Method and system for controlling water balance in metal air electrochemical cells

Abstract

A method of controlling water balance in an electrochemical cell during operation of the metal-air electrochemical cell includes controlling a ratio of a water vapor partial pressure in an intake air supply to a balanced water vapor pressure of an electrolyte of the metal-air electrochemical cell. Controlling the ratio is accomplished by controlling one or both of a water vapor partial pressure in the intake air supply and an equilibrium water vapor pressure of the electrolyte. The method may be performed by a system having a programmable controller programmed to perform the method and configured to receive signals from the sensors and control the relative humidity control device and the temperature control device based on the signals received from the sensors.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to USSN 63 / 715,244, filed November 1, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to electrochemical cells, and more particularly to metal-air electrochemical cells and systems and methods for controlling the water balance in metal-air electrochemical cells. Background Technology

[0004] In metal-air electrochemical cells (e.g., zinc-air cells), the air flowing through the cell can cause water to be carried away from or added to the electrolyte. This is particularly true for open and semi-open metal-air electrochemical cells. If the electrolyte absorbs or loses excessive water over time, the electrolyte concentration will deviate from optimal operating conditions, leading to performance degradation. If the cell loses excessive water over time, the electrolyte level may drop too low, exposing some electrodes. If some electrodes are not submerged in the electrolyte, ions cannot transfer within it, and therefore the unsubmerged sections of the electrodes cannot participate in the chemical reactions required for cell operation. If the cell absorbs excessive water over time, the electrolyte volume may increase beyond the cell tank's capacity, leading to overflow and electrolyte leakage. Therefore, maintaining a proper water balance in the electrolyte is desirable throughout the cell's operation.

[0005] There is a need for efficient systems and methods for controlling the water balance in metal-air electrochemical cells, especially zinc-air cells. Summary of the Invention

[0006] A method for controlling the water balance in a metal-air electrochemical cell during operation, the method comprising: controlling the ratio of the partial pressure of water vapor in the intake air supply source to the equilibrium water vapor pressure of the electrolyte in the metal-air electrochemical cell, wherein controlling the ratio is accomplished by controlling one or both of the partial pressure of water vapor in the intake air supply source and the equilibrium water vapor pressure of the electrolyte.

[0007] A system for controlling the water balance in an electrochemical cell during operation of a metal-air electrochemical cell, the system comprising: an air supply source; a metal-air electrochemical cell pneumatically connected to the air supply source to receive intake air from the air supply source; a humidity sensor configured to determine the relative humidity of the intake air; a first temperature sensor configured to determine the temperature of the intake air; a humidity control device and a temperature control device located between the air supply source and the metal-air electrochemical cell for controlling the relative humidity and temperature of the intake air; a second temperature sensor configured to determine the temperature of the electrolyte in the electrochemical cell; and a programmable controller programmed to perform the method and configured to receive signals from the sensors and control the relative humidity control device and the temperature control device based on the signals received from the sensors.

[0008] For metal-air electrochemical cells, proper vapor pressure balance between the electrolyte and the intake air supply is essential to ensure that the bulk electrolyte does not absorb or lose excessive water over time. The partial pressure of water vapor in the air depends on both temperature and relative humidity; therefore, relying solely on the determination of relative humidity and control of the intake air supply is insufficient. Furthermore, relying solely on the determination of the relative humidity in the air space near the electrolyte is also insufficient.

[0009] The equilibrium vapor pressure (also known as saturated vapor pressure, or simply vapor pressure) of an electrolyte describes its tendency to evaporate water, and this equilibrium vapor pressure varies with the electrolyte's temperature and concentration, both of which can change throughout the battery's charge and discharge cycles. A higher equilibrium vapor pressure indicates more evaporation, and a lower equilibrium vapor pressure indicates less evaporation. It has now been found that water balance is achieved when the partial pressure of water vapor in the intake air supplied to the electrochemical cell equals the equilibrium vapor pressure of the electrolyte.

[0010] To maintain proper water balance in the electrolyte of the electrochemical cell, the water vapor pressure of the air supply source (i.e., the partial pressure of water vapor in the air supply source) must be controlled, as this pressure varies with temperature and relative humidity. To determine how to control the water vapor pressure of the air supply source, it is compared to the equilibrium water vapor pressure of the electrolyte. The equilibrium water vapor pressure of the electrolyte is determined based on the electrolyte's temperature and concentration. If the water level in the electrolyte is to be maintained at its current level, the water vapor pressure of the air supply source is controlled to be the same as the equilibrium water vapor pressure of the electrolyte. If the water level in the electrolyte is too low, the water vapor pressure of the air supply source is controlled to be higher than the equilibrium water vapor pressure, allowing water to be added to the electrolyte from the air supply source. If the water level in the electrolyte is too high, the water vapor pressure of the air supply source is controlled to be lower than the equilibrium water vapor pressure, allowing water to be removed from the electrolyte into the air and discharged from the cell. Therefore, by determining the equilibrium water vapor pressure of the electrolyte in the battery cell, and then determining the relative humidity and temperature setpoints that the air supply source should be adjusted to based on the electrolyte vapor pressure, the water vapor pressure of the air supply source can be appropriately controlled, thereby appropriately controlling water loss and absorption in the battery cell and solving the problem of long-term water loss and absorption in the battery cell.

[0011] In some embodiments, the method uses the inlet air mass flow rate, electrolyte temperature, and electrolyte concentration to determine the desired water vapor pressure of the inlet air, and then determines the inlet air temperature setpoint and relative humidity setpoint. In some embodiments, the state of charge (SBC) of the electrochemical cell is used to estimate the electrolyte concentration. The SBC is used to determine the electrolyte concentration, which is combined with the electrolyte temperature to calculate the saturated vapor pressure of the electrolyte. Therefore, in some embodiments, the water balance in the metal-air electrochemical cell is controlled by adjusting the water vapor mass flow rate in the inlet air based on the cell's SBC (as a substitute for electrolyte concentration) and electrolyte temperature, according to a calibration curve derived experimentally or theoretically approximated between the water vapor mass flow rate in the inlet air stream and the rate at which water is added to / lost from the cell.

[0012] In some embodiments, a target cell water balance is identified; a target water vapor pressure difference between the water vapor pressure in the intake air supply source and the equilibrium water vapor pressure of the electrolyte is empirically determined based on a time-series dataset that correlates vapor pressure difference with cell water balance, the target water vapor pressure difference ensuring the achievement of the target cell water balance; and the temperature and relative humidity of the intake air supply source are adjusted based on temperature and relative humidity setpoints determined according to the target water vapor pressure difference. In some embodiments, the time-series dataset correlating vapor pressure difference with cell water balance is obtained from a continuous calibration curve using data from multiple tests. In some embodiments, the time-series dataset correlating vapor pressure difference with cell water balance is obtained from a series of discrete points of vapor pressure difference relative to cell water balance, these discrete points being recorded in a lookup table and interpolated to obtain cell water balance based on vapor pressure difference and vice versa.

[0013] This method can be implemented in systems including a programmable air system controller programmed to automatically adjust the water balance in the electrochemical cell based on this method. The ratio of the water vapor partial pressure in the air supply source to the equilibrium water vapor pressure of the electrolyte is determined using the temperature and relative humidity of the air supply source, as well as the temperature and concentration of the electrolyte. This informs the air system controller to change the air vapor pressure (via temperature and / or relative humidity control), thereby ensuring that excessive water is not removed from or added to the bulk electrolyte of the metal-air electrochemical cell over time. In this technology, the vapor pressure balance between the electrolyte and the air supply source is a characteristic of concern.

[0014] The air system includes various physical components, including a programmable controller in which the method is programmed as an algorithm. Other physical components of the system include, for example, at least one humidity control device, at least one temperature control device, and at least one flow rate device (e.g., a variable-speed air pump), which can be used to control the humidity, temperature, and mass flow rate of the air supply source. Additionally, the system may include various sensors, such as at least one humidity sensor, at least one air temperature sensor, and at least one pressure sensor configured to determine the relative humidity of the air. The programmable controller is configured to receive signals from at least one sensor and is configured to control at least one humidity control device, at least one temperature control device, and at least one flow rate device based on the signals received from at least one sensor according to the programmed method. In summary, the algorithm is based on a desired water balance outcome (i.e., clean water absorption, clean water loss, or clean water balance) control system to control the inflow / outflow of water into / outflow from the air supply source supplied to one or more electrochemical cells.

[0015] In some embodiments, the system includes a water recirculation subsystem that replenishes water to the electrolyte when the electrolyte level in the electrochemical cell is below an acceptable electrolyte level. In some embodiments, the water recirculation subsystem includes a level sensor for sensing the electrolyte level. In some embodiments, the water recirculation subsystem includes a water reservoir and a pump for pumping water from the water reservoir into the electrolyte in the cell. In some embodiments, the water recirculation subsystem includes a water reservoir and a valve operable to open and close to allow water from the water reservoir to replenish the electrolyte when the electrolyte level in the electrochemical cell is below an acceptable electrolyte level. In some embodiments, a programmable controller receives a signal from a level sensor and controls the pump or valve to replenish water to the electrolyte.

[0016] Methods and systems for controlling electrolyte balance can be applied to electrochemical systems with one or more cells. For multiple cells, if measuring the electrolyte temperature and concentration on each cell is not feasible, an estimated average across all cells can be used instead. In the absence of sensors on each cell, thermal modeling and state-of-charge algorithms can be used to estimate the electrolyte temperature and concentration separately. While it may not be possible to achieve perfect balance for all cells, the desired overall electrolyte balance outcome (absorption / leakage / equilibrium) can be achieved.

[0017] The metal-air electrochemical cell comprises a metallic material acting as the anode and an oxygen-containing material acting as the cathode. In some embodiments, the oxygen-containing material is provided as a gas (e.g., pure oxygen) or as a component of atmospheric air, which also contains nitrogen and other typical gases found in atmospheric air. In some embodiments, the metallic material comprises one or more of the following: lithium, sodium, potassium, zinc, magnesium, calcium, aluminum, copper, lead, and iron, typically in a low oxidation state (e.g., 0 valence oxidation state). In some embodiments, the electrochemical cell is a zinc-air electrochemical cell.

[0018] The electrolyte in a metal-air electrochemical cell typically comprises a liquid medium containing anions capable of reacting with oxidized anode materials to form anionic metal complexes. This liquid medium includes an aqueous phase. In some embodiments, the anion is a hydroxide ion, which may be present in a solution in the aqueous phase medium, for example, by dissolving an alkali metal hydroxide (e.g., NaOH, KOH) in water to form an aqueous hydroxide ion solution. When the anode material is oxidized, the hydroxide ions react with metal cations to form metal acid salt complexes. In a zinc-air electrochemical cell, the metal acid salt is a complex containing Na+ in the electrolyte solution. + or K + Zn(OH)4 counterion 2- In some embodiments, the electrolyte comprises potassium hydroxide.

[0019] In some embodiments, the electrochemical cell includes an open or semi-open system configuration. In some embodiments, the electrochemical cell is a battery.

[0020] Systems utilizing the methods described in this paper can operate unattended for extended periods at remote locations.

[0021] Other features will be described or become apparent in the following detailed description. It should be understood that each feature described herein can be used in any combination with any one or more of the other described features, and each feature does not necessarily depend on the presence of other features, except where it would be obvious to those skilled in the art. Attached Figure Description

[0022] To provide a clearer understanding, preferred embodiments will now be described in detail by way of example, in conjunction with the accompanying drawings, in which:

[0023] Figure 1 A schematic diagram of a zinc-air battery in a semi-open configuration is shown.

[0024] Figure 2 Depicting the control Figure 1 A block diagram of the water balance system in a zinc-air battery.

[0025] Figure 3 Depicting the control Figure 2 Used for control Figure 1 The flowchart of the algorithm for water balance in a zinc-air battery.

[0026] Figure 4 The saturated vapor pressure of the electrolyte (denoted as ) is depicted in the experiment used to determine the empirical water balance relationship in the zinc-air battery. The curve showing the relationship between kPa and time (h) and temperature (°C) and OH- concentration (M).

[0027] Figure 5 Depicting in relation to Figure 4 The graph shows the change of vapor pressure difference (kPa) over time (h) in the same experiment. The vapor pressure difference is the partial pressure of water vapor in the intake air. With respect to the saturated vapor pressure of the electrolyte The difference between them.

[0028] Figure 6 A graph depicting the cell water balance (grams of water per kilogram of humid air) during discharge in such a test process is presented.

[0029] Figure 7 This is a graph showing the change in cell water balance (number of grams of water per kilogram of humid air) with vapor pressure difference (kPa) during two zinc-air cell discharge experiments.

[0030] Figure 8A A schematic diagram of a zinc-air battery in a semi-open configuration, with a water recirculation subsystem as shown in the first embodiment, is depicted.

[0031] Figure 8B A schematic diagram of a zinc-air battery in a semi-open configuration, according to a second embodiment with a water recycling subsystem, is depicted. Detailed Implementation

[0032] refer to Figure 1The diagram schematically depicts a zinc-air battery 1 with a semi-open system configuration. The zinc-air battery 1 includes a ventilated or perforated housing 2 containing a potassium hydroxide aqueous electrolyte 3 in contact with a bed of metallic zinc 4 supported on top of a permeable membrane 5 forming the base plate of the housing 2. The housing 2 is located on top of an air chamber 6 through which a regulated air supply source 7 flows, along the sides and underside of the membrane 5. Oxygen (O2) diffuses from the air supply source 7 through the membrane 5 to contact the metallic zinc 4, subsequently oxidizing the zinc 4 supported on the membrane 5. Water vapor (H2O) permeates through the membrane 5 in two directions, but more H2O permeates from the direction of higher vapor pressure to lower vapor pressure. In this way, the air 7 flowing through the air chamber 6 exchanges mass with the battery 1 until the air exits the air chamber 6 as an exhaust gas 8 with a slightly different gas composition than the air supply source 7. In this configuration, long-term mass balance cannot be guaranteed.

[0033] Embodiments of the methods and systems described below involve appropriately conditioning air before it enters the battery in an effort to promote specific cell water balance behavior. The methods described below allow for determining how to set the conditions of the intake air supply source (e.g., temperature, humidity) to promote cell water balance over time in a semi-open configuration.

[0034] system

[0035] refer to Figure 2 Used for control Figure 1 The water balance system 10 in the zinc-air battery 1 includes a variable-speed air pump 12, a temperature controller 14 pneumatically connected to the air pump 12, a humidity controller 16 pneumatically connected to the temperature controller 14, and an intake manifold 18 pneumatically connected to the humidity controller 14. An air chamber 6 below the zinc-air battery 1 is pneumatically connected to the intake manifold 18, such that untreated air 11 (e.g., atmospheric air) is pumped by the air pump 12 to flow through the temperature controller 14 and humidity controller 16 for conditioning before flowing into the intake manifold 18, and then flows into the air chamber 6 as an intake air supply source 7. The system 10 further includes a programmable air system controller 20, which electronically communicates with the air pump 12, temperature controller 14, and humidity controller 16 to control their operation. The air system controller 20 includes an air pump sub-controller 25 for controlling the air pump 12. System 10 also includes multiple sensors 30 that communicate electronically with air system controller 20 to provide data input to the air system controller 20 about system 10, wherein control algorithm 50 uses the input data to determine how to control air pump 12, humidity controller 16 and temperature controller 14 to achieve a desired ratio of water vapor partial pressure in intake air supply source 7 to equilibrium water vapor pressure of electrolyte 3.

[0036] Sensor 30 includes a relative humidity sensor 32 for measuring the relative humidity in untreated air 11, a temperature sensor 33 for measuring the temperature in untreated air 11, an intake pressure sensor 34 for measuring the pressure 44 of the intake air supply source 7 at the intake manifold 18, an intake relative humidity sensor 35 for measuring the relative humidity in the intake air supply source 7 at the intake manifold 18, an intake temperature sensor 36 for measuring the temperature of the intake air supply source 7 at the intake manifold 18, and an electrolyte temperature sensor 37 for measuring the temperature 47 of the electrolyte 3 in the zinc-air battery 1.

[0037] The algorithm uses input data from relative humidity sensor 32 and temperature sensor 33 to calculate the partial pressure of water vapor 42 in the untreated air 11. The algorithm uses input data from relative humidity sensor 35 and temperature sensor 36 to calculate the partial pressure of water vapor 45 in the intake air supply source 7. Manual titration is used to determine the concentration 48 of the electrolyte 3 in the zinc-air battery 1. Intake pressure sensor 34 provides data to air system controller 20 to control pump 12 to provide the desired pressure 44 to the intake air supply source 7. The pressure 44 required to produce the desired air mass flow rate is determined based on experiments and modeling. The air mass flow rate of pump 12 at any given time is determined by any two of the pump pressure ratio (outlet pressure / inlet pressure), pump speed (rpm), and pump power consumption.

[0038] The programmed controller 20 receives measured, estimated, and calculated data inputs from system 10 and uses these data inputs to calculate the optimal temperature setpoint 52 and optimal humidity setpoint 53 for the intake air 7 (i.e., at what rate water needs to be added / removed from the intake air 7). The controller 20 then uses the air pump sub-controller 25 (based on the air mass flow rate setpoint 49), the temperature controller 14, and / or the humidity controller 16 to control the speed (e.g., rpm) 51 of the air pump 12 to achieve the vapor pressure difference between the intake air 7 and the electrolyte 3 that produces the desired battery water balance result (i.e., clean water absorption, clean water loss, or clean water balance). The air mass flow rate setpoint 49 is a predetermined parameter determined solely by the system discharge current and is considered an input when calculating the amount of water to be added / removed from system 10.

[0039] method

[0040] The control algorithm 50 controls the flow rate of water into / out of the air supply 7 supplied to the battery 1 based on the desired water balance result. The algorithm 50 is based on the vapor pressure difference between the air supply source 7 and the saturated vapor pressure of the electrolyte 3 according to equation (1), and the cell water balance according to equation (2).

[0041]

[0042] in, It is the partial pressure of water vapor in the intake air supply source 7, and It is the saturated vapor pressure of electrolyte 3;

[0043]

[0044] Cell water balance refers to the mass of water added to / removed from battery 1 when 1 kg of humid air is supplied. Since the mass flow rate of the air supply source 7 is determined by the state of the battery, cell water balance describes the rate at which water is added to / removed from battery 1.

[0045] refer to Figure 3 When supplying air to one or more metal-air batteries (such as zinc-air battery 1), algorithm 50 can be implemented as follows:

[0046] Step 1:

[0047] Determine the desired water quality to be added to / removed from the battery (if applicable).

[0048] Step 2:

[0049] In order to achieve a given air mass flow rate Next in time interval Adding or removing water Equation (3) can be used to solve for the target "cell water balance" value required to achieve this goal.

[0050]

[0051] Step 3:

[0052] Determine the saturated vapor pressure of the battery electrolyte (or the average saturated vapor pressure of many batteries receiving a common air supply).

[0053] One way to achieve this is to measure or estimate the temperature and concentration of the battery electrolyte and use these measurements to calculate the saturated vapor pressure based on empirically derived formulas. In this case, "concentration" refers to the amount of solute dissolved in the water-based electrolyte, which lowers the saturated vapor pressure of the solution relative to pure water. This concentration can be measured in various ways depending on the application (manual titration, derivation from state-of-charge estimates, electrochemical modeling, etc.).

[0054] The exact relationship between the predicted saturated vapor pressure of an electrolyte (e.g., temperature and concentration) will be unique for different electrolyte formulations. In one example, the saturated vapor pressure of the electrolyte is calculated based on temperature and concentration. The formula used for this calculation (e.g., equation (4)) described in Balej J. (1985) International Journal of Hydrogen Energy. 10(4), 233-243 is derived from empirical data, the contents of which are incorporated herein by reference. In the case of zinc-air battery 1 with electrolyte 3 including KOH, it is assumed that the KOH solution is adequately used as a substitute for electrolyte 3 containing other components (zinc, etc.). More accurate formulas for the saturated vapor pressure of a particular electrolyte formulation can be obtained through further experiments and / or electrochemical modeling.

[0055]

[0056] in, It is the saturated vapor pressure of water vapor above the solution (electrolyte), where M is the electrolyte (e.g., free OH-). - The equation is given by the concentration of ions (T) and the electrolyte temperature (T). The exact form of the equation will depend on the specific application.

[0057] Step 4:

[0058] Determine the partial pressure of water vapor in the air entering the system. One way to achieve this is to use the temperature and relative humidity of the untreated air entering the air system. To achieve this, obtain the saturated vapor pressure of water at the temperature of the untreated air. (For this purpose, there are many empirically derived formulas in the field, such as the formula in Huang J. (2018) Journal of Applied Meteorology and Climatology. 57:1265-1272, the entire contents of which are incorporated herein by reference) and multiply the saturated vapor pressure by the relative humidity of the air (%) ).

[0059]

[0060] Step 5:

[0061] Calculate the current “vapor pressure difference” using equation (1). This value represents the difference between the partial pressure of water vapor in the air entering the system and the saturated vapor pressure of the electrolyte under its current conditions.

[0062] Step 6:

[0063] Using the target "cell water balance" value determined in step 2, a lookup table (or similar) is used to determine the target "vapor pressure difference," which will inform the air temperature and humidity setpoints. The lookup table (or similar) is empirically determined through laboratory experiments using the scheme described in the following sections. It is important to note that this is not the only way to achieve this relationship. Theoretical calculations and / or computer modeling can also be used. When the target vapor pressure difference and the current vapor pressure difference are not the same, water must be added to or removed from the intake air to achieve the target vapor pressure difference.

[0064] Step 7:

[0065] Using the saturated vapor pressure of the electrolyte And the target "vapor pressure difference" obtained in step 6 is solved in equation (1). .

[0066] Step 8:

[0067] Use temperature controller 14 and / or humidity controller 16 to change the vapor pressure of the intake air 7 to be equal to that obtained in step 7. Typically, only the humidity controller 16 is needed to control the mass flow rate of water entering / leaving the air based on a target vapor pressure difference. However, the amount of water that air can hold is a function of air temperature. Thus, it may be necessary to change the air temperature before water can be added / removed. The humidity controller 16 performs both humidification and dehumidification; however, dehumidification is usually rarely required.

[0068] Step 9:

[0069] Once the target vapor pressure is achieved Therefore, the target vapor pressure difference and cell water balance are achieved, and these conditions are maintained for the time t used in step 2.

[0070] Experimental Procedure for Determining Empirical Relationships of Water Balance in Metal-Air Batteries

[0071] The following section details an example of how to empirically determine the relationship between the “vapor pressure difference” and the “cell water balance” used in step 6 of the algorithm. This example is specifically for a zinc-air battery 1 with electrolyte 3 comprising an aqueous solution of potassium hydroxide. However, this procedure can be generalized to any metal-air electrochemical cell with any suitable aqueous electrolyte.

[0072] The following experimental procedure for generating empirical data correlates the vapor pressure difference [kPa] (the difference between the partial pressure of water vapor in the intake air supplied to the battery and the saturated vapor pressure of the electrolyte in the metal-air battery) with the cell water balance [g / kg] (the rate at which the metal-air battery absorbs or loses water). The units here are grams of water per kilogram of supplied humid air.

[0073] The generated data can be in the form of equations or lookup tables, allowing the operator to determine the vapor pressure differential based on the "cell water balance" value, and vice versa. The first step requires a model that can predict the saturated vapor pressure of the electrolyte used across the entire range of operating conditions. As described in step 6 above, this is not the only way to achieve this relationship. Theoretical calculations and / or computer modeling can also be used.

[0074] To generate the desired relationship, an expression is needed that can predict the saturated vapor pressure of the electrolyte under its operating conditions. This data can be obtained in the literature if the electrolyte formulation has been established / sufficiently studied. Instead of data specific to a given electrolyte, publicly available data on the saturated vapor pressure of the main salt solution (in this case, a KOH solution) at different temperatures and concentrations can be used, assuming the main salt solution is sufficiently similar to the electrolyte for usefulness.

[0075] In this example, the following equation 6 is derived from a table of measured values:

[0076]

[0077] Where M is the free OH- ions in the electrolyte. - The molar concentration, and T is the electrolyte temperature.

[0078] In summary, any expression that accurately predicts the saturated vapor pressure of the electrolyte will work for any metal-air battery. Different batteries will have different electrolytes, and therefore different expressions, and may even have no different parameters. The saturated vapor pressure of novel electrolyte formulations can also be obtained using electrochemical modeling.

[0079] The experimental setup used in this paper includes:

[0080] 1. Zinc-air battery with no internal or external electrolyte leakage.

[0081] 2. A constant air supply.

[0082] 3. Sensors for measuring air temperature, pressure, and relative humidity.

[0083] 4. Electrolyte thermocouple.

[0084] 5. Electrolyte titration equipment.

[0085] Typically, air needs to be supplied to metal-air batteries only during discharge, therefore the tests described in this document are performed in this cell state. However, the same experiments can be performed during charging or idle periods. The same physical principles still apply as long as air flows to the battery. Air passing through the battery exchanges water with the electrolyte, so without an air supply, there is no water exchange. Testing during charging or discharging also has the benefit of subjecting the battery to the full range of temperatures and concentrations it may experience during operation, which is not the case when the cell is idle.

[0086] Before testing begins, sensors are installed to measure the temperature, pressure, and relative humidity of the air in the battery's intake and exhaust airflow. Furthermore, thermocouples are installed to measure the temperature of the electrolyte near the battery's permeable membrane. The electrolyte is titrated to determine the free OH- ions in the electrolyte. - The molar concentration (i.e., electrolyte concentration) is determined. A constant airflow is then initiated to the battery. The airflow rate is determined based on the stoichiometric requirements of the discharge reaction at a given current. The battery is then discharged (but can also be charged or left idle). In experiments, the battery typically discharges for 12–16 hours. At the end of the discharge, the electrolyte is titrated to determine the free OH- ions in the electrolyte. - Molar concentration.

[0087] Based on the electrolyte concentration measurements obtained at the beginning and end of the test, and knowledge of the occurring reactions, a continuous function of molar concentration versus time was interpolated to observe the changes in electrolyte molar concentration during the test. Here, the molar concentration of zincate was measured by titration, and then the molar concentration of free OH- ions was calculated based on this titration. Simultaneously, the electrolyte temperature was recorded throughout the test, and the saturated vapor pressure of the electrolyte was calculated using Equation 6. Figure 4 The figure illustrates how the electrolyte vapor pressure changes during discharge based on the electrolyte temperature and concentration in one such test.

[0088] Then, the partial pressure of water vapor in the intake air is calculated based on the air temperature and relative humidity throughout the test. Having obtained the mapping relationship between the saturated vapor pressure of the electrolyte and the partial pressure of water vapor in the intake air throughout the test, the change in vapor pressure difference over time can be obtained by subtracting the two. Figure 5 The diagram illustrates the relationship with Figure 4 The change in vapor pressure difference during the same test. The partial pressure of water vapor in the intake air minus the saturated vapor pressure, so a negative value means that the saturated vapor pressure of the electrolyte is greater than the partial pressure of water vapor in the intake air supply source, and vice versa.

[0089] The cell water balance is then calculated by subtracting the specific humidity of the air exiting the battery from the specific humidity of the air entering the battery. Specific humidity (grams of water per kilogram of moist air) is calculated based on air temperature, pressure, and relative humidity. A positive cell water balance indicates that the air leaving the cell is drier than the air entering the cell, therefore the cell has absorbed water. A negative value indicates that the air leaving the cell contains more water than the air entering the cell, indicating that the cell has lost water to the airflow. Figure 6 The cell water balance during one such test is shown.

[0090] Therefore, a time-series dataset was created, where each vapor pressure difference data point corresponds to a cell water balance data point. Plotting these data points on a scatter plot illustrates that the greater the difference between the water vapor partial pressure in the intake air supply source and the saturated vapor pressure of the electrolyte, the more water is absorbed or lost (depending on which one is larger). Figure 7 This relationship is illustrated. Figure 7 The graph plots two separate discharges on the same battery. The curve is expected to cross [0,0], because this means there is no difference in vapor pressure, and therefore no water absorption or loss. That is, there is no partial pressure difference that "drives" the transfer of purified water.

[0091] Having time-series datasets containing corresponding vapor pressure differentials and cell water balance values ​​facilitates the creation of models that predict cell water balance based on vapor pressure differentials using several methods. For example, regression can be used to generate continuous calibration curves using data from several tests. Alternatively, a series of discrete points can be recorded in a lookup table and interpolated to obtain cell water balance based on vapor pressure differentials, and vice versa. Thus, an empirical relationship between a given vapor pressure differential and cell water balance can be derived, and this empirical relationship is required in step 6 of the algorithm.

[0092] Water Recirculation Subsystem

[0093] One way to achieve long-term water balance in zinc-air electrochemical cells is to couple the algorithm with an electrolyte level sensor and a water reservoir. In this approach, the water balance algorithm is intentionally designed to bias one or more cells towards water loss over time, making the system more predictable. Specifically, the partial pressure of water vapor in the air supplied to the cells is controlled to be lower than the equilibrium partial pressure of water vapor above the electrolyte at the current temperature and concentration. As the electrolyte level drops due to water loss through the air cathode, the electrolyte level sensor detects when the electrolyte reaches a low threshold. In response to the sensor indicating that the low threshold has been reached, the controller instructs valves and / or pumps to direct liquid water stored in the water reservoir back into the cells until the level sensor determines that the electrolyte has recovered to an acceptable level. Liquid water from the water reservoir can be directed to the cells using hydrostatic pressure or a pump.

[0094] Figure 8A A schematic diagram of a zinc-air battery 61 in a semi-open configuration, representing a first embodiment with a water recirculation subsystem, is depicted. The zinc-air battery 61 includes a ventilated or perforated housing 62 containing a potassium hydroxide aqueous electrolyte 63 in contact with a bed of metallic zinc supported on top of a permeable membrane forming the base plate of the housing 62. The housing 62 is located on top of an air chamber 66 through which an regulated air supply source 67 flows, along the sides and underside of the membrane. Oxygen diffuses from the air supply source 67 through the membrane to contact the metallic zinc, which then oxidizes the zinc supported on the membrane. Water vapor permeates through the membrane in both directions, but more water permeates out of the housing 62 than enters it. In this way, the air 67 flowing through the air chamber 66 exchanges mass with the battery 1 until the air exits the air chamber 67 as an exhaust gas 68 with a water volume greater than that of the air supply source 67. This water infiltration caused the electrolyte level 63 to drop below the acceptable electrolyte level 83.

[0095] For this reason, the zinc-air battery 61 is associated with a water recirculation subsystem. In a first embodiment, the water recirculation subsystem includes a water reservoir 81 located below an acceptable electrolyte level 83 and a water pump 82 in fluid communication with the water reservoir 83 to pump water from the water reservoir 81 to a fill port 85 at the top of the housing 62 above the acceptable electrolyte level 83 when the electrolyte level is below the acceptable electrolyte level 83, thereby replenishing the electrolyte 63 with water. To determine when the electrolyte level is below the acceptable electrolyte level 83, the water recirculation subsystem includes a level sensor 84 located on the housing 62 at the acceptable electrolyte level 83. The level sensor 84 is in electronic communication with a programmable controller 80, and the signal from the level sensor 84 is processed by the controller 80. The controller 80 also communicates electronically with the pump 82, so when the controller 80 determines that the electrolyte level is too low, the controller 80 sends a signal to turn on the pump 82, which pumps water from the reservoir 81 through the fill port 85 into the housing 62 until the electrolyte level is equal to or higher than the acceptable electrolyte level 83. In practice, there is an acceptable margin around the acceptable electrolyte level 83, which has a lower level; below this lower level, the electrolyte level is too low, and above this lower level, the electrolyte level is too high. The water recirculation subsystem is operated to maintain the electrolyte level within the margin.

[0096] Figure 8B A schematic diagram of a zinc-air battery 61 with a second embodiment having a water recirculation subsystem is depicted. The second embodiment of the water recirculation subsystem is gravity-fed and does not involve a liquid pump. Figure 8BIn this system, the hydrostatic level of the water reservoir 81 is higher than the outer casing 62 by a distance Δh, and water flows from the reservoir 81 to the filling port 85 under the influence of gravity. The water recirculation subsystem includes a valve 86, instead of a pump, which can be electrically switched on and off via a switch 87. Switch 87 is controlled by a programmable controller 80 using a signal from a level sensor 84, in a manner similar to... Figure 8A The pump 82 is controlled in the same way in the illustrated embodiment.

[0097] By carefully reading the specification, the novel features will become clear to those skilled in the art. However, it should be understood that the scope of the claims should not be limited to the embodiments, but rather should be interpreted in the broadest possible sense as described in the text of the claims and the entire specification.

Claims

1. A method for controlling the water balance in a metal-air electrochemical cell during operation, the method comprising: During the operation of the metal-air electrochemical cell, the ratio of the partial pressure of water vapor in the air supply source to the equilibrium water vapor pressure of the electrolyte in the metal-air electrochemical cell is controlled, wherein the ratio is controlled by controlling one or both of the partial pressure of water vapor in the air supply source and the equilibrium water vapor pressure of the electrolyte.

2. The method as described in claim 1, wherein, The partial pressure of water vapor in the intake air supply source is controlled, and the partial pressure of water vapor in the intake air supply source is controlled by changing one or both of the temperature and relative humidity of the air in the intake air supply source.

3. The method as claimed in claim 1 or claim 2, wherein, The equilibrium water vapor pressure of the electrolyte is controlled, and the equilibrium water vapor pressure of the electrolyte is controlled by changing one or both of the temperature and concentration of the electrolyte.

4. The method of claim 1, wherein, The temperature and concentration of the electrolyte are determined, the equilibrium water vapor pressure of the electrolyte is determined based on the temperature and concentration of the electrolyte, and one or both of the temperature and relative humidity of the air in the air supply source are changed to add water to or remove water from the electrolyte or maintain the water in the electrolyte at the same level.

5. The method of claim 1, wherein, The water mass flow rate in the air intake source is controlled by changing the water vapor partial pressure in the air intake source based on the difference between the water vapor partial pressure in the air intake source and the saturated vapor pressure of the electrolyte. This difference will generate a target amount of water added to / from the battery cell and lost.

6. The method of claim 1, wherein, The ratio is controlled in the following way: Determine the temperature and concentration of the electrolyte; The equilibrium water vapor pressure of the electrolyte is determined based on the temperature and concentration of the electrolyte. Determine the temperature and relative humidity of the air intake supply source; The partial pressure of water vapor in the air intake supply source is determined based on the temperature and relative humidity of the air. Perform one of the following operations: For water purification absorption, the partial pressure of water vapor in the air supply source is increased to a level higher than the equilibrium vapor pressure of the electrolyte by increasing the temperature and / or relative humidity of the air supply source. To prevent water loss, the water vapor partial pressure in the air supply source is reduced to below the equilibrium vapor pressure of the electrolyte by lowering the temperature and / or relative humidity of the air supply source. For water balance, the water vapor partial pressure in the air supply source is changed to be equal to the equilibrium vapor pressure of the electrolyte by changing the temperature and / or relative humidity of the air supply source. as well as Continue supplying air until the target water volume is added, removed, or balanced.

7. The method of claim 5 or claim 6, wherein, The concentration of the electrolyte is determined by manual titration or state of charge estimation.

8. The method of any one of claims 5 to 7, further comprising determining the mass flow rate of the air supply source, and determining, based on the mass flow rate, the amount of water to be added to or removed from the electrolyte within a given time period.

9. The method according to any one of claims 5 to 8, wherein, The electrochemical cell is one of a plurality of electrochemical cells, and the temperature and concentration of the electrolyte include determining the average temperature and average concentration of the electrolyte on all the cells that receive air from the air supply source in the plurality of electrochemical cells, and determining the equilibrium water vapor pressure of the electrolyte based on the average temperature and average concentration.

10. The method of claim 1, wherein: Identify the target battery cell's water balance; Based on a time-series dataset that correlates the vapor pressure difference with the cell water balance, a target vapor pressure difference between the partial pressure of water vapor in the air supply source and the equilibrium vapor pressure of the electrolyte is empirically determined, and the target vapor pressure difference will enable the achievement of the target cell water balance. as well as The temperature and relative humidity of the air intake supply source are adjusted based on the temperature setpoint and relative humidity setpoint determined according to the target water vapor pressure difference.

11. The method of claim 10, wherein, The time-series dataset that correlates the vapor pressure difference with the cell water balance was obtained from the following: Continuous calibration curves using data from multiple tests; and / or The vapor pressure difference is relative to a series of discrete points of the cell's water balance. These discrete points are recorded in a lookup table and interpolated to obtain the cell's water balance based on the vapor pressure difference, and vice versa.

12. The method according to any one of claims 1 to 11, wherein, The electrolyte includes potassium hydroxide.

13. The method according to any one of claims 1 to 12, wherein, The electrochemical cell is a zinc-air electrochemical cell.

14. The method according to any one of claims 1 to 13, wherein, The electrochemical cell is a battery.

15. A system for controlling the water balance in a metal-air electrochemical cell during operation, the system comprising: Air supply source; A metal-air electrochemical cell, wherein the metal-air electrochemical cell is pneumatically connected to the air supply source to receive intake air from the air supply source; A humidity sensor configured to determine the relative humidity of the intake air; A first temperature sensor, configured to determine the temperature of the intake air; A humidity control device and a temperature control device are located between the air supply source and the metal-air electrochemical cell, and are used to control the relative humidity and temperature of the incoming air. A second temperature sensor is configured to determine the temperature of the electrolyte in the electrochemical cell. as well as A programmable controller, the programmable controller being programmed to perform the method as described in any one of claims 1 to 14, and being configured to receive signals from the sensor and control the relative humidity control device and the temperature control device based on the signals received from the sensor.

16. The system of claim 15, further comprising at least one pressure sensor for determining the pressure of the intake air.

17. The system of claim 15 or claim 16, further comprising a water recirculation subsystem, wherein the water recirculation subsystem replenishes water to the electrolyte when the electrolyte level in the electrochemical cell is lower than an acceptable electrolyte level.

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