Theoretical model predicts fuel cell performance
January 21, 2015
By José Tadeu Arantes
Agência FAPESP – A catalyst slowly deteriorates as its surface oxidizes, representing a critical factor in fuel cells. Fuel cells convert chemical energy into electricity and are used, for example, to power vehicles.
Until now, the only way to determine exactly how much longer a fuel cell would remain operational was to interrupt the process, open the cell, remove the catalyst, and analyze its surface. This procedure was impractical, to say the least.
A new methodology for the assessment of fuel cell durability that does not require characterization of the catalyst’s condition has been presented by a group of Brazilian and German researchers coordinated by Hamilton Varela, a professor at the São Carlos Chemistry Institute of the University of São Paulo (USP).
An article published by the group is featured on the cover of The Journal of Chemical Physics. The article reports the results and is entitled “Coupled slow and fast surface dynamics in an electrocatalytic oscillator: model and simulations”.
The article focuses on theory and computer simulations. The study presents a series of equations that can be used when the process of catalysis under oscillatory conditions is combined with slow poisoning of the catalyst. “We haven’t created any materials,” Varela told Agência FAPESP. “We’ve developed a new concept and a new methodology for its application.”
The starting point for the model’s development was a practical experiment involving the oxidation of formic acid on the surface of a platinum-tin catalyst.
In 2014, Varela and collaborators published the findings from this experiment, which was supported by FAPESP, in an article featured on the cover of ChemPhysChem and entitled “Long-lasting oscillations in the electro-oxidation of formic acid on PtSn intermetallic surfaces”.
“In this experiment, we found that if pure platinum was used as the catalyst, the system began to oscillate and rapidly died owing to oxidation of the metal. We, therefore, replaced the platinum catalyst with another made of platinum and tin,” Varela said.
“We observed that the addition of tin substantially boosted catalytic activity, retarding oxidation of the catalyst’s surface and stabilizing the reaction process for more than 2,200 oscillatory cycles. Previously, the process withstood only a few dozen cycles,” he explained.
The new model allows these types and other types of problems to be addressed far more comprehensively. “We left the specific behind and went to the general,” Varela said.
The secret is to conduct the catalytic reaction of interest under oscillatory conditions. “When we investigated the system’s long-term evolution, we noticed that the characteristics of the oscillations change as the surface of the catalyst is coated and loses activity,” he said.
“We then realized that if we measured changes in the oscillations, we could determine the state of the catalyst precisely and make predictions about its functioning without having to characterize its surface physically.”
“The change in the oscillatory pattern tells you about the surface of the catalyst. It’s an entirely new concept in terms of the characterization and evolution of systems.”
Combination of dynamics
The model is sufficiently comprehensive for application to a wide variety of phenomena, not only the operation of fuel cells. The required condition is that there must be a combination of two dynamics, one of which is oscillatory while the other is long-term.
“Couplings of processes that occur in completely different time scales are common in every area of nature. In the human body, for example, the heart beats dozens of times per minute, and this process is coupled with aging, which is a very slow process. Their speeds differ by a factor of about ten to the power of six,” Varela said.
Varela noted the affinity between the theoretical research that resulted in the new model and the work of German physicist Gerhard Ertl, who won the 2007 Nobel Prize in Chemistry for his studies of catalytic processes on metallic surfaces.
From 2000 to 2003, Varela studied for his PhD at the Department of Physical Chemistry, Fritz Haber Institute, Max Planck Society, Berlin (Germany) where Ertl is the department head. Varela received a grant from the Max Planck Society. He later earned a postdoctoral fellowship from FAPESP and became a founding member and managing scientist at the Ertl Center for Electrochemistry & Catalysis in Gwangju, South Korea. Markus Eiswirth, one of the co-authors of the article published in The Journal of Chemical Physics, is a vice-director of the Ertl Center.
In his Director’s Message to the 2009 opening symposium held by the center named in his honor, Ertl referred to the serious problems the world faces regarding the environment, climate, energy conversion, raw materials and food, highlighting the importance of chemistry, and in this connection, electrochemistry and catalysis, in the search for solutions to these problems.
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