16-1 Voltammetry Electrochemistry techniques based on current (i) measurement as function of voltage...
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Transcript of 16-1 Voltammetry Electrochemistry techniques based on current (i) measurement as function of voltage...
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