Spectroelectrochemical Tracking of Nickel Hydroxide Reveals Its Irreversible Redox States upon Operation at High Current Density

Transition-metal (oxy)hydroxides with an abundance of redox metal sites are important for the development of electrochromic devices, rechargeable metal-air batteries, pseudo-capacitors, and industrial electrolyzers. The robust charging reversibility of the redox metal sites ensures long-term durability of the devices but remains unachieved and usually ignored. Here, we use in situ UV-vis and Raman spectroscopies to track the redox states of a nickel hydroxide Ni(OH)(2) model catalyst for the oxygen evolution reaction (OER) during its lifetime in strong alkaline media. We show that at 200 mAcm(-2) in the 1.0 M KOH electrolyte, the reversible redox states of the catalytically active Ni sites gradually disappear when the catalyst converts into an irreversible and inactive phase. We found that after deactivation, the complete reduction of such oxidized Ni sites can not be achieved until a very negative reduction potential around 0.1 V RHE was applied owing to the structural amorphization/disordering of the layered Ni(OH)(2) catalyst. Our results suggest that the deactivated and unreduced Ni sites hardly re-hydrate/re-hydroxylate and thus obstruct the OER process. These findings provide direct evidence for elucidation of the origin of the oxygen evolution decay and contribute to a reference to extend the lifetime of 2D-layered transition-metal hydroxide catalysts by stabilizing the reversible redox metal sites.