Development of dual-layer hollow fiber with modified electrolyte for micro-tubular solid oxide fuel cell

Abstract

The excellent ionic conductivity at temperature ranges from 400-700 °C has made cerium gadolinium oxide (CGO) as one of promising alternative solid electrolyte materials for intermediate temperature solid oxide fuel cell (IT-SOFC) application. However, the requirement of high sintering temperature up to 1550 °C for densification of CGO electrolyte complicates the cell fabrication process as it reduces the porosity in the electrode layers, particularly in the co-sintering step of anode/electrolyte dual-layer hollow fiber (DLHF). Hence, the fabrication of the DLHF is challenging due to the different sintering behaviors of the each layer. The main objective of this study is to develop anode/electrolyte DLHFs with improved electrolyte properties with reduced co-sintering temperature for IT-SOFCs via a single-step phase inversion-based co-extrusion/co-sintering technique. The sintering properties of electrolyte flat sheet was studied by comparing two approaches, (i) using mix particle size electrolyte and (ii) addition of lithium oxide as sintering additive in the electrolyte. The DLHF with modified electrolyte was later fabricated by phase inversion based co-extrusion and co-sintered at temperature ranging from 1400 to 1500 °C. The DLHF was evaluated in term of the morphology, mechanical strength and gas-tightness as well as electrical conductivity, porosity and permeability of the anode layer. This study showed that a dense CGO flat sheet layer sintered at 1450 °C with the addition of 30% nano size CGO particles were obtained. Meanwhile, the doping of 2 mol% of lithium nitrate into CGO was found to reduce the sintering temperature to 1400 °C. When the co-sintering temperature increased, the mechanical strength, gas-tightness and electrical conductivity were increased, whereas the porosity and permeability of the anode layer were decreased. The DLHF that was co-sintered at 1450 °C showed sufficient properties and therefore, it was chosen for the construction of micro-tubular SOFC (MT-SOFC). When comparing the maximum power density of MT-SOFC namely nickel (Ni)-CGO/CGO (unmodified), Ni-CGO/30%nano-70%micron CGO (first approach) and Ni- CGO/lithium (Li)-CGO (second approach); it was found that the Ni-CGO/30%nano- 70%micron CGO cell performed the best. At 500 °C, the cell produced the highest maximum power density, which was 275 Wm-2 as compared to Ni-CGO/Li-CGO cell (60 Wm-2) and Ni-CGO/CGO cell (200 Wm-2). Porous anode in Ni- CGO/30%nano-70%micron CGO DLHF provided active reaction site, while dense electrolyte layer resulted from pore filling caused by the introduction of 30% nano sized CGO particles improved the gas-tightness of the electrolyte. Meanwhile, the closed pore caused by the migration of Li ions in anode sponge–like region of Ni- CGO/Li-CGO hindered the triple phase boundary region that impaired the cell performance. This limited the inclusion of Li in DLHF design. Nevertheless, the results from this study has proven the feasibilities to accelerate the densification of electrolyte as well as presented an advanced electrolyte material for MT-SOFC.