The Oxygen Reduction Reaction in Alkaline Solution

The oxygen reduction reaction (ORR) is one of the key reactions in electrochemical energy conversion devices like fuel cells [1-3]. Recently, as the high over-potential of ORR decreases the efficiency of these devices, various carbon-supported Pt catalysts have been studied in order to lower the over-potential of the ORR [3]. The aim of this work is to determine the influence of catalyst loading on ORR. The molybdenum carbide (Mo₂C) derived carbon supported Pt (20wt% metal) nanoclusters activated electrodes with catalyst loadings from 0.1 to 1.0 mg cm^-2 have been tested in 0.1M KOH solution. Commercially available unmodified and modified carbons (Vulcan XC72^®, 20wt% Pt- Vulcan XC72^® ) were used as reference materials.

Catalyst ink was prepared by suspending various powders in the mixture of Milli-Q water, isopropanol and Nafion dispersion solution (Aldrich). The loading was varied by adding different volumes of the suspending agents to the catalyst powder. The catalyst ink was pipetted onto the glassy carbon disk electrode (GCDE) surface and dried at room temperature. The flat cross-section surface (geometric) area of the composite electrode was 0.196 cm². The loading of a catalyst was 0.1; 0.2; 0.4 or 1.0 mg cm^-2.

The electrochemical measurements were carried out in a three-electrode electrochemical cell. Electrochemical characteristics for catalysts were established by cyclic voltammetry (CV) and rotating disc electrode (RDE) methods. Electrochemical impedance spectroscopy was used to obtain the high-frequency series resistance (R_s at ac f → ∞), i.e. the electrolyte resistance of a system. It was demonstrated that in 0.1M KOH solution the iR drop compensation is significant in the stages of mixed kinetic. Therefore, all currents presented are corrected against the ohmic potential drop.

RDE data were measured at rotation rates from 0 to 3000 rpm (v=10 mV∙s^-1) within the region of potentials from +0.31 to -0.55 V. The potentials were measured against Hg|HgO|0.1M KOH reference electrode. CVs were measured at potential scan rates (v mV/s) 5, 10, 20, 30, 50, 70, 100, 150 and 200. All RDE data and CVs were measured in both Ar and O₂ saturated solutions. The solutions were saturated with Ar or O₂ between the each measurement, respectively. All measured current densities were corrected for current densities measured in 0.1 M KOH solution saturated with Ar.

The kinetics of the ORR is generally analyzed using a Tafel-like relationship, shown in Figure 1. The slopes of the Tafel-like plots have been obtained ranging from -65 to -90 mV dec^‑1 for various 20wt%Pt-C(Mo₂C) catalyst loadings. The 20wt%Pt-C(Mo₂C) catalyst showed noticeably higher activity towards ORR compared with that established for the 20wt%Pt-C(Vulcan XC72^®) electrode.

There are very well expressed reduction current peaks in the j_c, E‑curves for cathodic potential sweep direction. For all materials studied the reduction peak current density values (|j_peak|) increase nearly linearly with v^1/2 (characteristic of diffusion limited process). Reduction peak current density values (|j_peak|) increase with catalyst loading, shown in Figure 2 (at potential scan rate v = 30 mV/s). Same tendency has been observed at all potential scan rates.

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. Authors thank I. Tallo for performing the high temperature chlorination synthesis of C(Mo₂C). 

References

[1]K.Kinoshita, Carbon: Electrochemical and Physicochemical Properties; John Wiley & Sons: New York, 1988, pp 293-387.

[2]R.Jäger, P.E.Kasatkin, E.Härk, E.Lust, Electrochemistry Communications, 35 (2013) 97.

[3]R.Jäger, E.Härk, P.E.Kasatkin, E.Lust, Investigation of a carbon-supported Pt electrode for oxygen reduction reaction in 0.1M KOH aqueous solution, Journal of The Electrochemical Society, (submitted Manuscript #JES-14-0609) (2014)