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

<h2>Summary</h2> 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 <i>ex situ</i> transmission electron microscopy. Amorphization of the nanoparticles is attributed to significant H₂O incorporation, with the oxygen intensity increasing both in <i>operando</i> and <i>ex situ</i> measurements.