graphene-based air electrodes for solid oxide...
TRANSCRIPT
Graphene-based Air Electrodes for Solid Oxide Electrochemical Cells
April 18, 2014
Prof. Min Hwan Lee School of Engineering
Graphene for electrochemical devices
• Electron conducting
• High surface area
• Catalytic
• Batteries
• Supercapacitors
• Fuel Cells
• Sensors
• …
Properties Applications
What is a fuel cell?
Electrochemical “Energy Conversion” Device
Fuel Cell Chemical Energy
Electrical Energy
Fuel : hydrogen, alcohols, hydrocarbons, etc.
Efficient and quiet
Fuel cell operation
Overall: H2 + ½ O2 → H2O + electricity
H2 O2 O2-
e- e-
H2 → 2H+ + 2e- (HOR) ½ O2 + 2e- → O2- (ORR)
Driving force?
Ans. The chemical potential difference between the reactants (H2 + O2) and the product (H2O)
H2 + O2
H2O Che
mic
al P
oten
tial
Catalyst
Cell losses
H2 O2 O2-
(HOR) (ORR)
Ohmic Loss
Activation Loss for ORR
Activation Loss for HOR
Solid Oxide Fuel Cell (SOFC)
• Advantages à Fuel Flexibility à Simpler System (No humidity control, etc.)
• Disadvantages à Material/Part selection à Durability à Limited applicability
High Operating Temperature > 800 °C
Lower Operating Temperature! (< 400 °C)
Both ohmic and activation loss ∝ exp(Ea/kT)
Reduction in T causes significant Losses!
H2 O2 O2-
(HOR) (ORR)
Ohmic Loss
Activation Loss for ORR
Activation Loss for HOR
To counteract the significant ohmic loss
Thinning the electrolyte < 100 nm
H. Huang et al. J. Electrochem. Soc., 154, B20, 2007 Y. B. Kim et al. Electrochem. Comm., 13, 403, 2011
To counteract the significant electrode loss
Conventional perovskite-based electrodes à Not active at low temperatures
Pt-based electrode à Expensive à Fast degradation
Need a totally new material system because…
Objective
New air electrode (cathode) materials for LT-SOFCs
Doped Graphene?
Why graphene as the cathode?
Qu et al. ACS Nano, 4, 1321, 2010
• Graphene (and its derivatives) – Extraordinary thermal and electrical conductivities – High specific surface area (theoretically 2630 m2/g for single-layer) – Strong mechanical strength and flexibility – Excellent catalytic activity (Doped Graphene)
N-doped Graphene as an ORR catalyst in Fuel Cell
Catalytic Activity Durability over cycle (200k cy.)
Qu et al. ACS Nano, 4, 1321, 2010
• Catalytically superior to Pt • Highly durable • Resistant to CO and methanol poisoning
How N-doping catalyze ORR ?
Active site: C with high spin density or high charge density
L. Zhang and Z. Xia, J Phys. Chem., 115, 11170, 2011
N-doping
Teflon-lined auto-clave
GO solution + dicyandiamide(DCDA) + D.I. water
sonication in the suspension
(90% water+10% MeOH)
Hydrothermal rxn. @ 140, 180°C
Solution & symmetric cell preparation procedure
NrGO (Nitrogen Doped Reduced Graphene Oxide)
GO (Graphene Oxide)
Flake graphite powder
Sonication for better dispersion
Centrifuging to remove unexfoliated
particles
Oxidization using KMnO4
Expansion of graphite sheets
Graphene oxide solution
Filtration with PTFE filter
Graphene-based electrode deposition
• rGO
• NrGO140
• NrGO180
YSZ preparation Tape Masking Plasma cleaning Dropping electrode ���solution
Drying and ���re-dropping
Removing the mask A symmetric cell
NrGO180 rGO
(b) (a)
500 nm 500 nm
Electrochemical Impedance Meas.
RL RH
CPEL CPEH
0 100 2000
50
100
150 E xperimenta l F itted
-‐Im(Z
) [x10
3 Ω]
R e(Z ) [x103 Ω]
0 2000 40000
2000
4000
30 mHz
0.7 MHz
10 kHz
curr
ent
colle
ctio
n Electrode
YSZ Electrode Au mesh
Au mesh
Air
Air
/U I% %
Oxygen Reaction Performance
rGO NrGO140 NrGO180
Au Pt
Y. Jee et al. submitted
Thermal Stability over Time
0 3 60
3
6
9RL / R
L-‐Init.
T ime [h]
Pt
NrGO
ASRct / ASRct-initial
Y. Jee et al. submitted
Thermal stability of Pt: accelerated agglomeration
Agglomeration of Pt
At 500°C for 5 hrs
• Ostwald ripening à Loss of active sites for ORR
Thermal stability: morphological
500nm 500nm
(d) (c)
as-deposited NrGO180 annealed NrGO180
350°C
(f) annealed NrGO (e) as-deposited
350°C
Thermal stability: defects & stoichiometry
Y. Jee et al. submitted
225 250 275 300 325 350 375 4000.9
1.0
1.1
1.2
1.3
1.4
1.5 rG O N rG O 180
intens
ity ratio (G/D
)A nnea ling tempera ture [¡ÆC ]
Raman XPS
As-dep. 250°C 350°C
Ato
mic
ratio
[%]
10
20 Oxygen Nitrogen
Thermal stability: d-spacing
NrGO180 (As - 350°C) : ~3.4Å
20 25 30 35
2? [°] (Co)
as-dep. 250°C 300°C 350°C 400°C
Y. Jee et al. submitted
Cell Integration
S. Ji et al. In preparation
H2
O2
AAO
Pt
YSZ+GDC
N-rGO Nitrogen-doped graphene: ~ micron
Anodic Aluminum Oxide (AAO): 80 nm pore
Gadolinia-doped Ceria (GDC): 200 nm
Yttria Stabilized Zirconia (YSZ): 80 nm Platinum (Pt): 300 nm
Open Circuit Voltage
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4 5 6 7 8 9
OCV [V]
Cell #
S. Ji et al. In preparation
# 1 2 3 4 5 6 7 8 9
Peak Power Density (mW/cm2) 0.1 0.25 - 0.08 0.08 0.02 0.04 0.13 0.09
Ohmic Resistance (Ωcm2) 319 328 - 275 289 199 - 174 313
Full Cell Performance
3 2 1
4 5 6
9 8 7
Most loss occurs at the electrolyte-NrGO interface
Summary
Ø LT-SOFC needs new air electrodes (Perovskites: poor catalysis; Pt: expensive and limited)
Ø N-doped graphene showed a great oxygen reaction performance and durability at < 400 °C
Ø Needs much engineering opportunity (interface w/ current collector; cheaper doping process, enhanced reaction sites, etc.)
Ø Good potential to be an alternative cathode material
for LT-SOFCs
Acknowledgement • Dr. Youngseok Jee • Andrew H. Moon • Alireza Karimaghaloo • Dr. Hidetaka Ishihara, Prof. Vincent C. Tung, UC Merced • Sanghoon Ji, Prof. Suk-Won Cha, Seoul National Univ.
• Dr. Jin-Woo Han @ NASA Ames • Dr. Jihwan An and Prof. Fritz Prinz @ Stanford
