Transcript ANODE CATALYSTS FOR LOW TEMPERATURE FUELL CELLS

ANODE CATALYSTS FOR
LOW TEMPERATURE FUELL
CELLS
Branimir N. Grgur
Faculty of Technology and Metallurgy,
University of Belgrade,
Karnegijeva 4, 11001 Belgrade, Serbia,
e-mail: [email protected]
• DaimlerChrysler B-Class F-Cell
(Hydrogen/Oxygen)
• Unvieled at the 2005 Geneva Motorshow
this new generation of F-Cell has a power
output of over 100 kW and a driving range
of over 400 km.
• Fuel Cell Functionality
• Fuel cells generate electricity from a simple electrochemical reaction in
which an oxidizer, typically oxygen from air, and a fuel, typically
hydrogen, combine to form a product, which is water for the typical fuel
cell. Oxygen (air) continuously passes over the cathode and hydrogen
passes over the anode to generate electricity, by-product heat and water.
The fuel cell itself has no moving parts – making it a quiet and reliable
source of power.
Anode Reaction: 2H2 = 4H+ + 4e
Cathode Reaction: O2 + 4H+ + 4e = 2H2O
Overall Cell Reaction: 2H2 + O2 = 2H2O
Hydrogen
- explosive
- cost of H2 production?
- difficult to storage
Possible source of H2, methanol
reforming?
1.2
O2-reduction
1.0
Cell Voltage, U / V
Low temperature fuel cells based on
polymer electrolytes (polymer electrolytes fuel
cells, PEFC) are today one of the most
promising electrochemical power sources for
application in transportation and portable
power generation. Hydrogen generated by
steam reforming of methanol, with a typical
reformer gas composition ~75 % H2, ~25 %
CO2 and 1-2 % CO, was widely considered as a
fuel in the low temperature fuel cells. A major
problem in the development of PEFC has been
the deactivation of Pt anode, as a best hydrogen
electrooxidation catalyst, by even trace level,
e.g. 5-10 ppm, of carbon monoxide. Without
CO in the gas mixtures, cell voltage, U, of the
system based on hydrogen oxidation and
oxygen reduction reaction on platinum catalyst
is between 1 and 0.8 V depending on the output
cell current, as shown in Fig. 1.Operating cell
voltage is smaller than theoretical one of a 1.23
V, mostly because of slow oxygen reduction
kinetics. In the presence of 0.1% CO in
hydrogen fuel, doted line in Fig 1, anode
voltage loss is 0.5 to 0.6 volts, and cell voltage
is reduced to only 0.4 to 0.5 volts, which is
unacceptable high loss.
0.8
0.6
0.4
UT=1.23 V
H2 (0.1% CO)-oxidation
U = 0.3-0.4 V
UI=0.8-0.9 V
0.2
H2-oxidation
0.0
Current / A
Fig. 1. Cell voltage - current
dependencies
of fuel cell reaction.
This problem arise due to the strong CO adsorption on the platinum surfaces (COads),
which lead to the “poisoning” of active sites for hydrogen oxidation reaction. Such strong
binding has been explained by Blyholder by electron donation from 5s carbon monoxide
orbital to metal, and subsequent transfer of two electrons from d metal atomic orbital to
the antibindig 2p* CO orbital, as shown on Fig. 2. This electron transfer is known as
beck-donation.
Energy


2p
+
2
+
5
2p
O
1
1
2s
-
4
-
+
3
2s


1s
1s
1
CO
+
5
2
C
C
de
de
e
O
Metal
Fig. 2. The energy level of carbon monoxide molecules,
and formation of metal-carbon monoxide bonding.
To remove COads from the surface it is necessary to generate some oxygenated
species that can react with COads producing CO2 and to release some free sites on
Pt surfaces for hydrogen oxidation reaction. The mechanism for the oxidative
removal of the COads from platinum anodes has been a topic of intense
investigation for the past 40 years. The overall reaction for removing COads is
COads + H2O = CO2 +2H+ + 2e
(1)
The mechanism of hydrogen oxidation in the H2/CO mixtures on platinum can be
given by the following sequences:
k
ads


COb + Pt
Pt-COads
Pt + H2O = Pt-OHads + H+ + e
k

 CO2 + H+ + 2Pt + e
Pt-COads + Pt-OHads ox
2Pt + H2 = 2Pt-2Hads
2Pt-2Hads = 2Pt + 2H+ + 2e
(4)
(5)
(6)
(7)
(8)
Hydrogen oxidation reaction occurs on the free sites liberated during the time
between COads oxidative removal, Eq. (6) and CO readsorption from solution, Eq.
(4). At the low potential (E< ~0.6 V) the rate constant of COb readsorption is
much higher then the rate constant for COads oxidation, and practically only the
infinitely small number of platinum sites could be liberated, for H2 oxidation.
The hydrogen oxidation reaction reaches maximum at the same potential
where COads is oxidized by Pt-OH (coverage with COads in that potential
region tends to zero). According to this, it is necessary to provide supply of
OH species to adjunct platinum atoms covered by COads by some other metal,
which does not adsorb CO. This catalyst is known as bi-functional catalyst.
Schematic representation of such catalyst is given in Fig. 3.
Pt-COads+M- OH =Pt + CO2+ H+ + M + e
CO2+H+
H2
H
O
O C
M
Pt
Fig. 3. Bi-functional catalyst
(schematic representation).
As the alloying metal (M) should provide –OH for the reaction with
COads:
Pt-COads + OH-M  Pt + M + CO2 + H+ + e
(9)
it is necessary to find such metal which can provide OH on the as low
as possible potential (e.g. near the hydrogen reversible potential).
Although, practically all transition metals are oxidized in the acid
solutions, the fact is that only few metals in alloy with platinum shows
certain activity for oxidation of H2/CO mixtures. In eighties,
encouraging catalytic performance has been reported for Pt-Ru, Pt-Sn
alloy electrodes in the electrooxidation of H2/CO mixtures.
Unfortunately, even with these catalysts and low level of CO, ~100
ppm in a H2/CO fuel, voltage losses in a real PEFC are too high, (for
PtRu alloy ~0.4 V at 0.6 A cm-2). This is due adsorption of CO on Ru
sites as well, blocking the sites for OH nucleation.
The inactivity of different Pt-M alloys could be searched in the metal to
oxygen bond strength, and in the corrosion behavior of alloys. Electronegative
alloying metals such are Ti, V, Nb, Ta ect. have to strong bond strength
(practically in the form of the metal oxides) so this metals cannot provide
oxygenated species for COads oxidation. Some noble metals could provide
oxygenated species which can oxidize COads (e.g. Ru, Ir, Au) but the potential
where this metals are in the form of oxy-hydroxide is to high for the
application as the anode in the fuel cells. From the other hand the transition
metals (Fe, Co, Ni, Mn) and some other metals and metalloids as Zn, Cd, Ga,
Bi, In, Sb, Ge are in the form of oxy-hydroxide at the low potentials ~ 0 V vs.
RHE, but unfortunately most of the platinum alloys with this metals are
corrosion unstable or during the thermal preparation of alloys strong
segregation and enrichment in platinum in to the surface layer occur, and
alloys have platinum like behavior. By analyzing the metal to oxygen bond
strength and thermodynamic data of metal oxidation in acid solution it has
been concluded that platinum alloys with molybdenum or tungsten could be
the metal of choice. Platinum-molybdenum alloy has been chosen for the
further investigation mostly because tungsten has to high melting point
(3410oC) and it is difficult to prepare the alloy with platinum.
On Fig. 4 the polarization
curves for 2% CO and 2%
CO/H2 mixtures on Pt, PtRu,
Pt3Sn, Pt3Mo and Pt3Re
electrodes are shown. It is
obvious that H2 oxidation
follows the CO oxidation,
which is in agreement with
above given discussion.
60
40
0.5M H2SO4,
T=333 K
2% CO/Ar
Pt
Pt3Sn
PtRu
Pt3Re
Pt77Mo23
-2
20
j / Acm
At the potential of interests
(~0.2 V), Pt-Mo shows the
best activity.
80
0
250
H2-Pt
200
150
~0.2 V
100
Fig. 4 Polarization curves for
2% CO/Ar and 2% CO/H2 mixtures
50
0
0.0
2% CO/H2
0.2
0.4
0.6
E / V (RHE)
0.8
Development of the anode fuel cell catalysts, few
steps should be follows:
1. Preparation and characterization of the catalysts.
2. Investigations of the hydrogen oxidation reaction.
3. Investigation of CO and CO/H2 oxidation reaction.
4. Preparation of supported catalysts and testing in the
laboratory and in the real fuel cell.
5. Mechanistic studies.
1. Preparation and characterization of the catalysts.
First step in development of the anode catalyst is preparation. Catalyst can be
prepared by melting, arc-melting in argon atmosphere, ion-beam implantation
ect. followed by the heat treatment for homogenization. After preparation alloy
sample should be mechanically grounded into a cylindrical shape to fit in
Teflon or Kel-F holder for rotating disk electrode (RDE).
After preparation, alloy sample should be characterized by means of some
surface and UHV techniques (AES, LEIS, XPS). Usually, X-ray diffraction is
the most common characterization to obtain bulk composition and structure. In
some cases due to the strong segregations during the heat treatment, surface
composition can differ from bulk composition. An example of that are PtMo
alloy. The bulk composition of alloy was 66 mol.% Pt and 33 mol.% Mo
(Pt2Mo). But after UHV annealing treatment at 970 K for 30 min. resulting
surface concentration of Mo was 23 mol.%, and after Ar ion (0.5 keV)
spattering 30 mol.%, as shown in Fig. 5.
970K 20 min
Pt-Mo
0.5 keV Ar
Pt237 C
Mo186
Pt
Pt43 64
x1
x2.5
Intensity / arb. units
dN / dE
Mo221
Intenzitet /
Arb.jed.
+
PtMoimp
O
18 mol % Mo
+
LEIS He 2 keV
235
230
1000
0.8
0.9
800
300
400
600
400
200
0
1.0
Mo 3d
500
Electron energy / eV
Fig. 5. Derivative mode AES spectrum of
the PtMo following sputter-cleaning and
annealing at 970 K.
Insert: LEIS spectra of the same surfaces.
Intensity / arb. units
200
225
B.E. / eV
E1 / E0
100
80 78 76 74 72 70 68 66
Mo3d
Pt
Mo
0.7
0
Pt4f
1
1'
2
3
2'
3'
240
235
230
225
220
Binding energy / eV
XPS- bulk composition, chemical
shifts, oxidation states of metals
2. Investigations of the hydrogen oxidation reaction.
The first condition that an catalysts could be used as fuel cell anode is a good
catalytic properties in hydrogen oxidation reaction. After recording the cyclic
voltammograms, the solution was saturated with H2 and hydrogen oxidation
reaction was examined, as shown in Fig 6. At Pt85Mo15 and Pt77Mo23 electrode
reaction rates are identical with the oxidation of hydrogen on pure Pt. The
overpotential/current relation closely follows that for pure diffusion control:
•
 = -2.303(RT/2F) log (1-j/jd)
(10)
where jd is the measured diffusion-limited current density at any rotation rate
and j is the observed current density at overpotential .
5
While
a
relatively
small
surface
concentration of Mo (15 and 23 mol.%)
4
2500
atoms
has
no
effect
(practically
unmeasurable by standard RDE methods) on
1600
3
3600
the hydrogen oxidation reaction, the HOR is
900
dramatically inhibited on the Pt70Mo30
2
surface. Figure 6 shows that below 0.4 V the
Pt Mo
900
polarization curve practically have no
1
rotation rate dependence, characteristic for
pure diffusion control. This implies reaction
Pt Mo
Pt
0
control with probably Tafel (chemical)
Pt Mo
reaction as a rate-determining step. Above
-1
0.4 V, some increase of the current density
0.1
Pt Mo
and rotation rate dependencies has been
-2
observed, but more likely as mixed
0.0
0.2
0.4
0.6
0.8
1.0
diffusion-reaction control. Above 0.6 V, the
E / V (RHE)
current density decrease again which is
connected with oxidation of Pt atoms on the
surface. This dramatic change in the shape of
Fig. 6. Cyclic voltammograms (50 mV s-1)
in deaerate 0.5 H2SO4 of the different surfaces polarization curve is strongly connected with
(market at the figure) and polarization curves the chemistry of Mo atoms at the surface.
j / mAcm
-2
 / rpm 3600
70
85
30
15
77
23
70
30
(1 mV s-1) in H2 saturated electrolyte at 333 K
at different rotation rates
3. Investigation of CO and CO/H2 oxidation reaction.
• The second condition that an catalysts could be used as fuel
cell anode is a good catalytic properties in oxidation of
CO/H2 mixtures. Figure 7 shows the quasi-steady state (1
mV/s) polarization curves of 0.1 % CO/H2 mixtures on the
three different PtMo bulk alloy surfaces and Pt in 0.5 M
H2SO4. In order to assure the equilibration of the electrode
surfaces with CO prior to the recording the polarization
curve, the electrode was held at 0.1 V for 3000 s (see the
insert of Fig. 8) at a rotation rate of 2500 rpm, followed by
potential scan from 0 V.
0.8
0.7
0.4
H2 / % CO
1
-2
Pt
Pt70Mo30
Pt77Mo23
Pt85Mo15
0.6
0.3
0.2
0.5 M H2SO4
T=333 K
=2500 rpm
v=1 mV/s
0.1
0.0
0.5
0.4
j / mA cm
E / V (RHE)
0.5
0.7
E / V (RHE)
0.6
0.1 % CO / H2
0.05
0.1
Pt (0.1% CO/H2)
0.01
0
1000
2
2000
t/s
-2
40 A cm
-2
106 A cm
0.1
1
-2
j / mA cm
Fig. 7. Polarization curves (1 mV s-1) for
the oxidation of 0.1% CO in hydrogen on
different PtMo surfaces and pure Pt in
0.5 M H2SO4 at 2500 rpm, 333 K, followed
by 3000 s of poisoning at 0.1 V.
H2 / % CO
2
0.1
0.05
0.1
-2
210 A cm
0.0
0.01
T=333 K
=2500 rpm
v=1 mV/s
0.1
0.3
0.2
Pt77Mo23
0.01
0.1
1
-2
j / mA cm
Fig. 8. Polarization curve for the oxidation of H2
on the Pt77Mo23 alloy surface containing various
levels of CO. Insert: Time dependence of the current
density at 0.1 V following a switch from pure
H2 to H2/CO mixtures.
4. Preparation of supported catalysts and testing in the
laboratory and in the real fuel cell.
Once, when some alloy shows potentially good activity, it should be prepared as
carbon supported catalyst and tested in the real conditions. Preparation methods
can differ depending on the alloy, and some principles can be found in Ref. [16]
[K. Kinoshita, Electrochemical Oxygen Technology, The Electrochemical
Society Series, Pennington, N. J., 1992]. For testing such catalyst in the
laboratory cell easy rotating disk method developed by Schmidt at al. [T.J.
Schmidt, H.A. Gasteiger, G.D. Stab, P.M. Urban, D.M. Kolb, R.J. Behm, J.
Electrochem. Soc. 145 (1998) 2354] can be applied. Briefly, the thin catalysts
layers were prepared by attaching ultrasonically redispersed catalysts
suspension in distilled water onto the glassy carbon or gold RDE, resulting in a
constant metal loading in the range of of 10-30 μg cm-2.After drying in flowing
high purity argon at room temperature, the deposited catalyst layer was then
covered with a diluted aqueous-methanol Nafion® solution.
10-30 l cm-2
Nafion 20 l
GC, Au
catalyst
suspension in
watter
Electrochemical cell
drying
RDE
Ultrasonic bath
drying
RDE
4
0.4
2500
0.2
3
1600
0.0
-2
PtMo 4:1
-0.6
v = 50 mV/s
-0.8
 = 400 rpm
1
-2
-1
60 C
0.5 M H2SO4
o
-1.0
0.0
0.1
0.2
0.3
0.4
0.5
2
0
0
0.6
E / V (RHE)
-1/2
0.077 mA cm rpm
4
0
-2
o
t = 60 C
0.5 M H2SO4
jd / mA cm
-0.4
j / mA cm
-2
j / mA cm
900
2
-0.2
10
20
30
40
50
1/2
( / rpm)
-2
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
CV
4

E / V (RHE p(H2)=p )
Hydrogen oxidation
100%
0.10
75%
45%
H2 % in H2 / N225%
-2
4
jd / mA cm
0
2
0.06
jk / mA mg

E / V (RHE, p(H2) = p )
j / mA cm
-2
2
0.08
-1
E = 40 mV
o
60 C
0.5 M H2SO4
10
3
slope = 1.1
10
2
0.1

1
p(H2) / p
0.04
0.02
ba = 33 mV dec
-1
0
0.0 0.2 0.4 0.6 0.8 1.0
p(H2) / p
0.0
0.1
0.2
0.00

0.3
10
0.4

2
10
3
10
-1
100 ( id j / jd-j) (j in mA mg (alloy))
E / V (RHE p(H2)=p )
Partial pressure effect
Kinetics study
4
Knowing the metal loading it is possible to simulate real fuel cell conditions
250
PtMo (4:1)
PtMo (3:1)
PtRu (1:1)
E / V (RHE)
200
150
75% H2
25% CO2
250 ppm CO
100
o
60 C
0.5 M H2SO4
2500 rpm
50
10 g cm
Pt/H2
-2
0
0
100
200
-1
300
-2
j / mA mg (alloy) cm
Second, but long term method is formation of small scale real fuel cell. In such cell cathode
should be the same for comparison, and anode should be exchangeable. By trying different
anode at the same operating conditions, as shown on Fig. 9, voluble conclusions can be
drown, like dependence of cell voltage on applied current density, possible deactivation of
catalytic properties etc.
1.2
100 ppm CO Fuel
R
Average Pt ELAT Standard-H2 Data
R
Standard Pt ELAT
Pt-Mo-Proprietary
Pt-Ru (1:1 a/o)
Pt-Sn (95:5 a/o)
Cell Voltage / Volts
1.0
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
1000
Current Density / mAcm
1200
1400
-2
2
Comparison of Anode Catalysts, 16 cm Active area-Nafion 115
-2
0
1.0 mgcm Metal Loading, Fuel/Air - 3.5/4.0 Bar, t=70 C
1600
5. Mechanistic studies.
Determination of the mechanism in the CO/H2 oxidation reaction is not necessary, but
could be very useful in complete understanding of catalyst behavior. One example is
possible mechanism of CO/H2 oxidation on PtMo alloy. Based on the experimental
evidences, the following mechanism for CO oxidation at low potential region on PtMo
alloy could be given as follows:
CO 
k
(CO)
ads

P t - Mo(OH)3 
 COads
kdes (CO)
COads - P t 
k (CO)
 P t  Mo(OH)3
ox

Mo(OH)3 
 
 CO 2
kred (CO2 )
kox
 P t  Mo(OH)2  H   e


P t  Mo(OH)2  H 2 O 

 P t  Mo(OH)3  H  e
(12)
(13)
(14)
kred
where: kads(CO) and kdes(CO) are rate constants for CO adsorption and desorption;
kox(CO) and kred(CO2) are rate constants for COads oxidation and CO2 reduction; kox
andi kred are rate constants for Mo(OH)2 oxidation and Mo(OH)3 reduction. Rate
determining step (r.d.s) reaction given by Eq. (13) is proposed.
For the steady state conditions the change of COads coverage in Eq. (14) can be given as:

CO
p (CO)
 kads (CO)
(1  CO  OH )  kox (CO)COOH  0

t
p
(15)
where kox(CO) is potentially dependent rate constant:
kox(CO)=k*ox(CO) exp
 FE 


 RT 
(16)
(k*ox(CO) is chemical rate constant and  is the transfer coefficient).
After rearranging Eq. (15) with neglecting the kred(CO2) and kdes(CO), the COads coverage is given
by:
CO 
kads (CO)[ p(CO) / p  ](1  OH )
 FE 

kads (CO)[ p(CO) / p  ]  kox
(CO)OH exp

RT


(17)
Considering a simple site-blocking model for the poisoning of H2 by adsorbed CO, given by:
j = j(H2) [1-(CO)]n
(18)
where j is the measured current in the presence of CO, j(H2) is the limiting diffusion current for
pure H2 oxidation,
so, j in the presence of CO can be given by substituting (CO) from (17) in (18):
n



kads (CO)[ p(CO) / p ](1  OH )


j  j( H 2 ) 1 






k
(
CO
)[
p
(
CO
)
/
p
]

k

exp

FE
/
RT
)

ads
ox OH


(19)
where n, depending of the hydrogen oxidation mechanism is 1 for
the slow Heyrowsky reaction or 2 for slow Tafel reaction.
Equation (19) only qualitatively describes the polarization curve
for oxidation of H2/CO mixtures, as it can be seen from Fig. 10,
where the simulation of Eq. (19) is given. Quantitative equation
should include coverage dependence of kads(CO), and potentially
dependence of different forms of Mo oxy-hydroxides from which
some are active and other are not for CO oxidation reaction, but
unfortunately this data are unknown in the presence.


k ads (CO)[ p(CO) / p ](1   OH )
j  j (H 2 )1 





k
(
CO
)[
p
(
CO
)
/
p
]

k

exp

FE
/
RT
)
ads
ox OH



1
4
5
4
1
2
3
4
2
3
j / mA cm
-2
3
0.1% CO
n=2
1 - pure H2
2
kads(CO)=100,
*
*
kads(CO) - kox(CO)
2 - 100-0.005
3 - 100-0.05
4 - 100-1
5 - 100-10
1
kox(CO)=0.05
1- pure H2
2- 2% CO
3- 0.1% CO
4- 0.05% CO
0
0.0
0.2
0.4
0.6
0.0
0.2
0.4
E / V (RHE)
0.6
2
To have the compete picture of complexity of CO oxidation reaction it should
be include in Eq. (19) the coverage dependencies of the rate constants for CO
adsorption reaction:
 ads G  (CO)  adsG(CO ) 

kads (CO )  k P(CO )exp 

RT


(24)
where k is the constant, P(CO) is the coverage dependent sticking probability
of CO adsorption, adsG(CO) Gibbs energy of adsorption for CO = 0, 
symmetry of activation barrier, and [adsG(CO)] coverage dependent part of
Gibbs energy of adsorption.
This equation includes the existence of weakly adsorbed states of CO, when
CO is higher then 0.8.
CONCLUSION
• Since the anode catalyst has been investigated for more than
forty years, only few alloys show some activity in the
oxidation of CO/H2 mixtures. This fact is discouraging, but
very challenging.
• One should try with new catalyst formulation including
preparation of new (secondary, ternary) alloys and variation of
the alloy composition.
• Other could try with different supports which can change alloy
properties.
• Understanding of thermodynamics and kinetics of alloys and
reactions could help in such search.
• Try with air Bleed Effect with reformat ?