In Situ Detection of Iron in Oxidation States ≥ IV in Cobalt-Iron Oxyhydroxide Reconstructed during Oxygen Evolution Reaction

Cobalt-iron oxyhydroxides (CoFeOOHx) are among the most active catalysts for the oxygen evolution reaction (OER). However, their redox behavior and the electronic and chemical structure of their active sites are still ambiguous. To shed more light on this, the complete and rapid reconstruction of four helical cobalt-iron borophosphates with different Co:Fe ratios into disordered cobalt-iron oxyhydroxides can be achieved, which are electrolyte-penetrable and thus most transition metal sites can potentially participate in the OER. To track the redox behavior and to identify the active structure, quasi in situ X-ray absorption spectroscopy is applied. Iron in high oxidation states >= IV (Fe4+) and its substantial redox behavior with an average oxidation state of around 2.8 to above 3.2 is detected. Furthermore, a 6% contraction of the Fe-O bond length compared to Fe3+OOH references is observed during OER and a strong distortion of the [MO6] octahedra is identified. It is hypothesized that this bond contraction is caused by the presence of oxyl radicals and that di-mu-oxyl radical bridged cobalt-iron centers are the active sites. It is anticipated that the detailed electronic and structural description can substantially contribute to the debate on the nature of the active site in bimetallic iron-containing OER catalysts.

Automated summary

The redox behavior and active structure of cobalt-iron oxyhydroxides catalysts for the oxygen evolution reaction (OER) are investigated. Quasi in situ X-ray absorption spectroscopy is used to track the redox behavior and identify the active structure, and it is found that iron exhibits substantial redox behavior with an average oxidation state between 2.8 and above 3.2. The contraction of the Fe-O bond length and distortion of the [MO6] octahedra during OER suggest the presence of oxyl radicals and the involvement of di-mu-oxyl radical bridged cobalt-iron centers as the active sites. These findings contribute to the understanding of bimetallic iron-containing OER catalysts.