Patent Publication Number: US-8980079-B2

Title: Electrolytic cell for ozone production

Description:
RELATED APPLICATIONS 
     This patent application claims priority from provisional U.S. patent application No. 61/419,574, filed Dec. 3, 2010, entitled, “Electrolytic Cell for Ozone Production,” and naming William J. Yost III, Carl David Lutz, Jeff Booth, Don Boudreau, and Nick Lauder as inventors, the disclosure of which is incorporated herein, in its entirety, by reference. 
    
    
     TECHNICAL FIELD 
     The present inventions relate to electrolytic cells, and more particularly, to ozone producing electrolytic cells having solid electrolyte membranes. 
     BACKGROUND ART 
     Electrolytic cells may be used for the production of various chemistries (e.g., compounds and elements). One application of electrolytic cells is the production of ozone. Ozone is an effective killer of pathogens and bacteria and is known to be an effective disinfectant. The U.S. Food and Drug Administration (FDA) approved the use of ozone as a sanitizer for food contact surfaces and for direct application to food products. Accordingly, electrolytic cells have been used to generate ozone and dissolve ozone directly into source water, thereby removing pathogens and bacteria from the water. As a result, electrolytic cells have found applications in purifying bottled water products and industrial water supplies. 
     SUMMARY OF THE EMBODIMENTS 
     In a first embodiment there is provided an electrolytic cell for producing ozone. The cell includes an anode including a free-standing diamond material, and a cathode spaced from the first electrode, and a proton exchange membrane. The proton exchange membrane is between the anode and the cathode and separates the anode and the cathode. 
     In some embodiments, the cathode also includes a free-standing diamond material, and the cell is configured to reverse polarity between the anode and the cathode. In some embodiments, the free-standing diamond material includes boron doped diamond material. 
     In some embodiments, the anode and the cathode are in fluid communication to receive water from a common source, and in some embodiments the cell is configured to split source water flow into a first water flow and a second water flow, where the first water flow is supplied to the anode and the second water flow is supplied to the second electrode. In some embodiments, the cell is configured so that the first water flow and the second water flow are joined after at least one of the first water flow and the second water flow is provided with ozone. In yet other embodiments, the joined water flow is supplied to a chamber containing water, where the water within the chamber is purified by the ozone. 
     In some embodiments, the cell is configured to be installed within a pipe. 
     In yet other embodiments, the cell is free of a catholyte solution and a catholyte reservoir. 
     In some embodiments, the free-standing diamond material includes boron doped diamond material with a thickness of between about 100 microns and about 700 microns. 
     Some embodiments also include a cylindrical housing, a first semi-circular frame member, and a second semi-circular frame member. In some such embodiments, the anode, cathode and membrane are sandwiched between the first semi-circular frame member and the second semi-circular frame member, and the anode, cathode, membrane, first semi-circular frame member and second semi-circular frame member are within the cylindrical housing. In yet other embodiments, at least one of the first semi-circular frame member and the second semi-circular frame member is extendable to produce a compressive force on the anode, cathode and membrane. 
     In another embodiment, a diamond electrode includes a free-standing diamond material having a first side, a second side opposite to the first side, and a thickness of at least about 100 microns. The electrode also includes a current spreader coupled to the first side of the free-standing diamond material. The current spreader has an electrical contact and may have a mesh configuration or a frame configuration. In such an embodiment, the electrode is capable of conducting a current density of at least about 1 ampere per square centimeter through the free-standing diamond material for several hours (i.e., a sustained current density) without degrading the electrical conduction capacity or ozone-producing capacity of the electrode. In another embodiment, the free-standing diamond material has a thickness of at least about 200 microns. 
     In another embodiment, a method of operating an electrolytic cell includes providing an electrolytic cell having a first electrode of diamond material, a second electrode of diamond material, and a membrane between and separating the first electrode and the second electrode. The embodiment further includes providing, at a first time, a voltage differential across the first electrode and the second electrode, where the voltage differential has a first polarity, and then reversing, at a second time after the first time, the polarity of the voltage differential across the first electrode and the second electrode. The voltage differential has a second polarity at the second time. The method then reverses, at a third time after the second time, the polarity of the voltage differential across first electrode and the second electrode, such that the voltage differential has the first polarity at the third time. 
     Some embodiments include periodically reversing the polarity of the voltage differential, such that the voltage differential periodically alternates between the first polarity and the second polarity. 
     In some embodiments, the voltage differential produces a current flow through the first diamond material, where the current flow through the first diamond material has a current density of at least about 1 ampere per square centimeter during the entire interval between the first time and the second time. 
     Some embodiments also supply water to the electrolytic cell, where all of the water is supplied from a single source, and separate the water into two streams, where a first stream contacts the first electrode and the second stream contacts the second electrode. The first stream and second stream are separated by the membrane. The method then introduces ozone into the first stream at the first electrode, and then combines the first stream and the second stream to produce a combined stream, after introducing the ozone. Some embodiments direct the combined stream to a holding chamber. Other embodiments also provide additional water to the holding chamber, where the additional water is purified by the ozone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of embodiments will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which: 
         FIGS. 1A and 1B  schematically illustrates an electrolytic cell according to a first embodiment; 
         FIG. 2  schematically illustrates an electrode with a free-standing diamond; 
         FIG. 3  schematically illustrates a prior art laminated electrode; 
         FIGS. 4A-4D  schematically illustrate varies views of a current spreader; 
         FIG. 5  schematically illustrates an electrolytic cell according to another embodiment; 
         FIG. 6  schematically illustrates an electrolytic cell according to another embodiment; 
         FIG. 7  schematically illustrates an embodiment of an electrolytic cell within a housing; 
         FIG. 8  schematically illustrates an alternate embodiment of an electrolytic cell within a housing; 
         FIG. 9  schematically illustrates an embodiment of an electrolytic cell within a tube; 
         FIG. 10  schematically illustrates an embodiment of an electrolytic cell within a system; and 
         FIG. 11  illustrates a method of operating an electrolytic cell. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     In accordance with one embodiment, an electrolytic cell for producing ozone in flowing water includes at least one free-standing diamond electrode. The free-standing diamond electrode is capable of handling appreciably higher power than previously-known electrodes, and among other things is capable of producing more ozone. 
     One embodiment of an electrolytic cell  100  is schematically illustrated in  FIG. 1A , and a cross-section of that cell  100  is schematically illustrated in  FIG. 1B , exposing the internal components of the cell  100 . 
     As shown in  FIG. 1B , the electrolytic cell  100  has two electrodes: an anode  101  and a cathode  102 . In this embodiment, the anode  101  is a boron-doped free-standing diamond anode, while the cathode  102  is a formed from titanium or another conductive material. The anode  101  and the cathode  102  may include through-hole features  110  to increase their surface area and to allow water to pass through them. 
     To form ozone, a water source is supplied to the cell  100  and a positive electric potential is applied to the anode while a different electric potential is applied to the cathode  102 , so as to create a voltage differential (or potential difference) across the anode  101  and cathode  102 . In the embodiment shown in  FIG. 1 , the electrical potential is applied via anode and cathode contacts  103 ,  104 . On the anode side of the cell  100 , the difference in electric potential breaks up water molecules into 1) oxygen and 2) hydrogen cations. The oxygen forms into ozone, which dissolves into the water. The hydrogen cations are pulled from the anode side of the cell to the cathode side by the negative electric potential applied to cathode  102 . Once on the cathode side of the cell, the cations form hydrogen bubbles. 
     To facilitate the movement of protons (e.g., hydrogen cations) from the anode  101  to the cathode  102 , in some embodiments, a solid membrane  105  is used as a solid electrolyte and placed between the anode  101  and cathode  102  (e.g., a proton exchange membrane (PEM), such as Nafion®). Additionally, in some cases, the membrane  105  is used as a barrier to separate the source water flow on the cathode side of the cell  100  from source water on the anode side of the cell. To provide structural integrity to the membrane  105 , the membrane may also include a supporting matrix (not shown). 
     As illustrated, the membrane  105  is between the electrodes  101  and  102  and the contacts  103  and  104 . Indeed, such a configuration may describe the membrane as being “sandwiched” between the electrodes, and the arrangement of electrodes  101 ,  102  and membrane  105 , and/or the arrangement of electrodes  101 ,  102 , membrane  105 , and contacts  103  and  104 , may be described as forming an electrode sandwich. The sandwich is not limited to these components, however, and various embodiments may include other components or layers in the sandwiched stack. 
     In the embodiment of  FIGS. 1A and 1B , the cell  100  includes an anode frame  106  and a cathode frame  107 . The frames  106 ,  107  both position the anode  101 , cathode  102 , anode contact  103 , cathode contact  104 , and membrane  105 , and provide structural integrity to the assembly. The frames  106 ,  107  also include one or more openings  108  through which source water can flow. The size and shape of the openings  108  can be varied to achieve different flow rates through the cathode or anode areas by varying the fluid resistance of the openings either by size, length, or some other aspect of geometry. In some illustrative embodiments, the electrolytic cell also includes an O-ring  109  about its outer periphery. When the electrolytic cell  100  is inserted into a pipe (which may be a tube or other housing), the O-ring  109  may help secure and seal the electrolytic cell  100  to the inside perimeter of the pipe. Alternately, or in addition, the O-ring  109  may also provide a compressive force against the frames  106 ,  107 , to help “clamp” the frames  106 ,  107  to one another. 
     An embodiment of a free-standing diamond electrode  200  is schematically illustrated in  FIG. 2 , and includes a current spreader  201  and a free-standing diamond  202 . 
     The free-standing diamond  202  has a first side  202 A, and second side  202 B opposite to the first side. The diamond also has a thickness  202 C, defined as the distance between the first side  202 A and the second side  202 B. In the embodiment in  FIG. 2 , the free-standing diamond has a substantially uniform thickness, which is to say that its thickness is substantially the same at all points. 
     As used herein and in any claim appended hereto, a “free-standing diamond” is a non-laminated doped diamond material with a thickness of greater than about 100 microns. For example, the free-standing diamond may have a thickness of 100 microns, 200 microns, 300 microns, 400 microns or more. Indeed, some embodiment may have a thickness of 500 microns, 600 microns, 700 microns or more. 
     These thick diamonds are beneficially capable of carrying current at high current densities for sustained periods of time without a significant deterioration in performance, and without incurring substantial damage. For example, in some embodiments, the free-standing diamond is capable of conducting sustained current density of at least about 1 ampere (or “amp”) per square centimeter, while other embodiments are capable of conducting sustained current density of at least about 2 amperes per square centimeter, for example. During tests, the inventors have operated a free-standing diamond electrode at a current density of about 2 amperes per square centimeter for periods of at least about 500 continuous hours, without damaging the electrode or degrading its current carrying or ozone-producing performance. Such electrodes may produce more ozone per square centimeter of surface area than previously known electrodes, and may therefore be made more compact than a prior art electrode configured to produce the same amount of ozone per unit time. Electrodes according to various embodiments may also have longer useful and productive lifetimes than previously-known electrodes. 
     In contrast, prior art electrodes include laminated thin-film diamond layer, such as a thin film diamond coating on a substrate. See, for example, a paper titled “ Electrochemical Ozone Production Using Diamond Anodes And A Solid Polymer Electrolyte ” by Alexander Kraft et al, Electrochemistry Communications 8 (2006), 883-886. An exemplary prior art electrode  300  is schematically illustrated in  FIG. 3 , and includes a substrate  301  and a thin-film diamond layer  302 . The thin-film diamond layer  302  may be grown on the substrate  302 ; such a diamond layer does not exist before it is grown, in contrast with a free-standing diamond which may exist independent of a current spreader. 
     The structural and electrical integrity of the electrode  300  depends on the physical contact between the diamond layer  302  and the substrate  301 . That contact, and therefore the integrity of the electrode  300 , is compromised if the diamond layer  302  begins to de-laminate from the substrate  301 . Such delamination may be caused, for example, by thermal stress within the electrode  300 , and particularly as such thermal stress is expressed at the interface of the diamond layer  302  and the substrate  301 . Thermal stress, in turn, may be caused by differences in the coefficient of thermal expansion of the diamond layer  302  and the substrate  301 . Further, the thermal stress increases with increasing thickness  303  of the diamond layer. 
     For this reason, the diamond layers used in previously known electrodes have been of limited thickness and limited current density ratings. Limiting the thickness of the diamond layer of a laminated electrode limits the thermal stress generated as a result of the difference in the respective thermal coefficients of expansion of the diamond material and the substrate. Generally, the thickness of the diamond layer has been limited to ranges of about 10 microns or less. 
     However, guarding the structural integrity of an electrode by limiting the thickness of a diamond layer comes at a cost. Such electrodes have limited current density capacity. For example, current densities of less than about 400 millamps per square centimeter were reported in the paper titled “ Electrochemical Ozone Production Using Diamond Anodes And A Solid Polymer Electrolyte ” mentioned above. Indeed, some manufacturers of laminate diamond electrodes recommend keeping current density below 0.5 amps per square centimeter. Greater current densities, particularly if maintained for minutes or hours, may damage such electrodes and/or cause performance degradation, such as by causing the diamond layer and substrate to begin delaminating. Such a limited current capacity limits the electrode&#39;s ozone production capacity. 
     Returning to  FIG. 2 , the current spreader  201  is affixed to, and electrically coupled to, the free-standing diamond  202 . In operation, a voltage supply may be coupled to the current spreader to connect the free-standing diamond  202  to a host system. For example, the current spreader  202  includes an extended portion  203 , which extended portion may be used as an electrical contact, such as a bond to which a wire may be soldered for example. As such, the current spreader  201  is electrically conductive. In some embodiments, the current spreader may include metal, such as titanium for example. 
     Various embodiments of current spreaders may take a variety of forms. For example, a current spreader may be a mesh or lattice configuration. An embodiment of a lattice current spreader  703  is schematically illustrated in  FIG. 7 , for example. 
     An alternate embodiment of a current spreader has a “frame” shape, so-called because a portion of the frame has a rectangular or square shape, and thereby resembles the shape of a picture frame. An embodiment of a frame configuration of a current spreader  400  is schematically illustrated in  FIGS. 4A-4D , for example. Specifically,  FIG. 4A  presents a perspective view of the current spreader  400 , while  FIG. 4B  presents a side view,  FIG. 4C  presents a top view, and  FIG. 4D  presents a bottom view. The currents spreader  400  is conductive, and may include titanium, for example. The dimensions in  FIG. 4D  are illustrative and not intended to limit various embodiments. 
     A frame portion  401  of the current spreader includes an aperture  402 . The aperture  402 , when coupled to a free-standing diamond (not shown in  FIG. 4 ), presents a large area of the free-standing diamond to water, thereby facilitating the production of ozone. If the perimeter of the frame portion  401  defines an area, then the aperture  402  occupies most of that area. For example, the aperture  402  may occupy about 80 percent, about 90 percent, or more of the frame portion  401 . 
     An alternate embodiment of an electrolytic cell  500  is schematically illustrated in  FIG. 5 , and has several features similar to the electrolytic cell  100  discussed above, such as contacts  503 ,  504 , membrane  505 , and O-ring  509 . Such features are not discussed again here. 
     The electrolytic cell  500  differs from the electrolytic cell  100 , however, at least because electrolytic cell  500  has two free-standing diamond electrodes  501 ,  502 . As such, it is not necessary to identify one electrode as the anode and another electrode as the cathode. Either of the electrodes  501 ,  502  are capable of acting as the anode, as the cathode, or indeed even alternating back and forth between the roles of anode and cathode. In some embodiments, the cell  500 , or a system hosting the cell  500 , may include circuitry to reverse the polarity of the voltages input to the electrodes. Such circuitry may include, for example, a switching network having a number of switches coupled between the input voltages and the electrodes  501  and  502  to selectively direct a first input voltage to the first electrode  501  and a second voltage to the second electrode  502 , and to controllably reverse the polarity of the input voltages so as to direct the first input voltage to the second electrode  502 , and the second input voltage to the first electrode  501 . As such, one electrode  501  acts as the anode and the other electrode  502  acts as the cathode when the input voltage has a first polarity. However, when the input voltage polarity is reversed (i.e., to a second polarity), the first electrode  501  then acts as the cathode, and the second electrode acts as the anode. 
       FIG. 6  schematically illustrates another embodiment  600  of a two-diamond electrolytic cell. In  FIG. 6 , the cell  600  includes a serial configuration of boron doped diamond electrodes  601 ,  602  located on the same side of the membrane  603  and connected to electrode contacts  604 ,  605 , respectively. As shown in  FIG. 6 , the membrane  603  is in contact with both of the diamond electrodes  601  and  602 . In this configuration, cations travel horizontally through the membrane  603  between the electrodes  601  and  602 . 
     Another embodiment of an electrolytic cell assembly  700  is schematically illustrated in  FIG. 7 . In this embodiment, the cell assembly  700  includes a housing  700 A with a cylindrical interior volume  700 B (which housing may be referred to as a cylindrical housing, irrespective of its outer shape), and the diamond electrodes  701 ,  702 , current spreaders  703 ,  704 , membrane  705 , and semi-circular frames  706  and  707  reside within the cylindrical interior volume  700 B. 
     In this embodiment, water is supplied to the electrodes  701 ,  702  via a water passage  710  which is part of the housing  700 A. As the water approaches the electrodes  701 ,  702 , it encounters a divider  711  within the water passage  710 . The divider effectively forms channels that split the water into a first stream (which may be referred to as a first water flow) and a second stream (which may be referred to as a second water flow). These channels in turn direct the first stream to the first electrode  701 , and the second stream to the second electrode  702 . The first and second streams then flow separately, and some of the water molecules in the stream that passes the anode (which could be either electrode  701  or  702 , depending on the polarity of the voltage supplied to the electrodes) will have their hydrogen atoms and oxygen atoms disassociated, and some of the oxygen atoms will then form ozone. As such, ozone is introduced into one of the streams. In some embodiments, the streams may be recombined at a point after the streams pass the electrodes  701  and  702 . 
     In some embodiments, at least one of frames  706  and  707  may be extendable to produce a compressive force on the electrode sandwich. For example, a frame  706  and/or  707  may include two parts that are spring loaded such that the spring pushes against the two parts to urge them apart, thereby expanding the frame. As such, one part of the frame pushes against the cylindrical interior of the housing, while another part of the frame pushes against the electrode sandwich. 
     Yet another embodiment of an electrolytic cell  800  assembly is schematically illustrated in  FIG. 8 . This embodiment includes a different housing  800 A, but also has a cylindrical interior volume  800 B. This embodiment  800  includes an electrolytic cell  801  within the cylindrical interior volume  800 B. Specifically, electrolytic cell  801  includes at least one frame-shaped current spreader  802 , which may be similar to current spreader  400  discussed above. 
       FIG. 9  schematically illustrates an embodiment of system  900  hosting an electrolytic cell  901 . The system  900  includes an electrolytic cell  901  that is installed within the inside perimeter of a tube  902 . In this embodiment, the electrolytic cell may be cell  100  as discussed above, or may be another embodiment of an electrolytic cell described herein, for example. In the embodiment of  FIG. 9 , the O-ring  109  prevents water from flowing between the cell  900  and the inside perimeter of the tube  901 . 
       FIG. 10  schematically illustrates another embodiment of system  1000  hosting an electrolytic cell  1000 .  FIG. 10  shows an electrolytic cell  100  within a housing  1001  in accordance with one embodiment of the present invention. The electrolytic cell  100  in this embodiment is the cell  100  described above, but could be selected from among other embodiments disclosed herein, such as electrolytic cell  500  to name just one example, or an entirely different cell. 
     The housing includes an inlet  1002 , an outlet  1003 , and a water passage (or “piping)  1004  connecting the inlet  1002  to the outlet  1003 . In illustrative embodiments, the inlet  1002  and/or the outlet  1003  include push-n-lock tube connections for easy connection of the housing  1001  to a source water supply. Examples of connections that could be used are provided in application Ser. No. 12/769,133, which is incorporated herein, in its entirety, by reference. 
     According to various embodiments of the present invention, source water flows into the inlet  1002  and through the water passage  1004 , the electrolytic cell  100 , and the outlet  1003  in the direction shown by arrow  1005  in  FIG. 10 . A portion of the source water flows through the anode side of the cell  100  while another portion of the source water flows through the cathode side of the cell  100 . 
     As the water flows through the electrolytic cell  100 , a positive electric potential is applied to the anode  101  while a negative electric potential is applied to the cathode  102 . The electrical potential is applied via the anode and cathode contacts  103 ,  104 , which are, in turn, connected to a power source via electrical leads  1006 . In illustrative embodiments, the anode and cathode contacts  103 ,  104  are formed from titanium mesh or a titanium frame current spreader that is spot welded onto the electrical leads  1006 . In this way, the anode and cathode contacts  103 ,  104  allow source water to make contact with the surfaces of the anode  101  and the cathode  102 . The electrical leads  1006  pass through walls of the water passage  1004  and, in exemplary embodiments, bushing screws  1007  and O-rings  1008  are used to prevent leakage of source water between the leads and the walls of the water passage. 
     As explained above, the water on the anode side of the cell  100  forms 1) oxygen and 2) hydrogen cations. The oxygen forms into ozone, which dissolves into the water, while the hydrogen cations are pulled towards the cathode side of the cell and form hydrogen bubbles. Using system  1000  as an example, the water on the cathode side of the cell  100  (including the hydrogen) and the water of the anode side of the cell (including ozone and other species) and are joined and then flow out of the output  1003 . 
     The inventors recognized that mixing the water from the anode side the cell  100  and the cathode side of the cell has disadvantages. When the products of the electrolytic reaction are mixed, they react and recombine. For example, the hydrogen on the cathode side of the cell recombines with the ozone, hydroxyl radicals, and other oxygen derivatives from the anode side to form other species of chemicals. In some cases, as much as about 30% of the ozone may recombine downstream of the electrolytic cell  100  and, thus, reduce the net ozone production of the cell  100 . 
     Yet, the inventors recognized that, in illustrative embodiments of the present invention, this disadvantage is outweighed by the simple and economical design of the electrolytic cell  100 . As shown in the design of  FIGS. 9 and 10 , only a single water supply is necessary to supply the anode and cathode side of the cell  100  In contrast, in many prior art systems, the anode is supplied by a water supply and the cathode is supplied by a catholyte solution from a reservoir. This prior art arrangement adds complexity and cost to the electrolytic cell. 
     Furthermore, the inventors realized that the disadvantages associated with mixing products such as hydrogen and ozone can be limited by minimizing the exposure time of the products to one another. More particularly, the inventors discovered that exposure time can be minimized by flowing the water and the products into a large chamber or reservoir  1020 . In the chamber, the buoyant hydrogen bubbles rise to the top and move away from the ozone and, thus, no longer react and recombine. In one exemplary embodiment of the invention, the products flow into a large chamber immediately after they are formed. Typically, the less time the products (ozone and hydrogen) spend within the turbulent flow of the water passages, the less they recombine to nullify the ozone production of the cell. 
     The inventors have also recognized that there are certain disadvantages associated with an electrolytic cell that does not have catholyte solution supplied from a reservoir. During the electrolytic reaction, scale (e.g., calcium carbonate) from source water builds up or deposits on the membrane  105  and other components of the cell  100 . Eventually, if it does build up as noted, the scale impedes the electrochemical reaction within the cell  100 . Such deposits within the electrolytic cell  100  can shorten useful cell life, or require disassembly and cleaning of internal components to restore cell performance and efficient production of target chemistries, such as ozone. To help prevent this problem, prior art systems use a reservoir of catholyte solution (e.g., water with sodium chloride and/or citric acid) and apply the solution to the surface of the membrane and the cathode of the prior art devices. The catholyte solution helps prevent the buildup of scale on the membrane and the cathode and, thus, improves cell efficiency. 
     Nonetheless, the inventors have recognized that, although the catholyte solution helps prevent the buildup of scale, it also requires the use of additional parts and further complicates and adds cost to the design of electrolytic cells and systems that use them. The inventors further recognized that, in illustrative embodiments of the present invention, the disadvantages associated with scale build up are outweighed by the simple and economical design of the electrolytic cell  100 . As shown in the design of  FIGS. 9 and 10 , for example, illustrative embodiments of the present invention do not include a reservoir or a catholyte solution—in other words, such embodiments are free of a reservoir and a catholyte solution. This economical and simple design of the cell  100  allows for it to be replaced once it is no longer efficient. 
     Illustrative embodiments of the present invention are particularly useful as disposable and low cost solutions for water purification. Whereas more expensive and complex prior art systems require replacement of catholyte solution and/or disassembly of the cell to restore efficiency, illustrative embodiments of the electrolytic cell are simply removed, disposed of, and replaced with a new cell assembly. Although illustrative embodiments of the cell may have limited life times (albeit longer lifetimes that previously known cells), it may be more cost effective to simply replace disposable cells instead of maintaining more complex prior art electrolytic cells. Such disposable electrolytic cells are particularly useful when the source water supply has low levels of impurities. In such circumstances, scale build up is low and further mitigates the need for a catholyte solution. Other factors may also be present that mitigate the need for a catholyte solution. 
     A method  1100  of operating an electrolytic cell is illustrated in  FIG. 11 . As mentioned above, in an electrolytic cell that has two free-standing diamond electrodes, it is not necessary to identify one electrode as the anode and another electrode as the cathode. Either of the electrodes is capable of acting as the anode, as the cathode, or indeed even alternating back and forth between the roles of anode and cathode. This characteristic allows the operation of an electrolytic cell in such a way as to mitigate the buildup of scale. 
     As such, the method begins with by providing an electrolytic cell including a first electrode having a diamond material and a second electrode having a diamond material (step  1101 ). The electrolytic cell may be similar to the cells described above, or may be of another design. In some embodiments, the diamond electrodes are free-standing diamonds, but in other embodiments the diamond electrodes may even include laminated diamond layers as known in the art. The electrolytic cell also includes a membrane between the first electrode and the second electrode and separating the first electrode and the second electrode. 
     In operation, water is supplied to the electrolytic cell (step  1102 ). As mentioned above, some embodiments separate the incoming water into first and second streams, and direct the first stream to an anode, and the second stream to the cathode. As such, some embodiments separate the water into such streams at step  1102 . As noted above, some embodiments to not require or use an electrolyte solution. As such, all of the water may be supplied from a common source, rather than have some water supplied from a water source, and an electrolyte solution supplied from a different source. Therefore, some embodiments, supply water to the electrolytic cell from a single or common source. 
     As mentioned above, an electrical potential difference is supplied across the electrodes when the cell is in operation. As such the method also provides, in step  1103  at a first time, a voltage differential across the first electrode and the second electrode, the voltage differential having a first polarity. 
     While in this configuration, scale may begin or continue to build up on the electrodes. To combat scale build up, the next step reverses the polarity of the voltages to the first electrode and the second electrode (step  1104 ). This step  1104  is performed at a second time later than the first time, and the voltage differential thereby has a second (opposite, or inverse) polarity at the second time. By reversing the polarity of the voltage, the forces of attraction between the electrodes and the scale is also reversed, such that an electrode that attracted scale under the first polarity, now repels scale under the second polarity. Repeated reversal of the polarity over time (e.g., first polarity; second polarity; first polarity; second polarity, etc.) may help mitigate scale buildup, and may even reverse previously built-up scale. 
     As such, the process includes another reversal of the voltage differential at a third time after the second time (step  1105 ). This new voltage differential has the first polarity at the third time. 
     This process or cycle of polarity reversal may be repeated periodically. The period of the cycle may be determined by the systems operator, and the chosen period may depend on such factors as the size of the electrolytic cell, the rate of water flow past the electrodes, and the content (e.g., impurity content) of the water, among other things. For example, the polarity may be reversed once each minute, once each hour, once each day, or periodically, periodically or even randomly at various intervals. 
     The applied voltage differential produces a current flow through the first diamond material. In illustrative embodiments, this current flow through the first diamond material has a current density of at least about 1 ampere per square centimeter during the entire of interval between the first time and the second time. For example, during this time, the current flow can have a current density of about 1.5 amperes per square centimeter, about 2 amperes per square centimeter, 3 amperes per square centimeters, or great amounts as determined by those skilled in the art. 
     Then, the method introduces ozone into the first stream at the first electrode at step  1106 . Finally, the method combines the first stream and the second stream to produce a combined stream at step  1107 , after introducing the ozone. 
     Some embodiments also direct the combined stream to a holding chamber (step  1108 ). Further, some embodiments provide additional water to the holding chamber, where the additional water is purified by the ozone (step  1109 ). The additional water may be provided before, after, or during the arrival of the combined stream of ozone-laden water to the holding chamber. 
     The embodiments of the invention described above are intended to be merely exemplary; numerous variations and modifications will be apparent to those skilled in the art. For example, but without limitation, some embodiments describe a system with a specified electrolytic cell, but generally any such system could be configured to use any of the cells described above. As another example, the method of  FIG. 11  includes both splitting the water stream, and reversing polarity of the voltage across the electrodes. However, a method that splits the water stream could be implemented without reversing the polarity of the voltage, and a method that reverses the polarity of the voltage could be implemented without splitting the water stream. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.