Highly Durable Carbon Nanotube Composite Support By Pyrolysis of Conductive Polymer for Polymer Electrolyte Membrane Fuel Cell

Recently, there has been renewed of great interest in polymer electrolyte membrane fuel cells (PEMFCs) as promising alternative power generation systems due to higher efficiency and power density, lower operating temperature, and much less release of environmental pollutants than conventional energy conversion devices. Despite significant advances of PEMFCs, low durability is still key issues that should be addressed prior to their commercialization. The long-term performance loss of PEMFC mostly stems from the degradation of PEMFC catalysts and carbon supports. Although several types of Pt-based catalysts with enhanced catalytic activity or durability have emerged, they are still in development [1].

Among various kinds of carbon materials, carbon nanotube (CNT) has emerged as a promising support for PEMFC catalysts due to its surface area, high electrical conductivity and excellent durability [2]. However, CNT has no enough binding sites for anchoring Pt particles, and the Pt/CNT catalysts thus have poor Pt dispersion and broad particle size distribution with large size, especially at high Pt loading [3]. One of facile approaches to solve this drawback is the use of heteroatom-doping to carbon supports, where the metal-carbon support interactions have been implicated in the enhanced electrochemical durability and activity in the modified system [4].

Herein we present the fabrication of the nitrogen-containing carbon and CNT (NC-CNT) composite for PEMFC electrocatalysts and the enhanced durability of the Pt/NC-CNT composites by nitrogen doping. The NC-CNT composite has been synthesized by the polymer wrapping of the CNT with poly(styrenesulfonic acid) and in situ polymerization of pyrrole on CNT surfaces. Pyrolysis was performed at temperatures ranged from 400~1000 ^oC in an inert atmosphere of N₂gas for 2 h. Uniform coating of polymer layer acing as linkers onto CNT surfaces was confirmed by UHR-FESEM/STEM. As shown in Figure 1(a), CNT was encapsulated by about 15 nm of coated polymer layers.

For the as-synthesized NC-CNT composite, a new peak located at ~399 eV is observed, which indicates the presence of nitrogen atom. The atomic percentage of N-element in the NC-CNT composite is in the rage of 3.1~12.7 at.% by controlling the amount of conductive polymer and the heat-treatment temperature. As shown in Figure 1(b), high resolution N1s core level spectra of the heat-treated NC-CNT composite contain multiple components due to the presence of different chemical states of nitrogen. The peak at 400.9 eV is attributed to the graphite-like N, which is the most stable nitrogen species during the pyrolysis. The content of graphitic-N is in the range of 8.8~36.1% for NC-CNT composites.

As shown in Figure 2(a), Pt nanoparticles were uniformly dispersed in the NC-CNT composites and their particle size was ~2.5 nm. Although Pt nanoparticles are distributed on CNT surfaces with a high density, they do not aggregate with each other as compared to Pt/unmodified CNT, resulting for the strong interaction with the NC-CNT composites via Pt-N-C bonding as well as the effective isolation of adjacent Pt nanoparticles by the coated polymer layers.

The electrochemical durability of Pt/C catalyst has a significant influence on their long-term performance. As shown in Figure 2(b), the ECSA of Pt/NC-CNT only decreased by 15.0%, while Pt/unmodified CNT and commercial Pt/C (Pt 46.7%, TKK) catalysts lost about 40% and 65% of the initial ECSA, respectively. The result clearly demonstrates that Pt/NC-CNT is much higher stable than Pt/unmodified CNT and commercial Pt/C catalyst. The interaction between Pt and N-doped CNT composite was also analyzed by the XPS (not shown here). As compared to Pt/unmodified CNT, the shift in the Pt4f peak to a higher binding energy (~1 eV) observed in the Pt/NC-CNT composite is attributed to the strong interaction between Pt and N-containing moieties, thus creating stabilizing effect against the aggregation and dissolution of Pt nanoparticles. It should be also noted that Pt/NC-CNT composites show better durability with higher N-doping level and more graphitic N contents.

In brief, the enhanced durability of the Pt/NC-CNT composite catalyst results from the polymer stabilization effect and strong interaction between Pt and support by nitrogen doping. The NC-CNT composites with different N-configuration can be also fabricated by controlling N-doping level and heat-treatment temperature. Our study also provides a design guide of highly durable Pt catalyst supports with a great potential for PEMFC.

References

[1] H. A. Gasteiger, N. Markovic, Science 324, 48 (2009).

[2] L. Qu, L. Dai, J. Am. Chem. Soc. 127, 10806 (2005).

[3] Y. L. Hsin, K. C. Hwang, C. T. Yeh,  J. Am. Chem. Soc. 129, 9999 (2007).

[4] Y. Zhou, K. Neyerlin, T. S. Olson, S. Pylypenko, J. Bult, H. N. Dinh, T. Gennett, Z. Shao, R. O’Hatre, Energy Environ. Sci. 3, 1437 (2010)