NH4+ quantification

The produced NH₃ was quantitatively determined using the indophenol blue method^10,69. Typically, a certain amount of electrolyte was taken out from the reaction cell and diluted to 2 ml. Then, 2 ml of 1 M NaOH (≥98%) solution containing sodium citrate (≥99%) and salicylic acid (≥99%) (stored at 4 °C) and 1 ml of freshly prepared 0.05 M NaClO was added. The mixed solution was shaken for few seconds. Finally, 0.2 ml of 1 wt.% sodium nitroferricyanide (≥99%) solution (stored at 4 °C) were added for the colour reaction. After keeping at room temperature for 1 h, the resulting solution was measured using an ultraviolet–visible (UV–Vis) spectrophotometer. The absorbance at ~655 nm was used to determine the concentration of NH₃. In order to quantify the amount of NH₃, a calibration curve was built using standard NH₄Cl (≥99.5%) solution in 0.1 M KOH.