The temperature effect and correction models for using electrical resistivity to estimate wood moisture variations

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Available online at www.sciencedirect.com

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Synthesis of ultrafine ruthenium phosphide nanoparticles and nitrogen/phosphorus dual-doped carbon hybrids as advanced electrocatalysts for all-pH hydrogen evolution reaction Qian Luo, Caili Xu, Qian Chen, Jie Wu, Yi Wang, Yun Zhang, Guangyin Fan* College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, 610068, China

highlights
Ruthenium

graphical abstract

phosphides

on

Nitrogen/Phosphorus-doped

car-

bon were prepared.
Small ruthenium phosphides were achieved with carbon matrix during pyrolysis.
SeRuP2/NPC

displays

electrocatalytic

excellent

activity

toward

HER at all pH ranges.

article info

abstract

Article history:

Pt-group metal phosphides are widely utilized as efficient electrocatalysts for hydrogen

Received 12 June 2019

evolution reaction (HER), whereas most of the synthetic strategies are complicated,

Received in revised form

dangerous, and toxic with the use of large amount of nitrogen (N) and/or phosphorus (P)

31 July 2019

sources. Here, we report the synthesis of ruthenium phosphide nanoparticles (NPs)

Accepted 5 August 2019

confined into N/P dual-doped carbon by pyrolyzing self-prepared ruthenium-organo-

Available online 30 August 2019

phosphine complex using 1,3,5-triaza-7-phosphadamantane (PTA) as the ligand and N/P sources. The achieved SeRuP2/NPC displayed excellent electrocatalytic activity (over-

Keywords:

potentials of 19, 37, and 49 mV in alkaline, neutral, and acidic media, respectively, at

Transition metal phosphides

10 mA cm2) toward HER at all pH ranges. The high performance of SeRuP2/NPC must be

Ruthenium phosphide

ascribed to the homogeneously distributed and P-rich RuP2 NPs with the diameter of

Nitrogen- and phosphorus-doping

3.29 nm on the NPC surface, which can considerably improve the atom utilization for HER.

Hydrogen evolution reaction

The present synthetic strategy not only avoids the use of additional N/P sources but also

* Corresponding author. E-mail address: [email protected] (G. Fan). https://doi.org/10.1016/j.ijhydene.2019.08.028 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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the generation of flammable and toxic PH3 gas. This synthetic strategy can be extended to prepare other traditional metal phosphides for electrocatalytic applications. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction As a clean energy carrier, hydrogen has shown considerable prospect in various applications. Electrochemical water splitting is a promising approach that realizes clean hydrogen production [1e6]. However, the high overpotential of hydrogen evolution reaction (HER) considerably restricts hydrogen production from water splitting. For efficient water electrolysis, a high-performance electrocatalyst is required to promote the kinetic for HER. Pt has been demonstrated as the best catalytically active site for HER, however, the high cost and low reserve hinder its large-scale application in electrochemical hydrogen production [7]. Developing efficient Pt-free materials as efficient electrocatalysts for HER is highly desirable. Transition metal phosphides (TMPs) have shown advanced electrocatalytic performance for HER [8e18]. Synthetic methods significantly affect TMP morphology, particle size, and microstructure. Two main synthetic approaches, namely, solution-phase and solid-state reactions, have been developed for TMPs in terms of HER catalysis. Solution-phase reactions with the use of large amounts of organic solvents and tri-n-octylphosphine as additional P source are flammable and corrosive, and the strict operation conditions inhibit large-scale application [19,20]. Solid-state reactions for TMP synthesis lack morphology and size control, and large irregular particles are always achieved during high-temperature pyrolysis [21e23]. Large amounts of N and P sources are introduced during synthetic procedures to enhance electrochemical performance, thereby, making the operation conditions tedious and complicated. Most synthetic strategies that utilize salt hypophosphite as P source are dangerous and toxic due to the inevitable release of flammable and toxic PH3 gas [24e26]. Therefore, the production of uniformly dispersed TMPs with efficient HER activities through a simple method is important but remains challenging. Here, we reported the construction of ultrasmall, uniformly dispersed ruthenium phosphide NPs and N/P dualdoped carbon hybrids through the thermal treatment of selfsynthesized ruthenium complex of 1,3,5-triaza-7phosphadamantane (PTA) and ruthenium (III) chloride. The developed strategy considerably simplified the commonly used methods for TMP-based composites without the use of additional reducing agents and organic compounds as N/P source. As expected, the achieved hybrid of SeRuP2/NPC displayed excellent electrochemical activity for HER in all pH ranges in terms of the low overpotential values of 19, 37, and 49 mV in alkaline, neutral, and acidic media, respectively, at 10 mA cm2. These results were comparable with those of many studies on heteroatom-doped Ru-based nanocatalysts. The SeRuP2/NPC catalyst also exhibited excellent durability in

all pH ranges. The present study offers an easy route for the synthesis of other TMPs for electrocatalytic applications.

Experimental Chemicals and materials RuCl3$xH2O (35%e42%) and H2SO4 were supplied by Aladdin Industrial Corporation. 1,3,5-Triaza-7-phosphaadamantane and Nafion (5 wt%) were purchased from Sigma-Aldrich. Carbon black (Vulcan XC-72R) was bought from Carbot Corp. KOH, K2HPO4, and KH2PO4 were acquired from Alfa Aesar. All purchased reagents were not further processed.

Synthesis of SeRuP2/NPC Typically, 189 mg PTA and 52 mg RuCl3$xH2O were dispersed in 30 mL of ethanol and then stirred at 60  C for 1 h to form a yellow precipitate. Afterward, 160 mg carbon black was added to the mixture above. The solvent was evaporated at 80  C to form a homogeneous powder. The resulting powder was annealed at 900  C for 2 h under a flowing Ar atmosphere at a heating rate of 5  C min1. After cooling down to room temperature, the black solid was collected, washed with ethanol several times, and dried in vacuum at 60  C overnight. Finally, black SeRuP2/NPC powder was obtained. The Ru content of SeRuP2/NPC was 4.8 wt% determined by inductively coupled plasma optical emission spectrometry (ICP-OES). To study the reproducibility, the amounts of chemicals and material were accurately weighed and the synthetic conditions were strictly controlled. The electrochemical performance of the control samples for HER exhibits almost the same linear sweep voltammetry (LSV) curves for HER in basic solution (Fig. S1), suggesting the good reproducibility of SeRuP2/NPC. For comparison, the Ru-PTA complex was directly pyrolyzed without carbon matrix under identical conditions and the resulting product was designated as L-RuP2/NPC. The Ru content of L-RuP2/NPC determined by ICP-OES was 10.1 wt%.

Characterizations The sample morphology and particle size were measured by transmission electron microscopy (TEM) systems (JEM-2100F). The crystal structure of the samples was measured using a Regaku D/Max-2500 X-ray diffractometer (XRD). The surface composition and elemental chemical states of the samples were examined by X-ray photoelectron spectroscopy (XPS) in a thermal ESCALAB 250 Axis Ultraspectrometer. The metal content was determined by ICP-OES on Opima 8000 instrument. Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation AUTOLAB

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PGSTAT302N. To calculate the values of charge transfer resistance (Rct), the circuit diagram was obtained by fitting Nyquist plots of SeRuP2/NPC and L-RuP2/NPC with Nova software.

Electrochemical tests All electrochemical measurements were performed on a standard three-electrode setup connected with a CHI 760E setup (Chenhua Corp., Shanghai, China). The electrochemical measurements in acidic (0.5 M H2SO4) and neutral (1.0 M PBS) were performed using a saturated calomel electrode as the reference electrode. Electrochemical measurements were performed in alkaline solution (1.0 M KOH) with Hg/HgO as the reference electrode. A graphite rod was used as the counter electrode in all measurements. A carbon cloth was cut into small pieces with the dimension of 0.5 cm  1 cm and used as the working electrode. Next, 2 mg catalyst and 5 mL of Nafion solution were dispersed into 400 mL of ethanol and underwent ultrasonic treatment for 0.5 h to obtain the catalyst ink. Then, the carbon cloth was dipped in 70 mL of the catalyst ink (1.4 mg cm2 catalyst) to cover an area of 0.25 cm2 and then was dried at room temperature. The overpotential of the sample was measured by LSV, and the scanning speed was 5 mV s1. All polarization curves were iR-corrected.

Results and discussion Scheme 1 illustrates the schematic fabrication of SeRuP2/NPC. First, the Ru precursor was synthesized via the coordination of PTA and ruthenium (III) chloride under stirring. Afterward, carbon black was added to absorb the resulting rutheniumorganophosphine complex. After the solvent was completely evaporated, the mixture was pyrolyzed at 900  C under an Ar atmosphere. During the synthetic process, the ligand PTA acted as P and N sources in synthesizing the desirable SeRuP2/ NPC product. The morphology and microstructure of as-prepared samples were characterized by TEM. As shown in the TEM image of SeRuP2/NPC, uniformly distributed NPs with the diameter of 3.29 ± 0.72 nm were observed on the carbon matrix (Fig. 1aeb). The high-resolution TEM (HRTEM) of a nanocrystal

revealed the distinct d-spacing of ~0.193 nm, which corresponded to the (220) plane of RuP2 (Fig. 1a inset). By contrast, the TEM image of the L-RuP2/NPC sample showed that the sizes of RuP2 NPs were much larger than those of SeRuP2/NPC, with the diameter of 8.54 ± 2.14 nm (Fig. 1c and d). The corresponding HRTEM image of the L-RuP2/NPC sample showed the lattice fringe of the (111) plane of RuP2 with a lattice spacing of ~0.230 nm (Fig. 1c, inset). XRD was used to characterize the crystalline phase of the two samples, and the corresponding patterns are shown in Fig. 2a. The diffraction peaks of SeRuP2/NPC and L-RuP2/NPC corresponding to the (110), (020), (101), (210), (121), (211), and (031) planes of the orthorhombic RuP2 phase (JCPDS No. 34-0333) were observed. The results suggest the successful formation of RuP2 NPs in the two samples, which match well with the TEM results. The weaker peak intensity and wider peak width of SeRuP2/NPC further indicated the smaller particle size of RuP2 NPs than that of L-RuP2/NPC [14]. These findings were consistent with the TEM results. Comparing the sizes of RuP2 NPs in SeRuP2/ NPC and L-RuP2/NPC showed that the addition of carbon matrix during the pyrolysis process was beneficial for the transformation of the Ru/PTA complex into small RuP2 NPs. The achieved highly dispersed and small RuP2 NPs firmly decorated on the carbon matrix can improve atom utilization and boost electrocatalytic activity [27,28]. XPS was further applied to determine the surface compositions and the elementary chemical states of SeRuP2/NPC. The XPS survey spectrum indicated the presence of N, P, O, C, and Ru elements in the sample (Fig. 2b). The atomic percentages of N, P, O, C, and Ru were 2.2%, 2.05%, 9.53%, 85.75%, and 0.47%, respectively. The N 1s spectrum shown in Fig. 2c had four fitted peaks at 402.3, 401.4, 400.0, and 398.0 eV, which could be ascribed to oxided-N, graphitic-N, pyrrolic-N, and pyridinic-N, respectively [29]. In the P2p spectrum in Fig. 2d, the deconvoluted peaks at 129.8 and 130.6 eV were assigned to the P 2p3/2 and P 2p1/2 in RuP2, respectively, whereas the peak at 133.7 eV was associated with the PeO bond [30,31]. The P 2p3/2 peak is negatively shifted by 0.3 eV compared with that of P (130.1 eV) [30], thereby indicating the formation of negatively charged P species. The PeO bond formation can be attributed to the formation of surface amorphous oxides, such as phosphates [32]. In the Ru 3p XPS spectrum (Fig. 2e), the Ru 3p1/2 and Ru 3p3/2 peaks had the binding energies of 485 and

Scheme 1 e Schematic illustration of the preparation procedure of SeRuP2/NPC.

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Fig. 1 e TEM images (inset: HRTEM images) and the particle size distributions for (a, b) SeRuP2/NPC and (c, d) L-RuP2/NPC.

462.7 eV, respectively. The binding energy of Ru 3p3/2 is positively shifted by 0.2 eV compared with that of Ru 3p3/2 of metal Ru (462.5 eV) [33]. The generation of positively charged Ru in SeRuP2/NPC must be assigned to the electron transfer from Ru to P. The high binding energies of Ru 3p at 465.5 and 487.8 eV were attributed to RuPx oxide species [34]. The C 1s spectrum shown in Fig. 2f could be divided into three peaks at 286.7, 284.7 and 285.2 eV, which were consistent with CeN, C]C/CeC, and CeP/CeOeP, respectively [30,35]. The XPS results successfully confirm the N/P dual-doping of the carbon framework. First, we studied the electrocatalytic activity of SeRuP2/ NPC for HER in alkaline solution (1.0 M KOH) with a scan rate of 5 mV s1 using a standard three-electrode device. For comparison, we also tested the catalytic activity of L-RuP2/ NPC and commercial Pt/C (20 wt%) under the same conditions. Fig. 3a shows the LSV curves of different catalysts at room temperature. SeRuP2/NPC clearly exhibited better HER performance than that of L-RuP2/NPC. Particularly, the overpotentials for HER over SeRuP2/NPC and L-RuP2/NPC were 19 and 51 mV at 10 mA cm2, respectively (Fig. 3c). This finding can be explained by the small and high dispersions of the RuP2 nanocrystals in SeRuP2/NPC in comparison with the large and wide size distribution of the RuP2 nanocrystals in L-RuP2/NPC. The performance of SeRuP2/NPC was slightly better than that of Pt/C (h10 ¼ 25 mV). To compare the catalytic activities of the tree samples further, we calculated their mass activities (Fig. 3a, inset). Owing to the low loading of Ru (4.8 wt%), the mass activity of SeRuP2/NPC was calculated to be 391 A g1, higher than that of L-RuP2/NPC (90 A g1) and commercial Pt/C

(300 A g1), thereby indicating the superior activity of SeRuP2/ NPC toward HER in basic solution. Our developed SeRuP2/NPC electrocatalyst remarkably outperformed many existing Rubased nanocatalysts, such as Ru@N-doped graphite carbon (h29 ¼ 65 mV) [36], RuP2@NPC(h10 ¼ 52 mV) [15], NiFeRuLDH(h10 ¼ 29 mV) [37], Ru NP/C(h10 ¼ 25 mV) [38], RueNiCoLDH(h10 ¼ 28 mV) [39], Ru@graphene nanoplatelets (h10 ¼ 22 mV) [40], Ni@Ni2PeRu (h10 ¼ 51 mV) [41], RueCN-0.16 (h10 ¼ 53 mV) [42], and Ru nanocrystal (h10 ¼ 81 mV) [43], while slight higher than those of Ru/CN-800(h10 ¼ 14 mV) [33], Ru2P/ RGO-20 (h10 ¼ 13 mV) [44], Ru/C-300(h10 ¼ 14 mV) [45], Ru@CQD (h10 ¼ 10 mV) [46], and Ru@C2N (h10 ¼ 17 mV) [47]. More comparison of metal nanocatalysts for HER can be found in Table S1. Furthermore, the intrinsic properties of SeRuP2/NPC and LRuP2/NPC for HER were investigated by Tafel slopes and turnover frequency (TOF), which were calculated from the Tafel curves (Fig. 3b) and the method illustrated in Supporting Information. Specifically, SeRuP2/NPC displayed a low Tafel slope of 41 mV dec1 and high TOF of 0.37 s1 at an overpotential of 40 mV, whereas L-RuP2/NPC possessed higher Tafel slope of 81 mV dec1 and lower TOF of 0.034 s1 at the same overpotential (Fig. 3b and c). Note that the Tafel slope of SeRuP2/NPC was also lower than that of Pt/C (56 mV dec1). The results suggest the more efficient kinetic of SeRuP2/NPC toward HER [48,49]. In addition to the catalytic activity of hydrogen evolution, durability is also an important factor in evaluating the performance of electrocatalysts. We used cyclic voltammetry (CV) to measure the durability of SeRuP2/ NPC. As shown in Fig. 3d, the LSV curves of SeRuP2/NPC

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Fig. 2 e (a) XRD patterns of SeRuP2/NPC and L-RuP2/NPC. (b) XPS full spectrum and high-resolution XPS spectra of (c) N 1s, (d) P 2p, (e) Ru 3p, and (f) C 1s for SeRuP2/NPC.

showed insignificant change before and after 3000 CV cycles. The Vet curve still showed that the SeRuP2/NPC had high HER stability during the long-term circulation in alkaline solution. The enhanced HER performance of SeRuP2/NPC was further analyzed. First, it is well-known that small metal NPs can provide more surface-active sites that are available for reaction. Metal dispersion is a critical parameter that reveals the ratio of the exposed surface-active sites of nanocatalysts for reaction. A large metal dispersion means that increased catalytically active sites available for HER [42]. Consequently, the metal dispersion values of SeRuP2/NPC and L-RuP2/NPC are 40.48% and 15.59%, respectively, which are roughly determined according to the average particle size of the metal NPs [42]. Therefore, SeRuP2/NPC, with higher metal dispersion, can supply more active sites for HER than that of L-RuP2/ NPC. As a demonstration, the electrochemical surface areas of SeRuP2/NPC and L-RuP2/NPC were evaluated by measuring the double layer capacitance (Cdl) of the catalysts. As shown in Fig. 4a and b, the CV curves at different scanning rates were

obtained in the non-Faraday region. As demonstrated in Fig. 4c, the Cdl values of SeRuP2/NPC and L-RuP2/NPC were 45.07 and 41.20 mF cm2, respectively. Therefore, SeRuP2/NPC has more surface active sites available for HER. Second, the electrical conductivity of an electrocatalyst also plays a critical role in improving the HER activity [16]. An electrocatalyst with poor conductivity produces additional overpotential for HER. The EIS testing results shown in Fig. 4d demonstrated that the charge transfer resistance of SeRuP2/NPC (Rct ¼ 54.5 U) was lower than that of L-RuP2/NPC (Rct ¼ 569 U), which agreed well with the HER catalytic activity. Third, previous results showed that P atoms in TMPs have extremely important roles in HER due to the transformation of electrons from metal atoms to electronegative P atoms [50,51]. Thus, these P species may function as bases to trap positively charged protons in the process during HER catalysis [16]. Consequently, the P-rich RuP2 NPs can efficiently enhance the performance of SeRuP2/ NPC toward HER catalysis [52]. Last, previous results indicated that heteroatom doping is beneficial for the electrochemical

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Fig. 3 e (a) LSV curves of SeRuP2/NPC, L-RuP2/NPC, and commercial Pt/C in 1 M KOH (Inset: mass activity). (b) Tafel plots. (c) Corresponding Tafel slopes and overpotentials at 10 mA cm¡2. (d) Polarization curves of SeRuP2/NPC before and after 3000 CV cycles (inset: V-t curve at 10 mA cm¡2 for SeRuP2/NPC).

Fig. 4 e (a) and (b) CV curves with different sweep rates and (c) Capacitive current vs scan rates for SeRuP2/NPC and L-RuP2/ NPC. (d) EIS Nyquist plots of SeRuP2/NPC and L-RuP2/NPC under 1.0 M KOH at overpotential of 40 mV.

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performance of carbon-based materials [53e56]. The positive influences of N-doping on carbon matrix for electrocatalysis could be ascribed to the following reasons: (1) N-doped carbon possessed high conductivity and enhanced the rapid transmission of electrons [57]. (2) N-doping could stimulate the positive charge density on adjacent carbon atoms and induces the carbon atoms to become effective catalytic active sites [57]. (3) N-doped carbon could effectively anchor metal NPs and prohibited their agglomeration, resulting in strong interaction between metal and N atoms [42]. Similarly, P-doping also can produce defects in carbon matrix and improve the electron delocalization because of the good electron-donating characteristics of P, thus boosting the active sites for electrochemical applications [58]. As a result, the high performance of present system for HER must be ascribed to coupled effects of high conductivity, favorable dispersity, heteroatom-doping, and P-rich features of SeRuP2/NPC. A catalyst that can electrolyze water to generate hydrogen in all-pH ranges is essential for practical applications [59]. Inspired by the impressive HER property in basic solution, we further investigated the HER activity of SeRuP2/NPC in neutral

and acidic media to assess the generality in a wide pH range. As shown in Fig. 5a, L-RuP2/PNC required a higher overpotential of 84 mV to achieve 10 mA cm2 current density, whereas SeRuP2/NPC only needed an overpotential of 37 mV to deliver the same current density under neutral conditions (1.0 M PBS, pH ¼ 7). Note that SeRuP2/NPC possessed relatively low overpotential compared with those of previous reported electrocatalysts, such as RuP2@NPC (h10 ¼ 57 mV) [15], RuP475(h10 ¼ 47 mV) [30], and MoP2 NS/CC (h10 ¼ 67 mV) [10] (Table S2), indicating the high electrochemical activity for HER in neutral conditions. Kinetic studies showed that a Tafel slope of 53 mV dec1 was obtained for HER catalyzed by SeRuP2/ NPC, whereas a high Tafel slope of 95 mV dec1 was achieved for L-RuP2/NPC in accordance with the catalytic activity (Fig. 5b). We also tested the durability of SeRuP2/NPC under neutral conditions. The results illustrated in Fig. 5c indicated the good stability of SeRuP2/NPC for HER under neutral conditions. SeRuP2/NPC also exhibited a high activity with a low overpotential (h10 ¼ 49 mV) and a small Tafel slope (50 mV dec1, Fig. 5dee), which was superior to the L-RuP2/NPC catalyst (h10 ¼ 67 mV, Tafel slope: 62 mV dec1) under acidic

Fig. 5 e LSV curves of SeRuP2/NPC, L-RuP2/NPC, and commercial Pt/C in (a) 1.0 M PBS and (d) 0.5 M H2SO4. (b) and (e) Corresponding Tafel plots of SeRuP2/NPC, L-RuP2/NPC, and commercial Pt/C. LSV curves of SeRuP2/NPC before and after 3000 CV cycles in (c) 1.0 M PBS and (f) 0.5 M H2SO4 (Inset: V-t curve at 10 mA cm¡2 for SeRuP2/NPC).

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conditions (0.5 M H2SO4 aqueous solution, pH ¼ 0). Nevertheless, the SeRuP2/NPC displayed relatively low catalytic activity toward HER in acidic medium, compared with those of RuP-475 (h10 ¼ 22 mV) [30], Ru2P@PNC/CC (h10 ¼ 15 mV) [29], and RuP2@NPC (h10 ¼ 38 mV) [15]. More comparison results could be found in Table S3. As shown in Fig. 5f, the durability tests indicated that SeRuP2/NPC maintained excellent durability for HER in acidic media. Note that the SeRuP2/NPC exhibited comparable HER activity compared with commercial Pt/C in the neutral (h10 ¼ 27 mV, Tafel slope: 42 mV dec1) and acidic (h10 ¼ 30 mV, Tafel slope: 39 mV dec1) media. All results clearly show that SeRuP2/NPC has outstanding hydrogen evolution activity and durability over a wide pH range.

Conclusions To sum up, we reported a novel hybrid SeRuP2/NPC electrocatalyst, which possessed uniformly dispersed and small RuP2 NPs anchored on N/P dual-doped carbon via a simple strategy by direct pyrolysis of a self-synthesized ruthenium-organophosphine complex of PTA and ruthenium (III) chloride. Benefitting from coupled effects of high conductivity, favorable dispersity, heteroatom-doping, and P-rich features, the SeRuP2/ NPC displayed excellent electrochemical activity with overpotentials of 19, 37, and 49 mV at 10 mA cm2 in 1.0 M KOH, 1.0 M PBS, and 0.5 M H2SO4 solutions, respectively. Compared with previous noble metal-based phosphide electrocatalysts, the present study has the following featured advantages. (1) Uniformly dispersed and small P-rich RuP2 NPs with the diameter of 3.29 nm were facilely synthesized via the thermal treatment of self-prepared ruthenium complex with PTA as both ligand and N/P source. To the best of our knowledge, this work is the first example to synthesize small P-rich RuP2 NPs through pyrolysis at a high calcination temperature. (2) The present synthetic strategy avoids the use of additional N/P sources, such as tri-noctylphosphine and salt hypophosphites (NH4H2PO2 and NaH2PO2), which enable the entire process simplified and timesaving. (3) The as-prepared SeRuP2/NPC functioned well as a pH-universal electrocatalyst with high long-term durability for HER. The present study will offer a fascinating approach in developing N/P dual-doped TMPs as catalytically active catalysts for other potential catalytic applications.

Acknowledgment We acknowledge the financial supports from the National Natural Science Foundation of China (21777109) and the Scientific Research Fund of Sichuan Provincial Education Department of Sichuan Province (17CZ0029).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.028.

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