L. Mogni

A perspective on low-temperature 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.

High Temperature Crystal Chemistry and Oxygen Permeation Properties of the Mixed Ionic–Electronic Conductors LnBaCo2O5 + δ ( Ln = Lanthanide )

The high temperature crystal chemistry and oxygen permeation properties of the cation-ordered LnBaCo 2 O 5+δ perovskite oxides [lanthanide (Ln) = Pr, Nd, and Sm] have been investigated in comparison with the cation-disordered La 0.5 Ba 0.5 CoO 3-δ perovskite. The LnBaCo 2 O 5+δ (Ln = Pr, Nd, and Sm) oxides exhibit a metal-insulator transition at T 350°C in air, as evidenced by an orthorhombic to tetragonal transition. At a given temperature, the oxygen permeation flux decreases from Ln = La to Nd to Sm due to the changes in crystal symmetry and lattice strain. The oxygen permeation mechanism in the Ln = Nd is bulk-diffusion-limited rather than surface-exchange-limited for membrane thickness L ≥ 1.1 mm.

Oxygen Reduction Reaction on Ruddlesden–Popper Phases Studied by Impedance Spectroscopy

The oxygen reduction mechanism of Ruddlesden-Popper phases Sr 3 FeMO 6+δ (M = Fe, Co, Ni) has been investigated by impedance spectroscopy at 500, 600, and 700°C under oxygen partial pressure pO 2 between 10 -5 and 1 atm using both He and Ar as gas carriers. Thick porous electrodes were sprayed on dense Ce 0.9 Gd 0.1 O 2-x and impedance spectra data were collected on symmetrical cells. An equivalent circuit was proposed considering the electrolyte resistances R el , a Warburg element W HF , and two parallel elements RCpe (RCpe IF and RCpe LF ). For the three compounds, W HF has been assigned to the oxygen vacancies diffusion in the bulk, the intermediate component, RCpe IF , to oxygen dissociative adsorption in the electrode surface, and the low frequency element, RCpe LF , to oxygen diffusion in the gas phases. In the case of the Sr 3 Fe 2 O 6+δ and Sr 3 FeCoO 6+δ compounds, the p0 2 dependence of Warburg high frequency component suggests a complex process involving both oxygen bulk diffusion and charge transfer. The results of Sr 3 FeMO 6+δ (M = Fe, Co, Ni) compared with those of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ perovskite electrodes, allowing us to discuss the effect of the crystal structure on the electrochemical behavior of these layered compounds.

The enhanced electrochemical response of Sr(Ti0.3Fe0.7Ru0.07)O3−δ anodes due to exsolved Ru–Fe nanoparticles

A mixed conducting oxide with a nominal composition Sr(Ti0.3Fe0.7Ru0.07)O3−δ (STFRu) is studied, in comparison with SrTi0.3Fe0.7O3−δ (STF) oxide, as an anode for solid oxide fuel cells. Exposing STFRu to reducing fuel conditions at 800 °C for 4 h results in the exsolution of essentially all of the Ru and a small fraction of the Fe from the oxide, and the formation of Ru1−xFex nanoparticles on the oxide surfaces. Most of the nanoparticles have the hexagonal structure expected for Ru-rich alloys, and thermogravimetric analysis suggests the composition x ∼ 0.2. A small fraction of bcc-structure, presumably Fe-rich, nanoparticles are also detected. Comparison of cells with STFRu and STF anodes shows that the presence of Ru induces a reduced polarization resistance and increases the maximum power density under most cell operating conditions, particularly at lower temperatures and hydrogen partial pressures. For example, at 700 °C and 30% H2 fuel, the maximum power density is 0.1 W cm−2 for STF compared to 0.3 W cm−2 for STFRu. There is also a significant change in the shape of the current–voltage curves and the pH2-dependence of the anode polarization resistances RP,A ∝ (pH2)−m, from m ∼ 0.5–1.0 for STF to m ∼ 0.11–0.29 for STFRu; these suggest that Ru1−xFex nanoparticles improve anode performance by promoting hydrogen adsorption.

Anomalous X-ray diffraction study of Pr-substituted BaCeO3 - δ.

The effect of Pr doping on the crystal structure and site occupancy was studied for the nominally synthesized BaCe1 - xPrxO3 - δ (x = 0, 0.2, 0.4, 0.6 and 0.8) perovskites using anomalous X-ray powder diffraction (AXRD) data and Rietveld analysis. Crystal structure parameters were accurately determined using 10,000 eV photons, and the Pr occupancy was refined using data collected with 5962 eV photons, close to the Pr LIII absorption edge. BaCe1 - xPrxO3 - δ crystallizes in the Pnma (No. 62) space group for all x values. Pr cations are mainly located at the Ce sites (perovskites B site), but a small fraction of them increasingly substitute some of the Ba ions at the A site as Pr content increases. The Pr doping introduces electronic defects (Pr(+3)/Pr(+4)) and oxygen vacancies needed for H2O incorporation and H-ionic conductivity. A decrease in the orthorhombic distortion would produce the opposite effects on the electronic and ionic mobility. The electronic mobility should increase due to an improvement in the overlap of the (Ce/Pr)4f-O2p orbital, while the proton mobility should decrease as a consequence of a larger hopping distance.

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.

Metal oxides: crystallographic characterizations for high-temperature electrochemistry applications

The strong relationship between the crystallographic structure and defects with the electronic and transport properties of a certain material is a key figure in most of the research conducted in materials science. The properties of an oxide in particular (transport, electrochemical, thermal, etc.) can be affected by several factors including the synthesis method and environmental conditions such as pressure, temperature, atmosphere, electrical current and field, magnetic field, etc. Therefore, it is important to go beyond the limited information obtained through the traditional ex-situ characterization techniques toward the more exhaustive ones provided by the in-situ or in-operando experiments, where it is possible the study of a device under nonambient working conditions.