Electrocatalytic properties of nickel and nickel-based alloys

Materials

Chemistry

and Physics,

22

(I 989) 12I ~ 148

121

ELECTROCATALXPIC PROPERTIES OF NICKEL AND NICKEL-BASED ALLOYS

A.G.l?SHENICHNIKOV A.N.Frumkin Institute of Electrochemistry, Academy of Sciences of the USSR, Moscow (USSR)

ABSTRACT The electrocatalytic properties of nickel electrodes are considered. The hydrogen and oxygen adsorption isotherms for nickel in alkaline electrolytes are described. The properties of the oxide layers on the surface of the nickel electrode are elucidated. The results of investigations of the processes with hydrogen and organic substances participating are discussed. A method of measuring the surface of metallic nickel in compact and dispersed catalysts directly in electrolyte solution is suggested. The hydrogen and oxygen adsorption isotherms for dispersed and compact nickel obtained by a combination of vacuum-electrochemical, ellipsometric and coulometric methods are described. It is shown that at ERRE= 0 V, 0~~0.75-0.80, Q. ~0; at E = 0.2 V, &$O, Qo~O.l. The properties of two types of oxide phases formed on the nickel surface during oxidation with oxygen and anodic polarieation are established. The developed concepts of the properties of nickel catalysts are used for interpretation of the results obtained in the investigation of electrooxidation of alcohols, electrohydrogenation of unsaturated hydrocarbons and hydrogen 0254-0584/89/$3.50

0 Elsevier Sequoia/Printed

in The Netherlands

122

evolution. Characteristics of hydrogen cathodes with surface skeleton catalysts based on nickel and its alloys are given. INTRODUCTION Nickel may be considered as a nonconsumable electrode (electrocatalyst) in solutions of pH>V.

But also in these solutions, in

a certain potential range nickel can undergo marked dissolution to form nickelite ions [I> Development of electrocatalysts for processes of practical importance involving hydrogen, oxygen and organic substances is based on the possibility of using nickel, in some cases, as a nonconsumable electrode. Electrodes based on nickel and its alloys are, or can be, used in such electrochemical devices as hydrogen-oxygen cells, fuel cells, cells for production of chlorine and alkalis, apparatus for electrosynthesis, electrochemical sensors, etc. As a result of the studies on nickel catalysts carried out in the past IO-15 years, it has become possible to consider, on the basis of a unified concept, the properties of compact and dispersed nickel, to offer a correct interpretation of the charging curves by comparing hydrogen and oxygen adsorptions from electrolyte solution and from the gas phase,and to elucidate the role of adsorbed solvent (water) in electrocatalytic processes.

TYPES NICKEL

OF NICKEL

CATALYSTS.

IN ELECTROLYTE

DETERMINATION

OF THE SPECIFIC

SURFACE

OF

SOLUTION

Both compact metals and dispersed catalysts can be used as electrodes. Low-porosity films deposited by evaporation under vacuum should be also grouped with compact metals. Dispersed nickel can be divided into three groupes of materials: those containing no additions, skeleton nickel catalysts and nickel catalysts on carriers.

Studies on the properties of nickel catalysts and hydrogen adsorption from the gas phase [2-61 suggest that there exist several (at least two) forms of hydrogen bond with the surface, which show up in different dependences of the work function and electrical resistance on coverage (smooth nickel), and also in the presence of well-defined peaks on the desorption rate vs - temperature curve (dispersed nickel). It is now an established fact (also for skeleton catalysts) that at pressures below atmospheric pressure, hydrogen does not undergo absorption in the metal bulk but is all present on the surface At the moment of adsorption molecular hydrogen dissociates readily [41 and is present on the surface mainly as atomic hydrogen. Hydrogen adsorption is described by Ternkin'sisotherm[6,8-II]. A smooth adsorption isotherm indicates that there is overlapping of the bond energies of different forms of adsorbed hydrogen. At pressures of several thousand atmospheres H2 dissolves in nickel [12J Similar conditions can be realised by high cathodic polarization of the nickel electrode in the presence of recombination inhibitors [13]. Taking account of hydrogen dissolution in the nickel bulk is important for interpretation of the charging curves after cathodic polarization of the electrode. It has been found that in the case of cathodic polarization the shift of the electrode exposure potential in the cathodic direction and increase of the exposure time lead to increased area under the anodic potentiodynamic charging curve, which is due to removal of adsorbed hydrogen [I4& To ensure the reproducibility of the nickel electrode surface a three-step pretreatment is suggested, consisting of cathodic reduction of the electrode at Em-O.2

V to reduce surface

oxides (I step), to abstract hydrogen dissolved in the metal bulk

124

at E-O.l5-0.20 V (down to the values of the current close to zero) (II step) and the electrode exposure at the required potential in the range of 0.0-0.5 V (III step). The necessary condition for the accuracy and comparability of measurements is the possibility of determing the true catalyst surface directly in solution. The specific surface (determined by the BET method and from oxygen adsorption from the gas phase at 2O'C) of different dispersed nickel specimens is compared with the amount of electricity expended in the section of the anodic charging curve in the potential range of 50-170 mV obtained by current interruption at each point after imparting to it a certain charge (Table I> [7]. Account was taken in measurements of the fact that oxygen is chemisorbed only on the surface free of oxide, in the amount of one O2 molecule per 1 nickel atom. The same recalculation constant was obtained by comparing the data on oxygen chemisorption from the gas phase with the amount of electricity under the potentiodynamic charging curve of a compact wire electrode [14-]. determines the nickel surface, while the difference, 02 'BET - '02 = 'ox' is the specific surface of the oxides. The nickel surface in alkaline solution can be determined by

Thus S

the formula: c //W ’

/cm2/ =

(1) 1120

In measuring the galvanostatic charging curves without current interruption, the anodic curve becomes nonlinear, whereas the cathodic curve measured immediately after switching off the current at E = 0.170 V is linear [15-16) It has been found that the slope of the anodic branch of the charging curve at E = 0.1 V is equal to that of the cathodic branch and to the mean integral

125

capacity in the potential range of 50-170 mV. Thus the pseudocapacity, C, in eqn.(l> should be understood to mean the pseudocapacity at E = 0.1 V on the anodic branch, the capacity equal to it on the linear cathodic branch, or the mean capacity in the range of 50-170 mV.

Table

I. Specific

Material

surface

of different

catalysts.

qs = Method of surface measurement Sax= BET from chemi- from sorption charging SBET- &/So* of 02 curve sO* 0.05'EL0.17V SBBT m2/g So m2/g @/cm* Coul/g m2, 2

nickel car bony1 powder the same, heating at 400°C (6 hr) 50% skel. nickel + 50% carb. nickel: per Ig.mix the same per Ig skel. catalyst pressed and sintered electrode from skel. nickel the same from 5% skel.+5% carb.nick.

0.45

52

0

0.34

0.32

39

0

45

8.8

102

36

1352

pseudocapacity CS JR cm2

1120

10

90

17.6

145

72.4

90

17.6

143

72.4

45

8.8

110

36.2

Regardless of the method of plotting the charging curve, in the range of 30-180 mV the plotting time should be no less than 15 minutes. This corresponds to the sweep rate of 0.2 mV/s (or current density of 0.2pA_/cmZ true surface under galvanostatic

126

conditions). With increasing sweep rate, due to the slowness of processes at a nickel electrode, the pseudocapacity decreases. Therefore, with high sweep rates one can not obtain accurate results. Thus at the sweep rate of IO mV/s the surface will be understated by a factor of 7. INTERPRETATION OF THE CHARGING CURVE. HYDROGEN AND OXYGEN ADSORPTION ISOTHERMS Interpretation of the charging curve in the case of nickel presents particular difficulties. As we know, for platinum metals the charging curve has well-defined hydrogen and oxygen sections, separated by the double layer region. In the case of nickel, due to the slowness of the adsorbed hydrogen oxidation and oxygen deposition, the hydrogen and oxygen sections overlap and the charging curve has no well-defined regions (Fig.1). The hydrogen and oxygen adsorption regions on nickel can be separated using a method permitting determination of the adsorp-

400

E, mV

Fig.1. Dependence of pseudocapacity on potential in alkaline electrolyte. I-Pt, 2- Ni, 2a- hydrogen region, 2b- oxygen region.

127

tion of one of these substances. For dispersed nickel this is the vacuum-electrochemical method. By this method it is possible to adsorb hydrogen directly from the gas phase, and by measuring the potential of the electrode immersed in the solution to plot the 'absolute' (in

contrast to electrochemical method) charging

curve [771 The adsorption isotherm from the gas phase (the pressures are recalculated in terms of potentials according to the Nernst equation) is given in Fig.2, curve 1.

%Wx 'H ,' 1.0

0.5

0 0.2

0.4

E, V Hydrogen (1,2) and oxygen (3) adsorption isotherms. Fig.2. x- dispersed nickel, I- gas phase, 2,3- electrolyte. x 0 o- compact nickel.

The inhomogeneity factor, f, of the isotherm is close to 14-15. Immersing the electrode in the solution (a new electrode being used at each point) we obtained the hydrogen adsorption isotherm in electrolyte (curve 2). The surface coverage with hydrogen at E = 0 V is 0.75-0.80 and is close

to zero at E = 0.2 V.

In the case of a compact nickel electrode it is possible to determine the oxygen adsorption isotherm by the ellipsometric method p7]

128

Considering the position of the hydrogen region on the charg3ng curve (isotherm) to be the same as for dispersed nickel (Fig.2, curve 2) we can assume that in the potential range of 0.2-0.5 V the whole amount of electricity is expended in oxygen deposition Ni + x OH-srNi(OH)x + e-

(2)

In this potential range the ellipsometric parameter,n, which characterizes the oxide amount on the electrode surface is proportional to the amount of electricity qo:

bA= &As,

(3)

Oc = 0.16 min.cm2/&Coul

Assuming eqn.(3) to be valid for more negative potentials, we obtained the qo(E) dependence for Ec0.2 V. Next from the difference d-q,, where d

is the total amount of electricity on the

charging curve with the condition GS25

- q, I 0 (making curves

coincident at Es0.25) we obtained the dependence of the amount of electricity corresponding to the hydrogen adsorption on potential (adsorption isotherm): (4)

The hydrogen adsorption isotherm determined by the ellipsometric method for compact nickel corresponds to that for dispersed nickel-(curve 2, Fig.2). Displacement of the hydrogen adsorption isotherm in the negative direction (along the potential axis) when passing from the gas phase to solution and decrease of the i~omo~eneity factor from 'I5to 7.5 are due to adsorption of water dipoles turned with their negative ends to the electrode, whose overall dipole moment depends on potential [8].

129

ELECTROCATALYTIC PROPERTIES OF THE SURFACE The properties of the surface of a metal (in this case nickel) electrode depend strongly on the presence on the surface of 'foreign' substances firmly bound to the metal. In aqueous solutions the fa phase oxide layer formation is of particular interest. The stoichiometric parameter X in eqn.(2) at E = 0.5 V becomes equal to 2. If we assume the adsorption layer to correspond to the stoichiometric formula Ni(OH)2, then at E = 0.5 V, Qox = 1 (Fig.2). When the electrode is exposed at EL-O,5 V and then reduced at 0 V, the adsorption layer is reversibly formed and removed from the surface [17] A marked strengthening of the oxide layer and its change into a phase oxide occurs at E71.0 the potential range 0.5~EL1.35

V. In

V the ellipsometric parameter A

increases BSj. If we assume eqn.(3) to be valid also for this potential range, then we shall have, from the ellipsometric data, that at E = 1.35 V the film thickness will increase to 3 monolayers at constant film composition. As E approaches to 1.35 V, the change in the ellipsometric parameters slows down greatly, and formation of a strong oxide layer (a 3-layer phase oxide Ni(OH)2) occurs. This layer is not reduced on electrode exposure at E = 0 V. One can draw similar conclusions from coulometric measurements but taking into consideration the fact that nickel can dissolve in 0.1 N KOH at E70.7

V b8].

When the electrode is oxidized at a certain anodic potential (e.g. at 1 V) and then reduced at different Ered, oxides with different properties are formed (Fig.3). The potentiodynamic curve obtained after electrode exposure at Ered = -0.05 V (curve 2) does not differ from the 'standard' curve 1 of the electrode without oxides on its surface. In the oxygen region, however, (a peak at E = 0.5 V) it lies lower. Curve 2 characterizes an

130

oxide of ty-peOX 1 on which hydrogen is adsorbed in the same manner as on pure nickel. This type of oxide was also found to be present on dispersed nickel b9] In this case the exchange current of hydrogen ionization on the surface covered with OX 1 was shown to be several times less than on an uncovered surface.

0.50

0

E,

1.00

V

Fig.3. Quasiequilibrium potentiodynamic curves, v = 0.2 mV/s: l- 'standard' curve; 2,5- control curves after electrode oxidation at E -1.1 V and reduction at Ered = -0.05 (2) and + 0.01 V(3)9”

Curve

3 of Fig.3 (Ered= +O.Ol V) characterizes a blocking oxide

(OX 21, which decreases the active surface of nickel. If we consider the blocking oxide as consisting of two OX 1 layers then taking into account the obvious relation (after electrode reduction at Ered) eox 2 + 0ox

1

+

(5)

QM = 1

where QM is the free metal surface, we can find the coverages 0ox 1 and Box 2: 0ox 2 = (#

eox 1

where

= 0.5

- QH>/ q - Q) 8

(6) - 5 eox 2

I

G = q,,and QH are the areas of the hydrogen sections of

131

the standard and control potentiodynamic curves, respectively: Q,,and Q are the total areas under the standard and control curves (Fig.5). Table II lists the results of calculation of %,

80X ,,

and 80X 2 for the conditions of Fig.3.

Table

II. State

treatment

of the nickel

0.5-0.7 1.0

1.0

surface

depending

on the

conditions.

Treatment conditions EOX' v

electrode

E

red, V

State of the surface *M

Qox 1

@ox 2

0.01

1.00

0.01

0.17

0.62

0.21

-0.05

0.55

0.45

0.00

-

The specific surface (total SBET and metallic surface SM) (Table I) can be considered as characteristic of the properties of the dispersed catalyst surface. The amount of oxides of the type of OX 1 can be estimated as shown above, or evaluated from the exchange current value with respect to the hydrogen ionization reaction. Measurement of the exchange current in the case of dispersed catalysts presents

certain

difficulties

since

it is not

easy to ensure equal accessibility of the surface. These difficulties can be overcome, however, through measuring thin catalyst layers f'l53The specific surfaces and the exchange currents of some catalysts are listed in Table III [16J As it follows from the Table III, the effective electrochemical activity of the catalyst i,.SM does not always correspond to catalysts with large specific surface.

132 Table III. Influence of the method of catalyst preparation on the specific surface and exchange current.

N=

type of catalyst

specific surface m2/g total,SRRT metallit, SM

exchange current

io.SM

ioXIOb, A/cm2

1

carbonyl nickel powder

0.80

0.78

3-8

30

2

the same as 1, heated at 450°C

0.40

0.38

3-8

15

3

skeleton nickel from Ni-Al alloy

90

65

0.9

58

4

the same as 3 air-oxidized

90

25

0.4-0.6

12

5

the same as 4, heated at 600°C in H2

90

19

3.6

68

PROPERTIES OF THE AQUEOUS LAYER ADJACENT TO THE ELECTRODE. ELECTROOXIDATION OF ALCOHOLS It is evident from the results described above that in studies on the adsorbability of organic compounds and the electrocatalytic processes with their participation which occur at nickel electrodes, one can use the methods developed for platinum metals. Due to the slowness of the processes involving hydrogen adsorbed on nickel, however, in using pulse methods, one has to make corrections for the amount of electricity expended in the oxidation (hydrogenation) of the organic substance approaching the electrode surface. It was demonstrated by the charging curves method [20-221 that some simple organic compounds (methanol, ethanol, ethylene) are

133

virtually not adsorbed on a nickel electrode. On the other hand, it is known that hydrocarbons are destructively adsorbed on nickel from the gas phase [231 This difference is supposed to be due to the presence in aqueous solutions of an aqueous layer adjacent to the electrode, which reduces the rate of hydrogen adsorption and inhibits the adsorption of a number of organic substances [24-23.Adsorption of organic substance (methanol) can occur only at a very small water content [2& On the other hand, it is shown that the catalytic activity of dehydrogenation in solution and of electrochemical oxidation of alcohols is associated with the activation of water molecules in the layer adjacent to the electrode k7-2&

the water acquiring

electron-acceptor (oxidizing) properties. The presence of electron-acceptor properties accounts for the possibility of detecting the photoemission of electrons from a nickel electrode into electrolyte without introducing into solution a special electronacceptor [293. Taking into account the possibility of an interaction of an alcohol molecule (corresponding anion) with the electrode surface and with the active water in the layer adjacent to the electrode (H20),, one can consider two paths of the alcohol oxidation reaction [28](schemeof Fig.4)

H20+ne-

H20+e-

b

nOHnH

Fig.4. Scheme of hydrogenation and electrooxidation of aliphatic alcohols on skeleton nickel catalyst in alkaline electrolyte.

134

Path 1 (interaction with electrode surface) can be written as (high electrolyte concentration): H

M + R3_nCHnO- -

n H ads + R;_nCO-

(8)

where R,,,R2, R 3-n are radicals of the fatty acid series; n* R3-nCo- is the n-fold anion radical. A characteristic feature of reaction (8) is the abstraction of hydrogen atoms in d-position. Then the anion radical interacts with water RFnCO-

+ (n+l)HOH -R+nC(oH)n+q

(9)

+ OH- + $32

which is followed by dehydration of the hydroxylated intermediate H20 + R2C=O

(n=l)

H20 + R-CC'OR

(n=2)

2H20 + CO2

(n=3)

I-

t

1

(10)

Adsorbed hydrogen formed by reaction (8) under anodic polarization can be oxidized on the electrode

n Hads

+ n OH--n

H20 + e-

(11)

Thus, the nature of the final products of electrochemical oxidation of aliphatic alcohols is determined by the number, n , of hydrogen atoms in the d -position. At n = 1, ketones are formed; at n = 2, acids; and at n = 3, carbon dioxide. For n = 2 and n = 3 this conclusion has been confirmed experimentally: methanol oxidation yields CO2 [3C$ The final product of ethylene glycol oxidation (n = 2x2) is oxalic acid [31+ Following path II, alcohol (anion) interacts with active water in the layer adjacent to the electrode. For example, (%')a

+CHOH3

3 H2 + CO2

(12)

135

Under the conditions hindering the removal of molecular hydrogen formed by reactions (9),(12)

(porous electrode, anodic po-

tential leading to decrease of Q..and to increase of the rate of destructive

d -hydrogen abstraction) it diffuses to the elec-

trode and becomes oxidized. The current along path I is made up of two parallel reactions (8)-(11) and (9>-(II)

(the currents iq and i,), while i

3

is

the

current along path II. The total current, i , is i

‘I,

=i 1 +i 2 +i 3 = l'i? i2 i3 > = 1 + 92, + Y3q + -q =

(I +-7

When molecular hydrogen evolution is completely excluded, which can be easily detected experimentally., v21=1, the fraction of alcohol molecules reacting with active water g3,, = 3 - 2, 9 = i/i,, is determined experimentally: i is the steady current at a given potential, i? is the de~~ogenation

current (reac-

tion (8) is slow), determined from the potential decay curve after current interruption. For 3 N KOH + 12 M CH3OH at 70°C v-2, i.e. the contribution of the process following path II at E70.03

V is negligible (Fig.4)

10

20

30

40

IO

20

30E

4omv

Fig.5. Comparison of the rate of the steady-state processes ('I> with the rate of the dehydrogenation step (2) in electrooxidation of methanol (a) and ethylene glycol (b). 7O"C, alcohol concentration 12 M.

136

The slow step of the overall process of electrooxidation of alcohols on nickel is de~~ogenat~on

(81, described by the kine-

tic equation p4j: (14) where n is the number of hydrogen atoms abstracted sirn~~o~s~~ and Kdeh is the dehydrogenation rate constant referred to one hydrogen atom. It has been found for a group of alcohols of different structure (methanol, ethanol, ethylene glycol, propanol, iso-propanol, glycerol) that it is the hydrogen atoms in the d-position

that undergo dehydrogenation, and Kdeh to the first

approximation does not depend on the structure of the alcohol molecule, The dehydrogenation mechanism does not change in the temperature range of 30-90°C. It should be noted that with increasing specific metallic surface of skeleton nickel catalysts, the dehydrogenation constant Kdeh increases: Kdehwconst

+ Sit where n

7

1.

This leads to

a drastic increase of the effective activity of catalyst (current density at a fixed potential) Kef = Kdeha Sg~const. Sy

[32.

Sokolrskij et al$3,34]established a close interrelationship between the liquid-phase and electrocatalytic hydrogenation processes. A common factor for these systems is the presence of the heterogeneous system electrode (catalyst) - solution. They showed that it is possible in principle to control the direction and kinetics of a multi-step hydrogenation process by maintaining a certain catalyst potential. As was pointed out previously, due to the presence of a blocking adsorbed water layer on the nickel electrode surface a number of organic substances, including ethylene, are not chemisorbed

137

on it. In this case addition of the hydrogen atom to organic substance follows the so-called 'striking' mechanism involving the interaction of the molecule of the substance undergoing hydrogenation with the adsorbed hydrogen atom. The general scheme of the process in the model system Ni + H2 + C2H4 can be represented as follows [35].

(15)

Roman numerals in the scheme (15)

denote the overall processes:

I - electrohydrogenation; II - liquid-phase - catalytic hydrogenation; III - hydrogen discharge-ionization; arabic numerals denote individual steps of overall processes: I) H2-

2 H

ads 2) Hads + OH-3) 2 Hads + C$$---

(hydrogen adsorption) H20 + e- (hydrogen electrooxidation)

(16)

C2H6

(hydrogenation)

(17)

It follows from scheme (15)

(18)

that all overall processes include

steps involving adsorbed hydrogen. Under the conditions when the potential determining reaction (step 2) is in equilibrium, the surface coverage with hydrogen is unambiguously determined by the potential in keeping with the equation of the hydrogen adsorption isotherm on nickel in alkaline electrolyt;e(Fig.2). (19) The kinetic equation for the process of the type 'IBias for a uniformly inhomogeneous surface can be written as [36]

(20)

(21)

where V is the process rate, K is a constant, a, is the adsorption coefficient, P is the equilibrium pressure of substance A, P is the partial pressure of substances B, 3 ichiometric coefficients of the reaction,&

and r are the stois the transfer co-

efficient sop = exp (f.49,)is the adsorption isotherm For the step 3 (eq.17): A = Hads, B = C2H4, B1 = C2H6,$ r= v3

(22) = 2,

1 and equation (21) assumes the form: = K3 PC2B2 . exp

'HI -I

(23)

For the hydrogen adsorption step (1) we have: v1 = Kl . PH2 . exp (-d. f

%I)

(24)

As it follows from [21], the steady-state process of electrochemical ethylene hydrogenation at an equally accessible electrode is limited by step (3) and described by the kinetic equation: lg v3 =a+bE

(25)

The set of equations (19), (23), (2%) and (25) describesquantitatively all the processes in scheme (15). If the limiting step of the overall liquid-phase-catalytic hydrogenation process is the hydrogen adsorption process (the ratio (PC2H4/ PH2 is large), the steady-state potential Eh that becomes established in the course of hydrogenation, is more positive than the equilibrium potential calculated by the Nernst equation from the value of PH . Taking into account the eq:ality of the rates of steps (I) and (31, using equations (23)-(25) we have

139

0

= 2/3 (lg K.,- a + 71 Eh

pH + 213 J-g e

(26)

From the experimental dependence d Eh, lg-

%2

(Fig.6) we can

pC2H4

determine the constant Kq.

, mv

AEh 60 40 20

0.4

1.2 - lg(P /PF> H2

Fig-G. Dependence of the steady-state hydrogenation potential Gh) on lg
The exchange current of hydrogen adsorption calculated from the obtained data is equal [2d to 1.4~10~~ A.cm-2 . In literature the value of the deviation of the steady-state potential of the electrode (catalyst) from the equilibrium hydrogen electrode potential in the same solution ( AE~)

is used as

a characteristic of the liquid-phase catalytic hydrogenation process and the dependence Eh, lg Wh (Nh is the hydrogenation sate) [24,27] is considered. Two limiting cases determine the slopes of a AEON’dWh [24$ If the step of the addition of hydrogen to ethylene (step 3) is

the

limiting one, aaEh/dWh

= 0 and the

potential is determined by the partial hydrogen pressure. If hydrogen adsorption (step I) is the limiting step

i3AEh

( a lgwh

I

)P = II2

0.43 (I -d)

f.B

(27)

140

and

1 043 .

nEh=A+

(1 -d)

-/9

lg

(28)

('h)PR_ =const L

where A = AE~ at WP =const =I (in corresponding units). *2 Formally this equation is similar to the Tafel equation and the quantity A characterizes the electrocatalytic activity of the catalyst. Thus in accordance with scheme (15), the kinetics of liquidphase hydrogenation can be examined with the use of the data for electrohydrogenation and hydrogen discharge-ionization processes. And ace

versa, the data on the kinetics of liquid-phase cataly-

tic hydrogenation can be employed for investigating the specific features of electrochemical processes.

HYDROGEN CATHODES FOR ALKALIRE CELLS Electrodes on the basis of nickel and its alloys (e.g. with a surface skeleton catalyst @SC) layer [38] are considered to be most promising for alkaline cells. Steel electrodes with a nickel coating can be used as cathodes. A possible scheme of the hydrogen evolution reaction is 2 H20 + 2 e- -2

Hads + 2 OH-

(b)

2 *ads -H2 H

ads

+ H20 + e- -

2H20+2

e- -

(a)

H2 + OHH2 + 2 OH-

(29) (c) (overall reaction)

The sequence of Volmer-Tafel reactions (29 a)-(29

b) is unlikely,

especially at high cathodic potentials (Es0.05 V), since in this case, as was pointed out earlier, 0H = 1. More likely is the sequence of Volmer-HeyrovsQ reactions (29 a)-(29 c) which leads to the overall reaction (29).

141

The

polarization characteristic of cathode under polari.ZatiOn

(E70.03 V) can be written as *K = b lg lo*

(30)

s=. yl

where i, is the exchange current density (calculated on the basis of true surface) of reactions (29 a> or (29 e), S is the specific surface of catalyst per 1 cm2 visible electrode surface, 'ly is the utilization factor of catalyst determined by the relation of the characteristic length of the psocess, to the active layer thickness of the surface skeleton catalyst. The characteristic length of the process is determined by the diffusion and ohmic limitations. The value of the Tafel slope 'b' depends on the reaction mechanism and parameter Y.

As was pointed out previously,

the exchange current can depend on the method of catalyst preparation and its dispersion. Thus the parameters i,, S and b are interrelated to a certain degree, and the polarization minimum AE

depends in a complicated manner on the catalyst properties

and the catalytic layer structure, One of the probable reasons for the decrease of the characteristic process length is the blocking of pores by the evolving gas bubbles. But by an appropriate structure of the porous layer, the presence both of large and fine pores, it is possible to avoid the blocking of pores [38]. This is explained by the fact that in fine pores bubbles are formed at high supersaturations. In

large pores bubbles are formed

at small supersaturations. Large pores thus act as gas channels and in fine pores dissolved gas diffuses to large pores. In porous layers, therefore, which have an optimum structure, the characteristic process length I, is determined by the decrease of local polarization across the layer due to ohmic losses (and not owing to diffusion limitations). At 70°C in 7 N KOH this characteristic length is equal to about 100 urn.In the electrode

142

layers formed, for example, on the wire of grid electrodes at the layer thickness less than IOOym.

yy-1 and the whole surface of

the catalyst acts under kinetic conditions. In this case for the sequence of reactions (29 a)-(29 c) taking into consideration the condition 0R

1, we can write [Yj:

(2F/RT)E . e (l;T+ I iE/iz + e (F/RT)E

(311

where ia and i0" are the exchange currents of reactions (29 a) 0 and (29 c). At high polarization values e (2F/RT)E<< ,,and e(F/RT)Eeia /ib 0

0'

Li

e

for large negative A E

(32)

IC

At not very negative E, but at E < - 0.025 V and under the condition ii4 iz, we can assume that e (2F/RT)Ee,, and e(F/RT)E,ia o /iz. We shall have:

32$ZE (33)

Ia = 2.i: . e

Figure

7

shows

the

polarization

curves

for

electrodes

with

diffe-

rent S values. When the current density i, is recalculated per I cm2 true surface, the curves coincide (y

= I>, i.e.

the

Cmrent

density is proportional to the surface S [39,4C$ The process characteristics are listed in Table IV. Enhanced cathode activity can be achieved by choosing suitable catalysts, by increase of S (to I = I,), temperature increase, structure refinement j41-443.

143

For materials readily adsorbing hydrogen the sequence of reactions (29 a)-(29 c) is characterized by an opposite effect of ,

the bond energy IVRe_K,on the exchange currents of the steps

(29 aF-G!Y c> [45]: aiz/aW'

E,

0

7

and

ai;/aB

< 0

I

v

0.12

0.07 0.02

I

2

Fig.7. Polarization curves of hydrogen evolution for electrodes with surface skelet2n c2talysts and spec. surface, S per unit visible surface (cm /cm ) l-2000; 2-4000; j5-5000;4-6000; '7N KOH, 7OV. Table IV. Kinetic parameters of the hydrogen evolution reaction at electrodes with surface skeleton catalyst (7 N KOH).

conditions

204C

213

OS06

2

0*37

70°C

2/3

0.25

2

2.5

% b = (~j$$-+/(~

) = the dtiensionaless Tafel slope

To make the first section of the Tafel plot, which has a lesser 'b' value, longer it would be advisable to use additions reducing the hydrogen adsorption energy (Cd, Fe, Co, S etc.)

(Fig,8).

0.10

0.05

0

I

c

1

1

2

f

3 Ig i, mA/cm2

Fig.8. Polarization characteristics of hydrogen cathodes VS active layer composition, 7 N HOH, 70°C: I- nickel (without active layer); ;I-5Ikle-SSC; Bbe: 2- Ni; 3- Ni-Fe; 4- Ni-Co; 5- Ni-S.

CONCZUSION The problem of the state of the nickel electrode surface in electrolyte

solution has been considered. The methods of deter-

mining the hydrogen and oxygen adsorption isotherms on nickel in electrolyte have been discussed. It has been shown that to the first approximation the isotherms describe the adsorption processes on a

uniformly in homogeneous surface (Ternkin'sisotherm).

It has been established that the aqueous adsorption layer has a blocking effect, decreasing (as compared to the gas phase) the hydrogen adsorption rate and hindering the adsorption of some organic substances (hydrocarbons, alcohols). On the other hand, the water molecules adsorbed on nickel exhibit electron acceptor (oxidizing) properties and participate actively in the oxidation process of alcohols. The kinetic features of the process of electrooxidation of alcohols, electrohydrogenation of ethylene

145

and hydrogen evolution have been established. The processes of liquid-phase-catalytic and electrocatalytic hydrogenation of hydrocarbons have been considered from a

unified point of view. It

has been found that some of the steps of these processes are common to all of them and this permits the data of electrochemical meas~ement

to be used for investigation of the liquid-phase pro-

cesses and vice versa. It has been shown that the deviation of the potential from that of the equilibrium hydrogen electrode during liquid-phase hydrogenation processes is due to the slowness of hydrogen adsorption. The shift of the potential during liquid-phase hydrogenation of ethyl.eneon dispersed nickel has been explained and a corresponding calculation has been made. It has been demonstrated that the hydrogen evolution reaction on nickel occurs under the conditions of complete coverage of the electrode surface with adsorbed hydrogen. For this reason the overall process can be represented as two consecutive steps: discharge and electrochemical desorption. The exchange currents of individual steps have been estimated per unit true surface of nickel. Some p0ssibl.eways of enhancing the exchange current of the electrochemical desorption reaction and increasing the length of the first Tafel section with the slope of 40 mV have been considered. The characteristics of different hydrogen electrodes activated by surface catalysts based on nickel and its compounds are given. REFEILENCES 1

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