These figures seem to imply that fresh lithium is required for the generation of hydrogen. This is based both on the vanishing ring current during prolonged formation (cycling) and relaxation after growth (where the disk potential is 0 V), and on the peak in ring current during nucleation and early growth, where the growing layer is expected to have the largest surface area . We further note that the generation of hydrogen is not limited by diffusion, based on the effect of H2O concentration and rotation rate (SI-6). Importantly, these results also indicate that hydrogen is continuously present at the interface during the electrodeposition process.