Relation between nickel crystalline structures and their electrocatalytic properties

J. ElectroanaL Chem., 96 (1979) 191--201 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

191

RELATION BETWEEN NICKEL CRYSTALLINE S T R U C T U R E S AND T H E I R ELECTROCATALYTIC PROPERTIES PART II. DETERMINATION OF THE N A T U R E OF ADSORPTION SITES FOR HYDROGEN

L. ANGELY, G. BRONOEL and G. PESLERBE

Laboratoire d'Electrolyse, C.N.R.S., Meudon (France) (Received 2nd February 1978; in final form 16th May 1978)

ABSTRACT It is possible to determine the function Q = f(E) which indicates for each potential value, the equilibrium-state hydrogen coverage. By preliminary determination of the electrode roughness from the differential capacity obtained by pulse techniques the maximum coverage of weakly bound Had s can be deduced. By 9ssuming that each Had s atom corresponds to one site, the number of sites per unit area of the interface is calculated. Besides, for electrodes characterized by different crystallite diameters, the number of atoms located in different positions (edges, crystallite faces, etc.) may be determined. By comparison of the theoretical estimate with the number of sites which can bind Hads, it can be concluded that the sites are defined only by the low coordination Ni atoms (crystallite edges and summits).

(1) INTRODUCTION

It has recently been shown (ref. 1, this issue pp. 183--190) that one can obtain nickel electrodes with different and well defined structures b y vacuum deposition. It is possible to establish relationships between the structural characteristics of these electrodes and their electrochemical properties, particularly for oxidation and reduction of hydrogen in alkaline solution. It should first of all be pointed out that while numerous literature references to hydrogen adsorption on the platinum-like metals exist in this field, few studies on nickel electrodes have actually been made. In this paper previous work on this subject is reviewed and new data obtained by double layer differential capacity measurements and potentiostatic charging curves are given. This group of results will enable us to define the nature of the adsorption sites for hydrogen on nickel. (2) DESCRIPTION OF THE SUBJECT

The first significant experiments on the rates of hydrogen electrode reactions on nickel may be attributed to Frumkin et al. [2,3] who were the first to show that the rate determining step in hydrogen evolution is the process: H 2 0 + e ~ O H - + Hads. In addition, since 1939 the existence of an anodic limiting cur-

192 rent related to concentration polarisation and to surface oxidation of the electrode has been established. More recent interpretations have been based mainly on Tafel plot analysis. Bockris and Potter [4] attempted to explain the deviations of the experimental plots from the theoretical lines by introducing H dissolution into the lattice, whereas Devanathan and Selvaratman [ 5] showed in 1960 that analysis of these curves could only be made when the value of the fraction of sites available (1 -- 0) is known as a function of potential, with correction of the experimental curves for the partial oxidation current obtained at low overpotentials. In parallel experiments workers of the Japanese school [6--9] following Horiuti [10] attempted to prove that the combination step (2 H -~ Hf) was rate limiting or that Na ÷ discharge occurred in the reaction sequence. The most complete analysis has been made by Makrides [11]. Discussing the results of Devanathan and Selvaratman he concluded that equilibrium coverages in Hags should probably be much higher (0 > 0.2) although the value given by double pulse measurements is very low (0eq ~- 0.05). He showed that the determination of the r.d.s, from the stoichiometric number was ambiguous in the case of hydrogen, and concluded that the process was limited by the slow discharge step, with a change in mechanism at high overpotential. The 1964 work of Weininger and Breiter [12] which provided supplementary information on the capacity of the double layer of the system, and which defined potential ranges of NiOH formation and Ni(OH)2 should be also mentioned. Finally, using the theory of Frumkin, T a m m et al. [13,14] have recently proposed that the r.d.s, is the slow discharge on the major part of the electrode surface, with a simultaneous limitation by the Hads combination step on certain areas. Published work on the study of relationships existing between hydrogen electrode kinetics and the crystalline structure of nickel electrodes is scarce. Ohmori and Matsuda [6a] used nickel obtained by vacuum deposition but did not determine a behavioural pattern different from that of solid nickel. However Baranowski and Smialowski [15] indicated that poisoning of nickel surfaces with small structural differences results in large changes in their behaviour, particularly for absorbed quantities of hydrogen. Work in this laboratory in 1967 [16] showed large changes for behaviour of vacuum deposite d nickel electrodes, particularly for He oxidation. In addition, Silk et al. [17] have noted major differences between charging curves on ex-carbonyl nickel and nickel obtained by electron beam fusion. The most systematic work has been carried out recently by Kudryashov et al. [18] who studied the influence of mechanical and thermal treatment on the nickel electrode on hydrogen evolution. It was shown that active sites are related to surface dislocations. As regards the hydrogen electrode reaction mechanisms on nickel, we assume in this study that certain data recently established for platinum [19,20] may be transposed to nickel. These data are in agreement with Makrides conclusions, notably regarding the existence of high Had s coverage near the equilibrium potential. However the reaction mechanism is more complex, since a chemisorbed molecular species is proposed, with interaction effects in 2 steps. Furthermore, we will show that a close relationship exists between these hypotheses and information concerning the adsorption of gaseous hydrogen on nickel, in particular for the identification of the active sites [21--29] and the influence of crystalline structure on these [30--36].

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194 (3) EXPERIMENTAL CONDITIONS

(3.1) Nature of the electrodes Using results previously obtained in a study of the structural properties of nickel electrodes obtained by vacuum deposition [1 ], we selected six typical structures (Table 1) for examination of electrochemical behaviour. The structures were produced b y vacuum deposition, with or without subsequent annealing treatment. As a comparison, the behaviour of two solid nickel electrodes was also examined. The first (g) was obtained b y cutting a small bar of nickel, followed by annealing for 50 h at l l 0 0 ° C and 10 -6 Tort. In this instance, the surface layers initially disturbed b y coldworking cannot be considered to be restructured. Electrode (h) was made b y electron beam fusion of spectroscopic grade nickel, which wax subsequently annealed for 50 h at 1100°C and 10 -6 Torr. The surface was never mechanically disturbed and X-ray examinations showed a highly ordered surface structure. Table 1 shows the structural and textural characteristics of these electrodes.

(3.2) Electrochemical cell All experiments were conducted in 1 M KOH at 22°C. The electrolyte was prepared from Merck KOH and water (18 M~2 resistivity) obtained by passage through active carbon + ion exchange resin filters (Millipore super Q). Prior to experiments carried o u t under inert atmosphere (ultra-high-purity grade nitrogen), the electrolyte was degassed by nitrogen bubbling for 24 h. The electrodes studied consisted of a film of nickel on a vitreous carbon pellet, with the exception of the massive nickel. They were then e m b e d d e d in a KOH-resistant resin to allow attachment to an axle to form a rotating disk electrode, which could be used at speeds up to 12,000 rpm. Using a similar platinum electrode and the redox system Fe3+/Fe 2+ it was verified that the Levich relationship was o b e y e d to within 1%. The counter electrode which was placed in a c o m p a r t m e n t separated from the working electrode by a fritted glass disk, consisted of a gold plate. Spectrographic analysis showed the absence of traces of platinum metals. A saturated calomel reference electrode with a Luggin capillary within 2 mm of the surface of the working electrode was used. (4) DETERMINATION OF SURFACE ROUGHNESS

(4.1) Capacitance measurement Devanathan and Selvanatman [ 5], Breiter and Weininger [ 12] and Makrides [11] have discussed the probable values of the differential capacity of double layer. A value of 16 p F / c m - 2 was presumed likely [ 5] (cf. values measured on mercury electrodes). In this case, a small electrode roughness factor is assumed. However, considering the m e t h o d of surface preparation (in the case of ref. 12, chromic + sulfuric acid oxidation followed b y cathodic reduction), it is probable that the surface consists of a microporous layer produced b y reduction of oxides. In this case, the differential capacity per cm 2, at unit roughness factor,

195

is probably overestimated. Finally, following the analysis given for platinum [ 19] the minimal value of differential capacity must be measured in a potential range where coverage in hydroxide and adosrbed hydrogen is minimal. Capacity measurement, by galvanostatic pulse (slope of V = f(t) line during the first 20 ps) on electrode (h) showed that a noticeable capacity minimum exists at 120 mV/RHE, a potential at which it is possible that a certain a m o u n t of strongly chemisorbed hydrogen'exists. Measurements conducted at more positive potentials would be even more ambiguous, due to the problem of non-negligible hydroxide coverage. In addition, comparison of 2 electrodes for which Haas coverages were found to be of the same order but which showed different differential capacities measured under the above conditions makes it clear that variations of capacity in this potential region are essentially due to the roughness factor. The capacity measurement was therefore considered to be a comparison assuming that interracial composition variations are small or have negligible effect on double layer capacity. However, a reference measurement on an electrode of unit roughness factor is required. This was considered to be the case for electrode (h) obtained by electron beam fusion.

(4.2) Results and discussion For electrode (h), a capacity of 10 pF cm -2 was measured. This value is less than that established by Weininger and Breiter [12]. However, the roughness of their electrode was probably higher than the value they supposed. In addition, their capacitive term, taking into account the measurement frequency could not uniquely apply to the double layer capacity. For our electrodes, results were as follows: Electrodes

(a)

(b)

(c) (d)

(e)

(f)

(g)

(h)

C/pF cm - 2

56

48

30 23

17

17

15

10

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5.6

4.8

3

2.3

1.7

1.7

1.5

1

Electrodes which had undergone annealing treatments at temperature greater than 500°C had a noticeably regular capacity indicating a roughness factor of about 1.7. In this case, micrographic data indicate the probability of a compact surface. The roughness factor (p) is then determined by the compact arrangement of the crystallites, taking into consideration their crystallization mode. Calculations made for compact arrangements of tetradecahedrons give p = 1.82 and for cubooctahedrons p = 1.63. Taking into account possible errors in capacity estimations, these values are in good agreement with those found experimentally. However, the crystallisation mode (cubooctahedron or tetracahedron) of electrode (f), treated at 660°C cannot be determined as the annealing temperature is higher than the temperature of crystallisation (TR ~ 600 ° C). On the other hand, for electrodes treated at a lower temperature, the most likely crystalline shape is tetracahedral [37]. For non-annealed electrodes, very high roughness factors are noted: 5.6, which are explained by the fact that a loosely packed aggregate is formed on the surface of the crystallites. The value p = 5.6 corresponds to an intercrystallite porosity (due to disorder of the crystallite level) affecting about

196

3 layers of crystallites. For electrodes given thermal treatment at 400 ° C (c) and 490°C (d) a noticeable reorganization of the deposit with a disappearence of intercrystallite channels was seen. However, the roughness factors determined, especially for electrodes treated at 100 ° C, result n o t only from the effect of crystallization mode and intercrystallite porosity, b u t also from the presence of microsteps on the crystallite surfaces. (5) CATHODIC CHARGING CURVES

(5.1) Description o f me thod The method employed is that of charging, which presents the advantage over the voltage sweep method, of making more evident the adsorption steps with a slow rate. Besides, the analysis of the information obtained with this m e t h o d needs less hypothesis (pseudo-equilibrium) than with voltage sweep m e t h o d because the integration of i = f(t) at each potential has for the limit condition i = 0 which is an experimental datum. As in the other experiments, the electrodes are rotated (rotation speed of the order of 3000 rpm). Electrodes were studied in deaerated 1 M KOH and maintained under inert atmosphere. In the first phase of the experimental programme electrodes were kept for 10 min at +120 m V / R H E corresponding to the minimum capacity. A potentiostatic pulse was then applied at a given potential Ead s (120 > Ead s > 0) for a variable length of time tad s. At this potential, the reaction H 2 0 + e - -> O H - + Hads occurs. An opposite pulse was then applied after time t a d s . On charge (application of pulse) or on discharge (return to E = 120 mV/RHE) the i = f(t) curve was recorded on an oscilloscope. Integration and subtraction of the double layer charge established the quantity QH, equivalent to the amount of hydrogen adsorbed during time tad s at the adsorption potential. (5.2) Results and discussion (a) Comparison between QH measured on charge and discharge. In all domains of the potential explored, the solid nickel electrodes (g, h) showed identical QH values on charge and discharge. Electrode (b) made b y condensation at 150°C behaved similarly. On the other hand, for all other electrodes, particularly those annealed between 400 and 600 ° C, QH was much greater on charge than on discharge. Thus for an adsorption time of 3 min at +50 m v / R H E , QH was 1100 pC cm -2 for electrode (c). Since the average number of surface nickel atoms is around 2 × 101~ cm -2 for relatively dense planes, complete monolayer Had s coverage should give a QH Value of 340 pC/cm -2. It is therefore probable that most of the hydrogen adsorbed on the surface penetrates into the lattice. This factor is more pronounced when the structure has a high density of vacancies, e.g. for electrodes (c) and to a lesser degree other specimens subjected to annealing treatment. For electrode (f), this effect was also observed b u t the quantities of hydrogen detected were much smaller. Assuming that the penetration of H affects the whole of the deposit (thickness ~ 1 pm) the concentration of H in the lattice would be around 1020 a t o m s c m - 3 . For electrodes exhibiting marked diffusion of hydrogen into the lattice, QH o n discharge is considerably smaller than on charge. In fact, we may suppose that QH values on discharge are essen-

197 tially due to surface hydrogen, the hydrogen dissolved in the lattice scarcely intervening. The latter hydrogen is probably quasi-irreversibly " t r a p p e d " in the vacancies.

(b) Experimental c u r v e s . QH = f(tads)Ead~= cte and Q H = f(Eads)tad~= cte. Only the discharge curves under conditions when H diffusion into the lattice is negligible are significant. Under these conditions QH = f(t) curves on discharge all show (Fig. 1) an obvious maximum corresponding to an electrosorption time of 30 s. The decrease of QH stretches over a time of the order of 2 min. Therefore it consists of a relatively slow phenomenon probably related to a parasitic adsorption and it may be supposed that after 30 s the coverage in impurities is still negligeable and that the values of QH taken at the maximum observable after 30 s are only slightly underestimated. For electrodes (b) and (h) the QH values on discharge and charge were identical. A QH = f(E) curve for a time tad~ of 30 S corresponding to the adsorption maximum is shown in Fig. 2. In the range of potential examined, t w o separate adsorption regions are observed. The first, at more positive potentials (E > 50 m V / R H E ) is distinguished b y a wave whose plateau is practically established at +70 m V / R H E . At less positive potentials, a further increase in QH Occurs. As for platinum for which a weakly b o u n d form of hydrogen is observed at ca. 100 m V / R H E [19], we may assume that the initial stage of the adsorption (120 mV > E > 50 mV) corresponds to the filling of a related group of weakbonding adsorption sites by H. The point of inflection of this curve (at around 75 mV/ RHE) corresponds in potential to a peak obtained under potentiodynamic condi-

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198 tions, indicating quasi-reversible adsorption. We may therefore suppose that strongly b o u n d hydrogen may exist at more positive potentials, where it is masked b y formation of Ni(OH)2 layers. Indeed, a study of the oxidoreduction of nickel [38] has evidenced that up to +150 m V / R H E the coverage in Ni(OH)2 is noticeable. The higher QH values at potentials around 0 m V / R H E , b y analogy with the Pt/H system, may be attributed to coverage of molecular type. For electrodes showing marked diffusion of hydrogen into the lattice part of the QIt value determined at potentials close to 0 / R H E may be due to adsorption coupled with lattice diffusion. The most important information which can be deduced from the Qu = f(E) curves is the value of QH determined b y the plateau of the first adsorption wave which corresponds to the filling up of the loosely b o u n d Had s sites. As the plateaus can be reached for most of the electrodes at ca. 75 mV, it is reasonable to think that the values of QH obtained do not include H2,ads, which may appear at less positive potentials. The values of QH were determined for 6 electrodes: Electrodes

(a)

(b) (c) (e) (g) (h)

Q~/pCcm - 2 140 75 96 30 10 8 (c) Significance of Oeq. Two possible methods exist for evaluating coverage. In previous experiments [1] it was assumed that the maximum number of sites existing on the surface was determined by the average number of nickel atoms present on a plane of I cm 2 area. In the case of solid nickel (for example the electrode (g), the coverage in loosely-bound Had s is 0 = 10/340 ~-- 0.03 using this assumption). This value is similar to that established by Devanathan and Salvanatman [ 5] for potentials approaching equilibrium. As we have seen Makrides [11] concluded that this value was much t o o small on theoretical grounds. We propose that the 0 value is determined b y the fact that the maxim u m number of Hads adsorption sites is much smaller than the total number of atoms present on the surface and that this maximum value is directly associated with the limiting value, Q~. Under these conditions the maximum number of active sites no is given by the relationship no = c~ Q~/F where no varies as a function of electrode structure. The QH = f(E) curves show that the plateau of the first wave is always reached well before 0 m V / R H E , the coverage 0eq at the equilibrium potential being very close to unity in every case. Under steady-state conditions, a variation of 0 as a function of electrode potential should be observed, depending on the rate of the different steps in the reaction mechanism. (d) Nature of adsorption sites. Using the above Q~ values, maximum numbers of no sites for hydrogen adsorption for the different electrodes studied may be determined: Electrodes

(a) (b) (c) (e) (g) (h)

10 - 1 3 n 0 / c m - 2

87 45

60 19 6

5

A relationship between no and crystallite diameter can n o w be obtained. For electrodes (a) and (b), which have similar crystallite diameters (30)k), a fundamental difference is observed. Electrode (a) prepared at 100°C has a large

199

number of superficial microsteps which is in contrast to electrode (b), prepared at 150 ° C. For electrodes prepared at 100 ° C, followed by annealing, we may quote the example of treatment at 400°C for 2 h (c) which results in an increase in crystallite diameter (40 £ ) and a reduction in surface microstep density. These two characteristics are further accentuated by annealing at 587°C (e) for two hours when crystallite diameter increases to 75 A, a value associated with a very low level of micxosteps. The massive nickel electrodes (g, h) has a crystallite diameter greater, than 1 pm. If we consider their respective methods of preparation, the dislocation density must be greater for electrode (g) than for electrode (h). If we consider electrodes possessing crystallite diameters less than 10 pm, it is possible to calculate theoretically the changes in density of different related groups of sites on tetradecahedral crystallites as a function of diameter. Figure 3 shows the number of different sites as a function of diameter, corresponding as follows to: (1) (2) (3) (4) (5)

atoms atoms atoms atoms atoms

at at at at at

exposed summits re-entrant edges exposed edges the re-entrant minima the crystal planes

We have also indicated the experimental curve (6) on this diagram which has a shape similar to that for the variation of sites at the edges and summits (curves 1--4). Consequently, if crystal plane atoms are not active sites, we may conclude that the only possible active sites correspond to atoms on the exposed edges and summits. For a tetradecahedral crystallite, the total coordination number for atoms on the planes (curve 5) is 13 and 14 respectively for (100) and (111). The total coordination number is valid for a f.c.c, system with nearest neighbours at a distance of "a"x/~/2 and "a". Atoms existing on re-entrant edges and summits have higher coordination numbers (respectively 18 and 11), and so cannot be identified with adsorption sites. However, the coordination number for atoms on exposed edges and summits is smaller (10 and 7). As a result, only the latter two related groups of atoms 1 and 3 can be considered in the adsorption sites. Consequently, the theoretical curve, (7) representing the total number of sites for the tetradecahedral crystallization mode, is given by the sum of curves (1) and (3). We show in Fig. 4 a curve giving the sum of the exposed edges and summits for tetradecahedral (curve 7) and cubooctahedral (curve 8) crystallites together with the experimental curve. It can be clearly seen that the difference between the experimental curve (6) and curve (8) is much greater than that for curve (7), indicating that the crystallites are most probably tetradecahedral. For the similar series of electrodes (a), (c) and (e), the difference between the experimental and theoretical curves (7) becomes less as crystallite diameter increases. Taking the structural parameters of the electrodes into account, this divergence must be essentially due to the presence of microsteps which are very numerous for (a), and in small quantity for (e). Ir~ addition, the divergence between point B, on the curve for electrode (b) (prepared at 150 ° C) and the theoretical curve (7) is less than that corresponding to point A, for electrode (a) (prepared at 100°C). We have shown above that electrode (b) has a microstep value inferior to that of electrode (a), thus confirming the microstep

200

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hypothesis. The active sites present on the surface of films obtained by vacuum deposition are therefore to be identified with the atoms existing on the exposed edges and summits of crystallites, together with the atoms on the exposed microedges of microsteps on the crystallite faces. For the solid nickel electrodes (g), and (h) with crystallite size greater than 1 #m, the active sites must be determined from a cubooctahedral rather than a tetradecahedral model. In this case, the theoretical number of sites corresponding to atoms on exposed summits and edges is considerably smaller (0.1 × 1013 cm -2) than the number determined experimentally (6 X 1013 for (g) and 5 × 1013 for (h)). Under these conditions, we may suppose that the active sites correspond to atoms on the edges of exposed dislocation. The quantities of sites determined experimentally are in good agreement with the dislocation density existing in solid metals (~ 1012 cm-2). In conclusion, two very different patterns of behaviour are observed concerning H a d s adsorption sites on nickel. For massive metals, with comparatively large crystallite diameter (>1 pm) active sites mainly correspond to atoms on exposed dislocation edges. The theory of Kudryashov et al. [ 18] is therefore confirmed. For nickel films having very small crystallites (¢ < 200 A) the active sites correspond to atoms on exposed edges and summits of crystallite together with atoms on the microsteps found on the crystallite faces. We also note then

201

that the large number of sites for a given crystallite diameter is in agreement with crystallization in the tetradecahedral rather than the cubooctahedral mode. The comparison with platinum discloses analogies and differences. In both cases, tightly and weakly bound forms for H can exist. For the platinum, it seems that these differences in bonds are due to the situation of all the atoms at the interface (external and internal sites). For the nickel, the sites corresponding to weakly bound H are determined by only the small number of atoms with a very low coordination (defects, edges and summits). REFERENCES 1 2 3 4 5 6 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

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