| 摘要: | 結合電阻式熔煉與自行開發的恆溫揮發鑄造程序(Isothermal evaporation casting process, IECP)，成功產製儲氫合金Mg2Ni- x mol% Mg3MNi2 (M=Mn, Al；x=0, 15, 30, 60, 100)電極材料，此製備法的特點是可調整Mn與Al元素比例生產不同含量的Mg3MnNi2與Mg3AlNi2，並使用IECP法純化Mg2Ni合金。本實驗將探討錳與鋁含量對於合金結構、放電容量、循環穩定性與表面元素組成之影響。 首先分析不同錳與鋁含量之合金組成比例與晶體結構，合金分別由Mg2Ni與新相Mg3MnNi2或新相Mg3AlNi2所組成。結果顯示，未含錳與鋁時(x=0)，合金呈現單一六方最密堆積結構之Mg2Ni，隨著錳與鋁含量增加，新相Mg3MnNi2或新相Mg3AlNi2於合金中晶出，當錳與鋁含量達到x=100時，合金中生成單一面心立方結構之新相Mg3MnNi2或新相Mg3AlNi2。 隨著錳與鋁元素的添加，合金中生成新相Mg3MnNi2與Mg3AlNi2，當新相含量逐漸增加，合金之最大放電容量隨之提高。合金之放電容量由Mg2Ni合金的17 mAh/g分別提升為Mg3MnNi2合金的166 mAh/g與Mg3AlNi2合金的110 mAh/g。此放電容量的提高歸因於新相Mg3MnNi2與Mg3AlNi2能提升合金表面之抗蝕性能，有效改善合金進行充放氫反應之活性。 添加錳與鋁元素於合金中生成新相Mg3MnNi2與Mg3AlNi2能有效減緩合金放電容量之衰退速率。在不同錳含量之合金中，Mg3MnNi2合金具有最佳循環穩定性，在第15次充放電循環時，仍保有最大放電容量之62.05%，此循環壽命的提升歸因於Mg3MnNi2合金具有抑制合金表面發生氧化反應之能力。在不同鋁含量的合金中，Mg2Ni- 15 mol% Mg3AlNi2合金則具有最佳耐蝕性，在第25次循環時，保有最大放電容量(50 mAh/g)之76%。此循環壽命提升原因為合金含有15 mol% Mg3AlNi2時，能降低合金在充放電循環過程中粉化速率，且表面生成氧化層Al2O3，減緩合金表面的腐蝕反應，提升合金之循環穩定性。 由上述結果可知新相Mg3MnNi2有效提升合金之放電容量與循環壽命，為進一步改善負極材料之電化學性能符合實際應用之需求。將Mg3MnNi2合金與Co、Ti金屬粉末混合球磨30分鐘製備Mg3MnNi2- x mol% M (M=Co, Ti; x=0, 200)合金。本研究將探討Co、Ti金屬元素對合成材料之結構、表面腐蝕行為與電化學性質之影響。 對於球磨處理後Mg3MnNi2合金，內部結構由多晶組織轉為奈米晶組織，而Co與Ti金屬元素促使Mg3MnNi2合金生成非晶相組織，此外，Co與Ti金屬元素能有效提升合金表面之抗腐蝕能力。Mg3MnNi2、Mg3MnNi2-2Co 與Mg3MnNi2-2Ti經球磨處理後，最大放電容量分別可達206 mAh/g, 238 mAh/g與209 mAh/g。 Mg3MnNi2-2Ti合金雖具有較低的最大放電容量，但此合金之電容量保持速率高於Mg3MnNi2與Mg3MnNi2-2Co合金。在十次充放電循環次數後，Mg3MnNi2、Mg3MnNi2-2Co 與Mg3MnNi2-2Ti合金之電容量保持速率分別為51%、59%與80%。結果顯示Co與Ti金屬元素添加能抑制Mg3MnNi2合金表面之氧化反應。含Ti合金的表面所生成TiO2氧化膜有效改善合金之循環穩定性。 使用Ni元素置換Mg2Cu合金中Cu元素改善合金的吸放氫性質之研究中，甚少研究針對Mg-Cu-Ni三元合金進行吸放氫反應過程中作結構分析。本研究將結合電阻熔煉法與恆溫揮發鑄造法(IECP)製備Mg2Cu1-xNix (x = 0, 0.2, 0.4, 0.6, 0.8, 1)合金，探討Ni元素置換對合金的結構、儲放氫性質與電化學性能之影響。 X光單晶繞射結果顯示晶體結構由FCO結構的Mg2Cu轉為HCP結構的含Ni合金，且晶格體積隨Ni含量增加而縮小。但吸氫速率與儲氫量隨Ni含量增加而提高，在溫度3000C與氫壓50 atm環境條件，Mg2Cu與Mg2Ni合金的儲氫量分別為2.54 wt%與3.58 wt%。 合金經活化處理後進行充氫反應可生成MgH2、MgCu2與Mg2NiH4化合物，當合金進行放氫反應時，結構回復至單相Mg-Cu-Ni三元化合物，其放氫平台壓相較Mg-Cu與Mg-Ni二元化合物為高。合金中Ni含量提高有效降低MgH2與Mg2NiH4兩相之放氫溫度。當合金添加微量銅能改善表面的抗蝕特性，Mg2Ni0.6Cu0.4合金具有最大放電容量(40 mAh/g)。 Mg2Ni- x mol% Mg3MNi2 (M=Mn, Al；x=0, 15, 30, 60, 100), the novel composite alloys employed for hydrogen storage electrode, have been successfully synthesized by a method combining electric resistance melting with isothermal evaporation casting process (IECP). The characteristic of synthesizing the composite alloys Mg2Ni- x mol% Mg3MNi2 (M=Mn, Al) with various Mg3MnNi2 and Mg3AlNi2 content could be applied to both formation proportions of Mg3MnNi2 and Mg3AlNi2 through adding Mn and Al element, and Mg2Ni alloy synthesis through IECP. Therefore, the composite alloys Mg2Ni- x mol% Mg3MNi2 (M=Mn, Al；x=0, 15, 30, 60, 100) have been synthesized by the preparation method. The influences of the Mg3MnNi2 and Mg3AlNi2 phases on the structure, surface chemical composition and electrochemical properties of the synthesized materials were studied. To confirm the purity of the composite alloys, the composition and structure were examined by electron probe X-ray microanalyzer (EPMA) and X-ray diffraction (XRD). According to EPMA analysis, the composite alloys are composed of Mg2Ni phase and the new Mg3MnNi2 phase or the new Mg3AlNi2 phase. XRD analysis results show that both formation phases of the composite alloys were Mg2Ni with hexagonal crystal structure and Mg3MnNi2 or Mg3AlNi2 with face-centered cubic crystal structure. It is found on the electrochemical studies that maximum discharge capacities of the composite alloys increase with the increasing content of the Mg3MnNi2 and Mg3AlNi2 phase. The discharge capacities of the electrode alloys are effectively improved from 17 mAh/g of the Mg2Ni alloy to 166 mAh/g of the Mg3MnNi2 alloy and 110 mAh/g of the Mg3AlNi2 alloy, respectively. Cyclic voltammetry (CV) results confirm that the increasing content of the Mg3MnNi2 phase and the Mg3AlNi2 phase effectively improves the reaction activity of the electrode alloys. Surface analyses indicate that the phases can enhance the anti-corrosive performance of the particle surface of the composite alloys. It is also found on the cyclic discharge stability studies that both Mg3MnNi2 phase and Mg3AlNi2 phase possess a positive effect on the retardation of cycling capacity degradation rate of the electrode materials. Among the Mn-containing alloys, Mg3MnNi2 alloy had the best cyclic discharge stability. This alloy has kept 62.05% of its maximum discharge capacity at 15th cycle. It referred the improvement in the cycle life to that Mg3MnNi2 alloy was used to inhibit the formation of corrosive reaction against the alloy surface in the alkaline solution. Among the Al-containing alloys, Mg2Ni-15 mol% Mg3AlNi2 composite had the best anti-corrosion performance. This composite has kept 76% of its maximum discharge capacity (50 mAh/g) at 25th cycle. The improvement of the cycling stability of the electrode alloy with 15 mol% Mg3AlNi2 can be ascribed to the decrease in the rate of pulverization of the alloy during cycling, leading to retard the corrosion reaction against the alloy surface due to the formation of a dense Al2O3 film. The results as mention above reveal that Mg3MnNi2 phase possesses the advantage of improving discharge capacity and cyclic stability of the electrode alloy. In order to further enhance the electrochemical performance of Mg3MnNi2 alloy, the prepared Mg3MnNi2 alloy was ball milled with metallic Co and Ti element for 30 min to produce Mg3MnNi2- x mol% M (M=Co, Ti; x=0, 200) alloys. The influences of the Co and Ti element on the structure, surface corrosion behavior and electrochemical properties of the synthesized materials were investigated. For the ball milled Mg3MnNi2 alloy, the characteristic peaks of the Mg3MnNi2 phase decreased in intensity and broadened, revealing that there was a phase transformation from polycrystalline to nanocrystalline state. X-ray diffraction (XRD) studies showed that Co and Ti facilitated the amorphization of Mg3MnNi2 alloy. Furthermore, the addition of Co and Ti elements are effective in enhancing the anti-corrosion ability of the alloys. The maximum discharge capacities of ball-milled Mg3MnNi2, Mg3MnNi2-2Co and Mg3MnNi2-2Ti alloys were 206 mAh/g, 238 mAh/g and 209 mAh/g, respectively. Although Mg3MnNi2-2Ti alloy had lower maximum discharge capacity, the capacity retaining rate of this alloy was much higher than those of Mg3MnNi2 and Mg3MnNi2-2Co alloys. After ten cycles charge/discharge, the capacity retention rates for ball-milled Mg3MnNi2, Mg3MnNi2-2Co and Mg3MnNi2-2Ti alloys are 51%, 59% and 80%, respectively. These results indicate that the positive effect on suppressing of the oxidation of the Mg3MnNi2 alloy surface was caused by the introduction of Co and Ti metallic elements. Combined with XPS depth profile analyses, it is suggested that the TiO2 oxide layer would be effective in improving the cyclic stability of the electrode alloy. For improving the hydriding/dehydriding properties of Mg2Cu alloy through the substitution of Ni for Cu, few ones on the structure analyses of Mg-Cu-Ni ternary alloy were investigated. The effect of Ni-substitution on the structure and hydrogen storage properties of Mg2Cu1-xNix (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys prepared by a method combining electric resistance melting with isothermal evaporation casting process (IECP) has been studied. The X-ray single-crystal diffraction results indicated that crystal structure transforms Mg2Cu with FCO into Ni-containing alloys with HCP structure, and it leads to a decrement of the cell volume with increasing Ni concentration. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg2Cu and Mg2Ni alloys absorbed 2.54 and 3.58 wt.% H, respectively, at 3000C under 50 atm H2. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH2, MgCu2 and Mg2NiH4 while the discharged samples contained ternary alloys of Mg-Cu-Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg-Cu and Mg-Ni alloys. With increasing nickel content, the effect of Ni is actually effective in MgH2 and Mg2NiH4 destabilization, leading to a decrease of the desorption temperature of these two phases. The partial substitution of Cu for Ni is slightly effective in improving the discharge capacity of Mg-Cu-Ni alloys, and Mg2Ni0.6Cu0.4 alloy reaches the maximum discharge capacity (40 mAh/g) among the alloys. |
