Hydrogen Permeation through Dense BaCe0.8Y0.2O3-δ - Ce0.8Y0.2O2-δ Composite-Ceramic Membranes

This report presents hydrogen permeation measurements through dense composite-ceramic membranes. The high-temperature operation of ceramic hydrogen-separation membranes makes them suitable in many hydrogen-production applications, such as catalytic partial oxidation of methane (CPOX), steam reforming of methane, the gasification of carbonaceous materials, and methane dehydroaromatization (MDA). Hydrogen permeation is measured through BaCe_0.8Y_0.2O_3-δ–Ce0_.8Y_0.2O_2-δ (BCY-YDC) membranes fabricated by solid-state reactive sintering with 1 wt.-% NiO and standard ceramic-processing methods. Hydrogen gas is incorporated into the membrane as protonic defects; proton transport must be compensated by electron transport in order for protons to re-associate into hydrogen gas on the opposite side of the membrane. BCY serves as the proton conductor, and YDC serves as the electronic conductor.

For hydrogen-permeation testing, the composite-ceramic membranes are hermetically sealed inside of a ceramic manifolding assembly utilizing a spring-compression system with vermiculite seals. Hydrogen and helium (for leak detection) are fed to the permeant (feed) side of the membrane using high-precision mass flow controllers, while argon is fed to the permeate (sweep) side of the membrane. All gases are humidified using a room-temperature bubbler, which results in ~3 mol.-% steam. The permeate gas composition is continuously measured using a gas chromatograph calibrated for low levels of hydrogen, helium, nitrogen, and oxygen (balance argon).  The hydrogen-permeation rate is quantified through the hydrogen mole fraction measured in the permeate exhaust. 

Hydrogen permeation rates are measured as a function of temperature and the hydrogen partial-pressure gradient. The permeation rate is found to increase exponentially with increasing temperature, and linearly with an increasing hydrogen partial-pressure gradient. The hydrogen permeation rate through a BCY-YDC membrane over several days is shown in Figure 1. No degradation in performance is observed at 900 °C and a 0.1 atm hydrogen partial-pressure gradient. However, when the gradient is increased to 0.5 atm, the permeation rate is found to decrease over time. Following testing, grain-boundary fractures and metallic nano-particles are observed in scanning electron micrographs of the membrane. This, in addition to performance degradation, may indicate that the materials are not stable in highly reducing environments. However, X-ray diffraction patterns of the membrane after testing do not reveal the formation of tertiary phases.  

Figure 1: Hydrogen permeation through a BCY-YDC membrane at 900 °C with a 0.1 and a 0.5 atm hydrogen partial-pressure gradient.