Wireless electrochemical fabrication of tungsten oxide nanoporous layers in closed bipolar cells

Abstract

In this work, the anodization of tungsten (W) foils using closed bipolar electrochemical cells is demonstrated for the first time. The anodization was done using three different electrolytes: (1) 1 M NH₄NO₃, 1 wt%. H₂O in ethylene glycol (EG); (2) 1 M (NH₄)₂SO₄, 75 mM NH₄F in H₂O; and (3) 170 mM NH₄ 1.5 wt%. H₂O in EG. Different square-wave potentials and frequencies were applied during the anodization. Among the tested electrolytes, electrolyte 1 produced the most well-defined and homogeneous WO₃ nanoporous (NP) layers. X-ray photoelectron spectroscopy confirmed the presence of multiple W oxidation states on the WO₃ NP layers using electrolytes 1 and 2, with W⁶⁺ and W⁵⁺ being the dominant species. The results demonstrate well-defined WO₃ NP layers with a high W⁶⁺ species concentration and less than 10 at.% W⁵⁺ is achieved using electrolyte 1. These findings provide valuable insights into the relationship between the electrolyte composition, W oxidation states, and the morphology of WO₃ NP layers.