Novel Cu-Based Catalysts for the ORR in PEM Fuel Cells

The sluggish oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC) imposes the need for a catalyst to increase the reaction rate and overall cell efficiency.  Pt, due to its high catalytic activity and relative stability in the PEMFC environment, is currently the most used catalyst.  However, Pt, as a precious metal, is expensive and a limited resource, greatly impacting the PEMFC cost and, as a consequence limiting its mass commercialization¹.

The search for non-precious metal catalysts (NPMC) to replace Pt for the ORR has led to the development of transition metal-based materials.   These NPMCs are generally synthesized by pyrolysis of a combination of metal salts, a nitrogen source (macromolecules or reactive gas) and a carbon support.   State of the art NPMCs are based on Fe and Co and are synthesized in elaborate multi-step processes combining high temperature treatment (900 ^oC to 1050 ^oC) with acid wash or reactive gas (NH₃) pyrolysis. These catalysts have proven to have good ORR activity but limited durability ^2,3.

In nature, Cu oxidases are known to reduce oxygen at approximately 1.2 V vs. RHE⁴, which makes Cu an obvious candidate for ORR catalysts. In our laboratory we have synthesized a new class of NPMCs for the ORR based on immobilized copper (Cu) complexes of substituted triazoles, supported on carbon black.  These catalysts are synthesized by covalently attaching ligands to the carbon surface via diazonium coupling, yielding a phthalocyanine-like molecule.  Figure 1 shows an example of a typical synthesis scheme for Cu triazolo-phthalocyanine (Cu-TrPc) catalyst.

Figure 1.  Synthesis scheme immobilized Cu-TrPc complexes.

The as-synthesized catalysts show low catalytic activity when compared to Pt, but offer a good starting point to understand the structured catalytic centers and the ORR mechanism.  Some of our Cu catalysts, tested via rotating ring disk electrode experiments (RRDE), have shown onset potentials as high as 0.6 V vs. RHE at pH 1 with less than 5 % H₂O₂ production. After a one step pyrolysis at 950 ^oC in inert atmosphere, the activity of the Cu-TrPc catalyst is significantly improved, reaching a RRDE onset potential of 0.82 V vs. RHE, as shown in figure 2. 

Figure 2. RDE plots comparing the catalytic activity of Cu-TrPc complexes without heat treatment (red) and after heat treatment under N₂ gas at 950 ^oC.  Experiments were conducted in 0.1 M H₂SO₄, at 1600 rpm and at a scan rate of 10 mV/s.

The samples were characterized using fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), thermo gravimetric analysis – mass spectrometry (TGA-MS) and x-ray powder diffraction (XRD).  RRDE experiments were used to study the effect of catalyst loading, to determine the reaction order and catalyst stability.  Optimization methods to enhance the catalytic performance will also be described.

Acknowledgements

We gratefully acknowledge the support of this work by the NSF-funded TN-SCORE program, NSF EPS-1004083, under Thrust 2.

References

1. Vielstich, W.; Gasteiger, H. A.; Yokokawa, H., Eds.  Handbook of Fuel Cells: Advances in Electocatalysis, Materials, Diagnostics & Durability. Wiley and Sons. 2009, Volume 5.
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3. Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Science, 2009, 324, 71-74.
4. N₄-Macrocyclic Metal Complexes.  Editors: J. Zagal, F. Bedioui. J.P. Dodelet. Springer 2006.