# Electrical Storage¶

Cells can either be:

• Galvanic - they produce electricity from chemical reactions
• Electrolytic - they produce chemical reactions from electricity

In power storage, we consider the energy density, rather than energy alone, that is that we look at $$\frac{Wh}{kg}$$.

Energy $$1Wh=3600\:J$$

Power $$1W=\frac{3600J\:}{60\min}=\frac{1\:J}{s}$$

Electrical storage fits primarily into three categories:

## Batteries¶

Utilise a controlled chemical reaction with finite reactants to produce electricity. They can be primary - (non rechargeable) or secondary (rechargeable). They can be single or multi cell, with each cell having the following components:

### Anode¶

The negative electrode. This gives up electrons to the circuit as it is oxidised

### Cathode (think cation - positive)¶

The positive electrode. This accepts electrons from the circuit and is reduced

### Electrolyte¶

Provides the medium for the charge transfer, as ions in the battery that will migrate between the cathode and the anode

### Separator¶

Keeps the two electrolytes from binding and forming a non reactive salt

Primary battery - Leclanché cell

#### Anode¶

$$\ce{Zn + 2NH4Cl + 2OH- -> Zn(NH3)2Cl2 + 2H2O + 2e-}$$

#### Cathode¶

$$\ce{2MnO2 + 2H2O + 2e- -> 2MnO(OH)2 + 2OH-}$$

#### Electrolyte¶

$$\ce{NH4Cl}$$ and/or $$\ce{ZnCl2}$$ in water

Primary battery - Alkaline cell

#### Anode¶

$$\ce{Zn + 2OH- -> Zn(OH)2 + 2e-}$$

#### Cathode¶

$$\ce{2MnO2 + 2H2O + 2e- -> 2MnO(OH)2 + 2OH-}$$

#### Electrolyte¶

Aqueous $$\ce{KOH}$$

Primary battery - Lithium Metal

#### Anode¶

$$\ce{Li -> Li+ + e-}$$

#### Cathode¶

$$\ce{MnO2 + Li+ + e- -> LiMnO2}$$

#### Electrolyte¶

Lithium salts in organic solvents

#### Anode¶

$$\ce{Pb + H2SO4 <=> PbSO4 + 2H+ + 2e-}$$

#### Cathode¶

$$\ce{PbO2 + H2SO4 + 2H+ + 2e- <=> PbSO4 + 2H2O}$$

#### Electrolyte¶

Aqueous $$\ce{H2SO4}$$

#### Anode¶

$$\ce{Cd + 2OH- <=> Cd(OH)2 + 2e-}$$

#### Cathode¶

$$\ce{2NiO(OH) + H2O + e- <=> Ni(OH)2 + OH-}$$

#### Electrolyte¶

Aqueous $$\ce{KOH}$$

Secondary battery - Nickel metal hydride

#### Anode¶

$$\ce{MH + OH- <=> H2O + M + e-}$$

#### Cathode¶

$$\ce{NiO(OH) + H2O + e- <=> Ni(OH)2 + OH-}$$

#### Electrolyte¶

Aqueous $$\ce{KOH}$$

Secondary battery - Lithium Ion

#### Anode¶

$$\ce{Li_xC6 <=> 6C + xLi+ + xe-}$$

#### Cathode¶

$$\ce{Li_{(1-x)}MO2 + xLi+ + xe- <=> LiMO2}$$

#### Electrolyte¶

Organic electrolyte

Secondary battery - Lithium Sulphur

#### Anode¶

$$\ce{Li <=> Li+ + e-}$$

#### Cathode¶

$$\ce{S_8 + 16e- <=> 8S^{2-}}$$

#### Electrolyte¶

Organic electrolyte

## Fuel Cells¶

Ae fundamentally similar to batteries, with the primary exception that they require a constant flow of the fuel, rather than having a self contained reserve.

Fuel Cell - Proton Exchange Membrane

#### Anode¶

$$\ce{2H2 -> 4H+ + 4e-}$$

#### Cathode¶

$$\ce{O2 + 4H+ + 4e- -> 2H2O}$$

#### Electrolyte¶

Organic electrolyte

## Supercapacitors¶

Store electricity as a potential between two charged surfaces. These are electrostatic storage mechanisms, not chemical and are designed for short, intense pulses of power, with charging in between, rather than a continuous current as would be provided by a typical battery.

The defining factor for a supercapacitor over a capacitor is that supercapacitors have very large, porous plates (large $$A$$) with very small dielectric mediums (small $$L$$), causing them to be able to build up a much larger charge compared to their regular counterparts.

In the application below, we can see that with a supercapacitor as opposed to a regular capacitor, in a system where a constant voltage is needed, when power pulses are required from the same power source, the voltage can be maintained to a much higher degree than with regular capacitors in their place.

## Fitting them all together¶

The ragone plot shows us how all of these technologies overlap in terms of their energy and power output.