Later on, in the charge step, half-way across the LS-FeC plateau at ~3.3 V, CO2 starts to evolve again, increasing until the cut-off voltage is reached. This is the onset of the impact that the PW electrode potential has on oxidizing electrolyte impurities. Since we are within the electrolyte stability window, electrochemical oxidation of EC or DEC is unlikely. Rather, impurities in the electrolyte/on the electrodes such as alcohols (e.g. forming aldehydes) and other -COxH groups oxidise forming CO2 . However, as soon as the cell switches over to the discharge step CO2 seems to immediately cease to evolve, which indicates that the PW surface structure is stabilized against electrolyte degradation upon re-sodiation of its surface. The lithium layered oxides (NMCs) observe similar behaviour, where CO2 evolution ceases abruptly upon lithiation and discharge . Therefore, another possible cause for the observed CO2 is the ring opening of EC due to nucleophilic attack from exposed CN groups at the PW surface. The likelihood of this increases the more Na + is removed from the material, explaining the dramatic drop in CO2 when the surface is re-sodiated upon discharge. Finally, the most dominant and remarkable gas evolution process is observed approaching the end of discharge at 2.50V where H2 is seen forming in each consecutive cycle. The same observation was previously made by Geisler when cycling HC against Na metal in the presence of EC:DEC based electrolyte . Dreyer et al. who only looked at PW half cells did not observe this phenomenon, suggesting it is solely associated with HC . Geisler postulated that this was the result of the fast and dynamic shrinkage of the HC when Na + is being de-inserted from the structure, which in turn releases trapped H2 that formed during the charge step due to Na plating in the HC pores . To our knowledge, this feature is unique for HC as the reaction between Na metal (after plating occurs) and carbonate-based electrolyte produces also other gaseous products, not just H2. Considering the molecular size difference it is however more likely that other gases, such as CO2, CO and C2H4 would be trapped and therefore observed at this point over H2 which is small and notoriously difficult to contain in carbonaceous materials at RTP .