Degradation tests of two phosphoric acid (PA) doped PBI membrane based HT-PEM fuel cells were reported in this paper to investigate the effects of start/stop and the presence of methanol in the fuel to the performance degradation. Continuous tests with H 2 and simulated reformate which was composed of H 2 , water steam and methanol as the fuel were performed on both single cells. 12-h-startup/12-h-shutdown dynamic tests were performed on the first single cell with pure dry H 2 as the fuel and on the second single cell with simulated reformate as the fuel. Along with the tests electrochemical techniques such as polarization curves and electrochemical impedance spectroscopy (EIS) were employed to study the degradation mechanisms of the fuel cells. Both single cells showed an increase in the performance in the H 2 continuous tests, because of a decrease in the ORR kinetic resistance probably due to the redistribution of PA between the membrane and electrodes. EIS measurement of first fuel cell during the start/stop test showed that the mass transfer resistance and ohmic resistance increased which can be attributed to the corrosion of carbon support in the catalyst layer and degradation of the PBI membrane. During the continuous test with simulated reformate as the fuel the ORR kinetic resistance and mass transfer resistance of both single cells increased. The performance of the second single cell experienced a slight decrease during the start/stop test with simulated reformate as the fuel.

Direct methanol fuel cells (DMFC) are becoming a choice of a power source in the field of power electronics, and portable devices because of their high energy density. The benefits of using a fuel cell towards the environment will be enhanced if the fuel used for its application comes from renewable sources such as ethanol. A method of modeling of the performance of DMFC was developed and validated with the experimental data obtained from a passive DMFC operated under varying methanol and ethanol concentrations. Impedance spectroscopy was employed to measure ohmic, activation and mass transport losses for all concentrations. Improved performance of the cell was observed when the concentrations of the solutions were closer to stoichiometric values. The model predicted results were compared to the corresponding experimental values and found satisfactory.

This paper presents results from an investigation concerning load-induced degradation, recovery, and control of solid oxide fuel cells (SOFCs). In this study, commercially available SOFCs were subject to extended over-current conditions, followed by periods of open-circuit operation. During times of current loading, degradation was observed in the cells’ electrical performance through polarization and electrochemical impedance spectroscopy (EIS) measurements. These measurements showed an increase in the polarization curve’s ohmic region slope, i.e. large-signal resistance, as well as an increase in the cell’s small-signal low-frequency impedance. The degradation was temporary however, as the electrical performance recovered during times of open-circuit operation. These results, attributed to electrochemically-induced oxidation and reduction of nickel in the anode, suggest the degradation phenomenon is controllable via the electrical terminals. As such, an additional test was performed on an SOFC powering a pulse-width modulated load, with the load’s duty-cycle negatively proportional to the cell’s large-signal resistance. Polarization and EIS measurements taken during this test showed that despite the controlled load, degradation occurred throughout the test. However, post-test scanning electron microscope images revealed cracks in the cell’s cathode along the boundary between the active and bulk layers. This type of cracking was not observed in the original degradation and recovery tests, suggesting that the degradation observed in the controlled load test was irreversible and caused by a separate phenomenon.

The proton exchange membrane fuel cell (PEMFC) is a promising substitute for classic energy converter machines. However, there are still concerns regarding reliability and durability of PEMFCs, and hence, various modeling methods have been employed. One of the effective methods used for analysis and diagnosis of the fuel cell and other electrochemistry systems is Electrochemical Impedance Spectroscopy (EIS) which has been well established due to its high speed and precision. In this paper, the temperature sensitivity of the EIS results at different current densities obtained for a high temperature PEMFC are studied using electrochemical impedances reported in the literature. The magnitude of different elements in the proposed equivalent circuit for different temperatures and current densities, and root mean squares of deviation of various measurements are extracted. Then, the significance of the variations for different temperatures is examined via t-test. Finally, the minimum temperature change at different currents that can be captured by EIS is determined considering the root mean squares of measured values.

An experimental analysis of cell temperature effects on an air-breathing, PEM fuel cell is presented. The cell was tested in three active area sizes of 5 cm 2 , 10 cm 2 , and 25 cm 2 . The cell’s design minimized the influence of self-heating by using a large thermal body in its construction which conducted heat away from the active area. This allowed for the use of a heater and controller to test a constant cell temperature uninfluenced by current density. Polarization and electrochemical impedance spectroscopy testing showed that at higher current density, elevated temperature increased the buoyancy of the air around the cell which improved open cell performance. However, the opposite is true for lower current density as membrane dehydration becomes more prevalent at higher temperatures. Schlieren imaging, in conjunction with the polarization and EIS data, shows how heated and more buoyant air boosts cell performance. Infrared imaging identifies temperature gradients on the active surface which may hinder cell performance slightly.

Heterogeneous dielectric material systems and their interfaces which are engineered at the local level (atomic/nano/micro scale) present special challenges for analysis and interpretation of their macroscopic functional behavior in response to specific operational histories. In addition to the chemical composition of the material, the local morphology plays a definitive role in the determination of the transport, electrochemical, and mechanical functions of the heterogeneous constituents, and especially in their interaction and collective action. The basic science of the effect of morphology on polarization, resulting multi-physics properties, and their evolution processes (which determine durability and life) are less well understood. The present paper will focus on a small part of this broader problem. Specifically, the effect of porous microstructure on the broadband dielectric characteristics of the solid oxide fuel cell (SOFC) will be discussed. Different variations of microstructure (such as porosity, pore connectivity, distribution, geometry etc) of YSZ based SOFC materials will be studied using a broadband dielectric spectroscopy system at different temperatures. A finite element based meso-to-continuum model (mm scale model with micron scale features) will be developed to predict dielectric characteristics (voltage, current, impedance distribution) utilizing actual morphology (captured via a 3D X-ray tomography microscope). The paper will summarize all preliminary results from the ongoing research and provide a pathway towards a robust analysis that will help design better microstructures for SOFC electrodes.

Ni-YSZ cermet is commonly used as the anode of a solid oxide fuel cell (SOFC) because it has excellent electrochemical performance and is a cost effective anode material for coal-syngas fuel. However, coal-syngas contains trace contaminants, such as phosphine (PH 3 ), hydrogen sulfur (H 2 S), arsine (AsH 3 ) and stibine (SbH 3 ), and these can cause degradation of the SOFC. Ni-YSZ anode-supported SOFCs were exposed to syngas and H 2 fuel, while co-feeding PH 3 and chlorine Cl 2 impurities under a constant current load at 800°C. The cell degradation was postponed in syngas and highly mitigated in H 2 . In another test, a Ni-based filter was used to remove the PH 3 impurity. The results show that the filter can effectively remove 20 ppm PH 3 to a level which does not significantly degrade the SOFC over 400 h. The poisoning effects were evaluated by current-voltage scans and impedance spectroscopy, in addition to thermodynamic and chemical analyses. The post-mortem analyses of the cell and filter were performed by means of XRD and SEM/EDS.

This paper presents a novel architecture of an impedance spectrometer dedicated to the characterization and diagnostic of large Fuel Cell (FC) stacks operated in galvanostatic mode. The validation tests are first performed on a single Proton Exchange Membrane Fuel Cell (PEMFC). Then, experiments are carried out on a twenty-cell PEMFC stack delivering more significant power levels. The proposed impedancemeter allows spectrum measurements on cells located in the middle of the stack, where common mode potentials are usually too high for commercial devices. Moreover, the impedances of different individual cells in the stack are acquired with a synchronous measurement reference (global stack impedance). This capability allows distinguishing any singular cell behavior or drift effect of operational parameters (e.g. stack temperature and polarization current).

In this work the effect of the MEA preparation techniques on the performance of DMFC was evaluated using three different methods of electrocatalyst deposition: i) catalyst coated membrane; ii) catalyst coated carbon paper; and iii) decal deposition. Optimization of the nafion content (5–15 wt. %) at anode and cathode sides of the MEA and the pressure (150–500 atm) were also performed. Activities of both supported and unsupported Pt and PtRu catalysts (Johnson Matthew) were compared in room temperature DMFC (RT-DMFC) using polarization curves. All MEAs prepared were also characterized by electrochemical (cyclic voltammetry, impedance spectroscopy) methods. It was shown that optimal nafion content is 5–10 wt. % at both anode and cathode sides, while the optimal pressure is in the 300–500 atm. range. The unsupported catalysts showed slightly higher power density at RT-DMFC (∼ 14 mW/cm2) as compared to the supported ones (∼10 mW/cm2) at the same Pt load. Variation of the wetness of MEAs upon mounting in DMFC allowed us to increase of the power density of RT-DMFC up to 32 mW/cm 2 .

This paper presents test results regarding the poisoning effects of CO and CO 2 on H 3 PO 4 /Polybenzimidazole (PBI) membrane based high temperature proton exchange membrane fuel cell (HT-PEMFC). Electrochemical impedance spectroscopy (EIS), which is a non intrusive diagnostic tool for electrochemical systems, has been used to investigate these effects. A single cell test setup consisting of an electrically heated single cell assembly with a PEMEAS CELTEC P membrane electrode assembly (MEA) of an active area of 45cm 2 and mass flow controllers for Air, H 2 , CO and CO 2 was constructed in the laboratory. All operational parameters as well as data acquisition are controlled by two LabView programs, running on two separate computers. The impedance spectrum of the fuel cell is recorded at different operating points and then an Equivalent Circuit (EC), proposed for modelling the cell impedance, is fitted to the spectrum in order to analyze and quantify the impact of the individual factors on HT-PEMFC performance. Results showed that CO poisoning has an effect on all the losses monitored. Intermediate frequency resistances showed higher increase with increasing contamination and decreasing temperature than high frequency resistances, which is attributable to the adsorption of CO on Pt catalyst.

Tunable diode laser absorption spectroscopy (TDLAS) is used to measure the local ac response of the water partial pressure to a sinusoidal current excitation during locally resolved impedance measurements in a sub-saturated polymer electrolyte fuel cell (PEFC). The experimental results, along with the locally resolved impedance data are compared to calculated data as obtained from a 1+1 dimensional ac impedance model. The detailed experimental data allows model validation and in situ parameter determination. The model results are in good agreement with the experimental results. This emphasizes the importance to consider the build-up of water partial pressure oscillations along the gas channels in ac impedance models for sub-saturated PEFCs. The results show that minor modifications of literature membrane parameters are needed for a good fit to the experimental data set.

Microjets have been implemented into a PEM fuel cell in an attempt to achieve even distribution of reactants and passive cooling of the fuel cell unit and a preliminary investigation of this application has yielded positive results. Unlike conventional reactant supply, the reactant microjets impinge on the back side of the flow field plate before wrapping around and traversing the flow field channels where they are involved in electrochemical activity. However, it was observed that the fuel cell was subject to significant flooding, which limited its continuous operation. This paper discusses the results from an experimental study that has been carried out in an attempt to reduce or eliminate the occurrence of flooding through the use of two different flow-field configurations. Measurements of these flow field configurations, identified as MJFC I and MJFC II, are presented here. MJFC I, the first generation prototype, uses a variant of parallel flow field while MJFC II, the second generation prototype, uses multiple independent serpentine flow field channels. For similar operating conditions, characteristic curves obtained showed that MJFC I is susceptible to flooding while MJFC II has superior water management capability. Secondary tests using Electrode Impedance Spectroscopy confirm these findings.

Electrochemical Impedance Spectroscopy (EIS) is a powerful and proven tool for analyzing AC impedance response. A conventional three electrode EIS method was used to perform the investigation in the present study. Saturated potassium chloride solution was used as the electrolyte and three different material rods were used as working electrodes. Different configurations of electrode area were exposed to the electrolyte as an active area to investigate electrode geometry effects. Counter to working electrode distance was also altered while keeping the working electrode effective area constant to explore the AC response dependence on the variation of ion travel distance. Some controlled experiments were done to validate the experimental setup and to provide a control condition for comparison with experimental results. A frequency range of 100 mHz to 1 MHz was used for all experiments. In our analysis, we have found a noteworthy influence of electrode geometry on AC impedance response. For all electrodes, impedance decreases with the increase of effective area of the electrolyte. High frequency impedance is not as dependent on geometry as low frequency response. The observed phase shift angle drops in the high frequency region with increased working electrode area, whereas at low frequency the reverse is true. Resistance and capacitive reactance both decrease with an increase of area, but resistance response is more pronounce than reactance. For lower frequencies, small changes in working area produce very distinctive EIS variations. Electrode material as well as geometry was systematically varied in the present study. From these and other studies, we hope to develop a fundamental foundation for understanding specific changes in local geometry in fuel cell (and other) electrodes as a method of designing local morphology for specific performance.

Electrochemical impedance spectroscopy (EIS) is a widely utilized experimental method for fundamental mechanism understanding and diagnosis of solid oxide fuel cells (SOFCs). However, EIS experimental results are in general interpreted using equivalent circuit model method. When EIS response induced by very complicated multi-physics processes in SOFC is approximated with a simple equivalent circuit, it will inevitably lead to ambiguity. In this research, first principle EIS simulation method is developed to link multi-physics processes of a button cell to EIS response. The EIS modeling method is validated using experimental data from open literature. The validated EIS model is employed to investigate SOFC performance. Simulation results indicate that EIS response contains a plethora of SOFC behavior information that collectively contributed by intrinsic SOFC material property, porous microstructure, as well as operating conditions. The simulation method developed in this research can be potentially utilized for interpretation and de-convolution of experimental EIS results.

In the present work, we are concerned with heterogeneous material systems that have multiple distinct materials and void phases, and associated interfaces, for which the geometric scale and specific morphology at the micro/nano-level play an essential role in the global properties, functional behavior, and material system performance. Such materials are at the heart of revolutionary advances in devices that convert and store energy (e.g., batteries, fuel cells, solar cells, capacitors, and many electro-optical devices), and other membrane-based devices used in chemical and fuel processing, sequestration, and extraction. In recent years, advances in a variety of additive manufacturing methods and techniques have made it possible to design, control, and fabricate specific micro- or nano-structure to achieve prescriptive functional performance of the material systems and devices in which they appear. However, systematic multiphysics analysis methods properly set on field equations that represent the local details is not available, so that first-principles understandings and designs of those materials are not properly founded. Recently, the doe established an energy frontiers research center for physics based nanostructure design and fabrication of hetero geneous f unctional m aterials, called the HeteroFoaM center, to address this and related questions. The present paper presents some initial findings of part of that effort related specifically to morphology. For the present study, a finite element model (FEM) was developed Using COMSOL MULTIPHYSICS to predict impedance behavior when the field equations are set on the local features of regular geometric micro-structures The results for are compared with experimental data obtained from impedance spectroscopy of single SOFC fuel cell elements. For the model, equivalent idealized geometric structures were assumed corresponding to the YSZ material morphology of the fuel cell. Continuously aligned pore structures were represented by rectangular extrusions and regular porosity was represented by geometric shapes such as circles, rectangles and triangles. For experimental data, fragments of a button cell were used with silver paste contacts on the electrodes. For the model, the geometric microstructure was varied by using different shapes i.e. circles, rectangles and triangles while keeping the total material quantity constant, and by using equivalent areas for each of the geometric shapes. Impedance response for the frequency range from 1Hz to 1MHz was obtained for both the models and the experiments. Observations and interpretations of morphology effects are presented.

Development of technology related to solid oxide fuel cells at Materials Group, BARC includes synthesis of materials viz. lanthanum strontium manganite (LSM), yttria stabilized zirconia (YSZ), NiO etc. using chemical methods followed by shaping components and integrating into single cells. The indigenously prepared materials are converted to single cells for studies on material characterization and cell performances. This paper presents results of studies on electrical performances of tubular solid oxide fuel cell under hydrogen and oxygen atmospheres and correlates them with the use of cathode functional layers. Studies on symmetrical cells comprising of YSZ electrolyte sandwiched between cathode layers were carried out using electrical impedance spectroscopy (EIS). The EIS results exhibit improvement of area specific resistance (ASR) on incorporation of functional layers. Based on EIS data, double functional layers with varying compositions were incorporated in button cells as well as tubular single cells. Two types of tubular cells were fabricated, electrolyte supported and cathode supported. The electrolyte supported tubular cells were fabricated by making one-end closed tubes of YSZ, followed by coating with porous LSM inside and porous NiO-YSZ layer outside the tubes. On the other hand, tubular cathode supported cells were fabricated by one-end closed porous LSM tubes, followed by subsequent coatings of electrolyte and anode. The cells were characterized for electrical performance from 800 to 1000°C. Electrical power output from electrolyte supported cells with cathode functional layers increased from 300 mW per cell to 500 mW per cell. On the other hand, cathode supported cells exhibited improvement in ASR from ∼ 2 Ωcm 2 to < 1 Ωcm 2 . The results were correlated with microstructure, area of triple phase boundaries and catalytic activity of cathode.

In this paper, the behavior of an anode-supported solid oxide fuel cell is studied by using voltage-current density measurement and electrochemical impedance spectroscopy. The cell total polarization obtained from electrochemical impedance spectroscopy results is shown to be consistent with the area-specific resistance calculated from the voltage-current density curve. An electrolyte-supported solid oxide fuel cell is then used to build an equivalent electrical circuit model using reference electrodes and electrochemical impedance spectroscopy. A four-constant phase element model is proposed to analyze the anode-supported solid oxide fuel cell. The model is used to evaluate an anode-supported solid oxide fuel cell under different cell voltages. The individual resistances are also studied as a function of applied voltage, and their physical meaning is explained in terms of reaction mechanisms occurring at the cathode and anode. It is shown that some of the obtained resistances are independent of diffusion while others have both a charge transfer and diffusion component.

Impedance characteristics of a 6 kW proton exchange membrane (PEM) electrolyzer stack are presented under various operating conditions. An electrolyzer stack was operated under room temperature and partial current range (0 to 80 A). The whole stack impedance spectrums were measured by three different power supply configurations. The total sweeping frequency range (0.5 Hz to 20 kHz) is divided into low frequency (0.5 to 20 Hz), middle frequency (20 Hz to 1 kHz), and high frequency (1 to 20 kHz). Each frequency range required a different measurement setup to measure the whole stack impedance data. In this study, the partial impedance spectrums at low and high frequency ranges are successfully measured and analyzed. The measured data is verified with Kramers-Kronig relations. Measurement issues at the middle frequency region are discussed.

Samarium (Sm) is a rare earth material that shows promise for use in cathodes of intermediate temperature-operating solid oxide fuel cells (IT-SOFCs). Perovskite-structured oxide containing Sm has very attractive electrocatalytic properties, and spinel-structured oxide generally exhibits low thermal expansion, indicating its suitability for application as a SOFC cathode. In this paper, the characteristics of the various Sm-based oxide materials (Sm-Sr-(Co,Fe,Ni)-O) deposited on Sm 0.2 Ce 0.8 O 1.9 (SDC) electrolyte pellets were investigated in terms of their microstructure, sinterability and electrochemical properties. The relationship between the composition and the sintering temperature was studied and discussed. Results show that the substitution of iron (Fe) and nickel (Ni) in Co-sites affects the sinterability, adhesion to the electrolyte and electrochemical activity, such that the different sintering temperatures for these compositions should be considered. The microstructure and sinterability of the cathodes were investigated using a scanning electron microscope (SEM). Area specific resistance (ASR) values for all cathode compositions were measured using AC electrochemical impedance spectroscopy (EIS).

Alkaline fuel cells are one of the low cost types of fuel cells. In this contribution, the performance of an alkaline fuel cell was investigated by varying different operational parameters. The cell was tested under four different electrolyte concentrations and three different levels of anode flow rates. The results of the test revealed that the efficiency of the cell increases with the increase in electrolyte concentration. Anode flow rate was not found to have a considerable impact on the cell performance. Impedance spectroscopy has been conducted to validate the mathematical model and further investigate ohmic resistance, anode and cathode activation losses and mass transport losses. The optimal level of electrolyte concentration and anode flow rate for an alkaline fuel cell has been deduced through modeling & statistical analysis.