fi Progress on rst-principles-based materials design for INAUGURAL ARTICLE hydrogen storage Noejung Parka, Keunsu Choib, Jeongwoon Hwangb, Dong Wook Kimc, Dong Ok Kimc, and Jisoon Ihmb,1 aInterdisciplinary School of Green Energy, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea; bDepartment of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea; and cHanwha Chemical R&D Center, Daejeon 305-804, Korea This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2011. Contributed by Jisoon Ihm, October 9, 2012 (sent for review July 30, 2012) This article briefly summarizes the research activities in the field of vehicles and limited forms of zero-emission cars, larger-scale hydrogen storage in sorbent materials and reports our recent works energy operations such as stationary back-up power systems and and future directions for the design of such materials. Distinct full commercial scale automotive applications are very difficult to features of sorption-based hydrogen storage methods are described realize through the pure electric power storage devices. Hydro- compared with metal hydrides and complex chemical hydrides. We gen storage systems have obvious advantages in certain important classify the studies of hydrogen sorbent materials in terms of two aspects (e.g., energy density and durability), and the search for key technical issues: (i) constructing stable framework structures safe and efficient hydrogen storage materials continues for such with high porosity, and (ii) increasing the binding affinity of hydro- wide application purposes. gen molecules to surfaces beyond the usual van der Waals interac- tion. The recent development of reticular chemistry is summarized as Hydrogen Storage Methods and Sorption-Based Storage a means for addressing the first issue. Theoretical studies focus Systems mainly on the second issue and can be grouped into three classes Methods for hydrogen storage may be divided into two categories: according to the underlying interaction mechanism: electrostatic physical confinement inside vessels and binding onto host mate- interactions based on alkaline cations, Kubas interactions with open rials. The physical confinement of hydrogen gas involves either SCIENCES transition metals, and orbital interactions involving Ca and other high-pressure compression or cryogenic liquefaction and has been nontransitional metals. Hierarchical computational methods to en- used for special purposes or on the laboratory scale. The safety (for APPLIED PHYSICAL able the theoretical predictions are explained, from ab initio studies high-pressure) and cost (for liquefaction) concerns associated with to molecular dynamics simulations using force field parameters. We such methods suggest that large-scale commercial applications also discuss the actual delivery amount of stored hydrogen, which of stored hydrogen might be unlikely via such physical storage depends on the charging and discharging conditions. The usefulness methods. Regarding the method to bind hydrogen onto an absor- and practical significance of the hydrogen spillover mechanism in bent medium, a variety of host materials have been investigated (3, increasing the storage capacity are presented as well. 5). These materials may be classified into three distinct groups depending on the microscopic mechanism related to the charging Renewable Energy and the Significance of Hydrogen and discharging properties: metal hydrides that contain atomic Storage hydrogen as a constituent of the bulk material, complex chemical he prosperity of modern civilization is inextricably based on en- hydrides that react chemically upon charging and discharging, and Tergy supply networks (on-grid or off-grid) that deliver petroleum, the adsorption of molecular hydrogen onto the surfaces of highly natural gas, and electricity to residential, public, commercial, and porous materials. Each method has its own advantages and dis- industrial facilities, as well as transport systems. Concerns are grow- advantages. For example, metal hydride systems (in particular the ing over the limited availability of natural resources and the envi- AB5-type heavy metal alloys) can provide greatly satisfactory vol- ronmentally hazardous byproducts of the energy conversion process, umetric storage density, easy kinetics, and good reversibility, but such as greenhouse gases. From this perspective, renewable energy the gravimetric density is fundamentally limited (6). The re- harvesting technologies based on natural energy flows, including versibility and reaction kinetics present the largest hurdle to solar power, wind power, geothermal energy, and tidal energy, are complex chemical hydride storage systems. In a sense, the com- increasingly gaining momentum. The storage of the harvested energy plex chemical hydride can perhaps be used with a compartment system that is subject to off-board regeneration processes (4, 7). remains a fundamentally important factor for the utilization of the A marked drawback of sorption-based storage in porous mate- renewable energy because the sources of the renewable energy usu- rials is the weak strength with which hydrogen is physisorbed ally undergo significant spatial and temporal variations (1). The issue onto surfaces. Without a particular enhancement of the ad- of hydrogen storage can be understood in this context. Among sorption strength, sorption-based storage systems are practically chemical fuels, pure hydrogen (i.e., in the gas form) has the highest operative only around the liquid nitrogen temperature (77 K). mass density but the poorest volumetric density (2, 3). As a result, Nevertheless, sorbent materials and sorption-based storage sys- hydrogen storage research traces a long history of studies toward the tems deserve a more intensive investigation because they present reversible condensation of hydrogen into a limited volume using a variety of inherent advantages over other storage systems. Mi- lightweight devices. In recent decades a globally recognized objective croscopically, adsorption does not require the dissociation of is the development of a stored hydrogen carrier that can power hydrogen molecules and thus does not involve energy barriers. vehicles through fuel cells (or, perhaps, combustion engines). For Macroscopically, this approach permits the rapid charging example, the guideline published by the US Department of Energy and discharging of hydrogen, and the corresponding thermal states that, for a commercially competitive vehicle, hydrogen storage systems need to achieve an overall capacity of 5.5 wt% hydrogen with a volumetric ratio of 40 g/L within a few years (4). Author contributions: D.W.K., D.O.K., and J.I. designed research; N.P., K.C., J.H., D.W.K., However, the objectives of hydrogen storage need to be defined D.O.K., and J.I. performed research; N.P., D.W.K., and D.O.K. contributed new reagents/ from a wider perspective of the energy storage and energy back-up analytic tools; N.P., K.C., and J.H. analyzed data; and N.P., K.C., and J.I. wrote the paper. system. Although supercapacitors and rechargeable batteries The authors declare no conflict of interest. have shown an optimistic outlook in the field of hybrid electric 1To whom correspondence should be addressed. E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1217137109 PNAS | December 4, 2012 | vol. 109 | no. 49 | 19893–19899 Downloaded by guest on September 25, 2021 management is expected to be easier than chemical or metal various MOFs and ZIFs. An extensive review of them is provided hydrides (8). A fundamental requirement for sorbent materials is in refs. 11 and 23. Furukawa et al. (10) described the inclusion of the presence of a stable framework structure with sufficient po- a long and slim linker as conveying ultrahigh porosity. They recently rosity and surface area. In this respect, recent experimental ad- reported the preparation of an extremely porous structure, named vancement in the field of reticular chemistry, that is, metal– MOF-210, which shows an extremely high specific surface area. In organic frameworks (MOFs) and covalent organic frameworks terms of the Brunauer–Emmett–Teller area, MOF-210 reaches the (COFs), is particularly noteworthy (9–11). On the other hand, highest value so far (6,240 m2/g), which exceeds that of other many theory-based studies have addressed the issues of increasing members of the family of MOFs [e.g., MOF-5 (3,800 m2/g) and binding affinity of hydrogen molecules onto surfaces by in- MOF-177 (5,640 m2/g)]. Its excess and total hydrogen uptakes (86 troducing various strategies, as will be described in more detail in mg/g and 176 mg/g at 77 K and 80 bar, respectively) are higher than later sections. those of other porous crystals (10). These results showed that an increase in the specific surface area of a porous material directly Computational Methods Applied to Hydrogen Storage benefits the uptake of hydrogen storage at low temperatures, and Problems also the storage of heavier gases like CO2 or methane. The gov- Several hierarchical computational methods have been used in this erning thermodynamics associated with the gas uptake by MOFs is field. An accurate evaluation of the binding strength between hy- derived from the physisorption of gas molecules onto a chemically drogen molecules and the host material surface is central to this inert MOF surfaces. The polarity of a building unit and the ap- study.