Hydrogen, methane, and other energy gases are vital and eco-friendly energy carriers for people today. However, the optimal storage of these gases poses a considerable constraint on their use.

    In the quest for better methods of storing and transporting hydrogen and methane, scientists worldwide have conducted extensive research. Metal-organic frameworks (MOFs) have emerged in the last 20 years as a new type of porous hybrid material that has attracted wide attention from material scientists.

    Recently, a research team led by Northwestern University designed and synthesized a new type of MOF with ultra-high porosity and surface area. Compared to traditional adsorbent materials, it can store more hydrogen and methane at safer pressures and lower costs.

    "We have developed better methods for storing hydrogen and methane gases for the next generation of clean energy vehicles," said Omar K. Farha, the study's leader and a chemistry professor at Northwestern University. "Using chemical principles, we designed a porous material with precise atomic arrangements, achieving ultra-high porosity."

    The study was published in the "Science" journal on April 17. Chen Zhijie, a postdoc from Omar Farha's laboratory, is the first author of the paper. Li Penghao, another postdoc from Northwestern University's chemistry department, Sir Fraser Stoddart, one of the 2016 Nobel Prize winners in Chemistry, and Ryther Anderson from the Colorado School of Mines are co-first authors. Researchers from the National Institute of Standards and Technology and other universities and institutions also participated. DeepTech interviewed Omar Farha and Chen Zhijie.

[Image caption] Screenshot of the research paper on the "Science" magazine official website. (Source: Science)

Why hydrogen and methane?

  If wind and photovoltaic power generation are the driving forces of the new energy industry, hydrogen is the future star of the energy field. Whether or not you are familiar with the periodic table, you should know that hydrogen is listed first, being the smallest known atom to humans. Perhaps more surprisingly, hydrogen is also the most widespread substance in the universe, constituting 75% of the universe's mass and belonging to secondary energy.

    Excluding nuclear fuel, hydrogen's calorific value is 2.5 times that of liquefied petroleum gas and three times that of gasoline, ranking it at the top among all fuels. Crucially, burning hydrogen does not produce carbon dioxide. Instead, it produces water, which can be electrolyzed to produce hydrogen, making it a recyclable clean energy source. As a result, hydrogen has always been considered one of the most ideal future energy carriers.

    Methane, on the other hand, boasts vast reserves on Earth and diverse sources. It has a relatively high energy density and clean combustion products, making it the most advantageous alternative energy for vehicular applications. However, the challenges in storage and transportation have hindered their widespread application.

    For motor vehicles, for example, hydrogen and methane-powered cars currently require high-pressure compression to operate. The pressure needed for hydrogen gas tanks is 700 bar or 10,000 Psi, equivalent to 300 times the pressure of a car tire. Furthermore, due to the low density of hydrogen, the cost of achieving this pressure is high. Given hydrogen's highly flammable nature, there are also safety concerns.

    Therefore, developing new adsorbent materials that can store hydrogen and methane gases in cars at lower pressures would help scientists and engineers achieve the goal of "developing the next generation of clean energy vehicles". To achieve this goal, the size and weight of vehicular fuel tanks also need to be optimized. In this study, the porous material balanced the volume and weight of hydrogen and methane, bringing researchers one step closer to their goal.

    The MOFs developed by Omar's laboratory can store a large amount of hydrogen and methane in their pores and deliver them to the car's engine at pressures lower than what current fuel cell vehicles require.

    "Adsorbent materials are porous solids that adsorb liquid or gas molecules on their surface," explained Omar Farha. "The MOF structure we designed, due to its special nanopores, means that a one-gram sample (roughly the size of six jelly beans) can cover a surface area equivalent to 1.3 football fields."

(Source: DeepTech)

 A Research That Could Revolutionize the Gas Storage Industry
    MOFs (Metal-Organic Frameworks) materials are composed of organic molecules and metal ions or clusters. The organic molecules usually contain elements such as carbon, oxygen, nitrogen, and phosphorus, most of which are aromatic carboxylic acids. They form multi-dimensional, highly crystalline, porous frameworks through self-assembly. The earliest prototypes of MOFs can be traced back to materials like Prussian Blue.
    "To visualize what a MOF is, you can imagine it as a set of Tinker Toys (circuit blocks similar to LEGO)," Omar explained to DeepTech. "Here, metal clusters or ions can be considered as the circular or square nodes of the blocks, while organic molecules are the connecting rods."
Omar’s research group has been studying how "atomically precise location" functional materials can solve exciting problems in chemistry and materials science, with application areas covering energy and the environment. They are committed to fundamentally understanding the role of three-dimensional structures in altering material functions, applying them to gas storage and separation, catalysis, and water pollution treatment.
    "In this latest research, we synthesized a MOF called NU-1500, based on past works. We used six organic linkers, combined with metal trimers of iron, aluminum, chromium, or thulium to construct NU-1500," Omar explained. These initial materials all showed good gas adsorption properties. However, their pore sizes and volumes are relatively small, limiting their weight performance.
    "The relationship between volumetric and gravimetric gas adsorption of these previously studied materials inspired our contact with collaborators. They can calculate the relationship between the structures and properties of MOF-like materials," Omar said.
Ryther Anderson from the Colorado School of Mines simulated the gas adsorption behavior of many MOFs with similar topological structures, pore sizes, and organic connectors. From their work, Omar's team discovered a new type of MOF that had never been synthesized before, anticipated to have an ideal balance of weight and volume for methane and hydrogen gas storage performance. They then synthesized this MOF in the lab and measured its storage capacity for methane and hydrogen under various conditions.

Dr. Penghao Li, a postdoc under Sir Fraser Stoddart, was responsible for the synthesis of the organic ligands. It's worth mentioning that Sir Fraser Stoddart, along with two other scientists, developed molecular machines, opening a new world for the development of chemistry by winning the Nobel Prize in Chemistry. These molecular machines have now been used in the research and development of new materials, novel sensors, and energy storage systems.

 Image | Organic-Inorganic Hybrid Porous Material with Extremely High Porosity and Surface Area (Source: Timur Islamoglu & Chen Zhijie)

    Regarding the synthesis method, Chen Zhijie stated that they used a solvent-thermal synthesis method to prepare this MOF—putting metal salts and organic linkers into a sealed container and baking it in a preheated furnace. "The material is activated by supercritical carbon dioxide, a common method we developed for activating MOFs with extremely high porosity and surface areas," he said.
    So, is it possible for this seemingly magical material to be mass-produced in the short term? Omar said that the current laboratory research focuses more on academically enhancing the basic understanding of such materials, and developing a knowledge base of their unique structural and property relationships. "However, from a commercial perspective, we believe that the most significant material cost for scaling up this special MOF comes from the synthesis of the organic ligands," he said. "This is because the metal trimers are based on cheap, and abundantly available metals like aluminum and iron. Of course, introducing a solvent recovery system could significantly reduce the cost of organic solvents used in the synthesis process."  Omar believes that these materials are very likely to be synthesized on a large scale commercially in the future. "I hope this research can be applied in the next few years," he said. "I believe that the first field to be entered will be the natural gas storage industry. In fact, there are already commercial applications using some MOFs with ION-X cylinders for storing toxic gases at relatively low compression pressures."
    In his view, the safety optimizations brought by MOF materials will change the way gases are stored, transported, and conveyed. "This is a study that might change the entire gas storage industry," Omar stated.

Main researchers and lead authors introduction

Image description | On the left is Dr. Chen Zhijie; on the right is Dr. Li Penghao (Source: Personal)

Chen Zhijie is currently conducting postdoctoral research in the chemistry department at Northwestern University in Omar Farha's laboratory. He began his studies at Shanghai Jiao Tong University's School of Chemistry and Chemical Engineering in 2008. After obtaining his bachelor's degree in 2012, he went to King Abdullah University of Science and Technology (KAUST) in Saudi Arabia for his doctoral studies. Since obtaining his Ph.D. in 2018, he has been conducting postdoctoral research at Northwestern University.

    His research mainly focuses on the controllable preparation of framework materials and the performance research of functionalized fabric-loaded composite materials. He has achieved significant academic results in the fields of the synthesis of ultra-high specific surface area porous materials, applications in hydrogen and methane storage, moisture absorption from the air, fabric-loaded MOFs/polymer composites for the degradation of nerve toxins, and the defects and evolution of MOFs.

    Li Penghao, another lead author, is also a postdoctoral researcher at Northwestern University and has been studying under the guidance of Nobel Prize in Chemistry laureate Sir J. Fraser Stoddart. He pursued his undergraduate studies in the Chemistry Department of Nankai University from 2007 to 2011 and then earned his master's and doctoral degrees from Boston University and the University of Oregon, respectively.

Since August 2017, he has been conducting postdoctoral research at Northwestern University. His main research topics include the synthesis of organic porous materials as well as their topological research and characterization.

-End-

References:

https://science.sciencemag.org/content/368/6488/297

https://sites.northwestern.edu/omarkfarha/

https://sites.northwestern.edu/omarkfarha/group-members-2/

https://www.in-en.com/article/html/energy-2283805.shtml

https://www.nobelprize.org/prizes/chemistry/2016/stoddart/facts/

The article is sourced from the WeChat Official Account 'DeepTech深科技'.