Redox flow batteries (RFBs) are rechargeable electrochemical systems in which two redox-active species, operating at disparate potentials, are dissolved or suspended in liquid electrolytes, stored in inexpensive tanks, and pumped through a power-converting electrochemical stack (reactor) where they are oxidized and reduced to charge and discharge the battery. We work on the science and engineering of proton-conducting and cross-over free solid-state ceramics for use in RFB for long-duration energy storage. Combining expertise in inorganic synthesis, solid-state chemistry, materials characterization, electrochemistry, electrochemical engineering, mechanical design, and techno-economic modeling, we will expand the library of fundamental knowledge on the behavior of solid-state proton conductors in contact with liquid electrolytes, establish design criteria for competitive embodiments, and pioneer proof-of-concept prototypes that illustrate the transformative potential of this approach.