New approach in catalyst design for electrochemical energy conversion

“The results confirmed that nano-scale mass transfer channels, which can be in situ constructed during the catalyst synthesis, can boost the catalytic activity of the catalyst although the active sites in the catalyst was decreased”Weiwei Cai

Researchers from the China University of Geosciences in Wuhan, China, with colleagues from Liaocheng University and Chongqing University of Arts and Sciences, have designed a strategy for facile selectively etching to ensure in situ adjusting of the mass transfer structure of a cobalt-iron (CoFe) catalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The graphene-encapsulated CoFe nanoclusters with relatively few graphene layers were etched to form hollow graphene cages and acted as mass transfer channels during electrocatalytic processes.

As reported in the journal Materials Today Energy [Wu et al. Mater. Today Energy (2020) DOI: 10.1016/j.mtener.2020.100438], CoFe encapsulated in a graphene cage was found to be highly active as a bi-functional catalyst, and that this mass transfer structure would have a major impact on the performance of zinc-air batteries (ZAB), which also exhibited three times the power density of noble metal catalysts. In addition, this metal-air battery could be a useful secondary battery technology due to its significant energy density.

In electrochemical energy conversion, mass transfer structure design has usually been conducted during fabrication of the electrode. However, by adjusting the acidic treatment condition, CoFe nanoclusters with relatively few graphene layers coated were selectively etched to form hollow graphene cages. These nanoclusters acted as catalytic active sites while the graphene cages acted as mass transfer channels during the ORR/OER processes.

Since the performance of energy conversion devices such as metal-air batteries, electrolyzed water and fuel cells are affected not only by catalyst activity but also the mass transfer structure of the electrodes, the team wanted to optimize the mass transfer structure in nano-scale during the catalyst design.

A problem with such catalysts used in the air-electrode, however, is that ZABs can underperform in terms of both power density and stability, so these new cost-effective catalysts with carbon-based materials offer the most promising electrocatalytic performance. As team leader Weiwei Cai said, “The results confirmed that nano-scale mass transfer channels, which can be in situ constructed during the catalyst synthesis, can boost the catalytic activity of the catalyst although the active sites in the catalyst was decreased”.

The team now hopes to demonstrate how hierarchical mass transfer channels can be designed and fabricated during the catalyst design to combine with the micro-scale mass transfer channels, which can be controllably designed during the electrode fabrication. They also hope to improve the power density and stability by exploring other active sites than CoFe for the bi-functional catalyst development with facilely constructed mass transfer structure.