Indeed, the cell containing added NaH evolved 16 times more H2 than the blank at the same potential. Assuming the NaH is not partaking in other H2 evolving side reactions at the same voltage, it is concluded that the H2 peak observed at 2.5 V (where HC is at ~600 mV vs Na + /Na) is partially (if not completely) caused by the oxidation of NaH in the SEI when HC is above 430 mV vs Na + /Na. Similar experiments were performed in a symmetric HC|HC cell configuration and confirms the observation of H2 evolution once the HC electrode is oxidized.