An Analysis on the Different Processes of Water Electrolysis |
Electrolysis is an electrochemical process in whichelectrical energy is the driving force of chemical reactions. Substances aredecomposed, by passing a current through them. The first observation of thisphenomenon was recorded in 1789. Nicholson and Carlisle were the first whodeveloped this technique back in 1800 and by the beginning of the 20th centurythere were already 400 industrial water electrolysis units in use. As mentioned before, water is decomposed to hydrogen andoxygen, by passing a current through it in the presence of suitable substances,called electrolytes. Electric current causes positively charged hydrogen ionsto migrate to the negatively charged cathode, where a reduction takes place inorder to form hydrogen atoms. The atoms formed then combine to form gaseoushydrogen molecules (H2). On the other hand, oxygen is formed at the otherelectrode (the positively charged anode). The stoichiometry of the reaction istwo volumes of hydrogen to one volume of oxygen. The most important part of theconstruction of electrolysis units is to use adequate electrodes to avoidunwanted reactions, which produce impurities in the hydrogen gas. Anothernecessary component of such a unit is a separating membrane that allows thepassage of ions, or electrons and not oxygen, or hydrogen atoms. This membraneallows the gases to be kept separate in order to avoid the risk of an explosivemixture being formed in the electrolysis unit. In the initial discovery of electrolysis, an acidic watersolution was used, but nowadays there is a trend towards alkaline electrolytessuch as potassium hydroxide (KOH). This technology offers the advantages ofmaterials which are cheaper and less susceptible to corrosion compared to thoserequired to handle acids. Electrolysis plants with normal or slightly elevatedpressure usually operate at electrolyte temperature of 70-90oC, cell voltage of1.85-2.05 V and consume 4-5 KWh / m3 of hydrogen, which is obtained at a purityof 99.8% and more. Pressure electrolysis units run at 6-200 bar and there is nosignificant influence on the power consumption. Because of its high energyconsumption and also of the quite substantial investment, water electrolysis iscurrently used for only 4% of world hydrogen production. Nowadays research and development into high efficiencyelectrolysers is flourishing in many areas. A way of improving electrolysisunits efficiency is by increasing the process temperature which lowers thevoltage required to electrolyse the water, but also requires more expensivematerials. Despite the fact that the total energy needed for theelectrochemical decomposition of water decreases only slightly with increasingtemperature, the reversible part of the energy requirement (ΔF), which issupplied as electrical energy, decreases considerably. Therefore an increasingamount of the total energy could be supplied as heat. At elevated temperatures(800-900oC) the electric power consumption is approximately only 3 kWh / m3 ofhydrogen. It must be noted that this technology is still in the developmentstage.