Perspectives on Technical Challenges and Scaling Considerations for Tubular Protonic-Ceramic Electrolysis Cells and Stacks

Abstract

Proton-conducting ceramics (protonic ceramics) form the basis for applications that include intermediate-temperature (e.g., $500$--$700$~$^\circ$C) fuel cells, electrolyzers, and membrane reactors. The electrolyte membranes are typically perovskites such as heterovalently doped barium cerates and zirconates (e.g., BaCe$_{1-x-y}$Zr$_x$Y$_y$O$_{3-\delta}$, BCZY; and BaCe$_{1-x-y-z}$Zr$_x$Y$_y$Yb$_z$O$_{3-\delta}$, BCZYYb). Although the materials are dominantly proton conductors, they are mixed ionic-electronic conductors (MIEC) with oxygen-ion and small-polaron mobility. The present paper is concerned primarily with steam-electrolysis applications with the reactors using tubular cell configurations. An important advantage of the protonic-ceramic cells is that they can produce nearly dry hydrogen. Each tubular cell is comprised of a negatrode (electrolysis cathode), proton-conducting electrolyte membrane, and a positrode (electrolysis anode). The tubular cells are typically supported on the relatively thick (order of one millimeter) composite negatrode, with thin (order tens of microns) external membrane and positrode layers. The paper explores considerations for scaling from laboratory-based demonstrations to deployable technology.