The electrochemical characteristics of the Zr-Cr-Ni based metal hydride electrodes for rechargeable batteries have been investigated in 30wt\% KOH solution. Zr-based electrodes have intrinsically long cycle lives, but their poor activation are improved by alloy modification which a small amount of light rare earth elements(La, Mm, Nd) is added to Zr-Cr-Ni system. In order to investigate the origin of the easier activation behaviors and the longer cycle lives of these electrodes, the surface properties are analyzed by XPS and AES. It is found that the surface consists of the La(rare earth elements), Zr-, Cr-oxide and metallic nickel. And the relative concentration of nickel at the surface increases gradually during the extended cycling. It is suggested that the improved activation is due to the easy penetration of hydrogen through the La-oxide which has a porous structure existed at the surface, and the long durability is due to the enriched nickel which can play a role of the protection from degradation. The electrochemical performances, such as discharge capacity, current density dependence and temperature dependence on the discharge capacity and so on, of the developing alloys are investigated in 30wt% alkaline solution. In order to find out guide lines for useful Ni-MH batteries, the selection criteria related to alloy design for practical Ni-MH batteries are discussed with results studied by previous researchers and in this work. Thus, it is concluded that the most important factors of alloy selection for useful Ni-MH batteries are the thermodynamic properties, especially, the hydride dissociation enthalpy and the surface properties which have specific surface area, degree of catalysis and properties of the oxide such as porosity and thickness. To improve the thermodynamic properties for MH electrodes, the substitution of Fe or Co is very effective. The replacement of Cr by Mn and/or V is very effective for improvement of surface properties. To increase the discharge capacity per unit weight and decrease MH alloy cost, the alloy system substituted Ti for Zr represents the best electrode performances. In our studies, the best electrode materials for a negative electrode of Ni-MH cell are the metal hydride alloys of $Zr_{1-x}Ti_x(CrMnVCo(and/or Fe)NiMm_{0.05}$ system. Where the range of Ti content(x) is 0 ~ 0.3.
Zr-Cr-Ni 계 수소저장합금 전극의 특성과 합금원소 치환에 따른 alloying element 효과와 potential MH 전극소재를 요약하여 나열하면,
1. 열역학적 특성
Stopped plateau 는 less critical parameter 이고 $Δ{H} \cong$ -20~ -40KJ/mole $H_2$ 를 갖는 수소저장합금 중 수소흡수량이 큰 재료.
2. 속도론적 특성
Zr 계 수소저장합금 전극은 방전이 주로 합금 내부에서 표면으로 이동하는 수소확산에 의해서 진행된다. 온도 및 전류에 따른 방전율을 향상시키기 위한 방법으로 effective surface area 를 증가시키는 것이 가장 효과적인 것으로 생각된다. 따라서 표면 조건의 변화로부터 전극의 방전기구도 변화시킬 수 있을 것으로 기대된다. 여기서 effective surface area 란, MH 합금의 surface area 가 크고, porous 한 표면 morphology 를 갖으며 표면에 oxide 가 적고, metallic nickel content 가 높은 표면 상태를 의미한다.
3. Specific element 의 역할 평형수소압 증가 혹은 수소화 반응 enthalpy 변화 : Fe, Co active surface 증가 : V, Mn
4. Potential Zr-Cr-Ni alloys
$Zr_{1-X}Ti_X(CrMnVCo)NiMm_{0.05}$
$Zr_{1-X}Ti_X(CrMnVCo)(and/or Fe)NiMm_{0.05}$, x＜0.5 합금들이고 $Zr_{0.7}Ti_{0.3}Cr_{0.5}(MnVCo)NiMm_{0.05}$ 전극의 방전 용량이 377mAh/g으로 가장 높았고 전류밀도 및 온도 변화에 따라 방전용량의 변화는 매우 적었다.
