Nanosponge Pt Modified Graphene Nanocomposites Using Silicon

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Nanosponge Pt modified graphene nanocomposites using silicon monoxides as a reducing agent: High efficient electrocatalysts for hydrogen evolution Fan Liao, Wen Shen, Yuyang Sun, Yanqing Li, Huixian Shi, and Mingwang Shao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03721 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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ACS Sustainable Chemistry & Engineering

Nanosponge Pt modified graphene nanocomposites using silicon monoxides as a reducing agent: High efficient electrocatalysts for hydrogen evolution Fan Liao, Wen Shen, Yuyang Sun, Yanqing Li*, Huixian Shi, Mingwang Shao* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, PR China * E-mail: [email protected] (YQ Li), and [email protected] (MW Shao)

KEYWORDS: Nanosponge; Platinum; Silicon monoxide; Graphene; Hydrogen evolution.

Modulating the morphology of Pt is an effective approach to augment the active sites and mass activities for hydrogen evolution reaction (HER). Here a facile one-step method is employed to fabricate Pt modified graphene nanocomposites (Pt-G) with the help of silicon monoxides (SiO) and hydrofluoric acid. SiO is introduced as a reducing agent for Pt ions and graphene oxides through Si-H bonds and promotes the catalytic performance of Pt-G. The obtained Pt on graphene shows a nanosponge structure due to the secondary nucleation and in-situ growth on the SiO. When the Pt-G nanocomposites are employed as electrocatalysts for HER, the optimal catalyst shows high mass activity (8.45 A·mg-1) as 1.6 and 1.9 times as those of commercial 20

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wt% Pt/C and Pt-G-gly (obtained by glycol reduction) at overpotential of 0.2 V. The current density of the catalysts exceeds that of commercial 20 wt% Pt/C at high overpotentials because SiO increases the desorption rate of hydrogen. Furthermore, the residual SiO could efficiently avoid the aggregation of Pt and restacking of the graphene, which increase the stability of the catalysts.


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INTRODUCTION The energy dilemma and environmental pollution boost the investigation of renewable sources of clean energy.1-4 Hydrogen, an eco-friendly and high energy density power source, is believed to be one of the most promising fuels to replace the traditional fossil fuels.5,6 Water electrolysis is a clean and sustainable way to produce hydrogen with high purity.7, 8 Noble metals, especially Pt, are considered to be the best catalysts for hydrogen evolution reaction (HER).9-11 Unfortunately, they face the disadvantages of high cost and low stability. Recently, noble metal-based composites supported on graphene are verified as an advanced strategy to augment the number of active sites of the catalysts and decrease the dosage of noble metals. Yet, these methods have shortcomings which are not satisfied for electrocatalysis: (1) the preparation process often needs several steps and is time-consuming; (2) the frequently-used additives in the fabrication procedure, such as linking agents and surfactants would block the active sites and therefore influence the catalytic performance; (3) these particles easily aggregate during the catalysis and thus deteriorate their activity. For example, when the salts of noble metals in the solution of graphene were reduced with a reducing agent like NaBH4, the strong π– π stacking interaction leads to aggregation and restacking of graphene, decreasing the catalysis performance.12 In order to solve these problems, many researchers suggest adding an oxide for anchoring nanoparticles on the graphene. Several oxides, such as MoO2,13 TiO2,14,15 CeO216 and SiO217,18 have been demonstrated to improve the electrocatalytic performance successfully. Silicon monoxide (SiO), as an important silicon oxide, is a useful material in lithium ion batteries, electronic devices, and optical fibers.19, 20 However, it has not been used as electrocatalysts for HER. SiO usually has an amorphous structure.21 It is widely-accepted that SiO have both Si-Si


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and Si-O bonds, as verified in the previous reports.22, 23 The physical properties of SiO are more akin to those of SiO2 than those of Si,24 so the silicon monoxide may have the same effect as SiO2 to increase the electrocatalytic stability. Furthermore, silicon in previous literature has been employed to support noble metals because the Si-H bonds, originated from the reaction with hydrofluoric acid (HF), can reduce noble metals ions and anchor metal nanoparticles on the surface of silicon.25, 26 Inspired by the literatures, it is a reasonable assumption that the Si-Si bonds in SiO may also have the same reducing capacity and anchor noble metal on the support. In this research, a facile one-step method was employed to synthesize Pt-graphene nanocomposites (Pt-G) with SiO as the reducing agent. No similar application of SiO has been reported before. The obtained Pt nanoparticles had a unique nanosponge structure due to the secondary nucleation and epitaxial in-situ growth. The possible growth mechanism of such morphology of Pt was discussed in detail. The Pt-G nanocomposites were employed as electrocatalysts for HER and showed high mass activity. Although the addition of SiO increased the onset overpotential compared with the commercial 20 wt% Pt/C, the current density of Pt-G nanocomposites exceeded that of 20 wt% Pt/C at high overpotentials, which is beneficial for industrial application. The excellent HER performance of Pt-G-3 is due to the synergistic effect among residual SiO, Pt and graphene. The nanosponge Pt provided the active sites, while graphene enhanced the conductivity; SiO linked Pt and graphene. With the increasing overpotentials, the hydrogen atoms cover the surface of Pt and reach to a threshold value. Then some adsorbed hydrogen atoms on Pt can migrate to the residual SiO, which is beneficial for the hydrogen evolution at high overpotential. Furthermore, the addition of SiO decreased the aggregation of Pt and graphene and increased the stability of the catalysts, which can also be applied in other catalyst synthesis.


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METHODS Synthesis of Pt-G with the reducing agent of SiO. The chemicals used are listed in the Supporting information. Before the fabrication process, the size of SiO was reduced by ballmilling in an ethanol medium. The fabrication of Pt-G with the reducing agent of SiO was conducted at room temperature (25 oC). In a typical procedure for Pt-G, the ball-milling treated SiO (10 mg), graphene oxide (5 mg) and different volume of H2PtCl6·6H2O solution (5 mM) were mixed and stirred to form a suspension. Then 2 mL HF aqueous solution (5%) was added into the above mixture and vigorously stirred for 20 min. The products were obtained by centrifugation, washed with double distilled water for several times, and dried via a freezedrying method. Due to the different amount of the added H2PtCl6·6H2O, the Pt-G fabricated by reducing agent of SiO are defined as Pt-G-1 to Pt-G-4. The names, the detailed dosage of raw materials and the Pt contents are listed in Table S1. The contrast Pt-G sample using glycol as the reducing agent is defined as Pt-G-gly and the detailed fabrication process is described in the Supporting Information.

Electrochemical measurements. The electrochemical measurements were performed on the CHI 750 E electrochemical workstation. A conventional three electrode system was employed at room temperature. The modified glass carbon electrode (GCE with diameter of 3 mm) was used as working electrode. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a carbon rod. All the measured potentials were against reversible hydrogen electrode (RHE) according to the equation: E (vs. RHE) = E (vs. SCE) + 0.245 + 0.059 × pH (1)


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The working electrode was fabricated as follows: catalysts (2 mg) were dispersed in 5: 1 v/v water-isopropanol mixed solvent (900 µL) and Nafion solution (100 µL, 0.5 wt %) and ultrasonically stirred to become homogeneous suspension. Then 5 µL of the suspension was dropped on the surface of GCE, and dried naturally. The calculated catalysts loading were 142 µg·cm-2.

RESULTS AND DISCUSSION Characterization of the Pt-G. In order to demonstrate the formation of Pt-G by the reducing agent of SiO, XRD of the Pt-G-3 (black curve) are shown in Figure 1a. Peaks at 39.8°, 46.2°, 67.5° and 81.3° are corresponding to the (111), (200), (220), and (311) planes of cubic Pt (JCDPS No. 04-0802). The reduction of GO to graphene is also proved due to the disappearing of the characteristic XRD peak of GO (Figure 1a, blue curve) at around 10°. The XRD pattern of SiO (Figure 1a, pink curve) showed no peaks, indicating its amorphous structure. No purities like Si or SiO2 can be indexed. It is reported that SiO is composed of a random network of Si-Si and Si-O bonds.27 So it is reasonable to consider that the formation of Pt-G is due to the Si-H bond on the SiO after the addition of HF. The reaction may be similar to the reduction of Pt on Si. The Si-Si bond can form Si-H bond with the addition of HF (Si3Si*– OH + 6HF → 3Si–H + H2SiF6 + H2O), and reduce Pt4+ to Pt (Si-H + 2H2O + Pt4+ → SiO2 + 0.5H2 + Pt + 4H+).28 To verify the formation mechanism, control groups without the addition of HF and with HCl are conducted (Figure S1). The Pt particles are not produced in such experimental conditions. XPS was further employed to investigate the elemental compositions of Pt-G. The survey scan spectrum (Figure S2) shows C 1s, O 1s, Si 2p and Pt 4f peaks. Figure 1b shows the C 1s XPS


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spectra for Pt-G-3. The peaks at 284.6 eV is assigned to sp2 bonded carbon and alkyl C network (C–C/C=C). The peaks at 286.8 and 288.6 eV are ascribed to the carbonyl C (C=O), and hydroxyl and epoxy groups C-O (includes epoxide and hydroxyl) bonds,17 respectively. Compared with GO (Figure S3), the peak of C=O and C(O)OH significantly decreased while the peak of C–C/C=C increased, demonstrating the partial deoxygenating of GO after reduction.

Figure 1. (a) XRD patterns of SiO, GO and Pt-G-3; and core-level XPS spectra of Pt-G-3 nanocomposite: (b) C 1s; (c) Pt 4f; and (d) Si 2p. The reduction process is also confirmed by FTIR and Raman (Figures S4 and S5). FTIR spectra (Figure S4a) of Pt-G-3, SiO and GO revealed the chemical structure change after reduction. The green curve is the FTIR spectrum of SiO. The peaks at around 1085 cm-1, 792 cm1

and 470 cm-1 were assigned to the Si-O stretching vibration, bending vibration and rocking

vibration, respectively.29 After the treatment of HF, the new peak at around 630 cm-1 (Figure S5,


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megenta curve) is attributed to rocking vibration of Si-H bonds.29 The black curve in Figure S4a is the FTIR spectrum of GO. After the reduction of SiO, the peaks related to carbon oxides disappeared or greatly decreased. For example, the peak at 3433 cm−1 belonging to stretching vibration of O–H groups decreased. The peak at 1728 cm−1 disappeared, which is due to the decrease of stretching vibrations of C=O. The peaks at 1625 cm−1 and 1113 cm−1, related with stretching vibrations of C–O and bending vibration of C-O-C greatly decreased.30 The FTIR spectra indicated that GO are partly reduced, in accordance with the XPS results. In addition, a small peak of Si-O stretching vibration at 1085 cm-1 in Pt-G was observed, showing that a small amount of SiO still existed in the Pt-G-3. The intensity ratio of the D band to G band means the ratio of sp2 domains and the structural disorder of grapheme.31 Compared the Raman spectra of GO and Pt-G (Figure S4b), the intensity ratios of D/G bands are 0.82 and 0.97. The enlarged ratio for the Pt-G implied the enhanced disorder of the graphene, owing to the interactions among graphene, Pt and SiO, which may decrease the aggregation of graphene during catalysis. In the Pt 4f XPS spectrum of Pt-G-3 (Figure 1c), the peaks at 71.2 eV and 74.5 eV are assigned to Pt (0), while the signals at 71.9 and 75.3 are attributed to Pt in (II) states.32 The Si 2p XPS spectrum is also shown in Figure 1d, five components include Si4+, Si3+, Si2+, Si1+, Si0 are decomposed, which is in good agreement with the results in the previous literature.33 The morphology of the Pt-G was further characterized by electron microscopy. The raw SiO is micron sized geometry (Figure S6a). After ball-milling, the size is greatly decreased and uniform (Figure S6b). Figure 2a shows the SEM image of Pt-G-3. SiO and reduced Pt are dispersed on the graphene and difficult to discern. EDS spectra in Figure S7 revealed the elemental ratio of C, Si, O and Pt, which is corresponding to the ICP-MS analysis.


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Figure 2. Electron microscopy characterization for Pt-G-3: (a) SEM; (b) TEM; (c) HRTEM image showing the crystallinity of Pt nanoparticles; (d) SAED pattern of Pt-G; (e) HAADFSTEM image and corresponding EDS mapping showing C, Si, O, and Pt distributions; and (f) Schematic illustration for the fabricating of nanosponge Pt-G: left is the morphology change along with reaction time and right is an enlarged top view of a nanosponge structure of Pt-SiO.


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Under TEM investigation, Pt nanoparticles are connected with each other and dispersed on the graphene like patches (Figure 2b). The TEM images of Pt-G-1 to Pt-G-4 are shown in Figure S8. The nanoparticles show a nanosponge structure with voids inside (Figures 2b and S9). A clear lattice fringes in the HRTEM image was observed and the interplanar distance is 0.23 nm, which confirmed the Pt (111) lattice planes (Figure 2c). The corresponding SAED pattern in Figure 2d exhibits two sets of diffraction patterns. One is polycrystalline rings in accordance with (111), (200), (220), and (311) faces of the face-centered cubic Pt. Another is diffraction spots consistent with (100) plane of graphene. Unfortunately, the residual SiO is unable to clearly distinguish under SAED pattern owing to its amorphous structure. To prove that the Pt are on the SiO surface, elemental mapping by EDS was performed under STEM (Figure 2e). An overlay of Pt and Si signals demonstrates that the dispersion of Pt is in accordance with the shape of SiO. Pt nanoparticles are reduced by SiO and supported by the residual SiO. As mentioned above, the Pt nanoparticles are not simply aggregated nanoparticles, but show a nanosponge structure with voids inside, which is an interesting phenomenon and deserve further investigation. In order to clarify the reaction mechanism, we used Pt-SiO without graphene as the model to show the morphology change (Figures S10 and S11). Without the addition of HF, the surface of SiO was smooth (Figure S10a). With the extended reaction time and the aid of HF, small Pt nanoparticles appeared (Figure S10b and S11a) due to the reduction of Si-H bonds.34 The small Pt nanoparticles provided the initial template and induced the secondary nucleation and in-situ growth, which constituted large Pt nanoparticles with spongy morphology (Figure S9). Along with the reduction of Pt, the SiO was etched by HF. The surface of SiO became rough and porous (Figures S10c and S11b). The size of SiO reduced (Figures S10d and S11c) and finally totally covered by Pt nanoparticles (Figures S10e and S11d). The Pt and the residual


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SiO worked together and finally led to the nanosponge structure. The residual SiO was acted as the skeleton and connected the Pt nanoparticles and graphene. This reaction process is schematically illustrated in Figure 2f. In order to prove the reaction process intuitively, we specially used SiO with a diameter of about 4 mm as raw material (Figure S12a) to conduct experiments. When the platinum was reduced by SiO and HF, the obtained Pt clearly exhibited spongy morphology (Figure S12b). For further comparison, the XRD and TEM image of the commercial 20 wt% Pt/C are shown in Figure S13.

Electrochemical evaluation for Pt-G in HER. To test the catalytic performance of Pt-G, the HER activities were measured in N2-saturated 0.5 M H2SO4 solution and the scan rate is 5 mV·s1

. The linear sweep voltammetry (LSV) curves of Pt-G catalysts with different Pt amount are

shown in Figures 3a and S14a. The current density at the same overpotentials increased along with the increasing Pt amount and reached to the top when the mass ratio of Pt is 14.1 wt%. Then the current density showed a decline because more Pt nanoparticles exhausted most SiO and tended to be aggregation. The best Pt-G-3 catalyst was compared with 20 wt% Pt/C, Pt-G-gly, Pt-SiO, and SiO-G (Figures 3b and S14b). SiO-G showed no HER activity. The overpotentials for other catalysts that afford the current densities of 1, 10, and 120 mA cm-2 were listed in Table S2. Although the overpotentials at 10 mA·cm-2 for Pt-G-3 was slightly larger than those of 20 wt% Pt/C (∆ = 13 mV) and Pt-C-gly (∆ = 4 mV), it exceeded these two catalysts when the current densities were 108 mA·cm-2 and -23·mA·cm-2, respectively.


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Based on their CVs in 0.1 M HClO4 solution in Figure S15, the electrochemically active surface areas (ECSAs) of Pt-G-3, 20 wt% Pt/C, and Pt-G-gly catalysts were calculated. The ECSA of Pt-G-3 is 68.96 m2·g-1, which is 1.3 and 4.8 folds than those of Pt/C (52.60 m2·g-1) and Pt-G-gly (14.32 m2·g-1), respectively. The specific surfaces of the catalysts are evaluated by Brunauer-Emmett-Teller (BET) in Figure S16. The BET surface areas of Pt-G-3, 20 wt% Pt/C and Pt-G-gly are 76.7319, 164.6438 and 53.6013 m2·g-1, respectively. Both Pt-G-3 and Pt-G-gly possess smaller areas than that of 20 wt% Pt/C, which may be due to the procedure of N2 adsorption-desorption measurement during the BET test causing substantial aggregation in graphene-based materials.35 It is interesting to note that the BET curve of Pt-G-3 has a hysteresis loop in the high relative pressure, indicating the existence of micropores and mesopores. From the pore size distribution, the pores with diameter of 3.57 nm were clearly observed, which also demonstrate the nanosponge structure of Pt nanoparticles. No obvious pores are observed for 20 wt% Pt/C and Pt-G-gly. Although the BET surface area of Pt-G-3 is smaller than that of 20 wt% Pt/C, the nanosponge structure of Pt may provide more active site for HER. We also calculated the specific activity of Pt-G-3 and Pt/C based on the ECSA, as shown in Figure S17. The specific activity of Pt-G-3 exceeded that of 20 wt% Pt/C at overpotential of 0.116 V.


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Figure 3. (a) LSV curves of Pt-G catalysts; (b) LSV curves of the compared catalysts including 20 wt% Pt/C, SiO-G, Pt-G-gly and Pt-SiO; (c) Mass activities of the compared catalysts; and (d) corresponding Tafel plots derived from (b).

The mass activities at overpotentials of 0.1 and 0.2 V for these seven Pt-based catalysts are shown in Figure 3c. The mass activity orders are as follows: Pt-G-1 > Pt-G-3 > Pt-G-2 > 20 wt% Pt/C > Pt-G-4 > Pt-C-gly > Pt-SiO. The advantages of Pt-G catalysts are not obvious at overpotential of 0.1 V. However, the gap is widening at overpotential of 0.2 V. It is interesting to note that Pt-G-1 has the largest mass activity of 11.17 A·mg-1 at overpotential of 0.2 V, 2.1 folds than 20 wt% Pt/C (5.41 A·mg-1). The mass activity of Pt-G-3 is 8.45 A·mg-1, 1.6 folds than that of 20 wt% Pt/C and 1.9 folds than that of Pt-G-gly (4.55 A·mg-1). Pt-G catalysts using SiO as the reduction agent increase the utilization of Pt at large overpotentials. The hydrogen evolution by


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the catalysis of Pt-G-3 at overpotential of 200 mV is tested as shown in Figure S18, with the Faradaic yield of 95.35%. Tafel curves (Figure 3d) can be derived from the polarization curves. The Tafel slope of 35.4 mV·dec-1 for Pt-G-3 confirms that the HER kinetics follows the Tafel mechanism, which is similar as 20 wt% Pt/C (30.0 mV·dec-1) and Pt-C-gly (34.6 mV·dec-1). Without the addition of graphene, Pt-SiO showed a Tafel slope of 44.8 mV·dec-1, corresponding to the Heyrovsky mechanism. The Tafel slope data and the calculated apparent exchange current densities (j0) are listed in Table S3. The pH dependent relation of HER was also shown in Figure S19, demonstrating the reaction order of the Pt-G-3 catalyst is 2.04.

Figure 4. (a) Nyquist data for Tafel slope calculation by varying the overpotential by 5 mV increment ranging from 30 mV to 55 mV; and (b) The plot of Log (1/(RP+Rct)) against overpotential showing the Tafel slope of HER reaction.

Electrochemical impedance spectra (EIS) tests for Pt-G-3 are shown in Figure 4a. The equivalent circuit of the 2TS model (Rs-(Rp||CPE1)-(Rct||CPE2)) is inserted in EIS and the corresponding parameters are listed in Table S4. Rs is the solution resistance, Rp||CPE1 is


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associated with the electrode porosity at high-frequency, and Rct||CPE2 is associated with HER kinetics at low-frequency. Rs and Rp do not change with the applied overpotential while Rct decreases with the increasing overpotential. The total Faradaic resistance of the electrode (Rp+ Rct) was carried out to calculate the Tafel slope according the equation:36 log ቀோ

ଵ

ು ାோ೎೟

௃

ఎ

ቁ = log ቀ బ ቁ + ௕ ௕

(2)

According to the slope shown in Figure 4b, the Tafel slope is 1/0.0281 = 35.6 mV·dec-1, in accordance with the result obtained from the Tafel plot (Figure 3d). The catalytic stability is an important indicator to evaluate the catalyst. CV method was first employed for the Pt-G-3 catalyst at a scan rate of 50 mV·s-1 in 0.5 M H2SO4. After a continuous 2000 CV cycles, the polarization curves before and after were compared, showing in Figure 5a. At the overpotential of 0.1 V, the corresponding current density decrease is only 5 mA·cm-2 while the decrease for 20 wt% Pt/C and Pt-G-gly are 17 mA·cm-2 and 9 mA·cm-2 (Figures S20 and S21a). In addition, the chronopotentiometry technique was conducted for 60000 s. Compared with an obvious downtrend of current density for 20 wt% Pt/C and Pt-G-gly (Figure S21b), the measured HER current density for Pt-G-3 (Figure 5b) remained stable during the testing process. The overpotential that afforded the current density of 10 mA·cm-2 only increased 1.5 %, while that of Pt/C increased 14.9 %. The TEM image of Pt-G-3 after the stability test is shown in Figure S22. No obvious change of the morphology was observed. The XPS spectra of Pt-G-3 after stability test (Figure S23) indicates that the element of Si is almost not change after the stability test, which demonstrate the high stability of SiO in the skeleton. The excellent stability in both the CV cycling and chronopotentiometry indicated the good quality of Pt-G-3 in practical applications.


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Figure 5. Electrochemical stability: (a) LSV curves of Pt-G-3 before and after 2000 CV cycles; and (b) V-t curves of Pt-G-3 at current density of 10 mA·cm-2 in 0.5 M H2SO4. According to the above electrochemical analysis, we can obtain the following information: First, although the onset overpotential is larger than those of 20 wt% Pt/C and Pt-G-gly, the increasing rate of current density for Pt-G-3 catalyst is faster. When employing SiO as the reductive agent, its reducing capacity is lower than other reducing agent like glycol or NaBH4. The Pt nanoparticles are successfully reduced but the grephene oxides are partly reduced as confirmed by the XPS spectra.37 Also there are some unreacted SiO existed in the Pt-G catalysts, which lead to the smaller current density at lower overpotentials. However, with the increasing overpotentials, some adsorbed hydrogen atoms on Pt can migrate to the residual SiO and accelerates the desorption process, which is beneficial for the hydrogen evolution.38 So the current density of Pt-G-3 surpasses those of 20 wt% Pt/C and Pt-G-gly at higher overpotentials. Secondly, the mass activities of Pt-G-1 to Pt-G-3 are all larger than those of 20 wt% Pt/C and PtG-gly. The calculated ECSA of Pt-G-3 is also 1.3 and 4.8 folds than those of Pt/C and Pt-G-gly. This may be due to the nanosponge structure of Pt nanoparticles reduced by SiO, the formation mechanism of which is explained in the material characterization part. The initial small Pt nanoparticles induce the secondary nucleation and epitaxial in-situ growth, which constitute large Pt clusters and produce vacancies by the neighboring nanoparticles with spongy


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morphology. The SiO act as a 3 dimensional skeleton in the Pt-G-3. From the BET test (Figure S16), the pores with diameter of 3.57 nm for Pt-G-3 were clearly observed, which indicting that the nanosponge structure of Pt may provide more active sites for HER. Finally, the durability of the Pt-G-3 catalyst precedes that of 20 wt% Pt/C due to the strong bonding interaction between nanosponge Pt and graphene through SiO. The residual SiO acts as the skeleton for Pt nanoparticles and avoids aggregation of the nanosponge Pt. The hydrogen bonds between SiO and graphene also reduce the impact of graphene aggregation caused by the strong π–π stacking interaction, and improve the stability of the Pt-G-3 catalyst.

CONCLUSIONS Pt-G nanocomposites were successfully synthesized by a one-pot method introducing SiO as the reducing agent. The uniformly dispersed Pt nanoparticles showed a nanosponge structure. The unique structure is due to the secondary nucleation and in-situ growth of Pt on SiO. When employed as HER catalysts, the Pt-G catalysts showed higher mass activity than that of the commercial 20 wt% Pt/C catalyst. The ECSA ot Pt-G-3 is 4.8 times larger than that of Pt-G-gly. Besides, the stability of Pt-G-3 is also better than that of 20 wt% Pt/C due to the small quantity of residual SiO. The participation of SiO makes the synthesis process convenient and efficient. SiO not only acts as a reducing agent that does not need after treatment, but also an accessory ingredient for HER catalyst. The design strategy may be adopted in other catalyst for energy conversion technologies.


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ASSOCIATED CONTENT Supporting Information. Characterization of materials, HER performance of catalysts and useful parameters.

ACKNOWLEDGMENT The project was supported by the National Key Research and Development Program of China (2017YFA0204800), the National Natural Science Foundation of China (No.61722404), Qing Lan Project, Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions.

REFERENCES (1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I. B.; Norskov, J. K.; Jaramillo, T. F. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355, 146–178. (2) Huang, T.; Chen, Y.; Lee, J. M. Two-dimensional cobalt/N-doped carbon hybrid structure derived from metal-organic frameworks as efficient electrocatalysts for hydrogen evolution. ACS Sustain. Chem. Eng. 2017, 5, 5646–5650. (3) Seger, B.; Laursen, A. B.; Vesborg, P. C. K.; Pedersen, T.; Hansen, O.; Dahl, S.; Chorkendorff, I. Hydrogen production using a molybdenum sulfide catalyst on a titaniumprotected n plus p-silicon photocathode. Angew. Chem. In. Ed. 2012, 51, 9128–9131.


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Table of Contents

Nanosponge Pt modified graphene nanocomposites are fabricated with help of SiO and show high mass activity and stability toward hydrogen evolution.


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Figure 1. (a) XRD patterns of SiO, GO and Pt-G-3; and core-level XPS spectra of Pt-G-3 nanocomposite: (b) C 1s; (c) Pt 4f; and (d) Si 2p. 85x65mm (300 x 300 DPI)



Figure 2. Electron microscopy characterization for Pt-G-3: (a) SEM; (b) TEM; (c) HRTEM image showing the crystallinity of Pt nanoparticles; (d) SAED pattern of Pt-G; (e) HAADF-STEM image and corresponding EDS mapping showing C, Si, O, and Pt distributions; and (f) Schematic illustration for the fabricating of nanosponge Pt-G: left is the morphology change along with reaction time and right is an enlarged top view of a nanosponge structure of Pt-SiO. 150x178mm (300 x 300 DPI)


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Figure 3. (a) LSV curves of Pt-G catalysts; (b) LSV curves of the compared catalysts including 20 wt% Pt/C, SiO-G, Pt-G-gly and Pt-SiO; (c) Mass activities of the compared catalysts; and (d) corresponding Tafel plots derived from (b). 85x67mm (300 x 300 DPI)



Figure 4. (a) Nyquist data for Tafel slope calculation by varying the overpotential by 5 mV increment ranging from 30 mV to 55 mV; and (b) The plot of Log (1/(RP+Rct)) against overpotential showing the Tafel slope of HER reaction. 85x37mm (300 x 300 DPI)


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Figure 5. Electrochemical stability: (a) LSV curves of Pt-G-3 before and after 2000 CV cycles; and (b) V-t curves of Pt-G-3 at current density of 10 mA•cm-2 in 0.5 M H2SO4. 85x33mm (300 x 300 DPI)


Nanosponge Pt Modified Graphene Nanocomposites Using Silicon

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