This image shows the structure of one of the cage crystals loaded with water. Image: University of Liverpool.Scientists at the University of Liverpool in the UK have made an important breakthrough that could lead to the design of better fuel cell materials. In a paper published in Nature Communications, they describe their synthesis of nanometer-sized cage molecules that can be used to transport charge in proton exchange membranes.
Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technology for clean and efficient power generation in the 21st century. PEMFCs contain a component called a proton exchange membrane (PEM), which carries positively-charged protons from the positive electrode of the cell to the negative one, while electrons travel round an external circuit to generate a current. Most PEMs are hydrated and the protons are transferred through networks of water inside the membrane.
To design better PEM materials, more needs to be known about how the structure of the membrane allows protons to move easily through it. However, many PEMs consist of amorphous polymers that don’t have a regular structure, making it difficult to study how protons are conducted through them.
As an alternative approach, scientists from the University of Liverpool’s Department of Chemistry synthesized molecules that enclose an internal cavity, forming a porous organic cage into which other smaller molecules can be loaded, such as water or carbon dioxide. When these cages come together, they form channels in which the small ‘guest’ molecules can travel from one cage to another.
The end result is a crystalline material in which the arrangement of the cages is very regular. This allowed the researchers to build an unambiguous description of the structure using crystallography, a technique that allows the positions of atoms to be located. The molecules are also soluble in common solvents, which means they could be combined with other materials and fabricated into membranes.
The scientists measured the protonic conductivity of these porous organic cages after loading the channels with water, to assess their viability as PEM materials. The cages exhibited proton conductivities of up to 10-3S/cm, comparable to some of the best porous framework materials in the literature.
In collaboration with researchers from the University of Edinburgh and the Defence Science and Technology Laboratory (DSTL) in the UK and the US National Institute of Standards and Technology (NIST), they used a combination of experimental measurements and computer simulations to build a rich picture of how protons are conducted by the cage molecules.
Two distinctive features of proton conduction in these organic cage crystals were highlighted as design principles for future PEM materials. First, the cages are arranged so that the channels extend in three dimensions. This means that the movement of the protons is not limited to a particular direction, as is the case with many porous materials tested so far.
Second, the cages direct the movement of the water molecules, which means that protons can be passed between them quickly. Also, the cages are flexible enough to allow the water to reorganize, which is important when protons are transported from one water molecule to the next over longer distances.
“In addition to introducing a new class of proton conductors, this study highlights design principles that might be extended to future materials,” said Ming Liu from the University of Liverpool, who led the experimental work. “For example, the ‘soft confinement’ that we observe in these hydrated solids suggests new anhydrous proton conductors where a porous cage host positions and modulates the protonic conductivity of guest molecules other than water. This would facilitate the development of high temperature PEMFCs, as water loss would no longer be a consideration.”
“The work also gives fundamental insight into proton diffusion, which is widely important in biology,” added Sam Chong, also from the University of Liverpool. Chong has recently been appointed as a lecturer in the university’s Materials Innovation Factory (MIF). Due to open in 2017, the £68M facility will revolutionize materials chemistry research and development through facilitating the discovery of new materials that have the potential to save energy and natural resources, improve health or transform a variety of manufacturing processes.
This story is adapted from material from the University of Liverpool, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.