(4mh) Insights into 6-e- Electrochemical Water Oxidation on Tin Oxide-Based Catalyst

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. 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 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 other 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 selective catalysts for EOP under normal operating conditions. Interestingly, un-doped tin oxide (SnO₂) and antimony doped tin oxide (Sb-SnO₂) do not generate O₃.

My thesis work investigates the mechanism of EOP on Ni/Sb-SnO₂. Electrochemical analysis demonstrates the existence of Ni in multiple oxidation states under reaction conditions. Selective radical probes reveal the presence of solution-phase hydroxyl radicals ( ^•OH) and hydroperoxyl radicals ( ^•OOH), with ^•OOH being uniquely linked to O₃ production. Further analysis demonstrates a simultaneous emergence of O₃, ^•OH, and ^•OOH at the same potential, which suggests transient anodic hydrogen peroxide (H₂O₂) as a common source for all three species. Based on these findings, we suggest a mechanism in which leached Ni cations facilitate homogenous pseudo-Fenton reactions with H₂O₂ to generate ^•OOH radicals, which ultimately get oxidized to form O₃. Conversely, we show that Sb is not catalytically active and mainly serves as an n-type dopant that increases the catalyst electrical conductivity.

Based on this mechanism, we establish a comprehensive design strategy to induce EOP activity in SnO₂. We show that selective O₃ production using SnO₂ – based catalysts is 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. To substantiate our hypothesis, we employ tantalum (Ta), and tungsten (W) as additional n-type dopants with cobalt (Co) and iron (Fe) as transition metal dopants. Our results confirm that properly co-doping SnO₂ yields EOP-active and selective catalysts. Furthermore, we 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, we delve into the observed 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. Furthermore, we demonstrate the presence of at least three O₃ isotopologues. Additional investigations suggest that the electrochemical corrosion of the catalyst itself yields Hâ‚‚Oâ‚‚, 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. Our work is the first to suggest that instability and electrochemical activity might be intrinsically linked through the formation of ROS.

My meet the post-doc candidate poster presentation at AiChE will delve deeper into the topics discussed here, with additional discussions on novel methods to enhance catalyst stability. The results presented in this work offer fundamental insights into the critical role of radical species in electrocatalysis, along with the thermodynamic limitations that influence catalyst stability. It further transforms these fundamental concepts into practical guidelines for catalyst and material design. The findings presented here hold significant promise for accelerating the development of sustainable electrochemical water treatment technologies.