Synthesis of Proton Conducting Ceramic Membranes via Seeded Surface Crystallization - Matthew Z. Yates
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URChE
Synthesis of Proton Conducting
Ceramic Membranes via
Seeded Surface Crystallization
Matthew Z. Yates
Department of Chemical Engineering
Laboratory for Laser Energetics
University of Rochester
URChE
Proton Exchange Membrane Fuel Cells
Fuel (H2) Oxidant (Air/O2)
e -
H2 H+ 1/2O2
H+
e- H2O
Anode Electrolyte Cathode
• Advantages: high efficiency, low emission, simplicity, and silence.
• Applications: portable, mobile, and stationery power sources.
URChE
Fuel Cell Types
PEMFC AFC PAFC p-SOFC MCFC SOFC
30 to 50% Molten
Proton
solution of 100% Ceramic carbonates Ceramic
conducting
Electrolyte potassium phosphoric YSZ/Gd- in LiAlO3, of 5YSZ
polymer
hydroxide acid (H3PO4) CeO2 Li2CO3/Na2C or 8YSZ
(Nafion)
(KOH) O3
Charge
carrier
H+ OH- H+ H+ CO32- O2-
Operating 80 to 23 to 150 to 300 to 700 to
~650°C
temperature 120°C 250°C 220°C 700°C 1300°C
Adapted from: Larminie, J.; Dicks, A., Fuel cell systems explained. 2nd ed.; 2003.
URChE
Research Needs for Fuel Cell Membranes
Power Density (W/cm2)
1.5
Breakthrough
1.0 Membranes SOFC
PEMFC
0.5
PAFC MCFC
0 200 400 600 800
Temperature (°C)
Ease of hydrocarbon reforming
Ease of fuel cell construction
PEFC: Polymer electrolyte fuel cells MCFC: Molten carbonate fuel cells
PAFC: Phosphoric acid fuel cells SOFC: Solid oxide fuel cells
Adapted from: Ito, et al J. Power Sources, 2005
URChE
Hydrogen Membrane Fuel Cell (Toyota)
On-board Air/O2
reformer
e-
H2 H+ 1/2O2
gases H+
e- H2O
Palladium
membrane
Anode Electrolyte Cathode
Ito, et al J. Power Sources 2005:
demonstrated 1.4 W/cm2 at 600°C using 700 nm thick ceramic electrolyteURChE Membrane Development for Intermediate
Temperature Fuel Cells
• Create new materials with high ion conductivity
--- O2- conductive ceramics: fluorite-, perovskite-, apatite-,
and brownmillerite-based oxides
--- H+ conductive ceramics: perovskite oxides, fluorite-related
binary oxides, and apatite phosphates
• Reduce existing SOFC membrane thickness to lower ohmic
resistance of ceramic electrolytes
• Engineer membrane microstructures (orientation of crystals,
grains, or grain boundaries) to optimize ion conduction
ObjectiveURChE Hydroxyapatite Ceramic as a
Proton Conducting Membrane
HAP: Ca10(PO4)6(OH)2
(a) (b) (c)
H+ c-axis
O2- of PO43-
Ca2+
OH-
b-axis
c-axis
a-axis
(a) typical shape of a HAP single crystal; (b) atomic environment around OH- ions;
(c) proton transportation along the c-axis of HAP.
Adapted from: Satoshi Nakamura, et al., J. Applied Phys. 2001, 89, 5386-5392.URChE Proposed Idealized Hydroxyapatite
Membrane Structure
c-axis c-axis
Ideal HAP membrane structure: the c-axes of crystal
domains span the membrane to optimize proton transport.URChE
Tertiary Growth Process for Creating
Idealized Hydroxyapatite Membrane
(a) (b) (c)
• Seeding: electrochemical deposition to seed HAP on Pd substrate;
• Secondary growth: hydrothermal deposition under conditions that favor
c-out-of-plane growth to yield oriented columnar crystals;
• Tertiary growth: hydrothermal deposition under conditions that favor a-
plane growth to obtain oriented continuous crystalline films.
Lai, Z. P., et al., Science, 2003, 300, 456.
Karanikolos, G. N., et al., Chem. Mater. 2007, 19, 792.URChE
Hydroxyapatite Seed Layer Grown
on Pd Membrane
Top-view Side-view
~1.5 µm
• Set-up: Pd-cathode, Pt-Anode, current=9.3mA/cm2, Pt Pd
deposited at T=95oC for 4 min.
• Electrolyte solution: 50mM Tris, 137.8mM NaCl, 2.5mM
CaCl2, 1.67mM K2HPO4, pH= 7.20 adjusted with 37% HCl.
Ban, et al., J. Biomed. Mater. Res. 1998, 42, 387.URChE Secondary Hydrothermal Growth of
Hydroxyapatite on Seeded Pd Membrane
Top-view Side-view
~7µm
• HAP solution: 0.1M Ca(NO3)2, 0.06M (NH4)2HPO4, PTFE liner
0.1M Na2EDTA, pH=10 adjusted with 28% NH4OH. HAP solution
• Set-up: HAP/Pd facing to the bottom of PTFE liner, PTFE plate
reaction at T=200oC for 15 hours. HAP/PdURChE Surfactant-Promoted a-axis Growth to
Create Dense Membranes
Anionic
c-axis surfactant -
-
- c - -
-
-
a +
b-axis
a-axis
Cationic + + +
+
surfactant
a-planes: positively charged
c-planes: negatively charged
Kawasaki, T., Journal of Chromatography 1991, 544, 147.URChE Surfactant-Modified Tertiary Growth
on Pd Membrane
Top-view Side-view
~25 µm
• HAP solution: 0.1M Ca(NO3)2, 0.06M (NH4)2HPO4,
Cetylpyridinium
0.1M Na2EDTA, 0.01M Cetylpyridinium Chloride, Chloride
pH=8 adjusted with 28% NH4OH.
• Set-up: HAP/Pd facing to the bottom of PTFE liner,
reaction at T=200oC for 15 hours. (repeat 3 times)URChE
XRD Patterns of Hydroxyapatite Membranes
rd
HAP 3 growth (002)
nd
HAP 2 growth
Intensity (a.u.)
HAP seeds (211)
HAP powder (300)
20 22 24 26 28 30 32 34 36 38
2 θ (degree)URChE
Proton Conductivity (σ) of 25 micron thick
Hydroxyapatite Membrane
-2
10
-3
10
-4 Introduce H2
10
-5
10
σ (s/cm)
-6
10
-7
10
-8 N2 atmosphere
10 H2 atmosphere
-9
10
0 200 400 600 800 1000
o
T ( C)
• Literature data: σ~ 5 x 10-7 s/cm-1 measured at 800°C on a sintered
~1 mm thick disc.
Yamashita, K., et al., Solid State Ionics 1990, 40-41, 918.URChE
Area Specific Resistance (ASR)
of Hydroxyapatite Membrane
N2 atmosphere-25μm thick
6
10 N2 atmosphere-2.5μm thick
N2 atmosphere-0.5μm thick
H2 atmosphere-25μm thick
4
10 H2 atmosphere-2.5μm thick
ASR (Ω cm )
2
H2 atmosphere-0.5μm thick
2
10
0 0.5(Ω
2
cm )
10
2
0.1(Ω cm )
-2
10
0 200 400 600 800 1000
o
T ( C)
Steele, B. C. H. and Heinzel, A., Nature 2001, 414, 345.URChE
Reducing Membrane Thickness
• Shorter HAP seeding time (2 min)
• Lower (NH4)2HPO4 concentration (0.01 M)
• Film thickness ~ 5µm
• Shorter HAP seeding time (1 min)
• Lower (NH4)2HPO4 concentration (0.01 M)
• Film thickness ~ 2.5µmURChE
Density of Thin Membranes
Top-view Bottom-view
Thin membrane (~2.5µm thick) prepared by
electrochemical and hydrothermal depositions.URChE Doped Hydroxyapatite to Enhance
Proton Conductivity
• Similar tertiary growth process applied to yttrium and fluorine
doped hydroxyapatite (shown by others to have enhanced
conductivity
Yttrium-substituted HAP Fluorine-substituted HAPURChE
Conclusions
• Continuous, dense, hydroxyapatite membranes of
tunable thickness can be grown directly onto palladium
hydrogen membranes
• Crystal growth conditions have been identified that
produce hydroxyapatite membranes with the crystal
domains aligned to promote proton conductivity.
• The optimized membrane structure results in significant
enhancement proton conductivity.URChE
Acknowledgments
• Support from the Department of Energy (DOE) (DE-
FG02-05ER15722)
• DOE through the Laboratory for Laser Energetics (DE-
FC03-92SF19460)
• Horton Fellowship in Laboratory for Laser Energetics
• Researchers: Dongxia Liu, Yong-Gu KimYou can also read
