The hydrogen storage properties of Mg-intermetallic-hydrides by ab initio calculations and kinetic Monte Carlo simulations

First-principle calculations and kinetic Monte-Carlo simulations were performed to study the hydrogen storage properties of the intermetallic hydrides MgNiH₃, MgCoH₃ and thier mixture namely MgCo_0.5Ni_0.5H₃.Based on the heat of formation, desoprtion temperature, activation energies computed from DFT calculations and KMC simulations, we show that the MgNiH₃ involves a fast kinetic while it is thermodynamically unstable (−9.96 kJ/mol.H₂; 76.61 K) whereas MgCoH₃ has a high thermodynamic stability (−73.32 kJ/mol.H₂; 560.97 K) which prevents their application for mobile hydrogen storage.On the other hand, the electronic structures show that the Ni weakens the strong CoH bonding of the mixture MgCo_0.5Ni_0.5H₃, which enhances significantly its stability and its desorption temperature (−45.92 kJ/mol.H₂ and 351.33 K) without reducing its high volumetric capacity 133.73 g.H₂/l. Kinetic Monte-Carlo simulations show that MgCo_0.5Ni_0.5H₃ exhibits a fast charging time (only 4.6 min at 400 K and 10 bar).Thermodynamic properties including entropy S, Gibbs free energy G and thermal expansion coefficient are predicted within the quasi-harmonic approximation. It is verified that crystal structure of MgCo_0.5Ni_0.5H₃ is stable.     

A comparative ab initio study of hydrogen abstraction from n-propyl benzene

Aromatics form an integral part of typical aviation fuels with n-propyl benzene selected as a representative molecule for inclusion in several EU and US surrogate blends used for design calculations. Despite the practical relevance, kinetic and thermodynamic data obtained using comparatively accurate ab initio methods have to date not been compared with currently used reaction class based estimates. The use of ab initio methods for comparatively complex molecules also necessitates an assessment of the relative benefits of higher levels of theory as it is typically desirable to balance the accuracy of the treatment of individual reactions with the need to consider more complete reaction sequences. The current study examines six hydrogen extractions, via the hydrogen or methyl radicals from the n-propyl side chain. Potential energy surfaces were determined using 10 different approaches, including state-of-the-art DFT (M06, M06-2X and M08-SO) and contemporary composite methods (G4, G4MP2, CBS-QB3 and CBS-4M). Results are presented relative to data obtained using the CCSD(T)/jun-cc-pVTZ//M06-2X/6-311++G(3df,3pd) coupled cluster based method. Rate parameters were determined using transition state theory combined with (i) small curvature tunnelling and energetics at the M06-2X/6-31G(2df,p) level and (ii) Eckart tunnelling corrections and energetics at the CCSD(T)/jun-cc-pVTZ level. Results were found to agree comparatively well with modest differences in rates for several reactions. However, it is also shown that substantial deviations can arise with respect to reaction class based estimation techniques.     

Ab initio calculations of thermal radiative properties: The semiconductor GaAs

Spectral reflectance of GaAs from infrared (IR) to ultra-violet (UV) bands is predicted using ab initio calculations. We first predict the spectral dielectric function. Two major mechanisms exist for different photon wavelength, namely, photon–electron coupling in the UV to near-IR region and photon–phonon coupling in the far-IR region. For the near-IR to UV band, the electronic band structure of GaAs is calculated, and the imaginary part of the dielectric function is determined from the band structure using the Fermi’s golden rule. The real part of spectral dielectric function is then derived from Kramer–Kronig transformation. For the far-IR region, ab initio calculations are used to determine the phonon modes, and the dielectric function is then predicted using the oscillator model. The spectral reflectance for the entire spectrum is then calculated using Fresnel’s law for a semi-infinite GaAs slab. The predicted results agree reasonably well with experimental data, demonstrating the capability of ab initio calculations to predict thermal radiative properties of semiconductor materials from their atomic structures.     

Hydrogen adsorption on Li metal in boron-substituted graphene: An ab initio approach

The characteristics of hydrogen adsorption on Li metal atoms dispersed on graphene with boron substitution is investigated including Li clustering, hydrogen bonding characteristics, and the open metal states of Li adatom using density functional theory calculations. It is found that Li atoms are well dispersed on boron-substituted graphene and can form the (2 × 2) pattern because clustering of Li atoms is hindered by the repulsive Coulomb interaction between Li atoms. One Li adatom dispersed on the double side of graphene can absorb up to 8 hydrogen molecules corresponding to a 13.2% hydrogen storage capacity. In addition, the adsorption behaviors of non-hydrogen atoms such as C and B are calculated to determine whether Li atoms can remain as the open metal state in boron-substituted graphene.     

Ca and K decorated germanene as hydrogen storage: An ab initio study

The hydrogen storage capacity and performance of Ca and K decorated germanene were studied using density functional theory calculation. The Ca and K adatoms were found to be sufficiently bonded to the germanene without clustering at the hollow site. Further investigation has shown an ionic bonding is apparent based on the charge density difference and Bader charge analysis. Upon adsorption of H₂ on the decorated germanene, it was found that the Ca and K decorated systems could adsorb 8 and 9 H₂ molecules, respectively. The adsorption energies of H₂ molecules were within the Van der Waals energy (400–435 meV), suggesting weak physisorption. The charge density profile revealed that the electron of H₂ moved toward the adatom decoration without leaving the local region of H₂. This suggests that a dipole-dipole interaction was apparent and consistent with the energy range found. Finally, the gravimetric density obtained from the adsorption of H₂ on the decorated germanene shows that this material is a potential for H₂ storage media.     

An ab initio study of spectroscopic and thermodynamic characteristics of MgH2 and TiC systems

In this investigation, structural, dynamical and thermodynamic characteristics of magnesium hydride (MgH₂) and titanium carbide (TiC) are performed through first principle calculations based on the density functional theory (DFT). The lattice dynamics were investigated using the finite displacement supercell approach and thermodynamic calculations were carried out using the harmonic approximation. The modes of vibrations were studied to explore the behavior of individual atoms. The minimum hydrogen release temperature was noted to be Tc = 719 K when only electronic and vibrational free energies of MgH₂ and TiC systems are considered at pressure of 1 bar. However, a remarkable reduction in hydrogen release temperature is noticed in MgH₂ and TiC system i.e. Tc = 321 K (fall of 398 K) by including all the contributions of free energies (i.e. electronic, translational, rotational and vibrational) of hydrogen molecule.     

Hydrogen capability of bimetallic boron cycles: A DFT and ab initio MD study

Bimetallic boron cycle, B₆C₂TM₂ (TM = scandium, titanium), was recently predicted to have high stability and aromaticity. The hydrogen capabilities of these clusters were studied in the present work. Our computational results indicate that the gravimetric hydrogen uptake capacity of B₆C₂Sc₂ and B₆C₂Ti₂ and clusters are 11.7% and 11.4%, respectively. The adsorption energies of H₂ molecules on B₆C₂Sc₂ and B₆C₂Ti₂ clusters are predicted with different calculational schemes to meet the criteria of reversible hydrogen storage. The interaction of H₂ with B₆C₂Ti₂ cluster is a little stronger than that with B₆C₂Sc₂. Ab initio molecular dynamics simulations indicate that H₂ molecules can be efficiently released from the metal sites of B₆C₂TM₂ clusters at room temperature. The bulk-like B₆C₂Sc₂ and B₆C₂Ti₂ tetramer can also efficiently adsorb H₂ molecules.     

Functionalized imidazolium ionic liquids as electrolyte components of lithium batteries

Some basic properties and compatibility toward lithium electrode for electrolytes based on substituted imidazolium ionic liquid have been investigated. The ionic liquids having imidazolium cation substituted by methylcarboxyl or cyano group suffers from low conductivity. However, reversible lithium deposition–dissolution process was observed in electrolytes based on these electrolytes. In particular, lithium salt solution in cyanomethyl-substituted imidazolium ionic liquid provided similar cycle efficiency to conventional organic solvent electrolyte at constant-current condition. The mixed ionic liquid electrolyte containing the cyanomethyl-substituted ionic liquid also provided good cycle performance despite of containing large amount of 1-ethyl-3-methyl imidazolium (EMI)-based ionic liquid. Such mixed electrolyte system serves both the stability of lithium electrode process and valid conductivity for practical use.     

An ab initio study of dissociative adsorption of H2 on FeTi surfaces

Dissociative adsorption of H₂ on clean FeTi (001), (110) and (111) surfaces is investigated via ab initio pseudopotential-plane wave method. Adsorption energies of H atom and H₂ molecule on Fe and Ti terminated (001) and (111) and FeTi (110) surfaces are calculated on high symmetry adsorption sites. It is shown that, top site is the most stable site for horizontal H₂ molecule adsorption on (001) and (111) surfaces for both terminations. The most favorable site for H atom adsorption on these surfaces however, is the bridge site. In (110) surface, the 3-fold hollow site which is composed of a long Ti–Ti bridge and an Fe atom, (Ti–Ti)_L–Fe, and again a 3-fold hollow site this time composed of a short Ti–Ti bridge and an Fe atom, (Ti–Ti)_S–Fe, are the most stable sites for H₂ and H adsorption, respectively. With the analysis of the above favorable adsorption sites, probable dissociation paths for H₂ molecule over these surfaces are proposed. Activation energies of these dissociations are also determined with the use of the dynamics of the H₂ relaxation and climbing image nudged elastic band method. It is found that H₂ dissociation on (110) and Fe terminated (111) surfaces has no activation energy barrier. On other surfaces however, activation energies are calculated to be 0.178 and 0.190 eV per H₂ molecule for Fe and Ti terminated (001) surfaces respectively, and 1.164 eV for Ti terminated (111) surface.     

Fabrication of a highly efficient polymer electrolyte membrane by embedding nanofibrous metal oxide in cell components

In this work, a high-performance polymer electrolyte membrane based on sulfonated polyether ether ketone (SPEEK) loaded with zirconium oxide nanofibres (ZrN) was fabricated. ZrN with an average diameter of 182 nm were prepared by pyrolysing electrospun polyacrylonitrile fibres embedded with a zirconium precursor. The weight percentage of added ZrN and the relevant performance of the hybrid membrane (e.g., proton conductivity, methanol permeability and fuel cell performance) were experimentally determined and verified. It was found that the SPEEK-ZrN hybrid membrane exhibited sufficient proton conductivity (25.9 mS cm⁻¹), low methanol permeability (1.64 × 10⁻⁷ cm² s⁻¹) and superior selectivity (15.8 × 10⁴ S s cm⁻³) at room temperature. Compared to a pure SPEEK membrane, the SPEEK-ZrNT-1.5 membrane showed 1.3 and 1.7 times higher current density and power density, respectively.     

Multiscale modeling of femtosecond laser irradiation on a copper film with electron thermal conductivity from ab initio calculation

By combining the ab initio quantum mechanics (QM) calculation and the Drude model, electron temperature- and lattice temperature-dependent electron thermal conductivity is calculated and implemented into a multiscale model of laser material interaction, which couples the classical molecular dynamics (MD) and the two-temperature model (TTM). The results indicated that the electron thermal conductivity obtained from ab initio calculation leads to faster thermal diffusion than that using the electron thermal conductivity from empirical determination, which further induces a deeper melting region, a larger number of density waves travelling inside the copper film, and more various speeds of atomic clusters ablated from the irradiated film surface.     

Cost-optimized design point and operating strategy of polymer electrolyte membrane electrolyzers

Green hydrogen is a key solution for reducing CO₂ emissions in various industrial applications, but high production costs continue to hinder its market penetration today. Better competitiveness is linked to lower investment costs and higher efficiency of the conversion technologies, among which polymer electrolyte membrane electrolysis seems to be attractive. Although new manufacturing techniques and materials can help achieve these goals, a less frequently investigated approach is the optimization of the design point and operating strategy of electrolyzers. This means in particular that the questions of how often a system should be operated and which cell voltage should be applied must be answered. As existing techno-economic models feature gaps, which means that these questions cannot be adequately answered, a modified model is introduced here. In this model, different technical parameters are implemented and correlated to each other in order to simulate the lowest possible levelized cost of hydrogen and extract the required designs and strategies from this. In each case investigated, the recommended cost-based cell voltage that should be applied to the system is surprisingly low compared to the assumptions made in previous publications. Depending on the case, the cell voltage is in a range between 1.6 V and 1.8 V, with an annual operation of 2000–8000 h. The wide range of results clearly indicate how individual the design and operation must be, but with efficiency gains of several percent, the effect of optimization will be indispensable in the future.     

Numerical simulation of electrolyte particles trajectory to investigate battery cover design characteristics

In this research, a numerical simulation has been conducted to investigate one of the important factors for the efficient design of a maintenance-free (MF) lead-acid battery lid. Baffles and splashguards on the battery vent plug and inside the double lid are generally utilized to reduce the electrolyte loss, in the form of electrolyte droplets, during lifetime of maintenance-free batteries. Gas flow inside the battery cell and double lid is initially solved by finite volume method (FVM); then discrete phase model, in a Lagrangian reference frame, is employed to trace the electrolyte particles formed by bursting gas bubbles on the electrolyte surface and electrolyte splashing and agitating. In addition, appropriate physical models for the boundary conditions of the released gases, electrolyte droplets interaction with the lid walls and the flame arrestor are considered.     

Changes in the wettability of polymer electrolyte fuel cells components during cationic contamination and mitigation

Foreign cations are shown to cause mass transport losses, in particular due to wettability changes in the micro-porous layers (MPL) and the carbon paper substrate, and have a major impact on the durability and the performance of polymer electrolyte fuel cells. We studied the effects of cationic impurities on fuel cell system performance, especially on the water management by employing in-situ and ex-situ contamination methods. Changes in the wettability of the carbon paper surface following the in-situ contamination injection were quantified using the Wilhelmy plate method. The CaSO₄ precipitation on the macro-pores of the carbon paper substrate after the contamination injection causes a higher wettability leading to increased flooding of the carbon paper substrate and consequent mass transport losses. An ex-situ cleaning with an acid solution is shown to be very effective in removing the salt deposits of the carbon paper substrate. During the mitigation, the highly hydrophobic MPL acts as a barrier to the transport of the recovery solution into the membrane-electrode assembly (MEA), therefore isopropanol (IPA) was added to both the contaminant solution and the recovery solution to increase the wettability of the MPL. Wetting force measurements confirm that the added IPA can alter the wettability of the MPL and can render it fully hydrophilic, enabling the transport of the recovery solution into the MEA.     

Electronic structure,thermomechanical and phonon properties of inverse perovskite oxide (Na3OCl): An ab initio study

Within first principles calculations, the electronic structure, thermodynamic, mechanical stability, magnetism, and phonon properties of the inverse perovskite (Na₃OCl) have been summed up. The Birch-Murnaghan derived lattice constant and bond-lengths are identical, when compared to the experimental data. A direct energy gap of 2.18 eV observed from the band structure reveals the semiconducting nature of the present oxide. Also, the application of strain on electronic properties predicts the decrease in bandgap with respect to compressive strain and vice versa. The constituent nonmagnetic atoms in its crystal propose the total magnetic moment to be zero and the same is supported by susceptibility data. In addition to the negative Cauchy's pressure, the small bulk modulus compared to Young's modulus determined from elastic constants, possibly claims it as a brittle material. Also, the temperature dependent Gruneisen parameter (1.58) and Debye temperature (382.27 K) are determined to reveal the lattice thermal conductivity (κ = 6.48 W/mK) at room temperature.     

Substitution of nickel in Mg2Ni and its hydride with elements from groups XIII and XIV: An ab initio study

We have performed ab initio calculations with equilibrium supercells of the Mg₂Ni compound and its hydride Mg₂NiH₄ doped with elements X = Al, Ga, In, Si, Ge and Sn. Two concentrations of X in both structures have been set: (1) every 16th, and (2) every fourth Ni atom has been substituted by X. Total energy calculations yielded the Mg₂NiH₄ hydrogen absorption enthalpy ΔH_abs according to the chemical reaction Mg₂Ni + 2H₂ → Mg₂NiH₄. Reduction of the hydrogen absorption enthalpy was reported for both concentrations of X. When doping the Mg₂NiH₄ hydride with X = In in a low concentration (1), the value of hydrogen desorption enthalpy decreases from 68.22 to 55.96 kJ(mol H₂)^?1. Doping with X = In in a high concentration (2) further decreases the hydrogen desorption enthalpy to 5.50 kJ(mol H₂)^?1. Further, the electronic structure of Mg₂(Ni–In)H₄ hydride with a low In concentration indicates weaker Ni–H bonds in comparison with the pristine Mg₂NiH₄. Attraction between H and In atoms induced enhanced bonding between Mg and H atoms compared to the pristine Mg₂NiH₄.     

Electronic and optical proeprties of neon-doped rutile TiO<Subscript>2</Subscript> from ab inito calculations

The electronic-structure and optical properties of neon-doped rutile TiO₂ have been investigated using density functional theory with Slater type orbitals basis set and correlation. This was done using the PBE method as implemented within the Hyper Chem 7.52 software package with Ne concentration approaching the low level may present in industrial samples of rutile TiO₂. Defect states involving substitution of an oxygen atom for a neon atom were studied along with the more stable configuration of one neon substitution. Neon change the band structure and lead to a reduce in the band gap in rutile. This make that neon doping brings the absorption edge into the visible range and therefore increase the photocatalytic activity.     

Nonlinear D-optimal design of experiments for polymer–electrolyte–membrane fuel cells

Using empirical models, parameters have to be estimated from experimental data. Experimental characterization of fuel cell stacks is an expensive and time-consuming task. Therefore it is very important to choose an experimental design, which maximizes the statistical quality of the resulting information. Box and Lucas (Biometrika 46 (1959)) showed that it is possible to optimize nonlinear experimental designs by the minimization of the covariance matrix of the least squares estimate. The aim of this work is to adopt this general method in order to investigate its ability for application in polymer–electrolyte–membrane fuel cell (PEMFC) characterization. Based on an empirical PEMFC model a D-optimal design criterion has been developed and validated. Numerical methods, evolutionary and heuristic are investigated with respect to fast and robust evaluation of the design criterion. For a given set of experimental data best results are achieved using a heuristic approach, a so-called sequential search. Based on that result an algorithm to obtain an optimal design of experiments (DoE) in a nondeterministic operating area is introduced. The proposed algorithm is able to take into account experimental limitations due to test facilities or examinee. The algorithm further allows to include existing and for reference needed experiments.     

Controller design for polymer electrolyte membrane fuel cell systems for automotive applications

Continuous developments in Proton Exchange Membrane Fuel Cells (PEMFC) make them a promising technology to achieve zero emissions in multiple applications including mobility. Incremental advancements in fuel cells materials and manufacture processes make them now suitable for commercialization. However, the complex operation of fuel cell systems in automotive applications has some open issues yet. This work develops and compares three different controllers for PEMFC systems in automotive applications. All the controllers have a cascade control structure, where a generator of setpoints sends references to the subsystems controllers with the objective to maximize operational efficiency. To develop the setpoints generators, two techniques are evaluated: off-line optimization and Model Predictive Control (MPC). With the first technique, the optimal setpoints are given by a map, obtained off-line, of the optimal steady state conditions and corresponding setpoints. With the second technique, the setpoints time profiles that maximize the efficiency in an incoming time horizon are continuously computed. The proposed MPC architecture divides the fast and slow dynamics in order to reduce the computational cost. Two different MPC solutions have been implemented to deal with this fast/slow dynamics separation. After the integration of the setpoints generators with the subsystems controllers, the different control systems are tested and compared using a dynamic detailed model of the automotive system in the INN-BALANCE project running under the New European Driving Cycle.     

Conceptual design of large scale water electrolysis plant using solid polymer electrolyte technology

In an energy conscious environment, the key to the applicability of water electrolysis as a means for generation of hydrogen in bulk quantities is the achievement of high efficiencies (i.e. over 90%) at high enough current densities to keep the capital costs within economic bounds. The solid polymer electrolyte (SPE) water electrolysis technology developed by the General Electric Company is now demonstrating these efficiencies at current densities up to 500 A ft⁻², and the results of recent laboratory testing show a potential for increasing this to 2000 A ft⁻² within the next 10 years. This capability now makes water electrolysis one of the most promising methods for generating hydrogen from nuclear, solar or other non-fossil fuel energy sources.

The performance and life test results are shown including laboratory cells on which future performance projections are based. The design and development status of a scaled-up electrolysis cell suitable for large-sized hydrogen generation plants is described. Estimated capital costs and operating costs are projected from which the resultant hydrogen costs are calculated.