Reliable hydrogen-selective membranes represent a major technological barrier to economically produce hydrogen from carbon-based fuels. While Pd-alloy membranes presently lead the technology in this area, amorphous alloy membranes composed primarily of Ni and early transition metals (ETMs) exhibit several advantageous characteristics compared to the Pd-based alloy membranes, namely cheaper raw materials and reliable fabrication process. These materials are therefore of particular interest for the large-scale production of hydrogen from carbon-based fuels.

Sustained operation at 400°C in a coal-derived syngas environment desired to combine a high-temperature water-gas-shift catalyst and a membrane into a single membrane reactor module places great demands on the physical stability of such membranes. The upper temperature limit for these membranes is limited by several factors, most notably the crystallization of the membrane, which decreases its hydrogen permeability and mechanical strength.

CSIRO is currently developing an amorphous alloy membrane for the separation of hydrogen from coal-derived syngas at 400°C. A comprehensive examination of the glass-forming ability, thermal stability and hydrogen permeability has been undertaken, and a series of design constraints have been identified. The ratio of Ni and ETM components in the amorphous alloy for maximum thermal stability is determined and a co-relation between the bond valence of the ETM component and the thermal stability of membrane determined.

The results show that the most suitable alloy for operation at 400°C lies within the Ni60Nb(40-X)ZrX system. Hydrogen permeability and thermal stability measurements for alloys in this system are presented with guidelines for designing membranes for different operating regimes.