Construction of tetrahedral CoO4 vacancies for activating the high oxygen evolution activity of Co3−xO4−δ porous nanosheet arrays

This study presents low-crystalline and non-stoichiometric cobalt oxide (Co3−xO4−δ) porous nanosheet arrays (PNAs) grown on carbon fiber cloth (CFC) (Co3−xO4−δ PNAs/CFC) by a facile in situ anodic oxidation strategy. We firstly verified that the above prepared low crystalline cobalt oxide contained tetrahedral CoO4 vacancies, resulting in the creation of O vacancies at adjacent octahedral CoO6 sites, allowing the generation of tetragonal–pyramidal CoO5 sites which were regarded as active sites and being accessible for the oxygen evolution reaction (OER) with different reaction mechanisms compared to that of traditional CoO6 sites in high-crystalline and stoichiometric Co3O4, thus endowing Co3−xO4−δ PNAs/CFC with significantly improved OER activity and superior stability compared to their crystalline counterparts (Co3O4 PNAs/CFC), as illustrated by experiments and density functional theory (DFT) calculations. This study will open up a new approach for the synthesis of defect-rich materials and provide new insight into the structure–property relationship of OER catalysts.