Understanding the Activity and Stability Relationship on Metal Oxide Surfaces during the Electrochemical Generation of Reactive Oxygen Species

Abstract

Inappropriate waste disposal is a significant anthropogenic source of water pollution. This action is responsible for the release of a variety of pollutants into water bodies, including heavy metals, inorganic and organic compounds, which can have severe consequences for aquatic ecosystems and human communities.[1] Particularly, some organic contaminants such as Bisphenol A (BPA) and Per- and Polyfluoroalkyl Substances (PFAS) commonly found in trace concentrations can remain in the environment due to their high resistance to breakdown.[2] Thus, the development of highly effective water remediation technologies is a challenge.Electrochemical methods stand out among the array of tools due to their ability to eliminate pollutants effectively and selectively in an environmentally friendly way. The advantage of the method is tied to the electrochemical advanced oxidation processes (EAOP), which involve the generation of highly reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), ozone(O3), and hydroxyl radicals (•OH). These species can indirectly mineralize organic pollutants to CO2 and H2O. The ROS generation is related to the electrocatalyst's nature, being limited for some classes of materials, such as metal oxides (PbO2, SnO2, TiO2, WO3), and Boron-Doped Diamond (BDD).[2-5] Despite their highly reactive nature, we still know very little about the impact of ROS on the surface stability of metal oxide surfaces that show a high overpotential for the oxygen evolution reaction (OER), and therefore, are believed to generate predominantly ROS at high electrode potentials (e.g., >1.7 V vs reversible hydrogen electrode, RHE). Concerning the electrocatalysts' stability, the eventual deactivation constitutes a pivotal parameter with significant implications for the operational cost to commercialize this water remediation approach.Hence, to understand the trends in surface stability of metal oxides and their correlation to the activity toward ROS formation, we have deployed a combination of in situ electrochemical methods that allow us to directly measure the reaction rates (current) simultaneously with metal dissolution rates using the Stationary Probe Rotating Disk Electrode-ICP-MS method, and selectivity for the formation of O2, H2O2. Ou study framed metal oxides typically considered non-active for oxygen evolution and capable of generating ROS, such as PbO2, SnO2, TiO2, and ZnO.[3] To establish a comparative benchmark, we also utilized IrO2, known as a good OER catalyst due to its high activity-stability factor (ASF).[6-8] Figure 1 shows a set of cyclic voltammetry data obtained from a PbO2 surface in 0.1 mol L-1 NaOH. Although we observe oxidation currents only above 1.9 V vs RHE, monitoring the ring current at two different potentials reveals close to 100% selectivity towards O2 evolution and below 0.2% for H2O2 up to 2.2 V, while the onset of metal dissolution occurs above 2.1V. This result suggests that O2 evolution is still present even on materials that require very high overpotentials, and that metal dissolution might be intrinsically related to the formation of O2 and ROS from the electrolyte’s adsorbed oxygen anions on the metal oxide surface and/or (ionically) bonded oxygen anions in the metal lattice as expected from thermodynamic considerations.[9] Such fundamental comprehension about how the dissolution process impacts the activity of the surface sites for ROS generation can guide the design of not only active but also stable materials for water remediation systems.[10]