In situ Mössbauer study of redox processes in a composite hydroxide of iron and nickel

In situ Mössbauer spectra revealed changes in the electronic and structural properties of Fe sites brought about by redox processes attributed formally to Ni sites in a composite iron/nickel hydroxide. Measurements on the oxidized form, polarized in 1 M KOH at 0.5 V vs. Hg/HgO, revealed a singlet with an isomer shift of 0.22 mm/s vs. α-Fe. After reduction and polarization at 0.0 V, a doublet with an isomer shift of 0.32 mm/s and a quadrupole splitting of 0.44 mm/s was observed. The spectral changes, which accompanied the formal oxidation of Ni sites at 0.5 V, indicated a partial transfer of electronic density away from trivalent Fe sites and structural rearrangements toward more symmetrical coordination about these sites. This highly oxidized iron species in the composite hydroxide may be involved in the electrocatalytic mechanism for oxygen evolution in alkaline media.