Optimized Chlor-Alkali Membrane Cell Cathode
Chlor-alkali membrane cells were first commercialized in 1982. This is the same technology used to produce Green Hydrogen. Prior to the membrane cell, chlor-alkali was produced with a diaphragm cell and with a mercury cell. In 1968, Henri Beer made a historic discovery which was commercialized as the Dimensionally Stable Anode (DSA®) or Dimensionally Stable Electrode (DSE®). Henri Beer’s invention of applying a mixed metal oxide coating to a titanium substrate allowed the production of a stable anode which in turn allowed the operation of the membrane cell. The use of an oxide coating on the anode was a perfect and eloquent solution for a problem which had perplexed scientists up to that point. Using an already oxidized coating in the highly oxidative anolyte chamber prevented the anode from becoming further oxidized and hence a stable anode was produced. Although the oxide coating is highly mud-cracked and porous, it does an adequate job at protecting the titanium substrate used for anodes and hence long life and stable anode coatings are possible.
With the advent of membrane cell technology, a nickel cathode structure was employed to avoid iron contamination observed in the previous mercury and diaphragm cell technologies which used steel cathode structures. While a nickel cathode alone can readily evolve hydrogen, it is preferred to utilize a cathode coating which has a lower overpotential for hydrogen evolution. OEM suppliers of chlor-alkali membrane cells offer cathodes coated with metal oxides to lower the overpotential for hydrogen evolution. This oxide coating was developed for use on anodes and is not optimum for cathodes. The oxide coating does a poor job at protecting the underlying nickel substrate and the coating adhesion is also poor. Often, new cathodes or cathodes stored in a humid environment will display a greenish discoloration. This discoloration is caused by corrosion products from the nickel substrate migrating through the oxide coating. The discoloration and other non-uniformities are signs of compromised coating adhesion and the inability of the oxide coating to protect the underlying nickel substrate. Tape testing OEM supplied oxide coatings also reveals the poor adhesion of the oxide coating.
OEM supplied oxide cathode coatings are an improvement over a bare nickel cathode, however they suffer from the following limitations:
- The highly mud-cracked and porous oxide coating does not adequately protect the nickel substrate.
- The oxide coating used on cathodes is much less adherent than the oxide coating used on anodes and the cathode coating wears away in the reducing environment of the catholyte.
- Oxide coatings are only applied to a portion of the available cathode surface.
- Oxide coatings require the substrate to be roughened via grit blasting and/or etching thus causing distortion and shortening the useful life of the substrate.
- Oxide coatings have such poor adhesion that often even washing with water will remove the coating.
A better cathode coating is available from Electrode Solutions. Cathode Optimizing Solution (COS) was designed specifically for chlor-alkali and Greeg Hydrogen membrane cathodes and not re-purposed from anodes. Cathode Optimizing Solution was designed to address the limitations of oxide coatings and provides the following advantages:
- COS provides a cathode coating which is stable in the reducing environment of the catholyte.
- COS provides a coating which is crack free and pore free and better protects the underlying nickel substrate. During power outages, highly aggressive species such as chlorates are formed which will attack and corrode exposed nickel surfaces. Oxide coatings from OEM’s allow these aggressive species to attack the underlying nickel structure and thus compromise the oxide coating. COS protects against this attack.
- COS provides a coating which is bonded to the nickel substrate and is much more wear resistant than oxide coatings. (No coating removal via a tape test)
- COS requires no roughening or etching of the substrate thereby preserving the integrity and longevity of the cathode substrate.
- COS delivers voltage savings by producing a cathode which operates at a lower hydrogen overpotential than oxide coatings.
- COS activates all areas of the cathode which can deliver up to an additional 200 mV of savings as compared to OEM supplied cathode coatings.
- COS corrects the efficiency loss and higher hydrogen in chlorine seen with zero gap cells.