| The development of Solid oxide fuel cell （SOFC） is expected to easehuman demand for power,get rid of or reduce dependence on oil imports，variouscountries compete to research and development SOFC due to it rarely emissionspollution gas and greenhouse gas and it has a higher power and high energyconversion efficiency. The electrochemical perform-ance testing is necessary ofmaterials in the process of continuously explore new SOFC materials. At present,the linear impedance spectroscopy measurement is widely used in the diagnosis ofelectrolyte electrochemical performance and evaluation of electrode and electrolyte,usually, we put small disturbance voltage to curb its nonlinear responses in acimpedance spectroscopy testing, According to linear impedance test in the smalldisturbance voltage we can analyze the electrode and the electrochemical process inthe SOFC. However, the electrode of actual solid oxide fuel cell electrode is anonlinear system, so the result of electrochemical analysis is just a similar in linearconditions, it not fully reflect the internal real electrochemical process of solidoxide fuel cell, for this reason, we carried out the investgation of electrodesnonlinear impedance test system.The main components of nonlinear impedance spectroscopy testing system ismulti-function digital lock-in amplifier,we test the performance of multi-functiondigital lock-in amplifier and established no source impedance test system thatcontains current sampling circuit and VB test software before the design ofnonlinear impedance test system. Using this system we tested resistance of wirewith the change of time and the resistance in high temperature and the impedancespectroscopy of resistor-capacitor unit.the test result shows, the resistance of wireincreased only0.25%in30min, so it is necessary that test resistance of thosetemperature sensitive material using small current;from the result of resistance inhigh temperature we can see the range of resistance can achieve10-3or less;fromthe impedance spectroscopy of resistor-capacitor unit we can see the result is eaqualto the result of electrochemical workstations.On this basis, in order to provide large current for electrodes,we designednonlinear impedance test circuits that contains DC power〠have certain ACfrequency band width and with constant voltage/constant current control poweroutput functions, using lock-in amplifier for the signal source and the measuringinstrument, writing the test software with VB language, in the end we formed acomplete spectrum of nonlinear impedance test system. Using the nonlinear impedance test system we first test the nonlinear characteristics of reverse connectin parallel diode circuit and the circuit of diode-capacitance in parallel, we get thehigh order current signal, The simulation line and completely coincidence withtheory curve, It shows that using nonlinear impedance test system to test the samplethat have nonlinear characteristics is a feasible and reliable and accurate method.Test the LSM cathode impedance spectroscopy in different disturbance voltages,The results shows that the polarization resistance reduced with the increase of thedisturbance voltage, according with Tafel equation.we get the high order currentsignal of LSM cathode, the measuring results show that at low frequency thecapacitance characteristics reducing and the nonlinear resistance characterincreasing led high order current value increasing of LSM cathode on frequencydecreases,the decrease of exchange rate of gas-solid interface is one of the reasonsthat lead to nonlinear phenomenon, gives different order nonlinear impedance withthe change of oxygen partial pressure, Give statement to the phenomenon.The research results show that no source impedance test system and nonlinearimpedance spectroscopy testing system that based on the lock-in amplifier arereliable test system, the method of nonlinear impedance test system for electrodes ofsolid oxide fuel cell testing is a promising means testing. |
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