Metal-air Batteries: RRDE and EC-SPM Studies of Electrode Kinetics and Electrode Structure

Abstract

Metal air batteries are the energy storage devices, which possess the advantages of low cost, non-toxic, non-polluting, high theoretical specific energy, etc. Consequently, they received extensive attention and are considered as a special kind of fuel cell (FC) and one of the representatives of a new generation of the green secondary batteries. In this field, most of the works are focused on the study of oxygen reduction reaction and oxygen evolution reaction at the cathode and the selection and optimization of electrolyte composition. However, it is still far from satisfactory for commercialization, because the most efficient catalysts still consist of noble metals, such as platinum, gold, palladium, etc. and there are still many problems in nonaqueous electrolyte, such as the instability of the electrolytes, uncontrollable side reactions, poor reversibility of the ORR/OER, etc. Therefore, it is necessary and vital to make a further study and exploration to promote their development.<br /> In alkaline electrolyte, we found Co₃O₄/Ag and Co/Ag catalyst demonstrating a synergistic effect that the catalytic activity of Ag and Co₃O₄ or Co for ORR and OER, respectively, is enhanced by each other. Therefore, Co₃O₄/Ag and Co/Ag bimetallic catalyst possessing a bifunctional catalytic activity and higher stability for both ORR and OER in alkaline electrolyte could be used as an excellent catalyst for ORR and OER in metal-air batteries.<br /> In nonaqueous electrolyte, ORR and OER were also investigated in Li⁺ and Mg²⁺-DMSO based electrolyte. The mechanism of these reactions and the morphology of their products on Au and Pt electrodes were studied by cycle voltammetry technique including RRDE and EC-SPM technique, respectively.<br /> Furthermore, we investigated the insertion of Mg into Sb in 0.5 M MgCl₂ + 0.5 M AlCl₃ in tetraglyme based electrolyte. It was found that the insertion of Mg into Sb starts 320 mV earlier than Mg deposition. The formed component was determined to be Mg₃Sb₂ according to the mole ratio of the Sb pre-deposited on Au electrode and the inserted Mg (~3:2). The diffusion coefficient of Mg in Sb layer was calculated to be 4.7×10^-14 cm² s^-1. Moreover, the electrochemical deposition behavior of Sb on Au(111) was studied by cyclic voltammetry and EC-STM. A double row structure was observed on the Sb bulk adlayers formed on the smooth Au(111) surface. However, this bulk adlayer structure is dependent on the roughness of the surface and it will tend to particle-like structure formation on the rough surface. We also investigated Mg deposition and dissolution in nonaqueous electrolyte. We obtained that the Coulombic efficiency is very poor, especially at initial cycles, and it increased with the increase of cycle number. In addition, in MACC/tetraglyme the particle-like deposits on Au(111) surface was observed by EC-STM and these deposits could be dissolved almost completely under a strictly controlled argon atmosphere condition. It is also found that all these electrolytes are very sensitive to air.