How to Select an Appropriate Electrode Coating in the Electrolysis Process

How to Select an Appropriate Electrode Coating in the Electrolysis Process

It is generally accepted that a good catalyst for the oxygen evolution reaction (OER) must have a relatively high electronic conductivity, high affinity for adsorbed hydroxide intermediates and the electrode must be resistant to corrosion. Noble metal oxides of Ru, Ir, Rh, etc. are considered to be good electrocatalysts for the OER. However, noble metal oxide coatings can dissolve when exposed to harsh industrial conditions such as higher current densities (30 mA cm-2), strong acids (pH < 2), and aggressive chloride environments.

The appropriate selection of electrode materials is a prerequisite to optimizing the parameters in the electrolysis process. The most important requirements for an electrode coating material are:
1. Long-term stability (both mechanically and chemically);
2. Good electrocatalytic properties;
3. Available at low cost;
4. High surface area;
5. High electrical conductivity;
6. Minimal trapping of gas bubbles;
7. Selective for designated reactions;
8. Safe to handle.

It is important that an industrial coating material meets the requirements listed. However, it is highly unlikely a coating material will have all the characteristics required. In practice, a coating material with optimum properties for a given application will be selected. Thus, a coating with optimum activity, stability, and low cost will usually be selected.

It has been established that the performance of some precious metal oxides is far better than their corresponding precious metals. Conducting oxides or semiconducting oxides that can be doped are usually added to the active coating to stabilize it. Ta2O5, TiO2, ZrO2, SnO2 are examples of such stabilizing oxides. The electrocatalytic activity of oxide electrode coatings is controlled by factors such as the chemical nature of the catalyst, morphology, stoichiometry, band structure of the oxide, surface electronic structure, etc. These may be grouped into chemical (based on chemical composition) and structural (based on morphological features) factors.