Rhodium NanosheetsâReduced Graphene Oxide Hybrids: A Highly

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Rhodium Nanosheets-Reduced Graphene Oxide Hybrids: A Highly Active Platinum-Alternative Electrocatalyst for the Methanol Oxidation Reaction in Alkaline Media Yong-qiang Kang, Qi Xue, Pujun Jin, Jia-Xing Jiang, Jing Hui Zeng, and Yu Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02163 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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Rhodium Nanosheets-Reduced Graphene Oxide Hybrids: A Highly Active Platinum-Alternative Electrocatalyst for the Methanol Oxidation Reaction in Alkaline Media Yongqiang Kang,† Qi Xue,† Pujun Jin,* Jiaxing Jiang, Jinghui Zeng and Yu Chen* Key Laboratory of Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, West Chang'an Avenue, Chang'an District, Xi'an 710119, China * Corresponding authors. E-mail: [email protected] (P. Jin); [email protected] (Y. Chen) ABSTRACT: For the large-scale commercialization of direct methanol fuel cells, developing Pt-alternative anode electrocatalyst with low cost and high activity plays an important role. In this work, a one-pot hydrothermal method has been developed for the direct synthesis of the Rh nanosheets (Rh-NSs) on the reduced graphene oxide (RGO). The newly-prepared Rh-NSs/RGO hybrids have great electrocatalytic activity for the methanol oxidation reaction (MOR) in alkaline media, much better than single-component Rh nanoparticles (Rh-NPs) and Rh nanoparticles/RGO (RhNPs/RGO) hybrids, originating from the two-dimensional structure of Rh nanosheets and excellent physical/chemical property of the RGO. Very importantly, cyclic voltammetry (CV) measurements show the onset oxidation potential of the MOR at the Rh-NSs/RGO hybrids negatively shift ca. 120 mV compared to the commercial Pt/C electrocatalyst. Meanwhile, CV measurements show that the MOR current at the Rh-NSs/RGO hybrids is 3.6 times bigger than that at the commercial Pt/C electrocatalyst at 0.61 V potential. Additionally, chronoamperometry measurement shows the Rh-NSs/RGO hybrids have excellent stability for the MOR. These electrochemical data demonstrate that the Rh-NSs/RGO hybrids are highly promising Pt-alternative anode electrocatalyst for the MOR in alkaline media.

KEYWORDS: rhodium nanosheet, reduced graphene oxide, methanol electrooxidation, electrocatalytic activity, alkaline media

INTRODUCTION Direct methanol fuel cells (DMFCs), which directly convert methanol from chemical energy into electrical energy, have received more attentions for the applications in portable electronics devices and residential clean energy power sources due to their high specific energy, low toxic, near room working temperature, and safe structure.1-3 For the well commercialization of DMFCs, developing anode electrocatalyst with high performance and low cost plays an important role. During the methanol oxidation reaction (MOR) process, the electrocatalyst is expected to furnish outperformance in its activity and stability. Although bi/trimetallic Pt-based electocatalysts (such as Pt-Ru,4 Pt-TiO2,5 Pt-Co,6 and Pt-Pd-Cu7) generally show improved electrocatalytic performance relative to Pt, these Pt-based anode electrocatalysts still suffer from the low activity and expensive cost, which urge us to explore more advanced and efficient electrocatalysts. The previous works have demonstrated that the Rh nanocrystals have electrocatalytic performance for the MOR in acidic media.8-10 However, the activity of the Rh nanocrystals for the MOR in acidic media is much lower than that of the Pt nanocrystals. Very recently, our group demonstrate for the first time the Rh nanocrystals are the highly promising Pt-alternative electrocatalysts for the MOR in alkaline media due to the lower cost and higher activity.11 Structural/electronic states of noble metal nanostructures electrocatalysts generally play an important part in their electrocatalytic activity.12-15 At present, the two-dimensional (2D) nanosheets can provide an atomic level bridge between the active

center and catalytic activity due to their flat facet, low coordination facets, and ultrahigh surface area.16-19 Great successes have been achieved in the Rh nanosheets, which effectively increase the catalytic sites of the surfaces atoms and consequently show the excellent catalytic activity for the various catalysis reactions.20-24 For example, Huang’s group found that Rh nanosheets were excellent electrocatalyst for the water splitting;21 Li’s group and Zheng’s group synthesized the single-atom-layered Rh nanosheets, which exhibited great catalytic performance for the hydrogenation reaction of phenol.23-24 Our group reported that ultrathin Rh nanosheet nanoassemblies with dendritic morphology performed the super activity for hydrolysis of ammonia borane.25 Although the high activity and stability of Rh nanosheets have been discovered, the utilization of Rh need be further improved. One way to improve the utilization of noble metal nanostructures is the usage of carbon supporting matrix. Given the excellent electron transport property, large surface area, and fast mass diffusion, graphene is regarded as an excellent supporting matrix, which effectively releases the intrinsic active sites of noble metal nanostructures and stabilizes noble metal nanostructures due to the strong interaction between graphene and noble metal nanostructures.14, 26-31 Meanwhile, the strong electronic coupling between graphene and noble metal nanostructures generally occurs due to their highly hybridized projected density of state.32 As a result, graphene/noble metal nanostructures generally perform the high activity and stability. For example, Dai’s group has developed nanostructures/graphene hybrids to adjust the chemical/electrical structures of supported nanostructures,33 which can facilitate the

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charge transfer and ionic interchange. In Xiong’s report, the graphene promotes the catalytic activity of the Pt-Pd nanostructures for the hydrogen evolution reaction due to the improved electrocatalyst-electrode electron transfer pathway.34 In this work, we designed and synthesized the Rh nanosheetsreduced graphene oxide (Rh-NSs/RGO) hybrids by one-pot hydrothermal synthesis. Thanks to the abundant activity sites, unique 2D structure, super large surface area of RGO, Rh nanosheets uniformly anchored on RGO surface. The resultant Rh-NSs/RGO hybrids show outstanding reactivity and stability for the MOR in alkaline media, even beyond the commercial Pt/C electrocatalyst.

EXPERIMENTAL SECTION Reagents and Chemicals. Polyethyleneimine (PEI, Scheme S1, MW = 600) and diethylene glycol (DEG) were purchased from Aladdin Industrial Corporation. Graphene oxide (GO) was purchased from Nanjing JCNANO Technology Co., LTD. Rhodium (III) chloride hydrate (RhCl3·3H2O) was obtained from Shanghai Jiu Ling Chemical Co., Ltd. The commercial 30 wt.% Pt/C electrocatalysts were purchased from Johnson Matthey Corporation. Other chemicals used in this work were of analytical reagent grade. Preparation of the Rh-NSs/RGO Hybrids. 0.6 mL of 0.05 M RhCl3 solution, 0.6 mL of 0.5 M PEI solution, and 0.6 mL DEG solution were added into 20.0 mL of deionized water. The 20 mg of GO was added into the above mixed solution (pH=7) and heated at 180 °C for 10 h to achieve the Rh-NSs/RGO hybrids. Then, the obtained Rh-NSs/RGO hybrids were separated by centrifugation, and washed with deionized water. The Rh nanoparticles on reduced graphene oxide (Rh-NPs/RGO) hybrids were synthesized in the absence of PEI under same experimental conditions. The singlecomponent Rh nanoparticles (Rh-NPs) were synthesized in the absence of GO under same experimental conditions. Characterization. The transmission electron microscopy (TEM), high-resolution TEM (HRTEM) were taken by TECNAI G2 F20 transmission electron microscope. The scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectrum were taken on a Hitachi SU-8020 emission scanning electron microscope. The X-ray diffraction (XRD) measurements were carried out a DX-2700 diffractometer with Cu-Kα radiation. The Xray photoelectron spectroscopy (XPS) measurements were recorded on an AXIS ULTRA spectrometer with Al Kα radiation as an excitation source, and the binding energy (BE) was calibrated with respect to adventitious carbon (C1s, 284.6 eV). The atomic force microscopy (AFM) measurement was performed in ScanAsyst mode in air with a Dimension Icon system. The Raman spectra were recorded on a Renishaw In Via Reflex Micro Raman Spectrometer using an infrared excitation laser source at the wavelength of 532 nm. Thermogravimetric analysis (TGA) was performed on a Thermoanalyzer Systems Q600 of TA company under an air atmosphere over a temperature range of 20-1000 °C with a ramp rate of 10 °C min-1. Electrochemical Measurements. Cyclic voltammetry (CV) and chronoamperometry measurements were tested in a three-electrode cell by a CHI760E electrochemical work station. A glassy carbon disk (3 mm in diameter) coated with the electrocatalyst was used as the working electrode. A saturated calomel reference electrode and a Pt wire were used as the reference and counter electrodes, respectively.

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All potentials were converted to the RHE with equation: ERHE = ESCE + 0.0592×pH + 0.241 V. The working electrode was prepared as follows: 10 mg of electrocatalyst was added into 5 mL deionizedwater by ultra-sonication for ca. 15 min to obtain a homogeneous ink. Then, 4 μL of ink was uniformly dropped onto the glassy carbon electrode. After drying, 2 μL of 5 wt% Nafion was covered on the electrode surface. Finally, the electrode was dried at 40 °C. All electrochemical experiments temperature was performed at 30 ± 1 °C. The electrocatalytic activity of electrocatalysts for the MOR was carried out in a 1 M KOH and 1 M CH3OH mixture solution. CV curves were tested by a linear potential scan under 50 mV s−1 sweep rate. Electrochemically active surface areas (ECSAs) of electrocatalysts were calculated from CV curves at 50 mV s−1 in 1 M KOH using the following equation: (1) ECSA=Q/(C×m) Where, Q was Coulombic charge of the H desorption peak area and calculated by integration of the hydrogen desorption/adsorption peaks, C was the hydrogen adsorption constants (polycrystalline Rh: 220 μC cm–2;35-37 polycrystalline Pt: 210 μC cm–2 38-40), and m was the metal mass on the electrode surface.

RESULTS AND DISCUSSION Preparation of the Rh-NSs/RGO Hybrids. The overall synthetic route of the Rh-NSs/RGO hybrids are depicted route a in Scheme 1, which is obtained by heating the mixture of RhCl3, PEI, DEG, and GO at 180 °C for 10 h. For comparison, the Rh-NPs/RGO hybrids are synthesized in the absence of PEI (route b in Scheme 1), and the single-component Rh-NPs were synthesized in the absence of GO (route c in Scheme 1).

Scheme 1. Schematic synthesis process of (a) the Rh-NSs/RGO hybrids, (b) the Rh-NPs/RGO hybrids, and (c) Rh-NPs. Characterization of the Rh-NSs/RGO Hybrids. The component and crystalline structure of the products were examined by EDX and XRD. The EDX spectrum reveals that the Rh-NSs/RGO hybrids are mainly composed of Rh, C and N elements (Figure 1A). XRD pattern of GO shows an intense and sharp diffraction peak at 2θ=10.24°, corresponding to the C (001) peak of GO (Figure 1B).41 For the Rh-NSs/RGO hybrids, the broad peak located at 2θ = 25.03° can be assigned to the (002) peak of stacked reduced graphene oxide (RGO) sheets, which means the GO has been reduced to RGO (Figure 1B).42-43 Additionally, a series of typical peaks at 41°, 48°, 70° and 84° corresponding to the (111), (200), (220) and (311) planes of the standard face-centered-cubic (fcc) Rh crystal (JCPDS ICDD card no. 05-0685) are observed (Figure 1B), indicating the


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generation of metallic Rh. XRD pattern of the Rh-NPs/RGO hybrids is very similar to that of the Rh-NSs/RGO hybrids (Figure S1), revealing the same crystal structure. The loading of Rh-NSs at the Rh-NSs/RGO hybrids was measured accurately through TGA, which is calculated to be 9 wt.% (Figure S2).

Figure 1. (A) EDX spectrum of the Rh-NSs/RGO hybrids. (B) XRD patterns of the GO and Rh-NSs/RGO hybrids. The morphology of the Rh-NSs/RGO hybrids was first investigated by electron microscope. SEM and EDX element maps exhibit the uniform distribution of C and Rh element across the sheet area (Figure 2A), indicating the uniform anchorage of the RhNSs on the RGO. TEM image shows that the RGO sheets are uniformly decorated with Rh-NSs (Figure 2B). The diffraction spots of the (111), (200), (220) and (311) facets of the flat surface are observed by selected-area electron diffraction (SAED), indicating the polycrystallinity property (insert in Figure 2B). The highresolution TEM (HRTEM) image clearly shows the lattice fringes (Figure 2C), revealing the good crystallinity. The magnified HRTEM image shows the lattice-spacing of 0.212 nm (insert in Figure 2C), corresponding to Rh (111) plane. The typical AFM image indicates that the thickness of the Rh nanosheets on the RGO sheets is about 3 nm (Figure 2D).

with EDX observation. The spectrum of Rh 3d in the Rh-NSs/RGO hybrids are consisted of the doublets of 3d3/2 and 3d5/2, with 3:2 peak area ratio theoretically (Figure 3A). The spectrum of Rh 3d is deconvolved into coupled satellite peaks, which are assigned to Rh0 and RhIII species, respectively. Rh 3d spectra in the Rh-NSs/RGO hybrids have BE values of 207.5 and 212.2 eV for the Rh0 in 3d3/2 and 3d5/2 coupled peaks, respectively, which are coincide to the values reported by Prof. Somorjai and Prof. Zhang.44-45 Compared with the Rh 3d values of the Rh-NPs/RGO hybrids, the Rh3d values of the Rh-NSs/RGO slightly shift to the lower binding energy (Figure S4), indicating the morphology of the Rh nanostructures slightly affect their electronic structure. Integrating the peak areas of Rh0 with RhIII species, the Rh0 percent in the Rh-NSs/RGO hybrids is calculated to be 81.6%. The small amount of RhIII species originates from the oxidation of Rh in air. The significant C signal of the GO contains three types of carbon groups: epoxy carbon at 288.8 eV, carbon in C-O at 286.9 eV, and nonoxygenated C at 284.6 eV (Figure 3B).46-47 For the RhNSs/RGO hybrids, the C/O ratio dramatically increases after reduction, where the peak intensity related to the C–O, C–O and C=O tremendously decreases (Figure 3C). These observations demonstrate that the GO sheets have been well deoxygenated, especially for epoxy carbon. The carbon structures of the GO sheets and Rh-NSs/RGO hybrids were investigated by Raman (Figure 3D). The Rh-NSs/RGO hybrids show two prominent peaks, where G band belongs to the carbon atoms about in-plane vibration of sp2bonded at 1585 cm-1, and the D band relates to the sp3 electronic vibrations of carbon atoms at 1351 cm-1.46 Compared to GO, the ID/IG ratio of the Rh-NSs/RGO hybrids increases slightly, which suggests an decrease of sp2-bonded, owing to increasing number of smaller graphene domains after the reduction of GO.48 Meanwhile, ID/IG ratio in the Rh-NPs/RGO hybrids is close to that in the RhNSs/RGO hybrids (Figure S5), which also indirectly indicates the reduction of the GO during the synthesis of hybrids. Generally, the generation of RGO is benefit to enhance the electrocatalytic activity of the Rh-NSs/RGO hybrids due to the improved electrical conductivity.

Figure 2. Representative (A) SEM image and corresponding SEMEDX elemental mapping of the Rh-NSs/RGO hybrids: Rh mapping in green, and C mapping in red. (B) TEM image of the RhNSs/RGO hybrids. Insert on right bottom corner is corresponding SAED pattern. (C) HRTEM of the Rh-NSs/RGO hybrids. Insert on right bottom corner is magnified HRTEM image from the red square region in Figure 2C. (D) AFM images of the Rh-NSs/RGO hybrids. The catalytic activity of the electrocatalysts depends mainly on the surface composition. XPS was performed to analyze the chemical composition and surface atom electronic structures of the RhNSs/RGO hybrids. XPS survey spectrum reveals the Rh-NSs/RGO hybrids contains Rh, C and N elements (Figure S3), in consistent

Figure 3. (A) Rh 3d XPS spectrum of the Rh-NSs/RGO hybrids; C1s XPS spectra of (B) the GO sheets and (C) Rh-NSs/RGO



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hybrids; (D) Raman spectra of the GO sheets and Rh-NSs/RGO hybrids. Since the generation of metal nanosheets is believed to be surfactant-directed,49-50 we investigate the effect of PEI on the morphology. In a controlled experiment without PEI, the obtained Rh-NPs/RGO hybrids consist of the irregular large nanoparticles with obvious aggregation and no Rh nanosheets are presented on the RGO sheet (Figure 4A). The fact indicates that PEI as a stabilizing agent that stops the aggregation of Rh nanoparticles and a facetselective agent that induces the generation of Rh nanosheets during the synthesis of the Rh-NSs/RGO hybrids. It is evident that PEI adsorbs mainly on Rh(111) facets due to the strong interaction of amine-Rh, which contributes to the generation of the Rh-NSs according to the confined growth mechanism.11, 25, 51-53 Besides, the single-component Rh-NPs without RGO only contain the small amount of Rh nanosheets (Figure 4B). This fact indicates that the existence of the GO facilitates the formation of the Rh nanosheets during the synthesis of the Rh-NPs/RGO hybrids. Meanwhile, we observe that the particle size of the Rh-NPs/RGO hybrids is smaller than that of the single-component Rh-NPs. This difference mainly originates from the negative charged GO with high surface area (zeta potential −48 mV at pH 7.0), which results in the strong absorption of PAH-RhIII complex with positive charge on the GO surface.54-55 Consequently, the GO provide the abundant nucleation and growth sites for the Rh nanoparticles, which results in the small nanosheets size and well distribution of the Rh-NSs/RGO hybrids (Figure 2B).

Figure 4. TEM images of (A) the Rh-NPs/RGO hybrids and (B) Rh-NPs. Methanol Electrooxidation Tests. The electrochemical property of electrocatalysts in a KOH solution is investigated by CV (Figure 5A). In the absence of methanol, two obviously oxidation peaks correspond to different electrochemical reaction occurring in the forward scan direction. The first peak at between 0.09 and 0.3 V potentials range is refers to the oxidation of Habs.15, 21 The ECSA of the Rh-NSs/RGO hybrids and Rh-NPs/RGO hybrids are 48.66 m2 g-1Rh and 40.09 m2 g-1Rh, respectively. The large ECSA of the RhNSs/RGO hybrids can be ascribed to their high dispersity and 2D structure of the Rh-NSs on the RGO surface. The second oxidation peak on electrocatalysts emerges at ca. 0.45 V, which is belongs to the oxidation of Rh. As observed, the formation potential of Rh oxide at the Rh-NSs/RGO hybrids is higher than that at the Rh-NPs/RGO hybrids, which will improve the MOR activity of the Rh-NSs/RGO hybrids, because the noble metal oxides are inactive for the MOR. The MOR activity of electrocatalysts is investigated by CV (Figure 5B). The current of CVs was normalized by the Rh mass. In presence of methanol, the hydrogen absorbed/desorbed regions are significantly suppressed. The methanol oxidation reaction starts at

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0.49 V. A current peak at 0.61 V is observed in the forward scan direction. Another current peak is also found at 0.35 V in the reverse scan direction. In the forward scan direction, the peak current is associated with the oxidation of methanol species produced by methanol adsorption. In the reverse scan direction, the peak current is related to the removal of methanol species that are not completely oxidized in the forward scan, rather than the oxidation of chemical adsorption of methanol species. The Rh-NSs/RGO hybrids exhibit a mass activity of 264 A g−1Rh for the MOR at 0.61 V, which is 1.5 times greater than the Rh-NPs/RGO hybrids (171 A g−1Rh), demonstrating the higher utilization efficiency of Rh metal. Meanwhile, the RhNSs/RGO hybrids show the bigger ECSA and higher MOR mass activity than the Rh-NPs (Figure S6), demonstrating the introduction of RGO improves the utilization efficiency of Rh metal.

Figure 5. (A) CVs of the Rh-NSs/RGO hybrids and Rh-NPs/RGO hybrids recorded in 1 M KOH solution with a scan rate of 50 mV s-1. (B) Rh mass-normalized and (C) ECSA-normalized CVs of the RhNSs/RGO hybrids and Rh-NPs/RGO hybrids recorded in 1M KOH + 1 M CH3OH solution at a scan rate of 50 mV s-1. (D) Chronoamperometric curves of the Rh-NSs/RGO hybrids and RhNPs/RGO hybrids recorded at recorded in 1 M KOH + 1 M CH3OH solution at 0.61 V potential. To evaluate the intrinsic activity of the Rh-NSs/RGO hybrids and Rh-NPs/RGO hybrids for the MOR, the current densities were normalized by the ECSA of electrocatalysts (Figure 5C). The specific activity of the Rh-NSs/RGO hybrids for the MOR is higher than that of Rh-NPs/RGO hybrids. Due to the unordinary 2D structure, the Rh–Rh coordination number in Rh nanosheets is much lower than that in conventional Rh nanoparticles.24 These Rh atoms with low coordination number generally are regarded as the highly active sites for various electrocatalytic reactions, which contributes to the high specific activity of the Rh-NSs/RGO hybrids. Chronoamperometry measurements were used to investigate the stability and possible poisoning of the Rh-NSs/RGO electrocatalyst under long-time continuous operation (Figure 5D), which performed in a 1.0 M KOH + 1.0 M CH3OH solution at a constant potential of 0.61 V. The MOR currents at the Rh-NSs/RGO hybrids and Rh-NPs/RGO hybrids are 41 mA mg-1Rh and 27 mA mg-1Rh at 6000 s, respectively. Meanwhile, the decay rate of the MOR at the Rh-NSs/RGO hybrids is much lower than that at the Rh-NPs/RGO.


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Thus, chronoamperometry measurements confirm that the RhNSs/RGO hybrids have higher electrocatalytic activity and stability compared to the Rh-NSs/RGO hybrids. Since commercial Pt/C electrocatalyst is state-of-the-art electrocatalyst for the MOR, we further investigate the MOR activity of commercial Pt/C electrocatalyst and compared with the RhNSs/RGO hybrids under same experimental conditions. CV measurements show the ECSA value (48.66 m2 g-1Rh) of the RhNSs/RGO hybrids is slightly lower than that (51.04 m2 g-1Pt) of the commercial Pt/C electrocatalyst (Figure 6A). Very importantly, the peak potential of the MOR at the Rh-NSs/RGO hybrids negatively shifts ca. 120 mV compared to commercial Pt/C electrocatalyst (Figure 6B). At 0.61 V potential, the MOR mass activity (264 A g-1Rh) at the Rh-NSs/RGO hybrids is 3.6 times bigger than that (73 A g-1Pt) at commercial Pt/C electrocatalyst, indicating the high activity of the Rh-NSs/RGO hybrids for the MOR. Meanwhile, the electrocatalytic activity of the Rh-NSs/RGO hybrids for the MOR is much higher than that of Pt-based1, 56-57 and Pd-based58-61 electrocatalysts in alkaline media (Table S1), indicating the Rh-NSs/RGO hybrids are highly promising Pt-alternative electrocatalysts for the MOR in alkaline media. In similar, the ECSA normalized CVs indicate that the Rh-NSs/RGO hybrids have higher specific activity than commercial Pt/C electrocatalyst (Figure 6C). The previous works have demonstrated that the peak current ratio of the forward to reverse scans (If/Ir) can be used to evaluate the poisoning tolerance of electrocatalysts in DMFCs. In general, a higher If/Ir ratio indicates the better resistance to poisoning of carbonaceous species. Herein, the If/Ir ratio of the MOR at the Rh-NSs/RGO hybrids (6.6) is much bigger than that at commercial Pt/C electrocatalyst (2.3), indicating the Rh-NSs/RGO hybrids can provide a better electrocatalytic activity for the MOR. Additionally, chronoamperometry measurements also confirm the Rh-NSs/RGO hybrids have higher activity and long-term stability than commercial Pt/C electrocatalyst (Figure 6D). Compared to commercial Pt/C electrocatalyst, the slower MOR current decay rate at the Rh-NSs/RGO hybrids may be ascribed to the better resistance to poisoning of carbonaceous species.

M KOH +1 M CH3OH solution at a scan rate of 50 mV s-1. (D) Chronoamperometric curves of the Rh-NSs/RGO hybrids and Pt/C electrocatalysts recorded in 1M KOH +1 M CH3OH solution at 0.61 V potential.

CONCLUSIONS In summary, the Rh-NSs/RGO hybrids were synthesized by a onepot hydrothermal method. The introduction of the PEI and GO facilitated the generation of the Rh nanosheets. The electocatalytic performance of the Rh-NSs/RGO hybrids for the MOR was evaluated by a series of electrochemical measurements. Thanks to 2D structure of Rh nanosheets and electron transport property of the RGO, the Rh-NSs/RGO hybrids revealed the super electrocatalytic activity for the MOR in alkaline media. Very importantly, the peak potential of the MOR at the Rh-NSs/RGO hybrids negatively shifted ca. 120 mV compared to the commercial Pt/C electrocatalyst, showing the extreme reactivity for the MOR in alkaline media. Considering that the price of Rh was only 2/3 of price of Pt at 2016, the present electrochemical data demonstrated that the RhNSs/RGO hybrids had great potential to alternate Pt/C electrocatalyst in DMFCs as an excellent anode electrocatalyst.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.XXX. The XRD spectrum of the Rh-NPs/RGO hybrids (Figure S1), TGA curve of the Rh-NSs/RGO hybrids (Figure S2), XPS spectrum of Rh-NSs/RGO hybrids (Figure S3), Rh 3d XPS spectrum of the RhNPs/RGO hybrids (Figure S4), Raman spectrum of the RhNPs/RGO hybrids (Figure S5), CVs and Rh mass-normalized CVs of Rh-NSs and Rh-NSs/RGO hybrids (Figure S6).

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (P. Jin) *Email: [email protected] (Y. Chen) Author Contributions †

Y. Kang and Q. Xue contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by Fundamental Research Funds for the Central Universities (GK201602002, GK201701007 and GK201703029).

REFERENCES Figure 6. (A) CVs of the Rh-NSs/RGO hybrids and Pt/C electrocatalysts recorded in 1 M KOH solution at a scan rate of 50 mV s-1. (B) Metal mass-normalized and (C) ECSA-normalized CVs of the Rh-NSs/RGO hybrids and Pt/C electrocatalyst recorded in 1

(1). Huang, W.; Wang, H.; Zhou, J.; Wang, J.; Duchesne, P. N.; Muir, D.; Zhang, P.; Han, N.; Zhao, F.; Zeng, M.; Zhong, J.; Jin, C.; Li, Y.; Lee, S.-T.; Dai, H., Highly active and durable methanol oxidation electrocatalyst based on the synergy of platinum–nickel hydroxide–graphene. Nat. Commun. 2015, 6, 10035.



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The Rh nanosheets/RGO hybrids show remarkable MOR activity in alkaline media compared to the commercial Pt/C electrocatalyst, demonstrating highly promising Pt-alternative anode electrocatalyst.


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Scheme 1. Schematic synthesis process of (a) the Rh-NSs/RGO hybrids, (b) the Rh-NPs/RGO hybrids, and (c) Rh-NPs. 55x39mm (300 x 300 DPI)



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Figure 1. (A) EDX spectrum of the Rh-NSs/RGO hybrids. (B) XRD patterns of the GO and Rh-NSs/RGO hybrids. 54x23mm (300 x 300 DPI)


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Figure 2. Representative (A) SEM image and corresponding SEM-EDX elemental mapping of the RhNSs/RGO hybrids: Rh mapping in green, and C mapping in red. (B) TEM image of the Rh-NSs/RGO hybrids. Insert on right bottom corner is corresponding SAED pattern. (C) HRTEM of the Rh-NSs/RGO hybrids. Insert on right bottom corner is magnified HRTEM image from the red square region in Figure 2C. (D) AFM images of the Rh-NSs/RGO hybrids. 51x33mm (300 x 300 DPI)



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Figure 3. (A) Rh 3d XPS spectrum of the Rh-NSs/RGO hybrids; C1s XPS spectra of (B) the GO sheets and (C) Rh-NSs/RGO hybrids; (D) Raman spectra of the GO sheets and Rh-NSs/RGO hybrids. 70x59mm (300 x 300 DPI)


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Figure 4. TEM images of (A) the Rh-NPs/RGO hybrids and (B) Rh-NPs. 38x18mm (300 x 300 DPI)



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Figure 5. (A) CVs of the Rh-NSs/RGO hybrids and Rh-NPs/RGO hybrids recorded in 1 M KOH solution with a scan rate of 50 mV s-1. (B) Rh mass-normalized and (C) ECSA-normalized CVs of the Rh-NSs/RGO hybrids and Rh-NPs/RGO hybrids recorded in 1M KOH + 1 M CH3OH with a scan rate of 50 mV s-1. (D) Chronoamperometric curves of the Rh-NSs/RGO hybrids and Rh-NPs/RGO hybrids recorded at recorded in 1 M KOH + 1 M CH3OH at 0.61 V potential. 61x48mm (300 x 300 DPI)


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Figure 6. (A) CVs of the Rh-NSs/RGO hybrids and Pt/C electrocatalysts recorded in 1 M KOH solution with a scan rate of 50 mV s-1. (B) Metal mass-normalized and (C) ECSA-normalized CVs of the Rh-NSs/RGO hybrids and Pt/C electrocatalyst recorded in 1 M KOH +1 M CH3OH with a scan rate of 50 mV s-1. (D) Chronoamperometric curves of the Rh-NSs/RGO hybrids and Pt/C electrocatalysts recorded in 1M KOH +1 M CH3OH at 0.61 V potential. 60x47mm (300 x 300 DPI)


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