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
Secondary battery - Lead Acid
Anode¶
\(\ce{Pb + H2SO4 <=> PbSO4 + 2H+ + 2e-}\)
Cathode¶
\(\ce{PbO2 + H2SO4 + 2H+ + 2e- <=> PbSO4 + 2H2O}\)
Electrolyte¶
Aqueous \(\ce{H2SO4}\)
Secondary battery - Nickel Cadmium
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.
