A highly active partially reduced graphite oxide electrode for the electro-oxidation of aqueous sulfur dioxide

A low-cost, stable electro-catalyst for SO2 electro-oxidation is highly desirable for applications like Bunsen reaction, SO2 sensing, etc. Here we show, that carbon, which is generally considered inactive, can be converted to a highly active material for the reaction. We used a two-step activation process wherein the carbon was first anodized to substantially increase the surface oxygen content, and then annealed in the nitrogen atmosphere. The XPS analysis showed that annealing not only reduced the surface oxygen content of the electrode from ∼21–26 atomic% to ∼3–5 atomic% but also allowed tailoring of the oxygen functionalities by preferentially increasing the percentage of hydroxyl groups. The electrode's charge transfer resistance for SO2 electro-oxidation, determined from impedance spectroscopy, showed a linear correlation with the ratio of the percentage of surface hydroxyl groups to the sum of the percentages of epoxy and carbonyl groups. The effect of the activation process on structural characteristics like crystal domain size and defect density was studied using X-ray diffraction (XRD) and Raman spectroscopy. For SO2 electro-oxidation, the activated carbon electrode showed superior performance than the Pt electrode both in terms of substantially lower onset potential (0.26 V versus Ag/AgCl reference electrode compared to 0.35 V for Pt) and 36-times higher exchange current density (6.16 μA cm−2 compared to 0.17 μA cm−2 for Pt).