Mark Cassidy

Advanced Electrochemical Properties of LnBa0.5Sr0.5Co2O5 + δ (Ln = Pr , Sm, and Gd) as Cathode Materials for IT-SOFC

Excellent area-specific-resistance (ASR) values have been exhibited by cathode materials with a Sr-doped layer perovskite type structure and therefore show themselves to be possible candidates for intermediate-temperature-operating solid oxide fuel cell (IT-SOFC, 600-800°C) applications. SmBa 0.5 Sr 0.5 Co 2 O 5+δ (SBSCO) electrode was sintered onto 10 mol % gadolinia-doped ceria (Ce 0.9 Gd 0.1 O 2 , CG091) at 1000°C to form symmetrical cells and exhibited an ASR value of 0.092 Ω cm 2 at 700°C. The lowest ASR value was observed when the composite cathode of 50 wt % of SBSCO and 50 wt % of CG091 (SBSC050) was used in conjunction with an interlayer of CGO91 applied between the electrode and 8 mol % Y 2 O 3 stabilized ZrO 2 electrolyte. These were 0.12 Ω cm 2 at 600°C and 0.019 Ω cm 2 at 700°C, respectively. The coefficient of thermal expansion (CTE) of SBSCO was 21.9 × 10 -6 K -1 at 700°C. However, the CTE of the composite cathode of SBSC050 was shown to be 13.6 × 10 -6 K -1 at 700°C, this being more compatable with the other components within the cell.

Highly efficient, coking-resistant SOFCs for energy conversion using biogas fuels

Solid oxide fuel cells (SOFCs) afford an opportunity for the direct electrochemical conversion of biogas with high efficiency; however, direct utilisation of biogas in nickel-based SOFCs is a challenge as it is subject to carbon deposition. A biogas composition representative of a real operating system of 36% CH4, 36% CO2, 20% H2O, 4% H2 and 4% CO used here was derived from an anode recirculation method. A BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BCZYYb) infiltrated Ni-YSZ anode was investigated for biogas conversion. The infiltration of BCZYYb significantly promoted the electrochemical reactions and the cells exhibited high power output at the operational temperatures of 850, 800 and 750 °C. At 800 °C, supplied with a 20 ml min−1 biogas, the cell with a BCZYYb-Ni-YSZ anode, generated 1.69 A cm−2 at 0.8 V with an optimal amount of 0.6 wt% BCZYYb, whereas only 0.65 A cm−2 was produced with a non-infiltrated Ni-YSZ in the same conditions. At 750 °C, a maximum power density of 1.43 W cm−2 was achieved on a cell with a BCZYYb-Ni-YSZ anode, a 3 μm dense YSZ film electrolyte, a Gd0.1Ce0.9O2 (GDC) buffer layer and a La0.6Sr0.4Co0.2Fe0.8O3–Gd0.1Ce0.9O2 (LSCF-GDC) composite cathode. The cell remained stable, while operating at 0.8 V for 50 hours with a current density of 1.25 A cm−2. A well-designed cell structure and selected components made it possible to obtain excellent performance at good fuel utilisation. The analysis of gases in open-circuit conditions or under various current loads suggested that the prevalent reaction was reforming of methane without coking. This study demonstrates that the BCZYYb-Ni-YSZ is a promising electrode for carbon-containing fuel.

Demonstration of high performance in a perovskite oxide supported solid oxide fuel cell based on La and Ca co-doped SrTiO3

Perovskite electrodes have been considered as an alternative to Ni-YSZ cermet-based anodes as they afford better tolerance towards coking and impurities and due to redox stability can allow very high levels of fuel utilisation. Unfortunately performance levels have rarely been sufficient, especially for a second generation anode supported concept. A-site deficient lanthanum and calcium co-doped SrTiO3, La0.2Sr0.25Ca0.45TiO3 (LSCTA-) shows promising thermal, mechanical and electrical properties and has been investigated in this study as a potential anode support material for SOFCs. Flat multilayer ceramics cells were fabricated by aqueous tape casting and co-sintering, comprising a 450 μm thick porous LSCTA- scaffold support, a dense YSZ electrolyte and a thin layer of La0.8Sr0.2CoO3−δ (LSC)-La0.8Sr0.2FeO3−δ (LSF)-YSZ cathode. Impregnation of a small content of Ni significantly enhanced fuel cell performance over naked LSCTA-. Use of ceria as a co-catalyst was found to improve the microstructure and stability of impregnated Ni and this in combination with the catalytic enhancement from ceria significantly improved performance over Ni impregnation alone. With addition of CeO2 and Ni to a titanate scaffold anode that had been pre-reduced at 1000 °C, a maximum powder density of 0.96 W cm−2 can be achieved at 800 °C using humidified hydrogen as fuel. The encouraging results show that an oxide anode material, LSCTA- can be used as anode support with YSZ electrolyte heralding a new option for SOFC development.

Thick Film Processing Challenges in the Realisation of a Co-Fired Solid Oxide Fuel Cell Roll

The Solid Oxide Fuel Cell Roll (SOFCRoll) is a novel design based on a double spiral. Combining structural advantages of tubular geometries with processing advantages of thick film methods, it utilises a single cofiring process. The initial concept used separate tape cast layers which were laminated before rolling. To optimise layer thickness to function, thinner screen printed layers were combined into the tape cast structure in 2nd generation cells. This presented several processing challenges, such as achieving dense electrolyte layers, maintaining porous electrode and current collecting layers and incorporation of integral gas channels. Performance has been promising with open circuit voltages close to 1V and cell power of over 400mW at 800°C, however cracking is still evident. Therefore further iterations are in development where thinner layers are sequentially cast, aiming to improve interfacial bonding and better match plasticity and burn out to reduce cracking. This paper reviews key aspects of understanding and development of the SOFRoll , the challenges that have been tackled and what challenges remain, along with future directions for development and potential applications for this device.

New Materials for Improved Durability and Robustness in Solid Oxide Fuel Cell

This chapter provides an overview of the considerations that must be made regarding new materials development for improved durability and robustness in solid oxide fuel cells (SOFCs). A number of recent development concepts are outlined for the core cell materials of anode, electrolyte, and cathode, in particular new catalytic approaches such as catalyst impregnation and exsolution on the anode to improve redox and fuel flexibility and reduced temperature cathodes. Some of the challenges of scaling up into larger stacks are also discussed. Here the interactions of cell materials with stack materials, in particular the interconnect, are summarized, such as chromium poisoning and cell to interconnect electrical contact, both of which feature prominently in SOFC stack lifetime issues. Barriers to new materials development are outlined along with the potential for accelerated testing.

Microstructure dependence of performance degradation for intermediate temperature solid oxide fuel cells based on the metallic catalyst infiltrated La- and Ca-doped SrTiO3 anode support

Anode-supported solid oxide fuel cells with the configuration of the La0.2Sr0.25Ca0.45TiO3 (LSCTA−) anode, YSZ electrolyte and La0.8Sr0.2Co0.2Fe0.8O3 (LSCF)–YSZ cathode were fabricated using tape casting and co-sintering techniques followed by pre-reduction and impregnation. In order to improve the performance, the active anodes were prepared via the wet impregnation of metallic catalysts (Ni or Ni–Fe solution). The impregnation of 3 wt% nickel significantly improved the fuel cell performance from 43 mW cm−2 for the bare LSCTA− anode to 112 mW cm−2 for the Ni–LSCTA− anode at 700 °C in humidified hydrogen containing 3 vol% H2O. More interestingly, the substitution of 25 wt% Fe to Ni further enhances the power density by a factor of 1.5, compared to the Ni-impregnated cell. The cell infiltrated with Ni–Fe solid solution shows a slower degradation than the other two cells after the first 20 h period. High-resolution back-scattered electron (BSE) and transmission electron microscopy (TEM) images performed on the cross section of the impregnated anodes with time after ion beam preparation show that the sintering of the catalyst particles on the scaffold surface and the interaction between backbone and catalyst are the predominant contributions for the degradation of cell performance.