Hydrogen evolution activity enhancement by tuning the oxygen vacancies in self-supported mesoporous spinel oxide nanowire arrays

The development of facile strategies to tune the oxygen vacancy (OV) content in transition metal oxides (TMOs) is paramount to obtain low-cost and stable electrocatalysts, but still highly challenging. Taking NiCo₂O₄ as a model system, we have experimentally established a facile calcination and electrochemical activation (EA) methodology to dramatically increase the concentration of OVs and provide theoretical insight into how the concentration of OVs affects the performance of spinel TMOs towards the electrochemical hydrogen evolution reaction (HER). A self-supported cathode of OV-rich NiCo₂O₄ nanowire arrays was found to exhibit higher HER activity and better stability in alkaline media than its counterparts with fewer OVs. The electrocatalytic HER activity was in good agreement with the increasing concentration of OVs in the studied samples. A large current density of 360 mA·cm^–2 was reached with an overpotential of only 317 mV. Additionally, such a facile strategy was able to efficiently generate OVs in other TMOs (e.g., CoFe₂O₄ and NiFe₂O₄) for enhanced HER performance. In addition, our theoretical results suggest that the increasing OV concentration reduces the adsorption energy of water molecules and their dissociation energy barrier on the surface of the catalyst, thus leading to performance improvement of spinel TMOs toward the electrochemical HER. This work may open a new avenue to increase the concentration of OVs in TMOs in a controlled manner for promising applications in a variety of fields.