Potentiometric sensors for high temperature liquids cques FOULETIER enoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France) mail: Jacques.Fouletier@ lepmi . inpg .fr ronique GHETTA SC, IN2P3-CNRS, 53 Avenue des Martyrs, 38026 GRENOBLE Cedex (France) mail: Veronique. Ghetta @ lpsc .in2p3.fr MATGEN-IV: International Advanced School on Materials for Generation-IV Nuclear Reacto Cargèse, Corsica, September 24 - October 6, 2007 ML 4-1 & ML 4-2
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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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Potentiometric sensors for high temperature liquids
Jacques FOULETIERGrenoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France)E-mail: [email protected]
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)
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
From chemical potential
toElectrochemical
potential
MatgenIV going away for Girolata
Chemical and electrochemical potentials
S
1 mole
€
μj = dGdnj
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟ T,P,ni≠j
= G j
Chemical potential:
Chemical potential: work for thetransfer of one mole of a neutralspecies within S
= 0
= 0
S
1 moleElectrochemical potential:
Electrochemical potential: work forthe transfer of one mole of ionswithin S at a potential
≠ 0
= 0
Chemicalcontribution
Electrostaticcontribution
€
˜ μ j = μ j + z j FΦ
Electrochemical chains:- Various types of electrodes (1st, 2nd types, etc.)
Analysis of a component X dissolved in a molten metal or a molten salt
Potentiometric sensor: Black box in contact with the analyzed mediumSensing phenomenon: Measurement of a electro-motive force (e.m.f.) between two output wires
Requirement: E = f(aX)
E
aX
The 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.
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.
EE = (+) - (-)
Electrode (+)Electrode (-)
Remark: the analyzed component can be dissolved in electrolyte 2 or 3 or in metal Me
• Junction: interface between two ionic conductors
Junctions
Ionic conducto
r
Ionicconducto
r
Simple ionic junction: exchange of only one type of ionExample: <<O2->> / ((O2-)) stabilized zirconia/oxide dissolved in molten chloride
Multiple ionic junction: exchange of several ionsExample: <KCl> / ((KCl)) exchange: K+ and Cl-
<NASICON, Na+> / ((Na+ - K+))
Interface
Complex ionic junction: solid electrolytes conducting by different ionsExamples: <<O2->> / <<Na+>> stabilized zirconia / -alumina
Equilibrium: O2- + 2 Na+ = Na2O
• Electrode: interface between an ionic conductor and an electronic one
Electronic conductor:- solid or liquid metals or alloys- mixed ionic-electronic conductors (MIEC)
Interface
• 1st kind electrode (metal/metal ion electrode) : M / Mn+
Equilibrium: Mn+ + 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
Equilibrium conditions between two phases: same carriers
j j
Exchange of one particle (ion or electron)
€
˜ μ jα = ˜ μ j
βEquilibrium:
€
μ jα + z j Fφα = μ j
β + z j Fφβ
φα − φβ = −1
z j Fμ j
α − μ jβ
( )
Galvani potential difference: no method for measuring
€
˜ μ jα = ˜ μ j
β and ˜ μ kα = ˜ μ k
β j j
Exchange of more than one particle
k k
€
φ −φ =− 1z j F
μ jα − μ j
β( ) = −
1z j F
μ kα − μ k
β( )
Flux of matter generally, no equilibrium
Equilibrium conditions between two phases: different carriers
€
˜ μ O2− + 2 ˜ μ Na+ = μ Na2O
Equilibrium: O2- + 2 Na+ = Na2O
€
μO2− − 2FφSZ + 2μ
Na+ + 2Fφβ = μNa2O
φβ − φSZ =1
2FμNa2O − 2μ
Na+ − μO2−( )
SZ
Stabilizedzirconia
-alumina
Na+O2-
€
12
μO2+ 2 ˜ μ e− = ˜ μ O2−
Equilibrium: 1/2 O2 + 2 e- = O2-
Electrode reaction
Pt
O2
ELECTROLYTEStabilizedzirconia
12μO2
+2μe− −2FφPt= μO2− −2FφSZ
φPt−φSZ=1
4FμO2
+12F
+2μe− −μO2−( )
φPt−φSZ=RT4F
lnPO2+
12F
+2μe− −μO2−( )
SZ
Pt
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
€
˜ μ Na+α ,Pyrex = ˜ μ
Na+β,Pyrex or ˜ μ
O2−α ,YSZ = ˜ μ
O2−β,YSZ
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 phaseson contact,
- same ionic carrier: MS1/Pyrex or MS2/Pyrex
- different ionic carrier: stabilized zirconia / -alumina
Reference electrodes:- for molten metals (Pb, Fe, Na)- for molten salts (chlorides, fluorides)
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 pressure
€
Y2O3 → 2 YZr' + 3OO
× + VO••ZrO2
€
KCl → 2KSr' + ClCl
× + VCl•SrCl2
• Only a few solid electrolytes are available: ZrO2-Y2O3, (ThO2-Y2O3), -Alumina, CaF2, AlF3, etc.
Examples of solid electrolytes
Y
OxygenOxygenvacancyvacancy
ZrO2 - Y2O3
ZrZr
O
Doping (ZrO2-Y2O3 9 mol.%):
€
Y2O3 → 2 YZr' + 3OO
× + VO••
ZrO2
Oxide ion conductor
NASICON (Na3Zr2Si2PO12)
Framework structure with three-dimensionalchannels suitable for sodium ion conduction
Cation conductors
-Alumina (NaAl11O17)
Solid electrolytes (case of oxides): Main characteristics
• However, electronic species may also be present due to equilibria between the electrolyte and the gaseous phase:
€
12
O2 + VO•• → OO
× + 2h• or OO× → VO
•• + 2e− +12
O2
The region (P, T) of predominantly ionicconduction is generally termed theELECTROLYTIC DOMAIN
Patterson diagram
Temperature
Log P
O2
Domain of predominant
ionic conduction(99%)
log P(O2)
log
ionique
i n i p
Variation of theelectrical conductivitywith partial pressure
At given T
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)
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)
• Corrosion
• Handling not easy
• Hygroscopicity
• Compatibility with solids (containers, separators, etc.)
However,
Reference electrodes:- for molten metals (Pb, Fe, Na)- for molten salts (chlorides, fluorides)
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 thesemipermeability flux: use of Cr/Cr2O3 for molten steel monitoring)
High temperature measurements
Main difficulties:• chemical reactivity• noticeable semipermeability flux• long term stability
Coexistence electrodes: M/MxOy
Low temperature measurements
Main difficulty:• electrochemical reversibility
- Coexistence electrodes: Pd/PdO- Gas electrodes, Pt/O2 or MIEC/O2
No universally accepted reference electrode is available for electrochemical studies although 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 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.
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 350-500°C)
Ionic Membrane Liquid junction
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
• 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.
Liquid junction
Reference electrodes for molten fluorides
BN
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
LiF-BeF2-ZrF4 LiF-NaF-KF NaBF4
Tmax ≈ 500°
Composé ionique
LaF3
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 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.
Ionic membrane
Reference electrodes for molten fluorides
Pseudo-reference electrodes
Relatively 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.
€
E = E° +RTF
lnNb(V)[ ]Nb(IV[ ]
• 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.
€
E = E'° +RT5F
ln Ta(V)[ ]
• An inert metal M in contact with a solutionExample : Pt / PtOx / O2-
A.D. Graves, D. Inman, Nature, 208 (1965) 481.
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
Reference electrodes for molten fluorides
N. 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.
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 toavoid contamination and consequent modification of the experimental conditions