EP0041365B1

EP0041365B1 - Improved electrolytic process for the production of ozone

Info

Publication number: EP0041365B1
Application number: EP19810302353
Authority: EP
Inventors: Peter C. Foller; Mark L. Goodwin; Charles W. Tobias
Original assignee: University of California
Current assignee: University of California
Priority date: 1980-05-29
Filing date: 1981-05-28
Publication date: 1985-09-04
Also published as: DE3172122D1; EP0041365A1

Description

This invention relates generally to the production of ozone and more particularly to the electrolytic production of ozone utilizing highly electronegative anions in the electrolyte to greatly increase the ratio of 0₃ to O2 in the anodic gaseous product. The selection of electrode materials is also important in the electrolytic production method.
Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. Because of this activity it finds wide application in disinfection processes. In fact, it kills bacteria more rapidly than chlorine, it decomposes organic molecules, and removes coloration in aqueous systems. Ozonation removes cyanides, phenols, iron, manganese, and detergents. It controls slime formation in aqueous systems, yet maintains a high oxygen content in the system. Unlike chlorination, which may leave undesirable chlorinated organic residues in organic containing systems, ozonation leaves fewer potentially harmful residues. There is evidence that ozone will destroy viruses. It is used for sterilization in the brewing industry and for odor control in sewage treatment and manufacturing. And ozone is employed as a raw material in the manufacture of certain organic compounds, e.g., oleic acid and peroxyacetic acid.
Thus, ozone has wide spread application in many diverse activities, and its use would undoubtedly expand if its cost of production could be reduced. In addition, since ozone is explosive when concentrated as either a gas or liquid, or when dissolved into solvents or absorbed into gels, its transportation is potentially hazardous. Therefore, it is generally manufactured on the site where it is used. However, the cost of generating equipment, and poor energy efficiency of production has deterred its use in many applications and in many locations.
On a commercial basis, ozone is currently produced by the silent electric discharge process, wherein air or oxygen is passed through an intense, high frequency alternating current electric field. The discharge process forms ozone through the reaction:
Yields in the discharge process generally range in the vicinity of 2% ozone, i.e.; the exit gas may be about 2% 0₃ by weight. Such 0₃ concentrations, while quite poor, in an absolute sense, are still sufficiently high to furnish useable quantities of 0₃ for the indicated commercial purposes.
Other than the aforementioned electric discharge process, there is no other commercially exploited process for producing large quantities of 03.
However, 0₃ may also be produced by the electrolytic process, wherein an electric current (normally D.C.) is impressed across electrodes immersed in an electrolyte, i.e., electrically conducting, fluid. The electrolyte includes water, which, in the process, dissociates into its respective elemental species, i.e., 0₂ and H_z. Under the proper conditions, the oxygen is also evolved as the 0₃ species. The evolution of 0₃ may be represented as:
It will be noted that the H° in the electrolytic process is many times greater than that for the electric discharge process. Thus, the electrolytic process appears to be at about a six-fofd disadvantage.
More specifically, to compete on an energy cost basis with the electric discharge method, an electrolytic process must yield at least a six-fold increase in ozone. Heretofore, the necessary high-yields have not been realized in any fore- seeably practical electrolytic system.
The evolution of 0₃ by electrolysis of various electrolytes has been known for well over 100 years. High yields up to 35% current efficiency have been noted in the literature. (Current efficiency is a measure of ozone production relative to oxygen production for given inputs of electrical current, i.e., 35% current efficiency means that under the conditions stated, the O_Z0₃ gases evolved at the anode are comprised of 35% 0₃ by volume). However such yields could only be achieved utilizing very low electrolyte temperatures, e.g., in the range of -30 to -65°C. Maintaining the necessary low temperatures, obviously requires costly refrigeration equipment as well as the attendant additional energy costs of operation.
An electrolytic process for the production of 0₃ has now been devised which greatly increases the production efficiency of 0₃ to an extent sufficiently high to compete with the prior art electric discharge process.
The present invention provides an electrolytic process for the production of 0₃. The invented process yields 0₃ with very 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 fluoroanions are among the most electronegative of all anions. The hexafluoro-anions are most preferred, and in particular, the hexafluoro-anions of phosphorus, arsenic, and silicon. The ozone is produced in an electrolytic cell utilizing an electrolyte consisting of water and the acids or salts of the fluoro-anions dissolved therein. The fluoro-anion electrolytes are capable of producing high yields of 0₃ in the practice of the invention method.
The electrolytic cells employ conventional techniques in their construction, taking into account the corrosive nature of the electrolytes. However, as will be subsequently disclosed, careful selection of anode materials is advisable to maximize the advantage of the hexafluoro-anion electrolytes. The proper anodes substantially contribute to the high yields of 0₃ from the cells. Proper selection of electrode materials also minimizes current consumption for a given 0₃ yield, and reduces deterioration of the electrodes from the corrosive action of the electrolytes. Electrode materials such as platinum and lead dioxide in the beta crystalline form are useful electrode, especially anode, materials. However, a special form of carbon, specifically vitreous, or glassy carbon has also shown excellent results when utilized as electrode materials in the invention process.
In addition, the process of the invention, unlike previous 0₃ electrolytic processes, may be carried out at ambient or only slightly lower temperatures, and no special refrigeration of the cells is required.
Power requirements and current densities are well within conventional electrolytic cell practice. In somewhat more detail, 0₃ in high relative yield, is produced from electrolytic cells of conventional construction. Anodes of platinum, lead dioxide, or glassy carbon are most preferred. Cathodes of platinum, nickel, carbon, glassy carbon, or materials coated with the platinum metals i.e., those materials exhibiting low hydrogen overvoltages are preferred if hydrogen is to be evolved as the cathodic process. Alternately, oxygen may be reduced at an air or oxygen depolarized cathode. (Substantial savings in cell voltage would result).
The cell electrolyte is of crucial importance. In particular, highly electronegative fluoro-anions, having a "composite electronegativity" (as is subsequently defined) of perhaps 17.5 or greater and particularly the hexafluoro-anions of phosphorus, arsenic, and silicon, are preferred.
The electrolyte consists of an aqueous solution of the highly electronegative anions and any suitable cationic component, most usually the acid form of the anion, i.e., H⁺A-; or a cation of the soluble salts thereof, e.g., alkali metals, especially Na⁺, which is usually the most soluble.
A D.C. current is impressed across the cell electrodes in the usual manner, whereby a mixture of 0₂ and 0₃ gas is generated at the anode and H₂ is generated at the cathode.

Detailed description of the invention

According to the method of the invention, 0₃ is produced in high current efficiency, by electrolyzing water in an electrolytic cell wherein monomeric anions of very high electronegativity are added to the aqueous electrolyte to be present during electrolysis.
The added highly electronegative anions and their accompanying cations first serve to carry an electric current between the electrodes of the cell, since water itself, the major component of the electrolyte is essentially non-conducting. Secondly, the added highly electronegative anions greatly increase the production of 0₃ at the cell anode at the expense of 0₂ which normally results when water is electrolytically decomposed.
The mechanism whereby 0₃ is produced at the expense of 0₂ is not fully understood, however, it undoubtedly is influenced by a number of factors including such considerations as the anion's ability to stabilize cationic species formed intermediate to the 0₃ formation process; and the anion's ability to absorb upon the surface of the anode to a limited and ideal extent during the electrolytic process. Whatever the mechanism, it has been found that when high electronegative anions are admixed with water in an electrolytic cell, the production of 0₃ in relation to O2 is significantly improved. Not only is the 0₃/0₂ ratio greatly improved, but the cell components, including the electrolyte, need not be refrigerated to temperatures below the freezing point of water as has been the case with some prior art methods.
The electrolyte should comprise a solution of the highly electronegative anions (and their accompanying cations) dissolved in water.
It is desirable that the anions be as electronegative as possible i.e., have a "composite electronegativity," as will subsequently be defined, of at least 17.5, and for this purpose, the fluoro-anions are eminently suitable. Fluorine is the most electronegative of all the elements. Fluorine has the further capability of complexing with other elements of the periodic table to form the most highly electronegative anions known. The group V-B elements of the periodic table, phosphorus and arsenic, form particularly desirable hexafluoro-anions. Other related non- metallic elements, such as silicon and antimony also form hexafluoro-anions. The phosphorus, arsenic, boron (BF4-) and silicon fluoro-anions are the preferred anions for addition to the aqueous electrolyte for the method of the invention. (Other members of the fluoro-anion class include P0₂F2^-, HTiF₆ ^- , NbF₇ ⁼, TaF₇ ⁼, NiF₆ ⁼, ZrF₆ ⁼, GeF₆ ⁼, FeF₆ ^-, and the polyhalogenated boranes).
The antimony hexafluoro-anion demonstrates anomalously low ozone yields when utilized in the method of the invention. It is believed that this anomaly occurs because of the fact that antimony hexafluoride-anion solutions dimerize to form Sb₂F₁₁ ^- ions. The dimerized antimony hexafluoride-anions have an extremely high composite electronegativity of 46.0 (SbF_a- being 26.0). The dimerized anion has an enormous electron withdrawing power and it is believed that its extremely high electronegativity totally stabilizes an intermediate cationic species and so effectively inhibits ozone formation.
In any event, with the exception of the antimony hexafluoro-anion, the remaining group V-B elements, P and As, as well as Si hexafluoro-anions and the BF₄- ion are most preferred for use in the electrolytes.
Both the arsenic hexafluoro-anion, the silicon hexafluoro-anion and the tetrafluorborate ion promote very high ozone yields when utilized in the present invention. However, the most preferred is the phosphorous hexafluoro-anion which achieves outstandingly high ozone yields.
The fluoro-anions may be added to the aqueous electrolyte solution either in the form of their respective acids or as water soluble salts. While the acid form of the fluoro-anions may be preferred because of their higher solubilities in water, it may at times be desirable to utilize the fluoro-anion salts e.g., of sodium or potassium, since aqueous solutions thereof produce higher pH's than does the acid form. As will be noted hereinafter, corrosion of the cell electrodes can be a problem because of the low pH and extremely corrosive nature of the fluoro-anions. Therefore in the event that corrosion problems become excessive, the alkali metal salts of the hexafluoro-anions may be utilized to increase the pH and thereby alleviate electrode corrosion. Alternately, mixtures of the aforementioned anions may be utilized in order to maximize ozone yield, while minimizing corrosion problems.
On the other hand, the reduced solubility of the salts of the hexafluoro-anion can also lead to reduced ozone yields and the concurrent reduction of ozone yields must be balanced against reduced electrode corrosion when the salts are utilized in the electrolyte solutions.
A measure of the "composite electro negativity" of the fluoro-anions utilized in the present invention can be calculated from compilations of the electronegativity of the various elements which can be found in any standard work on inorganic and/or electrochemistry. The value of the electronegativity of any of the anions is calculated, on a per charge basis, as the absolute value of a direct summation of atomic electronegativities on the Allred-Rockow scale (see for example, Anor- ganische Chemie, Cotton and Wilkinson, Verlag Chemie, Basel, 1980 or Advanced Inorganic Chemistry, Cotton and Wilkinson, 3rd Edn. Interscience, New York, 1972). The "composite electronegativity" of the phosphorus fluoro-anion PF6 - is the highest of those hexafluoro-anions noted, and its use in the electrolytic process of the invention results in the highest 0₃ current efficiencies.
As will be noted subsequently, ozone current efficiencies are generally in accordance with the composite electronegativity of the anions utilized in the electrolytes. As noted previously, the antimony hexafluoro-anion is an exception to the rule, apparently for the reasons noted.
In any event, the highly electronegative anions either in the acid or salt form, are dissolved in water to form the electrolyte for use in the method of the invention.
From an ozone current efficiency standpoint, it is desirable to increase the fluoro-anion concentration in the electrolyte to the maximum of solubility. Increasing the anion concentration in the electrolyte, increases the ozone current efficiency in all instances. It should be borne in mind, however, that as the anion concentration increases, problems associated with electrode corrosion (except for glassy carbon) also increase to the extent that reductions of anion concentration below the maximum possible may be desirable. In any event, however, increasing anion concentration results in increasing ozone current efficiency.
As an illustration of the effect of anion concentration on ozone current efficiency, some tests were conducted utilizing various concentrations of HPF₆ in a cell having a Pb0₂ anode. At a 2.25 molar concentration, ozone was produced in as high as 21 % current efficiency. At 5 molar concentration, ozone was produced in a current efficiency of as high as 34%. At 7.3 molar concentration (maximum solubility of commercially available HPF₆) ozone was produced at slightly over 50% current efficiency. However, at the 7.3 molar concentration the anode potentials measured were very high and erratic.
When platinum anodes were substituted for the lead dioxide anodes, the same trend of increased concentration leading to increased current efficiency was observed. For instance, at a 3 molar concentration, ozone was produced in slightly under 10% current efficiency. On the other hand, at the 7.3 molar concentration, ozone was produced in slightly greater than 50% current efficiency. Similar results were observed in the case of the related fluoro-anions.
Unlike prior art ozone electrolytic production processes, the electrolysis utilizing the present invention method may be conducted at ambient or moderately lower temperatures. Of course, the passage of electrical current through the cell results in heating effects on the electrolyte and the cell components. It is therefore desirable to provide some cooling of the electrolyte and the cell electrodes. For this purpose, the electrolyte may be circulated to an external heat exchanger in order to maintain the temperature at, or slightly below ambient temperatures.
Slightly refrigerated coolants may be supplied to the heat exchangers or to the internal passages within the electrodes so that the electrolysis can proceed at temperatures from ambient and ranging down to 0°C. Increase in current efficiency of ozone production may be realized by cooling the cell and its electrolyte below the noted temperatures, but energy penalties of such refrigeration will result.
Proper selection of the electrolytic cell electrodes is very important to maximize ozone yields.
It will be understood that when practicing the method of the invention, hydrogen may be produced at the cell cathode. Conventional cathode materials may be utilized taking into account the corrosive nature of acid solutions of the electrolyte as well as the advantage in employing materials which exhibit low hydrogen overvoltages. Utilizing the above criteria, it will be understood that conventional cathode materials for hydrogen evolution such as platinum, carbon (including glassy carbon), platinized metals and/ or nickel are satisfactory for use as the cell cathodes. Alternately an air or oxygen depolarized cathode could be used. The reaction at this cathode would be:
There are several advantages to the incorporation of an air cathode into the process. They are:

(1) The cell voltage would be substantially reduced. Replacing hydrogen evolution with the reduction of oxygen theoretically saves 1.23 v. (In actual practice a 0.8 v swing is likely to be achieved).
(2) A separator between anode and cathode is no longer required, as no hydrogen is evolved to depolarize the anode. Further, savings in cell voltage result as I-R losses are reduced.
(3) The overall cell process becomes oxygen in and ozone out. The need for periodic additions of water is reduced.
(4) 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.

Air cathode technology is highly developed due to recent interest in its application to fuel cells, metal-air batteries, and the chlor-alkali industry. The electrodes are generally composed of teflon- bonded carbon containing small amounts of catalytic materials. The cathodes may be readily purchased. Their incorporation into a process for ozone manufacture is regarded as little problem.
On the other hand, the selection of anode materials is quite important to the maximization of ozone production. The anode material must be stable to strong anodic polarization; that is, 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 current densities needed to achieve a sufficient anodic potential for ozone formation. The anode material must also be stable to the high interfacial acid concentrations produced by anodic discharge of water as well as the chemically corrosive nature of the highly electronegative anions. It has been determined that three materials eminently satisfy the criteria for anodes. These materials are platinum metal, lead dioxide, especially lead dioxide in the beta-crystalline form, and glassy carbon.
Electrolysis cells in which the production of the ozone is carried out may follow standard technology taking into consideration the corrosive nature of the fluoro-anion electrolytes and the high oxidizing power of the ozone gases. As oxygen and ozone are produced at the cell anode when hydrogen is produced at the cell cathode, additional precautions must be taken to ensure the separation of the cathode gases from the anode gases. Providing for the above-noted considerations, however, is well within current cell technology. If, however, oxygen is reduced as the cathodic process no such separation is necessary. More specifically, the electrolysis cell chamber should be constructed of materials which are inert to the highly corrosive electrolyte. The chamber should therefore be coated with inert polymeric materials, perhaps even polyfluorinated polymers, e.g., Teflon, which is resistant to oxidizing gases and has an excellent resistance to highly acid and corrosive solutions.
Provisions must also be made for separating the anode compartment of the cell chamber from the cathode compartment in order to fully separate the hydrogen evolved at the cathode (if this cathodic reaction is chosen) from the gases evolved at the anode. Such separators are well known in the art, with a particularly useful separator being constructed of "Nafion" a perfluorinated polymeric ion exchange material which is available from E. I. Dupont. Membranes of such material, while forming a liquid and gas barrier, permit electric current flow between electrolytes or electrodes in contact with the two sides of the Nafion. Provision of such a barrier or similar barriers prevents interaction of the electrode gases. Such a separator is not needed if oxygen is to be reduced in the cathodic process.
Incorporation of such a membrane as Nation may be advantageous for a second reason. The cell electrodes may be pressed against it, thus minimizing the inter-electrode gap, resulting in reduced ohmic losses during electrolysis. Such a measure would increase the overall energy efficiency of the electrolysis. In such a practice, the electrodes might consist of fine wire meshes, or powdered materials held in place by wire meshes. Water flows in from the back side of the mesh and is decomposed on its surface. The fluoro-anion necessary for efficient ozone evolution is supplied by this electrolyte. The anodic and cathodic gases also escape through the back of the mesh electrodes. This "solid polymer electrolyte" (S.P.E.) technology has been fully developed by General Electric Corp. as applied to fuel cells and to water electrolysis (producing hydrogen and oxygen). The concept is also applicable to electrolytic ozone generation.
Electrodes for use in the cells, whether in conventional or S.P.E. geometry must be carefully selected; with special care being given to the selection of the anode materials.
As noted, if hydrogen is to be evolved at the cell cathode, any cathode material which exhibits resistance to acidic electrolytes and which has a low hydrogen overvoltage is suitable. For instance, the platinum metals, nickel or carbon may be used. In addition, materials coated with the platinum metals may be utilized for the cell cathode. If oxygen is to be reduced, teflon- bonded carbon porous electrodes are used. These may be catalyzed with the platinum metals, or certain oxides.
The selection of the anode material on the other hand, is much more critical to the successful operation of an ozone electrolysis cell. It has been determined that several materials demonstrate excellent performance as anode materials in the presence of the highly electronegative fluoro-anion electrolytes. These materials are platinum metal, the two crystalline forms of lead dioxide, as well as glassy carbon.
Ozone current efficiencies in cells utilizing platinum anodes are quite excellent; and in addition, the platinum electrodes are relatively inert to the corrosive effects of the fluoro-anion electrolytes. Anodes constructed of lead dioxide, and specifically lead dioxide in the beta-crystalline form, demonstrate even higher ozone current efficiencies than do platinum anodes. On the other hand, lead dioxide anodes are more susceptible than platinum to the corrosive effects of highly concentrated fluoro-anion electrolytes. Thus, where platinum or lead dioxide anodes are contemplated for use, the selection is most advantageously determined by the desirability of the highest ozone current efficiencies expected from the cell in contrast to the rapidity with which the anode is corroded by the particular electrolyte under utilization. If extremely high ozone current efficiency is desired and corrosion of the anode is a a secondary consideration, then the logical anode material would be lead dioxide in the beta-crystalline form. On the other hand, if the higher ozone current efficiency is not of prime importance, but anode durability is, then the logical anode material would be platinum, or glassy carbon, as hereinafter discussed. Obviously, the cost of anode materials may also be important and the economics of cell materials is also a factor to be considered.
In any event, platinum, lead dioxide (especially in the beta-crystalline form), and glassy carbon are excellent materials for the anode in the electrolytic cells of the present invention. Platinum has traditionally been used in investigations of the ozone evolution process. Even at current 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. As will be noted subsequently, ozone current efficiencies utilizing platinum anodes are quite excellent at all current densities and electrolyte concentrations.
However, ozone current efficiencies in cells utilizing lead dioxide anodes are consistently higher than in those using platinum anodes. Beta lead dioxide anodes give better yields than platinum in all electrolyte systems at ordinary current densities at near ambient temperatures.
Lead dioxide has two common crystalline forms, denoted as alpha and beta. Either crystalline structure may be electrodeposited on a suitable substrate in a pure, glassy form by controlling the pH, temperature and current density in the deposition process. For the purposes of the present invention, the beta crystalline form is more highly desired than the alpha crystalline form.
The beta crystalline form of lead dioxide is a tetragonal rutile structure of unit cell dimensions 3.8, 4.94 and 4.94 angstroms. Beta lead dioxide has a higher oxygen overvoltage than alpha lead dioxide and in fact, has a greater overvoltage than that of platinum. Cells with beta lead dioxide anodes give very high yields of ozone in all electrolyte systems studied at near ambient temperatures.
Lead dioxide anodes for use in the electrolytic cells of the invention may be prepared as follows:
Lead dioxide is deposited anodically which limits the choice of substrate materials. Most metals dissolve when the deposition is attempted. However, the noble metals, carbon, titanium, and tantalum, are suitable as substrates for the anodes.
Titanium and tantalum when utilized as substrate materials are first platinized to eliminate passivation problems sometimes encountered with the uncoated substrates.
Carbon may be utilized as a substrate, however, lead dioxide adherence is a particular problem if the carbon has not been thoroughly degassed. The carbon is degassed by boiling in water for some time followed by vacuum drying over a period of days. When degassed, adherence is greatly improved with respect to thermal stress. Vitreous orglassy carbon does not appearto have the adherence problem. Vitreous or glassy carbon may make a good choice for anode substrate material; or as hereinafter noted, makes excellent anode material, per se.
Platinum is the most convenient substrate material to work with, gives most uniform deposits and does not present any additional problems. Thus it is the most suitable substrate material for lead dioxide anodes. However, its high cost may make other previously mentioned substrate materials'more practical for commercial use.
In any event, lead dioxide is plated onto substrates from a well known plating bath comprising essentially lead nitrate, sodium perchlorate, copper nitrate, and a small amount of sodium fluoride and water. The substrate material is set up as the anode in a plating bath. The pH of the bath is maintained between 2 and 4. Current densities of between 16 and 32 milliamperes per square centimeter give bright, smooth and adherent lead dioxide deposits. Bath temperature is most usually maintained at about 60°C at all times during deposition. The deposition is carried out with vigorous stirring of the electrolyte and rapid mechanical vibration of the anode to give consistently finely granular deposits free from pinholes or nodules. A surface active agent may be added to the plating solution to reduce the liklihood of gas bubbles sticking to the anode surface.
By such method as noted above, excellent beta lead dioxide anodes may be prepared for use in the cells of the invention. It has also been determined that anodes prepared from glassy carbon compare very favorably with the other anode materials, i.e., platinum and -lead dioxide.
Glassy carbon is a particular form of carbon prepared by the controlled pyrolysis of successive layers of organic solutions of long-chain polymeric precursors in an inert atmosphere. The random structure of the polymer is nearly preserved, with only sub-microscopic graphitic regions occurring. Extraordinary chemical and physical properties result from this process. A high degree of resistance to oxidation, even at elevated temperature, is achieved. In many circumstances where ordinary forms of carbon (such as graphite, the most generally inert) degrade, glassy carbon remains unaffected. The intergraphitic carbon intrusion mechanism of attack is inhibited due to the absence of long- range order in glassy carbon.
The physical, chemical and electrochemical properties of glassy carbon vary with the method of preparation. Several starting polymeric resins are used, and pyrolysis temperatures ranging from 600 to 3000°C are employed. The heat treatment time is also of influence on the ultimate properties. With these three variables it is possible to obtain varying proportions of _Sp² and _Sp³ coordination of individual atoms. This then determines density, chemical inertness, and electrical and electrochemical properties traceable to variations in band gap. In general, resistivities of 30 to 80_X10-' ohm-cm are encountered. With all preparation methods the carbons are extremely hard (6 to 7 Mohs scale), non-porous, and gas impermeable.
Glassy carbon is commercially available from such sources as the Tokai Mfg. of Japan, and LeCarbone-Lorraine of France. However, due to limited application, and time consuming preparation, glassy carbon remains expensive.
Since glassy carbon is extremely hard and , brittle, special techniques must be employed to shape and prepare it for use as an anode in the electrolytic cell. Fortunately the material can be ordered from the manufacturers in a great variety of sizes and shapes; and, in fact, can be pyrolyzed from the forming resin to most any size or shape specified by the consumer.
Electrical connection to the electrode can be by a number of means. Mercury contacts and electrically conductive epoxy pastes (silver filled) are several suitable types of connection of the electrode to the source of power.
The glassy carbon is isotropic and for this reason, unlike pyrolytically grown graphite, it does not require any definite orientation in the electrolytic cell. In addition, at least with BF4 and PF_-6 anion solutions, the glassy carbon anodes appear to be more corrosion resistant with increasing ionic and acidic concentrations.
The cathode and anode are positioned within the electrolytic cell with electrical leads leading to the exterior. The cell is also provided with appropriate plumbing and external structures to permit circulation of the electrolyte to a separate heat exchanger. Suitable inlet and outlet passages are also provided in the cell head space to permit the withdrawal of the gases evolved from the cathode (if hydrogen is to be evolved) and from the anode. The two gas removal systems are maintained separate in order to isolate the cathode gases (when hydrogen is chosen to be evolved) from the anode gases. Nitrogen and/or air may be pumped through the gas handling system in order to entrain the evolved cathode and anode gases and carry them from the cell to the exterior where they may be utilized in the desired application. Alternately, if a flow-through air (or oxygen) cathode is employed, its excess gases may be used for this purpose.
In order to maintain or cool the cell electrodes, heat exchange passages may be provided within the electrode structures. These coolant passages are connected to external sources of coolant liquid which can be circulated through the electrodes during the electrolysis process in order to maintain or reduce their temperatures.
The electrodes through the electrical leads are connected to an external source of electric power with, of course, the polarity being selected to induce the electrolyte anion flow to the anode and cation flow to the cathode.
In order to drive the electrolysis reaction, it is necessary to apply electric power to the cell electrodes. The power requirements are not appreciably different for those cells utilizing platinum anodes from those cells utilizing lead dioxide anodes or glassy carbon anodes. Electrical potentials in the order of from 2-3 volts D.C. are quite sufficient for the various cell configurations. The current requirements are most easily measured in the terms of current density and may vary from a low of perhaps a tenth of an ampere per square centimeter up to densities slightly beyond one ampere per square centimeter. The power requirements are not necessarily dependent upon the electrolyte concentrations, nor in particular upon the anode materials. Thus current densities of from about 0.1 A/cm² to about 1.5 A/cm² will produce maximum ozone current efficiencies subsequent to start up of the electrolytic process.
In the case of platinum anodes the rise-time to maximum ozone yield is about 30 minutes. Lead dioxide anodes on the other hand, require perhaps 90 minutes to reach maximum ozone production. The glassy carbon anodes, on the other hand, are independent of time when used in ozone production. That is, the current efficiencies remain constant over extended production runs. In some tests with glassy carbon, ozone current efficiencies remained constantly high over runs of 2 hours at current densities of 0.4 Alcm² and 0.8A/cm^2. These results were in contrast with the rise time behaviors of Pt and Pb0₂ anodes noted above.
In a group of tests with Pt and Pb0₂ anodes, ozone current efficiences were determined utilizing electrolytes, anodes, in accordance with the invention.
The results of these tests are set forth in Table I below:
As will be noted from the results above, ozone can be produced at current efficiencies above 50% in electrolytic cells having either lead dioxide or platinum anodes and utilizing hexafluorophosphate-anion electrolyte.
In an evaluation of glassy carbon as anodes in the 0₃ process, three different samples were used. These were: an analytical electrode, presumed to have been produced by Tokai Mfg. of Japan and distributed by Princeton Applied Research (PAR), and two plates supplied by the Gallard Schlesinger Co. and believed to have been made by LeCarbone-Lorraine, France.
The starting material of the PAR electrodes was either a furfuryl alcohol or phenol formaldehyde resin, the Gallard Schlesinger starting materials being proprietary. The heat treatment temperature (HTT) of the PAR material was unknown, whereas the two Gallard Schlesinger samples (GS V-10, GS V-25) differed only in their heat treatment. The GS V-10 sample was heat treated to 1000°C, and the GS V-25 material was heat treated to 2500°C. These differences give rise to variations in yield of ozone when the materials are employed as anodes.
For experimental testing the above electrode materials were machined into 1 to 2 cm²samples of approximately 1 mm thickness and pressfit into teflon holders. Silver epoxy connections were then made to the rear surface of the carbon samples within a hollow cavity of the teflon holders.
As an anode for the evolution of ozone, glassy carbon meets the required criteria of stability to high concentrations of strong acid and to anodic polarization at high current density. The over- potential for oxygen evolution is comparable to that of platinum and lead dioxide. A high oxygen overvoltage is necessary to inhibit the competitive reaction of oxygen evolution and thus enhance ozone yields. Yields on the order of 25 to 30% current efficiency have been regularly reproduced in 7.3 M HBF₄ (tetrafluoroboric acid) electrolyte at 0°C; as compared with yields of 18% with Pb0₂ and 5% with Pt under identical conditions. Pressed carbon black and graphite rapidly degrade under these circumstances, and evolve only traces of ozone.
The GS V-10 glassy carbon anode was tested at increasing current densities in various concentrations of tetrafluoroboric acid at 0°C. At a current density of about 0.24 A/cm², the ozone current efficiency (ratio of 0₃ gas evolved relative to O₂ gas evolved) was about 1 1/2% for 2M HBF₄, about 10% for 5M HBF₄, and about 21 % for 7.3M HBF₄. At a current density of about 0.56 A/cm2, the ozone current efficiency was about 2% for 2M HBF₄, about 15% to 5M HBF₄, and about 26.5% for 7.3M HBF₄. At a current density of about 0.86 A/cm², the ozone current efficiency of 2M HBF₄ remained at the 2% level, while 5M HBF₄ had increased to about 17%, and 7.3M HBF₄ had increased to about 28.5%. The current efficiencies remained at the same levels when current densities were increased further.
The electrode was visibly attacked at the 2M concentration, less at 5M, and apparently not at all at 7.3M, the highest concentration level of HBF₄ available commercially.
The GS V-10 and GS V-25 anodes were compared to test the effect attributable to the method of preparation of glassy carbon. When run in 7.5M HBF₄ at 0°C at various current densities, the GS V-10 anode yielded consistently higher ozone current efficiencies. At a current density of about 0.2 A/cm², the GS V-10 anode yielded about a 14% current efficiency, and the GS V-25 anode yielded about an 11% current efficiency. At 0.4 A/cm², the GS V-10 anode yielded about a 21% current efficiency, while the GS V-25 anode yielded about a 16% current efficiency. At a current density of 0.6 A/cm², the GS V-10 anode yielded about a 24% current efficiency, while the GS V-25 anode yielded about a 19% current efficiency. At 1.0 A/cm², the GS V-10 anode yielded about 24.5% ozone current efficiency, and the GS V-25 anode yielded about 22% ozone efficiency.
Both samples were inert to electrochemical or corrosive attack during the tests.
Further tests with the PAR glassy carbon anode indicated that ozone current efficiencies, as in the case of Pt and PbO₂ anodes, decrease as the electrolyte temperature increases. Nonetheless, ozone current efficiencies of about 25% were exhibited when the cell was run with water from the city mains (about 13°C) as the coolant.
When glassy carbon anodes were run in contact with electrolytes other than HBF₄ and HPF₆, ozone current efficiencies were poor. Yields in H₂SiF₆ and H₂SO₄ electrolytes gave only 1 to 2% ozone current efficiencies. In addition, anode corrosion was excessive. HPF₆ yields were comparable to those in HBF₄.
From the above tests it is apparent that glassy carbon is an anode material comparable to both Pt and Pb0₂ for use in electrolytic cells for the generation of ozone from aqueous electrolytes of highly electronegative fluoro-anions.

Claims

1. A method of improving the yield of ozone in relation to the yield of oxygen from the anodic decomposition of water in aqueous fluorine-containing electrolytes characterised in that a monomeric ion is present during the electrolysis which has four or more fluorine atoms and one or more of the elements P, As, Si, Sb, B, Ti, Nb, Ta, Ni, Zr, Ge or Fe.

2. The method of Claim 1, wherein the anion is in the acid or water soluble salt form.

3. The method of Claim 2, wherein the soluble salt is an alkali metal salt.

4. The method according to any one of the preceding Claims, wherein the fluoro-anion is a hexafluoro-anion.

5. The method of Claim 4, wherein the hexafluoro-anion is selected from the group of PF₆ ^-, AsFs-, and HSiF_s- and mixtures thereof.

6. The method according to any one of the preceding Claims, wherein the anode is platinum or beta-Pb0₂, or glassy carbon.

7. The method according to any one of the preceding Claims, wherein the water and admixed anion electrolyte is electrolyzed in the presence of an air or oxygen depolarized cathode.

8. The method according to any one of the preceding Claims, wherein a solid polymeric electrolyte is placed between the anode and cathode of the cell.

9. An electrolyte for producing ozone in an electrolyte cell, comprising water characterised in that it further comprises a monomeric anion as defined in any one of Claims 1, 4 and 5.

10. A method for producing ozone from an electrolytic cell comprising passing an electric current through a fluorine-containing aqueous electrolyte characterised in that the electrolyte comprises a monomeric anion as defined in any one of Claims 1, 4 and 5.

11. An electrolytic cell for the production of ozone by the electrolysis of aqueous solutions of electronegative fluoro-anions comprising glassy carbon electrodes in said cell.

12. An electrolytic cell according to Claim 11 further comprising a solid polymeric electrolyte between the anode and cathode of said cell.

13. A method for producing ozone from an electrolytic cell characterised in that said cell is provided with a glassy carbon anode and a corrosion resistant cathode and an electrolyte including water and electronegative fluoro-anions dissolved therein.

14. A method for improving the service life of electrodes and the ozone yield in an ozone producing electrolytic cell characterised in that the cell comprises an aqueous electrolyte of fluoro-anions as defined in any one of Claims 1, 4 and 5 and electrodes fabricated from glassy carbon.

15. A method according to any one of Claims 10, 13 or 14, wherein a solid polymeric electrolyte is placed between the anode and the cathode of the cell.