Chemical state of nickel nanoparticles during the oxygen evolution reaction in a carbonate-bicarbonate buffer solution

The chemical state of nickel anodes during the oxygen evolution reaction can impact their electrocatalytic performance. Here, X-ray photoelectron and absorption spectroscopies reveal the chemical state of nickel nanoparticles under oxygen evolution reaction conditions in a mildly alkaline carbonate-bicarbonate buffer solution. Ni²⁺ and Ni³⁺ species are observed at the reaction onset potential with a 7:4 ratio, with no remaining metallic nickel. These species include NiO, which increasingly converts to other Ni²⁺ and Ni³⁺ species once the potential is increased above the onset potential. Conversely, when a 20-nm-thick nickel film is used instead of nickel nanoparticles, a significant amount of metallic nickel remains in the inner layers. Nickel nanoparticles also undergo significant morphological and structural changes during the reaction, as evidenced by ex situ transmission electron microscopy. Amorphization of the nanoparticles is attributed to significant H₂O incorporation, with the oxygen intensity increasing both in operando and ex situ measurements.