Determination of adsorption and activation volumes and apparent transfer coefficients by pressure and potential modulation [Elektronische Ressource] / vorgelegt von Hanchun Wang

Determination of adsorption and activation volumes and apparent transfer coefficients by pressure and potential modulation [Elektronische Ressource] / vorgelegt von Hanchun Wang

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Determination of adsorption and activation volumes and apparent transfer coefficients by pressure and potential modulation Dissertation zur Erlangung des Doktorgrades (Dr. rer.nat) der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn vorgelegt von Hanchun Wang aus Lichuan, Hubei Province, China Bonn,2009 Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn Promotionskommission Erster Gutachter: Prof. Dr. Helmut Baltruschat Zweiter Gutachter: Prof. Dr. Klaus Wandelt Fachnaher Gutachter: Prof. Dr. Siegfried Waldvogel Fachfremder Gutachter: Prof. Dr. Karl Maier Tag der mündlichen Prüfung: 16.03.2010 Erscheinungsjahr: 2010 Ich versichere, dass ich diese Arbeit selbständig verfasst und keine anderen als die angegebenen Quellen und Hilfsmittel benutzt sowie die Zitate kenntlich gemacht habe. Bonn, 23.12.2009 Hanchun Wang For my parents, my wife and my son 献给我的父母、妻子和儿子! Contents 1 Introduction………………………………………………………………………………...………….1 1.1 Fundamentals……………………………………………………………………………………...…1 1.1.1 Potential sweep method and potential step method………………………………………………1 1.1.

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Determination of adsorption and activation
volumes and apparent transfer coefficients
by pressure and potential modulation



Dissertation
zur
Erlangung des Doktorgrades (Dr. rer.nat)
der
Mathematisch-Naturwissenschaftlichen Fakultät
der
Rheinischen Friedrich-Wilhelms-Universität Bonn




vorgelegt von

Hanchun Wang




aus

Lichuan, Hubei Province, China




Bonn,2009



Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der
Rheinischen Friedrich-Wilhelms-Universität Bonn








Promotionskommission

Erster Gutachter: Prof. Dr. Helmut Baltruschat
Zweiter Gutachter: Prof. Dr. Klaus Wandelt
Fachnaher Gutachter: Prof. Dr. Siegfried Waldvogel
Fachfremder Gutachter: Prof. Dr. Karl Maier





Tag der mündlichen Prüfung:

16.03.2010




Erscheinungsjahr: 2010




Ich versichere, dass ich diese Arbeit selbständig verfasst und keine anderen als die angegebenen Quellen
und Hilfsmittel benutzt sowie die Zitate kenntlich gemacht habe.

Bonn, 23.12.2009




Hanchun Wang




























For my parents, my wife and my son
献给我的父母、妻子和儿子!










Contents

1 Introduction………………………………………………………………………………...………….1
1.1 Fundamentals……………………………………………………………………………………...…1
1.1.1 Potential sweep method and potential step method………………………………………………1
1.1.2 Electrochemical impedance spectroscopy………………………………………………………3
1.1.3 Method of ac voltammetry……………………………………………5
1.1.4 Single crystals ………………………………………………………………………6
1.1.5 Charge transfer coefficient and Tafel plot………………………………………………………11
1.2 Introduction to fuel cells and CO oxidation………………………………………………………13
1.2.1 Introduction to Fuel cell …………………………………………………………………………13
1.2.2 CO oxidation mechanism………………………………………………………………………19
1.2.3 The determination of the Tafel slope or apparent transfer coefficient for CO oxidation on Pt and
the contradiction in literature…………………………………………………………..………...20
1.3 Volume measurement and surface volume excess…………………………………………………23
1.3.1 Basic volume measurement……………………………………………………………………23
+1.3.2 Partial molar volume in solution, especially H …………………………………………………24
1.3.3 Reaction volume…………………………………………………………………………………27
1.3.4 Activation volume………………………………………………………………………………27
1.3.5 Surface volume excess and adsorption volume…………………………………………………28
2 Materials, instruments and methods…………………………………………………………………33
2.1 Chemicals…………………………………………………………………………………………33
2.2 Glassware…………………………………………………………………………………………33
2.3 Electrochemical instrumentation…………………………………………………………………34
2.3.1 Potentiastat and Lock-in Amplifier ……………………………………………………………34
2.3.2 The cells…………………………………………………………………………………………34
2.3.3 The setup for pressure modulation ………………………………………………………………35
2.4 Experimental procedures…………………………………………………………………………36
2.4.1 Preparation of the single crystal electrodes……………………………………………………36
2.4.2 Preparation of reference electrodes: RHE and Ag/AgCl………………………………………37
2.4.3 CO oxidation experiment……………………………………………………………………38
2.4.4 Pressure modulation experiment…………………38
2.4.5 Calibration of the force sensor Pressure modulation experiment ………………………………38
3 Determination of the apparent charge transfer coefficient for CO oxidation on various Pt
surfaces……………………………………………………………………………………………….41
3.1 Principles and calculations …………………………………………………………………………41
3.1.1 Principles…………………………………………………………………………………………41
3.1.2 Correction for slow ion adsorption ………………………………………………………………41
3.2 Polycrystalline platinum…………………………………………………………………………46
3.2.1 Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)…………………46
3.2.2 Potential sweep measurement ……………………………………………………………………48
3.2.3 Potential step experiments………………51
3.3 Pt(111) ………………………………………………………………………………………………59
3.3.1 CV and EIS………………………………………………………………………………………59
3.3.2 Potential sweep measurement ……………………………………………………………………61
3.3.3 Potential step measurement………………61
3.4 Pt(665) ……………………………………………………………………………………………65
3.4.1 CV and EIS………………………………………………………………………………………65
3.4.2 Potential sweep measurement ……………………………………………………………………67
3.5 Pt(332) ………………………………………71
3.5.1 CV ……………………………………………………………………71
3.5.2 Potential sweep experiment………………………………………………………………………71
3.5.3 Potential step measurements ……………………………………………………………………73
3.6 General discussion for the mechanism of CO oxidation on Pt …………76
3.6.1 General review of our results ……………………………………………………………………76
3.6.2 About the contradiction in Tafel slope in literature…………………………………………78
3.6.3 About the chemical step as the rds and the detection of COOHad in liteterature………………80
3.6.4 About the origin of the prepeak…………………………………………………………………81
3.6.5 About the potential dependence of Tafel slope for CO oxidation in alkaline solution…82
3.7 The stepped Pt surfaces modified by Ru and Sn ……………………………………………………83
3.7.1 α' for CO oxidation on Ru step decorated Pt(665) ………………………………………………83
3.7.2 α'on Sn step decorated Pt(332) ………………………………………………85
3.8 Summary……………………………………………………………………………………………88
4 The surface volume excess of hydrogen adsorption on polycrystalline Pt and the effect of
cations …………………………………………………………………………………………..…91
3- - 4-4.1 Volume change for Fe(CN) + e ↔ Fe(CN) : a test experiment………………………………92 6 6
4.1.1 Principles for measuring reaction volume by the method of Pressure modulation………………92
4.1.2 Results and discussion……………………………………………………………………………93
4.2 Principles calculations for surface volume excess measurements……………………………94
4.2.1 Principles…………………………………………………………………………………………94
4.2.2 Corrections for the pressure dependence of the reference electrode……………………………..97
4.3 Results and discussion…………………………………………………………………………100
4.3.1 Cyclic voltammetry………………………………………………………………………… …..100
4.3.2 The measurement of ac voltammetry……………………………………………………103
4.3.3 The ac current arising from pressure modulation ……………………………………………104
4.3.4 Molar volume of adsorbed hydrogen on polycrystalline Pt …………………………………105
4.3.5 The effect of cations on hydrogen adsorption …………………………………………………107
4.4 Summary…………………………………………………………………………………………110
5 Activation volume for CO oxidation on polycrystalline Platinum …………………………………113
5.1 Principles…………………………………………………………………………………………113
5.2 Results and discussion…………114
5.2.1 Activation volume for CO oxidation on Pt(poly) ……………………………………………114
5.2.2 Explanation for the activation volume………………………………………………………117
5.3 Summary…………………………………………………………………………………………119
Conclusions………………………………………………………………………………………………120
Notations
i current Q CO oxidation charge
current phasor C Capacitance i
i dc current q surface charge dc
i ac current f Frequency ac
i real part of the ac current V Vo l u m e ac-re
i charge transfer current volume change ∆V ct
‡ i adsorption current activation Volume ∆Vad
i maximum current or peak current adsorption volume ∆Vmax ad
j current density v partial molar volume of species “i” i
j v potential sweep rate √-1
transfer coefficient v mean molar volume α m
apparent transfer coefficient a chemical activities of oxidized species α′ ox
k rate constant a chemical activities of reduced species red
apparent rate constant t Time k ′
K equilibirum constant t time elapsed at current maximum max
fractional coverage of species i c Concentration θi
the amount of adsorbed species i r Radius Γi
surface excess of entropy d Diameter Γ s
surface excess of volume Overpotential Γ η V
Phase angle between ac current and G Gibbs energy ϕ
voltage
‡b activation free energy Tafel slope, b = 2.303RT/ αnF ∆G
-1 -1n The number of electrons transferred R gas constant, 8.314 J ⋅K ⋅mol
Mole number resistance
-1 F H Enthalpy Faraday constant, 96485 C ⋅mol
E potential S Entropy
U ac voltage x mole fraction of species B ac B
ac voltage in phasor notation γ interfacial tension of electrode u ac
T absolute temperature chemical potential µ
Z impedance p Pressure
Z the real part of the impedance CPE constant phase element re
Z the imaginary part of the impedance angular velocity ω im


GGAbbreviations

CV Cyclic voltammetry FCs Fuel cells
LSV Linear sweep voltammetry AFC alkaline fuel cell
fcc face centered cubic PEMFCpolymer electrolyte membrane fuel
cell, Proton exchange membrane fuel
cell
bcc body centered cubic PAFC phosphoric acid fuel cell
hcp hexagonal close packed MCFC molten carbonate fuel cell
STM scanning tunneling microscope SOFC solid oxide fuel cell
EIS electrochemical impedance ORR Oxygen reduction reaction
spectroscopy
RE Reference electrode L-H Langmuir-Hinshelwood
WE Working electrode E-R Eley-Rideal
CE Counter electrPM Pressure modulation