U.S. patent application number 15/992332 was filed with the patent office on 2018-09-27 for high performance flow battery.
The applicant listed for this patent is Applied Materials, Inc.. Invention is credited to Joseph Grover GORDON, II, Alan J. GOTCHER, Godfrey SIKHA, Gregory J. WILSON.
Application Number | 20180277864 15/992332 |
Document ID | / |
Family ID | 44710032 |
Filed Date | 2018-09-27 |
United States Patent
Application |
20180277864 |
Kind Code |
A1 |
GORDON, II; Joseph Grover ;
et al. |
September 27, 2018 |
HIGH PERFORMANCE FLOW BATTERY
Abstract
High performance flow batteries, based on alkaline
zinc/ferro-ferricyanide rechargeable ("ZnFe") and similar flow
batteries, may include one or more of the following improvements.
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. Second, a flow of electrolyte, such as
zinc species in the ZnFe battery, with a high level of 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.
Inventors: |
GORDON, II; Joseph Grover;
(San Jose, CA) ; GOTCHER; Alan J.; (Incline
Village, NV) ; SIKHA; Godfrey; (Santa Clara, CA)
; WILSON; Gregory J.; (Kalispell, MT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Applied Materials, Inc. |
Santa Clara |
CA |
US |
|
|
Family ID: |
44710032 |
Appl. No.: |
15/992332 |
Filed: |
May 30, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14470779 |
Aug 27, 2014 |
10008729 |
|
|
15992332 |
|
|
|
|
13076337 |
Mar 30, 2011 |
|
|
|
14470779 |
|
|
|
|
61322780 |
Apr 9, 2010 |
|
|
|
61319248 |
Mar 30, 2010 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2/38 20130101; Y02E
60/50 20130101; H01M 8/20 20130101; Y02E 60/10 20130101; H01M
8/04186 20130101; H01M 8/188 20130101 |
International
Class: |
H01M 8/04186 20060101
H01M008/04186; H01M 8/20 20060101 H01M008/20; H01M 8/18 20060101
H01M008/18; H01M 2/38 20060101 H01M002/38 |
Claims
1. A method of charging a flow battery, comprising: providing a
uniform high current density across a low resistance positive
electrode and a low resistance negative electrode, the high current
density passing through a deposition region of a deposition surface
of a negative half-cell of the flow battery; generating a
super-saturated electrolyte flow through a flow channel of the
negative half cell with a high rate of mixing in a deposition
region proximate the deposition surface, wherein the
super-saturated electrolyte has a zinc ion concentration greater
than 0.4N; and maintaining a mass transfer coefficient of the flow
proximate the deposition surface sufficiently large to maintain a
sufficient electrolyte concentration proximate the deposition
surface for substantially uniform deposition in the region of the
deposition surface.
2. The method of claim 1, wherein the super-saturated electrolyte
has a sufficient concentration of zinc ions for deposition rates on
the deposition surface that sustains the uniform high current
density through the deposition surface during the charging.
3. The method of claim 1, wherein the super-saturated electrolyte
has a zinc solubility of greater than about 0.7M in a 4N NaOH
containing solution.
4. The method of claim 1, wherein the super-saturated electrolyte
has a zinc solubility of about 0.73M in a 4N NaOH containing
solution.
5. The method of claim 1, wherein the super-saturated electrolyte
is prepared by combining zinc oxide (ZnO) with NaOH pellets.
6. The method of claim 1, wherein the uniform high current density
is greater than 70 mA/cm2.
7. The method of claim 1, wherein a mass transfer coefficient of
the super-saturated electrolyte has a value in the approximate
range of 5.3.times.10-4 m/s to 12.4.times.10-3 m/s.
8. The method of claim 1, wherein the flow battery is a flow
battery selected from the group consisting of: a ZnFe flow battery,
a ZnHBr flow battery, a ZnBr flow battery, a CeZn flow battery; and
a ZnCl flow battery.
9. The method of claim 1, wherein the flow channel is configured to
provide a high rate of mixing of the super-saturated electrolyte in
the negative plating zone proximate the surface of the negative
electrode.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of co-pending
U.S. patent application Ser. No. 14/470,779, filed on Aug. 27,
2014, which is a continuation application of U.S. patent
application Ser. No. 13/076,337, filed on Mar. 30, 2011 and now
published as US 2011/0244277, which claims benefit of U.S.
Provisional Patent Application Ser. No. 61/319,248, filed on Mar.
30, 2010 and U.S. Provisional Patent Application No. 61/322,780,
filed on Apr. 9, 2010, all of which are incorporated herein by
reference in their entirety herein.
FIELD OF THE INVENTION
[0002] This invention relates to high performance electrochemical
cells and batteries, and more particularly to flow batteries.
BACKGROUND OF THE INVENTION
[0003] 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.
[0004] 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
[0005] 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/cm.sup.2, and more particularly in the range of 100 to
250 mA/cm.sup.2, are provided by various embodiments of the present
invention.
[0006] 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).
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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
[0012] 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:
[0013] FIG. 1 is a schematic diagram of a zinc redox flow
battery;
[0014] FIG. 2 is schematic diagram of a zinc redox flow battery,
according to some embodiments of the present invention;
[0015] FIG. 3 is a schematic perspective view of a flow cell,
according to some embodiments of the present invention;
[0016] 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;
[0017] 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;
[0018] 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;
[0019] 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;
[0020] 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;
[0021] 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;
[0022] 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;
[0023] 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;
[0024] 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;
[0025] 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
[0026] FIG. 14 is a cross-sectional representation of a flow
laminarization feature, according to some embodiments of the
present invention.
DETAILED DESCRIPTION
[0027] 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.
[0028] Embodiments of the present invention provide high
performance flow battery apparatus and methods for enhancing,
charging, operating and using flow batteries.
[0029] 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/cm.sup.2 and even higher.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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/cm.sup.2, 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.
[0036] 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.
[0037] 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.
[0038] 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 .about.1 to
.about.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.
[0039] 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).
[0040] 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).
[0041] 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.
[0042] 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.
[0043] 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.
[0044] Tables 1 and 2 below shows illustrative values of high
operating current density and associated average mass transfer
coefficient (k.sub.m) 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/cm.sup.2s) and the difference in
concentration between the bulk of the solution and at the electrode
surface (mol/cm.sup.3). 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 (Sh.sub.m) 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. i.sub.L
is the limiting current density, that is the current density at
zero ion concentration in mA/cm.sup.2 at the electrode surface (or
electrode solid interface). i.sub.app is the favorable cell
operating current density, defined for purposes of the examples in
Table 1 as approximately .about.2/3 times i.sub.L in mA/cm.sup.2
(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. C.sub.b 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 (V.sup.th
edition, 1993).
TABLE-US-00001 TABLE 1 Operational Range Examples with C.sub.b =
0.25 (mol/L) I.sub.app (mA/cm.sup.2) 70 100 150 200 250 400 C.sub.b
(mol/L) 0.25 0.25 0.25 0.25 0.25 0.25 i.sub.L (mA/cm.sup.2) 105 150
225 300 375 600 k.sub.m (cm/s) 2.3 .times. 10.sup.-3 3.1 .times.
10.sup.-3 4.6 .times. 10.sup.-3 6.2 .times. 10.sup.-3 7.8 .times.
10.sup.-3 12.4 .times. 10.sup.-3 Sh.sub.m 64 86 129 172 215 342
TABLE-US-00002 TABLE 2 Operational Range Examples with C.sub.b = 1
(mol/L) I.sub.app (mA/cm.sup.2) 70 100 150 200 250 400 C.sub.b
(mol/L) 1.0 1.0 1.0 1.0 1.0 1.0 i.sub.L (mA/cm.sup.2) 105 150 225
300 375 600 k.sub.m (cm/s) 5.3 .times. 10.sup.-4 7.7 .times.
10.sup.-4 1.2 .times. 10.sup.-3 1.5 .times. 10.sup.-3 1.9 .times.
10.sup.-3 3 .times. 10.sup.-3 Sh.sub.m 16 21 32 43 54 86
[0045] For cell operation in the approximate range of 70 to 400
mA/cm.sup.2 the desirable mass transfer coefficient is between
approximately 5.times.10.sup.-4 and 1.24.times.10.sup.-2 cm/s. For
cell operation in the approximate range of 70 to 400 mA/cm.sup.2
the desirable mean Sherwood number is between approximately 15 and
350.
[0046] Calculated zinc deposition thickness on the cell deposition
surface(s) for 8 hours of charging operation for the following high
current densities are shown in Table 3.
TABLE-US-00003 TABLE 3 Approximate Zinc Deposition Thickness for 8
Hours of Charging current density (mA/cm.sup.2) 100 200 400 deposit
thickness (cm) 0.17 0.34 0.68 deposit capacity (mAh/cm.sup.2) 800
1,600 3,200 mass of deposit (g/cm.sup.2) 0.976 1.951 3.902
[0047] Note that these thickness figures scale linearly with
current density and time. For example, for a current density of 100
mA/cm.sup.2, a growth rate of approximately 0.21 mm/hour is
calculated; for a current density of 200 mA/cm.sup.2, a growth rate
of approximately 0.43 mm/hour is calculated; and for a current
density of 400 mA/cm.sup.2 a growth rate of approximately 0.85
mm/hour is calculated.
[0048] Although the examples provided herein are of ZnFe redox flow
batteries, other redox flow batteries may be fabricated using the
teaching and principles of the present invention. For example, the
following batteries may be fabricated: ZnHBr; ZnBr; CeZn; and
ZnCl.
[0049] FIG. 2 shows a schematic representation of a flow battery
200, according to some embodiments of the present invention. FIG. 2
is an example of a ZnFe flow battery. The flow battery 200 has a
positive electrode 212, a negative electrode 222 on the surface of
which there is a zinc plating zone 224, and a membrane 210
separating a positive channel 211 and a negative channel 221. The
flow of electrolyte through the separate channels in the cell and
through the rest of the fluid circuits is indicated by arrows 213
and 223, for the posilyte and negalyte circuits, respectively. Each
fluid circuit includes a cell channel (211 and 221), an optional
flow calming feature 262 (such as shown in FIG. 14), an electrolyte
reservoir (240 which in this example contains the posilyte-sodium
ferrocyanide/ferricyanide solution and 250, which in this case
contains the negalyte sodium zincate solution), a pump 263 and flow
sensor(s) 264. The pumps 263, sensors 264 and pump controller 265
are configured to control the flow of electrolyte through the cell,
including control of the flow regime in the cell--the flow regime
may be laminar, mixing, and/or turbulent flow as described
throughout this application.
[0050] The amount of electrolyte pumped through each side of the
electrochemical cell is determined by calculation of the cell
channel volume, the rate of electrolyte flow and the amount of
zincate depletion desired at the cell exit chamber with
consideration for the energy storage duration and piping volumes
and pipe run lengths defining the electrolyte storage tank sizes.
For a given amount of electrolyte per cell stack, a pump size is
selected using materials of construction having durability and long
life under conditions of strong base (e.g. 2 to 5 N NaOH) or strong
acid depending upon the nature of the electrolyte used in the
electrochemical cell. Typically, two pumps are selected for each
cell stack, one for each electrolyte. Generally acceptable
materials of construction include polypropylene, polyethylene,
fluorinated polymers, polyetherketones, polysulfones, polyphenylene
sulfide and the like. Various sensors are selected to measure the
fluid velocity, direction of fluid flow, temperature, pressure and
other metrics at various locations in the storage tanks, piping,
pumps, entrance and exit points of the cell channel. The various
data signals from each sensor are transferred by signal wire or by
wireless transfer to a data control system. The data control system
records the data flow, and uses algorithms, set points and control
inputs to send data signals to fans (for cooling, if required),
valves and motors to control (e.g. increase, decrease or hold)
motor speeds and valve positions which in turn increase, decrease,
hold constant or change fluid directions on command. The data
control system may under certain conditions send alarm signals and
other performance data to remotely located control rooms. The
piping to and from the cell channel is designed and sized to
minimize shunt current loses and the materials of construction are
selected for durability under conditions of strong base or strong
acid depending upon the nature of the electrolyte. The
electrochemical system is generally sited inside a fluid
containment system comprising appropriate sensors and alarms to
indicate any electrolyte leaks.
[0051] FIG. 3 shows a schematic perspective view of a cell 300. The
cell 300 may be roughly 0.5 cm thick, with larger dimensions of
roughly 30 cm.times.30 cm up to 132 cm.times.67 cm, for example. A
cross-sectional view of the section X-X is shown in FIG. 7. FIG. 3
shows a cell 300 with bipolar structural elements on either side.
The cell has positive 211 and negative 221 channels separated by a
membrane 230. A negalyte is pumped through the negative channel and
a posilyte is pumped through the positive channel, as shown;
posilyte fluid flow and negalyte fluid flow are shown by arrows 213
and 223, respectively. Further details of the cell are provided
above with reference to FIG. 7.
[0052] FIG. 4 shows the cell 300 of FIG. 3 in a frame 410. The cell
300 is surrounded by the frame 410 that serves to hold the membrane
(separator) and bipolar structural elements in place, creating the
flow channels, sealing the edges of the flow channels, providing a
place to attach the electrolyte flow and return pipes and,
optionally, may contain electrolyte distribution manifolds and flow
calming features. Flow of posilyte and negalyte into and out of the
frame for provision to the cell 300 is indicated by arrows 213 and
223, respectively.
[0053] More detailed examples of cells according to some
embodiments of the present invention are provided in FIGS. 5-7. The
cells may have large dimensions of 30 cm.times.30 cm, 90
cm.times.90 cm, 60 cm.times.90 cm, 45 cm.times.90 cm, or 132
cm.times.67 cm, for example. Examples of the cross-sectional
dimensions of the components of the cells are provided in FIGS.
5-7. However, the present invention is not limited to these cell
dimensions and may be used with cells of smaller or larger
dimensions. The cells are shown in cross-section, and the section
is perpendicular to the larger surface of the cell. (For example,
see section X-X in FIG. 3.) The power density is a function of the
cell chemistry and the cell current density. For ZnFe at 200
mA/cm.sup.2, for example, the discharge power density is about 0.3
W/cm.sup.2. For the cell dimensions listed above, the resulting
power per cell will be approximately 274 W, 2.43 kW, 1.60 kW, 1.22
kW and 2.45 kW, respectively.
[0054] FIG. 5 shows a first example of a schematic cross-section of
a bipolar ZnFe redox flow battery cell of the present invention. A
single battery cell is shown comprising a negative half cell 220, a
positive half cell 210 and a bipolar structural element 270. The
bipolar structural element 270 separates the positive half cell
from the negative half cell of adjacent cells. (See FIG. 6.) The
bipolar structural element 270 in this example is a 50% graphite
fiber/PPS interconnector on which there is a cadmium metal strike
271. (PPS is polyphenylene sulfide. Other polymer materials can be
used in place of PPS in the construction of bipolar structural
elements, such as polyetherketones, polysulfones, polyethylenes,
polypropylenes and the like in combination with conductive fillers
such as graphite fiber or flakes, certain carbon powders and carbon
blacks, carbon nanotubes, conductive metal powders, and the like.)
The positive half cell 210 comprises a porous Ni mesh redox
electrode, which completely fills the positive channel 211--the
posilyte flows through the porous Ni mesh redox electrode. The
negative half cell 220 includes a Zn plating zone 224 of variable
thickness on the Cd metal strike 271 and a negalyte flow channel
221. The positive half cell and the negative half cell of an
individual cell are separated from each other by a membrane 230,
made of material such as Nafion-114 or another separator material.
The membrane 230 keeps the zincate and iron electrolytes separated,
but Na ions and water are able to move through the membrane. The
membrane material can be a separator material with or without
grafted ionic chemical species.
[0055] In order to operate the bipolar cell of FIG. 5 at a high
current density, for example a charging current density of 200
mA/cm.sup.2, a high mass transfer rate is generated in the negative
flow zone. This may be achieved by increasing the mixing rate
and/or by increasing the electrolyte fluid flow rate, above that of
prior art cells. This may be done by adding mixing elements to the
cell channel, or by increasing the velocity without reaching the
turbulent flow regime, or by increasing the velocity until
turbulent flow is achieved, or by introducing turbulence generating
elements as discussed below. Note that zinc deposition current
density is a function of fluid (electrolyte) velocity and Reynolds
number. See R. D. Naybour, "The Effect of Electrolyte Flow on the
Morphology of Zinc Electrodeposited from Aqueous Alkaline Solution
Containing Zincate Ions" J. Eletrochem. Soc. pages 520-525, April
1969. Note that the deposition operating current density is a
function also of the concentration of the active species.
[0056] FIG. 6 shows a second example of a schematic cross-section
of a bipolar ZnFe redox flow battery cell. The positive electrode
612 comprises a porous Ni mesh redox electrode attached to a
bipolar Ni/Cu electrode 272--the Ni mesh being attached to the Ni
face of the bipolar electrode. The posilyte flow zone is occupied
by the porous Ni mesh. The negative electrode 622 may comprise a
Cd, Sn or Pb coated high surface area Cu or Ni mesh, which is 60%
to 98% porous, for example. The coated Cu or Ni mesh is attached to
the Cu face of the bipolar electrode 272. The coated Cu or Ni mesh
occupies the negalyte flow zone and the mesh generates mixing in
the negalyte flow, without requiring high fluid velocity. The cell
is set up so that the coated Cu or Ni mesh may be plated with Zn up
to approximately 20% to 70% of volume. FIG. 6 also illustrates how
the bipolar electrode 272 (or the bipolar structural element 270 of
FIG. 5) separates the cell from adjacent cells and facilitates the
efficient and cost-effective construction of a cell stack. Adjacent
cells are shown in FIG. 6.
[0057] FIG. 7 shows a third example of a schematic cross-section of
a bipolar ZnFe redox flow battery cell. (FIG. 7 is section X-X in
FIG. 3.) The positive channel 211 comprises a porous Ni mesh redox
electrode attached to a bipolar Ni/Cu electrode 272--the Ni mesh
being attached to the Ni face of the bipolar electrode. The
posilyte flow zone is occupied by the porous Ni mesh. The negative
channel 221 includes a zinc metal plating zone 224 and features 280
configured to induce efficient mixing or turbulence. Examples of
features 280 are shown in FIGS. 8-13 and are described below. (Note
that features 280 may be positioned in the flow channel above the
deposition surface as shown in FIG. 7, or in other embodiments may
be positioned directly on the deposition surface, as shown in FIGS.
11-13--FIGS. 8-10 show structures that may be either positioned on
the deposition surface or above it.) These features are designed to
generate a high rate of mixing of the flow while not necessarily
requiring high velocity (and therefore high pumping power
dissipation). Zn metal is plated on the Cu face of the bipolar
electrode 272. The Cu face may alternatively also be coated with
Cd, Sn or Pb. Note that the cylinders, cones and pyramids shown in
FIGS. 11-13 will be made of non-conducting material and are shown
to have sharp edges and points. However, if it is desired to make
the cylinders, cones and pyramids of conductive material then they
should have blunt edges and ends rather than sharp edges and
points. (Note that in order to improve the uniformity of Zn
plating, the features used to induce mixing and/or turbulence
should not have sharp points or edges if they are conductive--sharp
points and edges are electric field concentrators and lead to
undesirable non-uniform plating and even dendrite formation.)
Furthermore, the features should not occupy too large a volume such
that flow through the anode channel is unduly impeded--see below
for further details.
[0058] Note that the conditions under which turbulent flow is
induced can be conveniently defined for a particular channel
geometry by using, for example Reynolds numbers. Those skilled in
the art are familiar with the calculation of Reynolds numbers,
including for channels that contain features such as those shown in
FIGS. 8-13. Roughly, for the cell illustrated in FIG. 7 with a
substantially rectangular channel, a Reynolds numbers of at least
approximately greater than 1,300 or preferably 2,000 may be used to
ensure turbulent flow where the characteristic length is defined
through hydraulic diameter. The hydraulic diameter for a narrow
flow channel (L>>W) is twice the thickness of the channel,
i.e. 2W, where L and W are the length and width of the flow
channel, as measured perpendicular to the direction of fluid flow.
(See FIG. 3.)
[0059] For the cell shown in FIG. 7 incorporating the features
shown in FIGS. 8-13, lower Reynolds number may suffice to ensure
efficient mixing or a high rate of mixing, for example Reynolds
numbers of at least approximately 8 or greater may suffice to
ensure efficient mixing. However, the specific Reynolds number will
vary with the cell and flow channel design and mixing feature.
[0060] FIG. 8 shows an example of a structure suitable for inducing
mixing or turbulence in the electrolyte flow over the surface of a
flow battery electrode. A small section of woven wire mesh 820 is
shown on part of the surface of the electrode 810. The direction of
electrolyte flow is indicated by the arrow; the flow being
generally parallel to the surface of the electrode 810. The wire
mesh 820 disrupts the fluid flow, inducing desirable mixing in
either laminar, semi-turbulent or turbulent flow over the electrode
surface. Note that a similar effect may also be achieved by having
the mesh close to, but not necessarily on, the surface of the
electrode 810. A suitable wire diameter will be between 20% and 50%
of the channel thickness.
[0061] In some embodiments the wire mesh is conductive and acts as
part of the electrode surface thereby increasing the total
electrode surface area (in addition to the mesh acting as a mixing
element). In other embodiments, the mesh is non-conductive and
mixes the flow on the surface of the planar electrode. A
non-conductive mesh can also be used to ensure a specified
electrode-to-membrane spacing when it is sandwiched between the
electrode and membrane. In yet other embodiments, there can be
several layers of mesh, some conductive and some non-conductive. In
one such example, a conductive mesh is adjacent to the electrode
and a non-conductive mesh is between the conductive mesh and the
membrane. The non-conductive mesh acts as a spacer to keep the
plating surfaces away from the membrane, as well as, acting as a
flow mixing structure. Non-conductive mesh can be made of plastic
or other non-conductive or low conductivity materials. In yet
further embodiments, there may be a series of adjacent meshes with
varying electrical conductivity. This structure will determine the
local electrical field which controls the local current
distribution and hence the plating uniformity.
[0062] FIG. 9 shows another structure suitable for inducing mixing
in laminar or turbulent electrolyte flow. A small section of
non-woven wire mesh 830 is shown on part of the surface of the
electrode 810. The direction of electrolyte flow is indicated by
the arrow; the flow being generally parallel to the surface of the
electrode 810. The wire mesh 830 disrupts the fluid flow, inducing
desirable mixing in laminar or turbulent flow over the electrode
surface. Note that a similar effect may also be achieved by having
the mesh close to, but not necessarily on, the surface of the
electrode 810. A suitable wire diameter will be between 10% and 50%
of the channel thickness. Multiple wires may be stacked or spaced
across the cell channel to further enhance performance in some
embodiments.
[0063] FIG. 10 shows yet another structure suitable for inducing
mixing in laminar or turbulent electrolyte flow. Parallel
wires/tubes 840 are shown on part of the surface of the electrode
810. The direction of electrolyte flow is indicated by the arrow;
the flow being generally parallel to the surface of the electrode
810 and perpendicular to the long axes of the wires/tubes. The
wires/tubes 840 disrupt the fluid flow, inducing desirable
turbulent (non-laminar) flow over the electrode surface. Note that
a similar effect may also be achieved by having the wires/tubes
close to, but not necessarily on, the surface of the electrode 810.
A suitable wire diameter will be between 10% and 90% of the channel
thickness.
[0064] FIG. 11 shows part of an array of features for inducing
mixing in laminar or turbulent electrolyte flow. An array of
cylinders 850 is shown on part of the surface of the electrode 810.
The direction of electrolyte flow is indicated by the arrows; the
flow being generally parallel to the surface of the electrode 810.
The array of cylinders 850 disrupts the fluid flow, inducing
desirable mixing in the flow over the electrode surface. The
cylinders as shown are formed of non-conductive material and may
have sharp edges. A suitable cylinder height is between 20% and
100% of the channel thickness. The spacing and diameter must be
such as to generate turbulence at the desired flow rate.
[0065] FIG. 12 shows part of another array of features for inducing
mixing in laminar or turbulent electrolyte flow. An array of cones
(or tapered cylinders) 860 is shown on part of the surface of the
electrode 810. The direction of electrolyte flow is indicated by
the arrows; the flow being generally parallel to the surface of the
electrode 810. The array of cones 860 disrupts the fluid flow,
inducing desirable mixing in laminar or turbulent flow over the
electrode surface. The tapered cylinders as shown are formed of
non-conductive material and may have sharp points. A suitable
cylinder height is between 20% and 100% of the channel thickness.
The spacing and diameter must be such as to generate mixing while
not unduly increasing the flow resistance as the channel is reduced
in thickness as the Zn deposit increases in thickness.
[0066] FIG. 13 shows part of yet another array of features for
inducing mixing in the electrolyte flow. This is to illustrate that
shapes other than cylinders are suitable. An array of pyramids 870
is shown on part of the surface of the electrode 810. The direction
of electrolyte flow is indicated by the arrows; the flow being
generally parallel to the surface of the electrode 810. The array
of pyramids 870 disrupts the fluid flow, inducing desirable mixing
of the flow over the electrode surface. The tapered features as
shown are formed of non-conductive material and may have sharp
edges and may have other cross-sections, for example triangular,
other polygon or oval. A suitable feature height is between 20% and
100% of the channel thickness. The spacing and diameter must be
such as to generate mixing at the desired flow rate. The taper may
be chosen to maintain a high rate of mixing while not unduly
increasing the flow resistance as the channel is reduced in
thickness as the Zn deposit increases in thickness.
[0067] FIGS. 8-13 provide a range of examples of features that may
be used to induce mixing in the electrolyte flow over the surface
of the electrode. However, these examples are not intended to be a
comprehensive listing, and further features suitable for inducing
mixing in laminar and/or turbulent flow will be apparent to these
skilled in the art after reading this disclosure. For example,
further features may include: combinations of the above described
features; conductive and non-conductive meshes; ribbons; foam
structures; and other arrangements of wires or tubes.
[0068] The arrays shown in FIGS. 11-13 are shown as regular arrays
of features; however, these arrays may also have randomly
positioned features, or partially randomly positioned features.
[0069] The features of FIGS. 8-13 are shown on the surface of a
flow battery electrode. However, such features may alternatively,
or in addition, be located in the electrolyte stream immediately
prior to the electrolyte flowing across the electrode; the features
may be attached, for example to the inner surface of the plumbing
that delivers the electrolyte into the half cell.
[0070] In commercial flow battery operation the power consumed by
the pumping system is an important factor in optimizing overall
productivity of the battery system. While high fluid pumping rates
induce higher degrees of mixing in the cell they also demand more
pumping power that ultimately detracts from the power and energy
delivered by the battery system. Higher pumping rates also causes
higher wear and therefore more frequent preventive maintenance.
Pump power, mixing and turbulence can be traded off against each
other in battery design. Laminar flow inside and outside of the
cell and through the pipes to and from the cell generally reduces
pumping power requirements. When operating with turbulence for
mixing the electrolyte in the flow channel of a cell, the
turbulence can be quenched for example by allowing for a lower
velocity region in which flow velocity is reduced and laminar flow
resumed, for example using structures such as illustrated in FIG.
14. Ensuring flow outside of any intentional turbulent region of
the cell is laminar or substantially laminar reduces pump power
consumption.
[0071] FIG. 14 shows a cross-section of a modified pipe at the exit
from a half cell, the pipe is designed to calm the turbulent flow
and provide a laminar flow as the electrolyte moves through the
rest of the plumbing. The turbulently flowing electrolyte flows
from the half cell into a first segment of pipe 1410. The
electrolyte then enters a second segment of pipe 1420 in which the
cross-section of the pipe increases. As the pipe 1420 cross-section
increases, the velocity of the electrolyte decreases and the
turbulent flow is calmed, resulting in a laminar flow. The laminar
flowing electrolyte then enters a third section of pipe 1430 in
which the pipe cross-section decreases, so as to funnel the calmed
electrolyte into the plumbing 1440 that continues the electrolyte
circuit. The direction of electrolyte flow is indicated by the
arrows.
[0072] The flow of electrolyte through the cell flow channel may be
reversed to improve mixing, uniformity of deposition and to avoid
depletion of electrolyte at the deposition surface.
[0073] Standoffs can be used to support a mesh (or screen) mixing
element in the flow channel to avoid it contacting the membrane or
electrode and generally to avoid bowing or buckling of the mesh due
to high flow rate, turbulence or temperature variation.
[0074] Those of skill in the art will be aware of many definitions
and measures of turbulence. Turbulence in this context generally
means variations in flow velocity (velocity being a vector, and
variations including both variations in the speed and direction of
flow) including to cause mixing of the electrolyte flow to avoid
depletion of the deposited ion at or close to the deposition
surface during deposition (charging) or removal during
discharge.
[0075] Definition of substantially uniform deposition on a
deposition surface of the cell during charging means deposition
without or with reduced dendrite formation during the charging
period. Those of skill in the art will recognize that some
variation of deposition thickness across the deposition surface and
across the cell (perhaps less than 20%) are inherent, particularly
when in operation at high current density and with high
concentration of ions in the electrolyte.
[0076] A high rate of mixing generally means mixing in laminar or
turbulent flow to avoid or minimize depletion of the plating ion
particularly at or close to the deposition surface. A high rate of
mixing for use with various embodiments can be achieved as follows:
(1) with a channel and off deposition surface mixing device; (2)
with a channel and on deposition surface mixing device; (3) with a
channel with both on and off deposition surface mixing devices; or
(4) with a channel, mixing device and high electrolyte
velocity.
[0077] First, a high rate of mixing may be achieved when the mixing
element or device is located at a distance from the electrode
deposition surface of at least approximately twice the diffusion
boundary layer thickness and has a cross-sectional area of
approximately 10% to 80% of the cell channel cross-sectional area,
desirably from approximately 25% to 60%, or a high rate of mixing
can be achieved when the mixing device is located at a distance
from the electrode deposition surface of at least approximately 125
microns and has a cross-sectional area of approximately 10% to 80%
of the cell channel cross-sectional area, preferably from
approximately 25% to 60%; or a high rate of mixing can be achieved
when the mixing element or device is located at a distance from the
electrode deposition surface of at least approximately twice the
diffusion boundary layer thickness and have a cross-sectional area
of approximately 10% to 80% of the cell channel cross-sectional
area, preferably from approximately 25% to 60%, where the mixing
device has a repeating feature (or approximately repeating feature)
across the cell channel width and along the cell channel length; or
a high rate of mixing can be achieved when the mixing element or
device is located at a distance from the electrode deposition
surface of at least approximately twice the diffusion boundary
layer thickness and have a cross-sectional area of approximately
10% to 80% of the cell channel cross-sectional area, preferably
from approximately 25% to 60%, where the mixing device has a
repeating feature (or approximately repeating feature) across the
cell channel width and along the cell channel length and the
spacing interval of the repeating feature along the cell channel
length is at least approximately 1.1 times the spacing interval of
the repeating feature across the channel width.
[0078] Second, a high rate of mixing can be achieved when a mixing
element or device or devices are attached to the electrode
deposition surface and the ratio of the mixing device leading edge
repeat distance is at least approximately five times the mixing
device height from the electrode deposition surface, and the shape
of the mixing device can be selected from the group comprising a
wire, mesh, screen, a semi-spherical, round, semi-round or
rectangular shape or other shapes or combinations.
[0079] Third, a good rate of mixing can be achieved when a mixing
element or device or devices are attached to the electrode
deposition surface in combination with a second mixing device that
is located at a distance from the electrode deposition surface of
at least approximately twice the diffusion boundary layer thickness
and has a cross-sectional area of approximately 10% to 80% of the
cell channel cross-sectional area, preferably from approximately
25% to 60%.
[0080] Fourth, a high rate of mixing can be achieved when the
mixing element or device is located at a distance from the
electrode deposition surface of at least approximately twice the
diffusion boundary layer thickness and have a cross-sectional area
of approximately 10% to 80% of the cell channel cross-sectional
area, preferably from 25% to 60%, and the electrolyte fluid
velocity is at least approximately 5 cm/s, preferably at least
approximately 25 cm/s and more preferably at least approximately 50
cm/s.
[0081] In some applications, flow batteries, such as some
embodiments of the present invention, may be used for frequency
regulation. Furthermore, some embodiments of the present invention
may be used for other short duration power needs such as UPS
(uninterruptable power system) or short response power backup. For
short duration power needs some embodiments of flow batteries of
the present invention may be operable at high charging and
discharging current densities, such as greater than roughly 200
mA/cm.sup.2.
[0082] In other embodiments the flow battery may include one or
more of the following: a Reynolds Number of the flow channel is
greater than approximately 1300; a Sherwood Number of the flow
channel is greater than approximately 21; the uniform high current
density is >100 mA/cm.sup.2; there is at least one mixing
element in the flow channel; there is at least one turbulence
inducing element in the flow channel; the mass transfer coefficient
is greater than approximately 7.7.times.10.sup.-4 m/s; and the
charging cycle is at least 5 minutes in duration or is at least one
hour in duration.
[0083] In some embodiments, the high performance flow battery may
comprise a stack of cells with a sustainable operating current
density in a region of a cell in the cell stack during a charging
cycle of >100 mA/cm.sup.2. In some embodiments, the high
performance zinc-based flow battery may comprise depositing zinc on
a deposition surface in a cell of the battery at a rate greater
than 0.1 mm per hour, and in other embodiments at a rate greater
than 0.2 mm per hour. Furthermore, in some embodiments a method of
charging a high performance ZnFe flow battery may comprise growing
or depositing zinc on a deposition surface of a cell in the battery
at a rate greater than 0.1 mm per hour, and in other embodiments at
a rate greater than 0.2 mm per hour or greater than 0.4 mm per
hour.
[0084] In another embodiment, a high performance redox flow battery
comprises at least one cell comprising a low resistance positive
electrode in at least one positive half cell and a low resistance
negative electrodes in at least one negative half cell where a
resistance of the low resistance positive electrode and a
resistance of the low resistance negative electrode is small enough
for uniform high current density across a region of a deposition
surface of the at least one cell, an electrolyte flow through a
flow channel of at least one half cell with a high rate of mixing
in a deposition region proximate the deposition surface where the
electrolyte has sufficiently high concentration of an active ion
species for deposition rates on the deposition surface that sustain
the uniform high current density through the deposition surface
during a charging cycle, a mass transfer coefficient of the flow
proximate the deposition surface at least enough to maintain
sufficient electrolyte concentration proximate the deposition
surface for substantially uniform deposition in the region of the
deposition surface.
[0085] In another embodiment, a method of charging a high
performance flow battery comprising delivering a sufficient supply
of electrical energy to the flow battery at a voltage higher than a
voltage of the flow battery, providing a uniform high current
density across a low resistance positive electrode and a low
resistance negative electrode, the high current density passing
through a region of a deposition surface of at least one half cell
of the flow battery, generating an electrolyte flow through a flow
channel of the at least one half cell with a high rate of mixing in
a deposition region proximate the deposition surface where the
electrolyte has sufficiently high concentration of an active ion
species for deposition rates on the deposition surface that sustain
the uniform high current density through the deposition surface
during the charging, maintaining a mass transfer coefficient of the
flow proximate the deposition surface sufficiently large to
maintain a sufficient electrolyte concentration proximate the
deposition surface for substantially uniform deposition in the
region of the deposition surface. The flow may be laminar or
turbulent. The high rate of mixing is in the range sufficient to
maintain the mass transfer coefficient greater than
7.7.times.10.sup.-4 cm/s.
[0086] Furthermore, the present invention includes a method of
optimizing a high performance redox flow battery comprising
engineering a flow rate and a flow channel of the flow battery
optimized to ensure one or more of the following parameters are
satisfied: uniform mass transfer rate across the deposition
surface, at optimal fluid velocity; local current density of less
than approximately 2/3.times.i.sub.L, but sufficiently high to
prevent mossy deposition (in other words a local current density
suitable for providing, a dense, adherent, non-dendritic
morphology); and concentration depletion along the flow channel (on
the Zn side) to approximately <10% of an inlet
concentration.
[0087] Experiments confirm (1) a zinc solubility of 0.73M in 4N
NaOH and (2) a limiting current density (at a rotating disk
electrode) for this solution of 121 mA/cm.sup.2 at a rotation speed
of 120 rpm at 40.degree. C.
[0088] The super-saturated zinc electrolyte--0.73M Zn++ in 4N
NaOH--was prepared as follows. Step 1: prepare a stock solution (1
M Zn.sup.++ in 5.5N NaOH) by combining 8.139 g of ZnO (m.w. 81.39
g/mol) with 30 gm of NaOH pellets (m.w. 40 g/mol) and making up to
100 ml with D.I. water under constant stirring. The resulting
solution is a 1M Zn.sup.++5.5N NaOH. Step 2: dilution of (1 M
Zn.sup.++/5.5N NaOH) to a 4N NaOH solution by taking 100 ml of (1M
Zn.sup.++ in 5.5N NaOH) stock solution and making it up to 137.5 ml
with D.I. water--the resulting solution is 0.73M Zn.sup.++ in 4N
NaOH. (Note that the reported solubility limit of Zn.sup.++ in 4N
NaOH is 0.37M.) Note that electrolytes with NaOH concentrations in
the range of 2-4N are found to provide satisfactory zincate ion
concentration in combination with tolerable ferrous ion
concentration and tolerable corrosive solution properties, whereas
NaOH concentrations above 4N result in rapidly reduced ferrous ion
concentrations along with an electrolyte which is more
corrosive.
[0089] Experiments confirm that 0.73M Zn.sup.++ in 4N NaOH and 0.4
Zn.sup.++ in 2.2N NaOH are stable for at least four weeks.
[0090] Although the present invention has been particularly
described with reference to certain embodiments thereof, it should
be readily apparent to those of ordinary skill in the art that
changes and modifications in the form and details may be made
without departing from the spirit and scope of the invention.
* * * * *