Hydrogen evolution and entry in palladium at high current density

dcta metall, mater. Vol. 39, No. I1, pp. 2519-2525, 1991 Printed in Great Britain.All rights reserved

0956-7151/91 $3.00+0.00 Copyright © 1991PergamonPress plc

HYDROGEN EVOLUTION AND ENTRY IN PALLADIUM AT HIGH CURRENT DENSITY P. C. SEARSON Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218, U.S.A (Received 30 November 1990; in revised form 8 May 1991)

AImract--The kinetics of hydrogen evolution and entry into palladium membranes has been studied at high current densities using the electrochemicalpermeation technique. A two stage permeation transient was observed corresponding to diffusionin the a phase and mixed a + ~ phase, respectively.At low current densities the hydrogen evolution reaction exhibited a rate determining step of proton dirt.barge followed by absorption of the adsorbed intermediate into the palladium lattice. At high current densities, after conversion of the surface layer to the B phase, a large increase in the recombination flux was observed with a concomitant decrease in the hydrogen entry flux. Since the composition of the entry surface was time dependent at constant charging current, the hydrogen evolution kinetics at the entry surface also exhibited a time dependence. R6mm6--La cin6tique du transfert de l'hydrog6ne dans le cas des membranes de palladium a 6t6 6tudi6e d de fortes densit6sde courant grace d la technique de p6n6tration 61ectrochimique.On observe un r6gime transitoire de p6n6tration en deux 6tapes correspondant respectivement~ la diffusion clans la phase aet dam la phase mixte a +//. Pour les faibles densit6s de courant, la r6.action d'6volution de rhydrog6ne pr6sente une vitesse qui d6termine dans un premier temps la d6charge du proton, puis rabsorption, dans le r6seau de palladium, de l'interm6diaire adsorb6. Pour les fortes densit6s de courant, apr6s la transformation de la couche superfieielle en phase //, on observe une forte augmentation du flux de recombinaison, aecompagn6e d'une diminution du flux d'entr6e de l'hydrog6ne. Comme la composition de la surface d'entr6e d6pend du temps pour un courant de charge constant, la cin6tique d'6volution de l'hydrog6ne d la surface d'entr6e est, elle aussi, fonction du temps. Zumuamenfamung~Mit dem Verfahren der elektrochemischen Permeation wird die Wasserstoffbildung und die Kinetik des Wasserstoffeindringensin Palladium-Membranen bei hohen Stromdichten studiert. Eine zweistutige Permeationstransiente wird beobachtet, die der Diffusion in der a-Phuse mad in der (,, + ~)-Mischphase entspricht, lki niedrigen Stromdichten zeigt die Reaktion der Wasserstoffbildung einen ratenbestimmenden Schritt der Protonenfreigabe, welcher gefolgt ist durch Absorption des adsorbierten Zwischenstadiumsin das Pailadiumgitter. Bei hohen Stromdichten wird nach der Konversion der Oberltichenschicht in die ~-Phase eine starke Zunahme des Rekombinationsflussesbeobachtet, der begieitet ist yon einem abnehmenden FIuByon eindringendem Wasserstoff. Da die Zusammensetzungder Eintrittst~che bei konstantem Baladungsstrom zeitabh~ingig ist, weist auch die Kinetik an der Eintrittsfl~iche eine Zeitabh~.ngigkeitauf.

INTRODUCTION

EXPERIMENTAL

The development of hydrogen energy sources has led to considerable research related to hydrogen storage in solids and hydrogen fuel cells. Electrochemical methods have been used to study the diffusion of hydrogen in a wide range of metals and alloys. Palladium has been considered a model system for the study of hydrogen transport due to the relatively high diffusion coefficient and solubility. However, most of the work reported in the literature has been limited to relatively low charging current densities corresponding to hydrogen entry surface concentrations below the limit of the • phase [1-6]. There has been very little work concerning the electrochemical entry and diffusion of hydrogen in palladium at high current density, at concentrations beyond the limit of the a-phase.

Experiments were performed in a two compartment electrochemical cell using the bielectrode technique of Dcvanathan and Stachurski [2]. Each glass half-cellcontained a platinum counter electrode and reference electrode connected via a L u u i n - H a b e r capillary. For all experiments reported here the electrolyte in both half-ceUs was 0.1 M NaOH prepared from analytical grade reagent and distilled water. The solutions were continuously deaerated during experiments by bubbling of purified nitroBen gas. The temperature in both cells was controlled to 25 + 0.1°C. Polycrystalline palladium foils (99.9975%) were obtained from Johnson Matthey. Unless otherwise indicated the foils were annealed in vacuum (5 x 10 -s tort) for 3 h at 900°C and etched in HCI/HNO~ (3 : 1

2519

2520

SEARSON:

HYDROGEN EVOLUTION AND ENTRY IN PALLADIUM

by volume) prior to each experiment. This method of sample preparation has been shown to give highly reproducible results in low current density electrochemical permeation experiments [5, 6]. The thickness of the palladium foils was 250 ~ m for all experiments reported here. For convenience, the palladium surface at which protons were reduced is referred to as the entry surface, and the corresponding surface at which the diffusing hydrogen was oxidized is designated the output surface. At the start of each experiment, the exit surface of the palladium membrane was polarized to 0.3 V (SHE) and the current monitored. In most cases the current decreased to a value of < 1/~A cm -2 within a few minutes and at this time a constant cathodic current was applied to the entry surface. The interfacial impedance of each side of the palladium electrode was measured by superimposing a small a.c. signal onto the d.c. current, or potential, of each potentiostat and using a transfer function analyzer to compute the real and imaginary components of the impedance.

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RESULTS AND DISCUSSION

Low current density Figure I shows an optical micrograph of the palladium after annealing and etching. From this micrograph it can be seen that the average grain size is about 200 #m. Figure 2(a) shows a typical current transient recorded at the output surface, and Fig. 2(b) shows the corresponding potential of the input surface as a function of time at a current density of 0.314 mA cm -2. For the case of a constant entry flux, the diffusion coefficient can be calculated from the lag time of the current transient [2] according to L2 D

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(1)

where L is the membrane thickness and t~ is the time at which the current transient reaches 0.63 of the

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Fig. 1. Optical micrograph ofpolycrystallinepalladium after annealing and etching.

Fig. 2. (a) Typical permeation transient at a charging current density of 0.318 mAcm -2. (b) Potential transient for the input surface of the palladium membrane at a charging current density of 0.318 mA era-2. steady state value. The diffusion coefficient calculated from the transient in Fig. 2(a) is 3.4 x 10-7 cm 2 s-~, identical to the results of Early [5] and Bowker and Piercy [6]. For all experiments carried out in the range 0.032-0.318 mA c m - ' , the average diffusion coefficient was 3.5 + 0.1 x 10-Tcm2s -]. The steady state permeation current was consistently between 0.98 and 0.99 of the value of the charging current density at the entry surface, indicating that almost all of the protons reduced at the entry surface were absorbed into the palladium lattice. The permeation behaviour was found to be sensitive to surface treatment in agreement with previous reports [7]. A palladium membrane that was etched but not annealed resulted in a decrease in absorption efficiency (i~/iou,) in the range 0.90--0.98. Membranes that were annealed but not etched resulted in no measurable permeation current over a period of 1000 s, indicating the presence of a barrier, possibly a surface oxide, that effectively inhibits hydrogen entry. At longer times of charging (t > 2500 s) the permeation currently gradually decreased and after 10Ss the permeation current was 0 . 2 6 m A c r o - ' , corresponding to an absorption efficiency of 0.82. This indicates that there is some change in the surface chemistry at the entry surface at long charging times causing an attenuation of the hydrogen flux into the membrane. This effect is probably due to reduction of contaminant species in the electrolyte onto the palladium entry surface resulting in a decreased flux of hydrogen.

SEARSON:

HYDROGEN EVOLUTION AND ENTRY IN PALLADIUM

Figure 2(b) shows the potential of the input surface of the palladium as a function of time. From this figure it can be seen that after an initial decay, the potential decreases linearly with time for about 8 x 103 s. After 8 x 103 s a second linear relationship is observed to about 2 x 104s at which point the potential decreases only slightly with time up to 1.1 x l0 s s. Under conditions of constant current at the input surface it is assumed that there is a constant flux of hydrogen into the lattice and that the concentration of hydrogen at the entry surface (within the bulk material) increases with time until a steady state concentration is reached. Since the equilibrium potential of the Pd/H electrode is dependent on the concentration of hydrogen in palladium [8], the electrode potential would be expected to shift in the negative direction as the hydrogen concentration increased. However, the potential continues to decrease well after a steady state permeation current is established; this may reflect an increase in the hydrogen overpotential due to codeposition of contaminants onto the electrode surface. The slope of the first linear region was about 17 x 10-6V s -~ and was characteristic for all experiments at low current density ( < 2 m A c m - 2). The concentration of hydrogen at the entry surface under steady state conditions, C~,, was calculated from Fick's first law: i~L c~ = --

DF

(2)

where i~, is the steady state permeation current and F is the Faraday constant. For the example shown in Fig. 2(a) and using D = 3 . 4 x 10-7cm2s -~, the near surface hydrogen concentration corresponds to an H/Pd ratio of 0.0021 (0.24 x 10 -3 mol cm-3), well below the limit of the ~ phase. The concentration of hydrogen at the entry surface as a function of time, shown in Fig. 3, was calculated from the permeation transient by assuming a constant entry flux, determined by the steady state permeation current. For this calculation it was assumed that the concentration profile through the sample was linear and although this assumption is an approximation, calculations at higher current density (see Fig. 6) indicate that the error is not too large. Assuming that the ct phase limit corresponds to H/Pd = 0.03 then the error for the example shown in Fig. 6 was I%, and in all cases was less than 5%. From Fig. 3 it can be seen that the H/Pd ratio at the input surface increases to a steady state value given by equation (2).

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Fig. 3. Near-surface H/Pd ratio at a charging current density of 0.318 mAcm -2. current transient shown in Fig. 4(a) exhibits two time constants reaching a maximum value of about 10.5 mA crn -2. At longer times, after the two rising transients, a decrease in the permeation current was observed. Figure 5(a) and (b) show typical permeation transients at entry surface charging current densities of 60 and 100 mAcrn -2, respectively. The transients shown in Fig. 5 exhibit the same features as described above, although the time constants associated with the components are clearly different. Stackelberg and Ludwig [9] reported a two stage transient for diffusion of hydrogen in a 300pm palladium membrane from 0.1 M NaOH at a charging current density of 67 m A c m -2. The permeation transient was recorded at short times up to the current maximum; longer time behaviour was not reported. I

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High current density For all experiments conducted at high charging current densities in the range 2-100 mA cm -2, a two stage permeation current transient was observed. Figure 4 shows the permeation current transient and the corresponding entry surface potential at a charging current density of 31.8mA cm -2. The

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Fig. 4. (a) Typical permeation transient at a charging current density of 31.83 mA cm-2. (b) Potential transient for the input surface of the palladium membrane at a charging current density of 31.83 mA era-2.

2522

SEARSON: HYDROGEN EVOLUTION AND ENTRY IN PALLADIUM

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Fig. 5. (a) Permeation transient recorded at a charging current density of 60 mA cm -2. Co) permeation transient recorded at a charging current density of 100 mA cm-'. From the permeation current transients it can be seen that the initial transient reaches a maximum value of about 2 mA cm -2. The entry surface concentration for this first transient calculated from equation (2) and using i. = 2 mA crn -2, corresponds to an H/Pd ratio of 0.013 close to the limit of the a phase at 1 atmosphere and 25°C [10]. Bowker and Piercy [6] in similar experiments on the same thickhess of palladium reported a limiting permeation current density of 1.8 mA cm -2 for charging current densities in the range 2-5 mA cm -2. Figure 6 shows the entry surface H/Pd ratio calculated from the transient shown in Fig. 4(a) assuming a constant entry flux equivalent to a current density of 10.5mAcm -2. This plot exhibits a change of slope at an H/Pd ratio of 0.03, corresponding to the limit of the a phase. The position of the inflection 0.06

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Fig. 6. Near-surface H/Pd ratio at a charging current density of 31.83mAcro-2 and assuming a maximum entry flux corresponding to a current density of 10.5 mAcm -2.

point indicates that these assumptions are reasonable and do not cause significant error in the calculation. Similar plots were obtained for all transients recorded at charging current densities in the range 2-100 mA cm -2. As a result it can be concluded that the first permeation transient is controlled by diffusion in the ~ phase. The second stage of the permeation transient [Figs 4(a), 5(a) and 5(b)] corresponds to an increasing fraction of the ~ phase in the near surface region. At the beginning of the second stage the surface composition corresponds to the ,, phase maximum, with further increase in the near surface hydrogen concentration regions of ~ phase nucleate at the surface resulting in a mixed ,, + / / phase with an average composition greater than the a phase maximum but below the // phase minimum (0.03 < H/Pd < 0.62). This is supported by examination of the electrode potential of the entry surface recorded simultaneously with the permeation transient. For a charging current density of 31.8 mA cm -z, the electrode potential shown in Fig. 4(b), increases in the positive direction, corresponding to increasing a phase in the near surface region. When the near surface concentration exceeds the ~ phase maximum the electrode potential reaches a plateau value of - 0 . 9 4 V. It is well known that the equilibrium potential of the H/Pal electrode is dependent on composition in the ~ phase and the // phase [8], however, the mixed ~ + / / p h a s e exhibits a characteristic plateau potential that is the basis of the H/Pal reference electrode. As a result, the difference between the plateau potential and the equilibrium potential corresponds to the hydrogen overpotential at the given charging current density. Determination of the diffusion coefficient for hydrogen diffusion in the//phase is difficult due to the poorly defined limit of the ,, phase in the permeation transient [Figs 4(a), 5(a) and 5(b)]. This is to be expected since there is no well defined phase transition between and a and ~ + ~ phases and during the course of an experiment the relative proportion of the // phase in the near surface region will increase with time. For all permeation transients recorded at high current density, the diffusion coefficients for the second stage transients were in the range 1-4 x 10-6cm2s -l. Referring to Fig. 4(b), it is seen that after the region of constant potential at the entry surface, the potential shifts in the negative direction. The time at which this shift occurs corresponds to complete transformation of the entry surface to the // phase. At this point considerable hydrogen gassing is also seen at the entry surface indicating a change in the reaction mechanism associated with increased recombination and decreased absorption. Following the complete transformation of the surface to the p phase, a decrease in the permeation current density is seen at the exit surface due to the decreased entry flux. Detailed analysis of this decreasing permeation

SEARSON:

HYDROGEN EVOLUTION AND ENTRY IN PALLADIUM

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transient at long times was precluded since the gassing at the entry surface resulted in a considerable temperature increase in the half-cell on the entry side of the palladium membrane. The time, tc, at which the entry surface is completely transformed to the ,8 phase, determined from the entry surface electrode potential, is expected to be related to the entry surface charging current density, as shown in Fig. 7. In this plot, t f ' increases linearly with charging current density showing that the time to convert the entry surface to the ,8 phase is a linear function of charging current density• The expansion of the palladium lattice associated with the formation of the ,8 phase at the entry surface resulted in considerable deformation of the entry surface. Figure 8 shows an optical micrograph of the entry surface after charging at 31.8 m A c m -2. This surface morphology was typical of all experiments at high current density• The increase in the lattice constant between pure palladium and the :t phase limit is only 0.01%, however, the increase in lattice constant between ~maxand ,8~, is 3.3% [I 1]. Since the fraction of the ,8 phase at the entry surface increases with time, the nucleation and growth of this phase from the surface results in the large degree of surface roughness seen in Fig. 8. Figure 9 shows a schematic diagram of the change in the concentration profiles within the palladium membrane as a function of time during charging at high current densities. At short times, the concen-

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Fig. 9. Schematic diagram of concentration profiles and permeation transient as a function of time. x = 0 corresponds to the entry surface and x = L corresponds to the exit surface where C = 0. tration at the entry surface increases until the ct phase limits is attained• At longer times further absorption of hydrogen into the lattice results in increasing fractional coverage of the ,8 phase with an average composition between the limits of the two phases. At a time, t~, the entry surface is completely converted to the ,8 phase and there exists a region of mixed composition within the membrane.

The hydrogen entry surface Analysis of the kinetics of hydrogen evolution at palladium electrodes is complicated by the fact that both the equilibrium potential for palladium and the reaction mechanism are dependent on the near-surface concentration of hydrogen• Figure 10 shows a Tafel plot for annealed and etched palladium in 0.1 M NaOH. As a result of the limitations described above, the current-potential data were obtained by sweeping the potential. For the data shown in Fig. 10 the sweep rate was 1 x 10 3 Vs-~; identical results were obtained at 10 × 10 -3 V s- '. The Tafel slopes at low and high current density were 0.12 and 0.21 V, respectively. Pentland et al. [12] reported a Tafel slope of0.112 V for Pd in 0.1 M NaOH at current densities less than l mA cm 2, in good agreement with the

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Fig. 8. Optical micrograph of the entry surface of a palladium membrane after charging at 31.83 mA cm -2.

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2524

SEARSON:

HYDROGEN EVOLUTION AND ENTRY IN PALLADIUM

data shown in Fig. 10. Pentland et al. [12] reported a stoichiometric number of 2 and concluded that the rate determining step was proton discharge from water H20 + e - --* H ~ + O H - .

(3)

Since the absorption efficiency at low current density and at short times is close to 1.0, it is clear that all adsorbed hydrogen is absorbed into the lattice and that the rate of recombination is negligible. The second step in the reaction is absorption of hydrogen into the lattice H,d, ~

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This reaction can be assumed to be in pseudoequilibrium, at least at short times, with k 2 k ' O ~ = k _ 2 C , b, where 0,~ is the fractional surface coverage and k ' is the concentration at 0~, = 1. Since C,~, corresponds to the near surface hydrogen concentration, which increases with time, as shown in Fig. 9, the surface coverage by adsorbed hydrogen, 0,~, would also be expected to increase with time. At high current density, as described above, the near surface hydrogen concentration increases with time until all of the surface is converted to the fl phase (Fig. 9). The reaction mechanism at the fl palladium surface is dominated by recombination with an attenuation in hydrogen absorption. The shift in Tafel slope above about 1 mAcro -2 to - 0 . 2 1 V corresponds closely with the results from the permeation measurements described above. Further analysis of the reaction kinetics is complicated by the potential dependent equilibrium potential. At low current density ( < I mA cm -2) the interfacial impedance of the palladium membrane at the input surface exhibited a single loop in the frequency range l kHz-O.01 Hz, as shown in Fig. 11 for a charging current density of 0.318mAcm -2. The low frequency limit of the resistive component was in good agreement with the slope of the currentpotential curve indicating that the response is due to double layer capacitance and charge transfer resistance. There is no apparent evidence of the 200

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relaxation of an adsorbed intermediate indicating that the coverage, 0ad,--* 1. At current densities > l m A c m -2 the interfacial impedance was very small, however, in most cases an additional impedance could be detected at low frequencies, as shown in Fig. 12 for a charging current of 3.18 mA cm -2. CONCLUSIONS At low current densities ( < 2 mA cm-2), a single permeation transient was observed from which a diffusion coefficient of 3.5 +0.1 x 10-Tcm2s -I was calculated. The maximum absorption efficiency at low current density was close to 1.0, although at longer times of charging an attenuation was seen. At high current densities ( > 2 m A c m - 2 ) , a two stage permeation transient was seen due to diffusion in the ct and mixed ~ + fl phases. The diffusion coefficient for hydrogen in the fl phase, calculated from the permeation transients, was in the range 1-4 x 10-rcm 2 s -~. At longer times the permeation transient decreased and was associated with visible gassing at the entry surface and a shift of the electrode potential of the entry surface in the negative direction. The formation of the ~ phase at the entry surface resulted in considerable deformation of the palladium membrane. The kinetics of hydrogen evolution at palladium electrodes was found to be dependent on the near surface hydrogen concentration. At short times, at surface concentrations below the ,, phase maximum, the slow step was proton discharge followed by absorption of adsorbed hydrogen in the palladium lattice. At high current density, the fractional coverage of the fl phase increased with time until all of the near surface region is in the fl phase. At this time the recombination flux increased significantly and the absorption flux was attenuated.

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Fig. 11. Complex plane impedance plot for entry surface of a palladium membrane at a charging current density of 0.318 mA era -2.

Acknowledgements--The experimental work was carried

out at SRI, Menlo Park, Calif. and was supported by the Electric Power Research Institute under contract RP-3170-01. The author would like to thank T. Harris for his comments on this work.

SEARSON:

HYDROGEN EVOLUTION AND ENTRY IN PALLADIUM

REFERENCES I. M. A. V. Devanathan and Z. Stachurski, Proc. R. Soc. A 270, 90 (1962). 2. S. Schuldiner and J. P. Hoare, J. Electrochem. Soc. 103, 178 (1956). 3. J. Volkl and G. Alefeld, in Diffusion m Solids (edited by A. S. Nowick and J. J. Burton), p. 231. Academic Press, New York (1975). 4. H. Zuchner and N. Boes, Berichte Bunsen-Gesell. 76, 783 (1972). 5. J. G. Early, Acta metall. 26, 1215 (1978). 6. J. Bowker and G. R. Piercy, Metall. Trans. 16A, 715 (1985).

2525

7. T. Harris and R. M. Latanislon, Int. J. Hydr. Energy 14, 623 (1989). 8. M. J. Vasile and C. G. Enke, J. Electrochem. Soc. 112, 865 (1965). 9. V. M. yon Stackelberg and P. Ludwig, Z. Naturforsch. 19A, 93 (1964). 10. F. A. Lewis, The Palladium Hydrogen System. Academic Press, New York (1967). 11. E. Wicke and H. Brodowsky, in Hydrogen m Metals, Topics m Applied Physics (edited by G. Alefeld and J. Volkl), Vol 29. Springer, Berlin (1978). 12. N. Pentland, J. O'M. Bockris and E. Sheldon, J. Electrochem. Soc. 104, 182 (1957).