Shunsuke Taniguchi

Effect of proton-conduction in electrolyte on electric efficiency of multi-stage solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are promising electrochemical devices that enable the highest fuel-to-electricity conversion efficiencies under high operating temperatures. The concept of multi-stage electrochemical oxidation using SOFCs has been proposed and studied over the past several decades for further improving the electrical efficiency. However, the improvement is limited by fuel dilution downstream of the fuel flow. Therefore, evolved technologies are required to achieve considerably higher electrical efficiencies. Here we present an innovative concept for a critically-high fuel-to-electricity conversion efficiency of up to 85% based on the lower heating value (LHV), in which a high-temperature multi-stage electrochemical oxidation is combined with a proton-conducting solid electrolyte. Switching a solid electrolyte material from a conventional oxide-ion conducting material to a proton-conducting material under the high-temperature multi-stage electrochemical oxidation mechanism has proven to be highly advantageous for the electrical efficiency. The DC efficiency of 85% (LHV) corresponds to a net AC efficiency of approximately 76% (LHV), where the net AC efficiency refers to the transmission-end AC efficiency. This evolved concept will yield a considerably higher efficiency with a much smaller generation capacity than the state-of-the-art several tens-of-MW-class most advanced combined cycle (MACC).

Recent Achievements of NEDO Durability Project with an Emphasis on Correlation Between Cathode Overpotential and Ohmic Loss

Long-term performance testes by CRIEPI (Central Research Institute for Electric Power Industry) on six industrial stacks have revealed an interesting correlation between cathode polarization loss and ohmic loss. To make clear the physicochemical meaning of this correlation, detailed analyses were made on the conductivity degradation of YSZ electrolyte in button cells and then on the ohmic losses in the industrial cells in terms of time constants which are determined from speed of the tetragonal transformation through the Y diffusion from the cubic phase to the tetragonal phase. In some cases, shorter time constants (faster degradations) were detected than those expected from the two-time-constant (with and without NiO reduction effects) model, suggesting that additional ohmic losses after subtracting the contribution from the tetragonal transformation must be caused from other sources such as cathode-degradation inducing effects. Main cathode degradations can be ascribed to sulfur poisoning due to contamination in air in the CRIEPI test site. An important feature was extracted as this cathode degradations became more severe when the gadolinium-doped ceria (GDC) interlayers were fabricated into dense film. Plausible mechanisms for cathode degradations were proposed based on the Sr/Co depletion on surface of lanthanum strontium cobalt ferrite (LSFC) in the active area. Peculiar cathode degradations found in stacks are interpreted in term of changes in surface concentration by reactions with sulfur oxide, electrochemical side reactions for water vapor emission or Sr volatilization, and diffusion of Sr/Co from inside LSCF.

Fuel Cells (SOFC): Alternative Approaches (Electroytes, Electrodes, Fuels)

While we may realize commercialization of SOFC systems in the very near future, continuous challenges will be needed to develop advanced fuel cells with higher efficiency, higher durability, better flexibility, as well as lower production cost. For realizing next-generation SOFCs, we may apply a broad range of knowledge related to (1) electrolyte and (2) electrode materials for cell development as well as (3) explore alternative fuels. This entry gives an overview on possible alternative approaches for these three important aspects to realize advanced high-performance SOFCs in a future generation.

Highly Efficient Biomass Utilization with Solid Oxide Fuel Cell Technology

Mankind has been consuming plants, i.e. biomass, as an energy source for living and developing on earth from the paleolithic period to early the modern period. Consumption of bio-energy does not change the atmospheric environment because carbon dioxide emitted by the use of bio-energies will be used by plants through the photosynthesis (Zuttel, 2008). Since 1769 James Watt significantly improved the steam engine, invented by Thomas Savery in 1698. The steam engine was widely introduced for producing mechanical work from chemical energy of fuels, i.e. mineral coal and wood. More practical heat engines, external and internal combustion engines, have served for developing of human society for almost two and a half centuries. Since the Otto-Langen engine was first introduced in 1867, human society has developed using the internal combustion engines (IC engines), which nowadays are used worldwide for transportation, manufacture, power generation, construction and farming. However, large consumption of fossil fuels may bring about environmental pollution and climate change. Fuel cells are electrochemical devices that convert chemical energy of fuels directly into electrical energy without the Carnot limitation that limit IC engines. Even in the smallest power range of less than 10 kW, fuel cells exhibit electrical efficiencies of 35-50 %LHV (lower heating value), while being silent, whereas engines and microturbines show low electrical efficiency of 25-30%LHV and high levels of noise. Therefore, the fuel cell which can be operated with very low environmental emission levels, is regarded as a promising candidate for a distributed power source in the next generation. Although most fuel cells operate with hydrogen as a fuel, solid oxide fuel cells (SOFCs) operated in a high temperature range between 600 and 900 oC accept the direct use of hydrocarbon fuels. Hydrocarbon fuels directly supplied to SOFCs are reformed in the porous anode materials producing H2-rich syngas, which is subsequently used to generate electricity and heat through electrochemical oxidation (Steele & Heinzel, 2001; Sasaki & Teraoka, 2003). This type of SOFC, so called internal reforming SOFC (IRSOFC), enables us to simplify the SOFC system. Electrochemical performances of IRSOFC have been reported for gaseous and liquid fossil fuels such as methane (Park et al., 1999), propane (Iida et al., 2007), n-dodecane

In situ transmission electron microscopic observations of redox cycling of a Ni–ScSZ cermet fuel cell anode

In situ transmission electron microscopy (TEM) observations of a Ni(O)-Sc2O3-stabilized ZrO2 (ScSZ; 10 mol% Sc2O3, 1 mol% CeO2, 89 mol% ZrO2) anode in a solid oxide fuel cell (SOFC) have been performed at high temperatures under a hydrogen/oxygen gas atmosphere using an environmental transmission electron microscope (ETEM); the specimens were removed from cross-sections of the real SOFC by focused ion beam milling and lifting. When heating the NiO-ScSZ anode under a hydrogen atmosphere of 3 mbar in ETEM, nano-pores were formed at the grain boundaries and on the surface of NiO particles at around 400°C due to the volume shrinkage accompanying the reduction of NiO to Ni. Moreover, densification of Ni occurred when increasing the temperature from 600 to 700°C. High-magnification TEM images obtained in the early stages of NiO reduction revealed that the (111) planes of Ni grew almost parallel to the (111) planes of NiO. In the case of heating Ni-ScSZ under an oxygen atmosphere of 3 mbar in ETEM, oxidation of Ni starting from the surface of the particles occurred above 300°C. All Ni particles became polycrystalline NiO after the temperature was increased to 800°C. Volume expansion/contraction by mass transfer to the outside/inside of the Ni particles in the anode during repeated oxidation/reduction seems to result in the agglomeration of Ni catalysts during long-term SOFC operation. We emphasize that our in situ TEM observations will be applied to observe electrochemical reactions in SOFCs under applied electric fields.