The Design, Construction and Research Application of a Differential Electrochemical Mass Spectrometer (DEMS) [Elektronische Ressource] / Sean Ashton. Gutachter: Matthias Arenz ; Ulrich K. Heiz ; Moniek Tromp. Betreuer: Ulrich K. Heiz

Sean Ashton

Informations

• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2011
• Nombre de lectures: 24
• Langue: English
• Poids de l'ouvrage: 10 Mo

Extrait

TECHNISCHE UNIVERSITÄT MÜNCHEN
Lehrstuhl für Physikalische Chemie

The Design, Construction and Research
Application of a Differential Electrochemical
Mass Spectrometer (DEMS)

Sean James Ashton

Vollständiger Abdruck der von der Fakultät für Chemie der Technischen Universität
München zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
genehmigten Dissertation.

Vorsitzender: Univ.-Prof. H. Gasteiger, Ph.D.
Prüfer der Dissertation:
1. Univ.-Prof. Dr. U. K. Heiz
2. Univ.-Prof. M. Tromp, Ph.D.
3. Prof. Dr. M. Arenz, University of Copenhagen / Dänemark
Die Dissertation wurde am 06.06.2011 bei der Technischen Universität München
eingereicht und durch die Fakultät für Chemie am 25.07.2011 angenommen.
Abstract
Electrochemical half-cell studies on industrial electrocatalysts contribute
significantly towards our understanding of fuel cell processes. However, the study
of complex, often overlapping reactions using standard methods is limited to the
interpretation of a single electrode current. Presented here are details of the
design, construction and characterisation of a differential electrochemical mass
spectrometer (DEMS) that enables the in-situ elucidation of electrode currents.
The capability of the instrument is demonstrated in two studies. In the first,
DEMS is used resolve the conversion of the methanol oxidation reaction to
carbon dioxide on high surface area carbon (HSAC) supported Pt and PtRu
catalysts, whilst the second focuses on the corrosion of industrial HSACs,
separating partial and complete oxidation processes. Despite that both systems
have long since been studied, new insights and understanding can be obtained
using DEMS.

Zusammenfassung
Untersuchungen industrieller Elektrokatalysatoren in elektrochemischen Halb-
Zellen tragen wesentlich zum Verständnis der Prozesse in Brennstoffzellen bei.
Die Untersuchung von komplexen – sich oft überlappenden - elektrochemischen
Reaktionen mittels Standardmethoden ist aber regelmäßig auf die Interpretation
einer einzigen „Stom-Antwort“ begrenzt. In der vorliegenden Arbeit werden das
Design, der Aufbau und die Anwendung eines differentiellen elektrochemischen
Massenspektrometers (DEMS) beschrieben, welcher es ermöglicht, die einzelnen
Komponenten des in der elektrochemischen Zelle fließenden Stromes zu
differenzieren. Die Wirkungsweise wird anhand zweier Systeme demonstriert, die
Methanol Oxidation auf Kohlenstoff geträgerten Pt und PtRu Katalysatoren
sowie die Korrosion industriell eingesetzter Kohlenstoff Trägern. Für beide
Systeme, welche schon seit Längerem untersucht werden, konnten mittels DEMS
neue Erkenntnisse erlangt werden.







„Electrochemistry is the science which deals with the conversion of matter to
electricity; and/or electricity to matter‟
Kyvstiakovsky (L.Antropov, Theoretical Electrochemistry) 1910.





Table of Contents

1 Introduction ...................................................................................................... 1
1.1 Background . 1
1.1.1 Fuel Cell Technology ........................................................................... 2
1.1.2 Electrocatalyst Development ............................... 5
1.2 Outline and Objectives ............................................................................... 9

2 Differential Electrochemical Mass Spectrometry ......................................... 10
2.1 Principle of Operation .............................................. 10
2.2 Instrument Design Solutions .................................... 13
2.2.1 Electrochemical Cells ........................................ 13
2.2.2 Membrane Interfaces ........................................ 22
2.2.3 Vacuum Systems & Mass Spectrometer ........... 25
2.3 Research Applications .............................................................................. 27
2.3.1 Radio-Isotope Labelled Experimentation ......... 27
2.3.2 Characterisation of Organic Adsorbates .......................................... 28
2.3.3 Study of the Electro-oxidation of Small Organic Compounds ......... 28
2.4 Conclusions ............................................................................................... 30

3 Design and Construction of the DEMS Instrument ...................................... 32
3.1 Design and Development Process ............................................................. 33
3.2 DEMS Instrument Overview .................................... 35
3.2.1 Principle Components ....................................... 37
3.2.2 Operating Hardware and Software .................................................. 38
3.4 Electrochemical Half-Cell Setup .............................. 40
iii 3.4.1 Dual Thin-layer Flow Cell Design ..................................................... 42
3.5 Membrane Interface ................................................. 48
3.6 Vacuum System Design ............................................. 52
3.6.1 Three-Stage Differential Pumping .................... 54
3.6.2 Tubular Aperture............................................................................... 57
3.7 Instrumentation, Control and Data Acquisition ...... 61
3.7.1 DEMS Measurement Setup ............................................................... 63
3.7.2 QMS Calibration Setup ..................................... 81
3.7.3 Labview Software Architecture ......................................................... 86
3.8 Data Analysis ............................................................. 95
3.9 Summary ................................................................... 99

4 Practical Aspects of the DEMS Instrument ................................................ 101
4.1 Electrochemical Cell ............................................... 102
4.1.1 Potential Control .............................................. 103
4.1.2 Effect of Electrolyte Flow Rate ........................ 109
4.2 Performance of the Membrane Interface Material ............................... 119
4.3 Optimisation of the QMS ........................................................................ 124
4.3.1 Ion Source Parameters..... 124
4.3.2 Quadrupole and SEM Parameters .................................................. 127
4.4 Calibration of the DEMS Instrument .................... 130
4.5 Calibration of the QMS .......................................................................... 133
4.6 Further Considerations ........... 137
4.6.1 Measurement Error ......................................................................... 137
4.6.2 Maintenance ..................... 138
4.7 Summary ................................................................................................. 140
iv
5 Methanol Oxidation on HSAC Supported Pt and PtRu Catalysts ............. 143
5.1 Introduction ............................................................................................ 144
5.1.1 Background ...................... 145
5.1.2 Motivation ........................................................................................ 148
5.2 Experimental ........................... 150
5.2.1 Measurement Procedure ................................................................. 152
5.3 Results and Discussion ............ 153
5.3.1 Electrochemical Surface Area Determination ................................ 153
5.3.2 Cyclic Voltammetry ......................................... 159
5.3.3 Chronoamperometry ....................................... 167
5.3.4 Three-Dimensional Voltammetry ................... 173
5.3.5 Tafel Slope........................................................................................ 179
5.3.6 Activity ............................. 183
5.3.7 Potential Dependent Conversion ..................................................... 185
5.4 Conclusions ............................................................. 189

6 The Electrochemical Oxidation of HSAC Catalyst Supports ..................... 192
6.1 Introduction ............................................................................................ 193
6.1.1 Motivation ........................ 196
6.1.2 Background ...................................................................................... 197
6.3 Experimental ........................... 200
6.3.1 Electrochemical Oxidation Procedure ............................................ 202
6.4 Results and Discussion ............................................ 204
6.4.1 Determination of the Apparent Double-Layer Capacitance .......... 204
6.4.2 Substrate Background Contributions ............................................. 210
v 6.4.3 Electrochemical Oxidation of HSAC Supports ............................... 212
6.4.5 Electrochemical Oxidation of HSAC Supported Pt Catalysts ........ 242
6.4.6 Future Applications of DEMS in the Study of the COR ................ 249
6.5 Conclusions.............................................................................................. 253

7 Summary ..........................