Alkaline water electrolysis

Alkaline water electrolysis 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]

Alkaline water electrolysis
Typical Materials
Type of Electrolysis:Alkaline Water Electrolysis
Style of membrane/diaphragmNiO
Bipolar/separator plate materialStainless steel
Catalyst material on the anodeNi/Co/Fe
Catalyst material on the cathodeNi/C-Pt
Anode PTL materialTi/Ni/zirconium
Cathode PTL materialStainless steel mesh
State-of-the-art Operating Ranges
Cell temperature60-80C[1]
Stack pressure<30 bar[1]
Current density0.2-0.4 A/cm2[1][2]
Cell voltage1.8-2.40 V[1][2]
Power densityto 1.0 W/cm2[1]
Part-load range20-40%[1]
Specific energy consumption stack4.2-5.9 kWh/Nm3[1]
Specific energy consumption system4.5-7.0 kWh/Nm3[1]
Cell voltage efficiency52-69%[1]
System hydrogen production rate<760 Nm3/h[1]
Lifetime stack<90,000 h[1]
Acceptable degradation rate<3 µV/h[1]
System lifetime20-30 a[1]

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.

Structure and materials
Scheme of alkaline water electrolyzers. The catalysts are added to the anode and cathode to reduce the overpotential.

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.[5] The diaphragm further avoids the mixing of the produced hydrogen and oxygen at the cathode and anode,[6][7] respectively.

Typically, Nickel based metals are used as the electrodes for alkaline water electrolysis.[8] 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.

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.[15] [16]

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.

Volmer step:
Tafel step:

Heyrovsky step:

Overall cathode reaction:

References
  1. 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. Colli, A.N.; et al. (2019). "Non-Precious Electrodes for Practical Alkaline Water Electrolysis". Materials. 12 (8): 1336. Bibcode:2019Mate...12.1336C. doi:10.3390/ma12081336. PMC 6515460. PMID 31022944.
  3. "Alkaline Water Electrolysis" (PDF). Energy Carriers and Conversion Systems. 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. S2CID 35846371.
  5. "AGFA Zirfon Perl Product Specification". Archived from the original on 2018-04-23. Retrieved 29 January 2019.
  6. Schalenbach, M; Lueke W; Stolten D (2016). "Hydrogen Diffusivity and Electrolyte Permeability of the Zirfon PERL Separator for Alkaline Water Electrolysis" (PDF). Journal of the Electrochemical Society. 163 (14): F1480–F1488. doi:10.1149/2.1251613jes. S2CID 55017229.
  7. 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.
  8. Zhou, Daojin; Li, Pengsong; et al. (2020). "Recent Advances in Non‐Precious Metal‐Based Electrodes for Alkaline Water Electrolysis". ChemNanoMat. 6 (3): 336–355. doi:10.1002/cnma.202000010. ISSN 2199-692X. S2CID 213442277.
  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. PMC 4077405. PMID 24991521.
  10. 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. S2CID 104106046.
  11. 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.
  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. Bibcode:1995JTST....4..185S. doi:10.1007/BF02646111. S2CID 137144045.
  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; 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. S2CID 103477803.
  15. Scott, Keith (2020). Electrochemical methods for hydrogen production. Cambridge: Royal Society of Chemistry. ISBN 978-1-78801-378-9.
  16. Diaz-Morales, Oscar; Ferrus-Suspedra, David; Koper, Marc T. M. (2016). "The importance of nickel oxyhydroxide deprotonation on its activity towards electrochemical water oxidation". Chemical Science. 7 (4): 2639–2645. doi:10.1039/C5SC04486C.
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