Tuning the confinement space of N-carbon shell-coated ruthenium nanoparticles: highly efficient electrocatalysts for hydrogen evolution reaction

Xianlang Chen , Jian Zheng , Xing Zhong *, Yihan Jin , Guilin Zhuang , Xiaonian Li , Shengwei Deng and Jian-guo Wang *
Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou 310032, China. E-mail: zhongx@zjut.edu.cn; jgw@zjut.edu.cn

Received 29th July 2017 , Accepted 18th September 2017

First published on 19th September 2017


Development of efficient and durable catalysts for the hydrogen evolution reaction (HER) in an alkaline system is vital for the transformation of renewable energy into hydrogen fuel. In this study, we report the difference in the activity of semi- and fully-encapsulated Ru catalysts based on the effect of confined space. The fully-encapsulated Ru catalyst with porous nitrogen-doped carbon (5.0% F-Ru@PNC-800) displayed outstanding HER performance, a low overpotential of only 28 mV at 10 mA cm−2, and excellent stability. The fully-encapsulated Ru catalyst performs better than the semi-encapsulated Ru catalyst (5.0% S-Ru@PNC-800). Density functional theory calculation revealed that the different space sizes of carbon layers affect the charge transfer of the Ru nanoparticles and the carbon surface, leading to different activities. This work demonstrates that the control of confined space is an important strategy for designing highly efficient catalysts for energy conversion.


Introduction

Hydrogen, a clean renewable energy source with a high specific energy density, exhibits potential to replace other fuels (e.g., coal, methane and gasoline) and alleviate energy shortage in the future.1–6 An effective and simple method for producing high-purity hydrogen by electrocatalytic splitting of water has attracted increasing attention because of some particular advantages.7–13 Although the reaction rate of hydrogen evolution reaction (HER) in acidic solutions is approximately two to three orders of magnitude higher than that in alkaline solutions,14 considering overall water splitting for practical applications, the HER and oxygen evolution reaction (OER) of electrocatalysts are generally performed in basic media.15,16 Thus, developing highly efficient HER catalysts applied in alkaline solutions is important. Platinum (Pt)-based catalysts are currently the best electrocatalysts for HER. However, the HER performance of Pt-based catalysts under alkaline conditions remains unsatisfactory.17 In addition, the low abundance and high price of these catalysts considerably affect the extensive application of Pt.18,19 Recently, a number of Pt replacement catalysts have been reported;20–25 such catalysts exhibit low overpotential and high current density. Meanwhile, selecting suitable electrocatalysts remains challenging because of the following: (1) the HER behavior is hampered by the limited surface active sites26 and (2) issues on the properties and structural stability of electrocatalysts in harsh environments.27 To address these limitations, researchers have focused on designing and fabricating ideal HER electrocatalysts with excellent catalytic activity.

As a cheap replacement for Pt,28,29 ruthenium (Ru) possesses a hydrogen bonding strength of −65 kcal mol−1,30 and may exhibit considerable potential for HER or may even be preferred over Pt/C catalysts.28,29,31,32 Ru-based catalysts easily dissolve in alkaline solutions, thereby exhibiting limited activity and stability, which restrict their application.33–35 Encapsulating metal nanoparticles in carbon is one of the most attractive strategies used to improve electrocatalytic properties36–39 because carbon layers can prevent metal loss and deactivation. Several encapsulated Ru-based catalysts have been introduced for HER and showed favorable performance with high activity and stability.28,31 However, several important issues are still unrevealed, for example, the encapsulated form and degree and the inherent mechanism for enhanced HER performance of encapsulated metal nanoparticles.

In this study, we employed a simple method for thermal polymerization synthesis of Ru nanoparticles encapsulated in different confined spaces of three-dimensional porous nitrogen-doped carbon (3D Ru@PNC). The as-obtained 3D Ru@PNC with different morphologies were used as efficient electrocatalysts for HER. The fully-encapsulated sample displayed higher activity, lower overpotential, and improved stability compared with the other sample. DFT calculations indicated that the improved HER performance could be due to the optimum hydrogen adsorption energy on Ru induced by the increased charge transfer between Ru and carbon within a small confined space. Therefore, the suitable confined space between the carbon shell and the encapsulated metal, the three-dimensional structure, and doped nitrogen are beneficial to the reaction, diffusion, and stability of the catalyst.

Experimental section

Materials

Ammonium sulfate (NH4(SO4)2) was purchased from Shanghai Epiphanius Chemical Reagent Factory. Nafion 117 solution of 5 wt% was bought from Sigma-Aldrich. Commercial 20 wt% platinum on activated carbon (Pt/C) was purchased from Alfa Aesar. Ferric chloride, glucose (C6H12O6) and dicyandiamide (C2H4N4) were purchased from TCI. Ruthenium chloride was purchased from Chinese Jinan Science and Technology. Commercial 5% Ru/C (Shanxi Kaida Chemical Reagent Factory), graphene nanopowder (98%) and carbon nanotubes were purchased from Graphene Supermarket (Graphene Laboratories Inc.). All the reagents were used without further purification.

Synthetic procedures

Preparation of F-Ru@PNC samples. Ammonium sulfate, glucose, ruthenium chloride and dicyandiamide (DCDA) with a mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0.1125 (0.225 or 0.3375)[thin space (1/6-em)]:[thin space (1/6-em)]40 were homogeneously mixed by grinding. The obtained homogeneous mixture was then placed in a quartz tube and heated to 800 °C for 1 h under N2 flow (40 mL min−1) with a heating rate of 3.3 °C min−1 and cooled to room temperature naturally. The obtained samples were labelled as 2.5% F-Ru@PNC-800, 5.0% F-Ru@PNC-800 and 7.5% F-Ru@PNC-800. Two additional samples heated at different temperatures (700 or 900 °C), with a ruthenium chloride mass ratio of 0.225, were also prepared and labelled as 5.0% F-Ru@PNC-700 and 5.0% F-Ru@PNC-900.
Preparation of H-PNC and 5.0% S-Ru@PNC-800 samples. On the basis of the preparation of 5% F-Ru@PNC-800, H-PNC was obtained using an iron compound instead of ruthenium chloride, which was then washed with 3 M HCl for 24 hours. As for 5.0% S-Ru@PNC-800, it was prepared on the basis of 5.0% F-Ru@PNC-800, which was injected with an iron-compound, whose mole ratio is 2 times that of ruthenium, and then washed with 3 M HCl for 24 hours.
Preparation of different precursor samples. The other conditions used for the preparation of different precursor samples are the same as those for the preparation of F-Ru@PNC. The sample without glucose was labelled as “no glucose”, that without ammonium sulfate was labelled as “no (NH4)2SO4”, and that without dicyandiamide was labelled as “no DCAD”.
Preparation of Ru/G, Ru/CNT and Ru/PNC samples. Ruthenium chloride and graphene (Ru[thin space (1/6-em)]:[thin space (1/6-em)]G = 0.05[thin space (1/6-em)]:[thin space (1/6-em)]1 wt) were added into 20 ml deionized water, stirred for about 12 h to obtain a uniform solution at room temperature, and then reduced by hydrogen for 2 h at 300 °C. The obtained product was labelled as Ru/G. The same process was used for the preparation of Ru/CNT and Ru/PNC, except that graphene was replaced by carbon nanotubes and PNC.
Physicochemical characterization. The morphology pictures of samples were obtained by transmission electron microscopy (TEM) with a Tecnai G2F30S-Twin microscope equipped with a field emission gun operated at 300 kV. Energy dispersive X-ray (EDX) mapping and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images were analysed using a FEI Titan G2 80-200 ChemiSTEM. The structure images of samples were obtained by scanning electron microscopy (SEM) with a Hitachi FE-SEM S-4700 operated at 15.0 kV. The phase purity of samples was measured by X-ray diffraction (XRD) with Cu Kα (λ = 1.54 Å) radiation. The Brunauer–Emmett–Teller (BET) method was used to measure the pore distribution and specific surface areas with an ASAP2460 analyser. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos AXIS Ultra DLD XPS system.
Electrochemical measurements. The electrocatalytic properties of the prepared samples were measured with a three-electrode system using a CHI 660E potentiostat. A Ag/AgCl (3 M KCl) electrode was used as the reference electrode, a carbon rod electrode as the counter electrode, and a commercial glassy carbon electrode (GCE, 4.0 mm in diameter) with a rotating disk as the working electrode in 0.1 M KOH. The reference electrode was calibrated with respect to the reversible hydrogen electrode (RHE) to get all potentials, and all polarization curves were adjusted for the iR contribution. For the preparation of samples for the HER test, about 4 mg of samples were dissolved in 900 μl ethanol with 100 μl Nafion to form a homogeneous solution by sonication. Then, 5 μL solution was loaded onto the GCE and dried in air. The electrochemical measurements were conducted in 0.1 M KOH, which was purged with H2 gas to remove air from the solution. Linear sweep voltammetry (LVS) data were collected on the GCE under 1600 rpm in 0.1 M KOH with a scan rate of 5 mV s−1 at room temperature. Cyclic voltammetry curves (CVs) were obtained between 0.165 and 0.565 vs. RHE with different scan rates from 20 to 120 mV s−1 to investigate the electrochemical double-layer capacitance. The electrochemical impedance spectroscopy (EIS) spectra were collected with an amplitude of 5 mV in the frequency range of 106 to 1 Hz. The Faradic efficiency of the HER electrocatalysts was defined as the ratio between the amount of H2 measured from the experiment and the theoretical amount of H2. Therefore, we used the displacement method to measure the hydrogen gas from the reaction, and then the number of moles of H2 was calculated based on gas laws.
Computational section. Calculation method: all the theoretical models were determined using the Vienna ab initio simulation package (VASP).40,41 To describe the exchange correlation function, the Perdew–Burke–Ernzerhof function was used in the calculations.42 The models of Ru encapsulated in a graphene layer consist of different numbers of carbon atoms (40, 50, and 60) in the graphene substrate. All models were fully relaxed to the spin-polarized state and the ground state was considered in all calculations. The convergence of the total energy is set to 10−4 eV with a force on each atom of 0.05 eV Å−1. The hydrogen binding energy ΔEH was calculated using the formula
ΔEH = EH-slabEslab − 1/2EH2

The free energies were obtained by calculating the Δ

ΔG(H*) = ΔE(H*) + ΔZPE − TΔP
where ΔE (H*), ΔZPE and ΔS are the binding energy, zero point energy change and entropy change of H* adsorption, respectively. Then the Gibbs free energy with the overall correction is taken as the Δ
ΔG(H*) = ΔE(H*) + 0.20 eV

Results and discussion

The three-dimensional porous nitrogen-doped carbon (PNC) encapsulated Ru nanoparticles with diverse morphologies are illustrated in Scheme 1. The catalyst was prepared using glucose and dicyandiamide (DCDA) as carbon and nitrogen sources, respectively. Ammonium sulfate was selected to obtain a porous structure, for easy and efficient loading of metal precursors (i.e., ruthenium trichloride) into the porous structure. The fully-encapsulated Ru-based catalyst was denoted as F-Ru@PNC. Ferric chloride was used to obtain H-PNC and the semi-encapsulated Ru-based catalyst (S-Ru@PNC). For comparison, Ru nanoparticles supported on PNC were prepared. The catalytic performance of these particles was compared with that of the encapsulated samples.
image file: c7cy01539a-s1.tif
Scheme 1 Schematic illustration of the synthesis of H-PNC, Ru/PNC, S-Ru@PNC, and F-Ru@PNC.

The morphologies of the prepared samples were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. The TEM images are shown in Fig. 1a–d. Four different morphologies, namely, H-PNC, Ru/PNC, S-Ru@PNC-800, and F-Ru@PNC-800 were successfully formed. The high-resolution TEM (HRTEM) analysis showed that the average diameters of Ru nanoparticles in S-Ru@PNC-800 and F-Ru@PNC-800 are approximately 3–5 nm. The clear and continuous lattice spacing was measured with a d-spacing of 0.208 nm, which is close to the (101) planes of the Ru phase. This finding indicates the successful formation of Ru nanoparticles on the porous carbon (Fig. S1). In addition, Fig. 1e and S2–S3 show the 3D porous network of the microstructures of F-Ru@PNC. We also conducted a blank experiment with one component lacking from the raw materials, and almost no porous structure was observed. Hence, each raw material plays a vital role in forming such a hierarchical porous structure (Fig. S4). The TEM images obtained under different conditions, such as pyrolysis temperature and concentration, are displayed in Fig. S5–S6. A non-uniform shape and agglomeration of Ru nanoparticles were observed at 700 °C and 7.5% metal content, respectively, indicating that 800 °C and 5% content are the optimal conditions for the proposed system. In addition, high-angle annular dark-field scanning TEM (HAADF-STEM) and elemental analyses of 5.0% F-Ru@PNC-800 were conducted (Fig. S7). The corresponding energy-dispersive X-ray spectroscopy (EDS) mappings revealed that the Ru, N, C, and O elements were uniformly distributed on the sample (Fig. 1f–i), verifying that Ru nanoparticles were embedded in nitrogen-doped carbon.


image file: c7cy01539a-f1.tif
Fig. 1 (a–d) TEM images of H-PNC, 5.0% Ru/PNC, 5.0% S-Ru@PNC-800, and 5.0% F-Ru@PNC-800. (e) SEM image of 5.0% F-Ru@PNC-800. (f–i) EDX elemental mapping of C, O, N, and Ru of 5.0% F-Ru@PNC-800.

The X-ray diffraction (XRD) spectra of the four different samples are illustrated in Fig. 2a. An obvious broad peak at 26° was found in all samples, indicating that the (002) diffraction belongs to graphite carbon. Moreover, an additional broad peak at 43.4° was found in 5.0% F-Ru@PNC-800 and 5.0% S-Ru@PNC-800; this peak is assigned to the typical (101) Ru lattice plane (JCDPS #88-1734). However, such a characteristic peak of Ru in 5% Ru/PNC was unclear, which demonstrated that the presence/absence of carbon shell encapsulation influences the Ru nanoparticles' existence state. Furthermore, other small Ru phases were present in 7.5% F-Ru@PNC-800 and 5.0% F-Ru@PNC-900 (Fig. S8). These results indicate that the phases were influenced by the high temperature and metal content. This finding is consistent with the TEM observation. Moreover, nitrogen adsorption–desorption analysis suggested that 5.0% F-Ru@PNC-800 exhibited a Brunauer–Emmett–Teller (BET) surface area of 411.40 m2 g−1, which is higher than that of H-PNC (390.8 m2 g−1). The isotherm of the above samples was classified as a typical type II pattern with a hysteresis loop, revealing the presence of a mesoporous structure (Fig. 2b).43


image file: c7cy01539a-f2.tif
Fig. 2 (a) XRD spectra of H-PNC, 5.0% Ru/PNC, 5.0% S-Ru@PNC-800, and 5.0% F-Ru@PNC-800. (b) BET of 5.0% F-Ru@PNC-800 and H-PNC. Inset of (b): pore-size distribution of 5.0% F-Ru@PNC-800 and H-PNC. (c) High-resolution XPS spectra of C 1s and Ru 3d of H-PNC, 5.0% Ru/PNC, 5.0% S-Ru@PNC-800, and 5.0% F-Ru@PNC-800. (d) N 1s of H-PNC, 5.0% Ru/PNC, 5.0% S-Ru@PNC-800, and 5.0% F-Ru@PNC-800.

The chemical state of the elements and the surface composition of the composite were determined by X-ray photoelectron spectroscopy (XPS) analysis. As observed in Fig. S9, the presence of Ru, O, C, and N elements was determined in the XPS survey spectra. Binding energies of 462.8 and 485.1 eV, which are assigned to Ru 3p3/2 and Ru 3p1/2 XPS signals, are observed in Fig. S9.44 For the Ru 3d peaks, the binding energy of Ru 3d3/2 is 284.8 eV, which overlapped with the C 1s signal (Fig. 2c).45 The peak at 280.7 eV can be ascribed to Ru 3d5/2, which exhibited a slight shift compared with the commercial Ru/C (Fig. S10); this phenomenon indicated that the changes in the electronic Ru structure were influenced by the carbon shell.46 As depicted in Fig. 2d, the N 1s XPS peak of 5.0% F-Ru@PNC-800 can be split into three peaks, namely, pyridinic N (398.7 eV), pyrrolic N (399.7 eV), and graphitic N (401.1 eV).47,48 For 5.0% S-Ru@PNC-800, the nitrogen peaks exhibit an obvious negative shift compared with 5.0% F-Ru@PNC-800, implying that the different confined spaces affected the nitrogen binding energy. As for Ru/PNC, the N 1s spectra demonstrate that three peaks of nitrogen binding energy are weaker than those in F-Ru@PNC-800, indicating that the nitrogen chemical bonding states were affected by the presence of Ru or its loading manner.49 The surface element contents of the other XPS samples were also analyzed in Table S1 and Fig. S11–S12, respectively.

The electrocatalytic HER performance of the samples was tested on a glassy carbon rotating disk electrode in 0.1 M H2-saturated aqueous KOH solution by using a typical three-electrode configuration. We initially investigated F-Ru@PNC with different Ru contents and calcined at different temperatures (Fig. S13). The results showed that 5% F-Ru@PNC-800 displayed excellent electrocatalytic ability. It was clear that the low calcination temperature (700 °C) caused the incomplete carbonization of the Ru@PNC precursor and failed to form high-quality crystals, leading to a dramatic reduction in activity (Fig. S3). At a high temperature (900 °C), the deep carbonization destroyed the three-dimensional structure (Fig. S3), revealing an unsatisfactory HER activity. Based on these findings, we further investigated 5.0% S-Ru@PNC-800 for comparison. As shown in Fig. 3a, the polarization curve of the prepared 5.0% F-Ru@PNC-800 exhibits a lower onset overpotential than those of 5.0% S-Ru@PNC-800, 5.0% Ru/PNC, and H-PNC. It is noteworthy that both 5.0% F-Ru@PNC-800 and 5.0% S-Ru@PNC-800 exhibited improved activities compared with commercial Pt/C (20%). From another aspect, 5.0% F-Ru@PNC-800 exhibited an overpotential (η) of 30 mV at a current density of 10 mA cm−2, which is superior to 5.0% S-Ru@PNC-800 (72 mV), 5.0% Ru/PNC (152 mV), and H-PNC (478 mV). Remarkably, the catalytic activity of the 5.0% F-Ru@PNC-800 catalyst is approximately 3.27 times greater than that of the Pt/C catalyst (3.06 mA cm−1).29,50 The small onset overpotential and the high-current density of 5.0% F-Ru@PNC-800 revealed that the fully-encapsulated structure displayed a high electrocatalytic activity for HER. As shown in Fig. S14, the regularity of electrocatalytic activity in 0.5 M H2SO4 and 1 M KOH solution is the same as that in 0.1 M KOH solution. To reveal the intrinsic reaction processes of HER, we acquired Tafel plots and presented them in Fig. 3b. For 5.0% F-Ru@PNC-800, we obtained a value of 28.5 mV dec−1, which is smaller than that of 5.0% S-Ru@PNC-800 (34.1 mV dec−1), commercial Pt/C (20%) (35.5 mV dec−1), 5.0% Ru/PNC (92.2 mV dec−1), and H-PNC (161.1 mV dec−1). Fig. S15 shows the Tafel slope values of the other catalysts. From the results that we have obtained, one can conclude that the low Tafel slope of 5.0% F-Ru@PNC-800 manifested its excellent HER activity. These results demonstrate that the HER catalyzed by these samples may occur via a Volmer–Heyrovsky mechanism, where hydrogen release is the rate-limiting process.51–54 To the best of our knowledge, 5.0% F-Ru@PNC-800 exhibited a lower overpotential (η = 30 mV) than other carbon-based, nonprecious and precious metal materials under similar conditions reported to date (Table S2).


image file: c7cy01539a-f3.tif
Fig. 3 (a) Polarization curves and (b) Tafel plots for H-PNC, 5.0% Ru/PNC, 5.0% S-Ru@PNC-800, and 5.0% F-Ru@PNC-800 catalysts in 0.1 M KOH. (c) Different scan rates to measure capacitive currents for H-PNC, 5.0% Ru/PNC, 5.0% S-Ru@PNC-800, and 5.0% F-Ru@PNC-800. (d) Time dependence of current density obtained for 5.0% F-Ru@PNC-800 at a static overpotential of −25 mV for 15 h. Inset is an enlargement of an area in (d). (e) Polarization curves of 5.0% F-Ru@PNC-800 initially and after 1000 cyclic voltammetry scans. (f) Amount of experimentally measured and theoretically calculated hydrogen versus time for the 5.0% F-Ru@PNC-800 catalyst in 0.1 M KOH.

Furthermore, the exchange current density (j0) values were calculated using a Tafel plot with extrapolation. The fabricated 5.0% F-Ru@PNC-800 exhibited a remarkable j0 of 89 μA cm−2, which is 17.8 times higher than that of H-PNC (5 μA cm−2), and better than the other catalysts tested in this work (Table S3). Furthermore, the number of active HER sites and the electrochemical active surface area (ECSA) were evaluated by measuring the double-layer capacitance (Cdl), which is a standard scale used to estimate the interfacial area between the electrolyte and the electrode surface. Cyclic voltammetry (CV) curves were acquired within 0.165 V to 0.565 V (vs. RHE) at different scan rates during the HER process (Fig. S16–S18). As shown in Fig. 3c, the Cdl of 5.0% F-Ru@PNC-800 is 34 mF cm−2, confirming that the ECSA of 5.0% F-Ru@PNC-800 is approximately 3.4 times larger than that of H-PNC (10 mF cm−2). This result confirmed that 5.0% F-Ru@PNC-800 possessed the largest active surface area among all samples. In addition, electrochemical impedance spectroscopy analysis was conducted during HER to investigate the electrocatalytic kinetics of the as-prepared electrocatalysts. The Nyquist plots are illustrated in Fig. S19. The charge transfer resistance value of 5.0% F-Ru@PNC-800 is smaller than those of the other composite catalysts. This result confirmed that 5.0% F-Ru@PNC-800 showed excellent HER activity, consistent with the polarization measurements (Fig. 3a). The low-charge transfer resistance is favorable to achieve highly efficient electron charge transfer and induce rapid reaction kinetics because of the synergistic effect from the Ru nanoparticles encapsulated by a carbon shell and the prominent electrical coupling to the conductive three-dimensional structure.

Moreover, the electrocatalytic stability of 5.0% F-Ru@PNC-800 was also studied by chronoamperometry at a static overpotential of −25 mV. Fig. 3d shows that the catalytic current density remained at ≈9 mA cm−2 over 15 h of continuous operation. The catalytic current density only slightly decreased after a long-term testing. A typical serrated shape is presented in the insert of Fig. 3d, because of the alternate accumulation and release processes of H2 bubbles on the electrode. The excellent durability of 5.0% F-Ru@PNC-800 was further evaluated by long-term potential continuous CV for 1000 cycles at a scan rate of 100 mV s−1 (Fig. 3e). The polarization curves obtained before and after 1000 cycles were compared. A negligible difference was observed between the two polarization curves after the measurement. Moreover, the recycled 5.0% F-Ru@PNC-800 structure was further analyzed by TEM after the durability measurement (Fig. S20). Impressively, no significant aggregation was observed on the Ru nanoparticles, confirming that 5.0% F-Ru@PNC-800 displayed excellent stability in HER. As illustrated in Fig. 3f, the Faraday efficiency of HER was tested during a constant current electrolysis. The amount of hydrogen generated during the experiment versus the theoretical value was calculated by charge transfer, procuring an approximately 99% Faraday efficiency.

The hydrogen adsorption on both nitrogen-doped carbon and the Ru cluster was investigated to understand the synergistic effect between the PNC shell and confined Ru nanoparticles. As shown in Fig. 4 and Table S4, it is found that the adsorption strength of hydrogen on nitrogen-doped carbon increased from −0.16, −0.36 to −0.52 eV with the smaller confined space. Meanwhile, the values on the Ru site decreased from −0.69 without the confined carbon to −0.53, −0.29 and −0.21 eV. This result indicates that the strong interaction between Ru and nitrogen-doped carbon occurs in a small confined space. From the Bader charge analysis, a good linear relationship was observed between the charge of Ru and carbon and their adsorption energies (Fig. 5a and Table S4). The charge density differences also clearly illustrated the trend of the effect of confined space on electron transfer (Fig. S21 and Table S5). Furthermore, the calculated ΔΔG (H*) diagrams of HER on these different models are shown in Fig. 5b. First, supported Ru without any confined carbon displays the largest ΔΔG (H*) value, which can confirm that the regular Ru catalysts exhibited poor HER performance. Second, with decreasing area of the confined space, the ΔΔG (H*) values become small, especially for the Ru@PNC-3 model, which exhibited an overpotential of 0.01 eV. Therefore, the theoretical simulation results are in good agreement with the experimental results. Fig. 5c shows the mechanism underlying the enhanced HER, that is, the charge transfer between Ru and nitrogen-doped carbon increases but the overpotential decreases with decreasing area of the confined space. So the improvement of the catalytic HER reaction is caused by the synergistic effect of Ru particles and N-doped carbon materials, and HER mainly occurs on the ruthenium surface.


image file: c7cy01539a-f4.tif
Fig. 4 Top and side views of the theoretical calculation models and the adopted adsorption sites of H* on the carbon and metal: (a) Ru/PNC, (b and e) Ru@PNC-1, (c and f) Ru@PNC-2, and (d and g) Ru@PNC-3. Grey, blue, white and green represents carbon, nitrogen, hydrogen and ruthenium.

image file: c7cy01539a-f5.tif
Fig. 5 a) Relationship between adsorption energy and charge transfer. Five-pointed star indicates Ru@PNC-3. Square indicates Ru@PNC-2. Red circle indicates Ru@PNC-1. pentagon indicates Ru/PNC. b) Gibbs free-energy profile of HER on Ru/PNC, Ru@PNC-1, Ru@PNC-2, and Ru@PNC-3. c) Performance mechanism of catalysts. The green ball indicates a hydrogen atom.

Conclusions

In summary, we reported a process for simple thermal polymerization synthesis of Ru nanoparticles encapsulated in three-dimensional porous nitrogen-doped carbon with different confined spaces. Electrochemical measurements implied that 5.0% F-Ru@PNC-800 displayed a better HER activity than 5.0% S-Ru@PNC-800. DFT calculations indicated that the confined space affects the catalytic performance by enhancing the charge transfer. Therefore, Ru@PNC exhibits considerable potential for applications in water-splitting devices. Moreover, adjusting the area of the confined space is a promising method for designing highly active catalysts for clean energy utilization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC-21625604, 21671172 and 21776251).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Additional characterization and electrochemical measurement results. See DOI: 10.1039/c7cy01539a

This journal is © The Royal Society of Chemistry 2017