In order to test this hypothesis, after charging a full PW|HC cell up to 4.0 V (HC pushed down to 100 mV vs Na + /Na) a finely grinded NaH powder was manually added between the separator and HC, i.e. to be in electronic contact with the negative electrode. Figure shows that the cells, both blank and the one containing added NaH, evolved H2 at ~2.5 V upon discharge.