Hongqian Wang

Tape Casting of High-Performance Low-Temperature Solid Oxide Cells with Thin La0.8Sr0.2Ga0.8Mg0.2O3−δ Electrolytes and Impregnated Nano Anodes

Low-temperature solid oxide cells (LT-SOCs), operating at 400 to 650 °C, have great potential for commercialization since they can provide lower cost and improved long-term durability. Low operating temperature can also enable high round-trip efficiency of SOCs as reversible energy storage devices. This paper describes Sr0.8La0.2TiO3-α (SLT) anode supported LT-SOC with thin La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte made by tape casting, with screen printed La0.6Sr0.4Fe0.8Co0.2O3-δ (LSCF) cathode and impregnated Ni anode. Optimization of the anode functional layers is described; the best anodes had 68 vol % LSGM and 12.3 vol % Ni and yielded maximum power density of 1.6 Wcm-2 with a cell area specific resistance (ASR) of 0.21 Ωcm2 at 650 °C. Most of the cell ASR was associated with the cathode. Reversible electrolysis and fuel cell operation yielded similar characteristics with both 50% H2-50% H2O and syngas fuel. Life testing over 500 h showed that the cathode impedance stabilized after an initial break-in period; the ohmic and anode resistances, though relatively small, increased slightly with time.

Degradation of nano-scale cathodes: a new paradigm for selecting low-temperature solid oxide cell materials

Oxygen electrodes have been able to meet area specific resistance targets for solid oxide cell operating temperatures as low as ∼500 °C, but their stability over expected device operation times of up to 50 000 h is unknown. Achieving good performance at such temperatures requires mixed ionically and electronically-conducting electrodes with nano-scale structure that makes the electrode susceptible to particle coarsening and, as a result, electrode resistance degradation. Here we describe accelerated life testing of nanostructured Sm0.5Sr0.5CoO3-Ce0.9Gd0.1O2 electrodes combining impedance spectroscopy and microstructural evaluation. Measured electrochemical performance degradation is accurately fitted using a coarsening model that is then used to predict cell operating conditions where required performance and long-term stability are both achieved. A new electrode material figure of merit based on both performance and stability metrics is proposed. An implication is that cation diffusion, which determines the coarsening rate, must be considered along with oxygen transport kinetics in the selection of optimal electrode materials.

Cobalt-substituted SrTi0.3Fe0.7O3−δ: a stable high-performance oxygen electrode material for intermediate-temperature solid oxide electrochemical cells

A key need in the development of solid oxide cells (SOCs) is for electrodes that promote fast oxygen reduction and oxygen evolution reactions at reduced operating temperature (≤700 °C), with sufficient durability to allow operation over desired 40 000 h lifetimes. A wide range of electrode materials have been investigated, with some providing resistance low enough for cell operation below 700 °C, but it is generally found that the electrode performance degrades over time. Here we demonstrate an oxygen electrode material, Sr(Ti0.3Fe0.7−xCox)O3−δ (STFC), that provides a unique combination of excellent oxygen electrode performance and long-term stability. The addition of a relatively small amount of Co to Sr(Ti0.3Fe0.7)O3−δ, e.g., x = 0.07, reduces the electrode polarization resistance by >2 times. The STFC electrode yields stable performance in both fuel cell and electrolysis modes at 1 A cm−2. The fundamental oxygen diffusion and surface exchange coefficients of STFC are determined, and shown to be substantially better than those of La0.6Sr0.4Co0.2Fe0.8O3−δ, the most widely used SOC oxygen electrode material. While other electrode materials have been shown to exhibit better oxygen transport coefficients than STFC, they do not match its stability.

Imaging of Fuel Cell and Battery Electrodes Using Focused Ion Beam Scanning Electron Microscopy

This paper reviews results on the use of Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM) for three-dimensional (3D) tomographic studies of fuel cell and battery electrode morphology. Electrodes are typically complex twoor three-phase (including porosity) structures where each phase transports a different species, and electrochemical reactions occur at the interfaces between phases. Thus, the electrode morphology plays an important role in determining the transport and reaction rates, and hence the electrochemical performance. 3D imaging has been used to understand electrochemical processes by determining macrohomogeneous parameters (surface areas, tortuosities, etc) and by utilizing measured 3D structures in 3D electrochemical simulations. Electrode electrochemical degradation has also been studied by relating observed morphological changes to degradation during accelerated testing – mechanistic models are developed to predict long-term performance behaviour.