Reversible Multivalent (Monovalent, Divalent, Trivalent) Ion Insertion in Open Framework Materials

The reversible electrochemical insertion of multivalent ions into materials has promising applications in many fields, including batteries, seawater desalination, element purification, and wastewater treatment. However, finding materials that allow for the insertion of multivalent ions with fast kinetics and stable cycling has proven difficult because of strong electrostatic interactions between the highly charged insertion ions and atoms in the host framework. Here, an open framework nanomaterial, copper hexacyanoferrate, in the Prussian Blue family is presented that allows for the reversible insertion of a wide variety of monovalent, divalent, and trivalent ions (such as Rb+, Pb2+, Al3+, and Y3+) in aqueous solution beyond that achieved in previous studies. Electrochemical measurements demonstrate the unprecedented kinetics of multivalent ion insertion associated with this material. Synchrotron X-ray diffraction experiments point toward a novel vacancy-mediated ion insertion mechanism that reduces electrostatic repulsion and helps to facilitate the observed rapid ion insertion. The results suggest a new approach to multi valent ion insertion that may help to advance the understanding of this complex phenomenon.