Hirofumi Sumi

Comparison Between Internal Steam and CO2 Reforming of Methane for Ni-YSZ and Ni-ScSZ SOFC Anodes

The performance and durability for Ni-YSZ and Ni-ScSZ anodes of solid oxide fuel cells were compared between internal steam and CO 2 reforming of methane in the carbon deposition conditions. With a supply of H 2 -CO-CO 2 mixtures to the anode, the polarization resistance increased with a rise in CO concentrations at 1023 K because of the difficulty of electrochemical oxidation of carbon monoxide. Carbon was produced by the disproportionation of carbon monoxide in CO―CO 2 mixture (CO:CO 2 = 95:5) at 1023 K, which led to low performance and much degradation of the cells. In comparison between steam and CO 2 reforming of methane, the performance and durability in a gaseous mixture at H 2 O/CH 4 = 0.5 were better than that at CO 2 /CH 4 = 0.5. The durability of the Ni-ScSZ anode was superior to that of the Ni-YSZ anode at 1273 K. Amorphous carbon covered the Ni-YSZ anode surface after power generation, which deactivated the nickel catalyst and inhibited gas diffusion. However, crystalline graphite with a rod morphology was grown on the Ni-ScSZ anode, which affected the catalytic activity less than amorphous carbon. The crystallinity and morphology of deposited carbon are important in determining the performance and durability of the cells at low H 2 0/CH 4 and CO 2 /CH 4 ratios.

Changes of Internal Stress in Solid-Oxide Fuel Cell During Red-Ox Cycle Evaluated by In Situ Measurement With Synchrotron Radiation

The internal stress in anode-supported solid-oxide fuel cells (SOFCs) was evaluated by in situ measurement using high-energy x-ray synchrotron radiation. The oxidized cell had a compression of ∼400 MPa in the c-ScSZ electrolyte thin film and a tension of 50-100 MPa in the NiO-YSZ anode substrate at room temperature. The internal stress decreased with increasing temperature, becoming approximately zero at 1000 K. Although the internal stress returned to its initial value after the thermal cycle, the stress decreased to ∼200 MPa in the electrolyte after the reduction cycle because of the decrease of the coefficient of thermal expansion mismatch between the electrolyte and anode. The red-ox cycle would be detrimental for anode-supported SOFC.

Low temperature densification process of solid-oxide fuel cell electrolyte controlled by anode support shrinkage

A low temperature densification process of an electrolyte for solid-oxide fuel cells (SOFCs) has been developed by utilizing controlled electrode microtubular support shrinkage. This technology enables reduction of the co-sintering temperature for the electrolyte and the electrode (anode) support to temperatures as low as 1250 °C, resulting in the realization of a new anode microstructure using conventional, commercially available materials, NiO and Sc stabilized ZrO2. The new microtubular SOFC has shown high fuel cell performances from 0.36, 0.52, and 0.56 W cm−2 and energy efficiencies of 31, 44 and 47% (lower heating value) at 600, 650, and 700 °C respectively. Impedance analysis has also shown that the main contributing factor of the cell performance varies depending upon the operating temperatures, which is of importance for further optimization of the cell structures.

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.