Alkaline water electrolysis

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]

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]

Electrolysis requires minerals to be present in solution. Tap, well, and ground water contains various minerals, some of which are alkaline while others are acidic. Water above a pH of 7.0 is considered alkaline; below 7.0 it is acidic. The requirement is that there must be ions in the water to conduct electricity for the water electrolysis process to occur.[5][6]

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.[7] The diaphragm further avoids the mixing of the produced hydrogen and oxygen at the cathode and anode,[8][9] respectively.

Typically, Nickel based metals are used as the electrodes for alkaline water electrolysis. Considering pure metals, Ni is the most active non-noble metal.[10] The high price of good noble metal electrocatalysts such as platinum group metals and their dissolution during the oxygen evolution[11] is a drawback. Ni is considered as more stable during the oxygen evolution.[12] 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[13] [14] and hot dip galvanized Ni meshes.[15] 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. 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.
  5. "USGS Water Science School". Retrieved 14 October 2014.
  6. "Argonne National Laboratory Newton Ask a Scientist". Retrieved 14 October 2014.
  7. "AGFA Zirfon Perl Product Specification". Archived from the original on 2018-04-23. Retrieved 29 January 2019.
  8. 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.
  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.
  10. 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.
  11. 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.
  12. 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.
  13. 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.
  14. 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.
  15. 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.
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