Comprehensive development of high performance solid oxide fuel cells for intermediate and low temperature applications

In order to develop high performance solid oxide fuel cells (SOFCs) operating at low to intermediate temperatures, the three main SOFC components---the anode, electrolyte, and cathode---were comprehensively studied.;In order to lower anodic polarization losses in anode-supported SOFCs, a novel composite anode functional layer (AFL) having bimodal (nano/micro) structure was developed. Application of this AFL involved a simple process where a precursor solution was coated onto a conventional submicron sized colloidally-deposited Ni-GDC AFL. Cells prepared in this manner yielded maximum power densities (MPD) of 1.29, 1.16, 0.7 and 0.38 W/cm2 at 650, 600, 550 and 500°C, respectively. Electrochemical impedance results showed a striking decrease in both ohmic and non-ohmic area specific resistances (ASRs) for these cells compared to those with either no AFL, or a conventional AFL.;In addition, the effect composition of the conventional submironsized AFL on performance was examined. The highest MPD (1.15 W/cm2 at 650°C) was achieved at a composition of 60wt% NiO. This composition had the best performance over the intermediate temperature range (450 to 650°C). For the potentio-static test, the cell exhibited stable performance over 200 hrs of operation at 1.1 W/cm2. It was also revealed that electrode ASR has an inverse linear relationship with maximum power density at 650°C.;To better understand the effect of AFL composition, microstructural features of AFLs having various Ni-GDC compositions were quantified by a 3 dimensional (3D) reconstruction technique using a FIB/SEM dual beam system. Of the compositions tested, the highest triple phase boundary (TPB) density was achieved at 60wt% NiO, which corresponds to a 1:1 volume ratio of Ni to GDC phase. The quantified TPB density showed an inverse proportionality to electrode ASR.;Using a wet chemical co-precipitation method, nano-sized ESB particles were successfully synthesized at temperatures as low as ∼ 500°C. Due to the high sinterability of this powder, a dense erbia stabilized bismuth oxide (ESB) layer was successfully formed on a gadolinia doped ceria (GDC) electrolyte by a simple colloidal coating method. A systematic study on the sintering behavior of ESB was conducted to determine the optimum sintering conditions for these materials. I-V measurement a cell using this bilayered electrolyte sytem showed a high power density (∼ 1.5 W/cm2) at 650°C due to an enhancement in OCP and a significant reduction in ASR when compared to a GDC single cell.;The performance of conventional (La0.80Sr0.20)MnO 3-delta (LSM) cathodes were dramatically improved at the IT range by combining it with a highly conductive ESB phase. The electrode ASR measured from a symmetric cell consisting of LSM-ESB electrodes on an ESB electrolyte was only 0.08 O-cm2 at 700°C which is ∼60% lower than that of LSM-ESB on GDC electrolytes (0.19 O-cm2). This exemplifies the synergetic effect the ESB phase has both in the cathode bulk and at the electrolyte/electrode interface. The MPDs of the anode-supported SOFCs with LSM-ESB cathodes on ESB/GDC bilayered electrolytes were ∼836 mW/cm2 at 650°C, which is the highest value reported for SOFCs using LSM-bismuth oxide composite cathodes.