Experimental analysis of internal gas flow configurations for a polymer electrolyte membrane fuel cell stack

The internal gas distribution system utilised for supplying fresh reactants and removing reaction products from the individual cells of a fuel cell stack can be designed in a parallel, a serial or a mixture of parallel and serial gas flow configuration. In order to investigate the interdependence between the internal stack gas distribution configuration and single cell as well as overall stack performance, a small laboratory-scale fuel cell stack consisting of identical unit cells was subject to operation with different gas distribution configurations and different operating parameters. The current/voltage characteristics measured with the different gas distribution configurations are analysed and compared on unit cell- as well as on stack-level. The results show the significant impact of the internal stack gas distribution system on operation and performance of the individual unit cells and the overall stack.     

Effect of cell compression on the performance of a non–hot-pressed MEA for PEMFC

In this study, the effect of cell compression on the performance of a non–hot-pressed membrane electrode assembly (MEA) for a polymer electrolyte membrane fuel cell (PEMFC) is presented. The MEA is made without hot pressing, by carefully placing the gas diffusion electrodes (GDEs) and a membrane in a fuel cell fixture. Cell performance is assessed at five different compression ratios between 3.6% and 47.8%. It has been shown that ohmic resistance of the cell, mass transport resistance of reactants, charge transfer resistance at electrode, and overall cell performance are strongly dependent on the cell compression. On increasing the cell compression gradually, cell performance improves initially, reaches the best, and then deteriorates. The cell performance is assessed at fully humidified condition and at dry condition. Optimum cell performances are obtained at compression ratios of 14.2% and 25.7% for 100% relative humidity (RH) and 50% RH, respectively. It is also found that the cell with proper compression and at fully humidified conditions can deliver similar performance to a conventional hot-pressed MEA. Finally, it is shown that after the tests, GDEs can be peeled out, and the membrane inspection can be done as a postexperimental analysis.     

High temperature electrolyzer based on solid oxide co-ionic electrolyte: A theoretical model

In the present work a theoretical model of a solid oxide electrolyzer based on an electrolyte having both oxygen ion and proton conductivity is considered. The main parameters of the electrolytic process and an electrolyzer (distribution of gas components, electromotive forces and current densities along the electrolyzer channel, average values of electromotive forces and current densities) were calculated depending on a proton transport number and mode of the reactants’ feeding (co- and counter-flow). The performed analysis demonstrates considerable influence of the mode of feeding on all parameters of the electrolyzer: operation under the counter-flow mode is preferable as regards the specific characteristics and uniformity of their distribution along the electrolyzer. It is shown that the electrolyser's specific characteristics increase with the increase of the proton transport number.     

An error in solid oxide fuel cell stack modeling

This paper points out an error in the literature and analyzes its effect on electrochemical models of solid oxide fuel cell stacks. A correction is presented.     

Pressurized solid oxide fuel cells: Experimental studies and modeling

The hybrid power plant project at DLR aims at investigating the fundamentals and requirements of a combined fuel cell and gas turbine power plant. A specific aim is to demonstrate stable operation of a plant in the 50 kW class. Prerequisite for the power plant realization is the detailed characterization of each subsystem and their interactions. The pressurized solid oxide fuel cell (SOFC) is an essential part of one main subsystem. A combined theoretical and experimental approach allows a thorough insight into nonlinear behavior. This paper focuses on the influence of pressurization on SOFC performance in the range from 1.4 to 3 bar. Conclusions are based on experimental V(i)-characteristics as well as on overpotentials derived from elementary kinetic models. Experiments are performed on planar, anode-supported 5-cell short stacks. The performance increases from 284 mW cm⁻² at 1.4 bar to 307 mW cm⁻² at 2 bar and 323 mW cm⁻² at 3 bar (at 0.9 V; anode: H₂/N₂ 1/1; cathode: air; temperature: 800 °C). The benefit of a temperature rise increases at elevated pressures. Moreover, the effect of gas variation is enhanced at higher pressures. The main conclusion is that pressurization improves the performance. Due to different effects interfering, operation of pressurized SOFC requires further detailed analysis.     

On the nanostructuring and catalytic promotion of intermediate temperature solid oxide fuel cell (IT-SOFC) cathodes

Solid oxide fuel cells (SOFCs) are highly efficient energy converters for both stationary and mobile purposes. However, their market introduction still demands the reduction of manufacture costs and one possible way to reach this goal is the decrease of the operating temperatures, which entails the improvement of the cathode electrocatalytic properties. An ideal cathode material may have mixed ionic and electronic conductivity as well as proper catalytic properties. Nanostructuring and catalytic promotion of mixed conducting perovskites (e.g. La_0.58Sr_0.4Fe_0.8Co_0.2O_3−δ) seem to be promising approaches to overcoming cathode polarization problems and are briefly illustrated here. The preparation of nanostructured cathodes with relatively high surface area and enough thermal stability enables to improve the oxygen exchange rate and therefore the overall SOFC performance. A similar effect was obtained by catalytic promoting the perovskite surface, allowing decoupling the catalytic and ionic-transport properties in the cathode design. Noble metal incorporation may improve the reversibility of the reduction cycles involved in the oxygen reduction. Under the cathode oxidizing conditions, Pd seems to be partially dissolved in the perovskite structure and as a result very well dispersed.     

Numerical analysis of the manipulated high performance catalyst layer design for polymer electrolyte membrane fuel cell

A two‐dimensional, multiphase, non‐isothermal numerical model was used to investigate the effect of the high performance catalyst layer (CL) design. Microstructure‐related parameters were studied on the basis of the agglomerate model assumption. A conventional CL design (uniform Pt/C composition, e.g., 40 wt%) was modified into two sub‐layers with two different Pt/C compositions (in this study, 40 and 80 wt%). The performance of sub‐layers with different CL designs is shown to be different. Simulation results show that substituting part of the Pt/C 40 wt% with Pt/C 80 wt% increases the cell performance. It was found that factors including proton conductivity, open circuit voltage, and sub‐layer thickness have a significant impact on overall cell performance. Different water distribution for different membrane electrode assembly designs was also observed in the simulation results. More liquid water accumulation inside the membrane electrode assembly is seen when the Pt/C 80 wt% sub‐layer is next to the gas diffusion layer. Finally, several key design parameters for the proposed high performance CL design including agglomerate radius, Nafion thin film thickness, and the Nafion volume fraction within the agglomerate in terms of CL fabrication were identified on the basis of our simulation results. Copyright © 2014 John Wiley & Sons, Ltd.     

Electrochemical evaluation of mixed ionic electronic perovskite cathode LaNi1-xCoxO3-δ for IT-SOFC synthesized by high temperature decomposition

The cobalt doped perovskite cathode material LaNi_1-xCo_xO_3-δ (x = 0.4, 0.6, 0.8) synthesized by cost effective high temperature decomposition is investigated as mixed ionic electronic conductor (MIEC) for intermediate temperature solid oxide fuel cell (IT-SOFC). LaNiO₃ is known for its high electronic conductivity and to introduce more oxygen vacancies for enhancing its ionic conductivity, Ni at B site is substituted by Co. XRD analysis showed perovskite structure for all samples with no additional phases, which was also confirmed by FTIR results. Microstructure analysis revealed well connected and porous structure for LaNi_1-xCo_xO_3-δ (x = 0.6) compared to other compositions. The elemental analysis using EDX confirmed presence of lanthanum, nickel, and cobalt within all samples. No prominent weight loss was observed during TGA analysis. The highest value of conductivity was obtained for LaNi_1-xCo_xO_3-δ (x = 0.6) due to its porous and networked structure of sub micrometric grains. The superior performance is attained for the cell based on LaNi_1-xCo_xO_3-δ (x = 0.6) cathode with maximum power density of 0.45 Wcm^?2 compared to other composition which can be attributed to its well connected and porous structure that caused enhanced electrochemical reaction at triple phase boundary (TPB). It was therefore deduced that LaNi_1-xCo_xO_3-δ (x = 0.6) is promising composition to be used as MIEC cathode for IT-SOFC.     

Improvement of fuel cell performances through the enhanced dispersion of the PTFE binder in electrodes for use in high temperature polymer electrolyte membrane fuel cells

In high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), it is important that the structure of the electrode catalyst layer is formed uniformly. To achieve this, the binder must be well dispersed; however, polytetrafluoroethylene (PTFE), which is commonly employed in the preparation of HT-PEMFCs, is difficult to disperse during electrode manufacture due to its high hydrophobicity. In this study, we fabricate electrodes containing a surfactant to improve the dispersion of the PTFE binder and to enhance reproducibility during electrode manufacture. The electrodes are commonly prepared via a bar coating method, which is known to exhibit poor dispersion due to the small amounts of solvent employed compared to the spraying method. We then compare the properties of the obtained electrodes prepared in the presence and absence of the surfactant through physical and electrochemical characterization. It is found that the electrode containing the surfactant is structurally superior, and its single cell performance is significantly higher (i.e., 0.65 V at 0.2 Am cm⁻²). The single cells are suitable for operation at 150 °C using H₂/air at atmospheric pressure and a total platinum loading of 2.0 mg cm⁻².     

Exergy analysis of the diesel pre-reforming solid oxide fuel cell system with anode off-gas recycling in the SchIBZ project. Part I: Modeling and validation

Solid oxide fuel cell (SOFC) systems with anode off-gas recirculation (AGR) and diesel pre-reforming are advantageous because they can operate with the current fuel infrastructure. In the SchIBZ-project, the prototype of such a SOFC system for maritime applications has already been commissioned. In this first paper, we model the system devices to conduct an exergy analysis of this real SOFC plant and validate them with experimental values from experiments in laboratory scale. The results of our simulation agree well with the experimental values. The calculations with the validated results may be closer to the real thermodynamic behavior of such system components than previous literature.     

Design and fabrication of stair‐step‐type electrolyte structure for solid oxide fuel cells

The design and the fabrication of novel stair‐step electrolyte based on yttria stabilized zirconia are presented. The novel electrolyte has gradually reduced oxide ion transport paths achieved by the stair‐step design. The mechanical and electrochemical performance of the novel electrolyte are investigated and compared to those of standard electrolyte support. Three‐point bending tests indicate that the fracture displacement and force measured for the novel electrolyte are 11% and 32% less than those of the standard electrolyte support, respectively. However, the cell based on the novel electrolyte exhibits 40% higher electrochemical performance than the standard electrolyte supported cell at an operation temperature of 700 °C. Impedance analyses revealed that the enhanced cell performance is mainly due to the decrease in the ohmic resistance of the cell achieved by the novel electrolyte design. In addition, the electrode resistances are found to be decreased due to the increased electrochemical reaction zones since the contact area between the novel electrolyte and both electrodes are increased by the novel electrolyte design. Moreover, the cell with novel electrolyte produced 0.47 Wcm^?2 peak power at 750 °C while the standard electrolyte supported cell shows almost the same power output at around 800 °C. Thus, novel designed electrolyte also offers some amount of reduction in the operation temperature of solid oxide fuel cells. Copyright © 2012 John Wiley & Sons, Ltd.     

Numerical modeling and parametric analysis of solid oxide fuel cell button cell testing process

In laboratory studies of solid oxide fuel cell (SOFC), performance testing is commonly conducted upon button cells because of easy implementation and low cost. However, the comparison of SOFC performance testing results from different labs is difficult because of the different testing procedures and configurations used. In this paper, the SOFC button cell testing process is simulated. A 2‐D numerical model considering the electron/ion/gas transport and electrochemical reactions inside the porous electrodes is established, based on which the effects of different structural parameters and configurations on SOFC performance testing results are analyzed. Results show that the vertical distance (H) between the anode surface and the inlet of the anode gas channel is the most affecting structure parameter of the testing device, which can lead to up to 18% performance deviation and thus needs to be carefully controlled in SOFC button cell testing process. In addition, the current collection method and the configuration of gas tubes should be guaranteed to be the same for a reasonable and accurate comparison between different testing results. This work would be helpful for the standardization of SOFC button cell testing.     

Performance analysis of a solid oxide fuel cell with reformed natural gas fuel

In the present study a two‐dimensional model of a tubular solid oxide fuel cell operating in a stack is presented. The model analyzes electrochemistry, momentum, heat and mass transfers inside the cell. Internal steam reforming of the reformed natural gas is considered for hydrogen production and Gibbs energy minimization method is used to calculate the fuel equilibrium species concentrations. The conservation equations for energy, mass, momentum and voltage are solved simultaneously using appropriate numerical techniques. The heat radiation between the preheater and cathode surface is incorporated into the model and local heat transfer coefficients are determined throughout the anode and cathode channels. The developed model has been compared with the experimental and numerical data available in literature. The model is used to study the effect of various operating parameters such as excess air, operating pressure and air inlet temperature and the results are discussed in detail. The results show that a more uniform temperature distribution can be achieved along the cell at higher air‐flow rates and operating pressures and the cell output voltage is enhanced. It is expected that the proposed model can be used as a design tool for SOFC stack in practical applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Improving open-cathode polymer electrolyte membrane fuel cell performance using multi-hole separators

We developed a new separator with a multi-hole structure (MHS) in the rib region for open-cathode polymer electrolyte membrane fuel cell (OC-PEMFC) stack to improve performance. The electrochemical current–voltage performance results clearly demonstrate that the performance of the OC-PEMFC stack using the MHS design was higher than that using the conventional parallel design at high current regions (i.e., over 7 A). The current increased by 11.24% at 12 V (i.e., 0.6 V/cell). The effects of supplying additional oxygen and removing generated water were identified as factors improving the performance. The individual cell voltages demonstrate that the initial value of standard deviation for the OC-PEMFC stack using MHS was somewhat high, but it exhibited better uniformity at higher current regions.     

An ion-plasma technique for formation of anode-supported thin electrolyte films for IT-SOFC applications

This paper describes a preparation method and structural and electrochemical properties of a thin bilayer anode-electrolyte structure for a solid oxide fuel cell operating at intermediate temperatures (IT-SOFC). Thin anode-supported yttria-stabilized zirconia electrolyte films were prepared by reactive magnetron sputtering of a Zr-Y target in an Ar-O₂ atmosphere. Porous anode surfaces of IT-SOFCs were modified by a pulsed low-energy high-current electron beam prior to film deposition; the influence of this pretreatment on the performance of both the deposited films and a single cell was investigated. The optimal conditions of the pulsed electron beam pretreatment were obtained. For the electrolyte thickness about 2.5 μm and the value of gas permeability of the anode/electrolyte structure 1.01 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹, the maximum power density achieved for a single cell at 800 °C and 650 °C was found to be 620 and 220 mW cm⁻² in air, respectively.     

Increasing the efficiency of a portable PEM fuel cell by altering the cathode channel geometry: A numerical and experimental study

Portable fuel cells are receiving great attention today mainly because their energy density is higher than any portable battery solution. Among other types, portable polymer electrolyte membrane (PEM) fuel cells are an established technology where research on increasing their efficiency is leading product development and manufacturing. The objective of this work was to study and evaluate the redesign of a commercial portable fuel cell, improving its efficiency. A three-dimensional model of the original PEM fuel cell with parallel plus a transversal flow channel design was developed using Comsol Multiphysics, including the effects of liquid water formation and electric current production. Using this model, the effects of different channel geometries and respective cathode flow rates on the cell’s performance, including the local transport characteristics, were studied. Laboratory tests with various fuel cell stacks using the new channels structure were effectuated for an evaluation of the fuel cell’s performance, showing improvements in its efficiency of up to 26.4%.     

Heat and mass transfer analysis in single cell of PEFC using different PEM and GDL at higher temperature

According to the H₂ and fuel cell road map in Japan, the target operating temperature of polymer electrolyte fuel cell (PEFC) should be 90 °C from 2020 to 2025. In this study, the impact of polymer electrolyte membrane (PEM) and gas diffusion layer (GDL)'s thickness on heat and mass transfer characteristics as well as power generation performance of PEFC is investigated at operating temperature of 90 °C. The in-plane temperature distributions on anode and cathode separator are also measured using thermograph. As a result, it is observed that the increase in power from 1 W to 5 W at the current density of 0.80 A/cm² as well as even temperature distribution within 1 °C can be obtained at operating temperature of 90 °C by decrease in GDL's thickness from 190 μm to 110 μm. In addition, the power is increased from 3 W to 4 W at the current density of 0.80 A/cm² operated at 90 °C by decrease in the PEM's thickness from 127 μm to 25 μm.     

The properties and performance of micro-tubular (less than 2.0 mm O.D.) anode suported solid oxide fuel cell (SOFC)

Tubular solid oxide fuel cells (SOFCs) have many desirable advantages compared to other SOFC applications. Recently, micro-tubular SOFCs were studied to apply them into APU systems for future vehicles. In this study, electrochemical properties of the micro-tubular SOFCs (1.6 mm O.D.) have been characterized. Electrochemical analysis showed excellent performance with a maximum power density of 1.3 W/cm² at 550 °C. The impedance information gained at cell operating temperatures of 450, 500, and 550 °C showed individual cell ohmic resistances of 1.0, 0.6, and 0.2 Ω respectively. Within the operating temperature range of 450-550 °C, the ceria based micro-tubular SOFCs (cathode length: 8 mm) were found to have power densities ranging between 0.263 and 1.310 W/cm². The mechanical properties of the tubes were also analyzed through internal burst testing and monotonic compressive loading on a c-ring test specimen. The two testing techniques are compared and related, and maximum hoop stress values are reported for each of the fabrication parameters. This study showed feasible electrochemical properties and mechanical strength of micro-tubular SOFC for APU applications.