Structure and performance of clay composite membranes with improved sodium conductivity for salinity gradient batteries

Low-cost cation exchange membranes with improved ionic conductivity and permselectivity are needed for the deployment of efficient large-scale energy storage technologies or separation technologies such as electrodialysis. In this work, a series of montmorillonite (Mt) clays and sulfonated poly(ether ether ketone) (SPEEK) composite membranes with 1 to 20 weight percentage (wt%) additives are studied. Two types of clays are investigated, a generic K30 Mt and an aluminum pillared (Al-pil) Mt with larger interlayer spacing owing to the inorganic crosslinks between the clay platelets. The addition of inorganic clays with two-dimensional geometries enables the formation of percolating sodium diffusing pathways with reduced tortuosity. As a result, the conductivity of the membranes increases with an increasing clay loading fraction, reaching up to 1.4 times that of the pure SPEEK with 20 wt% K30 Mt. The permselectivity of the native SPEEK membrane also improves with the addition of set amounts of K30 Mt, while the Al-pil Mt composites suffer from a slightly reduced permselectivity due to their higher water uptake. The voltaic efficiency of a concentration gradient flow battery shows that the addition of 20 wt% K30 Mt clay to the SPEEK polymer matrix can improve the voltaic efficiency by up to 10%.