Search

In this study, we investigated the long-term stability of anion exchange membrane water electrolyzers (AEMWEs) under various bias conditions. The cell performance was relatively stable under conditions of voltage cycling in a narrow range, constant voltage and constant current. On the other hand, a relatively dynamic condition, voltage cycling, in a wide range detrimentally affected the cell stability. Abnormally high negative and positive currents were observed when the cell voltage was switched between 2.1 and 0 V. Impedance results and post-material analyses indicated that the performance degradation was mainly due to anode catalyst detachments, which increased non-ohmic resistance in the wide range voltage cycling. An increase in ohmic resistance was also observed, which was due to the membrane dehydration that occurred in the frequent rest times. Thus, it can be said that the voltage cycling range as well as the frequency of rest times are critical operational parameters in determining the long-term stability of AEMWEs.

This study investigated the effect of H2S concentration (5, 10 and 50 ppm) on the degradation and performance of Ni-YSZ anode supported solid oxide fuel cells. When supplied with hydrogen fuel containing H2S, the cell voltage dropped rapidly, and with increasing H2S concentration, voltage drop % increased (due to higher sulfur coverage on the Ni surface) and saturated more rapidly. A high concentration (50 ppm) of H2S led to an additional, slow rate voltage loss. In all cases, cell performance did not completely recover even after being supplied with H2S-free hydrogen fuel, because of the incomplete desorption of sulfur from the Ni surface. After the performance tests, nickel sulfides were detected on the Ni surface by Raman spectra, which were produced by the reaction of the remaining adsorbed sulfur with Ni during the cooling process. This indicates that the formation of nickel sulfides was not responsible for the secondary voltage drop. SEM/EDS analyses combined with FIB revealed that the reason for the additional 2nd drop was Ni oxidation; at a high sulfur coverage ratio (50 ppm), the outer layer of the Ni particle was oxidized by oxygen ions transported from the electrolyte. This indicates that H2S concentration as well as current density is a critical factor for Ni oxidation, and gives rise to the second voltage drop (irreversible cell degradation). The present work showed that the degradation behavior and phenomenon can differ significantly depending on the concentration of H2S, i.e., permanent changes may or may not occur on the anode (such as Ni oxidation) depending upon H2S concentration.

In this study, phase synthesis and electrochemical properties of Sm1-xNdxBa0.5Sr0.5Co2O5+d (x = 0–0.9) oxide systems where neodymium and samarium were replaced at the A-site of SmBa0.5Sr0.5Co2O5+d layered perovskite are investigated for use as cathode materials in Intermediate Temperature-operating Solid Oxide Fuel Cells (IT-SOFCs). The structure of Sm1-xNdxBa0.5Sr0.5Co2O5+d (x = 0–0.9) oxide systems can exist in either an orthorhombic (x = 0–0.4) or tetragonal (x = 0.5–0.9) form. The maximum electrical conductivities in Sm1-xNdxBa0.5Sr0.5Co2O5+d (x = 0–0.9) oxide systems were obtained from Sm0.2Nd0.8Ba0.5Sr0.5Co2O5+d (SNBSCO8) and their values are 1280 and 280 Scm−1 at 50 °C and 900 °C, respectively. The area specific resistances (ASRs) of SBSCO are 3.019, 0.611, and 0.092 Ωcm2 at 500, 600, and 700 °C, respectively. However, SNBSCO8 single phase gives the lowest ASRs of 1.751, 0.244 and 0.044 Ωcm2 at the same temperatures tested. SNBSCO8 is thus a promising candidate cathode material for IT-SOFC applications.

Various current collection methods were investigated during performance tests of anode-supported solid oxide fuel cells under constant current and thermal cycling conditions. Additional voltage losses originating in the current collector mesh were distinguished using Pt voltage probes attached to the electrode surfaces. Contact resistance was substantial when a Crofer mesh was used without Pt contact paste, depending on the diameter of the Crofer mesh wire, while it was negligible for silver mesh with Pt contact paste. It was also observed that contact resistance decreased with growing contact area, and non-ohmic polarization resistance increased due to Cr poisoning, with constant current test time. Thermal cycling tests were conducted on cells containing Ni Co coated Crofer meshes with and without Pt contact paste. The cell performance without Pt contact paste was unstable, especially during deep and slow cycling. This was attributed to increasing contact resistance between the cell and the mesh. In contrast, the cell with Pt contact paste operated stably throughout the thermal cycles, maintaining the level of contact resistance as well as the performance of the cell itself. The present work determined that the type of current collecting method can substantially affect cell stability during long-term constant current mode and thermal cycling, and contact resistance can be measured in-situ using additional voltage probes.

The authors are grateful for the support of the Basic Science Research Program, part of the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (No. 2014R1A1A1004163). The electrochemical properties and long-term performance of an in-situ composite cathode comprised of SmBa0.5Sr0.5Co2O5+δ (SBSCO) and Ce0.9Gd0.1O2−δ (CGO91) are investigated for metal supported solid oxide fuel cell (MS-SOFC) application. The Area Specific Resistance (ASR) of an in-situ composite cathode comprised of 50 wt% of SBSCO and 50 wt% of CGO91 (SBSCO:50) is 0.031 Ω cm2 in the first stage of measurement at 700 °C; this value of ASR increases to 0.138 Ω cm2 after 1000 h. The ASR of SBSCO:50 (in-situ sample at 750 °C) is 0.014 Ω cm2 at the initial stage of measurement; the increase of ASR after 1000 h at 750 °C is only 0.067 Ω cm2. These results suggest that the optimum temperature for in-situ firing of an SBSCO:50 cathode sample of MS-SOFC is higher than 700 °C, ideally around 750 °C. Postprint

High performance solid oxide fuel cells (SOFCs) based on gadolinia-doped ceria (GDC) electrolyte are demonstrated for intermediate temperature operation. The inherent technical limitations of the GDC electrolyte in sinterability and mechanical properties are overcome by applying sol–gel coating technique to the screen-printed film. When the quality of the electrolyte film is enhanced by the additional sol–gel coating, the OCV and maximum power density increase from 0.73 to 0.90 V and from 0.55 to 0.95 W cm −2 , respectively, at 650 °C with humidified hydrogen (3% H 2 O) as fuel and air as oxidant. The impedance analysis reveals that the reinforcement of the thin electrolyte with sol–gel coating significantly reduces the polarization resistance. Elementary reaction steps for the anode and cathode are analyzed based on the systematic impedance study, and the relation between the structural integrity of the electrolyte and the electrode polarization is discussed in detail.

This article investigates the relationship between ionic conductivity and various processing methods for aliovalent-doped, ceria solid solution particles, as an intermediate temperature-solid oxide electrolyte to explain the wide range of conductivity values that have been reported. The effects of doping material and content on the ionic conductivity are investigated comprehensively in the intermediate temperature range. The chemical routes such as coprecipitation, combustion, and hydrothermal methods are chosen for the synthesis of ceria-based nanopowders, including the conventional solid-state method. The ionic conductivity for the ceria-based electrolytes depends strongly on the lattice parameter (by dopant type and content), processing parameters (particle size, sintering temperature and microstructure), and operating temperature (defect formation and transport). Among other doped-ceria systems, the Nd 0.2 Ce 0.8 O 2− d electrolyte synthesized by the combustion method exhibits the highest ionic conductivity at 600 °C. Further, a novel composite Nd 0.2 Ce 0.8 O 2− d electrolyte consisting of a combination of powders (50:50) synthesized by coprecipitation and combustion is designed. This electrolyte demonstrates an ionic conductivity two to four times higher than that of any singly processed electrolytes.

This paper is presented to investigate operational characteristics of a direct methanol fuel cell (DMFC) stack with regard to fuel and energy efficiency, including its performance and stability under various operating conditions. Fuel efficiency of the DMFC stack is strongly dependent on fuel concentration, working temperature, current density, and anode channel configuration in the bipolar plates and noticeably increases due to the reduced methanol crossover through the membrane, as the current density increases and the methanol concentration, anode channel depth, and temperature decreases. It is, however, revealed that the energy efficiency of the DMFC stack is not always improved with increased fuel efficiency, since the reduced methanol crossover does not always indicate an increase in the power of the DMFC stack. Further, a lower methanol concentration and temperature sacrifice the power and operational stability of the stack with the large difference of cell voltages, even though the stack shows more than 90% of fuel efficiency in this operating condition. The energy efficiency is therefore a more important characteristic to find optimal operating conditions in the DMFC stack than fuel efficiency based on the methanol utilization and crossover, since it considers both fuel efficiency and cell electrical power. These efforts may contribute to commercialization of the highly efficient DMFC system, through reduction of the loss of energy and fuel.

Air-blowing type direct methanol fuel cells (DMFCs) are becoming more attractive for portable electronic devices as alternatives to the currently used Li-ion batteries because they are quieter with less parasitic power loss than the active-type DMFCs used a compressor. However, the blower has difficulty in providing a uniform air supply with a high flow rate to the cathode manifolds of the stack. In this study, a design that allows accurate measurements of the flow distribution on the air-blowing DMFC stack is developed using a novel scientific approach and careful construction of the experimental apparatus. Using this novel experimental technique, a novel stack design is produced to improve the performance and stability of the DMFC system under air-blowing conditions. Furthermore, auxiliary devices, such as ducts, guide vanes, foams, membrane and wedges are integrated and evaluated to the stack to assist in uniform flow by the blower. In particular, the inlet foam, membrane and upper angle duct help improve the uniformity of the lateral and longitudinal flow distribution in the air-blowing stack. Finally, the air-blowing stack with these auxiliary devices shows high performance with operational stability.

High performance membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs) are developed by changing the coating process, optimizing the structure of the catalyst layer, adding a pore forming agent to the cathode catalyst layer, and adjusting the hot-pressing conditions, such as pressure and temperature. The effects of these MEA fabrication methods on the DMFC performance are examined using a range of physicochemical and electrochemical analysis tools, such as FE-SEM, electrochemical impedance spectroscopy (EIS), polarization curves, and differential scanning calorimetry (DSC) of the membrane. EIS and polarization curve analysis show that an increase in the thickness and porosity of the cathode catalyst layer plays a key role in improving the cell performance with reduced cathode reaction resistance, whereas the MEA preparation methods have no significant effects on the anode impedance. In addition, the addition of magnesium sulfate as a pore former reduces the cathode reaction transfer resistance by approximately 30 wt%, resulting in improved cell performance.

To expand the applications of direct methanol fuel cells (DMFCs), the testing time required to evaluate their durability must be shortened. In this article, we present a step-by-step, accelerated degradation test (ADT) procedure for simple application by fuel cell engineers to the membrane electrode assembly (MEA) in DMFCs. Using MEA degradation data obtained under high stress conditions, we provide a method to estimate the lifetime distribution for normal use conditions and derive optimal testing plans for further degradation tests. A bi-exponential model with random coefficients is introduced to represent the nonlinear deterioration path of the MEAs. Based on the lifetimes estimated from the bi-exponential model at higher temperatures, the lifetime distribution at normal use temperature is extrapolated using the Weibull–Arrhenius model as the lifetime-temperature relationship.

One of the major challenges in direct methanol fuel cells (DMFCs) is to design reliable and stable FC systems that satisfy the very high dynamic demand in various environmental conditions for portable devices. This paper provides an overview of several failure modes and effect analyses (FMEAs) which can have significant consequences on the durability and stability of DMFCs, including high and sub-zero temperature storage, dry and high humidification atmospheres, and fuel/oxidation starvation by breakdown of fuel/air supply components. Firstly, some characterization methods are discussed to investigate changes of membrane electrode assemblies (MEAs) in terms of their physiochemical and electrochemical properties after testing in various simulated failure modes. Secondly, possible mitigating solutions to minimize the hazards associated with them are suggested through a fundamental understanding and scientific approach. The relationship between the causes and symptoms in DMFC systems is determined by examining a variety of failure sources.

The effect of cathode gas diffusion media with microporous layers (MPLs) on direct methanol fuel cell (DMFC) performances is studied by combining electrochemical analysis and physicochemical investigation. The membrane electrode assemblies (MEAs) using MPL-modified cathode gas diffusion layers (GDLs, GDL-1) showed slightly better performances (117 mW cm −2 ) at 0.4 V and 70 °C than commercial GDL (SIGRACET ® product version: GDL-35BC, SGL Co.) DMFC MEAs (110 mW cm −2 ). This might be due to high gas permeability, uniform pore distributions, and low water transport coefficient including methanol crossover. For GDL-1, the air permeability was 31.0 cm 3 cm −2 s −1 , while the one for SGL 35BC GDLs was 21.7 cm 3 cm −2 s −1 . Also, the GDL-1 in the pore-size distribution diagrams had distinct peaks due to more uniform distributions of macropores and micropores with smaller holes between aggregates of carbon particles compared to GDL-35 BC as confirmed by SEM images. Furthermore, the MEA using GDL-1 for the cathode had a lower water transfer coefficient compared to an MEA with a commercial 35 BC GDL.

This article proposes a new prediction model to describe the nonlinear performance degradation paths of membrane electrode assemblies (MEAs) in direct methanol fuel cell (DMFC): a bi-exponential model with random coefficients. The bi-exponential model is constructed on a mathematical basis representing second-order kinetics. Performance variation between MEAs is incorporated by random coefficients in the proposed model. A likelihood ratio test is sequentially executed to select random effects in the nonlinear random-coefficients model. Analysis results indicate that the reliability estimation can be substantially improved by using the nonlinear random-coefficients model to incorporate two heterogeneous degradation characteristics of MEA performance during continuous operation of DMFC. Confidence intervals of failure-time distributions are obtained by the parametric bootstrap method.

Comprehensive and systematic direct methanol fuel cell (DMFC) membrane electrode assembly (MEA) lifetime tests are presented under a wide range of operating conditions including constant voltage, constant current, constant power, and cyclic voltage loading. In this work, the characteristics of the MEA lifetime are strongly dependent on how the DMFC system is managed in a certain operation mode. The degradation ratio of MEAs is strongly dependent on the set voltage/current level in the constant operation mode. In the case of the DMFC constant power operation mode, the trend of MEA performance degradation shows the combined effect of both constant voltage and constant current operation mode, due to the required current increases to maintain the specified power output with time, while the voltage has the downward tendency from the initial value. The DMFC performance degradation under cyclic voltage loading (on–off) mode is much more severe than that of continuous voltage and current operation, while there is little difference between continuous voltage and current mode designs. The potential degradation mechanisms are proposed at each operating condition and demonstrated by using various electrochemical analysis techniques including impedance spectroscopy and polarization curves for improving long-term operational stability through mitigating such deterioration mechanisms of MEAs.

This study examined the effects of ball-milling with an AB 5 type LmNi 4.1 Al 0.25 Mn 0.3 Co 0.65 (Lm: Lanthanum rich mischmetal) alloy on the electrode characteristics of a BCC type Ti 0.32 Cr 0.43 V 0.25 alloy in order to determine its potential as an anode material for Ni–MH batteries. A mixture of the BCC type Ti 0.32 Cr 0.43 V 0.25 alloy powder and various concentrations of the AB 5 type LmNi 4.1 Al 0.25 Mn 0.3 Co 0.65 alloy powder was ball-milled for various ball-milling times. The discharge capacity of the electrode made from the Ti 0.32 Cr 0.43 V 0.25 alloy ball-milled with the LmNi 4.1 Al 0.25 Mn 0.3 Co 0.65 alloy was superior to the electrode made from the Ti 0.32 Cr 0.43 V 0.25 alloy alone. The highest discharge capacity of 310 mAh g - 1 was obtained when 20 wt% LmNi 4.1 Al 0.25 Mn 0.3 Co 0.65 alloy was added to the Ti 0.32 Cr 0.43 V 0.25 alloy and ball-milled for 20 min.

To expand commercial applications of polymer electrolyte membrane fuel cells (PEMFCs), the evaluation time for their durability must be shortened. This article provides a straightforward accelerated degradation testing (ADT) procedure for PEMFC for easy and quick implementation of the procedure. The ADT procedure includes statistical modeling of degradation patterns of membrane electrode assemblies (MEAs) in PEMFCs under startup–shutdown cycling conditions. For this purpose, we propose a nonparametric degradation model to describe the nonlinear performance degradation paths of PEMFC MEAs. The analysis results indicate that the nonparametric approach provides more accurate estimates of the observed degradation data than other parametric approaches. Based on the nonparametric degradation model, we suggest a method to predict failure-times under normal operating conditions by estimating the time-scale factor under accelerated operating conditions.