frontiers in electrochemistry s. chandravathanam, research scholar national centre for catalysis...
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Frontiers in Electrochemistry
S. Chandravathanam, Research Scholar
National Centre for Catalysis ResearchDepartment of Chemistry
Indian Institute of Technology, Madras
Orientation Programme in Catalysis for Research Scholars, 2008, 1/12/08
Fundamentals
Frontier Applications of Electrichemistry
Batteries
Fuel Cells
Supercapacitors
Photoelectrochemical cells
Contents
- 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.
What is Electrochemistry?
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.
Electrified Interface
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.
- 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.
What is an Electrochemical reaction?
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 Voltaic pile: silver zinc
1836 Daniell cell: copper zinc
1859 Planté: rechargeable lead-acid cell
1868 Leclanché: carbon zinc wet cell
1888 Gassner: carbon zinc dry cell
1899 Junger: nickel cadmium cell
1946 Neumann: sealed NiCd1960s Alkaline, rechargeable NiCd1970s Lithium, sealed lead acid1990 Nickel metal hydride (NiMH)1991 Lithium ion1992 Rechargeable alkaline1999 Lithium ion polymer
HISTORY OF BATTERIES
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
Applications: Radios, toys, photo-flash applications, watches
Half cell reactionsZn + 2 OH- —> ZnO + H2O + 2 e-
2 MnO2 + H2O + 2 e- —>Mn2O3 + 2 OH-
The overall reactionZn + 2MnO2 —> ZnO + Mn2O3 E = 1.5 V
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 OH-
Overall reaction:
2Zn +O2 + 2H2O —> 2Zn(OH)2 E = 1.65 V
Applications: Hearing aids, pagers, electric vehicles
Charging Discharging
Lithium ion Cells
Applications: Laptops, cellular phones, electric vehicles
Anode: lithium ions in the carbon material Cathode: lithium ions in the layered material (lithium compound)
Cathode
LiCoO2+ Cn Li1-XCoO2 + CnLix
Anode
Li1-XCoO2+ CnLix LiCoO2 + Cn
The lithium ion moves from the anode to the cathode during
discharge and from the cathode to the anode when charging.
Real time graph -
charging the Pb-acid
battery battery
Behavior of the battery at different
discharging rate Pb-acid battery; 100
mA (1), 200 mA (2) and 300 mA (3)
Charge/discharge curve for Lead – acid Battery
Echarge = Erev + + IR
Edischarge = Erev - - IR
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 rechargable
Automative application
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)
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
Sir William Grove
W.R. Grove, On Voltaic Series and the Combination of Gases by Platinum; Phil. Mag. XIV, 127-130 (1839)
- 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.
What are Fuel Cells?
Chemical energy of fuels Electrical Energy
Thermal Energy Mechanical Energy
Fuel Cell
ICE-1
ICE-2
ICE-3
Comparison of Fuel Cells with Internal Combustion Engines
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
- Transportation applications
- Space application
- avoids the need of pure H2
- envisaged for stationary power plants
- high volumetric energy density
Fuel Cell Efficiency
-G = Wrev - PV
Wrev – Useful work
PV – Work of expansionIn Fuel cells, no moving parts andso no work of expansion
-G = Wrev
2H2 4H+ +4eO2 +4H+ + 4e 2H2O
Overall reaction 2H2 + O2 2H2O
For an electrochemical reaction,
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
Current-Potential curve for H2 - Air fuel cell at 80 °C
Activation polarization
Ohmic polarization
Mass-transport polarization
the practical obtainable maximum energy conversion efficiency ~ 65% ( 2 times that of heat engine)
Relationship between current densities for hydrogen
evolution and M – H Bond Strength
Why Pt ?Why Pt ?
13
M – H Bond Strength, KJ /mol
> go. > 0
H - 0
> go. > 0
H - 1
Fuel Reaction -G° (kJ/mol)
-H (kJ/mol)
Ve (V)Max. Efficiency (%)
Hydrogen H2 + ½ O2 2H2O 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
Standard Free Energy, Enthalphy Change and Maximum efficiency
for few possible Fuel Cell Reactions
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).
a) Helmholtz model b) Gouy-Chapman model of the
diffuse layer c) Stern's model, combining (a) and (b)
Models of the Double Layer Structure of Electrified Interface
Schematic of different ways of electricity storage
Types of Capacitors
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 1 1
— = — + —
Cdl 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.
cyclic voltammetry behavior of a
reversibly chargeable electrochemical
capacitor material RuO2
Pseudocapacitance of 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.
- 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.
Generation of bands in solids from atomic orbitals of isolated atoms
- Charge carriers in Semiconductor can be altered by doping.
- 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).
(a)
(b)
(c)
- 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 p-type Ge; majarity charge carrier - holes).
Schematic diagram of the energy levels of an a)
intrinsic semiconductor, b) an n-type semiconductor
and c) a p-type semiconductor
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.
Band bending for an n-
type emiconductor (a)
and a p-type
semiconductor b) in
equilibrium with an
electrolyte
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.
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