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Exploiting the Kubas Interactio Hydrogen Storage Materials By Tuan K. A. Hoang and David M. Antonell 1. Introduction The increase in crude oil consumption and the rapid exhaustion of petroleum reservoirs has led to a boom in research in the area of alternative fuel.[1] New sources of energy should be clean, efficient, low cost, and easy to transport. Some candidates are wind, solar, geothermal, biomass, nuclear energy, and hydro- gen.[2] Hydrogen is an attractive energy source for mobile applications such as vehicles because it has the highest energy density of any substance and produces only water as a waste product when used in a fuel cell.[3] Unfortunately hydrogen is difficult to store and transport. Therefore, to commercialize hydrogen technology, the bottleneck represented by the storage and delivery steps must be overcome. amounts of heat are low, as in the case cumbersome cryogen on the material. In s solid-state materials gravimetric storage, Numerous high su materials have been mechanism is domin have adsorption ent materials, such as organic frameworks high hydrogen physi adsorption capacities limited adsorption investigated the hy samples under ambie capacities ranged in 0.04–0.46wt%. This is because the 4–10 kJmol�1 binding enthalpies are too weak to hold the Calculations have oom temperature 20–30 kJmol�1, ol�1 is a chers are tween the es involve o bind the P R O G R E S S R E P O R T www.advmat.de [*] Prof. D. M. Antonelli, T. K. A. Hoang ls bl se � t cc s o e ter a g an b represent a median on the continuum between metal hydride sorption materials, and are becoming increasingly important as researchers learn more about their applications to hydrogen storage problems. DOI: 10.1002/adma.200802832 however some research has indicated that 15–20 kJm more suitable target. For this reason many resear pursuing strategies to strengthen the interaction be substrate and hydrogen. Many of these new approach the use of s–p H2 complexes (the Kubas interaction) t Department of Chemistry and Biochemistry University of Windsor Windsor, ON N9B 3P4 (Canada) E-mail: danton@uwindsor.ca hydrogen to the surface under these conditions. demonstrated that the ideal enthalpy for r operation of a hydrogen storage system is Hydrogen adsorption and storage using solid-state materia much current research interest, and one of the major stum realizing the hydrogen economy. However, no material yet re close to reaching the DOE 2015 targets of 9wt% and 80 kgm To increase the physisorption capacities of these materials, adsorption must be increased to �20 kJmol�1. This can be a optimizing the material structure, creating more active specie or improving the interaction of the surface with hydrogen. Th this progress report are recent advances in physisorption ma higher heats of adsorption and better hydrogen adsorption temperature based on exploiting the Kubas model for hydro (h2-H2)–metal interaction. Both computational approaches achievements will be discussed. Materials exploiting the Ku Adv. Mater. 2009, 21, 1787–1800 � 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei looked features of a hydrogen storage material is the enthalpy of adsorption. If this value is too high, as in metal hydrides which generally have enthalpies in the 80 kJmol�1 range, energy is required to drive off hydrogen for use and enormous released on recharging. If this value is too of physisorption materials, expensive and ic cooling is required to keep the hydrogen pite of this, physisorption of hydrogen on shows promising properties such as high fast adsorption, and desorption kinetics. rface area microporous and mesoporous studied in this application. The adsorption ated by weak van der Walls forces, which halpy between 4 and 10 kJmol�1. Typical carbon nanotubes, fullerenes, and metal- (MOFs) possess very high surface areas and sorption at cryogenic temperature, but their at room temperature are limited due to the enthalpy. For example, Zuttel et al.[7] drogen storage capacities of 60 carbon nt conditions resulting in reversible storage n in the Design of i* Significant advances have been made in the search for suitable materials for hydrogen storage. While compressed gas is still being explored as an option, materials that absorb hydrogen as carriers are also being inves- tigated. The two major technologies for chemical storage are metal hydrides, which store hydrogen as a discreteM–H bond, and high surface materials for cryogenic hydro- gen physisorption.[4] However, no material explored to date meets the Department of Energy (DOE) 2015 system target of 9wt% weight adsorption or 80 kgm�3.[5] New materials and concepts are currently being explored and new storage strategies are being developed.[6] One of the most over- is an area of ing blocks in arched comes 3 at this time. he heats of omplished by n the surface, main focus of ials exhibiting t room en binding: d synthetic as interaction s and physi- m 1787 P R O G R E S S R E P O R T www.advmat.de pores, titanium materials and surface titanium species that exhibit high heat of adsorption exploiting the Kubas binding strategy will also be discussed, however a recent review focusing exclusively on MOFs has just appeared while this report was in progress.[10] 2. Carbon Nanotube and Fullerene Modified Materials Carbonmaterials have beenmodified withmany different species to improve the hydrogen uptake capacities. The strategy here is to decorate the surface with metal species that either add more binding sites, improve the adsorption capacity of the surface through electronic interactions, or both. Computational studies suggest that transition metal atoms, especially early metals in low oxidation states, can bind to fullerene C60 or C48B12 via the Kubas interaction to form stable media for hydrogen storage.[11] Zhao et al.[11a] proposed the formation of stable organoscandium bulkyballs using Sc(0) as shown in Figure 1 (Sc(0) here becomes Sc(II) after oxidative addition of one dihydrogen to form a dihydride). In this hypothetical molecule each scandium species can bind up to 11 hydrogen atoms, nine of which are in the form of dihydrogen and can be adsorbed and desorbed reversibly. Species such as C60[ScH2]12 and C48B12[ScH]12 are stable and can 1788 David Antonelli completed his B.Sc. and Ph.D. in organome- tallic chemistry at the University of Alberta. He was an NSERC postdoctoral fellow in organo- metallic chemistry at Oxford University and at Caltech. He later studied nanoporous mate- rials as a postdoctoral associate at the Massachusetts Institute of Technology. He is currently a Professor in Chemistry at the University of Windsor, and directs a research group focus on the electronic and catalytic properties of nanoporous materials with variable oxidation states and conducting molecular devices in the pores. Tuan K. A. Hoang obtained his B.Sc. in 2002 and Master of Chemical Engineering in 2005 at Ho Chi Minh City University of Technology, Vietnam. After 4 years duration as an Assistant Lecturer and Researcher at Ho hydrogen to the active metal sites on the surface.[8] Hydrogen spillover using Pd is also thought to increase enthalpy of binding and this has been reviewed elsewhere.[9] The Kubas interaction stands midway between hydrides and physisorption in binding strength, and for this reason, may be ideal in hydrogen storage systems designed for room temperature applications. The initial discovery of Kubas andmany of the subsequent works in this area focus on stable dihydrogen organometallic complexes at ambient pressure and temperature, in which vacuum must be applied to remove the dihydrogen species. For mobile vehicle applications, hydrogen is preferred to be stored at moderate pressures of 50–100 bar (1 bar¼ 105 Pa). This allows for hydrogen to be released simply by opening a valve and reducing the pressure, rather than applying heat or vacuum. Thus compounds which possess stable dihydrogen ligands at room temperature and pressure may not be suitable for hydrogen storage, because these would require heat or vacuum to liberate the H2. Unsaturated coordination metal species which exhibit Kubas interactions with multiple hydrogen molecules at ambient temperature and higher pressures are thus preferred. In this report, we present recent developments in hydrogen adsorption and storage. Our focus is on newmaterials that exhibit higher adsorption enthalpies as well as some theoretical research on titanium decorated bulkyballs, nanotubes, and polymers. New MOFs containing unsaturated coordination metal sites and small reversibly adsorb additional hydrogen, resulting in the uptake of Chi Minh City University of Technology, he came to Canada in 2007. Currently, he is a Ph.D. Student of Dr. Antonelli. His research focuses on supported vanadium and chromium organometallic materials for Kubas-type hydrogen storage. Figure 1. Optimized atomic structures of a) C60[ScH2(H2)4]12, b) C48B12[ScH(H2)5]12, c) Cp[ScH2] chain, and d) [ScH3]3 (left) and � 2009 WILEY-VCH Verlag Gmb ScH3(H2)6 (right). For clarity, only part of each bulkyball is shown. A close-packed vdW solid formed from C48B12[ScH(H2)5]12 would have a volumetric storage density of 43 kgH2m �3 even without consideration of the possibility of interlocking using the large open spaces indicated by the rhombus in (a). Reproduced with permission from [11a]. Copyright 2005, American Physical Society. 7.0 and 8.77wt%, hydrogen, respectively. The binding energies of hydrogen molecules to these adsorbates range within 0.3–0.4 eV, which are suitable for ambient temperature applications (Table 1). Scandium species are stable on the fullerene surface due to the binding energy of 2.8 eV. C60[ScH2]12 can reversibly bind four dihydrogen molecules to one scandium atom (Fig. 1) to form H & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1787–1800 P R O G R E S S R E P O R T www.advmat.de Experimental preparations of transition metal decorated nanotubes have been carried out by vapor deposition, but it is not clear whether these would be ideal substrates for hydrogen storage. Using this technique, nanowires of Ti, Ni, and Pd were C60[ScH2(H2)4]12, corresponding to 7.0wt% hydrogen uptake capacity. The authors propose that modification of the carbon framework of the fullerene may lead to further storage benefits. Laser vaporization of graphite pellets containing boron nitride powder produces fullerenes in which one or more atoms in the hollow carbon cage are replaced by a boron atom.[12] This substitution doping leads to a decrease in material weight and enhancementofthestability.Boronatomsinthefullerenestructure should also draw more charge from the scandium, or any other transitionmetal,resultinginanincreaseinbindingenergybetween the Sc atom and the bulky balls. In the case of C48B12[ScH]12, the binding energy of Sc to the bulky balls is 3.6 eV and each Sc can reversibly bind up to five hydrogen molecules to form C48B12[ScH(H2)5]12, resulting in a reversible hydrogen uptake capacity of 8.77wt%. This is very close to the 2015 DOE goal. While organoscandium bulky balls have yet to be synthesized, fullerenes doped with titanium, vanadium, niobium, and tantalum have been synthesized via laser-assisted vaporization of metals. Titanium and vanadium are able to bind directly to the fullerene structure via carbide bonding.[13] Fullerenes doped with titanium or vanadium seem to be stable whereas niobium and tantalum severely destabilize the fullerene structure. Mass spectroscopy studies on the C60Vx species synthesized at low laser intensities show that the cluster of C60V62, C60V73, and C60V86 are particularly stable. Similar species of C60Tix and C70Tix have been observed using titanium instead of vanadium. These materials are considered as possible candidates for hydrogen storage, however no data is yet available. Hydrogen adsorption on carbon nanotubes decorated with titaniumatomswas studied computationally by Yldirim et at.[14] In this approach the nanotubes act as a scaffolding to support the Ti centers for Kubas binding of hydrogen. Calculations show a zero-energy barrier for adsorption. The first hydrogenmolecule is dissociated to two hydrides by oxidative addition to the titanium. During this process, the H–H distance increases from 0.75 to Table 1. Calculated consecutive binding energies of H2 molecules (in eV H2 �1). In the case of bulkyballs, 12 H2 were added per calculation. Reproduced with permission from [11a]. Copyright 2005, American Physical Society. 1st H2 2nd H2 3rd H2 4th H2 5th H2 Cp[ScH2] 0.29 0.28 0.46 0.23 C60[ScH2]12 0.30 0.35 0.42 0.26 C48B12[ScH2]12 0.31 0.35 0.30 0.33 0.24 2.71 A˚ and the binding energy of hydrogen atom to titanium atom is 0.83 eV (Fig. 2). The second, third, and fourth hydrogen molecules bind to titanium atom without dissociation in a Kubas type fashion. Calculations demonstrate that the system is stable until four hydrogen molecules are adsorbed. The H–H bonds elongate to 0.81 A˚ in the case of two adsorbed hydrogen molecules with adsorption energies of 0.45 eV H2 S1, and to 0.86 A˚ in the case of four adsorbed molecules with adsorption energies of 0.34 eV H2 S1. This strategy is thus similar to that above described for Sc coated fullerenes in that it uses a carbon support as a grafting site for Kubas binding of hydrogen (Fig. 3). Figure 2. Energy versus reaction paths for successive dissociative and mol- ecular adsorption of H2 over a single Ti coated (8,0) nanotube. Adv. Mater. 2009, 21, 1787–1800 � 2009 WILEY-VCH Verlag G a) H2þ t80Ti! t80TiH2. b) 2H2þ t80TiH2! t80TiH2-2H2. c) H2þ t80TiH2- 2-2H2! t80TiH2-3H2.Thezeroofenergy is takenasthesumof theenergiesof the two reactants. The relevant bond distances and binding energies (EB) are also given. Reproduced with permission from [14]. Copyright 2005, American Physical Society. mbH & Co. KGaA, Weinheim 1789 P R O G R E S S R E P O R T www.advmat.de 3. Bis(titanium)ethylene Complex The binding abilities of metals to ethylene through s donation have been widely studied.[16] Since ethylene is much lighter as a substrate than fullerenes or carbon nanotubes, it may represent an ideal support for metal binding for hydrogen storage systems exploiting the Kubas interaction. Main group metals and especially transition metals can bind to ethylene in several different ways. For example, calculations show that ethylene can complex with two Lithium atoms to form a stable compound with a binding energy of 0.70 eV Li�1. This complexexhibits apredicted hydrogen adsorption capacity of 16wt % when each lithium atom 1790 fabricated with widths of <10 nm and continuous length of up to tens of microns.[15] Metals were introduced into the nanotubes to increase the conduction properties or coated onto the nanotubes to create a thin metallic layer. Due to the weak interactions between the metal atoms and nanotube surface, the chemical vapor deposition technique tends to create discrete metallic nanoclusters rather than surface atomic species. For hydrogen storage applications, the metal centers should ideally exist on the nanotube surface in the atomic form and not as clusters in order to maximize the available binding sites for hydrogen adsorption. adsorbs two hydrogen molecules. Durgun et al.[17] performed calculations to show that a single ethylene molecule can form a stable complex with two titanium atoms. This new molecule Ti2-ethylene can adsorb up to ten hydrogenmolecules according to calculation, reaching a gravimetric storage of 14wt %. These high predicted capacities demonstrate that ethylene may indeed be a better support than C60 or nanotubes for this application. The first hydrogen molecule is adsorbed dissociatively by each titanium atom to form the complex C2H4(TiH2)2 with a binding energy of 1.18 eV H2 �1. Consecutive hydrogen molecules are adsorbed non-dissociatively around each titanium atom. When each titanium ligates two hydrogen molecules C2H4(TiH2–2H2)2 forms with a binding energy of 0.38 eV H2 �1 and an H–H bond length of 0.81 A˚. A third hydrogen molecule is energetically favored to bind at the top side of the TiH2 group with a binding energy of 0.40 eV and a 0.82 A˚ H–H bond lengths. Figure 3. a) Two different views of the optimized structure of t80Ti-4H2. The relevant structural parameters are H–H¼ 0.84 A˚, Ti–H¼ 1.9 A˚, Ti–C¼ 2.17 A˚, Ti–C/¼ 2.40 A˚. b) The PDOS at the G point contributed from Ti, four H2 molecules, and the six carbons of the hexagon on which Ti and H2molecules are bonded. c) The s* antibonding orbital of the tetra-H2 complex; d–f) isosurface of the state just below EF at three different values: at c¼ 0.08 it is mainly Ti-d orbital; at c¼ 0.04 there is hybridization between the d orbital, two carbon p orbitals, and the 4H2 s* antibonding orbitals. At c¼ 0.02 it is clear that the other four carbon atoms are also involved in the bonding. Reproduced with permission from [14]. Copyright 2005, American Physical Society. � 2009 WILEY-VCH Verlag Gmb Figure 4. Atomic configurations of an ethylene molecule functionalized by two Ti atoms, holding a) two dissociated H2 molecules, b) six H2 mol- ecules, and c) eight H2 molecules. Panel (d) shows a configuration where ten H2 molecules are bonded all as discrete ligands. Spin-polarized calculations gave energies (in eV) by 1.5, 0.37, 0.16, and 0.06 for the configurations shown in (a–d), respectively, suggesting a magnetic ground state in all cases with a moment of m� 2mB. Panels (e) and (f) show the bonding orbital for the top (e) and side-on hydrogen ligands, respectively. Note that the hydrogen s*-antibonding orbitals are hybridized with Ti-d orbitals, suggesting Kubas interaction for the H2–Ti bonding. Reproduced with permission from [17]. Copyright 2006, American Physical Society. H & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1787–1800 P R O G R E S S R E P O R T www.advmat.de carbon hydrate. Theoretical studies by Ihm and coworkers[20] show that simple polymers decorated with light transition metal atoms may be useful for hydrogen storage. cis-Polyacetylene decorated with titanium atoms can adsorb up to 12wt% of hydrogen. The formula for this material is (C4H4 � 2Ti � 10H2)n. In this complex, hydrogen molecules bind to titanium atoms in a Kubas fashion with binding energies ranging from 0.42 to 0.58 eV, slightly higher than the binding energies of hydrogen molecules bound to titanium and scandium decorated nanotubes and bulkyballs.[11a,14] Of the five adsorbed hydrogen per Ti, 3.16 hydrogen molecules could be desorbed reversibly, resulting in Acetylene Alkynyl 4.00 3.95 0.05/5 Poly(ether ether ketone) Carbonyl 4.99 4.17 0.82/6 Figure 4d proposes the adsorption of five hydrogen molecules per titanium atom, resulting in a hydrogen uptake capacity of 14wt %. In all cases, the H–H bond of the adsorbed H2 is elongated, consistent with the Kubas interaction scheme, and suggests the reversible adsorption of hydrogen on the adsorbent. While these molecules encouraging from the standpoint of theory, synthesis of such systems has still remained elusive. These studies do however point in the direction of using anchored alkenes as binding sites on surfaces for low coordinate metal species for hydrogen storage. The binding of Ti to many other support materials other than ethylene and carbon nanotubes has also been studied by theory. Lee et al. proposed complexes of titanium with six different types of functional groups by first principle density functional electric structure calculations (Table 2). The functional groups used were 2-mercaptoethyl sulfide, ethane-1,2-diol, acetylene, poly(ether ether ketone) (PEEK), benzonitrile, and methyl-isocyanate. The results showed that each complex can reversibly bind up to six hydrogen molecules per titanium atom (Fig. 5). Titanium atoms were shown to bind in a stable manner with binding energies of the