Materials Chemistry of Superprotonic Solid Acids

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Non-technical Summary Materials with the ability to transport protons are essential for a wide range of technologies, from electricity production to ammonia synthesis. This project, funded by the Solid State and Materials Chemistry Program in the Division of Materials Research, focuses on the discovery of new proton conducting materials with exceptional transport due to structural disorder. Such materials combine remarkable "liquid-like" conductivity with the mechanical integrity of solids. Aiming to discover the design rules for creating new superprotonic conductors that meet a broad range of application needs, Prof. Haile and her group create new knowledge about the fundamental features of superprotonic conductivity through this study. Insights from their research will help, for example, to improve fuel cells for electricity production, to improve electrolyzers for hydrogen production, and to find new routes for ammonia synthesis. Beyond the specific scientific principles that are elucidated, the project advances the education of students across all levels, from engaging K-12 students through outreach activities, to guiding undergraduates and graduate students in cutting edge research.Technical Summary With funding from the Solid State and Materials Chemistry Program in the Division of Materials Research, this project advances the science of superprotonic solid acids, materials which are of increasing technological importance in a range of applications. Solid acid compounds can be described by the generic stoichiometry MnHm(AX4)p, where M is an alkali metal; A is an element such as S, P, or Se; X is oxygen or hydrogen; and n, m, and p are integers. A superprotonic solid acid is one in which rapid reorientation of AX4 polyanion groups facilitates fast proton conductivity. Typically, this disordered state is encountered at slightly elevated temperatures, and the transition to the superprotonic phase is accompanied by 3-4 orders of magnitude increase in conductivity. Through targeted chemical modifications to known superprotonic conductors the origins of the superprotonic transition and the magnitude of the conductivity in the superprotonic phase are elucidated. The research encompasses: (1) synthesis and structural characterization of solid acids with controlled substitutions on cation and polyanion sites; (2) measurement of transport properties; and (3) determination of phase stability. The insights gained are used to develop advanced materials that overcome the remaining technological barriers facing known superprotonic conductors.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.