Elementarkinetische Modellierung der Sauerstoffelektrode im wässrigen Elektrolyt für Metall-Luft-Batterien

Abstract

The oxygen reduction reaction (ORR) is used in many different applications such as sensor technolo-gy, fuel cells or metal-air batteries. It is necessary to get a better understanding of its elementary reac-tion steps to reduce reaction overpotentials and enable the commercialization of rechargeable metal-air batteries. Several mechanisms of the ORR have been proposed in the literature. Therefore, we begin by review-ing some reaction mechanisms. Then we concentrate on two distinct reaction mechanisms in aqueous solution, one valid in acidic and one valid in alkaline solution. The parameters for our elementary ki-netic models are taken from published density functional theory (DFT) calculations on the surface Pt(111). The mechanism in acidic media [1, 2, 3] distinguishes between three pathways (OO, OOH and HOOH pathway). The alkaline reaction mechanism [4] consists only of one pathway ( path-way) comparable with the OOH pathway in acidic media. To make the alkaline reaction mechanism more general we add the OO pathway. From binding and activation energies we calculate the parameters needed for our elementary kinetic simulations. A coverage dependence of binding energies is added based on literature data [5]. Elemen-tary kinetic simulations were carried out with the simulation tool DENIS (Detailed Electrochemistry and Numerical Impedance Simulation). It includes detailed electrochemistry based on thermodynamic consistency. We treat the species adsorbed on active surfaces of the electrodes with mean-field theory. Our results from elementary kinetic modeling of the reaction mechanism on Pt(111) in acidic media show that for high cell potentials the ORR reacts via the OO pathway. With decreasing potential the ORR proceeds via the OOH pathway which is followed by the HOOH pathway for low potentials. This leads to the working range of the ORR on Pt(111) between 0.9 and 0.4 V. In this potential range we can observe a significant current density; the undesirable HOOH pathway where oxygen is not completely reduced to water does not take place. After discussing these reaction mechanisms on Pt(111), we extent the general reaction mechanism in acidic media to investigate the influence of different metal catalysts (Ag, Cu, Ir, Ni, Pd, Pt, and Rh). Thermodynamic data for these catalysts is taken from the literature [6]. Comparing the activities of different metals for different cell potentials, our simulations show that Cu, Pd and Pt are the best cata-lysts. The reaction mechanism proposed for alkaline media is used to investigate the ORR for different cell potentials and the discharge of a metal-air battery. The mechanism is implemented in a detailed model of a lithium-oxygen battery including diffusion, real mixture behavior, and the precipitation of lithium hydroxide monohydrate (LiOH H2O) (parameters from [7]). Our results show that only the pathway takes place. For low potential the surface is completely covered with *. We observe that overpotentials are smaller in alkaline than in acidic media.