Patent Publication Number: US-2016240880-A1

Title: Flow battery start-up and recovery management

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority to U.S. Provisional Application No. 61/432,552, entitled “Flow Battery Start-Up and Recovery Management”, filed on Jan. 13, 2011, the contents of which are herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention is related to flow battery start-up and recovery. 
     2. Discussion of Related Art 
     Reduction-oxidation (redox) flow batteries store electrical energy in a chemical form, and subsequently dispense the stored energy in an electrical form via a spontaneous reverse redox reaction. A redox flow battery is an electrochemical storage device in which an electrolyte containing one or more dissolved electro-active species flows through a reactor cell where chemical energy is converted to electrical energy. Conversely, the discharged electrolyte can be flowed through a reactor cell such that electrical energy is converted to chemical energy. Electrolyte is stored externally, for example in tanks, and flowed through a set of cells where the electrochemical reaction takes place. Externally stored electrolytes can be flowed through the battery system by pumping, gravity feed, or by any other method of moving fluid through the system. The reaction in a flow battery is reversible; the electrolyte can be recharged without replacing the electroactive material. The energy capacity of a redox flow battery, therefore, is related to the total electrolyte volume (i.e., the size of the storage tank). The discharge time of a redox flow battery at full power also depends on electrolyte volume and can vary from several minutes to many days. 
     The minimal unit that performs the electrochemical energy conversion is generally called a “cell,” whether in the case of flow batteries, fuel cells, or secondary batteries. A device that integrates many such cells, coupled electrically in series and/or parallel to get higher current, voltage, or both, is generally called a “battery.” However, it is common to refer to any collection of coupled cells, including a single cell used on its own, as a battery. As such, a single cell can be referred to interchangeably as a “cell” or a “battery.” 
     Redox flow batteries can be utilized in many technologies that require the storage of electrical energy. For example, redox flow batteries can be utilized to store night-time electricity that is inexpensive to produce, and to subsequently provide electricity during peak demand when electricity is more expensive to produce or demand is beyond the capability of current production. Such batteries can also be utilized for storage of green energy (i.e., energy generated from renewable sources such as wind, solar, wave, or other non-conventional sources). Flow redox batteries can be utilized as uninterruptible power supplies in place of more expensive backup generators. Efficient methods of power storage can be used to construct devices having a built-in backup that mitigates the effects of power cuts or sudden power failures. Power storage devices can also reduce the impact of a failure in a generating station. 
     Therefore, there is a need for better performing flow cell batteries. 
     SUMMARY 
     In accordance with some embodiments of the present invention a method of performing a start-up of a flow cell stack includes applying a current to the stack to plate a catalyst on an electrode while leaving circulation pumps off; and turning circulation pumps on after plating is completed. 
     In some embodiments, a flow cell system can include a flow cell stack; a first circulation pump fluidically coupled between a first electrolyte tank and the flow cell stack to circulate the a electrolyte through the flow stack; a second circulation pump fluidically coupled between a second electrolyte tank and the flow cell stack to circulate a second electrolyte through the flow stack; a first mixing valve fluidically coupled between the first circulation pump and the second circulation pump to allow for mixing of the first electrolyte and the second electrolyte in the second electrolyte tank; a second mixing valve fluidically coupled between the second circulation pump and the first circulation pump to allow for mixing of the second electrolyte and the first electrolyte in the first electrolyte tank; and a start-up controller electrically coupled to provide voltages and currents to the stack, to provide control signals to the first circulation pump and the second circulation pump, and to provide control signals to the first mixing valve and the second mixing valve. 
     These and other embodiments of the invention are further described below with respect to the following figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a reduction-oxidation (redox) system according to some embodiments of the present invention. 
         FIG. 2  illustrates a state function for performing plating according to some embodiments of the present invention. 
         FIG. 3  illustrates a stack configured according to some embodiments of the present invention. 
         FIG. 4  illustrates the results of performing a plating function upon startup. 
     
    
    
     In the figures, elements having the same designation have the same or similar functions. The figures are illustrative only and relative sizes and distances depicted in the figures are for convenience of illustration only and have no further meaning. 
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, specific details are set forth in order to provide a thorough understanding of some embodiments of the invention. However, it will be apparent that the invention may be practiced without these specific details. 
     In the flow battery there are times when the Flow battery goes into a Shutdown State due to a leak in the System, an intentionally shutting the System down, hibernation, or other action. In each of these cases, the electrolyte in the flow cell battery will stay in the stack. The Voltage of the Stack will drop slowly based on the Shutcurrent and Diffusion current in the Stack. This drop in voltage causes the Stack to deplate once the stack voltage drops below the deplating voltage. This deplating of the Stack will cause the System performance to deteriorate once the System starts. Hence a Plating cycle is warranted. 
     As used in the present specification, the following words and phrases are generally intended to have the meanings as set forth below, except to the extent that the context in which they are used indicates otherwise. 
     As described herein, the term “cell” refers generally to any unit capable of performing electrochemical energy conversion. Exemplary cells include, but are not limited to, redox flow batteries, fuel cells, and secondary batteries. 
     As described herein, the term “membrane” refers to any material that forms a barrier between fluids, for example between electrochemical half-cells (e.g., an anode compartment and a cathode compartment). Exemplary membranes may be selectively permeable, and may include porous membranes and ion-selective membranes. Exemplary membranes may include one or more layers, wherein each layer exhibits a selective permeability for certain species (e.g., ions), and/or affects the passage of certain species. 
     As described herein, the term “fluid communication” refers to structures which are in contact with, but not necessarily affixed to, one another, whereby a fluid or gas can pass from one structure to the other. For example, two structures may be in fluid communication with one another by a channel, conduit, opening, and/or valve, even if the communication includes a valve in a closed state but provided that the valve may be opened, whereby a fluid or gas may be moved from one of the structures to the other. In addition, two structures may be considered to be in fluid communication with each other even in circumstances where one or more intermediate structures divert and/or interrupt the flow of the fluid or gas from the first structure to the second structure, so long as flow of the fluid or gas from the one or more intermediate structures to the second structure is ultimately possible. 
     As described herein, the “chromium side” of a cell refers generally to the negative side of a Cr/Fe based redox flow cell. In some embodiments, the oxidation of chromium occurs at the chromium side of the cell. 
     As described herein, the “iron side” of a cell refers generally to the positive side of a Cr/Fe based redox flow cell. In some embodiments, the reduction of iron occurs at the iron side of the cell. 
       FIG. 1  illustrates a schematic drawing of a simplified redox flow cell battery system  100 . As shown, redox flow cell system includes redox flow cell stack  101 . Stack  101  is represented by a single flow cell, which includes two half-cells  108  and  110  separated by a membrane  106 . Typically, stack  101  will include a plurality of single flow cells. An electrolyte  124  is flowed through half-cell  108  and an electrolyte  126  is flowed through half-cell  110 . Half-cells  108  and  110  include electrodes  102  and  104 , respectively, in contact with electrolytes  124  and  126 , respectively, such that redox reactions occur at the surface of the electrodes  102  or  104 . In some embodiments, multiple redox flow cells are electrically coupled (e.g., stacked) either in series to achieve higher voltage or in parallel in order to achieve higher current to form stack  101 . The stacked cells are collectively referred to as a battery stack and flow cell battery can refer to a single cell or battery stack. As shown in  FIG. 1 , electrodes  102  and  104  are coupled across load/source  120 , through which electrolytes  124  and  126  are either charged or discharged. 
     When filled with electrolyte, half-cell  110  of redox flow cell  100  contains anolyte  126  and the other half-cell  108  contains catholyte  124 , the anolyte and catholyte being collectively referred to as electrolytes. Reactant electrolytes may be stored in separate reservoirs and dispensed into half-cells  108  and  110  via conduits coupled to cell inlet/outlet (I/O) pipes  112 ,  114  and  116 ,  118  respectively. In some embodiments, an external pumping system is used to transport the electrolytes to and from the redox flow cell. Electrolyte  124  flows into half-cell  108  through inlet pipe  112  and out through outlet pipe  114 , while electrolyte  126  flows into half-cell  110  through inlet pipe  116  and out of half-cell  110  through outlet pipe  118 . 
     At least one electrode  102  and  104  in each half-cell  108  and  110  provides a surface on which the redox reaction takes place and from which charge is transferred. Suitable materials for preparing electrodes  102  and  104  generally include those known to persons of ordinary skill in the art. Redox flow cell  100  operates by changing the oxidation state of its constituents during charging or discharging. The two half-cells  108  and  110  are connected in series by the conductive electrolytes, one for anodic reaction and the other for cathodic reaction. In operation (e.g., during charge or discharge), electrolytes  126  and  124  are flowed through half-cells  108  and  110  through I/O pipes  112 ,  114  and  116 ,  118  respectively as the redox reaction takes place. 
     Positive ions or negative ions pass through permeable membrane  106 , which separates the two half-cells  108  and  110 , as the redox flow cell system  100  charges or discharges. Reactant electrolytes are flowed through half-cells  108  and  110 , as necessary, in a controlled manner to supply electrical power or be charged by load/source  120 . Suitable membrane materials for membrane  106  include, but are not limited to, materials that absorb moisture and expand when placed in an aqueous environment. In some embodiments, membrane  106  may comprise sheets of woven or non-woven plastic with active ion exchange materials such as resins or functionalities embedded either in a heterogeneous (such as co-extrusion) or homogeneous (such as radiation grafting) way. In some embodiments, membrane  106  may be a porous membrane having high voltaic efficiency Ev and high coulombic efficiency and may be designed to limit mass transfer through the membrane to a minimum while still facilitating ionic transfer. In some embodiments, membrane  106  may be made from a polyolefin material and may have a specified thickness and pore diameter. A manufacturer having the capability to manufacture these membranes, and other membranes consistent with embodiments disclosed, is Daramic Microporous Products, L.P., N. Community House Rd., Suite 35, Charlotte, N.C. 28277. In certain embodiments, membrane  106  may be a nonselective microporous plastic separator also manufactured by Daramic Microporous Products L.P. A flow cell formed from such a membrane is disclosed in U.S. Published Patent App. No. 2010/0003586, filed on Jul. 1, 2008, which is incorporated herein by reference in its entirety. 
     In some embodiments, multiple redox flow cells may be stacked to form a redox flow cell battery system. Construction of a flow cell stack battery system is described in U.S. patent application Ser. No. 12/577,134, entitled “Common Module Stack Component Design” filed on Oct. 9, 2009, which is incorporated herein by reference in its entirety. Another stack design is described in U.S. patent application Ser. No. 13/350,424, filed on Jan. 13, 2012, claiming priority to provisional application 61/432,541, filed on Jan. 13, 2011, which is herein incorporated by reference in its entirety. 
     In some embodiments of redox flow cell  100  in  FIG. 1 , electrolyte  124  includes an aqueous acid solution. In some embodiments, the acidic solution includes aqueous hydrochloric acid. Electrolyte  124  further includes at least one metal salt (e.g., a metal chloride salt). In some embodiments, electrolyte  126  comprises an aqueous acid solution. In some embodiments, the acidic solution includes aqueous hydrochloric acid. Electrolyte  126  further includes at least one metal salt (e.g., a metal chloride salt). 
     In one embodiment, a redox flow cell battery system is based on a Cr/Fe redox pair. The remainder of the description will be based on a Cr/Fe redox flow cell battery, however, it should be understood that the concepts described herein may also be applied to other metals. In an embodiment of a Cr/Fe redox flow cell battery, both electrolytes  124  and  126  include a solution of FeCl 2  and CrCl 3  in aqueous HCl. 
     During starting the system, a catalyst (for example Bismuth) is plated onto electrodes  102  and  104  shown in  FIG. 1 . The catalyst promotes the chemical reactions involved in the charging and discharging reactions in the cell. The chromium reactions are sluggish. The problem of the speed of the chromium reaction has been overcome by the addition of catalysts, for example Bismuth. The catalyst is introduced in the electrolyte solution upon initial startup. The first time, the catalyst is deposited on the surface of the electrode in a plating process. The existence of the catalyst plated on electrode  104  facilitates the chromium reactions. Typically, the plating process is performed dynamically with electrolytes flowing through the stack. 
     However, upon shut down the catalyst slowly dissolves into the electrolyte and the catalyst is no longer plated. Therefore, when a System  100  goes into a shutdown mode, a hibernation mode, or is intentionally turned OFF there is a possibility for system  100  to deplate the catalyst, resulting in a need to avoid the deplating process or to replate electrodes  104 . 
     Upon system shut down or hibernation, stack  101  will self-discharge due to electrolyte cross diffusion through porous membrane  106  and consequently catalyst gets oxidized and dissolved (i.e., deplated). The reactions that occur during shutdown upon cross diffusion include
         Self Discharge: Fe3 + +Cr2 + -&gt;Fe2 + +Cr3 −     Deplating: 3Fe3 + +Bi-&gt;3Fe2 + +Bi3 + 
 
The deplating process results in high equivalent series resistance (ESR). During Startup after Shutdown, Hibernation or intentional shutoff, when catalyst is deplated, improper plating can be promoted if charging currents are not low. Improper plating promotes undesired side reactions like H 2  generation. However, there is often a need to restart the system in a timely manor without going through the often slow dynamic plating process.
       

       FIG. 3  illustrates a system  300  according to some embodiments of the present invention. As shown in  FIG. 3 , pipes  114  and  112  fluidically couples half-cell  108  with left tank  314  and pipes  116  and  118  fluidically couple half-cell  110  to right tank  316 . Left tank  314  and right tank  316  include electrolytes. Note that although stack  101  is illustrated with a single cell having half-cells  108  and  110 , stack  110  generally includes a stack of individual cells. As shown in  FIG. 3 , electrolyte  124  from left tank  314  is pumped through half cell  108  by circulation pump  318 . Electrolyte  128  is pumped from right tank  316  by circulation pump  320 . System  300  includes check vales  302  and  304  to prevent electrolyte  124  and  126 , respectively, from draining from half cells  108  and  110 , respectively, back into tanks  314  and  316 , respectively. Cross-mixing valves  306  and  308  can be utilized to mix electrolytes in tank  314  and tank  316  through stack  101 . 
     Although check valves  302  and  304  are shown between mixing valves  306  and  308  and half-cells  108  and  110 , respectively, other placements are possible. For example, in some embodiments check valves  302  and  304  can be provided between mixing valves  306  and  308  and pumps  318  and  320 , respectively. In general, check valves  302  and  304  can be placed in any position to prevent electrolytes  124  and  126  from flowing back into tanks  314  and  316 , respectively. 
     As is further shown in  FIG. 3 , a start-up controller  322  is electrically coupled to control voltages and currents on electrodes  102  and  104  and operation of circulation pumps  318  and  320 . Controller  322  can also control cross-mixing valves  306  and  308 . 
     In according with some embodiments of the present invention, controller  322  can apply a small current to stack  101  during start-up to re-plate the catalyst before system  100  is fully turned on. The current can be applied prior to turning the circulation pumps on so that the catalyst that remains within stack  101  can be re-plated onto electrodes  104 . 
     Upon electrolyte cross diffusion through membrane  106 , the dissolved catalyst stays in the porous felt of membrane  106  as long as circulation pumps  318  and  320  are off, although some catalyst may diffuse across membrane  106 . Therefore, the catalyst can be re-plated prior to turning pumps  318  and  320  on, since all dissolved catalysts remain in stack  101 . In this case current density may be very low (0.5 mA/cm2 to 10 mA/cm2) in a “Static Plating” process. At the negative electrode (the chromium side), the static plating process involves
         Bi3 + +3e − -&gt;Bi   Cr3 + +e − -&gt;Cr2 + .       

     At the positive (iron side) electrode, the reaction is
         Fe2 + -&gt;Fe3 + +e − .       

       FIG. 2  illustrates an embodiment of a static plating state function  200  that controller  322  can execute for plating the catalyst on electrodes  102  and  104  prior to start-up of system  100 . Plating state function  200  is divided into 4 States: static plating  202 , trickle plating  204 , pulse plating  206 , and Init plating. Each state has a provision to be forced out of the state through a remote request. 
     Static plating  200  happens when ever System  300  starts up, transitions from hibernation or from a shutdown state. During static plating  202 , pumps  318  and  320  are OFF in order to keep the dissolved catalyst stack  101  and a low current, for example 1.3 mA/cm2, is applied. Exit from Static plating  204  happens when a particular voltage, for example 0.85V/cell, is reached. Static plating state  202  can be skipped when a pristine stack is installed, a forced de-plate has occurred, or a remote request to skip static plating has been received. During Static plating  202  the plating material that is dislodged from stack  100  and captured in the Electrolyte is plated back to the Stack electrode  104 . As shown in  FIG. 3 , check valves  302  and  304  can prevent electrolytes  124  and  126  from draining from stack  101 , ensuring that the catalyst remains within stack  101  to be replated. 
     During the trickle plating state  204  system  300  is charged to a predefined voltage level. This level is lower than the plating potential. Trickle plating is performed to speed up the plating process. The reason for e trickle plating state  204  is to change from Ferric to Ferrous state. During trickle plating, pumps  314  and  316  are turned on. 
     In the pulse plating state  206  the plating occurs on the electrode of stack  101  through the catalyst found in the tank electrolyte. Controller  322  applies a plating current periodically, for example current is applied in 15 sec ON pulse and 15 sec OFF. This is done until a particular OCV, for example 0.8 V per cell, is achieved. During this process, electrolyte circulation pumps  318  and  320  are turned on. 
     During the exit from pulse plating state  206  to Init-plating state  208  a low current charging maintained. The initial charging process is performed to avoid any deplating that might happen when the system is switched directly to a high current charging mode. 
     A circuit, the Sys Dongle described in U.S. application Ser. No. 12/844,059, filed on Aug. 12, 2010, which is herein incorporated by reference in its entirety, keeps track of the Stack  100  that is installed in a particular system. When a Stack is changed the Sys Dongle gets updated with the new Stack serial number. The firmware also notes the change in the stack serial number and sets a flag to force out of Static pre-charge state. Therefore, a normal plating process is performed on a pristine stack rather than the static plating state function  200  described above. In the case of a pristine stack, the catalyst is distributed throughout the electrolyte solution and is not confined to the stack itself, as it is during a shutdown or shut off. 
     In some cases, a forced deplate process may be performed. This may happen when the plating is not uniform across the electrodes and a reason to re-plate the entire stack is found. If the System H2 generation goes up, there might be case where the Stack plating might be compromised.  FIG. 3  illustrates a stack configured to allow for a forced deplating process, complete with pumps  306  and  308  for cross-mixing the electrolytes. As shown in  FIG. 3 , electrolyte  126  typically originates from tank  316  and electrolyte  124  originates from left tank  314 . Valve  306  can allow electrolyte from left tank  314  through pipe  310  into pipe  116 , mixing electrolyte  126  and with electrolyte  124  in right tank  316 . Additionally, valve  308  can allow electrolyte from right tank  316  through pipe  312  into pipe  112 , mixing electrolyte  124  with electrolyte  126  in left tank  314 . Therefore, electrolyte mixing can readily occur when pumps  318  and  320  are ON and valves  306  and  308  are opened. 
     System  100  can be put into a Force-Deplate State. This can be achieved by sending a remote request to the System  100 . The System can be in Charge or Discharge State at this time. A Force-Deplate State entry happens after a Hibernate State. During the forced Deplate State the electrolytes from tanks  314  and  316  are cross mix through the Stack for a period of time, for example 0.5 hrs. After Cross-mixing, System  100  enters Static plating state function  200  described above. 
       FIG. 4  illustrates the results of performing a startup plating process as opposed to not having a startup plating process upon startup. As shown in  FIG. 4 , the graph provides hydrogen production for each start-up of a flow cell system  100 . Area  402  indicates production with a pristine stack that has been dynamically plated. Area  404  provides the results of a startup without a plating process. Area  406  provides the results of hydrogen product when a static plating function  200  is executed. As is observed, hydrogen production upon startup is much reduced with application of the static plating process. 
     As is further shown in  FIG. 3 , in some embodiments check valves  302  and  304  are provided to prevent drainage of electrolytes  124  and  126  during a shutdown. In case of an intentional or unintentional shutting OFF of System  100 , the Electrolyte is prevented from draining from the Stack by these check valve  302  and  304 . In that case, electrolyte with the catalyst remains in the stack during the shut-down and can be replated without turning circulation pumps on during startup. 
     Further embodiments can be envisioned to one of ordinary skill in the art after reading this disclosure. In other embodiments, combinations or sub-combinations of the above disclosed invention can be advantageously made. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims.