Recent worldwide interest in building a decentralized, hydrogen-based energy economy has refocused attention on the solid oxide fuel cell (SOFC) as a potential source of efficient, environmentally friendly, fuel-versatile electric power. Due to its high operating temperature, the SOFC offers several potential advantages over polymer-based fuel cells, including reversible electrode reactions, low internal resistance, high tolerance to typical catalyst poisons, production of high-quality waste heat for (among other uses) reformation of hydrocarbon fuels, as well as the possibility of burning hydrocarbon fuels directly. Today, SOFCs are much closer to commercial reality than they were 20 years ago, due largely to technological advances in electrode material composition, microstructure control, thin-film ceramic fabrication, and stack and system design. These advances have led to dozens of active SOFC development programs in both stationary and mobile power and contributed to commercialization or development in a number of related technologies, including gas sensors,1 solid-state electrolysis devices,2 and iontransport membranes for gas separation and partial oxidation.3 Many reviews are available which summarize the technological advances made in SOFCs over the last 15-35 yearssreaders who are primarily interested in knowing the state-of-the art in materials, design, and fabrication (including the electrodes) are encouraged to consult these reviews.4-12 This review focuses on the factors governing SOFC cathode performancesadvances we have made over † Dedicated to Brian Steele, 1929-2003. Researcher, Entrepreneur, Consensus Seeker. 4791 Chem. Rev. 2004, 104, 4791−4843

Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes

In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metal–air batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field.

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Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis

Progress in material selection for solid oxide fuel cell technology: A review

Electrolytes for solid oxide fuel cells

This article provides a perspective on solid oxide fuel cells operating at low temperature, defined here to be the range from ∼400 °C to 650 °C. These low-temperature solid oxide fuel cells (LT-SOFCs) have seen considerable research and development and are widely viewed as the “next generation” technology, following the 650–850 °C SOFCs that are currently undergoing commercialization. LT-SOFCs have potential advantages for conventional SOFC applications such as stationary power generation, and may be viable for new portable and transportation power applications, along with electrolytic fuel production and energy storage. The characteristics of electrolyte and electrode materials are reviewed, with a focus on materials that have demonstrated good properties and cell performance at low temperature. Only oxygen-ion-conducting electrolytes are considered here. Anode materials are discussed, primarily the various Ni–cermet anode compositions that yield good low-temperature performance. Mixed ionically and electronically conducting cathode materials are described in detail, reflecting the extensive research activity that has aimed at providing useful oxygen reduction kinetics at low operating temperature. Cell design, materials compatibility, processing methods, and resulting microstructures are discussed, along with their role in determining cell performance. Results from state of the art LT-SOFCs are presented, and future prospects are discussed.

A perspective on low-temperature solid oxide fuel cells

Equivalent circuit analysis was undertaken on a mixed conductor electrode/solid oxide electrolyte system. In a limited case where surface reaction is the predominant rate-controlling process, the equivalent circuit was simplified to parallel connection of a resistor (surface reaction resistance) and a capacitor (chemical capacitance due to oxygen nonstoichiometry). Equilibrium oxygen vacancy concentration was correlated with the chemical capacitance. The model was applied to a dense film of La 0.6 Sr 0.4 CoO 3-δ deposited on a sintered plate of Ce 0.9 Gd 0.1 O 1.95 by a laser ablation method. Frequency response of the electrochemical impedance was measured under dc bias at 873-1073 K in O 2 -Ar gas mixtures. The observed capacitance was extremely large, e.g., around 0.1 to I F cm -2 for the 1.5 μm thick film. The oxygen vacancy concentration in the film was calculated from the capacitance and compared with the literature data measured by thermogravimetry. The film was found to show smaller oxygen nonstoichiometry. The enthalpy for oxygen vacancy formation in the film was about 40 kJ mol -1 larger than the bulk.

Determination of Oxygen Vacancy Concentration in a Thin Film of La0.6Sr0.4CoO3 − δ by an Electrochemical Method

Enhancement of oxygen exchange at the hetero interface of (La,Sr)CoO3/(La,Sr)2CoO4 in composite ceramics

The preferential site for oxygen isotope exchange was investigated on layered films of La 2-x Sr x CoO 4 (x = 0.5,1.0)/La 0.6 Sr 0.4 CoO 3 prepared by pulsed laser deposition (PLD) on a polycrystalline Ce 0.9 Gd 0.1 O 1.95 . An isotope exchange experiment was performed at 773 K in 18 O-enriched oxygen of 0.2 × 10 5 Pa. Three-dimensional imaging of secondary ion mass spectrometry (SIMS) was carried out to visualize the isotope distribution. The SIMS images suggested that there are fast oxygen-incorporation paths along the hetero-phase interface of La 2-x Sr x CoO 4 /La 0.6 Sr 0.4 CoO 3 . The results have reproduced our former findings on the fast oxygen surface exchange with La 2-x Sr x CoO 4 /La 0.6 Sr 0.4 CoO 3 composite polycrystals.

https://iopscience.iop.org/article/10.1149/1.2928612/pdf

Enhancement of Oxygen Surface Exchange at the Hetero-interface of ( La , Sr ) CoO3 / ( La , Sr ) 2CoO4 with PLD-Layered Films

Interfacial reaction and electrochemical properties of dense (La,Sr) CoO3−δ cathode on YSZ (1 0 0)

In order to probe the mechanism of O 2 reduction on mixed-conducting oxides, we have measured the nonlinear harmonic impedance response of a dense thin-film electrode of La 0.6 Sr 0.4 CoO 3-δ . By combining a well-defined geometry with detection of small nonlinear harmonics, this approach reveals the nonlinear rate law governing O 2 reduction/oxidation. Comparison to various models based on nonequilibrium thermodynamics suggests that O 2 reduction on La 0.6 Sr 0.4 CoO 3- δ is limited by dissociative adsorption. To the authors' knowledge, this is the first isolation of the full nonlinear rate law governing O 2 exchange on a perovskite oxide.

Maya Sase

Papers

Determination of Oxygen Vacancy Concentration in a Thin Film of La0.6Sr0.4CoO3 − δ by an Electrochemical Method

Enhancement of oxygen exchange at the hetero interface of (La,Sr)CoO3/(La,Sr)2CoO4 in composite ceramics

Enhancement of Oxygen Surface Exchange at the Hetero-interface of ( La , Sr ) CoO3 / ( La , Sr ) 2CoO4 with PLD-Layered Films

Interfacial reaction and electrochemical properties of dense (La,Sr) CoO3−δ cathode on YSZ (1 0 0)

Measurement of Oxygen Exchange Kinetics on Thin-Film La0.6Sr0.4CoO3 − δ Using Nonlinear Electrochemical Impedance Spectroscopy