Atomic Layer Deposition of Transition Metal Oxide Coatings for Oxygen Catalysis at the Air Electrode in Zn-Air Batteries
Creators: Labbe, Matthew R.
Publisher: name: University of Alberta Library
Year: 2024

<p>Energy storage is a critical step in fully adopting renewable energy, such as wind and solar, and eliminating carbon emissions from fossil fuel use. Electrochemical batteries are a versatile energy storage system and, as the ubiquitous battery of today, Li-ion batteries (LIBs) have begun to penetrate the market for grid-level energy storage. Coupled with the rising demand in electric vehicles and their associated battery requirements, there is a quest for new battery technologies that are more sustainable than the flammable, resource-limited, and expensive LIB. Zn-air batteries (ZABs) are one possible alternative chemistry, boasting higher theoretical energy density and safer operation than LIBs. Furthermore, Zn is inexpensive and abundant and the secondary reactant, oxygen, is freely available. Yet, ZABs are plagued by rechargeability issues at both the Zn and air electrodes. Specifically, the air electrode suffers from poor oxygen reaction kinetics and instability during cycling. One particular issue is flooding of electrolyte into the air electrode, which disrupts the necessary balance of oxygen, electrolyte, and electrons for the electrochemical reactions.<br>
The objective of this work was to develop catalysts for the air electrode in ZABs that improve the oxygen reaction kinetics during discharge and recharge and to apply them on the air electrode such that they provide stable cycling performance. Precious metal-based catalysts (e.g., Pt or RuO2) display good catalytic activity for the oxygen reactions but are impractical for widespread adoption of ZABs. Instead, this work focuses on readily abundant transition metal oxide catalysts using the elements of Mn, Fe, Zn, and O. Using atomic layer deposition (ALD), these transition metal oxide catalysts are deposited onto, and within, ZAB air electrodes. The uniform and conformal nature of ALD coatings ensures that the porosity of the air electrode is preserved, the amount of catalyzed reaction sites is maximized, and the distribution of catalytic material is well dispersed throughout the thickness and porosity of the electrode. These attributes improve the utilization of catalyst material and mitigate the effects of flooding on ZAB performance. <br>
The first study in this work developed an ALD process to deposit Fe oxide (FeOx) coatings on the air electrode in ZABs, which was shown to be a saturating and conformal ALD process using atomic force microscopy and in situ spectroscopic ellipsometry (SE). Electron microscopy and energy dispersive X-ray analysis revealed that a thin film of FeOx encased the carbon particles of the air electrode and distributed the catalyst more than 10 µm within the air electrode porosity. Using half cell electrochemical testing, the FeOx coating showed promise as a catalyst towards the recharge reaction at the air electrode. <br>
The second study further analyzed the morphology and growth characteristics of the FeOx ALD process on the carbon air electrode surface. Instead of the anticipated pinhole-free layer-by-layer growth signature to ALD, a series of island layers were revealed by electron microscopy. Pinholes between coalesced islands were visible and appropriate modelling of SE data determined that six layers of islands formed over a period of 650 ALD cycles. <br>
The third study in this work combined the FeOx ALD process developed in this thesis with a previously developed Mn oxide (MnOx) ALD process from earlier work in our group. Based on electron diffraction and X-ray photoelectron spectroscopy, the optimized ALD supercycle process deposited spinel-type (Mn,Fe)3O4 coatings on the carbon particles of the air electrode. This catalyst layer showed bifunctional activity towards both the charge and discharge reactions in half cell and full cell electrochemical testing. In full cell ZAB cycling, the ALD coating maintained stable performance for 600 h (1565 cycles) at 10 mA cm-2. The retention of bifunctional energy efficiency after cycling was 84% for the ALD coating and only 66% for a precious metal comparison under the same cycling conditions. <br>
For the fourth study, an ALD process for Zn oxide was integrated with the other three ALD processes for FeOx, MnOx, and (Mn,Fe)3O4. Mixed oxide films were successfully developed and deposited on ZAB air electrodes, however, they did not generally improve ZAB performance compared with their non-Zn counterparts aside from some improvement for the FeOx coating after Zn addition.</p>