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

Abstract

Fabrication of CoNb 2 O 6 thin films was possible through spin-coating an alkali-free, aqueous solution of a polyoxoniobate ([Nb 10 O 28 ] 6− , Nb 10 ) and a cobalt nitrate (Co-(NO 3 ) 2 ), followed by annealing. The results showed that depending on the calcination conditions, minor phases of unreacted Co 3 O 4 and β-Nb 2 O 5 may form alongside the columbite CoNb 2 O 6 phase. However, optimizing the calcination conditions can lead to the formation of single-phase orthorhombic columbite CoNb 2 O 6 . 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 CoNb 2 O 6 . 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 (NbCl 5 or Nb(OC 2 H 5 ) 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.