Alkaline water electrolysis

Typical Materials
Type of Electrolysis:| Alkaline Water Electrolysis
Style of membrane/diaphragm| NiO
Bipolar/separator plate material| Stainless steel
Catalyst material on the anode| Ni/Co/Fe
Catalyst material on the cathode| Ni/C-Pt
Anode PTL material| Ti/Ni/zirconium
Cathode PTL material| Stainless steel mesh
State-of-the-art Operating Ranges
Cell temperature| 60-80C[1]
Stack pressure| <30 bar[1]
Current density| 0.2-0.4 A/cm2[1][2]
Cell voltage| 1.8-2.40 V[1][2]
Power density| to 1.0 W/cm2[1]
Part-load range| 20-40%[1]
Specific energy consumption stack| 4.2-5.9 kWh/Nm3[1]
Specific energy consumption system| 4.5-7.0 kWh/Nm3[1]
Cell voltage efficiency| 52-69%[1]
System hydrogen production rate| <760 Nm3/h[1]
Lifetime stack| <90,000 h[1]
Acceptable degradation rate| <3 µV/h[1]
System lifetime| 20-30 a[1]

Alkaline water electrolysis has a long history in the chemical industry. It is a type of electrolyzer that is characterized by having two electrodes operating in a liquid alkaline electrolyte solution of potassium hydroxide (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.[1][3] 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.[4]

In the context of decarbonisation of industry alkaline water electrolysis can be regarded as an important technology enabling efficient energy conversion and storage.[5]

## 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 and Polysulfone.[6] The diaphragm further avoids the mixing of the produced hydrogen and oxygen at the cathode and anode,[7][8] 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.[9] The high price of good noble metal electrocatalysts such as platinum group metals and their dissolution during the oxygen evolution[10] is a drawback. Ni is considered as more stable during the oxygen evolution,[11] but stainless steel has shown good stability and better catalytic activity than Ni at high temperatures during the Oxygen Evolution Reaction (OER).[2]

High surface area Ni catalysts can be achieved by dealloying of Nickel-Zinc[2] 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[12] [13] and hot dip galvanized Ni meshes.[14] 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:

1. Cheaper catalysts with respect to the platinum metal group based catalysts used for PEM water electrolysis.
2. Higher durability due to an exchangeable electrolyte and lower dissolution of anodic catalyst.
3. Higher gas purity due to lower gas diffusivity in alkaline electrolyte.

## References

1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Carmo, 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.
2. ↑ 2.0 2.1 2.2 2.3 Colli, A.N. (2019). "Non-Precious Electrodes for Practical Alkaline Water Electrolysis". Materials 12 (8): 1336. doi:10.3390/ma12081336. PMID 31022944. Bibcode: 2019Mate...12.1336C.
3. ↑ "Alkaline Water Electrolysis". Energy Carriers and Conversion Systems. http://www.eolss.net/sample-chapters/c08/e3-13-03-02.pdf. Retrieved 19 October 2014.
4. ↑ Schalenbach, M; Tjarks G; Carmo M; Lueke W; Mueller M; Stolten D (2016). "Acidic or Alkaline? Towards a New Perspective on the Efficiency of Water Electrolysis". Journal of the Electrochemical Society 163 (11): F3197. doi:10.1149/2.0271611jes. https://publications.rwth-aachen.de/record/681185.
5. ↑ "Electrocatalysts for the hydrogen evolution reaction in alkaline and neutral media. A comparative review" (in en). Journal of Power Sources 493: 229708. 2021-05-01. doi:10.1016/j.jpowsour.2021.229708. ISSN 0378-7753. https://www.sciencedirect.com/science/article/pii/S0378775321002494.
6. ↑ "AGFA Zirfon Perl Product Specification". http://www.agfa.com/specialty-products/solutions/membranes/zirfon-perl-utp-500/. Retrieved 29 January 2019.
7. ↑ Schalenbach, M; Lueke W; Stolten D (2016). "Hydrogen Diffusivity and Electrolyte Permeability of the Zirfon PERL Separator for Alkaline Water Electrolysis". Journal of the Electrochemical Society 163 (14): F1480–F1488. doi:10.1149/2.1251613jes. http://jes.ecsdl.org/content/163/14/F1480.full.pdf.
8. ↑ 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.
9. ↑ Quaino, P; Juarez F; Santos E; Schmickler W (2014). "Volcano plots in hydrogen electrocatalysis–uses and abuses". Beilstein Journal of Nanotechnology 42: 846–854. doi:10.3762/bjnano.5.96. PMID 24991521.
10. ↑ Schalenbach, M (2018). "The electrochemical dissolution of noble metals in alkaline media". Electrocatalysis 9 (2): 153–161. doi:10.1007/s12678-017-0438-y.
11. ↑ Cherevko, S (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.
12. ↑ 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. doi:10.1007/BF02646111. Bibcode: 1995JTST....4..185S.
13. ↑ 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.
14. ↑ Schalenbach, M (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.

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