Exploiting the Kubas Interaction in the Design of Hydrogen Storage Materials By Tuan K. A. Hoang and David M. Antonelli* 1. Introduction The increase in crude oil consumption and the rapid exhaustion of petroleum reservoirs has led to a boom in research in the area of alternative fuel. [1] New sources of energy should be clean, efficient, low cost, and easy to transport. Some candidates are wind, solar, geothermal, biomass, nuclear energy, and hydro- gen. [2] Hydrogen is an attractive energy source for mobile applications such as vehicles because it has the highest energy density of any substance and produces only water as a waste product when used in a fuel cell. [3] Unfortunately hydrogen is difficult to store and transport. Therefore, to commercialize hydrogen technology, the bottleneck represented by the storage and delivery steps must be overcome. Significant advances have been made in the search for suitable materials for hydrogen storage. While compressed gas is still being explored as an option, materials that absorb hydrogen as carriers are also being inves- tigated. The two major technologies for chemical storage are metal hydrides, which store hydrogen as a discrete M–H bond, and high surface materials for cryogenic hydro- gen physisorption. [4] However, no material explored to date meets the Department of Energy (DOE) 2015 system target of 9 wt% weight adsorption or 80 kg m 3 . [5] New materials and concepts are currently being explored and new storage strategies are being developed. [6] One of the most over- looked features of a hydrogen storage material is the enthalpy of adsorption. If this value is too high, as in metal hydrides which generally have enthalpies in the 80 kJ mol 1 range, energy is required to drive off hydrogen for use and enormous amounts of heat are released on recharging. If this value is too low, as in the case of physisorption materials, expensive and cumbersome cryogenic cooling is required to keep the hydrogen on the material. In spite of this, physisorption of hydrogen on solid-state materials shows promising properties such as high gravimetric storage, fast adsorption, and desorption kinetics. Numerous high surface area microporous and mesoporous materials have been studied in this application. The adsorption mechanism is dominated by weak van der Walls forces, which have adsorption enthalpy between 4 and 10 kJ mol 1 . Typical materials, such as carbon nanotubes, fullerenes, and metal- organic frameworks (MOFs) possess very high surface areas and high hydrogen physisorption at cryogenic temperature, but their adsorption capacities at room temperature are limited due to the limited adsorption enthalpy. For example, Zuttel et al. [7] investigated the hydrogen storage capacities of 60 carbon samples under ambient conditions resulting in reversible storage capacities ranged in 0.04–0.46 wt%. This is because the 4–10 kJ mol 1 binding enthalpies are too weak to hold the hydrogen to the surface under these conditions. Calculations have demonstrated that the ideal enthalpy for room temperature operation of a hydrogen storage system is 20–30 kJ mol 1 , however some research has indicated that 15–20 kJ mol 1 is a more suitable target. For this reason many researchers are pursuing strategies to strengthen the interaction between the substrate and hydrogen. Many of these new approaches involve the use of s–p H 2 complexes (the Kubas interaction) to bind the PROGRESS REPORT www.advmat.de [*] Prof. D. M. Antonelli, T. K. A. Hoang Department of Chemistry and Biochemistry University of Windsor Windsor, ON N9B 3P4 (Canada) E-mail: danton@uwindsor.ca DOI: 10.1002/adma.200802832 Hydrogen adsorption and storage using solid-state materials is an area of much current research interest, and one of the major stumbling blocks in realizing the hydrogen economy. However, no material yet researched comes close to reaching the DOE 2015 targets of 9 wt% and 80 kg m 3 at this time. To increase the physisorption capacities of these materials, the heats of adsorption must be increased to 20 kJ mol 1 . This can be accomplished by optimizing the material structure, creating more active species on the surface, or improving the interaction of the surface with hydrogen. The main focus of this progress report are recent advances in physisorption materials exhibiting higher heats of adsorption and better hydrogen adsorption at room temperature based on exploiting the Kubas model for hydrogen binding: (h 2 -H 2 )–metal interaction. Both computational approaches and synthetic achievements will be discussed. Materials exploiting the Kubas interaction represent a median on the continuum between metal hydrides and physi- sorption materials, and are becoming increasingly important as researchers learn more about their applications to hydrogen storage problems. Adv. Mater. 2009, 21, 1787–1800 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1787