Electrochemical Properties of Nanocrystalline Nickel and Nickel-Molybdenum Alloys

223 Pages·1998·9.35 MB·English

Electrochemical Properties of Nanocrystalline
Nickel and Nickel-Molybdenum Alloys
Shangyu Wang
A thesis submitted to the Department of Materials and Metallurgical Engineering in
conformity with the requirernentsf or the Degree of Doctor of Philosophy
Queen's University
Kingston, Ontario, Canada
Copyright O Shangyu Wang, 1997
,
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Abstract
This thesis deals with the corrosion behavior of nanocrystalline Ni as well as the
synthesis, structural characterization and electrocatalytic properties for the hydrogen
evolution reaction (FER) of nanocrystalline Ni-Mo alloys.
The corrosion evaluation was carried out potentiodynamicaily and potentiostaticaily at
ambient temperature in 3 wt% NaCl neutrai water and 30 wt% KOH alkaline solutions.
The corrosion properties determined in the neutral and alkaiine solutions are shown to
differ little fkom those which had been determined for the acidic environment. The study
showed that the high corrosion resistance of conventional polycrystalline Ni is conserved
in its electrodeposited nanocrystalline counterpart. While nanocrystalline nickel was
found to corrode more unifody with higher passive current densities, polycrystailine
nickel was observed to s a e r severe localized corrosion at higher anodic potentials. This
behavior was rnainiy attributed to the differences in intercrystalline surface fraction and
intercrydline defect site distribution for nanocrystailine and polycrystalline materials.
A modified Watts bath was used to prepare bulk nanocrystailine Ni-Mo alloys and
electrodeposited nanocrystalline Ni-Mo coatings on porous substrates used for corrosion
and electrocatalytic measurements. Nanocrystalline Ni-Mo alloys with Mo content up to
1 1- wt% and grain size down to 13 nanometers were successNly produced. Results of X-
ray difiaction analysis showed that al1 of the nanocrystailine Ni-Mo bulk solids and
coatings were of single phase structure. The addition of molybdenum in nickel
encouraged the formation of a cauliflower-like surface morphology resulting in higher
surface area.
Evaluation of the electrocatalytic properties for the HER of polished nanocrystalline
nickel, nanocry stalline nick el-mol ybdenum d oy s and pol ycry stali ine nickel were
conducted in 30 wt% KOH solution at 25 O C using potential polarkation methods.
Compared with its polycrystalline counterpart, lower hydrogen overpotentials were
observed on the surface of electrodeposited nanocrystailine Ni. Even lower hydrogen
overpotentiais were found for polished nanocrystalline Ni-Mo alloys. Tafiel parameters
detemined nom the q-Log(i) curves suggest a mechanism of water discharge as the rate-
determinhg step for most of nanocrystalline samples tested in this study. The best HER
properties were found for the electrodeposited nanocrystalline Ni-Mo coatings on porous
substrates. This was attributed to (i) grain size reduction resulting in hi& intercrystalline
surface fiaction; (ii) the presence of higher energy grain boundaries and triple junctions
which provide preferential sites for the HER; and (iii) synergistic effect for the HER of
Ni and Mo in binary alloys.
Acknowledgements
1 would Iike to express my sïncere gratitude to rny supervisors Dr. U. Erb, now at
the Department of Metailurgy and Materials Science at the University of Toronto, and Dr
P.R. Roberge of the Royal Military College at Kingston, Ontario, for their guidance and
suppoa provided throughout the course of this study.
Financial support provided through the School of Graduate Studies of Queen's
University, the Naturai Sciences and Engineering Council of Canada, and Ontario Centre
for Materials Research are also gratefully acknowledged.
I would also like to thank my wife Lichun and my son Pin for their support and
encouragement throughout this work.
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Table of Contents
Abswact. ..............................................................................................................................i
...
AcknowIedgements .......................................................................................................... ui
.............................................................................................................
Table of Contents iv
List of Figures .................................................................................................................... x
..
List of Tables ................................................................................................................. xvu
.........................~....~................................................................................
List of Symbols air
CHAPTER 1. ..................................................................................................................... 1
INTRODUC~OIN ................................................................................................................. 1
CHAPTER 2 ...................................................................................................................... 9
Comosro~B EHAVIOORF NANOCRYSTALLNIIN INE A QUEOUSSO LUTION...S.. .............. 9
2.1 Literature review .................................................................................................... 9
2.1 .O Introduction ...................................................................................................... 9
2.1 .1 Nanostructures produced fiom amorphous precursors .................................. 10
2.1.2 Nanocrystalline electrodeposits ..................................................................... 17
2.1.2.1 Nanocrystalline Ni in acidic media .......................................................... 17
2.1 .2.2 Nanocrystalline Ni in sait spray environment ......................................... 2 1
2.1.2.3 Other corrosion tests on nanocrystalline Ni ............................................ 2 3
2.1.2.4 Nanocrystalline Ni-P alloys in acidic media .......................................... 23
................................................................................
2.1.3 Sputîered nanostructures 25
2.2 Present study: Corrosion behavior of nanocrystalline Ni in alkaline and pH
.........................................................................................................
neutrai solutions 30
2.2.1 Corrosion behavior in aikaline solution ..................................................... 30

3.1 htercrystalline volume hction ...................................................................... 58
3 -2 Intercrystalline surface fiaction ............................................................................ 61
3.3 Highly distributed corrosion current on nanocrystalline materials ...................... 64
3.4 Effect of intercrystalline surface hction on corrosion current densities and
corrosion potentials (Ecorr) ........................................................................................ 69
3.5 Reduced microscopie heterogeneity of nanocrystailine materials ....................... 72
3.6 Analogy to bimetallic corrosion ........................................................................... 72
3 -7 Grain boundary energy considerations. ................................................................ 75
3.8 Summary .............................................................................................................. 82
3.9 References ..................... ............. . ................................................................... 84
.,
....................................................................................................................
CHAPTER 4 86
SYNTHESIASN DS TRUCTUREOSF ELECTRODEPOSINTAEDN OCRYSTALLNMt-EM O
ALLOYSA S CATALYSTFSO RH YDROGENE VOLUTIORNE ACTIONS. ............................... 86
4.1 Introduction .................. ...................................... ................................................. 86
4.2 Electrodeposition of bulk nanocrystalline Ni-Mo alloys .................................. 88
4.2.1 Electrodeposition parameters and screening design ...................................... 88
4.2.2 Electrolyte ...................................................................................................... 90
4.2.3 Electrodeposition apparatus and procedures ................................................. 91
4.3 Characterization methods ..................................................................................... 93
4.4 Results and discussion. ......................................................................................... 93
4.4.1 EDS results and X-ray difbction patterns ...................................................9 3
4.4.2 Molybdenum content vs . grain size ............................................................... 97
.....................................................................................
4.4.3 Surface morphology 101
..............................................................................................
4.4.4 Microhardness IO4
4.4.5 Overdl screening design results and response analysis ............................... 107
4.6 Summary ............................................................................................................ 111
4.7 References ............................... .. ..................................................................... 113
............................. ........................................................
CHAPTER 5 116
ELECTROCATALYTICP ROPERTIEOSF E LECTRODEPOSITEBDU LKN ANOCRYSTALLNNEI
ANDN t-Mo ALLOYSF OR HYDROGENE VOLUTIORNE ACTIONS( HERI)N ALKALME
SOLUTIONS.. .................................................................................................................I l6
5.1 Literature review ................................................................................................ 116
5.1.1 introduction. ................................................................................................. 116
5.1.2 Mechanism of HER ..................................................................................... 118
5.1.3 Experimental Techniques ............................................................................. 119
5.1.3.1 Hydrogen Overpotentials on Metai and Metal Based Electrodes .......... 119
5.1.3.2 Potential Decay Transients .................................................................... 124
5.1.3 -3 Electrochemical Impedance Spectroscopy (EIS) ................................... 125
5.1.4 Electrode properties and the HER ................................................................ 128
5.1.4.1 Surface morphology .......................... . ............................................... 128
5.1 .4.2 Electronic structure and crystal lattice energy .......................... .. . . .1 29
5.1.4.3 Surface composition and crystallite orientation .................................... 132
5.1.4.4 Temperature effects ............................................................................... 133
5.1 -5N anocrystailine materials ............................................................................. 135
5.2 Experimentai ...................................................................................................... 137
5.2.1 Sample Preparation ...................................................................................... 137
5.2.2 Electrochemical Ce11 and Measurement Set-up ........................................... 139
5 .2.3 Measurement Procedures and Data Acquisition .......................................... 140
...............................................................................
5.2.4 Sample characterization 142
5.3 Results and Discussion. ......................................................................................1 42
5.3.1 Electrode Stability. ....................................................................................... 142
5.3.2 Hydrogen Evolution Reactions .................................................................... 147
5.3 .2.1 Electrodeposited nanocrystdline Ni electrodes ..................................... 147
Poieniiodynnmic polarizorion ........................................................................ 247
HER thermodjnamic and equilibrium potential ........................................... 119
HER Kinetics and TafZ parameters ............................................................... ISI
5.3.2.2 Electrodeposited nanocrystailine Ni-Mo alloys ................................... 157
Results of HER siudies ................................................................................... 157
The role of rnolybdenum .............................................................................. 162
Main effect analysis for the HER overpotential on bulk solid nanocrystalline
Ni-Mo electrodes withj7at surfces ............................................................... 164
5.4 Summary ............................................................................................................ 166
5.5 References .......................................................................................................... 168
..................................................................................................................
CHAPTER 6 172
SYNTHESAISN D PROPERTIEOSF E LECTRODEPOSITNEADN OCRYSTALNLIM-MEO
COATINGOS N POROUSS UBSTRAT.E...S.. ................................................................... 172
6.1 Introduction ........................................................................................................ 172
6.2 Electrodeposition of nanocrystalline Ni-Mo alloys on porous substrates .......... 173
6.2.1 The substrate ................................................................................................ 173
6.2.2 Parameter settings for the electrodeposition of nanocrystailine Ni-Mo alloys
on porous substrates ..............................................................................................1 73
......................................................................................
6.3 Charactenzation results 175
. .
6.3.1 Composition. ................................................................................................1 75
6.3.2 X-ray diffkction pattern ....................................................................... 177
6.3- 3 Surface morphology ..................................................................................... 178
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