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
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)
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)
MatgenIV going away for Girolata
From chemical potential
to
Electrochemical potential
Chemical and electrochemical potentials
Chemical potential:
dG 

 j  
dn 
 j T,P,n
 Gj
i j

S
F = 0
Electrochemical potential:

˜ j   j  z j FF
Chemical
contribution
Electrostatic
contribution
Chemical potential: work for the
transfer of one mole of a neutral
species within S

S
F ≠ 0
F = 0
F = 0
Electrochemical potential: work for
the transfer of one mole of ions
within S at a potential F
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 medium
Sensing phenomenon: Measurement of a electromotive force (e.m.f.) between two output wires
aX
Requirement: E = f(aX)
E
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.
Electrochemical chains
Electrode (-)
Same electronic conductors
Electrode (+)
(-) 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.)
Cell e.m.f.
E
E = f(+) - f(-)
Remark: the analyzed component can be dissolved in electrolyte 2 or 3 or
in metal Me
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+))
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)
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 / ClEquilibrium:
AgCl + e- = Ag + Clreference 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
Types of electrodes (2)
ELECTROLYTE
METAL
Gas
GAS ELECTRODE
The overall reaction requires a Three Phase
Boundary (TPB) between an electrolyte,
a metal and a gas
Examples:
- Pt, O2 / stabilized zirconia
Equilibrium :
1/2 O2 + 2 e- = O2- Cg, Cl2 / molten chloride
Equilibrium :
1/2 Cl2 + e- = Cl-
Equilibrium conditions between two phases: same carriers
Exchange of one particle (ion or electron)
a
j

j
Equilibrium:

˜ aj  
˜ j
 aj  z j F f a   j  z j F f 
fa


1
f a  f  
 aj   j
zjF
f

Galvani potential difference: no method for measuring

Exchange of more than one particle
a
j

j
k
k


˜ aj  
˜ j and 
˜a


˜
k
k
f a  f  

Flux of matter




1
1

 aj   j  
a


k
k
zj F
zjF

generally, no equilibrium
Equilibrium conditions between two phases: different carriers
Stabilized
zirconia
-alumina
Equilibrium:
O2-
O2- + 2 Na+ = Na2O
Na+

˜ O 2  2 
˜ Na    Na 2O
fSZ

f
O
2
 2 F f SZ  2
Na

 2 F f    Na 2O

1
 Na 2O  2    2
Na
O
2F
f   f SZ

Electrode reaction
ELECTROLYTE
Stabilized
zirconia
Pt
O2
fSZ
fPt

Equilibrium:
1
O 2  2  e
2
1/2 O2 + 2 e- =
O2-
1
 O 2  2 
˜ e  
˜ O 2
2
 2 F f Pt   O 2  2 F f SZ


f Pt  fSZ 
1
1


 2  e    O 2
O 2
4F
2F
f Pt  fSZ 
RT
1
ln PO 2 
 2  e    O 2
4F
2F


Objective: measurement of
a(Na2O) in NaCl-KCl
E.m.f. calculation of an ideal chain:
(-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na2O / YSZ / Pt, O2 (+)
MS1
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 interfaces
CALCULATION RULES
1. Within each solid electrolyte, the electrochemical potential of the majority carrier is constant:
(YSZ or Pyrex)
,Pyrex

˜ a,Pyrex


˜


Na
Na
,YSZ
or 
˜ a,YSZ


˜
2
2
O
O
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:

˜ a,MS1
 
˜ a,Pyrex


Na
stabilized zirconia / -alumina
O2- + 2 Na+ = Na2O

Na

˜ O 2  2 
˜ Na    Na 2O
E.m.f. of an ideal chain
(-) Pt / Ag / AgCl / NaCl - KCl / Pyrex / NaCl - KCl - Na2O / YSZ / Pt, O2 (+)
Molten salt
Solid Molten salt
(-)
Pt
Ag AgCl
Solid
Main
carriers
e-
e-
Ag+
NaCl-KCl
Molten salt
Na+,K+,Cl-
Pyrex NaCl - KCl - Na2O YSZ
Molten salt
Na+
Na+,K+,Cl-,O2-
O2-
Pt
O2 (+)
e-
fPt
f
fYSZ
fPt
fAg fAgCl
fMS1
fPyrex
fMS2
E = fPt(+) - fPt(-)
E
The roman catholic church
Types of cells:
- Cells without membrane
- Concentration cell with a porous membrane
- Concentration cells with a solid electrolyte membrane
CELLS WITHOUT MEMBRANE:
(-)
Example: measurement of
a(PbO) in PbO-SiO2 mixture
Pt, Fe, Pb(L) / PbO - SiO2(L) / O2(g), Pt
(+)
+ SiO2
 G RT
RT
E f

lna PbO in (PbOSiO 2 ) 
lnPO
2
2F
2F
4F
Main difficulty: solubility of oxygen in lead
Concentration cells
R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44
CONCENTRATION CELLS: cell with membrane (1)
Cell which has identical electrodes and a membrane inserted between solutions
differing only in concentration.
Two cases:
- 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 (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
(+)
CONCENTRATION CELLS: cell with membrane (3)
(-)
Pt, Fe, Pb(L) / PbO - SiO2(L)
((PbO)) + 2 e- = Pb + O2-
a

/ YSZ / PbO(L) / Pb, Fe, Pt
(PbO) + 2 e- = Pb + O2-
a
 o(PbO)  2 
˜ Pt,()


˜
2
e
O

˜ a 2  
˜  2
O
O
 ((PbO))  2 
˜ Pt,()
 
˜  2
e
O
o
Pt
 (PbO)
 2 ePt,()  2 F f ()
 
˜ a 2
 
˜ a 2  
˜  2
O
O
O
o
 (PbO)
 RTlna((PbO))  2 
˜ ePt,()  
˜  2
O

Pt
Pt
E  f()
 f()

Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871
RT
lna((PbO))
2F
(+)
CONCENTRATION CELLS: cell with membrane (4)
(-)
Pt, Fe, Pb(L) / PbO - SiO2(L)
a

/ YSZ / PbO / Pb, Fe, Pt
Pt
Pt
E  f()
 f()

RT
lna((PbO))
2F

R. Sridhar, J.H.E. Jeffes, Trans. Inst. Mining Met., 76 (1967) C44
Z. Kozuka, C.S. Samis, Met. Trans., 1 (1970) 871
(+)
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)
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
Y2O 3
ZrO2
'

 2 YZr
 3 OO  VO
SrCl2

KCl  2 K'Sr  Cl Cl  VCl

- 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
• Only a few solid electrolytes are available: ZrO2-Y2O3, (ThO2-Y2O3), -Alumina,
CaF2, AlF3, etc.
ZrO2 - Y2O3
Examples of solid electrolytes
-Alumina (NaAl11O17)
O
Zr
Doping (ZrO2-Y2O3 9 mol.%):
Y2O 3
'

 2 YZr
 3 OO  VO
NASICON (Na3Zr2Si2PO12)
ZrO2
Oxygen
vacancy
Y
Framework structure with three-dimensional
channels suitable for sodium ion conduction
Oxide ion conductor
Cation conductors
Solid electrolytes (case of oxides): Main characteristics
• However, electronic species may also be present due to equilibria between the
electrolyte and the gaseous phase:
1
1


O2  VO
 OO  2 h or OO  VO
 2 e   O2
2
2
si
sn
si
Temperature
sp
sionique
At given T
Log PO2
log s
Variation of the
electrical conductivity
with
partial pressure
log P(O2)
The region (P, T) of predominantly ionic
conduction is generally termed the
ELECTROLYTIC DOMAIN
Domain of
predominant
ionic conduction
(99%)
Patterson diagram
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)
However,
• Corrosion
• Handling not easy
• Hygroscopicity
• Compatibility with solids (containers, separators, etc.)
Reference electrodes:
- for molten metals (Pb, Fe, Na)
- for molten salts (chlorides, fluorides)
Reference electrodes (1)
Molten metals (Pb, Fe, Na)
High temperature measurements
Low temperature measurements
Main difficulties:
• chemical reactivity
• noticeable semipermeability flux
• long term stability
Main difficulty:
• electrochemical reversibility
Coexistence electrodes: M/MxOy
Main criteria:
- Coexistence electrodes: Pd/PdO
- Gas electrodes, Pt/O2 or MIEC/O2
- 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)
Reference electrodes (2)
Molten metals
Examples
Intermediate-temperature sensors
Ref.: air, Pd-PdO , Ir-Ir2O3
Air
YSZ
One-reading probes for molten iron
Ref.: Cr/Cr2O3
Internal
reference:
Pd-PdO,
Ir-Ir2O3
Cr/Cr2O3
YSZ
YSZ
Cr/Cr2O3
Molten metal
Molten metal
Tubular
Plug
Sensor
Sensor
f = 6 mm f = 6 mm
Needle
Sensor
f = 2 mm
D. Janke, Met. Trans. B, 13 B (1982) 227.
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.
Reference electrodes in molten chlorides
Ionic Membrane
Liquid junction
All-glass reference electrodes
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
Ag/AgCl/Cl- electrode
Liquid junction
Reference electrodes for molten fluorides
Stability, durability, reversibility, reproducibility and fast response ?
Liquid junction (BN, graphite)
Ionic membrane
Pseudo-reference electrodes
Pulse in-situ electrode
R. Winand, Electrochim. Acta, 17 (1972) 251
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°
BN
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.
Reference electrodes for molten fluorides
 Composé ionique
Ionic membrane
BN
Ni
LaF3
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 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
Reference electrodes for molten fluorides
Pseudo-reference electrodes
Relatively stable reference point, provided no oxidizing or reducing species come into
contact with the electrode.
• Inert metal in contact with a redox system (Mn+/Mp+)
Example : Nb(V) / Nb(IV)
RT Nb(V)
E  E 
ln
F
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)
E  E' 
RT
lnTa(V)
5F
P. Taxil, J. Mahenc, J. Appl. Electrochem., 17 (1987) 261.
• An inert metalM in contact with a solution
Example : Pt / PtOx / O2A.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: Characterization and Analysis, Dekker, New York,
1969, p.537
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
POTENTIOSTAT
Fe
BN
Classical
Ni reference
electrode
Graphite
NaF-NiF2
30 open-circuit
relaxation transients
Melt: NaF
T = 1025°C
Galvanostatic anodic pulse (ca. 0.2 s) followed
by open-circuit relaxation.
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.
End of the first part