(532cg) Investigation of Active Sites for Electrochemical Bromine Evolution Using Nitrogen-Doped Carbon Nanostructures

Bromine (Br₂) is widely used to produce brominated flame retardants, which are ubiquitous in the electronics and plastics industries. The U.S. and Israel account for 80% of the global Br₂ production. Despite its ever-increasing demand, conventional Br₂ manufacturing employs outdated techniques requiring hazardous chlorine gas as the bromide oxidizing agent or expensive Platinum (Pt) based electrodes for hydrogen evolution reaction (HER) on the cathode (while the Br₂ evolution reaction (BER) takes place on the anode). We proposed a novel approach for more economical Br₂ production, by replacing HER with oxygen reduction reaction (ORR) on the cathode, using nitrogen doped carbon nanostructures (CNx) on both electrodes.

Recently, we reported CNx as a robust BER catalyst in comparison to commercial Pt/C. CNx is also a promising ORR catalyst and several chemical probes have been used previously to provide insights into its active sites for ORR. In particular, the phosphate anion (PO₄^-), was found to be the sole probe that poisoned CNx for ORR. In this work PO₄^- has been used to distinguish the sites responsible for ORR and BER over CNx. It was found, that while the ORR performance degraded with increasing PO₄^- concentration, the BER performance remained unaffected. Pyridinic nitrogen or the positively charged carbon atom adjacent to it, is widely reported to be correlated with ORR activity. Previously we have shown that the pyridinic nitrogen concentration of CNx decreased with PO₄^- adsorption. However, the lack of PO₄^- poisoning in case of BER in conjunction with density functional theory (DFT) calculations, show that BER active sites differ from ORR active sites. Additionally, post reaction characterization using X-ray photoelectron spectroscopy, further elucidated the nature of BER active sites. This work aids in the rational design of inexpensive catalysts for sustainable electrochemical Br₂ generation at low temperature, eliminating the need for hazardous reactants.