Synergistic Interface Engineering of RuO2/Co3O4 Heterostructures for Enhanced Overall Water Splitting in Acidic Media

Designing nanocomposites with heterointerface as bifunctional electrocatalysts is a potential strategy to overcome the intrinsic activity limitation of electrocatalytic water splitting in acidic media, but it remains challenging. Herein, the highly efficient RuO2/Co3O4 electrocatalyst with a uniform nanoflower structure is prepared by hydrothermal growth combined with interface engineering. Benefiting from the unique nanostructure, the migration of electrons and intermediates is optimized by the sufficient exposure of abundant micropores and defects. Moreover, the formation of strong electronic interaction at the RuO2/Co3O4 heterointerfaces boosts the electrochemical active surface area and accelerates the reaction kinetics, which effectively improve the catalytic activity and stability of the catalyst. Based on enhanced intrinsic activity and electron transfer, the as‐synthesized RuO2/Co3O4 displays impressive hydrogen evolution reaction and oxygen evolution reaction activity, which respectively require low overpotentials of 240 and 100 mV to achieve a current density of 10 mA cm−2 in 0.5 m H2SO4. As a bifunctional electrode, RuO2/Co3O4 exhibits a low operating voltage of 1.58 V at 10 mA cm−2 for overall electrochemical water splitting. This study demonstrates the importance of heterostructure engineering in providing an avenue to achieve acid‐stable bifunctional electrocatalysts for energy conversion applications.


Introduction
[3] Typically, green hydrogen is produced via electrochemical water splitting driven by renewable electricity. [2,4,5]The overall electrocatalytic water splitting can be divided into two half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), occurring at the cathode and the anode, respectively. [6,7]Notably, the proton exchange membrane (PEM) electrolyzer operated in acidic media and has been considered as a prospective technology to satisfy industrial hydrogen production due to its higher-voltage efficiency and current density than conventional alkaline media. [8]owever, the sluggish kinetics and the high applied voltage of the electrochemical reaction process remain major bottlenecks for boosting the overall efficiency of electrochemical water splitting. [9]Nowadays many excellent catalysts have been developed in alkaline solutions, [10] but most of these catalysts show inferior activities and durability in acidic media.As a result, catalysts suitable for acidic environments still mainly rely on noble metal/metal oxides (e.g., Pt and RuO 2 ).The industrial application of noble metal-based catalysts has been severely limited due to their high prices and scarce reserves.Meanwhile, these anodic and cathodic catalysts in electrochemical water splitting are often separated due to their single-catalytic function. [11]Hence, it remains a grand challenge to explore bifunctional catalysts with low cost, superior activity, and durability for acidic overall water splitting.
Recently, transition metal oxide (TMO) as an effective electrocatalyst has attracted the attention of researchers due to its cost effectiveness, superior corrosion resistance, and catalytic activity. [12]As a typical TMO, cobalt-based oxide has become a promising research hotspot in bifunctional electrocatalyst design because of its environmental friendliness, abundant content, and thermodynamic stability.However, CoO x exhibits poor overpotential and stability as compared with noble metals, resulting in unnecessary energy consumption and higher operating cost. [13,14]It is worthwhile to develop an efficient strategy to enhance both activity and durability of CoO x .The surface area and electronic states are vital prerequisites for determining the electrocatalytic activity of cobalt oxide.To increase the surface area, tremendous research efforts have been devoted to synthesizing CoO x materials with various structures, including nanosheets, [15] nanowires, [16] nanomembranes, [17] and flower-like structures. [18]Nanostructural engineering has been demonstrated to be an effective approach to expose greater active sites and defect sites.The porous structures are beneficial for the transportation of electrons/reactants, thus dramatically accelerating the chemical reaction kinetics. [4,19]Unfortunately, the performance of bare CoO x for both HER and OER still limits its practical applications due to the high overpotential and poor intrinsic activity.
In response, the design of the interfacial structure, which consists of two or more different closely linked components, has been employed as a medium to facilitate electrons or intermediates transfer between different components. [20]The physical and chemical properties of electrocatalysts can be regulated by reasonable control of the interfacial atomic arrangement. [21]onstructing a multiphase interface between CoO x and other active metals is a potentially effective strategy to increase the electrocatalytic performance.The interfacial electronic effects in heterostructure help to attain optimal adsorption energy of key intermediates, which can accelerate the adsorption/desorption of hydrogen and improve the catalytic activity of the watersplitting reaction. [22,23][26] With a relatively cheaper price and stable catalytic properties, RuO 2 is regarded as a benchmark OER electrocatalyst in acidic electrolytes. [27][30] Therefore, creating strong coupling effects between multicomponents could result in superior activity and stability of the catalyst. [31]Hence, the rational design of nanostructure and heterostructure engineering between cobalt-based oxides and RuO 2 is worth exploring to develop next-generation bifunctional electrocatalysts. [32]erein, we synthesized RuO 2 /Co 3 O 4 with strong heterointerfaces and flower-like nanoarray structure through a simple hydrothermal strategy as a bifunctional electrocatalyst for water splitting.The catalyst is endowed with abundant active sites, strong electronic interaction, and optimization of intermediate adsorption.This is attributed to the unique nanoflower morphology and strong electronic coupling induced by interfaces between RuO 2 and Co 3 O 4 .As a result, RuO 2 /Co 3 O 4 exhibits low overpotentials of 230 and 100 mV for OER and HER at a current density of 10 mA cm À2 in 0.5 M H 2 SO 4 , respectively.Furthermore, the assembled electrolytic cell only needs a voltage of 1.58 V to achieve 10 mA cm À2 under the same conditions.This work demonstrates that the rational construction of heterogeneous materials can exert the coupling effect of interface between multiple components for superior electrocatalytic activity compared with single material.

Results and Discussion
The simple preparation process of the RuO 2 /Co 3 O 4 heterostructure is illustrated in Figure 1a, which involves the growth of Ru/ Co(OH) x flower-like nanostructures and oxidation treatment.First, the hydroxide of Ru and Co is formed by hydrothermal reaction and then the dried black-purple Ru/Co(OH) x was calcined at 350 °C under air atmosphere for 3 h to form the final black powder named RuO 2 /Co 3 O 4 (Figure S1, Supporting Information).Co 3 O 4 and RuO 2 serve as references.The morphology and interface structure of the catalyst were investigated by a scanning electron microscope (SEM) and transmission electron microscope (TEM).SEM images showed that Ru/Co(OH) x formed 3Dl acicular hierarchical structures with smooth surfaces (Figure S2, Supporting Information).After calcining, the resulting RuO 2 /Co 3 O 4 interfacial structure inherits the previous morphology (Figure 1b), and plenty of defects and pores are observed on the rough surface of RuO 2 /Co 3 O 4 (Figure 1c,d).The porous structure not only benefits the adsorption/desorption of water molecules but also facilitates the exposure of active sites and rapid bubble release for the catalytic reaction. [18]Besides, Co 3 O 4 shared morphological similarities with RuO 2 /Co 3 O 4 , illustrating that the presence of RuO 2 does not affect the growth mechanism of this synthesis method (Figure S3, Supporting Information). [33]The Brunauer-Emmett-Teller (BET) adsorption-desorption isotherm indicates that the specific surface area of RuO 2 /Co 3 O 4 (63.44 m 2 g À1 ) is larger than Co 3 O 4 (38.36 m 2 g À1 ) and RuO 2 (12.69 m 2 g À1 ).This result verifies that the formation of heterointerface is conducive to boosting the specific surface area significantly (Figure S4, Supporting Information).The TEM images present that the nanorod with a diameter of 80-100 nm is constructed by abundant nanoparticles (Figure 1e,f S1, Supporting Information), the Ru content in RuO 2 /Co 3 O 4 is verified to be only 2.55%, which is consistent with the previous analysis.Comparing the products synthesized with different amounts of RuCl 3 , it is found that the Ru/Co ratio in RuO 2 /Co 3 O 4 products is optimal when 50 mg RuCl 3 was used (Figure S5, Supporting Information).These results demonstrate that the RuO 2 /Co 3 O 4 catalyst with special nanostructure and heterointerface can be synthesized successfully through a doublephase coupling strategy.
The crystal structure transformation of RuO 2 /Co 3 O 4 was analyzed by X-ray diffraction (XRD), with the pure Co 3 O 4 and RuO 2 serving as reference samples (Figure 2a).In the spectra of the RuO 2 /Co 3 O 4 sample, the main peaks at 36.8, 59.3, and 65.2 match well with the (311), (511), and (440) planes of Co 3 O 4 (JCPDS No. 74-2120), [26] while the diffraction peaks at 28.  [26] which confirms that the synthesized catalyst has strong chemical coupling effects arising from the interfacial interactions between RuO 2 and Co 3 O 4 . [34,35][38] To gain a deeper insight into the electron transfer behavior between RuO 2 and Co 3 O 4 , an ultraviolet photoelectron spectrometer (UPS) test was performed to calculate the work function of pure Co 3 O 4 and RuO 2 /Co 3 O 4 (Figure 2c). [39]As shown in Figure 2c, the measured secondary electron cutoff energy (E cutoff ) of RuO 2 /Co 3 O 4 and Co 3 O 4 are 16.98 and 16.66 eV, respectively.According to band theory, WF is calculated by subtracting (E Cutoff ÀE Fermi ) from the excitation energy of He I (21.22 eV). [40]As the work function is negatively correlated to the value of E Cutoff , the work function of RuO 2 /Co 3 O 4 is smaller than that of Co 3 O 4 , which suggests a higher electrical conductivity and a significant modulation of electronic structure on the heterointerface between RuO 2 and Co 3 O 4 . [41]-ray photoelectron spectroscopy (XPS) was employed to explore the surface chemical and states of RuO 2 / Co 3 O 4 .The XPS survey spectra confirm that Co, O, and Ru are the main elements in the sample (Figure S6, Supporting Information).The high-resolution Ru 3p spectra of RuO 2 and RuO 2 /Co 3 O 4 discover that the characteristic peaks of Ru 3p 3/2 and Ru 3p 1/2 can be observed at binding energies of 461.5 and 483.75 eV, and their two shakeup satellites are also observed (denoted as "sat"), [34] suggesting the valence state of Ru is þ4 (Figure 2d).The XPS results in Figure 2e show that the highresolution Co 2p spectrum of Co 3 O 4 /RuO 2 can be deconvoluted into six subpeaks, and the peaks at 794.7/779.5 and 780.89/ 796.67 eV match well with Co 3þ and Co 2þ ions, with two satellite peaks at 788.18 and 804.57eV.[8,42] Significantly, compared with pure Co 3 O 4 , the binding energy of Co in RuO 2 /Co 3 O 4 positively shifts by 0.6 eV, illustrating the decrease of electron density of Co.In contrast, a slight negative shift is observed in the Ru 3p for RuO 2 /Co 3 O 4 in comparison with the RuO 2 reference.This serves as a direct demonstration of the electron transfer from Co to Ru sites at the interface.[7,21,43] As shown in Figure 2f, two different oxygen species are observed in the O 1s spectra of RuO 2 and Co 3 O 4 , which are attributed to lattice oxygen and adsorbed oxygen (O ads ), [26] respectively.Typically, due to the partial doping of the Co atom, the oxygen vacancies (O v ) are discovered in the O 1s spectra of RuO 2 /Co 3 O 4 .O v serves as a balance between the Co charge and the Ru charge to maintain electrical neutrality, [44,45] which is consistent with the EPR results.For RuO 2 /Co 3 O 4 , the peaks related to lattice oxygen and adsorbed oxygen shift toward higher binding energy, indicating that the formation of the Ru─O─Co bond at the heterogeneous interface promotes the interaction between atoms.[46] Furthermore, the existence of oxygen vacancies promotes the transfer of electrons to adjacent oxygen vacancies and reduces the density of electron clouds, leading to the positive shift of binding energy as well.[42,46] Based upon the above results, the establishment of heterointerfaces and electron transfer mechanism of RuO 2 /Co 3 O 4 with interfacial chemical bonding is demonstrated in Figure 2g.
The electrochemical OER activities of as-prepared catalysts were evaluated in 0.    3c).To further study the electrocatalytic properties of RuO 2 /Co 3 O 4 , impedance spectroscopy was performed.The Nyquist plots show that RuO 2 /Co 3 O 4 has the smallest semicircle in the high-frequency charge transfer resistance, suggesting its higher electron transfer ability and lowest response charge transfer resistance (Figure 3d).Similarly, the Mott-Schottky tests prove that RuO 2 /Co 3 O 4 exhibits a faster electron conduction rate due to the larger slope compared with RuO 2 (Figure S12, Supporting Information).Then, the electrochemical active surface area is assessed by calculating the double-layer capacitance (C dl ), which correlates with the amount of accessible active sites (Figure S13, Supporting Information). [47]The highest C dl value of RuO 2 /Co 3 O 4 (23.39 mF cm À2 ) is beneficial from the formation of the heterointerface between RuO 2 and Co 3 O 4 , which creates more accessible active sites (Figure 3e).To clarify the enhanced OER performance, the turnover frequency (TOF) was calculated to evaluate the intrinsic activity of the RuO 2 / Co 3 O 4 (Figure S14, Supporting Information).As displayed in Figure S14, Supporting Information, RuO 2 /Co 3 O 4 possesses a higher intrinsic activity, which is consistent with the result of OER LSV curves normalized by the electrochemical surface area (ECSA) (Figure S15, Supporting Information).To verify the practicality of the catalyst, a long-time durability test was performed using a chronopotentiometric method.As shown in Figure 3f, RuO 2 /Co 3 O 4 still exhibited excellent stability with minuscule change after a long-time operation of 20 h at 10 mA cm À2 , which is superior to commercial IrO 2 (Figure S10b, Supporting Information).Moreover, the robustness of RuO 2 /Co 3 O 4 is also demonstrated by the almost perfectly overlapping OER polarization curves before and after 1000 cyclic voltammetry (CV) cycles (Figure 3f ).Compared to other recently reported OER catalysts, RuO 2 /Co 3 O 4 shows excellent performance and stability duration for OER (Figure 3g, Table S2, Supporting Information).Therefore, the rational design of heterostructure has been demonstrated to enhance catalytic activities toward acidic OER performance by creating more active sites and reducing charge transfer resistance.
To further understand the chemical and structural evolution of the electrocatalyst during the OER reaction process, the catalyst after 1000 cycles of CV under OER conditions (RuO 2 /Co 3 O 4after) was characterized.As presented in The SEM images show that compared with the initial RuO 2 / Co 3 O 4 , the morphology of the sample after CV tests still retained the original nanoflower structure (Figure 4b,c).The unchanged morphology during the OER reaction proves that the unique flower-like nanoarray plays an active role in the OER reaction process.To confirm the possible changes in interfacial structure, TEM and high-resolution TEM (HRTEM) were employed.As revealed in the TEM image (Figure 4d), RuO 2 /Co 3 O 4 after OER shows a flower-like nanostructure formed by interconnected nanorods as before.The HRTEM image shows the different lattice fringes identified as RuO 2 and Co 3 O 4 , consistently indicating that the heterostructures with uniform interfacial contacts are well preserved after the test (Figure 4e,f ). [13]Raman spectroscopy was employed to further explore the evolution of the catalyst surface after the OER reaction (Figure S16  OER process, which triggers the slight reduction in the intensity of Ru 3p XPS peaks after the OER reaction period (Figure 4g). [48]ince Ru is one of the vital active sites for OER in acidic environment, the increase in overpotential should be related to the dissolution of Ru, and the shift to the higher binding energy of Ru 3p peaks can be considered another evidence for the change in the electronic structure of Ru 4þ (Figure 4h). [31,49]s shown in Figure 4h, during the OER process, the ratio of Co 3þ to Co 2þ increases slightly, which indicates that the oxidation dissolution of Co accounts for the degradation of OER performance as well. [4]In Figure 4i, the intensity changes of lattice oxygen and hydroxyl oxygen show opposite trends along with that of the O v peak decreasing significantly, which may be caused by the multiple interface effects of the catalyst. [26,34]The above analyses prove that the heterogeneous catalyst with a double-phase coupling mechanism shows lasting durability in acidic media.
Apart from the prominent OER performance, RuO 2 /Co 3 O 4 further demonstrates outstanding HER activity in 0.5 M H 2 SO 4 (Figure 5a,b).Similarly, RuO 2 /Co 3 O 4 -50 was chosen for further testing due to its best HER electronic activity (Figure S18, Supporting Information).Impressively, RuO 2 / Co 3 O 4 exhibits an overpotential of 100 mV to reach a current density of 10 mA cm À2 in acidic media for HER, which is slightly higher than that of commercial Pt/C but lower than RuO 2 and Co 3 O 4 .As illustrated in Figure 5b, RuO 2 /Co 3 O 4 has a remarkably lower Tafel slope of 85.1 mA dec À1 than Co 3 O 4 in an acidic solution.The improvement of the HER activity can be attributed to chemical bonding across the heterointerface on the cathodic side, which helps to obtain better performance than bare RuO 2 and Co 3 O 4 . [50]Encouraged by the remarkable performance of the electrocatalyst for OER and HER, an electrolyzer comprising two electrodes was assembled using RuO 2 / Co 3 O 4 as both the anode and cathode to assess the activity of overall water splitting in an acidic system (Figure 5d).The electrolyzer with commercial Pt/C cathode coupled with RuO 2 anode was fabricated as a reference.The assembled cell equipped with RuO 2 /Co 3 O 4 electrodes only requires an applied voltage of 1.58 V to reach 10 mA cm À2 , which is just 40 mV higher than the reference (Figure 5c).In addition, RuO 2 /Co 3 O 4 displays high durability, showing only 8 mV decay after 1000 CV cycles (Figure 5e).The long-term stability of the RuO 2 /Co 3 O 4 was also tested through constant chronopotentiometry at 10 mA cm À2 (Figure 5d).After a continuous testing period of 10 h, the water electrolytic potential exhibited high stability with slight current decay in 0.

Conclusion
In summary, we developed an efficient catalyst with heterogeneous interfaces consisting of RuO ).The typical boundary area of RuO 2 and Co 3 O 4 can be observed from high-resolution transmission electron microscope images (HRTEM, Figure1g), displaying the heterogeneous interface between RuO 2 and Co 3 O 4 clearly.The lattice fringes with an interplanar spacing of 0.455 and 0.258 nm are indexed to the (111) and (101) planes of Co 3 O 4 and RuO 2 , respectively.The (111) facets of Co 3 O 4 and the neighboring (101) surfaces of RuO 2 constitute the interfaces in RuO 2 / Co 3 O 4 heterostructures.The energy-dispersive spectroscope (EDS) elemental mappings confirm the presence and uniform distribution of Co, Ru, and O on RuO 2 /Co 3 O 4 samples.Compared with Co, fewer Ru nanoparticles are observed in the same area, which proves the low Ru content of the catalyst.Additionally, through the inductively coupled plasma (ICP) detection (Table 1, 35.2, and 54.4 correspond to the (110), (101), and (211) planes of RuO 2 (JCPDS No. 71-2273).XRD results indicate that RuO 2 and Co 3 O 4 coexisted as heterostructures in RuO 2 /Co 3 O 4 , shows the set of paramagnetic signals centered around g = 2.003 corresponding to the oxygen vacancies of RuO 2 /Co 3 O 4 .Compared with Co 3 O 4 and RuO 2 , the spectrum of RuO 2 /Co 3 O 4 shows a clearer resonance signal, indicating that the heterogeneous interface can increase the amount of oxygen vacancies of RuO 2 /Co 3 O 4 nanocomplex and improve the defect concentration.
5 M H 2 SO 4 solution by a standard threeelectrode system.The linear sweep voltammetry (LSV) measurement of different Ru content reveals that RuO 2 /Co 3 O 4 -50 possesses the lowest overpotential in acidic environment, which can be further demonstrated by the lowest Tafel slope and
Figure 4a, similar to primitive RuO 2 /Co 3 O 4 , the XRD peak intensity of RuO 2 and Co 3 O 4 decreases but still possesses identical peaks in RuO 2 / Co 3 O 4 after OER, demonstrating the structural stability during the OER reaction.Besides, the absence of the high-valence state of Ru confirms the stabilization of RuO 2 after binding to Co 3 O 4 .
, Supporting Information).Compared with the pristine catalyst, no new phase was observed in RuO 2 /Co 3 O 4 after OER, which was consistent with previous results.In addition to the morphology, the change of active chemical states deserves more attention.The XPS survey spectra clarify the retention of Co, Ru, and O elements in RuO 2 /Co 3 O 4 -after and no new phases are produced (Figure S17, Supporting Information).The major drawback of the RuO 2 in acidic solution is the dissolution of Ru and Ru-O covalence caused by oxidation of the lattice oxygen during the Intensity (a.u.)
5 M H 2 SO 4 solution, indicating the outstanding durability of RuO 2 /Co 3 O 4 in industrial overall water splitting.With the demonstrated excellent electrochemical and durability, 2 /Co 3 O 4 represents a promising electrode material for industrial applications in PEM electrolyzers.
2 and Co 3 O 4 domains through a simple hydrothermal strategy.The favorable flower-like nanoarray morphology endows RuO 2 /Co 3 O 4 a large electrochemically active surface area and abundant pores.Furthermore, the strongly coupled interface between RuO 2 and Co 3 O 4 offers

Figure 5 .
Figure 5. HER and water splitting performance in 0.5 M H 2 SO 4 .a) HER LSV curves and b) corresponding Tafel slope plots for RuO 2 /Co 3 O 4 , Co 3 O 4 , RuO 2 , and commercial Pt/C.c) LSV of water splitting.d) Schematic illustration of the electrolyzer.e) Chronopotentiometric test of RuO 2 /Co 3 O 4 at 10 mA cm À2 .The inset shows the overall water-splitting polarization curves of the RuO 2 /Co 3 O 4 before and after 1000 cycles of CV scans.