Solid-oxide fuel cells (SOFCs) are unique in their ability to directly convert the chemical energy of a wide variety of fuels to electric power with unmatched energy conversion efficiency. The articles in this issue of MRS Bulletin highlight the enormous potential of, and recent progress toward, operating SOFCs at lower temperatures (<650°C). Lower temperatures dramatically increase the number of potential applications for this technology as well as provide the opportunity to incorporate a wider variety of materials in SOFC power generation systems with greater reliability and lower cost. The articles in this issue describe materials development and processing for low-temperature SOFCs, including the enabling of nanotechnology and microelectromechanical systems-based cell designs, the development of highly active electrodes and their three-dimensional microstructural characterization, as well as the use of novel proton-conducting electrolytes, all of which provide new avenues of research. New fabrication methods are also being applied in the development of micro-SOFCs and microtubular SOFCs with higher power densities. Finally, advances in lowering performance degradation rates, a critical commercialization issue, are described.

We present here performance of Li ion batteries with SiC nanoparticle-film anode, which is fabricated by a double multi-hollow discharge plasma chemical vapor deposition (CVD) method. The first cycle of charge/discharge property of the Li ion battery with the SiC nanoparticle-film anode shows a high capacity of over 4,000 mAh/g, which is 12 times higher than the Li ion battery with the conventional graphite anode. The discharge capacity shows high stability for first 10th cycle, and is 3750 mAh/g for the 10th cycle.

A metal-supported solid oxide fuel cell (SOFC) using Ce0.8Sm0.2O2 (Sm-doped ceria, SDC) buffer layer and La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte films showed a small degradation in the cell performance after a long-term operation because of La migration from the electrolyte to the buffer layer, resulted in a formation of a less conductive phase. Thus, various ceramic materials such as doped ceria and perovskite-related oxides were investigated for an effective buffer layer with respect to fabricating reliable metal-supported SOFCs using a LSGM electrolyte film. In particular, La-doped CeO2 (LDC) and Pr-doped LaCrO3 (LPCr) were investigated as buffer layer material since the materials showed chemical compatibility with the LSGM and anode materials. The cell using a LDC buffer layer showed a prior stability during the operation for 100 h at 973 K, while the power density of the cell was slightly low owing to the low electrical conductivity of LDC compared with that of SDC or LPCr. In contrast, the cell using a LPCr buffer layer revealed significantly low open circuit voltage (OCV) and power density, which were attributed to Pr decomposition in the LPCr caused by the reactivity with water vapor. However, the metal-supported cell with a multilayer electrolyte film including LSGM/LPCr/SDC layers showed an almost theoretical OCV and reasonably high power density with no degradation after a long-term operation for 100 h at 973 K, suggesting that the LPCr layer effectively prevented La migration and the SDC layer led to avoid the Pr decomposition. Thus, a LPCr is an effective buffer layer material for reliable metal-supported SOFCs using a LSGM electrolyte thin film.

La1.61GeO5 thin film was fabricated on porous NiO-Ce0.80Sm0.20O2-δ (SDC) substrates by pulsed laser deposition (PLD) under various deposition conditions and the deposition parameters were optimized for obtaining the dense La1.61GeO5-δ film for the electrolyte of SOFC. It has been found that the high substrate temperature and the low oxygen pressure in chamber are suitable for obtaining the dense La1.61GeO5-δ thin film on porous NiO-SDC anode substrate. Since the as-deposited film is amorphous state, post-anneal treatment is essential for obtaining the crystalline La1.61GeO5-δ film. Under the optimized deposition condition, a dense and La1.61GeO5-δ thin film with a thickness of 3 μm can be successfully obtained. The obtained La1.61GeO5-δ film exhibits the electromotive force, which is slightly smaller that that of the theoretical open circuit potential. The power generation property of the cell using the obtained La1.61GeO5-δ film was studied and it was found that the maximum output density of a SOFC single cell using a La1.61GeO5-δ thin film deposited on a porous NiO-SDC substrate reached to 1948.2 mW/cm2 at 1173 K and 424.4 mW/cm2 at 873 K.

An La1-xSrxGa1-y-zMgyCozO3-(x+y+z)/2 (LSGMCO) has attracted much attention because it can be useable as an electrolyte of a solid oxide fuel cell due to its high oxide ion conductivity. We prepared LSGMCO thin films on silica glass and LaAlO3 single crystal substrates by pulsed laser deposition and evaluated their properties. LSGMCO thin films deposited at 800°C were poly-crystal and the deposition pressure affected their surface morphologies. In the case of the LaAlO3 single crystal substrate, a c-axis oriented LSGMCO thin film was obtained. DC conductivity and complex impedance of LSGMCO thin films were measured in vacuum atmosphere to investigate the effect of the crystal orientation on the oxide ion conductivity. It was revealed that resistance at a grain boundary of films is more dominant compare with the grain interior.

In this study, doped lanthanum gallate (LSGM with the composition La0.9Sr0.1Ga0.8Mg0.2O3-δ, LSGMC with the composition La0.8Sr0.2Ga0.8Mg0.15Co0.05O3-δ) films for an electrolyte of the solid oxide fuel cell (SOFC) were prepared by pulsed laser deposition (PLD) technique. In the vacuum chamber, LSGM or LSGMC targets were set on the rotating target holder. A KrF excimer laser was introduced into the chamber at an incident angle of about 45 degree. The doped LaGaO3 film was deposited onto NiO substrates without heating in argon ambient gas. The NiO substrate can be used directly as an electrode in the fabrication of the SOFC. The deposited LSGM films were characterized by X-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS) and scanning electron microscopy (SEM). As-deposited films were amorphous. After post annealing at 1273K for 6-10 hours, crystalline LaGaO3 was obtained. Films with thickness greater than several 10 μm showed an uniform and dense morphology. No gas leakage was found using thick films, which is an important characteristic for an electrolyte in fuel cells. The composition of the deposited films was slightly different to that of the target.

Doping transition metal cation is known to enhance the electric conduction of solid electrolytes, however, the ionic conduction can be improved by doping the small amount of transition metal, in particular, doping Co is effective for improving the oxide ion conductivity. In this investigation, oxide ion conductivity of LaGaO3 based oxide doped with Co were investigated in detail. It was found that LaGaO3 doped with Co for Ga site (LSGMC) show a notable oxide ion conductivity over a wide range of oxygen partial pressures, although a hole conduction was appeared by addition of excess amount of Co. Considering the electrical conductivity and transport number of oxide ion, the optimized composition of LSGMC seems to be existed at La0.8.Sr0.2, Ga0.8,.Mg0.115 CO0.085O3. Power generation characteristics of fuel cells was greatly improved by using LSGMC for electrolyte and extremely large power density can be obtained on both H2- O2 and H2-air cells. In particular, the maximum power density was attained to a value of 1.53 and 0.50 W/cm2 at 1073 and 873 K, respectively, on H2–O2 cell when the thickness of electrolyte was 0.18 mm. Furthermore, almost similar large power density was attained when air was used as oxidant. The high power density of cell demonstrated in this study suggests that the operating temperature of SOFC can be decreased by using LSGMC for electrolyte.