ORIGINAL PAPER

Temperature influence on hydrogen sorption in palladiumlimited-volume electrodes (Pd-LVE)

Received: 24 July 2002 / Accepted: 26 November 2002 / Published online: 20 February 2003� Springer-Verlag 2003

Abstract The effect of temperature on hydrogen anddeuterium electrosorption into a palladium LVE (limit-ed-volume electrode) has been investigated. A decreasein hydrogen capacity (H/Pd ratio) with increasing tem-perature has been observed. Temperature strongly in-fluences the plots of measured H(D)/Pd values vs.potential scan rate. In addition, hydrogen absorptionwas found to be dependent on the composition of thesurrounding electrolyte solution. These results haveconfirmed the hypothesis that two different mechanismsof hydrogen desorption from the palladium electrodetake place, namely electrochemical oxidation and non-electrochemical recombination. Further, the ratio be-tween the rate constants for these two processes has beenfound to change with temperature.

Keywords Deuterium electrosorption Æ Hydrogenelectrosorption Æ Palladium

Introduction

Recent investigations into a palladium-hydrogen systemdetected unusual behavior of hydrogen electrochemicallyabsorbed in palladium. It was found that the amount of

hydrogen oxidized electrochemically during cyclic vol-tammetry depends on the potential scan rate [1, 2, 3, 4].With an increase in the potential scan rate (m) the mea-sured H/Pd value first increases to a maximum and thendecreases. This phenomenon was explained on the basisof the existence of dual mechanisms of hydrogen de-sorption [1, 2, 3, 4, 5]. Hydrogen can be desorbed fromPd and other hydrogen-absorbing materials by eitherelectrochemical reaction and/or a non-electrochemicalrecombination process. The former process proceeds viathe Volmer reaction, while in the latter process absorbedhydrogen is removed from the electrode by participationin the Tafel reaction, i.e. 2Habs fi 2Hads fi H2 [6]. Therelative importance of each mechanism in the overallhydrogen desorption process depends on the potentialscan rate. The shape of measured H/Pd vs. m plots wasstrongly influenced by the thickness of the Pd layer aswell as by the electrolyte composition [1, 2, 3, 4]. Gen-erally, the smaller the Pd layer thickness, the greater theamount of absorbed hydrogen. This effect was explainedon the basis of the existence of a layer of hydrogen dis-solved just beneath the surface, i.e. ‘‘subsurface hydro-gen.’’ The hydrogen dissolved in this layer behaves in adifferent way than that dissolved in the bulk of the metal.The differences concern both the concentration and thethermodynamics of the generation of this form of hy-drogen. According to the literature [7, 8, 9, 10, 11, 12, 13,14], the thickness of the layer of subsurface hydrogenvaries from one layer to a value of 200–500 A. Thisthickness as well as the concentration of hydrogen dis-solved in the subsurface layer depend on the adsorptionof ions and poisons on the electrode surface [11, 13, 14].This observation indicates that the existence of a sub-surface layer of absorbed hydrogen cannot be the effectof the existence of unique surface morphology, e.g. a Pd-black-like structure. Thus, subsurface hydrogen can betreated as a separate phase (Hsub), similar to the a and bphases of absorbed hydrogen. The following diagrampresents the general scheme of the hydrogen absorption/desorption process with participation of ‘‘subsurfacehydrogen’’ [2, 3]:

J Solid State Electrochem (2003) 7: 321–326DOI 10.1007/s10008-003-0352-5

Andrzej Czerwinski Æ Iwona Kiersztyn Æ Michał Grden

A. Czerwinski (&) Æ I. Kiersztyn Æ M. GrdenDepartment of Chemistry,Warsaw University, Pasteura 1,02-093 Warsaw, PolandE-mail: aczerw@chem.uw.edu.plFax: +48-22-8225996

A. CzerwinskiIndustrial Chemistry Research Institute,Rydygiera 8, 01-793Warsaw, Poland

Present address: I. KiersztynDepartment of Chemistry,University of Podlasie,Siedlce, Poland

The process of hydrogen desorption can be controlledeither by the rate of the surface reaction or by bulkdiffusion, depending on the electrode polarization, whilethe amount of hydrogen desorbed with charge transferdepends on the relative rate of the two processes de-scribed above, i.e. k1/k2. The values for k1 and k2 mightrepresent a function of the diffusion coefficient (for dif-fusion as the rate-determining step) or they might beequal to classical rate constants (for desorption limitedby the surface process). The temperature should theninfluence both values in a different manner, hencechanging the k1/k2 ratio.

In this paper, we report results from a study of theinfluence of temperature on hydrogen sorption in pal-ladium limited-volume electrodes (Pd-LVEs).

Experimental

All experiments were performed in 0.1 M LiOH and 0.5 M H2SO4solutions prepared using high purity water (Millipore or 99.9%D2O) and pure LiOH or H2SO4 (BDH). It has been shown [15, 16]that at this D/H ratio in electrolyte solution [D/(H+D)=0.991] theenrichment of absorbed hydrogen with the light isotope is negli-gible. A platinized platinum foil was used as an auxiliary electrode.The Ag/AgCl electrode was used as the reference electrode, but allpotentials in the paper are referred to RHE at room temperature(25 �C).

The working electrode was a gold wire (99.9%, diameter=0.5 mm) covered with palladium. Palladium was electrochemicallydeposited at a constant current density (j=1 mA cm)2) in an acidic(4 M HCl) PdCl2 (0.1 M) solution using a Pd wire as the anode.The length of the part of the wire covered with the Pd deposit was1 cm. The whole Pd deposit was immersed in solution without anycontact with gas above the solution level. The working cell wasequipped with a water jacket and the temperature controlled in therange 273–361 K via a thermostat (WLMU2C, Germany). The LVelectrodes used in this work were obtained under identical condi-tions, i.e. the geometry of the matrix electrode (gold) and the de-position parameters. The roughness factor of the electrodes usedwas calculated on the basis of the charge of the reduction of thesurface oxides, according to the method described elsewhere [17,18]. The obtained roughness factor values were ca. 10, with thedifferences between the electrodes studied not greater than 20%.

At the beginning of each experiment, the old palladium layerwas dissolved in concentrated nitric acid and a new palladiumdeposit was plated on the gold electrode. During the experimentsthe palladium electrode was first held at a constant potential(Eabs=)0.26 V vs. RHE) for 30 min in basic solution and 5 min(Eabs=)0.13 V vs. RHE) in acidic solution to complete the elec-trode saturation with hydrogen [15, 19]. This time was sufficient forthe saturation of all the electrodes used. The potential values werechosen to ensure full saturation of the electrodes with absorbedhydrogen in both solutions studied. To avoid the influence ofageing, the electrode was subjected to a few tens of cycles of hy-drogen absorption and desorption at the beginning of the mea-surements.

The amount of absorbed hydrogen was calculated from thecharge obtained from the integration of the anodic peak currents ofthe cyclic voltammogram taken after the saturation of the palla-dium electrode with hydrogen. CV curves were recorded at a po-tential sweep rate of 20 mV s)1 unless indicated otherwise. It wasstated before [5] that the amount of hydrogen adsorbed and ab-sorbed in/on the palladium electrode calculated from integration ofthe hydrogen oxidation peak is not influenced by the oxidation ofhydrogen gas formed during electrode polarization at potentials inthe hydrogen evolution region. The thickness of the deposited Pdlayer ranged between 0.2 lm to 1.6 lm (1000–8000 Pd layers).

The purity of the solutions used was checked using the platinumelectrode. Both continuous cyclic voltammetry and CV curves re-corded after a long period of cathodic polarization show that thesolutions were free from impurities. This method also confirmedthe effectiveness of solution deoxygenation. The amount of hy-drogen adsorbed and absorbed in/on the palladium electrode cal-culated from the oxidation peak is not influenced by the evolutionof hydrogen gas during the electrode polarization at applied po-tentials [3].

The measurements were performed with a CH Instrumentselectrochemical analyzer, model 604 (Cordova, USA), coupled withan IBM compatible computer.

Results and discussion

Temperature effect with continuous cyclingof the palladium electrode

Figure 1 presents cyclic voltammograms of the Pd elec-trode recorded at a sweep rate of 0.01 V s)1 in 0.5 MH2SO4 in the temperature range from 273 to 341 K. Thethickness of the palladium electrode was ca. 1.6 lm. Theelectrode potential was swept continuously without anyinterruption at any potential value for complete electrodesaturation with hydrogen. An increase in temperatureshifted the peak for oxidation of absorbed hydrogen tomore negative potential values. Moreover, the separationbetween peaks for hydrogen sorption and desorptiondecreased with increasing temperature. This effect sug-gests an increase in the reversibility of the hydrogen ab-sorption/desorption process. Both these effects are due tothe influence of temperature on the diffusion of hydrogenin palladium and the rate of electrochemical reaction.These results are in agreement with conclusion of Zhanget al. [20] that an increase in temperature accelerateshydrogen absorption into palladium under both galva-nostatic and potentiostatic charging conditions.

Influence of potential scan rate

A study of the influence of temperature on the amount ofelectrodesorbed hydrogen/deuterium (maximum

�k1! 1

2H2

Hb�Ha�Hsub�Hads !

�k2! Hþ þ e�ðor k2; Hads þOH� ! H2Oþ e�Þ

ð1Þ

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saturation) was performed for both hydrogen isotopesstudied with various Pd layer thicknesses, various elec-trolyte solutions and for various potential scan rates forhydrogen electrodesorption. The hydrogen and deuteri-um concentrations in palladium are shown as a H(D)/Pdratio. For the experiments presented in Figs. 2, 3, 4, 5 theelectrode was conditioned at a negative potential (5 or30 min, according to the procedure described in the Ex-perimental section) prior to the potential sweep to ensurefull saturation of the electrode with hydrogen and deu-terium. Then the electrode potential was swept in theanodic direction to 0.6 V vs. RHE and the hydrogen(deuterium) oxidation current peak was recorded. Onecan see that for both hydrogen isotopes an increase intemperature is connected with a decrease in the concen-tration of absorbed hydrogen (deuterium) in palladium

[H(D)/Pd value]. This effect is in agreement with ther-modynamic data [21, 22, 23, 24, 25]. The decrease inconcentration of hydrogen and deuterium in palladiumwith an increase in temperature is a general feature

Fig. 2 Influence of the potential scan rate on H/Pd ratios at varioustemperatures. 0.5 M H2SO4; thickness of Pd layer ca. 1.6 lm

Fig. 3 Influence of the temperature on the amount of absorbedhydrogen and deuterium. 0.5 M H2SO4; absorption potential)0.1 V vs. RHE

Fig. 4 The influence of the temperature on the amount of hydrogenabsorbed from basic solution in samples with various thickness.0.1 M LiOH; absorption potential )0.26 V vs. RHE

Fig. 5 Influence of the temperature on the amount of hydrogenabsorbed from various electrolyte solutions. Pd layer thickness ca.1.6 lm

Fig. 1 Cyclic voltammograms, at 0.01 V s)1, of a limited-volume(1.6 m thickness) palladium electrode (Pd-LVE) in 0.5 M H2SO4 atvarious temperatures. The electrode potential was not stopped atany value for Pd saturation with hydrogen

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characteristic of various thicknesses of Pd samples andvarious electrolyte solutions (vide infra).

Figure 2 shows the influence of the potential sweeprate on the amount of electrooxidized hydrogen re-corded in 0.5 M H2SO4. These plots were taken atvarious temperatures for a constant palladium layerthickness of 1.6 lm. The temperature ranged from 273to 346 K. Hydrogen was absorbed at a constant po-tential value (Eabs=)0.13 V vs. RHE). When the ab-sorption process was complete, the absorbed hydrogenwas electrooxidized from palladium with various po-tential sweep rates (from 0.1 to 0.01 V s)1). A charac-teristic feature of the H/Pd plots shown in Fig. 2 is themaximum visible at relatively slow sweep rates, i.e.below 0.02 V s)1. It has been shown [2, 3] that themaximum amount of hydrogen electrooxidized from aPd-LVE depends significantly on the thickness of the Pdlayer and the potential sweep rate. These experimentsshow that the maximum amount of hydrogen electro-oxidized from a Pd-LVE also significantly depends onthe temperature. With an increase in temperature, oneobserves a decrease in the difference between peak H/Pdvalues and those on the semi-plateau observed for highervalues of potential scan rates. For the highest tempera-tures studied, the peak is shifted towards lower m values.

Since the amount of hydrogen absorbed at a constantpotential must be scan rate independent, another hy-drogen desorption pathway must be introduced. As westated, the position of the peak on the H(D)/Pd vs. m plotdepends on the ratio of the two rate constants for theconcurrent hydrogen desorption reactions (k1/k2):

�k1! 1

2H2

Habs�Hads !

�k2! Hþ þ e�

ð2Þ

The decrease in the size of the peak observed in Fig. 2 atpotential sweep rates below 0.01 V s)1 suggests thatparticipation of both processes of hydrogen desorptionfrom the electrode varies with a change in the potentialsweep rate. The smaller the potential scan rate, thegreater the amount of hydrogen desorbed withoutcharge transfer and, as an effect, a lower H/Pd value iselectrochemically measured. This is explained by the factthat k2 increases with the electrode potential, while k1 ispotential independent. Thus, the decrease in potentialscan rate causes an increase in the k1/k2 ratio. Thismeans that more hydrogen is desorbed non-electro-chemically and hence the measured H/Pd value de-creases. In the vicinity of the peak the k1/k2 ratio reachesa minimum and most probably all the hydrogen is de-sorbed electrochemically. With a further increase in m,however, due to the slowness of the diffusion process,not all hydrogen reaches the electrode surface during thepotential scan. Thus, some part of the absorbed hydro-gen cannot be desorbed electrochemically and a decreaseof the H/Pd value is observed. These results support our

earlier conclusions regarding the existence of two pathsof hydrogen desorption [1, 2, 3, 4, 5]. The influence oftemperature on the shape of the H/Pd vs. m plots indi-cates changes in the importance of both desorptionpathways with temperature. We can propose the fol-lowing explanations of this effect:

1. The effect could be explained by changes of the k1/k2ratio, which is temperature dependent. This meansthat the rate constants describing the rate of elec-trochemical desorption and chemical desorption(without charge transfer) are affected by the temper-ature in a different manner.

2. The other explanation is based on the complexity ofthe overall mechanism of hydrogen desorption. Wehave presented (Eq. 1) a model of the overall hy-drogen desorption process. This is a simplifiedscheme and it is possible that other steps may be in-volved in the process. One could suggest that the laststage of desorption is more complex. If more thanone step is involved in desorption of Hads, it is pos-sible that the temperature can change the rate-deter-mining step.

3. The temperature can influence the energetics of var-ious forms of hydrogen adsorbed on the electrodesurface (Hads) and participating in the hydrogen de-sorption process. If energetically different forms ofHads participate in each of the desorption mecha-nisms, this effect could change the importance of bothhydrogen desorption pathways at various tempera-tures.

Isotopic effect on hydrogen absorption

Data in Fig. 3 show the influence of temperature on theamount of hydrogen and deuterium absorbed in palla-dium. For temperatures not greater than ca. 310 K, themaximum concentration of absorbed hydrogen wasslightly greater than deuterium, which agrees withknown thermodynamic data about the isotopic effect inPd/H2 and Pd/D2 systems (see e.g. [26, 27]). For tem-perature values greater than 310 K, an increase in tem-perature caused a smaller decrease in the D/Pd valuethan in the case of absorbed hydrogen. In this temper-ature range the difference between the concentration ofabsorbed deuterium and hydrogen increased with anincrease in temperature. Finally, at temperatures greaterthan 310 K the concentration of deuterium dissolved inPd was greater than the concentration of hydrogen.Studies of temperature influence on absorption of gas-eous hydrogen and deuterium in massive palladiumsamples much thicker than those used in our study haveshown the concentration of absorbed hydrogen to begreater than the concentration of deuterium in thetemperature range studied in our work (see e.g. [26, 27]).This comparison indicates that the temperature-related

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behavior of hydrogen and deuterium dissolved in thinPd layers (0.2–0.6 lm) differs from that observed in bulksamples. We propose an explanation for this phenome-non based on differences in the behavior of the subsur-face layer of hydrogen and deuterium. Theconcentration of both isotopes in the subsurface layercan vary, and, in addition, variations can occur withrespect to the thickness of the subsurface layer in pal-ladium saturated with hydrogen and deuterium. Sincethe behavior of hydrogen dissolved in very thin layers ofpalladium is determined mainly by the hydrogen presentin the subsurface region (see discussion for Fig. 2), thislayer should also be responsible for the observed isoto-pic effect. Thus, bearing in mind all the above-men-tioned factors and taking into account that the processof deuterium absorption in palladium is less exothermicthan the absorption of hydrogen [16, 26, 27, 28, 29], it ispossible that hydrogen/deuterium dissolved in relativelythin palladium layers behave in a different way than inmuch thicker samples, and that this absorption is morestrongly affected by temperature changes. Results fromthe following experiment confirm this.

Influence of thickness of the Pd layer

The influence of temperature on hydrogen absorption inpalladium samples with two different thicknesses isshown in Fig. 4. The experiments were carried out in0.1 M LiOH. In the temperature range 273–361 K for1.6 lm and 0.2 lm of Pd the maximal steady state ofhydrogen concentration decreases by ca. 14% and 27%,respectively. This decrease in H/Pd value for the thinnerPd layer suggests that in a thin layer of palladium agreater relative amount of hydrogen is absorbed moreexothermally than in the case of a thicker layer. Thishydrogen can be located in the subsurface layer wherethe concentration of dissolved hydrogen is greater thanin the bulk [8, 9, 12]. After comparison of the possiblethickness of the subsurface layer (see Introduction) withthe thickness of the overall Pd layer, it is obvious thatthe percentage of hydrogen absorbed in the subsurfacelayer with respect to the total amount of hydrogen ab-sorbed in the overall volume of the electrode is greaterfor thinner layers. Thus, this observation implies that theabsorption of hydrogen in the subsurface layer shouldbe more exothermic than the generation of other formsof hydrogen absorbed in the bulk of the palladium.

Influence of the electrolyte solution composition

Figure 5 shows the influence of temperature on hy-drogen absorption from various electrolyte solutions.In the temperature range 340–361 K the slope of thechange of the H/Pd value with an increase in temper-ature is very similar in both acidic (0.5 M H2SO4) andbasic (0.1 M LiOH) solutions (Fig. 5). For the lowertemperature values, however, a faster decrease in the

amount of absorbed hydrogen is observed in acidicsolution. In the discussion of Fig. 5, two effects mustbe considered:

1. Alkali metal atom/ion inclusion. It is known thatduring saturation of palladium in aqueous solutionsof alkali metal hydroxides the atoms or ions of thesemetals are irreversibly incorporated into the palla-dium lattice [30, 31, 32, 33, 34]. This effect isprobably responsible for most of the differencesoccurring during absorption of hydrogen in basicand acidic solutions. This process is very slow [30,31]. Thus, in the time scale of our experiments wecan assume that the concentration of included alkalimetal is far from the equilibrium state. This leads usto conclude that the profile of the concentration ofincorporated alkali metal atoms/ions presentedschematically in fig. 6 in [3] is governed mainly bythe kinetics of the process of incorporation. Al-though the rate-determining step of this process isnot clear, we can assume that both the rate constantand diffusion coefficient increase with an increase intemperature according to the Arrhenius formula.Thus, the rate of incorporation of alkali ions shouldalso increase. As a result, the concentration profilefor incorporated metal atom/ion should vary withtemperature. Thus, it is possible that, due to theincrease of the rate of incorporation of alkali ions,both the concentration of alkali metal atom/ion andthe depth of penetration in the bulk of palladiumincrease with an increase in temperature. Since mostof the incorporated alkali metal atoms/ions are lo-cated just beneath the surface, the changes in theirconcentrations must strongly influence the behaviorof the subsurface hydrogen. It is known that theinteraction between absorbed hydrogen and alkalimetal atoms present inside the Pd lattice increasesthe stability and the concentration of absorbed hy-drogen [35, 36]. Thus, increasing the concentrationof incorporated lithium with an increase in temper-ature can improve the stability of a greater amountof subsurface hydrogen and, therefore, below ca.350 K a smaller decrease in the H/Pd value is ob-served.

2. Thermodynamic factor. Hydrogen absorption inpalladium is an exothermic process [26, 27]; thus theincrease in temperature impedes hydrogen absorp-tion. It is likely that at higher temperatures the exo-thermic effect of hydrogen absorption prevails. As aconsequence, the decrease of H/Pd with an increase intemperature is similar in both solutions.

Conclusions

1. The temperature has a strong influence on each of thetwo mechanisms of removal of absorbed hydrogen/deuterium from palladium. These two mechanisms

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are: an electrochemical oxidation step, and a non-electrochemical recombination process.

2. The amount of absorbed hydrogen and deuterium inpalladium decreases with an increase in temperature.The influence of temperature on hydrogen and deu-terium absorption is more significant at small thick-nesses of the Pd-LVE. This is due to the moresignificant contribution of subsurface hydrogen to thetotal hydrogen concentration. Generation of thisform of absorbed hydrogen is probably more exo-thermic than formation of hydrogen absorbed in thebulk of the metal.

3. The decrease of hydrogen content with an increase intemperature is smaller in basic than in acidic solu-tions. This is probably due to variation with tem-perature of the concentration profile of the alkalimetal atoms/ions incorporated into the Pd latticeduring hydrogen electrosorption.

Acknowledgements This work was financially supported by thePolish State Committee for Scientific Research (KBN), grant no.3T09A 003 19.

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