Influence of Temperature on the Oxygen Electroreduction Activity at Nanoporous Carbon Support

The main requirements for PEMFC supports and catalysts are the high redox sustainability, energy and power densities, high efficiency and low/zero emission of contaminants. As the working temperature of the PEMFC is around 80^oC, the synthesized catalyst materials should be tested in the similar conditions. Therefore, the influence of temperature on the O₂ electroreduction characteristics has been studied and discussed.

For analyzing the effect of temperature on the electrocatalytic activity of the various catalysts, the polarization curves were recorded in O₂-saturated 0.1 M KOH solution using rotating disc electrode (RDE) method.

The non-conventional nanoporous carbide derived carbon powder, synthesized from molybdenum carbide (Mo₂C) at 750°C using the high-temperature chlorination method, has been used as the electrocatalyst support for oxygen electroreduction.

The electrochemical measurements were carried out in a three-electrode electrochemical jacketed glass cell connected to circulating temperature controlled water bath. Several important reaction parameters, such as the number of electrons involved in the ORR, the slope values of Tafel-like plots at different temperatures, and the kinetic rate constant for the ORR, were obtained from the RDE measurements.

Electrode potentials were measured against Hg|HgO, 0.1M KOH reference electrode. RDE data were measured at rotation rates from 0 to 3000 rpm (v=10 mV∙s^-1) and within the region of potentials from +0.31 to -0.71 V vs. Hg|HgO, 0.1M KOH. Cyclic voltammograms 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 solution. The solutions were saturated with Ar or O₂, respectively, between measuring of each voltammogram.

Due to the different temperature, the ohmic drop corrections have been made. Electrochemical impedance spectroscopy was used to obtain the high frequency series resistance (Rs at ac frequency f → ∞), i.e. the electrolyte resistance of a system. The solution resistances measured at 10, 22, 40, 60, 80 ^oC were 47, 37, 30, 23, 17 ohms, respectively, The respective resistance value was used to correct the measured potentials against ohmic potential drop in the solution phase between the electrode surface and the tip of the Luggin capillary.

The influence of temperature is significant within the region of quick increase of current density, thus, within the region of mixed kinetics - where both the charge transfer and diffusion rates are the rate limiting steps. Due to the increase of temperature, the diffusion coefficient values increase but the kinematic viscosity of the solution and the oxygen concentration decrease.

For all electrodes and at all temperatures, two different linear regions in Tafel-like plot have been observed. The slope value of Tafel-like plot increases with increasing temperature.

Acknowledgements: This work was supported by the European Spallation Source Project: Estonian Partition in ESS Instrument design, development and building and application for scientific research: SLOKT12026T; the Estonian target research: IUT20-13; the Estonian Centre of Excellence in Science: TK117T "High-technology Materials for Sustainable Development"; the Estonian Energy Technology Program: SLOKT10209T and the Materials Technology Project: SLOKT12180T.

 

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