Introduction

With the transition from traditional fossil fuels to clean and sustainable energy, lots of attentions have been paid on storage systems with environmental benignity, high efficiency and alternative energy conversion. Fuel cells have been considered as the most efficient and clean energy conversion device because fuels react with oxygen via mild electrochemical processes without combustion and the overall fuel-conversion efficiency is not limited by the Carnot cycle laws1,2. Designing bifunctional catalysts with good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities would be highly beneficial to the development of metal-air batteries. However, developing catalysts for ORR and OER with high activity at low cost remain great challenges3,4. Platinum-based materials are known to be the most active electrocatalysts for the ORR and the OER. However, the limited reserves of Pt, high cost, the activity deterioration with time and poor durability severely hinder the large-scale applications of Pt in ORR and OER5,6,7,8.

On the other hand, the determination of hydrogen peroxide (H2O2) has aroused more and more interests of researchers as its significance in the fields of applications in industry as well as biological reactions. Therefore, a rapid, accurate and reliable method to detect H2O2 is of highly demanded. Among various techniques for H2O2 detection, electrochemical H2O2 electrocatalysts are promising due to their high sensitivity, low cost, good selectivity, easy for automation and operational simplicity9,10,11,12. Catalysts for hydrogen peroxide reduction, oxygen reduction and oxygen evolution reactions are vital in biological assay and renewable-energy technologies including fuel cells and water splitting.

Recently, transition metal oxides including magnanimous oxide, cobalt oxide, iron oxide and nickel oxide as promising materials have received considerableattention due to their low cost, high abundance and perfect catalytic activity for the ORR, OER and immobilizing enzymes for further applications in fabrication of hydrogen peroxide biosensor13,14,15. Among them, Co3O4 with spinel crystal structure is beneficial to electron transportation between Co2+ and Co3+ ions, which has been extensively considered as an efficient electrocatalyst for OER and ORR16,17,18. Previous studies reported that the efficiency of cobalt oxide as an OER catalyst could be ascribed to the increasing population of CoIV centers at the oxide surface during electrochemical oxidation19. More interesting, Co3O4, which exhibits catalase-like activity for the decomposition of H2O2, can be applied to the detection of H2O2 in aqueous medium20. However, these Co3O4-based catalysts usually suffer from the poor electrical conductivity, short active site density and the dissolution or agglomeration during electrochemical processes. Co3O4 itself is a material with a little ORR, OER and H2O2 sensing activity and further studies exhibit that synergy between the carbon materials and Co3O4 can give a huge promotion of the electrocatalytic activity21,22. Therefore, lots of researches have used conductive carbon nanomaterials such as carbon nanotubes (CNTs), carbon foam and graphene etc. To improve the conductivity of Co3O4 based hybrid catalysts as well as obtain uniformly dispersed Co3O4 nanoparticles and thus to improve the electrocatalyst activity.

Graphene, a two-dimensional layer framework of sp2-hybridized carbon with outstanding chemical and physical properties, has attracted a lot of attention in the last years21,23,24. Graphene could be an attractive support for metal oxides to form a new class of nanocomposites for ORR due to their notable electronic conductivity and high surface area25,26. Dai’s group reported a hybrid material consisting of Co3O4 nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the ORR and OER27. Wang etc. synthesized a novel multifunctional nanohybrid by chemically coupling ultrafine metal oxide nanoparticles to reduced grapheme oxide (rGO) as an effective catalyst for oxygen reduction reaction28. Anchoring Co3O4 nanocrystals on carbon-based supports could significantly improve their electrocatalytic activity contributed by the small crystalline size and conductive support29. What’s more, chemical doping with hetero atoms is an efficacious method to regulate electronic properties and surface chemistry of assembled graphene by the modulation of the carbon-carbon bonds30,31. It has been also reported that nitrogen-doped graphene can promote the electrochemical reduction of H2O232. As previous study, the introduction of the Co−N4 complex onto the graphene basal plane facilitates the activation of O2 dissociation and the desorption of H2O during the ORR33. Nitrogen-graphene can produce the synergistic support effect because the reactive intermediates such as hydrogen peroxide are known to decomposed by nitrogen doped carbon nanostructures. However, so far, few researches have reported catalyst which has three functions for H2O2 reduction, ORR and OER.

We report herein the synthesis of Co3O4 nanoparticles anchored on nitrogen-doped reduced graphene oxide (Co3O4/N-rGO) through a simple and scalable method as tri-functional catalysts for H2O2 reduction, ORR and the OER, as shown in Fig. 1. Co3O4 anchored uniformly into laminar nitrogen-doped reduced graphene oxide was confirmed by scanning electron microscopy (SEM). Co3O4/N-rGO possesses a good electrocatalytic activity toward H2O2 reduction by enhancing the current response and decreasing H2O2 reduction over potential. The electrochemical results demonstrate that the Co3O4/N-rGO can exhibit higher activity for both the ORR and the OER and better durability than a commercial carbon-supported Pt catalyst. The strong coupling between Co3O4, nitrogen and reduced graphene oxide (rGO) is found to play an important role in the high electrocatalytic activities of the Co3O4/N-rGO. This synthesis route can be easily adopted for large-scale manufacturing due to its process simplicity and the accessibility of precursor materials.

Figure 1
figure 1

Schematic illustration of the synthesis of Co3O4/N-rGO.

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Results and Discussion

Characterization

Figure 2A and B illustrated field emission scanning electron microscopy (FE-SEM) images of Co3O4/N-rGO. We can clearly see from SEM images in Fig. 2B that Co3O4 nanoparticles are uniformly anchored on the rGO substrate with an approximate average diameter of 150 nm. This may be attributed that Co2+ ion was coordinated with negatively charged oxygen-containing functional groups on N-rGO sheets. During the hydrothermal process, Co2+ was oxidized into Co3+ by oxygen-containing groups, and crystallized to form Co3O4 nanoparticles anchored into N-rGO sheets34. However, Fig. 2C demonstrate Co3O4/rGO does not exhibit such a uniform morphology distribution of Co3O4. In addition, we only found that a typical corrugated structure in Fig. 2D, suggesting there is no Co3O4 particles nucleate on N-rGO surface. The oxygen-containing functional groups of rGO were beneficial for the nucleation and anchoring of nanocrystals on the sheets to achieve covalent attachments, which help to shape the uniform formation of Co3O435,36. In addition, these uniform structures of Co3O4 particles anchored into N-rGO can also be ascribed to NH3 together with oxygen-containing functional group coordinating with cobalt cations and thus reducing Co3O4 particles size and enhancing particles nucleation on N-rGO37. XRD was performed to investigate the phase structure of Co3O4/N-rGO. As shown in Fig. 3A, the diffraction peaks of the pristine Co3O4 was consistent with the standard Co3O4 (JCPDS card: 42-1467).

Figure 2
figure 2

Scanning electron microscopy image of Co3O4/N-rGO (A,B), Co3O4/rGO (C) and N-rGO (D), respectively.

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Figure 3
figure 3

XRD spectrum of Co3O4/N-rGO (A) and Raman spectra of Co3O4/N-rGO, N-rGO and Co3O4/rGO (B); The XPS full spectrum of Co3O4/N-rGO (C) and high resolution Co2p spectra of Co3O4/N-rGO (D); High-resolution N1s XPS spectra of Co3O4/N-rGO (E); The percentage of three nitrogen species in Co3O4/N-rGO (E).

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The major diffraction peaks of Co3O4/N-rGO were well in agreement with those of Co3O4 except for the broad (002) peak at approximately 25°, which can be ascribed to disordered stacked graphitic sheets30. This manifest that the original GO has been reduced to rGO during the hydrothermal process, again confirming we have successfully incorporated Co3O4 into rGO38. BET experiments was conducted to obtain specific surface area of as-prepared samples and the isotherms exhibit typical IV isotherms where the recorded BET surface area of Co3O4/N-rGO, N-rGO and Co3O4 are 103.9 m2/g, 139.7 m2/g, and 62.8 m2/g, respectively. These results indicate the change of N-rGO structure after doping with Co3O4.

Raman spectroscopy was carried out to extend the study for the carbon structures in Co3O4/N-rGO, N-rGO and Co3O4/GO hybrid which are shown in Fig. 3B, where the peaks of Raman spectrum of Co3O4 anchored on the N-rGO and Co3O4/GO hybrid at 193, 470 and 680 cm−1, can be attributed to the Eg, F2g and A1g modes of Co3O439. It is noted that there are two remarkable peaks around 1339 and 1591 cm−1 refer to the D-band (arising from the edge or defect sites of carbon) and G band (representing the sp2 carbon) of the graphene domain, respectively40.

X-ray photoelectron spectroscopic (XPS) measurements were performed to determine the surface element constitution in Co3O4/N-rGO. The sharp peaks in Fig. 3C are corresponded to the characteristic peaks of C1s, O1s, N1s and Co2p, indicating the existence of carbon, oxygen, nitrogen and cobalt elements in the prepared sample. The XPS spectrum for Co2p shown in Fig. 3D reveals two major peaks with binding energies at 780.1 and 795 eV, corresponding to Co2p3/2 and Co2p1/2, respectively, with a spin energy separation of 15 eV, which is attributed to the Co2+ oxidation state, indicating that a portion of Co3+ is reduced to Co2+ with generating oxygen vacancies17. These results again confirmed that Co3O4 nanoparticles have been anchored on N-rGO, Co2+ and Co3+ in the crystal structure of Co3O4 are being considered to be playing a vital role in improving catalytic performance of oxygen reduction reaction and oxygen evolution reaction37. Furthermore, the main beautiful structure of Co3O4 is the peculiar cation distribution in the face centered cubic (FCC) crystal where the Co2+ ions reside on the 1/8th of the tetrahedral A sites while the Co3+ ions occupy 1/2 of the octahedral B sites11, endow the system viable for electrocatalytic applications. The high-resolution N1s XPS spectrum of Co3O4/N-rGO was used primarily to determine the bonding configurations of N atoms in the composite, as seen in Fig. 3E, The peak deconvolution suggests four components were centered at about 398, 400, 401, and 403 eV, corresponding to pyridinic N, pyrrolic N, quaternary N, and oxidized N, respectively. N atom have the lone electron pairs which can hybridize with sp2 carbon atoms to celebrate oxygen reduction reaction. The performance of ORR depends on the bonding configuration of N atoms in carbon materials. It has been reported that the onset potential of a nitrogen-doped catalyst has strong relation with pyridinic form nitrogen, but little effect by pyrrolic nitrogen and oxidized type nitrogen41.

Electrocatalytic activity of Co3O4/N-rGO for H2O2 reduction

Though enzyme based electrochemical sensors have been widely developed to sensing H2O2 due to the advantages of high sensitivity and good selectivity, these sensors often suffer from unstable response due to the intrinsic nature of enzymes42. Therefore, it is necessary to develop a simple non-enzymatic strategy for sensing H2O2 with high sensitivity. To date, electrocatalysts for design H2O2 sensors with high sensitivity, good selectivity and easy regulation properties hold leading position among various sensors12. It has been proved that functional nano-structured transition-metal oxides exibit good electrocatalytic activity toward the H2O2 reduction, which provides valuable strategy for the nonenzymatic determination of H2O243,44. Among various kinds of transition metal oxides, Co3O4 shows attracting electronic and electrocatalytic properties. Particularly, its normal spinel crystal structure is favorable for electron transportation between Co2+ and Co3+ ions and Co3O4 possess catalase-like activity, which is benefit to sensing H2O2. Therefore, Co3O4 have been extensively explored as the sensing materials for developing enzyme-free H2O2 sensors. However, Co3O4-based catalysts usually suffer from the poor electrical conductivity, low active site density and the dissolution or agglomeration during electrochemical processes45,46. On the other hand, graphene has the ability to promote electron transfer rates and graphene-based modified electrode had much better electrocatalysis toward H2O247.

In our study, Co3O4 nanoparticles were incorporated into nitrogen doped graphene, leading to improved conductivity, enhanced catalytic activity and stability of the metal oxide nanocatalyst, and thus a better catalytic effect to H2O2 reduction due to the synergistic effect. To investigate the electrocatalytic characteristics to H2O2 reduction of Co3O4/N-rGO, voltammetric measurements were performed using the Co3O4/N-rGO, Co3O4/rGO and N-rGO modified GC electrodes in the presence of 5 mM H2O2 at a scan rate of 0.05 V s−1. Figure 4A exhibit that a distinct catalytic current peak at −0.40 V could be ascribed for Co3O4/N-rGO modified GC electrode. Figure 4B reveals the CVs of Co3O4/N-rGO modified GC in the presence of different concentration of H2O2 in 0.1 M PB solution (pH7.0) at the scan rate of 50 mV s−1. It demonstrates that the reduction current gradually increases with the increase of H2O2 concentration (from the top: 0.5 1, 2 and 5 mM), which manifest the Co3O4/N-rGO material have improved electrocatalytic activity to H2O2 and pave a route for quantitive analysis.

Figure 4
figure 4

Cyclic voltammograms of (A) Co3O4/N-rGO, N-rGO and Co3O4/rGO modified GC electrodes in 0.1 M PB solution (pH 7.0) containing 5 mM H2O2 and (B) Co3O4/N-rGO modified GC electrode in 0.1 M PB solution (pH 7.0) containing different concentration of H2O2 (from the top: 0, 0.5, 1, 2 and 5 mM). Scan rate 50 mV s−1.

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It is a significant way for amperometric technique to test the sensing property of Co3O4/N-rGO modified GC electrode. We studied the effect of the applied potential in order to improve the Co3O4/N-rGO modified GC electrode performance towards non-enzymatic H2O2 sensing. We investigated applied potential on the amperometric response on the Co3O4/N-rGO modified GC electrode towards sequential addition of 0.5 mM H2O2 by varying the potential between−0.6 V and −0.3 V. As shown in Fig. 5A and B, as amperometric response of the Co3O4/N-rGO modified GC electrode has the optimal sensitivity, the applied potential at −0.6 V was selected. Figure 5A shows a typical current-time plot of the Co3O4/N-rGO modified GC electrode on successive addition of H2O2 at an applied potential of −0.6 V. Catalytic currents showed linear response to H2O2 from 0.5 mM to 17.5 mM (R2 = 0.994) with a detection limit (S/N = 3) to be 0.1 mM, which are comparable to or even better than those of the other metal-free or enzyme based H2O2 biosensors9,32,48. To investigate the selectivity for H2O2 sensing, the amperometric responses of ascorbic acid (AA), dopamine (DA), uric acid (UA) and glucose (Glu) are investigated on the Co3O4/N-rGO modified GC electrode. As shown in Fig. 5D, when the Co3O4/N-rGO-modified GC electrode was polarized at −0.6 V, the addition of 0.2 mM AA, 0.02 mM DA, 0.2 mM UA and 5 mM Glu did not produce an observable current response while the addition of H2O2 induced obvious reduction currents reponse, indicating that the measurements of H2O2 are essentially interference-free from other relevant electroactive species. Therefore, the as-prepared the Co3O4/N-rGO modified GC electrode is a good candidate for the fabrication of stable and specific amperometric sensor for the nonenzymatic detection of H2O2. The excellent performance of H2O2 sensor can be ascribed to the well distributed and high loading amount of Co3O4 nanoparticles.

Figure 5
figure 5

The effect of applied potential (A,B) to the amperometric response of sequential addition of 2 mM H2O2 on the Co3O4/N-rGO modified GC electrode. Amperometric response of Co3O4/N-rGO to successive addition of H2O2. The inset is the plot of H2O2 peak current versus H2O2 concentration (C). Amperometric response of the Co3O4/N-rGO exposed to H2O2, AA, DA, UA and glucose. Applied potential: −0.6 V and supporting electrolyte: 0.1 M PB solution (pH 7.0) (D).

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The performance of oxygen reduction reaction

To evaluate the ORR catalytic activity of Co3O4/N-rGO, N-rGO and Co3O4/rGO, CV measurements were performed in both O2 and N2-saturated 0.1 M KOH solution. As shown in Fig. 6A, CV of N-rGO and Co3O4/rGO in the O2-saturated electrolyte shows a reduction peak at −0.34 V and −0.33 V respectively, suggesting their electrochemical catalytic activity for ORR. As for Co3O4/N-rGO composite modified electrode, a reduction peak at ca. −0.26 V is observed, which is more positive than those of N-rGO and Co3O4/rGO while it also has a highest current density, suggesting a great improvement of catalytic activity, which is better than tri-functional carbon materials in previous study32. Previous have reported that the electrocatalytic activity of Co3O4 was mainly affected by structure46. Co3O4 particles have a spinel structure and the direct Co-Co interactions across shared octahedral edges of its spinel framework can enhance the electronic conductivity which is beneficial to the ORR catalytic activity. On the other hand, the N-graphene also exhibited a much better electrocatalytic activity, long-term operation stability for oxygen reduction reaction22. Therefore, such an excellent electrocatalytic activity of the Co3O4/N-rGO toward ORR can be ascribed to the synergetic chemical coupling effects of Co3O4 and N-graphene18,49,50.

Figure 6
figure 6

(A) Cyclic voltammograms of Co3O4/N-rGO, Co3O4/rGO, N-rGO modified GC electrodes in an O2-saturated and in N2-saturated 0.1 M KOH at a scan rate of 10 mV s−1. (B) LSVs of Co3O4/N-rGO on RRDE in 0.1 M KOH with various rotation rates at a scan rate of 5 mV s−1.

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To investigate the oxygen reduction mechanism of Co3O4/N-rGO modified GC electrodes, the ORR was studied by RRDE technique via measurement of the yield of the generated intermediate H2O2. The RRDE technique was applied to quantitatively determine the n value toward ORR and the H2O2 generation rate by setting the potential of the ring electrode at 0.4 V.

The electron transfer number n of ORR and HO2− intermediate production percentage (HO2− %) were determined as

where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the Pt ring, which was determined to be 0.421,46. From the Fig. 6B, it was calculated H2O2% value for the Co3O4/N-rGO during ORR process is about 63.5–32.2% at potentials ranging from −0.3 to −0.8 V. The calculated n value for the Co3O4/N-rGO is about 2.9 to 3.4 from −0.3 to −0.8 V. These results reveal that the electrocatalytic process of Co3O4/N-rGO is an improved four-electron pathway and a two-electron transfer pathway occurred simultaneously for ORR.

Methanol poisoning and stability are key issues challenging the cathode materials in current fuel cell techniques51. The effect of methanol poisoning and stability on the Co3O4/N-rGO was investigated in Fig. 7A and B by current-time (i-t) chronoamperometry. As shown in Fig. 7A, when methanol was injected, a significant decrease (90.7%) in current was observed for the Pt/C electrode, whereas only a slight decrease (13.3%) was observed for the Co3O4/N-rGO, suggesting poor tolerance of Pt/C to methanol compared with the Co3O4/N-rGO material. Figure 7 Bshows that the amperometric response of ORR on the Co3O4/N-rGO which exhibits a very slow attenuation of relative current, after 2000 s i.e. a current loss of approximately 31.37%. In contrast, the Pt/C reveals degraded stability with a current loss (39.58%) after 2000 s, indicating the Co3O4/N-rGO has a better stability than Pt/C.

Figure 7
figure 7

(A) Chronoamperometric responses of Co3O4/N-rGO and Pt/C (20%) at −0.3 V in the O2-saturated 0.1 M KOH. The arrow indicates the addition of 3.0 M methanol into the O2-saturated electrochemical cell. (B) Chronoamperometric responses obtained at the Pt/C (20%) and Co3O4/N-rGO at −0.3 V in O2-saturated 0.1 M KOH.

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The catalytic property of oxygen evolution reaction

Previous studies have reported Co3O4 particles deposited on stable supporting and conducting substrates can be used as effective electrode materials for both the oxygen reduction (ORR) and evolution (OER) reactions via decreasing overpotential in fuel cells and water electrolyzers17,52. The good catalytic performance of OER can be ascribe to the small crystalline size and the mixed valences Co2+ and Co3+ of Co3O4 as well with conductive support substrates17. Sun and his groups synthesize Co3O4 nanorod–multiwalled carbon nanotube hybrid with a onset potential of about 0.47 V vs. Ag/AgCl and Tafel slope of 65 mV/dec37. In our work, a rotational disk electrode (RDE) tests were also carried out in alkaline solution to further evaluate the OER catalytic activity of the Co3O4/N-rGO. Figure 8A showed the typical linear sweep voltammograms using the RDE at an electrode rotating speed of 1600 rpm and a potential scanning rate of 5 mV s−1. From the OER region, the Co3O4/N-rGO afforded a sharp onset potential at 1.54 V, which is worse than that of RuO2 at 1.49 V and better than Pt/C. The OER over potential at current density of 10 mA cm−2 is close to that of RuO2, indicating the Co3O4/N-rGO has a good OER property. The Tafel slope is usually used to study the catalytic mechanism of electrocatalysis for OER. In Fig. 8B, The Tafel slope comparison showed that Co3O4/N-rGO has Tafel slope of 204 mV/dec which is much smaller than those for Pt/C (308 mV/dec) and is bigger than RuO2 (63 mV/dec), suggesting Co3O4/N-rGO has an improved performance of OER.

Figure 8
figure 8

(A) The OER polarization curves of Co3O4/N-rGO catalyst and commercial Pt/C (20%) at a sweep rate of 5 mV s−1 using RDE with a rotation speed of 1600 rpm and (B) corresponding Tafel plots.

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Conclusions

In summary, this study describes a facile and effective route to synthesize hybrid material consisting of Co3O4 nanoparticles anchored on nitrogen-doped reduced graphene oxide (Co3O4/N-rGO) as a high-performance tri-functional catalyst for ORR, OER and H2O2 sensing. Owing to the synergetic chemical coupling effects between Co3O4 and graphene, the Co3O4/N-rGO exhibited excellent electrocatalytic activity with a direct reduction to H2O2 at −0.6 V and sensing ability towards H2O2. Although Co3O4/rGO or N-rGO alone has little catalytic activity, the Co3O4/N-rGO exhibits high ORR activity with ORR peak potential to be −0.26 V (vs. Ag/AgCl) and the number of electron transfer number is 3.4, excellent tolerance to methanol crossover and exceptionally good stability to Pt/C (20%) in alkaline solutions. Catalytic studies of Co3O4/N-rGO for OER display a better onset potential, overpotential under the current density of 10 mA cm−2 and a smaller Tafel slope with Pt/C (20%). Due to the ease of synthesis and electrode fabrication, the method developed by this study could be used for large-scale synthesis of non-precious metal-based trifunctional metal catalyst for hydrogen peroxide reduction, ORR and OER.

Experimental

Chemicals and materials

Nafion perfuorinated resin solution (5 wt% in a mixture of lower aliphatic alcohols and water) and commercial platinum/carbon (Pt/C) 20 wt% (Pt loading: 20 wt%, Pt on carbon black) were obtained from Sigma-Aldrich. All other chemicals (analytical grade) were purchased from Beijing Chemical Reagent Company (Beijing, China) and used without further purification. Ultra-pure water was obtained with a Milli-Q plus water purification system (Milli-pore Co. Ltd., USA).

Materials characterization

Scanning electron microscopy (SEM) images were obtained on a Hitachi S-2600N scanning electron microscope. Elemental analysis data were obtained through Flash EA 1112. The X-ray photoelectronspectra (XPS) spectra were obtained using a VG Micro-tech ESCA 2000 using a monochromic 15 Al X-ray source. For rotating ring-disk electrode (RRDE) measurements, a bipotentiostat (CHI 832, Shanghai Chenhua Instrument Co. Ltd.) and a rotating ring-disk electrode with a rotating GC disk electrode and a platinum ring electrode (ALS RRDE-2) were used. The collection efficiency of the ring-disk electrode was evaluated with the Fe(CN)63−/4− redox couple and was calculated to be 0.4. Electrochemical measurements were performed with a computer-controlled Electrochemical analyzer (CHI600E, Chenhua, China) in a two-compartment electrochemical cell with as-prepared material modified on a glassy carbon electrode (3 mm in diameter) as working electrode, a platinum wire as counter electrode, and a Ag/AgCl (3 M KCl) electrode as reference electrode. All electrochemical experiments were performed at room temperature.

Preparation of Co3O4/N-rGO, N-rGO and Co3O4/rGO materials

16.5 mg oxidized graphene oxide (GO) were redispersed in 50 mL anhydrous ethanol to form GO anhydrous ethanol suspension with concentration to be 0.33 mg/mL. The first step to prepare Co3O4/N-rGO was performed by adding 3.6 ml of 0.2 M Co(Ac)2 aqueous solution to 72 ml of GO anhydrous ethanol suspension, followed by the addition of 1.8 ml of NH4OH (30% solution) and 2.1 ml of water, consequently. The reaction was kept at 80 °C with stirring for 10 h. After that, the reaction mixture from the first step was transferred to a 100 mL autoclave for hydrothermal reaction at 180 °C for 12 h. Co3O4/GO hybrid was made by the same steps without adding NH4OH (30% solution) in the first step26. N-rGO hybrid was also made by the same steps just as making Co3O4/N-rGO preparation without adding Co(Ac)2 aqueous solution.

The fabrication of as-prepared materials modified electrodes

A rotating ring-disk electrode (RRDE) with a rotating glassy carbon (GC) disk electrode (4 mm diameter) and a platinum ring electrode (ALS RRDE-2), and a GC electrode with a diameter of 3 mm working electrode were used as working electrode in this study. Prior to the surface modification, the delectrode were polished with 1.0, 0.3, and 0.05 μm alumina slurries, and finally rinsed with Milli-Q water under an ultrasonic bath for 1 min. A Co3O4/N-rGO modified GC electrode was prepared by casting the 4 μL of 2 mg/mL Co3O4/N-rGO suspension on the disk electrode surface and drying in air to evaporate the solvent. Similarly, 4 μLof 2 mg/mL N-rGO solution and 4 μL of 2 mg/mL Co3O4/GO suspension were dropped on GC electrodes, respectively and dried in air to evaporate the solvent for control experiment. Finally, 5 μL nafion (0.5%) solution (diluted 10 times with deionized water) was covered onto electrode surface and dried to form modified working electrode.

All of the electrochemistry experiments were performed at room temperature. The Co3O4/N-rGO modified GC electrode was pretreated by electrochemical oxidation in a phosphate buffered solution (pH =6.8) at a potential of 1.7 V (vs. Ag/AgCl) for 300 s at room temperature, followed by potential sweeping from 0.0 V to 1.4 V in 0.5 M H2SO4 until a stable voltammogram was achieved, the purpose of electrochemical oxidation in phosphate buffered solution and H2SO4 is increased more oxygen containing functional group in carbon materials to increase the active site in oxygen reduction reaction. some Co3O4 nanoparticles may dissolv in H2SO4 thus leaves more active sites on grapheme. For linear sweep voltammetry (LSV) from 0.2 to −1.0 V, The Co3O4/N-rGO modified GC was scanned at a scan rate of 10 mV·s−1 to measure the surface behavior of the ORR activity of the catalyst in O2-saturated 0.1 M KOH. For more quantitative measurements of the ORR activity, LSV was conducted on the catalyst-coated RRDE at a scan rate of 5 mV·s−1 in O2-saturated KOH solution at various rotation rates from 400 to 2025 r·min−1.

Additional Information

How to cite this article: Zhang, T. et al. Co3O4 nanoparticles anchored on nitrogen-doped reduced graphene oxide as a multifunctional catalyst for H2O2 reduction, oxygen reduction and evolution reaction. Sci. Rep. 7, 43638; doi: 10.1038/srep43638 (2017).

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