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Microbial Phosphorous Enabled Synthesis of Phosphides Nanocomposites for Efficient Electrocatalysts Tian-Qi Zhang, Jian Liu, Lin-Bo Huang, Xu-Dong Zhang, Yong-Gang Sun, XiaoChan Liu, De-Shan Bin, Xi Chen, An-Min Cao, Jin-Song Hu, and Li-Jun Wan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06123 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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Journal of the American Chemical Society

Microbial Phosphorous Enabled Synthesis of Phosphides Nanocomposites for Efficient Electrocatalysts Tian-Qi Zhang1,2, Jian Liu1,2, Lin-Bo Huang1,2, Xu-Dong Zhang1,2,Yong-Gang Sun1,2, Xiao-Chan Liu1,2, De-Shan Bin1, 2, Xi Chen3, An-Min Cao1,2*, Jin-Song Hu1,2*, Li-Jun Wan1,2* 1

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing 100190, China. 2

University of Chinese Academy of Sciences (UCAS), Beijing 100049, China

3

University of Science and Technology of China. (USTC), Hefei 230026, China

KEYWORDS. Biomass, Transition metal phosphides, Nanocomposite, Electrocatalysis, Hydrogen evolution reaction.

ABSTRACT: Transition metal phosphides have recently been identified as low-cost and efficient electrocatalysts which are highly active for the hydrogen evolution reaction. Unfortunately, to achieve a controlled phosphidation of nonprecious metals towards a desired nanostructures of metal phosphides, the synthetic processes usually turned complicate, high-cost, and even dangerous due to the reaction chemistry related to different phosphorus sources. It becomes even more challenging considering the integration of those active metal phosphides with the structural engineering of their conductive matrix towards a favorable architecture for optimized catalytic performance. Herein, we identified that the biomass itself could act as an effective synthetic platform for the construction of supported metal phosphides by recovering its inner phosphorous upon reacting with transition metals ions, forming well-dispersed, highly active nanoparticles of metal phosphides incorporated in the nanoporous carbon matrix, which promised high activity to catalyze the hydrogen evolution reaction. Our synthetic protocol not only provides a simple and effective strategy for the construction of a large variety of highly-active nanoparticles of metal phosphides, but also envisions new perspectives on an integrated utilization of the essential ingredients, particularly phosphorous, together with the innate architecture of the existing biomass for the creation of functional nanomaterials towards a sustainable energy development.

INTRODUCTION The increasing demand on energy has raised serious concern on the development of clean and sustainable energy sources, among which the production of H2 from the water splitting process becomes an appealing solution to ensure an affordable energy supply.1-3 To facilitate the hydrogen evolution, different catalysts, typically noble metals such as Pt and Ir,4,5 have been identified to promote the electrochemical water splitting with high activity and low overpotentials.6,7 Unfortunately, due to the high cost and scarcity of the noble metals, it has become a persistence pursuit while a formidable challenge for the development of highly-efficient catalysts which are favorably composed of earth-abundant elements.8 Recently, transition metal phosphides have been identified as a family of non-precious metal catalysts which were able to accelerate the hydrogen evolution reaction (HER).9-11 Particularly, well-dispersed nanoparticles of phosphides of transition metals including Co, Ni and Fe could exhibit high HER activity while maintain an extraordinary mechanical strength and chemical stability, showing promising characters towards a possible alternative for nonnoble metal catalysts.12-14 Accordingly, it has been under

intensive pursuit for the construction of different transition metal phosphides (TMPs) with designated nanostructures and functions so as to fully exploit the potential of TMPs for the clean energy production. Despite of low cost of the compositional elements in TMPs, it remains an open challenge to ensure an economic and environmental-friendly synthesis of the TMPs nanocatalysts considering the fact that the current phosphorous donor are usually related to expensive and dangerous precursors.15,16 For example, to achieve a successful phosphidation of transition metals, reactants such as trinoctyl phosphine (TOP) or PH3 have been widely used as effective phosphorous sources so as to achieve a desired structural and compositional control, which makes the synthetic process expensive, complicate and even dangerous.17,18 It has been highly demanded that new phosphidation strategy with highly-active alternatives of these precursors be identified for the preparation of TMPs catalysts. Equally importantly, the structural engineering of the nanosized TMPs particles should be favorably coupled with the architectural design of their conductive supports, typically carbon, so that the electrical conductivity and dispersity of the active components could be optimized to

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ensure an improved electrochemical performance.19 Accordingly, it becomes appealing that synthetic strategy be developed to integrate the TMPs formation together with the architectural tailoring of their supports, forming nanocatalysts with active nanoparticles of TMPs anchored in a highly-porous carbon matrix. It is also a prerequisite that the new synthetic protocol be low cost and environmental benign in terms of both the synthesis process and the phosphorous precursors. Biomass has long been qualified as raw carbon precursors for the production of different functional materials with designated structures, which have been found broad applications in different areas including catalysts, sorption materials, and biochemical.20-22 The biomass-based activated carbon showed attractive characters including but not limited to: First, the biomass production is usually related to environmental benign process with high quantity and quality, which guarantees a reliable feedstock of carbon production through a controlled carbonization process.23 Second, the transformation of the biomass precursors to active carbon showed a high flexibility in the structural design and architectural control of key morphological parameters such as the shape, size, and porosities.23,24 Third, the rich chemistry of the biomass leaves enormous room for the further functionalization and compositional control of the carbonized species. The abundant surface functional groups such as C-O, COOH-, and OH as well as the rich compositional elements nitrogen has endowed the biomass further possibility in tuning the physicochemical properties of activated carbons.22,25 In our research, we have a special interest in yeast, a microorganism widely used in the ethanol and baking industry, mainly due to its high content of phosphorous. As a main nutrition required for the yeast growth, phosphorous accounts for almost 0.3 wt% of the total weight.26 Interestingly, we found that the yeast itself can be developed into an ideal synthetic platform for supporting nanocatalysts with highly-dispersed metal phosphides nanoparticles embedded in mesoporous carbon matrix. We demonstrated that an in-situ phosphidation of the metal species could be well integrated with the structure engineering of the carbon matrix, resulting in a favorable architectures of supported catalysts applicable to a large variety of metal phosphides such as Co2P, Mn2P, Ni2P, and Zn3P2. We showed that the prepared nanocomposite could be used as a promising catalyst which was able to accelerate the hydrogen evolution reaction (HER) with high activity and stability. Our synthetic protocol not only provides an simple and effective strategy for the construction of a large variety of metal phosphides for morphological control and performance optimization, but also demonstrates the possibility of a full utilization of the essential ingredients as well as the innate architecture of the existing biomass in the creation of functional nanomaterials towards a sustainable energy development.

RESULTS AND DISCUSSION Scheme 1 shows the synthesis route we adopted for the construction of metal-phosphide-based nanocomposite.

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Firstly, the yeast was treated under a hydrothermal condition, which is a process widely applied to transform biomasses to carbonaceous species with rich surface functional groups.22 To ensure a satisfied utilization of the whole carbon matrix for further functionalization, we designed an etching process initiated by additives of NaCl and glutaraldehyde (GA) in the hydrothermal process. Accordingly, the yeast can be transformed into a hollow and porous architecture, making its inner structure easily accessible for the acquisition of the coexistent metal ions. The carbonaceous product after the hydrothermal reaction was then treated at a high temperature of 900 oC under an inert atmospheres, which simultaneously initiated a carbonization process of the organic species together with an efficient phosphidation of the inner metal ions by the constitutional phosphorous of yeast, forming a composite structure with uniform nanoparticles of metal phosphides dispersed across the highly-porous carbon matrix.

Scheme 1. Schematic illustration of the synthesis of Metal phosphide-C nanocomposite. Yeast particles (in light-yellow) are the biomass precursor. NaCl and glutaraldehyde can induce different pores in the formed carbonaceous matrix (in blue) during the hydrothermal reaction. Metal ions can be trapped inside the porous matrix and produce metal oxide nanoparticles (in red); The high temperature heating results in a conductive substrate (in deep blue) and a simultaneous formation of metal phosphides nanoparticles (in yellow ) accomplished by the biomass phosphorous.

Figure 1a shows a scanning electron microscopy (SEM) image of the cobalt-containing sample after the high temperature treatment. The final product exists as nearlyspherical microparticles with size around 2 μm. A close look on the particle surface (inset in figure 1a) reveals the existence of holes with different sizes on the wall. The transmission electron microscopy (TEM) analysis was then carried out to look inside the particles. Figure 1b shows a dark-field TEM image of a randomly-selected particle, which details a porous structure with different kinds of cavities distributed across the microparticle. Meanwhile, a careful TEM observation is able to locate highly-dispersed nanoparticles embedded inside the porous matrix. The X-ray diffraction (XRD) pattern of this cobalt-containing sample identifies the emergence of Co2P (PDF # 89-3030) with no extra impurities (figure 2a). Meanwhile, a blank test by excluding the Co2+ species produced only porous carbon (figure S1a-b), which are amorphous in nature from its XRD pattern (figure S1c). Therefore, it is reasonable to expect that these emerged nanoparticles are crystalline Co2P rooted from the Co2+ species. As a matter of fact, it is out of our expectation to confirm the existence of Co2P since its formation usually


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necessitates a phosphidation process, which is achieved by designated phosphorous reactants.27-30 The emergence of Co2P in our synthesis inspired us to revisit the yeast itself, which is a widely-researched biomass with a wellknown but easily-ignored fact of its high content of phosphorous. Clearly, the phosphidation of cobalt was with no doubt accomplished by the constitutional phosphorous of yeast during the heating, which enlightened us of an interesting tool to prepare metal phosphide without the involvement of those expensive and dangerous precursors as discussed above. High resolution TEM (HRTEM) analysis was carried out on these embedded nanoparticles of Co2P. Figure 1c showed the HRTEM image of a randomlyselected Co2P particle, which is crystalline as revealed by the lattice fringes. The lattice spacings are measured to be 0.272 nm, corresponding to the (111) planes of the orthorhombic Co2P.

Figure 1. SEM (a) and TEM (b) images of the Co2P-C sample prepared with 0.09 M Co(NO3)2 used in the hydrothermal reaction. The inset in figure 1a showed a high magnification SEM image; (c) High resolution TEM image of randomlyselected particles in figure 1b; (d-f) TEM images of Co2P-C nanocomposites synthesized by controlling the Co(NO3)2 concentrations at (d) 0.18 M, (e) 0.36 M, and (f) 0.54 M, respectively.

We found that our synthesis route showed a flexible capability to form and host an increased amount of Co2P particles by simply using higher concentrations of the cobalt precursor, namely Co(NO3)2, in the hydrothermal reaction. Figure 1d-f show the TEM images of different Co2P-carbon composites (denoted as Co2P-C) with Co2+ contents controlled at different levels. It is clear that more Co2P nanoparticles formed inside as we doubled the Co2+ concentration from 0.09 M (figure 1b) to 0.18 M (figure 1d). An increase to 0.36 M of Co(NO3)2 could further manifest such a trend (figure 1e) and finally the carbon matrix would be swamped with densely-populated Co2P particles as shown in figure 1f ( 0.54 M Co(NO3)2). Interestingly, we did not observe an obvious change in the shape and size for either the Co2P particles or the carbon matrix, showing a reliable capability for the structural control of the key components in the Co2P-C nanocomposite. The actual weight loading of the Co2P particles in each sample was listed in table S1.

Further investigation was carried out on the Co2P-C nanocomposite with a focus on its compositional distribution. Figure S2 shows the elemental mapping of a representative Co2P-C particle which was randomly picked for compositional analysis. The existence of three concerned elements, namely C, Co, P, is verified and their distributions are in good agreement with the shape character of the microsized particle. A detailed elemental analysis was then carried out on single Co2P nanoparticles with the help of the line-scan technique of the energydispersive X-ray spectroscopy (EDX). As shown in figure 2b, the profiles of Co and P fit well to the outlines of the nanoparticle, confirming the formation of Co2P nanoparticles from our synthetic approach. The highly porous structure of Co2P-C is further determined by the nitrogen adsorption-desorption isotherms (figure 2c), which shows a characteristic hysteresis loop in the curves and revealed a mesoporous nature of the carbon matrix as a result of the high temperature treatment. The result from the Barrett-Joyner-Halenda (BJH) pore size and volume analysis shows a relatively broad distribution of mesopores, which is in good agreement with the TEM observation. Furthermore, the specific surface area of the nanocomposite is measured to be as high as 632.8 m2/g, which is highly desirable for its further applications when mass transport is a concern, typically catalysis with a need for more accessible active sites. The Raman spectrum of this sample (figure 2d) shows a high intensity ratio of D-band over Gband, which indicates a relatively-low graphitic degree of the formed carbon, a phenomenon agrees well with the HRTEM observation on the carbon matrix. Further characterization is then carried out with the help of X-ray photoelectron spectroscopy (XPS), which provides detailed information on the surface property of the Co2P-C sample. As shown in figure S3, the signal of nitrogen is detected, indicating the existence of nitrogen doping in the carbon matrix after the high temperature treatment as a result of the residue from the constituent nitrogen in yeast.

Figure 2. (a) XRD pattern of the prepared Co2P-C product; (b) line-scan analysis of single Co2P particle (red: Co, yellow: P); (c) Nitrogen adsorption/desorption isotherms of Co2P-C;



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the inset showed the mesopore-size distributions in the sample; (d) Raman spectrum of the Co2P-C sample.

Intrigued by the effectiveness and simplicity of our synthetic route in forming Co2P-C, we took a close examination on the key steps so as to track the formation mechanism of this nanocomposite. Figure S4a and b show the TEM and SEM images of the yeast particles used for the reaction, which exist as solid particles with a larger particle size around 4 μm. For the precursor sample collected after the hydrothermal reaction (Denoted as p-Co2P-C), the TEM image shows the incorporation of tiny nanoparticles inside the biomass-derived carbonaceous matrix. Elemental analysis (inset in figure S5a) discloses the existence of both cobalt and oxygen inside. Its XRD curve in figure S6 showed tiny but discernible peaks of Co3O4 (PDF#43-1003). The HRTEM analysis on these cobaltcontaining nanoparticles confirms that there exist poorlydefined but clearly-discernible lattice fringes. A fast Fourier-transformation (FFT) of the HRTEM image (figure S5b) shows weak FFT points in the pattern and their inter-planar spacing are calculated to be 0.285 and 0.243 nm, which are in good agreement with the distances from the (220) and (311) planes of Co3O4, respectively. It is therefore expected that a hydrothermal treatment transforms the cobalt species in solution into poorlycrystallized Co3O4 nanoparticles, which is actually not surprising considering the fact that hydrothermal processes have been widely adopted as a reliable protocol for the preparation of Co3O4 nanocrystals.31 Our control experiments highlight the importance of NaCl as an effective cavity creator during the hydrothermal treatment. Without NaCl, the formed microparticles would be mostly solid in shape with no obvious inner cavities observed (figure S7a). The selection of NaCl in our synthesis is based on the fact that NaCl is able to damage the cell walls as a result of the salt stress.32 In this way, we are able to channel into the inner particle to form different holes inside, and accordingly make the whole particle accessible for mass transport of the cobalt species. To prevent the collapse of the yeast during the cavitation, GA is used to strengthen the skeleton of the yeast, which is a widely-used additive in yeast treatment due to its capability to improve the strength of the polysaccharide networks.22,33 For comparison, the synthesis without GA will form collapsed yeast particles into sticky and solid ones (figure S7b). We consider that a successful entrapment of cobalt species inside the carbonaceous matrix is probably benefiting from the following two facts: First, the porous and open structure can facilitate the transportation of the Co2+ species into its inner part. Otherwise only limited amount of nanoparticles can be found inside the solid microparticles, showing its inadequate capability in the acquisition of the coexistent metal species (figure S7c) as compared to the Co2P-C sample we produced in figure S7d. Second, the rich surface group can provide rich functionality for the anchor of Co2+. The Fourier transform infrared spectroscopy (FTIR) curve shown in figure S8 confirms the existence of different amphiphilic groups including hy-

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droxyl carbonyl, and carboxyl. It has been expected that such a hydrothermally-treated biomass surface are suitable for the adsorption of metal ions due to the electrostatic attraction,22,34 which explains well our observation that an efficient entrapment of cobalt species was achieved inside the carbonaceous matrix without forming a large amount of separate Co3O4 particles. Most importantly, we are able to achieve an in-situ phosphidation of the anchored metal species during the high temperature treatment, which is with no doubt ascribed to the rich phosphorous constituent in the carbon matrix since yeast itself is the only reactant source we have. Favorably, the high temperature treatment designated for the carbonization process also ensures a suitable inert environment for the phosphidation of the metals. The synthetic protocol demonstrated here successfully circumvents the challenge as discussed above on the preparation of metal phosphides, making it simple and effective to prepare highlydispersed nanoparticles through a revisit on the compositional chemistry of the microbial biomass.

Figure 3. Dark-filed TEM images for the samples of (a) Mn2P-C, (b) Ni2P-C, and (c) Zn3P2-C, respectively. The inset in each TEM image showed the line scan result of a randomly-selected particle from the corresponding sample. The XRD patterns of all these three different samples are shown in (d).

Considering that there is no specific requirements necessitated on the metal ions for the preparation of metal phosphides, we expect that such a synthetic route could be readily applicable for other metals ions as long as they can be absorbed and dispersed inside the carbonaceous matrix. Accordingly, a large variety of metal ions have been tested by following the same recipe. Figure 3 shows the TEM (figure 3a-c) and XRD (figure 4d) results of three representative samples when nitrates of manganese, nickel, and zinc are used instead of cobalt. Similar to the TEM observation on Co2P-C sample, all these samples show similar structural characters in which highly dispersed


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nanoparticles are embedded inside the porous carbon matrix. The inset in each TEM image shows a corresponding elemental analysis on single nanoparticles. The line scan results clearly confirm the formation of phosphides for different metals including manganese (figure 3a), nickel (figure 3b), and zinc (figure 3c), respectively. The XRD characterization confirms the formation of their crystalline phases with no impurity phases observed. The HRTEM analyses were also carried out on nanoparticles of different metal phosphides with the results shown in figure S9, showing the crystalline feature of the supported particles. It is also noted that these nanoparticles of metal phosphides are mostly incorporated into the carbon structures, showing the effectiveness of the adsorption and phosphidation strategy in construction the metal phosphide-C (MP-C) nanocomposite. The consistency in the structural and shape control also reveals the versatility and reliability of our synthetic protocol, which is critical for the further evaluation of their performance in different application areas.

Figure 4. (a) Polarization curves of three different samples including Co2P-C, Blank C, control sample and 20 wt% Pt-C −1 (tested in 0.5 M H2SO4 at a scan rate of 5 mV s ). (b) Tafel plots of both Co2P-C and 20 wt% Pt-C. (c) Durability test of Co2P-C sample tested in 0.5 M H2SO4. (d) Capacitive currents of Co2P-C tested at 0.164 V as a function of scan rates. Inset: −1 CV curves at different scan rates from 4 to 16 mV s .

The highly-dispersed nanoparticles of metal phosphides incorporated in a high-surface-area support have attracted increased attention as active catalysts for different reactions such as hydrogenation, desulfurization,35 and water electrolysis.36 The last of these potentials is demonstrated here by using the Co2P-C nanocomposite as a HER electrocatalyst. Figure 4a shows the HER polarization curves of different samples when tested in 0.5 M H2SO4. For comparison, catalytic performance for three different samples including the biomass-derived carbon, the 20 wt% Pt-C, and a Co2P control sample prepared by a conventional wet-chemistry protocol (Figure S10, see experimental section for details) are also evaluated. As ex-

pected, the pure carbon itself without Co2P nanoparticles inside shows barely any HER activity. On the contrary, the existence of Co2P in the carbon matrix shows an obvious boost on the catalytic activity. The overpotential of this Co2P-C composite is measured to be 96 mV at catholic current densities of 10 mA/cm2 as compared to 178 mV of the control sample, showing that the formed Co2P-C nanocomposite are highly active catalysts even (A detailed comparison with the reported results on Co2P is listed in table S2). Meanwhile, we also observe a rapid increase of the cathode current as the potential turned more negative, revealing a favorable capability of our Co2P-C nanocomposite in facilitating the hydrogen evolution from water.12,37 Figure 4b displays the Tafel plots, which are the graphs described in overpotential against the logarithm of the current density. The Tafel slope of Co2P-C is 68 mV dec−1, indicating that the hydrogen evolution on Co2P-C should follow the Volmer-Heyrovsky mechanism.38,39 To have a better understanding on the durability of the Co2P-C sample as a reliable catalyst, we have carried out further stability study through the accelerated degradation. The time-dependent current density curve under a static overpotential of 50 mV (figure 4c) showed that Co2P-C would maintain its catalytic activity for at least 8h. The inset in figure 4c recorded the overpotential values after 1000 continuous CV scans at a scan rate of 50 mV/s. The sample showed almost negligible difference in the overpotential upon long cycles, showing a high stability of the Co2P-C nanocomposite under the acid condition. To have a better understanding on the electrochemical surface area (ECSA) of the Co2P-C nanocomposite, the double-layer capacitance of the material was tested due to the positive correlation to the ECSA value.40,41 As shown in figure 4d, the calculated value was 24.75 mF, corresponding to 124.1 mF cm-2 normalized by the geometric area of the electrode, which was much higher the 6.03 mF from the control sample (Figure S11). A higher ECSA is known to be capable of providing enough active sites for the HER, which is expected to be a major factor to achieve a hitagh HER performance as observed on our Co2P-C nanocomposite.42

CONCLUSION In conclusion, we demonstrated that the phosphorouscontaining yeast can act as an effective synthetic platform for the preparation of active nanocatalysts with metal phosphides nanoparticles dispersed across the porous carbon matrix. We verified that NaCl can act as an effective etching agent for the yeast, which could be engineered into a highly-porous structure towards an effective entrapment of metal species inside by utilizing the rich surface functional groups. We demonstrated that an insitu phosphidation of the metal species could be well integrated with the structure control of the carbon matrix, forming a favorable architectures of supported catalysts applicable to a large variety of metal phosphides such as Co2P, Mn2P, Ni2P, and Zn3P2. Our preliminary test confirmed that the prepared nanocomposite could be promising to accelerate the hydrogen evolution reaction (HER)



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with high activity and stability. Such a synthetic protocol not only provide an simple and effective strategy for the construction of a large variety of metal phosphides nanocomposite, but also demonstrated the possibility of a full utilization of the essential ingredients as well as the innate architecture of the existing biomass in the creation of functional nanomaterials towards a sustainable energy

EXPERIMENTAL SECTION Preparation of Carbon microspheres. 1 g Yeast cells pre-washed with acetone were dissolved in 30 mL pure water under magnetic stirring. Subsequently, 5 mL Sodium chloride solution (100 mg/mL) and 100 μL glutaraldehyde aqueous solution were added to the Yeast suspensions, which was then placed in a 50 mL Teflon-sealed autoclave and maintained at 190 oC for 8 h. The solid products with a purple brown color were centrifuged, then washed by three cycles of centrifugation/washing/redispersion in pure water, and oven-dried at 80 oC for 10 h. Finally, the obtained product was prepared via further carbonize at temperature of 900 oC for 6h, respectively, by a tubular reactor in a flow of Argon. After naturally cooling to room temperature, black carbonized products were obtained. Preparation of Co2P-C, Mn2P-C, Zn3P2-C, Ni2P-C Microspheres.1g Yeast cells pre-washed with acetone were dissolved in 30 mL pure water under magnetic stirring. Subsequently, 5 mL Sodium chloride solution (100 mg/mL), 100 μL glutaraldehyde aqueous solution and 0.18 mol/L transition metal nitrate (cobaltous nitrate hexahydrate, manganese nitrate, zinc nitrate hexahydrate or nickel (II) nitrate hexahydrate) were added to the yeast suspensions, which was then placed in a 50 mL Teflonsealed autoclave and maintained at 190 oC for 8h. The solid products with a purple brown color were centrifugal separated, then washed by three cycles of centrifugation/washing/redispersion in pure water, and oven-dried at 80 oC for 10h. Finally, the obtained product was prepared via further carbonize at temperature of 900 oC for 6h, respectively, by a tubular reactor in a flow of Argon. After naturally cooling to room temperature, black carbonized products were obtained. Preparation of the Co2P control sample. The hollow microspheres were prepared by following the protocol reported in reference 43.43 0.5 g CoCl2 and 1.2 g yellow phosphorous was dispersed in 30 mL ethanol. The suspension was then added into the 40 mL Teflon-lined autoclave. The autoclave was heated to 200 oC for 30 h and the formed solid product was collected for further characterizations. Electrochemical measurement. All of the electrochemical measurements were performed in a threeelectrode system on an electrochemical workstation (CHI 660D) in 0.5 M H2SO4 electrolyte. Typically, 3 mg of catalyst and 30 μL Nafion solution (Sigma Aldrich, 5 wt %) were dispersed in 1.2 mL ethanol solution by sonicating for 1 h to form a homogeneous ink. Then 30 μL of the dispersion (containing 75 μg of catalyst) was loaded onto a glassy carbon electrode with 5 mm diameter (loading

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0.382 mg cm-2). While a Hg/HgCl2 (in 4.2 mol KCl solution) electrode and a graphite rod were served as the reference electrode and counter electrodes, respectively. All of the potentials were calibrated to a reversible hydrogen electrode (RHE).The working electrode was polished with Al2O3 powders with size down to 0.05 μm. Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] * [email protected]

ACKNOWLEDGMENT The authors acknowledge funding support from the National Natural Science Foundation of China (Grant No 51672282， 21373238,), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010101), and the major State Basic Research Program of China (973 program: 2013CB934000).

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