Electrochemical Features of Low Pt Electrocatalysts Deposited Over Doped Carbon Nanotubes

Abstract

Carbon nanotubes (CNT) are being utilized in PEM fuel cells electrocatalysts, due to their physical and chemical characteristics. Diverse studies have been reported on their use as support of Pt to improve the metal particles dispersion, as well as to provide a higher mechanical and electrochemical stability. As it is known, the development of highly efficient electrocatalysts with low platinum loading is crucial in order to achieve the massive application of PEM fuel cells. The electronic properties of CNT can still be improved, since they play an important role in the electrochemical performance. Recently, it has been reported that the incorporation of heteroatoms to the CNT structure (i.e. N, B, F, S, Si, etc.) can change positively their properties such as stability. Also, it is expected the dopant to have an effect on the Pt nanoparticles dispersion, affecting the electrochemical activity. The bonds form with the heteroatoms, as well as their concentration within the volumetric structure of the carbon, generally influence the properties of the material and change its structural geometry. The morphology and characteristics of the materials differ for each type of doping. For example, N-doping creates hollow sections separated by one or a few graphitic layers, while S-doping produces branched morphologies. Hence, it is of great interest to take advantage of the defects created by addition of heteroatoms to the CNT structure. In this research work low-Pt over doped CNT systems were obtained and evaluated. The doped carbon nanotubes were synthesized using a modified chemical vapor deposition (M-CVD) system in a high-temperature argon flow furnace. Toluene was used as a carbon source, ferrocene as a catalytic agent, adding different precursors of the heteroatoms. Low Pt loadings were deposited over pristine and doped CNT using a colloidal methodology. The structure of the obtained materials was analyzed by X-ray diffraction and Raman spectroscopy. Also, energy dispersive X-ray spectroscopy and scanning electron microscopy studies were done in order to obtain the chemical properties and the morphology. Electrochemical impedance spectroscopy (EIS) studies were done in the presence of 0.5 M H2SO4 and at an amplitude of 10 mV at different potentials vs AgAgCl (KCl sat), within a frequency range of 100 mHz to 100 KHz, in order to analyze the conductivity of the doped CNT used as support. In addition, electrochemical evaluation was performed by cyclic and lineal voltammetry in a conventional three electrode cell, in acidic electrolyte and using Ag/AgCl (KCl sat) as reference electrode, and Pt wire as a counter. In addition, in order to compare the same analyzes were carried out on reference materials such as vulcan carbon and nanotubes without dopant (pristine). The results of an extensive physical and chemical characterization confirmed the growth of doped CNT with different heteroatoms. It was possible to establish that the morphological characteristics of the obtained doped carbon nanotubes varied, depending on the dopant. Differences on the diameter, length, as well as the graphitization degree of the structure were found. Also, it was possible to confirm the low Pt loading over the supports. The evaluation of the electrochemical properties yielded interesting differences. Hence, results obtained of low-Pt loading over doped CNT electrochemical systems, will be presented and discussed at the conference.Acknowledgments: We thank the Fund "CONACyT-SENER-Sustainability Energy" (No. 254667) for the support provided for the development of this project.