Hydrogen electrodes with surface skeleton catalysts

0360--3199/92$5.00 + 0.00 PergamonPress Ltd. © 1992InternationalAssociationfor HydrogenEnergy.

Int. J. Hydrogen Energy, Vol. 17, No. 12, pp. 929-934, 1992.

Printed in Great Britain.

HYDROGEN ELECTRODES WITH SURFACE SKELETON CATALYSTS N. KOROVlN and E. UDRIS Moscow Power Engineering Institute, Moscow, Russia (Received for publication 8 June 1992)

Abstract--The technique of producing electrodes with a surface skeleton catalyst (SSC) has been described. Properties and qualities of the SSC electrodes have been considered. High activity of the SSC cathodes in the hydrogen evolution reaction has been shown. The SSCs modified by adatoms of Cd and other heavy metals and also by anodic oxidation appeared to be especially active. The mechanisms of the catalytic effect have been discussed. It has been shown that SSC electrodes were characterized by specific properties different from those of the base active metal, depending on the nature of the inactive component and on the method of SSC production.

INTRODUCTION Recently some new hydrogen evolution electrodes for alkaline solutions have been suggested [ 1 - 5 ]. The surface skeleton catalyst (SSC) cathode can also be classified as a new one. Although they were proposed in 1965 [6], their investigation began only in recent years. Taking into account their important advantages and the lack of information in the literature it seems useful to consider the SSC manufacturing technique, composition, structure and catalytic characteristics in this paper. THE ELECTRODE PREPARATION TECHNIQUE The principle of SSC production consists of creating, by different methods (chemical, electrochemical, spraying and rolling), one layer of an inactive component (Al, Zn, Mg and so on) on the catalytically active metal (Ni, Co, Pt and others) or its alloy, heating the system for the intermetallic formation, followed by the leaching of the inactive component. Zinc is usually deposited electrochemically from a sulphate solution [ 7, 8 ] on a nickel foil or net, or on a layer of nickel or its alloy. Aluminium is deposited on Ni by methods of electron beam, gas flame or plasma spraying. The binary or multicomponent system is then heated in an inert or air atmosphere. The temperature and the treatment time is chosen depending on the nature of the initial components. The intermetallic compounds of active and inactive components are formed during such thermal treatment. The sequential order and the number of phases formed obey a phase diagram [9], the intermetallic that appears first being the most rich in the easier-melting metal. The authors [ 10, 11 ] suggested producing SSC on the base of an N i - A l system by the aluminizing method, in which intermetallics were formed

in one stage. Electrodes are exposed in a mixture of AI, A1203 and NH4CI powders at 6 5 0 - 8 0 0 ° C for 1 h. In our laboratory AI coatings on Ni were obtained by electron beam spraying. After thermal treatment the inactive comportents are removed from the intermetallics by leaching as usual. As a result a porous layer of complicated composition is obtained. Unlike the Raney catalysts [ 12] the SSCs are formed directly on the substrate with its participation. In [ 1, 5] N i - A 1 coatings were plasma sprayed onto nickel substrates. The electrodes were then activated in concentrated boiling NaOH. Though the authors considered the electrodes as Raney type, they are similar to the SSC, taking into account their preparation technique. One can see the main principles of the SSC preparation in Table 1. The improvement of the preparation technique allowed the obtention of rather stable catalysts [8, 13-15]. SSC COMPOSITION The composition of the alloys formed after Ni and Zn thermal treatment and also of the SSC produced after Zn leaching have been studied by X-ray and derivatographic analyses, X-ray photoelectronic spectroscopy (RPES) and by anodic voltammetry [7, 14, 16]. According to the phase diagram [17] NiZns, Ni~Zn2t, NiZn3 and NiZn intermetallics can be formed in this system. In practice the phase composition strongly depends on the heating temperature. The phases of Zn, Ni and all the four intermetallics were found after the exposure of the deposits to 3 0 0 - 3 5 0 ° C in an inert atmosphere. After the treatment at 400°C zinc became included in the intermetallics almost completely. The fraction of NiZns and NisZn2t compounds was a maximum after heating at 400-500°C, where only the

929

N.

930

KOROVIN and E. UDRIS

Table 1. Principles of SSC preparation

SSC type Ni(Zn)-SSC Ni(AI)-SSC Ni(al)-SSC

Coating preparation method

Active layer thickness (/zm)

Heating temperature (°C)

Ref.

Zn electrodeposition Electron beam spraying Aluminizing

20-40 6 20-100

400-420 600-620 650-800

[6, 7] This work [10, 11]

NiZn phase was formed at 650°C [14]. The phase composition also depends on the treatment time. First, in the phase most rich in zinc, NiZn8 is formed. Then NisZn2t appears between the NiZns phase and the nickel base, and so on. Thus the catalytic layer has a flaky phase composition [7]. Japanese investigators have referred to a similar electrode as being lamellar [18]. The composition and the structure of the SSC itself are formed during the dissolving of the inactive component. While leaching, the phase more rich in Zn or AI is dissolved initially. For example zinc is removed nearly completely from the NiZn8 phase in alkaline solution, whereas NisZn2~ is dissolved only partially, and the phases NiZn and NiZn3 are left almost completely intact. Even an 80 h leaching of the NisZn2t phase in 6 M KOH solution at 80°C resulted in removing only 33 % (mass) of zinc [7]. That is why the most developed SSC surface is obtained from the phases with high Zn content. At the same time the non-destroyed phases, such as NisZn2~, act as a stabilizing factor in the SSC structure, preventing its recrystallization and maintaining the electrode potential constant at some level. The prepared SSCs have a complicated chemical composition that includes the initial components, intermetallics, metal oxides and hydrogen. The zinc content is about 12-20% (mass), distributed across the layer irregularily [7, 16]. Two maxima of zinc content in the catalytic layer have been found by means of X-ray fluorescence spectroscopy [ 16], the first peak observed on the interphase with the nickel layer (intermetallic), and the second near the surface, where zinc exists in the form of hydroxides. Also Ni oxides and hydroxides have been found on the SSC surface. At a depth of 10-15 nm and deeper Ni is mainly in the metallic state [16]. N i - S S C contains a considerable amount of hydrogen in adsorbed and dissolved forms. According to Ref. [19] the fresh-leached Ni(Zn)-SSC possessed 9 - 1 4 ml/g H2(4.9-6.8 atomic %), including about 1 - 2 ml/g of dissolved hydrogen.

pores have a volume share of about 40%, the surface area formed by them being approximately 10-15 % of the SSC general surface area. Besides there is a great number of micropores 5 - 1 0 nm in diameter, their volume share being about 20% of the general porosity, with the surface share being 8 0 - 8 5 % of the general surface area. The wide pores act as transport channels in the electrochemical reaction; due to this the mass transfer processes, especially gas removal, are essentially relieved. The narrow pores provide the highly developed surface. The SSC surface area has been estimated by BET, charging curves [7, 13], standard porosimetry [20] and potential decay curves [21 ]. The specific surface area per mass unit, volume unit and visible surface unit [roughness factor ~ ) ] have been determined. The charging curves method, applying to SSCs, has been developed [22]. The experiments have shown that the SSC specific surface values lay between 20 and 140 m2/g, or 10 and 100 me~ cm 3, depending on the preparation conditions, and were of the same order as for Raney catalysts [12, 23, 24]. However, the specific surface area of the porous cermet electrodes made from Raney and carbonile nickel is one order lower. The high specific surface area is an important SSC quality. The roughness factor values are in the interval from 200 to 10,000 depending on the SSC layer thickness and preparation technique. The specific quantity of the intermetallic rich in inactive removable component appeared to be the main factor determining the surface area. For example, the specific surface area of the SSC, obtained from NiZns, is one or two orders higher than the one prepared from NisZn2t. The

0.10

SSC POROUS STRUCTURE AND SURFACE AREA The SSC layer has a porous structure. For example, the total porosity of Ni(AI)-SSC lies between 62% and 71% [ 13]. The investigations by means of scanning and electron microscopes and by porosimeter [ 16, 20] have shown, that Ni(Zn-SSC) was cut by a net of long and rather wide (10 ~m and more) channels, expanding towards the surface. The volume share of the wide pores is about 40%. The blocks between the channels, 50-100/~m in width, contain mesopores 0 . 1 - 6 t~m in diameter (Fig. 1). These

0.05

JI

I

-8

-7

-6

-5

log (r/m) Fig. 1. Differential porometric curve for the Ni(Zn)-SSC.

931

HYDROGEN ELECTRODES WITH SSCs extreme dependence of the specific surface area on the SSC preparation temperature [14] can be explained by the temperature influence of the alloy phase composition. At the maximum temperature of 450°C the layer contains mainly NiZns. The decrease of the SSC specific surface area from 36 to 21 m2/g with the aluminizing temperature increase from 650 to 750°C [13] can be attributed to the decrease of the intermetallic rich in A1. CATHODIC HYDROGEN EVOLUTION ON SSCs Having a highly developed surface and porous structure, SSCs are active catalysts for cathodic hydrogen evolution. Thus the hydrogen evolution rate on Pt(Zn)-SSC = 5000) at the overvoltage of 0.1 V reaches 400 mA/cm2, * while on the Pt/Pt ~ = 1600) it equals 90 mA/cm ~ [25]. Besides that, Pt(Zn)-SSC electrodes appeared to be essentially more stable. After 500 h of working time at 100mA/cm 2 the overvoltage has increased by 90 mV for Pt/Pt and only by 6 mV for Pt-SSC. The hydrogen evolution polarization curves on Ni(AI)-SSC in 7 M KOH solution at 70°C consist of two straight lines with different slopes of about 40 and 120 mV and exchange current density of 2.5 × 10 -7 A/cm 2 and 0.5 × 10-6 A/cm 2 respectively [11, 13]. Based on the dependence of SSC coverage by hydrogen on the electrode potential obtained earlier, the authors carried out the kinetic treatment of experimental polarization curves. In consequence they concluded that the cathodic process was limited by the H20 discharge stage: H20 + e = Has + OHat low current densities (the first region), and by the electrochemical desorption stage: H20 + Has + e = H2 + OHat high current densities (the second region). The dependence of the reaction rate on alkali concentration has its maximum at 0.1 M KOH [ 11 ]. Ni(Zn)-SSCs also show high catalytic activity in the hydrogen evolution reaction. Thus, the overvoltage on such electrodes in 6 M KOH solution at 20°C and current density of 300 mA/cm 2 is 300-320 mV lower than on smooth Ni electrodes. Between 10 and 300 mA/cm 2 the slope of the polarization curves has a nearly constant value of 0.070-0.075 V. Taking into account high energetic inhomogeneity of the SSC surface one can assume the mixed reaction control with slow discharge on the larger area and slow electrochemical desorption on the rest surface, as was proposed for the smooth Ni [26]. The reaction activation energy is about 44 El mol and is of the same order as for the Ni wire electrode and the Raney Ni cathodes produced by plasma [4]. To explain low Tafel slopes and high catalytic activity of high-area nickel surfaces Conway et al. proposed a mechanism involving possible formation of a three*Here and further we mean visible, i.e. geometric surface area.

dimensional hydride [5]. According to the authors the hydrogen evolution reaction proceeds as described below: XNi + H20 4- e ~ NixH + OHNixH 4- H:O + e ~ (NixH)H~s + OH(NixH)H,as +

H 2 0 4- e - -

NixH + OH- + H2.

Later [ 1] the authors concluded that it was "difficult to comprehend hydride formation only on high surface area matrices and not on smooth electrodes." Analysing the reaction mechanism one must take into account that the catalytic behaviour of high surface area electrodes is determined mainly by their surface composition, which differs greatly from their bulky counterparts. For Ni-A1 Raney, for example, it was shown [27] that the more dispersed the surface was, the more it was oxidized and covered by Ni and A1 hydroxides. MODIFIED SSC ELECTRODES The SSC electrodes can be modified by additives during the stages of layer formation and leaching or into the readymade catalyst, or by the surface condition modification of the prepared SSC by means of impregnation followed by reduction, electrochemical replacement or deposition, chemical deposition, oxidation, etc. The cathodic hydrogen evolution rate can be considerably increased by using Ni alloys with phosphorus, sulphur, iron, cobalt and some other elements as active components. For example the reaction rate on NiS(Zn)-SSC at 20°C and 0.18 V overvoltage is three times higher than that on Ni(Zn)-SSC (Fig. 2) and approximately two times higher than on the steel cathodes with Ni-S electroplated coatings [28]. As one can see from the Fig. 2, the slope for NiS(Zn)-SSC is lower than that for Ni(Zn)-SSC, which may be caused by an alteration of the reaction mechanism. The electrodes with Ni-Co(Zn)-SSC (7% Co mass) appeared to be especially active (see Fig. 2) [27]. Systems

1

0.20

0.15 0.10 f

I

I 2 log li (mA cm'2)]

Fig. 2. Hydrogenevolutionpolarizationcurves in 6 M KOH solution at 20°C for the SSC systems: (1) Ni(Zn); (2) Ni(AI), (3) NiS(Zn); (4) Ni-Co(Zn).

N. KOROVINand E. UDRIS

932

such as Ni-Fe(al)-SSC and Ni-Co(al)-SSC have also shown a high activity for the reaction [28]. We supposed that the adatoms or clusters of heavy metals with high hydrogen overvoltage and low adsorption capacity have an influence upon the hydrogen adsorption energy and therefore on the hydrogen evolution rate. In this way the SSC modification has been carried out by means of immersion treatment in salt solutions of Cd, Hg, Pb, TI, Bi and some other metals [ 2 9 - 3 2 ] . The method consisted of immersion of Ni(Zn)-SSC after their leaching and washing into the sulphate or nitrate solutions of the specified metals and exposure of the electrodes in the solutions for a definite time interval. After careful washing off, the electrodes have been tested for the hydrogen evolution reaction in 6 M KOH solution. One considerable catalytic effect has been obtained as a result of the treatment described (see Fig. 3). The hydrogen overvoltage at current density of 300 mA/cm 2 decreased by 80-140 mV in comparison with initial samples of 20 #m thickness; the slope coefficient was reduced by up to 2 7 - 5 0 mV and the exchange current density became about an order of magnitude higher. The activation of nickel by cadmium has been confirmed by M. Jaksi~ and other investigators [2]. As one can see from Fig. 3, the hydrogen overvoltage value depends on the nature of the cation in the treatment salt. The highest effect has been achieved in the case of cadmium salts. The polarization curves for electrodes treated in cadmium nitrate solutions have slope and overvoltage values near those for Pt/Pt electrodes (Fig. 3, curves 4 and 5). The anion in the treatment salt also has its influence upon the hydrogen overvoltage. As deduced from Fig. 4, the catalytic effect is lower in the case of sulphate salts than that of nitrate, with the polarization curve slopes being about 45 mV and 27 mV, respectively. Increasing the adsorption time and salt concentrations, the hydrogen evolution rate rises approaching definite limits. However, in the cases of

mercury or bismuth nitrate solutions, the evolution rate upon treatment time dependence passes through a maximum. The catalytic effect after Ni(Zn)-SSC treatment with heavy metal salts has been observed not only in the KOH solution, but also in LiOH, NaC1 and others. The nature of the effect is not yet clear enough. The reaction acceleration cannot be explained by an increase in the surface area. According to the experimental data the roughness factor becomes even lower after the modification. It has been determined, by means of physical and chemical methods, that the metal adsorption took place on the SSC surface during the immersion treatment. The adatoms coverage after the modification reached, for example, 0.2 and 0.043 monolayers in the cases of Cd and Pb nitrates respectively. The mass content in the surface layer was 0 . 1 - 1 5 % for cadmium and 0.25-6.2% for lead. As displayed in Fig. 5, the Pb and Cd adsorption isotherms showed a logarithmic character, testifying to the energetic inhomogeneity of the SSC surface [32, 33]. The adsorption of those metals studied leads to hydrogen and oxygen desorption from the SSC surface. The hydrogen desorption causes a potential shift to more positive values (Fig. 6). The metals, Cd in particular, are adsorbed at an underpotential condition. The adsorption proceeds in time limits of 2 0 - 6 0 min in concentrated ( 0 . 5 - 2 M) solutions and of several hours in diluted ones. X-Ray analysis showed no N i - C d intermetallic phase formation [34]. The Cd adsorption can be described by the equation: Cd 2+ + ~ad~ = -'-UadsC"~(2-X0++ XH +. The Cd is adsorbed irreversibly and is not removed even under anodic polarization up to the oxygen evolution potentials. Probably, cadmium forms oxide-hydroxide structures together with nickel during the anodic polarization [34].

/4 s÷ ~ I

_

~+,¢~t ,p+

0.14

0.2

/ . / ./ / .,~3

t="

/'~ .+/

0.10

0.1

~]~ I 1

0.06 I 2 log [i (mA cm2)]

Fig. 3. Hydrogenevolutionpolarizationcurves in 6 M KOH solution at 20°C for Pt/Pt (5) and for the Ni(Zn)-SSC (1-4); nonmodified (1) and exposed to a 0.5 M solution of Pb(NO3)2 (2), TINO3 (3) and Cd(NO3)2 (4).

1.0

1.5

2.0

log [i (mA em'2)] Fig. 4. Hydrogenevolutionpolarizationcurves in 6 M KOH solution at 20°C on the electrodes modified in solutionsof Cd(NO3)2 (1), CdSO4 (3), KNO3 (4) and by electroplated Cd (2).

HYDROGEN ELECTRODES WITH SSCs

933

16

~

12 8

500

•

i

4¸ 0

log [C (tool 1 1)]

100 I

Fig. 5. The dependence of adsorbed metal coverage (mass%) on the solution concentration (the exposure time being up to the stationary condition): (1) Pb(NO3)2; (2) Cd(NO3)2.

I

f

0.05 0.I0 0.15 Surface coverage 8Cd Fig. 7. The dependence of the hydrogen evolution current density at 0.15 V polarization (1) and the adsorbed hydrogen amount (2) on the SSC surface coverage by cadmium.

1.g ~

0.4

0.8

2

0.6

'~

- 0.2

0.4

0.2 0

I 30

I 60 t (rain)

Fig. 6. The change of the cadmium content in SSC (1) and the stationary electrode potential (2) during its treatment in the 0.5 M CdSO4 solution.

The metal adsorption seems to be the main reason for the catalytic effect in the hydrogen evolution reaction. As one can see from Fig. 7, the hydrogen evolution rate increases with the rising SSC coverage by Cd. The catalytic influence of the electrodeposited Cd also testifies to the important role of metal on the SSC surface. It follows from Fig. 2 (curve 1) and Fig. 4 (curve 2) that the activity of SSC electrodes with the electrodeposited Cd is considerably higher than that of the initial one. The average SSC coverage by electrodeposited cadmium, assuming its uniform distribution, is near to one. However, considering its local character of deposition and non-uniform depth distribution, the real surface coverage is essentially lower. Thus one can consider that Cd and other heavy metals on the SSC surface are responsible for the catalytic effect of the first turn. But the mechanism of their catalytic influence is less clear. One possible way is the decrease of the amount

of adsorbed hydrogen, at first strongly bonded (Fig. 7, curve 2), which leads to the appearance of new hydrogen evolution centres with a different limitation stage. However, the hydrogen desorption is not the only cause of the catalytic effect because the sharp decrease of the hydrogen surface content is not followed by the same sharp increase in the evolution rate (Fig. 7). One can suppose the formation of metal adatom clusters with favourable electronic structure [2]. They become the catalytic centres for the hydrogen evolution reaction which proceeds by a different mechanism than on the initial SSC. The polarization curve slope values change from 70 to 4 0 - 5 0 mV, which can indicate the change in the slow stage of discharge. So, adatoms influence the M - H bond energy and change the limitation stage of the reaction. Further investigations of the element composition and component valency on the SSC surface are being carried out to give an answer about the role of an inactive component and anodic oxidation in SSC modification. REFERENCES 1. B. V. Tilak, A. C. Ramamurthy and B. E. Conway, Proc. Ind. Acad. Sci. (Chem. Sci.) 97, 359 (1986). 2. M. M. Jaksi~, Int. J. Hydrogen Energy 12, 727 (1987). 3. A. Nidola, Int. J. Hydrogen Energy 9, 367 (1984). 4. Y. Choquette, H. Menard and L. Brossard, Int. J. Hydrogen Energy 15, 21 (1990). 5. B. E. Conway, H. A. Kozlowska, M. A. Sattar and B. V. Tilak, J. Electrochem. Soc. 130, 1825 (1983). 6. N. V. Korovin, Invent. certificate 206094 on 1965 (USSR), Bull. Invent. 24 (1967); Invent. certificate 21883 on 1966 (USSR), Bull. Invent. 18 (1968). 7. N. I. Kozlova, Electrochimija 712, 19-24 (1972).

934

N. KOROVIN and E. UDRIS

8. D. V. Sokolskij, P. I. Zabotin and S. V. Druz, Electrochimija 24, 1323-1329 (1988). 9. V. Ch. Bugakov, Diffusion in Metals and Alloys. GITTI, Moscow/Leningrad Izdat. (1949). 10. R. Ch. Burshtein, A. Y. Pshenichnikov and S. F. Chero nyishov, in Second Meeting on Electrocatalysis (abstract), p. 21. Nauka, Moscow (1978). II. A. G. Pshenichnikov, Int. J. Hydrogen Energy 7, 5 1 - 5 9 (1982). 12. A. B. Fasman and D. V. Sokolskij, Structure and Physical- Chemical Properties of Skeleton Catalysts. Nauka, Alma-Am (1968). 13. A. G. Pshenichnikov, S, F. Chernjishov, Ju. I. Krjukov etal., Electrochimija 18, 1011 (1982). 14. D. V. Sokolskij, P. I. Zabotin and Ju. V. Pichugov, in Kinetika i mechanism electrodnjih reakzij, Vol. 11, pp. 81-90. Trudju IOKE KazSSR, Nauka, Alma-Am (1975). 15. N. V. Korovin and N. I. Kozlova, Electrochimija 21, 383-387 (1985). 16. D. V. Sokolskii, P. I. Zabotin and S. V. Druz, Kinetika i cataliz 28, 1105-1110 (1987). 17. M. Hansen and K. Anderko, Structure of Binary Alloys. Metallurgizdat, Moscow (1962). 18. T. Jasushita and T. Emshina, in Kataliticheskie Reakzii v Jidkoi Phase, pp. 288-291. Nauka, Alma-Am (1972). 19. D. V. Sokolskii, P. I. Zabotin and S. V. Druz, Electrochimija 15, 885-890 (1979). 20. E. S. Tverdohlebov et al., in Electrochemicheskie processi v perspektivnih istachnikah toka, Trudi MEI, Vol. 604, pp. 3 - 9 (1983). 21. D. V. Sokolskii, P. I. Zabotin and S. V. Druz, in Trudi

22. 23. 24. 25. 26. 27. 28. 29.

30.

31. 32. 33. 34.

IOKE, Vol. 11, pp. 7 1 - 7 3 . Nauka, Alma-Am, KazSSR (1975). O. S. Abramson, S. F. Chernishov and A. G. Pshenichnikov, Electrochimija 12, 1667-1671 (1976). R. I. Kokes and P. H. Emmet, J. Am. Chem. Soc. 81, 5032 (1959). J. P. Jamey, I. Trambouze and M. Prettre, Bull. Soc. Chim. France 10, 3139 (1966). N. V. Korovin and N. I. Kozliova, Electrochimija 8, 413-416 (1972). L. Tamm, Ju. Tamm, V. Past et al., Uchenije zapiski Tartu Univ. 378, 3 - 9 (1976). A. I. Savelov, A. B. Fasman, A. I. Liashenko et al., J. Phizicheskoj chimii 63, 1918 (1989). N. V. Korovin, O. N. Saveljeva and E. Ja. Udris, Electrochimija 23, 707 (1987). N. N. Kozlova, T. V. Lapshina and V. V. Chernova, in Electrochimija i chimicheskie istochniki toka, Vol. 365, pp. 103-105. Trudi MEI, Moscow (1978). A. G. Pshenichnikov, S. F. Chernishov, Ju, I. Krjukov et al., in Hydrogen Energy Progress VI, Proc. 7th Worm Hydrogen Energy Conf., Moscow, Vol. l, pp. 403-423. Pergamon Press, New York (1988). N. V. Korovin, N. I. Kozlova and O. N. Saveljeva, Electrochimija, 14, 491-495 (1978); 16, 585-587 (1980). N. V. Korovin, N. I. Kozlova, O. N. Saveljeva et al., J. Res. Inst. Catal. Hokk. Univ. 29, 2 9 - 3 3 (1981). N. V. Korovin, N. I. Kozlova, O. N. Save[ieva et al., Dokladi AS USSR 257, 149-156 (1981). N. V. Korovin, E. Ja. Udris and O. N. Saveljeva, Electrochimija 22, 354-364 0986).