Hu, Qiang

Denne afhandling beskriver udviklingen af en iltsensor med en intern reference elektrode (IROS) og undersøgelser af dens egenskaber. Den interne reference elektrode udnytter det veldefinerede oxygen partialtryk i den binære blanding af Ni og NiO som reference. IROSens måleelektrode er enten en metallisk platin eller en keramisk komposit af (La0.75Sr0.25)0.95MnO3±d(LSM25) og 8 mol % yttria-stabiliseret zirconia (8YSZ) med eller uden imprægnering af samaria-doteret ceria (SDC). Den keramiske måleelektrode udmærker sig ved både at have en lavere polarisationsmodstand og at kunne bruges i et større temperaturinterval – især ved lavere temperaturer - end platinelektroden. IROSen udmærker sig ved stor præcision, god stabilitet, hurtigt respons samt god tolerance over for cykliske temperatur- og pO2-variationer. Desuden kan sensoren nemt regenereres elektrolytisk, når referenceelektrodens Ni-indhold er brugt op. Cellerne er reproducerbare og kan fabrikeres med billige og fleksible metoder. På denne baggrund skønnes det, at denne nyudviklede IROS kan kommercialiseres i fremtiden. Der er foretaget teoretiske beregninger af den oxygen lækage, der skyldes elektronledningsevnen i en elektrolyt lavet af 8YSZ. Dette er udmøntet i grafer, der kvantitativt kan bruges til at designe en sensor med en forudbestemt levetid. Endimensionale numeriske simuleringer er udført for at studere cellespændingens tidsafhængighed ved gasskift. Her blev den asymmetriske opførsel mellem skift fra højt til lavt oxygenpartialtryk og tilbage tilskrevet potentialets logaritmiske afhængighed af oxygen partialtrykket. Fejlkilderne i pO2-målingerne er blevet analyseret. Temperaturafhængighederne af ligevægtspotentialerne af ti binære metal/metaloxid blandinger, der kan være kandidater til referenceelektroden, er blevet evalueret. Disse inkluderer Fe/FeO, Pb/PbO, Cr/Cr2O3, der alle er mindre følsomme overfor temperaturfluktuationer end Ni/NiO.Kinetikken i den elektrokemiske reduktion af NiO er undersøgt og kan beskrives med Avrami-ligningen, der beskriver faseomdannelser. Den elektrokemiske reduktion af NiO kan indeholde en induktionsperiode. Den maksimale reaktionshastighed opnås, når mellem 2 og 11 % af NiO er reduceret. Den stigende modstand under reduktionen kan tilskrives en enkelt bue i impedansspektrene. Den pO2-afhængige del af modstanden af den kompositte elektrode er bestemt. SDC-imprægnering kan i signifikant grad reducere polarisationsmodstanden af den kompositte LSM25/8YSZ-elektrode. Designs, der kan udvide mulighederne for anvendelsen af IROSen, er angivet. På grundlag af principperne for IROS’en, er det muligt at udvikle sensorer med interne referencer, der kan detektere andre gasser, som f.eks. hydrogen, klor og brom.The thesis describes the research on and development of an internal reference oxygen sensor (IROS). The IROS is potentiometric and uses the equilibrium pO2of the binary mixture of Ni/NiO as the reference pO2. The sensing electrode of the IROS are made from metallic Pt or the composite of (La0.75Sr0.25)0.95MnO3±δ(LSM25) and 8 mol % yttria stabilized zirconia (8YSZ), with/without samaria doped ceria (SDC) impregnation. The composite sensing electrodes are verified superior to Pt in terms of a lower polarization resistance and an extended working temperature range. Cell performance is evaluated comprehensively and the evaluation is targeted to the performance indicators that relate closely to the practical application. IROSes show high accuracy, good stability, fast response, good tolerance to thermal and pO2cycling and easy recoverability when Ni is depleted. Both cell fabrication and performance show good reproducibility. Apart from the excellent performance the IROSes are fabricated by an inexpensive and flexible method. Therefore, the IROSes present in this thesis may be commercialized in the future. Theoretical investigations on IROS are carried out. The electronic leak of an electrolyte made from 8YSZ is evaluated quantitatively and figures that may be used to design the depletion period of an IROS due to the electronic leak of 8YSZ are provided. One dimensional numerical simulations are performed to study the variation in cell voltage during the process of gas mixing, and the asymmetric behavior in cell response is revealed. The errors in pO2 measurement are analyzed. The temperature dependence of the equilibrium potentials of ten candidates of binary mixtures including Fe/FeO, Pb/PbO, Cr/Cr2O3are evaluated with respect their use in IROSes that are less sensitive to temperature fluctuations. Electrochemical reduction of NiO is studied. The kinetics of electrochemical reduction of NiO can be described by the Avrami equation. The electrochemical reduction of NiO may require an induction period. The maximum reaction rate is obtained when 2 - 11 % NiO is reduced. The increasing resistance in NiO reduction can be exclusively ascribed to one arc in impedance spectra. The pO2-dependent part of the resistances of the composite electrodes is determined. SDC impregnation reduces significantly the polarization resistance of the composite of LSM25/8YSZ. Designs that may extend the application of IROSes are provided. Based on the concepts and fundamentals of the IROS, internal reference sensors that detect other gas species such as hydrogen, chlorine and bromine may be developed

Online Research Database In Technology

Improved Internal Reference Oxygen Sensors Using Composite Oxides as Electrodes

http://orbit.dtu.dk/ws/files/77567528/Thesis_Qiang_Hu_Improved_IROS_Using_Composite_Oxides_as_Electrodes..PDF

Improved Internal Reference Oxygen Sensors Using Composite Oxides as Electrodes

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology