Papers

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pccp 2015

The catalyzed hydrogen sorption mechanism in alkali alanates

Züleyha Özlem Kocabas Atakli, Elsa Callini, Shunsuke Kato, Philippe Mauron, Shin-Ichi Orimo and Andreas Züttel

Physical Chemistry Chemical Physics, 2015,
DOI: 10.1039/C5CP01684C

The hydrogen sorption pathways of alkali alanates were analyzed and a mechanism for the catalytic hydrogen sorption was developed. Gibbs free energy values of selected intermediate steps were calculated based on experimentally determined thermodynamic data (enthalpies and entropies) of individual hydrides: MAlH4, M3AlH6, and MH. The values of the activation energies, based on the intermediates M+, H, MH, and AlH3, were obtained. The mechanism of the catalytic activity of Ti is finally clarified: we present an atomistic model, where MAlH4 desorbs hydrogen through the intermediates M+, H, MH, and AlH3 to the hexahydride M3AlH6 and finally the elemental hydride MH. The catalyst acts as a bridge to transfer M+ and H from MAlH4 to the neighboring AlH4, forming AlH63- and finally isolated MH, leaving AlH3 behind, which spontaneously desorbs hydrogen to give Al and 1.5H2. The proposed mechanism is symmetric in the direction of hydrogen desorption as well as readsorption processes.
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Surface Reactions are Crucial for Energy Storage

E. Callini, S. Kato, P. Mauron, A. Züttel

CHIMIA International Journal for Chemistry, Volume 69, Number 5, May 2015, pp. 269-273(5)

Reactions between gas molecules, e.g. H2 and CO2 and solids take place at the surface. The electronic states and the local geometry of the atomic arrangement determine the energy of the adsorbate, i.e. the initial molecule and the transition state. Here we review our research to identify the surface species, their chemical state and orientation, the interaction with the neighbouring molecules and the mobility of the adsorbed species and complement the experimental results with thermodynamic modelling. The role of the Ti was found to be a bridge between the charged species preventing the individual movement of the ions including charge separation. The Ti has no catalytic effect on the hydrogen sorption reaction in borohydrides. The physisorption of molecular hydrogen is too weak at ambient temperature to reach a significant hydrogen storage density. The addition of a hydrogen dissociation catalyst to a nanoporous material with a large specific surface area may potentially enable the spillover of hydrogen atoms from the metal catalyst to the surface of the porous material and chemisorb on specific sites with a much higher binding energy compared to physisorption. The intercalation of alkali metals in C60 fullerenes increases the interaction energy of hydrogen with the so-called metal fullerides significantly. Sterical diffusion barriers by partial oxidation of the surface of borohydrides turned out to redirect the reaction path towards pure hydrogen desorption and suppress the formation of diborane, a by-product of the hydrogen evolution reaction from borohydrides previously undetected. The combination of a newly developed gas controlling system with microreactors allows us to investigate complex reactions with small quantities of nano designed new catalytic materials. Furthermore, tip-enhanced Raman spectroscopy (TERS) will allow the investigation of the reactions locally on the surface of the catalyst and the near ambient pressure photoelectron spectroscopy enables analysis of the surfaces in ultra-high vacuum and in situ interaction with the adsorbates i.e. while the reaction takes place. This brings us in a unique position for the investigation of the heterogeneous reactive systems. The mechanism of the Ti catalysed hydrogen sorption reactions in alanates was recently established based on spectroscopic investigations combined with thermodynamic analysis of the transition states.  
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The catalyzed hydrogen sorption mechanism in alkali alanates

Züleyha Özlem Kocabas Atakli, Elsa Callini, Shunsuke Kato, Philippe Mauron, Shin-Ichi Orimo and Andreas Züttel

Physical Chemistry Chemical Physics, 2015, DOI: 10.1039/C5CP01684C

The hydrogen sorption pathways of alkali alanates were analyzed and a mechanism for the catalytic hydrogen sorption was developed. Gibbs free energy values of selected intermediate steps were calculated based on experimentally determined thermodynamic data (enthalpies and entropies) of individual hydrides: MAlH4, M3AlH6, and MH. The values of the activation energies, based on the intermediates were obtained. 
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Storage of Renewable Energy by Reduction of CO2 with Hydrogen

Züttel Andreas; Mauron Philippe; Kato Shunsuke; Callini Elsa; Holzer Marco; Huang Jianmei

CHIMIA International Journal for Chemistry, Volume 69, Number 5, May 2015, pp. 264-268(5)

The main difference between the past energy economy during the industrialization period which was mainly based on mining of fossil fuels, e.g. coal, oil and methane and the future energy economy based on renewable energy is the requirement for storage of the energy fluxes. Renewable energy, except biomass, appears in time- and location-dependent energy fluxes as heat or electricity upon conversion. Storage and transport of energy requires a high energy density and has to be realized in a closed materials cycle. The hydrogen cycle, i.e. production of hydrogen from water by renewable energy, storage and use of hydrogen in fuel cells, combustion engines or turbines, is a closed cycle. However, the hydrogen density in a storage system is limited to 20 mass% and 150 kg/m3 which limits the energy density to about half of the energy density in fossil fuels. Introducing CO2 into the cycle and storing hydrogen by the reduction of CO2 to hydrocarbons allows renewable energy to be converted into synthetic fuels with the same energy density as fossil fuels. The resulting cycle is a closed cycle (CO2 neutral) if CO2 is extracted from the atmosphere. Today’s technology allows CO2 to be reduced either by the Sabatier reaction to methane, by the reversed water gas shift reaction to CO and further reduction of CO by the Fischer–Tropsch synthesis (FTS) to hydrocarbons or over methanol to gasoline. The overall process can only be realized on a very large scale, because the large number of by-products of FTS requires the use of a refinery. Therefore, a well-controlled reaction to a specific product is required for the efficient conversion of renewable energy (electricity) into an easy to store liquid hydrocarbon (fuel). In order to realize a closed hydrocarbon cycle the two major challenges are to extract CO2 from the atmosphere close to the thermodynamic limit and to reduce CO2 with hydrogen in a controlled reaction to a specific hydrocarbon. Nanomaterials with nanopores and the unique surface structures of metallic clusters offer new opportunities for the production of synthetic fuels.  
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Solvent-free synthesis and stability of MgB12H12

Remhof, Arndt; Yan, Yigang; Rentsch, Daniel, Andreas Borgschulte, Craig M. Jensen and Andreas Züttel

JOURNAL OF MATERIALS CHEMISTRY A Volume: 2 Issue: 20 Pages: 7244-7249 Published: 2014

MgB12H12 has been widely discussed as an intermediate in the hydrogen sorption cycles of Mg(BH4)(2), but its properties such as stability and reactivity are still unknown. We achieved the synthesis of MgB12H12 via the reaction between Mg(BH4)(2) and B2H6 at 100 to 150 degrees C. When bulk Mg(BH4)(2) was used as the starting material, a yield of 10.2 to 22.3 mol% was obtained, which was improved to 92.5 mol% by using Mg(BH4)(2) nanoparticles. The as-synthesized MgB12H12 decomposed into boron between 400 and 600 degrees C, preceded by a possible polymerization process. The formation mechanism of MgB12H12 and its role in the decomposition process of Mg(BH4)(2) are discussed.  
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Dehydrogenation studies of the bimetallic borohydrides

Chong, M.; Callini, E.; Borgschulte, A., Züttel A., and Jensen C. M.

RSC ADVANCES Volume: 4 Issue: 109 Pages: 63933-63940 Published: 2014

One of the major issues associated with the use of borohydride complexes for hydrogen storage is the thermodynamic stability of these materials, with the Group I and II complexes also requiring the most demanding temperatures to facilitate dehydrogenation. In recent years, the idea of modulating thermodynamic properties by combining metals with different stabilities (as monocation borohydrides) has come to light. By incorporating a cation with ionic bonding characteristics, it has been proposed that the volatile Sc borohydrides can be stabilized to an extent that diborane release is prevented. We show, using in situ IR and TG analysis, that these complexes do indeed release significant quantities of diborane, which is an irrevocable barrier to reversibility. Our findings suggest that the Group I/Sc bimetallic borohydrides are not suitable for hydrogen storage.  
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The Role of Ti in Alanates and Borohydrides: Catalysis and Metathesis

Elsa Callini, Andreas Borgschulte, Cedric L. Hugelshofer, Anibal J. Ramirez-Cuesta, Andreas Züttel

JOURNAL OF PHYSICAL CHEMISTRY C Volume: 118 Issue: 1 Pages: 77-84 Published: JAN 9 2014

Ti catalyzes the hydrogen sorption reaction in alanates and allows the measurement of pressure composition isotherms, that is, the reaction equilibrates at each point of the isotherm. Although some effects of Ti compounds addition to borohydrides have been shown, our measurements show that the hydrogen desorption reaction from borohydrides is not catalyzed by Ti, when the system is exposed to a gas flow. The reabsorption of hydrogen by the products of the desorption reaction requires high temperatures and high pressures. Furthermore, the reaction pathway for the hydrogen desorption is different from the absorption one and as a consequence the borohydrides do not equilibrate with the gas phase during the hydrogen sorption reactions. Ti in borohydrides leads to the formation of stable and volatile Ti-containing species, for example, (Ti(BH4)(3)). This is a metathesis reaction, that is, a bimolecular process involving the exchange of bonds between the two reacting chemical species. In this paper, we have investigated via spectroscopy measurements the hydrogen sorption reactions of NaAlH4 and LiBH4 with Ti catalyst in view of the changes in the solid phase as well as in the gas phase.  
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Decoration of graphene with nickel nanoparticles: study of the interaction with hydrogen

Mattia Gaboardi, Andreas Bliersbach, Giovanni Bertoni, Matteo Aramini, Gina Vlahopoulou, Daniele Pontiroli, Philippe Mauron, Giacomo Magnani, Giancarlo Salviati, Andreas Züttel and Mauro Riccò

JOURNAL OF MATERIALS CHEMISTRY A Volume: 2 Issue: 4 Pages: 1039-1046 Published: 2014

Graphene obtained from thermal exfoliation of graphite oxide was chemically functionalized with nickel nanoparticles (NPs) without exposing the system to oxidizing agents. Its structural, physical and chemical properties have been studied by means of TEM, X-ray photoelectron and Raman spectroscopies, and SQuID magnetometry. The formation of 17 nm super-paramagnetic (SPM) monodispersed Ni NPs was observed. Nitrogen sorption experiments at 77 K yield a Brunauer-Emmet-Teller specific surface area (BET-SSA) of 505 m(2) g(-1) and helium adsorption at room temperature gives a skeletal density of 2.1 g cm(-3). The interaction with atomic hydrogen was investigated by means of Muon Spin Relaxation (mu SR) showing a considerable fraction of captured muonium (similar to 38%), indicative of strong hydrogen-graphene interactions. Hydrogen adsorption has been measured via pressure concentration isotherms demonstrating a maximum of 1.1 mass% of adsorbed hydrogen at 77 K and thus a 51% increased hydrogen adsorption compared to other common carbon based materials.  
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Probing hydrogen spillover in Pd@MIL-101(Cr) with a focus on hydrogen chemisorption

P. Á. Szilágyi, E. Callini, A. Anastasopol, C. Kwakernaak, S. Sachdeva, R. van de Krol, H. Geerlings, A. Borgschulte, A. Züttel and B. Dam

PHYSICAL CHEMISTRY CHEMICAL PHYSICS Volume: 16 Issue: 12 Pages: 5803-5809 Published: 2014

Palladium nanoparticles can split the dihydrogen bond and produce atomic hydrogen. When the metal nanoparticles are in intimate contact with a hydrogen-atom host, chemisorption of H-atoms by the host has been suggested to occur via the hydrogen spillover mechanism. Metal-organic frameworks were predicted to be able to act as effective chemisorption sites, and increased ambient-temperature hydrogen adsorption was reported on several occasions. The intimate contact was supposedly ensured by the use of a carbon bridge. In this work, we show that it is possible to introduce catalyst palladium particles into MOF’s pores and simultaneously ensuring good contact, making the employment of the carbon bridge redundant. The addition of Pd nanoparticles indeed increases the ambient-temperature hydrogen uptake of the framework, but this is found to be solely due to palladium hydride formation. In addition, we show that the hydrogen atoms do not chemisorb on the host framework, which excludes the possibility of hydrogen spillover.  
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Processing as a Route to the Clean Dehydrogenation of Porous Mg(BH4)2

Stadie, Nicholas P.; Callini, Elsa; Richter, Bo, Jensen Torben R, Borgschulte Andreas, Züttel Andreas

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Volume: 136 Issue: 23 Pages: 8181-8184 Published: JUN 11 2014

Compounds of interest for chemical hydrogen storage at near ambient conditions are specifically tailored to be relatively unstable and thereby desorb H-2 upon heating. Their decomposition must be performed in the absence of impurities to achieve clean dehydrogenation products, which is particularly challenging for an emerging class of microporous complex hydride materials, such as gamma-phase Mg(BH4)(2), which exhibits high surface area and readily adsorbs (sometimes undesired) molecular species. We present a novel strategy toward the purification of yMg(BH4)(2) using supercritical nitrogen drying techniques, (1) showing that clean hydrogen can be released from Mg(BH4)(2) under mild conditions and (2) clarifying the origin of diborane among the decomposition products of stable borohydrides, a topic of critical importance for the reversibility and practical applicability of this class of hydrogen storage compounds. This technique is also widely applicable in the pursuit of the high-purity synthesis of other porous, reactive compounds, an exciting future class of advanced functional materials.  
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Effect of composition and particle morphology on the electrochemical properties of LaNi5-based alloy electrodes

Mariana Spodaryk, Larisa Shcherbakova, Anatoly Sameljuk, Valentina Zakaznova-Herzog, Beat Braem, Marco Holzer, Philippe Mauron, Arndt Remhof, Yurii Solonin, Andreas Züttel

JOURNAL OF ALLOYS AND COMPOUNDS Volume: 607 Pages: 32-38 Published: SEP 15 2014

Al and Co substituted LaNi5-based intermetallic compounds were investigated as negative electrode materials in alkaline KOH-electrolyte. The structure, morphology and electrochemical properties of gas-atomised and cast LaNi4.5Al0.5 and LaNi2.5Co2.4Al0.1 alloys were investigated and the effect of the substitutions and the production method on the surface and bulk composition is compared to cast LaNi5 as a reference.
While the rapid solidification of the gas-atomised alloys affects the site energy distribution for hydrogen on the interstitial sites, the activation and degradation mechanism of the electrodes cycle life is dominated by the elemental composition of the alloy and especially the surface. Therefore, the alloys with the larger Al content activate within a few cycles and also degrade faster as compared to the alloys with a high Co content. Furthermore, the dissolution of Al leads to a highly active surface with a lower reaction resistance, which leads to an order of magnitude increased high rate dischargeability.  
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The role of MgB12H12 in the hydrogen desorption process of Mg(BH4)2

Yan, Yigang; Remhof, Arndt; Rentsch, Daniel; Züttel Andreas

CHEMICAL COMMUNICATIONS Volume: 51 Issue: 4 Pages: 700-702 Published: 2015

The presence of MgB12H12 has often been considered as the major obstacle for the reversible hydrogen storage in Mg(BH4)2. This communication provides evidence that the MgB12H12 phase (or [B12H12](2-) monomer) does not exist in the decomposition products of Mg(BH4)(2) at temperatures ranging from 265 to 400°C, and thereby it will not act as a dead end. 
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Description of the capacity degradation mechanism in LaNi5-based alloy electrodes

Mariana Spodaryk, Larisa Shcherbakova, Anatoly Sameljuk, Adrian Wichser, Valentina Zakaznova-Herzog, Marco Holzer, Beat Braem, Oleg Khyzhun, Philippe Mauron, Arndt Remhof, Yurii Solonin, Andreas Züttel

JOURNAL OF ALLOYS AND COMPOUNDS Volume: 621 Pages: 225-231 Published: FEB 5 2015

The mechanism of the capacity degradation of LaNi5-based alloy electrodes was investigated with a special focus on the influence of the alloy and surface composition, as well as the unique structure obtained by gas atomisation. The electrochemical properties, especially the cycle life curve (i.e. the capacity as a function of the cycle number of LaNi4.5Al0.5, LaNi2.5Co2.4Al0.1, (La + Mm) Ni3.5Co0.7Al0.35Mn0.4Zr0.05, and MmNi(4.3)Al(0.2)Mn(0.5) alloy electrodes), was analysed and modelled. The capacity degradation upon cycling is determined by the chemical state of the alloy elements and the solubility of their oxides. The cycle life curves for the alloy electrodes without Co exhibited a rapid activation (3-4 cycles to reach maximum capacity), as well as rapid degradation (130-180 cycles for 50% maximum discharge capacity). LaNi2.5Co2.4Al0.1 and (La + Mm) Ni3.5Co0.7Al0.35Mn0.4Zr0.05 alloy electrodes activated after 7-10 cycles and showed very stable discharge behaviour (more than 400 cycles). The Co-containing alloy electrodes primarily lose the cycle stability because of mechanical decrepitation, whereas the alloys without Co suffer from selective dissolution of the unstable elements in the potential window, which was shown by our model of alloy degradation and confirmed by means of SEM, WDX, and ICP-OES data.were obtained.