Co3O4 nanoparticles anchored on nitrogen-doped reduced graphene oxide as a multifunctional catalyst for H2O2 reduction, oxygen reduction and evolution reaction

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 oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and H2O2 sensing. Electrocatalytic activity of Co3O4/N-rGO to hydrogen peroxide reduction was tested by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry. Under a reduction potential at −0.6 V to H2O2, this constructing H2O2 sensor exhibits a linear response ranging from 0.2 to 17.5 mM with a detection limit to be 0.1 mM. Although Co3O4/rGO or nitrogen-doped reduced graphene oxide (N-rGO) alone has little catalytic activity, the Co3O4/N-rGO exhibits high ORR activity. The Co3O4/N-rGO hybrid demonstrates satisfied catalytic activity with ORR peak potential to be −0.26 V (vs. Ag/AgCl) and the number of electron transfer number is 3.4, but superior stability to Pt/C in alkaline solutions. The same hybrid is also highly active for OER with the onset potential, current density and Tafel slope to be better than Pt/C. The unusual catalytic activity of Co3O4/N-rGO for hydrogen peroxide reduction, ORR and OER may be ascribed to synergetic chemical coupling effects between Co3O4, nitrogen and graphene.

catalytic activity for the ORR, OER and immobilizing enzymes for further applications in fabrication of hydrogen peroxide biosensor [13][14][15] . Among them, Co 3 O 4 with spinel crystal structure is beneficial to electron transportation between Co 2+ and Co 3+ ions, which has been extensively considered as an efficient electrocatalyst for OER and ORR [16][17][18] . Previous studies reported that the efficiency of cobalt oxide as an OER catalyst could be ascribed to the increasing population of Co IV centers at the oxide surface during electrochemical oxidation 19 . More interesting, Co 3 O 4 , which exhibits catalase-like activity for the decomposition of H 2 O 2 , can be applied to the detection of H 2 O 2 in aqueous medium 20 . However, these Co 3 O 4 -based catalysts usually suffer from the poor electrical conductivity, short active site density and the dissolution or agglomeration during electrochemical processes. Co 3 O 4 itself is a material with a little ORR, OER and H 2 O 2 sensing activity and further studies exhibit that synergy between the carbon materials and Co 3 O 4 can give a huge promotion of the electrocatalytic activity 21,22 . Therefore, lots of researches have used conductive carbon nanomaterials such as carbon nanotubes (CNTs), carbon foam and graphene etc. To improve the conductivity of Co 3 O 4 based hybrid catalysts as well as obtain uniformly dispersed Co 3 O 4 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 years 21,23,24 . 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 area 25,26 . Dai's group reported a hybrid material consisting of Co 3 O 4 nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the ORR and OER 27 . 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 reaction 28 . Anchoring Co 3 O 4 nanocrystals on carbon-based supports could significantly improve their electrocatalytic activity contributed by the small crystalline size and conductive support 29 . 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 bonds 30,31 . It has been also reported that nitrogen-doped graphene can promote the electrochemical reduction of H 2 O 2 32 . As previous study, the introduction of the Co− N 4 complex onto the graphene basal plane facilitates the activation of O 2 dissociation and the desorption of H 2 O during the ORR 33 . 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 H 2 O 2 reduction, ORR and OER.
We report herein the synthesis of Co 3 O 4 nanoparticles anchored on nitrogen-doped reduced graphene oxide (Co 3 O 4 /N-rGO) through a simple and scalable method as tri-functional catalysts for H 2 O 2 reduction, ORR and the OER, as shown in Fig. 1. Co 3 O 4 anchored uniformly into laminar nitrogen-doped reduced graphene oxide was confirmed by scanning electron microscopy (SEM). Co 3 O 4 /N-rGO possesses a good electrocatalytic activity toward H 2 O 2 reduction by enhancing the current response and decreasing H 2 O 2 reduction over potential. The electrochemical results demonstrate that the Co 3 O 4 /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 Co 3 O 4 , nitrogen and reduced graphene oxide (rGO) is found to play an important role in the high electrocatalytic activities of the Co 3 O 4 /N-rGO. This synthesis route can be easily adopted for large-scale manufacturing due to its process simplicity and the accessibility of precursor materials.

Results and Discussion
Characterization. Figure 2A and B illustrated field emission scanning electron microscopy (FE-SEM) images of Co 3 O 4 /N-rGO. We can clearly see from SEM images in Fig. 2B that Co 3 O 4 nanoparticles are uniformly anchored on the rGO substrate with an approximate average diameter of 150 nm. This may be attributed that Co 2+ ion was coordinated with negatively charged oxygen-containing functional groups on N-rGO sheets. During the hydrothermal process, Co 2+ was oxidized into Co 3+ by oxygen-containing groups, and crystallized to form Co 3 O 4 nanoparticles anchored into N-rGO sheets 34 . However, Fig. 2C demonstrate Co 3 O 4 /rGO does not exhibit such a uniform morphology distribution of Co 3 O 4 . In addition, we only found that a typical corrugated structure in Fig. 2D, suggesting there is no Co 3 O 4 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 Co 3 O 4 35,36 . In addition, these uniform structures of Co 3 O 4 particles anchored into N-rGO can also be ascribed to NH 3 together with oxygen-containing functional group coordinating with cobalt cations and thus reducing Co 3 O 4 particles size and enhancing particles nucleation on N-rGO 37 . XRD was performed to investigate the phase structure of Co 3 O 4 /N-rGO. As shown in Fig. 3A, the diffraction peaks of the pristine Co 3 O 4 was consistent with the standard Co 3 O 4 (JCPDS card: 42-1467).
The major diffraction peaks of Co 3 O 4 /N-rGO were well in agreement with those of Co 3 O 4 except for the broad (002) peak at approximately 25°, which can be ascribed to disordered stacked graphitic sheets 30 . This manifest that the original GO has been reduced to rGO during the hydrothermal process, again confirming we have successfully incorporated Co 3 O 4 into rGO 38 . 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 Co 3 O 4 /N-rGO, N-rGO and Co 3 O 4 are 103.9 m 2 /g, 139.7 m 2 /g, and 62.8 m 2 /g, respectively. These results indicate the change of N-rGO structure after doping with Co 3 O 4 .
Raman spectroscopy was carried out to extend the study for the carbon structures in  X-ray photoelectron spectroscopic (XPS) measurements were performed to determine the surface element constitution in Co 3 O 4 /N-rGO. The sharp peaks in Fig. 3C are corresponded to the characteristic peaks of C 1s , O 1s , N 1s and Co 2p , indicating the existence of carbon, oxygen, nitrogen and cobalt elements in the prepared sample. The XPS spectrum for Co 2p shown in Fig. 3D reveals two major peaks with binding energies at 780.1 and 795 eV, corresponding to Co 2p3/2 and Co 2p1/2 , respectively, with a spin energy separation of 15 eV, which is attributed to the Co 2+ oxidation state, indicating that a portion of Co 3+ is reduced to Co 2+ with generating oxygen vacancies 17 . These results again confirmed that Co 3 O 4 nanoparticles have been anchored on N-rGO, Co 2+ and Co 3+ in the crystal structure of Co 3 O 4 are being considered to be playing a vital role in improving catalytic performance of oxygen reduction reaction and oxygen evolution reaction 37 . Furthermore, the main beautiful structure of Co 3 O 4 is the peculiar cation distribution in the face centered cubic (FCC) crystal where the Co 2+ ions reside on the 1/8 th of the tetrahedral A sites while the Co 3+ ions occupy 1/2 of the octahedral B sites 11 , endow the system viable for electrocatalytic applications. The high-resolution N 1s XPS spectrum of Co 3 O 4 /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 sp 2 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 nitrogen 41 .

Electrocatalytic activity of Co 3 O 4 /N-rGO for H 2 O 2 reduction. Though enzyme based electrochemical
sensors have been widely developed to sensing H 2 O 2 due to the advantages of high sensitivity and good selectivity, these sensors often suffer from unstable response due to the intrinsic nature of enzymes 42 . Therefore, it is necessary to develop a simple non-enzymatic strategy for sensing H 2 O 2 with high sensitivity. To date, electrocatalysts for design H 2 O 2 sensors with high sensitivity, good selectivity and easy regulation properties hold leading position among various sensors 12 . It has been proved that functional nano-structured transition-metal oxides exibit good electrocatalytic activity toward the H 2 O 2 reduction, which provides valuable strategy for the nonenzymatic determination of H 2 O 2 43,44 . Among various kinds of transition metal oxides, Co 3 O 4 shows attracting electronic and electrocatalytic properties. Particularly, its normal spinel crystal structure is favorable for electron transportation between Co 2+ and Co 3+ ions and Co 3 O 4 possess catalase-like activity, which is benefit to sensing H 2 O 2 . Therefore, Co 3 O 4 have been extensively explored as the sensing materials for developing enzyme-free H 2 O 2 sensors. However, Co 3 O 4 -based catalysts usually suffer from the poor electrical conductivity, low active site density and the dissolution or agglomeration during electrochemical processes 45,46 . On the other hand, graphene has the ability to promote electron transfer rates and graphene-based modified electrode had much better electrocatalysis toward H 2 O 2 47 . In our study, Co 3 O 4 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 H 2 O 2 reduction due to the synergistic effect. To investigate the electrocatalytic characteristics to H 2 O 2 reduction of Co 3 O 4 /N-rGO, voltammetric measurements were performed using the Co 3 O 4 /N-rGO, Co 3 O 4 /rGO and N-rGO modified GC electrodes in the presence of 5 mM H 2 O 2 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 Co 3 O 4 /N-rGO modified GC electrode. Figure  It is a significant way for amperometric technique to test the sensing property of Co 3 O 4 /N-rGO modified GC electrode. We studied the effect of the applied potential in order to improve the Co 3 O 4 /N-rGO modified GC electrode performance towards non-enzymatic H 2 O 2 sensing. We investigated applied potential on the amperometric response on the Co 3 O 4 /N-rGO modified GC electrode towards sequential addition of 0.5 mM H 2 O 2 by varying the potential between− 0.6 V and − 0.3 V. As shown in Fig. 5A and B, as amperometric response of the Co 3 O 4 /N-rGO modified GC electrode has the optimal sensitivity, the applied potential at − 0.6 V was selected. Figure 5A Fig. 6A, CV of N-rGO and Co 3 O 4 /rGO in the O 2 -saturated electrolyte shows a reduction peak at − 0.34 V and − 0.33 V respectively, suggesting their electrochemical catalytic activity for ORR. As for Co 3 O 4 /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 Co 3 O 4 /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 study 32 . Previous have reported that the electrocatalytic activity of Co 3 O 4 was mainly affected by structure 46 . Co 3 O 4 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 reaction 22 . Therefore, such an excellent electrocatalytic activity of the Co 3 O 4 /N-rGO toward ORR can be ascribed to the synergetic chemical coupling effects of Co 3 O 4 and N-graphene 18,49,50 .
To investigate the oxygen reduction mechanism of Co 3 O 4 /N-rGO modified GC electrodes, the ORR was studied by RRDE   where I d is the disk current, I r is the ring current, and N is the current collection efficiency of the Pt ring, which was determined to be 0.4 21,46 . From the Fig. 6B 51 . The effect of methanol poisoning and stability on the Co 3 O 4 /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 Co 3 O 4 /N-rGO, suggesting poor tolerance of Pt/C to methanol compared with the Co 3 O 4 /N-rGO material.   the oxygen reduction (ORR) and evolution (OER) reactions via decreasing overpotential in fuel cells and water electrolyzers 17,52 . The good catalytic performance of OER can be ascribe to the small crystalline size and the mixed valences Co 2+ and Co 3+ of Co 3 O 4 as well with conductive support substrates 17 . Sun and his groups synthesize Co 3 O 4 nanorod-multiwalled carbon nanotube hybrid with a onset potential of about 0.47 V vs. Ag/AgCl and Tafel slope of 65 mV/dec 37 . 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 Co 3 O 4 /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 Co 3 O 4 /N-rGO afforded a sharp onset potential at 1.54 V, which is worse than that of RuO 2 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 RuO 2 , indicating the Co 3 O 4 /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 Co 3 O 4 /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 RuO 2 (63 mV/dec), suggesting Co 3 O 4 /N-rGO has an improved performance of OER.

Conclusions
In summary, this study describes a facile and effective route to synthesize hybrid material consisting of Co 3 O 4 nanoparticles anchored on nitrogen-doped reduced graphene oxide (Co 3 O 4 /N-rGO) as a high-performance tri-functional catalyst for ORR, OER and H 2 O 2 sensing. Owing to the synergetic chemical coupling effects between Co 3 O 4 and graphene, the Co 3 O 4 /N-rGO exhibited excellent electrocatalytic activity with a direct reduction to H 2 O 2 at − 0.6 V and sensing ability towards H 2 O 2 . Although Co 3 O 4 /rGO or N-rGO alone has little catalytic activity, the Co 3 O 4 /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 Co 3 O 4 /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) 6 3− / 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.  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 Co 3 O 4 /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 NH 4 OH (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. Co 3 O 4 /GO hybrid was made by the same steps without adding NH 4 OH (30% solution) in the first step 26 . N-rGO hybrid was also made by the same steps just as making Co 3 O 4 /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 μ m alumina slurries, and finally rinsed with Milli-Q water under an ultrasonic bath for 1 min. A Co 3 O 4 /N-rGO modified GC electrode was prepared by casting the 4 μ L of 2 mg/mL Co 3 O 4 /N-rGO suspension on the disk electrode surface and drying in air to evaporate the solvent. Similarly, 4 μ Lof 2 mg/mL N-rGO solution and 4 μ L of 2 mg/mL Co 3 O 4 /GO suspension were dropped on GC electrodes, respectively and dried in air to evaporate the solvent for control experiment. Finally, 5 μ L 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 Co 3 O 4 /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 H 2 SO 4 until a stable voltammogram was achieved, the purpose of electrochemical oxidation in phosphate buffered solution and H 2 SO 4 is increased more oxygen containing functional group in carbon materials to increase the active site in oxygen reduction reaction. some Co 3 O 4 nanoparticles may dissolv in H 2 SO 4 thus leaves more active sites on grapheme. For linear sweep voltammetry (LSV) from 0.2 to − 1.0 V, The Co 3 O 4 /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 O 2 -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 O 2 -saturated KOH solution at various rotation rates from 400 to 2025 r·min −1 .