The theoretical specific energy of aluminum-air fuel cells 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 battery has great commercial potential in special, civil, and underwater power systems, backup power sources for telecommunication systems, and portable power supplies.
Overview of Metal Air Batteries
Lithium-ion batteries have high specific energy and are secondary batteries that are relatively mature in research and have been commercialized on a large scale. However, in the face of tremendous development in the fields of mobile electronic equipment and electric vehicles in recent years, lithium-ion batteries have been difficult to meet. The demand for its large capacity, 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, and lithium-air batteries.
Since the positive active material of this type of battery mainly comes from oxygen in the air, the theoretical amount of positive active material is infinite, 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.Also read:72v 200ah lithium battery pack
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 battery has great commercial potential in special, civil, and underwater power systems, backup power sources for telecommunication systems, and portable power supplies.
Aluminum-air battery structure and principle
From the analysis of the existing research results and battery characteristics, the aluminum-air battery has 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 products currently developed can reach 300-400Wh/kg, which is much higher than the specific energy of various types of batteries today.
(2) The specific power is medium. Since the working potential of the air electrode is far from its thermodynamic equilibrium potential, its exchange current density is very small, and the polarization of the battery is very large during discharge, resulting in the specific power of the battery only reaching 50-200W/kg.
3) Long service life. The aluminum electrode can be replaced continuously, so the lifespan of the aluminum-air battery depends on the working life of the air electrode.
(4) Non-toxic, 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, etc.
(5) Strong adaptability. The structure of the battery and the raw materials used can be changed according to the practical environment and requirements, and it has strong adaptability.
(6) Aluminum, the raw material of battery negative electrode, is cheap and easy to obtain. Compared with other metals, the price of metal aluminum is relatively low, and the manufacturing process of metal anodes is relatively simple.
Aluminum anode (negative electrode)
Aluminum (Al) is an ideal electrode material. The theoretical energy density of metal aluminum is 8.2W˙h/g, which is second only to 13.3W˙h/g of lithium among common metals. It is a light metal battery material with the highest mass specific energy other than lithium metal. 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.Also read:aimeno
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 and corrosion in a strong alkaline environment, which affects the electrode potential, and the product floats in the electrolyte to affect 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 "submerge" or "dry up", or even "climb alkali" or "leak liquid". Damage to the structure of the battery. In order to solve the above problems, scholars at home and abroad have carried out research from the following three aspects:
- Aluminum anode alloying
Industrial grade aluminum (99.0%) contains many impurities, such as iron (0.5%), silicon, copper, manganese, magnesium and zinc, which will aggravate the hydrogen evolution corrosion of aluminum at the phase interface, especially iron and aluminum will form localized galvanic cells, resulting in exponentially increased electrochemical corrosion. Alloy components that can improve 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: ① the melting point of the alloying element is lower than that of the metal Al; ② the solid state saturation in Al is higher; ③ the electrochemical activity is higher than that of Al; ④ the solubility in the electrolyte is relatively high. High; ⑤ has a high hydrogen evolution overpotential. In addition, processing the anode metal into an ultra-fine-grained material can further improve anode efficiency.
- Add a 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 acid, amine and amino acid slow-release agents and their inhibition efficiency on aluminum corrosion are shown in Table 1:
Researchers use natural substances as inhibitors of aluminum corrosion, and experiments have shown that organic amines, pyrroles, etc. have obvious inhibitory effects on aluminum corrosion. By adding organic compounds and water-soluble compounds to strong alkaline electrolytes, the electrochemical behavior of aluminum metal anodes was studied 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 best heat treatment process can be found by suitable orthogonal experiments.
Electrolyte
The electrolyte of aluminum-air battery 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 of the aluminum anode is seriously passivated, which reduces the working voltage, makes it difficult to increase the power and current of the battery, and also leads to 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, releasing a large amount of hydrogen, reduces the output power and anode utilization of the battery, and is more serious at high current density. If it is a simple solution to 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 inhibit the corrosion of aluminum and hydrogen evolution to solve the appeal problem.
Air Electrode (Positive)
The cathode is the reaction place of O2, which is breathable, conductive, waterproof, anti-corrosion and catalytic. It is also often called the air electrode. The air electrode is generally composed of a three-layer structure 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. Electrochemical active sites at the three-phase interface; the conductive current collector mainly plays the role of electrical conduction and mechanical support; the waterproof and breathable layer has a loose porous hydrophobic structure, which not only provides the gas required for the reaction for 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 and plays a decisive role in its electrochemical performance. The performance of the aluminum-air battery depends largely on the selected cathode catalyst. The performance of the air electrode can directly affect the electrode reaction balance. 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 include the following:
(1) Precious metal catalysts. 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 catalyst. Organometallic macrocycles have good catalytic activity for oxygen reduction, especially when they are adsorbed on large surface area carbons. 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 macrocyclic compounds include thermal decomposition method and precursor preparation method. 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 the application.
(3) Perovskite oxide catalysts. 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. It is currently the best preparation of perovskite oxides. Methods.Also read:48v lithium ion battery 200ah
(4) Inexpensive catalysts. The most important representative is manganese dioxide catalyst. Its biggest advantage is that it is rich in raw materials and low cost. It can be widely used in batteries with aqueous or non-aqueous electrolytes. However, the electrocatalytic activity of a single manganese dioxide is limited. Research has never stopped.
(5) AB2O4 spinel oxide catalyst. The crystal lattice of spinel is face-centered cubic. There are 32 close-packed O2-ions in the unit cell, 64 tetrahedral voids and 32 octahedral voids are occupied by metal ions. The dehydration activity of spinel is related to the fraction of B ions located in the tetrahedral voids. The larger the fraction, the more acidic the catalyst surface and the higher 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 the condition of anodic polarization in alkaline electrolyte. At the same time, nickel has the highest oxygen evolution efficiency among metal elements, so nickel is traditionally used as the 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 feasible catalyst direction for aluminum-air batteries.
(7) Composite catalyst. Combining two or more catalysts together can 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 used in industrial and civil fields, mainly due to the need to improve the material preparation technology and the understanding of the concept of 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 exist, including
(1) The self-corrosion and hydrogen evolution of the aluminum anode largely restrict its discharge efficiency, and the surface passivation of the aluminum anode affects its discharge response aging;
(2) The matching between the electrolyte and the anode can not only form a rapid anodic oxidation reaction mechanism with the aluminum electrode, but also maintain the high efficiency and stability of ion transfer, and the recyclability of oxidation products;
(3) The structure of the air electrode, the self-loss of the current in the conductive current collector, and the oxygen reduction capacity of the air electrode catalyst need to be further optimized and improved.
The concept of aluminum-air battery as a secondary battery: As a metal fuel cell, the general concept of aluminum-air battery is that it is a primary battery, which is a misunderstanding in the understanding of the charge-discharge cycle unit. As a classic secondary battery, the lithium-ion battery that we generally use now 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.
Pure aluminum or alloy undergoes anodization reaction (discharge) to form Al2O3˙nH2O, Al2O3˙nH2O is calcined to form Al2O3, and further electrolytic reduction (charging) can regenerate aluminum; Al2O3 and
Al2O3˙nH2O can also be used as a raw material for the preparation of chemical alumina.
The high specific energy of aluminum-air battery and its safety and harmless to the environment determine that it will have a broad development prospect. In the future, it may be first used in mobile equipment such as power vehicles and mining. The aluminum-air battery can be designed as an integrated battery pack, which is stored at a gas station or a special charging station like gasoline and other fuels. When the anode is used up during use, the battery pack can be directly replaced. The discharged battery pack is handed over to a professional technical company for the separation and recovery of Al2O3˙nH2O and the secondary assembly of the battery pack.