Kinetic Analysis of Oxygen Reduction Reaction on Different Self-Supported C-N-Me (Me = Fe, Co, Cu) Catalysts in Acidic Medium

Abstract

The use of Pt as preferred catalyst for the cathode of PEMFC is one of the bottlenecks to their widespread commercialization, due to its high cost and scarcity [1]. The development of non-precious metal catalysts with improved performance and durability represents a potential solution to Pt dependence problem. In this work, different Pt-free C-N-Me (Me = Fe, Co, Cu) electrocatalysts for oxygen reduction reaction (ORR) are synthesized using ordered mesoporous silica as hard template [2]. For catalysts synthesis, the silica template is mixed with a nitrogen containing molecule and a transition metal ion. Afterwards a heat treatment at 800 °C is performed under inert atmosphere. After etching with HF, the resulting material is a negative replica of the silica ordered mesoporous structure [2]. A kinetic study on different C-N-Me catalysts is performed using a rotating ring disk electrode (RRDE) setup (RRDE-3A ALS Model 2323) in acidic medium. Some important parameters of the ORR, such as Tafel slope, exchange current density, peroxide formation, oxygen adsorption, and reaction order for oxygen, are obtained. In addition, tests are performed at different temperatures, heating the RRDE electrochemical cell with a thermostatic bath, in order to determine the activation energies for the ORR, after appropriate corrections taking into account mass-transport limitation and the lower oxygen solubility at higher temperatures. For a purpose of comparison, tests have been performed for our C-N-Me catalysts and for a standard 20% Pt/Vulcan catalyst. From the Arrhenius plots obtained from the current densities measured at different temperatures in the kinetic control potential range , activation energies of 16.6 kJ mol-1 and 25.4 kJ mol-1were obtained for C-N-Fe and 20% Pt/Vulcan, respectively. These values are comparable with values found in the literature [3,4]. For Pt/C based materials, Tafel slopes suffer some variation at different temperatures [5], suggesting a change of reaction mechanism promoting the oxygen reduction reaction. This occurrence can be explained by coupling experimental data (kinetic parameters) with mathematical modeling (mass transfer, adsorption and charge phenomena) [6]. In the case of non-noble catalysts, this phenomena is not completely discussed until now, which is crucial to rational design more active electrocatalyst. The complete analysis of kinetic parameters coupled with analytical modelling, will lead to optimization of this type of catalyst and further cell-designing using Multiphysic Modelling. These data can be used as a starting point to be coupled with a mathematical model able to resolve this complex system and understand their electro-kinetic mechanism.