Identification of non-traditional coordination environments for iron ions in nickel hydroxide lattices

Abstract

The uniformity in electrochemical behavior of Fe_xNi_1−x(OH)₂ is remarkable given the diversity in published fabrication protocols, composition-dependent structural changes, and multitude of electrochemical reaction mechanisms proposed. We use complementary techniques to show that multiple unique Fe(III) coordination environments exist in Fe_xNi_1−x(OH)₂ synthesized by alkaline precipitation. Samples synthesized with formamide added to the reaction vessel yield Mössbauer and Raman spectra consistent with Fe(III) successfully incorporated into the Ni(OH)₂ lattice. Additional spectroscopic features emerge for samples synthesized in the absence of formamide, revealing a second Fe(III) coordination environment. Correlations in structural information derived from the numerous characterization techniques suggest that the secondary environment sits atop the 2-dimensional Ni(OH)₂ sheets. X-ray diffraction and X-ray absorption spectroscopy fail to resolve this second site, but show that Fe(III) induces a non-uniform structural contraction that distorts the Ni(OH)₂ lattice for all samples. Electrocatalytic oxygen evolution is qualitatively similar for both sample series, but samples synthesized in the presence of formamide consistently outperform those from conventional pH precipitation. Iron ions within the Ni(OH)₂ lattice are therefore deemed to be the catalytically relevant structural feature. The identification of a secondary iron site demonstrates that qualitative similarities in electrochemical behavior mask quantitative differences in structure and electrocatalytic competence.