Engineering news
Researchers have made an important development in proton exchange membrane technology that could lead to the design of better fuel cells.
The research, conducted at at the University of Liverpool and published this month in the journal Nature Communications, should help engineers understand better the processes happening in proton exchange membrane fuel cells (PEMFCs), also known as polymer electrolyte membrane fuel cells and could have applications in lithium battery technology and sensors for reactive chemicals.
PEMFCs are considered a promising technology for clean and efficient power generation. They contain a proton exchange membrane (PEM), which carries positively-charged protons from the positive electrode of the cell to the negative one. Most PEMs are hydrated and the charge is 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 lets protons move through it. However, most PEMs are made of amorphous polymers, so it is difficult to study how protons are conducted because the precise structure of amorphous polymers is not known.
For the research, scientists synthesised nanometre-sized “cage” molecules to transport charge in the proton exchange membrane (PEM).
These porous organic cages arrange to form channels between the electrodes, in which small ‘guest’ molecules, such as water or carbon dioxide, can travel from one cage to another. Crystallography was then used to precisely track atomic positions within the material.
The synthesised molecules are also soluble in common solvents, which means they could be combined with other materials and fabricated into membranes.
Dr Ming Liu who led the experimental work, said that the study could facilitate the development of high temperature PEMFCs, as water loss would no longer be a consideration.
The research identified some design principles for future PEM materials. The channels found extend in three dimensions, meaning the proton’s movement is not limited to a particular direction.
The cages directed 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 reorganise, which is important when protons are transported from one water molecule to the next over longer distances.
University of Liverpool chemist, Dr Sam Chong, told PE that it will take a few years to optimise the properties of the PEM materials before the technology can be commercialised.
PEMFCs were first used by General Electric in the early 1960s, but were restricted to expensive, niche applications such as space missions due to the requirement of high operating temperatures and very pure hydrogen and oxygen as fuel, which rendered them too expensive for commercial use. Nevertheless, because PEMFCs are lightweight they could be applied to transportation and could replace batteries in portable electronic devices.
The researchers worked with the University of Edinburgh, the Centre for Neutron Research at National Institute of Standards and Technology (NIST), and Defence Science and Technology Laboratory (DSTL), they used a combination of experimental measurements and computer simulations to build a picture of how protons are conducted by the cage molecules.
The research paper, titled Three-Dimensional Protonic Conductivity in Porous Organic Cage Solids can be found here.