Relation between nickel crystalline structures and their electrocatalytic properties

J. Electroanal. Chem., 96 (1979) 183--190

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© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

RELATION BETWEEN NICKEL CRYSTALLINE STRUCTURES AND THEIR ELECTROCATALYTIC PROPERTIES PART I. DETERMINATION OF STRUCTURAL CHARACTERISTICS OF NICKEL ELECTRODES OBTAINED BY VACUUM DEPOSITION

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 Different nickel configurations need to be defined and characterized. These different structures are obtained by vacuum deposition on several substrates. Apart from the nature of substrate, the influence of deposit and annealing temperature have been studied. We observe that the variations of the nickel structure concern: unit cell, diameter and shape of the crystallites, crystallization facies, stacking faults, microvacancies, vacancies, etc. Four films are described in detail.

(1) INTRODUCTION Nickel has useful advantages as an electrocatalytic electrode material for oxidation and reduction of hydrogen, for example in hydrogen-air fuel cells or for production of hydrogen in electrolyzers. Unfortunately, for hydrogen evolution, nickel-based cathodes are characterized by a greater polarization than platinoid electrocatalysts. In addition, nickel-based electrodes present irreversible inhibition phenomena in fuel cells at high anodic polarizations. It is possible that the electrocatalytic characteristics of nickel-based electrodes may be altered by changes in their crystalline structure. To verify this possibility, different nickel crystallite configurations need to be defined and characterized. One of the most successful methods of obtaining nickel specimens with varied structures is vacuum deposition under different condensation and annealing conditions. In this article, we describe the conditions under which nickel deposits were obtained by this method, and describe their structural characteristics as determined by visual and electron micrography and X-ray diffraction. (2) CONDITIONS UNDER WHICHELECTRODES WERE OBTAINED BY VACUUM DEPOSITION It is well known that parameters such as residual vacuum evaporation rate, evaporation time, substrate type and temperature may greatly influence deposit texture and structure. However, one of the most important variables is the

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nature of the substrate on which the electrode is to be formed. This substrate must possess high electrical conductivity, good thermal properties and must be electrochemically inert when the deposits are porous. One of the most difficult problems is in obtaining sufficient adhesion of the film to the substrate. After considering these factors, only composite araldite resin + powdered nickel and vitreous carbon were further considered as substrates. Experiments on structural characteristics were performed on electrodes with resin composite substrafes, but the majority of electrochemical studies were carried out on vitreous carbon based deposits. One of the advantages of vitreous carbon is that it permits annealing up to 800 ° C. For both types of substrates, the influence of temperature on deposit structures was studied. Apart from substrate temperature variation and annealing temperature, all other parameters for each t y p e of deposit were kept constant (system pressure: 10 -~ Tort; condensation rate: 30 nm min -1). The source consisted of a nickel block of spectrographic purity (99.998) and constant mass. Electron b o m b a r d m e n t was used to raise its temperature to 1500°C. The substrate was placed 16 cm from this source and maintained at a constant temperature between 10 and 800 ° C (+2 ° C) by a thermostat; the thermocouple being placed within the substrate at 0.2 mm from the condensation surface. All substrates were in the form of I cm diameter 0.2 cm thick pellets. (3) STRUCTURAL STUDY

A great deal of literature data now exists concerning crystalline structures of metal films obtained by vacuum deposition. However structural characterisations of nickel films data are rather scarce [1--13] and it is often difficult to establish the precise nature of the formation conditions for the deposits. Consequently it was necessary to carry out a series of studies to determine the structure of the different deposits obtained in this work.

(3.1) Deposit texture Most films obtained were about 1 pm thick. Electron diffraction showed the virtual absence of preferred orientation and the existence of a layer structure with layers parallel to the substrate surface. When deposits were obtained on a composite substrate (araldite + powdered nickel of 3 nm grain diameter) the surface roughness as measured using Talystep was considerable. The surface had a relief pattern with an average amplitude of 0.15 pm and a microscopic examination showed the presence of a " f l o c c u l e n t " agglomerate (Fig. 1). On vitreous carbon the surface roughness appeared more indistinct and homogeneous. Nevertheless, deposit compactness was only high for specimens which were either on substrates heated to over 150°C or which had been annealed at temperatures of over 500°C for 2 h. In other instances pores were f o u n d in the crystalline masses, coalescence occurring only to a small extent under these conditions.

(3.2) Structure (3. 2.1) Experimental The majority of results were obtained from Debye-Scherrer photographs

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Fig. 1. (a) SEM examination of nickel deposited on the substrate araldite + powdered nickel; (b) electron micrograph on nickel deposited on vitreous carbon. [Microphotography carried out b y Mrs. Miloche and Dr. Sella (Laboratoire de Physique des Mat~riaux, C.N.R.S., Bellevue, France.)]

with stripped deposits. They were placed in a cylindrical camera using monochromatic iron K a 1 radiation. The information obtained under these conditions may be used to determine three sets of parameters:

(a) Determination of the phases present and their specific characteristics. When

186 the unit cell dimension differs from that corresponding to the bulk unsupported metal the strains present in the deposit can be determined by assuming that the Young's modulus is close to that for the former.

(b ) Determination of the diameter and shape of crystalIites. These determina.I

,

tions were made by measuring the linewidth for the low index planes, using as a reference the linewidth for large nickel crystallites (nickel in a massive ~tate) (~-- 1 pm). After measuring the lines corresponding to (111), (220) and (200) planes, the outer shape of the crystallites can be reconstructed. Strictly speaking, linewidth depends not only on crystallite dimensions b u t also on lattice order and on the density of stacking faults. For the latter, the broadening effect was weak since the measurements were taken for the close packed planes~ For deposits with a high density of stacking faults, the crystallite diameter will be underestimated.

(c) Assessment of the number of vacancies and stacking fault vacancies. For this, an original method was used which consisted of analysis of the profile of t h e continuous background (i.e. diffused radiation). It is known that the intensity of the continuous background for each angular value depends on the energy scattered (coherent and incoherent scattering). The minimum for values of between 30 ° and 50 ° and the noticeable downward trend for angular values greater than 60 ° is explained b y the presence of microvacancies. It is possible to determine qualitatively the relative number of microvacancies by comparing the amplitude of the concavity at the minimum of the background (Fig. 2) with that for massive nickel.

(3.2. 2) Results (a) Phases present and cell dimensions. The determination of the planes present in the films, the cell dimensions and the presence of an hexagonal phase in a

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low concentration in the f.c.c, matrix were the subject of another publication [14].

(b ) Dimensions and shape of the crystallites. Figure 3a shows the average diameter of the crystallites as a function of substrate temperature during condensation of vitreous carbon substrates. Between 20 and 120°C the average diameter was virtually constant at around 26 ~. Since the density of stacking faults was high at around 100°C and since these faults increase X-ray linewidth, it is probable that crystallite dimensions for deposits obtained at temperatures near 100°C were about 30 ~. For deposits obtained at 100°C, then annealed for 2 h between 400 and 700 ° C, one can see an increase in the size of the crystallites (Fig. 3b), so that the actual diameter increases from 3 0 / ~ for a non-annealed deposit to 1 5 0 / ~ for one treated at 700 ° C. These average estimates were calculated with the assumption that the crystallites were spherical in shape. Analysis of the lines corresponding to the 3 principal axes as a function of substrate temperature shows (Fig. 4) that the crystallites actually change from an ellipsoidal shape at a low temperature to a more spherical shape at temperatures in excess of 100 ° C, the temperature of coalescence being ca. 100 ° C. As a function of annealing temperature a tendency towards a spheroidal shape is observed, though anomalous results occur at around 650 ° C. (c) Lattice vacancies and stacking fault vacancies. Figure 2 shows typical profiles of the continuous background registered on a D.S. diffraction-pattern for a sample of massive nickel and for a nickel deposit condensed at 100 ° C. The position of the arbitrary dimension (Z) used to determine the ratio r (T = Z/Zo) characteristic of the deviation from the normal diffraction pattern is shown. In

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Fig. 5, the ratio Z/Zo has been plotted as a function of substrate temperature (Fig. 5a) and as a function of annealing temperature (Fig. 5b) for samples deposited at 100°C on vitreous carbon substrates in both cases. A maximum value for substrate temperature of about 100 ° C was noted.

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It has been shown elsewhere [5] that the density of stacking faults is maxim u m at this temperature, consequently the density of microvacancy stacking faults is high. In addition the reduction of X-ray diffusion results mainly from the existence of microvacancies; double vacancies and intercrystallite microcavities cannot be responsible for a decrease in the X-ray scattering. A diminution of the density of lattice vacancies and stacking fault vacancies may be observed as a function of the annealing temperature, though total disappearance of these defects does not occur, in agreement with ref. 8. (4) S U M M A R Y O F R E S U L T S - - D E S C R I P T I O N O F T H E S T R U C T U R E A N D T E X T U R E OF DEPOSITS OBTAINED BY VACUUM DEPOSITION

By restricting work to a related group of deposits for which the substrate (vitreous carbon) remained constant, the morphological changes in deposits as a functibn of condensation and annealing temperature may be determined. This determination cannot however be considered complete if based only on the results given above. It is advisable to supplement the latter with literature data, particularly in regard to the definition of the exact crystallite shape, i.e. of deposit surface morphology, and to short-range lattice order. We have demonstrated that X-ray crystallographic results enable a determination of.the shape of the mean outer perimeter of the crystallites to be made. The exact shape is established by the crystallization mode. For large crystals in their equilibrium state we suppose that a f.c.c, lattice crystallizes in the cubooctahedral mode. For deposits with crystallite size < 1 5 nm, the crystallization mode should be tetradecahedral [2]. This was confirmed by high resolution electron microscopy (Fig. 1). With regard to the exact position of atoms in the lattice, analysis of the X-ray photographs does not give useful information since a slight deviation of position will be shown by a line broadening effect which also depends strongly on crystallite size and on the presence of stacking faults. It has been shown [2,5,9] that atomic mobility following condensation is low at a temperature of less than 100 ° C, and that lattice order becomes less as the temperature is reduced with tendency towards the formation of quasi-amorphous deposits. It is therefore apparent that the lattice, although still largely f.c.c., is badly ordered with each atom offset from its equilibrium position. ACKNOWLEDGEMENT

We wish to thank Electricit~ de France (E.D.F.) for financial support. REFERENCES 1 2 3 4 5

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