New materials hold the key to fundamental advances in energy conversion and storage, both of which are vital in order to meet the challenge of global warming and the finite nature of fossil fuels. Nanomaterials in particular offer unique properties or combinations of properties as electrodes and electrolytes in a range of energy devices. This review describes some recent developments in the discovery of nanoelectrolytes and nanoelectrodes for lithium batteries, fuel cells and supercapacitors. The advantages and disadvantages of the nanoscale in materials design for such devices are highlighted.

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Nanostructured materials for advanced energy conversion and storage devices

Fuel cells convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. However, before fuel-cell technology can gain a significant share of the electrical power market, important issues have to be addressed. These issues include optimal choice of fuel, and the development of alternative materials in the fuel-cell stack. Present fuel-cell prototypes often use materials selected more than 25 years ago. Commercialization aspects, including cost and durability, have revealed inadequacies in some of these materials. Here we summarize recent progress in the search and development of innovative alternative materials.

Materials for fuel-cell technologies

Pretreatments to enhance the digestibility of lignocellulosic biomass

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

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

Fuel cells constitute an attractive power-generation technology that converts chemical energy directly and with high efficiency into electricity while causing little pollution. Most fuel cells require hydrogen as the fuel, but viable near-term applications will need to use the more readily available hydrocarbons, such as methane. Present-day demonstration power plants and planned fuel-cell electric vehicles therefore include a reformer that converts hydrocarbon fuel into hydrogen. Operating fuel cells directly on hydrocarbons would obviously eliminate the need for such a reformer and improve efficiency. In the case of polymer-electrolyte fuel cells, which have been studied for vehicle applications, the direct use of methanol fuel has been reported, but resulted in fuel permeating the electrolyte1,2. Solid oxide fuel cells — promising candidates for stationary power generation — can also use hydrocarbon fuel directly to generate energy, but this mode of operation resulted in either carbon deposition at high temperatures or poor power output at low operating temperatures3,4,5. Here we report the direct electrochemical oxidation of methane in solid oxide fuel cells that generate power densities upto 0.37 W cm−2 at 650 °C. This performance is comparable to that of fuel cells using hydrogen6,7 and is achieved by using ceria-containing anodes and low operating temperatures to avoid carbon deposition. We expect that the incorporation of more advanced cathodes would further improve the performance of our cells, making this solid oxide fuel cell a promising candidate for practical and efficient fuel-cell applications.

A direct-methane fuel cell with a ceria-based anode

Oxygen transfer processes in (La,Sr)MnO3/Y2O3-stabilized ZrO2 cathodes: an impedance spectroscopy study

Effect of LSM-YSZ cathode on thin-electrolyte solid oxide fuel cell performance

Increased solid-oxide fuel cell power density using interfacial ceria layers

Solid‐oxide fuel cells (SOFCs) with electrolytes that provide high open‐circuit voltage (OCV) and low ohmic loss down to 550°C are described. The electrolytes were bilayers consisting of a 4 to 8 μm thick Y‐doped ceria (YDC) layer with a 1 to 1.5 μm thick Y‐doped zirconia (YSZ) layer on the fuel side. The cathode/supports were . The anodes consisted of thin YDC and Ni‐YSZ layers. The YDC/YSZ electrolyte SOFCs yielded 85 to 98% of the theoretical OCV, compared with ≈50% for YDC electrolyte SOFCs. The cathode overpotential, which was a main factor limiting SOFC power density, was lower for YDC/YSZ than YSZ electrolytes. The maximum power density at 600°C, 210 mW/cm2, is higher than for previously reported SOFCs.

Tsepin Tsai

Papers

A direct-methane fuel cell with a ceria-based anode

Oxygen transfer processes in (La,Sr)MnO3/Y2O3-stabilized ZrO2 cathodes: an impedance spectroscopy study

Effect of LSM-YSZ cathode on thin-electrolyte solid oxide fuel cell performance

Increased solid-oxide fuel cell power density using interfacial ceria layers

Low‐Temperature Solid‐Oxide Fuel Cells Utilizing Thin Bilayer Electrolytes