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Potentiometric sensors for high temperature liquids

ML 4-1 & ML 4-2. Potentiometric sensors for high temperature liquids. Jacques FOULETIER Grenoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France) E-mail: Jacques.Fouletier@lepmi.inpg.fr Véronique GHETTA

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Potentiometric sensors for high temperature liquids

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  1. ML 4-1 & ML 4-2 Potentiometric sensors for high temperature liquids Jacques FOULETIER Grenoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France) E-mail: Jacques.Fouletier@lepmi.inpg.fr Véronique GHETTA LPSC, IN2P3-CNRS, 53 Avenue des Martyrs, 38026 GRENOBLE Cedex (France) E-mail: Veronique.Ghetta@lpsc.in2p3.fr MATGEN-IV: International Advanced School on Materials for Generation-IV Nuclear Reactors Cargèse, Corsica, September 24 - October 6, 2007

  2. Potentiometric measurement of activities in molten salts and molten metals Part 1 Activity - Activity coefficient: - Activity coefficients, reference states - Henry’s and Raoult’ laws Electrochemical chains: - Various types of electrodes (1st, 2nd types, etc.) - Interface equilibrium - Ideal Cell e.m.f. calculation Types of cells: - Formation cells (without membranes) - Concentration cell with a porous membrane - Concentration cells with a solid electrolyte membrane Electrolytes: main characteristics of molten and solid electrolytes - structure - conductivity (ionic, mixed) - Electroactivity domains Reference electrodes: - for molten metals (Pb, Fe, Na) - for molten salts (chlorides, fluorides)

  3. Part 2 Sources of errors in potentiometric cells: - 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 potential Case studies: - Oxide ion activity in molten chlorides - Oxidation potential in molten fluorides - Monitoring of oxygen, hydrogen and carbon in molten metals (Pb, Na)

  4. MatgenIV going away for Girolata From chemical potential to Electrochemical potential

  5. Chemical potential: 1 mole F = 0 Chemical potential: work for the transfer of one mole of a neutral species within S S F = 0  Electrochemical potential: 1 mole F = 0 Electrochemical potential: work for the transfer of one mole of ions within S at a potential F S F ≠ 0 Chemical contribution Electrostatic contribution Chemical and electrochemical potentials

  6. Electrochemical chains: - Various types of electrodes (1st, 2nd types, etc.) - Interface equilibrium - Ideal cell e.m.f. calculation

  7. Potentiometric sensor: Black box in contact with the analyzed mediumSensing phenomenon: Measurement of a electro-motive force (e.m.f.) between two output wires aX Requirement: E = f(aX) E What is a potentiometric sensor? Analysis of a component X dissolved in a molten metal or a molten salt The objective of this lecture is to describe the components of this black box. These components are referred to as electrodes, membranes, electrolytes, etc. The whole components form an electrochemical chain.

  8. Same electronic conductors Electrode (+) Electrode (-) Cell e.m.f. E E = f(+) - f(-) Electrochemical chains (-) Me / Electrolyte 1 // Electrolyte 2 // Electrolyte 3 / Me’ / Me (+) 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 Ionic conductor Ionic conductor Simple ionic junction: exchange of only one type of ion Example: <<O2->> / ((O2-)) stabilized zirconia/oxide dissolved in molten chloride Complex ionic junction: solid electrolytes conducting by different ions Examples: <<O2->> / <<Na+>> stabilized zirconia / -alumina Equilibrium: O2- + 2 Na+ = Na2O Multiple ionic junction: exchange of several ions Example: <KCl> / ((KCl)) exchange: K+ and Cl- <NASICON, Na+> / ((Na+ - K+))

  10. Electrodes • Electrode: interface between an ionic conductor and an electronic one Interface Ionic conductor Electronic conductor Ionic 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. • 2nd kind electrode (coexistence electrode): Ag / AgCl / Cl- Equilibrium: AgCl+ e- = Ag + Cl- reference electrode Types of electrodes (1) • 1st kind electrode(metal/metal ion electrode) : M / Mn+ Equilibrium: Mn+ + n e- = M • 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 Phase Boundary (TPB) between an electrolyte, a metal and a gas METAL ELECTROLYTE Gas Examples: - Pt, O2 / stabilized zirconia Equilibrium : 1/2 O2 + 2 e- = O2- - Cg, Cl2 / molten chloride Equilibrium : 1/2 Cl2 + e- = Cl-

  13. Exchange of one particle (ion or electron) a b Equilibrium: j j   Galvani potential difference: no method for measuring Exchange of more than one particle a b j j k k Flux of matter generally, no equilibrium Equilibrium conditions between two phases: same carriers

  14. Stabilized zirconia -alumina Equilibrium: O2- + 2 Na+ = Na2O O2- Na+ SZ  Electrode reaction Equilibrium: 1/2 O2 + 2 e- = O2- Pt ELECTROLYTE Stabilized zirconia O2 SZ Pt Equilibrium conditions between two phases: different carriers

  15. (-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na2O / YSZ / Pt, O2 (+) MS1 MS2 1. Within each solid electrolyte, the electrochemical potential of the majority carrier is constant: (YSZ or Pyrex) CALCULATION RULES 2. Each junction is characterized by an equilibrium involving only the majority carriers of the phases on contact, - same ionic carrier: MS1/Pyrex or MS2/Pyrex - different ionic carrier: stabilized zirconia / -alumina O2- + 2 Na+ = Na2O Objective: measurement of a(Na2O) in NaCl-KCl E.m.f. calculation of an ideal chain: • 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 interfaces

  16. Main carriers e- e- e- Ag+ Na+,K+,Cl- Na+ Na+,K+,Cl-,O2- O2- Pt E YSZ  Pt Ag AgCl Pyrex MS1 MS2 E = Pt(+) - Pt(-) E.m.f. of an ideal chain (-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na2O / YSZ / Pt, O2 (+) Molten salt Molten salt Solid (-) Pt O2 (+) Pt YSZ Ag NaCl - KCl - Na2O Pyrex AgCl NaCl-KCl Molten salt Molten salt Solid

  17. The roman catholic church Types of cells: - Cells without membrane - Concentration cell with a porous membrane - Concentration cells with a solid electrolyte membrane

  18. + SiO2 Concentration cells CELLS WITHOUT MEMBRANE: Example: measurement of a(PbO) in PbO-SiO2 mixture (-) Pt, Fe, Pb(L) / PbO - SiO2(L) / O2(g), Pt (+) Main difficulty: solubility of oxygen in lead R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44

  19. - membrane permeable only to one ion (solid electrolyte) (-) Pt, Fe, Pb(L) / PbO - SiO2(L) / YSZ/ PbO(L) / Pb, Fe, Pt (+) <<O2->> Equilibrium: theoretical e.m.f. - membrane permeable to several ions (liquid junction) (-) Pt, Fe, Pb(L) / PbO - SiO2(L) / Porous / PbO(L) / Pb, Fe, Pt (+) oxide Flux of matter: no equilibrium CONCENTRATION CELLS: cell with membrane (1) Cell which has identical electrodes and a membrane inserted between solutions differing only in concentration. Two cases:

  20. CONCENTRATION CELLS: cell with membrane (2) (-) Pt, Fe, Pb(L) / PbO - SiO2(L) / YSZ/ PbO(L) / Pb, Fe, Pt (+) <<O2->> Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871

  21. (PbO) + 2 e- = Pb + O2- ((PbO)) + 2 e- = Pb + O2- CONCENTRATION CELLS: cell with membrane (3)   (-) Pt, Fe, Pb(L) / PbO - SiO2(L) / YSZ/ PbO(L) / Pb, Fe, Pt (+) Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871

  22. CONCENTRATION CELLS: cell with membrane (4)   (-) Pt, Fe, Pb(L) / PbO - SiO2(L) / YSZ/ PbO / Pb, Fe, Pt (+) R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44 Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871

  23. Electrolytes: main characteristics of molten and solid electrolytes - Structure - Conductivity (ionic, mixed) - Electroactivity domain Reference electrodes: - for molten metals (Pb, Fe, Na) - for molten salts (chlorides, fluorides)

  24. • 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 pressure ZrO2 SrCl2 Solid electrolytes: Main characteristics • Only a few solid electrolytes are available: ZrO2-Y2O3, (ThO2-Y2O3), -Alumina, CaF2, AlF3, etc.

  25. ZrO2 - Y2O3 -Alumina (NaAl11O17) O Zr Doping (ZrO2-Y2O3 9 mol.%): NASICON (Na3Zr2Si2PO12) ZrO2 Oxygen vacancy Y Framework structure with three-dimensional channels suitable for sodium ion conduction Cation conductors Oxide ion conductor Examples of solid electrolytes

  26. sisn sisp log s Variation of the electrical conductivity with partial pressure sionique Temperature log P(O2) Log PO2 At given T Domain of predominant ionic conduction (99%) The region (P, T) of predominantly ionic conduction is generally termed the ELECTROLYTIC DOMAIN Patterson diagram • However, electronic species may also be present due to equilibria between the electrolyte and the gaseous phase: Solid electrolytes (case of oxides): Main characteristics

  27. Solid electrolytes: Requirements for an ideal potentiometric cell• Conduction by only one ion• Negligible electronic conductivity (far lower than 1 %, if possible …)• Chemical stability Not 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. • 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 However, • Corrosion• Handling not easy• Hygroscopicity• Compatibility with solids (containers, separators, etc.) DRAWBACKS Molten electrolytes: Main characteristics Cf. lecture GL 11

  29. Reference electrodes: - for molten metals (Pb, Fe, Na) - for molten salts (chlorides, fluorides)

  30. Low temperature measurements High temperature measurements Main difficulties: • chemical reactivity • noticeable semipermeability flux • long term stability Main difficulty: • electrochemical reversibility • Coexistence electrodes: Pd/PdO • Gas electrodes, Pt/O2 or MIEC/O2 Coexistence electrodes: M/MxOy Reference electrodes (1) Molten metals (Pb, Fe, Na) Main criteria: - known thermodynamic data (calibration often necessary) - equilibrium oxygen pressure within the electrolytic domain (not always possible: 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 the semipermeability flux: use of Cr/Cr2O3 for molten steel monitoring)

  31. One-reading probes for molten iron Ref.: Cr/Cr2O3 Internal reference: Pd-PdO, Ir-Ir2O3 Cr/Cr2O3 Air YSZ YSZ YSZ Cr/Cr2O3 Tubular Sensor  = 6 mm Plug Sensor  = 6 mm Needle Sensor  = 2 mm Molten metal Molten metal D. Janke, Met. Trans. B, 13 B (1982) 227. Reference electrodes (2) Molten metals Examples Intermediate-temperature sensors Ref.: air, Pd-PdO , Ir-Ir2O3

  32. Reference electrodes in molten salts No universally accepted reference electrode is available for electrochemical studies although reference electrodes based on the Ag(I)/Ag(0) couple are undoubtedly the most common. Halogen electrode in halide melts are generally successful, but such electrodes are inferior 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 as containers or diaphragms, and also because of the undetermined liquid junction potentials: 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. Ionic Membrane Liquid junction Very thin glass (R less than 5 k in the range 350-500°C) J.O’M. Bockris, G.J. Hills, D. Inman, L. Young, J. Sci. Instr. Soc.33 (1956) 438 All-glass reference electrodes Ag/AgCl/Cl- electrode Liquid junction Reference electrodes in molten chlorides

  34. Liquid junction (BN, graphite) Pseudo-reference electrodes Pulse in-situ electrode Ionic membrane Reference electrodes for molten fluorides Stability, durability, reversibility, reproducibility and fast response ? R. Winand, Electrochim.Acta, 17 (1972) 251

  35. BN Reference electrodes for molten fluorides Liquid junction • Ni - NiF2 contained in a thin-walled boron nitride envelope. The electrode was developed for potential measurement in molten LiF-NaF-KF (42-11.5-46.5 mol.%) (FLINAK) at a working temperature of 500-550°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. BN Ni LaF3 Ni foam Reference electrodes for molten fluorides Composé ionique Ionic membrane • 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 electrode is more complicated while the crystal cracks after few experiments. LiF-BeF2-ZrF4 LiF-NaF-KF NaBF4 Tmax ≈ 500° H. R. Bronstein, D. L. Manning, J. Electrochem. Soc.,119(2) (1972) 125 F. R. Clayton, G. Mamantov, D.L. Manning, High Temp. Science, 5 (1973) 358

  37. • 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+ions Example : Ta(V) / Ta(0) P. Taxil, J. Mahenc, J. Appl. Electrochem., 17 (1987) 261. Reference electrodes for molten fluorides Pseudo-reference electrodes Relatively stable reference point, provided no oxidizing or reducing species come into contact with the electrode. According to Mamantov, Ni or Pt wires had a constant potential within ± 10 mV in molten fluorides over 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 solution Example : Pt / PtOx / O2- A.D. Graves, D. Inman, Nature, 208 (1965) 481.

  38. POTENTIOSTAT Classical reference electrode Ni Fe BN Graphite 30 open-circuit relaxation transients NaF-NiF2 Melt: NaF Galvanostatic anodic pulse (ca. 0.2 s) followed by open-circuit relaxation. T = 1025°C 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 reference electrode. The amount of foreign species introduced into the electrolyte must be very small to avoid contamination and consequent modification of the experimental conditions N. Adhoum, J. Bouteillon, D. Dumas, J.C. Poignet, J. Electroanal. Chem., 391 (1995) 63 Y. Berghoute, A. Salmi, F. Lantelme, J. Electroanal. Chem., 365 (1994) 171.

  39. End of the first part

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