Rhodium Nanosheets–Reduced Graphene ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10156-10162

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, Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), 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, P.R.China S Supporting Information *

ABSTRACT: For the large-scale commercialization of direct methanol fuel cells, developing Pt-alternative anode electrocatalysts 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 (RhNSs) on 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 (RhNPs) and Rh nanoparticles/RGO (Rh-NPs/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 a highly promising Pt-alternative anode electrocatalyst for the MOR in alkaline media. KEYWORDS: Rhodium nanosheets, 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 great attentions towards applications in portable electronics devices and residential clean energy power sources due to their high specific energy, low toxicity, near room working temperature, and safe structure.1−3 For the good commercialization of DMFCs, developing an anode electrocatalyst with high performance and low cost plays an important role. During the methanol oxidation reaction (MOR) process, the electrocatalyst is expected to show excellent performance 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 low activity and high cost, which urge us to explore more advanced and efficient electrocatalysts. Previous works have demonstrated that Rh nanocrystals demonstrate 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 demonstrated for the first time that Rh nanocrystals are highly promising Pt-alternative electrocatalysts © 2017 American Chemical Society

for the MOR in alkaline media due to the low cost and high activity.11 Structural/electronic states of noble metal nanostructure electrocatalysts generally play an important part in their electrocatalytic activity.12−15 At present, 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 electrocatalysts for water splitting;21 Li’s group and Zheng’s group synthesized singleatom-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 with superior activity for hydrolysis of ammonia borane.25 Received: June 30, 2017 Revised: August 21, 2017 Published: September 21, 2017 10156

DOI: 10.1021/acssuschemeng.7b02163 ACS Sustainable Chem. Eng. 2017, 5, 10156−10162

Research Article

ACS Sustainable Chemistry & Engineering

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 workstation. 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. 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 deionized-water by ultrasonication 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:

Although the high activity and stability of Rh nanosheets have been discovered, the utilization of Rh needs be further improved. One way to improve the utilization of noble metal nanostructures is the usage of a carbon supporting matrix. Given its excellent electron transport properties, 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 them due to the strong interaction of the two materials.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 with 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 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 nanosheets−reduced graphene oxide (Rh-NSs/RGO) hybrids by one-pot hydrothermal synthesis. Thanks to the abundant activity sites, unique 2D structure, and super large surface area of RGO, Rh nanosheets uniformly anchored on the 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.

■

ECSA = Q /(Cm)

(1)

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−238−40), 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 is depicted as route a in Scheme 1, which is obtained by heating the mixture

EXPERIMENTAL SECTION

Reagents and Chemicals. Polyethylenimine (PEI, Scheme S1, M.W. = 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. A 0.6 mL portion 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) and 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 X-ray 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 (C 1s, 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

Scheme 1. Synthesis process of (a) the Rh-NSs/RGO Hybrids, (b) the Rh-NPs/RGO Hybrids, and (c) Rh-NPs

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 singlecomponent Rh-NPs were synthesized in the absence of GO (route c in Scheme 1). 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). The 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 10157


Research Article


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). 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 Rh-NSs/RGO hybrids. XPS survey spectrum reveals the Rh-NSs/RGO hybrids contains Rh, C, and N elements (Figure S3), consistent with EDX observation. The spectrum of Rh 3d in the Rh-NSs/RGO hybrids consists of the doublets of 3d3/2 and 3d5/2, with 3:2 peak area ratio theoretically (Figure 3A).

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

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 facecentered-cubic (fcc) Rh crystal (JCPDS ICDD card no. 050685) are observed (Figure 1B), indicating the 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). 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 Figure 3. (A) Rh 3d XPS spectrum of the Rh-NSs/RGO hybrids. C 1s 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.

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 coincide with the values reported by Somorjai and Zhang.44,45 Compared with the Rh 3d values of the Rh-NPs/RGO hybrids, the Rh 3d 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 Rh-NSs/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 spectroscopy (Figure 3D). The RhNSs/RGO hybrids show two prominent peaks, where the G band belongs to the carbon atoms in-plane vibration with sp2bonding 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

Figure 2. Representative images. (A) SEM image and corresponding SEM-EDX 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. (inset) Corresponding SAED pattern. (C) HRTEM of the Rh-NSs/RGO hybrids. (inset) Magnified HRTEM image from the red square region in part C. (D) AFM images of the Rh-NSs/RGO hybrids.

anchorage of the Rh-NSs 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 high-resolution 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), 10158


Research Article

ACS Sustainable Chemistry & Engineering increases slightly, which suggests an decrease of sp2-bonding, owing to the increasing number of smaller graphene domains after the reduction of GO.48 Meanwhile, the ID/IG ratio in the Rh-NPs/RGO hybrids is close to that in the Rh-NSs/RGO hybrids (Figure S5), which also indirectly indicates the reduction of the GO during the synthesis of hybrids. Generally, the generation of RGO is beneficial to enhancing the electrocatalytic activity of the Rh-NSs/RGO hybrids due to the improved electrical conductivity. 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 irregular large nanoparticles with obvious aggregation and no Rh nanosheets are presented on the RGO sheet (Figure 4A). This indicates

Figure 5. (A) CVs of the Rh-NSs/RGO 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 and Rh-NPs/RGO hybrids recorded in 1 M KOH + 1 M CH3OH solution at a scan rate of 50 mV s−1. (D) Chronoamperometric curves of the Rh-NSs/RGO and Rh-NPs/RGO hybrids recorded in 1 M KOH + 1 M CH3OH solution at 0.61 V potential.

electrocatalysts emerges at ca. 0.45 V, which 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 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 RhNSs/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 Rh-NSs/ 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. 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

Figure 4. TEM images of (A) the Rh-NPs/RGO hybrids and (B) RhNPs.

that PEI is a stabilizing agent that stops the aggregation of Rh nanoparticles and a facet-selective agent that induces the generation of Rh nanosheets during the synthesis of the RhNSs/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 singlecomponent Rh-NPs without RGO only contain a 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 RhNPs. This difference mainly originates from the negatively charged GO with high surface area (zeta potential −48 mV at pH 7.0), which results in the strong absorption of the PAHRhIII complex with positive charge on the GO surface.54,55 Consequently, the GO provides abundant nucleation and growth sites for the Rh nanoparticles, which results in the small nanosheets size and good distribution of the Rh-NSs/RGO hybrids (Figure 2B). Methanol Electrooxidation Tests. The electrochemical property of electrocatalysts in a KOH solution is investigated by CV (Figure 5A). In the absence of methanol, two obvious oxidation peaks correspond to different electrochemical reactions occurring in the forward scan direction. The first peak between 0.09 and 0.3 V potentials refers to the oxidation of Habs.15,21 The ECSA values of the Rh-NSs/RGO hybrids and Rh-NPs/RGO hybrids are 48.66 and 40.09 m2 g−1Rh, respectively. The large ECSA of the Rh-NSs/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 10159


Research Article


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 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 that 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 over the Rh-NSs/RGO hybrids may be ascribed to the better resistance to poisoning of carbonaceous species.

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 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. Thus, chronoamperometry measurements confirm that the Rh-NSs/RGO hybrids have higher electrocatalytic activity and stability compared to the Rh-NSs/RGO hybrids. Since the commercial Pt/C electrocatalyst is the state-of-theart electrocatalyst for the MOR, we further investigate the MOR activity of the commercial Pt/C electrocatalyst and compared it with the Rh-NSs/RGO hybrids under same experimental conditions. CV measurements show the ECSA value (48.66 m2 g−1Rh) of the Rh-NSs/RGO hybrids is slightly lower than that (51.04 m2 g−1Pt) of the commercial Pt/C electrocatalyst (Figure 6A). Very importantly, the peak

■

CONCLUSIONS In summary, the Rh-NSs/RGO hybrids were synthesized by a one-pot hydrothermal method. The introduction of 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 the 2D structure of Rh nanosheets and electron transport properties of the RGO, the Rh-NSs/ RGO hybrids revealed super electrocatalytic activity for the MOR in alkaline media. Very importantly, the peak potential of the MOR with 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 two-thirds of the price of Pt in 2016, the present electrochemical data demonstrated that the Rh-NSs/RGO hybrids had great potential to be an alternate to the Pt/C electrocatalyst in DMFCs as an excellent anode electrocatalyst.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02163. XRD spectra of the Rh-NPs/RGO hybrids (Figure S1), TGA curves of the Rh-NSs/RGO hybrids (Figure S2), XPS spectra of Rh-NSs/RGO hybrids (Figure S3), Rh 3d XPS spectra of the Rh-NPs/RGO hybrids (Figure S4), Raman spectra of the Rh-NPs/RGO hybrids (Figure S5), CVs and Rh mass-normalized CVs of Rh-NSs and RhNSs/RGO hybrids (Figure S6) (PDF)

Figure 6. (A) CVs of the Rh-NSs/RGO hybrids and Pt/C electrocatalyst 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 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 electrocatalyst recorded in 1 M KOH + 1 M CH3OH solution at 0.61 V potential.

■

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) of the Rh-NSs/RGO hybrids is 3.6 times bigger than that (73 A g−1Pt) with commercial Pt/C electrocatalyst, indicating the high activity of the Rh-NSs/RGO hybrids for the MOR. Meanwhile, the electrocatalytic activity of the RhNSs/RGO hybrids for the MOR is much higher than that of Pt-1,56,57 and Pd-based58−61 electrocatalysts in alkaline media (Table S1), indicating that Rh-NSs/RGO hybrids are highly promising Pt-alternative electrocatalysts for the MOR in alkaline media. Similarly, the ECSA-normalized CVs indicate that the Rh-NSs/RGO hybrids have higher specific activity than the commercial Pt/C electrocatalyst (Figure 6C). The previous

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.J.). *E-mail: [email protected] (Y.C.). ORCID

Jiaxing Jiang: 0000-0002-2833-4753 Jinghui Zeng: 0000-0003-3279-0652 Yu Chen: 0000-0001-9545-6761 Author Contributions †

Y.K. and Q.X. contributed equally to this work.

Notes

The authors declare no competing financial interest. 10160


Research Article


■

(17) Sun, Y.; Gao, S.; Lei, F.; Xie, Y. Atomically-thin two-dimensional sheets for understanding active sites in catalysis. Chem. Soc. Rev. 2015, 44, 623−636. (18) Sun, Y.; Gao, S.; Lei, F.; Liu, J.; Liang, L.; Xie, Y. Atomically-thin non-layered cobalt oxide porous sheets for highly efficient oxygenevolving electrocatalysts. Chem. Sci. 2014, 5, 3976−3982. (19) Tan, C.; Zhang, H. Wet-chemical synthesis and applications of non-layer structured two-dimensional nanomaterials. Nat. Commun. 2015, 6, 7873. (20) Jang, K.; Kim, H. J.; Son, S. U. Low-temperature synthesis of ultrathin rhodium nanoplates via molecular orbital symmetry interaction between rhodium precursors. Chem. Mater. 2010, 22, 1273−1275. (21) Zhang, N.; Shao, Q.; Pi, Y.; Guo, J.; Huang, X. Solvent-mediated shape tuning of well-defined rhodium nanocrystals for efficient electrochemical water splitting. Chem. Mater. 2017, 29, 5009−5015. (22) Zhao, L.; Xu, C.; Su, H.; Liang, J.; Lin, S.; Gu, L.; Wang, X.; Chen, M.; Zheng, N. Single-crystalline rhodium nanosheets with atomic thickness. Adv. Sci. 2015, 2, 1500100. (23) Jiang, Y. Q.; Su, J. Y.; Yang, Y. A.; Jia, Y. Y.; Chen, Q. L.; Xie, Z. X.; Zheng, L. S. A facile surfactant-free synthesis of Rh flower-like nanostructures constructed from ultrathin nanosheets and their enhanced catalytic properties. Nano Res. 2016, 9, 849−856. (24) Duan, H.; Yan, N.; Yu, R.; Chang, C.-R.; Zhou, G.; Hu, H.-S.; Rong, H.; Niu, Z.; Mao, J.; Asakura, H.; Tanaka, T.; Dyson, P. J.; Li, J.; Li, Y. Ultrathin rhodium nanosheets. Nat. Commun. 2014, 5, 3093. (25) Bai, J.; Xu, G.-R.; Xing, S.-H.; Zeng, J.-H.; Jiang, J.-X.; Chen, Y. Hydrothermal synthesis and catalytic application of ultrathin rhodium nanosheet nanoassemblies. ACS Appl. Mater. Interfaces 2016, 8, 33635−33641. (26) Guo, S.; Wen, D.; Zhai, Y.; Dong, S.; Wang, E. Platinum nanoparticle ensemble-on-graphene hybrid nanosheet: one-pot, rapid synthesis, and used as new electrode material for electrochemical sensing. ACS Nano 2010, 4, 3959−3968. (27) Zeng, M.; Liu, Y.; Zhao, F.; Nie, K.; Han, N.; Wang, X.; Huang, W.; Song, X.; Zhong, J.; Li, Y. Metallic cobalt nanoparticles encapsulated in nitrogen-enriched graphene shells: its bifunctional electrocatalysis and application in zinc - air batteries. Adv. Funct. Mater. 2016, 26, 4397−4404. (28) Li, L.; Zhang, J.; Liu, Y.; Zhang, W.; Yang, H.; Chen, J.; Xu, Q. Facile fabrication of Pt nanoparticles on 1-pyrenamine functionalized graphene nanosheets for methanol electrooxidation. ACS Sustainable Chem. Eng. 2013, 1, 527−533. (29) Wang, L.; Dong, Y.; Zhang, Y.; Zhang, Z.; Chi, K.; Yuan, H.; Zhao, A.; Ren, J.; Xiao, F.; Wang, S. PtAu alloy nanoflowers on 3D porous ionic liquid functionalized graphene-wrapped activated carbon fiber as a flexible microelectrode for near-cell detection of cancer. NPG Asia Mater. 2016, 8, e337. (30) Jafari, Z.; Mokhtarian, N.; Hosseinzadeh, G.; Farhadian, M.; Faghihi, A.; Shojaie, F. Ag/TiO2/freeze-dried graphene nanocomposite as a high performance photocatalyst under visible light irradiation. J. Energy Chem. 2016, 25, 393−402. (31) Wang, Y.; Rong, Z.; Wang, Y.; Wang, T.; Du, Q.; Wang, Y.; Qu, J. Graphene-based metal/acid bifunctional catalyst for the conversion of levulinic acid to γ-valerolactone. ACS Sustainable Chem. Eng. 2017, 5, 1538−1548. (32) Seo, M. H.; Choi, S. M.; Seo, J. K.; Noh, S. H.; Kim, W. B.; Han, B. The graphene-supported palladium and palladium−yttrium nanoparticles for the oxygen reduction and ethanol oxidation reactions: Experimental measurement and computational validation. Appl. Catal., B 2013, 129, 163−171. (33) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 2011, 133, 7296−7299. (34) Bai, S.; Wang, C.; Deng, M.; Gong, M.; Bai, Y.; Jiang, J.; Xiong, Y. Surface polarization matters: enhancing the hydrogen-evolution reaction by shrinking Pt shells in Pt−Pd−graphene stack structures. Angew. Chem., Int. Ed. 2014, 53, 12120−12124.

ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (GK201602002, GK201701007, and GK201703029) and the 111 Project (B14041).

■

REFERENCES

(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. (2) Bu, L.; Guo, S.; Zhang, X.; Shen, X.; Su, D.; Lu, G.; Zhu, X.; Yao, J.; Guo, J.; Huang, X. Surface engineering of hierarchical platinumcobalt nanowires for efficient electrocatalysis. Nat. Commun. 2016, 7, 11850. (3) Jiang, K.; Bu, L.; Wang, P.; Guo, S.; Huang, X. Trimetallic PtSnRh wavy nanowires as efficient nanoelectrocatalysts for alcohol electrooxidation. ACS Appl. Mater. Interfaces 2015, 7, 15061−15067. (4) Velázquez-Palenzuela, A.; Centellas, F.; Garrido, J. A.; Arias, C.; Rodríguez, R. M.; Brillas, E.; Cabot, P.-L. Kinetic analysis of carbon monoxide and methanol oxidation on high performance carbonsupported Pt−Ru electrocatalyst for direct methanol fuel cells. J. Power Sources 2011, 196, 3503−3512. (5) Xiao, J.; Pan, Z.; Li, W.; Chen, X.; Wu, S.; Chen, C.; Lin, Y.; Hu, G.; Wei, Z.; Zheng, Y. Pt nanoparticles supported on one-dimensional (1D) titanium silicon nitride with high performance and stability for methanol electrooxidation. J. Mater. Sci. 2017, 52, 10686−10696. (6) Baronia, R.; Goel, J.; Tiwari, S.; Singh, P.; Singh, D.; Singh, S. P.; Singhal, S. K. Efficient electro-oxidation of methanol using PtCo nanocatalysts supported reduced graphene oxide matrix as anode for DMFC. Int. J. Hydrogen Energy 2017, 42, 10238−10247. (7) Chang, R.; Zheng, L.; Wang, C.; Yang, D.; Zhang, G.; Sun, S. Synthesis of hierarchical platinum-palladium-copper nanodendrites for efficient methanol oxidation. Appl. Catal., B 2017, 211, 205−211. (8) Sathe, B. R. Methanol electro-oxidation on nanostructured rhodium network. Energy Environ. Focus 2015, 4, 196−200. (9) Yuan, Q.; Zhou, Z.; Zhuang, J.; Wang, X. Tunable aqueous phase synthesis and shape-dependent electrochemical properties of rhodium nanostructures. Inorg. Chem. 2010, 49, 5515−5521. (10) Wu, Z. X.; Chen, W. L.; Liu, H. Y.; Zhai, P.; Xiao, C. X.; Su, D. S.; Liu, H. C.; Ma, D. Reconstruction of Rh nanoparticles in methanol oxidation reaction. Catal. Sci. Technol. 2015, 5, 4116−4122. (11) Kang, Y. Q.; Li, F. M.; Li, S. N.; Ji, P. J.; Zeng, J. H.; Jiang, J. X.; Chen, Y. Unexpected catalytic activity of rhodium nanodendrites with nanosheet subunits for methanol electrooxidation in an alkaline medium. Nano Res. 2016, 9, 3893−3902. (12) Huang, J. L.; Li, Z.; Duan, H. H.; Cheng, Z. Y.; Li, Y. D.; Zhu, J.; Yu, R. Formation of hexagonal-close packed (hcp) rhodium as a size effect. J. Am. Chem. Soc. 2017, 139, 575−578. (13) Chen, G.; Xu, C.; Huang, X.; Ye, J.; Gu, L.; Li, G.; Tang, Z.; Wu, B.; Yang, H.; Zhao, Z.; Zhou, Z.; Fu, G.; Zheng, N. Interfacial electronic effects control the reaction selectivity of platinum catalysts. Nat. Mater. 2016, 15, 564−569. (14) Cavallin, A.; Pozzo, M.; Africh, C.; Baraldi, A.; Vesselli, E.; Dri, C.; Comelli, G.; Larciprete, R.; Lacovig, P.; Lizzit, S.; Alfe, D. Local electronic structure and density of edge and facet atoms at Rh nanoclusters self-assembled on a graphene template. ACS Nano 2012, 6, 3034−43. (15) Yu, N. F.; Tian, N.; Zhou, Z. Y.; Huang, L.; Xiao, J.; Wen, Y. H.; Sun, S. G. Electrochemical synthesis of tetrahexahedral rhodium nanocrystals with extraordinarily high surface energy and high electrocatalytic activity. Angew. Chem., Int. Ed. 2014, 53, 5097−5101. (16) Lei, F.; Liu, W.; Sun, Y.; Xu, J.; Liu, K.; Liang, L.; Yao, T.; Pan, B.; Wei, S.; Xie, Y. Metallic tin quantum sheets confined in graphene toward high-efficiency carbon dioxide electroreduction. Nat. Commun. 2016, 7, 12697. 10161


Research Article


hybrid of MoS2 and graphene as an active catalyst for hydrogen evolution reaction. Chem. Mater. 2014, 26, 2344−2353. (54) Wang, Y.; Fugetsu, B.; Sakata, I.; Mao, W.; Endo, M.; Terrones, M.; Dresselhaus, M. Preparation of novel tetrahedral Ag3PO4 crystals and the sunlight-responsive photocatalytic properties using graphene oxide as the template. Carbon 2017, 119, 522−526. (55) Bai, J.; Han, S.-H.; Peng, R.-L.; Zeng, J.-H.; Jiang, J.-X.; Chen, Y. Ultrathin rhodium oxide nanosheet nanoassemblies: synthesis, morphological stability, and electrocatalytic application. ACS Appl. Mater. Interfaces 2017, 9, 17195−17200. (56) Ayán-Varela, M.; Ruiz-Rosas, R.; Villar-Rodil, S.; Paredes, J. I.; Cazorla-Amorós, D.; Morallón, E.; Martínez-Alonso, A.; Tascón, J. M. D. Efficient Pt electrocatalysts supported onto flavin mononucleotide− exfoliated pristine graphene for the methanol oxidation reaction. Electrochim. Acta 2017, 231, 386−395. (57) Sneed, B. T.; Young, A. P.; Jalalpoor, D.; Golden, M. C.; Mao, S.; Jiang, Y.; Wang, Y.; Tsung, C.-K. Shaped Pd−Ni−Pt coresandwich-shell nanoparticles: influence of Ni sandwich layers on catalytic electrooxidations. ACS Nano 2014, 8, 7239−7250. (58) Hsieh, C.-T.; Yu, P.-Y.; Tzou, D.-Y.; Hsu, J.-P.; Chiu, Y.-R. Bimetallic Pd−Rh nanoparticles onto reduced graphene oxide nanosheets as electrocatalysts for methanol oxidation. J. Electroanal. Chem. 2016, 761, 28−36. (59) Jurzinsky, T.; Bär, R.; Cremers, C.; Tübke, J.; Elsner, P. Highly active carbon supported palladium-rhodium PdXRh/C catalysts for methanol electrooxidation in alkaline media and their performance in anion exchange direct methanol fuel cells (AEM-DMFCs). Electrochim. Acta 2015, 176, 1191−1201. (60) Yuwen, L.; Xu, F.; Xue, B.; Luo, Z.; Zhang, Q.; Bao, B.; Su, S.; Weng, L.; Huang, W.; Wang, L. General synthesis of noble metal (Au, Ag, Pd, Pt) nanocrystal modified MoS2 nanosheets and the enhanced catalytic activity of Pd-MoS2 for methanol oxidation. Nanoscale 2014, 6, 5762−5769. (61) Li, X.; Niu, X.; Zhang, W.; He, Y.; Pan, J.; Yan, Y.; Qiu, F. Onepot anchoring of Pd nanoparticles on nitrogen-doped carbon through dopamine self-polymerization and activity in the electrocatalytic methanol oxidation reaction. ChemSusChem 2017, 10, 976−983.

(35) Jerkiewicz, G.; Borodzinski, J. J. Studies of formation of very thin oxide films on polycrystalline rhodium electrodes: application of the Mott-Cabrera theory. Langmuir 1993, 9, 2202−2209. (36) Housmans, T. H. M.; Feliu, J. M.; Koper, M. T. M. CO oxidation on stepped Rh[n (1 1 1) × (1 1 1)] single crystal electrodes: a voltammetric study. J. Electroanal. Chem. 2004, 572, 79−91. (37) Yau, S.-L.; Kim, Y.-G.; Itaya, K. In situ scanning tunneling microscopy of benzene adsorbed on Rh(111) and Pt(111) in HF solution. J. Am. Chem. Soc. 1996, 118, 7795−7803. (38) del Carmen Gimenez-Lopez, M.; Kurtoglu, A.; Walsh, D. A.; Khlobystov, A. N. Extremely stable platinum-amorphous carbon electrocatalyst within hollow graphitized carbon nanofibers for the oxygen reduction reaction. Adv. Mater. 2016, 28, 9103−9108. (39) Huang, Y.; Zhao, T.; Zeng, L.; Tan, P.; Xu, J. A facile approach for preparation of highly dispersed platinum-copper/carbon nanocatalyst toward formic acid electro-oxidation. Electrochim. Acta 2016, 190, 956−963. (40) Zhao, X.; Dai, L.; Qin, Q.; Pei, F.; Hu, C.; Zheng, N. Selfsupported 3D PdCu alloy nanosheets as a bifunctional catalyst for electrochemical reforming of ethanol. Small 2017, 13, 1602970. (41) Yang, Y.; Liu, Q.; Liu, X.-P.; Liu, P.-Z.; Mao, C.-J.; Niu, H.-L.; Jin, B.-K.; Zhang, S.-Y. Multifunctional reduced graphene oxide (RGO)/Fe3O4/CdSe nanocomposite for electrochemiluminescence Immunosensor. Electrochim. Acta 2016, 190, 948−955. (42) Fan, X.; Jiao, G.; Gao, L.; Jin, P.; Li, X. The preparation and drug delivery of a graphene-carbon nanotube-Fe3O4 nanoparticle hybrid. J. Mater. Chem. B 2013, 1, 2658−2664. (43) Vinayan, B. P.; Nagar, R.; Raman, V.; Rajalakshmi, N.; Dhathathreyan, K. S.; Ramaprabhu, S. Synthesis of graphenemultiwalled carbon nanotubes hybrid nanostructure by strengthened electrostatic interaction and its lithium ion battery application. J. Mater. Chem. 2012, 22, 9949−9956. (44) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Reaction-driven restructuring of Rh-Pd and Pt-Pd core-shell nanoparticles. Science 2008, 322, 932−4. (45) Guan, H.; Lin, J.; Qiao, B.; Yang, X.; Li, L.; Miao, S.; Liu, J.; Wang, A.; Wang, X.; Zhang, T. Catalytically active rh sub-nanoclusters on TiO2 for CO oxidation at cryogenic temperatures. Angew. Chem., Int. Ed. 2016, 55, 2820−2824. (46) Xu, G.-R.; Hui, J.-J.; Huang, T.; Chen, Y.; Lee, J.-M. Platinum nanocuboids supported on reduced graphene oxide as efficient electrocatalyst for the hydrogen evolution reaction. J. Power Sources 2015, 285, 393−399. (47) Wang, S.; Yu, D.; Dai, L.; Chang, D. W.; Baek, J.-B. Polyelectrolyte-functionalized graphene as metal-free electrocatalysts for oxygen reduction. ACS Nano 2011, 5, 6202−6209. (48) Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A.; Ventrice, C. A.; Ruoff, R. S. Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 2009, 47, 145−152. (49) Zhang, Z.-P.; Wang, X.-Y.; Yuan, K.; Zhu, W.; Zhang, T.; Wang, Y.-H.; Ke, J.; Zheng, X.-Y.; Yan, C.-H.; Zhang, Y.-W. Free-standing iridium and rhodium-based hierarchically-coiled ultrathin nanosheets for highly selective reduction of nitrobenzene to azoxybenzene under ambient conditions. Nanoscale 2016, 8, 15744−15752. (50) Huang, X.; Li, S.; Huang, Y.; Wu, S.; Zhou, X.; Li, S.; Gan, C. L.; Boey, F.; Mirkin, C. A.; Zhang, H. Synthesis of hexagonal close-packed gold nanostructures. Nat. Commun. 2011, 2, 292. (51) Furlan, S.; Giannozzi, P. The interactions of nitrogen dioxide with graphene-stabilized Rh clusters: a DFT study. Phys. Chem. Chem. Phys. 2013, 15, 15896−15904. (52) Wang, H.; Robinson, J. T.; Diankov, G.; Dai, H. Nanocrystal growth on graphene with various degrees of oxidation. J. Am. Chem. Soc. 2010, 132, 3270−3271. (53) Zheng, X.; Xu, J.; Yan, K.; Wang, H.; Wang, Z.; Yang, S. Spaceconfined growth of MoS2 nanosheets within graphite: the layered 10162


Rhodium Nanosheets–Reduced Graphene ... - ACS Publications

Recommend Documents