Testing Methanol-Based Fuel Cells Using FTIR Spectroscopy
Lynn M. Savage
Lithium-ion batteries are used to power devices ranging from cell phones to laptops, but improvements on battery technology are being sought to increase their efficiency and to reduce their environmental impact. Possible replacements for lithium-ion batteries are direct methanol fuel cells, which use liquid fuel as a source of protons and electrons. This type of fuel cell is promising because it provides twice the power density of lithium-ion cells at only ~20 percent efficiency and, if it could operate at 100 percent efficiency, might provide up to 10 times higher power density.
A schematic shows the construction of a direct methanol fuel cell. The polymer electrolyte membrane typically is made with Nafion, but this material allows too much methanol crossover to permit efficiency of more than 20 to 25 percent. Reprinted from Journal of Polymer Science B (Aug. 15, 2006, pp. 2201-2225) with permission from Wiley Periodicals Inc.
However, direct methanol fuel cells use a polymer electrolyte membrane for transporting protons from the methanol on the anode side to the cathode, and the most often used membrane material — Nafion, manufactured by DuPont in Fayetteville, N.C. — is problematic. A perfluorocarbon related to Teflon, Nafion permits too much of the methane to permeate it, reducing the output voltage as well as the cell’s lifetime.
To understand the toll that methanol permeation — also known as crossover — takes on membranes in general and on Nafion in particular, engineers must know how the chemicals transport with one another. Gravimetric and electrochemical techniques as well as nuclear magnetic resonance spectroscopy have been used to help elucidate the chemical underpinnings of direct methanol fuel cells but have been unable to clarify the transport mechanisms as methanol diffuses into a membrane and is absorbed by it. Now researchers at Drexel University in Philadelphia have used Fourier transform infrared spectroscopy and attenuated total reflectance (FTIR-ATR) to determine the diffusion and sorption of both methanol and water in Nafion.
The investigators — graduate student Daniel T. Hallinan Jr. and assistant professor Yossef A. Elabd — used an FTIR spectrometer made by Thermo Electron (now Thermo Fisher Scientific of Waltham, Mass.) and a ZnSe ATR crystal made by Specac Inc. of Cranston, R.I. They reported their findings in the Nov. 22 issue of Journal of Physical Chemistry B.
To evaluate methanol diffusion through the membrane, they placed a hydrated, precut section of Nafion onto the crystal inside the ATR chamber, allowed the membrane to dry out, then rehydrated it and flowed methanol through the chamber. The hydration-dehydration stages helped achieve common starting points for data collection. The scientists collected data using the spectrometer’s HgCdTe detector, taking 32 scans per spectrum at a resolution of 4 cm–1.
They observed that the spectra of interest were the C-O symmetric stretch associated with methanol at 1016 cm–1 and the H-O-H bending associated with water permeating the membrane at 1640 cm–1. The former band increased with time, indicating an accumulation of methanol within the Nafion near the interface between the membrane and the ATR crystal. According to the researchers, these experiments are the first to measure both multicomponent diffusion and sorption in Nafion in the presence of a concentration gradient.
They concluded that the sorption of methanol — rather than its diffusion — has the most significant ef-fect on methanol crossover and that the results should influence how chemists synthesize Nafion alternatives for methanol fuel cells. Elabd said that they also are interested in devising a way to apply an electric potential to the Nafion during diffusion and sorption.
Contact: Yossef A. Elabd, Drexel University, Philadelphia; e-mail: firstname.lastname@example.org.
MORE FROM PHOTONICS MEDIA