Stability and decomposition pathways of the NiOOH OER active phase of NiOx electrocatalysts at open circuit potential traced by ex situ and in situ spectroscopies

Abstract

Ni based materials are studied widely for their application as electrocatalysts for alkaline water electrolysis. To further understand and improve these materials, the study of the oxygen evolution reaction (OER) active NiOOH phase is necessary. However, the reversibility of the phase transition from Ni(OH)₂ to NiOOH renders the study of the active phase with ex situ techniques difficult. Therefore, we have investigated the pathways and time evolution of the NiOOH decomposition at open circuit potential after the chronamperometric polarization of sputtered NiO thin films. The samples are characterised in situ and ex situ using Raman and X-ray photoelectron spectroscopy, respectively. We show that the NiOOH phase formed under OER conditions (U > 1.5 V, 1 M KOH, room temperature) quickly degrades to Ni(OH)₂ when set to open circuit potential both inside and emersed from the electrolyte. We demonstrate that NiOOH can be detected ex situ, but only within a limited timeframe unless it is stabilised in a vacuum environment. H₂O adsorption in ultra-high vacuum reveals that the reduction of NiOOH requires protons and electrons, which need to be generated by an oxidative reaction. Our results emphasize the importance of using in situ methods for the characterization of the active phase of the electrocatalyst to observe and understand in detail the processes occurring when analyzing post-catalysis samples by ex situ techniques.