Stanford University

Michael Shandalov1, Shriram Ramanathan2, Changhyun Ko2

and Paul McIntyre1

1Department of Materials Science and Engineering, Stanford University2Division of Engineering and Applied Sciences, Harvard University

The goal of this project is to enhance the power density and low-temperature efficiency of solid oxide fuel cells (SOFC) manufactured by atomic layer deposition. These enhancements will be achieved by engineering the morphology of the electrolyte at the nanoscale, and microstructure-related conductivity of the thin film SOFC membranes will be studied.The efficiency of a fuel cell is limited by the loss mechanisms inherent to its operation. The power density of a fuel cell is limited by the area of its electrolyte membrane. These two operational parameters are related by the fact that thinner electrolytes not only limit the resistive losses within the fuel cell, but they also allow the incorporation of more active area into agiven stack volume [1].

IntroductionWhile an increase in membrane performance can be achieved via reduction of its thickness, the major increase in membrane area proposed by this research will be achieved by forming the electrolyte as an array of metal oxide nanotubes instead of planar films. In this part of our work we investigate novel membrane synthesis methods for increased SOFC power density by using HfO2 oxide nanotubes to produce an enhancement of the effective membrane surface area per planar area of the device.

Schematic illustration of fuel cell

basic function

Sketch made by Prof. Paul McIntyre

Atomic Layer Deposition (ALD)

Using ALD we are capable of making a pinhole-free ultra-thin electrolyte layers. Furthermore, ALD-grown layers are highly conformal to the substrate, even over rough surfaces. Using thisadvantage, we are able to produce HfO2 nanotubes on vertical Genanowire template. The chemical composition of the membrane was determined by the mix of precursor and water vapor admitted to the deposition chamber. It has been shown that the crystalline structure can be controlled by annealing at a specified temperature.

ALD process

HfO2

Precursor (Temp.) Hf[N-(CH3)2]4 (90oC)

Oxidant (Temp.) H2O (25oC)

Growth per cycle 0.85A

Film roughness 2A

Substrate temp. 300oC

Initially, 1.5µm-long Ge vertical nanowires were used as template for growth of HfO2 nanotubes

Deposition of 2-8nm-thick HfO2layer using ALD followed by anneal at 800oC for 3hrs in Ar

Deposition of 1.5µm-thick SiO2encapsulation layer using plasma enhanced chemical vapor deposition (PECVD)

Chemichal mechanical polishing (CMP) to expose HfO2 nanotubes

Ge selective wet etch out of HfO2nanotubes

Buffered oxide etch (BOE) of SiO2encapsulation layer

Processing conditions for ALD of HfO2 ultra-thin layers

HfO2nanotube

HfO2 film

Si(111)

SiO2

5nm

40 nm

High-resolution TEM cross section images of HfO2

nanotubes and films

5nmSi(111)

SiO2

Vertical, having ultra-thin walls, crystalline (except 2nm-thick sample) HfO2 nanotubeswere observed within SiO2 encapsulation layer. Additionally, HfO2 nanocrystallinefilms on Si(111)surface were characterized

Nanodiffraction patterns obtained from cross-section samples show (a) mixed tetragonal and monoclinic structure (for 3.1nm-thick wall sample) of and (b) monoclinic (for 7.8nm-thick wall sample) structure of HfO2 nanotubes. Selected area electron diffraction (SAED) obtained from plan-view samples confirmed (c) tetragonal and monoclinic structure (from 3.1nm-thick wall) and (b) monoclinic (from 7.8nm-thick wall) structure of HfO2 nanotubes.

HfO2 thickness Observed phase

1.8nm Amorphous

3.1nm Tetragonal

4.3nm Tetragonal

5.5nm Monoclinic

7.8nm Monoclinic

Amorphous

Tetragonal

Monoclinic

(a) (b)

Thic

knes

s

(111) tet

(-102) mono

(211) mono

(220) tet, mono

(c) (d)

(110) mono

(-111) mono

(111) mono

(020) mono

(022) mono

(-221) mono

Annealed HfO2

3.1nm

3.1nm

7.8nm

7.8nm

(b)(a)

50nm

HfO2 nanocrystalline film,exhibiting <111> texture on Si(111)Schematic energy map for HfO2 [2] by A. Navrotsky

HfO2 exhibits a number of crystalline modifications, with phases of higher symmetry becoming increasingly stable with increasing temperature. The surface energies of nanocrystalline oxides play a major role in phase stability. Metal oxides of identical bulk composition with different crystal structures can have vastly different surface energies. Figure (a) above shows results reported recently by A. Navrotsky [2] for the enthalpy of the different polymorphs of HfO2, referenced to the bulk enthalpy of the monoclinic phase. Because amorphous HfO2 has a lower surface energy than either the tetragonal or the monoclinic variants, it is predicted to be the most stable form of this oxide at large molar surface areas, e.g. in nanocrystalline films.

Thick-wall nanotubes – ultra large surface area SOFC membranes

Planned future process

Ge vertical nanowires will be used as template for growth HfO2nanotubes

Deposition of 2-8nm-thick HfO2layer using ALD followed by anneal at 800oC for 3hrs in Ar

Deposition of 1.5µm-thick SiO2encapsulation layer using PECVD, through-Si windows wet etch

Au nanoparticles dispersion on Si(111) surface

ALD of LSCO cathode

Ge selective wet etch out of HfO2nanotubes BOE of SiO2 encapsulation layer CVD of porous Pt anode

Thick-wall nanotubes – towards device fabrication

SEM images of hollow vertical HfO2 nanotubes:

plan view

50nm

First, Ge selective wet etching was applied to broken Ge nanowires on Si substrate covered by ALD HfO2. Ge is highly susceptible to aqueous corrosion as result of solubility of GeO2 in water. The results showed that Ge nanowire cores were dissolved after few minutes in 30% H2O2aqueous solution at 40oC in ultrasonic bath, without producing any etching of HfO2 nanotubes. SEM of the etched samples showed hollow broken HfO2 nanotubes. Unbroken HfO2 nanotubeswere not etched, since H2O2 solution could not reach encapsulated Ge core.

Thick-wall nanotubes – towards device fabrication

(b)(a)

10nm100nm

(a) TEM image of hollow HfO2 nanotubes synthesized by ALD of HfO2 on Ge NWs of 20nm diameter, and selective wet etching of the Ge in dilute H2O2; (b) cross-section view of a HfO2 nanotube – note that the facet structure of the NW surface is evident in the shape of the inner nanotube surface.

Thick-wall nanotubes – towards device fabrication

EDS analysis area

100nm

EDS - confirmation of Geremoval from HfO2 nanotubes

Ge

Energy dispersive spectroscopy (EDS) analysis shows absence of residual Ge inside HfO2 nanotubes after selective wet etching. Note that Cu contamination is from TEM grid after ion milling of the sample.

Conductivity measurements of HfO2 films on MgO substrates - varying temperature (by Changhyun Ko, Harvard)

Impedance can be represented as a complex number:

Nyquist plot represents complex impedance:

We assume that the equivalent circuit model for our sample system is composed of three components: contact resistance, film resistance and capacitor considering the shape of Z' vs Z'' plots is semi-circle. By fitting the plots based on this simple model, the values of film resistances are obtained. Then we can convert the film resistance into the conductivity since we know films thickness, width and length. In our measurements, the length is a distance between two parallel stripe electrodes made with Pt paste.

Conductivity measurements of HfO2 films on MgO substrates - varying temperature (by Changhyun Ko, Harvard)

MgO

HfO2

Glue

Single crystal 10% bulk YSZ

100nm

In the reduced environment, the range of P(O2) is 10-19 ~ 10-16 atm.5% FG means 5% Hydrogen forming gas.

Activation energies

TEM cross-section image showing 57nm thick HfO2 film on MgOisolative substrate

Plot above shows total (ionic and electronic) conductivity data of HfO2 nanocrystalline films as function of temperature. Thinner films show increased conductivity, compared to thicker films. The films exhibit increased conductivity in oxygen-rich environment (air), while different calculated activation energies indicate a different conduction mechanism in low and high oxygen partial pressure.

Conductivity measurements of HfO2 films on MgO substrates - P(O2) dependence

Conductivity measurements of HfO2 films as function of P(O2) are underway at this moment. These measurements are critical in determining the conductivity mechanism in HfO2 films. The future results may show oxygen ionic conductivity and/or electronic conductivity in these films.

Conclusions• We showed ability to fabricate both ultra-thin wall and thick wall HfO2

vertical nanotubes, which can be used as SOFC membranes

• Phase stability study of HfO2 nanotubes and films confirmed microstructure-related crossover in polymorph stability at the nanoscale

• Conductivity measurements suggest possible oxygen ionic conductivity in HfO2 films, making them attractive material for SOFC membranes

1. C. Ginestra, R. Sreenivasan, A. Karthikeyan, S. Ramanathan and P. McIntyre, Electrochem. Solid-State Lett., 10 (2007) B161.

2. A. Navrotsky, J. Mater. Chem., 15 (2005) 1883.

AcknowledgementsGCEP funding

Ge nanowire template fabrication:Irene Goldthorpe, Makoto Koto, Shu Hu

Assistance in ALD: Yasuhiro Oshima, Cynthia Ginestra, Andy Lin