ABSTRACT
Introduction ................................................................................................................................. 134 Classes of Complex Metal Hydrides ........................................................................................ 138
Metal Alanates ........................................................................................................................ 138 Synthesis of Metal Alanates .................................................................................................. 138 Metal Alanate Crystal Structures ......................................................................................... 139
LiAlH4 ................................................................................................................................. 139 NaAlH4 ................................................................................................................................ 140 KAlH4 .................................................................................................................................. 140 Mg(AlH4)2 ........................................................................................................................... 140 Ca(AlH4)2 ............................................................................................................................ 141
Hydrogen Storage Properties of Alanates .......................................................................... 142 LiAlH4 ................................................................................................................................. 143 NaAlH4 ................................................................................................................................ 143 KAlH4 .................................................................................................................................. 145 Mg(AlH4)2 ........................................................................................................................... 146 Ca(AlH4)2 ............................................................................................................................ 146 Na2LiAlH6 and LiMg(AlH4)3 ............................................................................................ 147
Metal Borohydrides .................................................................................................................... 147 Synthesis of Metal Borohydrides ......................................................................................... 148 Crystal Structures of Metal Borohydrides .......................................................................... 149
LiBH4 ................................................................................................................................... 149 NaBH4 .................................................................................................................................. 150 Mg(BH4)2 ............................................................................................................................. 150 Ca(BH4)2 .............................................................................................................................. 150
Hydrogen Storage Properties of Metal Borohydrides ...................................................... 150 LiBH4 ................................................................................................................................... 150 NaBH4 .................................................................................................................................. 153 Mg(BH4)2 ............................................................................................................................. 153 Ca(BH4)2 .............................................................................................................................. 154 Other Metal Borohydrides ............................................................................................... 155
Amides, Imides, Nitrides ........................................................................................................... 157 Li3N ........................................................................................................................................... 157 LiNH2 Modified with Mg ...................................................................................................... 161 LiMgN ...................................................................................................................................... 166
Mixed-Anion Complex Metal Hydrides .................................................................................. 169
Complex metal hydrides represent a class of compounds composed of metal cations (typically group I and II elements) and “complex” hydrogen-containing anions such as alanates (AlH4-), borohydrides (BH4-), and amides (NH2-). The IUPAC (International Union of Pure and Applied Chemistry) recommended names for the AlH4-and BH4-salts are tetrahydro aluminates and tetrahydroboronates, although these names are seldom used in the literature. Unlike the interstitial metal hydrides discussed in Chapter 5 by Ben Chao and Lennie Klebanoff, complex metal hydrides display hydrogen atoms covalently bound to Al, B, and N. Complex metal hydrides are of interest for hydrogen storage applications due to their light weight and high hydrogen content [1-4]. They release molecular hydrogen either by heating or by a chemical reaction, such as hydrolysis. In fact, many complex hydrides release hydrogen in the presence of water or aqueous solutions. However, such reactions are quite exothermic and are not easily reversible. Here, we consider explorations of complex metal hydrides with the goal of finding a reversible hydrogen storage system with higher gravimetric density than the interstitial hydride materials but retaining the other attractive features of fast kinetics, favorable thermodynamics, and release of very pure hydrogen gas. We seek a material that can release hydrogen and reabsorb it without having to remove the material or the tank from the vehicle or piece of equipment powered by hydrogen. In other words, we seek an “on-board-reversible” complex hydride material. Chemists have used complex hydrides for almost a century in organic syntheses involv-
ing reduction of esters, carboxylic acids, and organic amides. It was not until the mid1990s that complex metal hydrides were considered for hydrogen storage applications. The pioneering work of Bogdanovic and Schwickardi [5] charged the field when they demonstrated that H2 can be stored reversibly in sodium alanate (NaAlH4) doped with titanium [5]. Titanium addition has proven beneficial because it lowers the decomposition temperature of NaAlH4 but, more importantly, promotes full reversibility. This was a remarkable breakthrough and demonstrated for the first time that H2 release from a complex metal hydride could be reversible. Ti-catalyzed sodium alanate remains one of the better reversible complex metal hydrides known. Chapter 5 presented the van’t Hoff expression for the formation of metal hydrides
and how the thermodynamic changes in enthalpy ΔH and entropy ΔS are derived from pressure-composition-temperature (PCT) studies. As discussed in Chapter 5, for hydrogen storage applications, we seek a ΔH of dehydrogenation near 40 kJ/ mol, assuming
Destabilized Complex Metal Hydrides ................................................................................... 172 Theoretical Prediction of Complex Metal Hydride Materials .............................................. 179 Nanoscale Complex Metal Hydrides ....................................................................................... 185 Summary and Outlook ............................................................................................................... 198
Destabilized Materials ........................................................................................................... 198 Nanoconfinement ................................................................................................................... 199 Kinetics of Solid-State Reactions ..........................................................................................200 Effect of Additives on the Rates of Solid-State Reactions .................................................200 Borohydrides ........................................................................................................................... 201
References ..................................................................................................................................... 201
ΔS in the range of about 120 J/ K mol H2. For a hydride with an equilibrium pressure of 1 atm, a 10-kJ/ mol H2 variation in ΔH results in about an 80 K change in the decomposition temperature [6]. In general, higher values of ΔH suggest higher stability of the complex metal hydride, while lower ΔH values suggest lower stability. A majority of complex metal hydrides require heat to release H2, but there are also some important exceptions [7-14]. It is important to consider the kinetics of hydrogen absorption/ desorption for complex
metal hydride reactions. Solid-state reactions involving H2 often suffer from high kinetic barriers required for diffusion. The dependence of the rate constant k on the reaction temperature (in kelvin) and activation energy Ea is given by the Arrhenius equation,
where A is the preexponential factor. The activation energy originates from the barrier associated with bond breaking in the transition state of the potential energy surface between the reactants and products of a reaction. Higher temperatures typically result in accelerated H2 release reaction rates. Many reactions involving metal hydrides occur in the presence of catalysts. The role of
the catalysts is to accelerate a dehydrogenation or rehydrogenation reaction without modifying ΔG (standard Gibbs energy change) of the reaction or being consumed in the process. The catalysts do participate in the reaction and lower the activation energy for various processes; however, they are regenerated as the reaction proceeds to completion. Often, the role of the catalysts in reactions involving complex metal hydrides is to form activated species for rapid H2 release or aid the dissociation of H2 at the gas/ solid interphase and accelerate diffusion of atomic H. To extract values of the Ea, measurements are made of the variation of the rate constant k with temperature, as follows:
ln lnk E RT
Aa= − +
Plotting ln k vs. 1/T gives a line of slope -Ea/ R, with the preexponential factor obtainable from the intercept. With R = 8.314 J·mol-1·K-1, the units of activation energy are joules per mole. The Ea values for hydrogen release from complex metal hydrides are typically ≥100 kJ·mol-1, but catalysts can significantly reduce the activation energy (to tens of kilojoules per mole) [11-18], as discussed in the following sections. Although many metal hydrides display high volumetric and gravimetric hydrogen
densities, for true commercial viability, the complex metal hydrides need to satisfy many performance requirements. The requirements are especially stringent for light-duty hydrogen-powered vehicles with fuel cells or internal combustion engines (ICEs) [19]. Complex metal hydrides have high gravimetric and volumetric capacities and tunable thermodynamics, which can allow reversible H2 storage without removing the material from the H2 tank. On-board reversibility is one of the most challenging requirements driving recent complex hydride materials discovery [11, 13, 16, 17, 19-26]. To help focus and guide scientific research and development (R&D) in the field, the
U.S. Department of Energy (DOE) developed a Hydrogen Storage Multi-Year Research Development and Demonstration Plan (MRDDP) that identifies barriers to using solidstate hydrogen storage for hydrogen-powered cars. Some of these barriers are as follows:
A. Cost. We need low-cost materials and components for hydrogen storage systems, as well as low-cost, high-volume manufacturing methods.
