English

• 2015, a new type of oxide material emerged which is known as high entropy oxides (HEOs), possessing unique physical and chemical properties [1-3]. The concept of HEOs is derived from high entropy alloys (HEAs), referring oxides contain five or more cations with an equal or near-equal molar fraction leads to the formation of single-phase crystal structure [4-7]. When all the elements have the same molar fraction, mixing entropy reaches the maximum [8]. According to G = H − TS, Gibbs free energy reaches the minimum, indicating the phase of HEOs is the most thermodynamically stable and HEOs have excellent stability [9]. In HEOs, randomly and uniformly distributed of cations and there is no difference between solute and solvent. Due to HEOs possess unique properties, which have various potential applications. For example, (Co_0.2Ni_0.2Cu_0.2Zn_0.2Mg_0.2)O supported Pt was applied to CO oxidation [10], operating at 135 ℃ for 40 h without the loss of activity due to entropy-stabilization behavior. Moreover, transition metal-based HEO (TM-HEO) of (MgCoNiCuZn)_1−xLi_xO possesses high Li-ion conductivity [11], rock (Co_0.2Ni_0.2Cu_0.2Zn_0.2Mg_0.2)O was applied to reversible energy storage [5], providing a possibility to tailor electrochemical performance by changing element composition or element molar fraction. As a result of excellent stability and high coordination saturation, HEOs are difficult to apply to electrocatalysis. It is significant to activate surface atoms of HEOs to promote their application in the field of electrocatalysis.

  De-alloying is a promising strategy to activate surface atoms and decrease reaction overpotential [12]. Duan et al. through electrochemical de-alloying strategy converted PtNi alloy into jagged platinum nanowires (J-Pt NMs) with 13.6 A/mg_Pt for oxygen reduction reaction (ORR) [13]. Ultrahigh mass activity of J-PtNWs can be attributed to low coordination number of surface atoms, decreasing the binding energy of reaction intermediate and the activation energy of ORR rate determining step. Besides, constructing amorphous structure is another efficient strategy to activate surface atoms. Amorphous oxides usually exhibit greater electrocatalytic properties than their crystalline forms [14-16]. Amorphous NiFeMo oxide underwent a rapid surface self-reconstruction process [17], generating favorable electronic and geometric structure and creating active NiFe oxy(hydroxide) layer on the surface of oxide. More importantly, creating defects has been proven that it is an effective way to improve intrinsic activity of active sites. Oxygen vacancy is one of the most common defect types in transition metal oxides, being able to modulate their electronic structure, band gap, conductivity, and catalytic performance [18-20]. Oxygen vacancies can lower the Gibbs free energy associated with ^*OOH adsorption, enabling an easier release of oxygen [17]. It is well known that oxygen vacancies can greatly improve OER properties via regulating electronic property, changing the bonding strength of reactants or reaction intermediates, and reducing kinetics barrier [17, 21].

  Herein, we synthesized a low-crystallinity HEO with abundant oxygen vacancies via NaBH₄ reduction strategy. Low-crystallinity (Fe, Co, Ni, Zn, Mn)₃O₄ (HEO-Origin) nanosheet provides abundant reactive sites and accelerates electron transfer. Abundant oxygen vacancies in HEO-Origin promote the activation of surface atoms, tunes electronic structure, and ultimately enhance intrinsic activity of active sites. As expected, as-synthesized HEO-Origin as OER electrocatalyst showed an excellent electrocatalytic properties and stability.

  HEO-Origin was synthesized via a facile NaBH₄ reduction strategy. HEO-500℃-air was obtained by calcining HEO-Origin at 500 ℃ under air atmosphere. HEO-Origin displays curly nanosheets (Fig. S1a in Supporting information). With the increase of calcination temperature, the size of catalysts increased dramatically (Figs. S1b-f in Supporting information). As the temperature continues to rise to 900 ℃, the size of nanosheets ranges from hundreds of nanometres to micrometres. Transmission electron microscopy (TEM) also indicates HEO-Origin showed nanosheet feature in Figs. 1a and b. The thickness of HEO-Origin was about 40 nm and the width was 200 nm, which was confirmed by Atomic Force Microscope (AFM) in Fig. S2 (Supporting information). After thermal annealing at 500 ℃ under air atmosphere, Brunauer-Emmett-Teller (BET) surface area of HEOs decreased dramatically from 223.5 m²/g to 85.5 m²/g (Fig. S3 in Supporting information). High BET specific surface area of HEO-Origin provides abundant active sites, facilitating infiltration of electrolyte into catalyst interior and endowing catalyst with excellent catalytic performance [21].

  Figure 1

  

  Figure 1.  (a, b) TEM images and (c) HRTEM of HEO-Origin. (d) XRD patterns of HEO-Origin and HEO-500℃-air. (e) SAED pattern and (f) EDS element mappings of HEO-Origin.

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  X-ray diffraction (XRD) patterns in Fig. 1d confirmed that HEO-Origin exhibited two characteristic diffraction peaks of spinel oxide (CoFe₂O₄, JPCDS No. 22–1086). High-resolution TEM (HRTEM) in Fig. 1c revealed that HEO-Origin without obvious lattice fringes, implying HEO-Origin with low-crystallinity. Simultaneously, selected-area electron diffraction (SAED) pattern in Fig. 1e shows that HEO-Origin is only a weak reflect hole, reflecting catalyst is lack of long-range order in the third dimensionality and atomic lattice is irregular arrangement [22, 23]. Annealing HEO-Origin under air atmosphere from 300 ℃ to 900 ℃ (Fig. S4 in Supporting information), as-obtained catalysts all possess single phase spinel structure of CoFe₂O₄, which is similar to Fd-3 m structure of (Co, Cr, Ni, Mn, Fe)₃O₄ [24]. Raman spectroscopy results showed that HEOs possess five Raman active bands (A_1g + E_g + 3F_2g), locating at 155, 250, 337, 435 and 610 cm⁻¹ (Fig. S5 in Supporting information). HEOs Raman spectrum match well with ZnFe₂O₄, implying that spinel HEOs was successfully synthesized. During energy-dispersive X-ray (EDS) test, homogeneous ink was dropped on Cu grid. Fig. S6 (Supporting information) showed that Fe, Co, Ni, Mn, Zn and O were all presence in HEO-Origin. EDS element mappings in Fig. 1f confirms that all elements in HEO-Origin were uniformly distributed without phase separation. Elements uniformly distributed endows HEO-Origin with a great number of possible microstates numbers and high configurational entropy [8]. Molar fraction of metal elements was measured by inductively coupled plasma optical emission spectrometry (ICP-OES), which was listed in Table S1 (Supporting information).

  Linear sweep voltammetry (LSV) was performed on glassy carbon electrode to investigate electrochemical OER activities, polarization curves are shown in Fig. 2a. Onset potential of HEO-Origin is 1.456 V_RHE (reversible hydrogen electrode) far less than that of HEO-500℃ (1.514 V_RHE). HEO-Origin needs only 265 mV overpotential to achieve 10 mA/cm² lower than that of HEO-500℃-air (335 mV). Specific activity of HEO-Origin (0.0396 mA/cm_BET²) is 10 times higher than that of HEO-500℃-air (0.00395 mA/cm_BET²) at 1.525 V_RHE (Fig. S7 in Supporting information). OER activities decreased with the increase of heat treatment temperature in Fig. S8 (Supporting information), due to catalyst after thermal annealing exposes fewer reactive sites and possesses a lower surface defect. To verify the role of oxygen vacancies, HEO-Origin was calcined at 500 ℃ under N₂ atmosphere (HEO-500℃-N₂), corresponding XRD patterns and LSV polarization curve are presented in Fig. S9 (Supporting information). As expected, the OER performance of HEO-500℃-air is poorer than HEO-500℃-N₂, suggesting that oxygen vacancies play a significant role. Fig. S10 (Supporting information) shows that O 1s spectrum is deconvoluted into four characteristic oxygen peaks, O1 at 530 eV is typical for lattice oxygen, O2 at 530.88 eV is assigned to oxygen in hydroxyl groups on the outside of catalysts, O3 at 531.55 eV is associated with defect sites with a low oxygen coordinate, O4 at 532.47 eV is attributed to surface-adsorbed oxygen [25-27]. The content of O3 in HEO-Origin (57.1%) is highest in HEO-500℃-N₂ (33.8%) and HEO-500℃-air (21.6%). The excellent catalytic performance of HEO-Origin can be attributed to high specific surface area provides abundant active sites and promotes the infiltration of electrolyte into the interior of catalysts [28], abundant oxygen vacancies tailor electron distribution on the surface of the catalysts and accelate electron transfer [17, 29].

  Figure 2

  

  Figure 2.  (a) LSV curves, (b) Tafel slopes and (c) CV curves of HEO-Origin and HEO-500℃-air. (d) LSV curves of HEO-Origin before and after 3000 CVs cycles.

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  Electrochemical surface area (ECSA) was determined by cyclic voltammetry (CV) in Fig. S11 (Supporting Information). C_dI of HEO-Origin is 1.79 mF/cm² larger than that of HEO-500℃-air (1.28 mF/cm²). The decrease of ESCA indicates active area of catalyst is dramatically decreased after thermal annealing. After ECSA normalization (Fig. S11d in Supporting Information), OER current of HEO-Origin is still higher than that of HEO-500℃-air, indicating HEO-Origin possesses higher intrinsic activity. Tafel slope of HEO-Origin is 36.87 mV/dec lower than HEO-500℃-air (49.67 mV/dec) in Fig. 2b, implying HEO-Origin with excellent kinetics. In addition, OER is strongly affected by surface properties of catalysts. In Fig. 2c, oxidation peak and reduction peak of HEO-Origin located at 1.334 V and 1.241 V_RHE, respectively. Surface reconstruction of HEO-Origin occurs prior to OER. Broad oxidation peak within a potential ranging from 1.2 V_RHE to 1.4 V_RHE, promoting the oxidation of M²⁺ to M³⁺ or M⁴⁺, (Co²⁺/Co³⁺ peak at 1.1–1.3 V_RHE and Co³⁺/Co⁴⁺ peak at 1.24–1.54 V_RHE) [18, 22].

  LSV curve of HEO-Origin after 3000 CVs cycles presents negligible degradation in Fig. 2d. Simultaneously, the stability of HEO-Origin loaded on nickel foam was also measured by chronoamperometry in Fig. S12 (Supporting information). After 40,000 s of testing, the current density of HEO-Origin decreased a little from 22.4 mA/cm² to 21.53 mA/cm². These results indicate that HEO-Origin is promising as a stable and efficient catalyst for electrochemical OER, being able to survive harsh conditions. Constructing low-crystallinity HEO-Origin with rich oxygen vacancies and abundant active sites, endowing catalyst with excellent catalytic activity and stability. The electrocatalytic OER performance of HEO-Origin is even better than most of reported catalysts in Table S2 (Supporting information).

  Nyquist plots within a potential range from 1.15 V_RHE to 1.55 V_RHE are shown in Figs. 3a and b. The related equivalent circuit is presented in Fig. S13 (Supporting information). Table S3 and S4 (Supporting information) display the summary of corresponding fitting date. At high frequency region, the first semicircle can be attributed to R_film [30]. The second semicircle or straight line in low frequency region is assigned to charge-transfer resistance (R_ct) associated with the overall reaction rate of OER. Nyquist plots of HEO-Origin, HEO-500℃-N₂ and HEO-500℃-air at 1.49 V_RHE are presented in Fig. S14 (Supporting information), the smallest R_ct of HEO-Origin (37.93 Ω) reveals that HEO-Origin has the fastest electron transfer rate than HEO-500℃-N₂ (510.2 Ω) and HEO-500℃-air (2796 Ω) [31-33]. As a result of HEO-Origin with abundant oxygen vacancies, delocalized electrons close to oxygen vacancies can be excited to conduction band, improving conductivity and electrochemical OER activity [22].

  Figure 3

  

  Figure 3.  (a, b) Nyquist plots and (c, d) Bode phase plots of HEO-Origin and HEO-500℃-air at various voltages.

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  Bode phase plot of HEO-Origin presented in Fig. 3c, phase peak A (10³–10⁵ Hz) has low phase angle, indicating fast charge propagation [34]. Phase peak B (10¹–10³ Hz) is associated with surface species oxidation at tetrahedral (Td) sites. When potential higher than 1.35 V_RHE, phase peak B intensity of HEO-Origin is dramatically decreased, implying its surface is almost completely oxidized. Phase peak C (10⁻¹–10² Hz) can be observed When potential higher than 1.45 V_RHE. Phase peak C implys the occurrence of OER, which is consistent with its onset potential. However, HEO-500℃-air has no phase peak at low-frequency region in Fig. 3d, indicating that its poor OER activity.

  X-ray photoelectron spectroscopy (XPS) is a surface sensitive detective tool, which can be used to investigate valence state [35]. XPS spectrum before and after OER are presented in Figs. S15 and S16 (Supporting information). Figs. 4a and b indicate that the content of O3 is dropped from 57.1% (HEO-Origin) to 21.6% (HEO-500℃-air) after thermal annealing. After stability test, the content of O3 is dropped from 57.1% to 33.9%, still higher than that of HEO-500℃-air (21.6%). Co satellite peaks around 786.3 eV and 803.7 eV in Fig. 4c, implying the existence of high-spin Co²⁺ [36]. Co²⁺ (781.4 eV, 797.1 eV) and Co³⁺ (780.15 eV, 795.95 eV) coexist in HEO-500℃-air (Fig. 4d). However, cabalt only exists as Co²⁺ in HEO-Origin. The content of Co²⁺ in HEO-Origin (100%) is highest than HEO-500℃-N₂ (64.85%, Fig. S17 in Supporting information) and HEO-500℃-air (33.82%). After OER stability test, the binding energy of Co 2p shifts to lower energy indicates a portion of Co²⁺ was oxidized to Co³⁺. The content of Co²⁺ in HEO-Origin, HEO-500℃-N₂ and HEO-500℃-N₂ is decreased from 100% to 28.42%, 64.85% to 27.48%, 33.82% to 0%, respectively (Figs. 4c and d and Fig. S17 in Supporting information). Fe²⁺ (710.1 eV) and Fe³⁺ (711.5 eV) exist in both HEO-Origin and HEO-500℃-air in Figs. 4e and f. After calcination at 500 ℃ in air atmosphere, the content of Fe²⁺ is almost unchanged from 24.07% to 24.41%. Besides, Ni 2p spectra is fitted in Figs. 4g and h. In HEO-500℃-air, the binding energy of 854.8 eV and 872.5 eV are assigned to Ni²⁺, and the binding energy of 856.0 eV and 873.9 eV are attributed to Ni³⁺. However, nickel only exists as Ni³⁺ in HEO-Origin, due to Co³⁺ at Td sites of NiCo₂O₄ is thermodynamically unstable and prones to convert to Co²⁺, the existence of Ni³⁺ facilitates the adsorption of OH^* intermediate [37]. After stability test, the binding energy of Fe 2p and Ni 2p shift to lower binding energy, implying that a part of Fe³⁺ and Ni³⁺ were reduced to Fe²⁺ and Ni²⁺, respectively. Surprisingly, HEO-Origin surface was free of zinc cation after OER stability test, due to Zn²⁺ leaches out in KOH electrolyte under high oxidation potential, which is confirmed by Zn XPS spectrum in Fig. S16d (Supporting information). Some cation defect sites may be generated which is also conducive to OER. In comparison, we can detect Zn XPS single, Zn²⁺ in HEO-500℃-air is difficult to completely leaching or dissolving.

  Figure 4

  

  Figure 4.  Fitted (a, b) O 1s spectra, (c, d) Co 2p spectra, (e, f) Fe 2p spectra and (g, h) Ni 2p spectra of HEO-Origin (left) and HEO-500℃-air (right) before (above) and after (bottom) OER stability test.

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  The excellent OER performance of HEO-Origin originates from the presence of oxygen vacancies improve the conductivity, enhances OH⁻ adsorption capacity and serves as efficient active sites [38]. The oxidation of Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ in HEO-Origin occurs before OER (Fig. 2c) and the content of Co³⁺ is dramatically increased after OER stability test. The increased Co³⁺ indicates theformation of CoOOH, which is highly active for electrochemical OER [38]. Furthermore, Zn²⁺ can be etched at high potential in alkaline electrolyte to create cation vacancies. These synergistic effect together leads to the remarkable OER activity of as-synthesized HEO-Origin.

  In this work, we present a facile and effective method to activate surface atoms of HEOs. Constructing low-crystallinity HEO-Origin nanosheet with large specific surface area and rich oxygen vacancies. HEO-Origin requires an overpotential of 265 mV to achieve 10 mA/cm² far less than that of HEO-500℃-air, possessing excellent OER activity even better than plenty of reported catalysts. The excellent catalytic performance of HEO-Origin can be attributed to large surface area exposing a great number of active sites, oxygen vacancies efficiently alter catalyst surface electron distribution and accelate electron transfer. The method for activating surface atoms of HEOs provides a new opportunity to discover novel properties of HEOs and promote the application of HEOs in the field of electrocatalysis.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. U19A2017, 21902047, 51402100, 21825201, 21573066, and 21905088) and the Provincial Natural Science Foundation of Hunan (Nos. 2020JJ5044, 2022JJ10006).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.085.

  
  
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  Figure 1  (a, b) TEM images and (c) HRTEM of HEO-Origin. (d) XRD patterns of HEO-Origin and HEO-500℃-air. (e) SAED pattern and (f) EDS element mappings of HEO-Origin.

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  Figure 2  (a) LSV curves, (b) Tafel slopes and (c) CV curves of HEO-Origin and HEO-500℃-air. (d) LSV curves of HEO-Origin before and after 3000 CVs cycles.

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  Figure 3  (a, b) Nyquist plots and (c, d) Bode phase plots of HEO-Origin and HEO-500℃-air at various voltages.

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  Figure 4  Fitted (a, b) O 1s spectra, (c, d) Co 2p spectra, (e, f) Fe 2p spectra and (g, h) Ni 2p spectra of HEO-Origin (left) and HEO-500℃-air (right) before (above) and after (bottom) OER stability test.

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