Three-dimensional staggered snake-shaped flow channel structure and all-vanadium redox flow battery

Abstract

The invention discloses a flow channel structure of a redox flow battery, and particularly relates to a three-dimensional staggered snake-shaped flow channel structure and an all-vanadium redox flow battery, the flow channel structure comprises two parts, and the two parts are positioned on the two sides of an electrode in the thickness direction and are respectively processed on the side surface, close to an electrode, of a bipolar plate and the side surface, close to a diaphragm, of the electrode; an inlet of the runner structure is formed in the bipolar plate side part, and an outlet is formed in the electrode side part; the flow channel structure of each part is snake-shaped, and the flow channel structures of the two parts are distributed in a staggered manner in the plane direction of the electrode. The technical problem that the convective mass transfer effect of reactive ions in the thickness direction of the electrode is improved by changing pressure gradient distribution in the electrode is solved. Under the same electrolyte flow supply and different current densities, the flow channel structure promotes the mass transfer process of reactive ions in the electrode, the brought overpotential loss is lower than that of a traditional flow channel, and the limiting current density is higher. In a charge-discharge experiment, the flow battery disclosed by the invention is smaller in overpotential and higher in charge-discharge capacity.

Description

Technical Field

[0001] The present invention relates to a flow channel structure of a liquid flow battery, in particular to a three-dimensional staggered serpentine flow channel structure. Meanwhile, the present invention also relates to an all-vanadium liquid flow battery containing the three-dimensional staggered serpentine flow channel structure. Background Art

[0002] The all-vanadium liquid flow battery consists of a battery stack, electrolyte, electrolyte circulation system, battery management system, and power conversion system. The main active element is vanadium. During the battery charging and discharging process, the vanadium element changes in multiple valence states (V 4+ / V 5+ , V 3+ 、V 2+ Redox reaction (oxidation-reduction reaction) realizes the mutual conversion of electrical energy and chemical energy.

[0003] The battery stack of the all-vanadium liquid flow battery is mainly composed of electrodes, diaphragms, bipolar plates, current collectors, frames and seals. The electrode is the place where the electrochemical reaction occurs; the diaphragm is located between the positive and negative electrodes, and its main function is to prevent the vanadium ions in the positive and negative electrolytes from mixing with each other, while allowing hydrogen ions to pass through to transfer charge and maintain the ion balance and circuit conduction inside the battery; the current collector is used to collect the current generated on the electrode and transmit it to the external circuit; the bipolar plate plays the role of collecting and conducting current, connecting multiple single cells to form a battery stack, and the flow channel structure will be processed on the bipolar plate to promote the transfer of electrolyte in the porous electrode and reduce the pressure drop loss. It also provides mechanical support for the membrane and electrode to prevent the mixing of positive and negative electrolytes and direct contact of electrodes.

[0004] The energy loss of battery stack charging and discharging is one of the main reasons for the energy loss of liquid flow battery system. Voltage loss is one of its manifestations, and its size is generally expressed by voltage efficiency. Voltage efficiency refers to the ratio of the integral of the discharge voltage of the liquid flow battery system over time to the integral of the charging voltage over time during the charging and discharging process. The losses mainly include: resistance overpotential, the voltage loss caused by the resistance of the entire liquid flow battery; concentration overpotential, the voltage loss caused by insufficient supply of reaction ions on the carbon fiber surface inside the electrode due to limited mass transfer of reaction ions; reaction overpotential, the voltage loss caused by the electrochemical reaction.

[0005] Improving the mass transfer process of reactive ions can reduce the mass transfer overpotential loss of liquid flow batteries and increase the ultimate power density of liquid flow batteries, which is an important way to reduce the production cost of liquid flow batteries. As the first component that the electrolyte flows through after entering the battery, the flow channel structure will have an important impact on the mass transfer process of reactive ions in the liquid flow battery.

[0006] At present, the common flow channel structure designs of flow batteries include cross-type flow channels, serpentine flow channels, parallel flow channels, etc. Figure 1As shown in (a) to (c). These flow channel structures are all processed on the bipolar plate outside the porous electrode. From the overall structure, the complete flow channel has a two-dimensional distribution structural characteristic, that is, the entire flow channel is laid flat on the x-y plane of the porous electrode. This two-dimensional distribution makes the electrolyte infiltration and infiltration outlets between the flow channel and the porous electrode distributed on the bipolar plate side of the porous electrode. Since the pressure distribution in the electrode shows a trend of gradually decreasing from the local infiltration inlet to the local infiltration outlet, there is an obvious pressure gradient in the electrode plane direction, especially on the side close to the bipolar plate. In the electrode thickness direction, the pressure gradient gradually decreases from the bipolar plate side to the membrane side, which eventually leads to a gradual deterioration of convective mass transfer from the bipolar plate side to the membrane side in the electrode thickness direction.

[0007] Therefore, the present invention is proposed. Summary of the invention

[0008] One of the inventive objectives of the present invention is to provide a three-dimensional staggered serpentine flow channel structure design, which changes the pressure gradient distribution in the electrode by splitting the traditional flow channel structure into two parts and locating them on both sides of the electrode thickness direction, and processing them on the electrode side surface near the bipolar plate and the diaphragm side surface near the electrode respectively, thereby expanding the two-dimensional distribution of the pressure gradient in the electrode under the traditional flow channel structure to three dimensions.

[0009] Another object of the present invention is to provide an all-vanadium liquid flow battery containing the above-mentioned three-dimensional staggered serpentine flow channel structure.

[0010] In order to achieve the above-mentioned purpose, the present invention designs a three-dimensional staggered serpentine flow channel structure, which includes two parts and is located on both sides of the electrode thickness direction, and is processed on the electrode side surface of the bipolar plate and the diaphragm side surface of the electrode respectively;

[0011] The inlet of the flow channel structure is at the bipolar plate side portion, and the outlet is at the electrode side portion;

[0012] The flow channel structure of each part is serpentine, and the two parts of the flow channel structure are staggered in the direction of the electrode plane.

[0013] The three-dimensional staggered serpentine flow channel structure design provided by the present invention changes the two-dimensional distribution of the infiltration and infiltration outlets in the traditional flow channel structure into a three-dimensional staggered distribution. This distribution change directly affects the pressure distribution in the electrode. Among them, the two parts of the flow channel that are flatly distributed in the direction of the electrode plane can ensure the uniform supply of electrolyte in the direction of the electrode plane. The flow channel part connected to the inlet and the flow channel part connected to the outlet are respectively located on both sides of the electrode thickness direction, which can improve the convective mass transfer effect of the reaction ions in the direction of the electrode thickness. In addition, the staggered distribution of the inlet part and the outlet part in the plane direction is to avoid the flow mass transfer dead zone between adjacent parts caused by the direct parallel distribution of the inlet and outlet parts.

[0014] Specifically:

[0015] Influence of pressure distribution in the electrode: When the distribution of the inlet and outlet of the flow channel structure is adjusted, the flow path and pressure distribution of the electrolyte inside the electrode will be significantly changed. For the traditional flow channel, its inlet and outlet branches are in the same plane near the bipolar plate side of the electrode, resulting in a large pressure gradient in the plane direction near the bipolar plate side and a small pressure gradient in the thickness direction. The electrolyte flow path is U-shaped, and most streamlines have high flow velocities near the bipolar plate side, and the streamlines are sparse and have low flow velocities away from the thickness part of the side. The three-dimensional staggered serpentine flow channel of the present invention significantly changes the pressure distribution and streamline distribution of the flow battery electrode, because its infiltration and seepage outlets are on both sides of the electrode thickness direction, the pressure changes significantly in the electrode thickness direction, and there is also an obvious pressure gradient in the plane direction due to the accumulation of pressure loss, achieving a relatively obvious pressure gradient distribution in three dimensions, and the electrolyte flow path is curved and the overall distribution is uniform.

[0016] Influence of velocity distribution in the electrode: The velocity distribution of the traditional flow channel and the three-dimensional staggered serpentine flow channel in the electrode plane direction is relatively uniform, and both can achieve uniform distribution of the electrolyte in this direction. However, the electrolyte flow velocity in the flow battery electrode using the three-dimensional staggered serpentine flow channel is relatively large, and it is more obvious in the electrode thickness direction. In the flow battery using the traditional flow channel, the high flow velocity in the electrode thickness direction is only near the bipolar plate side, and the flow velocity in other areas is low; while in the flow battery using the three-dimensional staggered serpentine flow channel, due to the significant increase in pressure gradient, the electrolyte flow velocity in the electrode thickness direction, especially in the part away from the bipolar plate side, increases significantly.

[0017] Influence of reaction ion concentration distribution and overpotential distribution: In traditional channel flow batteries, only the electrolyte flow rate and ion concentration are high near the bipolar plate side. The ion concentration is low on the far side due to poor convective mass transfer and dominant diffusion mass transfer. In three-dimensional staggered serpentine channel flow batteries, the electrolyte flow rate in the thickness direction of the electrode is significantly increased and evenly distributed, the reaction ion concentration is high and evenly distributed, and the overall distribution of concentration overpotential is small.

[0018] The present invention provides an all-vanadium liquid flow battery, which is composed of a battery stack, an electrolyte, an electrolyte circulation system, a battery management system, and a power conversion system. The main active element is vanadium. During the battery charging and discharging operation, the vanadium element changes in multiple valence states (V 4+ / V 5+ , V 3+ 、V 2+The positive and negative electrolytes of the all-vanadium liquid flow battery are stored in different storage tanks. Driven by a pump, they circulate through the positive and negative electrodes of the flow battery, causing redox reactions, and finally realizing the charge and discharge of the flow battery. Among them, the battery stack of the all-vanadium liquid flow battery is mainly composed of electrodes, diaphragms, bipolar plates, current collectors, frames and seals, and its flow channel structure adopts the above-mentioned three-dimensional staggered serpentine flow channel structure.

[0019] Compared with the prior art, the three-dimensional staggered serpentine flow channel structure and the all-vanadium liquid flow battery obtained by the present invention have the following technical effects:

[0020] The three-dimensional staggered serpentine flow channel of the present invention has significant advantages in liquid flow batteries. Under the same electrolyte flow supply and different current densities, especially in the mass transfer control area with higher current density, its overpotential loss is lower than that of the traditional flow channel, and the limiting current density is higher. In the charge and discharge experiments, the liquid flow battery using the new flow channel has a smaller overpotential and a higher charge and discharge capacity. That is, the new flow channel brings higher voltage efficiency.

[0021] In addition, although the inlet and outlet pressure drop of the new flow channel is slightly higher than that of the traditional flow channel, especially in the area with higher flow rate, it improves the mass transfer effect of the reaction ions inside the electrode, making the energy efficiency of the flow battery system higher than that of the flow battery using the traditional flow channel. For example, compared with the traditional cross-type flow channel structure, the energy efficiency of the flow battery system is about 3.2% higher (the electrolyte inlet flow rate and current density are 20 mL / min and 100 mA / cm 2 ), and the current efficiency of the flow batteries with the two flow channel structures is basically similar. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the novel three-dimensional staggered serpentine flow channel structure of the present invention and the traditional flow channel: (a) cross type; (b) serpentine type; (c) parallel inlet and outlet type; (d) three-dimensional staggered serpentine overall structure; (e) three-dimensional staggered serpentine flow channel plane view; (f) three-dimensional staggered serpentine thickness direction cross-sectional view;

[0023] Figure 2 It is a schematic diagram of the basic structure of the single cell assembly in the liquid flow battery of the present invention;

[0024] Figure 3 Comparison of the results of three-dimensional staggered serpentine flow channel and traditional cross-type flow channel (electrode area 30×30cm, current density 100mA / cm 2 );

[0025] Figure 4 This is a display diagram of the flow battery single cell performance test bench.

[0026] In the figure:

[0027] Bipolar plate 1, electrode 2, flow channel structure 3, steel end cover 4, electrode frame 5, diaphragm 6, current collecting plate 7, PP end cover 8;

[0028] Liquid flow battery single cell 10, positive and negative electrolyte storage tanks 20, peristaltic pump 30, pressure gauge 40, signal acquisition instrument 50, electrochemical workstation 60. DETAILED DESCRIPTION

[0029] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field belong to the scope of protection of the present invention.

[0030] As an embodiment of the present invention, a three-dimensional staggered serpentine flow channel structure is provided in this embodiment, such as Figure 1 (d) and (f) shown.

[0031] The three-dimensional staggered serpentine flow channel structure described in this embodiment includes two parts and is located on both sides of the electrode 2 in the thickness direction, and is respectively processed on the surface of the bipolar plate 1 near the electrode 2 side and the surface of the electrode 2 (porous electrode) near the diaphragm 6 side (away from the bipolar plate 1); the inlet of the flow channel structure 3 is on the side of the bipolar plate 1, and the outlet is on the side of the electrode 2; the flow channel structure 3 of each part is serpentine, and the two parts of the flow channel structure 3 are staggered in the plane direction of the electrode 2. The staggered design of the inlet and outlet parts avoids the flow dead zone caused by direct parallel distribution.

[0032] In addition, the all-vanadium liquid flow battery provided in this embodiment is composed of a battery stack, an electrolyte, an electrolyte circulation system, a battery management system, and a power conversion system, and the main active element is vanadium. Among them, the battery stack of the all-vanadium liquid flow battery is mainly composed of electrodes, diaphragms, bipolar plates, current collectors, frames, and seals, and its flow channel structure adopts the above-mentioned three-dimensional staggered serpentine flow channel structure.

[0033] The present invention verifies the application of the above-mentioned flow channel structure in liquid flow batteries through experiments.

[0034] Experimental preparation stage:

[0035] The main materials are determined. The electrode material uses original carbon felt (SGL GF5) with an initial thickness of 5mm; the bipolar plate uses graphite plate; the stack end plate uses stainless steel end plate; the current collector uses copper plate; and the diaphragm (ion exchange membrane) uses Nafion117 cation exchange membrane.

[0036] The process and size parameters are determined.

[0037] For the flow channel of the bipolar plate, the flow channel structure is made by precision machining technology (such as CNC machining, etc.) to ensure that the width, depth, length and rib width of the flow channel meet the design requirements. 2 For example, the flow channel depth is set to 1 mm, the flow channel width is set to 1 mm, the flow channel rib width is set to 3 mm, and the branch length is set to 8 mm.

[0038] For the flow channel part of the electrode, the flow channel structure is made by laser processing technology, and it is also ensured that the width, depth, length and rib width of the flow channel meet the design requirements. The width, depth, length and rib width of the flow channel structure are consistent with the above.

[0039] Pretreatment. The compression rate of the electrode needs to be adjusted between 20% and 60%. The porous electrode pretreatment uses deionized water to rinse the electrode multiple times to remove impurities. Then, the electrode is immersed in the initial electrolyte for more than 12 hours to improve its wettability. The selective permeable membrane is first immersed in deionized water at 80°C for 1 hour to remove impurities in the membrane. Subsequently, the membrane is immersed in a 4 mol / L sulfuric acid solution at 80°C for 1 hour to ionize the hydrogen, and finally rinsed repeatedly with deionized water to remove the residual sulfuric acid solution. After the pretreatment, the membrane needs to be immersed in the initial electrolyte for another 12 hours to ensure that it is fully expanded and absorbs the electrolyte.

[0040] In this experiment, the liquid flow battery for improving the mass transfer effect of the reaction example is as follows Figure 2 The structure shown is assembled. During the assembly process, it is necessary to ensure good contact between the components to avoid the problem of excessive contact resistance.

[0041] The connection between the electrode and the flow channel uses raw tape to enhance the airtightness and prevent electrolyte leakage. In order to avoid the occurrence of side reactions, especially the generation of oxygen and hydrogen, the assembled flow battery needs to be protected by nitrogen. Nitrogen is continuously delivered to the liquid storage tanks of the positive and negative electrodes to prevent the redox reaction inside the battery.

[0042] Build Figure 4Liquid flow battery single cell detection system. The system includes key equipment such as liquid flow battery single cell, positive and negative electrolyte storage tanks, peristaltic pumps, pressure gauges, signal acquisition instruments and electrochemical workstations. The storage tank is used to store electrolytes and transport the electrolytes to the flow channel of the liquid flow battery single cell through a peristaltic pump. The electrolytes used in the experiment were 1.6 mol / L vanadium ion solution and 4 mol / L sulfuric acid solution. The peristaltic pump injects the electrolyte into the flow channel from the inlet of the battery at different flow rates such as 5mL / min and 20mL / min, so that it circulates between the flow channel and the electrodes to provide reactants for the battery's charge and discharge process. The flow control accuracy of the peristaltic pump has been verified, and the error is controlled within 0.5%. The pressure gauge and signal acquisition instrument are installed at the electrolyte inlet and outlet of the liquid flow battery to monitor the pressure changes of the electrolyte. The data is transmitted to the computer through the signal acquisition instrument for subsequent analysis. The electrochemical workstation is used to monitor and record the electrochemical performance data of the flow battery, including changes in voltage, current, temperature, etc. during the charging and discharging process.

[0043] During pre-charging, the electrolyte is charged by constant current until the set state of charge (SOC) is reached. The laboratory environment temperature is controlled at 25°C. The charge and discharge curve of the flow battery is obtained through constant current charge and discharge test. Set the appropriate current density, control the cut-off voltage of charge and discharge, record the changes in voltage and current, analyze the charge and discharge efficiency and coulomb efficiency of the battery, and obtain the polarization curve by measuring the voltage change of the flow battery at different current densities to further evaluate the resistance characteristics of the battery; the pressure gauge measures the electrolyte pressure drop at the inlet and outlet of the flow battery to evaluate the hydraulic performance of the flow battery.

[0044] The operating mechanism of the above-mentioned all-vanadium liquid flow battery is as follows: before the operation starts, the electrolyte is pumped into the flow channel system. The peristaltic pump transports the electrolyte from the storage tank to the flow channel of the battery according to the preset flow rate. The flow channel design of the liquid flow battery allows the electrolyte to circulate in the battery along a serpentine path, increasing the tortuosity of the flow to ensure the uniform distribution of the electrolyte. Ensure that the electrolyte can fully contact the electrode surface in the battery and avoid the generation of dead zones or short-circuit flow channels. The staggered design also optimizes the mixing effect of the fluid, reduces the boundary layer resistance of the liquid flow, and improves the uniformity and stability of the flow. In the electrolyte, ions migrate through a selectively permeable membrane to complete the charge and discharge reaction of the battery. During the charging process, an oxidation reaction occurs at the negative (anode) electrode, and vanadium ions (V 4+ ) is reduced to vanadium trivalent ions (V 3+ ), and release electrons, while the vanadium ions (V 4+ ) is oxidized to vanadium pentavalent ions (V 5+ ). Electrons flow from the negative electrode to the positive electrode through the external circuit, forming a current. During the discharge process, the vanadium trivalent ion (V 3+) is oxidized to vanadium tetravalent ion (V 4+ ), and release electrons; the vanadium pentavalent ion (V 5+ ) is reduced to vanadium tetravalent ion (V 4+ ). Electrons flow from the external circuit through the negative electrode to the positive electrode, completing the battery discharge process. Electrons inside the battery flow through the external circuit, from the negative electrode to the positive electrode, generating current. This current can be used to power an external load and complete energy output.

[0045] The experimental results are as follows Figure 3 shown.

[0046] The flow channel structure of this invention improves the pressure distribution in the flow battery electrode by adjusting the distribution of the inlet and outlet, so that the flow battery presents a large pressure gradient in both the electrode plane direction and the thickness direction, which can effectively improve the uniformity of convective mass transfer in the electrode thickness direction. Compared with the traditional cross-type flow channel structure, this invention improves the single cell energy efficiency and system energy efficiency of the flow battery by 4.2% and 3.2% respectively (the electrolyte inlet flow rate and current density are 20 mL / min and 100 mA / cm 2 ), which increased the battery's charge and discharge capacity by 40% and increased the flow battery's limiting current density by more than 60%.

[0047] In addition, based on the above embodiments, it is possible to consider designing an adjustable serpentine flow channel and optimizing the distribution of the fluid by changing the shape of the flow channel. Under different working conditions of the battery, the shape of the flow channel is adjusted according to the liquid flow rate and battery load (such as by adjusting the valve or the automatic adjustment mechanism of the flow channel) to maintain uniform flow of the fluid, so as to further improve the battery performance under the advantages of this design.

[0048] The above-described embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A three-dimensional staggered serpentine flow channel structure, characterized in that: The flow channel structure includes two parts and is located on both sides of the electrode thickness direction, and is processed on the electrode side surface of the bipolar plate and the diaphragm side surface of the electrode respectively; The inlet of the flow channel structure is at the bipolar plate side portion, and the outlet is at the electrode side portion; The flow channel structure of each part is serpentine, and the two parts of the flow channel structure are staggered in the direction of the electrode plane.

2. The all-vanadium liquid flow battery according to claim 1, characterized in that: The all-vanadium liquid flow battery comprises the three-dimensional staggered serpentine flow channel structure as described in claim 1 above.

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