Pomegranate-Inspired Design of Highly Active and Durable Bifunctional Electrocatalysts for Rechargeable Metal-Air Batteries
Authors: Li Ge , Wang Xiaolei , Chen Zhongwei 
Published: February 2020

The fast depletion of fossil fuels and severe deterioration of ecology have stimulated extensive research on the utilization and storage of clean and sustainable energy. Among various energy storage technologies, rechargeable metal-air batteries possess the highest energy density, making them excellent candidate for next generation electrical vehicles (EVs) and hybrid electrical vehicles (HEVs). In metal-air batteries, a metal anode is coupled with an air-breathing cathode that utilizes oxygen from the atmosphere as the reactant for the electrochemical reactions. Discharging and charging processes occur driven by the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reduction (OER). The major challenge associated with the commercialization of metal-air batteries resides in the sluggish kinetics of the ORR and OER resulting in large overpotentials. Therefore, developing efficient electrocatalyst with high catalytic activities is of great importance for high performance metal-air batteries.
Precious metal-based catalysts such as platinum (Pt), palladium (Pd), iridium (Ir) and alloys have been intensively studied showing superb catalytic properties and have been widely accepted as the most active electrocatalysts. Unfortunately, the scarcity and electrochemical instability of these catalysts have prevented their widespread use due to extremely high costs and unsatisfactory durability. Nonprecious transition metals (e.g. Fe, Co, Ni, Mn) and perovskite-based catalysts have been extensively explored. However, they suffer from inefficient catalytic activity due to self-accumulation and intrinsically poor electrical conductivity. To address these challenges, nanostructured materials with various morphologies (e.g. nanoparticles, nanowires, nanosheets, nanotubes) have been designed for effective exposure of catalytic planes and enhanced diffusion of reactive species, while conductive carbon materials (e.g. carbon nanotubes (CNTs) and graphene) are often introduced into catalysts to increase the conductivity and structural stability. For example, Co3O4 nanocrystals/reduced graphene oxide bi-functional catalyst exhibits unusual catalytic activity attributed to the synergetic chemical coupling effects. Despite numerous studies, rational design strategy and efficient development of desired high-performance electrocatalysts is yet limited. An efficient electrocatalyst is expected to (i) exhibit high catalytic activity with a large amount of active sites for ORR and OER processes; (ii) possess sufficient mass transfer pathways for fast electrode kinetics; and (iii) be chemically stable with robust material and/or electrode architecture for high durability.
Herein, we demonstrate the development of pomegranate-like electrocatalysts based on transition metal oxide nanocrystals embedded nitrogen-doped partially graphitized carbon framework with excellent catalytic activity for ORR and OER and outstanding durability. Nitrogen-doped has been reported to remarkably accelerate OER process by participating in the electrocatalytic reaction. To demonstrate the design concept, cobalt oxide (Co3O4) is chosen as a model material due to its abundance and theoretically high electrocatalytic activity. The Co3O4 nanocrystals embedded nitrogen-doped partially graphitized carbon framework (Co3O4/NPGC) with unique pomegranate-like composite architecture provides several major advantages: (i) low dimension of highly active Co3O4 nanocrystals seeds possess active sites for electrochemical reactions; (ii) the pomegranate-like structure efficiently prevents the metal oxide from self-accumulation and provide the mass transfer pathways, which further maintains the catalytic activity; (iii) graphitized carbon shell and framework is not only highly conductive which significantly increases electrical conductivity, but also chemically stable and highly robust which enhances the catalyst durability. Benefiting from the unique pomegranate-like architecture, the Co3O4-based composite electrocatalyst exhibits a high half-wave potential of 0.842 V for ORR, and a low overpotential of only 450 mV at the current density of 10 mA cm-2 for OER. Single-cell zinc-air battery is also fabricated with superior durability, holding great promise in the practical implementation of rechargeable metal-air batteries.

Figure 1