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 LiCoO 2 mixed with acetylene black as conductive filler and PVDF binder, electrolyte solution of ethylene carbonate (EC)/diethylcarbonate (DEC) (1:1 vol.) containing 1 M LiPF 6 , 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
An anode-supported honeycomb SOFC that can achieve high power density and enhanced durability was developed. Electrolyte layer of 8YSZ (8 mol% yittria stabilized zirconia) is employed by dip-coating an anode honeycomb substrate of Ni/YSZ, followed by being coated with cathode layer of La 0.7 Sr 0.3 MnO 3 . Current-voltage and current-power density characteristics of the cells having different anode and cathode flow channel configurations at 850℃ were measured. The results show promising performances of the cell as a starting point to develop stacks composed of multiple honeycomb cells.
We have investigated the behavior of an operating polymer electrolyte fuel cell (PEFC) with supplying a mixture of carbon monoxide (CO) and hydrogen (H2) gases into the anode to develop the PEFC diagnosis method for anode CO poisoning by reformed hydrogen fuel. We analyze the characteristics of the CO poisoned anode of the PEFC at 80°C including CO adsorption and electro-oxidation behaviors by current-voltage (I‐V) measurement and electrochemical impedance spectroscopy (EIS) to find parameters useful for the diagnosis. I‐V curves show the dependence of the output voltage on the CO adsorption and electro-oxidation. EIS analyses are performed with an equivalent circuit model consisting of several resistances and capacitances attributed to the activation, diffusion, and adsorption∕desorption processes. As the result, those resistances and capacitances are shown to change with current density and anode overpotential depending on the CO adsorption and electro-oxidation. The characteristic changes of those parameters show that they can be used for the diagnosis of the CO poisoning.
Water flooding under high current and humidity conditions is a main barrier to enhancing the performance of polymer electrolyte fuel cells (PEFCs). This study evaluated a double microporous layer (MPL) coated gas diffusion layer (GDL) consisting of a thin hydrophilic layer coated on a hydrophobic MPL coated GDL. An accurate measurement of the contact angle was introduced to assess the wettability of the MPL. Besides, the water breakthrough pressure and water vapor permeance values were measured to evaluate the water transport ability of the MPL. The oxygen transport resistance was measured using the limiting current density in polarization curves. Appropriate hydrophilic MPL containing 5% Nafion, 25% TiO 2, and carbon black in the double MPL enhanced the ability of the GDL to discharge water at the catalyst layer, effectively reducing water flooding. The total oxygen transport resistance obtained with the double MPL was reduced by about 20% compared to that obtained with a hydrophobic MPL. Moreover, the pressure-independent and pressure-dependent resistances were separated from the total oxygen transport resistance measured under various back pressure conditions. The double MPL exhibited a substantially reduced pressure-independent resistance at the interface between the MPL and the catalyst layer.
Liquid water accumulated in the catalyst layer (CL) and gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC, PEMFC) results in performance deterioration due to inhibition of oxygen transport to the cathode CL (Flooding). To enhance the drainage in the GDL, the application of a microporous layer (MPL) is effective to the CL side of the GDL substrate [ 1 , 2 ]. To elucidate the effects of two-phase flow on the oxygen transport in an MPL for its optimized designs, we model three dimensional porous structures of in-house MPLs with pore network model (PNM) [ 3 , 4 ] for convective air permeation and oxygen diffusion. Pore diameter distribution derived by a focused ion beam scanning electron microscope (FIB-SEM) is employed in the PNM. Air permeation measurements combined with oxygen diffusion measurements with changing wetting liquid saturation (Galwick, Porous Materials Inc., USA) by using gas chromatography validate the model. Acknowledgments The authors are indebted to graduate students, Messrs. Yuhang Liu, Kentaro Harano, Dingfeng Chen for help in the modeling and experiment. Thanks are offered to Professor Kohei Ito of Kyushu University for valuable discussions. The authors acknowledge Ms. Chie Uryu of International Research Center for Hydrogen Energy, Kyushu University for operating the FIB-SEM. References Nakajima H, Konomi T and Kitahara T. 2007. Direct water balance analysis on a polymer electrolyte fuel cell (PEFC): Effects of hydrophobic treatment and micro-porous layer addition to the gas diffusion layer of a PEFC on its performance during a simulated start-up operation. Journal of Power Sources 171: 457-463. Kitahara T, Konomi T and Nakajima H. 2010. Microporous layer coated gas diffusion layers for enhanced performance of polymer electrolyte fuel cells. Journal of Power Sources 195: 2202-2211. Gostick J, Aghighi M, Hinebaugh J, Tranter T, Hoeh MA, Day H, Spellacy B, Sharqawy MH, Bazylak A, Burns A, Lehnert W and Putz A. 2016. OpenPNM: A pore network modeling package. Computing in Science & Engineering 18: 60-74. http://openpnm.org/ . Accessed on September 14, 2020
We have analyzed the dependence of the concentration overpotentials on the properties of gas diffusion layers (GDLs) of a polymer electrolyte fuel cell (PEFC) using dual serpentine flow fields during a start-up operation. Moreover, we directly and separately analyze such dependence of the amount of the water accumulated at each component of a cell by measuring the weight of adherent water. As a result, the hydrophobic treatment with micro porous layer addition to a GDL substrate is found to be effective to improve the cell performance by suppressing the water accumulation at the electrode (catalyst layer) which increases the concentration overpotential. Suppressed water accumulation at the electrode also increases cumulative current that represents the power generation and calorific power important for warm-up. Besides, increase in the coarseness of the GDL substrate is not so effective for decreasing the concentration overpotential and increasing the cumulative current for the case of the serpentine flow field. Increase in the thickness of the GDL substrate increases the concentration overpotential and decreases the cumulative current. These results will offer proper design parameters of GDLs for improving the performance of PEFCs, in Particular during start-up.
