16-1

Voltammetry

• Electrochemistry techniques based on current (i) measurement as function of voltage (Eappl)

• Working electrode (microelectrode) place where redox occurs surface area few mm2 to limit current flow

• Reference electrode constant potential reference (SCE)

• Counter electrode inert material (Hg, Pt) plays no part in redox but completes circuit

• Supporting electrolyte alkali metal salt does not react with electrodes but

has conductivity

16-2

Voltammetry

• Potentiostat (voltage source) drives cell supplies whatever voltage needed between working

and counter electrodes to maintain specific voltage between working and reference electrode Almost all current carried between working and

counter electrodes Voltage measured between working and

reference electrodes Analyte dissolved in cell not at electrode surface

16-3

Method

• Excitation signal applied Wave response based on method

LinearDifferential pulseSquare waveCyclic

Developed current recorded

16-4

Signals

16-5

Electrodes

16-6

Potential ranges• Number of useful elements for

electrodes Pt Hg C Au

• Limits Oxidation of water

2H2O->4H+ +O2(g) + 4e-

Reduction of water 2H2O+ 2e- ->H2 + 2OH-

16-7

Overpotential

• Overpotential always reduces theoretical cell potential when current is flowing = Ecurrent - Eequilibrium

• Overpotential due to electrode polarization: concentration polarization - mass transport

limited adsorption/desorption polarization - rate of

surface attach/detachment charge-transfer polarization - rate of redox

reaction reaction polarization - rate of redox reaction of

intermediate in redox reaction• Overpotential means must apply greater potential

before redox chemistry occurs

16-8

Voltammograms• Current against applied voltage

Increase in current at potential at which analyte is reduced Reaction requires

electrons * supplied by

potentiostat• Half wave potential (E1/2) is close to

E0 for reduction reaction

• Limiting current proportional to analyte activity

16-9

Methods

• Current is just measure of rate at which species can be brought to electrode surface Stirred -

hydrodynamic voltammetry Nernst layer near

electrode

* Diffusion layer

* Migration

* convection

16-10

Methods

• Analyte (A) and product (P)

• In stirred solution convection dominates

16-11

Methods

• Current is a measure of how fast the analyte can go to electrode surface

16-12

Hydrodynamic• Single voltammogram can

quantitatively record many species Requires sufficient

separation of potentials

• Need to remove O2

16-13

Hanging Hg electrodePolarography

• Differs from hydrodynamic unstirred (diffusion

dominates) dropping Hg electrode

(DME) is used as working electrode

current varies as drop grows then falls off

16-14

Linear Scan• Advantages of DME

clean surface and constant mixing

constant current during drop growth

No H2 formation

• Disadvantages of DME: Hg easily oxidized cumbersome to use

16-15

16-16

16-17

Cyclic Voltammetry

• Oxidation and reduction

• Variation of rates

• Peak potentials Anode (bottom peak) Cathode (top peak)

Difference 0.0592/n

• Peak currents Cathode (line to peak) Anode (slope to bottom)

Peak currents equal and opposite sign

• Mechanisms and rates of redox

16-18

CV data

16-19

Molten Salt Processes

• Inorganic phase solventHigh temperature needed to form liquid phaseDifferent inorganic salts can be used as solvents

• Separations based on precipitationReduction to metal statePrecipitation

• Two types of processes in nuclear technologyFluoride salt fluidChloride eutectic

Limited radiation effectsReduction by Li

16-20

Molten Salt Reactor • Fluoride salt

BeF2, 7LiF, ThF4, UF4 used as working fluid

thorium blanketfuelreactor coolantreprocessing solvent

233Pa extracted from salt by liquid Bi through Li based reduction

Removal of fission products by high 7Li concentrationU removal by addition of HF or F2

16-21

Pyroprocesses• Electrorefining• Reduction of metal ions to metallic state• Differences in free energy between metal ions and salt• Avoids problems associated with aqueous chemistry

Hydrolysis and chemical instability• Thermodynamic data at hand or easy to obtain• Sequential oxidation/reduction

Cations transported through salt and deposited on cathodeDeposition of ions depends upon redox potential

16-22

Electrochemical Separations

• Selection of redox potential allows separationsCan use variety of electrodes for separation

• Developed for IFR and proposed for ATWDissolution of fuel and deposition of U onto

cathodeHigh temperature, thermodynamic dominateCs and Sr remain in salt, separated later

16-23

Electrorefining

Electrorefining

16-24

Reduction of oxide fuelInput

• 445 kg oxide (from step 1)

• 135 kg Ca

• 1870 kg CaCl2

Output

• 398 kg heavy metal (to step 3)

• To step 82 kg Cs, Sr, Ba189 kg CaO1870 kg CaCl2

• 1 kg Xe, Kr to offgas

Step 2

Metal

Operating ConditionsT= 1125 K, 8 hours4 100 kg/1 PWR assembly

16-25

Anode

Uranium SeparationInput

398 kg heavy metal (from step 2)• 385 kg U, 20 kg U3+(enriched, 6%)• 3.98 kg TRU, 3.98 kg RE• 188 kg NaCl-KCl

Output• 392 kg U on cathode• To step 4 (anode)

15 g TRU, 14 g RE, 2.8 kg U, 5 kg Noble Metal• Molten Salt to step 5

10 kg U, 3.9 kg TRU,

3.9 kg RE, 188 kg NaCl-KCl

Step 3

Operating ConditionsT= 1000 K, I= 500 A, 265 hours4 100 kg/1 PWR assembly

16-26

Anode

Polishing Reduces TRU DischargeInput from Anode #3• 5 kg Noble Metal, 2.8 kg U, 15 g TRU, 14 g RE,

1.1 kg U3+, 18.8 kg NaCl-KCl

Output

Anode• 5 kg Noble Metal, 0.15 g U, 0.045 g TRU, 0.129 g

RE

Cathode• 1.5 g Noble Metal, 2.9 kg U

Molten Salt (to #3)• 28 g Noble Metal, 1 kg U, 15 g TRU, 14 g RE, 18.8

kg NaCl-KCl

Step 4

Metal

Operating ConditionsT= 1000 K, I= 500 A, 2 hours1 PWR assembly

16-27

Electrowinning Provide Feed for Fuel

Input from molten salt from #3

• 10 kg U, 4 kg TRU, 4 kg RE, 4.3 kg Na as alloy, 188 kg NaCl-KCl

Output

Cathode

• U extraction 9.2 kg

• U/TRU/RE extraction, 1 kg U, 4 kg TRU, 0.5 kg RE

Molten Salt (to #7)

• 3.5 kg RE, 192 kg NaCl-KCl

Step 5

Metal

Operating ConditionsT= 1000 K, I= 500 A, 3.7 hours for U/TRU/RE, 6.2 hours for U1 PWR assembly

16-28

Reduction of Rare EarthsInput• Molten Salt from #5

3.4 kg RE• 1.7 kg Na as alloy• 188 kg NaCl-KCl

Output• Molten Salt (to step 3)

189 kg NaCl-KCl• Metal Phase

3.4 kg RE

Step 7

Metal

Operating ConditionsT= 1000 K, 8 hours

16-29

Recycle Salt: Reduction of OxideInput

• Chlorination189 kg CaO, 1870 kg CaCl2, 239 kg Cl2

• Electrowinning2244 kg CaCl2

Output

• Chlorination2244 kg CaCl2, 54 kg O2

• Electrowinning (to #2)1870 kg CaCl2, 135 kg

Ca, (239 kg Cl2)

Step 8

Operating ConditionsT= 1000 K, I= 2250 A, 80 hours

16-30

Anode TRU

U, TRU, and Fission Product SeparationInput• 45 kg from Step 9 (includes Zr)

Includes 9.5 kg TRU, 0.5 kg RE

Output• Anode

33 kg NM, 2 kg U• Molten Salt (to #11)

Small amounts of U, TRU, RE • Cathode (to #12)

Most TRU, RE

Step 10

Operating ConditionsT= 1000 K, I= 500 A, 6.7 hours

16-31

Electrowinning TRU for Salt Recycle

Input from molten salt from #10

• 1.7 kg U, 7.4 kg TRU, 0.5 kg RE, 2.8 kg Na as alloy, 188 kg NaCl-KCl

Output

Cathode (to #12)

• U/TRU/RE extraction, 1.7 kg U, 7.4 kg TRU, 0.1 kg RE

Molten Salt (to #13)

• 0.4 kg RE, 191 kg NaCl-KCl

Step 11

Metal

Operating ConditionsT= 1000 K, I= 500 A, 6.1hours for U/TRU/RE

Salt from 10 electrorefining systems

16-32

Reduction to Remove Rare EarthsInput• 0.4 kg RE (from #11), 188 kg

NaCl-KCl, 0.2 kg Na as alloy

Output• Molten Salt

188 kg NaCl-KCl• Metal Phase

0.4 kg RE

Step 13

Metal

Operating ConditionsT= 1000 K, 8 hours