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. 2023 Jun 28;9(26):eadh1320. doi: 10.1126/sciadv.adh1320

Atomically dispersed Ni activates adjacent Ce sites for enhanced electrocatalytic oxygen evolution activity

Zhihao Pei 1,†, Huabin Zhang 2,†, Zhi-Peng Wu 2, Xue Feng Lu 3, Deyan Luan 1,*, Xiong Wen (David) Lou 4,*
PMCID: PMC10306285  PMID: 37379398

Abstract

Manipulating the intrinsic activity of heterogeneous catalysts at the atomic level is an effective strategy to improve the electrocatalytic performances but remains challenging. Here, atomically dispersed Ni anchored on CeO2 particles entrenched on peanut-shaped hollow nitrogen-doped carbon structures (a-Ni/CeO2@NC) is rationally designed and synthesized. The as-prepared a-Ni/CeO2@NC catalyst exhibits substantially boosted intrinsic activity and greatly reduced overpotential for the electrocatalytic oxygen evolution reaction. Experimental and theoretical results demonstrate that the decoration of isolated Ni species over the CeO2 induces electronic coupling and redistribution, thus resulting in the activation of the adjacent Ce sites around Ni atoms and greatly accelerated oxygen evolution kinetics. This work provides a promising strategy to explore the electronic regulation and intrinsic activity improvement at the atomic level, thereby improving the electrocatalytic activity.

Atomically dispersed Ni together with adjacent Ce sites is highly active for the electrocatalytic oxygen evolution reaction.

INTRODUCTION

Electrocatalytic oxygen evolution reaction (OER) is crucial to the acquisition of renewable energy sources, thus helping address the imminent fossil fuel shortage and environmental pollution (1–4). However, because of the generation of energetic intermediates in the complex four-electron reaction process, the challenging sluggish reaction kinetics of OER greatly limits its practical applications (5, 6). To overcome these issues, it is essential to explore effective OER electrocatalysts, which can reduce the activation barrier, improve the reaction kinetics, and enhance the energy conversion efficiency (7–9). At present, much research has been devoted to nonprecious metal electrocatalysts because the high cost and scarcity of those noble metal-based catalysts (e.g., IrO2 or RuO2) have restricted their large-scale application for OER (10, 11). However, despite great efforts to develop cost-effective electrocatalysts, their intrinsic electrocatalytic activity remains unsatisfactory (12).

Recently, with the prosperous and rapid development of single-atom catalysts (SACs), the exploration of heterogeneous catalysis has been gradually extended to the atomic level (6, 13–21). Introducing the cation promoters has proved as an effective strategy for activating the relatively inert atoms via effective electronic manipulation, thus realizing the great improvement of the intrinsic activity (22–24). On the other hand, the performance of isolated active sites depends heavily on their structures, such as the configuration, dispersion states, and mutual interactions with the supports (25–27). Therefore, the intrinsic activity of catalysts can be further optimized by choosing the support for anchoring cation promoters in a particular configuration.

Here, atomically dispersed Ni is introduced over CeO2 nanoparticles (NPs) entrenched on hollow peanut-shaped nitrogen-doped carbon (NC) structures (denoted as CeO2@NC) via elaborately designed atom decoration and confinement process. Experimental and theoretical calculation results show that the electronic structures of Ni atoms and their adjacent Ce atoms could be regulated well by the electron transfer after the introduction of isolated Ni atoms, resulting in the greatly activated Ce sites and better intrinsic catalytic activity. Thus, the resultant atomically dispersed Ni anchored on CeO2@NC (a-Ni/CeO2@NC) catalyst exhibits excellent OER activity and kinetics with an overpotential of 286 mV to afford a current density of 10 mA cm−2 and a small Tafel slope of 49 mV dec−1, which is much better than that of several control samples including CeO2@NC calcinated at 300°C (denoted as CeO2@NC-300) and atomically dispersed Ni anchored on NC (denoted as a-Ni@NC).

RESULTS

Synthesis and structural characterization of a-Ni/CeO2@NC

The synthetic schematic of a-Ni/CeO2@NC is illustrated in Fig. 1. First, highly uniform peanut-shaped Fe2O3 particles are synthesized with an average length of about 1.1 μm (fig. S1). Then, a thin polydopamine (PDA) layer is grown on the surface of Fe2O3 particles by the self-polymerization process in an alkaline solution, forming the Fe2O3/PDA core-shell structure (fig. S2). These peanut-shaped Fe2O3/PDA particles are carbonized at a high temperature, followed by the acidic etching of Fe species, and, then, the hollow peanut-shaped NC structures can be obtained (fig. S3). After the growth of CeO2 NPs on the surface of hollow NC structures, isolated Ni atoms are decorated and confined over lattice of CeO2 NPs to form a-Ni/CeO2@NC. Moreover, slightly modified strategies have also been applied to the synthesis of a-Ni@NC (fig. S4), metal-free NC, and CeO2@NC loaded with different contents of Ni species.

Fig. 1. Schematic illustration of the synthesis of a-Ni/CeO2@NC.

Fig. 1.

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(I) Polydopamine (PDA) coating on the surface of peanut-shaped Fe2O3 particles to form Fe2O3/PDA core-shell structures. (II) Formation of hollow peanut-shaped NC structures through annealing and HCl etching. (III) Formation of CeO2@NC. (IV) Decoration of atomically dispersed Ni on CeO2@NC to form a-Ni/CeO2@NC.

In Fig. 2A, the field-emission scanning electron microscopy (FESEM) image shows the well-maintained peanut-shaped structure of CeO2@NC. X-ray diffraction (XRD) spectra (fig. S5), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) images show the uniform growth of CeO2 NPs on hollow peanut-shaped NC structures (Fig. 2, B and C). After the Ni decoration process, these atomically dispersed Ni species are well anchored on CeO2@NC, and, then, the resultant a-Ni/CeO2@NC could be formed (Fig. 2, D to F). Meanwhile, only the lattice fringes of CeO2 in the aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) image (Fig. 2G) and characteristic XRD peaks of CeO2 can be observed in the a-Ni/CeO2@NC sample (fig. S5). These results demonstrate that the Ni species are confined in the crystal lattice of CeO2, confirming no aggregation of Ni species in the form of NiOx or Ni particles. The energy-dispersive x-ray (EDX) spectrum also verifies that Ni and Ce elements coexist in the a-Ni/CeO2@NC sample (fig. S6). The Ni contents of CeO2@NC samples prepared by using different Ni precursor amounts are determined via the inductively coupled plasma optical emission spectrometer (ICP-OES), and the Ni content of a-Ni/CeO2@NC is found to be 0.96 weight % (wt %) (table S1). Meanwhile, the HAADF-STEM image and corresponding elemental mapping images of a-Ni/CeO2@NC show that isolated Ni atoms are uniformly distributed throughout the hollow peanut-shaped NC structure (Fig. 2, H and I). Textural information of the sample has been evaluated by the N2 sorption isotherms, in which typical type IV isotherms and mesoporous structures have been confirmed for a-Ni/CeO2@NC, CeO2@NC, and CeO2@NC-300 (fig. S7). Meanwhile, the characteristic peak assigned to the F2g vibration mode of CeO2 at 455.8 cm−1 has been observed in the Raman spectra of both CeO2@NC-300 and a-Ni/CeO2@NC, further demonstrating that the original structure of CeO2 has been well maintained after the Ni decoration (fig. S8) (28). In addition, the ultraviolet photoelectron spectroscopy (UPS) valence band spectra are measured to elucidate the electronic properties of the samples (fig. S9). The calculated valence band maximum values are about 3.66 eV for CeO2@NC-300 and 3.39 eV for a-Ni/CeO2@NC. The upshifted valence band to the Fermi level after the introduction of isolated Ni atoms coupled with the decreased work function of a-Ni/CeO2@NC (4.34 eV) compared to CeO2@NC-300 (4.44 eV) demonstrates the faster electron-transfer kinetics of a-Ni/CeO2@NC (29).

Fig. 2. Morphological and structural characterizations.

Fig. 2.

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(A) Field-emission scanning electron microscopy (FESEM), (B) transmission electron microscopy (TEM), and (C) high-resolution TEM (HRTEM) images of CeO2@NC. (D) FESEM, (E) TEM, and (F) HRTEM images a-Ni/CeO2@NC. (G) Aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) image of a-Ni/CeO2@NC. (H) HAADF-STEM and (I) corresponding elemental mapping images of a-Ni/CeO2@NC.

The chemical state and coordination environment of the samples are investigated by x-ray photoelectron spectroscopy (XPS) and x-ray absorption fine spectroscopy (XAFS) analyses. The high-resolution Ce 3d XPS spectra show that the binding energy of Ce 3d for a-Ni/CeO2@NC has an obvious negative shift (≈0.4 eV) compared with the CeO2@NC-300, verifying the effective electronic coupling between the decorated Ni species and Ce from CeO2 (fig. S10A) (30). Meanwhile, compared with CeO2@NC-300, the increased Ce3+ ratio in a-Ni/CeO2@NC indicates that CeO2 acts as an electron acceptor in the Ni-Ce coupling. It is commonly accepted that the presence of Ce3+ species is beneficial for the oxygen absorption and thus accelerating electrocatalytic reactions (31, 32). The high-resolution Ni 2p XPS spectra show that the Ni 2p3/2 peak of a-Ni/CeO2@NC has been positively shifted by ≈0.4 eV with respect to that of a-Ni@NC, further indicating the electron redistribution process after the Ni decoration and the effective electron transfer from Ni to CeO2 (fig. S10B) (33).

X-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) measurements are carried out to further understand the local coordination environment. The k2-weighted Fourier transform EXAFS of a-Ni/CeO2@NC shows a dominant Ni-O coordination at 1.62 Å, which is almost identical with the Ni-O peak of NiO (Fig. 3A). Besides, there is no Ni-Ni/Ni-O-Ni coordination detected in a-Ni/CeO2@NC, suggesting the existence of isolated Ni atoms anchored on the CeO2 support, which is also corroborated by the quantitative EXAFS curve fitting analysis of a-Ni/CeO2@NC (fig. S11 and table S2) (34, 35). Figure 3B shows the Ni K-edge XANES spectra of a-Ni/CeO2@NC and its reference material. The absorption threshold of a-Ni/CeO2@NC is positively shifted compared with Ni foil, indicating the oxidized nature of Niδ+ (δ > 0) (36). To further verify the above atomic coordination, the Ce L3-edge XAFS and XANES analyses are carried out as well. As shown in Fig. 3C, scattering paths of Ce centers for a-Ni/CeO2@NC in R space exhibit the dominant peak at about 1.9 Å related to Ce-O coordination (34). In addition, the absorption threshold position of a-Ni/CeO2@NC and pure CeO2 could be obtained by the analysis of their Ce L3-edge XANES spectra in Fig. 3D. The slightly decreased valence of Ce for a-Ni/CeO2@NC further demonstrates electron redistribution characteristics after the introduction of Ni. The wavelet transform (WT) EXAFS analysis is carried out to distinguish backscattering atoms (Fig. 3E). Compared with the WT contour plots of NiO (Ni-Ni scattering path at about 6.8 Å−1 and Ni-O scattering path at about 4.2 Å−1) and Ni foil (Ni-Ni scattering path at about 7.2 Å−1), one dominant peak assigned to the Ni-O scattering path (about 4.0 Å−1) can be observed in a-Ni/CeO2@NC, further verifying the existence of isolated Ni atoms anchored on CeO2.

Fig. 3. Structural characterization of the catalysts.

Fig. 3.

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(A) Fourier transform (FT) EXAFS spectra of Ni K-edge for a-Ni/CeO2@NC, Ni foil, and NiO. (B) Ni K-edge XANES spectra of a-Ni/CeO2@NC and Ni foil. (C) FT EXAFS spectra of Ce L3-edge for a-Ni/CeO2@NC and CeO2. (D) Ce L3-edge XANES spectra of a-Ni/CeO2@NC and CeO2. a.u., arbitrary units. (E) Wavelet transform (WT) EXAFS contour plots of Ni K-edge for Ni foil, a-Ni/CeO2@NC, and NiO.

Electrochemical performance evaluation

To evaluate the electrocatalytic performances of a-Ni/CeO2@NC and related reference materials, electrochemical OER tests are performed in 1.0 M KOH electrolyte. The linear sweep voltammetry (LSV) curve of the NC sample indicates its negligible contribution to the electrocatalytic OER activity (fig. S12). Meanwhile, although NC is inactive for OER, the specially designed hollow peanut-shaped structure could also play a key role for the excellent OER activity. To verify its effect, another sample of CeO2 NPs decorated with Ni species but without NC supports (denoted as Ni/CeO2) is synthesized (fig. S13) and its performance toward OER is also measured. The LSV curves show that, although Ni/CeO2 has excellent OER activity with an overpotential of about 320 mV at the current density of 10 mA cm−2, its activity is still worse than that of a-Ni/CeO2@NC (fig. S14). TEM observations and electrochemical impedance spectroscopy (EIS) analysis demonstrate that Ni/CeO2 without the hollow peanut-shaped NC support is easy to agglomerate with poor conductivity, which is not beneficial for enhancing the OER activity (figs. S15 and S16). Figure 4A shows that the a-Ni/CeO2@NC catalyst displays the smallest onset overpotential of about 232 mV, as well as the highest current density throughout the entire applied potential range. Meanwhile, the a-Ni/CeO2@NC catalyst only requires an overpotential of 286 mV to achieve the current density of 10 mA cm−2, outperforming those of a-Ni@NC (338 mV), CeO2@NC-300 (418 mV), and RuO2 (371 mV) (Fig. 4B). In particular, compared with the CeO2@NC-300, the overpotential for the current density of 10 mA cm−2 has been reduced by 132 mV after the decoration of Ni atoms, indicating that the OER activity has been greatly boosted in a-Ni/CeO2@NC. The OER kinetics of these catalysts has been further investigated by analyzing their Tafel plots. According to the calculated results, the a-Ni/CeO2@NC catalyst presents the smallest Tafel slope of 49 mV dec−1 (Fig. 4C). The excellent OER activity and kinetics enable a-Ni/CeO2@NC to be one of the most promising noble metal–free SACs for OER in the alkaline medium (table S3) (37–52).

Fig. 4. Electrocatalytic OER performance evaluation.

Fig. 4.

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(A) LSV plots; (B) the corresponding overpotentials at the current density of 10 mA cm−2; (C) Tafel slopes of a-Ni/CeO2@NC, a-Ni@NC, CeO2@NC-300, and RuO2. (D) Turnover frequency (TOF) values of a-Ni/CeO2@NC and a-Ni@NC. (E) Capacitive current density (ΔJ/2) at 0.95 V versus reversible hydrogen electrode (RHE) against the scan rate for a-Ni/CeO2@NC, a-Ni@NC, and CeO2@NC-300. (F) Potential-time curves at the current density of 10 mA cm−2 for a-Ni/CeO2@NC, a-Ni@NC, and CeO2@NC.

The turnover frequency (TOF) values based on the nickel species have been calculated from the current densities obtained from the LSV curves. As observed in Fig. 4D, the TOF values of a-Ni/CeO2@NC are nearly four times higher than that of a-Ni@NC in the overpotential range of 250 to 400 mV, validating the highly improved intrinsic activity of a-Ni/CeO2@NC. To determine the electrochemically active surface area (ECSA) of these catalysts, cyclic voltammetry (CV) curves are measured at the non-Faradaic region with different scan rates to calculate the electrochemical double-layer capacitance (Cdl) (figs. S17 and S18). As shown in Fig. 4E, the Cdl value of a-Ni/CeO2@NC is 3.33 mF cm−2, which is substantially larger than that of a-Ni@NC (1.99 mF cm−2) and CeO2@NC-300 (1.49 mF cm−2). Meanwhile, the ECSA-normalized LSV curves show that the a-Ni/CeO2@NC catalyst exhibits higher current density compared to its references, suggesting its more excellent intrinsic OER catalytic activity (fig. S19). In addition, the effect of Ni decoration on CeO2@NC with varied loading contents has also been investigated. The electrocatalyst with a loading amount of 0.96 wt % shows the highest activity toward OER (fig. S20). The ECSA values of catalysts with different Ni decoration contents are analyzed accordingly. The nonlinear relationship between the above two parameters suggests that Ni atoms are highly unlikely to be the only active sites of a-Ni/CeO2@NC for OER, indicating the synergistic activation of Ce sites (figs. S21 and S22) (53). In addition, from the EIS analysis, the a-Ni/CeO2@NC catalyst displays the smallest charge transfer resistance among these catalysts (fig. S23), which agrees with its smallest Tafel slope (Fig. 4C). In short, the considerably enhanced intrinsic catalytic activity and kinetics of a-Ni/CeO2@NC fully demonstrate the activation effect of the introduced Ni atoms to their adjacent Ce sites for the promoted OER activity. As an important parameter in the exploration of electrocatalysts, durability has been further evaluated by the long-term potential-time measurement (Fig. 4F). Compared with CeO2@NC, the a-Ni/CeO2@NC catalyst shows more outstanding OER durability with a negligible potential increase after the 100-hour OER operation in the alkaline medium. The excellent durability of a-Ni/CeO2@NC compared with CeO2@NC may originate from the interactions between single Ni atoms and the support to stabilize the metal active sites (54). Moreover, the excellent stability of a-Ni/CeO2@NC can be further demonstrated by the nearly unchanged morphology, electrochemical performances, and structural features before and after the OER test (figs. S24 to S28).

Theoretical calculations

To interpret the catalytic performance and explore the underlying reaction mechanisms for the greatly improved intrinsic activity after decorating isolated Ni atoms on the CeO2 support, density functional theory calculations have been carried out. First, an a-Ni@CeO2 model with different adsorption sites and two counterpart models (a-Ni@NC and CeO2) are constructed on the basis of the XAFS analysis and HAADF-STEM results (figs. S29 to S33). According to the partial density of state (PDOS) calculation results, there is a substantial sharing range from −5 to −2 eV of Ni 3d and Ce 5d for a-Ni@CeO2, which suggests the existence of strong electronic interactions between Ni and Ce atoms (Fig. 5A and fig. S34A). Besides, the PDOS of Ni 3d shows a rather strong intensity around the Fermi level (Ef). The intensity of Ce 4f/5d around the Ef is also slightly increased compared to that of CeO2, thus improving the conductivity and promoting the electron transfer of a-Ni/CeO2 (Fig. 5, A and B, and fig. S34). In addition, the free energies for each step of these models are calculated at two states including zero electrode potential (U = 0 V) and equilibrium potential (U = 1.23 V) to determine the rate-determining step (RDS) and their limiting reaction barrier during the OER process (Fig. 5C and fig. S35) (55). As shown in Fig. 5C, the formation of *O is the RDS for a-Ni@NC and CeO2, while the formation of both *O and *OOH acts as the RDS for a-Ni/CeO2 (Ce site) and a-Ni@CeO2 (Ni site). Meanwhile, the a-Ni/CeO2 (Ni site) exhibits the smallest thermodynamic limiting potential (1.42 V) and reaction energy barrier among its counterparts (figs. S35 and S36). Note that, after the introduction of the isolated Ni atom, the reaction energy barrier of the adjacent Ce site becomes much smaller than that of CeO2, which suggests that the isolated Ni atom could activate its adjacent Ce sites. Charge density difference analysis is carried out to study the electron transfer and the interactions of the structure. The top and side views of a-Ni/CeO2 exhibit the electron transfer from the Ni atom to its adjacent CeO2 (fig. S37). Meanwhile, the *O intermediate-adsorbed structural model exhibits the electron transfer from the Ni atom to the adsorbed O atom (Fig. 5D). The PDOS of Ni 3d for a-Ni@CeO2 before and after O adsorption is also analyzed accordingly. The upshifted d-band center (εd) of Ni 3d for O-adsorbed a-Ni@CeO2 to Ef also demonstrates the electron depletion of Ni atoms (fig. S38). The above electron transfer from Ni to adsorbed O indicates the strong electronic coupling between the Ni atom and the adsorbed O intermediate, which is also confirmed by the orbital overlaps of Ni 3d and O 2p from PDOS calculations (fig. S39). Theoretical calculation results indicate that the introduced isolated Ni atoms could manipulate the electronic structure of a-Ni/CeO2 effectively, thereby improving its thermodynamic reaction energetics and activating adjacent Ce sites for enhanced OER intrinsic activity.

Fig. 5. Density functional theory calculations.

Fig. 5.

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Calculated PDOS of (A) a-Ni/CeO2 and (B) CeO2. (C) Free energy diagram of OER for a-Ni/CeO2 (Ni site), a-Ni/CeO2 (Ce site), CeO2, and a-Ni@NC at U = 0 V. (D) Charge density difference of *O intermediates on the surface of a-Ni/CeO2. The yellow and cyan contours represent charge accumulation and depletion in the real space with an isosurface level of ±0.002 |e| Bohr3, respectively. The red and yellow spheres represent O and Ce atoms, respectively. The purple sphere represents the Ni atom.

DISCUSSION

In summary, an electrocatalyst consisting of a-Ni/CeO2@NC has been developed by a well-designed atom decoration and confinement strategy. The electrochemical measurement and theoretical calculation results demonstrate that the activation of Ce sites around isolated Ni atoms has been achieved by manipulating the electronic structures, which could greatly improve the OER kinetics and intrinsic activity of a-Ni/CeO2@NC effectively. As a result, the obtained a-Ni/CeO2@NC electrocatalyst exhibits reduced overpotential of 286 mV to achieve the current density of 10 mA cm−2 with a Tafel slope of 49 mV dec−1 and excellent stability for OER operation over 100 hours. This work could provide some valuable insight into improving the intrinsic electrocatalytic activity of heterogeneous catalysts by anchoring suitable single atoms to activate their adjacent metal atoms, thereby facilitating further practical applications in energy storage and conversion fields.

MATERIALS AND METHODS

Synthesis of peanut-shaped Fe2O3 particles

In a typical synthesis, 25 ml of FeCl3·6H2O solution (2.0 M) was heated at 75°C in an oil bath with the constant stirring for 5 min. Then, 25 ml of NaOH solution (5.4 M) was dropwise added with a rate of 10 ml min−1. After stirring at 75°C for another 15 min, 2.5 ml of Na2SO4 solution (0.6 M) was slowly added into the above mixed solution and stirred for about 5 min. Then, the resultant Fe(OH)3 gel was placed in a preheated oven at 100°C for about 96 hours. After being washed by deionized (DI) water and ethanol for at least 10 times until the filtrate was clear and dried at 70°C for 10 hours, the peanut-shaped Fe2O3 particles can be collected.

Synthesis of Fe2O3@PDA

The obtained Fe2O3 particles (350 mg) were first dispersed in 400 ml of Tris-buffer solution (10 mM) by stirring for 25 min, followed by adding 480 mg of dopamine hydrochloride. After stirring for about 8 hours at the room temperature, the obtained Fe2O3@PDA products can be collected by being centrifuged, washed by ethanol, and dried at 70°C.

Synthesis of peanut-shaped NC

The obtained Fe2O3@PDA was annealed in the N2 atmosphere at 450°C for 1 hour with the heating rate of about 1°C min−1. Then, 80 mg of the annealed product was added in 10 ml of HCl solution (12 M) for 10 hours to remove the template. To completely remove all the Fe species, the black powder after the HCl etching was added in 25 ml of HCl solution (1 M) for 10 hours at 180°C by the solvothermal reaction. Last, the obtained peanut-shaped NC particles were collected after being washed in ethanol and dried at 70°C for about 10 hours.

Synthesis of CeO2@NC

The obtained peanut-shaped NC particles (25 mg), Ce(NO3)2·6H2O (120 mg), and hexamethylenetetramine (70 mg) were added sequentially in a mixed solvent with 50 ml of ethanol and 50 ml of DI water with the stirring for about 30 min. Subsequently, the above solution was transferred into a 250-ml single-necked round-bottom flask in the preheated oil bath at 60°C under refluxing and constant stirring for 2 hours. Last, after being washed by ethanol and dried overnight, the resultant CeO2@NC samples could be obtained.

Synthesis of CeO2@NC-300

The obtained CeO2@NC sample was calcinated at 300°C in the N2 atmosphere for 1 hour. After the sample was naturally cool down to the room temperature, the CeO2@NC-300 sample was obtained.

Synthesis of a-Ni/CeO2@NC

The obtained CeO2@NC samples (10 mg) and Ni(NO3)2·6H2O (20 mg) were added into 10 ml of ethanol under stirring in a preheated oil bath at 70°C for 2 hours. Then, the obtained product was collected by being centrifuged, washed in ethanol, and dried at 70°C for about 10 hours. After drying, the Ni-adsorbed CeO2@NC black powder was annealed at 300°C in the N2 atmosphere for 1 hour. Last, the a-Ni/CeO2@NC sample was obtained.

Synthesis of different contents of Ni species anchored on CeO2@NC

The similar synthesis route to a-Ni/CeO2@NC was adopted except changing the adding amount of Ni(NO3)2·6H2O.

Synthesis of a-Ni@NC

The obtained peanut-shaped NC samples (5 mg) and Ni(NO3)2·6H2O (20 mg) were added into 10 ml of ethanol under stirring in a preheated oil bath at 70°C for 2 hours. Then, the obtained product was collected by being centrifuged, washed in ethanol, and dried at 70°C for about 10 hours. After drying, the Ni-adsorbed NC black powder was annealed at 300°C in the N2 atmosphere for 1 hour. Last, the a-Ni@NC sample was obtained.

Synthesis of Ni/CeO2

NaOH (9.6 g) and Ce(NO3)2·6H2O (868 mg) were mixed in 40 ml of DI water under the stirring for about 1 hour. Then, the mixed solution was placed in the Teflon-lined stainless autoclave at 100°C for about 24 hours in the oven. After the natural cooling down of the autoclave, the obtained product was collected by centrifugation, washed in ethanol, and dried at 70°C. After drying, about 10 mg of the light-yellow powder and 20 mg of Ni(NO3)2·6H2O were dissolved in 10 ml of DI water under the stirring at 70°C for 2 hours. Then, the obtained product was collected by centrifugation, washed in ethanol, and dried at 70°C. Last, the Ni-adsorbed CeO2 powder was annealed at 300°C in the N2 atmosphere for 1 hour. Last, the Ni/CeO2 sample was obtained.

Material characterizations

FESEM (JEOL-6701) and TEM (JEOL-2100F) were used to obtain the morphology of the samples. The structure of the samples was determined by XRD (Bruker D2 Phaser) with Cu Kα radiation. The compositions of the samples were analyzed by the FESEM equipment with an EDX spectroscopy. The STEM EDX spectroscopy was obtained and analyzed in the Oxford Aztec EDX system. The aberration-corrected JEM-ARM200F was used to obtain the atomic resolution HAADF-STEM images. The 3Flex Surface Characterization Analyzer (Micromeritics) was adopted to record the N2 adsorption-desorption isothermal curves of the samples at 77 K. Before the test, the samples were degassed in a vacuum environment at 423 K for 12 hours. XPS (Shimadzu Kratos AXIS Supra) was measured to analyze the detailed electronic structure of the samples. In addition, the binding energy of 284.8 eV was taken as the reference for the adventitious carbon (C 1s). UPS measurement was carried out by Kratos AXIS Supra. Raman spectra were investigated by a Raman spectrometer (DXR, Thermo Fisher Scientific, USA). ICP-OES (Thermo Fisher Scientific, IRIS Intrepid II XSP spectrometer) was used to determine the contents of Ni species in the samples. The XAFS spectra of Ni K-edge and Ce L3-edge were measured at the National Synchrotron Radiation Research Center, Taiwan.

Electrochemical measurements

The electrocatalytic OER performance evaluation of catalysts was performed by a standard three-electrode cell in the O2-saturated 1.0 M KOH (pH 13.8) solution. High-purity, semiconductor-grade KOH with a trace metals basis of 99.99% was used to prepare the electrolyte, which would further undergo a purifying treatment reported by Boettcher’s group before being used for the electrochemical measurements (56). The catalyst was dropped on a carbon fiber paper and served as the working electrode. In addition, a Hg/HgO reference electrode and a carbon rod counter electrode were used during the OER tests. To prepare the catalyst ink, typically, 5 mg of the catalyst was dispersed into the solution composed of 0.7 ml of ethanol, 0.25 ml of DI water, and 0.05 ml of 0.5 wt % Nafion solution followed by the constant ultrasonication for about 1 hour. The catalyst ink was then dropped on the carbon paper (0.5 cm by 3 cm), with a loading area of 0.5 cm by 0.4 cm. All potentials were calibrated to the reversible hydrogen electrode (RHE) based on the following equation: ERHE = EHg/HgO + 0.059 × pH + 0.098. The LSV curves were recorded during the potential range from 1.30 to 1.70 V versus RHE with a scan rate of 5 mV s−1 to evaluate the activity of catalysts toward OER. The overpotential (η) of OER could be obtained by the equation of η = ERHE – 1.23 V. To evaluate the reaction kinetics, the Tafel slope was obtained by the Tafel equation: η = b log j + a, where b denotes the Tafel slope and j is the current density. In addition, the EIS was tested in the frequency window from 0.1 to 106 Hz at 1.65 V versus RHE with an amplitude of 5 mV. The TOF was calculated by the following equation: TOF = j × A/(4 × F × n), where A denotes the area of the working electrode, F means the Faraday constant (96,485 C mol−1), and n is the number of moles of the Ni active sites assuming that all Ni atoms deposited on the electrode have been incorporated in the reaction. CV was used to determine the electrochemical double-layer capacitances of the samples. The catalysts were measured in the non-Faradaic range with the scan rates of 5, 10, 15, 20, and 25 mV s−1, respectively. The Cdl value of the catalyst was obtained by fitting the slope of the plot of the current density versus the scan rate (57). For the calculation of ECSA, 40 μF cm−2 is assumed as a moderate value of the specific capacitance for a flat surface (58). ECSA of the catalysts was calculated according to the following equation: ECSA = Cdl/40 μF cm−2 per cm2ECSA.

Acknowledgments

Funding: X.W.L. acknowledges the funding support from the Ministry of Education of Singapore through the Academic Research Fund (AcRF) Tier-2 grant (MOE2019-T2-2-049).

Author contributions: Z.P., D.L., and X.W.L. conceived the idea. Z.P. carried out the material synthesis and material characterizations. Z.P., H.Z., X.F.L., and X.W.L. analyzed the experimental data. Z.P. and Z.-P.W. carried out the theoretical calculation. Z.P., H.Z., D.L., and X.W.L. discussed the results and cowrote the manuscript. All authors read and commented on the manuscript.

Competing interests: The authors declare that they have no competing interests.

Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.

Supplementary Materials

This PDF file includes:

Supplementary Text

Figs. S1 to S39

Tables S1 to S3

Click here for additional data file. (2.4MB, pdf)

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