The effect of phosphate on iridium oxide electrochemistry

The rate of the oxygen evolution reaction (OER) at anodically formed iridium oxide and nano-crystalline iridium oxide powder is significantly lower in phosphoric acid and sulphuric acid solutions than in perchloric acid solutions in the temperature range 0–150 °C. The results are interpreted as due to strong adsorption of phosphate on the oxide blocking active sites for intermediates in the OER by phosphoric acid moieties. Anions also appear to be involved in the intercalation processes of both forms of the oxide. The mass changes observed for oxide powder during intercalation in phosphoric acid were twice those in perchloric acid.

► The rate of the oxygen evolution reaction on iridium oxide is substantially lower in phosphoric and sulphuric acids than in perchloric acid.
► This will lower the electrocatalytic activity of iridium oxide in applications where the electrolyte contains phosphate.
► This may limit the use of iridium oxide with phosphoric-acid doped polybenzimidazole membranes for high-temperature PEM water electrolysis.