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

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 NH 4 NO 3 , 1 wt%. H 2 O in ethylene glycol (EG); (2) 1 M (NH 4 ) 2 SO 4 , 75 mM NH 4 F in H 2 O; and (3) 170 mM NH 4 1.5 wt%. H 2 O 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 WO 3 nanoporous (NP) layers. X-ray photoelectron spectroscopy confirmed the presence of multiple W oxidation states on the WO 3 NP layers using electrolytes 1 and 2, with W 6+ and W 5+ being the dominant species. The results demonstrate well-defined WO 3 NP layers with a high W 6+ species concentration and less than 10 at.% W 5+ is achieved using electrolyte 1. These findings provide valuable insights into the relationship between the electrolyte composition, W oxidation states, and the morphology of WO 3 NP layers. • First ever anodization of W foils using closed bipolar electrochemical cell shown. • Complete surface transformation of metallic W into nanoporous (NP) WO 3 layers. • XPS confirmed multiple W oxidation states, predominantly W 6+ and W 5+ . • Demonstrated relationship of electrolytes / W oxidation states / NP morphology.