EPR study of nickel-zinc-iron electrodeposits for alkaline water electrolysis

Int. J. Hydrogen Energy, Vol. 14, No. 6, pp. 361-363, 1989. Printed in Great Britain.

0360-3199/89 $3.00 + 0.130 Pergamon Press plc. © 1989 International Association for Hydrogen Energy.

EPR* STUDY OF NICKEL-ZINC-IRON ELECTRODEPOSITS FOR ALKALINE WATER ELECTROLYSIS M. V. ANANTHand N. V. PARTHASARADHY CECRI Madras Unit, CSIR Complex, Madras 600113, India

( Received for publieation 16 November 1988) Abstract--EPR measurements on electrodeposited Ni-Zn-Fe alloys were carried out. The results were used to explain the electrocatalytic activity of the alloys in 30% KOH solution. The deposit obtained at pH 2.90 had a low hydrogen overvoltage, since the contribution to the conduction band by the nickel and zinc atoms was not masked by the d overlapping of iron atoms. The coating obtained at pH 4.83 had a lower electrocatalytic activity due to the significant spin orbit coupling of iron atoms. Thus the electrocatalytic activity of the coatings was found to be dependant on EPR parameters.

INTRODUCTION The hydrogen evolution characteristics of Ni-Fe alloy deposits in alkaline solutions were reported [1]. Likewise, N i - Z n electrocoated cathodes were patented [2]. It is, therefore, worthwhile to investigate the electrocatalytic behaviour of N i - Z n - F e codeposits. In fact, such a deposition was carried out due to the levelling action [3]. The codeposition of iron group metals with zinc has several unique features [4]. The pH of the bath exercises an irregular behaviour on the zinc content of the deposit. Paramagnetic resonance of conduction electrons in metals is known [5]. Hence, EPR could prove to be a powerful probe for studying the state of conduction electrons in codeposits. In this work, it is proposed to study the effect of bath pH on the electrocatalytic activity of N i - Z n - F e deposits and attempts to explain any anomalies in the light of EPR spectra. To the authors' knowledge, such an attempt is being made for the first time. EXPERIMENTAL The N i - Z n - F e alloy was electroplated from the sulphate bath in the pH range 2.90 to 6.00 (dectrometric). The bath used contained NiSO 4 • 6H20 300 gl-1; FeSO4 • (NH4)2SO4 • 6 H 2 0 15 gi -1 and H3BO 3 42 g 1-1 at 30°C and 30 m A cm -2. The cathode was a mild steel rod embedded in "Araldite", exposing a circular surface of 0.31 cm 2 geometrical area and the anode was a bagged graphite rod. Prior to electrodeposition the steel cathode was given the usual cleaning treatments. Hydrogen evolution characteristics were studied in a three compartment cell with graphite anode and a saturated calomel electrode (SCE) through a salt bridge * Electron Paramagnetic Resonance. 361

as a reference; 30% (wt vo1-1) K O H constituted the alkaline medium. The samples for EPR study were prepared by electrodepositing on stainless substrates, removing the deposits mechanically and grinding to fine powder. EPR measurements were carried out on x-band Varian E-line century series EPR spectrometer at ambient temperature and microwave frequency of 9.41 G Hz.

RESULTS A N D DISCUSSION Results are presented in Figs 1 and 2 and Table 1.

(a) Polarisation studies The influence of bath pH and duration of plating on the electrocatalytic activity on N i - Z n - F e codeposits in 30% K O H are demonstrated in Figs 1 and 2. The deposit obtained at pH 2.90 shows a striking reduction in hydrogen overvoitage. The latter increases with pH in the acidic region and decreases as the pH approaches neutral value. A comparison of the data reveals that the electrocatalytic activity can be enhanced with thicker deposits (i.e. increased plating duration). Since the deposit at pH 6.0 was cracked and operational problems occurred, EPR studies were restricted typically to deposits at pH valules of 2.90 (sample P) and 4.30 (sample Q).

(b) E P R measurements Table 1 gives the EPR parameters deduced from the E P R experiments. The curves obtained for both the samples were broad and the line widths can be attributed to the powderd polycrystalline nature of the samples. The "g" shifts in both the cases correlated well with the existence of ferro-magnetism in the material.

362

M . V . ANANTH AND N. V. PARTHASARADHY

Table 1. EPR parameters determined from experimental curves

Sample

Full line width AHp_p

/4o

Gauss)

(Gauss)

A/B

Deposit at pH 2.90

1280

660

1.10

Deposit at pH 4.83

1160

730

0.65

720

--

0.57

Deposit at pH 4.83 --iron contribution

•

Uncoated mild steel

o N i - Z n - F e electrodeposit at pH 4.45 n N i - Z n - F e electrodeposit at pH 5.15 1.7

1.7

N i - Z n - F e electrodeposit at pH 6.00

//

1.6 •

,,

•

•

1.6

~

1.5

0

1.5

1.4

0

1.4

•/~/Ym f--

• Uncoated mild steel

O~/

• N i - z n - F e electrodeposit at pH 4.16

et

r~ Ni-Zn-Fe electrodeposit at pH 4.83

1.3

o Ni-Zn-Fe electrodeposit at pH 2.90 I

100

200

300

400

500

I

I

I

I

I

I

I

600

100

200

300

400

500

600

Current density (mA cm -2)

Fig. 1. Hydrogen evolution characteristics of samples with a plating duration of 15 min.

Current density (mA crn -2 )

Fig. 2. Hydrogen evolution characteristics of samples with a plating duration of 30 min.

NICKEL-ZINC-IRON ELECTRODEPOSITS The curve for sample P was only very slightly asymmetric. The contribution due to the iron atoms in the conduction band is small enough to be observed at the gain setting and the curve is broader than that for sample Q. Its curie temperature was farther from the room temperature than for sample Q. Thus, based on EPR results, the deposit P is expected to exercise a lower hydrogen overvoltage, as the contribution due to the conduction band by the nickel and zinc atoms are not masked by the d overlapping of iron atoms. This expectation is supported by the polarisation results. The curve obtained for sample was highly asymmetric. The overlapping lines in the spectrum can be attributed to the contribution of iron atoms. It is well known that in a ferromagnetic material, the "g" shift is towards lower fields and is proportioned to 2:r M (where M is the magnetisation of the specimen) where the field is applied parallel to the plane of the specimen. Where the field is perpendicular, the "g" shift is towards higher fields and is proportional to 4:r M. The present observation may then be explained by postulating the existence of two sets of conduction electrons, one due to the host nickel matrix containing zinc and the other due to iron atoms, which give rise to two resonance lines. Since the samples were polycrystalline, the stronger lines may arise from species which are aligned parallel to the applied field, while a relatively small number from a plane lying at right angles to the applied field. The smaller of "g" shifts observed in the latter case arises

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from smaller magnetism due to a smaller number of species. The contribution due to the overlapping iron line is significant. Thus the EPR analysis of this sample predicts an increase in hydrogen overvoltage on cathodic polarisation, due to significant spin-orbit coupling of iron atoms. This is supported by the polarisation results. SUMMARY Exploratory experiments on the correlation between electrocatalytic properties of N i - Z n - F e deposits for alkaline water electrolysis and EPR spectra, are encouraging. An elaborate study of EPR parameters can yield considerable information. Detailed experiments are in progress.

Acknowledgement--The authors sincerely thank Prof. K. I. Vasu, Director of the Institute for his encouragement and interest in the work. REFERENCES 1. E. R. Gonzalez, L. A. Avaea, A. Carubelli, A. A. Tanaka and G. Tremiliosi-Fiiho, Int. J. Hydrogen Energy 9 689 (1984). 2. R. Peony, Mater Prot. 7-8, 27 (1968). 3. W. H. Safranek, German patent 1,016,527 (1957). 4. A. Brenner, Electrodeposition of Alloys, Vol, II, p. 194, Academic Press, New York (1963). 5. T. W. Griswold, A. F. Kip and C. H. Kittel, Phys. Rev. 88, 951 (1952).