- Email: [email protected]
SiNW nanocomposites
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Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites Wen Shen, Bin Wu, Fan Liao**, Binbin Jiang, Mingwang Shao* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, PR China
article info
abstract
Article history:
In recent years, platinum-based catalysts are the most promising candidates for hydrogen
Received 12 December 2016
evolution reaction (HER). Decrease of the Pt usage is an effective way to obtain low-cost
Received in revised form
hydrogen production. In order to achieve this goal, we synthesized Pt-Ag/SiNW
24 February 2017
composites with different dosages of Pt, using inexpensive Ag as a co-catalyst and
Accepted 20 March 2017
SiNWs as carriers, and tested their catalytic performance in HER. The results showed that
Available online xxx
the optimal composition of Pt: Ag: Si ¼ 4.1: 21.5: 74.4 in mass ratio. With such a low amount of Pt, the Pt-Ag/SiNW catalyst exhibited exciting HER performance, whose turnover fre-
Keywords:
quency is 6.3H2Pt1s1 at 0.2 V vs. RHE, 2.7 times as large as that of 40 wt% Pt/C. This
Hydrogen evolution reaction
design greatly increased the utilization ratio of Pt, which may open a new way for prep-
Silicon nanowire
aration of other Pt-based catalysts.
Nanocomposite
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Pt-based catalyst
Introduction Recently, we have to face the increasingly serious environmental pollution and the upcoming energy depletion on account of the high dependence on fossil fuels. Hydrogen has been considered as an ideal candidate in the future to replace fossil fuels due to its properties of carbon-free release, sustainability and renewability [1e4]. Nowadays, electrolysis of water, one of the simplest ways for hydrogen evolution reaction (HER), has drawn worldwide attention [5,6]. The key issue for HER is to search for the cheap electrocatalysts with excellent activity in order to produce hydrogen massively and efficiently.
Up to now, noble metals, such as platinum (Pt), have been demonstrated to be the most effective electrocatalysts for HER [7e9]. However, their scarcity and high cost prevented wide range application [10e12]. At present, inorganic catalysts including MoS2 [13e15], WS2 [16], WC [17], MoC2 and MoC [18] have become a focus of concern because of their abundance and low costs. Nevertheless, their electrocatalytic performance was still regarded as inferior to Pt-based catalysts, which makes decreasing the Pt usage an ultimate goal for the design request of electrocatalysts to achieve low-cost hydrogen production. Supported carriers are wildly adopted in the high efficient electrocatalysts for HER. For example, silicon nanowires (SiNWs), achieved via numerous methods [19,20], can effectively avoid the aggregation of nanoparticles
* Corresponding author. Fax: þ86 512 65882846. ** Corresponding author. Fax: þ86 512 65882846. E-mail addresses: [email protected] (F. Liao), [email protected] (M. Shao). http://dx.doi.org/10.1016/j.ijhydene.2017.03.110 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shen W, et al., Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.110
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grown on their surfaces, increasing the active sites of the catalysts [21,22]. Moreover, SiNWs-supported noble metals also turned out to show enhancement of the electrocatalytic activity due to the low desorption energy of hydrogen of SiNWs [22]. Here, we developed a method to dramatically decrease the usage of Pt via adding the low-cost Ag as a co-catalyst and the inexpensive silicon nanowires (SiNWs) as efficient electrocatalyst carriers. Although pure Ag has a low activity in the HER, it has good chemical stability and be adopted as excellent catalysts [23]. In addition, Ag has a lattice constant similar to that of Pt [24], which may be beneficial to reduce the stress of Pt-Ag bimetallic and improve their stability. Pt-Ag/SiNW composites with different dosages of Pt were synthesized and their catalysis in HER was conducted in the acidic medium. The results showed that the optimal composition of PtAg/SiNW composites is 4.1 (Pt): 21.5 (Ag): 74.4 (Si) in mass ratio. With such a low amount of Pt, the Pt-Ag/SiNW catalyst exhibited exciting hydrogen evolution reaction performance, whose turnover frequency (TOF) is 6.3H2Pt1s1 at 0.2 V vs. RHE, 2.7 times as large as that of 40 wt% Pt/C.
Experimental Materials The H2PtCl6$6H2O and AgNO3 were obtained from Aladdin Industrial Co.; Nafion (5 wt%) and commercial Pt/C catalysts (40 wt%) were purchased from Alfa Aesar Co. All other chemical reagents were of analytical grade without further purification. The doubly distilled water was used in the experiment.
Synthesis of Pt-Ag/SiNW, Ag/SiNW and Pt-Ag catalysts SiNWs (30 mg), synthesized via the high temperature method [19], immersed in 20 mL 1.5 mM AgNO3 solution with stirring for 30 min. 1.2 mL 5% HF aqueous solution was added for 1 h to remove the thin oxide layer and to form siliconehydrogen bonds, which can reduce the silver ions to Ag nanoparticles on the surface of SiNWs (Ag/SiNW). And then 1, 3 and 5 mL 1.5 mM H2PtCl6$6H2O solution were added for 1 h to obtained catalysts with different components, which are named Pt-Ag/ SiNW-1, Pt-Ag/SiNW-2, and Pt-Ag/SiNW-3, respectively. PtAg/SiNW-2 was immersed in the excessive 5% HF aqueous solution to etch the residual SiNWs to obtain pure Pt-Ag nanocomposites, whose mass ratio of Pt: Ag is 16: 84. In addition, SiNWs (30 mg) immersed in 20 mL distilled water and 3 mL 1.5 mM H2PtCl6$6H2O solution were added with stirring for 30 min. Next, 1.2 mL 5% HF aqueous solution was added for 1 h to prepare the catalyst with the same Pt loading on SiNWs as Pt-Ag/SiNW-2 supported.
Characterization The phase and crystallographic structure of all as-prepared samples were characterized by X-ray powder diffraction (XRD, Philips X'pert PRO MPD diffractometer) equipped with Cu Ka radiation (l ¼ 0.15406 nm). Transmission electron
microscope (TEM) image and high resolution TEM (HRTEM) image were obtained by a FEI Tecnai F20 transmission electron microscope with the accelerating voltage of 200 kV. The elemental contents of Ag and Pt in electrocatalysts were detected by Hitachi 180-80 atomic absorption spectrometer. The chemical states of the as-prepared samples were measured by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS UltraDLD ultrahigh vacuum surface analysis system with Al Ka radiation (1486 eV) as a probe and In foil as the sample holder.
Electrochemical measurements All electrochemical measurements were carried out in a traditional three-electrode system and the CHI 750 C electrochemical workstation was used to test the electrochemical performance. The glass carbon electrode (GCE) with the diameter of 3 mm, modified with the studied catalysts, was used as the working electrode. The saturated calomel electrode (SCE) was used as the reference electrode and a carbon rod was the counter electrode. The fabrication process of the working electrode is as follows: 2 mg catalysts were dispersed in the mixture of 900 mL watereisopropanol mixed solvent (VH2 O : Viso ¼ 5:1) and 100 mL Nafion solution (0.5 wt%), and ultrasonically stirred to form homogeneous suspension. Afterwards, 6 mL of suspension was loaded onto the GCE with loading of 0.170 mg cm2, and then dried naturally.
Results and discussion Characterization of Pt-Ag/SiNWs catalysts XRD patterns of SiNWs, Ag/SiNW, Pt-Ag/SiNW-2 and Pt-Ag nanocomposites are shown in Fig. 1A. All diffraction peaks in Curve a, which shows the XRD pattern of SiNWs, can be identified to pure phase of cubic Si. The cell parameter of SiNWs is calculated to be a ¼ 5.430 ± 0.002 A, corresponding with the theoretical value (a ¼ 5.430 A, JCPDS data No. 271402). Platinum and silver ions can be easily reduced by SieH bonds, which were produced by removing the thin oxide layer on the surface of SiNWs in the HF aqueous solution [21]. Therefore, Ag and Pt nanoparticles can grow on the SiNWs successfully. Compared with Curve a, new peaks appeared in Curve b with the cell parameter calculated as a ¼ 4.090 ± 0.002 A, which is in great agreement with reported value of Ag (a ¼ 4.086 A, JCPDS data No. 04-0783). There is no obvious peak of Pt in Curve c because of the small content of Pt in Pt-Ag/SiNWs. The crystallite size of catalysts were determined using the Scherrer equation: D ¼ 0.9 l/b cosq, where l is the X-ray wavelength for Cu Ka (0.15406 nm) and b is the full width half maximum (FWHM) in radians. The size of the Pt-Ag nanoparticles in the Pt-Ag/SiNW-2 is 17.7 nm, based on the (111) diffraction peak. After the removal of SiNWs, the peaks in Curves d were broadened due to the emergence of Pt peak. The locations of the diffraction peaks of Ag and Pt are too close to be distinguished. The XPS spectra of Pt-Ag/SiNWs were recorded in Fig. S1 (Supporting materials), which show the existence of Pt, Ag and Si elements.
Please cite this article in press as: Shen W, et al., Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.110
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Fig. 1 e (A) XRD patterns of SiNWs, Ag/SiNW, Pt-Ag/SiNW-2 and Pt-Ag; (B) The TEM image of Pt-Ag/SiNW-2 catalysts with the scale bar of 200 nm, the inset being the diameter distribution of 200 Pt-Ag nanoparticles; (C) The HRTEM image of Pt-Ag/ SiNW-2 with the scale bar of 2 nm; (D) HAADF-STEM image of Pt-Ag/SiNW-2 with the scale bar of 50 nm; and (E, F and G) its corresponding EDS mapping showing elemental distribution of Si, Ag and Pt, respectively.
TEM image (Fig. 1B) shows the morphology of Pt-Ag/SiNW2 in detail. A single SiNW, which has a diameter of 180 nm, was covered by Pt-Ag nanoparticles. The inset is the diameter distribution of 200 Pt-Ag nanoparticles, which shows that the average size of Pt-Ag nanoparticles is 17.8 nm, corresponding to the XRD results. High-resolution TEM (HRTEM) (Fig. 1C) displays Pt-Ag (111) plane with a lattice spacing of 0.23 nm and Si (111) plane consistent with the lattice fringe of 0.31 nm. High angle annular dark field scanning TEM (HAADF-STEM) image of Pt-Ag/SiNW catalysts is shown in Fig. 1D and the corresponding energy-dispersive X-ray spectroscopy (EDS) mapping (Fig. 1EeG) reveals the elemental distribution of silicon (orange), silver (green) and platinum (blue), respectively. Moreover, the TEM image (Fig. S2, Supporting materials) of PtAg nanoparticles reveals that they all aggregate together after the removal of SiNWs. Also the HRTEM image of the prepared pure Pt-Ag is displayed in Fig. S2 (Supporting materials).
Electrochemical characterization In a three-electrode configuration, the HER performance of different catalysts fabricated onto GCE were investigated by linear sweep voltammetry (LSV) in N2-saturated 0.5 M H2SO4 at a scan rate of 5 mV s1 (Fig. 2A). The 40 wt% commercial Pt/C is tested as a comparison, which exhibits superior HER
performance with a near-zero onset overpotential and high current density as expected. Ag/SiNW shows negligible HER activity. However, it is noted that the addition of small amount of Pt boosts the HER performance. Pt-Ag/SiNW-1, PtAg/SiNW-2, Pt-Ag/SiNW-3 and 40 wt% Pt/C require overpotential of 170, 135, 150 and 50 mV to afford current density of 10 mA cm2, respectively. The overpotential of 135 mV is much better than the Ag-based catalysts [25]. The electrocatalytic activity increased first with the increasing Pt content, and decreased. According to the metal contents of Pt-Ag/SiNW catalysts listed in Table 1, the optimal mass ratio of Pt: Ag: Si is 4.1: 21.5: 74.4, which is Pt-Ag/SiNW-2. Additionally, the pure Pt-Ag nanocomposites impair the HER performance with higher overpotentials than those of Pt-Ag/SiNW catalysts at the same current densities because the increased agglomeration tendency of Pt-Ag nanocomposites without the support of SiNWs may reduce the surface area [22]. The Tafel slopes of each catalysts derived from the polarization curves are shown in Fig. 2B, which were fitted into the Tafel equation (h ¼ b$log (j) þ a, where j is the current density and b is the Tafel slope). The Tafel slope of 40% Pt/C is 30 mV dec1, which is consistent with the reported value [26,27], therefore demonstrating the reliability of the electrochemical measurement we used. For a smaller Tafel slope, the catalysts will exhibit higher HER activity [28,29]. Pt-Ag/SiNW-2
Please cite this article in press as: Shen W, et al., Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.110
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Fig. 2 e (A) Polarization curves of Ag/SiNW, Pt-Ag, Pt-Ag/SiNW and 40 wt% Pt/C catalysts in oxygen-free H2SO4 for HER; and (B) The corresponding Tafel plots showing Tafel slopes of Pt-Ag, Pt-Ag/SiNW and 40 wt% Pt/C derived from Fig. 2A.
Table 1 e The raw materials and metal contents of Pt-Ag/ SiNW catalysts. Raw materials
Pt-Ag/SiNW-1 Pt-Ag/SiNW-2 Pt-Ag/SiNW-3
Catalysts
1.5 mM Agþ (mL)
1.5 mM Ptþ (mL)
Ag (wt%)
Pt (wt%)
20 20 20
1 3 5
21.8% 21.5% 21.1%
1.4% 4.1% 6.8%
exhibits a Tafel slope of 70 mV dec1, which is smaller than those of Pt-Ag/SiNW-1, Pt-Ag/SiNW-3 and Pt-Ag with the Tafel slopes of 79, 84 and 91 mV dec1, respectively, demonstrating that the appropriate mass ratio of Pt: Ag: Si in the Pt-Ag/SiNW catalysts may be beneficial to the HER performance. In the meantime, the Tafel slope can be used to determine the HER mechanism. The Tafel slope of Pt-Ag/SiNW-2 reveals that it belongs to a VolmereHeyrovsky mechanism. Moreover, the electrocatalytic performance of the catalyst with almost the same Pt loading as Pt-Ag/SiNW-2 is shown in Fig. S3 (Supporting materials). It requires an overpotential of 190 mV to afford a current density of 10 mA cm2 with the Tafel slope of 119 mV dec1. Both the overpotential and Tafel
slope are much larger than those of the Pt-Ag/SiNW-2, indicating the synergistic effect in Pt-Ag/SiNW-2 catalysts. Given the best HER performance of Pt-Ag/SiNW-2, the electrochemical impedance spectroscopy (EIS) of this catalyst was further investigated at different overpotentials with the frequency ranging from 0.01 Hz to 10,000 Hz to understand the electrode kinetics of HER. The Nyquist plot (Fig. 3A) with different overpotential shows a higher-frequency process related to the double-layer capacitance and a lower-frequency component represented the H species coverage. The inset is the equivalent circuit, in which Rs is the solution resistance, Rct is the charge-transfer resistance, Cdl is the double layer capacitance, Rsa is the surface adsorption resistance, Csa is the surface adsorption capacitance, and L is the surface adsorption inductance. The plot of log((Rct þ Rsa)1) vs. overpotential (Fig. 3B) reveals a Tafel slope of 69.9 mV dec1, which is in agreement with the value derived by the polarization curve for Pt-Ag/SiNW-2 [30]. The corresponding Bode plots were shown in Fig. S4 (Supporting materials), while the Nyquist plots and Bode plots of Pt-Ag/SiNW-1, Pt-Ag/SiNW-3 and Pt-Ag are in Fig. S5 (Supporting materials). And the apparent exchange current densities (j0, app) and real exchange current densities (j0, real) of Pt-Ag/SiNW catalysts are calculated and listed in Table S1 (Supporting materials), which show that Pt-Ag/ SiNW-2 has the largest apparent and real exchange current densities.
Fig. 3 e (A) Nyquist plots of Pt-Ag/SiNW-2 at various overpotentials. Symbols are the experimental data and solid lines are the fitted data. The inset displays the relevant equivalent circuit model. And (B) the plot of log ((Rct þ Rst)¡1) vs. overpotential reveals the Tafel slope of HER. Please cite this article in press as: Shen W, et al., Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.110
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Fig. 4 e (A) Tafel plots of Pt-Ag/SiNW-2 catalysts measured in x H2SO4 (x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5 M) for HER; (B) the curve of cathodic current density (log j) at ¡0.1 V vs. RHE against pH; (C) Tafel plots of Pt-Ag/SiNW-2 catalysts measured in 0.5 M H2SO4 at different temperatures (298, 308, 318, 328 and 338 K) for HER; and (D) Arrhenius plot.
The effects of pH and temperature on HER also have been measured. LSV curves were carried out in oxygen-free xM H2SO4 (x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5 M) with a sweep rate of 5 mV s1. The corresponding Tafel curves (Fig. 4A) derived from LSV curves shows the potential region of HER. It is obvious that the current density enlarges at the same overpotential with the increasing concentration of hydrogen ions. The relationship between cathodic current density of HER and pH is shown in Fig. 4B and the reaction order can be determined as 2.13. LSV curves in oxygen free 0.5 M H2SO4 solution at different temperatures ranging from 298 K to 338 K with the sweep rate of 5 mV s1 are tested to calculate the activation energy of the whole reaction. The corresponding Tafel curves are shown in Fig. 4C. The HER performance of Pt-Ag/SiNW-2 become better (lower onset potential and larger cathodic current density) with the increase of reaction temperature. Furthermore, the exchange current density (j0) can be derived from the Tafel curves. The Arrhenius plots showing the linear function of log j0 against T1 is presented in Fig. 4D. The electrochemical activation energy (DG0) is calculated to be 14.9 kJ mol1 according to the Arrhenius equation [20] (log j0 ¼ log (KFC)eDG0/ (2.303RT), where R is gas constant and the DG0 is the apparent activation energy). The small activation energy indicates the promising HER performance of Pt-Ag/SiNW-2 catalysts. The stability (Fig. 5) is measured in acidic electrolyte by jet method at 0.2 V (vs. RHE) because it is an important factor to evaluate the HER performance. The result shows that only a
slight loss of cathodic current can be observed for both Pt-Ag/ SiNW-2 and 40% Pt/C during 18,000 s. However, the cathodic current density of Pt-Ag decreases to almost zero. TEM images of the Pt-Ag nanocomposites, Pt-Ag/SiNW-2 and 40% Pt/C before and after the durability test were shown in Fig. S2 in the Supporting materials. Figs. S2A and S2C (Supporting materials) show that Pt-Ag/SiNW-2 keeps its previous morphology and 40 wt% Pt/C aggregates after
Fig. 5 e jet plots of Pt-Ag/SiNW-2, Pt-Ag and 40 wt% Pt/C catalysts at ¡0.2 V (vs. RHE) in oxygen-free 0.5 M H2SO4.
Please cite this article in press as: Shen W, et al., Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.110
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18,000 s' catalysis; Fig. S2E (Supporting materials) indicates that the pure Pt-Ag, aggregating after the removal of SiNWs, clusters more seriously after the stability test. The turnover frequency (TOF) is widely used in the molecular catalysis field to assess the efficiency of catalysts, which can also be employed for electrocatalytic reactions. The TOF is the mass of molecules reacting for a certain reaction per unit time, which can be calculated using the equation [31]: TOF ¼
jMPt 2FmPt
where j is the current density at a given potential, MPt is atomic weight of Pt, F is Faraday's constant, and mPt is the mass of per square centimeter of Pt in the catalyst. The calculated TOF for AgPt/SiNW-2 is 6.3H2Pt1s1 at 0.2 V (vs. RHE), which is 1.7 times larger than that of 40 wt% Pt/C (2.3H2 Pt1 s1).
Conclusions In this work, we synthesized an efficient Pt-Ag/SiNW catalyst for HER with a small amount of Pt, in which SiNWs were a good carrier with reasonably large surface areas. Pt-Ag/SiNW2 with the mass ratio of 21.5: 4.1: 74.4 (Ag: Pt: Si) exhibited exciting HER performance with a high TOF value (2.8H2Pt1s1 at 0.2 V vs. RHE), 2.7 times as large as that of 40 wt% Pt/C. The synergistic effect between two metals and the SiNWs with low H-desorption energy behavior could improve the HER performance, which can be widely applied in energy conversion.
Acknowledgments The project was supported by Major Research Plan of National Natural Science Foundation of China (No. 91433111), Qing Lan Project, Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.110.
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Please cite this article in press as: Shen W, et al., Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.110
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ScienceDirect journal homepage: www.elsevier.com/locate/he
Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites Wen Shen, Bin Wu, Fan Liao**, Binbin Jiang, Mingwang Shao* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, PR China
article info
abstract
Article history:
In recent years, platinum-based catalysts are the most promising candidates for hydrogen
Received 12 December 2016
evolution reaction (HER). Decrease of the Pt usage is an effective way to obtain low-cost
Received in revised form
hydrogen production. In order to achieve this goal, we synthesized Pt-Ag/SiNW
24 February 2017
composites with different dosages of Pt, using inexpensive Ag as a co-catalyst and
Accepted 20 March 2017
SiNWs as carriers, and tested their catalytic performance in HER. The results showed that
Available online xxx
the optimal composition of Pt: Ag: Si ¼ 4.1: 21.5: 74.4 in mass ratio. With such a low amount of Pt, the Pt-Ag/SiNW catalyst exhibited exciting HER performance, whose turnover fre-
Keywords:
quency is 6.3H2Pt1s1 at 0.2 V vs. RHE, 2.7 times as large as that of 40 wt% Pt/C. This
Hydrogen evolution reaction
design greatly increased the utilization ratio of Pt, which may open a new way for prep-
Silicon nanowire
aration of other Pt-based catalysts.
Nanocomposite
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Pt-based catalyst
Introduction Recently, we have to face the increasingly serious environmental pollution and the upcoming energy depletion on account of the high dependence on fossil fuels. Hydrogen has been considered as an ideal candidate in the future to replace fossil fuels due to its properties of carbon-free release, sustainability and renewability [1e4]. Nowadays, electrolysis of water, one of the simplest ways for hydrogen evolution reaction (HER), has drawn worldwide attention [5,6]. The key issue for HER is to search for the cheap electrocatalysts with excellent activity in order to produce hydrogen massively and efficiently.
Up to now, noble metals, such as platinum (Pt), have been demonstrated to be the most effective electrocatalysts for HER [7e9]. However, their scarcity and high cost prevented wide range application [10e12]. At present, inorganic catalysts including MoS2 [13e15], WS2 [16], WC [17], MoC2 and MoC [18] have become a focus of concern because of their abundance and low costs. Nevertheless, their electrocatalytic performance was still regarded as inferior to Pt-based catalysts, which makes decreasing the Pt usage an ultimate goal for the design request of electrocatalysts to achieve low-cost hydrogen production. Supported carriers are wildly adopted in the high efficient electrocatalysts for HER. For example, silicon nanowires (SiNWs), achieved via numerous methods [19,20], can effectively avoid the aggregation of nanoparticles
* Corresponding author. Fax: þ86 512 65882846. ** Corresponding author. Fax: þ86 512 65882846. E-mail addresses: [email protected] (F. Liao), [email protected] (M. Shao). http://dx.doi.org/10.1016/j.ijhydene.2017.03.110 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shen W, et al., Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.110
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grown on their surfaces, increasing the active sites of the catalysts [21,22]. Moreover, SiNWs-supported noble metals also turned out to show enhancement of the electrocatalytic activity due to the low desorption energy of hydrogen of SiNWs [22]. Here, we developed a method to dramatically decrease the usage of Pt via adding the low-cost Ag as a co-catalyst and the inexpensive silicon nanowires (SiNWs) as efficient electrocatalyst carriers. Although pure Ag has a low activity in the HER, it has good chemical stability and be adopted as excellent catalysts [23]. In addition, Ag has a lattice constant similar to that of Pt [24], which may be beneficial to reduce the stress of Pt-Ag bimetallic and improve their stability. Pt-Ag/SiNW composites with different dosages of Pt were synthesized and their catalysis in HER was conducted in the acidic medium. The results showed that the optimal composition of PtAg/SiNW composites is 4.1 (Pt): 21.5 (Ag): 74.4 (Si) in mass ratio. With such a low amount of Pt, the Pt-Ag/SiNW catalyst exhibited exciting hydrogen evolution reaction performance, whose turnover frequency (TOF) is 6.3H2Pt1s1 at 0.2 V vs. RHE, 2.7 times as large as that of 40 wt% Pt/C.
Experimental Materials The H2PtCl6$6H2O and AgNO3 were obtained from Aladdin Industrial Co.; Nafion (5 wt%) and commercial Pt/C catalysts (40 wt%) were purchased from Alfa Aesar Co. All other chemical reagents were of analytical grade without further purification. The doubly distilled water was used in the experiment.
Synthesis of Pt-Ag/SiNW, Ag/SiNW and Pt-Ag catalysts SiNWs (30 mg), synthesized via the high temperature method [19], immersed in 20 mL 1.5 mM AgNO3 solution with stirring for 30 min. 1.2 mL 5% HF aqueous solution was added for 1 h to remove the thin oxide layer and to form siliconehydrogen bonds, which can reduce the silver ions to Ag nanoparticles on the surface of SiNWs (Ag/SiNW). And then 1, 3 and 5 mL 1.5 mM H2PtCl6$6H2O solution were added for 1 h to obtained catalysts with different components, which are named Pt-Ag/ SiNW-1, Pt-Ag/SiNW-2, and Pt-Ag/SiNW-3, respectively. PtAg/SiNW-2 was immersed in the excessive 5% HF aqueous solution to etch the residual SiNWs to obtain pure Pt-Ag nanocomposites, whose mass ratio of Pt: Ag is 16: 84. In addition, SiNWs (30 mg) immersed in 20 mL distilled water and 3 mL 1.5 mM H2PtCl6$6H2O solution were added with stirring for 30 min. Next, 1.2 mL 5% HF aqueous solution was added for 1 h to prepare the catalyst with the same Pt loading on SiNWs as Pt-Ag/SiNW-2 supported.
Characterization The phase and crystallographic structure of all as-prepared samples were characterized by X-ray powder diffraction (XRD, Philips X'pert PRO MPD diffractometer) equipped with Cu Ka radiation (l ¼ 0.15406 nm). Transmission electron
microscope (TEM) image and high resolution TEM (HRTEM) image were obtained by a FEI Tecnai F20 transmission electron microscope with the accelerating voltage of 200 kV. The elemental contents of Ag and Pt in electrocatalysts were detected by Hitachi 180-80 atomic absorption spectrometer. The chemical states of the as-prepared samples were measured by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS UltraDLD ultrahigh vacuum surface analysis system with Al Ka radiation (1486 eV) as a probe and In foil as the sample holder.
Electrochemical measurements All electrochemical measurements were carried out in a traditional three-electrode system and the CHI 750 C electrochemical workstation was used to test the electrochemical performance. The glass carbon electrode (GCE) with the diameter of 3 mm, modified with the studied catalysts, was used as the working electrode. The saturated calomel electrode (SCE) was used as the reference electrode and a carbon rod was the counter electrode. The fabrication process of the working electrode is as follows: 2 mg catalysts were dispersed in the mixture of 900 mL watereisopropanol mixed solvent (VH2 O : Viso ¼ 5:1) and 100 mL Nafion solution (0.5 wt%), and ultrasonically stirred to form homogeneous suspension. Afterwards, 6 mL of suspension was loaded onto the GCE with loading of 0.170 mg cm2, and then dried naturally.
Results and discussion Characterization of Pt-Ag/SiNWs catalysts XRD patterns of SiNWs, Ag/SiNW, Pt-Ag/SiNW-2 and Pt-Ag nanocomposites are shown in Fig. 1A. All diffraction peaks in Curve a, which shows the XRD pattern of SiNWs, can be identified to pure phase of cubic Si. The cell parameter of SiNWs is calculated to be a ¼ 5.430 ± 0.002 A, corresponding with the theoretical value (a ¼ 5.430 A, JCPDS data No. 271402). Platinum and silver ions can be easily reduced by SieH bonds, which were produced by removing the thin oxide layer on the surface of SiNWs in the HF aqueous solution [21]. Therefore, Ag and Pt nanoparticles can grow on the SiNWs successfully. Compared with Curve a, new peaks appeared in Curve b with the cell parameter calculated as a ¼ 4.090 ± 0.002 A, which is in great agreement with reported value of Ag (a ¼ 4.086 A, JCPDS data No. 04-0783). There is no obvious peak of Pt in Curve c because of the small content of Pt in Pt-Ag/SiNWs. The crystallite size of catalysts were determined using the Scherrer equation: D ¼ 0.9 l/b cosq, where l is the X-ray wavelength for Cu Ka (0.15406 nm) and b is the full width half maximum (FWHM) in radians. The size of the Pt-Ag nanoparticles in the Pt-Ag/SiNW-2 is 17.7 nm, based on the (111) diffraction peak. After the removal of SiNWs, the peaks in Curves d were broadened due to the emergence of Pt peak. The locations of the diffraction peaks of Ag and Pt are too close to be distinguished. The XPS spectra of Pt-Ag/SiNWs were recorded in Fig. S1 (Supporting materials), which show the existence of Pt, Ag and Si elements.
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Fig. 1 e (A) XRD patterns of SiNWs, Ag/SiNW, Pt-Ag/SiNW-2 and Pt-Ag; (B) The TEM image of Pt-Ag/SiNW-2 catalysts with the scale bar of 200 nm, the inset being the diameter distribution of 200 Pt-Ag nanoparticles; (C) The HRTEM image of Pt-Ag/ SiNW-2 with the scale bar of 2 nm; (D) HAADF-STEM image of Pt-Ag/SiNW-2 with the scale bar of 50 nm; and (E, F and G) its corresponding EDS mapping showing elemental distribution of Si, Ag and Pt, respectively.
TEM image (Fig. 1B) shows the morphology of Pt-Ag/SiNW2 in detail. A single SiNW, which has a diameter of 180 nm, was covered by Pt-Ag nanoparticles. The inset is the diameter distribution of 200 Pt-Ag nanoparticles, which shows that the average size of Pt-Ag nanoparticles is 17.8 nm, corresponding to the XRD results. High-resolution TEM (HRTEM) (Fig. 1C) displays Pt-Ag (111) plane with a lattice spacing of 0.23 nm and Si (111) plane consistent with the lattice fringe of 0.31 nm. High angle annular dark field scanning TEM (HAADF-STEM) image of Pt-Ag/SiNW catalysts is shown in Fig. 1D and the corresponding energy-dispersive X-ray spectroscopy (EDS) mapping (Fig. 1EeG) reveals the elemental distribution of silicon (orange), silver (green) and platinum (blue), respectively. Moreover, the TEM image (Fig. S2, Supporting materials) of PtAg nanoparticles reveals that they all aggregate together after the removal of SiNWs. Also the HRTEM image of the prepared pure Pt-Ag is displayed in Fig. S2 (Supporting materials).
Electrochemical characterization In a three-electrode configuration, the HER performance of different catalysts fabricated onto GCE were investigated by linear sweep voltammetry (LSV) in N2-saturated 0.5 M H2SO4 at a scan rate of 5 mV s1 (Fig. 2A). The 40 wt% commercial Pt/C is tested as a comparison, which exhibits superior HER
performance with a near-zero onset overpotential and high current density as expected. Ag/SiNW shows negligible HER activity. However, it is noted that the addition of small amount of Pt boosts the HER performance. Pt-Ag/SiNW-1, PtAg/SiNW-2, Pt-Ag/SiNW-3 and 40 wt% Pt/C require overpotential of 170, 135, 150 and 50 mV to afford current density of 10 mA cm2, respectively. The overpotential of 135 mV is much better than the Ag-based catalysts [25]. The electrocatalytic activity increased first with the increasing Pt content, and decreased. According to the metal contents of Pt-Ag/SiNW catalysts listed in Table 1, the optimal mass ratio of Pt: Ag: Si is 4.1: 21.5: 74.4, which is Pt-Ag/SiNW-2. Additionally, the pure Pt-Ag nanocomposites impair the HER performance with higher overpotentials than those of Pt-Ag/SiNW catalysts at the same current densities because the increased agglomeration tendency of Pt-Ag nanocomposites without the support of SiNWs may reduce the surface area [22]. The Tafel slopes of each catalysts derived from the polarization curves are shown in Fig. 2B, which were fitted into the Tafel equation (h ¼ b$log (j) þ a, where j is the current density and b is the Tafel slope). The Tafel slope of 40% Pt/C is 30 mV dec1, which is consistent with the reported value [26,27], therefore demonstrating the reliability of the electrochemical measurement we used. For a smaller Tafel slope, the catalysts will exhibit higher HER activity [28,29]. Pt-Ag/SiNW-2
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Fig. 2 e (A) Polarization curves of Ag/SiNW, Pt-Ag, Pt-Ag/SiNW and 40 wt% Pt/C catalysts in oxygen-free H2SO4 for HER; and (B) The corresponding Tafel plots showing Tafel slopes of Pt-Ag, Pt-Ag/SiNW and 40 wt% Pt/C derived from Fig. 2A.
Table 1 e The raw materials and metal contents of Pt-Ag/ SiNW catalysts. Raw materials
Pt-Ag/SiNW-1 Pt-Ag/SiNW-2 Pt-Ag/SiNW-3
Catalysts
1.5 mM Agþ (mL)
1.5 mM Ptþ (mL)
Ag (wt%)
Pt (wt%)
20 20 20
1 3 5
21.8% 21.5% 21.1%
1.4% 4.1% 6.8%
exhibits a Tafel slope of 70 mV dec1, which is smaller than those of Pt-Ag/SiNW-1, Pt-Ag/SiNW-3 and Pt-Ag with the Tafel slopes of 79, 84 and 91 mV dec1, respectively, demonstrating that the appropriate mass ratio of Pt: Ag: Si in the Pt-Ag/SiNW catalysts may be beneficial to the HER performance. In the meantime, the Tafel slope can be used to determine the HER mechanism. The Tafel slope of Pt-Ag/SiNW-2 reveals that it belongs to a VolmereHeyrovsky mechanism. Moreover, the electrocatalytic performance of the catalyst with almost the same Pt loading as Pt-Ag/SiNW-2 is shown in Fig. S3 (Supporting materials). It requires an overpotential of 190 mV to afford a current density of 10 mA cm2 with the Tafel slope of 119 mV dec1. Both the overpotential and Tafel
slope are much larger than those of the Pt-Ag/SiNW-2, indicating the synergistic effect in Pt-Ag/SiNW-2 catalysts. Given the best HER performance of Pt-Ag/SiNW-2, the electrochemical impedance spectroscopy (EIS) of this catalyst was further investigated at different overpotentials with the frequency ranging from 0.01 Hz to 10,000 Hz to understand the electrode kinetics of HER. The Nyquist plot (Fig. 3A) with different overpotential shows a higher-frequency process related to the double-layer capacitance and a lower-frequency component represented the H species coverage. The inset is the equivalent circuit, in which Rs is the solution resistance, Rct is the charge-transfer resistance, Cdl is the double layer capacitance, Rsa is the surface adsorption resistance, Csa is the surface adsorption capacitance, and L is the surface adsorption inductance. The plot of log((Rct þ Rsa)1) vs. overpotential (Fig. 3B) reveals a Tafel slope of 69.9 mV dec1, which is in agreement with the value derived by the polarization curve for Pt-Ag/SiNW-2 [30]. The corresponding Bode plots were shown in Fig. S4 (Supporting materials), while the Nyquist plots and Bode plots of Pt-Ag/SiNW-1, Pt-Ag/SiNW-3 and Pt-Ag are in Fig. S5 (Supporting materials). And the apparent exchange current densities (j0, app) and real exchange current densities (j0, real) of Pt-Ag/SiNW catalysts are calculated and listed in Table S1 (Supporting materials), which show that Pt-Ag/ SiNW-2 has the largest apparent and real exchange current densities.
Fig. 3 e (A) Nyquist plots of Pt-Ag/SiNW-2 at various overpotentials. Symbols are the experimental data and solid lines are the fitted data. The inset displays the relevant equivalent circuit model. And (B) the plot of log ((Rct þ Rst)¡1) vs. overpotential reveals the Tafel slope of HER. Please cite this article in press as: Shen W, et al., Optimizing the hydrogen evolution reaction by shrinking Pt amount in Pt-Ag/SiNW nanocomposites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.110
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Fig. 4 e (A) Tafel plots of Pt-Ag/SiNW-2 catalysts measured in x H2SO4 (x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5 M) for HER; (B) the curve of cathodic current density (log j) at ¡0.1 V vs. RHE against pH; (C) Tafel plots of Pt-Ag/SiNW-2 catalysts measured in 0.5 M H2SO4 at different temperatures (298, 308, 318, 328 and 338 K) for HER; and (D) Arrhenius plot.
The effects of pH and temperature on HER also have been measured. LSV curves were carried out in oxygen-free xM H2SO4 (x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5 M) with a sweep rate of 5 mV s1. The corresponding Tafel curves (Fig. 4A) derived from LSV curves shows the potential region of HER. It is obvious that the current density enlarges at the same overpotential with the increasing concentration of hydrogen ions. The relationship between cathodic current density of HER and pH is shown in Fig. 4B and the reaction order can be determined as 2.13. LSV curves in oxygen free 0.5 M H2SO4 solution at different temperatures ranging from 298 K to 338 K with the sweep rate of 5 mV s1 are tested to calculate the activation energy of the whole reaction. The corresponding Tafel curves are shown in Fig. 4C. The HER performance of Pt-Ag/SiNW-2 become better (lower onset potential and larger cathodic current density) with the increase of reaction temperature. Furthermore, the exchange current density (j0) can be derived from the Tafel curves. The Arrhenius plots showing the linear function of log j0 against T1 is presented in Fig. 4D. The electrochemical activation energy (DG0) is calculated to be 14.9 kJ mol1 according to the Arrhenius equation [20] (log j0 ¼ log (KFC)eDG0/ (2.303RT), where R is gas constant and the DG0 is the apparent activation energy). The small activation energy indicates the promising HER performance of Pt-Ag/SiNW-2 catalysts. The stability (Fig. 5) is measured in acidic electrolyte by jet method at 0.2 V (vs. RHE) because it is an important factor to evaluate the HER performance. The result shows that only a
slight loss of cathodic current can be observed for both Pt-Ag/ SiNW-2 and 40% Pt/C during 18,000 s. However, the cathodic current density of Pt-Ag decreases to almost zero. TEM images of the Pt-Ag nanocomposites, Pt-Ag/SiNW-2 and 40% Pt/C before and after the durability test were shown in Fig. S2 in the Supporting materials. Figs. S2A and S2C (Supporting materials) show that Pt-Ag/SiNW-2 keeps its previous morphology and 40 wt% Pt/C aggregates after
Fig. 5 e jet plots of Pt-Ag/SiNW-2, Pt-Ag and 40 wt% Pt/C catalysts at ¡0.2 V (vs. RHE) in oxygen-free 0.5 M H2SO4.
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18,000 s' catalysis; Fig. S2E (Supporting materials) indicates that the pure Pt-Ag, aggregating after the removal of SiNWs, clusters more seriously after the stability test. The turnover frequency (TOF) is widely used in the molecular catalysis field to assess the efficiency of catalysts, which can also be employed for electrocatalytic reactions. The TOF is the mass of molecules reacting for a certain reaction per unit time, which can be calculated using the equation [31]: TOF ¼
jMPt 2FmPt
where j is the current density at a given potential, MPt is atomic weight of Pt, F is Faraday's constant, and mPt is the mass of per square centimeter of Pt in the catalyst. The calculated TOF for AgPt/SiNW-2 is 6.3H2Pt1s1 at 0.2 V (vs. RHE), which is 1.7 times larger than that of 40 wt% Pt/C (2.3H2 Pt1 s1).
Conclusions In this work, we synthesized an efficient Pt-Ag/SiNW catalyst for HER with a small amount of Pt, in which SiNWs were a good carrier with reasonably large surface areas. Pt-Ag/SiNW2 with the mass ratio of 21.5: 4.1: 74.4 (Ag: Pt: Si) exhibited exciting HER performance with a high TOF value (2.8H2Pt1s1 at 0.2 V vs. RHE), 2.7 times as large as that of 40 wt% Pt/C. The synergistic effect between two metals and the SiNWs with low H-desorption energy behavior could improve the HER performance, which can be widely applied in energy conversion.
Acknowledgments The project was supported by Major Research Plan of National Natural Science Foundation of China (No. 91433111), Qing Lan Project, Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.110.
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