Hydrogen storage materials are being considered for safe on-board vehicle storage of H2 gas used to power proton exchange membrane fuel cells. There are various methods for storing hydrogen,1 which include high pressure storage in aluminum containers,2 chemisorbed onto metal hydrides3 and physisorbed onto carbon-based materials.4 Metal hydrides are viewed as one of the most feasible storage method because they are low volume containment vehicles for H2 gas, and they show promise for H2 desorption under mild conditions.
Metal hydrides are compounds comprised of one or more metals that contain anionic hydrogen (H–) in their lattice. There are a vast number of metal hydrides that can be synthesized, but the prevalent scope of interest is in the lighter metal hydrides (the weight % of hydrogen increases as the molecular weight of the metal hydride decreases). Of these light metal hydrides, sodium aluminum hydride (NaAlH4, Product No. 357472), lithium aluminum hydride (LiAlH4, Product No. 199877), lithium hydride (LiH, Product No. 201049), and sodium hydride (NaH Product No. 223441) have high gravimetric storage capacity for hydrogen, but are also reactive in air and moisture-rich environments.5
Our research impacts on the hydrogen energy economy through the development of “smart” nanofilms for the protection of metal hydrides against air and moisture, while permitting release of hydrogen gas through these semi permeable nanofilms. Future generations of these films may have catalytic metals—known to enhance the dehydrogenation reaction6—embedded within for the purpose of controlled release of catalyst to the hydride particle surface. Our nanofilms for encapsulating metal hydrides are layer-bylayer electrostatic self-assembled films.
Layer-by-layer thin films are comprised of polyelectrolyte layers each on the order of 2 nm thickness.7,8 The layer-by-layer self-assembly technique can be used to deposit conformal, multilayer nanofilms onto planar surfaces and colloidal particles. Film growth is made possible by coulumbic attraction between the polycation and the polyanion at the surface to be coated. The working medium for self-assembly is typically water or acetone-water mixtures.9 However, water as a solvent cannot be used to coat the water-sensitive metal hydrides. For metal hydrides, pure formamide was used as a solvent for the self-assembly of films.10
Layer-by-layer electrostatic self-assembly coating of nanofilms was performed onto sodium aluminum hydride powders using formamide as the working medium. The polyanions are polystyrene sulfonate [(PSS) (MW 70,000) (Product No. 243051)] and the polycations are polyallylamine hydrochloride [(PAH) (MW 15000) (Product No. 283223)]. Figure 1 shows schematically the layer-by-layer self-assembly of PSS and PAH films over sodium aluminum hydride. For deposition onto colloidal sodium aluminum hydride, the powders were suspended in formamide using concentrations of 20 mg/mL. For self-assembly, PSS and PAH in concentrations of 2 mg/mL were added to the colloidal suspension in the following manner. The surface charge on bare sodium aluminum hydride in formamide is positive. Thus, sodium aluminum hydride particles contained in formamide suspension were first exposed to the polyanion (PSS) for a period of fifteen minutes and subsequently to the polycations (PAH) for a period of fifteen minutes.
Figure 1.The growth of polymer films on sodium aluminum hydride.
After each exposure (and deposition of each film layer), the particles were removed from suspension by centrifugation and rinsed in formamide for a period of fifteen minutes. Zeta potential (Zetaplus, BIC) measurements were taken to observe the alternation in the surface charge after each polyelectrolyte exposure and formamide rinse. Figure 2 shows schematically the process steps in the layer-by-layer self-assembly approach used. The processing steps were repeated until two bilayers of PSS and PAH (4 layers in total) overlaid the sodium aluminum hydride particle surface.
Figure 2.The process of coating “smart” nanofilms onto sodium aluminum hydride.
One key indicator of the successful layering of alternately charged polyelectrolytes over colloidal particle surfaces during layer-by-layer self-assembly is the measured alternation in surface charge gained using zeta potential measurement (Zetaplus, BIC). Figure 3 shows this surface charge reversal after each polyelectrolyte layer was deposited over sodium aluminum hydride. The surface charge measurements were taken after the formamide rinse step of our process. PSS is the polyanion and results in a zeta potential reading of a –60 mV after its first layering. PAH is the polycation resulting in a zeta potential reading of +16 mV after its first layering. The second bilayer comprised of PSS followed by PAH read surface charges of –75 mV and +16 mV, respectively.
Figure 3.Zeta potential measurement from sodium aluminum hydride having PSS and PAH overlay films (two polymeric bilayers).
An additional feature of the layer-by-layer self-assembly technique is its versatility in controlling total thickness of polyelectrolyte layers for the variety of polyelectrolytes that may be deposited over planar and colloidal particle surfaces. Inorganic clay flakes and nanoparticles of silica and titania may also be deposited over colloidal particles using layerby- layer self-assembly. Our own work explores the versatility of the layer-by-layer technique in order to develop films of a protective nature and having the ability to prevent air and moisture from interacting with the reactive metal hydrides— thereby improving shelf life of these materials.
The sodium aluminum hydride particles with two bilayers of PSS/PAH were carefully dried and imaged using an FESEM with energy dispersive spectroscopy (EDS) capabilities. The SEM image in Figure 4 (left) shows a single sodium aluminum hydride particle having PSS/PAH overlay films. Energy dispersive x-ray mapping of carbon (Figure 4, right) reveals the conformal coverage of the polymeric films over the sodium aluminum hydride particle. In order to confirm that no reaction between the sodium aluminum hydride particles and the formamide working solvent had occurred, x-ray diffraction (XRD) of bare sodium aluminum hydride particles after a 24-hour soak in formamide was measured. Figure 5 shows clearly that NaAlH4 crystallographic peaks are consistent with the NaAlH4 (indicated by red markers). Minor peaks occurring below 2ϑ=28° are from kapton tape used for protecting the sodium aluminum hydride during XRD measurement.
Figure 4.SEM and X-ray mapping of carbon (from nanofilms) on sodium aluminum hydride.
Figure 5.X-ray diffraction of NaAlH4 after 24 hour soak in formamide— the working solvent.
Thus highly reactive and corrosive hydrides can be protected from air and moisture by encapsulating them within smart polyelectrolyte films. Figure 6a clearly shows complete and even coverage of PSS and PAH films (deposited onto planar surfaces from an aqueous working solvent). These films have surface defects due to inclusions of dust particles that may be removed from the solvents by submicron filtration. Studies to test the stability and permeability of PSS and PAH films to the gases H2, O2, and H2O are underway and preliminary results are encouraging.
Figure 6.(a) (PSS/PAH)6 multilayers on silver QCM. (b) Montmorillonite clay in alternation with PAH.
Pure organic polymer films may not form very tight packing, so an alternative to forming air impermeable films is to introduce inorganic particles within the overlay films. Low permeability films over colloidal particles may be prepared from nanomaterials such as Montmorillonite clay flakes and inorganic TiO2 and SiO2 nanoparticles.
Montmorillonite clay flakes of 1 nm thick and 500 nm crosssections can be used to coat air and moisture sensitive metal hydrides. Figure 6b shows montmorillonite (negative surface charge) assembled through alternation with a polycation (PAH) providing a bilayer of 3.2 nm onto a Si substrate.11,12 Using montmorillonite clay flakes, one can build a very tight organicinorganic multilayer network. These clay/PAH films (Figure 6b) appear to be free of surface defects—compared with PSS/PAH films (Figure 6a). The internal structure of these multilayer films resembles paper mache—where gas diffusion may occur only by percolation through pores created by packing defects. Generally, such multilayers have a much lower permeability than organic films. The possibility for tunable permeability by controlling packing defects through varying self-assembly conditions is promising.
Water-sensitive metal hydrides may also be coated with inorganic nanoparticles in alternation with organic polymer films. Figure 7 demonstrates the coating of a 250 nm colloidal latex particle with 40 nm silica particles assembled in alternation with PSS/PAH.13 The bare colloidal latex particle and the silica coated latex particle are shown in Figure 7. The protective nature of such films is demonstrated by the ability of a similarly architectured film to stabilize nifedipine from photoreaction.
Figure 7.250 nm diameter bare latex particle (left) and covered with 40-nm silica particle shell (PAH/PSS/PAH/silica) (right).