1. Could you explain the significance of your article to the non-specialist? 

Petroleum and natural gas supply most of the energy consumed nowadays, but increasing worldwide demand is placing strain on petroleum supplies, and this promotes an active search for alternative fuels. Among them, hydrogen has been proposed as an energy source for both vehicles and stationary applications. However, widespread use of hydrogen as a fuel requires a means for its safe storage and transport; hydrogen adsorption on porous solids is an attractive possibility. Zeolites, and related porous materials, are known to be efficient gas adsorbents at a low temperature, and they have the advantage of low cost and high stability. That is why we are currently investigating hydrogen adsorption on zeolites. A major improvement would be to find adsorbents showing a relatively high interaction energy with the hydrogen molecule, which would facilitate hydrogen storage at a temperature closer to ambient. That has prompted us to look at zeolites containing highly charged cations, capable of an enhanced interaction with the adsorbed hydrogen molecule. Among such cations, Mg2+ came as a natural choice on account of its low cost (magnesium is very abundant) and also because of its low density, desirable for portable applications. We found that the interaction energy of hydrogen with magnesium-exchanged zeolites is about 20 times higher than the gas liquefaction heat, which is highly relevant for hydrogen storage.

2. What has motivated you to conduct this work? 

Use of hydrogen as an energy source necessitates finding means for safe (and cost-effective) hydrogen storage and transport, since storage in pressurized gas cylinders is expensive and risky. Reversible adsorption in lightweight porous materials constitutes one such means. Among adsorbents for hydrogen, active carbon, zeolites, and porous metal-organic frameworks have all been considered as promising candidates. Regarding zeolites, presence of extra-framework cations creating strong electric fields inside zeolite channels and cavities should favour hydrogen retention, on account of polarization of the adsorbed hydrogen molecule and consequent ion-dipole interaction. Zeolites also have the advantages of low cost, high stability and structural diversity. The versatility of zeolites regarding pore size and specific pore volume, and also their wide range of extra-framework (exchangeable) cations, prompted us to explore (in a systematic way) the possibility of using these materials for hydrogen storage. We started looking at alkali-metal exchanged zeolites (see, e.g., P. Nachtigall et al., Phys. Chem. Chem. Phys. 8 (2006) 2286) which, although promising, gave hydrogen interaction energies no greater than 10 kJ mol-1. The idea of choosing Mg2+ as the extra-framework cation crystallized out of two considerations: (i) the higher polarization power expected for Mg2+ (as compared to alkali-metal cations) and (ii) the light weight and low cost of magnesium.

3. Where do you see this work developing in the future? 

For economical reasons, the target (for the year 2010) of energy density in hydrogen storage systems for the automotive sector was set at 6.5 wt% by the US Department of Energy (DOE). Although substantial effort was made during recent years to find hydrogen storage materials capable of reaching the DOE target, no such material was yet found. Among the aspects to consider in the use of zeolites as hydrogen storage materials, maximum hydrogen loading capacity is one important parameter but another one is the strength of the gas-solid interaction. For an optimum delivery cycle the adsorption enthalpy should be neither too low (so as to enhance storage) nor to high (to facilitate release). Since the gas-solid interaction energy depends on the strength of intra-zeolite electric fields, which are determined by the number and kind of extra-framework cations and by the pore size, judicious choice of these parameters would help to attain the desired final properties. It should also be noted that other porous materials (and not only zeolites) can be potential candidates for hydrogen storage by physisorption. Among them, worth considering are porous metal-organic frameworks, which also show wide versatility in structure and porosity (see, e.g., U. Mueller et al., J. Mater. Chem. 16 (2006) 626).

4. Are there any particular challenges facing future research in this area? 

At present, developed technologies for hydrogen storage are compression in hydrogen gas cylinders and cryogenic storage of liquid hydrogen. Both systems present major safety concerns, and also they are expensive because of the energy needed for compressing the gas. Cost-effective means for hydrogen storage currently under active research include reversible hydrogen adsorption in suitable porous materials, formation of hydrogen clathrates and metal hydrides. They all have specific advantages and setbacks. For instance, reversible hydrogen storage by physisorption still needs a low temperature, since no good adsorbent at room temperature has yet been found. Metal hydrides, on the other side, tend to be too stable and they only release hydrogen (with a reasonable kinetics) at a temperature well over 100 degrees. So that, a major challenge is either to increase temperature at which physisoption can be achieved, or to decrease temperature for hydrogen release from chemisorption systems (metal hydrides and related materials).
Regarding storage in porous adsorbents, increasing gas adsorption enthalpy would facilitate hydrogen retention at a relatively high temperature, while synthesis of highly porous materials (i.e. solids having a very large specific pore volume and surface area) would increase hydrogen loading. These are the main lines along which our research group, and several others, are conducting research in this field.