PL236763B1 - High-capacity electrochemical capacitor - Google Patents

Abstract

The subject of the application is a high-capacity electrochemical capacitor consisting of a positive and negative electrode made of a carbon material with a developed specific surface of at least 200 m2 g-1, separated by a separator and immersed in an electrolyte which is a solution containing copper (II) ions and alkali metal bromides.

Description

Description of the invention

The subject of the invention is a high-capacity electrochemical capacitor made of two symmetrical carbon electrodes with a more specific surface, which is used in energy storage and conversion systems. In this electrochemical capacitor, the electrodes are separated by a semipermeable membrane, and a salt mixture containing copper (II) cations together with alkali metal bromides with a concentration in the range 0.1-2.0 M constitutes an electrolyte solution.

Electrochemical capacitors (Koetz R., Carlen M. "Principles and applications ot electrochemical capacitors" Electrochimica Acta 45 (2000) 2483-2498) are devices used to store energy capable of delivering it in a very short time, and therefore characterized by high power. Commonly used electrochemical capacitors are made of two electrodes (made of different materials, it can be activated carbon or metal oxides) separated by a separator (semi-permeable membrane) with a soaked electrolyte solution. The outstanding properties result from the characteristics of the electrode's polarizable material and the ion layer attracted to its surface. Electrochemical capacitors are used in many fields of electronics (Miller J.-R., Burke A. "Electrochemical capacitors: Challenges and opportunities for real-world applications" Electrochemical Society Interface 17 (2008) 53-57). They can be an alternative and support for electrochemical cells or batteries. Electrochemical capacitors are characterized by a high power density, a very long service life and a higher energy density than electrolytic and dielectric capacitors. However, efforts are still being made to increase their energy parameters, and due to the direct dependence of energy on the value of capacitance and operating voltage, methods are currently being sought to increase these values (Equation I).

(l) E = '/ rC'U ² where, E - energy / J, C - system capacity / F, U - operating voltage / V.

As mentioned earlier, the capacity of the system because of the properties of the active material, which is related primarily to the surface of the active S / m ^{2 1} g. Nowadays, there are many methods of determining the specific surface area, and for microporous carbon materials, nitrogen adsorption at 77 K or carbon dioxide at 273 K is most often used. After delineating the adsorption isotherm using the Brunauer-Emmet-Teller (BET) method (International standard of the specific, surface area ofsolids by gas adsorption - BET method ”(2010) ISO 9277: 2010) can calculate the value of the specific surface area of the test material (Equation 2).

where, X - amount of adsorbed nitrogen at a given relative pressure / cm ³ , Po / P - relative pressure / -, X _m - monolayer capacity / cm ³ and C - constant BET / -.

The size of the specific surface area depends on the porous structure of the tested material. It should be borne in mind that none of the methods of surface determination gives the direct absolute value of the surface that actively participates in the formation of the electrical double layer. It is this value that determines the amount of adsorbed charge at the electrode / electrolyte interface and indicates the capacity of the system. As presented in the literature, the size of the pores present in the material and the diameters of the ions must be correlated (Pohlmann S., Lobato B., Centeno TA, Balducci A. "The influence of porę size and surface area of activated carbons on the perfromance of ionic liquid based supercapacitors ”Physical Chemistry Chemical Physics 15 (2013) 17287-17294). However, often these discussions are based on model carbon systems, i.e. those with a specific and undifferentiated structure, and not on the group of activated carbons. Since this area of research has been widely researched, other methods of increasing the specific capacity of the material are being sought. One of them is the modification of activated carbons so that they can participate in charge transfer reactions, the so-called redox. It is an effective method due to the significant increase in the capacity value. However, such systems are characterized by reduced efficiency and shorter working time compared to systems that store a charge by electrostatic interactions only. Methods of modifying carbon material are related to the incorporation of heteroatoms such as oxygen or nitrogen into the graphene structure (Hulicova-Jurcakova D., Kodama, M., Shiraishi, S., Hatori, H., Zhu, ZH, Lu, GQ "Nitrogen-enriched nonporous carbon electrodes with extraordinary supercapacitance ”Advanced

PL 236 763 B1

Functional Materials 19 (2009) 1800-1809) or creating functional groups capable of donating or receiving electrons (Frackowiak E., Beguin F. "Electrochemical storage of energy in carbon nanotubes and nanostructured carbons" Carbon 40 (2002) 1775-1787). The increased capacity through redox reactions, either from the reaction of the electrode material or the electrolyte, is called pseudocapacity. One of the cheaper methods of introducing pseudo-capacity into the operation of the system is the use of active aqueous solutions as electrolytes.

As presented by Fic. K, Frackowiak "Electrochemical capacitor" P.413468 the use of copper (II) nitrate (V) solution as an electrolyte results in high operating voltage (1.6 V) and very good system life (5000 cycles at a current density of 1 Ag ^-1 ). Additionally, other redox solutions, such as bromides or iodides, were investigated (Menzel J., Fic K., Meller M., Frackowiak E. "The effect of halide ion concentration on capacitor performance" Journal of Applied Electrochemistry 44 (2014) 439-445 ). However, in these systems, a problem was observed due to the short-term redox activity of Br ^- or I ^- ions, which was caused by precipitation, i.e. the formation of complex ions, ie IO3 ^- with a low solubility product. As previously presented, the mixture of aqueous solutions with different characteristics allows for the improvement of operating parameters (Fic K., Frackowiak E., Lota G., Meller M. "Electrochemical capacitor working in solutions of dihydroxybenzenes and bromide ions" PAT.221355). For this reason, the following invention has been proposed, which aims to reduce the drawbacks of the above-presented systems while maintaining or improving the operating parameters.

The essence of the invention is a high-capacity electrochemical capacitor. It consists of a positive and negative electrode separated by a separator. The specific surface of the electrodes used is a highly developed surface, i.e. at least 200 m ² g ^-1 . The carbon electrodes with the membrane are placed in a solution containing copper (II) cations together with alkali metal bromides. It is preferable to use an aqueous solution of copper (II) nitrate and potassium bromide. These salts are most preferably used in a concentration of 0.5 M. Another preferred solution is the use of an aqueous solution of copper sulphate and potassium bromide. It is most preferred to use these salts at a concentration of 1 M and 0.5 M, respectively.

Due to the use of the capacitor according to the invention, the following technical and operational effects were obtained:

• possibility of extending the operating voltage of the system (> 1.2 V);

• possibility of obtaining long-term cyclic durability of systems at high voltage (1.2 V, 5000 cycles);

• very good preservation of the capacitance of the electrochemical basin on reversible redox reactions for galvanostatic charge / discharge in the 0.1-10 A g ^-1 regime;

• high reversibility of the charging / discharging process (> 90%);

• the symmetry of the system makes it cheap and easy to construct;

• the use of aqueous inorganic salts greatly facilitates the assembly of the system. It does not require work in an inert atmosphere, which significantly reduces the final cost of the device;

• pH in the range of 3-5 allows the use of stainless steel current collectors, which significantly reduces the production costs of the device.

The invention is presented in the drawings, where Fig. 1 shows a diagram of an electrochemical capacitor using copper (II) solutions and alkali metal bromides, Fig. 2 cyclic voltamperograms in the range of 0-0.8 V at a scanning speed of 5 mV s ^-1 , allowing compare systems working only with a solution containing Cu ²⁺ ions and in a mixture containing Cu ²⁺ ions and alkali metal bromides, in this case KBr, Fig. 3 a galvanostatic charge / discharge test allowing to compare systems working only with the solution containing ions Cu ²⁺ and in a mixture containing Cu ²⁺ ions and alkali metal bromides, in this case KBr: a) curve with a current density of 0.1 A g ^-1 ; b) dependence of capacitance on current density, Fig. 4 Nyquist curves recorded at a voltage of 0.8 V allowing to compare systems working only with a solution containing Cu ²⁺ ions and in a mixture containing Cu ²⁺ ions and alkali metal bromides, in this case KBr, Fig. 5 graphs of the dependence of capacitance on the frequency recorded at a voltage of 0.8 V allowing to compare systems working only with a solution containing Cu ²⁺ ions and in a mixture containing Cu ²⁺ ions and alkali metal bromides, in this case KBr, Fig 6 cyclic voltammetry test in a three-electrode system for a capacitor operating in a solution containing Cu ²⁺ ions and alkali metal bromides, in this case

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KBr, Fig. 7 cyclic voltamperograms recorded at various operating voltages, from 0.6 V to 1.3 V for a system operating in a solution containing Cu ²⁺ ions and alkali metal bromides, in this case KBr, for a travel speed of 2 mV s ^-1 .

The electrochemical capacitor is made of two symmetrical electrodes - the positively and negatively polarized electrode, which are the active material with a better developed surface. These electrodes are immersed in an electrolyte, which is a solution containing Cu ²⁺ ions and alkali metal bromides. The electrodes are separated by a membrane (separator), which prevents their contact, and at the same time is permeable to ions (Fig. 1).

The shape of the voltammetric curve of such a constructed capacitor using a solution containing Cu ²⁺ ions and alkali metal bromides as electrolyte shows a very high redox activity compared to the electrochemical capacitor using only copper (II) nitrate (Fig. 2). The capacitance values of a capacitor constructed in this way are in the range 576-222 F g ^-1 (capacity expressed in relation to the mass of one electrode), depending on the value of the scanning speed in the range 1 mV s ^-1 to 100 mV s ^-1 . In addition, the system has reached a high operating voltage. ie 1.2V compared to an unmodified solution with copper (II) cations present using the same carbon cloth. In this range, the reversibility of the charge / discharge processes, calculated from galvanostatic charge / discharge (0.1 A g ^-1 ), exceeds 70% for Coulomb efficiency and exceeds 50% for energy efficiency (Fig. 3). The behavior of the electrochemical capacitor was investigated using idle impedance spectroscopy and a voltage load of 0.8 V (Fig. 4). The addition of an alkali metal bromide solution improves the characteristics of the system, both at rest and at a voltage of 0.8 V. For a charging / discharging frequency of 1 Hz, the capacity of the system (converted for a single electrode) is 100 F g ^-1 (Fig. 5). The behavior of the individual electrodes presents a redox active cation Cu ^{+ 2} on the negative electrode and the anion bromide Br ^-1 at the positive electrode, which affects the expansion of the negative electrode potential-window (from 0.2 V to over 0.3 V) (Fig. 6). For the activated carbon used, cyclic voltamperograms were made with a travel speed of 2 mV s ^-1 for different rated voltages. From the efficiency of the charging / discharging processes, the maximum voltage value for this system was determined to be 1.2 V (Fig. 7).

The invention is illustrated by the following example:

P r z k ł a d I

The electrodes of the electrochemical capacitor are made of activated carbon. The actual surface area of the activated carbon used was 1550 m ² g ^-1 . The electrode material consists of 100% by weight. of carbon fabric. Discs with a diameter of 10 mm were cut from it. The prepared electrodes were separated by a glass fiber separator with a diameter of 12 mm and a thickness of 260 μm. The components were placed in an electrochemical vessel, which was then filled with electrolyte, which is a solution containing Cu ²⁺ ions, in this case copper (II) nitrate (V) and alkali metal bromides, in this case potassium bromide, preferably with a concentration of 0, 5 mol / l (M).

The capacitor constructed in this way was subjected to electrochemical tests: cyclic voltammetry (1-100 mV s ^-1 ), galvanostatic charge / discharge (0.1-10 A g ^-1 ), electrochemical impedance spectroscopy (1 mHz-100 kHz) and cyclic tests. The test results are shown in Figures 2-7.

Claims (3)

Patent claims 1. High-capacity electrochemical capacitor consisting of a positive and negative electrode separated by a separator and immersed in the electrolyte, the electrodes made of carbon material with a specific surface area of at least 200 m ² g ^-1 , characterized in that the electrolyte is a solution containing ions copper (II) and alkali metal bromides. 2. High-capacity electrochemical capacitor according to claim 1, characterized in that the solution is copper (II) nitrate and potassium bromide, the concentration of both salts being preferably 0.5 mol / l. 3. High-capacity electrochemical capacitor according to claim 1, characterized in that the solution is copper (II) sulfate and potassium bromide, the concentration of both salts being 0.1-2 M mol / l.