Molecular architecture: New class of materials for tomorrow's energy storage

Researchers at the Technische Universität Berlin (TUB) have created a new family of semiconductors, the properties of which were investigated by the Helmholtz-Zentrum Berlin (HZB). The researchers christened the first member “TUB75”. The material belongs to the class called metal-organic frameworks, or MOFs for short, and could open up new opportunities for energy storage. The work was published in Advanced Materials.

“TUB75 is the first semiconducting phosphonate MOF in the literature. It paves the way for a new family of semiconductors through its extremely rich structural chemistry”, says Gündoğ Yücesan. The chemist and his team from the Technische Universität Berlin designed and synthesised the new material. The research group headed by Konrad Siemensmeyer at the HZB Quantum Material CoreLab studied the magnetic properties – with surprising results.

“Materials scientists all over the world have been searching a very long time for organic magnetic materials”, says Siemensmeyer. “They are extremely rare and often only exhibit magnetic properties at very low temperatures.” TUB75 is different.

“We were able to detect one-dimensional spin chains. The spins, being the respective intrinsic angular momentum of each atom and thus the magnetic moment, are arranged in a row in this case.” This is very interesting for nanoelectronics, for example. No longer will electric charges be used for information processing, but instead magnetic spins. “The interactions are theoretically well delineated”, adds Siemensmeyer. A group of Canadian scientists has developed models for the newly discovered class of materials that explain the distribution of moments and interactions.

Even if the magnetic properties are surprising for a complex substance of this type, they are only one small aspect of the multi-faceted new class of materials. “Due to their rich structural chemistry and their exceptionally high thermal and chemical stability, the new family of materials could become the next-generation of electrode materials”, explains Yücesan. This is because the surface area can be designed to be larger compared to activated carbon-electrodes and adapted to the respective applications.

“We see great potential for supercapacitors, for example.” These are electrochemical energy storage devices with very high energy density per unit volume. They can be charged many times faster than conventional batteries. Since they also can withstand many more charging cycles, they are now widely used in power electronics, such as for energy recovery in buses and trains. However, they store far less energy per unit mass than batteries. New electrode materials – such as TUB75 – are intended to reduce this gap.

The secret lies in the pores

Metal-organic solids as such have been known for a long while. They consist of metal atoms that are directly bound to organic molecules. “In the past, these crystal structures were valued for their aesthetic beauty. Some of them actually resemble Moroccan tiles”, Yücesan says. “Until recently, however, metal-organic solids had no practical use.” This changed at the beginning of the 21st century when their microporosity was discovered. This means that they possess an extremely large surface area due to an extremely large number of tiny pores.

“The well-defined microporous architectures that MOFs can form are their most important feature”, says Yücesan. “This is because the pores can serve as storage compartments for small molecules.” This turns hydrogen into a storable and transportable fuel, for example. It can capture and bind carbon dioxide, thereby extracting this greenhouse gas from industrial processes and hence from the carbon cycle for a very long time. It can bind toxins and thus render them harmless. Or they can be utilised as active pharmaceutical agents that, when bound to a carrier, can enter the body and be released there in a specific manner.

The exciting things about this class are that the materials are made up of very basic chemicals in a modular fashion, and their pore size can be adjusted for the desired application. Materials researchers refer to this as a metal-organic framework (MOF), which they assemble from inorganic building units – the IBUs – and structure-guiding organic connecting molecules – the linkers. The organic struts are connected to each other via the inorganic building units. This facilitates formation of two- and three-dimensional nano-sized structures. “I like to call it molecular architecture – for creating functional microporous structures”, says Yücesan. The first materials based on this principle were synthesised in the 1990s.

Phosphonic-acid MOFs: semiconducting, magnetic and exceptionally stable

“However, due to their large surfaces, MOFs can absorb not only small molecules but also electric charges”, says the chemist. Of course, common materials such as activated carbon or graphite also do this. But compared to these, the frameworks have an advantage: “While the structure of conventional electrode materials can only be further developed to a very limited extent, MOFs offer us a platform with surfaces that can be optimised.”

There is a catch to the whole thing, though. The MOFs created thusfar have generally been insulators. “We wanted to change that and have concentrated on the phosphonic acid group to achieve this”, says Yücesan.

Phosphonic acid is a compound of phosphorus, hydrogen, and oxygen that forms white crystals under normal conditions. “On the one hand, it displays the most different metal binding modes, which increases the possible combinations for new materials”, the chemist explains as the reason for his choice. “And on the other hand, MOFs produced using phosphonic acids are little known and little studied so far.”

They therefore offer the chemist and his team a structurally versatile platform with plenty of scope for development. “At present, only a very limited number of phosphonate-based MOFs exist in the literature. Many properties are still unknown. But the few known materials already indicate an extraordinary thermal and chemical stability compared to conventional MOFs.”

The researchers have named the first member of the new family of materials after the Technische Universität Berlin (TUB). “It was relatively easy to crystallise TUB75 because we used a small, structurally rigid organic linker”, says Yücesan. “With more complex molecules, however, it can take years to synthesise the desired MOF and optimise its crystallisation conditions.”

In the meantime, he and his team have not only developed the know-how for the synthesis of new phosphonic-acid linkers, but have also optimised their crystallisation processes. “Nevertheless, we are still at the very beginning. There is still a lot of research to be done. But I believe that phosphonate-MOF research will become one of the research areas with the greatest growth in the near future.”