Broadly, hydrogen selection membranes (based on the materials used) can be categorized into four types: (i) polymer (organic), (ii) metallic, (iii) carbon, and (iv) ceramic.8 The latter three are called inorganic membranes. Inorganic membranes can be classified into two groups from the viewpoint of the raw material-metal membranes and ceramic membranes.21 Also, it could be divided into porous (meso- and microporous) and nonporous (dense) membranes.25 Dense inorganic membranes are typically made of a polycrystalline ceramic or metal. The table below illustrates some of the most important factors in membrane properties.
[pic]
[pic]
Table 15 provides a comparison of the classes of the H2 separation membrane materials presented throughout this review. When this information is compared against the 2015 targets, we quickly see that each individual class has its own distinct advantages and disadvantages

While polymer-based membranes are arguably the cheapest and easiest processed of the materials, they are less thermally robust and lack sufficient selectivity and flux capacities.

Dense type polymer membranes can be divided into glassy and rubbery polymeric membranes. The former have higher selectivity and lower flux, whereas the latter have higher flux but lower selectivity.19 According to Kluiters,19 operating temperatures for polymer membranes are _100 °C. Several key advantages are that they possess the ability to cope with high-pressure drops and low cost. However, limited mechanical strength, relatively high sensitivity to swelling and compaction, and susceptibility to certain chemicals such as hydrochloric acid (HCl), sulfur oxides (SOx), and CO2 make polymeric membranes less attractive.

Though carbon-based membranes show much promise in the area of light gas separations, they stillpossess problems that influence their introduction to market. First, they are very brittle and fragile. Second, their difficulty to process results in high expenses to fabricate. Third, they only demonstrated high selectivities for certain gas mixtures, with gases of molecular sizes smaller than 4.0-4.5 Å.

metallic This article focuses specifically on metallic membranes for the separation of H2.Hydrogen selectivity is typically very high in these systems, since the dense structure prevents the passage of large atoms and molecules such as CO, CO2, O2, N2, etc.). This high selectivity translates to very high purity H2 and the increased thermal stabilities allow higher operating temperatures. These are the primary advantages that metallic membranes offer over other materials. The metals which are most suitable for H2 separation membranes typically have high H2 permeabilities,25 high diffusivities or solubilities,26 and good thermal stability at elevated temperatures.27 These include but are not limited exclusively to tantalum, niobium, and vanadium, and unlike platinum and palladium, they are abundant and comparatively
[pic] [pic] Combined result of these composition and performance issues ultimately determines the cost and viability of a given material for application in commercial H2 separation technologies. The five performance targets for H2 separation set forth by the U.S. Department of Energy reflect the present capabilities and highlight the distinct research and development opportunities which are necessary components to fully realize the hydrogen economy.14 The specific targets are as follows: (1) higher H2 flux rates; (2) lower material costs; (3) improved durability; (4) lower parasitic power requirements; and (5) lower membrane production/fabrication costs (see Table 1 for target values).1

Regardless of which method is used, separation and purification of the nearly 6 Exajoules (1 EJ ) 1018 joules) of H2 which is produced industrially per year is a paramount task which the membrane research ommunity must endeavor to address before the “hydrogen economy” can become a reality.

[pic]
[pic]