Voltammetry

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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

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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

3
Method

• Excitation signal applied
• Wave response based on method
• Linear
• Differential pulse
• Square wave
• Cyclic
• Developed current recorded

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Signals
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Electrodes
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Potential ranges

• Number of useful elements for electrodes
• Pt
• Hg
• C
• Au
• Limits
• Oxidation of water
• 2H2O-gt4H O2(g) 4e-
• Reduction of water
• 2H2O 2e- -gtH2 2OH-

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Overpotential

• Overpotential h always reduces theoretical cell
  potential when current is flowing
• h 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

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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

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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

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Methods

• Analyte (A) and product (P)
• In stirred solution convection dominates

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Methods

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

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Hydrodynamic

• Single voltammogram can quantitatively record
  many species
• Requires sufficient separation of potentials
• Need to remove O2

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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

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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

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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

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CV data
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Molten Salt Processes

• Inorganic phase solvent
• High temperature needed to form liquid phase
• Different inorganic salts can be used as solvents
• Separations based on precipitation
• Reduction to metal state
• Precipitation
• Two types of processes in nuclear technology
• Fluoride salt fluid
• Chloride eutectic
• Limited radiation effects
• Reduction by Li

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Molten Salt Reactor

• Fluoride salt
• BeF2, 7LiF, ThF4, UF4 used as working fluid
• thorium blanket
• fuel
• reactor coolant
• reprocessing solvent
• 233Pa extracted from salt by liquid Bi through
  Li based reduction
• Removal of fission products by high 7Li
  concentration
• U removal by addition of HF or F2

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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
  cathode
• Deposition of ions depends upon redox potential

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Electrochemical Separations

• Selection of redox potential allows separations
• Can use variety of electrodes for separation
• Developed for IFR and proposed for ATW
• Dissolution of fuel and deposition of U onto
  cathode
• High temperature, thermodynamic dominate
• Cs and Sr remain in salt, separated later

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Electrorefining
Electrorefining
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Reduction of oxide fuel
Step 2

• Input
• 445 kg oxide (from step 1)
• 135 kg Ca
• 1870 kg CaCl2
• Output
• 398 kg heavy metal (to step 3)
• To step 8
• 2 kg Cs, Sr, Ba
• 189 kg CaO
• 1870 kg CaCl2
• 1 kg Xe, Kr to offgas

Metal
Operating Conditions T 1125 K, 8 hours 4 100
kg/1 PWR assembly
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Uranium Separation
Step 3

• Input
• 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

Operating Conditions T 1000 K, I 500 A, 265
hours 4 100 kg/1 PWR assembly
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Polishing Reduces TRU Discharge
Step 4

• Input 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

Metal
Operating Conditions T 1000 K, I 500 A, 2
hours 1 PWR assembly
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Electrowinning Provide Feed for Fuel
Step 5

• 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

Metal
Operating Conditions T 1000 K, I 500 A, 3.7
hours for U/TRU/RE, 6.2 hours for U 1 PWR assembly
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Reduction of Rare Earths
Step 7

• Input
• 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

Metal
Operating Conditions T 1000 K, 8 hours
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Recycle Salt Reduction of Oxide
Step 8

• Input
• Chlorination
• 189 kg CaO, 1870 kg CaCl2, 239 kg Cl2
• Electrowinning
• 2244 kg CaCl2
• Output
• Chlorination
• 2244 kg CaCl2, 54 kg O2
• Electrowinning (to 2)
• 1870 kg CaCl2, 135 kg
• Ca, (239 kg Cl2)

Operating Conditions T 1000 K, I 2250 A, 80
hours
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U, TRU, and Fission Product Separation
Step 10

• Input
• 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

Operating Conditions T 1000 K, I 500 A, 6.7
hours
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Electrowinning TRU for Salt Recycle
Step 11

• 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

Metal
Operating Conditions T 1000 K, I 500 A,
6.1hours for U/TRU/RE Salt from 10
electrorefining systems
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Reduction to Remove Rare Earths
Step 13

• Input
• 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

Metal
Operating Conditions T 1000 K, 8 hours