Electrocatalytic water splitting is regarded as a technology with great prospects for production of clean hydrogen energy [1-5]. However, there still exist great challenges to fabricate exceedingly efficient catalysts to accelerate the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in alkaline media, due to their sluggish kinetics and unseemly thermodynamics [6-10]. Especially for OER process, it contains complicated multistep proton-coupled electron transfer processes, responsible for a large overpotential [11]. At present, the commonly recognized OER and HER catalysts with prominent performance are Ir-based and Pt-based materials, respectively [12, 13]. But high price and limited reserve of these noble metals restrain their widespread application. Among all kinds of alternative catalysts for expensive materials, spinel-based oxides have gained enormous attention owing to rich element composition, variable valence state and good stability during electrocatalytic process [14-17]. Nevertheless, the electrocatalytic activity of spinel-based oxides without further modulating is still far from people's satisfaction.

Obviously, the catalytic activity of catalysts is highly associated with their elemental composition and structure traits [18, 19] Introducing heteroatoms into substrate materials has been certified a useful strategy to optimize materials' surface property and improve their catalytic performance [20, 21]. A critical point lies in the choice of heteroelement. It has been recorded that Ru has moderate H* and OH* intermediates absorption energy, due to the similar Ru-H bond energy (~65 kcal/mol) with Pt-H bond energy (~70 kcal/mol) and reasonable oxophilic character [22-25]. And the price of Ru is the cheapest among Pt-group metals [26]. Therefore, Ru has great potential to be used in electrocatalytic water splitting realm. For instance, Huang and co-workers reported the synthesis of Ru-doped CoCr LDHs as efficient a OER catalyst due to synergetic charge transfer [27]. Besides, Chen and co-workers designed RuCo nanoalloys catalyst, displaying impressive HER performance, with the overpotential of 28 mV being need to reach the current density of 10 mA/cm² [26]. More importantly, Qi and co-workers engineered Co MOF by introducing Ru, forming hollow (Ru-Co)O_x compound which can promote overall water splitting performance [28]. In addition, the morphology of catalysts also plays a pivotal role in catalytic process. The two-dimension (2D) nanomaterial with largely exposed surface atoms and edge sites is deemed as an appealing electrocatalyst for water splitting, which is not only in favor of the absorption of reactant species due to its large parts of superficial coordination-unsaturated atoms but also helpful to abridged diffusion path of reactants and products [21, 29, 30]. It has been reported that nanosheets array structure of various catalysts show excellent performance such as NiFe-LDH [21], CoOOH [31] and Co-NiS₂ [32]. Inspired by these relevant reports, it may be working to construct 2D spinel-based oxides catalysts integrated with Ru to achieve efficient overall water splitting.

Herein, we synthesized porous NiCo₂O₄ nanosheets (NSs) arrays coupled with Ru aligned on carbon fiber cloth (CFC) (denoted as Ru-NiCo₂O₄ NSs) through a facile impregnating method to introduce Ru. The obtained porous Ru-NiCo₂O₄ NSs features desirable elements composition and favorable structure virtues which showcase higher oxygen vacancy concentration, decreased charge transfer impedance and more accessible active sites. Moreover, the introduction of Ru greatly reduces the activation energy of OER process. Accordingly, Ru-NiCo₂O₄ NSs manifest superior OER performance with only an overpotential of 230 mV at the current density of 10 mA/cm² and long-term stability in alkaline media. Furthermore, Ru-NiCo₂O₄ NSs show superior HER activity than NiCo₂O₄ NSs counterpart. The integrated alkaline electrolyzer using Ru-NiCo₂O₄ NSs as bifunctional electrodes materials needs 1.60 V to reach the current density of 10 mA/cm². And the activity and durability of Ru-NiCo₂O₄ NSs overall water splitting system are both superior to Pt/C||RuO₂ system. This work presents a viable guide for developing efficient spinel-based oxides electrocatalysts.

The synthesis of NiCo₂O₄ NSs refers to the previous report [20]. Firstly, the precursor NiCo-LDH was synthesized by a facile electro-deposition method. 0.1 mol/L Ni(NO₃)₂·6H₂O (20 mL) and 0.1 mol/L Co(NO₃)₂·6H₂O (40 mL) were mixed as electrodeposition electrolyte. A piece of carbon fabric cloth (CFC) was used as electro-deposition substrate. The process was performed at −1 V (vs. Ag/AgCl) for 15 min. After finishing, the final product was rinsed with DI water several times, and then dried in 50 ℃ electro-oven. The NiCo₂O₄ NSs were obtained by calcining NiCo-LDH at 300 ℃ for 2 h with the heating rate (2 ℃/min) in air atmosphere. Then, the Ru-NiCo₂O₄ was synthesized by impregnating NiCo₂O₄ NSs in RuCl₃ solution (10 mg/mL) for 20 min, then dried in 50 ℃ electro-oven and further annealed at 300 ℃ for 2 h in air atmosphere.

X-ray diffraction (XRD) measurement was conducted on Rigaku MiniFlex 600 diffractometer from 10° to 90° under a constant voltage of 40 kV and constant current of 15 mA. The morphology characterization was obtained by ThermoFisher Apreo S filed-emission scanning electron microscopy (FESEM) with a voltage of 30 kV. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and EDS mapping pictures of samples was collected on a Tecnai G2 F30 filed emission transmission electron microscopy. Raman spectra were measured on a Horiba LabRAM HR Evolution spectrophotometer with a laser line of 532 nm. Electron spin resonance (ESR) experiments were made on JES-FA300 electron spin resonance spectrometer. X-ray photoelectron spectroscopy (XPS) was performed by a VG ESCALAB 220I-XL device. The data were corrected using C 1s line as standard. Ultraviolet photoelectron spectrometer (UPS) was conducted on Kratos Axis Supra device. The contact angle test was carried out on a Kruss DSA100 optical contact angle/surface tension meter.

Electrochemical measurements were performed on CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Corp., China) with a standard three-electrode configuration, with Pt plate and Hg/HgO as counter electrode and reference electrode, respectively, for OER test. While for HER test, the counter electrode was changed to a carbon rod. The catalysts grown on CFC were used as working electrodes directly. 1.0 mol/L KOH solution was used as electrolyte and Linear scan voltammogram (LSV) curves were carried out at a scan rate of 2 mV/s. All the electrode potential was converted to the reversible hydrogen electrode (RHE), using the equation E_RHE = E_Hg/HgO + 0.0591 × pH + 0.098 V.

Scheme 1 illustrates the synthesis procedure of catalysts. At first, the phases of samples obtained were analyzed by the X-ray diffraction (XRD) technique. Fig. 1a shows that the diffraction peaks of the pristine NiCo₂O₄ NSs can well match with cubic NiCo₂O₄ (JCPDS No. 20–781, space group F*3). And the Ru-NiCo₂O₄ NSs possess similar diffraction patterns with the pristine NiCo₂O₄ NSs without related Ru oxides phase being detected, implying Ru was doped successfully in NiCo₂O₄ NSs preliminarily. As shown in Fig. 1b and Fig. S1a (Supporting information), the original NiCo₂O₄ NSs presents interlaced nanosheets morphology grown on CFC characterized by scanning electron microscope (SEM). From the transmission electron microscope (TEM) image (Fig. 1c), it can be detected that the nanosheets present porous property. It can be seen from Figs. 1d and e and Fig. S1b (Supporting information), the Ru-NiCo₂O₄ NSs exhibit similar morphology feature with the pristine NiCo₂O₄ NSs, declaring the introducing of Ru does not ruin porous nanosheets structure of NiCo₂O₄ NSs. It has been documented that porous structure is in favor of accelerating mass transfer process and enhancing the accessibility of active sites [33-35]. Furthermore, high-resolution transmission electron microscopy (HRTEM) image of Ru-NiCo₂O₄ NSs exhibits lattice spacing of 0.24 nm, which can be assigned to (311) plane of NiCo₂O₄ (Fig. 1f). The high-angle annular dark-field scanning TEM image (Fig. 1g) and TEM energy-dispersive spectra (EDS) mapping images (Fig. 1h) show the uniform distribution of Ru, Ni, Co and O in the checked region. Furthermore, Fig. 1i presents the crystal structure of NiCo₂O₄.

Moreover, the X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectrometer (UPS), Raman spectra and electron spin-resonance spectroscopy (ESR) were carried out to further probe the structural information of catalysts obtained. As shown in XPS survey spectrums of Ru-NiCo₂O₄ NSs and NiCo₂O₄ NSs (Fig. S2 in Supporting information), the Ru 3p signal lays in Ru-NiCo₂O₄ NSs, while it does not appear in NiCo₂O₄ NSs, confirming Ru was introduced into NiCo₂O₄ NSs matrix successfully, which is consistent with the TEM-EDS result. The high-resolution O1s XPS spectra (Fig. 2a) can be deconvoluted into three parts: lattice oxygen, oxygen vacancy, or hydroxyl group and absorbed oxygen from low binding energy to high binding energy in sequence [36, 37] Compared with NiCo₂O₄ NSs, the binding energy of O 1s XPS spectra for Ru-NiCo₂O₄ NSs positively shifts ~0.2 eV, exhibiting a slight electron-deficiency O state. Furthermore, the binding energy of Ru 3p_3/2 in Ru-NiCo₂O₄ NSs is 463.7 eV (Fig. 2b), which is located between the reference samples RuCl₃ (Ru³⁺) and RuO₂ (Ru⁴⁺), implying that the average valence state of Ru is between +3 and +4 in Ru-NiCo₂O₄ NSs [27, 38]. Besides, there is no distinct difference between NiCo₂O₄ NSs and Ru-NiCo₂O₄ NSs concerning Co and Ni 2p XPS spectra, affirming the preservation of NiCo₂O₄ phase after Ru modification (Fig. S3 in Supporting information). In addition, the UPS was conducted to probe dynamics of electrons on the surface of the as-prepared catalysts [39]. As shown in Fig. 2c, the Ru-NiCo₂O₄ NSs and NiCo₂O₄ NSs exhibit different secondary electron cutoff energy with 16.34 eV for Ru-NiCo₂O₄ NSs and 16.22 eV for NiCo₂O₄ NSs and thus the corresponding work function are 4.88 eV and 5.00 eV for Ru-NiCo₂O₄ NSs and NiCo₂O₄ NSs, respectively. The smaller work function indicates that Ru modulation facilitates the electronic properties of Ru-NiCo₂O₄ NSs, which is contributed to enhancing catalytic activity [40]. Besides, Raman as a surface-sensitive spectrum technology was further applied to gain insights into NiCo₂O₄ NSs before and after Ru modification. As shown in Fig. 2d, the Ru-NiCo₂O₄ NSs showcases similar spectra patterns with the pristine NiCo₂O₄ NSs except for a slight blue shift, which is due to the doping of Ru. Furthermore, the Raman spectra of Ru-NiCo₂O₄ NSs exhibit different spectra feature compared with RuO₂ (Fig. S4 in Supporting information), which implies the Ru may exist as a highly dispersed state in NiCo₂O₄ NSs substrate and further indicating that Ru are doped into NiCo₂O₄ NSs successfully. Moreover, ESR spectra were conducted to investigate the change of oxygen vacancy. From Fig. 2e, it can be seen that Ru-NiCo₂O₄ NSs owns a stronger ESR signal originating from the electron trapped at oxygen vacancies than that of NiCo₂O₄ counterpart [41, 42]. Therefore, we can deduce that the Ru-NiCo₂O₄ NSs possess more oxygen vacancies than NiCo₂O₄ NSs, which is helpful to facilitate water dissociation [43]. Furthermore, the excellent hydrophilicity feature of Ru-NiCo₂O₄ NSs supported on CFC enables the easy accessibility of reactants and fast release of product molecules, thus accelerating catalytic process (Fig. 2f, the inset shows the contact angle of bare CFC) [44].

The electrocatalytic performance of catalysts prepared was evaluated by a standard three-electrode configuration using 1.0 mol/L KOH as electrolyte. Linear scan voltammogram (LSV) curves of OER (Fig. 3a) show that the required overpotentials for Ru-NiCo₂O₄ NSs, NiCo₂O₄ NSs and RuO₂ to reach the current density of 10 mA/cm² are 230 mV, 340 mV and 329 mV, respectively, manifesting excellent OER activity of Ru-NiCo₂O₄ NSs. Furthermore, the Fig. 3b illustrates Ru-NiCo₂O₄ NSs possesses a small Tafel slope (79 mV/dec) lower than that of NiCo₂O₄ NSs (95 mV/dec) and RuO₂ (82 mV/dec) counterparts, indicating the accelerated OER kinetic for Ru-NiCo₂O₄ NSs. Besides, the stability is also a vital aspect for assessing the performance of catalysts. From Fig. 3g (green line), it can be seen that Ru-NiCo₂O₄ NSs show nearly no activity decay in the period of 42 h OER test. To gain the insights about the origin of activity, the electrochemical impedance spectroscopy (EIS) of Ru-NiCo₂O₄ NSs and NiCo₂O₄ NSs were collected to investigate charge transfer ability. As shown in Fig. 3c, Ru-NiCo₂O₄ NSs exhibits a smaller radius than that of NiCo₂O₄ NSs counterpart, implying a lower charge transfer (R_ct) impedance of Ru-NiCo₂O₄ NSs for OER process, which is conducive to improving OER performance [45]. In addition, electrochemical double-layer capacitance (C_dl) was tested to access the electrochemical surface area (ECSA) of the as-prepared catalysts (Fig. S5 in Supporting information) [46]. The value of C_dl for Ru-NiCo₂O₄ NSs and NiCo₂O₄ NSs are 278 mF/cm² and 200 mF/cm² (Fig. 3h), respectively, suggesting the larger ECSA of Ru-NiCo₂O₄ NSs. The larger ECSA manifests more accessible active sites on Ru-NiCo₂O₄ NSs surface in catalytic process, thus boosting OER performance. Moreover, LSV curves at various temperature for Ru-NiCo₂O₄ NSs and NiCo₂O₄ NSs were analyzed to evaluate apparent kinetic barriers in OER process. Extended the obtained Arrhenius plots to the OER thermodynamic equilibrium potential, the value of apparent activation energy (E_a) can be obtained [47, 48]. As illustrated in Fig. S6 (Supporting information), the value of E_a of Ru-NiCo₂O₄ NSs is 26.6 kJ/mol while the E_a of NiCo₂O₄ NSs is 45.9 kJ/mol, implying Ru modification availably decrease E_a for OER course.

Due to the similar Ru-H bond energy to Pt-H bond energy [22], the HER performance of NiCo₂O₄ NSs after Ru modification may also be improved. As shown in LSV curves of HER (Fig. 3d), an overpotential of 313 mV is needed for NiCo₂O₄ NSs to reach current density of 10 mA/cm², while the value decreases to 117 mV for Ru-NiCo₂O₄ NSs. The Tafel slopes are analyzed to be 88 mV/dec and 97 mV/dec for Ru-NiCo₂O₄ NSs and NiCo₂O₄ NSs, respectively (Fig. 3e), declaring a more favorable HER kinetics for Ru-NiCo₂O₄ NSs. Regarding long-time stability, the chronopotentiometry test shows no obvious activity degradation of Ru-NiCo₂O₄ NSs after 42 h (Fig. 3g, blue line). Furthermore, EIS plots were collected to assess charge transfer feature of materials between electrode and electrolyte interface. As illustrated in Fig. 3f, the Ru-NiCo₂O₄ NSs possess a smaller R_ct than that of NiCo₂O₄ NSs, manifesting that Ru modification accelerates charge transfer in HER process, thus promoting HER activity. Fig. 3i displays a larger C_dl for Ru-NiCo₂O₄ NSs (21.3 mF/cm²) than NiCo₂O₄ NSs (2.2 mF/cm²) based on CVs test (Fig. S7 in Supporting information), thereby a larger ECSA of Ru-NiCo₂O₄ NSs for HER catalysis, which is also favorable to promote HER performance.

In situ Raman spectra characterization analysis was performed to reveal changes on Ru-NiCo₂O₄ NSs electrodes during the OER and HER processes. The peak located at ~470 cm⁻¹ and ~670 cm⁻¹ are attributed to E_g phonon modes and A_1g phonon modes of Ru-NiCo₂O₄ NSs, respectively (Fig. 4). Potentials from open circuit voltage (OCV) to 1.6 V vs. RHE were applied for OER in suit Raman measurements. The corresponding spectra result are shown in Fig. 4a, characteristic peaks associated with E_g and A_1g phonon modes exhibits no differentiable variation, implying good stability of Ru-NiCo₂O₄ NSs in OER process. For HER, we applied potential from OCV to −0.25 V vs. RHE. The corresponding spectra (Fig. 4b) obtained also show no obvious change, manifesting good HER stability of Ru-NiCo₂O₄ NSs. Besides, SEM characterization was conducted to probe the change of morphology of Ru-NiCo₂O₄ NSs after OER and HER. As shown in Fig. S8 (Supporting information), the morphology of Ru-NiCo₂O₄ NSs after OER and HER remains nanosheets structure. All evidences illustrate the good stability of Ru-NiCo₂O₄ NSs.

Based on the enhanced OER and HER performance of Ru-NiCo₂O₄ NSs, the overall water-splitting performance was assessed by utilizing Ru-NiCo₂O₄ NSs as both anode and cathode catalysts in 1.0 mol/L KOH solution. Fig. 5a illustrates the configuration of overall water splitting device. Fig. 5b showcases the LSV curves of Ru-NiCo₂O₄ NSs for the overall water splitting at various temperatures (25, 45 and 65 ℃). A cell voltage of 1.60 V was needed to reach the current density of 10 mA/cm² at 25 ℃ for Ru-NiCo₂O₄ NSs couple, which is lower than that of Pt/C||RuO₂ set (1.64 V) (Fig. 5c). The overall water splitting performance of Ru-NiCo₂O₄ NSs system can be further improve by increasing temperature. As exhibited in Fig. 5d, cell voltages needed to obtain the current density of 10 mA/cm² at 45 ℃ and 65 ℃ are 1.54 V and 1.49 V, respectively. In addition, Ru-NiCo₂O₄ NSs set displays good stability on a long-time testing at different current densities 10, 20, 40, 80 mA/cm² (Fig. 5e), while for Pt/C||RuO₂ system, the cell voltages enlarge constantly over time (Fig. 5f). All the results verify Ru-NiCo₂O₄ NSs can be used as efficient overall water splitting catalysts.

In summary, the porous NiCo₂O₄ NSs integrated with Ru through a facile impregnation method is devised. The obtained Ru-NiCo₂O₄ NSs catalysts exhibit superior OER activity with a low overpotential of 230 mV at 10 mA/cm², the enhanced HER activity and robust stability. The promotion of catalytic performance of Ru-NiCo₂O₄ NSs can be attributed to the low charge transfer impedance, more accessible active sites and higher oxygen vacancy concentration. Experimental results demonstrate that the introduction of Ru efficiently regulates the surface structure property of NiCo₂O₄ NSs. The optimized components and structural advantages facilitate synergistically OER and HER process. This work presents a practicable strategy for improving the catalytic performance of spine-based oxides for both 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

We acknowledge support from the National Natural Science Foundation of China (Nos. 21922105 and 21931001), the National Key R & D Program of China (2021YFA1501101) as well as the Special Fund Project of Guiding Scientific and Technological Innovation Development of Gansu Province (No. 2019ZX-04) and the 111 Project (No. B20027). We also acknowledge support by the Fundamental Research Funds for the Central Universities (Nos. lzujbky-2021-pd04, lzujbky-2021-sp41, lzujbky-2021-it12 and lzujbky-2021-37). J. Yin acknowledges the support of the China Postdoctoral Science Foundation (No. 2021M691375) and the China National Postdoctoral Program for Innovative Talents (No. BX20200157).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.12.058.