfaces. According to Tafel equation:
𝑉𝑉 = 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵|𝑖𝑖| [1-1]
where V, B, and i are the potential difference, Tafel slope, and current density, respectively. This
equation shows the behavior when we assume that only the activation polarization losses exists.
The reaction rate at the electrode is determined by the chemical reaction, and the reaction rate at
cathode is much lower than that at anode in a PEMFC. In order to increase the reaction rate at
cathode and decrease the loss, considerable studies on the electrode catalysts have been
conducted. Although platinum and platinum-ruthenium alloy have been found to be the most
effective catalysts, studies on new materials including tungsten, tin, and molybdenum have been
reported for more reasonable production costs [5]. The concentration polarization losses are
caused by the lack of reactants on the electrode surfaces. For these losses modeling and new flow
4
channel patterns to prevent the porous electrodes from flooding with water have been reported
[4,6]. The ohmic losses are dominantly caused by the resistance of the PEM. In order to reduce
its resistance, many studies for exploring new materials with high ionic conductivity have been
made.
1.1.2. Alkaline fuel cells
Alkaline fuel cells (AFCs) were developed for the space shuttles because they were
lighter than any other practical power generation systems that could be mounted in the limited
space in the shuttles. These AFCs produced water and electricity from pure oxygen and
hydrogen from the shuttle tanks [7]. The half-cell chemical reactions are as follows:
Cathode: 12
O2 + H2O + 2e− → OH−
Anode: H2 + 2OH− → 2H2O + 2e−
The reaction rate of the reduction reaction at the cathode is much faster than that in PEMFC, and
the cathode does not need a noble metal electrocatalyst. However, in usage on the earth they
showed poor performance and durability. One of the dominant reasons is that the CO2 in the air
reacts with the electrolyte of AFCs, potassium hydroxide (KOH), resulting in the deposition of
potassium carbonate (K2CO3), which causes a decrease of the cell performance [7]. The OH-
conductive solid electrolyte has gathered much attention in order to overcome this problem and
use AFC with air rather that with pure oxygen.
5
1.2 Ion exchange membranes
1.2.1 Basis of ion exchange membranes
An ion exchange membrane (IEM) is a membrane that allows the permeation of specific
ions through various mechanisms and prevents permeation of other ions and molecules. The
permeable ions are called “counter-ions” because the membrane itself is charged oppositely by
ions fixed inside the membrane. The ions in the electrolyte charged oppositely to counter-ions
are called “co-ions”. For example, when we use a Cl- exchange membrane in a NaCl aqueous
solution the counter-ion is Cl- and the co-ion is Na+. There are many IEMs with varying
properties, such as conductivity, permselectivity between like-charged ions, transport number,
ion exchange capacity (IEC), water uptake, mechanical strength, and so on. Taking these
parameters into consideration, the membrane should be chosen properly to suit the purpose of the
systems. For hydrogen PEMFCs, the IEM is used to provide fuel, in the form of protons, from
anode to cathode, and conductivity is important in order to avoid ohmic losses. In the case of an
electrolyzer that has many ionic species the membrane has to have a good permselectivity in
order to maintain its performance by keeping undesirable ions from permeating through the
membrane [8].
1.2.2 Development of IEMs for PEMFCs and AFCs
Many studies have been carried out in order to develop highly conductive ionic
membranes under various operating conditions. Nafion® 117 has a high conductivity of 120 mS
6
cm-1 at 120 oC in the low pressured gas phase and 90 mS cm-1 at 25 oC in deionized water [9,10].
It also shows tough mechanical strength that enables it to be used in commercial PEMFCs.
Nafion is made by E.I. du Pont de Nemours & Co. (Inc) and has been applied for a wide range of
electrochemical systems. It has ionomers with hydrophobic C-F chains and hydrophilic sulfonic
acid units (see Figure 1-3) [3].
(CF2－CF2)x (CF －CF2)y
O
CF2
FCーCF
O
CF2
CF2
SO3H
Figure 1-3: The structure of Nafion membrane.
Because of these properties, Nafion has been used in lower temperature conditions desirable for
PEMFCs. However, it cannot be used over 100 oC because heat treatment causes shrinkage of
the membrane and decrease of the water content [11]. Moreover, the morphology of the
membrane can be changed at high temperature [12]. In order to overcome this dilemma,
membranes with various unconventional materials are being developed, and the conductivities
are also being improved as shown in Table 1-1. In parallel with the improvement of organic
materials some inorganic materials and combinations of organic monomers and inorganic
7
particles, which are called “composite membranes,” have been developed for PEMFCs as shown
in Table 1-2. We found that the reported conductivities at both low temperature and high
temperature have been improved year by year. These kinds of efforts are also being conducted
for AFCs as shown in Table 1-3.
8
Table 1-1: The reported organic membrane examples and their ionic conductivities for PEMFCs.
Table 1-2: The reported inorganic material examples and their ionic conductivities for PEMFC
Reported Year
Materials Conductivity mS/cm
Temperature oC
Relative humidity
Measurement (Frequency range)
Source
2003 3-glycidoxypropyltri-methoxysilane(GPTS), sulfonated phenyltriethoxysilane (SPS), tetraethoxysilane(TEOS)
36 120 15 % Gas through-plane (10m-5MHz)
[17]
2004 Poly (styrene-co-methacrylate)—silica covalent by copolymerization of monomers (styrene and 2-hydroxyethyl methacrylate)
0.8
25 30 Gas through-plane (10-10MHz)
[18]
2005 Nafion doped with solid acidic inorganic material (ZrO2, SO4/ZrO2, ZrOH, ZrO2 sol-gel)
1.5/14 1.8/18.5
90 120
10/40 % Gas through-plane (10m-1MHz)
[19]
2006 1-butyl-3-methyl-imidazolium chloride (BMImCl) and 12-phosphotungstic acid (PWA)
2 30 96 % Gas through-plane [20]
2006 Sulfonated polyether ether ketone(SPEEK) /WO3·2H2O
7.5 19
30 50
100 % Gas through-plane (5-5MHz)
[21]
2009 Nafion–mesoporous zirconium phosphate 11 30 100 % Gas through-plane (10-1MHz)
[22]
2009 2-acrylamido-2-methyl-1-propane sulfonic acid (PAMPS) in diphenylsiloxane-silica (Ph2SiO–SiO2)
~2 10
30 80
80 % Gas through-plane (1-10MHz)
[23]
Reported Year
Materials Conductivity mS/cm
Temperature oC
Relative humidity
Measurement (Frequency range)
Source
1998 Nafion Perfluorosulfonic acid
28 55
25 81 % 100 %
Gas in-plane (0.1 - 20 kHz)
[13]
1998 bis[(perfluo-roalkyl)sulfonyl]imide ionome 8.9 100
25 31 % 100 %
Gas in-plane (0.1 - 20 kHz)
[13]
2000 Aromatic based Sulfonated polyphenylquinoxaline (S-PPQ)
9.8 20 80 % Not mentioned clearly
[14]
2003 Heteropolyacid (HPA)/sulfonated poly(arylene ether sulfone)
80 150
25 130
100 % Gas in-plane (10 Hz - 1 MHz)
[15]
2005 Sulfonated block copolyimides 350 80 100 % Gas in-plane (50 Hz - 50 kHz)
[16]
9
Table 1-3: The reported membrane examples and their ionic conductivities for AFCs
Reported Year
Materials Conductivity mS/cm
Temperature oC
Relative humidity
Measurement (Frequency range)
Source
2006 4-vinylpyradine (4-VP) and styrene (Initiator: 2,2’-azobisisobutyronitrile)
8 25 Liquid (Methanol)
Liquid through-plane
[24]
2007 1,4-diazabicyclo-[2.2.2]-octane (DABCO) and 1-azabicyclo-[2.2.2]-octane (Quinuclidine) on poly(epichlorydrin-allyl glycidyl ether) copolymer
2.5 13
20 60
98 % Gas through-plane (5 Hz–13 MHz)
[25]
2010 Random copolymer of poly(methyl methacrylate-co-butyl acrylate-co-vinylbenzyl chloride)
8.2 80 60 % Gas in-plane [26]
2010 Silica/poly(2,6-dimethyl-1,4-phenylene oxide) 12 35
30 90
100 % Gas in-plane [27]
(2006) Potassium chloride solution (KOH) 200 25 - (Conductivity meter with 50 kHz)
[28]
10
1.3 Problem statement and methodology
We found that the in-plane measurements with 4 electrodes are mostly used for the ionic
conductivity measurements of organic materials shown in Table 1-1. We can measure the ionic
conductivity for the in-plane direction with this method. However, the protons are conducted
from the anode to the cathode in the operating conditions of PEMFCs. This direction is through-
plane. In order to evaluate the ionic conductivities of PEMs more accurately, we need to conduct
a through-plane measurement.
Furthermore, in tables 1-1, 1-2, and 1-3 we found that the conductivity measurement
methods used in these reports vary depending on the materials. We need to evaluate these
measurement methods in order to compare these results. Thus, we measured the Nafion sample
with the two measurement methods, the in-plane and the through-plane in the gas phase. As for
the liquid phase measurements the conductivity was measured with a through-plane method.
Then the result was compared to the reference data measured with the in-plane method.
In addition to these studies, highly OH- conductive materials are in demand in order to
increase the utility and applications of AFCs. The materials reported so far have shown relatively
lower conductivities than those for PEMFCs. In addition, these organic materials are not stable in
higher temperature over 80 oC. Therefore, we have synthesized new inorganic materials. These
materials were characterized with scanning electron microscopy (SEM) and X-ray diffraction
(XRD), and their conductivities were examined with the through-plane conductivity
measurements as well.
11
Chapter 2 Theory and experimental technique
In this chapter, we will examine the membrane and ion exchange mechanism first. Then
we will describe the conductivity cells that are used in each of the three ionic conductivity
measurements. Finally, the impedance measurement technique will be explained.
2.1 Ion conducting mechanisms
2.1.1 Ion exchange mechanism by thermodynamics
We first look at the interface example between a cation exchange membrane and
electrolyte solution as shown in Figure 2-1.
+-
+
--
-
+
+
+
-Electrolyte solution
(Activity: a1)Membrane
+
+
-
-
+
a1 a2a1 a2
Electrolyte solution(Activity: a2)
Figure 2-1: The interface between the solution and the membrane.
12
a1, a1, a2, and a2 are the activities of permeable cations in the electrolyte1, in the membrane at
the electrolyte 1 side, in the electrolyte2, and in the membrane at the electrolyte 2 side,
respectively. The membrane has anions fixed inside of it. This makes its electrochemical
potential different from the bulk solution. This potential difference is termed the Donnan
potential [29]. It can be written as
𝐸𝐸𝐷𝐷𝐵𝐵𝐷𝐷 = 𝑅𝑅𝑅𝑅𝐹𝐹𝐵𝐵𝐷𝐷 𝑎𝑎1����
𝑎𝑎1 [2-1]
On the other side of the membrane the Donnan potential is
𝐸𝐸𝐷𝐷𝐵𝐵𝐷𝐷 ′ = 𝑅𝑅𝑅𝑅𝐹𝐹𝐵𝐵𝐷𝐷 𝑎𝑎2
𝑎𝑎�2 [2-2]
Also, there occurs the diffusion of ions inside the membrane. The diffusion potential is
𝐸𝐸𝐷𝐷𝑖𝑖𝐷𝐷𝐷𝐷 = 𝑅𝑅𝑅𝑅𝐹𝐹 ∫ ∑ 𝑡𝑡�̅�𝑖
𝑧𝑧𝑖𝑖𝑑𝑑 𝐵𝐵𝐷𝐷𝑎𝑎𝑖𝑖�
𝑎𝑎�2𝑎𝑎�1
[2-3]
Thus, its total membrane potential is
𝐸𝐸 = 𝐸𝐸𝐷𝐷𝐵𝐵𝐷𝐷 + 𝐸𝐸𝐷𝐷𝐵𝐵𝐷𝐷′ + 𝐸𝐸𝐷𝐷𝑖𝑖𝐷𝐷𝐷𝐷 [2-4]
𝐸𝐸 = 𝑅𝑅𝑅𝑅𝐹𝐹𝐵𝐵𝐷𝐷 𝑎𝑎1����
𝑎𝑎1+ 𝑅𝑅𝑅𝑅
𝐹𝐹𝐵𝐵𝐷𝐷 𝑎𝑎2
𝑎𝑎�2+ 𝑅𝑅𝑅𝑅
𝐹𝐹 ∫ ∑ 𝑡𝑡�̅�𝑖𝑧𝑧𝑖𝑖𝑑𝑑 𝐵𝐵𝐷𝐷𝑎𝑎𝑖𝑖�
𝑎𝑎�2𝑎𝑎�1
[2-5]
2.1.2 Ion exchange mechanism in Nafion membrane
As we saw in the Section 1.2.2 and Figure 1-3, Nafion has ionomers inside it. These
ionomers are supported Teflon that has high mechanical strength and chemical stability. Its
structure model is shown in Figure 2-2.
13
Figure 2-2: The structure model of Nafion membrane. [30]
These hydrophilic units retain moisture inside the membrane and pass protons around. Thus, we
can assume the proton conductivity of Nafion will increase when the humidity around the
membrane increases in the gas phase. The temperature also affects its conductivity [31]. When
the temperature increases, the conductivity of protons increases exponentially following the
equation
𝜎𝜎𝑅𝑅 = 𝜎𝜎0𝑒𝑒𝑒𝑒𝑒𝑒 �−𝐸𝐸𝑎𝑎𝑘𝑘𝑅𝑅� [2-6]
where σ, σ0, and k are the ionic conductivity and the pre-exponential factor, and the Boltzmann
constant, respectively. We see that the ionic conductivity increases as the temperature increases
exponentially.
Teflon
Perfluorosulfonic chains
14
2.2 Conductivity measurements
2.2.1 Conductivity cells
From the discussion above, we now recognize that we need to control temperature,
pressure, and humidity to evaluate the ionic conductivity of an IEM in the gas phase. For the
measurement in liquid phase the glass cell is appropriate in order to prevent its corrosion and
leakage of the electrolyte solutions. In our experiment, we used a glass conductivity cell as
shown in Figure 3-3.
Figure 2-3: The conductivity cell in liquid phase.
The sample membrane is held between two glass chambers and fixed with a clip. Then the
chambers are filled with sample solution. The membrane is in contact with solution by the cross
section area A, and its through-plane length is l as shown in Figure 2-4.
15
Figure 2-4: The cross section of the cell.
For the gas phase measurement we used a Bekktech conductivity cell (BT-112, Figure 2-
5) made of metal. It can flow inert nitrogen gas inside the cell so that we can avoid any
undesirable reactions. It is equipped with two platinum disk electrodes that sandwich the sample
membrane.
Figure 2-5: The conductivity cell in gas phase.
16
The membrane sample was cut into a circle and the Teflon sample holder fixes the sample and
electrodes. The length of the sample is l and the cross section is A as shown in Figure 2-6 below.
Figure 2-6: The cross section and the width of the membrane.
For the in-plane gas phase measurement, we changed this 2-electrode set into the 4-electrode one
as shown in Figure 2-7 below.
Figure 2-7: The sample and electrodes of in-plane method.
17
L, H, W are the distance between two reference electrodes, the height, and the width of the
sample, respectively. The sample should be cut into a rectangle. The outside working and
counter electrode apply DC voltage and a potential gradient occurs across the sample. At the
same time, the reference electrodes measure the potential difference of the sample, sensing the
flow of protons across the surface of the sample.
For the gas phase measurement the cell is connected to a BT-104 saturator by Bekktech and
a BT-301 pressurized deionized water system. With the saturator, we can control the relative
humidity (RH) which is expressed in a percentage. RH is a simple indicator of humidity, the ratio
of humidifier’s saturated vapor pressure to that in the conductivity cell.
RH% = (Saturated vapor pressure of the humidifier )(Saturated vapor pressure of the conductivity cell )
× 100 [2-7]
For example, when the temperature of the conductivity cell is 80 Co (vapor pressure, V.P. =
0.0467 MPa) and that of the humidifier is 60 Co (V.P.=0.020), RH = (0.020/0.0467)×100 = 42 %
[11]. The cell also has thermocouples to detect and control the temperature inside. PID
(proportional-integral-derivative) controllers attached to the thermocouples are used to keep the
temperature constant in each component, the conductivity cell, the humidifier, and the inlet. The
temperature of the inlet connecting the humidifier to the cell has to be higher than others set
point temperatures. Otherwise, there occurs vapor condensation inside the inlet, resulting in an
incorrect RH value.
18
2.2.2 Calculation for each measurement
In the liquid phase through-plane measurement the membrane’s conductivity can be
calculated by the equation,
𝑅𝑅𝑚𝑚𝑒𝑒𝑚𝑚 = 𝑅𝑅𝑡𝑡𝐵𝐵𝑡𝑡 − 𝑅𝑅𝑠𝑠𝐵𝐵𝐵𝐵 [2-8]
where Rmem, Rtot,and Rsol represent the resistance of the membrane, total resistance, and the
resistance of solution, respectively. Then we calculate its conductivity by the equation,
𝜎𝜎 = 1𝑅𝑅𝑚𝑚𝑒𝑒𝑚𝑚
∙ 𝐵𝐵𝐴𝐴
[2-9]
where σ, l, and A are the conductivity of the membrane, the length, and the area of the
membrane. This equation is also applicable to the gas phase through-plane measurement because
there is no liquid between electrodes and the membrane. For these through-plane measurements,
the resistance of the cell and membrane are measured by electrochemical impedance
spectroscopy (EIS). In the next section, the concept of EIS and its process will be described.
In gas phase in-plane measurement the length and area of the membrane is different from
those of through-plane as shown in Fig.2-7. In equation 2-9, l changes to L, and A is W・T.
Now the equation for conductivity is,
𝜎𝜎 = 1𝑅𝑅𝑚𝑚𝑒𝑒𝑚𝑚
∙ 𝐿𝐿𝑊𝑊×𝑅𝑅
[2-10]
For the in-plane measurement, it is not necessary to apply EIS with AC voltage. When we apply
DC voltage to the cell, it returns linear current, or an ohmic response. Its resistance can be
obtained from the slope of the i-V plot.
19
2.3 Electrochemical impedance spectroscopy
2.3.1 Modeling for equivalent circuit
Every electrochemical system has electrical double layers because there are always phase
boundaries between interfaces of electrodes and electrolytes (see Figure 2-8). This double layer
has two opposite electric charges between the boundaries which part two phases. This double
layer can be regarded as a capacitor with a certain amount of capacitance when we apply low
(lower than 10 mV) AC voltage, so we can assume that the system is linear.
+
++
++
CathodeAnode
- -
--
--
----
+
+
+
+
+
Electrolyte
Anions Cations
Interface Interface
Figure 2-8: The interface concept between the electrodes and the electrolyte.
Under this consideration, we can assume an electrical circuit to be equivalent to an
electrochemical system. For example, the boundary of Figure 2-8 is equivalent to the elements in
Figure 2-9 (a), or more simply expressed as (b).
20
Rsol
Rct Rct
Cdl Cdl
Rsol
Rct
Cdl=
(a) (b)
(a) Two interfaces (b) Simplified interface
Figure 2-9: The equivalent circuit.
Rsol, Rct, and Cdl are the resistance of the solution, the charge transfer resistance, and the double
layer capacitance. The charge transfer resistance stands for difficulty of charge transfer on the
surface of electrode which is unique to electrode materials and electrolytes. The double layer
capacitance is the capacitance at the electrode surface as shown in Fig 2-8. When we have this
equivalent circuit, we can easily evaluate it with electrochemical impedance spectroscopy (EIS),
identifying what kind of factors (capacitors, resistance, reactance) are in the system. Then we can
predict the behavior of some materials.
2.3.2 Basic theory of electrochemical impedance spectroscopy
Assuming that the capacitance of the double layer is Cdl, when we apply an AC voltage,
V, to the capacitor,
V = V0 exp(j2πft) [2-11]
where j, f, and t are imaginary unit, frequency of the voltage, and time by second. The current, i,
can be written as
𝑖𝑖 = 𝑑𝑑𝑑𝑑𝑑𝑑𝑡𝑡
= 𝐶𝐶𝑑𝑑𝐵𝐵𝑑𝑑𝑉𝑉𝑑𝑑𝑡𝑡
= (𝑗𝑗2𝜋𝜋𝐷𝐷𝐶𝐶𝑑𝑑𝐵𝐵 )𝐸𝐸0 𝑒𝑒𝑒𝑒𝑒𝑒(𝑗𝑗2𝜋𝜋𝐷𝐷𝑡𝑡) = (𝑗𝑗2𝜋𝜋𝐷𝐷𝐶𝐶𝑑𝑑𝐵𝐵 )𝑉𝑉 [2-12]
21
where Q stands for the total electrical charge passed. Now the impedance of the double layer, Z,
is,
𝑍𝑍 = 𝑉𝑉𝐼𝐼
= 1𝑗𝑗 (2𝜋𝜋𝐷𝐷𝐶𝐶𝑑𝑑𝐵𝐵 )
[2-13]
Thus, we can see that the impedance phases of an electrochemical system with a double layer
will change when we change the frequency. For a simple circuit as in Figure 2-9 the impedance
is
𝑍𝑍 = 𝑅𝑅𝑠𝑠𝐵𝐵𝐵𝐵 + 𝑅𝑅𝑐𝑐𝑡𝑡1+𝑗𝑗 (2𝜋𝜋𝐷𝐷𝑅𝑅𝑐𝑐𝑡𝑡 𝐶𝐶𝑑𝑑𝐵𝐵 )
= �𝑅𝑅𝑠𝑠𝐵𝐵𝐵𝐵 + 𝑅𝑅𝑐𝑐𝑡𝑡1+(2𝜋𝜋𝐷𝐷𝑅𝑅𝑐𝑐𝑡𝑡 𝐶𝐶𝑑𝑑𝐵𝐵 )2� − 𝑗𝑗 2𝜋𝜋𝐷𝐷𝑅𝑅𝑐𝑐𝑡𝑡 2𝐶𝐶𝑑𝑑𝐵𝐵
1+(2𝜋𝜋𝐷𝐷𝑅𝑅𝑐𝑐𝑡𝑡 𝐶𝐶𝑑𝑑𝐵𝐵 )2 [2-14]
From this equation, we see the impedance phase (the imaginary part) changes as the frequency
changes. Especially, when the frequency increases toward infinite (f → ∞), the impedance
approximates the solution resistance (Rsol). When the frequency increases toward zero (f → 0),
the impedance approximates the sum of the solution resistant and the charge transfer resistance
(Z → Rsol + Rct). Also, the imaginary part reaches its maximum when
2𝜋𝜋𝐷𝐷 = 1𝑅𝑅𝑐𝑐𝑡𝑡 𝐶𝐶𝑑𝑑𝐵𝐵
[2-15]
From these results, we can write its impedance plot as shown in Figure 2-10.
(a) The Nyquist plot (b) Bode plot
Figure 2-10: Typical impedance plots.
22
The impedance plot as (a) is called the Nyquist plot, and (b) is called the Bode plot. When we
obtain the Nyquist plot and the Bode plot of an electrochemical system we can compare these
plots with those modeled with an equivalent circuit. Then we can identify each element in the
actual electrochemical system and even predict the conducting mechanism of the measured
sample. The frequency range should be adjusted depending on the behavior of the material. The
applied voltage should be low (10 mV – 100 mV) in order not to have any undesirable chemical
reactions.
In the actual model fitting, we can use the constant phase element (CPE) that makes the
modeled Nyquist plot circle twisted so that the modeling fits the results of experiments. This
element corresponds with the surface roughness of the electrodes, a distribution of reaction rates,
varying thickness or composition of the membrane, and non-uniform current distribution [31-36].
The impedance of the CPE can be written as
𝑍𝑍𝐶𝐶𝐶𝐶𝐸𝐸 =1
𝑑𝑑𝐵𝐵(𝑗𝑗2𝜋𝜋𝐷𝐷)𝐷𝐷
where Qo and n represents the modified capacitance and the degree of phase difference. When Qo
is C and n is unit, ZCPE is equal to the impedance of a capacitor with its capacitance of C.
23
Chapter 3 Measurement for PEMs
3.1 In-plane measurement in gas phase
3.1.1 Experimental setup
We used an EIS system produced by Gamry Instruments in this experiment that has a
power supply and a frequency response analyzer. With this system, we can apply DC and AC
voltage and obtain the Nyquist plot under an arbitrary frequency range within 1.0 mHz – 300
kHz. The system has an inert nitrogen gas cylinder, humidity and PID temperature controls in
addition to the conductivity cell and EIS Gamry system used in the first measurement method in
the liquid phase (See Figure.3-1). Nitrogen gas is first injected into the humidifier and then to the
cell in order to prevent undesirable electrochemical reactions. The flow and pressure of nitrogen
gas is controlled to 18.3 psi and around 350 cm3 min-1 by a flow controller, flow meter, and two
gate valves.
Beaker
Water coolerWater trap
V-3
Conductivity cell
Nitrogen cylinder BT-104 Bekktechsaturator
BT-301 Pressurized Deionized water system
Pressure gauge
Valve
Valve
Flow meter
Thermocouples
Figure 3-1: Gas phase measurement setup.
24
The measurement was conducted in a procedure standardized by DOE and the Bekktech
Company to get conductivity under certain conditions of RH and temperature [35]. We used
25oC, 30 oC, 80 oC, and 120 oC as the temperature of the cell and 20 %RH -100 %RH as the
humidity. The process is as follows in Table 3-1.
Table 3-1: The experimental procedure
Time (min.) Relative humidity Saturator temperature for the 120 oC experiment
oC
Saturator temperature for the 80oC experiment
oC 0* 70 % 108.9 71.4 15 60 % 104.4 68.0 30 50 % 99.2 63.8 45 40 % 93.0 59.0 60 30 % 85.4 53.0 75 25 % 81.1 49.2 90 20 % 75.2 44.8 105 25 % 81.1 49.2 120 30 % 85.4 53.0 135 40 % 93.0 59.0 150 50 % 99.2 63.8 165 60 % 104.4 68.0 180 70 % 108.9 71.4 195 80 % 113.0 74.5 210 90 % 116.7 77.5 225 95 % 118.4 78.7 240 100 % 120.0 80.0
* The sample was calibrated at 70 %RH for 2 hours before measurements
As for the measurement in low temperatures (30 oC), the RH was kept at 100 % because
humidity controlling is difficult and may cause significant errors in the results.
25
The cell is equipped with four linear shaped electrodes, and the sample was cut to a
rectangle of 2.0 × 0.2 cm. The sample was put on electrodes and fixed with the holder. The cell
was connected to the Gamry system, the saturator, and the gas flow line. Then its ionic
conductivity was measured with the procedure in the previous section except for the usage of DC
voltage.
3.1.2 Results
The voltage-current plot (V-i plot) is shown in Figure 3-2. From the slope of this plot we
obtained the Rmem value and calculate the conductivity with equation 2-10. Ohmic responses and
conductivity data are shown in Figure 3-3, and Table 3-2 below.
(a) Ohmic response at 80 oC (b) Ohmic response at 120 oC.
Figure 3-2: Ohmic responses in the in-plane gas phase measurement.
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
-0.0002 -3E-18 0.0002
Current A
Voltage V30%50%80%90%100%
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
-0.0002 -1E-18 0.0002
Current A
Voltage V30%50%80%90%100%
26
(b) Conductivity at 80 oC.
(b) Conductivity at 120 oC.
Figure 3-3: Conductivity in the in-plane gas phase measurement.
1.0E+00
1.0E+01
1.0E+02
0 20 40 60 80 100
Cond
ucti
vity
σm
S/cm
Relative humidity %
Measured
Reference
80 oC
1.0E+00
1.0E+01
1.0E+02
0 20 40 60 80 100
Cond
ucti
vity
σ
mS/
cm
Relative humidity %
Measured
Reference
120 oC
27
Table 3-2: Conductivity data in the in-plane gas phase measurement [37].
Relative humidity
Conductivity at 80 oC mS cm-1
Reference data at 80 oC mS cm-1
Conductivity at 120 oC mS cm-1
Reference data at 120 oC mS cm-1 cm-1
70 % 46.13 52 62.06 66 60 % 42.45 49 41.46 48 50 % 39.23 29 27.96 31 40 % 20.98 19 18.84 21 30 % 15.79 12 9.439 12 25 % 9.435 7.5 7.080 7.1 20 % 4.542 4.9 4.333 4.1 25 % 6.145 7.2 6.206 6.2 30 % 8.226 8.5 8.454 8.9 40 % 10.81 15 17.89 18 50 % 19.63 23 26.65 29 60 % 22.83 32 41.95 47 70 % 42.66 45 61.33 65 80 % 50.71 62 79.74 89 90 % 58.37 89 121.7 140 95 % 72.07 110 186.6 175 100 % 105.1 130 214.8 200 Conductivity
at 30 oC mS cm-1 Reference data at 30 oC mS cm-1
1 100 % 81.73 82
The obtained conductivities are very close to the reference data provided by Bekktech. This
means that the system worked properly and has no problem with the cell and the sample. The
data at 80 oC deviate slightly from the reference data. The conductivities are increasing as the
RH decreases, and vice versa. The quality of data could be improved by equilibrating for more
time at each temperature so that the sample absorbs water molecules.
28
3.2 Through-plane measurement in gas phase
3.2.1 Experimental setup
Through-plane measurement uses the same system as Fig.3-1 except for the conductivity
cell and its electrodes. As for the sample, Ag paste was put on the membrane and dried out for 24
hours in order to ensure the electric connection between the sample and the electrodes. Then the
membrane was cut into a disk (d = 1.6 cm) and sandwiched between the electrode plates and held
in a Teflon bracket and the exterior of the conductivity cell. The cell was connected to the
nitrogen cylinder, and its leakage was checked with an observation of the flow meter and the
pressure gauge. Then the temperatures of the cell, the inlet, and the humidifier were increased up
to experimental value gradually. The ionic conductivities were measured, controlling the RH
inside the cell following the procedure in Table 3-1.
3.2.2 Results
Each impedance measurement was taken three times at each temperature to get the
average value, which is shown in Figure 3-4. From this plot, we read the Rmem value by fitting the
curve with the model as shown in Figure 3-5 and calculate conductivity with equation 2-9. The
obtained conductivity data are shown in Figure 3-6 and Table 3-3 below.
29
(a) Nyquist plot at 80 oC.
(b) Nyquist plot at 120 oC.
Figure 3-4: The Nyquist plot in the through-plane gas phase measurement.
0
500
1000
1500
2000
2500
3000
0 1000 2000 3000 4000 5000 6000
Im{Z
} Ω
Re {Z} Ω
30%
40%
50%
60%
70%
80%
0
20
40
60
80
100
120
140
0 50 100 150
Im{Z
} Ω
Re {Z} Ω
30%
40%
50%
60%
70%
80%
30
Rsol
Rct
Cdl CPE
Figure 3-5: The equivalent circuit model for the measurement.
(a) Conductivity at 80 oC.
1.0E-02
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
0 20 40 60 80 100 120
Cond
ucti
vity
σm
S/cm
Relative humidity %
Through-plane
In-plane
80 oC
31
(b) Conductivity at 120 oC.
Figure 3-6: Conductivity in the through-plane gas phase measurement.
1.0E-01
1.0E+00
1.0E+01
1.0E+02
1.0E+03
0 20 40 60 80 100 120Cond
ucti
vity
σm
S/cm
Relative humidity %
Through-plane
In-plane
120 oC
32
Table 3-3: Conductivity data in the through-plane gas phase measurement.
Relative humidity Conductivity at 80 oC mS cm-1
Conductivity at 120 oC mS cm-1
70 % 0.5019 3.606 60 % 0.4232 3.180 50 % 0.3270 2.973 40 % 0.2556 2.074 30 % 0.1880 1.541 25 % 0.1528 1.160 20 % 0.1362 0.9378 25 % 0.1461 1.003 30 % 0.1602 1.001 40 % 0.2001 1.086 50 % 0.1929 1.424 60 % 0.2345 1.952 70 % 0.2909 2.228 80 % 0.3186 3.346 90 % 0.3952 8.449 95 % 0.5387 18.04
100 % 0.8911 25.61
The results both at 80 oC and 120 oC are extremely low compared with the in-plane data,
although the slopes are almost the same, especially at 120 oC. However, the isotropic property of
Nafion has reported by Silva. et al [38]. I theory if the methods are equivalent, the measurement
results should be similar. Therefore, we can assume that the measurement procedure have some
problems. There are three clear challenges in the measurement. First, the electrical contact at the
interface between the electrodes and the membrane was not good. The silver paste between the
membrane and the electrodes might be lost somewhat because the paste was not absorbed in the
membrane. This occurred the interface resistance, and it made the membrane resistance increased.
Secondly, the membrane might not get reached equilibrium with the RH in the cell. For the in-
33
plane measurement, it is easier for the membrane to get equilibrated because the membrane
expose its surface. However, for the through-plane measurement, the surface of the membrane is
covered with the electrodes. The membrane needs more time to have water molecules inside it.
Finally, the extrapolating the membrane resistance from the Nyquist plot for through-plane
measurement could have larger inaccuracy than reading the ohmic slope for in-plane
measurement.
3.3 Through-plane measurement in liquid phase
3.3.1 Experimental setup
The setup consists of conductivity cells for liquid solutions and a measurement EIS
system. The measurement was done under atmospheric pressure and ambient temperature, 25
Co, with a frequency of 100 Hz - 100 kHz. The Nafion 117 sample was cut into circle (its
diameter, d, was about 15 mm) so that its diameter was larger than that of the conductivity cell.
Then it was immersed in sample solutions for 24 hours before the measurement. As for the
solutions, 2 mol L-1 (M) HCl was prepared. The conductivity cell was filled with the HCl
solution, and then titanium electrodes were put into the cell. The conductivity can be calculated
from its resistance with equation 2-9.
34
3.3.2 Results
The Nyquist plot of HCl solution with the membrane and HCl solution were shown in
Figure 3-7. The total resistance and the resistance of the solution was obtained by the curve
fitting with the same model as the one in the gas phase (Figure 3-5). The membrane resistance
was calculated with equation 2-8, and then the conductivity was calculated with equation 2-9.
The measured values of l and A were 0.0178 cm and 0.865 cm2, respectively.
Figure 3-7: Nyquist plot in liquid phase measurement.
As a result of the calculation, the ionic conductivity in the 2 M HCl aqueous solution was 84 mS
cm-1 in the through-plane direction. The conductivity of nafion 117 was reported to be 76 mS
cm-1 with the in-plane measurement in the liquid phase in 1 M H2SO4 aqueous solution [39,40].
These results shows that the direction of the conductivity measurement does not affect the ionic
conductivity significantly.
0.00E+00
2.00E+00
4.00E+00
6.00E+00
8.00E+00
0.00E+00 1.00E+00 2.00E+00 3.00E+00 4.00E+00
Im{Z
} Ω
Re {Z} Ω
2M HCl solution
HCl + membrane
35
3.3.4 PEM measurement summary
First, we obtained the ionic conductivities with the in-plane gas phase measurement. The
obtained results were very close to the reference data. Thus, we confirmed that our system
worked properly. However, the results with through-plane gas phase measurement were
significantly lower than the result of the first in-plane measurement even though the
measurement in the liquid phase shows very close conductivities to the reference data. We
assume that our measurement method has some challenges to be systematically addressed . In
order to conduct the ionic conductivity measurement for thin-film membranes, we need to make
sure if the measurement system works properly.
36
Chapter 4 Measurement for OH- conductive materials
4.1 Synthesis and characterizations of inorganic materials
4.1.1 Background on layered double hydroxides and hydroxyapatite
Two layered double hydroxides (LDHs) with Cl- anion in the interlayers were
investigated for chloride ion conductivity. LDHs consist of positively charged layers with two
different valences of metal cations and exchangeable hydrated anions such as Cl- in the
interlayers as shown in Figure 4-1. The LDHs are similar to the mineral hydrotalcite, [Mg6 Al2
(OH)16]2+CO32- • 4H2O [41], and hence they are also called hydrotalcite-like compounds. When
some of the divalent ions such as Mg2+ or Ca2+ are substituted by trivalent cations such as Al3+,
positive charge will be created on the layers. This positive charge on the layers needs to be
satisfied by anions with water molecules in the interlayers. LDHs can be represented by the
formula [42]:
[Mz+1-xM
3+ x (OH)2][Xn-
x/n•yH2O].
Most commonly, z = 2, and M2+ is a divalent cation, Ca2+, Mg2+, etc., and M3+ is a trivalent
cation, Al3+, Cr3+, etc, and a variety of anions, X such as Cl-, NO3-, CO3
2- etc. and y is the
number of moles of coordinated water molecules per formula weight.
37
One commercially available Mg-Al LDH with carbonate anions in the interlayers was
procured from Aluminum Company of America (Alcoa) located in Pittsburgh, PA. This material
has a Mg:Al ratio of 3:1.
Figure 4-1: Schematic representation of the layered double hydroxide structure [43].
38
Hydroxyapatite, Ca5(PO4)3OH
In the apatite structure, Ca5(PO4)3X, the X-- ions (X = OH, F, or C1) form one-
dimensional chains (shown in Figure 4-2) parallel to the c-axis [44,45]. The X-X distance is
typically 3.44Å for OH- and F- in apatite. Calcium can be substituted by many ions such as Sr,
Ni, Zn, Pb etc. while phosphate could also be replaced by carbonate. Thus, the apatite structure
is highly amenable for OH conductivity.
Figure 4-2: Hydroxyapatite structure projected on x, y plane [44].
39
4.1.2 Synthesis of sample materials
For the synthesis of 2Mg:Al and 2Ca:Al LDHs, magnesium chloride, calcium chloride,
aluminum chloride and sodium hydroxide were acquired from Aldrich chemical company.
Chloride containing Mg-Al LDH (JK-3)
The chloride form of LDH with magnesium and aluminum (Mg-Al LDH) was prepared
by coprecipitation of mixed Mg and Al chloride solution with a solution of NaOH. First, aqueous
solutions of magnesium chloride, MgCl2⋅6H2O (1.2M) and aluminum chloride, AlCl3⋅6H2O
(0.6M) were prepared to obtain a Mg/Al molar ratio of 2 using 50 ml of deionized water. An
aqueous solution of sodium hydroxide, NaOH (6M) was prepared separately in 50 ml of
deionized water. The mixed Mg-Al solution was then added drop by drop to the NaOH solution
while mixing at room temperature. This mixing led to precipitation of Mg-Al LDH phase. The
resulting precipitates were washed several times with deionized water and dried at 60 oC prior to
characterization by different techniques.
Chloride containing Ca-Al LDH (JK-4)
The chloride form of Ca-Al LDH (mineral name, hydrocalumite) was synthesized with
calcium and aluminum chlorides in a similar way to that of Mg-Al LDH. The chloride form of
Ca-Al LDH was made with calcium and aluminum chlorides after mixing them together in
sodium hydroxide solution. First, an aqueous solution of NaOH (2.5M) was prepared by
dissolving NaOH crystals in 50 ml of deionized water. Then, a 50 ml aqueous solution of Ca-Al
was made by dissolving CaCl2⋅2H2O (0.9M) and AlCl3⋅6H2O (0.45M) in a Ca/Al molar ratio of 2.
This 50 ml of mixed Ca-Al solution was slowly added drop by drop to 50 ml of sodium
40
hydroxide solution under vigorous stirring at room temperature. This mixing led to precipitation
of Ca-Al LDH phase. The resulting precipitates were washed several times with deionized water
and dried at 60 oC prior to characterization by different techniques.
Hydroxide containing Mg:Al LDH (YN 252)
The commercial LDH sample that was obtained from Alcoa was calcined at 500oC for 4h
in a tubular furnace to decompose carbonate to carbon dioxide and amorphize the LDH material.
This amorphized LDH was then equilibrated with NH4OH at room temperature to reconstitute
the LDH structure with hydroxide ions in the interlayers as charge balancing anions. Thus, the
LDH containing OH anions was synthesized by the above new approach. The resulting LDH
material was washed with deionized water and dried at 60 oC prior to characterization by
different techniques.
Synthesis of hydroxyapatite (YN 148)
For the synthesis of Ca-hydroxyapatite, (NH4)2HPO4 and Ca(NO3)2 containing15.7 %
H2O (Alfa Aesar) were used as precursors. The molar ratios of Ca: P for Ca- hydroxyapatite
synthesis was 5:3. In a typical synthesis, appropriate amounts of Ca(NO3)2 was dissolved in 20
ml of deionized water and then 40 ml of 0.375 M (NH4)2HPO4 solution was added to the Ca
solution. After mixing, the pH was adjusted to 10 using NH4OH. The mixture was treated in an
autoclave at 50oC for 4.5 h. The resulting precipitates were washed several times with deionized
water and dried at 60 oC.
41
4.1.3 Characterizations of sample materials
X-ray diffraction
Powder X-ray diffraction (XRD) patterns of synthesized LDHs and hydroxyapatite were
recorded with a Scintag diffractometer operated at 35 kV voltage and 30 mA current using CuKα
radiation. The X-ray diffraction patterns of each sample was recorded using a scanning rate of 5o
(2θ)/min and in the range of 5-45 degrees two theta. The X-ray peaks can be used to identify the
sample comparing it to a standard. XRD can tell us whether the samples are pure phases or
mixed with impurities. Powder XRD clearly showed that pure phases of all the LDHs and
hydroxyapatite were obtained.
Scanning Electron Microscopy
Some of the samples were characterized by scanning electron microscope (SEM) to
determine the particle size and morphology. Scanning electron microscopy was done using a
field emission scanning electron microscope (JSM-6700F, JEOL, Tokyo, Japan) on samples
coated with very thin carbon using a carbon coater. An accelerating voltage of 5 KV was used
for observation with the microscope. The materials were characterized with SEM and XRD. The
results are shown below in Figures 4-3 to 4-7.
42
Figure 4-3: Scanning electron micrograph showing the particle morphology and size of Chloride containing
Mg-Al LDH (JK-3).
43
Figure 4-4: Scanning electron micrographs showing the particle morphology and size of Chloride containing
Ca-Al LDH (JK-4) at two different magnifications.
44
Figure 4-5: X-ray diffraction patterns of (a) chloride containing Mg-Al LDH (JK-3) and
(b) chloride containing Ca-Al LDH (JK-4).
Figure 4-6: X-ray diffraction pattern of hydroxide containing Mg:Al LDH (YN 252).
45
Figure 4-7: X-ray diffraction pattern of hydroxyapatite (YN 148).
4.2 Conductivity measurement
4.2.1 Sample preparations and experimental setup
300 mg of inorganic particles were pressed into a 13 mm disk pellet with a pressure of
about 5 t cm-2. Then the pellet was annealed at 200 oC for 6 hours in a furnace heated and cooled
with the ramp rate of 1.5 oC min-1. The sample was then painted with Ag paste and dried for 24
hours in order to make sure that an electrical contact was achieved with electrodes of the
conductivity cells. The prepared samples were sandwiched and fixed with two platinum disk
electrodes of the through-plane conductivity cell in the same way of that of the measurements for
the PEMs. Then the cell was connected to the nitrogen cylinder as shown in Figure 3-1, and we
5 10 15 20 25 30 35 40Degrees 2θ(Cukα)
Inte
nsity 3.43Å
2.81Å
2.72Å
2.62Å2.26Å
3.08Å
3.87Å5.29Å
46
flowed the gas into the cell and controlled the pressure and the temperature. With regard to the
relative humidity inside the cell, we kept the RH constant until we obtained stable data.
4.2.2 Results
We obtained the Nyquist plots of each material as shown in Figure 4-8 and their bulk
resistances were estimated with their inflexion point or intersection point with real axis which
can be thought as the bulk resistance. Then the conductivities were calculated with the equation
[2-9]. The conductivities for each material are shown in Figure 4-9.
(a) Nyquist plot of Mg-Al LDH (JK-3)
0.00E+00
2.00E+00
4.00E+00
6.00E+00
8.00E+00
1.00E+01
1.20E+01
1.40E+01
1.60E+01
1.80E+01
2.00E+01
0.00E+00 1.00E+02 2.00E+02 3.00E+02 4.00E+02
Im{Z
} Ω
Re {Z} Ω
30 C
50 C
80 C
100 C
120 C
95 %RH JK-3
47
(b) Nyquist plot of Ca-Al LDH (JK-4)
(c) Nyquist plot of Mg:Al LDH (YN 252)
0.0E+00
1.0E+01
2.0E+01
3.0E+01
4.0E+01
0.0E+00 5.0E+01 1.0E+02
Im{Z
} Ω
Re {Z} Ω
30 C
50 C
80 C
100 C
120 C
95 %RH JK-4
0.0E+00
2.0E+01
4.0E+01
6.0E+01
8.0E+01
1.0E+02
0.0E+00 1.0E+02 2.0E+02
Im{Z
} Ω
Re {Z} Ω
30 C
50 C
80 C
100 C
120 C
95 %RH YN 252
48
(d) Nyquist plot of hydroxyapatite (YN 148)
Figure 4-8: Nyquist plots of inorganic materials at 95 %RH.
(a) OH- Conductivity at 120 oC.
0.0E+00
1.0E+02
2.0E+02
3.0E+02
4.0E+02
5.0E+02
0.0E+00 2.0E+02 4.0E+02 6.0E+02 8.0E+02
Im{Z
} Ω
Re {Z} Ω
30 C
50 C
80 C
120 C
95 %RH YN148
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
0 20 40 60 80 100
Con
duct
ivity
σS/
cm
RH %
JK3
JK4
YN252
YN148
120 oC
49
(b) OH- Conductivity at 95 %RH.
Figure 4-9: OH- Conductivities in inorganic materials.
The Nyquist plot shows the temperature and humidity dependence of the conductivity of
materials. Mg:Al LDH (YN 252) keeps the conductivity larger than 0.01 mS cm-1 in the wide
range of the RHs. Ca-Al LDH (JK-4) also keeps the conductivity in low RHs. It also shows high
conductivity in low temperature ranges.
Comparing these conductivities with those of reported materials in Table 1-3, our
materials do not have comparable conductivities in the high temperature and high RH ranges.
However, they have the possibility to be used in AFCs that are operated at low temperature
(below 100 oC). Also, what we measured in this experiment were the conductivities of particles
instead of synthesized membranes with inorganic particles. In order to make more precise
evaluations, we need to examine the correlations between the conductivity of particles and that
of synthesized membranes.
1.0E-01
1.0E+00
1.0E+01
0 20 40 60 80 100 120
Con
duct
ivity
σS/
cm
Temperature oC
JK3
JK4
YN252
YN148
95 %RH
50
Chapter 5 Conclusion
We have applied our through-plane gas phase conductivity cell for the organic PEM,
making use of the conductivity measurement for the inorganic materials. As a result, the
membrane in the gas phase showed significantly lower conductivities although the measurement
in the liquid phase of the same membrane showed a proper result. Thus, we conclude that our
measurement has some problems in the procesure and the measurement in the through-plane
direction should further be developed. At the same time, we have examined newly synthesized
inorganic OH- conductive materials. They were characterized with SEM and XRD and their OH-
ionic conductivities were measured. Although the materials showed relatively lower
conductivities than other materials recently reported, their usages in AFCs are worth studying
because they showed stable conductivities in the low temperature and humidity ranges.
51
BIBLIOGRAPHY
1. Barbir, F. PEM Fuel Cells: Theory and Practice; Academic Press, 2005.
2. Hydrogen and Our Energy Future, U.S. Department of Energy,
http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/hydrogenenergyfuture_web.pdf
3. Peter Atkins, J. d. P. Physical Chemistry; 8th ed.; Oxford University Press: New York, 2006.
4. Baschuk, J. J.; Li, X. H., Modelling of polymer electrolyte membrane fuel cells with variable
degrees of water flooding. Journal of Power Sources 2000, 86, (1-2), 181-196.
5. Gotz, M.; Wendt, H., Binary and ternary anode catalyst formulations including the elements
W, Sn and Mo for PEMFCs operated on methanol or reformate gas. Electrochimica Acta
1998, 43, (24), 3637-3644.
6. Su, A.; Chiu, Y. C.; Weng, F. B., The impact of flow field pattern on concentration and
performance in PEMFC. International Journal of Energy Research 2005, 29, (5), 409-425.
7. McLean, G. F.; Niet, T.; Prince-Richard, S.; Djilali, N., An assessment of alkaline fuel cell
technology. International Journal of Hydrogen Energy 2002, 27, (5), 507-526.
8. Sata, T.; Sata, T.; Yang, W. K., Studies on cation-exchange membranes having
permselectivity between cations in electrodialysis. Journal of Membrane Science 2002, 206,
(1-2), 31-60.
9. Casciola, M.; Alberti, G.; Sganappa, M.; Narducci, R., On the decay of Nafion proton
conductivity at high temperature and relative humidity. Journal of Power Sources 2006, 162,
(1), 141-145.
52
10. Halseid, R.; Vie, P. J. S.; Tunold, R., Influence of ammonium on conductivity and water
content of Nafion 117 membranes. Journal of the Electrochemical Society 2004, 151, (3),
A381-A388.
11. Sone, Y.; Ekdunge, P.; Simonsson, D., Proton conductivity of Nafion 117 as measured by a
four-electrode AC impedance method. Journal of the Electrochemical Society 1996, 143, (4),
1254-1259.
12. Kim, Y. S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y. T.; Harrison, W.; Zawodzinski,
T. A.; McGrath, J. E., Effect of acidification treatment and morphological stability of
sulfonated poly(arylene ether sulfone) copolymer proton-exchange membranes for fuel-cell
use above 100 degrees C. Journal of Polymer Science Part B-Polymer Physics 2003, 41, (22),
2816-2828.
13. Sumner, J. J.; Creager, S. E.; Ma, J. J.; DesMarteau, D. D., Proton conductivity in Nafion (R)
117 and in a novel bis[(perfluoroalkyl)sulfonyl]imide ionomer membrane. Journal of the
Electrochemical Society 1998, 145, (1), 107-110.
14. Kopitzke, R. W.; Linkous, C. A.; Anderson, H. R.; Nelson, G. L., Conductivity and water
uptake of aromatic-based proton exchange membrane electrolytes. Journal of the
Electrochemical Society 2000, 147, (5), 1677-1681.
15. Kim, Y. S.; Wang, F.; Hickner, M.; Zawodzinski, T. A.; McGrath, J. E., Fabrication and
characterization of heteropolyacid (H3PW12O40)/directly polymerized sulfonated
poly(arylene ether sulfone) copolymer composite membranes for higher temperature fuel cell
applications. Journal of Membrane Science 2003, 212, (1-2), 263-282.
53
16. Nakano, T.; Nagaoka, S.; Kawakami, H., Preparation of novel sulfonated block copolyimides
for proton conductivity membranes. Polymers for Advanced Technologies 2005, 16, (10),
753-757.
17. Li, S. W.; Liu, M. L., Synthesis and conductivity of proton-electrolyte membranes based on
hybrid inorganic-organic copolymers. Electrochimica Acta 2003, 48, (28), 4271-4276.
18. Aparicio, M.; Duran, A., Hybrid organic/inorganic sol-gel materials for proton conducting
membranes. Journal of Sol-Gel Science and Technology 2004, 31, (1-3), 103-107.
19. Thampan, T. M.; Jalani, N. H.; Choi, P.; Datta, R., Systematic approach to design higher
temperature composite PEMs. Journal of the Electrochemical Society 2005, 152, (2), A316-
A325.
20. Li, Z. Y.; Zhang, Q.; Liu, H. T.; He, P.; Xu, X. D.; Li, J. H., Organic-inorganic composites
based on room temperature ionic liquid and 12-phosphotungstic acid salt with high assistant
catalysis and proton conductivity. Journal of Power Sources 2006, 158, (1), 103-109.
21. Mecheri, B.; D'Epifanio, A.; Di Vona, M. L.; Traversa, E.; Licoccia, S.; Miyayama, M.,
Sulfonated polyether ether ketone-based composite membranes doped with a tungsten-based
inorganic proton conductor for fuel cell applications. Journal of the Electrochemical Society
2006, 153, (3), A463-A467.
22. Sahu, A. K.; Pitchumani, S.; Sridhar, P.; Shukla, A. K., Co-assembly of a Nafion-
Mesoporous Zirconium Phosphate Composite Membrane for PEM Fuel Cells. Fuel Cells
2009, 9, (2), 139-147.
54
23. Daiko, Y.; Sakakibara, S.; Sakamoto, H.; Katagiri, K.; Muto, H.; Sakai, M.; Matsuda, A.,
Formation of a High Conductivity Fuel Cell Electrolyte by Pressing Diphenylsiloxane-Based
Inorganic-Organic Hybrid Particles. Journal of the American Ceramic Society 2009, 92, (1),
S185-S188.
24. Huang, A. B.; Xia, C. Y.; Xiao, C. B.; Zhuang, L., Composite anion exchange membrane for
alkaline direct methanol fuel cell: Structural and electrochemical characterization. Journal of
Applied Polymer Science 2006, 100, (3), 2248-2251.
25. Stoica, D.; Ogier, L.; Akrour, L.; Alloin, F.; Fauvarque, J. F., Anionic membrane based on
polyepichlorhydrin matrix for alkaline fuel cell: Synthesis, physical and electrochemical
properties. Electrochimica Acta 2007, 53, (4), 1596-1603.
26. Luo, Y. T.; Guo, J. C.; Wang, C. S.; Chu, D., Quaternized poly(methyl methacrylate-co-butyl
acrylate-co-vinylbenzyl chloride) membrane for alkaline fuel cells. Journal of Power Sources
2010, 195, (12), 3765-3771.
27. Wu, Y. H.; Wu, C. M.; Varcoe, J. R.; Poynton, S. D.; Xu, T. W.; Fu, Y. X., Novel
silica/poly(2,6-dimethyl-1,4-phenylene oxide) hybrid anion-exchange membranes for
alkaline fuel cells: Effect of silica content and the single cell performance. Journal of Power
Sources 2010, 195, (10), 3069-3076.
28. Gilliam, R. J.; Graydon, J. W.; Kirk, D. W.; Thorpe, S. J., A review of specific conductivities
of potassium hydroxide solutions for various concentrations and temperatures. International
Journal of Hydrogen Energy 2007, 32, (3), 359-364.
55
29. Carl H. Hamann, A. H., Wolf Vielstich Electrochemistry; Second ed.; Wiley-vch: Weinheim,
2007.
30. Sata, T. Ion Exchange Membranes; The Royal Society of Chemistry, 2004.
31. Spiegel, C. PEM Fuel Cell Modeling and Simulation Using Matlab; Academic Press, 2008.
32. Mulder, W. H.; Sluyters, J. H.; Pajkossy, T.; Nyikos, L., TAFEL CURRENT AT FRACTAL
ELECTRODES - CONNECTION WITH ADMITTANCE SPECTRA. Journal of
Electroanalytical Chemistry 1990, 285 (1-2), 103-115.
33. Kim, C. H.; Pyun, S.; Kim, J. H., An investigation of the capacitance dispersion on the fractal
carbon electrode with edge and basal orientations. Electrochimica Acta 2003, 48 (23), 3455-
3463.
34. Schiller, C. A.; Strunz, W., The evaluation of experimental dielectric data of barrier coatings
by means of different models. Electrochimica Acta 2001, 46 (24-25), 3619-3625.
35. Jorcin, J. B.; Orazem, M. E.; Pebere, N.; Tribollet, B., CPE analysis by local electrochemical
impedance spectroscopy. Electrochimica Acta 2006, 51 (8-9), 1473-1479.
36. Oldham, K. B., The RC time "constant" at a disk electrode. Electrochemistry
Communications 2004, 6 (2), 210-214.
37. LLC, B. In-Plane Conductivity Testing To Characterize High Temperature Polymer
Electrolyte Membranes, 2006.
38. Silva, R. F.; De Francesco, A.; Pozio, A., Tangential and normal conductivities of
Nafion((R)) membranes used in polymer electrolyte fuel cells. Journal of Power Sources
2004, 134 (1), 18-26.
56
39. Yoshida, N.; Ishisaki, T.; Watakabe, A.; Yoshitake, M., Characterization of Flemion (R)
membranes for PEFC. Electrochimica Acta 1998, 43 (24), 3749-3754.
40. Slade, S.; Campbell, S. A.; Ralph, T. R.; Walsh, F. C., Ionic conductivity of an extruded
Nafion 1100 EW series of membranes. Journal of the Electrochemical Society 2002, 149
(12), A1556-A1564.
41. Reichle, W. T., Synthesis of Anionic Clay-Minerals (Mixed Metal-Hydroxides, Hydrotalcite).
Solid State Ionics 1986, 22, (1), 135-141.
42. Miyata, S.; Okada, A., Synthesis of Hydrotalcite-Like Compounds and Their
Physicochemical Properties - Systems Mg2+-Al3+-So42- and Mg2+-Al3+-Cro42-. Clays
and Clay Minerals 1977, 25, (1), 14-18.
43. Goh, K. H.; Lim, T. T.; Dong, Z., Application of layered double hydroxides for removal of
oxyanions: A review. Water Research 2008, 42, (6-7), 1343-1368.
44. Kay, M. I.; Young, R. A.; Posner, A. S., Crystal Structure of Hydroxyapatite. Nature 1964,
204, (496), 1050-&.
45. Eysel, W.; Roy, D. M., Hydrothermal Flux Growth of Hydroxyapatites by Temperature
Oscillation. Journal of Crystal Growth 1973, 20, (3), 245-250.