Composite, Thin Film, Supported — MIEC Membranes for Hydrogen Generation and Separation
Committee Members: Advisor: Srikanth Gopalan, MSE/ME; Uday Pal, MSE/ME; Soumendra N. Basu, MSE/ME; Xi Lin, MSE/ME; Appointed Chair: Catherine Klapperich, ME/ME
Abstract: Decomposition of steam under a chemical driving force at moderate temperatures (~900 oC) offers a convenient and economical way to generate hydrogen. A significant amount of hydrogen can be generated and separated by splitting steam and removing oxygen using a mixed ion-electron conducting (MIEC) membrane. In this work, Gd0.2Ce0.8O1.9-δ – Gd0.08Sr0.88Ti0.95Al0.05O3±δ MIEC membranes have been explored in which, Gd0.2Ce0.8O1.9-δ (GDC) functions as a predominantly oxygen ionic conductor, and Gd0.08Sr0.88Ti0.95Al0.05O3±δ (GSTA) functions as a predominantly n-type electronic conductor under the process conditions. During the hydrogen generation process, oxygen transports from the feed side to the permeate side through coupled diffusion of oxygen ions and electrons under an oxygen partial pressure gradient across membranes. This process results in a H2-rich product on the feed side and depleted fuel gases on the permeate side.
In this work, membrane architectures comprising self-standing thick membranes and thin membranes supported on porous supports of the same composition have been studied. The effect of membrane thickness on hydrogen generation has been studied by measuring the area-specific hydrogen generation rates at different experimental conditions. Experimental results have shown that the hydrogen generation process for the thick membranes was controlled by the oxygen bulk diffusion through membranes, while the hydrogen generation process for the dense thin membranes was controlled by both the surface exchange reactions and oxygen bulk diffusion process. The area-specific hydrogen generation rates of the supported dense thin membranes were significantly enhanced by applying a porous catalytic layer onto the surface of the membrane. Experimental results showed that the area-specific hydrogen generation rates were higher when the surface catalytic layer was exposed to the feed side rather than the permeate side. A mathematical model for calculation of the area-specific hydrogen generation rate has been developed that takes into account the measured oxygen partial pressures, gas compositions, and gas flow rates of the inlet and outlet gases on the feed side of the membrane. Based on these results, recommendations have been made for a system based on an MIEC membrane to generate hydrogen at practically attractive rates.