Collaborative Research: Elucidation of the Grotthuss Topochemistry in Reticular Electrodes for Fast Proton Batteries

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Non-Technical Summary With this collaborative project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, two research groups at the University of California Riverside and Oregon State University investigate fundamental aspects of how fast diffusion of hydrogen ions occurs in confined networks of water. When metal ions move though water, they push past the water molecules as they go. It is already known that hydrogen ions migrate in a completely different manner and faster. The researchers study in detail how this process works and what makes it many times faster than the diffusion of metal ions, for example what makes it faster than that of lithium ions in batteries. Several factors can enable very fast, hydrogen-ion batteries that have the potential to be charged and discharged for millions of cycles, and so present a remarkable opportunity to realize the Holy Grail of electrochemical energy storage: to achieve simultaneously the energy densities of batteries and the power and cycle life of capacitors. The abundance of hydrogen also makes hydrogen-ion batteries a promising candidate for the grid-level storage batteries that are needed to provide a continuous and dependable electricity supply from intermittent power sources such as wind and solar energy. Advancing knowledge and the associated technology in these areas aids the United States to remain economically competitive. Additionally, the project produces educational videos targeted to students and the broader public that present concepts in energy storage and its role in society. Training of graduate and undergraduate students with the skills needed to enter the workforce in the energy technology sector and additional outreach activities take place at both institutions.Technical Summary The existing knowledge of battery chemistry is built upon the understanding that the kinetics are dictated by desolvation and vehicular diffusion of the working ion. The research in this collaborative project, which is supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, explores a new paradigm of battery chemistry where by using protons as the working ions with an aqueous electrolyte, charge conduction does not rely on the long-range physical migration of ions through the host electrode but instead obtains long-range movement of charge via the Grotthuss mechanism of proton displacement along the crystal water network in an electrode. Transport of protons via the Grotthuss mechanism involves the movement of a quasiparticle defect in the bonding topology of water. It is fundamentally different from the vehicular transport of metal cations and could give rise to ultra-fast insertion kinetics and the ability to provide batteries that deliver high power at ultra-low temperatures. The project focuses on mechanisms of proton transport and storage in Turnbull blue and its family of analog compounds. These systems have an open and defected crystal structure that hosts an internal network of crystal water. The crystal water network provides pathways for Grotthuss diffusion, making the kinetics of proton transport and storage exceedingly fast, even at temperatures well below the freezing temperature of water. This project tests the central hypothesis that the transport performance of protons in this system depends on the topology imposed on the H-bonding network of crystal water by the surrounding host framework. To test this, the researchers a. determine the topological characteristics of the water network in Prussian blue analogs, from the atomic to the mesoscopic scale, and the role they play in Grotthuss topochemistry; b. elucidate the mechanisms of proton insertion, storage, and transport in the water network within Prussian blue analogs at all states of charge; and c. formulate testable design principles that can guide the development of new reticular materials for proton transport and storage.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.