The thinner membrane is contributed to higher hydrogen flux but rolled sheet of pure palladium is very expensive and has limitation of its thickness reduction.
To enhance hydrogen permeability, increase
separation efficiency and reduce membrane cost, various
methods, including chemical vapor deposition
(CVD)[5,6], sputtering[7], electroless plating[7–10] and
combining of these technique[11], were utilized to
make ultra-thin membrane. However, only the membrane
itself cannot withstand the pressure difference
which associates hydrogen atoms diffusing through
the membrane. The palladium membrane is thus always
prepared by depositing on the porous substrate.
Typical substrate materials for the composite membrane
include ceramic and stainless steel[12,13]. Nevertheless,
metallic substrate tends to form solid state
interdiffusion with membrane at elevated temperature
which would decline the hydrogen flux through the
palladium membrane. To solve that problem, it was
proposed to modify the metal substrate with intermediate
ceramic layer as a diffusion barrier[14]. On the
contrary, ceramic supports have preferential advantages
of being chemically inert and stable especially
at high temperature. Also, their structures have a
benefit effect on the stability of composite membrane.
Porosity, pore size and thickness present in the substrate
before metal deposition have an important influence
on the deposited film and hydrogen permeation
of composite membrane[15].