PL238546B1 - Primary charging method for a hybrid electrochemical capacitor - Google Patents

Abstract

The subject of the invention is a method of primary charging of a hybrid electrochemical capacitor operating in electrolyte solutions of different pH, consisting of a positive electrode (4) and a negative electrode (3) made of a carbon material with a developed specific surface area of at least 200 m2/g, characterized in that during the first charging a charge not exceeding 600 coulombs, preferably 400, should be applied to each gram of the electrode with a current density not exceeding 5 A/g, preferably 1 A/g in the form of an appropriate number of direct current pulses with a duration not exceeding 5 seconds, preferably 2 seconds, wherein the maximum operating voltage during the primary charging cannot exceed 2.4 V.

Description

Description of the invention

The subject of the invention is a method of primary charging of a hybrid electrochemical capacitor working in electrolyte solutions of different pH, consisting of two porous carbon electrodes with a developed surface, working in solutions of appropriate pairs of water electrolytes with different pH, each of which is a separate medium for individual electrodes. The negative electrode of such a capacitor should contain electrochemically adsorbed hydrogen. This type of capacitor is used as an energy storage device.

Due to the characteristic way of accumulating the charge, electrochemical capacitors are used wherever conventional electrochemical cells are not able to meet the requirements in terms of high power demand. Since the electric double layer charge and discharge mechanism is purely electrostatic in nature, it is possible to charge and discharge a capacitor in a very short time (on the order of milliseconds) while delivering enormous amounts of power. In order for this process to take place effectively, carbon with a developed surface (most often above 1000 m ² g ^-1 ) and appropriate porosity must be used as the electrode material, which has been presented in the literature: E. Frąckowiak, F. Beguin: Electrochemical storage of energy in carbon nanotubes and nanostructured carbons, Carbon, 2002, 40, 1775-1787 and E. Frąckowiak: Carbon materials for supercapacitor application, Physical Chemistry Chemical Physics, 2007, 9, 1774-1785.

Electrochemical capacitors are powerful, but their energy is much lower than that of common energy sources such as lithium-ion or nickel-metal hydride cells. In order to maintain a relatively high energy value (without losing power), it is preferable to increase the operating voltage of the capacitor. According to the following formula:

E = x C x U ^{2 it} can be seen that the energy is proportional to the capacitance C and the square of the operating voltage of the capacitor U, which primarily depends on the type of electrolyte that was used. The greatest possibilities are given by anhydrous organic electrolytes, because the system is not limited by the low voltage of water decomposition, which occurs according to thermodynamics at a voltage of 1.23 V. flammable and environmentally unfriendly electrolytes such as, for example, tetraethylammonium tetrafluoroborate in acetonitrile.

There are works that show capacitors operating in water electrolytes at a voltage much higher than allowed by thermodynamics. Thanks to the use of a 1 mol L ^-1 L12SO4 solution in combination with a suitable carbon material, it was possible to obtain a voltage of even 2.2 V, which was described in the publication: K. Fic, G. Lota, M. Meller, E. Frąckowiak: Novel insight into neutral medium as electrolyte for high-voltage supercapacitors, Energy & Environmental Science, 2012, 5, 5842-5850 and is the subject of the patent PL215699 obtained. In turn, in the literature of L. Demarconnay, E. Raymundo-Pinero, F. Beguin: A symmetric carbon / carbon supercapacitor operating at 1.6 V by using a neutral aqueous solution, Electrochemistry Communication, 2012, 12, 1275-1278, a capacitor was described which it can work with a very good cyclic charge / discharge efficiency at a voltage of 1.6 V. In this case, the authors used an aqueous solution of 0.5 mol L ^-1 L12SO4 (PCT / EP2011 / 054147 application). Another concept that allowed to increase the capacitor operating voltage to 1.6 V in 1 mol L ^-1 H2SO4 was to balance the masses of individual electrodes. This has been described in: V. Khomenko, E. Raymundo-Pinero, F. Beguin: A new type of high energy asymmetric capacitor with nanoporous carbon electrodes in aqueous electrolyte, Journal of Power Sources, 2010, 195, 4234-4241. Another example of an asymmetric capacitor is described in: T. Brousse, M. Toupin, D. Belanger: A Hybrid Activated Carbon-Manganese Dioxide Capacitor using a Mild Aqueous Electrolyte, Journal of Electrochemical Society, 2004, 151, A614-A622. The presented solution was based on the use of MnO2 as the positive electrode and active carbon as the negative electrode. An aqueous solution of 0.65 mol L ^-1 K2SO4 was an electrolyte in this case, which allowed to obtain a stable operating voltage of 1.5 V, even after 23,000 charge / discharge cycles. There are several patents, the subject of which concerns electrode materials working in appropriate solutions of water electrolytes, which in electrochemical capacitors allow to increase the capacity and extend the operating voltage of the entire system, e.g. PL215699 "High-voltage electrochemical capacitor", PL215046 "Electrochemical capacitor

PL 238 546 Β1 operating in iodide solution ", PL213083" Electrochemical capacitor operating in an electrolyte solution with the addition of surfactant ". It is also possible to use an asymmetric system in which the composition of the positive electrode is based on nickel compounds, and the positive electrode is a carbon material enriched with at least one redox pair. Thanks to this combination, the whole can work in a water electrolyte at a higher voltage. This solution is the subject of WO 03/088374 A2, 2003.

The electrochemical characteristics of capacitors working in a system where each electrode is in a solution with a different pH allowed for much higher capacitance values and voltage extension, compared to a system in which both electrodes worked in the same electrolyte. Taking into account the oxygen and hydrogen evolution potentials at individual electrodes in the acid, neutral and alkaline electrolyte, it becomes possible to safely expand the electrochemical window of the electrolyte. In the case of 5 mol L ^-1 L1NO3, the oxygen evolution potential on the positive electrode is E02 = 0.817 V. In turn, in a solution of 6 mol L ^-1 KOH on the negative electrode, hydrogen is released when the potential Eh 2 = -0.87202 V is exceeded. , in which the capacitor operates in two electrolyte solutions with different pH is the subject of the applications: P.406638 Krzysztof Fic, Mikołaj Meller, Elżbieta Frąckowiak "Electrochemical capacitor working in electrolyte solutions with different pH" and P.406648 Krzysztof Fic, Mikołaj Meller, Elżbieta Frąckowiak "Electrochemical capacitor working in electrolyte solutions with different pH".

In the above solutions, the concept of increasing the voltage was applied by appropriately balancing the overvoltage of hydrogen and oxygen evolution on individual electrodes, obtaining a high output voltage of the device. However, apart from increasing the voltage, the pseudo-capacitive effect can be obtained by using the phenomenon of hydrogen electrosorption on the negative electrode, increasing the energy of the capacitor while maintaining its high power.

The process of electrosorption of hydrogen in the <C> carbon material is based on the electrolytic decomposition of the electrolyte, which is most often an aqueous solution of potassium hydroxide KOH with a concentration of 6 mol / L:

H ₂ O + e'-H + OH- (1) and hydrogen sorption in statu nascendi, according to the equation:

<O + H-CH _{ad (2}

In sum, equations (1) and (2) can be written:

<C> + H ₂ O + e “^ <C> H _ad + OH“ (3)

This mechanism was proposed in the publication: K. Jurewicz, E. Frąckowiak, F. Beguin "Towards the mechanism of electrochemical hydrogen storage in nanostructured carbon materials" Applied Physics A. 78 (2004) 981-987. In the same work it was suggested that in addition to the above reactions, there are also other reactions that reduce the efficiency of electrosorption, related to the recombination of hydrogen atoms:

^CH ad ^{+ H} 2 ° ^{+ e} ”θ H ₂ + OH“ + C (4)

2 _H -H ₂ t ₍₅₎ < ⁶ >

It is also important that electrochemically sorbed hydrogen tends to form weak chemical bonds with carbon atoms with an energy of 110 kJ / mol, which was demonstrated in the work by: F. Beguin, M. Friebe, K. Jurewicz, C. Vix-Guterl , J. Dentzer, E. Frąckowiak "State of hydrogen electrochemically stored using nanoporous carbons as negative electrode materials in an aqueous medium" Carbon 44 (2006) 2392-2398. The potential for hydrogen evolution is closely related to the pH of the solution in which the electrolysis is carried out:

^Ε ^ = -θ · ⁰⁵⁹ Ρ ^Η (Ό

According to which in a 6 mol / L potassium hydroxide solution (pH = 14) it is -0.873 V resp. Normal Hydrogen Electrode. In fact, this potential is lower due to a certain value of the overpotential for active carbons amounting to about 0.2 V, most often related to the charge transfer resistance. By polarizing a carbon electrode with a current with a density above -250 mA / g, even lower potential values are achieved, and taking into account the fact that a redox process takes place on it, such a system may constitute an excellent negative electrode of the cell, which was presented in the work of K. Jurewicz, E. Frąckowiak , F. Beguin "Nanoporous H-sorbed carbon as anode of secondary cell" Journal of Power Sources 188 (2009) 617-620 and patent application: PCT / IB2006 / 000227.

The electrical capacity of such an electrode is closely related to the amount of stored hydrogen; 1 wt.% the hydrogen stored in the carbon material corresponds to a charge of 272 mAh / g. The ability to reverse hydrogen sorption of a given carbon material is mainly determined by its structural properties, while in the works of: F. Beguin, K. Kierzek, M. Friebe, A. Jankowska, J. Machnikowski, K. Jurewicz, E. Frąckowiak "Effect of various porous nanotextures on the reversible electrochemical sorption of hydrogen in activated carbons "Electrochimica Acta 51 (2006) 2161-2167 and D. Qu" Mechanism for electrochemical hydrogen insertion in carbonaceous materials "Journal of Power Sources 179 (2008) 310-316 has been shown, that nanoporous activated carbons with pore diameters below 0.6 nm have the highest sorption capacity. Unfortunately, no more than about 2.0 wt% hydrogen has been stored so far, corresponding to a capacity of 544 mAh / g. Further modification of the hydrogen sorption capacity in the carbon material can be achieved by the use of surfactants, both in the carbon material and in the electrolyte.

The essence of the invention is a method of primary charging of a hybrid electrochemical capacitor operating in electrolyte solutions with different pH, consisting of a positive and negative electrode made of carbon material with a developed specific surface of at least 200 m ² / g located in electrolytes, in which the positive electrode is located in electrolyte, which is a neutral environment in the range of pH 5-8, while the negative electrode is located in the electrolyte, which is an alkaline solution with a pH> 6. Electrolytes selected in this way constitute the so-called a hybrid system of electrolytes, one of which is responsible for the high over-potential of oxygen evolution (electrolyte for the positive electrode), while the other should be an appropriate environment, characterized by a high over-potential of hydrogen evolution. Due to the use of electrolytes with such specific electrochemical properties, it is possible to extend the operating voltage of the electrochemical capacitor, significantly exceeding the value of water decomposition, which according to thermodynamics is limited to 1.23 V. Electrolytes are separated by separators and a membrane previously soaked with a solution of The pH is neutral, preferably 5 mol L ^-1 LiNO3.

It is important that the negative electrode of such a capacitor contains electrochemically adsorbed hydrogen in an optimal amount. The first charging of the capacitor takes place in a pulsed manner and takes into account the designed electrode capacitance. In order to adsorb the optimal amount of hydrogen, a charge not exceeding 600 coulombs, preferably 400, should be applied to each gram of the electrode with a current with a density not exceeding 5 A / g, preferably 1 A / g in the form of an appropriate number of DC pulses with a duration not exceeding 5 seconds, preferably 2 seconds. . The maximum operating voltage during primary charging must not exceed 2.4 V.

Thanks to the solution according to the invention, the following technical and operational effects were obtained:

• the possibility of obtaining a higher gravimetric power density (approx. 1100 W kg ^-1 ) and energy (approx. 22 Wh kg ^-1 ) compared to a capacitor working only in 5 mol L ^-1 LiNO3, the energy density value of which is approx. 15 Wh kg ^-1 with a simultaneous power density of approx. 1000 W kg ^-1 ;

• the possibility of obtaining a higher gravimetric power density (approx. 1100 W kg ^-1 ) and energy (approx. 22 Wh kg ^-1 ) compared to a capacitor working only in 6 mol L ^-1 KOH, the energy density value of which is approx. 3 Wh kg ^-1 with a simultaneous power density of approx. 400 W kg ^-1 ;

• in the case of a capacitor operating in a lithium nitrate (V) solution as an electrolyte for the positive electrode and in a potassium hydroxide solution as an electrolyte for the negative electrode, a higher operating voltage of the capacitor was obtained compared to the capacitor operating separately in each of these electrolytes. The obtained voltage of 2.1 V exceeds the value of the water decomposition voltage.

PL 238 546 B1

The capacitor is shown in the figure, Fig. 1 shows a schematic diagram of the capacitor. Its operation is based on the simultaneous use of water electrolytes of different pH (neutral and alkaline) in the electrochemical condenser and the beneficial effect of hydrogen electrosorption. This means that the negative electrodes 3 and positive 4 of the capacitor work in different electrolytes 1 and 2, separated from each other by separators 5 and diaphragm 6. Positive electrode 4 should work in a neutral electrolyte, while the negative electrode 3 in an alkaline electrolyte.

As each of the electrodes 3 and 4 in connection with the respective electrolytes 1 and 2 constitute an independent, electrochemically active system, it is possible to use them efficiently and extend the capacitor voltage. Electrochemically adsorbed hydrogen also allows to increase the electric capacity and thus increase the energy of the capacitor.

The invention is illustrated by the following example in an embodiment. The electrodes of the electrochemical capacitor were made of active carbon, the real surface of which was 2522 m ² g ^-1 . Tablets with a diameter of 10 mm and a thickness of about 0.7 mm were obtained by pressing in a hydraulic press the mixture: 85% by weight. % carbon material, 10 wt. % binder (PTFE) and 5 wt. acetylene carbon black. Then, the electrodes prepared in this way together with separators (Munktell MGC) were transferred to vials containing solutions of appropriate electrolytes. The positive electrode was placed in the electrolyte, which is 5 mol L ^-1 LiNO3, while the negative electrode 3 was placed in the electrolyte 2, which was 6 mol L ^-1 KOH. Separators (Munktell MGC) were placed in a 5 mol L ^-1 LiNO3 solution. All vials were capped and left for one hour. After this time, the separators with the electrodes were transferred to the electrochemical vessel. In order to adsorb the optimal amount of hydrogen, a charge of 400 coulombs was applied to each gram of the electrode with a current of 1 A / g in the form of 200 two-second DC pulses. The maximum operating voltage was 2.27 V.

The capacitor constructed in this way and originally charged was subjected to electrochemical tests: cyclic voltammetry (1-20 mV / s), galvanostatic charge / discharge (200 mA / g 10 A / g). The stable operating voltage of the capacitor was 1.96 V. The capacity of such a capacitor for a discharge current density of 1 A g ^-1 was 179 F g ^-1 .

Claims (1)

Patent claim 1. The method of primary charging of a hybrid capacitor operating in electrolyte solutions of different pH, consisting of a positive and negative electrode made of carbon material with a developed specific surface of at least 200 m ² / g located in electrolytes, characterized in that the capacitor whose positive electrode ( 4) is located in the electrolyte (1), which is a neutral environment in the range of pH 5-8, while the negative electrode (3) is located in the electrolyte (2), which is an alkaline solution with pH> 6, the electrolytes (1) and (2) are separated by separators (5) and a membrane (6) previously soaked with a solution with a neutral pH, preferably 5 mol L ^-1 LiNO3, during the first one charged with a charge not exceeding 600 coulombs, preferably 400, for each gram of electrode with a current with a density not exceeding 5 A / g, preferably 1 A / g, in the form of a corresponding number of DC pulses with a duration not exceeding 5 seconds, preferably 2 seconds, the maximum voltage being operation during the primary charging does not exceed 2.4 V.