Electrolysis of water on silicides of some transition metals in alkaline solutions

Int. J. l'(vdrogen Energy, Vol. 17, No. 7, pp. 479-483, 1992. Printed in Great Britain.

0360-3199/92 $5.00 + 0.00 Pergamon Press Ltd. International Associationfor Hydrogen Energy.

ELECTROLYSIS OF WATER ON SILICIDES OF SOME TRANSITION METALS IN ALKALINE SOLUTIONS A. K. VIJH, G.BI~LANGER and R. JACQUES lnstitut de recherche d'Hydro-Qufbec, Varennes, Qu6bec, Canada J3X ISl

(Received for publication 26 February 1992) Abstract--The cathodic and anodic behaviour of FeSi2, FeSi, CoSi2, TiSi~, NiSi2, ZrSi~, CusSi, Cr3Si, VSiz, AgSi, MnSi, PtSi and PdSi electrodes during the electrolysis of water in alkaline solutions has been examined. All these silicides show excellent electrochemical stability as cathodes and can sustain high rates of the hydrogen evolution reaction (HER), albeit at rather high cathodic overpotentials. On the anodic side, silicides of Cu, Cr, Ti and V show visible corrosion in the alkaline solutions. Other silicides show an oxygen evolution region which leads to a limiting current characteristic of oxide growth, quasi-similar to some valve metals.

1. INTRODUCTION A principal focus of modern research in electrocatalysis is to discover electrode materials that exhibit excellent electrochemical stability and show interesting activity towards typical electrochemical reactions. It is also desirable that these materials be inexpensive, abundantly available, non-toxic, easy to manipulate and non-polluting. Such a class of materials, it has been shown [ 1 - 4 ] , is represented by metal silicides. Typically, these silicides are highly conducting with resistivity values falling in the range of 0.005 - 0 . 1 ohm cm [4]. Silicides of transition metals show metallic lustre and high chemical and electrochemical stability in electrolyte solutions. Also, a variety of electrode reactions, e.g. the hydrogen evolution reaction (HER) can be sustained at high rates on these electrode materials, albeit at rather appreciable overpotentials. Previously, we have reported on the excellent electrochemical stability and some electrocatalytic activity of iron silicides, both in acidic [ 1, 2] and alkaline [3] solutions. The electrochemical activity of a number of transition metal silicides in acidic solutions has also been published [4] from our laboratories. The object of the present paper is to explore the cathodic and anodic behaviour of some transition metal silicide electrodes in the electrolysis of water in alkaline solutions.

2. EXPERIMENTAL All experimental procedures, instruments, materials, etc., were as described recently for the study of transition metal silicides in acidic solutions [1, 2, 4]. The electrolyte solutions were 1 M KOH in deionized and doubly distilled water, the second distillation being over

alkaline potassium permanganate. The potassium hydroxide was Baker Analyzed Reagent meeting A.C.S. specifications and was obtained from J.T. Baker Co. of Phillipsberg, NJ, U.S.A. The electrochemical measurements were carried out in a three-compartment Pyrex cell, with compartments separated by solution-sealed stop-cocks. The solutions were constantly purged with nitrogen during the measurements. The working electrodes were sealed in a piece of Kel-F and were polished down to 600 grit on a silicon carbide wheel. The electrodes were then washed in doubly-distilled deionized water before using. The counter electrodes were made from a vitreous graphite rod mounted in heat shrinkable Teflon tubing. The reference electrode was Hg/HgO, with a reversible potential of - 9 2 6 mV against a reversible hydrogen electrode (RHE).

3. RESULTS AND DISCUSSION

3.1. Potentiodynamic profiles In order to explore the surface reactivity and general electrochemical characteristics of these materials in the alkaline media, potentiodynamic profil ~.s were performed on the following electrodes: FeSi2, FeSi, CoSi?, TiSi2, NiSi2, ZrSi2, CusSi, Cr3Si, VSi2, AgSi, MnSi2, PtSi, PdSi In general, a featureless profile was obtained without characteristic oxidation-reduction peaks, usually exhibiting an hysteresis in the ascending and descending curves as is often observed, e.g. on a graphite electrode. Some oxidation-reduction peaks were observed on CusSi, however (Fig. 1). A small cathodic peak was observed on AgSi whereas NiSi2 showed a small anodic peak; an hysteresis observed on PdSi contained some broad anodic and cathodic bumps approaching a peak. 479

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The main point to be drawn from these experiments is that no clear-cut peaks for the hydrogen d e p o s i t i o n - ionization on the electrode surfaces could be observed on these materials; on the Hg/HgO reference electrode scale used, these peaks should be seen, if present, around either side of the reversible hydrogen potential, i.e. - 9 2 6 m V (Hg/HgO). Also there were no peaks indicating formation-reduction of oxides in these profiles, except for the same anodic features mentioned above on Cu2Si, NiSi2 and PdSi.

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All silicides constitute stable electrode surfaces that sustain high rates of the hydrogen evolution reaction (HER), albeit at high overpotentials (Table 1). Although there are some differences in the electrocatalytic activities of different silicides for the HER, most of them exhibit an overpotential in the range of 8 0 0 - 1 0 0 0 mV approximately for the rate of HER of 100 mA cm-2. Further, no clear-cut Tafel lines are observed because of many factors, e.g. curvature in these lines, changes of slopes with bumpy transitions or steep lines resembling a " t r a n s i t i o n " region: where some linear region is observed, the "Tafel slopes" are quite high, i.e. generally greater than 150 mV decade ~. Hence a quantitative analysis of these data would perhaps not carry much significance. It is interesting, however, to observe and analyze qualitatively the main characteristics of the steady-state, potentiostatic polarization curves on these materials covering both the anodic and cathodic potential regions. In Fig. 2, it may be noted that on the anodic side, a transition region characteristic of anodic oxide formation is observed with some associated oxygen evolution at the higher anodic potentials. Much higher rates on the cathodic side, where the HER is occurring, can be seen quite clearly. Rates on NiSI2 are much higher than those on CusSi both on the anodic and cathodic side. Similar qualitative behaviour is also observed on Cr3Si, VSi2 and AgSi, shown in Fig. 3. When another set of silicides is examined in Fig. 4, it is seen, not unexpectedly that PtSi

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shows substantially higher activity than either MnSi2 or TiSi2. Similarly a noble metal silicide, namely, PdSi, exhibits higher activity than the non-noble metal silicides ZrSi2 and CoSi2 in Fig. 5, although the rate on CoSi2 is higher than that on PdSi at the highest cathodic potentials.

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Fig. 6. A potentiostatic anodic polarization plot on FeSi in 1 M KOH: the rate of change of potential is 0.8 mV S ~. The main features are an inhibition inflexion and a transition region so characteristic of surface oxide formation; rather similar behaviour is also observed on most other transition metal silicides of nonnoble and non-catalytic metals. It should be added that the data in Figs 2 - 5 have been separated into each of these figures merely for convenient representation in these graphs, i.e. without creating too much clutter; except in the foregoing sense, the presentation of silicides in various figures are rather arbitrary and do not denote any scientific basis for the particular groupings.

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Although Figs 2 - 5 show some anodic regions where oxide can be formed, the main focus in those runs was on the hydrogen evolution region. In this section, we present a more detailed situation regarding the anodic behaviour of these transition metal silicides in the oxygen evolution region. In this context, a variety of interesting features can be observed. First of all, silicides of Cu, Cr, Ti and V show visible corrosion when anodized to high anodic potentials [ - 3 . 0 V (Hg/HgO)] in KOH, although some corrosion must undoubtedly also occur on some other silicides. On FeSi, one observes a " t y p i c a l " anodic behaviour expected on a stable, conducting electrode (Fig. 6): a transition region between - 0 . 1 and 0.6 V Hg/HgO; in terms of RHE, - 1.026 and 1.526 V is followed by a more or less linear region of oxygen evolution reaction (OER) which shows an inhibition inflexion [5] at around 1.2 V (Hg/HgO), which leads to a limiting current region at higher anodic potentials characteristic of an oxide-growth

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type of process. Most of the silicides studied show this kind of qualitative behaviour (Fig. 6); silicides made of more catalytic metals (e.g. Co, Ni, Pt, Pd) show a somewhat different type of current -potential characteristic, as exemplified in Fig. 7 for PtSi: at lower potentials, an incipient transition region enters into the oxygen evolution line (which has a rather high slope and is somewhat curvy and bumpy) culminating, at higher anodic potentials, in a limiting current region so typical of oxygen evolution pro-

(1) Many transition metal silicides provide suitable surfaces for the cathodic evolution of hydrogen and the anodic evolution of oxygen, during electrolysis of water in alkaline solutions. (2) In general, clear oxidation-reduction peaks are not observed in the potentiodynamic profiles; only a hysteresis is usually observed. Only slight peaks are observed at CusSi, AgSi, NiSi2 and PdSi. (3) All silicides examined here sustain high rates of the cathodic hydrogen evolution reaction, albeit at high overpotentials. Excellent electrochemical stability of the electrodes is maintained under the conditions of the HER. (4) On the anodic side, silicides of Cu, Cr, Ti and V show visible corrosion in alkaline solutions, when anodized to high anodic potentials. Other silicides show anodic behaviour similar to that observed on FeSi: an oxygen evolution region that gives rise, at higher anodic potentials, to a limiting current region characteristic of anodic oxide growth.

REFERENCES 1. A. K. Vijh, G. B61anger and R. Jacques, Mater. Chem. Phys. 19, 215 (1988). 2. A. K. Vijh, G. B61anger and R. Jacques, Mater. Chem. Phys. 20, 529 (1988). 3. A. K. Vijh, G. B61anger and R. Jacques, Mater. Chem. Phys. 21, 529 (1989). 4. A. K. Vijh, G. B61anger and R. Jacques, Int. J. Hydrogen Energy 15, 789 (1990). 5. A. K. Vijh, Electrochemistry of Metals and Semiconductors, p. 78. Marcel Dekker, New York (1973).