Promising Proton Conductor for Intermediate-Temperature Fuel Cells: Li_13.9Sr_0.1Zn(GeO₄)₄

Abstract

Commercialization of fuel cell technologies hinges on the development of solid electrolytes of sufficient ionic conductivity at intermediate temperatures (200–600 °C). Here we report a novel proton conductor derived from Li13.9Sr0.1Zn(GeO4)4 (LSZG), demonstrating the highest protonic conductivity (0.034 S cm–1 at 600 °C) among all known proton conducting ceramics, which is much higher than those of several well-known oxygen ion conducting electrolytes (e.g., ∼0.009 and 0.018 S cm–1, respectively, for zirconia- and ceria-based oxide electrolyte at 600 °C). Interestingly, after fully replacing the mobile Li+ ions by H+ through proper ion exchange, the H+ conductivity increases from 0.034 to 0.048 S cm–1 at 600 °C. A simple but effective ab initio molecular dynamics simulation study suggests a unique H+/Li+ transport mechanism: the proton in LSZG moves freely in the Li+ interstitial space within the 3D Li+ transport network (i.e., 4c and 4a sites, as the occupancies of the Li1 and Li2 sites are 55% and 16%, respectively). In particular, a solid oxide fuel cell (SOFC) based on an LSZG electrolyte (∼40 μm thick) demonstrates high open circuit voltage (∼1.1 V) and good peak power density (377 mW cm–2) at 600 °C. The cell performance may be further improved if the electrode–electrolyte interface can be optimized. The new transport mechanism and excellent proton conductivity suggest that the LSZG represents an important family of electrolyte materials, which may be used as a proton-conducting membrane for intermediate-temperature SOFCs and hydrogen production or separation.