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• Due to worldwide overconsumption of fossil fuels, it is necessary to find environmental-friendly and renewable energy materials [1-3]. Hydrogen is recognized as one of the most promising green energy owing to its rich reservation on the earth [4, 5]. However, the efficiency of overall water splitting for harvesting of hydrogen largely lowers due to the kinetically sluggish OER. Thus, developing highly active and cost-effective catalysts is very essential to improve the energy conversion efficiency [6, 7]. Currently, some noble metals such as RuO₂/IrO₂ and Pt exhibit outstanding catalytic performances for OER and HER. However, the high cost and scarcity hinder their further applications. Therefore, much attention has been focused on alternative earth-abundant and inexpensive non-noble metals materials. Nevertheless, only a few catalyst materials possess the abilities to catalyze both HER and OER. Therefore, the development of bi-functional catalytic materials is a challenge in overall water splitting.

  Metal-organic frameworks (MOFs), which are formed by linking inorganic ions and organic chains through strong coordination bonds, have been concerned owing to their unique structures and intrinsically large surface area. MOFs present multi-functionalities for supercapacitors [8], lithium ion batteries [9], gas adsorption/ separation [10] and heterogeneous catalysis [11]. Moreover, MOFs can be transformed into metal oxides, porous carbons, carbon/metal and metal sulfides through postsynthesis treatment. Among them, Co-based derivatives (Co₃O₄) have attracted considerable attentions as water splitting catalysts due to their thermal stability, abundant reserves and environmental friendliness in alkaline electrolytes [12, 13]. However, MOF derivatives are mostly in powder form, where the usage of adhesives takes many disadvantages in catalytic performances. Meanwhile, it is challenging to synthesize them with controllable morphologies though MOFs precursors. Conducting polymers, such as PPy and PANI, usually combine with other transition metal compounds owing to their intrinsic high conductivity, facile preparation and superior environmental stability [14, 15].

  In this work, Co-based MOFs are synthesized by a facile approach on Ni foam substrate, and transformed to Co₃O₄ nanoleaves through thermal pyrolysis. Finally, Co₃O₄ nanoleaves are decorated using polypyrrole through an electrodeposition method. The hybrid Co₃O₄/PPy nanostructures exhibit remarkable electrocatalytic performances for OER and HER with low overpotential as well as remarkable durability. It might result from their enhanced conductivity and synergistic effect between Co₃O₄ and PPy samples.

  Prior to experiment, a piece of Ni foam was treated by 1 mol/L HCl solution to remove surface impurity, and then washed ultrasonically with deionized water and absolute ethanol several times, respectively. All other chemicals were used without any further purification.

  The details of procedure are shown as follows. 0.06 mol/L of Co(NO₃)₂·6H₂O and 0.3 mol/L of 2-methylimidazole (Hmim) powder were dissolved in 50 mL deionized water separately with magnetic stirring for 30 min at room temperature. Subsequently, Co(NO₃)₂·6H₂O pink solution was added into the Hmim solution quickly and the pretreated nickel foam (4 × 4 cm²) was completely immersed in the above solution for 2 h. Then the sample supported on nickel foam was rinsed with deionized water and absolute ethanol several times and dried overnight at 60 ℃ in an oven. Finally, the Co-MOF precursor was annealed at 300 ℃ for 1 h at a ramping rate of 1 ℃/min in muffle furnace to obtain Co₃O₄ nanoleaves.

  Composites were fabricated through a chronopotentiometry deposition method. To prepare polypyrrole, 1 g p-TSA was dissolved in 60 mL deionized water and stirred for 15 min. Then 0.2 mL pyrrole was dropped into the above solution slowly and stirred for 30 min to obtain clear solution. The above mentioned Co₃O₄ products were used as working electrode. Pt foil and Ag/AgCl electrode was acted as the counter and reference electrodes, respectively. The potential window is 0.95 V. After electrodeposition, Co₃O₄/PPy product was rinsed several times with deionized water and ethanol, and dried at 60 ℃ for 8 h.

  The phase purity and crystal structure of the as-fabricated products were analyzed by power X-ray diffraction analyzer (XRD, 7000, Shimadzu). Fourier transform infrared spectrum (FTIR, 4000–500 cm^-1) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi, Thermo Scientific) were used to study the composition of the as-prepared samples. The morphology of the product were characterized by scanning electron microscopy (SEM, Gemini 300-71- 31).

  Electrochemical measurements of the as-synthesized samples were studied using a CHI660E electrochemical workstation (Chenhua, Shanghai) in a typical three-electrode system. The as-prepared samples were used as working electrode, Ag/AgCl electrode and graphite rod as reference electrode and counter electrode, respectively. OER and HER performances were investigated in 1.0 mol/L KOH solution (pH 13.7). The measured potentials were converted to the reversible hydrogen electrode (RHE) according to Nernst equation: E_RHE = E_SCE + 0.198 + 0.059 × pH, where E_SCE refers the measured potentials.

  The overpotential (η) could be obtained through the equation: η = E_RHE - 1.23. To evaluate the performances of OER and HER, linear sweep voltammetry (LSV) polarization curves were measured at a scan rate of 2 mV/s. ECSA values of OER and HER were calculated via cyclic voltammetry (CV) curves in the non-faradaic voltage range. The potential range of OER is from 0.1 V to 0.2 V and that of HER is from -0.3 V to -0.2 V. As scan rates increases (10-50 mV/s), current density changes linearly. The slope of the line is considered twice of C_dl. Electrochemical impedance spectroscopy (EIS) curves were recorded in the same electrolyte ranging from 10^-1 Hz to 10⁵ Hz at 5 mV.

  X-ray diffraction (XRD) patterns are used to analyze the crystal structures of the as-prepared samples (Fig. 1a). The characteristic peaks of black curve at 44.6°, 52.0° and 76.6° belong to nickel foam (JCPDS No. 42-0712). The others at 2θ values of 31.3°, 36.9°, 59.4°, 65.2° are in accordance with (220), (311), (511) and (440) crystal planes of Co₃O₄ products (JCPDS No. 42-1467). It is confirmed that Co-MOF precursor is converted into Co₃O₄ products through thermal pyrolysis. It could be observed that the intensities of all the diffraction peaks in the red curve are obviously weakened, which indicates that Co₃O₄ products are successfully coated by polypyrrole. FTIR spectrum is used to further identify it, as shown in Fig. 1b. The absorption peak located at 1095 cm^-1 can be ascribed to N-H and C-H vibration modes in pyrrole rings. The peaks at 1637 cm^-1 and 3331 cm^-1 correspond to the vibration mode of absorbing water. The peaks at 2859 cm^-1 and 2930 cm^-1 are thought to be N-H bond of aromatic amines [16].

  Figure 1

  

  Figure 1.  Chemical composition characterization (a) XRD patterns of Co₃O₄ nanoleaves and Co₃O₄/PPy-120. (b) FTIR of Co₃O₄/PPy-120. (c) Full spectrum, (d) Co 2p, (e) O 1s and (f) N 1s XPS spectra of the Co₃O₄/PPy-120.

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  XPS measurements are also conducted to analyze surface composition of samples. The full spectra show Co, O, Ni and N elements in Fig. 1c, which is consistent with XRD and FTIR analysis. Fig. 1d is the high-resolution XPS spectrum of Co element. It shows two strong peaks of Co 2p_1/2 and Co 2p_2/3 and two satellite peaks which result from spin-orbital coupling of Co 2p_1/2 and Co 2p_3/2 orbits. The energy interval between strong peaks at 795.3 eV and 779.8 eV is about 15.5 eV, which is the characteristic of Co 2p_1/2 and Co 2p_3/2 orbits of Co₃O₄ phase. The peaks show slightly shift compared with those of pure Co₃O₄ samples [17], indicating electron transfer and strong interaction between Co₃O₄ and PPy samples. The fitting peaks at 795.8 and 780.7 eV are assigned to Co²⁺, and the peaks at 794.8 and 779.3 eV could be ascribed to Co³⁺. The main peak in the O 1s region is divided into three peaks at 530.4, 531.1 and 532.3 eV (Fig. 1e), which are associated with oxygen in Co₃O₄, hydroxide groups and surface lattice oxygen, respectively. Three peaks are separated in the deconvolution of the N 1s region centered at 397.9, 399.4 and 400.3 eV, which correspond to the C=N defects of PPy, the -NH- segment in PPy backbone and the connections of N-Co between PPy and Co₃O₄, respectively (Fig. 1f) [18].

  The morphologies of the as-synthesized samples are observed through SEM. Fig. 2a shows that Co₃O₄ samples are uniformly grown on nickel foam with a leaf-like two-dimensional shape. Through further observation (Fig. 2b), it can be clearly found that the width of each nanoleaf is approximately 400 nm with an average thickness of 20 nm. Figs. 2c and d exhibit that Co₃O₄ samples are uniformly covered by a layer of thin film, indicating that PPy was successfully deposited due to coordinating between cobalt ions and -NH- sections in polypyrrole chains. Co₃O₄ nanoleaves cannot be seen clearly, revealing a strong interaction between PPy and Co₃O₄ nanoleaves.

  Figure 2

  

  Figure 2.  Morphology characterization: (a, b) SEM images of Co₃O⁴ nanoleaves. (c, d) SEM images of the Co³O⁴/PPy-120.

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  The OER catalytic activities of Co₃O₄/PPy samples are studied by LSV polarization curves. A small hump before catalytic reaction corresponds to redox peak of Co₃O₄ at 1.1–1.2 V, in which the valence state of cobalt ions ranges from Co²⁺/Co³⁺ to Co³⁺/Co⁴⁺ as shown in Fig. 3a. Those active sites facilitate OH^- absorption for OER in alkaline solution. In addition, the contacting with electrolyte plays an important role for OER. In order to optimize the performances of catalysts, the samples by 30 s electrodepositing PPy on Co₃O₄ products are denoted as Co₃O₄/PPy-30 and 120 s (Co₃O₄/PPy-120) and 240 s (Co₃O₄/PPy-240), respectively. As shown in Fig. 3b, Co₃O₄/PPy-120 shows the overpotential of 220 mV at 10 mV/s, which is lower than Co₃O₄/PPy-30 (260 mV), Co₃O₄/PPy-240 (280 mV) and pure Co₃O₄ (320 mV). As expected, Co₃O₄/PPy-120 shows the best catalytic performance with an overpotential of 310 mV at 100 mV/s in comparison to the other three materials. The overpotentials are comparable to those of cobalt-based catalysts [19-22].

  Figure 3

  

  Figure 3.  OER electrocatalytic performances: (a) LSV curves. (b) Overpotential at 10 and 100 mA/cm². (c) Tafel plot. (d) CV curves. (e) ECSA analysis. (f) Chronoamperometric plot.

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  Tafel plots are used to analyze OER kinetics from polarization curves (Fig. 3c). Co₃O₄/PPy-120 samples present a Tafel slope of 57.7 mV/dec, which is smaller than Co₃O₄/PPy-30 (89.8 mV/dec), Co₃O₄/PPy-240 (101.3 mV/dec) and pure Co₃O₄ (118.6 mV/dec). These results reveal that the introduction of PPy can accelerate electrons transferring to the electrode surface due to its superior conductivity, resulting in great improvement of catalytic kinetic.

  Fig. 3d shows CV curves of Co₃O₄/PPy-120 samples between 1.0 V and 1.1 V in non-Faradaic region. It can be clearly found that no visible redox peaks appear, indicating a double layer capacitive behavior. The double-layer capacitance (C_dl) is proportional to the electrochemical surface area (ECSA) [23]. As shown in Fig. 3e, the C_dl of Co₃O₄/PPy-120 is 0.031 mF/cm², while those of pure Co₃O₄, Co₃O₄/PPy-30 and Co₃O₄/PPy-240 are 0.066 mF/cm², 0.038 mF/cm² and 0.010 mF/cm², respectively. ECSA of Co₃O₄/ PPy-120 sample is not highest in these samples. It shows that the intrinsic synergistic effect between Co₃O₄ and PPy results in a low overpotential.

  To assess durability and stability of the as-prepared samples, the long-term cycling of Co₃O₄/PPy-120 is tested at a current density of 10 mA/cm² for 28 h. Fig. 3f shows the curve of Co₃O₄/ PPy-120 is smoother than that of bare Co₃O₄ nanoleaves. Moreover, this curve does not decline almost even after 28 h, suggesting its excellent durability, which could be also confirmed through the inset in Fig. 3f. The morphology of Co₃O₄/PPy-120 almost not changes with negligible collapse and aggregation after 28 h cycling. The above results show the amorphous PPy film can effectively prevent Co₃O₄ from collapsing due to volume changing repeatedly in OER process.

  The HER electrocatalytic activities of the as-prepared samples were also investigated in the same electrolyte condition. Similar to OER, Figs. 4a and b show Co₃O₄/PPy-120 samples with an overpotential of 140 mV to deliver a current density of 10 mA/ cm², which is lower than those of Co₃O₄/PPy-240 (175 mV), Co₃O₄/ PPy-30 (214 mV) and Co₃O₄ nanoleaves (260 mV). The values of overpotential are comparable to those of cobalt-based catalysts reported recently [19, 24, 25].

  Figure 4

  

  Figure 4.  HER electrocatalitic performances: (a) LSV curves. (b) The overpotential at 10 mA/cm². (c) Tafel plot. (d) Nyquist plots. (e) ECSA analysis. (f) Chronoamperometric plot.

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  In addition, Co₃O₄/PPy-120 samples possess favorable HER kinetics with a lower Tafel slope of 83 mV/dec compared to Co₃O₄/PPy-240 (95 mV), Co₃O₄/PPy-30 (141 mV) and Co₃O₄ nanoleaves (115 mV). The lower Tafel slope shows an operative Volmer-Heyrovsky reaction pathway. Typically, in alkaline solution, HER reaction mechanisms can be described through the following three steps [26]:

  (1)

  (2)

  (3)

  The OH^- ions by water splitting might be prone to chemisorbing on Co₃O₄ sites at Co₃O₄/PPy interface due to strong electrostatic force with positive Co²⁺/Co³⁺ ions. While PPy sites could promote H chemisorptions, which corresponds to Volmer process [27, 28]. It can also make hydrogen-metal binding energy to a relatively appropriate level, and facilitate desorption/release of H_ads from active sites.

  The EIS measurements of Co₃O₄/PPy-120 and Co₃O₄ samples are also carried out to investigate the kinetics performance in alkaline conditions (Fig. 4c). The slope of the straight part in Co₃O₄/PPy-120 samples is close to that of vertical part, revealing its small electrocatalytic resistance. And its slope is larger than that of Co₃O₄ nanoleaves, indicating a reduced electrocatalytic resistance owing to the introduction of PPy. Moreover, it could be observed that a short semicircle with a small charge transfer resistance (R_ct) in inset, further proving its fast electron transfer process [29, 30]. The above results reveal that HER kinetics of electrodes are greatly promoted after covering of a layer of amorphous PPy, which shortens electrons diffusion/transfer paths due to the synergistic effects between PPy and Co₃O₄ materials. Fig. 4d shows EIS of two electrode materials. It demonstrates low resistance of the composite.

  The ECSA of the as-prepared samples in HER is calculated according to the same method as OER. As shown in Fig. 4e, The ECSA of Co₃O₄/PPy-120 samples is 0.0046 mF/cm², while the ones of Co₃O₄ nanoleaves, Co₃O₄/PPy-30, Co₃O₄/PPy-240 are 0.0054, 0.0042 and 0.0025 m; F/cm², respectively. Durability is very important to assess HER catalytic performance of electrodes. In the first 4 h, the curve drops a lot (about 23 % loss), and then nearly remains stable with negligible loss, suggesting its superior stability as shown in Fig. 4f.

  In view of excellent OER and HER catalytic activities of Co₃O₄/ PPy-120 samples, their overall water splitting performances are investigated in two-electrode configuration in 1 mol/L alkaline solution. The device is shown in Figs. 5a and b. It can be observed that some bubbles appear from the surface of working electrodes. It shows a catalytic activity with a cell voltage of 1.67 V at 10 mA/cm², which is only 0.10 V higher than the benchmark Pt/C||RuO₂ electrodes. Long-term stability of Co₃O₄/PPy-120 electrode is tested in 1 mol/L KOH at 0.58 V potential in Fig. 5d. After 28 h, the current density remains basically constant, indicating superior stability of water splitting.

  Figure 5

  

  Figure 5.  Overall water splitting of the Co₃O₄/PPy-120: (a, b) Photograph of twoelectrode configuration. (c) Polarization curves. (d) Chronoamperometric plot.

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  In summary, Co₃O₄/PPy products are fabricated on nickel foam by a facile method. The depositing time of PPy seriously affect catalytic performances of the electrodes. Co₃O₄/PPy-120 samples possess superior OER and HER electrocatalytic performance with a low overpotential and remarkable durability. Meanwhile, water electrolyzer exhibits an excellent performance, which mainly benefit from the unique structures and synergistic effect between Co₃O₄ and PPy materials. The introduction of PPy improves the conductivity of electrodes and facilitates the electronic transfer during OER and HER.

  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 project is supported by the Opening Project of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) (No. 2020-KF-12).

  
  
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• 

  Figure 1  Chemical composition characterization (a) XRD patterns of Co₃O₄ nanoleaves and Co₃O₄/PPy-120. (b) FTIR of Co₃O₄/PPy-120. (c) Full spectrum, (d) Co 2p, (e) O 1s and (f) N 1s XPS spectra of the Co₃O₄/PPy-120.

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  Figure 2  Morphology characterization: (a, b) SEM images of Co₃O⁴ nanoleaves. (c, d) SEM images of the Co³O⁴/PPy-120.

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  Figure 3  OER electrocatalytic performances: (a) LSV curves. (b) Overpotential at 10 and 100 mA/cm². (c) Tafel plot. (d) CV curves. (e) ECSA analysis. (f) Chronoamperometric plot.

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  Figure 4  HER electrocatalitic performances: (a) LSV curves. (b) The overpotential at 10 mA/cm². (c) Tafel plot. (d) Nyquist plots. (e) ECSA analysis. (f) Chronoamperometric plot.

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  Figure 5  Overall water splitting of the Co₃O₄/PPy-120: (a, b) Photograph of twoelectrode configuration. (c) Polarization curves. (d) Chronoamperometric plot.

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