Revealing the reactivity of the Iridium trioxide intermediate for the oxygen evolution reaction in acidic media

We report a strategy to isolate IrO<sub>3</sub> as an intermediate for the oxygen evolution reaction (OER). Its reactivity is studied using X-ray absorption spectroscopy, X-ray and neutron diffraction and X-ray photoelectron spectroscopy. Its stability is assessed by using on-line mass spectroscopy and inductively coupled plasma optical emission spectroscopy and presented herein. Upon reaction with water in acidic conditions, we could observe the formation of a new protonated iridate phase of composition H<sub>2</sub>IrO<sub>3</sub>. Coupling OER measurements and dissolution rate determination, we could show that its activity and stability are governed by a yet ill-described charge compensation mechanism enlisting reversible bulk proton insertion inside the catalyst structure. This singular property enables an enhanced activity and stability towards dissolution compared to the stellar IrO<sub>x</sub>/SrIrO<sub>3</sub> catalyst. Such a finding opens the route towards the design of new OER catalysts enlisting proton insertion that could be competitive for water splitting in acidic media.<br>