# Defect-Engineered High-Entropy Spinel Oxide@Onion-Like Carbon Catalysts for High-Areal-Energy Rechargeable Zinc–Air Batteries

## TL;DR

This paper introduces a new type of electrocatalyst for rechargeable zinc-air batteries that performs better than most existing options.

## Contribution

The novel contribution is the design and synthesis of defect-engineered high-entropy spinel oxide@onion-like carbon catalysts for efficient zinc-air batteries.

## Key findings

- The HESOx/OLCAT electrocatalysts achieved discharge areal energy densities exceeding the recommended threshold for real-world applications.
- Defect-rich HESOx/OLCAT outperformed most existing electrocatalysts for oxygen evolution and reduction reactions.
- The catalyst's performance is attributed to electronic modulation and weakened d-band centers of reaction intermediates.

## Abstract

Rechargeable zinc-air batteries (ReZAB) have emerged
as the next-generation
batteries with several advantages over the conventional lithium-ion
battery. In this work, single nanocrystals of inverse-type high-entropy
spinel oxides (HESOx, particle size of 10–12 nm) confined in
highly curved defective onion-like carbons (HESOx/OLCAT) as efficient electrocatalysts for oxygen evolution reaction (OER),
oxygen reduction reaction (ORR), and ReZAB, have been synthesized.
The HESOx materials were thoroughly characterized using several analytical
techniques, including X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), scanning transmission electron microscopy (STEM),
Raman, and electron paramagnetic resonance (EPR). HESOx/OLCAT catalyst was tested for ReZAB using literature-recommended parameters
that would allow for real technological application. These parameters
include a current loading of 10 mA cm–2 and a discharge
areal energy density of 35 mWh cmgeometric
–2, which maps a Li-ion battery pack-level specific energy of 120 Wh
kgpack
–1. HESOx/OLCAT electrocatalysts
allowed for continuous discharging and charging at a current loading
of 10 mA cm–2 with discharge areal energy densities
between 37 and 74 mWh cmgeometric
–2,
thus outperforming the recommended threshold of 35 mWh cmgeometric
–2. Considering that most studies (>90%) hardly
meet the recommended threshold for technological application of ReZAB,
the present work represents one of the top-performing electrocatalysts
for ReZAB. The excellent electrocatalytic properties of defect-rich
HESOx/OLCAT toward ORR/OER and ReZAB are governed by the
strong electronic modulation arising from d-π hybridization,
the availability of multiple catalytic sites for intermediates, and
weakened d-band centers of the rate-determining intermediates (i.e.,
*O adsorption for ORR and *OOH formation for OER) compared to the
pristine HESOx. This work introduces an effective approach for the
design and synthesis of single nanocrystals of high-entropy electrocatalysts
for the development of low-cost, robust, and technologically relevant
rechargeable zinc–air batteries.