In most instances separate electrocatalysts are needed to promote the reduction of O2 in the fuel cell mode and to generate O2 in the energy storage-water electrolysis mode in aqueous electrochemical systems operating at low and moderate temperatures (T greater than or equal to 200 C). Interesting exceptions are the lead and bismuth ruthenate pyrochlores in alkaline electrolytes. These catalysts on high area carbon supports have high catalytic activity for both O2 reduction and generation. Rotating ring-disk electrode measurements provide evidence that the O2 reduction proceeds by a parallel four-electron pathway. The ruthenates can also be used as self-supported catalysts to avoid the problems associated with carbon oxidation, but the electrode performance so far achieved in the research at Case Western Reserve University (CWRU) is considerably less. At the potentials involved in the anodic mode the ruthenate pyrochlores have substantial equilibrium solubility in concentrated alkaline electrolyte. This results in the loss of catalyst into the bulk solution and a decline in catalytic activity. Furthermore, the hydrogen generation counter electrode may become contaminated with reduction products from the pyrochlores (lead, ruthenium)

Prakash, Jai

Tryk, Donald

Yeager, Ernest

English

In most instances separate electrocatalysts are needed to promote the reduction of O2 in the fuel cell mode and to generate O2 in the energy storage-water electrolysis mode in aqueous electrochemical systems operating at low and moderate temperatures (T greater than or equal to 200 C). Interesting exceptions are the lead and bismuth ruthenate pyrochlores in alkaline electrolytes. These catalysts on high area carbon supports have high catalytic activity for both O2 reduction and generation (1,2). Rotating ring-disk electrode measurements provide evidence that the O2 reduction proceeds by a parallel four-electron pathway. The ruthenates can also be used as self-supported catalysts to avoid the problems associated with carbon oxidation, but the electrode performance so far achieved in the research at Case Western Reserve University (CWRU) is considerably less. At the potentials involved in the anodic mode the ruthenate pyrochlores have substantial equilibrium solubility in concentrated alkaline electrolyte. This results in the loss of catalyst into the bulk solution and a decline in catalytic activity. Furthermore, the hydrogen generation counter electrode may become contaminated with reduction products from the pyrochlores (lead, ruthenium). A possible approach to this problem is to immobilize the pyrochlore catalyst within an ionic-conducting solid polymer, which would replace the fluid electrolyte within the porous gas diffusion O2 electrode. For bulk alkaline electrolyte, an anion-exchange polymer is needed with a transference number close to unity for the Oh(-) ion. Preliminary short-term measurements with lead ruthenates using a commercially available partially-fluorinated anion-exchange membrane as an overlayer on the porous gas-fed electrode indicate lower anodic polarization and virtually unchanged cathodic polarization

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I. 
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0 
N89-2299 
ELECTROCATALYSTS FOR OXYGEN ELECTRODES I N  FUEL CELLS 
AND WATER ELECTROLYZERS FOR SPACE APPLICATIONS 
Jai Prakash, Donald Tryk, and Ernest Yeager 
Case Western Reserve University 
Cleveland, Ohio 
In most instances separate electrocatalysts are needed to promote the 
reduction of 02 in the fuel cell mode at to generate 02 in the energy storage - 
water electrolysis mode in aqueous electrochemical systems operating at low 
and moderate temperature ( T  2 O O 0 C > .  This situation arises because, even 
with relatively high performance catalysts, the 02 reduction and generation 
reactions are still quite irreversible with much overpotential. The 
potentials of the 02 electrode in the cathodic and anodic modes are separated 
by typically 0.6 V and the states of the catalyst surface are very different. 
Interesting exceptions are the lead and bismuth ruthenate pyrochlores in 
alkaline electrolytes. 
catalytic activity for both 02 reduction and generation (1.2). 
rotating ring-disk electrode measurements provide evidence that the 02 
reduction proceeds by a parallel four-electron pathway. The ruthenates can 
also be used as self-supported catalysts to avoid the problems associated with 
carbon oxidation, but the electrode performance so far achieved in the 
research at CWRU is considerably less. The possibility exists, however, o f  
supporting these catalysts on a stable electronically-conducting high area 
oxide support such as a perovskite. 
At the potentials involved in the anodic mode the ruthenate pyrochlores have 
substantial equilibrium solubility in concentrated alkaline electrolyte. This 
results in the loss of catalyst into the bulk solution and a decline in 
catalytic activity. Furthermore, the hydrogen generation counter electrode 
may become contaminated with reduction products from the pyrochlores (lead, 
ruthenium). 
These catalysts on high area carbon supports have high 
Furthermore, 
A possible approach to this problem is to immobilize the pyrochlore catalyst 
within an ionic-conducting solid polymer, which would replace the fluid 
electrolyte within the porous gas diffusion 02 electrode. For bulk alkaline 
electrolyte, an anion-exchange polymer i s  needed with a transference number 
close to unity,for the OH- ion. 
transport of the lead and ruthenium, which are expected to be in complex 
anionic forms. 
a commercially available partially-fluorinated anion-exchange membrane as an 
overlayer on the porou's gas-fed electrode indicate lower anodic polarization 
and virtually unchanged cathodic polarization. The leakage of soluble Pb and 
Ru species from the catalyst through the ionomer membrane will be evaluated. 
Other methods of applying polymer layers over the gas-fed electrodes or 
incorporating the polymer within the electrode will be discussed. 
Such a membrane may not block completely the 
Preliminary short-term measurements with lead ruthenates using 
Acknowledgement: 
Laboratory and the Department of Energy through a subcontract with Lawrence 
Berkeley Laboratory. 
This research has been supported by NASA through the Lewis 
21 
https://ntrs.nasa.gov/search.jsp?R=19890013626 2020-03-20T03:04:50+00:00Z
References:  
1 .  H .  S .  Horowi tz ,  J .  M. Longo, and H .  H .  Ho row i t z ,  
- 130, 1851 (1983).  
. Elect rochem. Soc., 
2. J .  Prakash, R. E .  Carbonio,  M .  Razaq, D. T ryk ,  and E .  Yeager, 173rd 
Meet ing  o f  t h e  E lec t rochemica l  S o c i e t y ,  A t l a n t a ,  Georg ia,  May 15-20, 
1988, A b s t r a c t  No. 503, and re fe rences  t h e r e i n .  
22 


Electrocatalysis for oxygen electrodes in fuel cells and water electrolyzers for space applications

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