Superprotonic Solid Acid Compounds for Sustainable Energy Technologies

AMRS Colloquium Series
Title: Superprotonic Solid Acid Compounds for Sustainable Energy Technologies
Speaker: Prof. Sossina M. Haile (Materials Science and Engineering, Northwestern University, Evanston, IL, USA)
Superprotonic solid acid electrolytes, materials with chemical and physical properties intermediate between conventional acids (e.g., H 3 PO 4 ) and conventional salts (e.g., Cs 3 PO 4 ), have emerged as attractive candidates for fuel cell and other electrochemical applications. Key characteristics of these materials, which include CsHSO 4 , Cs 3 H(SeO 4 ) 2 , CsH 2 PO 4 , and Cs 2 (HSO 4 )(H 2 PO 4 ), are tetrahedral oxyanion groups linked by hydrogen bonds and a polymorphic structural transition to a disordered state at moderate temperatures. In the high temperature state, rapid oxyanion reorientation and dynamic disorder of the hydrogen bond network facilitate high proton conductivity. The transition to the structurally disordered phase is accompanied by a jump in conductivity by 3-5 orders of magnitude, and the activation energy for proton transport drops to a value of ~ 0.35 eV. Of materials displaying such behavior, CsH 2 PO 4 is of particular technological significance due to its chemical stability against both oxidation and reduction in device-relevant environments. We present here an overview of the proton transport characteristics of CsH 2 PO 4 and the current status of electrochemical technologies in which it has been deployed. Material limitations translate into device limitations, motivating our efforts to develop and discover new superprotonic conductors. We show that dramatic changes in phase behavior and proton conductivity of the base phosphate can be induced by only minor changes in chemistry, suggesting routes for tuning behavior to achieve desired outcomes.