Highly efficient non-precious metal electrocatalyst for ORR in fuel cells and metal-air batteries

18 December 2012

a)Current–voltage and b) power–voltage curves of Zn–air cells with one of the nanotetrapod non-precious metal and 20% Pt/C catalysts. A gas diffusion layer without any catalysts was used as the baseline air electrode for comparison. Source: Lee et al. Click to enlarge.

A team of S. Korean and American scientists led by Dr. Jaephil Cho at Ulsan National Institute of Science and Technology (UNIST) reports on a newly developed, highly efficient non-precious metal electrocatalyst for the oxygen reduction reaction (ORR) in the journal Angewandte Chemie.

Inspired by the tetrapod structures of a breakwater, the novel material for electrodes is created from affordable melamine foam and carbon black. The unique porous architecture greatly facilitates rapid mass transport, while the N-doped ketjenblack and Fe/Fe₃C-functionalized surface of the framework significantly enhance the ORR activity of cathodes for fuel cells and metal-air batteries.

Architectural features of tetrapod structures, commercially available melamine foam (inset), and a cross-sectional view (SEM image) of a fractured melamine foam after pyrolysis. The interconnected large pores may facilitate fast mass transport. Lee et al. Click to enlarge.

The reaction that occurs at the cathodes of fuel cells and metal-air batteries is the electrochemical reduction of oxygen—the oxygen reduction reaction (ORR). This reaction is considerably inhibited because of its sluggish rate, and the efficiency of the cells is lower than it could be.

The catalytic cathode must ensure that oxygen reacts with water, taking up electrons to form OH^- ions in alkaline solution. The problem is that in a complex system involving solid, liquid, and gaseous reactants, transport processes are often too slow and inhibit the process, especially when discharging with higher current densities.

Cathodes made of a porous carbon support (carbon black) on which a catalytically active metal like platinum is finely dispersed can very effectively minimize this kinetic inhibition. However, they are expensive and not very stable, thus making them impractical for widespread application. The team from UNIST and the Georgia Institute of Technology aimed to develop a more economical alternative.

Tetrapods, whose four “feet” are pointed toward the corners of an imaginary tetrahedron, are constructed at the coast as well as near dams and piers to reduce the force of waves crashing against the shore. These structures also provide sanctuary for marine life forms in their many large cavities. When melamine foam is pyrolyzed and ground with a mortar and pestle, it forms microscopic fragments resembling tetrapods.

The scientists treated melamine foam with iron chloride and nitrogen-doped ketjenblack (conducting pellets of carbon black). They carbonized this product and extracted it with sulfuric acid. The resulting nanotetrapods studded with nanoparticles of carbon black have a very high specific surface area, a large number of catalytically active centers (Fe/Fe₃C, and CN groups), and many pores that allow for rapid mass transport.

Electrochemical performances of the low-cost catalysts are comparable to those of Pt-based catalysts at low overpotentials and even better at high overpotentials....Performance measurements using RRDE and Zn–air cells suggest that a large surface area of N-doped ketjenblack and Fe/Fe₃C of melamine carbon foam are responsible for the dramatically increased ORR activity. Good methanol tolerant and durability were also demonstrated. As commercially available melamine foam and ketjenblack carbon are relatively inexpensive and the facile synthetic method is amendable to mass production, the non-precious metal catalysts are a promising alternative for a new generation of low-cost and high-performance metal–air batteries and fuel cells.

—Lee et al.

Resources

• Lee, J.-S., Park, G. S., Kim, S. T., Liu, M. and Cho, J. (2012), A Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction: N-Doped Ketjenblack Incorporated into Fe/Fe₃C-Functionalized Melamine Foam. Angew. Chem. Int. Ed. doi: 10.1002/anie.201207193