冷变形对铝阳极材料电化学能的影响.doc

The Effect of Cold-rolling Deformation on the Electrochemical Properties of Al Anode MaterialsHE JunguangCorresponding author. Tel.: +86 fax: +86 379 64230597. E-mail address: (J. G. He).a,b，WEN Jiubab，LI Xudongaa School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, PR Chinab School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, PR ChinaAbstract: Based on as-cast Al-0.1Ga-0.1Sn-0. 5Bi (wt. %) aluminum anodes, the effect of cold-rolling deformation on the electrochemical properties of Al anode materials in 4 mol.L-1 KOH solution was investigated. The results indicate that cold-rolling deformation can improve the electrochemical properties of alloys compared with cast alloy. With the increase of the deformation, the self-corrosion rate and hydrogen evolution rate decreased firstly and then increased slightly. the self-corrosion rate and hydrogen evolution rate was lowest and discharge time was longest with the alloy of 80% cold rolling reduction. Microstructure and anodes corrosion dissolved mechanism of aluminum were analyzed with metallographic analysis technology and polarization curve, too.Keyword: Aluminum anodes, Cold-rolled deformation, Electrochemical properties, Metallographic structure1. IntroductionAluminum is undoubtedly suitable for the anodic material of metal/air batteries due to its advantages such as abundant, low cost, environmentally benign and metallurgically workable without special equipment, etc. Its electrochemical equivalent (2.98 Ah/g) and theoretical specific energy (8.1 kWh/kg) are second only to lithium. However, pure aluminum will forms a passive oxide film on its surface when it is exposed to the environment, which leads to a reduction of the anodic voltage efficiency. So, some trace elements such as gallium, tin, and bismuth, magnesium, etc. should be added for improving its performance.Much attention has been paid to adding different elements to improve the electrochemical properties of aluminum anode since 1960s 1-6. In addition, Despic et al. 7-9 investigated the influence of heat treatment on the electrochemical properties of aluminum. And a number of Al-air batteries in saline or alkaline electrolyte systems have been previously reported 10-11. However, the reports of effect of cold deformation on the electrochemical properties of alloy anode were few published. Nestoridi et al. 12 considered that aluminium alloys after cold rolling not heat treatment had the smallest corrosion rate. Robinson et al. 13 founded that alloy intergranular corrosion resistance declined with the increasing of deformation. Moreover, the researches for deformation process on electrochemical performance of aluminum anodes have seen relatively little.The experiments in this paper, based on the aluminum anode materials with additions of tin, gallium and bismuth, we studied the influence of the cold-rolling deformation on electrochemical properties for aluminum anodes.2. Experimental methods2.1. Material preparationThe nominal compositions of alloys in present experiment are 0.1%wt. Ga-0.1%wt. Sn-0. 5% wt. Bi-Al. Raw materials are high purity aluminium, tin, gallium and bismuth (99.9%) for casting the above anodes alloys. Raw materials were cut, dried, weighed the required amount of materials and melted in a corundum crucible in ZGJL0.01-4C-4 vacuum induction furnace with argonshield at 760 for 5 min. The molten alloy was poured in a preheated cast iron dye of dimensions 20 mm 140 mm with air cooling. The external part of ingot was eliminated to avoid segregation problems. Samples were cut transversely into 1.0 mm, 1.2mm, 2.5mm, 5mm, 10mm, respectively. They were then homogenized for 2h at 673 K, and cold rolled to a thickness of 1 mm using double roller mill apart from the specimen of 1 mm. The samples were further cut into 1 cm1 cm. The electrode was covered by AB gum (a commercially reagent) except working area (1cm2). The working area was polished step by step with different grain sizes of emery paper, the oil was removed with acetone, washed with distilled water, and dried in vacuum.2.2. Self-corrosion rateAnode samples were immersed in 4mol.L-1 KOH solution for 30 min followed by samples were taken out. The corrosion products of the samples were cleaned out in a solution of 68% HNO3 for about 510 min, then rinsed by deionized water, and dried. The weight of the sample before and after the immersion of solution was measured. Self-corrosion rate was calculated based on the measurement of weight loss. 2.3. Hydrogen evolution rateThe hydrogen evolution rate of the aluminum anode was measured with drainage method using a capacity flask with a graduated tube in 4 mol.L-1 KOH alkaline solution. The hydrogen evolution rate is represented by the volume of hydrogen that was released from the consumption of aluminum alloy. 2.4. Anode efficiencyAnode efficiency was carried out in 4mol.L-1 KOH solution at room temperature by Blue electric batteries testing system. Graphite was used as the cathode. The working area of the test anode is 1 cm2. The ratio of anode to cathode surface area is 1:12. Anode and cathode were immersed in 4mol.L-1 KOH solution at current density of 25 mA.cm-2 for 60 minutes. Anode efficiency was calculated based on the measurement of weight loss.2.5. Polarizing curve and ac resistance diagramThe anode polarizing curve and ac resistance diagram were carried out with three electrodes system at room temperature by CHI660C electrochemical test system (CHI Company, USA). A saturated calomel electrode (SCE) served as the reference electrode and a graphite electrode was used as the counter electrode. The scanning scope of the polarizing curve was -2.0-1.0V, while the scanning speed was 0.001Vs1. Ac impedance spectra were recorded when the open circuit potential was stabilised after immersed in 4 mol.L-1 KOH solution for 30 min, using an excitation voltage of 5 mV. The frequency range studied was between 10 kHz and 100 mHz.2.6. Surface determination of the aluminum ElectrodesThe samples were ground with emery paper (grade 400-800-1000-2000), polished with 2.5 and 1.5 m diamond paste, rinsed with ethanol in an ultrasonic cleaner, etched with 2.5% HN03 + 1.5% HCl + 1% HF solutions. Microstructure was observed on the surface of the polished samples using optical microscopy.3. Results and Discussion3.1. Electrochemical performanceTable 1 shows electrochemical performance of the aluminum anode alloys with various deformations. As shown in Table 1, the self-corrosion rate of the as-cast alloy was higher, after cold-rolling deforming, it became lower relative to as-cast alloy. With the increase of the deformation, the self-corrosion rate decreased firstly and then increased slightly. The self-corrosion rate was lowest with the alloy of 80% cold rolling reduction. Hydrogen evolution rate of anode alloy after deforming was lower than that of as-cast alloy. With the increase of the deformation, there was the same change tendency with self-corrosion rate. The reason may be attributed to the reducing of defects and the improvement of structures the alloy during cold-rolling. The results indicate that the cold rolling deformation of anode alloy is favor to the reducing of self-corrosion and hydrogen evolution, it is important significance for the improvement of the utilization rate of anode alloy and the extending battery discharge time. Comparing with as-cast alloy, open circuit potential of alloy after cold-rolling deforming shifted to the negative. Anode efficiency after deformation was higher than that of as-cast alloy. With the increase of the deformation, anode efficiency increased firstly and then increased slightly. The self-corrosion rate was lowest with the alloy of 80% cold rolling reduction.Table 1 Electrochemical performance for aluminum anodes alloys in different deformationAs-cast40%60%80%90% OCP/V(SCE)-1.6563-1.6449-1.6579-1.7241-1.6570Self-corrosion rate/mgcm-2min-10.090280.083330.076390.069440.0722hydrogen evolution rate/mlcm-2min-10.006430.005710.004800.004280.00630anode efficiency/%86.0389.4792.6696.1887.523.3. Polarizing curveFig.1 shows the Polarizing curve of different deformation of cold-rolling aluminum anodes at 1 mV.s-1 in 4 mol.L-1 KOH solution. The self-corrosion potential values of 60%, 80%, 90% cold rolling reduction were about in -1.724, -1.6579, -1.657 V (Vs SCE), respectively. The self-corrosion potential value of as-cast alloy was -1.656 V (Vs SCE). Comparing with as-cast alloy, cold-rolling deformation of alloy caused corrosion potential negative shift. Fig.1. Tafel polarization curve of different deformation alloy in 4mol.L-1 KOH solution3.4. Electrochemical impedance spectraEIS results for as-cast alloy and the processed alloys exposed to 4 mol.L-1 KOH solution for 30 min were plotted in Fig.2. It was found that Cold-rolling alloy exhibited higher impedance value than the as-cast material. Higher impedance value indicated better corrosion resistance. Corrosion resistance has improved for cold-rolling samples compared to the as-cast metal. Defects such as vacancy, dislocation, etc. may be formed during cold-rolling deformation process, Al (OH)3, electrode reaction product, would adhere in these defects, activate reaction were hindered effectively, increase the reaction resistance.Fig.2. EIS curve of different deformation alloy in 4mol.L-1 KOH solution3.2. Discharge curveFig.3 shows the Constant resistance discharge curve of different deformation of cold-rolling aluminum anodes at 100 in 4 mol.L-1 KOH solution. Obviously, discharging voltage was 0.29 volt for as-cast alloy, potential fluctuated with time significantly during discharge process, and discharge voltage decreased suddenly after 5 hours discharge, the battery function was lost. Comparing to the as-cast alloy, discharge time was longer obviously after cold rolling deformation alloy. With the increase of the deformation, the discharge time of anode alloy increased firstly and then decreased. The discharge time of anode alloy was longest reaching 20 hours with the alloy of 80% cold rolling reduction. This result has a good agreement with the electrochemical properties in Table. 1. According to the analysis above, the 80% of the anode alloy had lowest hydrogen evolution and self-corrosion rate, highest discharging voltage, longest discharge time and the good electrochemical properties.Fig.3. Constant resistance discharge curve of different deformation for cold-rolled aluminum anodes at 100 in 4 mol.L-1 KOH solution.3.3. MicrostructuresThe effect of cold-rolling deforming on the microstructures of aluminum anode alloys is shown in Fig.4. All alloys were mainly consisted of a-Al matrix with segregation on continuous or semi-continuous network of grain boundaries. There was a lot of dendritic structure and element enrichment at grain boundaries in Figure 4 (a). Element enrichment of low melting formed grain boundary segregation in the dendrite, potential at grain boundary was lower than crystal interior, and the corrosion micro-cells formed between the Al matrix and the segregation, which made self-corrosion seriously, resulting in lower anode utilization efficiency, larger fluctuates of discharge voltages and poor performance. After cold-rolling deformation, the dendrite structure of anode alloy were broken into smaller particles, grain distributed evenly. It leaded to lower self-corrosion which caused by the fall of grain. Metastructure can be found in the crystal interior in figure 4 (b), maybe due to dislocation multiplication during rolling process, dislocation cells occur after dislocation pile-up and tangling. Fig.4. Metallographof the aluminum anode. (a) as-cast (b) cold-rolling4. Conclusions1. As-cast anode alloy has higher self-corrosion rate and hydrogen evolution rate, and its discharge potential is unsteadily.2. After cold-rolling reduction, Al-Zn-Sn-Ga anode alloy has lower self-corrosion rate, hydrogen evolution rate, open circuit potential, higer discharge potential and longer discharge time in 4mol.L-1 KOH solution.3. With the increase of cold-roll