Relationships between texture, surface chemistry and performance of N-doped carbon xerogels in the oxygen reduction reaction

Abstract

Nitrogen-doped carbon xerogels (N-CX) are emerging materials for electrochemical technologies. Regarding this, the present work was focussed on studying the impact of texture and surface chemistry of N-CX on their electrochemical performance as metal-free electrocatalysts for ORR. Thus, a series of N-CX was prepared, followed by post-treatment with gaseous NH3. The electrocatalytic properties of N-CXs were studied as a function of the temperature applied during functionalization as well as the concentrations and types of oxygen and nitrogen functionalities in their structures. It was found that carboxylic acid groups were incorporated into the structure of N-CX during ball-milling prior to the electrode preparation. The electrochemical tests in basic medium revealed that the presence of these oxygen species enhances the ORR performance of these materials. Moreover, the diffusion-limited current density and the kinetic current were found to increase with the surface area of N-CX, while the onset potential and the number of transferred electrons were maximized by the highest ratio between quaternary (NQ) and pyrrolic nitrogen (N5) functionalities. An outstanding electrochemical performance was displayed by N-CX functionalized at 950 °C, which exhibited nearly a four-electron pathway for the ORR with a significant onset potential of 0.842 V.