Fabrication of Nano-structured Thin Film Solid Oxide Fuel Cell by Multi-deposition Methods

Abstract

The intention of this paper is to investigate the performance of thin films Solid oxide Fuel Cell fabricated by two different deposition processes namely, Atomic Layer Deposition (ALD) and Sputtering. The deposition processes have been studied to fabricate electrode and electrolyte of the thin film SOFC. ALD technique is able to deposit on complex nanostructure due to high step coverage and it is appropriate to increase triple phase boundary (TPB) of a fuel cell. Noble metal deposition processes have been developed for ALD to deposit the electrode of a cell. ALD processes for ruthenium, platinum, and palladium were studied. All the processes are based on the reaction of the metal precursor with reactant such as oxygen and formalin. Ruthenium films were grown from a Bis(cyclopentadienyl)ruthenium(II) precursor RuCp2 with oxygen as a reactant at 1Torr and 400°C. High quality platinum films were grown from Trimethyl(methylcyclopentadienyl)platinum(IV) precursor MeCpPtMe3 with oxygen at 1Torr and 400°C. Palladium was deposited using Pd(II) hexafluoroacetylacetonate precursor (Pd(hfac)2) with formalin or Palladium acetylacetonate precursor Pd(C5H7O2)2 with oxygen at 0.3Torr and 200°C. Discrete platinum and ruthenium nanoparticles were grown as fuel cell catalysts by controlling the number of ALD cycles. Nanoparticle size was 10~20nm on SiO2 wafer. Platinum nanoparticles were grown at 50 cycles, while ruthenium nanoparticles were grown at 200cycles. The ruthenium-platinum catalysts used in direct methanol fuel cells (DMFCs) where a bimetallic catalyst is required for optimal performance. The formation of bimetallic nanoparticles is complicated by the relative rates of nucleation and growth for platinum and ruthenium. The ruthenium-platinum bimetallic-nanoparticle catalyst which has 1:1 composition ratio was grown by controlling the number of ruthenium pulse and platinum pulse. Porous platinum nanocatalysts on anodic aluminum oxide (AAO) were fabricated by using a high-pressure sputtering technique in a gaseous Ar. The high performance of Pt nanocatalysts fabricated at a sputtering pressure of 80mTorr was due to formation of the porous catalyst layer with pores for gas channel and increase the amount of TPB. Yttria stabilized zirconia (YSZ) films were synthesized by ALD and Sputtering as electrolyte of the cell. Tetrakis-(dimethylamino)zirconium and tris(methylcyclopentadienyl)yttrium were used as ALD precursors with oxygen as oxidant. From X-ray photoelectron spectroscopy (XPS) compositional analysis, the yttria content was identified to increase proportionally to the pulse ratio of Y/Zr. Accordingly, the target stoichiometry ZrO2/Y2O3 ) 0.92:0.08 was achieved employing a mix of pulse numbers, Zr/Y=7:1. The deposition rate of YSZ was dependent on surface morphology of substrate. The growth rate ALD YSZ on flat substrate such as SiO2 wafer was about 1Å/cycle and the one on porous structured substrate such as porous platinum layer was about 0.6Å/cycle respectively. YSZ thin films with various target-substrate distance from 6cm to 12cm were deposited by using RF sputtering with metal alloy target of composition 92% Zr –8% Y. A plasma power of 200 Watts and working pressure of 5 mTorr with 8 to 2 argon-to-oxygen ratio were employed. The growth rate sputtered YSZ on flat substrate such as SiO2 wafer was about 1.5nm/min and the one on porous structured substrate such as porous platinum layer was about 1nm/min respectively. The YSZ layer fabricated in both processes (i.e. ALD and Sputter) was investigated to measure O2- ions conductivity. For this purpose, Impedance Spectroscopy Technique was adopted. The films grown were analyzed with various methods. The crystalline phases were identified by X-ray diffraction (XRD) and the film thickness and the surface roughness were determined by X-ray reflectivity (XRR). Energy dispersive X-ray spectroscopy (EDX, EDS) was the principal method used for the film thickness determination. The surface morphology of the films was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM) Using thin film deposition techniques (i.e. ALD and sputtering), Thin film solid oxide fuel cell was fabricated on nanoporous anodic aluminum oxide (AAO) templates. The porous Pt anode and cathode were deposited by sputter and bi-layered YSZ electrolyte was deposited by ALD. This structure served as a solid oxide fuel cell designed to operate at intermediate temperatures. Cell area supported by AAO template was extended compared to free-standing cell. Porous structure supported cell has an issue of electrical shortage and low open circuit voltages (OCVs) resulted from gas leakage through pinholes especially of columnar microstructure of the electrolyte by sputtering. Both OCV and IV performance were measured to investigate the effect of ALD YSZ interlayer using performance test station. As a result of performance measurement, bi-layered electrolyte by using both processes of ALD and sputtering reduced the thickness of electrolyte without electrical shortage and the cell area was increased using nanoporous AAO. AAO Supported cell which is improved by ALD YSZ interlayer shows good stability against degradation of anode and high performance due to thickness reduced electrolyte.