Fabrication\nand Electrocatalytic Evolution of Alkali-Free\nCoNb<sub>2</sub>O<sub>6</sub> Thin Films for Water Oxidation via Polyoxoniobates

Fabrication of CoNb<sub>2</sub>O<sub>6</sub> thin films was possible\nthrough spin-coating an alkali-free, aqueous solution of a polyoxoniobate\n([Nb<sub>10</sub>O<sub>28</sub>]<sup>6–</sup>, Nb<sub>10</sub>) and a cobalt nitrate (Co(NO<sub>3</sub>)<sub>2</sub>), followed\nby annealing. The results showed that depending on the calcination\nconditions, minor phases of unreacted Co<sub>3</sub>O<sub>4</sub> and\nβ-Nb<sub>2</sub>O<sub>5</sub> may form alongside the columbite\nCoNb<sub>2</sub>O<sub>6</sub> phase. However, optimizing the calcination\nconditions can lead to the formation of single-phase orthorhombic\ncolumbite CoNb<sub>2</sub>O<sub>6</sub>. The optimal annealing temperature\nwas evaluated by analysis of the powders made from the same solution\nannealed at different temperatures. The calcination temperature has\nbeen determined to have a significant impact on the phase purity of\nthe CoNb<sub>2</sub>O<sub>6</sub>. Furthermore, the powders and thin\nfilms exhibited superior catalytic activity and stability toward water\noxidation, with a low overpotential and a high current density. Fabrication\nof these thin films by the use of simple aqueous solutions, rather\nthan moisture-sensitive precursors (NbCl<sub>5</sub> or Nb(OC<sub>2</sub>H<sub>5</sub>)<sub>5</sub>) or energy-intensive methods (sputter\ndeposition/laser ablation), fosters environmentally benign and sustainable\noptions for various thin-film-based applications. This work provides\na promising avenue for the development of efficient, green and noble-metal-free\nelectrocatalyst thin films for water oxidation.