(504d) Chemical Stability and Hydrogen Permeation Properties of Zirconium- Doped Mixed Proton-Electron Conducting Strontium Cerate Membranes
Perovskite-structured ceramics with mixed protonic-electronic conductivity have attracted considerable attention in recent years for producing hydrogen of high purity (>99.99%) from coal or methane conversion at high temperatures (> 600 oC). Thulium (Tm) doped strontium cerate (SrCeO3) has a high protonic and electronic conductivity but rather poor chemical stability in CO2- containing atmospheres. Strontium zirconate (SrZrO3), in contrast, is a rather stable material but has low protonic and electronic conductivity. In the present work chemical stability and hydrogen flux of a solid solution between these compounds have been investigated, in an attempt to find a composition with both high electrical conductivity and good chemical stability against CO2. Compound of general formula SrCe0.95-xZrxTm0.05O3-ä where x ranged from 0 to 0.4 were prepared by citrate method and characterized by X-ray powder diffraction, thermal gravimetric analysis, differential scanning calorimetry in flowing CO2 and four- point- direct current (DC) method to measure total electrical conductivity at different partial pressure of O2. Hydrogen flux of the membranes was measured under various operating conditions. Doping of Zr into SrCe0.95Tm0.05O3-ä greatly enhanced its chemical stability; with 40% Zr doped material did not react with CO2. Not unexpectedly, introduction of Zr leaded to a decrease in the total electrical conductivity that was about three times less than that of basic SrCe0.95Tm0.05O3-ä without Zr doping. However, 40% Zr doped powders were difficult to sinter as they could reach only up to 90% of the theoretical density. Best Zr-doped membranes containing a pure perovskite phase, displaying good stability and high density were 30% Zr- doped SrCe0.65Zr0.30Tm0.05O3-ä. It appears to give a good compromise between conductivity and stability for H2 separation applications.