Storage of Renewable Energy by Reduction of CO2 with Hydrogen

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

Pressure and temperature dependence of the decomposition pathway of LiBH_4

The decomposition pathway is crucial for the applicability of LiBH_4 as a hydrogen storage material. We discuss and compare the different decomposition pathways of LiBH_4 according to the thermodynamic parameters and show the experimental ways to realize them. Two pathways, i.e. the direct decomposition into boron and the decomposition via Li_2B_(12)H_(12), were realized under appropriate conditions, respectively. By applying a H_2 pressure of 50 bar at 873 K or 10 bar at 700 K, LiBH_4 is forced to decompose into Li_2B_(12)H_(12). In a lower pressure range of 0.1 to 10 bar at 873 K and 800 K, the concurrence of both decomposition pathways is observed. Raman spectroscopy and ^(11)B MAS NMR measurements confirm the formation of an intermediate Li_2B_(12)H_(12) phase (mostly Li_2B_(12)H_(12) adducts, such as dimers or trimers) and amorphous boron

Experimental charge density of LiBD4 from maximum entropy method

We report on maximum entropy method study of the experimental atomic and ionic charges of LiBD4 in its low-temperature orthorhombic phase. Synchrotron radiation x-ray powder diffraction data, neutron powder diffraction data, and density functional calculations were used. The atomic and ionic charges were determined for both experimental and theoretical results using the Bader analysis for atoms in molecules. The charge transfer from the Li cation to the BD4 anion is 0.86(+/- 9) e, which is in good agreement with the ab initio calculated value of 0.895 e. The experimental accuracy was determined considering the differences between results obtained for data collected at 10 and 90 K, different experimental setups (high-resolution diffractometer or image plate diffractometer), and different structural models used for the prior density distributions needed for accurate maximum entropy calculations (refined using only synchrotron radiation x-ray powder diffraction data or combined with neutron powder diffraction data)

Hydrogen density in nanostructured carbon, metals and complex materials

The challenge in the research on hydrogen storage materials is to pack hydrogen atoms or molecules as close as possible. Hydrogen absorbed in metals can reach a density of more than 150 kg m⁻³ (e.g. Mg₂FeH₆ or Al(BH₄)₃) at atmospheric pressure. For metallic hydrides, however, due to the large atomic mass of the transition metals the gravimetric hydrogen density is limited to less than 5 mass%. Nanostructured carbon materials, e.g. carbon nanotubes or high surface area graphite absorb hydrogen at liquid nitrogen proportional to the specific surface area 1.5 mass%/1000m²g⁻¹. Light weight group three metals, e.g. Al, B, are able to bind four hydrogen atoms and form together with an alkali metal an ionic or at least partially covalent compound. The complex hydrides can only be cycled in as nanostructured materials

Stability and decomposition of NaBH₄.

We investigate the stability and hydrogen desorption of NaBH₄. Dynamic pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows are used to determine thermodynamic parameters of reaction. From the van’t Hoff equation the enthalpy and entropy of reaction, −108 ± 3 kJ mol⁻¹ of H₂ and 133 ± 3 J K⁻¹ mol⁻¹ of H₂ released, are obtained, respectively. This corresponds to a decomposition temperature of Tdec = 534 ± 10 °C at 1 bar of H₂. The decomposition thereby occurs in one step; i.e., only one plateau is visible in the pressure composition isotherms. Elemental Na is identified as the major solid component in the residue by X-ray diffraction. The experimental results are discussed on the basis of theoretical calculations using the density functional theory approach. Starting from the optimized structure of the cubic α-phase of NaBH₄, we discuss possible decomposition routes involving elemental Na and B as well as Na−H and Na−B binary compounds as residual products

Surface Reactions are Crucial for Energy Storage

Reactions between gas molecules, e.g. H-2 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 C-60 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