水光解

www.advmat.de www.MaterialsViews.com C O M M U N IC A TIO N Yung-Huang Chang , Cheng-Te Lin , Tzu-Yin C Wenjing Zhang , Kung-Hwa Wei , and Lain-Jon Highly Effi cient Electrocatalyti by MoS x Grown on Graphene- Academia Sinica, Taipei 10617, Taiwan, ROC Hydrogen energy is clean and serves as one of the most prom- ising candidates for replacing petroleum fuels in the future. Although the rare metals, such as platinum, have high effi ciency The MoS x layer was then formed after subsequent annealing at E-mail: lanceli@gate.sinica.edu.tw C.-L. Hsu, Dr. K.-H. Wei Department of Materials Science & Engineering National Chiao Tung University HsinChu 300, Taiwan, ROC Dr. L.-J. Li Department of Physics National Tsing Hua University Taiwan, ROC DOI: 10.1002/adma.201202920 various temperatures (100, 120, 170, 200, 250, 300 ° C) in a H 2 / Ar environment (500 torr; H 2 :Ar = 20:80) for 20 min. Figure 1 c shows SEM images of the graphene-protected Ni foam grown with the MoS x annealed at 120 ° C. These images suggest that the MoS x layer exhibits almost full coverage over the graphene surfaces, demonstrating that effi cient loading of MoS x on 3D Ni foam is achievable. The high resolution SEM image in Figure 1 d reveals that the surface of the deposited MoS x mate- rials is very rough, and the MoS x materials are composed of nanometer-scaled structures with large amounts of edges. The Dr. Y.-H. Chang, Dr. C.-T. Lin, T.-Y. Chen, Dr. Y.-H. Lee, Dr. W. Zhang, Dr. L.-J. Li Institute of Atomic and Molecular Sciences in the hydrogen evolution reaction (HER), their scarcity and high cost inhibit large scale applications. [ 1–6 ] Recently, inorganic catalysts such as nanometer-scaled MoS 2 and WS 2 have drawn great attention due to their low cost, high chemical stability, and excellent photocatalytic [ 7–24 ] and electrocatalytic properties in HERs. They are potentially useful if they can be tailored for the development of hydrogen energy devices. In order to enhance the effi ciency of inorganic catalysts, many research efforts have been made toward the modifi cation of material properties, [ 25 ] the formation of composite catalysts, [ 26–31 ] and the fabrication of the electrodes with nano-architecture. [ 30–33 ] Recently, MoS 2 / reduced graphene oxide catalyst composites have been success- fully made for enhancing the electrocatalytic HER effi ciency, where the reduced graphene oxide sheets serve the function of hosting MoS 2 as well as enhancing the conductance of the com- posites. [ 31 , 34 ] However, most of the reported electrode materials were still based on two-dimensional (2D) planar structures. To improve the electrocatalytic HER effi ciency, it is crucial to effectively increase the surface area for catalyst loading. Hence, the research into three-dimensional (3D) electrode structures is emergent. A three-dimensional graphene foam synthesized on the Ni foam skeleton by chemical vapor deposition (CVD) has been reported. [ 36 , 37 ] The graphene foam without the support of an Ni skeleton is brittle and is not able to serve as a 3D elec- trode for hosting catalysts. The 3D Ni foam is a low cost and conductive metal with a high surface area, which is ideal for use as a template to host catalysts for increasing the number of reaction sites. [ 38–40 ] However, it suffers from instability in acidic solutions, and thus is not suitable for the electrocatalytic HER. Here, we report that the graphene sheets grown on Ni foams © 2012 WILEY-VCH Verlag GAdv. Mater. 2012, DOI: 10.1002/adma.201202920 hen , Chang-Lung Hsu , Yi-Hsien Lee , g Li * c Hydrogen Production Protected 3D Ni Foams provide robust protection and effi ciently increase their stability in acid. The highly conductive 3D graphene/Ni foam structure also effectively increases the catalyst loading, leading to the enhancement in electrocatalytic HER effi ciency. Meanwhile, we formulated MoS x ( x ≥ 2) catalytic materials on graphene- protected Ni foam to form a rigid 3D electrocatalytic architec- ture, where the MoS x materials are grown by the thermolysis of ammonium thiomolybdates at different temperatures in a CVD chamber. The electrocatalytic HER of the MoS x /graphene/3D Ni foam was performed in a 0.5 M H 2 SO 4 solution. The HER current density for the MoS x /graphene/3D Ni foam, either nor- malized by geometrical area or electrochemical surface area (ESA), is higher compared with the MoS x on various planar carbon electrodes including carbon paper, carbon cloth, and graphene mats. X-ray photoelectron spectroscopy (XPS) anal- ysis of the materials reveals that the higher HER effi ciency is related to the presence of bridging S 2 2 − or apical S 2 − in amor- phous states. The three-dimensional Ni foam (110 ppi; thickness = 1.6 mm) was obtained from Nexcell battery Co. (Taiwan). The growth of a few layers of graphene on the Ni-foam by CVD has been reported elsewhere. [ 35 ] In brief, the Ni foams are reduced with H 2 fl ow (100 sccm) at 1050 ° C for half an hour before the CVD growth (gas ratio CH 4 :H 2 = 15:100; growth temperature 1050 ° C for 1 h; pressure 500 mtorr). Figure 1 a shows the scan- ning electron microscopy (SEM) images for the as-obtained Ni foam, where submillimeter pores can be clearly seen and the Ni grains of the skeletons are observable at a higher magnifi cation. Figure 1 b displays the SEM images after graphene layers are grown on the surfaces of Ni foam. The Ni surfaces are fully cov- ered with graphene layers and the graphene wrinkles are clearly identifi ed. The Raman spectrum in Figure S1, Supporting Information, proves that these graphene sheets are of a few layers. To grow MoS x catalysts on the graphene surfaces, the graphene-protected Ni foam was immersed in an ammonium thiomolybdate solution (5 wt% of (NH 4 ) 2 MoS 4 in DMF). The Ni foam was then backed on a hot plate at 100 ° C for 10 min. mbH & Co. KGaA, Weinheim 1wileyonlinelibrary.com Administrator 文本框 电催化的 Administrator 文本框 泡沫 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 2 www.advmat.de C O M M U N IC A TI O N wileyonlinelibrary.com © 2012 WILEY-VCH Verlag Gm electrocatalytic HER is normally performed in acidic solu- tions, where the Ni foam is not stable in acid. The dashed line in Figure 1 d displays the polarization curve of the as-obtained Ni foam electrode in a 0.5 M H 2 SO 4 solution, where the oxida- tive potential (positive potential vs reactive hydrogen electrode, RHE) induces a high current which indicates the dissolution Figure 1 . a) SEM images for the as-obtained Ni foam, where sub-mil- limeter pores can be clearly observed. b) SEM images of the Ni foam surfaces after graphene layers are grown. c) SEM images of the graphene- protected Ni foam grown with the MoS x annealed at 120 ° C. d) High resolution SEM image of the sample in shown in (c). e) The polarization curves of the as-obtained Ni foam electrode (dotted line), the graphene- protected Ni foam electrode (solid line), and the graphene-protected Ni foam electrode with MoS x grown at 120 ° C (dotted line). The measure- ments were performed in a 0.5 M H 2 SO 4 solution. The current was nor- malized by the geometrical area of the Ni foam. -0.3 -0.2 -0.1 0.0 0.1 0.2 -150 -100 -50 0 50 100 15 (e) 0 C u rr en t d en si ty ( m A /c m 2 ) Potential (V) vs. RHE Ni-foam Graphene/Ni-foam MoS 2 /Graphene/Ni-foam www.MaterialsViews.com of Ni in acidic solutions. After CVD graphene layers are depos- ited on Ni foam surfaces, the oxidative current is signifi cantly suppressed (solid line). This observation proves that the CVD graphene is able to protect the Ni foam from oxidative corro- sion in acidic environments. The dotted line in Figure 1 d dem- onstrates the polarization curve of the graphene-protected Ni foam electrode grown with MoS x (annealed at 120 ° C), where the negative current at a negative potential is the current associ- ated with the HER. The inset is a photograph taken during the electrocatalytic hydrogen reduction (potential: –0.2 V), where the large (millimeter size) H 2 bubbles are formed due to ease of merging of the evolved tiny H 2 gas bubbles in the pores of the 3D electrodes. Figure 2 a shows the polarization curves (measured current normalized by the geometrical area of the Ni foam) for the MoS x prepared at different annealing temperatures. It is observed that the HER effi ciency exhibits a maxima at T = 120 ° C and it decreases with the further increase in annealing temperature. The inset plots the current density at the applied potential of 0.2 V as a function of annealing temperature. It is noted that the geometrical size of the Ni foam and the loading amount of the MoS x are similar for each annealing temperature (see Table S1, Supporting Information, for details). Figure 2 b replots the polarization curves using the measured current normalized by loading weight of MoS x and the electrochemical surface area (ESA) (see Table S1 for details). The ESA was determined by the reported method. [ 41 , 42 ] The HER effi ciency still shows a maxima at T = 120 ° C, consistent with Figure 2 a. A Tafel plot is normally used to evaluate the effi ciency of the catalytic reaction. Table S2 and Figure S2, Supporting Information, show the HER activity of the graphene-protected Ni foam electrodes decorated with MoS x prepared at different temperatures. These values were derived from the polarization measurements. Figure 2 c shows that the Tafel slope for the electrodes decorated with the MoS x formed at 120 ° C is the smallest ( ≈ 42.8 meV dec − 1 ). The classical theory of hydrogen generation [ 34 , 43 , 44 ] suggests that a Tafel slope of ≈ 40 meV dec − 1 indicates a low surface coverage of adsorbed hydrogen and the reaction is as shown in Equation (1) and (2). The MoS x prepared at a higher annealing temperature, e.g., 300 ° C, results in a lower HER effi ciency and the reaction mechanism moves to a larger surface coverage and the reaction follows Equations (1) and (3). [ 34 , 35 ] Also, these results suggest that the different current density values actually originate from the various catalytic activities of MoS x catalysts formed at different temperatures. H3O + + e− + catalyst −→ catalyst-H + H2O (1) H3O + + e− + catalyst-H −→ catalyst + H2 + H2O (2) catalyst-H + catalyst-H −→ 2 catalyst + H2 (3) To understand the differences between the MoS x catalysts prepared at various temperatures, XPS was adopted to char- acterize the chemical bonding structures. Figure 3 displays the detailed XPS scans for the Mo and S binding energies for these MoS x catalysts. The MoS x prepared at 300 ° C exhibits two characteristic peaks at 232.1 and 228.9 eV, attributed to the bH & Co. KGaA, Weinheim Adv. Mater. 2012, DOI: 10.1002/adma.201202920 www.advmat.de C O M M U N www.MaterialsViews.com 0 5 (a) 0 200 oC 250 oC 300 oC 100 oC 120 oC 170 oC m A /c m 2 ) © 2012 WILEY-VCH Verlag Gm IC A TIO N Mo 3d 3/2 and 3d 5/2 binding energies for Mo 4 + . [ 45–47 ] The peaks, corresponding to the S 2p 1/2 and 2p 3/2 orbitals of divalent sulfi de ions (S 2 − ) are observed at 162.9 and 161.8 eV. [ 45–47 ] The stoichiometric ratio (S:Mo) estimated from the respective inte- grated peak area of XPS spectra is close to ≈ 2.09, suggesting Figure 2 . Polarization curves for the MoS x prepared at different annealing temperatures, where the current density is normalized by a) geometrical area of the Ni foam, and b) both the loading weight of MoS x and the electrochemical surface area (ESA). c) Tafel plot for the MoS x grown (at 120 ° C) on a graphene-protected Ni foam. -0.3 -0.2 -0.1 0.0 -150 -100 -50 (b) (c) 100 150 200 250 300 -50 -40 -30 -20 -10 0 C u rr en t d en si ty ( Potential (V) vs. RHE C u rr e n t d e n s it y (m A /c m 2 ) Temperature ( o C) -0.3 -0.2 -0.1 0.0 -15 -10 -5 0 5 100 150 200 250 300 -6 -4 -2 0 100 oC 120 oC 170 oC 200 oC 250 oC 300 oC C u rr en t d en si ty ( A /g ) Potential (V) vs. RHE C u rr e n t d e n s it y (A /g ) Temperature (oC) 1 10 0.0 0.1 0.2 0.3 0.4 slope = 42.8 mV/sec = 109-141 mV Current density (mA/cm2) O ve rp o te n ti al ( V ) 120 oC Adv. Mater. 2012, DOI: 10.1002/adma.201202920 that the structure is close to MoS 2 . [ 48 , 49 ] When the annealing temperature is lowered, in addition to the XPS peaks for the MoS 2 structure, other sets of peaks are also observed. The observation of Mo 3d 3/2 and 3d 5/2 binding energies at 233.1 and 230 eV suggests the presence of Mo 5 + ions. [ 45–47 ] Meanwhile, the S 2p 1/2 and 2p 3/2 energies at 164.3 and 163.2 eV suggest the existence of bridging S 2 2 − or apical S 2 − . [ 45 , 46 ] Although it is not possible to exclusively identify the ratio between these sulfur species due to their similar binding energies, the pres- ence of these higher energy peaks indicate that the active HER species are likely related to these species. It is noted that the HER effi ciency for highly crystalline MoS 2 obtained at 1000 ° C is very low (Figure S3, Supporting Information), indicating that MoS 2 is less active for electrocatalytic HER. The S:Mo atomic ratios for these samples are labeled in Figure 3 , from which we conclude that the structure of the MoS x obtained at lower temperatures such as 100, 120, and 170 ° C is stoichiometrically close to Mo 2 S 5 . The transmission electron microscopy (TEM) and X-ray diffraction analyses reveal that all the MoS x materials obtained in the temperature range of 100 to 300 ° C are basi- cally amorphous (data not shown). This evidence implies that the higher HER effi ciency is related to the presence of bridging S 2 2 − or apical S 2 − in amorphous states. We have also examined the XPS spectra for the MoS x sample prepared at 120 ° C after electrocatalytic hydrogen generation reaction (Figure S4, Sup- porting Information). Interestingly, the the content of Mo 6 + and Mo 5 + in a MoS x material is increased after electrocatalytic reac- tion even though the Mo 6 + oxidation state does not exist before the electrocatalytic reaction, in good agreement with the groups of Tang [ 30 ] and Vrubel. [ 45 ] However, the binding energies of S 2 − , located at 161.7 and 162.8 eV, and energies for bridging S 2 2 − or apical S 2 − , located at 163.2 and 164.3 eV, are still present after electrocatalytic reactions, not showing any apparent change in XPS peak profi le. Figure 4 shows the measured hydrogen gas evolution rate (mmol of H 2 normalized by the weight of the catalyst and the geometrical area of the graphene-protected Ni foam) for the MoS x prepared at various temperatures. The H 2 evolu- tion rate normalized by catalyst weight and ESA is shown in Figure S5, Supporting Information, for comparison. The highest hydrogen production rate we have achieved so far is around 13.47 mmol g − 1 cm − 2 h − 1 (302 mL g − 1 cm − 2 h − 1 ) at a potential of V = 0.2 V for the MoS x obtained by annealing at 120 ° C. Note that the current density for the sample operated at 0.2 V is around 45 mA cm − 2 . Moreover, Figure S6, Sup- porting Information, shows the current density as a function of hydrogen evolution time. The hydrogen production effi ciency is superior to several recent HER reports based on MoS 3 parti- cles, [ 45 ] amorphous MoS x prepared by electro-polymerization, [ 50 ] and MoS 2 /reduced graphene oxides. [ 34 ] The advantage of using a 3D Ni foam as an electrode is that the loading weight of the MoS x catalyst is larger than other carbon-based electrodes such as carbon paper, carbon cloth, and graphite mats as detailed in Table S3, Supporting Information. Figure 5 compares the polarization current density (normal- ized by ESA) for the MoS x grown on various carbon electrodes, where the graphene-protected 3D Ni foam exhibits the highest current density. The superior performance is attributed to the relatively high catalyst loading weight as well as the relatively 3wileyonlinelibrary.combH & Co. KGaA, Weinheim 4 www.advmat.de www.MaterialsViews.com C O M M U N IC A TI O N in graphene-based 3D electrodes may fur- ther advance the effi ciency of various elec- trocatalytic reactions, which warrants more investigations. Experimental Section Growth of Graphene on Ni Foam : Three-dimensional Ni foam (110 ppi; thickness = 1.6 mm) was obtained from Nexcell battery Co. (Taiwan). The Ni foams were reduced with H 2 fl ow (100 sccm) at 1050 ° C for half an hour in a CVD furnace before the CVD growth. For the growth of graphene layers, a mixture of CH 4 and H 2 gases (ratio 15:100; pressure 500 mtorr) was introduced to the furnace at 1050 ° C for 1 h. Thermolysis to Form MoS x Catalysts : The graphene- protected Ni foam was immersed in an ammonium thiomolybdate solution (5 wt% of (NH 4 ) 2 MoS 4 in DMF). The Ni foam was then backed on a hot-plate at 100 ° C for 10 min. The MoS x layer was then formed after subsequent annealing at various temperatures (100, protected Ni foam, an a lower resistance of the electrodes: the sheet resistance of these carbon electrodes is listed in Table S3. In summary, a 3D Ni foam deposited with graphene layers on its surface was used as a conducting solid support to load MoS x Figure 3 . XPS scans for the Mo and S binding energies of various MoS x cat wileyonlinelibrary.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Wein emission scanning e were recorded by an rate of 5 mV s − 1 in a using an Ag/AgCl ( a graphite rod as th foam samples as th electrolysis measurem reversible hydrogen e catalysts for electrocatalytic hydrogen evolution. The graphene sheets grown on Ni foams provide robust protection and effi - ciently increase its stability in acid. The hydrogen evolution rate reaches 302 mL g − 1 cm − 2 h − 1 (13.47 mmol g − 1 cm − 2 h − 1 ) at an overpotential of V = 0.2 V and the catalytic species were likely related to the bridging S 2 2 − or apical S 2 − . The developments Figure 4 . The measured hydrogen gas evolution rate normalized by the weight of the catalysts and the geometrical area of the graphene-pro- tected Ni foam. 0 5 10 15 20 25 30 35 0 1 2 3 4 5 100 oC 120 oC 170 oC 200 oC 250 oC 300 oC Time (min) V o lu m e (m m o l g -1 cm -2 ) Figure 5 . The polar MoS x (annealed at 1 graphene-protected -0.3 -80 -60 -40 -20 0 Cu rr e n t d en s ity (m A /c m 2 ) P d 3D MoS x /graphene/Ni foam was examined by a fi eld- lectron microscope (JSM-6500F). Polarization curves AUTOLAB pontentiostat (PGSTAT 302N) with a scan 0.5 M H 2 SO 4 solution. A three-electrode confi guration KCl saturated) electrode as the reference electrode, e counter electrode, and the 3D MoS x /graphene/Ni e working electrode was adopted for polarization and ents. In 0.5 M H 2 SO 4 , potentials were referenced to a lectrode (RHE) by adding a value of 0.21 V. ization current density (normalized by ESA) for the 20 ° C) grown on various carbon electrodes, where the 3D Ni foam exhibits the highest current density. -0.2 -0.1 0.0 otential (V) vs