- Email: [email protected]
Monodispersed and well-dispersed RhxP nanoparticles decorated on phosphorus-doped nitride carbon for efficient alkaline and acidic hydrogen evolution
Applied Surface Science 489 (2019) 796–801
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Monodispersed and well-dispersed RhxP nanoparticles decorated on phosphorus-doped nitride carbon for efficient alkaline and acidic hydrogen evolution Caili Xu, Qi Wang, Rong Ding, Yi Wang, Yun Zhang, Guangyin Fan
T
⁎
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal phosphide Rhodium Hydrogen evolution reaction Monodispersed
Metal phosphides (MPs) synthesized by solid-state reaction have shown considerable potential in hydrogen evolution reaction (HER) from water splitting compared with that of Pt. However, large-sized MP nanoparticles (NPs) with irregular morphologies are inevitably achieved via the solid-state reaction with high-temperature calcination, which lead to the reduced availability of the active sites. Here, a simple and straightforward strategy was developed to synthesize monodispersed, uniformly distributed, and small RhxP NPs decorated on phosphorus-doped nitride carbon (PNC) via the pyrolysis of rhodium(Ш) chloride, triphenylphosphine, and melamine (RhxP/PNC-m). The RhxP/PNC-m that served as a HER electrocatalyst displayed excellent performance with the overpotentials of 24 and 27 mV at 10 mA cm−2 in basic and acidic solutions, respectively. This catalyst also showed good long-term stability in both media. The high-performance of RhxP/PNC-m should be attributed to the small, monodispersed, and highly distributed RhxP NPs on PNC support, which can supply abundant catalytically active sites for HER. The developed strategy for the synthesis of small RhxP NPs may offer new opportunities toward the electrochemical applications of other metal phosphides.
1. Introduction Hydrogen has been recognized as a promising future energy carrier attributed to the concerns over the massive consumption of fossil fuels and environmental pollutions [1–4]. Electrochemical water splitting is receiving unprecedented attention due to its featured merits, such as high purity of hydrogen production and carbon-neutral nature [5–8]. The design and synthesis of effective electrocatalysts are important to accelerate hydrogen evolution reaction (HER) from electrochemical water splitting in both basic and acidic conditions [9,10]. Pt has been demonstrated as the state-of-the-art electrocatalyst with the highest exchange current density and the lowest Tafel slope in acidic conditions [11,12]. Nevertheless, Pt-based electrocatalysts still possess the disadvantages of high price, limited availability, and poor stability for HER. Considerable efforts have been exerted to develop cost-effective electrocatalysts with high electrochemical properties for HER. Unfortunately, most of the developed electrocatalysts show unsatisfactory performances compared with that of Pt. Thus, the exploration of Pt-free catalysts with high performance for electrochemical HER is highly desirable but remains a considerable challenge. Pt-group noble metal (i.e., Ru and Rh) phosphides (MPs) have been
⁎
investigated as potential electrocatalysts for HER and gained rapid developments because of their high performances, comparison with non-noble MPs [13–15]. Two main synthetic strategies, including wetchemical and solid-state reactions, have been developed to produce MPs. Although some of the developed electrocatalysts have shown stratified activity toward HER, the following drawbacks must be overcome. (1) With regard to wet chemical syntheses, a large number of capping agents are required to control the size and obtain monodispersed MPs. The purity of capping agents is a key factor to reproduce catalyst, however many agents, such as oleylamine, have impurities [16,17]. Additionally, the use of tri-n-octylphosphine as P source requires a strict air-free condition due to the generation of corrosive and flammable P species during MP synthesis. (2) Regarding solid-state method via the pyrolysis of metal precursors and P sources, controlling the morphology and size of MPs at high calcination temperature is difficult. Although phosphorization can be achieved using salt hypophosphite (NaH2PO2/NH4H2PO2) at low calcination temperature, the generation of hazardous PH3 gas is inevitable [18]. Therefore, the simple and straightforward synthesis of small, monodispersed, and highly dispersed MPs deposited on carbon support with comparable Pt performance for HER remains challenging.
Corresponding author. E-mail address: [email protected] (G. Fan).
https://doi.org/10.1016/j.apsusc.2019.06.004 Received 27 March 2019; Received in revised form 17 May 2019; Accepted 1 June 2019 Available online 03 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
Here, a simple and straightforward strategy was developed to synthesize small, monodispersed, and well-distributed RhxP nanoparticles (NPs) supported on phosphorus-doped nitride carbon (PNC) via the pyrolysis of rhodium(Ш) chloride, triphenylphosphine, and melamine. The as-prepared RhxP/PNC-m can be used as an effective electrocatalyst for HER catalysis with the low overpotentials of 24 and 27 mV at 10 mA cm−2 in 1.0 M KOH and 0.5 M H2SO4, respectively. The catalyst also showed good long-term stability in both media. The facile and straightforward preparation of such highly active RhxP/PNC can be extended to the production of other MPs for electrochemical applications. 2. Experimental
Scheme 1. Synthesis and electrocatalytic HER over the catalysts.
2.1. Materials and chemicals 2500 diffractometer with a Cu Kα radiation source. The surface compositions and chemical states of the elements on the prepared materials were obtained by using the X-ray photoelectron spectroscopy (XPS) carried out by a Thermo ESCALAB 250 Axis Ultra spectrometer. The LabRam HR Evolution spectrometer (532 nm laser wavelength) was used to collect Raman spectrum.
RhCl3·nH2O with Rh content of 39 wt% was supplied by Kunming Institute of Precious Metals, China. Triphenylphosphine powder (PPh3, 99%) and melamine were purchased from Alfa Aesar. Ethanol and urea were brought from Aladdin Industrial Inc., China. The obtained chemicals were directly used without further purification. De-ionized (DI) water (resistivity of 18.2 MΩ cm) was used for electrochemical measurements.
3. Results and discussion As shown in Scheme 1, two samples can be synthesized via a facile and straightforward process through the solid-state reaction by using rhodium(Ш) chloride as the metal precursor and PPh3 as P source in the presence of melamine or urea. The resultant products were applied as electrocatalysts for HER in both acidic and basic solutions. Fig. 1a–f shows the TEM images and corresponding particle size distributions of RhxP/PNC-m and RhxP/PNC-u, respectively. Polydispersed RhxP NPs with diameters of 11.8 nm were formed in RhxP/ PNC-u sample (Fig. 1a–c). Several bright rings made up of discrete spots are shown in the selective area electron diffraction (SAED) pattern of RhxP/PNC-u (Fig. 1g). The spots indexed to the (304) plane of the Rh3P4 and (422) plane of Rh2P were clearly detected, which matched well with the XRD pattern of RhxP/PNC-u with the peaks at 43° and 86.5° (Fig. S1), respectively. Note that the TEM images of RhxP/PNC-m synthesized with melamine as the N source displayed the presence of small and monodisperse RhxP NPs with a diameter of 4.1 nm that were uniformly decorated on the support (Fig. 1d–f). High-resolution TEM (HRTEM) images (Fig. 1d–e, inset) showed the well-crystallized NPs with the interplanar spacings of 0.281 and 0.196 nm, corresponding to the (401) plane of Rh3P4 and (220) plane of Rh2P, respectively. SAED pattern of RhxP/PNC-m (Fig. 1h) also exhibited weak diffraction rings corresponding to the (220) plane of Rh2P and (401) plane of Rh3P4. The STEM-EDX mapping images (Fig. 1i–n) also proved that C, O, N, P, and Rh were uniformly dispersed across the RhxP/PNC-m sample, which matched well with the energy dispersive X-ray spectroscopy analysis (Fig. S2). These results indicate that the use of melamine was the key to the synthesis of small, monodispersed, and well-dispersed RhxP NPs on PNC support. It is expected that such RhxP NPs in RhxP/PNC-m can provide abundant catalytically active sites for HER. Fig. 1o shows the XRD pattern of RhxP/PNC-m. The intense diffraction peaks at 2θ = 28.1, 32.5, 46.6, 58.0, 68.1, 77.5 and 86.5° were well-indexed to the (111), (200), (220), (222), (400), (420), and (422) planes of cubic Rh2P (JCPDS No. 77-0300). Besides, a low level of crystal phase of Rh3P4(JCPDS No. 89-3049) was also detected, which coincided well with TEM results. The results suggest the successful synthesis of monodisperse and well-distributed RhxP NPs on CN surface by the developed facile and straightforward strategy. Fig. S3 exhibits the XPS survey spectrum of RhxP/PNC-m, in which C, P, N, and Rh elements were distinctively detected near the sample surface. Note that a relative high content of N (3.79 atm%) was achieved even after the high-temperature pyrolysis. The high resolution C1s spectrum could be deconvoluted into three components with
2.2. Preparation of catalysts Typically, 200 mg of PPh3 was dissolved in 25 mL of ethanol to form a homogeneous solution. Then, 16.8 mg of RhCl3·nH2O was transferred into the solution, followed by ultrasonicating for another 20 min. After adding 600 mg of melamine, the mixture was continued to stir for 30 min. Finally, the solid was collected through evaporating the resultant solution at 80 °C. After cooling down, the solid was put into a porcelain boat and calcined at 900 °C for 2 h with a heating rate of 5 °C min−1 under Ar atmosphere. The collected solid was donated as RhxP/PNC-m. To study the generality, urea instead of melamine was employed to prepare MPs under the same conditions. The achieved product was designated as RhxP/PNC-u. To verify the P-doping of nitride carbon, the control samples of PNC-m and PNC-u were synthesized without the metal precursors. 2.3. Electrochemical measurements All electrochemical measurements for HER were carried out on a standard three electrode cell system by using a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Factory, China) and rotating ring disk electrode at room temperature. The counter electrode and reference electrode were carbon and Hg/HgO electrode (1.0 M KOH) or Hg/Hg2Cl2 electrode (0.5 M H2SO4), respectively. Glass carbon rotating disk electrode (RDE) electrode with a diameter of 4 mm was selected as the working electrode and polished with alumina polishing powder to obtain the clean surface. For the preparation of catalyst ink, 2.0 mg of the as-prepared catalyst was added into the mixture solution of 800 μL of ethanol and 10 μL of 5.0 wt% nafion. Then, the mixture was followed by ultrasonication for 30 min to form a uniform slurry. 30 μL of catalyst ink was pipetted onto the RDE. The RDE could be employed to test after drying under mild conditions. For comparison, 20 wt% Pt/C was dropped onto the RDE. All the LSV curves were iR corrected with a scan rate of 5 mV s−1 and all the overpotentials were calibrated to reversible hydrogen electrode (RHE). 2.4. Characterizations The morphology of the as-prepared samples was analyzed by transmission electron microscope (TEM, FEI Tecnai G2 20) with an operating voltage of 200 kV. The crystal structure of the catalysts was determined by X-ray diffraction (XRD) operated on a Regaku D/Max797
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
Fig. 1. TEM images (a, b) and corresponding size distribution (c) of RhxP/PNC-u. (d) and (e) TEM images of RhxP/PNC-m (Insets are HRTEM images of Rh3P4 (d) and Rh2P(e)). (f) Corresponding particle size distribution of RhxP/PNC-m. SAED patterns for RhxP/PNC-u (g) and RhxP/PNC-m (h). (i–n) STEM image and EDX mapping images of RhxP/PNC-m. (o) XRD pattern of RhxP/PNC-m.
during sample preparation and characterization [28]. Fig. S6 shows the Raman spectra for RhxP/PNC-m and RhxP/PNC-u. The two broad peaks located at 1347 and 1580 cm−1 were ascribed to the disordered sp3 carbon (D band) and graphitic sp2 carbon (G band), respectively [29,30]. The integrated intensity ratios (i.e., D bond to G bond: ID/IG) of RhxP/PNC-m and RhxP/PNC-u were 1.25 and 1.03, respectively, which indicated that RhxP/PNC-m had more disordered carbon than that of RhxP/PNC-u [31]. Decreased graphitization and highly disordered/defective carbon are beneficial in enhancing the electrical conductivity and electrochemical property of the carbon materials [9]. The successful synthesis of RhxP/PNC-m can be verified by these characterizations including TEM, XRD, XPS, and Raman. The electrocatalytic performances of the as-prepared catalysts were first investigated in alkaline solution, and commercial Pt/C was also tested for comparison (Fig. 3a). RhxP/PNC-m exhibited higher HER activity with an overpotential of 24 mV at 10 mA cm−2 compared with that of RhxP/PNC-u (30 mV) and comparable to that of 20 wt% Pt/C. The developed RhxP/PNC-m had considerably higher activity than that of many reported Rh-based catalysts, such as Rh2P [7], Rh NSs [32], Rh/SWNTs [33], and rGO/CoP-Rh-2.5 [34] (Table S1). It is well established that the dispersion of metal nanocatalyst plays an important role in improving the electrocatalytic performance for HER [11,35,36]. Generally, the larger dispersion of metal nanocatalyst means abundant
binding energies of 284.8, 285.6, and 288.7 eV, corresponding to CeC, C]N/C]O and CeC]O bonds (Fig. 2a) [19,20]. The P 2p spectrum could be fitted into four individual peaks, as shown in Fig. 2b. The characteristic peaks at 130.1 and 131 eV were assigned to metal phosphide. The deconvoluted peak at 132.5 eV should be ascribed to the PeC bond, indicating the successful P-doping of the nitride carbon framework [21]. It could be further verified by the SEM-EDS mapping images of control samples of PNC-m and PNC-u, where the N,P doping of carbon matrix was clearly detected [22] (Figs. S4 and S5). The peak (green area) located at 133.7 eV should also be ascribed to residual metal phosphates due to surface partial oxidation [17,23]. High resolution N 1s spectrum could be divided into three peaks at 398.5, 401.1, and 403.9 eV, which should be attributed to pyridinic-, graphitic-, and oxidized-N, respectively (Fig. 2c) [21,24]. As shown in Fig. 2d, the 3d region of Rh could be deconvoluted into four characteristic peaks, thereby corresponding to the metallic and oxidized states of Rh, respectively. Specifically, the peaks with the binding energies of 307.5 and 312.2 eV were assigned to metallic Rh [25,26]. The binding energy of Rh0 shifted to the high binding energy direction due to the formation of Rh-P coordination structure, which resulted in the negative shift of the binding energy of P (2p3/2) by 0.1 eV [21,27]. The two other peaks located at 309.0 and 313.7 eV should be ascribed to the oxidized Rh, which may originate from the inevitable contact of air 798
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
Fig. 2. XPS spectra of RhxP/PNC-m in the (a) C1s, (b) P 2p, (c) N 1s, and (d) Rh 3d regions.
Fig. 3. (a) Polarization curves for RhxP/PNC-m, RhxP/PNC-u and 20 wt% Pt/C with a scan rate of 5 mV s−1. (b) corresponding Tafel slops. (c) The comparison of overpotential (10 mA cm−2), Tafel slopes, Cdl values between RhxP/PNC-m and RhxP/PNC-u. (d) Chronopotentiometry curves (10 mA cm−2) for RhxP/PNC-m. All tests are in 1.0 M KOH. 799
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
Fig. 4. (a) Polarization curves and (b) Tafel slopes of for RhxP/PNC-m, RhxP/PNC-u and 20 wt% Pt/C in 0.5 M H2SO4. (c) Comparison results of overpotential (10 mA cm−2), Tafel slopes, Cdl values between RhxP/PNC-m and RhxP/PNC-u. (d) Chronopotentiometry curves (10 mA cm−2) for RhxP/PNC-m.
Fig. 3b). The Tafel slope range indicates that the two catalysts probably undergo similar Volmer−Tafel mechanism. To identify the different catalytic activities toward HER, we estimated the electrochemically active surface areas by measuring the capacitances of double layer (Cdl) in the non-faradaic region. The Cdl value of RhxP/PNC-u was 3.5 mF cm−2, whereas the corresponding value of RhxP/PNC-m was 24.8 mF cm−2 (Figs. 3c and S7). The results suggest that the high performance of RhxP/PNC-m must be ascribed to abundant catalytically active sites for HER in 1.0 M KOH. Good stability is a vital factor in electrocatalyst exploration. On the basis of the high-performance of RhxP/PNC-m, we further investigated its durability, and the result is shown in Fig. 3d. The decrease in current density was insignificant in the V–t curve, thereby indicating the long-term stability of RhxP/PNC-m toward HER. Considering the high performance of RhxP/PNC-m for HER in basic solution, the catalytic activity of the sample was also tested in 0.5 M H2SO4 solutions. The RhxP/PNC-u and 20 wt% Pt/C were included for comparison. As shown in Fig. 4a, RhxP/PNC-m also exhibited excellent catalytic behavior toward HER with an overpotential of 27 mV at 10 mA cm−2, which was also comparable to that of 20 wt% Pt/C. However, the overpotential of RhxP/PNC-u (36 mV) was higher than that of RhxP/PNC-m and 20 wt% Pt/C. Meanwhile, the Tafel slopes of RhxP/PNC-m and RhxP/PNC-u were 27.2 and 28.6 mV dec−1 (Fig. 4b–c), respectively, which were slightly higher than that of 20 wt % Pt/C (18.1 mV dec−1). To study the differences in catalytic activities for HER in acidic media, we also determined the Cdl values of the samples in acidic solution. As shown in Figs. 4c and S8, the Cdl value of RhxP/PNC-m for HER in acidic solution was 17.4 mF cm−2, which was higher than that of RhxP/PNC-u (1.4 mF cm−2). RhxP/PNC-m also showed good long-term stability in 0.5 M H2SO4 (Fig. 4d). Overall, these results clearly suggest the superior electrochemical performance for HER in basic and acidic conditions.
highly reactive surface sites for catalytic reaction and thereby the high catalytic performance [35]. The monodispersed metal NPs with small sizes are correlated to the high dispersion, which can improve the efficiency of atom utilization and promote the catalytic property of metal nanocatalyst. Therefore, the high performance of RhxP/PNC-m for HER must be correlated to the higher dispersion originated from the monodispersed and small RhxP NPs in comparison with the nonmonodispersed and large RhxP NPs in RhxP/PNC-u. Besides, the RhxP NPs in the two samples are composed of P-rich Rh3P4 and P-deficient Rh2P phases, respectively. Accordingly, the catalytically active nature of the two phases is further analyzed. Previous results have shown that the P-deficient MP exhibited better catalytic performance than that of the P-rich counterpart [37], because the electron delocalization in metal would be significantly prohibited with P-rich MP due to the strong electronegativity of P atoms [38]. Thus, P-deficient MP with smaller charge transfer impedance and better electron transfer ability possessed increased electrocatalytic performance for HER [37]. Additionally, it was also discovered that the high performance of Rh2P toward HER originated from moderate interaction between H and negatively charged P in Rh2P, consistent with the nearly zero Gibbs free energy as calculated by density functional theory (DFT) [39]. As a result, the Rh2P phase is probably the main catalytic active site for the HER catalysis, which is consistent with previous reported results [7,17,39,40]. Previous studies have shown that three typically fundamental pathways are involved in HER from electrochemical water splitting in the basic medium. The first step is Volmer reaction (Tafel slope: 120 mV dec−1), in which the H* species are generated via water dissociation. Then, the generated H* species may undergo two parallel reactions to produce H2, including Heyrovsky reaction (Tafel slope: 40 mV dec−1) and Tafel reaction (Tafel slope: 30 mV dec−1) [10,35,41]. The calculated Tafel slope of RhxP/PNC-m in the present study was 38.4 mV dec−1, which was comparable to that of 20 wt% Pt/C (38.1 mV dec−1, 800
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
4. Conclusions
[14] J.F. Callejas, C.G. Read, E.J. Popczun, J.M. McEnaney, R.E. Schaak, Nanostructured Co2P electrocatalyst for the hydrogen evolution reaction and direct comparison with morphologically equivalent CoP, Chem. Mater. 27 (2015) 3769–3774. [15] Y. Xu, R. Wu, J. Zhang, Y. Shi, B. Zhang, Anion-exchange synthesis of nanoporous FeP nanosheets as electrocatalysts for hydrogen evolution reaction, Chem. Commun. 49 (2013) 6656–6658. [16] S. Mourdikoudis, L.M. Liz-Marzán, Oleylamine in nanoparticle synthesis, Chem. Mater. 25 (2013) 1465–1476. [17] K. Wang, B. Huang, F. Lin, F. Lv, M. Luo, P. Zhou, Q. Liu, W. Zhang, C. Yang, Y. Tang, Y. Yang, W. Wang, H. Wang, S. Guo, Wrinkled Rh2P nanosheets as superior pH-universal electrocatalysts for hydrogen evolution catalysis, Adv. Energy Mater. 8 (2018) 1801891. [18] Y. Wang, Z. Liu, H. Liu, N.T. Suen, X. Yu, L. Feng, Electrochemical hydrogen evolution reaction efficiently catalyzed by Ru2P nanoparticles, ChemSusChem 11 (2018) 2724–2729. [19] B. Yuan, C. Bao, L. Song, N. Hong, K.M. Liew, Y. Hu, Preparation of functionalized graphene oxide/polypropylene nanocomposite with significantly improved thermal stability and studies on the crystallization behavior and mechanical properties, Chem. Eng. J. 237 (2014) 411–420. [20] Y. Chen, X. Zhang, P. Yu, Y. Ma, Stable dispersions of graphene and highly conducting graphene films: a new approach to creating colloids of graphene monolayers, Chem. Commun. (2009) 4527–4529. [21] Z. Pu, I.S. Amiinu, Z. Kou, W. Li, S. Mu, RuP2-based catalysts with platinum-like activity and higher durability for the hydrogen evolution reaction at all pH values, Angew. Chem. Int. Ed. 56 (2017) 11559–11564. [22] Y. Zheng, Y. Jiao, L.H. Li, T. Xing, Y. Chen, M. Jaroniec, S.Z. Qiao, Toward design of synergistically active carbon-based catalysts for electrocatalytic hydrogen evolution, ACS Nano 8 (2014) 5290–5296. [23] C.-C. Hou, Q. Li, C.-J. Wang, C.-Y. Peng, Q.-Q. Chen, H.-F. Ye, W.-F. Fu, C.-M. Che, N. López, Y. Chen, Ternary Ni–Co–P nanoparticles as noble-metal-free catalysts to boost the hydrolytic dehydrogenation of ammonia-borane, Energy Environ. Sci. 10 (2017) 1770–1776. [24] Y. Cao, S. Mao, M. Li, Y. Chen, Y. Wang, Metal/porous carbon composites for heterogeneous catalysis: old catalysts with improved performance promoted by Ndoping, ACS Catal. 7 (2017) 8090–8112. [25] M. Ming, Y. Ren, M. Hu, Y. Zhang, T. Sun, Y. Ma, X. Li, W. Jiang, D. Gao, J. Bi, G. Fan, Promoted effect of alkalization on the catalytic performance of Rh/alkTi3C2X2 (X=O, F) for the hydrodechlorination of chlorophenols in base-free aqueous medium, Appl. Catal., B 210 (2017) 462–469. [26] J. Bai, G.-R. Xu, S.-H. Xing, J.-H. Zeng, J.-X. Jiang, Y. Chen, Hydrothermal synthesis and catalytic application of ultrathin rhodium nanosheet nanoassemblies, ACS Appl. Mater. Interfaces 8 (2016) 33635–33641. [27] P. Jiang, Q. Liu, Y. Liang, J. Tian, A.M. Asiri, X. Sun, A cost-effective 3D hydrogen evolution cathode with high catalytic activity: FeP nanowire array as the active phase, Angew. Chem. Int. Ed. 53 (2014) 12855–12859. [28] D. Özhava, S. Özkar, Rhodium(0) nanoparticles supported on nanosilica: highly active and long lived catalyst in hydrogen generation from the methanolysis of ammonia borane, Appl. Catal., B 181 (2016) 716–726. [29] K. Zhou, J. He, X. Wang, J. Lin, Y. Jing, W. Zhang, Y. Chen, Self-assembled CoSe 2 nanocrystals embedded into carbon nanowires as highly efficient catalyst for hydrogen evolution reaction, Electrochim. Acta 231 (2017) 626–631. [30] T. Hümmer, J. Noe, M.S. Hofmann, T.W. Hänsch, A. Högele, D. Hunger, Cavityenhanced Raman microscopy of individual carbon nanotubes, Nat. Commun. 7 (2016) 12155. [31] M. Biswal, A. Banerjee, M. Deo, S. Ogale, From dead leaves to high energy density supercapacitors, Energy Environ. Sci. 6 (2013) 1249–1259. [32] N. Zhang, Q. Shao, Y. Pi, J. Guo, X. Huang, Solvent-mediated shape tuning of welldefined rhodium nanocrystals for efficient electrochemical water splitting, Chem. Mater. 29 (2017) 5009–5015. [33] W. Zhang, X. Zhang, L. Chen, J. Dai, Y. Ding, L. Ji, J. Zhao, M. Yan, F. Yang, C.R. Chang, S. Guo, Single-walled carbon nanotube induced optimized electron polarization of rhodium nanocrystals to develop an interface catalyst for highly efficient electrocatalysis, ACS Catal. 8 (2018) 8092–8099. [34] H. Zheng, X. Huang, H. Gao, W. Dong, G. Lu, X. Chen, G. Wang, Decorating cobalt phosphide and rhodium on reduced graphene oxide for high-efficiency hydrogen evolution reaction, J. Energy Chem. 34 (2019) 72–79. [35] J. Wang, Z. Wei, S. Mao, H. Li, Y. Wang, Highly uniform Ru nanoparticles over Ndoped carbon: pH and temperature-universal hydrogen release from water reduction, Energy Environ. Sci. 11 (2018) 800–806. [36] H. Wang, M. Ming, M. Hu, C. Xu, Y. Wang, Y. Zhang, D. Gao, J. Bi, G. Fan, J.-S. Hu, Size and electronic modulation of iridium nanoparticles on nitrogen-functionalized carbon toward advanced electrocatalysts for alkaline water splitting, ACS Appl. Mater. Interfaces 10 (2018) 22340–22347. [37] Q. Chang, J. Ma, Y. Zhu, Z. Li, D. Xu, X. Duan, W. Peng, Y. Li, G. Zhang, F. Zhang, X. Fan, Controllable synthesis of ruthenium phosphides (RuP and RuP2) for pHuniversal hydrogen evolution reaction, ACS Sustain. Chem. Eng. 6 (2018) 6388–6394. [38] S. Carenco, D. Portehault, C. Boissière, N. Mézailles, C. Sanchez, Nanoscaled metal borides and phosphides: recent developments and perspectives, Chem. Rev. 113 (2013) 7981–8065. [39] H. Duan, D. Li, Y. Tang, Y. He, S. Ji, R. Wang, H. Lv, P.P. Lopes, A.P. Paulikas, H. Li, S.X. Mao, C. Wang, N.M. Markovic, J. Li, V.R. Stamenkovic, Y. Li, High-performance Rh2P electrocatalyst for efficient water splitting, J. Am. Chem. Soc. 139 (2017) 5494–5502. [40] Z. Pu, I.S. Amiinu, D. He, M. Wang, G. Li, S. Mu, Activating rhodium phosphidebased catalysts for the pH-universal hydrogen evolution reaction, Nanoscale 10 (2018) 12407–12412. [41] M. Hu, M. Ming, C. Xu, Y. Wang, Y. Zhang, D. Gao, J. Bi, G. Fan, Towards highefficiency hydrogen production through in situ formation of well-dispersed rhodium nanoclusters, ChemSusChem 11 (2018) 3253–3258.
To sum up, we developed a facile and straightforward method in synthesizing small, monodispersed, and well-distributed RhxP NPs deposited on PNC support. The as-prepared RhxP/PNC-m catalyst showed excellent catalytic activity and stability toward HER in both acidic and basic solutions. To achieve 10 mA cm−2 current density, the overpotentials of 24 mV in 1.0 M KOH and 27 mV in 0.5 M H2SO4 are required, which exceeded those of many reported catalysts. Compared with reported catalytic systems, our developed catalyst possessed the following featured advantages. (1) RhxP/PNC-m catalyst was prepared via a solid-state reaction instead of wet-chemical method with large amount of capping agent, which made the entire process convenient, cost-effective, and time-saving. (2) Small and monodispersed RhxP NPs can be achieved via the solid-state reaction with high-temperature pyrolysis. (3) The obtained RhxP/PNC-m catalyst possessed excellent electrochemical activity and long-term stability for HER in both acidic and basic solutions. This work highlighted a simple strategy for the preparation of small RhxP-based particles toward highly effective HER and may create a new approach for the design of other metal phosphides for electrochemical applications. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21777109, U1433101 and U1533118). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.06.004. References [1] M. Rakap, Hydrogen generation from the hydrolytic dehydrogenation of ammonia borane using electrolessly deposited cobalt–phosphorus as reusable and cost-effective catalyst, J. Power Sources 265 (2014) 50–56. [2] A. Hung, S. Tsai, Y. Hsu, J. Ku, Y. Chen, C. Yu, Kinetics of sodium borohydride hydrolysis reaction for hydrogen generation, Int. J. Hydrog. Energy 33 (2008) 6205–6215. [3] C. Xu, M. Hu, Q. Wang, G. Fan, Y. Wang, Y. Zhang, D. Gao, J. Bi, Hyper-cross-linked polymer supported rhodium: an effective catalyst for hydrogen evolution from ammonia borane, Dalton Trans. 47 (2018) 2561–2567. [4] G. Fan, X. Li, Y. Ma, Y. Zhang, J. Wu, B. Xu, T. Sun, D. Gao, J. Bi, Magnetic, recyclable PtyCo1−y/Ti3C2X2 (X = O, F) catalyst: a facile synthesis and enhanced catalytic activity for hydrogen generation from the hydrolysis of ammonia borane, New J. Chem. 41 (2017) 2793–2799. [5] D. Yoon, B. Seo, J. Lee, K.S. Nam, B. Kim, S. Park, H. Baik, S. Hoon Joo, K. Lee, Facet-controlled hollow Rh2S3 hexagonal nanoprisms as highly active and structurally robust catalysts toward hydrogen evolution reaction, Energy Environ. Sci. 9 (2016) 850–856. [6] H. Zheng, X. Huang, H. Gao, G. Lu, A. Li, W. Dong, G. Wang, Cobalt-tuned nickel phosphide nanoparticles for highly efficient electrocatalysis, Appl. Surf. Sci. 479 (2019) 1254–1261. [7] F. Yang, Y. Zhao, Y. Du, Y. Chen, G. Cheng, S. Chen, W. Luo, A monodisperse Rh2Pbased electrocatalyst for highly efficient and pH-universal hydrogen evolution reaction, Adv. Energy Mater. 8 (2018) 1703489. [8] Y. Zhang, Y. Ma, Y.-Y. Chen, L. Zhao, L.-B. Huang, H. Luo, W.-J. Jiang, X. Zhang, S. Niu, D. Gao, J. Bi, G. Fan, J.-S. Hu, Encased copper boosts the electrocatalytic activity of N-doped carbon nanotubes for hydrogen evolution, ACS Appl. Mater. Interfaces 9 (2017) 36857–36864. [9] H. Wang, C. Xu, Q. Chen, M. Ming, Y. Wang, T. Sun, Y. Zhang, D. Gao, J. Bi, G. Fan, Nitrogen-doped carbon-stabilized Ru nanoclusters as excellent catalysts for hydrogen production, ACS Sustain. Chem. Eng. 7 (2019) 1178–1184. [10] X. Cheng, H. Wang, M. Ming, W. Luo, Y. Wang, Y. Yang, Y. Zhang, D. Gao, J. Bi, G. Fan, Well-defined Ru nanoclusters anchored on carbon: facile synthesis and high electrochemical activity toward alkaline water splitting, ACS Sustain. Chem. Eng. 6 (2018) 11487–11492. [11] C. Xu, M. Ming, Q. Wang, C. Yang, G. Fan, Y. Wang, D. Gao, J. Bi, Y. Zhang, Facile synthesis of effective Ru nanoparticles on carbon by adsorption-low temperature pyrolysis strategy for hydrogen evolution, J. Mater. Chem. A 6 (2018) 14380–14386. [12] J.K. Nørskov, T. Bligaard, A. Logadottir, J. Kitchin, J.G. Chen, S. Pandelov, U. Stimming, J. Electrochem. Soc. 152 (2005) 31–32. [13] E.J. Roberts, C.G. Read, N.S. Lewis, R.L. Brutchey, Phase directing ability of an ionic liquid solvent for the synthesis of HER-active Ni2P nanocrystals, ACS Appl. Energy Mater. 1 (2018) 1823–1827.
801
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Monodispersed and well-dispersed RhxP nanoparticles decorated on phosphorus-doped nitride carbon for efficient alkaline and acidic hydrogen evolution Caili Xu, Qi Wang, Rong Ding, Yi Wang, Yun Zhang, Guangyin Fan
T
⁎
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal phosphide Rhodium Hydrogen evolution reaction Monodispersed
Metal phosphides (MPs) synthesized by solid-state reaction have shown considerable potential in hydrogen evolution reaction (HER) from water splitting compared with that of Pt. However, large-sized MP nanoparticles (NPs) with irregular morphologies are inevitably achieved via the solid-state reaction with high-temperature calcination, which lead to the reduced availability of the active sites. Here, a simple and straightforward strategy was developed to synthesize monodispersed, uniformly distributed, and small RhxP NPs decorated on phosphorus-doped nitride carbon (PNC) via the pyrolysis of rhodium(Ш) chloride, triphenylphosphine, and melamine (RhxP/PNC-m). The RhxP/PNC-m that served as a HER electrocatalyst displayed excellent performance with the overpotentials of 24 and 27 mV at 10 mA cm−2 in basic and acidic solutions, respectively. This catalyst also showed good long-term stability in both media. The high-performance of RhxP/PNC-m should be attributed to the small, monodispersed, and highly distributed RhxP NPs on PNC support, which can supply abundant catalytically active sites for HER. The developed strategy for the synthesis of small RhxP NPs may offer new opportunities toward the electrochemical applications of other metal phosphides.
1. Introduction Hydrogen has been recognized as a promising future energy carrier attributed to the concerns over the massive consumption of fossil fuels and environmental pollutions [1–4]. Electrochemical water splitting is receiving unprecedented attention due to its featured merits, such as high purity of hydrogen production and carbon-neutral nature [5–8]. The design and synthesis of effective electrocatalysts are important to accelerate hydrogen evolution reaction (HER) from electrochemical water splitting in both basic and acidic conditions [9,10]. Pt has been demonstrated as the state-of-the-art electrocatalyst with the highest exchange current density and the lowest Tafel slope in acidic conditions [11,12]. Nevertheless, Pt-based electrocatalysts still possess the disadvantages of high price, limited availability, and poor stability for HER. Considerable efforts have been exerted to develop cost-effective electrocatalysts with high electrochemical properties for HER. Unfortunately, most of the developed electrocatalysts show unsatisfactory performances compared with that of Pt. Thus, the exploration of Pt-free catalysts with high performance for electrochemical HER is highly desirable but remains a considerable challenge. Pt-group noble metal (i.e., Ru and Rh) phosphides (MPs) have been
⁎
investigated as potential electrocatalysts for HER and gained rapid developments because of their high performances, comparison with non-noble MPs [13–15]. Two main synthetic strategies, including wetchemical and solid-state reactions, have been developed to produce MPs. Although some of the developed electrocatalysts have shown stratified activity toward HER, the following drawbacks must be overcome. (1) With regard to wet chemical syntheses, a large number of capping agents are required to control the size and obtain monodispersed MPs. The purity of capping agents is a key factor to reproduce catalyst, however many agents, such as oleylamine, have impurities [16,17]. Additionally, the use of tri-n-octylphosphine as P source requires a strict air-free condition due to the generation of corrosive and flammable P species during MP synthesis. (2) Regarding solid-state method via the pyrolysis of metal precursors and P sources, controlling the morphology and size of MPs at high calcination temperature is difficult. Although phosphorization can be achieved using salt hypophosphite (NaH2PO2/NH4H2PO2) at low calcination temperature, the generation of hazardous PH3 gas is inevitable [18]. Therefore, the simple and straightforward synthesis of small, monodispersed, and highly dispersed MPs deposited on carbon support with comparable Pt performance for HER remains challenging.
Corresponding author. E-mail address: [email protected] (G. Fan).
https://doi.org/10.1016/j.apsusc.2019.06.004 Received 27 March 2019; Received in revised form 17 May 2019; Accepted 1 June 2019 Available online 03 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
Here, a simple and straightforward strategy was developed to synthesize small, monodispersed, and well-distributed RhxP nanoparticles (NPs) supported on phosphorus-doped nitride carbon (PNC) via the pyrolysis of rhodium(Ш) chloride, triphenylphosphine, and melamine. The as-prepared RhxP/PNC-m can be used as an effective electrocatalyst for HER catalysis with the low overpotentials of 24 and 27 mV at 10 mA cm−2 in 1.0 M KOH and 0.5 M H2SO4, respectively. The catalyst also showed good long-term stability in both media. The facile and straightforward preparation of such highly active RhxP/PNC can be extended to the production of other MPs for electrochemical applications. 2. Experimental
Scheme 1. Synthesis and electrocatalytic HER over the catalysts.
2.1. Materials and chemicals 2500 diffractometer with a Cu Kα radiation source. The surface compositions and chemical states of the elements on the prepared materials were obtained by using the X-ray photoelectron spectroscopy (XPS) carried out by a Thermo ESCALAB 250 Axis Ultra spectrometer. The LabRam HR Evolution spectrometer (532 nm laser wavelength) was used to collect Raman spectrum.
RhCl3·nH2O with Rh content of 39 wt% was supplied by Kunming Institute of Precious Metals, China. Triphenylphosphine powder (PPh3, 99%) and melamine were purchased from Alfa Aesar. Ethanol and urea were brought from Aladdin Industrial Inc., China. The obtained chemicals were directly used without further purification. De-ionized (DI) water (resistivity of 18.2 MΩ cm) was used for electrochemical measurements.
3. Results and discussion As shown in Scheme 1, two samples can be synthesized via a facile and straightforward process through the solid-state reaction by using rhodium(Ш) chloride as the metal precursor and PPh3 as P source in the presence of melamine or urea. The resultant products were applied as electrocatalysts for HER in both acidic and basic solutions. Fig. 1a–f shows the TEM images and corresponding particle size distributions of RhxP/PNC-m and RhxP/PNC-u, respectively. Polydispersed RhxP NPs with diameters of 11.8 nm were formed in RhxP/ PNC-u sample (Fig. 1a–c). Several bright rings made up of discrete spots are shown in the selective area electron diffraction (SAED) pattern of RhxP/PNC-u (Fig. 1g). The spots indexed to the (304) plane of the Rh3P4 and (422) plane of Rh2P were clearly detected, which matched well with the XRD pattern of RhxP/PNC-u with the peaks at 43° and 86.5° (Fig. S1), respectively. Note that the TEM images of RhxP/PNC-m synthesized with melamine as the N source displayed the presence of small and monodisperse RhxP NPs with a diameter of 4.1 nm that were uniformly decorated on the support (Fig. 1d–f). High-resolution TEM (HRTEM) images (Fig. 1d–e, inset) showed the well-crystallized NPs with the interplanar spacings of 0.281 and 0.196 nm, corresponding to the (401) plane of Rh3P4 and (220) plane of Rh2P, respectively. SAED pattern of RhxP/PNC-m (Fig. 1h) also exhibited weak diffraction rings corresponding to the (220) plane of Rh2P and (401) plane of Rh3P4. The STEM-EDX mapping images (Fig. 1i–n) also proved that C, O, N, P, and Rh were uniformly dispersed across the RhxP/PNC-m sample, which matched well with the energy dispersive X-ray spectroscopy analysis (Fig. S2). These results indicate that the use of melamine was the key to the synthesis of small, monodispersed, and well-dispersed RhxP NPs on PNC support. It is expected that such RhxP NPs in RhxP/PNC-m can provide abundant catalytically active sites for HER. Fig. 1o shows the XRD pattern of RhxP/PNC-m. The intense diffraction peaks at 2θ = 28.1, 32.5, 46.6, 58.0, 68.1, 77.5 and 86.5° were well-indexed to the (111), (200), (220), (222), (400), (420), and (422) planes of cubic Rh2P (JCPDS No. 77-0300). Besides, a low level of crystal phase of Rh3P4(JCPDS No. 89-3049) was also detected, which coincided well with TEM results. The results suggest the successful synthesis of monodisperse and well-distributed RhxP NPs on CN surface by the developed facile and straightforward strategy. Fig. S3 exhibits the XPS survey spectrum of RhxP/PNC-m, in which C, P, N, and Rh elements were distinctively detected near the sample surface. Note that a relative high content of N (3.79 atm%) was achieved even after the high-temperature pyrolysis. The high resolution C1s spectrum could be deconvoluted into three components with
2.2. Preparation of catalysts Typically, 200 mg of PPh3 was dissolved in 25 mL of ethanol to form a homogeneous solution. Then, 16.8 mg of RhCl3·nH2O was transferred into the solution, followed by ultrasonicating for another 20 min. After adding 600 mg of melamine, the mixture was continued to stir for 30 min. Finally, the solid was collected through evaporating the resultant solution at 80 °C. After cooling down, the solid was put into a porcelain boat and calcined at 900 °C for 2 h with a heating rate of 5 °C min−1 under Ar atmosphere. The collected solid was donated as RhxP/PNC-m. To study the generality, urea instead of melamine was employed to prepare MPs under the same conditions. The achieved product was designated as RhxP/PNC-u. To verify the P-doping of nitride carbon, the control samples of PNC-m and PNC-u were synthesized without the metal precursors. 2.3. Electrochemical measurements All electrochemical measurements for HER were carried out on a standard three electrode cell system by using a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Factory, China) and rotating ring disk electrode at room temperature. The counter electrode and reference electrode were carbon and Hg/HgO electrode (1.0 M KOH) or Hg/Hg2Cl2 electrode (0.5 M H2SO4), respectively. Glass carbon rotating disk electrode (RDE) electrode with a diameter of 4 mm was selected as the working electrode and polished with alumina polishing powder to obtain the clean surface. For the preparation of catalyst ink, 2.0 mg of the as-prepared catalyst was added into the mixture solution of 800 μL of ethanol and 10 μL of 5.0 wt% nafion. Then, the mixture was followed by ultrasonication for 30 min to form a uniform slurry. 30 μL of catalyst ink was pipetted onto the RDE. The RDE could be employed to test after drying under mild conditions. For comparison, 20 wt% Pt/C was dropped onto the RDE. All the LSV curves were iR corrected with a scan rate of 5 mV s−1 and all the overpotentials were calibrated to reversible hydrogen electrode (RHE). 2.4. Characterizations The morphology of the as-prepared samples was analyzed by transmission electron microscope (TEM, FEI Tecnai G2 20) with an operating voltage of 200 kV. The crystal structure of the catalysts was determined by X-ray diffraction (XRD) operated on a Regaku D/Max797
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
Fig. 1. TEM images (a, b) and corresponding size distribution (c) of RhxP/PNC-u. (d) and (e) TEM images of RhxP/PNC-m (Insets are HRTEM images of Rh3P4 (d) and Rh2P(e)). (f) Corresponding particle size distribution of RhxP/PNC-m. SAED patterns for RhxP/PNC-u (g) and RhxP/PNC-m (h). (i–n) STEM image and EDX mapping images of RhxP/PNC-m. (o) XRD pattern of RhxP/PNC-m.
during sample preparation and characterization [28]. Fig. S6 shows the Raman spectra for RhxP/PNC-m and RhxP/PNC-u. The two broad peaks located at 1347 and 1580 cm−1 were ascribed to the disordered sp3 carbon (D band) and graphitic sp2 carbon (G band), respectively [29,30]. The integrated intensity ratios (i.e., D bond to G bond: ID/IG) of RhxP/PNC-m and RhxP/PNC-u were 1.25 and 1.03, respectively, which indicated that RhxP/PNC-m had more disordered carbon than that of RhxP/PNC-u [31]. Decreased graphitization and highly disordered/defective carbon are beneficial in enhancing the electrical conductivity and electrochemical property of the carbon materials [9]. The successful synthesis of RhxP/PNC-m can be verified by these characterizations including TEM, XRD, XPS, and Raman. The electrocatalytic performances of the as-prepared catalysts were first investigated in alkaline solution, and commercial Pt/C was also tested for comparison (Fig. 3a). RhxP/PNC-m exhibited higher HER activity with an overpotential of 24 mV at 10 mA cm−2 compared with that of RhxP/PNC-u (30 mV) and comparable to that of 20 wt% Pt/C. The developed RhxP/PNC-m had considerably higher activity than that of many reported Rh-based catalysts, such as Rh2P [7], Rh NSs [32], Rh/SWNTs [33], and rGO/CoP-Rh-2.5 [34] (Table S1). It is well established that the dispersion of metal nanocatalyst plays an important role in improving the electrocatalytic performance for HER [11,35,36]. Generally, the larger dispersion of metal nanocatalyst means abundant
binding energies of 284.8, 285.6, and 288.7 eV, corresponding to CeC, C]N/C]O and CeC]O bonds (Fig. 2a) [19,20]. The P 2p spectrum could be fitted into four individual peaks, as shown in Fig. 2b. The characteristic peaks at 130.1 and 131 eV were assigned to metal phosphide. The deconvoluted peak at 132.5 eV should be ascribed to the PeC bond, indicating the successful P-doping of the nitride carbon framework [21]. It could be further verified by the SEM-EDS mapping images of control samples of PNC-m and PNC-u, where the N,P doping of carbon matrix was clearly detected [22] (Figs. S4 and S5). The peak (green area) located at 133.7 eV should also be ascribed to residual metal phosphates due to surface partial oxidation [17,23]. High resolution N 1s spectrum could be divided into three peaks at 398.5, 401.1, and 403.9 eV, which should be attributed to pyridinic-, graphitic-, and oxidized-N, respectively (Fig. 2c) [21,24]. As shown in Fig. 2d, the 3d region of Rh could be deconvoluted into four characteristic peaks, thereby corresponding to the metallic and oxidized states of Rh, respectively. Specifically, the peaks with the binding energies of 307.5 and 312.2 eV were assigned to metallic Rh [25,26]. The binding energy of Rh0 shifted to the high binding energy direction due to the formation of Rh-P coordination structure, which resulted in the negative shift of the binding energy of P (2p3/2) by 0.1 eV [21,27]. The two other peaks located at 309.0 and 313.7 eV should be ascribed to the oxidized Rh, which may originate from the inevitable contact of air 798
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
Fig. 2. XPS spectra of RhxP/PNC-m in the (a) C1s, (b) P 2p, (c) N 1s, and (d) Rh 3d regions.
Fig. 3. (a) Polarization curves for RhxP/PNC-m, RhxP/PNC-u and 20 wt% Pt/C with a scan rate of 5 mV s−1. (b) corresponding Tafel slops. (c) The comparison of overpotential (10 mA cm−2), Tafel slopes, Cdl values between RhxP/PNC-m and RhxP/PNC-u. (d) Chronopotentiometry curves (10 mA cm−2) for RhxP/PNC-m. All tests are in 1.0 M KOH. 799
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
Fig. 4. (a) Polarization curves and (b) Tafel slopes of for RhxP/PNC-m, RhxP/PNC-u and 20 wt% Pt/C in 0.5 M H2SO4. (c) Comparison results of overpotential (10 mA cm−2), Tafel slopes, Cdl values between RhxP/PNC-m and RhxP/PNC-u. (d) Chronopotentiometry curves (10 mA cm−2) for RhxP/PNC-m.
Fig. 3b). The Tafel slope range indicates that the two catalysts probably undergo similar Volmer−Tafel mechanism. To identify the different catalytic activities toward HER, we estimated the electrochemically active surface areas by measuring the capacitances of double layer (Cdl) in the non-faradaic region. The Cdl value of RhxP/PNC-u was 3.5 mF cm−2, whereas the corresponding value of RhxP/PNC-m was 24.8 mF cm−2 (Figs. 3c and S7). The results suggest that the high performance of RhxP/PNC-m must be ascribed to abundant catalytically active sites for HER in 1.0 M KOH. Good stability is a vital factor in electrocatalyst exploration. On the basis of the high-performance of RhxP/PNC-m, we further investigated its durability, and the result is shown in Fig. 3d. The decrease in current density was insignificant in the V–t curve, thereby indicating the long-term stability of RhxP/PNC-m toward HER. Considering the high performance of RhxP/PNC-m for HER in basic solution, the catalytic activity of the sample was also tested in 0.5 M H2SO4 solutions. The RhxP/PNC-u and 20 wt% Pt/C were included for comparison. As shown in Fig. 4a, RhxP/PNC-m also exhibited excellent catalytic behavior toward HER with an overpotential of 27 mV at 10 mA cm−2, which was also comparable to that of 20 wt% Pt/C. However, the overpotential of RhxP/PNC-u (36 mV) was higher than that of RhxP/PNC-m and 20 wt% Pt/C. Meanwhile, the Tafel slopes of RhxP/PNC-m and RhxP/PNC-u were 27.2 and 28.6 mV dec−1 (Fig. 4b–c), respectively, which were slightly higher than that of 20 wt % Pt/C (18.1 mV dec−1). To study the differences in catalytic activities for HER in acidic media, we also determined the Cdl values of the samples in acidic solution. As shown in Figs. 4c and S8, the Cdl value of RhxP/PNC-m for HER in acidic solution was 17.4 mF cm−2, which was higher than that of RhxP/PNC-u (1.4 mF cm−2). RhxP/PNC-m also showed good long-term stability in 0.5 M H2SO4 (Fig. 4d). Overall, these results clearly suggest the superior electrochemical performance for HER in basic and acidic conditions.
highly reactive surface sites for catalytic reaction and thereby the high catalytic performance [35]. The monodispersed metal NPs with small sizes are correlated to the high dispersion, which can improve the efficiency of atom utilization and promote the catalytic property of metal nanocatalyst. Therefore, the high performance of RhxP/PNC-m for HER must be correlated to the higher dispersion originated from the monodispersed and small RhxP NPs in comparison with the nonmonodispersed and large RhxP NPs in RhxP/PNC-u. Besides, the RhxP NPs in the two samples are composed of P-rich Rh3P4 and P-deficient Rh2P phases, respectively. Accordingly, the catalytically active nature of the two phases is further analyzed. Previous results have shown that the P-deficient MP exhibited better catalytic performance than that of the P-rich counterpart [37], because the electron delocalization in metal would be significantly prohibited with P-rich MP due to the strong electronegativity of P atoms [38]. Thus, P-deficient MP with smaller charge transfer impedance and better electron transfer ability possessed increased electrocatalytic performance for HER [37]. Additionally, it was also discovered that the high performance of Rh2P toward HER originated from moderate interaction between H and negatively charged P in Rh2P, consistent with the nearly zero Gibbs free energy as calculated by density functional theory (DFT) [39]. As a result, the Rh2P phase is probably the main catalytic active site for the HER catalysis, which is consistent with previous reported results [7,17,39,40]. Previous studies have shown that three typically fundamental pathways are involved in HER from electrochemical water splitting in the basic medium. The first step is Volmer reaction (Tafel slope: 120 mV dec−1), in which the H* species are generated via water dissociation. Then, the generated H* species may undergo two parallel reactions to produce H2, including Heyrovsky reaction (Tafel slope: 40 mV dec−1) and Tafel reaction (Tafel slope: 30 mV dec−1) [10,35,41]. The calculated Tafel slope of RhxP/PNC-m in the present study was 38.4 mV dec−1, which was comparable to that of 20 wt% Pt/C (38.1 mV dec−1, 800
Applied Surface Science 489 (2019) 796–801
C. Xu, et al.
4. Conclusions
[14] J.F. Callejas, C.G. Read, E.J. Popczun, J.M. McEnaney, R.E. Schaak, Nanostructured Co2P electrocatalyst for the hydrogen evolution reaction and direct comparison with morphologically equivalent CoP, Chem. Mater. 27 (2015) 3769–3774. [15] Y. Xu, R. Wu, J. Zhang, Y. Shi, B. Zhang, Anion-exchange synthesis of nanoporous FeP nanosheets as electrocatalysts for hydrogen evolution reaction, Chem. Commun. 49 (2013) 6656–6658. [16] S. Mourdikoudis, L.M. Liz-Marzán, Oleylamine in nanoparticle synthesis, Chem. Mater. 25 (2013) 1465–1476. [17] K. Wang, B. Huang, F. Lin, F. Lv, M. Luo, P. Zhou, Q. Liu, W. Zhang, C. Yang, Y. Tang, Y. Yang, W. Wang, H. Wang, S. Guo, Wrinkled Rh2P nanosheets as superior pH-universal electrocatalysts for hydrogen evolution catalysis, Adv. Energy Mater. 8 (2018) 1801891. [18] Y. Wang, Z. Liu, H. Liu, N.T. Suen, X. Yu, L. Feng, Electrochemical hydrogen evolution reaction efficiently catalyzed by Ru2P nanoparticles, ChemSusChem 11 (2018) 2724–2729. [19] B. Yuan, C. Bao, L. Song, N. Hong, K.M. Liew, Y. Hu, Preparation of functionalized graphene oxide/polypropylene nanocomposite with significantly improved thermal stability and studies on the crystallization behavior and mechanical properties, Chem. Eng. J. 237 (2014) 411–420. [20] Y. Chen, X. Zhang, P. Yu, Y. Ma, Stable dispersions of graphene and highly conducting graphene films: a new approach to creating colloids of graphene monolayers, Chem. Commun. (2009) 4527–4529. [21] Z. Pu, I.S. Amiinu, Z. Kou, W. Li, S. Mu, RuP2-based catalysts with platinum-like activity and higher durability for the hydrogen evolution reaction at all pH values, Angew. Chem. Int. Ed. 56 (2017) 11559–11564. [22] Y. Zheng, Y. Jiao, L.H. Li, T. Xing, Y. Chen, M. Jaroniec, S.Z. Qiao, Toward design of synergistically active carbon-based catalysts for electrocatalytic hydrogen evolution, ACS Nano 8 (2014) 5290–5296. [23] C.-C. Hou, Q. Li, C.-J. Wang, C.-Y. Peng, Q.-Q. Chen, H.-F. Ye, W.-F. Fu, C.-M. Che, N. López, Y. Chen, Ternary Ni–Co–P nanoparticles as noble-metal-free catalysts to boost the hydrolytic dehydrogenation of ammonia-borane, Energy Environ. Sci. 10 (2017) 1770–1776. [24] Y. Cao, S. Mao, M. Li, Y. Chen, Y. Wang, Metal/porous carbon composites for heterogeneous catalysis: old catalysts with improved performance promoted by Ndoping, ACS Catal. 7 (2017) 8090–8112. [25] M. Ming, Y. Ren, M. Hu, Y. Zhang, T. Sun, Y. Ma, X. Li, W. Jiang, D. Gao, J. Bi, G. Fan, Promoted effect of alkalization on the catalytic performance of Rh/alkTi3C2X2 (X=O, F) for the hydrodechlorination of chlorophenols in base-free aqueous medium, Appl. Catal., B 210 (2017) 462–469. [26] J. Bai, G.-R. Xu, S.-H. Xing, J.-H. Zeng, J.-X. Jiang, Y. Chen, Hydrothermal synthesis and catalytic application of ultrathin rhodium nanosheet nanoassemblies, ACS Appl. Mater. Interfaces 8 (2016) 33635–33641. [27] P. Jiang, Q. Liu, Y. Liang, J. Tian, A.M. Asiri, X. Sun, A cost-effective 3D hydrogen evolution cathode with high catalytic activity: FeP nanowire array as the active phase, Angew. Chem. Int. Ed. 53 (2014) 12855–12859. [28] D. Özhava, S. Özkar, Rhodium(0) nanoparticles supported on nanosilica: highly active and long lived catalyst in hydrogen generation from the methanolysis of ammonia borane, Appl. Catal., B 181 (2016) 716–726. [29] K. Zhou, J. He, X. Wang, J. Lin, Y. Jing, W. Zhang, Y. Chen, Self-assembled CoSe 2 nanocrystals embedded into carbon nanowires as highly efficient catalyst for hydrogen evolution reaction, Electrochim. Acta 231 (2017) 626–631. [30] T. Hümmer, J. Noe, M.S. Hofmann, T.W. Hänsch, A. Högele, D. Hunger, Cavityenhanced Raman microscopy of individual carbon nanotubes, Nat. Commun. 7 (2016) 12155. [31] M. Biswal, A. Banerjee, M. Deo, S. Ogale, From dead leaves to high energy density supercapacitors, Energy Environ. Sci. 6 (2013) 1249–1259. [32] N. Zhang, Q. Shao, Y. Pi, J. Guo, X. Huang, Solvent-mediated shape tuning of welldefined rhodium nanocrystals for efficient electrochemical water splitting, Chem. Mater. 29 (2017) 5009–5015. [33] W. Zhang, X. Zhang, L. Chen, J. Dai, Y. Ding, L. Ji, J. Zhao, M. Yan, F. Yang, C.R. Chang, S. Guo, Single-walled carbon nanotube induced optimized electron polarization of rhodium nanocrystals to develop an interface catalyst for highly efficient electrocatalysis, ACS Catal. 8 (2018) 8092–8099. [34] H. Zheng, X. Huang, H. Gao, W. Dong, G. Lu, X. Chen, G. Wang, Decorating cobalt phosphide and rhodium on reduced graphene oxide for high-efficiency hydrogen evolution reaction, J. Energy Chem. 34 (2019) 72–79. [35] J. Wang, Z. Wei, S. Mao, H. Li, Y. Wang, Highly uniform Ru nanoparticles over Ndoped carbon: pH and temperature-universal hydrogen release from water reduction, Energy Environ. Sci. 11 (2018) 800–806. [36] H. Wang, M. Ming, M. Hu, C. Xu, Y. Wang, Y. Zhang, D. Gao, J. Bi, G. Fan, J.-S. Hu, Size and electronic modulation of iridium nanoparticles on nitrogen-functionalized carbon toward advanced electrocatalysts for alkaline water splitting, ACS Appl. Mater. Interfaces 10 (2018) 22340–22347. [37] Q. Chang, J. Ma, Y. Zhu, Z. Li, D. Xu, X. Duan, W. Peng, Y. Li, G. Zhang, F. Zhang, X. Fan, Controllable synthesis of ruthenium phosphides (RuP and RuP2) for pHuniversal hydrogen evolution reaction, ACS Sustain. Chem. Eng. 6 (2018) 6388–6394. [38] S. Carenco, D. Portehault, C. Boissière, N. Mézailles, C. Sanchez, Nanoscaled metal borides and phosphides: recent developments and perspectives, Chem. Rev. 113 (2013) 7981–8065. [39] H. Duan, D. Li, Y. Tang, Y. He, S. Ji, R. Wang, H. Lv, P.P. Lopes, A.P. Paulikas, H. Li, S.X. Mao, C. Wang, N.M. Markovic, J. Li, V.R. Stamenkovic, Y. Li, High-performance Rh2P electrocatalyst for efficient water splitting, J. Am. Chem. Soc. 139 (2017) 5494–5502. [40] Z. Pu, I.S. Amiinu, D. He, M. Wang, G. Li, S. Mu, Activating rhodium phosphidebased catalysts for the pH-universal hydrogen evolution reaction, Nanoscale 10 (2018) 12407–12412. [41] M. Hu, M. Ming, C. Xu, Y. Wang, Y. Zhang, D. Gao, J. Bi, G. Fan, Towards highefficiency hydrogen production through in situ formation of well-dispersed rhodium nanoclusters, ChemSusChem 11 (2018) 3253–3258.
To sum up, we developed a facile and straightforward method in synthesizing small, monodispersed, and well-distributed RhxP NPs deposited on PNC support. The as-prepared RhxP/PNC-m catalyst showed excellent catalytic activity and stability toward HER in both acidic and basic solutions. To achieve 10 mA cm−2 current density, the overpotentials of 24 mV in 1.0 M KOH and 27 mV in 0.5 M H2SO4 are required, which exceeded those of many reported catalysts. Compared with reported catalytic systems, our developed catalyst possessed the following featured advantages. (1) RhxP/PNC-m catalyst was prepared via a solid-state reaction instead of wet-chemical method with large amount of capping agent, which made the entire process convenient, cost-effective, and time-saving. (2) Small and monodispersed RhxP NPs can be achieved via the solid-state reaction with high-temperature pyrolysis. (3) The obtained RhxP/PNC-m catalyst possessed excellent electrochemical activity and long-term stability for HER in both acidic and basic solutions. This work highlighted a simple strategy for the preparation of small RhxP-based particles toward highly effective HER and may create a new approach for the design of other metal phosphides for electrochemical applications. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21777109, U1433101 and U1533118). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.06.004. References [1] M. Rakap, Hydrogen generation from the hydrolytic dehydrogenation of ammonia borane using electrolessly deposited cobalt–phosphorus as reusable and cost-effective catalyst, J. Power Sources 265 (2014) 50–56. [2] A. Hung, S. Tsai, Y. Hsu, J. Ku, Y. Chen, C. Yu, Kinetics of sodium borohydride hydrolysis reaction for hydrogen generation, Int. J. Hydrog. Energy 33 (2008) 6205–6215. [3] C. Xu, M. Hu, Q. Wang, G. Fan, Y. Wang, Y. Zhang, D. Gao, J. Bi, Hyper-cross-linked polymer supported rhodium: an effective catalyst for hydrogen evolution from ammonia borane, Dalton Trans. 47 (2018) 2561–2567. [4] G. Fan, X. Li, Y. Ma, Y. Zhang, J. Wu, B. Xu, T. Sun, D. Gao, J. Bi, Magnetic, recyclable PtyCo1−y/Ti3C2X2 (X = O, F) catalyst: a facile synthesis and enhanced catalytic activity for hydrogen generation from the hydrolysis of ammonia borane, New J. Chem. 41 (2017) 2793–2799. [5] D. Yoon, B. Seo, J. Lee, K.S. Nam, B. Kim, S. Park, H. Baik, S. Hoon Joo, K. Lee, Facet-controlled hollow Rh2S3 hexagonal nanoprisms as highly active and structurally robust catalysts toward hydrogen evolution reaction, Energy Environ. Sci. 9 (2016) 850–856. [6] H. Zheng, X. Huang, H. Gao, G. Lu, A. Li, W. Dong, G. Wang, Cobalt-tuned nickel phosphide nanoparticles for highly efficient electrocatalysis, Appl. Surf. Sci. 479 (2019) 1254–1261. [7] F. Yang, Y. Zhao, Y. Du, Y. Chen, G. Cheng, S. Chen, W. Luo, A monodisperse Rh2Pbased electrocatalyst for highly efficient and pH-universal hydrogen evolution reaction, Adv. Energy Mater. 8 (2018) 1703489. [8] Y. Zhang, Y. Ma, Y.-Y. Chen, L. Zhao, L.-B. Huang, H. Luo, W.-J. Jiang, X. Zhang, S. Niu, D. Gao, J. Bi, G. Fan, J.-S. Hu, Encased copper boosts the electrocatalytic activity of N-doped carbon nanotubes for hydrogen evolution, ACS Appl. Mater. Interfaces 9 (2017) 36857–36864. [9] H. Wang, C. Xu, Q. Chen, M. Ming, Y. Wang, T. Sun, Y. Zhang, D. Gao, J. Bi, G. Fan, Nitrogen-doped carbon-stabilized Ru nanoclusters as excellent catalysts for hydrogen production, ACS Sustain. Chem. Eng. 7 (2019) 1178–1184. [10] X. Cheng, H. Wang, M. Ming, W. Luo, Y. Wang, Y. Yang, Y. Zhang, D. Gao, J. Bi, G. Fan, Well-defined Ru nanoclusters anchored on carbon: facile synthesis and high electrochemical activity toward alkaline water splitting, ACS Sustain. Chem. Eng. 6 (2018) 11487–11492. [11] C. Xu, M. Ming, Q. Wang, C. Yang, G. Fan, Y. Wang, D. Gao, J. Bi, Y. Zhang, Facile synthesis of effective Ru nanoparticles on carbon by adsorption-low temperature pyrolysis strategy for hydrogen evolution, J. Mater. Chem. A 6 (2018) 14380–14386. [12] J.K. Nørskov, T. Bligaard, A. Logadottir, J. Kitchin, J.G. Chen, S. Pandelov, U. Stimming, J. Electrochem. Soc. 152 (2005) 31–32. [13] E.J. Roberts, C.G. Read, N.S. Lewis, R.L. Brutchey, Phase directing ability of an ionic liquid solvent for the synthesis of HER-active Ni2P nanocrystals, ACS Appl. Energy Mater. 1 (2018) 1823–1827.
801