The doping of solid phase precursors followed by pyrolysis or the copyrolysis of gas phase precursors has allowed us to produce catalysts with good activity toward oxygen reduction. Efforts are currently underway to better understand the reasons for the catalytic activity of the bulk doped catalysts with a view toward further improving their activity

Giner, J.

Lessner, P.

Manoukian, M.

Sarangapani, S.

English

NASA Technical Reports Server

N90-20470
NON-NOBLE ELECTROCATALYSTS FOR ALKALINE FUEL CELLS
S. Sarangapani, P. Lessner, M. Manoukian and J. Giner
Giner, Inc.
Waltham, Massachusetts 02254
INTRODUCTION
Noble metals, when used as electrocatalysts for oxygen reduction in metal-
air batteries and fuel cells, suffer from the disadvantages of high cost,
susceptibility to poisoning and sintering. Carbons activated with macrocyclics
have attracted increasing attention as alternative electrocatalysts for oxygen
reduction (Yeager, 1984). Initial activity of these catalysts is good, but
performance declines rapidly. Pyrolyzing the macrocyclic on the carbon support
leads to enhanced stability and the catalyst retains good activity (Wiesener,
1986). The exact nature of the catalytic sites after pyrolysis is still under
investigation (Scherson, et al., 1983; McBreen, et al., 1987).
Despite this promising method of treating macrocyclic activated carbon
catalysts, the performance decay is still too large to be acceptable. Part of
the stability problem occurs because the catalytic layer on the carbon surface
is, at most, a few monolayers thick. The approach described here is designed to
develop bulk doped catalysts with similar structures to pyrolyzed macrocyclic
catalysts. The transition metal and coordinated ligands are dispersed
throughout the bulk of the conductive carbon skeleton.
Two approaches to realizing this concept are being pursued, both involving
the doping of a carbon precursor followed by high temperature pyrolysis to form
a M-N-C catalyst. In one approach, the precursor is a solid phase ion exchange
resin. Two resins have been selected for doping. One is IRC-50 (Figure la)
which is a weak cation exchange resin that contains no nitrogen in its backbone
or exchange groups. The transition metal _nd nitrogen are introduced by either
exchanging a transition-metal (such as Fe +_ from FeCI_) and nitrogen (frQm
NH4OH) sequentially or exchanging a chelation compound (such as Fe(bipy)_+).
The other ion exchange resin chosen was Chelite-C (Figure ib) which contains a
nitrogen-containing iminodiacetic acid exchange group. Exchange of a simple
transition metal salt results in metal-nitrogen coordination before pyrolysis.
In the second approach, gas phase precursors are mixed in a reactor and
pyrolyzed to form the catalyst. Acetylene (C2H2) is used as the carbon source,
ammonia (NH3) as the nitrogen source, and an iron-containing organometallic as
the transition metal source.
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EXPERIMENTAL METHODS
Chelation compounds for doping the IRC-50 resin have included Fe(bipy)3SO4,
Co(bipy3)Cl 2, Fe(phen)3SO 4, Co(NH3)6CI 2 , and Co(en)3Cl 3. Doping is carried out
by mixing the resin with a solution of the dopant and agitating the mixture at
80°C for 3 hours. The doped resin is dried and then placed in a tube furnace.
The resin is pyrolyzed under an NH 3 atmosphere at temperatures near 900°C.
The gas phase precursors are pyrolyzed in a vertical tube furnace at
i
temperatures near llO0°C. The C2H2, NH3, and iron-containing organometallic are
introduced at the top of the furnace. Product is collected in a water trap at
the bottom of the furnace or inside the tube.
Electrodes are prepared from the bulk doped carbons by mixi_e catalyst
with PTFE and spreading onto a Ag-plated Ni screen.
Electrochemical tests are performed using the floating electrode technique
(Giner and Smith, 1967). The electrolyte is 30% KOH and the temperature is held
at 80°C. The electrodes are tested galvanostaticaily and the measurements are
corrected for external ohmic losses. Potentials are reported versus a dynamic
hydrogen electrode (DHE).
The catalysts are analyzed for their carbon, hydrogen, nitrogen, and metal
content. Some catalysts have also had BET surface areas determined using a
Micromeritics Flowsorb II surface area analyzer.
i
Mossbauer spectroscopy has been used in an attempt to identify the species
present in the iron-containing catalysts. This work is being done in
conjunction with Professor Daniel Scherson at Case Western Reserve University.
RESULTS AND DISCUSSION
E
A. Electrochemical Tests
Figure 2 compares the electrochemical performance of catalysts prepared from
IRC-50 doped from solutions of Fe(bipy)3S04, Co(en)3Cl3, Co(bipy)3SO4,.. _._
Co(NH3)6CI , and FeCI3-NH4OH with CoTMPP adsorbed and pyrolyzed on Vulcan XC-72.
There is a large improvement in performance on changing the dopant Solution from
FeCI3-NH4OH to Fe(bipy)3SO 4 with the same resin and pyrolysis conditions. The
Fe(bipy), Co_en), and Co(hex) systems show a performance that is 50-80 mV less
at I00 mA/cm z than the CoTMPP on Vulcan XC-72. The surface areas of Co(en),
Co(hex) and Fe(bipy), doped carbons are in the same range as CoTMPP doped
Vulcan XC-72. The differences in the kinetic region may be attributed to the
differences in the surface concentrations of the catalyst. The earlier dropoff
of performance may be due to either ohmic losses in the structure or hindrance
to oxygen transport. The Co(bipy) system shows somewhat poorer performance;
stoichiometric analysis of the Co(bipy) system showed that only 1% Co was incor-
porated in the final catalyst (compared to 10% for iron) and, therefore, doping
conditions need to be adjusted.
=
The next set of experiments examines two different approaches for the
synthesis of "Fe-N" complexes. Figure 3 compares the performance of the IRC-50
Fe(bipy) system with Chelite-C doped with FeCI 3. In the first system, the Fe is
166
already coordinated with nitrogen in the bypyridine complex. Introducing this
complex into the ion exchange resin, allows bulk-doping and ensures molecular
level interaction between the carbon and the "Fe-N" complex. In the second
system, the resin has a backbone that contains nitrogen and coordinates with the
iron. In addition to differences in the density of the exchange sites (which
determines the metal/nitrogen ratio) in these two systems, the molecular inter-
actions are bound to be different which may result in differences in oxygen
reduction performance. The carbon prepared from the chelating resin shows
essentially the same performance in the kinetic region, indicating that the
nature of the Fe-N complexes may be similar regardless of their method of prepa-
ration, as long as covalent type interactions are ensured as opposed to purely
ionic type interactions. At higher current densities, there are some differ-
ences in the performance of these systems, which may be attributed to the non-
optimized electrode preparation. It is also quite possible that the stabilities
of these complexes may vary widely. Preliminary examination of the catalysts
using Mossbauer spectroscopy indicates that the IRC-50 and Chelite-C systems
contain one species in common and one species each that is unique to that
catalyst.
Figure 4 shows the results of performance measurements on catalysts from
gas phase precursors. The undoped carbon prepared from gas pyrolysis shows very
poor performance. Addition of the Fe and N has a dramatic effect on perfor-
mance. The surface area of the Fe/N doped carbon in this figure, is only
19 m2/g and the nitrogen/metal mole ratio is 0.46. Studies are in progress to
improve both these factors.
B. Characterization of Catalysts
The catalysts are routinely analyzed for carbon, hydrogen, nitrogen, and
metal content. Table I shows the stoichiometry calculated for some typical
catalysts. There is still some unknown that is not accounted for in the C, H,
N, M. Some of this unknown is oxygen which comes from the COO- groups in the
resin and exposure of catalyst to the atmosphere. The oxygen is present in the
form of metal oxides. However, a calculation of the maximum weight percentage
of oxygen present in this form shows that it is not enough to account for all of
the unknown. The sample of IRC/Fe(bipy) was also sent for Si analysis. Some Si
is present, but Si or SiO 2 does not account for all of the unknown. Emission
spectroscopy was conducted to determine if any other heavy metals are present.
Only minor amounts were present. The sample was sent for sulfur analysis which
showed that some sulfur was present. This sulfur is probably from the sulfate
group in the Fe(bipy)3SO 4. This is because some of the dopant is imbibed in the
pores of the IRC-50 instead of being ionically bound to the COO" groups.
BET surface areas of catalysts prepared from the doped ion-exchange resin
precursors are in the range of 100-300 m2/g. When the IRC-50 resin with no
dopant was pyrolyzed, the surface area was less than 5 m2/g. This is consistent
with work in progress which indicates that the presence of the metal dopant has
a large effect on the nature of the pyrolysis process.
]67
CONCLUSIONS
The doping of solid phase precursors followed by pyrolysis or the co-
pyrolysis of gas phase precursors has allowed us to produce catalysts with good
activity toward oxygen reduction. Efforts are currently underway to better
understand the reasons for the catalytic activity of the bulk doped catalysts
with a view toward further improving their activity.
ACKNOWLEDGEMENT
This work is supported by the United States Department of Energy under ....
Contract No. DE-ACO2-87ER80500.M001. We would like to thank Professor
Daniel Scherson for providing the sample of CoTMPP adsorbed on Vulcan XCo72 and
performing the Mossbauer spectroscopy on the catalyst samples.
' Z
REFERENCES
Giner, J. and S. Smith. 1967. A Simple Method for Measuring the Polarization of
Hydrophobic Gas Diffusion Electrodes. Electrochem. Technol. 5:59. -;
McBreenl J., W.E. O'Grady, D.E. Sayers, C.Y. Yang, and K.l. Pandya. i987. An
EXAFS Study of Pyrolyzed Metal Macrocyclic Electrocatalysts. In Electrodes,
Materials and Processes for Energy Conversion and Storage, S. Srinlvasan, S.
=.
Wagner, H. wr0blowa (Eds.). Electrochemical Society Proceedings. 87-12.
Scherson, D.A., S.L. Gupta, C. Fierro, E.B. Yeager, M.E. Kordesch, J. Eldridge,
R.W. Hoffman, and J. Blue. 1983. Cobalt Tetramethoxyphenyl Porphyrin
Emission Mossbauer Spectroscopy and 02 Reduction Electrochemical Studies.
Electrochim. Acta. 28:1205 .....
Wiesener, K. 1986. N4-Chelates as Electrocatalyst for Cathodic Oxygen
Reduction. Eiectrochlm. Acta. 31:1073.
Yeager, E. 1984.
29:1527.
Electrocatalysts for 02 Reduction. Electrochim. Acta.
!
=
i
=
_=
=
=
z
TABLE I
Catalyst
IRC 50/Fe(bipy)
IRC 50/Co(bipy)
IRC 50/Co(en)
IRC 50/Co(hex)
Chelite-C/Fe
Gas Phase Carbon
Gas Phase/Ferro-
cene Derivative
Wt%
C H N M Unk
70.70 0.75 2.36 9.60 16.59
85.86 0.19 0.210 1.18 12.56
88.42 0.31 0.620 10.8 0
96.28 0.29 0.i0 2.63 0.7
79.6 0.22 0.190 13.5 6.4
99.00 0.5 ...... 0.5
78.34 0.34 1.38 12.00 7.90
Surface
Area
(m'/g)
310.5
87.3
150.4
205.3
191.1
19
168
CH 3
L
_CCH2CHCH 2
_CHCH 2
Figure la: Structure of IRC-50 Weak
Iii
I
D
>
>
E
v
.4-..-I
C
O
EL
Acid Ion Exchange Resin.
1100 ..... ,
1000
900
800
700
600
500
0
"[3
X
+
_CHCH2CHCH 2
_"\0,, - --CHCH E--"-"O/
Oo, ,o
Figure lb: Structure of Chelite-C.
• • • • • w w • I
O\oo Ooo o
V R o o_ OOooo
• _: •\ • • u oa_ °°°°°°oo
+++++ £A A
+ ++++
+
2 3 4 _ 6
400 ............... ' ........
1 10 1 O0 1000
Current Density mA/cm 2
FiXture 2 z Polarisation curves for o 2 reduction rich porous slootrodos in 7M KOH st 80"C.
Cuz've £z 4.8q v/w @o-TIqPP on XC-72 omrbon.
Cucve 2t IRC-S0 + Fo (added as bipyridyl.oomplox):..
Curve 3s ZRC-$0 + Co (added am the hezeuuno ohlor_ao_.
Curve 4:ZRC-S0 + Co (added am trim othylenealaline oobtlt(III) ohlortdo).
curve S: XRC-50 + Co (added am btpyridyl oonplsx).
Cil¢'Ve 6| XRC-50 + Fo (addod ms tho ohlorido) + H (added &8 HH4OR).
169
1100 ...... ' ........ " ....
W
I
D
(0
>
>
E
v
m
0
o
>
E
v
C
¢
0
Figure 4 :
1000
9OO
8OO
7OO
6OO
5OO
400
Figure 3:
1100
1o00
9OO
8OO
700
6OO
50O
'+
+
+
+
+ +
-_ + +
+ +
+ + +
+
+ + ++++ ++
+ ++++ ++++,hi-+,
++++ +
/
2
1
'+.t_
J.+
4-
I I0 1O0 1000
Current Density mA/cm 2
PolarJzatton Curves for 09 Reduction With Porous E]ectnxles in 7_ KOH at _)°C.
:urve 1: Chelite-C + Fe (_idded as the chloride) Pyro]yzed at 900-C.
;urve 2:IRC-50 +Fe (added as the bipyrtdyl complex), Pyro]yzed at 900°C.
• • • • .'1 . • • .Jig . . • ..
Lo
/%
/% /%
0
[]
2
/% AZl
0
0
0
/%/% A/% A /%A/%_ A/%/%_
0
[]
O
0
O
[]
0
400 ' " .....................
1 10 100 1000
Curr_tlDensitymA/cm _
Polarigation ourvel for O_ reduotion vith porou8 eleotrodea £n 7M KOE a¢ 8O'C.
C_To 1: Bleotrode was made with pyroZyaed
C2H 2 + Fe (added as dimethyZaminoethylferrooone).
Curve 2: pyrolyzad C2K 2 (_OO0"C).
m
]70


Non-noble electrocatalysts for alkaline fuel cells

Carbons activated with macrocyclics have attracted increasing attention as alternative electrocatalysts for oxygen reduction. Initial activity of these catalysts is good, but performance declines rapidly. Pyrolyzing the macrocyclic on the carbon support leads to enhanced stability and the catalysts retain good activity. The approach described is designed to develop bulk doped catalysts with similar structures to pyrolyzed macrocyclic catalysts. The transition metal and coordinated ligands are dispersed throughout the bulk of the conductive carbon skeleton. Two approaches to realizing this concept are being pursued, both involving the doping of carbon precursors. In one approach, the precursor is a solid phase carbon-containing ion-exchange resin. The precursor is doped with a transition metal and/or nitrogen, and the resulting mixture is pyrolyzed. In the other approach, the precursor is a gas-phase hydrocarbon. This is introduced with a transition metal species and nitrogen species into a reactor and pyrolyzed. Several studies have been conducted to determine if there is a synergistic effect between the transition metal and nitrogen and the effect of different methods of introducing the metal-nitrogen (M-N) coordination on performance. One approach was to introduce the metal and nitrogen separately, for example, by sequentially doping FeCl3 and NH4OH into the resin. Catalysts were prepared from an undoped ion-exchange resin, a resin doped only with N, a resin doped only with Fe, and a resin doped with both Fe and N. Introduction of nitrogen alone has no beneficial effect on the performance of the catalysts. The introduction of the Fe alone significantly improves the performance in both the high and low current density regions. When both Fe and N are introduced, the performance at lower current densities (catalytic activity) is increased beyond that of the Fe-doped carbon, but the performance at higher current densities is similar to the carbon containing only Fe. Catalysts prepared from resins-Fe(bipy)3SO4 precursors have performance that is only slightly less then CoTMPP adsorbed and pyroloyzed on Vulcan XC-72. Their performance is much better than carbons which have had the N and Fe introduced separately

e 
a 
e 
NON-NOBLE ELECTROCATALYSTS 
N89- 229 
FOR ALKALINE FUEL CELLS 
S. Sarangapani, P. Lessner, M. Manoukian, and J .  Giner  
GINER,  I N C .  
Waltham, Massachusetts 
Noble meta ls ,  when used as e l e c t r o c a t a l y s t s  f o r  oxygen r e d u c t i o n  i n  a l k a l i n e  
f u e l  c e l l s ,  s u f f e r  from t h e  d isadvantages o f  h i g h  cos t ,  s u s c e p t i b i l i t y  t o  
p o i s o n i n g  and s i n t e r i n g .  Carbons a c t i v a t e d  w i t h  macrocyc l i cs  have a t t r a c t e d  
i n c r e a s i n g  a t t e n t i o n  as a l t e r n a t i v e  e l e c t r o c a t a l y s t s  f o r  oxygen r e d u c t i o n .  
I n i t i a l  a c t i v i t y  of these c a t a l y s t s  i s  good, b u t  performance d e c l i n e s  
r a p i d l y .  P y r o l y z i n g  the  macrocyc l i c  on t h e  carbon suppor t  leads  t o  enhanced 
s t a b i  1 i t y  and the  c a t a l y s t s  r e t a i n  good a c t i v i t y .  
Desp i te  t h i s  p romis ing  method o f  t r e a t i n g  m a c r o c y c l i c - a c t i v a t e d  carbon 
c a t a l y s t s ,  t h e  performance decay i s  s t i l l  too l a r g e  t o  be accep tab le .  The 
approach desc r ibed  here i s  des igned to  develop b u l k  doped c a t a l y s t s  w i t h  
s i m i l a r  s t r u c t u r e s  to  py ro l yzed  macrocyc l i c  c a t a l y s t s .  
and coo rd ina ted  l i g a n d s  a re  d i spe rsed  th roughout  t h e  b u l k  o f  t h e  conduc t i ve  
carbon s k e l e t o n .  
The t r a n s i t i o n  meta l  
Two approaches t o  r e a l i z i n g  t h i s  concept a re  be ing  pursued,  b o t h  i n v o l v i n g  t h e  
dop ing  o f  carbon p recu rso rs .  I n  one approach, t h e  p r e c u r s o r  i s  a s o l i d  phase 
carbon-conta in ing  ion-exchange r e s i n .  The p recu rso r  i s  doped w i t h  a 
t r a n s i t i o n  meta l  and/or  n i t r o g e n ,  and t h e  r e s u l t i n g  m i x t u r e  i s  p y r o l y z e d .  I n  
the  o t h e r  approach, t h e  p recu rso r  i s  a gas-phase hydrocarbon.  Th is  i s  
i n t r o d u c e d  w i t h  a t r a n s i t i o n  meta l  spec ies and n i t r o g e n  species i n t o  a r e a c t o r  
and py ro l yzed .  Th is  paper w i l l  m a i n l y  d iscuss  work conducted u s i n g  t h e  s o l i d -  
phase p y r o l y s i s  approach. 
For t h e  so l id -phase p recu rso rs ,  ion-exchange r e s i n s  were  s e l e c t e d .  Weak a c i d  
cation-exchange r e s i n s  and c h e l a t i n g  r e s i n s  have been used. 
i n c l u d e d  Fe3+ (from FeC13>, Fe(bipy>$Oq and NH40H. 
temperatures have ranged from 700-900OC. 
Gas-phase p y r o l y s i s  i s  c a r r i e d  o u t  i n  a tube fu rnace .  The gas-phase p r e c u r s o r ,  
ammonia ( t h e  n i t r o g e n  source) ,  and t h e  i r o n - c o n t a i n i n g  o r g a n o m e t a l l i c  a r e  
i n t r o d u c e d  i n t o  the  r e a c t o r .  The heated zone o f  t h e  fu rnace  i s  ma in ta ined  a t  
a tempera ture  o f  1000°C. 
Dopants have 
T y p i c a l  p y r o l y s i s  
E lec t rochemica l  t e s t s  a r e  per formed u s i n g  the  f l o a t i n g  e l e c t r o d e  techn ique .  
The e l e c t r o d e s  a re  t e s t e d  a t  8OoC i n  7M KOH. 
Severa l  s t u d i e s  have been conducted t o  determine i f  t h e r e  i s  a s y n e r g i s t i c  
e f f e c t  between t h e  t r a n s i t i o n  meta l  and n i t r o g e n  and t h e  e f f e c t  o f  d i f f e r e n t  
methods of  i n t r o d u c i n g  t h e  M-N c o o r d i n a t i o n  on per formance.  
One approach was to  i n t r o d u c e  t h e  meta l  and n i t r o g e n  separa te l y ,  for example, 
by s e q u e n t i a l l y  doping FeC13 and NH40H i n t o  t h e  r e s i n .  
p repared from an undoped ion-exchange r e s i n ,  a r e s i n  doped o n l y  w i t h  N,  a 
r e s i n  doped o n l y  w i t h  Fe, and a r e s i n  doped w i t h  b o t h  Fe and N. I n t r o d u c t i o n  
of n i t r o g e n  a lone has no b e n e f i c i a l  e f f e c t  on t h e  performance o f  t h e  
c a t a l y s t s .  The i n t r o d u c t i o n  o f  t h e  Fe a lone s i g n i f i c a n t l y  improves t h e  
performance i n  b o t h  t h e  h i g h  and low c u r r e n t  d e n s i t y  r e g i o n s .  When b o t h  Fe 
C a t a l y s t s  were 
e 
25 
https://ntrs.nasa.gov/search.jsp?R=19890013628 2020-03-20T03:04:47+00:00Z
and N are introduced, the performance at lower current densities ("catalytic 
activity") is increased beyond that of the Fe-doped carbon, but the 
performance at higher current densities is similar to the carbon containing 
only Fe. 
Catalysts prepared from resi ns-Fe(bi py)3SO4 precursors have performance that 
is only slightly less then CoTMPP adsorbed and pyrolyzed on Vulcan XC-72. 
Their performance is much better than carbons which have had the N and Fe 
introduced separately. 
4 
This work is being supported by the U . S .  Department of Energy under Contract 
NO. DE-AC02-87ER80500.MOOl. 
26 


Non-noble electrocatalysts for alkaline fuel cells

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