Patent Publication Number: US-2021167433-A1

Title: In-situ electrolyte preparation in flow battery

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 14/892,586 filed Nov. 20, 2015; which is a National Phase of International Patent Application No. PCT/US2013/042174 filed May 22, 2013. 
    
    
     BACKGROUND 
     Flow batteries, also known as redox flow batteries or redox flow cells, are designed to convert electrical energy into chemical energy that can be stored and later released when there is demand. As an example, a flow battery may be used with a renewable energy system, such as a wind-powered system, to store energy that exceeds consumer demand and later release that energy when there is greater demand. 
     A typical flow battery includes a redox flow cell that has a negative electrode and a positive electrode separated by an electrolyte layer, which may include a separator, such as an ion-exchange membrane. A negative fluid electrolyte (sometimes referred to as the anolyte) is delivered to the negative electrode and a positive fluid electrolyte (sometimes referred to as the catholyte) is delivered to the positive electrode to drive electrochemically reversible redox reactions. Upon charging, the electrical energy supplied causes a chemical reduction reaction in one electrolyte and an oxidation reaction in the other electrolyte. The separator prevents the electrolytes from freely and rapidly mixing but permits selected ions to pass through to complete the redox reactions. Upon discharge, the chemical energy contained in the liquid electrolytes is released in the reverse reactions and electrical energy can be drawn from the electrodes. Flow batteries are distinguished from other electrochemical devices by, inter alia, the use of externally-supplied, fluid electrolyte solutions that include reactants that participate in reversible electrochemical reactions. 
     SUMMARY 
     Disclosed is a method of in-situ electrolyte preparation in a flow battery includes providing a vanadium-based electrolyte solution having vanadium ions of predominantly vanadium V 4+  to a first electrode and a second electrode of at least one cell of a flow battery. The vanadium V 4+  at the first electrode is converted to vanadium V 3+  and the vanadium V 4+  at the second electrode is converted to vanadium V 5+  by providing electrical energy to the electrodes. A reducing agent is then provided to the vanadium V 5+  at the second electrode to reduce the V 5+  to vanadium V 4+ . The vanadium V 3+  at the first electrode is then converted to vanadium V 2+  and the vanadium V 4+  at the second electrode is then converted to vanadium V 5+  by providing electrical energy to the electrodes. 
     Also disclosed is a method of preparing a vanadium-based electrolyte solution having vanadium ions of predominantly V 4+ . The method includes providing a first solution and a second solution. At least one of the solution and the second solution includes vanadium V 5+ . At least one of the first solution and the second solution includes a reducing agent, and a ratio of moles of the reducing agent to moles of the vanadium V 5+  is 2:1 or greater. The first solution and the second solution are then combined. The reducing agent reduces the V 5+  to V 4+ . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  illustrates an example flow battery. 
         FIG. 2  illustrates an example method of in-situ electrolyte preparation in a flow battery. 
         FIG. 3  illustrates an example method of preparing a vanadium-based electrolyte solution having vanadium ions of predominantly V 4+ . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows portions of an example flow battery  20  that can be used for selectively storing and discharging electrical energy. As an example, the flow battery  20  can be used to convert electrical energy generated in a renewable energy system to chemical energy that is stored until a later time when there is greater demand, at which time the flow battery  20  then converts the chemical energy back into electrical energy. The flow battery  20  can supply the electric energy to an electric grid, for example. 
     The flow battery  20  includes a fluid electrolyte  22  that has an electrochemically active specie  24  which, under charge and discharge conditions, functions in a redox pair with regard to an additional fluid electrolyte  26  that has an electrochemically active specie  28 . In this example, the electrochemically active species  24 / 28  are based on vanadium and the fluid electrolytes  22 / 26  are thus vanadium-based electrolyte solutions. The fluid electrolytes  22 / 26  are contained in a supply/storage system  30  that includes first and second vessels  32 / 34  and pumps  35 . 
     The fluid electrolytes  22 / 26  are delivered from the first and second vessels  32 / 34 , using the pumps  35 , to at least one cell  36  of the flow battery  20  through respective feed lines  38 . The fluid electrolytes  22 / 26  are returned from the cell  36  to the vessels  32 / 34  via return lines  40 . The feed lines  38  and the return lines  40  connect the vessels  32 / 34  with first and second electrodes  42 / 44  of the cell. Multiple cells  36  can be provided as a stack. 
     The cell or cells  36  each include the first electrode  42 , the second electrode  44  spaced apart from the first electrode  42 , and an electrolyte separator layer  46  arranged between the first electrode  42  and the second electrode  44 . For example, the electrodes  42 / 44  are porous carbon structures, such as carbon paper or felt. In general, the cell or cells  36  can include bipolar plates, manifolds and the like for delivering the fluid electrolytes  22 / 26  through flow field channels to the electrodes  42 / 44 . The bipolar plates can be carbon plates, for example. It is to be understood, however, that other configurations can be used. For example, the cell or cells  36  can alternatively be configured for flow-through operation where the fluid electrolytes  22 / 26  are pumped directly into the electrodes  42 / 44  without the use of flow field channels. 
     The electrolyte separator layer  46  can be an ionic-exchange membrane, an inert micro-porous polymer membrane or an electrically insulating microporous matrix of a material, such as silicon carbide (SiC), that prevents the fluid electrolytes  22 / 26  from freely and rapidly mixing but permits selected ions to pass through to complete the redox reactions while electrically isolating the electrodes  42 / 44 . In this regard, the fluid electrolytes  22 / 26  are generally isolated from each other during normal operation of the flow battery, such as in charge, discharge and shutdown states. 
     The fluid electrolytes  22 / 26  are delivered to the cell  36  to either convert electrical energy into chemical energy or, in the reverse reaction, convert chemical energy into electrical energy that can be discharged. The electrical energy is transmitted to and from the cell  36  through an electric circuit  48  that is electrically coupled with the electrodes  42 / 44 . 
     In the charge, discharge and shutdown state after charge or discharge, the vanadium in the first fluid electrolyte  22  has vanadium ions of V 2+ /V 3+  and the vanadium in the second fluid electrolyte  26  has vanadium ions of V 4+ /V 5+  (which can also be denoted as V(ii)/V(iii) and V(iv)/V(v), although the valences of the vanadium species with oxidation states of 4 and 5 are not necessarily 4+ and 5+), the concentrations of which depend upon the charge state of the flow battery  20 . In the illustrated example, however, the flow battery  20  is shown in an in-situ preparation state, prior to any charging or discharging of the fluid electrolytes  22 / 26 , for preparing the fluid electrolytes  22 / 26  from starting materials. In the in-situ preparation state, the fluid electrolytes  22 / 26  each have vanadium ions of predominantly V 4+ . The term “predominantly” and variations thereof used herein with reference to ions of a particular oxidation state means that the particular oxidation state is the highest concentration oxidation state among all oxidation states of the electrochemically active species. In further examples, equivalent amounts (by volume) or substantially equivalent amounts of the fluid electrolytes  22 / 26  are provided to the electrodes  42 / 44  such that there are also equivalent or substantially equivalent concentrations of V 4+  at the electrodes  42 / 44 . For example, the fluid electrolytes  22 / 26  are provided from the same source batch or starting material such that, once the starting material is divided, the fluid electrolytes  22 / 26  have equivalent or substantially equivalent concentrations of V 4+ . The fluid electrolytes  22 / 26  thus also have equivalent or substantially equivalent amounts (by moles) of V 4+ . The term “substantially equivalent” used herein with reference to amounts or concentrations means that the amounts or concentrations are within +/−5%. 
     The preparation of vanadium-based fluid electrolytes for flow batteries can be relatively expensive. For example, vanadium-based fluid electrolyte can be produced, ex-situ with respect to a flow battery, from vanadyl sulfate (VOSO 4 ) crystals. Vanadyl sulfate is expensive and thus greatly increases the cost of preparing a vanadium-based fluid electrolyte. As will be described, the flow battery  20  can be used for the in-situ preparation of the fluid electrolytes  22 / 26  from relatively inexpensive vanadium oxide (V 2 O 5 ) powder. 
       FIG. 2  illustrates an example method  50  of in-situ electrolyte preparation in a flow battery, such as the flow battery  20 . As shown, the method  50  generally includes steps  52 ,  54 ,  56  and  58 . The example method  50  will be described with reference to the flow battery  20 . However, it is to be understood that the method  50  is not limited to the illustrated configuration of the flow battery  20  disclosed herein and may be utilized with other flow batteries having different configurations. In this example, step  52  includes providing a vanadium-based electrolyte solution having vanadium ions of predominantly vanadium V 4+  to the first electrode  42  and the second electrode  44 . With reference to  FIG. 1 , the vanadium-based electrolyte solution can be provided in the vessels  32 / 34  and then pumped into the cell  36  to the respective first and second electrodes  42 / 44 . In one example, the vanadium ions provided to each of the first electrode  42  and the second electrode  44  has a concentration of 90% or greater, or alternatively 95% or greater, of V 4+ . 
     After providing the vanadium-based electrolyte solution to the first and second electrodes  42 / 44 , the vanadium V 4+  at the first electrode  42  and the second electrode  44  are converted, respectively, to vanadium V 3+  and vanadium V 5+  by providing electrical energy through the electric circuit  48  to the first and second electrodes  42 / 44 . 
     The electrical energy is then stopped and, at step  56 , a reducing agent is provided into the second fluid electrolyte  26  to reduce the vanadium V 5+  to vanadium V 4+ . In other words, the charging cycle of the flow battery  20  at step  52  converts the V 4+  to, respectively, V 3+  and V 5+ , while the reducing agent then converts the V 5+  back to V 4+ . At this stage in the method  50 , the vanadium-based electrolyte solution at the first fluid electrolyte  22  is thus predominantly V 3+  and the vanadium-based second electrolyte solution at fluid electrolyte  26  is predominantly V 4+ . 
     At step  58 , which represents a second charging cycle, electrical energy is again provided through the electric circuit  48  to the first and second electrodes  42 / 44  to convert the V 3+  at the first electrode  42  to V 2+  and convert the V 4+  at the second electrode  44  to V 5+ . Thus, after step  58 , the vanadium-based electrolyte solution at the first electrode  42  and the second electrode  44  are in a fully charged state. Moreover, because equal parts of the vanadium-based electrolyte solution are provided to the first and second electrodes  42 / 44  at step  52 , the concentration of the V 2+  at the first electrode  42  is equal to the concentration of the V 5+  at the second electrode  44  after step  58 . For example, the concentrations are equal within +/−5%. 
     In one example, the reducing agent that is added at step  56  includes an acid. In a further example, the acid is selected from oxalic acid, formic acid or combinations thereof. Alcohol can alternatively be used. In one example based upon the use of oxalic acid, a byproduct of the reaction between the electrolyte and the oxalic acid is the generation of carbon dioxide, which is not harmful to the flow battery  20 . Thus, the use of oxalic acid additionally provides the benefit of avoiding the generation of toxic chemicals or chemicals that would otherwise debit the performance of the flow battery  20 . 
       FIG. 3  illustrates an example method  60  of preparing the vanadium-based electrolyte solution having vanadium ions of predominantly V 4+ . As an example, the method  60  can be used to prepare the vanadium-based electrolyte solution that is used in the method  50 . The method  60  includes steps  62  and  64 . At step  62 , a first solution and a second solution are provided. At least one of the first solution and the second solution includes vanadium V 5+ . In one example, the vanadium V 5+  is the predominant vanadium ion. At least one of the first solution and the second solution includes a reducing agent, and a ratio of moles of the reducing agent to moles of the vanadium V 5+  is 2:1 or greater. 
     At step  64 , the first solution and the second solution are combined. Once combined, the reducing agent reduces the vanadium V 5+  to vanadium V 4+  and thus results in the production of the vanadium-based electrolyte solution with vanadium ions of predominantly V 4+ . 
     In a further example, the first solution includes the reducing agent and the second solution includes an acid. In a further example, the reducing agent includes oxalic acid, formic acid or a combination thereof, and the acid of the second solution includes sulfuric acid. The oxalic acid can be provided as oxalic acid dihydrate, for example. Alternatively, or in addition to the oxalic acid and formic acid, the reducing agent can include an alcohol. In one further example, the first solution includes the reducing agent and the vanadium V 5+ . 
     The one of the first solution or the second solution that includes the vanadium V 5+  can be prepared using vanadium oxide (V 2 O 5 ) powder. For example, the vanadium oxide powder can be combined with the reducing agent and water (e.g., deionized water) to form the first solution or can be combined with the acid of the second solution. 
     Equations I and II below illustrate the underlying chemical reactions of the reduction of the vanadium oxide powder to produce vanadium V 4+ . In Equation I, the reaction product of VO 2   +  represents the oxidation state of vanadium V 5+ . In Equation II, the reaction product of VO 2+  represents the oxidation state of vanadium V 4+ . The reduction of V 5+  to V 4+  in Equation II is an endothermic reaction. In method  60 , the combining of the first solution and the second solution provides heat to drive this endothermic reaction. For example, the dilution of the acid of the second solution is exothermic and thus provides heat to drive the reduction of V 5+  to V 4+ . Moreover, the use of the noted ratio of 2:1 provides a sufficient amount of reducing agent to reduce substantially all of the V 5+  to V 4+ . Thus, the resulting vanadium-based electrolyte solution has predominantly V 4+ . In one further example, step  64  is carried out at room temperature without the application of external heat. The exothermic reaction of the dilution of the acid of the second solution can heat the mixture of the first solution and the second solution to a temperature above room temperature (approximately 23° C.), such as about 60° C. Furthermore, although the method  60  can also be carried out in-situ in the flow battery  20 , the method  60  can alternatively be carried out ex-situ, separate from the flow battery  20 , with equal amounts of the resulting electrolyte solution having vanadium ions of predominantly V 4+  subsequently provided into the first and second electrodes  32 / 44  of flow battery  20  for execution of method  50 . 
       V 2 O 5(s) +2H + → 6 2VO 2   + +H 2 O  EQUATION I:
 
       2VO 2   + +H 2 C 2 O 4 +2H + →2VO 2+ +2CO 2 +2H 2 O  EQUATION II:
 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.