Electrochemical Generation of Ozone Using Solid Polymer Electrolyte - State of the Art
Indian Journal of Chemistry Vol. 43A, August 2004, pp. 1599-1614
Advances in Contemporary Research
Electrochemical generation of ozone using solid polymer electrolyte - State of the art
Sang-Do Han*", Jung Duk Kim", K C Singh*b & R S Chaudhar/ "Korea Inst itute of Energy Research, Yuseong-gu, Daejeon 305-343, Korea bDepartment of Chemistry, Maharshi Dayanand University, Rohtak, Haryana, India Received 22 September 2003; revised 1 June 2004
In this review article, up-to-date information about the electrochemical generation of ozone using solid polymer electrolyte, fabrication of the electrochemical cell, deposition and mechanism of the anode and cathode catalysts, preparation of current collectors and optimum conditions for the proton exchange membrane, cell assembly and high ozone cUlTent efficiency have been described.
IPC Code: lnt. Cl 7 COIB 13110; C25B 1/13
Dr. K. C. Singh did hi s Ph.D. from M. D. materials. Currently, he is working as Head of Sensors and University Rohtak, Haryana, in 1980. He Materials Research Center of KIER and is the Editor of th e has been working as a lecturer and Reader Journal of Korean Sensors Society. since 1980 in the same University. His areas of research are solutions thermodynamics, anodic oxide films & Prof. Chaudhary did his Ph.D. in 1972 and water electrolysis and hydrogen/oxygen/ then worked as faculty member at Banaras ozone production. He has published nearly Hindu University, Varanasi till 1989. He 90 research papers in the research journals joined Maharshi Dayanand University as of national and international repute. He worked as a visiting Professor in 1989. His research interests are ~c ientist at Korea Institute of Energy Research, Korea for one and in the fields of electrochemistry, cOlTosion half years in 1999, 2001 and 2003. The present work has been control and development of speciality can'ied out in Korea. chemicals for industries. He has developed many products, which have found applications in industries. Prof. Chaudhary has worked as visiting Mr. Jung-Duk Kim received hi s B.E. scientist at reputed laboratories like Max-Planck-Institute fuer degree from the Department of Chemical Eisenforschung, Dusseldorf, Germany, Technische Hogeschool Engineering, Seoul National University, Delft, University of Twente, Enschede, The Netherlands. He has Seoul, Korea, in 1976. He has been worked on nine sponsored projects -and has won several awards working at Korea Institute of Energy for hi s research work. He has been head of the Chemistry Research (KIER) since 1980. Currently he Department and Dean of the Faculty of Physical Sciences, is working as Senior Researcher at the Maharshi Dayanand University and the university honored him Sensors and Materials Research Center of with Award of Excellence. He has published 68 research papers in KIER. His major interest fields are: (i) hi ghly reputed journals and guided 12 Ph.D. students. water electrolysis and hydrogen/oxygen/ozone production, (i i) inorganic/organic nanophosphor materials and sensor materi als.
Ozone, 0 3, is a very powerful oxidant having an Dr. Sang-Do Han did his B.Sc. from oxidation potential of 2.07 volts. This potential makes Kyungpook National University in 1975, it the fourth strongest oxidizing chemical known. M.Sc. course from Chungnam National Ozone is present in large quantity in the upper University, Korea, in 1984, and received his atmosphere of the earth and provides protection from Ph.D. degree in Solid State Chemistry from University of Bordeaux, France in 1994. He harmful ultra violet rays of the sun. Due to its strong has worked at LG Semiconductor Co. Ltd. oxidation potential 0 3 has a very short li fe. Ozone, from 1978-1980, and is cUlTently working dissolved in water may decompose in about 20 at Korea Institute of Energy Research minutes. Ozone decomposes into secondary oxidants (KIER) since 1980. His areas of interest are: (i) electronic and electrolyte materials, (ii) chemical sensors, (i ii ) hydrogen, oxygen such as highly reactive hydroxyl (OHO) and peroxyl and ozone production, and (iv) inorganic/organic phosphor (H02°) radicals. These radicals are among the most 1600 INDIAN J CHEM. SEC A. AUGUST 2004 reactive oxidizing species known. They undergo fast of nitrogen. Further, when the ozone generated by free radical reactions with dissolved compounds. electric discharge is used for water treatment, varioLls Hydroxyl free radicals have an oxidation potential of disadvantages arise. In particular, because of its low 2.8 V which is higher than most oxidizing species concentration, the ozone dissolved in water is including 0 3. Most of the OH" radi cals are produced insufficient to treat the water and results in low in chain reactions where OH" itself or H02", act as operational efficiency. Additionally, dry ozone from 1Il ltlators. Hydroxyl radicals act on organic the electric discharge method takes a longer time to contaminants either by hydrogen abstraction or by dissolve in the water to be treated than wet ozone hydrogen addition to a double bond, the res ulting from the electrolytic process. Also, the ozone radicals disproportionate or combine with each other produced from the discharge process contain s formin g many types of intermediates which react impurities of the electrode material, which may be a further to produce peroxyls, aldehydes and hydrogen problem if ultra pure water is to be produced. To peroxides. avoid these disadvantages of electri c discharge
Due to its strong oxidative property, 0 3, has been generated ozone, the industry is shifti ng attenti on to recognized as a useful chemical in disinfection and the electrolytically generated ozone. sterilization processes. It ki lls micro-organisms, Nevertheless, th is method of ozone production has decomposes organic molecules, removes cyanide, been exploited widely and many equipments have phenols, iron, manganese, detergents and coloration been designed to use ozone for sterili zation, fro m aqueous systems . It is used to disinfect potable purification of water, treatment of sewage water, water, food, surgical equipment and to treat sewage laundry waste, soils, food products etc. It has been water, swimming pool etc.1-I6. Ozone is also used in observed that ozone yield increases when oxygen pressure is increased in the corona discharge industries such as semiconductor production, 17 ~ breweries, pharmaceuticals, bio-technology etc ., chamber . where ul tra pure water is required in the Ozone can also be produced by the electrolytic manufacturing processes. It may also be employed as process where water is used as electrolyte which a raw material in the manufacture of certain organic dissociates into oxygen and hydrogen at anode and compounds such as oleic acid, peroxyacetic acid etc. cathode respectively. Under certain suitable The use of ozone for purification of water is conditions , the oxygen is evolved as 0 3 species. particularly advantageous as it does not leave any harmful residuals in water as in the case of chlorine. However, the cost of ozone production is high due to poor energy efficiency of the manufacturing !J.Ho 298 value in the electrolytic process is almost six processes. Its application and use will certainly times that in corona discharge process. Thus, the expand if its cost of production may be reduced. electrolytic process appears to be at about six-times Further, the transportation of ozone is hazardous due disadvantageous. To compete wi th the electric to its explosive nature when concentrated either as a discharge process for 0 3, the electrolytic process must gas or liquid, or when dissolved into solvents or be at least six times more efficient. adsorbed into cells. Therefore, it is always preferred The evolution of 0 3 by electrolysis of vari oLls to generate ozone on the site where it is to be used. el ectrolytes has been known sin ce a long ti me and Commercially, ozone is produced by corona cunent efficiency as high as 35% of 0 3 by volume has l 8 discharge process, where oxygen or air is passed been reported in the literature . It means that O2 and through an intense high frequency A.c. electric field. 0 3 produced at the anode are comprised 35% 0 3 by The followi ng reaction occurs. volume. However, such high yields of 0 3 could only be achieved utilizing very low temperature of the !J.ff298 = 34.1 kcal electrolyte (-30 to -65°C). Maintaining the necessary low temperature, obviously requires costly equipment Ozone production efficiency in this process is and additional energy cost of the operation. appro~imately 2% (by weight) only, but it is still sufficiently high to furnish usable quantities of ozone Electrochemical cell assembly for the commercial purposes. Another disadvantage of Electrochemical cell for the production of 0 3 is just the corona process is the production of harmful oxides a common type of electrochemical cell consisting of HAN el al.: ELECTROCHEMICAL GENERATION OF OZONE USING SOLID POLYMER ELECTROLYTE 1601 anode, cathode and electrolyte(s). Oxidation occurs at the resistance of the circuit and the electrolyte and the anode and reduction at cathode. The electrons flow in overpotential required in order to make reactions (2) the external circuit from anode to cathode and to and (3) proceed at significant rates at 25°C, the actual complete the circuit of the cell, the charge is cell voltage will be of the order of 3.0 V. transferred by ionic conduction through the To lower this actual cell voltage and, hence, electrolyte in the cell. The electrolyte must be poor minimize the consumption of electrical energy, the electronic conductors to prevent internal short electrodes need to be placed as close as possible to circuiting of the cell. each other. It is possible by using an ion exchange A typical electrochemical cell will have a positively membrane such as Nafion of thickness in the range of charged anode and negatively charged cathode and 50-175 ~m. It will provide condition of minimum the electrolyte comprising water and certain salts, distance or zero gap between anode and cathode. acids or bases. The electrodes are connected through Further, the electrodes may be coated with the electrical leads to an external source of electric etectrocatalysts layers to speed up the rate of power with the polarity being selected to induce the hydrogen and ozone evolution. Several attempts have anions of the electrolyte to flow towards anode and been made to reduce the cost of electrocatalysts and to cations to the cathode of the cell. Oxygen gas and increase the ozone generation efficiency by replacing some ozone gas are evolved at the anode surface and them with alternative materials but much success has hydrogen gas at the cathode surface. Electrical not been achieved. The electrocatalysts not only face potential of 2 to 3 volts D.C. is sufficient for the a highly acidic environment but also high mechan ical various cell configurations. The CUrTent density 2 tension due to gas evolution. Therefore, much requirements may vary from 0.1 to 1.5 A/cm for attention has been devoted to the structure of the maximum ozone current efficiencies. el ectrodes; not only to reduce the noble metal loading The ceil is also provided with circulation of the but also to strenothen the bonding of the electrodes on electrolyte to a separate heat exchanger to keep b to the solid polymer electrolyte (SPE) membrane-'3-'--) temperature low and a system for withdrawal of gases Pl atinum is a good electrocatalyst for hydrogen separately from cathode and anode. Nitrogen and/or evolution and lead dioxide is effective for the air may be pumped through the gas handling system electrochemical formation of ozone from water. in order to entrain the evolved cathode and anode However, placing the electrodes as close as possible gases and carTY them fro m the cell to the exterior to each other and using tIle most effective anode and where they may be utilized for the desired application. cathode electrocatalysts may lower the cell voltage The following reactions occur at anode and cathode only by a few hundred milli volts. Consequently, in the electrolytic cell. using these approaches t may not be possible to reduce the cell voltage appreciably. Alternately we may think of some other cathodic reaction occurring in . the el ectrochemical cell at a higher positive .. . (2) potential than that at which hydrogen is evolved. For achieving this, cathodic dl~ polarizers may be added to EO is the standard electrode potential. the cathode chamber t(I reduce the overall cell If the nature of the catalytic surface of the anode is voltage. Chlorine, bromine, chlorine dioxide, changed, a competing · electrochemical water dinitrogen tetraoxide, oxygen, air, ferric chloride, oxidation reaction may become more favourable. This benzoquinone, hypobromus acid, hypochlorous acid, water oxidation reaction involves the liberation of sodium ferricyanide, sodium nitrate are some ozone gas and has a reversible potential of 1.51 V. important cathode depolarizers which may be utilized for decreasing the cell voltage. Air or oxygen .. . (3) depolarized cathode has been successfully employed for ozone production with several advantages 18. The Considering the above electrochemical equations, it cell voltage will be substantially reduced as now is apparent that the minimum cell voltage required to oxygen reduction will occur at the cathode in place of decompose the water electrochemically into hydrogen hydrogen evolution and it will theoretically reduce the and ozone under standard conditions is 1.51 V to be cell voltage by 1.23 volts. In actual practice, a 0.8 V applied belween anode and cathode. However, due to swing is likely to be achieved. A separator between 1602 INDIAN J CHEM, SEC A, AUGUST 2004
anode and cathode is no longer required, as no contribute to the high yields of 0 3 from the cells. hydrogen is evolved to depolarize the anode. The Careful selection of electrode materials also overall process becomes oxygen or air in and ozone minimizes cunent consumption for a given 0 3 yield, out and the need for periodic additions of water is also and reduces deterioration of the electrodes from the reduced. The cathodic reactions involving the conosive action of the electrolytes. Anode should be reduction of oxygen are: of that material which possesses a high value of oxygen overpotential in a particular electrolyte. The anode material must be stable to strong anodic polarization, i.e., it must be in its highest oxidation state, or be kinetically resistant to further oxidation. In addition to lowering of the cell voltage, Further, the anode must be highly conductive in order elimination of hydrogen evolution and replacing it to handle the cunent densities needed to achieve a with oxygen reduction reaction at the cathode in sufficient anodic potential for ozone formation. It ozone generation electrolytic systems provides certain should also work as an electrocatalyst for the benefits such as removal of explosive hydrogen gas, production of ozone. elimination of the possibility of reaction between Usually materials like platinum, graphite and ozone and hydrogen and production of hydrogen titanium have been used as anode depending upon the peroxide as a byproduct. To make these reactions to choice of electrolyte and other parameters of the occur efficiently, special gas diffusion electrodes are electrolytic cell. Two materials, platinum metal and ~ required. Thermodynamically, oxygen reduction lead dioxide satisfy the criteria for anodes. It has been 48 reactions are favoured in comparison to the formation observed , in electrochemical cells with anodes of hydrogen at the cathode. Hence the reduction of having a high overvoltage (Pt, Pb02), that in addition oxygen at the cathode will reduce the overall cell to the evolution of oxygen, ozone evolution also voltage, which in a way is the energy required to drive occurred as a by-product. this electrochemical system. Platinum anodes are relatively inert to the COITosive Ozone current efficiency is the measure of the effect of the electrolytes and have traditionally been proportion of the cunent supplied, which is utilized used in investigations of the ozone evolution process. for the production of ozone and is measured by Even at cunent densities of ten's of amperes per comparing the ideal ozone yield (assuming that all the square centimeter, the platinum electrode experiences cunent supplied is used for ozone production) with minimal weight loss. A protective film of PtO/Pt02 actual ozone yield. The value of the ideal ozone yield prevents further oxidation of the electrode material. can be calculated by the relation. Also, the oxygen overvoltage on bright platinum is among the highest observed. Ozone cunent It / n F = Ideal ozone yield in moles efficiencies utilizing platinum anodes are quite excellent at all cunent densities and electrolyte where I is the cell current in amperes; t is the time of concentrations. However, ozone cunent efficiencies electrolysis in seconds; n is the number of electrons in cells using lead dioxide anodes are consistently taking part in the reaction and F is the Faraday higher than in those using platinum anodes. ~-lead constant (96484 Coulombs). dioxide anodes give superior ozone yields in all The DC electrical energy requirement, J, for ozone electrolyte systems at ordinary current densities at production in kilowatt hours per kilogram (kWh/kg) near ambient temperatures. If extremely high ozone of ozone is given by the following expression: current efficiency is desired and corrosion of the anode is a secondary consideration, then the logical J = E Il F / 3600 N M anode material would be ~-lead dioxide. On the other where E is the cell voltage; n is the number of hand, if the highest ozone current efficiency is not of electrons released per mole of ozone formed; N is the prime importance, but durability is, then the preferred current efficiency of ozone production; M is the anode material would be platinum. molecular weight of ozone (48 g). The ~-crystalline form of lead dioxide is a Anode materials tetragonal rutile structure of unit cell dimensions 3.8, Various anode materials have been utilized for the 4.94, 4.94 angstroms. ~ -le ad dioxide has a higher production of ozone. Proper anodes substantially oxygen overvoltage than alpha lead dioxide and in HAN et al.: ELECTROCHEMICAL GENERA nON OF OZONE USING SOLID POLYMER ELECTROLYTE 1603 fact, has a higher overvoltage than that of platinum litres with distilled water; heated to boiling for 2- also. ~-Iead dioxide is deposited anodically which 3 minutes to dissolve any white precipitates; limits the choice of substrate materials. Most metals cooled and used. pH = 5. dissolve when the deposition of lead dioxide is 3. 269 ml of 69.9% nitric acid (266.5g HN03) , 1 attempted. Only platinum, titanium, tantalum and litre distilled water, and 472 g lead oxide, PbO; carbon are suitable substrates for the anodes. lead oxide added slowly to the diluted nitric acid Titanium and tantalum when utilized as substrate with stirring; diluted to 2 litres and heated to 75 materials are first platinized to eliminate passivation °C with stirring, cooled and filtered through problems sometimes encountered with the uncoated sintered glass. To this bath added 0.75g per litrer substrate. copper nitrate, Cu(N03h.3H20 and 0.75g per litre Igepal CO-880 (alkyl phenoxy polyoxyethylene Platinum electroplating on titanium base ethanol). The bath pH is about 3.5. 49 The following bath was proposed by Kitada et al • Grigger et al. 54 used tantalum as the base metal and H2Pt(OH)6, 20g/l; KOH, 50g/1 and potassium oxalate, deposited even more than 2 em thick layer of lead 30g/lt; T = 90°C, pH = 13.5 approx., current density = dioxide which were adherent and did not show any 2 3A1dm , deposition efficiency = 30mg/A.min, time = signs of erosion of the base metal. This plating on Ta 240 min, thickness 100~m, purity 99.9 wt %, and was unexpected, since Ta polarizes in most hardness = 350 Hv. electrolytes when operated as the anode. They used lead nitrate bath with addition of copper nitrate and Another useful bath for electroplating platinum on Igepal CO-880 at anode current density of 0.016 - porous Ti sheet is: 3 g, hydrgen hexa hydroxy 0.032 amp/cm2 at a temperature of 70°C. pH of the platinate(IV); 4 g, potassium acetate and 6g, bath drifts strongly to a lower value during potassium hydroxide, mixed and made into 100 ml electrolysis and causes corrosion of all the common solution with deioriized water; T=80°C, current metals. pH of the bath can be maintained in the range 2 density = 30 mAlcm , pH=12.5, and time = 10 min. of about 2-4 during electrolysis by the frequent Carbon may be used as substrate, however, lead addition of lead oxide. They have also suggested that dioxide adherence is a problem if the carbon has not electrical contact with lead dioxide should be taken by been degassed. Carbon is degassed by boiling in water spraying a 0.0002 cm thick coating of silver on the for some time followed by vacuum drying over a contact area of lead dioxide to avoid heating due to period of days. Vitreous or glassy carbon50 does not contact resistance during the use of lead dioxide as appear to have the adherence problem and can be used anode for ozone production. In order to protect the as anode substrate material but it is expensive. silver coating and to provide a rugged electrical Platinum is the most suitable anode substrate material contact to the lead dioxide, silver coated area was for lead dioxide anodes but its high cost may be a sprayed with a heavy coat of copper, 0.16 cm or more problem. Titanium, porous titanium and tantalum in thickness. sheets have also been used as substrate by many It has been observed that a change of surface workers for the deposition of ~-lead dioxide. texture of electrodeposited ~-Pb02 on a Ti anode in Several electrolytic baths have been reported in the HCI04 , H2S04 and near neutral phosphate buffer literature. Some of them which provide a good solution occurs when O2 and 0 3 were anodically 5l o53 55 deposition of ~-Iead dioxide are mentioned below . evolved . It was found that strong roughening of ~ Pb02 surface takes place in acid electrolytes leading 1. Potassium sodium tartarate, KNaC4H40 6.4H20 to a decrease in the 0 3 efficiency at constant current (100 g), sodium hydroxide (50 g) and lead oxide, densities. Phosphate buffer electrolyte with pH close PbO (96 g) dissolved in the order listed in to neutral is a more appropriate electrolyte for the distilled water to make 2 litres of solution; heated electrochemical production of ozone. to 60°C to make solution of lead oxide; cooled Silva and coworkers56 electrodeposited platinum and filtered through sintered glass, pH=13. layer on titanium support from H2PtCI6 solution 3 2 2. 108 ml of 60% perchloric acid; (100 g HCI04) , (0.002 mol dm- ) at 30 rnA cm- for 5 min. ~-Iead 167 ml distilled water and 111 g lead oxide, PbO dioxide was then deposited from Pb(N03)2 solution 2 3 dissolved in diluted perchloric acid; made up to 2 ([Pb +] = 0.2 mol dm- , pH = 2; T=60°C) at a constant 1604 INDIAN J CHEM, SEC A, AUGUST 2004
2 61 62 anodic current density of 20 rnA cm- for 30 min. The ozone . Amadelli et ai. have tried to explain the average thickness of the ~-lead dioxide was mechanism of oxygen and ozone evolution at fluoride approximately 40 /lm. modified lead dioxide electrodes in 1M sulphuric acid Scheler and Wabner57 have found that there is a solution. It was found that the current efficiency for ozone formation as a function of the amount of NaF decrease in the 0 3 yield with an increase in S2082- in added to the Pb0 deposition bath reaches a maximum the electrolyte containing 1.5 M H2S04 + 2.3 M 2 -3. (NH4hS04 used for the production of persulphate. for a concentration of 0.01 mol dm It is suggested 58 Velichenko el ai. ,59 have proposed that lead that low doping fluorine amounts reduce the rate of dioxide deposition onto Au substrate occurs in several oxygen evolution and persulphate formation, steps. The first step is the formation of an oxygen providing the best conditions for ozone production. 63 containing species such as OHads, chemisorbed on the Schultze and Beyer have also recommended the electrode. In the second stage, these particles interact use of ~-lead dioxide as an electrocatalyst for the with lead compounds forming a soluble intermediate anode in the production of oxygen and ozone by product, Pb(OH)2+ which is oxidized electrolyzing high purity water using a solid polymer electrochemically to form Pb02. The same electrolyte membrane. According to them ozone mechanism may be assumed for the deposition of lead production is not possible without the use of ~ -l ead dioxide on Ti substrate. dioxide as anode catalyst. In their study on eiectrodeposition of lead dioxide H20 ~ OHads + H+ + e on titanium substrate, Lee and co-workers64 have 2 reported that ~-lead dioxide phase 031 is dominantly Pb + + OHads ~ Pb(OHi+ deposited on Ti anode and it is in hydrated form as Pb(OH)2+ + H20 ~ Pb02 + 3H+ + e- Pb02.6H20. Amadelli and coworkers65 have shown the Electrodeposition of lead dioxide on Pt also obeys beneficial effects of doping lead dioxide anode with the same mechanism as is valid for an Au electrode. Fe3+, Co2+ to increase the efficiency for the The main difference between the two electrode electrogeneration of ozone. However, they carried out materials lies in the lower oxygen evolution the experiments at low current density values. In their overpotential (11) for Pt than for Au (llAu > llpt), earlier publication66 cathodic oxidation method of resulting in a lower Pb02 deposition potential and treating pollutants has been described in which 0 2/03 lower lead dioxide current efficiency at Pt. Only the generated at Pb02 anode are swept with oxygen first portions of Pb02 are electrodeposited directly at stream into the cathodic compartment of the same the surface of the electrode material. Further electrochemical containing polluting species. The , electrodeposition occurs at deposited Pb02 which H20 2 formed at the cathode along with 0 3 gives rise makes the growth independent of the electrode to a highly oxidising environment, which will oxidize material. the pollutants. 6o Later, Velichenko and co-workers further Koganezawa67 has pointed out that electrodeposited investigated the electrodeposition of Pb02 on Pt in the lead dioxide layer of anode catalyst is not smooth and presence of F and Fe(lII), separately as well as uniform, has poor adherence on the substrate and is together in the electrodeposition bath and assessed the subject to cracking and peeling off from the substrate. electrocatalytic activity of the resulting oxides for O2 Lead dioxide film formed by electrodeposition is and 0 3 evolution. Pb02 electrodeposited in presence liable to have irregular resistance at different points in of F favoured O2 evolution, whereas the presence of the assembly when it is pressed against proton Fe(III) in the electrodeposition bath showed better exchange membrane which in turn results in the electrocatalytic activity of Pb02 towards 0 3 evolution. generation of heat. In his ozone generating Pb02 grown from F and Fe(III) containing solutions electrolysis cell, he prepared the anode catalyst by gave the highest activity for 0 3 evolution and the mixing and kneading the mixture of lead dioxide large amount of iron incorporated into Pb02 in the powder, the PTFE dispersion (5% by weight of lead presence of F is proposed as a possible cause of the dioxide) and the volatile dispersion medium, enhanced activity for ozone formation. spreading it on a suitable plate and then dry it at a Persulphate formation on Pt was considered as a temperature of 100°C or below and then peeling it off side reaction during electrochemical production of the suitable surface. This peeled off dry layer is HAN et at.: ELECTROCHEMICAL GENERATION OF OZONE USING SOLID POLYMER ELECTROLYTE 1605 sandwiched between the anodic electrode plate and observed that peroxo compounds are not formed as the proton exchange membrane to form the ozone intermediates during electrochemical ozone generating catalyst layer. production at Pb02 electrodes as proposed by Chernik 68 74 Amadelli et ai. have found that B-lead dioxide is a et al. . The same conclusion was arrived at by Pavlov 75 poor electrode material for oxygen evolution, and for and Monahov who also discarded the formation of this reason it makes an interesting material for the peroxo compounds at Pb02 electrodes, considering electrochemical processes taking place at higher only (OHO)ads and (OO)ads as intermediates during positive potentials such as 0 3 production or the electrolysis in acid medium. oxidation of organic compounds. Various factors have Da Silva and coworkers76 have recently proposed been found to influence the electrochemical process the following mechanism for the simultaneous for oxygen evolution: Pre-treatment of the electrode evolution of oxygen and ozone on B-Pb02 electrode. increases the hydrous zones; temperature at which Electrochemical steps: Kinetics control electrodeposition is carried out has an effect on the hydration state of the surface and hence (a) electrocatalytic activity; electrolyte anions, (b) particularly SO/- and CF3S03 -, are adsorbed and they inhibit both water discharge and desorption of reaction intermediates and also undergo oxidation at Chemical steps: Efficiency control higher positive potentials. Fluoride added to the electrolyte is strongly adsorbed and suppresses and/or modifies the structure of hydrous layer with the consequence that water discharge is inhibited in the lower potential range and, prevalently at the higher posItive potentials, desorption of oxygen [1-S](02)ad s ~[1-B][1-S] (02)ads+B[1-S](02)* ads (e) intermediates is strongly retarded. A low electrolyte (0< B
Menth and Stucki 35 have described the details of binder for both coatings. A woven wire mesh, made the electrochemical ozone synthesis cell as shown in of platinized titanium, with aperture-width of 70 mesh Fig. 3. Nafion membrane of thickness 0.125 mm as served as current collector on both sides of the solid solid polymer electrolyte was used for production of polymer electrolyte. The open structure on top of the ozone. Cathode side of the membrane was coated with current collectors was made of expanded titanium. A a mixture of 85 % by weight of carbon and 15% by stainless steel sheet of 0.2 mm thickness was used as 2 weight of platinum and the loading was 2 mg/cm • On the bipolar plate of the assembly. The space between the anode side a coating of 13-lead dioxide powder was the bipolar plates and the solid electrolyte was 2 applied (4 mglcm ). A plastic polymer was used as a completely filled with water in which air was forced into the system at the rate of 20 lit/min. The cell was operated at 12°C temperature of the inlet water, at 1 mNcm2 current density and the resulting total voltage across the terminals of the electrolysis block was 2 12.2 V for each electrode area of 10 cm . The concentration of ozone was determined iodometrically, and amounted to approximately 0.01 gIl. A plurality of individual cells may be integrated together between end plates so th at the cells are electrically connected in series, hydrodynamically connected in parallel, and combined to form a block (Fig. 4). However, this cell could support only limited current density, which was not sufficient for increasing the efficiency of ozone production. 6 7 843348
Fig. 2-Schematic diagram of four electrolytic cells stacked in series
Oz,o, t
I 8 +
7
Fig. 4-A diagrammatic section through a device for providing ozone. 1- Nafion 125 membrane, 2- Cathode catalyst coating, 3- Anode catalyst coating, 4 & 5- Current collectors, 6- Bipolar plates, 7-End plates, 8 & 9- Electrical term inals 10- Distribution Fig. 3- A diagrammatic section through a portion of ozone box with inlet pipe, 11- Collector box wi th outlet pipe, 12- electrolysis cell Insulating frame HAN el al.: ELECTROCHEMICAL GENERATION OF OZONE USING SOLID POLYMER ELECTROLYTE 1611
Baumann and Stucki2 placed the Nafion membrane glass transition temperatures of the ion-exchange between lead dioxide anode and platinum cathode. membrane and the thin layer of Nafion, to get a better The current efficiency was increased by oxygenating bonding strength between electro-catalysts and the a water stream fed to the anode and the cathode as the ion-exchange membrane. oxygen was reduced to water at room temperature Shuji and coworkers46 have described an apparatus releasing an increased yield of ozone. for electrochemical ozone generation which Ni shiki et 01.36 have described the use of comprises electrically conductive porous anode preliminary roughening of the Nafion membrane carrying ozone generating catalyst, Nafion membrane either by filing with emery paper or by ion sputtering as solid polymer electrolyte and a cathode composed and then making an adhering layer of fine particles of of a gas electrode containing a catalyst, wherein the B-Iead dioxide onto the ion-exchange membrane gas electrode has both hydrophilic and hydrophobic before an electrodeposited layer of lead dioxide is properties and the catalyst is unevenly distributed, formed on the membrane. The adhering layer of lead being concentrated at the ion-exchange membrane dioxide prevents the active material from being side, while supplying an oxygen containing gas to the unevenly penetrated into th e ion-exchange membrane cathode side during electrolysis. during the process of electrodeposition thus avoiding It has been reported by Sawamoto and coworkers40 side reactions and improving the current efficiency. that ozone produced by electrolytic decomposition of They suggested the application of a slurry containing water using f1uororesin-type ion-exchange membrane B-lead dioxide powder of 100 to 425 mesh on the as a solid polymer electrolyte may contain a slight surface of SPE membrane and then dry at room or amount of fluorine-containing substances due to high temperature. Alternately, the adheri ng layer can decomposition of f1uororesin material by ozone. also be made by hot press method. Then B-lead When ozone containing gas is used in applications dioxide is electrodeposited from a lead nitrate bath to where high purity is required, e.g., in ultrapure water reinforce the adhering layer and to provide a larger production, there is a possibility that inclusion of electrode area. Nine different methods have been these impurities might be a serious problem. They reported for making the cell assembly and ozone found that if the ozone-containing gas containing efficiency between 13-16% at a current density of fluorine-containing substances is cooled to 20De or 1A/cm 2 and cell voltage 3.6 volts has been achieved. less, the fluorine-containing substances can be al most Shimamune and coworkers37 also used a similar completely removed. cell as described by Menth and Stucki to generate 15 Shal et al.47 studied electrochemical generation of wt % ozone electrolytically at the rate of 27 grams/hr ozone in a two compartment cell as a function of (about 20 l!hr) at 30De and used it for water treatment. current density, anode diaphragm gap width and a Dhar38 has deposited a proton conducting material distance between the anode lower edge and cell such as perfluorocarbon copolymer on top of the bottom. The results indicated that the current catalytic side of the porous gas diffusion electrodes efficiency ranged from 3.9-7.0%, voltage efficiency acting as anode and cathode. With sufficient deposits from 15-47%, while the power consumption and on both electrodes, it is th en possible to avoid the use energy efficiency ranged from 270-790 kWh/kg and of the electrolyte membrane which is used in the 0.62-2.1 4 %, respectively. common type of solid polymer electrolyte fu el cells. Pallav Tatpudi41 has explained that simultaneous Watnabe et 01.39 have disclosed the method for production of ozone and hydrogen peroxide can be intimate bonding between the membrane and th e carried out in the ozone production assembly using catalyst electrode thus minimizing the depth of the solid polymer electrolyte membrane thus decreasing ion-exchange membrane encroaching into the catalyst the overall cost as hydrogen peroxide is obtained as a electrode structure. A solution of Nafion in byproduct. cyclohexanol having viscosity of 3000 cpoise was It has also been reported42 that a porous, thin layer applied to one surface of Nafion membrane, 200 J..lm of Nafion can be made on the surface of Nafion thick, to form an electro-conductive thin layer of membrane with a suspension of Nafion powder and Nafion of 5 J..lm thickness and a lower glass transition heating at a temperature of 180 to 200De while 2 temperature than that of the Nafion membrane. This applying a pressure of 5 kg/cm . The apparent ion-exchange membrane was hot-pressed with th e thickness of this Nafion layer was 100 J..lm. When electrode catalysts at a temperature between the two alpha lead dioxide was first deposited on a porous 1612 INDIAN J CHEM, SEC A, AUGUST 2004 substrate obtained by compacting tantalum filaments r--r-- and sintering the compact, followed by the deposition t 14 d, ~25C of B-Iead dioxide, the electrode assembly did not '-'~ 12 o----o4OC /, I>-- -i:J. S5C show any change even after 1000 hour run and cell >-= ,, / <>-----.~ 70C voltage was 3.3 V with a current efficiency of 14.5%. ~ 10 ,I / 43 G V . Murphy et aL. have described in detail an J it 8 I I / electrolytic cell for the production of ozone utilizing ..... /' .... 6 / / / I"~; an anodic electrocatalyst and a membrane and I / ;",;' ~ I ..- electrode assembly formed by bonding a PTFE j I / ~ 4 " .... ' containing, proton exchange polymer-impregnated, u ,I / 2 i ./ gas diffusion cathode to a proton exchange ~:;.. membrane. Lead dioxide electrodeposited on a 1!7 ~ platinized substrate of sintered porous titanium was 0.4 0.8 1.2 · 1.6 2.0 2.4 2.8 used as anode and a commercially available platinum CURRENT ~ENSITY. (Acm-2,l - catalyzed gas-diffusion electrode (ELAT, E-TEK, Inc.) was used as the cathodic material. 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