刘源，李卫东，吴捍，卢思宇

郑州大学，化学学院，绿色催化中心，郑州 450000

1 Introduction

Hydrogen energy is considered as a new type of green energy in the future due to its advantages such as abundant resources,high calorific value of combustion and no pollution. Hydrogen evolution reaction (HER) from water splitting has attracted wide attention due to its low energy consumption, environmentally friendly process, high energy conversion efficiency, and product purity1–5. However, the slow kinetics and high overpotential of HER have severely restricted its application. Platinum (Pt) is the most excellent HER catalyst so far. However, the reserves of Pt are rare and the cost of Pt is expensive, which greatly limits its large-scale application in industry6,7. Compared with other precious metal catalysts such as Pd, Pt and Ir, ruthenium (Ru)possesses a similar bond strength with hydrogen, Ru also has obvious cost advantages8–11. Therefore, Ru-based HER catalysts are considered as ideal alternative materials and have become a research hotspot in the past few years. Although Ru catalyst has excellent HER performance, its cost is still high, and its stability in alkaline solution needs to be further improved.

Carbon nanomaterials, such as graphene oxide (GO), carbon nanotubes (CNTs), metal-organic framework (MOF) and carbon black have large specific surface area, excellent electronic transmission capability, acid and alkali corrosion resistance and environmental friendliness, which has been proved to be good carriers to improve the performance of metal catalysts12–19.Tsiakarasetal. reported the boron-doped RhFe alloy uniformly distributed on the carbon supportviaa one-pot method and exhibits excellent catalytic performance for HER20. The overpotential at current density of 10 mA·cm−2only increased about 4 mV after 1000 cycles’ durability tests. Baiet al. first synthesized the Pd nanosheets on rGO and then selectively wrapped Pt on the Pd nanocrystalsviareducing H2PtCl6inN,Ndimethylformamide (DMF). The Pt-Pd-rGO catalyst was prepared by a two-step method, which reduced the amount of Pt and achieved high-efficiency hydrogen production performance21.Laasonen’s group used electrochemical methods to dissolve platinum foil in sulfuric acid to form Pt ions, and then successfully dispersed Pt atoms on the sidewalls of single-walled carbon nanotubes by electrochemical methods to prepare SWNT/at-Pt samples, SWNT/ at-Pt shows excellent electrocatalytic activity for HER22. Therefore, carbon nanomaterial as a carrier can effectively accelerate the HER reaction. However, carbon nanotubes, carbon black and other carbon carriers are easy to oxidize and agglomerate. Moreover,these carbon carriers have high raw material costs, cumbersome processes and serious pollution during the preparation process.

As a new class of carbon nanomaterials, carbon dots (CDs)has stimulated a wide range of research interests due to their high electric conductivity, high good stability, low cost, and the flexibility of their surface modifications in recent years. What’s more, the abundant heteroatoms doping in CDs and abundant functional groups on the surface of CDs can act as favorable sites for fabricating photo- and electro-active metal catalyst23–27.Meanwhile, many metal ions with emptydorbitals can also coordinate with CDsviatheir functional groups to form a relatively stable CDs-metal ion coordination composite materials, which can be further transformed into carbon-based catalytic material loaded with highly dispersed heteroatom doped metal nanoparticles (NPs). In this process, the metal ions are confined between the carbon dots, forming ultra-fine nanocrystals with stable structure, effectively preventing the metal NPs from agglomerating and growing28,29. In addition, by selecting the appropriate reaction precursor molecule, the CDs can be simultaneously doped with different heteroatoms and controlled by different doping amounts, and the surface modification and functionalization can be performed during the synthesis of CDs30. Benefitting from these excellent properties,CDs show great potential to modify Ru NPs as a carrier to gain efficient electrocatalysts for HER. In our previous work, by using CDs as the building blocks, we designed metals@CDs or semiconductor@CDs electrocatalyst, and established a general route to prepare carbonized polymer dots and nanocrystalline composite materials and clarified the internal relationship between the micro-nano structure of composite materials and the catalytic performance. However, there is no in-depth discussion on the effect of the interactions between CDs and Ru on HER performance. In this work, we prepared various carriers including MOF, CNTs, GO, molecules, CDs and then combined them with RuCl3to get diverse Ru NPs loaded on different carriers. Among them, CDs as carrier could realize the controllable preparation of Ru NPs both in size and dispersibility, making the active sites effectively be exposed, and thereby improving the catalytic performance, indicating that CDs indeed have the advantages as building blocks to establish efficient electrocatalyst.

2 Experimental and computational section

Carbon nanotubes, graphene oxide, citric acid, melamine,lithium chloride, glacial acetic acid, acetic acid, acetic anhydride, 1,3,5-benzoic acid, 5% (w) Nafion solution, 20% (w)Pt/C, 5% (w) Ru/C, potassium hydroxide, potassium dihydrogen phosphate, and dipotassium hydrogen phosphate were purchased from Aladdin Reagent (Shanghai) Co., Ltd. Ethanol, acetone,hexane, and sulfuric acid were purchased from Sinopharm Group Chemical Reagent Co., Ltd. Carbon nanotubes and graphene were purchased from Nanjing Xianfeng Nanomaterial Technology Co., Ltd. Ruthenium trichloride hydrate(RuCl3·xH2O) was purchased from Guiyan Platinum Co., Ltd.All the reagents were analytical grade and utilized without further purification.

2.1 Synthesis of Ru@CDs

The CDs were synthesized by a previous report. Typically,citric acid (2.1014 g) and melamine (1.2612 g) was dissolved in a Teflon autoclave (250 mL) by deionized water (200 mL). Then heated at 200 °C for 8 h. After naturally cooling to room temperature, 300 mg RuCl3·xH2O was added and dissolved completely, reheated at 200 °C for 8 h. The resulting solution was concentrated by evaporation and freeze-dried. The solid was annealed in a tube furnace at 500 °C, 600 °C and 700 °C for 6 h under Ar atmosphere with a heating rate of 2 °C·min−1.

2.2 Electrochemical measurements

The electrochemical measurements were carried out using an electrochemical work station (CHI760E, Shanghai, China) in a conventional three-electrode cell system at room temperature where the graphite rod, saturated calomel electrode (SCE) and commercial glassy carbon electrode (GCE, 5 mm diameter,0.196 cm−2) are used as the counter electrode, reference electrode and working electrode, respectively. The recorded current density corresponds to the geometric surface area of the GCE. an ethanol suspension containing 500 μL of ethanol, 3 mg of catalyst, and 50 μL of 5% (w) Nafion solution was obtained by ultrasonic mixing for about 30 min. Then the obtained 15 μL catalyst ink suspension was coated on the polished GC electrode and dried in air (the catalyst loading on GCE is about 0.42 mg·cm−2). According toE(vs.RHE) =E(vs. SCE) +EөSCE+0.059pH, the potential measured with respect to the SCE electrode is converted into the potential with respect to the reversible hydrogen electrode (RHE). In order to compare the effects of the structure and components of the catalyst on the HER activity, the electrocatalytic activities of other catalysts were measured under similar conditions. Polarization curves were obtained from linear sweep voltammetry (LSV)measurements at a sweep rate of 2 mV·s−1. All measurements were corrected without IR compensation.

3 Results and discussion

The Ru@CDs could be obtainedviathe process illustrated in Fig. 1, where the N doped CDs were synthesized by hydrothermal citric acid and ethylenediamine. The transmission electron microscopy (TEM) is firstly used to expose the morphology of as-prepared CDs. As shown in Fig. 2a, the CDs were uniformly distributed with an average particle size of 3.88 nm (Fig. S1). The lattice fringes of CDs can be clearly observed from high-resolution transmission electron microscopy(HRTEM), where the lattice spacing of 0.21 nm correspondeds to the (100) crystal plane of graphite carbon (Fig. 2b)31. The Xray powder diffraction (XRD) can also show the structure of CD with the diffraction peak at 22.40° corresponds to the (100)crystal plane of carbon, which agrees with the above TEM results (Fig. 2c).

Fig. 1 Schematic illustration of the synthesis of the Ru@CDs electrocatalyst.

Fig. 2 (a) TEM image and (b) HRTEM image of as synthesized CDs, (c) XRD pattern, (d) FT-IR spectra, (e) excitation-emission matrix and (f) UV-Vis, PL emission, and PL excitation spectra of as synthesized CDs.

The functional groups and chemical composition of CDs were analyzed by Fourier transform infrared spectroscopy (FT-IR). As shown in Fig. 2d, the peak at 3341 cm−1corresponds to the stretching vibration of the N―H/O―H bond. The absorption peaks at 1667, 1455 and 1114 cm−1was attributed to the stretching vibration of C ＝C, C―N, and C―O bonds,respectively32. When excited with 370 nm, the CDs aqueous solution exhibited the strongest emission at 454 nm with a blue luminescence under the irradiation of ultraviolet light. As shown in the ultraviolet-visible absorption spectrum (UV-Vis), The peak at 238 nm was corresponded to theπ–π* transition of the C＝C bond (Fig. 1e,f)33. X-ray photoelectron spectroscopy(XPS) can clearly show the chemical composition on the surface of CDs surface. As shown in Fig. S2, the absorption peaks at 285.1, 530.9 and 400.2 eV corresponded to C 1s, O 1sand N 1s,respectively, which was consistent with the FT-IR spectrum results, indicating that there are some nitrogen and oxygen functional groups on the surface of the prepared CDs.

Fig. 3 (a) TEM image of the Ru@CDs 600; (b) HRTEM image of Ru@CDs 600; (c) EDX elemental mapping of Ru@CDs 600 (The scale bar is 50 nm).

Ru@CDs was prepared by a hydrothermal process, and the resulting product was calcined under a nitrogen atmosphere.During this process, the CDs self-crosslink to form a N-doped film-like structure due to those groups on the surface of CDs(such as ―NH2, ―COOH, ―OH,) and the van der Waals bonding between adjacent CDs, while confining Ru NPs. Fig. 3 showed the TEM images of Ru@CDs 600, it can be observed that Ru NPs were evenly dispersed in the film-like structure formed by CDs self-crosslinking (Fig. 3a). In addition, the HRTEM image (Fig. 3b) clearly showed good crystallinity with a plane spacing 0.21 nm, which belongs to the (101) crystal plane of Ru. During the second hydrothermal process, Ru3+induces CDs to crosslink into a nitrogen-doped graphene-like film-like structure, where small and uniform Ru NPs can be observed. As shown in Fig. 3c, the uniform distribution of C, N and Ru elements also be verified by the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM)and energy dispersive X-ray spectroscopy (EDX). In addition,the chemical state of Ru was analyzed by XPS. The XPS spectrum of Ru@CDs 600 (Fig. S3) showed the presence of several elements of C (285 eV), N (400 eV), O (531.8 eV), and Ru (464 eV). The crystallisation process was also explored,where Ru@CDs samples were treated at different annealing temperatures of 500 °C and 700 °C. As shown in Fig. S4,Ru@CDs 500 showed poor crystallization performance with an average particle size of 1.48 nm. At 600 °C, the crystallization of Ru NPs were clearly visible with an average size 2.08 nm.When the calcination temperature increased to 700 °C, the average size of Ru NPs grew to 3.61 nm and obviously aggregated, which could be not conducive to the effective contact between the catalyst and the solution, thus hinder the catalytic process. Their structures were further exposed using the XRD measurement. As shown in Fig. S5, the diffraction peak intensity of Ru gradually increased with the calcination temperature increasing, which indicated that the higher calcination temperature was beneficial to the crystallinity and growth of Ru NPs34.

In order to further explore the influence of carbon carriers, we prepared Ru-MOF, Ru@Molecule, Ru@CNTs and Ru@GO at an annealing temperature of 600 °C. First, the morphology of Ru NPs catalysts supported on different carbon-based supports was analyzed by TEM. As shown in Fig. 4a, the resulting Ru@CDs showed a film-like structure and small and uniform Ru NPs can be observed. Fig. 4b,c showed the composite structure of Ru@GO and Ru@CNTs, respectively. It can be seen that the Ru NPs were agglomerated and accumulated. Because Ru3+tends to adhere to the surface of the carrier, resulting in uneven distribution of Ru3+, which leads to aggregation of Ru NPs during the reaction. Fig. 4d showed the TEM image of Ru-MOF.It can be seen that the generated Ru-MOF has a cubic structure with clear edges and good crystallinity. In Ru@Molecule, as shown in Fig. 4e and Fig. 4f, a part of the CDs self-crosslink to form a film-like structure covering Ru NPs, but most of the CDs exist as monodisperse, and Ru NPs were partially agglomerated.This result showed that there is an intermediate state between the molecules and CDs, and the intermediate state had no confinement effect. Hence, the special space-confined effect,which originates from the coordination behavior between CDs and metal ions, which is the key to the formation of monodisperse Ru NPs during the annealing process.

Fig. 4 (a) TEM images of Ru@CDs 600; (b) TEM images of Ru@GO; (c) TEM images of Ru@CNTs; (d) TEM images of Ru-MOF;(e–f) TEM images of Ru@Molecule.

Fig. 5 (a) HER polarization curves for the Ru@CDs annealed at different temperature, (b) polarization curves, (c) Tafel plots and(d) Nyquist plots for the HER on RDE modified with Ru@CDs, Ru-MOF, Ru@Molecule, Ru@CNTs and Ru@GO annealed at 600 °C.

The electrochemical performance of the prepared electrodes toward the HER was initially evaluated using a standard threeelectrode system in an alkaline solution (1 mol·L−1KOH). As shown in Fig. 5a, Ru@CDs 500 showed poor catalytic performance, because of the poor crystallinity. Among them,Ru@CDs 600 exhibited the most excellent HER activity with a low overpotential of 22 mV at 10 mA·cm−2, indicating that the annealing conditions could significantly affect the crystallinity of Ru NPs and HER activity. However, when the annealing temperature was raised to 700 °C, the overpotential increased to 32 mV. Therefore, the optimal crystallization temperature of the Ru@CDs hybrid was 600 °C. This phenomenon may be due to the low crystallinity that reduced the interaction between CDs and Ru NPs, and the high temperature may cause Ru NPs aggregating, thus inhibited the electron transport process. To explore the effect of different carbon carriers on HER activity,we have prepared Ru-MOF, Ru@Molecule, Ru@CNTs, and Ru@GO at the same condition and evaluated their catalytic activity. As shown in Fig. 5b, Ru@Molecule had almost no catalytic activity and Ru-MOF showed relatively poor catalytic performance (121 mV at the current density of 10 mA·cm−2),while Ru@CNTs and Ru@GO showed similar catalytic properties with the overpotentials 55 mV and 61 mV at the current density of 10 mA·cm−2, respectively. We determined the Tafel slope by fitting the linear portion of the Tafel curve to study the reaction kinetics during the HER process35,36. As shown in Fig. 5c, the Tafel slope of Ru@CDs 600 was 50.72 mV·dec−1,which was smaller than that of Ru@CNTs 600 (63.39 mV·dec−1), Ru@GO 600 (63.28 mV·dec−1) and Ru-MOF 600(101.37 mV·dec−1). The smallest Tafel slope of Ru@CDs 600 indicated that the electrochemical desorption of active H and the generation of H2by H3O+were rate-determining steps,indicating that HER on Ru@CDs may occur through the Volmer-Heyrovsky mechanism37,38. The charge transfer impedance (Rct)affects the total reaction rate of HER. As shown in Fig. 5d, the radius of the semicircle of Ru@CDs 600 catalyst was the smallest, indicating that the charge transfer resistance of Ru@CDs 600 was the smallest than that of other catalysts,indicating that Ru@CDs 600 had the best electron transfer ability. The superior charge transport may be attributed to the faster electron transport capability between Ru NPs and CDs39,40. All the results proved that CDs were ideal electron acceptors and donors, which could promote the electron transport during the reaction, hence combining CDs with Ru NPs could significantly reduce the interface resistance.

The stability of Ru@CDs 600 was measured by cyclic voltammetry (CV). At the same condition, the cycle stability of Ru-MOF, Ru@CNTs and Ru@GO were also explored. As shown in Fig. 6a,b, Ru-MOF and Ru@GO showed a certain degree of decrease after 2000 cycles of CV, and Ru@CNTs exhibited an obvious attenuation (Fig. 6c). Ru@CDs exhibited a negligible attenuation before and after the 2000 cycles with the overpotential increased only 2 mV at current density of 10 mA·cm−2(Fig. 6c). The excellent stability of Ru@CDs may due to the formation of N-rich graphene skin during the annealing process, at the same time, the metal NPs were confined between CDs, forming ultra-fine nanostructures with stable structure crystals, effectively preventing the agglomeration and growth of metal NPs.

Fig. 6 (a) Ru-MOF; (b) Ru@GO; (c) Ru@CNTs and (d) Ru@CDs before and after 2000 CV cycles in 1 mol·L−1 KOH.

We have also explored the performance of Ru@MOF,Ru@Molecule, Ru@CNTs and Ru@GO for HER at other annealing temperatures. As shown in Fig. S6a, when the calcination temperature was 500 °C, Ru@CNTs and Ru@GO show excellent catalytic performance with the overpotential 50 mV and 44 mV at the current density 10 mA·cm−2, respectively.However, Ru@CDs showed poor catalytic performance due to low carbonization, and Ru@Molecule had almost no catalytic performance at 500 °C. It is worth noting that when the calcination temperature raised to 700 °C, the Ru@Molecule exhibited excellent catalytic performance (39 mV at the current density of 10 mA·cm−2, Fig. S6b), which is equivalent to that of Ru@CDs (29 mV at the current density of 10 mA·cm−2), and relatively better than Ru-MOF (121 mV), Ru@CNTs (67 mV)and Ru@GO (59 mV). We suggest that it required a higher reaction energy barrier during the CDs generation from the small molecule citric acid and melamine. When the calcination temperature was not enough, it just formed an intermediate state between the molecules and CDs, and with the increasing of calcination temperature, the intermediate state changed to CDs gradually. Therefore, the catalytic performance of Ru@Molecule was almost the same with Ru@CDs at 700 °C(Fig. S6b).

4 Conclusions

In summary, we have successfully synthesized Ru@MOF,Ru@Molecule, Ru@CNTs, Ru@GO and Ru@CDs, and explore the effects of different carbon-based carriers on HER.Among them, the catalysts with CDs as carrier could realize the controllable preparation of Ru NPs both in size and dispersibility, making the active sites effectively be exposed,and thereby improving the catalytic performance. Ru@CDs exhibited the highest activity (22 mV at the current density of 10 mA·cm−2, 50.72 mV·dec−1) and excellent stability in 1.0 mol·L−1KOH. We found that there is an intermediate state between the molecules and CDs, and with the increasing of calcination temperature, the intermediate state changed to CDs gradually, but the intermediate state had no confinement effect. The special space-confined effect, which originates from the coordination behavior between CDs and metal ions,which is the key to the formation of monodisperse Ru NPs during the annealing process. Therefore, CDs can be used as an ideal HER carrier to obtain the high activity, good stability,low cost and simple preparation electrocatalyst. Such approach could be extended to other metals and create a family of metal@CDs with high HER performance, and the combination of metal and semiconductor NPs with CDs also provide an acceptable strategy for constructing other efficient electrocatalyst for OER, ORR, CO2RR and so on. CDs-based hybrid materials will open a new field for energy conversion and storage.

Supporting Information:available free of chargeviathe internet at http://www.whxb.pku.edu.cn.