Physical, chemical and electrochemical properties of pure and doped ceria

The direct electrochemical oxidation of dry hydrocarbon fuels to generate electrical power has the potential to accelerate substantially the use of fuel cells in transportation and distributed-power applications1. Most fuel-cell research has involved the use of hydrogen as the fuel, although the practical generation and storage of hydrogen remains an important technological hurdle2. Methane has been successfully oxidized electrochemically3,4,5,6, but the susceptibility to carbon formation from other hydrocarbons that may be present or poor power densities have prevented the application of this simple fuel in practical applications1. Here we report the direct, electrochemical oxidation of various hydrocarbons (methane, ethane, 1-butene, n-butane and toluene) using a solid-oxide fuel cell at 973 and 1,073 K with a composite anode of copper and ceria (or samaria-doped ceria). We demonstrate that the final products of the oxidation are CO2 and water, and that reasonable power densities can be achieved. The observation that a solid-oxide fuel cell can be operated on dry hydrocarbons, including liquid fuels, without reforming, suggests that this type of fuel cell could provide an alternative to hydrogen-based fuel-cell technologies.

Direct oxidation of hydrocarbons in a solid-oxide fuel cell

Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

Fuel cells will undoubtedly find widespread use in this new millennium in the conversion of chemical to electrical energy, as they offer very high efficiencies and have unique scalability in electricity-generation applications. The solid-oxide fuel cell (SOFC) is one of the most exciting of these energy technologies; it is an all-ceramic device that operates at temperatures in the range 500–1,000 °C. The SOFC offers certain advantages over lower temperature fuel cells, notably its ability to use carbon monoxide as a fuel rather than being poisoned by it, and the availability of high-grade exhaust heat for combined heat and power, or combined cycle gas-turbine applications. Although cost is clearly the most important barrier to widespread SOFC implementation, perhaps the most important technical barriers currently being addressed relate to the electrodes, particularly the fuel electrode or anode. In terms of mitigating global warming, the ability of the SOFC to use commonly available fuels at high efficiency, promises an effective and early reduction in carbon dioxide emissions, and hence is one of the lead new technologies for improving the environment. Here, we discuss recent developments of SOFC fuel electrodes that will enable the better use of readily available fuels.

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Advanced anodes for high-temperature fuel cells

Solid oxide fuel cells (SOFCs) have the promise to improve energy efficiency and to provide society with a clean energy producing technology. The high temperature of operation (500−1000 °C) enables the solid oxide fuel cell to operate with existing fossil fuels and to be efficiently coupled with turbines to give very high efficiency conversion of fuels to electricity. Solid oxide fuel cells are complex electrochemical devices that contain three basic components, a porous anode, an electrolyte membrane, and a porous cathode. In this short review, a survey of the types and properties of materials that have been considered for each of these components is presented with an emphasis on the requirements for operation at intermediate temperature (500−800 °C). Some directions for future research are discussed.

Materials for Solid Oxide Fuel Cells

Thermal, Electrical, and Electrocatalytical Properties of Lanthanum-Doped Strontium Titanate

A solid oxide fuel cell with a gadolinia-doped ceria anode: preparation and performance

Electrolysis has long been used to dissociate water into its constituents of oxygen and hydrogen. Various electrolyzers have been developed and are commercially available today, including those based on proton exchange membranes, molten carbonate, phosphoric acid, alkaline, and solid oxide technology. 1-5 Some of these are reversible systems capable of operating both as a fuel cell and as an electrolyzer, although fuel cell and electrolyzer functions are carried out in separate subsystems. A reversible fuel cell can take advantage of excess electrical grid capacity during off-peak hours to produce hydrogen fuel, to be utilized later during periods of high electrical demand. The power unit fuel cell is sized for the peaking load in a practical reversible fuel cell, whereas the electrolyzer is rated at a power that can produce sufficient hydrogen to recharge the hydrogen storage capacity over the remaining hours of the day. If energy conversion, electrical to chemical and chemical to electrical, can occur in the same device with reasonable efficiencies, there could be significant overall cost benefits. For solid oxide electrolysis cells SOEC to be of commercial interest, the cost of the hydrogen produced must be competitive with that of other means of production. The cost of electricity is a significant factor in steam electrolysis, comprising 75% to 95% of that of electrolysis-derived hydrogen according to performance and cost

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Electrode Performance in Reversible Solid Oxide Fuel Cells

Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia-doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.

Nanoscale effects on ion conductance of layer-by-layer structures of gadolinia-doped ceria and zirconia

Direct electrochemical oxidation of methane was attempted on a gadolinia-doped ceria Ce 0.6 Gd 0.4 O 1.8 (CG4) electrode in a solid oxide fuel cell using a porous gold–CG4 mixture as current collector. Gold is relatively inert to methane in contrast to other popular SOFC anode materials such as nickel and platinum. CG4 was found to exhibit a low electrocatalytic activity for methane oxidation as well as no significant reforming activity implying that the addition of an electrocatalyst or cracking catalyst to the CG4 anode is required for SOFC operating on methane. The methane conversion observed at the open-circuit potential and low anodic overpotentials seems to be due to thermal methane cracking in the gas phase and on the alumina surfaces in the cell housing. At high anodic overpotentials, at electrode potentials where oxygen evolution was expected to take place, the formation of CO 2 increased sharply. No carbon deposition was observed on the CG4 electrode after operating at 1000°C during 350 h of total testing time with partial pressures of methane and steam in the range of 0.5–33 and 3–26 kPa, respectively.

Olga A. Marina

Papers

Thermal, Electrical, and Electrocatalytical Properties of Lanthanum-Doped Strontium Titanate

A solid oxide fuel cell with a gadolinia-doped ceria anode: preparation and performance

Electrode Performance in Reversible Solid Oxide Fuel Cells

Nanoscale effects on ion conductance of layer-by-layer structures of gadolinia-doped ceria and zirconia

High-temperature conversion of methane on a composite gadolinia-doped ceria–gold electrode