Discharge curves of a zinc-air battery with sheet anode of (a) pure Zn, (b) Zn 67 Al 33 

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... Zn powders not working for Zn-Al powders. Melting and rolling were designed to prepare the anodes, then the difficulties lay in, among others, the lack of porosity that was created through PTFE-bonded electrolytic Zn powders in conventional Zn-air batteries. The designed Zn-Al alloy anodes were intended for applications in mechanical recharge systems. There are several advantages of Zn and Al, including abundance of raw materials, low cost, low toxicity, easy handling and high specific energy. Three Zn-Al alloys (13.4, 33 and 41 atomic percent Al, respectively) were chosen according to the Al-Zn binary phase diagram [18]. Binary alloy ingots with nominal compositions of Zn 100 − x Al x (x= 13.4, 33 and 41 atomic percent, respectively) were prepared by melting the mixtures of pure Zn (purity> 99.98%) and Al beads (purity> 99.95%) in an electric furnace at 685 o C under air atmosphere. Anode sheets were prepared by rolling, using a conventional rolling mill. The final thickness of the sheets was controlled to within 0.28 mm + 0.03 mm. The sheets were annealed at 240 o C for 12 hours after rolling to remove residual stresses and to stabilize the two-phase structure. The ICP-AES analysis was carried out to analyze the composition of the anode sheets. The density of the alloys was determined by the Archimedes’ method for specimens that were polished, degreased in acetone, and precision weighed prior to the tests. Test cells, with an interior size of 42 mm x 24 mm x 6.09 mm, were assembled with an anode sheet taken from those described above, 6.6 M KOH electrolyte, a separator made of non-woven cloth, and a commercially available air cathode (Evionyx, Taiwan). A BAT-750 charge/discharge equipment (Acutech Systems Co., Ltd., Taiwan) was used to perform discharge char- acterization at different constant currents, and to monitor the OCV of the cells. The discharge currents applied were 100, 150 and 200 mA, respectively. During the cell tests, no forced con- vection of air was used. The specific anode capacity, in mAh g − 1 , was calculated from the amperage, hours at end-point of discharge and the weight of the thin anode sheet. Preferential etching of the Al-rich phase in the rolled sheets was carried out by conventional etching, using a KOH solution of pH 9, wherein Zn is electrochemically stable according to the Pourbaix diagram [19]. For the polarization tests, specimens were characterized with a three-electrode configuration using a reference electrode Ag/AgCl (3 M KCl), and a Pt-coated Ti-gauze as a counter electrode. The 6.6 M KOH electrolyte, maintained at 25 o C, was purged with Ar for two hours prior to testing. The polarization curves were acquired using an electrochemical work- station (Model 604A, CH Instruments, USA) under a constant voltage-sweep rate of 1 mV s − 1 . The crystal structure of the anode sheets was characterized by a Shimadzu XRD-6000 X-ray diffractometer using Cu K α 1 radi- ation at a wavelength of 0.154056 nm. The metallurgical microstructure of the anode sheets before and after the electrochemical tests was examined by using a scan- ning electron microscope (SEM, JEOL 5410) equipped with an energy dispersive spectrometer (EDS). The compositions of the alloy phases were quantitatively measured using an electron probe microanalyzer (EPMA, JXA-8800M, JEOL) with the aid of a ZAF program [20]. The ICP-AES analysis showed that all three alloys had the cor- rect compositions as per the design specs, within an error < 3.2%. The density of Zn 86 . 6 Al 13 . 4 , Zn 67 Al 33 and Zn 59 Al 41 , being 6.41, 5.57 and 5.38 g cm − 3 , respectively, decreases with the increasing Al content. Compared with 7.14 g cm − 3 of pure Zn, the studied anode alloys are 10.2%, 22.0% and 24.6% lighter, respectively. The low weight contributes to the increased specific anode capacity. Hence it is advantageous for 3C (computer, communication and consumer) batteries which demand both lightweight and compact size. Figure 1 shows the X-ray diffraction patterns (XRD) of the Zn- Al anode sheets. The sheets compose of a two-phase structure, the Zn-rich and the Al-rich phases as per the design specs. The lattice parameters were then calculated and listed in Table 1. The lattice parameters of the phases in the Zn-Al alloys are slightly affected by the composition. This is due to the fact that a small amount of Al becomes substituted into the Zn lattice, and vice versa. As a result the c/a values of the Zn-rich phase and the a values of the Al-rich phase slightly decrease with the increase in Al content, as shown in Table 1. The theoretical and practical open circuit voltages (OCV) of a zinc-air battery are 1650 mV and 1400 mV, respectively. The lower practical OCV value is a result of passivation, hence the incomplete reactions at the cathode and the anode. The drastic decrease in voltage then flattening upon loading is typical of zinc-air batteries. The typical discharge behaviors of the test cells are shown in Fig. 2. The resultant specific anode capacity (mAh g − 1 ) and the OCV values are listed in Table 2. All the cells performed similar discharge curves with a long flat plateau characteristic of alkaline batteries. For the discharge tests of a pure Zn anode, the specific anode capacity decreased with higher discharging currents. A 17% decrease in specific anode capacity occurs in cells with a pure Zn anode when the discharge current increases from 100 mA to 200 mA. This is as a result from the serious anodic passivation, which in turn lowers the anode utilization of the rolled Zn electrode [21]. The addition of Al not only raises the OCV value but also increases the specific anode capacity. The cell with a Zn 67 Al 33 anode had the highest OCV value of 1560 mV compared with 1460 mV for the cell with a pure Zn anode. The higher OCV value is attributed to the higher oxidation potential of the Zn-Al alloys. Pure Al possesses a higher theoretical electrochemical equivalence of 2980 mAh g − 1 compared with the 820 mAh g − 1 for Zn. The anode capacities of the test cells with Zn-Al anodes show substantially improved values compared with that of the pure Zn anode, due to the fact that the alloying element Al also serves as the fuel. The highest resultant specific anode capacity was 720 - 800 mAh g − 1 for the test cells with the Zn-Al anodes. It is worth noting that a close examination of Table 2 shows that the resultant specific anode capacity of a cell with an anode of Zn-Al alloys actually slightly increases with a higher current loading between 100 mA to 200 mA. This is due to the two-phase structure of the Zn-Al sheets and the preferential etching of the Al-rich phase that contin- ues to react with KOH, first to create surface porosity, which in turn refreshes the surface of the Zn-rich phase for a continuing better discharge. In fact the two phases discharge alternatively and simultaneously. With increasing the discharge current the specific surface area of the alloy anode becomes higher due to the quicker penetration of the discharged Al-phase, leading to a higher specific anode capacity. Because Al reacts more severe than Zn in a concentrated alkaline solution, the Al-rich phase dissolves and discharges more quickly than the Zn-rich phase. The different reaction speeds cause un-balanced etching of the anodes and result in a self- generated increased porous structure with a high surface area. With a higher discharge current, the deeper penetrating channels will appear as a matter of course during the discharge and dissolution processes. The metallurgical microstructure of Zn 86 . 6 Al 13 . 4 and Zn 67 Al 33 anode before the electrochemical test are shown in Fig. 3. The elemental distribution was examined by EPMA. In order to obtain reliable quantitative data, a deliberative task was carefully employed in the EPMA analysis by choosing the appropriate accelerating voltage, beam current, and focus-beam size. The reported compositions as listed in this study are the average of at least ten measured points. The composition of the gray region in Fig. 3 (a) (Zn 86 . 6 Al 13 . 4 alloy), region I (the matrix) was Zn-4.0 at.% Al which could be assigned to the Zn-rich phase; the dark region, region II was Al- 42 at % Zn which could be assigned to the Al-rich phase. The composition of the gray region in Fig. 3 (b) (Zn 67 Al 33 alloy), region III (the matrix) is Zn-3.6 at.% Al which is again assigned to the Zn-rich phase; the dark region, region IV is Al-22 at.% Zn which is assigned to the Al-rich phase. The metallurgical microstructure of the etched Zn 59 Al 41 anode shown in Fig. 4 is typical of abundant etched pores, which serve as channels for the electrolyte to flow in. Self-generated porosity allows the penetration of electrolyte deep into the anode surface to result in better utilization and hence a higher specific anode capacity. The anodic polarization curves of the Zn-Al sheets are shown in Fig. 5. By the three-electrode measurement, Fig. 5, the OCV values of the Zn-Al sheets possess more negative poten- tials than the pure Zn electrode. Among the three Zn-Al alloys, Zn 86 . 6 Al 13 . 4 and Zn 67 Al 33 exhibit more negative voltages than pure Zn at all current densities. From a previous research [22], the surface morphology of the post-discharged Zn anode can be classified into many different structures, such as carpet-like form, thistle-like structure, and boulder-type growth, etc. The formation of a carpet-like form and a thistle-like structure are attributed to the precipita- tion phenomena. During the discharge process, the discharge product Zn(OH) 2 dissolves into the KOH electrolyte to form zincate, K 2 Zn(OH) 4 . After saturation, ZnO precipitates on the surface of the anode and forms a thistle-like structure that coa- lesces into a solid film. Once the ZnO film blocks the electrolyte permeation, the discharge reaction will be forced to terminate. Figure 6 shows the surface morphology of ...