Presentation on theme: "Potentiometric sensors for high temperature liquids"— Presentation transcript:
1 Potentiometric sensors for high temperature liquids ML 4-1 & ML 4-2Potentiometric sensors for high temperature liquidsJacques FOULETIERGrenoble University, LEPMI, ENSEEG, BP 75, SAINT MARTIN D’HERES Cedex (France)Véronique GHETTALPSC, IN2P3-CNRS, 53 Avenue des Martyrs, GRENOBLE Cedex (France)MATGEN-IV: International Advanced School on Materials for Generation-IV Nuclear ReactorsCargèse, Corsica, September 24 - October 6, 2007
2 Potentiometric measurement of activities in molten salts and molten metals Part 1Activity - Activity coefficient:- Activity coefficients, reference states- Henry’s and Raoult’ lawsElectrochemical chains:- Various types of electrodes (1st, 2nd types, etc.)- Interface equilibrium- Ideal Cell e.m.f. calculationTypes of cells:- Formation cells (without membranes)- Concentration cell with a porous membrane- Concentration cells with a solid electrolyte membraneElectrolytes: main characteristics of molten and solid electrolytes- structure- conductivity (ionic, mixed)- Electroactivity domainsReference electrodes:- for molten metals (Pb, Fe, Na)- for molten salts (chlorides, fluorides)
3 Part 2 Sources of errors in potentiometric cells: Case studies: - Errors ascribed to the reference electrode- reversibility- reactivity- Errors due to the porous membrane- concentration modification- diffusion potential- Errors due to the solid electrolyte membrane- partial electronic conductivity- interferences- Errors due to the measuring electrode- buffer capacity- mixed potentialCase studies:- Oxide ion activity in molten chlorides- Oxidation potential in molten fluorides- Monitoring of oxygen, hydrogen and carbon in molten metals (Pb, Na)
4 From chemical potential Electrochemical potential MatgenIV going away for GirolataFrom chemical potentialtoElectrochemical potential
5 Chemical and electrochemical potentials 1 moleChemical potential:Chemical potential: work for thetransfer of one mole of a neutralspecies within SF = 0S1 moleElectrochemical potential:Electrochemical potential: work forthe transfer of one mole of ionswithin S at a potential FF ≠ 0F = 0ChemicalcontributionElectrostatic
6 Electrochemical chains: - Various types of electrodes (1st, 2nd types, etc.)- Interface equilibrium- Ideal cell e.m.f. calculation
7 What is a potentiometric sensor? Analysis of a component X dissolved in a molten metal or a molten saltPotentiometric sensor: Black box in contact with the analyzed medium Sensing phenomenon: Measurement of a electro-motive force (e.m.f.) between two output wiresRequirement: E = f(aX)EaXThe objective of this lecture is to describe the components ofthis black box. These components are referred to as electrodes, membranes, electrolytes, etc. The whole components form an electrochemical chain.
8 E Electrochemical chains E = f(+) - f(-) Same electronic conductors Electrode (+)Electrode (-)(-) Me / Electrolyte 1 // Electrolyte 2 // Electrolyte 3 / Me’ / Me (+)Cell e.m.f.EE = f(+) - f(-)Membranes• solid electrolyte (permeable to only one ion)• porous membrane (permeable to several ions, electrons, etc.)Remark: the analyzed component can be dissolved in electrolyte 2 or 3 or in metal Me
9 Junctions Junction: interface between two ionic conductors Interface Simple ionic junction: exchange of only one type of ionExample: <<O2->> / ((O2-)) stabilized zirconia/oxide dissolved in molten chlorideComplex ionic junction: solid electrolytes conducting by different ionsExamples: <<O2->> / <<Na+>> stabilized zirconia / -aluminaEquilibrium: O Na+ = Na2OMultiple ionic junction: exchange of several ionsExample: <KCl> / ((KCl)) exchange: K+ and Cl-<NASICON, Na+> / ((Na+ - K+))
10 ElectrodesElectrode: interface between an ionic conductor and an electronic oneInterfaceIonic conductorElectronic conductorIonic conductor:- aqueous solutions- molten salts (chlorides, fluorides, nitrates, carbonates, etc.)- solid electrolyte (anionic or cationic conductors)Electronic conductor:- solid or liquid metals or alloys- mixed ionic-electronic conductors (MIEC)
11 Types of electrodes (1)1st kind electrode (metal/metal ion electrode) : M / Mn+Equilibrium: Mn+ + n e- = M• 2nd kind electrode (coexistence electrode): Ag / AgCl / Cl-Equilibrium: AgCl + e- = Ag + Cl-reference electrode• 3rd kind electrode (formation of a new phase): O2,M / -Alumina (Na+)Equilibrium: 2 Na+ + 2 e- + 1/2 O2 = <<Na2O>>(-Alumina )Other types of electrode (not developed in this lecture):- ideally polarisable electrodes: C / MX (no electrochemical reaction)- ion blocking electrodes: exchange of electrons, no electrochemical reaction- electron blocking electrodes: exchange of ions, no electrochemical reaction- intercalation electrode: injection of ions in an electron conducting phase
12 Types of electrodes (2) GAS ELECTRODE The overall reaction requires a Three PhaseBoundary (TPB) between an electrolyte,a metal and a gasELECTROLYTEMETALGasExamples:- Pt, O2 / stabilized zirconiaEquilibrium : 1/2 O e- = O2-- Cg, Cl2 / molten chlorideEquilibrium : 1/2 Cl2 + e- = Cl-
13 a b j a b j k Equilibrium conditions between two phases: same carriers Exchange of one particle (ion or electron)Equilibrium:Galvani potential difference: no method for measuringabjExchange of more than one particlekFlux of matter generally, no equilibrium
14 Equilibrium conditions between two phases: different carriers Stabilizedzirconia-aluminaNa+O2-Equilibrium: O Na+ = Na2OSZElectrode reactionPtO2ELECTROLYTEStabilizedzirconiaEquilibrium: 1/2 O e- = O2-SZPt
15 E.m.f. calculation of an ideal chain: Objective: measurement of a(Na2O) in NaCl-KClE.m.f. calculation of an ideal chain:(-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na2O / YSZ / Pt, O2 (+)MS MS2• Each solid electrolyte is conducting by only one ion (the minority carriers are neglected)• The electronic conductivity of the solid electrolytes is negligible• No current is passing through the cell• Equilibrium at all the interfacesCALCULATION RULES1. Within each solid electrolyte, the electrochemical potential of the majority carrier is constant:(YSZ or Pyrex)2. Each junction is characterized by an equilibrium involving only the majority carriers of the phaseson contact,- same ionic carrier: MS1/Pyrex or MS2/Pyrex- different ionic carrier: stabilized zirconia / -aluminaO Na+ = Na2O
17 Types of cells: - Cells without membrane The roman catholic churchTypes of cells:- Cells without membrane- Concentration cell with a porous membrane- Concentration cells with a solid electrolyte membrane
18 CELLS WITHOUT MEMBRANE: Example: measurement of a(PbO) in PbO-SiO2 mixture(-) Pt, Fe, Pb(L) / PbO - SiO2(L) / O2(g), Pt (+)+ SiO2Main difficulty: solubility of oxygen in leadConcentration cellsR. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44
19 CONCENTRATION CELLS: cell with membrane (1) Cell which has identical electrodes and a membrane inserted between solutionsdiffering only in concentration.Two cases:(-) Pt, Fe, Pb(L) / PbO - SiO2(L) / YSZ / PbO(L) / Pb, Fe, Pt (+)<<O2->>Equilibrium: theoretical e.m.f.- membrane permeable only to one ion (solid electrolyte)(-) Pt, Fe, Pb(L) / PbO - SiO2(L) / Porous / PbO(L) / Pb, Fe, Pt (+)oxideFlux of matter: no equilibrium- membrane permeable to several ions (liquid junction)
23 Electrolytes: main characteristics of molten and solid electrolytes - Structure- Conductivity (ionic, mixed)- Electroactivity domainReference electrodes:- for molten metals (Pb, Fe, Na)- for molten salts (chlorides, fluorides)
24 Solid electrolytes: Main characteristics • The solid electrolyte are generally composed of host lattices (ZrO2, ThO2, PbCl2), doped with the introduction of cations with different valences (Ca2+, Y3+, K+, etc.): - formation of point defects (vacancy or interstitials) as charge-compensating defects the ionic conductivity is ascribed to only one ion - with sufficiently high doping concentrations (a few percents), the ionic conductivity can be assumed as independent on partial pressureZrO2SrCl2• Only a few solid electrolytes are available: ZrO2-Y2O3, (ThO2-Y2O3), -Alumina, CaF2, AlF3, etc.
25 Examples of solid electrolytes OxygenvacancyZrO2 - Y2O3ZrODoping (ZrO2-Y2O3 9 mol.%):ZrO2Oxide ion conductor-Alumina (NaAl11O17)NASICON (Na3Zr2Si2PO12)Framework structure with three-dimensionalchannels suitable for sodium ion conductionCation conductors
26 Solid electrolytes (case of oxides): Main characteristics • However, electronic species may also be present due to equilibria between the electrolyte and the gaseous phase:log P(O2)log ssioniquesi snsi spVariation of theelectrical conductivitywith partial pressureAt given TPatterson diagramTemperatureLog PO2Domain ofpredominantionic conduction(99%)The region (P, T) of predominantly ionicconduction is generally termed theELECTROLYTIC DOMAIN
27 Solid electrolytes:Requirements for an ideal potentiometric cell • Conduction by only one ion • Negligible electronic conductivity (far lower than 1 %, if possible …) • Chemical stabilityNot required conditions for an ideal potentiometric cell • The total conductivity can be very low (noticeably higher than the input impedance of the millivoltmeter) • The species exchanged at the electrodes can be different than the majority carrier of the electrolyte (pH electrode using a Li+ or Na+ glass, oxygen sensor using CaF2 or -alumina electrolytes) • The nature of the majority carrier in the electrolyte (anions or cations) doesn’t matter (oxygen sensor using oxide ions, fluoride ions or sodium ions)
28 Molten electrolytes: Main characteristics Cf. lecture GL 11• Large number of molten salts: chlorides, fluorides, carbonates, nitrates, etc. • Solid at room temperature • Temperature range: 150°C to more than 1000°C • Good stability • High electrical conductivity • High chemical and electrochemical reaction rates • Wide electrolytic domain (redox, acid-base)ADVANTAGES• Corrosion • Handling not easy • Hygroscopicity • Compatibility with solids (containers, separators, etc.)However,DRAWBACKS
29 Reference electrodes: - for molten metals (Pb, Fe, Na)- for molten salts (chlorides, fluorides)
30 Reference electrodes (1) Molten metals (Pb, Fe, Na)High temperature measurementsMain difficulties:• chemical reactivity• noticeable semipermeability flux• long term stabilityCoexistence electrodes: M/MxOyLow temperature measurementsMain difficulty:• electrochemical reversibilityCoexistence electrodes: Pd/PdOGas electrodes, Pt/O2 or MIEC/O2Main criteria:- known thermodynamic data (calibration often necessary)- equilibrium oxygen pressure within the electrolytic domain (not alwayspossible: Cr/Cr2O3 for molten steel monitoring)- long term stability- constant voltage in spite of possible disturbance (high buffer capacity)- equilibrium activity not too far from the measured one (reduction of thesemipermeability flux: use of Cr/Cr2O3 for molten steel monitoring)
32 Reference electrodes in molten salts No universally accepted reference electrode is available for electrochemical studiesalthough reference electrodes based on the Ag(I)/Ag(0) couple are undoubtedly themost common.Halogen electrode in halide melts are generally successful, but such electrodes areinferior in experimental convenience to those based on Ag(I)/Ag(0).The design of reliable reference electrodes in molten fluorides remains a major problem,due to the corrosive action on metal electrodes, and on glass or ceramics used ascontainers or diaphragms, and also because of the undetermined liquid junctionpotentials: use of quasi reference electrode, of in-situ pulse reference electrodes, etc.However, until yet, no totally satisfactory designs.G.J. Janz, in Molten Salts Handbook, Academic Press, London, 1967.
33 Reference electrodes in molten chlorides Ag/AgCl/Cl- electrodeLiquid junctionAll-glass reference electrodesJ.O’M. Bockris, G.J. Hills, D. Inman, L. Young,J. Sci. Instr. Soc. 33 (1956) 438Very thin glass (R less than 5 k in the range °C)Ionic MembraneLiquid junction
34 Reference electrodes for molten fluorides Stability, durability, reversibility, reproducibility and fast response ?Liquid junction (BN, graphite)Ionic membranePseudo-reference electrodesPulse in-situ electrodeR. Winand, Electrochim. Acta, 17 (1972) 251
35 Reference electrodes for molten fluorides Liquid junctionBN• Ni - NiF2 contained in a thin-walled boron nitride envelope. The electrode was developed for potential measurement in molten LiF-NaF-KF ( mol.%) (FLINAK) at a working temperature of °C. Boron nitride is slowly impregnated by the melt to provide ionic contact. The wetting occurs in about 6 hours in molten FLINAK. At higher temperatures, the BN appears to deteriorate permitting mixing of the melts. Furthermore, the boron nitride tube contained a boric oxide binder that dissolved contaminated the electrolyte, and changed the electrode potential.LiF-NaF-KF, LiF-BeF2-ZrF4≈ 15 jours, Tmax ≈ 500°H.W. Jenkins, G. Mamantov and D.L. Manning, J. Electroanal. Chem., 19 (1968) 385.H.W. Jenkins, G. Mamantov and D.L. Manning, J. Electrochem. Soc., 117 (1970) 183.P. Taxil and Zhiyu Qiao, J. Chim. Phys., 82 (1985) 83.
36 Reference electrodes for molten fluorides Composé ioniqueIonic membraneLaF3NiBNNi foam• The nickel-nickel fluoride reference electrode system exhibiting a membrane from a single crystal lanthanum trifluoride. Because of the solubility of the LaF3 in the fluorides melts, a nickel frit with fine porosity was used in order to protect the crystal. The system was tested for temperatures up to 600°C. On the other hand, the single crystal LaF3 is expensive, the assembling of the electrodeis more complicated while the crystal cracks after few experiments.LiF-BeF2-ZrF4 LiF-NaF-KF NaBF4Tmax ≈ 500°H. R. Bronstein, D. L. Manning, J. Electrochem. Soc., 119(2) (1972) 125F. R. Clayton, G. Mamantov, D.L. Manning, High Temp. Science, 5 (1973) 358
37 Reference electrodes for molten fluorides Pseudo-reference electrodesRelatively stable reference point, provided no oxidizing or reducing species come intocontact with the electrode.• Inert metal in contact with a redox system (Mn+/Mp+)Example : Nb(V) / Nb(IV)U. Cohen, J. Electrochem. Soc., 130 (1983) 1480.• A metal M in contact with a solution of Mn+ionsExample : Ta(V) / Ta(0)P. Taxil, J. Mahenc, J. Appl. Electrochem., 17 (1987) 261.According to Mamantov, Ni orPt wires had a constant potentialwithin ± 10 mV in molten fluoridesover a period of months.G. Mamantov, Molten Salts: Characteriza-tion and Analysis, Dekker, New York, 1969, p.537• An inert metal M in contact with a solutionExample : Pt / PtOx / O2-A.D. Graves, D. Inman, Nature, 208 (1965) 481.
38 Reference electrodes for molten fluorides Pulse reference electrode• Electrochemical generation of an in-situ redox couple for a very short time• Use this system as an internal redox probe to check periodically a classical referenceelectrode.The amount of foreign species introduced into the electrolyte must be very small toavoid contamination and consequent modification of the experimental conditionsT = 1025°CGraphiteMelt: NaFNiNaF-NiF2BNClassicalreferenceelectrodeFePOTENTIOSTATGalvanostatic anodic pulse (ca. 0.2 s) followedby open-circuit relaxation.30 open-circuitrelaxation transientsN. Adhoum, J. Bouteillon, D. Dumas, J.C. Poignet, J. Electroanal. Chem., 391 (1995) 63Y. Berghoute, A. Salmi, F. Lantelme, J. Electroanal. Chem., 365 (1994) 171.