Effect of electrochemical bromide doping on the performance of nitrogen-doped carbon nanostructures for oxygen reduction reaction

Abstract

Nitrogen-doped carbon nanostructures (CNx) are promising metal-free electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. A facile doping of bromide ions in the CNx structure by conducting bromine evolution reaction (BER) was shown to improve the ORR performance of these CNx catalysts. Electrochemical half-cell measurements demonstrated a significant enhancement in the ORR activity of CNx following BER treatment at > 1.07 V RHE . X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy were conducted to investigate the changes in CNx resulting in this enhancement in ORR activity. The Br 3d region of XPS confirms the formation of C-Br bonds and the incorporation of Br in the CNx structure during the BER treatment. The analysis of the N 1 s region in XPS and the N K-edge region in NEXAFS of BER-treated CNx shows an increase in pyridinic nitrogen content, which is believed to be a marker for ORR active sites. The increase in pyridinic nitrogen content was attributed to an increase in the defect density in the CNx structure during the BER treatment, confirmed by both Raman spectroscopy and NEXAFS results. The NEXAFS region corresponding to increased C-O functionalities during ORR on BER-treated CNx suggests that α-carbon neighboring the pyridinic nitrogen species could be the active sites for ORR on CNx. This study offers a facile electrochemical halogen doping strategy, which can be tuned for optimum ORR performance.