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Atomic Layer Deposition of Metal Oxides for the Preparation of High Performance Zn-Air Battery Electrodes
- Author / Creator
- Clark, Michael P.
As the world continues to implement renewable energy technologies such as wind and solar, comes the demand for grid scale energy storage. Zn-air batteries (ZABs) are a promising candidate for grid scale energy storage because of their high energy density, low cost, high safety, and low environmental impact. Current generation ZABs, however, suffer from poor efficiency due to the sluggish kinetics at the air electrode. Despite their excellent activity, noble metal catalysts such as Pt are far too expensive for widespread use; this has driven the development of inexpensive transition metal oxide (TMO) catalysts. Due to their poor electrical conductivity, TMOs are typically nanostructured and/ or mounted on a conductive support.
Atomic layer deposition (ALD) is a thin film fabrication technique capable of producing highly conformal films of a wide range of materials. Because of the highly uniform and conformal nature of ALD films, ALD is particularly well suited for coating of highly porous structures such as the air electrode used in ZABs. The purpose of this work is to develop a process to prepare high performance ZAB air electrodes by depositing MnOx directly into the porosity of the air electrode by ALD.
The first study in this work included a thorough study of the saturation behaviour of bis(ethylcyclopentadienyl) manganese ((EtCp)2Mn) and water, with and without a forming gas (FG) (5% H2, 95% N2) plasma step. Contrary to previously published literature, the deposition using only (EtCp)2Mn and water (W-MnOx) did not experience saturating reactions; this deposition did not follow an ALD mechanism. A saturating ALD mechanism was achieved for depositions that used a FG plasma step between the (EtCp)2Mn and water doses (FG-MnOx), resulting in a growth per cycle of 1.15 Å/cy within the temperature range of 100 – 200 °C. Porous carbon electrodes were coated with MnOx following both recipes. Scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDX) line scans of electrode cross sections showed that the saturating mechanism FG-MnOx resulted in deposition deeper within the porosity than the W-MnOx. Electrochemical testing showed that the FG-MnOx also had improved electrochemical surface area as well as activity towards the oxygen reduction reaction; this is attributed to the better porosity coverage of FG-MnOx over W-MnOx.
The second study in this work involved the preparation and testing of MnOx coated electrodes in a full cell ZAB. Three types of electrodes were prepared in this work, FG-MnOx, FG-MnOx + CoOx, and O2-MnOx. FG-MnOx was prepared using the same procedure used in the first study and the FG-MnOx + CoOx sample was prepared by depositing CVD CoOx on top of FG-MnOx. O2-MnOx was prepared using an oxygen plasma and did not follow a saturating deposition mechanism. Scanning transmission electron microscopy (STEM) revealed that the gas diffusion layer (GDL) particles were successfully coated with a uniform layer of MnOx. Electron diffraction and x-ray photoelectron spectroscopy (XPS) were used to identify FG-MnOx and O2-MnOx as hausmannite, Mn3O4. Full cell ZAB tests showed excellent performance for MnOx coated electrodes, out performing Pt/Ru-C at current densities larger than 100 mA cm-2. FG-MnOx and O2-MnOx electrodes had maximum power densities of 170 and 184 mW cm-2, respectively. With the catalyst distributed within the structure of the GDL, performance limitations associated with electrolyte flooding and air diffusion are reduced, improving discharge potential and cycling behavior. FG-MnOx + CoOx electrodes showed good cycling stability, both in a tri-electrode configuration and bifunctionally. When cycled at 20 mA cm-2 for 100 h (200 cycles), FG-MnOx + CoOx had initial and final discharge potentials of 1.18 and 1.15 V, respectively.
- Graduation date
- Spring 2020
- Type of Item
- Doctor of Philosophy
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