Tungsten carbide, which is known to be active for hydrogen oxidation and CO tolerant has a hexagonal structure. Titanium carbide is inactive and has a cubic structure. Four different samples of the cubic alloys Wx-1TixC were prepared and found to be active and CO tolerant. These alloys are of interest as possible phosphoric acid fuel cell catalysts. They also are of interest as opportunities to study the activity of W in a different crystalline environment and to correlate the activities of the surface sites with surface composition

Mcalister, A. J.

English

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https://ntrs.nasa.gov/search.jsp?R=19810009972 2020-03-21T15:10:53+00:00Z
(NASA—CR-165221) NON—NOBLE CATALYSTS AND	 N81-1849:
CATALYST SUPPORTS FOR PHOSPHORIC ACID FUEL
CELLS Quarterly Report, Aug. — Nov, 1980
Y	 (National Bureau of Standards) 13 p
	
Unclas
HC A02/MF A01
	
CSCL 10A G3/44 41533
CR 165221
Non—Noble Catalysts and Catalyst Supports
For Phosphoric Acid Fuel Cells
1st Quarterly Report
A. J. McAlister
Metallurgy Division
National Bureau of Standards
Washington, DC 20234
August — November 1980
Prepared for:
Natioral Aeronautics and Space Administration
Lewis Research Center
Under Agreement C-46229—D
For:
U.S. Depi..rtment of Energy
Energy Technology
Division of Fossil Fuel Utilization
Under Interagency Agreement D,-Ai- 0j-80ET 17068
t ,	 I
Introduction
Under this contract, work is being carried out on three
tasks:
I. Anode catalysts resistant to CO poisoning
II. Cathode supports for Pt catalysts
III. W—Ti carbide catalysts.
Under Task I, work has been principally directed toward the
possibility that by supporting highly active, CO sensitive Pt
upon weakly active, CO tolerant WC, an anode catalyst with
intermediate activity and CO tolerance might be achieved. This
possibility was suggested by observation by Bockris and
McHardy [1] of enhanced specific activity of cathodic reduction
of 02
 supported on sodium tungsten bronze, and by the
likelihood [Hamilton, 2] that Pt will segregate to the WC
surface, perhaps forming monolayer or sub—monolayer coverage.
Evidence that very highly dispersed Pt can be prepared on WC,
was presented in our Oct. 1 79—July 1 80 progress report.
Improved CO tolerance was indicated, but interpretation was not
definitive because, (a) the basis of comparison was Pt sheet,
not finely dispersed Pt, and (b) the Pt/WC electrodes obviously
contained both a very highly dispersed and a more coarsely
dispersed fraction of Pt, and only the overall reduction in
activity due to CO poisoning could be ascertained. Immediate
objecti°es in this task will be to prepare Pt/WC samples in
which the highly dispersed fraction only is achieved, or at
2
least maximized, and to study them in concentrated H 3PO4 at
elevated temperature. In connection with the objectives of
this task, it is of interest to note that the W1—xTixC
cubic alloys currently under study in Task III display
apparently complete tolerance to poisoning by 3 percent CO
included in the H 2 gas feed in measurements made in 1M
H3PO4 at 23*C.
Task II was motive, - pd by our past observation [3] of
exceptional stability of several refractory hard metals,
notably TaN and TaB, at high overpotential in H 3PO4 . These
offer the possibility of corrosion free performance as 02
reduction catalyst supports in acid fuel cells.
In Task III, the cubic alloys W 1—xTi xC are being
studied as anode catalysts over the range of solubility of W in
cubic TiC. TiC itself is inactive [4,5], while the alloys with
W have been found in the present investigation to be active and
CO tolerant. Work is in progress to determine the rate
constants as function of composition from polararization
studies performed on thin pororous electrodes. The work is of
interest from the standpoint of the catalysts themselves, as
well as the opportunity to observe the behavior of W in a
different crystalline environment (cubic as opposed to the
hexagonal structure of WC) and to correlate the activity with
the titration of surface sites with Ti.
3
_.....^
Progress During the Reporting Period
Task I — Two preparations of Pt/WC were carried out by
precipitation and reduction of chloroplatinic acid. Loadings
were 0.053 x 10-6 and 0.013 x 10-6
 gm. Pt per cm2 of WC
BET area. The WC support was of low specific area (0.38m2/gm
BET) and activity (i o
 - 0.6 x 10`6
 A/cm2
 at 23' in 1M
H3 PO4 ). The aim here is to increase the fraction of Pt
residing in the highly dispersed form. Thin porous electrodes
of each preparation were prepared in the manner described in
our Oct. 1 79 — July 1 80 progress: report, where the
precipitation/reduction procedure is also described.
Task 1I — An attempt was made to prepare dispersed Pt on
TaN by precipitation and reduction of chloroplatinic acid.
Upoil removal of the sample from the reduction chamber, it was
found that the H 2
 distribution tube had failed, and that the
internal residue normally found in the cool area adjacent to
the furnace was missing. A porous electrode of this material
was prepared and examined by linear sweep voltammetry and H2
stripping in 1M H 3PO4 ; indicated Pt content was nil. The
reduction apparatus has been repaired, and further preparative
attempts will be carried out.
Task III — Thin porous electrodes of four different samples
of the cubic alloy W1—xTi xC were prepared and examined in
N21' 	 and 3 percent CO/H 2 saturated 1M H3PO4 by
linear sweep voltammetry, at a sweep rate of 2.5 mV/sec. The
4
MEN
	
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l
%
	
	
compositions of the alloys are listed in Table 1. All of the
alloys were active for H2 oxidation and, within the
reproducibility of the measurements, unaffected by CO.
An attempt was made, within the framework of the thi'ii
porous electrode model [6]. assuming reaction control and Tafel
kinetics, to obtain the transfer coefficient and total exchange
current for each sample. Anomalous results were obtained
which, in the light of re—examination of the raw data which
showed high surface impedances, were in all likelihood due to
transient effects during the dynamic measurements. Steady
state measurements were then undertaken (and seriously
impeded	 by a sudden break dawn in our electronically
controlled data aquisition system). At the present time,
steady state polarization measurements have been made on three
of the four samples. Hydrogen oxidation and evolution have
been studied in the cell described in our Oct. 1 79—July 180
Progess Report. The rate parameters reported for
W.24 Ti . 76 C in our July Monthly Narrative were based on the
dynamic measurements and should, of course, be disregarded.
Transfer coefficients for H 2 evolution and oxidation for the
three samples now studied are listed in Table 1, as is the
exchange current density for the W^ 24Ti .76C sample (the	 A
only one of the four for which BET measurements are currently
available. Other samples have been sent to a commercial
laboratory for BET studies, and the results should be available
5
W.
soon.) Fig. 1 shows the results of the polarization studies on
W.24 Ti . 7fiC; the data on the other two samples are
comparable. Studies in 3 percent CO/H 2 have beeen carried
out in the steady state on two of the samples, and the CO
tolerance noted in the dynamic studies confirmed.
The assumption of Tafel kinetics is rather well born out by
Fig. I. The assumption of reaction control, on the other hand,
has not been adequately justified. We are confident, however,
that these electrodes are operating under reaction control, and
will outline our reasons for our confidence, as well as
measures now being undertaken to place this belief on firmer
ground.
Our assumption of reaction control is based largely on long
experience with electrodes of this type [7], fabricated of
relatively coarse powders of low activity catalysts lightly
bonded wth PTFE. We have had good success in modelling such
electrodes, using gross electrode densities and reasonable
estimates of the product of the H2 diffusion coefficient and
solubility. Moreover, pore volumes inferred fom the difference
between gross electrode density and bulk catalyst density have
been found in reasonable agreement with those inferred from
mass increase with acid pickup. Given the low oxidation
I
currents obtained in the present work, something very strange
would have to occur if our ere:ctrodes were not reaction
limited. We will of course explore the question further. Much
6
^L
thicker electrodeswill be prepared, and activation energy
measurements made, looking for diagnostic slope and activation
energy reductions.
It is of some interest to compare i o determined for the
1—x - 0.24 sample with that measured for a stoichiometric,
"oxygen free" WC sample studied under the same conditions.
For the WC, aox = - gle ar^d i o = 0.6 x 10-7 A/cm2.
Assuming approximately equal metal site surface densities on
the two materials and no depletion or enrichment of W on the
WTiC surface, multiplication of the WC i o value by the simple
dilution factor of .24 yields i o - 0.1 x 10-7 A/cm2,
while multiplication by the probability of adjacent W sites,
(.24) 2 , yields i o = 0.3 x 10-8 , quite close to the
observed 0.6 x 10-8 for the WTiC sample, and suggesting
similar active sites on the two materials. It will be of
considerable interest to see if this relation holds over the
entire range of W concentration in WTiC.
7
Current Problems
In this reporting period, two equipmnt failures (the above
mentioned electronic data aquisition system and reduction
system glassware problems) caused significant delays. Repairs
have now been made and no further problems of this type are
foreseen. More seriously, a family illness has curtailed the
investigator's efforts during most of November. This situation
appears to have stabilized and it is hoped that work can now be
fully resumed.
Work Planned
Task I — Characterization of the new Pt/WC preparation by
H 2 stripping and plating, and measurement of H. and 3
percent CO/H2 isotherms on these preparations will be carried
out as soon as possible.
Task II — Preparation of TaN and TaB supported Pt
catalysts has begun, and preliminary evaluation of these
catalysts will be the main object of the next quarter's work.
Task III — Steady stat% measurements will be carried out on
the remainirg WTiC sample, and the measures outlined above for
ascertaining whether the thin porous electrodes are in fact
truly reaction controlled pursued.
R
8
References
1. Bockris, J. 0. M. and McHardy, J., J. Electrochem. Soc. 120,
61 (1973).
2. Hamilton, J. C, Phys. Rev. Letters 42, 9189 (1979).
3. Bennett, L. H., Cohen, M. I., Dragoo, A. L., Franklin, A.
D., McAlister, A. J., and Young, K. F., Materials for Fuel
Cells, NBSIR 77-1270, 1977. (National Technical Infor-
mation Service, S pringfield, VA 22151).
5. Bethin, J. R., Ph.D. Thesis, University of Illino+s,
Urbana, I11. 1979.
6. Vogel, W., Lundquist, J., and Bradford, A., Electrochimica
Acta 17, 1735 (1972).
7. McAlister, A. J., Cohen, M. I., Electrochemica Acta, in
press, and previous reports to DoE in this series.
9
Table 1
Composition and apparent rate constants for H 2 oxidation and
evolution on W 1-xTi xC alloys in 1M H3PO4 at 23RC.
1-x	 x	
aox
	 Qevol
	
io (A/cm2)
.24 .76 .20 .57	 0.6 x 10-8
.31 .69 .15 .36	 -
.45 .55 .11 .59	 -
.47	 .53	 -	 -
Z
.
10
Figure Captions
Fig. 1. Current density (normalized to total electrode BET
area) for W .24Ti .7CC in N2 saturated 1M H3PO4 at
23°C, open circles; and the H2 oxidation current obtained as
the difference between the current densities oL.alned in H2
and N2 saturated acid at positive overpotentiai (vs. a
dynamic Pt reference electrode), filled circles.
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Non-noble catalysts and catalyst supports for phosphoric acid fuel cells

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