| Magnesium?Mg?presents very interesting properties as low electrode potential?-2.37 V vs.SHE?with high Faradic capacity of 2202 mAh·g-1,environmental friendliness and easy machining.These outstanding characteristics of Mg and its alloys make them attractive in battery field.For primary batteries,Mg–MnO2 batteries display many superior features,such as 0.3–0.4 V high work voltage,threefold higher capacity than that of a theoretical one,long service life compared with the resemble model of traditional Zn–MnO2 cell.However,the intrinsic problems of self-corrosions and negative difference effect?NDE?limit their practical current efficiency in aqueous solutions.The another major obstacle for magnesium alloy is delayed action,caused by hydroxide layer passive film of spontaneously formed on the electrode surface in the aqueous solution,owing to the higher chemical and electrochemical activity of Mg alloy.To overcome these problems related to the Mg alloys,different techniques have been employed such as composite electrolytes and inhibitors,AGO-PVB hybrid coating,pulse technology to improve the electrochemical properties of AZ31B Mg alloy in MgSO4-Mg?NO3?2(0.14 mol·L-1MgSO4,1.86 mol·L-11 Mg?NO3?2)composite solution.Furthermore,the solid nucleation model and the corresponding kinetic process of Mg alloy surface under constant pressure signal were investigated.A new method for the preparation of MnO2 was proposed to enhance the discharge performance of electrode materials.?1?The electrochemical method was adopted to investigate the influence of different additives on the effect of AZ31B Mg alloy in composite solution.The optimal combination of additives is 30 mmol·L-1 NaF-10 mmol·L-1Na3PO4.EIS tests indicate that NaF-Na3PO4 are excellent inhibitors with inhibition efficiency reaches 98.8%.Furthermore,the delayed time is only 0.08 s,which are 31 times less than that with the composite solution without additives.The as-formed electrolyte improves the specific discharge capacity to approximately 1539 mAh·g-1,which is 57%higher than that of the blank electrolyte of AZ31B Mg anode.Therefore,the present work offers a new way to enhance the performance and lifespan of Mg primary batteries.?2?A polyvinyl butyral?PVB?coating modified with graphene oxide which activation in cerium nitrate solution has been introduced to the AZ31B magnesium?Mg?alloy surface can obviously reduce the corrosion rate and delayed action of AZ31B Mg alloy in composite solution.EIS results indicate that the resistance of Mg alloy treated and untreated with AGO-PVB coating are 31.4 and 1.4 k?at the immersion time of 48h,the corrosion resistance increases by 25.8 times,and the inhibitor efficiency reaches approximately 95.5%.The R1-t fitted equations suggest that the AGO-PVB coating can serve as a long-term protective barrier for Mg alloy in aqueous solution.In addition,the Mg electrode with AGO-PVB coating shows an outstanding discharge behavior and the delayed time nearly can be neglected at a constant discharge density of 2.5 mA·cm-2.Therefore,the present work offers a new way to enhance the corrosion resistance and discharge properties of Mg-MnO2 batteries.?3?The effects of current pulse on the electrochemical behavior of AZ31B Mg alloy in composite aqueous solution are investigated using multi-current step and electrochemical impedance spectroscopy.The results indicate that by adjusting the pulse height and width in order to improve the delayed action of Mg anode in composite solution,and the optimum pulse combination is 25 mA-100 ms.Moreover,excellent discharge behavior obtained at this pulse condition can be well maintained even the immersion time is longer than 10 days.The evolutionary model of the surface film is suggested finally based on the experimental results and surface analysis methods.?4?The effects of pulse technique on the discharge property and polarization resistance of Mg alloy were detailed studied by using galvanostatic discharge and electrochemical impedance spectroscopy.The results indicate that AZ31B Mg alloy has a very low potential peak of-1.25 V and hardly can be neglected value of potential dip?0.06 V?of the pulse signal of-1.0 V-100 ms.Moreover,the ideal discharge characteristics of Mg electrode with pulse load can be maintained despite that immersion time length is extended to 10 days.The surface morphology and composition of Mg alloy are analyzed by SEM,FT-IR and XPS.The studies show that the voltage pulse does not influence the composition of the surface film,whereas,it affects the electrochemical properties by changing the surface image.?5?The initial nucleation/growth mechanism of the Mg alloy in the composite electrolyte is consistent with 3D instantaneous nucleation model by analyzing the collected pulse signal.The electrochemical results indicate that the enhanced overpotentiale increases the current signal and reduces the nucleation time,while does not change the nucleation mechanism.Furthermore,the rate constant K,and combined rate constant K2N0 linely increases with the overpotentiale,indicating that the available positions are increased and thus accelerating the electrocrystallization process.?6?GDY can be used as a promising substrate for growing MnO2 nanospheres,owing to their large surface area and highly conjugated electronic structure for the first time.Furthermore,GDYO,the oxidation from GDY,is proved to be an even excellent substrate for MnO2 growth to enhance the capacitor performance of MnO2.The MnO2@GDYO electrode has a high specific capacitance of 301 F·g-1 at 0.2 A·g-1 and98%initial capacitance retention is observed even after 3000 cycles.This high performance composite could act as an optimal candidate for the next generation electrochemical energy storage systems. |
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