1. NANYANG TECHNOLOGICAL UNIVERSITY
Nanoparticles For Small Molecule Electrocatalysis
Submitted in Partial Fulfillment of the Requirements
for the Degree of Bachelor of Engineering of
Nanyang Technological University
by Xing Dan
School of Materials Science & Engineering
2015
2. 2
Disclamer
This report contains confidential information that is the intellectual property of
research group of Prof. Xu Zhichuan of Nangyang Technological University.
Therefore, it is strictly for the intended audience only and no further distribution is
allowed without permission.
3. 3
Abstract
Non-precious metal electrodes, Ni and Co hydroxides and oxides, have been recently
found active towards oxygen evolution reaction (OER) in alkaline. In this report, a
first and complete study on composition dependence of Ni-Co hydroxides and oxides
for oxygen evolution reaction is presented. The stainless steel mesh (SSM) is used for
the electro-deposition of Ni-Co hydroxides electrodes. The atomic ratio of Ni /Co in
Ni-Co hydroxides is limited by changing the ratio of precursor concentration. Ni-Co
oxide electrodes are further achieved by annealing the Ni-Co hydroxides. By
measuring double layer capacitance with using cyclic voltammetry (CV), the
morphology factors of Ni-Co hydroxides and oxides are shown. Oxygen Evolution
Reaction (OER) performance of Ni-Co hydroxides and oxide electrodes is
investigated by CV, and electrochemical impedance spectroscopy (EIS) techniques at
room temperature (RT, ~25 °C). The high OER activity electrodes on Ni-Co
hydroxides and oxides with different Ni contents are presented. The OER
performance on Ni-Co hydroxides has little change while adjusting the ratio of Ni/Co.
when the ratio of Ni/Co is 7/3 in solution, OER exists the best performance on Ni-Co
oxides. Ni decreases onset potential and increases the tafel slope in Ni-Co hydroxides.
Ni decreases onset potential and decreases the tafel slope in Ni-Co oxides.
4. 4
Acknowlegement
This final year project has been an exciting learning journey for the student, but it
would not be possible without the guidance and help of the following people. The
student would also like to record his thankfulness to:
Prof. Xu Zhichuan (Nanyang Technological Universiy): My supervior who welcomed
me to join his research group and who is always willing to give me advises whenever
I have problems.
Dr. Sun Shengnan (Nanyang Technological University): My direct thesis mentor who
guided me through-out the whole project in his spare time out of his busy research
work. His invaluable knowledge in the area is the key to make the project possible.
5. 5
Tableof Contents
DISCLAMER.......................................................................................................................................................2
ABSTRACT............................................................................................................................................................3
ACKNOWLEGEMENT.................................................................................................................................4
TABLE OF CONTENTS................................................................................................................................5
LIST OF FIGURES...........................................................................................................................................6
KEY DEFINITIONS, ACRONYMS AND ABBREVIATIONS...................................................6
1. INTRODUCTION............................................................................................................................................9
1.1 BACKGROUND ..............................................................................................................................................9
1.2 PURPOSE AND SCOPE................................................................................................................................10
2. REVIEW OF THEORYAND PURPOSE OF WORK.......................................................................11
3. EXPERIMENT...............................................................................................................................................22
3.1 MATERIAL PREPARATION .......................................................................................................................22
3.2 MATERIAL CHARACTERIZATION.............................................................................................................24
3.3 ELECTROCHEMICAL TEST ........................................................................................................................24
4. RESULTS AND DISCUSSION.................................................................................................................25
5. CONCLUSION...............................................................................................................................................34
6. RECOMMENDATIONS.............................................................................................................................34
REFERENCE.......................................................................................................................................................35
6. 6
List of Figures
Figure1.1 Co3O4 CV 1 mV/s in 1.0 M KOH (pH 14)
Figure 1.2.1 Layer structure of (a) Co(OH)2, (b) CoO(OH), and (c) CoO2 . The main
difference among these structures is the number of protons between CoO2 layers.
Figure 1.2.2 The flow of OER circle
Figure 1.2.3 The flow of OER line
Figure 1.3.1 E-- 550mV vs Hg/HgO; 3.5 M KOH
Figure 1.3.2 AB2O4 spinel structure
Figure 1.4.1 X-ray diffraction patterns of cobalt hydroxide deposited
potentiostatically.
Figure 1.4.2 Eh/pH stability regimes (top) and predicted solubility limit (lower) and
for the aqueous cobalt system (25 ◦C, with 0.1MK(NO3) supporting electrolyte).
Cobalt ion concentrations in both plots are indicated with similar colored lines. (For
interpretation of the references to color in this figure legend, the reader is referred to
the web version of the article.)
Figure 1.5.1 the j vs the ratio of concertrations of Co2+ and Ni2+
Figure 1.5.2 Nyquist plots measured at E=0.58V corresponding Oxygen Evolution
Reaction on the Co+Ni mixed oxides electrodeposited from different Co2+/Ni2+ ratio
solutions, 1M NaOH, at 25oC.
Figure 3.1 The composition of stainless steel mesh
Figure 4.1.1 CV curves of Ni-Co hydroxides
Figure 4.1.2 CV curves of Ni-Co oxides
7. 7
Figure 4.1.3 CV curves of SSM at RT and 300
Figure 4.2.1 current at 0.80 V of Ni-Co hydroxides and oxides
Figure 4.3.1 potential of Ni-Co hydroxides at 10mA and 50mA
Figure 4.3.2 potential of Ni-Co oxides at 10mA and 50mA
Figure 4.4.1 potential of Ni-Co hydroxides and oxides at 10 mA and 50 mA
Figure 4.5.1 tafel slope from CV at 2 mV/s of Ni-Co hydroxides and oxides
Figure 4.5.2 tafel slope of Ni-Co hydroxides and oxides with IR correction
Figure 4.6.1 a) mass vs. Ni/Co ratio b) mass vs. passing charge c) current vs. passing
charge of Ni-Co hydroxides d) current vs. passing charge of Ni-Co oxides e) double
layer capacitance vs. passing charge of Ni-Co hydroxides f) tafel slope vs. passing
charge of Ni-Co hydroxides g) double layer capacitance vs. passing charge of Ni-Co
oxides h) tafel slope vs. passing charge of Ni-Co oxides
Figure 4.7.1 double layer capacitance cuvres of Ni-Co hydroxides and oxides
8. 8
Key definitions,Acronyms andAbbreviations
Oxygen Evolution Reaction(OER): Oxygen evolution is the process of generating
molecular oxygen through chemical reaction. Mechanisms of oxygen evolution
include the oxidation of water during oxygenicphotosynthesis, electrolysis of water
into oxygen and hydrogen, and electrocatalytic oxygen evolution
from oxides and oxoacids.
Stainless steel mesh (SSM)
Cyclic voltammetry (CV)
Electrochemical impedance spectroscopy (EIS)
9. 9
1. Introduction
1.1 Background
The Industrial Revolution was the transition to new manufacturing processes in the
period from about 1760 to sometime between 1820 and 1840. This transition included
going from hand production methods to machines, new chemical manufacturing and
iron production processes, improved efficiency of water power, the increasing use
of steam power, and the development of machine tools. It also included the change
from wood and other bio-fuels to coal.
After the Industrial Revolution, the manufacturing methods develop from hand
production methods to machine tools. The machine tools help industry to promote
productivity and throughout, but machine tools need energy to use and process. For
producing energy, the fossil fuel needs to heat and produce heat energy to make
machine tools process. Then later, the energy achieved is mainly from the combustion
of fossil fuel.
Due to the combustion of fossil fuel, it takes a significant influence to the
environment and climate, and the combustion of fossil fuel causes a low efficiency of
production of energy because of incomplete combustion of fossil fuel and the flow out
of heat energy.
Thinking of the issues caused by the combustion of fossil fuel, the scientists find out
the Oxygen Evolution Reaction of water by studying the small molecule
electrocatalysis. OER has a high efficiency of production of electrical energy and
positive effect to environment. The following issue is that what materials of catalysts
and which rate of catalysts can produce a high efficiency of production of electrical
energy by studying the nanoparticles for small molecule electrocatalysis.
10. 10
1.2 Purpose and Scope
For increasing demand for sustainable sources of energy, it makes solar energy to
drive the electrolysis of water to hydrogen and oxygen. The Oxygen Evolution
reaction of water has a high efficiency of production of electrical energy. But energy
efficiency is limited by the activities of the catalysts used at the anode and cathode.
So we study how to produce a high energy efficiency by studying the activities of the
catalysts and the ratio, the amount of catalysts and some other factors. A particular
challenge is to find highly active catalysts for the anodic oxidation of water, since the
over-potential for this electrode is a major contributor to the inefficiency of splitting
water electrochemically.
11. 11
2. Review of Theory and Purposeof Work
1. Oxygen Evolution Reaction(OER)
In the societal pursuit of sustainable energy, a critical element is decided by the
design of cost-effective and highly active catalysts for energy conversion and storage
applications. Among them, catalysts for oxygen evolution reaction (OER) and oxygen
reduction reaction (ORR) are the key of renewable-energy technologies including
water splitting and fuel cell. OER is an important process that enables many energy
storage options, like electricity-driven and direct-solar water splitting.
In water-alkali environment, the anodic reaction is a complex process, in which the
hydroxyl ions generated at the cathode are consumed at the anode to produce oxygen
and water molecules (4OH− ↔ O2 + 2H2O + 4e−, 2 H2O-> O2+4 H + +4 e- in acid
electrolyte or 4 OH- ->O2+2 H2O+4 e- in base electrolyte).
The oxygen activation involves a proton and electron transfer to form adsorbed
−OOH before the O−O bond is broken. This requires the catalyst could stabilize
−OOH moderately. After dissociation, adsorbed O and OH are formed on the surface
of the catalyst. At this time, a weak binding force between the catalyst and adsorbed
O and OH is necessary in order to desorb water quickly.
For OER, electrodeposited Co/Ni mixed oxide electrodes are prepared anodically for
oxygen evolution in alkaline media. From the literature, the reactions and their
standard electrode potential about oxidation of Co2+
and Ni2+
in neutral
medium are:
Ni2+
+ 2H2O - 2e-
-> NiO2 + 4H+
2Ni2+
+ 3H2O - 2e-
-> Ni2O3 + 6H+
12. 12
2Co2+
+ 3H2O - 2e-
-> Co2O3 + 6H+
3Co2+
+ 4H2O - 2e-
-> Co3O4 + 8H+
1.1 OER on Co3O4
The utility of alkaline solution is two-fold: Inhibit metal or oxide corrosion by
decreasing the requisite potential at the anode and Increase solution conductivity.
Maximal activity includes 1 mg/cm
2
, 10 mA/cm
2
at over-potential of 328 mV.
Figure1.1 Co3O4 CV 1 mV/s in 1.0 M KOH (pH 14)
1.2 How to generate O2
13. 13
Figure 1.2.1 Layer structure of (a) Co(OH)2, (b) CoO(OH), and (c) CoO2 . The main
difference among these structures is the number of protons between CoO2 layers.
Co+OH
-
→CoOH+e
-
CoOH + OH
-
→ Co(OH)2 + e
-
Co(OH)2 +OH-→CoO+H2O+e
-
Co(OH)2+2/3OH
-
→1/3Co3O4+4/3H2O+e
-
1/3 Co3O4 + 1/3 H2O + 1/3 OH
-
→ CoO(OH) + e
-
CoO(OH)+OH
-
→CoO2 +H2O+e
-
While there is general agreement that the oxidation of Co(III) to Co(IV) precedes the
onset of O2 evolution from cobalt electrodes covered with a layer of cobalt oxides, the
14. 14
details of the OER reactions remain a subject of discussion. The prevailing suggestion,
though, is that CoO2 reacts further with OH- anions to form adsorbed H2O2, which
then decomposes to form HOO species. The subsequent reaction of adsorbed HOO
species with OH anions results in the concurrent formation of O2 and H2O, along with
the release of an electron. Thus, irrespective of the initial stoichiometry of
Co-containing electrocatalysts, at the potential where OER occurs, the surfaces of
such catalysts consist of an oxide layer in which the principal oxidation state of Co is
Co(IV). Since CoO2 is not a stable phase, once the potential applied to the
electrocatalyst is removed, CoO2 rapidly reverts to Co3O4.
Figure 1.2.2 The flow of OER circle
15. 15
From above figure 1.2.2, a reaction circle contains four electron transfer steps. In the
first and third steps, it also releases one water molecule. In the fourth step, it releases
one oxygen molecule. So the net reaction can be written as 4OH- → 2H2O + 4e- +
O2.
It involves one metal center, which we assume to be in oxidation state 3+, surrounded
by a OH and two H2O ligands. The individual reaction steps in terms of the change in
ligands at one Co site are as follows.
Figure 1.2.3 The flow of OER line
1.3 OER on metal-doped Co
3
O
4
OER activity of the spinels increases relative to that of Co3O4, in the order:
Co3O4 < NixCo3-xO4 ≤ CuxCo3-xO4 < LixCo3-xO4.
Spinel oxides:
AB
2
O
4
=(A
1-x
B
x
)[A
x
B
2-x
]O
4
(A
1-x
B
x
) is tetrahedral [A
x
B
2-x
] is octahedral
Normal spinel structure: when x=0,
All A occupy tetrahedral site and All B occupy octahedral site.
16. 16
Inverse spinel structure: when x=1,
All A and half of B occupy octahedral site.
Figure 1.3.1 E-- 550mV vs Hg/HgO; 3.5 M KOH
17. 17
Figure 1.3.2 AB2O4 spinel structure
1.4 NiCo2O4
NiCo2O4 has much better electronic conductivity and higher electrochemical activity
than those of the two corresponding single component oxides as NiCo2O4 is a mixed
valence oxide and a pure spinel structure. For doping of Co3O4 with nickel, the Ni3+
ions are stabilized, and they substitute for Co3+ in the octahedral sites. Ni cations form
almost exclusively the octahedral sites and Co cations occupy evenly between the
tetrahedral and octahedral sites.
The magnetic properties of NiCo2O4 have not been used in applications but have been
investigated in the context of the mixed valencies of the Ni and Co cations in this
inverse spinel, where the Ni cations form almost exclusively the octahedral sites and
the Co cations occupy evenly between the tetrahedral and octahedral sites.
18. 18
Figure 1.4.1 X-ray diffraction patterns of cobalt hydroxide deposited
potentiostatically.
NiCo2O4 is generally regarded as a mixed valence oxide that adopts a pure spinel
structure. It has been reported to possess a much better electronic conductivity and
higher electrochemical activity than those of the two corresponding single component
oxides.
19. 19
Figure 1.4.2 Eh/pH stability regimes (top) and predicted solubility limit (lower) and
for the aqueous cobalt system (25 ◦C, with 0.1MK(NO3) supporting electrolyte).
Cobalt ion concentrations in both plots are indicated with similar colored lines. (For
interpretation of the references to color in this figure legend, the reader is referred to
the web version of the article.)
Cobalt hydroxide has been reported in two phases: -Co(OH)2 and -Co(OH)2. The
beta phase is a hexagonal layered hydroxide iso-structural to brucite and pink in
coloration (a=3.177Å, c = 4.653 Å). In accordance with the nomenclature of nickel
hydroxides, the -Co(OH)2 phase should indicate a complementary brucite-like phase,
but with water molecules inter-calatedin the sheet structure. Actually the presence of
Ni3+ at octahedral sites is indicated by the increased electric conductivity in
20. 20
NixCo3xO4. Ni doping resulted in the creation of new active sites with lower
activation energy.
1.5 OER on Co-Ni oxides
Figure 1.5.1 the j vs the ratio of concertrations of Co2+ and Ni2+
From figure 1.5.1, The current density of OER at E = 0.75V on the different Co+Ni
oxides electrodeposited from solutions containing different Co
2+
/Ni
2+
ratios.
21. 21
Figure 1.5.2 Nyquist plots measured at E=0.58V corresponding Oxygen Evolution
Reaction on the Co+Ni mixed oxides electrodeposited from different Co2+/Ni2+ ratio
solutions, 1M NaOH, at 25oC.
22. 22
3. Experiment
All consumable equipments and reagents used in the experiments were directly
obtained from Inorganic Service Lab of School of Materials Science and Engineering.
And all chemicals were guaranteed to be analytically pure and well-maintained
without futther purifications. Various experimental procedures have been conducted
in order to obtain the optimal product, but only the final selected optimal
experimental procedure is presented in this thesis. The typical experimental procedure
is as shown below.
3.1 Material Preparation
The stainless steel mesh(SSM)(500 mesh) substrates are washed in dilute HCl
solution ultrasonically for five minutes, then cleaned in acetone in five minutes and
ethanol for 15 minutes respectively. After cleaning, the SSM substrates are rinsed by
deionized water and then dry them in air. These preparation processes are for the
electrodeposition of the Ni-Co hydroxides. The traditional three-electrode method is
needed for carrying on of the electrodeposition. The working electrode is attached by
the SSM conductive substrate(~1.0 cm * 1.0 cm) and the reference electrode and
counter eletrode are attached respectively by a saturated calomel electrode (SCE) and
a Pt wire. The x M Ni(NO3)2 and (0.1-x) M Co(NO3)2 are used as the electrolyte,
when x=0, 0.03, 0.05, 0.07, 0.09, 0.1. A potential/galvanostat is used to keep a
constant potential -0.85 V against SCE until the passing charge is 1.2 C at room
temperature. The Ni-Co oxides are achieved by annealing at 300oC for 2 hours with a
ramping rate 2.5oC/min. The following equations are explained as the mechanism of
preparing Ni-Co hydroxides and oxides:
NO
-
+H O+2e
-
→NO
-
+2OH
-
23. 23
NO
-
+6H O+8e
-
→NH +9OH
-
NO
-
+6H O+6e
-
→NH
+
+8OH
-
6Co(OH)2 + O2 → 2Co3O4 + 6H2O
Ni(OH)2→ NiO + H2O
2Ni(OH)2 + 4Co(OH)2 + O2 → 2NiCo2O4 + 6H2O
The preparation method is Electrodeposition and Chronopotentiometry at 1.0mA.
Co(NO3)2 and Ni(NO3)2 at different Ni/Co ratio are used as the precusor. The
substrate is stainless steel mesh(SSM) that need the 10.0*10.0mm2 size. The passing
charge is limited as 1.2C at 1.0mA.
24. 24
Figure 3.1 The composition of stainless steel mesh
From above figure 3.1, the SSM consists of Ni element, and the Ni has a positive
contribution to the experiment.
3.2 Material characterization
Energy dispersive X-ray (EDX) data and field emission scanning electron microscopy
(FESEM) images are obtained by JSM-7600F and 6340F. The X-ray diffraction
(XRD) is taken for characteration of the samples’ crystal structure by using Shimadzu
(x2) with Cu Kα radiation. Thermo-gravimetric (TG) and differential
thermo-gravimetric (DTG) analyses are performed on TA 2950 (TA Instruments)
with a ramping rate of 5 °C min-1 to 600 °C in air.
3.3 Electrochemical test
The electrodeposition and electrochemical test are conducted with a Bio-Logic
electrochemical station (SP150) with a built-in EIS analyzer. The data are collected
by using EC-Lab and EC- Lab Express software package. CV and EIS examine the
working electrodes. A Hg/HgO (1 M KOH, aqueous) electrode is used as a reference
electrode and a Pt wire is used as a counter electrode.
25. 25
4. Results and Discussion
4.1 CV curves of OER in 1.0 M KOH at 10 mV / s
Figure 4.1.1 CV curves of Ni-Co hydroxides
Figure 4.1.2 CV curves of Ni-Co oxides
-0.2 0.0 0.2 0.4 0.6 0.8
-20
0
20
40
60
80
100
120
I/mA
E / V vs. Hg/HgO
Ni2+
/ Co2+
0:10
3:7
5:5
7:3
9:1
10:0
SS mesh
-0.2 0.0 0.2 0.4 0.6 0.8
0
20
40
60
80
100
Ni2+
/ Co2+
0:10
3:7
5:5
7:3
9:1
10:0
I/mA
E / V vs. Hg/HgO
SS mesh
annealing
26. 26
From figure 4.1.1 and 4.1.2, due to the different ratio of Ni2+/Co2+, the crests are
different, the crests in figure 4.1.1 are larger than those in figure 4.1.2. when the ratio
of Ni2+/Co2+ increases, the crest is closer the incline line which is formed after 0.6 V.
For Ni-Co hydroxides, the incline lines from different ratio of Ni2+/Co2+ after 0.6 V
are similar, but the incline line at ratio 7:3 of Ni2+/Co2+ is larger than other ratios of
Ni2+/Co2+ for Ni-Co oxides. The SSM annealed does not exist the crest and has a low
incline line after 0.6V.
From below figure 4.1.3, after 0.6 V, SSM exists an incline line, but the SSM at RT
has a more incline line than that at 300.
Figure 4.1.3 CV curves of SSM at RT and 300
4.2 Current at 0.80V vs. Hg/HgO
-0.2 0.0 0.2 0.4 0.6 0.8
0
20
40
60
80
I/mA
E / V vs. Hg/HgO
SS mesh RT
SS mesh 300
27. 27
Figure 4.2.1 current at 0.80 V of Ni-Co hydroxides and oxides
From figure 4.2.1, SSM annealed has a lower current than that at RT. Ni-Co
hydroxides has a higher current than Ni-Co oxides annealed and Ni-Co hydroxides
has little fluctuation but Ni-Co Oxides exists a little fluctuation.
4.3 Potential at 10 mA and 50 mA
Figure 4.3.1 potential of Ni-Co hydroxides at 10mA and 50mA
0:10 3:7 5:5 7:3 9:1 10:0
40
50
60
70
80
90
100
110
120
130
I/mA@0.80Vvs.Hg/HgO
Ni / Co in solution
Hydroxides
Oxides
No iR correction
SS mesh RT 76.73
SS mesh 300 55.37
0:10 3:7 5:5 7:3 9:1 10:0
1.48
1.49
1.50
1.51
1.52
1.53
1.54
E/Vvs.RHE@10mA
Ni / Co in solution
Hydroxides 10mA iR correction
Hydroxides 10mA no iR correction
0:10 3:7 5:5 7:3 9:1 10:0
1.48
1.50
1.52
1.54
1.56
1.58
1.60
1.62
1.64
1.66
E/Vvs.RHE@50mA
Ni / Co in solution
Hydroxides 50mA iR correction
Hydroxides 50mA no iR correction
28. 28
From Figure 4.3.1, the potential of Ni-CO hydroxides at 50 mA is higher than that at
10 mA, and the potential of Ni-Co hydroxides without IR correction is larger than that
with IR correction. At 50 mA, the potential has little fluctuation, but at 10 mA, the
potential decreases based on the change of ratio of Ni/Co.
Figure 4.3.2 potential of Ni-Co oxides at 10mA and 50mA
From Figure 4.3.1, the potential of Ni-CO oxides at 50 mA is higher than that at 10
mA, and the potential of Ni-Co oxides without IR correction is larger than that with
IR correction. At 50 mA, the potential has little fluctuation, but at 10 mA, the
potential increases firstly, then decreases from 3:7 to 9:1, finally increases based on
the change of ratio of Ni/Co.
4.4 Contrast between Ni-Co hydroxides and oxides
0:10 3:7 5:5 7:3 9:1 10:0
1.51
1.52
1.53
1.54
1.55
1.56
1.57
1.58
1.59
E/Vvs.RHE@10mA
Ni / Co in solution
Oxides 10mA iR correction
Oxides 10mA no iR correction
0:10 3:7 5:5 7:3 9:1 10:0
1.54
1.56
1.58
1.60
1.62
1.64
1.66
1.68
1.70
1.72
E/Vvs.RHE@50mA
Ni / Co in solution
Oxides 50mA iR correction
Oxides 50mA no iR correction
29. 29
Figure 4.4.1 potential of Ni-Co hydroxides and oxides at 10 mA and 50 mA
Based on several experimental process, from above figure 4.4.1, the potential at 50
mA is higher than that at 10 mA. The potential of Ni-Co oxides annealed is larger
than that of Ni-Co hydroxides. The potential of Ni-Co hydroxides has little fluctuation
but potential of Ni-Co oxides annealed has a little fluctuation. SSM at RT has a lower
potential than that at 300.
0:10 3:7 5:5 7:3 9:1 10:0
1.50
1.51
1.52
1.53
1.54
1.55
1.56
1.57
1.58
1.59
1.60
1.61
1.62
E/Vvs.RHE@10mA
Ni / Co in solution
Hydroxides 10mA no iR correction
Oxides 10mA no iR correction
SS mesh RT 1.566
SS mesh 300 1.598
0:10 3:7 5:5 7:3 9:1 10:0
1.58
1.60
1.62
1.64
1.66
1.68
1.70
1.72
1.74
SS mesh 300 1.677
SS mesh 300 1.711
E/Vvs.RHE@50mA
Ni / Co in solution
Hydroxides 50mA no iR correction
Oxides 50mA no iR correction
30. 30
4.5 Tafel slope from CV at 2 mV /s
Figure 4.5.1 tafel slope from CV at 2 mV/s of Ni-Co hydroxides and oxides
From figure 4.5.1, the potential of SSM at RT without Ni-Co hydroxides is the
highest from 0 to 10. From 0 to 10, the potential of SSM at 300 without Ni-Co oxides
is the largest but after 10, the potential of ratio of 3:7 of Ni-Co oxides is the highest.
1 10 100
1.47
1.48
1.49
1.50
1.51
1.52
1.53
1.54
1.55
st mesh RT
0:10
3:7
5:5
7:3
9:1
10:0
E-iR/Vvs.RHE
lg (i / mA)
Ni-Co hydroxides
1 10
1.51
1.52
1.53
1.54
1.55
1.56
1.57
st mesh 300
0:10
3:7
5:5
7:3
9:1
10:0
E-iR/Vvs.RHE
lg (i / mA)
Ni-Co oxides
31. 31
Figure 4.5.2 tafel slope of Ni-Co hydroxides and oxides with IR correction
From figure 4.5.2, the tafel slope of Ni-Co hydroxides increases from 0/10 to 9/1,
then decreases from 9/1 to 10/0, the tafel slope of Ni-Co oxides increases from 0/10 to
3/7, then decreases from 3/7 to 10/0, just at point of 9/1, the value of Ni-Co
hydroxides is larger than that of Ni-Co oxides.
4.6 The influence of mass loadings
0/10 3/7 5/5 7/3 9/1 10/0
20
25
30
35
40
45
SS mesh RT 30.60
SS mesh 300 36.30
Tafelslope/mVdec-1
Ni / Co in solution
Hydroxides
Oxides
iR correction
33. 33
Figure 4.6.1 a) mass vs. Ni/Co ratio b) mass vs. passing charge c) current vs. passing
charge of Ni-Co hydroxides d) current vs. passing charge of Ni-Co oxides e) double
layer capacitance vs. passing charge of Ni-Co hydroxides f) tafel slope vs. passing
charge of Ni-Co hydroxides g) double layer capacitance vs. passing charge of Ni-Co
oxides h) tafel slope vs. passing charge of Ni-Co oxides
From figure 4.6.1, the mass loadings has little influence to the double layer
capacitance and the tafel slope of Ni-Co hydroxides and oxides.
4.7 The influence of double layer capacitance
Figure 4.7.1 double layer capacitance cuvres of Ni-Co hydroxides and oxides
From figure 4.7.1, double layer capacitance of Ni-Co oxides has little influence,
double layer capacitnce of Ni-Co hydroxides increases based on the change of ratio of
Ni/Co.
0:10 3:7 5:5 7:3
20
40
60
80
100
120
140
160
180
Double-layeredcapacitance/mF
Ni / Co in solution
Hydroxides
Oxides
34. 34
5. Conclusion
In summary, the high OER activity electrodes on Ni-Co hydroxides and oxides with
different Ni contents are presented. In the process of electrodeposition of electrodes,
the stainless steel mesh is used. From the experiment and results, the OER
performance on Ni-Co hydroxides has little change while adjusting the ratio of Ni/Co.
when the ratio of Ni/Co is 7/3 in solution, OER exists the best performance on Ni-Co
oxides. Ni decreases onset potential and increases the tafel slope in Ni-Co hydroxides.
Ni decreases onset potential and decreases the tafel slope in Ni-Co oxides.
6. Recommendations
According to the experimental results and conclusions, the OER performance on
Ni-Co hydroxides has little change while adjusting the ratio of Ni/Co. when the ratio
of Ni/Co is 7/3 in solution, OER exists the best performance on Ni-Co oxides. Ni
decreases onset potential and increases the tafel slope in Ni-Co hydroxides. Ni
decreases onset potential and decreases the tafel slope in Ni-Co oxides. OER
performance and activities are effected by the ratio of Ni/Co and Ni function, so Ni
needs to be controlled for OER.
35. 35
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