Nanointegrated, High-Performing Cobalt-Free Bismuth-Based Composite Cathode for Low-Temperature Solid Oxide Fuel Cells

Scaffold Infiltrated Cathodes for Low-Temperature Solid Oxide Fuel Cells

Lowering the operating temperature of solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs) to reduce system cost and increase lifetime is the key to widely deploy this highly efficient energy technology, but the high cathode polarization losses at low temperatures limit overall cell performance. Here we demonstrate that by engineering a universal ceria-based scaffold with infiltrated nanoscale electrocatalysts, a low cathode polarization <0.25 Ω·cm2 with remarkably high performance 1 W/cm2 at 550 °C is achieved. The combination of low processing and operating temperature restrains the nanosized electrocatalysts, not only allowing fast oxygen transport but also providing an essential electronically connective network to facilitate electrochemical reactions without requiring the high-temperature processing of a separate cathode layer. Moreover, excellent SOFC durability was demonstrated for over 500 h. This work shows a promising pathway to reduce processing/system costs with all scalable ceramic processing techniques for the future development of low-temperature SOFCs and SOECs.

Bismuth-oxide nanoparticles: study in a beam and as deposited

Bi2O3 is a promising material for solid-oxide fuel cells (SOFC) due to the high ionic conductivity of some phases. The largest value is reached for its δ-phase, but it is normally stable at temperatures too high for SOFC operation, while nanostructured oxide is believed to have more suitable stabilization temperature. However, to manufacture such a material with a controlled chemical composition is a challenging task. In this work, we investigated the fabrication of nanostructured Bi2O3 films formed by deposition of free Bi-oxide nanoparticles created in situ. The particle-production method was based on reactive sputtering and vapour aggregation. Depending on the fabrication conditions, the nanoparticles contained either a combination of Bi-metal and Bi-oxide, or only Bi-oxide. Prior to deposition, the free particles were probed in the beam - by synchrotron-based photoelectron spectroscopy (PES), which allowed assessing their composition "on the-fly". The nanoparticle films obtained after deposition were studied by PES, scanning electron microscopy, transmission electron microscopy, and electron diffraction. The films' chemical composition, grain dimensions, and crystal structure were probed. Our analysis suggests that our method produced Bi-oxide films in more than one polymorph of Bi2O3.

Turning Detrimental Effect into Benefits: Enhanced Oxygen Reduction Reaction Activity of Cobalt-Free Perovskites at Intermediate Temperature via CO2-Induced Surface Activation

A minor amount of CO₂ in air usually causes a detrimental effect on oxygen activation over a solid oxide fuel cell (SOFC) cathode because insulating surface carbonate is easily formed, which inhibits charge transfer during the oxygen reduction reaction (ORR). In this study, we report that the detrimental effect due to the CO₂ interaction with perovskite oxide can be turned into a beneficial effect for facilitating ORR through tailoring the material composition of the perovskite. More specifically, for cobalt-free SrSc_0.025Nb_0.075Fe_0.9O_3-δ (SSNF), the exposure to the CO₂ atmosphere results in the formation of a minor amount of surface strontium carbonate mainly in the form of a nanofilm over the perovskite surface, which protects the electrode from further corrosion by CO₂, thus achieving a relatively stable performance even under a 10% CO₂-containing air atmosphere. When CO₂-free air is restored, the SrCO₃ is successfully decomposed at intermediate temperatures. As a result, the surface reaction kinetics is recovered to the initial degree while the charge transfer process is obviously improved. An area-specific resistance of only 0.07 Ω cm² is achieved at 650 °C after the CO₂-induced surface activation, much smaller than the original value of 0.13 Ω cm². In addition, the CO₂-treated electrode shows a fairly stable performance for ORR under a subsequent CO₂-free air atmosphere. To create such a beneficial effect, it is critical to tailor the degree of interaction of the perovskite surface with CO₂, while the benchmark Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) shows a too strong interaction with CO₂ with the formation of bulk-phase-like carbonate, which failed to decomposed even when restored with a CO₂-free atmosphere at intermediate temperatures, and as a result, worsened the ORR activity after the CO₂ treatment.

Enhancing Oxygen Reduction Reaction Activity and CO2 Tolerance of Cathode for Low-Temperature Solid Oxide Fuel Cells by in Situ Formation of Carbonates

Development of low-cost and cobalt-free efficient cathode materials for oxygen reduction reaction (ORR) remains one of the paramount motivations for material researchers at a low temperature (<650 °C). In particular, iron-based perovskite oxides show promise as electrocatalysts for ORR because Fe metal is cheaper and naturally abundant, exhibit matched thermal expansion with contacting components such as electrolytes, and show high tolerance in a CO₂-containing atmosphere. Herein, we demonstrated a new mechanism, the in situ formation of alkali metal carbonates at the cathode surface. This new mechanism leads to an efficient and robust cobalt-free electrocatalyst (Sr_0.95A_0.05Fe_0.8Nb_0.1Ta_0.1O_3-δ, SAFNT5, A = Li, Na, and K) for the application of low-temperature solid oxide fuel cells (LT-SOFCs). Our results revealed that the formation of Li\K carbonates boosts the ORR activity with an area-specific resistance as low as 0.12 and 0.18 Ω cm² at 600 °C, which show the highest performance of the cobalt-free single-phase cathode that has been ever reported so far. We also find that the chemical stability and tolerance of tested cathodes toward CO₂ poisoning significantly improved with alkali carbonates, as compared to the pristine SrFe_0.8Nb_0.1Ta_0.1O_3-δ (SFNT) at 600 °C. This work demonstrates the conclusive role of alkali carbonates in developing highly efficient and stable cobalt-free cathodes for LT-SOFCs and CO₂ neutralization.