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

Transcription

1 Voltammetry Electrochemistry techniques based on current (i) measurement as function of voltage (E appl ) Working electrode (microelectrode) place where redox occurs surface area few mm 2 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 16-1

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 16-2

3 Method Excitation signal applied Wave response based on method Linear Differential pulse Square wave Cyclic Developed current recorded 16-3

4 Signals 16-4

5 Electrodes 16-5

6 Number of useful elements for electrodes Pt Hg C Au Limits Oxidation of water 2H 2 O->4H+ +O 2 (g) + 4e- Reduction of water 2H 2 O+ 2e - ->H 2 + 2OH - Potential ranges 16-6

7 Overpotential Overpotential η always reduces theoretical cell potential when current is flowing η = E current -E equilibrium 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 16-7

8 Current against applied voltage Increase in current at potential at which analyte is reduced Reaction requires electrons * supplied by potentiostat Half wave potential (E 1/2 ) is close to E 0 for reduction reaction Voltammograms Limiting current proportional to analyte activity 16-8

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 16-9

10 Methods Analyte (A) and product (P) In stirred solution convection dominates 16-10

11 Methods Current is a measure of how fast the analyte can go to electrode surface 16-11

12 Single voltammogram can quantitatively record many species Requires sufficient separation of potentials Need to remove O 2 Hydrodynamic 16-12

13 Differs from hydrodynamic unstirred (diffusion dominates) dropping Hg electrode (DME) is used as working electrode current varies as drop grows then falls off Hanging Hg electrode Polarography 16-13

14 Advantages of DME clean surface and constant mixing constant current during drop growth No H 2 formation Disadvantages of DME: Hg easily oxidized cumbersome to use Linear Scan 16-14

15 16-15

16 16-16

17 Cyclic Voltammetry Oxidation and reduction Variation of rates Peak potentials Anode (bottom peak) Cathode (top peak) Difference /n Peak currents Cathode (line to peak) Anode (slope to bottom) Peak currents equal and opposite sign Mechanisms and rates of redox 16-17

18 CV data 16-18

19 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 16-19

20 Molten Salt Reactor Fluoride salt BeF 2, 7 LiF, ThF 4, UF 4 used as working fluid thorium blanket fuel reactor coolant reprocessing solvent 233 Pa extracted from salt by liquid Bi through Li based reduction Removal of fission products by high 7 Li concentration U removal by addition of HF or F

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 cathode Deposition of ions depends upon redox potential

22 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 16-22

23 Electrorefining Electrorefining 16-23

24 Input 445 kg oxide (from step 1) 135 kg Ca 1870 kg CaCl 2 Output Reduction of oxide fuel 398 kg heavy metal (to step 3) To step 8 2 kg Cs, Sr, Ba 189 kg CaO 1870 kg CaCl 2 1 kg Xe, Kr to offgas Step 2 Metal Operating Conditions T= 1125 K, 8 hours kg/1 PWR assembly 16-24

25 Input Uranium Separation Step kg heavy metal (from step 2) 385 kg U, 20 kg U 3+ (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 Anode Molten Salt to step 5 10 kg U, 3.9 kg TRU, Operating Conditions 3.9 kg RE, 188 kg NaCl-KCl kg/1 PWR assembly T= 1000 K, I= 500 A, hours

26 Polishing Reduces TRU Discharge Input from Anode #3 Step 4 5 kg Noble Metal, 2.8 kg U, 15 g TRU, 14 g RE, 1.1 kg U 3+, 18.8 kg NaCl-KCl Output Anode 5 kg Noble Metal, 0.15 g U, g TRU, 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 Anode Metal Operating Conditions T= 1000 K, I= 500 A, hours 1 PWR assembly

27 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 Operating Conditions Metal T= 1000 K, I= 500 A, 3.7 hours for U/TRU/RE, 6.2 hours for U 1 PWR assembly

28 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 Reduction of Rare Earths Step 7 Metal Operating Conditions T= 1000 K, 8 hours 16-28

29 Input Recycle Salt: Reduction of Oxide Chlorination 189 kg CaO, 1870 kg CaCl 2, 239 kg Cl 2 Electrowinning 2244 kg CaCl 2 Output Chlorination Step kg CaCl 2, 54 kg O 2 Electrowinning (to #2) 1870 kg CaCl 2, 135 kg Ca, (239 kg Cl 2 ) Operating Conditions T= 1000 K, I= 2250 A, 80 hours

30 U, TRU, and Fission Product Separation Input Step 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 Anode TRU Operating Conditions T= 1000 K, I= 500 A, 6.7 hours

31 Electrowinning TRU for Salt Recycle Step 11 Input from molten salt from # 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 16-31

32 Input Reduction to Remove Rare Earths Step 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 16-32