The theoretical specific energy of the aluminum-air fuel cell can reach 8100Wh/kg, and it has the advantages of low cost, high specific energy density and specific power density. As a special fuel cell, aluminum-air batteries have great commercial potential in special, civil, and underwater power systems, backup power sources for telecommunication systems, and portable power sources.
Metal Air Batteries Overview
Lithium-ion batteries have a high specific energy, which is a relatively mature secondary battery that has been commercialized on a large scale. However, in recent years, in the face of the huge development of mobile electronic devices and electric vehicles, lithium-ion batteries have been difficult to meet. Its large-capacity demand, especially the power battery system that is highly dependent on energy. Therefore, metal-air batteries with a specific capacity several times larger than lithium-ion batteries have emerged, such as zinc-air batteries, aluminum-air batteries, magnesium-air batteries, lithium-air batteries, etc.
Since the positive active material of this type of battery is mainly derived from oxygen in the air, the amount of theoretical positive active material is unlimited, so the theoretical capacity of the battery mainly depends on the amount of negative metal, and this type of battery has a larger specific capacity.
Among them, the theoretical specific energy of the aluminum-air fuel cell can reach 8100Wh/kg, which has the advantages of low cost, high specific energy density and specific power density. As a special fuel cell, aluminum-air batteries have great commercial potential in special, civil, and underwater power systems, backup power sources for telecommunication systems, and portable power sources.
Aluminum-air battery structure and principle
From the analysis of existing research results and battery characteristics, aluminum-air batteries have the following characteristics:
(1) High specific energy. Aluminum-air battery is a new type of high specific energy battery. The theoretical specific energy can reach 8100Wh/kg. The current research and development products have reached 300-400Wh/kg, which is much higher than the specific energy of various types of batteries today.
(2) The specific power is moderate. Since the working potential of the air electrode is far away from its thermodynamic equilibrium potential, its exchange current density is very small, and the polarization of the battery is very large when it is discharged, so that the specific power of the battery can only reach 50-200W/kg.
(3) Long service life. Aluminum electrodes can be constantly replaced, so the life of an aluminum-air battery depends on the working life of the air electrodes.
(4) Non-toxic and no harmful gas is produced. The electrochemical reaction of the battery consumes aluminum, oxygen and water to generate Al2O3˙nH2O, which can be used for drying adsorbents and catalyst carriers, grinding and polishing abrasives, ceramics and excellent precipitants for sewage treatment.
(5) Strong adaptability. The battery structure and raw materials used can be changed according to the practical environment and requirements, and have strong adaptability.
(6) Aluminum, the raw material of battery negative electrode, is cheap and easy to get. Compared with other metals, the price of metal aluminum is relatively low, and the manufacturing process of metal anodes is relatively simple.Also read:battery pack manufacturers
Aluminum anode (negative pole)
Aluminum (Al) is an ideal electrode material. The theoretical energy density of aluminum is 8.2W˙h/g, which is second only to lithium at 13.3W˙h/g among common metals. The light metal battery material with the highest mass-to-energy ratio other than metallic lithium. The mass specific energy of the aluminum-air battery can actually reach 450Wh/kg, and the specific power can reach 50~200W/kg. It has the advantages of high theoretical capacity, low consumption rate, light weight, negative potential, abundant resources and easy processing, and has been widely studied.
However, since aluminum is a very active amphoteric metal, the development of aluminum anodes is still affected by the following problems.
(1) There is a passivation film on the surface of aluminum, which affects the electrochemical activity of aluminum.
(2) Aluminum is an amphoteric metal element, which determines that it is prone to hydrogen evolution corrosion in a strong alkaline environment, affecting the electrode potential, and the product floating in the electrolyte affects the progress of the entire electrochemical reaction.
(3) The unique semi-open system of the air battery makes the air electrode vulnerable to the influence of external humidity, causing the aluminum anode to "flood" or "dry up", or even "alkali climbing" or "leakage". damage to the structure of the battery. In order to solve the above problems, scholars at home and abroad have conducted research from the following three aspects:
- Aluminum anode alloying
Industrial grade aluminum (99.0%) contains more impurities, such as iron (0.5%), silicon, copper, manganese, magnesium and zinc, etc., which will intensify the hydrogen evolution corrosion of aluminum at the phase interface, especially iron and aluminum will form a local primary battery, resulting in exponentially increased electrochemical corrosion. Alloying components that increase both chemical activity and corrosion resistance can be added to aluminum.
The elements that need to be added to the alloying of aluminum alloys need to meet the following conditions: ①The melting point of the alloying elements is lower than that of metal Al; ②The solid saturation in Al is higher; ③The electrochemical activity is higher than Al; ④The solubility in the electrolyte is relatively low. High; ⑤ has a high hydrogen evolution overpotential. In addition, processing the anode metal into an ultrafine-grained material can further improve the anode efficiency.
- Add slow-release agent to the electrolyte
Due to the cost of anode alloying, people often choose to add some slow-release agents to the electrolyte to ensure the performance of aluminum-air batteries. Some carboxylic acids, amines, amino acid slow-release agents and their inhibition efficiency on aluminum corrosion are shown in Table 1:
Researchers have used natural substances as inhibitors of metal aluminum corrosion, and experiments have proved that organic amines, pyrrole, etc. have obvious inhibitory effects on aluminum corrosion. The electrochemical behavior of the aluminum metal anode is studied by adding organic matter and water-soluble compounds to the strong alkaline electrolyte to reduce the corrosion rate of aluminum, thereby improving the performance of aluminum-air batteries.
- Heat treatment process
Heat treatment affects the properties of the alloy by changing the distribution of trace elements in the aluminum alloy and the microstructure of the alloy surface, which belongs to the research category of technology. The optimum heat treatment process can be found by suitable orthogonal experiments.
Electrolyte
The electrolyte of aluminum-air batteries is mostly neutral salt solution or strong alkaline solution. When a neutral electrolyte is used, the self-corrosion of the anode is small, but the surface passivation of the aluminum anode is serious, which reduces the working voltage, makes it difficult to increase the power and current of the battery, and also causes voltage hysteresis, and the product aluminum hydroxide colloid will also settle and block Electrolyte, so this type of battery can only be used as a low-power power output device.
When a strong alkaline electrolyte is used, the passivation of aluminum is reduced, and the lye can absorb a certain amount of reaction product aluminum hydroxide, and the performance of the battery is relatively good, but aluminum is an amphoteric metal, which will occur in a strong alkaline environment Strong hydrogen evolution corrosion, release a large amount of hydrogen, reduce the output power of the battery and the utilization rate of the anode, and it is more serious at a high current density. If it is simply to solve the above problems, you can choose to replace the electrolyte regularly and add additives to the electrolyte that can activate the surface of the aluminum anode and suppress the hydrogen evolution corrosion of aluminum to solve the appeal problem.
Air electrode (positive electrode)
The cathode is the reaction site of O2, which is breathable, conductive, waterproof, anti-corrosion and catalytic, and is also often called an air electrode. The air electrode is generally composed of a porous catalytic layer, a conductive current collector, and a waterproof and breathable layer. The porous catalytic layer is the main place where oxygen is reduced. Three-phase interface electrochemically active sites; the conductive current collector mainly plays the role of electrical conductivity and mechanical support; the waterproof and breathable layer has a loose, porous and hydrophobic structure, which not only provides the gas required for the reaction of the catalytic layer, but also prevents the electrolyte from diffusing the gas Channel flooded.
The catalytic layer is the most critical part of the air electrode, which plays a decisive role in its electrochemical performance. The performance of aluminum-air batteries depends largely on the selected cathode catalyst. The performance of the air electrode can directly affect the balance of the electrode reaction. Therefore, improving its performance can improve the utilization rate of the anode of the aluminum-air battery to a certain extent and inhibit the self-corrosion of the anode aluminum.
Commonly used catalysts for aluminum-air batteries are as follows:
(1) Noble metal catalyst. Platinum and silver are commonly used, and their catalytic activity and high performance are relatively stable, but their adoption rate is not high due to their high price and shortage of resources.
(2) Metal macrocyclic compound catalysts. Organometallic macrocycles show good catalytic activity for oxygen reduction, especially when they are adsorbed on carbons with large surface areas. And their activity and stability can be significantly improved by heat treatment. Therefore, it is expected to replace noble metal oxygen reduction catalysts. Common synthesis methods of metal macrocycles include thermal decomposition and precursor preparation. However, since the heat treatment process of the thermal decomposition method will cause the metal macrocyclic compound to react with the carbon matrix, the catalyst prepared by the precursor method has poor activity, so there are certain problems in application.
(3) Perovskite oxide catalyst. Perovskite oxides have high catalytic activity for the reduction and evolution of oxygen, and are inexpensive, so they have broad application prospects in aluminum-air batteries and fuel cells. The current research on perovskite oxygen electrode catalysts mainly focuses on improving the preparation method and finding new substitution elements to improve the catalytic performance. The amorphous precursor method, especially the malic acid precursor method, can prepare perovskite oxides with fine grains and large specific surface area, thereby greatly improving their catalytic activity, and is currently a better method for preparing perovskite oxides. Methods.
(4) Cheap catalysts. The most important representative is manganese dioxide catalyst. Its biggest advantage is that it is rich in raw materials and low in cost and can be widely used in batteries with aqueous or non-aqueous electrolytes. However, the electrocatalytic activity of a single manganese dioxide has certain limitations, so people here Research in this area has never stopped.
(5) AB2O4 spinel oxide catalyst. The crystal lattice of spinel is face centered cubic. There are 32 close-packed 02- ions in the unit cell, 64 tetrahedral voids and 32 octahedral voids occupied by metal ions. The dehydration activity of spinel is related to the fraction of B ions located in tetrahedral voids. The larger the fraction, the more acidic the surface of the catalyst and the greater the dehydration activity. Generally, this catalyst is not used in aluminum-air batteries.
(6) Other metal and alloy catalysts. Nickel is relatively cheap and has high corrosion resistance under anodic polarization conditions in alkaline electrolytes. At the same time, nickel has the highest oxygen evolution efficiency among metal elements, so nickel is traditionally used as an anode material for alkaline water electrolysis. Alloy catalysts such as nickel-iron and nickel-cobalt are also often used, which have good catalytic activity and corrosion resistance, and are also a catalyst direction that can be considered for aluminum-air batteries.
(7) Composite catalyst. Two or more catalysts are combined to better improve the catalytic activity of the air electrode of the aluminum-air battery.
Application prospect of aluminum-air battery
At present, aluminum-air batteries have not been widely applied in industrial and civil fields, mainly due to the need to improve the material preparation technology and the understanding of the concept of its secondary charge and discharge.
At the technical level: the measured specific energy and discharge efficiency of aluminum-air batteries are quite different from the theoretical values, and the main technical problems include:
(1) The self-corrosion and hydrogen evolution of the aluminum anode greatly restrict its discharge efficiency, and the surface passivation of the aluminum anode affects its discharge response aging;
(2) The compatibility between the electrolyte and the anode can not only form a rapid anodic oxidation reaction response mechanism with the aluminum electrode, but also maintain the efficiency and stability of ion transfer, and the recyclability of oxidation products, etc.;
(3) The structure of the air electrode, the self-loss of the current in the conductive current-collecting layer, and the oxygen reduction ability of the air electrode catalyst need to be further optimized and improved.
The idea of aluminum-air battery as a secondary battery: aluminum-air battery as a metal fuel cell is generally considered to be a primary battery, which is a misunderstanding of the charge-discharge cycle unit. The lithium-ion battery that we commonly use now is a classic secondary battery, which can realize instant charge-discharge conversion. If the aluminum-air battery can realize the industrialized charge-discharge process, it can also be regarded as a secondary battery from the perspective of this large cycle system. secondary battery, and this is one of the key technologies to solve its popularization and application.
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