**4. Applications of graphene–metal oxide nanohybrids as electrocatalysts**

Graphene–metal-oxide-based nanohybrid materials have a wide range of applications from electrochemical sensing to the ORR electrocatalysis, due to their superior properties and low cost [87, 88]. The new types of materials could open up a new window for superior electroca‐ talytic activity as well as selectivity and durability, which can act as promising electrode materials for various electrochemical reactions [89, 90, 91]. In this section, we present some examples for the applications of graphene–metal oxide nanohybrids for different kinds of electrocatalytic reactions.

## **4.1. Metal-oxide-decorated 2D graphene structures and electrocatalysis**

If metal oxide nanoparticles are randomly loaded on graphene sheets but with little control in size and structure [92], then it can result in make the poor interaction between nanoparticles and graphene sheet [93]. Recently, the methods have been continuously improved, to some degree with controlling their locations and amount. In this context, a number of metal oxides are synthesized and loaded onto graphene matrix used as nonenzymatic sensor materials [16, 29, 31, 35, 45, 94, 95]. For example, Sun and coworkers [96] made a presynthesized monodis‐ perse Co/CoO core/shell nanoparticles on the graphene surface (Fig. 12). In Co/CoO core/shell nanoparticles, Co core size and thickness can be tuned by controlling the oxidation conditions. In their work, they demonstrated the significance of Co/CoO size and graphene support for the tuning of electrocatalysts for efficient ORR with a selective 4e process (Fig. 12b,c).

**Figure 12.** (a) A TEM image of the Co/CoO core/shell NPs deposited on graphene surface. (b) ORR polarization curves of the G–Co/CoO NPs and commercial C–Pt catalyst. Scan rate: 10 mV s−1 and rotation rate 400 rpm. (c) The chronoam‐ perometric responses for the ORR on the G–Co/CoO NPs and commercial C–Pt catalyst at −0.3 V. Rotation rate: 200 rpm. The measurements were performed in O2-saturated KOH (0.1 M) solution. (Reproduced with permission from ref. 96 Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The optimized Co/CoO–graphene electrocatalyst exhibited reasonably high activity and better stability than the commercial Pt/C catalyst. It is also evident that the ORR activity of cobalt oxide–graphene hybrid electrocatalysts is significantly enhanced with the Co content and its coupling with N-doped graphene [97]. Cobalt oxides/graphene nanohybrid materials can also serve as the ORR catalysts used in Li–O2 batteries. Graphene sheets with Co3O4 nanofiber immobilized on both sides can act as a bifunctional catalyst for the ORR[98]. This excellent electrochemical performance relies on the facile electron transport and fast O2 diffusion between the porous Co3O4 nanofiber networks and the ultrathin graphene layer. Manganeseoxides-based graphene nanocomposites have also been used as a stable and low-cost cathode electrocatalysts for fuel cells and Li–air batteries [99, 101]. It is clearly evident that electroca‐ talytic performance of metal oxide–graphene nanocomposites is closely associated with morphology and size of metal oxide nanoparticles and metal oxide–graphene electronic couplings [102]. Qiao and coworkers showed a mesoporous structure of Mn3O4/graphene hybrid nanomaterials with good ORR activity, excellent stability, and high selectivity [103]. Kim and coworkers also showed the ORR mechanism of a system with a lower loading (19.2%) of Mn3O4 nanoparticles on graphene sheets is comparable to that of the Pt/C electrode with a 4e transfer, whereas the composite with a higher Mn3O4 content (52.5%) undergoes a conven‐ tional 2e process [104]. Also, other graphene-supported metal oxides, e.g., Fe3O4 [105], Fe2O3 [106], Cu2O [107, 108], and Ru2O [109] were also studied as electrocatalysts for ORR in fuel cells and Li–air batteries.

## **4.2. Metal-oxide-decorated 3D graphene structures and electrocatalysis**

**4.1. Metal-oxide-decorated 2D graphene structures and electrocatalysis**

398 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

If metal oxide nanoparticles are randomly loaded on graphene sheets but with little control in size and structure [92], then it can result in make the poor interaction between nanoparticles and graphene sheet [93]. Recently, the methods have been continuously improved, to some degree with controlling their locations and amount. In this context, a number of metal oxides are synthesized and loaded onto graphene matrix used as nonenzymatic sensor materials [16, 29, 31, 35, 45, 94, 95]. For example, Sun and coworkers [96] made a presynthesized monodis‐ perse Co/CoO core/shell nanoparticles on the graphene surface (Fig. 12). In Co/CoO core/shell nanoparticles, Co core size and thickness can be tuned by controlling the oxidation conditions. In their work, they demonstrated the significance of Co/CoO size and graphene support for

the tuning of electrocatalysts for efficient ORR with a selective 4e process (Fig. 12b,c).

**Figure 12.** (a) A TEM image of the Co/CoO core/shell NPs deposited on graphene surface. (b) ORR polarization curves of the G–Co/CoO NPs and commercial C–Pt catalyst. Scan rate: 10 mV s−1 and rotation rate 400 rpm. (c) The chronoam‐ perometric responses for the ORR on the G–Co/CoO NPs and commercial C–Pt catalyst at −0.3 V. Rotation rate: 200 rpm. The measurements were performed in O2-saturated KOH (0.1 M) solution. (Reproduced with permission from

The optimized Co/CoO–graphene electrocatalyst exhibited reasonably high activity and better stability than the commercial Pt/C catalyst. It is also evident that the ORR activity of cobalt oxide–graphene hybrid electrocatalysts is significantly enhanced with the Co content and its coupling with N-doped graphene [97]. Cobalt oxides/graphene nanohybrid materials can also serve as the ORR catalysts used in Li–O2 batteries. Graphene sheets with Co3O4 nanofiber immobilized on both sides can act as a bifunctional catalyst for the ORR[98]. This excellent electrochemical performance relies on the facile electron transport and fast O2 diffusion between the porous Co3O4 nanofiber networks and the ultrathin graphene layer. Manganeseoxides-based graphene nanocomposites have also been used as a stable and low-cost cathode electrocatalysts for fuel cells and Li–air batteries [99, 101]. It is clearly evident that electroca‐ talytic performance of metal oxide–graphene nanocomposites is closely associated with morphology and size of metal oxide nanoparticles and metal oxide–graphene electronic couplings [102]. Qiao and coworkers showed a mesoporous structure of Mn3O4/graphene hybrid nanomaterials with good ORR activity, excellent stability, and high selectivity [103]. Kim and coworkers also showed the ORR mechanism of a system with a lower loading (19.2%)

**a b c**

ref. 96 Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

3D structured graphene (e.g., graphene foam) is an ideal candidate for noble metal oxide catalyst support for electrocatalytic applications because of its high electron conductivity, large surface area, sufficient porosity, and thermal stability [110]. 3D graphene can offer high surface areas for higher loading of metal oxide nanoparticles, which can show enhanced electrocata‐ lytic activity. For example, Dong and coworkers [17] synthesized a 3D graphene–cobalt oxide nanohybrid material for high-performance supercapacitor and enzymeless glucose detection. Figure 13 shows the electrocatalytic oxidation of glucose in low-concentration alkaline solutions. Figure 13a shows the CV curves of the 3D graphene/cobalt oxide composite electrode obtained at different scan rates. Two pairs of redox peaks (I/ II and III/ IV) are observed from the CV. The redox peak currents increase with increasing scan rate proportionally, which implies a surface-controlled electrochemical process. Figure 13b shows that the oxidation current at peak III (at ∼0.58 V) started increasing with introduction of glucose, but the other peak remained almost constant. Figure 13c shows the amperometric responses of the gra‐ phene/Co3O4 composite electrode to glucose with various concentrations. The calibration curve of the amperometric response was plotted in Fig. 13d. This composite material showed an excellent sensitivity of 3.39 mA mM-1 cm-2, a relatively narrow linear range (up to 80 μM) and sub-100 nM lower detection limit (LOD).

Feng and Mullen have studied the controllable structural assembly of Fe3O4 nanoparticles on 3D N-doped graphene aerogel support [21]. Figure 14 (a-d) shows interconnected macropo‐ rous graphene hybrid network uniformly decorated with Fe3O4 nanoparticles. The Fe3O4/N– graphene aerogel network displayed excellent electrocatalytic activity for the ORR in alkaline electrolytes with a high current density, low ring current and H2O2 yield, being a four electron transfer number, and high stability (Fig. 14e,f). The electrocatalytic ORR has an onset potential of +0.16 V (vs Ag/AgCl) and a high current density of 1.46 mA cm−2, which is well-comparable with the performance by commercial Pt/C.

## **4.3. Heteroatom doped-graphene-materials and their electrocatalysis**

Nitrogen and sulfur are the mostly used elements for doping graphene[111, 114]. Specific doping on graphene could lead to remarkable increase in charge carrier concentration, specific surface area, and enhanced capacitance retention. The N- or S-doped graphene materials show new exciting properties compared to pristine graphene. For instance, the spin density and charge distribution of carbon atoms are modulated by the neighboring nitrogen dopants, which induce the "activation region" on the graphene surface. This kind of activated region can directly participate in electrocatalytic reactions such as ORR [115, 116] or anchor metal nanoparticles with specific catalytic activity desired [117].

**Figure 13.** Electrochemical sensing of glucose in 0.1 M NaOH solution using the 3D graphene/Co3O4 composite elec‐ trode. (a) CV curves measured at different scan rates (5, 10, 20, and 50 mV/s). (b) CV curves in the presence of different concentrations of glucose (0, 0.2, 0.4, 0.6, 0.8, and 1 mM), at the scan rate of 20 mV/s. (c) Amperometric data of the composite electrode (potential 0.58 V) upon addition of glucose to increasing concentrations. (d) Average dose re‐ sponse curve (amperometric current response *vs* glucose concentration) obtained from three different sensors, with a linear fitting at lower concentration range and an exponential fitting at higher concentration range. The error bars indi‐ cate the standard deviations. (Reproduced with permission from ref. 17. Copyright ACS 2012)

Sheng et al. [118] reported a facile catalyst-free method for the synthesis of N-doped graphene via thermal annealing graphene oxide with melamine for the electrocatalytic application in ORR. Figure 15 compares cyclic voltammograms (CVs) for the electrochemical reduction of O2 at a bare glassy carbon electrode (GCE), graphene/GCE, and NG/GCE in O2-saturated 0.1 M KOH solutions. The onset potential of ORR at the NG/GCE occurs at 0.1 V, which is about 0.1 V more positive than that of graphene/GCE. The electrocatalytic process of NG/GCE is a one-step four-electron pathway for ORR, which is almost twice as large as that for pristine graphene in the current density. Sulfur-doped graphene was successfully prepared using GO and benzyl disulfide as precursors under high temperature, and the as-prepared temperaturedependant S-Doped graphene was tested as a metal-free cathode catalyst for oxygen reduction. All the results further confirmed that the S-doped graphene is a promising material with high catalytic activity for ORR [119]. N and S co-doped graphene was also developed and used for ORR recently; compared to the single element doped graphene, co-doped graphene displayed even more efficient electrocatalysis toward ORR [120].

**Figure 14.** (a,b) Typical SEM images of Fe3O4/N-Gas, revealing the 3D macroporous structure and uniform distribution of Fe3O4 NPs in the GAs. The red rings in (d) indicate Fe3O4NPs encapsulated in thin graphene layers. Representative (c) TEM and (d) HRTEM images of Fe3O4/N-Gas, revealing an Fe3O4 NP wrapped by graphene layers. (e) CVs of Fe3O4/N-GAs in N2- and O2-staturated 0.1 M aqueous KOH electrolyte solution at a scan rate of 100 mV s–1. (f) LSVs of Fe3O4/N-GAs in O2-saturated 0.1 M KOH at a scan rate of 10 mV s–1 at different RDE rotation rates (in rpm). (Repro‐ duced with permission from ref. 21. Copyright ACS 2012)

Sheng et al. [118] reported a facile catalyst-free method for the synthesis of N-doped graphene via thermal annealing graphene oxide with melamine for the electrocatalytic application in ORR. Figure 15 compares cyclic voltammograms (CVs) for the electrochemical reduction of O2 at a bare glassy carbon electrode (GCE), graphene/GCE, and NG/GCE in O2-saturated 0.1 M KOH solutions. The onset potential of ORR at the NG/GCE occurs at 0.1 V, which is about 0.1 V more positive than that of graphene/GCE. The electrocatalytic process of NG/GCE is a one-step four-electron pathway for ORR, which is almost twice as large as that for pristine graphene in the current density. Sulfur-doped graphene was successfully prepared using GO and benzyl disulfide as precursors under high temperature, and the as-prepared temperaturedependant S-Doped graphene was tested as a metal-free cathode catalyst for oxygen reduction. All the results further confirmed that the S-doped graphene is a promising material with high catalytic activity for ORR [119]. N and S co-doped graphene was also developed and used for ORR recently; compared to the single element doped graphene, co-doped graphene displayed

cate the standard deviations. (Reproduced with permission from ref. 17. Copyright ACS 2012)

**Figure 13.** Electrochemical sensing of glucose in 0.1 M NaOH solution using the 3D graphene/Co3O4 composite elec‐ trode. (a) CV curves measured at different scan rates (5, 10, 20, and 50 mV/s). (b) CV curves in the presence of different concentrations of glucose (0, 0.2, 0.4, 0.6, 0.8, and 1 mM), at the scan rate of 20 mV/s. (c) Amperometric data of the composite electrode (potential 0.58 V) upon addition of glucose to increasing concentrations. (d) Average dose re‐ sponse curve (amperometric current response *vs* glucose concentration) obtained from three different sensors, with a linear fitting at lower concentration range and an exponential fitting at higher concentration range. The error bars indi‐

even more efficient electrocatalysis toward ORR [120].

400 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 15.** (a) Cyclic voltammograms (CVs) for ORR obtained at a bare GCE (a), graphene/GCE (b), and NG5/GCE (N % = 7.1%) (c) in O2-saturated 0.1 M KOH aqueous solution. (B) CVs for ORR at NGs, synthesized with different mass ratio of GO and melamine (1:1, 1:2, 1:5, 1:10, 1:50) at 800°C, modified GCE in O2-saturated 0.1 M KOH aqueous solu‐ tion. Scan rate: 100 mV/s. (Reproduced with permission from ref. 118 Copyright ACS 2011)
