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

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

Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges

Because of the generally lower activation energy associated with proton conduction in oxides compared to oxygen ion conduction, protonic ceramic fuel cells (PCFCs) should be able to operate at lower temperatures than solid oxide fuel cells (250° to 550°C versus ≥600°C) on hydrogen and hydrocarbon fuels if fabrication challenges and suitable cathodes can be developed. We fabricated the complete sandwich structure of PCFCs directly from raw precursor oxides with only one moderate-temperature processing step through the use of sintering agents such as copper oxide. We also developed a proton-, oxygen-ion–, and electron-hole–conducting PCFC-compatible cathode material, BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY0.1), that greatly improved oxygen reduction reaction kinetics at intermediate to low temperatures. We demonstrated high performance from five different types of PCFC button cells without degradation after 1400 hours. Power densities as high as 455 milliwatts per square centimeter at 500°C on H2 and 142 milliwatts per square centimeter on CH4 were achieved, and operation was possible even at 350°C.

https://science.sciencemag.org/content/sci/349/6254/1321.full.pdf

Readily processed protonic ceramic fuel cells with high performance at low temperatures

The increasing world population and the need to improve quality of life for a large percentage of human beings are the driving forces for the search for sustainable energy production systems, alternative to fossil fuel combustion. Among the various types of alternative energy production technologies, solid oxide fuel cells (SOFCs) operating at intermediate temperatures (400–700 °C) show the advantage of possible use both for stationary and mobile energy production. To reach the goal of reducing the SOFC operating temperature, proton-conducting oxides are gaining wide interest as electrolyte materials. This critical review provides a broad overview of the most recent progresses obtained tailoring the properties of proton-conducting oxides for fuel cell applications, analyzing and comparing the different strategies proposed to match high-proton conductivity with good chemical stability (170 references).

Materials challenges toward proton-conducting oxide fuel cells: a critical review

The performance of a single-chamber solid oxide fuel cell was studied using a ceria-based solid electrolyte at temperatures below 773 kelvin. Electromotive forces of ∼900 millivolts were generated from the cell in a flowing mixture of ethane or propane and air, where the solid electrolyte functioned as a purely ionic conductor. The electrode-reaction resistance was negligibly small in the total internal resistances of the cell. The resulting peak power density reached 403 and 101 milliwatts per square centimeter at 773 and 623 kelvin, respectively.

https://science.sciencemag.org/content/sci/288/5473/2031.full.pdf

A low-operating-temperature solid oxide fuel cell in hydrocarbon-Air mixtures

The performance of a single-chamber solid oxide fuel cell (SOFC) was studied between 350 and 900°C in flowing mixtures of methane, ethane, propane, or liquefied petroleum gas and air with a fuellair volume ratio of one, where their oxidation proceeded safely without explosion. Among all tested electrode materials, Ni-Ce 0.8 Sm 0.2 O 1.9 cermet and Sm 0.5 Sr 0.5 CoO 3 oxide functioned best as the anode and cathode, respectively, in various gas mixtures. A cell constructed from a La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3 electrolyte with the two electrodes generated >900 mV in a methane-air mixture between 600 and 800°C and in an ethane-air mixture between 450 and 650°C. A small electrode reaction resistance resulted in increasing power density with decreasing electrolyte thickness. The peak power density at 450°C increased from 34 to 101 mW cm -2 with decreasing electrolyte thickness from 0.50 to 0.18 mm. The working mechanism of the single-chamber SOFC at different temperatures was also studied by measuring the catalytic activities of the two electrodes for partial oxidation of the hydrocarbons.

Single‐Chamber Solid Oxide Fuel Cells at Intermediate Temperatures with Various Hydrocarbon‐Air Mixtures

Performance of a single-chamber solid oxide fuel cell was evaluated using a 0.15 mm thick Sm-doped ceria (SDC) electrolyte together with a 30 wt % SDC-Ni anode and a Sm 0.5 Sr 0.5 CoO 3 cathode at heating temperatures below 500°C in a flowing mixture of butane and air. A large quantity of reaction heat, which was evolved by the partial oxidation of butane by oxygen at the anode, caused a temperature rise of more than 100°C at the anode, followed by thermal conduction to the cathode through the electrolyte. Simultaneously, the cell generated a large electromotive force of ca. 900 mV between the two electrodes. The resulting peak power density reached 245, 180, 105, and 38 mW cm -2 at heating temperatures of 450, 400, 350, and 300°C, respectivcly. The comparison of the butane fuel with the other hydrocarbon fuels showed that the fuel cell performance became enhanced, especially at reducing temperatures, as the carbon number of the hydrocarbon increased, and the chain structure was branched

Jun-ichi Tokuno

Papers

A low-operating-temperature solid oxide fuel cell in hydrocarbon-Air mixtures

Single‐Chamber Solid Oxide Fuel Cells at Intermediate Temperatures with Various Hydrocarbon‐Air Mixtures

A Solid Oxide Fuel Cell Using an Exothermic Reaction as the Heat Source