Atomic Layer Deposition of a Manganese-Iron Mixed Oxide As a Bifunctional Oxygen Catalyst for Zinc-Air Batteries
Authors: Labbe Matthew , Clark Michael Patrick , Cadien Ken , Ivey Douglas G 
Published: December 2020

Carbon-free energy sources will become a necessary step in order to mitigate the effects of climate change. In order to integrate these renewable energy sources, which are usually intermittent and not on-demand, into the power grid, a significant amount of energy storage will be required. The popularity of lithium-ion batteries as an energy storage option has exploded in recent years; however, a safer and less expensive alternative may be zinc-air batteries.
Zinc-air batteries combine zinc, an abundant and safe metal, with oxygen from the air. Using oxygen as a battery component, however, presents issues because of the slow kinetics of both oxygen reduction and oxygen evolution, the necessary steps in battery discharge and charge, respectively [1]. To compensate, catalysts are utilized that improve reaction kinetics for both the charge and discharge reactions. The benchmark in oxygen reaction catalysis is rare and expensive noble metals, such as platinum and ruthenium, which hinders practicality. On the other hand, inexpensive and abundant transition metal oxides have also shown promising catalytic activity towards these oxygen reactions [2]. Unfortunately, transition metal oxide catalysts that improve the reaction kinetics of the oxygen reduction reaction are typically not active towards the oxygen evolution reaction, and vice versa. However, a ternary oxide of two transition metals may result in a bifunctional catalyst, which is active towards both charge and discharge reactions.
The aim of this work is to produce a bifunctional, mixed oxide catalyst through atomic layer deposition (ALD). ALD employs self-terminating surface reactions of a metal-containing gaseous precursor and an oxygen co-reactant to produce very conformal and thin oxide films. Highly porous carbon paper, which typically serves as the air electrode in metal-air batteries, acts as the substrate for ALD of these transition metal oxide films. ALD can conformally coat the high surface area substrate with minimal loss in porosity. Furthermore, ALD can penetrate deeper into the pores of the electrode as compared with other deposition techniques. Together this allows ALD to yield high surface area nanostructured catalysts that maximizes the three-phase region between gaseous oxygen, liquid electrolyte, and the solid catalyst, thereby improving battery performance [3].
Previously studies have shown manganese oxides to be active towards the oxygen reduction reaction, while iron oxide is a component of many oxygen evolution catalysts [3]–[5]. Therefore, a recipe to deposit iron oxide films using ethylferrocene and an oxygen plasma is combined with an already established manganese oxide deposition procedure, with the ultimate goal of creating a mixed transition metal oxide catalyst. Electrochemical characterization techniques are employed to evaluate the bifunctional activity of synthesized transition metal oxides films. The atomic-level control of ALD means that the mixed oxide can be grown in various different proportions. An optimization process to establish the best ALD sequence is showcased, resulting in a bifunctional efficiency of 55% at 20 mA/cm2 for a 10 nm catalytic film.
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