Short CommunicationOptimal isosteric heat of adsorption for hydrogen storage and delivery using metal–organic frameworks
Introduction
Recently, there has been an intense focus on hydrogen (H2) energy as a replacement for fossil fuels due to the rapid depletion of petroleum deposits and the air pollution caused by burning fossil fuels [1]. However, the absence of safe and economical techniques for H2 storage is a major hurdle for transportation applications. Storage methods such as high-pressure containers and liquid hydrogen have been considered, but these methods are energy-intensive and cost-prohibitive. Many researchers are trying to develop metal hydride storage systems, which promise high gravimetric and volumetric storage but suffer from slow kinetics [2]. In addition, the physisorption of H2 in porous materials is an attractive alternative due to the rapid uptake and release of H2. Numerous materials have been investigated for physisorption, including zeolites, activated carbons, carbon nanotubes, and metal–organic frameworks (MOFs). Among these, MOFs are the most promising due to their extremely large surface areas (up to 5600 m2/g) and low crystal densities [2], [3], [4], [5], [6], [7], [8]. Several MOFs were reported to meet the H2 storage targets proposed by US Department of Energy (DOE) but only at cryogenic temperatures [9], [10]. Currently, no MOFs can attain the DOE target at ambient temperatures due to the weak H2–MOF interactions. From a computational study, Frost and Snurr showed that the DOE targets could be attained at ambient temperatures if the isosteric heat of adsorption (Qst) could be increased for MOFs with large free volumes [11]. Thus, a main issue for H2 storage in MOFs is finding strategies for increasing the Qst without significant losses in free volume [6].
Most studies of H2 storage in MOFs have focused on the storage capacity at high pressures, but for practical applications, the amount adsorbed at the discharge pressure is also an important consideration. The discharge pressure is typically around 1.5 bar. As shown in Fig. 1, the deliverable capacity is the amount of hydrogen adsorbed at high pressure (e.g. 120 bar) minus the amount adsorbed at the discharge pressure (e.g. 1.5 bar). Until now, however, only a few studies used the deliverable capacity for judging the H2 storage capability of MOFs [1], [12], [13], [14], [15]. A pressure of 120 bar is consistent with the DOE targets.
Frost and Snurr showed that a large increase in the Qst leads to a considerable rise in the storage capacity even at high pressures [11]. However, a large Qst may also increase the H2 uptake at low pressures and could reduce the deliverable capacity (refer to Fig. 1). As a result, there must be an optimal Qst value for obtaining the maximum deliverable capacity. Bhatia and Myers used simple thermodynamic arguments to calculate that the optimal Qst for ambient temperature storage and delivery of H2 between 30 and 1.5 bar is 15.1 kJ/mol [15]. Areán and co-workers suggested that a considerably higher value (22–25 kJ/mol) was an optimal Qst for the same delivery conditions based on extrapolation of data for H2 adsorption in zeolites [16].
In this work, we study H2 storage and delivery between 120 and 1.5 bar in MOF materials using grand canonical Monte Carlo (GCMC) simulations. First, we compare the deliverable capacity and storage capacity at two different temperatures (77 and 298 K). Second, we test whether an optimal Qst exists for maximum H2 delivery at 298 K in MOFs. Finally, we explain the differences between the optimal Qst values from our GCMC simulations and the values from other groups.
Section snippets
Simulations
Eight MOFs with a wide range of free volumes and surface areas were selected: UMCM-1 [17], MOF-177 [18], Cu-BTC [19] and five isoreticular MOFs (IRMOFs-1, -9, -10, -15, -16) [20]. H2 adsorption isotherms and Qst in the eight MOFs were predicted up to 120 bar at 77 and 298 K from GCMC simulations. The same model reported in our previous studies was used for these simulations [11], [21], [22], and this model was already shown to reasonably predict the low-pressure H2 isotherms and Qst in IRMOFs-1
Results and discussion
Fig. 2 shows a comparison between the storage capacities (at 120 bar) and the deliverable capacities (from 120 to 1.5 bar) for H2 adsorption in the eight MOFs. At 77 K, the deliverable capacity is lower than the storage capacity due to the considerable adsorption at low pressures. An important observation is that some MOFs cannot attain the DOE targets in view point of the deliverable capacity although their storage capacities exceed the targets (Fig. 2a). At 298 K, however, the deliverable
Conclusions
In this study, the storage capacity and the deliverable capacity were compared from simulated H2 isotherms in MOFs. The results show that the deliverable capacity should be considered if there is a strong H2–MOF interaction and that stronger interactions are needed for reaching the DOE hydrogen storage targets. The relationship between the heat of adsorption Qst and the deliverable capacity between 120 and 1.5 bar at 298 K was investigated for eight MOFs. Interestingly, the results show that an
Acknowledgment
This work was supported by the Department of Energy under Award Number DE-FG36-08GO18137.
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