advances in oxygen evolution catalysis in solid polymer electrolyte water electrolysis

7
Int. 1. Hydrogen Energy, Vol. 6, pp, 159-165 Pergamon Press Ltd. 1981. Printed in Great Britain ~) International Association for Hydrogen Energy 0360-3199/81/030159--07 $02.00/0 ADVANCES IN OXYGEN EVOLUTION CATALYSIS IN SOLID POLYMER ELECTROLYTE WATER ELECTROLYSIS J. M. SEDLAK, R. J. LAWRANCE and J. F. ENOS Direct Energy Conversion Programs, General Electric Company, Wilmington, MA 01887, U.S.A. (Received for publication 16 June 1980) AImtraet---Continuous polarization of a stabilized RuO2 oxygen anode in a solid polymer electrolyte water electrolysis cell at 1072mA cm-z resulted in severe anode deterioration within 24-48 hr. In contrast, a new Ru-based mixed oxide catalyst, termed WE-3, exhibited stability in oxygen evolution over a 6700 hr period, with testing beyond that point still in progress. The activation energy for oxygen evolution on WE-3 is comparable to that observed on new RuO2 anodes. INTRODUCTION ELECTROLYTIC decomposition of water is of interest because the hydrogen produced can be used as a fuel, a power load leveling reactant, a chemical reagent, or for generator cooling. Another important electrolysis application is the utilization of the oxygen produced as a life support gas in spacecraft and submarines. Thin film cells have been developed based on electrolyte sheets of sulfonated fluorocarbon polymers [1, 2]. The attractive features of this solid polymer electrolyte technology include the following. First, only pure water, with no added electrolyte, is pumped to the cell. Second, the electrolyte is only 0.025 cm thick. The iR component of total cell voltage may be only 200 mV at a substantial current density, i.e., 1000 mA cm -2. A major source of irreversibility in the solid polymer electrolyte cell is the overvoltage at the oxygen evolution electrode. Platinum in small amounts (~ 1 mg cm -2) at the cathode is sufficient to catalyze the hydrogen evolution reaction nearly reversibly. Ruthenium dioxide is well-known as an excellent catalyst for oxygen evolution owing to its relatively low overvoltage characteristics [3]. Ruthenium is considerably less expensive than either platinum or iridium. In practical appli- cations, however, RuO2 is susceptible to severe deterioration during prolonged oxygen evolution. Various investigators [4-11] have ascribed performance degradation to one or more of the following: (a) formation of poorly conductive oxide films at the RuO2-substrate (e.g., Ti) interface, (b) dissolution of RuO2, (c) loss of inherent catalytic activity, and (d) fouling of separator membranes. It has also been demonstrated that even IrO2 undergoes slow corrosion in oxygen evolution [9]. There have been several major catalyst problems experienced in the development of commercial water electrolysis systems. First, the anode ideally must evolve oxygen with a total cell potential less than 2.00 V at relatively high current densities (e.g., 1000 mA cm-2). Current density dictates electrolyzer size and, consequently, affects capital costs. Second, the catalyst must exhibit stable performance over thousands of hours at full load. Test intervals noted in the literature frequently amount to a few days or less. Third, the cost of materials must be minimal. The objective of the present work was twofold: (1) quantitative measurement of voltage degradation of RuO2 oxygen anodes with time, and (2) demonstration of long-term performance stability of a new Ru-based oxygen anode, WE-3,* under development at the General Electric Company. It is recognized that the prospects of hydrogen generation will be significantly enhanced if lower cost, more stable and efficient oxygen evolution catalysts can be developed [1, 2, 8, 12]. EXPERIMENTAL Polarization measurements and endurance tests were carried out for both RuO2 and WE-3 (reduced metal oxide) anodes in a temperature-controlled cell with a 46.6 cm2 superficial electrode * Proprietary material, General Electric Company. 159

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Page 1: Advances in oxygen evolution catalysis in solid polymer electrolyte water electrolysis

Int. 1. Hydrogen Energy, Vol. 6, pp, 159-165 Pergamon Press Ltd. 1981. Printed in Great Britain ~) International Association for Hydrogen Energy

0360-3199/81/030159--07 $02.00/0

ADVANCES IN OXYGEN EVOLUTION CATALYSIS IN SOLID POLYMER ELECTROLYTE WATER ELECTROLYSIS

J. M. SEDLAK, R. J. LAWRANCE and J. F. ENOS

Direct Energy Conversion Programs, General Electric Company, Wilmington, MA 01887, U.S.A.

(Received for publication 16 June 1980)

AImtraet---Continuous polarization of a stabilized RuO2 oxygen anode in a solid polymer electrolyte water electrolysis cell at 1072 mA cm -z resulted in severe anode deterioration within 24-48 hr. In contrast, a new Ru-based mixed oxide catalyst, termed WE-3, exhibited stability in oxygen evolution over a 6700 hr period, with testing beyond that point still in progress. The activation energy for oxygen evolution on WE-3 is comparable to that observed on new RuO2 anodes.

INTRODUCTION

ELECTROLYTIC decomposition of water is of interest because the hydrogen produced can be used as a fuel, a power load leveling reactant, a chemical reagent, or for generator cooling. Another important electrolysis application is the utilization of the oxygen produced as a life support gas in spacecraft and submarines. Thin film cells have been developed based on electrolyte sheets of sulfonated fluorocarbon polymers [1, 2]. The attractive features of this solid polymer electrolyte technology include the following. First, only pure water, with no added electrolyte, is pumped to the cell. Second, the electrolyte is only 0.025 cm thick. The iR component of total cell voltage may be only 200 mV at a substantial current density, i.e., 1000 mA cm -2.

A major source of irreversibility in the solid polymer electrolyte cell is the overvoltage at the oxygen evolution electrode. Platinum in small amounts (~ 1 mg cm -2) at the cathode is sufficient to catalyze the hydrogen evolution reaction nearly reversibly. Ruthenium dioxide is well-known as an excellent catalyst for oxygen evolution owing to its relatively low overvoltage characteristics [3]. Ruthenium is considerably less expensive than either platinum or iridium. In practical appli- cations, however, RuO2 is susceptible to severe deterioration during prolonged oxygen evolution. Various investigators [4-11] have ascribed performance degradation to one or more of the following: (a) formation of poorly conductive oxide films at the RuO2-substrate (e.g., Ti) interface, (b) dissolution of RuO2, (c) loss of inherent catalytic activity, and (d) fouling of separator membranes. It has also been demonstrated that even IrO2 undergoes slow corrosion in oxygen evolution [9].

There have been several major catalyst problems experienced in the development of commercial water electrolysis systems. First, the anode ideally must evolve oxygen with a total cell potential less than 2.00 V at relatively high current densities (e.g., 1000 mA cm-2). Current density dictates electrolyzer size and, consequently, affects capital costs. Second, the catalyst must exhibit stable performance over thousands of hours at full load. Test intervals noted in the literature frequently amount to a few days or less. Third, the cost of materials must be minimal.

The objective of the present work was twofold: (1) quantitative measurement of voltage degradation of RuO2 oxygen anodes with time, and (2) demonstration of long-term performance stability of a new Ru-based oxygen anode, WE-3,* under development at the General Electric Company. It is recognized that the prospects of hydrogen generation will be significantly enhanced if lower cost, more stable and efficient oxygen evolution catalysts can be developed [1, 2, 8, 12].

EXPERIMENTAL

Polarization measurements and endurance tests were carried out for both RuO2 and WE-3 (reduced metal oxide) anodes in a temperature-controlled cell with a 46.6 cm 2 superficial electrode

* Proprietary material, General Electric Company.

159

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160 J.M. SEDLAK et al.

area. The electrolyte was simply a 0.025 cm thickness of Nation* membrane containing 0.82 meq. H + g-1 attached to immobile sulfonate anions. The cation transport number of this membrane is essentially unity. The water content was 28 w/o. On opposite faces of the solid polymer electrolyte, polytetrafluoroethylene (PTFE)-bonded anode and cathode catalysts were affixed at 4 mg cm -2 loadings. Cathodes were platinum in all cases. RuO: employed in this work had been "stabilized" by heating the catalyst at 500°C for one hour (cf. [7]).

During electrolysis, deionized water is pumped to the anode compartment. Protons formed by water decomposition migrate across the membrane to the cathode where they are reduced to molecular hydrogen. Water transported by electro-osmosis to the cathode, after separation of product hydrogen, is combined with anode effluent for recycling to the anode inlet. All data were obtained at atmospheric pressure. Complete details on the General Electric Company cell design for solid polymer water electrolysis are available in the literature [2, 12].

DISCUSSION

RuO2 performance degradation Instability of RuO2 anodes in oxygen evolution has been noted frequently in the literature

[4-11]. There has, however, been some indication in the literature [7] that RuO2 can be stabilized by heating for brief periods in air at -5000C. The present work involved quantitative measurement of the variation in electrochemical properties of a "stabilized" RuO: anode under continuous polarization at 1072 m A c m -2 and 82°C. This relatively high current density was selected on the basis of economic considerations. Current density has a large impact on electrolyzer size and, therefore, capital costs.

Figure 1 shows the total voltage across the solid polymer electrolyte cell at 0, 24 and 48 hr for current densities in the range 100-1600 mA crn -z. Initially, the total cell potential was 1.725 V at

2.10 , f , ,¢' 82"C j atm pressure 2. O0 / /

/

1 . 9 0 .o/ o gd ~ ;> ~" 1.80 Z ~ / / S / ~

1.60

1. 80

1.40 I I I ~oo 2oo 400 1ooo ~ooo

CURRENT DENSITY, mA/cm 2

FIG. 1. Total cell voltage vs current density for solid polymer water electrolysis: RuO2 anode and Pt cathode. Q, Initial polarization, rq, After 24 hr at 1072 mAcm- ' . A, After

48 hr at 1072 mA cm -2.

* Registered trademark of E.I. duPOnt deNemours.

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ADVANCES IN OXYGEN EVOLUTION CATALYSIS 161

1 , 5 5

I . 50

1 . 4 5

~2~C atm prcs~ure

,G /

/ /

- - I ! _ _ t r

100 200 400 1000 2 0 0 0

C U R R E N T D E I ~ I T y , r r ~ / c m 2

FIG. 2. /R-free polarization plots: RuO2 anode and Pt cathode. ®, Initial polarization. [] After 24 hr at 1072 mA cm -2. A, After 48 hr at 1072 mA cm -~.

1000 mA cm -2 and 82°C. This amounts to a water electrolysis efficiency of 86% based on the thermoneutral potential of 1.484 V. After 24 and 48 hr, the RuO2 cell efficiency had dropped to 83 and 79%, respectively. Clearly, the rate of performance degradation had accelerated with time. Earlier data obtained in this laboratory on a variety of thermally prepared RuO2 oxygen anodes consistently exhibited cell potentials above 2.0 V after only 2-5 days of continuous polarization at 1072 m A c m -2 and 82°C.

Figure 2 depicts the iR-free plots as a function of polarization time at 1072 mA cm -:. At time zero and 24 hr, Tafel behavior was observed but the slope increased from 97 to 120 mV. Addi- tionally, the cell resistance increased from 4.6 to 5.0 mfl. After 48 hr, Tafel behavior was no longer followed and cell resistance reached 5.5 mfl. RuO2 performance deterioration thus involves both electrical resistance and electrokinetic mechanism changes. Increased resistivity is attributed to RuO2 corrosion at the electrode interface, with subsequent loss of contact area. The rapid change of Tafel slope and eventual loss of linearity is indicative of the increasing dominance of corrosion processes. The formation of soluble oxidation products such as HERuO5 was evidenced on occasion by a slight brown discoloration of anode effluent water.

In a comprehensive study [6] of RuO2 anodic polarization in aqueous electrolytes over the pH range 0.5-13, it was found that the transition to the corrosion region occurred near 1.4 V at 80°C. Those investigators found two Tafel regions with a slope of 30--40 mV below 1.40 V, and a slope of 80 mV above 1.40 V (in acid media). It is evident that the immobilized solid polymer electrolyte behaves as a liquid acidic electrolyte with respect to RuO2 anode deterioration. Furthermore, we

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162 J . M . S E D L A K et al.

2.11 I

2. 0

1.9 ~-

L 8

1, 7

1.6

1.5

1 .4

1.3

I I t I I I 1 49~C

/ 60"C 71"C 82°C

9 3 ~

I I I I I I 20 40 100 200 400 1000 2000

CURRENT DENSITY, mA/cm 2

FIG. 3. Total cel l vo l tage vs current dens i ty as a function of temperature after 3000 hr at 1072 m A cm -2 and 82°C: W E - 3 and Pt cathode .

1. 70

1~ 65

O 1. 60

1. 55

1. 50

z.4s

1.40

1. 35

I I I

I 20 40

J t I I 49"C

~ ~ ~ 60"C 71"C

82'C

93"C

I f I I I I00 200 400 1000 2000

CURRENT DENSITY, mA/cm 2

Flo . 4 . / R - f r e e polarizat ion plots: W E - 3 an od e and Pt ca thode over 49-93°C.

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ADVANCES IN OXYGEN EVOLUTION CATALYSIS 163

have demonstrated that practical electrolyzer current densities (i.e. >500 mA c m -2) c a n n o t be attained using unalloyed RuO2 without exceeding the 1.4 V onset of corrosion. A 48 hr, continuous polarization is required in our experience to provide an initial assessment of RuO2 stability.

The oxygen evolution characteristics of a freshly prepared RuO2 anode remain as a goal to be achieved with a material which is stable over thousands of operating hours.

WE-3 anode performance This novel Ru-based oxide anode was run continuously in scaled hardware at 1072 mA cm -2

and 82°C for 3000 hr; the total cell polarization curves in Fig. 3 were then taken at 20-1600 mA c m -2 o v e r a temperature range of 49--93 °. Subsequently, the cell amassed a total of 6700 hr (tests still in progress). The inherent stability of WE-3 is evidenced by the relatively small voltage increase observed over this interval (~0.014 mV hr-1). It is important to note that we report total cell potential. Only a portion of this voltage variation can, therefore, be assigned specifically to the WE-3 anode.

The WE-3 cell potential after 3000 hr of 1.76 V compares very favorably with the potential of a n e w R u O 2 cell at 1.73 V (compared at 1000 mA cm-2). The/R-free polarization plots for WE- 3 in Fig. 4 indicate that Tafel behavior is maintained after 3000 hr at all temperatures. Tafel slopes for WE-3 remained within 112 -+ 7 mV. These findings are in sharp contrast to the loss of Tafel behavior after only 24 hr with an RuO2 anode.

3. O0

2. O0

1. O0

~ - I I ] l

I I I I 2. 7 2. 8 2. 9 3 .0 3 .1 3 .2

103/T, °K

PIG. 5. "Arrhenius plots" for WE-3 catalyst water electrolysis at constant overpotential: 0.3 and 0.4 V.

Activation energies were estimated for oxygen evolution on WE-3 at constant overpotential (7) in the same way as had been described previously [7] (b = Tafel slope):

AH = -2.303R(0 log J/O (1/T))~ + 2.303RTTI/b.

The kinetic plots according to this equation are shown in Fig. 5 for ~ = 0.3 and 0.4 V. These overpotential encompass all current densities of practical significance and are well into the published corrosion region for RuO2 [6]. In order to compute overpotentials, it wasfirst necessary to calculate

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164 J.M. SEDLAK et al.

1. 25

E 0

v .

1.20

1.1E I

i

1.10

I I I I I I

50 100 T, "C

150

FIG. 6. Standard potential for water decomposition vs temperature.

the standard potential, ~°, for water decomposition as a function of temperature. Since these %° values are of general usefulness they are presented in Fig. 6 over 25-150°C.

AH values for WE-3 were 64 -+ 5 kJ mo1-1. Previously, 63 kJ mol -] had been reported for oxygen evolution on RuO2 at r/= 0.2 V [7]. The close agreement with the earlier AH value for RuO2 is somewhat fortuitous. However, all results indicate comparable catalytic properties between WE-3 and fresh RuO2 anodes.

Continuing research efforts will be directed to improvement of WE-3 voltage performance by composition variations and operation of solid polymer electrolyte cells at T > 120°C. Preliminary data on WE-3 have shown that total cell voltages of 1.6-1.7 V can be achieved at 1072 mA cm -2 current density with temperatures in the range of 120-150 ° .

CONCLUSIONS

First, the voltage performance characteristics of the WE-3 oxygen anode are comparable to those of freshly prepared RuO2 anodes.

Secondly, WE-3 is highly resistant to oxidative corrosion as evidenced by its stability over 6600 hr at -1 .65 V (/R-free). By way of comparison, RuO2 deteriorates rapidly even at only 1.40 V. The excellent stability of WE-3 involves a novel solid state structure obtained through selective alloying of the Ru constituent.

Thirdly, WE-3 represents a significant advancement in terms of anode cost for non-alkaline water electrolysis. Even in its present stage of development, WE-3 is nearly four times less expensive than platinum or iridium.

REFERENCES

1. W . A . TH-r~RINGTON & J. F. AUSTIN, Extended Abs. Electrochemical Society, 146th Meeting (New York, 1974), pp. 576--577.

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ADVANCES IN OXYGEN EVOLUTION CATALYSIS 165

2. L. J. NLrITALL & J. H. RUSSELL, Proc. 2nd World Hydrogen Energy Conference (Zurich, 1978), pp. 391-402.

3. O. ROETSCHI & P. DELAHAY, J. chem. Phys. 23, 556 (1955). 4. S. TRASAI"H & G. BUZZANCA, J. electroanal. Chem. 29, App. 1 (1971). 5. L. D. BURKE & T. O. O'MEARA, J. chem. Soc., Faraday Trans. 1 68, 839 (1972). 6. D. V. KOUKOLINA, YU. I. KRASOVITZKAYA & T. V. IVANOVA, Elektrokhimiya 14, 470 (1978), 7. C. IWAKURA, K. HIRAO & H. TAMURA, Electrochim. Acta 22, 329, 335 (1977). 8. J. L. WEIN1NOER & R, R. RUSSELL, J. electrochem. Soc, 125, 1482 (1978). 9. D. N. BUCKLEY & L. D. BURKE, J. chem. Soc., Faraday Trans. 76 (II), 2431 (1977).

10. T. LOUCKA, J. appl. Electrochem. 7, 211 (1977). 11. J, LLOPIS & M. VAZQUEZ, Electrochim. Acta 11,633 (1966). 12. P. W. T. Lu & S. SRINIVASAN, J. appl. Electrochem. 9, 269 (1979).