Carbon Support Modification for Improving Platinum Performance in PEMFC

With high activities for anodic electro-oxidation reaction and cathodic electro-reduction reactions, Platinum (Pt) has been the general choice of catalyst for low-temperature fuel cells. However, the insufficient durability and catalytic activity remain major obstacles to the widespread commercialization of fuel cell technology.¹The catalytic activity and stability of Pt catalysts depend on many factors, among which the carbon supports play a crucial role.

    On Pt electrodes, since the electrocatalytic reactions are confined to the three phase boundary (TPB) regions, it is essential to build an efficient TPB. An efficient TPB, which ensures a good contact between reactants and catalysts and an easy transport of electron, ion, gas, and liquid, can simultaneously meet the requirements both of activity and utilization of Pt catalysts.² Recently, we have devoted our research to synthesize carbon nanostructure supported platinum electrode to build an efficient TPB.^3,4 Carbon nanostructure supports directly grown on the gas diffusion layer (GDL) ensures a fast electron, mass transport and low internal resistance creating an efficient TPB.³ Moreover, all the preparation processes were conducted in dry plasma atmosphere which could effectively avoid the cohesion of nanostructures induced by capillary force in the liquid-air interface of drying process.This integrated structure electrode which possesses ultra-low Pt loading, high Pt utilization and efficient TPB structure, is desirable for application in low-temperature fuel cells.

    In order to further improve the catalytic activities of the Pt catalysts, one of the strategies is creating strong catalyst-support interactions by functionalizing carbon support materials. Nitrogen (N) modification of carbon nanomaterial, which plays a critical role in improving Pt performance, has shown fascinating applications. We synthesized N-doped carbon nanofiber (CNF) supported platinum catalysts using an approach combined of plasma enhanced chemical vapor deposition and in situ plasma activation. This approach can successfully introduce nitrogen functionalities into carbon network, and at the same time, preserve the graphitic structure of carbon support. XPS results indicate that NH₃ plasma modification mainly creates pyridinic nitrogen functionalities, while N₂ plasma modification mainly increases the percentage of pyrrolic nitrogen in carbon network, suggesting a modification of certain nitrogen functionalities on carbon support. The oxidative CO-stripping peak potentials for Pt/CNF, Pt/CNF-N₂ and Pt/CNF-NH₃ are 0.67 V, 0.64 V and 0.62 V vs. SCE, respectively (Fig. 1). The negative shifts for peak potential suggest that the adsorbed CO can be more easily removed from Pt/CNF-NH₃ and Pt/CNF-N₂ electrodes than from Pt/CNF. The measurements of chronoamperometry were carried out and the results were in good agreement with the CO-stripping. The decay of the oxidation current densities on Pt/CNF-N₂ and Pt/CNF-NH₃ electrodes are slower than that of Pt/CNF, and the oxidation current densities at the end of the test on the Pt/CNF-NH₃ are larger than that on the Pt/CNF-N₂and Pt/CNF electrodes. N-modified Pt/CNF electrodes exhibit a better poisoning-resistance/durability ability than pristine Pt/CNF electrode, suggesting that the nitrogen functionalities in the carbon network may play an important role for anti-poisoning from intermediates.

Fig.1. CO-stripping voltammetry in 1 M H₂SO₄ solution for Pt/CNF, Pt/CNF-N₂, and Pt/CNF-NH₃ at a scan rate of 50 mV s^–1.

Acknowledgements

    This research is financially supported by the National Nature Science Foundation of Anhui province (No. 1308085QA09) and National Nature Science Foundation of China (Nos. 11205202, 21203204, 11175214).

References

1.  J. Hu, L. Jiang, C. Zhang, X. Zhang, Y. Meng, X. Wang, Appl. Phys. Lett., 104 (2014) 151602.

2. J. Hu, C, Zhang, L, Jiang, Y, Meng, , J. Phys. Chem. C, 117 (2013) 12902.

3. C. Zhang, J. Hu, X. Wang, X. Zhang, H. Toyoda, M. Nagatsu, Y. Meng, Carbon, 50 (2012) 3731.

4. C. Zhang, J. Hu, X. Shu, H. Toyoda, M. Nagatsu, S. Fang, Y. Meng,. Electrochim. Acta, 56 (2011) 6033.