Hydrogen storage using carbon adsorbents: past, present and future

Interest in hydrogen as a fuel has grown dramatically since 1990, and many advances in hydrogen production and utilization technologies have been made. However, hydrogen storage technologies must be significantly advanced if a hydrogen based energy system, particularly in the transportation sector, is to be established. Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H2, in metal hydrides, via chemical storage or by gas-on-solid adsorption. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that could be extremely energy efficient. Consequently, the hydrogen storage properties of high surface area “activated” carbons have been extensively studied. However, activated carbons are ineffective in storing hydrogen because only a small fraction of the pores in the typically wide poresize distribution are small enough to interact strongly with hydrogen molecules at room temperatures and moderate pressures. Recently, many new carbon nanostructured absorbents have been produced including graphite nanofibers and carbon multi-wall and single-wall nanotubes. The following review provides a brief history of the hydrogen adsorption studies on activated carbons and comments on the recent experimental and theoretical investigations of the hydrogen adsorption properties of the new nanostructured carbon materials. PACS: 81.07.De; 81.05.Uw; 68.43.h The decreasing fossil fuel supply and the growing number of densely populated metropolitan cities with poor local air quality have spurred an initiative to develop an alternative fuel. Hydrogen, which may be produced from renewable sources while burning pollution-free, has emerged as one of the most promising candidates for the replacement of the current carbon-based energy services. Although hydrogen could easily supply all of the world’s vehicular energy demands [1], ∗Corresponding author. a major impediment to the development of this new technology is the lack of a convenient, cost-effective on-board storage system. Possible current approaches to vehicular hydrogen storage include (i) physical storage via compression or liquefaction, (ii) chemical storage in irreversible hydrogen carriers (e.g. methanol, ammonia), (iii) reversible metal and chemical hydrides and (iv) gas-on-solid adsorption. Although each storage method possesses desirable attributes, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for personal transportation vehicles. The United States Department of Energy (DOE) has set target system energy densities at values of 6.5 wt % and 62-kg H2/m. Presently, a compact, lightweight hydrogen-storage system for transportation is not available. Hydrogen storage is therefore the key enabling technology that must be significantly advanced in terms of performance and cost effectiveness if hydrogen is to become an important part of the world’s energy economy. Recently, lightweight carbon adsorbent materials have become interesting for possible use in a hydrogen-storage system. Early work in this area in the 1960s, 1970s and 1980s focused on the H2-adsorption properties of various ‘activated’ carbon materials which were prepared from mineralogical or organic precursors. These materials were typically obtained by thermochemical processing and contained many different types of carbon structures that provided a variety of environments for binding hydrogen. Unfortunately, the vast majority of the sites for adsorption could not stabilize hydrogen above cryogenic temperatures. Recent advances in the science of carbon nanostructures have allowed new types of adsorbents to be ‘engineered’. Numerous studies on molecular hydrogen adsorption on graphite nanofibers (GNFs) and carbon multi-wall and single-wall nanotubes (MWNTs and SWNTs) have been reported. This review provides a brief history of the hydrogen-adsorption studies on activated carbons and also outlines the recent experimental and theoretical investigations on engineered nanostructured materials. Finally, we highlight some specific research areas that require further investigation, with particular attention to the case of carbon single-wall nanotubes.