Electrochemical testing of an innovative dual membrane fuel cell design in reversible mode

Solid oxide fuel Cells (SOFC) are intrinsically reversible which makes them attractive for the development of reversible devices (rSOC). The main hurdles that have to be overcome are the higher degradation in electrolyzer (EL) mode and the slow and difficult switching form mode to mode. This work aims at the development and experimental validation of a concept for rSOC based on a new dual membrane fuel cell (dmFC) design which can overcome the existing problems of the classical SOFC. The kernel of the system is additional chamber - central membrane (CM) for water formation/evacuation in FC mode and injection in El mode. Its optimization in respect of microstructure and geometry in laboratory conditions is carried out on button cells. The electrochemical performance is evaluated based on volt-ampere characteristics (VACs) combined with impedance measurements in different working points. The influence of a catalyst in the water chamber is also examined. The VACs which give integral picture of the cell performance are in excellent agreement with the impedance studies which ensure deeper and quantitative information about the processes, including information about the rate limiting step. The results from the optimization of the water chamber show that the combination of design and material brings to important principle advantages in respect to the classical rSOC \u2013 better performance in electrolyzer mode combined with instantaneous switching

Impedance Studies of the Reduction Process in NiO-YSZ SOFC Anodes

The aim of this study is the development of a methodology for deeper insight into the Ni-YSZ anode behavior by in situ impedance monitoring of the Ni network initial formation and changes during the redox process. The testing is performed on single bare anodes. This approach bypasses complications arising from the impedance data analysis of cells. For more complete description of the changes in the anode microstructure, gas permeability measurements are also introduced. The obtained results show that impedance can register the changes in the sample's conductivity during anode reduction, as well as during redox cycling performed in mild oxidation conditions. The oxidation rate is much slower than the reduction one. The combination of impedance with gas permeability and microstructural measurements gives new opportunities for anode optimization in respect to the ohmic and the concentration polarizatio

RECHARGEABLE COMPACT LI CELLS WITH LIXCR0.9V0.1S2 AND LI1+XV3O8 CATHODES AND ETHER-BASED ELECTROLYTES

The electrochemical parameters of compact disk cells with cathodes of Li1+xV3O8, LiCr0.9V0.1S2, and electrolytes based on cyclic ethers, are studied. It is shown that the decrease of discharge time from 10 to 1.3h has but a small effect on cathode utilization, which drops from 80% to about 70% for both cathode materials. The polarization resistance of freshly deposited Li, from electrolytes of ethers, and their mixtures with ethylene carbonate, are identical. Continuous cycling tests with maximum cathode utilization in the electrolyte of composition 1.5M LiAsF672MeTHF/THF(1:1)/0.2% 2MeF demonstrate a cycling efficiency of 96-97% for Li. A lower efficiency of about 94% is obtained in ethylene carbonate containing electrolytes. It is suggested that an increase in the thickness of the passive film formed on the Li electrode is responsible for the capacity decay at the end of the cycling life. The rate of self-discharge at room temperature for a fully charged cell is about 5-8% per month. This self-discharge is attributed to the Li electrode, which is passivated during storage in the ether-based electrolytes

Impedance studies of cathode/electrolyte behaviour in SOFC

This paper reports impedance studies of the cathode/electrolyte behaviour in solid oxide fuel cells (SOFC), based on comparative investigation of half-cells with yttria stabilized zirconia (YSZ) electrolyte and different cathode materials: lanthanum strontium manganite (LSM), and composite LSM/YSZ with low ionic conductivity as well as the electron conducting Ag, Pt and Au. For improved impedance data analysis the technique of the differential impedance analysis is applied. It ensures structural and parametric identification without preliminary assumptions about the working model. It is found that despite the low ionic conductivity of LSM, the cathode reaction of the oxide cathode materials is a two-step process including: (i) charge transfer with activation energy of the resistivity Ea increasing with the temperature and (ii) transport of oxygen ions through the bulk of the electrode (rate-limiting stage) with Ea independent on the temperature. For the metal (electron conducting) electrodes, the reaction behaviour is described with one step process with higher Ea at higher temperatures. The activation energy of the electrolyte conductivity decreases with the increase of the temperature. The observed changes in Ea for the electrolyte and the cathode reaction (the charge transfer step for the LSM-based electrodes) appear in the same temperature interval. This interesting coincidence suggests for correlation between the bulk (electrolyte) and surface conduction properties. Approaches for improvement of both the ionic conductivity and the supply with electrons in LSM should be also searched. \ua9 2007 Elsevier Ltd. All rights reserved

Electrochemical and microstructural studies of Ni-YSZ anode redox behaviour

A new approach for in-situ conductivity measurements of the Ni phase in Ni/YSZ cermet anode during its formation and redox cycling, applying impedance spectroscopy, is introduced. The electrochemical testing is combined with direct gases (H2, N2 and their mixtures) permeability measurements. The resistance of the samples, which has inductive behavior, reaches ? constant value of about 40 m\u3a9 in the first 10-15 minutes and does not change during further reduction. The resistance decreases with about 8-10% after the first oxidation-reduction cycle. In the next cycles it does not change, or slightly increases. The permeability of the redox cycled samples is lower. Although there is a general correlation between permeability and porosity, depending on the testing conditions, for samples with the same porosity, the permeability differs due to different tortuosity

Differential Resistance Analysis \u2013 a New Tool for Evaluation of Solid Oxide Fuel Cells Degradation

Solid Oxide Fuel Cells (SOFCs) are a promising technology that can provide efficient and clean energy production. The general barriers hindering their market entry are durability, i.e. resistance to aging, and costs. In parallel to the deeper insight into the different degradation sources and improved understanding of ageing factors and their interactions, work towards higher accuracy for the assessment and monitoring of real-world fuel cell ageing in necessary. The requirements for operational stability formulate the parameter degradation rate (DR). Most often long term durability tests are performed at constant current load and the decrease of the voltage is used for its definition. In this work a new approach based on analysis of the volt-ampere characteristics, named Differential Resistance Analysis (DRA), is presented. It operates with the differential resistance, i.e. with the derivative of the voltage in respect to the current (dU/dI = Rd) which is more sensitive to small deviations and thus increases the sensitivity of the analysis. Two performance indicators are derived (Rd, min and \u394U 17) with differing selectivity: \u394U 17 is more sensitive to activation losses and Rd, min - to transport hindrances. The application of the DRA is demonstrated on examples from measurements in fuel cell and in reverse (fuel cell/electrolyzer) mode, as well as on modeling data. The results show that the method is at least 10 times more sensitive to DR evaluation in comparison with the classical approach