Fabrication of CoNb₂O₆ thin films was possible through spin-coating an alkali-free, aqueous solution of a polyoxoniobate _{(Nb10 O 28} ^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.