Anchoring MnCo2O4 Nanorods from Bimetal-Organic Framework on rGO for High-Performance Oxygen Evolution and Reduction Reaction.

Oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are important reactions of energy storage and conversion devices. Therefore, it is highly desirable to design efficient and dual electrocatalysts for replacing the traditional noble-metal-based catalysts. Herein, we have developed a high-efficiency and low-cost MnCo2O4-rGO nanocomposite derived from bimetal-organic frameworks. For OER, MnCo2O4-rGO showed an onset potential of 1.56 V (vs reversible hydrogen electrode (RHE)) and a current density of 14.16 mA/cm2 at 1.83 V, being better than both pure MnCo2O4 and Pt/C. For ORR, MnCo2O4-rGO exhibited a half-wave potential (E 1/2) of 0.77 V (vs RHE), a current density of 3.33 mA/cm2 at 0.36 V, a high electron transfer number n (3.80), and long-term stability, being close to the performance of Pt/C. The high activity of MnCo2O4-rGO was attributed to the synergistic effect among rGO, manganese, and cobalt oxide. As a result, the resultant MnCo2O4-rGO has a great potential to be applied as a high-efficiency ORR and OER electrocatalyst.

Introduction

With the development of economy and society, there has been a growing demand for green energy because of the nonrenewable fossil fuel energy sources.[1−3] The metal–air battery has become the focus of research due to its higher energy density than the current state-of-the-art lithium-ion batteries, high conversion efficiency, environment friendliness, etc.[4,5] The cathode reactions in the metal–air battery, namely, the oxygen reduction reaction (ORR) in the discharge process and the oxygen evolution reaction (OER) in the charging process, play a key role in the energy conversion efficiency.[6−9] However, the kinetics of the ORR and OER are slow due to the high overpotential; therefore, the development of good bifunctional catalysts has become the key to improve the metal–air battery.[10−13] It is well known that platinum, iridium, and ruthenium are excellent catalysts for the ORR and OER. However, the disadvantages of high cost, limited reserves, poor durability, and rapid inactivation seriously hamper their large-scale applications in the ORR and OER.[14−20] Therefore, it is highly desirable to develop high-performance and inexpensive bifunctional non-noble-metal electrocatalysts.[21]

Cobalt-based binary spinel-type metal oxides as inexpensive bifunctional non-noble-metal electrocatalysts, such as MnCo2O4,[22−26] CuCo2O4,[27] NiCo2O4,[28] etc., show excellent performance in the ORR and OER because of their redox stability, the complementation and synergy of two metals, and their variable valence state.[22−28] Especially, MnCo2O4 has been widely utilized in metal–air battery, alkaline fuel cells, and solid oxide fuel cells.[22−26] At the same time, graphene is a famous carbonaceous material with a single layer of carbon atoms tightly packed into a two-dimensional honeycomb lattice structure.[28,29] Compared with other carbon materials, graphene has a larger specific surface area, excellent conductivity, and higher electrochemical stability, and produces a synergistic effect on binding with a metal oxide, which can improve the electrocatalytic activity of oxide and stability.[30,31] For example, Dai et al. reported spinel manganese-cobalt oxide/graphene nanocomposites with advanced oxygen reduction electrocatalytic activity because of their synergistic effect between cobalt oxide, managese oxide, and graphene.[15] Despite these efforts, studies on MnCo2O4/nanocarbon bifunctional catalysts for the ORR and OER are still scarce. Significant performance gaps remain unfilled to furnish bifunctional catalytic activity on a par with precious-metal-based catalysts.

On the other hand, metal-organic frameworks (MOFs) with various morphologies and architectures composed of metal ions and bridging ligands have been known as highly potential precursors or sacrificial templates to prepare a series of tailorable porous inorganic micro/nanomaterials,[32−34] such as porous hollow metal oxides. Apart from the more specific surface area, which could increase the contact surface between electrode material and electrolyte, the porous microstructure could provide more catalytic active sites, resulting in enhanced electrochemical activities.[35−38] However, there are less reports about MnCo2O4-rGO nanocomposites derived from bimetal-organic frameworks (MOFs)/graphene oxides.[39]

Herein, we report a MnCo2O4-reduced graphene oxide (rGO) nanocomposite derived from bimetal-organic frameworks (MOFs)/graphene oxides as the pyrolytic precursors with high-performance ORR and OER electrocatalysts, as bifunctional non-noble-metal electrocatalysts. The ORR electrocatalytic activity of the MnCo2O4-rGO catalyst in a 0.1 M potassium hydroxide solution is close to 20% of the commercial Pt/C, and the OER electrocatalytic activity in a 1 M potassium hydroxide solution is better than that of Pt/C, in terms of onset voltage and diffusion-limited current density. The high electrocatalytic activities of MnCo2O4-rGO for the OER and ORR are caused by its porous spherical structure, which promotes the transfer of electrons. The catalyst also shows excellent stability in alkaline media. The method not only provides a new route for synthesizing MnCo2O4-rGO but also has the advantages of low cost and high yield, and thus, it can be produced on a large scale and promote the development of metal–air battery.

Results and Discussion

Characterizations of MnCo2O4-rGO

Figure exhibits the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the MnCo2O4-rGO nanocomposites. As shown in Figure a, the as-prepared product consists of reduced graphene oxides and nanorods with a diameter of 100 nm. These nanorods appear to be wrapped between the rGOs. Figure b shows the TEM images of the MnCo2O4-rGO nanocomposites. It can be seen that the as-prepared product consists of rGOs and MnCo2O4 nanrods, which is consistent with the results of the SEM images. The selected area electron diffraction pattern in Figure c suggests that the MnCo2O4-rGO nanocomposites are polycrystalline. The diffraction rings match well with the (311) and (400) planes of spinel-structured MnCo2O4,[40] in a good agreement with the X-ray diffraction (XRD) results in the following part. To further probe the incorporation of the MnCo2O4/rGO nanocomposites, energy-dispersive system (EDS) spectroscopy was also used. Figure e–h displays the elemental mappings of Co (red), Mn (green), O (blue), and C (yellow), revealing the presence of the Co, Mn, O, and C elements in the MnCo2O4/rGO nanocomposites.

Figure 1

Characterizations of MnCo2O4-rGO nanocomposites: (a) SEM images; (b) TEM images; (c) SAED pattern; and (d–h) EDS mapping.

The Raman spectra of the MnCo2O4-rGO nanocomposites are shown in Figure a, which displays two prominent peaks around 1350 cm–1 (D band) and 1580 cm–1 (G band). It is well known that the intensity ratio of the D to G band (ID/IG) was used to probe the ordered and disordered crystal structures of carbon.[41−43] The smaller the ID/IG ratio, the higher the degree of ordering in the carbon material.[42] The ID/IG value of the MnCo2O4-rGO nanocomposites is 0.98. Hence, the Raman results correspond to the presence of rGOs in the MnCo2O4-rGO nanocomposites. Figure b presents the XRD pattern of the MnCo2O4-rGO nanocomposites. The characteristic peaks correspond to well-crystallized MnCo2O4 (JCPDS Card No. 30–0820) with the spinel structure of the (220), (311), (400), (422), (511), and (440) planes.[40] The structure of the MnCo2O4-rGO nanocomposites is considered as a mixed valence oxide with a cubic spinel structure, in which manganese and cobalt are distributed over the positions of eight faces and tetrahedra.

Figure 2

(a) Raman spectra of MnCo2O4-rGO. (b) XRD patterns of MnCo2O4-rGO.

To further determine the elemental composition and valence states, the MnCo2O4-rGO nanocomposites were investigated by the X-ray photoelectron spectroscopy (XPS). All of the binding energies were corrected for specimen charging by referring them to the C 1s peak (284.6 eV). Figure a shows the overall XPS spectra of the MnCo2O4-rGO nanocomposites, which clearly show the existence of C, O, Mn, and Co. The atom ratio between Mn and Co is determined to be 1:2, which is same as the theoretical value. The XPS spectrum of Co 2p can be fitted by considering two spin–orbit doublet characteristics of Co3+ and Co2+, indicating the existence of Co with different valence states (Co3+ and Co2+) (Figure b).[40] The two peaks located at 780.2 (Co 2p3/2) and 795.6 eV (Co 2p1/2) could be ascribed to Co3+, while the peaks located at 780.9 (Co 2p3/2) and 795.5 eV (Co 2p1/2) could be ascribed to Co2+. Similarly, as exhibited in Figure c, the spectrum of Mn 2p also indicates the presence of Mn2+ and Mn3+. The main peaks locates at 654.2 eV (Mn 2p1/2) and 642.8 (Mn 2p3/2) belong to Mn3+, while the two peaks located at 641.8 (Mn 2p3/2) and 652.4 eV (Mn 2p1/2) could be ascribed to Mn2+.[39] The existence of Mn3+/Mn2+ and Co3+/Co2+ solid-state redox couples could provide a notable electrochemical activity. The XPS spectrum of C 1s shows that three distinct C configurations coexist, including carbonyl carbon (C=O), carbon (C–O), and graphic carbon (C–C) (Figure d).[27]

Figure 3

(a) XPS survey scan of MnCo2O4-rGO nanocomposites. (b) High-resolution XPS spectra of the Co 2p peak. (c) High-resolution XPS spectra of the Mn 2p peak. (d) High-resolution XPS spectra of the C 1s peak.

Electrochemical Properties

To investigate the ORR performance of the as-prepared samples, the prepared working electrode was first placed in a N2-saturated and O2-saturated 0.1 M KOH solution for CV test. As can be seen in Figure a, the MnCo2O4-rGO nanocomposites exhibited a superior ORR activity, showing a more positive onset potential (∼0.89 V vs reversible hydrogen electrode (RHE)) and peak potential (∼0.72 V vs RHE) and a much higher peak current density (2.58 mA cm–2) than MnCo2O4 (∼0.83, 0.60 V, and 1.32 mA cm–2, respectively). The onset potential for the Pt/C catalyst (20 wt % Pt on Vulcan HCA-PT20) is located at 0.93 V vs RHE, only ∼40 mV more positive than that of the MnCo2O4-rGO nanocomposites, but the peak current density of the Pt/C catalyst (2.26 mA cm–2) is lower than that of the MnCo2O4-rGO nanocomposites. This indicates that the addition of graphene in MnCo2O4 can increase the ORR catalytic activity to close to commercial Pt/C in 0.1 M KOH. It should be noted that the electrocatalytic performance of pure rGO is limited according to previous reports.[44,45] Hence, pure rGO is not considered in further ORR study.

Figure 4

(a) CV curves of MnCo2O4, MnCo2O4-rGO, and Pt/C in O2-saturated (solid lines) and N2-saturated (dashed lines) 0.1 M KOH electrolyte solution at a scan rate of 50 mV/s. (b) linear sweep voltammograms (LSV) curves of MnCo2O4-rGO in O2-saturated 0.1 M KOH at a scan rate of 10 mV/s at different rotating- disk electrode (RDE) rotation rates (rpm). (c) LSV curves of MnCo2O4, MnCo2O4-rGO, and Pt/C in O2-saturated 0.1 M KOH with a sweep rate of 10 mV/s at 1600 rpm and (d) the corresponding Tafel slopes.

The rotating disk electrode (RDE) measurements were usually employed to delve into the ORR kinetics and mechanism of the MnCo2O4-rGO nanocomposites, and LSV polarization curves were recorded at different rotation rates in an O2-saturated 0.1 M KOH solution (Figure b), which exhibits an increase in the limiting current density with an increase in the rotation rate. The Koutecky–Levich (K–L) plots extracted from the above LSV polarization curves (between 0.6 and 0.4 V) (Figure b inset) exhibit a linear relationship between J–1 and ω–1/2. The fitting lines are almost parallel, indicating that the first-order reaction kinetics is related to the concentration of dissolved oxygen and has a similar electron transfer number (n) at different voltages of ORR. It is reported that the mechanism of electron transfer in the ORR is a direct and effective four-electron reduction reaction[46,47] [eq ] or two-electron reduction reaction[44] [eq ], and the electron transfer number (n) can be calculated on the basis of the Koutecky–Levich (K–L) equations[48]where J is the measured current density, JL and JK are the diffusion- and kinetic-limited current densities, respectively, ω is the angular velocity (rpm), n is the electron transfer number, F is the Faraday constant (96 485 C/mol), C0 is the bulk concentration of O2 (1.19 mol m–3 in 0.1 M KOH), D0 is the diffusion coefficient of O2 (1.88 × 10–9 m2/s in 0.1 M KOH), v is the kinematic viscosity of the electrolyte (10–6 m2/s in 0.1 M KOH), the constant 0.2 is adopted when the rotation speed is expressed in rpm, and k is the electron transfer rate constant. According to eqs –5, the average electron transfer number (n) of MnCo2O4-rGO was calculated as 3.8 on the basis of the Koutecky–Levich (K–L) equations, indicating a four-electron transfer process. Thus, H2O is directly formed as an intermediate product from oxygen by consuming four electrons according to the previous literature.[44] The four-electron reduction reaction pathway is favorable for increasing the reaction rate, so it has important applications in metal–air batteries.

Figure c displays the linear sweep voltammograms (LSV) of the MnCo2O4, MnCo2O4-rGO, and Pt/C at a rotating rate of 1600 rpm in O2-saturated 0.1 M KOH. It can be seen that the pure MnCo2O4 material exhibits poor electrocatalytic activity toward ORR in an alkaline solution, while MnCo2O4-rGO represents a higher electrocatalytic activity (onset potential of 1.11 V, current density of −3.33 mA/cm2 at 0.36 V) than MnCo2O4 (onset potential of 1.12 V, current density of −2.30 mA/cm2 at 0.36 V). Therefore, the combination of graphene and MnCo2O4 is essential to improve the electrochemical activity of ORR. On the other hand, the E1/2 value of MnCo2O4-rGO is 0.77 V, similar to that of Pt/C at 0.80 V and more positive than that of pure MnCo2O4 at 0.66 V, as shown in Figure c. The slope of the Tafel plots of MnCo2O4-rGO was −150.1 mV/dec, better than the ones of MnCo2O4 (−142.2 mV/dec) and Pt/C (−120.2 mV/dec) (Figure d). These results further confirmed that the MnCo2O4-rGO nanocomposite is an efficient electrochemical catalyst for the ORR and had very good kinetic processes.

As a bifunctional catalyst, excellent ORR and OER activities are both required. Therefore, the OER catalytic activity of MnCo2O4-rGO was also evaluated. Linear sweep voltammetry (LSV) was carried out in a N2-saturated 1 M KOH solution at a rotating rate of 1600 rpm for MnCo2O4, MnCo2O4-rGO, and Pt/C (Figure a). The onset potential and the current density are the main parameters to evaluate the catalytic activity of the sample on OER. The onset potential of MnCo2O4 is 1.63 V, and the current density is 6.12 mA/cm2 at 1.83 V. Furthermore, the onset potential of MnCo2O4-rGO is 1.56 V, and the current density reached 14.16 mA/cm2 at 1.83 V. The results showed that the addition of graphene could enhance the OER performance. It should be noted that the onset potential of MnCo2O4-rGO is lower than that of Pt/C (1.65 V) and the current density of MnCo2O4-rGO is higher than that of Pt/C (2.75 mA/cm2 at 1.83 V), which demonstrated that the OER activity of MnCo2O4-rGO is better than that of commercial Pt/C. The OER dynamics of these samples was also evaluated with the corresponding Tafel plots (Figure b).[20] The slope of MnCo2O4-rGO was 106.9 mV/dec, better than that of pure MnCo2O4 (130.0 mV/dec) and Pt/C (134.0 mV/dec). The results showed that the combination of MnCo2O4 and graphene effectively promoted the kinetic process. The high activity of MnCo2O4-rGO is ascribed to the synergistic effect between cobalt oxide, manganese oxide, and graphene.

Figure 5

(a) LSV curves of MnCo2O4, MnCo2O4-rGO, and Pt/C in N2-saturated 1 M KOH with a sweep rate of 10 mV/s at 1600 rpm. (b) Tafel plots of MnCo2O4, MnCo2O4-rGO, and Pt/C for OER.

The stability of the electrocatalyst is another important factor to evaluate its application. To investigate the electrochemical stability of catalysts, chronoamperometric analysis can be used. The chronoamperometric measurements of MnCo2O4, MnCo2O4/rGO catalyst, and Pt/C were carried out in an O2-saturated 0.1 M KOH solution at a fixed potential of 0.4 V at 1600 rpm. As shown in Figure S2, the retention rate of the current density of the MnCo2O4-rGO composite is much higher than that of MnCo2O4 and Pt/C, showing that the MnCo2O4-rGO composite has a higher retention rate and coulombic efficiency and a lower internal resistance than MnCo2O4 and Pt/C. In addition, the current does not decline. It could be concluded that the MoCo2O4-rGO composite has a long-term stability for electrocatalytic activity, indicating that MoCo2O4-rGO has structural and chemical stabilities after stability test.

Conclusions

In this work, a new bifunctional electrocatalyst with excellent ORR and OER activities was designed and synthesized by the combination of MnCo2O4 and graphene. MnCo2O4-rGO showed a higher activity than MnCo2O4 for both the ORR and OER due to a synergistic effect of the graphene and MnCo2O4. It should be noted that the OER activity of MnCo2O4-rGO is also better than that of the commercial Pt/C catalyst. On the whole, the MnCo2O4-rGO nanocomposite is an excellent bifunctional electrocatalyst for the OER and ORR, and is expected to be a good bifunctional oxygen electrode for metal–air batteries.

Experimental Section

Materials

All of the chemicals were directly used after purchase without further purification. Deionized water was used for the preparation of all solutions.

Synthesis of MnCo2O4/rGO Composite

Graphene oxide (GO) was synthesized from graphite by a modified Hummers’ method, similar to our previous reports.[49−51] In a typical synthesis, 0.414 g of manganese acetate and 0.881 g of cobalt acetate were dispersed in 30 mL of deionized water, then 50 mL of a GO solution (1 mg/mL) was added in it as solution A, and 0.752 g of fumaric acid was dispersed into 30 mL of ethanol as solution B. Then, solution B was added into solution A under continuous stirring for 30 min, and the reaction mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave. The hydrothermal reaction was carried out at 200 °C for 6 h. The product was collected by centrifugation and washed three times with water and ethanol before drying. Finally, the dried powder was put in a tubular furnace at 600 °C for 3 h at a 4 °C/min heating rate in nitrogen. For comparison, MnCo2O4 was synthesized under a similar condition without the presence of GO.

Material Characterizations

The crystal structures of the as-prepared products were determined by X-ray diffraction (XRD, XRD-6000) using the Cu Ka radiation (0.15406 nm). The microstructures and morphologies of the products were characterized using scanning electron microscopy (SEM, JEOL, JSM 6480) and transmission electron microscopy (TEM, Philips Tecnai 12) with energy-dispersive X-ray analysis (EDX). The Raman spectra of the as-prepared products were obtained by a Renishaw Raman spectrometer.

Electrochemical Measurements

All electrochemical tests were carried out on an electrochemical workstation (CHI 760E, Chenhua Ltd. Co., China) with a conventional three-electrode cell. A Pt foil and a saturated calomel electrode (SCE) were applied as the counter and reference electrodes, respectively. The working electrode was prepared by loading the catalysts on a glassy carbon electrode with a disk diameter of 5 mm (disk area = 0.1963 cm2 ) and as the rotating disk electrode. The sample ink was prepared by mixing 4.12 mg of material, 50 μL of 5 wt % Nafion solution, 200 μL of ethanol, and 800 μL of deionized water followed by 30 min of sonication, forming a homogeneous catalyst ink. Then, 20 μL of the homogeneous ink was dropped onto the glassy carbon with the corresponding mass loading and dried slowly in air. Similarly, 2.06 mg of 20 wt % Pt was used to prepare the ink and 20 μL was dropped onto the glassy carbon electrode with a mass loading of 0.20 mg/cm2.

Before test, a N2 or O2 flow was introduced into the electrolyte in the cell for 30 min to give a saturation state. The working electrode was cycled with a scan rate of 20 mV/s for at least 50 times before the data collection. Cyclic voltammetry (CV) experiments were done at a scan rate of 50 mV/s in a N2- or O2-saturated KOH solution (0.1 mol/L). The linear sweep voltammetry (LSV) experiments were conducted in an O2-saturated KOH (0.1 M) solution at different speed rates (400, 625, 900, 1225, 1600, 2025, 2500 rpm) at a scan rate of 10 mV/s. As for OER, the LSV experiments were different with ORR. They were conducted in a N2-saturated KOH (1.0 M) solution at a scan rate of 10 mV/s, and the data were collected after stabilization. The stability tests for ORR were carried out at 0.4 V in an O2-saturated 0.1 M KOH solution (ω = 1600 rpm) by the chronoamperometric method. The recorded potentials all use a negative scan pattern, and the potentials were referenced to the reversible hydrogen electrode (RHE) through RHE calibration (E(RHE) = E(Ag/AgCl) + 0.0951pH + 0.197). Furthermore, to ensure O2 saturation, the gas was bubbled into the electrolyte prior to the start of each experiment and maintained over the electrolyte during each measurement.