Nickel-iron electrocatalysts exhibit activities for oxygen evolution which are significantly higher than those of their end-members, but the source of enhanced activity is not well established. In our work, we aim to understand how the presence of iron influences the electrocatalytic activity of Ni-Fe. Electrochemically deposited Ni-Fe films containing 15-55% Fe exhibit the highest catalytic activity; in this composition range, the current densities for oxygen evolution are increased by two orders of magnitude compared to Ni and almost four orders compared to Fe. The Ni(OH)2/NiOOH equilibrium potential shifts linearly to anodic potentials with Fe content, indicating the influence of Fe on the energetics of the Ni(OH)2/NiOOH transformation.Two in situ spectroscopic techniques, confocal Raman spectroscopy and X-ray absorption spectroscopy, were employed to characterize Ni-Fe system as the oxygen evolution reaction is being driven. Both Raman and X-ray absorption spectroscopy reveal NiOOH and FeOOH to be present under operating conditions. In particular, X-ray absorption spectra indicate that Fe exhibits a contracted Fe-O bond as a consequence of its occupancy in the Ni(OH)2/NiOOH lattice. Interestingly, while X-ray absorption spectra indicate that Fe causes negligible changes in the local coordination of Ni in NiOOH, Raman spectroscopy shows clear changes in the relative intensities of the two characteristic Ni-O vibrations of NiOOH. These differences point to the complementary nature of these two techniques, that is, Raman spectroscopy may be able to probe subtle changes in Ni-O character due to the presence of ions and water molecules which intercalate into layered NiOOH compounds. We will discuss the implications of our observations and our path forward to understanding the Ni-Fe system.

Electrochemical and spectroscopic study of nickel-iron thin films for the oxygen evolution reaction

248th National Meeting (2014)

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