The Common Intermediates of Oxygen Evolution and Dissolution Reactions during Water Electrolysis on Iridium

Abstract

Understanding the pathways of catalyst degradation during the oxygen evolution reaction is a cornerstone in the development of efficient and stable electrolyzers, since even for the most promising Ir based anodes the harsh reaction conditions are detrimental. The dissolution mechanism is complex and the correlation to the oxygen evolution reaction itself is still poorly understood. Here, by coupling a scanning flow cell with inductively coupled plasma and online electrochemical mass spectrometers, we monitor the oxygen evolution and degradation products of Ir and Ir oxides in situ. It is shown that at high anodic potentials several dissolution routes become possible, including formation of gaseous IrO₃ . On the basis of experimental data, possible pathways are proposed for the oxygen-evolution-triggered dissolution of Ir and the role of common intermediates for these reactions is discussed.

Keywords: electrochemical mass spectrometry; iridium dissolution; oxygen evolution reaction; reaction mechanisms; water electrolysis.

Figure 1

a) Measured potential during 30 s of anodic polarization of Ir thermal oxide in 0.1 m HClO₄ at 5, 10, 15, and 20 mA cm⁻² at room temperature. b) Average rate of iridium dissolution as measured online with ICP‐MS. Mass spectra of c) O₂ (m/z 32) and d) IrO₃ (m/z 240) acquired online with OLEMS. The color gradient indicates the increase of applied current density from 5 mA cm⁻² to 20 mA cm⁻². The baselines in (c) and (d) show the O₂ (m/z 32) and IrO₃ (m/z 240) signals measured at the open circuit potential.

Figure 2

Dependence of a) the amount of dissolved Ir, b) the formation of IrO₃ and potential at the end of polarization on the current density obtained for metallic Ir (green), reactively sputtered IrO₂ (black) and thermal IrO₂ (red).

Scheme 1

Simplified scheme showing possible pathways of Ir dissolution during the OER. Green arrows indicate the mechanism that is preferable for electrocatalytically active Ir‐based materials where OER occurs at lower potentials. Red arrows present the dissolution route dominating at higher anodic potentials. Blue arrows show intermediate steps that take place regardless of the electrode material and potential. Corresponding equations can be found in the Supporting information.