Fuel cells that use hydrogen to generate power offer significant promise for a more environmentally friendly, efficient source of energy, but significant challenges to their widespread use remain.
Taking inspiration from nature, a Nebraska research team led by Shudipto Dishari is designing polymers to find a solution for one of those challenges by making charge-bearing ions run faster across the cells.
Hydrogen-based fuel cells generate electricity through a chemical reaction between hydrogen and oxygen that produces water as the only emission. During this process, the splitting of hydrogen not only produces electrons that give power, but also makes ions. The cell relies on electrolytes to carry these ions through the system to catalysts sitting on the electrode located at the opposite end of the cell.
In hydrogen fuel cells, the electrolyte is a permeable polymer, known as an ionomer because it conducts ions. But a key challenge is what’s known as ion transport limitations. A nano-thin ionomer layer coating the catalysts conducts the charged particles poorly. The slow ion transport within this thin catalyst-binding ionomer layer impairs the fuel cell’s performance.
“The current state-of-the-art ionomers fail to conduct protons efficiently in such thin layers over electrodes. So, we thought that maybe we have to find additional mechanisms for ion transport that works even in thin layers. Then we started to think, ‘Why don’t we dig holes into the films? A hole will probably allow the water to go through. And then, when the water is going through, maybe it will take ions with it,’” Dishari said.
That’s proving to be true.
Dishari’s research aims to develop new, biological ion channel-inspired ionomers to boost the movement of those charges and improve the performance of clean energy technologies to meet the cost-performance-durability targets set by the Department of Energy.
This approach creates channels known as macrocyclic cavities in ionomers, through which water and ions can move more quickly, thus making energy production more efficient. The method with which Dishari has experimented appears to be about 13 times more conductive than the currently widely used ionomer.
“While it is difficult to experimentally quantify the relative contribution of cavity and conventional proton hopping pathways exterior to cavities, we showed that a non-macrocycle-containing analog of a macrocycle-containing ionomer could have several orders of magnitude weaker proton conductivity.” Dishari said.
“These ionomers can also be stacked favorably to control the direction of ion movement.” Dishari said.
Hydrogen’s potential as an energy source is considerable. Since 2015, hydrogen-powered cars have been offered for sale from three different car companies; they are pricey but could be harbingers. Ultimately, the team’s research could both reduce the cost and improve the energy efficiency of hydrogen-based fuel cells and related energy conversion and storage devices.
The added advantage of these ionomers is that they are free from perfluoroalkyl and polyfluoroalkyl substances, commonly known as PFAS. Many companies are aiming to reduce the use of these manmade chemicals in their products. Eco-friendly devices made using eco-friendly materials could bring a huge advantage, Dishari said.
Different aspects of this ionomer design research are funded by Dishari’s National Science Foundation CAREER Award and the U.S. Department of Energy Office of Science Early CAREER Award.