• Energy Research

Nitrogen electrode reaction has been investigated in a LiCl-KCl-CsCl melt containing Li 3 N. The following reaction, N 3 - → ½N 2 + 3e - , is confirmed by quantitative analysis of anodically evolved gas. The Nernst relation holds for the rest potential of Ni electrodes at nitrogen gas pressure, p N 2 , of 0.1-1.0 atm and the anion fraction of N 3 - ion, x N 3-, of 0.003-0.020 (anion fraction). Then the standard formal potential of N 2 /N 3 - , E 0 ' N 2 / N 3-, is evaluated to be 0.193 ′ 0.003 V vs. Li + /Li (p N 2 = 1 atm,x N 3- = 1) at 673 K. The dependence of E 0 ' N 2 / N 3- upon the temperature (605-721 K) gives a linear relation, whose slope is (-0.790 ′ 0.096) X 10 - 3 V K - 1 . Thermodynamic quantities for formation of Li 3 N in the melt are also estimated.

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.

Infrared spectroscopy of molten LiCl-KCl-LiH at 673 K provided absorption band in the wavenumber region of for several ion concentrations. This absorption band is ascribed to the vibration of ion pair in the melt. The absorption band ascribed to the vibration of ion pair was not observed in the melt. The existence of the Li-H interaction shows a strong attractive force between ion and ion in the melt. © 2004 The Electrochemical Society. All rights reserved.

Current distribution of a practical solid oxide fuel cell (SOFC) has been analyzed from a thermal analysis combined with surface temperature measurements. Electrochemical impedance spectroscopy with two-electrode set-up is employed on an anode-supported microtubular SOFC. This cell is an intermediate temperature SOFC composed of an Ni/(ZrO2)0.9(Y2O3)0.1 cermet anode, an La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte, and an (La0.6Sr0.4)(Co0.2Fe0.8)O3 cathode. The impedance spectra give the resistances at the anode and cathode, and the cell Ohmic resistance. By numerically integrating these resistances, overpotentials are evaluated. The overpotentials and the single electrode (electrochemical) Peltier heats, at the anode and cathode provide individual heat production rates. Since the energy balance equations incorporating these heat production rates determined by current yield the surface temperatures of the cell, local current densities are obtained so that this calculated and measured temperatures by thermocouples at several positions in the anode and cathode surfaces coincide.

Abstract Enhancement of the performance of polymer electrolyte fuel cells (PEFCs) requires an appropriate water balance between the conservation of membrane humidity and the discharge of excess water produced in the cell. In the present study, a novel triple microporous layer (MPL) coated gas diffusion layer (GDL), in which a hydrophilic layer was coated on a hydrophobic double MPL, was developed to enhance the PEFC performance under both low and high humidity. The thin hydrophilic layer in the triple MPL is effective at conserving the humidity of the membrane electrode assembly (MEA) under low humidity, while the hydrophobic double MPL between the hydrophilic layer and the carbon paper substrate prevents removal of water from the hydrophilic layer. This results in a significant enhancement of the ability of the GDL to prevent dehydration of the MEA. The triple MPL coated GDL, where the polytetrafluoroethylene (PTFE) content in the hydrophobic MPL in contact with the hydrophilic layer is set to 30 mass% and that in contact with the substrate is set to 10 mass%, is effective at expelling excess water from the catalyst layer, which results in much higher PEFC performance under high humidity than that for a conventional hydrophobic MPL coated GDL.

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 contaminated metal particles incorporated into the positive electrode in manufacturing process. Thus, we have investigated the dissolution­/deposition behavior of metals such as copper, nickel, iron, and stainless steel so far1. We also proposed a diagnosis method for the incorporation of the metal particles by electrochemical impedance spectroscopy, taking advantage of characteristic changes of the impedance spectra (phase angle) between 10-­1 Hz1. In the present study, we investigate the time variation of the impedance spectra associated with the growth of the metal deposits at the negative electrode. Test cells were assembled with positive electrodes of metal plates of iron, nickel and stainless steel, electrolyte solution of ethylene carbonate (EC)/diethylcarbonate (DEC) (1:1 vol.) containing 1 M LiPF6, and negative electrodes of graphite mixed with PVDF binder. Potential between the metal plates and the negative electrode was maintained at 4.2 V by a potentio/galvanostat to electrochemically dissolve the metal plate at the positive electrode and deposit the metals at the negative electrode, while the negative electrode was maintained at 0.3 V against a lithium wire in the test cell by an auxiliary potentio/galvanostat to reproduce the potentials in a lithium ion battery. We observe the metal deposits at the negative electrodes from the iron, nickel and stainless steel plates by FE-SEM and EDX. Time variations of the phase angle in the impedance spectra between 10­-1 Hz for the above metal deposits are correlated with the metal growth, taking into account the impedance of the SEI layer at the metal surface. 1. Hironori Nakajima, and Tatsumi Kitahara, Diagnosis Method to Detect the Incorporation of Metallic Particles in a Lithium Ion Battery, ECS Trans., Vol. 68, 2, 59-74 (2015).

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

Abstract The thermoelectric power of the Li + /Li couple was measured in a LiCl–KCl eutectic melt at 633–773 K. The obtained value, 0.089 mV K −1 , gives the single electrode Peltier heat for the reaction Li + + e − ⇌ Li as − 5.8 kJ F −1 at 673 K (endothermic cathodic reaction). The single electrode Peltier heat for the reaction 1 2 H 2 + e − ⇌ H − in a molten LiCl–KCl–LiH system has been calculated using the temperature dependence of the equilibrium potential of the H 2 / H − couple measured with reference to the Li + /Li potential, which gives 5.8 kJ F − 1 (exothermic cathodic reaction) under the condition of hydrogen pressure of 1.0 atm, H − ion concentration of 0.01 (anion fraction) and 673 K. The single electrode Peltier heats for formation of Pd–Li alloys have been also evaluated.

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.