Alkaline water electrolysis has a long history in the chemical industry. It is a type of

electrolyzer In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...

that is characterized by having two

electrodes An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials dep ...

operating in a liquid alkaline electrolyte solution of

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

(KOH) or sodium hydroxide (NaOH). These electrodes are separated by a diaphragm, separating the product gases and transporting the hydroxide ions (OH⁻) from one electrode to the other. A recent comparison showed that state-of-the-art nickel based water electrolyzers with alkaline electrolytes lead to competitive or even better efficiencies than acidic polymer electrolyte membrane water electrolysis with platinum group metal based electrocatalysts. In the context of decarbonisation of industry alkaline water electrolysis can be regarded as an important technology enabling efficient energy conversion and storage.

Structure and materials

The electrodes are typically separated by a thin porous foil (with a thickness between 0.050 to 0.5 mm), commonly referred to as diaphragm or separator. The diaphragm is non-conductive to electrons, thus avoiding electrical shorts between the electrodes while allowing small distances between the electrodes. The ionic conductivity is supplied by the aqueous alkaline solution, which penetrates in the pores of the diaphragm. The state-of-the-art diaphragm is Zirfon, a composite material of

zirconia Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant sta ...

and Polysulfone. The diaphragm further avoids the mixing of the produced hydrogen and oxygen at the cathode and anode, respectively. Typically, Nickel based metals are used as the electrodes for alkaline water electrolysis. Considering pure metals, Ni is the least active non-noble metal. The high price of good noble metal electrocatalysts such as platinum group metals and their dissolution during the oxygen evolution is a drawback. Ni is considered as more stable during the oxygen evolution, but stainless steel has shown good stability and better catalytic activity than Ni at high temperatures during the Oxygen Evolution Reaction (OER). High surface area Ni catalysts can be achieved by dealloying of Nickel-Zinc or Nickel-Aluminium alloys in alkaline solution, commonly referred to as Raney nickel. In cell tests the best performing electrodes thus far reported consisted of plasma vacuum sprayed Ni alloys on Ni meshes and hot dip galvanized Ni meshes. The latter approach might be interesting for large scale industrial manufacturing as it is cheap and easily scalable.

Advantages compared to PEM water electrolysis

In comparison to polymer electrolyte water electrolysis, the advantages of alkaline water electrolysis are mainly: # Cheaper catalysts with respect to the platinum metal group based catalysts used for PEM water electrolysis. # Higher durability due to an exchangeable electrolyte and lower dissolution of anodic catalyst. # Higher gas purity due to lower gas diffusivity in alkaline electrolyte.

References

{{Reflist Chemical processes Electrochemistry Electrolysis Industrial gases Hydrogen production