Dual alkali-carbonate activated nitrogen-doped carbons as oxygen reduction reaction electrocatalysts: a study of porosity structure effects

Abstract

Nitrogen-doped carbons with consistent N-content and N-functionality distribution but variable porosity and surface area were prepared and studied to evaluate textural property effects on oxygen reduction reaction (ORR) electrocatalysis independent of chemical composition. By employing various characterization techniques, the similar N-content (1–2.5 at%) and N-motif distribution were confirmed between samples, as was variability in porosity and surface area. From electrochemical testing, a larger micropore volume associated with pores < 1 nm (>0.25 cm³ g⁻¹) and surface area (>approximately 1000 m² g⁻¹) correlated with increased onset potential (E_onset), half-wave potential (E_1/2), and an electron transfer number, n, of approximately 4, wherein the most significant effect of porous structure on electrochemical performance was observed for E_1/2. With respect to E_onset and n, more subtle effects were observed regarding micropore volume (<1 nm) and surface area, as the kinetics of the ORR are predominantly tuned by the presence and type of active N-sites. An increased micropore volume and surface area realized slight increases in E_onset and n owing to improved access to these active sites. Thus, textural properties primarily affect mass transport for ORR electrocatalysis with effects on kinetic parameters only arising regarding accessibility to active sites located within micropores.