Graphene Oxide Based Non Precious Metal Catalysts for Oxygen Reduction Reaction in Alkaline Media

The commercialization and widespread of polymer electrolyte fuel cells (PEFCs) has been limited by the high cost of Pt-based catalysts used in both PEFC anode and cathode. Recently, non-precious metal catalysts (NPMCs) have demonstrated high volumetric activity, low peroxide yield, and stability^[1-2] and appear as potential candidates for the replacement of Pt-based oxygen reduction reaction (ORR) catalysts in the PEFC cathode.

State-of-the-art NPMCs are typically synthesized from highly heterogeneous precursors^[1,3,4], making understanding of the ORR active site in such catalysts particularly arduous. Catalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as good chemical stability, excellent conductivity, and more importantly, can potentially be functionalized in a controlled manner^[5]. GO inherently has a high defect density and can be modified with various functional moieties, e.g. carboxyl, hydroxyl, and epoxy groups using various synthesis techniques such as Hummers or Hoffmans methods. The desire to have a high defect density arises from the possibility that defect sites may serve as niches for the chemical doping of heteroatoms, such as nitrogen (N) that have been proven to play a critical role in the overall performance of NPMCs. Further, the N-doped sites might serve as anchors for the coordination of transition metal atoms such as Fe, leading to the formation of the ORR active sites.

In this work, we have synthesized GO using different chemical routes and, subsequently, treated it with different drying strategies and finally doped it with ammonia at various temperatures (500°C-900°C). The electrochemical performance of the synthesized catalysts in oxygen reduction reaction was measured using RDE. In a 0.5 M H₂SO₄ electrolyte, a half-wave potential of 0.70 V and low peroxide formation were obtained. Conversely, in a 0.1 M NaOH electrolyte, a half-wave potential of 0.85 V was observed. Lower peroxide yields were observed when a drying treatment was part of the synthesis process. A detailed ICP-MS study identified ppm levels of Mn and Fe in the sample, possibly due to the intrinsic GO synthesis method used. Results obtained using other characterization techniques, such as Raman and XPS, will be presented to further understand the role of the metal and nitrogen doping in the performance of GO-based catalysts.

These first results indicate high performance of treated GO-based NPMCs in both acidic and alkaline media. Although, we have demonstrated effective oxygen reduction activity and have a better understanding of GO-based catalysts, we are now focusing our efforts on the progressive addition of different metal precursors such as Co and Ni.

Acknowledgement

Financial support from the Los Alamos National Laboratory, Laboratory-Directed Research and Development (LDRD) is gratefully acknowledged.

References:

[1] Wu, G., More, K. L., Johnston, C. M., Zelenay, P., Science, 332, 443-447 (2011).

[2] Bashyam, R., Zelenay, P., Nature, 443, 63-66 (2006).

[3] Chung, H. T., Johnston, C. M., Artyushkova, K., Ferrandon, M., Myers, D. J., Zelenay, P., Electrochem. Commun., 12, 1792-1795 (2010).

[4] Wu, G., Nelson, M., Ma, S. G., Meng, H., Cui, G. F., Shen, P. K., Carbon, 49, 3972-3982 (2011).

[5] Qu, L., Liu, Y., Baek, J.-B., Dai, L., ACS Nano, 4, 1321-1326 (2010).