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Vibrational Spectroscopy of Ion Exchange Membranes

A thesis presented

by

Dunesh Kumari

to

The Department of Chemistry and Chemical Biology

In partial fulfillment of the requirements for the degree of Master of Science

in the field of

Chemistry

Northeastern University

Boston, Massachusetts

April, 2011

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Vibrational Spectroscopy of Ion Exchange Membranes

by

Dunesh Kumari

ABSTRACT OF THESIS

Submitted in partial fulfillment of the requirements

for the degree of Master of Science in Chemistry

in the Graduate School of Arts and Sciences of

Northeastern University, April, 2011

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ABSTRACT

Infrared Spectroscopy (IR) and density functional theory (DFT) calculations were used to

study Nafion, a sulfonated tetrafluoroethylene ionomer used as the electrolyte material of choice

for polymer electrolyte membrane fuel cells (PEMFCs). A methodology is described for

assignment of infrared peaks in terms of mechanically coupled internal coordinates of near

neighbor functional groups.

This work demonstrates (chapter 2- 4) the use of ionomer functional group internal

coordinate coupling analysis to assign two key Nafion peaks formerly assigned as the sulfonate

symmetric stretch (1056 cm-1

) and a COC (A) vibrational mode (971 cm-1

). The experiments and

theory complement each other to show that the dominate motions of the 1056 cm-1

and 971 cm-1

modes are attributed to the COC (A) and the sulfonate stretch respectively, exactly reverse of the

convention used for decades. The salient point is that both peaks result from mechanically

coupled internal coordinates of both functional groups. This explains why the 1056 cm-1

and 971

cm-1

peaks shift together with changes in the sulfonate group environment (i.e., ion exchange or

membrane dehydration). The assignments, correlated with extensive literature data, and new data

showing both peaks vanishing upon rigorous dehydration (i.e. conversion of a C3V deprotonated -

SO3- to a C1 -SO3H) of the membrane, were based on the correlation of observed IR peaks with

animations of mechanically coupled internal coordinates obtained by DFT calculations. Further,

the above methodology was augmented with polarization modulated infrared reflection-

adsorption spectroscopy (PM-IRRAS) to elucidate the Nafion ionomers functional groups that

participate in self-assembly of Nafion onto Pt surfaces. A model for Nafion adsorption onto Pt

shows that the Nafion side-chain sulfonate and CF3 co-adsorbates are structural components of

the Nafion-Pt interface. The DFT-spectroscopy method of assigning peaks in terms of

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mechanically coupled internal coordinates was augmented with ion-exchange-induced shifts for

extensive assignments of Nafion IR peaks (explicit and deconvoluted).

Chapter 5 focuses on the durability of the membrane electrode assembly (MEAs) in

direct methanol fuel cells (DMFCs). The MEA catalyst-polymer interfacial region was examined

for degradation modes.

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ACKNOWLEDGEMENTS

Upon completion of this work, I would like to express my sincere appreciation to my

advisor Dr Eugene Smotkin for his excellent guidance, constant encouragement, and supervision

throughout the course of my research. I extend my gratitude to him for his constant availability;

his enthusiasm and creativity have always inspired me in my educational pursuits. I would also

like to thank my past and present colleagues, who definitely had a positive influence on me and

they include Aracelis Rivera, Mathew Webber, Adam Yakaboski, Ian Kendrick, Joseph Bedard

and Michael Bates. Special acknowledgement is given to Dr. Nickolas Dimakis for his assistance

in this project.

Appreciation is expressed to the members of my graduate committee, Dr Sanjeev

Mukerjee and Dr Max Diem for their assistance and invaluable advice in completing this work.

Finally, I would like to take an opportunity to thank the rest of the faculty and staff from the

chemistry and chemical biology department who provided assistance and direction during the

course of my graduate study.

On a personal note, I want to thank all my family and friends for their love, trust, and

support during my pursuit to Masters. I want to thank my parents, Subhash and Ajodhya, for

always being there, supporting me in my academics and life pursuit. I am enormously grateful

for love and care my brothers, Naresh and Rahul has given me. I would like to express my

special gratitude to my brother-in-laws, Dev and Hari for their continuous support and

encouragement.

Finally, my thanks, with love and excitement, goes to my husband, Dr. Harsh Chauhan

for being the most wonderful part of my life. Thank you.

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Table of Content

ABSTRACT ................................................................................................................................... 3

ACKNOWLEDGEMENTS ......................................................................................................... 5

TABLE OF CONTENTS ............................................................................................................. 6

LIST OF FIGURES AND TABLES............................................................................................ 9

LIST OF ABBREVIATIONS .................................................................................................... 11

CHAPTER 1: Introduction ........................................................................................................ 14

1.1 Ionomers ................................................................................................................................ 14

1.2 Fuel Cells................................................................................................................................ 14

1.3 Research Objectives .............................................................................................................. 16

1.3.1 Aim 1: Mechanically coupled Internal coordinates of Ionomer Vibrational Modes: ...... 19

1.3.2 Aim 2: Elucidating the Ionomer-Electrified Metal Interface ........................................... 19

1.3.3 Aim 3: IR spectroscopy of the Ion Exchange Membrane ................................................. 19

1.3.4 Aim 4: Durability studies on the performance of MEAs in DMFCs ................................ 19

CHAPTER 2: Mechanically Coupled Internal Coordinates of Ionomer Vibrational Modes

...................................................................................................................................................... 21

2.1 Introduction ........................................................................................................................... 21

2.2 Experimental ......................................................................................................................... 21

2.2.1 Transmission Spectroscopy .............................................................................................. 23

2.2.2 Computational method ..................................................................................................... 23

2.3 Result and Discussions.......................................................................................................... 24

2.4 Conclusions ............................................................................................................................ 24

CHAPTER 3: Elucidating the Ionomer-Electrified Metal Interface ..................................... 30

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3.1 Introduction ........................................................................................................................... 30

3.2 Experimental ......................................................................................................................... 31

3.2.1 Attenuated total reflectance (ATR) spectroscopy ............................................................. 31

3.2.2 Preparation of arc-melt Pt* ............................................................................................. 31

3.2.3 Polarization modulated infrared reflection absorption spectra (PM-IRRAS)* ............... 32

3.2.4 Computational method ..................................................................................................... 32

3.3 Results and Discussion.......................................................................................................... 33

3.3.1 Analysis of IR spectra ....................................................................................................... 33

3.3.2 Nafion/Pt adsorption model ............................................................................................. 33

3.4 Conclusion ............................................................................................................................. 37

CHAPTER 4: IR Spectroscopy of Ionomers ............................................................................ 41

4.1 Introduction ........................................................................................................................... 41

4.2 Experimental ......................................................................................................................... 41

4.2.1 Attenuated total reflectance (ATR) spectroscopy ............................................................. 42

4.2.2 Computational method ..................................................................................................... 42

4.3 Results and Discussion.......................................................................................................... 43

4.4 Conclusion ............................................................................................................................. 43

CHAPTER 5: Durability studies on performance degradation of MEAs in DMFCs .......... 56

5.1 Introduction ........................................................................................................................... 56

5.2 Experimental section ............................................................................................................ 58

5.2.1 MEA preparation.............................................................................................................. 58

5.2.2 Cross-section sample preparation ................................................................................... 58

5.2.3 ATR microscopy ............................................................................................................... 59

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5.2.4 Scanning Electron Microscope ........................................................................................ 60

5.2.5 X-Ray Diffraction ............................................................................................................. 60

5.3 Results and Discussion.......................................................................................................... 61

5.3.1 ATR-FTIR microscopy...................................................................................................... 61

5.3.2 Scanning Electron Microscopy ........................................................................................ 61

5.3.3 X-Ray Diffraction ............................................................................................................. 67

5.4 Conclusions ............................................................................................................................ 67

CHAPTER 6: Future Research ................................................................................................. 70

6.1 Dehydration Study of Anion Exchange Membrane using Infrared Spectroscopy and

Density Function Theory ............................................................................................................ 70

6.1.1 Introduction ...................................................................................................................... 70

6.1.2 Hypothesis and objectives ................................................................................................ 70

Reference ..................................................................................................................................... 72

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List of Figures and Tables

Figure 1.1: PEMFC Schematic.51

The Nafion PEM is sandwiched between the anode and

cathode catalysts. .......................................................................................................................... 18

Figure 2.2. NSC λ= 3 (Left), NSCB anion (Right) (Red= O, White= H, Grey= C, Purple= S,

Green= F, Yellow dotted lines = H-bonds) ................................................................................... 24

Figure 2.3. Transmission IR spectra of Nafion 112 showing the evolution of 1415 cm-1

and 908

cm-1

bands upon dehydration ........................................................................................................ 25

Figure 2.4. Proton dissociation threshold and the formation of H3O+ and C3V local symmetry.

Top: λ= 0, left; λ=3, center; λ=4, right. Bottom: λ= 7, left; λ=9, center; λ=10, right. (Red= O,

White= H, Grey= C, Purple= S, Green= F, Yellow dotted lines are hydrogen bonds. ................. 26

Figure 2.5: Maestro animation snapshots of DFT calculated modes of the full side chain and

backbone ....................................................................................................................................... 28

Figure 3.1. DFT calculated normal modes (black lines) and Nafion ATR spectrum (red). ........ 33

Figure 3.2. Theoretical and experimental spectra. ATR of hydrated Nafion (red); PM-IRRAS of

Nafion-H on Pt (grey); PM-IRRAS of Nafion-Li on Pt (blue); Selected DFT peaks (black lines 1-

6). .................................................................................................................................................. 34

Figure 3.3. Normal mode coordinate animation snapshots of the Nafion side-chain anion and

backbone fragment (see scheme 1). .............................................................................................. 36

Figure 3.4. Gaussian 03 Viewer Nafion-Pt interface model. Oxygen (red), Sulfur (yellow),

Fluorine (light blue), Carbon (grey), Pt (dark blue). ..................................................................... 39

Figure 4.1. DFT calculated normal modes (black lines) and Nafion 117 ATR spectrum (red) ... 43

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Figure 4.2. (a) ATR-IR spectra of dehydrated cation exchanged Nafion 117, (b) Ion exchange

induced ν shift in hydrated and dehydrated Nafion™ & (c) Peak width at half height (left scale)

and η (right scale) vs. ions. The absolute hardness of the H+ ions is ∞. ....................................... 45

Figure 4.3. DFT calculated normal modes (red lines) and Nafion ATR spectrum (black ............ 46

Figure 4.4. Ion exchange induced ν shift on deconvoluted ATR peaks (region 940-1020 cm-1) (a)

992 cm-1 (b) 981 cm-1 & (c) 971 cm-1. ....................................................................................... 48

Figure 4.5. (a) Ion exchange induced ν shifts for 1057 cm-1 ATR peak. The relevant DFT

calculated peaks (red line 3) are superimposed upon the deconvoluted peaks. (b) Normal mode

coordinate animation snapshots of the Nafion side-chain anion and backbone fragment ............ 49

Figure 4.6. Left panel (a-h)Ion exchange induced ν shift on deconvoluted ATR ........................ 52

Figure 4.7. (a) Ion exchange induced ν shift on shoulder peaks in region 1280-1330 cm-1

. The

relevant DFT calculated peaks are superimposed (red lines 12 &13) on the ATR spectra. (b & c)

....................................................................................................................................................... 53

Figure 5.1 left: Membrane Electrode Assembly (MEA). Middle: Epoxy mounted cross cut MEA.

Right: Automated ATR mapping for cross sectional studies. ....................................................... 60

Figure 5.3 ATR-FTIR spectrum (900-1400 cm-1) of Nafion obtained for MEAs before and after

lifetime operation in a fuel cell ..................................................................................................... 64

Figure 5.4: Mean peak intensity value for IR absorption bands of Nafion Vs operation time of the

MEAs. ........................................................................................................................................... 64

Figure 5.5: SEM electrograph comparing the images for unused, 950 hrs and 1515hrs operated

MEAs in low mode at 20 kV accelerating voltage at X180 magnification. ................................. 66

Figure 5.6. A comparison of XRD pattern obtained from the catalysts for unused and used

MEAS: (a) PtRu black from anode layer (b) Pt black from cathode layer ................................... 68

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Table 2.1: Calculated s(A1) mode of C3V (i.e., d or greater) sulfonate………………………..25

Table 3.1: PM-IRRAS and DFT IR adsorption peaks and assignments…...................................35

Table 3.2: Average partial charges of selected Nafion segments……………………………….36

Table 4.1: ATR and DFT IR adsorption peaks and assignments…………………………….52

Table 5.1: Membrane thickness measured from the ATR mapping and SEM image…...............64

Table 5.2: Mean particle sizes and Lattice parameters for the anode and cathode catalyst

evaluated from XRD measurements……………………………………………………………66

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LIST OF ABBREVIATIONS

DMFC – Direct Methanol Fuel Cell

MEA – Membrane Electrode Assembly

ATR– Attenuated Total Reflectance

GDL – Gas Diffusion Layer

fcc – Face Centered Cubic

ORR – Oxygen Reduction Reaction

PEM – Polymer Electrolyte Membrane

PEMFC – Polymer Electrolyte Membrane Fuel Cell

IRAS – Infrared Reflection Absorption Spectroscopy

NASA – National Aeronautics and Space Administration

SAXS – Small Angle X-ray Scattering

NMR - Nuclear Magnetic Resonance

DFT –Density Functional Theory

NSC – Nafion side Chain

NSCB – Nafion side chain with a PTFE backbone

PTFE - Polytetrafluoroethylene

PM-IRRAS – Polarization Modulated Infrared Reflection Absorption Spectroscopy

XRD – X-ray Diffraction

SEM – Scanning Electron Microscopy

FTIR – Fourier Transform Infrared

FER – Fluoride Emission Rate

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XPS – X-ray Photoelectron Spectroscopy

TEM – Transmission Electron Spectroscopy

EPR – Electron Paramagnetic resonance

AMFCs – Alkaline Membrane Fuel Cells

AEM – Anion Exchange Membrane

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CHAPTER 1: Introduction

1.1 Ionomers

Ionomers, a class of polymers have fascinated the polymer scientists for the last 40 years.

The wide range of compositions, molecular architectures, and morphologies present in the

ionomers, makes them of high interest.1 Ionomers have applications in analytical chemistry,

2

environmental remediation,3 as reactive solvents for organic reactions,

4-6,7-10 sensors,

11 batteries,

and fuel cells.12-15

Ionomers are polymers with pendant ionic groups (e.g., sulfonate, carboxylate,

phosphonates, quaternary ammonium, etc.) attached to homopolymer, random, or block

copolymer backbones. The scope of this thesis deals with Nafion, a sulfonated

tetrafluoroethylene copolymer which is a benchmark ionomer due to its ionic conductivity and

electrochemical/chemical resistance.16

E. I. DuPont de Nemours first manufactured Nafion in

1960s and since then it has been available in different forms for studies. Nafion and the side

chain derivative of the Nafion are used in practical fuel cell systems and are commercially used.

The general properties of the Nafion are summarized below.

Scheme 1.1 shows the Nafion “repeat” structure with pendant side chains of perfluoronated

vinyl-ethers terminated by a sulfonic acid group. The polytetrafluoroethylene (-CF2-CF2-CF2-)

backbone is responsible for the high chemical, mechanical and thermal stability, 17, 18

while the

inductive withdrawing effect of the perfluorocarbon chain on the sulfonic acid group is

responsible for high proton conductivity.18, 19

The hydronium ion in the ion exchange polymer

can be readily exchanged for metal ions like Na+, Li

+, Ca

2+ etc. by soaking in an appropriate

aqueous electrolyte solution. An interesting and important practical aspect of Nafion is its ability

to sorbs relatively large amounts of water and solvents depending upon the polymer weight,

counterion form, and temperature of the equilibration.

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Scheme 1.1. A repeat unit of Nafion with key functional group called out for later discussion.

Because of the technological importance of the Nafion as well as the novel structural

features, a wide range of tools has been used to study the structure. The structure and

morphology of Nafion is complex. With some crystallinity, Nafion structure contains two non-

crystalline regions, hydrophobic backbone phase, and the hydrophilic ionic areas. The ionic

groups at the end of the side chains of the perfluorinated backbone form clusters within the

polymer matrix. Their properties, governed by ionic interactions within discreet regions of ionic

aggregates, are sensitive to solvents, water content, degree of neutralization and temperature.20

The properties of Nafion in the protonated and cation exchanged form, with and without sorbed

solvents have been probed by a wide variety of methods including broadband dielectric

spectroscopy, DFT, electron paramagnetic resonance, nuclear magnetic resonance (NMR),

small-angle x-ray scattering (SAXS), infrared and Raman spectroscopy.16, 17, 21-42

Nevertheless,

an understanding of ionomer aggregate structure, even in the most well studied Nafion, has been

elusive. The “ionic peak,” often used to deduce ionic aggregate size, lacks the definitive features

required for development of unambiguous morphological models:43, 44,45

SAXS is complemented

by other techniques, such as spectroscopy.

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1.2 Fuel Cells

The worldwide demand for clean power generation is rapidly increasing. Today, world

markets relies on the combustion of fossil fuel and renewable sources like wind and solar for

energy production with some dependence on nuclear power. The efficiency (of the traditional

energy use) is too low and the energy carriers are exhaustible, create pollution and takes up large

amounts of land. It seems foreseeable that in future one cannot rely any longer alone on the

possibilities used so far. Fossil fuels reserves are limited, and it has been predicted that

production of fossil fuel will peak around 2020 and then decline.46

Although the light-water

based nuclear power is clean, it produces toxic solid waste with a half life of 24, 110 years.47

Therefore, hopes are pinned to hydrogen as a clean and safe alternative for energy production.

Fuel cell science and technology are emerging as a key component of an economy based on

renewable energy.13-15

A fuel cell is an electrochemical device, which converts the free energy of a chemical

reaction into usable electrical energy. Due to high efficiency of energy conversion, low pollution

level, low noise, and low maintenance, costs render fuel cells preferable over other energy

conversion devices. When fuel cells are powered by hydrogen derived from renewable sources

(solar, wind, biomass etc.) or with alcohols such as methanol and ethanol, has negligible adverse

impact on the environment and economy sector.48

Thus, there are considerable research efforts

by industrial developers and world governments to develop and commercialize fuel cell

technology.48, 49

Sir William Grove invented the fuel cell in 1839, but it was in the middle of the

twentieth century when Bacon’s pioneering work led to the use of fuel cell in the space mission

(Gemini and Apollo space program by NASA). Today fuel cells are explored as energy

conversion devices for stationary, automotive, portable, and military power applications.

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There are many different types of fuel cells including proton exchange membrane fuel

cells (PEMFCs), direct methanol fuel cell (DMFC), solid oxide fuel cells, phosphoric acid fuel

cells and alkaline fuel cells as well as biofuel cells. A simplistic view of a fuel cell is shown in

Figure 1.1. The membrane electrode assembly (MEA) is a key component of a fuel cell

comprised of the polymer electrolyte membrane (PEM) sandwiched between gas diffusion

layers, and catalysts. The catalyst layer (typically platinum or platinum alloys) act as electrodes,

anode, and cathode to which the fuel and oxidants are supplied respectively. Gas diffusion layers

are carbon fiber cloths or paper that acts as a medium to disperse the fuels evenly across the

polymer electrolyte membrane. The polymer electrolyte membrane acts as a proton conductor

and prevents the mixing of fuels across the electrodes. The power is generated via coupling of

complimentary oxidation-reduction reactions via an electrolyte. Fuel is oxidized at the anode to

produce protons and electrons. The electrons travel through the external circuit where they can

power an electrical load on the way to the cathode. Protons travel through PEM and the electrons

from the external circuit react with oxygen at the cathode to form water. The proton across PEM

migrates via the Grotthuss hopping mechanism, which explains the unusual fast diffusion of

protons through a PEM.50

Therefore, the fuel cell performance is governed by the microstructure

properties of MEA.

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Figure 1.1: PEMFC Schematic.51

The Nafion PEM is sandwiched between the anode and

cathode catalysts.

Fuel cell have two interrelated major drawbacks; cost and lifetime.48

The state-of-the-art

catalysts are platinum and platinum alloys, which are extremely expensive. Most widely used

membrane electrolyte in PEMFC and DMFC applications is Nafion. The research also focuses to

investigate the degradation of the MEA components by postmortem analysis of any changes in

the membrane and catalyst over long time run in fuel cells.

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1.3 Research Objectives

The scope of this thesis focuses on IR spectroscopy of the Nafion and the durability study

on the performance degradation of MEA in DMFCs.

1.3.1 Aim 1: Mechanically coupled Internal coordinates of Ionomer Vibrational Modes52

: Key

Nafion IR peaks are unambiguously assigned in terms of mechanically coupled internal

coordinates of near neighbor functional groups. Chapter 2 discusses the technique developed for

correlating experimental IR peaks to the density functional theory (DFT) calculated peaks and

assignment of those peaks by visualization of mechanically coupled internal coordinates. 16, 52

This enabled the proper analysis of the IR spectra of Nafion and an incomplete understanding of

correlations between the exchange group environment (e.g. extent of hydration, nature of

exchange group cation, etc) and frequencies.

1.3.2 Aim 2: Elucidating the Ionomer-Electrified Metal Interface53

: Chapter 3 focuses on the

augmentation of the above methodology with polarization modulated infrared reflection-

adsorption spectroscopy (PM-IRRAS) to elucidate the ionomers functional groups that

participate in self assembly of Nafion onto Pt surfaces.

1.3.3 Aim 3: IR spectroscopy of the Ion Exchange Membrane54

: The objective of the chapter 5

is the augmentation of the DFT-spectroscopy method developed in chapter 1 with ion-exchange-

induced shifts for extensive assignments of Nafion peaks (explicit and deconvoluted).

1.3.4 Aim 4: Durability studies on the performance of MEAs in DMFC: The objective of

chapter 5 is to understand the origin and location of degradation and factors that determine the

lifetime of the fuel cell. The major goal is to characterize the catalyst-polymer interfacial region

and identify degradation modes.

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Alkaline membrane fuel cells (AEMFCs) are attracting attention due to their potential to

replace the acidic PEMFCs because of improved kinetics at the cathode side. The use of anion

exchange membranes (AEMs) in AFMCs improves the electrochemical kinetics by operating at

elevated pH, which eliminates the use of platinum-based catalysts. Future research is suggested

to study the structure and hydration properties of AEM fragments at different degrees of

hydration. The correlations of the experimental and theoretical techniques will present useful

information for understanding OH- dissociation and transfer in exchange membranes.

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CHAPTER 2: Mechanically Coupled Internal Coordinates of Ionomer Vibrational Modes

2.1 Introduction

Nafion, a sulfonated tetrafluoroethylene copolymer, and its short-side-chain derivative

revolutionized low temperature fuel cell development. Nevertheless, after over 7500 publications

on Nafion since 1970,55

the proper assignment of Nafion vibrational modes (and ionomers in

general), free-standing or interfaced to metal surfaces, has been elusive. The focus of this chapter

is the definitive assignment of key infrared (IR) peaks, including those associated with the SO3-1

exchange group and the ether linkages. Nafion and relevant derivatives are schematized in

scheme 2.1.

Scheme 2.1: Structures of Nafion and derivatives, R: -SO3H (Nafion), -SO2F (sulfonyl

fluoride), and -SO2N(H)SO2CF3 (-sulfonyl imide).

The highlights of reported interpretations of selected IR spectra (Fig. 2.1a-f) provides context

for this chapter. The attenuated total reflectance (ATR) spectra of hydrated Nafion (a) and the

short-side-chain ionomer (b) (i.e., scheme 2.1) focus on the ~1060 cm-1

and a multiplet that

includes a shoulder at 995 cm-1

and two peaks ~983 and 970 cm-1

, hereafter referred to as

and , respectively. The νhf and νlf have been conventionally assigned to ether groups in

proximity to the backbone and the sulfonate group, respectively. Cable et al.2 associated to the

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ether linkage closest to the sulfonate group because of its enhanced sensitivity to ion exchange

and the fact that the persists in the Dow short-side chain ionomer spectrum. The short-side

chain ionomer has only one ether group, positioned adjacent to the sulfonate group. The sulfonyl

fluoride precursor was also compared to Nafion. In the sulfonyl fluoride spectrum (c),

diminishes concurrently with the ~1060 cm-1

. These observations were reconciled by invoking

solvation effects as responsible for the sensitivity of to ion exchange because in hydrated

Nafion, the sulfonate group is embedded in an aqueous phase. Therefore, the ether group in

closest proximity to the sulfonate group may be subject to solvation as well and thus sensitive to

ion exchange. The , which is essentially insensitive to ion exchange, has been attributed to the

ether link distant from the sulfonate group. Further, Cable2 concluded that the concurrent loss of

the 1060 cm-1

peak is due to the loss of the SO3- symmetric mode. Byun et al.

4 also reported the

same loss of the upon substitution of the sulfonic acid group for a sulfonyl imide (fig 2.1 f),

and assigned as did Cable et al. (Fig. 2.1e, f).

Figure 2.1. (a) Nafion-H, (b) short-side-chain ionomer (c) sulfonyl fluoride, (d) Nafion-H, (e) H+

form of Nafion, (f) sulfonyl imide Spectra a-d adapted from Cable et al. Spectra e and f adapted

from ref. 4

The association of 1060 cm-1

peak and , solely with SO3- and ether link modes,

respectively, precludes proper analysis of the spectra. However, if the mechanical coupling of

23

the internal coordinates of the SO3- and its near-neighbor COC are considered, the analysis of

1060 cm-1

and peaks of Figure 2.1 can be reconciled without the need for invoking solvation

of the ether link vide infra. Warren and McQuillan3 noted the importance of the considering

vibrational contributions from more than one functional group when assigning IR absorptions of

fluoropolymers. This motivated us to examine internal coordinate coupling of ionomer

functional groups, resulting in the reassignment of the two important Nafion peaks.

2.2 Experimental

2.2.1 Transmission Spectroscopy

Transmission infrared spectra of Nafion 112 were obtained on a Bruker Vertex 80V

Spectrometer (Bruker Optics Inc, Billerica, MA) under dry air or vacuum. The spectra were

signal averaged from 100 scans at 4 cm-1

resolution with a dry-air purge at ambient temperature.

The Nafion samples were dehydrated on a vacuum line at 10-2

torr (under nitrogen) at 135 °C for

several hours. Samples were transferred to a dry box for sample holder installation in order to

minimize atmospheric exposure.

2.2.2 Computational method

Unrestricted DFT5,6

with the hybrid X3LYP7 functional was used for geometry

optimization and calculations of the normal mode frequencies and corresponding IR spectra of

triflic acid, the Nafion side chain (NSC), and the NSC with a PTFE backbone segment (NSCB).

The calculations were done at water/sulfonate ratios ()8 from zero to 10. The option to include a

dielectric in the calculation was not used because the effect of such an option would be a small

perturbation over the effects due to sequential addition of water molecules to the solvation

sphere. The X3LYP extension of the B3LYP9 functional yields more accurate heats of

formation. The all-electron 6-311G**++ Pople triple- basis set is used in all calculations (“**”

24

and “++” denote polarization10

and diffuse11

basis set functions, respectively). Jaguar 6.5

(Schrodinger Inc., Portland, OR) uses the pseudospectral method

12 for calculation of time-

consuming integrals with the same accuracy as the fully analytical DFT codes. Images of the

geometry optimized NSC and NSCB anion are shown in Figure 2.2.

Figure 2.2. NSC λ= 3 (Left), NSCB anion (Right) (Red= O, White= H, Grey= C, Purple= S,

Green= F, Yellow dotted lines = H-bonds)

2.3 Result and Discussions

Figure 2.3 shows the transmission spectra of Nafion 112 in hydrated and dehydrated

state. The transmission spectra show a concurrent loss of intensity of 1062 cm-1

and due to

dehydration of the membrane, simultaneous with evolution of peaks at 1415 and 908 cm-1

. The

transition of the dehydrated (red) to the hydrated (blue) spectrum is attributed to a change in the

point group symmetry of the sulfonic acid group vide infra. The following density functional

theory (DFT) calculations show that as the proton dissociates from the sulfonic acid group (e.g.,

with hydration), the local point group symmetry changes from C1 to C3V. The transition to C3v

symmetry with hydration is confirmed by transmission IR spectroscopy.

25

Figure 2.3. Transmission IR spectra of Nafion 112 showing the evolution of 1415 cm-1

and 908

cm-1

bands upon dehydration

Figure 2.4 shows DFT optimized structures of the triflic acid exchange site as water

molecules are sequentially added. The triflic acid calculations reveal a threshold λ (λd) where the

SO-H bond dissociates (Fig. 2.4 top right), and a λi-o, where the H3O+ loses a direct hydrogen

bond to the sulfonic acid anion (Fig. 2.4 bottom right.)

26

Figure 2.4. Proton dissociation threshold and the formation of H3O+ and C3V local symmetry.

Top: λ= 0, left; λ=3, center; λ=4, right. Bottom: λ= 7, left; λ=9, center; λ=10, right. (Red= O,

White= H, Grey= C, Purple= S, Green= F, Yellow dotted lines are hydrogen bonds.

Although hydrated Nafion has C1 symmetry overall, it has regions of local symmetry –

namely the -SO3–

(C3V) and the ether groups (C2V). Thus, smaller molecules composed of these

fragments were optimized, and an understanding of their properties was transferred to Nafion.

Fundamental vibrational modes and IR intensities have been calculated for all optimized

geometries. Maestro (Schrodinger Inc., Portland, OR) converts Jaguar output files to vibrational

mode animations. The full animations of selected calculated modes are in the supplemental

material (Movie 2.1-2.5). Snapshots of the CF3SO3- symmetric stretch (s(A1)) and the CF3OCF3

asymmetric (as(B2)) and rocking modes (r(B2)) and associated frequencies are shown (Fig. 2.5,

top row).

27

Hereafter the s(A1), as(B2), and r(B2) modes are referred to as “pure modes.” The equilibrium

positions and vibrational mode extrema of NSCB modes corresponding to the bands at 969 cm-1

( ) and 1060 cm-1

(Fig. 2.2) are shown in Figure 2.5. The animations enable visualization of

how the pure mode internal coordinates mechanically couple to yield the NSCB modes. The

center row snapshots show a 983 cm-1

NSCB mode1 that results from the coupling of the s(A1),

as(B2), and r(B2) with the dominate mode being the s(A1). The full animations show that the

dominate pure mode of the 1060 cm-1

peak is actually the CF3OCF3 (as)(B2) mode with a much

weaker contribution from the s(A1) of triflic acid. The 1060 cm-1

is primarily a as(B2) mode

mechanically coupled to the internal coordinates of the s(A1) of the SO3- group. The key point is

that the ether link nearest the exchange group has internal coordinates that are mechanically

coupled to the s(A1) mode: The 1060 cm-1

and peaks cannot be purely ascribed to the SO3-1

and COC modes, respectively.

In fact, , conventionally assigned as an ether mode, derives from the triflic acid SO3-1

s(A1) mode with a calculated average of 974 cm-1

(see Table 2.1).

Table 2.1. Calculated s(A1) mode of C3V (i.e., d or greater) sulfonate.

1 This calculated value corresponds to the experimental 969 cm

-1 not to be confused with the experimental 983 cm

-1

H2O/SO3- (λ) Triflic acid SO3

- vs (cm-1)4 9785 9656 9807 9648 9799 980

10 975

28

Figure 2.5: Maestro animation snapshots of DFT calculated modes of the full side chain and

backbone: Top row: Small molecule “pure” modes; Middle row: 983 cm-1

; equilibrium positions

in center panel. Bottom row: 1060 cm-1

; equilibrium positions in center panel. Extrema at left

and right panels of center panels. AVI animations in supplemental material.

The above analysis obviates the need to invoke ether link solvation for analysis of the Figure

1 spectra. Because the internal coordinates of the 1060 cm-1

and peaks are mechanically

coupled, they always shift (upon ion exchange of the SO3- group) or diminish together (upon

dehydration, Fig. 2.3).

Reconsider the spectra of Figure 2.1 in light of mechanically coupled internal coordinates.

The is not in the short-side-chain spectra (b) because was backbone the ether link that was

not mechanically coupled to the SO3-. In the short-side-chain derivative, the remaining is now

the backbone ether link mechanically coupled to the SO3- group (i.e., the functional groups

29

responsible for the mechanically coupled s(A1), as(B2), and r(B2) are now in close proximity

to the backbone). In the sulfonyl fluoride spectra (c), the C3V symmetry of the SO3- is lost.

Thus, similar to the case of dehydration (Fig. 2.3), the and 1060 cm-1

peaks vanish. The

sulfonyl imide spectrum (f) behaves similarly to that of the sulfonyl fluoride. The remaining

peak at 1069 cm-1

is not inconsistent with our analysis. Korzeniewski confirmed this peak as the

asymmetric S-N-S stretch.

Warren and McQuillan3 recognized that Nafion vibrational modes have contributions from

multiple functional groups using DFT at the B3LYP/6-311G+(d,p) level of theory. Their

calculated 929 cm-1

mode was assigned to a coupling of C-S stretching and SO3- symmetric

stretching to explain the loss of the 971 cm-1

band upon dehydration. Okamoto19

calculated peaks

at 989 and 1060 cm-1

as SO3- symmetric and COC (nearest the head group) asymmetric

stretching, respectively for a model side chain of Nafion in its anion form,

(CF3)2CFOCF2CF(CF3)OCF2CF2SO3- , using B3LYP/6-31G(d,p)++.

2.4 Conclusions

The 1060 cm-1

and peaks result from the mechanical coupling of the internal

coordinates of SO3- and the COC “pure” modes. The 1060 cm

-1 mode is dominated by an ether

link mode. The calculated mode at 983 cm-1

, a major contributor to the peak, is dominated by

the SO3- s(A1) mode. The consideration of mechanically coupled internal coordinates is essential

for the analysis of infrared spectra of ionomers, and correlation of those spectra with the effects

of ion exchange and state of hydration.

30

CHAPTER 3: Elucidating the Ionomer-Electrified Metal Interface

3.1 Introduction

Nafion, a perfluorosulfonic acid ionomer have dominated the modern-day polymer

electrolyte membrane fuel cells (PEMFC) industry as the electrolyte of choice.56

Their properties

are paramount to the successful operation and commercialization of fuel cells. Besides from

conducting protons and acting as a fuel separator between the electrodes, Nafion enhances

electrocatalytic activity of the catalyst ink in the PEMFCs.56, 57

A solubilized version of Nafion58

(ionomer solution) is often used to prepare catalyst “inks” in PEMFCs to support the

electrocatalytic layers.59

The ionomer-metal interface formed after evaporation of the ink

solvent is central to PEMFC electrocatalysis. Although the ionomer-metal interface plays crucial

roles in the fuel cells, little is known about ionomer-metal interfacial structure.

Markovic and co-workers probed Pt-electrolyte interfaces by measurements of CO oxidation

currents, in sulfuric, perchloric and KOH solutions, synchronized with IR absorption-reflection

spectroscopy of linear (νCOl) and bridge bound (νCO

b) COads on Pt(111).

60 More recently their

electrochemical studies on Pt(hkl)-Nafion interfaces suggest that the Nafion sulfonate group

adsorbs onto the Pt surface.61

That the active state of a catalyst only exists during catalysis62

,

operando infrared spectroscopic techniques was used by Kendrick et. al characterization of the

catalyst-ionomer interface.53

The Stark tuning curves of CO on Pt electrode53

, similar to those

acquired in an electrochemical cell by Markovic and co-workers61

, demonstrated an adsorption

phenomenon onto Pt despite the lack of mobile ions typical of dilute sulfuric acid solutions. This

suggests a need for elucidation of Nafion functional groups responsible for modulation of CO/Pt

interactions (i.e., Stark tuning).53

The focus of this chapter is to correlate the attenuated total

31

reflectance spectroscopy of Nafion 117, and polarization modulated IR reflection absorption

spectroscopy of Nafion spin-coated onto Pt with density functional theory calculated Nafion

spectra to elucidate Nafion functional group co-adsorption responsible for the Stark tuning of

COads on high surface area fuel cell electrodes. A model is suggested for a Nafion adsorption

onto Pt based with appropriate dihedral and native bond angles, consistent with experimental and

calculated spectra.

3.2 Experimental

3.2.1 Attenuated total reflectance (ATR) spectroscopy

A surface pressure of 815 psi was maintained over the 1.8 mm diameter ATR crystal.

Spectra were obtained using a Bruker™ Vertex 70 and Vertex 80V vacuum FTIR spectrometer

(Bruker, Billerica, MA). A MIRacle™ ATR accessory (Pike Technologies Spectroscopic

Creativity, Madison, WI) with a ZnSe ATR crystal was used. The spectra were signal averaged

from 100 scans at 4 cm-1

resolution with a dry-air purge at ambient temperature. Atmospheric

compensation (to eliminate H2O and CO2 interference in the beam path) was used in all

measurements. Data processing for all infrared data was done with the Bruker™ OPUS 6.5™

software.

3.2.2 Preparation of arc-melt Pt*

The preparative method for arc melted electrodes has been described.63

Briefly, the arc-

melter (Materials Research Furnaces, Sun Cook, NH) was charged with 3 mm Pt shot (99.9+%,

Sigma-Aldrich, St. Louis, MO) The chamber was evacuated to -29 psig and purged with argon

three times. The Pt was arc-melted at 75 amps under an Ar bleed. The chamber was vented to

Note- * Work done by Adam Yakaboski

32

flip and arc-melt the sample three times. The Pt slug was epoxied (Devcon HP250, Danvers,

MA) to a modified glass syringe barrel, cut flat using a diamond cut-off saw (Buehler IsoMet

1000, Lake Bluff, IL), and finally polished to a mirror finish using 0.05 µm aluminum oxide

(Magner Scientific, Dexter, MI). The electrode was sonicated in Nanopure™ water (Milli-Q,

Billerica, MA) for 10 min. Nafion ionomer solution (20µL) was pipetted onto the Pt electrode

assembly mounted on an inverted electrode rotator (Pine Instrument Company, Grove City, Pa)

and then rotated (1000 rpm, one min).

3.2.3 Polarization modulated infrared reflection absorption spectra (PM-IRRAS)*

The Vertex 80V spectrometer was equipped with a Hinds II/ZS50 photoelastic modulator

(Hinds Instruments, Hillsboro, OR), SR830 lock-in amplifier (Stanford Research Systems,

Sunnyvale, CA) and a D3131\6 MCT detector (Infrared Associates, Stuart, FL). The angle of

incidence was 60° and the photoelastic modulator frequency was 50.14 kHz. The PM-IRRAS

cell design have been reported.64, 65

Spectra were averaged (710 scans; 4 cm-1

resolution). Li-

exchanged Nafion was prepared by soaking Nafion samples in 0.1M salt solutions.

3.2.4 Computational method

Unrestricted DFT66, 67

with the X3LYP68

functional was used for geometry optimization

and calculations of the normal mode frequencies and corresponding IR spectra of the

deprotonated and protonated NSC and NSCB. Output files were converted to vibrational mode

animations using Maestro (Schrodinger Inc). Details concerning the computational strategy have

been reported in chapter 2.52

Note- * Work done by Adam Yakaboski

33

3.3 Results and Discussion

3.3.1 Analysis of IR spectra

The DFT calculated spectrum of a 55-atom Nafion side-chain and backbone segment

provides 159 normal mode frequencies and intensities. Figure 3.1 shows the theoretically derived

peak positions and intensities (black lines) superimposed upon the ATR spectrum (red line) of

hydrated Nafion.

Figure 3.1. DFT calculated normal modes (black lines) and Nafion ATR spectrum (red).

PM-IRRAS enhances (relative to the ATR) vibrational modes of functional groups

ordered by the Pt surface. Figure 3.2 shows the ATR spectrum (red), the PM-IRRAS spectra of

34

Nafion-H/Pt interface (grey line), Nafion-Li/Pt interface of Li+ exchanged Nafion (blue line) and

6 selected (from the 159 calculated) DFT calculated frequencies and intensities.

Figure 3.2. Theoretical and experimental spectra. ATR of hydrated Nafion (red); PM-IRRAS of

Nafion-H on Pt (grey); PM-IRRAS of Nafion-Li on Pt (blue); Selected DFT peaks (black lines 1-

6).

The low-frequency ATR band (Fig. 3.2) at 971 cm-1

(corresponding to theoretical 984

cm-1

; line-1) and the 1056 cm-1

band (corresponding to theoretical 1059 cm-1

; line-3) are due to

the mechanically coupled internal coordinates of sulfonate and COC (A) ether link .52

Thus

these peaks shift concertedly with changes in the sulfonate environment. Consider the ATR and

PM-IRRAS spectra of the protonic form of Nafion (Fig. 3.2, grey line). The 1056 cm-1

and 971

cm-1

peaks concertedly shift to higher frequencies in the PM-IRRAS because of the interaction of

the sulfonate functional group with the Pt surface. A similar effect is observed with Li+

exchange of the adsorbed Nafion (blue line).

A convention for correlating PM-IRRAS enhanced peaks to the calculated DFT peaks

enabled the identification of functional groups ordered by the Pt surface: The association of

observed PM-IRRAS peaks with DFT peaks, assigned by visualization of mechanically coupled

35

internal coordinates,16, 52

provides the basis for such a convention. Normal mode coordinate

animations (generated by Maestro from DFT output files) explicitly show how neighbor

functional groups (called out in scheme 1.1) are mechanically coupled. The calculated internal

coordinates are viewed in the context of calculated normal modes of relevant small molecules

(e.g., triflic acid, CF3OCF3, 10-carbon CF2 backbone, etc.) hereafter are referred to as “pure

modes,” which serve as the basis-elements for assigning DFT calculated normal modes

associated with observed peaks. Figure 3.3 shows the assignments of the 6 selected DFT peaks

and snapshots of the corresponding Maestro animations. The atoms contributing to the

dominating motion (black circles) and the next most significant atom motions (dotted circles)

comprise pure modes that form the basis for the assignments. An alternate strategy for

determining the dominant mode is to consider the contribution to the potential energy surface on

an atom by atom basis.69

While this may change the selection of the dominant mode, it does not

alter what pure modes contribute to the assignments. The correlation of the DFT to PM-IRRAS

peaks (Fig. 3.3) and the resulting assignments in terms of the mechanically coupled modes are

tabulated in Table 3.1.

36

Figure 3.3. Normal mode coordinate animation snapshots of the Nafion side-chain anion and

backbone fragment (see scheme 1). Left and right views are extrema positions of the vibrational

mode. Functional groups associated with the dominant internal coordinates and next most

significant motions are designated by solid and dotted boundary lines respectively.

The pure mode peak assignments (Table 3.1) elucidate functional groups ordered by the

Pt surface. Animations of the pure modes and the internal coordinates of the 6 selected peaks are

in Movies 3.1-3.12 and Movies 3.13-3.18 respectively in Supporting Information as .AVI files.

The rational for the key functional group assignments (Table 3.1) is supported by the overlap of

the DFT calculated peak positions with the PM-IRRAS peaks. Consider the DFT and PM-

IRRAS peaks in the contexts of the bulk-Nafion ATR and the report by Cable et al.21

that the

1056 cm-1

and 971 cm-1

peaks shift with alterations of the sulfonate group environment. The bulk

ATR peak at 1056 cm-1

(red), the PM-IRRAS peak of protonated Nafion adsorbed on Pt (grey

line) at 1061 cm-1

and the PM-IRRAS peak of lithiated Nafion adsorbed on Pt (blue line) at 1077

2. 992 cm-1 CF2

(BBdef) + COC(B) δs

6. 1322 cm-1 CF2 δs (BBdef)

1. 984 cm-1 SO3

- s + COC (A) as + COC (B) ρr

3. 1059 cm-1 COC (A) as + SO3

- s

4. 1168 cm

-1 CF2 δs (BBdef) + CF2 ρr (SCdef) + COC(A)

5. 1254 cm-1 CF3

as + COC (A) δs + COC (B) as

37

cm-1

(Fig. 3.3) confirm that Pt surface atoms induce frequency shifts, as do extent-of-hydration52

and ion exchange of Nafion.21

Thus the PM-IRRAS enhances bulk-Nafion-modes that are

shifted due to functional group interactions with Pt. Less explicit than the 1056 cm-1

peak, are

PM-IRRAS peaks derived from bulk-Nafion-modes that are convoluted within the Nafion ATR

broad envelop region (1100 - 1300 cm-1

), in particular the 1164 and 1260 cm-1

PM-IRRAS peaks.

Di Noto et al.70

extensively deconvoluted the broad envelop region. Their resulting peak library

includes 1148, 1245 cm-1

, which could be reconciled with an association of the DFT peaks (Fig.

3.3, line-4 and line-5 ) with the shifted PM-IRRAS peaks at 1164 and 1260 cm-1

.

Symmetric stretch, s; Asymmetric stretch, as

Wagging, ; Scissoring, δs; Twisting, ; Rocking, ρr

Backbone deformation, BBdef; Side-chain deformation, SCdef; Backbone Stretching, BBstre

Table 3.1. PM-IRRAS and DFT IR adsorption peaks and assignments.

3.3.2 Nafion/Pt adsorption model

The animation of the theoretical peak at 1254 cm-1

(Fig. 3.3, line-5), associated with PM-

IRRAS peaks at 1260 cm-1

(blue and grey lines), suggests that the CF3 internal coordinates

dominate the normal mode. The insensitivity of the 1201 cm-1

peak, to ion exchange, suggests

Wavenumber (cm-1) Pure Mode Components

PM-IRRAS DFT

1 971 984 SO3- s + COC(A) as + COC(B) ρr

2 984 992 CF2 (BBdef) + COC(B) δs

3 1061 1059 COC(A) as + SO3- s

4 1164 1168 CF2 δs (BBstre) + CF2 (BBdef) ρr + COC(A)

5 1260 1254 CF3 as + COC(A) δs + COC(B) δs

6 1322 1322 CF2 δs (BBdef)

38

that the internal coordinates are not substantially coupled to the sulfonate group. The 1260 cm-1

band-intensity is over an order-of-magnitude greater than that the cluster of peaks (i.e.,

associated with theoretical lines 1 and 2) that are mechanically coupled to the sulfonate pure

mode: The CF3 functional group is a co-adsorbate of comparable importance to the sulfonate

exchange group in formation of the Nafion/Pt interface. Further support for this model is

provided by Mulliken population71

analysis. Atomic charges of the 55 Nafion fragment atoms

were calculated. Table 3.2 shows the average charges of the backbone, side chain and CF3

group fluorine atoms and the average charges on the sulfonate oxygen atoms. The charges for

chemically equivalent atoms (e.g., CF3 fluorine and sulfonate oxygen atoms) differ because the

calculations are done for the lowest energy Newman projections where the atomic environments

are different for chemically equivalent atoms because of the absence of symmetry in the full

molecule. The chemically equivalent atoms have smaller charge standard deviations as would be

expected. The average charge of the CF3 fluorine atoms are the highest amongst the three

classes of fluorine atoms (Table 3.2) and are about 18% that of the sulfonate oxygens.

Table 3.2. Average partial charges of selected Nafion segments

A Gaussian 03 Viewer (Gaussian, Wallingford, CT) used to construct a 2-equivalent

(1100 g/equiv) model of Nafion 117, enables rotation of dihedral angles while maintaining the

native bond angles associated with each functional group. The CF3 and SO3- groups, oriented

with the two planes defined by the CF3 fluorine and sulfonate oxygen atoms parallel to a Pt

Segment Backbone (F) Side-chain (F) CF3 (F) Sulfonate (O)

13 atoms 8 atoms 3 atoms 3 atoms

Avg. Partial Charge -0.0665 -0.0816 -0.0876 -0.4879

Standard Deviation 0.027 0.055 0.013 0.012

39

surface, effect ordering of the CF2 backbone segments with respect to the Pt surface. Ordering of

the CF2 groups would be expected to yield PM-IRRAS peaks. The PM-IRRAS peak at 1164 cm-

1 is associated with the theoretical peak (line-4) at 1168 cm

-1. The line-4 animation shows that

CF2 backbone internal coordinates dominate the 1168 cm-1

mode, supporting the suggestion of

ordered CF2 groups. Figure 3.4 is the Gaussian View model resulting from orienting the CF3 and

SO3- groups for adsorption to the Pt surface. The numbers (yellow) associate DFT calculated IR

peaks (line-1 – 6, Fig. 3.3) and associated PM-IRRAS peaks with regions of order induced by the

CF3 and SO3- functional group adsorbates.

Figure 3.4. Gaussian 03 Viewer Nafion-Pt interface model. Oxygen (red), Sulfur (yellow),

Fluorine (light blue), Carbon (grey), Pt (dark blue). *

The ordering of the backbone CF2 groups in the Gaussian model is a natural consequence

of adjusting the dihedral angles of the anchoring groups for adsorption, while maintaining

functional group native bond angles. From this evolves a model shown in figure 3.4 that

confirms comparable importance of the roles of the Nafion side-chain CF3 and sulfonate groups

Note- * Work done by Ian Kendrick

40

as co-adsorbing functional groups. The details of exactly how adsorbed CF3 functional groups

influence the operando Stark tuning curves are not yet established. The low density of

functional group adsorption sites, relative to the number of backbone CF2 groups suggests an

explanation as to why Nafion is observed to enhance electrode processes.4, 56

3.4 Conclusion

This work elucidates the Nafion ionomers functional groups that participate in self

assembly of Nafion onto Pt surfaces. PM-IRRAS of Nafion-Pt interfaces, and ATR

spectroscopy of Nafion, correlated with DFT calculated normal mode frequencies confirm that

Nafion side-chain sulfonate and CF3 co-adsorbates are structural components of the Nafion-Pt

interface. These “anchoring” functional groups reduce degrees of freedom available for

backbone and side-chain CF2 dynamics. The partial ordering of Nafion CF2 groups is supported

by observed PM-IRRAS and DFT calculated peaks possessing vibrational internal coordinates

dominated by, and mechanically coupled to side-chain CF2 group motions.

41

CHAPTER 4: IR Spectroscopy of Ionomers

4.1 Introduction

Infrared (IR) spectroscopy is most widely used analytical technique to study the

ionomers. It is a very powerful tool for the nondestructive characterization and measurement of

the physical and chemical properties of a polymeric system and provides better understanding of

the relation between the molecular structure and its macroscopic properties. Bower and

Maddams explain how infrared spectroscopy (IR) can elucidate purely organic polymer

structures at a number of levels from specific normal modes to molecular configurations in

crystalline and amorphous regions.72

IR methods benefit from the relative simplicity of spectra

that result from the repetitive nature of the polymer chain: the number of the peaks is about 3n,

where n is the number of atoms in the repeat unit, rather than 3N, where N is the number of

atoms in the whole molecule. The intensity and widths of the bands provide information

regarding macro conformations of the polymer.72

Ionomers provide an interesting twist to the application of infrared spectroscopy for

physical structure determination. One might expect that the pendant exchange group increases

the complexity of the analysis, and indeed assignment of exchange-group functional groups have

been the source of a great deal of confusion.33

In fact, ion exchange properties can be used

advantageously for assignment of ionomer peaks. This work shows how ion-exchange-induced

shifts of IR bands (including deconvoluted bands) simplify the correlation of those bands to

theoretically calculated normal modes. Alterations of the exchange group environment are

mechanically coupled to neighboring functional groups.52

This mechanical coupling is quantified

by density functional theory (DFT) calculation of the ionomer repeat group IR spectrum. The use

42

of ion-exchange-induced shifts enables the assignment of all 3n ionomer normal modes.

Eventually, analysis of peak widths and intensities will provide ionomer macro conformational

information. Therefore the focus of this chapter is the extensive assignments of Nafion IR peaks

(explicit and deconvoluted) in terms of mechanically coupled internal coordinates of near

neighbor functional groups. The assignments are based on the correlation of the ion-exchange-

induced shifts in observed ATR peaks with the DFT spectroscopy method.

4.2 Experimental

4.2.1 Attenuated total reflectance (ATR) spectroscopy

The protonated form of Nafion 117 (1100 eq. wt, .007 in.), obtained from E. I. DuPont

was pretreated for attenuated total reflectance (ATR) spectroscopy. Nafion-117 was immersed in

boiling ~8 M nitric acid for 20 min, rinsed with Nanopure™ water, and finally immersed in

boiling water for one hr. The Nafion samples were ion-exchanged in 0.1M salt solutions of the

respective cations under ambient conditions. The spectra were obtained in both the hydrated and

dehydrated form (e.g., in vacuum at 115°C for 3 hrs.). Spectra were obtained using a Bruker™

Vertex 70 and Vertex a 80v vacuum FTIR spectrometer (Bruker, Billerica, MA). A MIRacle™

ATR accessory (Pike Technologies Spectroscopic Creativity, Madison, WI) with a ZnSe ATR

crystal was used.

Spectra were fit to 100% Lorentz + Gaussian peaks using the Curve Fit program of

OPUS 6.5. The local least square algorithm followed by Levenberg-Marquardt algorithm73

was

used to minimize the variance between the raw and the fitted curves. The broad bands at 1204

cm-1

and 1148 cm-1

and the multiplet at 900-1000 cm-1

(Fig. 3) were deconvoluted. The spectra

were baseline corrected and an initial curve fit was used to estimate the numbers of bands using

the peak positions visually selected based on the location of DFT normal modes in the broadband

43

region. The initial guesses are then auto fitted to yield calculated maxima and intensities. The

calculated intensities are discarded and the maxima obtained from the fit are then used to initiate

the fitting routine. The fitting routines were repeated to minimize the RMS. In the case of the

two broad peaks at 1204 and 1148 cm-1

, 8 peaks were visually selected (from DFT calculated

peaks and experimental work) and deconvoluted using above procedure. Similarly, the medium

sized envelope in the region of 940-1020 cm-1

was deconvoluted into three peaks. This algorithm

provides identical initial guesses amongst the variety of researchers on the project.

4.2.2 Computational method

Same as in Chapter 2.

4.3 Results and Discussion

Figure 4.1 shows the 3n calculated normal modes (excluding those with negligible

intensity) of the 55 atom Nafion repeat group superimposed upon the Nafion ATR IR spectrum.

The theory and experimental data exemplifies the simplicity of polymer spectra as described by

Bower and Maddams.72

Figure 4.1. DFT calculated normal modes (black lines) and Nafion 117 ATR spectrum (red)

44

Figure 4.2 (a) shows the ATR spectra (900-1300 cm-1

) of dehydrated cation exchanged

Nafion. Ion exchange alters the polarization of the sulfonate S-O bonds, leading to frequency

shifts and changes in the absorption intensities.20

Waters of solvation shield both the sulfonic

acid group and the cations, thus diminishing ion-sulfonate pairing. Thus, the degree of hydration

and cation type has a significant effect on vibrational mode internal coordinates that include the

SO3- group.

17, 74, 75 The ion-paring strength depends on the cation size and the sulfonate affinity

for the cation.75

Ion-paring affinities are correlated by Pearson’s hard-soft acid-base theory: 76, 77

η= (I-A)/2 (1)

where η, the absolute hardness, is defined as one-half of the difference between the ionization

energy (I) and the electron affinity (A). The η for H+ is defined as infinity. Sulfonate, a hard

base, strongly interacts with hard acids resulting in vibrational frequency dispersion of peaks

with internal coordinates mechanically coupled to the SO3- group (e.g., 1056 cm

-1 and peaks

within the 981 cm-1

multiplet relative to those of protonated Nafion). Figure 4.2 shows the effect

of hydration and cation type on the peak position (Fig. 4.2b) and peak width at half height

(PWHH; Fig. 4.2c) for the 1056 cm-1

band.52

The large absolute hardness of the H+, Li

+ and Ca

2+

ions correlate with substantial polarization of the sulfonate S-O bonds, leading to frequency

shifts and peak widths broadening. However, the transition metal ions (e.g., Pt2+,

Co2+,

and Ru3+)

having low η values are found away from the ionic region and thus polarizes the S-O dipoles to

a lesser extent, hence very less or no variations in the peak position (Fig. 4.2b) and PWHH (Fig.

4.2c) is observed.

45

Figure 4.2. (a) ATR-IR spectra of dehydrated cation exchanged Nafion 117, (b) Ion exchange

induced ν shift in hydrated and dehydrated Nafion™ & (c) Peak width at half height (left scale)

and η (right scale) vs. ions. The absolute hardness of the H+ ions is ∞.

The sensitivity of peak positions and widths to hydration and cation type vary with extent

of coupling with the sulfonate group coordinates. The distant polymer backbone modes are least

sensitive to ion exchange. In order to estimate the sensitivity of the peak position to the cation

type the standard deviation (σ) for hard acid ions namely H+, Li

+ and Ca

2+ were calculated. The

threshold value for the sensitivity of the peak position is calculated to be σ =1.5 cm-1

, any peak

Hydrated

Dehydrated η∞

(a)

(c)(b)

46

with σ above 1.5 cm-1

is considered to be coupled with the sulfonate group coordinates. The

correlation of peak sensitivities, to density functional theory calculated internal coordinates

provide an avenue for assignment of IR peaks due to explicit and deconvoluted peaks.

Figure 4.3. DFT calculated normal modes (red lines) and Nafion ATR spectrum (black).

Deconvoluted peaks (grey) overlay ATR spectrum. Theoretical peaks selected for best overlap

with deconvolution.

Figure 4.3 shows theoretically derived peaks and intensities (red lines) superimposed

upon the ATR spectrum (black line) of hydrated Nafion over the range of interest to this study

along with deconvoluted peaks. Normal mode coordinate animations for each of the 13 lines

were from DFT output files generated by Maestro (Schrodinger Inc., Portland, OR). The

animated internal coordinates are viewed in the context of animated modes of relevant small

molecules (e.g. triflic acid, CF3OCF3, 10-carbon CF2 backbone, etc.) hereafter are referred to as

“pure modes.” These pure modes serve as basis-elements for assigning DFT calculated normal

modes associated with observed explicit and deconvoluted peaks. The ion exchange induced ν

ATR-IR spectrum

Deconvoluted peaks

DFT calculated line spectrum

47

shifts of the 13 selected ATR peaks are correlated to the calculated DFT absorption peaks is

shown in Figure 4.4, 4.5 and 4.6. The resulting peak assignments are made in terms of the

mechanically coupled modes derived from snapshots of the corresponding Maestro animation.

The atoms contributing to the dominating motion (black circles) and the next most significant

atom motions (dotted circles) comprise pure modes that form the basis for the assignments.

Figure 4.4 shows the effect of counterion type on the frequency of the deconvoluted ATR

peaks (region 900-1000cm-1

) and the sensitivity of frequency shift is reported in terms of the σ.

The lower frequency peak at 971 cm-1

sensitive to ion exchange have σ of 2 cm-1

, whereas the

higher frequency peak at 981cm-1

and at 992 cm-1

are essentially insensitive to ion exchange with

σ lower than 1 cm-1

. The sensitivity of the 971 cm-1

peak, to ion exchange, suggests that the

internal coordinates is substantially coupled to the sulfonate group consistent with the previous

observations. 52, 53

The animation of the theoretical peak at 981 cm-1

(Fig. 4.4 e, line-1), associated with

deconvoluted ATR peaks at 971 cm-1

(Fig. 4.4 c), shows that the peak results from the

mechanically coupled internal coordinates of the sulfonate and COC (A) ether link. The

animation of the theoretical peak at 992 cm-1

(line-2; Fig. 4.4 d), associated with deconvoluted

ATR peak at 981 cm-1

(Fig. 4.4 b), suggests that the peak results from internal coordinate

coupling of backbone CF2 modes. Because the internal coordinates of the theoretical peaks at

992 cm-1

are not mechanically coupled to the distant side chain SO3- group, it is insensitive to ion

exchange effect.

The assignment of the deconvoluted peak at 992 cm-1

still remains open for discussion.

Figure 4.4 shows that the peak position is found to be independent of the cation exchange. The

peak lacks an associated calculated peak and its corresponding normal mode animation file,

48

precludes the assignment of this peak. It is not associated with any of the calculated DFT normal

modes. X-Ray and thermal techniques have confirmed that the perfluorosulfonates and their

precursors are partially crystalline, with a crystal structure similar to that of

polytetrafluoroethylene (PTFE). 20, 78, 79

The appearance of the peak at 992 cm-1

can be taken as

evidence for the formation of the ordered domains in the methylene groups of the Nafion in a

transconfiguration. Bower and Maddams explains that when a polymer is allowed to crystallize

or is subject to physical deformation, sometimes the peaks get broadens or split and sometimes

totally new peaks appear. These changes in the IR peaks are either due a change in the

interaction between the identical chemical repeat units had occurred or conformational change.72

Figure 4.4. Ion exchange induced ν shift on deconvoluted ATR peaks (region 940-1020 cm-1) (a)

992 cm-1 (b) 981 cm-1 & (c) 971 cm-1. The relevant DFT calculated peaks (red lines 1 & 2) are

superimposed upon the deconvoluted peaks. (d & e): Normal mode coordinate animation

snapshots of the Nafion side-chain anion and backbone fragment (see scheme 1) for relevant

DFT calculated peaks (red lines 1 & 2). Left and right views are extrema positions of the

vibrational mode. Functional groups associated with the dominant internal coordinates and next

most significant motions are designated by solid and dotted boundary lines respectively.

(a) (c)(b)

(e)(d)

σ: 2.3σ: 0.6 σ: 0.8

2 992 cm-1

CF2 (BBdef) + COC(B) δs

Wavenumber (cm-1

) Pure Mode Components

ATR DFT

1 972±1m 983 SO3- s + COC(A) as + COC(B) ρr

2 982±1m 992 CF2 (BBdef) + COC(B) δs

1 983 cm-1

SO3- s + COC (A) as + COC (B) ρr

49

The vibrational band at 1056 cm-1

is highly sensitive (σ: 7.2) to the ion exchange (Fig.4.

5 a). The snapshot image associated normal mode animation (red line 3, Fig. 4.5 b ) suggests that

the vibrational band arises from the mechanically coupled coordinates of the sulfonate and COC

(A) ether link.

Figure 4.5. (a) Ion exchange induced ν shifts for 1057 cm-1 ATR peak. The relevant DFT

calculated peaks (red line 3) are superimposed upon the deconvoluted peaks. (b) Normal mode

coordinate animation snapshots of the Nafion side-chain anion and backbone fragment (see

scheme 1) for relevant DFT calculated peaks (red line 3). Left and right views are extrema

positions of the vibrational mode. Functional groups associated with the dominant internal

coordinates and next most significant motions are designated by solid and dotted boundary lines

respectively.

The maxima at 1148 cm-1

and 1200 cm-1

have been conventionally assigned to symmetric

and antisymmetric -CF2 stretching modes respectively, precludes proper analysis of the spectra.

COC(A) as + SO3-s

σ: 7.23

(a)

(b)

3 1059 cm-1

COC (A) as + SO3- s

50

Additional modes are expected for the C3v SO3- group and C2v COC groups including the

symmetric and antisymmetric stretches convoluted within the broad bands of the “CF2” groups.

Hietner-Wirguin80

reported shifts in absorption bands of a shoulder at 1300 cm-1

that varied with

cation type. They associated the peak to the high-energy part of the split antisymmetric vibration

of SO3-, although the high intensity of the nearby CF2 vibrations made it difficult to locate the

exact position of the shoulder. Blanchard and Nuzzo81

also reported these perturbations in the

Nafion difference spectra of films loaded with different counterions. They also observed spectral

changes occurring in 1300-1200 cm-1

region and at 1150 cm-1

and assigned the former to the

SO3- antisymmetric stretching mode and the latter as variations in the CF2 stretching modes

induced by structural rearrangements.81

Korzeniewski et al.32

also reported similar shifts in the

Nafion IR difference spectrum during hydration in 1300-1100 cm-1

region. The strongest features

at 1159 and 1235 cm-1

were attributed to C-F stretching modes of CF2 groups and the 1275-1249

cm-1

region was associated with the SO3- antisymmetric stretching mode.

32 However they were

not able to explain the changes in the CF2 IR absorption band. Gruger et al.19

reported a SO3-

antisymmetric stretching mode at 1135 cm-1

and a mixed CF2/ SO3- stretching mode at 1204-

1235 cm-1

.81

However, deconvolution of the two maxima into several peaks and correlation of

the ion-exchange effect with DFT spectroscopy, explicitly assign peaks in term of mechanically

coupled coordinates of neighbor functional group.

Figure 4.6 shows the effect of ion exchange on the 8 deconvoluted ATR peaks from the

maxima at 1148 cm-1

and 1200 cm-1

. The relevant DFT calculated peaks (red lines 4-13) are

superimposed upon the deconvoluted peaks. The assignments of the selected DFT peaks (red

lines 4-13) and snapshots of the corresponding Maestro animations are also shown in Figure 4.6

(left panel). Figure 4.6 a, b & c (left panel) shows that the deconvoluted ATR peaks at 1130 cm-

51

1, 1146 cm

-1 and 1176 cm

-1 are insensitive to the ion exchange with σ lower than 1.5 cm

-1. The

insensitivity of these peak to ion exchange, suggests that the internal coordinates are not

substantially coupled to the sulfonate group. The snapshot images and animation of the

associated theoretical peaks at 1139 cm-1

, 1146 cm

-1 and 1177 cm

-1 (line 4-6; Fig.4.6 a, b & c

right panel), suggests that the peaks are due to mechanically coupled internal coordinates of

backbone CF2 stretching modes. In agreement with the theoretical peak assignment based on

animations of normal mode coordinate and ion exchange sensitivity (Fig. 4.6 f & g), the

deconvoluted peaks at 1215 cm-1

(σ: 3.0) and 1224 cm-1

(σ: 3.5) are primarily assigned as

internal coordinates of SO3- stretching modes mechanically coupled to nearby COC(A), CF2 and

CF3 functional groups respectively. The variations in the peak position observed are due to direct

sulfonate-cation interaction within the ionomer.

The deconvoluted peaks at 1192 cm-1

, 1202 cm-1

and 1242 cm-1

(Fig. 4.6 d, e & h left

panel) are also insensitive to ion exchange with σ value of 1, 0.6 and 1.2 cm-1

respectively. The

snapshot images and animation of the associated theoretical peaks (line 7, 8, & 11; Fig. 4.6 d, e

& h right panel) suggests that the peaks are primarily due to the mechanically coupled internal

coordinates of the side chain CF2 and CF3 stretching mode.

The vibrational peaks (very short in intensity) at 1304 and 1318 cm-1

are insensitive to

the ion exchange as shown in the Figure 4.7 (a) with σ lower than the threshold value of 1.5 cm-1

, the peaks are not associated with the internal coordinates of the SO3- group. The animation of

the DFT peaks (red line 12 & 13 figure 4.7 b &c) suggests that the lower frequency peak at 1304

cm-1

is primarily due to mechanically coupled modes of backbone C-C stretches and peak at

1318 cm-1

is due to the backbone CF2 stretching modes.

52

Figure 4.6. Left panel (a-h)Ion exchange induced ν shift on deconvoluted ATR peaks (1120-1260

cm-1

region) (a) 1130 cm-1

(b) 1146 cm-1

(c) 1176 cm-1

(d) 1192 cm-1

(e) 1202 cm-1

(f) 1215 cm-1

(g) 1224 cm-1

& (h) 1254 cm-1

. The relevant DFT calculated peaks (red lines 4-11) are

superimposed upon the deconvoluted peaks. Right panel (a-h): Normal mode coordinate

animation snapshots of the Nafion side-chain anion and backbone fragment (see scheme 1) for

relevant DFT calculated peaks (red lines 4-11). Left and right views are extrema positions of the

vibrational mode. Functional groups associated with the dominant internal coordinates and next

most significant motions are designated by solid and dotted boundary lines respectively.

(a)

(g)

(f)

(e)

(d)

(c)

(b)

(h)

Ab

so

rban

ce U

nit

Ab

so

rban

ce U

nit

Ab

so

rban

ce U

nit

Ab

so

rban

ce U

nit

Ab

so

rban

ce U

nit

Ab

so

rban

ce U

nit

Ab

so

rban

ce U

nit

Ab

so

rban

ce U

nit

Wavenumber (cm-1)

σ: 1.2

σ: 1.5

σ: 0.6

σ: 1

σ: 0.6

σ: 3

σ: 3.5

σ: 1.2

4 1139 cm-1

CF2 s (BBstre) + CF2as (BBstre)

5. 1146 cm-1

CF2 s (BBstre) + CF2 δs (BBdef) + COC(B) ρr

6. 1177 cm

-1 CF2 as (BBstre) + COC(B) δs + COC(B)

7. 1196 cm

-1 COC(A) as + CF3 as + CF2 (SCdef)

8. 1200 cm

-1 CF3 as + CF2 δs (BBdef) + COC(B)

9. 1216 cm-1

SO3- as + CF2 as (SCstre) + COC(A)

10. 1224 cm-1

SO3- as + CF2 as (SCstre) + CF3 as + COC(A)

11. 1254 cm-1

CF3 as + COC (B) as + COC (A) δs + CF2 δs

(BBdef)

53

Figure 4.7. (a) Ion exchange induced ν shift on shoulder peaks in region 1280-1330 cm-1

. The

relevant DFT calculated peaks are superimposed (red lines 12 &13) on the ATR spectra. (b & c)

Normal mode coordinate animation snapshots of the Nafion side-chain anion and backbone

fragment (see scheme 1) for relevant DFT calculated peaks (red lines 12 and 13). Left and right

views are extrema positions of the vibrational mode. Functional groups associated with the

dominant internal coordinates and next most significant motions are designated by solid and

dotted boundary lines respectively.

12 1303 cm-1

C-C (BBstre) + C1-C2 + C3-CF3 + COC (A) ρr

(a)

(c)

(b)

σ: 1.1σ: 0.5

13 1322 cm-1

CF2 δs (BBdef)

54

Note- * Deconvoluted ATR peaks

Symmetric stretch, s; Asymmetric stretch, as; Wagging, ; Scissoring, δs; Twisting, ;

Rocking, ρr

Backbone deformation, BBdef; Side-chain deformation, SCdef; Backbone Stretching, BBstre

Very strong, vs; Medium, m; Weak, w; Shoulder, sh

Table 4.1. ATR and DFT IR adsorption peaks and assignments.

The assignment of all the selected 13 peaks in the region from 900-1350 cm-1

(Fig. 4.2) in

terms mechanical coupling of pure modes is tabulated in Table 4.1. Animations of the pure

modes and the internal coordinates of the selected 13 peaks are in Movies 3.1-3.12 and Movies

4.1-4.13 respectively in Supporting Information as .AVI files

4.4 Conclusion

The correlation of the DFT to ion-exchange effect enables the assignments of explicit and

deconvoluted ATR peaks. The shift due to metal ion exchange is greatest for the internal

coordinates of peaks mechanically coupled to SO3- site and dampens with distance. The peaks

due to mechanically coupled internal coordinates of backbone CF2 stretching modes are

Wavenumber (cm-1) Pure Mode Components

ATR DFT

1 971 m 984 SO3- s + COC(A) as + COC(B) ρr

2 981 m 992 CF2 (BBdef) + COC (B) δs

3 1056 m 1059 COC(A) as + SO3- s

4 1130* sh 1139 CF2 s (BBstre) + CF2 as (BBstre)

5 1147 vs 1146 CF2 as (BBstre) + CF2 δs (BBdef)+ COC(B) ρr

6 1176* sh 1177 CF2 as (BBstre) + COC(B) δs + COC(A)

7 1192* vs 1196 COC(A) as + CF3 as + CF2 (SCdef)

8 1202 vs 1200 CF3 as + CF2 δs (BBdef) + COC(B)

9 1215* vs 1216 SO3- as + CF2 as (SCstre)+ COC(A)

10 1224* sh 1224 SO3- as + CF2 as (SCstre) + CF3 as + COC(A)

11 1242* sh 1254 CF3 as + COC(B) as + COC(A) δs + CF2 δs (BBdef)

12 1304 w 1303 C-C (BBstre) + C1-C2 + C3-CF3 + COC(A) ρr

13 1318 w 1322 CF2 δs (BBdef)

55

insensitive to the ion exchange. The methodology of assigning IR bands in the context of

mechanically coupled internal coordinates of neighboring functional groups, and correlating

those assignments to functional groups interactions with ion exchange, has broad applications

towards characterization of new ionomers. The strategies for optimization of new ionomers

would benefit from methods for vibrational mode assignments that do not require the laborious

preparation of derivatives (structural or isotopic) for structure elucidation.

56

CHAPTER 5: Durability studies on performance degradation of MEAs in DMFCs

5.1 Introduction

PEMFCs are an electrochemical device that is receiving worldwide attention as a highly

efficient and environmentally clean energy conversion device. Direct methanol fuel cells

(DMFCs) are subcategories of PEMFCs, which converts methanol directly to electricity without

using a reformer, and have the advantages of a higher power density than the reformer based

hydrogen fueled fuel cells.82

The performance of the DMFCs has improved markedly over the

last decade but still daunting lifetime degradation process needs to be mitigated.48, 49

Advancements required for the commercialization include improvement in the electro catalytic

activity and stability, PEM durability and resistance to methanol crossover and water-heat

management. Nafion based membrane are widely used as an electrolyte, while supported or

unsupported Pt and PtRu based catalysts are commonly used as electrocatalysis. 17, 83, 84

The cell

performance is greatly affected by the microstructure properties of MEA.85,84

MEA failure

mechanism includes i) electrocatalytic sintering via coalescence and growth of the carbon

support and Ostwald ripening; ii) growth of catalyst particles; iii) corrosion of catalyst particles;

iv) electro catalyst poisoning by accumulated intermediates from methanol oxidation or

impurities,and v) degradation of the ion conducting polymer in the reaction layer due to free

radical species generated in the interface. All these degradation processes are a strong function of

operating conditions such as temperature, partial pressure, relative humidity, overpotential etc. 82,

86, 87

57

So far most degradation studies rely on the chemical analysis of fuel cell effluents or the

solutions after ex-situ Fenton’s test. Measurement of fluoride concentration in the effluent water

was used as a standard parameter to quantify degradation levels. The decomposition mechanism

of the perflurosulfonic acid (PFSA) electrolyte induced by crossover reactants gases and the

effect of the relative humidity (RH) in the mixed reactant gases on the fluoride emission rate

(FER) has been discussed in the literature.88-94

Various spectroscopic techniques like broadband

dielectric spectroscopy, electron paramagnetic resonance (EPR), fourier transform infrared

(FTIR), raman techniques, solid state and X-ray photoelectron spectroscopy (XPS) have been

used to study the correlation between the cell performance and microstructure of MEA and to

identify degradation products. 17-21,95

X-Ray diffraction (XRD), Transmission electron

spectroscopy (TEM), scanning electron microscopy (SEM) has also been used to investigate any

changes in the catalyst structure and morphology, as well as particle size and chemical

composition.94, 96-100

Although Fenton’s test is straightforward and has been used as benchmark

for PEM durability evaluation and deterioration of the membrane does not involve electrode

processes and is incomparable with variation in the fuel cell operating conditions.

Despite the necessity of the long-term stability for the successful commercialization of DMFCs,

only a few publications has examined the catalyst-membrane activity under a long-term lifetime

operated condition of fuel cell85, 101-106

. The purpose of this study is to investigate the degradation

of the MEA components by postmortem analysis for any changes in the membrane and catalyst

over long-term operation in a fuel cell. We investigate the effects of long-term performance on

the structure and durability of catalyst-membrane interfacial region using ATR-IR microscopy,

Electron microscopy, and XRD.

58

5.2 Experimental section

5.2.1 MEA preparation

Nafion-117 (E.I. DuPont De Nemours & Co.) was pretreated and the catalyst inks were

prepared as previously described.106

Briefly, Pt black and PtRu (Johnson Matthey) black were

used at the cathode and anode, respectively. The PtRu black and 5 wt% Nafion solutions were

mixed in isopropanol solution to form a dispersion of catalyst black ink. The cathode catalyst ink

was prepared similarly with Pt black, although PTFE dispersion was included in the ink. The

inks were deposited onto the GDLs by paintbrush at loading of 4mg cm−2

for both electrodes.

The carbon cloth (E TEK,ELAT/NC/DS/V2 double sided ELAT electrode, carbon only, no

metal, 20% wet proofed) was used as the gas diffusion layer (GDL) and backing layer in the

cathode. The carbon paper (Toray paper TGPH 060) was used as the anode GDL. The MEA was

formed by hot pressing the anode and cathode diffusion layers onto the Nafion film.

5.2.2 Cross-section sample preparation

The cross-sectional samples of Nafion membrane extracted from MEA lifetime studies

were prepared. A group of reproducible and identical MEA’s from the DMFCs operated for

different lengths of time namely, unused, 50 hrs, 100 hrs, 950 hrs and 1515hrs were removed,

labeled and treated with liquid nitrogen were used for subsequent studies. The MEA samples

were cut into small pieces and mounted in the epoxy. The cross sections of the epoxy-mounted

samples were cut exposing the sample using Buehler Isomet™ 100 precision saw and then were

polished using different size alumina powder on MetaServ 2000 variable speed grinder /polisher.

MEA cutting process neither disperse Pt particles inside the membrane nor creates defects on the

MEA; however it may lead to poor bonding between gas diffusion layer (GDL) and the

membrane.107

Also in this method, there is some amount of alumina that sticks on to the

59

membrane while polishing. The samples prepared were characterized by using the ATR

microscopy, Electron microscopy, and XRD.

5.2.3 ATR microscopy

ATR spectra of cross-sectional samples Nafion along with the GDL were obtained using

a Bruker™ Hyperion 2000 microscope attached to Bruker™ Vertex 70 Fourier transform

infrared spectrometer (FTIR) (Bruker, Billerica, MA). A MIRacle™ ATR accessory (Pike

Technologies Spectroscopic Creativity, Madison, WI) with a ZnSe ATR crystal was used. ATR

is a surface technique with a penetration depth of ~ 10 micrometers. The spectra range of 600 to

2000 cm-1

was investigated by signal average of 100 scan and 2 cm-1

of resolution with dry air as

purging gas at ambient temperature. Background spectra were taken over the same period as the

sample spectra. Atmospheric compensation (to eliminate H2O and CO2 interference in the beam

path) for all the measurements was performed.

Cross-sectional samples were visualized with the help of the 20X ATR-objective (Bruker,

Billerica, MA). The visual image was taken as snapshot and then measurement spots were

defined as a rectangular grid as shown in Figure 5.1 and 3-D video assisted measurements were

taken using ATR. The 3-D spectral images of all the measurement spots aligned together were

obtained. ATR data was processed with OPUS_6.5™ software from Bruker™. 2-D spectra’s

were extracted from the 3-D spectra obtained and was averaged for anode, middle, and cathode

respectively.

60

Figure 5.1 left: Membrane Electrode Assembly (MEA). Middle: Epoxy mounted cross cut MEA.

Right: Automated ATR mapping for cross sectional studies.

5.2.4 Scanning Electron Microscope

Epoxy mounted MEA samples were surface coated with a thin layer of graphite by

sputtering and SEM images were obtained using HitachiS4800 microscope (Hitachi High Tech.

Ontario, Canada). The HitachiS4800 is equipped with secondary electron detector and a

backscattered electron detector. Both are engineered to image electrons at low accelerating

energies. SEM is very useful in analyzing the morphology of the carbon support, catalyst and the

membrane. The images were obtained at an accelerating voltage of 20 kV.

5.2.5 X-Ray Diffraction

Catalyst layers were peeled off from the MEAs and were characterized. The structural

information of MEAs was obtained by XRD. XRD measurements were recorded on a Rigaku

Ultima III X-ray diffractometer system (Rigaku MSC, Woodlands, TX) using a graphite crystal

counter monochromator that filtered Cu K_ radiation. The X-ray source was operated at 46 kV

and 40 mA. The patterns, recorded in the 2θ range of 30–140◦, were obtained using high

precision and high resolution parallel beam geometry in the step scanning mode at 1 deg min−1

.

The identification of phases was made by referring to the Joint Committee on Powder Diffraction

Catalytic layer

Cross-section sample

Cathode

Nafi

on

mem

bran

e

Anode

165 μm

Membrane electrode

Assembly (MEA)Microscope image of MEA

Automated ATR mapping for cross-sectional studies

MEA embedded in epoxy

61

Standards International Center for Diffraction Data (JCPDS-ICDD) database. Lattice parameters

were calculated using JADE 7 Plus software (Rigaku). Grain sizes were determined from the

Scherrer equation using the Pseudo-Voigt profile function.

5.3 Results and Discussion

5.3.1 ATR-FTIR Microscopy

Infrared absorption studies, has been used widely to elucidate the nanostructure of the

Nafion membrane. Nafion has strong vibrational bands associated with -CF2, -COC and -SO3-

functional groups. The local molecular symmetry of the functional groups enables prediction of

allowed spectroscopic transitions. The -SO3- and -CF3 groups have C3ν symmetry, which gives

rise to symmetric (A1) and antisymmetric (E) doubly degenerated S-O stretching modes.16

Both

the -CF2 and -COC groups have C2ν local symmetry which gives rise to symmetric and

antisymmetric stretching modes.16

Table 4.1 indicates the assignments of various explicit and

deconvoluted vibrational bands of Nafion. Any change in the band intensities for 1056 cm-1

and

971cm-1

peak due to the coupled mode of sulfonate and ether link, indicates the extent of

degradation for the side chain fragments in Nafion. Whereas the deconvoluted ATR peak at 981

cm-1

is due to the internal coordinate coupling of backbone CF2 modes any changes, any changes

would indicates the preferential cleavage in the fluorocarbon backbone region. Although the

region from 1130-1275 cm-1

contains vibrational modes contributed by the coupling of various

functional group coordinates as shown in the chapter 4 (table 4.1), the integrated peak intensities

for the two broadband at 1148 and 1204 cm-1

were compared to study any changes in this region.

Figure 5.2 shows the 3D ATR-FTIR spectrum of all the measurement spots aligned together

across the membrane spanning from anode to cathode (as shown in figure 5.1) for unused and

1515 hrs operated Nafion membrane. Color gradient for the peaks intensity is uniform for unused

62

and 1515 hrs operated as we move from anode to cathode, indicating no significant changes in

the peak intensity of the IR absorption bands of Nafion while spanning across the membrane.

Figure 5.2. 3D spectra of Nafion peak intensities across the membrane width (anode to cathode)

for (a) unused and, (b) 1515hrs operated sample.

Further, 2 D spectra were extracted for mid membrane to study changes in the peak

intensities of Nafion over the lifetime operation. Figure 5.3 compares the 2D ATR-FTIR spectra

of the unused and lifetime operated (238, 382, 403, 420, 950 and 1515 hrs) Nafion for the middle

portion of the membrane. The spectra under different lifetime operation looked very similar to

the unused sample with respect to both peak position and intensity for different functional

groups. Figure 5.4 (a & b) compares the mean peak intensity Vs operation time for different IR

absorption bands of Nafion. The confidence interval (CI) for the mean of 20 replicate

measurements of the intensity was calculated using the student’s t-test statistics for 95% CI. The

CI for the intensity means overlaps indicates that there is no significant difference observed in

any of the various IR absorption bands of Nafion. Even after 1515hrs of operation in the

(a) (b)

63

DMFCs, no chemical deterioration of the membrane occurs. This clearly indicates that after 1500

hrs of operation in the fuel cell, membrane is probably not affected by the active oxygen radical

species (*OH & *OOH). These active radical spices are thought to be formed by the homolytic

cleavage of H2O2 catalyzed by the metal impurities like Fe2+

and Cu2+

at the anode or by the

radical formation as intermediate in the oxygen reduction reaction (ORR) at the cathode.106, 108-

110 The contamination by the minor cationic impurities like Pt

2+ , Ru

3+, Ca

2+, Fe

2+/3+ , Cu

2+ etc

from the carbon support, gas diffusion layers, catalyst layer or fuel cell hardware are also

inevitable in the fuel cell.106, 110

The 1056 cm-1

and 971 cm-1

bands, having mechanical coupled

vibrational internal coordinates originating from the adjacent sulfonate and COC (A) ether link,52

shift concurrently upon ion exchange and are sensitive to the state of polarization of the SO3-

anion. Since no changes in the peak position of the vibrational frequencies at 972cm-1

and 1056

cm-1

is observed (fig. 5.3), demonstrate that in the DMFCs the chemical degradation of the

membrane due to cationic impurities probably doesn’t occur till 1500 hrs of operation.

64

Figure 5.3 ATR-FTIR spectrum (900-1400 cm-1) of Nafion obtained for MEAs before and after

lifetime operation in a fuel cell.

Figure 5.4: Mean peak intensity value for IR absorption bands of Nafion Vs operation time of the

MEAs.

0.26

0.28

0.30

0.32

0.34

0.36

Unused Unused 238 hrs 382 hrs 403 hrs 420hrs 420hrs 950 hrs 950 hrs 950 hrs 1515hrs

Ab

so

rba

nc

e U

nit

Membrane electrode assemblies operation time

1208 cm-1

1148 cm-1

0.07

0.07

0.08

0.08

0.09

0.09

Unused Unused 238 hrs 382 hrs 403 hrs 420hrs 420hrs 950 hrs 950 hrs 950 hrs 1515hrs

Ab

so

rba

nc

e U

nit

Membrane electrode assemblies operation time

1057 cm-1

970 cm-1

982 cm-1

(a)

(b)

65

5.3.2 Scanning Electron Microscopy

SEM is used to study any morphological changes in the surface of MEAs microstructure

and the particle size of the catalysts with its distribution on any solid support such as carbon.

Figure 5.5 shows the SEM electron micrograph of the cross sectional samples of of an unused

and used MEAs (950 and 1515 hrs) obtained at 20 kV accelerating voltage in the low mode at

X180 magnification. The catalyst layer of the MEAs appear brighter than the membrane since

they contain heaviest element in the specimen (i.e., Pt, Ru), scattering the incident electron beam

more efficiently than the lighter elements from the PEM and the carbon support (F, C, O and S).

The thicknesses of the membrane measured from the SEM and ATR micrographs are reported in

Table 5.1. Although form ATR microscopy studies, even after 1515 hrs of operation no chemical

degradation of the membrane is observed, substantial thinning of the membrane is observed from

the SEM and ATR micrograph measurements. This suggests that the membrane is dissolving at

electrode-membrane interfacial region over the lifetime operation i.e. chemical degradation is

occurring mainly at the edges.

The cathode catalyst for the unused sample is uniformly distributed and is intact at its

place but after hours of operation in the fuel cells, restructuring and redistribution of the cathode

catalyst occurs (fig. 5.5). Such a process is likely to occur following electrochemical/chemical

corrosion of platinum leading to formation of ionic species. Similarly, at anode, Pt-Ru catalyst

undergoes reduction depending on the fuel cell conditions (current load, temperature, and

relative humidity).107, 111-114

The reduced Pt2+

and Ru3+

ionic species are redistributed in the

catalyst layer, and across the membrane. There have been number of reports for the dissolution

and crossover of these ionic species across the membrane. The charged species generated at the

66

electrodes acts as a center of free radicals that chemically degrades the membrane, leading to

increased porosity, gas crossover and ultimate membrane thinning and MEA failure.

Figure 5.5: SEM electrograph comparing the images for unused, 950 hrs and 1515hrs operated

MEAs in low mode at 20 kV accelerating voltage at X180 magnification.

Table 5.1. Membrane thickness measured from the ATR mapping and SEM images

MEA Unused

Cathode Catalyst layer Cathode Catalyst layer

MEA 950 hrs

MEA 1515hrs

Parameter:S-4800 Hitachi

Accelerating voltage -20kV

Magnification- 180X

Observations

Thickness of the membrane decreases

Cathode layer dissolving

Uneven redistribution of catalyst

Anode

Cathode

165μm

Anode

Cathode

123μm

Anode

Cathode

123μm

Anode

Cathode

123μm

Anode

Cathode

110μm

Anode

Cathode

110μm

Anode

Cathode

110μm

Sample

Name

Membrane thickness (μm)

ATR Mapping

Membrane thickness (μm)

SEM micrographs

Unused 165 ± 3 165

950 hrs 120 ± 3 123

1515 hrs 105 ± 2 110

67

5.3.3 X-Ray Diffraction

XRD patterns of anode and cathode catalyst layer separated from unused and used MEAs

(50 & 950 hrs) were measured (Fig. 5.6). The Pt and PtRu black forms a face centered cubic

(fcc) structure. The major characteristic peaks corresponding to the Platinum (111), (200), (311),

(222) and (400) were identified from both the Pt black (cathode) and PtRu black (anode). The

mean particle size and lattice parameters of catalysts for used and unused MEAs were calculated

from the XRD patterns by using JADE software and are listed in Table 5.2.

The strongest peak corresponding to Platinum (111) in cathode appeared less sharp after the

lifetime testing (Fig. 5.6), implying a decrease in the particle size. At anode the characteristics

peaks of the platinum (111) are broader and shifts towards the higher degree due to the alloying

with the smaller size Ru and formation of face-centered cubic PtRu alloy particles. Table 5.2

shows that the particle size and lattice parameters of anode catalyst slightly increase after hours

of operation. However, the cathode catalyst experiences an increase of almost 2.5 nm in particle

size; a significant decrease in the lattice parameters is observed. Previous studies has reported

that the deviation of the lattice parameters of the catalyst towards higher value than predicted by

the Vegards law indicates phasing out of the Ru from the fcc crystals of PtRu alloys, which does

not give rise to diffraction peaks.82

Therefore, the slight increase in the lattice parameter is

presumably due to the oxidation, dissolution, and separation of Ru metal atoms in the Pt

crystalline form. A slight increase in the particle size of the anode catalyst results in the

reduction of active surface area of catalyst, adversely affecting the performance of the reaction

kinetics and ultimately degrading the performance. The probable reason for the decrease in the

particle and lattice size of cathode catalyst is the dissolution of the catalyst due to the highly

68

Pt-Ru black Anode Pt black Cathode

Sample Name Grain Size (nm) Lattice Parameter (Å) Grain Size (nm) Lattice Parameter (Å)

Unused 2.9±0.1 3.8767±0.0024 10.3±0.2 3.9168±0.0036

50 hrs 3.0±0.0 3.8796±0.0118 9.6±0.4 3.9140±0.0045

950 hrs 3.7±0.1 3.8852±0.0084 7.8±0.6 3.8154±0.0433

corrosive environment of the high water content, low pH (<1), high temperature (50–90 ◦C), and

high potentials (0.6–1.2 V) coupled with substantial oxygen partial pressures. 82, 115

Figure 5.6. A comparison of XRD pattern obtained from the catalysts for unused and used

MEAS: (a) PtRu black from anode layer (b) Pt black from cathode layer

Table 5.2. Mean particle sizes and Lattice parameters for the anode and cathode catalyst

evaluated from XRD measurements

2θ

Pink- Unused

Blue- 50 hrs UsedYellow- 950 hrs used

(111)

(200)

(220)(311)

(331) (420)(222)

(400)

(111)

(200)

(220) (311)

(331) (420)

(222)

(400)

Pink- Unused

Blue- 50 hrs UsedYellow- 950 hrs used

2θ

(A)

(B)

(a)

(b)

69

5.4 Conclusions

In this study, we have investigated lifetime performance and degradation of MEAs after

long hours (100,238, 382, 420, 950, and 1515) of operation in fuel cells to understand the

changes in the microstructure, surface composition, the interfacial region of the MEAs and the

membrane chemical composition. This study demonstrate that, although no chemical aging of the

membrane is observed, substantial changes in the microstructure of the catalyst and interfacial

region is seen which adversely affect the performance of MEAs. Aged MEAs undergo

redistribution of most components of the active layers coupled to: a) changes in the shape and

size of Pt particle (cathode) and PtRu alloy particle (anode), b) redistribution of the ionic species

like Pt and Ru into the membrane, and c) thinning of the ionomers membrane. The membrane at

the edges is being dissolved due to the highly corrosive environment and generation of the ionic

species, which leads to chemical degradation.

70

CHAPTER 6: Future Research

6.1 Dehydration Study of Anion Exchange Membrane using Infrared Spectroscopy and

Density Function Theory

6.1.1 Introduction

Alkaline membrane fuel cells (AMFCs) have attracted a lot of attention due to their

potential to replace the acidic proton exchange membrane fuel cells (PEMFCs).116

The main

reason for the AMFCs popularity is the fast kinetics at the cathode and anode; use of cheaper

non-noble metal catalysts and reduced cost.117, 118

The use of anion exchange membranes

(AEMs) in AFMCs improves the electrochemical kinetics by operating at elevated pH which

eliminates the use of platinum based catalysts.116, 119

The improvement in AEMs and ionomers

used for AMFCs is a key challenge in order to achieve the practical performance of fuel cells but

till date there are only few reported membrane and ionomers for AFMCs. There has been a

considerable work carried out in developing AEMs with higher ionic conductivity and stability

to improve the efficiency and performance of AEMs. To better understand and develop

membranes with desired properties for AEM based materials, there is a need to understand their

chemical behavior, microstructure, and the interactions-taking place within the ionic domains of

the membrane in a working environment.

6.1.2 Hypothesis and objectives

Infrared spectroscopy and ab initio calculations can be used to study the structure and

hydration properties of AEM fragments at different degrees of hydration (λ = 1to 10). AEMs are

functionalized with quaternary ammonium cation that uses hydroxide or carbonate as the

71

conductive ions. The benzyl trimethyl ammonium head group [C6H5-CH2- (CH3)3N] +

is present

in the ionic domain of AEMs are used as a model cation for the chemical stability studies. These

head groups are considered as the center of activity within the polymer membrane containing

water in a working environment involving interactions between the head group quaternary

ammonium cation and the dissociation of OH-. The dissociation of the OH

- ions results in a

change in the point group symmetry of the cluster. This transition in point group symmetry will

be manifested in the FTIR spectra. The optimized geometry, harmonic vibrational frequencies,

and infrared intensities of the modeled compound, as it transitions through point group

symmetries will be studied.

72

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