Toward Transformative Ambient Rare Earth Metal Electrorefining: A Perspective on Electrolyte Design and Circular Processing

Abstract

Ambient electrochemical production of rare earth metals (REMs) in organic electrolytes has emerged as a promising alternative to conventional high-temperature and chemically intensive processes. Over the past two decades, a limited number of research groups have demonstrated the feasibility of REM electrodeposition under room temperature conditions. However, practical implementation remains limited by challenges in metal stability, selectivity, electrolyte performance, and scalability. This perspective critically assesses reported ambient REM electrochemical systems, including ionic liquid, solvent-based, ligand-coordinated, and mediated strategies. Particular attention is given to efforts toward coupling REM production with rare earth element recovery, including direct and indirect electrorefining of spent NdFeB magnets. While conceptually promising, these approaches reveal key limitations, such as complex hydrometallurgical processing, limited anodic dissolution and loss of REM selectivity. Another central focus is the insufficient validation of electrodeposition product quality and performance, as reported characterization techniques often provide inconclusive evidence regarding purity and metallic REM formation. Future progress requires integrated advances in coordination chemistry, interfacial electrochemistry, electrolyte and electrode design, alongside process engineering and technoeconomic assessment. Within this framework, a conceptual M2REM (Magnet-To-Rare Earth Metal) is illustrated as an example of integrated electrorefining platform for the direct and ambient transformation of spent magnets into REMs, paving the way for energy-efficient and circular rare earth metal processing.

Significance

Rare earth metals are universally regarded as indispensable for modern technologies, playing a pivotal role in driving electrification, advancing digitalization, and powering breakthroughs in advanced manufacturing. − Their unique electronic structure and magnetic properties underpin high-performance applications in permanent magnets, electric vehicle traction motors, wind turbines, microelectronics, and energy storage systems. Among these, Nd–Fe–B magnets dominate as the leading high-performance magnetic materials, incorporating neodymium with strategic additions of dysprosium and terbium to enhance thermal stability and coercivity. As global electrification accelerates, demand for REMs, particularly Nd and Dy, continues to rise sharply, with neodymium demand alone exceeding 100,000 tons annually and projected to grow significantly in the coming decades. From a techno-economic and geopolitical perspective, the supply of REMs remains highly constrained and unevenly distributed. Mining, separation, and refining capacities are concentrated in a limited number of regions, creating supply vulnerabilities, price volatility, and strategic dependencies ,, These challenges are further amplified by increasing demand from electric vehicles, renewable energy systems, and defense technologies. In parallel, secondary resources, such as end-of-life electronics and Nd–Fe–B magnet-containing devices, represent a rapidly growing yet underutilized reservoir of rare earth elements. − Despite their potential, these resources remain largely untapped due to the absence of efficient, scalable, and economically viable recovery and refining technologies. Consequently, securing a resilient, sustainable, and domestically adaptable supply chain for REMs has emerged as a critical priority for both industrial advancement and national security.

The current industrial production of rare earth metals relies on a sequence of energy- and chemically intensive processes that are inherently fragmented and resource-intensive. ,, As illustrated in Figure , conventional supply chains follow a linear pathway beginning with mining and beneficiation of rare earth ores, followed by complex hydrometallurgical operations for extraction and separation into rare earth oxides (REOs), and culminating in high-temperature metallurgical conversion to metals. These chemical processes involve aggressive lixiviants, multistage solvent extraction, and extensive purification steps, contributing to high operational costs and significant environmental burden. − For example, the production of one ton of rare earth oxides can generate thousands of tons of mining waste, substantial volumes of wastewater, and radioactive byproducts, in addition to large quantities of sulfur dioxide emissions. Despite the growing interest in circular economy strategies, current recycling pathways largely mirror this linear paradigm. End-of-life products, such as spent NdFeB magnets from electronic devices and electric vehicles, are typically processed through similar hydrometallurgical routes, yielding mixed oxide intermediates that must re-enter the same energy-intensive downstream processing chain. As a result, only a small fraction of rare earths is effectively recycled today, and the potential for closing the material loop remains largely unrealized. The final conversion of REOs into metallic form is dominated by high-temperature metallurgical processes, particularly molten salt electrolysis and calciothermic reduction (Eq ). − Though molten salt electrolysis enables the production of high-purity metals and alloys, it requires elevated temperatures (often >800–1000 °C) and corrosive fluoride-based electrolytes, leading to high energy consumption and hazardous emissions. While both oxide and fluoride-based routes rely on cathodic reduction of RE³⁺ to REM, oxide system generates CO/CO₂ evolution at carbon anode, whereas fluoride systems produce fluorinated emissions such as CF₄ and C₂F₆ (Eq to Eq ). These centralized, capital-intensive processes are inherently difficult to decentralize or integrate with distributed recycling streams. Consequently, the current REM value chain remains inefficient, environmentally taxing, and poorly aligned with emerging circular and sustainable manufacturing paradigms.

Calciothermic reduction:

$${\mathrm{RE}}_2{\mathrm{O}}_3+3\mathrm{Ca}\to 2{{\mathrm{RE}}^0}_{\mathrm{Metal}}+3\mathrm{CaO}$$ 1

Molten salt electrolysis:

$$\mathrm{Cathodic}:{\mathrm{RE}}^{3+}+3{\mathrm{e}}^{-}\to {{\mathrm{RE}}^0}_{\mathrm{Metal}}$$ 2

Anodic (using carbon anode):

$${\mathrm{REF}}_3:4{\mathrm{F}}^{-}+\mathrm{C}\to {\mathrm{CF}}_4+4{\mathrm{e}}^{-};2\mathrm{C}+6{\mathrm{F}}^{-}\to {\mathrm{C}}_2{\mathrm{F}}_6+6{\mathrm{e}}^{-}$$ 3

RE₂O₃:

$$\mathrm{C}+2{\mathrm{O}}^{2‐}\to {\mathrm{CO}}_2+4{\mathrm{e}}^{-};\mathrm{C}+{\mathrm{O}}^{2-}\to \mathrm{CO}+2{\mathrm{e}}^{-}$$ 4

1.

Schematic representation of the circular economy in rare earths. Reprinted with permission from ref . Copyright 2023 Springer Nature.

In response to these limitations, significant research efforts have been directed toward developing alternative electrochemical approaches capable of producing REMs under ambient or room temperature (<100 °C) conditions. Among these room-temperature electrodeposition in nonaqueous media, particularly ionic liquids (ILs) has emerged as a promising pathway to reduce energy consumption and environmental impact. − These systems offer key advantages, including wide electrochemical windows, intrinsic ionic conductivity, low volatility, and tunable solvation environments, enabling the stabilization and reduction of highly electropositive rare earth ions that cannot be processed in aqueous electrolytes. From a broader systems perspective, ambient electrochemical processes hold the potential to fundamentally reshape REM production by enabling modular, lower-energy, and potentially distributed processing platforms. Such approaches could facilitate the direct integration of rare earth recovery from secondary resources, including magnet recycling streams, into localized production systems. This shift would not only reduce reliance on centralized, high-temperature infrastructure but also support the development of circular supply chains aligned with sustainability goals. Despite these advantages, ambient electrochemical REM production and electrorefining remains at an early stage of development and has yet to achieve the performance, scalability, and economic competitiveness required for industrial adoption. Nevertheless, the continued exploration of these systems reflects a growing recognition that transformative advances in rare earth downstream processing will require new paradigms that integrate electrochemistry, materials design, and process engineering. In this context, ambient electrochemical approaches represent a high-impact research direction with the potential to address critical challenges in energy, sustainability, and global supply security.

Ambient Electrolyte Design and Strategies

Since the first demonstration of rare earth metal electrodeposition in nonaqueous media in the mid-2000s, the field of ambient electrochemical REM production has remained relatively niche, with sustained contributions from a limited number of research groups worldwide. Over the past two decades, a relatively small but active research community has shaped this field, with significant contributions from established groups and other emerging teams with intermittent activities. Key advancements in this field are summarized in Table . Collectively, these efforts have established the feasibility of REM electrodeposition at or near room temperature, while simultaneously revealing persistent challenges related to electrolyte design, interfacial stability, and metal speciation.

1. Major Reported Ambient Organic Electrolytes and Their REM Electrodeposition Performance.

Class of Organic Electrolytes	Electrolyte System	Reported Metrics (Efficiency % or RE Composition %)	Reference
Conventional Solvents and IL	Dy(OTf)3 in DMF/PyrrolidiniumNTf2	CE: 93%	Lodermeyer
Conventional ILs	Dy(NTf2)3 in PhosphoniumNTf2	CE:75%	Matsumiya
F-free IL	Nd(NO3)·6H2O in Pyrrolidinium Dicyanamide	CE: 60%	Pozo-Gonzalo
Water-IL	Nd(NTf2)3 in PhosphoniumNTf2	RE%: 9% (EDS)	Pozo-Gonzalo; Rudnev
RE-Solvate IL	DMI-Nd(OTf)3-LiNO3	RE%: 94% (EDS)	Zhang
Neutral ligand	Sm(NTf2)3 in TriMethyl Phosphate	RE%: 37% (EDS)	Jayakumar; Dai
Borohydride-based	DyCl3 in LiBH4 in 1,2-dimethoxyethane or 2-methyltetrahydrofuran	RE%: 35% (EDS)	Binnemans ,
Silylamide-based	NdCl3 with Lithium bis(trimethylsilyl)amide in THF	FE%: 90% (ICP) RE%: 93% (EDS)	Atifi

A wide range of electrolyte systems has been explored to enable the reduction of highly electropositive rare earth ions, which require very negative potentials (about – 3 V vs Fc/Fc⁺) and therefore challenge the electrochemical stability of most ambient organic media. Significant efforts focused on conventional organic solvents as electrolyte media. , Lodermeyer and co-workers reported the electrodeposition of dysprosium using solvents such as acetonitrile, propylene carbonate, dimethylformamide (DMF), as well as mixtures with pyrrolidinium triflate ILs containing rare earth triflate salts. These systems offered reduced viscosity relative to neat ILs and improved mass transport. Among the solvents explored, DMF provided relatively high salt solubility and a sufficiently wide electrochemical window to enable observable deposition. However, most of these systems generally suffered from low current densities, limited stability, and, critically, the absence of conclusive evidence for metallic REM formation, highlighting the limitations of purely solvent-based approaches.

Among the different electrolyte systems explored for ambient REM production, conventional ionic liquids have received the most sustained attention because of their wide electrochemical windows, negligible vapor pressure, nonflammability, and highly tunable physicochemical properties. − Their ability to stabilize highly electropositive metal ions made them attractive candidates for overcoming the limitations of aqueous systems, where hydrogen evolution and rapid metal oxidation dominate electrochemical behavior.

Matsumiya and co-workers were among the earliest groups to systematically investigate ambient rare earth electrochemistry in ionic liquid environments. ,− Their studies examined multiple IL families based on pyrrolidinium, ammonium, and phosphonium cations paired with triflate (OTf^–) and bis­(trifluoromethanesulfonyl)­imide (NTf₂ ^–) anions for deposition of rare earth metals such as Nd and Dy. These studies demonstrated that rare earth ions could undergo electrochemical reduction in such media and provided some of the earliest evidence supporting the feasibility of ambient REM deposition. Building upon these studies, subsequent investigations further explored pyrrolidinium-based IL systems, focusing on the influence of electrolyte composition, metal-ion speciation, and interfacial behavior on electrochemical performance and deposition characteristics. An additional strategy within IL systems involves the use of water as a promoter. Significant studies proposed that controlled addition of water can enhance reduction kinetics, shift deposition potentials, and increase apparent current densities. − However, these benefits are accompanied by significant drawbacks, including hydrogen evolution, metal oxidation, and reduced product purity. This approach has been critically evaluated, demonstrating that while water enhances kinetics, it compromises selectivity and stability, making such systems inefficient for metallic REM production. Collectively, these efforts established important foundations for understanding electrochemical reduction pathways in IL systems.

Despite these promising features, conventional ILs systems exhibit several intrinsic limitations that have constrained their broader application and scalability. The most significant challenge is the high viscosity of ionic liquids, which is one or 2 orders of magnitude higher than aqueous electrolytes, directly impacting transport properties and metal electrodeposition performance. Under ambient conditions, the diffusion coefficients (D) of rare earth species in ILs are often substantially lower (D in the range of 10^–11 m²/s) than in conventional molecular solvents (D in the range of 10^–7 m²/s), resulting in slow transport of electroactive species toward the electrode surface. , Consequently, many reported systems exhibit low achievable current densities and limited deposition rates (about 1 mA/cm²). Under such conditions, concentration polarization near the electrode surface can become dominant, causing local depletion of electroactive rare earth species and producing electrochemical responses that are strongly transport-limited rather than kinetically controlled. Apparent decreases in current density or deposition efficiency may therefore not necessarily reflect intrinsic electron-transfer limitations, but rather limitations arising from limited transport of metal ions within the IL electrolytes. Another limitation of conventional IL systems involves relatively low metal loading capacity and challenges in maintaining stable rare earth speciation within a fluidic environment. As such, strong ion pairing and complex coordination environments may alter reduction mechanism, making interpretation of electrochemical behavior difficult.

In addition to transport limitations, IL-based systems frequently exhibit challenges associated with interfacial stability and parasitic reactions. − Rare earth deposition occurs at highly negative potentials, often approaching or exceeding the electrochemical stability limits of several ionic liquid components. Under these conditions, electrolyte decomposition can occur through reduction of solvent species, cation decomposition, or anion breakdown. Such reactions frequently result in the formation of passivating surface layers, which progressively cover the electrode surface and disrupt sustained metal deposition. Although such interfacial layers can sometimes provide beneficial stabilization in battery systems, their formation in rare earth electrochemistry often introduces additional resistance and limits deposition efficiency. While the nature and origin of these passivating layers remain poorly understood, their composition likely results from a combination of electrolyte decomposition products, residual impurities, reaction intermediates, and metal-containing species. The presence of trace water or oxygen contamination may further complicate these interfacial processes through formation of oxide or hydroxide species. Despite these limitations, conventional ILs have played a critical role in establishing the feasibility of ambient REM electrochemistry and continue to provide valuable mechanistic insights into rare earth reduction processes.

The field has subsequently shifted toward water and fluorine-free, ligand-based electrolyte systems that leverage coordination chemistry to stabilize rare earth species and tune electrochemical behavior. − Trimethyl phosphate (TMP)-based systems have been explored to enhance solubility and modify the coordination environment of rare earth ions. , Following demonstrations of calcium deposition and stripping in borohydride systems, Binnemans and co-workers investigated similar electrolytes, using Li-, Na-, and tetrabutylammonium-based borohydride salts, for Nd and Dy electrodeposition. , While the anhydrous borohydride-based systems mitigate water and fluorine-driven degradation, they exhibit challenges including passivation layer formation, codeposition of alkali metals, and limited anodic stability, especially at higher deposition rates. , A silylamide-based electrolyte has recently been introduced, representing a distinct and robust class of nitrogen-coordinating ligand systems for REM electrodeposition. This designer electrolyte facilitated high rare earth loading, enhanced chemical stability, and more controlled electrochemical behavior, while efficiently enabling simultaneous dual REM stripping and deposition. More broadly, these ligand-based developments reflect a transition toward rational electrolyte design, in which coordination chemistry is used to modulate reduction thermodynamics, kinetics, and interfacial stability.

Beyond direct electrodeposition, mediated electrochemical approaches could also facilitate rare earth reduction under milder conditions. In these systems, a redox-active mediator would enable indirect reduction of rare earth ions. Inspired by metallothermic reduction processes, particularly calcium reduction (calciothermic) of rare earth salts at high temperatures, calcium-mediated systems within borohydride electrolyte have been explored. While this approach offers the potential to decouple electron transfer from direct deposition, it introduces additional complexity in controlling selectivity, particularly when reduction potentials of mediator and rare earth species are closely spaced. Additional hybrid electrolyte systems have been explored by Baoguo Zhang and co-workers, including aluminum chloride (AlCl₃)-based systems combined with organic solvents such as 1,3-dimethyl-2-imidazolidinone, as well as lithium nitrate-containing formulations. , While these systems offer unique coordination environments and electrochemical behavior, they face similar challenges related to metal stability and selectivity. Taken together, these diverse electrolyte systems and strategies highlight the breadth of approaches pursued in ambient REM electrochemistry. Despite notable progress, no single system has yet achieved the combination of stability, selectivity, efficiency, and scalability required for practical implementation. The continued evolution of the field points toward the need for integrated design principles to overcome the intrinsic limitations of existing electrolyte approaches.

Coupled Ambient REM Production with REE Recovery

The integration of electrochemical rare earth metal production with rare earth element electrochemical recovery represents a critical step toward developing truly circular and efficient rare earth processing technologies. While most reported ambient electrochemical studies focus on REM electrodeposition from prepurified salts, emerging efforts have begun to explore the coupling of upstream recovery with downstream metal production. These approaches can be broadly categorized into indirect strategies, where precursors REE salts are supplied from external processes, and direct strategies, where complex feedstocks such as spent magnets are used as the source material. Indirect approaches remain the dominant paradigm in ambient REM electrochemistry. ,, In these systems, REE salts, typically chlorides, triflates, or other soluble complexes, are used as the electrolyte feedstock. , These salts are generally obtained through conventional hydrometallurgical processes involving mining or recycling of RE-containing materials, followed by extraction, separation, and purification. As a result, the electrochemical step is effectively decoupled from the upstream recovery process, addressing only the final metal reduction stage. While this approach simplifies electrochemical control and enables systematic study of deposition mechanisms, it inherently retains the complexity, cost, and environmental burden associated with multistep chemical processing. Another promising indirect strategy would involve the use of electro-immobilized REE species within electrode architectures; a step explored in several previous studies. − In these isolated systems, REE ions are electrochemically preincorporated (chemisorption, physisorption, dialysis) into electrode materials mostly in RE rich concentration electrolytes, which can be theoretically employed as anode system that releases RE ions in the metallization electrolyte bath. While conceptually appealing, this approach is limited by low REE loading capacity and challenges in maintaining uniform distribution and accessibility of active species. Moreover, the presence of residual impurities, particularly water and competing ions, can lead to cross-contamination, parasitic reactions, and reduced product purity. These limitations restrict the scalability and practical applicability of electro-immobilized systems for bulk REM production

REM anodic dissolution:

$$\mathrm{NdFeB}\to {\mathrm{Nd}}^{3+}+\mathrm{FeB}+3{\mathrm{e}}^{-}$$ 5

Iron anodic co-dissolution:

$$\mathrm{NdFeB}\to {\mathrm{Fe}}^{2+}+\mathrm{NdB}+2{\mathrm{e}}^{-}$$ 6

Direct approaches aim to integrate REE recovery and REM production within a single electrochemical framework by using complex solid feedstocks, such as spent Nd–Fe–B magnets, as the source of rare earth elements. In the context of the circular economy, such electrorefining strategies represent a critical opportunity to bypass conventional linear processing routes and enable a direct magnet-to-metal conversion pathway, thereby shortening the value chain and reducing material, energy, and environmental burdens (Figure ). However, only a limited number of studies have explored anodic dissolution of REM-containing materials in room-temperature organic electrolytes. ,,,,, Simultaneous REM electrowinning, using NdFeB magnet as anode, has been initially explored in phosphonium-based ionic liquids. A more recent report also explored the direct electrorefining of NdFeB magnets using a dimethylformamide (DMF)-based electrolyte containing NdCl₃. While these works demonstrated the conceptual feasibility of coupling anodic dissolution with cathodic deposition, several key ambiguities remain. The anodic REM stripping (Eq ) in the employed electrolyte system is not clearly established, without definitive analytical (voltammetric) assignment of REM versus background or transition metal contributions. More fundamentally, significant codissolution of iron (Eq ) remains a major limitation. Due to the multiphase microstructure of NdFeB magnets, where Nd-rich grain boundary phases are embedded within a Fe-rich matrix, uncontrolled anodic conditions lead to simultaneous dissolution of both rare earth and transition metal components. This loss of selectivity not only reduces the purity of the resulting metal deposit but also undermines process efficiency and recyclability. Thus, despite the strong conceptual alignment of direct electrorefining with circular processing strategies, current implementations remain at an early stage.

2.

Flowchart of the typical electrochemical techniques and one-pot selective electrochemical leaching/deposition at room temperature for recycling REE from spent Nd–Fe–B magnets. Reprinted ref . Copyright 2024 American Chemical Society.

Performance and Characterization in Ambient REM Electrochemistry

A critical barrier to progress in ambient rare earth metal electrochemistry is not only the intrinsic chemical and electrochemical complexity of these systems, but also the lack of standardized, rigorous, and meaningful analytical methodologies used to evaluate and report performance. Across several reports of the above outlined literature, key metrics, including faradaic efficiency, stripping efficiency, purity, and oxidation state, are often reported in ambiguous or inconsistent ways, leading to potential misinterpretation of results and overestimation of process viability.

The widespread neglect of stripping voltammetric analysis remains a major limitation in the current ambient REM electrochemistry literature. In principle, the ratio of oxidative (stripping) charge to reductive (deposition) charge provides useful electrochemical insight into the fraction of deposited material that remains electrochemically recoverable in the metallic zerovalent RE⁰ state over a given voltammetric time scale (scan rate). While this ratio should not be interpreted as a strict standalone metric for the ultimate efficiency of bulk REM production, it nevertheless represents an important comparative descriptor for evaluating the ability of different electrolyte systems to support and preserve electrochemically recoverable REM metallization. Many studies report cathodic reduction currents and elemental characterization of deposits, yet significantly fewer provide systematic evaluation of anodic stripping behavior, despite stripping analysis being one of the most direct electrochemical methods for assessing reversibility and stability of the deposited metal. ,− ,− Cathodic reduction features are often interpreted as evidence of successful metallic REM deposition, although the corresponding anodic stripping signals are weak, poorly resolved, or entirely absent.

This issue becomes particularly important at highly reducing potentials where substantial cathodic current may originate not only from rare earth reduction, but also from electrolyte decomposition and codeposition, hydrogen evolution, and chemically coupled interfacial reactions. Under such conditions, cathodic current alone cannot be considered sufficient evidence of stable metallic REM formation. Significant cathodic current and surface deposition may still occur even when a substantial fraction of initially generated RE⁰ undergoes rapid chemical reoxidation or interfacial consumption. Consequently, surface deposition does not necessarily imply sustained metallization in the strict sense of preserving stable electrochemically recoverable RE⁰ species. As illustrated in Figure a, several reported ionic liquid systems exhibit large cathodic responses accompanied by weak or poorly defined stripping features. In conventional ionic liquid electrolytes based on triflate and bistriflimide systems, broad cathodic waves are often observed near the electrochemical stability limits of the electrolyte. However, the corresponding oxidative stripping currents are frequently small relative to the cathodic charge, suggesting limited preservation of metallic RE⁰ species. Such behavior may indicate that a substantial fraction of the cathodic current originates from irreversible side reactions, electrolyte decomposition, passivation processes, or formation of electrochemically inactive surface species rather than sustained stabilization of deposited RE⁰ metal.

3.

(a) Cyclic voltammograms in Nd-loaded/free phosphonium-NTf₂ IL with water additives. Reprinted from ref . Copyright 2019 American Chemical Society. (b) Cyclic voltammograms in Nd-loaded/free silylamide-based electrolyte in THF. Reprinted from ref . Copyright 2025 American Chemical Society. (c) Voltammetric stripping efficiency in the silylamide-based electrolyte in THF. Reprinted from ref . Copyright 2025 American Chemical Society.

The interpretation of electrochemical behavior becomes even more complex in water-promoted ionic liquid systems. Addition of water often leads to significantly enhanced cathodic current densities and shifts in reduction features, which have frequently been interpreted as evidence of improved deposition kinetics or enhanced REM reduction. − However, the corresponding anodic stripping signals in many of these systems remain weak or poorly resolved. Such behavior raises the possibility that water addition may simultaneously promote competing parasitic processes, including hydrogen evolution, altered interfacial chemistry, accelerated electrolyte decomposition, and increased chemical reactivity of deposited REM species. Without clear comparison to background (i.e., control and baseline experiments) signal, assignment of cathodic and anodic features becomes highly ambiguous. As illustrated by the recently reported silylamide-based electrolyte system shown in Figure (b and c), significant evolution of the cathodic and anodic responses is observed over repeated voltammetric cycles collected at different concentrations. Independent of the exact mechanistic interpretation of the reduction process (i.e., direct vs mediated reduction), the appearance and progressive evolution of anodic stripping features over multiple cycles provide tangible evidence for stabilization of electrochemically recoverable REM (or mixed with alkali metal deposit) species under voltametric time scale. Notably, stripping efficiencies approaching ∼ 60% under voltammetric conditions represent a relatively high degree of chemical reversibility and indicate that a substantial fraction of deposited metal remains stable in the zerovalent state over the voltammetric time scale. Such behavior suggests that the electrolyte environment is sufficiently robust to suppress extensive chemically coupled reoxidation and decomposition reactions following deposition. Conversely, systems exhibiting weak stripping responses indicate that a significant fraction of deposited RE⁰ is either chemically consumed, passivated, electrochemically isolated, or otherwise rendered inaccessible for reoxidation. The evolution of stripping efficiency over repeated cycles may provide additional insight into interfacial conditioning processes analogous to those commonly observed in battery systems, where formation of a stabilizing interphase progressively suppresses parasitic reactions. The increase in stripping efficiency observed in Figure c is consistent with such behavior and suggests progressive stabilization of deposited metal.

Closely related, Faradaic or current efficiency (FE or CE) is a commonly reported metric in metals electrodeposition; however, its determination in ambient REM systems is frequently oversimplified. These efficiency metrics are often calculated based solely on the total rare earth content measured in the deposit, typically using elemental analysis techniques such as Inductively Coupled Plasma (ICP) or Energy Dispersive X-ray Spectroscopy (EDS), versus the theoretical amount based on Coulombic charge during electrodeposition. Given the high reactivity (in situ and/or post deposition) of rare earth metals and their strong tendency toward oxidation, this assumption is often unjustified. As a result, reported FE values may significantly overestimate the true efficiency of metallic REM production.

Generally, the assessment of the deposition product purity and quality presents a critical challenge at small bench scale. While elemental EDS and ICP characterization methods provide valuable information on RE distribution and composition, they do not distinguish between metallic and oxidized forms of rare earth species, nor do they capture the presence of electrolyte-derived RE ions residues. Consequently, reported purity values may not accurately reflect the true chemical state or functional quality of the deposited material. As such, the most significant source of ambiguity arises in the determination of the oxidation state of the deposited material. X-ray photoelectron spectroscopy (XPS) is widely employed as a primary tool for assessing the chemical state of rare earth deposits; however, its interpretation in these systems is highly nontrivial and often ambiguous. ,,, The intrinsic multiplet structure of rare earth core-level spectra, particularly the 3d and 4f regions, combined with strong sensitivity to surface oxidation, charging effects, and sputtering-induced modifications, complicates spectral analysis and can lead to misleading assignments. ,, As a result, distinguishing between metallic (RE⁰) and oxidized (RE³⁺) states based solely on simplified and isolated XPS peak signal is inherently challenging and requires careful calibration, appropriate reference standards, and complementary analytical techniques. This complexity is discussed in a recently reported XPS analyses of neodymium electrodeposit obtained in a borohydride-based electrolyte. For instance, the characteristic Nd 3d₅/₂ binding energy for metallic Nd has been inconsistently assigned over a relatively broad range (≈978–981 eV). While shifts in binding energy upon sputtering are often interpreted as evidence for increased metallic character in the bulk, a more rigorous examination suggested that alternative interpretations are equally plausible (Figure ). Attempts to deconvolute spectra using linear combinations of reference Nd³⁺ and Nd⁰ components-derived, for example, from Nd oxide and metallic foil, frequently fail to produce physically meaningful fits. This discrepancy may arise from differences in morphology (e.g., crystalline Nd foil versus dendritic or amorphous electrodeposits), the presence of mixed or intermediate chemical states, or unresolved spectral contributions from ligand-bound or surface-modified species. Furthermore, sputter cleaning, commonly employed to probe bulk composition, can introduce artifacts, including preferential sputtering, reduction, or structural damage, thereby altering the chemical state of the material and complicating curve resolution. Robust analysis requires fitting across a wide spectral window that captures the full set of multiplet, and satellite features associated with each oxidation state. Narrow fitting ranges or omission of key fingerprint components can lead to unsupported conclusions regarding the presence and relative abundance of metallic versus oxidized species. Given these challenges, quantitative determination of Nd valence distributions from standard XPS analysis alone remains unreliable.

4.

XPS sputtering analysis of obtained Nd electrodeposit (ED) in a borohydride-based electrolyte, and peak fittings of standard Nd foil before and after sputtering. Reprinted from ref . Copyright 2023 American Chemical Society.

Similarly, X-ray diffraction (XRD) is also used as evidence for the formation of metallic REM phases; however, its applicability at proof concept stage is limited. ,, In many cases, reported XRD patterns are inconclusive or insufficient to definitively confirm metallic REM formation. While many studies report nanoscale or dendritic deposits, it remains unclear whether these systems can produce bulk metallic forms, such as dense films or ingots, under ambient conditions. The typically small quantity (per bench scale batch) of electrodeposited material, often below the detection threshold of conventional XRD (on the order of ∼ 0.1 g), makes phase identification difficult. Moreover, rare earth metals are highly reactive and can oxidize rapidly upon exposure to air, potentially altering the phase composition prior to analysis. Accurate XRD characterization therefore requires carful inert handling and transfer protocols, which are not routinely implemented. Together, these characterization challenges highlight a need for standardized and rigorous analytical frameworks in ambient REM electrochemistry. Without such rigor, it will remain difficult to distinguish true advances from artifacts or partial reductions, hindering progress toward scalable and practical technologies.

Outlook for Transformative Ambient REM Electrorefining

The development of ambient electrochemical routes for REM production remains a high-risk, high-reward frontier, with the potential to fundamentally transform rare earth supply chains. However, realizing this vision requires addressing a set of deeply coupled scientific and engineering challenges that extend well beyond incremental improvements in electrolyte formulation or electrochemical performance. Progress in this field demands a coordinated, multidisciplinary effort, calling on researchers from coordination chemistry, interfacial electrochemistry, materials engineering, and process design to collaboratively advance these systems from proof-of-concept to scalable and practical technologies.

A primary challenge in ambient rare earth electrorefining lies in achieving and maintaining the stability of rare earth metals in organic environments. Ensuring the formation and persistence of zerovalent metallic states therefore requires electrolyte systems that not only enable reduction at extremely negative potentials but also suppress reoxidation and chemical decomposition. This demands rigorously controlled (i.e., water, fluorine-free) environments with well-defined coordination chemistry, tailored ligand frameworks, and minimal reactive impurities. ,,,, From a transport and process perspective, mass transport limitations remain a major bottleneck for both deposition rate and scalability. The high viscosity of many nonaqueous electrolytes, combined with complex speciation and low diffusion coefficients, restricts achievable current densities and throughput. Overcoming these limitations will require the codesign of electrolytes with improved transport properties and electrode architectures that enhance reactant accessibility.

At a more fundamental level, the field remains underdeveloped in its understanding of the interplay between coordination chemistry and interfacial electrochemical processes. In contrast to battery systems, where significant efforts have been devoted to correlating ion solvation, transport, and interfacial behavior, primarily for light elements such as lithium and sodium, ambient REM electrochemistry remains in its infancy. , The chemistry of lanthanides, characterized by large ionic radii, high charge density, and heavier electronic structure, is expected to give rise to significantly richer and more complex behavior than that observed in lighter alkali metal systems. Key questions remain open regarding the nature of metal–ligand bonding in organic electrolytes, including the extent of ionic versus covalent character and the role of core orbitals in coordination and solvation. − The influence of ligand identity and coordination environment, particularly the distinction between oxygen- and nitrogen-donor ligands, on electrochemical metal reduction pathways is a critical area requiring systematic investigation. These interactions directly impact reduction thermodynamics, intermediate stability, and interfacial kinetics. In particular, the possibility of intermediate oxidation states formation, such as RE­(II), and the pathways through which they form, whether via direct multielectron transfer or mediated processes involving alkali or alkaline earth metals (e.g., Li, Na, Ca), remain poorly understood. ,, Looking forward, the integration of computational and data-driven approaches offers significant opportunities for accelerating electrolyte discovery and optimization. The use of quantum chemical calculations to unveil metal–ligand interactions, combined with machine learning frameworks for screening electrolyte compositions and ligand structures, can provide valuable insights into design principles that are difficult to access experimentally. Such approaches will be particularly important given the vast chemical space associated with rare earth coordination environments and solvent systems.

Nucleation and growth processes represent an additional area where existing models may be insufficient to capture the complexity of ambient REM electrodeposition. Current interpretations often rely on classical frameworks such as instantaneous or progressive nucleation; however, the highly reducing potentials, complex interfacial chemistry, and potential formation of metastable phases suggest that more advanced models are needed. − Understanding how these factors influence morphology, crystallinity, and reactivity of the deposited metal is critical, particularly given the strong coupling between deposition structure and subsequent stability. Interfacial stability, including the formation and evolution of solid electrolyte interphase (SEI), also plays a central role in determining system performance. Unlike battery systems, where SEI formation has been extensively studied, its nature and function in rare earth metal electrochemical systems remain largely unexplored. The composition, stability, and reactivity of these interphases under highly reducing conditions are expected to strongly influence both REM deposition efficiency and product quality.

Equally important is the need to address the analytical and methodological limitations outlined above, particularly in the electrochemical interpretation and validation of REM metallization, as well as oxidation-state characterization. Many reported studies emphasize cathodic reduction currents and postdeposition elemental analysis while providing only limited discussion of anodic stripping reversibility, baseline electrolyte behavior, or cycle-to-cycle electrochemical evolution. However, these electrochemical aspects are critical for distinguishing transient reduction processes and surface coating phenomena from true sustained metallization involving preservation of electrochemically recoverable RE⁰ metallic species. Future studies should place greater emphasis on stripping efficiency and voltammetric reversibility over multiple electrochemical cycles, as comparative electrochemical descriptors of electrolyte robustness and the ability to preserve deposited REM in the zerovalent state. Systematic evaluation of stripping behavior as a function of scan rate, cycling scans, and electrolyte composition could provide a valuable framework for comparing electrolyte stability and metallization robustness across the diverse ambient REM electrochemistry systems. Similarly, interpretation of XPS data remains highly challenging due to the complex multiplet structure of lanthanide spectra, sensitivity to surface oxidation, charging effects, and sputtering-induced artifacts, all of which can lead to ambiguous assignment of metallic versus oxidized REM species. These limitations become particularly significant for air-sensitive and structurally heterogeneous electrodeposits, where morphology, oxidation, and interfacial composition may evolve rapidly during handling and analysis. The establishment of standardized metrics, rigorous validation protocols, and unified reporting guidelines is therefore essential for meaningful comparison across studies and for identification of genuine advances in ambient REM electrochemistry. A useful precedent can be found in the field of ambient nitrogen reduction to ammonia, where, after years of inconsistent reporting, community-wide analytical guidelines were proposed to define best practices for experimental validation and performance assessment. A similar analytical effort will be critical for improving reproducibility, strengthening scientific credibility, and accelerating the transition of ambient REM electrorefining from exploratory proof-of-concept studies toward scientifically rigorous and technologically relevant platforms.

Process design for ambient rare earth electrorefining may need to evolve toward multifunctional systems capable of integrating selective anodic dissolution with cathodic metal recovery within a stable electrochemical environment. In this context, a SMART M2REM (Scalable, Multifunctional, Ambient, Recyclable, and Tuned Magnet-to-Rare Earth Metal) concept is proposed as an example framework for the future development of integrated rare earth processing technologies. Rather than representing a fully demonstrated process, SMART M2REM serves as a conceptual platform intended to connect emerging advances in electrolyte design, electrode engineering, and electrochemical process architecture into a unified strategy for direct magnet-to-metal conversion. As schematically illustrated in Figure , the SMART M2REM framework illustrates an integrated dual-function electrochemical architecture in which selective anodic extraction and cathodic metallization are coupled within a single electrochemical system. Unlike conventional rare earth processing routes that separate extraction, purification, and metallization into multiple unit operations, the illustrated framework seeks to integrate these functions into a continuous and electronically controlled process. Within this architecture, spent NdFeB magnets are envisioned as engineered electrochemical interfaces capable of directly participating in material recovery and transformation pathways.

5.

Conceptual SMART M2REM electrorefining process, illustrating the dual function of REM recovery and production using a microporous spent magnet anode and silylamide designer electrolyte system.

The SMART M2REM framework is driven by several limitations identified across current ambient rare earth electrochemical systems, including poor stripping selectivity, transport limitations within complex feedstocks, electrolyte instability, and the disconnect between rare earth recovery and metal production. Two key design elements are envisioned as potential strategies to address these challenges. The first involves restructuring spent NdFeB magnets into microporous electrochemical anodes to increase accessibility to rare-earth-rich phases, shorten diffusion pathways, increase electrochemically active interfacial area, and potentially mitigate depletion-driven Fe codissolution. The second is the use of multifunctional ligand-designed electrolytes, such as recently reported silylamide-based systems, to stabilize anodically stripped rare earth species, support high metal loading, and suppress parasitic interfacial reactions and passivating layer formation. Importantly, the SMART M2REM framework involves a broader processing perspective in which complex feedstocks and multiple electrochemical functions can be integrated within a single electrolyte environment, enabling simultaneous electro-lixiviation of complex materials and cathodic metallization under ambient conditions. The illustrated framework is therefore intended not only as a strategy for NdFeB magnet recycling, but also as a broader design philosophy for future electrochemical processing of secondary critical-material resources.

Despite its potential advantages, SMART M2REM remains a conceptual framework, and several scientific and engineering challenges must be addressed before practical implementation can be realized. The heterogeneous composition of spent NdFeB magnets, including variations in rare earth content, oxidation state, coatings, and contaminant species, may introduce substantial variability in electrochemical behavior and process reproducibility. Although microporous restructuring is expected to improve electrolyte accessibility and mass transport, excessive porosity may compromise mechanical integrity and electrical conductivity, requiring careful optimization of pore architecture. Selective anodic stripping may also remain challenging due to depletion-driven activation of Fe-rich domains and potential Fe codissolution during prolonged operation. In parallel, the long-term stability of multifunctional electrolytes under simultaneous anodic and cathodic operation remains largely unexplored, particularly with respect to impurity accumulation, electrolyte decomposition, transport limitations at high metal loading, recyclability, and nonvolatility. While these scientific and engineering challenges remain, the SMART M2REM concept illustrates how coordinated advances in electrolyte chemistry, electrode architecture, and electrochemical design may provide pathways toward simplified and circular rare earth metal manufacturing systems.

Finally, any proposed ambient REM electrochemical platform must ultimately be evaluated within a technoeconomic context. Comparative analysis against established molten salt and chemical processing routes, taking into account energy consumption, capital and operational costs, scalability, and environmental impact, will be essential to determine the practical viability of emerging technologies. Without such analysis, it will remain difficult to assess whether proposed ambient electrochemical approaches can move beyond laboratory demonstrations to industrial implementation. In summary, the path toward transformative ambient REM electrorefining requires a paradigm shift from empirical exploration to integrated, mechanistically informed design. By addressing fundamental gaps in coordination chemistry, interfacial processes, transport phenomena, and analytical rigor, and by leveraging advances in computation and process engineering, the field can progress toward scalable, energy-efficient, and sustainable solutions for rare earth metal electrorefining.

The author declares no competing financial interest.