A team of researchers from Ulsan National Institute of Science and Technology (UNIST) has developed a new type of solid electrolyte materials that can enhance the performance and efficiency of hydrogen fuel cells. The new materials are based on metal-organic frameworks (MOFs), which are porous structures composed of metal clusters and organic ligands. The researchers introduced zwitterionic sulfamic acid as a guest molecule into the pores of MOFs, which increased the conductivity of hydrogen ions within the solid electrolyte. The findings have been published in the journal Angewandte Chemie International Edition and selected for the back cover.
Hydrogen fuel cells: eco-friendly power sources
Hydrogen fuel cells are devices that convert chemical energy from hydrogen and oxygen into electrical energy, without producing any harmful emissions. They are considered as promising eco-friendly power sources for various applications, such as vehicles, portable devices, and stationary power plants. However, hydrogen fuel cells face some challenges, such as high cost, low durability, and limited operating temperature range.
One of the key components of hydrogen fuel cells is the electrolyte, which is a material that allows the transport of hydrogen ions (protons) between the electrodes. Currently, most hydrogen fuel cells use a polymer membrane called Nafion as an electrolyte, which has high proton conductivity and good stability. However, Nafion also has some drawbacks, such as high water uptake, low thermal stability, and high sensitivity to impurities. Moreover, the mechanism of proton transport in Nafion is not fully understood, which limits the optimization of its performance.
Metal-organic frameworks: potential alternatives
To overcome these limitations, the researchers from UNIST explored the use of metal-organic frameworks (MOFs) as alternative electrolyte materials for hydrogen fuel cells. MOFs are materials that consist of metal clusters connected by organic ligands, forming a three-dimensional porous structure. MOFs have several advantages over Nafion, such as high surface area, tunable pore size and shape, adjustable chemical functionality, and thermal stability.
The researchers used two types of MOFs, namely MOF-808 and MIL-101, which have different pore sizes and structures. They introduced zwitterionic sulfamic acid (HSA) as a guest molecule into the pores of MOFs, using a simple impregnation method. HSA is a low-acidity amphoteric ionic substance that has both positive and negative charges on the same molecule. HSA can form strong hydrogen bonds with various species, which makes it an effective medium for transferring protons.
Enhanced proton conductivity and durability
The researchers measured the proton conductivity of the MOF-HSA composites using an electrochemical impedance spectroscopy technique. They found that the proton conductivity of the composites was significantly higher than that of the pure MOFs, reaching values of 10^-1 S cm^-1 or higher at room temperature and under humidified conditions. The proton conductivity also increased with increasing HSA loading in the MOFs, indicating that HSA plays a crucial role in facilitating proton transport.
The researchers also tested the durability of the MOF-HSA composites by exposing them to air for 30 days. They found that the composites maintained their proton conductivity over time, showing no signs of degradation or loss of HSA. This suggests that the composites have excellent stability and robustness under ambient conditions.
The researchers attributed the enhanced proton conductivity and durability of the MOF-HSA composites to the synergistic effects between the MOFs and HSA. They proposed that HSA molecules form a continuous network within the pores of MOFs, which provides a pathway for proton hopping. Moreover, HSA molecules can interact with both the metal clusters and the organic ligands of MOFs, which stabilizes their structure and prevents their decomposition.
Implications and future work
The research team has demonstrated that MOF-HSA composites are promising solid electrolyte materials for hydrogen fuel cells, with high proton conductivity and stability. The team believes that their work can pave the way for developing more efficient and eco-friendly power sources based on MOFs.
“Our study shows that MOFs can be used as versatile platforms for designing novel solid electrolyte materials with high performance and durability,” said Professor Myoung Soo Lah, who led the study. “We hope that our findings can inspire further research on MOF-based materials for various energy applications.”
The team plans to continue their research on optimizing the properties and performance of MOF-HSA composites for hydrogen fuel cells. They also aim to explore other types of guest molecules that can enhance proton transport in MOFs.