Effects of external field treatment on the electrochemical behaviors and discharge performance of AZ80 anodes for Mg-air batteries

doi:10.1016/j.jmst.2019.07.043

Abstract

Abstract:

In this work, the effects of external field treatment on electrochemical behaviors and discharge performance of as-cast Mg-8%Al-0.5%Zn (wt%) anodes for Mg-air batteries are systematically investigated in 3.5 wt% NaCl solution. The external field treatment particularly the variable-frequency ultrasonic field can dramatically refine the α-Mg grains and β phases. The grain size decreases from 1594.8 ± 43 μm (untreated billet) to 140.6 ± 15 μm after variable-frequency ultrasonic field treatment. The values of open circuit potential, electrochemical activity and corrosion resistance of Mg-8%Al-0.5%Zn anode are improved with external field treatment, which should be attributed to refined grains and dispersive β phase. The external field treatment especially the variable-frequency ultrasonic field improves stability and the value of cell voltage, and enhances the discharge performance of the Mg-8%Al-0.5%Zn anode. The variable-frequency ultrasonic field treated anode has the best discharge capacity and anodic efficiency at current density of 45 mA cm^-2, with the values of 1417 mA h g^-1 and 63.3%, respectively. The ultrasonic field vibration changes the dissolution behavior of α-Mg matrix during the discharge process, showing the oriented surface morphologies.

Key words: Mg-air batteries, Magnesium alloy, Discharge performance, Electrochemical behaviors, Ultrasonic field, Electromagnetic field

Xingrui Chen, Shaochen Ning, Qichi Le, Henan Wang, Qi Zou, Ruizhen Guo, Jian Hou, Yonghui Jia, Andrej Atrens, Fuxiao Yu. Effects of external field treatment on the electrochemical behaviors and discharge performance of AZ80 anodes for Mg-air batteries[J]. J. Mater. Sci. Technol., 2020, 38: 47-55.

Figures/Tables 11

Table 1 Chemical composition of AZ80 alloy (wt%).

Al	Zn	Mn	Si	Fe	Ni	Cu	Ca	Mg
8.45	0.611	0.405	0.006	0.003	0.004	0.002	0.001	Bal.

Fig. 1. Schematic of the experimental procedure: (a) the semi-continuous casting for the Mg anodes; (b) the AZ80 billet; (c) Mg-air battery test.

Fig. 2. Microstructures of as-cast AZ80 alloys by different methods: (a) traditional DC method; (b) with FUF treatment; (c) with LEF treatment; (d) with VUF treatment; (e) statistic of grain size.

Fig. 3. SEM images of as-cast AZ80 anodes with different field treatment: (a) traditional DC method; (b) with FUF treatment; (c) with LEF treatment; (d) with VUF treatment; (e) statistic of area fraction of β phase.

Fig. 4. Open circuit potential (a) and polarization curves (b) of investigated as-cast AZ80 magnesium alloys treated by different external fields in 3.5 wt% NaCl solution.

Table 2 Fitting results of the polarization curves.

Anode	E_OCP (V_SCE)	I_corr (A cm^-2)	E_corr (V_SCE)
DC	-1.592	5.88×10^-5	-1.463
FUF	-1.599	3.02×10^-5	-1.477
LEF	-1.597	4.78×10^-5	-1.475
VUF	-1.612	1.66×10^-5	-1.490

Fig. 5. Electrochemical impedance spectra of as-cast AZ80 alloys with different external field treatment in 3.5 wt% NaCl solution: (a) Nyquist plots; (b) equivalent circuit; (c) charge transfer resistance and induction.

Fig. 6. Discharge curves of AZ80 anodes casted with different field treatment in 3.5 wt% NaCl solution at current density of (a) 1.5 mA cm-2, (b) 15 mA cm-2, (c) 30 mA cm-2 and (d) 60 mA cm-2.

Table 3 Average discharge potential during the discharge tests.

Current (mA cm^-2)	DC	FUF	LEF	VUF
1.5	1.401	1.432	1.424	1.471
15	1.249	1.276	1.279	1.330
30	1.137	1.165	1.176	1.210
60	0.954	1.029	1.022	1.088

Fig. 7. Discharge properties of assembled Mg-air batteries with different anodes: (a) discharge capacity and average cell voltage; (b) anodic efficiency.

Fig. 8. Surface morphologies of (a, b) DC, (c, d) FUF, (e, f) LEF, (g-i) VUF AZ80 anodes after the discharge in 3.5 wt% NaCl solution at 1.5 mA cm-2 for 5 h after removing of discharge products.

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