Electroreduction of Er³⁺ in nonaqueous solvents

Abstract

The electroreduction of Er³⁺ in propylene carbonate, N,N-dimethylformamide, or a variety of quaternary ammonium ionic liquids (ILs) was investigated using [Er(OTf)₃] and [Er(NTf₂)₃]. Systematic variation of the ILs' cation and anion, Er³⁺ salt, and electrode material revealed a disparity in electrochemical interactions not previously seen. For most ILs at a platinum electrode, cyclic voltammetry exhibits irreversible interactions between Er³⁺ salts and the electrode at potentials significantly less than the theoretical reduction potential for Er³⁺. Throughout all solvent–salt systems tested, a deposit could be formed on the electrode, though obtaining a high purity, crystalline Er⁰ deposit is challenging due to the extreme reactivity of the deposit and resulting chemical interactions, often resulting in the formation of a complex, amorphous solid–electrolyte interface that slowed deposition rates. Comparison of platinum, gold, nickel, and glassy carbon (GC) working electrodes revealed oxidation processes unique to the platinum surface. While no appreciable reduction current was observed on GC at the potentials investigated, deposits were seen on platinum, gold, and nickel electrodes.

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1 Introduction

Lanthanide (Ln) materials have quietly become critically important in today's high tech society, where they are found in advanced electronics for transportation, optical, magnetic, and luminescent materials.¹ At the same time, reliable supplies of these elements are becoming more difficult to secure.² The ability to recover these materials from industrial and commercial waste products has the potential to eliminate the need to mine for these extremely useful materials. One possible, cost-effective route to recover Ln metals involves reduction of their constituent ions from solution. Such recovery cannot be accomplished in aqueous media as hydrogen is evolved prior to Ln⁰ reduction. Many previous reports on the reduction of Ln³⁺ or Ln²⁺ have used molten salts (e.g., LiCl–KCl) to successfully form high-purity, crystalline Ln⁰ films.^3–7 In contrast, a limited number of research groups have reported electroreduction of Ln³⁺ or Ln²⁺ at room temperature using organic solvents; however, annealing at higher temperature was necessary to crystallize the final Ln⁰ films.^8,9 Ionic liquids (ILs), however, present an attractive alternative, possessing the requisite potential window for Ln^3+/2+ reduction, while avoiding both the corrosiveness and high temperatures (300–500 °C) of molten salts and the volatility of organic solvents.¹⁰

Several previous reports have attempted to use IL electrolytes to deposit various Ln⁰ or Ln-containing alloys, including lanthanum,^11,12 cerium,¹³ samarium,^13–15 europium,^13–17 dysprosium,^18–20 and ytterbium.^14,15,21,22 Most of these reports, however, were limited to one or two particular ILs and note organic contamination in the film. Without further annealing or alloying elements (e.g., Al), crystalline films could not be obtained. In order to successfully recover high purity metal or electroplate electronic grade films, further advances in the electroreduction of the Ln³⁺ must be pursued.

In this report, the electrodeposition of Er⁰, an electrochemically understudied lanthanide, was investigated in two organic solvents and a systematic array of ILs, summarized in Table 1. The organic solvents propylene carbonate (PC) and N,N-dimethylformamide (DMF) were selected as they represent useful battery solvent systems and a standard organic synthesis solvent. The electroreduction of Er³⁺ in these solvents was compared to several quaternary ammonium ILs used in the literature. Here we systematically varied the cations butyltrimethylammonium (BTMA), 1-propyl-1-methylpiperidinium (PMP), 1-butyl-1-methylpyrrolidinium (BMP), the anions: bis(trifluoromethanesulfonyl)imide (NTf₂⁻), trifluoromethanesulfonate (OTf⁻), dicyanimide (N(CN)₂⁻), and the electrode material: platinum, gold, nickel, and glassy carbon (GC).

Table 1 Electrolyte–salt–electrode systems evaluated in this work. Erbium salt concentration was 100 mM for all systems except for C and D, where it was limited by solubility to 50 mM. Peak potentials for processes R_a, R_b, O_a, O_b were recorded at 100 mV s⁻¹

System	Electrolyte	Erbium salt	Electrode	Peak potential vs. Fc/Fc^+/V
				Ra	Rb	Oa	Ob
A	PC + 0.1 M n-Bu4NBF4	[Er(OTf)3]	Platinum	—	−2.535	—	—
B	DMF + 0.1 M n-Bu4NBF4	[Er(OTf)3]	Platinum	—	−2.039	—	−1.169
C	BTMA-NTf2	[Er(OTf)3]	Platinum	−1.472	−2.440	0.126	−0.713
D	PMP-NTf2	[Er(OTf)3]	Platinum	−1.423	−2.795	0.097	−0.716
E	BMP-NTf2	[Er(OTf)3]	Platinum	−1.345	−2.771	0.046	−0.978
F	BMP-N(CN)2	[Er(OTf)3]	Platinum	—	−2.460	—	−0.486
G	BMP-OTf	[Er(OTf)3]	Platinum	−1.432	−2.862	−0.158	−0.607
H	BMP-NTf2	[Er(NTf2)3]	Platinum	−1.327	−2.775	0.048	−0.870
I	BMP-NTf2	[Er(NTf2)3]	Gold	−1.572	−2.780	—	—
J	BMP-NTf2	[Er(NTf2)3]	Nickel	—	−2.423	—	—
K	BMP-NTf2	[Er(NTf2)3]	Glassy carbon	—	—	—	—

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The source of the Er³⁺ was the commercially available [Er(OTf)₃].²³ For several studies [Er(NTf₂)₃] was synthesized and used in order to match the anion of the specific IL. It should be noted that while many of these anions are considered “non-coordinating,” allowing more facile electrodeposition, recent reports have shown this is not always the case when the Ln cations are involved.²⁴

The following results demonstrate the influence of the electrolyte–salt–electrode system on the electrochemical response and resulting film morphology. None of these systems produced a 100% pure, crystalline Er⁰ deposit. Nevertheless, significant variation in interaction between electrode and electrolyte was noted, which is an important first step to understanding the processes that are occurring during electrodeposition.

2 Experimental

2.1 Materials

The following chemicals were purchased from (i) Sigma-Aldrich: propylene carbonate (PC, 99.7%, anhydrous) and N,N-dimethylformamide (DMF, 99.8%, anhydrous), tetrabutylammonium tetrafluoroborate (n-Bu₄NBF₄, 99%), 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate (BMP-OTf, 99%) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-NTf₂, >98.5%), [Er(OTf)₃] (98%), concentrated hydrochloric acid (HCl), nitric acid (HNO₃), potassium hydroxide (KOH), ethanol (anhydrous), silver bis(trifluoromethanesulfonyl)imide [AgNTf₂] (97%), ferrocene (98%), erbium foil (99.9%), (ii) Iolitec, Inc.: butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (BTMA-NTf₂, 99%), 1-propyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide (PMP-NTf₂, 99.5%), 1-butyl-1-methylpyrrolidinium dicyanamide (BMP-N(CN)₂, >98%); (iii) Acros Organics: bis(trifluoromethanesulfonyl)imide (H-NTf); (iv) Alfa Aesar: Er⁰ rod (99.9%), erbium chloride (ErCl₃, 99.9%).

PC and DMF were further dried over activated 3 Å sieves and kept in an argon filled glovebox containing less than 0.5 ppm water or oxygen. n-Bu₄NBF₄ was added as supporting electrolyte for both the PC and DMF experiments. All ILs, n-Bu₄NBF₄, and [Er(OTf)₃] were evacuated on a Schlenk line with all glass or glass/Teflon connections (no rubber tubing was used) to less than 0.5 mTorr and heated to 100 °C with constant stirring for 48 h. The hot evacuated flasks were sealed under vacuum and immediately transferred to the glovebox. To prevent possible contamination, all glassware was cleaned in a base bath (KOH/ethanol) for 12 h, rinsed with deionized water, soaked in fresh aqua regia (3 : 1 conc. HCl : HNO₃) for 15 min, rinsed 10 times with deionized water, and dried overnight at 100 °C in an oven. Hot glassware was transferred directly to the glovebox. All deionized water was purified to 18.2 MΩ cm.

2.2 [Er(NTf₂)₃] synthesis

A procedure analogous to previously reported syntheses of other Ln(NTf₂)₃ salts was developed.^18,25 In general, on the benchtop, H-NTf₂ (0.660 g, 2.35 mmol) was dissolved in deionized water (1.5 mL). To this Er⁰ foil (0.130 g, 0.78 mmol) was carefully added and a vigorous reaction occurred that dissolved the foil and left a pink solution and a small grey precipitate. The pink mother liquor was carefully decanted from the precipitate (which was discarded) and transferred to a Schlenk flask. The reaction mixture was evacuated (<0.5 mTorr) and then heated under vacuum to 100 °C for 48 h on an all glass Schlenk line (no rubber tubing was used). The resulting pink, flaky powder (0.501 g, 68.6% yield) was loaded directly into the glovebox. The pink flaky solid was identified as [Er(NTf₂)₃]. Crystals of Er(H₂O)₅(Tf₂N)₃ were isolated by rotary evaporation of water from the solution above.²⁶ The unit cell parameters of an experimental crystal from the reaction and the literature reported values were found to be in agreement.²⁶

2.3 Electrochemical measurements

All electrochemical measurements were recorded in a glovebox using a standard three electrode electrochemical cell. For cyclic voltammetry (CV), the working electrode was platinum, gold, nickel, or GC 1.6 or 3.0 mm in diameter (Bioanalytical Systems, Inc, West Lafayette, IN, USA). Working electrodes were successively polished in 1.0 μm and 0.05 μm alumina (Buehler), rinsed and sonicated in deionized water, then dried at 100 °C in an oven overnight. The hot electrodes were directly loaded into the glovebox. For potentiostatic depositions, the working electrode consisted of 1–2 cm² areas of platinum film sputtered onto silicon (100 nm Pt/40 nm Ti/400 nm SiO₂/Si (100)). The counter electrode was a coiled platinum wire for CVs, or a freshly polished Er⁰ rod for potentiostatic depositions. For the IL systems, the reference electrode (nonaqueous reference electrode kit, Bioanalytical Systems) consisted of a silver wire in 10 mM [AgNTf₂] in the same ILs as the test cell. For PC and DMF, the reference electrode was Ag/AgCl wire in 0.1 M ErCl₃ in PC or DMF, respectively. Reference electrode potentials were calibrated against ferrocene (Fc) for each electrolyte, and all potentials are reported against the Fc/Fc⁺ couple.

2.4 Surface analysis

Samples were interrogated using a Zeiss Supra 55VP scanning electron microscope (SEM) at 5 kV and 4 mm working distance. Energy dispersive X-ray spectroscopy (EDS) measurements were recorded with an Oxford Instruments detector at 15 kV and 10 mm working distance. Crystallinity was evaluated using X-ray diffraction (XRD) with a Bruker D2 Phaser system set in the traditional Bragg–Brentano geometry with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were recorded using a Kratos Axis Ultra DLD instrument with a monochromatic Al Kα (1486.6 eV) source. Full survey spectra were collected with an analyzer pass energy of 160 eV and a step size of 1 eV. High resolution spectra were collected with an analyzer pass energy of 20 eV and step sizes of 0.1 eV. The analyzer was used in hybrid mode with a large spot size of 300 μm by 700 μm, elliptically. Binding energies were calibrated by setting the major C 1s peak to 284.6 eV. In order to remove any residual IL prior to surface analysis, samples were rinsed three times, soaked overnight, rinsed again three times using DMF, and the residual solvent was allowed to evaporate overnight in the glovebox.

3 Results and discussion

3.1 Variation of electrolyte and erbium salt – voltammetry

The electrochemical response of a platinum working electrode in electrolyte–salt systems A–H was interrogated using CV (Table 1). Representative CVs using electrolyte with and without the Er³⁺ salt are presented in Fig. 1. As calculated from molten salt systems and commonly listed in standard reduction potential tables, the standard reduction potential of Er³⁺ to Er⁰ is −2.73 V vs. Fc/Fc⁺.²⁷ This provides an approximate potential as to where Er³⁺ reduction should occur with the exact value influenced by the particular ions in solution.

Fig. 1 (A–H) CVs of platinum in solvent systems A–H from Table 1, without (dotted blue lines) and with (solid red lines) erbium salt added. Each CV is the 5^th scan at 100 mV s⁻¹.

As can be observed in Fig. 1 for systems A–H, the reduction currents were notably higher and distinct redox peaks appeared after the Er³⁺ salt was added. The locations of these peaks, to be discussed in detail later, are compiled in Table 1. It was found that an erbium-containing film had been produced (vide infra) after application of −2.90 V for 2 h. This indicates that the addition of an Er³⁺ complex increased reduction currents and enabled the deposition of an erbium-containing film onto platinum at potentials where Er³⁺ reduction is known to be favorable. Several differences were noted in the electrochemical behavior of the Er³⁺ systems and these are discussed below for (i) organic solvents and (ii) ILs.

3.1.1 Organic. The two organic systems investigated used (i) PC and (ii) DMF solvents to determine the CV behavior of 0.1 M [Er(OTf)₃]. For PC in Fig. 1A, a large reduction wave was observed at potentials less than −1.5 V, with no corresponding oxidation peak. In contrast, a smaller reduction wave was seen in the DMF system (Fig. 1B) at potentials less than −2.0 V, but an oxidation peak was present near −1.1 V. It is not clear if this oxidation peak is simply a shift in the broad oxidation peak from the neat solution or whether a unique oxidation process has occurred. For both systems, a thin film was observed on the electrode and subsequent application of 0.0 V was unable to remove it. This implies that any possible redox process is not fully reversible. The comparatively small reduction current for DMF implies that DMF is not an ideal solvent for Er³⁺ reduction.

3.1.2 Ionic liquids. For the IL systems (Table 1, systems C–H) more complicated behaviors were observed in CV in comparison to the organic solvents. In particular, multiple pairs of reduction and oxidation peaks were present, as seen in Fig. 2C–H. The first reduction peak (R_a) was located near −1.4 V while the second peak (R_b) was near −2.7 V. Variation of the IL cation resulted in a shift of the R_a peak potentials by ±75 mV; changing the anion to N(CN)₂⁻ suppressed the R_a peak. Additionally, the R_b peak was observed at higher potentials when N(CN)₂⁻ was used.

Fig. 2 (A) Influence of scan rate on CVs of platinum in 0.1 M Er(OTf)₃ in BMP-NTf₂ (system E). CVs are the 5^th scan and scan rates varied over (innermost) 25, 50, 100, 250, 500 mV s⁻¹ (outermost). (B) Linear fits of peak current vs. the square root of scan rate for peaks O_a, O_b, and R_a, indicating diffusionally controlled processes. (C) Linear fits of peak potential vs. the logarithm of scan rate for O_a (R² = 0.982), but not for O_b or R_a (R² = 0.739, 0.913), implying an ideally irreversible process for O_a.

Likewise, one oxidation peak (O_a) was observed near 0.0 V for each system (C–H), while the second (O_b) was observed at −1.0 V. Through variations of the anion of the Er³⁺, the peak potential of O_a could be shifted by up to 0.2 V; in the case of N(CN)₂⁻ the O_a peak was completely suppressed. The peak potential for O_b was found to vary by 0.5 V, with the NTf₂⁻ anion giving the lowest potential and N(CN)₂⁻ giving the highest potential. This suggests that Er³⁺ complexes with OTf⁻ and NTf₂⁻ are electrochemically stable at similar potentials, while Er³⁺ complexes with N(CN)₂⁻ favor more noble potentials.

To better understand the kinetics of these redox processes, CVs were recorded for a variety of scan rates over 25–500 mV s⁻¹. From this dataset, it was determined that R_a, O_a, and O_b are diffusion controlled, as they display a linear relation between peak current and the square root of the scan rate (plotted in Fig. 2B for system E). Squared Pearson correlation coefficients (R²) values were calculated to be 0.982, 0.999, and 0.973 for R_a, O_a, and O_b, respectively. By comparing the peak potential to the logarithm of the scan rate, it was determined that peak O_a (R² = 0.982) represents an ideally irreversible reaction; however, R_a and O_b (R² = 0.913, 0.739) do not follow this trend.²⁸ Furthermore, at fast scan rates (500 mV s⁻¹) 85% of the reduction charge of process R_a could be reoxidized by process O_a, while the coulombic efficiency dropped to 26% at 25 mV s⁻¹. Similarly, 26% of the charge passed from process R_b could be oxidized by O_b at 500 mV s⁻¹, declining to 9% at 25 mV s⁻¹.

The relation between these redox processes (R_a, R_b, O_a, O_b) can be better understood if the cathodic window of the CV is gradually widen, as plotted in Fig. 3A for system E. Fig. 3A shows the results of such studies, where five scans were performed at each potential window before further opening the window. In order to observe peak O_a, it was found that the potential must be swept to a potential less than that of peak R_a. Thus, the reactant for process O_a is generated by process R_a. Similarly, the potential must be swept to R_b in order to observe O_b; the product of R_b is the reactant of O_b.

Fig. 3 (A) Influence of cathodic limit on CVs of 0.1 M Er(OTf)₃ in BMP-NTf₂ (system E). Each CV is the 5^th scan at 100 mV s⁻¹. (B) Anodic linear sweeps (100 mV s⁻¹) after potentiostatic deposition at −2.90 V vs. Fc/Fc⁺ for 10, 30, 60, 120, or 180 s.

An additional experiment was undertaken where the potential was held at −2.9 V for select times and then swept to +1.0 V at the same scan rate as in the CVs. From this approach, it was found that the relative intensity of peaks O_a and O_b could be varied, as shown in Fig. 3B. At short times (10 s) peak O_a is larger, while at longer times (180 s) peak O_b is larger. This indicates that process R_b consumes the reactant for process O_a, while generating the reactant for process O_b. Because it was previously shown that the product of R_a is the reactant for O_a, it can be deduced that R_b consumes the product of R_a.

If a film is deposited onto the platinum surface at process R_b, then the electrode removed from solution, rinsed, and immersed in neat IL, peaks R_a and O_a are suppressed, while peaks R_b and O_b decrease in intensity with each scan until the CV appears similar to that of the neat IL. This suggests that species generated by R_a and O_a are in solution, with only those still absorbed to the electrode surface present to irreversibly react.

For the ionic liquid systems with NTf₂⁻ or OTf⁻ anions (C–E, G, H), it was determined that process R_a is the first electrochemical step in a reduction pathway that may generate an erbium-containing film. However, depending on the subsequently applied potential, the product of process R_a may be irreversibly oxidized by process O_a (not yielding a film) or further reduced by process R_b to form the film. Once reduced by process R_b, these species may be oxidized by process O_b. The fact that peak O_b grows at the expense of peak O_a in Fig. 3B suggests that the species generated by process O_b are not the same as those generated by process R_a, as they cannot be further oxidized by process O_a. More generally, the film deposited on the electrode surface by process R_b cannot be removed by application of potentials up to +1.0 V; deposition of the erbium-containing film is not reversible.

System F (Fig. 1F, BMP-N(CN)₂) is unique from the other ILs in that only one oxidation and reduction peak were observed. This reduction peak occurred at potentials expected for Er³⁺ reduction and the observed current was nearly twice that of the other systems at equivalent potentials. Thus, the N(CN)₂⁻ anion appears to be a superior choice for electroreduction of Er³⁺, in that it does not suffer from competing electrochemical processes.

Overall, process R_a appears more prevalent in systems where the NTf₂⁻ anion is present (i.e., systems C, D, E, H). When the NTf₂⁻ anion is absent, process R_a is either reduced in intensity (system G) or absent (systems A, B, F).

In summary, the product of process R_a was determined to be a solution-bound intermediate that was irreversibly produced during formation of the erbium-containing film. No film was observed if potential R_a is applied. The product of O_a is an irreversibly oxidized form of R_a. The product of O_a is also solution-bound; however, application of R_a followed by O_a does not yield a deposit on the electrode surface. The product of R_b is the deposited erbium-containing film, as discussed throughout this report. The product of O_b is an oxidized form of the film generated by R_b. Process O_b will not remove the film generated by R_b.

3.2 Variation of electrolyte and Er³⁺ salt – film analysis

For each of the systems A–H, films were grown onto platinized silicon wafers for 2 h at −2.90 V, cleaned per the Experimental section, and transferred directly from glovebox to SEM. SEM, EDS, XPS, and XRD analysis performed in an attempt to identify the deposited material. The resulting film microstructure is presented in the characteristic SEM micrographs of Fig. 4. The variety of microstructures observed was unexpected. For PC in Fig. 4A, a microstructure with features on the order of 10 nm was observed and the platinum film delaminated in circular areas 10–100 μm across. For DMF (Fig. 4B) a nearly uniform, “mud-cracked” film appeared, with fine cracks scattered about larger pieces tens of micrometers across. In the case of the ILs (Fig. 4C–H) a uniform, porous film coated the substrate. In many cases brighter, more crystalline looking regions appeared in this porous film. These brighter regions correspond to higher atomic masses and erbium concentrations, as confirmed in backscattered electron imaging and EDS mapping.

Fig. 4 Plan-view SEM micrographs of erbium-containing films deposited in various electrolytes onto platinum for 2 h at −2.90 V vs. Fc/Fc⁺. Electrolytes in (A–G) contained [Er(OTf)₃], while (H) used 0.1 M [Er(NTf₂)₃]. Subfigures (A–H) correspond to electrolyte–salt–electrode systems outlined in Table 1.

EDS spectra were recorded for each of the films in Fig. 4. A typical EDS spectrum is provided in Fig. 5 for system F. All spectra appeared similar, with major peaks from Er, peaks from the substrate (Pt, Ti, Si, O), and minor peaks from organic impurities (C, F, N, O, and possibly S, Cl). To compare the relative purity of the samples, all EDS spectra were normalized to the same background, and the relative peak intensities (within a given spectrum) after background subtraction were tabulated in Table 2. The organic solvents, DMF and PC, showed relatively higher intensities of organic impurities compared to the ionic liquids. System B, BTMA-NTf₂, was a notable exception, showing the highest intensities of carbon and nitrogen. It is likely that the ammonium group in BTMA-NTf₂ is more susceptible to reduction compared to the other pyrrolidinium or piperidinium-based ionic liquids, resulting in increased concentration of carbon and nitrogen on the electrode surface.

Fig. 5 (A) EDS spectrum of the film in Fig. 4F. (B) Zoomed in spectrum from (A). EDS spectra of all films from Fig. 4 appeared similar, with major peaks from Er, peaks from the substrate (Pt, Ti, Si, O), and minor peaks from impurities (C, F, N, O).

Table 2 EDS peak intensities scaled to Er M within each spectrum of erbium-containing films on platinized silicon from Fig. 4. Pt, Ti, Si are omitted as they are attributed solely to the substrate

System	Relative EDS peak intensity
	Er M	C Kα	F Kα	N Kα	O Kα
A	1.00	3.24	0.46	0	1.80
B	1.00	2.44	0.31	0.07	1.80
C	1.00	4.33	1.93	0.28	0.98
D	1.00	1.59	1.05	0.17	1.31
E	1.00	0.40	0.17	0.11	0.72
F	1.00	0.74	0.03	0.14	0.45
G	1.00	0.41	0.15	0	0.70
H	1.00	0.31	0.10	0	0.83

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Additionally, some samples' surfaces were characterized by XPS. Survey spectra, plotted in Fig. 6A were consistent with EDS measurements; in addition to erbium, traces of the anions were observed: fluorine, oxygen, nitrogen, and carbon. The presence of platinum is attributed to cracks in the film which exposed the platinum bottom electrode. A high resolution Er 4d spectrum from sample B, plotted in Fig. 6B, is compared against that of an erbium foil. While the peak shape of the Er 4d is not easily fit due to complex multiplet splitting, the presence of a sharp peak at 167.3 eV is indicative of metallic erbium, while a broad peak at 168.9 suggests Er³⁺.^29–31 The metallic erbium foil of Fig. 6B shows this sharp peak, as expected, and also shows the broad peak indicative of surface oxidation. Sample B reveals erbium solely in the 3+ oxidation state, as evidenced by sole broad peak at 168.9 eV. It is unlikely that this is simply adsorbed erbium salt, as this would readily dissolve in DMF during the soak and rinse after electrodeposition. As these films readily oxidize in air, the 10 second exposure to atmosphere upon transfer to the XPS chamber would likely oxidize any Er⁰. Electrochemical results indicate that some species were reduced at potentials known to thermodynamically favor Er³⁺ reduction, and only when erbium salts were present, thus formation of a reduced erbium species is likely. Surface compositions (atomic%), as determined using the relative sensitivity factors built into CasaXPS were 4.8 ± 0.8% Er, 56.4 ± 6.9% C, 33.6 ± 4.1% O, 1.6 ± 1.4% N, 4.1 ± 1.4% F.

Fig. 6 (A) XPS survey spectra typical of erbium-containing films deposited from systems A–H in Fig. 4. (B) Er 4d spectrum from erbium-containing film deposited from system B (DMF) compared to that of an erbium foil.

All films deposited onto platinized silicon show no detectable crystallinity and were deemed amorphous. Plotted in Fig. 7 are log scale X-ray diffraction θ–2θ patterns of a typical sample and a clean substrate. All observed peaks are attributed to the platinized silicon substrate. The thickness of the erbium-containing film varied from sample to sample, but was on the order of 0.5–1.0 μm, providing sufficient erbium-containing material to detect any crystallinity, if present. This lack of crystallinity is consistent with previous literature reports of lanthanum deposition in ionic liquids.¹²

Fig. 7 Log scale X-ray (Cu Kα) diffraction patterns from erbium-containing films deposited onto platinum in electrolyte–salt systems A–H for 2 h at −2.90 V vs. Fc/Fc⁺. Throughout all samples, only peaks attributed to the platinized silicon substrate were observed.

3.3 Variation of electrode material

The influence of the working electrode material was evaluated in 0.1 M [Er(NTf₂)₃] in BMP-NTf₂ for working electrodes made of platinum, gold, nickel, or GC. Representative CVs are presented in Fig. 8H–K, corresponding to systems H–K in Table 1. For gold (Fig. 8I), reduction peaks R_a and R_b are observed at potentials similar to those on platinum (Fig. 8H). No appreciable change in electrochemical response is observed on GC compared to the neat IL. Thus, GC is a poor choice of substrate for the electroreduction of Er³⁺ in these electrolyte–salt systems, at the potentials tested. Surprisingly, the oxidation peaks O_a and O_b present on platinum and described in detail in Fig. 2 and 3, are absent on gold and nickel (and GC of course). The superior catalytic ability of platinum may enable processes O_a to occur at lower potentials, while other electrode materials require potentials outside of those interrogated. The exact products of processes R_a and R_b may be different on gold and platinum, or they may be the same as the products produced on platinum, but these products might not be as easily oxidized as they are on platinum. Either way, gold and nickel offer a way to avoid the ideally irreversible process O_a, allowing the products of R_a to be subsequently reduced to an erbium-containing film even if more oxidizing potentials were previously applied.

Fig. 8 Influence of working electrode material on CVs of neat BMP-NTf₂ (dotted blue lines) and with 0.1 M Er(NTf₂)₃ added (solid red lines). CVs are the 5^th scan at 100 mV s⁻¹. Subfigure letters correspond to electrolyte–salt–electrode systems outlined in Table 1.

4 Conclusion

A variety of organic solvents and ILs were investigated as electrolytes for the electrochemical reduction of Er³⁺. While all of the electrolytes evaluated have the requisite potential window for theoretical Er³⁺ reduction, irreversible interactions between electrolyte and Er³⁺ salt during electrodeposition resulted in the formation of a complex, amorphous erbium-containing solid–electrolyte interface. The choice of electrolyte was found to greatly influence the microstructure of the deposited erbium-containing film. Through a systematic investigation of ILs at a platinum electrode, the N(CN)₂⁻ anion was found to give slightly larger reduction currents and avoided an intermediate reduction reaction (R_a), as compared to OTf⁻ or NTf₂⁻ anions. Comparison of different electrode materials in BMP-NTf₂ revealed oxidation processes unique to platinum surfaces, and the absence of any appreciable reduction current on GC. Further research, especially into N(CN)₂⁻ anions and nickel electrodes, may enable room temperature electrochemical removal of valuable erbium from rare earth-containing waste streams.

Acknowledgements

The authors thank Ms. B. Mckenzie for assistance with the scanning electron microscopy, and Dr M. Brumbach for recording XPS data. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

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