investigations into the electrochemical synthesis of hydrogen peroxide

139 Pages·2010·2.79 MB·English

H O
O + 2H + + 2e−
2(g) Co catalyst 2 2

INVESTIGATIONS INTO THE

ELECTROCHEMICAL SYNTHESIS OF

HYDROGEN PEROXIDE

Maria Inês Q. R. Campos

Masters in Chemistry

Department of Chemistry

FCTUC

July 2010

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Cover: Cobalt catalyst powder,
a rotating ring disk electrode,
and a fuel cell stack.
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“It takes about 50 years for a new idea to break
through and become vogue; no one likes an intruder,
particularly when he is upsetting the commonplace”

Kallen H, Mann A.
The one and the many
University Press of Chicago, 1979
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INVESTIGATIONS INTO THE
ELECTROCHEMICAL SYNTHESIS
OF
HYDROGEN PEROXIDE

Maria Inês Q. R. Campos
Dissertation presented for the Master in Chemistry,
Specialisation in- Quality Control and Environment
Supervisor Prof. Christopher Brett

July 2010

Universidade de Coimbra

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TABLE OF CONTENTS

TABLE OF SYMBOLS...................................................................................... xi
ABBREVIATIONS..........................................................................................xiii
ABSTRACT..................................................................................................xv
RESUMO...................................................................................................xvii
ACKNOWLEDGEMENTS..................................................................................xix
AGRADECIMENTOS.......................................................................................xxi

CHAPTER 1 - Introduction ..............................................................................23
1.1. CONTEXT.............................................................................................................................23
1.1.1. Hydrogen peroxide.........................................................................................................23
1.1.2. Fuel cells........................................................................................................................25
1.1.3. Non-precious metal catalysts for fuel cells...................................................................28

1.2. H2O2 SYNTHESIS – HISTORY AND STATE-OF-THE-ART.........................................................31
1.2.1. Large-Scale Processes...................................................................................................32
A. Autoxidation methods – Anthraquinone auto-oxidation..............................................32
B. Alcohol Autoxidation.....................................................................................................35
C. Electrolytic methods (The Huron-Dow Process)..........................................................35
D. Comparison of Large-Scale production methods.........................................................36
1.2.2. Emerging alternatives....................................................................................................36
A. Direct Synthesis.............................................................................................................37
B. Fuel Cells.......................................................................................................................38

1.3. FUNDAMENTALS OF THE TECHNIQUES USED.........................................................................40
1.3.1. Thermodynamic concepts..............................................................................................42
A. Oxygen reduction reaction (ORR).................................................................................42
B. Electrode reactions in hydrogen fuel cells....................................................................43
C. Reaction rate, current density and Tafel analysis.........................................................43
D. Mass transfer.................................................................................................................45
1.3.2. Methods for catalyst activity evaluation........................................................................45
A. Conventional 3-electrode cells.......................................................................................45
A.1 Cyclic voltammetry..........................................................................................................45
A.2 Rotating disk electrode....................................................................................................48
A.3 Rotating ring-disk electrode............................................................................................52
B. Single cell (PEMFC).....................................................................................................55
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B.1 Test station..........................................................................................................................56
B.2 Polarization curve...............................................................................................................56

1.4. OBJECTIVES..........................................................................................................................59

CHAPTER 2 - Experimental.............................................................................61
2.1 MATERIALS..........................................................................................................................61
2.1.1. Reagents.........................................................................................................................61
2.1.2. Equipment......................................................................................................................62

2.2 CONVENTIONAL 3-ELECTRODE ELECTROCHEMICAL CELL....................................................62
A. Electrode preparation....................................................................................................63
B. Set-up.............................................................................................................................64
2.2.1 Cyclic voltammetry.........................................................................................................65
2.2.2 Rotating disk electrode...................................................................................................65
2.2.3 Rotating ring disk electrode...........................................................................................66
A. Collection efficiency......................................................................................................66

2.3 SINGLE CELL (PROTON EXCHANGE MEMBRANE FUEL CELL).................................................67
2.3.1 MEA preparation...........................................................................................................68
2.3.2 Single Cell set up...........................................................................................................70
2.3.3 Dependence of the humidifier temperature..................................................................71
2.3.4 Dependence of the total gauge pressure........................................................................73
2.3.5 Polarization curves........................................................................................................73
A. Flow rates.......................................................................................................................73
B. Volume collected (Co catalyst)...........................................................................................74
2.3.6 Ohmic losses – current interrupt...................................................................................74
2.3.7 Hydrogen peroxide quantification................................................................................75
2.3.8 Open circuit voltage (OCV)...........................................................................................76

CHAPTER 3 - Results and Discussion .................................................................77
3.1 CHARACTERIZATION MEASUREMENTS IN THE 3-ELECTRODE CELL.......................................77
3.1.1 Cyclic voltammetry.........................................................................................................77
A. Carbon/baseline voltammetry........................................................................................77
B. Pt catalyst voltammetry..................................................................................................78
C. Co catalyst voltammetry.................................................................................................78
D. Active surface of metals (EPSA and ECSA).................................................................79
3.1.2 Rotating disk electrode...................................................................................................80
A. Pt hydrodynamic voltammograms.................................................................................81
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B. Co hydrodynamic voltammograms................................................................................81
3.1.3 Ink composition study – amount of Nafion...................................................................83
A. Pt catalyst.......................................................................................................................84
A.1 Mass activity (Pt catalysts).............................................................................................86
A.2 Koutecky-Levich equation.............................................................................................90
A.3 Number of electrons transferred...................................................................................93
A.4 Tafel analysis.................................................................................................................93
B. Co catalyst......................................................................................................................95
B.1 Levich equation..............................................................................................................96
B.2 Koutecky-Levich equation.............................................................................................96
B.3 Number of electrons.......................................................................................................98
B.4 Tafel analysis.................................................................................................................98
3.1.4 Rotating ring disk electrode...........................................................................................99
A. Collection efficiency....................................................................................................100
A.1 Collection efficiency – layer influence........................................................................101
B. Pt catalyst.....................................................................................................................102
C. Co catalyst....................................................................................................................103
C.1 Negative scan limits.....................................................................................................105
D. H O current efficiency %...........................................................................................105
2 2

3.2 PEM SINGLE CELL MEASUREMENTS...................................................................................107
3.2.1. Dependence of the humidifier temperature................................................................107
A. Theoretical model analysis..........................................................................................107
B. Testing of the theoretical model..................................................................................108
B.1 Reproducibility.............................................................................................................110
3.2.2. Dependence of the pressure gauge..............................................................................110
3.2.3. Polarization curves......................................................................................................111
A. Pt catalyst.....................................................................................................................112
B. Co catalyst....................................................................................................................114
B.1 H O maximum percentage produced.........................................................................116
2 2
B.2 Efficiency of the catalyst..............................................................................................118
3.2.4. Open circuit voltage (OCV).........................................................................................119
A. Theoretical calculations....................................................................................................119
B. Pt catalyst..........................................................................................................................120
C. Co catalyst.........................................................................................................................120
3.2.5. Electrode kinetics – Tafel analysis..............................................................................121
A. Pt catalyst..........................................................................................................................121
B. Co catalyst.........................................................................................................................121
3.2.6. Gains O /Air.................................................................................................................122
2
A. Pt catalyst..........................................................................................................................122
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B. Co catalyst.........................................................................................................................123
3.2.7. Parameters interfering in the Single Cell...................................................................124
3.2.8. Comparison between RDE and SC..............................................................................126

CHAPTER 4 - Conclusions.............................................................................131
REFERENCES.............................................................................................134

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