Dr. Vishnumurthy K A

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ELECTROCHEMICAL ENERGY SYSTEMS
Introduction
Electrochemistry is the branch of science, which deals with transformation of chemical
energy into electrical energy and vice versa. Electric current is a flow of electrons generated
by a battery, when the circuit is completed. A substance which allows electric current to pass
through is called a conductor, e.g., all metals, graphite, fused salts aqueous solutions of acids,
bases and salts; while insulator or non-conductor is a substance which does not conduct the
electric current. The conductors are of two types
1.Metallicconduction
2.Electrolytic induction
Metallic conduction
1. It involves the flow of electrons in a conductor.
2. No change in chemical properties of the conductor as a result of metallic conduction
3. It does not involve any transfer of matter.
4. Generally, metallic conduction shows an increase in resistance as the temperature is
raised.
Electrode Potential
When a metal is placed in the solution of its own salt, then oxidation or reduction takes place. Each electrode usually consists of a metal in contact with a solution of its own ion. Since a cell is a combination of two electrodes, each electrode is referred to as a single electrode or half- cell. A layer of positive ions or negative ions is formed around the metal. This layer is called as Helmholtz electrical double layer. A difference of potential is consequently set up between the metal and the solution. The potential difference will persist as long as the charge is allowed to remain on the metal. At equilibrium the potential difference between the metal and solution becomes a constant value. The equilibrium potential difference so established is
Thus standard electrode potential of a metal is the measure of tendency of a metallic
Measurement of electrode potential:
It is impossible to know the absolute value of a single electrode potential.
We can only determine the relative value of electrode potential if we can fix
arbitrarily the potential of one electrode. For this purpose the potential of
standard hydrogen electrode (SHE) has been arbitrarily fixed as zero.
This electrode is represented as Pt, H2 (1atm); H+(1M) Whenever,
the potential of any electrode say M|Mn+ is to be measured experimentally,
it is combined with the standard hydrogen electrode
Galvanic cell

Calomel electrode

The calomel electrode consists of a glass vessel containing a layer
of Hg over which is placed a paste of an Hg, Hg2Cl2 and KCl.
Above this there is a solution of KCl saturated with the Mercurous salt.
A platinum wire is fused in the glass tube for electrical connection.
A salt bridge is used to couple with other half-cell.





Nernst equation
A quantitative relationship between electrode potential and concentration of the substance involved.
This expression is known as Nernst equation.

 

 

 

 

ION SELECTIVE ELECTRODE
Possesses the ability to respond only to certain specific ions, thereby developing potential w.r.t that species only in a mixture and ignoring the other ions totally. In other words, the potential developed by an ion-selective electrode depends only on the concentration of species or ions of interest. For egs. Glass membrane is only H+ ions selective.
Glass electrode

 

 

 

 

 

 

 

MEMBRANE POTENTIAL AND NERVE CONDUCTION
You may recall the phenomena of osmosis and osmotic pressure that are observed when two solutions having different solute concentrations are separated by a thin film or membrane whose porosity allows small ions and molecules to diffuse through, but which holds back larger particles. If one solution contains a pair of oppositely-charged ionic species whose sizes are very different, the smaller ions may pass through the semipermeable membrane while the larger ones are retained. This will produce a charge imbalance between the two solutions, with the original solution having the charge sign of the larger ion. Eventually the electrical work required to bring about further separation of charges becomes too large to allow any further net diffusion to take place, and the system settles into an equilibrium state in which a constant potential difference (usually around a volt or less) is maintained. This potential difference is usually called a membrane potential or Donnan potential after the English chemist who first described this phenomenon around 1930.
Origin of Membrane Potential : If the smaller ions are able to diffuse through the membrane but the larger ions cannot, a potential difference will develop between the two solutions. This membrane potential can be observed by introducing a pair of platinum electrodes

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Lithium ion battery
Lithium-ion batteries were designed to overcome the safety problems associated with the highly reactive properties of Lithium metal. The essential feature of the Lithium ion battery is that at no stage in the charge-discharge cycle should there be any Lithium metal present. Rather, Lithium ions are intercalated into the positive electrode in the discharged state and into the negative electrode in the charged state and move from one to the other across the electrolyte. Lithium-ion batteries thus operate based on what is sometimes called the "rocking chair" or "swing" effect. This involves the transfer of Lithium ions back and forth between the two electrodes. The anode of a Lithium-ion battery is composed of Lithium, dissolved as ions,
into a carbon or in some cases metallic Lithium. The cathode material is made up from Lithium liberating compounds, typically the three electro-active oxide materials, Lithium Cobalt-oxide LiCoO2, Lithium Manganese-oxide LiMn2 O4 , and Lithium Nickel-oxide LiNiO2 Lithium salt constitutes the electrolyte. The origin of the cell voltage is then the difference in free energy between Li + ions in the crystal structures of the two electrode materials.
Lithium-ion cells have no memory effect and have long cycle life and excellent discharge performance. For safety reasons, charge control circuitry is required for virtually all Lithiumion applications. Lithium-ion technology uses a liquid or gel type electrolyte. This cell chemistry and construction permits very thin separators between the electrodes which can consequently be made with very high surface areas. This in turn enables the cells to handle very high current rates making them ideal for use in high power applications. Some early cells used flammable active ingredients which required substantial secondary packaging to safely contain these potentially hazardous chemicals. This additional packaging not only increased the weight and cost, but it also limited the size flexibility. Modern cell chemistries and additives have essentially eliminated these problems.
Lithium Cobalt LiCoO2:
Lithium Cobalt is a mature, proven, industry-standard battery technology that provides long cycle life and very high energy density. The polymer design makes the cells inherently safer than "canned" construction cells that can leak acidic electrolyte fluid under abusive conditions. The cell voltage is typically 3.7 Volts. Cells using this chemistry are available from a wide range of manufacturers. The use of Cobalt is unfortunately associated with environmental and toxic hazards

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FUEL CELLS

DEFINITION:
A fuel cell is an electrochemical device, which can continuously convert the chemical energy of a reducing agent and an oxidant fuel stored externally by a process involving an essentially invariant electrode electrolyte system.
A fuel cell consists of two electrodes and an electrolyte. However, the fuel and the oxidizing agents are continuously and separately supplied to the two electrodes of the cell, at which they undergo reactions. These cells are capable of supplying current as long as they are supplied with the reactants.
A fuel cell may be represented as : Fuel/electrode/electrolyte/electrode/oxidant
At the anode fuel undergoes oxidation and at the cathode oxidant undergoes reduction.

Differences between a battery and a fuel cell:
Battery Fuel cell
1. Store chemical energy. Do not store chemical energy.
2. Reactants are within the cell. Reactants are supplied continuously.
3. Products remain in the cell. Products are continuously removed from the cell

ADVANTAGES:
Savings in fossil fuels due to the high efficiency of electrochemical energy conversion.
Low pollution level, no noxious exhaust gases formed.
Production of water of drinking quality in hydrogen-oxygen system.
only a small number of moving parts.
Low noise level.
Low maintenance, exchangeable parts.
No need of charging.
Theoretically, the efficiency can be 100%. In practice, the efficiency is 50-80% which is high compared to conventional methods

DISADVANTAGES:
High initial cost of the system (catalyst, membranes etc).
Large weight and volume of gas fuel storage systems.
High price of clean hydrogen.
Lifetimes of the cells are estimated but not accurately known (40,000hrs for acidic and 10,000hrs for alkaline cells).
Present lack of infrastructure to distribute hydrogen.

CLASSIFICATION:
They are classified as

1. Indirect fuel cells: Use organic fuels or biochemical substance, which is decomposed by using enzymes to a simple fuel like hydrogen. Egs. Reformer fuel cells and biochemical fuel cells Direct fuel cells: The products of the reaction are discarded.

2. Direct fuel cells are classified as

• Low temperature fuel cell: egs H2-O2, N2 compound –O2 and Metal-oxygen fuel cells etc [<100oc]

• Intermediate temperature fuel cell: egs. Organic compound - O2 and H2-O2 fuel cells etc [100-500oc]

• High temperature fuel cell: egs. H2-O2, CO-O2 fuel cells etc [500-1000oc]

• Very high temperature fuel cell: egs, H2-O2, CO-O2 fuel cells etc [> 1000oc]

CLASSIFICATION BASED ON THE TYPE OF ELECTROLYTE USED:

1.Alkaline fuel cells:
Alkaline fuel cells use an aqueous solution of potassium hydroxide as electrolyte. The electrodes can be built from low-cost carbon and plastics. They are used in emergency and portable power generation.

2.Phosphoric acid fuel cells:
These fuel cells use 98% H3PO4 as electrolyte. Platinum particles dispersed on carbon act as catalyst at both anode and cathode. They are used to provide light and heat in large buildings.

3.Molten carbonate fuel cells:
Here, molten mixture of carbonates is used as electrolyte. The anode is porous Ni and cathode is porous NiO. These cells find use in chlor alkali and aluminium industries.

4. Solid oxide fuel cells:
In solid oxide fuel cells, Y2O3 - stabilized ZrO2 is used as the electrolyte. The anode is Co-ZrO2 or Ni-ZrO2. The cathode is Sr-doped LaMnO3. These cells are suitable for electric vehicles.

5. Solid polymer electrolyte fuel cells:
These cells use a polymer membrane as electrolyte. The electrodes are made of porous carbon impregnated with Pt

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