Solid and acid electrolytes for Al-air batteries based on xanthan-HCl hydrogels

This paper presents first investigations on solid and strongly acid electrolytes for Al-air batteries. These electrolytes are prepared starting from a "green" polysaccharide (xanthan gum) and HCl solutions (between 4 and 24 wt%). The gelling capability of xanthan is used to obtain real solid products characterized by ionic conductivities of practical interest (10(-2) S cm(-1)) in electrochemical cells. The adsorption properties of xanthan on metal Al are exploited to control anode self-corrosion and realize Al-air cells with very high anodic efficiencies (> 80%). The behavior of Al-air cells is studied utilizing the weight loss technique, electrochemical impedance spectroscopy, potentiodynamic polarization curves, scanning electron microscopy coupled to energy-dispersive spectroscopy, and discharge tests at constant current (1-5 mA) with Pt/C-based air cathodes. The best overall performance is observed with electrolytes prepared starting from HCl at 24% and gel solid/liquid ratio of 1.40 g ml(-1). The hydrogels obtained in this work permit for the first time the operation of an Al-air galvanic cell based on solid and strongly acid electrolytes with high anodic efficiency and limited dendrite formation.