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Potentiometric sensors for high temperature liquids Jacques FOULETIER Grenoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France)

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Presentation on theme: "Potentiometric sensors for high temperature liquids Jacques FOULETIER Grenoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France)"— Presentation transcript:

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

2 Potentiometric measurement of activities in molten salts and molten metals Electrolytes: main characteristics of molten and solid electrolytes - structure - conductivity (ionic, mixed) - Electroactivity domains Types of cells: - Formation cells (without membranes) - Concentration cell with a porous membrane - Concentration cells with a solid electrolyte membrane 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 Part 1 Reference electrodes: - for molten metals (Pb, Fe, Na) - for molten salts (chlorides, fluorides)

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

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

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

6 Electrochemical chains: - Various types of electrodes (1 st, 2 nd 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 salt Potentiometric sensor: Black box in contact with the analyzed medium Sensing phenomenon: Measurement of a electro- motive force (e.m.f.) between two output wires Requirement: E = f(a X ) E aXaX 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 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.) Same electronic conductors Cell e.m.f. E E =  (+) -  (-) Electrode (+) Electrode (-) Remark: the analyzed component can be dissolved in electrolyte 2 or 3 or in metal Me

9 Junction: interface between two ionic conductors Junctions Ionic conductor Ionic conductor Simple ionic junction: exchange of only one type of ion Example: > / ((O 2- )) stabilized zirconia/oxide dissolved in molten chloride Multiple ionic junction: exchange of several ions Example: / ((KCl))exchange: K + and Cl - / ((Na + - K + )) Interface Complex ionic junction: solid electrolytes conducting by different ions Examples: > / > stabilized zirconia /  -alumina Equilibrium: O Na + = Na 2 O

10 Electrode: interface between an ionic conductor and an electronic one Electrodes 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) Interface

11 1st kind electrode (metal/metal ion electrode) : M / M n+ Equilibrium: M n+ + n e - = M Types of electrodes (1) 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 2nd kind electrode (coexistence electrode): Ag / AgCl / Cl - Equilibrium: AgCl + e - = Ag + Cl - reference electrode 3rd kind electrode (formation of a new phase): O 2,M /  -Alumina (Na + ) Equilibrium: 2 Na e - + 1/2 O 2 = > (  -Alumina )

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 Gas ELECTROLYTE Examples: - Pt, O 2 / stabilized zirconia Equilibrium :1/2 O e - = O 2- - C g, Cl 2 / molten chloride Equilibrium :1/2 Cl 2 + e - = Cl -

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

14 Equilibrium conditions between two phases: different carriers Equilibrium: O Na + = Na 2 O  SZ  Stabilized zirconia  -alumina Na + O 2- Equilibrium: 1/2 O e - = O 2- Electrode reaction Pt O2O2 ELECTROLYTE Stabilized zirconia  SZ  Pt

15 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 CALCULATION RULES 1. 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 phases on contact, - same ionic carrier: MS1/Pyrex or MS2/Pyrex - different ionic carrier: stabilized zirconia /  -alumina O Na + = Na 2 O Objective: measurement of a(Na 2 O) in NaCl-KCl (-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na 2 O / YSZ / Pt, O 2 (+) MS1 MS2

16 E.m.f. of an ideal chain (-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na 2 O / YSZ / Pt, O 2 (+) Pt Ag AgClNaCl-KCl Pyrex NaCl - KCl - Na 2 O YSZ PtO2O2 (-)(+) e-e- e-e- e-e- O 2- Na + Ag + Na +,K +,Cl - Na +,K +,Cl -,O 2- Main carriers   Ag  AgCl  MS1  Pyrex  MS2  Pt  YSZ E E =  Pt (+) -  Pt (-) Solid Molten salt Solid Molten salt

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

18 R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44 (-) Pt, Fe, Pb (L) / PbO - SiO 2(L) / O 2(g), Pt (+) CELLS WITHOUT MEMBRANE: Main difficulty: solubility of oxygen in lead Concentration cells + SiO 2 Example: measurement of a(PbO) in PbO-SiO 2 mixture

19 CONCENTRATION CELLS: cell with membrane (1) Cell which has identical electrodes and a membrane inserted between solutions differing only in concentration. Two cases: (-) Pt, Fe, Pb (L) / PbO - SiO 2(L) / Porous / PbO (L) / Pb, Fe, Pt (+) oxide Flux of matter: no equilibrium - membrane permeable to several ions (liquid junction) (-) Pt, Fe, Pb (L) / PbO - SiO 2(L) / YSZ / PbO (L) / Pb, Fe, Pt (+) > Equilibrium: theoretical e.m.f. - membrane permeable only to one ion (solid electrolyte)

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

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

22 R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44 Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871  (-) Pt, Fe, Pb (L) / PbO - SiO 2(L) / YSZ / PbO / Pb, Fe, Pt (+) CONCENTRATION CELLS: cell with membrane (4)

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 Solid electrolytes: Main characteristics The solid electrolyte are generally composed of host lattices (ZrO 2, ThO 2, PbCl 2 ), doped with the introduction of cations with different valences (Ca 2+, Y 3+, 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 ZrO 2 SrCl 2 Only a few solid electrolytes are available: ZrO 2 -Y 2 O 3, (ThO 2 -Y 2 O 3 ),  -Alumina, CaF 2, AlF 3, etc.

25 Examples of solid electrolytes Y Oxygenvacancy ZrO 2 - Y 2 O 3Zr O Doping (ZrO 2 -Y 2 O 3 9 mol.%): ZrO 2 Oxide ion conductor NASICON (Na 3 Zr 2 Si 2 PO 12 ) Framework structure with three-dimensional channels suitable for sodium ion conduction Cation conductors  -Alumina (NaAl 11 O 17 )

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: The region (P, T) of predominantly ionic conduction is generally termed the ELECTROLYTIC DOMAIN Patterson diagram Temperature Log PO 2 Domain of predominant ionic conduction (99%) log P(O2) log   ionique  i    n  i    p Variation of the electrical conductivity with partial pressure At given T

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 CaF 2 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 M olten 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) Corrosion Handling not easy Hygroscopicity Compatibility with solids (containers, separators, etc.) However,

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

30 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/Cr 2 O 3 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/Cr 2 O 3 for molten steel monitoring) High temperature measurements Main difficulties: chemical reactivity noticeable semipermeability flux long term stability Coexistence electrodes: M/M x O y Low temperature measurements Main difficulty: electrochemical reversibility - Coexistence electrodes: Pd/PdO - Gas electrodes, Pt/O 2 or MIEC/O 2

31 Reference electrodes (2) Molten metals Examples Intermediate-temperature sensors Ref.: air, Pd-PdO, Ir-Ir 2 O 3 Needle Sensor  = 2 mm One-reading probes for molten iron Ref.: Cr/Cr 2 O 3 Tubular Sensor  = 6 mm Plug Sensor  = 6 mm D. Janke, Met. Trans. B, 13 B (1982) 227. YSZ Cr/Cr 2 O 3 Air Internal reference: Pd-PdO, Ir-Ir 2 O 3 YSZ Molten metal

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 Reference electrodes in molten chlorides Ag/AgCl/Cl - electrode Liquid junction All-glass reference electrodes J.O’M. Bockris, G.J. Hills, D. Inman, L. Young, J. Sci. Instr. Soc. 33 (1956) 438 Very 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) Pseudo-reference electrodes Pulse in-situ electrode Ionic membrane R. Winand, Electrochim. Acta, 17 (1972) 251

35 Ni - NiF 2 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-BeF 2 -ZrF 4 ≈ 15 jours, T max ≈ 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. Liquid junction Reference electrodes for molten fluorides BN

36 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 LiF-BeF 2 -ZrF 4 LiF-NaF-KF NaBF 4 T max ≈ 500°  Composé ionique LaF 3 Ni BN Ni foam The nickel-nickel fluoride reference electrode system exhibiting a membrane from a single crystal lanthanum trifluoride. Because of the solubility of the LaF 3 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 LaF 3 is expensive, the assembling of the electrode is more complicated while the crystal cracks after few experiments. Ionic membrane Reference electrodes for molten fluorides

37 Pseudo-reference electrodes R elatively stable reference point, provided no oxidizing or reducing species come into contact with the electrode. Inert metal in contact with a redox system (M n+ /M p+ ) Example : Nb(V) / Nb(IV) U. Cohen, J. Electrochem. Soc., 130 (1983) A metal M in contact with a solution of M n+ ions Example : Ta(V) / Ta(0) P. Taxil, J. Mahenc, J. Appl. Electrochem., 17 (1987) 261. An inert metal M in contact with a solution Example : Pt / PtO x / O 2- A.D. Graves, D. Inman, Nature, 208 (1965) 481. 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 Reference electrodes for molten fluorides

38 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. 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 T = 1025°C Graphite Melt: NaF Ni NaF-NiF 2 BN Classical reference electrode Fe POTENTIOSTAT Galvanostatic anodic pulse (ca. 0.2 s) followed by open-circuit relaxation. 30 open-circuit relaxation transients

39 End of the first part


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