gupea_2077_34583_1

gupea_2077_34583_1
The Oxygen Reduction Reaction in Nonaqueous Electrolytes: Li-Air Battery
Applications.
Current Density (Acm-2)
6,00E-04
4,00E-04
2,00E-04
0,00E+00
-2,00E-04
-4,00E-04
-6,00E-04
-8,00E-04
-3,00
-2,00
-1,00
0,00
1,00
Potential V vs Ag/Ag+
MD. KHAIRUL HOQUE
Degree project for Master of Science 30 hec
Department of chemistry and Molecular Biology
Electrochemistry Group
University of Gothenburg
2013/09
The Oxygen Reduction Reaction in Non-aqueous
Electrolytes: Li-Air Battery Applications
MD. KHAIRUL HOQUE
Department of Chemistry and Molecular Biology
Electrochemistry Group
University of Gothenburg
Gothenburg, Sweden, 2013
Abstract
Lithium air battery is one of the most promising power technologies of future because it has
theoretical specific energies 100 times that of the state of the art Li-ion battery. One of the
main obstacles in the development of Li-air battery technology is the stability of electrolyte.
The focus of research work presented in this thesis is on the investigation of the oxygen
reduction reaction (ORR) in non-aqueous electrolytes relevant for Li-air batteries. The
oxygen reduction reaction mechanisms and kinetics are elucidated by using the
electrochemical techniques such as cyclic voltammetry. Dimethyl sulfoxide (DMSO) and
acetonitrile
(MeCN)
were
chosen
as
solvents
whereas
tetrabutylammonium
hexafluorophosphate (TBAPF6), lithium perchlorate LiClO4 and lithium hexafluorophosphate
(LiPF6) as supporting electrolytes. By using the glassy carbon electrode as working and
platinum mesh as counter electrode it was found that the ORR is quasi-reversible in
TBAPF6/DMSO as well as in TBAPF6/MeCN. In the case of lithium based supporting
electrolytes (LiPF6 and LiClO4) the ORR in DMSO as well as MeCN was irreversible with a
follow-up chemical reaction. These results show that the interaction of small highly charged
Li+ with surrounding solvent and counter ions is markedly different than large bulky TBA+ ion
and such interactions strongly affect the reaction mechanism of oxygen reduction and
oxygen mobility through the electrolytes. Moreover, the differences seen in the reversibility
of the ORR in TBA+ compared with Li+ containing electrolytes is probably due to the
formation of insulating Li2O/Li2O2 on cathode during the discharging process. The knowledge
of the ORR mechanism inferred from these results will be useful for the selection of
appropriate organic electrolytes and for a rapid development of the rechargeable Li-air
battery for automotive industry.
Key words: Cyclic voltammetry (CV) and rotating disk voltammetry (RDE), the oxygen
reduction reaction (ORR).
i
Table of content
Abstract .................................................................................................................................................... i
Chapter1 .................................................................................................................................................. 1
1.1. Motivation and aim of the study ...................................................................................................... 1
1.2 Introduction ....................................................................................................................................... 1
1.2.1. Overview of Li-air battery .......................................................................................................... 1
1.2.2 Lithium ion batteries .................................................................................................................. 4
1.2.3 Metal air Batteries ...................................................................................................................... 4
1.2.4 Drawbacks of air cathodes-electrolyte....................................................................................... 6
1.2.5 Progress regarding Li-air cathode............................................................................................... 9
Chapter 2 ............................................................................................................................................... 11
2 General theory of electrochemistry ................................................................................................... 11
2.1 Oxidation-reduction potentials ................................................................................................... 11
2.2 Mass transport ............................................................................................................................ 11
2.3 Essential electrode Reaction ....................................................................................................... 12
2.4 Heterogeneous rate constant ..................................................................................................... 12
2.5 Cyclic Voltammetry ..................................................................................................................... 12
2.5.1 Scan rates ............................................................................................................................. 13
2.5.2 Reversible systems ............................................................................................................... 14
2.5.3 Irreversible and Quasi-Reversible Systems .......................................................................... 15
2.6 Uncompensated Resistance ........................................................................................................ 16
2.7 Rotating Disk Electrode (RDE) ..................................................................................................... 16
Chapter 3 ............................................................................................................................................... 20
3 Experimental ...................................................................................................................................... 20
3.1 Materials...................................................................................................................................... 20
3.2 Potentiostats ............................................................................................................................... 20
3.3 Cells and electrode setup ............................................................................................................ 20
3.4 Measurements Procedure ........................................................................................................... 21
3.5 Electrochemical Experiments ...................................................................................................... 21
Chapter 4 ............................................................................................................................................... 23
4 Results and discussion ........................................................................................................................ 23
4.1 Oxygen Reduction in 0,1M TBAPF6/DMSO .................................................................................. 23
4.2 Oxygen Reduction in 0.1M TBAPF6/MeCN .................................................................................. 29
ii
4.3 Oxygen Reduction in 0,1M LiPF6/DMSO ...................................................................................... 33
4.4 Oxygen Reduction in 0,1M LiPF6/MeCN. ..................................................................................... 36
4.5 Oxygen Reduction in 0,1M LiClO4/DMSO .................................................................................... 37
4.6 Oxygen Reduction in 0,1M LiClO4/MeCN .................................................................................... 40
4.7 Oxygen Reduction in mixed LiPF6 and LiClO4 system in DMSO ................................................... 40
4.8 Oxygen Reduction in mixed LiPF6 and TBAPF6 system in DMSO ................................................. 41
4.8 Comparison of kinetics properties for different electrolytes ...................................................... 42
Chapter 5 ............................................................................................................................................... 45
Conclusions and Future works .............................................................................................................. 45
Appendix................................................................................................................................................ 46
Acknowledgements ............................................................................................................................... 48
References ............................................................................................................................................. 49
iii
Chapter1
1.1. Motivation and aim of the study
This project is a part of a big project ‘’Testing and Exploration of Metal Air Battery
Technology’’. The metal air battery technology is quite new for Swedish automotive
industries. The whole project consists of different parts such as literature survey of the
different metal air technologies, modeling of metal air cell, fundamental understanding of
electrode reaction mechanisms, construction of Swagelok design Li-air cell and preliminary
testing of available metal air batteries. A vital part of the project is related with the studies
of reaction mechanisms of Li-air battery. The results from this project will further be used in
performing simulations of full Li-air cell.
The aim was to gain fundamental understanding of oxygen reduction reactions by using the
electrochemical techniques. Non-aqueous based Li–air system was chosen due to its
advantage over aqueous system. Literature survey was done to find most appropriate
organic solvents and salts currently used in Li-air battery systems. DMSO and MeCN were
found as good solvents because they have shown good results compared with other
solvents. Therefore DMSO and MeCN were chosen for further studies of reaction kinetics (1).
Electrochemical techniques such as cyclic voltammetry (CV) and Rotating Disk Electrode
(RDE) were used to elucidate the kinetics of these reactions.
1.2 Introduction
1.2.1. Overview of Li-air battery
Lithium air battery is usually defined as a battery consists of Lithium metal-based anode and
porous carbon based air-cathode, which continuously extracts oxygen from air (2). Current
Li-ion batteries are not satisfactory for the practical application of electric vehicles, because
of their electrode materials having intercalation chemistry. Solvent co-intercalated into
graphite cathode. Electric vehicles need high current supply and this problem is more in high
current application. Therefore, Li-air batteries have received significant attraction due to
1
their energy density, which is higher than current Li-ion battery. Li-air batteries can be
categorized based on the type of electrolyte used,
i) Aprotic (organic) solvents
ii) Aqueous solvents
iii) Hybrid (non-aqueous/aqueous) solvents, and
iv) All solid-state electrolyte.
A typical design for non-aqueous or aprotic lithium air batteries is shown in figure 1a, which
is composed of a metallic lithium anode, lithium salt in an organic solvent, and a porous O2
breathing cathode composed of large surface area carbon particles and catalyst particles.
The most common technique of preparing cathode materials for non-aqueous system is to
mix carbon black (Ketjen Black, active coal), a polymer binder and an organic solvent to form
slurry which is coated on a metal grid. The resulting air cathode should have higher surface
area with reasonable pore volume (3).
2
Figure 1: Four different architectures of Li-air batteries in which Li metal is used as anode
material. Aprotic, aqueous, mixed aprotic/aqueous are liquid electrolyte based architecture
(4).
The architecture of aqueous Li-air cell is shown schematically in (Figure 1b). Metallic lithium
is used as anode covered with Li-ion conducting ceramic film, which prevents vigorous
reaction of metallic lithium with water. The aqueous electrolyte consists of lithium salts
dissolved in water. Catalyst is needed with positive electrode that reduce activation energy
barrier for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER).
The main advantage is that discharge reaction product is soluble in water. The disadvantage
is that energy density of aqueous system is much lower than that of conventional Li-ion
batteries because of narrow electrochemical window of water (3).
Solid-state Li-air batteries composed of lithium metal as anode, glass-ceramic electrolyte,
and a porous carbon cathode (Figure 1c). The anode and cathode are separated from the
solid electrolyte by the polymer ceramic membrane. The polymer ceramic composites are
3
used to reduce overall impedance. For example ,18.5Li2O6.07Al2O337.05GeO237.05P2O5,
LAGP and two layers of polymer (ethylene oxide) (PEO) incorporated with Li-salt LiN
(SO2CF2CF3)2 (LiBETI). Advantage of Solid state Li-air battery system is LAGP could completely
prevents the reaction of H2O or CO2 with the negative Li electrode. However, the main
drawback is its low conductivity (5).
Metallic lithium is used as anode in mixed aqueous/aprotic Li-air battery system (Figure 1d).
One part of the electrolyte is aqueous and another part is aprotic. The porous cathode is in
contact with aqueous electrolyte and the Li-metal anode with non-aqueous electrolyte. Two
electrolytes are separated with a lithium conducting ceramic (5).
1.2.2 Lithium ion batteries
Lithium ion batteries are rechargeable batteries in which Li-ion moves from anode to
cathode during discharging and back to anode during charging process. Intercalated lithium
compounds are usually used as electrode materials. In lithium-ion batteries aqueous
electrolyte, organic electrolyte or composite electrolyte can be used. Due to high reactivity
to water non-aqueous or aprotic solvent are preferred. Non-aqueous electrolytes consist of
Li-salts e.g., LiPF6, LiBF4 or LiClO4 in organic solvent (ethylene carbonate, dimethyl carbonate,
and diethyl carbonate).Organic solvent decomposes during charging and form solid
electrolyte inter phase (SEI). Room temperature ionic liquids are alternative solvents to limit
the flammability and volatility of aprotic solvents. In Li-ion batteries, various lithium
compounds such as lithium cobalt oxide (LiCoO2), Lithium iron phosphate (LFP), lithium
manganese oxide (LMO), lithium nickel manganese cobalt oxide (NMC), Lithium nickel cobalt
aluminum oxide (NCA) and lithium titanate (LTO) etc are used as anode materials (6).
Electrochemical reaction of cobalt based Li-ion batteries is given in table 1.
1.2.3 Metal air Batteries
Metal air batteries (lithium-air, iron-air, aluminum–air, magnesium-air and zinc–air) have
recently attracted much attention due to their high energy density. Cathode of metal air
battery utilizes oxygen from air (ambient) and air act as reactant in the electrochemical
reaction. Recently, it has been shown that theoretically it is possible that Li-air battery can
4
have specific energy of 11,680 Wh/Kg, which is close to gasoline (figure 2). Suggested cell
reaction and theoretical energy density, practical energy density of different batteries are
given in table 1. Littauer and Tsai (7) in 1976 introduced the concept of Li–air chemistry.
Abraham in 1996 presented non-aqueous Li-O2 battery and Bruce discovered the
reversibility of the system in 2006. Their work attracted great attention and triggered
numerous research projects on this system.
Figure 2: The gravimetric energy densities (Wh/kg) for various types of rechargeable
batteries compared to gasoline (7).
Figure 3: The schematic figure of a non-aqueous Li–air battery and the porous cathode
structure (7).
5
Li–air battery is a key research area for next-generation power sources which would ideally
can turn the entire vehicle into electric to reduce emission to the environment. At present
practical energy density of Li-air battery is far from its theoretical energy density. In order to
improve the Li-air battery, we need to understand its reaction mechanisms.
A schematic diagram of non-aqueous Li-air battery system is given in figure 3. Where
oxidation reaction occurs at the anode and electron flow through external circuit and Li+ ion
react with oxygen and form Li2O2 in the cathode. During charging Li2O2 decomposed to Li+
and oxygen.
Table 1: Electrochemical reactions and energy densities of the various rechargeable batteries
(8).
Types
Cell reactions
Pd +PdO2+2HSO4−+2H+→2PdSO4+2 H2O
Theoretical
energy density
(Wh/Kg)
170
Practical
energy density
(Wh/Kg)
30–50
Lead–
Acid
Ni–Cd
Ni–MH
Li-ion
Li–S
2NiO(OH)+Cd+2H2O→2Ni(OH)2+Cd(OH)2
xNi(OH)2+M→xNiOOH+MHx
LiCoO2+C→LixC+Li1−xCoO2
xLi++S8+e→Li2Sx
245
280
400
2600
45–80
60–120
110–160
Zn–air
Li2Sx+ Li++e→Li2S2 or Li2S
2Zn +O2→2ZnO
1084
∼400
Li–air
2Li+O2→Li2O2
11,680
∼2000
∼400
1.2.4 Drawbacks of air cathodes-electrolyte
Understanding the mechanisms of the ORR in non-aqueous solutions is the main key to
develop high efficiency and power capability of Li-air battery (9). Li-metal which is high
capacity electrode contains the ionic charge carriers. Li metal reacts with electrolyte and
leads to formation of unstable decomposition layer. When Li metal is immersed in an organic
solvent, it spontaneously reacts to form Li-ion conducting film on its surface. The reaction
between lithium and solvent takes place and a multi layer deposition of Li-salt creates mass
diffusion barrier which inhibits the reaction kinetics (4). The air cathode provides an
interface where O2 from the air is reduced on the surface of the cathode. Carbon with or
6
without catalyst enhances the rate of O2 reduction. The product of the discharge in the Li-air
cell is Li2O and Li2O2 which are not soluble in organic electrolyte solutions (10).
Finding a suitable and stable electrolyte for a Li–air battery is a major challenge as well as
cost effective catalysts to reduce over-potentials for the discharge and charge reactions.
Development of nanostructured air cathode materials can optimize transport of all reactants
to active surface of the cathode and provide sufficient space for discharge product. In order
to supply contaminants free O2 to the system a high throughput air breathing membrane
which can separate O2 from air and can avoid H2O, CO2 and other contaminants needs to be
developed (4). To achieve high energy density, a high positive electrode capacity needs to be
developed for Li-air cell. The major capacity limiting factors are passivation, pore blockage,
and O2 transport limitations. Passivation of electrode surface by electronically insulating
discharge products limit the Li-air battery capacity as low discharge rate. Blockage of micro
pore cathode by Li oxides and other byproducts can limit accessibility of electrode surface
for electrochemical reaction (3).
Another problem associated with air cathode electrode in non-aqueous electrolyte is the
deposition of insoluble reaction product Li2O2 at active sites. Once a dense product layer is
formed on the entire active surface, ionic or electronic transport become limited through
the product layer. Due to poor electronic conductivity of the product Li2O2, the discharge
current density decreases with the increase in the thickness of the product layer, which
eventually leads to the termination of electrode reaction. Another problem is linked with Li–
air battery is that the use of catalysts in electrode surface to enhance the electrode kinetics
and reduce the energy loss associated with the discharge–charge polarization. Proper
distribution and loading influence the cathode reaction as well as performance of the Li–air
battery. Oxygen transport is another challenge for Li-air battery research. Sufficient porosity
and minimal tortuousness is needed for proper oxygen transport to the active sites of
electrode with minimum energy loss. The resistance to the transport of O2 and Li-ions
through the pores decrease and electron transport (11).
Nano structured electrodes are used for Li-air battery to get better performance but due to
complex synthesis process fabrication cost becomes high. Nano structured electrodes have
7
large surface area which allows to have undesired side reactions between the electrode and
electrolyte (11).
Layered cathode can be operated at high voltages and have exhibited high capacities (12).
One example spinel LiMn1.5Ni0.5O4, olivine LiCoPO4, and olivine LiNiPO4 which can be
operated at high voltage and showed high capacity. However, the major difficulty is that
these cathodes are unstable in organic electrolytes. Like LiPF6 in 1:1 ethylene carbonate
(EC)/diethyl carbonate (DEC) electrolyte form an SEI layer on the cathode surface during
discharge which becomes severe and aggressive at elevated temperatures (∼550C) after
subsequent cycles. These reactions degrade the electrolyte and cathode, which results in the
capacity failure. Reactivity of highly oxidizing cathode surface could also be problem also for
long term stability and life cycle of electrolyte (12).
Olivine LiFePO4 (1-d structure) used as cathode showed low electronic and Li+ ion
conductivity. Small particle size and carbon coating is needed to realize high rate capability,
which results in high processing cost. If layered LiCoO2 cathode (2-d structure) used as
cathode, only 50% of the theoretical capacity can be utilized due to safety concern (12).
Another problem is that porous carbon is flooded while contacting with non-aqueous
electrolyte and discharge product (Li2O2, Li2O) insoluble in electrolyte are precipitated into
cathode pores and this restricts the transport of oxygen towards the pores where reaction
takes place. Electrolyte’s ability to properly transport the oxygen depends on electrolyte
parameters such as oxygen solubility and oxygen diffusion (13).
Solvent plays an important role in determining cycling characteristics and efficiency of the
rechargeable Li-air battery. Recent studies revealed that organic carbonates, esters and
ethers are not good candidates as electrolytes for Li-air battery due to decomposition during
discharging and Lithium carbonate (LiCO3) and lithium alkyl carbonate (RO-(C=O)-OLi) were
identified after few cycles. The reason for carbonate species generation is chemical reaction
between Li2O2 and carbonate base electrolyte. Research proved that Li2O2 is highly reactive
against carbonate solvents, moisture and CO2 gas (1). However, esters and ethers based
electrolytes are relatively more stable than organic carbonates (14), (15), (16). The dominant
decomposition pathway was found in the O-alkyl carbon atom of organic carbonates where
super oxides attack nucleophilically. Computational studies have revealed that lithium
8
superoxide, also lithium peroxide itself can act as degradation agent for carbonate and
esters based solvents. For example, it has been shown that in the presence of lithium
peroxide, propylene carbonate (PC) is irreversibly decomposed (17). The main drawback
regarding carbonate based organic electrolytes is that the attack of the solvent molecule by
superoxide radical anion. XRD results of the discharge air electrodes showed that lithium
propylenedicarbonate (LPDC), or lithium ethylenedicarbonate (LEDC) and lithium carbonate
(Li2CO3) are constantly the main discharge products rather than lithium peroxide (Li2O2) or
lithium oxide (Li2O). In situ GC-MS analysis indicates that Li2CO3 and Li2O can’t be oxidize at
potential as high as 4.6V vs. Li/Li+. However, other discharge products are readily oxidized
.The superoxide attack on the solvent molecules at faster rate so that the formation of LiO2
from superoxide radical anion and the Li-ion is slower than super oxide attack, results
solvent degradation (18).
1.2.5 Progress regarding Li-air cathode
Recent studies have been considering ethers and glymes as solvents for Li-air batteries
because these are more stable than organic carbonates against nucleophilic attack by
superoxide. However, for long term cycling of rechargeable Li-O2 battery ethers and glymes
are unstable (19).
1:1 (EC:PC) with lithium salt, LiTFSI showed higher discharge capacity than all the electrolytes
containing ethers and glymes. The PC–DME based electrolytes have even high capacity and
oxygen solubility than PC-EC. TPFPB is a good additive which can partially dissolve Li2O and
Li2O2 and oxygen solubility also increases when TPFPB added (20).
Most recent study revealed that ionic liquid N-methyl-N-propylpiperidinium bis
(trifluoromethansulfonyl) amide (PP13TFSA) is an appropriate candidate as a solvent.
Quantitative analysis was carried out by gas chromatography to measure the amount of
evolved gases from different organic electrolytes, which are listed in table 2.
9
Table 2: Compositions and their normalized concentration of various gases stored during the
initial charge by gas chromatography. Amounts of the evolved gases were normalized by the
electrical quantity during charging (21).
Electrolyte solvent
PP13TFSA
EC-DEC
PC
GBL
TEGDME
Classification
Norm. gas conc. [L/Ah]
–
Carbonate
Carbonate
Lactone
Ether
H2
0.311
0.114
0.035
0.579
0.197
CO
0.000
0.028
0.014
0.079
0.007
CO2
0.000
0.555
0.676
0.826
0.001
Studies on the use of catalysts have revealed that over potential of charging process can be
reduced. MnO2 is the best studied metal oxide, which is used as catalyst to promote
oxidation of Li2O2. However, its efficiency as catalyst depends on the structure and
morphology. Among the different metal catalysts the manganese-catalyzed air cathodes
have shown the highest specific energy about 4000mAh/g. Another study found that
Au/carbon cathode promotes the ORR process and Pt/carbon cathode promotes the OER
process. But metals Au, Pt are economically unfeasible (22). By using CeO2 as catalyst a
smooth increase in the ORR rate and reduction in the polarization was seen. Capacity of
2128mAhg-1 displayed when CeO2 was used as catalyst. Not only surface area but also crystal
structure plays important role on the elctrocatalytical performance of different catalysts in
the Li-air batteries (23).
Recent studies have revealed that the presence of redox mediator with a lower potential in
the electrolyte of the rechargeable non-aqueous Li-O2 battery could recharge at higher
current density (1 mA/cm2). The tetrathiafulvalene (TTF) molecule is oxidized to TTF+ at the
cathode surface which in turn oxidizes the insulating solid (Li2O2) and TTF+ reduces back to
TTF. Effective oxidation of Li2O2 leads to complete reversibility of Li air battery. Here, this
mediator act as an electron hole transfer agent that permits effective oxidation of Li2O2.
However, the absence of redox mediator leads to severe polarization on charging (24).
10
Chapter 2
2 General theory of electrochemistry
Electrochemistry is the branch of chemistry which deals with the interrelation of electrical
and chemical effects. A large part of this field deals with the study of chemical changes
caused by the passage of an electric current and the production of electrical energy by
chemical reactions. In Electrochemistry electron transfer reactions take place at the solid
solution interface. The solid is the electron conductor i.e., the electrode and ionic conductor
is the electrolyte. In this section most relevant and important equations and theory will be
discussed (25).
2.1 Oxidation-reduction potentials
Oxidation and reduction involves the transfer of electron between substances. Both
processes take place simultaneously. If one substance loose electron, another substance
gain that electron. Equilibrium potential of redox reactions are directly related to
thermodynamics and also specify at which potentials reduction and oxidation reaction take
place in the absence of kinetic limitations. This potential depends on pH and can be
measured as relative to a reference electrode placed in the solution.
2.2 Mass transport
There are three kinds of mass transport processes such as diffusion, convection and
migration, which can influence electrochemical reactions. Diffusion occurs in solution when
relative concentration of a reagent is dense. Diffusion can be defined as movement of
species under the influence of chemical potential gradient (i.e concentration gradient).
Action of force on solution generates convection. This action can be pump, a flow of gas or
even gravity. Fluid flow occurs because of natural convection. It is caused by density
gradient. Force convection can be characterized by stagnant regions, laminar flow and
turbulent flow. Final form of mass transport is considered as migration. Migration caused
due to a gradient of electrical potential (26).
11
2.3 Essential electrode Reaction
An electrode reaction can be characterized by the Nernst equation. The Nernst equation
illustrates the relation between the concentration of the redox species at the electrode
surface and applied potential on electrode. In general case O is capable of being reduced to
R at the electrode by the following reversible electrochemical reaction.
Eq1
Where C*R and C*0 are the bulk concentrations of reduced and oxidized species respectively,
and E° is the formal potential.
If the system follows the Nernst equation the electrode reaction is often said to be
thermodynamically or electrochemically reversible. A process can either reversible or not
depends on time dependent measurements, the rate of change of driving force and a speed
at which the system can establish equilibrium. A given system can behave reversibly in one
experiment and irreversibly in another under different experimental conditions (25).
2.4 Heterogeneous rate constant
Redox reactions in non-aqueous solvent involved electron transfer (ET) process. Solvent has
remarkable influence on the ET process that occurs either homogenously in the solution or
heterogeneously at the electrode surface. The rate constant of ET process which occurs
heterogeneously is called heterogeneous rate constant. ET rate constant is proportional to
the exponential of the applied voltage. Simple electrochemical methods such as CV, RDE can
be used to determine rate constant of an ET process (27).
2.5 Cyclic Voltammetry
Cyclic voltammetry is one of the most versatile and commonly used techniques for studying
the electrochemical reactions. In the cyclic voltammetry (CV) potential on the working
electrode is scanned linearly backward and forward within the pre defined limit. The
12
resulting current on the working electrode is measured as function of the applied potential.
Figure 4 shows potential input in cyclic voltammetry (25). CV can be used for different
purposes. For example, it can be used to acquire qualitative information about
electrochemical reaction but also quantitative information about reaction kinetics and mass
transport. From cyclic voltammetry the type of reaction and rate of reaction can be
determined by using some equations such as the Randles-Sevcik equation, Nicholson and
Shain equation 25).
Figure 4: (a) Potential as a function of time and (b) current as a function of voltage for cyclic
voltammetry (25).
2.5.1 Scan rates
Dependence of the oxidation/reduction peak potentials on scan rate is good indicator if an
electrochemical reaction is reversible, quasi-reversible or irreversible. For reversible
reactions, the oxidation and reduction peak potentials do not change with scan direction, for
Quasi or irreversible reactions peak potentials shift in the scan direction (figure 5). Current
density also depends upon the scan rate and increases at high scan rate because the
concentration of electroactive species increases in diffusion layer. Reversible
electrochemical reactions has some characteristics such as, peak position do not change with
scan rate, ratio of oxidation and reduction peak current should be one and peak current is
proportional to square root of the scan rate (25).
13
Figure 5: Scan rate and rate constant dependence of cyclic voltammetry curve
2.5.2 Reversible systems
A theoretical expression of peak current for a reversible cyclic voltammogram is derived as a
function of the scan rate which is called Randles-Sevcik expression. According to this
expression, the dependence of peak current Ip on scan rate v can be written as shown below,
ip= 2.69 × 105n3/2AD1/2Cv1/2
Eq2
Where, ip= peak current, A
n= electron stoichiometry
A= electrode area, cm2
D= diffusion coefficient, cm2s-1
C= concentration, molcm-3
v= scan rate, Vs-1
ip increases with v1/2 and is directly proportional to concentration. This expression is very
important in the study of electrochemical mechanisms. The ratio of anodic and cathodic
peak currents should be close to one.
For a reversible electrochemical reaction, the number of electron transferred in the
electrode reaction can be determined by the anodic and cathodic peak potentials
14
Eq 3
Where ∆Ep is potential separation, Epa is anodic peak potential and Epc is cathodic peak
potential (25).
2.5.3 Irreversible and Quasi-Reversible Systems
For irreversible system individual oxidation and reduction peaks are reduced in size and
separated widely. Totally irreversible systems are characterized by shift of peak potential
with the scan rate:
Eq 4
at 250C
Eq 5
Where α is the transfer coefficient, and n is the number of electrons involved in the charge
transfer step, R is the gas constant, F is Faraday constant, T is temperature. The potential Ep
occurs at a higher value than E0 with the over potential related to k0 and α.
Charge transfer coefficient is a measure of the symmetry of the energy barrier. Charge
transfer coefficient is independent of any mechanistic reflection and based on experimental
data and is used for the elucidation of electrode kinetics.
Nicholson and Shain equation is usually used to analyze Irreversible and Quasi-Reversible
systems. According to Nicholson and Shain, peak current is given by,
=2.99×106n (nα)1/2ACD1/2v1/2
Eq 6
Where area is A, concentration C, diffusion coefficient is D, α charge transfer coefficient
Here peak current is still proportional to bulk concentration.
For quasi-irreversible systems the current is controlled by mass transfer and charge transfer.
The standard heterogeneous rate constant range for quasi-irreversible systems is: 10-1> ko >
10-5 cm/s. Peak separation is quite larger than reversible system. The expression which is
used to calculate rate constant of quasi-irreversible systems,
15
Eq 7
Where Ψ equivalent parameter, D0 is oxidation diffusion coefficient, DR is reduction diffusion
coefficient, Faraday constant F, scan rate v (28).
2.6 Uncompensated Resistance
If potential profile is considered in solution between the working and auxiliary electrodes,
the solution between these electrodes can be regarded as potentiometer. Uncompensated
resistance can affect the measured value of current or potential. It is denoted by Ru .It also
depends on the electrode size. This can be minimized by the use of three electrode system.
2.7 Rotating Disk Electrode (RDE)
RDE is one of the most popular hydro dynamic methods used in a three electrode system.
The electrolytes are forced by convection to a rotating disk electrode. When the rotating
speed increases the species flux to the surface increases by convection and current also
increases. The rotating disk electrode (RDE) is one of the few convective electrode systems
where hydrodynamic equations and the convective equations have been solved rigorously.
This technique is very simple to construct and consists of a disk of the electrode material
inserted in a rod of an insulating material.
16
Figure 6: Schematic view of Rotating disk electrode.
A schematic view of RDE is given in figure 6. At stationary electrodes the diffusion layer can
grow independently. By using convective methods such as RDE, the growth of diffusion layer
can be restricted. The role of mass transport on electrode reaction kinetics can be elucidated
from the kinetic parameters obtained from CV and RDE experiments (25).
Dr. Benjamin Levich illustrated first the mathematical treatment of convection and diffusion
for rotating disk electrode,
Eq8
IL is the current limited in voltammogram
n is the number of electrons transferred,
F is the Faraday constant
A is the electrode area
D0 is the diffusion coefficient
ω is rotation speed
v is the kinematic viscosity of the solution and
17
C0 is the concentration of the electroactive species
This equation only applies to the mass transfer limited condition at RDE and assumes IL is
proportional to
and ω1/2.
is also called levich constant.
For totally irreversible one step one electron reaction is analyzed by the Koutecky-Levich
equation which is given below,
Eq9
Where iK represents the current in the absence of any mass-transfer effects, iL is the current
limited in voltammogram, n is the number of electrons transferred and F is the Faraday
constant, A is the electrode area, D0 is the diffusion coefficient, ω is rotation speed, ϑ is the
kinematic viscosity of the solution and C0 is the concentration of the electroactive
species.
is a constant only when iK is very large (28).
Studies showed that the current is often exponential to the over potential η, that is given by
Tafel in 1905,
Eq 10
It is a successful model of electrode kinetics, known as Tafel equation where
and
Eq11
Where I0 is exchange current which can be considered as idle current. A plot of Log Ik (kinetic
current) vs η (over potential) known as Tafel plot, is a useful technique for estimating kinetic
parameters. A schematic diagram of tafel plot is given bellow,
18
Figure 7: Tafel plots for anodic and cathodic branches of the current over potential curve.
A Tafel relationship cannot be observed for the systems where the mass-transfer effects on
the current are absent. At these points Tafel behavior is an indicator of totally irreversible
kinetics. Tafel slope can also be used to determine either the reaction is reversible or not.
For one electron reversible reaction slope of Tafel plot should be 120mV/dec. (25).
19
Chapter 3
3 Experimental
The main focus of the project is to utilize electrochemical methods for analyzing reaction
mechanism. Here overview of setup, experimental procedure and electrochemical
techniques are described.
3.1 Materials
Dimethyl Sulfoxide (DMSO)(Anhydrous,99,9%), acetonitrile (MeCN)Anhydrous,99,9%,
tetrabutylammonium hexafluorophosphate (TBAPF6) (electrochemical grade,
≥99, 0%), lithium hexafluorophosphate (LiPF6) (battery grade, >99,9%), Lithium perchlorate
(LiClO4) (battery grade, dry 99, 99%) were purchased from Sigma Aldrich. All chemicals were
immediately stored in glove box filled with purified argon where the moisture and oxygen
content was less than 1ppm.Medium size glove bag was purchased from Sigma Aldrich.
3.2 Potentiostats
Potentiostats used in these experiments are as follows,

Gamry Reference 600TM , Gamry Analyst 5.6 Potentiostats/Galvanostat and

Atutolab, NOVA 1.8
3.3 Cells and electrode setup
For all experiments three electrode cell system was used (figure 5). Glassy carbon was used
as working electrode (dia-5mm), pt mesh was used as counter electrode and Ag/AgCl,
Ag/Ag+ was used as reference electrode. The Ag/AgCl electrode was prepared by oxidation
of Ag wire in saturated KCl in H2O. The Ag/Ag+ electrode was prepared by oxidation of Ag
wire in 0.001M AgNO3/MeCN and outer junction was filled with working solution (MeCN or
DMSO). The surface area of counter electrode was much larger than working electrode to
restrict the limitations of the processes occurring at the working electrode.
20
Figure 8: Schematic setup of three electrode system used in all experiments.
3.4 Measurements Procedure
All solution preparations were carried out inside of glove box filled with dry Argon, where
the moisture and oxygen content was less than 1ppm.Measured salt and solvent were mixed
by a stirrer at 600rpm. Experiments were carried out inside the glove bag filled with dry
Argon. Before start of the experiments the glove bag was evacuated two times in order to
remove moisture and air from the glove bag.
3.5 Electrochemical Experiments
The electrochemical experiments were performed with two different setup, Gamry
Reference 600TM, Gamry Analyst 5.6 potentiostats/Galvanostat and an Autolab (Ecochemie
Inc., model-PGSTAT 12) equipped with electrochemical cell. The electrochemical cell was
built in house consisted of traditional three electrode system utilizing Ag/Ag reference
electrode. The cell had inlet and outlet valves for oxygen or argon purging. The glassy carbon
of 5mm diameter used as working electrode was polished with 0,3μm alumina paste and
rinsed thoroughly with milli-Q water and dried carefully prior to the experiments. All the
21
cyclic voltammetry was performed in argon filled glove bag (atomsbag two-hand non-ster,
slide closer, size M 39’’×48’’, sigma-aldrich) where H2O and O2 concentration were kept
bellow 1ppm at room temperature. For RDE experiments glassy carbon electrode was
rotated with RDE rotor. For the ORR measurement the solution were purged with pure O2.
The effect of sweep rate on the voltammograms were observed by using same concentration
of electrolyte at different scan rates 25, 50, 100, 200, 300, 400, 500 mVs-1. Initial condition
was set as step size 2mV, equilibrium time 5 s and I/R range was kept fixed for all CV scan.
Different potentials range (±V) was chosen for each scan. Open circuit potential and
impedance of each electrolyte was measured. For impedance measurements, the spectra
were measured in the frequency range from 1Hz to 100000 Hz at an open circuit potential.
For all RDE measurements I/R range was kept as auto module. Rotation speed of 300,750,
1000, 1250, 1700, 2000, 2500, 3000, 3550, 4000, 5000 RPM used for the RDE experiments.
22
Chapter 4
4 Results and discussion
Non-aqueous solvents are best media for investigating the oxygen reduction reactions (ORR)
relevant for the Li-air battery system. Literature studies revealed that three possible O2
reduction products such as LiO2, Li2O2, Li2O are formed and these are highly polar. In order to
dissolve these products and avoid their passivation of electrode surface, polar solvents are
required. There are several aprotic solvents which were studied for Li-air battery research. In
the study dimethyl sulfoxide (DMSO) and acetonitrile (MeCN) were chosen to investigate the
fundamental reaction mechanisms of Li-air battery. Three different salts i.e., LiPF6, LiClO4
and TBAPF6 were chosen for this study. The properties of DMSO and MeCN are given in
Table 3.
Table 3: Chemical and physical properties of solvents (29).
Solvent
Dielectric
Donor Numbers
viscosity
constant (250C)
(kcal/mol)
η (cP)
oxygen
solubility
(mM/cm3)
DMSO
48
29.8
1.948
2.1
MeCN
36.64
14.1
0.361
8.1
4.1 Oxygen Reduction in 0.1M TBAPF6/DMSO
The role of TBAPF6 on the reduction properties of oxygen in DMSO were studied by using
cyclic voltammetry (CV) and rotating disk electrode (RDE) voltammetry. Glassy carbon was
used as working electrode, because real cathode materials are also made of carbon. Figure 9
displays cyclic voltammetry (CV) for the reduction of oxygen in a 0.1M TBAPF6/DMSO
electrolyte. The reference electrode which was used in the CV experiments was Ag/Ag+.
Peak potentials separation between anodic (Epa= -1.19V) and cathodic (Epc= -1.125V) is
65mV, which is close to 60mV. Peak current ratio is close to unity. These results indicate that
O2 reduction in the presence of TBA+ ions is reversible.
23
6,00E-04
Epa
4,00E-04
Current Density (Acm-2)
2,00E-04
0,00E+00
Argon
-2,00E-04
TBA/DMSO
-4,00E-04
-6,00E-04
-8,00E-04
-3,00
Epc
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potential V vs Ag/Ag+
Figure 9: Cyclic voltammograms for the reduction of oxygen in 0.1M TBAPF6/DMSO on a
glassy carbon working electrode at a scan rate of 100mVs-1.The values are IR corrected. The
blue curve is argon background.
In Figure 10 cyclic voltammograms for the reduction of oxygen-saturated TBAPF6/DMSO at
different scan rates (50mV to 400mV) are shown. Reduction seems to be reversible at all
sweep rates. However there is slight shift in peak position. The Randles-Sevcik plots
presented in Figure 11 by using equation 2 are linear, which indicates a fast diffusion
controlled electrochemical process.
24
2,00E-04
1,50E-04
1,00E-04
Current A
5,00E-05
0,00E+00
400mv/s
-5,00E-05
50mv/s
-1,00E-04
100mv/s
200mv/s
-1,50E-04
300mv/s
-2,00E-04
Argon background
-2,50E-04
-3,00E-04
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potential V vs Ag/Ag+
Figure 10: Cyclic voltammograms for the reduction of oxygen saturated 0.1 M TBAPF6/DMSO
on GC electrode at various scan rates.
3,00E-04
2,00E-04
Currentr A
1,00E-04
reduction
oxidation
0,00E+00
-1,00E-04
-2,00E-04
-3,00E-04
0,17
0,27
0,37
0,47
SQRT(scan rate) (Vs-1)1/2
0,57
0,67
Figure 11: Randles-Sevcik plot of peak current vs square root of the scan rate in 0.1 M
TBAPF6/DMSO.
25
Diffusion coefficient for oxidation and reduction was 9.1×10-6 cm2s-1 and 9.1×10-6 cm2s-1
respectively. These values were calculated from Randles-Sevcik slope by using equation 2.
These experimental values are very close to the values reported in literature 9.7 ×10-06 cm2s1
for oxygen reduction in DMSO (29). Kinetics of the reaction was analyzed by using eq 2 and
are shown in figure 12, where Ψ is wave shape factor calculated from this equation Ψ=24/(EE0 ) . Figure 12 shows the variation of shape factor as function of ((D0/DR)1/4/(∏D0vF/RT)1/2).
Rate constant (k0) was calculated by rearranging the equation 7 where k0 is considered as
the slope of the figure 12. Rate constant for oxygen reduction in presence of 0.1M
TBAPF6/DMSO is 0.012cms-1.
1,4
1,2
1
Ψ
0,8
0,6
0,4
0,2
0
0
20
40
60
80
100
(D0/DR)1/4/(∏D0vF/RT)1/2
120
Figure 12: Variation of Ψ with scan rate, in this plot k0 is the slope of the curve.
26
140
5,00E-04
0,00E+00
Current Density (Acm-2)
-5,00E-04
300RPM
-1,00E-03
750RPM
1260RPM
-1,50E-03
1750RPM
2150RPM
-2,00E-03
2550RPM
3050RPM
-2,50E-03
3550RPM
-3,00E-03
-3,50E-03
-4,00E-03
-2,25
-2,00
-1,75
-1,50
-1,25
-1,00
-0,75
-0,50
-0,25
0,00
Potential V vs Ag/Ag+
Figure 13: Rotating Disk Electrode voltammograms collected at 5mV s−1 in oxygenated 0.1
MTBAPF6 /DMSO electrolyte at various rotation rates.
Figure 13 displays RDE voltammograms collected at 5 mV s−1 in oxygen saturated 0.1
MTBAPF6 /DMSO electrolyte at various rotation rates. Rotating disk electrode is
hydrodynamic technique which uses convection as mode of mass transport. RDE data was
analyzed by using the Levich equation, which establishes a relation between current at the
rotating disk and angular frequency. In the eq 5 limiting current density (Ilim ) and n the
number of electron transferred, F is the Faraday constant (96500 Cmol-1), ν is the kinematic
viscosity of the solution (1.9×10-3cm2s-1) (29), c is concentration of oxygen (2.1×10-6 molcm-3)
(29) and (
is the angular frequency, Diffusion co-efficient can be calculated from the
Levich equation. Levich plot given in Figure 14 indicates a mass transfer controlled electrode
process. Diffusion coefficient of 9.8×10-6 cm2s-1 calculated from Levich equation is very close
to value given in literature 9.7×10-6 (29).
27
-2,00E-04
-3,00E-04
Current A
-4,00E-04
TBAPF6/DMSO
-5,00E-04
-6,00E-04
-7,00E-04
5,00
7,00
9,00
11,00
13,00
15,00
ω1/2(rads-1)1/2
17,00
19,00
21,00
Figure 14: Levich plot of limiting current vs square root of rotation in 0.1M TBAPF6/DMSO.
Scan rate 5mV/s.
Reaction kinetics can be further investigated using the eq 10 & 11.
A plot of kinetic current (Ik) vs over-potential should be linear and exchange current density
and over-potential can be determined from this plot. Kinetic current (Ik) can be calculated
from equation 12 as given below.
eq12
Where ¡ is the measured current during oxygen reduction and Ilim diffusion limited current
from levich plot. The slope is very close to 120mVdec-1 .
Diffusion coefficient calculated from both experimental cyclic voltammogram (9.1×10-6 cm2s1
) and rotating disk voltammogram (9.8×10-6 cm2s-1) was very close to literature value
(9.7×10-6 cm2s-1) (29). Cyclic voltammograms seem to be scan rate dependent, which is
characteristic of quasi-reversible reaction. From the heterogeneous rate constant and
28
voltammograms peak separation it can be deduced that oxygen reduction is quasi-reversible
process with high reaction rate.
4.2 Oxygen Reduction in 0.1M TBAPF6/MeCN
Influence of TBAPF6 on the reduction of Oxygen in MeCN was studied using CV and RDE
methods. Ag/Ag+ reference electrode was used in these CV and RDE experiments. Cyclic
voltammogram CV scanned from -2 to 0.5 V for the reduction of oxygen in 0.1M
TBAPF6/MeCN are presented in figure 15. Figure 15 shows the reduction of oxygen in a 0.1M
TBAPF6/MeCN electrolyte at 100mVs-1 scan rate. No appreciable current was observed under
argon saturated system. The peak currents ratio for oxygen saturated voltammogram is
close to unity. Oxidation peak and reduction peaks are separated by 85mV. Theoretical peak
separation for one electron reversible system is 59 mV.
3,00E-03
Current Density(Acm-2)
2,00E-03
1,00E-03
0,00E+00
-1,00E-03
TBA/MeCN
-2,00E-03
Argon
-3,00E-03
-4,00E-03
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
Potential V vs Ag/Ag+
Figure 15: Cyclic voltammograms for the reduction of oxygen in 0.1M TBAPF6/MeCN on a
glassy carbon working electrode at a scan rate of 100mVs-1.The values are IR corrected. The
black curve is argon background.
In Figure 16 the cyclic voltammograms for the reduction of oxygen-saturated TBAPF6/MeCN
at different sweep rates are shown. The peak position is shifted with the scan rates, which is
one of the characteristics of quasi reversible reaction. CV data was further analyzed by
29
Randles-Sevcik equation. Randles-Sevcik plot is linear which indicates the diffusion
controlled electrochemical process (figure1, appendix).
1,00E-03
5,00E-04
Current A
0,00E+00
-5,00E-04
25mv/s
400mv/s
50mv/s
200mv/s
300mv/s
100mv/s
-1,00E-03
-1,50E-03
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
Potential V vs Ag/Ag+
Figure 16: Cyclic voltammograms for the reduction of oxygen saturated 0.1 M TBAPF6/MeCN
on GC electrode at various scan rates (IR corrected).
Diffusion coefficient for oxygen reduction was 1.6×10-5 cm2s-1 and for oxidation was 1.6×10-5
cm2s-1. These values were calculated from Randles-Sevcik slope using equation 2.These
experimental values are very close to the literature value 2.1 ×10-5 cm2s-1 (30). Kinetics of the
reaction was analyzed by eq 2 and is shown in appendix figure 2. Rate constant for oxygen
reduction in presence of 0.1M TBAPF6/MeCN is 0.0066cms-1.
30
2,00E-03
0,00E+00
Current Density (Acm-2)
-2,00E-03
-4,00E-03
440RPM
900RPM
1550RPM
2050RPM
2550RPM
-6,00E-03
-8,00E-03
-1,00E-02
-1,20E-02
-1,40E-02
-1,92
-1,72
-1,52
-1,32
-1,12
-0,92
-0,72
-0,52
Potential V vs Ag/Ag+
Figure 17: RDE voltammograms collected at 5mVs−1 in oxygenated 0.1 M TBAPF6 /MeCN
electrolyte at various rotation rates.
In Figure 17 the typical steady-state voltammograms for O2 reduction on a RDE in oxygensaturated 0.1 M TBAPF6 solution at various rotation rates are shown. Figure 17 shows that
current generation by this hydrodynamics method is higher compared to CV under diffusion
control. Limiting current for the system increased with the rotation rates. From these
voltammograms, it appears that there is a significant increase in the cathodic current
whereas anodic current is negligible. These RDE voltammograms were further analyzed by
Levich equation (eq8), where ϑ is the kinematic viscosity of the solution (4.4 × 10-3 cm2 s-1)
(30) and C is the concentration of oxygen in solution (8.1 mM) (30). In Figure 18 Levich plot
for the reduction of oxygen from the RDE data of limiting current density at various rotation
rates is shown. Diffusion coefficient of 1.6×10-5 cm2s-1 was calculated from eq 8 and which is
very close to the value given in literature 2.4×10-5 cm2s-1 (30). Reaction rate can be further
investigated by Tafel plot. A Tafel slope for this system is very close to 120Dec-1.
31
-5,0000E-04
-1,0000E-03
Current A
-1,5000E-03
-2,0000E-03
TBAPF6/MeCN
-2,5000E-03
-3,0000E-03
6,00
8,00
10,00
12,00
14,00
16,00
18,00
ω1/2(rads-1)1/2
Figure 18: Levich plot of limiting current vs square root of rotation in 0.1M TBAPF6/MeCN.
Scan rate 5mVs-1.
The oxygen reduction reaction was analyzed for 0.1M TBAPF6/MeCN system. Experimental
value of diffusion coefficients of both CV and RDE method is very close to literature value
-5
2 -1
(2.4×10 cm s ) (29). Cyclic voltammograms seem to be scan rate dependent and which is a
characteristic of a typical quasi-reversible reaction. Therefore, oxygen reduction to form
superoxide is one electron quasi-reversible process. Heterogeneous rate constant value is
close to the value found for 0.1M TBAPF6/DMSO system.
32
4.3 Oxygen Reduction in 0.1M LiPF6/DMSO
Influences of LiPF6 on the reduction of Oxygen in DMSO were studied using CV and RDE
methods using glassy carbon as working and Ag/Ag+ as reference electrodes. Oxygen
reduction mechanism in LiPF6 containing electrolytes was different from TBAPF6 containing
electrolytes. Figure 19 illustrates O2 reduction in 0.1M LiPF6/DMSO at 100mVs-1 scan rate. No
appreciable current was observed under argon saturated system. However, in O2 saturated
system very large reduction peak current is seen compared to the oxidation peak current.
The reduction peak potential is -1.27V and the oxidation peak potential is -0.316V. Peak
separation is very large compared to the TBAPF6/DMSO, despite the fact that both systems
contain same solvent. The calculated half peak potential is -1.18V. For one electron
reversible process, potential difference between cathodic peak and half-peak potential
(potential at the half-value of the peak current) is 56.5 mV (25). For the investigated
LiPF6/DMSO system the potential difference is about 90mV at 100mVs-1 scan rate, which
indicates more complex processes occurring than just reversible processes.
5,0000E-05
Current A
0,0000E+00
-5,0000E-05
100mv/s
-1,0000E-04
-1,5000E-04
-2,0000E-04
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potentiual V vs Ag/Ag+
Figure 19: Cyclic voltammograms for the reduction of oxygen in 0.1M LiPF6/DMSO on a
glassy carbon working electrode at a scan rate of 100mVs-1.The values are IR corrected. The
black curve is argon background.
33
3,00E-04
2,00E-04
Current A
1,00E-04
0,00E+00
500mv/s
-1,00E-04
400mv/s
300mv/s
200mv/s
-2,00E-04
100mv/s
50mv/s
-3,00E-04
-4,00E-04
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potential V vs Ag/A+
Figure 20: Cyclic voltammograms for the reduction of oxygen saturated 0.1 M LiPF6/DMSO
on GC electrode at various scan rates.(IR corrected).
Figure 20 shows, the cyclic voltammograms for the reduction of oxygen-saturated 0.1M
LiPF6/DMSO at different sweep rates. The peak position is shifted with the scan rates. Scan
rate dependency which is observed in this system is one of the characteristics of quasi
reversible system or irreversible system. CV data was further analyzed by the Nicholson &
Shain relationship (eq 6), where, transfer coefficients α (0, 5), diffusion coefficient of the
oxygen D (1.6×10-5cm2s-1) were used (29). Nicholson & Shain plot is linear and number of
electron involved in this reduction process is one which is determined from eq 6 (figure 21).
However, in these voltammograms more than one reduction and oxidation peaks are
apparent which are more obvious at lower scan rate than higher scan rates. The first
reduction process could be the formation of superoxide, which further reduced to another
reduced form of oxygen.
34
-1,50E-04
-1,70E-04
-1,90E-04
Current A
-2,10E-04
-2,30E-04
-2,50E-04
-2,70E-04
-2,90E-04
reduction
-3,10E-04
-3,30E-04
y = -0,0004x - 3E-05
R² = 0,9873
-3,50E-04
SQRT(scan rate) ( V/s)1/2
Figure 21: Peak current vs. square root of the scan rate in 0.1 M LiPF6/DMSO.
1,00E-04
Current Density (Acm-2)
0,00E+00
-1,00E-04
5000rpm10mvs-1
-2,00E-04
4000rpm10mvs-1
-3,00E-04
3000rpm10mvs-1
-4,00E-04
2000rpm10mvs-1
-5,00E-04
-6,00E-04
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potential V vs Ag/Ag+
Figure 22: Rotating Disk Electrode voltammograms collected in oxygenated 0.1 M LiPF6
/DMSO at various rotation rates.
35
Further investigation of the reduction process was conducted via rotating disk electrode
(RDE) voltammetry and results are shown in Figure 22. These RDE experiments were
conducted at 10mVs-1 scan rate. Excessive amount of insulating product deposited after
every run on the working electrode and that deposit had to be removed before further use.
Moreover, cyclic voltammograms show scan rate dependency. Because several peaks
appeared in the cyclic voltammograms. Here, oxygen reduction is one electron reduction
accompanied with chemical follow-up reactions. In presence of Li+ ion L2O2, Li2O could form
during discharge process. These insulating products deposit on electrode surface and block
the reaction sites of electrode surface. These discharge products cannot be removed from
the electrode surface by electrochemical cycling. Therefore, the oxidation process is even
more complex and difficult to analyze.
4.4 Oxygen Reduction in 0.1M LiPF6/MeCN.
In Figure 23 the cyclic voltammograms for the reduction of oxygen-saturated 0.1M
LiPF6/MeCN at different sweep rates are shown. The peak position is shifted with the scan
rates and no significant oxidation peak is observed. Oxygen reduction in presence of
LiPF6/MeCN is very slow and the process seems to be irreversible.
36
2,00E-04
Current Density (Acm-2)
1,00E-04
0,00E+00
-1,00E-04
200mv
100mv
-2,00E-04
500mv
-3,00E-04
-4,00E-04
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potential (V)
Figure 23: Cyclic voltammograms for the reduction of oxygen saturated 0.1 M LiPF6/MeCN
on GC electrode at various scan rates.
4.5 Oxygen Reduction in 0.1M LiClO4/DMSO
Results of the ORR in 0.1M LiClO4/DMSO solution are shown in Figure 24. The oxidation peak
current is very low compared to the reduction peak current. The reduction peak potential is 1.2 V and oxidation peak potential is 0.2V . Peak separation is very large compared to TBA
system and even larger than LiPF6 in DMSO which leads to irreversible oxygen transfer
reaction. Half peak potential of the system is -1.12V . The potential difference between
cathodic peak and half peak potential is 80mV. For one electron reversible reaction the
potential difference between cathodic peak and half-peak potential (potential at the halfvalue of the peak current) is 56.5mV (25). This 80mV of potential difference demonstrates
complex process than a reversible process.
37
2,00E-04
1,50E-04
1,00E-04
Current A
5,00E-05
0,00E+00
-5,00E-05
-1,00E-04
-1,50E-04
-2,00E-04
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
1,50
Potential V Ag/Ag+
Figure 24: Cyclic voltammograms for the reduction of oxygen in 0.1M LiClO4/DMSO on a
glassy carbon working electrode at a scan rate of 100mVs-1.The values are IR corrected.
3,00E-04
2,00E-04
Current A
1,00E-04
0,00E+00
400mv/s
-1,00E-04
300mv/s
-2,00E-04
200mv/s
100mv/s
-3,00E-04
-4,00E-04
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
Potential V vs Ag/Ag+
0,50
1,00
1,50
Figure 25: Cyclic voltammograms for the reduction of oxygen saturated 0.1 M LiClO4/DMSO
on GC electrode at various scan rates.
38
Figure 25 shows, the cyclic voltammograms for the reduction of oxygen-saturated 0.1M
LiClO4/DMSO at different sweep rates. As seen in the case of LiPF6/DMSO the peak position
is shifted with the scan rates which is one of the characteristics of quasi-reversible or
irreversible system. CV data was further analyzed by the Nicholson & Shain relationship (eq
6), Experimental plot is linear and number of electron involved in this reduction process is
one (figure 3, appendix).
1,00E-03
Current Density (Acm-2)
5,00E-04
0,00E+00
1000RPM
2000RPM
4000RPM
5000RPM
-5,00E-04
-1,00E-03
-1,50E-03
-2,00E-03
-3
-2,5
-2
-1,5
-1
-0,5
0
0,5
1
Potential (V)
Figure 26: Rotating Disk Electrode voltammograms collected at 10mVs−1 in oxygenated 0.1 M
LiClO4 /DMSO electrolyte at various rotation rates.
In Figure 26 the reduction process analyzed via rotating disk electrode (RDE) voltammetry is
shown. These RDE experiment were conducted at 10mVs-1 scan rate and various rotation
rates. Cyclic voltammograms seem to be scan rate dependent. The oxygen reduction process
seems to be one electron reduction with follow-up chemical reactions. The oxidation process
is complex and difficult to analyze. Passivation is associated with the ORR in the presence of
Li+ ion in DMSO solvent. Due to the deposition of insoluble reduction products on to the
surface of the working electrode blocks reaction sites and these products cannot be
removed by electrochemical cycling. A continuous decrease of the reduction peak current
was obtained over multiple cycles.
39
4.6 Oxygen Reduction in 0.1M LiClO4/MeCN
Figure 27 shows, the cyclic voltammograms for the reduction of oxygen-saturated 0.1M
LiClO4/MeCN at different sweep rates. The peak position is shifted with the scan rates. No
significance oxidation peak is observed. Reduction of oxygen in presence LiPF6/MECN is very
slow. No oxidation peak was observed and the process seems to be irreversible.
5,00E-04
4,00E-04
Current Density (Acm-2)
3,00E-04
2,00E-04
1,00E-04
0,00E+00
100mv
-1,00E-04
200mv
-2,00E-04
400mv
-3,00E-04
-4,00E-04
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potential (V) vs Ag/Ag+
Figure 27: Cyclic voltammograms for the reduction of oxygen saturated 0.1 M LiClO4/MeCN
on GC electrode at various scan rates.
4.7 Oxygen Reduction in mixed LiPF6 and LiClO4 system in DMSO
Mechanism of oxygen reduction for mixed Lithium salts 0.1M (70%LiClO4+30% LiPF6, 80%
LiClO4+20% LiPF6, 90% LiClO4+10% LiPF6) in DMSO was investigated. In Figure 28 CV curves of
pure and mixed systems are compared. The value of the reduction peak current for mixed
system is lower compared to 0.1M LiPF6/DMSO and 0.1MLiClO4/DMSO but secondary
reduction peaks disappeared. Oxidation process seems to be more complex according to CV
curves. Moreover, the reduction peak currents were gradually increased with the increase of
LiClO4 concentration.
40
1,5000E-04
1,0000E-04
5,0000E-05
Current A
0,0000E+00
-5,0000E-05
70%LiClO430%LiPF6
80%LiClO420%LiPF6
90%LiClO410%LiPF6
LiPF6
LiClO4
-1,0000E-04
-1,5000E-04
-2,0000E-04
-2,5000E-04
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potential V vs Ag&Ag+
Figure 28: Cyclic voltammograms for the reduction of oxygen saturated 0.1 M Li based mixed
salt/DMSO on GC electrode at 200mVs-1 scan rate.
4.8 Oxygen Reduction in mixed LiPF6 and TBAPF6 system in DMSO
Oxygen reduction in DMSO was also investigated for mixtures of TBAPF6 and LiPF6 salts.
Mixed systems are 0.1M (80%TBA+20%20LiPF6), 0.1M (70%TBA+30%20LiPF6) and 0.1M
(50%TBA+50%LiPF6). Figure 29 displays, CV comparison of mixed TBA, LiPF6 systems with
pure TBAPF6, LiPF6 systems. The reduction peak current for 0.1M (50%TBA+50%LiPF6) is
higher compared to pure systems as well as compared to other mixed systems. On the other
hand, the reduction peak currents for other mixed systems are lower compared to pure Li
systems. However, secondary reduction peaks disappeared. Oxidation peaks are similar for
pure Li and mixed systems.
41
6,00E-04
4,00E-04
2,00E-04
Current Density (Acm-2)
0,00E+00
-2,00E-04
0.1MTBAPF6
80%TBA,20%LipF6
50%TBA,50%LiPF6
70%TBA,30LiPF6
0.1MLiPF6
-4,00E-04
-6,00E-04
-8,00E-04
-1,00E-03
-1,20E-03
-1,40E-03
-3,00
-2,50
-2,00
-1,50
-1,00
-0,50
0,00
0,50
1,00
Potential V vs Ag/ag+
Figure 29: Cyclic voltammograms for the reduction of oxygen saturated 0.1 M mixed
(LiPF6+TBAPF6 salt) in DMSO on GC electrode at 200mvs-1 scan rate.
4.8 Comparison of the oxygen reduction reaction kinetics in different
electrolytes
The ORR mechanism in TBAPF6/DMSO and in TBAPF6/MeCN is one electron reduction of
oxygen to superoxide (O2-) and subsequent re-oxidation of superoxide to oxygen. Reaction
mechanism of oxygen reduction can be described by reaction scheme shown below;
Reduction: O2 + e- ⇾ O2-
Oxidation: O2- ⇾ O2 + eIn Table 2 kinetic properties of different systems investigated in this study are compared.
According to the values of heterogeneous rate constant the oxygen reduction reaction is
quasi-reversible in DMSO and also in MeCN. However, in the presence of TBA+ ion in DMSO
42
the value of heterogeneous rate constant of oxygen reduction is higher than that in MeCN
and these calculated values are close to literature values (29). In the presence of LiPF6 and
LiClO4 oxygen reduction seems to be slower. Tafel slopes for TBA in DMSO and MeCN were
closer to 120Dec-1.
Table 4: Kinetics properties of oxygen saturated different electrolytes.
electrolyte
0,1M TBAPF6 in
Experiment
Heterogeneo
Literature
Literature value of
al diffusion
us rate
diffusion
Heterogeneous rate
coefficient
constant ko
coefficient
constant ko (cms-1)
(cm2/s)
(cms-1)
(cm2/s)
9.8×10-6
0.012
9.7×10-6 (29)
(*4×10-2 - 4×10-3)(29)
1.6×10-5
6.6×10-3
2.4×10-5(29)
(*3×10-3 - 6×10-4)(29)
DMSO
0,1M TBAPF6 in
MeCN
Oxygen reduction in mixed Li electrolytes in DMSO solvent is more complex than pure Li
systems. Several reduction peaks disappeared although the value of reduction current peaks
decreased. Oxidation process is even more complex than the reduction process in mixed
electrolyte systems.
Rate constant not only depends on diffusion coefficient but also on the peak potential
separation. Comparing the CV of TBAPF6/DMSO (figure 10) and TBAPF6/MeCN (figure 16)
with LiPF6/DMSO (figure 20 ) and LiPF6/MeCN (figure 23) it is clear that peak separation of
TBA based electrolyte is quite less than Li based electrolytes. More clear information about
the rate constant can be obtained according to the suggested zone boundaries for rate
constants by Matsuda and Ayabe’s.
Reversible Ʌ ≥15; k0= 0,3 ϑ1/2 cms-1
Quasi-reversible 15≥ Ʌ ≥10-2(1+α); 0,3 ϑ1/2 ≥2×10-5ϑ1/2 cms-1
Totally irreversible Ʌ ≤ 10-2(1+α) ; k0 ≤ 2×10-5ϑ1/2 cms-1(26).
43
Oxygen reduction rate constant value of 0.1M TBAPF6 in DMSO and MeCN is in the quasireversible range, their peak potentials are scan rate dependant. That’s why, oxygen
reduction in presence of TBA+ ion is quasi-reversible. Heterogeneous rate constant for the
ORR of oxygen in the presence of Li+ ion could not be determined, because reduction
process was accompanied with multiple reduction and chemical follow-up reactions.
Deposition of discharge product on electrode surface blocked the reaction sites which
resulted in a decrease of discharge capacity for subsequent cycles.
44
Chapter 5
Conclusions and Future works
Oxygen reduction mechanism was investigated in organic solvents which are relevant for the
rechargeable Li-air battery. Role of different supporting electrolytes i.e., TBAPF6, LiPF6 and
LiClO4 on oxygen diffusivity and reduction mechanism in DMSO and MeCN was analyzed.
Oxygen reduction and subsequent oxidation strongly influenced by the type of solvent and
also by the nature of cation. Oxygen reduction in DMSO in the case of TBA+ PF6- ions in
DMSO as well as in MeCN was quasi-reversible with fairly high heterogeneous rate constant.
This indicates a profound effect of cation interaction with the solvent on the oxygen
diffusivity. LiClO4 has pronounced drawback of forming passivation layer on electrode. It was
not possible to analyse the oxidation process due to the occurrence of multiple processes
during oxidation. Oxygen reduction in mixed supporting electrolytes (LiClO4+LiPF6) was quite
complex. Extra reduction peaks disappeared in the cyclic voltammetry of mixed salts based
electrolytes. It might be the case that mixed electrolytes may have influence in the dissolving
of reduction products.
In future studies should be performed with appropriate catalyst to improve oxidation
process. Catalyst can help to dissolve the reduction products and accelerate oxygen
reduction process. Pd mixed with MnO2 and CeO2 could be used as catalysts. As we have
seen in this work that nature of electrolytes have great influence on the oxygen reduction
process, therefore new class of non-aqueous electrolytes need to be developed. Ionic liquid
could be a good choice to improve the reversibility of the Li-air battery.
45
Appendix
1,50E-03
1,00E-03
Current A
5,00E-04
0,00E+00
oxidation
reduction
-5,00E-04
-1,00E-03
-1,50E-03
0,10
0,20
0,30
0,40
0,50
0,60
SQRT(scan rate) (Vs-1)1/2
0,70
Figure 1: Randles-Sevcik plot of peak current vs square root of the scan rate in 0.1 M
TBAPF6/MeCN. Blue rectangle line is experimental oxidation plot and red rectangle line is
experimental reduction plot.
0,45
0,4
0,35
Ψ
0,3
0,25
0,2
Ψ
0,15
0,1
0,05
0
0
20
40
(D0/DR
60
80
100
)1/4/(∏D
1/2
0vF/RT)
Figure 2: A plot of variation of wave shape factor Ψ with scan rate, in this plot k0 is the slope
of the curve for in 0.1 M TBAPF6/MeCN.
46
-1,50E-04
Current A
-2,00E-04
-2,50E-04
-3,00E-04
LiClO4/DMSO
-3,50E-04
0,25
0,30
0,35
0,40
0,45
0,50
0,55
0,60
0,65
0,70
SQRT(scan rate) (V/s)1/2
Figure 3: Peak current vs. square root of the scan rate in 0.1 M LiClO4/DMSO. Red rectangle
line is theoretical plot of one electron reduction.
47
Acknowledgements
This work has been done in the Laboratory of electrochemistry. I would like to sincerely
thank my supervisor associate professor Zareen Abbas for his constructive help in theoretical
and practical studies. Without his help it would not have been possible to successfully done
this project. I would also like to thank my examiner Professor Elisabet Ahlberg for giving me
an opportunity to work in electrochemistry group and helping me to analyze the kinetics of
reactions.
I would like to thanks PHD students Gert Göransson, Adriano S. O. Gomes and Kristopher
Hedenstedt for helping me with the instrumentation and software. Without their help it
would be difficult for me to work in the lab.
I also would like to thanks my supervisor from Volvo Technology Senior Research Engineer
Istaq Ahmed for assisting me with the requirements as well as giving me the support needed
for this project. Last but not least, I would like to special thanks to Volvo for giving me fund
for this project.
48
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