In-situ X-ray Absorption Spectroscopic Study of Fe Incorporation in Ni and Ni/Fe-based OER Catalysts Ultra-dispersed in Graphene Aerogels: Addressing the identified challenges

It is previously known that the presence of Fe impurities causes the electrochemical activation of Ni catalysts for the oxygen evolution reaction (OER). However, the active species and the catalytic sites are still under investigation. In order to address this crucial issue, our group has prepared highly dispersed Ni-based catalysts in graphene aerogels that are postulated as ideal materials to study the effect of Fe incorporation in Ni catalysts. This material exhibits enhanced sensitivity to Fe incorporation, potentially leading to a greater contribution of Ni-Fe species after conditioning, compared to previously reported bulk Ni materials. This increased presence of Ni-Fe species, which are the focus of our analysis, may results in more representative and thus clearer data. Our group also confirmed that even graphene aerogels containing Ni and Fe are positively affected by the presence of Fe in the electrolyte, emphasizing not only the necessity of having Fe in the sample, but also the importance of the in-situ formed Ni-Fe species to achieve higher OER activity. X-ray absorption spectroscopy experiments were conducted with these samples at CLAESS beamline (AV-2023027444-2, 24-26 May). Ex-situ analysis shows the presence of NiO in rGO-Ni, while rGO-NixFe(100-x) samples seem to present a mixture of Ni-Ni and Ni-Fe bonding with a much smaller portion of Ni-O bonds. After data processing, we can preliminarily conclude that significant changes were detected during the OER experiment, such as the oxidation of Ni sites or Fe incorporation, as indicated by a shift in the XANES region. To validate these promising results, it is essential to conduct more key operando experiments (e.g., under total Fe absence, both electrolyte and sample) and to measure standards that better correlate with our samples.