The transportation sector represents one of the largest greenhouse gas emissions sources worldwide. Owing to this situation, alternative fuels like hydrogen and methane are essential in searching for cleaner energy sources for this sector. The implementation of gaseous fuels faces a significant challenge: storage! The current gas storage technologies employ high pressure, representing explosion risks in vehicles in case of an accident.

Besides the safety concerns, there is a need to increase the driving range (due to the lower volumetric energy density of gaseous fuels compared to liquid fuels), meaning that more gas must be stored in as low volume as possible. To this end, porous materials are implemented as an adsorption medium to store gases. In this blog, we will talk about the adsorbed gas technology (AGT) and the importance of materials to meet its implementation requirements in the transportation sector.

What's needed for the transportation sector?

According to the U.S. Department of Energy, there is a minimum capacity storage requirement to be economically and practically viable. For methane, the targets are a gravimetric storage capacity of 0.5 g/g and a volumetric storage capacity of 263 cm3 (standard temperature and pressure, STP) cm−3. For hydrogen, the targets are a gravimetric storage capacity of 4.5 weight (wt)% and a volumetric storage capacity of 30 g/L.

Highly porous materials are thought to meet gas storage requirements for cleaner fuel sources' future implementation. For this matter, adsorbent materials such as porous carbons, covalent organic frameworks, porous organic polymers, and metal-organic frameworks (MOFs) are commonly used. MOFs outstand among these materials due to their highly ordered nature with the highest surface area. Besides, they offer several advantages like:

• Tailorable chemistry
• Pore geometry manipulation
• Rational design for specific applications

The implementation of adsorbed gas technology allows the storage at moderate pressures (5-100 bar), while compressed gas technology uses high pressure (250 bar). In AGT, the pressure starts at 100 bar when the tanks are at their maximum capacity; after fuel consumption, the pressure drops to 5 bar. A fraction of the gas remains adsorbed at the lower pressure and cannot be released. The amount of gas that can be released is known as the deliverable capacity and represents a critical design parameter.

Meeting the storage requirements

Several MOFs have been studied for their implementation in gas storage systems. MOFs like the HKUST-1 exhibit a high volumetric storage capacity and a moderate gravimetric methane uptake of 281 cm3 (STP)/cm3 and 0.23 g/g−1 at 100 bar/ 298 K; this MOF is considered as a benchmark material for methane uptake. Other examples are ultra-porous materials are MOF-210, NU-110, and DUT-60. After several studies, a compromise has been found between the volumetric and gravimetric storage capacities; materials with high volumetric capacity storage often show low to moderate gravimetric capacity and vice-versa.

However, as we said earlier, MOFs offer the advantage of making a rational design. An example of this approach was performed in recent work at the National Institute for Standards and Technology (NIST). They synthesized a new aluminum-based MOF material (Al-MOF) with deliverable capacities of ~0.29 g/g (202 cm3 (STP)/cm3) and~0.32g/g (224 cm3 (STP)/cm3) between 5 and 100 bar for methane. The deliverable capacity of the Al-MOF is comparable with those of benchmark MOFs like MOF-905 (203 cm3 (STP)/cm3; 5 to 80 bar at 298K), and HKUST-1 (207 cm3 (STP)/cm3; 5 to 100 bar at 298 K).

Hydrogen adsorption studies showed a deliverable capacity of 8.2 wt % (44.6 g/L) under combined temperature and pressure swing conditions. With these results for hydrogen storage capacity, the Al-MOF exceeds the U.S. Department of Energy's design conditions.

Increasing the storage capacity

After a redesign process, a new MOF was created, again based on Aluminum. The new Al-MOF increased its gravimetric methane uptake to 0.60 g/g at 270 K and 80 bar compared to the previous Al-MOF. Deliverable capacities between 5 and 100 bar are 0.50 g/g (198 cm3 (STP)/cm3 at 296 K) and ~0.60 g/g (238 cm3 (STP)/cm at 270 K). Therefore, the new Al-MOF is among the best porous crystalline materials for methane storage, surpassing the gravimetric capacity storage target of the U.S. Department of Energy.

The new MOF also showed an increased capacity for hydrogen adsorption studies similar to methane storage. Both the absolute uptake at 77 K/100 bar and the deliverable capacities for H2 are higher than those of the previous version, along with similar volumetric uptake and capacities (14.0 vs. 8.2 wt % and 46.2 g/L vs. 44.6 g/L).

We will publish a case study on methane storage applications in our success stories shortly. If you are looking to explore MOFs' benefits, please contact us!