CO - Enea

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Sviluppo di Catalizzatori Anodici per il Funzionamento
Multifuel di Celle a Combustibile ad Ossidi Solidi
Antonino S. Aricò, Massimiliano Lo Faro,
Marco Ferraro, Vincenzo Antonucci
CNR-ITAE Institute, Via Salita S. Lucia sopra Contesse, 5 – 98126 Messina Italy
mailto: [email protected]
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Solid Oxide Fuel Cells
Internal reforming:
CH4 + H2O € 3H2 + CO (S/C > 2)
H2 + O2- € H2O + 2eCO + O2- €CO2 + 2e-
Cathodic reaction:
O2 + 4e- € O2-
Anodo
cermet di nichel e
zirconia stabilizzata
con ittria
Elettrolita
zirconia stabilizzata
con ittria
Direct oxidation:
CH4 + 4O2- € 2H2O + CO2 + 8e-
Catodo
manganito di
lantanio drogato
con stronzio
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Ossidazione diretta di combustibili organici (metano, gas
naturale, LPG) e biocombustibili (alcoli, glicerolo, biogas) a
temperature intermedie
• Lenta cinetica di ossidazione
• Processo utilizzabile a partire da 500 °C
• Catalizzatori metallici o ossidi, promotori
• Formazione di carbone
• Stabilità ai cicli redox
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Fuel processing
Desulphurization
NG
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Direct electrochemical oxidation of organic fuels
Working Temperature ≤ 750°C
Fuel: CH4, LPG, biofuels
Oxidant (air)
Cathode
Membrane
Electrolyte
Anode
External
electric
circuit
Fuel
Products
Cost Reduction
1) Substitution of expensive
materials by stainless steel
for electrical interconnectors
2) No reforming process required.
(CH4 + H2O → CO +3H2)
3) System simplification
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Direct electrochemical oxidation of organic fuels in SOFCs
The direct utilization of alcohol fuels into the anode compartment of a SOFC
appears as a suitable option. Typically, air or steam has to be supplied with the
fuel to the anode compartment to prevent coking. An anode materials resistant
toward coke formation under dry conditions can significantly simplify the SOFC
system
e-
Ceria‐based electrolyte
Oxide‐based electrocatalysts
O2-
CO2 + H2O
Air
Exhaust
H2, CH4, GPL,
CO, alcohols
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Methanol-fed SOFC
Methanol is a liquid fuel of strategic interest because of its suitable reactivity and the high energy density: 6 kWh kg‐1
Methanol may represent a “carbon dioxide‐neutral” fuel if produced from biomasses. Presently, it can be obtained from biomass fermentation at 66% efficiency.
Methanol does not require a new infrastructure for fuel distribution like hydrogen and it shows advantages in terms of storage and
handling.
It may allows a reduction of the size, complexity as well as cost of the SOFC system especially for remote generation, auxiliary power
units, and portable power sources
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Need of alternative anodes for direct utilization of organic fuels in SOFCs
Ni-cermet (Ni-YSZ 60-40% vol) is the preferred electro-catalyst for SOFCs. Yet, Ni is also a
good catalyst for the hydrocarbon cracking reaction. The direct feed of dry organic fuels
in at Ni-cermet anode would result in carbon deposition and rapid degradation.
Modification of the electronic density of Ni particles by effect of oxide-supports can
significantly affect the activity and tolerance to carbon deposition.
Oxides with mixed ionic and electronic conductivity (MIEC) may modify the chemical and
electronic properties of dispersed Ni particles and their propensity to form carbon deposits
under anhydrous conditions.
In this work, we have conducted an electrochemical study of the direct oxidation process of
pure methanol (99.99%) in an SOFC based on a Ni-modified La0.6Sr0.4Fe0.8Co0.2O3 anode
catalyst in the presence of ceria-based electrolytes.
We have prepared a highly dispersed Ni-catalyst on the electro-conductive perovskite and
successive thermal treatment in air and reduction in hydrogen.
The rate of the electrochemical oxidation of methanol was investigated and compared with
that of synthesis gas.
Ex-situ catalytic studies were carried out to get insights about catalytic activity and
chemical stability.
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ANODE POWDER PREPARATION
•La0.6Sr0.4Fe0.8Co0.2O3 (LSFCO) powder was impregnated at 50 °C with a solution of Ni nitrate in water (0.5 M).
•The powder was first dried and then calcined in oven at 500 °C for 5 h (heating rate, 2 °C min‐1; cooling rate: 2 °C min‐1). Afterwards, a mixture of a 70% wt. Ni/LSFCO catalyst and 30 % wt. Ce0.9Gd0.1O2
(CGO, Praxair) powders was prepared by ball milling in ethanol for 20 h. The resulting Ni content was 10 % wt. on LSFCO as determined by X‐ray fluorescence
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Anode powder structure
morphology characterization
d
š206
100
„200
š211
š116
š107
š213
d
d
200
d
š200
300
š006
š114
400
„111
š103
š110 d
500
Intensity / a.u.
•X‐ray diffraction analysis of the thermally activated at 1100 °C and subsequently reduced at 800 °C showed, the occurrence of metallic Ni [JCPDS 04‐0850] and La‐depleted perovskite Sr(Fe0.5Co0.5)O2.88, as well as La2NiO4 [JCPDS 33‐
712]. 600
300
200
surface
Co Kα
400
100
Ni Kα
500
Ni Kα
FeKα Fe Kα
600
Co Kα
30
La L
700
La L
Counts / counts per second
0
bulk
0
4.5
5
5.5
6
KeV
6.5
7
7.5
35
40
45
50
55
60
65
70
2 Theta / degree
•The morphology of Ni particles on the electroconductive perovskite support consists of well dispersed spherical Ni particles with a size ranging between 10 nm and 35 nm on the surface of the electroconductive perovskite; segregated on the surface as confirmed by EDAX analysis
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Surface area characteristics of the anode powder
3
Volume / cm g
-1
16
14
12
10
8
0.035
14
0.030
12
0.025
10
0.020
8
0.015
6
0.010
4
0.005
2
0.000
10
Relative Volume / %
18
Cumulative Volume / cm 3 g -1
20
0
1000
100
Diameter / Å
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
•Pore size distribution (PSD) in the catalyst shows two broad PSDs. •The cumulative pore volume in the present case is 0.0325 cm3 g‐1. •The specific surface area for the calcined catalyst calculated by using the BET equation was 5.51 m2 g‐1. Temperature programmed reduction
of anode powder in hydrogen
α2
1500
Rate of H2 consumption / cps
1250
α1
1000
750
α4
α3
250
0
100
200
300
400
500
600
•TPR analysis showed a complex reduction profile for Ni, probably because of the different metal‐support interaction for various sites. •A reduction peak centered at about 500 °C was observed for Ni species with two shoulders at about 412° and 577 °C. •Comparing such data with the total amount of metallic Ni from XRD, it appears that only 67 % of the metallic Ni is formed below 650 °C. By assuming a surface atomic density for Ni of 6.5 Å/atom and a particle size of 27 nm, a value of MSA equal to 0.8 m2 g‐1 of nickel catalyst was calculated. 500
0
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700
800
Temperature / °C
α1 / 412 °C
α2 / 498 °C
α3 / 577 °C
µmol H 2 g-1
percentage of
metallic Ni
µmol H 2 g-1
percentage of
metallic Ni
µmol H 2 g-1
21.54
20.62 %
33.07
33.89 %
11.78
percentage of
metallic Ni
12.07 %
α4 / above 660
°C
percentage of
metallic Ni
33.42 %
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Selectivity to syngas / %
Catalytic studies
100
The catalytic behavior of the anode powder
with regard to the conversion of methanol and
selectivity to syngas and H2 was studied in the
range of temperature 500 °C- 800 °C under ATR,
SR and POX.
(a)
95
POX
90
SR
85
ATR
80
400
500
600
700
800
900
The catalyst started to convert methanol
immediately under all reaction conditions.
Reaction temperature / °C
Selectivity to H2 / %
80
Reaction
(b)
SR (S/C=2.5)
75
SR
ATR (S/C= 2.5; O/C=0.5)
70
ATR
POX
65
POX (O/C=0.5)
T (°C)
500
600
700
800
500
600
700
800
500
600
700
800
Selectivity to Syngas
(%)
91.32
87.88
87.59
86.84
87.42
88.08
81.40
81.70
91.54
91.36
91.90
92.02
Selectivity to
H2 (%)
72.45
72.66
72.76
72.38
64.92
67.55
68.53
67.95
63.60
64.53
64.99
65.77
CO/CO2
2.17
1.26
1.20
1.13
1.79
1.72
0.71
0.77
3.48
3.48
3.62
4.23
60
400
500
600
700
800
Reaction temperature / °C
900
The presence of oxygen in the reaction stream
contributes to decrease the selectivity to hydrogen
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Catalytic studies
40
30
Time / h
80
100
120
Product composition with time on stream
in ATR of methanol at 700 °C; >100 h test
60
65
70
2 theta / degree
5000
(d)
4000
ATR @ 800°C
3000
2000
1000
POX @ 800°C
d220
z400
60
55
z311
d211
z222
40
50
„200
20
45
„111
d200
z220
0
40
d111
10
35
š103
d110
z200
CO2
CO
20
d220
z400
SR @ 700°C
0
30
0
z311
d211
z222
1000
POX @ 700°C
„200
d210
2000
„111
d200
z220
50
ATR @ 700°C
3000
d111
60
4000
d110
z200
H2
70
Intensity / a.u.
80
Intensity / a.u.
Product concentration / %
The catalyst also showed a good stability over 100 h with no deactivation neither
significant loss of catalytic activity towards H2. It maintained good selectivity and
5000
yield.
(c)
SR @ 800°C
0
30
35
40
45
50
55
60
2 theta / degree
65
70
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Electrocatalyst discharged after the ATR catalytic study
(a)
(b)
La2NiO4
perovskite
Carbon
(c)
2
Signal Intensity / mV
CGO
Carbon
1.8
CHNS-O analysis
1.6
1.4
1.2
N2
1
0
200
400
600
Time / sec
•Post-operation CHNS-O analysis carried out on
the catalyst after the endurance test (ca 10 h)
under ATR of methanol at 700 °C revealed a
carbon content of only 1.13% wt, also envisaged
from TEM analysis.
Fuel cell experiments
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•The electrochemical performance approached a power density of 350 mW cm-2 at 700
°C with both direct methanol feed and syngas fuels, the activation losses were
moderate.
•The OCV approaches 0.760 V in both cases. This is lower than that reached by
conventional hydrogen fed yttria-stabilized zirconia (YSZ) based SOFC devices mainly
due to the mixed ionic-electronic conductivity of the CGO electrolyte.
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Ac-impedance experiments
•Methanol oxidation results in a more evident high frequency arc than syngas.
•The low frequency semicircle is similar in both experiments being the same
electrocatalyst used at the cathode.
•The series resistance was around 80% of the total cell impedance as expected for a
250 μm thick electrolyte based cell.
•Methanol oxidation or in-situ conversion to syngas is slower than syngas oxidation
determining a slight increase of polarization resistance.
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0.8
Potential / V
0.4
0.7
0.3
0.6
Hydrogen
Methanol 0.2
Syngas
Glycerol 0.1
Propane
0.5
0.4
0.3
0.0
0.3
0.6
0.9
1.2
1.5
-2
Current density / A cm
0.0
Power density / W cm
0.5
-2
Fuel flexibility characteristics fo the perovskite supported Ni anode:
Fuel cell polarizations in the direct utilization mode
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Electrochemical Impedance Spectroscopy
0.08
0.06
0.04
Hydrogen
Methanol
Syngas
Glycerol
Propane
-Z'' / ohm cm
-Z'' / ohm cm
2
0.10
2
•EIS plots collected for the SOFC fed with various pure fuel consisted of a small highfrequency depressed arc overlapping to a large low-frequency arc.
•The series resistance (Rs) derived from the high frequency intercept on the real axis of
the Nyquist plot, observed in the case of SOCF fed with hydrogen was due mainly to
the electrolyte thickness. The value of series resistance changed significantly with the
kind of fuel.
•The values of Rs for these experiments appeared to be consistent with the hydrogen
production levels observed in the ATR experiments.
0.2
0.1
0.0
0.2
0.3
0.4
0.5
Z' / ohm cm
0.6
2
0.7
0.02
0.00
0.20
0.25
0.30
Z' / ohm cm
0.35
2
0.40
Testing fuel flexibility
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Post-operation
SEM analyis
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Alternative catalysts for direct methane oxidation: NiCu alloys-ceria cermets
2 nm
Current density / A·cm
-2
0.60
0.50
0.40
Ba-doped NiCu / CGO,
0.6V, 750°C
0.30
0.20
NiCu / CGO, 0.6V, 750°C
0.10
Direct oxidation dry CH4: Time study
0.00
0.0
50.0
100.0
150.0
200.0
250.0
Time / h
Improved stabilization of the time-profile after Ba doping
Conclusions
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A highly dispersed Ni catalyst on the surface of a MIEC oxide may represent a
promising approach for direct utilization of organic fuels.
The autothermal reforming studies used to simulate organic fuel reaction in SOFCs
indicate the occurrence of direct oxidation to CO2.
In the presence of ionic oxygen (O2-) dry organic fuels may be directly oxidized to CO2
at the SOFC anode.
The different profiles of ac-impedance spectra at OCV for the SOFC fed with dry syngas
or methanol seem to confirm the occurrence of direct oxidation.
Power densities ranging from 290 to 350 mW cm-2 were achieved with an electrolyte
supported SOFC depending on the type of organic fuel.
No significant carbon formation for direct oxidation of dry organic fuel in SOFCs was
observed for this catalyst. This represents an advantage with respect to the
conventional Ni/YSZ anode electrocatalyst that usually needs of a specific fuel
processor for the utilization of organic fuels in SOFCs.
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ACKNOWLEDGMETNS
ACCORDO DI PROGRAMMA CNR – MSE
Linea progettuale 2 Sviluppo di materiali per celle a combustibili ad ossidi solidi operanti a
temperature intermedie (IT-SOFC)
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Grazie per la vostra attenzione !
Institute CNR-ITAE – Messina (Italy)