Ryuji Kikuchi

Impedance Analysis of Internal Resistance Affecting the Photoelectrochemical Performance of Dye-Sensitized Solar Cells

To clarify the relationships between the performance of dye-sensitized solar cells and their internal resistances, we applied ac impedance spectroscopy to the analysis of the cells. Impedance and I-V measurements of solarcells were carried out for various cell compositions and measurement conditions such as different kinds of conductive glass substrates, two kinds of counter electrodes, calcination temperature, applied bias, and distance between electrodes; in order to separate internal resistances which occur at different interfaces. Impedance spectra of dye-sensitized solar cells showed that their internal resistance consisted of at least five components. It was found that the resistance of a conductive glass substrate appeared as ohmic resistance, and that electron transfer at F-doped SnO 2 (FTO)/TiO 2 , TiO 2 /TiO 2 , Pt/electrolyte, and TiO 2 /electrolyte interfaces was identified as the internal resistances determinative to the performance of dye-sensitized solar cells. Internal resistances at FTO/TiO 2 , Pt/electrolyte and the resistance of conductive glass affected the fill factor. Internal resistances at TiO 2 /TiO 2 and TiO 2 /electrolyte mainly affected the photocurrent. Particularly, the TiO 2 /electrolyte was significant as the interface controlling photocurrent of the dye-sensitized solar cell. The impedance stemming from the diffusion of the electrolyte had little effect on the performance of the cells.

Fuel flexibility in power generation by solid oxide fuel cells

Power generation characteristics of solid oxide fuel cell (SOFC) with internal steam reforming of hydrocarbons were investigated. Steam reforming reaction over a Ni-YSZ cermet catalyst attained almost the equilibrium conversion and selectivity in the fixed bed reactor at 1000 °C. The conversion of internal reforming of hydrocarbons was incomplete because of the limited contact time with a thick layer of the Ni cermet electrode. Therefore, the fuel cell supplied with pre-reforming gas to the anode always gave rise to a lower terminal voltage because of the insufficient conversion of fuel compared with that supplied with post-reforming gas at a given current density. Methane internal reforming proceeded without deterioration with time, whereas the power generation with ethane and ethylene suffered from carbon deposition even at high steam-to-carbon ratio. Carbon deposition region and equilibrium partial pressure of oxygen in the C–H–O diagram were estimated from the thermodynamic data. The effect of the gas composition in the power generation characteristics, especially, difference in reactivity between H2 and CO, was investigated. The H2–H2O and CO–CO2 fuel systems led to almost the same open circuit voltage at the same H2/H2O and CO/CO2 ratios at 1000 °C, as expected from the thermodynamic equilibrium. The output voltage in a discharge condition was always higher for H2–H2O than for CO–CO2 at every current density.

Effect of precious metal addition to Ni-YSZ cermet on reforming of CH4 and electrochemical activity as SOFC anode

Abstract Various kinds of precious metals were added to the Ni-Y2O3-stabilized zirconia (Ni-YSZ) cermets, and the relation between steam reforming of CH4 and the electrochemical activity as a solid oxide fuel cell (SOFC) anode was investigated. Ru and Pt additions promoted the reforming and suppressed the coke depositions. The electrochemical activity of the SOFC anode was enhanced by the addition of Ru and Pt, indicating that these precious metals effectively functioned as the anode catalysts. The impedance related to gas diffusion was greatly reduced, indicating that stability of the anode catalyst of SOFC was considerably improved since coke was hardly deposited.

Internal Reforming of SOFCs Carbon Deposition on Fuel Electrode and Subsequent Deterioration of Cell

Several cermet anode materials were investigated for direct internal reforming operation of solid oxide fuel cells. The effects of steam and discharge conditions were examined for internal reforming operation with propane as a hydrocarbon fuel. Even at high steam-to-carbon (S/C) ratio, power generation characteristics with Ni-yttria-stabilized zirconia (YSZ) were deteriorated during internal reforming operation with propane at 1000°C, possibly due to carbon formation at low current densities, whereas stable generation behavior was observed for the cells with Ni-scandia-stabilized zirconia (ScSZ) or Ni-samaria-doped ceria (SDC) anode by feeding propane at low S/C = 0.8. The carbon deposition rate was measured for several cermet anode materials by feeding dry or humidified methane by the gravimetric technique. The carbon deposition rate over Ni-ScSZ was lower than Ni-YSZ, leading to better generation performance with propane at low S/C. Ni-SDC showed the highest carbon deposition rate among the cermets investigated, even for humidified methane. This implies that carbon formed over Ni-SDC could be effectively removed upon power generation.

Intermediate-Temperature Fuel Cell Employing CsH2PO4 ∕ SiP2O7-Based Composite Electrolytes

The proton-conductive electrolytes based on CSH 2 PO 4 /SiP 2 O 7 composites with various molar ratios were synthesized, and their structural and electrochemical properties were investigated at intermediate temperatures. Theinterfacial chemical reaction between CsH 2 PO 4 and SiP 2 O 7 took place during the heat-treatment at 220°C, resulting in the formation of CsH 5 (PO 4 ) 2 phase for the composition at CsH 2 PO 4 :SiP 2 O 7 = 2:1-1:5. The resulting composites showed high proton conductivity with no conductivity jump inherent in CsH 2 PO 4 , and the maximum conductivity achieved was 66 mS cm - 1 at 272°C. The fuel cell employing CsH 2 PO 4 /SiP 2 O 7 -based composite electrolyte (molar ratio 1:2, thickness 1.8 mm) was demonstrated at 220°C and showed good performance. It was possible to generate electricity up to 150 mA cm - 2 , and the maximum power density was 44 mW cm - 2 .

Low-temperature methane oxidation over oxide-supported Pd catalysts: inhibitory effect of water vapor

Abstract The influence of water vapor on the activity for low-temperature methane oxidation over oxide-supported catalysts such as Pd/Al 2 O 3 , Pd/SnO 2 , and Pd/Al 2 O 3 -36NiO was studied. It was found that Pd/Al 2 O 3 was deactivated most significantly due to water vapor, and that Pd/Al 2 O 3 -36NiO was most insensitive to water vapor. The catalytic activity of Pd/Al 2 O 3 decreased monotonically as water vapor concentration increased, whereas Pd/SnO 2 and Pd/Al 2 O 3 -36NiO showed almost constant activity under higher water vapor concentrations. The catalytic activity at high steam concentration was in the following order: Pd/SnO 2 >Pd/Al 2 O 3 -36NiO>Pd/Al 2 O 3 . Kinetic analysis with methane adsorption as the rate-limiting step was applied to evaluate the water inhibiting effect. Pd/Al 2 O 3 displayed the most negative value of the enthalpy of water adsorption, while Pd/SnO 2 and Pd/Al 2 O 3 -36NiO exhibited similar water adsorption enthalpy. Deactivation and regeneration of Pd/SnO 2 and Pd/Al 2 O 3 catalysts were investigated by cyclic feed of water vapor. Both the catalysts were deactivated rapidly upon switching on water feed, and then they regenerated gradually to the initial activity after the water feed was switched off.

Current-voltage characteristics and impedance analysis of solid oxide fuel cells for mixed H2 and CO gases

Current-voltage (I-V) characteristics and electrode impedance of a tubular-type solid oxide fuel cell (SOFC) were analyzed for mixed fuel gases, consisting mainly of CO, H 2 , H 2 O, and a carrier gas, as simulated reformed gas of hydrocarbons or coal gas. I-V characteristics of a single cell were measured as a function of various operational parameters including the H 2 -to-CO ratio, the type of carrier gas such as He, N 2 , and Ar, the temperature, the fuel-to-carrier gas ratio, and the water vapor concentration. It has been experimentally confirmed that the use of CO-rich gases results in comparable performance to that of H 2 -rich gases and thus mixed gas such as coal gas is useful as a SOFC fuel. We have found, for the first time, that the I-V characteristics depend on the carrier gas indicating the importance of gas transport in porous anodes for anodic polarization. The change in cell voltage hy varying fuel compositions was mainly caused by the change in anode impedance associated with a low frequency semicircle in a Cole-Cole impedance plot at 1000°C. The fuel gas compositions in thermodynamic equilibrium were calculated and compared with the initial gas compositions, suggesting the importance of water vapor concentration to control the equilibrium H 2 -to-CO ratio for CO-rich fuel gases.

Diagnosis of chaotic dynamics of bubble motion in a bubble column

Abstract The motion of bubbles in a two-dimensional bubble column was characterized by the deterministic chaos analysis of time-series data of bubble frequency, measured locally by an optical transmittance probe with a narrowed He Ne laser beam. To diagnose the dynamics of bubble motion, three different methods were applied to the series of time intervals between two successive bubble passages: (i) the correlation dimensions were estimated from the reconstructed attractors, (ii) using the short-term predictability analysis, the deterministic and random features in the bubble motion were characterized in terms of Mann-Whitney statistic, and (iii) the value of the Hurst exponent was computed using the rescaled range ( R/S ) analysis. All three parameters were found to show consistent and significant changes at the transition between homogeneous bubble flow and heterogeneous churn-turbulent flow. These results indicate that at low velocity the motion of bubbles in the bubble column is deterministic chaotic whereas at high gas velocity the random-like motion of bubbles becomes significant.

Mechanistic study and catalyst development for selective carbon monoxide methanation

Selective CO methanation has been attractive as a CO removal technique from reforming gases in polymer electrolyte fuel cell systems. The catalysts for the title reaction require the following two features: (i) high CO methanation activity at low temperatures and (ii) low CO2 methanation activity at high temperatures. In this review, we surveyed numerous studies of selective CO methanation using heterogeneous catalysts, and discussed its plausible mechanism. Furthermore, we summarized how the activity and selectivity of CO methanation can be affected by the particle size of active metals, support materials, and additives.

Preparation of Monodisperse Chitosan Microcapsules with Hollow Structures Using the SPG Membrane Emulsification Technique

We describe herein successful preparations of monodisperse chitosan microcapsules with hollow structures using the SPG membrane emulsification technique. Two preparation procedures were examined in this study. In the first method, monodisperse calcium alginate microspheres were prepared and then coated with unmodified chitosan. Subsequently, tripolyphosphate treatment was conducted to physically cross-link chitosan and solubilize the alginate core at the same time. In the second method, photo-cross-linkable chitosan was coated onto the monodisperse calcium alginate microspheres, followed by UV irradiation to chemically cross-link the chitosan shell and tripolyphosphate treatment to solubilize the core. For both methods, it was determined that the average diameters of the chitosan microcapsules depended on those of the calcium alginate microparticles and that the microcapsules have hollow structures. In addition, the first physical cross-linking method using tripolyphosphate was found to be preferable to obtain the hollow structure, compared with the second method using chemical cross-linking by UV irradiation. This was because of the difference in the resistance to permeation of the solubilized alginate through the chitosan shell layers.