Chuancheng Duan

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

Cooler ceramic fuel cells Ceramic ion conductors can be used as electrolytes in fuel cells using natural gas. One drawback of such solid-oxide fuel cells that conduct oxygen ions is their high operating temperatures (at least 600°C). Duan et al. have made a proton-conducting ceramic fuel cell with a modified cathode material that exhibits high performance on methane fuel at 500°C (see the Perspective by Gorte). Science, this issue p. 1321; see also p. 1290 A proton-conduction cathode and simpler fabrication enable lower-temperature operation of methane-fueled ceramic fuel cells. [Also see Perspective by Gorte] 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.

Zr and Y co-doped perovskite as a stable, high performance cathode for solid oxide fuel cells operating below 500 °C

Zr and Y co-doped perovskite BaCo0.4Fe0.4Zr0.1Y0.1O3−δ (BCFZY0.1) was recently developed as a promising new cathode for protonic ceramic fuel cells (PCFCs). Here, it is applied for the first time as a cathode for low-temperature solid oxide fuel cells (LT-SOFCs). It exhibits large lattice parameters, high oxygen reduction reaction (ORR) activity, exceptional low-temperature performance, long-term stability, and excellent chemical compatability with ceria-based SOFC electrolytes. When BCFZY0.1 is used as the cathode in Ce0.8Gd0.2O2−δ (GDC20)-based SOFCs, it enables a peak power density of 0.97 W cm−2 at 500 °C with 2500 hours stable performance and complete recoverability without any degradation after more than 80 fast thermal ramping cycles. Even at 350 °C, peak power density reaches 0.13 W cm−2. It also shows good H2O and CO2 tolerance.

Highly durable, coking and sulfur tolerant, fuel-flexible protonic ceramic fuel cells

Protonic ceramic fuel cells, like their higher-temperature solid-oxide fuel cell counterparts, can directly use both hydrogen and hydrocarbon fuels to produce electricity at potentially more than 50 per cent efficiency1,2. Most previous direct-hydrocarbon fuel cell research has focused on solid-oxide fuel cells based on oxygen-ion-conducting electrolytes, but carbon deposition (coking) and sulfur poisoning typically occur when such fuel cells are directly operated on hydrocarbon- and/or sulfur-containing fuels, resulting in severe performance degradation over time3–6. Despite studies suggesting good performance and anti-coking resistance in hydrocarbon-fuelled protonic ceramic fuel cells2,7,8, there have been no systematic studies of long-term durability. Here we present results from long-term testing of protonic ceramic fuel cells using a total of 11 different fuels (hydrogen, methane, domestic natural gas (with and without hydrogen sulfide), propane, n-butane, i-butane, iso-octane, methanol, ethanol and ammonia) at temperatures between 500 and 600 degrees Celsius. Several cells have been tested for over 6,000 hours, and we demonstrate excellent performance and exceptional durability (less than 1.5 per cent degradation per 1,000 hours in most cases) across all fuels without any modifications in the cell composition or architecture. Large fluctuations in temperature are tolerated, and coking is not observed even after thousands of hours of continuous operation. Finally, sulfur, a notorious poison for both low-temperature and high-temperature fuel cells, does not seem to affect the performance of protonic ceramic fuel cells when supplied at levels consistent with commercial fuels. The fuel flexibility and long-term durability demonstrated by the protonic ceramic fuel cell devices highlight the promise of this technology and its potential for commercial application.Tests on a versatile protonic ceramic fuel cell resistant to carbon deposition and sulfur poisoning show that its durability and the wide range of fuels it can accept make it suitable for use in industry in the near future.

Ce-doped La0.7Sr0.3Fe0.9Ni0.1O3−δ as symmetrical electrodes for high performance direct hydrocarbon solid oxide fuel cells

La0.7Sr0.3Fe0.9Ni0.1O3−δ (LSFNi) and La0.6Ce0.1Sr0.3Fe0.9Ni0.1O3−δ (CLSFNi) are synthesized and applied for use as symmetrical electrodes in direct-methane solid oxide fuel cells (SOFCs). In a symmetric SOFC, the same electrode material is used for both the anode and cathode and must therefore remain active in both oxidizing and reducing atmospheres. LSFNi and CLSFNi retain their stable perovskite phase at high temperature in both oxidizing and moderately reducing environments, with a minor amount of SrLaFeO4 phase (K2NiF4 structure) present under reducing conditions. Symmetric SOFCs incorporating either LSFNi or CLSFNi electrodes give excellent peak power densities of ∼900 mW cm−2 at 850 °C in wet H2/air (3% H2O). In wet CH4/air (3% H2O), the CLSFNi electrode greatly outperforms the LSFNi electrode (522 mW cm−2vs. 221 mW cm−2) due to the enhanced methane reforming activity imparted by the cerium doping. The performance and stability of the CLSFNi symmetric cell under direct CH4 operation are among the best reported to date in the literature, indicating that CLSFNi is a promising electrode material for symmetrical SOFCs.