[China Aluminum Network] The five elements of the aluminum fuel cell mentioned by the author here refer to the five major components of the battery: anode, cathode, electrolyte, catalyst, and product Al(OH)3. Now a comprehensive and simple introduction to all aspects of them.
Aluminum plate
The anode of the aluminum fuel cell is made of aluminum alloy, and pure aluminum cannot be used. The first reason is that the passivation of pure aluminum is very fast and the polarization is serious. The passivation film on the surface of pure aluminum, namely the oxide film (of course also on the aluminum alloy) is only about 5nm thick, but it is dense, resulting in an increase in negative polarization, positive shift in potential, and voltage lag. The oxidation (passivation) process of aluminum can be divided into three stages: the formation of amorphous Al2O3, the formation of crystalline oxides, and the extremely slow oxidation process.
Another reason why pure aluminum cannot be used is that aluminum will undergo corrosion in the electrolyte, also known as self-discharge, and precipitates hydrogen (6Al+6H2O=2Al(OH)3+3H2↑), reducing the coulombic efficiency, mixing, and addition of the electrode. Large mass transfer coefficient.
Industrial pure aluminum, Fe, Cu, Si and other impurities make a significant increase in self-corrosion, Mn can offset the adverse effects of iron, no manganese, Fe and Al to form Al3Fe, aluminum positive for the matrix; there are Mn when the formation of Al6FeMn Its potential is almost equal to that of the substrate aluminum. The addition of Mg leads to the cathodic polarization of aluminum in the substrate, which causes the negative shift of aluminum potential and less corrosion. At the same time, if the Mg content is large, Mg2Si can be produced. Its electrochemical properties are similar to those of aluminum, which reduces the difference in electrochemical properties and reduces aluminum. Corrosion.
The corrosion rate and electrochemical activity of aluminum electrodes in alkaline electrolytes are related to their crystal structure: Pure aluminum crystals are anisotropic, with few active sites, and the corrosion is not uniform across the surface of the whole electrode. The addition of certain alloying elements can weaken The anisotropy or disappearance of the original crystal, so that the entire electrode surface is uniformly eroded, but once the content of the alloying element is too large, a new phase will be formed, enriched in the grain boundary, preferential dissolution of the grain boundary, resulting in uneven electrode Corrosion and corrosion rate increase. The finer and more uniform the aluminum alloy grains, the more uniform the aluminum anode surface dissolution, the higher the aluminum electrode current efficiency.
The dense, passive oxide film on the surface of the aluminum cell increases the internal resistance of the cell, limiting its electrochemical activity. Therefore, high-quality aluminum anodes must have an activated oxide film layer and high corrosion resistance. Current research shows that elements such as Ga, Bi, Pb, Sn, In, Mg, Ti, Mn, Ce, and Si can both Increasing the activity of the aluminum oxide film can also inhibit the aluminum corrosion. For example, tin ion Sn+4 enters into the alumina film and replaces Al+3 ions, and holes are generated, so that the resistance of the aluminum oxide film is significantly reduced, the potential of the aluminum electrode is significantly negatively shifted, and the current is significantly increased; Ga is deposited on aluminum. The surface of the anode is activated, when the Al-Sn-Ga alloy anode is dissolved, Sn and Ga dissolve into the solution, Sn+4 deposits on the surface of the aluminum electrode, and Ga ions are subsequently deposited in the vacancy created by the tin, so that the surface of the electrode continuously generates new The active site, and thus the Al-Sn-Ga alloy has a high activation characteristic.
As mentioned earlier, Mn can counteract the adverse effects of Fe. Moreover, when the anode of Al-Mn alloy is polarized, a double oxide film is formed due to the enrichment of Mn, the inner layer is mainly Al2O3, but there are also Mn, and the outer layer is Mn rich layer. The potential of Mn is more negative and increases the activity of the alloy.
The electrochemical performance of Al-0.3Ga-0.3Bi-0.45Pb quaternary aluminum alloy developed by Harbin Institute of Technology is a good material for the anode of aluminum fuel cells; the potential in the 4mol/L NaOh solution was developed by China Shipbuilding Industry Corporation. The Al-Pb-Ga aluminum alloy of -1.37 V (for Hg/HgO electrode); the Al-0.1Sn-0.05Ga aluminum alloy developed by Nestoridi et al. have an open circuit potential of -1.5 in NaCl solution. V, current density> 0.2 A/cm2 (for SCE electrode); Wenjiba et al. of Henan University of Science and Technology made in-depth study on the microstructure and properties of Al-Zn-In alloy, and developed Al-5Zn-0.03In- Anode performance of 5Mg-0.05Ti-(0.1Si, 0.5Mn, 0.5Ce) anode alloys.
Ga, In, Zn, Sn, and the like of the above alloy are some elements that can form a low-melting point eutectic with Al, and can make the aluminum electrode meet the requirement of large-current discharge, and they constitute a eutectic in the low-melting point eutectic alloy. The battery is in a molten state at the operating temperature, and the passivation oxide film becomes a microporous structure, thereby increasing the area of ​​the electrolyte and the aluminum substrate, improving the discharge performance, and negatively shifting the electrode potential. After addition of alloying elements that can form low melting point eutectics in pure aluminum, the open circuit voltage of the material can generally move more than 500mV in negative direction, and its electrochemical performance is greatly improved.
The corrosion of aluminum in the electrolyte is always accompanied by the hydrogen evolution effect, and the corrosion behavior of aluminum can be suppressed by suppressing the hydrogen evolution reaction. Since the hydrogen evolution reaction is related to the hydrogen overpotential of the electrode, the addition of high hydrogen overpotential elements can greatly reduce the corrosion and increase its utilization. Elements that increase the hydrogen overpotential include Bi, In, Pb, Hg, Cd, Sn, Tl (é“Š), Zn, and the like.
We know that the current efficiency and corrosion morphology of aluminum anodes depend on its microstructure. This microstructure, in addition to being affected by alloying, also has a lot to do with the heat treatment of the material. The effect of heat treatment on the anodic aluminum alloy is: uniform treatment, negative potential, and low polarization; annealing, slightly positive shift in potential, polarization is also small; quenching, polarization increases, uneven surface anodic dissolution; quenching and aging, materials Will contain thermal defects, uneven corrosion. In these four kinds of heat treatment, the current efficiency of the Al-Zn-In alloy anode is uniform and the annealing has a higher current efficiency, which can reach 94%-98%; the latter two treatments will be in the microscopic view of the material. Thermal defects (fault grooves) were induced in the tissue and local dissolved corrosion occurred. The anode current efficiency was about 69%.
The shape of the aluminum anode also has a certain influence on the battery performance. A suitable electrode shape can reduce the corrosion of the aluminum anode and increase the battery power and discharge density. There have been studies on aluminum anodes of different shapes, such as cylindrical, flat, wedge-shaped, etc. It is difficult to say which shape is better now, but much more is used today.
Air electrode and catalyst
The core of the aluminum fuel cell is the air electrode, ie, the cathode, which consists of a gas permeable membrane and a catalyst. The catalyst is platinum pt because of its good activity, stability and selectivity. The oxidant (oxygen) is stored in the external container of the battery. When it is needed, it will flow into the battery cathode or be pumped into it under pressure. Actually, most batteries use air, with only a small amount of pure oxygen. Before the air enters the cathode, it should pass through. Purification treatment.
Researches on oxygen electrodes mainly focus on two aspects: optimization of electrode structure, improvement of the gas-phase mass transfer rate of oxygen, high-efficiency catalysts and lower-cost catalysts, overcoming severe electrochemical polarization in the oxygen reduction process. In the production cost of aluminum fuel cells, the precious metal pt catalyst occupies a large proportion, and the precious metal catalyst is very sensitive to poisoning and sintering. In 2016, the world’s platinum production was 189.8 tons, China’s output was 3.3 tons, and the Shanghai Gold Exchange’s annual average price was 221.81 yuan/g. The carbon itself as a catalyst carrier also has a certain catalytic effect.
The early catalyst research focused on precious metals such as Ni, Ag, Pt, etc. They are not only expensive, but also do not fundamentally solve the problem of catalytic activity. In recent years, certain achievements have been made in the study of organic catalysts and metal complex oxide catalysts, particularly in the case of calcium iron oxide complex oxide catalysts. The cheap MnO2 has a certain catalytic effect on the oxygen reduction process, and the rare earth oxides are acidic, and they are chemically active and have an osmotic effect. The La0.6Ca0.4CoO3 perovskite type oxide catalyst prepared by the sol-gel method has a good catalytic effect on the oxygen cathode of the aluminum fuel cell, and its stimulatory effect is stronger when its content is 25%. The catalytic performance of the mixed catalyst is often better than that of a single catalyst, for example, 5% La0.6CoO3+15%CaO, 10%La0.6Ca0.4CoO3+10%CaO, 5%La0.6Ca0.4CoO3+15%ZnO, 10%La0 .6Ca0.4CoO3+10%MnO2 and so on have very good electrochemical performance.
Very New Energy Technology Co., Ltd. successfully prepared a continuous production process of high-efficiency oxygen reduction catalytic materials and air diffusion electrodes. They used a new type of catalyst formulation. The prepared air electrode is not only low in cost but also superior in performance, and its discharge density is similar to that of foreign countries. In 2013, the company built the first continuous semi-automated fuel cell air cathode production line in China.
In order to meet the needs of the continuous development of smart grids, mobile communications, electric vehicles and emergency rescue, the Ningbo Materials Research Institute of the Chinese Academy of Sciences successfully developed a kilowatt-class aluminum air battery power generation system based on a graphene air cathode in May 2017, with an energy density as high as 510Wh/kg, capacity 20kWh, output power 1000W, this system can supply power for 1 TV, 1 PC, 1 fan and 10 60W lighting bulbs at the same time. The research team is actively developing 5kW-class high-power aluminum fuel cell systems for communication base station backup power supplies and electric vehicle rangers. In 2016, China had about 6 million communication base stations and used more than 10 million sets of lead-acid battery packs. It was urgently needed to replace them with performance-based aluminum fuel cells.
Electrolyte (liquid) and product Al(OH)3
At present, the electrolyte used in aluminum fuel cells is alkaline and neutral, but it is mainly alkaline, because it can remove the oxide passivation film on the aluminum anode, generate large current, accompanied by severe hydrogen evolution corrosion, Commonly used NaOH or KOH solution. Neutral electrolyte is mostly NaCl solution. Although hydrogen evolution corrosion decreases, the battery reaction will produce Al(OH)3, which will reduce the electrolyte conductivity, and it will accumulate and be released in time. Otherwise, the electrolyte becomes paste or even semi-solid. There are other methods of treatment, such as periodically changing the electrolyte, circulating the electrolyte, or adding seeds to the electrolyte to precipitate flocculent Al(OH)3. The performance of the acidic H2SO4 electrolyte solution is better than that of the neutral NaCl solution because Cl- ions can initiate uniform pitting on the aluminum anode surface, enhancing the research in this area, or the electrolyte with better performance will appear.
In addition, according to GMWu et al., a solid electrolyte composed of Poly Ving'alcoho (PVA) and Polyacrylic acid (PAA) has good hydrophilicity and uniform structure, and is excellent in alkaline solution. In terms of catalytic performance, anode utilization can reach 90% at PVA:PAA=10:75.
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