Alkaline water electrolysis is a type of
electrolyser that is characterized by having two
electrodes operating in a liquid alkaline electrolyte. Commonly, a solution of
potassium hydroxide (KOH) or
sodium hydroxide (NaOH) at 25-40 wt% is used.[6] These electrodes are separated by a diaphragm, separating the product gases and transporting the hydroxide ions (OH−) from one electrode to the other.[4][7] A recent comparison showed that state-of-the-art nickel based water electrolysers with alkaline electrolytes lead to competitive or even better efficiencies than acidic
polymer electrolyte membrane water electrolysis with platinum group metal based electrocatalysts.[8]
The technology has a long history in the chemical industry. The first large-scale demand for hydrogen emerged in late 19th century for
lighter-than-air aircraft, and before the advent of
steam reforming in the 1930s, the technique was competitive.[citation needed]
Hydrogen-based technologies, which have evolved significantly since the initial discovery of hydrogen and its early application as a buoyant gas approximately 250 years ago. In 1804, the Swiss inventor Francois Isaac de Rivaz secured a patent for the inaugural hydrogen-powered vehicle. This prototype, equipped with a four-wheel design, utilised an internal combustion engine (ICE) fuelled by a mixture of hydrogen and oxygen gases. The hydrogen fuel was stored in a balloon, and ignition was achieved through an electrical starter known as a Volta starter. The combustion process propelled the piston within the cylinder, which, upon descending, activated a wheel through a ratchet mechanism. This invention could be viewed as an early embodiment of a system comprising hydrogen storage, conduits, valves, and a conversion device.[9]
Approximately four decades after the military scientist Ritter developed the first electrolyser, the chemists Schoenbein and Sir Grove independently identified and showcased the fuel cell concept. This technology operates in reverse to electrolysis around the year 1839. This discovery marked a significant milestone in the field of hydrogen technology, demonstrating the potential for hydrogen as a source of clean energy.[9]
Structure and materials
Scheme of alkaline water electrolyzers. The catalysts are added to the anode and cathode to reduce the overpotential.[10]
The electrodes are typically separated by a thin porous foil, 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.
Asbestos diaphragms have been used for a long time due to their effective gas separation, low cost, and high chemical stability; however, their use is restricted by the
Rotterdam Convention.[11] The state-of-the-art diaphragm is Zirfon, a composite material of
zirconia and
Polysulfone.[12] The diaphragm further avoids the mixing of the produced hydrogen and oxygen at the cathode and anode,[13][14] respectively. The thickness of asbestos diaphragms ranges from 2 to 5 mm, while Zirfon diaphragms range from 0.2 to 0.5 mm.[11]
Typically, Nickel based metals are used as the electrodes for alkaline water electrolysis.[15] Considering pure metals, Ni is the least active non-noble metal.[16] The high price of good noble metal electrocatalysts such as platinum group metals and their dissolution during the oxygen evolution[17] is a drawback. Ni is considered as more stable during the oxygen evolution,[18] but stainless steel has shown good stability and better catalytic activity than Ni at high temperatures during the
Oxygen Evolution Reaction (OER).[5]
High surface area Ni catalysts can be achieved by dealloying of Nickel-Zinc[5] 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[19][20]
and hot dip galvanized Ni meshes.[21] The latter approach might be interesting for large scale industrial manufacturing as it is cheap and easily scalable, but unfortunately, all the strategies show some degradation.[22]
Electrochemistry
Anode reaction
In alkaline media oxygen evolution reactions, multiple adsorbent species (O, OH, OOH, and OO–) and multiple steps are involved. Steps 4 and 5 often occur in a single step, but there is evidence that suggests steps 4 and 5 occur separately at pH 11 and higher.[23][24]
Overall anode reaction:
Where the * indicate species adsorbed to the surface of the catalyst.
Cathode reaction
The hydrogen evolution reaction in alkaline conditions starts with water adsorption and dissociation in the Volmer step and either hydrogen desorption in the Tafel step or Heyrovsky step.
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 electrolytes.
Disadvantage
One disadvantage of alkaline water electrolysers is the low-performance profiles caused by the commonly-used thick diaphragms that increase ohmic resistance, the lower intrinsic conductivity of OH− compared to H+, and the higher gas crossover observed for highly porous diaphragms.[25]
References
^Divisek, J.; Schmitz, H. (1 January 1982). "A bipolar cell for advanced alkaline water electrolysis". International Journal of Hydrogen Energy. 7 (9): 703–710.
doi:
10.1016/0360-3199(82)90018-0.
^David, Martín; Ocampo-Martínez, Carlos; Sánchez-Peña, Ricardo (June 2019). "Advances in alkaline water electrolyzers: A review". Journal of Energy Storage. 23: 392–403.
doi:
10.1016/j.est.2019.03.001.
hdl:2117/178519.
S2CID140072936.
^
abcdefghijklmnCarmo, M; Fritz D; Mergel J; Stolten D (2013). "A comprehensive review on PEM water electrolysis". Journal of Hydrogen Energy. 38 (12): 4901.
doi:
10.1016/j.ijhydene.2013.01.151.
^Zeng, Kai; Zhang, Dongke (June 2010). "Recent progress in alkaline water electrolysis for hydrogen production and applications". Progress in Energy and Combustion Science. 36 (3): 307–326.
doi:
10.1016/j.pecs.2009.11.002.
^
abSmolinka, Tom (2021). Electrochemical Power Sources: Fundamentals, Systems, and Applications: Hydrogen Production by Water Electrolysis. Elsevier.
ISBN978-0-12-819424-9.
^Haug, P; Koj M; Turek T (2017). "Influence of process conditions on gas purity in alkaline water electrolysis". International Journal of Hydrogen Energy. 42 (15): 9406–9418.
doi:
10.1016/j.ijhydene.2016.12.111.
^Schalenbach, M; et al. (2018). "The electrochemical dissolution of noble metals in alkaline media". Electrocatalysis. 9 (2): 153–161.
doi:
10.1007/s12678-017-0438-y.
S2CID104106046.
^Cherevko, S; et al. (2016). "Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability". Catalysis Today. 262: 170–180.
doi:
10.1016/j.cattod.2015.08.014.
^Schiller, G; Henne R; Borock V (1995). "Vacuum Plasma Spraying of High-Performance Electrodes for Alkaline Water Electrolysis". Journal of Thermal Spray Technology. 4 (2): 185.
Bibcode:
1995JTST....4..185S.
doi:
10.1007/BF02646111.
S2CID137144045.
^Schiller, G; Henne R; Mohr P; Peinecke V (1998). "High Performance Electrodes for an Advanced Intermittently Operated 10-kW Alkaline Water Electrolyzer". International Journal of Hydrogen Energy. 23 (9): 761–765.
doi:
10.1016/S0360-3199(97)00122-5.
^Schalenbach, M; et al. (2018). "An alkaline water electrolyzer with nickel electrodes enables efficient high current density operation". International Journal of Hydrogen Energy. 43 (27): 11932–11938.
doi:
10.1016/j.ijhydene.2018.04.219.
S2CID103477803.
^Esfandiari, N; et al. (2024). "Metal-based cathodes for hydrogen production by alkaline water electrolysis: Review of materials, degradation mechanism, and durability tests". Progress in Materials Science. 143: 101254.
doi:
10.1016/j.pmatsci.2024.101254.
^Scott, Keith (2020). Electrochemical methods for hydrogen production. Cambridge: Royal Society of Chemistry.
ISBN978-1-78801-378-9.