Alkaline-electrolysis (AEL) is a known and developed technology used for production of hydrogen from water and is currently the main route used to produce electrolytic hydrogen. It is considered more developed than competing electrolysis technology Proton Exchange Membrane (PEM) (Weeda, 2018).
Electrodes in AEL are made of nickel or of porous metal structures (NOW 2018). Hydrogen ions move towards the cathode and hydroxide ions move towards the anode. A diaphragm is used to separate the two electrode compartments. Gas receivers are then used to collect the formed hydrogen and oxygen gases. To ensure good conductivity the used electrolyte should consist of high-mobility ions. Potassium hydroxide (KOH) is normally preferred over sodium hydroxide (NaOH) because of higher conductivity (Santos, Sequeira, & Figueiredo, 2013).
Cathodic reaction: 2 H2O + 2e- => H2 + 2OH-
Charge carrier: OH-
Anodic reaction: 2OH- => 0.5 O2 + H2O + 2e-
AEL operates at a temperature of around 60-70 degrees C (Weeda, 2018) and can produce hydrogen at a pressure of 30 bar (De Vita, et al., 2018), although installations that operate at atmospheric pressure also exist (ECN, 2018). This is expected to increase to 40 bar by 2030 and 70 bar by 2050 (NOW, 2018).
Alkaline Electrolysis Cells have a limited ability to respond to load changes, which is essential when flexibility is required by the power system.
The current start-up time is around 50 minutes (NOW, 2018).
The electrolysis takes place in cells, which can be stacked (called a ‘stack’). An installation can consist of multiple stacks.