Fuel cells directly and efficiently convert chemical energy to electrical energy. Of the various fuel cell types, solid-oxide fuel cells (SOFCs) combine the benefits of environmentally benign power generation with fuel flexibility. However, the necessity for high operating temperatures (800–1,000 °C) has resulted in high costs and materials compatibility challenges. As a consequence, significant effort has been devoted to the development of intermediate-temperature (500–700 °C) SOFCs. A key obstacle to reduced-temperature operation of SOFCs is the poor activity of traditional cathode materials for electrochemical reduction of oxygen in this temperature regime2. Here we present Ba_(0.5_Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-delta) (BSCF) as a new cathode material for reduced-temperature SOFC operation. BSCF, incorporated into a thin-film doped ceria fuel cell, exhibits high power densities (1,010 mW cm^(-2) and 402 mW cm^(-2) at 600 °C and 500 °C, respectively) when operated with humidified hydrogen as the fuel and air as the cathode gas. We further demonstrate that BSCF is ideally suited to 'single-chamber' fuel-cell operation, where anode and cathode reactions take place within the same physical chamber. The high power output of BSCF cathodes results from the high rate of oxygen diffusion through the material. By enabling operation at reduced temperatures, BSCF cathodes may result in widespread practical implementation of SOFCs.

A high-performance cathode for the next generation of solid-oxide fuel cells

Dry (CO2) reforming of methane (DRM) is a well-studied reaction that is of both scientific and industrial importance. This reaction produces syngas that can be used to produce a wide range of products, such as higher alkanes and oxygenates by means of Fischer–Tropsch synthesis. DRM is inevitably accompanied by deactivation due to carbon deposition. DRM is also a highly endothermic reaction and requires operating temperatures of 800–1000 °C to attain high equilibrium conversion of CH4 and CO2 to H2 and CO and to minimize the thermodynamic driving force for carbon deposition. The most widely used catalysts for DRM are based on Ni. However, many of these catalysts undergo severe deactivation due to carbon deposition. Noble metals have also been studied and are typically found to be much more resistant to carbon deposition than Ni catalysts, but are generally uneconomical. Noble metals can also be used to promote the Ni catalysts in order to increase their resistance to deactivation. In order to design catalysts that minimize deactivation, it is necessary to understand the elementary steps involved in the activation and conversion of CH4 and CO2. This review will cover DRM literature for catalysts based on Rh, Ru, Pt, and Pd metals. This includes the effect of these noble metals on the kinetics, mechanism and deactivation of these catalysts.

A review of dry (CO2) reforming of methane over noble metal catalysts

Hydrogen production reactions from carbon feedstocks: fossil fuels and biomass.

Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation

Lewis Acids: From Conventional Homogeneous to Green Homogeneous and Heterogeneous Catalysis

Due to the potential employment of ammonia as a hydrogen source for proton exchange membrane (PEM) fuel cell applications, an experimental study has been undertaken to determine the ammonia decomposition rates over a Ni-Pt/Al 2 O 3 catalyst under conditions likely to be employed in such an application. Using differential analysis of integral data, intrinsic rate data have been obtained between temperatures of 520 and 690 °C and at ammonia pressures between 50 and 780 Torr. It is shown that a first-order rate expression provides an adequate fit of the experimental data over the entire range of pressures and temperatures, although hydrogen inhibition effects may be significant at the lowest temperature. The activation energy was found to be on the order of 50 kcal/mol as opposed to the very low values (5–10 kcal/mol) obtained previously by others. The data are analyzed in the context of previous studies that were conducted under markedly different reaction conditions and differences are attributed to the types of catalysts employed and the fact that previous studies have primarily dealt with very low ammonia concentrations. Consequently, it does not appear as though NH bond cleavage is the rate determining step under these conditions. It is also shown that the rate expression is capable of predicting, to within 5%, the high ammonia conversions required for fuel cell applications.

Ammonia decomposition kinetics over Ni-Pt/Al2O3 for PEM fuel cell applications

Oxygen permeation rates through ion-conducting perovskite membranes

A reaction kinetic study of the formation of mullite from single-phase aluminosilicate gels has been performed using dynamic X-ray diffraction. In situ measurements of conversion and separate infrared spectra data indicate that the amorphous single-phase precursors form crystalline mullite at temperatures as low as 1213 K. All of the data are consistent with a nucleation rate-controlled mechanism, unlike previous work with diffusion-limited powders and diphasic gels. At temperatures below 1400 K, the conversion data are adequately fit with a binary model that can approximate a uniform distribution of activation energies. At higher temperatures, evidence indicates that diffusion mechanisms become the rate-limiting step.

Mullite Formation Kinetics of a Single‐Phase Gel

The effects of steam and carbon dioxide on calcite decomposition using dynamic X-ray diffraction

A series of ion-conducting perovskites of the form [La 1-x A x ][Co 0.2 Fe 0.8 ]O 3-δ (x=0.4, 0.6 for A=Sr; x=0.8 for A=Ba) were investigated for their use as a catalytic membrane for the oxidative coupling of methane (OCM). A-site cations consisting of La 0.4 Sr 0.6 and La 0.2 Ba 0.8 produce materials with the highest oxygen fluxes and result in C 2+ selectivities of 50% at 1098 K, which are significantly higher than those achieved with a powdered catalyst in a packed-bed reactor configuration. Selectivities in these materials appear to be limited by high oxygen ion recombination rates that compete for oxygen with the desired coupling reaction(s). The results also indicate that oxygen fluxes are not limited by diffusion, but by surface exchange rates at the oxygen-lean side of the membrane. The study on stability of these materials showed that all three perovskites could be reduced in a pure methane or ethane stream at 1023 K, but they were totally stable under reaction conditions where oxygen is present.

William J. Thomson

Papers

Ammonia decomposition kinetics over Ni-Pt/Al2O3 for PEM fuel cell applications

Oxygen permeation rates through ion-conducting perovskite membranes

Mullite Formation Kinetics of a Single‐Phase Gel

The effects of steam and carbon dioxide on calcite decomposition using dynamic X-ray diffraction

Perovskite-type oxide membranes for the oxidative coupling of methane