The Electrolysis Of Water
2H2O(l) → 2H2(g) + O2(g)
Electrolysis of water can be observed by passing direct current from a battery or other DC power supply through a cup of water (in practice a salt water solution increases the reaction intensity making it easier to observe). Using platinum electrodes, hydrogen gas will be seen to bubble up at the cathode, and oxygen will bubble at the anode. If other metals are used as the anode, there is a chance that the oxygen will react with the anode instead of being released as a gas, or that the anode will dissolve.
For example, using iron electrodes in a sodium chloride solution electrolyte, iron oxides will be produced at the anode. With zinc electrodes in a sodium chloride electrolyte, the anode will dissolve, producing zinc ions (Zn2+) in the solution, and no oxygen will be formed. When producing large quantities of hydrogen, the use of reactive metal electrodes can significantly contaminate the electrolytic cell – which is why iron electrodes are not usually used for commercial electrolysis. Electrodes made of stainless steelcan be used because they will not react with the oxygen.
The energy efficiency of water electrolysis varies widely. The efficiency is a measure of what fraction of electrical energy used is actually contained within the hydrogen. Some of the electrical energy is converted to heat, a useless by-product. Some reports quote efficiencies between 50% and 70% This efficiency is based on the Lower Heating Value of Hydrogen. The Lower Heating Value of Hydrogen is total thermal energy released when hydrogen is combusted minus the latent heat of vaporisation of the water. This does not represent the total amount of energy within the hydrogen, hence the efficiency is lower than a more strict definition. Other reports quote the theoretical maximum efficiency of electrolysis as being between 80% and 94%. The theoretical maximum considers the total amount of energy absorbed by both the hydrogen and oxygen. These values refer only to the efficiency of converting electrical energy into hydrogen’s chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency is more likely to be between 25% and 40%.