Electrocatalytic Generation of Ozone on Nickel- and Antimony- Doped Tin Oxide Electrodes for Water Purification

Ozone is recognized as a safe and powerful disinfectant in water that leaves no harmful residuals because it rapidly decays into oxygen. The current method of ozone generation is cold corona discharge, which is unfavorable because it creates harmful nitrogen oxides in air and yields low concentrations of ozone when dissolved in water, lowering operational efficiency. A possible solution to this problem is generating ozone electrochemically, which can be done by choosing catalytic electrode materials that are much more selective in oxidizing water into ozone than oxygen. Nickel- and antimony-doped tin dioxide (NATO) has proven to be an electrode material active for ozone generation; however, its current efficiency (i.e. selectivity of ozone instead of oxygen) has not exceeded 50% in literature [1] and the material has poor stability.

Typically, NATO electrodes are synthesized as a sol-gel from a coating of metal precursors decomposed thermally in air. In this study, we electrodeposit thin films of NATO to control catalyst loading and compare the effects of synthetic routes on the ozone generation reaction. From baths containing either chloride or nitrate ions, we deposit either metallic or oxide films. With chloride ions, Sn(II) is reduced directly to Sn(0) on the substrate, and then is thermally oxidized to SnO2. With nitrate ions, reduction of nitrate ions increases local pH and causes SnO2 to precipitate onto the substrate. The films are characterized by X-ray diffraction and Scanning Electron Microscopy/Energy Dispersive X-ray analysis, and ozone selectivity is measured with spectroelectrochemistry. We observe that thermal oxidation yields higher current efficiencies than electrochemical oxidation. Annealing time does not affect ozone current efficiency, but high annealing temperatures increase both SnO2 crystallinity and ozone current efficiency. Increasing the concentration of Sb and Ni in the electrodeposition bath increases both the amount of dopant present and the ozone current efficiency. This electrodeposited NATO synthesis method has yet to be optimized, but achieves ozone current efficiencies of 35-45%, comparable to the sol-gel method. Efforts to relate the structural and material properties of NATO to ozone selectivity and activity are ongoing.

Sources:

[1]. Christensen, P. A. “Room Temperature, Electrochemical Generation of Ozone with 50% Current Efficiency in 0.5M Sulfuric Acid at Cell Voltages < 3V.” Ozone: Science and Engineering, no. 31, 2009, pp. 287–293., doi:10.1080/01919510903039309.