Low-Temperature Direct Ammonia Fuel Cell with a Hydrophilic Anode Diffusion Layer

Abstract

The present invention discloses a low-temperature direct ammonia fuel cell employing a hydrophilic anode diffusion layer. The technical solution is as follows: The diffusion layer of the anode diffusion electrode uses a carbon fiber porous material treated by hydrophilic treatment. The anode catalyst is directly coated on the surface of one side of the diffusion layer. The hydrophilic anode diffusion electrode is composed of a hydrophilic anode diffusion layer and a relatively hydrophobic catalytic layer, such that the hydrophilicity of the overall electrode from the diffusion layer to the catalytic layer decreases. The carbon fiber substrate in the hydrophilic anode diffusion electrode is treated by hydrophilic treatment, and its carbon fiber morphology, porosity and conductivity do not change. With the strengthening of the hydrophilic treatment of the hydrophilic anode diffusion electrode, the range of decrease in the water contact angle on the diffusion layer side is from 125° to 90°. The present invention can effectively improve the electrochemical performance of the battery and significantly reduce the current fluctuation, and improve the stability of the battery output performance. It enables the anode diffusion electrode to have a stronger liquid storage capacity, which has a positive effect on maintaining the wetness of the anion exchange membrane and promoting the transmembrane transport of anions.

Description

Technical Field

[0001] The present invention belongs to the field of fuel cells, and particularly relates to a low-temperature direct ammonia fuel cell. Background Art

[0002] As a hydrogen-rich carbon-neutral fuel, ammonia has the advantages of high energy density, perfect preparation, storage and transportation technologies, and high commercialization degree. It is expected to solve the hydrogen energy industry chain problems faced by fuel cells from the perspective of fuel storage and transportation. At present, the technology of low-temperature direct ammonia fuel cell (DAFC) is in the stage of rapid development after early exploration and has attracted more and more attention in the energy field. In a low-temperature direct ammonia fuel cell, humidified oxygen is introduced into the cathode of the cell and undergoes an oxygen reduction reaction (ORR) to generate hydroxide ions. Ammonia, as the anode fuel, combines with the hydroxide ions generated at the cathode and undergoes an ammonia oxidation reaction to generate nitrogen and water. Among them, ammonia fuel is usually supplied to the anode in the form of a mixed solution of liquid ammonia / potassium hydroxide. The reactions at both electrodes of the cell usually occur in the temperature range of 60-100°C.

[0003] In terms of water and heat management, the conversion between gas / liquid two phases is involved in both the anode and cathode of DAFC. In this context, the structures and properties of the diffusion layer and catalytic layer of the cathode and anode of DAFC are particularly important. For the anode diffusion layer (ADL), it is based on carbon fiber and needs to play the most basic role of conducting electrons and then undertaking the Faraday reactions at both electrodes. Secondly, since ammonia is supplied to the anode of the cell in the form of a solution, the anode diffusion layer needs to have a certain hydrophilicity so that the mixed solution of ammonia water / potassium hydroxide can reach the catalytic layer more easily, and then the ammonia oxidation reaction (AOR) occurs at the anode. The reaction product nitrogen of AOR will generate in the form of bubbles in the catalytic layer, resulting in the appearance of a three-phase interface of nitrogen-ammonia water-solid catalyst particles. This three-phase interface will greatly hinder the contact between ammonia water and catalyst particles, thus affecting the cell performance. It is worth mentioning that this kind of influence is not long-term. As the AOR reaction proceeds, the nitrogen bubbles will gradually increase, be squeezed, and finally discharged from the anode flow channel through the ADL. The whole process from bubble generation to discharge only lasts for a few seconds, and the influence on the cell performance is manifested as the current (voltage) fluctuation in the constant voltage (current) mode, that is, the fluctuation of the output power. Therefore, the ADL used for low-temperature direct ammonia fuel cells should have the functions of hydrophilicity, liquid storage, and exhaust (bubbles).

[0004] Carbon paper is the most commonly used material for the diffusion layers of both the anode and cathode, and is widely used in various types of fuel cells. Commercially available carbon paper is basically not subjected to hydrophilic or hydrophobic treatment, and only provides the structural substrate of carbon fiber. The substrate itself exhibits hydrophobicity. On the one hand, this will increase the resistance to the diffusion of the anode solution of the DAFC to the catalytic layer, resulting in insufficient supply of AOR reactants, thereby reducing the battery output performance and at the same time reducing the fuel utilization rate. On the other hand, the hydrophobicity of the carbon fiber causes the solution adhesiveness in the diffusion layer to decrease. When the AOR product nitrogen is discharged through the diffusion layer, it squeezes the liquid in the pores, which will cause the liquid disconnection phenomenon between the diffusion layer and the catalytic layer, thereby causing fluctuations in the battery performance.

[0005] Based on this, the present invention proposes a direct ammonia fuel cell using a hydrophilic anode diffusion electrode, which can effectively improve the anode mass transfer problem faced by low-temperature DAFC, thereby effectively improving the battery performance and significantly reducing the output performance fluctuations of the battery under operating conditions, and enhancing the battery stability. Summary of the Invention

[0006] The object of the present invention is to propose a low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion electrode, which can improve the mass transfer resistance of the battery anode on the basis of effectively improving the battery power density, thereby significantly reducing the output performance fluctuations of the battery under operating conditions and enhancing the battery performance stability.

[0007] The composition structure of the low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer includes: anode current collector plate, anode flow field plate, anode diffusion electrode, anion exchange membrane (only coated with cathode catalyst), cathode gas diffusion layer, cathode flow field plate, and cathode current collector, etc. Its technical characteristics are: the diffusion layer of the anode diffusion electrode uses a carbon fiber porous material treated with hydrophilic treatment, and the anode catalyst is directly coated on the surface of one side of the diffusion layer. The hydrophilic anode diffusion electrode is composed of a hydrophilic anode diffusion layer and a relatively hydrophobic catalytic layer, so that the hydrophilicity of the entire electrode from the diffusion layer to the catalytic layer decreases. The carbon fiber substrate in the hydrophilic anode diffusion electrode is treated with hydrophilic treatment, and its carbon fiber morphology, porosity and conductivity do not change. And with the strengthening of the hydrophilic treatment, the water contact angle on the diffusion layer side of the hydrophilic anode diffusion electrode decreases to 125° to 90°.

[0008] Low-temperature direct ammonia fuel cells generally form a membrane electrode by laminating a catalyst-coated membrane on two diffusion layers. The similarities and differences between the hydrophilic anode diffusion electrode proposed in the present invention and the ordinary membrane electrode are as follows: (1) Both electrodes use carbon paper with a carbon fiber structure as the diffusion layer substrate, and are prepared by spraying a catalyst slurry on the diffusion layer, and the compositions of their catalytic layers are the same; (2) The anode catalyst of the hydrophilic anode diffusion electrode is directly covered on one side of the anode diffusion layer, and the catalytic layer of the hydrophilic anode diffusion electrode is in contact with the membrane by pressing; (3) The hydrophilic diffusion layer obtained by modifying the substrate will not change its carbon fiber morphology size, porosity and electrical conductivity; (4) After modifying the substrate, the hydrophilicity of the hydrophilic diffusion layer is significantly enhanced, thereby greatly improving its liquid storage and gas exhaust capabilities; (5) The hydrophilicity gradient from the diffusion layer to the catalytic layer in the hydrophilic diffusion electrode decreases, that is, the catalytic layer has relatively hydrophobicity, which is conducive to the discharge of AOR product water and avoids excessive solution aggregation in the catalytic layer.

[0009] Under this technical solution, the ammonia / KOH solution supplied by the anode enters the anode diffusion electrode through the anode flow channel and diffuses to the anode catalytic layer through the diffusion layer. Under the action of catalyst particles, ammonia combines with hydroxide ions generated at the cathode to undergo an oxidation reaction, generating nitrogen and water. During this process, on the one hand, the carbon fibers of the anode diffusion electrode act as conductors to transfer electrons to the anode current collector plate, and on the other hand, store a large amount of ammonia / KOH solution to ensure sufficient supply of reactants to the catalytic layer direction, and at the same time discharge the AOR product nitrogen in the reverse direction. The relative hydrophobicity of the catalytic layer can promote the discharge of AOR reaction product water, avoiding the overall excessive solution aggregation phenomenon of the anode diffusion electrode caused by the enhanced hydrophilicity of the diffusion layer, which will significantly increase the ohmic impedance of the battery. This structural method can improve the anode solution supply from the diffusion layer to the catalytic layer, reduce the influence of the ammonia-water-nitrogen-catalyst particle three-phase interface of the anode on the battery performance, and effectively improve the battery performance stability.

[0010] The characteristics and beneficial effects of the present invention are:

[0011] (1) The biggest feature is that it does not change the composition structure of the low-temperature direct ammonia fuel cell, but only treats the anode diffusion layer, so that the liquid storage and gas exhaust capabilities of the anode diffusion electrode after hydrophilic treatment are significantly enhanced, which can effectively improve the mass transfer problem of the ammonia-water-nitrogen-catalyst particle three-phase interface of the anode catalytic layer, realize good water and heat management of the DFAC, and thus improve the battery performance stability.

[0012] (2) The diffusion layer carbon fiber material of the hydrophilic anode diffusion electrode for low-temperature DAFC has stronger hydrophilicity, the combination of the battery anode diffusion layer and the catalytic layer is closer, and the ammonia / KOH solution supplied by the anode is more likely to reach the catalytic layer through the anode diffusion layer for AOR reaction, thereby improving the battery power density.

[0013] (3) The anodic diffusion electrode pair after hydrophilic treatment keeps the anion exchange membrane moist, which plays a positive role in promoting the transmembrane transport of anions. Moreover, the water produced by the AOR reaction can be discharged reversely from the catalytic layer, avoiding the overall excessive solution aggregation phenomenon of the electrode caused by the enhanced hydrophilicity of the diffusion layer. Description of the Drawings

[0014] Figure 1 It is a schematic diagram of the structure of a low-temperature direct ammonia fuel cell using a hydrophilic anodic diffusion layer.

[0015] Figure 2 (a) is the photographed effect diagram of the hydrophilic anodic diffusion layer side of the embodiment of the present invention.

[0016] Figure 2 (b) is the photographed effect diagram of the hydrophilic anodic diffusion layer side of the embodiment of the present invention.

[0017] Figure 3 (a) is the water contact angle of the hydrophilic anodic diffusion layer without hydrophilic treatment.

[0018] Figure 3 (b) and (c) are the water contact angles of the hydrophilic anodic diffusion layer after hydrophilic treatment.

[0019] Figure 4 It is a performance comparison diagram of a low-temperature direct ammonia fuel cell using a hydrophilic anodic diffusion electrode.

[0020] Figure 5 (a) is a comparison diagram of the current uniformity of two types of cells with and without hydrophilic treatment.

[0021] Figure 5 (b) is a comparison diagram of the current fluctuations of two types of cells with and without hydrophilic treatment under the constant voltage mode. Detailed Embodiments

[0022] The structure of the present invention will be further described below in conjunction with the drawings and specific embodiments. It should be noted that this embodiment is narrative rather than restrictive, and does not limit the protection scope of the present invention.

[0023] The composition structure of a low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer includes: an anode current collector plate, an anode flow field plate, an anion exchange membrane (only coated with a cathode catalyst), a cathode gas diffusion layer, a cathode flow field plate, and a cathode current collector, etc. The innovative structure lies in that the diffusion layer of the anode diffusion electrode adopts a carbon fiber porous material treated by hydrophilic treatment, and the anode catalyst is directly coated on the surface of one side of the diffusion layer. The hydrophilic anode diffusion electrode is composed of a hydrophilic anode diffusion layer and a relatively hydrophobic catalytic layer, which reduces the hydrophilicity of the overall electrode from the diffusion layer to the catalytic layer. The carbon fiber substrate in the hydrophilic anode diffusion electrode is treated by hydrophilic treatment, and its carbon fiber morphology, porosity, and conductivity do not change. With the strengthening of the hydrophilic treatment of the hydrophilic anode diffusion electrode, the range of the water contact angle reduction on the diffusion layer side is from 125° to 90°.

[0024] The anode flow field plate circulates ammonia / KOH solution; the cathode flow field plate circulates air.

[0025] The diffusion layer of the anode diffusion electrode adopts a carbon fiber porous material treated by hydrophilic treatment, and a small amount of hydrophobic particles are added to the catalytic layer to make the catalytic layer have hydrophilic characteristics.

[0026] The specific embodiments of the present invention are as Figure 1 shown: The anode current collector plate, the anode flow field plate, the hydrophilic anode diffusion electrode, the anion exchange membrane (only coated with the cathode), the cathode diffusion layer, the cathode flow field plate, and the cathode current collector plate are combined into one. Among them, the anode diffusion electrode composed of the anode diffusion layer and the catalytic layer, the anion exchange membrane coated with the cathode catalytic layer, and the cathode diffusion layer are pressed together to form a membrane electrode (MEA), which is located in the center of the battery, and the remaining components are sequentially connected by bolts.

[0027] The specific technical solution is: The diffusion layer obtained by hydrophilic modification treatment is prepared into a diffusion electrode by ultrasonic spraying, and used as the anode diffusion electrode of the low-temperature direct ammonia fuel cell. The structure of the low-temperature DAFC under this scheme is: anode current collector plate - anode (ammonia / KOH solution) flow channel - anode diffusion electrode - anion exchange membrane (only coated with the cathode) - cathode diffusion layer - cathode (air) flow channel - cathode current collector plate. Among them, the anode diffusion electrode, the anion exchange membrane, the cathode catalytic layer, and the cathode diffusion layer are pressed together to form the core component membrane electrode of the DAFC, and the remaining components are sequentially connected by bolts.

[0028] The diffusion layer in the hydrophilic anode diffusion electrode is treated by acid etching, graphene oxide cross-linking, and high-temperature sintering. Without changing the carbon fiber morphology size, porosity, and conductivity of the diffusion layer substrate, the hydrophilicity of the substrate material can be increased, and it has stronger liquid storage and exhaust capabilities, thereby effectively improving the three-phase interface situation of ammonia water - nitrogen - catalyst particles at the anode of the battery, and also playing a positive role in keeping the anion exchange membrane of the battery moist and improving the ionic conduction ability of the membrane.

[0029] Preparation of hydrophilic anode diffusion electrode: Select a certain type of carbon paper as the carbon fiber substrate of the anode diffusion electrode, and the substrate thickness is 200 um. Immerse the substrate completely in a sulfuric acid solution with a certain concentration for acid etching to increase the number of oxygen-containing functional groups carried by the carbon fiber. Add graphene oxide to crosslink with the acid-etched carbon fiber, and form a hydrophilic anode diffusion layer available for DAFC through high-temperature sintering. Mix platinum / iridium / carbon in a certain mass ratio, select isopropanol as the solvent, and PTFE solution as the adhesive to prepare the anode catalyst slurry. Spray the slurry evenly on the surface of one side of the diffusion layer by ultrasonic spraying, and then the hydrophilic anode diffusion electrode for low-temperature DAFC can be obtained.

[0030] Figure 2 It is a real-shot picture of the diffusion layer side and the catalytic layer side of the anode diffusion electrode after preparation. The diffusion layer side is the carbon fiber treated hydrophilically, and its porous structure is obvious; the catalytic layer side is the particle structure composed of platinum / iridium / carbon, and its structural characteristics are relatively dense.

[0031] Figure 3 It is a comparison chart of the water contact angles of the diffusion layer substrate before and after hydrophilic treatment. The water contact angle is one of the indexes to characterize the hydrophilicity of materials. The water contact angle of the diffusion layer substrate without hydrophilic treatment is 126.4°, and with the improvement of the hydrophilic treatment degree, the water contact angle of the diffusion layer substrate (decreases) changes to 120.9° and 100.9°.

[0032] By using the carbon paper substrates without hydrophilic treatment and with hydrophilic treatment respectively to prepare membrane electrodes and anode diffusion electrodes, and assembling low-temperature direct ammonia fuel cells according to the corresponding structures for full-cell performance testing. During the testing process, control the battery operating temperature at 80 °C through a fixture. Pass a certain concentration of ammonia water / KOH solution into the anode at a fixed flow rate, and pass 100% humidified air into the cathode at a fixed flow rate under normal pressure. Measure the polarization curves of the two groups of batteries under the above working conditions to evaluate the electrochemical performance of the batteries. In addition, record the current fluctuation conditions of the two groups of batteries in the mode of constant battery output voltage, and take 350 data points within 3 min for data processing, and calculate the absolute current fluctuation value (mA) and relative volatility (%) of the battery to evaluate the performance stability of the battery.

[0033] Figure 4 It is a comparison of the polarization curves and power density curves of the two groups of batteries. The results show that the use of hydrophilic anode diffusion electrodes effectively improves the battery power density.

[0034] Figure 5 It records the current fluctuation conditions of the two groups of batteries at different voltages. The output performance fluctuation problem of the direct ammonia fuel cell using hydrophilic anode diffusion electrodes has been greatly improved, and the battery performance stability has been improved.

[0035] A low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion electrode, the hydrophilic anode diffusion electrode uses carbon paper as a substrate, and the diffusion layer is hydrophilically treated by adding oxygen-containing functional groups and finally prepared into a hydrophilic anode diffusion electrode for a low-temperature direct ammonia fuel cell. The hydrophilic anode diffusion electrode ensures the effective diffusion of the anode solution without changing the porosity of the original diffusion layer substrate, enabling it to have better liquid storage and exhaust performance, thereby effectively improving the battery power density and significantly reducing the performance fluctuation under operating conditions, and the stability of the battery performance is verified through examples.

Claims

1. A low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer, the composition structure of the fuel cell comprising: An anode current collector plate, an anode diffusion electrode, an anode flow field plate, an anion exchange membrane, a cathode gas diffusion layer, a cathode flow field plate, and a cathode current collector, characterized in that: the diffusion layer of the anode diffusion electrode is made of a carbon fiber porous material treated by hydrophilic treatment, the anode catalyst is directly coated on the surface of one side of the diffusion layer, and a hydrophilic anode diffusion electrode is composed of a hydrophilic anode diffusion layer and a relatively hydrophobic catalytic layer, so that the hydrophilicity of the whole electrode from the diffusion layer to the catalytic layer decreases. The carbon fiber substrate in the hydrophilic anode diffusion electrode is treated by hydrophilic treatment, and its carbon fiber morphology, porosity and conductivity do not change. The anode flow field plate circulates ammonia / KOH solution; the cathode flow field plate circulates air.

2. The low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer according to claim 1, wherein: With the strengthening of the hydrophilic treatment, the water contact angle of the hydrophilic anode diffusion electrode decreases to 125° to 90° on the diffusion layer side of the hydrophilic anode diffusion electrode.

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