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A Study On The Improvement Of Cycling Stability Of Ball-milled Mg-based Hydrogen Storage Electrode Alloys By Means Of Multi-component Alloying

Posted on:2003-02-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y ZhangFull Text:PDF
GTID:1101360062975899Subject:Materials science
Abstract/Summary:PDF Full Text Request
In this thesis, domestic and foreign literatures on Mg-based hydrogen storage electrode alloys were exhaustively reviewed first. Advantages and drawbacks of all hydrogen storage electrode alloys were examined, compared and discussed. The high cycling capacity degradation rate of the Mg-based alloys was unanimously regarded as the chief obstacle for the commercialization of the Mg-based alloys in spite of their many attractive merits. The object of this study is to improve the cycling capacity degradation of the Mg-based electrode alloys. On the basis of the literature review, the research scheme for improving the cycling stability of the ball-milled binary Mg5oNi5o alloy by means of multi-component alloying was chosen. In addition to the finding of some alloys with better performance, a clearer understanding of the corrosion and passivation mechanism leading to the high-rate capacity degradation and the roles of each alloying element played in the capacity degradation were also the objectives of this research. The method of step by step optimization was adopted.For binary Mg-Ni electrode alloys, the cycling capacity degradation was found consistently very high. XRD and TEM investigation revealed that the corrosion product was Mg(OH)2 in the form of fiber protruding out from the original surface, hence very loose and gave very little protection to the alloy from further corrosion. AES study revealed that the surface layer containing of the flbred corrosion product was rather thick (>600nm) and grew constantly on further cycling. The increase on Ni content was found helpful in decreasing the rate of corrosion and the surface polarization resistance but made the discharge capacity of the alloy decreased markedly. Microencapsulation of the binary MgsoNiso with Cr, Ti, Y and Ni lowered the cycling capacity degradation to some extent in the beginning, but as the protective passivation layer was still very loose and the oxide particles of the encapsulated elements crumbled away easily from the surface of the alloy.From the cycling degradation curves of the ternary alloys Mg45M5Niso (M = Ti, Zr, Y, Fe, Cu) alloys, the cycling process was divided into two stages. In the first stage (the initial 20 cycles), the corrosion of Mg was the main cause for the cycling degradation. From the experimental data of corrosion current versus cycling time, atmathematic formula 'cOr~ (t) I~ exp(--) was formulated, from which the cyclingIcapacity retention against time formula= a + b exp(---) was worked out, thisC7.curve conincided very well with the experimental data of CN acquired during cycling.IIIcurve conincided very well with the experimental data of CN acquired during cycling. This coincidence affirmed once more the cause for cycling degradation in this stage was the corrosion of Mg. In the second stage after 20 cycles, the rate of cycling capacity degradation of alloys could be divided into two categories. For the first kind, namely the alloys with Sat.% of Fe and Cu, the rate of cycling degradation even increased. The reason was believed to be the complete dissolution of the oxides of Fe and Cu once formed ox' the alloy surface into the alkaline solution and made the alloy even less resistant to corrosion. The second kind of alloys which contained Ti, Zr and Y as substitution elements had stable oxides formed on its surface and became more and more resistant against corrosion as the oxide particles grew into a corrosion resistant passivation film gradually as cycling went on. We named this stage the dissolution and passivation stage.From previous experiment results, as a next step of study, we chose Ti as the substituting element for Mg and studied systematically the electrochemical performance of the alloys Mg5~~Ti~Ni5O (x 5. 10, 15). It was found in the investigations that with the increase of Ti content, the cycling stability of the alloy was increasing gradually, yet the maximum discharge capacity, the high rate discharge-ability and the exchange current densi...
Keywords/Search Tags:Ball-milled Mg-based hydrogen storage electrode alloys, Cyclingcapacity degradation, Multi-component alloying, Cycling stability, Passivation film, Corrosion current density, Polarizaion resistance, Exchange current density
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