CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Applications Ser. Nos. 61/319,248 filed Mar. 30, 2010 and 61/322,780 filed Apr. 9, 2010, incorporated by reference in their entirety herein.
FIELD OF THE INVENTION
This invention relates to high performance electrochemical cells and batteries, and more particularly to flow batteries.
BACKGROUND OF THE INVENTION
The “greening” of the energy economy, increasing demand and use of renewable energy sources such as wind and solar, and the expected proliferation for example of plug-in hybrid vehicles and all electric vehicles, increasingly strain the electricity distribution grid. High capacity electrical energy storage technologies such as pumped hydroelectric can play an important role in grid load balancing, time shifting renewable energy sources from time of generation to peak time of use, however, geography and cost limit their use, particularly on a local level.
Existing high capacity battery technologies, for example flow batteries, are too expensive for widespread adoption because the effective cost of the resulting energy and/or power delivered is well above market prices. There exists therefore a substantially unmet need for a low-cost, high-capacity, efficient and high performance battery technology.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide high performance flow battery apparatus and methods for enhancing, charging, operating and using flow batteries. High current density charging rates and discharging rates in the range of approximately 70 to 400 mA/cm2, and more particularly in the range of 100 to 250 mA/cm2, are provided by various embodiments of the present invention.
Embodiments of the high performance, alkaline zinc/ferro-ferricyanide rechargeable (“ZnFe”) flow batteries of the present invention are based on a number of improvements over the prior art. These embodiments are also applicable to other flow batteries that incorporate the plating of a metal to store energy (such as: ZnHBr; ZnBr; CeZn; and ZnCl).
First, the battery design has a cell stack comprising a low resistance positive electrode in at least one positive half cell and a low resistance negative electrode in at least one negative half cell, where the positive electrode and negative electrode resistances are selected for uniform high current density across a region of the cell stack—that is with a resistance across the electrodes sufficiently low to ensure small voltage variations across the electrode and hence uniform current flow out of the electrode and across the cell stack.
Second, a flow of electrolyte (for example, zinc species in the ZnFe battery) with a high level of mixing (also referred to herein as a “high rate of mixing” and “high mixing”) through at least one negative half cell in a Zn deposition region proximate a deposition surface where the electrolyte close to the deposition surface has sufficiently high zinc concentration for deposition rates on the deposition surface that sustain the uniform high current density. The electrolyte flow and mixing of the flow in the negative half cell are engineered to provide a mass transfer coefficient sufficient to support the high current density and to provide substantially uniform deposition of, for example zinc, over the deposition surface of a cell. Furthermore, some embodiments have been flow engineered to provide zinc deposition at less than a limiting current, where the deposited zinc has a dense, adherent, non-dendritic morphology.
Third, the zinc electrolyte has a high concentration and in some embodiments has a concentration greater than the equilibrium saturation concentration—the zinc electrolyte is super-saturated with Zn ions. Different embodiments of the present invention combine one or more of these improvements.
Electrolyte flow with high mixing through the cell may be due to high fluid velocity in a parallel plate channel. However, the mixing in the flow may be induced by structures such as: conductive and non-conductive meshes; screens; ribbons; foam structures; arrays of cones, cylinders, or pyramids; and other arrangements of wires or tubes used solely or in combination with a planar electrode surface. Use of such structures may allow for high mixing of the electrolyte with laminar flow or with turbulent flow at high or low fluid velocity. Furthermore, structures for calming the turbulent flow may be included in the electrolyte fluid circuit immediately after the cell.
According to embodiments of the present invention, methods for operating a flow battery may include flowing electrolyte with high mixing in a laminar flow regime, or turbulent flow regime, through at least one negative half cell in a Zn deposition region proximate a deposition surface. Furthermore, some embodiments include depositing Zn with a dense, adherent, non-dendritic morphology. The high mixing flow may be utilized during charging and/or discharging of battery cells.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:
FIG. 1 is a schematic diagram of a zinc redox flow battery;
FIG. 2 is schematic diagram of a zinc redox flow battery, according to some embodiments of the present invention;
FIG. 3 is a schematic perspective view of a flow cell, according to some embodiments of the present invention;
FIG. 4 is a schematic perspective view of the cell of FIG. 3 contained within a frame, according to some embodiments of the present invention;
FIG. 5 is a schematic cross-sectional representation of a first example of cell configurations for a redox flow battery, according to some embodiments of the present invention;
FIG. 6 is a schematic cross-sectional representation of a second example of cell configurations for a redox flow battery, according to some embodiments of the present invention;
FIG. 7 is a schematic cross-sectional representation of a third example of cell configurations for a redox flow battery, according to some embodiments of the present invention;
FIG. 8 is an example of a mixing inducing woven wire mesh feature on the surface of a flow battery electrode, according to some embodiments of the present invention;
FIG. 9 is an example of a mixing inducing non-woven wire mesh feature on the surface of a flow battery electrode, according to some embodiments of the present invention;
FIG. 10 is an example of a mixing inducing wire/tube feature on the surface of a flow battery electrode, according to some embodiments of the present invention;
FIG. 11 is an example of a mixing inducing array of cylinders on the surface of a flow battery electrode, according to some embodiments of the present invention;
FIG. 12 is an example of a mixing inducing array of cones on the surface of a flow battery electrode, according to some embodiments of the present invention;
FIG. 13 is an example of a mixing inducing array of pyramids on the surface of a flow battery electrode, according to some embodiments of the present invention; and
FIG. 14 is a cross-sectional representation of a flow laminarization feature, according to some embodiments of the present invention.
Embodiments of the present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of some embodiments of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration.
Embodiments of the present invention provide high performance flow battery apparatus and methods for enhancing, charging, operating and using flow batteries.
FIG. 1 shows an example of a prior art redox flow battery 100. See, for example, Wu et al. Indian Journal of Technology, vol. 24, July 1986, pp 372-380. The flow battery comprises positive and negative half cells 110 and 120, respectively, separated by separator 130. Electrolyte for the half cells is stored in tanks 140 and 150 and is pumped through the half cells, as shown by the arrows. The flow battery shown in FIG. 1 is a Zn/Fe redox flow battery; the posilyte is an Fe complex and the negalyte is a zincate salt. However, prior art flow batteries do not operate at high enough current densities and are not efficient enough to be economically viable for large scale energy storage. The present invention provides improvements to flow batteries that will allow high current density operation with high efficiency at low cost. For example, some embodiments of the present invention will provide redox flow batteries with charging current densities of 70, 80, 90, 100, 125, 150, 200 mA/cm2 and even higher.
The alkaline zinc/ferro-ferricyanide (“ZnFe”) rechargeable battery system of some embodiments of the present invention is intended for utility load leveling, load following, area regulation services, transmission & distribution deferral applications, wind and solar integration applications amongst other megawatt energy storage applications having an energy storage capacity from a few minutes, such as 15 minutes up to and exceeding 24 hours duration. The ZnFe battery is a hybrid redox flow battery in which the active materials (zinc oxide and sodium ferrocyanide) are stored in reservoirs external to the cell and brought to the site of electrochemical reaction as saturated solutions in a sodium hydroxide electrolyte.
During charge, energy is stored in the form of zinc metal deposited upon the zinc electrode substrate and as ferricyanide formed by anodic oxidation of the ferrocyanide reactant. When the demands of the load require, energy may be drawn from the cell by anodically dissolving the zinc to form zinc oxide with the simultaneous reduction of ferricyanide ions to ferrocyanide. These processes are highly reversible and selective, enabling the cell to operate with the advantages of high cycling efficiency, high cell voltage, random cycling and switch times of less than 5 ms from load to isolation or from isolation to full load.
Prior art flow batteries, especially Zn based, have problems with dendrite growth particularly as operating current density is increased during charging (deposition). For example, zinc dendrites may form during the deposition (charging) process in a zinc-based battery due to various causes. Zinc dendrites can cause problems in zinc-based batteries including a reduction in performance, cell short circuits and reduced operating lifetime all of which increase effective operating costs.
Embodiments of the present invention will provide higher performance (and thereby a lower operating cost) for zinc and other flow batteries by increasing the sustainable operating current density for charging and discharging of cells with reduced or minimized growth of dendrites. Flow battery embodiments of the invention, particularly for grid storage applications, generally will have power outputs approximately in the range from 20 kW to 25 MW and greater and energy outputs approximately in the range of 5 kWh to 600 MWh or discharge durations from 5 to 15 minutes to over 24 hours for a given power rating of the flow battery although higher and lower power and energy outputs can be used. Generally, the charge and discharge times are defined by the market application for a specific flow battery product. Typical discharge times are 15 minutes, 1, 2, 4, 8, 12, 16 and 24 hours. The ratio of charge to discharge time is generally in the range from 2 to 1 or 1 to 1 or 1 to 2, with approximately a 1 to 1 charge to discharge ratio being desirable.
Embodiments of high performance flow batteries, for example a ZnFe flow battery, of the present invention are based on a number of improvements over the prior art that will allow operation at high current densities and/or that lower battery overall operating costs.
First, the battery design has a cell comprising a low resistance positive electrode in at least one positive half cell and a low resistance negative electrode in at least one negative half cell, where the positive electrode and negative electrode resistances are selected for uniform high current density across a region of the cell stack—that is with a resistance across the electrodes sufficiently low to ensure small voltage variations across the electrode and hence uniform current flow out of the electrode and across at least a region of the cell (for example, voltage variations typically less than 5 to 10 mV where the resistance across a cell results in less than 200 mV loss at an operating current density of 100 mA/cm2, corresponding to a variation in current density of less than 20%.) Cells are often assembled together in series in a cell stack that includes multiple cells. The electrical connection between cells in the cell stack can be in the form of a bipolar electrode or other electrode designs including using wires to connect cells together in series and or parallel to make a cell stack. Typically multiple cell stacks are combined to make a battery system.
Second, a flow rate of electrolyte (for example, zinc species in the ZnFe battery) with a high rate of mixing is induced through at least one negative half cell in a Zn deposition region proximate a deposition surface where the electrolyte solution has sufficiently high zinc concentration for deposition rates on the deposition surface that sustain the uniform high current density across a cell or across substantially all of the cells in a cell stack. The flow in the negative half cell is engineered to provide substantially uniform deposition of zinc over the deposition surface. Furthermore, some embodiments are flow engineered to provide zinc deposition, where the zinc has a dense, adherent, non-dendritic morphology. The flow may be laminar with mixing elements or the mixing may be achieved through turbulent flow at high velocity or turbulent flow at lower velocity with turbulence elements added to a flow channel of the cell.
Third, the zinc electrolyte has a high concentration and in some embodiments has a concentration greater than the equilibrium saturation concentration, that is the zinc electrolyte is super-saturated with zinc ions. Different embodiments of the present invention combine one or more of these improvements.
The flow battery operating current density is a function of the concentration of active ion species. Some embodiments of the invention provide a super-saturated electrolyte to increase the concentration of ions particularly during charging. Zincate electrolyte can be manufactured with super-saturated zinc (Zn) ions through a chemical or electrochemical route. For example zincate electrolyte can be manufactured with approximately ˜1 to ˜1.9 Molar zinc ions, which remains stable for in excess of one day. See Dirkse, Journal of the Electrochemical Society, Volume 128 (No. 7), July 1987, pp 1412-1415; Dirkse, Journal of the Electrochemical Society, Volume 134 (No. 1), January 1987, pp 11-13; and Debiemme-Chouvy & Vedel, Journal of the Electrochemical Society, Volume 138 (No. 9), September 1991, pp 2538-2542. Note that it is permissible to have zincate particles in the electrolyte provided that the particle size is small relative to the size of the electrolyte channel, that is, the flow channel of the cell. Furthermore, the electrolyte chemistry for the ZnFe flow battery has the added advantage of providing basic (high pH) electrolytes, which are less corrosive than many of the alternative electrolyte chemistries which are more acidic. A basic chemistry is advantageous for the initial cost and longevity of components of the flow battery such as the plumbing and pumps used to feed the electrolyte flow to and from the cell stack of the flow battery.
High operating current density across the cell deposition surface and through the cell stack lowers the effective cost per unit power or energy output of the battery and lowers overall operating costs. Embodiments of the invention will provide sustainable higher operating current density by ensuring that dendrite growth is avoided or minimized particularly during charging (deposition).
Dendrite growth will be avoided or minimized by ensuring generally uniform operating current density across the deposition surface in the cell and by ensuring there is always an adequate, generally uniform and high concentration of ions in the electrolyte available at or close to the cell deposition surface where the ion concentration is consistent with the high operating current density and sufficient or greater than the concentration required to sustain the current density through deposition surface(s).
High current density operation with laminar flow of electrolyte through the cell flow channel without adequate mixing results in reduced ion concentration in the diffusion boundary layer at or close to the deposition surface which results in non-uniform deposition and dendrite growth. Operating the cell with an electrolyte flow regime that results in mixing (either with laminar flow or with turbulence) in the electrolyte flow through the cell flow channel increases the mass transfer coefficient and decreases the diffusion boundary layer thickness at the deposition surface which in turn increases the availability of ions for deposition. High availability of ions (for example zinc ion concentration in zincate in a ZnFe battery) allows higher current density operation without significantly depleting the electrolyte concentration in the uniform region of the cell deposition surface(s) and as a result with little or no dendrite growth.
The combination of both increased zincate ion concentrations in the electrolyte and increased mixing of the electrolyte in the cell flow channel near the deposition surface, both relative to prior art cells, will reduce or eliminate the formation of dendrites. This will allow sustainably increased high current density operation and will result in a smaller sized cell, smaller overall cell stack and smaller overall module which will decrease the cell, stack and module costs and overall operating costs for a given power and/or current output. These resulting capabilities will provide a more economic battery system and will lower the overall cost of energy and power output of a battery system.
Cell performance is enhanced by engineering the electrolyte flow and cell flow channel geometry to generate sufficient mixing or turbulence to reduce the diffusion boundary layer thickness at the deposition surface.
Tables 1 and 2 below shows illustrative values of high operating current density and associated average mass transfer coefficient (km) estimates for the flow in the cell flow channel according to embodiments of the present invention. The mass transfer coefficient relates the rate of mass transfer to the electrode surface (mol/cm2·s) and the difference in concentration between the bulk of the solution and at the electrode surface (mol/cm3). Mixing in the cell flow channel for increased operating current density can also be described in terms of the Sherwood Number or mean Sherwood Number (Shm) defined as the dimensionless mass transfer coefficient, also defined as the ratio of convective transport to diffusive transport of ions in the electrolyte. Note that the examples of Sherwood numbers in the tables below are calculated based on correlations for flow through 3D turbulent structures; however, other calculation methods may be used within the spirit and scope of the present invention. iL is the limiting current density, that is the current density at zero ion concentration in mA/cm2 at the electrode surface (or electrode solid interface). iapp is the favorable cell operating current density, defined for purposes of the examples in Table 1 as approximately ˜⅔ times iL in mA/cm2 (although those of skill in the art will recognize that other values or definitions may be used within the spirit and scope of various embodiments of the invention). v is the average flow velocity in cm/s of the electrolyte flowing through the cell flow channel. Cb is the bulk concentration, i.e. the active ion concentration outside the diffusion boundary layer, mol/l. Tables 1 and 2 below also provide illustrative examples of these parameters. While these parameters and terms are familiar to those skilled in the art, additional details can be found in text books such as for example “Advanced Transport Phenomenon: Fluid Mechanics and Convective Transport” by L. Gary Leal Chapter 9, published by Cambridge University Press, in 2007, and “Unit Operations of Chemical Engineering” by Warren L . McCabe, Julian C. Smith and Peter Harriot, Chapter 21, published by McGraw Hill Inc (Vth edition, 1993).
Operational Range Examples with Cb = 0.25 (mol/L)