electrochemical generation of ozone using solid polymer...

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. University Rohtak, Haryana, in 1980. He has been working as a lecturer and Reader since 1980 in the same University. His areas of research are solutions thermodynamics, anodic oxide films & water electrolysis and hydrogen/oxygen/ ozone production. He has published nearly 90 research papers in the research journals

of national and international repute. He worked as a visiting ~c ientist at Korea Institute of Energy Research, Korea for one and half years in 1999, 2001 and 2003 . The present work has been can'ied out in Korea.

Mr. Jung-Duk Kim received hi s B.E. degree from the Department of Chemical Engineering, Seoul National University, Seoul, Korea, in 1976. He has been working at Korea Institute of Energy Research (KIER) since 1980. Currently he is working as Senior Researcher at the Sensors and Materials Research Center of KIER. His maj or interest fields are: (i)

water electrolysis and hydrogen/oxygen/ozone production, (i i) inorganic/organic nanophosphor materials and sensor materi als.

Dr. Sang-Do Han did his B.Sc . from Kyungpook National University in 1975 , M.Sc. course from Ch ungnam National Univers ity, Korea, in 1984, and received his Ph .D. degree in Solid State Chemistry from Un iversity of Bordeaux, France in 1994. He has worked at LG Semiconductor Co. Ltd. from 1978-1980, and is cUlTently working at Korea Institute of Energy Research

(KIER) since 1980. Hi s areas of interest are: (i) electronic and electrolyte material s, (ii) chemical sensors, (i ii ) hydrogen, oxygen and ozone production, and (iv) inorganic/organic phosphor

materials. Currently, he is working as Head of Sensors and Materials Research Center of KIER and is the Editor of the Journal of Korean Sensors Society.

Prof. Chaudhary did his Ph.D. in 1972 and then worked as faculty member at Banaras Hindu University, Varanas i till 1989. He joined Maharshi Dayanand University as Professor in 1989. His research interests are in the fields of electrochemistry, cOlTosion control and development of speciality chemicals for industries. He has developed many products, which have found

applications in industries. Prof. Chaudhary has worked as visiting scientist at reputed laboratories like Max-Planck-Institute fuer Eisenforschung, Dusseldorf, Germany, Techni sche Hogeschool Delft, University of Twente, Enschede, The Netherlands. He has worked on nine sponsored projects -and has won several awards for hi s research work. He has been head of the Chemistry Department and Dean of the Faculty of Physical Sciences, Maharshi Dayanand University and the univers ity honored him with Award of Excellence. He has published 68 research papers in highly reputed journals and guided 12 Ph.D. students.

Ozone, 0 3, is a very powerful oxidant having an oxidation potential of 2.07 volts . This potential makes it the fourth strongest oxidizing chemical known . Ozone is present in large quantity in the upper atmosphere of the earth and provides protection from harmful ultra violet rays of the sun . Due to its strong oxidation potential 0 3 has a very short li fe. Ozone, dissolved in water may decompose in about 20 minutes. Ozone decomposes into secondary oxidants such as highly reactive hydroxyl (OHO) and peroxyl (H02°) radicals. These radicals are among the most

1600 IN DIAN J CHEM. SEC A. AUGUST 2004

reactive oxidizing species known. They undergo fast free radical reactions with dissolved compounds. Hydroxy l free radicals have an oxidation potential of 2.8 V which is higher than most oxidizing species including 0 3. Most of the OH" radi cals are produced in chain reactions where OH" itself or H02", act as 1Il ltlators. Hydroxyl radicals act on organic contaminants either by hydrogen abs traction or by hydrogen addition to a double bond, the resulting rad icals disproportionate or combine with each other forming many types of intermediates which react further to produce peroxyls, aldehydes and hydrogen peroxides.

Due to its strong oxidative property, 0 3, has been recognized as a useful chemical in disinfec tion and sterilization processes. It ki lls micro-organisms, decomposes organic molecules, removes cyanide, phenols, iron , manganese, detergents and coloration fro m aqueous systems . It is used to disinfect potable water, food, surgical equipment and to treat sewage water, swimming pool etc.1-I6. Ozone is also used in industries such as semiconductor production, breweries, pharmaceuticals, bio-technology etc ., where ul tra pure water is required in the manufacturing processes. It may also be employed as a raw material in the manufacture of certain organic compounds such as oleic acid, peroxyacetic acid etc. The use of ozone for purification of water is 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 processes. Its application and use will certainly expand if its cost of production may be reduced. Further, the transportation of ozone is hazardous due to its explosive nature when concentrated either as a gas or liquid, or when dissolved into solvents or adsorbed into cells . Therefore, it is always preferred to generate ozone on the site where it is to be used.

Co mmercially, ozone is produced by corona discharge process, where oxygen or air is passed through an intense high frequency A.c. electric field . The followi ng reaction occurs.

!J.ff298 = 34.1 kcal

Ozone production efficiency in this process is appro~imately 2% (by weight) only, but it is still sufficiently high to furnish usable quantities of ozone for the commercial purposes. Another disadvantage of the corona process is the production of harmful oxides

of nitrogen. Further, when the ozone generated by electric discharge is used for water treatment, varioLls disadvantages arise. In particular, because of its low concentration, the ozone dissolved in water is insufficient to treat the water and results in low operational efficiency. Additionally, dry ozone from the electric discharge method takes a longer time to dissolve in the water to be treated than wet ozone from the electrolytic process. Also, the ozone produced from the discharge process contains impurities of the electrode material, which may be a problem if ultra pure water is to be produced. To avoid these disadvantages of electric discharge generated ozone, the industry is shifti ng attention to the electrolytically generated ozone.

Nevertheless, th is method of ozone production has been exploited widely and many equipments have been designed to use ozone for sterili zation, purificatio n of water, treatment of sewage water, laundry waste, soils, food products etc. It has been observed that ozone yield increases when oxygen pressure is increased in the corona discharge

17 ~ chamber .

Ozone can also be produced by the electrolytic process where water is used as electrolyte which dissociates into oxygen and hydrogen at anode and cathode respectively. Under certain suitable conditi ons , the oxygen is evolved as 0 3 species .

!J.Ho298 value in the electrolytic process is almost six times that in corona discharge process . Thus, the electrolytic process appears to be at about six-times disadvantageous. To compete wi th the electric discharge process for 0 3, the electrolytic process must be at least six times more effici ent.

The evol ution of 0 3 by electrolysis of varioLls el ectrolytes has been known since a long ti me and cunent efficiency as high as 35% of 0 3 by volume has been reported in the literature l 8

. It means that O2 and 0 3 produced at the anode are comprised 35% 0 3 by volume. However, such high yields of 0 3 could only be achieved utilizing very low temperature of the electrolyte (-30 to -65°C). Maintaining the necessary low temperature, obviously requires costly equipment and additional energy cost of the operation.

Electrochemical cell assembly Electrochemical cell for the production of 0 3 is just

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 anode and reduction at cathode. The electrons flow in the external circuit from anode to cathode and to complete the circuit of the cell, the charge is transferred by ionic conduction through the electrolyte in the cell. The electrolyte must be poor electronic conductors to prevent internal shortcircuiting of the cell.

A typical electrochemical cell will have a positively charged anode and negatively charged cathode and the electrolyte comprising water and certain salts, acids or bases . The electrodes are connected through the electrical leads to an external source of electric power with the polarity being selected to induce the anions of the electrolyte to flow towards anode and cations to the cathode of the cell. Oxygen gas and some ozone gas are evolved at the anode surface and hydrogen gas at the cathode surface. Electrical potential of 2 to 3 volts D.C. is sufficient for the various cell configurations. The CUrTent density requirements may vary from 0.1 to 1.5 A/cm2 for maximum ozone current efficiencies .

The ceil is also provided with circulation of the electrolyte to a separate heat exchanger to keep temperature low and a system for withdrawal of gases separately from cathode and anode. Nitrogen and/or air may be pumped through the gas handling system in order to entrain the evolved cathode and anode gases and carTY them fro m the cell to the exterior where they may be utilized for the desired application .

The following reactions occur at anode and cathode in the electrolytic cell.

.. . (2)

EO is the standard electrode potential. If the nature of the catalytic surface of the anode is

changed, a competing · electrochemical water oxidation reaction may become more favourable. This water oxidation reaction involves the liberation of ozone gas and has a reversible potential of 1.51 V.

.. . (3)

Considering the above electrochemical equations, it is apparent that the minimum cell voltage required to decompose the water electrochemically into hydrogen and ozone under standard conditions is 1.51 V to be applied belween anode and cathode. However, due to

the resistance of the circuit and the electrolyte and the overpotential required in order to make reactions (2) and (3) proceed at significant rates at 25°C, the actual cell voltage will be of the order of 3.0 V.

To lower this actual cell voltage and, hence, minimize the consumption of electrical energy, the electrodes need to be placed as close as possible to each other. It is possible by using an ion exchange membrane such as Nafion of thickness in the range of 50-175 ~m. It will provide condition of minimum distance or zero gap between anode and cathode. Further, the electrodes may be coated with etectrocatalysts layers to speed up the rate of hydrogen and ozone evolution. Several attempts have been made to reduce the cost of electrocatalysts and to increase the ozone generation efficiency by replacing them with alternative materials but much success has not been achieved. The e lectrocatalysts not only face a highly acidic environment but also high mechan ical tension due to gas evolution . Therefore, much attention has been devoted to the structure of the electrodes; not only to reduce the noble metal loading but also to strenothen the bonding of the electrodes on

b '3-'-to the solid polymer electrolyte (SPE) membrane- -)

Pl atinum is a good electrocatalyst for hydrogen evolution and lead dioxide is effective for the electrochemical formation of ozone from water. However, placing the electrodes as close as possible to each other and using tIle most effective anode and cathode electrocatalysts may lower the cell voltage only by a few hundred milli volts. Consequently, 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 electrochemical cell at a higher positive potential than that at which hydrogen is evolved. For achieving this, cathodic dl~polarizers may be added to the cathode chamber t(I reduce the overall cell voltage. Chlorine, bromine, chlorine dioxide, dinitrogen tetraoxide, oxygen, air, ferric chloride, benzoquinone, hypobromus acid, hypochlorous acid, sodium ferricyanide, sodium nitrate are some important cathode depolarizers which may be utilized for decreasing the cell voltage. Air or oxygen depolarized cathode has been successfully employed for ozone production with several advantages 18. The cell voltage will be substantially reduced as now oxygen reduction will occur at the cathode in place of hydrogen evolution and it will theoretically reduce the cell voltage by 1.23 volts. In actual practice, a 0.8 V 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 hydrogen is evolved to depolarize the anode. The overall process becomes oxygen or air in and ozone out and the need for periodic additions of water is also reduced. The cathodic reactions involving the reduction of oxygen are:

In addition to lowering of the cell voltage, elimination of hydrogen evolution and replacing it with oxygen reduction reaction at the cathode in ozone generation electrolytic systems provides certain benefits such as removal of explosive hydrogen gas, elimination of the possibility of reaction between ozone and hydrogen and production of hydrogen peroxide as a byproduct. To make these reactions to occur efficiently, special gas diffusion electrodes are required. Thermodynamically, oxygen reduction reactions are favoured in comparison to the formation of hydrogen at the cathode. Hence the reduction of oxygen at the cathode will reduce the overall cell voltage, which in a way is the energy required to drive this electrochemical system.

Ozone current efficiency is the measure of the proportion of the cunent supplied, which is utilized for the production of ozone and is measured by comparing the ideal ozone yield (assuming that all the cunent supplied is used for ozone production) with actual ozone yield. The value of the ideal ozone yield can be calculated by the relation.

It / n F = Ideal ozone yield in moles

where I is the cell current in amperes; t is the time of electrolysis in seconds; n is the number of electrons taking part in the reaction and F is the Faraday constant (96484 Coulombs) .

The DC electrical energy requirement, J, for ozone production in kilowatt hours per kilogram (kWh/kg) of ozone is given by the following expression:

J = E Il F / 3600 N M

where E is the cell voltage; n is the number of electrons released per mole of ozone formed ; N is the current efficiency of ozone production; M is the molecular weight of ozone (48 g).

Anode materials Various anode materials have been utilized for the

production of ozone. Proper anodes substantially

contribute to the high yields of 0 3 from the cells. Careful selection of electrode materials also minimizes cunent consumption for a given 0 3 yield, and reduces deterioration of the electrodes from the conosive action of the electrolytes. Anode should be 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. Further, the anode must be highly conductive in order to handle the cunent densities needed to achieve a sufficient anodic potential for ozone formation . It should also work as an electrocatalyst for the production of ozone.

Usually materials like platinum, graphite and titanium have been used as anode depending upon the choice of electrolyte and other parameters of the electrolytic cell. Two materials, platinum metal and ~lead dioxide satisfy the criteria for anodes. It has been observed48

, in electrochemical cells with anodes having a high overvoltage (Pt, Pb02), that in addition to the evolution of oxygen, ozone evolution also occurred as a by-product.

Platinum anodes are relatively inert to the COITosive effect of the electrolytes and have traditionally been used in investigations of the ozone evolution process. Even at cunent densities of ten's of amperes per square centimeter, the platinum electrode experiences minimal weight loss. A protective film of PtO/Pt02 prevents further oxidation of the electrode material. Also, the oxygen overvoltage on bright platinum is among the highest observed. Ozone cunent efficiencies utilizing platinum anodes are quite excellent at all cunent densities and electrolyte concentrations. However, ozone cunent efficiencies in cells using lead dioxide anodes are consistently higher than in those using platinum anodes . ~-lead dioxide anodes give superior ozone yields in all electrolyte systems at ordinary current densities at near ambient temperatures. If extremely high ozone current efficiency is desired and corrosion of the anode is a secondary consideration, then the logical anode material would be ~-lead dioxide. On the other hand, if the highest ozone current efficiency is not of prime importance, but durability is , then the preferred anode material would be platinum.

The ~-crystalline form of lead dioxide is a tetragonal rutile structure of uni t cell dimensions 3.8, 4.94, 4.94 angstroms. ~-lead dioxide has a higher 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 also. ~-Iead dioxide is deposited anodically which limits the choice of substrate materials. Most metals dissolve when the deposition of lead dioxide is attempted. Only platinum, titanium, tantalum and carbon are suitable substrates for the anodes. Titanium and tantalum when utilized as substrate materials are first platinized to eliminate passivation problems sometimes encountered with the uncoated substrate.

Platinum electroplating on titanium base The following bath was proposed by Kitada et al49

•

H2Pt(OH)6, 20g/l; KOH, 50g/1 and potassium oxalate, 30g/lt; T = 90°C, pH = 13.5 approx., current density = 3A1dm2

, deposition efficiency = 30mg/A.min, time = 240 min, thickness 100~m, purity 99.9 wt %, and hardness = 350 Hv.

Another useful bath for electroplating platinum on porous Ti sheet is: 3 g, hydrgen hexa hydroxy platinate(IV); 4 g, potassium acetate and 6g, potassium hydroxide, mixed and made into 100 ml solution with deioriized water; T=80°C, current density = 30 mAlcm2

, pH=12.5, and time = 10 min. Carbon may be used as substrate, however, lead

dioxide adherence is a problem if the carbon has not been degassed. Carbon is degassed by boiling in water for some time followed by vacuum drying over a period of days. Vitreous or glassy carbon50 does not appear to have the adherence problem and can be used as anode substrate material but it is expensive. Platinum is the most suitable anode substrate material for lead dioxide anodes but its high cost may be a problem. Titanium, porous titanium and tantalum sheets have also been used as substrate by many workers for the deposition of ~-lead dioxide.

Several electrolytic baths have been reported in the literature. Some of them which provide a good deposition of ~-Iead dioxide are mentioned below5l o53

.

1. Potassium sodium tartarate, KNaC4H40 6.4H20 (100 g), sodium hydroxide (50 g) and lead oxide, PbO (96 g) dissolved in the order listed in distilled water to make 2 litres of solution; heated to 60°C to make solution of lead oxide; cooled and filtered through sintered glass, pH=13.

2. 108 ml of 60% perchloric acid; (100 g HCI04) ,

167 ml distilled water and 111 g lead oxide, PbO dissolved in diluted perchloric acid; made up to 2

litres with distilled water; heated to boiling for 2-3 minutes to dissolve any white precipitates; cooled and used. pH = 5.

3. 269 ml of 69.9% nitric acid (266.5g HN03) , 1 litre distilled water, and 472 g lead oxide, PbO; lead oxide added slowly to the diluted nitric acid with stirring; diluted to 2 litres and heated to 75 °C with stirring, cooled and filtered through sintered glass. To this bath added 0.75g per litrer copper nitrate, Cu(N03h.3H20 and 0.75g per litre Igepal CO-880 (alkyl phenoxy polyoxyethylene ethanol). The bath pH is about 3.5.

Grigger et al. 54 used tantalum as the base metal and deposited even more than 2 em thick layer of lead dioxide which were adherent and did not show any signs of erosion of the base metal. This plating on Ta was unexpected, since Ta polarizes in most electrolytes when operated as the anode. They used lead nitrate bath with addition of copper nitrate and Igepal CO-880 at anode current density of 0.016 -0.032 amp/cm2 at a temperature of 70°C. pH of the bath drifts strongly to a lower value during electrolysis and causes corrosion of all the common metals. pH of the bath can be maintained in the range of about 2-4 during electrolysis by the frequent addition of lead oxide. They have also suggested that electrical contact with lead dioxide should be taken by spraying a 0.0002 cm thick coating of silver on the contact area of lead dioxide to avoid heating due to contact resistance during the use of lead dioxide as anode for ozone production. In order to protect the silver coating and to provide a rugged electrical contact to the lead dioxide, silver coated area was sprayed with a heavy coat of copper, 0.16 cm or more in thickness.

It has been observed that a change of surface texture of electrodeposited ~-Pb02 on a Ti anode in HCI04 , H2S04 and near neutral phosphate buffer solution occurs when O2 and 0 3 were anodically evolved55

. It was found that strong roughening of ~Pb02 surface takes place in acid electrolytes leading to a decrease in the 0 3 efficiency at constant current densities. Phosphate buffer electrolyte with pH close to neutral is a more appropriate electrolyte for the electrochemical production of ozone.

Silva and coworkers56 electrodeposited platinum layer on titanium support from H2PtCI6 solution (0.002 mol dm-3

) at 30 rnA cm-2 for 5 min. ~-Iead dioxide was then deposited from Pb(N03)2 solution ([Pb2+] = 0.2 mol dm-3

, pH = 2; T=60°C) at a constant


anodic current density of 20 rnA cm-2 for 30 min. The average thickness of the ~-lead dioxide was approximately 40 /lm.

Scheler and Wabner57 have found that there is a decrease in the 0 3 yield with an increase in S2082- in the electrolyte containing 1.5 M H2S04 + 2.3 M (NH4hS04 used for the production of persulphate.

Velichenko el ai.58,59 have proposed that lead

dioxide deposition onto Au substrate occurs in several steps. The first step is the formation of an oxygen containing species such as OHads, chemisorbed on the electrode. In the second stage, these particles interact with lead compounds forming a soluble intermediate product, Pb(OH)2+ which is oxidized electrochemically to form Pb02. The same mechanism may be assumed for the deposition of lead dioxide on Ti substrate.

H20 ~ OHads + H+ + e

Pb2+ + OHads ~ Pb(OHi+

Pb(OH)2+ + H20 ~ Pb02 + 3H+ + e-

Electrodeposition of lead dioxide on Pt also obeys the same mechanism as is valid for an Au electrode. The main difference between the two electrode materials lies in the lower oxygen evolution overpotential (11) for Pt than for Au (llAu > llpt), resulting in a lower Pb02 deposition potential and lower lead dioxide current efficiency at Pt. Only the first portions of Pb02 are electrodeposited directly at the surface of the electrode material. Further electrodeposition occurs at deposited Pb02, which makes the growth independent of the electrode material.

Later, Velichenko and co-workers6o further investigated the electrodeposition of Pb02 on Pt in the presence of F and Fe(lII), separately as well as together in the electrodeposition bath and assessed the electrocatalytic activity of the resulting oxides for O2

and 0 3 evolution. Pb02 electrodeposited in presence of F favoured O2 evolution, whereas the presence of Fe(III) in the electrodeposition bath showed better electrocatalytic activity of Pb02 towards 0 3 evolution. Pb02 grown from F and Fe(III) containing solutions gave the highest activity for 0 3 evolution and the large amount of iron incorporated into Pb02 in the presence of F is proposed as a possible cause of the enhanced activity for ozone formation.

Persulphate formation on Pt was considered as a side reaction during electrochemical production of

ozone61. Amadelli et ai. 62 have tried to explain the

mechanism of oxygen and ozone evolution at fluoride modified lead dioxide electrodes in 1M sulphuric acid solution . It was found that the current efficiency for ozone formation as a function of the amount of NaF added to the Pb02 deposition bath reaches a maximum for a concentration of 0.01 mol dm -3 . It is suggested that low doping fluorine amounts reduce the rate of oxygen evolution and persulphate formation, providing the best conditions for ozone production.

Schultze and Beyer63 have also recommended the use of ~-lead dioxide as an electrocatalyst for the anode in the production of oxygen and ozone by electrolyzing high purity water using a solid polymer electrolyte membrane. According to them ozone production is not possible without the use of ~-l ead dioxide as anode catalyst.

In their study on eiectrodeposition of lead dioxide on titanium substrate, Lee and co-workers64 have reported that ~-lead dioxide phase 031 is dominantly deposited on Ti anode and it is in hydrated form as Pb02.6H20.

Amadelli and coworkers65 have shown the beneficial effects of doping lead dioxide anode with Fe3+, Co2+ to increase the efficiency for the electrogeneration of ozone. However, they carried out the experiments at low current density values. In their earlier publication66 cathodic oxidation method of treating pollutants has been described in which 0 2/0 3

generated at Pb02 anode are swept with oxygen stream into the cathodic compartment of the same electrochemical containing polluting species. The H20 2 formed at the cathode along with 0 3 gives rise to a highly oxidising environment, which will oxidize the pollutants.

Koganezawa67 has pointed out that electrodeposited lead dioxide layer of anode catalyst is not smooth and uniform, has poor adherence on the substrate and is subject to cracking and peeling off from the substrate.

Lead dioxide film formed by electrodeposition is liable to have irregular resistance at different points in the assembly when it is pressed against proton exchange membrane which in turn results in the generation of heat. In his ozone generating electrolysis cell, he prepared the anode catalyst by mixing and kneading the mixture of lead dioxide powder, the PTFE dispersion (5% by weight of lead dioxide) and the volatile dispersion medium, spreading it on a suitable plate and then dry it at a temperature of 100°C or below and then peeling it off 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 the proton exchange membrane to form the ozone generating catalyst layer.

Amadelli et ai. 68 have found that B-lead dioxide is a poor electrode material for oxygen evolution, and for this reason it makes an interesting material for the electrochemical processes taking place at higher positive potentials such as 0 3 production or the oxidation of organic compounds. Various factors have been found to influence the electrochemical process for oxygen evolution: Pre-treatment of the electrode increases the hydrous zones; temperature at which electrodeposition is carried out has an effect on the hydration state of the surface and hence electrocatalytic activity; electrolyte anions, particularly SO/- and CF3S03 -, are adsorbed and they inhibit both water discharge and desorption of reaction intermediates and also undergo oxidation at 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 intermediates is strongly retarded. A low electrolyte temperature also inhibits O2 evolution mainly by inhibition of the desorption reaction intermediates in the whole potential range investigated, in contrast to the selective effect of fluoride.

Babak et ai.70 proposed an electrode mechanism for the evolution of oxygen and ozone in which a parallel route for 0 3 formation is considered as the result of the encounter between O2 and 0° adsorbed on the electrode surface. The step responsible for 0 3

formation can be considered fast and diffusion controlled, excluding ozone formation as rate determining step.

Kotz and Stucki7l have given a schematic representation of the possible parallel routes leading to O2 evolution and 0 3 production on Pb02 electrodes. Based on the experimental evidence provided by Thanos and coworkers72

, it is assumed that partial current efficiency due to persulphate formation at BPb02 electrode during oxygen evolution and electrochemical ozone production processes can be considered negligible.

Wabner and Grambow73 experimentally verified the formation of free (OHO)ads radicals during water electrolysis at Pb02 electrodes and suggested that hydroxyl radicals formation is essential for the production of ozone at Pb02 electrodes. They also

observed that peroxo compounds are not formed as intermediates during electrochemical ozone production at Pb02 electrodes as proposed by Chernik et al.74

. The same conclusion was arrived at by Pavlov and Monahov75 who also discarded the formation of peroxo compounds at Pb02 electrodes, considering only (OHO)ads and (OO)ads as intermediates during electrolysis in acid medium.

Da Silva and coworkers76 have recently proposed the following mechanism for the simultaneous evolution of oxygen and ozone on B-Pb02 electrode.

Electrochemical steps: Kinetics control

(a)

(b)

Chemical steps: Efficiency control

[1-S](02)ads~[1-B][1-S] (02)ads+B[1-S](02)* ads (e) (0< B<I)

Oxygen evolution:

(f)

Ozone formation:

S(O°)* ads + B[1-S] (02) * ads ~ [S + B[I-S](03)ads (g)

[S+B[1-S](03)ads ~ 0 3 i (h)

Sand B are the partial surface coverages describing the competition between O2 and 0 3 evolution processes while '*' represents the surface coverage by oxygenated species leading to 0 3 formation and ,0,

represents the free radicals. According to thermodynamics, 0 3 evolution occurs

at high overpotentials (E>1.5 1 V(vs. RHE)) simultaneously with O2 production via steps (h) and (f), respectively. In the above mechanism, initially the electrochemical reaction proceeds via 'electrochemical steps' (steps (a) and (b)) where the anodic current is sustained by the oxidation of the absorbed water molecule, with concomitant release of two H+ ions, resulting in an electrode surface covered by 0° and a very low interface pH. Continuation of


the electrode process proceeds via 'chemical steps', which control the efficiency with respect to 0 3 and O2 evolution processes.

Factors such as electrode surface inhomogeneity (e.g. non-stoichiometry, roughness/porosity77, nature of the electrode material73

, and anion adsorption7J,77,78

can change the chemical nature of the electrode/electrolyte interface, thus affecting the competition between O2 and 0 3 evolution. Therefore, in step (c) the influence of these factors on the electrode process is accounted for, separating the total (O·)ads coverage into two distinct active surface sites, originating 8 and [1-8], which at the end lead to ozone and oxygen evolution, respectively. In a subsequent stage of the electrode process oxygen formation takes place(step(d)). Now, the electrode surface is covered by both (O·)ads and (02)ads species. Step (e) represents the separation of the [1-8](02)"ds coverage in two additional fractions, represented by [1-~] and ~,

which are necessary to the simultaneous occurrence of the O2 and 0 3 evolution processes : (i) [l-~][ 1-8] describes the amount of adsorbed O2 that detaches from the electrode surface without reacting with 0·, originating the oxygen evolution step (step(f)); (ii) ~[1-8] describes the amount of adsorbed O2 that maintains an intimate contact with O· (step (g)). The last stage of the electrochemical process called ozone formation is represented by steps (g) and (h). In such a process, 0 3 formation, requires successful encounters between adsorbed O2 and O'-species, depends upon surface concentration of the active centers leading to 0 3 formation represented by the [8 + ~(l-8) ] coverage.

Further, the adsorption of anions having a high electronegativity, as is the case of f1uoro-anions , induces a stabilization in O· -coverage, thus increasing the surface concentration of the active centres leading to ozone evolution. Introduction of f1uoro-anions in the base electrolyte (3.0 mol dm-3 sulphuric solution) provokes an increase in the activation barrier for oxygen evolution on the high overpotential domain where ozone evolution becomes significant.

Comparison of the oxygen evolution current efficiency as a function of electrolyte composition shows that electrode performance increases in the following sequence:

Base electrolyte (BE) < BE + NaF < BE + HBF4 < BE + KPF(i

There is some rise-time necessary to produce maximum ozone current efficiencies subsequent to start-up of the electrolytic process. In case of platinum anodes the rise-time is about 30 minutes. Lead dioxide anodes on the other hand, require perhaps 60-90 minutes to reach maximum ozone production.

Cathode materials At cathode hydrogen is evolved and platinum or

platinized metals are the most widely used cathode materials. Some times, when cost is the consideration, graphite may also be employed as cathode.

Alternately an air or oxygen depolarized cathode could also be used. The same air (or oxygen) fed to the air cathode could also serve to dilute and carry off the ozone that is anodically evolved by flowing through the cathode. In air cathode technology, the electrodes are generally composed of teflon bonded carbon contall1l11g small amounts of catalytic materials like platinum or certain oxides. Such cathodes are easily available in the market and can be incorporated in the process for ozone production. The air cathodes also require a metallic substrate for conductivity. It is desirable that the substrate be inert to corrosion due to aggressive electrolyte ions . Usually, the substrate may be formed by plating of conductive materials such as silver or nickel. A small protective current of 1-10 rnA per cm2 may be required when the ozone generator is shut down to prevent corrosion or change in the characteristics of the air/electrolyte interface within the partially hydrophobic porous cathode structures 79.

Generally, proton exchange membrane, Nafion, is coated with platinum on one side which acts as cathode. Electroless plating of platinum is carried out by exposing one face of the Nafion membrane to tetra amine platinum chloride hydrate, Pt (NH3) Ch.H20 , solution for some time and then reducing the platinum ions deposited on the surface of Nafion membrane by a dilute solution of sodium borohydride (NaBH4)' Her et ai. 80

-83 have found that the Nafion membrane

should be impregnated with 0.6 rnM Pt(NH3)4Ch.H20 solution for 40 min followed by reduction in lrnM NaBH4 solution for 2 hours. The temperature for impregnation and reduction is 50°C and Pt-Ioading obtained varied from 0.4 to 0.6 mg/cm2

.

Millet and coworkers34 used 0.01 M solution of Pt(NH3)4CI2 for 15 min and reduction was performed in 0.3% NaBH4 solution at 25°C for 2 hours . They observed a platinum loading of l.l3 mg/cm2 on Nafion membrane. Sakai et ai. 84 carried out the

HAN et at.: ELECTROCHEMICAL GENERATION OF OZONE USING SOLID POLYMER ELECTROLYTE 1607

platinum coating in two cycles using the same reagents and achieved a loading of 1.5 mg to 2.5 mg/cm2

.

Electrolytic deposition of platinum has also been achieved by Nagel et al.8s first dipping Nafion membrane in 0.5% solution of Pt(NH3MN02h complex at 90°C for 30 min and then pressing it between platinum anode and graphite cathode. The electrolysis was carried out for 1 hour at 0.5A/cm2

current density to get 0.7 mg/cm2 deposition of platinum on the Nafion membrane.

Kitada and Yarita49 have described several baths for electroplating of platinum. Platinum electrochemical bath consists of (i) one compound selected from the group consisting of chloroplatinic acid, chloro platinates of alkali metals, hydrogen hexa hydroxoplatinate and hex a hydroxo platinates; (ii) a hydroxylated alkali metal (20-100gll) and (iii) a soluble carboxylate. Typical composition of a good Pt electroplating bath is: hydrogen hexa hydroxoplatinate, H2Pt(OH)6 30g/l; potassium acetate, CH3COOK 40 gil and potassium hydroxide 60 gil.

Electrolytes Foller'8 has described the use of some electrolytes

which yields 0 3 with high current efficiencies , in some instances as high as 52%. Such current efficiencies are achieved by employing very highly electronegative anion constituents in the electrolyte. The fluoro-anions are among the most electronegative of all anions. The hexafluoroanions are most preferred, and in particular, the hexafluoro anions of phosphorous, arsenic and silicon and the tetrafluoroborate ions. Ozone is produced in an electrolytic cell utilizing an electrolyte consisting of water and the acids or salts of the fluoro-anions dissolved therein. Preferred anode materials for use in the electrolytic cell are either platinum or ~-lead dioxide. The fluoro-anion electrolytes are capable of producing high yields of 0 3. Corrosion of the cell electrodes can be a problem because of the low pH and extremely corrosive nature of the fluoro-anions. Parts of the cell in contact with the corrosive electrolyte may, therefore, be constructed from , or coated with, an inert material such as polyvinyl chloride or polytetrafluoroethylene. Ozone current efficiency increases with increase in the concentration of the fluoro-anions but the corrosive action of the electrolyte on the electrodes also increases at the same time. Phosphorous hexafluoro-anion provides the maximum ozone current efficiency at a concentration

of 7.3M of HPF6. Other members of the fluoro-anion class include P02F2- , HTiF6-, NbF7-, TaF7- , NiF6- , ZrF6- , GeF6- , FeF6- and the polyhalogenated boranes. However, antimony hexafluoride anion provides anomalously low ozone yield which may be due to the fact that antimony hexafluoride anion solutions dimerise to form Sb2F ,,- ions having extremely high electronegativity which completely stabilizes the intermediate cationic species and thus effectively inhibits ozone formation.

The use of the above described anions containing fluorine is also distinguished by the toxic nature of the electrolyte, which requires that care must be taken to ensure a clean separation between the electrolyte and the gas mixture which is produced.

Solid polymer electrolyte

Solid polymer electrolyte (SPE) membranes have been widely used in fuel cell technology, hydrogen

d · d d . '9-22 Th I pro uctlOn an oxygen pro uctlOn . ese po ymer membranes have excellent mechanical and chemical stability, high ionic conductivity and good gas impermeability. However, such ion-exchange membranes are costly and due to its strong acidity, the choice of electrocatalysts also becomes limited only to costly platinum group metals or oxide electrocatal ysts.

SPE membranes or proton exchange membranes are manufactured by various companies such as Du Pont, Dow Chemicals, Asahi Chemical Industries, Tokuyama Soda Company, Tosoh Corporation etc. These may consist of polymer materials having sulphonate functional groups contained on a fluorinated carbon backbone (perfluorosulphonate polymer), sulphonated polymer having a nonfluorinated carbon backbone (polystyrene sulphonate), polymer having carboxylate functional groups attached to a fluorinated carbon backbone, polymers based on perfluoro bis-sulfonimides or perfluoro phosphonic acids. However, 'Nafion' membranes marketed by DuPont are generally preferred and composed of poly tetrafluoroethylene (PTFE) structure in which sulphonate ion exchange groups are attached to the terminating end of the polymer side chains. Membranes of various thicknesses and equivalent weights are available. 'Nafion' 117 and 'Nafion' 115 and Dow Chemical ' s experimental PEM XUS-13204.20(also containing perfluorinated suI phonic acid) show a very high resistance to chemical attack. Linkous et al. 87

,88 have studied several polymeric materials like aromatic


polyesters, poly benzimidazoles, polyphenylene sulphides, polysulphones, polyethersulphones, polyketones and polyimides to develop a new solid polymer electrolyte. However, a cheap and better alternative of 'Nafion' is still awaited86.

The water absorption capacity of Nafion depends upon the heat treatment given to the membrane before soaking in water26 . If it is immersed in water at room temperature, it absorbs up to 17 wt % of water whereas when boiled in water for 30 min water uptake increases to 30 wt %. The permeability coefficients of gases through Nafion depend greatly on the water content, the cation form and the ion exchange

. 27-30 Th . h h h capacity . e gas permeatIOn rate t roug t e same sample varies with temperature, pressure and membrane thickness. Permeability of hydrogen gas is almost double of that of oxygen . When membrane absorbs water, the narrow channels in the structure are fi lled with water. The gas diffuses through water, and when its diffusion approaches the value of diffusion of hydrogen and oxygen in water, diffusion becomes constant.

It has been proposed by Gierke et al.26 that Nafion membrane has a cluster network model in which polymeric ions and absorbed water exist in almost spherical domains as ionic clusters, separated from the PTPE matrix. It is assumed that these clusters are connected by short narrow channels which have a diameter of about lOA. The cluster size grows with increasing amount of absorbed water only up to a certain limit.

Before using it in the electrochemical cell, Nafion membrane is prepared by first soaking it in hot water for about 30 min and then soaking it in 10% HCl to ensure that the entire membrane is in the H+ form. The membrane has to be kept wet at all times as it acts as a conductor only when it is wet. It is preferred that the proton exchange membrane be pretreated with an aqueous solution of sulphuric acid followed by rinsing the membrane with pure water, rinsing with hydrogen peroxide solution and finally rinsing with pure water at a temperature between 50-100°C and under pressure.

These membranes have been successfully used in SPE cells by numerous workers. Hydrated hydrogen ions are the charge carriers in the membrane which move through the solid electrolyte by passing from one fixed sulphonic acid group to the adjacent one. The two faces of the membrane are bonded to the electrocatalysts required for operating the cell. Anodic and cathodic reactions occur at these electrocatalytic

materials. Pure deionized water is invariably used as the material which is electrolysed producing oxygen gas, hydrogen ions and electrons in the anode chamber. The hydrogen ions move through the SPE and recombine electrochemically with electrons, which pass via external circuit to form hydrogen gas in the cathode chamber.

SPE cells have also been employed to generate ozone along with the oxygen at the anode and hydrogen at the cathode. Anode should have material capable of generating high percentage of ozone such as lead dioxide and cathode material should be of high hydrogen-generating capability such as platinum. However, it is highly sensitive to poi soning by the metallic ions impurity, which may be present in the deionized water circulated in the cell due to slow but continuous corrosion of steel piping in the assembly of ozone generator. Andolfatto et al. 89 have suggested the incorporation of ion exchangers in the circulating water circuits of the assembly to take care of this problem.

It is essential that electrolytic ozone production be carried at high current densities because the efficiency of ozone generation is low at low cun'ent densities. At high current densi ties , a higher proportion of the electrical current goes to the desired ozone formation reaction at the expense of other reactions forming oxygen. At high current density, the energy cost per unit amount of ozone generated is at minimum and the size of the electrodes, which determines the overall equipment cost, can be kept minimum for the optimum efficiency of ozone generation.

Current collectors and gaskets The transfer of electrons from anode to cathode in

the outer circuit takes place through current collectors, which are separately pressed onto the electrocatalysts working as anode and cathode. Current collectors should have high electric conductivity, low contact resistance with the electrocatalysts, high corrosion resistance, low resistance to the flow of water in the direction parallel to the principle plane of symmetry of the planar structure and ensure a good current distribution over the entire area of the membrane. These can be in form of porous plate, a fine woven wire mesh, open structure from expanded metal or woven wire or porous metal sheet. Various materials for current collectors have been proposed by researchers28.3 1-34. Tantalum, niobium, zirconium, titanium and graphite may be used as current collector. However, the most preferred material is titanium or titanium alloy and

HAN et al.: ELECTROCHEMICAL GENERA nON OF OZONE USING SOLID POLYMER ELECTROLYTE 1609

has been widely used in electrochemical generation of ozone. Hydrogen embrittlement is generally observed in case of titanium current collectors. However, titanium current collectors coated with platinum layer do not suffer from hydrogen embrittlement.

Nakanori32 used sintered titanium fibre plate electroplated with platinum as anode and sintered stain less steel fibre plate electroplated with gold as cathode because stainless steel fibre is more resistant to hydrogen embrittlement than titanium fiber. Even platinum current collectors made up of Pt gauze (196 mesh cm-2

) welded onto a perforated Pt foil (0.2 mm) thick have been used by Millet and co-workers34

•

Takenaka et a/. 31 have developed a new type of porous carbon sheets and made special graphite filters as cathodic current collectors.

On either side of the SPE membrane, two nonconducting and chemically resistant gaskets are placed which have well defined cutouts to fit around the perimeter of the porous titanium plates (current collectors) in cathodic and anodic chambers of the electrochemical cell. When the cell assembly is put together, the gaskets are compressed and do not allow the leakage of any liquid or gas from the cell. Gasket material for the cathodic chamber can be selected from the group consisting of neoprene, silicone rubber elastomer materials, Viton, Kalrez and urethanes. Due to highly oxidizing aggressive environment encountered in the anodic side of the electrochemical cell, gasket should be selected from fluorocarbonbased polymeric materials such as polytetrafluoroethylene (PTFE or Teflon), chlorotrifluoroethylene, polytetrafluoroethylene containing organic fillers, copolymer of tetrafluoroethylene and hexafluoropropylene, polyvinylidene fluoride and fluorocopolymers containing vinylidene fluoride and hexafluoropropene43

.

Electrochemical cell design A schematic diagram of the electrochemical cell for

the production of ozone, using sold polymer electrolyte membrane, is shown in Fig.l.

When various single electrochemical cells are put together, current collector also serves as a separator between the anode chamber of one cell and the cathode chamber of the adjacent cell in a bipolar configuration. In such a multiple cell arrangement, individual cells are put together in a filter-press type arrangement and connected in a series electrical circuit. The assembly is designed in such a manner that it facilitates the flow of fluid over all the

electrodes surface area. One side of the bipolar plate placed between each of the individual electrolytic celis, is in electrical contact with the anode of the first adjacent cell and the other side in electrical contact with the cathode of the second adjacent cell. The bipolar plate has sections removed for internal manifolding to allow fluid flow between adjacent cells. The assembly is clamped together (with nuts, bolts, screws and grippers) tightly with two end plates having electrical connection means, a water inlet port, cathode product outlet port, anode product outlet port, oxygen or air inlet port (if required) and cell water temperature measurement facility. End plates can be made from corrosion resistance materials like stainless steels, nickel alloys (monel, inconel, hastelloy), titanium, tantalum, hafnium, niobium and zirconium. Again, titanium is preferred over other materials. The inside surfaces of the end plates can be plated with platinum or any other noble metal to avoid the formation of highly corrosion resistant oxide films. A schematic representation of the two individual cells connected in series is shown in Fig. 2.

Numerous designs of the electrochemical ozone generation assembly have been reported in the literature. Attempts have been made to minimize the electronic resistance and to achieve a high ozone current efficiency by improving the deposition characteristics of f)-lead dioxide and using air or oxygen as cathode depolarizer to reduce the cell voltage.

Anode [+) Cathode H

6754321345 6

1. N afion membr'lne

2. 8 eta lead dioxide layer

3. Platinum layer

4. Porous titanium plate

5. Channels

6. End plates/Current collectors

7. Gasket

8. Bipolar plate

Fig. I-Schematic diagram of water electrolysis cell for ozone production


Menth and Stucki 35 have described the details of the electrochemical ozone synthesis cell as shown in Fig. 3. Nafion membrane of thickness 0.125 mm as solid polymer electrolyte was used for production of ozone. Cathode side of the membrane was coated with a mixture of 85% by weight of carbon and 15% by weight of platinum and the loading was 2 mg/cm2

• On the anode side a coating of 13-lead dioxide powder was applied (4 mglcm2

). A plastic polymer was used as a

6 7 843348

Fig. 2-Schematic diagram of four electrolytic cells stacked in series

Oz,o,

t I

Fig. 3- A diagrammatic section through a portion of ozone electrolysis cell

binder for both coatings. A woven wire mesh, made of platinized titanium, with aperture-width of 70 mesh served as current collector on both sides of the solid polymer electrolyte. The open structure on top of the current collectors was made of expanded titanium. A stainless steel sheet of 0.2 mm thickness was used as the bipolar plate of the assembly. The space between the bipolar plates and the solid electrolyte was 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 12.2 V for each electrode area of 10 cm2

. 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 that 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.

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 box with inlet pipe, 11- Collector box wi th outlet pipe, 12-Insulating frame

HAN el al.: ELECTROCHEMICAL GENERATION OF OZONE USING SOLID POLYMER ELECTROLYTE 1611

Baumann and Stucki2 placed the Nafion membrane between lead dioxide anode and platinum cathode. The current efficiency was increased by oxygenating a water stream fed to the anode and the cathode as the oxygen was reduced to water at room temperature releasing an increased yield of ozone.

Nishiki et 01.36 have described the use of preliminary roughening of the Nafion membrane either by filing with emery paper or by ion sputtering and then making an adhering layer of fine particles of B-Iead diox ide onto the ion-exchange membrane before an electrodeposited layer of lead dioxide is formed on the membrane. The adhering layer of lead dioxide prevents the active material from being unevenly penetrated into the ion-exchange membrane during the process of electrodeposition thus avoiding side reactions and improving the current efficiency. They suggested the application of a slurry containing B-lead dioxide powder of 100 to 425 mesh on the surface of SPE membrane and then dry at room or high temperature. Alternately, the adheri ng layer can also be made by hot press method. Then B-lead dioxide is electrodeposited from a lead nitrate bath to reinforce the adhering layer and to provide a larger electrode area. Nine different methods have been reported for maki ng the cell assembly and ozone efficiency between 13-16% at a current density of 1 A/cm2 and cell voltage 3.6 volts has been achieved.

Shimamune and coworkers37 also used a similar cell as described by Menth and Stucki to generate 15 wt % ozone electrolytically at the rate of 27 grams/hr

(about 20 l!hr) at 30De and used it for water treatment.

Dhar38 has deposited a proton conducting material such as perfluorocarbon copolymer on top of the catalytic side of the porous gas diffusion electrodes acting as anode and cathode. With sufficient deposits on both electrodes, it is then possible to avoid the use of the electrolyte membrane which is used in the common type of solid polymer electrolyte fuel cells.

Watnabe et 01. 39 have disclosed the method for intimate bonding between the membrane and the catalyst electrode thus minimizing the depth of the ion-exchange membrane encroaching into the catalyst electrode structure. A solution of Nafion in cyclohexanol havi ng viscosity of 3000 cpoise was applied to one su rface of Nafion membrane, 200 J..lm thick, to form an electro-conductive thin layer of Nafion of 5 J..lm thickness and a lower g lass transition temperature than that of the Nafion membrane. This ion-exchange membrane was hot-pressed with the electrode catalysts at a temperature between the two

glass transition temperatures of the ion-exchange membrane and the thin layer of Nafion, to get a better bonding strength between electro-catalysts and the ion-exchange membrane.

Shuji and coworkers46 have described an apparatus for electrochemical ozone generation which comprises electrically conductive porous anode carrying ozone generating catalyst, Nafion membrane as solid polymer electrolyte and a cathode composed of a gas electrode containing a catalyst, wherein the gas electrode has both hydrophilic and hydrophobic properties and the catalyst is unevenly distributed, being concentrated at the ion-exchange membrane side, while supplying an oxygen containing gas to the cathode side during electrolysis.

It has been reported by Sawamoto and coworkers40

that ozone produced by electrolytic decomposition of water using f1uororesin-type ion-exchange membrane as a solid polymer electrolyte may contain a slight amount of fluorine-containing substances due to decomposition of f1uororesin material by ozone. When ozone containing gas is used in applications where high purity is required, e.g., in ultrapure water production, there is a possibility that inclusion of these impurities might be a serious problem. They found that if the ozone-containing gas containing fluorine-containing substances is cooled to 20De or less, the fluorine-containing substances can be al most completely removed.

Shal et al.47 studied electrochemical generation of ozone in a two compartment cell as a function of current density, anode diaphragm gap width and a distance between the anode lower edge and cell bottom. The results indicated that the current efficiency ranged from 3.9-7.0%, voltage efficiency from 15-47%, while the power consumption and energy efficiency ranged from 270-790 kWh/kg and 0.62-2.1 4 %, respectively.

Pallav Tatpudi41 has explained that simultaneous production of ozone and hydrogen peroxide can be carried out in the ozone production assembly using solid polymer electrolyte membrane thus decreasing the overall cost as hydrogen peroxide is obtained as a byproduct.

It has also been reported42 that a porous, thin layer of Nafion can be made on the surface of Nafion membrane with a suspension of Nafion powder and heating at a temperature of 180 to 200De while applying a pressure of 5 kg/cm2

. The apparent thickness of this Nafion layer was 100 J..lm. When alpha lead dioxide was first deposited on a porous


substrate obtained by compacting tantalum filaments and sintering the compact, followed by the deposition of B-Iead dioxide, the electrode assembly did not show any change even after 1000 hour run and cell voltage was 3.3 V with a current efficiency of 14.5%.

Murphy et aL.43 have described in detail an electrolytic cell for the production of ozone utilizing an anodic electrocatalyst and a membrane and electrode assembly formed by bonding a PTFE containing, proton exchange polymer-impregnated, gas diffusion cathode to a proton exchange membrane. Lead dioxide electrodeposited on a platinized substrate of sintered porous titanium was used as anode and a commercially available platinumcatalyzed gas-diffusion electrode (ELAT, E-TEK, Inc.) was used as the cathodic material. The anodic and cathodic electrocatalysts layers were impregnated with a coating of the solution of Nafion in lower aliphatic alcohols. Thus prepared gas-diffusion cathode was bonded on one side of proton exchange membrane (Dow Chemical's PEM XUS-13204.20) by hot press method and anode was placed on the other side of the proton exchange membrane. Pure oxygen gas as a cathodic depolariser was supplied to the cathode at a pressure of 40 psi and water was continuously recirculated over the surface of the anode at a flow rate of 200 ml per min from the reservoir. The cell voltage and ozone current efficiency were measured as a function of temperature and applied current density (Figs 5 and 6). It was observed that the cell voltage increased linearly with increasing current density and decreased with increasing temperature at any selected applied current density. For all current densities, the ozone current efficiencies were highest for the lowest cell temperature (25°C) and decreased for any given current density with increasing temperature. A maximum current efficiency of the order of 15% was achieved at a current density of approximately 2.5 A cm-2 at 25°C.

Later in 2002, Murphy and Hitchens45 designed an electrochemical ozone sterilizer which can be used for the sterilization of surgical equipment and medical waste, oxidize the organics found on waste water, clean laundry, breakdown contaminants in soil, kill micro-organisms in food products by using pressurized, humidified, and concentrated ozone.

In US Patent 5607562, 1997, Shimamune et al.44

have given the details of an electrolytic ozone generator comprising cation-exchange resin Nafion 117 as solid polymer electrolyte, B-Iead dioxide

t 14

~ '-' 12

>-= ~ 10

G it 8 ..... .... 6 ~ ~ 4 u

2

r--r- -

d , /,

/ , ,

V I , /

J .

/ I / I /' / / I"~;

I / ;",;'

j I / I ..-

i I

./ " .... ' ,

/

1!7 ~:;..

~

0.4 0.8 1.2 · 1.6 2.0 2.4 2.8

CURRENT ~ENSITY. (Acm-2,l -

~25C o----o4OC I>- - -i:J. S5C <>-----.~ 70C

Fig. 5-Variation of ozone current efficiency with current density at various temperatures

8 r----r--,--,--.---.----.. .-! -. ...... .. _.-9 12C

7 t--t--f--+-+--I--+--::/,----I o----<J 25C

t--t---r--+-~---+--4----

O .................................................... ..l....L..i........J...... ............ L ............... ..l..i,~ o 0.4 0.8 1.2 1.6 2.0 2.4 2.8 _ __ _

CURRENT DENSITY. (Acm-2) --

Fig. 6-Variation of cell voltage with current density at variolls temperatures

coated on a substrate consisting of sintered titanium fibre coated with TilTa and platinum as anode and a porous cathode structure made up of graphite with a coating of platinum black. Ai r was supplied to the cathode chamber and the electrolysis was carried out at 20°C, at a current density of 100 Ndm2 and the cell voltage was 3.0 v. It was found that the supply of air to the cathode results in a saving of about 0.2 V in the cell voltage and that the temperature of the electrolytic solution can be maintained at the initial level without equipping the cell with a separate cooling arrangement. However, they have not clearly mentioned about the ozone current efficiency obtained with their ozone generator, which is a very important parameter.

Andrews and Murphl9 have given the design of an ozone generating system that combi nes single-usc

HAN et [fl.: ELECTROCHEMICAL GENERATION OF OZONE USING SOLID POLYMER ELECTROLYTE 1613

elements or segments with an extended use fixture that is used to activate the single-use elements. Anode catalyst applied on one side of the proton exchange membrane may be divided into segment or patches where each segment represents the limited-use portion of the ozone generator. Each segment is further advanced into a fixture that provides the balance of the electrochemical system required for operation of ozone generator.

A compact and portable device for generating mixture of ozone and oxygen has been described by Schulze et 01.63 in which special care has been taken to avoid poisoning of the high purity water used and also to improve the cooling of the electrochemical cells where ~-lead dioxide has been employed as the anode catalyst and perfluorinated ion exchange membrane as the electrolyte.

Most of the electrochemical cells mentioned above have many similarities and common features . Major difference lies in the method of preparation and or deposition of anode and cathode electrocatalysts on solid polymer electrolyte membrane. There still exist many problems, which have not been explored, understood and answered completely. The cost of materials required for making an ozone generator is high. The energy requirement for operating the ozone electrochemical generator is still not economically viable. Almost no efforts have been made to scale up the production of ozone and commercialization of the electrochemical process for ozone production is still awaited.

Future requirements The requirement of ozone is bound to increase

several times in the future due to its remarkable property to disinfect without leaving any harmful residual as in case of other disinfectants. Ozone, at present, is being used for special applications only where the cost of ozone is either a secondary or no consideration. Strict regulations lay more emphasis on sterilization of articles and commodities, which are meant for human use or consumption. Ozone is perhaps the most potential candidate as disinfectant of future. Ozone requirement in certain specific industries where it is used for making ultra pure water or in the process will also increase. The cost of ozone production is also one of the main hindrances in making ozone a popular disinfectant. Solid polymer electrolyte, platinum electrocatalyst and titanium current collectors required for ozone electrochemical cell make thc initial investment abnormally high.

The present knowledge of ozone production by water electrolysis is not yet complete and the problem is still being tackled at R&D stage. Whatever work has been reported till now is just sufficient to make a small assembly for the production of ozone for a specific application of it and that too at a high cost and, low ozone current efficiency. More consistent research efforts are required in the direction of development of ne\V low cost polymeric materials to be used as solid polymer electrolyte, cost effective and more efficient alternatives for platinum electrocatalyst and titanium current collectors. There is also a need to develop an anode catalyst, which can provide higher ozone yield than ~-lead dioxide. The problem of mass production of the parts , of electrochemical ozone generator also re,qui.r~s. some

, ! ,

serious attention.

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