In situ Fe3+ Incorporation vs. Compounded Fe Doping of NiO and Ni(OH)2 Catalysts for the Alkaline Oxygen Evolution Reaction

Abstract

Electrocatalysis research of Ni-based materials has demonstrated the effect of Fe impurites in alkaline electrolytes on the oxygen evolution reaction (OER) actvity. A significant body of research is focused on the study of amorphous NiOx(OH)y thin films as well as nanoparticulate Ni-oxides in various crystal structures, where the surface structure of the synthesized pre-catalyst change dynamically during the OER. Therefore, understanding of the dependence of the crystal structure and its capability to incorporate Fe ions as actve sites is challenging. In this study, we extended our work of faceted NiO(111) and Co- and Mn-doped NiO(111) to Fe dopants. The known activity enhancement of due to the presence of Fe3+ ions in the electrolyte was studied dependent on well-defined pre-catalysts that serve as host structures. In particular, α-Ni(OH)2 as well as NiO with (111) and (100) pre-dominant faceting were tested in pure electrolyte and after addition of Fe3+ (Figure 1), resulting in a substantial improvement of the OER activity. Depended on the host material, a reduction of the overpotential up to 180 mV where achieved.

The in-situ incorporation of Fe3+ was compared to that of synthesized Fe-doped hydroxides and oxides based on a solvothermal approach. The results revealed that the compounded materials exhibit higher overpotentials than that of in situ Fe3+-doped materials. One possible explanation is attributed to conductivity limitations of the compounded Fe-doped samples. It was found that FeOxHy films are less conductive than NiOxHy. On the other hand, in situ incorporated Fe3+ ions are expected to be surface enriched without influencing the conductivity. The materials were first studied in a rotating disc electrode set-up and were transferred to half-cell measurement for application relevant temperatures and current densities, utilizing porous transport electrodes that resemble anion exchange membrane water electrolyzer anodes.