Applying metal-organic frameworks (MOF) in electrochemical systems is a currently emerging field owning to the rich metal nodes and high specific surface area of MOF. However, the poor electrical conductivity and low ion transport of MOF hinders their widespread application. 

Encapsulating nanoparticles into hollow mesoporous carbon sphere (HMCS) is a classical design helpful to stabilize catalytic active sites, increase electrical conductivity, and reduce mass transport length. It is believed that the elaborate combination of MOF with HMCS to construct a yolk-shell structured hybrid material will effectively overcome the shortcoming of MOF in electrocatalysis field. 

In a study published in National Science Review, the research team led by Prof. CAO Rong and LI Hongfang from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences designed a yolk-shell structured ZIF-67@HMCS hybrid material by using ZIF-67 as core and HMCS as shell. 

The researchers well controlled the particle size of ZIF-67 by utilizing the spatial confinement effect from HMCS, which shortens the diffusion path and enhances the ion transportation. Encapsulating ZIF-67 in HMCS also increases its conductivity prominently.  

Moreover, the typical hierarchical pore structures of HMCS guarantee the diffusion of reactive species to the exposed active sites of ZIF-67 quickly and efficiently, and thus improve the electrochemical activity.  

The ZIF-67@HMCS hybrid material exhibits superior bifunctional electrocatalytic activity towards both ORR and OER. What’s more, the assembled Zn-air battery by ZIF-67@HMCS as air-cathode also presents impressive performance and long-term stability. 

This study paves a new way toward designing stable MOF used directly as high efficiency electrochemical catalysts in promising energy storage devices to meet the growing demand of stable energy supply. 

The synthetic procedure and images for ZIF-67@HMCS hybrid material. (Image by Prof. CAO’s group)