Novel material outperforms standard catalysts in zinc-air batteries

Using thin sheets of a new trimetallic material as catalysts in zinc-air batteries enhances the batteries’ performance, increasing their energy density (Nano Lett. 2020, DOI: 10.1021/acs.nanolett.0c00717). Made of three common metals—nickel, iron, and manganese—the new material could be an alternative to precious-metal catalysts, making Zn-air batteries less expensive and thus allowing them to be used more widely. What’s more, the material retains its performance even when the battery, shaped into a fiber, is twisted.

“Our work may open a completely new paradigm of devising efficient electrocatalysts for various metal-air batteries,” says study coauthor Guoxiu Wang of the University of Technology Sydney. They could also be used in fuels cells and in reactions that split water to generate hydrogen, the authors say.

Zn-air batteries use a constant flow of oxygen from the air to oxidize a zinc electrode and generate current. During charging, the reverse happens: zinc is reduced, and oxygen is generated. The batteries have the potential to store energy at high density and are environmentally friendly, making them potentially useful in applications such as electric vehicles.

The batteries also contain catalysts to help increase the efficiency of the reactions involving oxygen. But it’s rare to find catalysts that can efficiently boost both the reduction of oxygen during discharge and the evolution of oxygen during charging.

Transition-metal nitrides are promising catalysts in metal-air batteries because they have high electrical conductivity and form different complexes easily. Nickel, iron, and manganese nitrides have shown good performance in catalyzing the oxygen evolution reaction, though their oxygen reduction rates are poor. Wang, Fengxia Geng of Soochow University, and colleagues speculated that a nitride made with the three metals should have better performance. Making such a trimetallic nitride into thin sheets would allow exposure of surface atoms and unsaturated coordination sites, greatly improving the oxygen reduction reaction.

But making thin sheets of NiFeMn nitride is next to impossible because the compound tends to form thick clumps. So to make stable sheets of the compound, the researchers mixed a titanium carbide (Ti₃C₂) and NiFeMn hydroxide. The two oppositely charged materials self-assembled to form a monolayer of hydroxide on a sheet of Ti₃C₂. Annealing in an ammonia atmosphere transformed the hydroxide into the nitride.

“The authors innovatively demonstrated a new method to well address the two problems simultaneously,” says Chunyi Zhi of the City University of Hong Kong who studies energy storage materials and was not part of the study.

The researchers then made a Zn-air battery using the new catalyst, which showed an energy density of 693 (W h)/kg, compared with about 550 (W h)/kg for Zn-air batteries today, and had good stability for 120 charge-discharge cycles. Compared with this, a standard platinum and ruthenium–based catalyst showed a large voltage gap between the charge and discharge curves, indicating poor stability, even at 60 cycles.

In addition, the thin sheets allowed the team to make a Zn-air battery in the form of fibers. The battery showed equally good performance when the fibers were twisted or bent. Three such fibers connected together powered light-emitting diodes and an electroluminescent panel, opening up possibilities for use in flexible electronics.