Performance of tubular Solid Oxide Fuel Cell at reduced temperature and cathode porosity

The effect of decreasing the inlet temperature and the cathode porosity of tubular Solid Oxide Fuel Cell (SOFC) with one air channel and one fuel channel is investigated using Computational Fluid Dynamics (CFD) approach. The CFD model was developed using Fluent SOFC to simulate the electrochemical effects. The cathode and the anode of the cell were resolved in the model and the convection and conduction heat transfer modes were included. The results of the CFD model are presented at inlet temperatures of 700 °C, 600 °C and 500 °C and with cathode porosity of 30%, 20% and 10%. It was found that the Fluent-based SOFC model is an effective tool for analyzing the complex and highly interactive three-dimensional electrical, thermal, and fluid flow fields associated with the SOFCs. It is found that the SOFC can operate in the intermediate temperature range and with low porosity cathodes more efficient than at high temperatures given that the transport properties of the cathode, anode and the electrolyte can be kept the same.     

Three-dimensional multiphase modeling of alkaline anion exchange membrane fuel cell

Alkaline anion exchange membrane (AAEM) fuel cell is becoming more attractive because of its outstanding merits, such as fast electrochemical kinetics and low dependence on non-precious catalyst. In this study, a three-dimensional multiphase non-isothermal AAEM fuel cell model is developed. The modeling results show that the performance is improved with more anode humidification, but the improvement becomes less significant at higher humidification levels. The humidification level of anode can change the water removal mechanisms: at partial humidification, water is removed as vapor; and for full humidification, water is removed as liquid. Cathode humidification is even more critical than anode. Liquid water supply in cathode has a positive effect on performance, especially at high current densities. With more liquid water supply in cathode, liquid water starts moving from channel to CL, rather than being removed from CL. Liquid water supply in cathode is needed to balance the water amounts in anode and cathode. Decreasing the membrane thickness generally improves the cell performance, and the improvement is even enhanced with thinner membranes, due to the faster water diffusion between anode and cathode, which reduces the mass transport losses.     

Numerical analysis of the single electrode heat effect in molten carbonate fuel cells: temperature analysis of the electrolyte plate by applying irreversible thermodynamics

A temperature analysis model of a molten carbonate fuel cells (MCFC) stack is used to calculate the single electrode heat effects. The magnitude of heat which evolves from the cathode and absorbed at the anode is large, and in similar value to the electrical output of a MCFC. This suggests that the heat evolution of a single electrode causes a temperature difference between the electrodes. The temperature distribution in the electrolyte plate is evaluated to establish more accurate results concerning the temperature analysis model of the stack. The temperature distribution in the electrolyte plate is studied by applying irreversible thermodynamics. When the operating current density is less than 3000 A m^?2 and the thermal conductivity of the electrolyte is more than 2 W m^?1 K^?1, the temperature difference between cathode and anode is estimated to be less than approximately 1 K. This result proves that the temperature difference between the electrodes can be supposed constant in constructing the temperature analysis model of the MCFC stack. This results also allows us to construct a two‐dimensional heat production distribution in the cell plane and discrete heat production distribution in the stacking direction for the practical use of the temperature analysis model. Copyright © 2004 John Wiley & Sons, Ltd.     

Modeling of two-phase transport in the diffusion media of polymer electrolyte fuel cells

A three-dimensional model of polymer electrolyte fuel cells (PEFCs) is developed to investigate multiphase flows, species transport, and electrochemical processes in fuel cells and their interactions. This two-phase model consists of conservation principles of mass, momentum, species concentration and charges, and elucidates the key physicochemical mechanisms in the constituent components of PEFCs that govern cell performance. Efforts are made to formulate two-phase transport in the anode diffusion media and its coupling with cathode flooding as well as the interaction between single- and two-phase flows. Numerical simulations are carried out to investigate multiphase flow, electrochemical activity, and transport phenomena and the intrinsic couplings of these processes inside a fuel cell at low humidity. The results indicate that multiphase flows may exist in both anode and cathode diffusion media at low-humidity operation, and two-phase flow emerges near the outlet for co-flow configuration while is present in the middle of the fuel cell for counter-flow one. The validated numerical tools can be applied to investigate vital issues related to anode performance and degradation arising from flooding for PEFCs.     

An easy-to-approach and comprehensive model for planar type SOFCs

An easy-to-approach and comprehensive mathematical model for planar type solid oxide fuel cells is presented in the current work. It provides a tool for researchers to conduct parametric studies with less-intensive computation in order to grasp the fundamentals of coupled mass transfer, electrochemical reaction, and current conduction in a fuel cell. In the model, the analysis for the mass transfer polarization at a known average fuel cell operating temperature is based on an average mass transfer model analogous to an average heat transfer process in a duct flow. The effect of the species' partial pressure at electrode/electrolyte interfaces is therefore included in the exchange current density for activation polarizations. An electrical circuit for the current and ion conduction is used to analyze the ohmic losses from anode current collector to cathode current collector. The three types of over-potentials caused by different polarizations in a planar type solid oxide fuel cell can be identified and compared. The effects of species concentrations, properties of fuel cell components to the voltage–current performance of a fuel cell at different operating conditions are studied. Optimization of the dimensions of flow channels and current-collecting ribs is also presented. The model is of significance to the design and optimization of solid oxide fuel cells for industrial application.     

Electrochemical impact of the carbonate in ceria-carbonate composite for low temperature solid oxide fuel cell

Ceria-carbonate composite has been suggested as a promising electrolyte for solid oxide fuel cells operating at low temperatures. However, the roles of carbonate in the enhancement of the superionic conductivity and fuel cell performance of ceria-carbonate composite electrolytes are not yet confirmed. In this work, we look into the chemical and ionic state, transmission and segregation of carbonate and alkali cations under normal and electrochemical conditions. The XRD measurement confirms that there are not any carbonate crystals in the sample electrolyte. It is interesting to see that part of the carbonate and alkali ions are not formed into the stoichiometric carbonate in sample materials under the typical electrical field condition from the EDS and XPS analysis. Instead, carbonate (CO₃^2?) and alkali ions accumulate in the cathode side, which we believe, was caused by the electrochemical “catalyst” of CO₂ and alkali ions that accelerated the electrochemical oxygen reduction reaction, while the CO₃^2? ions as one of the charge carriers which diffuse from fuel cell cathode to anode are on account of the concentration gradient. Those together contribute to the excellent electrochemical performances of ceria-carbonate composite electrolyte for low temperature solid oxide fuel cell.     

Simulation of species transport and water management in PEM fuel cells

A single phase computational fuel cells model is presented to elucidate three-dimensional interactions between mass transport and electrochemical kinetics in proton exchange membrane (PEM) fuel cells with straight gas channels. The governing differential equations are solved over a single computational domain, which consists of a gas channel, gas diffusion layer, and catalyst layer for both the anode and cathode sides of the cell as well as the solid polymer membrane. Emphasis is placed on obtaining a basic understanding of how three-dimensional flow and transport phenomena in the air cathode impact the electrochemical process in the flow field. The complete cell model has been validated against experimentally measured polarization curve, showing good accuracy in reproducing cell performance over moderate current density interval. Fully three-dimensional results of the flow structure and species profiles are presented for cathode flow field. The effects of pressure on oxygen transport and water removal are illustrated through main axis of the flow structure. The model results indicate that oxygen concentration in reaction sites is significantly affected by pressure increase which leads to rising fuel cells power.     

Three-dimensional modeling of water transport in PEMFC

A three dimensional two phase flow model is proposed to study transport phenomena in a PEMFC. In order to capture the effects of liquid water on the performance of the fuel cell, all regions are modeled from the anode to the cathode as having finite thickness. The geometry of the bipolar plate is modeled in detail to capture the effect of liquid water accumulation under the channel rib. This model takes into account the effect of temperature and inlet RH of both the anode and cathode. The three-dimensional model uses the finite volume method to solve the equations of mass conservation, momentum, energy, species transfer and protonic potential. These equations include the effect of liquid water on the transport properties as well as the electrochemical source. The effects of water on ohmic losses are presented for different humidity conditions of the anode and cathode at various fuel cell temperatures.     

Improved molten carbonate fuel cell performance via reinforced thin anode

The effects of anode thickness on electrochemical performance and cell voltage stability of molten carbonate fuel cell (MCFC) were examined using single cell test. It was found that supported thin nickel-aluminum (Ni–Al) anode with small pore size enhanced cell performance by reducing its mass transfer resistance and crossover. The stability of cell voltage was also observed. This was achieved after 0.25 mm thick anode was reinforced with Ni 60 mesh. Unsupported 0.3 mm thick anode yielded poor performance due to deformation and cracks after a long thermal exposure. The performance was improved significantly after all the anodes were reinforced with Ni mesh.     

Corrosion of anode current collectors in molten carbonate fuel cells

Corrosion of metallic parts is one of the life-time limiting factors in the molten carbonate fuel cell. In the reducing environment at the anode side of the cell, the corrosion agent is water. As anode current collector, a widely used material is nickel clad on stainless steel since nickel is stable in anode environment, but a cheaper material is desired to reduce the cost of the fuel cell stack. When using the material as current collector one important factor is a low resistance of the oxide layer formed between the electrode and the current collector in order not to decrease the cell efficiency. In this study, some candidates for anode current collectors have been tested in single cell molten carbonate fuel cells and the resistance of the oxide layer has been measured. Afterwards, the current collector was analysed in scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). The results show that the resistances of the formed oxide layers give a small potential drop compared to that of the cathode current collector.     

Modeling of a planar solid oxide fuel cell based on proton‐conducting electrolyte

A 2D computational fluid dynamics (CFD) model is developed to study the performance of an advanced planar solid oxide fuel cell based on proton conducting electrolyte (SOFC‐H). The governing equations are solved with the finite volume method (FVM). Simulations are conducted to understand the transport phenomena and electrochemical reaction involved in SOFC‐H operation as well as the effects of operating/structural parameters on SOFC‐H performance. In an SOFC based on oxygen ion conducting electrolyte (SOFC‐O), mass is transferred from the cathode side to the anode side. While in an SOFC‐H, mass is transferred from the anode to the cathode, which causes different velocity fields of the fuel and oxidant gas channels and influences the distributions of temperature and gas composition in the cell. It is also found that increasing the inlet gas velocity leads to an increase in the local current density and a slight decrease in the SOFC‐H temperature due to stronger cooling effect of the gas species at a higher velocity. Another finding is that the electrode structure does not significantly affect the heat and mass transfer in an SOFC‐H at typical operating voltages. Copyright © 2009 John Wiley & Sons, Ltd.     

Modeling of assisted cold start processes with anode catalytic hydrogen–oxygen reaction in proton exchange membrane fuel cell

Catalytic hydrogen–oxygen reaction is a potentially effective way to help start up proton exchange membrane fuel cells (PEMFCs) from sub-zero temperatures. In this study, the anode hydrogen–oxygen catalytic reaction is implemented in a three-dimensional multiphase cold start model. It is found that successful cold start from −20 °C can be achieved with the assist of the catalytic reaction in galvanostatic mode. With anode catalytic reaction, the start-up current density must be moderate, because a high current density lowers the assisted heating effect, and a low current density slows down the start-up process. The temperature difference between the anode and cathode catalyst layers (CLs) is negligible, which indicates that the heating location in the electrodes for the catalytic reaction makes no significant difference. The humidification of anode due to the catalytic reaction also reduces the ohmic resistance of the membrane, leading to enhanced performance during the start-up processes.     

Numerical study of thermoelectric characteristics of a planar solid oxide fuel cell with direct internal reforming of methane

A three-dimensional mathematical thermo-fluid model coupling the electrochemical kinetics with fluid dynamics was developed to simulate the heat and mass transfer in planar anode-supported solid oxide fuel cell (SOFC). The internal reforming reactions and electrochemical reactions of carbon monoxide and hydrogen in the porous anode layer were analyzed. The temperature, species mole fraction, current density, overpotential loss and other performance parameters of the single cell unit were obtained by a commercial CFD code (Fluent) and external sub-routine. Results show that the current density produced by electrochemical reactions of carbon monoxide cannot be ignored, the cathode overpotential loss is the biggest one among the three overpotential losses, and that the proper decrease of the operating voltage leads to the increase of the current density, PEN structure temperature, fuel utilization factor, fuel efficiency and power output of the SOFC.     

A three-dimensional mathematical model for liquid-fed direct methanol fuel cells

A three-dimensional, single-phase, multi-component mathematical model has been developed for a liquid-fed direct methanol fuel cell (DMFC). The traditional continuity, momentum, and species conservation equations are coupled with electrochemical kinetics in both the anode and cathode catalyst layer. At the anode side, the liquid phase is considered, and at the cathode side only the gas phase is considered. Methanol crossover due to both diffusion and electro-osmotic drag from the anode to the cathode is taken into consideration and the effect is incorporated into the model using a mixed-potential at the cathode. A finite-volume-based CFD technique is used to develop the in-house numerical code and the code is successfully used to simulate the fuel cell performance as well as the multi-component behavior in a DMFC. The modeling results of polarization curves compare well with our experimental data. Subsequently, the model is used to study the effects of methanol crossover, the effects of porosities of the diffusion layer and the catalyst layer, the effects of methanol flow rates, and the effects of the channel shoulder widths.     

Heat and mass transfer effects in a direct methanol fuel cell: A 1D model

Models are a fundamental tool for the design process of fuel cells and fuel cell systems. In this work, a steady-state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in the DMFC, is presented. The model output is the temperature profile through the cell and the water balance and methanol crossover between the anode and the cathode. The model predicts the correct trends for the influence of current density and methanol feed concentration on both methanol and water crossover. The model estimates the net water transfer coefficient through the membrane, α, a very important parameter to describe water management in the DMFC. Suitable operating ranges can be set up for different MEA structures maintaining the crossover of methanol and water within acceptable levels. The model is rapidly implemented and is therefore suitable for inclusion in real-time system level DMFC calculations.     

A direct carbon fuel cell with (molten carbonate)/(doped ceria) composite electrolyte

A composite electrolyte containing a Li/Na carbonate eutectic and a doped ceria phase is employed in a direct carbon fuel cell (DCFC). A four-layer pellet cell, viz. cathode current collector (silver powder), cathode (lithiated NiO/electrolyte), electrolyte and anode current collector layers (silver powder), is fabricated by a co-pressing and sintering technique. Activated carbon powder is mixed with the composite electrolyte and is retained in the anode cavity above the anode current collector. The performance of the single cell with variation of cathode gas and temperature is examined. With a suitable CO₂/O₂ ratio of the cathode gas, an operating temperature of 700 °C, a power output of 100 mW cm⁻² at a current density of 200 mA cm⁻² is obtained. A mechanism of O²⁻ and CO₃²⁻ binary ionic conduction and the anode electrochemical process is discussed.     

Comprehensive one-dimensional,semi-analytical,mathematical model for liquid-feed polymer electrolyte membrane direct methanol fuel cells

Polymer electrolyte membrane direct methanol fuel cells (PEM-DMFCs) have several advantages over hydrogen-fuelled PEM fuel cells; but sluggish methanol electrochemical oxidation and methanol crossover from the anode to the cathode through the PEM are two major problems with these cells. In the present work, a comprehensive one-dimensional, single phase, isothermal mathematical model is developed for a liquid-feed PEM-DMFC, taking into account all the necessary mass transport and electrochemical phenomena. Diffusion and convective effects are considered for methanol transport on the anode side and in the PEM, whereas only diffusional transport of species is considered on the cathode side. A multi-step reaction mechanism is used to describe the electrochemical oxidation of methanol at the anode. Stefan–Maxwell equations are used to describe multi-component diffusion on the cathode side and Tafel type of kinetics is used to describe the simultaneous methanol oxidation and oxygen reduction reactions at the cathode. The model fully accounts for the mixed potential effect caused by methanol crossover at the cathode. It shows excellent agreement with literature data of the limiting current density for different low methanol feed concentrations at different operating temperatures. At high methanol feed concentrations, oxygen depletion on the cathode side, due to excessive methanol crossover, results in mass-transport limitations. The model can be used to optimize the geometric and physical parameters with a view to extracting the highest current density while still keeping a tolerably low methanol crossover.     

Validation of H/O conduction in doped ceria–carbonate composite material using an electrochemical pumping method

A hydrogen and oxygen electrochemical pump technique has been employed to elucidate the conduction of proton and oxygen ion in a doped ceria–carbonate composite electrolyte for intermediate temperature solid oxide fuel cells. The composite material shows efficient conductivities of both of the two ions at 650 °C. The molten carbonate phase is important for the migration of both of the two ions. The mechanism of the conduction of proton and oxygen ion is also discussed.     

Lifetime expectancy of molten carbonate fuel cells: Part II. Cell life simulation using bench and coin-type cells

The lifetime of molten carbonate fuel cells is simulated in terms of electrolyte loss rate, voltage reduction rate, and activation energy using 7 cm² coin- and 100 cm² bench-type molten carbonate fuel cells. Arrhenius plots are used to determine the temperature dependence of the anode gas-phase mass transfer resistance, cathode gas and liquid-phase mass transfer resistances and electrolyte loss rate. The gas-phase mass transfer resistance of the anode has positive activation energy, indicating more substantial resistance at higher temperatures. The cathodic gas-phase mass transfer resistance has small and negative activation energy. In contrast, the cathode shows negative and positive activation energies at the mass transfer resistance of superoxide ion (${\text{O}}_2^{?})$ and CO₂ in the liquid electrolytes, respectively. The negative value indicates a lower overpotential at higher temperatures and vice versa. The Arrhenius plot of the electrolyte weight loss rate shows positive activation energy, indicating that an increase in temperature causes a simultaneous increase in electrolyte weight loss. The cell life of a molten carbonate fuel cell is predicted using a factor that relates the voltage reduction and electrolyte loss rates. A lower value of the factor gives a longer cell life.     

Simulation of the performance for the direct internal reforming molten carbonate fuel cell. Part I: distributions of temperature,energy transfer and current density

This study examined the temperature distributions of the anode and cathode gases of the cell body as well as the current density distributions at each point of the direct internal reforming molten carbonate fuel cell (DIR‐MCFC) using numerical modelling. The model was based on assumptions and experimental data from a 5 cm × 5 cm sized unit cell operation. The results showed there was an approximately 13°C temperature difference between the initial point (0, 0) and end point (1, 1) of the cell body and the temperature increased steadily along with the direction of the anode gas flow. The temperature distribution of the anode gases showed a similar trend to those of the cell body. The temperature of the anode gases was an average 11°C lower than that of the cell body. The temperature distributions of cathode gases were relatively higher than those of the anode gases and the cell body. The temperature distributions at each point of the cell body, including the anode and cathode gases, could be explained by the different rates of the electrochemical, methane steam reforming and water–gas shift reactions at each point in the cell body. The current density distribution at the entrance of the cell was the highest at 290 mA cm^?2, and decreased steadily to 150 mA cm^?2 at the exit. These results were also confirmed by the amount of hydrogen reacted in the electrochemical reaction (referred to Part II). Finally, modelling simulations showed a non‐uniform distribution of the temperature and current density throughout the DIR‐MCFC were observed. In addition, it was confirmed that the distributions of the reaction rates and gas compositions at each point of the cell also showed a great deal of difference throughout the DIR‐MCFC. The non‐uniformity of these temperature distributions can lead to deterioration in the cell performance. These might provide the necessary information for solving these problems. Copyright © 2005 John Wiley & Sons, Ltd.