Furthermore, the hydrogen peak occurs at a specific voltage and is relatively sharp (i.e., rapid evolution) suggesting the reaction is more likely triggered by an electrochemical process rather than a physical one associated with electrode shrinkage, which should result in a more extended peak. The sudden release of H2 during discharge of this PW|HC full cell is more likely a result of the upwards sloping potential of the HC at this point, which rapidly changes the environment around the anode from reductive to oxidative. SEI species that formed reductively on the HC while it was sitting at ~100 mV vs Na + /Na could be re-oxidised. Since the present understanding of the SEI on HC in Na-ion systems is mostly limited to contain NaF and NEDC, both of which are not expected to release H2, a more likely candidate is however NaH, a hydride salt. The reaction between Na and H has a negative standard Gibbs free energy of formation below, implying its formation is favourable . Vilá et al. showed through cryogenic electron microscopy that LiH is present within the SEI on a lithium metal anode in LIBs, outlining how it forms when a lithium-ion half-cell contains trace hydrogen gas . It is not unlikely, therefore, to extend these findings to the Na-ion system since both hydrogen (Eq. 1 & Eq. 6) and Na in a close-to-metallic state in internal pores of HC at 100 mV are present . The formed NaH at the anode surface would then be oxidised according to Eq. 8 at ~0.43 V vs Na + /Na (based on the assumption that such a reaction occurs at a similar potential to LiH, which has been shown to oxidise at 0.76 V vs Li + /Li) .