(509bl) Sustainable Bromine Generation By Electrochemical Halogen Evolution Using Nitrogen-Doped Carbon Nanostructures

Recently, electrochemical halide oxidation processes have garnered much attention to replace commercial methods for halogen production. Currently, large scale bromine manufacturing requires transportation and handling of hazardous chlorine gas as the bromide oxidant making this process unsafe, thereby creating the need for a novel approach for bromine evolution. Traditionally, electrochemical bromine production involves bromine evolution reaction (BER) on the anode while hydrogen evolution reaction (HER) occurs on the cathode. However, high energy requirements and expensive noble metal cathode catalysts make this process energy intensive and economically unattractive. Therefore, we propose replacing HER with oxygen reduction to water at the cathode i.e. using oxygen depolarized cathodes (ODCs) to increase energy savings of electrocatalytic bromine evolution, based on the lowered thermodynamic cell potential. This requires active ORR electrocatalysts, tolerant to corrosive electrolytes and halide poisoning from ions which may cross over from the anode. To that end, we report nitrogen-doped carbon nanostructures (CN_x) as robust platinum group metal free electrocatalysts for ORR and BER in acidic medium.

For ORR (cathode reaction) conducted in presence of bromide ions, CNx exhibited superior stability compared to 10% Pt/C demonstrating high resistance to halide poisoning. For BER (anode reaction), CNx showed higher current densities than 10% Pt/C even at low bromide concentration at low overpotentials. CNx has also been shown to be stable using chronoamperometry under high oxidative potentials up to 18 hours without carbon corrosion. Post reaction characterization using X-ray photoelectron spectroscopy, and Raman spectroscopy in conjunction with computational density functional theory (DFT) calculations are used to elucidate the nature of BER active sites and propose the BER mechanism occurring over CNx. These results demonstrate the applicability of CNx as a promising noble metal free electrocatalyst for ODC technology assisted bromine production at room temperature eliminating the need for toxic chlorine gas as the oxidant.