Nanostructured Spinel Oxides as Bi-functional Electrocatalysts for Rechargeable Metal-air Batteries

Download or read book Nanostructured Spinel Oxides as Bi-functional Electrocatalysts for Rechargeable Metal-air Batteries written by Dong Un Lee and published by . This book was released on 2017 with total page 125 pages. Available in PDF, EPUB and Kindle. Book excerpt: Due to continuously increasing energy demands, particularly with the emergence of electric vehicles (EV), smart energy grids, and portable electronics, advanced energy conversion and storage systems such as fuel-cells and metal-air batteries have drawn tremendous research and industrial attention. Even though the lithium-ion battery technology is the most developed and widely distributed energy device for a wide range of applications, some researchers view its energy density insufficient for fulfilling the ultimate requirements of highly energy intensive applications such as EVs. Recently, zinc-air batteries have re-gained research attention since the initial development in the 1970s due to their remarkably highly energy density and the potential to be electrically rechargeable. However, some technological hurdles such as low charge/discharge energy efficiency, and insufficient cycle stability have hampered commercialization and introduction of rechargeable zinc-air batteries to the market. The mentioned hurdles are currently the main challenges of rechargeable zinc-air battery developed, and they stem from the fact that the reaction kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are intrinsically very sluggish. The two are the main electrochemical reactions that govern the charge and discharge processes of a rechargeable metal-air battery at the air electrode, and these oxygen reactions must be facilitated by active electrocatalysts in order to progress them at practically viable and stable rates. Currently, the best known catalysts for ORR and OER are carbon supported platinum (Pt/C) and iridium (Ir/C), respectively. However, the use of these precious metal based catalysts for large scale applications like EVs and energy storage systems is prohibitively expensive. Additionally, the durability of these catalysts have been reported to be insufficient for long-term usage under normal device operating conditions. Perhaps most importantly, the precious metal based catalysts are strongly active towards only one of the two oxygen reactions required for rechargeable applications. For example, Pt/C is a strong ORR active catalyst, while Ir/C is a strong OER active catalyst. Recently in the literature, a simple physical mixture of these two catalysts have been used to render bi-functionality, but this method is very rudimentary and still requires two separate syntheses for each catalyst. This suggests that future bi-functionally active catalysts must not only be non-precious (inexpensive), but also a single active material capable of catalyzing both ORR and OER over the same active surface. Having said above, non-precious catalyst research, specifically for bi-functional ORR and OER electrocatalyses, has increased dramatically beginning in the 90's with a very popular and positive belief in the energy community that rechargeable lithium-air batteries could potentially replace lithium-ion batteries. This wave of interest has also picked up research in rechargeable zinc-air batteries since the electrochemical oxygen reactions that take place at the air electrodes are fundamentally very similar. Additionally, the use of zinc metal as the anode, which is one of Earth's most abundant elements, and the water-based (aqueous) solutions as the electrolyte (as opposed to organic ones) made the rechargeable zinc-air battery development very attractive and seemingly easy to scale-up. Moreover, primary (non-rechargeable) zinc-air batteries have already been commercialized and are available in the market as hearing aid batteries, leading many researchers to believe that a simple tuning of the current technology would lead to a successful secondary (rechargeable) zinc-air battery development. However, there are a set of technical difficulties specific to rechargeable zinc-air batteries that have slowed the development for the past few decades. Therefore, the work presented in this thesis aims to address the challenges of rechargeable zinc-air batteries particularly from the active bi-functional electrocatalyst standpoint to make them as commercially viable as possible.