Potentiometric sensors for high temperature liquids

Potentiometric sensors for high temperature liquids

Jacques FOULETIERGrenoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France)E-mail: [email protected]

Véronique GHETTALPSC, IN2P3-CNRS, 53 Avenue des Martyrs, 38026 GRENOBLE Cedex (France)E-mail: [email protected]

MATGEN-IV: International Advanced School on Materials for Generation-IV Nuclear ReactorsCargèse, Corsica, September 24 - October 6, 2007

ML 4-1 & ML 4-2

mailto:[email protected]










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)

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.)

- Interface equilibrium- Ideal cell e.m.f. calculation

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 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

Electrodes

Ionic conducto

r

Electronic conducto

r

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

• 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

• 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 )

Types of electrodes (2)

GAS ELECTRODEThe overall reaction requires a Three PhaseBoundary (TPB) between an electrolyte,a metal and a gas

METAL

Gas

ELECTROLYTE

Examples:- Pt, O2 / stabilized zirconiaEquilibrium : 1/2 O2 + 2 e- = O2-

- Cg, Cl2 / molten chlorideEquilibrium : 1/2 Cl2 + e- = Cl-

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

O2- + 2 Na+ = Na2O

€

˜ μ O2− + 2 ˜ μ Na+ = μ Na2O

€

˜ μ Na+α ,MS1 = ˜ μ

Na+α ,Pyrex

Objective: measurement of a(Na2O) in NaCl-KCl

(-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na2O / YSZ / Pt, O2 (+)MS1 MS2

E.m.f. of an ideal chain

(-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na2O / YSZ / Pt, O2 (+)

Pt Ag AgCl NaCl-KCl Pyrex

NaCl - KCl - Na2O YSZ Pt O2(-) (+)

e-e- e-O2-Na+Ag+ Na+,K+,Cl- Na+,K+,Cl-,O2-Maincarriers

AgAgCl

MS1Pyrex

MS2

Pt

Pt

YSZ E

E = Pt(+) - Pt

(-)

Solid Molten salt Molten salt

Solid Molten salt Molten salt

Types of cells:- Cells without membrane- Concentration cell with a porous membrane- Concentration cells with a solid electrolyte membrane

The roman catholic church

R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44

(-) Pt, Fe, Pb(L) / PbO - SiO2(L) / O2(g), Pt (+)

CELLS WITHOUT MEMBRANE:

Main difficulty: solubility of oxygen in lead Concentration cells

€

E = −Δf G°

2F−

RT2F

lnaPbO in(PbO−SiO2 ) +RT4F

lnPO2

+ SiO2

Example: measurement of a(PbO) in PbO-SiO2 mixture

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) / Porous / PbO(L) / Pb, Fe, Pt (+)

oxide

Flux of matter: no equilibrium

- membrane permeable to several ions (liquid junction)

(-) 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)

Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871



<<O2->>

Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871



€

μ (PbO)o + 2 ˜ μ e

Pt,(+) = ˜ μ O2−α

˜ μ O2−α = ˜ μ

O2−β

μ ((PbO)) + 2 ˜ μ ePt,(−) = ˜ μ

O2−β

(PbO) + 2 e- = Pb + O2-((PbO)) + 2 e- = Pb + O2-

€

μ(PbO)o + 2μe

Pt,(+) − 2Fφ(+)Pt = ˜ μ

O2−α

˜ μ O2−α = ˜ μ

O2−β

μ(PbO)o + RTlna((PbO)) + 2 ˜ μ e

Pt,(−) = ˜ μ O2−β

€

E = φ(+)Pt − φ(−)

Pt =RT2F

lna((PbO))

R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871

(-) Pt, Fe, Pb(L) / PbO - SiO2(L) / YSZ / PbO / Pb, Fe, Pt (+)

€

E = φ(+)Pt − φ(−)

Pt =RT2F

lna((PbO))


Electrolytes: main characteristics of molten and solid electrolytes

- Structure- Conductivity (ionic, mixed)- Electroactivity domain


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 (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 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

Reference electrodes (2)Molten metals

Examples

Intermediate-temperature sensorsRef.: air, Pd-PdO , Ir-Ir2O3

NeedleSensor

= 2 mm

One-reading probes for molten ironRef.: Cr/Cr2O3

TubularSensor

= 6 mm

PlugSensor

= 6 mm

D. Janke, Met. Trans. B, 13 B (1982) 227.

YSZYSZ

Cr/Cr2O3

Cr/Cr2O3

Air

Internalreference:Pd-PdO,Ir-Ir2O3YSZ

Molten metal Molten metal

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 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


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


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


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.


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

T = 1025°C

Graphite

Melt: NaF

Ni

NaF-NiF2

BN

Classicalreferenceelectrode

Fe

POTENTIOSTAT

Galvanostatic anodic pulse (ca. 0.2 s) followedby open-circuit relaxation.

30 open-circuitrelaxation transients

End of the first part

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Potentiometric sensors for high temperature liquids