Hydrogen and its competitors, 2004

34Risø Energy Report 3Hydrogen storage5.2T cß-Phase8010060P eq [bar]10α-Phaseα+ß-Phase4020E 0 [mV]10-200.10.00.20.4 0.6 0.8 1.02.8 3.2 3.6C H [H/M] T –1 [10 –3 K –1 ]Figure 12: Pressure-concentration-temperature (pcT) plot and Van't Hoff curve (logarithm of the equilibrium pressure vs. the reciprocal temperature)for LaNi 5 . The vertical axis indicates the corresponding hydrogen pressure or the equivalent electrochemical potential.to the metal either as molecules, in the gas phase, or asatoms from an electrolyte (Figure 11). In the former case,hydrogen molecules dissociate at the surface beforeabsorption; desorption releases hydrogen atoms, whichrecombine to form H 2 .The thermodynamics of hydride formation from gaseoushydrogen are described by pressure-compositionisotherms or pcT curves (Figure 12). The host metaldissolves some hydrogen as a Sieverts-type solid solutionphase. As the hydrogen pressure increases, so does theconcentration of hydrogen atoms in the metal (c H ). Atsome point, H-H interactions become locally importantand a hydride phase starts to nucleate and grow.While the solid solution and the hydride coexist, theisotherms show a plateau whose length determines howmuch hydrogen can be stored and recovered by means ofsmall pressure changes. In the pure hydride phase, thehydrogen pressure rises steeply with concentration.The plateau or equilibrium pressure p eq (T) dependsstrongly on temperature. The enthalpy of hydride formation(∆H) can be found from the slope of a Van't Hoffplot of the logarithm of the plateau pressure against 1/T.For an equilibrium pressure of 1 bar at 300K, ∆H shouldbe 19.6 kJ/mol hydrogen. The operating temperature ofa metal hydride system is fixed by the plateau pressure inthermodynamic equilibrium and by the overall reactionkinetics [13,14].Hydrogen is stored in the interstices of the host metallattice as atoms, never as molecules. As hydrogen isabsorbed, the lattice expands and often loses some of itssymmetry. The co-existence of the non-expanded solidsolution phase and the anisotropically expanded hydridephase gives rise to lattice defects and internal strainfields, ultimately causing the decrepitation of brittle hostmetals such as intermetallics. The hydrogen atomsvibrate about their equilibrium position, move locallyand also undergo long-range diffusion.In terms of electronic structure, the hydrogen atom'snuclear proton attracts the electrons of the host metal.As a result, the metal's electron bands are lowered inenergy and hybridise with the hydrogen band to formlow-lying bands 6-8 eV below the Fermi level. The Fermilevel itself is shifted, and various phase transitions(metal-semiconductor, magnetic-nonmagnetic, reflecting-transparent,order-disorder) may occur.The metal-hydrogen bond has the advantages ofproviding very high hydrogen storage density atmoderate pressure, and desorption of all the storedhydrogen at the same pressure.Which metallic systems are appropriate for hydrogenstorage? Hydrides can be formed from many metals,including palladium (PdH 0.6 ), the rare earth metals(REH 2 and REH 3 ) and magnesium (MgH 2 ). However,none of these are in the pressure-temperature rangeattractive for mobile storage: 1-10 bar and 0-100°C,corresponding to ∆H in the range 15-24 kJ/mol hydrogen.The discovery of hydrogen sorption by intermetalliccompounds created a flurry of R&D effort worldwide, inthe hope that some of these compounds would be suitablefor practical hydrogen storage systems (Table 9).Alloys based on LaNi 5 have some very promising properties,including fast and reversible sorption with littlehysteresis, a plateau pressure of just a few bars at roomtemperature, and good cycling life. The volumetric (crystallographic)density of hydrogen in LaNi 5 H 6.5 at 2 bar isequal to that of gaseous molecular hydrogen at 1,800 bar,and all this hydrogen is desorbed at only 2 bar. In prac-