The Au POSCs were prepared from a Au wire with a diameter of 1.0 mm (purity: 5N, MaTecK). The 0.01 M KOH electrolyte was prepared from KOH pellets (Sigma-Aldrich, 99.99 %) and Milli-Q water (18.2 MΩ cm, TOC ≤ 3 ppb). Aqua regia was used for cleaning and etching the POSCs, which was prepared from fuming HCl (37 %, Merck, for analysis) and HNO 3 (65 %, Merck, for analysis) with a ratio of 3:1.