Materials: Pt wires with a diameter of 0.5 mm and a purity of 99.99 % from MaTecK were utilized as AE and PWE. As electrochemical counter electrode a Pt-sheet (10 × 7.5 mm) and as plasma counter electrode a stainless steel plate (20 mm × 20 mm × 3.5 mm size) were used, respectively. The 0.01 M KOH solution was prepared from KOH pellets (99.99 %, Sigma-Aldrich) and Milli-Q water (18.2 MΩ cm, TOC ≤ 3 ppb).