The Impact of Pt-Nanocluster Deposition and Nafion® Content on the Oxygen Electroreduction Kinetics on Molybdenum Carbide Derived Carbon Synthesized at 1000°C

Carbide derived carbons (CDCs) are unique materials in which pore size, shape, uniformity and other parameters can be controlled in exact manner. Therefore these materials are interesting objects to be used as a catalyst support in polymer electrolyte membrane fuel cells (PEMFC) [1-3]. The structure of carbon material influences greatly electrochemical activity and therefore optimization of carbon material properties must be carried out in order to develop high power density low temperature fuel cells [4]. CDCs with incorporated platinum nanoparticles are highly active towards oxygen electroreduction (ORR) even at low Pt loadings. It has been suggested that microporous-mesoporous CDCs are viable fuel cell catalyst supports that are capable of realizing the full activity of Pt-nanoparticles with superior corrosion stability [1-3, 5].

Carbon derived from Mo₂C has been prepared using chlorination method at 1000°C. Pt was deposited onto carbon material using NaBH₄ reduction method [1-3]. The structure of studied materials has been characterized using XRD, HRSEM, EDX and gas adsorption/desorption measurements. Electrochemical measurements were carried out in three-electrode cell. The kinetics of ORR has been analyzed using cyclic voltammetry, rotating disc electrode and electrochemical impedance spectroscopy methods in 0.5M H₂SO₄solution. The data of electrochemical and EDX analysis were in good agreement and demonstrated that the Pt content in the synthesized material was 10 wt%. It was shown that even the low Pt loading improves the electrochemical activity significantly by reducing the overpotential of ORR nearly by 0.8V. Number of electrons transferred per one oxygen molecule electroreduction calculated from Koutecky- Levich equation increases from two to four when activating the carbon material with Pt-nanoparticles.

The content of Nafion®^®was varied in the range 5-20 wt%. Results established show that the highest kinetic currents of ORR were observed with 15% Nafion®, but the percentage of Nafion® has only slight influence on the electrochemical activity. Thus, it was concluded that the Nafion® ionomer disperses very uniformly on the material studied and does not hinder the mass transport. This study demonstrates that investigated material is attractive for PEMFC applications.

Figure 1. SEM image of Pt-activated carbon derived from Mo₂C synthesized at 1000°C.

Acknowledgements.

This work was supported by Estonian target research project SF0180002s08, Estonian Centre of Excellence Project 3.2.1101.12-0019, Estonian Energy Technology Programme project SLOKT 10209T and ESF Grant ETF8267.

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