Highly Dispersive MoP Nanoparticles Anchored on Reduced

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Highly Dispersive MoP Nanoparticles Anchored on rGO Nanosheets for Efficient Hydrogen Evolution Reaction Electrocatalyst Yufei Zhang, Jun Yang, Qiuchun Dong, Hongbo Geng, Yun Zheng, Yunlong Liu, Wenjun Wang, Cheng-Chao Li, and Xiaochen Dong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07133 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Applied Materials & Interfaces

Highly Dispersive MoP Nanoparticles Anchored on rGO Nanosheets for Efficient Hydrogen Evolution Reaction Electrocatalyst Yufei Zhang,a‡ Jun Yang,b‡ Qiuchun Dong,b Hongbo Geng,a* Yun Zheng,a Yunlong Liu,c Wenjun Wang,c Cheng Chao Li,a* Xiaochen Dongb* a

School of Chemical Engineering and Light Industry, Guangdong University of Technology,

Guangzhou 510006, China b

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211800, China c

School of Physical Science and Information Technology, Liaocheng University, Shandong,

252059, China *Corresponding authors. E-mail addresses: [email protected]; [email protected]; [email protected] ‡

These authors contributed equally.

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ABSTRACT Electrochemical water splitting with non-noble metal catalysts provides an eco-friendly strategy for renewable production of hydrogen. In this study, the MoP@C@rGO composite was prepared via mild reactions through chemical bath and post annealing process. With the assistance of citric acid, the MoP@C@rGO composite containing ultrafine MoP nanoparticles with size of 3 nm anchored on two-dimensional C/rGO nanosheets has been obtained. The chelation effect with citric acid and the merits of rGO not only lead to affordable active sites but also improved the electrical conductivity and the stability at the same time. Serving as hydrogen evolution reaction (HER) electrocatalyst, the MoP@C@rGO composite presents a small overpotential of 168.9 mV at 10 mA cm-2. Its durability is more stable than that of the pure MoP, comparing samples of MoP@C and MoP@rGO. The relative high activity, stable performance as well as the simple preparation process make the MoP@C@rGO composite promising HER electrocatalyst.

KEYWORDS: ultrafine MoP nanoparticles; two-dimensional nanosheets; rGO; ative sites; hydrogen evolution reaction

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INTRODUCTION With the ever growing demands for green and renewable resources, a great deal of researches have been focused on sustainable energy conversion or storage systems, including batteries, supercapacitors, hydrogen generation.1-4 Nowadays, hydrogen has been considered as an ideal substitute for non-renewable fossil fuels. However, scalable production of hydrogen with high efficiency and low cost still remains challenging. Electrochemical water splitting, which offers scalable hydrogen production with eco-friendly character, draws considerable research attention.5-7 To this end, the development of active electrocatalysts with lower overpotential, resulting in the energy-cost reduction during the water-splitting process, has become the main obstacle for the study on HER.8-9 Presently, precious Pt exhibits the highest active HER capability in fuel cells, but suffers from high cost, scarcity in supply and poor durability for scalable applications.10 Thus, more and more researches have been concentrated on the innovation of non-noble metal based catalysts to fulfill the demands of low-cost and high activity.11 Transition-metal compounds, especially mixed oxides, have been extensively studied as HER electrocatalysts in recent years.12-14 Nevertheless, their relatively sluggish electron kinetics due to the inherently low electrical conductivity significantly reduces the HER activity.15-16 Recently, molybdenum-based compounds such as MoS2,17 MoSe2,18-19 MoB,20 MoN,21 are found to be attractive alternatives with promising HER activity. Among a variety of reported candidates, molybdenum phosphides have emerged as promising HER electrocatalysts. Owing to the reduced metal-hydrogen interaction offered by Mo-P bonds, hydrogen becomes much easier to release from the active sites.22-23 It is also reported that 3



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the mono-phosphorus phase MoP often shows metallic characteristics, which could afford relatively high electronic conductivity and play positive role in enhancing catalyst efficiency.24 Moreover, the lone-pair electrons provided by phosphorous atom can effectively sustain the surface charge state.25 Thus, molybdenum phosphide (MoP) is expected to be promising catalysts in HER for its relatively high electrical conductivity, metallic behavior and excellent thermal stability.24 It is widely acknowledged that nanosized morphology is ideal for HER electrocatalyst since its exciting surface structure could lead to more exposed active sites. Porous structure together with the high electrical conductivity is also beneficial for transfer of electrons and reactants inside the electrocatalysis. Therefore, it is promising to take these considerations into the design of MoP. On the other hand, graphene oxide (rGO) has been widely investigated as an effective conducting support in energy related research, because of the merits of 2D structure, high conductivity and structural stability.26-28 It is also reported that substrates, including porous carbon and graphene, could effectively avoid the aggregation of nanoparticles.29 Meanwhile, the resulting porous structure could supply large surface area for electrolyte and active materials, resulting in enhanced catalytic behavior. Although tremendous efforts have been devoted to coupling graphene with different transition metal compounds, ultrafine MoP nanoparticles with graphene support for HER have not been realized so far. Inspired by above considerations, a two-step strategy to fabricate MoP@C@rGO hybrid nanosheets was developed. Firstly, citric acid was used to ensure the uniform distribution of Mo precursors on rGO substrate. Subsequently, the precursors were converted to 3 nm MoP nanoparticles via annealing with the citric acid decomposed to carbon and buffering the 4


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MoP nanoparticles simultaneously. Hybridizing the rGO can not only effectively enhance the electrical conductivity of MoP, but also improve the diffusion of the electrolyte and the products due to the two-dimensional nanostructures. When tested as HER electrocatalyst, the achieved MoP@C@rGO obtained low overpotential of 168.9 mV (10 mA cm-2) with a Tafel slope of 79 mV dec-1 in acidic media, indicating promising HER activity. EXPERIMENTAL SECTION Synthesis of the MoP@C@rGO composite, MoP@C composite, MoP@rGO composite and pure MoP Graphene oxide (GO) was achieved according to reference with simple modification.30-31 A GO suspension with concentration of 5 mg mL-1 was prepared for further usage via dispersing certain amount of GO in DI water. 0.2 mmol (NH4)6Mo7O24·4H2O, 1.4 mmol (NH4)2HPO4 and 2.8 mmol citric acid were dissolved in 40 mL distilled water. Then, 10 mL of GO suspension (5 mg/mL) was added under stirring. After ultrasonication for 30 min, the solution was reacted in heating mantle at 90 oC for 6 h. The resulted gel was collected and dried at 120 oC for 2 h. Finally, MoP@C@rGO composite was obtained by calcination at 750 oC for 6 h with Ar/H2 flow. The synthetic procedures of MoP@C composite, MoP@rGO composite and pure MoP are similar with the above reaction with minor modification. The MoP@C composite was achieved without GO suspension. The MoP@rGO was prepared without adding citric acid. And pure MoP was synthesized directly with 0.2 mmol (NH4)6Mo7O24·4H2O and 1.4 mmol (NH4)2HPO4. Characterization Field-emission scanning electron microscope (FESEM, JEOL, JSM-7600F) and 5



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transmission electron microscope (TEM, JEOL 2010F) were used for exploring the morphology information of the MoP@C@rGO, MoP@C, MoP@rGO and MoP. The X-ray power diffraction patterns were recorded on Shimadzu 6000 diffractometer with radiation of Cu Kα1 (λ = 0.15406 nm). The surface area and pore information were investigated by nitrogen adsorption/desorption isotherms using the porosimetry system (ASAP 3020). The BET surface area of the MoP@C@rGO were determined via the Brunauer-Emmett-Teller method. Electrochemical measurements The electrochemical characterizations were conducted in 0.5 M H2SO4 with three-electrodes system on an electrochemical workstation (CHI 760E, CHI Inc., USA). Ag/AgCl electrode, Pt and a glassy carbon electrode were employed as the reference, the counter and the working electrode, respectively. Working electrode was prepared by mixing 5.0 mg the achieved sample with carbon black (1.0 mg), Nafion solution (50 µL, 5.0 % Nafion in ethanol) and DI water (450 µL). The mixture was ultrasonicated to form uniform dispersion before use. Finally, 5.0 µL suspension was dropped onto working electrode with 3 mm in diameter. After fully dried, the working electrode was cycled by cyclic voltammetry test in an Ar-saturated electrolyte. The working electrode can be used for further test until reproducible CV curves was obtained. Commercial Pt/C catalyst was tested under the same condition for comparison. RESULTS AND DISCUSSION The MoP@C@rGO composite was prepared via a facile chemical bath method and a following high temperature phosphidation process. The XRD pattern of the MoP@C@rGO 6


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composite is shown in Figure 1a. The peaks shown in the diffraction pattern can be indexed to hexagonal MoP phase (JCPDS No.24-0771) without confusion. Specifically, the eight characteristic peaks located at 27.9°, 32.2°, 43.1°, 57.5°, 64.9°, 67.9°, 74.3° and 85.7° can be assigned to [001], [100], [101], [110], [111], [102], [201] and [112] planes of the hexagonal MoP phase. However, characteristic peaks of rGO and carbon cannot be directly detected in the XRD pattern, which may be attributed to the high density loading of MoP. For comparison, pure MoP, MoP@C and MoP@rGO composites were synthesized at the same conditions except with or without the addition of GO and citric acid. From the XRD patterns (Figure S1A, Figure S2A, Figure S3A), all the diffraction peaks of MoP, MoP@C and MoP@rGO composites can be assigned to the same phase as MoP@C@rGO composite, noted as MoP (JCPDS No. 24-0771). Owing to the low carbon content in the composite, no specific peaks can be assigned to carbon phase. Figure 1B-C shows the structure of hexagonal MoP. As can be known form the figure that six P atoms trigonal coordinate with one Mo atom and thus forming a tungsten carbide-type structure. Figure 2A and B show the SEM images of MoP@C@rGO composite. The TEM image taken from such nanosheet is shown in Figure 2C. It can be clearly observed that numerous MoP nanoparticles with uniform size are well distributed on the C@rGO matrix. TEM images in high magnification further demonstrate the diameter of the MoP nanoparticle is around 3 nm (Figure 2 D and E). A high-resolution TEM (HRTEM) image in the inset of Figure 2E was taken on one single nanoparticle. The interplanar distance of 0.28 nm was achieved from the presented clear lattice fringes, which can be indexed to the (100) plane of MoP. The EDX elemental mapping of MoP@C@rGO composite (Figure 2F-I) indicates 7



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that all the composed elements, including Mo, C and P are homogeneously distributed over the C@rGO substrate, suggesting MoP nanoparticles are well-proportioned and coated with carbon layers. The structure of pure MoP was also investigated by using SEM and TEM. As shown in Figure S1B-D, MoP particles exhibit serious aggregation when without carbon supporting matrix. TEM images (Figure S1E and F) further identify that the diameter of the MoP particles is larger than 100 nm, which is thirty times that of MoP nanoparticles in MoP@C@rGO composite. Thus, it can be proposed that the citric acid and GO support are vital for the uniform formation of extra small MoP nanoparticles during the synthesis. To clearly investigate the morphology influence offered by the additions of citric acid and rGO, detailed structure information of MoP@C and MoP@rGO composite was explored. As demonstrated in Figure S2B and C, MoP particles in MoP@C composite are fully encapsulated by citric derived carbon. SEM and TEM images (Figure S3B and 3C) of MoP@rGO composite exhibit that MoP nanoparticles with a diameter of about 100 nm are wrapped by rGO nanosheets. By comparison with MoP, MoP@C, and MoP@rGO composite, it can be concluded that only by synergistic effects from citric acid and rGO can the 3 nm sized MoP nanoparticles be uniformly distributed on C@rGO substrate. This may be attributed to the capping agent effect provided by citric acid, which can chemically bond with Mo, leading to the homogeneously scattered nucleation sites. To further identify the elemental compositions and the corresponding valence states of MoP@C@rGO composite, X-ray photoelectron spectroscopy (XPS) was conducted. The full survey XPS spectrum of MoP@C@rGO reveals the coexistence of Mo, P and C elements (Figure S4). The high-resolution spectra of the composing elements are depicted in Figure 3. As shown in 8


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the Mo 3d spectrum (Figure 3A), it can be fitted into three corresponding peaks. The peak located at 228.5 eV can be assigned with Mo 3d5/2.32 Another two peaks centered at 236.2 and 232.8 eV are associated with the Mo6+ 3d3/2 and 3d5/2 of MoO3, respectively. This may arise from the partly oxidation of the sample surface when exposed to air.33 Moreover, for the P 2p spectrum (Figure 3B), obvious peaks at 129.4 and 130.3 eV are found, which could be assigned to P2p1/2 and P2p3/2 in MoP. Another prominent peak at 130.2 eV fitted with peaks centered at 133.6 eV and 134.5 eV relating with the characteristic peak of P-C and P-O, indicating the interaction of MoP with carbon and the partly oxidation on the sample surface when exposure to air.34 Figure 3C, the C 1s spectrum can be assigned to three peaks. The obvious peak centered at 284.8 eV corresponded with graphitic C atoms. Other two peaks at 285.9 and 288.6 eV represent the existence of C–O and O–C=O.35-37 Nitrogen adsorption-desorption (Figure S5) was carried out to characterize the inner structure of the MoP@C@rGO. Determined by BET method, the specific surface area of the MoP@C@rGO is calculated to be 73.3 m2 g-1 (Figure S5A). As shown in Figure S5B, the pore size distribution falls in the range of 5-10 nm, which is in agreement with the TEM results. The relatively large specific surface area may provide extra active sites and facilitate the penetration of electrolyte, which is an ascendant feature when acting as electrocatalyst. In order to fully explore the structural advantages of MoP@C@rGO for HER, the electrocatalytic performances of MoP@C@rGO, MoP, MoP@C and MoP@rGO were evaluated via linear sweep voltammetry (LSV) technique. Three-electrode system was tested in 0.5 M H2SO4 acidic solution at a scan rate of 2 mV s-1. For comparison, the HER 9



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activity of the commercial Pt/C catalyst was also tested under the same condition. As shown in Figure 4A, Pt/C exhibits high HER catalytic performance with an extraordinarily low overpotential and ultra large current density. Apart from Pt/C, the synthesized MoP@C@rGO composite exhibits the best electrocatalytic HER activity among the four samples, demonstrating a very low onset potential of ~130 mV. However, MoP, MoP@C and MoP@rGO show much inferior HER activities, with an onset potential of 191, 230 and 150 mV, respectively. As indicated in Figure 4B, an overpotential of 168.9 mV is needed for MoP@C@rGO electrode when achieving 10 mA cm-2. In comparison, MoP, MoP@rGO and MoP@C display poor performance and overpotentials of 280.5 mV, 223.5 mV and infinity when approaching 10 mA cm-2, respectively. Comparing the above results, it is obvious that carbon cooperation plays a vital role for performance enhancement since bare MoP shows poor activity. However, the exposed active sites of MoP are blocked when only combining with citric acid derived amorphous carbon, resulting in extraordinary low activity. In the case of the MoP@C@rGO composite, ultrafine MoP nanoparticles are anchored on C/rGO substrate, leading to the overpotential of 168.9 mV for achieving the same catalytic current density. Thereby, the combination of citric acid and the GO shows synergistic effects, which could help improve the conductivity of MoP and enlarge the active surface of the material. The mechanism for the HER process in acidic electrolytes can be divided into three principal steps, including the Volmer, the Heyrovsky, and the Tafel steps. In order to study the mechanism, the corresponding Tafel plots for MoP@C@rGO, MoP, MoP@C, MoP@rGO and commercial Pt/C electrocatalysts can be plotted according to the Tafel 10


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equation η=blogj+a, where b represents Tafel slope and j is the current density, as presented in Figure 4C. The Pt/C shows an extra low Tafel slope of 45 mV dec-1, indicating its high activity. The Tafel slope of 79 mV dec-1 is achieved for MoP@C@rGO, which is much smaller than those of MoP (94 mV dec-1), MoP@C (166 mV dec-1) and MoP@rGO (98 mV dec-1). It suggests that the hydrogen evolution process of these composite follows the Volmer-Heyrovsky mechanism, maybe because that the discharge reaction is the rate limiting step.38 Moreover, with the cooperation of rGO and carbon matrix, kinetics during the discharge step is enhanced, resulting in the significant improvement of Tafel slope. The exchange current density (j0) of the MoP@C@rGO composite can be gained by extrapolating the Tafel plot, which is calculated to be 0.67 mA cm−2. The aforementioned results indicate the high HER activity which include relative low overpotential, low Tafel slope, and high exchange current density of the MoP@C@rGO composite electrode in acidic solution. The long-term stability performance of the MoP@C@rGO composite is carried out for 17 h in 0.5 M H2SO4 with a current density of ~10 mA cm-2. It can be clearly seen that a stable potential of around -0.17 V was observed through the 17 h of continuous operation (Figure 4D), implying good durability under HER condition. The slightly fluctuation of the curve may due to the generated hydrogen cannot diffuse from the surface of the catalyst immediately, which may block the effective active sites and hinder the performance. In addition to the exhibited high activity, the MoP@C@rGO electrode also presents stable cyclability for HER as indicated in Figure 5A. No decay in cathodic current density was observed for the polarization curve even after repeatedly testing for 17 h. The catalytic 11



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activity becomes even better after cycles in acidic electrolyte. The electrochemical impedance spectroscopy (EIS) of MoP@C@rGO, MoP, MoP@C and MoP@rGO were measured for exploring the structure advantages and the corresponding Nyquist plots are shown in Figure 5B. It can be seen obviously that the semicircle of MoP@C@rGO composite in high frequency range is the smallest among the four electrodes, indicating the lowest charge transfer resistance. In addition, vertical lines in the low-frequency region correspond with the diffusion resistance. It can be obviously seen that the MoP@C@rGO composite obtains the most vertical line, suggesting the MoP@C@rGO electrode possesses reduced electrical resistivity and ionic resistances when compared with other electrodes. CONCLUSION In summary, we have developed a facile strategy to prepare ultrafine MoP nanoparticles anchored on rGO assisted by citric acid. The obtained MoP@C@rGO exhibits significant enhancement in HER activity with a low onset potential (130 mV), a small overpotential (168.9 mV at 10 mA cm-2) and superior durability when compared with MoP, MoP@C and MoP@rGO. The excellent HER performance of the MoP@C@rGO can be ascribed to the following reasons. Firstly, with the assistant of citric acid, highly dispersive MoP nanoparticles with a size of 3 nm were achieved. The nanosized MoP particles afford relative short diffusion length for electrolyte penetration, thus leading to the enhancement in diffusion rate. Secondly, the combination with rGO and the carbon resulting from high temperature phosphorization of citric acid not only improves the electrical conductivity of the electrode but also increases the specific area, which can provide more catalytic sites. This work also presents a facile solution for the construction of high-performance catalysts 12


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with nanosized phosphide/carbon composite. SUPPORTING INFORMATION The XRD patterns, SEM and TEM images of pure MoP, MoP@C, MoP@rGO are available. The integrated XPS spectrum, the N2 adsorption-desorption isotherm and the pore size distribution of MoP@C@rGO are also given. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61525402, 61775095, 51771058), Characteristic Innovation Project of Guangdong Province Education Department Grant (2016KTSCX032). The Project also supported by Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme. References (1) Goodenough, J. B.; Park, K. S. The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 2013, 135, 1167-1176. (2) Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332-337. (3) Fan, H.; Yu, H.; Zhang, Y.; Guo, J.; Wang, Z.; Wang, H.; Zhao, N.; Zheng, Y.; Du, C.; Dai, Z.; Yan, Q.; Xu, J. 1D to 3D Hierarchical Iron Selenide Hollow Nanocubes Assembled from FeSe2@C Core-Shell Nanorods for Advanced Sodium Ion Batteries. Energy Storage Mater. 2018, 10, 48-55. (4) Lin, Y.; Qiu, Z.; Li, D.; Ullah, S.; Hai, Y.; Xin, H.; Liao, W.; Yang, B.; Fan, H.; Xu, J.; Zhu, C. NiS2@CoS2 Nanocrystals Encapsulated in N-Doped Carbon Nanocubes for High Performance Lithium/Sodium Ion Batteries. Energy Storage Mater. 2018, 11, 67-74. 13



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Figure 1. (A) XRD pattern of MoP@C@rGO composite. (B-D) Corresponding MoP crystal structure.

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Figure 2. (A, B) SEM images of MoP@C@rGO, insets showing the cross-section SEM images. (C-E) TEM images of the MoP@C@rGO (inset of (E) shows a HRTEM image). (F) STEM image and (G-I) elemental mapping images of Mo, P and C of MoP@C@rGO.

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Figure 3. XPS spectra of (A) Mo 3d, (B) P 2p and (C) C 1s regions of MoP@C@rGO.

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Figure 4. (A) Polarization curves, (B) the histogram of overpotential, and (C) corresponding Tafel plots of MoP@C@rGO, MoP, MoP@C, MoP@rGO and Pt/C in 1 M H2SO4. (D) Time-dependent current density curve under a static overpotential of 170 mV for 17 h of MoP@C@rGO.

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Figure 5. (A) Catalyst stability test for MoP@C@rGO before and after stability test. (B) Electrochemical impedance spectroscopy (EIS) of MoP@C@rGO, MoP, MoP@C and MoP@rGO.

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Highly Dispersive MoP Nanoparticles Anchored on Reduced

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