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Hydrogen permeation through palladium-nickel-hydroxide film bilayer
ELSEVIER
1.
Inlrodudion
Hvdmgen permeation through metal membranes has generally proved 10 hc a” effective method ,o snalyle the effect of hydrogen lrapping occurring in metals on the hydrogen transport [l-3]. However. one can hardly apply the pemveation method to oxides. primarily because of their brittleness and low hydmgen diffusivity. Yoon and Pyun [4] and Pyun et al. [5] approached the problem by adopting a bilayer configuration which is composed of a titanium oxide film chemical vapor deposited onto a Pd foil acting as a support as well as a hydrogen absorber. Electrosynthesized nickel hydroxide (ESN) designated as a-Ni(OH), has been found to have superior electrochemical properties compared with chemically prepared p-Ni(OH), [6]. The electrochemical pmpenies of ESN are found ,o improve on the addition of Co. and several studies in the literature concerning the
effect of cobalt on the mechanical [7,X], chemical [9] and elecrrochemical (lo-121 pmpenics of ESN have bee” reported. ESN has hco” know” (131 to have small grain size and large grain houndary area. Owing 10 the large grain boundary area. no, only hydrogen diffusion through the Ni(OH), grain interior, but also hydrogen diffusion along the grain boundaries is believed to play a” importa”, ,otc in the hydrogen transport through ESN. Thus. the hydrogen diffusion along the grain boundaries should be considered in the enalvsis of the hydrogc” transport through the Ni(OH), &I. This work considers the two hydrogen diffusion paths involving grain boundaries and grain interior of the Ni(OH), film. For this pwpose. hydrogen permeation experiments were eanied out on bilayers of palladium substrate covered with a him of pore nickel hydroxide (Pd-Ni(OH),) and with a film of mixed hydroxide of nickel and cobal, (Pd-Ca(OH),-incorporated Ni(OH),). Hydrogen trapping at the grain houndark??and the amount of the hydrogen injected into the Ni(OH), film were discussed in ferms of
apparent buik-up
hydrogen diffusivity and decay penneation
determined from current transicnir.
the
The bilaye, composite specimen consists of Pd fni! electrodeposited with nickel hydroxide. The Pd ful substrate (Aidrich Chem. Co.. 99.98% purity) was annealed under high vacoam of IO-‘ Pa at 650°C for 2h. followed by furnace-coo:mg. The annealed foil substrate was mechanicatly polished with #I200 SIC emery paper to eliminate surface oxide films and then chemically etched in concentrated nitric acid for 30 s. The tina! thickness of Pd foil was controlled to about IW &Ill. Three kinds of pure nickel hydroxide (Ni(OH)?), I”%- and 30%cobalt hydroxide (Co(OH),)-incarporated nickel hydroxide films were electrodeposited onto a Pd substrate gatvanostatically with a current density of OSmAcm-’ far 16M)s in 0.05 M nickel nitrate solution containing 0. 10 and 30 ~01% of 0.05 M cobalt nitrate solution. The mole iraction of the incorporated Ca(OH), in the Ni(OH), film was assumed to he same as Ihc mole fraction of the Co in the dcoosition solution. The resultine film thlcknesa was calculated to be about 1.2 pm, I.6 am and 1.0 ,un for pure Ni(OH),, IO mol% and 30 mol% Co(OH),-incorporated Ni(OH), films respectively. if the density of the active material is assumed to be 3.5 gem-’ [l4]. The electrochemical permeation double cell, which allows electrachcmical hydrogen injection on the cathodic side of ihe bilaye, (Pd) and electrochemical detection on the anodic side (hydroxide). as already used by Devanathsn and Stachurski [15]. was used in this study. The Pd-Ni(OH), film bilayer specimen separated two electrochemical compartments and was in contact with 1 M KOH solution. The solution was deaerated by bubbling with purified nitrogen gas before and during the experiments. The apparent surface area of the electrode exposed to electrolyte was 1.89 cm’ on both sides. A platinum wire and a saturated calomel electrode (SCE) were used as a c”““tc, electrode and a reference electrode respectivelv. Ah-r holdmg an anodic potential ranging from 0 to 0.35 V fSCEI at the exit side (nickel bvdroxide) for 24 h to’ obt& low background anodic current ‘densities, the cathodic side (Pd) was cathodically polarized. From th!: aoment, the resulting hydrogen buildup permeation current transient was recorded until the steady-u& current was erained. Then, the cathod;, polarization was switched to the open circuit state. At the same time, the hydrogen decay permeation current transient was measured. Hydrogen was galvanostatically injected into the Pd
side (cathodic compartment) by application of a cu,rent density of I50 PA cm-‘, using a Hewlett Packard 6177~ precision constant current source. and potentiostatically extracted from the Ni(OH), side (anadtc compartment) at various applied anodic potentials ranging from 0 to 0.35 V (S&), using a W&king LT 7X potcntiostat. All electrochemical experiments were performed at room temperature.
3. Results It is necessary to know the diffusivity of hydrogen in the Pd layer for the analysis of the permeation current transient; of the Pd-Ni(OH), film bilaye,. For this purpose, a hydrogen permeation experiment was performed on the 1W tun thickness Pd layer alone. By integrating the build-up permeation current transient cmve. the quantity of hydrogen which has permeated through can be obtained at va,ious times. An extrapolation of the plot of quantity against time gives the time lag I,_ which is related to the apparent diffusivity of hydrogen D,, by [IS]
, ^ tL=L-/(6D,,)
(1)
where L is the thickness of the Pd layer. The 1,. value estimated from the build-up current transient was 240 s. and the apparent diffusivily of hydrogen b, in the Pd layer was calculated from the tL value to be about 3x IO-’ cm’s-’ at room temperature. The value calculated is in good agreement with the values of about 1 x10.’ to 6xltl~‘cm’s~ at ,~om temperitwe previously reported in the literature [l&16]. Fig. 1 demonstrates typical hydrogen build-up permeation current transients for the bilayers of Pdfilm containing various fractions of Ni(OH)Z Co(OH),. The time to steady-state permeation CUT-
rent is reduced with increasing fraction of Co(OH), incorporated into the Ni(OH), film. It shouid he noted that for the three kinds of hilaver comoosite specimen, the permeation GUI :nt stee$y rises’’ after the arrival of the tint traces of hydrogen on the anodic side of the membrane. The per&at~on current of the Mayer of Pd-pure Ni(OH), tilm increases gradually after the steep rise, indicating that there is significant hydrogen trapping in the film. It should he noted that the lime to arrival 0: the first traces of ltydrogen on the anodic side of the composite sprcimen is at least nine times longer than that for the Pd monolayer. indicating that no portion of the Pd is exposed directly to the electrolvte in the enodic camoartmcnt. Fig. 2 shows typical itydrogen &cay permeation current transients for the bilayers of Pd-Ni(OH), lilm containing various fractions of Co(OH):. For the bitavers of Pd-owe NitOH), film and Pd_3O% Co(OH)L-incorporatcd f&In the permcation current decays more rapidly than that of the P&IO% Co(OH),-incorporated film hilayer. For the three kinds of bilayer composite specimen, the current in the decay transients approaches steady-state more rapidly than in the build-uo transients. Fig. 3 presents piot of the time lag ti. against anode ootential imoored on the wit side (NIIOH).). determined from the charge passed during the hiild-up current transient. The tL value decreases with increaring fraction of Co(OH), in the Ni(OH), film. Also. the f, value decreases wtth increasine applied anodic pate&l for the bilayers of Pd-pure Ni(OH): film azd Pd-10% f:ofDH),-in~ornorated Mm. The t. value of the Pd130%‘Co(dH)2-incorpuratcd tilk bilayer is independent of applied anodic potential. In the case of electron insulating oxides. such as TiO,, one must consider the electric field across the
.
~,.
_
oxide film in the analysis of the hydrogen transport through the oxide. However, the electric field across the Rim becomes negligibly low as the electronic conductivity in an oxide or a hydroxide film increases. Pure and dbped Ni(OH), films~osed in this study seem to have relatively higher electronic eonductivities. Therefore. the electric field effect on the ‘lydrogen transport through the Ni(OH), film was disregarded. In the absence of electric field. tL for the bilayer under the galvanostatic hydrogen injection condition is giveo by Song and Pyun [l’l] 1.; ‘1 =zD,+-
Li
KL,L, +7
(2)
where L, and D, respectively denote the thickness and diffusivity of hydrogen for the first layer (Pd). Subscript 2 represents the corresponding quantity for the second layer (Ni(OH),). K refers to the ratio of the hydrogen concentration of the Pd side to that of the Ni(OH), side just near the Pd-Ni(OH), tilm interface. This is a consequence of Henry’s law, assuming that at alI times the ratio of the concentrations acres, the interface is equal to the ratio of the corresponding equilibrium solubilities. Fig. 4 exhibits plots of apparent hydrogen diffusivity 6, against anodie potential imposed on the exit side (Ni(OH),). determined from the I, value of the build-up current transient. The determined L& value increases from about an order of IO-” to IO ” cm’ SK’ in magnitude with iraeasing fraction of c~,rnH). in the Ni(OH), film. .&n&g that hydrogen existing only in the Ni(OH), film contributes to the decay permeation current. one can also cakulate the apparent hydrogen diffusivity from the slope of the plot of IO&,,,, i)& vs. l/r which is given as 1181
Ihrcc kinds of Ni(OH), film. Howrvcr. ,he Ij,, value is an order of 10~” cm2 s ’ in magnitude and there is no aignincan, difference in ,hc 6,, value hclwcrn purr Ni(OH), film and Co(OH),-incorporalod film.
4. Discus*iun
curren, density. i the where i.,_,, is ,hc slcady-stale current dcnaity during ,hc decay transients. I the lime, F Ihe Faraday cons&m,, and AC remcsents the hvdmgen conccn,ralGn difference between the wetface Pd-Ni(OHI, and the exit side. The linear plot of vs. l/r was readily made from the log(L.,,,~ -i).‘j decay permcalion current transients shown in Fig. 2. Fig. 5 shows plots of apparrn, hydrogen diffusivily agains, anodic po,cn,ial imposed on the exi, side (Ni(OH),). The V&K varies slightly from (3.1 t the 0.56)X 10 ‘I 10 (h.Z?,,.53)X 10 ‘I cm’s
’ for
‘” ‘----l
ESN has hccn found 1131 lo have wry high specific rurlace arca. of an order 01 ahou, 2Om’ g ‘, I-L‘. wry fine grain six. Thus, the hydrogen transport through ESN is govcmcd by the hydrugcn diffusmn no, only lhruugh the grain interior bu, also along the groin boundaries. The three kinds of Ni(OH)? lihn used in Lhis study are conaidcrcd fo ~151 in ,he form of (3.Ni(OH), due IO a long ageing lime of about 24 h. It was reported [IS] that the water content for 0. Ni(OH), varied I:.irn 0 to 0.3 of a mole fract,on depending upon the preparative expcrimcntal conditions. Several invesrigatora [IY-211 nobd tha, the water in Ni(OH), can cilhcr he adsorbed or s,ruc,urally bonded between the Ni(OH), laltices. Mani and de Neufville [22] reported that in P-phase Ni(OH)?. only ahsorhcd water exists a, ,hr grain houndaries. unaccumpanicd by structurally bound war. The abrupt rise in permealion current afwr the arrival of the tint ,races of hvdroren on the anodic side of the mcmhrane. as shown in Fig. 1, seems 10 he a,,ribu,ahle ,G ,h; Lc, tha, ,hc hydrogen transport across the Ni(OH),leleclmlyte interface occurs only after the hydrogen transferred from Pd has accunw kited in the grain interior and boundaries of the Ni(OHL film. This behavior is orohahlv rrlaled 10 the well k&n sell-discharge of ;he Ni(6H), electrode [2324]. Mao c, al. 1231 reported. from a microcalorimetric study of the self-discharge of the NiOOH-Ni(OH)2 eleclrodc in a hydrogen environment. that either a direct chemical reaction between NiOOH and dissolved hydrogen in the elrctroly,c or an eleclruchcmical oxidation of hydrogen a, the NiOOH surface is Lhe dominant mechanism. They found that ,he setf-discharge of the NiOOH electrode is an exothermic reaction with a very large enthalpy change of about -145kJmolK’. During the build-up permeation current transient. the rl. value is expected to decrease with decreasing amoun, of hydrogen injected into Lhe Ni(OH), grain interior. thereby increasing the 6,, value. The increased 6,, value determined from the build-up transients with increasing fraction of Co(OH), in the Ni(OH), film. as shown in Fig. 4, suggests Ihal, at a given hydrogen content, the amount of hydrogen injecred into the Ni(OH), grain interior is reduced by the Co(OH), incarporalion inlo the Ni(OH), tilm.
This is consistent with the repart 1231 lhal metal additives with high ovcrpolenlinls for hydrogen oxidation. such as Cd and Co, give u much lower cnrhalpy change. and hence may resuh in skwer >elf-discharge kinetics. Cunsidcring the large cnlbalpy cbangc fur Ihc ruaction bcrwcen NiOOH and dissolved bydrogcn. hydrogen injected inlo the Ni(OH), grain mlerior doea not seem lo be extracted wring the decay pcrmcation currcnf trunsienl. Thus. the hydrogen transport through Ihe Ni(OH), lilm during Ihe decay pcrmeation current tranricnt t’i convdercd 10 hc governed by the hydrogen diffusion along the grain houndanes.
hydrogen corwn:. the amount of lhe hydrogen inIcctcd into Ihe Ni(OH), grain interior is reduced by ColOH,, mcorpnration. (2) The ri,, value delermined horn the decay pcrmcarwn currum transicm was an order of 10 18cm>, maeoitudc and there was no sienifi-
m
0.35 V (SCEl investigated. This re~uh suggest>&at Ihe hydrogen transport through the lilm is governed by hydrugcn trapping ilf Ihr: trap sites prcscnt at the grain hmmdaricc rather ihan Ihusr existing in the grain inrcrinr.
Acknowledgments The rcceim of research eranls under the moeram fir Developmem ‘of hew Elcctrochromic hl;~terials” (contract No. 93.03W-01. 01.3. IYY3i IWh) from the Korea Sciencr and Enqmuermz Foundation. Korea, is palefully acknowludpcd. -
~~Fundamcn~al Resexch
_
around
0,ll~-l~.2tl~
(SCE)
with
i&acing
anodic
potential imposed cm Ihc lilm. Ncvcrthclos.
thcrc is no sieniticafit differences in the D,, value dclurmmed from the decdy permcatmn current lrimbients hctwccn pure Ni(OH), lilm and Co(Otl),-incorporated tilm. irrcspcctivc of ax) hydroxide and hydroxide phases involved in the potential range inve&patcd This rcsub suggeststhat Lhc hydrogen transport through the Ni(OH), film during the decay transient is dcoxmined hv hvdroeen trappine. al the Erain boundaries rather than. the-comp&tio~ and &lal ~1ruc1uru of Lhr Ni(OH), grain int,rior. This argument is consislcnl wifh ihe report of Ywn and Pyun [ZSl.
5. Conclusions The hilavcrs 01 Pd-ourc Ni(OHl. film and Pd& Co(OH)&orporatcd ‘Ni(OHi, lil; were satisfactorily prepared by electrodepositing Ni(OH), film unto a Pd metal support. From the hilayer composite specimens. reliable hydrogen build-up and decay permeation current Iramieets were obtained. (1) From the increased annarent hvdroren diffusivity ‘&, value determined from the build-up permeation current transients with mcreasing fraction of Co(OH), in the Ni(OH), film. ir is suggested that. 81 a given
._
5”
Y. (?
Y,r,,,,.
.r -I
Pur,,
/ ,on,md
co,*,,n,r
.“d
Cnozp,rmd.
Z-11 ,,vxI,
d.5
$0
1.
Inlrodudion
Hvdmgen permeation through metal membranes has generally proved 10 hc a” effective method ,o snalyle the effect of hydrogen lrapping occurring in metals on the hydrogen transport [l-3]. However. one can hardly apply the pemveation method to oxides. primarily because of their brittleness and low hydmgen diffusivity. Yoon and Pyun [4] and Pyun et al. [5] approached the problem by adopting a bilayer configuration which is composed of a titanium oxide film chemical vapor deposited onto a Pd foil acting as a support as well as a hydrogen absorber. Electrosynthesized nickel hydroxide (ESN) designated as a-Ni(OH), has been found to have superior electrochemical properties compared with chemically prepared p-Ni(OH), [6]. The electrochemical pmpenies of ESN are found ,o improve on the addition of Co. and several studies in the literature concerning the
effect of cobalt on the mechanical [7,X], chemical [9] and elecrrochemical (lo-121 pmpenics of ESN have bee” reported. ESN has hco” know” (131 to have small grain size and large grain houndary area. Owing 10 the large grain boundary area. no, only hydrogen diffusion through the Ni(OH), grain interior, but also hydrogen diffusion along the grain boundaries is believed to play a” importa”, ,otc in the hydrogen transport through ESN. Thus. the hydrogen diffusion along the grain boundaries should be considered in the enalvsis of the hydrogc” transport through the Ni(OH), &I. This work considers the two hydrogen diffusion paths involving grain boundaries and grain interior of the Ni(OH), film. For this pwpose. hydrogen permeation experiments were eanied out on bilayers of palladium substrate covered with a him of pore nickel hydroxide (Pd-Ni(OH),) and with a film of mixed hydroxide of nickel and cobal, (Pd-Ca(OH),-incorporated Ni(OH),). Hydrogen trapping at the grain houndark??and the amount of the hydrogen injected into the Ni(OH), film were discussed in ferms of
apparent buik-up
hydrogen diffusivity and decay penneation
determined from current transicnir.
the
The bilaye, composite specimen consists of Pd fni! electrodeposited with nickel hydroxide. The Pd ful substrate (Aidrich Chem. Co.. 99.98% purity) was annealed under high vacoam of IO-‘ Pa at 650°C for 2h. followed by furnace-coo:mg. The annealed foil substrate was mechanicatly polished with #I200 SIC emery paper to eliminate surface oxide films and then chemically etched in concentrated nitric acid for 30 s. The tina! thickness of Pd foil was controlled to about IW &Ill. Three kinds of pure nickel hydroxide (Ni(OH)?), I”%- and 30%cobalt hydroxide (Co(OH),)-incarporated nickel hydroxide films were electrodeposited onto a Pd substrate gatvanostatically with a current density of OSmAcm-’ far 16M)s in 0.05 M nickel nitrate solution containing 0. 10 and 30 ~01% of 0.05 M cobalt nitrate solution. The mole iraction of the incorporated Ca(OH), in the Ni(OH), film was assumed to he same as Ihc mole fraction of the Co in the dcoosition solution. The resultine film thlcknesa was calculated to be about 1.2 pm, I.6 am and 1.0 ,un for pure Ni(OH),, IO mol% and 30 mol% Co(OH),-incorporated Ni(OH), films respectively. if the density of the active material is assumed to be 3.5 gem-’ [l4]. The electrochemical permeation double cell, which allows electrachcmical hydrogen injection on the cathodic side of ihe bilaye, (Pd) and electrochemical detection on the anodic side (hydroxide). as already used by Devanathsn and Stachurski [15]. was used in this study. The Pd-Ni(OH), film bilayer specimen separated two electrochemical compartments and was in contact with 1 M KOH solution. The solution was deaerated by bubbling with purified nitrogen gas before and during the experiments. The apparent surface area of the electrode exposed to electrolyte was 1.89 cm’ on both sides. A platinum wire and a saturated calomel electrode (SCE) were used as a c”““tc, electrode and a reference electrode respectivelv. Ah-r holdmg an anodic potential ranging from 0 to 0.35 V fSCEI at the exit side (nickel bvdroxide) for 24 h to’ obt& low background anodic current ‘densities, the cathodic side (Pd) was cathodically polarized. From th!: aoment, the resulting hydrogen buildup permeation current transient was recorded until the steady-u& current was erained. Then, the cathod;, polarization was switched to the open circuit state. At the same time, the hydrogen decay permeation current transient was measured. Hydrogen was galvanostatically injected into the Pd
side (cathodic compartment) by application of a cu,rent density of I50 PA cm-‘, using a Hewlett Packard 6177~ precision constant current source. and potentiostatically extracted from the Ni(OH), side (anadtc compartment) at various applied anodic potentials ranging from 0 to 0.35 V (S&), using a W&king LT 7X potcntiostat. All electrochemical experiments were performed at room temperature.
3. Results It is necessary to know the diffusivity of hydrogen in the Pd layer for the analysis of the permeation current transient; of the Pd-Ni(OH), film bilaye,. For this purpose, a hydrogen permeation experiment was performed on the 1W tun thickness Pd layer alone. By integrating the build-up permeation current transient cmve. the quantity of hydrogen which has permeated through can be obtained at va,ious times. An extrapolation of the plot of quantity against time gives the time lag I,_ which is related to the apparent diffusivity of hydrogen D,, by [IS]
, ^ tL=L-/(6D,,)
(1)
where L is the thickness of the Pd layer. The 1,. value estimated from the build-up current transient was 240 s. and the apparent diffusivily of hydrogen b, in the Pd layer was calculated from the tL value to be about 3x IO-’ cm’s-’ at room temperature. The value calculated is in good agreement with the values of about 1 x10.’ to 6xltl~‘cm’s~ at ,~om temperitwe previously reported in the literature [l&16]. Fig. 1 demonstrates typical hydrogen build-up permeation current transients for the bilayers of Pdfilm containing various fractions of Ni(OH)Z Co(OH),. The time to steady-state permeation CUT-
rent is reduced with increasing fraction of Co(OH), incorporated into the Ni(OH), film. It shouid he noted that for the three kinds of hilaver comoosite specimen, the permeation GUI :nt stee$y rises’’ after the arrival of the tint traces of hydrogen on the anodic side of the membrane. The per&at~on current of the Mayer of Pd-pure Ni(OH), tilm increases gradually after the steep rise, indicating that there is significant hydrogen trapping in the film. It should he noted that the lime to arrival 0: the first traces of ltydrogen on the anodic side of the composite sprcimen is at least nine times longer than that for the Pd monolayer. indicating that no portion of the Pd is exposed directly to the electrolvte in the enodic camoartmcnt. Fig. 2 shows typical itydrogen &cay permeation current transients for the bilayers of Pd-Ni(OH), lilm containing various fractions of Co(OH):. For the bitavers of Pd-owe NitOH), film and Pd_3O% Co(OH)L-incorporatcd f&In the permcation current decays more rapidly than that of the P&IO% Co(OH),-incorporated film hilayer. For the three kinds of bilayer composite specimen, the current in the decay transients approaches steady-state more rapidly than in the build-uo transients. Fig. 3 presents piot of the time lag ti. against anode ootential imoored on the wit side (NIIOH).). determined from the charge passed during the hiild-up current transient. The tL value decreases with increaring fraction of Co(OH), in the Ni(OH), film. Also. the f, value decreases wtth increasine applied anodic pate&l for the bilayers of Pd-pure Ni(OH): film azd Pd-10% f:ofDH),-in~ornorated Mm. The t. value of the Pd130%‘Co(dH)2-incorpuratcd tilk bilayer is independent of applied anodic potential. In the case of electron insulating oxides. such as TiO,, one must consider the electric field across the
.
~,.
_
oxide film in the analysis of the hydrogen transport through the oxide. However, the electric field across the Rim becomes negligibly low as the electronic conductivity in an oxide or a hydroxide film increases. Pure and dbped Ni(OH), films~osed in this study seem to have relatively higher electronic eonductivities. Therefore. the electric field effect on the ‘lydrogen transport through the Ni(OH), film was disregarded. In the absence of electric field. tL for the bilayer under the galvanostatic hydrogen injection condition is giveo by Song and Pyun [l’l] 1.; ‘1 =zD,+-
Li
KL,L, +7
(2)
where L, and D, respectively denote the thickness and diffusivity of hydrogen for the first layer (Pd). Subscript 2 represents the corresponding quantity for the second layer (Ni(OH),). K refers to the ratio of the hydrogen concentration of the Pd side to that of the Ni(OH), side just near the Pd-Ni(OH), tilm interface. This is a consequence of Henry’s law, assuming that at alI times the ratio of the concentrations acres, the interface is equal to the ratio of the corresponding equilibrium solubilities. Fig. 4 exhibits plots of apparent hydrogen diffusivity 6, against anodie potential imposed on the exit side (Ni(OH),). determined from the I, value of the build-up current transient. The determined L& value increases from about an order of IO-” to IO ” cm’ SK’ in magnitude with iraeasing fraction of c~,rnH). in the Ni(OH), film. .&n&g that hydrogen existing only in the Ni(OH), film contributes to the decay permeation current. one can also cakulate the apparent hydrogen diffusivity from the slope of the plot of IO&,,,, i)& vs. l/r which is given as 1181
Ihrcc kinds of Ni(OH), film. Howrvcr. ,he Ij,, value is an order of 10~” cm2 s ’ in magnitude and there is no aignincan, difference in ,hc 6,, value hclwcrn purr Ni(OH), film and Co(OH),-incorporalod film.
4. Discus*iun
curren, density. i the where i.,_,, is ,hc slcady-stale current dcnaity during ,hc decay transients. I the lime, F Ihe Faraday cons&m,, and AC remcsents the hvdmgen conccn,ralGn difference between the wetface Pd-Ni(OHI, and the exit side. The linear plot of vs. l/r was readily made from the log(L.,,,~ -i).‘j decay permcalion current transients shown in Fig. 2. Fig. 5 shows plots of apparrn, hydrogen diffusivily agains, anodic po,cn,ial imposed on the exi, side (Ni(OH),). The V&K varies slightly from (3.1 t the 0.56)X 10 ‘I 10 (h.Z?,,.53)X 10 ‘I cm’s
’ for
‘” ‘----l
ESN has hccn found 1131 lo have wry high specific rurlace arca. of an order 01 ahou, 2Om’ g ‘, I-L‘. wry fine grain six. Thus, the hydrogen transport through ESN is govcmcd by the hydrugcn diffusmn no, only lhruugh the grain interior bu, also along the groin boundaries. The three kinds of Ni(OH)? lihn used in Lhis study are conaidcrcd fo ~151 in ,he form of (3.Ni(OH), due IO a long ageing lime of about 24 h. It was reported [IS] that the water content for 0. Ni(OH), varied I:.irn 0 to 0.3 of a mole fract,on depending upon the preparative expcrimcntal conditions. Several invesrigatora [IY-211 nobd tha, the water in Ni(OH), can cilhcr he adsorbed or s,ruc,urally bonded between the Ni(OH), laltices. Mani and de Neufville [22] reported that in P-phase Ni(OH)?. only ahsorhcd water exists a, ,hr grain houndaries. unaccumpanicd by structurally bound war. The abrupt rise in permealion current afwr the arrival of the tint ,races of hvdroren on the anodic side of the mcmhrane. as shown in Fig. 1, seems 10 he a,,ribu,ahle ,G ,h; Lc, tha, ,hc hydrogen transport across the Ni(OH),leleclmlyte interface occurs only after the hydrogen transferred from Pd has accunw kited in the grain interior and boundaries of the Ni(OHL film. This behavior is orohahlv rrlaled 10 the well k&n sell-discharge of ;he Ni(6H), electrode [2324]. Mao c, al. 1231 reported. from a microcalorimetric study of the self-discharge of the NiOOH-Ni(OH)2 eleclrodc in a hydrogen environment. that either a direct chemical reaction between NiOOH and dissolved hydrogen in the elrctroly,c or an eleclruchcmical oxidation of hydrogen a, the NiOOH surface is Lhe dominant mechanism. They found that ,he setf-discharge of the NiOOH electrode is an exothermic reaction with a very large enthalpy change of about -145kJmolK’. During the build-up permeation current transient. the rl. value is expected to decrease with decreasing amoun, of hydrogen injected into Lhe Ni(OH), grain interior. thereby increasing the 6,, value. The increased 6,, value determined from the build-up transients with increasing fraction of Co(OH), in the Ni(OH), film. as shown in Fig. 4, suggests Ihal, at a given hydrogen content, the amount of hydrogen injecred into the Ni(OH), grain interior is reduced by the Co(OH), incarporalion inlo the Ni(OH), tilm.
This is consistent with the repart 1231 lhal metal additives with high ovcrpolenlinls for hydrogen oxidation. such as Cd and Co, give u much lower cnrhalpy change. and hence may resuh in skwer >elf-discharge kinetics. Cunsidcring the large cnlbalpy cbangc fur Ihc ruaction bcrwcen NiOOH and dissolved bydrogcn. hydrogen injected inlo the Ni(OH), grain mlerior doea not seem lo be extracted wring the decay pcrmcation currcnf trunsienl. Thus. the hydrogen transport through Ihe Ni(OH), lilm during Ihe decay pcrmeation current tranricnt t’i convdercd 10 hc governed by the hydrogen diffusion along the grain houndanes.
hydrogen corwn:. the amount of lhe hydrogen inIcctcd into Ihe Ni(OH), grain interior is reduced by ColOH,, mcorpnration. (2) The ri,, value delermined horn the decay pcrmcarwn currum transicm was an order of 10 18cm>, maeoitudc and there was no sienifi-
m
0.35 V (SCEl investigated. This re~uh suggest>&at Ihe hydrogen transport through the lilm is governed by hydrugcn trapping ilf Ihr: trap sites prcscnt at the grain hmmdaricc rather ihan Ihusr existing in the grain inrcrinr.
Acknowledgments The rcceim of research eranls under the moeram fir Developmem ‘of hew Elcctrochromic hl;~terials” (contract No. 93.03W-01. 01.3. IYY3i IWh) from the Korea Sciencr and Enqmuermz Foundation. Korea, is palefully acknowludpcd. -
~~Fundamcn~al Resexch
_
around
0,ll~-l~.2tl~
(SCE)
with
i&acing
anodic
potential imposed cm Ihc lilm. Ncvcrthclos.
thcrc is no sieniticafit differences in the D,, value dclurmmed from the decdy permcatmn current lrimbients hctwccn pure Ni(OH), lilm and Co(Otl),-incorporated tilm. irrcspcctivc of ax) hydroxide and hydroxide phases involved in the potential range inve&patcd This rcsub suggeststhat Lhc hydrogen transport through the Ni(OH), film during the decay transient is dcoxmined hv hvdroeen trappine. al the Erain boundaries rather than. the-comp&tio~ and &lal ~1ruc1uru of Lhr Ni(OH), grain int,rior. This argument is consislcnl wifh ihe report of Ywn and Pyun [ZSl.
5. Conclusions The hilavcrs 01 Pd-ourc Ni(OHl. film and Pd& Co(OH)&orporatcd ‘Ni(OHi, lil; were satisfactorily prepared by electrodepositing Ni(OH), film unto a Pd metal support. From the hilayer composite specimens. reliable hydrogen build-up and decay permeation current Iramieets were obtained. (1) From the increased annarent hvdroren diffusivity ‘&, value determined from the build-up permeation current transients with mcreasing fraction of Co(OH), in the Ni(OH), film. ir is suggested that. 81 a given
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