Pass the Proton, Please
Simulations help scientists see what's happening inside fuel cells
Results: Too many choices, not enough time. These words haunt those selecting materials for fuel cells—devices that use hydrogen to generate electricity, but not pollutants—for everything from locomotives to laptop computers. To focus on the most promising materials for fuel cells, designers need to know what is happening at the molecular level.
"We can't say ‘We'll try this one. That didn't work. We'll try another one,'" said Pacific Northwest National Laboratory's Ram Devanathan. "So, we have to understand the molecular chemistry." Devanathan, Dupuis, and their team built and analyzed computer simulations of what is happening near the cell membrane to reach this goal.
In a fuel cell, the membrane is sandwiched in the middle. Hydrogen enters the cell and a catalyst splits it into electrons and protons. The membrane forces the electrons to move through a circuit, generating power. The protons pass through the membrane. The electrons join the protons along with oxygen and forms water.
The effect of the membrane's water content, or hydration, on proton movement was a mystery. But, by combining theory with the power of supercomputers, the team now knows how protons move through the fuel cell membrane at different levels of hydration.
Why it matters: Clean, plentiful energy can significantly enhance the quality of life for large parts of the planet. But, as fossil fuel use increases, so do environmental issues. One alternative to burning fossil fuels is fuel cells, which run on hydrogen available from renewable sources. While specialized devices use fuel cells, more information is needed before the devices are ready for broad use.
Methods: For scientists, protons moved across a fuel cell membrane so fast that conventional experiments cannot reveal the details of proton transport. So, the team built a computer simulation of protons interacting with a Nafion® membrane. The simulation let the team slow down the interactions for study.
Nafion was created by DuPont in the 1960s. It moves protons quickly and doesn't break down easily. However, it is expensive. And, it doesn't work well at the high temperatures found inside fuel cells.
Detailed information on Nafion and water was added into the simulations. Running these simulations required powerful computers. So, the team used supercomputers at the National Energy Research Scientific Computing Center (NERSC) and the U.S. Department of Energy's EMSL, a national scientific user facility at PNNL. The team wrote all of the data analysis algorithms for the research.
The simulations and analysis yielded insights into behavior of Nafion observed but unexplained in experiments. For example, experiments showed that protons moved through the cell better when more water was added. The simulations showed the reason for this change was the water molecules formed a bucket brigade. They simply passed the protons from one water molecule to the next.
Another insight was why protons did not diffuse through the membrane when little water was available, i.e., low water content. The simulations showed that protons became ensnared by the water-loving groups on the end of Nafion molecules.
What's next? The team will systematically vary the structure and chemistry of Nafion-like polymers to see how changes affect proton transfer. The funding for this research was renewed by the U.S. Department of Energy's Office of Basic Energy Sciences.
Acknowledgments: This research was funded by the U.S. Department of Energy's Office of Basic Energy Sciences within the Office of Science. The work was done by Ram Devanathan, Arun Venkatnathan, and Michel Dupuis at PNNL.
This work supports PNNL's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.
Devanathan, R, A Venkatnathan, and M Dupuis, "Atomistic simulation of Nafion membrane: 1. Effect of hydration on membrane nanostructure." J. Phys. Chem. B 111:8069.
Devanathan, R, A Venkatnathan, and M Dupuis. 2007. "Atomistic simulation of Nafion membrane. 2. Dynamics of water molecules and hydronium ions." J. Phys. Chem. B 111:13006.