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Transcript
Frontiers in Electrochemistry
S. Chandravathanam, Research Scholar
National Centre for Catalysis Research
Department of Chemistry
Indian Institute of Technology, Madras
Orientation Programme in Catalysis for Research Scholars, 2008, 1/12/08
Contents
 Fundamentals
 Frontier Applications of Electrichemistry
 Batteries
 Fuel Cells
 Supercapacitors
 Photoelectrochemical cells
What is Electrochemistry?
- all about the study of Electrified Interfaces
and its consequences.
What is an Electrified Interface?
It is the two dimentional geometrical
boundary surface separating the two
phases.
What is an Electrified Interphase?
It is the three dimensional region of
contact between the two phases in
contact at their boundary.
Electrified Interface
Electrified Interface
Whenever an uncharged metal or electron conductor
contacts with an ionic solution manifests an excess
surface electric charge on both sides of the interphase;

- Creates a gigavolt per meter (107 V/cm)field in the interface
region, with the electroneutrality of the bulk metal.

The effect of this enormous field at the electrode
interface is the essence of electrochemistry.
Examples of Electrified Interfaces
 Why do Colloidal particles move under electric fields?
The electrified interface between the Colloidal particle
and the medium causes a potential difference in the interface,
which interacts with the externally applied electric field 
lies the basis for coating of metals.
 Is the friction between two solids in presence of liquid film
an Electrified interface?
Yes  the efficiency of a wetted rock drill depends
on the double layer structure at the metal/drill/aqueous
solution interface.
 The mechanism by which a nerves carry messages from
brain to muscles is based on the potential difference across
the membrane that separates a nerve cell from the
environment.
What is an Electrochemical reaction?
- it is the chemical transformation involving the
transfer of electrons across an interface.
Examples are,
2H+ + 2e  H2
C2H4 + 4 H2O  2 CO2 + 12 H+ + 12 e
Remarkable distinction from the chemical reaction
is the controlled way in which a chemical substance
produce another substance.
Batteries
What are Batteries?
- Electrical Energy Storage Device
- Store the electricity produced else where by driving the
charging reaction through the free energy hill by splitting the
reaction in two parts, each to take place on each electrode.
- as soon the electrodes are connected the charged reactants are
ready to react together to down the free energy hill by discharging
the electricity.
HISTORY OF BATTERIES
1800
1836
1859
1868
1888
1899
Voltaic pile: silver zinc
Daniell cell: copper zinc
Planté: rechargeable lead-acid cell
Leclanché: carbon zinc wet cell
Gassner: carbon zinc dry cell
Junger: nickel cadmium cell
HISTORY OF BATTERIES
1946
1960s
1970s
1990
1991
1992
1999
Neumann: sealed NiCd
Alkaline, rechargeable NiCd
Lithium, sealed lead acid
Nickel metal hydride (NiMH)
Lithium ion
Rechargeable alkaline
Lithium ion polymer
Battery Types
 Non-Chargeble (Disposable) Batteries - Primary
 Chargeble Batteries
- Secondary
Primary (Disposable) Batteries





Leclanché Cells (zinc carbon or dry cell)
Alkaline Cells
 Mercury Oxide Cells
 Zinc/MnO2 Cells
 Aluminum / Air Cells
Lithium Cells
 Liquid cathode lithium cells
 Solid cathode lithium cells
 Solid electrolyte lithium cells
Lithium-Iron Cells
Magnesium-Copper Chloride Reserve Cells
Secondary (Rechargeable) Batteries

Lead–acid Cells

Zinc/MnO2 Cells (Mechanical Recharging)

Nickel/Cadmium Cells

Nickel/Metal Hydride (NiMH) Cells

Lithium Ion Cells

Rechargeable Alkaline Manganese Cells
Alkaline Cells
Half cell reactions
Zn + 2 OH- —> ZnO + H2O + 2 e2 MnO2 + H2O + 2 e- —>Mn2O3 + 2 OH-
The overall reaction
Zn + 2MnO2 —> ZnO + Mn2O3 E = 1.5 V
Applications: Radios, toys, photo-flash applications, watches
Storage density about twice that of the carbon-zinc cell, but more expensive
Lead–acid Cells
Applications: Motive power in cars, trucks, standby/backup systems
Can be recharged hundreds of times and very cheap, but bulky
and environmentally noxious
Zinc/Air Cells
Anode: Amalgamated zinc powder
Cathode: Oxygen (O2)
Electrolyte: Potassium hydroxide (KOH)
Half-reactions:
Zn + 2OH- —> Zn(OH)2
1/2 O2 + H2O + 2e —> 2 OHOverall reaction:
2Zn +O2 + 2H2O —> 2Zn(OH)2 E = 1.65 V
Applications: Hearing aids, pagers, electric vehicles
Lithium ion Cells
Charging
Discharging
Anode: lithium ions in the carbon material
Cathode: lithium ions in the layered material (lithium compound)
Anode
Li1-XCoO2+ CnLix  LiCoO2 + Cn
Cathode
LiCoO2+ Cn  Li1-XCoO2 + CnLix
The lithium ion moves from the anode to the cathode during
discharge and from the cathode to the anode when charging.
Applications: Laptops, cellular phones, electric vehicles
Charge/discharge curve for Lead – acid Battery
Real time graph charging the Pb-acid
battery battery
Echarge = Erev +  + IR
Edischarge = Erev -  - IR
Behavior of the battery at different
discharging rate Pb-acid battery; 100
mA (1), 200 mA (2) and 300 mA (3)
Comparison of some Batteries
Battery Type
Specific
Energy
(Wh/kg)
Specific
Power
(W/kg)
Life Cycles
Application
Lead acid
35 – 40
180
300 - 400
as a Booster power for
start-up in internal
combustion engine
Nickel cadmium
45 - 55
150
700 - 1200
Toys
Zn - MnO2
8 - 64
25
Most of solid state
devices like hearing aid,
flash light batteries,
portable TV, computer,
etc.
Zn - Air
200
30
Mechanically Automative application
rechargable
Ni - MH
150-200
250-1000
700 – 1200
Automative application
Li ion
100-200
400-1200
Laptops, cell phones
Fuel Cells
History of Fuel Cells
Discovery – Sir William Grove – a
British Judge (1839)
Sir William Grove
Rediscovery – Francis Thomas Bacon
– an Engineer working in a turbine
Company (1932) – behind NASA’s use
of fuel cells in space flights (as
auxillary power source for low weight/
unit of energy)
Francis Thomas Bacon
W.R. Grove, On Voltaic Series and the Combination of Gases by Platinum;
Phil. Mag. XIV, 127-130 (1839)
What are Fuel Cells?
- are energy conversion devices, convert the
free energy change of a chemical reaction directly
into electricity (electrochemical energy
conversion) and not as heat in a chemical reaction.
Comparison of Fuel Cells with Internal Combustion
Engines
ICE-2
Thermal Energy
Mechanical Energy
ICE-3
ICE-1
Chemical energy of fuels
Fuel Cell
Electrical Energy
Schematic of energy conversion in Fuel cells and
Internal Combustion Engines (ICE)
Comparison of Batteries and Fuel Cells
Batteries – Energy Storers
(Utilize the electricity produced else where to drive
the charging reaction through the free energy hill).
(Effectiveness of batteries encompasses situations where it
would be impractical to store fuel to make electricity on the
spot, for example in portable equipments like telephones, tape
recorder etc.)
Fuel Cells – Energy generators
( Electricity is generated as a result of spontaneous
chemical reaction spilt into two half reactions)
Types of Fuel Cells
- Space application
- Transportation applications
- high volumetric energy
density
- avoids the need of pure H2
- envisaged for
stationary power
plants
Fuel Cell Efficiency
-G = Wrev - PV
Wrev – Useful work
PV – Work of expansion
In Fuel cells, no moving parts andso no work of expansion
 -G = Wrev
For an electrochemical reaction,
Overall reaction
2H2 4H+ +4e
O2 +4H+ + 4e  2H2O
2H2 + O2  2H2O
The electrical work in transporting these 4e across the potential
difference Ve, = 4FVe
Ve – thermodynamic equillibrium potential of the reaction
-G = 4FVe
For an n electron transport,
-G = nFVe
Maximum amount of useful
electrical work obtainable from a
chemical reaction
or
Intrincically available electrical
work of a chemical reaction
But H is the total energy change of the reaction,
including the the entrophy change for ordering and
disordering of reactants and products.
Efficiency of electrochemical energy conversion = G / H
= -nFVe / H
is not 100% efficient.
But has the theoretical maximum of 90%;
But heat engine has the theoretical maximum of 25 – 40 %,
based on the workable temperature range.
Efficiency of heat engine = (T1 – T2) / T1
Performance limitations of Fuel Cells

Activation
polarization

Ohmic polarization

Mass-transport
polarization
Current-Potential curve for H2 - Air fuel cell at 80 °C
the practical obtainable maximum energy conversion efficiency
~ 65% ( 2 times that of heat engine)
Why Pt ?
> go. > 0
> go. > 0
H - 0
H - 1
M – H Bond Strength, KJ /mol
Relationship between current densities for hydrogen
evolution and M – H Bond Strength
13
Standard Free Energy, Enthalphy Change and Maximum efficiency
for few possible Fuel Cell Reactions
Fuel
Reaction
Hydrogen H2 + ½ O2  2H2O
Ve (V)
-G°
-H
(kJ/mol) (kJ/mol)
Max.
Efficiency
(%)
56.69
68.32
1.229
83
Methane
CH4 + 2O2  CO2 + 2H2O
195.50
212.80
1.060
92
Methanol
CH3OH + 3/2O2  CO2 +
2H2O
168.95
182.61
1.222
93
Advantages of Fuel Cells
 Higher intrinsic efficiency
 Lesser CO2 accumulation in the atmosphere
Second Fuel Cell Principle – Electroregenerative Synthesis of
materials
Advantage
- energy production is the by-product
Eg., Electroregenerative synthesis of dichloroethylene
Anode reaction,
C2H2 + 2Cl-  C2H4Cl2 + 2e
Cathode reaction,
Cl2 + 2e  2Cl-
Super Capacitors
What are Supercapacitors?
- are the electrochemical storage devices, storing electricity
in the form of Electrochemical double layer.
- different from batteries (elctricity stored as chemical), or
dielectric capacitors or parallel plate condensors (electricity
is stored electrostatically in a dielectric material between
two metal plates).
Models of the Double Layer Structure of Electrified Interface
a) Helmholtz model b) Gouy-Chapman model of the
diffuse layer c) Stern's model, combining (a) and (b)
Types of Capacitors
Schematic of different ways of electricity storage
Comparison of Supercapacitors with Batteries
Supercapacitors have very
 high Specific Power of 102 kW/Kg (100 - 1000 times
higher than batteries),
 uncomparable cycle life of 105,
 less Specific Energy ( 40 Wh/Kg)
 store and deliver electricity by electrostatic charging
takes place at the two dimensional interface without any
irreversible or slow chemical phase change,  exhibit fast
charging and longer cycle life.
 no serious disposal and safety hazard
Ragone plot for various energy storage and conversion devices
Ragone plot showing energy density vs. power density
for various devices along with discharge time.
Capacitance of the Capacitors
The capacity of the parallel plate condensor
C (in farads or coulombs per volt) = A ε / 4 π d
A- Area of the contact plates
d- distance between the plates
ε – dielectric constant of the medium between the plates
The relation between capacitance "C" and the inter-plate voltage "V"
arises from accumulation of a charge "q“ is,
C = q/V or q = CV
Capacitance of the Double-layer Capacitor
The charge density "q" (coulomb/cm2) of electrons and ions at the
interface is dependent on the potential difference, ΔΦ, across this
double layer so that a differential capacitance "Cdl" arises, is
determined by,
Cdl = dq/d(ΔΦ) or Δq/ΔΦ
The difference of potential extends beyond the immediate layer of
solvated ions in the compact, capacitor-like (Helmholtz) region, out
into solution, so that a further diffuse-layer capacitance "Cdiff"
arises. It combines with the capacitance of Helmholtz region "CH"
in series, electrically, so that,
1
— =
Cdl
1
1
— +—
CH Cdiff
Applications of Supercapacitance
 Booster for hybrid vehicles with fuel cell or battery during
start-up or acceleration.
 Regenerative braking can be used to charge the
Supercapacitor for its fast charging rate.
Pseudocapacitance
- Double-layer capacitance "C" or "Cdl" is non-faradaic or
electrostatic .
Pseudocapacitance "CΦ“ is faradaic (Capacitance due to charge
transfer process)
- when the extent of faradaically admitted charge "q" depends
linearly, or approximately linearly, on the applied voltage "V". For
such a situation, there is a mathematical derivative, dq/dV that would be
constant, which is equivalent to, and measurable as, a capacitance.
- The pseudocapacitance can increase the capacitance of an
electrochemical capacitor by as much as an order of magnitude over that
of the double-layer capacitance.
Pseudocapacitance of RuO2
cyclic voltammetry behavior of a
reversibly chargeable electrochemical
capacitor material RuO2
Limitation of Capacitance of Double layer Capacitor
- charging of the high-area, porous-electrode structures that are
required for achieving large capacitance densities (farads/g)
encounters limitations of rate due to the distributed electrolytic and
contact resistances within the pore structure of such materials.
Photoelectrochemical Cells
What is Photoelectrochemistry?
- Generation of current following the
exposure of Semiconductor electrodes
to electromagnetic radiation.
Generation of bands in solids from atomic orbitals of isolated atoms
- metals do not absorb solar radiation.
- insulators also cannot absorb as the band gap is so
high (> 5 eV), the energy of the solar radiation is not
sufficient to excite electron from valence band (VB) to
the conduction band (CB).
- Semiconductors have the band gap not as large, 
promotion of electron is possible with the solar radiation.
Schematic diagram of the energy levels of an a)
intrinsic semiconductor, b) an n-type semiconductor
and c) a p-type semiconductor
(a)
- Charge carriers in Semiconductor can be
altered by doping.
(b)
(c)
- Addition of Group V element (P. As) into
Group IV element (Si, Ge) introduces
occupied energy levels into the band gap
close to the lower edge of CB, thereby
allowing facile promotion of electrons into
the CB (n-type Si, or n-type Ge; majority
charge carriers - e).
- Addition of Group III elements (Al. Ga) into
Group IV elements introduces vacant energy
levels into the band gap close to the upper
edge of the VB, which allows the facile
promotion of e from the VB (p-type Si, or ptype Ge; majarity charge carrier - holes).
Fermi level is defined as the energy level at which the probability of
occupation by an electron is ½;
- for an instrinsic semiconductor the Fermi level lies at the mid-
point of the band gap.
- Doping changes the distribution of electrons within the solid, and
hence changes the Fermi level.
- For a n-type semiconductor, the Fermi level lies just below the
conduction band, whereas for a p-type semiconductor it lies just
above the valence band.
- In addition to doping, as with metal electrodes, the Fermi level of
a semiconductor electrode varies with the applied potential; for
example, moving to more negative potentials will raise the Fermi
level.
Model of the Semiconductor-Electrolyte interphase
Metal-Electrolyte
Interface
Idealized interface between a semiconductor electrode / electrolyte
solution.
 If the redox potential of the solution and the Fermi level do not lie at
the same energy, movement of charge between the semiconductor
and the solution takes place in order to equilibrate the two phases.
 Excess charge located on the semiconductor does not lie at the
surface as it would for a metallic electrode, but extends into the
electrode for a significant distance (100-10,000 Å) - space charge
region.
 Hence, there are two double layers to consider: the interfacial
(electrode/electrolyte) double layer, and the space charge double
layer.
For an n-type semiconductor electrode at open
circuit, the Fermi level is higher than the redox
potential of the electrolyte, hence electrons will be
transferred from the electrode into the solution
 positive charge associated with the space
charge region, and is reflected in an upward
bending of the band edges  as majority charge
carrier is removed from this region, this region is
referred to as a depletion layer.
Band bending for an ntype emiconductor (a)
and a p-type
semiconductor b) in
equilibrium with an
electrolyte
For a p-type semiconductor, the Fermi layer is
lower than the redox potential, hence electrons
must transfer from the solution to the electrode
 generates negative charge in the space
charge region, causes a downward bending in
the band edges. Since the holes in the space
charge region are removed by this process, this
region is again a depletion layer.
Mechanism of production of photocurrent by an p-type
photocathode
Mechanism of production of photocurrent by an n-type photoanode
Intensity of Solar Energy Absorbtion by Semiconductors of different
band gaps energies
- low band gap materials absorb more of solar
radiation, but are easily photodegradable.
Applications of Photoelectrochemistry
 Substitution of gasoline and natural gas by H2 produced from
photoelectrochemical splitting of water; Solving CO2 build-up.
 Carrying out commercially important organic reactions (e.g.,
oxidation of toxic wastes, Kolbe-reaction, etc.)
Summary
 Electrochemistry leads to the sustainable
Future Energy Technologies (production,
Storage, Conversion and Application) .
References
1. Modern Aspects of Electrochemistry, J. O’M. Bockris and
A. K. N. Reddy, Kluwer Academic, 2000.
2. Electrochemistry, Prof. B. Viswanathan et al.,
S.Viswanathan Publishers, 2007
3. Electrochemistry of Semiconductors, Adrian W. Bott,
Current Separations 17 (1998) 87 – 91.
4. Electrochemical capacitors, Brian E. Conway,
http://electrochem.cwru.edu/ed/encycl
Thank You