Getting a charge out of MOFs

August 24, 2018

3D structure of the iron–pyrazolate chain.

image: 3D structure of the iron–pyrazolate chain. The iron chain acts like a wire capable
of conducting
electricity in an iron benzenedipyrazolate MOF crystal. (Credit: Berkeley Lab)

UC Berkeley researchers have developed a modular metal-organic framework with the highest electron charge mobilities ever observed.

Metal-organic frameworks (MOFs) are highly customizable porous network solids featuring cages that can come in many sizes and can attract and hold a variety of chemical components, such as carbon dioxide, methane, and hydrogen gases. And it’s this versatile specificity that gives MOFs so much potential for applications in next-generation batteries and in carbon capture, among a growing list. Despite their many positive traits, their open, porous structure – which holds on to electrons – isn’t ideal for applications that require electrons to freely flow with ions (charged particles) through a device to create an electric current.

Now, a research team led by Jeffrey Long, UC Berkeley professor of chemistry and chemical and biomolecular engineering and a senior faculty scientist at the Lawrence Berkeley National Laboratory has developed a technique for making an electrically conductive MOF that could also be used to improve the conductivity of other MOFs. The work was reported in Nature Materials.

To work around MOFs’ inherently low electrical conductivity, the researchers added a potassium chemical mix to an iron benzenedipyrazolate (BDP) MOF. The extra electrons produced during this reaction can then enter the MOF’s iron center and conduct electricity by hopping along the length of one crystal axis of the rod-shaped crystals. The iron center acts like a wire capable of conducting electricity.

Most MOFs degrade when exposed to potassium, but the iron BDP MOF features robust triangular channels that held up during a series of tests where each reaction boosted the material’s electron count until maximum conductivity for that material was reached, resulting in a MOF that conducts electricity up to 10,000 times better than before it underwent the potassium reactions. “It’s amazing that this architecture, once incorporated into a micron-size transistor-like device, allowed us to measure the electron count as it increased with each successive potassium reaction,” said Jeffrey Long, senior faculty scientist in the Materials Sciences Division at Berkeley Lab and professor of chemistry and chemical and biomolecular engineering at UC Berkeley who served as the study’s lead author.

Another challenge in this study was growing the MOFs so that their atoms are first perfectly aligned ­– electrons need to travel in a straight path to generate electricity – and then wiring up these micron-size devices to measure their conductivity. “This was incredibly difficult to do,” said Long. “We weren’t able to grow very large crystals of this MOF, and the size and shape in which the crystals grow made it difficult to wire them up into a device. But we found a way around that.”

Working with the lab of Peidong Yang, a senior faculty scientist in the Materials Sciences Division at Berkeley Lab and professor of chemistry and materials science and engineering at UC Berkeley, the researchers placed platinum contacts on each side of the MOF crystal, which are just 10 microns long – the length of two red blood cells lined up side by side. The newly created MOF is a continuation of work first reported by Long’s lab in 2009.

Read the Berkeley Lab announcement here>

More information:

Nature Materials