Oxygen Evolution and Dissolution of Iridium Based Water Splitting Anodes

Abstract

Renewable energy sources are expected to take up an important portion of produced electric energy in the near future. Production of hydrogen by proton exchange membrane water electrolysis (PEMWE) is nowadays considered as one of the most attractive ways to store an unavoidable excess of renewable energy. Produced hydrogen can be used in automotive applications. However, owing to high capital costs and a relatively low efficiency, this technology is still not a competitive alternative to traditional hydrogen production from fossil fuels. In particular issues related to sluggish oxygen evolution reaction (OER) kinetics and low stability of most of the catalysts in acidic environment must be solved1. Despite its high price and scarcity, iridium based oxides are the only materials considered as anode catalysts in the PEM electrolyzers2, as only they can provide the required longevity at relatively low overpotential of the OER. However, even most stable catalysts such as rutile IrO2 undergoes dissolution under conditions of the OER3,4. Therefore, optimization of the electrolyzer performance and design of novel more efficient iridium based catalysts is demanded, which cannot be achieved without a deep understanding of the OER itself and iridium degradation mechanisms. While the OER on iridium based electrodes was numerously discussed in the literature5, data on iridium electrochemical dissolution is limited. In the present work, a detailed and systematic study of iridium based oxides, hydrous Ir, rutile IrO2, etc, corrosion and the OER kinetics in acidic media is performed. Initial dissolution rates of iridium are measured online with the OER using a scanning flow cell (SFC) connected to an inductively coupled plasma mass spectrometer (ICP-MS). To obtain additional information on equilibrium concentration of dissolved iridium, long-term measurements in the h-cell with divided anodic and cathodic compartments are performed. The observed differences in the electrochemical activity and stability of iridium hydrous oxide and rutile iridium dioxide are correlated with diversity in chemical structure. Evaluation of surface composition of the oxides triggered by the OER is studied using X-ray photoelectron spectroscopy and its further effect on anode activity-stability relationships is analyzed. On the basis of the obtained results a general mechanism of the involved reactions is proposed and discussed.