by Patrick Schwager
Since the gradual replacement of conventional, fossil energy sources by renewable energies, which strongly depends on environmental conditions, there have increasingly been calls for high-power energy storage devices. Compared to lead batteries, redox-flow batteries, Nickel-based systems or state of the art Lithium-ion batteries, Lithium-air batteries are of particular interest due to its outstanding energy density. The functionality is based on the oxygen reduction reaction (ORR) within a gas diffusion electrode (GDE). Oxygen is reduced to O2•- which coordinates with Li+ to form LiO2•- . On the one hand this intermediate can adsorb at the electrode surface and disproportionate or undergoes a second reduction step to form the final solid ORR product Li2O2 which is suspect to clog the GDE thus reduces the devices capacity. On the other hand LiO2•- is preferably dissolved up to a certain extent in the electrolyte. This strongly depends on the solvents properties and can increase the performance of the battery .
The focus of interest of our research group lies on electrochemical processes within the GDE. The mechanisms of charging and discharging in organic electrolytes are investigated using positionable microelectrodes. Short-lived intermediates as well as the mass transport in porous gas diffusion electrodes are characterized by means of SECM (scanning electrochemical microscopy). In our approach we developed a setup to monitor the oxygen diffusivity and permeability of the gas diffusion electrode. A microelectrode is positioned in a micrometer range distance above the gas diffusion electrode to detect oxygen. Since no steady-state current for oxygen reduction is reachable in Li-containing electrolytes a pulsed potential is applied to the microelectrode to eliminate the effect of surface passivation . Furthermore this technique is used to monitor the clogging of the gas diffusion electrode while discharging (ORR).
(*) To calculate the charge and energy density only active material of anode and cathode were considered. Furthermore a porosity of 70% and a maximum of pore filling by the discharge product of 50% were assumed for the GDE.
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