New Oxygen Electrocatalysts Enable High-Performance Zinc-Air Batteries
by Emily Lewis
Materials Research Society | Published: 18 July 2013
Zinc-air batteries, which operate by reducing oxygen from the air and oxidizing a zinc anode, have been suggested as a promising energy storage technology. These cells are built from earth-abundant materials, and because air is one of the reactants, they produce more energy by weight than several other batteries currently in use, including lithium ion batteries. One of the limitations in the development of zinc-air batteries is the activity, durability, and cost of catalyst materials for the oxygen electrode. A team lead by Stanford’s Professor Hongjie Dai have now overcome some of these limitations by applying their groundbreaking air electrocatalyst technology to the development of new zinc-air batteries with outstanding energy output and stability.
The batteries employ a new oxygen reduction catalyst in the cathode, which consists of cobalt oxide nanoparticles that are anchored to nitrogen-doped carbon nanotubes. Prof. Dai explains that the non-precious metal catalyst is able to overcome “sluggish oxygen electrochemistry,” the major hurdle for zinc-air batteries, which is due to the high barrier of reducing oxygen. The team attributes the catalyst’s performance to the intimate chemical and electrical coupling between the cobalt oxide and the carbon nanotubes; this coupling is due to the covalent bonding of the cobalt oxide to oxygen functional groups on the nanotube surfaces. Prof. Dai states that the importance of the coupling is two-fold as it “can afford an optimization of both the activity and durability of the oxygen reduction catalysts.”
With this catalyst, the group has produced both primary (non-rechargeable) and rechargeable zinc-air batteries with excellent performance. In primary configuration, the cell outperformed the benchmark platinum catalyst and exhibited a ~15% increase in maximum power output over previous non-precious metal catalysts. The battery also demonstrated 22 hours of discharge without loss and provided twice the total energy per weight than current lithium-ion technology.
The rechargeable cell was configured with three electrodes, and employed a different catalyst for discharging (oxygen reduction) and charging (oxygen evolution). A nickel iron layered double hydroxide catalyst was used for the oxygen evolution reaction, as it had a higher activity for water oxidation to produce oxygen. Separating the oxygen reduction and oxygen evolution catalysts on two electrodes prevented degradation of the cobalt oxide-carbon nanotube catalyst at the high recharging voltage. Due to the stability and activity of both catalysts in this system, the batteries were found to operate at 65% efficiency for the full charge-discharge cycle (compared to previous reports of 50%) and were shown to operate without activity loss over hundreds of hours.
According to Prof. Dai, primary zinc-air batteries “could power electric vehicles to compete with current lithium-ion technology.” It has been previously suggested that electric vehicles can be powered by primary zinc-air batteries, which can quickly be refueled with the addition of fresh zinc to the cell. Although the secondary batteries still face problems, such as a limited zinc anode cycle life, he asserts that the development of advanced oxygen catalysts is an important step in making these cells commercially viable.
Read the abstract from Nature Communications here.
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