Electrocatalytic cogeneration of reactive oxygen species for synergistic water treatment

Abstract

This study examines the co-generation effect of hydroxyl radicals (OH) via water oxidation and H2O2 via O2 reduction in electrocatalytic processes with Sb-doped SnO2 (SS) anode and carbon nanotube (CNT) cathode pairs facing each other as a function of applied SS/CNT cell voltage and purging gas (O2 vs. N2). Prior to coupling the electrodes, both electrodes are examined for the generation kinetics and current efficiency of reactive oxygen species as well as the decomposition kinetics and total organic carbon (TOC) removal of phenol, as a function of applied half-potential (ESS or ECNT) and the purging gas. Regardless of the purging gas, a stepwise increase in ESS enhances the OH generation and phenol decomposition, yet inversely decreases the current efficiency of OH (max. ∼30%), owing to competitive O2 evolution. Similar to this, the current efficiency of H2O2 is high at a less negative ECNT, reaching ∼80% with O2 purging. However, phenol decomposition with CNT is trivial, owing to the limited reactivity between H2O2 and phenol. With N2 purging, H2O2 is not produced nor is phenol decomposed. Compared to the sum of the performance of the two half-electrodes, the SS/CNT pair exhibits similar OH generation kinetics regardless of the purging gas, yet significantly enhanced H2O2 generation kinetics. The phenol decomposition and TOC removal with the pair are more than double their sums of the two half-electrodes at all Ecells with O2 purging, whereas such a synergistic effect is found only at high Ecells when N2 is purged. The simultaneous evolution of O2 microbubbles appears to create an in situ oxic environment between the SS and CNT, leading to a synergistic effect through the production of H2O2, even under anoxic and anoxic-like conditions.