The graphene-based catalyst Fe-N-rGO has a much higher oxygen reduction reaction catalytic activity than those based on carbon black or oxidized carbon black.
MIT scientists, doing part of their work on beamline X11 at the National Synchrotron Light Source, have made a promising graphene-based catalyst to improve fuel cells.
Fuel cells convert hydrogen and oxygen into water, making electricity in the process. They are a source of quiet, efficient and clean energy, with the potential to replace combustion-based technologies in transportation and power applications. Starting in the 1960s, the U.S. put alkaline fuel cells on board spacecraft to produce electricity and water. Promising fuel-cell technologies of today include polymer-electrolyte-membrane fuel cells, also known as proton-exchange-membrane fuel cells (PEMFCs).
PEMFCs have the highest energy density of all fuel-cell types. They also have a relatively low operating temperature (ranging from 60-80 degrees Celsius), which means they warm up quickly – and begin generating electricity. That makes PEMFCs especially appealing for use in vehicles and in portable- and backup-power applications. Because they typically use platinum as a catalyst, however, the high cost of PEMFCs inhibits commercial development. To bring down the cost, research is focused on developing a non-precious-metal catalyst made of iron, nitrogen and carbon (Fe-N-C).
The team from MIT – Hye Ryung Byon, Jin Suntivich, and Yang Shao-Horn – prepared a graphene-based Fe-N-C catalyst (graphene is a monolayer of carbon) with high oxygen reduction reaction (ORR) activity, plus stability in acid. The method involves heat treatment of a mixture of Fe salt, graphitic carbon nitride and chemically reduced graphene (rGO).
The graphene-based catalyst exhibits reduction activity approaching those of the state-of-the-art, non-noble-metal catalysts reported to date, which highlights the opportunities of using the unusual surface chemistry of rGO to create active Fe-N sites and develop an improved catalyst.
Our approach is uniquely different from other groups,” said MIT’s Yang Shao-Horn, who is the Gail E. Kendall Associate Professor of Mechanical Engineering at the university. “We start from molecular building blocks and precisely control the surface chemistry of graphene as we build the catalyst.”
The researchers examined the surface chemical composition of Fe-N-rGO by x-ray photoelectron spectroscopy (XPS) and studied the atomic coordination of Fe by extended x-ray absorption fine structure (EXAFS). XPS and EXAFS of the Fe-N-rGO sample provided evidence for the incorporation of Fe ion and N into the rGO upon annealing.
Characterizing the Fe-N functionalization is experimentally very difficult, explained Shao-Horn, and x-ray absorption is one of the few techniques that can accomplish this task. “We use the X11 beamline at NSLS, where we have excellent support,” she added. “We are extremely fortunate to have this collaboration.”
According to Shao-Horn, ongoing work includes examination of Fe-N-rGo’s performance and lifetime in a more realistic fuel-cell configuration.