Materials: Five separate batches of the IL [MPPip][TFSI] were purchased from two different suppliers whose names are available upon reasonable request with a stated purity of either 99% or 99.9%. Details about the origin, purity, and purchase date are given in Table . Ultrapure water (18.2 MΩ cm) from a Sartorius Arium Pro water system was used for cleaning the electrochemical equipment. The water added to the ILs was also taken from this system. The working electrode consists of a Au(111) single crystal with a diameter of 12 mm purchased from MaTecK. Electrolyte preparation: Batches 3, 4 and 5 were used as received, without any further purification steps unless explicitly specified. In contrast, batches 1 and 2 were always vacuumdried. The drying process of the ILs under vacuum were done with a rotary vane pump at the minimal pressure of 10 -3 mbar and 60 °C overnight, while stirring the electrolyte. In some experiments, a molecular sieve (4 Å, Merck ) was added to the IL prior to the purification with vacuum and heat treatment, as described before. Details about the water content and preparation procedure of each measurement are summarised in Tables S1-2 of the SI. The ILs were prepared in an MBRAUN LABStar glove box with a N 2 (5.0, MTI Indus-trieGase AG) atmosphere (O 2 and H 2 O content ≤ 0.5 ppm). Only the addition of water was done under atmospheric conditions. After water addition, the vial with the IL was exposed to vacuum with a rotary vane pump at the minimal pressure of 10 -3 mbar for 3-5 cycles of approximately 30 s and consecutive flushed with N 2 (5.0, MTI IndustrieGase AG) to deaerate the electrolyte. All water contents were determined by Karl-Fischer-titration using a Karl-Fischer coulometer 851 Titrando by Metrohm.