Electrically conductive hydrogels for flexible energy storage systems

To power wearable electronic devices, various flexible energy storage systems have been designed to work in consecutive bending, stretching and even twisting conditions. Supercapacitors and batteries have been considered to be the most promising energy/power sources for wearable electronics; however, they need to be electrochemically sustainable and mechanically robust. Electrically conductive hydrogels (ECHs), combining the electrical properties of conductive materials with the unique features of hydrogels, are ideal frameworks to design and construct flexible supercapacitors and batteries. ECHs are intrinsically flexible to sustain large mechanical deformation; they can hold a large amount of electrolyte solution in a 3D nanostructured conducting network, providing an extremely high surface area for the required electrochemical reactions. To date, nanostructured three-dimensional ECHs have exhibited high performance when applied as active electrode materials for supercapacitors and lithium-ion batteries. Future research may attempt to develop functional ECHs with controllable size, composition, morphology, and interface. This review summarizes the material design and synthetic approach of ECHs, demonstrating the advances of percolation theory in ECH materials, and subsequently presents their effective application in flexible energy storage systems and discusses the challenges and opportunities in this field.