Electrochemical etching of a niobium foil in methanolic HF for electrolytic capacitor

•Surface enlargement of Nb foil can be achieved by electrochemical etching in methanolic HF.•Electrolytic capacitor of etched niobium foil exhibits a capacitance of 350 μF cm−2.•The method provides a way of developing commercially viable process.

Electrochemical etching of niobium foil in order to enlarge the surface area for the application in electrolytic capacitor was carried out in a methanolic electrolyte. We found that the pit density and depth are not linearly proportional to concentration of HF and applied potential: there is the optimal concentration of HF at each applied potential. The optimal etching condition was obtained at 50 V in 0.99 vol.% HF, which exhibited the capacitance of 350 μF cm−2. Pit density and depth of pits on electrochemical etched Nb foil under different conditions were counted from SEM images and electrochemical impedance spectroscopy (EIS) of the etched Nb foils was carried out for the capacitance measurement. Equivalent circuit model showing less than 5% error was suggested for applying to the etched niobium foil.