(223d) High Temperature Metal Membranes for Hydrogen Permeation without Platinum Group Metals

Group V metal membranes (tantalum, vanadium, niobium and their alloys) have extremely high theoretical permeability for dissociated hydrogen[1], but are unable to self-catalyze the hydrogen dissociation reaction.  Such membranes need to be coated with an active catalyst layer, typically palladium or a Pd-alloy, increasing the materials cost of the membrane.  Additionally, the use of a metallic surface catalyst layer limits the upper operating temperature of the membrane, as temperatures higher than 623 K tend to promote alloying, which causes loss of flux.  As group V metals are also prone to low-temperature hydrogen embrittlement, this limits their applicability.

Transition metal carbides, nitrides, and sulfides have been studied as substitutes for platinum group catalysts in applications requiring hydrogen dissociation, such as the water-gas shift reaction.  The goal of this study was to determine if these lower cost materials could be used as catalyst layers for group V metal membranes.

Molybdenum carbide (Mo₂C) was chosen as the test transition metal catalyst, and vanadium as the group V bulk metal.  Magnetron sputtering was used to apply layers of Mo₂C to both sides of commercial cold-rolled vanadium foils with thicknesses ranging from 50 to 125 microns.  Membranes sputtered at temperatures of 473 K or below had a cubic structure, and were the most permeable to hydrogen, while higher-temperature (873 K) sputtering produced an orthorhombic lattice[2].

These variations in catalyst structure produced corresponding variations in performance.  All membranes were single-gas tested in hydrogen and helium at temperatures from 823 to 1073 K and feed gas pressures up to 690 kPa gauge (100 psig).  No membranes had any detectable helium leakage through defects, and membranes without Mo₂C layers had no hydrogen flux, demonstrating that all hydrogen dissociation activity was caused by the catalyst. 

At temperatures below 1023 K, the structure and thickness of the carbide catalyst layer had the most influence on the hydrogen permeability.  As temperature increased, the permeability reached a maximum at 1023 K, and then decreased as predicted by Steward[1] due to decreasing hydrogen solubility in the vanadium foil.  The highest H₂ permeability was 5.9 x 10^-8 mol.m/m².s.Pa^0.5 at 1023 K, about 70% higher than pure Pd.  We believe these are the first measurements of hydrogen permeability for vanadium at high temperature.  The hydrogen flux for this 50 micron thick membrane at a differential feed pressure of 690 kPa was 0.7 mol/m².s.  Membranes display long term stability (> 150 hrs) at high temperature with no loss of flux and TEM analysis after testing confirms that the Mo₂C did not reduce or otherwise alloy with the vanadium foil.  Effects of process parameters such as V foil thickness, catalyst layer thickness, sputter conditions, and permeation temperature will be discussed and compared to Steward's predictions[1]. 

1.            Steward, S.A., Review of Hydrogen Isotope Permeability Through Materials, 1983, Lawrence Livermore National Laboratory.

2.            Gade, S.K., S.J. Chmelka, S. Parks, J.D. Way, and C.A. Wolden, Dense Carbide/Metal Composite Membranes for Hydrogen Separations Without Platinum Group Metals. Advanced Materials, 2011. 23(31): p. 3585–3589.