Recent progress in graphene-based and ion-intercalation electrode materials for capacitive deionization

Capacitive deionization (CDI) has attracted considerable interest as an energy-efficient and cost-effective approach for water desalination. The fundamental working principle of CDI relies on ion electrosorption on the surface of a pair of electrically charged electrodes. Therefore, the development of more efficient electrode materials with high salt adsorption capacity is at the heart of CDI technology, and has motivated considerable efforts over the past decade. In particular, various carbon materials, including conventional activated carbon and various forms of porous carbon and graphene, have been intensively investigated as the CDI electrode materials for their electrochemical stability and large surface area, which typically relies solely on ion absorption and the formation of Helmholtz electrical double layer. It has more recently been shown that significant improvements in salt adsorption capacity can be further achieved either by functionalization of carbon surface with redox-active groups, or by introducing electrochemically active transition metal oxides or conductive polymers into the carbon matrix. Herein, we briefly summarize the fundamental principle of CDI technology, and review the recent progress in the design and development of high performance CDI electrode materials. We will conclude with a brief perspective on the current challenges and future opportunities in CDI technology.