NANYANG TECHNOLOGICAL UNIVERSITY

Nanoparticles For Small Molecule Electrocatalysis

Submitted in Partial Fulfillment of the Requirements

for the Degree of Bachelor of Engineeringof

Nanyang Technological University

by Xing Dan

School of Materials Science & Engineering

2015

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.

2

AbstractNon-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.

3

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.

4

Table of Contents

DISCLAMER..................................................................................................................................... 2

ABSTRACT.......................................................................................................................................... 3

ACKNOWLEGEMENT.................................................................................................................. 4

TABLE OF CONTENTS................................................................................................................. 5

LIST OF FIGURES.......................................................................................................................... 6

KEY DEFINITIONS, ACRONYMS AND ABBREVIATIONS.............................................8

1. INTRODUCTION........................................................................................................................... 9

1.1 BACKGROUND...............................................................................................................................................9

1.2 PURPOSE AND SCOPE...............................................................................................................................10

2. REVIEW OF THEORY AND 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

5. CONCLUSION............................................................................................................................... 34

6. RECOMMENDATIONS...............................................................................................................34

REFERENCE..................................................................................................................................... 35

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).

6

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

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

7

Key definitions, Acronyms and AbbreviationsOxygen 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)

8

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.

9

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.

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.

10

2. Review of Theory and Purpose of 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

11

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+

2Co2+ + 3H2O - 2e- -> Co2O3 + 6H+

3Co2+ + 4H2O - 2e- -> Co3O4 + 8H+

1.1 OER on Co3O4

12

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 includes1 mg/cm2, 10 mA/cm2at 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

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-

14

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

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.

15

Figure 1.2.2 The flow of OER circle

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.

16

Figure 1.2.3 The flow of OER line

1.3 OER on metal-doped Co3O

4

OER activity of the spinels increases relative to that of Co3O4, in the order:

Co3O4 < NixCo3-xO4 ≤ CuxCo3-xO4 < LixCo3-xO4.

Spinel oxides:

AB2O

4 =(A

1-xB

x)[A

xB

2-x]O

4 (A

1-xB

x) is tetrahedral

[AxB

2-x] is octahedral

Normal spinel structure: when x=0,

All A occupy tetrahedral site and All B occupy octahedral site.

Inverse spinel structure: when x=1,

All A and half of B occupy octahedral site.

17

Figure 1.3.1 E-- 550mV vs Hg/HgO; 3.5 M KOH

Figure 1.3.2 AB2O4 spinel structure

18

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. Fordoping 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.

19

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.

20

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

21

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 Co2+/Ni2+ ratios.

22

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.

23

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.

24

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-

NO -+6H O+8e

- →NH +9OH

-

NO -

+6H O+6e-

→NH +

+8OH-

6Co(OH)2 + O2 → 2Co3O4 + 6H2O

Ni(OH)2→ NiO + H2O

25

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.

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.

26

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.

27

4. Results and Discussion

4.1 CV curves of OER in 1.0 M KOH at 10 mV / s

-0.2 0.0 0.2 0.4 0.6 0.8

-20

0

20

40

60

80

100

120

I / m

A

E / V vs. Hg/HgO

Ni2+ / Co2+

0:10 3:7 5:5 7:3 9:1 10:0

SS mesh

Figure 4.1.1 CV curves of Ni-Co hydroxides

-0.2 0.0 0.2 0.4 0.6 0.8

0

20

40

60

80

100Ni2+ / Co2+

0:10 3:7 5:5 7:3 9:1 10:0

I / m

A

E / V vs. Hg/HgO

SS meshannealing

28

Figure 4.1.2 CV curves of Ni-Co oxides

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.

-0.2 0.0 0.2 0.4 0.6 0.8

0

20

40

60

80

I / m

A

E / V vs. Hg/HgO

SS mesh RT SS mesh 300

Figure 4.1.3 CV curves of SSM at RT and 300

4.2 Current at 0.80V vs. Hg/HgO

29

0:10 3:7 5:5 7:3 9:1 10:040

50

60

70

80

90

100

110

120

130

I / m

A @

0.8

0V v

s. H

g/H

gO

Ni / Co in solution

Hydroxides Oxides

No iR correction

SS mesh RT 76.73

SS mesh 300 55.37

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

0:10 3:7 5:5 7:3 9:1 10:01.48

1.49

1.50

1.51

1.52

1.53

1.54

E / V

vs.

RHE

@ 1

0 m

A

Ni / Co in solution

Hydroxides 10mA iR correction Hydroxides 10mA no iR correction

0:10 3:7 5:5 7:3 9:1 10:01.48

1.50

1.52

1.54

1.56

1.58

1.60

1.62

1.64

1.66

E / V

vs.

RHE

@ 5

0 m

A

Ni / Co in solution

Hydroxides 50mA iR correction Hydroxides 50mA no iR correction

Figure 4.3.1 potential of Ni-Co hydroxides at 10mA and 50mA

30

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.

0:10 3:7 5:5 7:3 9:1 10:01.51

1.52

1.53

1.54

1.55

1.56

1.57

1.58

1.59

E / V

vs.

RHE

@ 1

0 m

A

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 / V

vs. R

HE

@ 50 m

A

Ni / Co in solution

Oxides 50mA iR correction Oxides 50mA no iR correction

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

31

0:10 3:7 5:5 7:3 9:1 10:01.501.511.521.531.541.551.561.571.581.591.601.611.62

E / V

vs.

RH

E @

10

mA

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 10:01.58

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 / V

vs.

RH

E @

50

mA

Ni / Co in solution

Hydroxides 50mA no iR correction Oxides 50mA no iR correction

Figure 4.4.1 potential of Ni-Co hydroxides and oxides at 10 mA and 50 mA

32

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.

4.5 Tafel slope from CV at 2 mV /s

33

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 - i

R /

V vs

. RH

E

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 - i

R /

V vs

. RH

E

lg (i / mA)

Ni-Co oxides

Figure 4.5.1 tafel slope from CV at 2 mV/s of Ni-Co hydroxides and oxides

34

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.

0/10 3/7 5/5 7/3 9/1 10/020

25

30

35

40

45

SS mesh RT 30.60

SS mesh 300 36.30

Tafe

l slo

pe /

mV

dec-1

Ni / Co in solution

Hydroxides Oxides

iR correction

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

35

0.6 0.8 1.0 1.2 1.4 1.6 1.80.2

0.4

0.6

0.8

1.0

1.2M

ass

/ mg

Passing charge / C

Ni7Co3OH Ni7Co3O

0:10 3:7 5:5 7:3 9:1 10:0

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

Mas

s / m

g

Ni / Co in solution

Hydroxides Oxides

0.6 0.9 1.2 1.5 1.8

100

105

110

115Ni7Co3OH

I / m

A

Passing charge / C 0.6 0.9 1.2 1.5 1.8

80

85

90

95

100I /

mA

Passing charge / C

Ni7Co3O

0.6 0.9 1.2 1.5 1.8

80

100

120

140

160

180

200

220

240

260Ni7Co3OH

Dou

ble

laye

r cap

acita

nce

/ mF

Passing charge / C 0.6 0.9 1.2 1.5 1.8

26

28

30

32

34

36Ni7Co3OH

Tafe

l slo

pe /

mV

dec-1

Passing charge / C

Tafel slope / mV dec-1

36

0.6 0.9 1.2 1.5 1.815

20

25

30

35

40

45

50

55 Ni7Co3OD

oubl

e la

yer c

apac

itanc

e / m

F

Passing charge / C 0.6 0.9 1.2 1.5 1.8

32

34

36

38

40

42 Ni7Co3O

Tafe

l slo

pe /

mV

dec

-1

Passing charge / C

Tafel slope / mV dec-1

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

37

0:10 3:7 5:5 7:3

20

40

60

80

100

120

140

160

180D

oubl

e-la

yere

d ca

paci

tanc

e / m

F

Ni / Co in solution

Hydroxides Oxides

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.

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.

38

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.

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