[en] Highlights: • The oxidation mechanism of Crofer 22 APU was investigated at high temperatures. • The growth rate of chromia increased abruptly due to oxygen that penetrates the Cr-Mn oxide. • Two-step thermal behavior for the growth of the scale was proposed. • A thickness prediction model for the scale of Crofer 22 APU was established. - Abstract: The oxidation mechanism of Crofer 22 APU, one of the most preferred interconnects for solid oxide fuel cells (SOFCs), was explored in air. Crofer 22 APU showed two-step thermal behavior during thermal exposure, based on the varying growth rate of the chromia scale. The growth rate of the chromia scale abruptly increased at a certain annealed time, suggesting that its formation kinetics changed during thermal exposure. A thickness prediction model for the entire scale of Crofer 22 APU, based on the observed two-step oxidation mechanism, was established using the parabolic rate law.

[en] Highlights: • High-performance lanthanum nickelate-based cathode is fabricated. • The sinterability of cathode is enhance by compositional modification. • Harmful chemical reactions are suppressed by lowering processing temperatures. • The cell with lanthanum nickelate cathode outperforms state-of-the-art cells. -- Abstract: The Ruddlesden-Popper phase lanthanum nickelate, La₂NiO_4+δ (LNO), offers excellent material properties as a cathode for solid oxide fuel cells (SOFCs). However, taking full advantage of its intrinsic properties is difficult in realistic cells because of its high chemical reactivity with the electrolyte at elevated temperatures. Herein, we demonstrate high-performance SOFCs with an LNO-based cathode fabricated by a low-temperature processing route that suppresses harmful chemical reactions. The sintering capability of the composite cathode composed of LNO and gadolinia-doped ceria (GDC) was enhanced by mixing Fe-based sintering additive with GDC, which formed reliable interfacial bonding with the electrolyte at a temperature ~200 °C below the typical processing temperature. Because no interdiffusion between cathode and electrolyte occurs at such low temperatures, the cell is successfully fabricated without diffusion blocking layer, which simplifies the cell structure and manufacturing process. The cell with the LNO-based cathode outperformed state-of-the-art cells, particularly at lower operating temperatures. These results highlight that the processing parameters strongly affect the electrochemical performance of this LNO-based cathode and must be carefully engineered to fully exploit its superior intrinsic properties.

[en] Highlights: • The fuel flexibility of thin-film-based solid oxide fuel cell is studied. • Sputtering and infiltration are combined to add Pd at different anode positions. • Thin-film-based cells were operated over butane direct utilization of at 600 °C. • Pd appears to contribute to water-gas shift and steam reforming reaction. • Pd improves electrochemical reaction through increasing hydrogen supply. -- Abstract: Fuel flexibility, which is one of the most important advantages of the solid oxide fuel cell, can be compromised at lower operating temperatures. Thus in this study, normal butane is selected as the fuel and multiscale-architectured thin-film-based solid oxide fuel cells are operated in direct butane utilization mode at T = 600 °C. Palladium (Pd) is chosen as the secondary catalyst to assist the reforming of the butane and is inserted at different positions at the anode. By combining two different Pd insertion methods, sputtering and infiltration, four different thin-film-based solid oxide fuel cells were prepared: (1) the cell without Pd (Ref-cell); (2) the cell with Pd at the anode functional layer, which was fabricated by alternating sputtered Pd layers with pulsed-laser deposited NiO/yttria-stabilized zirconia layers (Pd-S-cell); (3) the cell with Pd at the anode support, which was fabricated by infiltration (Ref-I-cell); and (4) the cell with Pd at both the anode functional layer and anode support (Pd-S-I-cell). As expected, different Pd distributions were observed along the thickness of the anode. The Pd-S-I-cell showed significant enhancement in performance and durability. Approximately three times cell performance enhancement for the best case is observed in comparison with that of the Ref-cell. The Pd distribution, not only at the anode functional layer but also at the anode support, appears to have accelerated the electrochemical and thermochemical reactions. In addition, a lesser degree of carbon deposition was observed at the anode of the Pd-S-I-cell as compared with the case of the others.

[en] Highlights: • Electrolyte with a thickness less than grain size was fabricated by cost-effective method. • Bamboo-structured thin electrolyte effectively reduces ohmic resistance of protonic ceramic fuel cell. • Electrode reactions were analyzed by distribution of relaxation time method. • Surface diffusion of an adsorbed oxygen to the triple phase boundaries at cathode is the most probable rate determining step. -- Abstract: High-performance and cost-effective fabrications should be simultaneously achieved for practical applications of fuel cells. Unfortunately, protonic ceramic fuel cells, which are considered next-generation solid oxide fuel cells operating at lower temperatures (≤600 °C), do not satisfy the requirements. While thin electrolyte and rapid reactions at electrode/electrolyte interfaces are crucial for cell performance, the thickness of the electrolyte via cost-effective ceramic processes is still not satisfactory (currently capable of >10 μm) and the electrode reaction(s) are yet to be clarified. Here we demonstrate the fabrication of a columnar-structured thin electrolyte (∼2.5 μm) of BaCe_0.55Zr_0.3Y_0.15O_3-δ, in which no perpendicular grain boundaries exist against the current direction, through a low-cost screen printing method. A high open-cell voltage of 1.10 V ensures that the thin electrolyte is sufficiently dense for gas-tightness, thereby achieving an extraordinary maximum power density of 350 mW/cm² at 500 °C. The electrode reactions are investigated by distribution of relaxation time method based on electrochemical impedance spectroscopy as a function of oxygen partial pressure and hydrogen partial pressure at 500 °C, suggesting that the reaction step corresponding to the surface diffusion of an adsorbed oxygen to the triple phase boundaries at the cathode is most probably the main contributor to the overall polarization resistances.

[en] A strategy for improving the stability of nickel-based solid oxide fuel cell (SOFC) anodes via compositional and microstructural engineering is presented. Ni content was reduced to 2 vol%, and nanosized Ni particles were uniformly dispersed in a mixed ionic-electronic conducting matrix comprising gadolinium-doped ceria (GDC) using a thin-film technique. Remarkable stability with no performance deterioration even after 100 reduction-oxidation cycles could be observed for the optimized nanostructured anodes. Cell performance at 500°C was enhanced, exceeding 650 mW/cm². This study offers valuable insights for enhancing the durability, performance, and productivity of SOFCs.