Electrochemical and Physical Characterization of Ru Activated Carbon Supported Electrodes in Alkaline Solution

Polymer electrolyte fuel cells (PEMFC) are promising energy sources for stationary and portable fuel cell applications [1-3]. However, the slow oxygen reduction reaction (ORR) on different catalysts is one of the most limiting factors in the energy conversion efficiency of PEMFC. Improved cathode catalysts could have a dramatic impact on the fuel cell efficiency. Therefore platinum-ruthenium and other alloys deposited onto various carbon supports have been studied to find materials with higher ORR activity [2, 4-6].

The following work examines the impact of ruthenium nanoparticles (20 wt%), deposited onto various carbon supports prepared from α-WC and Vulcan, on ORR kinetics in 0.1 M KOH aqueous solution. Experiments show that there is an important contribution of the nanoparticles to the electroreduction process of O_2.

Scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), high resolution transmission electron microscopy (HRTEM) and low temperature N₂ adsorption experiments were carried out to characterize the structure of the prepared materials.

EDX studies show that Ru nanoclusters have been deposited into/onto carbon support quite uniformly. A comparison of the N₂ adsorption data (table) and SEM images show that ruthenium forms mesoporous clusters on the surface of the supporting material (figure). SEM images show also that small ruthenium nanoparticles were uniformly observed on the support material, which means that deposition doesn’t take place quite homogenously.

Electrochemical measurements were carried out in a three-electrode electrochemical cell. Electrochemical characteristics for various Ru-modified micromesoporous carbons have been established by cyclic voltammetry (CV) and rotating disc electrode (RDE) methods. RDE data were measured at rotation rates from 0 to 3000 rpm (v=10 mV∙s^-1) and in the region of potentials from +0.17 to -0.50 V vs. Hg|HgO|0.1 M KOH. CVs were measured at potential scan rates (v mV/s) 5, 10, 20, 30, 50, 70, 100, 150 and 200, in both Ar and O₂ saturated solutions. The solutions were saturated with Ar or O₂, respectively, between measuring of each voltammogram. A glassy carbon disk electrode (GCDE) was used as a catalyst support for electrochemical measurements. Catalyst ink was prepared by suspending the catalyst powders in isopropanol, Milli-Q⁺ water and Nafion dispersion solution (Aldrich) mixture (5 wt%) and pipetted onto GCDE.

It was established that the ORR activity for 20%Ru-C(WC) is higher but comparable to 20%Ru-Vulcan. Ru-metal loadings play a dominant role in the kinetics of the ORR on both electrodes in alkaline solution. After correction for Ar saturated supporting electrolyte current densities, very nice diffusion current plateaus have been observed at E < -0.28 V vs. Hg|HgO|0.1 M KOH for 20%Ru-C(WC) and 20%Ru-Vulcan systems. CV data demonstrate that 20%Ru-C(WC) exhibits higher capacitance values than 20%Ru-Vulcan, which is in a good agreement with N₂ adsorption data.

Acknowledgements: This work was supported by the Estonian target research project SF0180002s08, the Estonian Centre of Excellence in Science Project TK117T "High-technology Materials for Sustainable Development", the Estonian Energy Technology Program project SLOKT10209T, the Materials Technology project SLOKT12180T.

References

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