Hollow carbon nanoreactors integrating NiFe-LDH nanodots with adjacent La single atoms for efficient oxygen electrocatalytic reactions

Abstract

Optimizing both mass transport and electronic structure of the active component is of interest to obtain electrocatalysts with superior oxygen evolution reaction (OER) performance. Here, we miniaturized the classical NiFe-layered double hydroxides (NiFe-LDHs) and integrated them into S/N co-doped hollow hierarchical porous carbon (SNHPC) loaded with rare earth La single atoms (La SAs) to obtain nanoreactors. The unique carbon framework induced uniform deposition of LDH nanodots and ensured adequate exposure during electrocatalysis. The advantages of the carbon carrier for the local electric field and interfacial OH⁻ layer density in the catalytic process were confirmed by finite element simulations. The well-designed NiFe-LDH@La SNHPC exhibited satisfactory activity (overpotential of 251 mV at 10 mA cm⁻²) and stability in alkaline media, exceeding those of commercial RuO₂. Impressively, a cathode catalyst combining NiFe-LDH@La SNHPC with Pt/C can be stabilized in rechargeable zinc–air batteries (ZABs) for more than 350 h. Theoretical calculations indicated that the introduction of La SAs modified the electronic structures of the NiFe-LDH nanodots, activated lattice oxygen activity, optimized the adsorption strength of the intermediates, and reduced rate-determining step energy barriers in OER. This study provides guidance for the preparation and design of sub-microreactors and information on the strong electron interaction effects induced by rare earth species.

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New concepts

This work introduces a carbon nanoreactor integrating miniaturized NiFe-layered double hydroxides (LDH) and rare-earth La single atoms, specifically designed for efficient electrocatalytic oxygen evolution reaction (OER). Unlike conventional rare-earth doping in LDH, the stepwise integration strategy avoids potential structural distortions in the host matrix caused by direct La doping while enabling effective electronic interactions between the two components. This concept highlights the intrinsic contributions of the unique electronic structure of rare-earth elements, thereby circumventing additional structural strain effects. Combining experimental and theoretical approaches, the study provides detailed insights into the localized coexistence of La single atoms and NiFe-LDH nanodots and their crucial role in reducing the energy barriers of OER. Additionally, the research validates the potential of the nanoreactor's abundant mesoporous and cavity structures in enhancing the mass transport of multiple reactants and products during OER. This new concept provides a promising strategy for the rational design of high-performance LDH catalysts using rare-earth elements and will further inspire the research and application of nano-framework materials in the field of green energy.

1. Introduction

The electrocatalytic oxygen evolution reaction (OER) is an important half-reaction in zinc–air batteries (ZABs) and overall water splitting and has an irreplaceable role in a variety of sustainable electrochemical energy conversion systems.^1,2 Unfortunately, OER reactions with four-electron transfer processes exhibit sluggish reaction kinetics and high theoretical overpotentials, greatly hindering the efficiency of the associated electrochemical devices.^3–5 A variety of electrocatalysts, especially the facilely prepared LDHs, are outstanding candidates due to their remarkable inherent catalytic activity toward OER.^6–8 Although various strategies, such as hetero-element doping, anion exchange, and construction of composite heterostructures, have been employed to further enhance the intrinsic activity of LDH and have led to significant progress, their low conductivity, limited exposure of active sites and self-aggregation tendency still need to be optimized.^9,10

Integration with carbon has been shown to positively impact LDHs. Porous carbon enriched with heteroatoms (N, O, etc.) was used to further improve their electrical conductivity and induce high exposure.^11–13 Notably, OER occurring at the gas–liquid–solid three-phase interface involves diffusion and mass transfer processes of multiple reactants, intermediates, and products.^14,15 Therefore, rational design of doped multilayer carbon nanostructures with porous shell layers and internal spaces is crucial. Achieving LDHs with anchored active components and exposed sites is beneficial for realizing electrolyte permeation and creating abundant mass diffusion pathways.^16,17 This is a very promising strategy for maximizing the utilization of LDH sites and improving the catalytic performance.

Furthermore, doping additional guest metal species to induce more flexible electronic structures in LDHs to modulate local charge transfer is an effective way to enhance their intrinsic activity.^18–20 The strong electronic interactions between the host metal ions in the LDH framework and the guest metal species have been shown to manipulate the d-band center of LDHs, resulting in near-optimal binding energies of oxygenated intermediates during OER processes.²¹ Among many alternative guest metal species, rare earth (RE) elements with varying degrees of 4f orbital electron filling have received much attention.^22–25 The unique localized 4f orbitals of RE species offer more possibilities to modulate the electronic structure of the active component by overlapping and hybridizing with the orbitals.^26,27 However, the introduction of RE species into the LDH lattice is prone to induce additional unavoidable structural strain effects, which are caused by the large ionic size of rare earths.^28–30 The LDH-related structural changes may inevitably interfere with the intrinsic contribution of electronic effects. Therefore, the rational construction of well-defined and easily investigated RE-LDH hybridization systems remains challenging.

Taking inspiration from the considerations mentioned above, we stepwise integrated ultra-miniaturized NiFe-LDH nanodots and atomically dispersed RE sites (La-N₄ motifs) into a S-doped hierarchical porous hollow carbon framework to construct a nanoreactor that can be used for efficient OER. The carbon support was obtained by simple one-step pyrolysis of zeolitic imidazolate framework-8 (ZIF-8) with melamine. Its hollow cavity, enriched mesopores, and highly open-space structure can facilitate the utilization of active sites and electrolyte mass transfer in OER processes. The effectiveness of the designed nanoreactor was demonstrated by finite element method (FEM) simulations. Furthermore, theoretical calculations revealed the significance of additional S sites in the substrate in inducing uniform deposition of NiFe-LDH nanodots and enhancing the overall charge transfer efficiency of the nanoreactor. More importantly, the stepwise integration strategy avoided the possible structural distortion caused by the direct doping of NiFe-LDH with La because of its ionic radius, while achieving the effective electronic interactions between the two. These rational designs will improve electrocatalytic performance of the nanoreactor for OER. The final sample, NiFe-LDH@La SNHPC, required only 251 mV overpotential to reach a current density of 10 mA cm⁻² in 1.0 M KOH medium, exceeding the performances of commercial RuO₂ catalysts and a series of comparison samples. In addition, its reliable electrochemical activity and long-term cyclability of more than 350 h were verified in rechargeable ZABs.

2. Results and discussion

2.1 Synthesis and characterization

As schematically outlined in Fig. 1a, the specific synthesis of NiFe-LDH@La SNHPC involved three steps of carbon carrier design, La SAs loading, and assembly of NiFe-LDH nanodots. First, classical ZIF-8 nanocrystals were prepared and subsequently subjected to high-temperature carbonization (1100 °C) with the assistance of melamine. As shown in the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images in Fig. S1 (ESI†), N-doped hollow porous carbon (NHPC) spheres with wrinkled surfaces of about 300 nm in diameter were successfully obtained thanks to the corrosive effect of gases, such as ammonia, generated by melamine at high temperature. In contrast, the NC obtained by merely pyrolyzing the ZIF-8 precursor still maintains its original ortho-dodecahedral morphology, except for the obvious shrinkage in size. The X-ray diffraction (XRD) patterns of both observed diffraction peaks are attributed to typical graphitic carbon (002) and (101) only at ∼24° and ∼44° (Fig. S2, ESI†).^31,32

Fig. 1 (a) Schematic illustration of the synthesis route to NiFe-LDH@La SNHPC. (b) XRD patterns for different samples. (c) TEM image of NiFe-LDH@La SNHPC at low magnification. (d) Dark-field TEM image and corresponding EDS elemental mapping of NiFe-LDH@La SNHPC.

Further in-depth studies were carried out to characterize the porosity of two carbon matrices with different morphologies using N₂ adsorption–desorption measurements. As shown in Fig. S3a and c (ESI†), NC displayed a typical type-I isotherm with sharply increased nitrogen uptake at relatively low relative pressure, indicating the dominant microporosity.^33,34 In contrast, NHPC revealed a type-IV isotherm with a well-defined hysteresis loop at a relative pressure of 0.6 < P/P₀ < 1.0 and an abrupt increase in adsorption volume at low relative pressures (P/P₀ < 0.02), implying the simultaneous existence of micropores and mesopores.¹⁷ Further, pore size distribution curves revealed the hierarchical porous structure of NHPC (Fig. S3d, ESI†), with mesopores concentrated at 4.1 nm and 13.3 nm. NC is dominated by a microporous structure with only a few mesoporous structures (Fig. S3b, ESI†). Even though NHPC has a smaller Brunauer–Emmett–Teller specific surface area (679.9 m² g⁻¹) than NC (1147.9 m² g⁻¹), its unique accessible hierarchical porous structure both provided space for the subsequent anchoring and domain limitation of the NiFe-LDH nanodots and facilitates the exposure of more active sites in the subsequent electrocatalytic process.^9,35 Moreover, it is worth noting that the large specific surface area and porous hollow structure of the nanoreactor may facilitate the rapid transport and removal of the gaseous products of the OER at the three-phase interface. On the one hand, the abundant hierarchical porous structure can serve as a channel for gas diffusion, helping O₂ bubbles to diffuse to the electrode surface and reducing the aggregation of bubbles on the catalyst surface. On the other hand, the larger specific surface area also helps to disperse the bubbles on the catalyst surface, reducing the probability of bubble coalescence and making it easier for bubbles to be removed from the catalyst surface.^36–38 Raman spectra of NHPC and NC samples were used to reveal the degree of defects and graphitization (Fig. S4, ESI†). Two main peaks are shown at 1351 cm⁻¹ and 1585 cm⁻¹, attributed to D-band (defective carbon) and G-band (graphitic sp² carbon) features, respectively. The degree of defects was estimated from the integrated intensity (I_D/I_G) of the D- and G-bands.^39–41 NHPC and NC exhibited close I_D/I_G values, implying the presence of abundant defective carbon in both, which provided effective anchoring sites for the adsorption of Ni²⁺/Fe³⁺ during the subsequent growth of NiFe-LDH nanodots.¹¹ The relatively high I_D/I_G values of NHPC may be related to more carbon defects caused by corrosive gases from melamine pyrolysis.⁴² The specificity of the structure of the NHPC sample motivated us to use it as a framework for a nanoreactor in subsequent procedures.

In the next stage, the La SAs were immobilized on NHPC by simple La³⁺ adsorption, secondary pyrolysis, and acid leaching treatments. This is facilitated by the evaporation of low-boiling Zn species in ZIF-8 during the initial carbonation process, leaving abundant of N sites for subsequent anchoring of additional metals.^43,44 Notably, inspired by theoretical calculations, we introduced additional heteroatoms, S, into the process. The density of states (DOS) of La SNHPC demonstrated the presence of more electronic states near the Fermi level, which facilitates the electron transfer during the reaction processes and enhances its catalytic activity (Fig. S5, ESI†).^45–47 Detailed structural characterization implies a successful homogeneous loading of atomically dispersed La on the SNHPC skeleton, with additional metal aggregates not detected (Fig. S6–S10, ESI†). The presence of S species did not adversely affect the formation of La SAs.

Finally, NiFe-LDH nanodots were then grown in situ from solution on the La SNHPC to obtain the ultimate nanoreactor (NiFe-LDH@La SNHPC). Detailed preparation processes are described in the ESI† Experimental section. The samples containing NiFe-LDH nanodots exhibited distinct diffraction peaks located at 11°, 23°, 35°, and 60°, which are attributed to (003), (006), (012), and (110) planes of hydrotalcite-like NiFe-LDH (Fig. 1b and Fig. S11, ESI†), respectively.⁹ The TEM image and dark-field TEM image showed that the nanoreactor retained the inherent hollow morphology of the precursor and was observed with distinct bright spots that were never observed in NHPC and La SNHPC (Fig. 1c and d). These uniformly dispersed bright spots in the substrate can be referred to as NiFe-LDH nanodots, further validating the feasibility of our carefully designed synergistic strategy of hierarchical pore restriction and S-site-assisted deposition. Upon comparing the TEM images in Fig. S12 and S13 (ESI†), significant aggregation of LDH nanodots was observed in the sample without S. In contrast, the growth of NiFe-LDH nanodots on the S-containing carbon substrate (SNHPC) was widely and homogeneously dispersed compared to that on NHPC. The S-assisted deposition was further supported by theoretical calculations (Fig. S14, ESI†), which showed binding energies between NiFe-LDH and the substrates with and without S. The results indicated that introducing S leads to a stronger interaction between NiFe-LDH and the substrate, effectively inhibiting the aggregation of NiFe-LDH and yielding more dispersed and homogeneous products. The energy dispersive X-ray spectroscopy (EDS) mapping exhibited a uniform distribution of Ni, Fe, La, S, O, and N on the carbon framework, implying that both NiFe-LDH nanodots and La SAs are highly dispersed in the carbon-based nanoreactor (Fig. 1d). The microscopic coexistence of the two was further observed by aberration-corrected high angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM). Irregular nanoparticles of about 5–10 nm can be observed in Fig. 2a (outlined with green dashed lines). The lattice fringes with an interplanar spacing of 0.24 nm were ascribed to the (006) facet of NiFe-LDH (bottom of Fig. 2b). Additionally, abundant La single atoms (outlined with yellow dashed circles) were observed around the NiFe-LDH nanodots, which exhibit larger sizes and higher brightness (top of Fig. 2b). In addition to coexisting with each other in the plane depending on the carbon substrate (region 3 in Fig. 2a), NiFe-LDH nanodots and La SAs were found to be in direct spatial contact (region 4 in Fig. 2a). This may be attributed to the fact that the nucleation and growth of LDH nanodots on the carbon support is stochastic. The corresponding 3D fitting plot more visually illustrates the coexistence of the two previously mentioned states (Fig. 2c). The La sites with higher signal strength are marked with arrows in the figure. The localized effective coexistence state of the two species was further strongly evidenced in the EDS mapping images at higher magnification (Fig. 2d). Furthermore, Fig. S15 (ESI†) shows the microstructure of NiFe-LDH@SNHPC without La, where only the presence of NiFe-LDH nanodots was observed. The content of metals Ni, Fe, and La in NiFe-LDH@La SNHPC were determined to be 15.2 wt%, 4.7 wt%, and 0.5 wt%, respectively, by inductively coupled plasma-optical emission spectroscopy analysis (ICP-OES).

Fig. 2 (a) AC-HAADF-STEM image of NiFe-LDH@La SNHPC. (b) and (c) Integrated pixel intensity profiles (b) and 3D fitting map (c) of different regions marked in Fig. 2a. (d) AC-HAADF-STEM image and the corresponding EDS elemental mapping of NiFe-LDH@La SNHPC at high magnification. (e) La L₃-edge Fourier transformation of the EXAFS spectra in R space for NiFe-LDH@La SNHPC and La₂O₃. (f) La L₃-edge EXAFS fitting curves of the NiFe-LDH@La SNHPC. Inset: Proposed architecture. The blue, gray, red, and yellow balls represent N, C, La, and S atoms, respectively. (g) and (h) High-resolution XPS spectra of Ni 2p (g) and Fe 2p (h) for different samples.

To further affirm the local coordination environments of La sites in NiFe-LDH@La SNHPC catalysts, the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were exploited and analysed. The La L₃-edge XANES spectra (Fig. S16, ESI†) verified that the valence of La single atoms was close to that of La₂O₃ (+3). Fig. 2e shows the Fourier transformation (FT)-EXAFS (R space) for the La L₃-edge of NiFe-LDH@La SNHPC and La₂O₃ references. The La–N bonds were located at 2.1 Å and no La–La bond (3.7 Å) was observed in NiFe-LDH@La SNHPC, further verifying the atomic dispersion of La, consistent with the results of the AC-HAADF STEM. The wavelet transform EXAFS (WT-EXAFS) contour plots of NiFe-LDH@La SNHPC exhibited a major peak near 3.0 Å⁻¹, unlike the La₂O₃, providing further evidence that La is atomically dispersed and coordinated to the light element N (Fig. S17, ESI†). According to the detailed fitting results (Table S1 and Fig. S18, ESI†), a single La atom was effectively anchored by four N atoms in NiFe-LDH@La SNHPC to form the La–N₄ motifs (inset of Fig. 2f). There was no significant La–Ni/Fe bonding.

X-ray photoelectron spectroscopy (XPS) was used to further analyze the surface chemical state of the sample and possible electronic interactions. The chemical states of C, N, O, S, and La in NiFe-LDH@La SNHPC are explained in Fig. S19 (ESI†). The C 1s (Fig. S19a, ESI†) spectrum can be split into five configurations containing C–S (284.4 eV), sp²-C (284.8 eV), sp²-C (285.5 eV), C–N (286.6 eV), and C–O (288.8 eV), confirming the S doping.⁴⁸ The N 1s spectrum (Fig. S19b, ESI†) can also be deconvoluted into four types of N species including pyridinic N (398.1 eV), pyrrolic N (399.3 eV), graphitic N (400.5 eV), and oxidized-N (402.2 eV).³⁹ The O 1s spectrum (Fig. S19c, ESI†) shows that the strong peak at 531.1 eV in NiFe-LDH@La SNHPC represents the metal–hydroxide (M–OH) bond in NiFe-LDH.¹⁸ Furthermore, the S 2p spectra (Fig. S19d, ESI†) can be attributed to the S 2p_3/2 (163.2 eV), S 2p_1/2 (164.5 eV), and sulfate species (C–SO_x).⁴⁹ The lack of characteristic metal–S bond peaks implied that the foreign S atoms tend to bind to the carbon substrate rather than directly to the La atoms in the center, which is consistent with the EXAFS results. Notably, due to the lower content of La and the encapsulation effect of the carbon carrier, it was difficult to accurately identify the oxidation state of the La atoms using the attenuated photoelectron signal (Fig. S19e, ESI†). Focusing on the influence of the La sites on the electronic structure of Ni and Fe in LDH nanodots, the detailed fitting results indicated that Ni species were mainly present in the NiFe-LDH nanodots as Ni²⁺, while the trivalent cation positions were mainly contributed to by Fe species (Fig. 2g and h).^9,50–52 The Ni 2p and Fe 2p peaks in NiFe-LDH@La SNHPC were positively shifted by 0.3 and 0.5 eV, respectively, compared to NiFe-LDH@SNHPC, suggesting that electrons were transferred from NiFe-LDH nanodots to La SAs, by a notable amount. This suggested that based on the two coexisting states of La SAs and NiFe-LDH nanodots mentioned earlier, the charge transfer between the two contributes to increasing the oxidation state of Ni and Fe, thus enhancing their intrinsic activity towards OER.^18,53 The calculated charge density difference plots of the NiFe-LDH@La SNHPC heterostructure (Fig. S20, ESI†) show that the charge accumulates on the La SNHPC side and is depleted on the NiFe-LDH side, which indicated spontaneous electron migration from the NiFe-LDH side to the substrate. Specifically, based on the Bader charge analysis, NiFe lost 0.013 e⁻ per atom after the introduction of La sites and results in the increase of oxidation states, which agrees well with the XPS analysis.

2.2 Electrocatalytic evaluation

Electrochemical OER measurements were carried out in a three-electrode configuration in a 1.0 M KOH aqueous solution for the obtained samples, using commercial RuO₂ catalysts as comparisons. The linear scanning voltammetry (LSV) curves of each sample after iR compensation are presented. It can be seen that the OER activity of La SNHPC or La NHPC alone was almost negligible (Fig. S21a, ESI†). However, the samples containing the NiFe-LDH nanodots component showed significant OER catalytic activity (Fig. 3a). This proved that NiFe-LDH nanodots were effective active sites in this material system. The overpotential was defined as the difference between the potentials at different current densities and the equilibrium potential (1.23 V). As shown in Fig. 3b, compared to NiFe-LDH@NHPC, NiFe-LDH@La NHPC in the presence of La has a significantly lower overpotential at a current density of 10 mA cm⁻², which should be attributed to the possible electronic interactions between NiFe-LDH and La SAs. The introduction of S in the carbon framework contributed to a further reduction of the overpotential. NiFe-LDH@La SNHPC required only a 251 mV overpotential to provide a current density of 10 mA cm⁻², exceeding the performance of commercial RuO₂ catalysts. As discussed previously, this may be related to the fact that S enriched the orbital distribution of the system at the Fermi energy level, which in turn improves the charge transfer efficiency. The advantage of the overpotential persisted at higher current densities and was more pronounced compared to commercial RuO₂ catalysts The activity-enhancing effect of S on the catalyst as a whole was also demonstrated in La SAs-free samples (Fig. S21b, ESI†). Based on the LSV curves, we can obtain the corresponding Tafel curves to describe the reaction kinetics of the catalyst. From Fig. 3c, the Tafel slope for NiFe-LDH@La SNHPC was 53.2 mV dec⁻¹, which was close to that of NiFe-LDH@La NHPC (54.5 mV dec⁻¹) and much smaller than those of NiFe-LDH@NHPC (67.5 mV dec⁻¹) and RuO₂ (102.3 mV dec⁻¹). Apparently, the performance of NiFe-LDH@La SNHPC in an alkaline medium was advanced among non-precious metal-based OER electrocatalysts (Fig. 3f and Table S2, ESI†). The solid NC substrate obtained by direct carbonization of ZIF-8 was also used for loading La SAs and NiFe-LDH nanodots in an equivalent process. It exhibited the same physical and morphological structure as NiFe-LDH@La SNHPC, except for differences in the carriers (Fig. S22, ESI†). However, NiFe-LDH@La SNC had a higher overpotential and a significantly larger Tafel slope (Fig. S23, ESI†). This proved that careful design of the carbon framework with a hierarchical pore-cavity structure favoring electrochemical mass transfer was necessary to ensure the reactivity and reaction kinetics of the catalysts.

Fig. 3 (a) LSV curves of OER. (b) Overpotentials. (c) Tafel plots. (d) The electrostatic field distribution near the channels of NiFe-LDH@La SNHPC (left) and NiFe-LDH@La SNC (right). (e) The OH⁻ concentration distributions near the channels of NiFe-LDH@La SNHPC (left) and NiFe-LDH@La SNC (right). (f) OER performance comparison with other previously reported NiFe-based catalysts. (g) i–t curves for NiFe-LDH@La SNHPC and commercial RuO₂ (the inset shows the percentage of current retained for NiFe-LDH@La SNHPC).

Further finite element analysis, based on the morphology and dimensions of the catalyst, showed that NiFe-LDH@La SNHPC exhibited significantly better localized OH⁻ enrichment and electrostatic field strength enhancement in the catalyst microenvironment than NiFe-LDH@La SNC with a single microporous solid structure. This improvement can be attributed to its unique cavity and hierarchical pore structure (Fig. 3d and e). The higher electrostatic field strength promoted the adsorption of water molecules on the catalyst surface and further enhanced the local electron transport, which directly benefited the OER kinetics.^54,55 In addition, the accumulated OH⁻ can effectively alleviate the delayed replenishment of the heavily consumed OH⁻ reactants during the OER process.^56–58 It is worth emphasizing that the gas produced after the OER can be transported through the channels inside the nanoreactor and the intermediate chamber, and the mass transfer process can be further accelerated.

The double-layer capacitance (C_dl) calculated from cyclic voltammetry (CV) curves at different scan rates in the non-Faraday current region was positively correlated with the electrochemical active surface area (ECSA), which was an important indicator of the actual number of active centers exposed to the electrocatalyst and represented the inherent activity level of the catalyst (Fig. S24, ESI†).^59–61 The C_dl value for NiFe-LDH@La SNHPC is 3.25 mF cm⁻², which is higher than those for NiFe-LDH@La NHPC (2.53 mF cm⁻²) and NiFe-LDH@NHPC (1.38 mF cm⁻²), implying that NiFe-LDH@La SNHPC has a larger ECSA. A larger ECSA was thought to be favorable for providing an abundance of more exposed active sites in the OER and higher intrinsic activity.²⁸ The charge transfer process of the electrode reaction was further evaluated using electrochemical impedance spectroscopy (EIS). The radius of curvature was directly proportional to the charge transfer resistance and inversely proportional to the charge transfer capacity in the low-frequency portion of Fig. S25 (ESI†). The NiFe-LDH@La SNHPC catalyst also exhibited the lowest values in these samples, supporting its efficient charge transfer capacity.⁵⁵ The durability of the catalyst was also evaluated. After running the chronoamperometric i–t measurement at a current density of 50 mA cm⁻² for up to 30 h, NiFe-LDH@La SNHPC still retained 91.1% of the initial current, which was significantly better than RuO₂ (Fig. 3g). It also shows only 25.6% performance loss at a higher current density 100 mA cm⁻² (Fig. S26, ESI†). The LSV curves before and after the 2000 CV cycles in Fig. S27 (ESI†) also showed only slight changes from each other, indicating excellent OER durability in alkaline solutions. In addition, its original morphological characteristics and uniform elemental distribution were maintained after the stability test, and no obvious structural collapse and active component aggregation were observed (Fig. S28 and S29, ESI†). XPS spectra were recorded after 2000 CV cycles to assess the elemental electronic states (Fig. S30, ESI†). Ni 2p high-resolution spectra exhibited characteristic Ni³⁺ peaks that differed from those of the pristine catalysts, indicating that the surface metal Ni species had been rearranged and oxidized during the OER process. The peak positions of the Fe 2p XPS spectra showed little shift after cycling. In addition, the fact that the changes in the elements contained in the carbon framework could not be observed implied overall structural stability during the catalytic process.

Inspired by the excellent OER performance of NiFe-LDH@La SNHPC, a two-electrode electrolyzer is assembled using commercial 20% Pt/C and the NiFe-LDH@La SNHPC sample as the cathode and anode, respectively, in 1.0 M KOH for the overall water splitting. The NiFe-LDH@La SNHPC‖Pt/C couple achieved a prominent water splitting performance with low cell voltages of 1.55 and 1.73 V at 10 and 50 mA cm⁻², respectively, lower than that of RuO₂‖Pt/C (1.58 V@10 mA cm⁻², 1.84 V@50 mA cm⁻²) (Fig. S31a, ESI†). Multi-step chronopotentiometry tests showed that NiFe-LDH@La SNHPC at different current densities exhibits lower voltages than commercial RuO₂. In contrast to the obvious current density decay of RuO₂, the change for NiFe-LDH@La SNHPC was minimal (Fig. S31b, ESI†). Furthermore, its good durability was also reflected in the stable operation for up to 30 h at a current density of 50 mA cm⁻² (Fig. S31c, ESI†).

The performance of NiFe-LDH@La SNHPC in a rechargeable ZAB was also evaluated. Prior to this, the oxygen reduction reaction (ORR) performance of NiFe-LDH@La SNHPC in 0.1 M KOH was investigated and commercial 20% Pt/C was used as a comparison sample. The LSV curves showed that NiFe-LDH@La SNHPC possessed a half-wave potential (E_1/2) of 0.76 V (Fig. S32a, ESI†), lower than that of Pt/C (0.85 V). The Koutecky–Levich (K–L) plots obtained from the LSV curves of the sample at different rotation rates showed that the ORR reaction proceeded by a four-electron process (Fig. S32b, ESI†). The ORR stability was also subsequently assessed (Fig. S32c, ESI†).

2.3 Device evaluation

Limited by the undesirable ORR activity, NiFe-LDH@La SNHPC was mixed with 20% Pt/C as a ZAB cathode catalyst with RuO₂ + Pt/C as a reference (Fig. 4a and Fig. S33, ESI†). The NiFe-LDH@La SNHPC + Pt/C-based ZAB showed an open-circuit voltage (OCV) of 1.43 V (Fig. 4b), which was significantly better than the ZAB assembled with Pt/C + RuO₂ (1.38 V). This may stem from the inherent ORR activity of NiFe-LDH@La SNHPC itself as well as the facilitating effect of high porosity and large cavities on the dispersion and exposure of active components of the commercial Pt/C catalysts after mixing. From the polarization curves in Fig. 4c, it can be seen that the ZAB driven by NiFe-LDH@La SNHPC + Pt/C possessed a good charging and discharging performance, and a smaller potential gap with respect to RuO₂ + Pt/C was observed throughout the voltage window. In addition, the peak power density of the ZAB assembled using NiFe-LDH@La SNHPC + Pt/C catalysts, calculated from the discharge polarization curves was 168.3 mW cm⁻² (Fig. 4d). Finally, the long-term rechargeability of both the ZABs was tested by continuous constant-current discharge–charge at 5 mA cm⁻². As shown in Fig. 4e and f, the NiFe-LDH@La SNHPC + Pt/C-based ZAB can be operated stably for more than 350 h and maintained a higher round-trip efficiency and narrower charge/discharge potential gap than the RuO₂ + Pt/C-based ZAB without significant degradation throughout the cycle. The above results supported the potential application of NiFe-LDH@La SNHPC in energy storage and conversion systems.

Fig. 4 (a) Schematic of a rechargeable ZAB. Open-circuit voltage (b), charge and discharge polarization curves (c), discharge polarization curves and power density plots (d), galvanostatic discharge/charge cycling performance (e) and corresponding voltage efficiency graphs (f) for ZAB-Pt/C + NiFe-LDH@La SNHPC and ZAB-Pt/C + RuO₂.

2.4 Theoretical calculation

To gather further mechanistic insights, we conducted theoretical calculations of the OER reaction pathways and electronic structure of NiFe-LDH@La SNHPC. First, we considered various OER pathways, including the adsorbate evolution mechanism (AEM), lattice oxygen mechanism (LOM), O–OH, and O–O coupling pathways, as depicted in Fig. S34 (ESI†). After examining the energetics and optimized structures for NiFe-LDH@La SNHPC, we found that the AEM pathway is unlikely to occur (Fig. S35, ESI†), and the O–OH and O–O coupling pathways exhibited relatively high energy barriers of 0.83 and 0.71 eV, respectively (Fig. S36 and S37, ESI†). Thus, the LOM pathway appeared more favorable and dominated the OER process. The lattice oxygen activity was a critical factor in the LOM pathway, which can be elucidated through electronic structure analysis. The projected density of states (PDOS) profiles for NiFe-LDH@SNHPC and NiFe-LDH@La SNHPC are presented in Fig. 5a and b. The O p-band centers (ε_Op) are calculated to be −1.67 eV for NiFe-LDH@SNHPC and −1.59 eV for NiFe-LDH@La SNHPC (Fig. 5c), indicating a positive shift toward the Fermi level upon the introduction of La sites. The position of the O p-band center serves as a crucial descriptor for oxygen activity. A higher energy level facilitates electron transfer through oxygen sites under anodic potential and promotes the release of oxygen from the lattice and the formation of oxygen vacancies.^62–66 In addition to the O p-band center, the Mott–Hubbard splitting in the d-orbitals was studied. Due to the strong d–d coulomb interactions, the NiFe d-orbitals are split into a lower Hubbard band (LHB) filled with electrons and an empty upper Hubbard band (UHB). The center positions of these bands are determined by the distribution of metal d-orbitals below and above the Fermi level in the DOS profiles.^62,66–68 As illustrated in Fig. 5d, the energy difference between the centers of the LHB and UHB was defined as ΔU and increased from 6.27 eV to 6.38 eV following the introduction of La, resulting in an upward shift of the UHB from 2.65 to 2.68 eV and a downshift of the LHB from −3.62 to −3.70 eV. The downward shift of the LHB center, along with the upward shift of the O p-band center leads to reduced overlap between NiFe d-orbitals and O p-orbitals, resulting in a weaker metal–oxygen bond. This should probably be attributed to the interactions between atomically dispersed La sites and local microscopic electrons of NiFe-LDH nanodots. This enhanced oxygen activity promotes the formation of oxygen vacancies. The results of crystal orbital Hamilton populations (COHP) analysis (Fig. 5e and f and Fig. S38, ESI†) were consistent with the band alignments, demonstrating a less negative value of the integrated COHP for Ni–O and Fe–O bonds in NiFe-LDH@La SNHPC, indicating greater anti-bonding character and weaker metal–oxygen bonding strength. Furthermore, the reaction energy trends of OER were compared between NiFe-LDH@La SNHPC and NiFe-LDH@SNHPC. Fig. 5g and h provide a schematic illustration and free energy diagram of the LOM pathway, revealing that NiFe-LDH@La SNHPC exhibited weaker intermediate binding and a lower energy barrier (0.27 eV) for the rate-determining step (RDS) compared to NiFe-LDH@SNHPC (0.38 eV). The above results imply a multidimensional role of La SAs, which effectively coexist locally with the NiFe-LDH nanodot active component with the help of carbon nanocage carriers, for optimizing the local electronic structure, increasing the oxygen activity, weakening the metal–oxygen bonding, and lowering the reaction energy barriers. These findings demonstrate the essential role of rare earth sites in enhancing the catalytic activity.

Fig. 5 Projected density of states of (a) NiFe-LDH@SNHPC and (b) NiFe-LDH@La SNHPC. (c) Band center positions of the lower Hubbard band (LHB) and upper Hubbard band (UHB) of the metal elements, and the p-orbital of oxygen for NiFe-LDH@La SNHPC and NiFe-LDH@SNHPC. (d) Schematic illustration of the energy levels of the bands. (e) Crystal orbital Hamilton populations (COHP) of the Ni–O bond on NiFe-LDH@La SNHPC and NiFe-LDH@SNHPC and (f) corresponding integrated COHP values. (g) Schematic illustration and (h) free energy diagram (U = 1.23 V/RHE) of the LOM pathway on NiFe-LDH@La SNHPC and NiFe-LDH@SNHPC. The coffee, blue, red, yellow, silver, brown, and green balls represent the C, N, O, S, Fe, Ni, and La, respectively.

3. Conclusion

In summary, a novel nanoreactor that enabled an efficient electrochemical water oxidation process was constructed through three steps: creating a hollow porous carbon carrier, loading La single atoms, and assembling NiFe-LDH nanodots. The well-defined multi-channel nanoreactor structure empowered the catalyst with the ability to assemble OH⁻ and enhance the electric field to facilitate the OER mass transfer process. The introduction of additional S sites synergized with the abundant micro- and mesoporous structures in the carbon framework to induce and confine NiFe-LDH growth and improve its effective exposure. In addition, DFT calculations indicated that the S site also provides an opportunity for enhancing the orbital distribution at the Fermi energy level across the nanoreactor and further improving the charge transfer efficiency. Further theoretical computational analyses revealed the critical role of electronic interactions between integrated La SAs and ultra-miniaturized NiFe-LDH with the help of carbon species in regulating the oxygen activity and weakening metal–oxygen bonds, which is advantageous for lowering the reaction energy potential of the rate-determining step in the OER process. The above rational design ensured the excellent OER and ZAB activities and stability of the target catalysts under alkaline conditions. This study provides an effective paradigm for the synergistic construction of high-performance electrocatalysts with multiple strategies.

Author contributions

All authors contributed to the collection and discussion of the content. All authors helped to revise the manuscript before submission.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We gratefully acknowledge the support from the National Science Foundation for Distinguished Young Scholars of China (22425503), National Natural Science Foundation of China (22371131), Industrial Technology Innovation Program of IMAST (No. 2024RCYJ02001), the 111 Project (B18030) from China, the Outstanding Youth Project of Tianjin Natural Science Foundation (20JCJQJC00130), and the Key Laboratory of Rare Earths, Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00313j

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