To further investigate the growth process we use a rotating ring disk electrode (RRDE) with a Cu disk (Adisk = 0.196 cm 2 ) and a Pt ring electrode, with a collection efficiency of 25.6%. The ring potential is fixed at 3.5 V, which is expected to be sensitive only to molecular hydrogen , and the electrode is rotated at 100 rpm. Figure shows the response of the ring current as the disk potential is cycled at 5 mV/s from OCP to 0.1 V at different concentrations of added water. We note however that a greyish deposit was observed on the disk after subsequent electroplating for the dry electrolyte (SI-6), such that this case might not be comparable one to one with the stationary electrode (Figure and methods). The disk current shows similar features to what was observed on the stationary electrode in Figure . The ring current concurrently displays clear peaks at ~0.5 and 1.3 V, with a minor increase near 2 V. Furthermore, both the disk and the ring current at 1.3 V increase with increasing water concentration, while at the 0.5 V peak they are both highest for the dry electrolyte. It is worth mentioning that the same behavior was observed when using a LiClO4 salt (SI-6). To understand the voltammogram it is insightful to consider the fraction of the current generating hydrogen at the disk, or efficiency, by plotting | 𝐼 𝑟𝑖𝑛𝑔 𝐼 𝑑𝑖𝑠𝑘 * 0.256 |, as shown in Figure for the downward scan. As this representation is sensitive to noise at low Idisk we mainly consider the positions around the peaks in disk current, indicated by the arrows. As such, the first (2 V) peak actually results in a dip in the partial current, which corresponds to a H2 generating partial current of ~0.1 for both 25 mM, and 75 mM of added H2O. At the second (1.3 V) peak, the partial current is clearly dependent on concentration of H2O, rising to approximately half the total current at the highest concentration. Importantly, the efficiency plateaus with a slight decrease as the scan progresses, indicating that all reactions passivate at similar speeds. The observation of the plateau is also crucial at 0.5 V. In fact, while a peak is observed in the ring current, the efficiency dips before recovering. As such, we do not attribute the peak in ring current to an additional H2-generating reaction, but rather to be due to a different process that catalyses the reaction starting at the 1.3 V peak. As the peak at 0.5 V is typically associated with Li underpotential deposition (UPD) process , this can potentially be attributed to modification of the surface. We therefore consider 2 reactions generating H2, with onsets near ~2 V and ~1.3 V.