Chemical changes from N-doped graphene and Metal-Organic Frameworks to N-G/MOF composites for improved electrocatalytic activity

Abstract

Integrating N-doped graphene (N-G) with Metal-Organic Frameworks (MOFs) enhances catalytic activity for oxygen reduction reaction (ORR), often exceeding both the performances of their precursors and, in some cases, even Platinum group metal (PGM)-based catalysts. However, the factors driving this improved catalytic activity in N-G/MOF composites remain unexplored, particularly from the perspective of the chemical changes. To investigate the chemical changes in N-G and MOF upon their integration and the implications of these changes on ORR catalytic activity, an N-G/MOF was synthesized from N-G with a MOF (ZIF-8) following a mechanochemical wet ball milling process. The N-G, ZIF-8, and N-G/MOF samples were examined for changes in elemental composition, chemical state of carbon, different nitrogen and carbon bonds, and other chemical interactions. In the N-G/MOF catalyst, compared to its N-G and ZIF-8 precursors, the relative oxygen content increased, indicating the formation of additional oxygen-containing groups. The C 1s peak shifted to a lower binding energy in N-G/MOF, suggesting changes in the overall chemical or oxidation state of the carbon atoms. Besides, the increase in pyridinic-N functional groups in N-G/MOF points to the formation of additional active sites. Furthermore, the formation of C–Zn bonds in N-G/MOF suggests the probable emergence of single-atom Zn sites, while the increase in CO bonds points to the formation of carboxyl or carbonyl groups. These chemical changes could be linked to the enhanced electrocatalytic activity of the N-G/MOF composite for ORR. This study may also be beneficial for other research focused on developing composite catalysts involving various N-G and MOFs-based materials.