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Master’s Thesis (석사 학위논문)

Development of Air Cathode Materials for Metal-

Air Batteries

A. Vignesh

Department of

Energy systems Engineering

DGIST

2016

Abstract

Fluctuation in oil price and the effect of global warming forced us to look for the replacement of

this fossil fuel by advanced high energy density metal air batteries. To bring this advanced

technology to commercialize market we need to have noble metals (such as Pt/C) and metal

oxides (such as RuO2 and MnO2) as catalysts in cathode to facile the oxygen reduction reaction

while discharging process and oxygen evolution reaction when recharging the battery. However,

the replacement of these noble metal-based catalysts is due to often suffer from multiple

disadvantages, including high cost, low selectivity, poor stability and detrimental environmental

effects.

We developed one dimensional LaCo1-xNixO3-δ perovskites by simple electrospinning approach.

We show that the progressive replacement of Co by Ni in the LaCo0.97O3- δ perovskite structure

greatly altered the OER electrocatalytic activity and the La(Co0.71Ni0.25)0.96O3-δ composition

exhibited the lowest overpotential of 324 at 10 mAcm-2

in 0.1M KOH. Subsequently as prepared

La(Co0.71Ni0.25)0.96O3-δ nanostructured was used as cathode catalyst for aqueous zinc-air battery,

which delivers high capacity of 705 mAh.g-1

zinc in primary zinc-air battery. The rechargeable

battery discharges with low overpotential of 0.792 V and 0.696 V at low capacity mode (400

mAh.g-1

catalyst) and high capacity mode (2500 mAh.g-1

catalyst), respectively, this value is much

lower than LaCo0.97O3-δ nanotube catalyst and precious Pt/C catalyst.

MS/ES

201424001

A. Vignesh. Development of air cathode materials for metal-air batteries.

Department of Energy systems Engineering. 2015. 103p. Advisor. Prof. Sangaraju

Shanmugam, Co-Advisor Dr. Yiseul Park .

(i)

In the second part of thesis, we synthesized a thin film carbon coated SiO2 nanosphere by

hydrothermal method and we utilized this material as cathode catalyst for lithium-air battery, it

delivers a capacity of 18588 mAh.g-1

of catalyst material at high current density of 150 mAh.g-1

,

which is nearly twice the capacity of nitrogen doped carbon material at same current density, and

for rechargeable battery it last for ten cycles at a cutoff capacity of 1000 mAh.g-1

. Later to

elucidate the working mechanism behind this metalloid oxide catalyst, we also reported the post

mortem analysis of cathode material of lithium-oxygen battery.

Key word: Air cathode catalyst, LaCo1-xNixO3-δ perovskites, zinc-air battery, SiO2/NC, lithium-

air battery.

(ii)

Contents

Abstract…………………………………………………………………………………………………………………………………………………i

Contents………………………………………………………………………………………………………………………………………………iii

List of Tables…………………………………………………………………………………………………………………………………………vii

List of Figures………………………………………………………………………………………………………………………………………viii

Chapter 1: Introduction………………………………………………………………………………………………………………….……1

1.1. Forward…………………………………………………………………………………………………………………………….……1

1.2. Status and complication of fuel cell technology………………………………………………………………..……4

1.3. PEM fuel cell…………………………………………………………………………………………………………………………..5

1.4. Lithium-ion battery technology………………………………………………………………………………………………7

1.5. Metal-air battery………………………………………………………………………………………………………………..….9

1.6. Aqueous system……………………………………………………………………………………………………………..……10

1.6.1.Zinc-air battery…………………………………………………………………………………………………………10

1.6.2.Zinc metal anode………………………………………………………………………………………………………11

1.6.3.Electrolyte………………………………………………………………………………………………………………..11

1.6.4.Air cathode catalyst………………………………………………………………………………………………….12

1.7. Non-aqueous system………………………………………………………………………………………………………..….13

1.7.1.Lithium-air battery…………………………………………………………………………………………………….13

(i)

(iii)

Chapter 2: Air cathode catalyst for aqueous zinc-air battery……………………………………………………………16

2.1. Literature survey………………………………………………………………………………………………………………16

2.2 Scope of the work…………………………………………………………………………………………………………….19

2.3. Experimental section………………………………………………………………………………………………………..20

2.3.1.Electrospinning approach……………………………………………………………………………………….20

2.3.2.Chemicals required…………………………………………………………………………………………………20

2.3.3. Method………………………………………………………………………………………………………………....20

2.3.4.Sol-gel approach……………………………………………………………………………………………..……..21

2.3.5.Chemicals required………………………………………………………………………………………………..21

2.3.6.Method…………………………………………………………………………………………………………….……21

2.4. Material characterization…………………………………………………………………………………………..……..23

2.5. Electrochemical characterization……………………………………………………………………………………...23

2.5.1.Electrochemical setup……………………………………………………………………………………………..23

2.5.2.Electrochemical active surface area …………………………………………………………………..……24

2.6.Zinc-air battery setup………………………………………………………………………………………………..……..25

2.7.Results and Discussion……………………………………………………………………………………………..……….27

2.7.1.Structural characterization…………………………………………………………………………….…………27

2.7.2.Electrochemical studies……………………………………………………………………………………………39

(iv)

2.7.3.Zinc-air battery performance……………………………………………………………………………….…..51

2.8.Summary…………………………………………………………………………………………………………………………..60

Chapter 3: Nitrogen doped carbon coated SiO2 as air cathode catalyst for non-aqueous lithium-oxygen

battery………………………….…………………………………………………………………………………………………..…………....61

3.1.Literature survey………………………………………………………………………………………………………….…….61

3.2.Scope of the work……………………………………………………………………………………………………………..63

3.3.Experimental section…………………………………………………………………………………………………………63

3.3.1.Hydrothermal approach……………………………………………………………………………………..………63

3.3.2.Chemicals Required……………………………………………………………………………………………………64

3.3.3.Synthesis procedure of SiO2 nanosphere……………………………………………………………………64

3.3.4.Synthesis procedure of SiO2/NC nanosphere………………………………………………………………65

3.4.Material characterization……………………………………………………………………………………………………65

3.5.Electrochemical characterization…………………………………………………………………………………………66

3.5.1.Electrochemical setup…………………………………………………………………………………….…………66

3.6.Lithium oxygen cell assembly and testing…………………………………………………............................67

3.6.1.Air cathode preparation…………………………………………………………………………………………….67

3.7.Results and discussion…………………………………………………………………………………………..…………….69

3.7.1.Structural analysis………………………………………………………………………………………..……………69

(v)

3.7.2.Electrochemical studies…………………………………………………………………………………..…………73

3.7.3.Lithium-oxygen battery performance………………………………………………………………………..76

3.8. Summary…………………………………………………………………………………………..…………………………………83

Conclusions………………………………….……………………………………………………………………………………………………84

Reference………………………………………………………………………………………………………………………………………………86

Acknowledgement……………………………………………………………………………………………………………………………….105

(vi)

List of tables

Table 2.1. Elemental composition ratio of Lanthanum, Cobalt and Nickel metals in perovskite

nanostructure and electrospun perovskite fiber mat. With 5 % error deviation…………………36

Table 2.2. Porous LaCo1-xNixO3- δ nanostructure OER activity parameters on 0.1M KOH

aqueous electrolyte……………………………………………………………………………….50

Table 2.3. Comparison of oxygen evolution catalytic activity of perovskite nanotubes, with

various catalysts………………………………………………………………………………….51

Table 2.4. Summary of charge (C)-discharge (D) potential gap of cathode catalysts at a current

density of 5 A/g…………………………………………………………………………………..57

(vii)

List of figures

Figure 1. Energy storage device for hybrid electric vehicle compared with driving range and

specific energy density…………………………………………………………………………….3

Figure 1.2. Scheme of a proton-conducting fuel cell……………………………………………..4

Figure 1.3. Schematic representation of Li-ion battery and its intercalation mechanism ………..8

Figure1.4. Theoretical and practical energy density of various types of metal-air battery……….9

Figure 1.5. Schematic polarization curves of zinc-air cell. The equilibrium potential of the zinc-

air cell (black line) is 1.65 V, but the practical voltage (red line) in discharge is lower than 1.65

V due to the sluggish ORR. A large potential is needed to charge zinc-air battery, higher than the

equilibrium potential (blue line)…………………………………………………………………12

Figure 1.6. Four types of Li-air batteries based on electrolytes…………………………………14

Figure 2.1.Schematic representation of perovskite nanostructure synthesize by electrospinning22

Figure 2.2. Schematic representation of synthesizing bulk perovskite by sol-gel method……..22

Figure 2.3. Swagelok setup based zinc-air battery system (MTI korea)………………………...26

Figure 2.4. Catalyst loaded gas diffusion layer…………………………………………………26

Figure 2.5. Rietveld refined XRD patterns of porous LaCo1-xNixO3-δ nanotube samples. The

black line indicates observed, blue line indicates calculated data and red line indicates difference

of both observed and calculated. La2O3 impurity is marked with asterisk, (b) Schematic

representation of perovskite structure……………………………………………………………28

(viii)

Figure 2.6. XRD patterns of NiCo2O4, La2O3 nanostructures and LaCo0.75Ni0.25O3 (bulk)

indicated by blue, red and black lines, respectively……………………………………………29

Figure 2.7. SEM images of LaCo1-xNixO3-δ nanostructure sample: (a) LaCo0.97O3-δ; (b)

La(Co0.71Ni0.25)0.96O3-δ; (c) La(Co0.45Ni0.51)0.96O3-δ; (d)La(Co0.22Ni0.74)0.96O3-δ; (e) LaNi0.99O3-δ. (f)

LaCo0.75Ni0.25O3 (bulk)…………………………………………………………………………..31

Figure 2.8. TEM and HR-TEM images of perovskite samples: (a & c) LaCo0.97O3-δ; (b &f)

La(Co0.71Ni0.25)0.96O3-δ; (d) LaCo0.45Ni0.51O3-δ; (e) LaNi0.99O3-δ…………………………………32

Figure 2.9. (a) Bright field TEM image of porous La(Co0.71Ni0.25)0.96O3-δ nanostructure and

corresponding elemental mapping analyses for (b) La, (c) Co, (d) Ni and (e) O and (f) overlay

image. ……………………………………………………………………………………………33

Figure 2.10. High resolution XPS spectra of Co2p in (a) LaCo0.97O3-δ; (b)La(Co0.71Ni0.25)0.96O3- δ;

and Ni3p spectrum in (c) La(Co0.71Ni0.25)0.96O3………………………………………………….35

Figure 2.11. Nitrogen adsorption-desorption isotherms and pore size distribution of (a, b)

LaCo0.97O3-δ; (c, d) La(Co0.71Ni0.25)0.96O3-δ; (e, f) La(Co0.45Ni0.51)0.96O3-δ; (g, h)

La(Co0.22Ni0.74)0.96O3-δ; (i, j) LaNi0.99O3-δ…………………………………………………….….39

Figure 2.12. Double-layer capacitance measurements for determining electrochemically active

surface area for (a, b) LaCo0.97O3-δ, (c, d) La(Co0.71Ni0.25)0.96O3-δ, (e,f)La(Co0.45Ni0.51)0.96O3-δ, (g,

h) La(Co0.22Ni0.74)0.96O3-δ, (i, j) LaNi0.99O3-δ nanostructure catalyst from voltammetry in 0.1 M

KOH…………………………………………………………………………………………..….43

Figure 2.13. LSV plot of ORR activity of porous perovskite nanostructure catalyst………..….43

(ix)

Figure 2.14. (a) OER activities of porous LaCo1-xNixO3-δ nanostructure measured in 0.1 M

KOH at 1600 rpm at a scan rate of 10mVs-1

after iR correction. (b) Tafel plot of OER activity for

corresponding samples……………………………………………………………………….…..45

Figure 2.15. (a)Mass activity of as synthesized perovskite samples at various potential. (b) Over

potential and Tafel slope variation with respect to increasing in nickel content in LaCo1-xNixO3-δ

nanotube sample………………………………………………………………………………...46

Figure 2.16. (a) Chronoamperometry test of porous La(Co0.71Ni0.25)0.96O3-δ catalyst on 0.1 M

KOH at constant potential of 1.6 V vs RHE for the duration of 10 hours. (b) Accelerated OER

stability of La(Co0.71Ni0.25)0.96O3-δ at rotation speed of 1600 rpm in 0.1 M KOH at a sweep rate of

10 mVs-1

after iR correction…………………………………………………………………….49

Figure 2.17: Specific discharge capacity of catalysts at a current density of 5A/g based on

consumption of zinc anode………………………………………………………………………53

Figure 2.18. Low capacity mode galvanostatic discharge (D) and charge(C) polarization curves

of at a constant current density of 5A/g (a) La(Co0.71Ni0.25)0.96O3-δ, (b) LaCo0.97O3-δ…………...55

Figure 2.19. (a) Comparison of Discharge and Charge plot of low capacity mode dotted line

represent LaCo0.97O3-δ and solid line represent La(Co0.71Ni0.25)0.96O3-δ, (b) Overpotential

comparison chart of two catalysts compared with initial and final cycle……………………….56

Figure 2.20. High capacity mode galvanostatic discharge (D) and charge(C) polarization curves

of at a constant current density of 5A/g (a) La(Co0.71Ni0.25)0.96O3-δ, (b) LaCo0.97O3-δ…………...58

(x)

Figure 2.21. (a) Comparison of Discharge and Charge plot of high capacity mode dotted line

represent LaCo0.97O3-δ and solid line represent La(Co0.71Ni0.25)0.96O3-δ, (b) Overpotential

comparison chart of two catalyst compare with initial and final cycle…………………………..59

Figure 3.1. High pressure hydrothermal reactor along with Teflon lined autoclave……………64

Figure 3.2. Handmade lithium-oxygen battery setup using open coin cell configuration……...68

Figure 3.3. XRD pattern of SiO2/NC and NC indicated by red and black line, respectively….69

Figure 3.4. SEM images of as synthesized sample: (a, b) SiO2; (c, d) SiO2/NC and (e) NC…...71

Figure 3.5. Deconvoluted XPS data of SiO2/NC (a) Carbon 1s spectra (b) Nitrogen 1s spectra.72

Figure 3.6. Rotating disk voltammograms of SiO2/NC catalyst in O2 saturated 0.1 M KOH with

sweep rate of 5mVs-1 at the different rotation rates indicated. The insets in show corresponding

Koutecky–Levich plots (J−1

versus ω−0.5

) at different potential…………………………………74

Figure 3.7. LSV polarization curve of SiO2/NC, NC and compared along with commercial 20

wt % Pt/C catalyst…………………………………………………………………………….....75

Figure 3.8.Discharge capacity plot of cathode sample prepared using SiO2/NC catalyst using

various electrolytes in the current density of 150mAh.g-1

………………………………………76

Figure 3.9. Discharge capacity plot of cathode sample using various catalyst in the current

density of 150mAh.g-1

…………………………………………………………………………..77

Figure 3.10. Cycling performance of the as prepared SiO2/NC catalyst with the capacity cutoff

of 1000 mAh.g-1

in the current density of 150mAh.g-1

…………………………….……………78

(xi)

Figure 3.11. XRD patterns analysed at different state of electrode (a) SiO2/NC catalyst and (b)

NC catalyst……………………………………………………………………………………….80

Figure 3.12. FESEM images of SiO2/NC catalyst electrode (a,b) pristine electrode (c,d)

discharge electrode and (e,f) after recharge electrode………………………………………..….81

Figure 3.13. FESEM images of NC catalyst electrode (a,b) pristine electrode (c,d) discharge

electrode and (e,f) after recharged electrode…………………………………………………….82

(xii)

1

Chapter 1. Introduction

1.1.Forward

There is a great deal of information and enthusiasm today about the development and increased

production of our global energy needs from alternative energy resources. Solar energy, hydrogen

energy, lithium-ion battery and Advanced Air batteries are all advanced resources for alternative

energy that are making progress [1-3].

From the alternatives, many researchers focusing on solar energy because of its first and

foremost reason behind this is more eco-friendly than non-renewable sources like oil, which

releases harmful greenhouse gases, carcinogens and carbon dioxide into the air [4] . However, it

outweigh their benefits by having primary disadvantage that it cannot produce energy under

darkness. Solar panels rely on significant levels of sunlight to operate efficiently and therefore

are unable to produce electricity in the darkness of the night sky, and in addition to that its

efficiency is quite low when compared with other energy resources.

The electrochemical devises like PEM fuel cell can extract energy from a hydrogen fuel in

anode compartment and earth abundant oxygen fuel source for cathode compartment leads to the

increase in efficiency of electrochemical conversion over internal combustion engine. Moreover,

we fuelled by pure ―clean‖ hydrogen, fuel cells produce only pure water as exhaust. Even when

powered by hydrocarbon fuels, they produce far less pollution than conventional technologies.

However, the infrastructure of hydrogen energy using in anode compartment is not up to date for

us, owing to its fact super-light hydrogen is hard to transport in a reasonable fashion [5]. It is

2

very expensive to move anything more than small amounts of it, making it impractical for most

functions.

The electrochemical energy storage devices, the fast growing technology is lithium ion battery,

which has become well proven and understood for powering small electronics like laptops or

cordless tools, and has become increasingly common in these applications – edging out the older

Nickel-Cadmium rechargeable battery chemistry due to lithium-ion battery many advantages

likely intercalation mechanism, lithium as light weight metal. However, the commercialization of

this technology to automotive application is quite challenging because of usage of well-known

commercialized very low weight density cathode material as Lithium-cobalt-oxide, Lithium iron

phosphate, which leads to have a slightly lower energy density [6,7].

To overcome the obstacles in these technologies, we need to look for fusion technology of

Fuel cell and lithium ion battery, fuse to get high energy density metal-air battery [8,9]. To

illustrate the superior properties of the metal-air batteries among energy storage and conversion

devices, a comparison of battery systems for providing energy for a hypothetical vehicle

powered by various battery systems is shown in Fig.1.

3

Figure 1.1 Energy storage device for hybrid electric vehicle compared with driving range and

specific energy density (Based on Phinergy Co.Ltd estimated range).

4

1.2. Status and Complication of Fuel cell technology

Fuel cell vehicles (FCVs) have the potential to significantly reduce our dependence on foreign

oil and lower harmful emissions that contribute to climate change. FCVs mainly run on Polymer

electrolyte (PEM) fuel cell or proton conducting fuel cell [10], which uses hydrogen as gas rather

than gasoline and emit no harmful tailpipe emissions. Several challenges must be overcome

before these vehicles will be competitive with conventional vehicles, but the potential benefits of

this technology are substantial. In order to bring the technology for commercialization we need

to look over following compartments.

1. Anode Catalyst

2. Membrane as electrolyte

3. Cathode Catalyst

Figure 1.2. Scheme of a proton-conducting fuel cell (source from Wikipedea)

5

Mechanism:

Anode: H2 (g) 2H+

+ 2 e- ERHE = 0 V

Cathode: ½ O2 + 2H+ +2e

- H2O ERHE = 1.23 V

Overall reaction: H2 + ½ O2 H2O ERHE = 1.23 V

1.3.PEM Fuel cell

PEM fuel cells are electrochemical devices that generate direct current (DC) electricity by

converting the chemical energy of fuel and oxidant to electrical energy. The basic set up of a fuel

cell consists of three compartments, which are the anode, cathode and electrolyte. In a fuel cell,

the fuel hydrogen is oxidized at the anode, while the oxidant oxygen is reduced at the cathode.

Produced protons travel from anode electrode to cathode compartment with the help of an

electrolyte, which is basically a charge carriers. A cell produces DC voltage when the electron

transfer occurs and is collected via an external circuit.

(i) Anode catalyst:

The hydrogen gas is oxidized at anode to H+ by using a suitable electrocatalyst, and transport as

a proton in the proton exchange membrane. A potential problem which arises from this system is

the production of small amount of impurities particularly carbon monoxide, which strongly

adsorb on the Pt catalyst, usually employed in the anode, blocking the sites for the hydrogen

6

adsorption and oxidation. Therefore, Pt is modified with other elements such as Ru or Mo for the

PEMFCs so as to search for enhanced CO tolerant materials [11]. Unfortunately, the relative

high cost and insufficient durability of these elements still hinder the large scale

commercialization of PEMFCs.

Ticianelli et al. reported that the platinum based tungsten carbide as anode electrocatalyst for

enhancing the hydrogen oxidation reaction rate by means of high CO tolerance, long durability

and low cost of this material [12].

(ii) Membrane as electrolyte

The hydrophobic tetrafluoroethylene backbone with pendant side-chains of perfluoronated vinyl-

ethers terminated by ion-exchange groups, i.e. sulfonic groups, acidic membrane named as

nafion is presently dominates the fuel cell market because of its high proton conductivity and

exchange capacity leads to increase in fuel cell performance [13].

The serious drawbacks of these membrane is low water retention capability to exchange protons

because it work under the temperature window between 0oC and 100

oC, secondly Nafion has

poor mechanical and chemical stability at elevated temperatures and in addition to that Nafion

cannot withstand under peroxide radical ion attack, it results in decrease in membrane

conductivity and performance of fuel cell [14].

7

(iii) Cathode catalyst

To date, platinum supported nanoparticle on high-surface-area carbon represent state-of-the-art

electrocatalysts used to catalyze the oxygen reduction reaction (ORR) with a high efficiency.

However, the prohibitive price, scarcity and insusceptibility in acidic media are some of the

limitations of Pt catalysts preventing the large-scale market penetration of PEM fuel cell

[15]. Recent studies on transition metal oxides, chalcogenides, carbon nitride, and conducting

polymer derived materials have been investigated for the ORR as an alternative to Pt-based

catalysts [16]. However, insufficient activity and low stability, particularly in acidic media, limit

their wide application in PEM fuel cell technology.

1.4. Lithium-ion battery technology

Compared to other battery systems, the Li-ion battery offers the greatest potential for

EV/HEV’s [17]. This is mainly because lithium has a low electronegativity (0.98 on Pauling

scale) and lowest atomic weight (6.94 g mol-1

) of all metals.

Li-ion battery is an engineered, electrochemical energy storage device composed of

various materials and components. The basic setup is a positive and negative electrode separated

within an organic electrolyte. The positive and negative electrodes are referred to as the cathode

and anode during discharge and vice versa during charge

Mechanism of Li-ion battery:

Energy is stored in or extracted from the Li-ion battery based on red-ox reactions

occurring at the positive and negative electrodes, while the energy is supplied to or delivered

8

from an electrical device or power source, respectively. The charging process for a Li-ion battery

with Li 1-x CoO2 as the positive electrode material and graphite as the negative electrode

material.

Positive: Li1–xCoO2 + x Li+ + x e

– LiCoO2 (ERHE = 0.6 V )

The lithium-ions for the intercalation process come from delithiation of the LixC6 anode:

Negative: LixC6 x Li+ + x e

– + 6 C (ERHE = -3.0 V )

The left side shows the layered structure of the Li1-xCoO2 and the right side the graphite sheets.

During charging, the Li-ions migrate from the positive electrode to the negative electrode and

vice versa during discharging.

During battery charge, the reverse reactions take place. Accordingly, the net cell

reactions during its charge-discharge process are:

Net Reaction: LiC6 + Li1-xCoO2 Li1-xC6 + LiCoO2 (Ecell = 3.6 V)

Figure 3. Schematic representation of Li-ion battery and its intercalation mechanism

discharge

charge

9

1.5.Metal-Air battery

Metal-air batteries have attracted much attention as a possible alternative, because their energy

density is extremely high compared to that of other energy storage and conversion devices. The

air battery system differs by other energy storage system by means of open cell configuration or

on other words we can say it is combination of batteries and fuel cell system.

There are several kinds of metal-air batteries based on different metal anodes; their reaction

mechanisms are variable, resulting in requests for different types of cell components. Typically,

metal-air batteries are divided into two types according to their electrolytes. One is a cell system

using an aqueous electrolyte; such a system is not sensitive to moisture. The other is a water-

sensitive system using an electrolyte with aprotic solvents. This system is degraded by moisture

[18].The theoretical and practical energy density of various type of rechargeable metal-air

battery is shown in figure 4 based on type of metal used as anode in the system.

Figure 4. Theoretical and practical energy density of various types of metal-air battery

10

1.6.Aqueous system

Metals such as Ca, Al, Fe, Cd, and Zn are appropriate for the aqueous system. Zinc-air batteries

in particular have powerful potential for use as alternative energy storage devices. Al can be

more easily corroded than Zn in alkaline solution, although Al-air cells have a much greater

energy density than zinc-air cells [19]. Also, Zn has various advantages such as low cost,

abundance, low equilibrium potential, environmental benignity, a flat discharge voltage, and a

long shelf-life. The most important merit of alkaline zinc-air batteries, however, is that non-noble

metal catalysts can be used for the oxygen reduction reaction. The theoretical specific energy

density of Zn-air batteries is 1084 Wh·kg−1

.

1.6.1.Zinc-Air battery

The mechanism of zinc air system is in the anode side dissolution of zinc metal into zinc ion in

the electrolyte and in later cathode side oxygen reduction reaction (ORR) reaction to form

hydroxyl radical in the electrolyte which travel to anode side through separator and combine with

Zn2+

ions and form zinc hydroxyl radical. In case of recharging zinc metal deposition from zinc

hydroxyl radical in the anode side and Oxygen evolution reaction (OER) occurs from the

hydroxyl radical presence in cathode side.

Anode reaction: Zn Zn2+

+ 2e-

Zn2+

+ 4OH- Zn(OH)4- ERHE = -1.24 V

Cathode reaction: O2 + 2H2O + 4e- 4OH- ERHE = 0.40 V

Overall reaction: Zn2+

+ 2H2O + 4e- O2 + Zn(OH)4- Ecell = 1.64 V

11

Zinc-Air battery system composed of three major components

1. Zinc Metal anode

2. Electrolyte

3. Air cathode catalyst.

1.6.2.Zinc metal anode

Zinc-air battery system use pure zinc metal as active material, While discharging oxidation of

zinc metal occurs to form zinc ions in the electrolyte medium, the corrosion of zinc metal rate is

much slower than other metals like aluminum and iron in the alkaline medium. As mentioned

above, because zinc metal participates in the anode reaction during discharge, the most practical

method of improving the performance of the zinc anode is to increase the surface area of the zinc

particles so that the zinc can react with the alkaline electrolyte more efficiently. Many

researchers are focusing on replacing the zinc metal with zinc nanopowder and alloying of zinc

metal with Ni or Al leads to increase in reactive sites of anode compartment and obviously

results in increase in performance of battery [20].

1.6.3.Electrolyte

The alkaline electrolyte of potassium hydroxide has been widely used for zinc-air battery

because of its superior ionic conductivity. Mclarnon et al reported that the 6M KOH shows

maximum ionic conductivity at the concentration [21]. In addition to that problem faced by this

electrolyte is CO2 contamination from the atmosphere leads to formation of potassium carbonate

that hinders the performance of zinc-air system. Many researchers suggest in adding the

inhibiting agents like LiOH, soda lime to prevent the potassium carbonate formation [22,23].

12

1.6.4.Air cathode catalyst

An air electrode is divided into two parts: one is the gas diffusion layer, composed of carbon

material and a hydrophobic binder such as polytetrafluoroethylene (PTFE) as a wet proofing

agent, which makes gas diffusion layer permeable to only air but not water. The second part is a

catalytic active layer which consists of catalysts, carbon materials, and binder. ORR takes place

in the catalytic active layer. A bi-functional catalyst is essential for a rechargeable zinc-air

battery for facile the electrochemical reactions namely, ORR while discharging and OER in case

of recharging the battery. To date non precious electrocatalyst like Pt-Ru/C and Pt-Ir/C have

been considered as universal choice of bifunctional catalyst, nevertheless these materials suffered

from scarcity and high cost [24,25].

Figure 5. Schematic polarization curves of zinc-air cell. The equilibrium potential of the zinc-air

cell (black line) is 1.65 V, but the practical voltage (red line) in discharge is lower than 1.65 V

due to the sluggish ORR. A large potential is needed to charge zinc-air battery, higher than the

equilibrium potential (blue line).

13

1.7.Non-Aqueous System

Alkali metals namely Na, Li and Mg has widely used for non-aqueous system. This is owing to

the fact these metal is explosively reactive with water. Among these Lithium-air system should

be appropriate choice because of its high energy density of 11,140 Wh.kg-1

, this is mainly

because of huge open circuit voltage of 2.96 V against Lithium metal. In addition to that size of

lithium metal i.e., ionic radii is small and its density is light compare with other alkali metals.

1.7.1.Lithium-Air battery

Lithium-air batteries have shown 5–10 times more energy density than a standard Li-ion battery.

The specific energy density of a Li-air battery is 5200 Whkg−1

or 18.7 MJkg−1

when the mass of

oxygen is included. However, Li-air batteries did not attract wide attention when they were

introduced, although they have higher energy densities than those of conventional Li-ion battery

systems (200–250 Whkg−1

) [26]. The estimation of theoretical capacity and energy of Li-air

batteries is in debate. Theoretical gravimetric and volumetric energy densities using aqueous

electrolytes have been investigated based on Li metal anode, air electrode, and electrolyte. It was

determined that the maximum theoretical gravimetric and volumetric energy densities are 1300

Whkg−1

and 1520 WhL−1

in basic electrolyte, and 1400 Whkg−1

and 1680 WhL in acidic

electrolyte, respectively [27]. The above specific capacity and energy projections are the

theoretical maximum limitation and are based on active materials including only Li metal, air

electrode, and electrolyte. However, in practical cells, some excess electrolyte must be in the cell

in order to provide ionic conductive media during the entire discharge process. In addition, other

necessary materials, including current collectors, membrane, and package materials, will further

14

reduce the cell specific capacity and energy by 20–30% [27]. Li-air battery is comprised of a

lithium metal foil anode, a Li+ conductive electrolyte and a thin carbon composite air cathode.

Different types of Li-air battery system is

i) aprotic/non-aqueous electrolytic type,

ii) aqueous electrolytic type,

iii) mixed/ hybrid electrolytic type, and

iv) solid-state electrolytic type.

All Li-air battery systems use lithium metal as the anode and oxygen gas as the cathode

material, but their electrochemistry differs according to the electrolyte types.

Figure 1.6. Four types of Li-air batteries based on electrolytes

15

1.8. Objective of this work

In general towards technology advancement, the aqueous and non-aqueous based metal-air

battery system applicable for hybrid electric vehicle development, owing to the fact that the both

this aqueous and non-aqueous air battery system energy density is much higher than the

commercialized lithium-ion battery. However, the performance of such device is mainly limited

by the slow kinetics of the oxygen reduction and evolution reactions on the cathode during

battery discharge and recharge. Consequently, the development of an active, stable, and

affordable bifunctional catalyst is absolutely critical for the commercialization of this technology.

Up to date, much research has been focused on the development of precious metal catalyst like

Pt or alloy based Pt-Ru, Pt-Au used for enhancing these sluggish reaction.

The main objective of this work is to develop the inexpensive transition metal oxides in the

aligned manner of one dimensional nanostructured oxygen deficient perovskite material. Due to

unique structure, oxygen deficient sites and most importantly presence of transition metal in the

octahedral site leads to improvement in ORR and OER performance in a facile manner, we

further explore this one dimensional nanostructured perovskite catalyst for aqueous zinc-air

battery system.

In the second part of the work is to, develop a thin film heteroatom doped carbon coated on the

SiO2 nanosphere. And we use the as synthesized catalytic material for non-aqueous lithium-

oxygen battery system and studied the battery performance.

16

Chapter 2. Air Cathode Catalyst for Aqueous Zinc-Air Battery

2.1.Litrature survey

At present, stable bifuctional catalyst based on noble metal oxides, such as Pt-Ru-Ir/C, Pt-Ru/C

and RuO2 [28,29] are considered as the universal choice of catalyst for ORR and OER.

Nevertheless these materials are scarce and expensive, which currently makes this system unable

to be used. As a result, it is necessary and of great technological significance to explore

inexpensive material that surpasses the performance of noble metal oxide catalysts. The first row

transition metal oxide structures such as hematite, ferrites, and spinels [30-32] have, received a

significant amount of interest as promising bifunctional catalytic materials because their long

term corrosion resistance in alkaline solution [33-35]. However these materials underperform

relative to the noble metal oxides. For the past few decades, the cobalt and nickel based materials

have received great attention as bifunctional electrocatalysts due to their good electronic

structural property, distinct oxidation state, eg filling, and o-p band center [36-39].

Among these materials, Mn, Fe, Co and Ni based perovskite oxide with ABxB’1-xO3 structure

shows promising properties of electrocatalysis towards oxygen reduction and evolution [40]. In

addition to that, molecular orbital theory indicated that transition metal based perovskites are

competitive candidate for electrocatalysis reaction [41,42]. This is mainly because of redox

behaviour of hybridization of the transition metal 3d and the oxygen 2p orbitals can give rise to

bond with significant covalent character, and degree of covalence influences the electrocatalytic

activity [43]. The lanthanum based perovskite materials like La(Co0.55Mn0.45)0.99O3,

LaTi0.65Fe0.35O3, LaMO3 (M = Ni, Co, Fe, Cr and Mn) receiving a great attention towards oxygen

17

electrocatalysis reactions [44-46]. Perovskite materials differ from other materials like metal

oxides and spinels by means of accommodate more than one A-site and B-site cation species.

The great tolerance of lattice mismatch and lattice heterogeneity confers perovskite with good

electrical, catalytic property and stability [44]. Hong et al. reported that preferable eg value

occupancy slightly less than 1 for better ORR and the value greater than 1 for OER catalytic

activity, and these eg occupancy directly relate with the transition metal and oxygen bond

strength in perovskite structure [47]. Till date Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF5582) that has eg

occupancy of nearly 1.2 exhibits highest OER activity among perovskite family [48]. Recently

Jung et al. reported that incorporation of rhombohedral LaCoO3 nanoparticle in surface of

BSCF5582 structure results in 2 to 3-fold increase in OER catalytic activity [49].

Gasteiger et al. reported that the partial substitution of B sites in perovskite oxides resulted in

oxygen vacancy or B-site ion valence changes, which enhanced the oxygen evolution activity

[41]. This fluctuation in activity is due to the variation of bulk B site cation’s 3d electrons, as

described by Otagawa and Bockris [50]. This fluctuation can also be attributed to the change in

the conduction band position of material, as described by Matsumoto et al. [51]. A drastic

enhancement of oxygen evolution was observed by tuning the morphology of MnO2 material

from particles to nanotube with well-aligned particles [52]. The catalytic activity of material can

be tuned by modifying the physical properties such as tuning the surface area, electrochemical

active surface area (ECSA), and electrode roughness factor. Chemical characteristics such as the

oxidation state and shifts in the material’s conduction band should also be concern. Such

material properties can be accomplished by tuning the morphology and also by substituting them

18

with multi-valent transition metal ions that are reflected in the electrocatalytic activity of the

material [53].

The active sites of the catalyst have been improved by enhancing the surface to volume ratio of

perovskite material. In general synthesis techniques such as electrospinning and hydrothermal

the material can change to different zones in morphology [54,55]. Among these techniques,

electrospinning is a versatile, low cost, and simple one-step technique that prepares one

dimensional nanostructures with different morphologies such as nanofibers, nanobelts and

nanotubes with a porous nature. Madhavi et al. [56] reported that the size and diameter of

nanotubes can easily be controlled by varying different parameters such as solution viscosity,

flow rate, applied potential and distance between the syringe and collector.

To put it in a nut shell, performance of zinc-air battery mainly depends on the catalytic material

to minimize the overpotential recent times [57-60]. In agreement with our findings, prior studies

have also shown that nickel or cobalt based metal oxides exhibit high intrinsic activity for

reducing the overpotential between ORR and OER [61, 62]

19

2.2.Scope of the work

In this study, for the first time, we have synthesized a series of porous LaCo1-xNixO3-δ

nanostructure with interconnected particles by adopting a simple electrospinning technique. The

effect of nickel content in the cobalt octahedral sites of the LaCo0.97O3-δ structure towards the

performance of the oxygen evolution reaction was systematically investigated, and also to

understand the morphology effect we synthesized LaCo0.75Ni0.25O3 (bulk) perovskite by citrate

method and investigate the ORR and OER activity.

In addition, the different OER activity of catalysts was elucidated with respect to nickel

substitution in porous perovskite nanostructure by calculating the turnover frequency of each

catalyst and correlating them with the surface properties of the catalysts.

We have applied as prepared highly oxygen deficient perovskite nanostructured materials as a

cathode catalyst in zinc-air battery and studied its performance based on the nickel substitution

effect in perovskite structure and also the morphological effect of this material.

20

2.3.Experimental Section

2.3.1.Electrospinning approach

Electrospinning is a widely used technique for the electrostatic production of nanofibers, during

which electric power is used to make polymer fibers with diameters ranging from 2 nm to several

micrometres from polymer solutions or melts. This process is a major focus of attention because

of its versatility and ability to continuously produce fibers on a scale of nanometres, which is

difficult to achieve using other standard technologies.

Electrospinning is a relatively simple way of creating nanofiber materials, but there are several

parameters that can significantly influence the formation and structure of produced nanofibers.

2.3.2.Chemicals Required

Polyacrylnitrile (PAN), Dimethylformamide (DMF), Lanthanum acetylacetonate, Cobalt

acetylacetonate and nickel acetylacetonate.

2.3.3.Method

The perovskites one-dimentional nanostructures were prepared by a two-step simple

electrospinning approach. Firstly, an equimolar ratio of lanthanum acetylacetonate, cobalt

acetylacetonate and nickel acetylacetonate was prepared in 3g of dimethylformamide (DMF)

solvent as solution A, followed by 10 weight percentage of Polyacrylnitrile (PAN) in 6g of DMF

as solution B used as a template for getting a tubular morphology structure. Then the solution A

and Solution B was stirred together with slight heating of 80oC overnight. The clear solution was

placed in a 12 mL plastic syringe attached with flat ended steel needle with 23G inner diameter.

21

Nanofibers were fabricated at 10kV of applied voltage at a distance of 10cm between needle and

rotating drum with solution feed rate was maintained at 1mLh-1

. After electrospinning all the

fibrous were calcination under air atmosphere in the tubular furnace at 250oC for 1h in order to

get an interconnected particle nature followed by 600oC for 1h to remove the polymer template

at a heating rate of 3oc/min.

2.3.4.Sol-gel Method

This method widely used for synthesizing metal oxide in bulk state, in this process mainly the

gelation formation with the metal precursor with some chelating agent and followed by

calcination in open air atmosphere leads to formation of bulk size material.

2.3.5.Chemicals Required

Lanthanum nitrate, cobalt nitrate, nickel nitrate, ethanol and D.I water.

2.3.6.Method

We have synthesized LaCo0.75Ni0.25O3 (bulk) by using sol-gel method. Equimolar ratio of

lanthanum nitrate, cobalt nitrae and nickel nitrate dissolved in D.I water and ethanol (1:3 v/v),

followed by vacuum drying over 6 hours and then calcination in muffle furnace at 6000C for 3

hours.

22

Figure 2.1. Schematic representation of perovskite nanostructure synthesize by electrospinning.

Figure 2.2. Schematic representation of perovskite bulk synthesize by sol-gel method.

23

2.4.Material Characterization

Phase formation of the material has been confirmed by X-ray diffractometer (Rigaku, Miniflex

600). The morphology of the samples was carried out by means of field-emission scanning

electron microscopy (FE-SEM, Hitachi, S-4800II, 3kV) and a thin osmium coating has given

before analyzing the samples. Transmission electron microscope image, elemental composition

of each material observed by Hitachi HF-3300kV instrument. The sample preparation was done

by ultrasonically dispersed the sample in absolute ethanol, placing a drop of solution in copper

grid coated with an amorphous carbon film, and then drying under open air condition. In order to

study the oxidation state and understand the bonding nature of the material, the samples were

characterized by X-ray photoelectron spectroscopy using a JEOL JPS-900 in an ultra-high

vacuum X-ray photoelectron spectrometer with an Al monochromatized cathode source at 25 W.

Furthermore to understand the surface area and porous nature of the material by using Brunauer–

Emmett–Teller (BET), Barrett–Joyner–Halenda (BJH) and Harkins–Jura (H–J) models with a

Micromeritics Tristar ASAP 2020.

2.5.Electrochemical characterization

2.5.1.Electrochemical setup

A conventional three electrodes setup was used to assess the oxygen evolution reaction and

electrode stability test by rotating disk electrode linear sweep voltammetry (LSV) and cyclic

voltammetry (CV), respectively in alkaline medium 0.1M KOH electrolyte under oxygen

saturated condition. The computer-controlled potentiostat (Bio-logic) was used to carry out the

electrochemical studies and all experiments were carried out at a 10mVs-1

scan rate.

24

(i).Catalyst ink preparation

The catalyst ink was prepared by 5 mg of material dispersed in the solvent containing 160 µl

isopropyl alcohol, 30 µL deionized water and 10 µL of 5 wt% Nafion as binder. The contents

were thoroughly sonicated using an ultrasonicator for 30 minutes to obtain a homogeneous

suspension. Two microliters of catalyst ink was dropped on the surface of rotating disk electrode

with the area of 0.07065 cm2 with the catalyst loading of 0.70mg.cm

-2. The three electrode cell

setup consists of Pt foil, saturated calomel along with glassy carbon rotating disk electrodes were

used as counter, reference and working electrodes, respectively. All potentials reported in this

work were against reversible hydrogen electrode (RHE), which was converted from a saturated

calomel electrode (SCE) based on conversion equation [63] ERHE=ESCE+0.241 V+0.059× pH-iR,

with the pH of 0.1 M KOH aqueous electrolyte being 13. The potential region of oxygen

evolution reaction monitored from 1 to 1.8 V and for ORR monitored from 1 to -0.2 V against

RHE with a rotation speed of 1600 rpm.

2.5.3. Electrochemical active surface area

To understand the impact of active sites in catalyst, we have studied the electrochemical active

surface area measurement from double layer capacitance in faradaic region using cyclic

voltammetry as reported by Mccrory et al.[64] The electrochemical setup have been arranged

with electrocatalyst loaded glassy carbon as working electrode, platinum is counter electrode and

standard calomel electrode used as reference. We have measured the double layer capacitance in

the potential region in which non faradaic process occur. This range is typically a 0.1 V potential

window centered at the open circuit potential (OCP) of the system. The working electrode was

25

held at each potential vertex for 10s before beginning the next sweep. The specific formula for

calculating ECSA is given below

Ic = v.Cdl

ECSA = Cdl/Cs

Where, Ic – charging current, Cdl – double layer capacitance, v – scan rate, Cs – specific

capacitance of 0.1M KOH is 0.060mFcm-2

[64,65].

2.6.Zinc-Air battery setup

Zinc-air battery cell configuration used in this study was split cell (EQ-STC-MTI-Korea) packed

with glass Whatmann microfiber separator sandwiched between polished zinc anode and catalyst

coated gas diffusion layer (GDL). The cathode catalyst ink prepared by 7mg catalyst and 3mg of

activated carbon are ultrasonically dispersing with 1 mL Isopropyl alcohol and 67 µl 5 wt %

Nafion ionomer for 30 minutes then catalyst ink has been coated in GDL by using brush coating

and then dried in a vacuum oven for overnight at 80oC. Discharge and charge cycle was done in

constant current mode using Battery analyser (BST8-3). While charging-discharging cycles, 1

atm oxygen continuously fed with constant flow rate in the cathode compartment. All

experiments were carried out at room temperature.

26

Figure 2.3. Swagelok setup based zinc-air battery system (MTI Korea)

Figure 2.4. Catalyst loaded gas diffusion layer

27

2.7.Results and Discussion

2.7.1.Structural Characterization

(i) Phase Analysis by X-Ray diffraction pattern

The X-ray diffraction patterns of perovskite samples presented in Figure 2.5 were subjected to

Rietveld refinement on the basis of well-matched rhombohedral structured LaCo1-xNixO3-δ, with

a space group of R-3CH and a small impurity La2O3. The slightest deviation in a doublet peak

located in the region of 2Ө at 33o, 40

o, 58

o and 69

o respectively, with respect to changes in the

ratio of cobalt and nickel ion on the B site of the LaCo0.97O3-δ, indicates that the crystal structure

was almost a cubic structure. It was observed that the lattice parameter of LaCo0.97O3-δ decreases

with an increase in substitution of nickel content due to the different ionic radius of cobalt and

nickel [66]. The XRD pattern of La0.75Ni0.25O3 (bulk), and La2O3 shown in Figure 2.6.

28

Figure 2.5. Rietveld refined XRD patterns of porous LaCo1-xNixO3-δ nanotube samples. The

black line indicates observed, blue line indicates calculated data and red line indicates difference

of both observed and calculated. La2O3 impurity is marked with asterisk, (b) Schematic

representation of perovskite structure.

29

Figure 2.6. XRD patterns of La2O3 nanostructures and LaCo0.75Ni0.25O3 (bulk)

30

(ii) Morphology analysis by SEM and TEM

The SEM images shown in Figure 2.7, clearly depict the formation of a nanotubular structure

with the outer and inner diameters of 100-150 and 50-70 nm, respectively, and a length of

several microns. The images also show the presence of a porous structure due to the

decomposition of polyacrylonitrile at 600oC, which results in the formation of one dimensional

morphology [67,68]. In order to understand the detailed mechanism behind this nanostructure

formation, it can be presumed that the metal precursor uniformly distributed along with polymer

fiber mat during the electrospinning process. At the second step, when we start to calcination

under open air atmosphere at 250 oC leads to stabilization of polymer and makes the strong

interaction of metal ion on the surface of polymer fiber, when we further calcination at 600 oC

leads to formation of perovskite structure by aggregation of metal ions in the form of

nanostructure with the help of polymer as the soft template [54].

Catalysts with such morphology could be useful for the efficient movement of ions through a

one dimensional porous structure with enormous active sites exposed to the substrate leads to a

facile movement, thus expecting an improved activity [69]. The TEM morphology analyses of

catalysts reveal that the porous structure has one dimensional nanostructure, as shown in Figure

2.8. Further, Figure 2.8 clearly shows the interconnected nanoparticles joined together forming

one dimensional nanotubes. Figure 2.8 c, further confirms that the structural integrity and

particles are in intimate contact with each other, which facilitate the electron transport rate, good

electrical conductivity, and matter diffusion. The TEM EDS mapping results depict the presence

of La, Co, Ni, and oxygen in a tubular structure, as shown in Figure 2.9.

31

Figure 2.7. SEM images of LaCo1-xNixO3-δ nanostructure sample: (a) LaCo0.97O3-δ; (b)

La(Co0.71Ni0.25)0.96O3-δ; (c) La(Co0.45Ni0.51)0.96O3-δ; (d)La(Co0.22Ni0.74)0.96O3-δ; (e) LaNi0.99O3-δ. (f)

LaCo0.75Ni0.25O3 (bulk)

32

Figure 2.8. TEM and HR-TEM images of perovskite samples: (a & c) LaCo0.97O3-δ; (b &f)

La(Co0.71Ni0.25)0.96O3-δ; (d) LaCo0.45Ni0.51O3-δ; (e) LaNi0.99O3-δ.

33

Figure 2.9. (a) Bright field TEM image of porous La(Co0.71Ni0.25)0.96O3-δ nanostructure and

corresponding elemental mapping analyses for (b) La, (c) Co, (d) Ni and (e) O and (f) overlay

image.

34

(iii) X-ray Photoelectron Spectroscopy (XPS):

The XPS (Figure 2.10) data evidently point out some active sites like Co2+

, Co3+

, Ni2+

, Ni3+

in

the perovskite material such as LaCo0.97O3-δ and LaCo0.71Ni0.25O3-δ where Co2p spectrally

differentiates the presence of Co2+

and Co3+

ions with respective of binding energy values of

779.8 and 781.71 eV in 2p3/2. On the other hand, the binding energy values are 794.8 and 796.5

eV for 2p1/2 of cobalt spectra [70-73]. The quantification of nickel from the Ni3p peak is not

possible, due to the overlapping binding energy values for Ni2p and La3d observed in the same

region [74]. Therefore, in order to eliminate ambiguous quantification, we have identified the

contribution of nickel as Ni2+

and Ni3+

with respective binding energy values of 66.9 eV and 71

eV [75]. It was reported that Ni doping in Co sites could lead to the creation of new active sites

with lower activation energy for the oxygen bond formation [76]. This also creates the mixed

valence states of cobalt and nickel content with Co3+

, Co2+

, Ni2+

and Ni3+

in LaCo0.71Ni0.25O3-δ.

Spin states in the B site of perovskite play an important role for the oxygen evolution catalyst

[77], The intermediate spin of cobalt cation (Co3+

) directly reflects OER activity as suggested by

Harvey et al. [78]. The presence of Co and Ni in the octahedral sites with variable oxidation

states of Co2+

, Co3+

, Ni2+

, and Ni3+

, as well as the creation of oxygen vacancy in the crystal

structure of metal oxides can be beneficial for enhancing the catalytic activity [79,80]. And also

these XPS studies reveal that the optimized ratio of Co3+

and Ni2+

in LaCo0.71Ni0.25O3-δ samples

is one of the main reasons for such an excellent performance.

35

Figure 2.10. High resolution XPS spectra of Co2p in (a) LaCo0.97O3-δ; (b)La(Co0.71Ni0.25)0.96O3- δ;

and Ni3p spectrum in (c) La(Co0.71Ni0.25)0.96O3-δ

36

(iv) Inductive coupled plasma-Mass spectroscopy (ICP-MS):

The chemical compositional analyse for all the perovskite nanostructure was performed using

inductive coupled plasma mass spectroscopy (ICP-MS; Perkin Elmer, optima 4300 DU). This

result confirms the ratio of La, Co and Ni metals present in each perovskite structure shown in

Table 2.1.

Table 2.1. Elemental composition ratio of Lanthanum, Cobalt and Nickel metals in perovskite

nanostructure and electrospun perovskite fiber mat. With 5 % error deviation.

Metal salts/Polymer mat

Perovskite

La

(at%)

Co

(at%)

Ni

(at%)

LaCo0.97O3-δ /Polymer 1 0.955839 ---

La(Co0.71Ni0.25)0.96O3-δ/Polymer 1 0.688733 0.256899

La(Co0.45Ni0.51)0.96O3-δ /Polymer 1 0.467054 0.511089

La(Co0.22Ni0.25)0.96O3-δ /Polymer 1 0.236528 0.744619

LaNi0.99O3-δ /Polymer 1 --- 1.03396

LaCo0.97O3-δ 1.05125 0.97395 --

La(Co0.71Ni0.25)0.96O3-δ 1.03001 0.71866 0.255906

La(Co0.45Ni0.51)0.96O3-δ 1.05036 0.45693 0.514882

La(Co0.22Ni0.74)0.96O3-δ 1.028349 0.22513 0.74842

LaNi0.99O3-δ 1.07254 --- 0.9959

37

(v) Brunner-Emmett-Teller (BET)

The catalytic surface area of materials is normalized by the BET. Thus, the BET surface area of

all catalysts was measured. The nitrogen adsorption and desorption isotherms and pore size

distribution of all porous nanostructure are shown in Figure 2.11; the BET specific surface areas

of all the five catalysts are listed in Table 2.2. The hierarchical perovskite catalysts exhibited

comparable BET surface areas of 18 to 22 m2.g

-1 with a pore size distribution of 20 to 40 nm.

38

(Continue)

39

Figure 2.11. Nitrogen adsorption-desorption isotherms and pore size distribution of (a, b)

LaCo0.97O3-δ; (c, d) La(Co0.71Ni0.25)0.96O3-δ; (e, f) La(Co0.45Ni0.51)0.96O3-δ; (g, h)

La(Co0.22Ni0.74)0.96O3-δ; (i, j) LaNi0.99O3-δ

40

2.7.2. Electrochemical studies:

(i) Electrochemical active surface area (ECSA):

The electrochemical active surface area of the as-synthesized electrocatalysts has been examined

using the electrochemical double layer capacitance, i.e., measuring the non-faradaic capacitance

current associated with double layer charging at different scan rates in 0.1M KOH electrolyte by

cyclic voltammetry. Figure 2.12 shows the CV of the La(Co0.71Ni0.25)0.96O3-δ catalyst in a non-

faradaic potential region and the CVs for the remaining catalysts are given in Figure 2.12. We

observe that the increase in electrochemical active surface and other consequent parameters,

namely double layer capacitance, roughness factor, and specific activity mainly originate from

the substitution of nickel content in octahedral sites of perovskite material. This substitution

results may in the creation of oxygen vacancies in the electrocatalyst, which provide better

assistance for the enhanced ORR and OER activity [59]. However, the LaNi0.99O3 catalyst

exhibits low active sites among the five catalysts but its ORR and OER activity is comparably

higher than the LaCo0.97O3 catalyst. This indicates the presence of nickel in octahedral sites,

which not only increases the active sites, but also changes the eg filling and O p-band center and

weakening the bonding strength of OH- species on the perovskite structure and come up with a

good bifunctional catalytic activity. Table 2.2 shows an overview of OER performance in 0.1M

KOH of our perovskite nanostructure and also from previous literature.

41

(Continue)

(a) (b)

(c) (d)

42

(Continue)

(g)

(e)

(h)

(f)

43

Figure 2.12. Double-layer capacitance measurements for determining electrochemically active

surface area for (a, b) LaCo0.97O3-δ, (c, d) La(Co0.71Ni0.25)0.96O3-δ, (e,f)La(Co0.45Ni0.51)0.96O3-δ, (g,

h) La(Co0.22Ni0.74)0.96O3-δ, (i, j) LaNi0.99O3-δ nanostructure catalyst from voltammetry in 0.1 M

KOH.

(ii) ORR activity of the catalyst

The electrocatalytic activity of as prepared all perovskite nanostructure was studied using RDE

technique in 0.1M KOH electrolyte at a rotation speed of 1600 rpm with a scan rate of 10mV.s-

1.The ORR onset potential and limiting current density of all catalyst shown in the LSV plot

shown in Figure 2.13. From this we can elucidate the onset potential of La(Co0.71Ni0.25)0.96O3-δ is

0.81 V, which is best performing ORR catalyst among all these perovskite nanostructure and its

limiting current density reaches the value of 2.25mA.cm-2

. This followed by LaNi0.99O3 catalyst,

which has the E1/2 potential of 20mV less than the best performing catalyst and then remaining

three perovsktie nanostructure. This shows that the nickel substitution in the octahedral site of

(i) (j)

44

perovskite structure leads to increase in ORR limiting current density as well as the onset

potential of the catalyst.

Figure 2.13. LSV plot of ORR activity of porous perovskite nanostructure catalyst.

(iii) OER activity of the catalyst

Oxygen evolution polarization traces obtained in aqueous alkaline 0.1 M KOH by using rotating

disk electrode with a scan rate of 10 mV.s-1

at 1600 rpm and all the results were plotted after iR

correction. Among the five catalysts, La(Co0.71Ni0.25)0.96O3-δ shows the lowest over potential of

324 mV at a 10 mAcm-2

current density. In addition, LaCo0.97O3-δ shows considerable

performance with an overpotential of 406 mV and also we studied the performance of La2O3

phase alone, it is not at all active for OER, from that we can claim that the small impurity of

La2O3 in perovskite structure doesn’t influence for OER performance. Apart from the oxygen

evolution reaction, the presence of nickel in the octahedral site of perovskite catalyst depicts the

45

changes in the nickel metal oxidation state (Ni2+

to Ni3+

) at a potential of 1.36 V. This leads to an

increase in the electrochemical active surface area and the catalyst’s roughness factor, which also

increases the OER activity [81,82]. The La(Co0.71Ni0.25)0.96O3-δ catalyst showed a lower

overpotential of 324 mV among the perovskite catalysts, as well as among the well-known

precious metal oxide catalysts, such as IrO2 (380 mV) and Ru/C (410 mV) [83] and also with the

recently reported non-precious metal oxide based catalysts listed in Table 2.3. And also in order

to understand the morphology effect of the catalyst we studied the performance of

LaCo0.75Ni0.25O3 (bulk), it shows overpotential of 410mV which is higher than the

La(Co0.71Ni0.25)0.96O3-δ nanostructure, this is mainly because of porous one dimensional nature

with interconnected particles makes the perovskite nanostructure as lead among the water

oxidation catalysts. In addition to that our catalysts showed a considerable OER performance

when compared with the various precious and non-precious OER catalysts [83].

The detailed OER electrochemical kinetics in 0.1M KOH over perovskite catalysts were

determined by using the Tafel plot (Figure 2.14 b), which was derived from the polarization

curves that used the Tafel equation ɳ = b log (j/j0) where ɳ is the overpotential, b is the Tafel

slope, j is the current density and j0 is the exchange current density. The Tafel slope of the

La(Co0.71Ni0.25)0.96O3-δ catalyst was found to be 37 mV/decade, which is much lower than the

noble metal catalysts such as IrO2 (79 mV/decade) and RuO2 (84 mV/decade) [84,85]. Also the

mass activity of all catalysts was derived from OER polarization plots and depicted in Figure

2.15 a. Based on the OER mass activity, it is evident that the La(Co0.71Ni0.25)0.96O3-δ catalyst

exhibits the highest mass activity of all the overpotentials, which is due to the existence of mixed

valence states of transition metal (Co2+

,Co3+

,Ni2+

and Ni3+

) with the optimized composition as

46

revealed by XPS results. Also the one-dimensional structure was combined with the presence of

the mesopore that maximizes the diffusion of electroactive sites, which enhanced the mass

transport of electrolyte ions and significantly promoted the OER performance.

Figure 2.14. (a) OER activities of porous LaCo1-xNixO3-δ nanostructure measured in 0.1 M

KOH at 1600 rpm at a scan rate of 10mVs-1

after iR correction. (b) Tafel plot of OER activity for

corresponding samples.

(a)

(b)

47

Figure 2.15. (a)Mass activity of as synthesized perovskite samples at various potential. (b) Over

potential and Tafel slope variation with respect to increasing in nickel content in LaCo1-xNixO3-δ

nanotube sample

(b)

(a)

48

(iv) OER stability:

Due to extraordinary OER performance, we investigate electrochemical OER stability of the best

catalyst was investigated by chronoamperometry in the 0.1M KOH solution at a constant

potential of 1.6 V vs RHE, as shown in Figure 2.16a. The durability of the perovskite

nanostructure was evaluated for a period of 10 h and it was found that all perovskite

nanostructure performing stable for a period of 10 h. And at the end of the 10 h durability study,

the current decay was 12.5 % for best performing catalyst which suggests the excellent durability

of La(Co0.71Ni0.25)0.96O3-δ nanostructure. Furthermore, the stability of La(Co0.71Ni0.25)0.96O3-δ

nanotubes was assessed by continuous cycling voltammetry in the RDE test with the same 0.1M

KOH. The OER overpotential of the catalyst remained stable until 114 cycles, as shown in

Figure 2.12 b. The stability of the material was retained due to the strong bonding nature of La

having bigger ionic radii than by varying the oxidation state of cobalt and nickel metal oxide in

the BO6 position. In addition, a well oriented crystal structure of material also was prominent

reason for the catalyst’s good stability.

49

Figure 2.16. (a) Chronoamperometry test of porous La(Co0.71Ni0.25)0.96O3-δ catalyst on 0.1 M

KOH at constant potential of 1.6 V vs RHE for the duration of 10 hours. (b) Accelerated OER

stability of La(Co0.71Ni0.25)0.96O3-δ at rotation speed of 1600 rpm in 0.1 M KOH at a sweep rate of

10 mVs-1

after iR correction.

(b) (i)

(ii)

50

Table 2.2. Porous LaCo1-xNixO3- δ nanostructure OER activity parameters on 0.1M KOH

aqueous electrolyte

Catalyst ECSA

(cm2)

BET

Surface

Area

(m2.g

-1)

Roughness

factor

Mass

activity

at 350mV

(mA.mg-1

)

Specific

activity

at 350mV

(mA.cm-2

)

LaCo0.97O3-δ 1.77 18.4 14.09 7.65 0.542

La(Co0.71Ni0.25)0.96O3-δ 5.16 20.2 41.20 20.33 0.493

La(Co0.45Ni0.51)0.96O3-δ 3.01 19.1 24.36 12.46 0.511

La(Co0.22Ni0.74)0.96O3-δ 2.09 19.5 16.60 12.48 0.750

LaNi0.99O3-δ 1.17 22.1 9.23 12.63 1.368

51

Table 2.3. Comparison of oxygen evolution catalytic activity of perovskite nanotubes, with

various catalysts.

2.7.3. Zinc-Air Battery Performance

(i) Primary Zinc-Air battery

The primary zinc-air battery studies have been evaluated using our perovskites nanotubular

catalyst loaded on carbon paper as gas diffusion layer as the air cathode and paired along with a

polished zinc metal in a glass fiber membrane soaked on 6M KOH electrolyte. It had an open

Family Electrocatalyst

Potential

( V vs

RHE)

Overpotential

(mV)

Catalyst

Loading

(mg.cm-2

)

Ref.

IrO2 1.61 380 / *

Noble metals Ru/C 1.62 390 / [83]

Pt/C 2.02 790 / [83]

Co3O4/mMWCNT 1.62 390 0.96 [86]

Spinels 1-D NiCo2O4 1.62 390 0.89 [57]

CoMn2O4/N-rGO

1.64

410

0.89

[58]

CoS2/NS-Gr 1.62 390 / [87]

Chalcogenides CoSe2 1.72 490 0.20 [88]

NiCo2S4/NS-rGO 1.70 470 0.28 [89]

LaNiO3/NC 1.66 430 0.15 [90]

Ba0.5Sr0.5Co0.8Fe0.2O3 1.60 370 0.36 [91]

LaCo0.97O3-δ 1.636 406 0.70 *

Perovskites La(Co0.71Ni0.25)0.96O3-δ 1.554 324 0.70 *

La(Co0.45Ni0.51)0.96O3-δ 1.597 367 0.70 *

La(Co0.22Ni0.74)0.96O3-δ 1.598 368 0.70 *

LaNi0.99O3-δ 1.601 371 0.70 *

LaCo0.75Ni0.25O3 (bulk) 1.640 410 0.70 *

52

circuit voltage of 1.45 V, near to theoretical voltage. The results shown in Figure 2.17 represents,

the cell voltage decreases from the open circuit voltage point and remain stabilized for a certain

period of hours for the two nanotubular perovskite catalysts such as LaCo0. 97O3-δ and

La(Co0.71Ni0.25)0.96O3-δ. The specific capacity of La(Co0.71Ni0.25)0.96O3-δ catalyst based battery is

705 mAhgzinc-1

as normalized by the mass consumed by zinc anode, corresponding to specific

energy density about 846 mwhgzinc-1

, followed by the LaCo0.97O3-δ nanotube catalyst with the

value of 710 mwhgzinc-1

. These results are shown that substitution of Nickel in the perovskite

structure increases the active sites of the catalyst results in huge fluctuations in capacity and

energy density of such a promising air battery system. The overall reduction mechanism of zinc-

air battery is

Cathode: O2 + 2H2O +4e- 4OH

-

Anode: Zn Zn2+

+2e-

Zn2+

+ 4OH- Zn(OH)4

2-

As formed zinc hydroxides in anode side is not a stable phase, overall reaction that governs

the transformation of Zn hydroxides to ZnO. The reason behind this transformation state due to

ZnO is chemically most stable state in the entire pOH/pH as theoretically explained by

Rossmeisl et al [92], using a Pourbaix diagram of zinc.

The obtained discharge capacity value of our perovskite nanotubular catalysts is comparable

to primary zinc-air batteries constructed by hybrid catalyst CoMn2O4/NrGO [58] and nitrogen,

phosphorus co- doped mesoporous carbon foam based cathode catalyst materials [93]. However

the observation of platue region for La(Co0.71Ni0.25)0.96O3-δ catalyst is at 1.2 V which is higher

53

than perovskite catalyst of LaCo0.97O3-δ having 1.1 V, this is mainly due to the substitution effect

of nickel in the octahedral site leads to creation of oxygen vacancy as well as change in

electronic conductivity of the material.

Figure 2.17: Specific discharge capacity of catalysts at a current density of 5A/g based on

consumption of zinc anode

(ii) Secondary Zinc-Air battery

To understand the effect of nanotubular catalyst in rechargeable zinc-air battery system, we cut

down the capacity of the battery at low capacity mode of 400 mAh/g and high capacity mode

with 2500mAh/g based on mass loading of perovskite catalyst in cathode compartment at the

current density of 5A/g. The charge and discharge cycle of perovskite catalyst performed over

20 cycles at low capacity mode and 10 cycles at high capacity mode. The mechanism behind the

54

secondary zinc air battery is while discharging, zinc gets oxidized in anode compartment,

whereas the oxygen is reduced to hydroxide ion in the cathode compartment. Then, the

hydroxide ion reacts with Zn2+

to form a soluble zincate (Zn (OH) 4 2−

) ion, which is further

dehydrated to form ZnO insulating material, this product leads the secondary zinc air battery for

fewest numbers of cycles and complete death of the cell due to dendrite formation at anode and

formation of ZnO as a complete passive layer on the surface of the anode. While in case of

recharging plating of zinc from (Zn(OH)4 2-

) in electrolyte and oxygen evolution reaction take

place in cathode compartment [94].

(iii) Low capacity mode

In low capacity mode (Figure 2.18 a & b) La (Co0.71Ni0.25) 0.96O3-δ nanotube catalyst shows very

low overpotential of 0.529 v at first cycle and the increase in overpotential comes mainly from

the discharge profile that is an oxygen reduction reaction and the charging profile remain stable

until 15 cycles and observation of a slight increase at 20th

cycle with overpotential of 0.792 V.

Secondly LaCo0.97O3-δ observed similar phenomenon and having an overpotential at 1st and 20

th

cycle is 0.878 V and 0.920 V respectively. This is little higher than nickel substituted perovskite

catalyst and it is mainly due to low catalytic active sites with material. For the comparative

study we have shown the discharge and charge voltage of LaCo0.75Ni0.25O3 (bulk) in Table 2.4.

for both high and low capacity mode.

55

Figure 2.18. Low capacity mode galvanostatic discharge (D) and charge(C) polarization curves

of at a constant current density of 5A/g (a) La(Co0.71Ni0.25)0.96O3-δ, (b) LaCo0.97O3-δ

(b)

(a)

56

Figure 2.19. (a) Comparison of Discharge and Charge plot of low capacity mode dotted line

represent LaCo0.97O3-δ and solid line represent La(Co0.71Ni0.25)0.96O3-δ, (b) Overpotential

comparison chart of two catalyst compare with initial and final cycle.

(b)

(a)

57

(iv)High capacity mode

Similarly to understand the cyclability and increase of overpotential for long discharge time, we

used high capacity mode (Figure 2.20 c & d). In this study, similar way La(Co0.71Ni0.25)0.96O3-δ

nanotube catalyst performs much better than LaCo0.97O3-δ nanotube catalyst and all catalysts lasts

for 10 cycles with varying overpotential. The calculated energy efficiency for La (Co0.71Ni0.25)

0.96O3-δ nanotubular catalyst was 67.8 % at low capacity mode and maintained 69.8 % at the high

capacity mode due to extraordinary ORR and OER activity of electrocatalyst. These reported

performances are far better than the precious Pt/C metal catalyst shown in Figure 3 e & f.

Table 2.4. Summary of charge (C)-discharge (D) potential gap of cathode catalysts at a current

density of 5 A/g

Catalyst

High capacity mode (2500 mAh.g-1catalyst) Low capacity mode (400 mAh.g-1

catalyst)

1st cycle 10th cycle 1st cycle 20th cycle

D

(V)

C

(V)

Over

Potential C-D (V)

D

(V)

C

(V)

Over

Potential C-D (V)

D

(V)

C

(V)

Over

Potential C-D (V)

D

(V)

C

(V)

Over

Potential C-D (V)

La(Co0.71Ni0.25)0

.96O3-δ

1.206 1.702 0.496 1.003 1.699 0.696 1.212 1.741 0.529 1.017 1.809 0.792

LaCo0.97O3-δ

1.223 1.870 0.716 1.067 1.875 0.808 1.044 1.922 0.878 0.991 1.911 0.920

Pt/C 1.115 1.995 0.880 0.958 2.198 1.240 1.109 1.980 0.871 1.011 2.118 1.107

LaCo0.75Ni0.25O3

(bulk)

1.124 1.904 0.780 1.098 1.994 0.896 1.112 1.942 0.830 0.951 2.054 1.103

58

Figure 2.20. High capacity mode galvanostatic discharge (D) and charge(C) polarization curves

of at a constant current density of 5A/g (a) La(Co0.71Ni0.25)0.96O3-δ, (b) LaCo0.97O3-δ

(b)

(a)

59

Figure 2.21. (a) Comparison of Discharge and Charge plot of high capacity mode dotted line

represent LaCo0.97O3-δ and solid line represent La(Co0.71Ni0.25)0.96O3-δ, (b) Overpotential

comparison chart of two catalyst compare with initial and final cycle.

(b)

(a)

60

2.8.Summary

We reported on a simple electrospinning technique for the synthesis of porous one dimensional

structured LaCo1-xNixO3-δ perovskites. The substitution of 25% of nickel in the B site of

LaCo0.97O3-δ structure achieved promising oxygen reduction reaction and oxygen evolving

catalytic activity and due to extra ordinary OER activity, we elucidate the OER stability which

last for 10h with 8% loss of current density. The unusual catalytic performance was due to the

differential oxidation states of cobalt and nickel. In addition, the alignment of different particle

natures aided to facilitate facile the electron transfer channels followed by the creation of

enormous catalytic active sites of material, which are much larger than precious metal catalysts

like IrO2, Ru/C and Pt/C.

In addition to that, we investigated the air cathode performance of three perovskite

electrocatalysts LaCo0.97O3-δ and La(Co0.71Ni0.25)0.96O3-δ nanotubes. The activity of these

materials shows as a promising material for air cathodes in both primary and secondary zinc air

battery systems. The extraordinary performance of La(Co0.71Ni0.25)0.96O3-δ can be attributed to the

substitution of nickel in the octahedral site of LaCo0.97O3-δ perovskite structure leads to increase

in electrocatalytic active sites of the material.

61

Chapter 3. Nitrogen doped Carbon Coated SiO2 as Air Cathode Catalyst for Non-aqueous

Lithium-Oxygen Battery

3.1. Literature survey

Environmental calamity urged the researcher to look for energy storage device with much greater

energy density, beyond from the present scenario lithium-ion battery technology in transportation

sector [6]. While the endorsement of 20th

century lithium-air battery invention has been get clear

focused by scientific community, owing to its extremely high energy density up to 3kWhkg-1

.

This results from usage of highly abundant oxygen as cathode material in the open cell

configuration.

The fore bidden mechanism behind the rechargeable lithium-air battery comprise of oxidation

of lithium at the anode side and the reduction of oxygen by sensible catalyst when discharging,

which leads to formation of Li2O2 or Li(OH) in the counter cathode side based on the electrolyte

used. While charging the vice versa Li2O2 or Li(OH) dissociate to go as Li ions to anode and

evolution of oxygen at the cathode compartment. This phenomenon has been allegedly

confronted by means of delineating x-ray diffraction pattern, in-situ Raman spectroscopy

techniques by many research groups [95-97].

The most widely used catalyst material in lithium air battery is carbon, owing to the fact that

highly tendency to reduce the oxygen molecules indeed in non-aqueous electrolyte with

enormous porosity, wide surface area nature, good electronic conductivity and charge carrier

mobility [98-100]. Nevertheless the carbon material has been quite less disappointed to perform

while in the case of recharging the battery, due to formation of undesirable carbon related by

62

products like Li2CO3 while discharging, and leads to disruption of carbon material at high

voltage instead of oxygen evolution reaction [101,102]. Ever since in order to reduce high

potential charging, numerous significant efforts has been carried out like functionalizing the

carbon material by means of doping highly electronegativity atoms like N, B, F and secondly

tuning the morphology along with controlling the porosity nature of carbon material [103-105].

In recent trends transition metals or metal oxides like platinum, gold, MnO2, Co3O4, NiCo2O4,

LaNiO3 and Pb2Ru2O7 has been vividly performing as a good catalyst for lithium-air system

[106-11]. Meitin et al. reported that MPd (M: Co, Ni, Cu) alloyed nanoparticles supported

reduced graphene oxide has been used as a electrocatalyst and achieved a high capacity and

good rate capability. CoFe2O4 spinel nanoparticles supported with Vulcun XC carbon performed

under low overpotential and good cyclic life time [113]. These reports shows that metal oxides

are well performed electrocatalysts for ORR and OER, but their low electrical conductivity,

limits their potential usage in metal-air batteries. To resolve that carbon materials are used as

conducting media and well dispersion agents. However, weak interface between carbon and

transition metal oxide leads to formation of Li2CO3, which results in degradation of catalytic

material and in addition to that their high cost, limited supply due to finite reserves, weak

durability, and detrimental environmental effects hinder commercialization of those precious and

non-precious transition metal oxides [102].

On the other hand choosing of electrolyte for non-aqueous rechargeable lithium air battery has

been a big challenge because of the electrolyte should be stable in precise voltage window, high

oxygen solubility, diffusivity as well as sufficient Li+ ion conductivity and the foremost thing is

high resistance to oxygen reduction species. Gasteiger et al. reported that the alkyl carbonate

63

based electrolytes form Li2CO3 and lithium alkyl carbonates due to nucleophilic attack on the

discharge product Li2O2, which results in hectic in oxidation of lithium carbonates even at 4.6 V

while charging [114]. Presently many research groups use lithium tetrafluorosulfonimide

(LITFSI) as lithium ion conducting salt along with tetraethylglycoldimethyl ether (TEGDME) as

solvent, yet the discharging voltage has been immensely lower than the open circuit voltage

owing to low resistance to oxygen reduction species. Recently, Bruce et al. reported that the

DMSO solvent based Lithium-air battery setup shows good stability with TiC as cathode

material [115].

3.2. Scope of this work

In this work, we synthesized nitrogen doped thin film carbon coated SiO2 nanosphere, we

denoted as SiO2/NC using guanine as the carbon and nitrogen source by adopting the

hydrothermal approach. To analyze the practical energy storage application, we perform the

lithium-air battery setup by using SiO2/NC as electrocatalyst, with LITFSI/DMSO as electrolyte

and to understand the in depth mechanism of SiO2 nanosphere we have carried out a post

analysis study of the catalyst loaded electrode.

3.3. Experimental

3.3.1. Hydrothermal method

Hydrothermal method to produce different chemical compounds and materials using closed-

system physical and chemical processes following in aqueous solutions at temperatures above

64

100°C and pressures above 1 atm. The main parameters of hydrothermal synthesis, which define

both the processes kinetics and the properties of resulting products, are the initial pH of the

medium, the duration and temperature of synthesis, and the pressure in the system. The synthesis

is carried out in autoclave that is sealed steel cylinders that can withstand high temperatures and

pressure for a long time.

Figure 3.1. High pressure hydrothermal reactor along with Teflon lined autoclave

3.3.2. Chemicals Required

Tetraethylorthosilicate, ammonia, guanine, absolute ethanol, D.I water

65

3.3.3. Synthesis procedure of SiO2 nanosphere

The SiO2 nanospheres were synthesized according to the well-known Stöber method [15].

absolute ethanol 158 mL, ammonia 7.8 mL, and distilled water 2.8 mL were introduced in a 250

mL roundbottom flask and heated to 50 °C under stirring, then 5.8 mL tetraethyl orthosilicate

was added into the solution and stirred at 50 °C for 24 h; SiO2 spheres were obtained by drying

the white solution at 70 °C for 24 h.

3.3.4. Synthesis procedure of SiO2/NC nanosphere

SiO2/NC composite were synthesized by thin film coating of nitrogen doped carbon on the

surface of SiO2 nanosphere by means of guanine as carbon and nitrogen source via hydrothermal

approach. One gram of guanine was dissolved in 75mL of D.I water under continuous stirring for

30 minutes, subsequently 0.5g of as prepared SiO2 nanosphere was added into the above mixture.

After this, the mixture was kept continuously stirring for 12 h. The resulting solution was

transferred to 100ml hydrothermal reactor flask and maintained at a temperature of 150oC for 10

hours. Then it was treated by filtration and washed with ethanol and distilled water for several

times, and dried at 80 °C under vacuum for 12 h. Finally, the SiO2/NC nanocomposites were

obtained by carbonizing in a furnace at 800oC 2 h under Ar atmosphere.

3.4.Material Characterization

Phase formation of the material has been confirmed by X-ray diffractometer (Rigaku, Miniflex

600). The morphology of the samples was carried out by means of field-emission scanning

electron microscopy (FE-SEM, Hitachi, S-4800II, 3kV) and a thin osmium coating has given

before analyzing the samples. In order to study the oxidation state and understand the bonding

66

nature of the material, the samples were characterized by X-ray photoelectron spectroscopy

using a JEOL JPS-900 in an ultra-high vacuum X-ray photoelectron spectrometer with an Al

monochromatized cathode source at 25 W. Furthermore to understand the surface area and

porous nature of the material by using Brunauer–Emmett–Teller (BET), Barrett–Joyner–Halenda

(BJH) and Harkins–Jura (H–J) models with a Micromeritics Tristar ASAP 2020.

3.5.Electrochemical characterization

3.5.1.Electrochemical setup

A conventional three electrodes setup was used to assess the oxygen evolution reaction and

electrode stability test by rotating disk electrode linear sweep voltammetry (LSV) and cyclic

voltammetry (CV), respectively in alkaline medium 0.1M KOH electrolyte under oxygen

saturated condition. The computer-controlled potentiostat (Bio-logic) was used to carry out the

electrochemical studies and all experiments were carried out at a 10mVs-1

scan rate.

(i) Catalyst ink preparation

The catalyst ink was prepared by 5 mg of material dispersed in the solvent containing 200 µL

ethanol, and 30 µl of 5 wt% Nafion as binder. The contents were thoroughly sonicated using an

ultrasonicator for 30 minutes to obtain a homogeneous suspension. Two microliters of catalyst

ink was dropped on the surface of rotating disk electrode with the area of 0.07065 cm2 with the

catalyst loading of 0.40mg/cm2. The three electrode cell setup consists of Pt foil, saturated

calomel along with glassy carbon rotating disk electrodes were used as counter, reference and

working electrodes, respectively. All potentials reported in this work were against reversible

hydrogen electrode (RHE), which was converted from a saturated calomel electrode (SCE) based

67

on conversion equation ERHE=ESCE+0.241 V+0.059× pH-iR, with the pH of 0.1 M KOH aqueous

electrolyte being 13. The potential region of ORR monitored from 1 to 0.2 V against RHE with a

rotation speed of from 400 to 2025 rpm.

3.6. Lithium oxygen cell assembly and testing

The Li-oxygen cell was then assembled in an Ar-filled dry glove box using an coin cell (MTI

Korea) configuration with openings allowing oxygen to enter the cathodic part. A lithium disc

(MTI Korea) was used as the anode and whatman-GF/A was used as the separator. The separator

was saturated with the LiTFSI-based electrolyte either with organic or LiPF6 based as solvent.

The disks of oxygen electrode had an area of 2.54 cm2. The cells were galvano-statically cycled

by an MTI Korea battery tester at room temperature between 4.5-2.0 V vs. Li+/Li at a current

density of 150mA.g-1

. During the tests, pure O2 was continuously circulated at the O2-cathode at

the rate of 1 atm pressure.

3.6.1. Air cathode preparation

The air cathode was prepared as a thin film over carbon paper GDL based current collector. A N-

methyl-2-pyrrolidone (NMP) slurry of previously prepared catalyst was mixed with super p

carbon black (Daejung, Korea) as electronic conductor and poly-(vinylidenefluoride) (PVdF,

Solvay Solef-6020) as binder in the weight ratio of 70:20:10 respectively, was deposited over

GDL using doctor blade technique. This film was dried at 55 °C overnight to obtain a composite

cathode with an electrode mass of about 1 mg.cm-2

. A dual pore oxygen cathode was prepared

thus it was composed of two interconnected porosity systems: a catalysed and a non catalysed

side. The non-catalyzed system (carbon paper GDL with a micro-porous layer) can allow the

68

transport of oxygen into the electrode even when the catalyzed system becomes eventually

blocked by reaction precipitates.

Figure 3.2. Handmade Lithium-oxygen battery setup using open coin cell configuration

69

3.7. Result and Discussion:

3.7.1. Structural analysis

(i) Phase analysis

The as synthesized catalyst was analyzed by powder XRD. As shown in Figure 3.3 the strong

and broad peak in the range of 2 theta value of 15o to 35

o can be assigned to amorphous silica. At

the same time, no obvious carbon related diffraction peaks can be detected, and the carbon

content of SiO2/NC materials is confirmed to be completely amorphous nature.

Figure 3.3. XRD pattern of SiO2/NC and NC.

70

(ii) Morphology analysis

The morphology of SiO2 and composite SiO2/NC nanosphere was analyzed by FE-SEM. The

monodisperse SiO2 exhibits spherical morphology with an average diameter of 350 nm (Figure

3.4 a,b). After hydrothermal reaction and carbonization, the as prepared SiO2 nanospheres were

decorated with a thin film coating of carbon and agglomerated together shown in Figure 3.4 (c,d).

(Continue)

(a) (b)

(c) (d)

71

Figure 3.4. SEM images of as synthesized sample: (a, b) SiO2; (c, d) SiO2/NC and (e) NC

(ii) Xray Photoelectron spectroscopy (XPS)

The element compositions of the prepared SiO2/NC are determined by XPS measurement and the

content of different elements are carbon, nitrogen and oxygen as 75.5, 10.24 and 8.44 percentage,

respectively. Based on the XPS results, N-doping of the prepared SiO2/NC is confirmed. The

SiO2/NC has N and O content is in the range of 10.24, and 8.44 at atomic percentage basis,

respectively. In case of C1s, four different groups of C=C/C-C in sp2-hybridized domains, C-

O/C-N (epoxy, hydroxyl or pyrrolic N), C=O/C=N (carbonyl, pyridinic N or quaternary N), and

O=C-O (carboxyl) groups, are determined by the peaks at 284.4, 285.3 and 286.7 eV,

respectively. In case of N1s, four types of nitrogen-containing groups could be observed. The

peaks at about 398.2, 400.4, 401.2, and 402.5–403.4 eV could be attributed to pyridinic N (N-6),

pyrrolic N (N-5), quaternary N (N–Q), and pyridine-N-oxide (N–X), respectively [116, 117].

(e)

72

Figure 3.5. Deconvoluted XPS data of SiO2/NC (a) Carbon 1s spectra (b) Nitrogen 1s spectra.

(a)

(b)

73

3.7.2. Electrochemical studies

(i) ORR catalysis

The LSV measurement on a RDE set up were conducted to assess the ORR activity of SiO2/NC

in O2-saturated 0.1 M KOH solution. As shown in Figure 3.6, SiO2/NC samples were tested with

various rotating speeds from 400 rpm to 2025 rpm. Furthermore, in the corresponding j−1

vs.

ω−1/2

plots, which shows an excellent agreement with the linear relationship at different voltages

described in Koutecky-Levich (K-L) equation as follows

j−1

= jK−1

+ jL−1

= jK−1

+ (Bω1/2

)−1

where B = 0.62nFCO(DO)2/3

ν−1/6

where j, jK and jL are the measured current density, kinetic current density and limiting

diffusion current density, respectively; ω is the rotation speed of the working electrode, n is the

transferred electron number per oxygen molecule during the ORR process, F (F = 96485 C mol−1

)

is the Faraday constant, CO (CO = 1.2 × 10−6

mol cm−3

) is the bulk concentration of O2,

DO (DO = 1.9 × 10−5

cm2 s

−1) is the diffusion coefficient of O2 in the 0.1 M KOH solution,

ν(ν = 0.01 cm2 s

−1) is the kinematic viscosity.

In order to compare the ORR activity of the catalyst, we evaluate the ORR activity of NC

sample and elucidate that the SiO2/NC has high ORR catalytic activity when compared with NC

this is mainly because of thin film deposition of nitrogen doped carbon on the surface of SiO2

74

leads to increase in mass transfer region. The E1/2 and onset value of SiO2/NC is 0.81 V and 0.76

V, respectively, which is much higher than NC. Moreover from Koutecky Levich plot it exhibits

the 4 electron transfer ORR reaction.

Figure 3.6. Rotating disk voltammograms of SiO2/NC catalyst in O2 saturated 0.1 M KOH with

sweep rate of 5mVs-1 at the different rotation rates indicated. The insets in show corresponding

Koutecky–Levich plots (J−1

versus ω−0.5

) at different potential.

75

Figure 3.7. Comparative LSV polarization curves of SiO2/NC, NC with commercial 20 wt %

Pt/C catalyst at a scan rate of 10mV.s-1

, measured in 0.1M KOH electrolyte

76

3.7.3.Lithium-oxygen battery performance

(i) Effect of solvent

To examine the preferable solvent for testing the performance of lithium-oxygen battery, we

used tetraethylglycoldimethylether(TEGDME) and dimethyl sulfonyl oxide (DMSO) by using

same catalyst of SiO2/NC and NC at the discharge current of 150mA.g-1

. Using DMSO solvent

as an electrolyte the discharge capacity of SiO2/NC catalyst shows 18,588 mAh.g-1

, this is much

higher than TEGDME solvent electrolyte and it capacity lasts for 15,655 mAh.g-1

. In addition to

that the discharge voltage of DMSO and TEGDME electrolyte base Lithium oxygen battery

shows a platue voltage of 2.76 V and 2.51 V respectively, this means the ORR is much facile in

DMSO compare with TEGDME electrolyte.

Figure 3.8.Discharge capacity plot of cathode sample prepared using SiO2/NC catalyst using

various electrolytes in the current density of 150mAh.g-1

77

(ii) Catalytic activity performance

In the present study, the full discharge study of lithium-oxygen battery has been evaluated using

SiO2/NC and NC catalysts with a current density of 150mA.g-1

, and it shows the discharge

capacity of 18588 mAh.g-1

and 9544 mAh.g-1

respectively. The increase in capacity of SiO2/NC

is mainly because of incorporation SiO2 in the catalytic part and which results in adsorption of

hydroxyl molecule from the electrolyte which results in increase of mass transfer region of

oxygen reduction reaction.

Figure 3.9. Discharge capacity plot of cathode sample using various catalyst in the current

density of 150mAh.g-1

78

(iii) Rechargeable Li-O2 battery

For rechargeable Lithium-oxygen battery, we examine the catalytic activity of the SiO2/NC by

reducing the cut off capacity to 1000 mAh.g-1

and evaluated the lithium-oxygen battery

cyclability performance at the current density of 150mA.g-1

. The SiO2/NC catalyst shows the

overpotential of 1.51 V at first cycle and in case of further cycle observation of decrease in

overpotential, this significantly shows the SiO2/NC catalyst get activating by each rechargeable

cycles, by transformation from SiO2 get reduced to Si material leads to activation of catalytic

material and further decrement in overpotential.

Figure 3.10. Cycling performance of the as prepared SiO2/NC catalyst with the capacity cutoff

of 1000 mAh.g-1

in the current density of 150mAh.g-1

79

(iv) Cathode post-mortem analysis

We conducted XRD and FESEM measurements to identify the discharge products of Li–

O2 batteries with SiO2/NC and NC electrodes. XRD patterns of the SiO2/NC electrodes at

different states for the first cycle at a current of 150 mA gtotal−1

are shown in Figure 3.11. As

compared with the XRD pattern of the fresh electrode, new diffraction peaks were observed for

the discharged electrode. Although these peaks were very strong, they can be reasonably

assigned as the Li(OH).H2O (as highlighted in Figure 3.11). These peaks indicate that

LiOH.H2O is a major crystalline discharge product, and also observation of shift in the XRD

pattern, this is mainly because of oxygen deficiency observed in the SiO2, which tells that the

transformation of SiO2 nanosphere to Si.

Lately, the diffraction peaks of LiOH.H2O disappeared and the shift became relocated as the

pristine fresh electrode when the battery was recharged to 4 V, which suggests that the discharge

product LiOH.H2O is decomposed in the charging process.

The Li(OH).H2O formed on the surface of SiO2/NC nanosphere in the discharging process were

directly observed by FESEM image and again disappearance of those product while recharged to

4 V.

In case of NC based electrode, the formation of both Li(OH).H2O and Li4CO3 as the discharge

product, and the formation of Li4CO3 is strongly formed on the electrode surface shown by XRD

pattern and FESEM image, while in the recharged electrode, the Li4CO3 discharge product is not

decomposed under charging process because of strong bond formation of lithium ion with the

carbonate leads to crack in the electrode surface when goes to high voltage.

80

Figure 3.11. XRD pattern analysed at different state of electrode (a) SiO2/NC catalyst and (b)

NC catalyst

81

Figure 3.12. FESEM image of SiO2/NC catalyst electrode (a,b) pristine electrode (c,d) discharge

electrode and (e,f) after recharged electrode.

82

Figure 3.13. FESEM image of NC catalyst electrode (a,b) pristine electrode (c,d) discharge

electrode and (e,f) after recharged electrode.

83

3.8. Summary

We have successfully prepared the SiO2/NC nanosphere with thin film coated nitrogen-doped

carbon on the surface of the material, by using combined sol-gel and hydrothermal approach.

Secondly, we have applied this material as the electrocatalyst for lithium-oxygen battery, for that

we have studied the ORR and OER catalytic activity in alkaline electrolyte, later we have applied

this catalyst for lithium-oxygen battery and its full discharge capacity of 18588 mAh.g-1

at the

extracted current density of 150 mAh.g-1

. Then in case of rechargeable battery, it last for 10

cycles with cut off capacity of 1000 mAh.g-1

at the current density of 150 mAh.g-1

.

To understand the mechanism behind the catalytic performance, we elucidate the post analysis

by XRD and FESEM of discharged and recharged electrode and confirmed the formation of and

disappearance of Li(OH).H2O product while discharge and recharge, respectively.

84

Conclusions

Alternative energy storage devices namely Zinc-air battery and lithium-oxygen batteries have

been studied to replace the fossil fuels in automotive application. In this upcoming technology

highly active and durable electrocatalysts on the cathode side are required to catalyse oxygen

reduction reaction during discharge and oxygen evolution reaction during charge for

rechargeable batteries.

Here we developed primary and rechargeable zinc-air batteries with novel one dimensional

nanstructured perovskite material as electrocatalyst. Firstly, we report the synthesis of a series of

porous perovskite nanostructure, LaCo0.97O3- δ, with systematical Ni substitution in Co octahedral

sites. The bifunctional electrocatalaytic activity of material was studied in alkaline based

electrolytes. We show that the progressive replacement of Co by Ni in the LaCo0.97O3- δ

perovskite structure greatly altered the OER electrocatalytic activity and the

La(Co0.71Ni0.25)0.96O3-δ composition exhibited the lowest overpotential of 324 at 10 mAcm-2

in

0.1M KOH. To understand the real scale application we used the as prepared La(Co0.71Ni0.25)

0.96O3 nanostructured as cathode catalyst for aqueous zinc-air battery, which delivers high

capacity of 705 mAh.g-1

zinc in primary zinc-air battery. The rechargeable battery discharges with

low overpotential of 0.792 V and 0.696 V at low capacity mode (400 mAh.g-1

catalyst) and high

capacity mode (2500 mAh.g-1

catalyst), respectively, this value is much lower than LaCo0.97O3-δ

nanotube catalyst and precious Pt/C catalyst.

Secondly, we developed ultra-high energy density non-aqueous lithium-oxygen battery using

thin film nitrogen doped carbon coated SiO2 material as cathode catalyst to enhance the

85

performance of lithium-oxygen battery. We are the firstly reporting metalloid material as cathode

catalyst for lithium-oxygen battery and it delivers a capacity of 18588 mAh.g-1

of catalyst

material at high current density of 150 mAh.g-1

, which is nearly twice the capacity of nitrogen

doped carbon material at same current density, and for rechargeable battery it last for ten cycle at

the cutoff capacity of 1000 mAh.g-1

. Later to elucidate the working mechanism behind this

metalloid oxide catalyst, we reported post mortem analysis of cathode material of lithium-

oxygen battery.

86

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국문 요약문

불안정핚 유가와 지구온난화는 화석연료를 대체핛 수 있는 높은 에너지 밀도의 금속 공기

전지의 발전을 야기하였다. 이 진보된 기술을 상용화 하기 위해서는 충·방전 시 양극 및

음극에서 산소홖원반응 및 산소발생반응을 일어나게 하기 위하여 귀금속(Pt/C) 및 금속

산화물(RuO2, MnO2) 촉매가 필요하다. 그러나 이러핚 귀금속 기반의 촉매는 가격이

비싸며 낮은 선택성과 낮은 내구성, 홖경오염 이라는 문제점들을 지니고 있다.

우리는 간단핚 전기방사법을 통해 일차원의 LaCo1-xNixO3-δ 페로브스카이트 촉매를

개발하였다. LaCo1-xNixO3-δ 페로브스카이트 구조 내에서 코발트를 니켈로 치홖했을 때

OER 홗성을 향상시킬 수 있으며, La(Co0.71Ni0.25)0.96O3-δ 구성 시 0.1M 의 KOH 에서 10

mAcm-2 의 전류를 얻기 위해서는 324 mV의 낮은 과전압이 요구됨을 확인하였다. 이를

전지에 홗용 시, La(Co0.71Ni0.25)0.96O3-δ 를 음극 촉매로 사용핚 수용성 일차 아연-공기

전지에서는 높은 용량(705 mAh.g-1

zinc)의 성능을 얻을 수 있었다. 재생 가능핚 이차 아연-

공기 전지로 제작 시, 전지 방전 과정에서 낮은 용량모드(400 mAh.g-1

catalyst) 에서는 0.792 V,

높은 용량모드(2500 mAh.g-1

catalyst) 에서는 0.696 V 로 LaCo0.97O3-δ 나노 튜브 구조 기반

촉매와 귀금속 촉매인 Pt/C를 사용하였을 때 보다 훨씬 낮은 과전압을 필요로 함을 확인핛

수 있었다.

104

학위논문의 두 번째 파트에서는 수열합성법을 통해 카본이 얇게 코팅된 SiO2

나노스피어(nanosphere)를 합성하였고, 리튬 공기 전지에 적용하여 150 mAh.g-1 의 높은

전류밀도에서 18588 mAh.g-1 의 용량을 지닌 음극 촉매로 사용하였다. 이는 같은 전류

밀도에서 질소가 도핑된 탄소보다 거의 2배의 용량을 보였으며, 2차 전지로 제조 시 1000

mAh.g-1 의 차단용량에서 10 사이클이 지속되는 것을 확인하였다. 본 학위논문에서는

준금속 산화물 촉매의 작동 매커니즘을 설명하기 위해 리튬 공기 전지 음극 물질에 대핚

분석 또핚 함께 논의되었다.

키워드: 공기극 촉매, LaCo1-xNixO3-δ 페로브스카이트, 아연-공기 전지, SiO2/NC, 리튬-공기

전지

105

Acknowledgement

I would like to express my sincere gratitude to my professor, Dr. Sangaraju Shanmugam for

letting me to fulfil my master’s research work of being a student here. I would also like to thank

the committee members professor, Dr. Seung-Tae Hong and Dr. Yiseul Park to give a good

guidance about my project. To all my lab mates and seniors, I am extremely grateful for your

timely help and suggestion throughout my dissertation project work during this two year

master’s program.