Journal of Membrane Science, 61 (1991) 157-165

Elsevier Science Publishers B.V., Amsterdam 157

Metal and metal oxides based membrane composites for solid polymer electrolyte water electrolysers*

A. Michas”g** and P. Milletb

“C.R.E.M.G.P.-B.P. 75, Domaine Uniuersitaire, 38042 Saint Martin d’HGres Cedex (France)

DRF/SPh/PCM-CEA, Centre d%tudes NuclCaires de Grenoble, 85X, 38041 Grenoble Cedex

(France)

‘DTEISTTILASP-CEA, Centre d%tudes Nucldaires de Grenoble, 85X, 38041 Grenoble Cedex

(France)

(Received October 29,1989)

Abstract

The electrocatalytic behaviour of thermally prepared anhydrous RuO.JNafion@ (prepared by pyrolysis of a RuCl, solution on a porous titanium substrate which is then pressed on to the membrane) and platinum/Nafion (prepared by in situ chemical precipitation of platinum parti- cles obtained by treatment of the Pt-exchanged membrane with a NaBH, solution) composite electrodes is compared towards the hydrogen evolution reaction in solid polymer electrolyte (SPE) water electrolysis. The metal oxide based composites show moderate electrocatalytic activity com- pared with the platinum based composites. In contrast, they are not sensitive to poisoning by foreign metal deposition. The RuO, based composite electrodes can be alternative cathodic ma- terials for SPE water electrolysers.

Keywords: electrochemistry; composite membrane, metal oxide based

Introduction

There is considerable interest in developing hydrogen and oxygen electro- catalysts as active components of composite electrodes for SPE (SPE stands for Solid Polymer Electrolyte and is a trade mark of Hamilton Standard) water electrolysis applications.

Historically, the U.S. firm General Electric Company first developed SPE water electrolysers for large scale hydrogen production [l] as well as for mil- itary purposes [ 21.

*Paper presented at the 6th International Symposium on “Synthetic Membranes in Science and Industry”, Ttibingen, September 4-8,1989. **Present address: Chemistry Department, Trinity College, Dublin 2, Ireland.

0376-7388/91/$03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

158

The Swiss firm Asea Brown Boveri has developed its own technology and marketed its MembrelTM process [ 3 1.

More recently we have described a new simple procedure for the preparation of platinum based SPE composite electrodes which show good electrochemical performance when used for water electrolysis (4,5].

Perfluorosulphonated ion-exchange Nafion membranes are chosen as Solid Polymer Electrolyte (Nafion is a trademark of E.I. du Pont de Nemours for its perfluorosulfonic acid polymer membranes). This material, which is widely used in the chlor-alkali industry, exhibits excellent electrochemical and me- chanical stability, high protonic conductivity and low gas permeability [6]. The highly acidic environment produced by the sulfonic acid groups of the membrane requires the use of noble metals and their oxides as electrocatalysts.

In a SPE configuration, the membrane is used as solid electrolyte and sep- arator. Water fed to the anode is decomposed into gaseous oxygen, hydrogen ions and electrons. The hydrogen ions diffuse through the membrane to the cathode while the electrons pass through the external circuit. Approximately 4 molecules of water are transported with each hydrogen ion. At the cathode, hydrogen ions and electrons recombine to produce hydrogen gas.

SPE water electrolysers present several advantages over conventional al- kaline water electrolysers. Only pure water is used and corrosion problems can be handled more easily; cells can be operated at high current densities (l-2 A- cm-‘); very compact electrolysis units can be designed [ 71.

Although metallic platinum is often used as cathodic material for the hydro- gen evolution reaction, its high sensitivity to poisoning by underpotential de- position (UPD) of other metals such as Cu and Pb may lead to a large increase in the cell voltage and thus to high losses in cell efficiencies.

Fig. 1. Electron transmission micrograph of a Pt-Nafion composite.

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Fig. 2. Scanning electron micrographs of (a) porous Ti electrode and (b) porous Ti-RuOp electrl ode.

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Recently, ruthenium dioxide sputtered on titanium foils has been shown to be insensitive to poisoning [ 81.

This work presents the preparation, characterization and electrocatalytic behaviour of platinum and ruthenium dioxide (prepared by thermal decom- position of chloride salts) composite electrodes towards the hydrogen evolu- tion reaction in SPE water electrolysis.

Preparation and characterization of membrane electrode assemblies

When a membrane exchanged with a platinum cationic species [ Pt ( NH3 )i’ ] is treated with a NaBH, solution, platinum particles are precip- itated in situ. This route leads to a concentration profile of platinum with the particles concentrated near the two membrane surfaces [ 51. Figure 1 shows a transmission electron micrograph of a Pt-Nafion composite. Typical metal loadings of approximately 1 mg-cm-’ are obtained.

This procedure exploits the particular internal structure of the Nafion mem- brane, which includes hydrophilic zones where the ionic sulphonate groups are clustered. Metal precipitation takes place inside these hydrophilic zones and the particle size is apparently limited by the size of the clusters. By choosing the conditions of preparation it is possible to control the size and distribution of the particles inside the membrane. Finely divided catalysts supported by a stable matrix make a composite which may act efficiently even under severe electrolysis conditions.

Hydrated ruthenium dioxide was previously prepared according to the same procedure using ruthenium cationic species instead of platinum complexes and KOH solution instead of NaBH, [ 91, and RuO,*xH,O-Nafion composites were tested for chlorine evolution. The stability and electronic conductivity of the catalyst are unfortunately poor because the oxide is hydrated [lo].

Anhydrous RuOz has been prepared by thermal decomposition of RuCl, [ 111 at 450°C on a porous titanium substrate. The electrode may then be pressed on to a Nafion membrane. Solutions and gels of Nafion can be used in order to obtain porous SPE composites of high electroactive area [ 121. This route may take advantage of the good electrocatalytic properties of RuO, while avoiding the poor stability shown by the hydrated dioxide prepared by the in situ method.

Figure 2 shows scanning electron micrographs of a porous Ti electrode (Fig. 2a) and a porous Ti electrode coated with RuOz (Fig. 2b). Typical RuO, load- ings of approximately 3 mg-cm-’ are realised.

Electrochemical performance

The SPE composites were mounted in an appropriate cell for the electro- chemical tests. The cell, described elsewhere [ $71, allows the separate study of the cathodic and anodic behaviour of the composites. All experiments were

161

10-

50-

60-

70. I I I I I I -0.2 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

POTENTIAL (v vs. DHE )

Fig. 3. Cyclic voltammogram for a Pt-Nafion composite; sweep rate 50 mV/sec.

6- I I

0 0.2 0.4 0.6 0.6 1.0 1.2

Potential (VVS DHE)

It: i

Fig. 4. Cyclic voltammograms for a porous Ti-RuO* electrode in 1M H,SO, solution (---) and for the same electrode pressed on to a Nafion membrane (---); sweep rate 20 mV/sec.

performed at room temperature using a dynamic hydrogen electrode (DHE) as the reference electrode.

Figure 3 shows the cyclic voltammogram obtained for a Pt-Nafion compos- ite. By integration of the area under the curve between + 50 and + 400 mV, we can calculate the real surface area of platinum and thus obtain the roughness factor R of the catalyst, which is the ratio between the real surface area and the geometric surface area.

Figure 4 shows the cyclic voltammogram obtained for a porous Ti-RuO, electrode in 1M H,SO, solution (solid line ) and the cyclic voltammogram ob-

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Current density (A Cd)

Fig. 5. Cathodic overpotential vs. current density plot for a Pt-Nafion composite and for a RuO,-

Nafion composite; sweep rate 1 mV/sec. The plots are iR corrected, and R denotes the dimension-

less roughness factor.

tained for the same electrode pressed on to a Nafion membrane (broken line). The real surface area of the oxide can be estimated using the area measured by the BET method [ 131. Approximately the same real surface area is obtained in the two experiments, apparently showing that no catalyst is lost when the electrode is pressed on to the Nafion membrane. This can be justified by the fact that the dioxide is localized in the upper part of the porous titanium sub- strate (Fig. 2).

Figure 5 shows the cathodic overpotential versus current density plot for a Pt-Nafion composite and a RuO,-Nafion composite. Much higher overpoten- tials are observed for RuO, in spite of its high roughness factor.

Poisoning

Figure 6 shows the cyclic voltammogram obtained for a Pt-Nafion compos- ite equilibrated with a 10W2 M Cu (ClO,), solution. The voltammogram clearly shows the underpotential deposition of copper, which makes the platinum very sensitive to poisoning due to the formation of a monolayer on the catalyst surface.

Figure 7 shows the cyclic voltammogram obtained for a RuO,-Nafion com- posite equilibrated with a 10e2 M Cu(ClO,), solution. Only bulk copper de- position is observed at approximately 0.3 V. This fact makes the dioxide less sensitive to poisoning because in bulk deposition the surface of the catalyst is not completely covered.

Figure 8 shows a current density versus time plot for a Pt-Nafion composite and for a RuO,-Nafion composite when a lo-” M copper solution is added to

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I I / I I I I 0 0.4 0.8 1.2

potential (V..DHE)

Fig. 6. Cyclic voltammogram for a Pt-Nafion composite equilibrated with 10e2M Cu(ClO,), SO-

lution; sweep rate 50 mV/sec.

15-

?O- E ;5-

Eo-

lo-&

;5-

0 01 0.2 0.3 0.4 0.5 0.6 ( Potential fv YS DHE)

Fig. 7. Cyclic voltammogram for a RuO*-Nafion composite equilibrated with a lo-’ M CU (ClO,),

solution; sweep rate 50 mV/sec.

the feed water. A dramatic drop of current density is observed for the Pt- Nafion composite, whereas the current density for the RuO,-Nafion composite remains constant.

164

01 0

I I I / I 30 60

Time ml” )

Fig. 8. Current density vs. time plot for a Pt-Nafion composite and for a RuO,-Nafion composite when a lo-’ M copper solution is added to the feed water.

Conclusions

The results obtained here show that thermally prepared anhydrous Ru02 is not sensitive to poisoning copper metal deposition when used as active cath- odic material in SPE composite electrodes.

The performances of a RuO, based cathode for SPE water electrolysis could be improved by etching the porous titanium substrate and by incorporating more catalyst in order to increase the roughness factor. The procedure for mak- ing the porous Ti-RuO, SPE electrode is simple. More sophisticated assem- blies can be prepared using solutions and gels of Nafion in order to improve the contact between catalyst, substrate and SPE. This is currently under in- vestigation in our laboratories.

Alternatively, this “thermal” route can be applied to the preparation of mixed metallic oxide based and metallic alloy based electrodes, which are known to be active electrocatalysts for oxygen evolution.

Acknowledgement

This work was partially supported by the 86 300 283 FR 18 PUJl stimula- tion EEC grant.

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References

1 General Electric Co., Solid polymer electrolyte water electrolysis technology development for large-scale hydrogen production, Rep. DOE/ET/26 202-1,1981.

2 L.J. Nuttall and J.M. Russell, Solid polymer electrolyte water electrolysis - development status, Int. J. Hydrogen Energy, 5 (1980) 75.

3 G. Scherer, H. Devantay, R. Oberlin and S. Stucki, Wasserstoff und Ozonerzeugung durch Membrel Wasserelektrolyse, Dechema Monographien, Band 98, Verlag Chemie, 1985.

4 P. Millet, R. Durand and M. Pineri, Preparation of new solid polymer electrolyte composites for water electrolysis, Proc. 7th World Hydrogen Energy Conf., Moscow, Vol. 1, 1988.

5 P. Millet, M. Pineri and R. Durand, New solid polymer electrolyte composites for water electrolysis, J. Appl. Electrochem., 19 (1989) 162.

6 A. Eisenberg and H.L. Yeager (Eds.), Perfluorinated Ionomer Membranes, ACS Symp. Ser., No. 180, American Chemical Society, Washington, DC, 1982.

7 P. Millet, Preparation et optimisation d’ensembles electrode-membrane-electrode. Appli- cation a l’klectrolyse de l’eau, Thesis, Institut National Polytechnique de Grenoble, 1989.

8 E.R. K&z and S. Stucki, Ruthenium dioxide as a hydrogen-evolving cathode, J. Appl. Elec- trochefn., 17 (1987) 1190.

9 A. Michas, J.M. Kelly, R. Durand, M. Pineri and J.M.D. Coey, Preparation, characterization and catalytic properties of perfluorosulphonated ion-exchange membranes containing sur- face-concentrated, hydrated ruthenium oxide particles, J. Membrane Sci., 29 (1986) 239.

10 A. Michas, Precipitation d’oxydes metalliques dans les membranes perfluorosulfonees: mi- crostructure et proprietes Blectrocatalytiques, Thesis, Institut National Polytechnique de Grenoble, 1986.

11 S. Trasatti and G. Lodi, Properties of conductive transition metal oxides with rutile-type structure, in: S. Trasatti (Ed.), Electrodes of Conductive Metallic Oxides, Part A, Elsevier, Amsterdam, 1980, p. 301.

12 P. Aldebert, P. Millet, C. Doumain, R. Durand, F. Novel-Cattin and M. Pineri, Preparation and Characterization of SPE composites for electrolyzers and fuel cells, Solid State Ionics, 35 (1989) 3.

13 L.D. Burke and O.J. Murphy, Cyclic voltammetry as a technique for determining the surface area of RuOz electrodes, J. Electroanal. Chem. Interfacial Electrochem., 96 (1979) 19.