Mesoporous Surface-Sulfurized Fe–Co3O4 Nanosheets Integrated with N/S Co-Doped Graphene as a Robust Bifunctional Electrocatalyst for Oxygen Evolution and Reduction Reactions

Playing a significant role in electrochemical energy conversion and storage systems, heteroatom-doped transition metal oxides are key materials for oxygen-involving reactions. Herein, mesoporous surface-sulfurized Fe–Co3O4 nanosheets integrated with N/S co-doped graphene (Fe–Co3O4–S/NSG) were designed as composite bifunctional electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Compared with the Co3O4–S/NSG catalyst, it exhibited superior activity in the alkaline electrolytes by delivering an OER overpotential of 289 mV at 10 mA cm−2 and an ORR half-wave potential of 0.77 V vs. RHE. Additionally, Fe–Co3O4–S/NSG kept stable at 4.2 mA cm−2 for 12 h without significant attenuation to render robust durability. This work not only demonstrates the satisfactory effect of the transition-metal cationic modification represented by iron doping on the electrocatalytic performance of Co3O4, but it also provides a new insight on the design of OER/ORR bifunctional electrocatalysts for efficient energy conversion.


Introduction
Driven by the urgent demand for renewable energies, developing electrochemical energy conversion and storage systems has become a worldwide priority recently, e.g., metal-air batteries, water-splitting systems, and fuel cells, which possess superior environmental friendliness, and high energy efficiencies for the conversion between chemical energies and electric energies [1][2][3][4]. Among them, the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) processes play a significant role, especially in the charge/discharge of rechargeable zinc-air batteries. Nevertheless, due to the complex multi-electron and proton transfer processes, OER/ORR is usually faced with sluggish reaction kinetics which seriously hinders the operation of electrochemical energy conversion and storage systems [5,6]. The precious metals Ru-and Ir-based materials are the most active commercial electrocatalysts for OER, while Pt is the most active ORR electrocatalyst [7,8]. Nonetheless, the scarce storage, high cost, and insufficient stability caused by the aggregation trends greatly impede the large-scale applications of these precious metal-based electrocatalysts. Therefore, it is necessary to develop cheaper alternatives to achieve efficient and durable OER/ORR bifunctional electrocatalytic kinetics.
proves that the Fe element can modulate the configuration of Co3O4. The lattice edge of Fe-Co3O4-S/NSG (0.263 nm) is assigned to the (311) plane of Fe3O4. In addition, porous structures were also observed in the high-angle annular dark field-STEM (HAADF-STEM) image (Figure 1f), which corresponded to the previous SEM results. Elemental mapping images of selected areas of the Fe-Co3O4-S /NSG samples demonstrated a uniform distribution of C, O, Fe, Co, S, and N, indicating that the S and NSG were successfully adsorbed on the material surface ( Figure 1g).  [25,26]) and Fe3O4 (PDF#26-1136 [27,28]), indicating that the synthesized catalyst was a mixture of Co3O4 and Fe3O4, and the surface sulphuration did not alter its crystal structure.   [27,28]), indicating that the synthesized catalyst was a mixture of Co 3 O 4 and Fe 3 O 4 , and the surface sulphuration did not alter its crystal structure. It is noteworthy that the additional characteristic peaks of Fe 3 O 4 were not observed, which could be attributed to the small amount of iron doping. Additionally, the peak intensity of Fe-Co 3 O 4 -S/NSG was lower than that of Co 3 O 4 -S/NSG due to the decrease in the proportion of the Co element. The magnified XRD data showed the (311) peaks of 37.02 • and 36.98 • for Co 3 O 4 -S/NSG and Fe-Co 3 O 4 -S/NSG, respectively, indicating that the incorporation of iron shifted the lattice negatively, causing the lattice spacing to become larger [29] (Figure 2b). These results were consistent with the results of the HRTEM images. The Raman spectra of Fe-Co 3 O 4 -S/NSG and Co 3 O 4 -S/NSG demonstrated a typical D band (sp 3 hybridized carbon) at 1326.1 cm −1 and a G band (sp 2 graphitic carbon) at 1582.6 cm −1 (Figure 2c). The intensity ratios of the D band and the G band (I D /I G ) were calculated to obtain the degree of graphitization of the electrocatalysts [30]. It is noteworthy that the additional characteristic peaks of Fe3O4 were not observed, which could be attributed to the small amount of iron doping. Additionally, the peak intensity of Fe-Co3O4-S/NSG was lower than that of Co3O4-S/NSG due to the decrease in the proportion of the Co element. The magnified XRD data showed the (311) peaks of 37.02° and 36.98° for Co3O4-S/NSG and Fe-Co3O4-S/NSG, respectively, indicating that the incorporation of iron shifted the lattice negatively, causing the lattice spacing to become larger [29] (Figure 2b). These results were consistent with the results of the HRTEM images. The Raman spectra of Fe-Co3O4-S/NSG and Co3O4-S/NSG demonstrated a typical D band (sp 3 hybridized carbon) at 1326.1 cm -1 and a G band (sp 2 graphitic carbon) at 1582.6 cm -1 (Figure 2c). The intensity ratios of the D band and the G band (ID/IG) were calculated to obtain the degree of graphitization of the electrocatalysts [30].   [31,32]. Specifically, Co3O4-S/NSG possessed an SSA of 41.82 m 2 g -1 and a pore volume of 0.27 cm 3 g -1 . When the material was doped with iron, the SSA was 40.81 m 2 g -1 , which was almost the same as the Co3O4-S/NSG results, indicating little change in the SSA. Ho the pore volume increased to 0.39 cm 3 g -1 . The results prove that the incorporation of iron can increase the number of porous structures in the material. Additionally, the inset reveals that the size of the pores was mainly distributed in the range of 2-10 nm. The mesoporous structure can provide abundant channels and highly electrochemically active surfaces, facilitating rapid mass transfer in OER/ORR reactions [33].  [31,32]. Specifically, Co 3 O 4 -S/NSG possessed an SSA of 41.82 m 2 g −1 and a pore volume of 0.27 cm 3 g −1 . When the material was doped with iron, the SSA was 40.81 m 2 g −1 , which was almost the same as the Co 3 O 4 -S/NSG results, indicating little change in the SSA. Ho the pore volume increased to 0.39 cm 3 g −1 . The results prove that the incorporation of iron can increase the number of porous structures in the material. Additionally, the inset reveals that the size of the pores was mainly distributed in the range of 2-10 nm. The mesoporous structure can provide abundant channels and highly electrochemically active surfaces, facilitating rapid mass transfer in OER/ORR reactions [33].
XPS spectra were obtained to determine the elemental composition of the catalysts' surface and the electronic state of these elements. The C 1s spectrum of Fe-Co 3 O 4 -S/NSG showed four peaks at 284. 15 The data proves that the nitrogen/sulphur co-doped graphene (NSG) was successfully synthesized. In the high-resolution N 1s spectrum of Fe-Co 3 O 4 -S/NSG, peaks at 398.28, 399.78, and 401.28 eV confirmed the existence of the pyridine N, pyrrole N, and graphite N, respectively (Figure 3b). A previous study has shown that a high pyridine N content is beneficial to the ORR process [34]. Therefore, compared with Fe-Co 3 O 4 -S/NSG, Co 3 O 4 -S/NSG with a slightly higher pyrrole N content possesses a better ORR starting potential, but the difference is not significant. The Fe 2p spectrum of Fe-Co 3 O 4 -S/NSG could be subdivided into two peaks at 717.28 and 713.78 eV, confirming the existence of Fe 3+ and Fe 2+ , which aligned with the XRD patterns ( Figure 3c). In the Co 2p spectra, the absorption peaks of the materials at 781.58/797.58 eV and 779.18/794.18 eV proved the presence of Co 2+ and Co 3+ (Figure 3d). Two satellite peaks could be discerned at 786.98 eV and 803.38 eV. Significantly, the peak intensity of Co 2+ decreased while the peak intensity of Co 3+ increased after the incorporation of iron, showing that the oxidizing agent Fe 3+ could convert Co 2+ into Co 3+ partially. The increase in the content of Co 3+ can raise the ratio of Co 3+ to Co 2+ in the catalyst, which is conducive to the catalytic reaction [35]. XPS spectra were obtained to determine the elemental composition of the catalysts' surface and the electronic state of these elements. The C 1s spectrum of Fe-Co3O4-S/NSG showed four peaks at 284.15, 284.80, 285.44, and 286.81 eV, which are the characteristics of the C-S bond, C-C bond, C-N/C-O bond, and C-N/C=O bond, respectively (Figure 3a). The data proves that the nitrogen/sulphur co-doped graphene (NSG) was successfully synthesized. In the high-resolution N 1s spectrum of Fe-Co3O4-S/NSG, peaks at 398.28, 399.78, and 401.28 eV confirmed the existence of the pyridine N, pyrrole N, and graphite N, respectively (Figure 3b). A previous study has shown that a high pyridine N content is beneficial to the ORR process [34]. Therefore, compared with Fe-Co3O4-S /NSG, Co3O4-S/NSG with a slightly higher pyrrole N content possesses a better ORR starting potential, but the difference is not significant. The Fe 2p spectrum of Fe-Co3O4-S/NSG could be subdivided into two peaks at 717.28 and 713.78 eV, confirming the existence of Fe 3+ and Fe 2+ , which aligned with the XRD patterns (Figure 3c). In the Co 2p spectra, the absorption peaks of the materials at 781.58/797.58 eV and 779.18/794.18 eV proved the presence of Co 2+ and Co 3+ (Figure 3d). Two satellite peaks could be discerned at 786.98 eV and 803.38 eV. Significantly, the peak intensity of Co 2+ decreased while the peak intensity of Co 3+ increased after the incorporation of iron, showing that the oxidizing agent Fe 3+ could convert Co 2+ into Co 3+ partially. The increase in the content of Co 3+ can raise the ratio of Co 3+ to Co 2+ in the catalyst, which is conducive to the catalytic reaction [35]. The spin-orbit transitions of S 2p1/2 and S 2p3/2 concentrated respectively at a binding energy of 163.08 eV and 161.58 eV in Fe-Co3O4-S/NSG (Figure 3e). Moreover, the peaks centred at 164.58 eV, 168.08 eV, and 169.28 eV were indexed to the characteristic peaks of sulphur oxides, which are located by surface oxidation during vulcanization. The S 2p3/2 orbital area of Fe-Co3O4-S/NSG was significantly higher than that of Co3O4-S/NSG, while the peak intensity of the sulphur oxides decreased after the doping of iron. This result indicates that the incorporation of iron can make sulphur atoms combine with Fe ions into the interior of the material, resulting in the decrease of the sulphur content on the surface of the material. Since transition metal sulfides have greater catalytic activity than transition metal oxides, Fe-Co3O4-S/NSG demonstrates superior OER/ORR performance. The O 1s spectra disclosed three absorption peaks, located at 532.48 eV for adsorbed oxygen, 531.38 eV for oxygen vacancies, and 529.18 eV for lattice  (Figure 3e). Moreover, the peaks centred at 164.58 eV, 168.08 eV, and 169.28 eV were indexed to the characteristic peaks of sulphur oxides, which are located by surface oxidation during vulcanization. The S 2p 3/2 orbital area of Fe-Co 3 O 4 -S/NSG was significantly higher than that of Co 3 O 4 -S/NSG, while the peak intensity of the sulphur oxides decreased after the doping of iron. This result indicates that the incorporation of iron can make sulphur atoms combine with Fe ions into the interior of the material, resulting in the decrease of the sulphur content on the surface of the material. Since transition metal sulfides have greater catalytic activity than transition metal oxides, Fe-Co 3 O 4 -S/NSG demonstrates superior OER/ORR performance. The O 1s spectra disclosed three absorption peaks, located at 532.48 eV for adsorbed oxygen, 531.38 eV for oxygen vacancies, and 529.18 eV for lattice oxygen, respectively (Figure 3f). After the incorporation of iron, the peak area of the typical metal-oxygen bond (M-O) at 529.18 eV increased obviously, which further proves that the iron was successfully doped. Meanwhile, the peak area of adsorbed oxygen also increased significantly because the oxygen in the air filled the surface vacancy caused by sulphur entering the internal position of the catalyst [36]. This result is consistent with that obtained from the high-resolution XPS spectra of S. Additionally, ICP-MS is used to detect the concentration of the elements within the materials. The results display that the proportion of Co and Fe in Fe-Co 3 O 4 -S/NSG is 34.87% and 3.16%, respectively, approximating the synthetic phase.
To evaluate the electrochemical properties of the as-prepared samples, LSV and CV tests were conducted in alkaline electrolytes using a three-electrode system. The mixture of commercial electrocatalysts Pt/C and Ir/C (mass ratio = 1:1) was included for comparison. The OER polarization curves of Fe-Co 3 O 4 -S/NSG, Co 3 O 4 -S/NSG, and Pt/C + Ir/C showed that at low current density, the overpotential of Fe-Co 3 O 4 -S/NSG was just lower than that of Co 3 O 4 -S/NSG. As the current density increased, the overpotential of Fe-Co 3 O 4 -S/NSG was even lower than that of Pt/C + Ir/C catalyst (Figure 4a). When the current density was 10 mA cm −2 , Fe-Co 3 O 4 -S/NSG exhibited an overpotential of 289 mV, which was lower than that of Co 3 O 4 -S/NSG (322 mV) (Figure 4b). When the current density was 200 mA cm −2 , Fe-Co 3 O 4 -S/NSG exhibited an overpotential of 594 mV, which was lower than that of Co 3 O 4 -S/NSG (624 mV), and Pt/C + Ir/C (626 mV). The Tafel slope curve of Fe-Co 3 O 4 -S/NSG showed a slope of 62.8 mV dec −1 , which was better than that of the commercial noble metal Pt/C + Ir/C catalyst (66.7 mV dec −1 ), while Co 3 O 4 -S/NSG delivered the largest slope of 86.4 mV dec −1 (Figure 4c). The ORR LSV curve of Fe-Co 3 O 4 -S/NSG gave an E 1/2 of 0.77 V, which was higher than that of Co 3 O 4 -S/NSG (0.75 V) to show superior ORR performance (Figure 4d). Intriguingly, Fe-Co 3 O 4 -S/NSG exhibited a small Tafel slope of 89.7 mV dec −1 , which outperformed Co 3 O 4 -S/NSG (90.7 mV dec −1 ) and was close to Pt/C + Ir/C (88.9 mV dec −1 ), demonstrating the fast kinetics of ORR on Fe-Co 3 O 4 -S/NSG (Figure 4e). The results confirm that Fe-Co 3 O 4 -S/NSG has excellent ORR/OER activity compared to the un-Fe-doped sample. The robust performance could be attributed to the synergistic effect between the alloy component and the defect-rich carbon carrier, which helps to expose more active sites and accelerate electron transport during the ORR/OER process [37][38][39]. The chronoamperometry (CA) measurement can be employed to test the stability [40,41]   Because of the positive relationship, an electrocatalyst with a higher Cdl value possesses a larger ECSA, which usually displays better electrocatalytic activity [42,43]. According to the CV curves of Co3O4-S/NSG and Fe-Co3O4-S/NSG at different scanning rates (20-100 mV s −1 ), the shapes of the CV curves remained stable while their area changed with the increase in the scanning rate (Figure 5a,b). The Cdl values for Co3O4-S/NSG and Fe-Co3O4-S/NSG were 18.07 and 19.88 mF cm -2 , respectively (Figure 5c). Com- Because of the positive relationship, an electrocatalyst with a higher C dl value possesses a larger ECSA, which usually displays better electrocatalytic activity [42,43]. According to the CV curves of Co 3 O 4 -S/NSG and Fe-Co 3 O 4 -S/NSG at different scanning rates (20-100 mV s −1 ), the shapes of the CV curves remained stable while their area changed with the increase in the scanning rate (Figure 5a,b). The C dl values for Co 3 O 4 -S/NSG and Fe-Co 3 O 4 -S/NSG were 18.07 and 19.88 mF cm −2 , respectively (Figure 5c). Combined with the BET results, Fe-Co 3 O 4 -S/NSG possessed a larger ECSA due to the incorporation of iron. The EIS test was carried out at an overpotential of 298 mV, when the semicircular diameter of Fe-Co 3 O 4 -S/NSG was smaller than that of Co 3 O 4 -S/NSG (Figure 5d). The inset shows the corresponding equivalent circuit, where R s indicates the solution resistance, CPE is the constant phase element and R ct is the charge transfer resistance [44,45] After Z-view fitting, the equivalent circuit fit resulted in a measured charge transfer resistance of Fe-Co 3 O 4 -S/NSG measured to be 4.674 Ω, which was lower than that of

Material Synthesis
Fe-Co 3 O 4 -S/NSG was fabricated by a three-step process, including hydrothermal synthesis of the precursor nanosheets, calcination to obtain Fe-Co 3 O 4 , and finally integration with NSG, along with surface vulcanisation to obtain the target product ( Figure 6). The specific synthesis process is as follows.
Graphene oxide (GO) was synthesized by using modified Hummer methods. An amount of 0.1 g GO and 0.5 g hydrophilic SiO2 nanoparticles (12-15 nm) were uniformly dispersed in 500 mL ethanol by ultrasonication. The ethanol was then evaporated using a rotary evaporator at 80 °C to obtain a flake of GO/SiO2 solid. Subsequently, 0.5 g melamine and 0.5 g dibenzyl disulfide (BDS) were added to the solids and ground into fine powder. The mixture was heated at 900 °C for 1 hour in a nitrogen-filled atmosphere with a heating rate of 5 °C min -1 . After that, nitrogen/sulfur co-doped graphene (NSG/SiO2) loaded with silica was synthesized. Next, the obtained NSG/SiO2 was placed in hydrofluoric acid (HF) solution for 12 h to remove the silica. The product was then cleaned several times with ultrapure water and ethanol, followed by drying at 70 °C under the condition of oxygen isolation. After drying, NSG was synthesized.

Synthesis of Co3O4-S/NSG and Fe-Co3O4-S/NSG
Fe-Co3O4-S/NSG or Co3O4-S/NSG was prepared by ultrasonic mixing. NSG and Fe-Co3O4 or Co3O4 were mixed in a mass ratio of 1:4 and sonicated in a 0.4 mol L -1 solution of sodium sulfide solution for 2 h and then left to stand for 24 h. Subsequently, the solids were washed and dried at 60 °C for 8 hours using a filtration technique to obtain the target product Fe-Co3O4-S/NSG or Co3O4-S/NSG electrocatalyst.

Physicochemical Characterizations
The structural and morphological characteristics of the synthetic material were determined with a scanning electron microscope (SEM) (TESCAN MIRA LMS, Brno, Czech Republic) and a scanning transmission electron microscopy (STEM) (FEI Tecnai G2 F20,

Synthesis of Co 3 O 4 and Fe-Co 3 O 4
Fe-Co 3 O 4 was synthesized by hydrothermal method and calcination methods. The specific process was as follows: 4.5 mM Co(NO 3 ) 2 ·6H 2 O and 0.5 mM Fe(NO 3 ) 3 ·9H 2 O were dissolved in 35 ml ultrapure water. Subsequently, 10 mM urea and 20 mM ammonia solution were added as precipitants. The obtained solution was stirred using a magnetic stirrer for 20 min and followed by sonication for 5 min. After that, the mixed solution was placed in a PTFE-lined autoclave at 170 • C for 9.5 h. Once cooled down to room temperature, the obtained solution was slowly washed with ethanol and ultrapure water, respectively. After drying, the pink powder was obtained. The synthesis of Co 3 O 4 was carried out by dissolving 5 mM Co(NO 3 ) 2 ·6H 2 O in 35 mL of ultrapure water. The remainder of the steps were the same as for the synthesis of Fe-Co 3 O 4 .

Synthesis of NSG
Graphene oxide (GO) was synthesized by using modified Hummer methods. An amount of 0.1 g GO and 0.5 g hydrophilic SiO 2 nanoparticles (12-15 nm) were uniformly dispersed in 500 mL ethanol by ultrasonication. The ethanol was then evaporated using a rotary evaporator at 80 • C to obtain a flake of GO/SiO 2 solid. Subsequently, 0.5 g melamine and 0.5 g dibenzyl disulfide (BDS) were added to the solids and ground into fine powder. The mixture was heated at 900 • C for 1 h in a nitrogen-filled atmosphere with a heating rate of 5 • C min −1 . After that, nitrogen/sulfur co-doped graphene (NSG/SiO 2 ) loaded with silica was synthesized. Next, the obtained NSG/SiO 2 was placed in hydrofluoric acid (HF) solution for 12 h to remove the silica. The product was then cleaned several times with ultrapure water and ethanol, followed by drying at 70 • C under the condition of oxygen isolation. After drying, NSG was synthesized.

Physicochemical Characterizations
The structural and morphological characteristics of the synthetic material were determined with a scanning electron microscope (SEM) (TESCAN MIRA LMS, Brno, Czech Republic) and a scanning transmission electron microscopy (STEM) (FEI Tecnai G2 F20, Hillsboro, OR, USA). The crystal phase analysis of the synthetic catalysts was elucidated by X-ray powder diffraction (XRD) (Rigaku Smartlab 9 kW, using an X-ray diffractometer over the range of 10 • to 90 • 2θ, Tokyo, Japan). The specific surface areas of the materials were calculated by a Brunauer-Emmett-Teller (BET) (Micromeritics APSP 2460, 77k, Norcross, GA, USA). Adopting the Barrett-Joyner-Halenda (BJH) method, pore size distribution was calculated from the desorption branch of the N 2 desorption isotherm. The chemical states and composition of materials were tested by X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha+ spectrometer, Shanghai, China). All spectra were calibrated using the C 1s peak energy of 284.8 eV binding energy standard peak. The elemental content of materials can be precisely detected by inductively coupled plasma mass spectroscopy (ICP-MS) (Agilent 7700s, Beijing, China). The synthesized samples were qualitatively analysed by using Raman spectra (LabRam HR Evolution, using an Ar-ion laser beam λ = 514 nm Shanghai, China).

Electrochemical Measurements
Utilizing a three-electrode system, electrochemical measurements were conducted in CHI 760E electrochemical workstation. The instrument was subjected to 95% iR compensation before the test. Taking 1 mol L −1 potassium hydroxide solution in an oxygen atmosphere as the electrolyte in the OER tests, a rotating disc-shaped glassy carbon electrode (RDE) with a diameter of 5 mm and an area of 0.19625 cm 2 was employed as the working electrode, a carbon rod was utilized as the counter electrode, and a Hg/HgO electrode was used as the reference electrode. During the test, the RDE was coated with different catalyst slurries. A high-speed rotator (Pine Instruments) was used in the ORR tests, the RDE coated with catalyst ink was utilized as the working electrode, a platinum wire was used as the counter electrode, and a Hg/HgO electrode was employed as the reference electrode. The electrolyte was 0.1 mol L −1 KOH solution in an oxygen sufficient atmosphere. The obtained potentials (E Hg/HgO ) were converted to an RHE scale utilizing the following Nernst equation: E RHE = E Hg/HgO + 0.098 + 0.059 × pH (1 M KOH, pH~14; 0.1 M KOH, PH~13), where E RHE represents the reversible potential and E Hg/HgO is the potential measured against the reference electrode.
To prepare the catalyst ink, 5 mg of Co 3 O 4 -S/NSG and 5 mg of Fe-Co 3 O 4 -S/NSG were dispersed in a solution containing 570 µL of isopropanol, 570 µL of anhydrous ethanol, 285 µL of ultrapure water, and 75 µL of 5 wt% Nafion, respectively. The suspension was then sonicated until it became a homogeneous ink-like consistency. An amount of 20 µL of the prepared catalyst was taken and added dropwise to the working electrode and 2.5 mg Pt/C and 2.5 mg Ir/C were dispersed in a solution containing 1425 µL of anhydrous ethanol and 75 µL of 5 wt% Nafion, which was then sonicated until it became homogeneous and ink-like. Subsequently, an amount of 5 µL of the prepared noble metal catalyst was taken and added dropwise to the working electrode. The Tafel plots were derived from the analysis of the linear sweep voltammetry (LSV) test with a scanning rate of 5 mV s −1 . The Tafel slopes were calculated by the formula η = a ± b log |j|, where η is the overpotential, j represents the current density, a is the overpotential at a current density of 1 mA cm −2 , and b represents the Tafel slope. Additionally, electrochemical impedance spectroscopy (EIS) tests were used to investigate the properties of materials and electrode reactions in the frequency range of 0.1 Hz to 100 kHz. Since the electrical double-layer capacitor (C dl ) is proportional to the electrochemically active surface area (ECSA), cyclic voltammetry (CV) measurements were conducted to study the reaction mechanisms within the static non-faradaic region. The CV tests were carried out between −0.9 and −0.8 V (vs. SCE) at the different scan rates of 20, 40, 60, 80, and 100 mV s −1 , respectively. The C dl was calculated by selecting the current density difference at a potential of -0.85 V at different rates and fitting the current density difference value to the sweep speed linearly. Half of the slope of the fitted straight line was the C dl . The chrono-current measurement was adopted to access the stability of the materials at 1.524 V for 12 h.

Conclusions
An efficient and durable OER/ORR bifunctional electrocatalyst (Fe-Co 3 O 4 -S/NSG) was developed by integrating sulfur/iron co-doped Co 3 O 4 (Fe-Co 3 O 4 -S) and nitrogen/sulfur co-doped graphene (NSG). The resulting material, Fe-Co 3 O 4 -S/NSG, had a homogeneous porous nanosheet structure, which could increase the number of active sites and accelerate electron transfer. Furthermore, the addition of NSG further increased the specific surface area of the material and improved the conductivity of the catalyst. In terms of performance, the Fe-Co 3 O 4 -S/NSG electrocatalyst demonstrated excellent bifunctional activity with an OER E 10 of 289 mV and an ORR E 1/2 of 0.77 V vs. RHE. Additionally, it could be stabilized at 4.2 mA cm −2 for 12 h, which is superior to the commercial Pt/C + Ir/C electrocatalyst. This work demonstrates that sulphated Iron-cobalt bimetallic oxides combined with nitrogen and sulfuric doped graphene can improve the overall OER and ORR performance of the catalyst. This provides a new approach to the design of low-cost and high-performance electrocatalysts for use in energy conversion and storage.