Patent Description:
Quality drinking water is an ever-growing need as world population increases. Yet, sources of fresh water on land are limited, with some currently being depleted significantly. The water quality of other fresh water sources may be compromised by industrial and agricultural processes and the expansion of cities.

Considering the foregoing, technologies are being developed to obtain fresh water from abundant sea and ocean water sources. However, these water sources are saline water that contain high concentrations of dissolved salt, which render the water unsuitable for human consumption, agricultural use, or industrial processes. Saline water requires desalination to lower its concentration of dissolved solids (e.g. salt) so that it can be utilized as a source of drinking water.

Efforts to desalinate water date back thousands of years. For example, first recorded attempts include evaporation of salt water conducted by sailors at sea. The first large-scale modern desalination process of multi-stage flash distillation was developed during the middle of the <NUM>th century. Yet, common problems persist with desalination processes. These problems include without limitation relatively high energy demands, environmental concerns, and material issues related to corrosion of membranes. These problems have prevented a widespread use of desalination to provide drinking water from saline water resources.

Cells for removing ions from a flow of water are disclosed in <CIT>, <CIT>, <CIT> and <CIT>.

According to one embodiment, a desalination cell with the features of claim <NUM> is disclosed.

Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, "parts of," and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed.

The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

It must also be noted that, as used in the specification and the appended claims, the singular form "a," "an," and "the" comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.

As used herein, the term "substantially," "generally," or "about" means that the amount or value in question may be the specific value designated or some other value in its neighborhood. These terms may be used to modify any numeric value disclosed or claimed herein. Generally, the term "about" denoting a certain value is intended to denote a range within ± <NUM>% of the value. As one example, the phrase "about <NUM>" denotes a range of <NUM> ± <NUM>, i.e. the range from <NUM> to <NUM>. Generally, when the term "about" is used, it can be expected that similar results or effects according to the invention can be obtained within a range of ± <NUM>% of the indicated value. The term "substantially" may modify a value or relative characteristic disclosed or claimed in the present disclosure. In such instances, "substantially" may signify that the value or relative characteristic it modifies is within ± <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>% or <NUM>% of the value or relative characteristic.

It should also be appreciated that integer ranges explicitly include all intervening integers. For example, the integer range <NUM> to <NUM> explicitly includes <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. Similarly, the range <NUM> to <NUM> includes <NUM>, <NUM>, <NUM>, <NUM>,. <NUM>, <NUM>, <NUM>, <NUM>. Similarly, when any range is called for, intervening numbers that are increments of the difference between the upper limit and the lower limit divided by <NUM> can be taken as alternative upper or lower limits. For example, if the range is <NUM>. to <NUM> the following numbers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> can be selected as lower or upper limits.

In the examples set forth herein, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus <NUM> percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In a refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus <NUM> percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In another refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus <NUM> percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples.

For all compounds expressed as an empirical chemical formula with a plurality of letters and numeric subscripts (e.g., CH<NUM>O), values of the subscripts can be plus or minus <NUM> percent of the values indicated rounded to or truncated to two significant figures. For example, if CH<NUM>O is indicated, a compound of formula C(<NUM>-<NUM>)H(<NUM>-<NUM>)O(<NUM>-<NUM>). In a refinement, values of the subscripts can be plus or minus <NUM> percent of the values indicated rounded to or truncated to two significant figures. In still another refinement, values of the subscripts can be plus or minus <NUM> percent of the values indicated rounded to or truncated to two significant figures.

As used herein, the term "and/or" means that either all or only one of the elements of said group may be present. For example, "A and/or B" means "only A, or only B, or both A and B". In the case of "only A", the term also covers the possibility that B is absent, i.e. "only A, but not B".

The term "comprising" is synonymous with "including," "having," "containing," or "characterized by. " These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps.

The phrase "consisting of" excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

The phrase "consisting essentially of" limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter.

With respect to the terms "comprising," "consisting of," and "consisting essentially of," where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.

The term "one or more" means "at least one" and the term "at least one" means "one or more. " The terms "one or more" and "at least one" include "plurality" as a subset.

Description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description and does not necessarily preclude chemical interactions among constituents of the mixture once mixed. First definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

The Earth's increasing population has created an ever-growing need for clean fresh water for human consumption, agricultural purposes, and industrial purposes. Fresh water refers to a water solution having a low salt concentration (e.g. less than <NUM> %). With limitations on fresh water sources, numerous attempts have been made to produce fresh water from abundant sea and ocean waters by desalination. Desalination is a process of removing mineral components from saline water. Removal of salt and other chemicals from the saline water requires electric or thermal energy to separate the saline water into two streams. The two streams are a freshwater stream containing a low concentration of dissolved salts and a second stream of concentrated brine having a high concentration of dissolved salts.

Various desalination technologies have been developed, for example evaporation, freezing, distillation, reverse osmosis, ion exchange, and electrodialysis. Yet, these technologies have certain drawbacks that prevent their widespread use and limit their success. For example, reverse osmosis typically requires a large input of electrical energy, which makes this technology quite expensive. Additionally, reverse osmosis utilizes selective membranes which are susceptible to fouling or unwanted accumulation of mineral deposits on the membrane surfaces. The membranes thus need frequent replacement which contributes to maintenance demands and increased cost.

Electrodialysis is another membrane desalination technology implementing ion exchange membranes. Electrodialysis may be costly and does not have a barrier effect against micro bacterial contamination. Yet, membrane-free technologies present other challenges. For example, freeze-thaw typically relies on extended periods of natural sub-zero temperatures and is therefore limited to certain climatic conditions. Multi-effect distillation utilizes several stages or effects during which feed water is heated by steam in tubes onto which saline water is being sprayed. But this technology presents high operating costs unless waste heat is available for the desalination process, and high temperatures may increase corrosion and scale formation.

Among the newly developed concepts are electrochemical approaches to desalination such as a desalination battery or an electrochemical device. Desalination batteries use an electric energy input to extract sodium and chloride ions, as well as other impurity ions from saline water to generate fresh water. The battery thus presents dual-ion electrochemical deionization technology, including sodium and chloride dual-ion electrochemical electrodes to which voltage is applied to bring about separation of saline water into fresh water having a relatively low concentration of dissolved salts and a concentrated brine stream.

It would be desirable to provide a water treatment system utilizing a desalination cell. A non-limiting example of a water treatment system utilizing a desalination battery may include a container to retain a liquid solution such as saline water or desalinated water, two electrodes, a power source, a saline water inlet, and a freshwater outlet. Additional components such as additional inlets, outlets, and the like are contemplated. The two electrodes may be separated by an exchange membrane. The exchange membrane may be either anion or cation exchange membranes. The exchange membrane may include a separator on either or both sides.

The container may be a container, compartment, housing, vessel, can, canister, tank, or the like of any shape, size, or configuration capable of obtaining, retaining, holding, and/or releasing a liquid solution such as saline water, brackish water, sea water, ocean water, fresh water, sweet water, drinking water, desalinated water, contaminated water, industrial water, etc. The container is spacious enough to house an adequate amount of a water solution undergoing desalination. Accordingly, dimensions thus differ based on a specific application. The container may be large enough to serve industrial applications. The container may be made from different materials capable of withstanding corrosion, temperature fluctuations, changing pH, varying pressure, and be resistant to other chemical, mechanical, and/or physical conditions.

The container may be made from glass, plastic, composite, metal, ceramic, or a combination of materials. The container may feature one or more protective coatings. The container may be made from a material configured to minimize the occurrence of water contamination. The container may be made from one or more materials that are nontoxic and comply with drinking water standards.

The electrodes are arranged within the battery to be in fluid communication with the water present in the container. The electrodes are at least partially submerged in the water solution. The electrodes may be fully submerged in the water solution. The electrodes may be placed on the opposite sides of a container, placed centrally in the container, or both be located on the same side of the container. The electrodes may be located next to each other or be separated by a distance with the presence of a separators and exchange membrane (either anion exchange membrane or cation exchange membrane).

The electrodes may be made from the same or different material, depending on the operating condition and device design. The first electrode, the second electrode, or both electrodes may be made from expanded graphite. Graphite is a crystalline allotrope of carbon and is an example of a semimetal. Graphite presents the most stable form of carbon under standard conditions. Graphite is an electric conductor with highly anisotropic acoustic and thermal properties and is self-lubricating. Graphite has a layered, planar structure. Graphite's individual layers are called graphene. The electrode may include expanded graphite having an interlayer distance sufficient to accommodate Na+ ions. The expanded graphite may be formed by modifying and/or expanding the interlayer distance of the pristine graphene layers.

As a result of the expanded interlayer distance, expanded graphite can uptake cations and anions from saline water, seawater, brackish water, or the like. Expanded graphite can uptake cations including, but not limited to Na+, Mg<NUM>+, Al<NUM>+, Si<NUM>+, K+, Ca+, Sc<NUM>+, Ti<NUM>+<NUM>+ <NUM>+, V<NUM>+/<NUM>+/<NUM>+/<NUM>+, Cr <NUM>+/<NUM>+, Mn<NUM>+/<NUM>+/<NUM>+, Fe<NUM>+/<NUM>+, Ni<NUM>+/<NUM>+/<NUM>+, Cu<NUM>+, Zn<NUM>+, Sn<NUM>+/<NUM>+, Pb<NUM>+, etc. and anions including, but not limited to, single anion species such as F-, Cl-, Br, I-, S-/<NUM>-, anion complexes such as ClO<NUM>-, ClO<NUM>-, ClO<NUM>-, BrO<NUM>-, BrO<NUM>-, SO<NUM><NUM>-, SiO<NUM><NUM>-, CN-, metal-containing anions such as MXyOzn- (where M = Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Cu Zn, Mo, Sn, Cs, and Pb; X = F, Cl, Br, I, N, and P; and <NUM><y≤<NUM>; <NUM>≤z≤<NUM>; <NUM>≤n≤<NUM>), and the like.

The electrodes of the desalination cell may be configured to function as intercalation hosts. Intercalation refers to reversible inclusion of one or more ions into materials with layered structures. The spaces between layers may serve as a temporary storage for one or more types of ions (e.g. alkali and alkali earth metal ions). An electrode may include an intercalation active material so that the electrode functions as an intercalation host. The intercalation active material may be configured to intercalate cations, anions or both.

Besides the active material, one of the electrodes or both may include one or more conductivity agents, one or more polymeric binders, and/or other components. The electrode(s) may include intercalation active material in the amount of about <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, or <NUM> to <NUM> wt. %, based on the total weight of the electrode. An electrode may include one or more conductivity agents in the amount of about <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, or <NUM> to <NUM> wt. %, based on the total weight of the electrode. An electrode may include one or more polymeric binders in the amount of about <NUM> to <NUM> wt. %, <NUM> to <NUM> wt. %, or <NUM> to <NUM> wt.

A non-limiting example of a conductivity agent may include carbon black, conductive carbon black, amorphous carbon, carbon fibers, quaternary ammonium salt(s), alkyl sulfonate(s), halogen-free cationic compound(s), the like, or a combination thereof.

A non-limiting example of a polymeric binder may be polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), polyethylene glycol (PEO), polyimide, polydopamine, poly(ethylene glycol) diacrylate, polymethylpentene, nylon, metalaramid, polyether imide, copolyester, polyetherketone, carboxymethyl cellulose, styrene-butadiene rubber (SBR), copolymers and blends such as poly(vinylidenefluoride-hexafluoropropylene) (PVdF-HFP), poly(vinylidenefluoride-chlrotrifluoroethylene) (PVdF-CTFE), poly(methyl methacrylate-vinyl acetate) (PMMA-VAc), poly(ethylene glycol) diacrylate (PEGDA), poly(methyl methacrylate-acrylonitrile-vinyl acetate) (PMMA-AN-VAc), poly(methyl methacrylate-co-butyl acrylate) (PMMA-co-BA), poly(<NUM>,<NUM>-ethylenedioxythiophene) polystyrene sulfonate-co-polyethylene glycol (PEDOT-co-PEG), the like, or a combination thereof.

A buildup of calcium carbonate (CaCO<NUM>) on the surface of an electrode of a desalination cell can make transport of alkali and alkali earth metal ions in and out of an intercalation host structure more difficult. One of the reactions that can lead to the formation of CaCO<NUM> may involve calcium bicarbonate (Ca(HCO<NUM>)<NUM>). Calcium bicarbonate decomposes into water (H<NUM>O) and carbon dioxide (CO<NUM>). The calcium is typically present in the water source in ionic form (otherwise referred to as "hard water"). Hard water may lead to decreased performance of the desalination cell.

Accordingly, it is beneficial to soften hard water in the desalination cell to remove calcium ions according to the following reaction (<NUM>):.

<NUM> Ca + HCO<NUM> → <NUM>. 5CaCO<NUM> + <NUM>. 5CO<NUM> + <NUM><NUM>O     (<NUM>).

Prussian blue (PB) compounds and Prussian blue analogue (PBA) compounds have a significant amount of void space where mono and/or divalent cations can be easily inserted. PB compounds and PBA compounds are generally referred to as PB compounds. When brine or hard water is inserted into the desalination system to be cleaned, it is possible to push Na+, K+, Ca<NUM>+, and/or Mg<NUM>+ ions into electrodes including one or more PB compounds, leaving clean water that can be collected and reused.

A non-limiting example desalination cell <NUM> for use in a water treatment device is depicted in <FIG>. Desalination cell <NUM> includes electrodes <NUM> and <NUM> and anion exchange membrane (AEM) <NUM> placed between electrodes <NUM> and <NUM>. AEM <NUM> separates individual water compartments <NUM> and <NUM>. Electrodes <NUM> and <NUM> are connected to voltage source <NUM>. Battery cell <NUM> also includes one or more water inlets <NUM> and water outlets <NUM>.

One or more inlets <NUM> and one or more outlets <NUM> may be used to bring in or release saline or desalinated water. The number of one or more inlets <NUM> and one or more outlets <NUM> per compartment <NUM> and <NUM> may be the same or different. For example, water compartment <NUM> may have one more inlet than water compartment <NUM>. One or more inlets <NUM> may be located between AEM <NUM> and electrodes <NUM> and <NUM>. One or more inlets <NUM>, one or more outlets <NUM>, or both may be located centrally between AEM <NUM> and electrodes <NUM> and <NUM>. One or more inlets <NUM> may be located directly across from one or more outlets <NUM>. Alternatively, one or more inlets <NUM> and one or more outlets <NUM> of the same compartment <NUM> or <NUM> may be staggered such that one or more inlets <NUM> and one or more outlets <NUM> are not aligned or are not placed on the same axis. One or more inlets <NUM>, one or more outlets <NUM> or both may have the same or different diameter. One or more inlets <NUM>, one or more outlets <NUM>, or both may connect battery <NUM> with reservoir <NUM>, reservoir <NUM>, or both.

Desalination cell <NUM> and additional battery cells disclosed herein may be connected to one or more water reservoirs <NUM> for storing saline or desalinated water, one or more pumps <NUM> configured to control water flow rate to and from desalination cell <NUM>, one or more valves <NUM> connected to one or more pumps <NUM>, and/or one or more devices <NUM> configured to check, determine, or monitor water quality such as a pH meter, water softener, etc. Reservoir <NUM> may be a container, compartment, housing, vessel, can, canister, tank, or the like of any shape, size, or configuration capable of obtaining, retaining, holding, and/or releasing a liquid solution such as saline water, brackish water, sea water, ocean water, fresh water, sweet water, drinking water, contaminated water, industrial water, etc. One or more pumps <NUM> may be automatic, manual, or both. One or more pumps <NUM> may be in one or more inlet pipes, one or more outlet pipes, a stream connected to water reservoirs <NUM> and/or <NUM>, or a combination thereof.

Desalination cell <NUM> may include two symmetrical electrodes <NUM>, <NUM> including the same or similar chemistry and loading of the electrode material. Alternatively, desalination cell <NUM> may include an asymmetric electrode configuration such that electrode <NUM> is made at least partially or entirely from a different material than electrode <NUM>. The electrode materials may share similar structural characteristics such as same space group, but the concentration of ions such as Na+, Ca<NUM>+, or Mg<NUM>+ may differ.

Desalination cell <NUM> may be operated in the following manner. A positive voltage V may be applied to desalination cell <NUM> to release cations such as Na+ from one of electrodes <NUM> and <NUM>. The cations are dispersed with the saline water in one of water compartments <NUM> and <NUM>, specifically brine compartment <NUM> including the saline water solution having a first concentration c1 of dissolved salts. The saline water in brine compartment <NUM> may be supplied through one of the water inlets <NUM>. As cations cannot travel through AEM <NUM>, the concentration of Na+ in brine compartment <NUM> increases. Anions such as Cl- become attracted and travel through the AEM <NUM> to neutralize the cations in brine compartment <NUM>. At the same time, cations such as Na+ ions intercalate into the other side of electrodes <NUM> and <NUM> due to charge neutrality and the applied voltage bias. This process creates clean water compartment <NUM> including a fresh or desalinated water solution having a second concentration c2 of dissolved salts on the opposite side of AEM <NUM> such that c1 > c2.

Desalination cell <NUM> may operate in cycles (intercalation and de-intercalation), where the water flows continuously. Under the continuous flow, the desalinated water from clean water compartment <NUM> may be stored in a reservoir <NUM>. Alternatively, desalination cell <NUM> may operate as a batch desalination device, where a limited amount of water may be supplied to a compartment to be cleaned in a smaller scale operation. Alternatively, or in addition, a semi-continuous flow of water may be supplied to desalination cell <NUM> such that water compartments <NUM> and <NUM> may be refilled with additional saline water and may operate in the reverse direction in the next cycle. In an alternative embodiment, desalination cell <NUM> may be designed as a cylindrical tubular cell. Both compartments <NUM> and <NUM> may be used for water purification in reverse operating direction.

In a non-limiting example, a continuous collection of clean water in successive cycles may be provided by utilizing clean water reservoir <NUM> and a recycling loop for water purification. During the start-up, electrodes <NUM> and <NUM> are at similar state-of-charge (for example <NUM>%), then electrode <NUM> is discharged (toward <NUM>%) and electrode <NUM> is charged toward <NUM>% SOC. In the first cycle, the first target ions including Na+, K+, Mg<NUM>+, Ca<NUM>+, and Pb<NUM>+, and the like may be removed from electrodes <NUM> and <NUM> including an intercalation host material. Anions are added to brine compartment <NUM> due to the cation-anion attraction (neutrality). Clean water compartment <NUM> thus contains desalinated water that may be collected. The next cycle allows to flush ions out of the electrodes <NUM> and <NUM>, expelling wastewater. Electrodes <NUM> and <NUM> may be also available for the next water purification cycle.

Because a desalination cell is a type of electrochemical cell where voltage is applied, it is possible for the inserted ions to react with other species that are present to form undesirable byproducts. PB compounds have a cyanide group (CN-), where six cyanide groups are typically coordinating a transition metal such as Fe, Cu, Ni, Mn, etc. to form a local octahedron coordination. As describe above, CaCO<NUM> formation can take place because the desalination device is designed to collect Ca<NUM>+ from hard water (with high Ca and Mg contents), when voltage is being applied to the electrochemical cell. The Ca<NUM>+ ions inserted into the intercalation host electrodes may lead to undesired side reactions (e.g. limescale formation) when in contact with PB compounds. The source of Ca<NUM>+ ions may be from Ca<NUM>+ ions already present in the intercalation host or Ca<NUM>+ ions dissolved in water residing in the desalination cell. Limescale buildup at the intercalation host electrodes can lead to electrode surface passivation, which can slow down or stop ion and/or electron transport that is necessary to the functioning of the electrochemical cell. Another potential negative side effect is CO<NUM> gas evolution. If a carbon source is generated from decomposition of one or more PB compounds in an electrode from a cyanide (CN) group, as opposed to from a carbonate that is dissolved in a water source, then associated electrode degradation may lead to a decreased capacity of the desalination cell. Typically, limescale formation is a larger issue at high temperatures, where the reduced solubility of carbonate causes a precipitate of calcium carbonate, which builds up into an undesired limescale coating.

Considering the foregoing, what are needed are electrode materials including one or more PB compounds that are relatively stable against calcium bicarbonate. The stability reduces the likelihood that unwanted side reactions take place between the PB compounds and other compositions present in the desalination cell, including other electrode materials. These side reactions take place between calcium ions and the PB intercalation hosts to form unwanted reaction products (e.g. limescale and CO<NUM> gas evolution). Reducing limescale buildup may be applied to many particular applications of desalination cells such as without limitation human consumption, whole residential applications, coffee makers, dishwashers, agricultural purposes, and industrial purposes.

<FIG> is graph <NUM> plotting reaction data for reactions between FeCo<NUM>(CN)<NUM> with calcium bicarbonate, which are not comprised by the present invention. X axis <NUM> of graph <NUM> is the relative molar fractions of FeCo<NUM>(CN)<NUM> and calcium carbonate given by the equation x in x·CaH<NUM>C<NUM>O<NUM> + (<NUM>-x)· FeCo<NUM>C<NUM>N<NUM>. For instance, if x equals <NUM> then the molar fraction of calcium carbonate is <NUM>% and the molar fraction of FeCo<NUM>(CN)<NUM> is <NUM>%. In other words, when x equals <NUM>, the mixture is pure FeCo<NUM>(CN)<NUM>. As another example, if x equals <NUM> then the molar fraction of calcium carbonate is <NUM>% and the molar fraction of FeCo<NUM>(CN)<NUM> is <NUM>%. In other words, when x equals <NUM>, the mixture is pure calcium carbonate. Y axis <NUM> of graph <NUM> shows the reaction enthalpy in eV/atom.

The interface reactions toolkit, available from open-access materials database (https://materialsproject. org/#apps/interfacereactions) of the Materials Project, is used to analyze stable chemical reactions between FeCo<NUM>(CN)<NUM> and Ca(HCO<NUM>)<NUM> (calcium bicarbonate) as shown in <FIG> that can yield CaCO<NUM>. Table <NUM> shows the results of this analysis. According to this analysis, CaCO<NUM> first forms as a decomposition product with a reaction enthalpy (Erxn) of -<NUM> eV/atom, when mole fraction (x) is <NUM> (i.e., <NUM>% PB compound reacting with <NUM>% calcium bicarbonate).

The framework of the analysis made in <FIG> and Table <NUM> is used to further examine other PB compounds with the first or second general compound disclosed above. The analysis includes determining the mole fraction (x) where CaCO<NUM> first appears as a decomposition product. If the decomposition reaction includes more calcium bicarbonate (CB) to be consumed, relative to the amount of PB compound, this electrode material would be more desired against limescale formation (i.e., increase CB/PB ratio), because less of the electrode material would degrade per unit CB. The analysis may also include analyzing the reaction enthalpy (Erxn) for the given reaction. A relatively more positive Erxn makes the limescale reaction (CaCO<NUM> formation) more difficult to take place. The analysis may also look at other relevant decomposition products. Other decomposition products (e.g. Ca(FeO<NUM>)<NUM>) may have the same or similar blocking or build-up effect compared with CaCO<NUM>, thereby hindering ionic transport. The analysis may also examine whether undesirable CO<NUM> gas evolution may occur in certain PB compounds. When CaCO<NUM> formation is reduced, the other resulting byproducts (e.g. transition metal carbonates, metal oxide, etc.) may be examined.

Table <NUM> shows characteristics of PB compounds available from the Materials Project. The space group of the PB compounds of Table <NUM> are found to be either F-<NUM> or Fm-<NUM> (which are both cubic structures). The hull distance (Ehull) of most PB compounds are significantly different from other stable compounds in the Materials Database such as oxide, nitrides, carbides, metals, etc. When the value of Ehull is zero, the given compound is thermodynamically "stable" at T equals <NUM>. Because they lie on the convex hull of a given chemical compounds, such compounds do not decompose to other chemical species because they lie on the convex hull of the given chemical compound. A compound with Ehull less than <NUM> meV/atom can be accessed at room temperature via experimental synthesis. However, as can be seen in Table <NUM>, almost all the compounds listed have very high Ehull values since their predicted decomposed species include very stable metal nitrides, carbon, and/or nitrogen (N<NUM>). While many of the compounds listed in Table <NUM> are accessible using experimental synthesis and are found to be stable in reality, the reported Ehull for these compounds are quite high due to the presence of more stable phase mixtures listed in the Table <NUM>. Lastly, Table <NUM> provides the DFT bandgap for the compounds identified in Table <NUM>, where most of the compounds except K<NUM>FeNiC<NUM>N<NUM> are found to be highly conducting with zero or small bandgap (less than <NUM> eV), thereby enabling electron transport during the intercalation electrochemical reaction. This characteristic can be used to appropriately adjust or reduce the number of additives in the electrode composition.

As shown below in Table <NUM>, spacegroup, hull distance (Ehull), stable decomposition phase mixture, and bandgap (Eg) are provided for selected PB compounds. PB compounds contain two to three transition metal cations (i.e., Fe, Co, Cu, and Ni), zero to two alkali metals (i.e., Li, Na, and K), and six cyanide (CN-) groups.

While the DFT band gap of ~<NUM> eV (semi-conducting) is reported for K<NUM>FeNiC<NUM>N<NUM> in the Materials Project, the reported DFT bandgap may have some error. According to one theory, when K<NUM>FeNiC<NUM>N<NUM> loses some of the K+ ions, it may become more conducting (i.e., semiconductor-to-metal transition).

Other PB compounds, which are not currently available in the Materials Project database, may be analyzed. These PB compounds are listed in Table <NUM>. PB compounds including alkali metals such as Li, Na, or K eventually lose the alkali metals during the activation step of desalination cell (e.g. the charging process). Therefore, three different PB compositions (i.e. FeCo(CN)<NUM>, FeCu(CN)<NUM>, and FeNi(CN)<NUM>) without alkali metalsare included in Table <NUM>. In addition, Table <NUM> includes Fe<NUM>[Fe(CN)<NUM>]<NUM> for analysis.

In Table <NUM>, the chemical reactivity of the compounds listed in Table <NUM> with calcium bicarbonate is examined. Instead of listing all decomposition products (similar to Table <NUM>) at the given molar fraction (x) where CaCO<NUM> first appears, only the relevant decomposition species such as CaCO<NUM>, MCO<NUM>, Ca(FeO<NUM>)<NUM>, CO<NUM>, metal, and metal oxides are listed in Table <NUM>. Also, Table <NUM> also provides the CB/PB ratio and the reaction enthalpy. Also, in the chemical reactions shown in Figure <NUM>, all PB compounds are abbreviated as "PB" and calcium bicarbonate is abbreviated as "CB". In Table <NUM>, PB compound reactions with calcium bicarbonate (CB; Ca(HCO<NUM>)<NUM>, equivalent to Ca + 2HCO<NUM> → H<NUM>O + CaCO<NUM> + CO<NUM>) to form CaCO<NUM>, are reported, where interface reactions toolkit of the Materials Project database are used.

As an observation from Table <NUM>, the change in alkali metal ions (i.e., K vs. Na) can significantly affect the final reaction products. Based on this observation, it is anticipated that all alkali metal ions, Li+, Na+, and K+ are removed from the electrode host during the activation cycle. For example, it is observed that Na<NUM>FeCuC<NUM>N<NUM> leads to the formation of Na<NUM>Ca<NUM>(CO<NUM>)<NUM> and K<NUM>FeCuC<NUM>N<NUM> leads to the formation of K<NUM>Ca<NUM>(CO<NUM>)<NUM>. It is also difficult to determine whether Na<NUM>Ca<NUM>(CO<NUM>)<NUM> builds-up at the electrode surface, similar to CaCO<NUM>. It is expected that both Na<NUM>FeCuC<NUM>N<NUM> and K<NUM>FeCuC<NUM>N<NUM> loses Na+ and K+ from the intercalation host, not affecting the outcome of this analysis, regardless of the products being formed when reacting with calcium bicarbonate, Ca(HCO<NUM>)<NUM>, as listed in Table <NUM>.

In Table <NUM>, FeCoC<NUM>N<NUM>, FeCuC<NUM>N<NUM>, and FeNiC<NUM>N<NUM> are additionally analyzed in combination with re-listing information from Table <NUM> for FeCo<NUM>C<NUM>N<NUM>, FeCu<NUM>C<NUM>N<NUM>, and FeNi<NUM>C<NUM>N<NUM> PB compounds to directly compare FeCoC<NUM>N<NUM>, FeCuC<NUM>N<NUM>, and FeNiC<NUM>N<NUM> versus FeCo<NUM>C<NUM>N<NUM>, FeCu<NUM>C<NUM>N<NUM>, and FeNi<NUM>C<NUM>N<NUM> (with an additional transition metal in PB compounds). Here are the comparisons based on the analysis of the data from Table <NUM>: (<NUM>) (FeCo<NUM>C<NUM>N<NUM> versus FeCoC<NUM>N<NUM>) FeCo<NUM>C<NUM>N<NUM> leads to less CaCO<NUM> and CoCO<NUM> formation; (<NUM>) (FeCu<NUM>C<NUM>N<NUM> versus FeCuC<NUM>N<NUM>) same amount of CaCO<NUM> and CO<NUM>, but, more Cu formation for FeCu<NUM>C<NUM>N<NUM>; and (<NUM>) (FeNi<NUM>C<NUM>N<NUM> versus FeNiC<NUM>N<NUM>) Similar amount of carbonates and CO<NUM>.

Initial analysis suggests that FeCo<NUM>C<NUM>N<NUM> is a beneficial compounds to mitigate CaCO<NUM> formation and restrict CO<NUM> gas formation. The CB/PB ratio and Erxn for these six compounds are similar, varying from <NUM> to <NUM> and -<NUM> to -<NUM> eV/atom, respectively. Based on the analysis of Table <NUM>, reducing Co contents (FeCo<NUM>C<NUM>N<NUM> to FeCoC<NUM>N<NUM>) trigger more CaCO<NUM> and CoCO<NUM> formations. According to Table <NUM>, FeCuC<NUM>N<NUM>, FeNiC<NUM>N<NUM>, FeCu<NUM>C<NUM>N<NUM>, and FeNi<NUM>C<NUM>N<NUM> lead to CO<NUM> gas formation. In addition, Cu-containing PB compounds have a metallic Cu as a final decomposition product. This partly explains instability of the Cu-containing PB compounds, where Cu can further oxidize to CuO and/or ionize to Cu<NUM>+. While Ni-containing PB compounds in Table <NUM> do not lead to Ni formation (i.e., improved stability versus Cu- containing PB), it is predicted that the Ni-containing PB compounds lead to CO<NUM> gas evolution. Lastly, FeMn<NUM>C<NUM>N<NUM> and FeMnC<NUM>N<NUM> are analyzed in Table <NUM>. While the Mn-substituted PB compounds lead to significantly more CaCO<NUM> (and, MnCO<NUM>), the CB/PB ratios and Erxn increased significantly for FeMn<NUM>C<NUM>N<NUM>. While FeMn<NUM>C<NUM>N<NUM> forms MnO as a byproduct, FeMnC<NUM>N<NUM> does not form MnO. Based on the analysis in Table <NUM>, FeMn<NUM>C<NUM>N<NUM> may be used as a compound to reduce or eliminate production of MnO. While Mn-containing compounds may lead to higher carbonate formation, we find that both CB/PB ratio and reaction enthalpy are higher for FeMn<NUM>C<NUM>N<NUM> than other candidates listed in Table <NUM>. The analysis predicts that FeCo<NUM>C<NUM>N<NUM> forms least amount of CaCO<NUM>, while some of Co may transform to CoCO<NUM>, in contact with calcium bicarbonate.

Fe<NUM>[M(CN)<NUM>]<NUM> compounds, where M = Fe, Ni, Cu, Co, and Mn are analyzed, as shown in Table <NUM>. Substituting Fe with Ni or Cu may lead to less CaCO<NUM> but more Ca(FeO<NUM>)<NUM> and CO<NUM> gas formations. In addition, substituting Ni suppresses CaCO<NUM> formation, but leads to a very high CO<NUM> gas release. In addition, Fe<NUM>[Cu(CN)<NUM>]<NUM> forms Cu as one of byproducts, which can further oxidize to CuO or Cu<NUM>+ especially for the environment similar to desalination cell (i.e., aqueous, abrupt pH change, and/or when voltage is being applied). Substituting Fe with Co or Mn produces less CaCO<NUM> and no CO<NUM>, but increased amount of MCO<NUM> (CoCO<NUM> and MnCO<NUM>). Analysis of the decomposition products suggests that Co and Mn substitutions are beneficial against degradation for desalination applications. Based on an analysis of Table <NUM>, CB/PB ratio for Fe<NUM>[Fe(CN)<NUM>]<NUM> is high. However, Fe<NUM>[Fe(CN)<NUM>]<NUM> leads to the highest amount of CaCO<NUM> (and, also CO<NUM> gas forms as a byproduct) as shown in Table <NUM>.

Table <NUM> shows an analysis of Fe<NUM>[M(CN)<NUM>]<NUM> compounds, where M = Fe, Ni, Cu, Co, and Mn. The chemical reactions of these compounds with calcium bicarbonate (CB; Ca(HCO<NUM>)<NUM>, equivalent to Ca + 2HCO<NUM> → H<NUM>O + CaCO<NUM> + CO<NUM>) that forms CaCO<NUM> are examined using interface reactions toolkit in the Materials Project database.

As shown in Table <NUM>, other metal substitution options are explored. Other 3d metals from the periodic table, including Sc, Ti, V, Cr and Zn, as shown in Table <NUM> are explored. Table <NUM> shows the analysis of FeMx(CN)<NUM> (where x = <NUM> or <NUM>) and Fe<NUM>[M(CN)<NUM>]<NUM> compounds, where M = Sc, Ti, V, Cr, or, Zn.

Among the chemical compounds analyzed in Table <NUM>, FeScC<NUM>N<NUM>, FeTiC<NUM>N<NUM>, or FeCrC<NUM>N<NUM> produce acceptable amounts of CaCO<NUM> and no CO<NUM>, but form Sc<NUM>O<NUM>, TiO<NUM>, or Cr<NUM>O<NUM> as a byproduct. It is improbable that Sc<NUM>+ and Ti<NUM>+ further oxidize, but Cr<NUM>+ may oxidize toward Cr<NUM>+ or Cr<NUM>+. FeZnC<NUM>N<NUM> leads to increased CaCO<NUM> amount, but no Zn metal or binary oxide formation occurs. Similar phenomenon also occurs for Fe<NUM>[M(CN)<NUM>]<NUM> compounds where M = Sc, Ti, and Cr, that form Sc<NUM>O<NUM>, TiO<NUM>, and Cr<NUM>O<NUM>, respectively. Sc or Cr substitution effectively suppresses CaCO<NUM> formation. While Fe<NUM>[Zn(CN)<NUM>]<NUM> produces more carbonates (i.e., CaCO<NUM> and ZnCO<NUM>), it does not form any Zn metal or binary oxide as the final decomposition products similar to FeZnC<NUM>N<NUM>, which is a desirable compound.

Claim 1:
A desalination cell comprising:
an electrode including a material having at least one compound of the following formula:

        AxMIyMIIz(CN)<NUM>,

wherein A is Na, Li or K, <NUM> ≤ x ≤ <NUM>, MI is a first metal, MII is a second metal, <NUM> ≤ y, and z ≤ <NUM>,
and wherein the first metal is Fe, Mn, Co, Ti, Cr or Zn,
characterized in that
the second metal comprises between <NUM> to <NUM> molar percent of the total molar concentration of the first and second metals, and
the second metal is Sc so that the material having Sc as the second metal is configured to reduce calcium carbonate formation and carbon dioxide gas formation during operation of the desalination cell.