Intrinsic Mechanical Effects on the Activation of Carbon Catalysts

Abstract The mechanical effects on carbon-based metal-free catalysts (C-MFCs) have rarely been explored although the C-MFCs have attracted worldwide interest as alternatives to the noble metal catalysts. Stress is everywhere, but a specialized study is strongly limited because the stress usually intermingles with other structural variables, including the dopants, defects, and interfaces in catalysis. Herein, we report a proof-of-concept study by establishing a platform to apply strain to a highly oriented pyrolytic graphite (HOPG) lamina continuously and collecting the electrochemical signals simultaneously. For the first time, the correlation between the surface strain of a graphitic carbon and its oxygen reduction reaction (ORR) activation effect is established. Results show that the in-plane and edge carbon sites in HOPG could not be further activated by applying tensile strain, but when the in-plane defects were involved in the structure, a strong and repeatable dependence of the catalytic activity on the tensile strain was observed, wherein ~ 35.0% improvement in ORR current density was realized by applying ~ 0.6% tensile strain. The density function theory (DFT) simulation shows that appropriate strain on the specific defect can optimize the adsorption of reaction intermediates, and the Stone-Wales defect on graphene correlates with the mechanical effect. Moreover, the effect was further authenticated by preparing a powdered graphene-based catalyst with varied strain-involved, which showed an apparent improvement of the ORR activity with ~ 0.4% surface strain. This work clarifies some basic principles of strain effects on graphitic carbon’s catalytic activities towards ORR, and may lay the foundation for developing carbon-based mechanoelectrocatalysis.