RU2520183C2 - Method to operate electrochemical capacitors - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention refers to the field of electric engineering and to a method of electrochemical capacitors operation. The suggested method includes connection of a capacitor to a current source, its charge up to the preset voltage, cassation of charge, and discharge, at that the temperature of the capacitor is measured preliminarily and against this temperature the maximum operating voltage of the charge excluding gas release is defined, and a calculation is made of the maximum charging voltage U_max, which is limited as per the formula U_max=k·t+b, where k and b are coefficients determined experimentally and depending on peculiarities of the capacitor design, t is the temperature, at that current of floating charge is calculated to measure coefficients k and b.

EFFECT: invention allows increase in the capacitor power and service life at operational safety ensured by optimisation of its charging conditions, which is recognised as a technical result of the invention.

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Description

The invention relates to the operation of electrochemical capacitors (hereinafter - capacitors or EC). It relates to capacitors with an aqueous alkaline electrolyte, a porous separator, and electrodes, one of which contains activated carbon material as the active material, and the other contains nickel hydroxide as the active material [1].

The invention equally relates to circuits consisting of capacitors connected in series, in parallel or in a mixed manner.

The invention is aimed at achieving high discharge power, long life and high energy output in the temperature range from plus 60 ° C and up to the freezing point of the electrolyte (about -60 ° C).

The invention describes both the method of operating capacitors and batteries from them, and the method of determining some parameters that are important during operation.

The invention can be used, in particular, when operating electrochemical capacitors when starting internal combustion engines at low temperatures, including when starting in winter conditions of car engines, diesel locomotives, diesel power plants, etc.

The invention can also be used to ensure safety and long life during operation of capacitors and modules of them in conditions of elevated temperatures up to plus 60 ° C, avoiding loss of electrolyte and thermal acceleration.

The use of the invention allows in all cases to achieve a combination of high energy and power delivered with safety and a long service life of capacitors and modules.

The energy intensity of capacitors is determined, in addition to the design features and the selected electrochemical system, the electrolyte decomposition voltage and the double layer capacity, which are inherent in the used electrolyte. In the case of an aqueous electrolyte, the theoretical decomposition voltage is 1.23 V, but the practical value of the decomposition voltage is always greater than the specified value and depends on a number of factors, including the nature of the electrode surfaces, current density, temperature, etc. The difference between theoretical and practical electrolyte decomposition voltages is called overvoltage.

It is known that with increasing temperature the overvoltage of electrode reactions decreases. The most important is the overvoltage of oxygen evolution at the positive electrode and the overvoltage of hydrogen evolution at the negative electrode. Reducing overvoltage with increasing temperature can lead to an increase in the rate of self-discharge, and when trying to keep a high voltage on the capacitor it can cause both loss of electrolyte due to electrolysis and cause “thermal acceleration” due to a progressive increase in the temperature of the device.

It is known that with decreasing temperature of any electrolyte its specific conductivity decreases, which leads to an increase in the internal resistance of the capacitor. It is also known that when the temperature decreases, the electrostatic capacitance of the capacitor decreases due to an increase in the degree of hydration and, as a result, an increase in the ionic radius of electrolyte ions.

A decrease in capacitance and an increase in the internal resistance of capacitors at low temperatures can lead to poor capacitor performance. The lower the temperature, the greater the energy during the discharge is dissipated on the internal resistance of the capacitor and does not reach the consumer.

Since the energy of the capacitor E is connected with the capacitance C and voltage U by the ratio

, $$E=\frac{C\cdot U^2}{2}$$

,

and the maximum power N is connected with the voltage and with the internal m resistance R by the ratio

, $$N=\frac{U^2}{\mathrm{four}R}$$

,

It becomes clear that increasing the voltage across the capacitor at low temperatures makes it possible to increase both the energy supplied and the power. From here it also becomes clear the need to charge the capacitor before it is discharged to a voltage close to the maximum allowable.

Determining the optimal voltage across the capacitor as a function of temperature is a difficult technical task, since this dependence is determined not only by the temperature, but also by a number of design features of the capacitors: the geometry of the electrode unit, the chemical composition of the active materials, current collectors and electrolyte, etc.

The closest in technical essence to the proposed solution is a method of charging and discharging a capacitor with a double electric layer, including connecting the capacitor to a current source, conducting its charge to a given voltage, terminating the charge and then discharging [2]. The operating voltage of such a capacitor is 1.3-1.5 V.

However, at a temperature of, for example, 50 degrees, such a capacitor at a voltage of 1.5 V has an unacceptably large value of the leakage current, and at a temperature of, for example, minus 30 degrees, its energy intensity is significantly reduced compared to the energy intensity at room temperature, mainly due to an increase in internal resistance.

The purpose of the invention is achieved by optimizing the conditions of its charge.

The technical result in the proposed method for the operation of electrochemical capacitors with an aqueous alkaline electrolyte, a porous separator and electrodes, one of which contains activated carbon material, and the other contains nickel hydroxide as an active material, including connecting the capacitor to a current source, holding its charge to a given voltage, cessation of charge and discharge, is achieved, according to the invention, by first measuring the temperature of the capacitor, and determine the maxim the total working voltage of the charge, at which decomposition of the electrolyte and concomitant gas evolution are excluded.

The invention also includes a method for determining the maximum voltage at which outgassing is excluded.

According to the invention, the maximum allowable voltage is considered the highest voltage at which there is no gas evolution from the capacitor or an increase in pressure in the gas space of the capacitor.

Continuous gas evolution and a concomitant increase in pressure are not possible if the continuous charge current i does not exceed the limiting diffusion current of oxygen released on the positive electrode. The fact that it is diffusion rather than kinetic restrictions that determine the possibility of recharging without gas evolution is due to the fact that the large specific surface area of the negative electrode and its potential value, which are much more negative than the equilibrium potential of the oxygen electrode, determine the high rate of oxygen ionization. Permissible continuous charge current can be calculated by the equation

$$i\le \frac{\mathrm{four}\cdot F\cdot S\cdot D\cdot \lambda \cdot a}{d\cdot {\lambda }_0},\left(\mathrm{one}\right)$$

where i is the continuous charge current of the capacitor. BUT;

F is the Faraday number, 96500 pendant / mol;

S is the area of the interelectrode gap, m;

D is the diffusion coefficient of oxygen in an alkaline electrolyte, 0.6 · 10 ^-9 m ² / s;

a is the solubility of oxygen in alkali, 0.16 mol / m ³ [3, p. 93-95];

d is the thickness of the separator, m;

- отношение удельных электропроводностеи сепаратора, пропитанного электролитом, и электролита. $$\frac{\lambda }{{\lambda }_0}$$

- the ratio of the electrical conductivity of the separator impregnated with the electrolyte, and the electrolyte.

Equation (1) is well known in electrochemistry [3] and describes the stationary diffusion process, when the reaction rate is limited by the diffusion transport of the reagent and is constant in time.

Subject to relation (1), oxygen released at the positive electrode is completely transported in dissolved form through the porous separator and ionized at the negative electrode.

Since the current of oxygen evolution under stationary conditions, provided that it is completely ionized, is equal to the total current, and the oxygen ionization current is equal to the current of its generation, it is impossible to produce hydrogen on a negative electrode under these conditions.

The above numerical values of the constants a and D are indicated for a temperature of 20 ° C. Experience and calculations show that with increasing temperature, the solubility of oxygen in the electrolyte decreases, and the diffusion coefficient of oxygen increases, while the product Z of the constants a and D remains approximately constant and amounts to about 10 ^-10 mol · m ^-1 · s ^-1 . Thus, in a wide temperature range, the relation

$$i\le \frac{\mathrm{four}\cdot F\cdot S\cdot Z\cdot \lambda }{d\cdot {\lambda }_0},\left(2\right)$$

where Z = 10 ^-10 mol · m ^-1 · s ^-1 .

According to the invention, the maximum voltage U _max at which no gas evolution occurs in the capacitor and the temperature t are related by the relation

$$U_{\max }=k\cdot t+b,\left(3\right)$$

where k and b are experimentally determined coefficients that depend on the design features of the capacitor: the type and thickness of the separator, the material of the current collectors of the electrodes, etc.

According to the invention, the coefficients k and b are determined as follows:

- calculate the value of i according to equation (2) for a given design of the capacitor;

- thermostat the capacitor at a temperature t1;

- maintain the capacitor at a temperature t1, charging it with a current i until a constant voltage U1 is established in time;

- cool the condenser to a temperature t2 less than t1, but not lower than to the freezing temperature of the condenser;

- maintain the capacitor at a temperature t2, charging it with a current i until a constant voltage U2 is established in time;

- calculate the constants k and b for equation (3) or find them graphically.

When conducting patent research, no solutions were found that are identical to the claimed, and, therefore, this invention meets the criterion of "novelty."

The invention does not follow explicitly from the known solutions, and, therefore, the proposed solution meets the criterion of "inventive step".

The invention can be illustrated by the following examples.

Example 1

Electrochemical capacitors (EC) were used as the initial test object, in which positive electrodes with nickel hydroxide as an active material, structurally similar to electrodes of aviation alkaline batteries, are used. Negative electrodes were made of carbon cloth. Current collectors of negative electrodes are made of nickel foil. The separator is made of non-woven polypropylene material, 0.2 mm thick. The electrolyte is a 6N KOH solution. The total overall surface of each half-block of electrodes was 0.5 m.

Using equation (2) and knowing the thickness of the separator, its electrical conductivity, and also the area of the interelectrode gap, we calculated the maximum allowable continuous charge current. He was 0.1 A.

Capacitors (2 pcs.) Were placed in a cold chamber and kept at a temperature of 0 ° С for 6 hours, then, without removing them from the chamber, a charge of 0.1 A was applied to stabilize the voltage.

Then, without stopping the charge, the capacitors were cooled to a temperature of -25 ° C and again held until the voltage stabilized.

In both cases, the duration of reaching the stationary voltage was about a day.

Table 1 shows the temperature dependence of the stationary voltage across the capacitors.

Table 1 Dependence of stationary voltages for EC on temperature under current 100 mA

Temperature EC No. 1 EC No. 2 Stress 0 1,682 1,69 -25 1,830 1,833

It can be seen from the table that the difference in voltage for the capacitors is very small and amounts to no more than 8 mV at the same temperature.

By linear approximation, the equation for the dependence of voltage on temperature is obtained:

$$U=-0.0057\cdot t+\mathrm{1.71,}\left(\mathrm{four}\right)$$

Figure 1 shows the voltage dependence calculated according to equation (4), at which and below which gas evolution from the capacitor is excluded, on temperature:

Despite the constant recharging, the pressure in the EC, measured by a water manometer connected through the valve hole to the gas space of the capacitors, did not change. This confirms the absence of gas evolution.

Example 2

In order to determine how effective the increase in the operating voltage to increase the useful energy delivered to the load was, climate low-temperature tests of a module of 50 series-connected capacitors similar to those described above were performed.

The module was charged to voltages of 62, 65, 70 V (which corresponds to the average voltage at the capacitors of 1.24, 1.3, 1.4 V). Then, discharges to a load of 0.1 Ohm were performed at temperatures of 25, -20, -30, -40 and -50 ° C. At temperatures of -40 and -50 ° C, the charge was also performed up to voltages of 90 V, which corresponds to a voltage of 1.8 V.

It was shown that increasing the voltage at low temperatures allows a very significant increase in the energy given to the load (Fig. 2). At temperatures of -40 and -50 ° C, an increase in the final charge voltage of a set of modules from 70 to 90 V leads to an increase in energy by 4.2 and 3.5 times, respectively. It is obvious that the voltage at temperatures of -40 and -50 ° C could be increased to 100 V: this is evidenced by the test results, at temperatures of 0-25 ° C, described above. In this case, the energy delivered to the load could be even lower at negative temperatures than at normal temperature.

Technically, the use of the possibility of increasing the voltage across the capacitors at low temperatures can be realized, for example, using DC-DC converters. The EC battery can be installed together with the rechargeable battery, which will ensure that a certain energy reserve is stored in the EC, will reduce the charge time before start-up, and will allow monitoring the condition of the EC battery. Before starting, the EC battery must be disconnected from the standard battery and charged to a voltage determined by the temperature of the EC. The temperature should be controlled by a sensor. When charging an EC, the state of the battery (level of charge, serviceability, value of internal resistance) does not play a significant role, because EC recharge current may be small. After reaching the required voltage, the engine is started from the EC, without using a standard battery of batteries.

It is obvious that the method for optimizing the voltage at charge described above is also suitable for electrochemical capacitors based on other electrochemical systems in which there is an aqueous electrolyte and a gas cycle is possible. In particular, this applies to capacitors with an acid electrolyte and a positive PbSO ₄ / PbO ₂ electrode. In this case, it is only necessary to clarify the value of the constant Z.

Sources of information taken into account:

1. Capacitor with a double electric layer. U.S. Patent 5,986,876.

2. Application WO 97/07518, cl. H01G 9/04, 1995

3. Chomskaya EA, Burdanova NF, Gorbacheva NF Gas-liquid flow control when charging batteries. - Saratov: Publishing house of Sarat. Univ., 1998 .-- 120 s.

Claims (1)

A method of operating electrochemical capacitors with an aqueous alkaline electrolyte, a porous separator and electrodes, one of which contains activated carbon material, and the other contains nickel hydroxide as the active material, which includes connecting the capacitor to a current source, conducting its charge to a predetermined voltage, terminating the charge and discharging characterized in that the temperature of the capacitor is preliminarily measured, by which the maximum working charge voltage is determined at which r zovydelenie, wherein the maximum charge voltage limit U _max according to the equation
U _max = k · t + b
where k and b are the coefficients determined experimentally and depending on the design features of the capacitor, t is the temperature,
and for measuring the coefficients k and b, the continuous charge current i is calculated according to the equation
$$i\le \frac{\mathrm{four}\cdot F\cdot S\cdot Z\cdot \lambda }{d\cdot {\lambda }_0}$$

,
where i is the continuous charge current of the capacitor, A;
F is the Faraday number, 96500 pendant / mol;
S is the area of the interelectrode gap, m ² ;
d is the thickness of the separator, m;
Z is a constant, 10 ^-10 mol · m ^-1 · s ^-1 ;
$$\frac{\lambda }{{\lambda }_0}$$

- the ratio of the electrical conductivity of the separator impregnated with the electrolyte and the electrolyte;
then determine the coefficients k and b, for which
- thermostat the capacitor at a temperature t1 and charge it with current i until a constant voltage U1 is established in time;
- thermostat the capacitor at a temperature t2 lower than t1, but not lower than the freezing temperature of the electrolyte in the capacitor and charge it with current 1 until a constant voltage U2 is established in time;
- calculate the constants k and b for equation (2) or find them graphically.

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