Stanford University Michael Shandalov1, Shriram Ramanathan2, Changhyun Ko2 and Paul McIntyre1 1Department of Materials Science and Engineering, Stanford University 2Division of Engineering and Applied Sciences, Harvard University
The goal of this project is to enhance the power density and lowtemperature 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 a given stack volume [1].
Introduction While 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 HfO 2 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 this advantage, we are able to produce HfO 2 nanotubes on vertical Ge nanowire 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
Initially, 1.5µm-long Ge vertical nanowires were used as template for growth of HfO 2 nanotubes Chemichal mechanical polishing (CMP) to expose HfO 2 nanotubes Processing conditions for ALD of HfO 2 ultra-thin layers Deposition of 2-8nm-thick HfO 2 layer using ALD followed by anneal at 800 o C for 3hrs in Ar Ge selective wet etch out of HfO 2 nanotubes Deposition of 1.5µm-thick SiO 2 encapsulation layer using plasma enhanced chemical vapor deposition (PECVD) Buffered oxide etch (BOE) of SiO 2 encapsulation layer HfO 2 Precursor (Temp.) Hf[N-(CH 3 ) 2 ] 4 (90 o C) Oxidant (Temp.) H 2 O (25 o C) Growth per cycle 0.85A Film roughness 2A Substrate temp. 300 o C
HfO 2 nanotube 5nm High-resolution TEM cross section images of HfO 2 nanotubes and films HfO 2 film 40 nm SiO 2 SiO 2 Si(111) 5nm Si(111) Vertical, having ultra-thin walls, crystalline (except 2nm-thick sample) HfO 2 nanotubes were observed within SiO 2 encapsulation layer. Additionally, HfO 2 nanocrystalline films on Si(111)surface were characterized
(a) 3.1nm (b) 7.8nm HfO 2 thickness Observed phase 1.8nm Amorphous 3.1nm Tetragonal 4.3nm Tetragonal 5.5nm Monoclinic 7.8nm Monoclinic (c) 3.1nm (d) 7.8nm Annealed HfO 2 Monoclinic (111) tet (-102) mono (110) mono (-111) mono (111) mono (020) mono Thickness Tetragonal (211) mono (220) tet, mono (022) mono (-221) mono Amorphous 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 HfO 2 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 HfO 2 nanotubes.
(a) (b) 50nm Schematic energy map for HfO 2 [2] by A. Navrotsky HfO 2 nanocrystalline film, exhibiting <111> texture on Si(111) HfO 2 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 HfO 2, referenced to the bulk enthalpy of the monoclinic phase. Because amorphous HfO 2 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 Au nanoparticles dispersion on Si(111) surface Ge vertical nanowires will be used as template for growth HfO 2 nanotubes Deposition of 2-8nm-thick HfO 2 layer using ALD followed by anneal at 800 o C for 3hrs in Ar ALD of LSCO cathode Deposition of 1.5µm-thick SiO 2 encapsulation layer using PECVD, through-si windows wet etch Ge selective wet etch out of HfO 2 nanotubes CVD of porous Pt anode BOE of SiO 2 encapsulation layer
Thick-wall nanotubes towards device fabrication SEM images of hollow vertical HfO 2 nanotubes: plan view 50nm First, Ge selective wet etching was applied to broken Ge nanowires on Si substrate covered by ALD HfO 2. Ge is highly susceptible to aqueous corrosion as result of solubility of GeO 2 in water. The results showed that Ge nanowire cores were dissolved after few minutes in 30% H 2 O 2 aqueous solution at 40 o C in ultrasonic bath, without producing any etching of HfO 2 nanotubes. SEM of the etched samples showed hollow broken HfO 2 nanotubes. Unbroken HfO 2 nanotubes were not etched, since H 2 O 2 solution could not reach encapsulated Ge core.
Thick-wall nanotubes towards device fabrication (a) (b) 100nm 10nm (a) TEM image of hollow HfO 2 nanotubes synthesized by ALD of HfO 2 on Ge NWs of 20nm diameter, and selective wet etching of the Ge in dilute H 2 O 2 ; (b) cross-section view of a HfO 2 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 100nm EDS analysis area EDS - confirmation of Ge removal from HfO 2 nanotubes Ge Energy dispersive spectroscopy (EDS) analysis shows absence of residual Ge inside HfO 2 nanotubes after selective wet etching. Note that Cu contamination is from TEM grid after ion milling of the sample.
Conductivity measurements of HfO 2 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 HfO 2 films on MgO substrates - varying temperature (by Changhyun Ko, Harvard) TEM cross-section image showing 57nm thick HfO 2 film on MgO isolative substrate 100nm Glue Activation energies HfO 2 MgO Single crystal 10% bulk YSZ In the reduced environment, the range of P(O 2 ) is 10-19 ~ 10-16 atm. 5% FG means 5% Hydrogen forming gas. 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 HfO 2 films on MgO substrates - P(O 2 ) dependence Conductivity measurements of HfO 2 films as function of P(O 2 ) are underway at this moment. These measurements are critical in determining the conductivity mechanism in HfO 2 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 HfO 2 vertical nanotubes, which can be used as SOFC membranes Phase stability study of HfO 2 nanotubes and films confirmed microstructure-related crossover in polymorph stability at the nanoscale Conductivity measurements suggest possible oxygen ionic conductivity in HfO 2 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. Acknowledgements GCEP funding Ge nanowire template fabrication: Irene Goldthorpe, Makoto Koto, Shu Hu Assistance in ALD: Yasuhiro Oshima, Cynthia Ginestra, Andy Lin