Ph.D. Oral Examination- Joon Hyung Shim - July 23rd (Wed) @10am
shimm at stanford.edu
Wed Jul 16 23:25:45 PDT 2008
Nanoscale Thin Film Ceramic Fuel Cells
Joon Hyung Shim
Department of Mechanical Engineering
Advisor: Fritz B. Prinz
Wednesday, July 23rd, 2008
10:00AM (Refreshments served @9:45AM)
MERL Conference Room (203)
The ceramic fuel cell (CFC) refers to fuel cells employing solid state ceramic electrolytes, including two types of fuel cells: the solid oxide fuel cell (SOFC) and the proton-conducting oxide fuel cell (PCOFC). Ion conducting ceramics require high operation temperatures of about 700~1000˚C for reasonably active charge transfer reactions at the electrode-electrolyte interface and ion transport through the electrolyte. This high operational temperature has limited CFC applications due to thermal instability of equipped devices. The goal of this study is to minimize Ohmic losses and activation losses at the electrolyte and electrode-electrolyte interface respectively by engineering CFC components to run fuel cells at reduced temperatures.
As a method of engineering SOFC electrolytes, we proposed to fabricate ceramic membranes at the nanometer scale. We have successfully fabricated free-standing 60 nm yttria-stabilized zirconia (YSZ) films, the most common electrolyte material for SOFCs, using atomic layer deposition (ALD). In fuel cell tests with porous platinum electrodes, ALD YSZ showed a maximum power density of 270mW/cm2 at 350˚C, which is a significant improvement from the expected performance estimated from reference values. We found that the performance enhancement originated from an increase in exchange current density at the electrode-electrolyte interface. An oxygen isotope (O18) experiment was used to trace oxide ion diffusion, and it was found that the oxygen-oxide ion exchange rate at the ALD YSZ surface was enhanced compared to the rate in bulk YSZ single crystals, while diffusivity in ALD YSZ films was found to be equal to that of bulk YSZ. This enhancement in the ALD YSZ films was also confirmed in O18 spatial mapping measured by nano-resolution secondary ion mass spectrometry (NanoSIMS).
We have also studied nanoscale yttrium-doped barium zirconate (BYZ) electrolytes as electrolytes for PCOFCs. Thin BYZ films were grown epitaxially on MgO(100) single crystals using pulsed laser deposition (PLD), and conductivity was measured using electrochemical impedance spectroscopy (EIS). X-ray diffraction (XRD) and atomic force microscopy (AFM) data confirmed that BYZ becomes polycrystalline with the formation of grains that negatively affect proton conduction as it grows thicker, and grain growth decreases conductivity by 2~3 times. Freestanding 130nm thick PLD BYZ was tested as an electrolyte in a PCOFC with porous platinum electrodes. A maximum power density of 120mW/cm2 at 450˚C was measured, which is lower than the expected performance based on reference data. This low performance is due to a low exchange current density rooted in low catalytic activity at cathodes. To solve this problem, we performed ALD of BYZ and found that ALD leads to improved quality of the BYZ surface, showing a higher exchange current density and exhibiting an improved maximum power density of 140mW/cm2 at 400˚C. We also confirmed that ALD BYZ fuel cells can operate with methanol as a fuel.
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