The control and optimization of macro/micro-structure of ion conductive membranes for energy conversion and storage

Ion conductive membranes (ICMs) are frequently used as separators for energy conversion and storage technologies of fuel cells, flow battery, and hydrogen pump, because of their good ion-selective conduction and low electronic conductivity. Firstly, this feature article reviews the recent studies on the development of new non-fluorinated ICMs with low cost and their macro/micro-structure control. In general, these new non-fluorinated ICMs have lower conductivity than commercial per-fluorinated ones, due to their poor ion transport channels. Increasing ion exchange capacity (IEC) would create more continuous hydrophilic channels, thus enhancing the conductivity. However, high IEC also expands the overall hydrophilic domains, weakens the interaction between polymer chains, enhances the mobility of polymer chains, and eventually induces larger swelling. The micro-scale expansion and macro-scale swelling of the ICMs with high IEC could be controlled by limiting the mobility of polymer chains. Based on this strategy, some efficient techniques have been developed, including covalent crosslinking, semi-interpenatrating polymer network, and blending. Secondly, this review introduces the optimization of macro/microstructure of both per-fluorinated and non-fluorinated ICMs to improve the performance. Macro-scale multilayer composite is an efficient way to enhance the mechanical strength and the dimensional stability of the ICMs, and could also decrease the content of perfluorosulfonic acid resin in the membrane, thereby reducing the cost of the per-fluorinated ICMs. Long side chain, multiple functionalization, small molecule inducing micro-phase separation, electrospun nanofiber, and organic–inorganic hybrid could construct more efficient ion transport channels, improving the ion conductivity of ICMs.