Rare earth metal-based oxygen electrocatalysts: insights from mechanisms and designs to applications

Abstract

Rare earth metal-based oxygen electrocatalysts show great potential in the field of energy conversion and storage, especially in the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), due to their unique electronic structures and excellent catalytic properties. This paper reviews research progress in rare earth metal-based oxygen electrocatalysts, focusing on synthesis strategies, evaluation indexes, design strategies, and prospects for applications in fuel cells, metal–air batteries, and the electrolysis of water. Firstly, the reaction mechanisms of rare earth metal catalysts in the ORR and OER are analysed, and the intrinsic mechanism of rare earth metals to enhance the reaction rate by regulating the redox behaviour and optimising the surface structure is further revealed with the help of in situ characterisation and other techniques. Secondly, four design strategies for catalysts are introduced, including doping, alloying, compounds, and single atomisation. Finally, the prospects and challenges for the development of rare earth metal-based catalysts are discussed.

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1 Introduction

The oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are critical processes in various energy conversion and storage systems. However, both the ORR and OER face significant challenges due to high potential energy barriers and slow kinetics, which inevitably reduce the reaction rates. Therefore, catalysts are essential for enhancing the kinetics of these reactions. Currently, state-of-the-art catalysts for the ORR and OER primarily consist of noble metal-based materials, such as platinum (Pt) and ruthenium (Ru). However, their scarcity, limited activity, and poor stability hinder their widespread industrial application.^1–3 Many researchers have explored the potential of using metal materials as electrocatalysts for the ORR^4–8 and OER,^9–13 including precious metals, transition metals, carbon-based catalysts^14–17 etc. Among these metal materials, the electronic orbital structures of rare earth elements^18–20 exhibit excellent electron transfer capabilities, which can reduce the band gap and the energy barriers associated with the rate-limiting steps, thereby further accelerating the electron transfer process in electrocatalytic reactions. In the ORR/OER process, the 4f electrons of lanthanides (La–Lu) among rare earth metals are shielded by the outer orbitals, forming a unique electron cloud distribution, which can precisely regulate the d-band centre position to balance the oxygen intermediate adsorption energy, and at the same time stabilise the reaction intermediates through multivalent cycling. Currently, although there are reviews focusing on lanthanide elements,²¹ this paper extends the scope of rare earth elements beyond lanthanides to include Sc and Y, providing a more detailed explanation for the role of rare earth elements in this type of catalyst. Whereas Sc and Y have no 4f orbitals, their 3d/4d electrons participate in the formation of oxides with excellent oxygen vacancy formation ability.^22–24 This ability can activate O₂ molecules (ORR) or promote reactions involving lattice oxygen (OER) to accelerate electron transfer. Therefore, rare earth metal catalysts are widely recognised as some of the most promising electrocatalysts for the ORR and OER (Fig. 1).

Fig. 1 (a) Cumulative annual number of ORR and OER rare earth-based genus catalysts published in the last 8 years. (b) The proportional distribution of different catalyst types within the analyzed literature corpus.

In recent years, significant progress has been made in the application of rare earth metal-based electrocatalysts for the ORR and OER. Tang et al.²⁵ designed an ORR electrocatalyst wrapped in situ in N-doped carbon nanofibers (Fe₃O₄/CeO₂@N-CNFs), and density-functional theory (DFT)^26,27 calculations showed that the Fe₃O₄/CeO₂ heterojunction-induced coupling of 3d–4f orbitals could cause a downward shift of the Fe-site d-band centre, which weakened the adsorption affinity of the O-containing intermediates and reduced the overall oxygen reduction pathway energy barrier, thus significantly increasing the intrinsic activity of the ORR. This provided new research ideas for the optimisation of ORR catalytic systems. While making the above progress in ORR research, rare earth metal-based electrocatalysts also demonstrated excellent application potential in the OER. Our team²⁸ developed a biomimetic electrocatalyst inspired by cell membranes, featuring a nickel–iron layered double hydroxide (NiFeLDH) with frustrated Lewis pair sites (FLPs) located in the outer layer. The gradient orbital coupling system among Ni 3d, O 2p, and Ce 4f, in conjunction with the FLP sites, enhanced the covalent bonding between Ni and O. This configuration provided additional orbital nodes to replace LHBs as electron donors, thereby maintaining the catalyst's stability and improving the scalar relationship between *OH and *OOH during the OER. In addition to demonstrating excellent performance in single ORRs and OERs, rare earth metal-based electrocatalysts also exhibited unique advantages in catalytic systems that combined both ORR and OER functions. Xu et al.²⁹ developed a highly active and stable catalyst featuring a d–f double-band redox center by anchoring a single cerium (Ce) atom to cobalt oxide (CoO). Their findings indicated that the mobility of 4f electrons introduced an enhanced spin–orbit coupling effect, promoting optimal σ/π bonding and facilitating flexible adsorption between the Ce/Co active site and *O. Additionally, the introduction of localized Ce 4f electrons enhanced the orbital bonding capacity of cobalt (Co), effectively suppressing the dissolution of Co sites and improving the structural stability of the cathode material. This effectively enhances both ORR and OER dual-function reactivity.^30–33 The gradient orbital coupling (GOC) framework proposed by Fu et al.³⁴ provided a rational understanding of rare earth-induced electrocatalytic mechanisms from the perspectives of molecular orbital theory and energy band structure. Although there were reviews that explored the application of rare earths in electrocatalysis from the perspective of orbital gradient coupling,³⁴ there are very few reviews systematically reviewing the synthesis, mechanisms, modification strategies,^35–38 and future challenges of rare earth-based catalysts, covering a wider range and better highlighting their role as oxygen electrocatalysts.

This paper aims to provide a comprehensive understanding of the recent advances and current applications of rare earth metal-based catalysts in the ORR and OER. Firstly, the fundamental principles of rare earth metal-based catalysts for the ORR and OER are elaborated, along with relevant characterization techniques and performance evaluation methods. Subsequently, the article reviews the synthesis strategies for rare earth metal-based oxygen electrocatalysts, summarizes their research progress, and highlights their applications as dopants,^39,40 alloys,^41,42 compounds,⁴³ and single-atom^44,45 catalysts. Finally, a rational perspective is provided for the future development and application of rare earth metal-based materials in electrocatalysis (Fig. 2).

Fig. 2 The main discussion directions of this paper.^46–49 Reproduced from ref. 46 with permission from Wiley, copyright 2025. Reproduced from ref. 47 with permission from the American Chemical Society, copyright 2024. Reproduced from ref. 48 with permission from the American Chemical Society, copyright 2025. Reproduced from ref. 49 with permission from Wiley, copyright 2023.

2 Oxygen reduction/evolution reaction

The ORR is defined as the process where oxygen, under particular conditions, gains electrons to undergo a reduction reaction. The OER is essentially the process of water or hydroxide ions losing electrons and ultimately producing oxygen. The effective implementation of both reactions is a key prerequisite for the efficient transformation and sustainable utilization of energy, and holds a crucial position in the advancement of new energy technologies.

2.1 Mechanism

The ORR and OER have long been regarded as the fundamental electrochemical processes that govern the performance of energy conversion and storage devices. Nevertheless, both reactions are characterized by complicated multi-electron transfer steps, sluggish kinetics, and considerable overpotentials. Hence, clarifying their underlying mechanisms is considered essential for the rational development of highly efficient electrocatalysts. On this basis, the reaction pathways and key influencing factors of the ORR and OER are systematically explained below.

The reaction mechanism of the ORR involves three sequential stages: (1) oxygen diffusion through the electrolyte, (2) oxygen adsorption onto the electrode surface, and (3) subsequent chemical decomposition of adsorbed oxygen species. This process fundamentally constitutes a competitive–synergistic interaction between oxygen and water molecules, wherein both species vie for occupation of active sites on the electrode surface.^50,51

The ORR electrocatalytic process is mainly through the 4e⁻ pathway or the 2e⁻ pathway⁵² (Fig. 3a), which react differently in alkaline and acidic media.^53–57

Fig. 3 (a) Schematic mechanism of the 2e⁻ and 4e⁻ pathways of the ORR. Reproduced from ref. 52 with permission from Wiley, copyright 2025. Schematic illustrations of the OER mechanisms: (b) AEM and (c) LOM. Reproduced from ref. 64 with permission from Nature Publishing Group, copyright 2024.

In an acidic environment,

4e⁻ pathway:

O₂ + 4H⁺ + 4e⁻ → 2H₂O E₀ = 1.23 V (1)

  2e⁻ pathway:

O₂ + 2H⁺ + 2e⁻ → H₂O₂ E₀ = 0.70 V (2)

2e⁻ peroxide reduction:

H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O E₀ = 1.76 V (3)

In an alkaline environment,

4e⁻ pathway:

O₂ + 2H₂O + 4e⁻ → 4OH⁻ E₀ = 0.40 V (4)

2e⁻ pathway:

O₂ + 2H₂O + 2e⁻ → HO₂⁻ + OH⁻ E₀ = 0.06 V (5)

2e⁻ peroxide reduction:

HO₂⁻ + 2H₂O + 2e⁻ → 3OH⁻ E₀ = 0.86 V (6)

Generally for metal–air batteries^58–60 or fuel cells,^61–63 the oxidation reaction occurs at the anode through the loss of electrons and oxygen molecules are reduced to H₂O by gaining electrons at the cathode. The manifestation of the ORR is contingent not merely upon O₂ but also on the intricate interplay between O₂ and the electrocatalytic surface. The direct reduction pathway of O₂ molecules involves a four-electron transfer process, whereas indirect reduction constitutes a two-electron reaction predominantly yielding H₂O₂ as a product. This latter mechanism, paradoxically, serves as a viable strategy for the targeted synthesis of H₂O₂. However, the electrocatalytic process optimized via the 4e⁻ direct reaction route demonstrates superior ORR efficacy, primarily because the 2e⁻ pathway, when applied in fuel cell contexts, generates H₂O₂ ^62,63 as a byproduct. This byproduct not only diminishes energy conversion efficiency but also initiates the degradation of proton-conducting polymer electrolytes.

The OER process usually involves the adsorption evolution mechanism (AEM) and the lattice oxygen participation mechanism (LOM) (Fig. 3b and c).⁶⁴ Understanding the reaction mechanism is essential for the theoretically informed design of more efficient catalyst materials. The OER process involves a complex four-electron transfer process, including the adsorption–dissociation of OH⁻, the formation of O–O intermediates, and the desorption of O₂, and it reacts differently in acid and alkaline environments.^65–67

In an acidic environment,

2H₂O → O₂ + 4H⁺ + 4e⁻ E₀ = 1.229 V (7)

  In an alkaline environment,

4OH⁻ → O₂ + 2H₂O + 4e⁻ E₀ = 0.401 V (8)

2.2 In situ characterisation/DFT to probe the source of modified catalyst activity

In situ techniques capture the structural evolution of catalysts under reaction conditions in real time, while DFT pinpoints active sites by simulating electronic structures.⁶⁸ The combination of the two can reveal the adsorption–desorption kinetics of complex reaction intermediates and elucidate the physicochemical nature of the active origins. In situ characterization techniques are indispensable for understanding redox electrocatalytic processes, encompassing methods such as infrared spectroscopy,⁶⁹ Raman spectroscopy,⁷⁰ X-ray absorption spectroscopy (XAS),⁷¹ X-ray photoelectron spectroscopy (XPS),⁷² and transmission electron microscopy (TEM).⁷³ Compared to traditional electrochemical methods, in situ characterization techniques provide a more comprehensive understanding of the reaction mechanisms (Fig. 3a). Raman spectroscopy, recognized as a non-destructive analytical method, operates based on the fundamental principle of light–matter interactions with chemical bonds inherent to the material. This technique offers comprehensive insights into the sample's chemical architecture, encompassing phase composition, morphological features, crystallographic quality, and intermolecular interactions. By enabling real-time monitoring of catalysts, reactants, and reaction intermediates,^74,75 in situ characterization provides unprecedented insights into the dynamic processes of the ORR and OER, thereby facilitating a more precise understanding of the underlying catalytic mechanisms.^76,77 For example, Hu and colleagues⁷⁸ presented a method for the synthesis of hollow carbon loaded gadolinium (Gd) SA catalysts using low and secondary coordination sphere engineering techniques. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) was employed to investigate the key reaction intermediates, revealing distinct absorption bands at 1635 cm⁻¹ and 3400 cm⁻¹, which were assigned to the O–H bending and stretching vibrations, respectively. These spectral features exhibited a significant increase in intensity as the applied potential decreased, suggesting the progressive accumulation of hydroxyl groups at the catalytically active metal sites. This observation is aligned with previous findings (Fig. 4b and c). In addition, the negative peak at 1230 cm⁻¹ was enhanced with decreasing potential, indicating its rapid conversion during the ORR process, and for O₂ at 1030 cm⁻¹, the peak of the O–O stretching mode, also increased with decreasing potential, highlighting the dynamic oxygen interactions at the catalyst surface that promoted the formation and decomposition of oxygen intermediates, thus improving the ORR performance. Du and colleagues⁷⁹ synthesized single-atom Ce catalysts supported on phosphorus, sulfur, and nitrogen co-doped hollow carbon matrices (Ce SAs/PSNC) for the ORR. Extended X-ray absorption fine structure (EXAFS) analysis revealed a dominant Ce–N(O) coordination peak at approximately 2.03 Å, while no Ce–Ce scattering path was observed near 3.63 Å (Fig. 4d). This absence of metal–metal interactions confirmed the atomic dispersion of Ce sites on the PSNC substrate. More importantly, considering that no significant absorption peaks other than that of Ce–N(O) are present, it can be reasonably inferred that S and P are not directly bonded to Ce in the first coordination shell layer. The fitted curves (Fig. 4e) were almost consistent with experimental data. In addition, the wavelet transform can provide strong resolution maps in k-space and R-space (Fig. 4f), and the results demonstrate that the Ce SAs/PSNC contour map has only one intensity maximum at ≈2.90 Å⁻¹, suggesting that the N elements with lower atomic numbers in the Ce SAs/PSNC samples are also coordinated to Ce with respect to the C–O bonds in CeO₂. In situ characterization techniques are essential for identifying reactive intermediate species and elucidating chemical reaction mechanisms. These techniques are also instrumental in revealing the OER mechanism of hydroxides and the development of cost-effective OER electrocatalysts. Fu et al.⁸⁰ addressed the limitations of the AEM scaling relationship during the OER process, proposing a novel rare-earth-site substitution strategy to modulate the lattice oxygen redox activity of spinel oxides. This approach enables the LOM pathway to exhibit dynamic surface state changes. To systematically investigate these surface potential-dependent dynamic changes, surface potential-dependent in situ Raman spectroscopy was employed, as illustrated in Fig. 4g and h. NiCo₂O₄ displayed three distinct peaks at 681.3, 529.3, and 476.8 cm⁻¹ belonging to the A_1g, F_2g, and E_g modes, respectively. Two peaks at 1355.2 and 157.7 cm⁻¹ correspond to the Disorder (D) and Graphite (G) bands of the carbon-carbon bonds, respectively. The intensity of the E_g and F_2g modes increased as the potential shifted, suggesting a greater freedom of movement for the connection to the T_d metal site. Li et al.⁸¹ developed a novel plasma (P)-assisted strategy to synthesize atomically dispersed Ce on CoO (P-Ce SAs@CoO), which was employed to investigate the origin of the OER activity in rare-earth transition metal oxide (RE-TMO) systems. To elucidate the surface transformations of P-Ce SAs@CoO and CoO during the OER process, they systematically investigated the effects of different applied potentials. Fig. 4i showed the contour plots of the Raman spectra of CoO. At a working bias potential of 0.2 V vs. Ag/AgCl, CoO exhibited the E_g, F_2g and A_1g vibrational peaks of Co–O with good signal intensity. When the bias potential was increased to around 0.45 V per Ag per AgCl, the signal intensity of the Co–O vibrational peaks showed an obvious trend of rapid decay, which might be related to surface remodeling, lattice oxygen precipitation, and dissolution–redeposition processes of the catalysts during the OER process.

Fig. 4 (a) Schematic diagram of common in situ characterization techniques for ORR and OER electrocatalysts. In situ ATR-SEIRAS spectra (b) recorded on Gd@N/P-HC for the ORR. (c) Corresponding contour plots. Reproduced from ref. 78 with permission from Wiley, copyright 2024. (d) Spectral analysis employing Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) characterization. (e) Resulting EXAFS R-space fitting curves derived from the experimental data. (f) Wavelet transform (WT). Reproduced from ref. 79 with permission from Wiley, copyright 2023. (g and h) Potential-shifted in situ Raman spectra of NiCo₂O₄ and Ce–NiCo₂O₄. Reproduced from ref. 80 with permission from Wiley, copyright 2024. (i) The corresponding contour plot of CoO. Reproduced from ref. 81 with permission from Wiley, copyright 2023.

In addition, the choice of transition adsorption states^82,83 and reaction pathways⁸⁴ can be understood by DFT calculations. Liu et al.⁸⁵ utilized graphene as the base material for rare earth single-atom electrocatalysts while also creating graphene supermonomers. The rare earth atoms were fastened at the middle of double vacancy defects present in the graphene support. Through modifying the coordination configuration of active sites at the defect locations, researchers developed a set of YbN_xC_y–graphene (YbN_xC_y-gra) catalyst models. Among these models, YbN₂C₂-pentagon (YbN₂C₂-pen) is characterized by a five-membered ring structure, which arises from two neighboring coordinated nitrogen atoms. DFT calculations were applied to study the activity of this catalyst. The plotted dynamic simulations (Fig. 5a) revealed that the electrocatalysts were dynamically simulated for 2500 steps at 298.15 K. The energies of all electrocatalyst configurations varied dynamically within a small range, further verifying its stability. PDOS of YbN₂C₂-pen are shown in Fig. 5b. The overlapping peaks of p–d and p–f orbitals of all catalysts were concentrated below the Fermi energy level, indicating the strong interaction between the ligand atoms and the active site atoms. The free energy diagrams of the reaction for the YbN₂C₂-pen electrocatalysts in an acidic environment are shown in Fig. 5c. The YbN₂C₂-pen catalysts showed better ORR catalytic activity on path 2 with overpotentials of 0.53 V and 0.65 V. The other configurations showed poorer ORR and OER activities. As shown in Fig. 5d, the horizontal coordinates indicated ΔG_OH* and the vertical coordinates indicated OOH*, 2OH* and O*. Due to the complexity of the YbN₂C₂-oppo-gra structure, it is not possible to obtain stable O* intermediates, so we do not consider their data points for linear fitting. The results showed a strong linear relationship. Zhao et al.⁸⁶ fabricated cerium-based single-atom catalysts (Ce SACs) featuring tailored electronic spin configurations to optimize the neutral ORR performance. Computational analyses revealed that the edge-hosted CeN₄(OH)₂ configuration demonstrated optimal energy stabilization for the four elementary reaction steps at 0 V (vs. RHE), outperforming both pristine and defect-containing models (Fig. 5e). For the pristine system, the primary rate-limiting process involved OH⁻ desorption, exhibiting an energy barrier exceeding 1.0 eV. In contrast, the edge-modified catalyst shifted the rate-determining step to OOH* formation, achieving a significantly reduced barrier of 0.2 eV. The enhanced binding affinity of OH* to CeN₄(OH)₂ (−3.63 eV) effectively mitigated active-site occlusion by adsorbed intermediates, thereby facilitating improved ORR kinetics. Analysis of the Ce 4d and 4f orbitals’ PDOS demonstrated that the elevated spin configuration originated from electron transfer from the d_z² and d_x²−y² orbitals to the Ce 4f orbital (Fig. 5f). This electronic redistribution induced a shift in the valence configuration from 4d¹⁰4f¹ to 4d⁸4f³, thereby stabilizing the high-spin state. Wen et al.⁸⁷ prepared a rare earth-based single-atom catalyst (CeSA) supported on three-dimensional porous nitrogen-doped carbon (3DCeSA-N-WS) and used DFT calculations to study the adsorption capacity of WS and 3DCeSA-N-WS-1 for Li₂S₆, as shown in Fig. 5g. Since the O coordination on the surface of CeSA originates from the adsorption of O in air, and the desorption of O and adsorption of Li₂S₆ occur during the electrochemical process, the Ce–N₄ coordination was used to construct the adsorption model of CeSA for calculation. Post-optimization adsorption energy analyses revealed that the 3DCeSA-N-WS-1 system (−2.71 eV) exhibited a significantly higher binding affinity for Li₂S₆ compared to the WS framework (−1.87 eV), validating the superior immobilization capability of cerium single-atom sites (CeSAs) toward lithium polysulfides (LiPSs). Differential charge density analysis further demonstrated electron redistribution upon CeSA–Li₂S₆ interaction, as characterized by reduced electron density around sulfur atoms and charge accumulation near CeSA centers. These observations were aligned with XPS spectral data (Fig. 5h), corroborating the electronic modulation mechanism. Guo et al.⁸⁸ proposed that incorporating lanthanides (Ln) with sequential variation in electronic structures could systematically tune the Ru–O bond covalency; this was attributed to the shielding properties of 5s/5p orbitals. This covalent modulation directly influenced the Ru vacancy formation energy (ΔG_{Ru vacancy}), which rose from 2.58 eV in pristine RuO₂ to 3.49 eV (Ho–RuO_x), 3.78 eV (Er–RuO_x), and 3.44 eV (Tm–RuO_x) (Fig. 5i). At 1.23 V vs. RHE, distinct structural configurations between ErRuO_x and RuO₂ were identified (Fig. 5j), while theoretical models revealed a volcano-type correlation between Ru–O covalency strength and catalytic activity. Notably, ErRuO_x exhibited a 35.5-fold enhancement in stability over RuO₂, emerging as the optimal catalyst. Experimental validation demonstrated that an Er–RuO_x-based electrolyzer achieved a current density of 3 A cm⁻² at 1.837 V, alongside sustained operation for 100 h at 500 mA cm⁻² with minimal degradation of 37 μV h⁻¹.

Fig. 5 (a) Molecular dynamic simulations of YbN₂C₂-pen. (b) Partial density of states for YbN₂C₂-pen. (c) Free energy diagram for YbN₂C₂-gra. (d) Linear relationship between ΔG_OH* and OOH*, 2OH*, and O*. The corresponding contour plot of CoO. Reproduced from ref. 85 with permission from the American Chemical Society, copyright 2024. (e) Gibbs free-energy diagrams of the perfect, defective and edge models at U = 0 V. (f) PDOS of d_z² and d_x²−y² orbitals of the edge model. Reproduced from ref. 86 with permission from Wiley, copyright 2023. (g) Structural configurations and adsorption energy values for Li₂S₆ molecules adsorbed onto WS and 3DCeSAN-WS-1 substrates. (h) Charge density difference (CDD) visualization of the optimized Li₂S₆ adsorption configuration on the 3DCeSA-NWS-1 catalyst surface. Reproduced from ref. 87 with permission from Elsevier, copyright 2024. (i) Variation in Ru vacancy formation energy (ΔG) plotted against integrated −ICOHP values for lanthanide-doped RuO_x systems. (j) Potential-dependent reaction pathways for electrochemical processes occurring on ErRuO_x and RuO₂ catalysts. Reproduced from ref. 88 with permission from Elsevier, copyright 2024.

3 Preparation and evaluation of catalysts

In the study of rare earth metal-based oxygen electrocatalysts, preparation methods and performance evaluation are critical steps in achieving a transition from mechanistic analysis to practical application. Therefore, we focus on common synthesis strategies, including the water/solvothermal method, the electrochemical deposition method, the vapour deposition method, the template assisted method, the pyrolysis method, the sol–gel method, and the solid-state reaction method. Additionally, we systematically review methods for evaluating electrochemical performance, such as onset potential (E_onset), half-wave potential (E_1/2), overpotential, Tafel slope, and electrochemical impedance, emphasising the role of experimental design in determining catalyst activity.

3.1 Synthesis method

Electrocatalytic synthesis methods can effectively improve the activity and selectivity of catalysts by means of the rational design and construction of active sites, surface modification, and modulation of electron transfer. Common synthesis methods for ORR and OER electrocatalysts include the hydrothermal water/solvothermal method,⁸⁹ electrochemical deposition,⁹⁰ vapor deposition,⁹¹ templating,⁹² the pyrolysis⁹³ method, the sol–gel method, and the solid-state reaction method (Fig. 6).

Fig. 6 Schematic representation of established synthesis protocols for rare earth metal-based materials.^94–100 Reproduced from ref. 94 with permission from Elsevier, copyright 2024. Reproduced from ref. 95 with permission from Nature Publishing Group, copyright 2020. Reproduced from ref. 96 with permission from Elsevier, copyright 2024. Reproduced from ref. 97 with permission from Wiley, copyright 2024. Reproduced from ref. 98 with permission from Wiley, copyright 2019. Reproduced from ref. 99 with permission from Wiley, copyright 2025. Reproduced from ref. 100 with permission from Nature Publishing Group, copyright 2024.

3.1.1 Water/solvothermal method. The solvothermal method involves fully dissolving and chemically reacting the reactants in the solvent in a closed solvent system at high temperature and high pressure, to generate electrocatalysts with specific structures and properties by controlling the reaction conditions.^101–103 Chen et al.⁴⁶ proposed a sustainable method for producing H₂O through a two-step hydrothermal treatment to synthesize hierarchical hollow samarium phosphate (SmPO₄) nanospheres with open channels. This hollow compound featured intrinsic open channels, optimal metal-atom spacing, and outstanding structural and compositional stability, demonstrating excellent 2e⁻ ORR performance under both neutral and alkaline conditions. Unique NiFe-layered double hydroxide (LDH) nanosheet arrays doped with lanthanides (NiFeSm-LDH, NiFeCe-LDH, and NiFeLa-LDH) were developed as high-performance electrocatalysts for the OER through hydrothermal synthesis by Du et al.¹⁰⁴ It is noteworthy that NiFeSm-LDH demonstrated exceptional performance, registering the lowest overpotential value of 203 mV at a current density of 10 mA cm⁻² during the OER. Although the hydrothermal reaction can grow crystals with very few defects^105–107 and good orientation under low temperature and isobaric conditions, which are suitable for the preparation of high-quality electrocatalysts, the hydrothermal method has a more limited industrial application, and the output of catalysts is limited by equipment and the process, which makes it difficult to meet the demands for large-scale production.¹⁰⁸

3.1.2 Electrochemical deposition method. Electrochemical deposition¹⁰⁹ methods generally include constant potential deposition, constant current deposition, and pulsed electrodepositions. Heterostructures composed of Eu₂O₃ and NiCo were synthesized through electrodeposition by Tang et al.¹¹⁰ Their research confirmed that strong electronic coupling within the Eu₂O₃–NiCo heterostructure alters the electronic structure at the heterogeneous interface via local electronic reconstruction. This modification significantly enhanced the electrocatalytic properties of the Eu₂O₃–NiCo catalyst. Fang et al.¹¹¹ fabricated Pr-incorporated Co(OH)₂ catalysts through a straightforward electrochemical deposition technique, wherein the rare-earth praseodymium (Pr) was homogeneously introduced into hexagonal Co(OH)₂ lattices. This structural modification remarkably amplified the inherent electrocatalytic activity of the Co(OH)₂ matrix. The doping of Pr significantly increased the electrochemically active area of the catalyst, providing more active sites and smaller charge transfer resistance, which was conducive to the catalytic performance of HMFOR. Zhang et al.¹¹² synthesized rare-earth Ce-doped, carbon cloth-loaded Ni–Fe–Se/CC self-supported electrode catalytic materials through in situ electrodeposition. By doping with rare-earth elements, which possess a distinct structural configuration compared to Ni and Fe and offer abundant electronic energy levels, they effectively modulate the properties of Ni–Fe selenide. This modification significantly enhanced the OER catalytic activity of Ni–Fe selenide. Compared to hydrothermal synthesis, electrodeposition is a simple process, low cost and more suitable for large-scale preparation.

3.1.3 Vapor deposition method. Vapor-phase deposition¹¹³ constitutes a technological approach that employs physical and chemical phenomena taking place within the gaseous phase to deposit a film or coating, endowed with particular attributes, onto a solid substrate. Zhou et al.¹¹⁴ successfully synthesized a novel 2D RE luminescent material, EuOCl, with a thickness of 1.1 nm and an ultra-narrow linewidth of 1.2 meV at room temperature using a chemical vapor deposition (CVD) technique. This luminescent 2D EuOCl sheet paved the way for the application of 2D rare-earth materials in luminescent devices. Zhou et al.¹¹⁵ reported the epitaxial growth of a 2-inch monolayer of a six-membered medium-entropy (ME) alloy on c-plane sapphire using CVD. A combination of techniques confirmed that rare earth, tungsten, molybdenum, indium, sulfur, and selenium atoms were uniformly distributed throughout the ME alloy monolayer lattice. Notably, the ME alloy demonstrated excellent electrocatalytic performance for hydrogen production^116,117 when exposed to visible and near-infrared light.

3.1.4 Template-assisted method. The template-assisted method^118,119 uses the specific spatial restriction and orientation provided by the template to enable the target material to grow, assemble and undergo other processes in the specific location and space of the template, to obtain a material with a specific shape, size and structure. Calcite-type oxides¹²⁰ are regarded as highly promising candidates for OER and ORR catalysis, with the potential to serve as alternatives to noble metals. However, conventional synthesis methods for these oxides are often associated with high reaction temperatures and prolonged reaction times, which result in materials that are characterized by large particle sizes and limited specific surface areas. These factors significantly hinder their electrocatalytic performance. Salt-assisted methods enable a high degree of control over the structure and morphology of 2D materials by modulating the structure and shape of the salt. Huang et al.¹²¹ greatly increased the electrochemical surface area (ECSA) of LaCaMnO₃ by an electrochemically-induced Ca leaching strategy, which exhibited better electrocatalytic ORR performance in alkaline solution than that of commercial platinum on carbon (Pt/C) catalysts. The three-dimensionally ordered macroporous (3DOM) Ni-based rare-earth catalysts developed by Feng et al.¹²² using the poly (methyl methacrylate) (PMMA) hard template method exhibited a large specific surface area and an ordered physical structure. This approach effectively reduced the size of Ni particles and enhanced their dispersion within the loaded catalysts, which facilitated the adsorption and efficient conversion of CO₂, thereby aiding the low-temperature CO₂ methanation reaction. The electrocatalysts synthesised by the template method have a high specific surface area and good pore structure, which can significantly improve the catalytic efficiency. However, the template method usually involves a multi-step synthesis process, including the selection of the template, the loading of active components, and the removal of the template, which makes the process cumbersome and increases the difficulty of preparation; this needs to be optimised further in the future for a wider range of applications.

3.1.5 Pyrolysis method. Pyrolysis¹²³ can effectively improve the stability and performance of electrocatalysts.¹²⁴ Tian et al.⁸ reported the synthesis of the Fenton-activated rare earth metal La–N–C, which features efficient dual active sites for the ORR. This material was prepared via the pyrolysis of a mixed complex using 1,10-phenanthroline as the ligand, with LaCl₃ and MgCl₂ serving as activators. The resulting La–N–C material possessed a rich microporous structure, featuring atomically dispersed LaN₄O molecules as new active sites, and demonstrated an excellent ORR performance. Zhang et al.¹²⁵ developed pseudo-amorphous IrO_x catalysts (IrO_x−3Nd) featuring a locally ordered rutile structure and unique defect sites using a pyrolysis–leaching method that involved rare earth metals (such as Nd, La, Pr, Sm, and Eu). This approach resulted in an ultra-low overpotential of 206 mV at 10 mA cm⁻², exceptional long-term stability of 2200 h, and a high hydrogen production efficiency of 1.68 V at 1 A cm⁻² in PEM water electrolysis (PEMWE). Pyrolysis can effectively improve the performance of catalysts, but during the pyrolysis process, metal atoms tend to aggregate to form nanoparticles, which may reduce the activity and stability of the catalysts.

3.1.6 Sol–gel method. The sol–gel approach, a wet chemical technique, involves the preparation of solid materials by way of hydrolyzing metal alkoxides or inorganic salts to generate a sol, with subsequent steps of gelation, drying, and sintering. Cai et al.¹²⁶ fabricated a heterostructure composite nanofibre cathode, PBSC-CPO-ES, which consisted of PrBa_0.5Sr_0.5Co₂O_5+δ and Ce_0.8Pr_0.2O_1.9, via a one-pot sol–gel approach. The distinctive nanofiber microstructure of this composite cathode, together with its abundant heterojunction interfaces, exerted a vital function in boosting charge transfer, oxygen surface exchange, and bulk diffusion. Chen et al.¹²⁷ prepared nanoparticle-shaped RE-RuO₂ catalysts using the sol–gel method. By adjusting the electronic redistribution of the Ru centre through the asymmetric configuration of Ru–O–La, they improved the activity and stability of RuO₂ for the acidic OER. The strengths of the sol–gel technique encompass the high purity of products, accurate regulation of the chemical makeup, gentle reaction conditions that aid low-temperature synthesis, and the capacity to produce homogeneous composites, films, or nanoscale materials. Nevertheless, it has drawbacks including that shrinkage and cracking tend to occur during gel drying, costs are high because organic compounds serve as precursors, and certain processes give off toxic by-products.

3.1.7 Solid-state method. A solid-phase reaction is defined as a procedure where solid materials undergo direct chemical reactions under specific temperature, pressure, and other conditions. It depend exclusively on the migration and diffusion of atoms, ions, or molecules in the solid state, along with lattice rearrangement, to generate new solid products. Tang et al.¹²⁸ put forward a sacrificial template approach for fabricating mesoporous perovskite oxide nanosheets (MPONs) with a large specific surface area through a solid-state reaction. In detail, glycine and metal salts were blended in a crucible before being subjected to annealing in a muffle furnace that had been preheated to 450 °C. When LaFeO₃ MPONs doped with the rare earth element europium (Eu) served as OER catalysts, an overpotential of merely 267 mV enabled the attainment of a current density of 10 mA cm⁻². Both theoretical computations and experimental investigations demonstrated that the superior OER performance of A-LaFeO₃ MPONs stems from not only their favorable porous architecture but also the increased involvement of lattice oxygen brought about by rare earth doping. Nevertheless, it has obvious limitations. For instance, reactions generally demand high-temperature environments, which not only result in high energy consumption but also make it hard to precisely regulate the morphology and microstructure of the products. Thus, efforts should be directed toward integrating and innovating with other preparation techniques to further enhance the process performance of this method.

In conclusion, different synthesis methods each have their unique advantages and limitations. In the actual process, it is necessary to combine specific material requirements and preparation conditions to reasonably select or optimize the synthesis method in order to achieve the efficient preparation of the target electrocatalytic product (Table 1).

Table 1 Comparison of the advantages and disadvantages of different synthesis methods

Method	Advantages	Disadvantages
Water/solvothermal method	Crystal growth can be achieved at relatively low temperatures	Reaction conditions are demanding
	Easy to control morphology and size	Difficult to prepare on a large scale
Electrochemical deposition method	Simpler preparation process	Limited range of applicable materials
	Low cost	Film uniformity depends on deposition conditions
	Suitable for large-scale preparation
Vapour deposition method	High product purity and good crystal quality	Expensive equipment, high energy consumption
	Suitable for precise control of product layer thickness and structure	High requirements for uniformity of large-area deposition
Template assisted method	Highly controllable morphology, enabling the production of catalysts with a high specific surface area and excellent pore structure	The template removal process is complicated
		Templates are expensive and limit scalability
Pyrolysis method	Simple process	Easily leads to particle agglomeration
	Easy to prepare on a large scale	Difficult to precisely control the morphology and crystal structure
Sol–gel method	Can form a high specific surface area and porous structure	Complex precursor preparation
	Complex oxides can be obtained at lower temperatures	Gel drying and sintering processes are prone to cracking
Solid-state reaction method	Suitable for large-scale powder production	High energy consumption due to high-temperature reactions
	Low cost and wide range of raw materials	High difficulty in controlling morphology and structure

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3.2 Performance testing of electrocatalysts for the ORR and OER

3.2.1 Generic evaluation indicators. In order to evaluate the performance of electrocatalysts, materials need to be subjected to a series of electrochemical tests. The main testing methods for the ORR and OER include cyclic voltammetry (CV), linear scanning voltammetry (LSV), Tafel slope, ORR electron transfer number (n), OER electrochemical impedance, etc.^129–131 The LSV method is most commonly used in electrochemical testing for evaluating the performance indexes of electrocatalysts. The ORR polarization curve features three critical parameters: E_onset, E_1/2, and J_L. E_onset is situated within the dynamic control zone of the mass transfer process, while E_1/2 is governed by the hydrodynamic region, encompassing mass transfer, electrode surface reactions, and diffusion control. J_L represents the area where diffusion is the dominant controlling factor (Fig. 7a). The overpotential in the OER polarization curve is an important metric for evaluating the performance of OER catalysts.^132–134 For different catalysts, the smaller the overpotential at the same current density, or the higher the current density at the same potential, the better the performance (Fig. 7b). Secondly, the slope of the Tafel curve reflects its dynamic properties. The smaller the Tafel slopes of the ORR and OER, the faster the kinetic rate of the reaction and the better the reaction performance (Fig. 7c). These parameters depend on multiple variables including electrode material composition, surface morphology,¹³⁵ electrolyte chemistry, and thermal conditions. The kinetic behavior of the system can be quantitatively derived from the Tafel slope magnitude, which reflects the relationship between overpotential and reaction rate. Accelerated durability assessments for ORR catalysts were conducted through prolonged CV cycling. The electrochemical performance retention was evaluated by comparing LSV polarization profiles acquired before and after 10 000 CV cycles. In addition, the durability of the ORR and OER electrocatalysts can be measured by the electrochemical method of i–t testing (Fig. 7d) by applying constant potentials to the surface of the catalysts/current and observing the corresponding degree of current/potential decay after a long test period.

Fig. 7 The performance parameters of OER catalysts evaluated based on (a) typical linear scanning voltammetry curves for the ORR, (b) typical linear scanning voltammetry curves for the OER, (c) Tafel slope, (d) stability, (e) J_k, (f) number of transferred electrons, (g) hydrogen peroxide yield, (h) electrochemical impedance, and (i) turnover frequency.

3.2.2 Unique evaluation indicators. J_k serves as a quantitative descriptor for the ORR dynamics, reflecting the intrinsic electrocatalytic activity of the system (Fig. 7e). This parameter is derived through the following computational relationships:

Y = ax + b (1)

B = 0.62nF(D₀)^2/3ν^−1/6C₀ (2)

a = 1/B (3)

b = 1/J_k (4)

In ORR electrochemical testing, the number of electrons transferred, n, needs to be calculated in order to better study the reaction mechanism (Fig. 7f). There are usually two ways of determining this: one is to use the Koutecky–Levich (K–L) equation, and the other is to calculate the K–L equation by means of a ring current test (RRDE).

K–L equations:

B = 0.62nF(D₀)^2/3ν^−1/6C₀ (6)

RRDE test:

The production efficiency of H₂O₂ generally increases when the electron transfer count reaches 2; otherwise, the reaction proceeds through a 4e⁻ pathway. By integrating these two analytical approaches, researchers can more accurately quantify the electrons transferred and subsequently identify the dominant reaction mechanism, providing critical insights for advancing ORR studies (Fig. 7g). The OER electrochemical impedance spectroscopy (EIS) technique entails introducing a low-amplitude alternating current (AC) perturbation to the electrode and analyzing the system's steady-state response dynamics. A characteristic impedance spectrum typically presents as a semicircular feature in the high-frequency region, with the semicircle radius corresponding to the charge-transfer resistance (R_ct) occurring at the electrode–electrolyte interface. This parameter exhibits an inverse relationship with catalytic performance: reduced semicircle dimensions signify lower R_ct values, which are indicative of accelerated charge-transfer kinetics and consequently enhanced catalytic efficiency for the electrocatalyst. In addition, by fitting the impedance spectrum, the equivalent circuit can be obtained and the solution resistance R can be calculated for IR compensation (Fig. 7h). The catalytic activity of OER electrocatalysts can be characterized by the turnover frequency (TOF) (Fig. 7i), which is calculated using the current density (j) and active site density (N) according to eqn (9). However, various factors, such as electrical conductivity and catalyst loading, can influence the current density. Therefore, it is essential to maintain consistent potentials when making comparisons.

TOF_app = J/(nFC) (9)

4 Modification strategies for rare earth metal-based ORR/OER electrocatalysts

Strategies for the modification^136,137 of electrocatalysts are key to enhancing their performance. This paper mainly reviews four strategies, namely, doping, single-atoms, alloys and complexes, each of which has its unique advantages and application prospects, and the combination and optimization of these strategies will provide a broader space for the development of electrocatalysts (Fig. 8).

Fig. 8 Schematic overview of advanced strategies for enhancing the ORR and OER catalytic performance in electrocatalyst systems.^138–141 Reproduced from ref. 138 with permission from Nature Publishing Group, copyright 2022. Reproduced from ref. 139 with permission from Wiley, copyright 2025. Reproduced from ref. 140 with permission from Wiley, copyright 2024. Reproduced from ref. 141 with permission from Elsevier, copyright 2024.

4.1 Doping

Doping^142,143 is a strategy that can enable new properties in a wide range of materials, and the use of rare earth ions as active dopants in inorganic lattices has been widely investigated since the 18th century because of their unique optical, magnetic and electrical properties. Fu et al.¹⁴⁴ selected rare-earth Nd atoms with unique 4f orbitals as heteroatoms to construct atomic-level Nd-doped Co sites anchored on N-doped carbon (Nd/Co@NC) via a MOF derivatisation strategy as an efficient bifunctional oxygen electrocatalyst (Fig. 9a). The fabricated catalysts demonstrated a remarkable electrocatalytic activity, reaching a maximum half-wave potential value of 0.85 V during the ORR (Fig. 9b and c). Operando electrochemical Raman spectroscopy provided critical insights into dynamic surface processes by monitoring spectral changes associated with Co–OOH intermediate formation. DFT calculations revealed that the adsorption of oxygen-containing intermediates achieved thermodynamic equilibrium through intense orbital interactions between Co-3d and Nd-4f electrons residing below the Fermi level (Fig. 9d), thereby optimizing the catalytic cycle through synergistic electronic coupling effects. The catalytic effectiveness was further substantiated in solid-state rechargeable Zn–air batteries. Shu et al.¹⁴⁵ used a simple non-stoichiometric strategy to prepare the LaMnO_3−δ perovskite with abundant A-site cation defects. The LaMnO₃ perovskite oxide showed excellent oxygen electrocatalytic activity. Both experimental results and DFT calculations showed that the introduction of La defects could significantly regulate the electronic structure of L_0.7MO, and fundamentally improve the inherent adsorption capacity of L_0.7MO to LiO₂ and O₂, thus ultimately regulating the morphology and distribution of Li₂O₂. A study by Du et al.¹⁴⁶ systematically examined a series of epitaxially grown rare-earth nickelate (RNiO₃) thin films featuring controllably substituted covalent ligands at the A-site. Employing a three-electrode electrochemical configuration in O₂-saturated 0.1 M KOH aqueous solution, the research evaluated the interplay between the films’ structural characteristics, physicochemical properties, and their bifunctional ORR/OER catalytic activities, enhancing the fuel cell performance. Among the chalcogenide oxides, rare-earth nickelates (RNiO₃, abbreviated here as RNO, where R denotes a rare-earth lanthanide element) have attracted great interest in recent years because of their remarkable properties and potential applications in electronics, catalysis, and energy storage. Covalent substitutions of the R ions in RNO affect the rotations, tilings, and distortions of the NiO₆ octahedra, and thus their structural, physical, and chemical properties (Fig. 9e). In contrast to LaNiO₃ (LNO), which maintained metallic conductivity across all measured temperatures, other members of this nickelate family demonstrated temperature-dependent transitions from metallic to insulating states (MITs) upon heating. Reciprocal space mapping (RSM) analysis, conducted near the STO(103) reflection (Fig. 9f), revealed coherent epitaxial strain in all RNO films, evidenced by their identical in-plane lattice constants with the SrTiO₃ (STO) substrate. Notably, the observed non-monotonic evolution of out-of-plane lattice parameters (Fig. 9g) could not be attributed to structural relaxation mechanisms. Chao et al.¹⁴⁷ demonstrated through systematic investigation that cerium doping exerted a profound enhancement on the ORR durability and resistance to corrosion for electrocatalysts operating in aggressive alkaline environments, while simultaneously accelerating the electrocatalytic reaction kinetics, as evidenced in Fig. 9h. Advanced in situ Raman spectroscopic analysis coupled with synchrotron-based X-ray absorption fine structure measurements (Fig. 9i) revealed that cerium incorporation facilitated the precise modulation of the Co³⁺/Co²⁺ ratio within the Co₃O₄ lattice. This strategic valence state engineering, as illustrated in Fig. 9j, optimized the adsorption/desorption equilibrium of reaction intermediates at the Co_oh³⁺ active sites. The catalyst exhibited exceptional operational stability over 290 h, coupled with a substantial energy storage capacity reaching 876.3 Wh kg⁻¹.

Fig. 9 (a) Schematic of the fabrication of Nd/Co@NC nanocomposites. (b) SEM of Nd/Co@NC nanostructures. (c) E_onset and E_1/2 values. (d) Operando Raman spectroscopic analysis of Nd/Co@NC during the OER. Reproduced from ref. 144 with permission from Wiley, copyright 2022. (e) Potential-dependent evolution of oxygen-containing intermediates during OER catalysis for Nd/Co@NC. (f) Phase diagram for RNiO₃ mapping temperature against rare-earth ionic radius. Red data points mark metal-to-insulator transition temperatures during heating cycles for bulk RNiO₃. (g) RSM measurements near the (103) crystallographic plane for LNO, NNO, SNO, and GNO epitaxial films on STO substrates. The horizontal dashed line represents the lattice parameter of the STO substrate. Reproduced from ref. 146 with permission from Wiley, copyright 2018. (h) The ORR LSV curves of Ce–Co₃O₄ and control samples. (i) O K-edge and Co L-edge EELS spectra among different regions throughout cycled Ce–Co₃O₄. (j) Modified schematic diagrams of DOS for Co₃O₄ before and after Ce doping. Reproduced from ref. 147 with permission from Wiley, copyright 2023.

Fu et al.⁸⁰ used NiCo₂O₄ as a model, and the doping of Ce at the octahedral position induced the formation of Ce–O–M (M = Ni, Co) bridges, which triggered charge redistribution within NiCo₂O₄. The developed Ce–NiCo₂O₄ catalyst exhibited significant OER activity, a low overpotential, good electrochemical stability, and good utility in anion-exchange membrane aqueous electrolyzers. TEM analyses were carried out on Ce–NiCo₂O₄, and the acicular subunits of Ce–NiCo₂O₄ exhibited porous and rough surfaces (Fig. 10a), and with the replacement of surface Ni by Ce, due to symmetry breaking, three oxygens with CUS, O_A, O_B, and O_C, and two kinds of Co, α-Co and β-Co, were produced (Fig. 10b). As shown in Fig. 10c, the LSV curve of Ce–NiCo₂O₄ showed a low overpotential of 270 mV at 10 mA cm⁻², which was lower than that of NiCo₂O₄ (310 mV) and commercial RuO₂ (350 mV). In conclusion, the LOM pathway for spinel oxides during the OER process was optimized by the introduction of oxygen-friendly rare earth sites. Guo et al.⁸⁸ theoretically elucidated that the incorporation of lanthanide elements with progressively varying electronic structures enabled continuous modulation of Ru–O covalent interactions through the orbital shielding effects contributed by 5s/5p electrons. Computational modeling substantiated that the stability of Ln-doped ruthenium oxides followed a volcanic relationship with respect to Ru–O covalency. During OER cycling, the dissolution of Ln species prompted the in situ generation of defective architectures characterized by Ln vacancy sites. Notably, Er-doped RuO_x exhibited the most significant vacancy formation energy (ΔG_{O vacancy} = 0.33 eV), surpassing RuO₂ by 0.29 eV, Ho–RuO_x by 0.02 eV, and Tm–RuO_x by 0.12 eV under vacancy-rich conditions (Fig. 10d and e). In comparison with the benchmark commercial RuO₂ catalyst (105 mV dec⁻¹), the engineered Er–RuO_x material exhibited a significantly decreased Tafel slope of 45 mV dec⁻¹, as visualized in Fig. 10f. This reduction demonstrates accelerated reaction kinetics during the oxygen evolution process. The polarization curves measured without iR-compensation (Fig. 10g) further reveal that the Er–RuO_x-integrated PEMWE outperforms the conventional RuO₂||Pt/C configuration. To elucidate the theoretical basis of the catalytic performance, a three-dimensional volcanic plot was constructed (Fig. 10h), systematically correlating the overpotential requirements with the Gibbs free energy differences associated with the adsorption of O* and OH* intermediates. Furthermore, this device exhibited exceptional operational durability when sustaining 500 mA cm⁻² for 100 h, with a minimal voltage decay rate of 37 μV h⁻¹ observed during extended testing. The development of non-precious metal electrocatalysts with superior activity and stability under acidic OER conditions represented a critical requirement for advancing hydrogen production via PEM water electrolysis. Li et al.¹⁴⁸ proposed a strategy to synergistically enhance the catalytic activity by erbium doping, thus enhancing the intrinsic OER activity and stability of Co₃O₄. EIS of Er–Co₃O₄ were recorded at 1.333 V (set voltage) and 100 kHz to 0.01 Hz, and the minimum R_ct of Er–Co₃O₄ was 11.3 Ω. The fastest electron transfer efficiency correlated with the catalyst's peak performance metrics (Fig. 10i). Analysis of the Pourbaix diagram was implemented to systematically assess the surface electrochemical states under various pH conditions and applied potentials. Notably, the oxygen intermediate (O) coverage on the 11/12 ML pre-oxidized Co₃O₄(311) surface was observed to be significantly lower than that on pristine Co₃O₄ during OER catalysis (Fig. 10j). To further elucidate the reaction mechanism, a state-of-the-art microkinetic volcano model was developed by mapping the oxygen evolution pathway against the Gibbs free energy difference. The OER activity of the erbium-doped Co₃O₄(311) catalyst was computationally evaluated using a volcano plot analysis at a current density of 10 mA cm⁻² (Fig. 10k). This theoretical assessment revealed that the strategic incorporation of erbium into the Co₃O₄(311) lattice substantially enhanced the surface electrocatalytic performance toward oxygen evolution. Two-dimensional (2D) sheet-shaped metal–organic frameworks (MOFs) found extensive use in electrocatalysis, thanks to their adjustable structures, abundant porosity, and large surface area. A boundary-engineered heterojunction, as reported by Wang and colleagues,¹⁴⁹ was formed through a controllable electrodeposition process—driven by the boundary effect—whereby Ce–Ni(OH)₂ was loaded onto the edges of Co-MOF nanosheets. In 1 M KOH, Co–MOF/Ce–Ni(OH)₂@CC displayed excellent OER activity, with the observation that it required an extremely low overpotential of merely 140 mV to achieve a current density of 10 mA cm⁻². Li and colleagues¹⁵⁰ put forward a strategy that combined morphology and electronic structure engineering to fabricate Co-MOF nanostructures with a ship-like morphology (denoted as CoCe-MOF/CP) grown on rare earth-doped carbon paper. This material demonstrated superior OER performance, requiring an overpotential of 267 mV at a current density of 10 mA cm⁻², and also displayed remarkable long-term stability exceeding 100 h.

Fig. 10 (a) AC-HAADF-STEM image of Ce–NiCo₂O₄. (b) Schematic model of the Ce–NiCo₂O₄ slab. (c) LSV curves of catalysts in 1 M KOH. Inset shows overpotentials at different current densities. Reproduced from ref. 80 with permission from Wiley, copyright 2024. (d) ΔG_{Ru vacancy} plotted against integrated −ICOHP values for Ln–RuO_x. (e) Volcano-type activity plot correlating catalytic performance of various electrocatalysts with their structural descriptors. (f) Comparison of Tafel slopes of home-made RuO₂, Er-RuO_x and Com. RuO₂. (g) Polarization characteristics of PEMWE based on the Er–RuO_x catalyst and commercial RuO₂ benchmark, measured at 80 °C. (h) Computed projected density of states (PDOS) for Ru d, O p, and Er f in Er-doped RuO_x (Er–RuO_x) system. Reproduced from ref. 88 with permission from Nature Publishing Group, copyright 2024. (i) EIS Nyquist plots of 4% Er–Co₃O₄, ErCoO₃, and Co₃O₄ at 1.333 V, the inset shows the equivalent circuit. (j) Phase diagram illustrating computational Pourbaix analysis for two-dimensional surface oxidation states. (k) Kinetic OER activity volcano plot at 10 mA cm⁻² correlated with Gibbs free energy differences (G_O − G_HO). Reproduced from ref. 148 with permission from the American Chemical Society, copyright 2024.

Therefore, doping can effectively modulate the electronic structure and physicochemical properties of catalysts and optimize the exposure of active sites, thus significantly improving catalytic activity and selectivity. However, the specific mechanism of the dopant's action in the reaction and its stability issues in real environments, such as the anti-dissolution ability in seawater electrolysis, still need to be further explored and improved.

Xing et al.¹⁵¹ took Gd as an example and introduced RE metal-bonded subsurface oxygen atoms (RE-O dipoles) into Pt-based ORR catalysts to address the stability issue of Pt. They investigated the ORR catalytic performance of Gd–OPt₃Ni in a three-electrode setup using a rotating disk electrode (RDE) technique in 0.1 M HClO₄ electrolyte. Compared with commercial Pt/C, the peak potentials related to surface oxidation and oxide reduction of Gd–O–Pt₃Ni shifted positively, indicating weakened Pt–OH_ads interactions due to the introduction of Gd–O dipoles (Fig. 11a). Following ADTs, the mass activity (MA) of the Gd–O–Pt₃Ni catalyst exhibited a sustained high performance at 1.04 A mg_Pt⁻¹. This value notably represented a 2.4-fold enhancement over the projected 2025 benchmark of 0.44 A mg_Pt⁻¹ and demonstrated superior initial activity (Fig. 11b). This work addressed the critical roadblocks to the widespread adoption of PEMFCs. Li et al.¹⁵² developed a high-performance non-precious metal OER catalyst for PEMWE, featuring single-atom Co sites anchored on amorphous Mo–Ce oxide supports and encapsulated within bamboo-like carbon nanotubes (denoted as CoSA-MoCeO_x@BCT). The material, characterized by abundant defects in the Mo–Ce oxide matrix, enables efficient in situ transformation of Co²⁺ species into low-coordination Co³⁺–O active sites under low-pressure conditions. The electrochemical characterization revealed that CoSA-MoCeO_x@BCT exhibited a remarkably low overpotential of 239 mV at a current loading of 10 mA cm⁻² (Fig. 11c). DFT further elucidated the catalytic mechanism, where analysis of the material's electronic structure (Fig. 11d) showed enhanced bonding orbital interactions at the Co–MoCeO_x interface. This covalent orbital reinforcement facilitates efficient charge transfer during intermediate adsorption processes. The energy paths of the acidic OER on MoO₃, MoCeO_x and CoSAMoCeO_x were calculated by the bimetallic site mechanism of LOM (Fig. 11e). DFT (Fig. 11f) revealed that the vacancy formation energy for Mo atoms in the CoSAMoCeO_x structure (8.72 eV) exceeded those of both MoO₃ (4.70 eV) and MoCeO_x (6.34 eV) by significant margins. Such structural reinforcement not only preserves the catalytic surface integrity but also contributes to the exceptional electrochemical stability observed in the CoSA-MoCeO_x@BCT electrocatalyst. Du et al.¹⁵³ demonstrated a general synthesis scheme for rare earth substituted LaCoO₃ (RE-LCO) perovskites (Fig. 11g). The introduction of cerium at the A-site narrowed the unit cell from 111.5 ų (LCO) to 111.4 ų (0.2Ce-LCO) (Fig. 11h). In particular, after loading RuO₂, the prepared RuO₂:0.2Ce-LCO hybrid structure exhibited an OER performance with a low overpotential of 135 mV at 10 mA cm⁻² under 1.0 M KOH conditions (Fig. 11i), and it also showed excellent long-term stability. Therefore, RuO₂-loaded Ce-LCO has higher OER activity, highlighting the positive role of rare earth addition in enhancing the OER performance of RuO₂-based catalysts. As shown in Fig. 11j, the RuO₂:0.2Ce-LCO catalyst demonstrated outstanding long-term stability.

Fig. 11 (a) Electrochemical characterization via cyclic voltammetry for the Gd–O–Pt₃Ni catalyst and commercial Pt/C benchmark under N₂-saturated conditions. (b) Comparative analysis of catalytic stability retention and MA post-accelerated stress testing (AST) cycles between Gd–O–Pt₃Ni and state-of-the-art electrocatalysts reported in the literature. Reproduced from ref. 151 with permission from Wiley, copyright 2023. (c) Polarization behavior comparison among BCT, MoCeO_x, CoSA-MoCeO_x, and CoSA. (d) Theoretical models and simulated electron density distributions for the CoSA-MoCeO_x catalyst architecture. (e) Schematic of the proposed OER mechanism illustrating dual-metal-site synergy in LOM catalysis. (f) DFT-calculated molybdenum vacancy formation energies within the MoO₃ crystalline framework. Reproduced from ref. 152 with permission from the Royal Society of Chemistry, copyright 2024. (g) Schematic of the synthesis methodology for Ce-LCO nanomaterial fabrication. (h) Atomic crystal structures of 0.2Ce-LCO. (i) OER polarization curves of LCO and Ce-LCO. (j) Chronoamperometry curves (i–t) of RuO₂:0.2Ce-LCO. Reproduced from ref. 153 with permission from Wiley, copyright 2024.

4.2 Alloys

Alloying the active metal by adding a second metal can significantly change the electronic and geometrical states of the active metal, and in addition, the alloyed catalysts^154,155 are more adaptable to the reaction conditions and are able to maintain a stable catalytic performance over a wider range of temperatures and pressures. A recent investigation by Pak et al.¹⁵⁶ demonstrated the synthesis of ternary alloy nanocatalysts through polyol-mediated incorporation of rare-earth elements (Y, Sc, La) into Pd–Ir matrices. Among various compositions, the Pd₄IrY_0.1/C catalyst achieved remarkable performance metrics, including an optimized ECSA of 26.46 m² g⁻¹ Pd and a half-wave potential of 0.626 V under ORR conditions. Consequently, to enhance the ORR activity of palladium-based alloy catalysts, alloying strategies aimed at enhancing the metallic palladium content should be developed through precise adjustment of the rare earth element concentrations. The catalyst functioned well in low-temperature fuel cells. The significant dependence on costly platinum-group metals (PGMs) as cathode electrocatalysts poses a critical barrier to the scalable implementation and economic viability of PEMFC technology. Pt₅La intermetallic compounds synthesized (Fig. 12a) by a new and facile method were designed by Xing et al.¹⁵⁷ Electrochemical characterization (Fig. 12b and c) revealed that the Pt₅La alloy catalyst, benefiting from intense electronic coupling between platinum and lanthanum, demonstrated superior catalytic performance. Specifically, the material achieved a half-wave potential of 0.92 V and delivered a mass activity of 0.49 A mg_Pt⁻¹, with reaction kinetics adhering strictly to the 4e⁻ transfer mechanism. La was successfully introduced into the Pt lattice by an atomically dispersed precursor-assisted synthesis method. Strong charge transfer between La and Pt atoms in the alloy nanocrystals significantly altered the electronic structure of Pt, which in turn led to a decrease in the adsorption strength of the adsorbed species. Chen et al.¹⁵⁸ tuned the electronic structure of platinum by introducing rare-earth atoms, and the d-band centre of platinum–rare-earth alloys was altered compared to platinum (Fig. 12d). In addition, the energy barrier for Pt demetallisation was enhanced by introducing rare earth elements into the Pt lattice (Fig. 12e). The synthesis strategy for the catalyst, which contributes to OH* adsorption behaviour and accelerates ORR kinetics (Fig. 12g), is shown in Fig. 12f. The ORR durability was significantly improved. The optimized Pt₃Y catalyst demonstrated remarkable catalytic performance enhancements across multiple operational parameters. Challenges associated with the ORR have traditionally restricted the operational efficiency of low-temperature fuel cells.^159–161 Addressing this critical limitation, Chorkendorff et al.¹⁶² conducted comprehensive studies on Pt_xY model nanocatalysts synthesized via gas-phase aggregation methodologies (Fig. 12h and i). Specifically, the optimized catalysts achieved a remarkable mass activity of 3.05 A mg_Pt⁻¹ at 0.9 V, demonstrating superior ORR performance metrics that surpassed conventional benchmarks.

Fig. 12 (a) Quantitative determination of nanoparticle Pt-shell dimensions was achieved by analyzing ten cross-sectional line profiles per particle. Reproduced from ref. 157 with permission from Wiley, copyright 2023. (b) LSV responses for Pt₅La alloy and Pt/C-JM catalysts in oxygen-saturated 0.1 M HClO₄ electrolyte, recorded at a scan rate of 20 mV s⁻¹. (c) Comparative electrochemical performance analysis of catalysts, including half-wave potentials and mass-normalized activities at 0.9 V. Reproduced from ref. 158 with permission from Tsinghua University Press, copyright 2022. (d) Charge density redistribution maps for the Pt₃Y alloy system (yttrium: orange spheres; platinum: blue spheres) with differential electron accumulation (yellow, 0.01 e Å⁻³) and depletion (green, −0.01 e Å⁻³) regions visualized. (e) Schematic representation of the hypothesized ORR catalytic pathway for metallic alloy systems. (f) Computed Gibbs free energy profiles under standard electrochemical conditions (U = 1.23 V) for catalytic reactions. (g) HAADF-STEM images. Reproduced from ref. 150 with permission from Elsevier, copyright 2023. (h) Spatial distribution of EDS intensity profiles mapped over the hyperspectral data cube along the manually defined purple linear paths. Reproduced from ref. 162 with permission from Springer Nature, copyright 2024. (i) EDS intensity line profiles extracted from the spectrum image data cube along the purple lines drawn on h.

Jiang et al.¹⁶³ rapidly synthesised Y alloy nanocatalysts, which exhibited excellent OER activity in aqueous oxidative environments, by a heat shock method. By incorporating dispersed Y into the Ir metal lattice, the catalyst exhibited an overpotential of 255 mV at a current density of 10 mA cm⁻² and showed excellent catalytic stability in acidic electrolytes for more than 500 h at a high current density of 100 mA cm⁻². TEM analysis showed that the prepared IrY nanoparticles were uniformly dispersed on carbon paper (Fig. 13a), and Fig. 13b provides insights into the electrocatalytic performance through the Tafel slope, which is very narrow for this alloy, at 54.8 mV dec⁻¹, suggesting that the Herovsky step is the rate-determining step in the OER process. The charge transfer resistance was the lowest, compared to Ir and commercial IrO₂, indicating enhanced OER kinetics (Fig. 13c). These results highlight the superior efficiency and kinetics of rare earth alloys in the OER. The catalyst also showed great potential for application in PEM water electrolysis. Gao et al.¹⁶⁴ proposed a lanthanide microalloying strategy (Fig. 13d) to prepare LaRuIr nanocrystals with strain-wave characteristics of oxide skins by rapid crystalline nucleation using heat-assisted sodium borohydride reduction in aqueous solution at 60 °C. As the density increases, the LaRuIr catalyst demonstrated a lower cell voltage. Compared to the RuIr and IrO₂ catalysts tested, this performance is evident in Fig. 13f. The atomic stacking model was further investigated, and the inverse fast Fourier transform (IFFT) results are presented in Fig. 13e. Figure 13g is a model of LaRuIr catalyst. The material exhibited an ABAB-type stacking sequence, suggesting the presence of a hexagonal close-packed structure. Under tensile strain conditions, the strong oxygen affinity of La in the La–Ir/Ru bond facilitated a stable surface dynamic reconstruction via lattice oxygen vacancy repopulation, thereby ensuring ultra-high long-term stability during the acidic OER. This work contributed to the development of advanced catalysts with unique strains for application in PEMWE. Zhu et al.¹⁶⁵ developed a multimetallic FeCoNiMnRuLa/CNT catalyst system demonstrating improved electrocatalytic activity toward water splitting through lattice engineering. By incorporating La into the FeCoNiMnRu matrix, the electronic configurations were modulated by reducing the concentration of unpaired d-electrons in the transition metal complex. This structural modification facilitated more efficient spin state transitions in oxygen-containing reaction intermediates, specifically promoting singlet-to-triplet oxygen conversion pathways. Electrochemical characterization via LSV for the OER revealed that the catalyst's enhanced performance—particularly pronounced under elevated current density conditions (Fig. 13h)—could be primarily attributed to the synergistic effect introduced by La doping. Furthermore, the La-induced lattice distortion facilitated d–d orbital electron transfer, resulting in a spin state transition of Fe–CoNi–Mn ruthenium from high spin (HS) to medium spin (MS). This spin state modulation enabled optimal d–p orbital hybridization in the MS configuration, thereby enhancing the OER activity (Fig. 13i and j).

Fig. 13 (a) TEM image of IrY and the corresponding size distribution histogram (inset) (scale bar: 200 nm). (b) Tafel slopes of IrY, Ir and Com-IrO₂. (c) Nyquist plots of IrY, Ir, and Com-IrO₂. Reproduced from ref. 163 with permission from Wiley, copyright 2024. (d) Schematic of the synthesis strategy for La, Ce, or Pr microalloying RuIr nanocrystals fabricated by using thermally assisted sodium borohydride reduction at 60 °C. (e) The IFFT images of the marked area. (f) Cell voltage–current curves in 0.5 M H₂SO₄. (g) Model of the thin (RuIr)O₂ oxide structure. (g) Electrochemical activity of FeCoNiMnRu/CNT, FeCoNiMnRu/CNT, CNT, and RuO₂ catalysts evaluated via LSV in 1 M KOH electrolyte during OER testing. Reproduced from ref. 164 with permission from Wiley, copyright 2024. (h) LSV curves of FeCoNiMnRu/CNT and control samples in 1 M KOH for the OER. (i) Dynamic visualization of spin-exchange interactions between parallel-spin electrons presented in a four-dimensional spatiotemporal framework. (j) Distinct spin-state configurations exhibited by four-electron systems during triplet oxygen formation processes. Reproduced from ref. 165 with permission from Elsevier, copyright 2025.

In the future, the alloy method will show greater potential and application prospects in electrocatalyst modification strategies. The design of alloy catalysts will be more refined, and the catalytic activity, selectivity and stability of alloys will be further enhanced by regulating their components, structures and interfacial properties. The alloy method will provide an important way to reduce the cost of electrocatalysts¹⁶⁶ and achieve large-scale industrial applications.

4.3 Compounds

The compound oxygen electrocatalyst^167,168 is a catalyst composed of a metal oxide,¹⁶⁹ a perovskite,^170–172 a spinel^173,174 and other compound materials. Qian et al.¹⁷⁵ reported a rare-earth metal–oxide engineering strategy by incorporating a heterojunction in N,O-doped carbon nanospheres (Fe₃O₄/La₂O₃@N,O-CNSs) to form Fe₃O₄/La₂O₃ heterostructures for efficient oxygen reduction electrocatalysis. Fig. 14a shows that charge is aggregated on the La₂O₃ side (yellow region) and depleted on the Fe₃O₄ side (green region); this represents the spontaneous migration of electrons from Fe₃O₄ to the La₂O₃ side. According to the above theoretical results, Fe₃O₄/La₂O₃@N,O-CNSs showed weaker adsorption of O₂ molecules and easier desorption of oxygen compounds in a series of subsequent reaction steps, highlighting the fact that La₂O₃ efficiently modulated the chemisorption behavior of the Fe active sites (Fig. 14b) and enhanced the ORR activity (Fig. 14c). The integrated catalyst system developed demonstrated exceptional performance metrics under realistic anion exchange membrane fuel cell (AEMFC) operation, achieving a power density of 148.7 mW cm⁻². However, the long-term stability of the composite materials may be affected by the interactions between the components, which are prone to structural decomposition or performance degradation. In addition, the synergistic effect between different materials is difficult to fully predict and optimize, which also puts higher requirements on the design of composite catalysts. Liu et al.¹⁷⁶ synthesized CeO₂-decorated MnOOH nanowire hybrids (CeO₂@MnOOH NWs) through a straightforward solvothermal approach (Fig. 14d). The integration of CeO₂ nanoparticles into the MnOOH matrix (Fig. 14e) induced strong electronic coupling between the three-dimensional electronic configurations of MnOOH and the 4f orbital states of CeO₂ at the Fermi level (E_F). Combined experimental characterization and DFT calculations revealed a subtle negative shift in the d-band center of Mn active sites (Fig. 14f). This electronic modulation resulted in accelerated charge carrier dynamics and enhanced electrical conductivity. Huang et al.¹⁷⁷ demonstrated the fabrication of a ternary electrocatalyst comprising cobalt phosphide (Co₂P) nanoparticles, nitrogen–phosphorus co-doped carbon (NPC), and CeO₂ nanosheets (designated Co₂P–NPC–CeO₂ hybrid). The optimized catalyst configuration exhibited superior ORR activity coupled with exceptional stability and robust resistance to methanol crossover effects. Jia et al.¹⁷⁸ synthesised CeO₂-modified N-doped C-based materials by a salt template method and a high temperature calcination method and optimized the synthesis conditions. The specific surface area of the materials was enhanced by modifying the pore structure using MOFs and sodium chloride (NaCl) templates. Additionally, the incorporation of Ce significantly improved the electrical conductivity of the catalysts, which exhibited comparable activity and superior stability compared to commercial Pt/C catalysts. Cai et al.¹⁷⁹ impregnated Pr(NO₃)₃ solution on the surface of PSCFN_0.1 by a spin-coating technique to form a PrO₂/PSCFN_0.1 heterostructure (Fig. 14g), which significantly improved the ORR kinetics. The polarization resistance of PrO₂–PSCFN_0.1 was only 0.016 Ω cm² at 800 °C, which was 56.8% lower than that of PSCF, and had good ORR kinetics (Fig. 14h and i). Wang et al.¹⁸⁰ fabricated mixed heterostructures consisting of lanthanum oxide and hydroxide (denoted as Co–LaMOH) through the spontaneous pyrolysis of cobalt-doped La-MOF-NH₂. Its hollow architecture and ample oxygen vacancies functioned synergistically to create a greater number of active sites, which in turn boosted electrical conductivity and enhanced the overall performance. The resulting 3Co–LaMOH/O_V@NC material, acting as a bifunctional oxygen electrocatalyst, displayed remarkable activity in the ORR. Additionally, the ZAB exhibited favorable stability over a 100 h period. Sun et al.¹⁸¹ fabricated a collection of carbon-supported rare earth oxides (RE_xO_y: Gd₂O₃, Sm₂O₃, Eu₂O₃, and CeO₂) combined with Pd nanoparticles, using rare earth MOFs as mediators. With Pd–Gd₂O₃/C serving as a typical example, the Pd–O–Gd bridging structure, formed through the creation of a Pd–Gd₂O₃ heterogeneous interface, induced charge redistribution. This process allowed for the modulation of the electronic configuration at Pd active sites. The resulting Pd–Gd₂O₃/C catalyst demonstrated a favorable ORR performance, featuring a high half-wave potential (0.877 V_RHE) and outstanding stability.

Fig. 14 (a) The Fe₃O₄/La₂O₃ interface characterized by differential charge density mapping and Bader charge distribution analysis, with electron-rich regions visualized in yellow and electron-deficient zones in green. (b) Morphological and compositional contrast between the synthesized Fe₃O₄/La₂O₃@N,O-CNS and Fe₃O₄@N,O-CNS nanostructures. (c) Electrochemical performance comparison via LSV between Fe₃O₄/La₂O₃@N,O-CNS control samples. Reproduced from ref. 175 with permission from Wiley, copyright 2023. (d) Schematic illustration of the step-by-step synthesis protocol for CeO₂@MnOOH nanocomposite fabrication. (e) HRTEM micrographs and corresponding SAED patterns of CeO₂@MnOOH. (f) Calculated PDOS for Mn-3d orbitals in the CeO₂@MnOOH heterojunction. Reproduced from ref. 176 with permission from Elsevier, copyright 2024. (g) SEM micrograph depicting the surface morphology of the PSCFN_0.1 sample. (h) EIS spectra recorded for PrO₂-modified PSCFN_0.1 composite. (i) Temperature-dependent polarization resistance (R_p) values for PrO₂–PSCFN_0.1 and pristine PSCFN_0.1 measured between 600 and 800 °C. Reproduced from ref. 179 with permission from Elsevier, copyright 2024.

The OER is one of the key steps in electrochemical water decomposition.^182,183 Layered CeO₂/NiCo hydroxides for the electrocatalytic OER were prepared by Yan et al.¹⁸⁴ As shown in Fig. 15a, XRD analysis revealed that the characteristic diffraction peaks of ZIF-67 exhibited a significant reduction, while those corresponding to CeO₂ became increasingly prominent. This observation suggested that the mechanism of the solvothermal reaction involved Ce and HMT, as schematically illustrated in Fig. 15b. Specifically, prior to the temperature reaching the critical point required for Ce ion hydrolysis, the coordination interaction between HMT and Ce ions not only facilitated the surface enrichment of Ce ions on ZIF-67 but also effectively retarded the subsequent hydrolysis process of Ce ions to a certain extent. As shown in Fig. 15c, DS-CeO₂/NiCo LDH required η values of around 282 and 373 mV to achieve current densities of 10 and 100 mA cm⁻², respectively; these are superior to other control samples. In addition, DS-CeO₂/NiCo LDH showed excellent stability and maintained the microcube morphology under the protection of the CeO₂ shell (Fig. 15d). In summary, the introduction of CeO₂ and its double-shell product enhanced the intrinsic activity of DSCeO₂/NiCo LDH and accelerated the electrocatalytic kinetics, thus improving the activity and stability of DSCeO₂/NiCo LDH. Tang et al.¹⁸⁵ proposed a new strategy to improve the OER performance by modifying the catalyst with CeO₂ while designing the non-homogeneous phase interface and tuning the phase composition. Experimental findings demonstrated that introducing rare earth oxides made it possible to regulate the phase transition from nitrides to metals. When Ni₃FeN/Ni₃Fe was modified with CeO₂, the resulting Ni₃FeN/Ni₃Fe/CeO₂-5% catalyst displayed superior activity. As shown in Fig. 15e and f, the overpotential of Ni₃FeN/Ni₃Fe/CeO₂-5% NPs was as low as 249 mV at a current density of 10 mA cm⁻². The excellent performance suggested its potential application in the OER. The in situ Raman spectra of Ni₃FeN/Ni₃Fe/CeO₂-5% NPs shown in Fig. 15g and h indicate that the characteristic NiOOH peaks start to appear at an applied potential of only 500 mV. This phenomenon suggested that the active NiOOH phase was more easily formed under the synergistic effect of CeO₂. This finding further explained the excellent OER activity of Ni₃FeN/Ni₃Fe/CeO₂-5% NPs. Yan et al.¹⁸⁶ presented a rare earth modification strategy to enhance AWE anode catalysis. Their approach leveraged the oxygenophilic properties of rare earths to increase OH coverage on the NiS₂ surface (Fig. 15i). This modification promoted the formation of active sites for efficient catalytic reactions. Density functional theory calculations were conducted to validate this mechanism, demonstrating that rare earths could effectively increase surface OH coverage (Fig. 15j). This modification strategy offered a novel pathway to overcome the limited surface hydroxyl (OH⁻) coverage challenges inherent in AWE anode systems.

Fig. 15 (a) The Co/Ce atomic ratio extracted from EDS (top) and XRD intensity of the strongest peak around 5 degree (bottom) with the increasing amount of Ce (green) and Hexamethylenetetramine (HMT, brown). (c) Comparison of LSV and CV between DS-CeO₂/NiCo LDH and Comparative Samples. (d) Potential–time curves and FESEM of DS-CeO₂/NiCo LDH at different durations (insert, all scale bars are 500 nm. Reproduced from ref. 184 with permission from Wiley, copyright 2024. (e) LSV curves of NiFe-LDH/CeO2–5% and comparative samples. (f) In situ Raman spectra of Ni₃FeN/Ni₃Fe/CeO₂-5% NPs at applied potential versus Ag/AgCl. (g) Spatial distribution diagrams illustrating the morphological features of Ni₃FeN/Ni₃Fe/CeO₂-5% nanoparticles. (h) Corresponding contour plots of Ni₃FeN/Ni₃Fe/CeO₂-5% NPs. Reproduced from ref. 185 with permission from Wiley, copyright 2024. (i) Potential-dependent variations in OH surface coverage and O₂ evolution rates during Ni-based electrocatalysis, as characterized by ATR-FTIR spectroscopy coupled with online mass spectrometry. (j) DFT-derived energy profiles for Eu₂O₃/NiS₂ and pristine NiS₂ under varying degrees of OH adsorption coverage. Reproduced from ref. 186 with permission from American Chemical Society, copyright 2024.

4.4 Single atoms

The monoatomic oxygen electrocatalyst^187,188 is a kind of highly efficient catalyst composed of single metal atoms dispersed on the support. Rare earth metal Ce-based SACs consisting of monoatomic Ce sites and CeO₂ nanoparticles (Fig. 16a) were constructed using a simple gas-phase migration strategy by Han et al.¹⁸⁹ Theoretical calculations showed that the synergistic effect of the introduction of CeO₂ and the change to the coordination structure of the monoatomic sites favoured lowering of the energetic barriers of the hydrogenation step of *OH on the monoatomic Ce sites, which led to the improved ORR performance with a positive half-wave potential of 0.88 V being achieved at 0.1 M KOH (Fig. 16b). The assembled CeNC-M-0.6 and Pt/C-based ZABs in Fig. 16c had excellent open-circuit potentials of 1.48 V and 1.51 V, respectively. The ZAB configuration fabricated demonstrated exceptional performance metrics, achieving a power density of 107 mW cm⁻² while maintaining robust operational durability over 400 cycles at 5 mA cm⁻². Du and colleagues¹⁹⁰ employed an axial coordination strategy and nanostructure design to construct La–Cl SAs/NHPC single atoms with good oxygen reduction reactivity (Fig. 16d). Different LaN₄Cl₂ motif structures and hierarchical porous carbon substrates promoted the maximum utilization of metal atoms, ensuring a high half-wave potential (0.91 V) (Fig. 16e) and remarkable performance in alkaline battery media. Fig. 16f shows a schematic diagram of a semi-solid flexible ZAB assembled with an organogel as the electrolyte based on La–Cl SAs/NHPC and commercial 20% Pt/C, both of which showed good battery performances. The main advantage of single-atom catalysts lies in the extremely high atom utilization, where each metal atom can act as an active centre, thus significantly improving the catalytic efficiency. The research group further demonstrated⁷⁹ the immobilization of Ce SAs on phosphorus-, sulfur-, and nitrogen-triply doped hollow carbon supports (Ce SAs/PSNC) via atomic-level dispersion techniques for enhanced ORR catalysis (Fig. 16g). Characterization revealed that the optimized Ce SAs/PSNC catalyst achieved a half-wave potential of 0.90 V during electrochemical evaluation (Fig. 16h). Performance benchmarking against state-of-the-art catalysts demonstrated superior catalytic efficiency (Fig. 16i), with the triply coordinated Ce SAs/PSNC system delivering a significantly elevated TOF of 52.2 s⁻¹ and MA of 36.1 A mg⁻¹ Ce, marking a 27-fold enhancement relative to its Ce SAs/NC counterpart (Fig. 16j). DFT calculations elucidated distinct electronic band structures near E_F, where Ce SAs/PSNC exhibited pronounced bonding/antibonding orbital interactions compared to Ce SAs/NC (Fig. 16k and l). The Ce SAs/PSNC-engineered ZABs demonstrated robust electrochemical characteristics, showcasing a consistently high open-circuit voltage of 1.49 V and achieving a peak power density measurement of 212 mW cm⁻².

Fig. 16 (a) Schematic diagram of the synthesis procedure for CeNC-M. (b) LSV curves. (c) Open circuit voltage measurements (inset: schematic illustration of zinc–air batteries). Reproduced from ref. 189 with permission from Wiley, copyright 2024. (d) Schematic of La SAs with different active centers and oxygen accessibility. (e) LSV curves at 1600 rpm in O₂-saturated 0.1 M KOH solution. (f) Flexible ZAB structural model diagram. Reproduced from ref. 190 with permission from Wiley, copyright 2024. (g) Schematic of the synthesis procedure for Ce SAs/PSNC. (h) Polarization curves in O₂-saturated 0.1 M KOH solution. (i) J_k and E_1/2 benchmarked against recently developed catalytic systems and Ce SAs/PSNC. (j) Quantitative comparison of mass-specific activity and TOF metrics between Ce SAs/PSNC and Ce SAs/NC catalysts. (k) Three-dimensional spatial mapping of electronic density distributions for the Ce SAs/PSNC architecture. (l) Corresponding electronic density contour analysis for the Ce SAs/NC reference material. Reproduced from ref. 79 with permission from Wiley, copyright 2023.

Fu et al.¹⁹¹ constructed a series of RE single atoms with modulated oxygen states on MnO₂ nanosheets via an effective and versatile Ar plasma (P)-assisted strategy (Fig. 17a). The engineered P-Gd SAs@MnO₂ material exhibited remarkable OER activity, achieving a low overpotential of 281 mV at 10 mA cm⁻² (η_j10) coupled with exceptional durability (Fig. 17b–d). Fu et al.⁸¹ fabricated atomically dispersed cerium species on cobalt oxide supports through an innovative plasma-enhanced synthesis protocol, yielding P-Ce SAs@CoO nanocomposites as depicted in Fig. 17e and f. Electrochemical evaluations demonstrated that this material achieved a notably low overpotential of 261 mV at 10 mA cm⁻² (Fig. 17g), while exhibiting superior operational durability compared to bare CoO. The enhanced performance originated from synergistic orbital interactions within the Ce(4f)–O(2p)–Co(3d) tripartite active centers, as revealed by theoretical calculations. Electrochemical water splitting powered by renewable electricity was a promising green pathway to realizing large-scale hydrogen production. Ciucci et al.¹⁹² demonstrated the synthesis of isolated iridium atoms with out-of-plane coordination to dimethylimidazole (MI) ligands anchored on CoFe-layered hydroxide nanostructures, denoted as Ir₁/(Co,Fe)–OH/MI (Fig. 17h). Electrochemical characterization via LSV revealed exceptional current densities exceeding 700 mA cm⁻² (Fig. 17i). Notably, the Ir₁/(Co,Fe)–OH/MI configuration exhibited superior catalytic efficiency (Fig. 17j), with overpotentials being significantly reduced compared to its counterparts: (Co,Fe)–OH/MI, Ir₁/(Co,Fe)–OH/MI, (Co,Fe)–OH, and the commercial IrO₂ benchmark at equivalent current densities. DFT calculations correlated this enhancement to electronic structure modulation, where the introduction of MI ligands induced a pronounced decrease in electron density at both Ir and Co active sites, as evidenced by differential charge density maps and Bader charge analysis (Fig. 17k). This electronic redistribution produced an upward shift in the d-band center positions of Ir and adjacent Co atoms, optimizing the binding energy of OER intermediates (Fig. 17l).

Fig. 17 (a) Schematic route to the synthesis of P-Gd SAs@MnO₂ nanosheets. (b) EPR spectra of P-Gd SAs@MnO₂ and control samples. (c) Polarization curves. (d) Comparative analysis of OER activity metrics for alternative P-RE SAs supported on MnO₂ substrates. Reproduced from ref. 191 with permission from Elsevier, copyright 2024. (e) Illustration of the synthesis protocol for preparing P-Ce SAs anchored on CoO supports. (f) Structural characterization of the CoO lattice: (f₁ and f₂) Moiré interference patterns derived from fast Fourier transform (FFT) analysis, (f₃) atomic configuration model for the CoO [110] crystallographic plane, (f₄) corresponding SAED pattern. (g) Electrocatalytic performance evaluation of P-Ce SAs@CoO and control catalysts by comparative LSV. Reproduced from ref. 81 with permission from Wiley, copyright 2023. (h) Three-dimensional visualization of atomic column overlap intensity distributions. (i) Comparative LSV. (j) Overpotential requirements at current density benchmarks: 10, 100, 300, and 600 mA cm⁻². (k) Charge density redistribution maps for catalytic surfaces. (l) Proposed 4e⁻ OER mechanism at cobalt active sites adjacent to iridium in Ir₁/(Co,Fe)–OH/metal interface (MI) architectures. Reproduced from ref. 192 with permission from Springer Nature, copyright 2024.

In addition, the four modification strategies for rare earth metal-based catalysts have provided a strong impetus for the development of the catalytic field, and a comparison of results from the latest research on ORR and OER is summarized in Tables 2 and 3.

Table 2 The influence of different modification strategies of rare earth metal-based catalysts on ORR performance

Design strategies	Electrocatalyst	Synthesis method	Eonset [V]	E1/2 [V]	JL [mA cm^−2]	Tafel slope [mV dec^−1]	Stability [h]	Ref.
Doping	Ce–Co3O4	Sol–gel method	—	0.69	4.83	64	290	147
	Nd/Co@NC	Pyrolysis method	1.03	0.85	—	68.2	24	144
Single atoms	CeNC-M-0.6	Pyrolysis method	0.98	0.88	—	55.7	—	189
	La–Cl SAs/NHPC	Vapour deposition method	0.81	0.91 V	6.75	50.4	230	190
Compounds	Fe3O4/La2O3@N,O-CNSs	Template assisted method	1.05	0.88	—	40	100	175
	2.5Co2P–NPC–CeO2	Complexing coprecipitation method	0.88	0.827	5.63	—	—	177
Alloys	Pt5La	Wet chemistry method	0.998	0.92	6	—	—	157
	Pt–RE (Pt15Y5Al80, Pt15La5Al80, Pt15Ce5Al80, and Pt15Sm5Al80)	Melt spinning method	—	0.89	—	—	—	158

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Table 3 The influence of different modification strategies of rare earth metal-based catalysts on OER performance

Design strategies	Electrocatalyst	Synthesis method	Overpotential [mV]	Tafel slope [mV dec^−1]	Stability [h]	Ref.
Doping	Er–Co3O4	Electrode modification/catalyst loading method	321	75.9	250	148
	Ce–NiCo2O4	Hydrothermal method	270	85	—	80
Single atoms	P-Gd SAs@MnO2	Hydrothermal method	281	161.9	—	191
	Ir1/(Co,Fe)–OH/MI	Wet chemistry method	179	24	120	192
Compounds	Ni3FeN/Ni3Fe/CeO2-5%	Co-precipitation method/pyrolysis method	249	73.1	30	185
	Eu2O3/NiS2	Electrochemical deposition method	260	—	240	186
Alloys	LaRuIr	Wet chemical method	184	49.73	60	164
	FeCoNiMnRuLa/CNT	Liquid phase mixing–Joule heating method	281	47.5	200	165

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Current research focuses on the exploration of modification strategies^193,194 at the experimental level, while the in-depth understanding of the catalyst modification^195,196 mechanism is still insufficient. Future research can explore more innovative modification strategies, strengthen the combination of theoretical calculations and experimental studies, and deepen our understand of the impact of modification strategies on catalyst performance through methods such as DFT.^197–199

5 Conclusions and outlook

5.1 Conclusions

To enhance the electrochemical performance of metal catalysts for oxygen reactions, rare earth metal compounds can serve as emerging functional materials that integrate the unique orbital structures and catalytic properties of rare earth elements into metal matrices. These rare earth-containing oxygen electrocatalysts provide opportunities to tailor electronic characteristics, regulate the transport of charge carriers, and synergize surface reactivity, which could significantly enhance catalytic performance and stability. Despite their critical importance, reviews focusing on rare earth metal-based oxygen electrocatalysts and related topics are limited. This review systematically summarizes the latest advances in rare earth metal-based catalysts for the ORR and OER, with a focus on catalytic reaction mechanisms. It emphasizes the crucial role of in situ characterization and theoretical calculations in uncovering the underlying mechanisms, both of which offer vital support for clarifying how rare earth metals modulate redox properties, refine surface structures, and boost reaction rates. The study also presents a detailed account of catalyst synthesis approaches and four design strategies. This paper also focuses on the synthesis strategies, evaluation indexes, design strategies, and prospects for applications in fuel cells, metal–air batteries, and the electrolysis of water.

5.2 Exploration of response mechanisms

In order to further explore the mechanism of rare earth metal-based oxygen electrocatalysts, the study should provide an in-depth understanding of the active sites and reaction pathways of the catalysts. Advanced in situ characterization techniques should be employed in experiments to monitor in real time the structural evolution of the catalysts and changes to the active centers during the reaction process; these provide a reliable basis for theoretical models. In addition, interdisciplinary collaborations that combine theoretical,^200–202 experimental and engineering applications to promote the long-term stability and high efficiency of rare earth metal-based electrocatalysts in practical installations may also provide a solid foundation for achieving commercial applications of sustainable energy technologies.

5.3 Deepening of the combination of theoretical calculations and experiments

The catalytic mechanisms and the determination of active sites of rare earth metal-based electrocatalysts still have large uncertainties. Although theoretical calculations play an important role in predicting the catalytic activity and optimizing the catalyst design, their integration with experimental results still needs to be further strengthened. In the future, a deeper understanding of the properties of rare-earth metals and their mechanisms of action in electrocatalytic reactions should be achieved through more accurate theoretical models combined²⁰³ with experimental data. Meanwhile, the theoretical calculations should pay more attention to the dynamic changes to catalysts in the actual experimental environment to better guide experimental design.

5.4 More applications for rare earth metal-based catalysts

Rare earth metal-based catalysts exhibit a wide range of potential applications in the field of electrocatalysis, especially in reactions involving energy conversion and storage, and have made great contributions to the hydrogen evolution reaction (HER), in addition to showing great potential in the ORR and OER.^204–206 Wang et al.²⁰⁷ proposed rare earth oxychloride (REOCl) as a novel additive to enhance the HER electrocatalytic activity of ruthenium in alkaline electrolytes. The strong coupling between REOCl and Ru facilitates enhanced charge transfer, resulting in the Ru/REOCl catalyst exhibiting superior HER performance compared to those of both Pt and Ru catalysts. Fu et al.²⁰⁸ developed a novel rare-earth single-atom modified MoS₂ (Tm SA–MoS₂) catalyst and demonstrated the critical role of Tm single atoms in enhancing the HER activity of MoS₂. In addition to electrocatalysis, rare earth metal-based catalysts have also made progress in photochemistry. Rare-earth double–single-atom (SA) catalysts containing ErN₆ and NdN₆ groups were prepared by atomic confinement and coordination methods by Yan et al.²⁰⁹ The efficient synthesis of C_nH_2n+1OH (n = 1, 2) via photochemical CO₂ reduction was expected to be carbon neutral. Duan et al.²¹⁰ demonstrated the application of single-atom yttrium (Y₁/TiO₂) supported on TiO₂ as an efficient photocatalyst for the activation of phenylpropane C(sp³)–H bonds at ambient temperature. This study highlighted the significant potential of rare-earth single-atom catalysts for activating C(sp³)–H bonds under mild conditions, powered by renewable solar energy. Rare earth metal-based catalysts will continue to play an important role in the future, and through technological innovation and interdisciplinary cooperation, it is expected that greater breakthroughs will be made for efficient catalysis^211–213 and resource utilization.

5.5 Challenges and outlook

Rare earth metal-based electrocatalysts have become attractive non-precious metal-based ORR/OER electrocatalysts, but they still face challenges such as insufficient activity and poor stability in practical applications.^214–216 Therefore, the development of ideal electrocatalysts still requires in-depth investigation of their reaction mechanisms and the promotion of sustainable catalyst development (Fig. 18). On this basis, the paper further examines the challenges confronting such oxygen electrocatalysts and prospective research directions, including intensifying investigations into catalytic mechanisms, facilitating the integration of theoretical computations and experimental studies, and extending their uses in other catalytic areas. As research advances, the application scenarios for rare earth-based electrocatalysts are set to expand continuously, thus advancing the development of the energy conversion and storage sector.

Fig. 18 Prospects for improving the reactivity of rare earth metal-based seawater electrocatalysts (red ball: O; blue ball: H).

Author contributions

Haiyan Wang: investigation, data curation, conceptualization, formal analysis, validation, writing – original draft. Yifan Xia: conceptualization, writing – review & editing, supervision, funding acquisition. Ruiteng Sun: conceptualization, formal analysis. Xiaobin Liu: formal analysis, conceptualization, validation. Jianping Lai: data curation, conceptualization. Jingqi Chi: supervision, validation, conceptualization. Lei Wang: writing – review & editing, funding acquisition, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (52072197, 52174283 and 22301156), the Key R. & D., the Natural Science Foundation of Shandong Province (ZR2024QB012), the Qingdao Natural Science Foundation (24-4-4-zrjj-16-jch), and the Shandong Province “Double-Hundred Talent Plan” (WST2020003).

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