the study of hydrogen sorption in palladium limited volume electrodes (pd-lve): part ii. basic...

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Journal of Electroanalytical Chemistry 492 (2000) 128–136

The study of hydrogen sorption in palladium limited volumeelectrodes (Pd-LVE)

Part II. Basic solutions

Andrzej Czerwinski a,b,*, Iwona Kiersztyn a,1, Micha l* Grden a

a Department of Chemistry, Warsaw Uni6ersity, Pasteura 1, 02-093 Warsaw, Polandb Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland

Received 14 April 2000; received in revised form 17 June 2000; accepted 20 July 2000

Abstract

The study of hydrogen and deuterium electrosorption into the palladium limited volume electrode (Pd-LVE) has beenperformed in basic solutions. The results are compared with data obtained earlier in an acidic solution. As in the acidic solution,the amount of electrosorbed hydrogen, H(D)/Pd, measured electrochemically in a basic solution, depends significantly on thesweep rate in cyclic voltammetric experiments and also on the thickness of the LVE. Two different mechanisms of hydrogendesorption which operate in both basic and acidic solutions, namely the electrochemical oxidation and the non-electrochemicalrecombination occurring simultaneously within the Pd-LVE, have been postulated. © 2000 Elsevier Science B.V. All rightsreserved.

Keywords: Palladium; Hydrogen; Deuterium; Sorption in palladium; Limited volume electrodes (LVE)

1. Introduction

It has been demonstrated in recent papers [1,2] thatthe amount of hydrogen absorbed, H/Pd, in the palla-dium limited volume electrode (Pd-LVE), calculatedfrom the charge of the hydrogen oxidation peak, de-pends significantly on the rate of the potential sweepand the thickness of the deposited layer of palladium.The explanation of the sweep rate influence on thehydrogen desorption process is based on two simulta-neous processes, one electrochemical and the othernon-electrochemical (chemical), occurring on the elec-trode [1–3]. Earlier it has been postulated [3] thathydrogen can also desorb from the electrode and dif-fuse to the bulk solution due to the surface recombina-tion reaction, 2Habs�2Hads�H2 without chargetransfer. This recombination reaction is preceded by thediffusion of hydrogen from the bulk of the metal to the

electrode surface. Our explanation of the observed ef-fects can now be supported theoretically by a recentpaper of Zhang et al. [4] who postulated that at lowercurrent densities the process of sorbed hydrogen re-moval from the Pd surface can go also through theTafel mechanism. Under our experimental conditions,the lower current densities can be translated to lowsweep rate of electrode polarization. When the thick-ness of the Pd-LVE is small, the hydrogen sorbed in asubsurface layer contributes significantly to the totalhydrogen concentration in the metal. This effect de-creases with the increase of the electrode thickness. Theresults have strongly supported earlier hypothesesabout hydrogen absorbed in palladium as a subsurfacelayer phase [5] which is supposed to be more hydrogenrich than the a- and b-phases of absorbed hydrogen [6].The formation of a subsurface layer of hydrogen hasbeen postulated by Breiter [5] as the first step of hydro-gen incorporation into palladium during the absorptionprocess.

We have suggested [2] that during the desorptionprocess, hydrogen present in the subsurface layer isresponsible for the generation of hydrogen adsorbed on

* Corresponding author. Fax: +48-22-8225996.E-mail address: [email protected] (A. Czerwinski).1 Present address: Department of Chemistry, University of Pod-

lasie, Siedlce, Poland.

0022-0728/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S 0022 -0728 (00 )00291 -6

A. Czerwinski et al. / Journal of Electroanalytical Chemistry 492 (2000) 128–136 129

the palladium surface, which can be directly oxidizedelectrochemically. The equilibria of other forms of hy-drogen, i.e. the a- and b-phases contribute indirectly(supplying subsurface layers) to the total rate of desorp-tion of hydrogen which results in rather unusual H(D)/Pd versus 6 plots. Generally, the scheme of hydrogenremoval from the palladium electrode in acid solutionshas been proposed [2] as follows:

HblHalHsublHads�

�k1

1/2H2

�k2

H++e−

(1)

The process of hydrogen desorption can be con-trolled either by the rate of the surface reaction or bybulk diffusion, depending on the relative rate of elec-trode polarization, while the amount of hydrogen des-orbed with charge transfer depends on the relative rateof two processes presented in the scheme above, i.e.k1/k2.

In this paper, we report our recent results of thestudy of both hydrogen desorption and absorption inthe Pd-LVEs in basic solutions with various thicknessesof a thin palladium layer. If, indeed, the two processesof hydrogen desorption, i.e. the electrochemical andnon-electrochemical step, occur on Pd-LVE, the sweeprate dependence of the amount of desorbed hydrogenshould be different for various thicknesses of the palla-dium layer. Moreover, this effect should be similar inacidic and basic solutions in spite of the fact thatmechanisms of hydrogen adsorption are different: pro-ton or deuteron reduction in acidic solution and waterdecomposition (H2Oads+e− [ Hads+OH−) in basicor neutral solutions.

2. Experimental

The experiments were performed mainly in 0.1 MLiOH and 0.1 M LiOD solutions prepared using highpurity water (Millipore® or 99.9% D2O, Institute forNuclear Studies, Swierk, Poland) and pure LiOH.Heavy water was distilled four times, including distilla-tion in the presence of permanganates. As was foundfor H2O, this procedure was appropriate for obtainingwater with purity comparable to the water preparedusing a Millipore® system. Also, a comparative studyhas been performed in 0.1 M solutions of NH4OH,LiOH, KOH and RbOH. A platinized platinum foilwas used as the auxiliary electrode. A Ag � AgCl elec-trode was used as the reference electrode, but all poten-tials are referred to the RHE in this paper which isclose (910 mV) to the hydrogen equilibrium potentialof the working electrode.

The working electrode was a gold wire (99.9%) cov-ered with palladium. Palladium was electrochemicallydeposited at a constant current density ( j=1 mAcm−2) in an acidic (4 M HCl) PdCl2 solution using aPd wire as the anode. The LV electrodes used in thiswork were obtained under identical conditions, i.e.geometry of electrode and deposition parameters, andthe roughness factor did not vary significantly, ca.920%. Other experimental errors might include thedifferences in the morphology of the palladium surfaceand the entrapment of hydrogen gas in the electrodepores.

At the beginning of each experiment, the old palla-dium layer was dissolved in concentrated nitric acidand a new palladium deposit was plated on the goldelectrode. During the experiments the palladium elec-trode was first held at a constant potential (Eabs= −0.40 V versus RHE) for 30 min to saturate the electrodewith hydrogen [7]. This time was sufficient for thesaturation of all the electrodes used [7]. It was statedearlier [2] that the amount of hydrogen sorbed in/on thepalladium electrode is not influenced by the evolutionof hydrogen gas during the electrode polarization atapplied potentials.

The amount of absorbed hydrogen was calculatedfrom the charge obtained by the integration of theanodic peak current on the cyclic voltammogram takenafter the saturation of the palladium electrode withhydrogen. The thickness range of deposited Pd wasbetween 0.2 and 1.0 mm. After taking into account thestructure of the elementary cell of palladium which is offace cubic centered symmetry with crystal dimensions,aPd=3.89 A, , in 0.2 mm we can expect about 500 layersof the same structure (only centered squares or onlysquares) and 1000 layers of mixed structures (squaresand centered squares, interchangeably). A typical cyclicvoltammogram for Pd-LVE (0.2 mm) in 0.1 M LiOHsolution is demonstrated in Fig. 1.

Fig. 1. Cyclic voltammograms, at 25 mV s−1, of the limited volume(0.2 mm) palladium electrode in 0.1 M KOH solution.

A. Czerwinski et al. / Journal of Electroanalytical Chemistry 492 (2000) 128–136130

Fig. 2. The flow-through palladium electrode. (1) palladium black, (2)RVC®, (3) 0.1 M LiOH solution, (4) glass tube, (5) gold wire.

In order to compare the results obtained with cyclicvoltammetry the experiments using the chronocoulo-metric technique were also performed. After hydrogenabsorption was completed the potential of the electrodewas changed to 0.55 V, where all hydrogen was elec-trooxidized. At this potential neither ad- nor absorbedhydrogen exists on palladium.

The measurements were performed with a CH Instru-ments, Model 604 (Cordova, USA) electrochemical an-alyzer coupled with an IBM PC compatible computer.

Lithium introduction into palladium during hydro-gen electrolysis in a 0.1 M LiOH solution was confi-rmed in the following experiment. A 0.1 M LiOHsolution was passed through palladium black (JohnsonMatthey) polarized at −0.50 V (versus RHE). A palla-dium black powder-electrode was placed in a 10 mmdiameter glass tube. The scheme of the experiment isdemonstrated in Fig. 2. The palladium black was im-mobilized in reticulated vitreous carbon (RVC®) whichserved simultaneously as an electric collector and wascontacted with a platinum wire. The solution waspassed through the palladium black with controlledflow rate of ca. 10 cm3 min−1 for a period of 21 days.The reference and the counter electrodes were placed inthe vessel where the flowing solution was collected.

After the experiment was finished the palladiumblack was separated from the RVC® foam, rinsed withwater (Millipore®) and dissolved in concentrated nitricacid. The clear solution containing only palladiumblack (and free of RVC® particles) was analyzed usingthe atomic absorption spectroscopy (AAS) method(Perkin–Elmer 4100 ZL spectometer). In this way, onlythe concentration of lithium incorporated into palla-dium was measured, without any influence of lithiumpossibly incorporated into RVC®.

3. Results and discussion

3.1. Cyclic 6oltammetry of Pd-LVE

Fig. 3 presents a cyclic voltammogram, at 5 mV s−1,for a 0.2 mm thick palladium electrode. Before theanodic potential sweep, the electrode was conditionedat negative potentials (−0.47 V for 30 min.) to ensurefull saturation of the electrode with hydrogen. Then,the potential of the electrode was scanned positivelyand the hydrogen oxidation current was recorded. Thiswas followed by cathodic polarization, which resultedin a cathodic wave. These oxidation and reductioncurrents represent the desorption and sorption of hy-drogen from and into the Pd-LVE in basic solution,respectively.

It can be seen from Fig. 3 that when the upper limitpotential in cyclic voltammetric experiments is limitedto 0.6 V, only partial oxidation of palladium surface

It should be noted that in the acidic solutions studiedpreviously, the region of hydrogen sorption/desorptionis clearly separated from the region of the palladiumoxide formation, which is contrary to the situation inalkaline solutions where these two processes overlapslightly [2,8,9,19]. Also, it has to be noted that in basicsolutions the amount of electrooxidized hydrogen fromthe palladium electrode can be calculated only from theanodic current peak. In the cathodic part of the voltam-mogram the currents from hydrogen adsorption–ab-sorption and hydrogen evolution overlap strongly.Although the currents of hydrogen and surface oxida-tion partially overlap in the positive scan, the error inthe estimation of charge needed for hydrogen oxidationshould not exceed 1%. Theoretically, for the thinnestlayer studied (ca. 1000 atomic layers), the ratio of thecharge needed for complete surface oxidation to PdO ata roughness factor ca. 10–20 [10] to the charge neededfor oxidation of absorbed hydrogen in the bulk of theelectrode does not exceed 4%. Under our experimentalconditions, the roughness factor of the Pd electrodeswas ca. 10. It has to be noted that during our experi-ments hydrogen oxidation is completed before the sur-face of Pd is totally oxidized which further decreasesthis ratio significantly. At our experimental conditionsonly a small part of the electrode was covered withoxides. The fact that the behavior of the palladiumelectrode observed in basic solutions, i.e. the shape ofH(D)/Pd vs. 6 plots, is similar to that reported previ-ously in acidic solutions [2] where no overlapping be-tween both oxidation processes occurs, confirms thatunder our present experimental conditions (scan raterange between 0.001 and 0.05 Vs−1) the influence of theprocess of surface oxidation on the charges obtainedfrom the hydrogen oxidation peak is rather negligible.It should be noted that with the increase of the scanrate both oxidation peaks overlap more and their sepa-ration is more difficult. Therefore, the range of scanrates used in the experiment had to be limited. Amaximum scan rate of 0.05 s−1 was chosen.


occurs. This effect is observed on LVE as well as onmassive palladium electrodes [7,11]. Therefore, in theexperiments performed in acidic solutions, two pro-cesses, namely the oxidation of sorbed hydrogen and

the surface Pd oxides generation are well separated [11].The Pd-LVE in acidic solutions behaved as a typicalthin layer electrode. The shape of the cyclic voltam-mogram shown in Fig. 3 suggests that in basic solutionit is possible to estimate the amount of hydrogen elec-trooxidized from palladium only during the positivescan. The calculation of the amount of hydrogen intakein palladium during the negative scan is impossible dueto overlapping of hydrogen adsorption, absorption andevolution currents.

3.2. Sweep rate dependence of hydrogen and deuteriumsorption in Pd-LVE of 6arious thickness

Fig. 4A and B show the sweep rate dependence of theabsorption capacity of hydrogen, H/Pd, and deuterium,D/Pd, in the palladium electrode determined from theanodic oxidation of sorbed hydrogen in 0.1 M LiOHand 0.1 M LiOD, respectively. These plots present datafor different palladium layer thicknesses. The electrodethickness ranged from 0.2 to 3.2 mm for both hydrogenand deuterium. Hydrogen was sorbed at a constantpotential (Eabs= −0.40 V). When the absorption pro-cess was completed, the absorbed hydrogen was oxi-dized from palladium at various sweep rates (from0.001 to 0.050 V s−1). The values of H(D)/Pd shown inFig. 4A and B represent average values, each takenfrom five to seven experiments.

A characteristic feature of the H(D)/Pd plots shownin Fig. 4A and B is the maximum visible at relativelyslow sweep rates, i.e. below 0.02 V s−1, and for rela-tively thin (0.2–3.2 mm) palladium films. The maximumratio of hydrogen (or deuterium) electrooxidized onPd-LVE to palladium atoms depends significantly onthe thickness of the Pd layer and this ratio changesfrom 0.88 (90.05) to 0.65 (90.05) for hydrogen andfrom 0.86 (90.05) to 0.66 (90.05) for deuterium.Generally, the thinner the Pd layer, the higher is theH(D)/Pd ratio. The maximum value of H(D)/Pd vs.sweep rate shifts toward lower sweep rates with anincrease of the electrode thickness. This is similar to theeffect observed in acidic solutions [2].

The amount of sorbed hydrogen (or deuterium) doesnot increase proportionally to the layer thickness. Sincethe amount of Pd increases when the Pd layer getsthicker, the H(D)/Pd ratio decreases. It can be ex-pected, therefore, that for a certain thickness of thepalladium electrode this maximum will not be seen atall. Indeed, when thick Pd layers are used, i.e. \3.2mm, this maximum disappears and the highest H(D)/Pdvalue does not exceed 0.73 (90.05) for hydrogen and0.68 (+0.05) for deuterium. The higher values of theH(D)/Pd ratio for thin palladium layers in comparisonto those obtained with solid palladium electrodes(wires, rods, foils, etc.) suggest that hydrogen (or deu-terium) is accumulated in higher concentration in a thin

Fig. 3. Cyclic voltammograms, at 5 mV s−1, of the limited volume(0.2 mm) palladium electrode in 0.1 M KOH solution. Before thepositive scan the electrode was held for 30 min at −0.47 V.

Fig. 4. (A) H/Pd ratios vs. polarization sweep rate for palladiumelectrodes with various thicknesses: (1) 0.2; (2) 0.8; (3) 1.6; (4) 3.2 mm.Solution 0.1 M LiOH. (B) D/Pd ratios vs. polarization sweep rate forpalladium electrodes with various thicknesses: (1) 0.2; (2) 0.8; (3) 1.6;(4) 3.2 mm. Solution 0.1 M LiOD.


Fig. 5. Influence of the cation on H/Pd ratios vs. sweep rate forpalladium. Solutions: (1) 0.1 M LiOH; (2) 0.1 M KOH; (3) 0.1 MRbOH, (4) 0.1 M NH4OH.

potentials where the electrode is free of both adsorbedand absorbed hydrogen, i.e. at potentials in the doublelayer charging region. The amount of electrooxidizedhydrogen obtained from chronocoulometry measure-ments performed according to the procedure describedabove, was the same (95%) as that obtained from theCV curve for the sweep rate at which the maximum onthe H/Pd plot was observed. Assuming that duringchronocoulometry experiments there is no hydrogenloss without charge transfer, we can conclude that themaximum values of H/Pd observed in Figs. 4 and 5correspond to the real amount of hydrogen absorbed inthe metal. This kind of measurement has been done inthe whole range of potentials of hydrogen absorption inpalladium and results obtained with CC and CV wereequal. This clearly indicates that reliable informationabout the amount of absorbed hydrogen might beobtained from cyclic voltammetry experiments and thistechnique might be useful for investigation of the hy-drogen sorption/desorption process if certain conditionsare fulfilled, namely Pd layer thickness and scan rate.

It is possible that the processes which occur at thepotential close to the reversible hydrogen potential (lowanodic overpotential, i.e. at the earlier stage of slowpotential scan) participate in the overall process ofnon-electrochemical hydrogen recombination, aroundthe minimum on the H(D)/Pd versus 6 plot, especiallywhen the surface coverage with adsorbed hydrogen andthe amount of absorbed hydrogen are high. During CAor CC experiments, this potential region is omitted andthese effects, which are, in our opinion, of great impor-tance, cannot be observed.

A significant decrease of the H(D)/Pd ratio at veryslow sweep rates, below 0.02 V s−1, also depends to agreat extent on the thickness of the Pd layer. The lowestamount of hydrogen (0.5590.05) or deuterium (0.6590.05) per amount of Pd is seen for thicker Pd layers inthis region. It is opposite to the results obtained inacidic solutions [2] where this effect was visible ratherfor thinner Pd layers. The depletion of the electrodefrom hydrogen or deuterium at very slow sweep ratescan be explained by the presence of an additionalnon-electrochemical removal of each of the hydrogenisotopes due to the recombination processes: 2H�H2

and 2D�D2. The differences between acidic and basicsolutions are probably due to interactions betweensorbed hydrogen and lithium in the subsurface layer ofthe electrode, moreover different mechanisms of hydro-gen reduction/oxidation with participation of protonsor water molecules have to be taken into account. Thisresult points again to the role of a subsurface layer ofhydrogen or deuterium in the Pd electrode.

The non-electrochemical recombination reactionmentioned above should be less important when thesweep rate is increased due to the fact that less time isallowed for the recombination process to occur. This

layer situated close to the electrode surface and sup-ports our view on the existence of a subsurface layer inthe Pd electrode [2].

The H(D)/Pd values in the range of sweep rateswhere maximum saturation of the thinnest layer isobserved are close to 0.9 for both hydrogen isotopes.This value is similar to the one obtained in acidicsolutions [2]. The H/Pd and D/Pd ratios reach the valueof 0.88 and 0.86 (9 0.05), respectively, for palladiumlayer thicknesses 50.2 mm. Contrary to acidic solu-tions, the value of H(D)/Pd coefficient found in basicsolution (lithium hydroxide) exceeds values obtained insolid electrodes only for the thinnest palladium layers[7–9]. The highest H(D)/Pd values obtained in thiswork for the thick Pd layers correspond to the valuescited in the literature, i.e. between 0.7 and 0.8 forhydrogen and deuterium [7–11,14]. This means that thesubsurface layer of absorbed hydrogen in basic solu-tions is thinner and/or the concentration of subsurfacehydrogen is different from that in acidic solution. It isdue to incorporated lithium interaction with absorbedhydrogen. It cannot be excluded that different gradientsof the stresses and defects produced in palladium dur-ing hydrogenation might influence the electrode’s be-havior in a different way for various electrodethicknesses. The distribution of stresses and defects canbe different just under the palladium surface and insidethe metal.

One can avoid the influence of the sweep rate on theamount of electrodesorbed hydrogen when a differentpotential program is applied to the electrode, namelychronocoulometry or chronoamperometry instead ofthe cyclic voltammetry. In this situation, the potential isshifted abruptly from a value at which hydrogen ab-sorption takes place to a positive value where theoxidation of hydrogen occurs and the oxidation chargeor current is measured. For complete removal of ab-sorbed hydrogen, it is necessary to desorb hydrogen at


would be in agreement with the values of the H(D)/Pdratio obtained at a sweep rate up to ca. 0.05 V s−1,seen in Fig. 4A and B for thin, 0.2 mm, Pd layers. Afurther decrease of the H(D)/Pd ratio with the increaseof the sweep rate can be explained by diffusion limita-tions, i.e. a slow diffusion of hydrogen from the bulk ofthe electrode to the subsurface layer and then to thesurface.

3.3. Sweep rate dependence of hydrogen and deuteriumsorption in Pd-LVE from basic solutions with differentcations

The influence of cations in basic solution on theH/Pd value is shown in Fig. 5.

One can see that for the group of cations studiedwith the exception of NH4

+, the H/Pd value decreaseswith the increase of the size of the cation. This effectsupports our earlier results [14] and it was interpretedin terms of different interactions between lithium groupmetals and hydrogen in the subsurface layer. Thehighest H/Pd ratio was found for an ammonia solutionhaving the same cation concentration as the metalconcentrations in other basic solutions.

The experiments with the flow-through Pd electrode,described in the experimental part, confirmed thatlithium is introduced irreversibly into palladium duringhydrogen electrolysis from 0.1 M LiOH. It was foundafter 21 days of electrolysis in 0.1 M LiOH at −0.50 Vthat the total lithium atomic concentration in palladiumwas ca. 0.7 atomic%. Taking into account the ratherlow value of the diffusion coefficient (D) of lithium inpalladium (between ca. 10−16 cm2 s−1 according toYamazaki et al. [15] and ca. 10−10 cm2 s−1 accordingto Astakhov et al. [20]) we can expect that in thesubsurface layer of the palladium electrode the concen-tration of lithium is significantly higher than in the bulkof the metal. It is likely that the value of D for lithiumdepends on the concentrations of absorbed hydrogenand incorporated lithium in a manner similar to thehydrogen diffusion coefficient. The results of Yamazakiet. al. [15] show that maximum Li concentration inpalladium electrode is reached at a depth ca. 25 nmbelow the surface. It can be supposed that the behaviorof other alkali metals is similar. It should be noted thatincorporated alkali metals can influence the process ofhydrogen sorption not only through direct interactionwith hydrogen (bond formation) but also throughchanges of the stress fields generated during hydrogensorption. Those changes depend on the amount ofincorporated metals and their atomic size. Hence, vari-ous metals might produce different stress effects.

It is proposed in the literature that in the case of themetals which can adsorb and absorb hydrogen thepresence of stresses generated during hydrogen sorptioncan promote entrance of the alkali metals into the

electrode. There, the system is further stabilized byinteraction of the incorporated metal with absorbedhydrogen and Pd, by constituting ternary alloys, likePd–Li–H or Pt–Rb–H [21,22]. The surface layer ofalkali metals generated during upd should be thermo-dynamically unstable in contact with water. The disso-lution of alkali metals in water might generateconcentration profiles like those reported by Yamazakiet al. [15], where the concentration of the alkali metal isdepleted near the surface.

Apart from small differences, the plots of H(D)/Pdversus 6 for both isotopes of hydrogen in our experi-ments are similar and, in our opinion, the differences inshapes are rather due to differences in the electrochem-ical behavior of H2O (H+) and D2O (D+) and thethermodynamic behavior of PdHUPd–H–Li andPdDUPd–D–Li systems, especially in the subsurfacelayer. It has to be noted that the diffusion coefficientsof hydrogen and deuterium in the a- and b-phases aredifferent [12,13] and can be influenced by the presenceof lithium in the subsurface layer. The value of thediffusion coefficient in palladium electrodes dependssignificantly on hydrogen concentration in the bulk ofthe metal [16]. Since the maximum on the H(D)/Pdplots depends significantly on the thickness of Pd inPd-LVE (which points to the existence of the subsur-face layer), we also suggest a different behavior ofhydrogen and deuterium in such a subsurface layerformed just beneath the electrode surface.

3.4. A model of hydrogen desorption from Pd-LVE ina basic solution

Contrary to results obtained in the acidic solution, adetailed interpretation of the results for basic solutionspresented in Fig. 4A and B is difficult. It is additionallycaused by the effects of incorporation of alkali metalsin palladium. We suggested [2] for the acidic solutionthat the last stage of hydrogen desorption might pro-ceed via two different pathways: (a) the electrooxida-tion of the sorbed hydrogen, which in basic solutionfollows the reaction scheme: Hads+OH−�H2O+e−;and (b) the hydrogen desorption via recombinationwithout any oxidation step and diffusion of H2 into thesolution, 2Hads�H2.

The involvement of a non-electrochemical recombi-nation step is clearly shown in work [2] on the differ-ence in the charges of sorbed and desorbed hydrogen(Section 3.1) as well as the dependence of the H(D)/Pdratio on the sweep rate and the thickness of the palla-dium layer (Section 3.2). Thinking similarly, in basicsolutions, the decrease of this ratio at very low sweeprates is evidence of the participation of the non-electro-chemical process in the desorption of hydrogen. It hasto be noted that at the open circuit potential electro-


sorbed hydrogen diffuses spontaneously out of fullysaturated bulk palladium [3,17,18]. This is caused by alow partial pressure (close to zero) of hydrogen dis-solved in the solution, which shifts the equilibriumbetween absorbed, adsorbed and gaseous hydrogen inthe direction of desorption. Therefore, hydrogen shouldimmediately escape from the thin layer of palladiumLVE [2]. Absorbed hydrogen in palladium has a strongtendency to leave the metal without any strong anodicelectrochemical polarization. This strongly supports ourexplanation of obtained results. Under open circuitconditions the process of hydrogen loss proceeds prob-ably mainly through the Tafel recombination reaction.However, participation of other, electrochemical reac-tions, e.g. the Heyrovsky reaction accompanied withthe respective oxidation process, especially at low posi-tive potentials close to the reversible hydrogen poten-tial, cannot be excluded.

The experimental results presented in this work sug-gest that the H(D)/Pd ratio, calculated for the elec-trooxidation process, depends not only on thedifference between the rates of electrochemical andnon-electrochemical (recombination) process, but alsoon the rate of hydrogen diffusion from the bulk of themetal towards the palladium surface. This, in turn, is afunction of the alkali metal concentration in the subsur-face layer. The rate of electrooxidation of absorbedhydrogen depends on the positive potential of the palla-dium electrode, i.e. on the sweep rate of the electrodepotential. Because the recombination reaction does notdepend on the potential the relative contribution ofthese processes should change with the sweep rate in aCV experiment. An increase of the amount of elec-trooxidized hydrogen with the increase of a sweep ratein a certain range of sweep rates is observed in Fig. 4Aand B.

In the model of hydrogen electrooxidation from thepalladium electrode in acidic solution we have proposedthat hydrogen dissolved in the outer layers of palladium(subsurface hydrogen) plays an important role duringelectrooxidation (also electrosorption). During the firststep of desorption, hydrogen diffuses from the bulk ofthe metal to the subsurface layers where it is in equi-librium with adsorbed hydrogen (Had at the palladiumsurface), and finally is removed from the electrode byelectrooxidation or chemical recombination. The pres-ence of lithium or other alkali metals in the bulk ofpalladium, especially in the subsurface layer, and itsinfluence on hydrogen sorption–desorption has to betaken into account. In this model also the transitionbetween a- and b-phases of sorbed hydrogen shouldplay an important role:

HblHalHsublHsub(Li)lHads ��OH−

H2O+e− (2)

One can expect that, at high sweep rates, the rate ofsubsurface layer hydrogen removal by electrooxidation

is higher than that of a subsurface layer formation.Under these conditions, the rate-limiting step of hydro-gen oxidation is the subsurface layer formation fromhydrogen present in the bulk of palladium as a- andb-phases. The equilibrium between all these forms ofabsorbed hydrogen is influenced by incorporatedlithium or other alkali metals. These metals in palla-dium probably generate hydrides which are in equi-librium with all forms of absorbed hydrogen. Wesuggest that this interaction between the sorbed metaland hydrogen occurs mainly in the subsurface layer ofpalladium LVE. In all these cases, the supply of hydro-gen to the subsurface layer at higher sweep rates islimited. In our opinion this diffusion limitation is themain reason for the presence of the ‘plateau’ in plots ofH(D)/Pd ratio versus 6 observed at sweep rates higherthan typical for the appearance of the maximum. Inthis region of sweep rates the rate determining step isthe equilibrium formation between absorbed hydrogenand hydrogen sorbed in the subsurface layer. Fig. 6presents a general model of hydrogen removal frompalladium with respect to potential sweep rate.

The model explains how the mechanism of hydrogendesorption is influenced by electrochemical changes in-duced when the sweep rate is varied. The ‘plateau’ isobserved on the plots for both isotopes of hydrogenabsorbed in thinner layers of deposited palladium, al-though for hydrogen this plateau appears at lowersweep rates than for deuterium. In this range of sweeprate the sorbed hydrogen electrooxidation is limitedonly by the equilibrium between absorbed hydrogenand hydrogen in the subsurface layer. For thicker palla-dium films, at higher sweep rates, the transport ofhydrogen from the bulk of the metal to the subsurfacelayer is limited by the rate of diffusion. Therefore, somedecrease of the H(D)/Pd ratio should be observed forfaster polarization, which is clearly seen in Fig. 4A andin B (curves 4).

The general scheme of hydrogen desorption frompalladium in a basic solution can be written as follows:

HblHalHsublHsub(Me)lHads ��OH−

�k1

1/2H2

�k2

H2O+e−

(3)Further studies of hydrogen and deuterium sorption

in palladium LVE (temperature effects), which are inprogress, will give us more data for a full explanationof the effects reported in this paper.

4. Summary

1. During electrooxidation of absorbed hydrogenand deuterium from palladium in basic and acidicsolutions both hydrogen isotopes could leave the elec-


Fig. 6. A model showing the influence of the sweep rate on the mechanism of hydrogen desorption from the palladium electrode. The possiblealkali metal concentration profile (in arbitrary units) based on Ref. [15] is included.

trode according to the two pathways: the electro-chemical oxidation step and the non-electrochemicalrecombination process. The contribution of bothmechanisms depends on sweep rate of electrode polar-ization.

2. When the thickness of the Pd-LVE is small, thehydrogen sorbed in a subsurface layer contributes sig-nificantly to the total hydrogen concentration in themetal. The thickness of the subsurface layer generatedin palladium in a basic solution is thinner than in anacidic solution. This is caused by the presence of al-kali metal in palladium which enters there during hy-drogen electrolysis. This effect decreases with theincrease of the electrode thickness.

3. Hydrogen present in the subsurface layer is re-sponsible for the generation of hydrogen adsorbed onthe palladium surface, which can be directly oxidizedelectrochemically. In basic solutions this reaction isinfluenced by metals incorporated with hydrogen inpalladium.

Acknowledgements

This work was supported financially by the PolishState Committee for Scientific Research (KBN) grantno. 3T09A 003 19.

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the study of hydrogen sorption in palladium limited volume electrodes (pd-lve): part ii. basic...

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