Date of Award

12-15-2014

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Fanglin (Frank) Chen

Abstract

Solid oxide fuel cells (SOFCs) have been considered as one of the most promising technologies for future energy conversion since they can in principle be operated with fuels ranging from H2 to any hydrocarbon fuel. However, the system cost and coking (when using hydrocarbon as fuel) issues for the state-of-art electrode materials/designs often limit their further application. The objective of this Ph.D dissertation is aiming at overcoming these problems and accelerating SOFC commercialization. One approach to cost reduction is lowering the SOFC operating temperature to below 800 or even 600 oC, so that inexpensive materials can be used and quick start-up and SOFC durability can be improved. However, the overall electrochemical performance of an SOFC system will significantly decrease with a reduced operating temperature due to increased ohmic resistance of the electrolyte and polarization resistances of the electrode reactions. Ohmic resistance could be reduced by decreasing thickness of the electrolyte or using electrolyte with high ionic conductivity. Polarization resistance could be reduced by applying novel microstructured electrodes. Here we have fabricated a novel hierarchically oriented porous anode-supported solid oxide fuel cell with thin Gd0.1Ce0.9O2 (GDC) electrolyte by freeze-drying tape-casting and drop-coating. Three dimensional (3D) X-ray microscopy and subsequent analysis have demonstrated that the substrate has a graded open and straight pore/channel structure. The diameter of pore size on the bottom and top surface, porosity distribution along thickness direction and tortuosity factor have been determined by SEM and calculation with help of Matlab. The novel microstructure is expected to facilitate gas diffusion in the anode during fuel cell operation. The cell performance at low temperature ranging from 500-600 oC has been evaluated systematically. SOFCs with such Ni-GDC anode, GDC film (30 μm) electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3-GDC (LSCF-GDC) cathode show significantly enhanced cell power output of 1.021 W cm-2 at 600 °C using H2 as fuel and ambient air as oxidant. Since cathode has been the center of the focus in the electrode development largely because oxygen reduction is more difficult to activate in SOFCs operating at commercially relevant temperature. Consequently, it is critically important to develop new cathode material or novel cathode microstructures with low polarization loss to maintain sufficient high electrochemical activity to enable SOFC operating at temperatures of below 600 oC. We have prepared SOFCs with hierarchically porous nano cathode network by a novel vacuum-free infiltration and subsequent freeze-drying combustion. The straight open GDC cathode skeleton and NiO-GDC anode substrate prepared by freeze-drying tape-casting facilitate mass transport while the nano cathode catalyst promotes the electrochemical reactions. The cell with straight open electrodes and hierarchically porous cathode network demonstrates a maximum power density of 0.65 Wcm-2 at 500 oC and impressive stability for more than 500 h at 400 oC using H2 as fuel and ambient air as oxidant. The simple and cost-effective fabrication process is expected to significantly impact the SOFC operability and accelerate its commercialization.

Another advantage of SOFCs compared with other energy conversion systems is the capability of direct utilization of hydrocarbon fuel. However, carbon species adsorb strongly on Ni surface and thus blocks the active site for electrochemical reactions, resulting in rapid performance degradation. In this research, we present an innovative and simple design for enhancing the coking resistance of the conventional nickel cermet SOFC anode. A thin nanoscale samaria doped ceria (SDC) catalyst layer has been deposited on the wall surface of the Ni-yttria-stabilized zirconia (Ni-YSZ) anode internal gas diffusion channel (5-200 μm in size) via a combination of freeze-drying tape-casting and vacuum-free infiltration. The efficiency for catalyst infiltration has been significantly improved by using hierarchically porous anode structure with open and straight channels. Single cells with nanoscale SDC layer show very stable cell performance and a peak power density of 0.65 Wcm-2 at 800 oC using methane as fuel, more than one order of magnitude higher than that for the cells using Ni-YSZ anode without the SDC catalyst layer. High resolution transmission electron microscopy (HRTEM) analysis indicates that nanoscale SDC layer can prevent the formation or growth of nickel carbide (onset of coking).

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