Lanthanide-Driven Electronic and Defect Engineering in Spinel Co₃O₄: Unraveling the Structure–Activity Synergy for Bifunctional Oxygen Electrocatalysis

Abstract

Highlights

• Citric acid-assisted sol–gel method is adopted to synthesize a series of lanthanide-doped Co₃O₄ catalysts.

• Sm-Co₃O₄ delivers the smallest potential gap and dominates the high-efficiency 4-electron ORR pathway.

• Sm doping triggers lattice distortion and abundant oxygen vacancies, elevating the Co²⁺/Co³⁺ ratio to 0.59.

• Lanthanide ion doping can simultaneously tune electronic structure and lattice defects of spinel Co₃O₄ to enhance oxygen electrocatalytic activity.

• A structure–activity relationship linking lattice defects, Co valence states and catalytic performance is established.

Abstract

The sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at air cathodes severely restrict the practical application of rechargeable zinc–air batteries (ZABs). Herein, equimolar lanthanide-doped spinel Co₃O₄ bifunctional electrocatalysts were synthesized via a citric acid-assisted sol–gel method. Among Ce-, Pr-, La-, and Sm-doped catalysts, Sm-Co₃O₄ exhibits the optimal electrocatalytic performance with a high ORR half-wave potential of 0.72 V and superior OER activity with an overpotential of 1.65 V at 10 mA cm⁻², achieving a minimal potential gap ΔE of 0.93 V. Rotating ring-disk electrode (RRDE) measurements and Koutecky–Levich (K–L) analyses confirm the exclusive 4e⁻ ORR pathway. Characterizations reveal that Sm doping modulates the electronic structure and lattice distortion of Co₃O₄, raises the Co²⁺/Co³⁺ ratio (0.59) and creates abundant oxygen vacancies, thereby significantly lowering the overpotentials for both ORR and OER. This study provides new insights for designing high-performance spinel-based bifunctional electrocatalysts for ZABs.

1. Introduction

The excessive exploitation of fossil fuels has precipitated a dual crisis of energy supply and ecosystem degradation, posing severe constraints on global sustainable development [1,2]. Consequently, the exploration of clean, renewable energy systems and the innovation of efficient energy storage and conversion technologies have emerged as pivotal research frontiers and core objectives of national carbon-neutrality strategies [3]. Among emerging electrochemical energy storage technologies, metal–air batteries (MABs) exhibit theoretical energy densities substantially surpassing those of lithium-ion batteries, coupled with intrinsic safety and cost competitiveness, rendering them one of the most promising candidates [4]. In particular, zinc–air batteries (ZABs) utilize earth-abundant zinc metal as the anode, enabling stable charge/discharge cycling in alkaline electrolytes with remarkable economic viability and safety profiles [5,6]. Additionally, ZABs possess a high theoretical gravimetric energy density (1086 W h kg⁻¹) and a flat operating voltage plateau, offering substantial potential for large-scale energy storage and portable power applications. Nevertheless, the research advancement and commercialization of ZABs have been persistently hindered by a critical bottleneck: insufficient oxygen electrocatalytic performance at the air cathode [7]. The charge and discharge processes of ZABs rely on two pivotal oxygen electrocatalytic reactions at the cathode: the ORR during discharge and the OER during charge. Both reactions involve multi-electron transfer with complex mechanistic steps, exhibiting intrinsically sluggish kinetics and substantial overpotentials [8]. Therefore, the development of highly efficient bifunctional catalysts is imperative to accelerate reaction kinetics, mitigate polarization losses, and thereby enhance energy conversion efficiency and cycling durability.

Spinel-type Co₃O₄ has attracted considerable attention as a promising catalytic material for ORR and OER owing to its abundance and superior redox properties [9,10]. The catalytic performance of spinel Co₃O₄ predominantly originates from the coexistence of Co²⁺ and Co³⁺ within the spinel framework. Specifically, Co²⁺ with moderate electron-donating capability facilitates O₂ adsorption and activation, thereby improving ORR activity, whereas Co³⁺ has been demonstrated to correlate with OER performance [11]. However, pristine Co₃O₄ is often constrained by inadequate electrical conductivity and insufficient active site density, resulting in catalytic performance far below theoretical expectations. Elemental doping represents an effective strategy for modulating the electronic structure and defect chemistry of metal oxides, with lanthanide doping garnering increasing attention in recent years. The distinctive properties of lanthanides stem from their progressively filled 4f orbitals, which are spatially confined within the atom and shielded by fully occupied 5s² and 5p⁶ orbitals, rendering the 4f electrons highly localized and minimally involved in chemical bonding [12]. Beyond the characteristic +3 oxidation state, certain lanthanides can stabilize alternative valence states, including +2 (Sm, Eu, Yb) and +4 (Ce, Pr, Tb). This flexible multivalence renders them excellent media for electron storage and transfer [13]. Furthermore, lanthanide ions typically exhibit relatively large ionic radii, and their incorporation into host metal oxide lattices induces significant lattice distortion, serving as a critical driving force for oxygen vacancy formation [14]. Such oxygen vacancies are essential for adsorbing reaction intermediates and regulating the local electronic environment. On the electronic level, the unique 4f orbital of lanthanide ions generates strong 4f–2p–3d orbital hybridization with Co–O units, which adjusts the d-band center of cobalt active sites to achieve moderate adsorption strength for all oxygen-containing reaction intermediates, avoiding the excessively strong or weak binding that restricts reaction kinetics [15]. Structurally, lanthanide substitution introduces controllable lattice strain and abundant oxygen vacancies, which provide extra unsaturated active sites and accelerate the rapid transfer of electrons and electrolytes [16,17,18]. Meanwhile, the reversible valence conversion of lanthanide ions can dynamically regulate the Co²⁺/Co³⁺ redox balance during cyclic oxygen electrocatalysis, effectively inhibiting structural collapse and active site degradation [19,20]. The above electronic modulation and structural optimization synergistically accelerate the reaction kinetics of both ORR and OER, thereby significantly improving the bifunctional catalytic activity of the lanthanide-doped Co₃O₄ electrocatalyst. Moreover, coupled reversible Sm²⁺/Sm³⁺ and Co²⁺/Co³⁺ redox pairs accelerate interfacial electron transport to overcome the poor conductivity of pristine Co₃O₄, while the robust integrated RE–Co–O lattice inhibits structural degradation during long cycling in Zn–air batteries for sustained catalytic performance [19]. In studies of La-doped Co₃O₄ catalysts, La³⁺ substitution for octahedral-site Co³⁺ has been shown to induce lattice distortion, alter the local chemical environment, and enhance oxygen vacancy concentration, synergistically improving redox performance, oxygen species activity, and reaction intermediate conversion efficiency [21].

In this work, a citric acid-assisted sol–gel method was employed to synthesize spinel-type Co₃O₄ catalysts doped with equimolar ratio of Ce, Pr, La, or Sm, respectively. The effects of different lanthanide dopants on the ORR and OER catalytic activities of Co₃O₄ were systematically compared, and the catalysts were evaluated as cathode materials for zinc–air batteries. The intrinsic origins of the enhanced catalytic activities were elucidated through comprehensive characterization, investigating the phase composition, morphology, and specific surface area, with particular emphasis on oxygen vacancy content and the Co²⁺/Co³⁺ ratio.

2. Materials and Methods

Ce-, Pr-, La-, and Sm-doped Co₃O₄ catalysts were separately fabricated via a citric acid-assisted sol–gel method. Briefly, 1.92 mmol Co (NO₃)₂·6H₂O (Aladdin Reagents Co., Ltd., Shanghai, China) and 4 mmol citric acid (Aladdin Reagents Co., Ltd.) were dissolved in 2.5 mL deionized water with the addition of 0.08 mmol Ce (NO₃)₃·6H₂O (Adamas Reagent Co., Ltd., Shanghai, China), Pr (NO₃)₃·6H₂O, (Adamas Reagent Co., Ltd.) La (NO₃)₃·6H₂O (Macklin Biochemical Co., Ltd., Shanghai, China) or Sm (NO₃)₃·6H₂O (Adamas Reagent Co., Ltd.) (nominal molar ratio of M/(M + Co) = 1%), respectively. The mixed solution was magnetically stirred at room temperature for 30 min to obtain a homogeneous precursor solution. Subsequently, the above solution was heated to 80 °C under continuous stirring and kept in an 80 °C water bath until a viscous gel was produced. The as-obtained gel was dried overnight in an oven at 120 °C. The brick-red porous dried product was fully ground into fine powder, which was then transferred to a muffle furnace and calcined at 500 °C for 4 h with a heating rate of 2 °C min⁻¹. The final black powders were denoted as M-Co₃O₄ (M = Ce, Pr, La, Sm) corresponding to various lanthanide dopants. For comparison, pure undoped Co₃O₄ was synthesized using identical procedures except that only 2 mmol Co (NO₃)₂·6H₂O was used, which served as the reference catalyst. Additional details of characterizations and electrocatalytic measurements are provided in the Supplementary Materials.

3. Results

3.1. Characterization Results

The morphological features of the Co₃O₄ and Sm-doped Co₃O₄ catalysts were characterized by FE-SEM. As displayed in Figure 1a, pristine Co₃O₄ exhibits bulky block-like morphology with smooth surfaces. Such a microstructure facilitates the exposure of highly active crystal facets and the construction of continuous conductive networks, thereby accelerating electron transport. Notably, compared with bare Co₃O₄ (Figure 1b), the particle size of Sm-Co₃O₄ is remarkably reduced (Figure 1c). Although slight nanoparticle aggregation can be observed visually, the refined primary grains form abundant intergranular mesopores within the aggregates rather than dense stacking. The diminished grain size enlarges the specific surface area, exposes abundant accessible catalytic active sites, and ultimately boosts electrocatalytic performance.

Figure 1.

SEM images of (a,b) Co₃O₄ and (c) Sm-Co₃O₄.

Figure 2a displays the XRD patterns of Co₃O₄ and M-Co₃O₄ series catalysts. All catalysts exhibit consistent characteristic diffraction peaks, which can be unambiguously assigned to the (111), (220), (311), (222), (400), (422), (511), (440) and (533) crystal planes of spinel-type Co₃O₄ (JCPDS PDF#98-000-0166). No diffraction peaks corresponding to other metal oxides are detected in M-Co₃O₄, revealing that Mⁿ⁺ may enter the Co₃O₄ lattice, replace cobalt ions within the lattice or occupy interstitial sites, without generating separate impurity oxide phases [22]. Figure 2b presents the magnified view of the (311) characteristic diffraction peak ranging from 35° to 39° 2θ for intuitive comparison. Evidently, all rare-earth doped samples show obvious peak broadening (increased FWHM) and slight peak shift toward higher 2θ angles relative to pure Co₃O₄. Among La-, Ce-, Pr- and Sm-doped samples, Sm-Co₃O₄ delivers the largest positive offset of the (311) peak. The shift to higher diffraction angles indicates reduced interplanar spacing caused by lattice compressive strain, while peak broadening confirms grain refinement after rare-earth substitution. In accordance with the Scherrer equation, their grain sizes tend to shrink and crystallinity declines, which is consistent with the SEM results.

Figure 2.

(a) XRD patterns of Co₃O₄ and M-Co₃O₄ catalysts in the 2θ ranges of 10~90° (b) and 35~39° and (c) Raman spectra of Co₃O₄ and M-Co₃O₄ catalysts.

Raman spectroscopy was employed to characterize the local chemical environment and defect structure of each catalyst, as displayed in Figure 2c. Co₃O₄ exhibits five characteristic peaks at approximately 195, 479, 520, 617 and 687 cm⁻¹, which are assigned to the ${\mathrm{F}}_{2\mathrm{g}}^{\left(1\right)}$, E_g, ${\mathrm{F}}_{2\mathrm{g}}^{\left(2\right)}$, ${\mathrm{F}}_{2\mathrm{g}}^{\left(3\right)}$ and A_1g vibrational modes of the spinel structure, respectively [23,24]. Among them, the ${\mathrm{F}}_{2\mathrm{g}}^{\left(1\right)}$ and E_g modes are associated with the bending vibration of O–Co–O at tetrahedral sites (Co²⁺–O²⁻), while the other three modes correspond to the stretching vibration of Co–O at octahedral sites (Co²⁺–O²⁻) [14]. The M-Co₃O₄ catalysts also display analogous characteristic peaks corresponding to these five vibrational modes. Combined with the XRD results (Figure 2a), it can be verified that lanthanide metal doping does not disrupt the fundamental spinel structure of Co₃O₄. Notably, all characteristic peaks of the M-Co₃O₄ catalysts show obvious broadening and red shifts compared with pure Co₃O₄. This phenomenon can be attributed to local lattice expansion and elongated bond lengths induced by the incorporation of lanthanide metal ions with larger radii, which weakens Co–O bonds and reduces their force constants [25]. Weakened Co–O bonds facilitate the generation of oxygen vacancies, and abundant oxygen vacancies are regarded as a critical factor for enhancing ORR/OER catalytic activity [26]. Peak broadening indicates an increased density of lattice defects in the catalyst, which favors the formation of more oxygen vacancies and other catalytic active sites [27]. The Sm-Co₃O₄ catalyst presents the most prominent red shift and broadening of the A_1g peak, signifying that it possesses the highest concentration of lattice defects and the weakest Co–O bonds.

Nitrogen physisorption measurements were conducted on the M-Co₃O₄ catalysts to investigate their specific surface area and pore structure characteristics. As shown in Figure 3a, the adsorption–desorption isotherms of all catalysts exhibit Type-IV features with distinct H3 hysteresis loops in the medium-to-high relative pressure (p/p₀) range, indicating that the catalysts possess typical mesoporous structures dominated by slit pores and intergranular mesopores formed by particle stacking [28]. The closure point of the hysteresis loop for the Sm-Co₃O₄ catalyst shifts toward a lower relative pressure, accompanied by a narrower pore size distribution (Figure 3b). This demonstrates that Sm doping optimizes the porous structure of the catalyst and generates mesopores with a narrower pore size [29]. As summarized in Table S1, all rare-earth doping treatments boost the specific surface area and total pore volume of Co₃O₄ while reducing average pore size, among which Sm-Co₃O₄ displays the optimal porous parameters. Such dramatic porosity modulation of Sm dopant is unique compared with La, Pr and Ce in sol–gel synthesis. Owing to the moderate ionic radius of Sm³⁺, Sm substitution introduces the strongest homogeneous lattice strain to suppress high-temperature grain sintering. Meanwhile, Sm³⁺ forms stable uniform citrate gel precursors, which decompose synchronously during calcination to generate abundant tiny intergranular mesopores. Excessively large La³⁺ and Ce³⁺ cause precursor instability and rare-earth segregation, while Pr³⁺ induces weaker lattice distortion. Benefiting from the above dual effects, Sm-Co₃O₄ obtains the maximum specific surface area (67.2 m² g⁻¹), highest pore volume (0.19 cm³ g⁻¹) and minimum pore diameter (10.2 nm), providing richer exposed active sites and faster mass transfer channels for ORR/OER.

Figure 3.

(a) N₂ adsorption–desorption isotherms and (b) pore-size distributions of Co₃O₄ and M-Co₃O₄ catalysts; (c) O₂-TPD profiles of Co₃O₄ and M-Co₃O₄ and (d) enlarged view.

The surface oxygen species of M-Co₃O₄ catalysts were characterized via O₂-TPD (Figure 3c). The desorption temperature and peak intensity collectively reflect the type, relative content and binding strength of surface oxygen species on catalysts, and such information is critical for uncovering the origin of catalytic activity toward ORR and OER. The low-temperature desorption peaks (Figure 3d) within the temperature range of 200–400 °C are attributed to weakly adsorbed superoxide species (${\mathrm{O}}_2^{-}$) and peroxide species (O⁻), which are closely associated with ORR activity [30,31]. Notably, the desorption peaks of M-Co₃O₄ emerge at lower temperatures in this temperature range, demonstrating that doping endows surface oxygen species with higher intrinsic activity and better activation ability, which facilitates the enhancement of ORR activity. The desorption peaks ranging from 400 to 600 °C correspond to surface lattice oxygen (O²⁻) strongly bound to Co³⁺ sites, and this species is correlated with OER catalytic activity [32,33]. M-Co₃O₄ catalysts exhibit intensified desorption peaks and enlarged peak areas in this temperature window, revealing that appropriate lanthanide metal doping can drastically boost the content of surface oxygen species. The intense peak at high temperatures (ca. 800 °C) originates from the desorption of bulk lattice oxygen, a process possibly accompanied by partial decomposition of metal oxides (Figure 3c). It can be observed that M-Co₃O₄ catalysts deliver stronger desorption signal with peak positions also shifting toward lower temperatures in this region, which further verifies the weakened Co–O bonds and improved overall reducibility of the catalysts. These findings indicate that the lattice oxygen migration capacity of theses catalysts is enhanced beneficially from the generation of abundant oxygen vacancies [30]. In summary, O₂-TPD results confirm that M-Co₃O₄ delivers favorable conditions for outstanding bifunctional catalytic performance by modulating the content and reactivity of surface oxygen species, among which Sm-Co₃O₄ achieves the optimal modification effect.

The wide-scan XPS spectra in Figure S1 verify that characteristic peaks assignable to Ce, Pr, La and Sm are detected over Ce-Co₃O₄, Pr-Co₃O₄, La-Co₃O₄ and Sm-Co₃O₄ apart from C, O and Co signals, respectively, demonstrating the successful incorporation of each lanthanide dopant. Figure 4a,b and Figure S2a–c displayed the deconvoluted O 1s XPS spectra of pristine Co₃O₄ and M-doped Co₃O₄ catalysts. Three distinct peaks located at 529.2, 530.5 and 531.9 eV can be resolved, which correspond to lattice oxygen (O_L), oxygen vacancies (O_V) and chemisorbed oxygen species (O_C), respectively [34]. Quantitative analysis in Figure 4c reveals that the Sm-Co₃O₄ catalyst delivers the highest proportion of surface oxygen vacancies. Moreover, all M-Co₃O₄ composites exhibit larger relative oxygen vacancy concentrations than bare Co₃O₄. This result indicates that rational lanthanide doping efficiently modulates surface chemical environments and generates abundant active oxygen vacancies and reactive oxygen species, which exert favorable impacts on both ORR and OER. Figure 4d,e and Figure S2d–f present the Co 2p XPS spectra of pristine Co₃O₄ and M-doped Co₃O₄ catalysts. Two dominant characteristic peaks appear at approximately 779.8 eV and 795.1 eV, which are assigned to the spin–orbit split Co 2p_3/2 and Co 2p_1/2 orbitals, respectively [35]. Peak deconvolution identifies two sets of doublets corresponding to Co²⁺ (781.3 and 796.5 eV) and Co³⁺ (779.6 and 794.7 eV), alongside two satellite shake-up peaks [36]. Quantitative analysis in Figure 4f reveals that all lanthanide-doped M-Co₃O₄ catalysts possess higher Co²⁺/Co³⁺ ratios than bare Co₃O₄. The Co²⁺/Co³⁺ values follow the descending order: Sm-Co₃O₄ (0.59) > Pr-Co₃O₄ (0.51) > La-Co₃O₄ (0.46) > Ce-Co₃O₄ (0.43) > Co₃O₄ (0.38), which matches the electrocatalytic activity trends. These observations confirm that lanthanide heteroatom doping facilitates the partial reduction of surface Co³⁺ to Co²⁺. Previous literature has documented that elevated Co²⁺ concentration serves multiple beneficial functions for bifunctional oxygen catalysis. Co²⁺ centers act as primary active sites for ORR, facilitating O₂ adsorption and cleavage of O=O bonds to boost the selectivity of the four-electron ORR pathway. Meanwhile, abundant Co²⁺ also creates a flexible redox environment favorable for OER [37].

Figure 4.

O 1s XPS spectra of (a) Co₃O₄ and (b) Sm-Co₃O₄; (c) O_V/O_L+O_V+O_C ratio; Co 2p XPS spectra of (d) Co₃O₄ and (e) Sm-Co₃O₄; (f) Co²⁺/Co³⁺ ratio.

Figure 5a displays the Ce 3d XPS spectrum of the Ce-Co₃O₄ catalyst, which can be divided into two main envelopes assigned to Ce 3d_3/2 and Ce 3d_5/2. After peak deconvolution, the two peaks located at 885.4 and 902.8 eV are attributed to Ce³⁺, while the other five peaks correspond to Ce⁴⁺ [38,39,40]. Figure 5b presents the Pr 3d XPS spectrum of the Pr-Co₃O₄ catalyst. Two primary peaks at 933.4 and 953.0 eV originate from Pr 3d_5/2 and Pr 3d_3/2, accompanied by their corresponding satellite peaks at 928.3 and 949.4 eV. These results reveal that Pr species coexist in both oxidation states of +3 and +4 [41]. The La 3d XPS spectrum of the La-Co₃O₄ catalyst is illustrated in Figure 5c. Two deconvoluted peaks at binding energies of 834.7 and 851.5 eV are characteristic of La³⁺. The peaks at 838.2 and 854.9 eV are assigned to the satellite peaks of La 3d_5/2 and La 3d_3/2, respectively [21,42]. Figure 5d shows the Sm 3d XPS spectrum of the Sm-Co₃O₄ catalyst, where two dominant peaks emerge at 1083.3 and 1110.5 eV, corresponding to Sm 3d_5/2 and Sm 3d_3/2 [19,43,44]. The Sm 3d_5/2 signal is centered at approximately 1083.3 eV, with a spin–orbit splitting energy of ΔE = 27.2 eV. The broad and asymmetric profiles of these two peaks verify the presence of Sm³⁺ [45,46,47]. The radii of lanthanide Mⁿ⁺ ions in M-Co₃O₄ catalysts are all larger than those of Co²⁺ and Co³⁺. Benefiting from moderate ionic size matching, Mⁿ⁺ cations readily substitute for tetrahedral Co²⁺ sites within the Co₃O₄ lattice. The incorporation of large-radius dopant ions into the crystal lattice induces local lattice strain, which can be relieved via the generation of abundant oxygen vacancies [48]. Meanwhile, doping with high-valence metal cations introduces positive charge defects. To maintain overall electrical neutrality of the material system, partial reduction of Co³⁺ to lower-valence Co²⁺ is triggered [49]. This synergistic process elevates the relative Co²⁺/Co³⁺ ratio and simultaneously creates a higher concentration of oxygen vacancies [50].

Figure 5.

(a) Ce 3d XPS spectrum of Ce-Co₃O₄; (b) Pr 3d XPS spectrum of Pr-Co₃O₄; (c) La 3d XPS spectrum of La-Co₃O₄; and (d) Sm 3d XPS spectrum of Sm-Co₃O₄.

Collectively, lanthanide heteroatom doping endows Co₃O₄ with synergistic electronic and spin modulation effects verified by systematic characterizations and previous theoretical reports. On the electronic level, 4f–2p–3d orbital hybridization redistributes surface charge, elevates the Co²⁺/Co³⁺ ratio and tunes the d-band center for moderate oxygen intermediate adsorption; lattice strain weakens Co–O bonds and generates abundant oxygen vacancies, simultaneously improving intrinsic conductivity and active site density. In terms of spin regulation, lanthanide-induced lattice distortion breaks octahedral crystal field symmetry and triggers spin polarization of Co 3d orbitals to lower the energy barriers of ORR/OER rate-determining steps. Among La, Ce, Pr and Sm dopants, Sm³⁺ delivers the strongest orbital coupling and spin polarization due to the optimal degree of ionic radius mismatch, which provides structural and electronic theoretical foundations for the subsequent electrochemical analysis of various rare-earth modified catalysts.

3.2. Electrocatalytic Performance for ORR and OER

The ORR and OER electrocatalytic performances of all catalysts are displayed in Figure 1. Figure 6a shows the ORR polarization curves of pristine Co₃O₄ and M-Co₃O₄ catalysts. Among all, Sm-Co₃O₄ delivers the highest half-wave potential, while Ce-, Pr-, and La-doped counterparts also exhibit markedly elevated half-wave potentials compared with undoped Co₃O₄. Figure 6b presents the LSV curves recorded during the OER process. At a current density of 10 mA cm⁻², Sm-Co₃O₄ possesses the lowest overpotential, and all doped catalysts outperform pure Co₃O₄. These results reveal that appropriate lanthanide doping can synergistically optimize both ORR and OER catalytic activity of Co₃O₄, while Sm doping exhibits the optimal modification effect. To intuitively evaluate the bifunctional electrocatalytic activity, the potential gap ΔE (ΔE = E₁₀ − E_1/2) was calculated, where E₁₀ refers to the OER potential at 10 mA cm⁻² and E_1/2 represents the ORR half-wave potential. The corresponding histogram of ΔE values is plotted in Figure 6c. A smaller ΔE signifies superior bifunctional catalytic performance. Sm-Co₃O₄ simultaneously achieves the maximum E_1/2 of 0.72 V, the minimum E₁₀ of 1.65 V, and the smallest ΔE of 0.93 V, directly verifying its outstanding bifunctional ORR/OER electrocatalytic activity.

Figure 6.

(a) LSV curves of ORR over Co₃O₄ and M-Co₃O₄ in oxygen-saturated electrolyte at 1600 rpm; (b) OER polarization curves of Co₃O₄ and M-Co₃O₄; (c) potential difference (ΔE).

To uncover the origin of enhanced ORR activity over lanthanide-doped catalysts, rotating ring-disk electrode (RRDE) measurements were carried out for all catalysts. As displayed in Figure 7a, the ring current for ${\mathrm{H}\mathrm{O}}_2^{-}$ oxidation is markedly lower than the disk current, revealing that the ORR preferentially proceeds via an efficient 4e⁻ pathway to produce OH⁻ rather than a 2e⁻ pathway generating ${\mathrm{H}\mathrm{O}}_2^{-}$ [51]. As illustrated in Figure 7b, all M-Co₃O₄ catalysts deliver electron transfer numbers close to 4.0 together with extremely low ${\mathrm{H}\mathrm{O}}_2^{-}$ yields, substantially outperforming pristine Co₃O₄. This demonstrates that appropriate lanthanide doping greatly boosts the catalyst selectivity toward the desirable 4e⁻ ORR pathway. The previous literature has verified that the 4e⁻ pathway possesses superior energy efficiency and faster reaction kinetics [52,53]. Additionally, this pathway avoids the accumulation of detrimental ${\mathrm{H}\mathrm{O}}_2^{-}$ intermediates, which further facilitates the ORR process.

Figure 7.

(a) RRDE curves and (b) electron transfer number (n) and the relative concentration of peroxides intermediate (${\mathrm{H}\mathrm{O}}_2^{-}$) over the Co₃O₄ and M-Co₃O₄ catalysts.

Figure 8a,c display the LSV curves of Co₃O₄ and Sm-Co₃O₄ recorded at various RDE rotation speeds, respectively. The limiting diffusion current density rises with the increasing rotation speed. This phenomenon originates from the shortened oxygen diffusion distance for ORR at higher rotation rates, which accelerates the mass transport. As presented in Figure 8b,d, the Koutecky–Levich (K–L) plots at different potentials exhibit well-defined linear and nearly parallel features, confirming first-order reaction kinetics with O₂ diffusion as the mass-transfer-limited step [36,54]. Calculated from the classic K-L equation, the average electron transfer number of Sm-Co₃O₄ reaches 3.98, higher than that of Co₃O₄ (3.67), which matches the RRDE results.

Figure 8.

LSV curves of (a) Co₃O₄ and (c) Sm-Co₃O₄ in oxygen-saturated 0.1 mol L⁻¹ KOH at different rotation rates, K-L plots of (b) Co₃O₄ and (d) Sm-Co₃O₄ at different potentials ranging from 0.30 to 0.50 V vs. RHE.

Charge transfer resistance (R_ct) was acquired via Electrochemical impedance spectroscopy (EIS) measurements. As shown in Figure 9a,b, Sm-Co₃O₄ delivers the smallest R_ct values for both ORR and OER, which are 82.44 Ω and 32.59 Ω, respectively. This reveals its favorable electrical conductivity and rapid faradaic reaction kinetics, thereby endowing superior electrocatalytic activity. The outstanding conductivity of Sm-Co₃O₄ accelerates electron transfer across the electrode–electrolyte interface. In addition, the Tafel slope serves as a critical indicator to evaluate ORR and OER kinetics. As depicted in Figure 9c,d, Sm-Co₃O₄ exhibits much lower Tafel slopes (52.64 mV dec⁻¹ for ORR, 91.25 mV dec⁻¹ for OER) in comparison with pristine Co₃O₄ (131.39 mV dec⁻¹ for ORR, 185.20 mV dec⁻¹ for OER). A smaller Tafel slope corresponds to faster reaction kinetics, demonstrating that Sm-Co₃O₄ possesses the optimal catalytic efficiency toward both ORR and OER. The ORR stability of the catalyst was assessed by comparing the LSV curves acquired before and after 2000 CV cycles (Figure 10a,b). These results reveal that the Sm-Co₃O₄ catalyst exhibits a negligible negative shift in E_1/2 of only ca. 11.9 mV, which is significantly smaller than Co₃O₄ (42.4 mV). In terms of OER performance, the overpotential of Sm-Co₃O₄ only positively shifts by 25.1 mV after 2000 CV cycles (Figure 10c), while pure Co₃O₄ suffers a much larger positive shift of 45.3 mV (Figure 10d). These contrastive results fully demonstrate the superior bifunctional cycling durability of Sm-doped Co₃O₄. Collectively, the ORR and OER results demonstrate that the Sm-Co₃O₄ catalyst exhibits an excellent bifunctionally electrocatalytic activity, outperforming the pristine Co₃O₄.

Figure 9.

EIS spectra of (a) ORR and (b) OER over Co₃O₄ and M-Co₃O₄; Tafel plots derived from the LSV curves of (c) ORR and (d) OER over Co₃O₄ and M-Co₃O₄.

Figure 10.

ORR-LSV curves of (a) Sm-Co₃O₄ and (b) Co₃O₄ before and after 2000 CV cycles, OER-LSV curves of (c) Sm-Co₃O₄ and (d) Co₃O₄ before and after 2000 CV cycles.

4. Discussion

In this study, equimolar Ce-, Pr-, La-, and Sm-doped spinel Co₃O₄ bifunctional oxygen electrocatalysts were separately fabricated via a citric acid-assisted sol–gel route. Electrochemical measurements demonstrate that lanthanide metal doping synchronously enhances the ORR and OER catalytic activities of the bare Co₃O₄, with Sm-Co₃O₄ delivering the optimal overall performance. Sm doping strengthens the selectivity of the four-electron ORR pathway, accelerates interfacial electron transport, and optimizes the kinetics of oxygen electrocatalysis. Physicochemical characterizations reveal the structure–activity mechanism regulated by doping, where Sm doping refines grain size and constructs uniform mesoporous structures to enrich exposed active sites. The lattice substitution of large-radius lanthanide ions generates lattice strain, weakens Co–O bonds and increases defect density, thus inducing abundant oxygen vacancies that act as intrinsic active sites for ORR and OER. Meanwhile, lattice positive charge defects promote the conversion of Co³⁺ to Co²⁺ and raise the surface Co²⁺/Co³⁺ ratio. This heteroatom doping strategy enables precise regulation of lattice defects, oxygen vacancy concentration and Co valence distribution in spinel Co₃O₄, offering a universal design guideline for developing high-performance spinel-based bifunctional oxygen electrocatalysts.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/ma19153188/s1. Figure S1: Wide-scan XPS spectra of Co₃O₄ and M-Co₃O₄; Figure S2: O 1s XPS spectra of (a) Ce-Co₃O₄, (b) Pr-Co₃O₄ and (c) La-Co₃O₄, Co 2p XPS spectra of (d) Ce-Co₃O₄, (e) Pr-Co₃O₄ and (f) La-Co₃O₄; Table S1: Textural properties of the catalyst derived from the nitrogen physisorption measurements.

Author Contributions

Conceptualization, T.C.; methodology, T.C.; validation, T.C.; formal analysis, T.C., H.C. and J.L.; resources, C.Z.; data curation, T.C.; writing—original draft preparation, T.C.; writing—review and editing, C.Z.; supervision, C.Z. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding Statement

This work was financially supported by the Natural Science Foundation of Shandong province (ZR2025MS164) and the National Natural Science Foundation of China (No. 22076088).