Nanostructured ti0 7m0 3o2 (MMo, ru) supports with novel cocatalytic functionality for pt advanced nanoelectrocatalysts for fuel cells

Abstract Thanks to the high energy yield and low environmental impact of hydrogen oxidation, the Proton Exchange Membrane Fuel Cell PEMFC and Direct Methanol Fuel Cells DMFCs represent p

National Taiwan University of Science and Technology

Department of Chemical Engineering

PhD Dissertation

Student ID: D9706814

Nanostructured Ti0.7M0.3O2 (M: Mo, Ru) Supports with

Nanoelectrocatalyst for Fuel Cells

Graduate Student : Van Thi Thanh Ho

July 2011

Abstract

Thanks to the high energy yield and low environmental impact of hydrogen oxidation, the Proton Exchange Membrane Fuel Cell (PEMFC) and Direct Methanol Fuel Cells (DMFCs) represent promising energy conversion technologies available today At present, carbon black supported platinum (Pt) catalyst is used for both fuel and air electrodes in PEMFCs and DMFCs at anode and cathodes However, several critical issues still need to be addressed before such cells can be commercialized for automotive applications For example, the oxygen reduction reaction (ORR) is kinetically limited at the cathode and instability of Pt on the cathode such as the loss

of Pt electrochemical surface area (ECSA) over time, because of Pt dissolution/aggregation/Oswald ripening side became the major contributors to the degradation of fuel cell performance Additionally, the predominance of weak interactions between the carbon support and the catalytic metal nanoparticles leads to the sintering of the catalytic metal nanoparticles and a consequent decrease in the

active surface area with long-term operation More important, the high potentials that

accelerate both electrochemical carbon corrosion and the dissolution of the active elements under normal operating conditions, are issues impacting on fuel cell durability that remain unresolved Furthermore, it is well-documented that at room

suffer from poor reaction kinetics are vulnerable to poisoning from CO-like intermediates formed during the oxidation of methanol and their relatively low stability under the acidic conditions may become catalytically inactive over time

In the search for improved catalyst materials, much research with different strategies have been used that addressed to find more active and stable catalysts for the fuel cell

Mo, Ru) Support with Novel Co-catalytic Functionality for Pt that Used as Advanced Nanoelectrocatalyst for Fuel Cells This work present a new approach by

can modify surface electronic structure of Pt, owing to a shift in the d-band centre of

high stability of Pt/Ti0.7M0.3O2 during potential cycling, which is attributable to the

enhancing the inherent structural, chemical stability and the corrosion-resistance of

activity which underlies a ‘bifunctional mechanism’ due to the high proton

interaction between Pt and intermediate species, which additionally enhances both CO-tolerance and the oxidation of methanol in the Methanol Oxidation Reaction

(able to support the same mass of Pt as traditional catalysts), resulting in significantly

membrane-electrode-assembly (MEA)

Furthermore, the improvement durability of Pt-based oxygen reduction reaction (ORR) catalysts was described in this work by changing the morphology of Pt-nanostructure from 0-dimension to 1-dimensional Pt nanowires structure that owing to their inherent structural characteristics, i.e high stability, preferential

exposure of highly active crystal facets, and easy electron transport with superior catalytic and stability for both ORR and MOR We demonstrate that these novel

durability over commercial Pt/C (E-TEK) and PtCo/C (E-TEK) catalysts for ORR for potential application in PEMFC and Pt/C (E-TEK), PtRu/C (JM) for MOR in DMFC This new approach opens a reliable path to the discovery advanced concept in designing new catalyst that can replace the traditional catalytic structure and motivate further researches in the field

KEY WORD: Nanostructured, Cocatalyst, Multifunctional, Fuel Cells

Acknowledgement

First and foremost, my deepest and sincerest appreciation goes to my advisor Prof Bing-Joe Hwang for suggesting the problem, supervising the work and being a potential source of inspiration at each stage of this thesis research work The completion of this thesis has been possible, only due to his intellectual support

I would like to thank my parent and husband for giving me the drive, ability and patience to seek the completion of this dissertation

I also thank my colleagues and professors in the Department of Chemical Engineering for their cooperation and supporting during the research period I would like to thank National Synchrotron Radiation Research Center and Peter, Shawn for providing XANES, XAS, EXAFS, XRD characterizations of my samples Thanks to the Department of Material for facilitating high resolution transmission electron microscopy measurements Finally, I gratefully acknowledge three years financial support from National Taiwan University of Science and Technology

Table of Contents

Abstract……….………I Acknowledgement……… IV Table of contents……… V

List of tables……… IX

List of scheme……… X

List of figures……….………XI

Chapter 1 Introduction………1

1.1 Overview about fuel cell technology………1

1.1.1 Proton exchange membrane fuel cells (PEMFCs)……… …4

1.1.1.1 Overview about Proton Exchange Membrane Fuel Cell………5

1.1.1.2 The Operating Principle of Proton Exchange Membrane Fuel Cell…… …….6

1.1.2 Direct methanol fuel cells (DMFCs)……… 9

1.2 Other cells……… 12

Chapter 2 Current Challenges for the Polymer Electrolyte Membrane Fuel Cell and

Direct Methanol Fuel Cell……….………14

2.1 Challenges for the Polymer Electrolyte Membrane Fuel Cell……… 14

2.1.1 Degradation and durability issues in catalyst layers……….… 14

2.1.1.1 Platinum degradation……… ……14

2.1.1.2 Carbon support degradation ……… …19

2.1.1.3 Ionomer degradation and interfacial degradation……… ….23

2.1.2 Cost of Pt metal catalyst……… … 25

2.1.3 ORR activity issues……… … 26

2.2 Challenges for the Direct Methanol Fuel Cells……….… 28

2.2.1 Slow electro-oxidation kinetics……….……28

2.2.2 Methanol crossover……….… 29

2.2.3 Gas management on anode side……….…… 30

2.2.4 Electrode structure……… 30

2.2.4 1 Electrocatalysis ……… ………… 30

2.2.4 2 Carbon-Support Oxidation ……… 32

Chapter 3 Motivation and Objective of the Present Research……… 37

Chapter 4 Materials and Methods……… ……… 41

4.1 Materials……….41

4.2 Methods……… 41

4.2.1 X-ray diffraction (XRD) measurements……….………… 41

4.2.2 Transmission electron microscopy (TEM) measurements……… 41

4.2.3 Determining of metal (s) loading……….………42

4.2.4 Surface area measurements ……….………42

4.2.5 Conductivity measurements ……… ………42

4.2.6 Proton conductivity measurements ……….………43

4.2.7 X-ray absorption spectra (XAS) measurements ……… ………43

4.2.8 Electrode Preparation and Electrochemical Measurements ………43

4.2.9 MEA Fabrication and Single-Cell Test………45

4.2.10 DFT simulation to model Ti0.7Ru0.3O2 structure and its XRD pattern…… 46

Chapter 5 Nanostructured Ti0.7Mo0.3O2 Support Enhances Electron Transfer to Pt: High Performance Catalyst for Oxygen Reduction Reaction 47

5.1 Introduction……….………47

5.2 Experiment section……….49

5.2.1 Synthesis of Ti0.7Mo0.3O2 nanoparticles ……… ………49

5.2.2 Synthesis of Pt/Ti0.7Mo0.3O2 catalyst 50

5.3 Results and discussions………51

5 3.1 Characterization of Ti0.7Mo0.3O2 support material……….51

5.3.2 Characterization of Pt/Ti0.7Mo0.3O2 catalyst……… 56

5.3.3 Electrochemical properties of Pt/Ti0.7Mo0.3O2 catalyst 64

5.4 Conclusions……… 71

Chapter 6 Robust non-Carbon Ti 0.7 Ru 0.3 O 2 Support with co-Catalytic Functionality for Pt : Enhances Catalytic Activity and Durability for Fuel Cells 73

6.1 Introduction……….73

6.2 Experiment section……….75

6.2.1 Synthesis of Ti0.7Ru0.3O2 nanoparticles ……….……… 75

6.2.2 Synthesis of Pt/Ti0.7Ru0.3O2 catalyst 76

6.3 Results and discussions………77

6.4 Conclusions……….…….93

Chapter 7 Direct Growth One-dimensional Pt Nanowires Supported on Ti 0.7 Ru 0.3 O 2 : Used as an Advanced Nanoelectrocatalyst for Methanol Oxidation and Oxygen Reduction Reaction 95

7.1 Introduction……….95

7.2 Experiment section……….98

7.2.1 Synthesis of Ti0.7Ru0.3O2 nanoparticles ……… 98

7.2.2 Growth of 50 wt% Pt NW supported on Ti0.7Ru0.3O2 ……… 98

7.2.3 Preparation of 50 wt% Pt nanoparticles supported on carbon black ……… 99

7.3 Results and discussions………99

7.4 Conclusions………116

Chapter 8 Summary and Conclusions……… 117

Supporting information……… 120

References……….126

Cuuriculum vitae of author……….140

List of publishcations……… 141

List of patents……… 142

List of conferences/workshops………143

List of Tables

Table 1.1 Summary of major types of fuel cells………3 Table 6.1 Properties of different support materials … ……… …80 Table 6.2 Electrochemical properties of Pt/Ti0.7Ru0.3O2 and commercial catalysts … …… 83

Table 7.1 Electrochemical properties of Pt NW/Ti0.7Ru0.3O2, Pt NW/C and Pt NP/C catalyst 10

List of Schemes

Scheme 2.1 Schematic presents the CO-poisoning of Pt cataltst………20 Scheme 2.2 Methanol crossover phenomenon……….31 Scheme 3.1 Traditional approach to improve the performance of catalyst at low temperature

fuel cells……… ……….…31

Scheme 3.2 New approaches to improve the performance of catalyst at low temperature fuel

cells……… ………31

Scheme 5.1 Schematic illustration of synthesis of Ti0.7Mo0.3O2 by low temperature

synthesis……… ……….31

List of Figures

Figure 1.1 Sir William Grove-credited with first electrochemical H2/O2 reaction to create

energy ……… 1

Figure 1.2 The applications of fuel cells vary depending of the type of fuel cell to be used with producing power anywhere in the 1 Watt to 10 Kilowatt range………4

Figure 1.3 Schematic of a polymer electrolyte membrane fuel cell……… 5

Figure 1.4 Schematic representations of processes inside a PEMFC……… … 7

Figure 1.5 Polymer Electrolyte Membrane (PEM) fuel cell stack……… … 9

Figure 1.6 Schematic of a direct methanol fuel cell………10

Figure 1.7 Direct Methanol Fuel Cell: Potential Uses………12

Figure 2.1 Pt particle growth is accelerated by a number of operational and design issues 15

Figure 2.2 Deposited Pt on membrane after cycles………16

Figure 2.3 Cross-sectional TEM images of a MEA after 1.0 V was applied for 87h ……… ……17

Figure 2.4 Illustration of the effect of CO on a proton exchange membrane fuel cell ……… …18

Figure 2.5 Polarization for steady-state performance showing the effects of 5 ppm NO2/air……….19

Figure 2.6 Carbon corrosion………20

Figure 2.7 MEA performances before and after the corrosion tests using ………20

Figure 2.8 The change in current–voltage performance of PEMFC by cell reversal experiment ……… …22

Figure 2.9 TEM image showing model structure of Pt/ionomer/C microstructure that creates the critical 3-phase interfaces……… ………24

Figure 2.10 The membrane degradation under cycles………24 Figure 2.11 The plot presents the production of precious metal, the application of Pt for

industry and the platinum typically rises in price during periods……… ………25

Figure 2.12 Internal short circuit created by crossover……… ………30 Figure 2.13 Performance of single cell before and after the life test………… ………34 Figure 2.14 SEM images of MEA in cross-section: (a) before the 75 h life test; (b) after the

75 h life test……… ………35

Figure 2.15 TEM images of electrocatalysts: as-received 30% PtRu/C and 30% PtRu/C after

the 75 h life test………35

Figure 5.1 X-ray diffraction pattern of Ti0.7Mo0.3O2 support material and Pt/Ti0.7Mo0.3O2

catalyst ……… 52

Figure 5.2 TEM image of Ti0.7Mo0.3O2 support material ……… 53

Figure 5.3 Nitrogen adsorption/desorption isotherms of Ti0.7Mo0.3O2 support and its corresponding Barrett–Joyner–Halenda (BJH) pore size distribution….……… 53

Figure 5.4 (A) Mo K near-edge spectra of Ti0.7Mo0.3O2 and molybdenum oxides used as

position.……… 55

Figure 5.5 (A) TEM image of Pt/ Ti0.7Mo0.3O2 catalyst and (B) HRTEM image of Pt/

Figure 5.6 Mo K near-edge spectra of Ti0.7Mo0.3O2, Pt/ Ti0.7Mo0.3O2 and Molybdenum oxides

Figure 5.7 (A) Pt LIII-edge XANES spectra, and (B) variation in unfilled d-states for Pt foil and different catalyst samples (denoted in the figure).………59

Figure 5.8 XAS spectra of Ti L2,3-edge of Ti0.7Mo0.3O2, Pt/Ti0.7Mo0.3O2 samples ……… 61

Figure 5.9 (A) The k2-weighted Pt L3-edge extended X-ray absorption fine structure

oscillations ……… 63

Figure 5.10 Cyclic voltammograms for (A) 20 wt% Pt/C (E-TEK), (B) 30 wt% PtCo(1:1)/C

Figure 5.11 (A) Polarization curves showing the ORR current of Pt/Ti0.7Mo0.3O2 catalyst and the two kinds of commercial catalysts (B) Comparison of the ORR activities on the three types of catalysts ……… 66

Figure 5.12 Polarization curves for ORR performance on Pt/Ti0.7Mo0.3O2catalysts recorded at

Figure 5.15 Polarization curves for Pt/MoO2 before and after 500 cycles and 1000 cycles 71

Figure 6.1 X-ray diffraction pattern: inset in Figure 6.1 X-ray diffraction patterns of

Figure 6.2 (A) Ru K-edge XANES spectra, and (B) Ru K-edge FT-EXAFS spectra for

catalyst.……….……… 78

Figure 6.3 TEM of Ti0.7Ru0.3O2 support material………79

Figure 6.4 TEM of Pt/Ti0.7Ru0.3O2 catalyst ……… 81

Figure 6.5 The oxidation of pre-adsorbed CO as measured by CO-stripping voltammetry in

Figure 6.6 (A) Pt LIII-edge XANES spectra of Pt/Ti0.7Ru0.3O2 and commercial Pt/C (E-TEK),

-edge XANES white line (B) Variation in unfilled d-states for Pt foil and different catalyst

Pt/C (E-TEK) catalysts ……… 85

Figure 6.7 Cyclic voltammograms for methanol oxidation, sweep rate = 10 mV s-1 in N2

Figure 6.8 (A) TGA curve of Ti0.7Ru0.3O2 support material The material was measured

Figure 6.9 Steady-state anodic polarization curves for Pt/Ti0.7Ru0.3O2 and commercial

Figure 6.10 Polarization curves and power densities for DMFCs using Pt/Ti0.7Ru0.3O2 and carbon-supported commercial catalysts (denoted in the figure) used as anode Measurements

anode and cathode sides ………91

Figure 6.11 Polarization curves and power densities for PEMFCs using Pt/Ti0.7Ru0.3O2 and carbon-supported commercial catalysts (denoted in the figure) used as anode Measurements

sides……… ………92

Figure 7.1 (A) and (B) TEM micrograph of Pt nanowires grown on the surface of

nanowires with a growth direction along the <111> axis ……… 100

Figure 7.2 (A) and (B) TEM images of Pt nanowires grown on the surface of carbon black,

(C) HRTEM image of Pt NWs grown on carbon black… ……… 101

Figure 7.3 X-ray diffraction patterns of Ti0.7Ru0.3O2 and Pt NW/Ti0.7Ru0.3O2 ………… 102

Figure 7.4 Ru K-edge XANES spectra for different samples: Ti0.7Ru0.3O2 support, RuO2

Figure 7.5 (A) Cyclic voltammograms for methanol oxidation, sweep rate = 10 mV s-1.(B)

Figure 7.6 The CO-stripping voltammetry in N2-saturated 0.5 M H2SO4 solution for (A) Pt

Figure 7.7 Pt LIII-edge XANES spectra of Pt NW/Ti0.7Ru0.3O2 and Pt NP/C, Pt NW/C catalysts and Pt foil ……… 110

Figure 7.8 Cyclic voltammograms at sweep rate = 20 mV s-1 of Pt NW/Ti0.7Ru0.3O2 and Pt NP/C, Pt NW/C catalysts.……… 111

Figure 7.9 Polarization curves showing the ORR current of Pt NW/Ti0.7Ru0.3O2 and Pt NP/C,

Pt NW/C catalysts The current was normalized to the geometric area of electrode (0.1964

cm2), and the electrode rotation rate was kept at 1600 rpm, sweep rate of 1 mV s-1… … 113

Figure 7.10 Stability characterization of the Pt NW/Ti0.7Ru0.3O2 before and after stability by 2000 potential cycles and the electrode rotation rate was kept at 1600 rpm, sweep rate of 1 mV s-1 All measurements were measured in 0.5 M H2SO4 at 25 oC with the Pt loading 0.22 mg cm-2……….……….115

Figure S1 EDX spectrum of Ti0.7Mo0.3O2 support material……… 121

Figure S2 EDX spectrum of Pt/Ti0.7Mo0.3O2 catalyst ……… 121

Figure S3 EDX elemental mapping of Pt/Ti0.7Mo0.3O2 catalyst… ……… 122

Figure S4 EDX spectrum of Ti0.7Mo0.3O2 support material ……… 123

Figure S5 Comparison of XRD diffraction patterns of Ti0.7Ru0.3O2 support: (Black) Experimental; (Blue) DFT simulated Inset in figure S5 shows Ball-and-stick model of simulated stable structure of Ti0.7Ru0.3O2 with Ru at corners O, Ti and Ru atoms are shown as red, grey and green balls, respectively….……… 123

Figure S6 EDX spectrum and elemental mapping of Pt/Ti0.7Ru0.3O2 catalyst……… 124

Figure S7 Chronoamperometry curves of (a) the 20 wt% Pt/Ti0.7Ru0.3O2 and (b) commercial 20 wt% Pt-10 wt% Ru/C (JM) and (c) 20 wt% Pt/C (E-TEK) catalysts in 10 v/v% CH3OH in 0.5 M H2SO4 solution at 25 C for 1 hr The oxidation potential was kept at 0.6 V vs NHE……… ……… 125

Chapter 1 Introduction

1.1 Overview about fuel cell technology

As early as 1839, William Grove discovered the basic operating principle of fuel cells

by reversing water electrolysis to generate electricity from hydrogen and oxygen The principle that he discovered remains unchanged today

A fuel cell is an electrochemical “device” that continuously converts chemical energy into electric energy (and some heat) for as long as fuel and oxidant are supplied Fuel cells therefore bear similarities both to batteries, with which they share the electrochemical nature of the power generation process, and to engines which-unlike batteries-will work continuously consuming a fuel of some sort Here is where the analogies stop, though Unlike engines or batteries, a fuel cell does not need recharging, it operates quietly and efficiently, and -when hydrogen is used as fuel-it generates only power and drinking water Thus, it is a so-called zero emission

Figure 1.1 Sir William Grove-credited with first electrochemical H2/O2 reaction to

create energy He built a gas battery with 50 cells and found that 26 cells were the minimum needed to electrolyze water In this figure, four cells are shown

William Grove

The main advantages of fuel cells with respect to traditional energy converters are:

 A high conversion efficiency: Moreover, that efficiency

increases with diminishing load, a very interesting characteristic for the transportation sector where part load operation is the rule and ICEs run

at reduced efficiency in low load conditions

 Very low emissions: The actual emission level depends on the

fuel– for cells fed by pure hydrogen; true zero-emission performance is achieved since the only reaction product is water Even if natural gas

will be lower than a comparable internal combustion engine (ICE) due

to the fuel cell’s higher efficiency Additionally, no toxic nitrogen

 Low noise levels: Since the electrochemical reaction is a

conversion process that requires no moving parts, operation of the fuel cell is completely silent A fuel cell system, with all the necessary systems for cooling, power conversion and air and fuel supply will emit some noise, mainly due to the air compressor

 System scalability: Due to their construction, fuel cell systems

are modular power generators Efficient systems can be built for power levels from several Watts to several Mega Watts

Since then, different types of fuel cells have been developed They are typically

classified by either their operating temperature or the type of electrolyte Table 1.1

gives an overview of the main classes of fuel cells with their associated fuels, operating temperatures and electrolyte types

Table 1.1 Summary of major types of fuel cells [1, 2]

Charge

Alaline fuel cell

Direct methanol

Ion exchange Membrane (e.g., Nafion)

The applications of fuel cells vary depending of the type of fuel cell to be used Since fuel cells are capable of producing power anywhere in the 1 Watt to 10 Megawatt range they can be applied to almost any application that requires power On the smaller scale they can be used in cell phones, personal computers, and any other type

of personal electronic equipment In the 1kW - 100kW range a fuel cell can be used to power vehicles, both domestic and military, public transportation is also a target area for fuel cell application, along with any APU application And finally, in the 1MW - 10MW range fuel cells can be used to convert energy for distributed power uses (grid

Figure 1.2 The applications of fuel cells vary depending of the type of fuel cell to be

used with producing power anywhere in the 1 Watt to 10 Kilowatt range

1.1.1 Proton exchange membrane fuel cells (PEMFCs)

Proton Exchange Membrane Fuel Cells (PEMFCs) are believed to be the best type of fuel cell as the vehicular power source to eventually replace the gasoline and diesel internal combustion engines First used in the 1960s for the NASA Gemini program, PEMFCs are currently being developed and demonstrated for systems ranging from 1W to 2kW PEM fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte This polymer is permeable to protons when it is saturated with water, but

it does not conduct electrons

1.1.1.1 Overview about Proton Exchange Membrane Fuel Cell

Figure 1.3 Schematic of a polymer electrolyte membrane fuel cell

The fuel for the PEMFC is hydrogen and the charge carrier is the hydrogen ion (proton) At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons The hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power Oxygen, usually in the form of air, is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water Compared to other types of fuel cells, PEMFCs generate more power for a given volume or weight of fuel cell This high-power density characteristic makes them compact and lightweight In addition, the operating temperature is less than 100ºC, which allows rapid start-up These traits and the ability to rapidly change power output are some of the characteristics that make the PEMFC the top candidate for automotive power applications

Other advantages result from the electrolyte being a solid material, compared to a liquid The sealing of the anode and cathode gases is simpler with a solid electrolyte,

and therefore, less expensive to manufacture The solid electrolyte is also more immune to difficulties with orientation and has fewer problems with corrosion, compared to many of the other electrolytes, thus leading to a longer cell and stack life One of the disadvantages of the PEMFC for some applications is that the operating temperature is low Temperatures near 100ºC are not high enough to perform useful cogeneration Also, since the electrolyte is required to be saturated with water to operate optimally, careful control of the moisture of the anode and cathode streams is

1.1.1.2 The Operating Principle of Proton Exchange Membrane Fuel Cell

Within the PEMFC, hydrogen and oxygen are converted into water while generating electricity A schematic diagram of the processes occurring in a PEMFC is shown in Figure 1.4

The electrochemical reaction of those processes can be noted as:

H2 + ½ O2  H2O (1.1) This overall reaction takes place in two, spatially separated, half reactions, shown on the left and right hand side of Figure 1.4 respectively and given by:

In which (1.3) is typically termed the Hydrogen Oxidation Reaction (HOR) and (1.2)

is composed of two reactions: the Oxygen Reduction Reaction (ORR) (1.4) and the reaction recombining the ionized species into product water (1.5):

Figure 1.4 Schematic representations of processes inside a PEMFC

In order to technically exploit such a reaction, a fuel cell is composed of an anode compartment containing the fuel (typically gaseous hydrogen) and a cathode compartment filled with the oxidant gas (typically oxygen contained in ambient air) The two chambers are separated through an electrically insulating (i.e no electron conduction) and gas impermeable membrane – the electrolyte – that is capable of conducting protons The area in contact with the membrane is covered with a platinum catalyst both on the anode and cathode side and is therefore referred to as a

Adjacent to the catalyst layer on both sides of the membrane is a porous, electrically conducting gas diffusion layer (GDL) It allows reaction gases (i.e hydrogen and oxygen) to flow to the reaction sites on the catalyst layer and product water to flow back out The gas diffusion layers are held in place by gas impermeable, electrically conducting plates Gas channels are machined into these so called bipolar plates in order to distribute hydrogen and oxygen evenly over the membrane surface

From the anode inlet, hydrogen is distributed over the whole cell surface from where

it diffuses across the gas diffusion layer to the catalyst layer Upon contact with the

and an electron (e") The electron cannot cross the membrane and leaves the fuel cell

as electric current through the gas diffusion layer and the bipolar plate The proton on the other hand is transported across the membrane to the cathode side On the cathode side, oxygen enters the fuel cell, is distributed over the cell surface by the gas channels, and diffuses to the catalyst layer and–again through the presence of the platinum catalyst – the oxygen atom recombines with the proton and the electron to form water The product water then diffuses back through the gas diffusion layer and

The heat generated during the electrochemical reaction is transferred to the bipolar plates through conduction From there, it is either transmitted to the ambient air through specially designed cooling fins or it is evacuated from the system through a cooling liquid (CL) circulating within the bipolar plates Depending on the current density (i.e the current per unit of cell surface area), electrical voltages between 1 V and 0.5 V 1 are typically achieved The fuel cell’s characteristics and operating conditions determine the maximum current density and power that can be achieved

single cells are stacked to form the so-called fuel cell stack Such a configuration corresponds to an electrical series connection of the single elements The reaction gases and cooling liquid are supplied to the cells through a parallel network of supply channels

Figure 1.5 Polymer Electrolyte Membrane (PEM) fuel cell stack

1.1.2 Direct methanol fuel cells (DMFCs)

Direct methanol fuel cells (DMFC) employ a polymer membrane as an electrolyte The system is a variant of the polymer electrolyte membrane (PEM) cell however; the catalyst on the DMFC anode draws hydrogen from liquid methanol This action eliminates the need for a fuel reformer and allows pure methanol to be used as a fuel [1, 2]

Figure 1.6 Schematic of a direct methanol fuel cell

The pure methanol is mixed with steam and fed directly in to the cell at the anode Here, the methanol is converted to carbon dioxide and hydrogen ions The electrons are then pushed round an external circuit to produce electricity (before returning to the cathode) whilst the hydrogen protons pass across the electrolyte to the cathode, as occurs in a standard PEM fuel cell At the cathode, the protons and electrons combine with oxygen to produce water

Thermodynamics of direct methanol fuel cell:

Overall reaction: 3/2 O2 + CH3OH  CO2 + 2 H2O (1.9)

The operating temperature for DMFCs is in the range of 60-130 oC but is typically

portable applications and have been used in a wide variety of portable electronic products such as mobile phones and laptop computers Due to the low temperature conversion of methanol to hydrogen and carbon dioxide the DMFC system requires a noble metal catalyst The cost associated with this catalyst is outweighed by the ability of the unit to function without a reforming unit By using liquid methanol as a fuel some of the storage problems related to hydrogen are eliminated In addition, liquid methanol is often considered to be easier to transport and supply to the public

Some of these (CO-like) species are irreversibly adsorbed on the surface of the electrocatalyst and severely poison Pt for the occurrence of the overall reaction, which has the effect of significantly reducing the efficiency for fuel consumption and the power density of the fuel cell Thus, it is vitally important to develop new electrocatalysts to inhibit the poisoning and increase the electro-oxidation rate by at least two-three orders of magnitude Other species may be released with a consequent decrease of fuel efficiency, whereas, an efficient catalyst must allow a complete

and formic acid Methyl formiate and other substances have been found in traces.DMFCs are characterized by two slow reactions i.e methanol electro-oxidation and oxygen reduction with the further drawback of the presence of a mixed potential

at the cathode determined by the methanol crossover On the other hand, there are specific advantages in using methanol as direct fuel especially for what concerns costs, simplicity of design, large availability, easy handling and distribution Another

concern is that, even though significant progresses have been made in enhancing the electrocatalysis of the four-electron transfer oxygen reduction reaction at low temperatures, the overpotential of this reaction at desired current densities (e.g 500

mA cm-2) is still about 400 mV in H2/air fuel cells and increases by about 50-100

mV in DMFCs because of the effect caused by methanol, which crosses over from

cars, the DMFC will likely be used in smaller electronics, show below Figure 1.7

Figure 1.7 Direct Methanol Fuel Cell: Potential Uses

1.2 Other Cells

In addition to the major fuel cell types described above, there are other fuel cells that are mentioned in scientific journals from time to time, and also cells that are described as ‘fuel cells’, but are not really so A fuel cell is usually defined as an electrochemical device that converts a supplied fuel to electrical energy (and heat)

continuously, so long as reactants are supplied to its electrodes The implication is that neither the electrodes nor the electrolyte are consumed by the operation of the cell Of course, in all fuel cells the electrodes and electrolytes are degraded and subject to ‘wear and tear’ in use, but they are not entirely consumed in the way that happens with two of the three types of cells briefly shown below, both of which are sometimes described as ‘fuel cells’

Chapter 2 Current Challenges for the Polymer Electrolyte Membrane Fuel Cell and Direct Methanol Fuel

Cell

2.1 Challenges for the Polymer Electrolyte Membrane Fuel Cell

In proton exchange membrane fuel cells, cost, reliability and durability are important issues that need to be solved before their commercialization Their performance decrease during operation is attributed, amongst others, to the loss of electrochemical surface area occurring during long-term ageing, after transients or after an incident (faulty operation) These losses are mainly due to catalyst metal degradation and carbon-support corrosion, which are continuous irreversible processes that can

2.1.1 Degradation and durability issues in catalyst layers

2.1.1.1 Platinum degradation

Under a combination of different aggressive conditions (such as nano-scale particles, strong acidic environments, oxidizing conditions, reactive intermediates, durative flow of liquid and gas, high electric currents, and large potential gradients), the catalyst layer (CL) components tend to experience subtle changes and function losses during the operation of PEM fuel cells For example, Pourbaix diagrams show that Pt

is unstable at a potential range of 1.0–1.2V vs SHE (Standard Hydrogen Electrode)

accumulate and result in a gradual decline in power output during the longterm

operation of a PEM fuel cell.

Platinum degradation mechanisms

Direct reasons for Pt catalyst degradation include: (1) Pt particle agglomeration and particle growth, (2) Pt loss and redistribution, and (3) poisonous effects aroused by contaminants All these effects will lead to either a loss of effective catalytic active sites or a loss of electronic contact with conductors, resulting in apparent activity loss

in the CL during long-term operation

As demonstrated by many researchers, agglomeration and particle growth of the nanostructure of Pt is the most dominant mechanism for catalyst degradation in PEM fuel cells First of all, it is believed that nano-sized structural elements are able to

the inherent tendency to agglomerate into bigger particles to reduce the high surface energy As particles grow, their surface energy decreases and the growth process slows The preparation method of the membrane electrode assembly (MEA) could be

a cause of agglomeration and particle growth This can be proven by examining the

Figure 2.1 Pt particle growth is accelerated by a number of operational and design

issues

electrically charged species, is the main reason for particle growth in Pt/C catalysts,

where the Pt is transported through the liquid and/or through the ionomer and the

mechanisms are predominately responsible for degradation during the potential cycling process: (1) Pt particles detaching from the support and dissolving into the electrolyte without re-deposition, and/or (2) a combination of Pt particle coalescence

and Pt solution/re-precipitation within the solid ionomer (Figure 2.2)

Figure 2.2 Deposited Pt on membrane after cycles

Whatever mechanism the particle growth follows, dissolution of Pt is an important step during the catalyst degradation process The lower the Pt ion concentration, the lower the degradation kinetics for the Pt/C catalyst, different electrode aging process may reveal different dissolution reactions taking place at the anode and the cathode Using the rotating ring-disk electrode (RRDE) experiment following different sweep

with a charge transfer number of ca

Pt loss during operation is another major source of CL degradation This can be caused by many factors such as Pt dissolution and wash-out Further, Pt migration

within MEA has been observed to have the same effect as Pt loss Many groups have reported the presence of Pt particles inside the PEM as well as enrichment of Pt in the

particles originate from the dissolved Pt species, which diffuse in the ionomer phase and subsequently precipitate in the ionomer phase of the electrode or in the membrane The precipitation occurs via the reduction of Pt ions by hydrogen that has crossed over from the anode, and thus it is called the “micrometer-scale diffusion process”

Figure 2.3 Cross-sectional TEM images of a MEA after 1.0 V was applied for 87 h

(a) TEM image near the interface between the cathode catalyst layer and the PEM, (b) TEM image of the PEM 10 nm from the cathode layer, (c) TEM image near the interface between the anode catalyst layer and the PEM

Another likely cause of severe degradation of the CL in PEMFCs is contamination In general, contamination can be categorized into two groups based on the sources: the

system-derived contaminants, such as trace amounts of metallic ions or silicon from

system components (e.g., bipolar metal plates, membranes, and sealing gaskets) [23]Depending on effective time, poison dose, and the reversibility of the poison effect, different contaminants exhibit different poisonous characteristics on Pt catalysts in PEM fuel cells In literature, the most extensively investigated contaminant is CO CO

these sites and decrease the activity of the catalyst Even trace amounts of these impurities from the reactant gas are likely to reduce fuel cell performance due to

of a PEMFC is illustrated in Scheme 2.1 and Figure 2.4

Scheme 2.1 Schematic presents the CO-poisoning of Pt cataltst

Figure 2.4 Illustration of the effect of CO on a proton exchange membrane fuel cell

influence on the fuel cell is reversible Fuel cell performance decreases as a result of

contaminated air CV spectra of the clean and poisoned MEAs indicated that the

NO2 [26]

Figure 2.5 Polarization for steady-state performance showing the effects of 5 ppm

NO2/air [26]

2.1.1.2 Carbon support degradation

Nano-scaled Pt particles are usually distributed on carbon support materials to obtain

a maximum utilization ratio and to decrease the cost of fuel cells However, under prolonged operation at high temperatures, high water content, low pH, high oxygen concentration, existence of the Pt catalyst and/or high potential, carbon support is prone to degrade both physically and chemically, which is called carbon oxidation (or carbon corrosion) Carbon oxidation weakens the attachment of Pt particles to the carbon surface, and eventually leads to structural collapse and the detachment of Pt particles from the carbon support, resulting in declines of the catalyst active surface

area and fuel cell performance (Figure 2.6, 2.7) In Sato et al.’s study [27] it was proven that no performance degradation occurred when a Pt-black catalyst was applied as the anode electrode catalyst Under the same hydrogen starvation operation, however, the Pt/C catalyst based fuel cell experienced severe degradation Generally, carbon corrodes under three conditions: (1) normal operating potentials, (2) gross fuel starvation at the anode, and (3) partial hydrogen coverage at the anode

Figure 2.6 Carbon corrosion

Figure 2.7 MEA performances before and after the corrosion tests using

Measurements were performed at a cell temperature of 75 °C under ambient pressures

Carbon corrosion mechanisms

Carbon corrosion may occur as a chemical or an electrochemical process More specifically, carbon oxidation takes place along two pathways that are believed to

incomplete oxidation leading to the formation of surface groups (Eqs (2.1) and (2.2);

The subscript “s” denotes the surface species

Carbon corrosion is sensitive to many factors such as potential, carbon surface area, and relative humidity within the fuel cell Among these factors, potential is the most

1.0 V at room temperature Therefore, carbon corrosion on the cathode is more serious than that on the anode during normal steady-state operation This was proven

by an observed thickness decrease after long periods of operation, especially at high

 Gross fuel starvation

When the fuel is insufficient to provide the expected current for the PEM fuel cell (called fuel starvation), the potential value of the anode increases With anode potential increasing, the cell potential can decrease to a value substantially below normal and even drive the cell into reverse operation, where the anode potential is

higher than the cathode potential Once the potential of the anode rises to above 0.207

V or further to over 1.23 V with the fuel consumption, water electrolysis and carbon oxidation at the anode will occur to provide the required protons and electrons for the

These reactions can be observed when the fuel cell experiences bad flow distribution, gas blockages, or sudden current changes due to heavy load under transient conditions The electrocatalyst degradation caused by fuel starvation and found severe surface area losses of the electrocatalyst, as well as a drop in cell performance due to

carbon support corrosion after cell reversal (Figure 2.8)

Figure 2.8 The change in current–voltage performance of PEMFC by cell reversal

experiment: (a) before experiment; (b) after experiment for 3min; and (c) after

 Corrosion due to air/fuel boundary

Carbon corrosion can also arise from a non-uniform distribution of fuel on the anode side (partial hydrogen coverage) and from crossover of reactant gas through the membrane Both start up and shut down, as well as local fuel starvations, can cause this type of carbon corrosion When oxygen is present on the anode side, the ORR will occur in this area The carbon oxidation reaction (2.4) and water electrolysis reaction (2.5) will occur on the corresponding cathode side According to the experimental results, the most pronounced corrosion damage was found within the first 30 cycles, when air and fuel were alternately fed to the anode The cathode

2.1.1.3 Ionomer degradation and interfacial degradation

Typically, carbon supported Pt or modified/alloyed Pt catalysts are partially embedded

in a proton conducting polymeric ionomer,such as a recast Nafion ionomer, which mediates proton migration as well as water transport inside its pore system A model

of a Pt/ionomer/C microstructure that creates the critical 3-phase interface is shown

Figure 2.9 Except the Pt catalyst and the carbon support, Nafion ionomer also plays

an important role in the CL to influence the structure and performance The distribution of the ionomer, as well as its content in the CL, can directly impact the

and dissolution of the recast ionomer in CL could lead to a decrease in ionic/electronic conductivity and mass transport ability of the MEA Another aspect concerning the ionic/electronic conductivity and mass transport on a macroscopic scale is the interfacial components alternation of CL/PEM and CL/GDL during the aging process, which is also a source of performance degradation of PEM fuel cell, especially under extreme conditions