I. Hydrogen Storage

  • Hydrogen atom is the smallest one in the Periodic Table. I think that hydrogen is also the most weird of the elements, because its properties in its three oxidation states (–1, 0 and +1) differ to the largest extent possible for an element as regards chemical properties. To take an extreme, H–1 has very diffuse orbitals and it is a strong reducing agent (i.e. an electron–density donor), while H+1 is very small species, a naked nucleus in fact, and it desires electron density (it is a powerful electron–density acceptor). Poor the alone proton in the hydrogen nucleus, which experiences such huge changes of the electron density in its surrounding! Using Pearson’s language we may say, that the derivative of the chemical potential upon the electron density is the largest for hydrogen, indeed. This makes it difficult to manipulate electron density around H nucleus in a desired manner. As we think based on the avoided crossing model of superconductivity, superconducting hydrides and materials for hydrogen storage are strongly related to each other by two equations:

(1a) Xn+ + H–1 –> X(n–1)+ + H0 and (1b) Xn– + H+1 –> X(n–1)– + H0.

These equations, especially Eq. (1a), are the firm basis for our current design of these fascinating materials of the future.

  • We have recently shown (W. Grochala, Peter P. Edwards, CHEM REV, 104(3): 1283-1315 2004), that – surprisingly enough – the temperature of thermal decomposition (Tdec, kinetic parameter) for a variety of binary hydrides MHn monotonically follows the standard redox potential for the involved Mn+/M0 redox pair (E0, thermodynamic parameter) in an impressive range of Tdec and E0 values. The value of Tdec can be further manipulated by the deliberate choice of a second element, E, and a desired stoichiometry of a ternary hydride  ExMyHz. This means that various properties of hydrogen (such as size, polarizability, hardness, charge density etc.) could be nicely tuned in metal hydrides (POL J CHEM, 79(6): 1087-1092 2005, J ALLOY COMP 404-406: 31-34 2005). Based on this discovery, we now investigate experimentally several complex but promising systems, which would liberate molecular H2 from a hydrogen storage material at some 60-120 oC, which is necessary if our hydrogen store is to cooperate with a low-temperature alkaline or polymer electrolyte membrane (PEM) H2/O2 fuel cells.
  • We also progress in theoretical analysis of conditions which need to be fulfilled if the reabsorption of H2 to the hydrogen store is viable at moderate pressures (2-20 bar), i.e. it reagents do not face large barrier along the reaction path (PHYS CHEM CHEM PHYS 8(11): 1340-1345 2006). This is an important practical condition for reversibility of H2 absorption/desorption cycle, and together with the previous condition of low Tdec value, a crucial target for the successful hydrogen reservoir.
  • Recently, we hale expanded our studies to the proton/hydride H2 stores and to catalysis of a heterolytic split of dihydrogen (CHEM COMMUN, (18) 2330-2332 2005, ADV FUNCT MATER, in press 2006).