metal oxide nanotubes and photometal oxide nanotubes and … · 2007. 10. 3. · soso d o de ue ce...

GCEP Symposium 10/3/07

Metal Oxide Nanotubes and Photo Excitation EffectsMetal Oxide Nanotubes and Photo-Excitation EffectsNew Approaches for Low-Temperature Solid Oxide Fuel Cells for Low

GWG-Emission Transportationp

Start Date: 3/1/07 (Stanford); 9/1/07 (Harvard)

PI’s: Paul McIntyre1 & Shriram Ramanathan2PI s: Paul McIntyre & Shriram Ramanathan

Students: Cynthia Ginestra,1 Andy Lin,1 & Annamalai Karthekeyan2, Masaru Tsuchiya

1. Department of Materials Science and Engineering, Stanford University2. School of Engineering, Harvard University

P.C. McIntyre & S. Ramanathan1


Trends in Personal Mobilitye ds e so a ob ty

• Transportation accounts for ~ 24% of GWG emissions currently• Increasing relative impact in future due to growing adoption of automotive transportation world-wide (e.g. personal automobile ownership increasing ~ 20% perannum in China)


Source: GCEP Advanced Transportation Assessment, Spring 06

annum in China)


Reduced GWG Automobile PropulsionReduced-GWG Automobile Propulsion Technologies

• Improved internal combustion engines (evolutionary)-hybrids-flexible-fuel IC engines burning hydrogen-richhydrocarbons

All l t i hi l• All electric vehicles

• Low temperature fuel cells

Major potential impact, but breakthoughs needed → esp. in materials & catalysts

⟩



Fuel Cell Basicsue Ce as cs

• Example: PEM fuel cell

• Hydrogen is typical fuel for PEM cells, but this is not necessarily the case for othernecessarily the case for otherfuel cell types

• Key idea: electrochemical d ti d id ti tireduction and oxidation reactions

on either side of an ion-conductingmembrane set up a steady-state voltage difference across the cellvoltage difference across the cell

-voltage difference (EMF) can do work (e.g. move a vehicle)


P.C. McIntyre & S. Ramanathan


Fuel Cell Typesue Ce ypes

SOFC’s – high efficiency flex fuel simple operation but high temperature


Source: Hirschenhofer et al., Fuel Cell Handbook, 4th Edition (1998)

SOFC s – high efficiency, flex fuel, simple operation, but high temperature


Solid Oxide Fuel CellsSo d O de ue Ce sOperating Principle

Siemens-Westinghouse Tubular SOFCSiemens Westinghouse Tubular SOFC• High operating temperatures

- target fixed power generationYSZ is a crystalline alloy of ZrO2 and Y2O3• Oxygen fast ion conductor


• Electronic insulator Source: S.C. Singhal, presented at Pan American Advanced Studies Institute, Rio de Janero, Brazil, Oct. 21, 2003


Fuel Cell Power Outputue Ce o e Output

OCV i th ti l• OCV is theoreticalmaximum EMF avail-able to drive electron current from anode to Actual EMF is less than OCV at a given current densitycurrent from anode to cathode (through interconnect) and do work.

• Actual EMF is less than OCV at a given current density• Portion of OCV is consumed in resistance loss across YSZ membrane and charge transfer losses at electrodes

• Reducing these losses is critical to lowering SOFC


• Reducing these losses is critical to lowering SOFC operating temperature


Our ApproachOu pp oacReduce SOFC operating temperature while maintaining highefficiency, power density, and fuel flexibility

Methods under investigation• Lower Ohmic resistance loss across YSZ membrane by

depositing ultrathin YSZ films via atomic layer deposition (ALD)- decrease membrane thickness from 10’s of mm to 10’s of nm.

• Study the possibility of reducing Ohmic and activation losses by using current output to produce UV light, which may enhance bulk conductivity and the rate of interface oxygen exchange between electrode & electolyte

• Increase power density by exploring high aspect ratio membranes• Increase power density by exploring high-aspect ratio membranes, including YSZ nanotubes



UV Illumination to Reduce Losses• UV illumination may change the surface structure and improve the catalytic properties of the fuel cell electrodes

UV Illumination to Reduce Losses

properties of the fuel cell electrodes

• Electrode/electrolyte exchange kinetics also can be affected

M lt h t t f i t d f t i YSZ l h i it i t

atomic oxygen

• May alter charge state of point defects in YSZ layer, changing its resistance

Example: UV-ozone oxidation of ultrathinmetals forms high quality metal oxideatomic oxygen

Ometals forms high quality metal oxide layers at room temperature at oxidation ratesmany orders of magnitude greater than forthermal oxidation

O2 + hν’ = 2OO + O2 = O3O + hν” = O + O


molecularO2

O3 + hν = O2 + O


YSZ Nanotube Synthesis & PropertiesS a otube Sy t es s & ope t es

Nanotube Forest

YSZ Membrane

(Oxidizer Side)

Forest (Fuel Side)

Perovskite Electrode

SiO2 Support

(b) Metal Oxide Nanotube Detail (a) Nanotube Array

Top left: vertical Ge nanowire array (40 nm diameter)Top left: vertical Ge nanowire array (40 nm diameter)

Bottom left: conformal ALD-HfO2 coating on Ge nano-wire (40 nm diameter)

Top right: Schematic of high aspect-ratio YSZ membranecomposed of metal oxide nanotubes

-note: Ge etches readily in dilute H2O2/H2O solution


note: Ge etches readily in dilute H2O2/H2O solution-YSZ functions as an etch stop


ALD: Ultrathin, Conformal Metal Oxide Film DepositionU t at , Co o a eta O de epos t o

• ALD: cyclic surface-saturatingh i l ti t dchemical reactions separated

by inert gas purges• Capable of producing fully conformal ultrathin films over

ALD Nanolaminate Approach:

arbitrary substrate surfaces

ALD Nanolaminate Approach:



Recent Results: Long-Period YSZ Nanolaminates

As-grown Annealed

SIMS Chemical Depth Profile(raw data by EAG Labs, Sunnyvale, CA)

• 20 nm total thickness • Tetragonal ZrO2

• Amorphous Y2O3• Appreciable, yet incomplete interdiffusion

• Raw Zr and Y counts calibrated using standard 8% YSZ

p 2 3 incomplete interdiffusion• Interfacial SiO2 growth

1 nm 15 nm

• Y:Zr content ratio plotted vs sample depth

• Further evidence of layer i diff i


interdiffusionC. Ginestra et al., Electrochem. Solid State Lett. (2007).


Recent Results: Short-Period YSZ Nanolaminates (3% Y2O3)As-grown Annealed

RMS roughness <1.8 nmfor 20 nm as-grown

3YSZ

RMS roughness <0.37 nmfor 20 nm annealed 3YSZ

3YSZ

• Amorphous structure• YSZ containing 2-4 mol% Y2O3

• Amorphous structure• 20-35 nm total thickness

• Polycrystalline tetragonal structure (same as bulk at thiscomposition)

• Columnar grain structure that spans entire film thickness ( i di )

C. Ginestra et al.,


(5–25 nm grain diameter)• Density increase from 5.8 to 6.1g/cc

Electrochem. Solid State Lett. (2007).


Recent Results: Electrical Characterization of ALD-YSZRecent Results: Electrical Characterization of ALD YSZConductivity vs FrequencyConductivity vs Frequency

Conductivity vs TemperatureConductivity vs Temperature

• 30 nm thick sample • Additional 6 hour anneal (900°C)

• ~10x higher than bulk polycrystalline 3YSZ 6• ~15x higher than polycrystalline 2YSZ 7

Sample conductivitySample conductivity

• Point defect equilibration• SiO2 interface layer thickens

• Voltage measurements (1Hz–300kHz)

g p y y• Comparable to bulk cubic YSZ (8-10 mol% yttria)

C. Ginestra et al., Electrochem. Solid State Lett. (2007).


• Total conductivity from complex impedanceC. Ginestra et al., Electrochem. Solid State Lett. (2007).


Recent Results: Thickness Effects on Structural Evolution in Evaporated YSZ Films

32nm thick YSZ film 75nm thick YSZ film


Change in crystalline structure on in-situ heating from room temperature to 600°C.


f S SPhase Transformation Sequence: Size Dependence

32nm thick YSZ film 75nm thick YSZ film


Tsuchiya et al., Phil. Mag. (in press, 2007)


Oxygen Ambient Effects in Post-Deposition Anneals

(a) Cubic YSZ

In-Situ c (220)

c (111)c (200)Oxygen annealing (“ex-

situ” condition) leads to tetragonal phase f ti i b In Situ

32 nm ( )

c (222)c (311)formation in e-beam

deposited YSZ films.

(b)We will investigate similar processes in ALD-laminate derived YSZ thin films

Tetragonal YSZ

t (112)t (220)

t (111)t (200)

Ex-Situ

YSZ thin films.

Tsuchiya et al.,


t (220)

t (222)t (311)

Ex Situ32 nm Phil. Mag. (in press, 2007)


Summary and Future WorkSummary and Future Work• Initial results on ALD-YSZ nanolaminates suggest this is a promising approach for making ultra-thin SOFC membranes

• Reason for enhanced total electrical conductivity of nanolaminate-derived samples and composition tuning of oxygen ion conductivity are now under investigationunder investigation

• Perovskite SOFC electrode deposition studies by ALD are under way

• Next year’s effort at Stanford will focus on electrode studies and developmentof microfabrication techniques to prepare thin film fuel cell arrays

• Postdoc arriving at Stanford Nov. 1 will lead fuel cell microfabrication work

• Ongoing work at Harvard emphasizes UV illumination studies on oxygen


transport in YSZ and across YSZ/electrode interfaces

metal oxide nanotubes and photometal oxide nanotubes and … · 2007. 10. 3. · soso d o de ue ce...

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