Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2019, 10, 289−294
Water-Facilitated Electrodeposition of Neodymium in a Phosphonium-Based Ionic Liquid Laura Sanchez-Cupido,† Jennifer M. Pringle,‡ Amal L. Siriwardana,† Ainhoa Unzurrunzaga,† Matthias Hilder,‡ Maria Forsyth,‡ and Cristina Pozo-Gonzalo*,‡ †
Fundación Tecnalia Research and Innovation, Paseo Mikeletegi 2, 20009 San Sebastián, Spain Institute for Frontier Materials, Deakin University, Melbourne, Victoria 3125, Australia
‡
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S Supporting Information *
ABSTRACT: Rare-earth metals are considered critical metals due to their extensive use in energy-related applications such as wind turbines and nickel−metal hybrid batteries found in hybrid electrical vehicles. A key drawback of the current processing methods includes the generation of large amounts of toxic and radioactive waste. Thus the efficient recovery of these valuable metals as well as cleaner processing methods are becoming increasingly important. Here we report on a clean electrochemical route for neodymium (Nd) recovery from [P6,6,6,14][TFSI], trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide which is amplified three times by the presence of water, as evidenced by the cathodic current density and thicker deposits. The role of Nd salt concentrations and water content as an additive in the electrochemistry of Nd3+ in [P6,6,6,14][TFSI] has been studied. The presence of metallic neodymium in the deposits has been confirmed by X-ray photoelectron spectroscopy.
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REM electrodeposition has also been considered in recent times using ionic liquids (ILs), which can simplistically be considered as low-temperature molten salts.16,34−48 Electrodeposition is a less complex and cleaner method for producing metals compared with other existing processes. However, REMs are highly reactive metals with standard electrode potentials close to Na and Li (e.g., − 2.34 V (Ce3+/Ce0) vs SHE for Ce and −2.66 (Nd3+/Nd0) and −2.41 V (Nd2+/Nd0) vs SHE for Nd). Therefore, traditional electrolyte solvents are not an option. Furthermore, it would be expected that waterfree conditions would be necessary for electrochemical processing. Ammonium-, pyrrolidinium-, and imidazolium-based ionic liquids have been successfully used in the electrochemical recovery of rare earths such as lanthanum, yttrium, gadolinium, ytterbium, and dysprosium.41−43,47,49,50 Electrochemical deposition of neodymium has also been attempted in several ionic liquids but has only been partially successful in phosphoniumbased ILs.34,38,40,44 For example, electrochemical recovery of Nd has been demonstrated by Kondo et al.34,38 using a phosphonium-based IL, triethylpentylphosphonium bis(trifluoromethylsulfonyl)amide, [P2,2,2,5][TFSI]. However, the current densities obtained during the electrodeposition from this IL were relatively low despite the high operation temperature (0.67 mA cm−2 at 150 °C). Additionally, the electrodeposition reaction was hindered by the presence of water, further limiting application to strictly controlled atmospheric conditions, which
are earths are a key component of many modern technologies including hard disk drives, electric motors, nickel−metal hydride batteries that fuel the majority of hybrid vehicles such as the Toyota Prius, and, increasingly, wind turbines at the center of many renewable power-generation plants.1,2 Recently, rare-earth metals (REMs) have been included in the top-14 critical raw materials, as established by the European Commission.3,4 Therefore, there is a growing concern about the reliable future access to these materials, and they have been targeted as a major material for reclaiming and secondary processing (i.e., recycling) in the so-called “circular economy”.5 Moreover, the primary extraction and processing methods for rare earths are based on traditional hydro- and pyrometallurgical procedures that have several drawbacks such as energy-intensive multistage processes, high working temperatures, and more alarming, large generation of toxic and radioactive waste due to the coextraction of thorium and uranium.6−11 Considering that the main producer and consumer of REM is dominated by one single region (i.e., China), this can further endanger future material availability and price. Therefore, the research and development of more sustainable processing and production as well as recycling of these important raw materials is critical for a sustainable ongoing use of rare-earth-based technologies. Electrochemical deposition of metals is a well-established process for many important materials such as aluminum12−17and nickel18−23 and has been considered for more precious metals such as titanium24−28 and tantalum29−33 but is usually achieved from high-temperature molten salt electrolytes at temperatures >500 °C. © XXXX American Chemical Society
Received: October 19, 2018 Accepted: December 21, 2018
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DOI: 10.1021/acs.jpclett.8b03203 J. Phys. Chem. Lett. 2019, 10, 289−294
Letter
The Journal of Physical Chemistry Letters
Figure 1. Cyclic voltammograms in (a) [P6,6,6,14][TFSI] containing different concentrations of Nd(TFSI)3 on glassy carbon, (b) 0.1 mol kg−1 Nd(TFSI)3/[P6,6,6,14][TFSI] containing 0.4 wt % H2O and control experiment in the absence of Nd(TFSI)3 on glassy carbon, and (c) 0.1 mol kg−1 Nd(TFSI)3/[P6,6,6,14][TFSI] containing 0.4 wt % H2O on copper working electrode. Scan rate: 100 mV s−1. Working temperature: 75 °C.
The mixtures containing 0.1 and 0.25 mol kg−1 Nd salt presented two reduction processes, C1 at −2.3 V and C2 at −3.3 V (Figure 1a), attributed to the reduction of Nd3+, as this process appears in the CV upon the addition of Nd(TFSI)3 salt. The absence of the oxidation processes demonstrates that it is an irreversible process, as has been reported in the literature for different lanthanides in ionic liquids and organic solventbased electrolytes.37,40−43,50,53,54 The current density of the second cathodic process (C2) became more significant with current densities up to −1.97 mA cm−2 with increasing salt concentration up to 0.35 mol kg−1. However, a more in-depth study would be necessary to confirm the nature of such processes, C1 and C2. Kondo et al. also reported an irreversible cathodic process at −2 V vs Fc+/Fc0 in 0.5 mol kg−1 Nd(TFSI)3/[P2,2,2,5][TFSI], which was assigned to the reduction of Nd3+ to Nd0.34 However, the maximum current density of −0.35 mA m−2 is lower than the one reported here in our [P6,6,6,14][TFSI]-based IL despite the fact that they used a smaller cation and higher working temperatures (150 °C), which presumably would favor higher mass transport and higher charge- transfer kinetics. The observed differences could be related to the different ionic liquid cation ([P2,2,2,5]+ vs [P6,6,6,14]+), the different working electrode materials (GC vs Pt), or the different water contents in the electrolyte. Figure 1a also depicts a progressive decrease in the current density of both cathodic processes upon increasing the Nd(TFSI) 3 concentration up to 0.5 mol kg −1 . This phenomenon could be attributed to limited mass transport upon salt addition due to the formation of large aggregates of
is detrimental toward the development of a commercial electrodeposition process. Thus the electrochemical recovery of Nd using ILs has been scarcely investigated, and a fundamental understanding of the electrochemistry of Nd in ILs is still lacking. Here we report a systematic electrochemical study of Nd salt in trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide, [P6,6,6,14][TFSI], focusing on the impact of Nd salt concentration and water as an additive to increase the mass transport rate, control speciation, and promote electrodeposition. This specific ionic liquid was selected due to its high electrochemical stability to ensure neodymium recovery from the electrolyte and hydrophobic properties to ease the working conditions by mitigating significant absorption of atmospheric water. The results indicate a promising route to Nd electrodeposition without the need for a dry atmosphere, anhydrous electrolytes, or excessive temperatures. The neodymium cathodic processes were studied across a range of Nd3+ concentrations in [P6,6,6,14][TFSI] containing 0.1 wt % H2O, and the electrochemical behavior was compared with the neat IL (Figure 1c). It is important to highlight that the possible electrochemical cathodic reactions involved in the production of Nd metal could follow a one-step (Nd3+ + 3e− → Nd0) or two-step pathway (Nd3+ + e− → Nd2+, and Nd2+ + 2e− → Nd0).51,52 This Letter does not attempt to describe in depth the deposition mechanism of neodymium metal, involving different steps such as Nd3+ mass transport, electron transfer, and deposition on the electrode, but only evaluate the redox processes of Nd3+ in [P6,6,6,14][TFSI] ionic liquid as a function of Nd salt and water concentration. 290
DOI: 10.1021/acs.jpclett.8b03203 J. Phys. Chem. Lett. 2019, 10, 289−294
Letter
The Journal of Physical Chemistry Letters
Figure 2. Electrodeposits obtained on copper working electrode in 0.1 mol kg−1 Nd(TFSI)3/[P6,6,6,1,4][TFSI] with (a) 0.1 wt % H2O and (b) 0.4 wt % H2O at −3.2 V vs Fc+/Fc0 at 75 °C, (c) curve-fitted Nd 3d3/2 XPS high-resolution spectra (18 nm) from the deposit attained from the mixture containing 0.4 wt % H2O in the XPS cell at −5.2 V vs Fc+/Fc0 at RT, and (d) quantification of the identified Nd species of the same deposit as panel c.
Upon increasing the H2O content from 0.1 to 0.4 wt %, the cathodic process C2, ascribed to the reduction of Nd3+, changed significantly, showing an almost three-fold increase in current density (to ca. −5 mA cm−2), and showed a typical nucleation and growth loop (Figure 1b).57 No anodic process was attained under those conditions, as in other works that report the successful electrodeposition of rare earth in ionic liquids.41,43 Water concentrations of 0.1, 0.3, 0.4, 0.5, and 2.1 wt % have also been tested, but there is not a constant increase in cathodic current with water content. An increase of such extent (three-fold) only occurred from 0.1 to 0.4 wt %. At 2.1 wt % water, we started to see detrimental effects on the cyclic voltammogram, indicating changes in the deposition mechanism, and thus 0.4 wt % was used for the further studies. Control experiments, in the absence of Nd salt, with two different water contents in the ionic liquid were performed to discard the origin of the cathodic processes as water reduction (Figure 1a,b). The controls showed several reduction processes corresponding to the breakdown of the ionic liquid, such as the reduction of the cation at −3.5 V and the possible decomposition of the TFSI anion at a more positive value (ca. −3 V) (Figure 1a), as reported in the literature.58 This effect is more acute in the presence of a larger amount of water (0.4 wt %), leading to two cathodic processes (ca. −1 and −3 V) (Figure 1b). Thus the ionic liquid breakdown processes in
slower mobility, making the system more viscous. Electrochemical impedance spectroscopy experiments support this hypothesis by showing a significant increase in impedance with concentrations >0.35 mol kg−1 (ca. 7000 ohm for 0.35 mol kg−1 Nd(TFSI)3 and 27 000 ohm for 0.5 mol kg−1 Nd(TFSI)3 at 75 °C) (Figure S2). Another effect can be related to a change in the speciation at high concentrations of Nd(TFSI)3. For example, the formation of coordination polymers of lanthanides in the presence of some ligands55 and in the case of Ti(II) at high concentrations has been reported,24 producing a decrease in the current density due to a decrease in the diffusion coefficient of the polymeric complexes. We also cannot eliminate the importance of metal speciation on the electrochemical behavior of the metal; a well-known example of this effect can be seen in the electrodeposition of aluminum from solutions of AlCl4− compared with Al2Cl7− from molten salts or ILs.17,56 The water effect on the electrochemical behavior of the Nd electrolyte systems was also assessed; the electrochemical behaviors of 0.1 mol kg−1 Nd(TFSI)3/[P6,6,6,14][TFSI] electrolyte in the presence of 0.1 and 0.4 wt % were compared (Figure 1a,b). This specific neodymium concentration was chosen for its optimal electrochemical performance in comparison with the other compositions in terms of current density and kinetics. 291
DOI: 10.1021/acs.jpclett.8b03203 J. Phys. Chem. Lett. 2019, 10, 289−294
Letter
The Journal of Physical Chemistry Letters
in the presence of free H2O, as reported in literature.58 The current density of these processes diminishes in the presence of Nd(TFSI)3 due to the coordination of water to Nd(III) and hence the decreased decomposition of the TFSI anion. Figure 2a,b shows the scanning electron microscopy (SEM) images of the deposits corresponding to 0.1 mol kg−1 Nd(TFSI)3/[P6,6,6,14][TFSI] containing 0.1 and 0.4 wt % H2O. Whereas the electrode surface in the presence of 0.1 wt % H2O shows only limited deposits, the deposits became more abundant and continuous upon increasing the H2O concentration. This trend is in good agreement with the increase in current densities observed in the cyclic voltammograms discussed above upon water addition (Figure 1a,b). The more abundant deposit attained from the electrolyte with 0.4 wt % H2O was further studied by energy-dispersive Xray spectroscopy (EDS), which showed Nd and O as the main elements (9 and 66 at %, respectively, bearing in mind that these are only semiquantitative values). The presence of oxygen could be attributed to handling the samples in air prior to the characterization and oxidation of the copper substrate. Copper from the substrate was also detected by EDS, suggesting that the deposit’s thickness should be
