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.     

Performance comparison of cocurrent and countercurrent flow solid oxide fuel cells

Solid oxide fuel cell (SOFC) is a complicated system with heat and mass transfer as well as electrochemical reactions. The flowing configuration of fuel and oxidants in the fuel cell will greatly affect the performance of the fuel cell stack. Based on the developed mathematical model of direct internal reforming SOFC, this paper established a distributed parameters simulation model for cocurrent and countercurrent types of SOFC based on the volume-resistance characteristic modeling method. The steady-state distribution characteristics and dynamic performances were compared and were analyzed for cocurrent and countercurrent types of SOFCs. The results indicate that the cocurrent configuration of SOFC is more suitable with regard to performance and safety.     

Current density distribution in an HT-PEM fuel cell with a poly (2, 5-benzimidazole) membrane

High-temperature proton exchange membrane (HT-PEM) fuel cells were more useable than traditional low-temperature proton exchange membrane fuel cells. To investigate the current density distribution in a single HT-PEM fuel cell with a poly (2, 5-benzimidazole) membrane, a modified current distribution measuring device was developed. This device included not only a current distribution measuring gasket to collect local current but also a segmented gas diffusion layer (GDL) to hinder electron transfer in the GDL along the gas flow direction. The effects of this device installation configuration and operating conditions on the current density distribution were analyzed. One of the important findings was that proton transfer along an in-plane direction in the membrane and electron transfer along an in-plane direction in the GDL really occur in HT-PEM fuel cells. These results were very helpful for the optimization of the flow field and operating parameters of the HT-PEM fuel cells.     

Calculations of transport phenomena and reaction distribution in a polymer electrolyte membrane fuel cell

The performance of Polymer Electrolyte Membrane fuel cells depends on the design of the cell as well as the operating conditions. The design of the cell influences the complex interaction of activation effects, ohmic losses, and transport limitations, which in turn determines the local current density. Detailed models of the electrochemical reactions and transport phenomena in Polymer Electrolyte Membrane fuel cells can be used to determine the current density distribution for a given fuel cell design and operating conditions. In this work, three-dimensional, multicomponent and multiphase transport calculations are performed using a computational fluid dynamics code. The computational results for a full-scale fuel cell design show that ohmic effects due to drying of polymer electrolyte in the anode catalyst layer and membrane, and transport limitations of air and flooding in the cathode cause the current density to be a maximum near the gas channel inlets where ohmic losses and transport limitations are a minimum. Elsewhere in the cell, increased ohmic losses and transport limitations cause a decrease in current density, and the performance of the fuel cell is significantly lower than that which could be attained if the ohmic losses and transport limitations throughout the cell were the same as those near the gas channel inlets. Thus overall fuel cell design is critical in maximizing unit performance.     

Experimental investigation of temperature distribution of planar solid oxide fuel cell: Effects of gas flow,power generation,and direct internal reforming

The temperature distribution of an operating planar solid oxide fuel cell (SOFC) is experimentally investigated under direct internal reforming conditions. An in situ measurement is conducted using a cell holder and an infrared (IR) camera. The effects of the gas flow configuration, exothermic power generation reaction, and endothermic steam–methane reforming reaction are examined at a furnace temperature of 770 °C. The fuel flow and airflow are set to a coflow or counterflow configuration. The heat generation and absorption by the reactions are varied by tuning the average current density and the concentration of methane in the supplied fuel. The maximum value of the local temperature gradient along the cell tends to increase with increasing internal reforming ratio, regardless of the gas flow configuration. From the view point of a small temperature gradient, the counterflow configuration clearly shows better characteristics than that of the coflow, regardless of the internal reforming ratio.     

Reactions of low and middle concentration dry methane over Ni/YSZ anode of solid oxide fuel cell

Directly using methane in solid oxide fuel cells (SOFC) requires the knowledge of the reaction of methane over the anode. The reactions of low and middle concentration dry methane were studied over the anode of solid oxide fuel cell with Ni/yttria-stabilized zirconia (YSZ) anode and YSZ electrolyte. The production rates of different types of gas at anode outlet were measured at different current density. Mass balance and relationships between production rates and reaction rates were used to analyze the chemical and electrochemical reactions that took place in parallel. When dry methane is in low concentration, methane decomposition and deposited carbon oxidation occurs at low current density with the overall reaction being partial oxidation of methane (POM). With increased current density, hydrogen oxidation and carbon monoxide oxidizing to carbon dioxide take place simultaneously, and the overall reaction becomes the direct oxidation of methane (DOM). When DOM occurs, a portion of methane participates the POM. However, the rate of POM decreases with increased current density. At medium methane concentration, only partial oxidation of methane takes place. Carbon deposition was found in all the tests across the concentration range investigated.     

Study of the characteristics of temperature rise and coolant flow rate control during malfunction of PEM fuel cells

In actual PEM fuel cell systems, the coolant flow rate is generally controlled to maintain a preset temperature at the coolant outlet. This implies that a change in coolant supply flow rate is a good early indicator of a malfunctioning PEM fuel cell stack and system components. In this study, various fuel cell malfunctions are simulated based on the practical coolant flow control strategy by using a three-dimensional, two-phase, multiscale PEM fuel cell model developed in our previous studies. The focus is on analysis of the characteristics of coolant flow rate change along with voltage degradation in various fuel cell malfunction cases. The model predictions show that in general, the coolant flow rate tends to increase proportionally with the degree of voltage degradation, but the increase in temperature inside the membrane electrode assembly (MEA) is not always related to the voltage drop and is influenced more directly by local current density distribution. Although the present numerical comparison between the normal and malfunctioning cases is conducted at the low current density of 0.3 A cm^?2, the general cell behavior will not be altered at higher current densities due to inverse relationship between cell performance and waste heat generation. The present work elucidates the complex interplay among increase in coolant flow rate, increase in MEA temperature, voltage drop, and change in local current density distribution when a PEM fuel cell malfunctions.     

Analysis of heat and mass transport in a miniature passive and semi passive liquid-feed direct methanol fuel cell

A two-dimensional, transient, multi-phase, multi-component, and non-isothermal model has been developed to solve the heat and mass transport in a passive and semi passive liquid-feed direct methanol fuel cell (DMFC). A semi passive DMFC uses channel at the cathode side to facilitate the oxidant transport. The transient characteristics of the temperature, methanol concentration, methanol crossover, useful current density and methanol evaporation are investigated. The results indicate that the temperature in the fuel cell increases during operation as much as 10 °C, due to the heat generation by internal phase change and the electrochemical reactions. However, it is revealed that the temperature distribution is nearly uniform at any time through all porous layers including the fuel cell and fuel delivery system. The effect of using an active feeding system in the cathode and passive methanol feeding in the anode (semi passive system) on the performance of a fuel cell is also studied. The active oxidant feeding to the cathode catalyst layer in the semi passive cell improved the fuel cell performance compared to that in a passive one. However, in general, the performance of passive cell is better than that in a semi passive one because of more temperature increase in the passive system.     

A novel technique for measuring current distributions in PEM fuel cells

A novel and simple technique was developed to measure current distribution in PEM fuel cells with serpentine flow fields. In this technique, a specially designed measuring gasket was inserted between the flow field plate and the gas diffusion layer, and the current at each sub-area of the fuel cell was measured by each of the current collecting strips on the measuring gasket. The current distribution measurement gasket was independent of PEM fuel cells, and can be used in any fuel cell without the need of a special fuel cell or modification of any component of an existing fuel cell. More importantly, this technique can be easily used to measure current density distribution in any cell or every cell in a fuel cell stack. In addition, this technique is very inexpensive, with the only additional cost being that of the measuring gasket. In this work, this measurement gasket technique was used to study the influences of humidification temperatures, cell operating temperatures, reactant flow rates, and operating pressures on current distributions in a PEM fuel cell. Local membrane hydration, reactant depletion and possible cathode flooding can be deduced from the measurement results, and some potential improvements in fuel cell designs are suggested.     

Current density distribution in air-breathing microfluidic fuel cells with an array of graphite rod anodes

Scale-up is required in the practical application of microfluidic fuel cells. Using an array of electrodes is demonstrated as a promising way. However, the non-uniform current density distribution in array anodes will significantly limit the power output. In this study, current density distribution in air-breathing microfluidic fuel cells with an array of graphite rod anodes is tested under acidic and alkaline conditions. The array anode is divided into four layers according to their distance to cathode. Current density of each layer is recorded individually. The cell performance under alkaline media is better than that under acidic media and various current density distributions are found under different media. When the air-breathing microfluidic fuel cell is operated under acidic media, at current densities lower than 50 mA cm^?3, current densities of two-layer anodes far from the cathode are higher than that of the other layers, while the reverse happens at current densities higher than 50 mA cm^?3. This is mainly due to the enhanced fuel transport caused by CO₂ bubbles and the lower ohmic resistance. Moreover, the generated CO₂ bubbles lead to fluctuation of discharging densities especially at low voltages. However, for the air-breathing microfluidic fuel cell operated under alkaline media, two-layer anodes far from the cathode are main contributor to the total current density at all operation voltages.     

Two-dimensional modeling for physical processes in direct flame fuel cells

Commercial direct flame fuel cells (DFFC) need larger cell surface area for higher power output. In such cases, multi-dimensional effects play significant roles on cell performances. In this work, a two-dimensional numerical model is developed to illustrate physical behaviors associated with the multi-dimensional effects in DFFCs. It is revealed that DFFCs suffer from the negative consequences of non-uniform distributions of temperature, species and voltage in radial direction. Non-uniform distributions of temperature and species results in the decrease of current density at edge regions of DFFCs, owing to lower ionic conductivities and fuel species concentration. And the non-uniform voltage distribution in radial direction causes the decreases of current density at center regions of DFFCs due to the lower over-potential there. Therefore, current density distributions in electrolytes are likely to be M-shaped. The multi-dimensional effects become progressively important with increasing the size of solid oxide fuel cells. Comparing with the DFFC with a SOFC with small cell radius (6.5 mm), a DFFC with a SOFC with large cell radius (33.75 mm) has 25–30% lower maximum power density. We also reveal that cross-over electronic currents in samaria-doped-ceria electrolytes and fuel species starvation due to the secondary oxidation are dominant factors on the cell performance loss at high cell temperatures (∼1000 K).     

Performance improvement in direct methanol fuel cells using a highly porous corrosion-resisting stainless steel flow field

Power generation with direct methanol fuel cell (DMFC) systems requires only simple equipment, and has the important advantage of using a liquid fuel with higher energy density and easier handling characteristics than hydrogen. However, the power output of DMFC is lower than hydrogen fuel cells. To improve the power output of DMFC it is very important to reduce diffusion polarization at higher current density conditions. This research used a corrosion-resisting type porous stainless steel developed based on the technology for metal–hydride battery electrodes in the separator flow fields for reactants and products in a single cell DMFC and analyzed its influence on performance characteristics.     

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

Modeling plays a very important role in the development of fuel cells and fuel cell systems. The aim of this work is to investigate the electrochemical processes of a Solid Oxide Fuel Cell (SOFC) and to evaluate the performance of the proposed SOFC design. For this aim a three-dimensional Computational Fluid Dynamics (CFD) model has been developed for an anode-supported planar SOFC with corrugated bipolar plates serving as gas channels and current collector. The conservation of mass, momentum, energy and species is solved by using the commercial CFD code FLUENT in the developed model. The add-on FLUENT SOFC module is implemented for modeling the electrochemical reactions, loss mechanisms and related electric parameters throughout the cell. The distributions of temperature, flow velocity, pressure and gaseous (fuel and air) concentrations through the cell structure and gas channels is investigated. The relevant fuel cell variables such as the potential and current distribution over the cell and fuel utilization are calculated and studied. The modeling results indicate that, for the proposed SOFC design, reasonably uniform distributions of current density over the active cell area can be achieved. The geometry of the cathode gas channel has a substantial effect on the oxygen distribution and thus the overall cell performance. Methods for arriving at improved cell designs are discussed.     

Numerical simulation analysis of the performance on the PEMFC with a new flow field designed based on constructal-theory

The design of the flow field structure has an important impact on the performance of PEMFC. An excellent design of the flow field will optimize the gas-liquid distribution inside the fuel cell, and enhance the diffusion of the reactant gases while reducing problems such as water flooding or uneven mass transfer of reactants, thus improving the overall performance of the cell. A new form of flow field based on the design ideas of Constructal-theory and Murray's Law was proposed in this paper. In this study, the PEMFC with the new and conventional flow fields were compared under the same conditions, and it is proved that the cell with the new flow field has a more balanced performance on output power and global pressure drop in contrast with conventional flow fields. In this study, the output power density of the PEMFC with the new flow field increased by an average of 1.35% compared to the PEMFC with Parallel flow field and Single Channel Serpentine flow field, and the pressure drop was reduced by 47.67% and 90.06% respectively compared to the PEMFC with the Single Channel Serpentine flow field and Double Channel Serpentine flow field. Meanwhile, the distribution of current density characteristics in a PEMFC with the new flow field was investigated and optimization of its structure size is analyzed. The reason for its non-uniform distribution of current density was revealed in this study, and an improvement scheme was proposed to improve the uniformity of current density, and the results of structural optimization research will have a certain guiding effect on practical applications.     

Behavior of PEMFC in starvation

Starvation is a vivid word to describe the operation condition of a fuel cell in sub-stoichiometric reactants feeding. In starvation, a fuel cell could not present its best performance; moreover, there might also be safety issue because of cell reversal. In this paper, current density distributions of proton exchange membrane fuel cell (PEMFC) in hydrogen and air starvation were studied with a segmented single fuel cell. Experimental results show that the polarization curves of the overall cell are different for anode and cathode starvation. And the current density distribution results show that for anode starvation, current density of the starved region drops sharply to zero, while for cathode starvation, there is no zero current density region observed. Numerical simulations give similar results.     

Experimental evaluation and mathematical modeling of a direct alkaline fuel cell

The performance of a direct alkaline fuel cell (AFC) is studied separately using methanol, ethanol and sodium borohydride as fuel. Potassium hydroxide solution was used as an electrolyte. Pt-black and manganese dioxide catalyst were used to prepare the anode and cathode electrodes. Ni mesh was used as current collector. The direct alkaline fuel cell was constructed with the prepared anode and cathode electrodes and Ni mesh. The current density–cell voltage characteristics of the fuel cell were determined by varying load and at different experimental conditions, e.g., electrolyte concentration, fuel concentration and temperature. The fuel cell performance increases initially with the increase in electrolyte (KOH) concentration and then decreases with further increase of the same. The cell performance increases initially and then no appreciable improvement noticed with the increase in fuel concentration. The performance of the fuel cell increases with increase in temperature in general with the exception to NaBH₄ alkaline fuel cell. A mathematical model for the direct alkaline fuel cell is developed based on reaction mechanism available in the literature to predict the cell voltage at a given current density. The model takes into account activation, ohmic, concentration overpotentials and other losses. The model prediction is in fair agreement with the experimental data on current–voltage characteristics and captures the influence of different experimental conditions on current–voltage characteristics.     

Two-dimensional dynamic simulation of a direct internal reforming solid oxide fuel cell

This study presents a two-dimensional mathematical model of a direct internal reforming solid oxide fuel cell (DIR-SOFC) stack which is based on the reforming reaction kinetics, electrochemical model and principles of mass and heat transfer. To stimulate the model and investigate the steady and dynamic performances of the DIR-SOFC stack, we employ a computational approach and several cases are used including standard conditions, and step changes in fuel flow rate, air flow rate and stack voltage. The temperature distribution, current density distribution, gas species molar fraction distributions and dynamic simulation for a cross-flow DIR-SOFC are presented and discussed. The results show that the dynamic responses are different at each point in the stack. The temperature gradients as well as the current density gradients are large in the stack, which should be considered when designing a stack. Further, a moderate increase in the fuel flow rate improves the performances of the stack. A decrease in the air flow rate can raise the stack temperature and increase fuel and oxygen utilizations. An increased output voltage reduces the current density and gas utilizations, resulting in a decrease in the temperature.     

Performance analysis of a proton exchange membrane fuel cell using tree-shaped designs for flow distribution

The performance of a proton exchange membrane (PEM) fuel cell is directly associated to the flow channels design embedded in the bipolar plates. The flow field within a fuel cell must provide efficient mass transport with a reduced pressure drop through the flow channels in order to obtain a uniform current distribution and a high power density. In this investigation, three-dimensional fuel cell models are analyzed using computational fluid dynamics (CFD). The proposed flow fields are radially designed tree-shaped geometries that connect the center flow inlet to the perimeter of the fuel cell plate. Three flow geometries having different levels of bifurcation were investigated as flow channels for PEM fuel cells. The performance of the fuel cells is reported in polarization and power curves, and compared with that of fuel cells using conventional flow patterns such as serpentine and parallel channels. Results from the flow analysis indicate that tree-shaped flow patterns can provide a relatively low pressure drop as well as a uniform flow distribution. It was found that as the number of bifurcation levels increases, a larger active area can be utilized in order to generate higher power and current densities from the fuel cell with a negligible increase in pumping power.     

Composition and performance modelling of catalyst layer in a proton exchange membrane fuel cell

The composition and performance optimisation of cathode catalyst platinum and catalyst layer structure in a proton exchange membrane fuel cell has been investigated by including both electrochemical reaction and mass transport process. It is found that electrochemical reactions occur in a thin layer within a few micrometers thick, indicating ineffective catalyst utilization for the present catalyst layer design. The effective use of platinum catalyst decreases with increasing current density, hence lower loadings of platinum are feasible for higher current densities of practical interest without adverse effect on cell performance. The optimal void fraction for the catalyst layer is about 60% and fairly independent of current density, and a 40% supported platinum catalyst yields the best performance amongst various supported catalysts investigated. An optimal amount of membrane content in the void region of the catalyst layer exists for minimum cathode voltage losses due to competition between proton migration through the membrane and oxygen transfer in the void region. The present results will be useful for practical fuel cell designs.     

Dynamic modeling of the effect of water management on polymer electrolyte fuel cells performance

Water management in Polymer Electrolyte Fuel Cell (PEFC) is a key factor in fuel cell performance, and it is an important contributor to the proton exchange membrane durability. Water droplet accumulation in the channel causes non-uniform distribution of gas pressure and spatial inhomogeneity of the local current density in potentiostatic mode. These spatial and temporal fluctuations in the operating conditions imply unequal use of the membrane surface and the catalyst layer, producing uneven degradation and aging of the Membrane Electrode Assembly (MEA). In order to study the dynamic and spatial performance of the fuel cell, a three-level model has been developed. The model is composed of a two-phase, where steam and liquid water drops movement are considered in the channel model; liquid water and gas diffusion are considered in Gas Diffusion Layers (GDLs) model; and finally, the electrochemical reactions are represented in the electrochemical model. The complete model provides a wider understanding of the effect of water on PEFCs and allows to analyze the local current density and the water distribution in response to experimental set-up parameters such as anode and cathode gas flows, total current or channel geometries. The model has been validated using neutron images and segmented cells technique to evaluate the spatial distribution of liquid water and current density in the cell. The developed model and the simulation procedure proposed in this paper allow obtaining long-term dynamic simulations with low computational effort.