Gas formation was investigated in an EL-CELL (PAT-Cell-Press) with a cup (polypropylene) containing the electrolyte mixture and sodium pieces. The cell was handled in an argon filled glove box and a PEEK sealing was used to close the cell tightly. The pressure was observed over a period of time and afterward the gases were extracted with a gas-tight syringe. The procedure is described in Ref 29 in more detail.