NETL is Developing Advanced Gas Separation Membranes

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	Membrane mechanisms, illustrated here for hydrogen separation.

Membranes, to most people, are associated with biology; cell membranes, mucous membranes, or tympanic membranes (ear drums) probably come to mind. NETL researchers are developing synthetic membranes that are very different from these biological membranes, with the goal of effective gas separation. Synthetic membranes can be made from organic or inorganic materials including metals, ceramics, polymers, and liquids.

Membrane technologies offer potential advantages for gas separation – they operate passively, with no moving parts; they can be designed to withstand chemical contaminants; they are energy efficient, with low operating costs; and they are modular and require little space. NETL is attempting to develop advanced, efficient membrane technologies that can be used to generate hydrogen (H₂) from synthetic gas (syngas), or oxygen (O₂), for oxyfuel combustion, or to separate carbon dioxide (CO₂) from power plant exhaust gases.

NETL’s Membrane Separation Team, composed of DOE, URS, Carnegie Mellon University, University of Pittsburgh, and Virginia Tech researchers, are investigating H₂ separation membranes fabricated from various materials, such as crystalline and amorphous metallic alloys, ceramics, and combinations including metallic and non-metallic parts called composites. The goal of NETL’s hydrogen separation membrane research is to develop cost-effective gas separation technologies to facilitate H₂ production from fossil fuels. Membranes already exist that can be used to separate H₂ and CO₂, producing high purity H₂, which can be used as fuel, and high purity CO₂, which is ready for sequestration, but these membranes are expensive and vulnerable to common contaminant gases, such as hydrogen sulfide (H₂S).  In order to overcome these problems, NETL researchers have developed a variety of techniques to facilitate new membrane materials development. They use both computational and rapid screening methods in order to speed up development of effective hydrogen membranes and shorten the time needed to create solutions to the world’s global warming problem.

Using Membranes to Produce Hydrogen

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	A scanning electron microscope image of a palladium alloy membrane that has been exposed to flue gas containing H2S.

Syngas can be produced by the gasification of coal, possibly combined with biomass for a more favorable carbon footprint. This gas can be reacted with water vapor to yield a product containing primarily H₂ and CO₂, with minor amounts of contaminants, such as H₂S.  Hydrogen selective membranes can be used to separate the H₂ from the CO₂ and other components, producing a high purity H₂ product that can be used directly as a fuel or as a reactant in subsequent processes, including the production of liquid fuels. The remaining high pressure CO₂ stream containing the other minor components is ready for geological sequestration or other uses.

The 2015 goals for H₂ separation membranes set by the U.S. Department of Energy stipulate performance criteria such as H₂ flux levels, working temperature, sulfur tolerance, H₂ purity, durability, and cost. With these goals in mind, complementary computational and experimental research is being conducted to design and develop effective membrane technologies.  A multi-scale approach is being applied that links research at the atomic/molecular, membrane, and device scales, with system-level analysis for the integration of membranes into power plants. 

The atomic/molecular scale includes computational chemistry approaches designed to probe material characteristics, such as the crystalline phase structure and surface chemistry of complex metal alloys.  The crystalline phase structure helps determine the H₂ permeability of a metal.  The surface chemistry (what is actually happening at the atomic level interface of the solid membrane and the gas stream) influences H₂ permeability and the membrane’s resistance to damage by impurities, which, in severe cases, can result in failure through corrosion. This type of information can be used to identify alloy compositions with the potential for improved performance.

At the membrane scale, membrane samples are synthesized and performance tested in the laboratory, usually as thin foils for metals.  Baseline performance is determined using clean H₂ over a range of temperatures and pressures. The resistance to degradation by H₂S is determined for promising compositions.  Following testing, the effects of exposure are studied using techniques such as scanning electron microscopy and x-ray diffraction.  NETL has state-of-the art facilities for conducting lab-scale membrane performance tests.  NETL is also designing and building new devices to streamline the identification of new membrane materials via a combinatorial approach.  Typically, materials are produced and tested individually; therefore, for a multi-component alloy, the test results are a snapshot of a single composition out of a large range of possible compositions.  An approach under development uses a “compositionally spread alloy film” (CSAF) that can contain all possible ratios of the individual metal components of the alloy in a single, small sample.  In this technique, researchers use a method called chemical vapor deposition to create metal plates that contain all the possible mixtures of two or even three metals. These plates can then be quickly characterized to reveal the properties of all the different mixtures. This approach enables the efficient screening of a property of interest over a very large range of compositions.  Any promising compositions that are identified can them be studied in more detail using more conventional techniques.

Development of CO₂-selective Mixed Matrix Membranes

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	Optical micrograph of MOF crystals suitable for inclusion in mixed matrix membranes.

Metal organic frameworks (MOFs) are a good example of a crystalline material that has the potential for excellent membrane performance and hasn’t yet been utilized because of difficulties in fabrication. MOFs are special materials that are made by using organic compounds called linkers to connect metal atoms in a lattice.  The result is a crystal that has perfectly regular pores and extremely high surface area. By changing the linkers and metal atoms, the size of the pores can be controlled to allow smaller gas molecules to pass through while preventing larger ones from passing through, a process called molecular sieving.  Since nitrogen (N₂) is slightly larger than carbon dioxide (CO₂), molecular sieving can be used to remove CO₂ from power plant exhaust.

Although many scientists and engineers have chosen to study MOFs for gas separation in recent years, almost all those studies have considered them as sorbents, materials that can absorb CO₂ and then release it when temperature or pressure changes.  Only a few researchers have attempted to create MOF-based membranes.  Most of these efforts have focused on mixed matrix membranes (MMMs), which combine the MOFs with a robust membrane polymer.  Attempts to invent MOF MMMs have usually failed due to poor compatibility between the MOF and polymer.  If the surface of the MOF crystal isn’t carefully chosen to easily associate with the polymer chains, voids form between the crystal and the polymer.  These voids allow gases to bypass the MOF crystals and prevent the membrane from efficiently separating the gases.  To prevent these voids from forming, NETL researchers are developing new polymers and MOFs that are designed to “lock” together.  The combination of the two new materials should allow the fabrication of membranes with excellent CO₂ capture properties.