Improvement of H2 absorption of LaNi5 by LiOH pretreatment

\ PERGAMON

International Journal of Hydrogen Energy 13 "0888# 768Ð772

Improvement of H1 absorption of LaNi4 by LiOH pretreatment Haru!Hisa Uchida\ Kaoru Suzuki\ Sakie Kubo\ Hiromi Kondo Department of Human Development\ Tokai University\ Hiratsuka\ 148!0181 Japan

Abstract The e}ect of LiOH pretreatment on the H1 absorption by LaNi4 was examined using a high pressure volumetric apparatus[ In the course of H1 absorption and desorption cycles\ air was introduced into the reaction bed and oxidized for 4Ð14 min after each evacuation[ Absorption kinetics after oxidation were measured[ The absorption reaction was signi_cantly disturbed in case when no pretreatment had been applied\ however\ the LiOH pretreated sample reacted with H1 drastically even after the signi_cant oxidations[ A similar e}ect was also observed after a H1O pretreatment[ Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[ Keywords] Alkaline treatment^ Hydrogen absorption kinetic^ Oxidation^ LaNi4

0[ Introduction In NiÐH batteries\ the charge!discharge characteristic can be signi_cantly stabilized by the pretreatment with alkaline hydroxides ð0\ 1Ł[ The reason for this e}ect may be attributed to the existence of alkaline metal element in the electrode surface region\ which accelerates the surface electrochemical reaction[ The surface e}ect on the elec! trochemical reaction modi_ed by the pretreatment should appeared also in H1 dissociative reaction in gas phase[ In this study\ therefore\ similarly pretreated LaNi4 by LiOH was examined using high pressure Sieverts| type appar! atus[

1[ Experimental The sample of LaNi4 was prepared by arc melting process and subsequent annealing for homogenization[ The block sample was pulverized by H1 sorption cycles and smaller than 59 mm powder was placed under dry air for longer than 0 month before use[ With this powder three kinds of pretreated samples were prepared[ Some

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sample powder was placed in 2 M LiOH solution for 2 h at room temperature "sample with LiOH#[ Additionally\ water was added and washed some powder samples "sample with LiOH¦H1O#[ For the comparison of LiOH pretreatment of water treatment\ water treatment with! out prior LiOH treatment was also applied for some powder samples "sample with H1O#[ High pressure of 39 bar H1 was used for the initial activation[ After 2 times absorption and desorption cycles using 6N H1\ the _rst oxidation by air was carried out for 4 min[ After the oxidation the reaction bed was evacuated by rotary pump for 09 min\ the subsequent H1 absorption was recorded for 09 min[ In the second and further oxidation 1 and more times 4 min oxidations were carried out[ The absorption cycles with oxidation were carried out with 09Ð39 bar H1[ Block samples with pretreatment were also prepared for depth pro_ling by Auger electron spectro! scopy "AES\ PHI!04!044\ 2kV\ 09 mA#[

2[ Results and discussion 2[0[ Initial activation In Fig[ 0\ typical initial activation curves were plotted[ While the sample without any pretreatment reacted with 39 bar H1 after 1 h in this case\ the sample with LiOH

9259!2088:88:,19[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved PII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 0 5 1 ! 0

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H[!H[ Uchida et al[ : International Journal of Hydrogen Energy 13 "0888# 768Ð772

Fig[ 0[ Initial activation of LaNi4 with and without pretreatment by LiOH\ LiOH¦H1O and H1O at 187 K under 39 bar H1[

or LiOH¦H1O pretreatment readily absorbed just after introducing H1[ The initial incubation time "the time without reaction# usually depends on the surface oxide stability and thickness under the same H1 pressure[ The activation occurs by the degradation and generation of clean surfaces of the alloy\ which are attributed to the hydride phase formation by increasing concentration of H in metal permeated through oxide layer[ The LiOH treatment\ therefore\ is considered to enable a faster reac! tion through the sample surface oxide layer[ In com! parison of pretreated samples\ the sample with only LiOH\ however\ reacted with less H1 than the sample with LiOH¦H1O treatment^ for example\ after 1 h of reaction\ more than 1 times the di}erence was observed in H:LaNi4[ This di}erence may be attributed to existing LiOH crystal overlayer on the surface\ which was formed during drying of the sample after soaking in LiOH solu! tion[ 2[1[ Absorption kinetics with air oxidation The cyclic H1 absorption reactions with air oxidation are shown in Fig[ 1[ With increasing amounts of oxidation\ the sample is more deactivated in cases with! out pretreatment[ Absorption was drastically disturbed by the oxidation by air "see Fig[ 1"a##[ On the other hand\ the samples with LiOH and LiOH¦H1O pretreatment reacted with H1 even after each oxidation "Fig[ 1"b# and "c##[ The water treatment after the LiOH treatment keeps almost the same reactivity of the sample even after the signi_cant oxidation[ For better understanding\ pre! treatment e}ect of water was also tested against the oxi! dation[ As shown in Fig[ 1"d#\ a similar e}ect to the LiOH¦H1O was observed[ This means that the water

pretreatment also prevents the deactivation by the surface oxidation[ 2[2[ Considered reaction mechanisms Figure 2 shows the depth pro_ling of the block sample pretreated like the powder samples[ The surface without any pretreatment shows that the surface is covered with La rich oxide which was formed by air oxidation[ The LiOH¦H1O treated surface was analyzed and it was found that not only the surface was covered by oxide\ but also Li element was observed in accordance with oxygen distribution[ Similar alkaline elemental dis! tribution in the surface oxide layer was also observed for the sample pretreated with KOH or NaOH ð0\ 1Ł[ It is considered that during and after the LiOH pretreatment Li ions di}used into the oxide layer more easily through dislocations or grain boundaries of the oxide because of the small ion diameter[ The surface with only H1O pretreatment was di}er! ently analyzed\ however[ On the La rich oxide layer\ a small Ni peak was found[ In the course of H1O treatment La may be signi_cantly oxidized and also removed from the surface[ The remaining Ni forms a thin concentrated layer in subsurface[ According to the thermodynamic data ð2Ł\ oxidized Ni can be easily reduced by H1 with higher pressure at room temperature[ Molecule H1 may be therefore easily dissociated at the concentrated Ni layer\ which enables a faster reaction even after the sig! ni_cant oxidation[ In Fig[ 3\ the considered reaction model is shown sche! matically[ The original LaNi4 powder particle is covered with the La rich oxide[ After each pretreatment\ the par! ticle surfaces are covered by di}erent layers as analyzed

H[!H[ Uchida et al[ : International Journal of Hydrogen Energy 13 "0888# 768Ð772

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Fig[ 1[ Absorption curves of H by LaNi4 under 39 bar H1 with increasing amount of air oxidation "a# without LiOH^ "b# with LiOH^ "c# with LiOH¦H1O^ "d# with H1O pretreatment[

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H[!H[ Uchida et al[ : International Journal of Hydrogen Energy 13 "0888# 768Ð772

Fig[ 2[ Elemental depth pro_ling by AES for block samples "a# without LiOH^ "b# with LiOH¦H1O^ "c# with H1O pretreatment[

by AES[ Through the activation\ the pulverization occurs and clean surface is generated by the hydrogenation[ However\ after oxidation by air the generated clean sur! face must be deactivated "see Fig[ 1"a##[ As shown in Fig[ 1"b# and "c#\ LiOH pretreated surfaces indicated the activity for the H1 absorption[ This means\ the original surface with Li element must be still activated even after signi_cant oxidation[ The small deactivation by the oxi! dation shown in Fig[ 1 "b# and "c#\ therefore\ is attributed to the decrease of original surface area by the degradation at the hydrogenation[

A similar reaction mechanism can be also explained for the surface with Ni rich subsurface layer prepared by the H1O treatment[ Metallic Ni can accelerate the dissociation of H1 ð3\ 4Ł[

3[ Conclusion In this paper the e}ect of LiOH pretreatment on the H1 absorption by LaNi4 was demonstrated with several signi_cant oxidations during the absorption cycles[ The

H[!H[ Uchida et al[ : International Journal of Hydrogen Energy 13 "0888# 768Ð772

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other hand\ the reaction rate determining step on the sur! face with Li element is still not well understood[ Further investigation must be continued[ From the technical point of view\ the results obtained indicate that the pretreat! ment with LiOH and:or H1O enlarge the possibility of application of LaNi4\ and thus reducing the cost by allow! ing the use of H1 with larger amount of impurities or under air mixing possibilities[ Acknowledgements This work was supported by the Tokyo Ohka Foun! dation for the Promotion of Science and Technology[ Fig[ 3[ Considered reaction mechanisms of the samples with and without pretreatments[

References

existence of Li in the surface oxide layer should accelerate the H1 dissociation of H permeation through the oxide layer[ Therefore\ under signi_cantly oxidizing conditions\ the powder sample of LaNi4 can easily react with H1[ A similar e}ect was also observed for the sample pretreated with H1O\ whose surface was characterized by the enriched Ni subsurface layer[ In this case the Ni sub! surface layer accelerated the dissociation of H1[ On the

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