This chapter intends to give insight into the use of carbon-based hydrogels for energy storage and/or conversion, which are becoming critical, given the ongoing need for decarbonization and growing renewable energy production. The ability to use sustainable materials and the added advantage to incorporate tailor-made, nontoxic, cheap, and electrochemically active functional moieties during their synthesis make them frontrunner materials in the field of green chemistry and energy applications. Here, after a brief introduction of graphene-, synthetic polymer-, and natural polymer-based hydrogels and many possibilities for their hybrid combination, a particular emphasis is given to their application to energy storage, energy conversion, and energy efficiency enhancement systems. Application examples concerning metal-air batteries, fuel cells, water electrolyzers, microbial fuel cells, photoelectrochemical water splitting, supercapacitors, lithium-ion batteries, and other metal-ion batteries, desalination, and water purification through solar vapor generation are highlighted.
