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