The hydrogen reaction at equilibrium on the metals Ni, Fe, Cu, Ag and Au

ht.

J. Hydro,qen

Energy, Vol. 22, No. I, pp. 669-673, 1997

rj 1997 International

Pergamon

for Hydrogen Energy Elsevier Science Ltd All rights reserved. Printedin Great Britain 0360-3199/97 $17.00+0.00

PII: SO360-3199(96)00205-d

THE HYDROGEN

REACTION

AT EQUILIBRIUM Cu, Ag AND Au

Association

ON THE METALS

Ni, Fe,

T. VALAND Agder College, Grooseveien 36, N-4890 Grimstad, Norway

(Receiaed,for publication 24 Sepfemher 1996)

Abstract-The hydrogen reaction on different metals has been treated with respect to the exchange current density as a function of pH and the equilibrium potential. The study shows that there is no significant difference between the different metals. They all follow the sameequations expected with respect to the effect of pH and equilibrium potential on the exchange current density. The “formal transfer coefficient” /I has the expected value 0.5 in the whole pH range 0 to 14. ‘0 1997International Association for Hydrogen Energy

Due to the stability of Ni in alkaline solutions, this metal finds wide application as cathodes in the alkaline water electrolysis and as anodes in alkaline fuel cells. In order to increase the hydrogen reaction rate on Ni based electrodes, the surface is made porous resulting in an increased surface area [l-3]. A thin layer of a catalytic material may also be formed on the surface. This layer may well be a Ni based alloy. Of special interest are the NIP, and the NiS, amorphous layers [4-71. Due to the importance of Ni, it is of interest to compare this metal with the other metals in the group.

NOMENCLATURE Equilibrium potential Standard equilibrium potential in acid solutions Standard equilibrium potential in alkaline solutions Anodic hydrogen oxidation rate Cathodic hydrogen reduction rate Exchange current density Standard exchange current density in acid solutions Standard exchange current density in alkaline solutions Dissociation constant of water Anodic reaction order of H,O + Cathodic reaction order of H,O + “Formal transfer coefficient” R, T and F have their usual meaning

METHOD

OF TREATMENT

In acid solutions the total hydrogen reaction is: H,O++e-

:= l/2H2+H20.

(1)

In alkaline solutions the total reaction is: H,O+e-

INTRODUCTION

= 1/2H?+OH

(2)

These reactions can be further divided into at least three partial reactions, i.e. the Volmer Reaction, the Tafel Reaction and the Heyrowsky Reaction. One of these will be the rate determining step (rds). Even though the reaction order is given by the rds, the above total reactions indicate that in acid solution the cathodic hydrogen evolution reaction is dependent on the H,O+ concentration and the anodic hydrogen oxidation is dependent on the partial pressure of hydrogen. In alkaline solutions, however, the anodic hydrogen oxidation is probably dependent on the OH - concentration in addition to the partial pressure of hydrogen. The cathodic hydrogen evolution reaction in alkaline solu-

The hydrogen reaction (her) is one of the most important and most studied reactions in electrochemistry. Together with the oxygen reduction reaction, it is the main cathodic reaction which determines the corrosion rate of metals. An extensive amount of work has been performed in connection with hydrogen production by water electrolysis and in connection with fuel cells. The platinum metals are known to have strong catalytic influence on the hydrogen reaction. On the other hand, the hydrogen evolution on metals like Hg and Pb is very slow. In between these two groups, Ni, Fe, Cu, Ag and Au form a group of metals which has an intermediate influence on the leer. 669

T. VbiLAND

670

tions is expected to be pH independent and independent of the partial pressure. These assumptions will have an implication on the exchange current density and thereby on the overvoltage of the hydrogen reaction. Current density data on different metals and in different solutions are widespread in the literature. From these data the exchange current density can be calculated. In order to get an overview of these important data, some of them are collected and treated together with the equilibrium potential. An overview of the data which give mechanistic information can be obtained by plotting the exchange current density versus pH and the equilibrium potential. The equilibrium potential of the hydrogen reaction is either obtained directly or calculated from the solution composition using the Nernst equation, taking into account the activity coefficients. If it is assumed that the anodic hydrogen oxidation reaction rate is given by the equation: j, = jp[H,O+]”

Also in this case a linear relationship is expected between log j, and pH, but this time the slope is p. A relationship between the equilibrium potential (E,) and the exchange current density (j,) can also be obtained from equations (3) and (4): ---logjix Y-X

logj, = Llogjiy-x

F - 2.303(y-x)RT

-PM?1

L~-P1~5,.

F Y 2.303RT y-x

(8)

When x = 0 and y = 1 (acid solutions), equation (8) is reduced to: -K-E,*0

2.303RT j, = (1 -B)F lwJT

which gives a linear relationship between E,, and log j,. When the temperature is 25°C the slope is

K;” exp

0.059 l-/Y

j, = -j:[H,0f14exp 1

(4)

a connection between the exchange current density j,, and pH can be obtained. x and y are the reaction orders of the H30f in the anodic and cathodic direction, respectively. They are determined by the rds, and are not necessarily connected to equations (1) and (2). Ei, Et, J: and jF are the standard equilibrium potentials and standard exchange current densities, respectively for Reactions 1 and 2. K, is the dissociation constant of water and fl is the “formal transfer coefficient”, which may, or may not be equal to the symmetry factor c(. When E = EO, j, = -j, =j,. In that case, equations (3) and (4) can be combined, giving the following relationship between the exchange current density j, and pH: log j, = log A - [(x - y)l+ y]pH -xl

log K, - B, (5)

where: -fl)logj:

b@:+~(l +-

and the cathodic rate by:

1ogA = /3logjp+(l

____ xy logK, y-x

and

If x = 0 and y = 1, which is most likely in acid solutions, equation (5) is reduced to: logj, = log A - (1 - /3)pH - B

(6)

which shows that a linear relationship is expected between log j, and pH. The slope is - (1 - fl). In alkaline solutions, x = - 1 and y = 0 are likely to be observed. In that case equation (5) is reduced to: log j, = log A + /JpH + B log K, - B.

(7)

In alkaline solution, where x = - 1 and y = 0 are expected, the equation is reduced to: E,-E;

2.303RT = - -logPF

giving a linear relationship slope equal to

j, J::

between E, and log j,, with a 0.059 PV

when the temperature is 25°C. RESULTS Exchange current densities for the hydrogen reaction at different pH values are widespread in the literature. On iron, Hurlen [8] has given values for the hydrogen reaction from pH = 0 to 14. Parsons [9] has given exchange current density values on different metals in different solutions. Taking the activity coefficient into account, the pH of these solutions can be estimated. Machado and Avaca [lo] have given the exchange current density for the hydrogen evolution on Ni at different temperatures in 0.5 M NaOH solutions. In Fig. 1 some of the data are plotted in a pH versus log j,, plot. Two lines are drawn through the data, one with a slope equal to 2 and the other with a slope equal to -2. Even though the data are a bit scattered, the lines fit the data reasonably well. There seems to be no significant difference between the metals. Some of the data on silver in acid solution are low, but others fit well, indicating that the deviation is due to experimental error. In Fig. 2 the same data as in Fig. 1 are plotted in a log

HYDROGEN

REACTION

ON METALS

16

.-

14

i

d pH/d logj, =2

12 10 i*Ni mFe

8

rcu

I P

oAg AAU

6

_J

-7.5

-7

-5.5

-6

-6.5

log&,/A

-5

-4.5

-4

cm-‘)

Fig. 1. The pH dependenceof the exchange current density of the hydrogen reaction on different metals. j0 versus E plot. In this figure two lines with a slope d E/d logj equal to -0.12 V and 0.12 V are plotted. As can be seen, the data fit the lines quite well. As expected,

the silver data deviating in Fig. 1, deviate in Fig. 2 as well. In fact, the spread in data on the right hand side of the plot, i.e. those corresponding to acid solutions, are

-3 -3.5 -4 -4.5

d Ed d log j0=0.12V

\\ ~ d Ed d log j0 =-0.?2 V

Q

,’

-6.5 -7 -7.5 -1

-0.8

-0.6

-0.4

-0.2

0

0.2

EN(she)

Fig. 2. The equilibrium potential/exchange current density dependency for the hydrogen reaction on different metals.

612

T. VALAND

more pronounced in this plot than in the pH versus logj plot, indicating that this plot is more sensitive than that in Fig. 1.

j, = jp exp &S-E?)

(15)

1

and the cathodic rate equation is: DISCUSSIONS

j, = -ji[H,O+]exp

Equation 5 shows that a linear relationship between expected. The slope and pH is 1% h dpH/dlogj,, = - l/[(x---y)/I+y], where x and y are the reaction orders of H30 + in the anodic and cathodic direction, respectively. /I is the “formal transfer coefficient”. As pointed out above, the expected values of x and y in acid solution are 0 and 1, respectively. In that case the slope should be equal to - l/(1 -/I). The line in the acid part of Fig. 1 is drawn with a slope equal to -2. This shows that /I = 0.5 seems to fit the data well. In alkaline solutions, where x = - 1 and y = 0 is expected, the slope is equal to l//I. The line in the alkaline part of Fig. 1 is drawn with a slope equal to 2, which confirms that /I = 0.5. The intersection of the two lines in Fig. 1 can be found by combining equations (6) and (7) and setting j, = jb”’ in both equations. jy’ is the exchange current density at the intersection. As a result, the pH at the intersection is given by the simple equation: pH’“’ = /I log K,.

(11)

Since /I = 0.5 and K, = 10-14, pH’“’ = 7 is expected. This fits the data in Fig. 1. The exchange current density jb”’ can be obtained by combining equations (11) and (6): logjo”’ = /Ilogjg+(l

-jI)logji

+(l-PM logKFrom the Nernst equation the following E,*Ois obtained:

E$'=E:+

2.303RT -log F

expression for

K,.,.

(13)

By introducing this equation into equation (12), the following simple expression for ft’ can be obtained: log jr’ = /I log jg + (1 - p) log]:.

(14)

In the particular case when p = 0.5, jb”’ = m is obtained, i.e. the exchange current density at the intersection is equal to the geometric mean of the two current densities ji and j$“. ji and jp can be found by extrapolating the lines in Fig. 1 to pH = 0 and pH = 14, respectively. The values 10-5.5 A/cme2 and 10-‘2.5 A/cm-’ are then obtained, which have a geometric mean of 10e9 A/cmm2. This value corresponds well with the point of intersection. The analysis of the data has shown that in acid solutions the cathodic rate equation is equal to:

-&T(~-~:)

1

(16)

In alkaline solutions the anodic and the cathodic rate equations are respectively given as j, = jp[H30+]-‘K;’

exp gTW

@I]

(17)

and j,=

-jiexp

-&GE-E:)

1

(18)

The partial pressure of hydrogen is not taken into account here. These equations are to be expected if the surface to a large extent is covered by adsorbed hydrogen. This may indicate that the treated metal at the hydrogen equilibrium potential is covered by adsorbed hydrogen. Another explanation could be that the literature data are obtained to a large extent by extrapolating data obtained by large cathodic overvoltages to the equilibrium potential. In that case high coverage is to be expected. CONCLUSIONS The study has shown that the exchange current density is not significantly dependent on the metal when Ni, Fe, Cu, Ag and Au are considered. The exchange current density of the hydrogen reaction can be estimated by simple equations as a function of pH and the equilibrium potential. The “formal transfer coefficient”, fi for the hydrogen reaction is equal to 0.5 in the vicinity of the equilibrium potential for all the metals. As expected, the anodic reaction is pH independent in acid solutions, while a first order dependence is found in alkaline solutions. The cathodic reaction is pH dependent in acid solutions and independent of pH in alkaline solutions. The change in mechanism takes place at pH = 7. REFERENCES 1. Borucinsky, Th., Rausch, S. and Went, H., Raney nickel activated &-cathodes. Part II: Correlation of morphology and effective catalytic activity of Raney-nickel coated cathodes J. Appl. Electrochem. 1992, 22, 1031. 2. Went, H., Preparation, morphology and effective activity of gas evolving and gas consuming electrodes. Electrochim. Acta 1994, 39, 1749. 3. Machado, S. A. S., Tiengo, J., De Lima Neto, P. and Avaca, L. A., The influence of H-absorption on the cathodic response of high area nickel electrodes in alkaline solutions. Electrochim. Acta 1994.39, 1757. 4. Harang, H., Electrolyte cell active cathode with low overvoltage. Patent, Norsk Hydro, Oslo. 5. Vandenborre, H., Vermeiren, Ph. and Leysen, R., Hydrogen

HYDROGEN evolution

REACTION

at nickel sulphide cathodes in alkaline medium.

Electrochim. Acta 1984,29,291. 6. Paseka, I., Sorption of hydrogen and kinetics of hydrogen evolution on amorphous Ni-S, electrodes. Electrochim. Acta 1993,38, 2449. 7. Paseka, I., Evolution of hydrogen and its sorption on remarkable active amorphous smooth Ni-P(x) electrodes. Electrochim. Acta 1995,40, 1633.

ON METALS

613

8. Hurlen, T., Anodic behaviour of iron in alkaline solutions. Electrochim. Acta 1963, 8, 609. 9. Parsons, R., In Handbook of Electrochemical Constants.Butterworths Scientific Publications, 1959. 10. Machado, S. A. S. and .4vaca, L. A., The hydrogen evolution reaction on nickel surfaces stabilized by H-absorption. Electrochim. Acta 1994,39, 1385.