Probing the Exchange Site Environment of Nafion and SPEEK Membranes Using FTIR and
Computational Methods
by Erin Kingston
B.S. in Chemistry, Rochester Institute of Technology
A thesis submitted to
The Faculty of
The College of Science of
Northeastern University
in partial fulfillment of the requirements
for the Degree of Master of Science
December 1, 2014
Thesis directed by
Eugene Smotkin
Professor of Chemistry and Chemical Biology
ii
Dedication
“When everything goes to hell, the people who stand by you without flinching- they are your
family.” Jim Butcher
This is for my family: Marcia and Russell Kingston, Anna Kingston, William Shufro, Sir Oliver
Bolliver, Carrie Young, Tracey Baird, Lindsay Wolf, Katherine Weisenburg, Jennifer
Swartzenberg, and Kristianne Clifford.
iii
Acknowledgements
I am very grateful for the opportunity to pursue my graduate degree at Northeastern University.
My journey was not completed alone and I wish to acknowledge all of the help and support I
have received through the years.
I would like to sincerely thank my advisor Dr. Eugene Smotkin whose invaluable advice
and encouragement made all of this possible. He not only answered my innumerable
questions but spent many late nights brainstorming and guiding my research. His passion
for learning and knowledge is something I will take with me wherever I go.
I would like to thank my collaborator Dr. Nicholas Dimakis who spent many hours
running calculations for me.
I would like to thank the entire Smotkin research group past and present but especially
Ian Kendrick and Jonathan Doan who offered advice and encouragement when I needed
it most.
I would like to thank those who encouraged my love of science: Dr. Diem, Dr.
Rosenberg, Dr. Connelly, Patricia Ornt, Bette Bovenzi, Stephen Palmer, Bonnie Dery,
and the many others who have encouraged me along the way.
Last but certainly not least my family and friends.
iv
Abstract of Thesis
Nafion is not only used in fuel cells: It has applications in many fields including but not
limited to: batteries, chlor-alkali production, catalysts for organic synthesis, and in both sensors
and bio-sensors. This has facilitated thousands of studies on Nafion, a Perfluorosulfonated
membrane. Despite the many studies of Nafion there is still confusion over the assignment of
bands in the infrared spectrum. State-of-hydration studies performed in conjunction with
Density Functional Theory (DFT) analysis has assisted in clearing up this confusion.
Ion exchange studies in Nafion reveal that the exchange site-metal ion interaction is
governed by the enthalpies of hydration and the charge to volume ratio. Metal ions with large
enthalpies of hydration (i.e. above 2000 kJ/mol) do not bind to the exchange site at high levels of
hydration. In fact membranes exchanged with metal ions that have large enthalpies of hydration
were unable to be dehydrated enough to see binding without destruction of the membrane. Metal
ions with enthalpies of hydration in the transition region (i.e. between 1000-2000 kJ/mol) bind in
a state-of-hydration dependent manner. These ions do not bind at high states of hydration but do
strongly bind when λ is at low levels. Metal ions with small enthalpies of hydration (i.e. below
600 kJ/mol) are bound to the exchange site at all states-of-hydration. Ions in the low region and
Ca2+
ions in the transitional region bind to the exchange site in such a way as to retain local C3V
symmetry. Crystal Orbital Overlap Population (COOP) calculations show that lithium and
sodium ions bind to the exchange site with covalent bonding between the metal ion and the
sulfur as well as electrostatic binding between the metal ion and the exchange site oxygen atoms.
All other metal ions in the study were restricted to electrostatic interactions between the metal
ion and the exchange site oxygen atoms.
v
Sulfonated Poly Ether Ether Ketone (SPEEK) is being studied as a possible replacement
for Nafion in fuel cells. SPEEK is more stable than Nafion, able to operate at higher
temperatures and is less expensive to manufacture. The morphology of SPEEK is very different
from Nafion and has not been studied as extensively. State-of-hydration studies together with
DFT assist in assigning transmission infrared bands of SPEEK. SPEEK is a weaker acid than
Nafion and as such may behave differently when exchanged with metal ions. Future work will
include studying how the SPEEK exchange sites interact with metal ions.
vi
Table of Contents
Dedication ii
Acknowledgements iii
Abstract iv
Table of Contents v
List of Figures vi
List of Tables vii
List of Abbreviations and Symbols viii
Glossary of Terms ix
Chapter 1.
1.1 Motivation for work 1
1.2 Membrane Electrolytes 2
1.3 Infrared Spectroscopy 6
1.4 Density Functional Theory 9
1.5 Crystal Orbital Overlap Populations 10
Chapter 2. Hydration / Dehydration studies of Nafion and SPEEK
2.1 Introduction 12
2.2 Experimental Methods 14
vii
2.3 Results and Discussion
2.3.1 Rehydration of Nafion 15
2.3.2 Time Dependent Dehydration of Nafion 18
2.3.3 Time Dependent Dehydration of SPEEK 22
2.4 Conclusions 24
Chapter 3. Ion Exchange Studies of Nafion 212
3.1 Introduction 25
3.2 Experimental Methods 29
3.3 Results and Discussion
3.3.1 ATR vs. Transmission Spectroscopy 31
3.3.2 Transmission FTIR of Li+ Exchanged Nafion 33
3.3.3 Bonding Scheme 35
3.3.4 FTIR of K+, Na
+, Li
+, H
+, Ca
2+, Ni
2+ & Al
3+ Nafion. 37
3.4 Conclusions 41
Chapter 4. Future Work
4.1 Ion Exchange Studies of SPEEK 43
4.2 Experimental Methods 43
viii
4.3 Preliminary Results
4.3.1 H+ and K
+ exchanged SPEEK 44
4.3.2 Li+ and Na
+ exchanged SPEEK 47
4.3.3 Al3+
exchanged SPEEK 48
4.4 Early Conclusions and Hypotheses 50
References 51
ix
List of Figures
1.1 Repeat unit of Nafion. 3
1.2 Repeat unit of SPEEK. 4
1.3 Kreuer’s proposed morphology of Nafion versus SPEEK. 5
2.1 Exchange site local symmetry model. 12
2.2 Time dependent transmission spectra of rehydrating Nafion. 16
2.3 Peak-intensity time-dependence during rehydration of Nafion. 17
2.4 Transmission spectra of Nafion. 18
2.5 Peak intensity time-dependence of Nafion during dehydration. 20
2.6 Normal mode animation snapshots of Nafion exchange site group modes. 21
2.7 Transmission spectra of fully sulfonated SPEEK. 22
2.8 Normal mode animation snapshots of SPEEK exchange site group modes. 23
2.9 Peak-intensity time-dependence during dehydration of SPEEK. 23
3.1 Chemical repeat units of Nafion with exchange site local symmetry dependent 26
group modes.
3.2 Time dependent transmission spectra of Nafion overlaid with DFT calculated 27
stick spectra.
3.3 Transmission spectra of hydrated Nafion immersed in 1M LiCl solution, 31
time dependent.
x
3.4 IR spectra of hydrated lithiated Nafion (ATR vs. Transmission). 32
3.5 ATR spectra of hydrated Nafion; protonated vs. lithiated. 33
3.6 Left: Nafion-Li DFT calculated spectrum superimposed on transmission 34
spectrum of dehydrated lithiated Nafion 212. Right: DFT normal mode animation
snapshots of lithiated Nafion.
3.7 COOP values calculated by Jaguar. 36
3.8 Transmission spectra of fully hydrated ion-exchanged Nafion with enthalpies 37
of hydration (-∆Hhyd).
3.9 Transmission spectra of Ca2+
and H+ exchanged Nafion. 38
3.10 Transmission spectra of K+ and Na
+ exchanged Nafion. 39
3.11 Transmission spectra of Ni2+
and Al3+
exchanged Nafion. 40
3.12 Models depicting the exchange site interactions. 41
4.1 Transmission spectra of commercial SPEEK. 44
4.2 Transmission spectra of CSPEK potassium exchanged. 46
4.3 Transmission spectra of CSPEEK lithium and sodium exchanged. 47
4.4 Transmission spectra of CSPEEK aluminum exchanged. 49
4.5 Schematic of an Al3+ ion complexed to SPEEK. 50
xi
List of Tables
2.1 Measured and calculated vibrational modes of Nafion 16
2.2 Measured and calculated vibrational modes of SPEEK 24
3.1 DFT assignments and measured bands of Nafion-H+ and Nafion-Li
+ 34
3.2 COOP integrals calculated by AOMIX 35
4.1 DFT band assignments of Hydrated SPEEK 45
4.2 DFT band assignments of Hydrated K+ exchanged SPEEK 47
xii
List of Abbreviations and Symbols
AO Atomic Orbitals
ATR Attenuated Total Reflectance
COOP Crystal Orbital Overlap Populations
DFT Density Functional Theory
DOS Density of States
FTIR Fourier Transform Infrared Spectroscopy
HOMO Highest Occupied Molecular Orbital
IP in-plane
LDA Local Density Approximation
NCDC National Climactic Data Center
NOAA National Oceanographic & Atmospheric Administration
OOP out of plane
PEFC Polymer Electrolyte Fuel Cell
PEM Proton Exchange Membrane
PBI Polybenzimidazoles
SPI Sulfonated Polyimide
xiii
SPEEK Sulfonated Poly Ether Ether Ketone
νs symmetric stretching
νas asymmetric stretching
δ bending (vibrational mode)
ϕ ring mode deformation
λ number of water molecules per exchange site
q charge
v volume
-∆Hhyd enthalpy of hydration
1
Chapter 1. Introduction to Fuel Cells
1.1 Motivation for work
Worldwide demand for energy is ever increasing. Our current economy is heavily dependent
on the combustion of fossil fuels supplemented with small contributions from renewable
resources such as solar, wind, and hydro-electric. According to the U.S. Energy Information
Administration production of domestic crude oil is expected to peak in 2016 at 7.5 million
barrels per day, will remain steady at this rate until 2020 and then begin to decline. Commercial
vehicle use is expected to continue to increase steadily through 2020 and exponentially increase
from 2020 to 2040.4 As a result of the increasing needs and the eventual decrease in oil
production it is imperative that we have other fuel sources ready and available.
Emerging fuel cell science and technology is a critical component to the creation of
economies based on renewable energy. The many benefits of fuel cells make these devices
preferable over other sources of renewable energy (wind, solar, and water). These benefits
include: low pollution outputs, high energy conversion efficiency, easy portability and low
maintenance requirements.5 A basic fuel cell consists of an anode, electrolyte, cathode, and fuel
sources. The fuel is pumped into the anode where it is oxidized and split into positive ions and
electrons. The positive ions then travel across the proton exchange membrane (PEM) to the
cathode and the electrons are forced along an external circuit which performs the work. At the
cathode an oxidant is pumped in (usually oxygen) which recombines with the protons and
electrons to form water.6 There are several types of fuel cells utilizing different membrane types,
catalysts, operating temperature ranges and fuel types. The most common types are the Polymer
Electrolyte fuel cell (PEFC), Direct Methanol and alkaline fuel cells.7
2
Polymer electrolyte fuel cells have several advantages including operation at lower
temperatures (typically in the range of 60 to 80° C) which allows the fuel cell to start up rapidly.
Also PEFCs have no liquids present other than water (therefore no corrosion issues) are capable
of high current densities and do not require any exotic material.6 Some of the downsides to
PEFCs include a challenging thermal management problem particularly at high current densities,
the poisoning of the cell by very small amounts of contaminants including carbon monoxide and
ammonia, and finally the membranes require a delicate balance of water to maintain optimum
operation.7
1.2 Membrane electrolytes
The largest determining factor in the output of the cell is the choice of polymer electrolyte
membrane.8 The polymer electrolyte membrane must have the ability to conduct protons across
while serving as a barrier to electrons and preventing the mixing of fuel and air at the electrodes.9
Thus fuel cell performance is dominated by the ability of the membrane to transport protons and
therefore the microstructural properties of the membrane.
Ionomers are polymers with pendant ionic groups (i.e. sulfonate, phosphates, ammonium,
etc).10
Ionomers come in many types (simple ionic, ampholytes, betaines, ionenes and
telechelic)11
however this work focused solely on simple ionic ionomers. Ionomers have
applications in many other fields aside from fuel cells including water purification, analytical
chemistry, batteries, and sensors.12-14
3
Nafion is currently the most studied of the ionomers available; as of November 6th 2014 a
search using the key words “Nafion Membrane” gave over 15,800 results on the web of
science.15
Despite the many publications there is still much to learn about this versatile
membrane.
Nafion (Fig.1), a sulfonated tetrafluoroethylene polymer, is a commercially available
membrane with many advantages including permeability to water and a high cationic activity.16
In the sulfonic acid form Nafion is a superacid with a PKa of around -6 which contributes to the
high conductivity.12
It is generally accepted that Nafion has three phases: a hydrophobic, a
hydrophilic and a fuzzy interphase. Many models have been proposed to explain the
morphology of Nafion including Yeager and Steck three-phase model,17
the core-shell model by
Fujimura et al.18
, the rod like model of Rubatat19
and others. Work by Falk showed that the
majority of water in hydrated Nafion is located in clusters and is not well dispersed in the
fluorocarbon medium. Falk soaked sheets of Teflon (i.e. the backbone of Nafion) in water and
COC-B
COC-A
Figure 1.1 Repeat Unit of Nafion
Hydrophilic exchange site
4
was unable to detect any water spectroscopically.20
No matter which model you subscribe to it is
clear that the exchange site sulfonate group is crucial to the transport of the protons.21-22
Conductivity of these membranes is directly related to both the number of sulfonate
exchange sites and the amount of water per exchange site, λ, where λ is the number of water
molecules per exchange site. Once the sulfonate group is converted to sulfonic acid the proton
can travel through the membrane utilizing the Grotthuss hopping mechanism.16, 23-24
Previous
studies by Webber et al. demonstrated that the proton leaves the exchange site at λ = 4.25
Studies
by Hwang et al. confirm these finding as they report a large increase in the conductivity at λ ~ 4
which is consistent with the proton leaving the exchange site.26
Nafion has some drawbacks it is expensive to manufacture (a 0.3 x 0.3 meter sheet of
Nafion from Ion power Inc. is $331.00 as of November 20, 2014), requires a platinum catalyst
(also driving up costs), degrades via an unzipping mechanism27
and is difficult to dispose of.
These drawbacks have led us to search for an alternative to Nafion. SPEEK (sulfonated poly
ether ether ketone) is one of many membranes being studied as a possible replacement for
Nafion because it is much cheaper to mass produce (a 0.20 x 0.30 m sheet of SPEEK from
Fumatech Inc is $87.77 as of November 20, 2014) and is more stable. SPEEK has an aromatic
backbone with the exchange site sulfonate groups attached directly to the backbone (Fig. 2).
5
Figure 1.2 Repeat Unit of SPEEK.
The lack of side chains changes the morphology of SPEEK as compared to Nafion.
Figure 1.3 shows the proposed morphology of Nafion as it compares to SPEEK. SPEEK has
narrower channels, dead end pockets and widely spaced exchange groups. Kreuer’s work
indicates that SPEEK has a larger hydrophobic-hydrophilic interface which results in a larger
Figure 1.3 Kreuer’s Proposed morphology of hydrated Nafion (left) and hydrated SPEEK
(right).2 (Reproduced with permission from: Kreuer, K. D., On the development of proton
conducting polymer membranes for hydrogen and methanol fuel cells. Journal of Membrane
Science 2001, 185 (1), 29-39.
6
separation of the exchange sites. Further the dielectric constant of hydrated SPEEK is only ~20
as compared to ~64 in hydrated Nafion.28
The drawbacks to SPEEK include decreased conductivity (as compared to Nafion with
the same degree of sulfonation)29
and increased sensitivity to hydration.5, 14, 30
The decreased
conductivity can be offset by increased sulfonation,31-32
however this can lead to excessive water
uptake and polymer swelling,33-34
in fact 100% sulfonated SPEEK can dissolve in water. 5
This thesis focuses on studying the exchange site groups using FT-IR time-dependent
hydration-dehydration studies, ion-exchange techniques, density functional theory calculations,
and crystal orbital overlap populations.
1.3 Infrared Spectroscopy
Infrared (IR) spectroscopy uses absorption of radiation from 10 to 12800 cm-1
and the
resulting excited states to gain chemical information from molecules. Photons with a specific
energy are shined on a sample and if the energy of the photon is an exact match to the difference
in molecular energy the photon may be absorbed.35
Infrared spectroscopy has been widely used
to study ionomers. Nafion has strong vibrational bands associated with the backbone, ether links
and the sulfonate functional groups. The functional group when in the acidic form SO3- has a
local symmetry of C3V and as the acidic form converts to the sulfonate form, SO3H the C3V
symmetry is lost and the exchange site becomes C1 (i.e. no symmetry). SPEEK contains the
same sulfonate/sulfonic acid functional groups and thus the same local symmetry. SPEEK is
further similar to Nafion in that it contains two similar ether groups. This loss of symmetry from
C3V to C1 causes bands in the IR arising from this symmetry to appear and disappear depending
on state-of-hydration.
7
The number and kinds of vibrations in a molecule depend on the number of atoms and the
degrees of freedom. To locate a point it is necessary to have 3 coordinates. Each coordinate is
considered to be one degree of freedom.36
Thus for a molecule with N atoms we have 3N
degrees of freedom. Three degrees of freedom are lost due to the motion of the entire molecule
through space (translational motion) and another 3 degrees are lost to the rotation of the entire
molecule. Thus we can calculate the total number of vibrations in a non-linear molecule as 3N-
6.35
If we consider a large molecule, such as a polymer, that has thousands of atoms we might
think that the spectrum would have thousands of bands. Fortunately polymers such as Nafion
and SPEEK consist of repeat units that contain less than 100 atoms. To further simplify each
individual repeat unit has dimensions on the order of 10-9
m and the wavelength of IR light is on
the order of 2 x 10-6
m.10, 36
Thus each wavelength encompasses many repeat units and only a
fraction of the normal modes of vibrations are those in which all repeat units vibrate in phase.
When the vibrations are similar but not in phase the waves cancel out along the length of the
chain and are IR inactive.36
When dealing with polymers one must consider both the chemical
and physical repeat units. Chemical repeat units are typically used in molecular modeling
calculations and are the unit which when connected end to end comprises the polymer. The
physical repeat unit varies with manufacturing process and environment.36
Since the polymer
has many physical repeat units we see broadening of the IR peaks due to group vibrations. This
broadening arises from similar groups of atoms in slightly different environments with slightly
different frequencies of vibration. Thus we expect in an infrared spectrum to see 3n or less
peaks, where n is the number of atoms in the chemical repeat unit instead of 3N where N is the
number of atoms in the whole molecule.36
8
A normal mode is one in which all atoms move with the same frequency and phase.37
In
infrared spectroscopy we make use of group modes. Group modes are the modes which
dominate any given band. For example the 983 cm-1
band in Nafion is dominated by νas of the
ether group furthest from the backbone and νs of the ether group closest to the backbone.25
The
backbone stretching as well as the exchange group stretches contribute very little intensity to this
band. Thus we can assign this band to COC-A νas and COC-B νs.
Infrared spectroscopy comes in many forms. The two types of IR used in these studies
were transmission and attenuated total reflectance (ATR). The principles of ATR are based on
reflection. When a beam of radiation passes from a denser to a less dense substance reflection
occurs. The amount of the incident beam that is reflected is dependent on the angle of incidence
(i.e. as the angle becomes larger the amount of reflection increases until a critical angle is
reached above which all radiation is reflected).35
During the reflection the beam penetrates a
small distance into the substance before it is reflected. How far the beam penetrates is dependent
on the wavelength of the beam, the angle of the beam with respect to the interface, and the index
of refraction of both materials.35
The penetrating radiation is called the evanescent wave. At
wavelengths where the less dense medium absorbs the evanescent wave the beam becomes
attenuated and thus we have attenuated total reflectance or ATR. ATR spectra are similar to
transmission spectra however ATR is a surface technique and the bands acquired are usually of
much smaller intensity. Unlike transmission spectra, ATR spectra are independent of sample
thickness because the beam only penetrates a few micrometers into the sample. How far the
beam penetrates, dp, depends on two factors: (1) the refractive indices of the crystal and the
sample, and (2) the angle at which the radiation strikes.35
Penetration depth can be calculated
using equation 1.1
9
𝑑𝑝 =𝜆𝑐
2𝜋[𝑠𝑖𝑛2𝜃− (𝑛𝑠𝑛𝑐
)2
]
12⁄ Eqn. 1.1
λc is the wavelength of the radiation in the crystal, θ is the angle of incidence, ns is the refractive
index of the sample and nc is the refractive index of the crystal. Unlike ATR, transmission
spectroscopy provides information about the full expanse of the sample. This means that the
sample must be thin enough to allow the radiation to penetrate all the way through and reach the
detector. Nafion 117 used in previous studies, with a thickness of 183 μm, precluded the use of
transmission spectroscopy and thus necessitated ATR as the method of choice. The advent of
thinner membranes such as Nafion 112 with a thickness of approximately 50 μm allowed us to
finally use transmission spectroscopy to probe the full width of Nafion membranes.
1.4 Using DFT to calculate Vibrational Frequencies:
Density Functional Theory (DFT) is a molecular modeling tool that allows us to calculate
the vibrational frequencies of molecules. This tool is most useful when used in conjunction with
experimental methods. DFT uses the harmonic approximation, bond lengths that minimize the
energy of the molecule and the second derivative of the energy with respect to the atoms position
to calculate vibrational frequencies. This is relatively simple for a small molecule like water but
becomes a much more complex problem when dealing with a polymer like Nafion. These
complexities result in the DFT calculations often being off by as much as 2%.3 In a previous
study we showed that the calculated band at 786 cm-1
in dehydrated Nafion corresponded to the
experimental 910 cm-1
band a difference of 124 cm-1
.3 This difference comes from the harmonic
treatment of the vibrations, the approximations necessary to solve the equations with the level of
computation available today and the choice of modelling unit.23, 38
As the molecules we study
become more complex the computational resources necessary to complete the calculations
10
becomes exponentially larger. Therefore a compromise must be made between numerical
accuracy of our calculations and the increased computational time. The two most important
considerations in DFT calculations are the choice of exchange correlation functional and the
choice of repeat unit. There are several levels of functionals available from the simplest local
density approximation (LDA) to the most complex Hyper-GGA (generalized gradient
approximation).38
Our calculations used a Hyper-GGA functional known as the X3LYP
functional. This is an extension of the B3LYP functional but it includes more accurate heats of
formation.1 When choosing a repeat unit it is important to realize that polymers do not have
fixed molar masses. Polymers contain a distribution of molar masses and chain lengths that
depends on the manufacturing conditions and polymerization techniques.10
This distribution
creates geometric irregularities and macro formations that cannot yet be computed using the
computational resources available today.1 An ideal model would include a hydrophobic phase, a
hydrophilic phase, a fuzzy interphase and irregular chains lengths. Instead all Nafion DFT
calculations were carried out using a 55 atom repeat unit (56 for protonated and ion exchanged
models) while SPEEK calculations used an 86 atom repeat unit (88 for protonated and ion
exchanged models). Our SPEEK repeat unit used two monomer units plus an additional ring as
opposed to Nafion where only one monomer unit was used. The use of two monomer units gives
a slightly more realistic calculated spectrum: Instead of one calculated band for each vibrational
mode there are two – four calculated bands per mode of vibration mimicking the broader bands
in an IR spectrum).
1.5 Crystal Orbital Overlap Populations Theory:
Crystal Orbital Overlap Populations (COOP) is a modeling method that enables us to
predict covalent bonding in molecules. COOP calculations provide no information about ionic
11
interactions or long range interactions.39
The COOP is a combination of Density of State (DOS)
plots that are combined with overlap populations. A Density of State (DOS) diagram is a
histogram of the number of energy levels as a function of the energy (peaks occur at energies
with high densities of energy levels).40
The energy bands in a DOS diagram give information
about the number of crystal orbitals at a given level of energy.41
The integral of the DOS with
respect to the energy results in a number of energy levels. Thus the integral of the DOS up to the
Fermi level equals the number of occupied crystal orbitals and when this integral is doubled it
results in the number of electrons giving information on the energetic distribution of the
electrons.40
The summation of the integral over all electrons present is the overlap population
for the atomic orbital (AO) pair and provides information on the bonding and anti-bonding
interactions between these two AOs.40
12
Chapter 2. Theoretical & Experimental IR Spectra of Hydrated/Dehydrated Nafion and
SPEEK
2.1 Introduction:
Every molecule has a set of symmetry operations that describes the molecule’s overall
symmetry. This set of symmetry operations is called the point group of the molecule.37
Group
theory is the mathematical treatment of the properties of groups and it can be used to determine
the vibrations (and hence the IR spectrum), molecular orbitals and other properties of molecules.
42 All molecules can be described in terms of symmetry even if only to say a molecule has no
symmetry. The full molecule Nafion has no symmetry (i.e. C1) however if we look at the local
symmetry of the exchange site we see that it is C3V in the sulfonate form and C1 in the sulfonic
acid form (Fig 2.1).
Despite the vast library of research available on Nafion there is still much debate over
both the morphology of the membrane and the assignment of certain bands in the IR spectra of
Nafion. In particular the bands at 969 cm-1
(the C3V,LF) and 1083 cm-1
(C3V,HF)have been the
source of much debate.43
In much of the literature these bands have been assigned to the ether
group nearest the exchange site (969 cm-1
), hereafter COC-A, and the sulfonate group (1083 cm-
C3v
C3v
C1
Figure 2.1 Exchange site local symmetry model. Left: Dehydrated protonated Nafion,
Middle: Hydrated Nafion. Right: Nafion-Li
13
1) despite the fact that both bands are sensitive to state of hydration and ion exchange.
44-49 This
has been explained by suggesting that the COC-A is solvated in the fully hydrated state. Webber
et al. used ATR and transmission spectroscopy in conjunction with DFT to show that the C3V,LF
arises in fact from the symmetric stretch of the sulfonate group coupled with the COC-A
stretch.25
The pre-treatment history of the membrane can affect the morphology of the
membrane.32, 50-52
Therefore all membranes in these studies were treated identically (i.e. all
Nafion and SPEEK membranes were pre-cleaned and stored using the methodology outlined in
the experimental section).
Rehydration studies confirm that Nafion rehydrates rapidly in atmosphere. Our
rehydration study showed that within 1 minute the C1 bands substantially diminish and the C3V
bands grow. Thus the set-up of a high vacuum line used in conjunction with the vacuum FTIR
spectrometer enabled the conduction of a time-dependent state-of-hydration study of samples
that were never exposed to ambient conditions. We showed the C1 and C3V group modes coexist
in the spectra at all but extreme states of hydration (i.e fully hydrated or fully dehydrated).
The morphology of SPEEK is still an enigma which led us to apply the same methods to
elucidate the assignments of bands in the IR that are associated with the exchange site. Since
SPEEK has an identical exchange site to Nafion it also has an exchange site local symmetry of
C3V or C1 depending on the state-of-hydration. The time dependent state-of-hydration studies of
SPEEK revealed an interesting deviation from the Nafion study. SPEEK C3V and C1 modes
coexist at all states-of-hydration.
14
2.2 Experimental Methods:
A. Membrane Preparation: Nafion 112 (Ion Power Inc., New Castle, DE) membranes were
cleaned by boiling in 7.9 M HNO3 (20 min) followed by rinsing in Nanopure™ water
(Nanopure™ water hereafter referred to as water) 6 times and finally boiling in water (1 h).
SPEEK membranes were cleaned by soaking in 5% by volume H2SO4 at 80°C for 12 hours
followed by rinsing in water 6 times and finally soaking in water at 80°C (1 h).
B. Membrane Transmission FTIR Spectroscopy: Nafion and SPEEK membranes were
stored in water (after cleaning). Membranes were removed from water, pat-dried with
ChemWipes™, and placed in the Vertex 70 IR spectrometer to obtain the spectra of the fully
hydrated membrane (ambient conditions). The sample is then transferred to the Vertex 80V
vacuum (1.00 hPa) spectrometer (Bruker, Billerica, MA) for time dependent spectra during
membrane dehydration. The Vertex 80V is located in a specially built glove box allowing the
membrane to be removed from the sample chamber, placed into a sealed vessel, and attached to
the vacuum line without being exposed to the atmosphere. Spectra were taken every hour until a
steady dehydrated state was reached (24 - 60 hrs). Dehydration was continued on a vacuum line
(100°C, 5 days for Nafion and 60°C, 2 days for SPEEK) equipped with a Welch 1402 DuoSeal
vacuum pump, a glass oil diffusion pump (Ace Glass, Vineland, NJ) and a liquid N2 trap. After
dehydration, the sample bulb was transferred to the FTIR glove box, and the membrane
positioned back into the Vertex 80 sample chamber. A final spectrum of the exhaustively
dehydrated membranes was taken. All spectra were signal averaged (50 scans, 4 cm-1
resolution)
15
using a DLaTGS detector. Data processing was completed using Bruker™ OPUS 6.5™
software.
C. Molecular Modeling Calculations: Unrestricted DFT53-54
with the X3LYP[53] functional
was used for geometry optimizations and calculations of the normal mode frequencies of
protonated and deprotonated Nafion. The X3LYP is an extension to the B3LYP55
functional
providing more accurate heats of formation. The 55 atom deprotonated repeat unit consists of
one Nafion monomer. The repeat unit backbone was capped with CH3 groups to prevent
computational interference from the side chain CF3 group. Jaguar 8.0 (Schrodinger Inc.,
Portland, OR) was used with the all-electron 6-311G**++ Pople triple- basis set (“**” and
“++” denote polarization56
and diffuse57
basis set functions, respectively). Output files were
converted to vibrational mode animations using the Maestro graphical user interface
(Schrodinger Inc.). Calculations were carried out on the high performance computing cluster at
the University of Texas, Pan American with 72 nodes of Dual 2.67Ghz processors, each node
with 48GB RAM and 250GB Disk. Crystal Orbital Overlap Populations (COOP)58
were
calculated by Jaguar using the AOMIX program. AOMIX processes output files from a variety
of quantum mechanical packages and generates densities-of-states and COOP spectra in terms of
constituent chemical fragments.58-59
DFT calculated normal mode peaks are denoted by
superscript (*) e.g. 983* cm-1
.
16
2.3 Results and Discussion
2.3.1 Rehydration of Nafion.
Hydrated Dehydrated
Figure 2.2 Transmission FTIR Rehydration study: Fully Dehydrated Nafion (left); Nafion
exposed to air 1 minute (center); Nafion exposed to air 10 minutes (right).
Measured
(cm-1
)
DFT
calculated
(cm-1
)
Band Assignment Exchange Site
Local symmetry
Hydrated 969 983* νs SO3-; νas COC-A C3V,LF
1060 1059* νas COC-A; νs SO3- C3V,HF
1708 N/A δ H2O
2550 – 3750 N/A ν OH
Dehydrated 920 786* νs SO3H; νs COC-A C1,LF
1414 1405* νas SO3H; νas COC-A C1,HF
Table 2.1 Measured and calculated (*) vibrational modes of Nafion. Measured modes fully
hydrated (collected on Vertex 70); dehydrated after 5 days at 100°C on vacuum line.
17
A Nafion 212 membrane was dehydrated on the vacuum line for 5 days under heat (100°
C). A spectrum was collected on the Vertex 80 (under vacuum) to confirm dehydration of the
membrane. The membrane was then placed into the Vertex 70 with the sample compartment
open to the environment and spectra were collected every 60 seconds until equilibrium was
reached (22 minutes). After 22 minutes spectra were collected every 10 minutes for a total of 20
hours. Finally the membrane was soaked in water for 5 minutes and a final spectrum collected.
Membranes were rehydrated on 1/28/2013 at 10 am, temperature outside was recorded at -
1.05°C, with a dew point of -18°C and a relative humidity of 27.25 (calculated using information
obtained from National Climactic Data Center (NCDC) and National Oceanographic and
Atmospheric Administration (NOAA)).
The C1 group mode (dehydrated Nafion) intensities immediately diminish when exposed
to an ambient relative humidity of 27% (Fig. 2.2). Within 1 min, the C1,LF and C1,HF bands lose
over 33% and 19.9% of the intensities respectively (Fig. 2.3), concurrent with appearance of the
923 cm-1
1414 cm
-1
1061 cm-1
Figure 2.3 Transmission IR spectroscopy peak-intensity time-dependence during Nafion
rehydration in Vertex 70.
18
bulk water peak at 1708 cm-1
(figure 2.2, middle). The 1708 cm-1
band has been assigned to
bending modes of water molecules in close proximity to dissociated protons (i.e. H3O+ and
H5O2+).
47, 60-61 DFT calculations suggest site values of at least 4 when the 1708 cm
-1 band first
appears (where λ = # of H2O molecules per exchange site).25
Within 10 min, the C1 bands vanish
while the C3V,HF band reaches a steady state under ambient conditions concurrent with the
emergence of a low wavenumber side of the 1708 cm-1
band (Fig. 3.3, right). The low
wavenumber side has been assigned to a second level of hydration including the outer solvation
sphere and the formation of continuous water channels in the membrane.12, 60
DFT calculations
suggest that the per site λ values are at least 10 when the low wavenumber side emerges.3, 25
The
almost instantaneous rehydration of Nafion previously dehydrated on a high vacuum line, at
relative humidities as low as 30%, renders state-of-hydration studies under ambient conditions
impossible. Thus a sample chain-of-transfer from high vacuum to an FTIR vacuum sample
compartment with no exposure to the environment was vital to our ability to study the local
exchange site environment as a function of state-of-hydration.
19
2.3.2 Time Dependent Dehydration of Nafion
The spectrum of hydrated Nafion has two bands arising from water vibrations. The first
very broad band falls between 2550 cm-1
and 3750 cm-1
has been assigned as O-H stretch.12, 20, 62
While this band is dominated by stretching modes of the water molecules as λ falls below 4 the
exchange site sulfonate groups become sulfonic acid and thus contribute O-H stretch to the band.
The contribution from the sulfonic acid O-H ensures that this band cannot be used to definitively
confirm full dehydration of the membrane. The second water band at 1641 cm-1
and its
associated shoulder at 1726 cm-1
have been assigned as bending modes of water.46, 62
This band
disappears completely in the dehydrated spectrum of Nafion (Fig 2.4, right). Dehydration of the
membranes was only considered complete when 3 criteria were met (1) the 1414 cm-1
and 920
cm-1
bands stopped growing (2) bands stopped shifting and (3) the 1641-1726 cm-1
band was
completely gone. Full dehydration of protonated Nafion 112 was determined to require at least 3
Figure 2.4 Transmission spectra of Nafion. Red: Hydrated; Blue: Dehydrated. Left: Region
of exchange site local symmetry; Right: water region
1414 cm-1
920 cm-1
969 cm-1
1060 cm-1
20
days under vacuum at room temperature (Vertex 80) plus an additional 5 days on the high
vacuum line at 100°C (Fig. 2.5). Time dependent dehydration of Nafion reveals that the 969 cm-
1 and 1060 cm
-1 band disappear concurrently with formation of bands at 910 cm
-1 and 1414 cm
-1
appear (Fig. 2.4).3 DFT normal mode animation snapshots reveal that these modes are local
symmetry dependent (Fig. 2.6). Bands that fall between 1100 cm-1
and 1350 cm-1
are difficult to
analyze due to the overwhelming vibrations of the backbone in this region. Therefore to analyze
the effects of hydration and dehydration on the exchange site we focus our efforts on the bands
that fall outside of the backbone region.
21
FIGURE 2.5 Nafion 112 transmission IR spectroscopy peak-intensity time-dependence during Nafion dehydration. Left column: Emerging C1 bands during dehydration. Right column: Diminishing C3V bands during dehydration.3
22
The incomplete disappearance of the 1060 cm-1 band, despite rigorous dehydration,
confirms that trace amounts of water still exist in the membrane. The C1 modes (fig 2.5, left
column) gradually grow and shift with dehydration. This smooth shifting is consistent with DFT
calculations that show a variation in the atom to atom distances over a range of λ values from 1
– 3. This variation in differences causes the C1 bands to shift with dehydration.
786* cm-1: νs SO3H
1405* cm-1: vas SO3H
C1 Group Modes C3V Group Modes
Figure 2.6 Normal mode animation snap-shots of hydrated and dehydrated Nafion.
983* cm-1: νs SO3-
1059* cm-1: νs SO3-
23
2.3.3 Time Dependent Dehydration of SPEEK
The dehydration of SPEEK revealed similar bands that grow and diminish with state-of-
hydration however the changes in the spectra were not as pronounced (Fig. 2.7). Further SPEEK
membranes dehydrate more rapidly than Nafion. SPEEK membranes reached equilibrium in 11
hours on the Vertex 80 (under vacuum and at room temperature). After two days on the high
vacuum line at 60°C no further shifting or diminishing of the bands was observed (after 5 days
on the vacuum line with heat SPEEK membranes begin to crack). The bands that diminish as the
membrane dehydrates, at 1080 and 1020 cm-1
, correspond to modes involving the symmetric
stretch of the exchange site (Fig 2.8). The bands that grow with dehydration at 898 and 1365 cm-
1 correspond to modes involving the sulfonic acid stretch (Fig. 2.8).
Figure 2.7 shows that the C3V bands at 1080 cm-1
and 1020 cm-1
do not disappear with
rigorous dehydration. Further the 898 cm-1
band that we assigned as a C1 band (Fig. 2.8) is
1362 cm-1
1080 cm-1
1020 cm-1
898 cm-1
Figure 2.7 Transmission spectra of SPEEK. Red: Hydrated; Blue: Dehydrated.
24
present even in the fully hydrated state (Fig. 2.7). This suggests that there is a smaller difference
between the hydrophobic and hydrophilic phases of SPEEK as compared to Nafion.1
25
Similar to the behavior of the spectra of dehydrating Nafion, SPEEK C1 bands shift with
increasing intensity. Unlike Nafion the C3V bands in SPEEK do not shift as they diminish.
C1 Group Modes
769* cm-1: νs SO3H
1374* cm-1: vas SO3H
C3V Group Modes
981* cm-1: νs SO3-
1068* cm-1: νs SO3-
Figure 2.8 Normal mode animation snapshots Hydrated and Dehydrated SPEEK.
Figure 2.9 Left: Growth and shift of 1362 cm-1
C1 band, Middle: Diminishing intensity of
1023 & 1081 C3V bands, Right: Growth and shift of 898 cm-1
C1 band.1
26
Table 2.2 Measured and calculated (*) vibrational modes of SPEEK. Measured modes fully
hydrated (collected on Vertex 70); dehydrated after 2 days at 60°C on vacuum line.
2.4 Conclusions
The use of DFT calculated bands in conjunction with state-of-hydration studies enabled
the assignment of bands in both Nafion and SPEEK as group modes rather than as single
functional group assignments. Despite rigorous dehydration using a high vacuum line and heat
trace amounts of water remain in the membranes.
Time dependent peak-intensity measurements indicate that at low levels of hydration a
range of λ values exists. Variations in bond length of the sulfonic acid oxygen and the hydrogen
cause shifting of the C1 bands with dehydration.3
SPEEK exchange site group modes coexist at all states-of-hydration unlike Nafion where
they do not coexist at extreme states-of-hydration.
Measured
(cm-1
)
DFT (cm-1
) Band assignment Exchange
site local
symmetry
Hydrated
622
1023
1081
1238
1255
610*
981*
1068*
1179*
1199*
φa OOP, φb OOP, δu SO3
φa IP, νs SO3
φa IP, νas CHa, νs SO3
φb IP, νas CHb νas SO3
νas CHa φa IP, νas SO3
C3v
C3v
C3v
C3v
C3v
Dehydrated 898
1080 1230
1362
769*
1037* 1159*
1374*
φa OOP, φb OOP, νs SO3H
φa IP, νas SO3H
φa IP, νas CHa, νas SO3H
φa IP, φb IP, νas SO3H
C1
C1 C1
C1
27
Chapter 3. Ion Exchange Studies of Nafion
3.1 Introduction
Fuel cells continue to garner global interest as the impact of climate change, today and in
coming decades, is better understood. An untenable anthropogenic factor is distributed CO2
emissions.63
Unlike point sources, such as coal fired plants, the sequestration of CO2 in highly
distributed emissions is impossible.64-66
Emission free automobiles can be realized with
advancements in polymer electrolyte (ionomer) membrane batteries and fuel cells.
Nafion is the dominant ionomer membrane used in fuel cell membrane electrode
assemblies. It is a perfluorosulfonated ionomer under extensive study since 1975 due to its super
acidity. Nafion is expensive to manufacture, decomposes via a radical ion catalyzed unzipping
mechanism, and yields toxic fluorinated gases when incinerated for recovery of electrode
assembly catalysts.67-70
The development of better ionomers hinges on understanding the interrelationships
between ionomer morphology, ion exchange and state-of-hydration. Nafion morphology models
have advanced since the sphere and cylinder model of Gierke and Hsu.71-72
More recent models
incorporate transitional interphases separating the hydrophobic backbone from hydrophilic
phases lined with ion exchange groups. Examples include the Yeager and Steck three-phase
model (presenting a lower degree of order without Gierke and Hsu strict geometrical
definitions),17
the core-shell structure of Fujimura et al,18, 73
the bi-layer model of Starkweather,74
the sandwich model of Haubold75
and the rod-like model of Rubatat.19
Irrespective of the choice
of model, the driving force for ionomer phase segregation is minimization of free energy of the
dissociated exchange site environment.
Infrared (IR) spectroscopy is ideal for ion-exchange and state-of-hydration studies of
28
ionomer membranes.76-77
Although it is impractical to articulate the contribution of every
oscillator fragment in a normal mode band assignment, the overly simplistic assignment of bands
to only one functional group has been the source of decades of confusion.45, 78-79
Our
experimental state-of-hydration studies combined with density functional theory (DFT)
calculated normal mode analysis resolved much of the controversies by (1) the proper
assignments of the Nafion fingerprint bands as group modes by use of normal mode eigenvector
animations and (2) the categorization of group modes involving the exchange site in terms of its
local symmetry.3, 25
Figure 3.1 shows the Nafion 55-atom chemical repeat units used to computationally
model the dehydrated (left) and hydrated (right) membrane. All 3N-6 normal modes were
calculated for both models. The backbone segments terminate with –CH3 to prevent interference
with theoretical modelling that probes the role of the –CF3 in structure function relationships.80
Figure 3.1 Chemical repeat units of Nafion with exchange site local symmetry dependent
group modes.
C1 C3V
Hydrated Nafion-H Dehydrated Nafion-H
HF
1414 cm-1
SO3H νas COC-A νas
LF
910 cm-1
SO3H νs COC-A νs
HF 1061 cm
-1
COC-A νas SO3- νs
LF 969 cm
-1
SO3- νs COC-A νas
COC-B
COC-A
29
The 969 cm-1
(C3V,LF)1 and 1060 cm
-1 (C3V,HF) bands of fully hydrated Nafion-(H
+) are
both group modes arising from the same COC-A (Fig. 3.1) and the exchange site. As Nafion is
dehydrated from a fully hydrated state (Fig. 3.2, right) the C3V,LF and C3V,HF bands (red),
gradually vanish with the emergence of C1 bands that finalize at 910 cm-1
and 1414 cm-1
(blue).
The shift from Nafion-(H+) to Nafion-H
+ occurs when λ (water per exchange site,
𝑛𝐻2𝑂
𝑛𝑒𝑥) falls
below 4. The classification of the these pairs of group modes as C3V and C1 modes has also
correlated spectral changes with exchange site derivatizations that have puzzled investigators for
years.25, 81
The C3V,LF band in proton exchanged Nafion had commonly been attributed to only the
COC-A ether link.45, 82-83
Figure 3.1 provides group mode assignments with functional groups
ranked in order of motional contribution to the mode. The assignments were based on density
functional theory calculated normal mode eigenvector animations, augmented with ion exchange
and state-of-hydration experimentation. This theory-experiment approach confirmed that the
1 LF: low frequency; HF: high frequency
Figure 3.2 Transmission spectra of Nafion 212 and DFT calculated normal modes. Left: Dehydrated
Nafion, DFT (drop lines) Middle: Partially dehydrated Nafion. Right: Fully hydrated Nafion, DFT (drop
lines).
1070 cm-1 1414 cm-1
910 cm-1 1412 cm-1
1066 cm-1
921 cm-1
1061 cm-1
969 cm-1
Dehydrated Hydrated
30
DFT calculated2 983* cm
-1 (correlated to the experimental 969 cm
-1 C3V,LF) is primarily due to
SO3- νs, which is mechanically coupled to COC-A νas. (Fig. 3.1). The sulfonate group dominates
the 983* cm-1
group mode, thus resolving long standing controversies concerning the effects of
variation of state-of-hydration and ion exchange on Nafion spectra.3, 82, 84
The experimentally
observed 983 cm-1
band, insensitive to state-of-hydration and ion exchange (Fig. 3.2), is
associated with a 972* cm-1
band: The eigenvector animations show motional contributions
from COC-A and the COC-B with almost no exchange site contribution.
Scaling factors are used because DFT-calculations typically overestimate vibrational
frequencies by a couple of percent,85
which can encompass the width of some of the observed
narrow bands (e.g., the 970 - 995 cm-1
multiplet). Another consideration is the use of a chemical
repeat unit rather than a physical repeat unit. A physical repeat unit contains geometric
irregularities and macro-formations that cannot be represented by a chemical repeat unit. 36
The
scaling factor along with the use of the chemical repeat unit explains why observed bands cannot
be directly correlated to DFT calculated lines solely on the basis of proximity. The effect of the
DFT scaling factor may be modest in comparison to the use of the chemical repeat unit.3
We observe that the C1 bands substantially diminish within seconds after exposure to
ambient conditions. Thus reliable state-of-hydration studies require a high vacuum line for
sample preparation and enclosure of the IR spectrometer within a glove box. In this work the
transmission spectra of the full width of ion exchanged Nafion (i.e., 50.8 μm), shows for the first
time that K+, Na
+ and Li
+ remain in the 3-fold symmetric sulfonate pocket at all states of
hydration.
2 DFT calculated normal mode bands denoted by * e.g. 983* cm
-1.
31
3.2 Experimental Section
Membrane Preparation. Nafion 212 (Ion Power Inc., New Castle, DE) membranes were
cleaned by boiling in 7.9 M HNO3 (20 min) followed by rinsing in Nanopure water
(Nanopure™ water hereafter referred to as water) 6 times and finally boiling in water (1 h).
Nafion Ion Exchange. Ion exchanged membranes were obtained by soaking the cleaned Nafion
in 50 mL of 1M salt solutions (48 h).
Membrane Transmission FTIR Spectroscopy. The Nafion was removed from solution, pat-
dried with ChemWipes, and placed in the Vertex 70 IR spectrometer to obtain the spectra of
the fully hydrated lithiated membrane (ambient conditions). The sample is transferred, via the
antechamber, into to the Vertex 80V vacuum (1.00 hPa) spectrometer (Bruker, Billerica, MA) for
time dependent spectra during membrane dehydration. Spectra were taken every hour until a
steady dehydrated state was reached (48 - 60 hrs). Dehydration was continued on a vacuum line
(100°C, 5 days) equipped with a Welch 1402 DuoSeal vacuum pump, a glass oil diffusion pump
(Ace Glass, Vineland, NJ) and a liquid N2 trap. After dehydration, the sample bulb was
transferred to the FTIR glove box, and the membrane positioned back into the Vertex 80 sample
chamber. A final spectrum of the exhaustively dehydrated Nafion was taken. All spectra were
signal averaged (50 scans, 4 cm-1
resolution) using a DLaTGS detector. Data processing was
completed using Bruker™ OPUS 6.5™ software.
Membrane ATR Spectroscopy. The same membranes used in the transmission FTIR study were
placed in fresh 1M LiCl solution (50 mL) for 12 to 48 hours. Membranes were removed from
solution, pat-dried with ChemWipes™ and placed immediately in the spectrometer. After initial
measurements were collected, membranes were rinsed (5 s) in water, pat-dried with
ChemWipes™ and placed back in the instrument for a second ATR measurement. All
32
measurements were signal averaged (50 scans, 4 cm-1
resolution) on the Vertex 70 spectrometer.
Data processing was completed using Bruker™ OPUS 6.5™ software.
Molecular Modeling Calculations. Unrestricted DFT86
with the X3LYP87
functional was used
for geometry optimizations and calculations of the normal mode frequencies of protonated and
deprotonated Nafion as well as lithiated Nafion repeat units. The X3LYP is an extension to the
B3LYP88
functional providing more accurate heats of formation. The 55-atom deprotonated
repeat unit consists of one Nafion monomer. The repeat unit backbone was capped with CH3
groups to prevent computational interference from the side chain CF3 group. Jaguar 8.0
(Schrodinger Inc., Portland, OR) was used with the all-electron 6-311G**++ Pople triple- basis
set (“**” and “++” denote polarization56
and diffuse57
basis set functions, respectively). Output
files were converted to vibrational mode animations using the Maestro graphical user interface
(Schrodinger Inc.). Calculations were carried out on the high performance computing cluster at
the University of Texas, Pan American with 72 nodes of Dual 2.67 Ghz processors; each node
with 48 GB RAM and 250 GB disk. Crystal Orbital Overlap Populations (COOP)89
were
calculated by Jaguar using the AOMIX program. AOMIX processes output files from a variety of
quantum mechanical packages and generates densities-of-states and COOP spectra in terms of
constituent chemical fragments.89
DFT calculated normal mode peaks are denoted by superscript
(*) e.g. 983* cm-1
.
33
3.3 Results and Discussion:
3.3.1 ATR vs. Transmission Spectroscopy
Transmission spectroscopy of lithiated Nafion showed, surprisingly, the presence of the
C3V,HF mode with a complete absence of the C3V,LF band at all states of hydration. This is in stark
contrast with previous reports: ATR spectra of lithiated Nafion reported by Lowry and Mauritz,
and transmission spectra of Jin et al. show the presence of both the C3V,LF and the C3V,HF.79, 90
The
explanation for the absence of the C3V,LF band in all of our transmission spectra required
experiments focusing on three key aspects of the previously reported results: (1) the length of
time the membrane was immersed in the exchange solution, (2) the “leaching of the membrane in
distilled water to remove excess caustic” and (3) the sensitivity of ATR to the membrane
surface.79
Figure 3.3 Transmission spectra of hydrated Nafion immersed in 1M LiCl. Solid red: 12
hours, Dashed Red: 24 hours, Dotted Red: 48 hours.
34
Spectra (Fig. 3.3) of membranes immersed (1M LiCl) for 12 hrs (solid line), 24 hrs
(dashed) and 48 hrs (dotted) show a diminishing C3V,LF band: Immersion times on the order of 24
hrs yield membranes that are not fully exchanged. Beyond 48 hours the slight shoulder (dotted
red line) persists.
Figure 3.4 shows the ATR (black) and transmission (red) spectra of hydrated Nafion-Li+.
All samples were soaked (48 hrs) in 1M LiCl. Spectra were taken without a rinse (solid,
black/red) and after a 5 s rinse in Nanopure water (dashed, black/red). The C3V,HF and C3V,LF
bands appear in the ATR spectra whether the membrane rinsed or not. The rinsed membrane
(dashed black) shows a more defined C3V,LF. The entire ATR spectrum of the rinsed, lithiated
membrane (dashed black) perfectly overlays that of Nafion-H+ (Fig. 3.5), confirming that rinsing
the membrane causes surface delithiation. ATR spectroscopy probes a surface region (i.e., ~10
μm evanescent wave) that would be most sensitive to de-lithiating caused by rinsing.
In the transmission mode the C3V,LF band is absent whether there is or is not a rinse. The effect of
the rinse is to shift the C3V,HF band several wavenumbers lower. This shift of the C3V,HF towards
the C3V,HF of purely protonated Nafion (1060 cm-1
) suggests surface depletion of Li+ during the
Figure 3.4 IR spectra of hydrated lithiated Nafion. Solid lines: No rinse; Dashed lines: 5 s
rinse. Transmission (red), ATR (black)
35
rinse. The loss of C3V,LF with retention of the C3V,HF band suggests the question of how an
exchange site with a 3-fold axis of symmetry have only one of a pair of C3V group modes? The
loss of the C3V,LF group mode intensity, which includes an SO3-1
symmetric stretch, likely results
from the shifting of the band to higher wavenumbers where it would be obscured by other bands
e.g. 983 cm-1.
3.3.2 Transmission FTIR of Nafion-Li+.
In order to identify relevant normal modes in the lithiated Nafion DFT calculated
spectrum, the normal vibrational mode animations were viewed. First, all vibrational modes with
a normalized intensity below 3% of the largest peak (1215 cm-1
, 642 absorbance units) were
disregarded. Then the animations of each normal mode are viewed in the Maestro program with
focus on the exchange group. Displacements with a stretching distance between the sulfur and
one of the oxygen atoms above 0.3 Å are considered, while smaller displacements are ignored.
The Maestro settings for the animations are Amplitude - 2 and Speed - 1. The calculated normal
modes that meet these conditions are shown in Table 3.1.
Figure 3.5 ATR of hydrated Nafion. Solid line: proton exchanged; Dashed line: lithium
exchanged after 5 s rinse.
36
Table 3.1 DFT calculated bands for Nafion-H+ and Nafion-Li
+ and normal mode assignments.
Why does the C3V,LF vanish when Nafion is exchanged with Li+? Recall that the C3V,HF
and C3V,LF bands are group modes where the sulfonate exchange group symmetry is C3V. The
absence of the C1 bands confirms that Li+ complexes with the exchange site in such a manner
that retains C3V local symmetry (Fig. 3.6). This is further supported by crystal overlap integrals to
λ
Measured
(cm-1
)
DFT
(cm-1
) Normal mode components
Band
assignm
ent
Nafion-(H+) >10 969 983* SO3
- νs; COC-A νas C3v,LF
1062 1059* COC-A νas; SO3- νs C3v,HF
Nafion-H+ <1 923 786* SO3H νs; COC-A νs C1,LF
1423 1405* SO3H νas; COC-A νas C1,HF
Nafion-Li+ >10 983
1073
Nafion-Li+ <1 940* SO3Li νs
983 982* COC-A νas
1080 1037* COC-A νs ;SO3Li νs C3v,HF
1162* SO3Li νas, COC-A νs, COC-B νas
1197*
SO3Li νas, COC-A νs, COC-B νas
B. 1037* (1073) cm-1: COC-A νs, SO3Li
νs
Figure 3.6 Left: Nafion-Li DFT calculated spectrum superimposed on transmission spectrum of
dehydrated lithiated Nafion 212. Right: DFT normal mode animation snapshots of lithiated
Nafion
C3v,HF
(1073 cm-1
)
Absence of C3v,LF
37
be discussed later. The Nafion-H+
C3V,HF band shifts from 1060 cm-1
to 1073 cm-1
in Nafion-Li+.
The resultant SO3-Li+ bond maintains the C3V symmetry, therefore the C3V,HF is still present. A
study of the normal mode animations of lithium exchanged Nafion reveals that there is a DFT
calculated band at 940* cm-1
with an intensity of 100 units. This band is not observed at 940 cm-
1 in the spectrum of lithiated Nafion. As discussed previously DFT spectra cannot simply be
correlated to the nearest experimental band because the calculated bands are often off by as much
as 2%. Thus if we consider the shift of the C3V,HF to higher wavenumbers it raises the possibility
that the 969 cm-1
band has also shifted to higher wavenumbers (possibly into the 983 cm-1
) and is
no longer visible.
These findings starkly conflict with predictions of previous molecular modeling studies
where either monodentate or bidentate complexation was predicted.91
A monodentate
configuration with the metal ion coordinated to one sulfonate oxygen atom or bidentate
(coordinated to two sulfonate oxygen atoms) would reduce the exchange site local symmetry to
C1. Elimination of the C3V,HF group mode, and emergence of C1 group modes would be expected.
Instead we observe retention of the C3V,HF and loss of the C3V,LF. This can be reconciled by (1) a
bonding scheme with orbital overlap between the lithium ion, the pocket of oxygen atoms and
the sulfur atom with a 3-fold axis of symmetry (C3V) and (2) a shift of the 969 cm-1
band to
higher wavenumbers.
3.3.3 Bonding Scheme.
Metal (M)
COOP M-S M-O3 M-(SO3)
M-S bond lengths (Å)
Li+ 0.9314 -0.5556 0.3758 2.21 Ca2+ 0.0158 -0.3182 -0.3024 2.67 Na+ 0.4757 -0.3287 0.1470 2.54 K+ -0.0794 -0.0736 -0.1530 2.97
Table 3.2 COOP integrals calculated by AOMIX; Metal-Sulfur bond lengths calculated using
Maestro.
38
Figure 3.7 shows the Crystal Orbital Overlap Populations (COOP) for the metal-sulfur
(dashed blue), metal-oxygen (dashed red) and metal-sulfonate (solid black) complexes. The
COOP between two atoms can be used as a measurement of the corresponding covalent bond
strength, where positive and negative COOP values indicate bonding and anti-bonding patterns,
respectively. COOP should not be used for ionic bonds and long-range interactions, where
minimal overlap is observed irrespective of the bond strength.39
Thus, if the calculated COOP
overlap is small, there is little covalent character to the bond with no information about the
extent of ionic interactions between the atoms. The lithium-sulfur COOP value of 0.9314 (Table
3.2) confirms a strong covalent bond between Li and sulfur (for comparison the carbon-carbon
COOP for the –CF2 groups between the ether and the exchange site is 0.96). The COOP
calculations also suggest that the lithium-oxygen interactions are limited to electrostatic forces
(Fig. 3.7). In summary, lithium has a multi-centered covalent bond that maintains the 3-fold axis
of the exchange local symmetry. Moreover, the strongest bonding is between the lithium and
sulfur.
Figure 3.7 COOP values calculated by Jaguar. Dashed vertical line: HOMO level. Black:
M-SO3; Dashed Red: M-O3; Dashed Blue: M-S
39
The Na-S bond has a smaller COOP value of 0.4757 which is half the value of the C-C
bond between the ether and the exchange site (0.96) and verifies quantitatively that this bond is
covalent (Table 3). COOP values for K+ and Ca
2+ indicate that the exchange site bond, while still
C3V in nature, is exclusively ionic in character (Table 3.2).
3.3.4 Transmission FTIR: K+, Na
+, Li
+, H
+, Ca
2+, Ni
2+, Al
3+ exchanged Nafion.
The transmission spectra (Fig. 3.8) ordered from lowest to highest heat of hydration (-
∆Hhyd) suggests three clusters of -∆Hhyd value: less than 550 kJ/mole, between 550 and 2000
kJ/mol and above 2000 kJ/mol.
Figure 3.8 Transmission spectra of fully hydrated, ion exchanged Nafion with -∆Hhyd.
4665 kJ/mol
2105 kJ/mol
1577 kJ/mol
1130 kJ/mol
520 kJ/mol
406 kJ/mol
322 kJ/mol
Al3+
H+
Li+
Na+
K+
Ni2+
Ca2+
40
Ions with ∆Hhyd below the first demarcation (K+, Na
+, Li
+) are symmetrically positioned
within the C3V exchange site pocket at all states-of-hydration (Fig 3.9) The C3V,LF mode is never
observed for K+, Na
+, Li
+ exchanged Nafion likely due to a shift in the band to higher
wavenumbers similar to the shift seen with the 1060 cm-1
band (Fig. 3.11).
Figure 3.10 focuses on ions with intermediate ∆Hhyd values. Although H+
and Ca2+
do not
interact with the exchange site when the membrane is fully hydrated, they strongly interact at
low states of hydration. In the case of protons, formation of the sulfonic acid exchange site
destroys the 3-fold axis of symmetry and thus both C3V group modes vanish. In the case of Ca2+
,
at low states of hydration, the ion sits within the exchange site pocket with maintenance of the
C3V,HF. The C3V,LF vanishes because, similar to the case of Li+, the binding of the metal ion causes
the band to shift and become obscured.
Figure 3.9 Hydrated (red); dehydrated (blue) ion exchanged Nafion transmission spectra.
Left: K+; Right Na
+
41
Metal ions with ∆Hhyd above 2000 kJ/mol (i.e. Al3+
and Ni2+
) are solvated only by water
at all state-of-hydration (the exchange sites never enter the first solvation sphere of Al3+
and
Ni2+
) . Spectra of Al
3+ and Ni
2+ exchanged membranes after dehydration on the vacuum line at
100°C for 10 days continue to display the water peak at 1730 cm-1
(Fig. 3.11). Further
dehydration of these membranes resulted in damage to the membrane (cracking, breaking into
pieces). Despite rigorous dehydration the membranes continue to exhibit both the C3V,HF and
C3V,LF with no evidence of complexation of the metal to exchange site. Sulfonate oxygens are
poor ligands due to the powerful inductive withdrawing effects of nearby fluorine atoms. This is
H+
C3V, HF
C3V, LF
Ca2+
C3V,HF
C3V,LF
C1,HF
C1,LF
Ca2+
C3V,HF
Dehydrated Hydrated
Figure 3.10. Transmission spectra of Ca2+
and H+ exchanged membranes. Top: Ca
2+
exchanged. Bottom: H+
exchanged.
H+
42
why this complex bonding scheme is governed by the ∆Hhyd of the counter ions exchanged into
the membrane.
In summary the interactions of metal counter ions with the exchange site can be
characterized by both the enthalpies of hydration and the charge/volume ratios. Figure 3.12
illustrates the three clusters. Ions with ∆Hhyd of less than 550 kJ/mol (Fig. 3.12 bottom) are
complexed to the exchange site at all states-of-hydration. Ions with enthalpies in the mid region
(i.e. 550 – 2000 kJ/mol) complex only when λ falls to low values (Fig 3.12 middle). Membranes
exchanged with metals whose ∆Hhyd exceeds 2000 kJ/mol cannot be dehydrated enough to bind
to the exchange site without destroying the membrane (Fig 3.12 top).
Al3+
1631 cm-1
970 cm-1
1059 cm-1
Ni2+
969 cm-1
1065 cm-1
1633 cm-1
Figure 3.11 Hydrated (red); dehydrated (blue) ion exchanged Nafion transmission
spectra. Left: Al3+
; Right Ni2+
43
3.4 Conclusions
The almost instantaneous rehydration of Nafion previously dehydrated on a high vacuum line, at
relative humidities as low as 30%, renders state-of-hydration studies under ambient conditions
impossible. A sample chain-of-transfer from high vacuum to an FTIR vacuum sample
compartment with no exposure to the environment was prerequisite to the categorization of
exchange site interaction of metals in terms of their enthalpies of hydration. This method
revealed three categories: (1) ions that are always bound (2) ions with state-of-hydration
dependent binding (3) ions that never bind. Transmission spectra reveal that K+, Na
+, and Li
+,
metal ions with ∆Hhyd of less than 550 kJ/mol, fall into the first category. Spectra of Ca2+
and H+
Figure 3.12 Models depicting exchange site interactions with ions. Q: charge; V: volume
Vq
-∆HHyd
>2000 kJ/mol
1130 kJ/mol
<550 kJ/mol
= 0
K+, Na+, Li+
H+
= 0
Al3+, Ni2+
= 0 unattainable
Ca2+
= 0 1157
kJ/mol
State-of-hydration dependent
44
exchanged Nafion, with ∆Hhyd from 550 – 2000 kJ/mol, confirm that the ions only complex to
the exchange site at low λ values.25
Metal ions with ∆Hhyd exceeding 2000 kJ/mol (e.g., Al3+
,
Ni2+
) do not bind to the sulfonate group at any attainable state-of-hydration (third category).
Loss of the C3V,LF band combined with retention of the C3V,HF band for metal exchanged Nafion
is resolved by a bonding scheme with the alkali M-S bond normal to the equilateral triangular
plane formed by the 3 oxygen atoms and a shift of the C3V,LF to higher wavenumbers where it is
obscured.
45
Chapter 4. Future Work
4.1 Ion Exchange Studies of SPEEK
There are several bands in the transmission spectra of SPEEK membranes that contain
contributions from the exchange site. Some of these exchange site group mode bands in SPEEK
have not been previously assigned. DFT calculations in conjunction with COOP can help us
assign these bands. In our previous study we used COOP calculations to show that lithium and
sodium ions bound to the exchange site of Nafion in a manner that not only kept the local C3V
symmetry but also included a covalent bond between the metal ion and the sulfur atom. We also
showed that potassium and calcium bound to the exchange site of Nafion in a manner that
maintained the C3V local symmetry but was limited to electrostatic interactions between the
metal ion and the exchange site oxygen atoms. SPEEK does not have nearby fluorine atoms to
withdraw electron density away from the exchange site and therefore is a weaker acid than
Nafion. We expect that this will influence how the metal ions bind to the exchange site.
4.2 Experimental Methods
Commercially available SPEEK membranes (referred to as CSPEEK) were pre-cleaned
following the procedure outlined in the previous chapter. After cleaning the membranes were
soaked in 50 mL of the following 1M salt solutions for 48 hours: KI, NaCl, LiCl, NiCl2, AlCl3.
The membranes were then removed from solution, pat-dried and placed into the Vertex 70
spectrometer (open to environment)for an initial fully hydrated spectrum. Next spectra were
collected every hour under vacuum in the Vertex 80V spectrometer for the time-dependent state-
of-hydration studies. Once the membranes reached equilibrium under vacuum (time varies
depending on ion) they were placed on the vacuum line under heat for at least 48 hours and a
46
final dehydrated spectrum was collected (time required on vacuum line depends on the ion).
Potassium exchanged SPEEK dehydrated on the Vertex 80V in 12 hours and did not dehydrate
further on the vacuum line. Sodium exchanged membranes required 26 hours on the Vertex 80V
and did not dehydrate further on the vacuum line. Lithium required 48 hours on the Vertex 80V
and two days on the vacuum line under heat. Nickel and Aluminum exchanged membranes
required 48 hours on the Vertex 80V plus over 48 hours on the vacuum line. Aluminum and
Nickel exchanged membranes were never fully dehydrated as the membranes cracked into small
pieces after 4 days on the vacuum line. Ion exchanged spectra were compared to the protonated
form of the membrane to confirm previous assignments and highlight differences. DFT
calculations along with COOP are currently being performed using the same parameters as
outlined in the previous chapter. These DFT and COOP will be compared with the existing
experimental data to study the interaction between the ion and the exchange site.
4.3 Preliminary Results
1089 cm-1
1026 cm-1
898 cm-1
767 cm-1
1365 cm-1
615 cm-1
Figure 4.1 Transmission spectra of CSPEEK. Red: Hydrated; Blue: Dehydrated.
47
4.3.1 H+ and K+ Exchanged CSPEEK
As we can see in Figure 4.1 there are four bands that diminish as the dehydration
progresses and two bands that grow with dehydration in SPEEK that are not obscured by the
backbone vibrations. In our previous work we assigned 5 of these six bands. Table 1 shows the
nine DFT calculated bands for hydrated SPEEK that include a significant contribution from the
exchange site. The bands in bold have previously been assigned by us. An examination of the
DFT calculations of the ion exchanged membranes in conjunction with the ion exchanged
spectra will assist in confirming the assignments of these bands. The 767 experimental band that
diminishes with dehydration is most likely correlated to the DFT calculated 695* cm-1
band.
Three other calculated bands include significant contributions from the exchange site; however
these bands are obscured by the backbone region. That is not to say that these bands cannot be
identified: Unlike Nafion, the backbone region of SPEEK is somewhat less complex and with
further ion-exchange and DFT calculations these bands may be able to be assigned.
Table 4.1 DFT calculated bands for Hydrated SPEEK.
DFT
Band
Intensity Experimental Normal Mode Assignments
610* 232* 620 δu SO3-, φa IP, COC, φb IP
695* 51* 767 φa IP, φb OOP, δu SO3-
981* 185* 1023 νs SO3
-, φa IP
1068* 107* 1068 νas SO3-, φa IP, νas COC
1175* 152* νas SO3-, φa IP, νas COC
1179* 480* 1238 φb IP, νs CHb, νas C=O, νas COC, νas
SO3- 1186* 206* νas SO3
-, φa IP, νas COC
1190* 147* νas SO3-, φa IP, φb IP,
1199* 745* 1255 νas SO3-, φa IP, νs Cha, νas COC
48
DFT calculations have been completed for K+ exchanged SPEEK, hydrated SPEEK,
dehydrated SPEEK and PEEK. An examination of the spectra of the hydrated and dehydrated
K+ exchanged membrane reveals that the previously identified C1 band at 898 cm
-1 is not
growing with dehydration (Fig 4.2, blue). Further the two bands assigned as C3V bands (at 1027
cm-1
and 1088 cm-1
) are not diminishing (Fig. 4.2, blue) or shifting with state-of-hydration. The
retention of the 1027 cm-1
and 1088 cm-1
suggests that the potassium ion binds to the exchange
site in a manner that retains C3V symmetry. The lack of shifting in these bands indicates that the
ion is bound at all states-of-hydration similar to Nafion. Once the COOP calculations have been
completed they will reveal the exact nature of the bond between the ion and the exchange site.
Table 2 shows the DFT calculated normal mode assignments for bands in K+ exchanged SPEEK
that include significant contributions from the complexed exchange site (modes in bold
correspond to bands previously assigned). In order for a band to be considered to have
767 cm-1
898 cm-1
1027 cm-1
1088 cm-1
1366 cm-1
619 cm-1
Figure 4.2 Transmission spectra of K+ exchanged CSPEEK. Red: Hydrated; Blue:
Dehydrated
49
“significant” contributions the stretch must be at least 0.3 angstroms (smaller displacements are
not included) and the calculated intensity of the band must be at least 3 percent of the maximum
calculated intensity. A study of the animations revealed that there are 8 group modes that
involve significant contributions from the exchange site. Three of these group modes confirm
our previous assignments as the 619, 1026 and 1088 cm-1
bands.
DFT
Bands
Intensity Experimental Normal Mode Assignments
614* 64 619 δu SO3K, φa IP, COC,
φb IP 636* 75* 767 δu SO3K, φa IP, COC, φb
IP 707* 47* φa IP, φb OOP, δu SO3K
993* 148* 1027 νs SO3K, φa IP
1082* 62* 1088 φa IP, νs SO3K
1143* 254 νas SO3K, φa IP
1149 299 νs SO3K, φa IP, CHa
1154 269 νs SO3K,
Table 4.2 DFT calculated bands containing SO3-K
+ contributions for K
+ exchanged SPEEK.
4.3.2 Li+ and Na
+ Exchanged SPEEK
Li+ Na+
Figure 4.3 Transmission spectra of CSPEEK. Left: Li+; Right: Na
+. Red: Hydrated; Blue:
Dehydrated.
50
The 1203 cm-1
band in the protonated form does not diminish or shift with dehydration
therefore we would not expect this band to have significant contributions from the exchange site.
An examination of the spectra of both the Li+ and Na
+ form of the membrane (Fig. 4.3) show that
the 1201 cm-1
does shift and/or diminish with state-of-hydration. The relocation or loss of the
1201 cm-1
(Li+) and 1202 cm
-1 (Na
+) would seem to suggest that this band involves a normal
mode arising from vibrations of the exchange site. The retention of this band in the protonated
form would seem to suggest the opposite i.e. this band does not involve local exchange site
symmetry. This conundrum will be addressed in future studies.
4.3.3 Al3+
exchanged SPEEK
The transmission spectra of the Al3+
exchanged membranes seen below (Fig 4.4) reveals
something not seen in the Nafion ion exchange study. In the previous study we were unable to
drive enough water from around the aluminum ion to see complexation without the membrane
cracking into pieces. In the CSPEEK aluminum exchanged membrane we were unable to drive
off all of the water before the membrane crumbled but we did drive off enough to see the effect
of the metal ion on the exchange site. Interestingly we see the 1026 cm-1
and 1088 cm-1
C3V
bands diminishing in intensity and shifting to lower wavenumbers (Fig 4.4, right) similar to the
behavior of the protonated membrane. The diminishing C3V bands suggest that the metal ion
would bind to the exchange site in a manner similar to the proton (i.e. with C1 local symmetry) if
we could remove enough water for binding to occur. Further the growth of the 1365 cm-1
C1
band and the shifting to lower wavenumbers of the C3V 620 cm-1
to 613 cm-1
support this
hypothesis. The 1201 cm-1
band behaves the same as it does in the protonated form (i.e. it
doesn’t shift or diminish with state-of-hydration).
51
The dehydrated spectrum of Al3+
exchanged CSPEEK (Fig. 4.4, blue) exhibits changes in
the backbone region between 1200 cm-1
and 1400 cm-1
. Aluminum is a trivalent cation and the
sulfonate exchange site has a formal charge of negative one therefore we might expect that the
ion would require multiple exchange sites and/or water molecules to stabilize it. Therefore the
possibility arises that these changes in the backbone region are caused by crosslinking between
multiple sulfonate exchange sites and each Al3+
ion. As the membrane dehydrates this would
cause the backbone to contort in order to maintain complexation (Fig 4.5). Further studies
increasing the levels of sulfonation and therefore decreasing the level of contortion necessary
would assist in verifying this hypothesis.
1026 cm-1
1088 cm-1
620 cm-1
1360 cm-1
Figure 4.4 Left: Transmission spectra of Al3+
exchanged CSPEEK; Right: expanded view of
C3V bands. Red: Hydrated; Blue: Dehydrated.
1016 cm-1
1080 cm
-1
52
Figure 4.5 Schematic of an Al3+
ion complexed to SPEEK.
4.4 Early Conclusions and Hypotheses:
Potassium, lithium and sodium exchanged SPEEK membranes bind to the exchange site
in a manner that retains C3V local symmetry. COOP calculations will help define the character of
these binding interactions. Aluminum ions only bind to the exchange site when λ is at low levels
and once bound do so in a C1 manner that may include crosslinking between exchange sites.
Further studies using SPEEK membranes of various degrees of sulfonation will assist in
confirming this hypothesis.
53
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