105 ,\lu Jials Cliemistry and Physics, 22 (1 989) I O S 1 19‘

CHLOR-ALKALI ACTIVATED CATHODES

t : . TACCAUD, F. LEROUX and J.C. MILLET A-kdc’lem Research Center, 69310 Pierre-B6nite (France)

new activated cathode for chlor-alkali electrolysis is des- ld. It is a composite coatlng consisting in a first Ru02 ther-

ma- layer, on a nickel support, modified by a second layer made of elcciroless nickel. This electrocatalyst permits a 2 5 0 mV saving in ‘lydrogen overpotential compared to smooth nickel : this impro- vement is quite stable for a ldng period of time in membrane cell Conditions and is not affected by ironimpurity in the catholyte. Clcctrochemical kinetic study ,reveals a Ru02- like behavior with additional surface area effect.

INTRODUCTION In chlor-alkali diaphragm cells, hydrogen evolution occurs on

1;llld steel cathodes. With the development of the new membrane tech-

fiology, higher caustic strength imposes the use of nickel cathodes. These two materials exhibit high hydrogen overpotential ( s 300

mv at 2 - 3 kA.m-2) ; a saving of close to 10 % in electrical ener-

9Y Could be achieved with a proper choice of cathodic catalysts. The aim of this paper is to describe a new proprietary coating

which is now at a pilot level in an Atochem plant. We will give, at first, the general requisites for a good cata-

1YStcandidateond abrief review of the known technology in that field ;

ntochem activated cathodes. After the description of its behavior then we will emphasize on the preparation process of

electrolysis, we will tentatively analyse the electrochemical kinetic parameters of this material.

0254-05 84/89/$3 SO 0 Elscvicr Sequoia/Printed in The Netherhnds

- - . 106

What is a good industrial electrocatalyst ?

Two main properties must be assigned to a catalyst : activity and stability : these qualities must be attained at a reasonable cost both in investment and manufacture.

An ideal set of properties may be the following :

- low hydrogen overvoltage at industrial current density, - no potential drift with time, - good chemical and electrochemical stability : long lifetime and no release of process-deleterious products,

- high adhesion to the support, - low sensitivity to poisoning by impurities, - low sensitivity to current shut down (short-circuit) or modulation, - no safety or environmental problems in the manufacture process, -

- easy to prepare at a low cost/life time ratio. One understands why there are many candidates at the laborato-

ry scale and very few elected at the industrial one.

Review -of the known technoloqy for activated cathodes. This is only a brief survey'of the known cathode activation

processes. It has been established that the hydrogen evolution reac- tion (HER) is a surface sensitive reaction like the oxygen evolu- tion reaction (OER). That is to sayl surface area, surface struc- ture and texture are important parameters. It is the reason why there are two main classes of activated cathodes : high area me- tallic coated and pure electrocatalytic coated.

The high area cathode is generally of the Raney nickel type : it is not the result of catalysis but o'nly of the considerable de- crease of the true current density : the process is Ni-Zn, Ni-A1, Ni-Cd electroplating, followed by chemical lixiviation.

Among the numerous catalysts described in the patent literatu- re the most frequently encountered are the followinq : - non precious metal oxides (NiO, Cr203.. . ) rl, 21 I

- precious metals and metal oxides (Pt, Ru02)- (3, 4, 51 , - association of non precious and precious metals (Ni-Ru, Ni-Ir ... )

; s f 3 RuOZ.. . ) [8, 91 .

- association of non precious metal and precious metal oxides (Ni-

The support is very often nickel sheet. It is usually advanta- geous to obtain a catalyst with a high surface area (10, 111 .

One gene : lyst with , -jood cata

3CHEM NEW ~~~~roductio~i The new ( (

zte and tal deriv<< ;i electroll

r -,isisting I I

T:h the su 2 preciou .*e and wi

r ier. The kh : the prep)

1 the cathl kAm-2 an1

) the effe,

1 Zparationi A surfac,

corundum fo> The deposit lution of 2' of rutheniL at 23OC). 1 air at 500' ving in ail A 0 , 5 5 mg/c For the co - solution 1 g PdC12 - solution - solution ( m4 ) 2 ~ ~ 4 by weight

talyst : activity at a reasonable -

ing : density,

long lifetime and

rcuit) or modulation ufacture process,

s at the laborato- one.

hodes. ode activation 3gen evolution reac- the oxygen evolu- 3, surface struc- the reason why : high area me-

ney nickel type : he considerable de- s is Ni-Zn, Ni-Al, viation. e patent literatu- owing :

- 21 1

2 1 4 1 4 r

Is (Ni-Ru, Ni-Ir ...I

s metal oxides (Ni-

s usually advanta- area [IO, 111 .

- - . _ 107

One generally observes that activated coatings associate a ca- talyst with some sort of cement because it is difficult to find a ood catalyst with a good adhesion to the substrate [121 .

A: CHEM NEW ACTIVATED CATHODE

12 -goduction The new cathode is made up of an electrically conducting subs- e and a coating containing a precious metal and/or a precious 1 derivative. This cathode is characterized by the fact that

ti electroconducting substrate carries a heterogeneous coating <isting of at least two coats g and b_. The coat g in contact I the substrate is made up of at least one compound chosen from precious metal group. The coat in contact with the electro-

1 + and with the coat 2 , is made up of a metal of high covering F. rzr. The experimental work presented and discussed here deals w ‘h :

a ) the preparation of Ru02/electroless Ni coatings, 0 ’ the cathodic behavior in a,33 % NaOH solution at 90°C under 3

kAm-’ and, c I the effect of iron poisoning.

PLeparation of RuO /electroless Ni coatings 2 A surface treatment of the nickel substrate is carried out using

corundum for sand-blasting (equivalent mean bead diameter : 170 vm). The deposition of the precursor of coat g is made by brushing a so- lution of 2 g of RuCl .y H20 containing approximately 38 % by weight of ruthenium metal, in 2 cm3 of ethanol (this solution was prepared at 23°C). The precursor is then converted by a heat treatment in air at 5OO0C durant 30 minutes ; this treatment is preceded by sto- ving in air at 12OoC durant 30 minutes to eliminate the solvent. A 0,55 mg/cm2 Ru02 deposit is obtained. For the coat &, it is necessary to prepare three solutions at 23’C :

- solution A : aqueous solution, containing 4 cm3 of 33 % HC1, and 1 g PdC12 per liter. - solution B : aqueous solution containing 50 g/1 NaH2P02,H20. - solution C : aqueous solution containing 20 g/1 NiS047H20 ; 30 g/1

3 (NH4)2S04 ; 30 g/1 NaH2P02,H20 ; 10 g /1 sodium citrate, 10 cm /1 20 %

by weight NH40H.

3

,

I

. I i

- - . .

108

The Ru02 covered substrate is successively immersed in solution A at ambient temperature for 1 min, in solution B at 3OoC for 1 min and then in 200 cm3 of solution C at 3 0 ° C , for 120 min. A 5,3 mg/cm Ni deposit in the form of a nickel - phosphorus alloy containing less than 15 at % of phosphorus is thus obtained. Visible hydrogen evolution occurs during the treatment.

Nature of the coating Chemical composition For this analysis, the coating is made on a glass substrate.

X-Ray diffraction indicates the presence of both nickel metal and ruthenium oxide. The weight % composition of nickel/phosphorus al- loy obtained by chemical analysis is Ni between 95 and 97 %,

P between 2,2 and 2,6 % .

Surface morphology SEM examination of a standard coated cathode (Ru02/electroless

:

nickel) shows that the surface is covered with nodules of nickel ; 1-2 um wide cracks can be seen through the nickel coating (see Fig. [lj ) .

Fig. 1. SEM examination of a standard coated cathode (RuO2/e1ectro- less nickel).

Cc IODIC BEHAVIOR c yriment under s 8

i % NaOH, 8 5 " C ,

,n expanded nic- el, is used as, membrane elect

'he electrochemi d Jotential and WI a -31 illustrate - 10 /electroless

r -

2 'a1 of - 1240 mP ;.C.E.) during

- gain of 250 mV ithode ,

- re coating weig:: .ter 100 days.

F i g . 2. Cathodic &- @Test 1 + Test 2

immersed in solution n B at 3OoC for 1 min 120 min. A 5,3 mg/cm2

5 alloy containing 2d. Visible hydrogen

glass substrate. :h nickel metal and .ckel/phosphorus al-

le (Ru02/electroless 1 nodules of nickel ; :kel coating (see -

sthode (Ru02/electro-

- - . _ 109

~xp_-.iment under standard membrane cell conditions ( 3 : ; NaOH, 85'C, 3 kAm-2)

2 i expanded nickel disc (0.5 dm ) , coated with RuO2/electro1ess 1, is used as the cathode of a sodium chloride aqueous solu-

ti( membrane electrolysis cell. ':e electrochemical behavior includes performance curves (catho- \tential and weight loss) E. operation time. The Figs r21 de

anr i-3J illustrate the following points : -.-

/electroless nickel operates at a constant electrode poten- 2 1 of - 1240 mV relative to the saturated calomel electrode

j C.E. ) during 2 years, - _ rain of 250 mV is obtained when compared to an uncoated nickel

hode, 2 coating weight loss is stabilized at 15 % (%a. 0 .9 mg/cm )

- .--er 100 days.

t u m t e d Nickel i sa

RuOUelectroless Hi I

mJ I

a ma tine (days) ..

Pig. 2. Cathodic potential (Ec) vs. operating time (t). @Test 1 t. Test 2

B time (days) 888

Fig. 3. Coating weight loss vs. operating time (t). O T e s t 1 + Test 2

\ The effect of iron poisoning

These experiments were carried out in small lab cells (cathode area 0.5 dm 2 ! by adding the iron impurity to the catholyte in the

form of nitrate salt and operating under standard membrane cell conditions. We observed that the voltage is stable during 200 days with the same value measured on a cell without iron addition.

ELECTROCHEMICAL CHARACTERIZATION OF THE COMPOSITE CATHODE-KIPIETIC STUDY

Experimental Method To obtain steady-state curves, a potential-staircase function

(sweep rate < 1 mV/s) is applied to the working electrode by a com- puter-controlled potentiostat. At each step, the potential is stored

I - 1

t

b

t

I

i I

I

i i

2 soon as the m 11 luded in the i that this m

,hly preparec i current d e r

.- irization of o lined during 1 X/cmL (geomett

In all cases,, f ion polarizatt +, electrode SI-

Potentials an ~n from the work+ 1 ' 2 have not k

i .nd to be negU All potential

C '.E. referencE the caustic

2 partial presc 1 Table I. All

'I 51e I. Thermoc -.

% w. NaOH

2 0 30 35 4 0 45

All current I

are apparent va the electrode b study of the hy

at electroless Electrodes Coatings and

cording to the a 1 cm2 geometi-

t M h s tii

b

tab cells (cathode catholyte in the

rd membrane cell )le during 200 days iron addition.

PE CATHODE-KINETIC

iaircase funct Zlectrode by a ? potential is

on com- stored

111 .. . _

as 3oon as the measured current satisfies some stabilization criteria in-:luded in the acquisition program. Tests were carried out to make SL-'? that this method gave equilibrium polarization results. Using fr .;hly prepared electrodes for each curve, a forward sweep in the hi !I current density range is first applied. That corresponds to a pr arization of several minutes. Then, the results presented here are oh,ained during the succeeding backward sweep between 600 mA/cm2 and 1 -,\/cm2 (geometric area).

In all cases, the electrolyte is strongly stirred to avoid dif- fh;ion polarization and to minimize the hold-up of gas bubbles on tkc electrode surface.

Potentials are measured using a Luggin probe positioned about 2 r", from the working electrode to minimize IR drop. Results reported h [ J . e have not been corrected for ohmic drop because this has been fc-:nd to be negligible at all current densities.

All potentials mentioned here are quoted with respect to the S.1.E. reference electrode. Thermoaynamic potentials for the HER V J . the caustic concentration were calculated using activities and H 2 partial pressures upon caustic soda [13] . They are summarized in Table I. All experiments are carried out in NaOH solutions at 85°C.

Table I. Thermodynamic potentialssfor HER NaOH concentration.

% w. NaOH a -1 (NHE)/V Eth (SCE)/V conc . / g . 1 - Eth NaOH

20 6 .1 6 .09 - 0.884 - 1.129 30 18 9.96 - 0.929 - 1.174 35 25 -47 11.25 - 0.951 - 1.196 40 46.42 14.3 - 0.982 - 1.227

~ 45 81.02 16.32 - 1.013 - 1.258

All current densities and exchange current densities reported are apparent values, being based on the geometric surface area of the electrode being studied. The method is applied to a comparative study of the hydrogen evolution reaction (HER) in alkaline solutions

at electroless Nil Ru02 electrodes and at the composite coating. Electrodes Coatings and composite electrodes were prepared on a Ni plate ac-

Cording to the method described previously. All electrodes were of a 1 cm2 geometric area.

- - . _ 112

Cell and auxiliary electrodes The electrochemical cell of about 100 cm3 was made of double-

walled glass, providing thermostatic control of the electrolyte temperature. The counter-electrode consisted of a 3 cm2 Pt sheet. Newly formed H2 was continuously removed from the electrode surface by bubbling an inert gas (N2).

The limiting diffusion currents associated with reduction of re- sidual traces of oxygen or of Pt corrosion products are assumed to be negligible.

Electrolyte Aqueous NaOH solutions (20, 30, 35, 40 and 45 weight % ) were

made up from Normapur R Prolabo NaOH dissolved in the required vo- lume of distilled water. In all experiments, the forward sweep was applied to pre-treat the system.

Measuring and recording apparatus Potentiometer : Tacussel Solea PJT 24/1, Interface : Tacussel Solea INTI, Microcomputer : Victor VPC I1 + Atochem acquisition/calculation

programs.

Results and discussion Ni electrode Examples of the steady-state polarization curves obtained for

HER with an electroless Ni electrode are shown in Fig. [4b] . In Fig. [4aJ , data are reported as a function of the current density logarithm.

Tafel plots for HER are linear over the current density range to 1 A The different kinetic parameters obtained by a

linear regression are presented in the following table :

% NaOH io x 10; A. cm a

b mV

20 30 35 40 45

25.27 90.62 94.84

120.8 167.3

0.5117 139 0.5876 121 0.5616 126 0.5417 131 0.5217 136

The reaction transfer coefficient is unaffected by varying the causticconcentration over the range 30 - 45 % w. i

,w .e -1!'

. j / 2 4 2

'ig. 4a. Plots ' L ( K ) electrodii , and 5 repre"

Fig. 4b. Exper Hydrogen Evolu' ( 3 5 8 K ) ( 1 ) : 2 0

- - .. 113

lectrolyte m2 Pt sheet.--- ctrode surface I duction of re- ,

1 re assumed t

ht % ) were required vo rd sweep wa:

n/calculatior

>tained €or --- I [4b] . In -rent density

sity range ained by a

b mV

139 121 126 131 136

iryiny the

e . A

-1: ‘q * 0 -1550.8 -1400.0 -1258.0 3 -*

r’ y. 4a. Plots of cathodic potentials against log(i) for the HER at ?. ( K ) electrode for the five caustic concentrations (curves 1,2,3, 4, and 5 represent the same data as in fiy.la).

E ;Iv - l!

T I

I I

Fig. 4b. Experimental stationary overpotential-current curves of Hydrogen Evolution Reaction at Ni(K) in aqueous alkaline solutions (358K) (1):20 %w (2):30 %w (3):35 %w ( 4 ) : 4 0 %w ( 5 ) : 4 5 %w NaOH.

On the basis of the values for the Tafel slope ( 2 140 mV/decade at 85OC) and for the reaction order with respect to pH ( 2 0 ) ana- lysis, the predominant mechanism on the Ni electrode appears to be the rate-determining discharge of water (Volmer reaction) fol- lowed by either a chemical or electrochemical desorption step.

R*2 Polarization curves obtained on these type of electrodes are

and composite electrodes

shown in Fig. [5] .

E mV I I

-1390.0 -1260.0

-100

Fig. 5. Plots of cathodic potentials against log(i) for the HER at Ru02electrode in aqueous alkaline solutions (358K) (2) : 30 %w ( 3 ) : 35 %w (4) : 40 %w ( 5 ) 4 5 8 1 . 7 h i m n u

(1) : 20 %W

Ru02 has a better electroactivity for the HER than the composi- te electrode but both exhibit two-slope Tafel relations, characte- ristic of a change of limiting step in a consecutive two-step path- way and/or a change of adsorption behavior of the intermediate

In the hicc I i are closcc

;qe s t s t ha 11 3p in the E In the ran and - 39 rr erefore, cx rature 114- , electrockt In fact pU

on of the opes which 3 . b ' . 7

- 1

Pig. 6a. Log. Of the NaOH electrode.

( 2 140 mV/decade 3 pH ( 2 0 ) ana- le appears to --- reaction) fol- -ption step.

d ectrodes are

t

f o r the HER at j

I I

(1) : 20 %w H.

an the composi- ions, characte- F? two-step path- itermediate

I

- - . _ '!

115

n the high current density range ( > 3 0 0 mA/cm 2 ) the Tafel slo-

De- are close to - 140 mV per decade for both cathodes. This slope s u L jests that discharge of water could be the rate-controlling s t ' i ) in the HER at the two electrode surfaces in this region.

n the range of 10 to 100 mA/cm 2 , the kinetic parameters are - 3 6 2nd - 3 9 mV for, composite and Ru02 electrodes respectively.

efore, considering that Tafel slopes may not change with tem- ture 114-1 , the above results suggest that in this current ran-

-gc, electrochemical H desorption micght control the HER rate.

in fact plots of the logarithm of the current density as a func- I

ti ! of the NaOH activity logarithm for the two electrodes exhibit es which are not typical of one limiting step mechanism. Fig.

6 - 5' .

\ A 0 . 4 4

&- 11 8 Om\

\ x - 1.1 - l\

I PnaOH - A I L 3 4 \

Fig. 6a. Logarithm of the absolute current density as a function Of the NaOH activity logarithm for different HER potentials at Ru02 electrode.

ity as a function of 3en potentials at

I !

orders (dlni/dlna i the potential in

Lectrodes cannot ap- id an equilibrated lutions, the mecha- 3 composite electro-

- - . - 117

Tt has been pointed out that the overall log i vs. E curve for

LS mechanism contains two linear sections and that the reaction c -ier with respect to OH- ions depends on the potential - .

Comparison electrodes The main difference in polarization behavior is the existence a low-slope region for Ru02-based electrodes that is not obser-

a 1 for the Ni electrode in the investigated current density range. the other hand, the Ru02 and composite electrodes exhibit the

Figure 7 shows that the activity ratio between composite and

of polarization behavior of Ru02'i and composite_

Ir:e two slope Taf el behavior.

,'2 electrodes is about 0.7.

r the hydrogen UPD peaks indicate that the real surface area is -obably smaller for Ru02 than for the composite electrode (Fig.8.

However charges determined by cyclic voltammetry in 1M NaOH un-

E # I

-1688.8 -1458. 8 -1W.8 11 -1158.8

/ J

Fig. 7 . Comparative polarization plots for the HER on Ni(K), RuOZ and composite electrodes at 358K !n 35 8w aqueous NaOH (I):Ni(K) (2):Ru02 (3):Ru02+Ni(K).

bPn(i)(mA/cm 2 )

I E=-1200mV

sloDes -

- 0 . 3 3

- 0 . 4

- 0.52

- 0 .8 '

1.55

I PnaOH ~

4 2 3

pig. 6b. Logarithm of the absolute current density as a function of the N a O H activity logarithm for different Hydrogen potentials at composite electrode (355K).

In particular, we observe experimental reaction orders (dlni/dlna NaOH) which have fractional values.and vary with the potential in the investigated current density range.

Thus, the overall rate of the HER on these electrodes cannot ap- parently be separated into a rate controlling and an equilibrated step but is under mixed control. In alkaline solutions, the mecha- nism for describing the H2 evolution on Ru02 and composite electro- des is then probably the following : - - M -!- H20 + e,gI?M - H + OH- (Volmer), - M -H + H20 +e>/H2 + OH- + M (Heyrovsky).

It has his mecha -der withi Compar iL e lectro) The mai.

f a low-s, cd for th! n the othtt ?me two s

Figure "

g o 2 elect11 However

?r the hycc robably sm

,nv -2680.6

Fig. 7. C o n and composi' (2):Ru02 ( 3

I L A

IO and composi Is? (1) :Ni(K)

les.

YROLESS NICKEL

IDIC ANODIC <GE CHARGE

155 0.033

ves are presen :annot be attri

- - * .

119

In conclusion, the presence of a cracked nickel coating on an R $ - ' ~ electrode seems to keep unchanged the mechanism of the HER ol,erved on a pure Ru02 cathode but significantly increases the r'L-1 surface area and consequently the overall rate of HER.

C' .ERAL CONCLUSION .i composite electrode obtained by electroless nickel deposit on hermal Ru02 coating is a highly efficient catalyst for the hy-

-c' "rjen evolution reaction in strong caustic solution. This cathode 2 stable for long time operation and is resistant to iron poiso- I J. Its activity seems to be due both to catalytic and efficient s 1 >-ace area effects.

a

I 3OWLEDGEMENTS '?he authors would like to thank Prof. S . Trasatti for helpful

d'rcussions and advice.

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655. 15 A . Saraby - Reintjes, J. Chem. SOC., Faraday Trans. I , g (1986)

3343.

In c o r

io2 e l e c

)served -a1 s u r f f

'NERAL C

A comF

t herma U -ogen ew , stable "9. I t a rface a

XNOWLEC:

The a w s c u s s i c

- 'FERENCE

'. as ah^

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58 12 3 J. Me?

(198C'

1 1.c.n 5 DOW, 6 0 . Sc.

(198C 7 BASF, 8 C h l o i

9 A t o c l

1 0 Dupoi

11 O l i n ,

us P

1 2 Atoc'

1 3 J. B 14 J. 0

655.

1 5 A . S

3 3 4 3