Electrochemical water splitting to produce hydrogen has attracted great interest as an environmentally-friendly renewable fuel. While cathodic hydrogen evolution (HER) is a relatively fast process that produces a valuable chemical, the anodic oxygen evolution reaction (OER) is a slow process that adds little to no economic value to water splitting.^1,2 Generating a high-performance oxidizer such as ozone instead of oxygen could make water splitting more economically feasible because of the added value of ozone. However, electrochemical ozone production (EOP) catalysts are typically hindered by low current efficiencies, poor selectivity, low stability, and high energy demands, which limit the industrial application of this reaction.^3,4

Further improvements in catalyst performance could be achieved by better understanding the mechanism of ozone production. Nickel and antimony doped tin oxide (Ni/Sb-SnO₂, NATO) is currently reported to have the highest EOP current efficiency at room temperature. However, the mechanism of EOP on NATO electrodes has not yet been established. A primary complication when studying the mechanism of EOP using NATO electrocatalysts in water is that oxygen atoms in the ozone molecule can originate from sources other than water, such as dissolved molecular oxygen or the electrocatalyst oxide lattice.^1,5,6

In this work, lattice oxygen participation in EOP is investigated by replacing water with acetonitrile, a polar aprotic solvent without oxygen atoms. Our results show that ozone can be generated in acetonitrile in similar quantities as aqueous conditions.² These quantities are inconsistent with a 6-electron process based on calculated current efficiencies. Furthermore, the addition of small quantities of water is shown to have a negative impact on ozone generation. The origin of this impact is thought to not be mechanistic in nature. Instead, we suggest that adding water to the mixture leads to the generation of hydroxide ions which act as ozone scavengers.

To our knowledge, this is the first report of electrochemical ozone production in a non-aqueous solvent. Future work will more conclusively determine the origin of oxygen atoms using isotopic labeling. Furthermore, the ability of nonaqueous solvents to stabilize reactive oxygen species and impact selectivity will be investigated. Utilizing the knowledge gained by studying ozone generation in nonaqueous solvents, it might be possible to design a better EOP system which could enhance the applicability of this reaction.

(1) Lees, C. M.; Lansing, J. L.; Morelly, S. L.; Lee, S. E.; Tang, M. H. Ni- and Sb-Doped SnO2 Electrocatalysts with High Current Efficiency for Ozone Production via Electrodeposited Nanostructures. J. Electrochem. Soc. 2018, 165 (16), E833. https://doi.org/10.1149/2.0051816jes.

(2) James L. Lansinga±, Lingyan Zhaob, Tana Siboonruanga, N. Harsha Attanayakea, Angela B. Leob, Peter Fatourosb, So Min Parkc, Kenneth R. Grahamc, John A. Keithb, Maureen Tang*a. Gd-Ni-Sb-SnO2 Electrocatalysts for Active and Selective Ozone Production.

(3) Christensen, P. A.; Attidekou, P. S.; Egdell, R. G.; Maneelok, S.; Manning, D. A. C.; Palgrave, R. Identification of the Mechanism of Electrocatalytic Ozone Generation on Ni/Sb-SnO ₂. J. Phys. Chem. C 2017, 121 (2), 1188–1199. https://doi.org/10.1021/acs.jpcc.6b10521.

(4) Wang, Y.-H.; Chen, Q.-Y. Anodic Materials for Electrocatalytic Ozone Generation. Int. J. Electrochem. 2013, 2013, 1–7. https://doi.org/10.1155/2013/128248.

(5) Jiang, W.; Wang, S.; Liu, J.; Zheng, H.; Gu, Y.; Li, W.; Shi, H.; Li, S.; Zhong, X.; Wang, J. Lattice Oxygen of PbO ₂ Induces Crystal Facet Dependent Electrochemical Ozone Production. J. Mater. Chem. A 2021, 9 (14), 9010–9017. https://doi.org/10.1039/D0TA12277G.

(6) Feng, J.; Johnson, D. C.; Lowery, S. N.; Carey, J. J. Electrocatalysis of Anodic Oxygen‐Transfer Reactions: Evolution of Ozone. J. Electrochem. Soc. 1994, 141 (10), 2708–2711. https://doi.org/10.1149/1.2059184.