The Effect of Palladium Coating on Hydrogen Storage Alloy Electrodes for Nickel / Metal Hydride Batteries

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 hidrogênio..

At 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 fits in the re duc tion of en viron men tal pol lu tion 1 .
In the se lec tion of ap pro pri ate intermetallic com pounds for hy dro gen stor age, sta bil ity is ob vi ously one of the most impor tant fac tors, and thus the choices of can di date ma te ri als are dis cussed mainly based on their af fin ity for hy dro gen.The em pir i cal model for mu lated by Miedema 2,3 , and commonl y re ferred to as "the rule of re versed sta bil ity", is gen er ally used as a guide for the pre dic tion of the heat of for ma tion of intermetallic com pounds and al loys.Ac cord ingly, since the sta bil ity of a ter nary hy dride, AB x H y , in creases with the in creas ing sta bil ity of the intermetallic com pound, AB x, the pla teau pres sure of a can di date hy dro gen stor age al loy can be ad justed by an ap pro pri ate choice of its mod i fi ers.In ad di tion, the hy poth e sis that an intermetallic com pound will more readily ac com mo date hy dro gen at oms ( i.e. , its free en ergy of for ma tion will be come more neg a tive, as the vol ume of their in ter sti tial sites in creases) can be used as a com ple men tary tool in se lect ing can di date hydriding alloys.For ex am ple, start ing with a lan tha num-based hydriding intermetallic al loy, a sub sti tu tion of ce rium, for ex am ple for lan tha num, will make the intermetallic compound less sta ble and thus in crease the pla teau pres sure.Fur ther more, mod i fi ers are se lected ac cord ing to their prop er ties as hy dro gen ab sorb ers, cat a lytic ef fects for enhanced hy dro gen ab sorp tion ki net ics, sintering in hi bi tion, and re sis tance to ox i da tion and at tack from the cor ro sive elec tro lyte en vi ron ment [4][5][6][7][8][9][10][11][12] .
In this re search, the ef fect of Pd coat ing on the intermetallic com pound MmNi 3.5 Co 0.8 Mn 0.4 Al 0.3 , where Mm (misch metal): 43.1 wt% La, 3.5 wt% Ce, 13.3 wt% Pr, and 38.9 wt% Nd was stud ied in or der to im prove both the ki net ics of hy dro gen ab sorp tion in the al loy and its life cycle char ac ter is tics.

Ex per i men tal
The work ing elec trodes were pre pared by mix ing 130 mg of sieved (< 12 0 µm) MmNi 3.5 Co 0.8 Mn 0.4 Al 0.3 al loy pow der, pre vi ously coated by Pd -electroless de pos ited by the tech nique de scribed else where 13 , with 370 mg of copper pow der which was used as the bind ing ma te rial.The mixed pow ders were then cold pressed to 150 kg/cm 2 onto a 1 cm 2 cur rent col lec tor con structed from a po rous nickel mesh.The ad di tion of cop per pow der re duces the for mation of cracks upon elec trode cy cling, which af fects the mechan i cal in teg rity of the elec trode 14 .The geo met ric area of the Pd -coated al loy work ing elec trode was about 2 cm 2 .The coun ter elec trode (sintered NiOOH) had a much larger geo met ric area than that of the work ing elec trode.The electro lyte, 1 M KOH, was pre pared from re agent grade KOH and deionized wa ter.A Hg/HgO elec trode con tain ing the same elec tro lyte was used as a ref er ence elec trode.All exper i ments were made at 25 °C.Elec tro chem i cal hy dro gen stor age ca pa bil i ties of the Pd -coated al loy elec trodes were mon i tored by mea sur ing the charge / dis charge re sponse of the elec trode to cur rents de liv ered by a bat tery cycler made in our lab o ra tory.The work ing elec trodes were pre vi ously ac ti vated for hy dride for ma tion by con tin u ous galvanostatic cy cling un til a con stant value of the hy dro gen stor age ca pac ity was ob tained.In all elec tro chem i cal tests, the dis charge cut-off po ten tial was -0.6 V vs. Hg/HgO refer ence elec trode.The elec tro chem i cal mea sure ments also in cluded the de ter mi na tion of the life cy cle and rate ca pabil ity of the Pd -coated al loy elec trodes.
The charg ing-discharging pro cess of the metal hy dride elec trode is de scribed by the over all re ac tion: and the elec tro chem i cal stor age ca pac ity is pro por tional to the hy dro gen con tent of the metal hy dride.Ac cord ingly, the spe cific dis charge ca pac ity, C d , was calcu lated from the amount of elec tric ity, Q, dis charged from the elec trode, de fined per unit weight of the al loy electrode,W, ac cord ing to: where C d is ex pressed in mAh/g.

Voltammetric stud ies
Pd -coated al loy elec trodes were cy cled at 1 mV/s between -0.4 V and -1.3 V (Fig. 1).The first po ten tial cy cle shows electroreduction cur rent peaks due to the pres ence of cop per ox ides pres ent in freshly pre pared work ing elec trodes.As cop per ox ides can not be re formed dur ing the for ward scanning in the pos i tive po ten tial di rec tion un der the ex per i men tal con di tions used in this work 15 , the electroreduction cur rent peaks were no lon ger ob served in the sub se quent cy cles, where only the cur rent peaks re lated to the hy dro gen electrode re ac tion could be de tected.Fur ther more, dur ing the re verse scan ning, the cur rent in creased rap idly when the po ten tial de creased be low -0.9 V ( vs. Hg/HgO), due to the con tri bu tion of the hy dro gen evo lu tion re ac tion.Dur ing the for ward scan, a broad an odic cur rent peak re lated to the elec tro-oxidation of ab sorbed hy dro gen was ob served 4,16 .As the num ber of re duc tion -ox i da tion cy cles in creases, hy dro gen electroformation oc curs ear lier, and cor re spondingly the hy dro gen elec tro-oxidation cur rent peak also increases.These pro cesses, as can be in ferred from the voltammograms in Fig. 1, are fur ther fa vored when the Pd con tent in creases from 3% to 6%.The ac ti va tion ef fect by po ten tial cy cling can be at trib uted to the re duc tion of metal sur face ox ides which in creases the re ac tion sur face area.Oth er wise, the re ac tion sur face area also in creases dur ing the ac ti va tion pro ce dure due to the con tin u ous ex pan sion and con trac tion of the metal al loy sur face pro duced by the for ma tion of the hy dride phase and the sub se quent dehydriding pro cess.
The po si tion of the broad an odic cur rent peak cor respond ing to the dehydriding re ac tion: re flects the overpotential for this re ac tion and pro vides in forma tion about the ef fect of Pd on the re ac tion ki net ics.Thus, the voltammograms in Fig. 1 show that the an odic cur rent peak po ten tial shifts from ca. -0.60 V to ca. -0.68 V on in creas ing the Pd con tent from 3% to 6%.Fur ther more, for the al loy electrode with out Pd coat ing, the an odic cur rent peak ap pears at ca.-0.57V.As the dif fu sion co ef fi cient of hy dro gen re ported for the La-based AB 5 al loy is around 10 -7 cm 2 s -1 17 , and taking into ac count that this value is large enough and on the or der of that of Pd 18 , it is rea son able to as sume that Pd plating has a cat a lytic ef fect on the charge trans fer step on the sur face of the elec trode, which con trols the over all elec tro-oxidation pro cess of absorbed hy dro gen 4 .

The in flu ence of the ac ti va tion treat ment on the elec trochem i cal be hav ior of Pd-coated al loy elec trodes
Pd-coated al loy elec trodes with dif fer ent Pd con tents were sub jected to an ac ti va tion treat ment com prised of cy cling the elec trodes in 1M KOH us ing a con stant cur rent at C/10 for 14 h in the charge, and at C/4 in the dis charge, up to the cut -off poten tial, where C/x de notes the cur rent that would com pletely dis charge the fully charged elec trode in x h.The cor re spond -ing elec trode po ten tial vs. dis charge ca pac ity plots (Fig. 2) show that af ter the first cy cles the al loy elec trode with out Pd coat ing ex hib its both high overpotential and low discharge ca pac ity.As the num ber of charge-discharge cy cles in creases, the overpotential de creases and higher dis charge ca pac i ties are at tained as a con se quence of the above-described ac ti va tion ef fect.On the other hand, Pdcoated elec trodes pres ent low overpotentials, and high discharge ca pac i ties are ob tained af ter a few cy cles.The Pd-coated al loys were sub jected to charge-discharge cy cling at C/2 rates.The cor re spond ing dis charge ca pac i ties were de ter mined at -0.6 V.The num ber of cy cles to ac ti vate the al loy elec trodes and to at tain full ca pac ity de creased with an in crease in the Pd con tent (Fig. 3).Full ca pac ity was obtained af ter 6-10 cy cles for Pd-coated al loy elec trodes with a Pd con tent ≥ 6%.Fur ther more, the Pd-coated al loy elec trode with 6% Pd pre sented the high est dis charge ca pac ity, which was on the or der of 285 mAh/g (Fig. 4).It should be noted that af ter pro longed cy cling, e.g. , more than 40-50 cy cles, the alloy elec trode un der goes some me chan i cal de te ri o ra tion due to the con sec u tive ex pan sion and con trac tion of the binder lat tice dur ing cy cling.

The ef fect of Pd con tent on the dis charge rate ca pa bil ity of Pd-coated al loy elec trodes
Plots of the dis charge rate ca pa bil i ties for Pd -coated al loy elec trodes with dif fer ent Pd con tents are pre sented in Fig. 5.The elec trodes were first charged at C/2 for 2 h 20 min, and then dis charged at dif fer ent rates.Fig. 5 shows that the Pdcoated al loy elec trodes with a Pd con tent ≥ 6% have high discharge ca pac i ties at low dis charge rates, and they re tain these high ca pac i ties even at higher rates of dis charge, whereas for a Pd con tent ≤ 3%, the Pd -coated al loy electrodes ex hibit a high de cay in their ca pac i ties at high discharge rates.

Con clu sions
Pd-coated al loy elec trodes ex hib ited ex cel lent hy drogen stor age be hav ior and good per for mance as metal hydride elec trodes in al ka line so lu tion.The Pd coat ing was very ef fec tive for in creas ing both the dis charge ca pac ity and rate ca pa bil ity and de creas ing the galvanostatic cycling time for ac ti vat ing the al loy elec trodes.The high est dis charge ca pac ity was at tained af ter 6-10 charge -discharge cy cles for al loy elec trodes with a 6% Pd con tent.The im proved per for mance of Pd-coated elec trodes is attrib uted to the cat a lytic ef fect of Pd on the charge trans fer step on the elec trode sur face.

Ac knowl edg ments
This re search pro ject was sup ported by the Consejo Nacional de Investigaciones Científicas y Técnicas and the Fundación Antorchas of Ar gen tina.Spe cial thanks are also

Fig ure 1 .
Fig ure 1. Cy clic voltammograms at 1 mV/s for Pd -coated al loy electrodes with dif fer ent Pd con tent: (a) 3%; (b) 6%.The cy cle num ber is in di cated in the fig ure.

Fig ure 2 .
Fig ure 2. Plots of elec trode po ten tial vs. dis charge ca pac ity for Pdcoated al loy elec trodes with dif fer ent Pd con tent : (a) 0%; (b) 3%; (c) 6%.The cy cle num ber is in di cated in the fig ure.

Fig ure 3 .Fig ure 4 .
Fig ure 3. Life cy cle be hav ior of Pd -coated al loy elec trodes with differ ent Pd con tent.

Fig ure 5 .
Fig ure5.Dis charge rate ca pa bil i ties for Pd -coated al loy elec trodes with dif fer ent Pd con tent.