• Energy Research

Abstract In-plane permeability of gas diffusion backing (GDB) of proton exchange membrane fuel cells (PEMFCs) was investigated experimentally. Toray-paper and SGL-paper were selected as GDB test samples. Several Toray-papers were treated in-house with polytetrafluoroethylene (PTFE) using the immersion technique, dried either under atmospheric or vacuum pressure, and then sintered. The dependence of PTFE distribution in the through-plane direction on the PTFE drying conditions was examined using scanning electron microscopy (SEM)-based energy dispersive X-ray spectroscopy (EDS) imaging. The EDS image maps revealed that the PTFE distribution strongly depended on the drying condition, and PTFE drying under vacuum pressure yielded a relatively uniform PTFE distribution. The measured in-plane permeability suggests that the homogeneous distribution of PTFE achieved by the vacuum drying produces a porosity-leveling effect. In addition, the relationship between the in-plane permeability and porosity of the Toray-paper samples followed the Kozeny–Carman relation, whereas due to non-fibrous solids such as binder, that of the SGL-paper samples did not.

Abstract Numerical models play a vital role in the SOFC (solid oxide fuel cell) research field; nonetheless, one can never rely on a non-validated model. Numerous models exist in the literature; however, they are utmost validated with the conventional I V (current-voltage) curves, whereas the temperature variations are almost never validated. In this study, we present spatial currents and temperatures computed by a numerical model and measured in-situ by the electrode-segmentation method in microtubular-SOFCs. By exploiting these numerical and experimental data, we evaluate the accuracy of the current distribution predicted by a numerical model validated with the conventional I V curve. This evaluation shows that the numerical model underestimates the current variations. Secondly, we assess the reliability of the temperature distribution predicted by the model verified with the conventional I V curve. This assessment reveals that the numerically computed temperatures substantially differ from the experimental results with the rising current density. Thirdly, we analyze the accuracy of the model-validation based on the I V curves and the temperature variations. This double validation approach improves the accuracy of the model.

Lithium ion batteries have the issue of internal short circuiting by the electrodeposition of metal at the negative electrode. This deposition follows the dissolution of metal particles incorporated into the positive electrode in manufacturing process. Thus, we have investigated the dissolution-deposition behavior of metals such as iron and copper using cyclic voltammetry and SEM/EDX. We have also addressed diagnosis method for the incorporation of the metal particles by electrochemical impedance spectroscopy. Test cells were assembled with positive electrodes of LiCoO2 mixed with acetylene black as conductive filler and PVDF binder, electrolyte solution of ethylene carbonate (EC)/diethylcarbonate (DEC) (1:1 vol.) containing 1 M LiPF6, and negative electrodes of graphite mixed with PVDF binder. Lithium metal was used as a reference electrode. Cyclic voltammetry was carried out with positive electrodes of iron plate and copper foil as working electrodes. Current onset potentials originating in the anodic dissolution of iron and copper were around 2.4 V and 3.6 V (vs. Li+/Li), respectively[1]. Hence those metal dissolve during charging of a cell. The anodic current for copper was larger than that for iron． Iron particle with a diameter of 150 μm, and copper particle with a diameter of 100 μm were adhered to the positive electrode surface. The fully charged cell by 1C with the iron particle exhibited no rapid voltage drop for one week whereas the cell with the copper particle showed rapid voltage drop before full charge, resulting in the short circuit much earlier than iron in agreement with the larger anodic dissolution current shown by the cyclic voltammetry. SEM and EDX observations of the surface of the negative electrode after charging showed iron and copper depositions in torus-shape. We also repeatedly maintained the cells with those metal particles under fully charged state (SOC:100%) for 24 hours before a discharging/charging cycle to assess the time variation of impedance spectra. As a result, we find characteristic change of the Bode plot for the negative electrode in the case of iron particles between 10-1 Hz as shown in Fig. 1. Since time variation in the impedance spectra was detected, electrochemical impedance spectroscopy is effective diagnosis method to detect the metal particle incorporation. [1] C. Iwakura, Y. Fukumoto, H. Inoue, S. Ohashi, S. Kobayashi, H. Tada, and M. Abe, J. Power Sources, 68, pp. 301-303 (1997). Fig. 1 Bode plots for the negative electrode (a) with and (b) without the Fe particles. Figure 1

Gas diffusion layers (GDLs) coated with a hydrophobic microporous layer (MPL) have been commonly used to improve the performance of polymer electrolyte fuel cells (PEFCs). In the present study, a GDL coated with an MPL containing hydrophilic carbon nanotubes (CNTs) was developed to achieve further enhancement of the PEFC performance under both low and high humidity conditions. The less hydrophobic pores formed primarily by the CNTs are effective at conserving the membrane humidity, which enhances the performance under low humidity conditions. The MPL containing the CNTs is also effective at expelling excess water from the catalyst layer. This allows the maximum pore diameter to be decreased to 5 μm without reducing the ability to prevent flooding, resulting in a much higher performance under high humidity conditions compared with that for a hydrophobic MPL coated GDL.

Introduction Major advantages of solid oxide fuel cells (SOFC), such as employing non-precious metals as catalyst, tolerating various fuels, and releasing high quality waste heat, are virtues of high operation temperature (873-1273 K). On the other hand, thermo-mechanical stresses, as the primary problem of the SOFC systems develop due mainly to the heat treatment and operation at high temperatures. For both planar and tubular designs, a number of numerical studies addressing the thermo-mechanical stresses have been published; the thermal expansion coefficient mismatch, residual stresses, and the spatial surface temperature variations during the operation, have been stated to be the primary reasons [1-3]. To estimate the stresses stemming particularly from the spatial temperature variations, researches have been relying on the local temperatures computed numerically with thermo-electrochemical models [1,2,4,5,6]. However, as repeatedly stated, due to the difficulty in conducting experiments at high temperatures, the models were hardly validated in terms of surface temperatures. We thus measured the local current and temperatures along a microtubular SOFC with the electrode-segmentation method. Given the surplus heat generated during the cell operation is commonly removed by excess air flow method, and this method likely affects the temperature distribution along the cells, we conducted experiments for co- and counter-flow configurations under various air flow rates. Experimental Setup The tubular anode substrate of the cell was made of NiO/YSZ (65:35 wt %). Upon coating with the 8YSZ electrolyte colloid by dip-coating, we sintered the tube at 1693 K for two hours. With a special mask designed for the segmentation of the cell, we brush-coated the La0.7Sr0.3MnO3/YSZ cathode slurry (10:3 wt %) on the electrolyte surface by a cotton swab and subsequently sintered at 1323 K for two hours. Eventually, we connected the silver wires and thermocouples to the regarding segments for local current/voltage, and temperature measurements. As depicted in Fig. 1, we fed fuel and air to the anode and cathode, respectively, in the co-flow configuration. For counter-flow configuration, we exchanged the fuel inlet and outlet, retaining inlet and outlet of the air flow. Note that we keep the segment labels as on in Fig. 1 throughout the discussion, despite the switch of the flow configuration. Though the inlet air temperature was 298 K, the fuel flow was pre-heated before entering the cell. Prior to fuel supply, we sustained the cathode surface temperatures of the segments at 1073 K by an electric furnace. Results and Discussion The temperature distribution profile along the cell alters with the gas flow configuration. With the rising air flow rate at 0.6 V in the co-flow configuration, the midstream and downstream segment temperatures arise, while the upstream segment temperature decreases with a relatively steeper slope. This entails a larger maximum temperature difference along the cell. We observe the same trend in the local currents, however, the slope of the current variations in all the segments are rather small and resemble each other. With the increasing air flow rate, at 0.6V in the counter-flow configuration, all the segment temperatures drop down, where the slope of the upstream segment is relatively higher than the other segments. As a result, the maximum temperature along the cell becomes larger. The midstream and downstream segment currents increase with rather small slopes, whereas the upstream segment current remains nearly constant. Conclusions The in-situ identified current/temperature distribution profiles for co- and counter –flow configurations differ from each other considerably. With the increasing air flow rate, the maximum temperature difference along the cell grows in both flow configurations; where the total current output variation is rather small. The counter-flow configuration exhibits larger maximum temperature difference along the cell comparing to the co-flow case. With these findings, we can deduce larger thermo-mechanical stresses at high air flow rates in the counter-flow configuration; which we will explore numerically on the basis of local temperatures in-situ measured here. References [1] K. Fischer et al., J. Fuel Cell Sci. and Technol. 6 (2009) 1-9. [2] A. Selimovic et al., M. Kemm, T. Torisson, and M. Assadi, J. Power Sources 145 (2005) 463-469. [3] O. Razbani et al., Applied Energy 105 (2013) 155-160. [4] C.-K. Lin et al., J. Power Sources 164 (2007) 238-251. [5] Nishino et al., J. Fuel Cell Sci. and Technol., 3 (2006) 33-44. [6] M. Suzuki et al., J. Power Sources, 180 (2008) 29-40. Fig. 1. Schematic of the experimental setup Figure 1

The lubrication problems of wear, friction, oil consumption and scuffing in internal combustion engines are closely related with the oil-film thickness between piston rings and cylinder liners. The piston rings usually operate under the condition of starved lubrication for the pack slide. We analyze on the oil-film characteristics of a piston ring pack, taking account of the interaction between rings. The experiments to examine the oil-film behaviour of ring specimens sliding under oil starvation are conducted using reciprocating test equipment. The oil-film characteristics of starved lubrication are clarified from the results of calculations and experiments.

While generating power by an SOFC, reactants hydrogen and oxygen are consumed; simultaneously, hydrogen is diluted with the product water-vapor. Namely, concentrations of the reactants and product vary over the electrochemical active area along the respective flow fields. The chemical potentials in the anode and cathode therefore change along the flow fields, giving rise to the reversible Nernst-loss. When the concentration variations along the anode flow field are too large, i.e., the oxygen pressure is much higher than that of hydrogen, which is likely to occur in the downstream provided that the air is supplied at sufficient rates, nickel particles, the conventional catalysts in the anode, tend to re-oxidize. As a result, the length of the three-phase boundary would shorten, limiting the electrochemical performance. Besides, the anode microstructure would expand due to the larger volume of nickel-oxide, resulting in stresses and micro-cracks [1]. To prevent the nickel re-oxidation, concentration variations are desired to be identified and mitigated. Spatial concentration variations give also rise to spatial current and temperature variations. Current variations result in performance degradation, reducing the electric efficiency of the power generation. Given that the overpotentials are released as the waste heat, temperature variations develop in relation with the involving heat transport processes, e.g., convective and radiant heat transfer processes. The temperature variations induce thermal stresses into the cell components, and they affect the current variations through the overpotentials as well. It was shown that the concentration and temperature variations couple in the counter-flow configuration, resulting in larger variations in comparison with the co-flow configuration [2]. Spatial current and temperature variations are hence of great importance from both the energy conversion efficiency and mechanical durability aspects. Spatial characterization of concentration, current and temperature variations is rather challenging. The high operation temperature (773-1273 K) of SOFCs makes the spatial characterization more difficult. Vibrational Raman Spectroscopy [3] and IR Thermography [4] can be employed for diagnosing the spatial concentration and temperature, respectively; however, both of them are quite expensive and they require transparent materials for the gas distribution plate. Although the segmentation method is easy to implement on tubular-SOFCs [2], it is quite laborious to apply on planar-SOFCs [5]. These challenges can be circumvented by numerical tools. In principle, numerical tools are obliged to be verified by benchmark experimental data for assuring the reliability of investigations. For verifying SOFC models, we need to consider in situ measurable properties, such as voltage, current, and temperature. Among these properties, I-V (current-voltage) validation and temperature validation appear to be the most practical options, which are to ensure the computation-reliability of concentration. Even though the conventional I-V curves provide a good basis for the model-validation, they may not ensure the accurate computation of the spatial variations. It is a fact that an I-V validated model might predict a number of distinct temperature fields depending on the incorporated heat transfer processes. Thereby, the computation-accuracy of the electrochemical performance is expected to be highly affected by the inaccurate temperature fields. This study is hence devoted to investigating the role of temperature variations on the reliability of the numerical tools for computing the associated properties. Herein we present the spatial variations in the characteristic properties of a microtubular-SOFC, firstly calculated by the model validated with only the conventional I-V curve, and secondly by the model verified with temperature variations, in addition to validating with the conventional I-V curve. For these evaluations, we exploit the experimentally and numerically obtained spatial current and temperature variations in a microtubublar-SOFC. We in situ acquired the experimental data by applying the segmentation method on a microtubular-SOFC, whereas we computed the numerical data by a two-dimensional model developed for the respective experimental conditions. References [1] A. Faes et al. Membranes, vol.2, yy.2012, pp.585 [2] Ö.Aydın et al., J. Electrochem. Soc., vol.163, yy.2016, pp.F216 [3] G. Schiller et al., Appl. Phys. B, vol.111, yy.2013, pp.29 [4] Y. Takahashi et al., Solid State Ionics, vol.225, yy.2012, pp.113 [5] P. Metzger et al., vol.177, yy.2006, pp.2045

The influence of microporous layer (MPL) design parameters for gas diffusion layers (GDLs) on the performance of polymer electrolyte fuel cells (PEFCs) was clarified. Appropriate MPL design parameters vary depending on the humidification of the supplied gas. Under low humidification, decreasing both the MPL pore diameter and the content of polytetrafluoroethylene (PTFE) in the MPL is effective to prevent drying-up of the membrane electrode assembly (MEA) and enhance PEFC performance. Increasing the MPL thickness is also effective for maintaining the humidity of the MEA. However, when the MPL thickness becomes too large, oxygen transport to the electrode through the MPL is reduced, which lowers PEFC performance. Under high humidification, decreasing the MPL mean flow pore diameter to 3 μm is effective for the prevention of flooding and enhancement of PEFC performance. However, when the pore diameter becomes too small, the PEFC performance tends to decrease. Both reduction of the MPL thickness penetrated into the substrate and increase in the PTFE content to 20 mass% enhance the ability of the MPL to prevent flooding.