Multi-functional engineering of rare earth-based catalysts for high-efficiency water splitting

HER

HER is a half-reaction that occurs at the cathode during electrochemical water splitting. Depending on the electrolyte conditions, the HER proceeds via two possible pathways: Volmer-Tafel and Volmer-Heyrovsky mechanisms [Equations (1)-(6)]:

In acidic media, the reaction pathway is given by^[28]:

Volmer step:

Heyrovsky step:

Tafel step:

In alkaline and neutral media, the reaction pathway follows^[29]:

Volmer step:

Heyrovsky step:

Tafel step:

OER

OER is the half-reaction that occurs at the anode during electrochemical water splitting. Depending on the electrolyte conditions, from a theoretical perspective, Nørskov proposed the following four-step reaction pathway under acidic conditions, as given in^[30]:

In alkaline media, the reaction pathway is expressed by^[31]:

Doping

Doping is a widely adopted strategy to enhance the catalytic activity of electrocatalysts. The main objective is to regulate the electronic structure of the parent material by introducing heteroatoms. Such modifications can optimize the adsorption free energy of active sites and reaction intermediates, while significantly improving charge transport efficiency, thereby accelerating catalytic reaction kinetics^[48,49]. In recent years, REE doping has emerged as a powerful method for tuning the crystal structure and electronic state at the atomic scale, offering new approaches to overcoming these limitations. For example, Fu et al. proposed a TM doping strategy by incorporating Fe into LaNiO₃ nanocrystals^[50]. The resulting Fe-doped LaNiO₃ exhibited significantly enhanced OER activity in 1 M KOH. The introduction of Fe not only induced lattice distortion in LaNiO₃ and increased oxygen vacancy concentration, but also elevated the charge density on Ni sites, thereby promoting *OOH intermediate formation and facilitating O-O bond cleavage [Figure 2A]. Further investigation into the structure-activity relationship and underlying mechanism revealed that Fe⁴⁺ doping in LaNiO₃ enhances the rate-determining O → OH step in the OER process by facilitating deprotonation through its unique electronic properties [Figure 2B]. This study demonstrates that doping TMs into a rare earth-based host material can synergistically modulate its electronic structure and surface reactivity.

Figure 2. (A) Schematic model of epitaxial growth of Sr and Fe-doped LaNiO₃ perovskite thin films on STO (001) substrate; (B) Schematic representation of the electronic coupling between Ni³⁺ and Fe⁴⁺ in SrLNFO-0.2. This figure is quoted with permission from Fu et al.^[50]; (C) HAADF-STEM image and elemental mappings of La-RuO₂. This figure is quoted with permission from Zhu et al.^[54]; (D) HER polarization curves of CeNiFe-MOF‖CeNiFe-MOF and Pt/C‖IrO₂ for water electrolysis; (E) OER polarization curves; (F) The free energy diagram for the HER process. This figure is quoted with permission from Yang et al.^[55]. STO: SrTiO₃; HAADF-STEM: high-angle annular dark-field scanning transmission electron microscopy; MOF: metal-organic framework; OER: oxygen evolution reaction; HER: hydrogen evolution reaction.

Beyond their role as host materials, REEs are also frequently employed as functional additives in other electrocatalysts. In these roles, they modulate the electronic structure, interfacial adsorption behavior, or reaction pathways of intermediates, further enhancing the overall synergistic effect in electrocatalytic processes^[51]. Unlike bulk doping, these strategies emphasize the auxiliary role of rare earth ions at interfaces or in heterostructures, for example, by inducing surface reconstruction, adjusting band gaps, or providing electron buffering zones. Thus, rare earth ions not only participate in electron transfer processes but also contribute to interface engineering and reaction coordination^[52,53]. For instance, La doping has been shown to introduce oxygen vacancies and modulate the surface electronic structure of RuO₂, enhancing electron accumulation and accelerating charge transfer [Figure 2C], which in turn significantly improves its electrocatalytic activity for water splitting^[54]. Similarly, Yang et al. doped cerium (Ce) into a NiFe-metal-organic framework (MOF) catalyst to construct a CeNiFe-MOF/nickel foam (NF) system with synergistic electronic effects^[55]. Under alkaline conditions, this material demonstrated markedly improved performance for both HER and OER (HER: 113 mV; OER: 224 mV at 10 mA cm^-2), and exhibited excellent stability in both freshwater and simulated seawater environments [Figure 2D and E]. In situ X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed that Ce doping induced redistribution of electron density in the Ni-Fe framework, facilitated dynamic adsorption-desorption of key intermediates (e.g., H*, OH*), and formed a multiphase interfacial synergy mechanism that significantly lowered reaction energy barriers [Figure 2F]. Compared to traditional bimetallic systems, the incorporation of rare earth ions endows the catalyst with superior charge transfer capability and faster intermediate response, highlighting their immense potential as cocatalysts.

Currently, the study of doping strategies in rare earth-based materials remains in a stage of rapid development. Key challenges include understanding the effects of doping type (n-type/p-type), dopant concentration, spatial site selection, and the cooperative mechanisms at heterointerfaces. Moving forward, the integration of multiscale computational modeling with in situ electrochemical and spectroscopic characterization is expected to unveil the microscopic roles of dopants in dynamic reaction environments, thereby advancing the application of rare earth-based catalysts in clean energy conversion systems.

Defect engineering

In addition to doping regulation, defect engineering has emerged as another crucial structural tuning strategy with great potential for enhancing the performance of rare earth-based catalysts. Due to their variable valence states and highly coordinated crystal structures, REEs are prone to the formation of oxygen vacancies, lattice distortions, and non-stoichiometric coordination environments under reaction conditions^[56]. This approach does not require the introduction of foreign elements but instead relies on techniques such as atmosphere control, plasma etching, or thermal treatment to induce lattice reconstruction and optimize the electronic structure, all while preserving the primary framework of the material. As a result, interfacial reaction kinetics and electron transport pathways are effectively improved^[57-59].

Huang et al. employed an atmosphere-controlled strategy by annealing Fe₂O₃-CeO₂ in a reducing Ar atmosphere, successfully inducing the formation of abundant and stable oxygen vacancies on the surface [Figure 3A]^[60]. This treatment enhanced both electronic conductivity and intermediate adsorption capacity. The resulting Fe₂O₃-CeO₂ nanoparticles exhibited significantly superior performance in alkaline OER compared to conventional CeO₂. The performance enhancement was attributed to the preferential formation of oxygen vacancies on the CeO₂ surface, which increased the Ce³⁺ ratio and optimized the electronic structure of Fe₂O₃ through charge transfer. This strengthened the interfacial interaction between Fe₂O₃ and CeO₂, facilitating electron transfer from CeO₂ to Fe₂O₃, thereby promoting O₂ desorption and markedly boosting the intrinsic catalytic activity.

Figure 3. (A) Schematic illustration of the synthetic process for Fe₂O₃@CeO₂-O_V. This figure is quoted with permission from Huang et al.^[60]; (B) Schematic diagram of the fabrication procedures; (C) LSV curves. This figure is quoted with permission from Wang et al.^[61]; (D) The schematic synthesis process of BiLaO-3 catalysts. This figure is quoted with permission from Wang et al.^[62]. LSV: Linear sweep voltammetry.

In addition to atmosphere control, plasma-induced defect engineering has emerged as a popular strategy in recent years. As shown in Figure 3B, Wang et al. employed a low-temperature NH₃/Ar plasma treatment to modify the surface of CeO₂ nanosheets, introducing abundant shallow defects without damaging the bulk lattice^[61]. This significantly enhanced the HER activity, reducing the overpotential to 65 mV at 10 mA cm^-2 [Figure 3C]. The plasma treatment effectively modulated the surface charge distribution and exposed more active sites, thereby promoting the adsorption/desorption of H* intermediates. Thermal treatment-induced structural reconstruction has also been widely applied in rare earth-based catalysts. As depicted in Figure 3D, Zou et al. synthesized oxygen-vacancy-rich La₂O₃/C composites through high-temperature pyrolysis of MOF precursors^[62]. The resulting catalysts exhibited excellent bifunctional HER/OER performance in alkaline media. Thermal treatment not only enhanced the electronic coupling between the carbon layer and rare-earth oxides, but also induced the formation of defect-rich layers around rare-earth sites, improving interfacial conductivity and the density of active sites. The resulting catalyst exhibited a current density of more than 200 mA cm^-2 at -1.06 V [vs. Reversible Hydrogen Electrode (RHE)].

Comprehensive studies have shown that defect engineering enhances the catalytic performance of rare earth-based catalysts through a dual mechanism: on the one hand, it introduces abundant active centers, and on the other, it modulates the electronic structure and interfacial energy states to optimize the reaction process both thermodynamically and kinetically. Specifically, various defect-engineering strategies not only create additional catalytic sites but also reconstruct the electronic configuration, enhance charge carrier mobility, and regulate the binding energies of reaction intermediates. These synergistic effects underscore the critical role of defect engineering in the development of high-performance electrocatalytic systems.

Alloy

Alloys are materials composed of two or more chemical elements, with at least one being a metal. In rare-earth alloys, charge transfer induced by metallic bonding not only significantly tunes the electronic band structure and catalytic activity of the catalyst, but also reshapes the spatial distribution of active components through geometric configuration changes such as lattice strain modulation and lattice constant alteration^[63,64]. Alloying REEs further allows for precise regulation of the microstructure evolution and phase distribution of the metallic matrix, making it an effective strategy for optimizing the performance of rare earth-based catalysts^[65]. Compared with late TM systems, rare-earth intermetallic compounds exhibit strong d-f orbital coupling, resulting in a negative enthalpy of alloy formation. This provides an intrinsic driving force for reducing lattice reconstruction energy barriers and stabilizing metastable phases thermodynamically^[66]. As shown in Figure 4A, Zhang et al. synthesized Rh₃Sc/Y nanostructured alloys via a sodium vapor reduction method^[67]. Compared to Rh/C, all X-ray diffraction (XRD) peaks of Rh₃Sc/C and Rh₃Y/C shift to higher 2θ angles, indicating an increase in alloy lattice spacing and the formation of Rh-Sc and Rh-Y alloys. Energy-dispersive X-ray spectroscopy (EDS) results show a uniform distribution of Rh and Sc/Y elements in the Rh₃Sc/C and Rh₃Y/C samples, confirming the formation of Rh-RE nanoalloys. Alloying active metal rhodium (Rh) with REEs scandium (Sc) and yttrium (Y) enabled directional charge transfer through metallic bonding, significantly regulating both the electronic structure and spatial configuration of the catalyst, thus synergistically enhancing the HER performance. The low electronegativity of REEs leads to electron enrichment at Rh active sites via a ligand effect, reducing the electron binding energy and increasing surface electron density. This weakens the adsorption of hydrogen intermediates (H*) and accelerates desorption kinetics. Meanwhile, the relatively large atomic radii of Sc and Y introduce considerable lattice strain, causing lattice expansion and shifting the d-band center of Rh downward, which further reduces the reaction energy barrier. Zhang et al. employed a solid-state synthesis strategy to fabricate ternary rare-earth Pt_3-xIr_xSc alloy nanoparticles [Figure 4B]^[68]. XRD characterization typically confirms the formation of well-defined cubic Pt-Sc alloy phases such as Pt₃Sc, indicating effective alloying between Pt and Sc. Corresponding scanning electron microscopy (SEM) analyses often reveal surface structures dominated by the (100) crystal plane, where ordered Pt/Ir and Pt/Ir-Sc atomic arrangements are observed. Upon acid treatment, partial leaching of Sc can occur, resulting in a Pt/Ir-enriched noble metal surface. Such surface reconstruction has been reported to enhance electrochemical stability and corrosion resistance under acidic operating environments, thereby improving the long-term durability of Pt-based catalysts. The low electronegativity of Sc facilitates electron transfer from Sc to Pt and Ir, resulting in a negatively charged Pt surface. This charge redistribution downshifts the d-band center of Pt, weakens the adsorption energy of H* intermediates, and optimizes HER kinetics. Additionally, the strong orbital overlap between Sc and Pt/Ir reduces the alloy formation enthalpy and enhances structural stability, enabling excellent durability under acidic, neutral, and alkaline conditions. The optimized Pt₂IrSc showed enhanced HER activity and stability across all pH values. To achieve a current density of 10 mA cm^-2, it only required low overpotentials of 13 mV, 18 mV, and 25 mV in 0.5 M H₂SO₄, 1 M phosphate buffered saline (PBS), and 1 M KOH, respectively.

Figure 4. (A) Schematic illustration of the synthesis of Rh₃RE alloys on Ketjen black. This figure is quoted with permission from Zhang et al.^[67]; (B) Schematic illustration of the synthesis of Pt_3-xIr_xSc alloys on graphene. This figure is quoted with permission from Zhang et al.^[68]; (C) Schematic diagram for the synthesis of the PdCeMoCuRu HEA. This figure is quoted with permission from Zhang et al.^[64]. OPDA: o-Phenylenediamine; HEA: high-entropy alloy; NC: nanocrystal; RE: rare earth.

High-entropy alloys (HEAs), a recently emerging multi-metal construction strategy, have also been adopted to build rare-earth-involved synergistic electrocatalytic platforms. As illustrated in Figure 4C, Zhang et al. synthesized HEA nanoparticles comprising Ce, Ni, Fe, Co, and Cu, and used rapid annealing to induce microphase separation and lattice stress field optimization. In comparison with the reference XRD pattern, the PdCeMoCuRu HEA exhibits a slight shift of diffraction peaks toward higher angles, suggesting that Ce incorporation compresses the Pd-Pd interatomic spacing, resulting in lattice contraction and compressive strain, which may further trigger dynamic structural reconstruction under electrocatalytic conditions. The resulting catalyst exhibited an HER overpotential of 89 mV and an OER overpotential of 274 mV in 1 M KOH, with long-term stability at 500 mA cm^-2 for over 100 h. The study revealed that Ce incorporation facilitates d-f orbital coupling, enhances electron transport efficiency, and generates internal electric fields via lattice distortion at the interface, thereby accelerating the reaction kinetics of H*/OH* intermediates^[64].

Heterostructures

The construction of heterostructures has emerged as another vital approach to enhance the performance of rare earth-based electrocatalysts. By integrating rare-earth materials with highly active components such as TMs, metal oxides, or sulfides, heterojunction interfaces can be formed that effectively overcome inherent limitations in catalytic structure and activity^[69]. These heterostructures often exhibit unique band bending effects, interfacial charge transfer pathways, and localized electric field modulation, which enable surface electronic reconstruction, increased active site density, and accelerated intermediate reaction kinetics, ultimately leading to synergistic enhancement in key reactions such as the HER and OER^[70,71].

Song et al. designed a nitrogen-doped carbon encapsulated Co/CeO₂/Co₂P/CoP heterostructure (Co/CeO₂/Co₂P/CoP@NC) by embedding CeO₂ nanowires into a zeolitic imidazolate framework-67 (ZIF-67) precursor followed by carbonization and phosphidation^[72]. The strong interfacial coupling between CeO₂ and Co₂P/CoP facilitated interfacial electron transfer, which optimized the adsorption strength of key intermediates and reduced the kinetic barriers for both OER and HER [Figure 5A-C]. Similarly, Chen et al. synthesized a bifunctional electrocatalyst composed of a MoO₂-CeO_x heterostructure using interfacial engineering^[73]. The heterointerface enabled efficient electron transfer and electronic structure modulation, accelerating overall charge transport. Additionally, the valence flexibility of Ce⁴⁺/Ce³⁺ enhanced electron mobility, while unoccupied orbital centers in MoO₂ promoted favorable H* adsorption and desorption. As a result, the MoO₂-CeO_x/NF catalyst demonstrated outstanding HER activity [Figure 5D and F].

Figure 5. (A) TEM images of Co/CeO₂/Co₂P/CoP@NC heterostructures; (B) HER performance of LSV polarization curves of Co/CeO₂/Co₂P/CoP@NC heterostructures; (C) OER performance of LSV polarization curves. This figure is quoted with permission from Song et al.^[72]; (D) TEM image of MoO₂-CeO_x/NF; (E) LSV curves of MoO₂-CeO_x/NF and Pt/C at 10.0 mA cm^-2 in 0.5 M H₂SO₄; (F) PDOS programs of MoO₂, Vo-MoO₂, CeO_x and Vo-MoO₂-CeO_x. This figure is quoted with permission from Chen et al.^[73]; (G) TEM image of the CeO₂@C₆₀ core-shell hybrid at 200 nm; (H) LSV polarization curves of pure CeO₂/SS, C₆₀/SS and CeO₂@C₆₀/SS core-shell hybrid in 1.0 M KOH; (I) chronoamperometric curves. This figure is quoted with permission from Munawar et al.^[74]. TEM: Transmission electron microscopy HER: hydrogen evolution reaction; LSV: linear sweep voltammetry; OER: oxygen evolution reaction; NF: nickel foam; PDOS: projected density of states.

Core-shell heterostructures further improve interface stability, corrosion resistance, and charge transport by coating rare-earth materials with active phases or protective outer layers. For instance, Munawar et al. employed ultrasonic assembly to fabricate a CeO₂@C₆₀ core-shell structure [Figure 5G]^[74]. The C₆₀ shell formed a three-dimensional (3D) conductive network that significantly accelerated charge migration. Interfacial electronic coupling induced a Ce³⁺/Ce⁴⁺ mixed valence state and abundant oxygen vacancy defects, which together reduced the *OOH intermediate adsorption/desorption barrier. Consequently, the OER overpotential was lowered to 312 mV at 10 mA cm^-2, and the catalyst exhibited excellent long-term stability, maintaining constant potential operation in 1 M KOH for over 45 h [Figure 5H and I].

Through rational design of heterostructured interfaces, it is possible to simultaneously improve active site utilization, enhance interfacial synergy, and optimize catalytic kinetics. This provides a multidimensional, tunable material platform for advancing HER, OER, and other critical electrocatalytic reactions.

Other strategies

Lanthanum-based

The 4f orbitals of Lanthanum can dynamically modulate the electron occupancy of the 5d orbitals and the charge distribution of neighboring atoms, thereby creating effective electron-accepting sites. This property significantly enhances the surface charge transfer capability of the material, leading to improved HER catalytic activity. Wei et al. synthesized a novel La-doped molybdenum phosphide nanoparticle (La-MoP@NC) encapsulated in a nitrogen-doped carbon matrix^[78]. As shown in Figure 6A, the transmission electron microscopy (TEM) image reveals uniformly cross-linked carbon-coated nanoparticles with an average diameter of approximately 15 nm. The La-doped catalyst exhibited excellent electrocatalytic activity and durability for HER over a wide pH range. The overpotentials of the La-MoP@NC electrode were 129.3 mV in 1.0 M KOH, 142.18 mV in 0.5 M H₂SO₄, and 235.27 mV in 1.0 M PBS [Figure 6B]. Detailed structural characterizations and DFT calculations confirmed that the La dopants effectively modulated the electronic density around Mo and P atoms, optimizing the Gibbs free energy for hydrogen adsorption on MoP and enhancing its intrinsic HER activity. Subsequently, Ye et al. synthesized La-MoP@N/C with varying La doping ratios, which also showed outstanding HER durability and electrocatalytic performance^[79]. The optimized nitrogen-doped carbon encapsulated La-doped MoP (La_0.025-Mo_0.975P@N/C) achieved a low overpotential of 113 mV at a current density of 10 mA cm^-2.

Figure 6. (A) TEM images of La-MoP@NC; (B) The iR-corrected LSV curves of La-MoP@NC in alkaline, acidic, and neutral media. This figure is quoted with permission from Wei et al.^[78]; (C) SEM images of the surface of Ho-LaNi₅/NF; D: HER polarization curves of Ho-LaNi₅/NF, LaNi₅/NF, Ni foam, Ni plate and commercial Pt/C. This figure is quoted with permission from Shi et al.^[80]; (E) iR-Corrected polarization curves of La-NMS@NF in 1.0 M KOH. This figure is quoted with permission from Jin et al.^[82]; (F) The OER and HER properties of HEAs were tested on NF using a three-electrode system; G: Distribution of each element in FeCoNiMnRuLa/CNT under the bright field conditions. This figure is quoted with permission from Zhang et al.^[83]; (H) Polarization curves of catalysts for HER. This figure is quoted with permission from Wu et al.^[84]. RHE: Reversible hydrogen electrode; NF: nickel foam; NP: nickel plate; CNT: carbon nanotube; HAADF: high-angle annular dark-field; TEM: transmission electron microscopy; SEM: scanning electron microscopy; HER: hydrogen evolution reaction; OER: oxygen evolution reaction; LSV: linear sweep voltammetry; iR: internal resistance; HEA: high-entropy alloy.

Lanthanum (La)-doped nickel-based metal foams combine the structural advantages of NFs with the unique catalytic properties of lanthanum, resulting in enhanced catalytic activity and outstanding thermochemical stability. As shown in Figure 6C, Wu et al. fabricated a hierarchical honeycomb-structured LaNi₅ alloy (Ho-LaNi₅/NF) on NF^[80]. This self-supported electrode exhibits excellent electrical coupling and conductivity between the NF and LaNi₅, forming a 3D self-supported heterostructure with remarkable electrocatalytic performance and durability for both the HER and OER. At a high current density of 100 mA cm^-2, it achieved an overpotential of 1.86 V, comparable to commercial IrO₂//Pt/C coupled electrodes (1.85 V) [Figure 6D]. Additionally, Li et al. synthesized a La-doped Ni₃S₂/MoS₂ heterostructure catalyst (La-NMS@NF) on NF^[81]. The incorporation of La dopants induced the formation of nanoflower-like structures with abundant porous channels, which promoted mass transfer and exposure of active sites. In 1.0 M KOH electrolyte, the catalyst achieved a HER overpotential of only 154 mV at 100 mA cm^-2 [Figure 6E]. High-entropy effects, lattice distortion, sluggish diffusion, and the cocktail effect collectively endow HEAs with excellent structural stability, abundant active sites, and enhanced intrinsic activity^[82]. Wang et al. reported a FeCoNiMnRuLa/CNT HEA catalyst, in which the introduction of La disrupted the lattice order of FeCoNiMnRu, enabling efficient electrocatalytic water splitting^[83]. La doping reduced the number of unpaired electrons in FeCoNiMnRu, thereby accelerating the transition from singlet oxygen-containing intermediates to triplet oxygen. During HER, the FeCoNiMnRuLa/CNT catalyst exhibited a low overpotential of 50 mV at 10 mA cm^-2 [Figure 6F]. The XRD patterns exhibit distinctive diffraction peaks for multi-walled carbon nanotubes (MWCNTs) at 26.1°, 43.1°, 53.7°, and 78.4°, which remain largely unaltered following the activation process. EDX in bright-field mode confirmed the uniform distribution of all metal elements within the HEA nanoparticles and the successful incorporation of La [Figure 6G]. The catalyst maintained excellent performance even after 200 h of stability testing.

RuO₂ tends to form RuO₄ and dissolve under acidic water oxidation conditions, which limits its commercial viability due to chemical instability. To address this issue, Wu et al. synthesized La-doped RuO₂ nanocrystals through hydrothermal and annealing processes^[84]. La doping accelerated the charge transfer rate of electrons or holes, enabling the La-doped RuO₂ nanocrystals to exhibit superior electrocatalytic performance in acidic electrolytes. In 0.5 M H₂SO₄, the catalyst showed a low overpotential of 71 mV at 10 mA cm^-2, along with excellent long-term stability. Compared with pure RuO₂, its HER overpotential was reduced by 15 mV [Figure 6H]. Rodney et al. synthesized La-doped copper oxide nanoparticles (Cu_1-xLa_xO) via a co-precipitation method and deposited them on a stainless steel substrate by drop casting^[85]. Testing in 1.0 M KOH alkaline electrolyte revealed that 1% La-doped CuO achieved a HER potential of 173 mV at 10 mA cm^-2 (vs. RHE).

Cerium-based

Cerium dioxide (CeO₂), a rare-earth metal oxide, is widely utilized as a promoter in electrocatalysis due to its abundant oxygen vacancies and the flexible redox transition between Ce³⁺ and Ce⁴⁺. Guo et al. synthesized a CeO₂-modified bimetallic phosphide catalyst, (CeO₂)-NiCoP^[86]. The surface modification by CeO₂ effectively regulated the electronic distribution on the bimetallic phosphide, promoting electron transfer from NiCoP to CeO₂. This shifted the d-band center of the metal active sites negatively, optimizing the binding strength with hydrogen intermediates and significantly enhancing HER activity. The overpotentials at current densities of 10, 500, and 1000 mA cm⁻² were as low as 84, 202, and 242 mV, respectively, which are substantially lower than those of unmodified NiCoP [Figure 7A].

Figure 7. (A) Polarization curves and the corresponding Tafel plots of CeO₂-NiCoP, NiCoP and commercial Pt/C catalysts. This figure is quoted with permission from Yu et al.^[86]. (B) XRD pattern of Fe₂P-CoP/CeO₂-20; (C) LSV curves of Fe₂P-CoP/CeO₂; (D) ΔG_H* profiles a on CoP, Fe₂P, Fe₂P-CoP, and Fe₂P-CoP/CeO₂. This figure is quoted with permission from Ding et al.^[87]; (E) SEM image; (F) Illustration of the HER mechanism on Y, Co-CeO₂. This figure is quoted with permission from Liu et al.^[89]; (G) HER performance examined in 1.0 M KOH of CeO₂-I, CeO₂-II, and CeO₂-III. This figure is quoted with permission from Liu et al.^[90]; (H) DF-TEM image of Ce-doped CoP, and element mapping images of Co, Ce and P, respectively; (I) Polarization curves (with iR corrections) of CoP, Ce-doped CoP and Pt/C catalysts with a scan rate of 5 mV/s in 0.5 M H₂SO₄. This figure is quoted with permission from Gao et al.^[91]. RHE: Reversible hydrogen electrode; XRD: X-ray diffraction; LSV: linear sweep voltammetry; SEM: scanning electron microscopy; LSV: linear sweep voltammetry; iR: internal resistance; DF-TEM: dark-field transmission electron microscopy.

Ding et al. constructed a novel Fe₂P-CoP/CeO₂ ternary heterojunction via interface engineering and selective phosphorization^[87]. After phosphorization under an N₂ atmosphere, CoFe-Prussian Blue Analogue-derived/CeO₂-20 (CoFe-PBA/CeO₂-20) nanocubes transformed into Fe₂P-CoP/CeO₂ composites. XRD confirmed the successful formation of the ternary heterojunction [Figure 7B]. Electrochemical results demonstrated excellent HER performance for Fe₂P-CoP/CeO₂-20, with overpotentials of 45 mV at 10 mA cm⁻² and 100 mV at 50 mA cm⁻² [Figure 7C]. DFT calculations revealed that CeO₂ coupling at the Fe₂P/CoP interface facilitated electron redistribution across the three-phase boundary, optimized the Gibbs free energy for H* adsorption, and significantly reduced the energy barrier for water dissociation, thereby enhancing alkaline water splitting [Figure 7D].

Guo et al. designed a CeO₂@CoSe₂ nano-needle catalyst supported on carbon cloth (CeO₂@CoSe₂/CC) via interface engineering^[88]. The catalyst exhibited outstanding HER performance, attributed to the synergistic effects of CeO₂ in promoting H₂O dissociation and CoSe₂ in facilitating H* adsorption. Additionally, Liu et al. developed a Y and Co co-doped cerium dioxide (Y, Co-CeO₂) catalyst to improve the performance of CeO₂ in alkaline HER^[89]. As shown in the SEM image [Figure 7E], the catalyst exhibited a nanosheet morphology. Lattice compression in CeO₂ increased oxygen vacancy concentration, which activated neighboring sites for hydrogen adsorption and significantly improved HER kinetics. The catalyst achieved an ultra-low overpotential of only 27 mV at 10 mA cm^-2, surpassing even Pt-based electrocatalysts [Figure 7F].

Recent studies have increasingly focused on two-dimensional (2D) structures due to their high surface exposure and compatibility with frustrated Lewis pair (FLP) active site designs. Liu et al. synthesized 2D CeO₂ nanosheets with varying oxygen vacancy concentrations and found that those with the highest oxygen vacancy concentration showed the best HER performance, achieving an overpotential of 132 mV at 10 mA cm^-2 [Figure 7G]^[90]. This work provides a theoretical and technical foundation for designing advanced FLP-type HER catalysts.

Cerium can also be doped into host catalysts to enhance HER performance. Gao et al. combined theoretical and experimental approaches to demonstrate that doping Ce into CoP adjusts the electronic structure and lowers the hydrogen adsorption free energy^[91]. Dark-field TEM and EDS mapping [Figure 7H] showed that Ce is not only stably incorporated but also uniformly distributed with Co and P, confirming successful doping without disrupting the CoP matrix. Compared to undoped CoP, Ce-doped CoP exhibited lower overpotentials, Tafel slopes, and charge transfer resistance, along with higher electrochemical surface area and turnover frequency, resulting in excellent catalytic activity and stability. Notably, in both acidic and alkaline media, the optimized Ce-doped CoP catalyst exhibited ultra-low overpotentials of 54 mV and 92 mV, respectively, at 10 mA cm⁻² [Figure 7I]. Finally, Zhang et al. synthesized a Ce-doped NiCoP catalyst (Ce-NiCoP) supported on nickel foam via a simple solvothermal method^[92]. The presence of Ce promoted the in situ formation of a hydrophilic NiCoP phase, resulting in outstanding HER performance.

Erbium-based

Erbium (Er), a REE with a unique 4f electronic configuration, can generate oxygen vacancies and lattice defects when doped into oxide structures. These defects provide additional active sites for electrocatalytic reactions, thereby enhancing both catalytic activity and stability, especially under acidic conditions for the HER^[93]. As shown in Figure 8A, Zhang et al. synthesized Er-doped CoP nanomeshes that exhibited larger specific surface areas and higher double-layer capacitance, indicating more exposed active sites^[94]. This enhancement was attributed to electronic structure modulation, particularly the optimization of the d-band center of Co sites, which brought the adsorption free energy of hydrogen intermediates (ΔG_H*) closer to the ideal value. As a result, the Er-doped CoP nanomesh demonstrated excellent HER performance in both acidic and alkaline media, with overpotentials of just 52 mV and 66 mV at 10 mA cm^-2, respectively, and maintained stability over 25 h [Figure 8B]. XPS analysis confirmed the successful incorporation of Er into the CoP lattice, evidenced by the appearance of a new peak at 171 eV in the Er-doped sample compared to pristine CoP [Figure 8C]. Additionally, Zhang et al. prepared vertically aligned Er-doped NiCoP nanowire arrays (Er-NiCoP/NF) grown on conductive NF^[95]. These electrodes displayed remarkable bifunctional catalytic activity, with an HER overpotential of only 46 mV at 10 mA cm^-2 [Figure 8D]. XPS analysis revealed a new Er 4d orbital peak at 168.8 eV [Figure 8E], confirming Er incorporation. DFT calculations further showed that Er doping adjusted the electronic structure of NiCoP, lowering the d-band center of Ni and Co atoms relative to the Fermi level. This shift optimized the Gibbs free energy for HER intermediates, thereby accelerating water-splitting kinetics [Figure 8F].

Figure 8. (A) TEM images of the Er-doped CoP nanomesh; (B) Polarization curves of bare CC, CoP, Er-doped CoP and Pt/C in 1.0 M KOH; (C) XPS full spectra of CoP and Er-doped CoP, respectively. This figure is quoted with permission from Zhang et al.^[94]; (D) LSV polarization curves of Er-NiCoP/NF, NiCoP/NF, bare NF, and commercial Pt/C for HER; (E) Calculated free-energy diagram of NiCoP and Er-NiCoP for HER; (F) XPS analysis of survey scan and high resolution. This figure is quoted with permission from Zhang et al.^[95]. (G) Aberration-corrected (AC) HAADF-STEM image of Pt-Er/h-NC; (H) Polarization curves of Pt-Er/h-NC and the references; (I) The theoretical volcano of HER on Pt atoms of various Pt-based catalysts. This figure is quoted with permission from Chen et al.^[97]. RHE: Reversible hydrogen electrode; TEM: transmission electron microscopy; XPS: X-ray photoelectron spectroscopy; LSV: linear sweep voltammetry; NF: nickel foam; HER: hydrogen evolution reaction; HAADF-STEM: high-angle annular dark-field scanning transmission electron microscopy; NC: nitrogen-doped carbon.

Nadeem et al. synthesized a bimetallic Pt-Er composite (Pt-Er@PCN920) by thermally decomposing a Zn-based metal-organic framework (Zn-MOF) under an inert atmosphere to obtain a nitrogen-doped porous carbon support^[96]. The resulting material possessed a porous structure and high surface area, enabling efficient HER activity^[96]. Chen et al. developed a dual-function catalyst (Pt-Er/h-NC) featuring Pt-Er alloy clusters, atomically dispersed Pt, and single Er atoms [Figure 8G]^[97]. This nanoarchitecture introduced abundant heterojunctions and boundary sites, providing more active centers for HER. Pt-Er/h-NC achieved an ultra-low overpotential of 25 mV at 10 mA cm^-2 in acidic media [Figure 8H]. The synergy among the nanostructured active sites, combined with Er-induced stabilization, significantly improved catalytic performance. DFT calculations [Figure 8I] revealed that Er modulates the hydrogen adsorption energy at Pt sites toward ~ 0 eV, resulting in higher current densities compared to pure Pt catalysts under the same conditions. Fatima et al. fabricated a highly efficient bifunctional catalyst by ultrasonically anchoring Er onto V₂CT_x MXene nanosheets, forming Er@V₂CT_x nanocomposites^[98]. At an optimal Er-to-V₂CT_x mass ratio of 0.12:1, the composite exhibited excellent HER performance in 1.0 M KOH, with an overpotential of 174 mV^[98].

Other rare earth-based catalysts

Beyond La, Ce, and Er, other REEs have also been explored for enhancing the performance of HER electrocatalysts. Xu et al. investigated the HER activity of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-σ (BSCF), a widely studied perovskite oxide in electrocatalysis^[99]. As shown in Figure 9A, its general formula ABO₃ (where A is a rare earth or an alkali; B is a transition metal; and O is oxygen) allows for rare-earth or alkali metals at the A-site and TMs at the B-site. Through simple A-site doping with praseodymium (Pr), the surface electronic structure and catalytic performance were effectively modified. The resulting Pr_0.5Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-σ (Pr_0.5BSCF) demonstrated enhanced HER activity and durability in alkaline media, requiring an overpotential of only 237 mV at 10 mA cm^-2, more than 100 mV lower than undoped BSCF (342 mV) [Figure 9B], with a Tafel slope of just 45 mV dec^-1. Raman spectroscopy confirmed that Pr_0.5BSCF maintained a cubic perovskite structure with Pm-3m symmetry [Figure 9C]. Moreover, the Pr_0.5BSCF-based electrode sustained stable hydrogen production at a high current density of 50 mA cm^-2 for 25 h, outperforming many other bulk or nanoscale non-Pt HER catalysts under strong alkaline conditions^[99]. Subsequently, Liu et al. developed a novel Ni-Ce-Pr-Ho electrocatalyst using a one-step constant-current electrodeposition method [Figure 9D]^[100]. Characterization and electrochemical tests revealed that this multicomponent system displayed excellent HER activity in 1 M KOH, achieving an overpotential as low as 78 mV at 10 mA cm^-2.

Figure 9. (A) The structure of ABO₃ perovskite oxide (where A is a rare earth or an alkali, blue; B is a transition metal, olive; and O is oxygen, red); (B) Polarization curves of BSCF, Pr_0.5BSCF and commercial Pt/C catalysts. The background HER activity of a conductive-carbon-supported GC electrode is shown for reference; (C) Raman spectra of BSCF and Pr_0.5BSCF. This figure is quoted with permission from Xu et al.^[99]; (D) Schematic illustration of the one-step electrodeposition for Ni-Ce-Pr-Ho catalyst. This figure is quoted with permission from Liu et al.^[100]; (E) HER LSV curves of various electrocatalysts. This figure is quoted with permission from Xu et al.^[101]; (F) Schematic of the AEMWE; G: Polarization curve in AEMWE. This figure is quoted with permission from Liu et al.^[102]. RHE: Reversible hydrogen electrode; NF: nickel foam; BSCF: Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-σ; HER: hydrogen evolution reaction; LSV: linear sweep voltammetry; AEMWE: anion exchange membrane water electrolysis.

Wang et al. reported a bifunctional “armored” catalyst consisting of NiGd nanoparticles encapsulated within a nitrogen-doped carbon shell (NiGd@N-C). Owing to its distinctive core-shell architecture, this catalyst exhibited outstanding HER performance in alkaline media, with an overpotential of only 45 mV at 10 mA cm^-2 [Figure 9E] and remarkable stability over 60 h. This was significantly better than the Gd-free counterpart (112 mV) and comparable to commercial Pt/C (26 mV). DFT calculations showed that the introduction of Gd, which has lower electronegativity than Ni, promoted electron penetration from the NiGd alloy core to the N-C shell, leading to interfacial charge redistribution and electron accumulation on the N-C shell surface. This optimized electronic configuration significantly reduced the ΔG_H* to near-zero values, lowering the reaction energy barrier, accelerating HER kinetics, and enhancing charge transfer. The synergy between the physical protection of the core-shell structure and the Gd-induced electronic effects is key to the high activity and long-term stability of the NiGd@N-C catalyst^[101].

To bridge the gap between laboratory-scale demonstrations and industrial deployment, incorporating REEs into electrocatalysts must prioritize scalability, cost-effectiveness, and compatibility with real-world electrolyzer systems. A key challenge lies in translating the enhanced activity and stability observed in small-scale tests to large-area electrodes operating under high current densities, where mass transport limitations, ohmic losses, and thermal management become pronounced. For practical implementation, REE-based catalysts should be evaluated in full-cell configurations, such as anion exchange membrane water electrolysis (AEMWE) and proton exchange membrane water electrolysis (PEMWE), which are pivotal for industrial hydrogen production due to their high efficiency and compact design. Liu et al. demonstrated the industrial relevance of Nd₂O₃@Ru heterojunctions supported on carbon nanotubes (Nd₂O₃@Ru/CNT) as a cathode catalyst in an AEMWE setup for overall water splitting^[102]. The AEMWE device, assembled with Nd₂O₃@Ru/CNT as the cathode, commercial RuO₂ as the anode, and a Fuma FAA-PK-130 anion exchange membrane [Figure 9F], also achieved 1.494 V at 100 mA cm^-2 and maintained operational stability for over 100 h at this current density without significant degradation. This REE-engineered material exhibited exceptional performance, requiring cell voltages of only 1.665 V and 1.806 V to achieve high current densities of 500 mA cm^-2 and 1,000 mA cm^-2 [Figure 9G], respectively, in 1 M KOH electrolyte. The superior performance and durability arise from the asymmetrical neodymium (Nd)-oxygen (O)-Ru bridge, where oxygen’s p-orbital mediates f-d orbital coupling between Nd and Ru, optimizing water adsorption, hydrogen evolution kinetics, and resistance to nanoparticle agglomeration under high-current conditions, as supported by in situ Raman spectroscopy and DFT calculations. This example highlights the potential of REE heterostructures to enable AEMWE operations at industrially relevant high current densities, promoting efficient and scalable green hydrogen production while reducing reliance on precious metals such as Pt. Extending such orbital coupling strategies to PEMWE systems could further harness the versatility of REEs across acidic and alkaline environments, facilitating broader commercialization.

In summary, rare earth-based catalysts exhibit diverse structural features and electronic properties depending on the specific element incorporated, host material, and synthetic strategy. Across La-, Ce-, Er-, and other rare-earth systems, these catalysts demonstrate varying degrees of enhancement in HER activity, stability, and intermediate adsorption behavior. Representative catalysts are systematically summarized in Table 1. An intuitive comparative illustration is further presented in Figure 10.

Figure 10. Comparison of HER electrocatalytic performance across different REE types. NC: Nitrogen-doped carbon; N/C: CNT: carbon nanotube; BSCF: Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-σ; NF: nickel foam; HER: hydrogen evolution reaction; REE: rare earth element.

Lanthanum-based

Compared to HER, OER is a more complex four-electron transfer process. Its reaction pathway involves four key intermediates: OH*, O*, OOH*, and O₂*, encompassing multiple consecutive elementary steps^[103,104]. Lanthanum (La)-doped nickel-based catalysts enhance OER efficiency and stability by tuning the d-band center of Ni, thereby optimizing the interaction between the catalyst and oxygenated intermediates, and lowering the reaction energy barriers. This doping strategy not only improves catalytic activity but also significantly enhances durability, offering a novel approach to designing high-performance OER catalysts. Wang et al. reported a ternary NiFeLa catalyst containing unique M-NiFe (M: 4d-Mo/5d-La) units with distinct d-orbitals^[105]. Both experimental results and theoretical calculations demonstrated that La doping optimized the hybridization between the d-orbitals of NiFeLa and the O-2p orbitals, enhancing the adsorption strength toward oxygen intermediates and reducing the energy barrier of the rate-determining step, thus boosting OER performance. The synthesized NiFeLa catalyst required only 1.58 V overpotential to achieve a high current density of 1,000 mA cm^-2 in an anion exchange membrane electrolyzer, maintaining excellent long-term stability over 600 h. Furthermore, Arumugam et al. investigated the influence of La doping on the catalytic activity and durability of CoMoO₄ in high pH media for overall water splitting^[106]. By doping La at various concentrations into the CoMoO₄ lattice, the electrocatalytic performance was regulated. The optimal performance was achieved with 5% La-doped CoMoO₄, which exhibited a low overpotential of 272 mV at 20 mA cm^-2 [Figure 11A]. Notably, La doping stabilized the lattice structure, optimized electronic distribution, and accelerated surface reconstruction into a stable γ-CoOOH phase, mitigating the severe MoO₄^2- leaching issue in the electrolyte and preserving structural integrity. DFT calculations revealed that La doping altered the electronic structure of CoMoO₄, effectively optimizing the adsorption energies of reactive hydrogen and oxygen intermediates and enhancing the intrinsic OER activity of CoMoO₄ [Figure 11B]. Song et al. also fabricated a La-doped nickel-iron layered double hydroxide/nickel sulfide heterostructure (La-NiFeLDH/NiS/NF) on nickel foam using a solution-based chemical etching method followed by one-step electrodeposition^[107]. As shown in Figure 11C, XPS confirmed the incorporation of La into NiFeLDH. This electrocatalyst demonstrated outstanding OER performance, requiring only 290 mV to reach 1,000 mA cm^-2 [Figure 11D], with remarkable durability over 200 h. Both theoretical and experimental results indicated the formation of strong Ni-S(O)-Ni(Fe) coupling interfaces within the heterostructure. DFT calculation results further showed that La doping facilitated electron transfer from NiS to NiFeLDH, tuned the d-band center of Fe atoms, optimized the binding strength to oxygen intermediates, and lowered the activation energy of the reaction [Figure 11E]. In addition, Nguyen et al. reported a novel La-based high-entropy perovskite oxide (HEPO) electrocatalyst for OER^[108]. The B-site of the HEPO lattice was composed of five successive 3d TMs: Cr, Mn, Fe, Co, and Ni. In the five non-equimolar HEPO samples, one TM in each sample had double the concentration of the others. All HEPOs outperformed conventional single-component perovskite oxides. The optimized La(CrMnFeCo₂Ni)O₃ HEPO delivered a low overpotential of 325 mV at 10 mA cm^-2 and exhibited excellent electrochemical stability over 50 h.

Figure 11. (A) LSV responses of different La-doped CoMoO₄ catalysts toward the OER in 1 M KOH; (B) Free energy plots of OER intermediates on the Co-site in the 5% La-CoMoO₄ system under different thermodynamic potentials in alkaline media. This figure is quoted with permission from Arumugam et al.^[106]; (C) XPS spectrum of La-NiFe LDH/NiS/NF La 3d; (D) Polarization curves of different electrodes; (E) Free energy diagram. This figure is quoted with permission from Song et al.^[107]; (F) LSV curves at 10 mA cm^-2 and current densities at 1.6 V of commercial RuO₂, RuO₂, and La_xRu_1-xO₂; (G) Reaction free energy of the acidic OER on RuO₂ and La_0.1Ru_0.9O₂. This figure is quoted with permission from Li et al.^[111]; (H) XPS spectra of O 1s for La₃IrO₇ and La₃IrO₇-SLD; (I) Polarization curves of La₃IrO₇-SLD and commercial IrO₂ at a scan rate of 5 mV s^-1. This figure is quoted with permission from Chen et al.^[112]; (J) Schematic of the PEM electrolyzer device and I-V curves of PEM electrolyzer using La-RuO₂. This figure is quoted with permission from Zhang et al.^[113]. RHE: Reversible hydrogen electrode; NF: nickel foam; LSV: linear sweep voltammetry; OER: oxygen evolution reaction; XPS: X-ray photoelectron spectroscopy; PEM: proton exchange membrane; SLD: surface La-defective.

Hao et al. designed ultrasmall (2 nm average diameter) Pt and La co-doped IrO₂ nanoparticles (Pt_0.1La_0.1IrO₂@NC) highly dispersed on nitrogen-doped carbon (NC)^[109]. In a 0.5 M H₂SO₄ solution, Pt_0.1La_0.1IrO₂@NC exhibited a record-low OER overpotential of only 205 mV at 10 mA cm^-2, along with excellent long-term stability of 135 h. Theoretical calculations indicated that co-doping Pt and La into the IrO₂ lattice effectively tuned the d-band center and lowered the energy barrier of the rate-determining step (O to OOH*) in the OER pathway. Gan et al. synthesized a La and sulfur co-doped FeOOH/NiOOH multiphase electrocatalyst^[110]. FeOOH in the system reduced the formation energy of NiOOH and enhanced its stability as an active HER site. The formed abundant oxygen vacancies increased the number of active sites, optimized intermediate adsorption, and improved conductivity. Moreover, La and S co-doping modulated the electronic structure of FeOOH. As a result, the resulting catalyst exhibited low overpotentials of 210/450 mV at 100/1,000 mA cm^-2, a small Tafel slope (32 mV dec^-1), and outstanding stability up to 60 h at 1,000 mA cm^-2. Additionally, in 30 wt% KOH at 60 °C, the catalyst achieved excellent OER performance with only 180 mV overpotential at 250 mA cm^-2, maintaining catalytic durability for 135 h at this current density.

Li et al. developed a facile sol-gel method to construct asymmetric Ru-O-La structures within RuO₂, which modulate electron redistribution and suppress Ru over-oxidation^[111]. In acidic media, the optimal La_0.1Ru_0.9O₂ catalyst required only 188 mV to achieve 10 mA cm^-2 [Figure 11F] and exhibited prolonged stability of 63 h at 1.6 V vs. RHE, far surpassing the 8 h stability of standard RuO₂. Combined experimental and theoretical analyses revealed that the Ru-O-La asymmetric structure facilitated electron transfer from La to Ru, increasing the electron density around Ru and preventing over-oxidation. Moreover, this electron redistribution shifted the Ru 4d-band center, thereby optimizing the adsorption and desorption of oxygen intermediates [Figure 11G]. Qin et al. also reported a surface La-defective IrO_x catalyst (La₃IrO₇-SLD) prepared via electrochemical activation, which outperformed commercial IrO₂ and most reported Ir-based catalysts in both OER activity and durability^[112]. As shown in Figure 11H, comparisons of XPS spectra before and after activation revealed a significant increase in surface hydroxylation, correlating with La leaching. After OER testing, the main O 1s XPS peak at 531.6 eV (assigned to Ir-OH species) broadened notably, indicating a surface reconstruction with a high hydroxyl content (71.7% vs. 34.9% in pristine La₃IrO₇), thus exposing more Ir active sites. Meanwhile, the La 3d and 4d peaks were significantly reduced post-OER, confirming La leaching and formation of La-deficient structures. As shown in Figure 11I, La₃IrO₇-SLD achieved 10 mA cm^-2 at an overpotential of only 296 mV, outperforming IrO₂ (316 mV). DFT calculations revealed that the Ir-centered lattice oxygen mechanism (LOM) endowed La₃IrO₇-SLD with superior catalytic activity and stability, as the Ir 5d orbitals are positioned at a more favorable energy level relative to the O 2p orbitals. In contrast, the free energy barrier for *OOH formation via the adsorbate evolution mechanism (AEM) pathway reaches 2.73 eV, which is significantly higher than those of IrO₂(110) and La₃IrO₇ (001). This result is inconsistent with the electrochemical observations, suggesting that La₃IrO₇-SLD is more likely to follow a lattice-oxygen-involved reaction pathway.

As a compelling case study, Zhang et al. demonstrated the industrial relevance of La-doped RuO₂ nanorods grown in situ on titanium mesh (La-RuO₂@TM) as an anode catalyst in a PEMWE setup^[113]. This REE-engineered material exhibited exceptional performance, requiring a cell voltage of only 1.815 V to achieve a high current density of 1 A cm^-2, while maintaining operational stability for over 120 h at 60 °C without significant degradation. Notably, this outperformed both undoped RuO₂@TM and commercial IrO₂ benchmarks under identical conditions, with lower voltages also recorded at 0.5 A cm^-2 (1.692 V) and 2 A cm^-2 (2.051 V). The enhanced durability stems from La doping, which forms a La-O-Ru local structure that modulates intermediate adsorption, mitigates Ru leaching, and reduces lattice oxygen loss, as corroborated by DFT calculations. This example underscores the potential of REE doping to enable PEMWE operations at industrially relevant high current densities, facilitating efficient green hydrogen production at scale. Extending such strategies to AEMWE systems could further leverage the electronic tunability of REEs in alkaline environments, potentially reducing reliance on precious metals and lowering overall system costs for widespread commercialization.

Despite the progress of La doping strategies in reducing reliance on noble metals (e.g., Ir-based systems) and improving performance at industrial-level current densities, large-scale application remains constrained by complex synthesis procedures (e.g., multicomponent precursor control in HEAs, precision interface engineering in heterostructures) and unclear dynamic stability mechanisms (e.g., oxygen vacancy evolution, surface reconstruction). Future research should focus on developing non-noble metal La-based high-entropy systems, employing in-situ characterization techniques to unravel dynamic active site evolution, and exploring green synthesis routes to promote practical application of La-doped catalysts in water electrolytic oxygen production.

Cerium-based

Ce-based catalysts can enhance their OER activity through strategies such as doping, interface engineering, and the construction of heterostructures. Wang et al. reported a mechanism based on interfacial electronic redistribution, in which electron transfer from Ce to Ni and Co in the CeO₂/NiCo₂S₄ heterostructure leads to the stabilization of metal-sulfur (M-S) bonds^[114]. This effect effectively suppresses sulfur leaching during OER. As shown in Figure 12A, the XRD pattern of CeO₂/NiCo₂S₄ corresponds well with the cubic phase CeO₂, respectively, with no impurity peaks observed. This confirms the successful formation of the heterostructure without chemical reactions between phases. Moreover, the well-modulated interface enhances the adsorption affinity toward oxygen intermediates and reduces the energy barrier for OER. Consequently, the CeO₂/NiCo₂S₄ catalyst exhibits outstanding OER performance, delivering ultralow overpotentials of only 146 and 271 mV at 10 and 100 mA cm^-2, respectively [Figure 12B]. More importantly, it demonstrates exceptional durability, maintaining stable performance at 500 mA cm^-2 for over 200 h - surpassing pristine NiCo₂S₄ and most reported TMC-based catalysts. Zhi et al. also proposed a new strategy, introducing a CeO₂ layer onto S-Co(OH)₂ to stabilize catalysts operating via the LOM^[115]. Explicit experimental identification of the switched reaction pathway to LOM in S-Co(OH)₂/CeO₂ has been achieved through pH-dependent OER activity measurements and tetramethylammonium cation (TMA) probe experiments. In situ XPS, Raman spectroscopy, and other studies indicate that CeO₂ acts as an electron buffer during OER, either accepting or releasing electrons at different stages. Specifically, at the initial voltage, CeO₂ extracts electrons from Co(OH)₂, promoting the generation of high-valence Co^δ+ species, thereby accelerating surface reconstruction and forming active CoOOH. With increasing potential, the reconstructed CoOOH evolves into CoO₂ with higher work function, which then extracts electrons from the CeO₂ “reservoir” rather than from M-O bonds, ensuring stability throughout the OER process. Additionally, the introduced S atoms enhance Co-O covalency and trigger the LOM pathway. As a result, the S-Co(OH)₂/CeO₂ hybrid delivers excellent OER performance in 1.0 M KOH. The inter requires only 227 mV to achieve 10 mA cm^-2, and operates stably over 100 h at 1000 mA cm^-2 in a self-assembled anion exchange membrane electrolyzer, demonstrating the pivotal role of the electron reservoir concept in the development of sustainable LOM-based electrocatalysts. Song et al. constructed a RuO₂/CeO₂ heterostructured catalyst via lattice-matched Ru-O-Ce bridges, which can effectively facilitate electron transfer between Ru and Ce species, and further induce lattice strain that distorts the local crystalline structure of RuO₂^[116]. This catalyst demonstrates remarkable stability, with negligible performance degradation after 1,000 h of continuous OER operation in 0.5 M H₂SO₄, while maintaining high activity with an overpotential of only 180 mV at 10 mA cm^-2. In situ attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS), differential electrochemical mass spectrometry (DEMS), and DFT calculations reveal that RuO₂ sites at the interface and non-interface promote the oxide path mechanism (OPM) and the enhanced AEM, respectively. You et al. constructed a 0D/2D heterostructure by decorating SrIrO₃ nanosheets with CeO₂ quantum dots, aiming for durable and efficient OER in strongly acidic conditions^[22]. The fast Fourier transform (FFT) image [Figure 12C] confirms the formation of the CeO₂/SrIrO₃ heterostructure. Theoretical calculations reveal that CeO₂ with electron rearrangement not only reduces the OER energy barrier but also mitigates Sr leaching. Benefiting from the dual role of CeO₂ in activation and stabilization, the CeO₂@SrIrO₃ heterostructure delivers excellent performance with an overpotential of only 238 mV to achieve 10 mA cm^-2, maintaining operation for over 50 h, outperforming most perovskite-based electrocatalysts [Figure 12D].

Figure 12. (A) XRD images of CeO₂/NiCo₂S₄; (B) LSV curves of as-prepared catalysts. This figure is quoted with permission from Wang et al.^[114]; (C) FFT pattern of 4CeO₂@SrIrO₃; (D) LSV curve of the commercial IrO₂, SrIrO₃, 2CeO₂@SrIrO₃, 4CeO₂@SrIrO₃ and 8CeO₂@SrIrO₃ electrocatalysts. This figure is quoted with permission from You et al.^[22]; (E) HRTEM image, the marked area (orange line) presents the defects; (F) LSV curves of RuO₂ and the prepared ZIF-67@Co(OH)₂, Co@Co(OH)₂, Ce-Co(OH)₂ and Ce_0.21@Co(OH)₂ in 1.0 M KOH. This figure is quoted with permission from Zhou et al.^[118]; (G) Mo 3d spectra for Ru₃MoO_x and Ru₃MoCeO_x; (H) Ce_3d spectra for Ru₃CeO_x and Ru₃MoCeO_x. Erbium-based; (I) LSV curves for RuO₂, Ru₃CeO_x, Ru₃MoO_x, Ru₃MoCeO_x, and Ru₃MoCe₂O_x. This figure is quoted with permission from He et al.^[120]. RHE: Reversible hydrogen electrode; XRD: X-ray diffraction; LSV: linear sweep voltammetry; FFT: fast Fourier transform; HRTEM: high-resolution transmission electron microscopy; ZIF: zeolitic imidazolate framework.

Ce-doping could shift the reaction mechanism from the conventional AEM to the LOM, thereby lowering the overall reaction barrier^[117]. Zhou et al. synthesized Ce_0.21-Co(OH)₂ to facilitate rapid electron transfer and improve OER activity and stability^[118]. As shown in Figure 12E, high-resolution TEM (HRTEM) images reveal a lattice spacing of 0.314 nm matching the (111) plane of CeO₂, indicating the embedding of CeO_x into the Co(OH)₂ matrix. The core-shell structure features vertically aligned porous Co(OH)₂ nanosheets that expose abundant active sites while enabling efficient Ce doping. Utilizing a high-pressure microwave-assisted technique, Ce was homogeneously incorporated into the matrix, forming tightly bound Ce_x-Co(OH)₂ with excellent phase dispersion. The interaction between Ce and Co species greatly accelerates surface electron transfer. Additionally, Ce promotes the formation of Co-superoxide intermediates and facilitates O₂ desorption, both of which are crucial for OER rate determination. Linear sweep voltammetry (LSV) curves show an overpotential of only 300 mV at 10 mA cm^-2 [Figure 12F]. He et al. developed Ce-NiO@MoO₂ catalysts using MoO₂ nanorods as supports^[119]. The introduction of Ce triggered the formation of flower-like NiO on the MoO₂ surface. Ce doping induced the generation of abundant high-valence, unsaturated Ni coordination sites as active centers, while tuning the Ni 3d band center upward, thereby enhancing the adsorption of oxygen intermediates and significantly lowering the energy barrier of the rate-determining OH* → O* step. As a result, Ce-NiO@MoO₂Ni₃ catalysts exhibited excellent OER performance, operating stably at 100 mA cm^-2 with a low overpotential of only 313 mV and sustaining over 200 h of continuous operation. This performance is attributed to the abundance of unsaturated Ni active sites and the highly ordered hierarchical structure that facilitates charge and mass transport. He et al. co-doped RuO₂ with Mo and Ce, both featuring relatively large ionic radii, to modulate the local electronic structure of Ru-O bonds, thereby simultaneously enhancing catalytic activity and stability. XPS analysis confirms the successful incorporation of Mo and Ce into RuO₂ [Figure 12G and H]^[120]. The resulting Ru₃MoCeO_x catalyst shows outstanding activity, with an overpotential of only 164 mV at 10 mA cm^-2 and stable performance in acidic medium for over 100 h at 100 mA cm^-2 [Figure 12I]. DFT and microscopic analysis reveal that Mo and Ce dopants effectively modulate the covalency of Ru-O bonds, tune electronic correlations, and lower the reaction barrier from 0.78 eV to 0.60 eV. These features make Ru₃MoCeO_x a promising anode catalyst for PEM electrolyzers. Ni-Fe layered double hydroxides (NiFe-LDHs) are widely regarded as the most active OER catalysts in alkaline media due to their distinctive layered structure and physicochemical properties. Liao et al. proposed a doping strategy to introduce Ce into NiFe-LDH on carbon paper (NiFeCe-LDH@CP), inducing structural disorder and unique lattice distortion^[121]. This distortion increases accessible surface area and generates more oxygen vacancies, accelerating OER via electronic structure modulation and intermediate adsorption optimization. The optimized NiFeCe-LDH@CP exhibits excellent durability over 70 h and achieves a high current density of 100 mA cm^-2 at a low overpotential of only 267 mV, 41 mV lower than that of pristine NiFe-LDH@CP. Theoretical calculations suggest that lattice distortion modulates the electronic structure of Ni at the active sites and lowers the energy barrier, thus significantly enhancing OER activity.

Erbium-based

Pan et al. proposed a strategy to synergistically enhance catalytic activity through erbium (Er) doping, aiming to improve the intrinsic OER activity and stability of Co₃O₄^[122]. As shown in Figure 13A, the Co³⁺/Co²⁺ ratio in 4% Er-doped Co₃O₄ (Er-Co₃O₄) reaches 0.97, significantly higher than the 0.59 observed in undoped Co₃O₄, which positively correlates with OER activity in acidic media. This increase in Co³⁺ content can be attributed to Er doping, which induces structural modification and promotes electronic interactions between Co and Er. The low-cost Er-Co₃O₄ catalyst with 4% Er exhibits notably enhanced performance, delivering an overpotential of 321 ± 5 mV at 10 mA cm^-2 [Figure 13B], along with excellent long-term stability exceeding 250 h. Moreover, Er incorporation into the spinel Co₃O₄ structure introduces structural defects and oxygen vacancies, and markedly elevates the Co³⁺/Co²⁺ ratio. The catalytic activity of this material is strongly influenced by the ratio of octahedrally coordinated Co³⁺ and tetrahedrally coordinated Co²⁺ cations, which constitute the spinel structure. Microkinetic modeling and theoretical calculations further confirm that Er-Co₃O₄ achieves a significantly optimized Gibbs free energy difference (ΔG_O*-ΔG_OH*), bringing its OER activity at target current densities close to the theoretical maximum [Figure 13C]. In addition, Zhu et al. enhanced the OER performance of layered double hydroxides (LDHs) by constructing Er-doped nickel-iron LDH catalysts supported on nickel foam (Er-NiFe-LDH@NF)^[123]. As illustrated in Figure 13D and E, operando Raman spectroscopy revealed that OH^- adsorption at Ni sites was markedly enhanced in Er-NiFe-LDH prior to 450 mV, compared to the undoped catalyst. This is evidenced by the increase in Raman signal intensity, indicating that Er doping facilitated the transformation from Ni-OH to Ni-OOH, thereby accelerating OER kinetics. The optimized Er-NiFe-LDH@NF demonstrated a remarkably low overpotential of just 191 mV to achieve 10 mA cm^-2 [Figure 13F], outperforming its undoped counterpart. Theoretical calculations suggest that Er incorporation optimizes the d-band center of NiFe-LDH, promotes spin crossover of valence electrons, and brings ΔG_*O-ΔG_*OH closer to the peak of the OER volcano plot. This balances the binding strengths of oxygen and hydroxyl intermediates, resulting in superior catalytic performance.

Figure 13. (A) Ratio of Co²⁺: Co³⁺ in Co₃O₄ and 4% Er-Co₃O₄; (B) LSV curves of different catalysts at 1 mV/s scan rate and negative scan after iR correction; (C) Microkinetic OER activity volcano model at 10 mA cm^-2 as a function of GO-GHO. This figure is quoted with permission from Pan et al.^[122]; (D) The in situ Raman spectra of NiFe-LDH@NF and (E) Er-NiFe-LDH@NF for OER in 0.1 M KOH solution; (F) Comparison of the OER activity of catalysts’ polarization curves in 1.0 M KOH solution. This figure is quoted with permission from Zhu et al.^[123]. RHE: Reversible hydrogen electrode; LSV: linear sweep voltammetry; iR: internal resistance; OER: oxygen evolution reaction; GO-GHO: the difference between the Gibbs free energy of O* and OH*; LDH: layered double hydroxide; NF: nickel foam.

Recent progress in REE-doped electrocatalysts for the OER has primarily focused on modulating electronic structures to optimize intermediate adsorption energies and suppress detrimental lattice oxygen participation, which often leads to catalyst degradation in harsh environments. While numerous studies have reported enhanced performance through doping strategies, the field has seen a proliferation of empirical listings of materials without sufficient emphasis on the underlying logical connections between dopant selection, structural modifications, and mechanistic improvements. For instance, doping with high-valence REEs such as Er or La can downshift the O 2p band center relative to the Fermi level, increasing the energy barrier for oxygen vacancy formation and thereby mitigating metal dissolution, which is a critical factor in acidic media. This not only stabilizes the catalyst but also fine-tunes the d-band center of TMs, aligning adsorption free energies closer to the Sabatier optimum for OER intermediates. Building on this framework, Hao et al. demonstrated the efficacy of co-doping W and Er into RuO₂, where charge redistribution lowers the O 2p band center from -3.31 eV to -4.12 eV, raising the oxygen vacancy formation energy from 0.67 eV to 2.29 eV^[124]. This electronic tuning inhibits lattice oxygen oxidation, preventing the formation of soluble Ru^x>4 species and enabling a low overpotential of 168 mV at 10 mA cm^-2 with exceptional stability of 500 h in 0.5 M H₂SO₄. Furthermore, such mechanisms provide a blueprint for PEMWE applications, where the catalyst sustained 120 h at 100 mA cm^-2 without structural collapse, highlighting how electronic modulation translates to practical durability under high-current conditions.

Other rare earth-based catalysts

In addition to La, Ce, and Er, researchers have also explored the use of other REEs to enhance OER performance. Wang et al. developed a series of uniquely doped lanthanide-based nickel-iron layered double hydroxide (NiFe-LDH) nanosheet arrays - namely, NiFeSm-LDH, NiFeCe-LDH, and NiFeLa-LDH - as high-performance electrocatalysts for the OER^[125]. Notably, NiFeSm-LDH exhibited outstanding performance, with the lowest overpotential of only 203 mV at a current density of 10 mA cm^-2 [Figure 14A]. Structural analyses, in situ Raman spectroscopy, and DFT calculations revealed that the superior catalytic activity arises from a synergistic functionality [Figure 14B and C]. Firstly, due to the lanthanide contraction effect, samarium (Sm) in NiFeSm-LDH can attract more electrons from the outer 3d orbitals of Ni compared to cerium (Ce) in NiFeCe-LDH and lanthanum (La) in NiFeLa-LDH. This leads to a higher Ni valence state and an elevated d-band center. Secondly, in situ Raman spectroscopy shows that the overpotential required to form the NiOOH phase on NiFeSm-LDH is lower (~ 1.32 V) than that on NiFeCe-LDH (1.32-1.37 V) and NiFeLa-LDH (~ 1.37 V), indicating faster reaction kinetics for the Ni²⁺ to Ni³⁺ transition in NiFeSm-LDH. Additionally, Wang et al. proposed a sacrificial template-induced nanostructure regulation strategy to fabricate mesoporous perovskite oxide nanosheets (MPONs) with controllable atomic doping [Figure 14D]^[126]. Among them, Eu-doped LaFeO₃ mesoporous perovskite nanosheets exhibited a low overpotential of 267 mV at 10 mA cm^-2, outperforming most reported perovskite oxides [Figure 14E]. The excellent catalytic activity of Eu-LaFeO₃ MPONs was attributed to their highly porous structure, which increases the density of active sites and enhances lattice oxygen participation, thereby improving intrinsic activity. Wu et al. also found that doping trivalent metals such as gadolinium (Gd), neodymium (Nd), and praseodymium (Pr) into rutile IrO₂ significantly enhanced its OER activity under acidic conditions. This enhancement is due to the increased Ir-O covalency^[127]. In contrast, doping with higher-valent metals (e.g., +5 or above) weakens the Ir-O covalency and deteriorates OER activity. As shown in the XRD patterns of Figure 14F, A-IrO_2-γ exhibits a pure rutile phase. Experimental and theoretical analysis indicates that enhanced Ir-O covalency activates lattice oxygen and facilitates a LOM, thereby accelerating OER kinetics. pH-dependent OER activities were conducted for prepared catalysts, revealing that LOM electrocatalysts exhibit a pH-dependent effect. This is further supported by the linear relationship between the natural logarithm of intrinsic activity and Ir-O covalency described by charge transfer energy. By tuning the Ir-O covalency, the Gd-doped IrO_2-σ catalyst achieves an overpotential of only 260 mV at 10 mA cm^-2, and demonstrates impressive stability over 200 h in 0.5 M H₂SO₄ solution [Figure 14G]. Due to their highly tunable electrochemical properties and excellent reactivity, high-entropy materials represent a promising class of next-generation water-splitting catalysts. Zhang et al. successfully prepared a novel high-entropy pyrochlore, (Y_0.2Ho_0.2Dy_0.2Gd_0.2Pr_0.2)₂Ru₂O₇ (HE-YRO), by equimolar co-doping of REEs - holmium (Ho), dysprosium (Dy), praseodymium (Pr), and gadolinium (Gd) - into the A-site of Y₂Ru₂O₇^[128]. As shown in Figure 14H, TEM-EDS mapping confirmed the homogeneous distribution of Ru, Y, Ho, Dy, Gd, and Pr, with atomic ratios of approximately 1: 0.18: 0.22: 0.21: 0.22: 0.23, closely matching the nominal stoichiometry, indicating the formation of a stable high-entropy phase. This catalyst displayed an ultra-low overpotential (η₁₀ = 200 mV) and no observable degradation during stability testing at 10 mA cm^-2 [Figure 14I]. Through comparative analysis of pyrochlore oxides, several features contributing to the high OER performance of HE-YRO were identified. First, compared with YRO, HE-YRO contains more low-valent Ru³⁺ species and oxygen vacancies, which enhance activity. Second, lattice distortion suppresses grain coarsening, resulting in a larger surface area and more accessible active sites for OER. These synergistic effects induced by equimolar multi-element doping at the A-site endow HE-YRO with excellent performance and chemical durability for acidic water oxidation. There are countless reports on rare earth-based OER catalysts. We have listed some representative catalysts and summarized them in Table 2. An intuitive comparative illustration is also presented in Figure 15.

Figure 14. (A) CV curves in O₂-saturated 1 M KOH solution at a scan rate of 2 mV/s with 85% iR correction; (B) AEM and (C) LOM pathways for NiFe-LDH, NiFeSm-LDH, NiFeCe-LDH and NiFeLa-LDH. This figure is quoted with permission from Wang et al.^[125]; (D) TEM image from the LaFeO₃ MPONs; (E) LSV polarization curves of LaFeO₃ particles and LaFeO₃ MPONs. This figure is quoted with permission from Wang et al.^[126]; (F) XRD patterns of A-IrO_2-𝛿 (A = Gd, Pr, Nd, Mo, W) and HM-IrO₂; (G) OER performance of the prepared electrocatalysts in 0.5 M H₂SO₄ LSV curves. This figure is quoted with permission from Wu et al.^[127]; (H) EDS spectrum of HE-YRO. The signals of Cu and C in the spectrum originated from the copper grid coated by carbon membrane; (I) Polarization curves of HE-YRO and YRO catalysts. This figure is quoted with permission from Zhang et al.^[128]. RHE: Reversible hydrogen electrode; CV: cyclic voltammetry; iR: internal resistance; AEM: adsorbate evolution mechanism; LOM: lattice oxygen mechanism; LDH: layered double hydroxide; TEM: transmission electron microscopy; MPON: mesoporous perovskite oxide nanosheet; LSV: linear sweep voltammetry; XRD: X-ray diffraction; OER: oxygen evolution reaction; HM: home-made; EDS: energy-dispersive X-ray spectroscopy. HE-YRO: high-entropy (Y_0.2Ho_0.2Dy_0.2Gd_0.2Pr_0.2)₂Ru₂O₇.

Figure 15. Comparison of OER electrocatalytic performance across different REE types. TM: transition metal; SLD: surface La-defective; NC: nitrogen-doped carbon; LDH: layered double hydroxide; NF: nickel foam; LSFN: La and S co-doped nickel-iron-based multiphase catalyst; OER: oxygen evolution reaction.

Overall, the electrochemical stability of rare-earth oxides plays a decisive role in determining their practical applicability in OER systems. Pourbaix analysis indicates that La₂O₃ is stable mainly under alkaline conditions but prone to dissolution in acidic media, whereas CeO₂ exhibits superior robustness across a wider pH range owing to its fluorite structure and reversible Ce⁴⁺/Ce³⁺ redox couple that enables oxygen-vacancy formation without bulk degradation. Er₂O₃ demonstrates improved stability in neutral to alkaline environments due to enhanced lattice binding from lanthanide contraction, yet may still undergo partial oxidation or reconstruction at high anodic potentials. To precisely assess and mitigate such degradation, inductively coupled plasma mass spectrometry (ICP-MS) tracking of dissolved REE and TM species during durability testing is strongly recommended. Moreover, as carbon supports are susceptible to anodic corrosion during OER, the adoption of non-carbon supports (such as TiO₂ or conductive oxides) or REE-oxide surface functionalization offers an effective pathway to enhance structural stability and prolong catalyst lifetime.