This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 20937--20942 20937 Cite this: Phys. Chem. Chem. Phys., 2013, 15, 20937 Methane storage capabilities of diamond analogues† Maciej Haranczyk,* a Li-Chiang Lin, b Kyuho Lee, bc Richard L. Martin, a Jeffrey B. Neaton c and Berend Smit abd Methane can be an alternative fuel for vehicular usage provided that new porous materials are developed for its efficient adsorption-based storage. Herein, we search for materials for this application within the family of diamond analogues. We used density functional theory to investigate structures in which tetrahedral C atoms of diamond are separated by –CC– or –BN– groups, as well as ones involving substitution of tetrahedral C atoms with Si and Ge atoms. The adsorptive and diffusive properties of methane are studied using classical molecular simulations. Our results suggest that the all-carbon struc- ture has the highest volumetric methane uptake of 280 V STP /V at p = 35 bar and T = 298 K. However, it suffers from limited methane diffusion. Alternatively, the considered Si and Ge-containing analogies have fast diffusive properties but their adsorption is lower, ca. 172–179 V STP /V, at the same conditions. 1. Introduction Natural gas, composed primarily (70–90%) of methane, is being investigated as a potential alternative fuel for vehicular usage due to its growing supply and lower CO 2 emissions comparing to traditional fuels. A critical step towards its widespread usage as a fuel for motor vehicles is to develop an on-board system that occupies about the same volume as a gasoline tank, and likewise stores enough natural gas to deliver about the same amount of energy. However the volumetric energy density of methane is relatively low (0.038 MJ L À1 methane at standard temperature and pressure, compared with 46.4 MJ L À1 for gasoline). 1 In order to minimize the size of the on-board natural gas storage system, the volumetric energy density of methane must be significantly increased. The US Department of Energy (DOE) set a target for a material’s CH 4 uptake in this application: at external CH 4 pressure of 35 bar and temperature of 298 K, the material should hold at least 180 volumetric units of CH 4 at the standard temperature and pressure per unit volume of the material (V STP /V). There are several approaches to accomplishing this: compression, liquefaction and adsorp- tion on the surface of a porous material. The first two pose significant challenges due to safety and weight requirements on tank systems. The latter option poses challenges due to require- ments on the porous material used for storage. There has been, however, tremendous progress made in this area, with many new classes of porous materials reported in recent years. New classes of adsorbent materials include both crystal- line materials such as metal–organic frameworks (MOFs), 2–4 and noncrystalline materials such as porous polymer networks 5 (PPNs, also porous aromatic frameworks 6 (PAFs)). MOFs are typified by crystalline coordination polymers comprising multi- topic organic ligands bound to metal ions or metal ion clusters; the most notable members of this class of materials exhibit unprecedented levels of porosity as evidenced by relatively high surface areas (as high as 7100 m 2 g À1 ), 7 and record-breaking gas sorption performance. However, MOFs have not yet been commercialized as natural gas sorbents due to their high cost and low chemical stability. PPNs are another class of porous materials that are being examined as potential methane sorbents. These materials are composed of covalently linked monomeric units that form tunable polymers exhibiting porosity. 8 For example, Zhou and co-workers at Texas A&M have developed a series of these porous polymers which exhibit superior chemical stability, and surface areas that approach those of some of the most porous MOFs. 5,8 Determining the optimal structure of methane storage materials poses a significant challenge. It has to compromise two factors: on one hand the material needs to be highly porous (e.g. low density) to provide volume to store methane, on the other hand it needs adsorbent material (e.g. high density) that provides surface for the guest molecule to interact with and adsorb on. So far, identification of the optimal balance between these factors, that is identification of materials with high methane a Lawrence Berkeley National Laboratory, One Cyclotron Road, MS 50F-1650, Berkeley, CA 94720-8139, USA. E-mail: mharanczyk@lbl.gov; Fax: +1 510 486 5812; Tel: +1 510-486-7749 b Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720, USA c Molecular Foundry, Lawrence Berkeley National Laboratory, One Cyclotron Road, MS 50F-1650, Berkeley, CA 94720-8139, USA d Department of Chemistry, University of California, Berkeley, CA 94720, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53814a Received 7th September 2013, Accepted 30th October 2013 DOI: 10.1039/c3cp53814a www.rsc.org/pccp PCCP PAPER Published on 31 October 2013. Downloaded by University of California - Berkeley on 18/02/2014 18:58:36. View Article Online View Journal | View Issue