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Title: Advanced Electrochemical Separations of Actinide/Fission Products via the Control of Nucleation and Growth of Electrodeposits

Abstract

It is well known that according to current knowledge the pyroprocessing of spent nuclear fuels invariably leads to the dendritic form of electrodeposits, which from technological point of view is undesirable for many reasons. The aim of this research was to study and preclude the formation of electrodeposits with dendritic morphology (of essential importance to FC-1 Program). This project is characterized with four novel study categories: (1) exploring electrodeposit morphology control by controlling the electrochemical deposition reaction rate, (2) the use of rotating disk electrode technique to distinguish the electrodeposition control modes (reaction rate vs. diffusion), (3) morphology studies of lanthanides (La) and actinides (U) in characteristic stages of nucleation and growth, (4) exploring the need for separation of electrochemical cell in compartments for cathodic and anodic reactions. All these study categories were applied on two distinct electrolyte systems (a) high temperature ionic liquids, i.e. molten LiCl-KCl salts, and (b) room temperature ionic liquids (compounds exclusively made of organic cations, and organic/inorganic anions). Because the quality (dense and smooth) of electrodeposits is a direct function of the electrochemical reduction rate (slower the better), the major expectation was that addition of organic molecules would provide the desired slowing down of electroreductionmore » effect, avoiding the undesired dependence on mass transfer. What was learned Electrodeposition is controlled by reaction rate (active region) The PI decided to use the rotating disk electrode technique to study the electrodeposition in molten salts. Although this technique is regarded as one of the most informative toward the answers on reaction mechanisms it is almost absent from literature on electrochemistry from molten salt electrolytes. What this research has found, by using the rotating disk electrode technique (RDE), that electrodeposition from molten salt is actually always in the electrochemical reaction rate control mode, not mass transfer controlled. If electrodeposition were a function of mass-transfer then electrodeposition would be a function of rotation speed. This is because faster rotation rates would produce smaller liquid film thickness at electrode, which would lead to higher concentration gradient, i.e. faster reaction rate. That is not what was found on the examples of electrodeposition of lanthanum and uranium. By running the effect of rotation speed on cyclic voltammetry for particular scan rate it was found that the speed of rotation was effective only at very small scan rates, 5-20mV/s and lower. The independence from rotation speed above 20 mV/s scan rate tells that the mass transfer is not an important parameter because the electrodeposition itself is electrochemical reaction rate controlled. It is interesting to note that all published papers on the subject of electrochemistry of lanthanides and actinides in molten salts in unison conclude that electroreduction is reversible/quasi-reversible (mass transfer controlled) at scanning rates up to about 100mV/s. The contradictory conclusion is most likely caused by experimental conditions used by others, working electrode being of cylindrical geometry as the result of exclusive use of a wire for a working electrode. Four stages of electrodeposition from molten salts In contrast to stationary wire electrode used in almost all published literature, in this project with RDE technique, it was found that electrodeposition of lanthanides (La), and actinides (U) is characterized with four distinct stages. The first two stages are nucleation and growth of nuclei until thin, dense and uniform film is produced. These first two stages are independent of electrochemical potential scanning rate and the speed of electrode rotation. The independence from these two kinetic parameters can be explained by direct conversion of already adsorbed layer of LaCl 3 and UCl 3 to corresponding metallic state. In the first two stages, we have nucleation of new solid phase (La, or U) on the substrate of different metal (W) used as working electrode. Once the working electrode is covered with the film of metal being electroreduced then further metal electrodeposition happens on the substrate with matching crystalline lattice (La on La, U on U). These are third and fourth stages and they are strictly governed by surface area changes of the produced metal nuclei and nuclei aggregates Very large electrodeposition currents at slow scanning rates In cyclic voltammetry, if the rates were controlled by mass transfer then the slower scanning rates would respond with smaller currents, i.e. smaller rates, because of smaller concentration gradient in the vicinity of electrode surface. In this study, opposite was found that the smaller scanning rates produced very large electrodeposition currents. Furthermore, only at very slow scanning rates the speed of electrode rotation was effective, indicating the introduction of mass transfer rate as contributing factor in the overall rate of electrodeposition. The finding of dependence of electrodeposition on mass transfer at low scan rates contradicts the general knowledge in electrochemistry that at slow scanning rates the mass transfer is fast enough to satisfy the rate of reaction demanded by the particular electrochemical potential. Obviously, some additional phenomena are influencing the electrodeposition (to be proposed below). The current densities of 2.5-5.0A/cm 2 can readily be produced during electrodeposition of lanthanum at scanning rate of 5mV/s and electrode rotation rate of 800 rpm (rotation rate per minute). The high current densities present in electrodeposition of lanthanum (and uranium) surpass by far the typical current density of industrial aluminum production from molten salts, 1.0A/cm 2. While the high current density in aluminum production can be supported by high concentration of aluminum in molten salt (about 12 wt% of AlF3) this is not the case in typical electrochemical studies with lanthanides and actinides, which are about ten times lower. Therefore, the exceptionally high current density observed during cyclic voltammetry of lanthanum at slow scan rate can be supported only by participation of additional electrochemical reduction reaction. In molten LiCl-KCl-AnCl 3 systems the supporting reaction can be only reduction of lithium chloride. The influence of underpotential deposited lithium Although the reduction of lithium requires potentials more cathodic than the reduction of lanthanum, lithium can still be deposited by so-called underpotential deposition mechanism. The co-reduction of lithium has received no attention in available literature sources and in PI’s opinion it must be addressed as perhaps this is the key reaction phenomenon that could guide the morphology of lanthanide and actinide deposits. Co-deposition of lithium during electrodeposition of lanthanum studies was confirmed by the stability of ceramic material that was used as shroud for working electrode. One more important notice on the reaction rate control and its effect on morphology of deposits. As stated above, the effect of electrode rotation speed was present only for slow scanning rates in cyclic voltammetry, but it is critical to state that even though the reaction rate increased with the increase of rotation speed the reaction never went into mass transfer (diffusion) controlled region Why are so high current densities experienced while working with rotating disk electrodes and are not as high when working with cylindrical geometry, universally used in published literature? The explanation must come again from the role of lithium. If lithium is produced during lanthanum deposition, buy underpotential deposition mechanisms, it stays trapped under electrode disk. Its accumulation under the disk leads to increase of reaction surface area, which in turn leads to faster reaction rate. It is in the form of droplets. In universally used wire-type electrodes lithium is still produced but vertical orientation of wire allows liquid lithium to readily float upward the electrode wall toward the surface of molten salt. This is not possible under horizontally oriented disk electrode. Morphology of electrodeposits in each four stages of deposition The PI is reporting scanning electron micrographs of metallic lanthanum and uranium not only as the final metal electrodeposit but is able to show the morphology of the nuclei and aggregates at each of the four cyclic voltammetry stages discussed above. To the PI’s knowledge, this type of information is not available in open literature, and it is appearing for the first time here in this final report. Electrodeposition of La and Nd from room temperature ionic liquids Electrodeposition of La was studied in 1-Ethyl-3-methylimidazolium dicyanamide (EMIM-DCA), while for Nd electrodeposition 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) was used. The research with room temperature ionic liquids was fully successful. The reaction mechanisms were studied by using the rotating disk electrode technique (as with high temperature molten salts). The reaction mechanisms were determined for each studied system. What was found is that electrodeposition from of La and Nd from ionic liquids was irreversible and controlled by mixed kinetics (mass transfer and electrochemical reaction rates). What was learned is that although the room temperature ionic liquids may find the practical application in various fields of chemical industry, but the PI is not convinced that they would be feasible for electrowinning of metals on industrial scale. The main drawback from application on industrial scale is their poor solubility for metallic compounds (actinide and lanthanide chlorides, for example) and their high viscosity. High costs are another negative property. On the other hand, compared to electrodeposition from high temperature ionic liquids, such as molten LiCl-KCl salts, the room temperature ionic liquids, as the name suggests, offer low temperature operation temperatures. Also, they are not as sensitive to the presence of minor quantities of water (and oxygen) as molten LiCl-KCl do. Electrodeposition of uranium in the presence of additives to molten salts Addition of organic molecules, in the form of charged ionic liquids, and neutral lubricants and greases, unfortunately did not work. This is because the proposed ionic liquids and ultra-high temperature greases showed no chemical stability when added to molten LiCl-KCl. The chemical instability was expressed in the form of produced smoke, and in case of molten salt containing UCl 3, immediate change of color from purple to gray. When high temperature grease was used, all uranium separated from molten salt and floated to the top of molten salt as a solid, black, crust (salt itself became colorless). This performance from ionic liquids and greases as additives to the molten salt electrolytes was not expected so the research had to refocus on the use of room temperature ionic liquids as the electrolytes to study the electrodeposition. Future work The future research should focus on the underpotential deposition of lithium and its effect on the morphology of lanthanides and actinides electrodeposits in molten LiCl-KCl electrolytes. This area of research is of importance for understanding the conditions for prevention of dendritic electrodeposits.« less

Authors:
Publication Date:
Research Org.:
Univ. of Iowa, Iowa City, IA (United States)
Sponsoring Org.:
NEUP
OSTI Identifier:
1595683
Report Number(s):
16-10813
16-10813
DOE Contract Number:  
NE0008556
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS; actinides; lanthanides; molten salts; ionic liquids; electrochemistry of uranium; electrochemistry of lanthanum; nucleation and growth; morphology of uranium electrodeposits

Citation Formats

Pesic, Batric. Advanced Electrochemical Separations of Actinide/Fission Products via the Control of Nucleation and Growth of Electrodeposits. United States: N. p., 2020. Web. doi:10.2172/1595683.
Pesic, Batric. Advanced Electrochemical Separations of Actinide/Fission Products via the Control of Nucleation and Growth of Electrodeposits. United States. doi:10.2172/1595683.
Pesic, Batric. Wed . "Advanced Electrochemical Separations of Actinide/Fission Products via the Control of Nucleation and Growth of Electrodeposits". United States. doi:10.2172/1595683. https://www.osti.gov/servlets/purl/1595683.
@article{osti_1595683,
title = {Advanced Electrochemical Separations of Actinide/Fission Products via the Control of Nucleation and Growth of Electrodeposits},
author = {Pesic, Batric},
abstractNote = {It is well known that according to current knowledge the pyroprocessing of spent nuclear fuels invariably leads to the dendritic form of electrodeposits, which from technological point of view is undesirable for many reasons. The aim of this research was to study and preclude the formation of electrodeposits with dendritic morphology (of essential importance to FC-1 Program). This project is characterized with four novel study categories: (1) exploring electrodeposit morphology control by controlling the electrochemical deposition reaction rate, (2) the use of rotating disk electrode technique to distinguish the electrodeposition control modes (reaction rate vs. diffusion), (3) morphology studies of lanthanides (La) and actinides (U) in characteristic stages of nucleation and growth, (4) exploring the need for separation of electrochemical cell in compartments for cathodic and anodic reactions. All these study categories were applied on two distinct electrolyte systems (a) high temperature ionic liquids, i.e. molten LiCl-KCl salts, and (b) room temperature ionic liquids (compounds exclusively made of organic cations, and organic/inorganic anions). Because the quality (dense and smooth) of electrodeposits is a direct function of the electrochemical reduction rate (slower the better), the major expectation was that addition of organic molecules would provide the desired slowing down of electroreduction effect, avoiding the undesired dependence on mass transfer. What was learned Electrodeposition is controlled by reaction rate (active region) The PI decided to use the rotating disk electrode technique to study the electrodeposition in molten salts. Although this technique is regarded as one of the most informative toward the answers on reaction mechanisms it is almost absent from literature on electrochemistry from molten salt electrolytes. What this research has found, by using the rotating disk electrode technique (RDE), that electrodeposition from molten salt is actually always in the electrochemical reaction rate control mode, not mass transfer controlled. If electrodeposition were a function of mass-transfer then electrodeposition would be a function of rotation speed. This is because faster rotation rates would produce smaller liquid film thickness at electrode, which would lead to higher concentration gradient, i.e. faster reaction rate. That is not what was found on the examples of electrodeposition of lanthanum and uranium. By running the effect of rotation speed on cyclic voltammetry for particular scan rate it was found that the speed of rotation was effective only at very small scan rates, 5-20mV/s and lower. The independence from rotation speed above 20 mV/s scan rate tells that the mass transfer is not an important parameter because the electrodeposition itself is electrochemical reaction rate controlled. It is interesting to note that all published papers on the subject of electrochemistry of lanthanides and actinides in molten salts in unison conclude that electroreduction is reversible/quasi-reversible (mass transfer controlled) at scanning rates up to about 100mV/s. The contradictory conclusion is most likely caused by experimental conditions used by others, working electrode being of cylindrical geometry as the result of exclusive use of a wire for a working electrode. Four stages of electrodeposition from molten salts In contrast to stationary wire electrode used in almost all published literature, in this project with RDE technique, it was found that electrodeposition of lanthanides (La), and actinides (U) is characterized with four distinct stages. The first two stages are nucleation and growth of nuclei until thin, dense and uniform film is produced. These first two stages are independent of electrochemical potential scanning rate and the speed of electrode rotation. The independence from these two kinetic parameters can be explained by direct conversion of already adsorbed layer of LaCl3 and UCl3 to corresponding metallic state. In the first two stages, we have nucleation of new solid phase (La, or U) on the substrate of different metal (W) used as working electrode. Once the working electrode is covered with the film of metal being electroreduced then further metal electrodeposition happens on the substrate with matching crystalline lattice (La on La, U on U). These are third and fourth stages and they are strictly governed by surface area changes of the produced metal nuclei and nuclei aggregates Very large electrodeposition currents at slow scanning rates In cyclic voltammetry, if the rates were controlled by mass transfer then the slower scanning rates would respond with smaller currents, i.e. smaller rates, because of smaller concentration gradient in the vicinity of electrode surface. In this study, opposite was found that the smaller scanning rates produced very large electrodeposition currents. Furthermore, only at very slow scanning rates the speed of electrode rotation was effective, indicating the introduction of mass transfer rate as contributing factor in the overall rate of electrodeposition. The finding of dependence of electrodeposition on mass transfer at low scan rates contradicts the general knowledge in electrochemistry that at slow scanning rates the mass transfer is fast enough to satisfy the rate of reaction demanded by the particular electrochemical potential. Obviously, some additional phenomena are influencing the electrodeposition (to be proposed below). The current densities of 2.5-5.0A/cm2 can readily be produced during electrodeposition of lanthanum at scanning rate of 5mV/s and electrode rotation rate of 800 rpm (rotation rate per minute). The high current densities present in electrodeposition of lanthanum (and uranium) surpass by far the typical current density of industrial aluminum production from molten salts, 1.0A/cm2. While the high current density in aluminum production can be supported by high concentration of aluminum in molten salt (about 12 wt% of AlF3) this is not the case in typical electrochemical studies with lanthanides and actinides, which are about ten times lower. Therefore, the exceptionally high current density observed during cyclic voltammetry of lanthanum at slow scan rate can be supported only by participation of additional electrochemical reduction reaction. In molten LiCl-KCl-AnCl3 systems the supporting reaction can be only reduction of lithium chloride. The influence of underpotential deposited lithium Although the reduction of lithium requires potentials more cathodic than the reduction of lanthanum, lithium can still be deposited by so-called underpotential deposition mechanism. The co-reduction of lithium has received no attention in available literature sources and in PI’s opinion it must be addressed as perhaps this is the key reaction phenomenon that could guide the morphology of lanthanide and actinide deposits. Co-deposition of lithium during electrodeposition of lanthanum studies was confirmed by the stability of ceramic material that was used as shroud for working electrode. One more important notice on the reaction rate control and its effect on morphology of deposits. As stated above, the effect of electrode rotation speed was present only for slow scanning rates in cyclic voltammetry, but it is critical to state that even though the reaction rate increased with the increase of rotation speed the reaction never went into mass transfer (diffusion) controlled region Why are so high current densities experienced while working with rotating disk electrodes and are not as high when working with cylindrical geometry, universally used in published literature? The explanation must come again from the role of lithium. If lithium is produced during lanthanum deposition, buy underpotential deposition mechanisms, it stays trapped under electrode disk. Its accumulation under the disk leads to increase of reaction surface area, which in turn leads to faster reaction rate. It is in the form of droplets. In universally used wire-type electrodes lithium is still produced but vertical orientation of wire allows liquid lithium to readily float upward the electrode wall toward the surface of molten salt. This is not possible under horizontally oriented disk electrode. Morphology of electrodeposits in each four stages of deposition The PI is reporting scanning electron micrographs of metallic lanthanum and uranium not only as the final metal electrodeposit but is able to show the morphology of the nuclei and aggregates at each of the four cyclic voltammetry stages discussed above. To the PI’s knowledge, this type of information is not available in open literature, and it is appearing for the first time here in this final report. Electrodeposition of La and Nd from room temperature ionic liquids Electrodeposition of La was studied in 1-Ethyl-3-methylimidazolium dicyanamide (EMIM-DCA), while for Nd electrodeposition 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) was used. The research with room temperature ionic liquids was fully successful. The reaction mechanisms were studied by using the rotating disk electrode technique (as with high temperature molten salts). The reaction mechanisms were determined for each studied system. What was found is that electrodeposition from of La and Nd from ionic liquids was irreversible and controlled by mixed kinetics (mass transfer and electrochemical reaction rates). What was learned is that although the room temperature ionic liquids may find the practical application in various fields of chemical industry, but the PI is not convinced that they would be feasible for electrowinning of metals on industrial scale. The main drawback from application on industrial scale is their poor solubility for metallic compounds (actinide and lanthanide chlorides, for example) and their high viscosity. High costs are another negative property. On the other hand, compared to electrodeposition from high temperature ionic liquids, such as molten LiCl-KCl salts, the room temperature ionic liquids, as the name suggests, offer low temperature operation temperatures. Also, they are not as sensitive to the presence of minor quantities of water (and oxygen) as molten LiCl-KCl do. Electrodeposition of uranium in the presence of additives to molten salts Addition of organic molecules, in the form of charged ionic liquids, and neutral lubricants and greases, unfortunately did not work. This is because the proposed ionic liquids and ultra-high temperature greases showed no chemical stability when added to molten LiCl-KCl. The chemical instability was expressed in the form of produced smoke, and in case of molten salt containing UCl3, immediate change of color from purple to gray. When high temperature grease was used, all uranium separated from molten salt and floated to the top of molten salt as a solid, black, crust (salt itself became colorless). This performance from ionic liquids and greases as additives to the molten salt electrolytes was not expected so the research had to refocus on the use of room temperature ionic liquids as the electrolytes to study the electrodeposition. Future work The future research should focus on the underpotential deposition of lithium and its effect on the morphology of lanthanides and actinides electrodeposits in molten LiCl-KCl electrolytes. This area of research is of importance for understanding the conditions for prevention of dendritic electrodeposits.},
doi = {10.2172/1595683},
journal = {},
number = ,
volume = ,
place = {United States},
year = {2020},
month = {1}
}