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A novel numerical methodology for predicting the electrochemical characteristics of a practical-size solid oxide fuel cell (SOFC) is proposed. The core idea is to divide the practical-size cell into independent one-dimensional (1D) unit cells and to evaluate the characteristics of the practical-size cell as an aggregation of the performance of local unit cells. The proposed methodology consists of two steps. The first is to construct a database of the electrochemical performance of the unit cells under a comprehensive range of operating conditions. For this purpose, a 1D numerical simulation model is developed and validated by comparing the predicted values with experimental data. It is confirmed that not only the current–voltage characteristics but also the impedance characteristics are accurately reproduced by the developed unit cell model. The second is to reproduce the performance of the practical-size cell using the constructed database. The cell is considered as an assembly of unit cells and the electrochemical performance of each unit cell is determined by referring to the constructed database. Subsequently, the macroscopic characteristics of the entire cell are evaluated by using the derived theoretical expressions correlating the macroscopic characteristics with the local electrochemical performance. The predicted cell characteristics fairly agree with the experimental data found in the literature. Since the macroscopic characteristics of the practical-size cell is evaluated with simple arithmetic expressions, the computational cost is significantly reduced, which realizes an exhaustive parametric study of the practical-size cell to identify the optimal operation conditions.

A hydrogen-electrode-supported solid oxide cell (SOC) is fabricated by phase-inversion tape casting technique and its electrochemical performance is investigated with particular focus on high fuel utilization operation and reversible operation. Structure of the hydrogen-electrode support layer is also analyzed. Electrochemical performance of the cell is measured and compared with that obtained from a conventional cell having homogenous porous structure in the hydrogen electrode support. Equivalent circuit fitting analysis is performed for quantitative understanding of impedance spectra. The results indicate that the finger-like pores in the hydrogen-support layer introduced by the phase-inversion process can significantly enhance gas diffusion property of the layer and hence improve cell performance particularly at lower reactant concentrations. Also, asymmetry in cell performance between fuel cell and electrolysis cell modes is suppressed, which is desirable for reversible operation of SOCs.

To control the temperature distribution in the Ni–YSZ (yttria-stabilized zirconia) anode of solid oxide fuel cells (SOFCs) by efficiently utilizing the heat generated by electrochemical reactions, the supply of methane–ammonia mixed fuel is proposed. The reaction characteristics of reforming/decomposition of the mixed fuel on a Ni–YSZ catalyst are experimentally investigated. A mixture gas of methane, steam, ammonia, and balance argon is supplied to a packed bed catalyst placed in a quartz tube in an electric furnace. The crushed Ni–YSZ anode of SOFCs is used as the catalyst. The exhaust gas composition is analyzed by gas chromatography and the streamwise temperature distribution of the catalyst bed is measured by an infrared camera. It is found that ammonia decomposition preferentially proceeds and steam methane reforming becomes active after sufficient ammonia has been consumed. A low-temperature region is formed by steam methane reforming owing to its strongly endothermic nature. Its position moves downstream while its magnitude decreases as the ammonia concentration in the fuel increases. This shows that the local temperature distribution can be controlled by tuning the ratio of methane to ammonia in the mixed fuel. It is also found that, at a certain mixture ratio, the mixed fuel realizes a hydrogen production rate higher than that for only methane or ammonia.

The presented research shows that the long-term operation of a solid oxide fuel cell can lead to substantial anisotropic changes in anode material. The morphology of microstructure in the investigated stack was observed before and after the aging test using electron nanotomography. The microstructural parameters were estimated based on the obtained digital representation of the anode microstructure. Anisotropy was discovered in two of the three phases that constitute the anode, namely nickel and pores. The third component of the anode, which is yttrium-stabilized zirconia, remains isotropic. The changes appear at the microscale and significantly affect the transport phenomena of electrons and gasses. The obtained results indicate that the reference anode material that represents the microstructure before the aging test has isotropic properties which evolve toward strong anisotropy after 3800 h of constant operation. The presented findings are crucial for a credible numerical simulation of solid oxide fuel cells. They indicate that all homogeneous models must adequately account for the microstructure parameters that define the anisotropy of transport phenomena, especially if microstructural data is taken from a post-operational anode.

Anode ridge structures are fabricated by printing anode pastes having different rheological properties to increase the electrode–electrolyte interfacial area in anode-supported solid oxide fuel cells. Homogeneous anode pastes are prepared by varying solid loadings, then their rheological characteristics are evaluated. It is confirmed that the shear stress at the same shear rate increases with increasing solid loading. The prepared pastes are printed onto flat anode substrates and then the width and height of the printed pastes are observed before and after drying and after sintering. As increasing solid loading in the paste, not only the width and height become smaller and larger, respectively, but also the shrinkage rate of the aspect ratio becomes smaller. Consequently, a large extension of the interfacial area of about 285% is obtained by printing the paste with the solid loading of 80%.

A simple but physicochemically meaningful numerical model of a solid oxide fuel cell (SOFC) anode is developed as a tool to understand and reproduce its impedance spectra. The model considers conservations of electrons, ions and gas species within the anode, which are coupled by a non-linear electrochemical reaction model. The developed model is first calibrated by impedance spectra that are experimentally obtained at the open circuit conditions at several temperatures, by adjusting the ionic conductivity, exchange current density and electrostatic capacitance. All of these parameters are confirmed to be within an acceptable range of values found in literature. Also, real anode microstructural parameters obtained by three-dimensional FIB-SEM imaging is implemented to eliminate any other arbitrary fitting parameters from the model. Then, the model is used to reproduce impedance spectra obtained from another anode with different microstructure and also those measured at polarized conditions and at different temperatures, and gives good agreement with the experimental results. Considering the computational resources required to reproduce impedance spectra, the developed model is expected to be used as an alternative to the conventional equivalent circuit fitting.

Ni/yttria-stabilized zirconia anodes of solid oxide fuel cells are fabricated by a wet infiltration technique and the ability of the infiltration technique to control the anode microstructure is quantitatively demonstrated by a detailed three-dimensional microstructural analysis. The microstructural analysis reveals favorable aspects of the infiltrated anodes, such as larger triple-phase boundary density and sufficiently large pore size, and they are mostly unachievable by the conventional powder-mixing and sintering approaches. The improved controllability of the infiltration technique is expected to be useful to tailor porous microstructures to meet the multiple requirements for transport and electrochemical reactions within the anodes.

The steam-methane reforming reaction on a Ni-YSZ (yttria-stabilized zirconia) cermet was experimentally investigated in the temperature range of 650 to 750°C. We examined the effects of the partial pressures of methane and steam in the supply gas on the reaction rate. The porous microstructure of the Ni-YSZ cermet was quantified using an FIB-SEM technique. A power-law-type rate equation was obtained on the basis of the unit surface area of the Ni-pore contact surface in the cermet. The kinetics indicated a strong positive dependence on the methane partial pressure and a negative dependence on the steam partial pressure, in good agreement with the literature. The obtained rate equation successfully predicted the reaction characteristics of Ni-YSZ cermets having different microstructures.

An overpotential model for planar solid oxide fuel cells (SOFCs) is developed and applied to a stack numerical simulation. Charge-transfer distribution within the electrodes are approximated using an exponential function, based on which the Ohmic loss and activation overpotential are evaluated. The predicted current-voltage characteristics agree well with the experimental results, and also the overpotentials within the cell can reproduce the results obtained from a numerical analysis where the distribution of the charge-transfer current within the electrodes is fully solved. The proposed model is expected to be useful to maintain the accuracy of SOFC simulations when the cell components, consisting of anode, electrolyte and cathode, are simplified into one layer element.

A three-dimensional numerical model of a single solid oxide fuel cell (SOFC) considering chromium poisoning on the cathode side has been developed to investigate the evolution of the SOFC performance over long-term operation. The degradation model applied in the simulation describes the loss of the cathode electrochemical activity as a decrease in the active triple-phase boundary (TPB) length. The calculations are conducted for two types of cell: lanthanum strontium manganite (LSM)/yttria-stabilized zirconia (YSZ)/Ni-YSZ and LSM-YSZ/YSZ/Ni-YSZ. Their electrode microstructures are acquired by imaging with a focused ion beam scanning-electron microscope (FIB-SEM). The simulation results qualitatively reproduce the trends of chromium poisoning reported in the literature. It has been revealed that the performance degradation by chromium is primarily due to an increase in the cathode activation overpotential. In addition, in the LSM-YSZ composite cathode, TPBs in the vicinity of the cathode–electrolyte interface preferentially deteriorate, shifting the active reaction site towards the cathode surface. This also results in an increase in the cathode ohmic loss associated with oxide ion conduction through the YSZ phase. The effects of the cell temperature, the partial pressure of steam at the chromium source, the cathode microstructure, and the cathode thickness on chromium poisoning are also discussed.

A simulation model of a small-scale solid oxide iron-air battery system was developed to clarify its fundamental characteristics and feasibility from the view point of energy efficiency. The energy flow in one cycle of charge/discharge operations was evaluated under a quasi-state assumption with 0-dimensional models of the system components, i.e., a solid oxide electrochemical cell, an iron (Fe) box and heat exchangers. Special care was taken when considering thermal aspects; not only a simple system but also a more complicated system with thermal recirculation by three heat exchangers was investigated. It was found that the system round-trip efficiency reaches 61% under the base conditions in this study. The results also show that several limitations exist for the operation parameters and conditions in view of practical applications. In particular, higher and lower limits exist for the fuel and air utilization factors under which the system operates effectively because of constraints such as the maximum allowable fuel-blower temperature and no heat input during the discharge operation.

In this paper, we propose an empirical formula for i0, TPB, the exchange current density per unit triple-phase boundary (TPB) length, for porous lanthanum strontium manganite (LSM) cathodes of solid oxide fuel cells (SOFCs); the evaluation of i0, TPB is of crucial importance in numerical simulations of electrodes based on reconstructed microstructures obtained by a dual beam focused ion beam scanning electron microscopy (FIB-SEM) and tomography techniques. To derive a widely applicable empirical formula for i0, TPB, electrochemical measurements of porous LSM cathodes are conducted under various oxygen partial pressures (0.05–0.25 atm) and temperatures (800–950 °C). By comparing the derived formula with that derived from a thin and dense patterned LSM electrode used in previous studies, it is found that at an air temperature of 800 °C, i0, TPB derived from a porous LSM cathode is approximately 40% smaller than that for the patterned electrode. This can be attributed to the fact that the electrochemical reaction in thin and dense electrodes can occur not only at the TPBs but also at the LSM surface owing to the non-negligible ionic conductivity of LSM. The derived formula is also applied to a three-dimensional numerical simulation to confirm its validity.

Here we propose an empirical formula for the exchange current density per unit TPB (three-phase boundary) length for porous LSM (lanthanum strontium manganite) cathodes for solid oxide fuel cells, which is often required for numerical modeling of cathode performance based on reconstructed microstructures obtained with tomography techniques. Electrochemical measurement of LSM cathodes was conducted under various oxygen partial pressure (0.05-0.25 atm) and temperature (800-950 °C) conditions. The derived values were compared with those from a thin and dense patterned LSM electrode in literature, and were found to be small at 800 °C. One of the possible reasons is that the charge transfer can be occurred at not only TPB but also two-phase (LSM-Pore) boundary in the case of thin patterned electrode.

A time-dependent 2-D numerical simulation was performed on a solid oxide iron-air battery (SOIAB) to reveal the fundamental characteristics of this new system. The SOIAB is a rechargeable battery consisting of a solid oxide electrochemical cell (SOEC) and iron as a redox metal. A simple battery configuration was employed assuming a system with a small capacity. A simulation model for a unit element was developed considering heat and mass transfer in the system, taking both electrochemical and redox reactions into account. The numerical results showed the spatial and temporal changes in the temperature field in the charge and discharge operations, which were due to the combined effects of heat generation/absorption by the electrochemical and redox reactions and heat exchange with the air supplied through convective heat transfer. As the reaction rates are functions of the local temperature, the predicted results show the importance of considering the heat transfer phenomena in this system. It was also found that the active reaction region in the redox metal evolves with time. The nonuniform distribution of iron utilization is affected by the effective gas diffusion coefficients in the porous redox metal, and consequently the change in the current density distribution in the SOEC.

Three-dimensional numerical analysis of solid oxide fuel cell (SOFC) anode polarization is conducted with a microstructure obtained by a focused ion beam and scanning electron microscope (FIB-SEM). Electronic, ionic and gaseous transports with electrochemical reaction are considered in the porous anode. A sub-grid scale (SGS) model is newly developed and effectively used to consider the structural information whose characteristic scale is smaller than calculation grid size. The proposed SGS models are designed to keep the quality of the structural information which is inevitably lost by resampling process in grid generation. Through comparisons between the simulation results and the experimental data, it was found that the SGS model can either improve the simulation accuracy under a given calculation grid system or reduce computational load for the same degree of simulation accuracy.

Three-dimensional numerical simulation of SOFC anode polarization is conducted with a structure obtained by a focused ion beam and scanning electron microscope (FIB-SEM). Electronic, ionic and gaseous transports with electrochemical reaction are considered. A sub-grid scale model is newly developed and effectively used to evaluate the transport flux in the porous structure. The proposed SGS model shows its potential to reasonably evaluate the transport flux considering the microstructure smaller than the grid size.