Electron Transfer Kinetics on Non-Precious Group Metal Catalyst in Non-Aqueous Electrolytes for Li-air Batteries.

摘要

For a clean, sustainable, and secure energy future, rechargeable Li-air batteries attract a great deal of interest for the next generation energy storage and conversion devices because of their extremely high theoretical energy density (up to 2--3 kWh/kg). This technology promises to bring the electrical vehicles to the market with a car that can travel 500 miles range on a single charge, which is roughly 10 times higher than current state of the art Li-ion batteries. However, the practical realization of this technology is still hindered by many challenges. One of the most challenging tasks in developing Li-air batteries is the lack of fundamental understanding of oxygen reduction reaction (ORR) and oxygen evaluation reaction (OER) on the oxygen electrode, especially when catalysts are used. The role of the catalytic material and the electrolyte must also be understood for improving the stability, kinetics, and activity of the batteries. In addition, for Li-air batteries to be economically competitive, inexpensive non-precious group metal catalysts must be developed.;Chapter 1 provides an introduction to current state of the art energy storage devices and their energy density comparison with Li-air batteries. An introduction to Li-air batteries, and the limiting factors for the performance of a reversible Li-air battery is provided. In light of the hard soft acid base (HSAB) theory, the effect of the electrolyte composition on battery performance is discussed. Inner & outer sphere electron transfer kinetics is discussed for supported catalytic systems. This chapter also covers the literature review on the non-precious metal catalysts for Li-air batteries.;Chapter 2 discusses the performance of a Fe-based metal-organic framework (MOF) catalyst for non-aqueous Li-air batteries. The synthesis and characterization of the Fe-based MOF catalyst are described. The effect of the MOF catalyst on the electron transfer kinetics for ORR studied in two different non-aqueous solvents, namely dimethyl sulfoxide (DMSO), tetraethylene glycol dimethyl ether (TEGDME) are described. The results indicate that, in high-donor number solvents such as DMSO, the solvent acts as a catalyst and promotes outer sphere electron transfers. However, in the presence of catalyst the system promotes inner sphere electron transfer occurring concurrently with outer sphere transfer via Fe active centers of the catalyst. This slightly increases the ORR activity of the system. On the other hand, in low-donor number solvents such as TEGDME, the catalyst significantly changes the ORR activity from the inner Helmholtz layer. These results are discussed according to hard soft acid base (HSAB) theory. In addition, a three-electrode electrochemical cell was designed, fabricated and used for the in-situ Raman spectroscopy observation of reduction products. The reduction products such as Li2O2 and LiO2 and the active center of the catalyst were identified in DMSO.;Chapter 3 provides a conclusion and offers insights toward future directions for the investigation of the non-precious catalysts for Li-air battery applications and their in-situ spectroscopic observation.