Patent Publication Number: US-2020292902-A1

Title: Process and apparatus for switching redoxactive cells

Description:
The present invention relates to a process, an apparatus and a system for switching electrochromic cells, wherein the voltages are controlled in order not to overstress the cells. 
     Electrochromic cells comprise electrochromic material which changes its optical properties when ions and electrons are inserted into it under the influence of an electric field caused by a voltage applied. In particular, the electrochromic material can be switched between a coloured and a decoloured state. 
     For example, electrochromic cells are used as switchable glazing or windows to prevent a room or an area which is equipped with such glazing from heating-up by sunlight. In particular, an energy management of a whole building can be influenced by windows comprising electrochromic cells. 
     For using electrochromic cells in windows, the electrochromic material is imbedded as a lamination layer in laminated glass of the window. Therefore, the requirements regarding the lifetime of the materials are very stringent. Preferably, a lifetime is desired that is comparable to conventional windows. 
     However, lifetime of electrochromic cells depend on the magnitude of the applied voltages and on the amount of charge inserted into the electrochromic layers of the electrochromic cell. The range of voltages which may be applied between the electrode layers for switching, without causing device degradation is often referred to as the redox stability range. The redox stability range is defined as the range between a positive and a negative redox voltage limit. 
     Consequently, voltage and charge limits have to be considered. Thus, voltage and charge limits have to be determined by experimentation. The redox stability range may be determined, for example, by cyclic voltammetry experiments at various temperatures. 
     The applied voltage may then be limited accordingly, thereby ensuring that the maximum voltage between the electrode layers does not exceed the limits of the redox stability range for that particular system. However, the consequence of simple limiting the voltage will lead to very low currents in different states of the switching process which reduces the switching speed significantly. 
     Further, switching with high currents allows higher switching speed or lower switching times but results in higher inhomogeneity of colouration or decolouration of the electrochromic material. The reason for the inhomogeneity is that the distribution of electrical voltages between the electrode layers of a cell depends inherently on the resistance of the electrode layers and the cell current. 
     High currents cause a greater internal voltage drop across the electrode layers which results in a less homogeneous voltage distribution. 
     Consequently, the object of the invention is to find a method for switching an electrochromic cell, wherein it has to be ensured that the potential between the electrode layers is always between safe redox limits. Further, it is an object of the invention to limit the cell current for optimisation of switching speed and transmission homogeneity. 
     The present invention solves the problems identified in the prior art as described above. 
     Therefore, the invention comprises a process and an apparatus for switching an electrochromic cell. The electrochromic cell comprises at least a first electrode layer and a second electrode layer each capable of reversibly inserting ions. Further, the cell comprises an ion-conducting layer that separates the first electrode layer and the second electrode layer. 
     Moreover, a temperature sensor is comprised for measuring a temperature in or on or in the vicinity of the electrochromic cell. 
     Further, a first contact member is electronically connected with the first electrode layer and a second contact member is electronically connected with the second electrode layer. The first and the second electrode layer are counter electrodes to each other. 
     Furthermore, the at least said first electrode layer comprises an organic polymer matrix and, an electrochromic material, electronically conductive nanoobjects and an electrolyte dissolved in a solvent are dispersed within said organic polymer matrix. 
     For switching the electrochromic cell, the invention comprises the step of measuring the current i C  flowing through the cell if a voltage is applied to the electrode layers. Consequently, a voltage U C  is applied to the contact members and varied as a function of current. The voltage U C  is preferably set by a controller. Thereby, the voltage generated between the electrode layers is kept within predetermined temperature dependent safe redox limits U EC  and such that the cell current is kept within predetermined temperature-dependent limits. 
     In particular, the applied voltage U C  is only increased if the cell current i C  is less than a maximum cell current, determined according to: 
         i   max   =j   max ×Area+( T−T   0 )× F  
 
     In the above equation, j max  is a predetermined maximum current density, Area is the active cell area, T is the temperature of the electrochromic cell measured with the temperature sensor, and T 0  is a reference temperature. However, the factor F allows the modification of the current according to temperature. Thereby, the factor F allows the modification of switching speed with respect to temperature. 
     As it is not possible to measure the voltage between the electrode layers directly, because the two electrode contacts are on opposite sides of the cell, it is only possible to directly measure the applied contact voltage U C  and estimate the voltage between the electrode layers. 
     However, the voltage between the electrode layers varies significantly over the area of the cell depending on the distance from the two electrode contacts. In particular, the largest potential difference between electrode layers always occurs at the edges of the cell, adjacent to the electrode contacts. Therefore, it is not necessary to know the complete voltage distribution of the cell under a given set of conditions. 
     It was found that the relationship between the applied contact voltage and the maximum voltage generated between the electrode layers may be described by a simple equation, involving cell current and a constant resistance of the cell, wherein the resistance is only dependent on cell width and height and on material properties of the electrode layer. 
     The resistance may then be calculated from w and h which are cell width and height in centimetres. The height corresponds to the length of the contacted cell edges. Further, a factor k which is a constant representative of the material used for the electrode layer in electrochromic devices has to be considered. Consequently, the resistance is calculated as follows: 
         R   Eff =( w/h )× k  
 
     Further, the maximum voltage generated between the electrode layers U f,max  occurring at the cell edges adjacent to the electrode contacts can be calculated using the formula: 
     
       
      
       U 
       f,max 
       =U 
       C 
       −i 
       c 
       R 
       Eff  
      
     
     where U C  is the potential applied to the cell contacts, i C  is the cell current and R Eff  is the effective resistance of the cell. Further, a safe redox limit U EC  is predetermined for a given switching process from electrochemical studies. Consequently, the applied contact voltage can be limited appropriately using the following calculation: 
     
       
      
       U 
       C,max 
       =U 
       EC 
       +i 
       C 
       R 
       Eff  
      
     
     If the voltage applied at the cell contacts U C  is maintained below the maximum limit U C,max , then it is indirectly ensured that the maximum voltage between the electrode layers U f,max  does not exceed its corresponding safe redox limit U EC . 
     Consequently, it was found that if the applied voltage U C  is only increased if the cell current i C  is less than a maximum cell current, determined according to: 
         i   max   =j   max ×Area+( T−T   0 )× F  
 
     the maximum voltage between electrode layers U C,max  does not exceed the temperature-dependent safe electrochemical limit U EC , wherein a voltage U C  is applied which is always as high as possible to ensure the maximum possible switching speed. 
     It has to be noted that the invention is described with respect to switching an electrochromic cell comprising the cases of colouration and decolouration of the cell. Consequently, the applied voltage U C  and the current i C  flowing through the cell as well as the other values can be distinguished as positive during colouration and negative during decolouration or vice versa depending on the polarity of the devices for measurement. 
     Consequently, to avoid confusion in the description of this invention, the values, for example the voltage U C  and the current i C , are considered as positive values, only. These values are representative of one of the different switching case. 
     Accordingly, the safe redox range characterized by the safe redox limits, namely a positive and a negative safe redox limit, will be considered with respect to the maximum value of the safe redox limit, namely the positive safe redox limit. 
     According to a first embodiment of the invention, the current flowing through the cell is measured in a non-continuous way. However, switching a window with an electrochromic cell will take several minutes. Therefore, the current will not significantly change in short intervals, like millisecond. Therefore, measuring the current in a non-continuous fashion, namely in time intervals, can be easy handled by a relatively cheap controller or microcontroller with a slow clock frequency without running the risk to exceed the save redox limits. 
     According to a further embodiment, the applied voltage is increased in a linear fashion if the cell current is less than the maximum cell current and the voltage generated between the electrode layers is within predetermined temperature dependent safe redox limits. 
     Thus, no stepwise change in the voltage occurs. A stepwise voltage change would however result in current peaks as it was found that this special electrochromic cell will behave as a capacitor for fast switching. Consequently, a stepwise change of the voltage can result in high current peaks which can reduce the lifetime of the cell significantly. However, increasing the voltage in a linear fashion will reduce the risk of high current peaks. 
     According to a further embodiment, the current flowing through the cell is measured over the time for calculating the charge inserted into the electrode layers. Therefore, the amount of charge inserted into the electrochromic cell can be calculated easily to switch of the voltage in the case the cell is switched in predetermined fashion or reaches a predetermined stage. 
     For example, if the cell should not be coloured or decoloured completely, the value for the amount of charge for the desired stage can be deposit in a memory. If the value is reached, the voltage can be switched off. 
     Further, for switching the cell completely, namely in a fully coloured or decoloured stage, the voltage can be switched off at the right time to ensure not to overcharge the cell. Therefore, an overcharge of the cell leading to the risk of reduced cycle time can be prevented. 
     According to a further embodiment, the applied voltage is increased or decreased depending on a further input of the controller, wherein the controller preferably has a loop-controller or a PID controller. The output of the controller therefore gives the value for the voltage. On the other hand, the controller has an input to measure the voltage at the contact members and increases or decreases the output so that the substantially exact voltage is applied to the contacts. Thus, the risk of voltages which pass over the safe redox limits will be eliminated. 
     According to a further embodiment, the leakage current of the cell is determined. The leakage current is defined as the current due to electrons flowing between the electrodes arising from the non-perfect electrical insulating behavior of the electrolyte layer. The leakage current is preferably measured in the fully colored or fully decoloured state by applying a constant DC voltage smaller than the voltage used for coloration/decoloration. The resulting current is measured over time and the value to that the current is converging is an estimation for the leakage current. To determine the leakage current is necessary to calculate the charge that is inserted into the electrochromic layers correctly. Only measuring the current leads to an overestimation of the inserted charge as the measured current is the sum of current due to ion movement and the leakage current. 
     Further, the invention comprises an apparatus for switching an electrochromic cell. The apparatus comprises at least a first and a second electrode layer which are each capable of reversibly inserting ions. The layers are separated by an ion-conducting layer. Further, the apparatus comprises a temperature sensor for measuring a temperature in or on or in close vicinity of the electrochromic cell. 
     Moreover, the apparatus comprises a first contact member which is electronically connected with the first electrode layer and a second contact member which is electronically connected with the second electrode layer. The first and the second electrode layer are counter electrodes to each other. 
     Furthermore, at least said first electrode layer comprises an organic polymer matrix and dispersed within said organic polymer matrix an electrochromic material, electronically conductive nanoobjects and an electrolyte dissolved in a solvent. 
     Further, the apparatus comprises means for applying a voltage to the contact members and a controller connected to the means for applying a voltage. In addition, the apparatus comprises an ammeter, adapted to measure the cell current and to send the measured values of the cell current to the controller. The controller is adapted to calculate the magnitude of the electrical voltage to be applied to the cell contact members based on values of temperature, electrochromic voltage limits and cell current. 
     Further, the controller is adapted to increase the applied voltage as a function of current, such that the voltage generated between the electrode layers is kept within predetermined temperature-dependent safe redox limits and such that the cell current is kept between predetermined temperature-dependent limits. 
     The controller is adapted to increase the applied voltage only if the cell current is less than a maximum cell current determined according to equation as already discussed in relation to the inventive process, namely: 
         i   max   =j   max ×Area+( T−T   0 )× F.  
 
     According to an embodiment of the apparatus, the ammeter is adapted to measure the current flowing through the cell in a non-continuous way. Further, according to another embodiment the controller is adapted to increase the applied voltage in a linear fashion, if the cell current is less than the maximum cell current and the voltage generated between the electrode layers is within predetermined temperature dependent safe redox limits. 
     In another embodiment of the apparatus, the ammeter is adapted to measure the current flowing through the cell over the time for calculating the charge inserted into the electrode layers. According to a further embodiment, the apparatus comprises a loop-controller or a PID controller, adapted to increase or decrease the applied voltage depending on the measured voltage at the contact members. Further, according to another embodiment, the controller is adapted to determine the leakage current of the cell. 
     According to an embodiment of the apparatus, the electrochromic material is present in the form of nanoobjects, preferably nanoparticles. 
     Providing the electrochromic material in the form of nanoobjects, preferably nanoparticles, allows for uniform distribution and secure immobilization of the electrochromic material within the organic polymer matrix of the electrode layer. Furthermore, electrochromic material in the form of nanoobjects, preferably nanoparticles, readily interacts with an electronically conductive network formed of electronically conductive nanoobjects, preferably nanowires, thus allowing uniform electronic contact to the electrochromic material throughout the electrode layer, and due to the small dimensions of the nanoobjects of the electrochromic layer, electrons do not need to travel over large distances in regions exhibiting low electronic conductivity. 
     According to a preferred embodiment of the apparatus, the electronically conductive nanoobjects are nanowires, preferably silver nanowires. 
     Electronically conductive nanowires are capable of imparting appropriate electronic conductivity to the electrode layer by forming an interconnected network at low concentration. Since their diameter is in the nanoscale (below 50 nm, preferably between 20 nm and 35 nm), nanowires are not visible or substantially not visible and do not distract from any visual appearance of the device. 
     According to a further embodiment, said first electrode layer is disposed on a first optically transparent electronically conductive layer, and said first contact member contacts said first optically transparent electronically conductive layer. Moreover, said second electrode layer is disposed on a second optically transparent electronically conductive layer, and said second contact member contacts said second optically transparent electronically conductive layer. Furthermore, said first optically transparent electronically conductive layer is disposed on a first electrically insulating optically transparent substrate and said second optically transparent electronically conductive layer is disposed on a second electrically insulating optically transparent substrate. Further, said first electrically insulating optically transparent substrate and/or second electrically insulating optically transparent substrate is glass or organic polymer. 
     Disposing the electrode layers on optically transparent layers which are electronically conductive enables uniform current distribution over the whole area of the electrode, thus ensuring uniform and fast colour change or the electrochromic material in the electrode layer. 
     According to a further embodiment, said first electrode layer is disposed on a first electrically insulating optically transparent substrate, and said first contact member contacts the edge of said first electrode layer. Moreover, said first electrically insulating optically transparent substrate is glass or organic polymer. Further, said second electrode layer is disposed on an optically transparent electronically conductive layer, and said second contact member contacts said optically transparent electronically conductive layer. Finally, said optically transparent electronically conductive layer is disposed on a second electrically insulating optically transparent substrate and said second electrically insulating optically transparent substrate is glass or organic polymer. 
     In another embodiment, said first electrode layer is disposed on an electrically insulating optically transparent substrate, and said first contact member contacts the edge of said first electrode layer. Further, said first electrically insulating optically transparent substrate is glass or organic polymer. Said second electrode layer comprises an organic polymer matrix and dispersed within said organic polymer matrix an electrochromic material, electronically conductive nanoobjects and an electrolyte dissolved in a solvent. 
     Moreover, said second electrode layer is disposed on an electrically insulating optically transparent substrate, and said second contact member contacts the edge of said second electrode layer. Finally, said second electrically insulating optically transparent substrate is glass or organic polymer. 
     If the electronic in-plane conductivity of the first electrode layer or of both electrode layers is sufficiently high, there is no need to provide optically transparent electronically conductive layer(s) for contacting said electrode layer(s), and the electrode layer(s) can be disposed directly on the electrically insulating optically transparent substrate(s). Doing so reduces complexity of the device, facilitates manufacturing thereof and reduces costs. Appropriate high in-plane conductivity of the electrode layer may be achieved by means of incorporating electronically conductive nanowires into the electrode layer. 
     Further, the invention comprises a system for switching at least one electrochromic cell comprising a master unit and at least one apparatus comprising an electrochromic cell and a controller according to any of the prior embodiments of the apparatus. 
     The master unit is coupled to the at least one apparatus and is adapted to supply a trigger signal to the controller of the at least one apparatus, wherein the controller of the at least one apparatus is adapted to switch the electrochromic cell of the at least one apparatus in response the trigger signal. 
     Consequently, the system can be integrated in a building, wherein the master controller can generate the trigger depending on the sun light irradiating on the building. Then the controller of the apparatus switches the cell and taking into account the parameters to ensure a fast switching while the safe redox limits are considered. 
     According to a further embodiment of the system, the controller of the at least one apparatus is adapted to store at least one of the measured parameters of the at least one apparatus. Therefore, the master unit can load the stored parameters, i.e. the temperature measured with the temperature sensor, to use this parameters for deciding if a trigger is send or not. 
     According to a further embodiment of the system, the controller of the at least one apparatus is in bidirectional communication with said master unit. A communication in both directions between the controller and the master unit ensures that the master unit can monitor the parameters and the stage of the controller on the one hand and on the other hand to send—beside the mentioned trigger—further instructions to control the colouration or decolouration, i.e. the stage of colouration or decolouration. 
     According to a further embodiment of the system, the master unit is adapted to monitor the stored parameter of the at least one apparatus and to generate the trigger depending on the monitored parameter. Thus, there is no need for extra sensors connected to the master unit, because the master unit can use the integrated temperature sensors of the apparatuses to decide if a trigger needs to be generated. 
    
    
     
       Further features and advantages of the invention arise from the following description of preferred embodiments, wherein reference is made to the drawings: 
         FIG. 1  shows an embodiment of an electrochromic cell; 
         FIG. 2  an embodiment of the apparatus and 
         FIG. 3  an embodiment of the system. 
     
    
    
       FIG. 1  shows an electrochromic cell  100  which comprises a first contact member  101  and a second contact member  102 . Two conductive layers  103 ,  104  are connected with the first  101  and second contact member  102 , respectively. At least one of these conductive layers  103 ,  104  is transparent. Further, a first electrode layer  106  and a second electrode layer  108  are shown which are separated with an ion-conducting layer  110 . 
     The electrode layers  106 ,  108  comprise an electrochromic material and electronically conductive nanowires  112 . These nanowires form an interconnected mesh throughout each of the electrode layers  106 ,  108  and also touch the conductive layers  103 ,  104 . Thus, these wires impart electronic conductivity throughout the organic polymer matrix of the respective electrode layer and improve the performance efficiency of the electrode. At least the first electrode layer  106  comprises an electrolyte  114  dissolved in a solvent. 
     Since nanowires are thin, these are still optically transparent. Further, the electrochromic particles in electrode  106  may be large particles or nanoparticles and may be of any shape. These particles may be rod like, spherical, disc like cubes, etc. It is not necessary that conductive nanowires  112  are used for both electrode layers  106 ,  108 , as an example if the electrolyte is opaque for a display use, and all the visual change is coming from layer  106  as one looks through the first conductive layers  103 , then one can use a carbon based counterelectrode as layer  108  which may have sufficient electronic conductivity. 
     Preferably, a first support layer is attached to the surface of the first substrate facing away from the first electrode layer and a second support layer is attached to the surface of the second substrate facing away from the second electrode layer. In this regard, it is particularly preferred that the first and second substrate comprise materials from the group of organic polymers and are in the form of foils, films, webs, and the first and second support layer comprise glass. 
     Furthermore, it is preferred that a third support layer is attached to the surface of the first support layer facing away from the first substrate and/or a fourth support layer is attached to the surface of the second support layer facing away from the second substrate. In this regard, it is particularly preferred that a third support layer is attached to the surface of the first support layer facing away from the first substrate and a fourth support layer is attached to the surface of the second support layer facing away from the second substrate. In this regard, it is particularly preferred that the first, second, third and fourth support layer comprise glass. 
       FIG. 2  shows a simplified block diagram of the apparatus  200  with the electrochromic cell  100 . A controller  202  controls a voltage source  204  to apply the voltage U C  to the contact members  206 ,  208  of the electrochromic cell  100 . In parallel, the controller measures the current i C  with an ammeter  210  and the voltage applied to the contacts  206 ,  208  with inputs  212 ,  214  of the controller  202 . 
     The controller  202  has a memory and is pre-programmed with the values for the effective resistance of the cell R Eff  and the maximum redox safe voltage U EC . Thus, the controller  202  calculates the maximum voltage U C,max  as follows: 
     
       
      
       U 
       C,max 
       =U 
       EC 
       +i 
       C 
       R 
       Eff  
      
     
     This voltage U C,max  is the maximum value which the controller  202  controls the voltage source  204  to apply to the contacts  206 ,  208 . Moreover, the maximum cell current i max  is calculated as follows: 
         i   max   =j   max ×Area+( T−T   0 )× F  
 
     Further, the controller  202  is pre-programmed with the Area, in particular 100 cm×50 cm of the cell and a factor F, in example F is 1, for the desired switching speed. Moreover, j max  is calculated as the maximum charge density for colouration divided by the desired time for a complete switching from a decoloured to a coloured state of the cell  100 . 
     Further, when the process of switching is initiated, the temperature T of the cell is measured with a temperature sensor  216  and a starting voltage, in example of 5% of U C,max , is applied to the contacts  206 ,  208 . Moreover, beginning from this starting voltage, the applied voltage U C  is increased if the measured cell current i C  is less than the maximum cell current i max . 
     Furthermore, the controller monitors the current i C  over time and calculates the charge of the cell  100 . If a desired amount of charge is reached and therefore, the cell  100  has a desired stage of colouration, the voltage U C  is switched off. 
       FIG. 3  shows a system  300  with four apparatuses  200 . The system  300  comprises a master unit  302  which is connected to the controllers  202  (see  FIG. 2 ) of the apparatuses  200  by data links  304 ,  306 ,  308 ,  310 . The master unit  302  requests the temperature T of each of the temperature sensors  216  of the apparatuses  200 , preferably in intervals of seconds or minutes. 
     In the case any of the apparatuses  200  transfers a temperature value which is above a first predetermined values, in example 35° C., the master unit  302  sends a trigger to the controller  202  of the respective apparatus  200  which has transferred the temperature value above the predetermined value. Preferably, the master unit  302  sends one or more further triggers to the controllers  202  of one or more apparatuses  200  which are associated with the apparatus  200  which has transferred the temperature value above the predetermined value. Each trigger then causes the controller  202  of the respective apparatus  200  to switch the cell  100  of the respective apparatus  200  according to an embodiment of the inventive process. 
     
       
         
           
               
             
               
                   
               
               
                 List of reference numbers 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 100 
                 Electrochromic cell 
               
               
                   
                 101 
                 First contact member 
               
               
                   
                 102 
                 Second contact member 
               
               
                   
                 103 
                 First transparent 
               
               
                   
                   
                 layer 
               
               
                   
                 104 
                 Second transparent 
               
               
                   
                   
                 layer 
               
               
                   
                 106 
                 First electrode layer 
               
               
                   
                 108 
                 Second electrode layer 
               
               
                   
                 110 
                 Ion-conducting layer 
               
               
                   
                 112 
                 Electronically 
               
               
                   
                   
                 conductive nanowires 
               
               
                   
                 114 
                 Electrolyte 
               
               
                   
                 200 
                 Apparatus 
               
               
                   
                 202 
                 Controller 
               
               
                   
                 204 
                 Voltage source 
               
               
                   
                 206, 208 
                 Contact memberss of the 
               
               
                   
                   
                 electrochromic cell 
               
               
                   
                 210 
                 Ammeter 
               
               
                   
                 212, 214 
                 Inputs 
               
               
                   
                 216 
                 Temperature sensor 
               
               
                   
                 300 
                 System 
               
               
                   
                 302 
                 Master unit 
               
               
                   
                 304, 306, 
                 Data links 
               
               
                   
                 308, 310 
               
               
                   
                 i C   
                 Cell current 
               
               
                   
                 i max   
                 Maximum cell current 
               
               
                   
                 F 
                 Factor 
               
               
                   
                 R Eff   
                 Cell 
               
               
                   
                 T 
                 Temperature 
               
               
                   
                 U C   
                 Voltage 
               
               
                   
                 j max   
                 Predetermined 
               
               
                   
                   
                 maximum current density 
               
               
                   
                 Area 
                 Active cell area 
               
               
                   
                 T 0   
                 Reference temperature 
               
               
                   
                 U C, max   
                 Maximum voltage 
               
               
                   
                 UEC 
                 Maximum redox safe 
               
               
                   
                   
                 voltage