Electrochemical ozone generation-A review Özge SİVRİOĞLU*, Taner YONAR *, *Department of Environmental Engineering, Engineering Faculty, Uludag University, 16059, Bursa, TURKEY (E-mail: ozge@uludag.edu.tr,yonar@uludag.edu.tr) Ozone, which is an allotropic form of oxygen, is known as one of the most powerful disinfectant and it is well known method for oxidizing several different organic or inorganic pollutants It has several applications including disinfection, sterilization, green oxidation of pollutants, deodorization of drinking and industrial process water, water and wastewater treatment since the discovery of ozone by Schonbein in. Ozone is unstable gas and standard redox potential can be between 1,51 to 2,07 depending on the dissolved oxygen concentration. Nowadays ozone replaces the chlorine in many applications because of chlorines by products and ozone’s oxidation . Introduction The ICOEST’2014 - SIDE OZONE GENERATION Ozone production in nature occurs in two ways: (i) Lightning, (ii) UV Light. Ozone was produced first electrochemically (Foller and Tobias 1982), currently ozone ozone can be generated by following technologies: (i) Corona discharge (dielectric barrier discharge); (ii) UV irradiation (photochemistry) and (iii) Electrochemical ozone production (electrolysis of aqueous solutions). Corona Discharge Corona Discharge (CD) is the most common technology for ozone production. In this process ozone is generated by passing dry oxygen or pure oxygen through a high energy electric field (Figure 1). The quantity of ozone production is dependent on some factors such as oxygen source, gas temperature and presence of impurities in the gaseous phase. Ultraviolet (UV) Irradiation This is a photochemical process and presents a very high specific energy demand due to its low efficiency .In this process ozone is generated by passing air over a UV lamp in the presence of 185 nm UV Light so a small part of the oxygen atoms then react with other oxygen molecules to form ozone by using high energy radiation. Development and optimisation of alternative technologies for pollution control in different contaminates environment is important in modern society. Ozone application in wastewater treatment is a promising technology. Ozone is commercialy generated using Corona Technology. Electrochemical generation of ozone is rising because it has high advanages of high gaseous ozone concentration, high dissolving efficiency in water, and no harmful NO x formation. In electrochemical ozone generation anodic material is the key factor which affect the performance of the system. High ozone current efficiencies (around 20– 50%) can be routinely obtained at certain anodes; Sn/Sb/Ni and BDD anodes showed current efficiency of over 30%. It’s clear that electrochemical ozone generation is going to be the most useful and cheaper way to pruce ozone. 2nd INTERNATIONAL CONFERENCE ON ENVIRONMENTAL SCIENCE AND TECHNOLOGY 14 – 17 May 2014, Side, Antalya, TURKEY Lightning passing through the air creates ozone from oxygen. Oxygen, from the air, is passed through a high voltage. Oxygen turns into ozone after it is hit with 185 nm UV light from a UV bulb. HOW OZONE MADE IN NATURE HOW DO WE MADE OZONE Electrochemical Ozone Production Last decades attention is focused on the electrochemical ozone production (EOP) because of the advantages of this method. Electrochemical ozone production basis is to oxidize water to ozone on the anode material in other words it is formed by electrolytic decomposition of water at the anode . In direct water electrolysis standard electrode potentials and ozone formation reactions can be seen below. Anode: 2H 2 O O 2 +4H + +4e - E 0 =1,229 V (1) 3H 2 O O 3 +6H + +6e - E 0 =1,511 V (2) O 2 +H 2 O O 3 +2H + +2e - E 0 =2,075 V (3) Cathode: 2H + +2e - H 2 E 0 =0 V (4) These are the fundemental reactions during electrolytic decomposition. In Eq. (2) ozone gas evolves by a six electron reaction at higher voltages of 1,511, accompanied by oxygen evolution. When we increase voltage above 2,075 its expected that oxidation of O 2 gas form O 3 as shown in Eq.(3).The oxygen evolution reaction always occur simultaneously during EOP due to its lower standard potential Electrode materials, cell configurations, current density, electrolyte, anodic potential affects the efficiency of electrochemical ozone production and among these factors anodic material is one of the most important parameter cause it must be carefully chosen to achieve appreciable ozone evolution with considerable efficiency. Several anode materials have been investigated, Pt, DSA, Au, Pd, PbO 2 , SnO 2 , boron doped diamond (BDD) and glassy carbon are some of them but only Pt and PbO 2 showed significiant current efficiency. The researchers and their works are given in Table 1. Table 1. Comparison of anode types Figure 1. Ozone Generation Figure 2. Simplest Cell Configuration Figure 3. Proton Exchange Membrane More research groups involved in devoloping this technology by employing new cell configurations and anode modifications because of the increasing attention in electrochemical ozone generation. In the simplest cell configuration, the electrodes are immersed directly in the electrolyte and the gases evolved at anode and cathode (Figure 2). Separated cells employ an inert separator such as glass, wetted Teflon or Nafion and the anode and cathode gases are vented separately. To overcome the acid electrolyte corrosion researchers devoloped proton exchage membrane configuration (Figure 3). In this system during electrolysis water was oxidized on anode to form ozone and release protons. These protons were than transferred to cathode and reduced to oxygen. In this way ozone can be produced without electrolyte. Oxygen in the presence of 185 nm UV light creates ozone. Pt ANODES Researchers C 0 Electrolyte Efficiency Putnam et al. 1948 - 53 HClO 4 % 25,9 Seader and Tobias 1952 -63,5 H 2 SO 4 % 32,4 Foller and Tobias 1982 0 H 2 SO 4 % 1,7 BORON DOPED DIAMOND Researchers C 0 Electrolyte Efficiency Fujishima et al. 2005 20-24 % 24 Kraft et al. 2006 20-24 % 50 Arihara et al. 2007 20-24 % 47 PbO 2 ANODES Researchers C 0 Anode Type Electrolyte Efficiency Foller and Tobias 1981,1982 0 PbO 2 H 2 SO 4 % 4 - % 7 Foller and Tobias 1982 0 PbO 2 +HPF 6 H 2 SO 4 % 50 Stucki et al.. 1985,1986 0 PbO 2 Nafion 120 % 15 - % 20 Amadelli et al. 1999 PbO 2 +Fe,Co H 2 SO 4 %15-20 Sn/ Sb/ Ni ANODES Researchers C 0 Anode Type Electrolyte Efficiency Cheng and Chan 2004 20-24 Sn+Sb HClO 4 % 15 Wang et al.. 2005 20-24 Sn+Sb+Ni H 2 SO 4 % 36,3 Wang et al. 2006 20-24 Sn+Sb+Ni MEA % 15,2 Christensen and Lin 2009 20-24 Sn+Sb+Ni H 2 SO 4 % 50 REFERENCES Basiriparsa, J., Abbasi, M. 2012. High efficiency ozone generation via electrochemical oxidation of water using Ti anode coated with Ni- Sb-SnO 2 . Journal of Solid State Electrochemistry, 16, 1011-1018. Christensen P.A., Yonar T., Zakaria, K. 2013. The electrochemical generation ozone:A rewiev. Ozone: Science & Engineering, 35,(3),149-167. Da Silva, L.M., Jardim, W.F. 2006. Trends and strategies of ozone application in environmental problems. Quimica Nova, 29, (2), 310-317. Wang Y.H., Chen Q,Y. 2013. Anodic materials for electrocatalytic ozone generation. International Journal of Electrochemistry,Vol 2013,Article ID 128248,1-7. Postma, S. 2012. Definitive Guide for Understanding Ozone. Ozone Solutions, Inc. 1-23 Anode: 2H 2 O O 2 +4H + +4e - E 0 =1,229 V 3H 2 O O 3 +6H + +6e - E 0 =1,511 V O 2 +H 2 O O 3 +2H + +2e - E 0 =2,075 V Cathode: 2H + +2e - H 2 E 0 =0 V