Electrochemical energy

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Content – Forms of energy

 


 
Electrochemical energy is what we normally call the conversion of chemical energy into electrical energy or vice versa. This includes reactions transferring electrons, redox reactions (reduction- oxidation).

Reduction, when a substance receives one electron. Oxidation when a substance gives away one electron.

There always has to be a balance of substances that give away and substance that receives electrons since electrons cannot exist on their own without any bindings. This means that if a reduction is talking place also an oxidation has to take place.

Example “Redox” process

Reduction: Cu2+ + 2e- → Cu (Copper)
Oxidation: (Zink) Zn → Zn2+ + 2e-

Electrochemical cells

There are basically to types of cells used for electrochemical conversion.
1) The galvanic cell that converts chemical energy into electrical energy, by a spontaneous reaction.
2) The electrolytic cell that converts electrical energy into chemical energy.
Electrical energy is used to fuel the reaction.

Electrolysis

A process whereby electrical energy is converted directly into chemical energy is called electrolysis; i.e., an electrolytic process. By virtue of their combined chemical energy, the products of an electrolytic process often react spontaneously with one another, reproducing the substances that were reactants and were therefore consumed during the electrolysis.

Electrochemical energy storage

Electrochemical energy storage is a method used to store electricity in a chemical form. This storage technique benefits from the fact that both electrical and chemical energy share the same carrier, the electron. This common point allows limiting the losses due to the conversion from one form to another.

Common forms for electrochemical storage and conversion

  • Batteries and accumulators
  • Capacitors
  • Fuel cells

Batteries and accumulators

Lead-acid accumulator:

Used for many purposes in particular in road vehicles such as automobiles, trucks, buses etc.

Typical reaction of a lead acid accumulator:
Pb(solid) + PbO2(solid) + 2 H2SO4(liquid) → 2 PbSO4(solid) + 2 H2O(liquid)

Discharge →
← Charge

Dry cell battery

Numerous applications among others for home appliances such as flash lights and small electronics.

The “dry cell” is not really a dry cell since the electrolyte used is NH4CI paste.

Typical reaction of dry cell battery:
Zn → Zn2+ + 2e–

2 MnO2 + 2H+ + 2e– → Mn2O3 + H2O

Lithium cell battery

Used in various appliances such as cameras, wristwatches, power tools and different types of other electronics. More recently also used as energy supply/storage for electrical automobiles.

They come in both non-rechargeable and chargeable versions.

Modern lithium cells operate by transporting Li+ ions between electrodes into which the ions can be inserted or intercalated. Cathodes are lithium transition-metal oxides such as LiCoO2, while anodes are lithium-containing carbon, LiC6. The species that undergoes oxidation-reduction is not lithium, but the transition metal.

Typical reaction of a lithium cell battery:
C + LiCoO2 ↔ LiC6 + Li0.5CoO2

Various materials are being used and tested to overcome lifetime issues and to increase the performance and capacity of the Lithium cells.

Capacitors

A capacitor or a condenser is an electrical component used to store energy electrostatically. There are many forms of capacitors.

All capacitors contain two or more conductor plates separated by an insulator that can store the energy (a dielectric material). A capacitor stores energy in the form of an electrostatic field between the plates.

The prime use for capacitors is in electronic circuits for blocking direct current while allowing alternating current to pass or in electric power transmission systems, where they will stabilize the voltage and the power flow.

If there is a potential difference across the capacitor`s conductors (e.g., when a capacitor is attached across a battery), an electric field develops across the dielectric, causing positive charge to collect on one plate and negative charge to collect on the other. If a battery has been attached to a capacitor for a sufficient amount of time, no current can flow through the capacitor. If a time-varying voltage is applied across the leads of the capacitor, a displacement current can flow.

Energy of an electric field

The work done in establishing the electric field, and hence the amount of energy stored, is:

W= \dfrac{1}{2}CV^2 + \dfrac{1}{2}VQ

Q = Charge stored
V = Voltage across the capacitor
C = Capacitance

Fuel cells

Fuel cells are different from batteries in that they require a continuous source of fuel and oxygen or air to sustain the chemical reaction, whereas in a battery the chemicals present in the battery react with each other to generate electricity. Batteries either has to be replaced or recharged when discharged.

Fuel cells can produce electricity continuously for as long as these inputs are supplied.

In most cases fuel cell refers to a reactor where hydrogen ions are transferred between the eletrodes. The fuel would be hydrogen or another hydrogen rich substance such as hydrocarbons; diesel, methanol or another natural gas component.

The anode and cathode contain catalysts that cause the fuel to undergo oxidation that generate positive hydrogen ions and electrons. The hydrogen ions are drawn through the electrolyte after the reaction. At the same time, electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. At the cathode, hydrogen ions, electrons, and oxygen react to form water (H2O).