Session W33: Physics of Hydrogen Production, Storage, Delivery

Boron Doping Carbon Structures Using Decaborane? A Theoretical Study

Boron-doped carbon materials have been shown to improve hydrogen storage. Boron-doped activated carbons have been produced using a novel process involving the pyrolysis of a boron containing compound and subsequent high-temperature annealing. A model for the boron doping process based on a Langmuir isotherm is presented. A theoretical study of the interaction of the boron containing compound with the undoped carbon precursor will be presented. Ab-initio calculations of the potential energy surface and the Langmuir isotherm parameters derived from them are also presented. The theoretical study outlines the unique capabilities and limits of this doping procedure.   

Mass Transport in the Dehydrogenation Reaction of B$_{20}$H$_{16}$

The compound B$_{20}$H$_{16}$ has been predicted to decompose directly into 20B and 8H$_{2}$ with favorable hydrogen release (6.9 wt. \%) and equilibrium temperature ($T$ = 20 $^{\circ}$C at a pressure of 1 bar H$_2$) [W.Q. Sun, et. al. Phys. Rev. B. \textbf{83}, 064112, 2011]. The segregation of B and H during this reaction is investigated using density functional theory assuming that this process is mediated by the diffusion of native point defects in solid B$_{20}$H$_{16}$. Using the calculated formation energies under relevant chemical conditions, those defects that form in the largest concentrations, and thus those that facilitate mass segregation, are identified. These results are used to gain insight into the possible kinetic limitations of this hydrogen storage reaction.   

Theoretical prediction of intermediates in the decomposition of Mg(BH$_4$)$_2$

We have studied the decomposition pathway of Mg-borohydride using density-functional theory (DFT) calculations of the free energy (including vibrational contributions) in conjunction with a Monte Carlo-based crystal structure prediction method, the prototype electrostatic ground state (PEGS) method. We find that a recently proposed Mg(B$_3$H$_8$)$_2$ intermediate [Chong, etc, {\it Chem. Commun.} {\bf 47}, 1330, (2011)] is energetically highly unfavorable with respect to decomposition into MgB$_{12}$H$_{12}$. We systematically search for low-energy structures of Mg-triboranes [Mg(B$_3$H$_8$)$_2$, MgB$_3$H$_7$, and Mg$_3$(B$_3$H$_6$)$_2$], closo-borane MgB$_{\rm n}$H$_{\rm n}$ (n=6,7,8,9,10,11), and Mg(B$_{11}$H$_{14}$)$_2$ compounds using PEGS+DFT simulations. We find that only the reaction enthalpy to Mg$_3$(B$_3$H$_6$)$_2$ is close to the stable MgB$_{12}$H$_{12}$ pathway, and falls within the thermodynamic conditions for reversibility [e.g., $\Delta$ H = 20$\sim$50 kJ/(mol H$_2$)]. Careful control over experimental conditions might allow for Mg$_3$(B$_3$H$_6$)$_2$ as a possible intermediate in the decomposition of Mg(BH$_4$)$_2$, and might allow Mg$_3$(B$_3$H$_6$)$_2$ to be rehydrided back to Mg(BH$_4$)$_2$ under modest H$_2$($T,p$) conditions.   

Industrial Scale Measurements of Hydrogen Uptake and Delivery in KOH Activated Carbons

The Alliance for Collaborative Research in Alternative Fuel Technologies (ALL-CRAFT) has been producing high surface area activated carbons. Here we will investigate the hydrogen adsorption characteristics of these activated carbons using a custom built 10 liter hydrogen adsorption apparatus filled with 4 kg of activated carbon. We will discuss problems and solutions specific to filling and delivering hydrogen from industrial scale systems. Results show that activated carbons can produce a significant but surmountable, amount of impedance to hydrogen flow. The 10 liter hydrogen storage system measures adsorption at temperatures between -78 Celsius and 100 Celsius and at pressures between zero and 100 bar. The 10 liter hydrogen adsorption uptakes are compared against results obtained with the Hiden Isochema HTP1 volumetric gas analyzer.   

Reversible Storage of Hydrogen and Natural Gas in Nanospace-Engineered Activated Carbons

An overview is given of the development of advanced nanoporous carbons as storage materials for natural gas (methane) and molecular hydrogen in on-board fuel tanks for next-generation clean automobiles. High specific surface areas, porosities, and sub-nm/supra-nm pore volumes are quantitatively selected by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. Tunable bimodal pore-size distributions of sub-nm and supra-nm pores are established by subcritical nitrogen adsorption. Optimal pore structures for gravimetric and volumetric gas storage, respectively, are presented. Methane and hydrogen adsorption isotherms up to 250 bar on monolithic and powdered activated carbons are reported and validated, using several gravimetric and volumetric instruments. Current best gravimetric and volumetric storage capacities are: 256 g CH4/kg carbon and 132 g CH4/liter carbon at 293 K and 35 bar; 26, 44, and 107 g H2/kg carbon at 303, 194, and 77 K respectively and 100 bar. Adsorbed film density, specific surface area, and binding energy are analyzed separately using the Clausius-Clapeyron equation, Langmuir model, and lattice gas models.   

The Stationary States of Adsorbed Hydrogen

In order to investigate the impact of quantum effects on hydrogen adsorption, it is important to understand the stationary states occupied by adsorbed hydrogen molecules. We present experimental inelastic neutron scattering spectra which provide evidence for significant mixing of degrees of freedom which are normally decoupled in free space. Results suggest that simultaneous treatment of translational and rotational degrees of freedom and consideration of the potential corrugation are necessary for improved theoretical understanding of the problem. Numerical calculations of the full five dimensional single body potential are used to understand the origins of the experimentally observed stationary states. In addition, we briefly discuss how our results may be used to understand the shape of hydrogen adsorption isotherms.   

Measured Enthalpies of Adsorption of Boron-Doped Activated Carbons

There is significant interest in the properties of boron-doped activated carbons for their potential to improve hydrogen storage.\footnote{Multiply Surface-Functionalized Nanoporous Carbon for Vehicular Hydrogen Storage, P. Pfeifer et al. DOE Hydrogen Program 2011 Annual Progress Report, IV.C.3, 444-449 (2011).} Boron-doped activated carbons have been produced using a process involving the pyrolysis of decaborane (B$_{10}$H$_{14})$ and subsequent high-temperature annealing. In this talk, we will present a systematic study of the effect of different boron doping processes on the samples' structure, hydrogen sorption, and surface chemistry. Initial room temperature experiments show a 20{\%} increase in the hydrogen excess adsorption per surface area compared to the undoped material. Experimental enthalpies of adsorption will be presented for comparison to theoretical predictions for boron-doped carbon materials. Additionally, results from a modified version of the doping process will be presented.   

Performance of Carbon Hydrogen Storage Materials as a Function of Post-Production Thermal Treatment

High-surface-area activated carbons for hydrogen storage were investigated as a function of post-synthesis surface treatment. Thermal treatment of the initial carbon in high vacuum at temperatures 200-1000 \r{ }C leads to materials with significantly different surface chemistries and hydrogen storage capacities. Results from nitrogen pore-structure analyses, FT-IR spectroscopy before and after the treatment, and thermogravimetric analysis and mass spectroscopy of volatile reaction products during treatment, are reported. For treatment at 600 \r{ }C, excess hydrogen adsorption at 80 K and 303 K is found to be 20-30{\%} higher than for the untreated sample. At temperatures below 450 \r{ }C, volatiles are mostly water and air; volatiles above 450 \r{ }C are mostly carbon dioxide and carbon monoxide. The results are interpreted as that high-temperature treatment produces materials with a large fraction of high-binding-energy sites.   

Size-scaling behavior of hydriding phase transformations in nanocrystals

By partnering data obtained from a novel in-situ luminescence-based probe with a statistical mechanical model we derive size-scaling laws for hydriding phase transformations relevant for hydrogen storage. We conclude that the observed experimental trends are consistent with thermally-driven nucleation, and derive scaling relations that reveal the fundamental size-dependence of nucleation barriers in nanocrystals for first-order phase transformations: near the critical point, the barrier to nucleation is controlled directly by the size of the nanocrystal. Consequently, phase transformation can occur in a nanocrystal even at the critical point, in stark contrast to the classical bulk scenario. Our results provide a detailed framework for understanding how nanoscale interfaces impact broad classes of thermally-driven solid-state phase transformations of relevance for hydrogen storage, catalysis, batteries, and fuel cells.   

Hydrogen affinity and structural stability of Mg-films on substrates

Using first-principles density functional theory we investigated the binding mechanism of hydrogen to thin Mg films and alloyed films. In ultrathin Mg films the stability of hydrides is much lower than in the corresponding bulk systems and it can be modified by metal alloying. We calculated the chemical potential of hydrogen in Mg films for different dopant species and film thicknesses while including all vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature and pressure, we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption/desorption. Experimentally those films were synthesized on the Si substrate using MBE technique, where we observed the formation of both Mg and Mg silcides. We studied the competing mechanism of Mg/Mg$_{2}$Si formation and their structural stabilities on the substrate.   

Hydrogen storage through spillover at sun-2nm Pt nanoparticle - support interfaces

Hydrogen generation and storage are essential components in the increasingly important field of energy storage. Electrochemical generation of Hydrogen atoms at the surface of Pt like metals at select potentials is a widely studied phenomenon. However, moving these adsorbed Hydrogen atoms to high surface area support systems (primarily Carbon) for storage is an issue. We show spillover of these adsorbed Hydrogen atoms to the conducting transition-metal oxide support for sub-2 nm Pt nanoparticles sputtered on fluorine doped tin oxide (FTO) using cyclic voltammetry. The sub-2 nm Pt nanoparticles are deposited on oxide and carbon support systems using a unique tilted target sputtering (TTS) system developed by our lab. The resultant Pt nanoparticles are highly homogeneous, have high number density and are crystalline in nature. We propose integrating this sub-2 nm Pt nanoparticle-FTO system with different carbon structures to see if the spilled over hydrogen can be stored reversibly on adjacent carbon support systems and study the involved hydrogen spillover and storage mechanisms.   

Activated carbon monoliths for methane storage

The use of adsorbent storage media for natural gas (methane) vehicles allows for the use of non-cylindrical tanks due to the decreased pressure at which the natural gas is stored. The use of carbon powder as a storage material allows for a high mass of methane stored for mass of sample, but at the cost of the tank volume. Densified carbon monoliths, however, allow for the mass of methane for volume of tank to be optimized. In this work, different activated carbon monoliths have been produced using a polymeric binder, with various synthesis parameters. The methane storage was studied using a home-built, dosing-type instrument. A monolith with optimal parameters has been fabricated. The gravimetric excess adsorption for the optimized monolith was found to be 161 g methane for kg carbon.   

A DFT Study of Atomic Hydrogen and Oxygen Chemisorption on the $\gamma $-U Surface

Generalized gradient approximation to density functional theory has been used to compute O and H atomic adsorption properties on the (100) surface of $\gamma $-U. The computational method used is the all-electron full-potential linearized augmented plane wave plus local orbitals basis method as implemented in the WIEN2k code. The adatom was allowed to approach the five-layer slab surface at the top, center and bridge sites. The bridge site was found to be most stable with chemisorption energies of 8.43 and 3.76 eV for O and H, respectively, spin-orbit coupling (SOC) included. The optimized distances to the surface for the O atom was 1.98 at the top, 0.75 at center and 1.32 {\AA} and at the bridge sites. For H, these distances were found to be 2.07, 0.57, and 1.40 {\AA} at the corresponding sites, respectively. Inclusion of SOC has significant effects on the energies, changing the chemisorption energy by as much as 0.63 eV for O and 0.43 eV for H. Possible changes in work function, magnetic moment, and charge density distributions have been investigated and will be presented. We will also present results on interstitial sites, by incorporation of O and H atoms inside the slab. Different properties were observed for the O adatom as compared to the H atom.   

Effect of Nitrogen Doping on the Electronic and Optical Properties of TaON

TaON is considered as a potential candidate as a visible-light responsive photocatalyst. We report the results of ab initio studies of electronic structure of TaON in monoclinic and hypothetical cubic phases using VASP code. Specifically, we show that the position of conduction and valence band can be modified by varying the nitrogen (N) concentration in TaO1+xN1-x. The bandgap decreases monotonically with the increase of N concentration from near 2.7eV to just over 1.1eV (i.e. by 230{\%}) when N concentration is reduced from x=0.5 to 1.5. The bandgap reduction is mostly associated with the change in the position of the valence band, while the conduction band is not sensitive to nitrogen content. We calculated the optical absorption spectra and discuss the effect of nitrogen doping on the photocatalytic activity of oxinitrides.   

Determination of crystal structure and the study of electronic properties of AgBiW$_{2}$O$_{8}$ by density functional theory

AgBiW$_{2}$O$_{8}$ shows promising features in solar energy to hydrogen conversion through photoelectrochemical (PEC) approach because of moderate band gap, stability in the liquid solution, suitable band edge positions and low production cost. However, there is not much study available to determine its crystal structure. Using mineral database of relevant oxides and density functional theory (DFT) total energy calculation, we have determined the crystal structures of AgBiW$_{2}$O$_{8}$ to be a wolframite structure, which contradicts a previous report. Our theoretical electronic structure and optical properties calculation agrees well with the recent experimental result.