11 Electrochemistry of Tm ( III ) and Yb ( III ) in Molten Salts

Pyrochemical processes appeared today gives an interesting option for future nuclear fuel cycles in several aspects. These latter will have to provide high recovery yields for actinides elements, (taking into account the sustainability requirement) to be safe, resistant versus proliferation risks, and cost-effective. This lead to a rather prolific research today, with many innovative concepts for future reactors, future fuels, and obviously future processes. Pyrochemical processes seems in this context to offer significant-established or presumed-advantages: (i) low radiolytical effects versus solvent processes (which increases the ability to process high burn-up, short-time cooled hot fuels); (ii) ability to dissolve new ceramic or dense fuel compounds; (iii) presumed compactness of technology (low number of transformation steps, small size of unit operations) [Uozumi, 2004; Willit, 2005]. Partitioning and transmutation (P&T) concept is nowadays considered as one of the strategies to reduce the long-term radiotoxicity of the nuclear wastes [Kinoshita et al., 2000]. To achieve this, the efficient recovery and multi-recycling of actinides (An), especially TRU elements, in advanced dedicated reactors is essential. Fuels proposed to transmute the actinides into short-lived or even stable radionuclides will contain significant amounts of Pu and minor actinides (Np, Am, Cm), possibly dissolved in inert matrices (U free), and will reach high burn-ups. Pyrochemical separation techniques offer some potential advantages compared to the hydrometallurgical processes to separate actinides from fission products (FP) contained in the irradiated fuel. The high radiation stability of the salt or metallic solvents used, resulting in shorter fuel cooling times stands out. The aim of the separation techniques which are currently being investigated, both hydrometallurgical and pyrometallurgical ones, is to optimize the recovery efficiency of minor actinides minimizing at the same time the fission products (FP) content in the final product. Special attention is devoted to rare earth elements (REE) mainly due to its neutronic poison effect and the high content into the spend fuel. In addition, REE have similar chemical properties [Bermejo et al., 2006, 2007, 2008a, 2008b; Castrillejo et al., 2005a, 2005b, 2005c, 2009; De Cordoba et al., 2004, 2008; Kuznetsov et al., 2006; Novoselova &

to those of actinides [Fusselman et al., 1999;Morss, 2008;Osipenko et al., 2010Osipenko et al., , 2011;;Roy et al., 1996;Sakamura et al., 1998;Serp et al., 2004Serp et al., , 2005aSerp et al., , 2005bSerp et al., , 2006;;Serrano & Taxil, 1999;Shirai et al., 2000] hence separation between these groups of elements is very difficult.For this reason, a good knowledge of the basic properties of REE in the proposed separation media is very important.The goal of these investigations is to determine the electrochemical and thermodynamic properties of some fission products (Tm and Yb), their mass transfer, and behavior in different fused solvents using transient electrochemical techniques, and potentiometric method (emf).

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Second, dry LnCl 3 was prepared by using the operation of carbochlorination of crystalline hydrate during heating in CCl 4 stream vapor in horizontal furnace.The obtained lanthanide chlorides (LnCl 3 ) were kept into glass ampoules under atmosphere of dry argon in inert glove box.Ln 3+ ions were prepared by direct addition of anhydrous LnCl 3 to the fused electrolytic bath.

Transient electrochemical technique
The experiments were carried out under inert argon atmosphere using a standard electrochemical quartz sealed cell using a three electrodes setup.Different transient electrochemical techniques were used such as linear sweep, cyclic, square wave and semiintegral voltammetry, as well as potentiometry at zero current.The electrochemical measurements were carried out using an Autolab PGSTAT30 potentiostat-galvanostat (Eco-Chimie) with specific GPES electrochemical software (version 4.9).The inert working electrode was prepared using a 1mm metallic W wire (Goodfellow, 99.9%).It was immersed into the molten bath between 3 -10 mm.The active surface area was determined after each experiment by measuring the immersion depth of the electrode.The counter electrode consisted of a 3 mm vitreous carbon rod (SU -2000).The Cl -/Cl 2 electrode is the most convenient reference electrode because it can be used for the direct thermodynamic calculations.It standard construction is the following.The quartz tube with porous membrane in the bottom and molten solvent in it has the graphite tube for chlorine gas introduction into the system.The chlorine gas is bubbling through the melt during the experiment [Smirnov, 1973].
The lanthanide concentrations were determined by taking samples from the melt which were dissolved in nitric acid solutions and then analyzed by ICP-MS.

Direct potentiometric method
The potentiometric study was carried out using an Autolab PGSTAT30 potentiostat-galvanostat (Eco-Chimie) with specific GPES electrochemical software (version 4.9).The electrochemical techniques were used such as potentiometry (zero current) and coulometry methods.The electrochemical set-up for potentiometric investigations is shown in Fig. 1.The inert working electrode was prepared using a 5 mm vitreous carbon rod (SU -2000) which was located in BeO crucible with the investigated melt.It was immersed into the molten bath between 3 -5 mm.During the experiments Ln 3+ ions were electrochemically reduced to Ln 2+ ions up to ratio Ln 3+ /Ln 2+ equals one.The counter electrode consisted of a 3 mm vitreous carbon rod (SU -2000) which was placed in quartz tube with porous membrane in the bottom with solvent melt and located in vitreous carbon crucible (SU -2000) with pure solvent without lanthanide chlorides.The Cl -/Cl 2 electrode was used as reference electrode.  .It shows a single cathodic peak at a potential of -1.762V vs. Cl -/Cl 2 and its corresponding anodic peak at -1.566V vs. the Cl -/Cl 2 .Similar behaviour for the reduction of Yb(III) ions has been observed in the fused equimolar NaCl-KCl mixture (Fig. 3), NaCl-KCl-CsCl eutectic (Fig. 4) and CsCl (Fig. 5).These figures show the linear sweep and the cyclic voltammograms obtained in the above systems with YbCl 3 at several scan rates, respectively.
The square wave voltammetry technique was used to determine the number of electrons exchanged in the reduction of Yb(III) ions in different molten compositions.Fig. 6 shows the bell-shaped symmetric cathodic wave obtained in the LiCl-KCl-YbCl 3 melt at 723 K.The number of electrons exchanged is determined by measuring the width at half height of the reduction peak, W 1/2 (V), registered at different frequencies (6-80 Hz).W 1/2 is given by the following equation, valid for reversible systems:   At low frequencies a linear relationship between the cathodic peak current and the square root of the frequency was found.Under these conditions the system can be considered as reversible and equation 1 can be applied [Bard & Faulkner, 1980].The number of electrons exchanged was close to 1.The same results were obtained in NaCl-KCl, NaCl-KCl-CsCl and CsCl media.Potentiostatic electrolysis at potentials of the cathodic peaks for all systems studied did not show the formation of the solid phase of tungsten surface after polarization.There is no plateau on the dependences potential -time.Also the working electrode did not undergo any visual change.X-ray analysis of the surface of the working electrodes after experiments also show an absence of formation of solid phase.The results obtained allow concluding that the reduction of Yb(III) ions takes place in a single step with the exchange of one electron and the formation of a soluble product, according to the following reaction: The reaction mechanism of the soluble-soluble Yb(III)/Yb(II) redox system was investigated by analyzing the voltammetric curves obtained at several scan rates.It shows that the cathodic and anodic peak potential (E p ) is constant and independent of the potential sweep rate (Fig. 7).On the other hand the cathodic and anodic peak current (I p ) is directly proportional to the square root of the polarization rate (υ) (Fig. 8).A linear relationship between the cathodic peak current density and the concentration of YbCl 3 ions in the melt was observed (Fig. 9).From these results and according to the theory of linear sweep voltammetry technique [Bard & Faulkner, 1980] it is concluded that the redox system Yb(III)/Yb(II) is a reversible and controlled by the rate of the mass transfer.From the transient electrochemical techniques applied we concluded that the potential of the system [Yb(II)/Yb(0)] can not be observed in the molten alkali chlorides media because it is more negative than the potential of the solvent Me(I)/Me(0), being Me: Li, Na, K and Cs, (Fig. 1).

Diffusion coefficient of Yb (III) ions
The diffusion coefficient of Yb(III) ions in molten chloride media was determined using the cyclic voltammetry technique and applying the Randles-Sevčik equation, valid for reversible soluble-soluble system [Bard & Faulkner, 1980]: where S is the electrode surface area (in cm 2 ), C 0 is the solute concentration (in mol cm -3 ), D is the diffusion coefficient (in cm 2 s -1 ), F is the Faraday constant (in 96500 C mol -1 ), R is the ideal gas constant (in J K -1 mol -1 ), n is the number of exchanged electrons, v is the potential sweep rate (in V s -1 ) and T is the absolute temperature (in K).
The values obtained for the different molten chlorides tested at several temperatures are quoted in Table 1.
The diffusion coefficient values have been used to calculate the activation energy for the diffusion process.The influence of the temperature on the diffusion coefficient obeys the Arrhenius's law through the following equation: where E A is the activation energy for the diffusion process (in kJ mol -1 ), D o is the preexponential term (in cm 2 s -1 ) and Δ is the experimental error.
From this expression, the value of the activation energy for the Yb(III) ions diffusion process was calculated in the different melts tested (Table 1).
The diffusion coefficient of ytterbium (III) ions becomes smaller with the increase of the radius of the cation of alkali metal in the line from Li to Cs (Table 1).Such behaviour takes place due to an increasing on the strength of complex ions and the decrease in contribution of D to the "hopping" mechanism.The increase of temperature leads to the increase of the diffusion coefficients in all the solvents.
The average value of the radius of molten mixtures ( ) R r + was calculated by using the following equation [Lebedev, 1993]: where i c is the mole fraction of i cations; i r is the radius of i cations in molten mixture, consist of N different alkali chlorides, nm.

Apparent standard potentials of the redox couple Yb(III)/Yb(II)
The apparent standard potential of the Yb(III)/Yb(II) system was determined from the cyclic voltammograms registered in YbCl 3 solutions in the different alkali metal chlorides tested at several temperatures.According to the theory of linear sweep voltammetry the following expressions, including the anodic and cathodic peak potentials and the half-wave potential, can be applied in the case of a soluble-soluble reversible system [Bard & Faulkner, 1980 It is known that for concentrations of electroactive species lower than 3 to 5•10 -2 in mole fraction scale, their activity coefficient is almost constant.In these conditions, it is more convenient using the apparent standard redox potential concept ( * () ( ) ) expressed as follows [Smirnov, 1973]: The formal standard redox potentials of (1.915 0.005) (3.5 0.2) 10 , (2.031 0.005) (3.7 0.2) 10 , (2.192 0.016) (4.3 0.2) 10 , (2.262 0.004) (4.2 0.2) 10 , The variation of the apparent standard potential of the redox couple Yb(III)/Yb(II) as a function of the reverse radius of the solvent cation (r) and the temperature was calculated.The relation obtained is: complex ions are present in the melts [Novoselova et al., 2004].Their relative stability increases with the increase of the solvent cation radius, and the apparent standard redox potential shifts to more negative values.Our results are in a good agreement with the literature ones [Smirnov, 1973].

Thermodynamics properties
Using the values of the apparent standard redox potentials the formal free Gibbs energy changes of the redox reaction was calculated according to following expression: Its temperature dependence allows calculating the enthalpy and entropy of the YbCl 3 formation by means of the relation [Bard & Faulkner, 1980]: The apparent standard Gibbs energy of formation of YbCl 3 in the different solvents tested can be expressed as: The changes of the thermodynamic parameters of the redox reaction ( 19) versus the radius of the solvent cation show the increasing in strength of the Yb-Cl bond in the complex ions [ ] 3 6 YbCl − in the line from LiCl to CsCl.

Voltammetric studies on active electrodes
Linear sweep voltammograms for the reduction of Yb(III) solution at inert tungsten (1) and active aluminum (2) electrodes at 873 K are presented in Fig. 10.The voltammogram on aluminum working electrode show the existence of two cathodic peaks at the potentials approximately -1.92 V and -2.92 V vs. Cl -/Cl 2 instead of one on tungsten electrode.Potentiostatic electrolysis at potential -1.92 V did not show the formation of solid phase on tungsten and aluminum surfaces after polarization.So we can suppose passing the reaction (2) at this potential on inert and active electrodes.
where n is equal 2, 3. Potentiostatic electrolysis allow to deposit Al 3 Yb or the mixture of Al 2 Tm and Al 3 Tm alloys as a thin films on the aluminum surface.

Electromotive force method 3.2.1 Apparent standard potentials of the redox couple Ln(III)/Ln(II)
The typical dependences of the redox potential of the couple Yb 3+ /Yb 2+ with different ratio Yb(III)/Yb(II) versus the duration at the temperature 818 K in NaCl-KCl-CsCl-YbCl 3 melt are presented in Fig. 13.The same type of the pictures was obtained for Tm 3+ /Tm 2+ and Yb 3+ /Yb 2+ systems in all investigations solvents.The equilibrium potential were fixed after 30-90 minutes after finishing of the electrolysis and depends from the conditions of the experiment.If the value www.intechopen.com of potential is constant during 30-40 minutes within the limits of ± 0.001 V then it is possible to say that the investigation system is in equilibrium conditions.The value of the apparent redox potential is determined by: Ln Ln E ++ is the equilibrium potential of the system, V; 32 * Ln Ln E + + is the apparent standard redox potential of the system, V; n is the number of exchange electrons; [Ln 3+ ] and [Ln 2+ ] are the concentrations of lanthanide ions in mole fraction.Variation of the equilibrium potential of the couple Ln 3+ /Ln 2+ as a function of the napierian logarithm ratio of concentrations [Ln 3+ ] and [Ln 2+ ] in fused LnCl 3 solutions on vitreous carbon indicated electrode at 818 K (NaCl-KCl-CsCl eutectic) and at 973 K (CsCl) is shown ) [Lebedev, 1993] for thulium (-2.720V) and ytterbium (-1.796V) at 973 K show the natural shift of the potential values to more negative region in line LiCl-CsCl.LnCl − [Papatheodorou & Kleppa, 1974;Yamana et al., 2003].Their relative stability increases with increasing of solvent cation radius from Li + to Cs + and the apparent standard redox potentials are shifted to more negative values.These results are in good agreement with literature data concerning the second coordination sphere influence on apparent standard redox potentials.

Thermodynamics properties
Using the values of the apparent standard redox potentials the formal free Gibbs energy changes and the apparent equilibrium constants of the redox reaction (38): ln eq GR T K Δ= − (40) The temperature dependence of the Gibbs energy change can be described by the following equation: The experiment data were fitted to the following equations using Software Origin Pro version 7.5: The activity coefficients of YbCl 3 in fused salts was determined from the difference between the apparent Gibbs free energy derived from the experimental measurements and the standard Gibbs free energy for pure compounds obtained in the literature [Barin, 1994]: The dependence of the activity coefficient of YbCl 3 versus the reverse temperature is given by the expressions (50, 51).Database for thulium compounds is absent in the literature [Barin, 1994].
It is also possible to estimate the equilibrium chlorine gas pressure above an alkali metal chloride melts containing hulium or ytterbium tri-and dichlorides for the reaction (52) by LnCl 3(l) ⇔ LnCl 2(l) + ½ Cl 2(g) (52) well-known equation ( 53) [Smirnov, 1973].Such kind of calculations were done for the concentration ratio of [Ln 3+ ]/Ln 2+ ] equals one in fused NaCl-KCl-CsCl eutectic and individual CsCl.The calculated values are summarized in Table 2.The average value of the radius of these molten mixtures in this line, pro tanto, is 0.137; 0.165 nm [Lebedev, 1993].From the data given in E + + in molten alkali metal chlorides were also determined by emf method.The basic thermodynamic properties of the reactions TmCl 2(l) + ½ Cl 2(g) ⇔ TmCl 3(l) and YbCl 2(l) + ½ Cl 2(g) ⇔ YbCl 3(l) were calculated.
The influence of the nature of the solvent (ionic radius) on the thermodynamic properties of thulium and ytterbium compounds was assessed.It was found that the strength of the Ln-Cl bonds increases in the line from Li to Cs cation.Barbanel, Ya.A. (1985).Coordination Chemistry of f-elements in Melts, Energoatomizdat, Moscow, Russia.

Fig. 1 .
Fig. 1.Experimental set-up for potentiometric studyThe total lanthanide concentrations were determined by taking samples from the melt which were dissolved in nitric acid solutions and then analysed by ICP-MS.The concentration of the reduced form of lanthanides was determined by volumetric method.

Table 1 .
Diffusion coefficient of Yb(III) ions in molten alkali metal chlorides at several temperatures.Activation energy for the ytterbium ions diffusion process The variation of the logarithm of the diffusion coefficient as a function of the reverse radius of the solvent cation (r) and reverse temperatures is given by the following expression: ]:

Table 2 .
Table 2 one can see that the relative stability of lanthanides(III) complexes ions is naturally increased in the line (NaCl-KCl-CsCl) eut.-CsCl.The comparison of the base thermodynamic properties of Tm and Yb in molten alkali metal chlorides at 973 K. Apparent standard redox potentials are given in the molar fraction scale YbCl − ions in fused alkali metal chlorides was investigated.It was found that the reduction of Yb(III) to Yb(II) ions is a reversible process being controlled by the rate of the mass transfer.The diffusion coefficient of [ ] 3

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YbCl − ions was determined at different temperatures in all investigation systems.The apparent standard electrode potential of the redox couple