The performance characteristics of Pd-coated mischmetal-based alloy electrodes containing either copper powder or Teflonized carbon additives were evaluated in alkaline solution at 2 5°C. The Pd coating was very effective for increasing both the discharge capacity and rate capability and for decreasing the galvanostatic cycling time for activating the alloy electrodes. The best performances were attained with electrodes with 6 wt % Pd content. The electrodes with the Teflonized carbon additive also exhibited greater stability and longer cycle life than those with the copper additive. The improved performance of Pd-coated alloy electrodes over that of an uncoated alloy electrode can be attributed to the catalytic effect of Pd on the charge-transfer step at the electrode surface.lnfroduction During recent years, extensive research into the rechargeable nickel-metal hydride battery using hydrogen storage alloys as negative electrode materials has been conducted because of its high energy density, high rate capability, long cycle life, and low level of toxins.12 The performance of the negative electrode is influenced significantly by the structure and conductivity of the alloy surfaces.3-5 Surface coating of LaNi5 and mischmetal (Mm)-based alloy particles with various metals (Cu, Ni, Pd, Co)3'°'3 has been effective for improving electrode properties such as discharge capacity, cycle life, and rate capability. Microencapsulation of the alloy particles with metal coatings improves the performance of the negative electrode in alkaline solution by preventing the deterioration of the disintegrated alloy particles and maintaining high electrical conductivity.36'6'9 It has also been found that the rate of activation during the charge-discharge cycling of mischmetal-based alloy electrodes is enhanced remarkably by electrodeposited Co and Pd coatings, which has been attributed to the improved electrocatalytic activity for the hydrogen electrode reaction.'°I n this work, a detailed study was carried out to assess the influence of electroless Pd coatings on the performance characteristics of Mm Ni35Co99Mn04A103 (Mm: 43.1 wt % La, 3.5 wt % Ce, 13.3 wt % Pr, and 38.9 wt % Nd) alloy electrodes containing either copper powder or Teflonized carbon additives in alkaline solution at 25°C. ExperimentalMmNi35Co08Mn04Al03 alloy powder with a particle size of less than 100 m was prepared by passing it through sieves after mechanical pulverization. The alloy particles were coated with Pd by electroless deposition using hypophosphite as the reducing agent. Electrolyte baths, which have components that either inhibit or poison the charge/discharge reactions of the metal hydride electrode, were excluded. The alloy powder was immersed in an electroless plating solution containing 2 g/L PdC13, 3 mL/L HC1 (38%), 160 mL/L NH4OH (with 28% NH3 content), 27 gIL NH4C1, and 10 g/L NaH3PO3H3O (55°C, pH 10). The electroless Pd plating was carried out by using different Pd-to-alloy weight ratios. A detailed electroless Pd coating procedure is described e...

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Este trabalho apresenta estudos de carga/descarga realizados sobre ligas metálicas multi componentes ("misch metal al loys") recobertas com Paládio e que podem ser utilizadas como eletrodos em baterias de níquel/hidreto metálico. Foi determinado o efeito do recobrimento nas características voltamétricas, no ciclos de vida e na capacidade sob diversos re gimes de descarga para os eletrodos de liga metálica. O número de ciclos necessários para ativar os eletrodos de liga metálica diminui quando o conteúdo de Pd aumenta. Os resultados mostram também que as ligas recobertas com Pd apresentam uma maior capacidade de carga e um desempenho melhor que aquelas sem o Pd. Essas melhores características podem ser atribuídas ao efeito catalítico do Pd sobre a reação de hidr ogênio. .Charge / dis charge stud ies car ried out on Pd -coated misch metal -based al loys for use in nickel-metal hy dride bat ter ies are pre sented. The ef fect of Pd coat ing on the voltammetric ch arac ter is tics, life cy cle be hav ior, and rate ca pa bil ity of the al loy elec trodes was de ter mined. The num ber of cy cles re quired to ac ti vate the al loy elec trodes de creases with an in crease in t he Pd con tent. The re sults also show that Pd -coated al loys ex hibit higher stor age ca pac i ties an d better per for mance than bare al loys.This im proved per for mance can be at trib uted to the cat alytic ef fect of Pd on the hy dro gen elec trode re ac tion. Key words: metal hy dride elec trodes, hy dro gen stor age ca pac ity, pal la dium coat ing In tro duc tionAt pres ent there is a re newed in ter est in re search on hy drogen re vers ible stor age re sult ing from the rec og ni tion of the many po ten tial ad van tages of such a sys tem for use in elec trochem i cal en ergy stor age de vices. For ex am ple, in power sources that uti lize hy dro gen as a fuel, the chem i cal stor age of hy dro gen as a metal hy dride may lead to a con sid er able re duction in op er at ing pres sures and vol ume. Vol u met ric en ergy den si ties com pa ra ble to those of cryo genic hy dro gen can be at tained for ap pli ca tions where the weight is less im por tant, such as in the ter res trial stor age of so lar-electrolytic hy dro gen, and when the bi-functional hy dro gen elec trode is em ployed in al ka line bat ter ies. When fully de vel oped, Ni-MH bat ter ies, for ex am ple, will ad e quately re place the ex ist ing Ni-Cd bat teries for sev eral sta tion ary (re serve and emer gency power sources, emer gency light ing in pub lic build ings such as schools, hos pi tals, etc.) and por ta ble (video cam eras, cel lu lar tele phones, lap-top com put ers, etc.) ap pli ca tions. The ad vantages of Ni-MH bat ter ies over the Ni-Cd sys tem are: (i) a higher cell en ergy den sity, and (ii) the elim i na tion of toxic cad mium thereby pre vent ing en vi ron men tal dam age. Fur thermore, the de vel op ment of Ni-MH bat ter ies will make them poten tial can di dates for trans por ta tion ap pli ca tions, such as in elec tric ve hi cles, with large ben e ...

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