Mechanistic studies of the hydrogen evolution reaction on tungsten under water electrolysis conditions

0360-3199/86 $3.00 + 0.00 Pergamon Journals Ltd. © 1986 International Association for Hydrogen Energy.

Int. J. Hydrogen Energy, Vol. 11, No. 7, pp. 455--458, 1986. Printed in Great Britain.

M E C H A N I S T I C STUDIES OF THE H Y D R O G E N E V O L U T I O N R E A C T I O N ON TUNGSTEN U N D E R W A T E R ELECTROLYSIS CONDITIONS A. A. TANAKA, E. R. GONZALEZ and L. A. AVACA Instituto de Ffsica e Quimica de S~o Carlos, USP, C.P. 369, 13560 S~o Carlos, SP, Brazil

(Received for publication 26 November 1985) Abstract--Tungsten (W), electrodeposited on mild steel from aqueous solutions, has shown to behave similarly to electroplated Ni when used as a cathode in 28% KOH at 60°C and current densities of the order of 135 mA cm-2. When compared with bare mild steel the W cathodes present an overpotential 50 mV higher, but this is largely compensated by the very much higher chemical stability of the deposits in the electrolyte. This is particularly important when the electrolyzer is to be used in an intermittent fashion. In the present work, the H2 evolution reaction was studied on pure and electrodeposited W electrodes in alkaline solutions through the recording of steady-state polarization curves. By comparison of the experimental electrochemical parameters with those predicted by theory, it was established that the mechanism of the reaction is of the Volmer-Heyrowsky type, with the electrochemical desorption reaction being the rate-determining step. The adsorption process is controlled by an activated Temkin isotherm for both pure and electrodeposited W.

HER b io r/ o~ 13 A 0 CD

NOMENCLATURE Hydrogen evolution reaction Tafel slope exchange current density overpotential symmetry factor for electron transfer symmetry factor for the hydrogen adsorption barrier 2.303 RT/F degree of coverage current density.

INTRODUCTION In the last decade, many significant advances have been made in water electrolyzers. Still, the traditional unipolar type accounts for a substantial fraction of the world market for H 2 production [1]. Under typical operating conditions, conventional electrode materials show overpotentials of the order of 350 mV for the cathodic as well as for the anodic reactions. Because of this, efforts have been concentrated in developing low cost materials showing smaller overpotentials together with a suitable chemical stability in the electrolyte solution. In particular, there are economical advantages when the new materials can be applied as electrodeposits on inexpensive base metals. Previous studies carried out in this laboratory [2] have shown the advantages of having a detailed mechanistic knowledge of the electrode reaction when evaluating new materials. This knowledge is usually obtained through studies in dilute solutions and low CDs, but it has been shown possible [3] to correlate those results with the behavior observed under water electrolysis conditions, i.e. concentrated KOH solutions and high CDs. Tungsten, electrodeposited on mild steel from aqueous solutions, was found to behave similarly to elec-

troplated Ni when used as a cathode in 28% KOH solutions at 60°C and CDs of the order of 135 mA c m -2 [4]. This means that W cathodes present an overpotential 50mV higher than mild steel. This disadvantage is largely compensated by the higher chemical stability of the deposits in the electrolyte, an important fact for electrolyzers that work in an intermittent fashion. In the present work, the mechanism of the H E R was studied in electrodeposited and pure W electrodes in alkaline solutions through the recording of steady-state polarization curves. Apart from its intrinsic value the present work will serve as basis for the future examination of the H E R on codeposits containing W. EXPERIMENTAL The substrates for the electrodeposition of W were constructed using Cu or brass plates mounted in an epoxy resin which proved to be inert under the working conditions. The electrode area was about 0.5 cm 2. Initially, all electrodes were mechanically polished using emery paper down to grade 600, chromic oxide and alumina. After careful washing with alcohol and water the electrodes were immersed under polarization into the electrodeposition bath. Several aqueous and nonaqueous baths were tried under different conditions for the electrodeposition of W [5-8]. Of these, only the phosphate bath [8] gave uniform deposits when the surface of the substrate was carefully prepared. The following formulation and conditions were used: H2WO4, 85 g 1-1; Na3PO4.12H20 , 625 g1-1; temperature, 80-90°C; CD, 100mA cm-2; electrolysis time 2-3 h. The experiments on pure W were done using a W wire (Varian, 99.95%) mounted on a glass capillary, leaving an exposed area of about 0.2 cm 2.

455

456

A. A. TANAKA, E. R. GONZALEZ and L. A. AVACA

A H g / H g O / 2 8 % aq. K O H electrode was used as reference. All potentials quoted in this work are given with respect to this reference electrode. Overpotentials were calculated using existing values in the literature [9] for the H g / H g O / O H - electrode vs H2/Pt, extrapolated to the required temperatures. Auxiliary electrodes were constructed using Pt foil. The steady-state polarizations were carried out in a three-compartment H-type glass cell of 300 ml capacity. The anode and cathode compartments were separated by a glass frit and the cathode compartment had a side entrance for a salt bridge formed by a Luggin capillary, a tap and an upper cup in which the reference electrode was immersed. The cell had also suitable entrances for passing gases through and over the solution. The cell was immersed in a water thermostat and provided with magnetically driven stirrers. The electrolytic solutions were prepared using K O H in triply distilled water. In order to obtain adequate reproducibility in the kinetic experiments related to the H E R it was found necessary to preelectrolyse the solution, at room temperature, for about 90 h at 1 A cm -z using Pt electrodes of 2 cm 2 total area [4]. The experimental procedure for the recording of cathodic curves was as follows: the cell was filled with preelectrolyzed solution and electrodes, similar to those to be used for the measurements, set into place. After achievement of the equilibrium temperature a current of 1 A cm -2 was passed for several hours for a final purification of the solution. A fresh cathode was then introduced into the catholyte, now saturated with H, and subjected to a cathodic pretreatment at 1 A cm -2 for 20 min. This pretreatment was shown to be necessary in order to obtain reproducible polarization curves free from hysteresis. The current-potential characteristic was then recorded point by point by fixing the current with a stabilized source and measuring the potential with a digital voltmeter. A steady value of this potential was usually reached after 2 min. The ohmic drop between the trip of the Luggin capillary and the electrode surface was evaluated in several experiments by measuring the potential, at fixed CD,

E/V

40 -2 \

i/Acrn-2,

~

:~

"~22..1225V

~7~1=-115o~ .10-4

900 I

I100 I

1300 pH

I

I

I

i

Fig. 2. Current density as a function of pH at constant electrode potential. as a function of the positioning of the Luggin capillary. For CDs up to 300 m A cm -2 and distances between 1 and 5 mm the cathodic potentials were independent of this parameter. RESULTS Figure 1 shows the Tafel plots for the H E R on electrodeposited W at pHs 9.55, 10.53, 11.55, 12.55 and 13.54. These results were used to construct the currentpH plots at constant potential and potential-pH plots at constant current shown in Figs. 2 and 3, respectively. From Fig. 1 the electrochemical parameters for the H E R on electrodeposited W, presented in Table 1, were obtained.

--1400 E/V

~ " i2,30xlO_3Acm_ 2

--I 3 0 0

--I / . . . ~ i 1 , 5 0 xIO'~Acm-2

c.,~c)~.~

--D- n 55 + 1255 + 1354 i0-zl i/Acre-2 10-2

--0900

I04 I

l

I Illlll

I

I

I I tltll

I

I

1 I Itlll

Fig. 1. Tafel plots for the HER at 25°C on electrodeposited W at different pHs.

.-t000 900 I

I

I100 I

]

1300 pH I

Fig. 3. Electrode potential as a function of pH at constant CD.

457

H2 EVOLUTION ON TUNGSTEN IN WATER ELECTROLYSIS Table 1. Electrochemical parameters for the HER on electrodeposited tungsten Low CD

i

High CD

(mV)

io (A cm 2)

b2 (mV)

io (A cm -2)

90 90 85 90 85

2.9 × 10-8 5.6 × 10 -8 6.2 × 10-8 2.2 X 10 -7 3.5 × 1 0 -7

120 125 122 120 122

3.3 x 10 7 5.6 x 10-7 7.2 × 10 -7 1.2 × 10 6 2.0 × 10 -6

bl

pH 9.55 10.53 11.55 12.56 13.54

Table 2. Electrochemical parameters for the HER on pure tungsten Low CD pH

bl (mV)

9.55 10.53 11.55 12.56 13.54

80 78 70 70 73

High CD

io (A cm -2) 4.9 5.8 6.6 1.0 1.8

b2 (mV)

× 10 9 X 1 0 -9

× 10-9 × 10-8 × 10-8

io (A cm -2)

117 115 112 110 110

2.0 2.8 4.4 6.0 9.2

l ~- 5 O0

x x × x x

10-7 10-~ 10-7 10 7 10 7

A similar analysis was done for pure W and the results are presented in Table 2. The effect of t e m p e r a t u r e on the H E R was evaluated by recording Tafel plots at the temperatures of 25, 40, 60 and 80°C. Figure 4 shows the results for electrodeposited W in a K O H solution of p H 13.54. F r o m these results, the Arrhenius plots for high and low C D s shown in Fig. 5. were constructed. The energies of activation were evaluated as being 42.5 and 20.5 kJ mo1-1 for the regions of low and high CD, respectively. In order to correlate the results at low CDs with those under water electrolysis conditions, Tafel plots were recorded in 28% K O H at 60°C. Figure 6 shows the result for electrodeposited W. In the region o f CDs < 1 0 - 2 A cm -2 there is a coincidence with the corresponding curve in Fig. 4, both having the same slope ( 1 2 0 m V d e c a d e - I ) . For CDs > 10 -1 A cm -2 a third

- 600 log io/Acm-2

~_.~_

o

--700 280 _ _

300

L

j

320

~ _ _

,

T-I/IO-3 k-t

l

,

Fig. 5. Arrhenius plots at pH = 13.54 for electrodeposited W at high ( t ) and low (O) CDs. linear region appears with a slope of 175 m V decade -1. Similar results were observed for pure W. DISCUSSION The difficulties involved in the electrodeposition of W are well known. Using aqueous or nonaqueous baths the deposits are very thin and usually nonuniform. This is due to the characteristics of the electrodeposition process, summarized as follows: (i) 99% of the current is used to evolve H2 [6] (ii) in many cases the potential is so negative that the solvent is decomposed. Alternatively, an oxide film may develop [10] (iii) W does not deposit over W [8]. In this work, satisfactory deposits were obtained only with a phosphate bath and for those substrates whose surfaces had been carefully prepared. The H E R on W has been studied mainly in acid solutions [11-16]. Only a few works have considered the reaction in alkaline solutions [13-15]. According to the results presented in Figs. 1, 4 and 6, Tafel plots for electrodeposited W in different K O H concentrations and different temperatures are charac-

--1600

--q 400

E/V

j

--1400

~

-

..

-A- 60 (3-80

--0.800 1

10-4 I

I

I tllli

10-5 I

I

l

I Illli

i/Acm'2 I

I

I

10-2

1 I IIII

Fig. 4. Tafel plots for the HER at pH = 13.54 on electrodeposited W at different temperatures.

--100(3 L

001 ~

I

J

I # '1'.

OIO ~

,

I

i

,,li

i/Acm"2 I

~

k

I

I

I O0 IIII

Fig. 6. Tafel plot for the HER on electrodeposited W in 28% KOH at 60"C.

458

A. A. TANAKA, E. R. GONZALEZ and L. A. AVACA

terized by three linear regions, with transitions taking place at about 10 -3 and 10 -2 A cm -2. Values of the Tafel slopes at low and intermediate CDs were in general in agreement with those observed previously in alkaline as well as in acid solutions. Some discrepancy exists with the values reported by A m m a r et al. [15], but they are within the expected range [11]. Following the classical paper of Conway and Salomon [17] on the mechanistic interpretation of the H E R in acid media, Peraldo Bicelli [18] presented an equivalent treatment for alkaline solutions. A comparison of the values of Tafel slopes (Fig. 1 and Table 1), (alog i/apH)e in Fig. 2 and (aE/OpH)i in Fig. 3 with those predicted by theory leads to the conclusion that the two Tafel regions correspond to the same mechanism. This involves an initial fast discharge step (Volmer reaction) followed by a slow electrochemical desorption step (Heyrowsky reaction) with adsorption of H under activated Temkin conditions. The same conclusions can be drawn from the analysis of the results obtained for pure W electrodes. For the proposed mechanism (0log i/0pH)E = fl, (a~l/apH)i = -tr.A/(ot+ fl) and the Tafel slope is equal to A / ( t r + f l ) , where ~r is the symmetry factor for the electron transfer barrier, flis the symmetry factor for the hydrogen adsorption barrier and A = 2.3 RT/F. Table 3 shows the values of a~ and fl for electrodeposited and pure W calculated from the previous analysis. This means that the experimentally observed change in Tafel slope with CD is not due to a change in mechanism but to small changes in the symmetry factors, in particular for ft. This could be related to an increase in coverage, within the Temkin region, which may have an effect on fl not accounted for by the kinetic equations. In general terms the analysis above shows that electrodeposited W retains the electrochemical properties of the pure metal except, perhaps, for small differences in the exchange currents, which could be explained by assuming a somewhat larger effective area for the electrodeposits. A comparison of the results obtained in dilute solutions at 60°C (Fig. 4) with those obtained under water electrolysis conditions (Fig. 6) shows that within experimental error the Tafel curves are coincident in the region 10-3-10 -2 A cm -2. Thus, it is reasonable to assume that the proposed mechanism holds under these experimental conditions up to 10 -1 A cm -2. It is important to note that a straightforward interpretation of the 120mV decade -1 Tafel Table 3. Symmetry factors for the HER on tungsten at 25°C Electrodeposited tungsten Low CD High CD tr 0.27 fl 0.40

0.18 0.30

Pure Tungsten Low CD High CD 0.29 0.50

0.22 0.30

slope in Fig. 6 would have led to the conclusion that in this region the mechanism involves a slow discharge step (Volmer reaction) under Langmuir adsorption conditions. For larger CDs there is a further increase in Tafel slope (Fig. 6) which cannot be supported by mechanistic studies. Nevertheless, if it is accepted that H coverage increases with CD, then 0---~ 1 and the adsorption will be described by a Langmuir isotherm, making (alog i/apH)E = 0. Under these conditions the Tafel slope (175 mV decade -1) equals A/o: resulting in a value of 0.34 for tr. Two important conclusions can be drawn from this mechanistic analysis. Firstly, a change in Tafel slope does not necessarily mean a change in the mechanism of the reaction and secondly, the interpretation of Tafel slopes obtained by single experiments under water electrolysis conditions could be misleading. Acknowledgements--This work was carried out under contract with the Financiadora de Estudos e Projetos (FINEP), Brazil. The authors thank the Conselho Nacional de Desenvolvimento Cientifico e Tecnol6gico (CNPq), Brazil, for research grants. REFERENCES 1. R. L. LeRoy and A. K. Stuart. In S. Srinivasan, F. J. Salzano and A. R. Landgrebe (eds), Industrial Water Electrolysis, p. 117. The Electrochemical Society, Princeton (1978). 2. L. A. Avaca, E. R. Gonzalez, A. A. Tanaka and G. Tremiliosi Filho. In T. N. Veziroglu, W. D. Van Vorst and J. H. Kelley (eds), Hydrogen Energy Progress IV, Vol. 1, p. 251. Pergamon Press, Oxford (1982). 3. L. A. Avaca, E. R. Gonzalez and A. Carubelli. In T. N. Veziroglu, W. D. Van Vorst and J. H. Kelley (eds), Hydrogen Energy Progress IV, Voi. 1, p. 327. Pergamon Press, Oxford (1982). 4. L. A. Avaca, A. Carubelli and E. R. Gonzalez. In T. N. Veziroglu, K. Fueki and T. Ohta (eds), Hydrogen Energy Progress, Vol. 1, p. 127. Pergamon Press, Oxford (1980). 5. L. F. Yntema, J. Am. Chem. Soc. 54, 3775 (1932). 6. L. F. Fink and F. L. Jones, Trans. Am. Electrochem. Soc. 59, 461 (1931). 7. A. L. Levinskas, Sooiet Electrochem. 1, 96 (1965). 8. M. L. Holt and L. Kahlemberg, Metal Ind. 31, 94 (1933). 9. T. S. Lee, J. Electrochem. Soc. 118, 1278 (1971). 10. A. D. Graves and D. Inman, Electroplg Metal Finish. 19, 314 (1966). 11. J. O'M. Bockris, I. A. Ammar and A. K. M. S. Huq, J. Phys. Chem. 61,879 (1957). 12. J. O'M Bockris and D. F. Koch, J. Phys. Chem. 65, 1941 (1961). 13. J. O'M. Bockris and S. Srinivasan, Electrochim. Acta. 9, 31 (1964). 14. J. O'M. Bockris, E. Gileadi and R. Haynes, J. Electroanal. Chem. 19, 446 (1968). 15. I. A. Ammar, S. Darwish and R. Salim, J. ElectroanaL Chem. 52, 443 (1974). 16. C. M. Marschoff and M. D. J. Casanova, Electrochirn. Acta. 21,849 (1976). 17. B. E. Conway and M. Salomon, Electrochim. Acta 9, 1599 (1964). 18. L. Peraldo Bicelli, Chim. hzd. (Milan) 55,792 (1973).