Insights Into Six-Electron Water Oxidation On Tin Oxide Based Electrodes
Authors:   , Alaufey Rayan , Snyder Joshua 
Published: January 2025

Rapid global urbanization has created a pressing need for clean and accessible water supplies. Traditional water treatment methods are often limited by the increasing demands and complexities associated with human population growth and urban expansion. A critical factor exacerbating the gap between industrial progress and environmental sustainability is the inability of current water treatment technologies to effectively address the growing issue of water resources contamination. Advancements in electrochemical water treatment methods could provide a promising path towards bridging this gap and offer sustainable solutions to global water scarcity. Among electrochemical water treatment methods, the 6-e⁻ electrochemical ozone production (EOP) reaction stands out. EOP offers a significant advantage because it allows for the on-site generation of ozone (O₃), a powerful oxidizer with a minimal environmental footprint compared to traditional disinfectants. However, EOP is plagued by selectivity issues due to the competing and thermodynamically favored oxygen evolution reaction (OER). Nickel and antimony doped tin oxide (Ni/Sb-SnO₂) is one of the most known selective catalysts for EOP under normal operating conditions. Interestingly, un-doped tin oxide (SnO₂), antimony doped tin oxide (Sb-SnO₂), and nickel doped tin oxide (Ni-SnO₂) do not generate O₃. This work investigates the mechanism of EOP on SnO₂. Electrochemical analysis suggests the existence of leached Ni(II), Ni(III), and Ni(IV) under reaction conditions. Selective radical probes reveal the presence of solution-phase hydroxyl radicals (•OH) and hydroperoxyl radicals (•OOH) under reaction conditions, with •OOH being uniquely linked to O₃ production. Further analysis demonstrates a simultaneous emergence of O₂, O₃, •OH, and •OOH at the same potential, which suggests transient anodic hydrogen peroxide (H₂O₂) as a common source for all four species. Based on these findings, I suggest a mechanism in which leached Ni(IV) cations facilitate homogenous pseudo-Fenton reactions with H₂O₂ to generate •OOH radicals, which ultimately get oxidized to form O₃. Conversely, I show that Sb is catalytically inert and mainly serves as an n-type dopant that increases the electrical conductivity of the catalyst, which leads to higher electrochemical output. Based on the suggested mechanism, I establish a co-doping design strategy to induce EOP activity in SnO₂, which is otherwise EOP inactive. I show that selective O₃ production using SnO₂-based catalysts is broadly achievable by co-doping with two elements: First, n-type dopants that enhance electrical conductivity. Second, transition metal dopants that leach and catalyze the generation of •OOH from H₂O₂. To substantiate this hypothesis, I employ tantalum (Ta), and tungsten (W) as additional n-type dopants with cobalt (Co) and iron (Fe) as additional transition metal dopants. The results confirm that properly co-doping SnO₂ yields EOP-active and selective catalysts. Furthermore, I demonstrate that the relationship between EOP activity/selectivity and electrical conductivity exhibits and intermediate maximum value before decaying, which can be explained as a competition between homogenous radical production, ultimately leading to EOP, and heterogenous OER. Additionally, I delve into the lack of catalyst stability in the context of the proposed mechanism for EOP. Oxygen-anion chemical ionization mass spectrometry (CIMS) and isotopic product analysis demonstrate that O₃ forms corrosively from the catalyst's oxide lattice without lattice oxygen regeneration. Moreover, I demonstrate the presence of at least three O₃ isotopologues. Additional investigations suggest that the electrochemical corrosion of the catalyst itself yields H₂O₂, which is subsequently catalyzed to form O₃ and O₂. These proposed pathways provide insights into both the roles of dopants in enhancing activity and the observed lack of stability of the catalysts. This work is the first to suggest that instability and electrochemical activity might be intrinsically linked through the formation of reactive oxygen species.