Fabrication and Electrocatalytic Evolution of Alkali-Free CoNb₂O₆ Thin Films for Water Oxidation via Polyoxoniobates

Fabrication of CoNb2O6 thin films was possible through spin-coating an alkali-free, aqueous solution of a polyoxoniobate ([Nb10O28]6–, Nb10) and a cobalt nitrate (Co(NO3)2), followed by annealing. The results showed that depending on the calcination conditions, minor phases of unreacted Co3O4 and β-Nb2O5 may form alongside the columbite CoNb2O6 phase. However, optimizing the calcination conditions can lead to the formation of single-phase orthorhombic columbite CoNb2O6. The optimal annealing temperature was evaluated by analysis of the powders made from the same solution annealed at different temperatures. The calcination temperature has been determined to have a significant impact on the phase purity of the CoNb2O6. Furthermore, the powders and thin films exhibited superior catalytic activity and stability toward water oxidation, with a low overpotential and a high current density. Fabrication of these thin films by the use of simple aqueous solutions, rather than moisture-sensitive precursors (NbCl5 or Nb(OC2H5)5) or energy-intensive methods (sputter deposition/laser ablation), fosters environmentally benign and sustainable options for various thin-film-based applications. This work provides a promising avenue for the development of efficient, green and noble-metal-free electrocatalyst thin films for water oxidation.