Dense cermet membranes for hydrogen separation☆
Introduction
The Office of Fossil Energy (FE) at the U.S. Department of Energy (DOE) sponsors a wide range of research, development, and demonstration programs to maximize the use of the vast domestic fossil resources and to ensure a fuel-diverse energy sector while responding to global environmental concerns. Cost-effective, membrane-based reactor and separation technologies are of considerable interest to DOE’s clean coal program to develop advanced coal-based power and fuel technologies. Argonne National Laboratory is developing dense hydrogen transport membranes (HTMs) for separating hydrogen from mixed gases, particularly product streams generated during coal gasification and/or methane reforming. Hydrogen separation with these membranes is nongalvanic (i.e., it does not use electrodes or an external power supply to drive the separation), and hydrogen separated from the feed stream is of high purity, so post-separation purification steps are unnecessary.
Materials development for the HTM at Argonne has followed a three-pronged approach. In one approach, we investigated single-phase mixed (ionic/electronic) conducting perovskite ceramics [1], [2]. The single-phase membranes gave a low hydrogen flux due to their poor electronic conductivity [3], [4]. In our second approach, we developed cermet (i.e., ceramic–metal composite) membranes that contained mixed-conducting perovskite ceramics combined with a metallic component [5], [6], [7], [8], [9]. In these cermets, the metal enhanced the hydrogen flux of the ceramic phase by increasing the electronic conductivity of the cermet. In our third approach, we dispersed a hydrogen transport metal, i.e., metal with high hydrogen permeability (Pd, Pd–Ag), in a thermodynamically and mechanically stable ceramic matrix (Al2O3 or yttria-stabilized ZrO2, YSZ) [10]. The cermets, made by our third approach, exhibit the highest hydrogen flux [11], [12].
In this paper, we report hydrogen flux measurements for Pd/YSZ cermet membranes as a function of temperature. Good chemical stability is a critical requirement for HTMs due to the corrosive nature of product streams from coal gasification and/or methane reforming. Hydrogen sulfide (H2S) is a particularly corrosive contaminant that HTMs are expected to encounter. When H2S reacts with a Pd/YSZ cermet membrane, palladium sulfide (Pd4S) forms on the membrane surface. Because Pd4S impedes hydrogen permeation through the membrane, the chemical stability of HTMs was evaluated by determining the conditions under which Pd4S forms. The Pd/Pd4S phase boundary was determined in the temperature range ≈450–650 °C using various feed gases that contained 10–73% H2 and ≈8–400 ppm H2S. To assess the effect of water vapor on hydrogen permeation through HTMs, the chemical stability of the cermet membrane was tested in the presence of steam by measuring its hydrogen flux in feed gas that contained 0.03–0.49 atm H2O. Finally, we report here the hydrogen flux of the cermet membrane versus time (up to 120 days) during exposure to simulated syngas containing H2, CO, CO2, and H2O. In addition, it is anticipated that the cermet-type membranes can overcome some drawbacks of pure Pd or Pd alloy membranes, i.e., reduce the cost and improve the mechanical strength of the membrane at high temperature (>700 °C).
Section snippets
Experimental
The powder mixture for fabricating the cermet membranes was prepared by mechanically mixing ≈50 vol.% Pd (average particle size ≈1.5 μm) with Y2O3-stabilized ZrO2 (average particle size, ≈1.0 μm). This powder mixture was pressed into disks and sintered at ≈1400 °C for ≈5 h in ambient air. For hydrogen permeation tests, both sides of the disks were polished with 600-grit SiC paper to obtain the desired thickness and produce faces that were flat and parallel to one another. Thin films of cermet
Results and discussion
Fig. 1 shows the temperature dependence of hydrogen flux through an ≈18-μm-thick Pd/YSZ cermet membrane on a porous alumina substrate disk made by the paste-painting method [8]. The feed gas was 90% H2/He and the sweep gas was N2. The thickness of the porous alumina substrate was ≈1.65 mm. The flow rate of both feed and sweep gas was adjusted to ≈500 mL/min to minimize concentration polarization. The feed and sweep gases were at ambient pressure. This membrane gave record high flux values of ≈26 cm
Conclusions
We have developed dense Pd/YSZ cermet membranes for separating hydrogen from mixed gases, particularly product streams generated during coal gasification and/or steam methane reforming. The highest measured hydrogen flux was ≈52 cm3[STP]/min-cm2 for a ≈20-μm-thick membrane on a porous substrate at 900 °C using 100% H2 at ambient pressure as the feed gas. The effect of hydrogen partial pressure on hydrogen flux indicates that the flux is limited by the bulk diffusion of hydrogen through the Pd
Acknowledgment
Work supported by the U.S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory’s Advanced Fuels Program, under Contract DE-AC02-06CH11357.
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The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.