Matthew Z. Yates - University of Rochester

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 (H 2 )
H 2
H +
H +
e -
e -
H 2 O
Oxidant (Air/O 2 )
1/2O 2
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
Electrolyte
Proton
conducting
polymer
(Nafion)
30 to 50%
solution of
potassium
hydroxide
(KOH)
100%
phosphoric
acid (H 3 PO 4 )
Ceramic
YSZ/Gd-
CeO 2
Molten
carbonates
in LiAlO 3 ,
Li 2 CO 3 /Na 2 C
O 3
Ceramic
of 5YSZ
or 8YSZ
Charge
carrier
H + OH - H + H + CO 3
2-
O 2-
Operating
temperature
80 to
120°C
23 to
250°C
150 to
220°C
300 to
700°C
~650°C
700 to
1300°C
Adapted from: Larminie, J.; Dicks, A., Fuel cell systems explained. 2 nd ed.; 2003.

URChE
Research Needs for Fuel Cell Membranes
Power Density (W/cm 2 )
1.5
1.0
PEMFC
0.5
Breakthrough
Membranes
SOFC
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
reformer
H 2
gases
e -
H +
H +
e -
H 2 O
Air/O 2
1/2O 2
Palladium
membrane
Anode Electrolyte Cathode
Ito, et al J. Power Sources 2005:
demonstrated 1.4 W/cm 2 at 600°C using 700 nm thick ceramic electrolyte

URChE
Membrane Development for Intermediate
Temperature Fuel Cells
• Create new materials with high ion conductivity
--- O 2- 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
Objective

URChE
Hydroxyapatite Ceramic as a
Proton Conducting Membrane
HAP: Ca 10 (PO 4 ) 6 (OH) 2
(a) (b)
H +
c-axis
O 2- of PO
3-
4
Ca 2+
OH -
(c)
b-axis
a-axis
c-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/cm 2 ,
deposited at T=95 o C for 4 min.
• Electrolyte solution: 50mM Tris, 137.8mM NaCl, 2.5mM
CaCl 2 , 1.67mM K 2 HPO 4 , pH= 7.20 adjusted with 37% HCl.
Pt
Pd
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(NO 3 ) 2 , 0.06M (NH 4 ) 2 HPO 4 ,
0.1M Na 2 EDTA, pH=10 adjusted with 28% NH 4 OH.
• Set-up: HAP/Pd facing to the bottom of PTFE liner,
reaction at T=200 o C for 15 hours.
PTFE liner
HAP solution
PTFE plate
HAP/Pd

URChE
Surfactant-Promoted a-axis Growth to
Create Dense Membranes
c-axis
- c
Anionic
surfactant
-
-
-
-
-
a +
-
b-axis
a-axis
a-planes: positively charged
c-planes: negatively charged
Cationic
surfactant
+
+
+
+
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(NO 3 ) 2 , 0.06M (NH 4 ) 2 HPO 4 ,
0.1M Na 2 EDTA, 0.01M Cetylpyridinium Chloride,
pH=8 adjusted with 28% NH 4 OH.
• Set-up: HAP/Pd facing to the bottom of PTFE liner,
reaction at T=200 o C for 15 hours. (repeat 3 times)
Cetylpyridinium
Chloride

URChE
XRD Patterns of Hydroxyapatite Membranes
HAP 3 rd growth
(002)
Intensity (a.u.)
HAP 2 nd growth
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
10 -2
10 -3
10 -4
Introduce H 2
σ (s/cm)
10 -5
10 -6
10 -7
10 -8
N 2
atmosphere
H 2
atmosphere
10 -9
0 200 400 600 800 1000
T ( o 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
N 2
atmosphere-25μm thick
10 6 0.5(Ω cm 2 )
N 2
atmosphere-2.5μm thick
N 2
atmosphere-0.5μm thick
ASR (Ω cm 2 )
10 4
10 2
10 0
0.1(Ω cm 2 )
H 2
atmosphere-25μm thick
H 2
atmosphere-2.5μm thick
H 2
atmosphere-0.5μm thick
10 -2
0 200 400 600 800 1000
T ( o C)
Steele, B. C. H. and Heinzel, A., Nature 2001, 414, 345.

URChE
Reducing Membrane Thickness
• Shorter HAP seeding time (2 min)
• Lower (NH 4 ) 2 HPO 4 concentration (0.01 M)
• Film thickness ~ 5µm
• Shorter HAP seeding time (1 min)
• Lower (NH 4 ) 2 HPO 4 concentration (0.01 M)
• Film thickness ~ 2.5µm

URChE
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 HAP

URChE
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 Kim