Patent Application
20170098851
The present disclosure generally relates to flow batteries. More specifically, the present disclosure relates to configurations and designs of the flow battery cell or stack.
Flow batteries having the potential for increased energy density are desirable for a variety of end uses. Redox (oxidation-reduction) flow batteries (RFB's) may be suitable candidates for grid-scale electrical energy storage (EES) and electrical vehicles (EV), due, at least in part, to their ability to separate power and energy, flexible layout, safety aspects, and potential cost effectiveness.
In conjunction with the flexibility allowed by the flow battery, relative cost-effectiveness of the battery's chemical components may be another desirable attribute. A typical RFB includes a stack of flow battery cells, each having an ion-exchange membrane disposed between a cathode and an anode. During operation, a catholyte flows through the cathode, and an anolyte flows through the anode. The catholyte and anolyte solutions electrochemically react separately in the reversible reduction-oxidation (“redox”) reactions. During the redox reactions, ionic species are transported across the ion-exchange membrane, and electrons are transported through an external circuit to complete the electrochemical reactions.
A commonly known RFB is a zinc-bromine flow battery. Another potential example may be a zinc-chlorate flow battery. The use of a multielectron chlorate cathode may result in a higher energy density and lesser safety issues, as compared to the use of other energy-dense cathodes, for example, bromine. The electrochemical reaction of the zinc chlorate flow battery requires precise pH control to maintain high reaction efficiency. However, in currently available flow battery configurations, the use of proton exchange membranes may be inefficient in selective transportation of protons, which results in a pH imbalance during the electrochemical reaction in the cells. Efforts have been made to achieve the optimal pH, for example by the addition of a buffer to the anolyte. However, these techniques may affect some other performance features of the flow batteries.
Thus, there is a need for new configurations of flow battery cells or stacks, which may, for example, allow for pH balance in the flow batteries.
One embodiment of the invention is directed to a flow battery cell. The flow battery cell includes a first electrode configured for charging a discharged catholyte, a second electrode configured for charging and discharging an anolyte, and a third electrode configured for discharging a charged catholyte. The second electrode is disposed between the first electrode and the third electrode. Each of the first electrode and the third electrode is separated from the second electrode by a bipolar membrane. A first bipolar membrane and a second bipolar membrane are disposed, respectively, between the first electrode and the second electrode, and the second electrode and the third electrode.
One embodiment is directed to a flow battery stack that includes an electrode array. The electrode array includes a plurality of first electrodes configured for charging a discharged catholyte; a plurality of second electrodes configured for charging and discharging an anolyte; and a plurality of third electrodes configured for discharging a charged catholyte. Each first electrode in the plurality of the first electrodes is disposed in an alternating manner with respect to each third electrode in the plurality of the third electrodes, and each second electrode in the plurality of second electrodes is disposed between a first electrode and a third electrode pair. The flow battery stack further includes a plurality of first bipolar membranes, wherein each first bipolar membrane in the plurality of the first bipolar membranes is disposed between a first electrode and a second electrode pair in the electrode array; and a plurality of second bipolar membranes, wherein each second bipolar membrane in the plurality of second bipolar membranes is disposed between a second electrode and a third electrode pair in the electrode array.
In one embodiment, a method for operating the flow battery stack is provided. The method includes charging the flow battery stack by contacting the discharged catholyte with at least one first electrode in the plurality of first electrodes and the anolyte with at least one second electrode in the plurality of second electrodes. The method further includes discharging the flow battery stack by contacting the anolyte with at least one second electrode in the plurality of second electrodes and the charged catholyte with at least one third electrode in the plurality of third electrodes. The at least one first electrode, the at least one second electrode, and the at least one third electrode constitute at least one flow battery cell in the flow battery stack.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.
In the following specification and claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable.
The term “catholyte” as used herein refers to an electrolyte disposed adjacent to a cathode in an electrolytic cell, and the term “anolyte” as used herein refers to an electrolyte disposed adjacent to an anode of the electrolytic call. The catholyte and the anolyte usually include one or more electrochemically active species that are oxidized or reduced under battery cell conditions. As used herein, the terms “charged catholyte” and “discharged catholyte” refer, respectively, to the oxidized and reduced forms of the electrochemically active species that are present in the catholytes in the charged state and the discharged state of the cell.
In a flow battery (sometimes also referred to as a flow-assisted battery), the cathode and anode of a battery cell, usually include a catholyte and an anolyte, respectively, separated by an ion-permeable membrane such as a proton exchange membrane (PEM). One example of a flow battery is a metal-halate battery. In the charged state of a metal-halate battery, the catholyte may include a solution of at least one halate salt in the cathode, and the anolyte may include a metal or metal alloy in combination with a source of an anion which is needed to form a metal salt upon the anolyte electrochemical discharge. The metal or metal alloy may be disposed on or detached (for example, in slurry form) from the anode. The cell chemistry of a metal-halate cell is usually based on a reversible redox (reduction-oxidation) reaction that involves the conversion of the halate to the corresponding halide ion. The metal or metal alloy is capable of being dissolved into the metal salt, for example a metal halide or metal acetate, during the redox reaction.
In a metal-chlorate cell, it is possible to use the same metal cation in both the anode and cathode, due to the high solubility of metal chlorates and chlorides. On the catholyte side, a metal chlorate is converted to the corresponding metal chloride (chlorate-to-chloride conversion) during discharging, while the chloride-to-chlorate reaction occurs during charging the cell. On the anolyte side, metal ions are converted to the respective metal itself (i.e., elemental metal) during charging; while the metal is dissolved into a corresponding salt, such as the chloride salt or the acetate salt, during discharge of the cell. Non-limiting example of such a halate/halide battery is described in a previously filed application, Publication No. WO2014197842.
Further, during discharging, the halate ion (for example, chlorate) consumes six electrons and six protons to generate a halide ion (for example, chloride) and three water molecules as shown in equation 1. During charging, the reaction proceeds in the reverse direction (E°=1.45 V, in the case of chlorate/chloride).
(ClO3)−+6H++6e−<==>Cl−+3H2O (Equation 1)
By way of example, the overall cell reaction can be expressed by equation 2 for a zinc-chlorate cell,
Zn(ClO3)2+6Zn+12HCl<==>7ZnCl2+6H2O (Equation 2)
That is, the reversible redox reaction involves the formation of six protons per one chloride ion oxidized during charging, and consumption of six protons per one chlorate ion reduced during discharging. As mentioned earlier, pH control during charging and discharging, may be desirable to maintain a high reaction rate and cell efficiency. For example, the chloride oxidation is desirably performed at a pH of about 6.7 while the chlorate reduction is desirably performed at a pH of about 1.0.
In case of an ideal proton exchange membrane, the protons are typically transferred from the catholyte to the anolyte during charging, and returned to the catholyte during discharging, thereby maintaining the pH balance. However, for proton exchange membranes typically employed in flow batteries (for example, Nafion® membranes), the competition between metal and hydrogen cations to be transferred across the membrane, may cause a pH imbalance under operating conditions. For example, the transfer rate of a metal cation Mn+ at 3-4M (molar concentration) of the metal chloride is significantly higher (by orders of magnitude) than the transfer rate of the protons at pH ˜6-7 (10−6-10−7M). Under these conditions metal cations carry most of the charge across the membrane. As a result, the pH decreases significantly in the catholyte during charging and rapidly increases during discharging. For example, in case of a 3M sodium chloride solution in the catholyte, the pH in the catholyte rapidly decreases to less than 2.0 during charging, as shown in
Aspects of the invention described herein address the noted shortcomings of the state of the art. Some embodiments of the invention present a flow battery cell and a flow battery stack that use a bipolar membrane to separate a catholyte and an anolyte. Use of the bipolar membrane between the catholyte and the anolyte may aid in maintaining and controlling pH of the catholyte, and may thus enable pH balance under the operating conditions of the flow battery cell.
The term “bipolar membrane” as used herein refers to an ion exchange membrane including a layered ion-exchange structure. A bipolar membrane typically includes a proton exchange layer and an anion exchange layer attached to each other. The proton exchange layer side of the membrane is usually referred to as cationic side, and the anion exchange layer side of the membrane is referred to as anionic side. As known to skilled in the art, cations cannot cross over through the anionic side and anions cannot cross over through the cationic side of the membrane. In some embodiments, small amounts of the cations and anions may pass through the anionic side and cationic side, respectively, of the bipolar membrane. The bipolar membrane splits water to protons and hydroxyl ions under the influence of an applied electric field, and protons and hydroxyl ions migrate out of the bipolar membrane in opposite directions.
The cell chemistry of the present flow battery cell and flow battery stack is based on the reversible redox (reduction-oxidation) reaction that involves the conversion of a halate to the corresponding halide ion in a similar manner as discussed above in context of the conventional cell. During discharging, the halate ion (for example, chlorate) consumes six electrons and six protons to generate a halide ion (for example, chloride) and three water molecules. Water may diffuse into the bipolar membrane and split to provide protons and hydroxyls on the interface of the proton and anion exchange layers of the bipolar membrane. In order to split water, a cationic side, that is the cation exchange layer of the bipolar membrane may be required to face the catholyte, during discharging. Furthermore, during discharging, the anionic side of the bipolar membrane may also be required to face the anolyte so that the protons generated at the cationic side of the membrane neutralize the hydroxyls formed upon the reduction of the halate. On the other hand, during charging, the halide ion oxidizes in the catholyte and generates six protons and six electrons. The pH of the catholyte may be maintained by neutralizing protons formed during charging with hydroxyls formed upon the water splitting. In order to split water, an anionic side, that is the anion exchange layer of the bipolar membrane may be required to face the catholyte, during charging.
To accommodate these features, some embodiments of the invention present a flow battery cell that includes a three-electrode configuration (also referred to as three-chamber configuration, in some embodiments), as described in greater detail below. The three-electrode configuration enables the use of the bipolar membrane between the catholyte and anolyte in the cell, and may thus aid in pH balancing during electrochemical reaction in the flow battery cell that involves generation/consumption of protons.
In one embodiment, the flow battery cell includes a first electrode configured for charging a discharged catholyte, a second electrode configured for charging and discharging an anolyte, and a third electrode configured for discharging a charged catholyte. The second electrode is disposed between the first electrode and the third electrode. Each of the first electrode and the third electrode is separated from the second electrode by a bipolar membrane. A first bipolar membrane and a second bipolar membrane are disposed, respectively, between the first electrode and the second electrode, and the second electrode and the third electrode.
The present disclosure also encompasses embodiments of, a flow battery stack that includes at least one flow battery cell and a method for operating the flow battery stack. The terms, “flow battery cell” and “cell” are used herein interchangeably, throughout the specification. The terms, “flow battery stack” and “flow battery” are used herein interchangeably, throughout the specification.
In some embodiments, an aqueous solution of a halate salt of at least one metal may be used as a charged catholyte. Non-limiting examples of suitable metals include sodium, lithium, calcium, zinc, nickel, copper or combinations thereof. The term “halate” refers to a salt of halogen oxoacid. Usually, the oxoacid compound conforms to the general formula HXO3, where X is chlorine, bromine, or iodine. The corresponding salts are halate salts, for example the chlorate salt, the bromate salt, and the iodate salt. In some cases, the term “halate” may be used to describe any of the chlorates, bromates, iodates, or combinations thereof. In the case of chlorine, the corresponding salt of chloric acid (i.e, the chlorate) may be selected from the group consisting of sodium chlorate, potassium chlorate, lithium chlorate, calcium chlorate, magnesium chlorate, barium chlorate, zinc chlorate, copper (II) chlorate, and combinations thereof. In the case of bromine, the corresponding salt of bromic acid (i.e., the bromate) may be selected from the group consisting of sodium bromate, potassium bromate, lithium bromate, calcium bromate, magnesium bromate, zinc chlorate, and combinations thereof. In the case of iodine, the corresponding salt (i.e., the iodate) may be selected from the group consisting of potassium iodate, sodium iodate, and combination thereof.
As discussed above, in some embodiments, during a redox reaction, the metal halates are reduced to metal halides, and the metal halides are oxidized to metal halates. The energy density of a catholyte is usually determined by the molar solubility of the electrochemically active species (for example, the metal halate and the metal halide) and the number of electrons involved in the redox reaction. Due to high solubility of chlorates in water (up to about 5.5-7M) and large number of electrons transfer during the halate reduction to the halide, flow batteries based on halate/halide catholytes may have the energy density as high as 300 Wh/kg.
In some embodiments, the discharged catholyte and the charged catholyte include different anionic forms (that is, a metal halate or a metal halide). In some embodiments, the charged catholyte includes one or more metal halates (as discussed above) during discharging the flow battery cell. In certain embodiments, the charged catholyte includes a solution of at least one metal chlorate (for example, zinc chlorate). In some embodiments, the discharged catholyte includes one or more metal halides during charging the flow battery cell. In certain embodiments, the discharged catholyte includes a solution of at least one metal chloride (for example, zinc chloride).
In some embodiments, the anolyte includes an aqueous solution of at least one metal salt, when the cell is in the discharged state. Non limiting examples of suitable metal salts include a zinc salt, cobalt salt, copper salt, iron salt, manganese salt, chromium salt, vanadium salt, titanium salt, or combinations thereof. The metal salt is capable of generating a metal or a metal alloy during charging the cell. The metal or the metal alloy may be present in the form of a slurry in the anolyte of the flow battery cell, or as a sheet or layer of material attached to a surface of the second electrode. The anolyte may optionally include a buffer, for example, an ionic buffer such as an ammonia compound, or an acetate. The metal or metal alloy is capable of being dissolved in the anolyte during discharging of the cell to regenerate the metal salt.
In some embodiments, as shown in
Referring again to
As illustrated in
Referring to
The first electrode 12, the second electrode 14 and the third electrode 16 may include an electrically-conductive substrate. Non-limiting examples of suitable electrically-conductive substrates may include carbon (in a conductive form, for example graphite), a metal, or a combination thereof. Suitable metals include, but are not limited to, ruthenium, tantalum, lead, titanium, nickel, platinum, palladium, or combinations thereof. In some embodiments, the electrically-conductive substrate includes a metal oxide. Suitable metal oxide examples include, but are not limited to, ruthenium oxide, tantalum oxide, lead oxide, titanium oxide, or combinations thereof. In some embodiments, an electrocatalyst may be deposited on the electrically-conductive material in combination with an ionomer to form a liquid diffusion layer. Non-limiting examples of electrocatalysts include polyoxometalate-based materials, platinum, palladium, ruthenium, rhodium, or various alloys or compounds of the aforementioned metals. One or both of the composition and the structure of the first electrode 12 and the third electrode 16 may be the same or different. In some embodiments, the first electrode 12 and the third electrode 16 may be composed of the same composition. In certain embodiments, the first electrode 12 includes ruthenium oxide. In certain embodiments, the third electrode 16 includes ruthenium compounds. The ruthenium compounds may be desirably insoluble in aqueous solutions with a pH from about 0.8 to about 9.0.
In some embodiments, the second electrode 14 includes an electrically conductive substrate that is electrochemically inert in the electrochemical environment of the flow battery cell 10. In some embodiments, the second electrode 14 may include a three-dimensional (3D) mesh. The 3D mesh form of the second electrode 14 may be desirable so that the metal plated in the first portion 31 of the second chamber 32 during charging of the cell, is available for dissolution in the second portion 33 of the second chamber 32 during discharging of the cell.
As discussed previously, during the cell reaction of a chlorate/chloride cell, the chlorate salt is converted to a chloride salt during discharging, while the chloride-to-chlorate reaction occurs during charging. On the anolyte side, metal ions are converted to the respective metal during charging, while the metal is dissolved into a corresponding salt, such as the chloride salt, during discharging.
Referring to
Similarly, during discharging, the third chamber 34 includes the charged catholyte 15 and the second chamber 32 includes the anolyte 17, as shown in
As will be apparent by the description above, in accordance with some embodiments of the invention, in the cell 10, the second chamber 32 is typically used for charging and discharging of the cell; however, different catholyte chambers (i.e., the first chamber 30 and the third chamber 34) are used for charging and discharging the cell.
Some embodiments of the invention are directed to a flow battery stack that includes an electrode array. The electrode array includes a plurality of first electrodes configured for charging a discharged catholyte; a plurality of second electrodes configured for charging and discharging an anolyte; and a plurality of third electrodes configured for discharging a charged catholyte. Each first electrode in the plurality of the first electrodes is disposed in an alternating manner with respect to each third electrode in the plurality of the third electrodes, and each second electrode in the plurality of second electrodes is disposed between a first electrode and a third electrode pair. The flow battery stack further includes a plurality of first bipolar membranes, wherein each first bipolar membrane in the plurality of the first bipolar membranes is disposed between a first electrode and a second electrode pair in the electrode array; and a plurality of second bipolar membranes, wherein each second bipolar membrane in the plurality of second bipolar membranes is disposed between a second electrode and a third electrode pair in the electrode array.
In one embodiment, the flow battery cells may be arranged in series. The number of cells in series depends on the voltage expected to be generated by the battery stack and the open circuit voltage (OCV) of the individual cell. For example, to achieve a 24 Volt battery having 2V OCV, the number of cells in the series may be 12.
The stack 40 further includes a plurality of first bipolar membranes 18 and a plurality of second bipolar membranes 24. In the electrode array 41, each first bipolar membrane 18 is disposed between a pair 44 of the first electrode 12 and a second electrode 14 (also referred to as a first electrode and second electrode pair 44) such that the first proton exchange layer 20 faces the second electrode 14 and the first anion exchange layer 22 faces the first electrode 12. Similarly, each second bipolar membrane 24 is disposed between a pair 46 of the second electrode 14 and the third electrode 16 (also referred to as a second electrode and third electrode pair 46) such that the second proton exchange layer 26 faces the third electrode 16 and the second anion exchange layer 28 faces the second electrode 14. In the flow battery stack 40, the at least one first electrode 12, the at least one second electrode 14, and the at least one third electrode 16 constitute at least one flow battery cell 10. In this configuration, each electrode (the first electrode 12, the second electrode 14, and the third electrode 16), except for the terminal electrodes, faces a bipolar membrane (the first bipolar membrane 18 or the second bipolar membrane 24) from both the sides. Further, as illustrated in
As described earlier with respect to the single cell 10 of
In some embodiments, as shown in
Another embodiment of this invention is directed to a method of operating the flow battery stack 40 (as shown in
When the battery stack 40 is discharged as shown in
Those skilled in the art understand that the battery stack 40 may include various other features and components in addition to the components described above. Non-limiting examples of additional components include current collectors, electrolyte storage tanks, and a casing. The discharged catholyte, the charged catholyte and anolyte storage tanks may be arranged in communication (e.g., liquid communication), respectively, with the plurality of first electrodes, the plurality of third electrodes, and the plurality of second electrodes. Other features of the flow battery stack may include pumps (not shown) for circulating the catholyte and anolyte solutions through the stack, via tubes or conduits. Conventional pumps can be used. Other methods for circulating the solutions are also possible, e.g., gravity-based systems.
Other examples of features and devices for the battery include sensors for pH monitoring, pressure measurement and control, gas flow, temperature, and the like. Batteries of this type will also include associated electrical circuitry and devices, e.g, an external power supply; as well as terminals for delivering battery output when necessary.
As mentioned above, in some embodiments, the flow batteries as described herein may be used as part of an electrical grid system, i.e., an interconnected network for delivering electricity from suppliers to consumers. For example, multiple flow batteries can be interconnected by known techniques, to allow storage of electricity on a large scale within the power grid. Those involved with electrical power generation on a commercial scale are familiar with various other features of the grid, e.g., power generation stations, transmission lines, and at least one type of power control and distribution apparatus. The flow batteries described herein may be able to provide the increased energy density, along with low battery costs, which may make them an attractive alternative for (or addition to) other types of grid storage units or systems, in accordance with some embodiments.
The flow batteries described herein can also be used for electrical vehicles, trucks, ships, and trains, as well as for other applications, such as submarines and airplanes. Electric vehicles include electric cars and hybrid electric cars. In some embodiments, the flow batteries could be incorporated as part of an electric powertrain, alone or supporting an internal combustion system. The flow batteries could also be used as independent electric source for the vehicle, e.g., for lighting, audio, air conditioning, windows, and the like.
Those skilled in the art are familiar with battery pack designs suitable for a given type of EV; as well as techniques for incorporating the battery into the drivetrain or other systems of the vehicle. As alluded to previously, the flexibility of the flow battery, including the ability to locate catholyte and anolyte sources in different places of the vehicle, may represent a considerable design advantage. The benefits of increased energy density arising from use of the halogen oxoacid salts may also enhance the battery profile of the electric vehicle or other device.
It should be understood that the battery configuration and design, as described herein, are not limited to flow batteries, and it will be understood that the descriptions and figures are not limited to metal halate flow batteries. The embodiments described herein may be utilized for any catholyte-anolyte chemistry that includes the proton generation and consumption, or requires pH control. Further, the embodiments described herein may be utilized for an electrochemical cell configuration, where charging and discharging require different electrode materials.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
