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Carbon Quantum Dots Modulated NiMoP Hollow Nanopetals as Efficient Electrocatalysts for Hydrogen Evolution Lin Tian,*,† Guofeng Qiu,† Yanchao Shen,† Xiang Wang,† Ju Wang,† Peng Wang,† Ming Song,† Jing Li,† Tongxiang Li,*,‡ Wenchang Zhuang,*,† and Xihua Du*,† †
College of Chemistry and Chemical Engneering, Xuzhou University of Technology, Xuzhou 221018, PR China College of Food (Biology) Engineering, Xuzhou University of Technology, Xuzhou 221018, PR China
‡
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S Supporting Information *
ABSTRACT: Although transition metal materials are extensively studied for hydrogen evolution reaction (HER) because of their intrinsic electrochemical properties, their practical applications have been hindered by low conductivity, unsatisfactory activity, and poor stability. To develop an approach for practical applications, it is imperative to improve their electrocatalytic performance. Herein, we report the fabrication of hollow NiMoP nanopetals embedded with Ndoped carbon dots (CDs) by a simple hydrothermal method. The addition of CDs can effectively modulate the morphology, reduce aggregation, and maintain long-term stability. Impressively, the optimal Ni5Mo3P@CDs3 exhibits excellent HER performance, in terms of a low overpotential of 183 mV at 10 mA cm−2, small Tafel slope of 41.04 mV dec−1, high conductivity, and remarkable long-term stability in acidic media for HER. This work opens an effective avenue to construct excellent property and accessible electrocatalysts for HER with the assistance of CDs.
1. INTRODUCTION The environmental destruction and depletion of traditional fossil fuel have compelled researchers to develop clean and highly efficient energy.1 Hydrogen, as a clean and sustainable energy source, is considered to be an alternative energy carrier to replace the conventional fossil fuel based energy.2 Hydrogen production from water splitting by electrocatalysis is an efficient method,3 while the catalyst is regarded as a key factor in the process of electrolysis of water.4 Currently, Pt-based nanomaterials are still the state-of-the-art electrocatalysts for HER.5−7 However, their exorbitant price and limited resources severely hinder their large-scale application.8 Therefore, it is imperative to develop cheap and efficient electrocatalysts for hydrogen production. Within the past few years, transition metals (TMs) have been studied as efficient electrocatalysts due to its excellent property.9,10 A large number of transition metal phosphides (TMPs) have been designed and synthesized, including FeP,10,11 NiP,12,13 CoP,14 and MoP.15 However, their acid instability and unsatisfactory activity have kept them from further applications.16,17 In order to solve these problems, carbon material supported TMPs hybrid materials have been constructed to improve the properties and stability of electrocatalysts for HER.18,19 So far, graphene,20 carbon cloth,21,22 and carbon nanotubes22 have been applied for enhancing the activity and stability of TMPs toward HER. Unfortunately, the tedious processing method mentioned above for carbon materials coupled with their relatively expensive price have further limited their large-scale © XXXX American Chemical Society
application. Carbon quantum dots (CDs), quasi-spherical zero-dimensional nanoparticles with diameters of less than 10 nm, discovered in 2004 should be efficient carbon-based conductors based on their high conductivity, simple synthesis, and lower cost.23,24 Recently, CDs−metal-based hybrid nanomaterials have been developed as potential electrocatalysts for energy conversion and storage.25 In our previous study, CDs−metal hybrid materials were successfully applied in electrocatalysis for the oxygen evolution reaction (OER) since CDs showed good conductivity, wide pH tolerance, and large specific surface area.26 Based on the above considerations, we proposed to further broaden the applied scope of CDs in water splitting and employed the CDs as the ideal catalyst support to synthesize the CDs−metal hybrid materials for efficient hydrogen production. Herein, we successfully constructed hollow NiMoP@CDs nanopetals for improving the electrocatalytic activity of HER by a feasible hydrothermal method. Interestingly, CDs not only efficiently modulated the morphology of NiMoP from solid nanoflowers to highly open NiMoP hollow nanopetals to improve the specific surface areas of catalysts but also significantly enhanced their conductivity. The efficient and accessible electrocatalysts were achieved by optimizing the compositions of NiMoP@ Received: Revised: Accepted: Published: A
April 8, 2019 May 31, 2019 July 3, 2019 July 3, 2019 DOI: 10.1021/acs.iecr.9b01899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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linear sweep voltammetry (LSV) for HER in 0.5 M H2SO4 solution at a scanning rate of 5 mV s−1. The stability test was performed by the chronopotentiometric (CP) method at a constant current density of 10 mA cm−2 for 40 h. For comparison, Ni5Mo3P nanopetals were also used as the baseline catalysts. It is worth noting that all the potentials measured were calibrated to a reverse hydrogen electrode.
CDs, which exhibited superior HER activity with a low overpotential of 183 mV at 10 mA cm−2 in 0.5 M H2SO4 solution, even better than many reported catalysts.27−30 Notably, the hollow NiMoP@CDs nanopetals also showed long-time stability for 40 h with negligible potential variation at a current of 10 mA cm−2.
2. EXPERIMENTAL SECTION 2.1. Preparation of CDs. CDs were prepared by a facile solid-phase thermal treatment method according to the reported literature.31 Experimentally, 0.30 g of dicyandiamide (DCD) and 0.45 g of citric acid (CA) were accurately weighed and ground in an agate mortar, and the mixture was put into tube furnace for heating from room temperature to 170 °C at a rate of 10 °C/min with a continuous nitrogen flow and sustained for 90 min. After being cooled to room temperature naturally, the as-prepared products were dispersed into deionized water with centrifugation at 10000 rpm for 15 min to remove large particles. Then the supernatant was further purified with a 0.22 μm filter membrane. Finally, a yellow CD powder was obtained by freeze-drying. 2.2. Preparation of Solid Ni3Mo3P Nanoflowers. Thirty milligrams of NiCl2·6H2O, 30 mg of Na2MoO4·2H2O, 20 mg of NaH2PO2·2H2O, and 60 mg of urea were dissolved in 10 mL of deionized water under vigorous stirring for 10 min, and the solution was added into a Teflon-lined stainless autoclave and maintained for 10 h at 180 °C. After being cooled to room temperature, the solid Ni3Mo3P nanopetals were obtained by centrifugation and washed several times with ethanol and acetone. The Ni1Mo3P and Ni5Mo3P were prepared by the same method but the amount of NiCl2·6H2O was changed to 10 and 50 mg, respectively. 2.3. Preparation of Hollow Ni5Mo3P@CDs3 Nanopetals. The Ni5Mo3P@CDs3 was synthesized through the same approach as solid Ni3Mo3P nanoflowers but an extra 3 mg of CDs was added in the 10 mL deionized water solution. The other products were prepared by changing the concentration of CDs from 3 mg/mL to 1 and 5 mg/mL, respectively, while the volume of deionized water was still 10 mL. These two obtained products were respectively labeled as Ni5Mo3P@CDs1 and Ni5Mo3P@CDs5. 2.4. Characterizations. Powder X-ray diffraction (PXRD) patterns were collected using an Rigaku UIV X-ray powder diffractometer equipped with a Cu radiation source (λ = 0.15406 nm). Scanning electron microscope images were carried out using a XL30 ESEM FEG scanning electron microscope. The morphologies of samples were conducted on a JEOL-2100 transmission electron microscope (TEM), which operated at 100 kV. X-ray photoelectron spectroscopy spectra (XPS) were conducted on a JEOL JPS-9010 MC spectrometer. The Brunauer−Emmett−Teller (BET) surface areas of the products were determined using a Quadrasorb-S1 physical adsorption apparatus, and the pore-size distribution was estimated by the Barrett−Joyner−Halenda method. Thermogravimetric analysis (TAQ50) was performed to study the thermal properties of the products. 2.5. HER Measurements. All of the electrochemical measurements were performed using a CHI 760E electrochemical analyzer with a three-electrode system, the glass carbon electrode modified by catalyst was employed as the working electrode, a Ag/AgCl electrode was used as the reference electrode, and a graphene rod was used as the counter electrode. The polarization curves were obtained using
3. RESULTS AND DISCUSSION As schematically shown in Scheme 1, the preparations of solid NiMoP nanoflowers and hollow NiMoP@CDs nanopetals Scheme 1. Synthesis Scheme of Solid NiMoP Nanopetals and Hollow NiMoP@CDs Nanopetals
were carried out by a facile hydrothermal method in the absence and presence of CDs, respectively. First, the nanosheets were formed under hydrothermal condition, subsequently, amounts of CDs were adhered on the surface of the NiMoP nanosheets because the abundant of −OH and -NH2 groups that distributed on the surface of CDs could firmly fix Mo6+ and Ni2+.32,33 And the adhered −OH and -NH2 groups could hindered the further accumulation of nanosheets, leading to the generation of hollow NiMoP@CDs nanohybrids. Conversely, the solid MoNiP nanoflowers were formed via self-assembled by lots of NiMoP nanosheets without CDs under the same reaction conditions. The as-obtained NiMoP nanopetals and NiMoP@CDs were thoroughly characterized by means of powder X-ray diffraction (PXRD), transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray photoelectron energy spectrometer (XPS), thermogravimetric (TGA), elemental mapping, and autosorb-iQ micromeritics. TEM tests were carried out to characterize the morphologies of the as-prepared materials. As shown in Figure 1a, CDs show a typical quasispherical structure with a diameter of about 4−6 nm (Figure S1). Figure S2a elucidates a detailed morphology of Ni5Mo3P; a uniform flower-like structure with a diameter of about 1.1 μm can be clearly observed (Figure S2b). After detailed observation, the surface of the NiMoP was found to be composed of many nanosheets (Figure S3), confirming the above speculated forming process of NiMoP. As CDs were added in the process of synthesized Ni5Mo3P, the center of the products became obviously brighter than the edge, which manifested the hollow structure of Ni5Mo3P@CDs (Figure 1c,d). The unique hollow structure of Ni5Mo3P@CDs3 not only allowed the aggregation (Figure S4) to enhance the surface areas and expose more surface active sites but also facilitated mass transfer during the HER process.34 Parts e and f of Figure 1 further depicted the clear morphology of Ni5Mo3P and Ni5Mo3P@CDs3 by SEM; as observed, the solid and hollow structures can be clearly observed from these two images. Moreover, the other composites showed the typical B
DOI: 10.1021/acs.iecr.9b01899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. TEM images of (a) CDs, (b) Ni5Mo3P, and (c, d) Ni5Mo3P@CDs3. SEM images of (e) Ni5Mo3P and (f) Ni5Mo3P@CDs3. (g) corresponding EDS mapping showing Ni, Mo, P, C. (h) XRD patterns of Ni5Mo3P@CDs3 and Ni5Mo3P.
between Ni5Mo3P@CDs3 and Ni5Mo3P, which can be attributed to the synergistic effect of NiMoP and CDs, indicating the Ni5Mo3P@CDs3 has been successfully constructed. Moreover, the clear lattice fringe illustrated by the HRTEM image is 0.264 Å, corresponding to the (611) planes of Mo2Ni6P3 (Figure S6), further revealing the singlecrystalline feature of the Mo2Ni6P3 nanosheets. In order to obtain insight into the texture properties and the pore-size distribution of Ni5Mo3P and Ni5Mo3P@CDs3, N2
hollow structure as Ni5Mo3P@CDs3 (Figure S5), demonstrating that this method is feasible for the fabrication of NiMoP@ CDs with tunable compositions.35 The EDX elemental mapping analysis of N5iMo3P@CDs3 suggests that Ni, Mo, P, and C elements were uniformly distributing throughout the whole hollow Ni5Mo3P@CDs3 nanopetals (Figure 1g). The XRD diffraction peaks of NiMoP@CDs can be well indexed to the Mo2Ni6P3 (JCPDF no. 89-2742), suggesting the formation of NiMoP. Meanwhile, a slight shift can also be observed C
DOI: 10.1021/acs.iecr.9b01899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. (a) N2 adsorption−desorption isotherms and pore size distribution of Ni5Mo3P and Ni5Mo3P@CDs3. (b) TGA curves of Ni5Mo3P and Ni5Mo3P@CDs3.
the peaks located at 132.37 and 133.13 eV were assigned to P 2p3/2 and P 2p1/2, respectively,44 and the peak appearing at 133.79 eV in the higher region could be ascribed to the P−O, which was attributed to the phosphide oxidation.45 The C 1s peak spectrum in Figure 3e can be divided into three peaks located at 284.77 eV for C−O, 285.88 eV for CC, and 288.29 eV for N−CO,46 respectively, confirming that the NCDs contained amounts of −OH, −NH2 and CO groups embedded into the surface of
[email protected] Besides, the O 1s signals of Figure 3f can be fit into two peaks located at 530.91 eV for CO and 531.90 eV for C−O, indicating the existence of abundant −OH in the surface of CDs. To obtain insight into the role of N-CDs on improving the electrocatalytic property, a series of electrochemical experiments of NiMoP and NiMoP@CDs were carried out. The HER activities were first evaluated by LSV using a standard three-electrode system in 0.5 M H2SO4 at a sweep rate of 5 mV/s. Figure 4a exhibited the polarization curves of NiMoP with different ratios of Ni:Mo without adding CDs, all of which showed unsatisfactory HER activity. Subsequently, the HER activity of the optimized Ni5Mo3P that modulated with CDs was further investigated. The HER activities were recorded in Figure 4b, where the Ni5Mo3P@CDs1, Ni5Mo3P@CDs3, and Ni5Mo3P@CDs5 showed significantly improved electrocatalytic performance toward HER when compared with Ni5Mo3P. Some CDs displayed poor electrocatalytic activity, which needed more than 500 mV to achieve the current density of 10 mA cm−2. These results indicated that the synergistic effect of CDs and Ni5Mo3P could greatly improve the HER activity. As seen in Figure 4c, the Ni5Mo3P@CDs3 exhibited excellent electrocatalytic activities and its overpotential was 183 mV at 10 mA cm−2, which was lower than solid Ni5Mo3P nanoflowers and many reported electrocatalyst. In addition, we have also conducted the electrochemical tests in in neutral (pH = 7) and alkaline media (pH = 14). As shown in Figure S7, the Ni5Mo3P@CDs3 required the overpotential of only 262 mV to achieve the current density of 10 mA cm−2 in alkaline media (pH = 14), further confirming the high electrocatalytic HER performance of Ni5Mo3P@CDs3. The Tafel slopes of all composites and Ni5Mo3P derived from the polarization curves were also obtained in Figure 4d. With the addition of N-CDs, the obtained Tafel slopes for NiMoP@CDs were obviously decreased, manifesting the fine HER kinetics and fast electron transfer rate modulated by the
adsorption−desorption analyses were performed. The hysteresis between the adsorption and desorption isotherm indicates the mesoporous nature of the sample with a BET surface area of Ni5Mo3P@CDs3 reaching 274.78 m2 g−1 (Figure 2a), which is 1.4 times as large as that of bare Ni5Mo3P (193.05 m2 g−1). It is found that the large surface area of Ni5Mo3P@CDs3 is ascribed to its unique hollow nanopetal structure. Compared to Ni5Mo3P with smaller pore volume (0.45 m3 g−1), Ni5Mo3P@CDs3 with obvious larger pore volume (0.74 m3 g−1) possesses a porous structure, which is beneficial for providing more contacts between active sites and electrolytes and increasing the sufficient pathways for the transport of electron or proton during HER process.36 In addition, Figure 2b depicts the thermal property of the Ni5Mo3P and Ni5Mo3P@CDs3 in the air. Although the tendency of their weight loss curves was approximately similar, the thermodynamic stability of Ni5Mo3P was remarkably improved after the incorporation of CDs, indicating the Ni5Mo3P@CDs3 was successfully prepared. There exist three main weight losses during the calcination process, where the early weight loss of about 8% for Ni5Mo3P@CDs3 observed at around 320 °C is due to the water molecules and partial phosphorus element loss37 and the weight loss of Ni5Mo3P is about 17%. Subsequently, a rapid weight loss of about 6% between 320 and 390 °C can be found in the weight loss curve of Ni5Mo3P@CDs3, which is mainly attributed to the pyrolysis of weak hydrogen interaction, phosphorus element, as well as the collapse of the carbon skeleton.38 Finally, the gradual weight loss at 390−580 °C is due to the further skeleton collapse of the materials. XPS measurements were performed to comprehensively analyze the electric interactions and valence states of Ni5Mo3P@CDs3. In Figure 3a, the XPS full spectrum of Ni5Mo3P@CDs3 established the existence of P, Mo, C, N, O, and Ni, indicating the N-CDs have successfully bonded to Ni5Mo3P. The XPS spectroscopy of Ni 2p is shown in Figure 3b, and the peak deconvolution of Ni 2p in Ni5Mo3P@CDs3 displays two typical peaks located at 856.03 and 873.50 eV, corresponding to the Ni 2p3/2 and Ni 2p1/2, along with two satellite peaks of 861.74 and 880.08 eV, respectively,39−41 The Mo 3d region of Ni5Mo3P@CDs3 is shown in Figure 3c, which can be deconvoluted into two typical spin−orbit doublets, consistent with the peak of typical Mo 3d.42,43 As shown in Figure 3d, the XPS peak of P 2p consists of three peaks, where D
DOI: 10.1021/acs.iecr.9b01899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 3. XPS spectra of (a) survey scan, (b) Ni 2p, (c) Mo 3d, (d) P 2p, (e) C 1s, (f) O 1s.
CDs.48 After a careful observation, the Tafel slopes of Ni5Mo3P with different contents of CDs were also demonstrated to be different, indicating that the quality of CDs added would affect their reaction kinetics. In order to study active sites and point out the essence of excellent HER performance, electrochemical active surface areas (ECSAs) of catalyst were evaluated by double layer-capacitance (Cdl) (Figure S8). All of the composites showed a larger double-layer capacitance (Cdl) value and ECSA than Ni5Mo3P, indicating CDs could expose abundant active sites available for intermediates.49 The Ni5Mo3P@CDs3 has the largest Cdl value of 7.06 mF cm−2 and largest ECSA of 117.7 cm2 (Figure 4e), which implies that the dopant amount of CDs also affects the active sites of composites, leading to the excellent HER activity. For the sake of gaining further insight into the reaction kinetics and HER properties, electrochemical impedance spectroscopy (EIS) analysis was also recorded as Figure 4f. The results indicate
that Ni5Mo3P@CDs showed a smaller diameter of impedance arc (DIA) than other samples in the high-frequency region and indicate the addition of CDs will promote the charge transfer and reaction. The Ni5Mo3P@CDs3 occupied the smallest DIA, implying appropriate addition of CDs will further accelerate the mobility of electron.50,51 In addition, the stability is another momentous parameter for HER properties of catalyst, which is usually evaluated by chronopotentiometry technique at the current density of 10 mA cm−2. In Figure 5, it is worth noting that the Ni5Mo3P@ CDs3 can keep constant potential for at least 40 h with no obvious attenuation in 0.5 M H2SO4, while the decrease of the chronopotentiometry of CDs and Ni5Mo3P can be observed at about 11 and 36 h, implying the CDs can efficiently improve the stability of HER of Ni5Mo3P. The synergistic effects between CDs and Ni5Mo3P are also favorable for promoting the HER stability. Moreover, the Ni5Mo3P@CDs3 could also E
DOI: 10.1021/acs.iecr.9b01899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. HER activity tests. (a) Polarization curves of Ni6Mo3P, Ni5Mo3P, and Ni3Mo3P in 0.5 M H2SO4 solution at a scan rate of 5 mV s−1. (b) Polarization curves of CDs, Ni5Mo3P, Ni5Mo3P@CDs1, Ni5Mo3P@CDs3, and Ni5Mo3P@CDs5 in 0.5 M H2SO4 solution at the scan rate of 5 mV s−1. (c) Overpotentials at the current density of 10 mA cm−2, (d) Tafel slopes, (e) double layer current vs scan rates plots, and (f) EIS of Ni5Mo3P, Ni5Mo3P@CDs1, Ni5Mo3P@CDs3, and Ni5Mo3P@CDs5.
Ni5Mo3P@CDs3 showed excellent HER property in 0.5 M
maintain the typical nanopetal structure with limited chemical composition variation after long-term tests (Figures S9 and S10), further confirming the high structure stability.
H2SO4, which was obviously superior to other composites and Ni5Mo3P. The chronopotentiometry technique revealed that
4. CONCLUSIONS In summary, a class of advanced Ni5Mo3P@CDs3 electrocatalysts was constructed using a facile hydrothermal method. The formation of a hollow structure was ascribed to the CDs that hurdle the assembles of nanosheets into solid material. Meanwhile, the addition of CDs can impede the aggregation of electrocatalysts, significantly enriching the surface active sites and facilitating charge transfer, leading to the large promotion of HER performance. A series of experiments showed that the
Ni5Mo3P@CDs3 occupied excellent stability with negligible potential variation over 40 h. More studies are in progress to further improve the water-splitting activity using this method. This work provides a sophisticated strategy for designing rational electrocatalysts by CD doping with excellent structural and electronic engineering to be applied in energy storage and conversion. F
DOI: 10.1021/acs.iecr.9b01899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 5. Prolonged CP of CDs, Ni5Mo3P, and Ni5Mo3P@CDs3 for 40 h.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01899. TEM and SEM images of the as-obtained samples; size distribution histogram and CV curves of different catalysts modified electrode for HER (PDF)
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AUTHOR INFORMATION
Corresponding Authors
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Lin Tian: 0000-0002-4345-5414 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Jiangsu Province Key R & D Projects of China (BE2016648), Key University Science Research Project of Jiangsu Province (16KJA210001), and the Natural Science Foundation of Jiangsu Province (BK20171169).
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REFERENCES
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Industrial & Engineering Chemistry Research
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DOI: 10.1021/acs.iecr.9b01899 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
