A method and device for recovering heat energy by using a bipyridinium salt flow battery

Abstract

The application discloses a method and device for realizing heat energy recovery by using a bipyridine salt liquid flow battery; the device is provided with end plates, gaskets, current collectors and graphite felt structures which are symmetrically arranged from outside to inside at two ends of a middle interlayer; the graphite felt structure is a plate provided with through holes, and the through holes are filled with graphite felt; the outer sides of the two end plates are respectively provided with storage tanks; the two storage tanks are respectively provided with positive electrolyte and negative electrolyte, and the negative electrolyte active material is N, N'-dicarboxypropyl-4, 4'-bipyridine salt; each storage tank is connected with the graphite felt through a circulating loop formed between the electrolyte circulating pipeline and the graphite felt; each storage tank is further connected with a heat exchanger through a heat exchange pipe; and the heat exchanger is connected with a heat source. The application uses a thermoelectric material as a negative active material, converts heat energy into electric energy through a low-temperature charging and high-temperature discharging process, and realizes low-cost and high-efficiency heat energy recovery.

Description

TECHNICAL FIELD

[0001] The present application relates to heat energy recovery, and more particularly to a method and device for heat energy recovery using a bipyridinium salt flow battery. BACKGROUND

[0002] Low-grade heat energy is widely present in nature and industrial production and life, and its distribution presents the characteristics of universality, dispersion, volatility, low grade and regionality. Compared with traditional temperature difference power generation technology (such as thermoelectric materials), certain chemical energy conversion systems (such as thermochemical batteries, redox flow batteries, etc.) have significant advantages, and can theoretically achieve higher energy conversion efficiency, especially under low temperature difference conditions. This is because the driving force of chemical reaction depends not only on the temperature difference, but also on the Gibbs free energy change of chemical substances.

[0003] At present, a variety of devices can directly convert low-grade heat energy into electrical energy, including thermo-electrochemical cells (TGC) and thermal regeneration electrochemical cycles (TREC). TGC generates a voltage difference at different temperatures through a single liquid aqueous redox couple, which has attracted attention due to its high Seebeck coefficient and low cost. TREC uses the voltage difference generated by the positive and negative redox couples at different temperatures to convert low-grade heat energy into electrical energy, and can simultaneously realize energy storage and heat-electricity conversion. However, the above two direct heat-electricity conversion technologies are not enough to drive portable devices, and the system efficiency is low.

[0004] In the prior art, Battistel et al. (Reynard D, Dennison C R, Battistel A, et al. Efficiency improvement of an all-vanadium redox flow battery by harvesting low-grade heat [J]. Journal of Power Sources, 2018, 390: 30-37.) found a temperature effect in the redox couple of the all-vanadium redox flow battery (VFB), which promoted the development of TREC-VFB. The positive electrode of the all-vanadium redox flow battery is composed of VO 2+ / VO2 couple, and the negative electrode is composed of V 2+ / 3+ / VO2 couple. By measuring the formal potential of the couple at different temperatures, the negative temperature coefficient is -1.013 mV / K, and the coulombic efficiency of charging and discharging is only 94%, indicating that there is self-discharge of the positive and negative electrodes. After the thermal regeneration cycle, the discharge energy density is increased by 11% (compared with charging and discharging at 20℃). However, this technology is an inorganic system, which is single in structure and difficult to control, and depends on metal resources. SUMMARY

[0005] The application provides a method and device for recovering heat energy by using a bipyridine salt flow battery as a negative active material, converting heat energy into electric energy through a low-temperature charging and high-temperature discharging process, and realizing low-cost and efficient heat energy recovery.

[0006] The application achieves the purpose by the following technical solutions.

[0007] A device for recovering heat energy by using a bipyridine salt flow battery; comprising an electric pile, a storage tank, a diaphragm pump, a heat exchanger and a heat source; the electric pile comprises a diaphragm, an end plate, a gasket, a current collector and a graphite felt structure; the diaphragm is provided with the end plate, the gasket, the current collector and the graphite felt structure in the middle from outside to inside in a symmetrical manner; the graphite felt structure is a plate material provided with a through hole, and the through hole is provided with the graphite felt; the outer sides of the two end plates are respectively provided with the storage tank; the two storage tanks are respectively provided with a positive electrolyte and a negative electrolyte, and the negative electrolyte active material is N,N'-dicarboxypropyl-4,4'-bipyridine salt; each storage tank is connected with the graphite felt through an electrolyte circulation pipeline to form a circulation loop; each storage tank is further connected with the heat exchanger through a heat exchange pipe to form a circulation loop; and the heat exchanger is connected with the heat source.

[0008] To further achieve the purpose of the application, preferably, the plastic end plate, the gasket and the current collector are all plate structures.

[0009] Preferably, the through hole is arranged in the middle of the gasket on the two sides of the diaphragm; the through hole and the graphite felt are square or circular.

[0010] Preferably, the through hole and the graphite felt are square with a size of 2-100cm*2-100cm or circular with a diameter of 2-100cm; and the thickness of the through hole and the graphite felt is 2-6mm.

[0011] Preferably, the electrolyte circulation pipeline is connected with the graphite felt in the graphite felt structure through the metal end plate, the plastic end plate, the gasket and the current collector.

[0012] The electrolyte circulation pipeline is provided with a diaphragm pump.

[0013] The two storage tanks are connected with the same heat exchanger through the heat exchange pipes, or the two storage tanks are connected with different heat exchangers through the heat exchange pipes.

[0014] Preferably, the end plate is composed of a metal end plate and a plastic end plate; the metal end plate is arranged outside the plastic end plate; the metal of the metal end plate is titanium, iron, copper, or aluminum alloy; the plastic of the plastic end plate is selected from polytetrafluoroethylene; the material of the gasket is selected from silica gel or fluorine glue; the diaphragm is selected from a Daramic diaphragm, a CSO diaphragm, a Nafion diaphragm, or a Nafion modified diaphragm; the current collector is selected from a graphite carbon felt, a conductive carbon layer, or a metal current collector; and the heat source is water heated by solar heat, industrial waste heat, or geothermal heat, and the temperature of the water is less than 100°C.

[0015] A method for recovering heat energy by using the redox flow battery of the above device: the positive electrolyte and the negative electrolyte are respectively added to two different storage tanks; the positive electrolyte and the negative electrolyte are pumped into the graphite felt of the battery on both sides of the diaphragm through the electrolyte circulation pipeline and the diaphragm pump, and charging is performed at room temperature through the current collector; the heat source exchanges heat with the heat exchanger, and the negative electrolyte and the positive electrolyte from the storage tank are heated; the negative electrolyte and the positive electrolyte are heated while circulating between the storage tank and the graphite felt; after being heated to 30-70°C, high-temperature discharging is performed through the current collector, and an electric current is output.

[0016] The positive electrolyte is composed of a positive active material and a supporting electrolyte; the negative electrolyte is composed of a negative active material and a supporting electrolyte; and the negative active material is N,N'-dicarboxypropyl-4,4'-bipyridinium salt.

[0017] Preferably, the concentration of the positive active material is 0.01M-1.5M, the concentration of the supporting electrolyte in the positive electrolyte is 0.5-6M; the concentration of the negative active material is 0.01M-1.5M, and the concentration of the supporting electrolyte in the negative active material is 0.5-6M.

[0018] Preferably, the positive active material is a halide salt, an iron salt, a TEMPO derivative, or a ferrocyanide.

[0019] The supporting electrolyte in the positive electrolyte and the negative electrolyte is an electrolyte composed of a cation and an anion; the cation is selected from potassium ion, sodium ion, ammonium ion, and lithium ion; and the anion is selected from halide ion, nitrate ion, sulfate ion, and oxalate ion.

[0020] Preferably, the halide salt is potassium iodide, cesium iodide, sodium iodide, or ammonium iodide.

[0021] The supporting electrolyte in the positive electrolyte and the negative electrolyte is potassium chloride, potassium oxalate, potassium sulfate, potassium nitrate, sodium chloride, sodium oxalate, sodium sulfate, sodium nitrate, ammonium chloride, ammonium oxalate, ammonium sulfate, ammonium nitrate, lithium chloride, lithium oxalate, lithium sulfate, lithium nitrate, potassium bromide, or sodium bromide.

[0022] Compared with the prior art, the application has the following advantages and beneficial effects:

[0023] 1) The bipyridine salt in the application not only has good solubility, but also has excellent electrochemical stability, high temperature coefficient and significant potential change affected by temperature.

[0024] 2) The bipyridine salt negative electrolyte in the application has the function of improving the energy efficiency of the system by using recovered low-grade heat energy, thereby improving the energy utilization efficiency.

[0025] 3) The application can be used as a liquid flow battery negative electrolyte, has stable circulation, few side reactions, and good charge and discharge capacity. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The structure of the device for realizing heat energy recovery by using the bipyridine salt liquid flow battery of the application.

[0027] Figure 2 The cycle charge and discharge graph of 11 mol / L N,N'-dicarboxypropyl-4,4'-bipyridine salt applied to the liquid flow battery in the example.

[0028] Figure 3 The charge and discharge graph of 11 mol / L N,N'-dicarboxypropyl-4,4'-bipyridine salt applied to the thermal liquid flow battery at different temperatures in the example.

[0029] Figure 4 The charge and discharge curve and efficiency graph of 1 mol / L N,N'-dicarboxypropyl-4,4'-bipyridine salt applied to the low-temperature charging and high-temperature discharging liquid flow battery in Example 1.

[0030] Figure 5 The charge and discharge graph of 0.1 mol / L N,N'-dicarboxypropyl-4,4'-bipyridine salt applied to the thermal liquid flow battery at different temperatures in Example 2.

[0031] Figure 6 The charge and discharge curve and efficiency graph of 0.1 mol / L N,N'-dicarboxypropyl-4,4'-bipyridine salt applied to the low-temperature charging and high-temperature discharging liquid flow battery in Example 2.

[0032] Figure 7 The charge and discharge graph of 0.5 mol / L N,N'-dicarboxypropyl-4,4'-bipyridine salt applied to the thermal liquid flow battery at different temperatures in Example 3.

[0033] Figure 8 The charge and discharge curve and efficiency graph of 0.5 mol / L N,N'-dicarboxypropyl-4,4'-bipyridine salt applied to the low-temperature charging and high-temperature discharging liquid flow battery in Example 3.

[0034] Figure 9 Cyclic voltammogram of N,N'-dicarboxypropyl-4,4'-bipyridinium salt in Example 5 with 0.5 M potassium chloride as supporting electrolyte

[0035] Figure 10 Temperature coefficient fitting plot of different pyridine salt molecules in Example 5.

[0036] Figure 11 Bar chart of temperature coefficient of N,N'-dicarboxypropyl-4,4'-bipyridinium salt with different counterions in Example 5. DETAILED DESCRIPTION

[0037] In order to more clearly illustrate the technical solutions of the present application, the present application is further described below in combination with the drawings and examples.

[0038] In the field of thermal energy recovery, the existing technology of all-vanadium redox flow battery inorganic system has the problems of single structure and difficulty in regulation, and relies on metal resources. Organic oxidation active material is composed of inorganic elements with high abundance, has the advantages of adjustable structure and high solubility, and the development of organic material redox flow battery is the development direction.

[0039] Among organic materials, the core structure of bipyridine compounds (also known as viologen compounds) is formed by connecting two pyridine rings through a carbon atom. This class of compounds can regulate their properties and functions by introducing different substituents (such as alkyl, aryl, etc.) on the nitrogen atoms of bipyridine. This class of compounds is mainly used in the fields of catalysts and ligands, luminescent materials, analytical chemistry, redox flow batteries and organic synthesis. The pyridine ring provides electron deficiency, which makes it exhibit good redox capacity in electrochemical reactions. This feature makes bipyridine compounds have high solubility and strong redox capacity, and can be used as negative electrode materials for organic redox flow batteries. However, the current research on redox flow batteries mainly focuses on improving the stability, solubility and other electrochemical properties of redox couples, and there is no existing technology that focuses on the application of viologen compounds in the field of thermal energy recovery.

[0040] Viologen molecules have significant thermal response characteristics, and their application potential as new thermal energy recovery electrode materials has been verified through tests.

[0041] The structural formula of N,N'-dicarboxypropyl-4,4'-bipyridinium salt is However, the existing technology of this compound is mainly used for preparing photochromic semiconductors. There is a nitrogen cation on the pyridine ring of N,N'-dicarboxypropyl-4,4'-bipyridinium salt. This structural feature endows the compound with unique redox properties, and the chemical reaction equation of electron gain and loss is as follows:

[0042]

[0043] When N,N'-dicarboxypropyl-4,4'-bipyridine salt gets an electron, a cation radical will be formed, and the original carbon-carbon double bond is composed of a sigma bond and a pi bond. When an extra electron enters the conjugated system, the electron cloud density of the pi bond increases. The electrons are distributed between the conjugated structures, so that the electron cloud density of the whole molecule is balanced.

[0044] The present application finds that N,N'-dicarboxypropyl-4,4'-bipyridine salt has a high temperature coefficient, and the temperature coefficient of the compound is mainly due to the dimerization phenomenon of its reduced state. By measuring its dimerization equilibrium constant, compared with other bipyridine molecules, N,N'-dicarboxypropyl-4,4'-bipyridine salt has a stronger dimerization equilibrium constant, which is the advantage that makes it exhibit a high temperature coefficient.

[0045] It is worth emphasizing that by applying N,N'-dicarboxypropyl-4,4'-bipyridine salt as a negative electrolyte active material, the present application realizes a breakthrough in the field of electrolyte systems, and for the first time achieves ionic regulation. Tests show that the electric pair of the bipyridine salt flow battery applied in the present application exhibits a temperature coefficient as high as -0.89 mV / K.

[0046] Redox flow batteries have the ability to store thermal energy in the form of chemical bond energy, realizing the storage and on-demand release of energy. The present application integrates heat recovery and power output in a redox flow battery, and converts heat into electrical energy during the discharging process, which can effectively alleviate the problems of volatility, intermittency and low direct utilization efficiency of low-grade thermal energy, thereby improving the stability and reliability of energy supply. Compared with other devices in the prior art that directly convert thermal energy into electrical energy, the present application provides continuous power output, pipe heat recovery and energy decoupling properties.

[0047] On the basis of the above mechanism, the present application provides a device for recovering thermal energy by using a bipyridine salt flow battery; comprising an electric pile, a storage tank, a diaphragm pump, a heat exchanger and a heat source; the electric pile comprises a diaphragm, an end plate, a gasket, a current collector and a graphite felt structure; the diaphragm is arranged in the middle, and the end plate, the gasket, the current collector and the graphite felt structure are symmetrically arranged from outside to inside at both ends of the diaphragm; the graphite felt structure is a plate material provided with through holes, and the through holes are filled with graphite felt; the outer sides of the two end plates are respectively provided with storage tanks; the two storage tanks are respectively provided with positive electrolyte and negative electrolyte, and the negative electrolyte active material is N,N'-dicarboxypropyl-4,4'-bipyridine salt; each storage tank is connected with the graphite felt through a circulating loop formed between the electrolyte circulating pipeline and the graphite felt; each storage tank is also connected with the heat exchanger through a circulating loop formed by a heat exchange pipe; and the heat exchanger is connected with the heat source.

[0048] The technology is based on using N,N'-dicarboxypropyl-4,4'-bipyridine salt as a negative electrolyte active material, and in combination with a stack structure, a stack is developed to cooperate with a heat exchanger and a heat source to realize heat energy recovery. The shape and size of the through hole and the graphite felt in the device can be adjusted according to the application needs, and the through hole and the graphite felt are preferably square with a size of 2-100cm*2-100cm or circular with a diameter of 2-100cm; the thickness of the through hole and the graphite felt is 2-6mm. In order to facilitate installation, the plastic end plate, the gasket and the current collector are preferably plate structures.

[0049] In the present application, the electrolyte circulation pipeline can be directly connected to the graphite felt from the outside, and the electrolyte circulation pipeline is preferably connected to the graphite felt in the graphite felt structure through the metal end plate, the plastic end plate, the gasket and the current collector. A diaphragm pump is installed on the electrolyte circulation pipeline for better circulation.

[0050] The present application can use one heat exchanger or two heat exchangers. Preferably, two storage tanks are connected to the same heat exchanger through heat exchange pipes, or two storage tanks are connected to different heat exchangers through heat exchange pipes. The connection mode of the storage tank has certain flexibility, which can be connected to two different storage tanks through the pipeline to independently perform heat exchange operation; or two storage tanks can be heated through the heat exchange pipe 11 connected to the same heat exchanger, thereby meeting different operation requirements and improving the operability and applicability of the system.

[0051] In the present application, the end plate can be one, two or even more, preferably two, and the end plate is composed of a metal end plate and a plastic end plate; the metal end plate is arranged on the outside of the plastic end plate. The metal of the metal end plate is titanium, iron, copper or aluminum alloy; the plastic of the plastic end plate is polytetrafluoroethylene; the main function of the gasket is to prevent the electrolyte from leaking to ensure the sealing and safety of the system; the material of the gasket is selected from silicone or fluorine glue; the diaphragm is selected from Daramic diaphragm, CSO film, Nafion film or Nafion modified film; the current collector is selected from graphite carbon felt, conductive carbon layer or metal current collector; the heat source is low-grade position heat energy involved in the present application, and the heat source is preferably water heated by sunlight, industrial waste heat and geothermal heat with a temperature less than 100℃.

[0052] The method for realizing heat energy recovery by using the bipyridine salt liquid flow battery of the above device is as follows: the positive electrolyte and the negative electrolyte are respectively added into two different storage tanks, the electrolyte circulation pipeline and the diaphragm pump are used to pump the positive electrolyte and the negative electrolyte into the graphite felt of the stack on both sides of the diaphragm, and the graphite felt is charged at room temperature through the current collector; the heat source exchanges heat with the heat exchanger to heat the negative electrolyte and the positive electrolyte from the storage tank, the negative electrolyte and the positive electrolyte are circulated between the storage tank and the graphite felt at the same time, and the negative electrolyte and the positive electrolyte are heated to 30-70℃ and then discharged at high temperature through the current collector to output current.

[0053] The positive electrolyte is composed of positive active material and supporting electrolyte; the negative electrolyte is composed of negative active material and supporting electrolyte; the negative active material is N,N'-dicarboxypropyl-4,4'-bipyridinium salt.

[0054] The concentration of the positive active material is preferably 0.01M-1.5M, and the concentration of the supporting electrolyte in the positive electrolyte is 0.5-6M; the concentration of the negative active material is 0.01M-1.5M, and the concentration of the supporting electrolyte in the negative active material is 0.5-6M. The positive active material is preferably halide salt, iron salt, TEMPO derivative or ferrocyanide.

[0055] The supporting electrolyte in the positive electrolyte and the negative electrolyte is preferably composed of cation and anion; the cation is selected from potassium ion, sodium ion, ammonium ion and lithium ion; the anion is selected from halogen ion, nitrate ion, sulfate ion and oxalate ion.

[0056] The halide salt is preferably potassium iodide, cesium iodide, sodium iodide or ammonium iodide; the supporting electrolyte in the positive electrolyte and the negative electrolyte is preferably potassium chloride, potassium oxalate, potassium sulfate, potassium nitrate, sodium chloride, sodium oxalate, sodium sulfate, sodium nitrate, ammonium chloride, ammonium oxalate, ammonium sulfate, ammonium nitrate, lithium chloride, lithium oxalate, lithium sulfate, lithium nitrate, potassium bromide or sodium bromide.

[0057] As Figure 1As shown, a device for recovering heat energy by using a bipyridinium salt flow battery; including an electric pile 10, positive and negative electrolyte, storage tank 6, electrolyte circulating pipeline 9, diaphragm pump 8, heat exchange pipe 11, heat exchanger 7 and heat source 12; the electric pile 10 is mainly composed of diaphragm, metal end plate 1, plastic end plate 2, gasket 3, current collector 4 and graphite felt structure 5; the diaphragm is provided in the middle, and the metal end plate 1, the plastic end plate 2, the gasket 3, the current collector 4 and the graphite felt structure 5 are symmetrically arranged from outside to inside; the metal end plate 1, the plastic end plate 2, the gasket 3, the current collector 4 and the graphite felt structure 5 are preferably plate structures, the graphite felt structure 5 is preferably provided with a through hole in the plate, and the through hole is provided with graphite felt; further preferably, the through hole is arranged in the middle of the plate; the through hole and the graphite felt are square with a size of 2.5cm*2.5cm, and the plate is preferably made of plastic material. The outer sides of the two metal end plates 1 are respectively provided with storage tanks 6; the two storage tanks 6 are respectively provided with positive and negative electrolytes; each storage tank 6 is connected with the graphite felt in the graphite felt structure 5 through 2 electrolyte circulating pipelines 9 arranged at intervals; each storage tank 6 is also connected with the heat exchanger 7 through 2 heat exchange pipes 11; the heat exchanger 7 is connected with the heat source 12; one of the 2 electrolyte circulating pipelines 9 connected with one storage tank 6 is provided with a diaphragm pump 8; preferably, the electrolyte circulating pipeline 9 passes through the metal end plate 1, the plastic end plate 2, the gasket 3, the current collector 4 and the graphite felt in the graphite felt structure 5. The symmetric components of the electric pile 10 are arranged and fixed by bolts at the two metal end plates 1. The storage tank 6 is preferably a jacket structure, such as a jacket beaker, which is convenient for exchanging heat with the outside.

[0058] Example 1

[0059] Utilize Figure 1 The device, a method for recovering heat energy by using a bipyridinium salt flow battery, comprises the following steps:

[0060] 1) Configuration of bipyridinium salt flow battery:

[0061] Preparation of positive electrolyte: mix the solutions of positive active material potassium iodide and positive supporting electrolyte potassium chloride configured respectively to form 12ml positive electrolyte, control the concentration of positive active material to be 3M, and the concentration of positive supporting electrolyte to be 2M.

[0062] Preparation of negative electrolyte: mix the solutions of negative electrolyte active material N,N'-dicarboxypropyl-4,4'-bipyridinium salt and negative supporting electrolyte potassium oxalate configured respectively to obtain 6ml negative electrolyte, wherein the concentration of negative electrolyte active material is 1M, and the concentration of negative supporting electrolyte is 4M.

[0063] 2) Operation of bipyridine salt flow battery: The positive electrolyte and negative electrolyte are added to two different storage tanks 6 respectively. The positive electrolyte and negative electrolyte are pumped into the graphite felt structure 5 of the two stacks 10 on both sides of the diaphragm using the electrolyte circulation pipeline 9 and the diaphragm pump 8 respectively. The current collector 4 is a graphite plate. The flow rates of the positive electrolyte and negative electrolyte are equal.

[0064] The entire bipyridine salt flow battery was subjected to the following flow battery cycle test and heat recovery under a nitrogen atmosphere.

[0065] The battery testing system's test clips are clamped onto the current collectors 4 of the positive and negative terminals respectively. The cyclic curve test consists of the following steps: rest for 5 seconds, constant current and constant voltage charging, rest for 5 seconds, and constant current and constant voltage discharging. The constant current and constant voltage charging is set according to the following parameters: charging current density is 20 mA / cm². 2 The charging cutoff voltage is 1.05V, and the cutoff current is 10mA. That is, when charging to 1.05V with a constant current, the constant voltage charging will continue until the current drops below 10mA, at which point this step ends and proceeds to the next step. After charging is complete, allow 5 seconds for rest, then perform constant current and constant voltage discharge according to the following parameters: discharge current density is 20mA / cm³. 2 The discharge cutoff voltage is 0.45V and the cutoff current is 10mA. That is, when the constant current charging voltage discharges to 0.45V, the constant voltage discharges to 0.45V until the current is below 10mA, and the process ends. Four processes are completed to form one cycle, and the next cycle continues to repeat the first process. The number of cycles is 50.

[0066] Record the charging capacity and voltage for each charge cycle, and the discharging capacity and voltage. Coulombic efficiency is the ratio of discharged capacity to charged capacity, reflecting charge utilization. Voltage efficiency is the ratio of average discharge voltage to average charge voltage, reflecting energy conversion efficiency considering polarization. The battery's energy efficiency is the product of coulombic efficiency and voltage efficiency. These factors comprehensively evaluate battery performance and are plotted as follows: Figure 2 The curve, Figure 2 The capacity-voltage curve of the battery is shown. According to the test data, the initial discharge capacity is 125.4mAh. After 80 charge-discharge cycles, the coulombic efficiency remains above 99.6%, and the energy efficiency remains at 76%. The discharge capacity on the 80th cycle is 117.7mAh, with a capacity retention rate of 93.9% and a capacity decay rate of 0.076% per cycle.

[0067] 3) Heat recovery: In this embodiment, the heat source 12 is water at 66°C. The heat source 12 exchanges heat with the heat exchanger 7 to heat the negative or positive electrolyte from the storage tank 6 (jacketed beaker). Simultaneously, the heating of the negative or positive electrolyte continuously circulates between the storage tank 6 and the graphite felt. When the negative or positive electrolyte is at 25°C, the charging current density is set to 20 mA / cm².2 , the charge cut-off voltage is 1.05 V, and the cut-off current is 10 mA; after the charging is completed, heat exchange is performed through the heat source 12 and the heat exchanger 7, and when the temperature of the negative electrolyte or the positive electrolyte is stabilized at 60°C, constant-current constant-voltage discharging is performed, and the discharging current density is controlled to be 20 mA / cm 2 , the discharging cut-off voltage is 0.45 V, and the cut-off current is 10 mA. The charging capacity and voltage and the discharging capacity and voltage are recorded, and a Figure 3 curve is drawn, and the three graphs show the capacity-voltage curve of the bipyridinium salt flow battery.

[0068] According to the measurement results, the energy loss of the N,N'-dicarboxypropyl-4,4'-bipyridinium salt / potassium iodide flow battery of the present embodiment is 21.9 mWh when charged at 25°C and discharged at 25°C, and is 11.9 mWh when charged at 25°C and discharged at 60°C. The reduced heat loss energy is the combined result of the conversion of heat into work and the reduction of internal resistance loss, and by calculation, the energy converted from heat to electricity accounts for 4.6% of the discharging energy.

[0069] As shown in Figure 4 , when tested in the charging and discharging mode of charging at 25°C and discharging at 60°C, the Coulombic efficiency of the battery is 98.32%, the voltage efficiency reaches 92.36%, and the energy efficiency reaches 90.81%, and at this high concentration, the performance of the battery is excellent.

[0070] The prior art document "A Sulfonate-Functionalized Viologen Enabling Neutral Cation Exchange, Aqueous Organic Redox Flow Batteries toward Renewable Energy Storage" discloses a sulfonate-functionalized viologen molecule 1,1'-bis(3-sulfopropyl)-4,4'-bipyridinium salt and potassium iodide to form a flow battery (hereinafter referred to as "comparative document viologen"), and is used in a neutral cation exchange aqueous organic redox flow battery. The positive active material used in the prior art document is consistent with the system of the present application.

[0071] Using the same test method as in the present embodiment, the temperature coefficient of the viologen of the prior art document is less than 0.4 mV / K, and the voltage of the flow battery formed is 1.0 V. A 300-cycle test is performed at a current density of 60 mA / cm 2 , using 0.5 M of the comparative document viologen as the active material, and the battery capacity retention rate is 94.1% (the average capacity retention rate per cycle is 99.99%), and the average energy efficiency is 58%. When the concentration of the viologen molecule is 1 M, the energy efficiency is 76%.

[0072] It can be seen that the viologen of the present embodiment has a significantly higher temperature coefficient (higher than 0.9 mV / K). By introducing carboxyl groups and improving the dimerization constant, the present application has achieved a significant improvement in the temperature coefficient. More importantly, this feature enables the flow battery containing the viologen of the present application not only to have higher energy efficiency during conventional charging and discharging, but also to further improve the overall energy output of the electrical system by recovering heat energy.

[0073] Example 2

[0074] Utilizing Figure 1 The device, a method for recovering heat energy by using a bipyridinium salt flow battery, comprises the following steps:

[0075] 1) Configuration of the bipyridinium salt flow battery:

[0076] Preparation of the positive electrolyte: mix the solutions of the positive active material potassium iodide and the positive supporting electrolyte potassium chloride to form 12 ml of the positive electrolyte, and control the concentration of the positive active material to be 0.5 M and the concentration of the positive supporting electrolyte to be 0.5 M.

[0077] Preparation of the negative electrolyte: mix the solutions of the negative electrolyte active material N,N'-dicarboxypropyl-4,4'-bipyridinium salt and the negative supporting electrolyte potassium oxalate to obtain 6 ml of the negative electrolyte, wherein the concentration of the negative electrolyte active material is 0.1 M and the concentration of the negative supporting electrolyte is 0.5 M.

[0078] 2) Operation of the bipyridinium salt flow battery: add the positive electrolyte and the negative electrolyte into two different storage tanks 6, respectively, and use the electrolyte circulation pipeline 9 and the diaphragm pump 8 to pump the positive electrolyte and the negative electrolyte into the graphite felt of the graphite felt structure 5 of the two stacks 10 on both sides of the diaphragm, the current collector 4 uses high-purity titanium sheet, and the flow rates of the positive electrolyte and the negative electrolyte pumped in are equal.

[0079] The entire bipyridinium salt flow battery is subjected to the following flow battery circulation test and heat energy recovery in a nitrogen atmosphere.

[0080] 3) Heat energy recovery: in the present embodiment, the heat source 12 is derived from solar energy, the heat source 12 exchanges heat with the heat exchanger 7, the negative electrolyte or the positive electrolyte from the storage tank 6 (jacketed beaker) is heated, and at the same time, the negative electrolyte or the positive electrolyte is circulated between the storage tank 6 and the graphite felt. When the negative electrolyte or the positive electrolyte is at 25℃, the charging current density is set to 20 mA / cm 2, the charge cut-off voltage is 1.05 V, and the cut-off current is 10 mA; after the charging is completed, heat exchange is performed through the heat source 12 and the heat exchanger 7, and when the temperature of the negative electrolyte or the positive electrolyte is stabilized at 60°C, constant-current constant-voltage discharging is performed, and the discharging current density is controlled to be 20 mA / cm 2 , the discharge cut-off voltage is 0.45 V, and the cut-off current is 10 mA. The charging capacity and voltage and the discharging capacity and voltage are recorded, and a curve is drawn. Figure 5

[0081] Figure 5 The battery performance test results of the battery at different temperatures are shown. According to the measurement results, the energy loss of the N,N'-dicarboxypropyl-4,4'-bipyridinium salt | potassium iodide flow battery when charging and discharging at 25°C is 1.8 mWh, and the energy loss when charging at 25°C and discharging at 60°C is 1.1 mWh. The reduced heat loss energy is the result of the combined action of heat conversion into work and the reduction of internal resistance loss, and the energy converted from heat into electricity accounts for 3.9% of the discharging energy by calculation.

[0082] Figure 6 The Coulomb efficiency of the battery is 99.98%, the voltage efficiency reaches 92.27%, and the energy efficiency reaches 92.25% when charging at 25°C and discharging at 60°C, and the performance of the battery is excellent at this low concentration.

[0083] Example 3

[0084] Using Figure 1 The device, a method for recovering heat energy using a bipyridinium salt flow battery, comprises the following steps:

[0085] 1) Configuration of the bipyridinium salt flow battery:

[0086] Preparation of the positive electrolyte: mix the solutions of the positive active material potassium iodide and the positive supporting electrolyte potassium chloride configured respectively to form 12 ml of the positive electrolyte, and control the concentration of the positive active material to be 2M and the concentration of the positive supporting electrolyte to be 0.5M.

[0087] Preparation of the negative electrolyte: mix the solutions of the negative electrolyte active material N,N'-dicarboxypropyl-4,4'-bipyridinium salt and the negative supporting electrolyte potassium oxalate configured respectively to obtain 6 ml of the negative electrolyte, and control the concentration of the negative electrolyte active material to be 0.5M and the concentration of the negative supporting electrolyte to be 0.5M.

[0088] ​2) Bipyrithium salt flow battery operation: the positive electrolyte and the negative electrolyte are added into two different storage tanks 6, and the positive electrolyte and the negative electrolyte are pumped into the graphite felt of the graphite felt structure 5 of the two stacks 10 on both sides of the separator by using the electrolyte circulation pipeline 9 and the diaphragm pump 8, and the current collector 4 adopts graphite plate, and the flow rates of the positive electrolyte and the negative electrolyte pumped in are equal.

[0089] The whole bipyrithium salt flow battery is subjected to the following flow battery cycle test and heat energy recovery in a nitrogen atmosphere.

[0090] 3) Heat energy recovery: in this embodiment, the heat source 12 is derived from flue gas waste heat, and the heat source 12 exchanges heat with the heat exchanger 7 to heat the negative electrolyte or the positive electrolyte from the storage tank 6 (jacketed beaker), while the negative electrolyte or the positive electrolyte is heated and circulated between the storage tank 6 and the graphite felt. When the negative electrolyte or the positive electrolyte is at 25℃, the charging current density is set to 20 mA / cm 2 , the charging cutoff voltage is 1.05 V, and the cutoff current is 10 mA; after the charging is completed, the heat source 12 exchanges heat with the heat exchanger 7, and when the temperature of the negative electrolyte or the positive electrolyte stabilizes at 60℃, constant-current constant-voltage discharge is carried out, the discharge current density is controlled to be 20 mA / cm 2 , the discharge cutoff voltage is 0.45 V, and the cutoff current is 10 mA. The charging capacity and voltage and the discharging capacity and voltage are recorded to draw a Figure 7 curve.

[0091] Figure 7 The battery performance test results of the battery at different temperatures are shown. According to the measurement results, the energy loss of the N,N'-dicarboxypropyl-4,4'-bipyridinium salt / potassium iodide flow battery when charged and discharged at 25℃ is 7.7 mWh, and the energy loss when charged at 25℃ and discharged at 60℃ is 3.5 mWh. The reduced heat loss energy is the combined result of the conversion of heat into work and the reduction of internal resistance loss, and the energy converted from heat into electricity accounts for 5.5% of the discharging energy by calculation.

[0092] Figure 8 The Coulomb efficiency of the battery is 98.27%, the voltage efficiency reaches 91.39%, and the energy efficiency reaches 89.81% when the battery is charged at 25℃ and discharged at 60℃, and the heat recovery efficiency of the battery at this concentration is excellent.

[0093] The separators described in embodiment 1, embodiment 2, and embodiment 3 all use the modified membrane formed by coating PVDF@carbon black on Nafion 115.

[0094] Embodiment 4

[0095] The Figure 1The device is a method for recovering heat energy by using a bipyridinium salt flow battery, comprising the following steps:

[0096] 1) Configuration of the bipyridinium salt flow battery:

[0097] Preparation of the positive electrolyte: mix the solutions of the positive active material 4-hydroxy TEMPO and the positive supporting electrolyte potassium chloride respectively to form 12 ml of the positive electrolyte, and control the concentration of the positive active material to be 0.1 M and the concentration of the positive supporting electrolyte to be 0.5 M.

[0098] Preparation of the negative electrolyte: mix the solutions of the negative electrolyte active material N,N'-dicarboxypropyl-4,4'-bipyridinium salt and the negative supporting electrolyte potassium oxalate respectively to obtain 6 ml of the negative electrolyte, wherein the concentration of the negative electrolyte active material is 0.1 M and the concentration of the negative supporting electrolyte is 0.5 M.

[0099] 2) Operation of the bipyridinium salt flow battery: add the positive electrolyte and the negative electrolyte into two different storage tanks 6 respectively, and use the electrolyte circulation pipeline 9 and the diaphragm pump 8 to pump the positive electrolyte and the negative electrolyte into the graphite felt of the graphite felt structure 5 of the two electrode stacks 10 on both sides of the diaphragm, the current collector 4 adopts a graphite plate, the diaphragm adopts an anion exchange membrane, and the flow rates of the positive electrolyte and the negative electrolyte pumped in are equal.

[0100] Example 5

[0101] Temperature coefficient test of N,N'-dicarboxypropyl-4,4'-bipyridinium salt under different counterions:

[0102] In an electrochemical system, the mutual conversion of chemical energy and electrical energy is realized through electrode reactions. The essence of electrode reactions is redox reactions, and their characteristics are determined by the redox couple involved in the reactions. The temperature coefficient is an index for measuring the change in the equilibrium electrode potential of the redox couple per unit temperature change (e.g., per Kelvin). Its sign indicates the direction of the change in the electrode potential with temperature, and the greater the absolute value, the more sensitive the electrode potential is to temperature changes, i.e., the greater the change in open-circuit voltage per unit temperature change. Its expression is as follows:

[0103]

[0104] The measurement of temperature coefficient was performed by cyclic voltammetry (CV) test. First, 4 mM of N,N'-dicarboxypropyl-4,4'-bipyridinium salt was dissolved in 0.5 mM of different supporting electrolyte ions solution, and the prepared solution was added into a jacketed beaker. This method used a three-electrode system, including a glassy carbon electrode as the working electrode, a silver-silver chloride electrode as the reference electrode, and a platinum electrode as the auxiliary electrode, to achieve accurate measurement of current and potential. The cyclic voltammogram curves were measured at different temperatures using a three-electrode system. After reading the formal potential of the cyclic voltammogram, linear fitting was performed to obtain the temperature coefficient.

[0105] Figure 9 The cyclic voltammogram curves of N,N'-dicarboxypropyl-4,4'-bipyridinium salt at different temperatures were shown. The voltammogram curves obtained by cyclic voltammetry can be used to determine the formal potential of N,N'-dicarboxypropyl-4,4'-bipyridinium salt at different temperatures. With the change of temperature, the cyclic voltammogram curve showed a negative moving trend, and the formal potential also moved towards the negative potential direction, indicating that with the increase of temperature, the redox potential of N,N'-dicarboxypropyl-4,4'-bipyridinium salt moved negatively.

[0106] Similarly, the temperature coefficients of various viologen derivatives were measured under the condition of 0.5 M KCl supporting electrolyte. First, for the basic bipyridinium salt, the substituents on the two nitrogen atoms are methyl and ethyl, and the temperature coefficient of methyl viologen is -0.5 mV / K. According to the literature, the temperature coefficient of ethyl viologen is -0.3 mv / K [Direct Conversion of Phase-Transition Entropy into Electrochemical Thermopower and the Peltier Effect], based on the mechanism of temperature coefficient generation, the molecule with carboxylate group introduced on the side chain of viologen has a higher temperature coefficient. The temperature coefficients of such substances are as follows: Figure 10As shown, the temperature coefficient of N,N'-dicarboxyethyl-4,4'-bipyridinium ((CEt)2V) is -0.68 mV / K, the temperature coefficient of N,N'-dicarboxybutyl-4,4'-bipyridinium ((CBu)2V) is -0.45 mV / K, and the temperature coefficient of N,N'-dicarboxypropyl-4,4'-bipyridinium ((CPr)2V) is -0.85 mV / K. Although N,N'-dicarboxyethyl-4,4'-bipyridinium and N,N'-dicarboxybutyl-4,4'-bipyridinium also have temperature coefficients, the solubility of their one-electron reduced states is low, and they are prone to precipitate during the charging and discharging process of the thermoelectrochemical cell, which can cause the flow pipeline to be blocked, thereby reducing the stability of the cell and the thermal energy conversion efficiency. In contrast, N,N'-dicarboxypropyl-4,4'-bipyridinium not only has a higher absolute value of the temperature coefficient, but also has a higher solubility, and therefore, this molecule is more suitable as a negative active material for a thermal conversion flow battery.

[0107] Figure 11 The effect of different counterions on the temperature coefficient of N,N'-dicarboxypropyl-4,4'-bipyridinium is shown. Under a variety of supporting electrolytes, this compound exhibits a high absolute value of the temperature coefficient. Therefore, N,N'-dicarboxypropyl-4,4'-bipyridinium has potential application value in a liquid flow battery system with variable counterions.

Claims

1. A device for heat energy recovery using a bipyridine salt flow battery; characterized in that, The system includes a fuel cell stack, storage tanks, a diaphragm pump, a heat exchanger, and a heat source. The fuel cell stack comprises a diaphragm, end plates, gaskets, current collectors, and a graphite felt structure. The diaphragm is positioned in the middle, with end plates, gaskets, current collectors, and graphite felt structures symmetrically arranged from the outside to the inside at both ends. The graphite felt structure is a plate with through-holes, each containing graphite felt. Storage tanks are located on the outer sides of the two end plates. Each storage tank contains a positive electrolyte and a negative electrolyte, respectively. The active material for the negative electrolyte is N,N'-dicarboxypropyl-4,4'-bipyridine salt. Each storage tank is connected to the graphite felt structure via an electrolyte circulation pipeline. A circulating loop is formed between the tanks; each tank is also connected to a heat exchanger via heat exchange pipes; the heat exchanger is connected to a heat source; the positive and negative electrolytes are pumped into the graphite felt of the fuel cell stack on both sides of the diaphragm using an electrolyte circulation pipeline and a diaphragm pump, respectively, and charged at room temperature through a current collector; the heat source exchanges heat with the heat exchanger to heat the negative and positive electrolytes from the tanks, and the negative and positive electrolytes circulate between the tanks and the graphite felt while being heated, and after being heated to 30-70℃, they are discharged at high temperature through the current collector to output current.

2. The apparatus for heat recovery using a bipyridine salt flow battery according to claim 1; characterized in that, The end plate, gasket, and current collector are all made of sheet metal.

3. The apparatus for heat recovery using a bipyridine salt flow battery according to claim 1; characterized in that, The through holes are located in the middle of the gaskets on both sides of the diaphragm; the through holes and the graphite felt are square or round.

4. The apparatus for heat recovery using a bipyridine salt flow battery according to claim 3; characterized in that, The through holes and graphite felt are square with a diameter of 2-100cm × 2-100cm or circular with a diameter of 2-100cm; the thickness of the through holes and graphite felt is 2-6mm.

5. The apparatus for heat recovery using a bipyridine salt flow battery according to claim 1; characterized in that, The electrolyte circulation pipeline passes through a metal end plate, a plastic end plate, a gasket, a current collector, and connects to the graphite felt in the graphite felt structure. A diaphragm pump is installed on the electrolyte circulation pipeline; The two storage tanks are connected to the same heat exchanger via heat exchange tubes, or the two storage tanks are connected to different heat exchangers via heat exchange tubes.

6. The apparatus for heat recovery using a bipyridine salt flow battery according to claim 1; characterized in that, The end plate is composed of a metal end plate and a plastic end plate; the metal end plate is disposed on the outside of the plastic end plate; the metal of the metal end plate is titanium, iron, copper or aluminum alloy; the plastic of the plastic end plate is polytetrafluoroethylene; the material of the gasket is silicone or fluoropolymer; the diaphragm is Nafion membrane or Nafion modified membrane. The current collector is selected from graphite carbon felt, conductive carbon layer or metal current collector; the heat source is water heated by solar heat, industrial waste heat or geothermal heat with a temperature of less than 100°C.

7. A method for recovering heat energy using the apparatus of claim 1, which utilizes a bipyridine salt flow battery; characterized in that: The positive and negative electrolytes are added to two different storage tanks. The positive and negative electrolytes are pumped into the graphite felt of the fuel cell stack on both sides of the diaphragm using electrolyte circulation pipelines and diaphragm pumps. The fuel cell is charged at room temperature through a current collector. The heat source exchanges heat with the heat exchanger to heat the negative and positive electrolytes from the storage tanks. The negative and positive electrolytes circulate between the storage tanks and the graphite felt while being heated. After being heated to 30-70°C, the fuel cell discharges at high temperature through the current collector to output current. The positive electrode electrolyte is composed of a positive electrode active material and a supporting electrolyte; the negative electrode electrolyte is composed of a negative electrode active material and a supporting electrolyte; the negative electrode active material is N,N'-dicarboxypropyl-4,4'-bipyridine salt.

8. The method according to claim 7; characterized in that, The concentration of the positive electrode active material is 0.01M to 1.5M, and the concentration of the supporting electrolyte in the positive electrode electrolyte is 0.5M to 6M; the concentration of the negative electrode active material is 0.01M to 1.5M, and the concentration of the supporting electrolyte in the negative electrode active material is 0.5M to 6M.

9. The method according to claim 7; characterized in that, The positive electrode active material is a halosalt or a TEMPO derivative; The supporting electrolyte in the positive and negative electrode electrolytes is an electrolyte composed of cations and anions; The cations selected are potassium ions, sodium ions, ammonium ions, and lithium ions; the anions selected are halide ions, nitrate ions, sulfate ions, and oxalate ions.

10. The method according to claim 9; characterized in that, The halosalts mentioned are potassium iodide, cesium iodide, sodium iodide, or ammonium iodide; The supporting electrolytes in the positive and negative electrode electrolytes are potassium chloride, potassium oxalate, potassium sulfate, potassium nitrate, sodium chloride, sodium oxalate, sodium sulfate, sodium nitrate, ammonium chloride, ammonium oxalate, ammonium sulfate, ammonium nitrate, lithium chloride, lithium oxalate, lithium sulfate, lithium nitrate, potassium bromide, or sodium bromide.

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