Oxygen-Electrode-Supported Solid Oxide Cells with High Fuel and Steam Utilization

Abstract

Oxygen-electrode-supported solid oxide cell (SOC) designs are potentially advantageous for electrolysis and reversible energy-storage applications. In a reversible SOC application, the oxygen produced during electrolysis can be stored and then used during fuel cell operation. In this case, the presence of pure oxygen at the oxygen electrode mitigates the significant gas diffusion limitations encountered with thick oxygen electrode supports when using air. Furthermore, supporting the cell mechanically with the oxygen electrode allows the fuel electrode to be significantly thinner, which alleviates gas diffusion limitations that make it difficult to achieve high steam utilization during electrolysis operation, and high fuel utilization in fuel cell operation. Nevertheless, development of oxygen-electrode-supported cells has been hampered by processing limitations, particularly the difficulty in co-firing oxygen electrode materials with higher melting temperature electrolyte materials to achieve a fully densified electrolyte while retaining satisfactory electrode porosity, surface area, and triple-phase boundary density (TPB), and avoiding undesirable interfacial reactions or interdiffusion. Electrolyte sintering aids can help reach desirable co-firing temperatures. In previous work, sintering aids were used for single-step co-firing temperature of fuel-electrode supported SOCs and resulted in cells with desirable microstructure, free of significant interdiffusion, and relatively good performance, but also with low TPB density and therefore high polarization resistance in the oxygen electrode compared to cells fabricated by a two-step firing process. In this work, we demonstrate successful fabrication of oxygen-electrode-supported SOCs consisting of LSM current collector, LSM-YSZ oxygen-electrode, YSZ electrolyte, and NiO-YSZ fuel electrode, using a reduced-temperature single-step firing process. Microstructural and chemical characteristics of the cell components are studied using scanning electron microscopy and energy dispersive spectroscopy. The electrochemical characteristics are studied using current-voltage measurements and impedance spectroscopy, with both air and pure oxygen at the oxygen electrode and with different H2/H2O contents at the fuel electrode. These cells achieve much better fuel or steam utilization than anode-supported cells due to the relatively thin fuel electrode, and the presence of pure oxygen in the oxygen-electrode support mitigates mass transport limitations. Cells operating in fuel cell mode tested with hydrogen and air produce a maximum power density (Pmax) of 325 mW/cm2 at 800 °C, with a limiting current density of 0.9 A/cm2. Pmax increases to 575 mW/cm2 when using pure oxygen as oxidant, due to the improved gas diffusion and a larger exchange current density in the cathode. LSM-YSZ electrodes of some cells are infiltrated with (Sm0.5Sr0.5)CoO3 (SSC), and enhancement of the electrode in this manner is critical for obtaining high cell power density. Incorporation of SSC nanoparticles increases Pmax to 580 mW/cm2 for a cell operating in fuel cell mode in hydrogen and air, with much smaller initial resistance compare to the non-infiltrated cell. Limiting current is similar to the non-infiltrated cells, suggesting large gas diffusion limitation due to the thick oxygen electrode. Changing the oxidant from air to pure oxygen results in significant improvement in cell performance, with Pmax of 1400 mW/cm2 and no evidence of limiting current up to 6 A/cm2, the highest current measured. Life tests were carried out on SSC-infiltrated cells operating at a direct current of 1 A/cm2 and temperature of 800°C in both fuel and electrolysis modes. Significant degradation was observed, primarily due to coarsening of the infiltrated SSC. Recent results using a different infiltrant, Sr(Ti,Fe)O3, which show better stability, will be presented. Finally, initial results on another method for reducing concentration polarization, via the use of patterned electrode supports made using an extrusion-based 3D-printing process, will be presented.