A better understanding of the H2 release can be obtained considering the cross-talk between both electrodes. Oxidation of the electrolyte or other electrode components like the conductive carbon leads to protic species which can diffuse to the sodium metal counter electrode. Reduction of these protic species at the counter electrode then leads to the release of H2, i.e. the formation of H2 is due to a cross-talk between both electrodes. This was nicely demonstrated by Metzger et al. for Li-cells using a DEMS setup in which cross-talk between the electrodes was eliminated by implementing a solid electrolyte membrane . Similar results were reported by Michalak et al. using a LiMNO|Graphite full cell. They found an onset for CO2 and H2 evolution when Ni is completely converted to Ni 3+ and Ni 4+ starts to form, and that the H2 evolution is mainly caused by the counter electrode . Comparison of these findings with the present results