(560gj) A Waste-to-Value Technology for Sustainable Bromine Production Using Heteroatom-Doped Carbon Nanostructures

Anthropogenic activities including extraction and utilization of fossil fuels such as coal and natural gas have been recently identified as the cause for increasing bromide (Br^-) concentration in surface water, resulting in elevated levels of carcinogenic brominated by-products in potable water supplies. It is, therefore, necessary to develop technologies to effectively and economically remove bromide ions from waste water by oxidizing them into bromine gas which is a speciality chemical extensively used in the manufacture of several indispensable compounds. An electrochemical Br^- oxidation system with bromine evolution reaction (BER) on the anode and hydrogen evolution reaction (HER) on the cathode has high energy requirements, thereby reducing the economic feasibility of the technology.

To significantly reduce the energy input for electrocatalytic Br^- oxidation we have proposed the use of oxygen depolarized cathodes (ODCs) where oxygen is reduced to form water instead of the traditional HER cathodes. Successful operation of ODCs depends on the activity of the catalyst towards oxygen reduction reaction (ORR) and stability after exposure to Br^- ions that can migrate from the anode to the cathode side. We have identified that nitrogen-doped carbon nanostructures (CN_x) are promising alternatives to precious metals, as both anode and cathode catalysts for electrocatalytic Br^- removal from waste water using ODC technology. At Br^- concentration as low as 0.025 M, the BER activity of CN_x is found to be comparable to that of state-of-the-art 10% Pt/C under acidic conditions, even at very low overpotentials. Chronoamperometry measurements demonstrate the stability of CN_x under BER conditions without significant destabilization or carbon corrosion. The effect of changing pH and presence of impurities in the electrolyte on the BER activity of CN_x is also examined. A combination of density functional theory (DFT) calculations, electrochemical measurements and characterization using X-ray photoelectron spectroscopy and Raman spectroscopy is used to investigate the nature of BER active sites and elucidate BER mechanism on CN_x catalyst. Additionally, for the cathodic oxygen reduction reaction, CN_x catalyst demonstrates high resistance to deactivation in the presence of Br^- ions, in contrast with commercial Pt/C sample. This study therefore shows the promise of using CN_x catalyst for electrocatalytic conversion of Br^- ions from waste water into bromine gas using ODC technology.