CN118522927A - Low temperature direct ammonia fuel cell employing hydrophilic anode diffusion layer - Google Patents

Abstract

The present invention discloses a low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer. The technical solution is as follows: the diffusion layer of the anode diffusion electrode adopts a carbon fiber porous material that has been hydrophilically treated, 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 catalyst layer, so that the hydrophilicity of the electrode as a whole from the diffusion layer to the catalyst layer decreases. The carbon fiber substrate in the hydrophilic anode diffusion electrode is hydrophilically treated, and its carbon fiber morphology, porosity and conductivity are not changed. The water contact angle of the hydrophilic anode diffusion electrode decreases from 125° to 90° on the diffusion layer side as the hydrophilic treatment is strengthened. 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. The anode diffusion electrode has a stronger liquid storage capacity, which has a positive effect on keeping the anion exchange membrane moist and promoting the transmembrane transmission of anions.

Description

Low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer

Technical Field

The invention belongs to the field of fuel cells, and in particular relates to a low-temperature direct ammonia fuel cell.

Background Art

As a carbon-neutral fuel rich in hydrogen, ammonia has the advantages of high energy density, perfect preparation, storage and transportation technology, and high degree of commercialization. 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, low-temperature direct ammonia fuel cell (DAFC) technology is in a rapid development stage after early exploration, and is receiving more and more attention in the energy field. In a low-temperature direct ammonia fuel cell, humidified oxygen is passed into the battery cathode and an oxygen reduction reaction (ORR) occurs to generate hydroxide ions. Ammonia is used as an anode fuel to combine with the hydroxide ions generated at the cathode and undergo an ammonia oxidation reaction to generate nitrogen and water. Among them, the anode is often supplied with ammonia fuel in the form of a liquid ammonia/potassium hydroxide mixed solution. The reaction at the two poles of the battery usually takes place in a temperature range of 60 to 100°C.

In terms of hydrothermal management, both poles of DAFC involve the mutual conversion of gas/liquid phases. In this context, the structure and properties of the diffusion layer and catalyst layer of the cathode/anodal electrode 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 undertake the Faraday reaction at both poles. Secondly, since ammonia is supplied in the form of a solution at the anode of the battery, the anode diffusion layer needs to have a certain hydrophilicity so that the ammonia/potassium hydroxide mixed solution can more easily reach the catalyst layer, and then carry out the ammonia oxidation reaction (AOR) at the anode. The nitrogen product of the AOR reaction will be produced in the catalyst layer in the form of bubbles, causing the appearance of a three-phase interface of nitrogen-ammonia-solid catalyst particles. This three-phase interface will greatly hinder the contact between ammonia and catalyst particles, thereby affecting battery performance. It is worth mentioning that this effect is not long-term. As the AOR reaction proceeds, the nitrogen bubbles will gradually grow, be squeezed, and eventually be discharged from the anode flow channel through the ADL. The entire process from bubble generation to discharge only lasts a few seconds. The impact on battery performance is manifested as current (voltage) fluctuations in constant voltage (current) mode, that is, fluctuations in output power. Therefore, the ADL used in low-temperature direct ammonia fuel cells should have the functions of hydrophilicity, liquid storage, and exhaust (bubble).

Carbon paper is the most commonly used material for the cathode and anode diffusion layers and is widely used in many different types of fuel cells. Commercial carbon paper is basically not treated with hydrophilicity and hydrophobicity, and only provides a structural substrate of carbon fiber, which itself exhibits hydrophobicity. On the one hand, this will increase the resistance of the anode solution of DAFC to diffuse into the catalyst layer, resulting in insufficient supply of AOR reactants, thereby reducing battery output performance and fuel utilization. On the other hand, the hydrophobicity of carbon fiber reduces the viscosity of the solution in the diffusion layer, and the AOR product nitrogen squeezes the liquid in the pores when it is discharged through the diffusion layer, which will cause the liquid in the diffusion layer and the catalyst layer to be cut off, causing fluctuations in battery performance.

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 battery performance and significantly reducing the output performance fluctuation of the battery under operating conditions, thereby improving battery stability.

Summary of the invention

The purpose 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 greatly reducing the output performance fluctuation of the battery under operating conditions and improving the battery performance stability.

The composition structure of the low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer includes: an anode current collector plate, an anode flow field plate, an anode diffusion electrode, 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. Its technical features are: the diffusion layer of the anode diffusion electrode adopts a carbon fiber porous material that has been hydrophilically treated, the anode catalyst is directly coated on the surface of one side of the diffusion layer, and the hydrophilic anode diffusion electrode is composed of a hydrophilic anode diffusion layer and a relatively hydrophobic catalyst layer, so that the hydrophilicity of the electrode from the diffusion layer to the catalyst layer decreases, and the carbon fiber substrate in the hydrophilic anode diffusion electrode is hydrophilically treated, and its carbon fiber morphology, porosity and conductivity are not changed. And with the strengthening of the hydrophilic treatment, the water contact angle on the diffusion layer side of the hydrophilic anode diffusion electrode is reduced to 125° to 90°.

Low-temperature direct ammonia fuel cells generally use a bipolar diffusion layer pressed together with a catalyst-coated membrane to form a membrane electrode. The differences and similarities between the anode hydrophilic diffusion electrode and the common membrane electrode proposed in the present invention 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. The catalyst layer components of the two electrodes 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 catalyst layer of the hydrophilic anode diffusion electrode is in contact with the membrane by pressing; (3) The hydrophilic diffusion layer obtained after modifying the substrate will not change its carbon fiber morphology, porosity and conductivity; (4) The hydrophilicity of the hydrophilic diffusion layer is significantly enhanced after modifying the substrate, thereby greatly improving its liquid storage and exhaust capacity; (5) The hydrophilicity gradient from the diffusion layer to the catalyst layer in the hydrophilic diffusion electrode decreases, that is, the catalyst layer has a relatively hydrophobic property, which is beneficial to the discharge of AOR product water and avoids excessive solution aggregation in the catalyst layer.

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 catalyst layer through the diffusion layer. Under the action of the catalyst particles, ammonia combines with the hydroxide produced by the cathode to undergo an oxidation reaction to generate nitrogen and water. In this process, the carbon fiber of the anode diffusion electrode acts as a conductor to transfer electrons to the anode current collector on the one hand, and on the other hand, stores a large amount of ammonia/KOH solution to ensure sufficient supply of reactants to the catalyst layer, while discharging the AOR product nitrogen in the reverse direction. The relative hydrophobicity of the catalyst layer can promote the discharge of the AOR reaction product water, avoiding the phenomenon of excessive solution aggregation of the anode diffusion electrode as a whole due to the enhancement of the hydrophilicity of the diffusion layer, which will significantly increase the battery ohmic impedance. This structural method can improve the supply of anode solution from the diffusion layer to the catalyst layer, reduce the influence of the anode ammonia-nitrogen-catalyst particle three-phase interface on the battery performance, and effectively improve the stability of battery performance.

The characteristics and beneficial effects of the present invention are:

(1) The biggest feature is that the composition structure of the low-temperature direct ammonia fuel cell is not changed, and only the anode diffusion layer is treated. After the hydrophilic treatment, the liquid storage and exhaust capacity of the anode diffusion electrode are significantly enhanced, which can effectively improve the mass transfer problem of the ammonia-nitrogen-catalyst particle three-phase interface of the anode catalyst layer, realize good hydrothermal management of DFAC, and thus improve the performance stability of the battery.

(2) The diffusion layer carbon fiber material of the hydrophilic anode diffusion electrode used in low-temperature DAFC is more hydrophilic, and the battery anode diffusion layer and catalyst layer are more tightly combined. The ammonia/KOH solution supplied by the anode can more easily pass through the anode diffusion layer to the catalyst layer for AOR reaction, thereby improving the battery power density.

(3) The hydrophilic treated anode diffusion electrode plays a positive role in maintaining the wetness of the anion exchange membrane and promoting the transmembrane transport of anions. In addition, the water produced by the AOR reaction can be discharged from the catalyst layer in reverse, avoiding the phenomenon of excessive solution aggregation in the electrode due to the enhanced hydrophilicity of the diffusion layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the structure of a low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer.

FIG. 2( a ) is a photographic effect diagram of the hydrophilic anode diffusion layer side of an embodiment of the present invention.

FIG. 2( b ) is a side-shot effect diagram of the hydrophilic anode diffusion layer according to an embodiment of the present invention.

Figure 3(a) is the water contact angle of the hydrophilic anode diffusion layer without hydrophilic treatment.

Figure 3 (b) and (c) are the water contact angles of the hydrophilic anode diffusion layer after hydrophilic treatment.

FIG4 is a performance comparison chart of low-temperature direct ammonia fuel cells using a hydrophilic anode diffusion electrode.

FIG5( a ) is a comparison of the current uniformity of two batteries with and without hydrophilic treatment.

FIG5( b ) is a comparison of the current fluctuations of the hydrophilic treated and untreated batteries under constant voltage mode.

DETAILED DESCRIPTION

The structure of the present invention is further described below in conjunction with the accompanying drawings and specific embodiments. It should be noted that this embodiment is descriptive rather than restrictive, and the protection scope of the present invention is not limited thereto.

The low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer is composed of an anode current collector, an anode flow field plate, an anion exchange membrane (only cathode catalyst is coated), a cathode gas diffusion layer, a cathode flow field plate, and a cathode current collector. The innovative structure is that the diffusion layer of the anode diffusion electrode adopts a carbon fiber porous material that has been hydrophilically treated, 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 catalyst layer, so that the hydrophilicity of the electrode from the diffusion layer to the catalyst layer is reduced. The carbon fiber substrate in the hydrophilic anode diffusion electrode has been hydrophilically treated, and its carbon fiber morphology, porosity and conductivity have not changed. As the hydrophilic treatment is strengthened, the water contact angle of the hydrophilic anode diffusion electrode decreases from 125° to 90° on the diffusion layer side.

The anode flow field plate flows an ammonia/KOH solution; the cathode flow field plate flows air.

The diffusion layer of the anode diffusion electrode adopts a carbon fiber porous material that has been hydrophilically treated, and a small amount of hydrophobic particles are added to the catalyst layer to make the catalyst layer have hydrophilic characteristics.

The specific embodiment of the present invention is shown in FIG1: an anode current collector plate, an anode flow field plate, a hydrophilic anode diffusion electrode, an anion exchange membrane (only cathode coated), a cathode diffusion layer, a cathode flow field plate, and a cathode current collector plate are combined into one. The anode diffusion electrode composed of an anode diffusion layer and a catalyst layer, the anion exchange membrane coated with a cathode catalyst 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 connected in sequence by bolts.

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

The diffusion layer in the hydrophilic anode diffusion electrode is treated with acid etching, graphene oxide cross-linking and high-temperature sintering. The hydrophilicity of the base material can be increased without changing the morphology, size, porosity and conductivity of the carbon fiber base of the diffusion layer. It has a stronger liquid storage and exhaust capacity, thereby effectively improving the three-phase interface of ammonia water-nitrogen-catalyst particles at the battery anode, and also plays a positive role in maintaining the wetness of the battery anion exchange membrane and improving the ion conductivity of the membrane.

Preparation of hydrophilic anode diffusion electrode: A certain type of carbon paper is selected as the carbon fiber substrate of the anode diffusion electrode, and the substrate thickness is 200um. The substrate is completely immersed in a sulfuric acid solution of a certain concentration, and acid etching is performed to increase the number of oxygen-containing functional groups carried by the carbon fiber. Graphene oxide is added to cross-link the acid-etched carbon fiber, and a hydrophilic anode diffusion layer that can be used for DAFC is formed after high-temperature sintering. Platinum/iridium/carbon are mixed in a certain mass ratio, isopropanol is selected as the solvent, and PTFE solution is used as the adhesive to prepare the anode catalyst slurry. The slurry is evenly sprayed on the surface of one side of the diffusion layer by ultrasonic spraying, and a hydrophilic anode diffusion electrode for low-temperature DAFC can be obtained.

Figure 2 is a photo of the diffusion layer and catalytic layer of the anode diffusion electrode after preparation. The diffusion layer is made of hydrophilic carbon fiber with a distinct porous structure, while the catalytic layer is made of a granular structure composed of platinum/iridium/carbon with a relatively dense structure.

Figure 3 is a comparison of the water contact angle of the diffusion layer substrate before and after the hydrophilic treatment. The water contact angle is one of the indicators to characterize the hydrophilicity of the material. The water contact angle of the diffusion layer substrate that has not been hydrophilicized is 126.4°, and as the degree of hydrophilic treatment increases, the water contact angle of the diffusion layer substrate changes (decreases) to 120.9° and 100.9°.

The membrane electrode and the anode diffusion electrode were prepared by using carbon paper substrates that were not hydrophilic and those that were hydrophilic, respectively, and a low-temperature direct ammonia fuel cell was assembled according to the corresponding structure for full battery performance testing. During the test, the battery operating temperature was controlled to 80°C by a fixture, a certain concentration of ammonia/KOH solution was passed into the anode at a fixed flow rate, and 100% humidified air was passed into the cathode at a fixed flow rate under normal pressure. The polarization curves of the two groups of batteries were measured under the above conditions to evaluate the electrochemical performance of the batteries. In addition, the current fluctuations of the two groups of batteries were recorded separately in the mode of constant battery output voltage, and 350 data points within 3 minutes were taken for data processing, and the absolute current fluctuation value (mA) and relative fluctuation rate (%) of the battery were calculated to evaluate the performance stability of the battery.

Figure 4 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.

Figure 5 records the current fluctuations of the two groups of batteries at different voltages. The output performance fluctuation problem of the direct ammonia fuel cell using a hydrophilic anode diffusion electrode has been greatly improved, and the battery performance stability has been improved.

A low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion electrode is prepared by using carbon paper as a substrate, and the diffusion layer is hydrophilized by adding oxygen-containing functional groups to finally prepare 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, so that it has better liquid storage and exhaust performance, thereby effectively improving the battery power density and greatly reducing the performance fluctuation under operating conditions. The stability of the improved battery performance has been verified through examples.

Claims (3)

1. A low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer, the battery structure comprising: an anode current collector plate, 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 adopts a carbon fiber porous material that has been hydrophilically treated, the anode catalyst is directly coated on the surface of one side of the diffusion layer, and the hydrophilic anode diffusion electrode is composed of a hydrophilic anode diffusion layer and a relatively hydrophobic catalyst layer, so that the hydrophilicity of the electrode as a whole from the diffusion layer to the catalyst layer is reduced, and the carbon fiber substrate in the hydrophilic anode diffusion electrode is hydrophilically treated, and its carbon fiber morphology, porosity and conductivity are not changed. 2. The low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer according to claim 1 is characterized in that the water contact angle of the hydrophilic anode diffusion electrode decreases from 125° to 90° on the diffusion layer side as the hydrophilic treatment is strengthened. 3. The low-temperature direct ammonia fuel cell using a hydrophilic anode diffusion layer according to claim 1 is characterized in that: the anode flow field plate flows an ammonia/KOH solution; and the cathode flow field plate flows air.