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.     

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.     

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.     

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.     

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.     

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.     

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.     

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%.     

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.     

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.     

Preparation and electrochemical properties of a Sm2−xSrxNiO4 cathode for an IT-SOFC

Cathodic materials Sm_2−xSr_xNiO₄ (0.5 ≤ x ≤ 1.0) for an IT-SOFC (intermediate temperature solid oxide fuel cell) were prepared by the glycine-nitrate process and characterized by XRD, SEM, ac impedance spectroscopy and dc polarization measurements. The results showed that no reaction occurred between the Sm_2−xSr_xNiO₄ electrode and the Ce_0.9Gd_0.1O_1.9 (CGO) electrolyte at 1100 °C, and the electrode formed good contact with the electrolyte after sintering at 1000 °C for 2 h. The electrochemical properties of these cathode materials were studied using impedance spectroscopy at various temperatures and oxygen partial pressures. Sm_1.0Sr_1.0NiO₄ exhibited the lowest cathodic overpotential. The area specific resistance (ASR) was 3.06 Ω cm² at 700 °C in air.     

An easy parameter estimation procedure for modeling a HT-PEMFC

Fuel cells are a very complex system in which many phenomena of different nature occur simultaneously and within a small space, so a truthful measurement of some variables is not feasible using state-of-the-art technology. If a deep knowledge of the unit is desired, modeling can be of great help when it is properly used as it is possible to calculate the value of the variables of interest by adjusting experimental data. However, when models are complicated, it is not trivial to identify in which way a certain parameter alters the model results and then it is necessary to resort to sensitivity analysis before approaching an adequate parameter estimation procedure. In this work, a parameter estimation procedure has been proposed with the results obtained from the sensitivity analysis applied to a high temperature proton exchange membrane fuel cell (HT-PEMFC) model. The procedure has been demonstrated to be straightforwardly applicable as well as effective, which makes it suitable to be used as engineering tool to obtain a first consistent value of the parameters that define the main characteristics of a HT-PEMFC.     

Operation of a proton exchange membrane fuel cell under non-humidified conditions using a membrane–electrode assemblies with composite membrane and electrode

Composite membranes with hydrophilic substances can retain water and allow the operation of proton exchange membrane fuel cells (PEMFCs) under non-humidified conditions. In this work, thin Nafion composite membranes with silica are prepared to operate a PEMFC with dry fuel and oxidant. In addition, the role of silica in the catalyst layer as a water retainer is studied. In particular, the anode and the cathode are modified separately to elucidate the effect of silica. The incorporation of silica in the membrane and the catalyst layer enhances single-cell performance under non-humidified operation. The cell performance of membrane–electrode assemblies using the composite membrane and electrode is higher than that of a MEA using commercial Nafion 111 membrane under non-humidified conditions.     

State-of-health diagnosis based on hamming neural network using output voltage pattern recognition for a PEM fuel cell

This work investigates a pattern recognition-based diagnosis approach as an application of the Hamming neural network to the identification of suitable fuel cell model parameters, which aim to diagnose state-of-health (SOH) for a polymer electrolyte membrane (PEM) fuel cell. The fuel cell output voltage (FCOV) patterns of the 20 PEM fuel cells were measured, together with the model parameters, as representative patterns. Through statistical analysis of the FCOV patterns for 20 single cells, the Hamming neural network is applied for identification of the representative FCOV pattern that matches most closely of the pattern of the arbitrary cell to be measured. Considering the equivalent circuit fuel cell model, the purpose is to select a representative loss ΔR_d, defined as the sum of two losses (activation and concentration losses). Consequently, the selected cell’s ΔR_d is properly applied to diagnose SOH of an arbitrary cell through the comparison with those of fully fresh and aged cells with the minimum and maximum of the ΔR_d in experimental cell group, respectively. This avoids the need for repeated parameter measurement. Therefore, these results could lead to interesting perspectives for diagnostic fuel cell SOH.     

Computational fluid dynamics modeling of a solid oxide electrolyzer cell for hydrogen production

A 2D computational fluid dynamics (CFD) model was developed to study the performance of a planar solid oxide electrolyzer cell (SOEC) for hydrogen production. The governing equations for mass continuity, momentum conservation, energy conservation and species conservation were discretized with the finite volume method (FVM). The coupling of velocity and pressure was treated with the SIMPLEC (Semi-Implicit Method for Pressure Linked Equations – Consistent) algorithm. Simulations were performed to investigate the effects of operating/structural parameters on heat/mass transfer and the electric characteristics of a planar SOEC. It is found that the gas velocity at the cathode increases significantly along the main flow channel, as the increase in H₂ molar fraction decreases the density and viscosity of the gas mixture at the cathode. It is also found that increasing the inlet gas velocity can enhance the SOEC performance. Another important finding is that the electrode porosity has small effect on SOEC performance. The results of this paper provide better understanding on the coupled heat/mass transfer and electrochemical reaction phenomena in an SOEC. The model developed can serve as a useful tool for SOEC design optimization.     

Quantifying multi-ionic conduction through doped ceria-carbonate composite electrolyte by a current-interruption technique and product analysis

A composite electrolyte consisting of a samarium doped ceria and a binary eutectic carbonate phase is investigated in this work. It has been found that O²⁻/H⁺ conductions take place when H₂ and O₂ used as the reactants. The presence of CO₂ in the cathode gas leads to the appearance of CO₃²⁻ conduction. The overall conductivity of the composite electrolyte is measured with a current-interruption technique and the ions transferred by O²⁻/H⁺/CO₃²⁻ respectively are obtained by a quantitative measurement of the reaction products, i.e. H₂O and CO₂. The change of the carbonate content in the composite electrolyte presents a great influence on the conductivity of each ion. According to these experimental facts, the pathways for the individual ionic conductions are proposed.