N-Doped Carbon Nanofibers on

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Ru2P Nanoparticle Decorated P/N-Doped Carbon Nanofibers on Carbon Cloth as a Robust Hierarchical Electrocatalyst with Pt-Comparable Activity toward Hydrogen Evolution Tingting Liu, Bomin Feng, Xiuju Wu, Yanli Niu, Weihua Hu, and Chang Ming Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00334 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Energy Materials

Ru2P Nanoparticle Decorated P/N-Doped Carbon Nanofibers on Carbon Cloth as a Robust Hierarchical Electrocatalyst with Pt-Comparable Activity toward Hydrogen Evolution Tingting Liu, †‡ Bomin Feng, †‡ Xiuju Wu, †‡ Yanli Niu, †‡ Weihua Hu†‡* and Chang Ming Li †‡*

†

Institute for Clean energy & Advanced Materials, Faculty of Materials & Energy, Southwest

University, Chongqing 400715, China ‡

Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies,

Chongqing 400715, China * Corresponding author. E-mail: [email protected] (W. H. Hu); [email protected] (C. M. Li).

ACS Paragon Plus Environment

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ABSTRACT: It is desirable yet challenging to develop highly active and durable hydrogen evolution reaction (HER) electrocatalysts with Pt-comparable activity for future energy devices. In this work, we report Ru2P nanoparticle decorated P, N dual-doped carbon nanofibers on carbon cloth (Ru2P@PNC/CC-900) as a highly efficient and durable hierarchical HER electrocatalyst in both acidic and alkaline media. Electrochemical tests show that this Ru2P@PNC/CC-900 possesses Pt-comparable HER activity to support 10 mA cm−2 HER current density at low overpotential of 15 and 50 mV in acidic and alkaline condition, respectively. Density functional theory calculations reveal that coupling Ru2P nanoparticles with heteroatomdoped carbon fibers leads to enhanced intrinsic HER activity. The integrative hierarchical architecture further endows high surface areas with good mechanical robustness to support abundant catalytically active sites, and possesses excellent electrical conductivity and efficient access for mass transportation to facilitate the HER process.

KEYWORDS: Ruthenium phosphide; N, P dual-doped carbon; polyaniline nanofiber; hydrogen evolution reaction; hierarchical electrocatalyst


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1. INTRODUCTION Hydrogen is considered as an ideal energy carrier in the hydrogen-economy because of its high energy density and zero emission of CO2.1-2 Water electrolysis offers a practical route to large-scale production of hydrogen fuels, and efficient hydrogen evolution reaction (HER) catalysts are needed for electro-reduction of protons into molecular hydrogen.3 Pt is the benchmark HER catalyst, but the high cost, rarity, and electrochemical stability severely hinder its large-scale implementation in electrochemical hydrogen generation. As such, a large number of non-Pt catalysts have been in-depth studied.4-12 However, the catalytic performance of those catalysts is still far from satisfaction if comparing to Pt. As a result, it is worthwhile yet challenging to search for a highly-effective HER catalyst. Ru-based materials such as oxide and nitride have been utilized to catalyze water oxidation.13-14 In recent years high HER activity has been reported for some Ru-based catalysts.15-24 For example, Mu’s group synthesized N, P dual-doped carbon-encapsulated RuP2 nanoparticles (RuP2@NPC) with high HER activity.17 Well-dispersed ultrasmall Ru nanoparticles were loaded on graphene-like carbon as an efficient HER electrocatalyst in acidic condition.15 Ru/N atommically-doped carbon HER electrocatalyst was successfully synthesized.25 Ru nanoparticles dispersed within two-dimensional carbon structure was reported to be highly active HER catalyst in both acidic and alkaline media.16 Ru-Co nanoparticle encapsulated in N-doped graphene and Ru-graphitic carbon nitride complex supported on carbon also demonstrate impressive HER activity in alkaline media.18,

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Despite great efforts, all these catalysts were synthesized in the form of powder; tedious operations including mixing the catalysts with polymer binders (such as PTFE and


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Nafion), and fixing the catalyst slurry on current collector are necessary before HER operation. For an electrochemical process involving gas generation such as HER, it is highly favorable to directly integrate the nanostructured catalyst on current collector to form a hierarchical 3D electrode.27 This binder-free architecture not only offers huge specific surface areas to maximize the utilization efficiency of catalytic active sites and to facilitate efficient mass transport of reactant (H+ ion) and gaseous product (H2), but also allows for smooth electron communication and long-term operation due to the intimate contact endowed low series resistance and stable integrative interfaces.28-30 In this study, we report Ru2P nanoparticle decorated P, N dual-doped carbon nanofibers on carbon cloth (Ru2P@PNC/CC-900) as an efficient 3D hierarchical HER electrocatalyst with excellent activity and good stability in both acidic and basic condition. This catalyst is synthesized via rational structure design and composition tailoring involving subsequent nanostructuring in electrochemical polymerization, metal impregnation in wet chemical bath and solid state reaction at high temperature. Electrochemical tests show that this Ru2P@PNC/CC-900 possesses Pt-comparable HER activity, outperforming most of HER catalysts reported to date. Density functional theory (DFT) calculations further reveal that the high activity originates from the heteroatom doping induced modification of hydrogen adsorption free energy on Ru2P surface.

2. EXPERIMENTAL SECTION 2.1 Chemicals and Materials: H2SO4 and KOH were purchased from Beijing Chemical Works Ltd. RuCl3·3H2O, phytic acid, aniline and ethanol were purchased from Aladdin Reagents Ltd. HCl and Pt/C (10 wt% Pt) were purchased from Sigma-Aldrich. CC was provided by Hongshan


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District, Wuhan Instrument Surgical Instruments business. All the reagents in the experiment were analytical grade and used as received. The deionized water (DI) used throughout all experiments was purified through a Millipore system (resistivity ≥ 18.2 MΩ). 2.2 Preparation of Ru(III)@PA-PANI/CC precursor: Carbon cloth (CC, 1 cm× 2 cm) was firstly cleaned by ultrasonic with acetone, water and ethanol several times. To synthesize phytic acid incorporated polyaniline nanofibers on CC (denoted as PA-PANI/CC), aniline (4.6 mL), phytic acid (1.55 mL), and HCl (8 mL) were dissolved in 85 mL DI and stirred for 30 min at room temperature. The three-electrode cell consists of the cleaned CC as the working electrode, graphite plate as the counter electrode and Ag/AgCl/saturated KCl as the reference electrode by a CHI 760E electrochemical analyzer (CH Instruments, Inc. Shanghai). Electrodeposition was carried out at a constant potential of 0.65, 0.70, 0.75 and 0.80 V vs. the Ag/AgCl reference electrode for 30 min at 30 ℃. After deposition, the CC was removed from the compression cell and rinsed first with ethanol, H2O and then dried at 60 ℃ for 6 h. Ru(III)@PA-PANI/CC precursor was obtained by immersing the PA-PANI/CC (electrodeposited at 0.80 V) in 0.1 M RuCl3·3H2O to allow the adsorption of Ru ions, followed by washing and drying. 2.3 Preparation of Ru2P@PNC/CC-900: The Ru(III)@PA-PANI/CC precursor was heated at 900 ℃ for 60 min in Ar atmosphere, and then cooled to ambient temperature. As-prepared sample was named as Ru2P@PNC/CC-900. 2.4 Preparation of Ru2P@PNC/CC-1000: The Ru(III)@PA-PANI/CC precursor is heated at 1000 ℃ for 60 min in Ar atmosphere, and then cooled to ambient temperature. As-prepared sample was named as Ru2P@PNC/CC-1000.


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2.5 Preparation of Ru@PNC/CC-700: The Ru(III)@PA-PANI/CC precursor is heated at 700 ℃ for 60 min in Ar atmosphere, and then cooled to ambient temperature. As-prepared sample was named as Ru@PNC/CC-700. 2.6 Preparation of Ru@PNC/CC-800: The Ru(III)@PA-PANI/CC precursor is heated at 800 ℃ for 60 min in Ar atmosphere, and then cooled to ambient temperature. As-prepared sample was named as Ru@PNC/CC-800. 2.7 Preparation of Ru2P: Commercial RuO2 (300 mg) and 3 g NH4H2PO2 were grounded to form homogeneous powder. The powder was then annealing at 900 ℃ for 1 h under Ar atmosphere. After cooled to room temperature, the black products were collected, washed by centrifugation with deionized water several times to remove the residue of reactants. Finally, the product was freeze-dried dried. 2.8 Preparation of Ru@N-CC-900: PANI was electrochemically polymerized on CC with the similar synthetic procedure as in Section 2.2 but without phytic acid added. As-prepared PANI@CC was soaked in 0.1 M RuCl3·3H2O to allow the adsorption of Ru ions, followed by washing and drying. After annealing at 900 ℃ for 60 min in Ar atmosphere, Ru@N-CC-900 was obtained. 2.9 Characterizations: Field emission scanning electron microscopy (FE-SEM) images were recorded on a JSM-7800F system with EDS from JOEL. X-ray powder diffraction (XRD) pattern was conducted on a powder X-ray diffractometer (RIGAKU, D/MAX 2550 VB/PC, Japan). Transition electron microscopy (TEM) measurements were performed on a HITACHI H-8100 electron microscopy (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was performed on Model ARCOS FHS12 (SPECTRO Analytical


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Instruments Inc., Germany). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALABMK II X-ray photoelectron spectrometer using Mg as the exciting source. 2.10 Electrochemical measurements: Electrochemical measurements were performed with a CHI 760E potentiostat (CH Instruments, China). A conventional one-component three-electrode cell was used with a graphite plate (5 cm×3 cm) as counter electrode. The CC with synthesized catalyst was cut to small pieces (geometric surface area: 0.5 cm× 0.5 cm) and each was used directly as working electrode. The acidic (0.5 M H2SO4) electrochemical measurements were performed using a saturated calomel electrode (SCE) as the reference electrode. The alkaline (1.0 M KOH) electrochemical measurements were performed using a Hg/HgO as the reference electrode. A graphite plate was used as the counter electrode in all measurements. Polarization curves were obtained using LSV curves conducted in acid with a scan rate of 5 mV s-1. Electrochemical impedance spectroscopy measurements were carried out in the frequency range of 100 kHz–0.01 Hz and then corrected by the iR loss according to the following equation: Ecorr = Emea – iR (where Ecorr is the iR-compensated potential, Emea is the experimentally measured potential, and R is the solution resistance. In all measurements, SCE was calibrated with respect to RHE. 2.11 DFT Computation: The Plane-wave DFT calculations are conducted using the CASTEP module (an Ab Initio Total Energy Program) of Materials Studio 8.0, with hydrogen binding energy calculated from different active sites.31 We use the PerdewBurke-Ernzerhof (PBE) functional on the generalized gradient approximation (GGA) method to treat the electron exchange correlation (EEC) interaction. A Monkhorst-Pack


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grid k-points of 1×1×1 and a plane wave basis set cut-off energy of 300 eV are used for the Brillouin zone integration. The structures are optimized for force and energy convergence set at 2.0×10−5 eV and 0.05 eV Å−1, respectively. The self-consistence field (SCF) is 2.0×10−6 eV/atom. To consider the influence of van der Waals interaction, the semi-empirical DFT-D force-field approach is applied.32-33 The Gibb’s free energies for hydrogen absorption ∆GH* are calculated from the given equation: ∆GH* = ∆EH* + ∆EZPE – T∆S Where ∆E is the binding energy, ∆EZPE is the change in zero-point energy, T is the Temperature, and ∆S is the entropy change of the system. The T∆S and ∆EZPE are proposed as previously reported by Norskov et al.34 Thus, we adopted the approximation that the vibrational entropy of hydrogen in the adsorbed state is negligible, in which case ∆SH ≈ SH* –1/2(SH2) ≈ –1/2(SH2), where SH2 is the entropy of H2(g) at standard conditions, and TS(H2) is ∼0.41 eV for H2 at 300 K and 1 atm.34-35

3. RESULTS AND DISCUSSION The Ru2P@PNC/CC-900 was synthesized via a simple three-step procedure (Figure 1). First, PA-PANI/CC precursor was prepared by growing polyaniline nanofibers (PANI: C and N source) on carbon cloth (CC, see its optical photographs, FE-SEM image and XRD pattern in Figure S1) by electrochemical polymerization in presence of phytic acid (PA: C and P source). In this process, PA was incorporated into the PANI nanorods through the hydrogen bonding and electrostatic interaction.36, 37 In second step, this PA-PANI/CC was immersed in RuCl3 solution to load Ru3+ via metal complexation to form Ru(III)@PAPANI/CC. Finally, the Ru2P@PNC/CC-900 was obtained by calcination of Ru(III)@PA-


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PANI/CC at 900 ℃ under inert atmosphere. (see optical photographs of Ru(III)@PAPANI/CC and Ru2P@PNC/CC-900 in Figure S1) The potential used for electrochemical polymerization of PA-PANI nanofibers was first optimized. As shown in Figure 2, only potential higher than 0.65 V (vs. Ag/AgCl/saturated KCl) is able to drive the formation of nanofibers (Figure 2A). With more positive potential, the nanofibers become longer with smaller diameter, as shown as the FE-SEM images in Figure 2B-D. At 0.8 V, the CC was fully covered with dense and interconnected PA-PANI nanofibers (Figure 2D) and thus 0.8 V was chosen as the optimal potential. These nanofibers possess length of several micrometres and their diameters are around 300 nm. Notably, each nanofiber demonstrates serrated surface with dense prominences, which offers large surface to load metal ions. After the loading of Ru3+, the nanofibers still maintain their morphology according to the FE-SEM image in Figure S2. The effect of annealing temperature on the product was further investigated. As revealed by the XRD patterns in Figure 3, when relatively low annealing temperature (e.g., 700 or 800 ℃) was applied, metallic Ru nanoparticle were formed on carbon cloth. Their XRD patterns demonstrate the characteristic peaks of metallic Ru (JCPDS No. 060663).16,

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If the annealing temperature rises to 900 or 1000 ℃, orthorhombic Ru2P

(JCPDS No. 65-2382) was formed according to the XRD patterns in Figure 3C and 3D.38 This result suggests that the annealing temperature also significantly influences the speciation of Ru. On Raman spectrum (Figure S3), the ID/IG ratio increases with the elevation of the annealing temperature, suggests more defects formed in the catalysts synthesized at higher temperature, which is consistent with previous observations.37


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Phytic acid plays a critical role as P source in the synthesis of Ru2P@PNC/CC-900. Control experiments suggest that if no phytic acid added, metallic Ru nanoparticles, rather than Ru2P nanoparticles, are formed on N-doped carbon nanorods on CC (denoted as Ru@N-CC-900, see its synthetic details in Section 2.8 and its XRD pattern, FE-SEM image, TEM image, and XPS spectrum in Figure S4-S6). The Ru2P@PNC/CC-900 catalyst was characterized in details. FE-SEM images show that it inherits the fibroid nanostructure of PA-PANI on CC with slightly shrunk size to form wheat ears-like structure (Figure 4A, B). The nanofibers interconnect with one another to form a network on the CC. The energy-dispersive X-ray spectrum (EDS) reveals the existence of Ru, P, N and C (Figure 4C) and the atomic percentages of various elements are provided in Figure 4C. ICP-OES analysis suggests a nearly 2:1.15 atomic ratio for Ru/P. Figure 4D and 4E present the TEM images of the nanofibers taken from Ru2P@PNC/CC-900, showing that there are abandunt Ru2P nanoparticles formed on the nanofibers and the diameter of most nanoparticles is below 50 nm. Statistical analysis of ca. 80 nanoparticles indicates a mean diamter of 20.19±9.62 nm with a median value of 18.0 nm, as shown as the size distribution in Figure 4F. The scanning TEM image and the corresponding EDS elemental mapping images (Figure 4G) further confirm that P, N and C elements are uniformly distributed throughout the nanofiber, suggesting the successful N, P co-doping on the carbon fiber; the Ru element is scattering on the N, P doped carbon nanofibers, and its distribution is consistent with the distribution of Ru2P nanoparticles observed in corresponding TEM image. The high-resolution TEM (HRTEM) image of the Ru2P@PNC/CC-900 (Figure 4H) shows well-resolved lattice spacing with interplanar distances of 2.35 Å and 3.37 Å, corresponding to the (112) and (011) planes of the Ru2P,


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respectively. The Ru2P nanoparticles are partially encapsulated by the carbon layer, which confers excellent stability to the catalyst. All these results suggest that this Ru2P@PNC/CC-900 catalyst is composed of Ru2P nanoparticles anchored on carbon nanofibers on CC. The TEM images of other samples are provided in Figure S7-S8, showing similar fibrous structure. The XPS further confirms the presence of Ru (2.03 at%), P(1.87 at%), N(0.89 at%), C (77.26 at%) and O (17.95 at%) elements in the Ru2P@PNC/CC-900 (Figure 5A). The atomic ratio for Ru/P is 2.03:1.87, which slightly differs from that obtained from EDX analysis (16.6:12.4) and from ICP-OES (2:1.15), possibly because of the different working principles of three analytical techniques. In the C 1s spectrum shown in Figure 5B, the deconvoluted subpeaks at 288.7, 285.8 and 284.5 eV are consistent with the CC=O, C=N/C-P/C=O and C=C, respectively, indicating successful N, P co-doping on the carbon lattice.39-41 The peaks at 284.7 and 280.2 eV could be ascribed to overlapped Ru 3d3/2 and Ru 3d5/2, respectively.17 In the Ru 3p spectrum in Figure 5C, the subpeaks at 484.0 and 461.7 eV are associated with the Ru 3p1/2 and Ru 3p3/2 of Ru2P, respectively.17 The P 2p spectrum of Ru2P@PNC/CC-900 (Figure 5D) could be deconvoluted to four subpeaks, of which the two peaks at 130.5 and 129.8 eV could be assigned to the P 2p1/2 and P 2p3/2 in Ru2P, respectively.42 The peaks at 134.3 and 133.3 eV can be assigned to PO and P-C species in the carbon fiber, respectively.43 The high-resolution N 1s region (Figure 5E) shows four peaks of oxidized-N (402.3 eV), graphitic-N (401.4 eV), pyrrolicN (400.1 eV), and pyridinic-N (398.0 eV).44-45 These observations are consistent with the C 1s spectrum and the EDX elemental mapping images, indicating the successful formation of N, P co-doped carbon fibres.


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The HER performances of the Ru2P@PNC/CC-900 (catalyst loading: 1.5 mg cm-2, equals to a Ru2P loading of ca. 0.43 mg cm-2) was evaluated in acidic solution in comparison to other catalysts. Figure 6A shows the iR corrected linear scan voltammetry (LSV) curves of different catalysts at scan rate of 5 mV s-1. It can be seen that bare CC has negligible HER activity, while commercial Pt/C loaded on CC exhibit outstanding HER catalytic activities with a near 0 mV onset potential. Remarkably, Ru2P@PNC/CC900 shows high activity even comparable to Pt/C to drive a HER current density of 10 mA cm-2 at small overpotential of 15 mV (Figure 6B). Although this overpotential is slightly higher than that for Ru@C2N (13.5 mV)16 and RuCoP clusters (11 mV),46 it compares favourably to most Ru-based catalysts including Ru/GLC (35 mV),15 RuP2@NPC (38 mV),17 Ru/C3N4/C (75 mV),18 Ru/NG-750 (8 mV),19 Ru-MoO2 nanocomposites (55 mV),21 Ru nanosheets (20 mV),22 RuPx@NPC (51 mV)38 and Ru2P/RGO-20 (22 mV)47 in acidic solution. It is also displayed in Figure 6A and B that this Ru2P@PNC/CC-900 catalyst is far superior to either Ru@N-CC-900 (111 mV@10 mA cm-2) or Ru2P (102 mV@10 mA cm-2) catalyst (see its XRD pattern and FE-SEM image in Figure S9). Meanwhile, it surpasses the Ru@PNC/CC-700 (22 mV@10 mA cm2

) and Ru@PNC/CC-800 (27 mV@10 mA cm-2) and Ru2P@PNC/CC-1000 (86 mV@10

mA cm-2). The Tafel plots for Ru2P@PNC/CC-900, Ru@N-CC-900, Ru2P and Pt/C on CC were further plotted and compared in Figure 6C. The Tafel slope of 28 mV dec-1 for Ru2P@PNC/CC-900 is lower than that for Pt/C (29 mV dec-1), Ru2P nanoparticles (86 mV dec-1), Ru@N-CC-900 (117 mV dec-1) and other rivals (Figure 6D), confirming the outstanding HER activity of Ru2P@PNC/CC-900. It also suggests that the HER on


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Ru2P@PNC/CC-900 occurs through a Volmer(RDS)−Tafel mechanism with a discharge step to form adsorbed hydrogen (H*) under acidic condition.48-50 This comparison implies that the high HER activity of Ru2P@PNC/CC-900 may originate from the rational combination of the compositional and structural merits from both Ru2P nanoparticle and P, N-doped carbon nanofiber. The long-term durability of Ru2P@PNC/CC-900 was investigated. As shown in Figure 6E, the Ru2P@PNC/CC-900 shows slightly decay of cathodic current density after 1000 continuous cyclic voltammetry (CV) scanning, which points to good stability. When a static overpotential (21 mV) was applied, the current density of Ru2P@PNC/CC-900 could be kept for at least 10 h with only a decrease of less than 20%. XRD (Figure S10) and FE-SEM (Figure S11) characterizations show that the Ru2P@PNC/CC-900 maintains its crystalline phase and fibroid network nanostructure after long-term electrolysis. The Ru abundance in Ru2P@PNC/CC-900 does not significantly decrease according to the XPS spectrum (Figure S12), suggesting negligible detachment or dissolving of Ru2P nanoparticles after electrolysis. The recursitivity of Ru2P@PNC/CC-900 for HER was further investigated by applying a successive cathodic potential polarization. As shown in Figure 6F, when overpotential of 15 mV was applied, the cathodic HER current density immediately levels off at ca. 10 mA cm-2 and remains quite stable for the rest 500 s. The Ru2P@PNC/CC-900 electrode is able to response immediately to a more negative potential with a larger cathodic current density. At the same time, it is clearly observed that the current density at same overpotential remains quite consistent even after multiple potential steps and long-time electrolysis.

This

measurement

demonstrates

the

excellent

recursitivity

of


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Ru2P@PNC/CC-900 for practical hydrogen production, due to its rational combination of catalytically active components with a hierarchical 3D architecture with excellent mechanical robustness, electrical conductivity, and efficient mass transportation. The HER activity of Ru2P@PNC/CC-900 was further studied in 1.0 M KOH in comparison to Ru@N-CC-900, Pt/C on CC and bare CC (Figure 7A). It is clearly observed that blank CC has poor HER catalytic activity for clean background; Ru2P@PNC/CC-900 needs an overpotential of 50 mV to drive a HER current density of 10 mA cm-2, which is only 16 mV more than that of commercial Pt/C (34 mV) and much smaller than that of Ru@N-CC-900 (85 mV). Literature survey further suggests that although this overpotential is higher than that of reported Ru/NG-750 (8 mV),19 Ru@C2N (17 mV),16 Ru/NC (21 mV),25 RuCoP clusters (23 mV)46 and Ru2P/RGO-20 (13 mV),47 it is lower than that of RuP2@NPC (52 mV),17 Ru-MoO2 nanocomposites (29 mV),21 and RuPx@NPC (74 mV)38 in alkaline media. The Tafel plots of Ru2P@PNC/CC-900 (66 mV dec-1), is also lower than that of Ru2P nanoparticles (88 mV dec-1) and Ru@N-CC-900 (87 mV dec-1) and is close to that of Pt/C on CC (52 mV dec-1), as shown in Figure 7B, indicating the excellent HER activity of Ru2P@PNC/CC-900 in alkaline media. It suggests that the rate-limiting step for the HER on the Ru2P@PNC/CC-900 is the Tafel step rather than the prior Volmer step.25 Polarization curves and chronoamperometric measurement in Figure 7C and 7D further verify the long-term catalytic stability of Ru2P@PNC/CC-900 electrode in alkaline solution. To elucidate the high catalytic performance of Ru2P@PNC/CC-900, we carried out DFT calculations by using Gibbs free energy of hydrogen adsorption (△GH*) as a descriptor of HER. The Ru2P belongs to orthorhombic phase and its crystalline structure


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is shown in Figure 8A. Ru-Ru-Ru hollow site in this structure is the most favorable H adsorption site according to the DFT calculation (Figure 8B). As shown in Figure 8C, the Ru2P has a free energy of proton adsorption (△GH* = -0.56 eV), which is in line with previous work suggesting that the metal atoms in a metal phosphide has a partial positive charge (δ+) while the P has a partial negative charge (δ-) due to weak electron density transfer from metal to P;51-53 When coupled on P, N doped carbon, the △GH* of Ru2P can be further decrease to a more desirable value of 0.31 eV (Figure 8C), which is in good agreement with the experimental obervation that the HER activity of Ru2P nanoparticles is inferior to the Ru2P@PNC/CC-900. The N/P dual doping could boost the capacity of proton adsorption and reduction on carbon to enhance HER activity.25,54 As a result of integrating Ru2P on P, N doped carbon, the H-binding strength is neither too strong nor too weak, making Ru2P@PNC/CC-900 a higher electrocatalytic performance for HER.51 We further evaluated the effective electrochemical surface area of Ru2P@PNC/CC-900 and other catalysts by measuring their double layer capacitances (Figure S13). Figure 8D exhibits the capacitive currents as a function of scan rate for five catalysts. The Ru2P@PNC/CC-900 shows largest capacitance of 92.7 mF cm-2 compared to Ru@PNC/CC-700 (36.8 mF cm-2), Ru@PNC/CC-800 (31.9 mF cm-2), Ru2P@PNC/CC1000 (31.0 mF cm-2), Ru2P nanoparticles (26.7 mF cm-2) and Ru@N-CC-900 (12.8 mF cm-2), suggesting that Ru2P@PNC/CC-900 offers high surface areas to support abundant and accessible catalytically active Ru2P nanoparticles to facilitate HER process.

4. CONCLUSION


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In summary, Ru2P nanoparticle decorated P, N dual-doped carbon nanofibers on carbon cloth has been synthesized as a hierarchical 3D HER electrocatalyst with high catalytic activity and excellent durability. To approach a HER current density of 10 mA cm-2, this Ru2P@PNC/CC900 catalyst only demands overpotentials of 15 and 50 mV with small Tafel slope of 28 and 66 mV dec-1 in acidic and alkaline conditions, respectively. The high HER performance of Ru2P@PNC/CC-900 could be ascribed to its rational combination of compositional and structural merits from both Ru2P nanoparticle and P, N-doped carbon nanofiber. On the one hand, Ru2P nanoparticles supported on P, N doped carbon demonstrate high intrinsic HER activity due to the synergistic effect two components according to the DFT calculation; meanwhile, the integrative 3D hierarchical architecture offers high surface areas with good mechanical robustness to support abundant catalytically active Ru2P nanoparticles, and possesses excellent electrical conductivity and efficient access for mass transportation to facilitate the HER process. Our study explores Ru2P@PNC/CC-900 as a promising HER catalyst, and may also offer possible new clues on the design and synthesis of efficient HER catalysts for future energy applications.

■ ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Optical photographs of CC, PA-PANI/CC, Ru(III)@PA-PANI/CC and Ru2P@PNC/CC-900; FE-SEM images of CC, Ru(III)@PA-PANI/CC, PANI/CC, Ru(III)@PANI/CC, Ru@N-CC-900, and Ru2P; XRD patterns of CC, Ru@N-CC-900, Ru2P and post-electrolysis Ru2P@PNC/CC-900;


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XPS spectra of Ru@N-CC-900 and post-electrolysis Ru2P@PNC/CC-900 ; Raman spectra of Ru@PNC/CC-700, Ru@PNC/CC-800, Ru2P@PNC/CC-900 and Ru2P@PNC/CC-1000; TEM images of Ru@PNC/CC-800, and Ru2P@PNC/CC-1000; CV curves (PDF).

■ AUTHOR INFORMATION Corresponding Author * Email: [email protected] (W. H. Hu); [email protected] (C. M. Li). ORCID W. H. Hu: 0000-0001-6278-9551; C. M. Li: 0000-0002-4041-2574 Notes The authors declare no competing financial interests.

■ ACKNOWLEDGEMENT We would like to gratefully acknowledge the financial support from Natural Science Foundation Project of CQ CSTC (cstc2016jcyjA0493), Fundamental Research Funds for the Central Universities (XDJK2018B001), and Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies.


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Figures and Captions

Figure 1. Schematic illustration of preparation for Ru2P@PNC/CC-900.


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Figure 2. FE-SEM images of PA-PANI/CC electrodeposited at a potential of (A) 0.65 V, (B) 0.70 V, (C) 0.75 V and (D) 0.80 V vs. Ag/AgCl/ saturated KCl.


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Figure 3. XRD patterns of (A) Ru@PNC/CC-700, (B) Ru@PNC/CC-800, (C) Ru2P@PNC/CC900 and (D) Ru2P@PNC/CC-1000. The asterisks refer to the peaks from CC.


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Figure 4. FE-SEM images (A, B), EDS spectrum (C), TEM images (D, E), nanoparticle size distribution (F), STEM image and EDS elemental mapping images (G) and HRTEM image (H) of Ru2P@PNC/CC-900.


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Figure 5. XPS survey spectrum (A) and high-resolution XPS spectra of (B) C 1s and Ru 3d; (C) Ru 3p; (D) P 2p, and (E) N 1s of Ru2P@PNC/CC-900.


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Figure 6. (A) LSV curves of Ru@PNC/CC-700, Ru@PNC/CC-800, Ru2P@PNC/CC-900, Ru2P@PNC/CC-1000, Ru@N-CC-900, Ru2P, Pt/C on CC and bare CC with a scan rate of 5 mV s-1. (B) Overpotentials at 10 mA cm-2 vs. RHE. (C, D) Corresponding Tafel plots and slopes. (E) Polarization curves for Ru2P@PNC/CC-900 before and after 1000 CV cycles (inset: Timedependent current density curve of Ru2P@PNC/CC-900 under static overpotential of 15 mV for 10 h). (F) The multi-potential step process of Ru2P@PNC/CC-900 for HER. All data measured in 0.5 M H2SO4.

Figure 7. (A) LSV curves of Ru2P@PNC/CC-900, Ru@N-CC-900, Pt/C on CC and bare CC in 1.0 M KOH with a scan rate of 5 mV s-1. (B) Tafel plots for Ru2P@PNC/CC-900, Ru@N-CC900 and Pt/C on CC. (C) Polarization curves for Ru2P@PNC/CC-900 before and after 1000 CV


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cycles. (D) Time-dependent current density curve of Ru2P@PNC/CC-900 under static overpotential of 50 mV for 10 h (without iR correction).

Figure 8. Theoretical models used in DFT calculations and the adopted adsorption sites of H* on the surface of these models: (A) Ru2P and (B) Ru2P@PNC. (C) The calculated free-energy diagram of HER at equilibrium potential for Ru2P@PNC, Ru2P nanoparticles and PNC. (D) Capacitive currents as a function of scan rate for Ru2P@PNC-900 and Ru2P.


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Table of Content (TOC)

We herein report Ru2P nanoparticle decorated P, N dual-doped carbon nanofibers on carbon cloth (Ru2P@PNC/CC-900) as a highly efficient and durable hierarchical HER electrocatalyst in both acidic and alkaline media.


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