Dual solid electrolytes for aluminium-air batteries based on polyvinyl alcohol acidic membranes and neutral hydrogels

Abstract

Water-based electrolytes are generally preferable to organic electrolytes due to their low cost and higher safety standard. However, their widespread use is generally limited by recharging issues. In the case of Al-air cells, water on the anode prevents aluminium deposition during recharging by producing hydrogen at more positive standard potentials. This promotes parasitic reactions that consume the anode and shorten the cell shelf life. On the cathode side, depending on the pH, water allows efficient oxygen reduction or oxygen evolution reactions. To produce electrolytes with different water contents on the two electrodes, we prepared solid electrolytes using polyvinyl alcohol (PVA) and aqueous HCl solutions with ionic conductivity ranging from 0.7 to 2.2 × 10−3 S cm−1 and electrochemical windows wider than 2.2 V. The new materials resembled a thin plastic membrane (0.6–0.9 mm thick) and were able to oxidise the Al-air cell anode. We also explored the possibility of using a dual electrolyte to allow the presence of water mainly on the cathode side. Specifically, cells assembled with two electrolytes in contact, with an acidic PVA membrane as anolyte and saline hydrogel as catholyte without separator, showed discharge capacity more than one order of magnitude greater than cells with single acidic PVA electrolytes. Electrochemical impedance spectroscopy (EIS) characterisation verified that the improved cell discharge capacity was attributed to the soft aqueous catholyte on the cathode-electrolyte interface and to the membrane conductivity stabilisation during cell use through the saline gel water. Cyclic voltammetry (CV) characterisation suggested the possibility of recharging the dual electrolyte cells thanks to the substantial absence of water on the anode side.