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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

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Published 01 January 2011
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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 ...................................................................................................... 256

8 References .................................................................................................... 260

9 Appendix ...................................................................................................... 277
9.1 A ............... 277
9.2 B ............................................................................................................... 281
vi List of Figures
Figure 1-1: A diagram illustrating the principles behind operation of the
hydrogen fed PEMFC. ....................................................................... 2
Figure 1-2: A graph highlighting the origins of PEMFC performance losses. ....... 4
Figure 1-3: An outline of the experimental setup and numerous parameters that
must be controlled during an in-situ MEA experiment for the
characterisation of electrocatalyst materials. .................................... 5
Figure 1-4: An illustration of an ex-situ electrochemical half-cell RDE setup that
may be employed to characterise the performance of a PEMFC
electrocatalyst material ex-situ. ......................................................... 7
Figure 2-1: An illustration of the typical components of a DEMS instrument. .... 11
Figure 2-2: An illustration of the „classic‟ DEMS cell construction. ..................... 13
Figure 2-3: A sketch of the thin-layer flow cell (Reproduced by permission of the
American Chemical Society) [40]. ................................................... 16
Figure 2-4: A drawing of the dual thin-layer flow cell (Reproduced by permission
of The Electrochemical Society) [28]. .............. 18
Figure 2-5: An illustration of the capillary inlet DEMS electrochemical cell. ...... 20
Figure 2-6: An illustration of the vacuum membrane distillation process through
a microporous PTFE membrane (Reproduced by permission of
Elsevier) [54]..................................................................................... 22
Figure 3-1: A diagram presenting an overview of the various components
contributing to the whole DEMS instrument setup. ....................... 35
Figure 3-2: Photograph of the DEMS instrument constructed as part of this thesis.
.......................................................................................................... 36
Figure 3-3: A technical drawing of the DEMS instrument construction
highlighting the electrochemical cell, membrane interface and
vacuum system including QMS. ...................................................... 37
Figure 3-4: A sketch of the three-electrode DEMS electrochemical flow cell setup.
.......................................................................................................... 40
Figure 3-5: Photographs of the DEMS electrochemical flow cell setup constructed
as part of this thesis. ......... 41
Figure 3-6: Exploded drawing of the modular DEMS electrochemical flow cell
design and membrane interface constructed as part of this thesis. 42
vii Figure 3-7: Drawing of the electrolyte flow regime through the dual-cyclone thin-
layer electrochemical flow cell. ........................................................ 44
Figure 3-8: Illustration of the electrolyte flow regime through the DEMS linear
dual thin-layer electrochemical cell flow through block. ................ 45
Figure 3-9: An illustration of the membrane interface. ......................................... 50
Figure 3-10: An illustration highlighting the chambers of the 3-stage differentially
pumped vacuum system. .................................. 52
Figure 3-11: Exploded diagram of the DEMS instrument vacuum system & QMS.
........................................................................... 53
Figure 3-12: Illustration of the original 3-way flange QMS vacuum chamber
housing (A) and modified 5-way QMS vacuum chamber housing
(B). ..................................................................................................... 56
Figure 3-13: A sketch of the DEMS vacuum chamber giving the respective
pumping speeds (S), conductances (C) and operating pressure (P) of
the 3-stage differentially pumped vacuum system when the DEMS
instrument is in operation. ............................................................... 56
Figure 3-14: Illustration of the DEMS adjustable aperture (left) and exploded
image (right). .................................................................................... 57
Figure 3-15: Illustration of the QMS cross-beam ion source highlighting possible
molecular beam inlet orientations. ................... 58
Figure 3-16: Technical drawing indicating the position and orientation DEMS
vacuum aperture with respect to the cross-beam ion source. ......... 59
Figure 3-17: Overview of instrumentation, control and data acquisition hardware
and software employed in this DEMS instrument. ......................... 63
Figure 3-18: Image of DEMS electrochemical potentiostat hardware setup. ....... 65
Figure 3-19: Image of custom built negative impedance device (NID) highlighting
the potentiostat and cell connections. .............................................. 66
Figure 3-20: A schematic of the original NID circuit design and electrochemical
cell setup............................................................................................ 67
Figure 3-21: A schematic of an enhanced NID circuit design allowing adjustable
iR-compensation developed for the DEMS instrument as part of this
thesis. ................................................................................................. 68
Figure 3-22: Images depicting the QC 422 oscilloscope connections (left) and NI
DAQ card used for QMS data acquisition (right). .......................... 71
viii