We perform an extensive combinatorial search for optimal nanostructured hydrogen storage materials among various metal-decorated polymers using first-principles density-functional calculations. We take into account the zero-point vibration as well as the pressure-and temperature-dependent adsorption-desorption probability of hydrogen molecules. An optimal material we identify is Tidecorated cis-polyacetylene with reversibly usable gravimetric and volumetric density of 7.6 weight percent and 63 kg/m 3 respectively near ambient conditions. We also propose "thermodynamically usable hydrogen capacity" as a criterion for comparing different storage materials.PACS numbers: 68.43. Bc, 71.15.Nc Hydrogen storage is a crucial technology to the development of the hydrogen fuel-cell powered vehicles [1,2]. Recently, nanostructured materials receive special attention because of potentially large storage capacity (high gravimetric and volumetric density), safety (solidstate storage), and fast filling and delivering from the fuel tank (short molecular adsorption and desorption time) [3,4,5]. However, when the thermodynamic behavior of the gas under realistic environments is taken into account, the usable amount of hydrogen with these nanomaterials falls far short of the desired capacity for practical applications and search for novel storage materials continues worldwide [6,7,8,9]. It is to be emphasized that hydrogen storage in nanostructured materials utilizes the adsorption of hydrogen molecules on the host materials and its thermodynamic analysis is distinct from that of metal or chemical hydrides. Each adsorption site on the nanomaterial behaves more or less independently and the probability of the hydrogen adsorption follows the equilibrium statistics which is a smooth function of the pressure and temperature. There is no sharp thermodynamic phase transition between the gas and the adsorbed state of H 2 , in contrast to the case of metal or chemical hydrides where an abrupt phase transition occurs at well-defined pressure at a given temperature [10].With this caveat, a general formalism applicable to the hydrogen adsorption on nanomaterials was derived in the present study from the grand partition function with the chemical potential determined by that of the surrounding H 2 gas acting as a thermal reservoir. As each site can adsorb more than one H 2 molecule, information on the multiple adsorption energy is necessary. (The situation is analogous to the O 2 adsorption and desorption on hemoglobin which can bind up to 4 O 2 molecules.) In equilibrium of the H 2 molecules between the adsorbed and desorbed (gas) states, the occupation (adsorption) number f is obtained from f=kT ∂lnZ/∂µ, where Z is the grand partition function, µ is the chemical potential of H 2 in the gas phase at given pressure p and temperature T, and k is the Boltzmann constant. Here, f per site is reduced towhere ε l is the adsorption energy per H 2 molecule when the number of adsorbed molecules is l and g l is the multiplicity (degeneracy) of the co...

We investigate the structure, topology, and dynamics of liquid water at the interface with natively hydroxylated (001) surfaces of InP and GaP photoelectrodes. Using ab initio molecular dynamics simulations, we show that contact with the semiconductor surface enhances the water hydrogen-bond strength at the interface. This leads to the formation of an ice-like structure, within which dynamically driven water dissociation and local proton hopping are amplified. Nevertheless, the structurally similar and isovalent InP and GaP surfaces generate qualitatively different interfacial water dynamics. This can be traced to slightly more covalent-like character in the binding of surface adsorbates to GaP, which results in a more rigid hydrogen-bond network that limits the explored topological phase space. As a consequence, local proton hopping can give rise to long-range surface proton transport on InP, whereas the process is kinetically limited on GaP. This allows for spatial separation of individual stages of hydrogen-evolving, multistep reactions on InP(001). Possible implications for the mechanisms of cathodic water splitting and photocorrosion on the two surfaces are considered in light of available experimental evidence.

We report the results of total-energy density functional theory and ab initio molecular dynamics simulations of (001) surfaces of InP and GaP in contact with gas-phase and liquid water. Both pristine and oxygen-rich surfaces (representing a submonolayer native surface oxide) are considered. We find that gas-phase binding of water on pristine mixed-dimer δ(2×4) reconstructions of InP/GaP(001) is comparable to the solvation energy of liquid water, and that the barriers for room-temperature dissociation are high. In the presence of a submonolayer surface oxide, water binding and dissociation instead become strongly exothermic and proceed with almost no barrier. In this case, the surface chemistry at the interface with liquid water differs significantly from that of gas-phase water adsorption due to the formation of strong, low-barrier hydrogen bonds between surface adsorbates and water molecules. Water dissociation on the oxygen-rich surface is accompanied by extremely rapid local proton hopping between hydrogen-bonded surface adsorbates.