optical switching of y-hydride thin film electrodesalexandria.tue.nl/openaccess/metis246681.pdf ·...

3348 .1 Electrochem, Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, Inc.

Pennington, NJ (1995).5. B. Lochel, A. Maciossek, H.-J. Quenzer, and B. Wagner,

This Journal, 143, 237 (1996).6. A. Maciossek, B. Lochel, S. Schulze, J. Noetzel, H.-J.

Quenzer, and B. Wagner, Microelectron. Eng., 27, 503(1995).

7. B. Lochel, A. Maciossek, M. Konig, H.-J. Quenzer, andH.-L. Huber, ibid., 23, 455 (1994).

8. G. Engelmann, 0. Ehrmann, R. Leutenbauer, H.Schmitz, and H. Reichi, in Workshop Digest ofMicroMechanics Europe, N. de Rooij, Editor, p. 85,Swiss Foundation for Research in Microtechnology,Neuchatel, Switzerland (1993).

9. C. Beuret, G.-A. Racine, J. Gobet, R. Luthier, and N. F.de Rooij, Proc. MEMS'94, 1994, Oiso, IEEE94CH3404-1, p. 81-85.

10. Ch. H. Ahn, Y. J. Kim, and M. G. Allen, in Digest ofTechnical Papers of Transducers '93, Yokohama,p. 70, Tokyo Institute of Technology, Tokyo (1993).

11. B. Lochel, A. Maciossek, M. KUnig, H.-J. Quenzer, andB. Wagner, Proceedings of ACTUATOR'94, H.Borgmann and K. Lenz, Editors, p. 109, AXONConsult. GmbH, Bremen, Germany (1994).

12. G. Engelmann and H. Reichi, Microelectron. Eng., 17,303 (1992).

13. B. Lochel, A. Maciossek, M. Konig, H.-L. Huber, andG. Bauer, ibid., 21, 463 (1993).

14. Ibid., 23, 455 (1994).15. B. LOchel and A. Maciossek, in Proceedings of the

Micromachining and Microfabrication ProcessTechnology Conference, Vol. 2639 p. 174, SPIE(1995).

Optical Switching of Y-Hydride Thin Film ElectrodesA Remarkable Elecfrochromic Phenomenon

P. H. L. Noltent

Philips Research Laboratories, 5656 AA Eindhoven, The Netherlands

M. Kremers and R. Griessen

Faculty of Physics and Astronomy of Vrije Universiteit, 1081 HV Amsterdam, The Netherlands

ABSTRACT

The optical appearance of yttrium thin film electrodes can be electrochemically switched from mirrorlike to highlytransparent by making use of the hydride-forming properties of Y. A strong alkaline solution is a suitable electrolyticenvironment for obtaining stable electrochromic electrodes. The presence of a thin Pd layer covering the Y electrode isessential in providing a sufficiently high electrocatalytic activity for the electrochemical charge-transfer reaction.Hydrogen is irreversibly bound in Y-dihydride and reversibly bound in Y-trihydride. The optical changes are reversibleand are induced within a narrow hydrogen concentration range, making this typical electrode material interesting forapplication in a new type of electrochromic devices.

IntroductionIt has long been recognized that metal-to-semiconduc-

tor and vice versa transitions can be induced by makinguse of the hydride-forming properties of rare-earthmetals.' The pronounced reversible changes in electricalconductivity, which are often accompanied by crystallo-graphic phase transitions, are induced by changing the hy-drogen content in the crystal lattice and have been attrib-uted to changes of the electronic structure of the material.Recently, Huiberts et al. reported that, in Y films, thisdramatic change in electrical conductivity is accompaniedby a remarkable optical effect.'-4 By changing the hydro-gen concentration within the solid by gas-phase loading,they showed that the optical appearance changes frommirrorlike f or the dihydride composition to highly trans-parent for the trihydride compound.

A Pd top coating was reported to be essential for load-ing and unloading the Y films with hydrogen gas outsidethe ultrahigh-vacuum (UHV) deposition chamber.2'3 Thesecoatings act as protective layers, preventing oxidationand/or poisoning of the underlying Y films in air.Furthermore, they act as catalytic layers, providing thedissociative adsorption and associative desorption ofhydrogen molecules at a sufficiently high rate. This reac-tion can be represented by

H, 2Had

Subsequently, the adsorbed hydrogen atoms (Had) can betransported into the solid by solid-state diffusion, whichresults in absorbed hydrogen (Hai)

* Electrochemical Society Active Member.

Had [2]

The Pd top coating has an additional advantage in that itforms a hydride itself and can act as a transporting medi-um of hydrogen toward the underlying Y film. Dependingon the hydrogen concentration within the solid, two phasetransitions reportedly occur in the case of Y. By changingthe H/Y ratio from 0 to 3, hexagonal Y is converted subse-quently into cubic Y-dihydride and hexagonal Y-trihy-dride." The complex hexagonal structure of the trihydrideform has been determined just recently.'

An alternative way of hydrogen loading is via elec-trolytic reduction of a proton donating species, such aswatet This electrochemical charge-transfer reactionoccurs at the solid/electrolyte interface and can be repre-sented by

H,O+eHd+OW [3]It has been shown that the kinetics of this reaction

depends strongly on the nature of the electrode surface.6Strikingly, the kinetics of this charge-transfer reactionwere found to be very fast at Pd.6 Once the adsorbed hy-drogen atoms are formed at the electrode surface they canbe absorbed by the Pd and, subsequently, by the Y elec-trode. Evidently, this latter process is identical to that

[1] occurring in the gas phase as represented by Eq. 2.To investigate whether reversible optical switching of Yfilms can be accomplished by electrochemical means, bothgalvanostatic and potentiostatic measurements are con-ducted. The electrochromic properties are investigatedsimultaneously by measurtng the electrode transparency. Inaddition, cyclovoltammetric measurements are performed

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J. Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, Inc. 3349

to examine the hydride formation and decomposition reac-tions in more detail.

ExperimentalThe Pd-coated Y films were produced by conventional

evaporation techniques (10-8 mbar vacuum level) on round(2 cm diam) polished quartz substrates. The Y thicknesswas varied between 200 and 500 nm and that of Pdbetween 0 and 20 nm. The layer thickness was accuratelycontrolled through quartz microbalance measurementsduring the evaporation process. In addition, the amount ofdeposited Y was checked afterward by chemical analysis(inductively coupled plasma emission spectroscopy; accu-racy 3%) and was in agreement with the microbalancemeasurements. Electrodes were made by contacting thesurface with a lead using a conducting adhesive (E-solderconductive adhesives No. 3021 from IMI). The contactswere shielded from the electrolyte by a chemically inertisolating lacquer. The electrode surface area was deter-mined by optical microscopy prior to the electrochemicalmeasurements.

The electrodes were tested in a conventional three-elec-trode configuration, i.e., in an open rectangular cuvette,using a Pt counterelectrode and a Hg/HgO/6 M KOH refer-ence electrode. All potentials in this work are given withrespect to this Hg/HgO electrode. All electrochemicalmeasurements were performed with a P05 73 potentio-scan from Wenking. The optical transmission of the filmswas investigated in situ by illuminating the electrodesfrom the back of the sample and detecting the light inten-sity at the front, using a white light source (Schott lampKL 1500) in combination with a photodetector (EG&G).The optical properties reported here have been investi-gated by measuring the integrated light intensity passingthrough the electrode during electrochemical operationand are presented on a linear scale in arbitrary units.However, they have also been investigated spectroscopic-ally and reveal additional interesting features. These re-sults will be reported in a separate paper.7

Since Hall measurements have shown that Y-trihydrideis an n-type semiconductor with a bandgap around2.3 eV, an optical red filter was used to prevent electrodeillumination with light of photon energies larger than2.11 eV (586 nm), in order not to exceed this bandgap.High energy light must be avoided since it generates holeswithin the valence band of an n-type semiconductor, mak-ing the electrode electrochemically unstable.8 Such a pho-toanodically induced dissolution process has been foundexperimentally in our investigations when using unfil-tered light. Although the chemical stability in an alkalinemedium is shown to be acceptable when highly energeticlight is excluded, the electrodes sometimes suffer frommechanical stability problems after a few weeks operationin the present alkaline solutions. This results in contactproblems. Severe hydrogen evolution was the main causeof these mechanical problems and therefore should beavoided as much as possible.

To insure a rigid experimental setup, all parts, includingthe electrochemical cell, were installed on an optical rail.All experiments were conducted at room temperature.

Results and DiscussionA most important requirement for the Y electrodes is

their chemical stability in the electrolyte. It is well knownthat Y and other rare-earth metals are extremely basemetals, with very negative electrode redox potentials.9 Aproper selection of the electrolyte composition is thereforeessential. On the basis of the Pourbaix diagram it isexpected that Y dissolves into acidic solutions easily.9 Thishas been confirmed experimentally. Alkaline solutions aremore promising in this respect, as hydroxides are expect-ed to be formed at the electrode surface which may act aspassivating layers. To provide a sufficiently stable electro-chemical environment in which the Y dissolution rate islow, and the ionic conductivity between the Y workingelectrode and counterelectrode is high, a concentrated

alkaline solution of 6 M KOH has been selected to performall electrochemical experiments.

Soon after immersing a freshly prepared Pd coated(5 nm) Y (500 nm) electrode in the alkaline electrolyte theopen-circuit potential (E0) stabilizes close to 0.0 V. Thisopen-circuit potential is determined by the electrochemi-cal nature of the electrode surface in contact with the elec-trolyte. In the present case, E,,, is determined by the pres-ence of both Pd and Y at the electrode surface and byhydrogen and oxygen dissolved in the aqueous electrolyte.The thin Pd top layers do not form a fully closed coating.An example of the surface morphology for a 5 nm Pd-coat-ed Y film is shown in Fig. 1. Although the contributionof the various electrochemical systems to E,,, cannot beestablished completely it seems likely, as is argued below,that the hydrogen content stored in the Y films is mainlyresponsible for the established open-circuit potential. Inthis respect it has been reported that the as-produced Yfilms already contain a significant amount of hydrogen.3

The absorption/desorption properties of hydride-f orm-ing metals/compounds are generally characterized bypressure-composition isotherms. There is a direct relation-ship between the equilibrium pressure (PH2) for absorptionand desorption of hydrogen gas and the equilibriumpotential (Er) of a hydride-forming electrode, according tothe Nernst equation

E, = — ln PH2 — 0.926 [V vs. Hg/HgO]nF [4]

where R is the gas constant, T the temperature, F theFaraday constant, and n the number of electrons involvedin the hydrogen evolution reaction. According to Eq. 4 apressure PH2 of 1 bar corresponds to —0.926 V vs. Hg/HgOat room temperature and every decade change in hydrogenpressure induces E,, to vary by approximately 30 mV. Anopen-circuit potential of 0.0 V corresponds therefore to anextremely low hydrogen pressure of approximately 10-32bar. This value is in good agreement with iO bar deter-mined from the measured enthalpy of formation of thedihydride, .H = —228 kJ/mol H2, and entropy of forma-tion, .S = —130.8 J/K mol H2, by means of the van 't Hoffrelation.'°

The result of a galvanostatic (constant reduction currentof 1 mA) charging experiment is shown in Fig. 2. The elec-trode potential is plotted as —E for comparison with thecorresponding pressure-composition isotherms. Threeregions can be distinguished in the electrode potentialtransient curve a: (i) a more or less constant plateau at

Fig. 1. SEM photomicrograph, revealing the surface morphologyof a Pd-coated (5 nm) Y thin film (500 nm) electrode after the elec-trochemically induced absorption/desorption experiments in a 6 MKOH alkaline solution.

1pm


3350 J. Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, Inc.

— x in YH0 1 2 3

Fig. 2. Development of the electrode potential E (solid curve a)and the in situ measured optical electrode transmission T (dashedcurve b) as a function of time for a 500 nm Y film electrode coveredwith a 5 nm Pd top layer during galvanostatic hydriding (1 mA) in6 M KOH.

—E —850 mV during the first 17 mill; (ii) a slopingplateau at somewhat more negative potentials, centredabout 24 mill; and (iii) a steep increase toward more nega-tive potentials at the end of charging followed by a level-ing off when hydrogen gas evolution starts. Taking the filmlayer thickness, the electrode surface area, and the inte-grated charge into account it can be calculated, usingFaraday's law, that these transitions correspond to hydro-gen contents of about YH17, YH27 and YH, respectively(see upper axis in Fig. 2). Although the determination ofthe film layer thickness was accurate, the exact hydrogencontents are still subject to some uncertainties as both theinitial hydrogen contents of the as-produced films, theoxide layer thickness formed in the electrolyte, and theamount of hydrogen desorbed into the electrolyte are notknown exactly. Evidently, the highest H-content of YH34 ishighly unreliable as part of the supplied charge is notstored in the form of hydrides but is converted into hydro-gen gas.

The simultaneously measured optical signal is repre-sented by curve b of Fig. 2 and reveals that the Y electroderemains optically closed, i.e., very low transmission inten-sities are measured up to high hydrogen contents. After—26 mm charging, the electrode starts to become trans-parent (transmission intensity increases sharply). Thisprocess is completed within several minutes. When opticalswitching is defined to take place within a region wherethe transparency changes between 10 and 90%, it is clearthat switching occurs within a narrow composition rangeat relatively high hydrogen content, i.e., between YH27 andYH30 (see Fig. 2). The semiconducting electrode propertiesinduced by hydrogen in this high concentration range3likely contributes to the sharp voltage change occurring atthe end of the charging process.

Figure 3 shows the potential relaxation on current inter-ruption, immediately after the first charging cycle. Theopen-circuit potential (curve a) slowly increases towardmore positive values during more than 15 h and changesmore abruptly after —950 mm. At the end of this relax-ation process a stable electrode potential close to 0 V isagain attained. The corresponding optical signal (curve b)also shows some remarkable features. Initially, thetransparency drops rapidly to become more or less stablefor almost 100 mm. Subsequently, the electrode slowlyturns back to its original optically closed state.

After the potential relaxation process, shown in Fig. 3,was completed the electrode was recharged. Figure 4shows the second galvanostatic charging curve a and the

Fig 3. Development of the electrode potential E,, (solid curve a)and the corresponding transmission (dashed curve b) of a Pd-coat-ed (5 nm) Y (500 nm) electrode under open-circuit (OC) conditions,immediately after galvanostatic (1 mA) hydriding.

corresponding transparency changes (curve b). A closeresemblance of the potential development and the report-ed pressure-composition isotherm is found.'1 For compari-son, the first charging curve is indicated (curve c), takingthe pronounced potential change at which H2 evolutionstarts as a synchronous point. Comparison of curves a andc clearly shows that during the previous open-circuit peri-od (Fig. 3) only the higher hydride compositions have beendecomposed, i.e., charging was resumed close to the Y-dihydride composition. This hydrogen content is similar tothe lowest composition of YH1, measured in the gasphase.3 This implies that in the present electrochemicalcase only the higher hydride forms can be formed/decom-posed reversibly and that once the lower hydride forms areformed they cannot be decomposed due to their extremelystable character. This latter phenomenon is expressed bythe extremely low hydrogen plateau pressures measuredup to the Y-dihydride composition.3 The electrode trans-parency change (curve b) again takes place within a rathernarrow hydride composition range, similar to the opticaldependence observed during the first charging cycle (seecurve b of Fig. 4).

Fig. 4. Development of the electrode potential (a) and the corre-sponding transmission (b) of a Pd-coated (5 nm) Y (500 nm) elec-trode during the second galvanostatic (1 mA) charging cycle. Thefirst galvanostatic hydriding curve, as represented by curve a inFig. 1, is also shown in curve c, for comparison.

1.5

-E LVI

1.0

-E0 [VI

0.5

C

(b))

[a.u.J

[au.]

—..t [mini

0 200 400 600 800 1000—..t [mm]

x in YH2

-E [V]

11.,

T[a.u.J

time -J10 mm



-E [VI

—.4 [mini

The origin of the hydride loss, occurring during the pre-vious open-circuit period (Fig. 3) has still to be discussed.Two processes may be responsible for this unstable behav-ior. In the first place, hydrogen may be released from theelectrode in the form of gas. This chemical associationreaction occurs at the electrode surface according to

YH —YH + y/2H2 [5]The produced hydrogen gas may be released in the form ofgas bubbles or dissolve directly into the electrolyte. How-ever, during the open-circuit conditions of the present ex-periment, no hydrogen gas evolution has been observed.Alternatively, the hydride may be consumed by an electro-chemical process. Such an electroless process occurs at theopen-circuit potential and can be represented by two sep-arate electrochemical reactions. The first step is possiblyinitiated by the reduction of oxygen, which is known todissolve easily into alkaline solutions

02 + 2H20 + 4e—40W [6]The electrons to be consumed by the oxygen reductionreaction can be delivered by the YH hydride, which itselfis oxidized according to

YH, + yOW - YH6 + yH2O + ye [7]This reaction sequence is well known to occur in recharge-able nickel-metal hydride batteries and is responsible forkeeping the oxygen pressure inside sealed batteries low.6On the basis of the amount of reversibly stored hydrideand the time needed to fully discharge the Y-trihydride, an02 reduction current of the order of 15 iA can be calculat-ed, which is in fair agreement with the expected reductioncurrents of a few microampere per square centimeter inalkaline solutions.

Since the kinetics of the hydride oxidation reaction7 ismuch better than that for the oxygen reduction reaction 6,E0 can be considered, according to Eq. 4, as a measure forthe surface hydrogen concentration. Furthermore, as thesereactions take place very slowly, a steady-state situationcan be assumed during the complete discharge process,i.e., E0 Ee. Although the development of the hydrogenconcentrations within the solid is not known exactly, agood indication of the desorption pressure of the isothermcan be obtained from the open-circuit potential curve a ofFig. 3. Assuming the presence of oxygen to be responsiblefor the unstable character of the Y-trihydride, it is evidentthat this problem can be solved by excluding oxygen fromthe electrochemical cell. In that case it is evident thatelectrochemical measurements are a strong tool for deter-mining absorption/desorption isotherms for hydride-forming materials. This is especially useful when extreme-ly low plateau pressures are involved that are generallyinaccessible by conventional gas-phase methods.

Galvanostatic discharging of a fully charged Y-electrodewith a relatively low current (0.2 mA) again shows thetwo-stage dehydriding process (Fig. 5). This not only holdsfor the development of the potential (curve a) but is evenmore pronounced for the optical transparency (curve b).Discharging was interrupted at 0 V. The potential recoveryafter this discharging process is followed during the sub-sequent 30 mm and reveals some remarkable features (seeFig. 6). Initially, the electrode potential (curve a) is quick-ly changing toward more negative potentials followed by aslower increase to the same end potential of 0 V observedbefore, while the optical signal (curve b) is not influencedat all. A possible explanation for this complex behaviormay be as follows. Assuming that at the end of thedischarge process, diffusion of H atoms within the elec-trode becomes rate-limiting, a sharp hydrogen concen-tration gradient is established at the electrode surface andthe potential increases significantly once the hydrogensurface concentration has become very low. This concen-tration gradient is balanced again after current interrup-tion, resulting in a higher surface concentration and, con-sequently, in a decreasing electrode potential. Meanwhile,

[a.u.]

Fig. 5. Development of the electrode potential (a) and the corre-sponding transmission (b) of a Pd-coated (5 nm) Y (500 nm) elec-trode during galvanostotic dehydriding (0.2 mA).

due to the presence of dissolved oxygen near the electrodethe hydride is slowly oxidized under electroless condi-tions, according to Eq. 6 and 7. As a result, the open-cir-cuit potential rises again to the value expected for the sta-ble dihydride.

The electrode properties have been investigated furtherby dynamic current-potential measurements. An exampleof such a plot is shown in Fig. 7a. Starting from 0 V, withY-dihydride, the electrode potential is scanned towardmore negative values with a low scan rate of 0.1 mV/s toensure quasi steady-state conditions. The low reductioncurrent flowing in this negative potential region mustagain be attributed to oxygen reduction. The cathodic cur-rent density starts to rise at —0.6 V and a single hydride-formation peak is found with its maximum at —0.85 V vs.Hg/HgO. Clearly, the electrode becomes transparent in thepotential region where the reduction current starts todecrease again, i.e., where the main part of the hydride-formation process has been completed (Fig. 7b). This is inagreement with the earlier described observations thatoptical switching takes place in a high but narrow con-centration range (see Fig. 4). Reversing the scanning direc-tion leads to oxidation of the previously formed hydrides.In contrast to the hydride-formation reaction, two sepa-rate stages can be distinguished in the oxidation process,which are clearly accompanied by distinctive opticalchanges. During the first oxidation stage — —0.8 V, only a

Ea.u.I

0 10 20 30t [mm]

Fig. 6. Development of the electrode potential (a) and the caries-ponding transmission (b) of a Pd-coated (5 nm) V (500 nm) elec-trode as a function of time under open-circuit conditions, imniedi-ately after galvanostattc (0.2 mA) dehydriding with cutoff voltageof 0 V vs. Hg/llgO.

T-E0 [VI

1.0 -

0.5-

0

-'--—(a) \. —I--


3352 J. Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, Inc.

'c QxA/cm2]-100

T[a.u.)

I

Fig. 7. Current-potential curves (a) and corresponding transmis-sion signal (b} of a Pd-coated (5 nm) Y (500 nm) electrode measuredin 6 M KOH with a scan rate of 0.1 mV/s. Due to previous charg-ing procedure the electrode is in the dihydride state at 0 V.

minor part of the reversible hydride is oxidized. However,this has a large impact on the electrode transparency. Theoptical properties are characterized by a clear hysteresis inthis potential range. The main part of the hydrides is oxi-dized at more positive potentials. In contrast to the firstdecomposition stage, this second oxidation step has amuch smaller impact on the optical electrode properties. Ithas been reported that, in the case of Y, the hydride for-mation/decomposition reaction induces the occurrence ofcrystallographic phase transformations.2'3 The presentresults suggest that phase transformation may also beimportant for the electrochemical behavior.

To investigate whether the hysteresis effects describedabove are related to kinetical limitations, potentiostaticexperiments have been performed. Starting again with a Yelectrode in the dihydride state, the electrode potentialwas initially switched to —0.2 V. During the equilibrationperiod, the cathodic current decreases to a low steady-state value, generally a low reduction current, generatedby the oxygen transformation reaction.6 Once steady statehas been attained, the electrode potential was made morenegative in successive steps of 100 mV The equilibrationtime was strongly dependent on the applied electrodepotential. The optical steady-state signal is shown in Fig. 8as a function of the applied electrode potential. Since onlyequilibrium conditions are considered in this experiment,the electrode potentials can again be converted, accordingto Eq. 4, into equilibrium hydrogen pressures. This pres-sure equivalent is plotted in Fig. 8. Preceding the completeoptical switching in the potential range of —0.9 to —1.0 V,a small intensity depression is always found at —0.8 V,making the electrode even less transparent than at more

Fig. 8. Potentiostatically measured steady-state transmission sig-nal of a Pd-coated (5 nm) Y (500 nm) electrode during hydriding (S)and dehydridin9 (0). The calculated hydrogen equilibrium pres-sure, corresponding to the steady-state electrode potential (Ej, isalso indicated.

positive potentials. This unexpected suppression, whichcan be recognized after a more detailed inspection of thetransient curves shown in Fig. 4, has been found in thegas-phase loading experiments. This blackening effect hasbeen attributed to the disappearance of the transparencyof the dihydride.2 This interesting phenomenon is de-scribed in more detail in a future paper.7 Furthermore, theelectrode transparency still changes significantly at nega-tive potentials, i.e., at high hydrogen partial pressures.Strikingly, on the reversed scan a small but stable intensi-ty plateau is again visible, indicating that the oxidationreaction is a two-step process.

Figure 8 shows that optical switching can be accom-plished by stepping the electrode potential from positivepotentials to, for example, —1.0 V and vice versa. Limitingthe potential range has the advantage that severe hydro-gen evolution can be avoided at negative potentials andthe basic material stability, becoming important at morepositive potentials, can be controlled. Figure 9 shows thedevelopment of the electrode transparency after appli-cation of such potential steps. As the Pd layer thickness isexpected to have a positive influence on the switchingspeed, the results of a 500 nm Y electrode covered with a20 nm Pd top-coating are presented in Fig. 9. Starting withan equilibrated electrode in the dihydride form at 0 V, theelectrode voltage was switched to —1.0 V During thisprocess the reduction current is sharply decaying. Theelectrode transparency is represented by curve a. It takes afew minutes to switch the electrode from mirrorlike for thedihydride composition to transparent for the trihydride.The reverse process is much faster, of the order of secondsas curve b reveals. The second stage of the dehydridingprocess, as already discussed in relation to, e.g., Fig. 7,takes a much longer time, as is indicated by the t = levelin Fig. 9. The time scale difference between the absorptionand desorption processes is likely related to the large dif-ferences in established overpotentials. The optical elec-trode appearance changes at relatively high hydrogen par-tial pressures (see,e.g., Fig. 8), i.e., at negative potentials,which implies that the overpotential at —1.0 V becomesvery low during the absorption process, whereas the over-potential is close to 1 V during the reversed process at 0 V.

a [iiNcm2i

I

log PH2

T [a.

I

-1.0 -0.8 -0.6 -0.4 -0.2Ee (V)

-0.5— E(V)



T [au.]

U

Fig. 9. Optical transient behavior of a Pd-coated (20 nm) Y(500 nm) electrode under potentiostatic control. The optical changesinduced by absorption (at —1.0 V) and desorption (0 V) are repre-sented by curves a and b, respectively. Note the difference in timescales.

Another factor which may contribute to this time scaledifference may be related to the fact that the opticalappearance is strongly dependent on the characteristicsacross the electrode thickness. For example, in hydrideformation the entire electrode thickness contributes to theoverall electrode transparency properties, whereas in thecase of hydride decomposition only a thin surface layer maybe responsible for optically closing the entire electrode.

ConclusionsReversible hydrogen loading and, consequently, revers-

ible optical switching of Pd-coated Y electrodes can beaccomplished by electrochemical means. Chemically sta-ble electrodes are obtained when electrochemical chargingand discharging is carried out in a strong alkaline (6 MKOH) solution. A distinction can be made between irre-versibly bound hydrogen, which is denoted as Y-dihydrideand reversibly bound hydrogen, formed at higher hydro-gen content. Reversible optical switching takes place in anarrow hydrogen concentration range at relatively highhydrogen content, i.e., in the range of YH27 and YH30, atrelatively high hydrogen partial pressures. The rate ofoptical switching is history dependent: switching from atransparent to mirrorlike appearance can be accomplishedwithin a few seconds, whereas the reverse process takesmuch more time, of the order of minutes, which is arguedto be related to the applied overpotential. This indicatesthat the rate of the electrochemical surface reaction playsan important role.

The reversible hydride-formation was a single-stepprocess, whereas both the electrochemical and opticalresults point to a two-step decomposition reaction. Thetransparent form of the Y-hydride is unstable under open-circuit conditions. The loss of the reversible part of the

stored hydrogen under these conditions has been attrib-uted to an electroless reaction of oxygen dissolved in theelectrolyte and the hydride. The chemical stability is poorwhen the Y electrodes are exposed to highly energeticlight, suggesting that the higher hydride forms, which areassigned to have n-type semiconducting properties, arephotoanodically converted into the oxidic form of the Yelectrode.

The results described here are dealing with the elec-trochromic characteristics of YH. However, is seems like-ly that other rare-earth type hydride-forming electrodes,including the group III transition metals, may reveal asimilar optical effect. A preliminary study confirmed thatsuch typical electrochromic behavior can be electrochem-ically induced in other Pd-coated rare-earth thin filmelectrodes. These and other results are presented in detailin a separate paper.12

AcknowledgmentsThe authors thank H. Donkersloot and J. Kerkhof for

preparation of the Y films, C. Geenen for the SEM investi-gations, and F. den Broeder, P. Duine, P. Kelly, M. Ouwer-kerk (Philips Research Laboratories), J. N. Huiberts, andR. Wijngaarden (Vrije Universiteit, Amsterdam) for stimu-lating discussions.

Manuscript submitted April 9, 1996; revised manuscriptreceived June 28, 1996.

Philips Research Laboratories assisted in meeting thepublication costs of this article.

REFERENCES1. P. Vajda, in Handbook on the Physics and Chemistry of

Rare Earths, Vol. 20, p. 207, K. A. Gschneider, Jr. andL. Eyring, Editors, Elsevier Science, By, London(1995).

2. J. N. Huiberts, R. Griessen, J. H. Rector, R. J. Wijn-gaarden, J. P. Dekker D. G. de Groot, and N. J.Koeman, Nature, 380, 231 (1996).

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5. T. J. Udovic, Q. Huang, and J. J. Rush, J. Phys. Uhem.Solids, 57, 423 (1996).

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7. M. Kremers, P. H. L. Notten, and R. Griessen, To bepublished (1996).

8. P H. L. Notten, J. E. A. M. van den Meerakker andJ. J. Kelly, Etching of 111-V Semiconductors: AnElectrochemical Approach, Elsevier, London (1991).

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10. R. Griessen and T. Riesterer, Top. Appi. Phys., 63, 219(1988) and references cited therein.

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12. M. Ouwerkerk, P H. L. Notten, M. Kremers, andR. Griessen, To be published (1996).

0 2 4 6 8

(a)

0 2 4 6 8 'co"- t[sI


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