1

OPERANDO SPECTROSCOPY OF ELECTROCHEMICAL ENERGY CONVERSION

DEVICES

A thesis presented

by

Emily Anne Lewis

to

The Department of Chemistry and Chemical Biology

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

in the field of

Chemistry

Northeastern University

Boston, Massachusetts

April, 2009

2

OPERANDO SPECTROSCOPY OF ELECTROCHEMICAL ENERGY CONVERSION

DEVICES

by

Emily Anne Lewis

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, 2009

3

ABSTRACT

A fuel cell for operando FTIR and x-ray absorption spectroscopy (XAS) was

designed and constructed. The cell is based on the combination of operando cells

previously developed in the Smotkin group. Special consideration was put into the

design to enable both transmission XAS, fluorescence XAS, and diffuse reflectance FTIR

spectroscopy. Operando x-ray fluorescence spectroscopy of the Pt cathode was

accomplished at Argonne national labs.

The x-ray absorption near edge structure (XANES) region of the fluorescence

XAS spectra was analyzed by subtractive normalization (∆-XANES). Using this

technique, the trends in the data became apparent: there were both time- and potential-

dependence in the reduction of surface and sub-surface Pt-oxides. The time constant for

the reduction of these oxides was greater than two hours at 530 mV, much longer than

previously expected.

Theoretical ∆-XANES (∆-XANES signatures) of oxygen adsorbates on Pt clusters

were modeled using FEFF8 software. Two new models were developed to generate the

∆-XANES signatures: the isolated all-atoms Janin cluster, and the embedded all-atoms

Janin cluster. The progressive development of these models provides insight concerning

the relative charge depletion of surface and subsurface atoms, and the compensation of

charge transfer by platinum-reservoir electron density.

Future directions of the operando spectroscopy cell include using electrochemical

promotion of organic chemistry (EPOCH) to assist epoxide ring-opening reactions for

pharmaceutical building blocks, and the study of fuel cell poisoning by common

atmospheric pollutants.

4

ACKNOWLEDGEMENTS

First I would like to thank Dr. Eugene Smotkin for initially taking me into his lab

as an undergraduate. Dr. Smotkin’s support and guidance have pushed me to explore the

limits my capabilities as a scientist. His enthusiasm and creativity have inspired me in

my educational pursuits. I would also like to thank the Smotkin research group for their

help and encouragement throughout my time in the lab.

I am very grateful to my thesis committee, Dr. Mabrouk and Dr. Mukerjee, whose

comments helped shape my thesis into its final form. I would also like to thank Dr.

Mabrouk for her guidance beyond my thesis. Finally, I would like to thank the rest of the

faculty and staff from the department who provided assistance and direction during my

time at Northeastern.

Finally I would like to acknowledge the support from my family and friends. My

parents, Theresa and Kevin, have always been there for me through all of my academic

and extracurricular endeavors. I would also like to thank all of my friends at Sunday

night dinner for being my home away from home.

5

TABLE OF CONTENTS

Abstract 2

Acknowledgements 4

Table of Contents 5

List of Figures 7

List of Abbreviations 8

Chapter 1: Introduction 10

1.1 PEM Fuel Cells 10

1.2 Operando Spectroscopy 13

1.2.1 Operando XAS 15

1.2.2 Operando FTIR 18

1.3 Research Objectives 19

Chapter 2: Operando Cell Design 21

2.1 Introduction 21

2.1.1 Legacy cells 21

2.2 Multi-Spectroscopic Cell Design Features 23

2.3 Experimental 25

2.4 Results and Discussion 26

2.5 Conclusions 29

Chapter 3: Time- and Potential-Dependent XAS 30

3.1 Introduction 30

3.2 Experimental 31

3.3 Results & Discussion 32

6

3.3.1 Time dependent Δ-XANES 32

3.3.2 Potential dependent Δ-XANES 33

3.4 Conclusions 35

Chapter 4: Theoretical Modeling of ∆-XANES Spectra 36

4.1 Introduction 36

4.2 Methods 40

4.2.1 Janin all-atoms signatures 40

4.2.2 Embedded all-atoms signatures 40

4.3 Results & Discussion 41

4.4 Conclusions 47

Chapter 5: Future Directions - Operando FTIR 48

5.1 Poisoning Studies 48

5.1.1 Introduction 48

5.1.2 Hypothesis and Goals 48

5.2 Epoxide Ring-Opening with EPOCH 49

5.2.1 Introduction 49

5.2.2 Hypothesis and Goals 50

References 51

7

LIST OF FIGURES

Figure 1.1: Polymer electrolyte membrane fuel cell schematic

Figure 1.2: An illustration of the scattering events of XAS

Figure 2.1: Legacy cells for operando spectroscopy

Figure 2.2: New multi-spectroscopic cell for operando spectroscopy

Figure 2.3: Illustrated exploded view of the cell

Figure 2.4: Photo of the cell in line at Argonne National Laboratory

Figure 2.5: Photo of the cell interface with an FTIR

Figure 2.6: Polarization curve obtained with the multi-spectroscopic cell

Figure 2.7: Comparison of XAS data acquired with the Viswanathan cell the new multi-

spectroscopic cell

Figure 3.1: Time-dependent operando fluorescence XAS fingerprints

Figure 3.2: Potential-dependent operando fluorescence XAS fingerprints

Figure 3.3: The error distribution through a ∆µ fingerprint

Figure 4.1: Janin cluster with adsorbed oxygen

Figure 4.2: A Janin cluster with oxygen adsorbed configuration demonstrating charge

transfer in the cluster

Figure 4.3: A Janin cluster embedded in an fcc Pt lattice

Figure 4.4: Comparison of ∆µ signatures created using different models

8

LIST OF ABBREVIATIONS

∆-XANES – subtractively normalized XANES

ATR-SEIRAS – attenuated total reflection surface enhanced infrared reflection

absorption spectroscopy

DMFC – direct methanol fuel cell

DRIFTS – diffuse reflectance infrared Fourier transform spectroscopy

EPOCH – electrochemical promotion of organic chemistry

EXAFS – extended x-ray absorption fine structure

fcc – face centered cubic

FTIR – Fourier transform infrared spectroscopy

GDL – gas diffusion layer

hcp – hexagonal close packed

ICE – internal combustion engine

IRAS – infrared reflection absorption spectroscopy

MEA – membrane electrode assembly

MFTIRS – microscopy Fourier transform infrared spectroscopy

NASA – national aeronautics and space administration

NEMCA – non-Faradaic electrochemical modification of catalytic activity

NHE – Normal hydrogen electrode

ORR – oxygen reduction reaction

PEM – polymer electrolyte membrane

PEMFC – polymer electrolyte membrane fuel cell

SAXS – small angle x-ray scattering

9

XANES – x-ray absorption near edge structure

XAS – x-ray absorption spectroscopy

10

CHAPTER 1: INTRODUCTION

1.1 PEM Fuel Cells

There is an immediate need for alternative (renewable, non-fossil fuel) energy

sources due to the unprecedented amount of anthropomorphic carbon dioxide in the

Earth’s atmosphere. Fuel cells are emerging as a key component of an economy based on

renewable energy. A fuel cell is a galvanic electrochemical cell that takes chemical

energy (in the form of a fuel) and converts it into usable electrical energy. This process

of electrochemical conversion is more efficient than the conversion of fuels to

mechanical energy through combustion; an average combustion engine runs at about 1/3

efficiency due to Carnot cycle limitations.1 Fuel cells that use hydrogen and oxygen

(denoted H2/air fuel cells) are of particular interest because the only product formed

through the electrochemical conversion is water. Because they operate at low

temperatures, the electrochemical combustion of fuels does not generate greenhouse

gases or atmospheric pollutants typical of internal combustion engines.

All electrochemical cells work through the coupling of complimentary oxidation-

reduction reactions via an electrolyte. In a H2/air fuel cell, hydrogen is oxidized into two

protons and two electrons at the anode. The protons migrate through the electrolyte to

the cathode, while the electrons travel through the external circuit where they can power

an electrical load on the way to the cathode. The protons, electrons and oxygen from air

combine at the cathode to form water. The combination of the half reactions leads to the

overall cell reaction (equation 1).

11

Equation 1:

2H2 → 4H+ + 4e

- E

0 = 0 V

4e- + 4H

+ + O2 → 2H2O E

0 = 1.23 V

2H2 + O2 → 2H2O Vcell = 1.23 V2

The energy associated with the overall cell reaction is 1.23 V. This is the theoretical

maximum potential the fuel cell can achieve.

One of the most promising types of H2/air fuel cells is the polymer electrolyte

membrane fuel cell (PEMFC). PEMFCs had their major debut as auxiliary power

sources aboard NASA’s Gemini space flights. Today, they are involved in about 90% of

all fuel cell research.3 H2/air PEMFCs have a number of advantages over other fuel cells:

they run at low temperatures, they are relatively lightweight, and they have very high

power densities. These properties make them excellent candidates for portable power

applications, automotive use, and stationary power sources. PEMFCs are unique because

of their polymer electrolyte or proton exchange membrane (PEM). When the hydrogen is

split into two protons at the anode, the protons cross a proton conductive membrane,

usually a sulfonated fluoropolymer such as Nafion, via the Grotthuss hopping

mechanism.4 The Grotthuss hopping mechanism explains the unusually fast diffusion of

protons through a PEM compared that of Brownian motion, which is one reason PEMFCs

are so viable. A fuel cell schematic is shown in Figure 1.1.

12

Figure 1.1: PEMFC Schematic from Liu 2002.5 The Nafion PEM is sandwiched between

the anode and cathode catalysts. Outside of the catalysts are the GDLs and flow fields.

To lower the activation energy of the cell reaction, catalysts are required. The

standard catalyst for both the anode and the cathode in a H2/air fuel cells is carbon

supported platinum (Pt/C), which has platinum nanoparticles dispersed onto activated

carbon particles. The active surface area of Pt/C varies with the amount of platinum, but

with nanometer particle size and good dispersion, it can be as high as 120 m2/g. When

the particle diameter is greater than 2 nm, the active surface area is directly proportional

to the reactivity of the catalyst.3 On average, Pt/C has about three times the specific

activity of unsupported catalysts.6 The disadvantage of platinum as a catalyst is that it is

a very expensive noble metal. Even with the sub-mg/cm2 platinum loadings achievable

with supported catalysts, the cost of a platinum MEA is prohibitive to fuel cell

marketability. The platinum loadings at the cathode side tend to be much larger than the

Hydrogen

fuel

H2O

4e- 4e

-

H+

13

loadings at the anode because of the sluggish electrode kinetics of the oxygen reduction

reaction (ORR). Thus the greatest impact on catalyst loading would result from

improving the ORR kinetics on Pt based catalysts, or finding non precious metal group

catalysts. Thus the primary focus of Department of Energy fuel cell R&D is cathode

catalysis.

Equation 1 shows that the ideal cell voltage of a H2/air fuel cell is 1.23 V;

however, the expected potential is about 1 V at best. Kinetic, ohmic, and mass transport

losses keep the cell from reaching its ideal potential.7 Most of the polarization losses are

attributed to the cathode (> 400 mV) because the hydrogen oxidation overpotential is in

the 10s of millivolts even during high performance. Ohmic losses are due primarily to

the resistance protons encounter as they migrate through the MEA. These losses are

usually associated with imperfect water management.8 Kinetic losses are due to the

activity of the catalyst, and its activation overpotential. Mass transport limitations are

due to the availability of the catalyst active sites, which can be blocked by cathode

flooding or by persistent adsorbates (poisons). Therefore, in addition to researching cost-

effective catalysts, new catalysts that circumvent kinetic and mass transport limitations

are also being explored.

1.2 Operando Spectroscopy

Although many spectroscopy techniques have been applied to the study of fuel

cell electrocatalysts, our approach emphasizes performing spectroscopy under entirely

realistic operating conditions. We call this approach “operando” spectroscopy. Operando

spectroscopy of catalysts is important because it examines the active state of the catalyst,

14

which only exists during catalysis.9 In fuel cells, operando spectroscopy differs from in

situ spectroscopy because it requires entirely realistic operating conditions, including

typical catalyst loadings. High loadings are avoided because they can cause a z-

dependence (normal to the plane of the catalyst layer) in the current and potential

profiles.10, 11

Additionally, the fuel cell is run with normal reactant flow conditions, in the

absence of additional acidic electrolytes (e.g., H2SO4 or HClO4), which is essential if

typical fuel cell operating temperatures are required, and if poisoning by mobile ions is to

be avoided. Although these prerequisites make operando spectroscopy substantially

more challenging than in situ spectroscopy, it allows for the most realistic examination of

the solid-gas catalytic interfaces.

Operando spectroscopy of the solid-gas interface is ideal for examining catalyst

adsorbates. Adsorbates effect dynamic changes on the catalyst surface such as surface

restructuring where surface atoms can be entirely displaced in order to make stronger

surface-adsorbate bonds.12

These changes can be observed, in operando and in situ

spectroscopy, as changes in the vibrational modes of either the surface or the adsorbate

atoms. For example, Shao et al. were able to study the potential dependence of the ORR

intermediates via in situ attenuated total reflection surface enhanced infrared reflection

absorption spectroscopy (ATR-SEIRAS).13, 14

Smith et al. were able to observe potential-

dependent platinum particle growth due to oxide formation and Ostwald ripening with in

situ small-angle x-ray scattering (SAXS).15

More common techniques such as Fourier

transform infrared spectroscopy (FTIR) and XAS are also emerging as powerful tools for

in situ and operando spectroscopy of electrochemical systems.

15

The biggest challenge in performing operando spectroscopy of fuel cells is cell

design. There are restrictions as to what components can be metallic because they can

interfere with XAS if their work function is close to that of the catalyst. The cell should

accommodate both transmission and glancing angle measurements. Fluorescence

measurements obtained at glancing angles mitigate interference due to the other cell

components. In the IR region, most fuel cell materials are completely non-transmissive,

so the operando cell must accommodate special windows to enable reflection techniques.

IR spectroscopy is also sensitive to MEA materials such as the carbon GDL, which

necessitates fabrication of MEA with gas diffusion layers modified to provide access of

the beam to the catalytic surface. Operando spectroscopy of fuel cells requires careful

water management because variable flooding causes fluctuations in the overall system

absorption coefficient. This reduces the amount of data available for signal averaging.

Finally, the cell geometry must accommodate the configuration of the optical components

of the spectrometry system.

1.2.1 Operando XAS

X-ray absorption spectroscopy has grown from a relatively esoteric technique to a

widely used analytical method over the last few decades. This is partially because of the

advancements in x-ray absorption near edge structure (XANES), which began in the

1970s. The first of these advancements was the development of synchrotron radiation

that has the ability to provide high intensity, polarized radiation with a wide spectral

range, predictable fluctuations, and small beam divergence.16

Another advancement was

the development of x-ray fluorescence techniques, which occurred after the development

16

of detectors sensitive to this energy range.17

Other advancements include the advent of

computer processing, which enables XAS spectra to be taken in minutes (as opposed to

hours), and the ability to model experimental data theoretically using full multiple

scattering software.16

To perform XAS spectroscopy, x-rays are tuned with a crystalline

monochromator to the work function of the sample. When these x-rays impinge on the

metal surface, a core electron is excited, and ejected as a photoelectron either into the

valence band of the metal (XANES) or into the continuum (EXAFS – x-ray absorption

fine structure).18

This wave is scattered by surrounding atoms creating the XAS

spectrum. The XANES region, within

50 eV of the work function, is most sensitive to changes in oxidation state of the metal

because it looks at multiple scattering events (Fig 1.2a). The EXAFS region of the

spectrum is a powerful analytical technique that can be used to determine the atomic

spacing of a crystal lattice (Fig. 1.2b).19

For H2/air fuel cells, the x-rays are commonly

tuned with a Si (111) crystal monochromator to the platinum LII-III edges at 13.731 and

11.92 keV, respectively.20

These transitions correspond to 2p1/2 or 2p2/3 to 5d

transition.21, 22

17

Figure 1.2: An illustration of the scattering events of XAS. The black spheres represent

absorbing atoms and the white spheres are scattering atoms. a.) the multiple scattering

events that occur in the XANES region; b.) the single scattering event of EXAFS. (From

Agarwal 1991 pg 255)19

Many studies have been done using in situ XAS 22-28

and operando XAS29-35

techniques to study catalyst properties. A few studies were done to look at the oxidation

state of the catalyst metals.29-32

All determined that at fuel cell operating potentials

conditions, platinum is purely metallic. The oxidation of CO was studied both on Pt22, 23

and PtRu27, 28, 33, 34

(the anode catalyst of a direct methanol PEMFC), and the electronic

benefits of ruthenium were exemplified. Finally, adsorption of ORR intermediates was

studied on fuel cell catalysts, and it was found that adsorption is both potential-dependent

and site-specific.25, 26, 33, 36

Subtractively normalized XANES (∆-XANES) is emerging as a new technique

for XANES analysis. 25-27, 33, 34, 37

Delta XANES takes the XANES of the substrate under

adsorbate-free conditions and uses it as a spectroscopic blank for XANES obtained under

conditions conducive to adsorption (e.g., by altering the potential or the chemical

environment). Although the quantitative assessment of these signatures is under debate,

they are excellent for qualitative analysis. The ∆-XANES signatures provide easy-to-

18

interpret spectral features that show smooth variations in intensity as a function of the

desired variable. Using computational software, theoretical ∆-XANES spectra can be

calculated, and used to interpret experimental data.

1.2.2 Operando FTIR

Unlike XAS, which observes the spectra of the catalyst, operando IR studies the

spectra of adsorbates. Operando FTIR allows for the unique properties of adsorbed

species to be clearly observed. Stark tuning is induced by the electrical field formed at

the electrode surface. It can allow forbidden vibrations to be seen in the IR spectra by

inducing a dipole moment in the adsorbed molecule.38, 39

The study of CO adsorption is

particularly important to fuel cells because of its persistence and its detrimental effects on

the catalyst. FTIR allows the site specificity of CO on the crystalline catalyst surface to

be observed via the change in vibrational frequency of the molecule.40-45

CO binds

preferentially to catalyst step sites because the platinum atoms are under coordinated, and

therefore, electron deficient. The terrace sites are only filled after so much CO is

adsorbed that the step sites are full. Finally the potential-dependence this site-specificity

as well as the potential dependence of adsorption configuration (e.g., atop, or bridge) and

total coverage can be seen using in situ FTIR.46-51

The characteristic IR vibrational bands of molecules can also be used to study fuel

cells in situ and in operando. Studies of direct methanol fuel cell (DMFC) anodes have

been done to look at the methanol oxidation reaction.52-63

Using FTIR techniques, it is

easy to distinguish between adsorbed methanol, gaseous methanol, and intermediates of

the reduction reaction. In situ FTIR can also be used to study the effects of atmospheric

19

pollutants on the catalytic surface. Quijada et al. used in situ infrared reflection

adsorption spectroscopy (FR-IRRAS) to study the irreversible adsorption of SO2 on

platinum electrodes by looking at its characteristic band at 1220 cm-1

.64-66

Another use of

operando FTIR is to study organic reactions promoted by non-Faradaic electrochemical

modification of catalytic activity (NEMCA) in a fuel cell environment.67-70

Reflectance FTIR techniques must be used to acquire operando spectra because

an IR beam cannot be transmitted through the components of the membrane electrode

assembly housing. A few techniques including diffuse reflection FTIR (sometimes

referred to as DRIFTS), IR reflection absorption spectroscopy (IRAS or IRRAS),

microscope FTIR reflection spectroscopy (MFTIRS), attenuated total reflectance (ATR),

and surface-enhanced IR absorption spectroscopy (SEIRAS).71

Diffuse reflectance FTIR

uses a special accessory to the usual FTIR spectrometer. The accessory directs the IR

beam to the sample through a series of mirrors. The beam is then scattered by the sample

and re-combined with an integrating mirror and sent to the detector. Diffuse reflectance

spectra exhibit the same peaks as regular absorbance spectra, but some of the more minor

peaks observed in the absorbance spectra are magnified.72

1.3 Research Objectives

The objectives of this research include development of a versatile operando

spectroscopy fuel cell that enables FTIR and XAS analysis of fuel cell catalytic surfaces

to study the time and potential dependent dynamics of fuel cell catalyst structures.

Chapter 2 discusses the design of the multi-spectroscopic operando fuel cell. Special

considerations were made to design a cell that could be interrogated by transmission

20

XAS, fluorescence XAS and FTIR spectroscopy. Chapter 3 discusses operando XANES

acquired with the new cell. The platinum LII-III edges were examined for changes in

oxidation state. The XANES data were analyzed by ∆-XANES technique for ease of data

trend observations. Chapter 4 discusses theoretical modeling of the operando ∆-XANES

using the full multiple scattering software FEFF8. Two models of platinum cluster

adsorbates were given special attention to show how charge transfer selectively depletes

charge from near neighbor atoms, and how this charge depletion is compensated for by

embedding the cluster into a larger reservoir of Pt atoms. Chapter 5 discusses the future

prospects for operando spectroscopy.

21

CHAPTER 2: OPERANDO CELL DESIGN

2.1 Introduction

Previous operando spectroscopy fuel cells have been reported for FTIR54-56, 73

and

XAS.29-35

There is an advantage to combining the form and functions of the two types of

operando spectroscopic cells. The multi-spectroscopic cell was designed to

accommodate FTIR and XAS studies of the electrode surface of operating fuel cells

membrane electrode assemblies. The advantage of a multi-spectroscopic cell is that

reaction conditions can be duplicated using the exact same electrode surface in either the

FTIR or XAS mode. This enables the acquisition of a diverse range of spectroscopic

information while minimizing the effects of sample variation. The complex humidity-

and air-sensitive parameters required of the fuel cell environment makes this feature

particularly attractive.

Another consideration in the design of the multi-spectroscopic cell was easy

interfacing to the commercially available diffuse reflection accessories for FTIR systems,

and orientation independent operation for easy manipulation in the cramped environment

of synchrotron beamline hutches.

2.1.1 Legacy Cells

Operando spectroscopy cells for fuel cell analysis, developed in the Smotkin lab,

have been extensively reported. Fan et al. designed an operando diffuse IR cell56

(Fig.

2.1a), and Viswanathan et al. designed an operando XAS cell35

(Fig. 2.1b). The Fan cell

was designed to fit into a 1990’s model Harrick Praying Mantis (Pleasantville, NY)

22

diffuse IR accessory. The cell had a number of useful design elements that were helpful

in developing the new cell: the IR window, the flow field design, and the gas inlet/outlet

features.

Figure 2.1: Legacy cells for operando spectroscopy. Top photos: Operando FTIR cell by

Fan et al. The right image is a top-view of the cell, which shows the CaF window. The

left image shows a side-view of the cell, which shows the gas inlets. Bottom photo:

Operando XAS cell by Viswanathan et al. The size of the cell is demonstrated, and the

window that allows spectroscopy is shown.35

The Viswanathan cell incorporated a graphite lower flow field with a posterior

cavity to eliminate signal attenuation in transmission XAS. This feature was integrated

into the new design. The new cell design was also built to optimize the fluorescence

XAS mode in addition to transmission mode, which would eliminate signal attenuation

due to travel length entirely. The new cell incorporated many design features from the

legacy cells, building upon their strengths, and improving their weaknesses.

2.2 Multi-Spectroscopic Cell

The new cell, show

transmission XAS, and fluorescence XAS (

fluorescence XAS mode, the top plate can be removed so that only the windowed

graphite

Figure 2.2: New multi-spectroscopic cell for operando spectroscopy. Top photo: Cell

disassembled. The top piece is the housing slider. The midd

contains the lower flow field, upper flow field, and the top plate. The bottom row

contains an MEA. Bottom photo: Assembled cell.

upper flow field is between the beam and the MEA. This is advantageous because the

stainless steel top contains iron, nickel and chromium

are similar to the edge energies of the catalyst components.

fluorescence XAS is that beam intensity is not attenuated by

material. The use of graphite flow fields (black plates in

lower flow field beneath the MEA

either mode of operation.

bevel on the underside to allow for transmission XAS to be

(Fig. 2.4).

Spectroscopic Cell Design Features

shown in Figure 2.2, enables operando diffuse reflectance FTIR,

transmission XAS, and fluorescence XAS (Fig. 2.3) of fuel cell catalytic layers

mode, the top plate can be removed so that only the windowed

spectroscopic cell for operando spectroscopy. Top photo: Cell

disassembled. The top piece is the housing slider. The middle row from left to right

contains the lower flow field, upper flow field, and the top plate. The bottom row

contains an MEA. Bottom photo: Assembled cell.

upper flow field is between the beam and the MEA. This is advantageous because the

contains iron, nickel and chromium, which fluoresce at energies that

are similar to the edge energies of the catalyst components. The advantage of

orescence XAS is that beam intensity is not attenuated by the lower flow field

The use of graphite flow fields (black plates in Figure 2.3), and recessing the

lower flow field beneath the MEA (in a similar fashion to the Viswanathan cell)

. To accommodate transmission XAS, the slider has a wide

l on the underside to allow for transmission XAS to be recorded at an

23

operando diffuse reflectance FTIR,

talytic layers. In

mode, the top plate can be removed so that only the windowed

spectroscopic cell for operando spectroscopy. Top photo: Cell

le row from left to right

contains the lower flow field, upper flow field, and the top plate. The bottom row

upper flow field is between the beam and the MEA. This is advantageous because the

, which fluoresce at energies that

The advantage of

the lower flow field graphite

), and recessing the

(in a similar fashion to the Viswanathan cell), enables

accommodate transmission XAS, the slider has a wide

an optimum angle

24

Figure 2.3 Illustrated exploded view of the cell. The red arrow represents the beam path

of reflectance FTIR or fluorescence XAS modes. The yellow arrow represents

transmission XAS mode.

A compact cell design is required because the beamline hutch is densely packed

with equipment. In the air-breathing fluorescence XAS mode, the cell is operated on its

side (Fig. 2.4), which can cause water droplets to form and pool at the bottom of the flow

field. Wicking material is used on the upper flow field to remove water formed at the

cathode by the ORR. Although the flow fields have no set polarity, these studies have

used the upper flow field as the cathode, which is most prone to the effects of flooding.

Figure 2.4: Photo of the cell in line at Argonne National Laboratory. The cell must be

operated on its side to accommodate the x-ray beam path.

25

For FTIR, the slider design allows for easy alignment below the integrating

mirrors of a Pike Technologies (Madison, WI) diffusIR accessory (Fig. 2.6). The original

accessory was designed so that the sample holder slides into position under the

integrating mirror. The cell design integrated this feature by including a housing slider

that easily aligned the cell in the accessory. The slider assembly incorporates wiring for

temperature and potentiostatic control, as well as manifolding for the reactant gases.

Swagelok fittings for of gas entry and exhaust, and a DE9 plug for potentiostatic and

temperature control are positioned at a terminal end of the slider for ease of insertion into

the commercially available diffuse refection accessory. The wide bevel angle of 158º on

the top plate maximizes the collection of the scattered signal. The upper flow field

employs a pins style-flow field to optimize flow distribution around the CaF2 window

inset. The wicking is still effective at controlling flooding in the upright operation mode

for FTIR, which is especially important in the water-sensitive IR region.

Figure 2.5: Photo of the cell interface with an FTIR. The cell is aligned within the Pike

diffusIR accessory, which connects to a Bruker (Billerica, MA) Vertex 70 FTIR

Spectrometer.

26

2.3 Experimental

Polarization curves were obtained with an EZstat potentiostat/galvanostat

(NuVant Systems, Inc. Crown Point, IN). The cell was operated as a H2/air fuel cell with

an air-breathing cathode at 50 oC with humidified gases. The membrane electrode

assembly (MEA) was prepared using standard procedures.74

The cathode side was

loaded with 30% PtNi/C acid washed E-Tek catalyst (special made E1030523) at 0.9

mg/cm2. The anode side was prepared with Pd/C Alfa Aesar catalyst (stock # 38309, lot #

C02R014) using a 1.0 mg/cm2 loading. Palladium was used on the anode because its L

edge energy is much lower than that of Pt and its K edge is twice as large as the Pt LII-III

edge that was being studied. The Ni K edge is so low that it enables separate

examination of Pt and Ni atoms.

The operando XAS were acquired at Advanced Photon Source (Argonne National

Laboratory, IL) at the MRCAT beamline 10-ID-B. The cell was aligned for fluorescence

mode between the source and the Lytle detector (Fig. 2.3). The operando spectra were

collected by tuning the beamline undulator to the Pt LIII edge energy range using the 2nd

harmonic of a Si (111) crystal monochromator. Higher harmonics were rejected using a

Rh reflection mirror. The incident ion chamber was filled with N2 gas.

2.4 Results and Discussion

A polarization curve was acquired after conditioning the cell for about an hour

(Fig. 2.6). The sweep shows hysteresis that is commonly observed during fuel cell cyclic

voltammetry. The return anodic sweep shows that cell performance is regained after

surface oxides are reduced.

27

Figure 2.6: Polarization curve obtained with the multi-spectroscopic cell.

Operando fluorescence XAS acquired with the new cell showed a step

improvement in signal-to-noise over the Viswanathan cell in transmission mode with

similar catalyst loadings, as exemplified by the Ni and Pt edge data of a PtNi catalyst

(Fig. 2.7). The nickel edge data shows a much cleaner EXAFS spectral region and the

absence of observable pre-edge noise. At the Pt edge, the difference in signal-to-noise

ratio is not as distinct, but there is less fluctuation in the EXAFS region. The improved

spectral quality is due to the use of the new fuel cell’s fluorescence-mode.

500

550

600

650

700

750

800

850

900

-35-30-25-20-15-10-50

Po

ten

tia

l (m

V)

Current (mA)

28

Figure 2.7: Comparison of XAS data acquired with the Viswanathan cell (red line) and

data acquired with the new multi-spectroscopic cell (green line). a.) Ni K edge of the

Pt3Ni cathode. b.) Pt LIII edge Pt3Ni cathode.

29

2.5 Conclusions

This work has demonstrated a new operando multi-spectroscopic fuel cell capable

of diffuse reflectance FTIR, fluorescence XAS, and transmission XAS. The cell was

operated under normal operating conditions and a standard polarization curve was

acquired. Fluorescence XAS data shows a significant decrease in signal noise compared

to the previous operando transmission XAS cell. The Ni K edge demonstrates that the

increased signal-to-noise ratio is more dramatic at lower energies. More operando

catalyst studies with this cell are sure to come.

30

CHAPTER 3: TIME- AND POTENTIAL-DEPENDENT ∆-XANES

3.1 Introduction

Fuel cell degradation can be ascribed to short-term and long-term degradation

modes.75

Our previous work has shown that in both cases, adsorption plays a key role.

For example in the case of direct methanol fuel cells, it has been shown that the anode is

highly susceptible to adverse adsorbates such as CO. The poisoning of an electrode

surface by CO occurs on a very short time scale. Recovery has been effected by fuel

starvation of the anode, which causes a positive excursion of the electrode potential that

oxidatively cleans the surface.75

There are also longer term degradation modes that occur

over periods of hours to hundreds of hours. The degradation of direct methanol fuel cell

cathodes exemplifies long term degradation modes. This includes time-dependent

poisoning of the cathode by Ru crossover from the anode, and deep oxide formation and

passivation of the cathode.75-79

The study of time-dependent degradation processes can

most effectively be studied by operando spectroscopy.9, 29, 32, 35, 55, 56, 73

The ability to monitor the time- and potential-dependence of oxygen adsorbates

on cathode catalysts is important because of their inevitable formation over the life of the

cell. Oxides cause detrimental kinetic and mass transfer losses, thus an understanding of

their persistence is essential to better catalyst design. Smith et al. recently studied Pt

growth due to oxides and Ostwald ripening with in situ SAXS.15

A number of other

studies have been done using in situ XAS 25-27, 31, 36

and operando XAS 29, 30, 32-34

techniques to study fuel cell catalysts. Towards this end, the multi-spectroscopic cell was

taken to a synchrotron source to acquire high-quality operando fluorescence XAS data.

31

These spectra were analyzed by the ∆-XANES technique for the easiest observation of

oxide trends.

3.2 Experimental

The cell was operated as an air-breathing H2/air cell. The membrane electrode

assembly (MEA) was prepared using standard procedures.74

The cathode side was loaded

with 20% Pt/C Alfa Aesar catalyst (HiSpec 3000 stock # 35849, lot # A305023) at 1.2

mg/cm2. The anode side was prepared with Pd/C Alfa Aesar catalyst (stock # 38309, lot #

C02R014) using a 1.0 mg/cm2 loading. The cell was operated at 50

oC with humidified

gases. Polarization curves were obtained with an EZstat potentiostat (NuVant Systems,

Inc. Crown Point, IN). The cathode was left open to enable the collection of high quality

XAS in fluorescence mode with no signal attenuation.

Pt LIII edge XAS data were collected at the MRCAT 10-ID-B beamline at the

Advanced Photon Source (Argonne National Laboratory, IL). The beamline undulator

was tuned to the Pt LIII edge energy range using the 2nd

harmonic of the Si (111)

monochromator crystal. A Rh reflection mirror was used to reject higher harmonics. The

incident ion chamber was filled with N2. XAS data were collected at different fuel cell

potentials: 0 mV (short circuit), 530, 600, 710, 780, 820, 900 mV vs. NHE, and at open

circuit voltage (OCV). The cell was held at each potential for about 2 hours while 32

scans were run. In addition, ex situ XAS data were collected on a similarly prepared dry

MEA unexposed to the fuel cell humidified environment.

32

3.3 Results and Discussion

3.3.1 Time-dependent ∆-XANES

After the cell is conditioned29, 35

for 1 hour, a reference XANES was obtained at 0

volts versus a normal hydrogen electrode (NHE). Time-dependent XAS were then

acquired and subtractively normalized to the reference spectrum to yield ∆µ fingerprints.

Figure 3.1 shows time dependent fingerprints acquired at 530 mV. The fingerprints are

the running average of three consecutive scans (Fig. 3.1). These initial cathode spectra

demonstrate the time-dependence of a peak at about 11568 eV. The time dependence is

likely related to a variety of time constants, including surface and sub-surface catalyst

restructuring induced by adsorption/desorption of oxygen. The fingerprint peak is

observed to decrease in magnitude as the potential is held constant. This is coincident

with the fact that 530 mV is a reducing potential for platinum oxide. The observed time

constant is on the order of hours, which is much longer than expected for simple surface

oxide reduction. The reduction of sub-surface oxygen species, which have been

incorporated into the platinum lattice, may be responsible for the long time constant. To

our knowledge, this is the first instance of time-dependent operando XAS of a fuel cell.

33

Figure 3.1 Time-dependent operando fluorescence XAS fingerprints highlighting an

oxide peak at 11568 eV.

3.3.2 Potential-dependent ∆-XANES

The fingerprint obtained at 204 minutes was used as the lowest potential plot in

the potential dependent data of Fig. 3.2. The rest of the potential-dependent signatures

are running averages of 30 scans taken at the given voltages. The peak shown at 11568

eV is the same oxide peak shown in the time-dependent signatures (Fig. 3.1). As the

potential was increased from 600 – 900 mV, the signature oxide peak increased. At

OCV, the peak intensity increased dramatically. The ex situ dry MEA signature shows

that the cathode is completely oxidized before exposure to the fuel cell environment,

consistent with the results of Stoupin et al.80

11550 11560 11570 11580 11590 11600Energy eV

530 mV vs Time

84 mins

96 mins

108 mins

120 mins

144 mins

168 mins

204 mins

34

Figure 3.2: Potential-dependent operando fluorescence XAS fingerprints highlighting an

oxide peak at 11568 eV.

The potential-dependent data shows a negative dip at about 11564 eV that evolves

at 780, 820, and 900 mV. This negative dip has been hypothesized to be a manifestation

of the ligand effect, which is the withdrawal of electron density from nearby atoms due to

adsorption of an electronegative atom or molecule.81

Charge transfer can shift a XANES

edge to higher energy if there is charge depletion. Therefore, when a reference spectrum

is subtracted from a blue-shifted spectrum, the result is a negative dip in the ∆-XANES

signatures.

However, this negative dip should be interpreted with caution. There are inherent

errors in creating accurate ∆µ fingerprints. To create reproducible fingerprints, the

reference and sample spectra need to be reproducibly aligned within 0.01 eV.

Furthermore, the Si (111) crystal monochromator that controls the beam frequency is

only accurate to ± 0.01 eV. Consequently, even if two experimental spectra could be

accurately and reproducibly aligned to 0.01 eV, the error inherent in the data is too large.

11550 11560 11570 11580 11590 11600Energy eV

Potential vs Energy

530 mV - 204 min

600 mV

710 mV

780 mV

820 mV

900 mV

ocv

dry

35

The manifestation of this error in not equally distributed through the whole spectrum, the

greatest impact is in the lower energy region of the signatures (Fig. 3.3).

Figure 3.3: The error distribution through a ∆µ fingerprint. Error is represented by the

shaded area.

3.4 Conclusions

Excellent quality operando fluorescence XAS has been acquired with high signal-

to-noise ratios using a new operando spectroscopy cell. The time-dependent data,

obtained with the cell, elucidates a time constant for surface and sub-surface oxide

removal that is on the order of hours. XAS taken at increasing potentials exhibit an

increase in the magnitude of the oxide peak. The potential-dependent spectra also exhibit

a negative dip prior to the oxide peak, which could be the manifestation of the ligand

effect. However, alignment errors in XANES and instrumental uncertainty in XAS

instrumentation make the analysis of fine structure fingerprint features challenging.

36

CHAPTER 4: THEORETICAL MODELING OF ∆-XANES SPECTRA

4.1 Introduction

The potential-dependence of adsorbate coverages on a distribution of sites is

likely the most important determining factor in electrocatalysis. An important example is

the ORR. The sluggishness of the ORR kinetics is the primary barrier to the

commercialization of PEMFCs. Diffusing dioxygen must compete with mobile anions

and tenaciously adsorbed oxygen for free Pt sites. The competition with mobile anions

can be mitigated by using electrolytes with anchored anions, such as Nafion.82

The

competition with tenaciously adsorbed oxygen is a more difficult issue. A strategy for

improving ORR catalysts is the alteration of the electronic band structure (by alloying) to

increase the Pt oxidation potential.83-88

A number of mixed metal catalysts have been

studied as alloys and more recently as core shell structures. Model systems such as Pt

skin alloys suggest that the key issue is electronic structure, with no need for alloying

components on the surface.89-91

De-alloying of Pt alloys is another strategy.92-97

The

experimental ORR work has been complemented with computational modeling based

primarily on cluster or periodic density functional theory (DFT).98-111

Oxygen adsorption is an elementary step in anode fuel cell reactions as well. In

DMFCs, oxygen adsorption is required for the bifunctional methanol oxidation

mechanism.112-114

Although Pt, a good C-H activator, oxidatively adsorbs methanol to

yield adsorbed CO, it cannot provide the oxygen required for complete oxidation to CO2

within the practical DMFC anode potential window (200 – 350 mV vs. NHE). Thus

practical DMFC anode catalysts are binary or ternary alloy systems where the alloying

37

components chemisorb oxygen at lower potentials (e.g., Os, Ru, W, etc). 47, 112-118

In spite

of a massive collective effort, the improvements in both anode and cathode kinetics over

the past few decades have been incremental. Much of the improvements can be attributed

to better preparative methods for MEAs.

The ability analyze adsorption site coverage dependencies is fundamental to

understanding electrocatalysis. Towards that end subtractively normalized x-ray

absorption near edge spectroscopy (∆-XANES) has been under development for analysis

of site-specific adsorption in a variety of electrochemical systems.24-27, 33, 34, 119

The

XANES of the substrate under adsorbate-free conditions is used as a subtraction standard

for XANES obtained under conditions conducive to adsorption (e.g., by altering the

potential or chemical environment). The challenge is interpretation of the experimental

∆-XANES (∆µ fingerprints). Ramaker uses theoretical XANES of model clusters such

as the Pt6 Janin cluster120

(Fig. 4.1), with and without adsorbates, to calculate theoretical

∆µ fingerprints (∆µ signatures).24-27, 33, 34, 119

Figure 4.1: Janin cluster with adsorbed oxygen. Clockwise from top left: atop adsorption,

bridged adsorption, hexagonal close-packed (hcp) adsorption, and face-centered cubic

(fcc) adsorption.

0 1

2 3

4 5

0 1

2 3

4 5

0 1

2 3

4 5 54

2 3

10

38

The 6-atom Janin cluster is an ideal model because it can accommodate all four oxygen

absorption sites: atop, bridged, fcc, and hcp.120

The ∆µ signatures are calculated using

FEFF8, a full multiple scattering code that employs self-consistent-field calculations of

local electronic structure.121, 122

To date, the Ramaker ∆µ signatures for oxygen

adsorption have been limited to the scattering of photoelectrons originating from only one

adsorbing Pt atom (atom-0 in Fig.4.1?).24, 26, 33

However, Ankudinov et al.123

showed that

changes in the near neighbor Pt-scattering potentials due to adsorbate-Pt bonds (i.e.,

changes in the Pt electronic structure) are sometimes more important than the effect of Pt-

adsorbate photoelectron multiple scattering. For example, the Pauling electronegativity

value for oxygen is 1.3 units larger than for Pt. Thus charge transfer from the Pt lattice to

the adsorbate oxygen would be expected. The withdrawal of electrons from near

neighbor Pt atoms should cause a blue shift in their respective XANES spectra. Such blue

shifts would be manifested as a negative dip at the low energy side of the ∆-XANES. In

order to account for the ligand effect, 81, 124

our first iteration in the development of a

more meaningful signature involved the inclusion of x-ray absorption events from all of

the atoms of the Janin cluster. These signatures are referred to as all-atoms signatures.

The metallic cluster has delocalized electrons that can participate in charge

transfer during modification of the cluster chemical environment (e.g., adsorption

processes) (Fig. 4.2).

39

Figure 4.2 A Janin cluster with oxygen adsorbed in the atop configuration. The orange

arrows represent the direction of charge transfer through the cluster.

Since XAS probes all atoms in the cluster, the photoelectrons from all of the adsorbate-

free Pt atoms and adsorbing atoms, should be included in the ∆µ calculations (i.e., all-

atoms model). However, the use of isolated Janin cluster atoms for an all-atoms model of

oxygen adsorption overemphasizes Pt charge depletion of the Janin cluster atoms because

they are artificially under-coordinated. A more realistic model would result from

embedding the Janin cluster within a larger reservoir of Pt atoms. This work shows how

progressive improvement of the ∆µ signatures models (i.e., from the limited-absorber

model of Ramaker, to an all-atoms model, and finally to an embedded-all-atoms model)

elucidates the importance of charge transfer effects induced by the adsorbate. Moreover

the analysis of the isolated all-atoms model provides insights into the relative

contribution to charge transfer from each Janin cluster atoms. That information is masked

by charge compensation from the Pt-reservoir in the embedded-all-atoms model. Thus

observations extracted from progressively increasing the number of atoms contributing

0 1

2 3

4 5

40

photoelectrons (i.e., from limited-absorber atom model to the embedded all-atoms-model)

contribute to the understanding of the ligand effect.

4.2 Methods

4.2.1 Janin all-atoms signatures

All-atoms signatures were computed for the atop and bridged adsorption

configurations using FEFF8, a full multiple scattering code.125

The XANES for the

adsorbate-free cluster and adsorption configurations were computed on a per-atom

basis (0-5 Fig. 4.1). These simulated XANES were used to calculate per-atom ∆µ

signatures, which could be averaged without weighting or configurationally averaged

to yield isolated all-atoms signatures. The criteria for the method of averaging will be

discussed later.

4.2.2 Embedded all-atoms signatures

When the atop-adsorbed Janin cluster is embedded into the fcc Pt reservoir, the

cluster symmetry is increased to C4v with the Pt – O bond as the C4 axis (Fig. 4.3).126

The

C4 operation creates four equivalent atom-1 sites about atom-0, and atom-2, -3, -4, and -5

(the subsurface atoms) become equivalent. The increased symmetry allows for

reassignment of the six available FEFF8 potentials to accommodate atoms of the Pt

reservoir. Four unique potentials are distributed between the oxygen, atom-0, the atom-

1s, and the subsurface atoms. The final two potentials are assigned to the reservoir. Each

of the four non-reservoir (Janin atoms) potentials yields a per-atom signature. The per-

atom signatures of a cluster configuration are configurationally averaged to yield an

41

embedded all-atom signature. The configurational average weights the per-atom

signatures by the multiplicity generated by relevant symmetry operations (i.e., the atom-1

per-atom signature and the subsurface atom signatures are both weighted by four because

their rotation about the C4 axis).

Figure 4.3: A Janin cluster embedded in an fcc Pt lattice

The bridged adsorption configuration yields a C2v symmetry upon insertion into

the Pt reservoir. The σv plane bisects the oxygen, atom-3, and atom-5. The σv reflection

makes atom-1 and -0 equivalent. Additionally, atom-2 and -4 have equal counterparts

opposite of the plane. The σv´ plane bisects the oxygen, atom-1, and atom-0. Reflection

through the σv´ plane equates the atom-2s to the atom-4s, and atom-3 to atom-5. As

before, per-atom signatures are generated from the unique potentials of atom-0/1s, atom-

2/4s, and atom-3/5s. Configurational-averaging weights atoms-2/4 by four, atoms-3/5 by

two, and atoms-1/0 by two as per the relevant symmetry operations.

4.3 Results & Discussion

Figure 4.4a shows the isolated per-atom ∆µ signatures for the atop (top) and

42

bridged (bottom) adsorption configurations. The atom-0 ∆µ signatures for the atop

and bridged configurations (dotted blue lines) correspond to the Teliska limited-

absorber signatures, which only consider x-ray photoelectrons from atom-0.26

The

primary effect of atop oxygen adsorption is an increase in the white line of the

adsorbing atom-0. There is a slight increase in edge absorption of atoms-2, -4, -3 and

-5. The slight increase in absorption in the atom-1 ∆µ signature will be discussed in

the context of the embedded-atop ∆µ signatures. The lower panel of Fig. 4.4a shows

the per-atom ∆µ signatures for bridged oxygen adsorption on the isolated Janin

cluster. In the bridged configuration atoms-2, -4, -3, and -5 exhibit a negative dip on

the low-energy side of the peak. This is a manifestation of the ligand effect. These

atoms experience substantial charge depletion upon adsorption of oxygen over atom-0

and -1. This causes the blue-shift in the XANES that is responsible for the negative

dip in the ∆µ signature. Negative dips are a direct consequence of a ligand effect. It

is noteworthy that the atom-0 and atom-1 signatures are not exactly the same. This

would be better discussed in the context of the embedded clusters, where the atom-0

and atom-1 signatures are exactly identical.

Figure 4.4b compares the embedded per-atom ∆µ signatures of the atop (top

panel) and bridged (lower panel) configurations. The atop atom-0 signature intensity

decreases significantly when the cluster is exposed to the Pt reservoir electron density.

Atoms-2, -4, -3, and -5 exhibit no change in the XANES, relative to the clean cluster,

upon atop adsorption. The embedded atom-1 atop signature is similar to that of the

isolated atom-1 atop signature except that the negative dip and the high intensity region

are more sharply defined. The well-defined negative dip in the embedded signature

43

suggests that the Pt reservoir is unable to fully charge-compensate surface atoms in

proximity to the adsorbing atom. In the embedded-bridged configuration, atoms-2, -4, -3,

and -5, now subsurface atoms, are almost completely charge-compensated (i.e., atoms-

3/5 still shows a small negative dip) by the reservoir electron density. The signatures of

atom-1 and atom-0 are now exactly the same, as would be predicted by increased

symmetry of the embedded cluster.

Figure 4.4c compares the configurational averages of the embedded (solid lines)

and isolated (dashed lines) signatures for both adsorption configurations. In the atop-

embedded signature, peak intensity is substantially decreased relative to the isolated

cluster: The subsurface-atom embedded signatures contribute no intensity to the

weighted average. It is noteworthy that the atom-1 charge depletion is completely

masked by the magnitude of the atom-0 signature intensity. However, the higher-energy

peak in the atom-1 signature is more prominent in the configurationally averaged

embedded signature. That peak appears as a shoulder in the configurationally averaged

isolated-cluster signature. In the bridge configuration, all of the charge depletion that

results from charge transfer from atoms-2, -3, -4, and -5, are completely compensated for

when those atoms are made subsurface by embedding the cluster. The negative dip in the

isolated signature disappears completely, the signature intensity is decreased, and the

profile is dominated by the atom-0/1 signature.

44

Figure 4.4a: Isolated per-atom signatures. Top: Signatures obtained with the atop

configuration; Bottom: Signatures obtained with the bridged configuration.

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

-20 0 20Energy (eV)

Isolated Bridged

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-20 0 20Energy (eV)

Isolated Atop

45

Figure 4.4b: Embedded per-atom signatures. Top: Signatures obtained with the atop

configuration; Bottom: Signatures obtained with the bridged configuration.

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

-20 0 20Energy (eV)

Embedded Bridged

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-20 0 20Energy (eV)

Embedded Atop

46

Figure 4.4c: Configurational averages of the per-atom signatures – dashed lines are

isolated all-atoms signatures, solid lines are embedded all-atoms. Top: Signatures

obtained with the atop configuration; Bottom: Signatures obtained with the bridged

configuration.

-0.02

0.00

0.02

0.04

0.06

-20 -10 0 10 20 30

Energy (eV)

Atop: Isolated vs. Embedded

configurationally averaged

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

-20 -10 0 10 20 30

Energy (eV)

Bridged: Isolated vs. Embedded

configurationally averaged

47

4.4 Conclusions

Limited-absorber ∆µ modeling precludes the ability to sense charge transfer

effects concomitant with adsorption. Adsorbed oxygen on a Pt6 Janin cluster modifies

the x-ray absorption properties of the adsorbate-free Pt atoms. ∆µ signatures calculated

on per-atom basis show that all Pt-atom photoelectrons must be included in FEFF8

simulations if charge transfer effects are to be modeled. Although the isolated Janin all-

atoms signatures overemphasize charge depletion (because Janin atoms are under-

coordinated relative bulk Pt atoms) they do determine the per-atom contributions to

charge transfer. Overemphasis of charge depletion can be mitigated by embedding the

Janin cluster into a larger reservoir of Pt atoms. The embedded per-atom ∆µ signatures

demonstrate that only the surface atom-1 in the atop configuration remains charge

depleted after change compensation from the Pt reservoir. All of the charge depletion

resulting from charge transfer effects from sub-surface atoms is compensated for by the

Pt reservoir. Thus this work shows how the progressive improvement of the signature

models, from the limited-absorber model of Ramaker, to an all-atoms model, and finally

to an embedded-all-atoms model, elucidates the importance of charge transfer effects

induced by the adsorbate. Moreover the analysis of the all-atoms model provides insight

into the source of charge transfer from near neighbor atoms, which are masked by charge

compensation from the Pt-reservoir electron density as evidenced by the embedded all-

atoms model.

48

CHAPTER 5: FUTURE DIRECTIONS – OPERANDO FTIR

5.1 Poisoning Studies

5.1.1 Introduction

In order for H2/air fuel cells to be a viable option for automotive power sources,

they need to be able to withstand the dynamic environment of the roadway. Assuming

that fuel cells will be gradually phased into to automotive power, they will

simultaneously be on the road with internal combustion engines (ICEs). Consequently,

even though H2/air fuel cells are clean forms of energy, they will be subject to a number

of air pollutants formed by ICEs such as NOx and SOx gases.

In situ voltammetric studies have shown these pollutants to have detrimental

effects on the performance of H2/air fuel cells.127-134

In situ FTIR studies show that SO2 is

irreversibly adsorbed onto platinum catalysts in aqueous solutions.64-66

However, the

effect of these contaminants have never been studied in a fuel cell by operando

techniques. These gases, and their reduction intermediates have distinct vibrational

modes in the IR region,20

which make them excellent candidates for study with the multi-

spectroscopic cell by diffuse reflectance FTIR. The fingerprints of the adsorbed species

and their reduction intermediates will enable mechanistic analysis of how these poisons

adversely affect fuel cell performance.

5.1.2 Hypothesis and objectives

Based on the results of operando CO adsorption studies, it is hypothesized that

there will be potential and site-specific coverage dependence of these adsorbates. It will

49

be interesting to see if SOx and NOx are adsorbed preferentially to CO or other common

poisons. Examination of the potential dependence of the adsorbates, in tandem with fuel

cell performance analysis, will provide information as to how adsorbate coverages affect

performance. An understanding of these adsorption mechanisms is required if successful

mitigation strategies are to be developed.

5.2 Epoxide Ring Opening with EPOCH

5.2.1 Introduction

The non-Faradaic electronic promotion of organic chemistry (EPOCH) is an ideal

catalytic system for epoxide ring-opening reactions, which represent an ideal synthetic

route to a number of pharmaceutical building blocks. EPOCH conversion of epoxides to

aldehydes and ketones provides a form of “green” chemistry in that atom efficiency is

enhanced compared to conventional syntheses (e.g., the oxidative cleavage of alkenes).

Moreover, solid polymer electrolytes obviate the need for liquid solvent waste disposals.

EPOCH occurs by a proton-spillover mechanism: hydrogen oxidized at the anode of a

PEM cell yields protons that migrate through the MEA to the cathode. The protons

spillover onto the cathode catalyst surface and become co-adsorbates with the target

organic reactants. The spillover protons have sufficient lifetime to catalyze isomerization

of the adsorbate; a transistor-like base current of spillover protons amplifies organic

transformations at the cathode.67-70

50

5.2.2 Hypothesis and objectives

The multi spectroscopy cell will be utilized to monitor EPOCH conversion of

epoxide ring-opening reactions via operando FTIR. Previous studies have examined

EPOCH reactions through the use of in-line spectroscopy techniques to monitor the

reaction effluent.67, 69

These reactions are excellent candidates to be studied via operando

FTIR because the reactants are adsorbed on the surface, reduced by spillover protons and

then desorbed. Each of these steps will have a unique fingerprint in the FTIR spectra.

The efficiency of the reaction can then be measured as a function of potential.

Furthermore, the reaction intermediates can be studied, elucidating the multi-step

reduction.

51

REFERENCES

1. Gilbert, T.; Kirss, R.; Foster, N.; Davies, G., Chemistry: The Science in Context.

Second ed.; W.W Norton & Company: New York, 2009.

2. Bard, A. J.; Faulkner, L. R., Electrochemical Methods: Fundamentals and

Applications. Second Edition ed.; John Wiley & Sons: New York, 2001; p 833.

3. Barbir, F., PEM Fuel Cells: Theory and Practice. Elsevier: 2005.

4. Agmon, N., The Grotthuss mechanism. Chemical Physics Letters 1995, 244,

(5,6), 456-62.

5. Liu, R.; Smotkin, E., Array membrane electrode assemblies for high throughput

screening of direct methanol fuel cell anode catalysts. Journal of Electroanalytical

Chemistry 2002, 535, 49-55.

6. Liu, L.; Pu, C.; Viswanathan, B.; Fan, Q.; Liu, R.; Smotkin, E., Carbon supported

and unsupported Pt-Ru anodes for liquid feed direct methanol fuel cells. Electrochimica

Acta 1998, 43, (24), 3657-3663.

7. Srinivasan, S., Fuel Cells: From Fundamentals to Applications. Springer Science:

New York, 2006.

8. Zawodzinski, T. A., Jr.; Derouin, C.; Radzinski, S.; Sherman, R. J.; Smith, V. T.;

Springer, T. E.; Gottesfeld, S., Water uptake by and transport through Nafion 117

membranes. Journal of the Electrochemical Society 1993, 140, (4), 1041-7.

9. Topsoe, H., Developments in operando studies and in situ characterization of

heterogeneous catalysts. Journal of Catalysis 2003, 216, (1-2), 155-164.

10. Bernardi, D. M.; Verbrugge, M. W., A mathematical model of the solid-polymer-

electrolyte fuel cell. Journal of the Electrochemical Society 1992, 139, (9), 2477-91.

11. Dawn M. Bernardi, M. W. V., Mathematical model of a gas diffusion electrode

bonded to a polymer electrolyte. AIChE Journal 1991, 37, (8), 1151-1163.

52

12. Somorjai, G. A., Introduction to Surface Chemistry and Catalysis. John Wiley &

Sons, Inc.: New York, NY, 1994.

13. Shao, M. H.; Adzic, R. R., Spectroscopic Identification of the Reaction

Intermediates in Oxygen Reduction on Gold in Alkaline Solutions. Journal of Physical

Chemistry B 2005, 109, (35), 16563-16566.

14. Shao, M.-h.; Liu, P.; Adzic, R. R., Superoxide Anion is the Intermediate in the

Oxygen Reduction Reaction on Platinum Electrodes. Journal of the American Chemical

Society 2006, 128, 7408-7409.

15. Smith, M. C.; Gilbert, J. A.; Mawdsley, J. R.; Seifert, S.; Myers, D. J., In Situ

Small-Angle Scattering Observation of Pt Catalyst Particle Growth During Potential

Cycling. Journal of the American Chemical Society 2008, 130, (26), 8112-8113.

16. Andrews, P. T.; Collins, I. R.; Dose, V.; Fink, J.; Fuggle, J. C.; Godby, R. W.;

Inglesfield, J. E.; Schneider, R.; Somers, J.; Vvedensky, D. D.; Zaanen, J.; Zeller, R.,

Unoccupied Electronic States. Springer-Verlag: 1992; Vol. 69.

17. Stohr, J., NEXAFS Spectroscopy. Springer-Verlag: Berlin, 1992; p 403.

18. Fay, M. J.; Proctor, A.; Hoffmann, D. P.; Hercules, D. M., Unraveling EXAFS

spectroscopy. Analytical Chemistry 1988, 60, (21), 1225A-1226A, 1228A, 1230A-

1234A, 1236A-1238A, 1240A-1243A.

19. Agarwal, B. K., X-Ray Spectroscopy. Second ed.; Springer-Verlag: Berlin, 1991.

20. Patnaik, P., Dean's Analytical Chemistry Handobook. Second ed.; McGraw-Hill:

New York, 2004.

21. Neutron and X-ray Specroscopy. Springer: Dordrecht, The Netherlands, 2006.

22. Singh, J.; Tromp, M.; Safonova, O. V.; Glatzel, P.; Bokhoven, J. A. v., In Situ

XAS with High-Energy Resolution: The Changing Structure of Platinum During the

Oxidation of Carbon Monoxide. Catalysis Today 2009, in press.

53

23. Maniguet, S.; Mathew, R. J.; Russell, A. E., EXAFS of carbon monoxide

oxidation on supported Pt fuel cell electrocatalysts. Journal of Physical Chemistry B

2000, 104, (9), 1998-2004.

24. Teliska, M.; O'Grady, W. E.; Ramaker, D. E., Determination of O and OH

Adsorption Sites and Coverage in Situ on Pt Electrodes from Pt L2-3 X-ray Absorption

Spectroscopy. Journal of Physical Chemistry B 2005, 109, (16), 8076-8084.

25. Teliska, M.; O'Grady, W. E.; Ramaker, D. E., Determination of H Adsorption

Sites on Pt/C Electrodes in HClO4 from Pt L2-3 X-ray Absorption Spectroscopy. Journal

of Physical Chemistry B 2004, 108, (7), 2333-2344.

26. Teliska, M.; Murthi, V. S.; Mukerjee, S.; Ramaker, D. E., Site-Specific vs

Specific Adsorption of Anions on Pt and Pt-Based Alloys. Journal of Physical Chemistry

C 2007, 111, (26), 9267-9274.

27. Scott, F. J.; Mukerjee, S.; Ramaker, D. E., CO Coverage/Oxidation Correlated

with PtRu Electrocatalyst Particle Morphology in 0.3 M Methanol by In Situ XAS.

Journal of The Electrochemical Society 2007, 154, (5), A396-A406.

28. Russell, A. E.; Maniguet, S.; Mathew, R. J.; Yao, J.; Roberts, M. A.; Thompsett,

D., In situ X-ray absorption spectroscopy and X-ray diffraction of fuel cell

electrocatalysts. Journal of Power Sources 2001, 96, (1), 226-232.

29. Viswanathan, R.; Hou, G.; Liu, R.; Bare, S. R.; Modica, F.; Mickelson, G.; Segre,

C. U.; Leyarovska, N.; Smotkin, E. S., In-Situ XANES of Carbon-Supported Pt-Ru

Anode Electrocatalyst for Reformate-Air Polymer Electrolyte Fuel Cells. Journal of

Physical Chemistry B 2002, 106, (13), 3458-3465.

30. Wiltshire, R. J. K.; King, C. R.; Rose, A.; Wells, P. P.; Hogarth, M. P.;

Thompsett, D.; Russell, A. E., A PEM fuel cell for in situ XAS studies. Electrochimica

Acta 2005, 50, (25-26), 5208-5217.

31. Roth, C.; Martz, N.; Buhrmester, T.; Scherer, J.; Fuess, H., In-situ XAFS fuel cell

measurements of a carbon-supported Pt-Ru anode electrocatalyst in hydrogen and direct

methanol operation. Physical Chemistry Chemical Physics 2002, 4, (15), 3555-3557.

54

32. Stoupin, S.; Chung, E.-H.; Chattopadhyay, S.; Segre, C. U.; Smotkin, E. S., Pt and

Ru X-ray Absorption Spectroscopy of PtRu Anode Catalysts in Operating Direct

Methanol Fuel Cells. Journal of Physical Chemistry B 2006, 110, (20), 9932-9938.

33. Roth, C.; Benker, N.; Buhrmester, T.; Mazurek, M.; Loster, M.; Fuess, H.;

Koningsberger, D. C.; Ramaker, D. E., Determination of O[H] and CO Coverage and

Adsorption Sites on PtRu Electrodes in an Operating PEM Fuel Cell. Journal of the

American Chemical Society 2005, 127, (42), 14607-14615.

34. Scott, F. J.; Roth, C.; Ramaker, D. E., Kinetics of CO Poisoning in Simulated

Reformate and Effect of Ru Island Morphology on PtRu Fuel Cell Catalysts As

Determined by Operando X-ray Absorption Near Edge Spectroscopy. Journal of Physical

Chemistry C 2007, 111, (30), 11403-11413.

35. Viswanathan, R.; Liu, R.; Smotkin, E. S., In situ x-ray absorption fuel cell.

Review of Scientific Instruments 2002, 73, (5), 2124-2127.

36. Teliska, M.; Ramaker, D. E.; Srinivasamurthi, V.; Mukerjee, S., In situ

determination of O(H) adsorption sites on Pt based alloy electrodes using X-ray

absorption spectroscopy. Proceedings - Electrochemical Society 2005, 2003-30,

(Fundamental Understanding of Electrode Processes), 212-216.

37. Ramaker, D. E.; Qian, X.; O'Grady, W. E., `Atomic' X-ray absorption fine

structure: a new tool for examining electronic and ionic polarization effects. Chemical

Physics Letters 1999, 299, (2), 221-226.

38. Korzeniewski, C.; Pons, S., In situ infrared spectroelectrochemistry. Journal of

Vacuum Science & Technology, B: Microelectronics and Nanometer Structures 1985, 3,

(5), 1421-4.

39. Korzeniewski, C.; Shirts, R. B.; Pons, S., Field-induced infrared absorption in

metal surface spectroscopy: the electrochemical Stark effect. Journal of Physical

Chemistry 1985, 89, (11), 2297-8.

40. Lebedeva, N. P.; Rodes, A.; Feliu, J. M.; Koper, M. T. M.; Santeen, R. A. v., Role

of Crystalline Defects in Electrocatalysis: CO Adsorption and Oxidation on Stepped

Platinum Electrodes as Studies by In Situ Infrared Spectrpscopy. Journal of Physical

Chemistry B 2002, 106, 9863-9872.

55

41. Kim, C. S.; Korzeniewski, C.; Tornquist, W. J., Site specific co-adsorption at

Pt(335) as probed by infrared spectroscopy: structural alterations in the CO adlayer under

aqueous electrochemical conditions. Journal of Chemical Physics 1994, 100, (1), 628-30.

42. Kim, C. S.; Korzeniewski, C., Vibrational Coupling as a Probe of Adsorption at

Different Structural Sites on a Stepped Single-Crystal Electrode. Analytical Chemistry

1997, 69, (13), 2349-2353.

43. Shin, J.; Korzeniewski, C., Infrared Spectroscopic Detection of CO Formed at

Step and Terrace Sites on a Corrugated Electrode Surface Plane during Methanol

Oxidation. Journal of Physical Chemistry 1995, 99, (11), 3419-22.

44. Kim, C. S.; Tornquist, W. J.; Korzeniewski, C., Infrared spectroscopy as a probe

of carbon monoxide adsorption at platinum(335) under aqueous electrochemical

conditions. Journal of Physical Chemistry 1993, 97, (24), 6484-91.

45. Kim, C. S.; Tornquist, W. J.; Korzeniewski, C., Site-dependent vibrational

coupling of CO adsorbates on well-defined step and terrace sites of monocrystalline

platinum: mixed-isotope studies at Pt(335) and Pt(111) in the aqueous electrochemical

environment. Journal of Chemical Physics 1994, 101, (10), 9113-21.

46. Garcia, G.; Silva-Chong, J. A.; Guillen-Villafuerte, O.; Rodriguez, J. L.;

Gonzalez, E. R.; Pastor, E., CO Tolerant Catalysts for PEM Fuel Cells

Spectroelectrochemical Studies. Catalysis Today 2006, 116, 415-421.

47. Liu, R.; Iddir, H.; Fan, Q.; Hou, G.; Bo, A.; Ley, K. L.; Smotkin, E. S.; Sung, Y.

E.; Kim, H.; Thomas, S.; Wieckowski, A., Potential-Dependent Infrared Absorption

Spectroscopy of Adsorbed CO and x-ray Photoelectron Spectroscopy of Arc-Melted

Single-Phase Pt, PtRu, PtOs, PtRuOs, and Ru Electrodes. Journal of Physical Chemistry

B 2000, 104, (15), 3518-3531.

48. Kardash, D.; Huang, J.; Korzeniewski, C., Surface Electrochemistry of CO and

Methanol at 25-75 DegC Probed in Situ by Infrared Spectroscopy. Langmuir 2000, 16,

(4), 2019-2023.

49. Leung, L.-W. H.; Wieckowski, A.; Weaver, M. J., In Situ Infrared Spectroscopy

of Well-Defined Single-Crystal Electrodes: Adsorption and Electrooxidation of CO on

Pt(111). Journal of Physical Chemistry 1988, 92, 6985-6990.

56

50. Jin, J. M.; Lin, W. F.; Christensen, P. A., The Effects of the Specific Adsorption

of Anion on the Reactivity of the Ru(0001) Surface Towards CO Adsorption and

Oxidation: In Situ FITRS Studies. Physical Chemistry Chemical Physics 2008, 10, 3774-

3783.

51. Stamenkovic, V. R.; Arenz, M.; Lucas, C. A.; Gallagher, M. E.; Ross, P. N.;

Markovic, N. M., Surface chemistry on bimetallic alloy surfaces: adsorption of anions

and oxidation of CO on Pt3Sn(111). Journal of the American Chemical Society 2003,

125, (9), 2736-2745.

52. Kardash, D.; Korzeniewski, C., Temperature Effects on Methanol Dissociative

Chemisorption and Water Activation at Polycrystalline Platinum Electrodes. Langmuir

2000, 16, (22), 8419-8425.

53. Sanicharane, S.; Bo, A.; Sompalli, B.; Gurau, B.; Smotkin, E. S., In situ 50 DegC

FTIR spectroscopy of Pt and PtRu direct methanol fuel cell membrane electrode

assembly anodes. Journal of the Electrochemical Society 2002, 149, (5), A554-A557.

54. Tkach, I.; Panchenko, A.; Kaz, T.; Gogel, V.; Friedrich, K. A.; Roduner, E., In

situ study of methanol oxidation on Pt and Pt/Ru-mixed with Nafion anodes in a direct

methanol fuel cell by means of FTIR spectroscopy. Physical Chemistry Chemical Physics

2004, 6, (23), 5419-5426.

55. Fan, Q.; Pu, C.; Ley, K. L.; Smotkin, E. S., In Situ FTIR-Diffuse Reflection

Spectroscopy of the Anode Surface in a Direct Methanol/Oxygen Fuel Cell. Journal of

the Electrochemical Society 1996, 143, (2), L21-L23.

56. Fan, Q. B.; Pu, C.; Smotkin, E. S., In situ Fourier transform infrared-diffuse

reflection spectroscopy of direct methanol fuel cell anodes and cathodes. Journal Of The

Electrochemical Society 1996, 143, (10), 3053-3057.

57. Vijayaraghavan, G.; Gao, L.; Korzeniewski, C., Methanol Electrochemistry at

Carbon-Supported Pt and PtRu Fuel Cell Catalysts: Voltammetric and in Situ Infrared

Spectroscopic Measurements at 23 and 60C. Langmuir 2003, 19, 2333-2337.

58. Rice, C.; Tong, Y.; Oldfield, E.; Wieckowski, A.; Hahn, F.; Gloaguen, F.; Leger,

J.-M.; Lamy, C., In Situ Infrared Study of Carbon Monoxide Adsorbed onto Commercial

Fuel-Cell-Grade Carbon-Supported Platinum Nanoparticles: Correlation with 13

C NMR

Results. Journal of Physical Chemistry C 2000, 104, 5803-5807.

57

59. Igarashi, H.; Fujino, T.; Zhu, Y.; Uchida, H.; Watanabe, M., CO Tolerance of Pt

Alloy Electrocatalysts for Polymer Electrolyte Fuel Cells and the Detoxification

Mechanism. Physical Chemistry Chemical Physics 2001, 3, 306-314.

60. Korzeniewski, C.; Basnayake, R.; Vijayaraghavan, G.; Li, Z.; Xu, S.; Casadonte,

D. J., Methanol electrochemical oxidation at nanometer-scale PtRu materials. Surface

Science 2004, 573, (1), 100-108.

61. Huang, J.; Korzeniewski, C., A temperature controlled cell for in situ infrared

spectroelectrochemical measurements and its use in the study of CO isothermal

desorption. Journal of Electroanalytical Chemistry 1999, 471, (2), 146-150.

62. Yang, Y. Y.; Zhou, Z. Y.; Sun, S. G., In situ FTIRS studies of kinetics of

HCOOH oxidation on Pt(110) electrode modified with antimony adatoms. Journal of

Electroanalytical Chemistry 2001, 500, (1-2), 233-240.

63. Bo, A.; Sanicharane, S.; Sompalli, B.; Fan, Q.; Gurau, B.; Liu, R.; Smotkin, E. S.,

In Situ Stark Effects with Inverted Bipolar Peaks for Adsorbed CO on Pt Electrodes in

50oC Direct Methanol Fuel Cells. Journal of Physical Chemistry B 2000, 104, 7337-

7381.

64. Quijada, C.; Rodes, A.; Huerta, F.; Vazquez, J. L., In situ FT-IRRAS study of

SO2 adlayers formed on Pt(111) electrodes from open-circuit adsorption in acidic media.

Electrochimica Acta 1998, 44, (6-7), 1091-1096.

65. Quijada, C.; Rodes, A.; Vazquez, J. L.; Perez, J. M.; Aldaz, A., Electrochemical

behavior of aqueous sulfur dioxide at polycrystalline Pt electrodes in acidic medium. A

voltammetric and in-situ FT-IR study. Part II. Promoted oxidation of sulfur dioxide.

Reduction of sulfur dioxide. Journal of Electroanalytical Chemistry 1995, 398, (1-2),

105-15.

66. Quijada, C.; Rodes, A.; Vazquez, J. L.; Perez, J. M.; Aldaz, A., Electrochemical

Behaviour of Aqueous SO2 at Pt Electrodes in Acidic Medium. A Voltammetric and In

Situ Fourier Transform IR Study Part I. Oxidation of SO2 on Pt Electrodes with Sulphur-

Oxygen Adsorbed Species. Journal of Electroanalytical Chemistry 1995, 394, 217-227.

67. Ploense, L.; Salazar, M.; Gurau, B.; Smotkin, E. S., Spectroscopic study of

NEMCA promoted alkene isomerizations at PEM fuel cell Pd-Nafion cathodes. Solid

State Ionics 2000, 136-137, 713-720.

58

68. Ploense, L.; Salazar, M.; Gurau, B.; Smotkin, E. S., Proton spillover promoted

isomerization of n-butylenes on Pd-black cathodes/Nafion 117. Journal of the American

Chemical Society 1997, 119, (47), 11550-11551.

69. Salazar, M.; Smotkin, E. S., Electrochemically promoted olefin isomerization

reactions at polymer electrolyte fuel cell membrane electrode assemblies. Journal of

Applied Electrochemistry 2006, 36, (11), 1237-1240.

70. Vayenas, C. G.; Bebelis, S.; Ladas, S., Dependence of catalytic rates on catalyst

work function. Nature (London, United Kingdom) 1990, 343, (6259), 625-7.

71. In Situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis.

First ed.; Elsevier: 2007.

72. Skoog, D. A.; Holler, F. J.; Crouch, S. R., Principles of Instrumental Analysis.

Sixth ed.; Thompson Brooks/Cole: 2007.

73. Fan, Q.; Pu, C.; Smotkin, E. S. In In-situ FTIR-diffuse reflection study of

methanol oxidation mechanisms on fuel cell anodes, Washington, DC, USA, 1996; IEEE,

Piscataway, NJ, USA: Washington, DC, USA, 1996; pp 1112-1116.

74. Gurau, B.; Smotkin, E. S., Methanol crossover in direct methanol fuel cells: a link

between power and energy density. Journal of Power Sources 2002, 112, (2), 339-352.

75. Zhenbo, W.; Rivera, H.; Wang, X.-P.; Zhang, H.-X.; Feng, P.-X.; Lewis, E.;

Smotkin, E. S., Catalyst failure analysis of a direct methanol fuel cell membrane

electrode assembly. Journal of Power Sources 2008, 177, (2), 386-392.

76. Zelenay, P.; Kim, Y. S., Performance Degradation of DMFC MEAs and Methods

of Improving Their Longevity. In Fuel Cells Durability Workshop, Washington, DC,

2005; p 29.

77. Ren, X.; Zelenay, P.; Thomas, S.; Davey, J.; Gottesfeld, S., Recent advances in

direct methanol fuel cells at Los Alamos National Laboratory. Journal of Power Sources

2000, 86, (1-2), 111-116.

59

78. Piela, P.; Eickes, C.; Brosha, E.; Garzon, F.; Zelenay, P., Ruthenium Crossover in

Direct Methanol Fuel Cell with Pt-Ru Black Anode. Journal of the Electrochemical

Society 2004, 151, (12), A2053-A2059.

79. Eickes, C.; Piotr Piela, J. D., Piotr Zelenay, Recoverable Cathode Performance

Loss in Direct Methanol

Fuel Cells. Journal of The Electrochemical Society 2006, 153, (1), A171-A178.

80. Lewis, E. A.; Jia, Q.; Rivera, H.; Grice, C.; Stoupin, S.; Fuccillo, M.; Segre, C.;

Smotkin, E. S., Time and potential dependent in situ XANES and FTIR of operating fuel

cells. In Presentation at 214th Meeting of the Electrochemical Society, Honolulu, HI,

2008.

81. Frelink, T. Ruthenium Promotion for the Electrochemical Oxidation of Methanol.

Technische University Einhoven, The Netherlands, 1995.

82. Liu, L.; Viswanathan, R.; Liu, R.; Smotkin, E. S., Methanol oxidation on Nafion

spin-coated polycrystalline platinum and platinum alloys. Electrochemical and Solid-

State Letters 1998, 1, (3), 123-125.

83. Mukerjee, S. Platinum and Platinum Alloy Electrocatalysts for Oxygen Electrodes

in Proton Exchange Membrane Fuel Cells: Electrochemical and X-Ray Absorption

Spectroscopic Investigation. Texas A & M University, 1994.

84. Mukerjee, S.; Srinivasan, S., Enhanced electrocatalysis of oxygen reduction on

platinum alloys in proton exchange membrane fuel cells. Journal of Electroanalytical

Chemistry 1993, 357, (1-2), 201-24.

85. Mukerjee, S.; Srinivasan, S.; Soriaga, M., Role of Structural and Electronic

Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction. Journal of the

Electrochemical Society 1995, 142, (5), 1409.

86. Murthi, V. S.; Urian, R. C.; Mukerjee, S., Oxygen Reduction Kinetics in Low and

Medium Temperature Acid Environment: Correlation of Water Activation and Surface

Properties in Supported Pt and Pt Alloy Electrocatalysts. Journal of Physical Chemistry B

2004, 108, (30), 11011-11023.

60

87. Srinivasamurthi, V.; Urian, R. C.; Mukerjee, S., ORR kinetics of Pt based alloys

using a RDE and a correlation with their fuel cell performance. 2005, 2002-31, (Proton

Conducting Membrane Fuel Cells III), 99-110.

88. Ziegelbauer, J. M.; Murthi, V. S.; O'Laoire, C.; Gulla, A. F.; Mukerjee, S.,

Electrochemical kinetics and X-ray absorption spectroscopy investigations of select

chalcogenide electrocatalysts for oxygen reduction reaction applications. 2008, 53, (17),

5587-5596.

89. Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N.

M., Effect of Surface Composition on Electronic Structure, Stability, and Electrocatalytic

Properties of Pt-Transition Metal Alloys: Pt-Skin versus Pt-Skeleton Surfaces. Journal of

the American Chemical Society 2006, 128, (27), 8813-8819.

90. Stamenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M., Surface

segregation effects in electrocatalysis: kinetics of oxygen reduction reaction on

polycrystalline Pt3Ni alloy surfaces. Journal of Electroanalytical Chemistry 2003, 554-

555, 191-199.

91. Stamenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M., Surface

Composition Effects in Electrocatalysis: Kinetics of Oxygen Reduction on Well-Defined

Pt3Ni and Pt3Co Alloy Surfaces. Journal of Physical Chemistry B 2002, 106, (46),

11970-11979.

92. Koh, S.; Hahn, N.; Yu, C. F.; Strasser, P., Effects of Composition and Annealing

Conditions on Catalytic Activities of Dealloyed Pt-Cu Nanoparticle Electrocatalysts for

PEMFC. Journal of the Electrochemical Society 2008, 155, (12), B1281-B1288.

93. Koh, S.; Strasser, P., Electrocatalysis on Bimetallic Surfaces: Modifying Catalytic

Reactivity for Oxygen Reduction by Voltammetric Surface Dealloying. Journal of the

American Chemical Society 2007, 129, (42), 12624-12625.

94. Liu, Z.; Koh, S.; Yu, C.; Strasser, P., Synthesis, Dealloying, and ORR

Electrocatalysis of PDDA-Stabilized Cu-Rich Pt Alloy Nanoparticles. Journal of the

Electrochemical Society 2007, 154, (11), B1192-B1199.

95. Srivastava, R.; Mani, P.; Hahn, N.; Strasser, P., Efficient Oxygen Reduction Fuel

Cell Electrocatalysis on Voltammetrically Dealloyed Pt-Cu-Co Nanoparticles13.

Angewandte Chemie International Edition 2007, 46, (47), 8988-8991.

61

96. Strasser, P., Fuel cell catalyst particles have platinum-rich shell, copper core.

Advanced Materials & Processes 2008, 166, (1), 13-13.

97. Strasser, P.; Koh, S.; Greeley, J., Voltammetric surface dealloying of Pt bimetallic

nanoparticles: an experimental and DFT computational analysis. Physical Chemistry

Chemical Physics 2008, 10, (25), 3670-3683.

98. Janik, M. J.; Taylor, C. D.; Neurock, M., First-Principles Analysis of the Initial

Electroreduction Steps of Oxygen over Pt(111). Journal of the Electrochemical Society

2009, 156, (1), B126-B135.

99. Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.;

Rossmeisl, J.; Greeley, J.; Norskov, J. K., Changing the activity of electrocatalysts for

oxygen reduction by tuning the surface electronic structure. Angewandte Chemie,

International Edition 2006, 45, (18), 2897-2901.

100. Zhang, J. L.; Vukmirovic, M. B.; Sasaki, K.; Nilekar, A. U.; Mavrikakis, M.;

Adzic, R. R., Mixed-metal Pt monolayer electrocatalysts for enhanced oxygen reduction

kinetics. Journal of the American Chemical Society 2005, 127, (36), 12480-12481.

101. Zhang, J. L.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R.,

Controlling the catalytic activity of platinum-monolayer electrocatalysts for oxygen

reduction with different substrates. Angewandte Chemie-International Edition 2005, 44,

(14), 2132-2135.

102. Jacob, T.; Muller, R. P.; Goddard, W. A., Chemisorption of Atomic Oxygen on

Pt(111) from DFT Studies of Pt-Clusters. Journal of Physical Chemistry B 2003, 107,

(35), 9465-9476.

103. Anderson, A. B.; Roques, J.; Mukerjee, S.; Murthi, V. S.; Markovic, N. M.;

Stamenkovic, V., Activation Energies for Oxygen Reduction on Platinum Alloys: Theory

and Experiment. Journal of Physical Chemistry B 2005, 109, (3), 1198-1203.

104. Nilekar, A. U.; Mavrikakis, M., Improved oxygen reduction reactivity of platinum

monolayers on transition metal surfaces. Surface Science 2008, 602, (14), L89-L94.

105. Nilekar, A. U.; Xu, Y.; Zhang, J.; Vukmirovic, M. B.; Sasaki, K.; Adzic, R. R.;

Mavrikakis, M., Bimetallic and Ternary Alloys for Improved Oxygen Reduction

Catalysis. Topics in Catalysis 2007, 46, (3-4), 276-284.

62

106. Hyman, M. P.; Medlin, J. W., Mechanistic Study of the Electrochemical Oxygen

Reduction Reaction on Pt(111) Using Density Functional Theory. Journal of Physical

Chemistry B 2006, 110, (31), 15338-15344.

107. Jacob, T.; Goddard, W. A., III, Adsorption of atomic H and O on the (111)

surface of Pt3Ni alloys. Journal of Physical Chemistry B 2004, 108, (24), 8311-8323.

108. Greeley, J.; Norskov, J. K., Combinatorial Density Functional Theory-Based

Screening of Surface Alloys for the Oxygen Reduction Reaction. Journal of Physical

Chemistry C 2009, 113, (12), 4932-4939.

109. Hansen, H. A.; Rossmeisl, J.; Norskov, J. K., Surface Pourbaix diagrams and

oxygen reduction activity of Pt, Ag and Ni(111) surfaces studied by DFT. Physical

Chemistry Chemical Physics 2008, 10, (25), 3722-3730.

110. Karlberg, G. S.; Rossmeisl, J.; Norskov, J. K., Estimations of electric field effects

on the oxygen reduction reaction based on the density functional theory. Physical

Chemistry Chemical Physics 2007, 9, (37), 5158-61.

111. Jinnouchi, R.; Anderson, A. B., Electronic structure calculations of liquid-solid

interfaces. Combination of density functional theory and modified Poisson-Boltzmann

theory. Physical Review B 2008, 77, (24), 245417/1-245417/18.

112. Watanabe, M.; Motoo, S., Electrocatalysis by ad-atoms. III. Enhancement of the

oxidation of carbon monoxide on platinum by ruthenium ad-atoms. Journal of

Electroanalytical Chemistry and Interfacial Electrochemistry 1975, 60, (3), 275-83.

113. Watanabe, M.; Motoo, S., Electrocatalysis by ad-atoms. II. Enhancement of the

oxidation of methanol on platinum by ruthenium ad-atoms. Journal of Electroanalytical

Chemistry and Interfacial Electrochemistry 1975, 60, (3), 267-73.

114. Watanabe, M.; Uchida, M.; Motoo, S., Preparation of highly dispersed platinum +

ruthenium alloy clusters and the activity for the electrooxidation of methanol. Journal of

Electroanalytical Chemistry and Interfacial Electrochemistry 1987, 229, (1-2), 395-406.

115. Ley, K. L.; Liu, R.; Pu, C.; Fan, Q.; Leyarovska, N.; Segre, C.; Smotkin, E. S.,

Methanol oxidation on single-phase Pt-Ru-Os ternary alloys. Journal of the

Electrochemical Society 1997, 144, (5), 1543-1548.

63

116. Smotkin, E. S.; Diaz-Morales, R. R., New Electrocatalysts by Combinatorial

Methods. Annual Review of Materials Research 2003, 33, (1), 557-579.

117. Reddington, E.; Sapienza, A.; Gurau, B.; Viswanathan, R.; Sarangapani, S.;

Smotkin, E. S.; Mallouk, T. E., Combinatorial electrochemistry: a highly parallel, optical

screening method for discovery of better electrocatalysts. Science (Washington, D. C.)

1998, 280, (5370), 1735-1737.

118. Guerin, S.; Hayden, B. E.; Lee, C. E.; Mormiche, C.; Russell, A. E., High-

Throughput Synthesis and Screening of Ternary Metal Alloys for Electrocatalysis.

Journal of Physical Chemistry B 2006, 110, (29), 14355-14362.

119. Oudenhuijzen, M. K.; vanBokhoven, J. A.; Miller, J. T.; Ramaker, D. E.;

Koningsberger, D. C., Three-Site Model for Hydrogen Adsorption on Supported

Platinum Particles: Influence of Support Ionicity and Particle Size on the Hydrogen

Coverage. Journal of the American Chemical Society 2005, 127, (5), 1530-1540.

120. Janin, E.; von Schenck, H.; Göthelid, M.; Karlsson, U. O.; Svensson, M., Bridge-

bonded atomic oxygen on Pt(110). Physical Review B 2000, 61, (19), 13144.

121. Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D., Real-space multiple-

scattering calculation and interpretation of x-ray-absorption near-edge structure. Physical

Review B 1998, 58, (12), 7565.

122. Ankudinov, A. L.; Nesvizhskii, A. I.; Rehr, J. J., Dynamic screening effects in x-

ray absorption spectra. Physical Review B 2003, 67, (11), 115120.

123. Ankudinov, A. L.; Rehr, J. J.; Low, J.; Bare, S. R., Effect of Hydrogen Adsorption

on the X-Ray Absorption Spectra of Small Pt Clusters. Physical Review Letters 2001, 86,

(8), 1642.

124. Dimakis, N.; Iddir, H.; Diaz-Morales, R. R.; Liu, R.; Bunker, G.; Chung, E.-H.;

Smotkin, E. S., A Band Dispersion Mechanism for Pt Alloy Compositional Tuning of

Linear Bound CO Stretching Frequencies. Journal of Physical Chemistry B 2005, 109,

(5), 1839-1848.

125. Rehr, J. J.; Ankudinov, A. L., Progress in the theory and interpretation of

XANES. Coordination Chemistry Reviews 2005, 249, (1-2), 131-140.

64

126. Cotton, F. A., Chemical Applications of Group Theory. Third ed.; John Wiley &

Sons: New York, NY, 1990.

127. Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.;

Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.;

Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota,

K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.;

Kimijima, K.-I.; Iwashita, N., Scientific aspects of polymer electrolyte fuel cell durability

and degradation. Chemical Reviews 2007, 107, (10), 3904-3951.

128. Yang, D.; Ma, J.; Xu, L.; Wu, M.; Wang, H., The effect of nitrogen oxides in air

on the performance of proton exchange membrane fuel cell. Electrochimica Acta 2006,

51, (19), 4039-4044.

129. Knights, S. D., Jia, Nengyou, Chuy, Carmen, Zhang, Jiujun, , Fuel Cell Reactant

Supply - Effects of Reactant Contaminants. In Fuel Cell Seminar 2005, Palm Springs,

CA, 2005.

130. Mohtadi, R.; Lee, W. K.; Van Zee, J. W., Assessing durability of cathodes

exposed to common air impurities. Journal of Power Sources 2004, 138, (1-2), 216-225.

131. Cheng, X.; Shi, Z.; Glass, N.; Zhang, L.; Zhang, J.; Song, D.; Liu, Z.-S.; Wang,

H.; Shen, J., A review of PEM hydrogen fuel cell contamination: Impacts, mechanisms,

and mitigation. Journal of Power Sources 2007, 165, (2), 739-756.

132. St-Pierre, J.; Jia, N.; Rahmani, R., PEMFC Contamination Model: Comptetitive

Adsorption Demonstrated with NO2. Journal of the Electrochemical Society 2008, 155,

(4), B315-B320.

133. Garsany, Y.; Baturina, O. A.; Swider-Lyons, K. E., Impact of Sulfur Dioxide on

the Oxygen Reduction Reaction at Pt/Vulcan Carbon Electrocatalysis. Journal of the

Electrochemical Society 2007, 154, (7), B670-B675.

134. Moore, J. M.; Adcock, P. L.; Lakeman, J. B.; Mepsted, G. O., The effects of

battlefield contaminants on PEMFC performance. Journal of Power Sources 2000, 85,

(2), 254-260.