Scalable synthesis of ruthenium-based electrocatalyst as a promising

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Scalable synthesis of ruthenium-based electrocatalyst as a promising alternative to Pt for hydrogen evolution reaction Zhen Zhang, Ping Li, Qi Feng, Bing Wei, Chenglong Deng, Jiantao Fan, Hui Li, and Haijiang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10502 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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ACS Applied Materials & Interfaces

Scalable synthesis of ruthenium-based electrocatalyst as a promising alternative to Pt for hydrogen evolution reaction Zhen Zhang †,‡,||, Ping Li ⊥,||, Qi Feng †,‡, Bing Wei † #, Chenglong Deng †, Jiantao Fan †, Hui Li *,† , Haijiang Wang *,§

† Department of Materials Science and Engineering, Southern University of Science

and Technology, Shenzhen, 518055, China ‡ School of Materials Science and Engineering, Harbin Institute of Technology,

Harbin, 150001, China § Department of Mechanical and Energy Engineering, Southern University of Science

and Technology, Shenzhen, 518055, China ⊥ Department of Physics, Soochow University, Suzhou 215006, China

# School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China

* Corresponding author: Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China E-mail address: [email protected]

Keywords: Mo2C, Ru, popcorn, electrocatalyst, hydrogen evolution reaction

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Abstract Designing highly active, stable and cost-efficient electrocatalysts as alternatives to replace Pt is extremely desirable for the hydrogen evolution reaction (HER). Despite much progress that has been made based on complete non-precious metals (NPM), very few NPM catalysts have showed comparable performance to Pt-based catalysts. Herein, a cost-efficient, environmentally-friendly and scalable method to synthesize novel ruthenium(Ru)-doped transition metal carbide (Mo2C) hybrid catalyst was proposed. The hybrid nanoparticles were uniformly distributed and strongly embedded in biomass-derived highly porous N-doped carbon framework. In particular, Mo2C@Ru exhibited a Pt-like remarkable electrocatalytic performance for HER and it only required an extremely low overpotential of 24.6 mV to reach the current density of 10 mA cm-2. Furthermore, our density functional theory (DFT) calculations indicated that the nanocomposite exhibits improved metal-hydrogen binding and favorable hydrogen adsorption energy which is comparable to that of Pt. The facile and scalable synthesis methodology, the relatively low cost and the excellent electrochemical HER performance comparable to that of commercial Pt/C, suggest the Mo2C@Ru electrocatalyst is a promising alternative to Pt for large-scale hydrogen production.

1. Introduction As a renewable energy carrier, hydrogen has been proposed as a promising candidate to replace fossil fuels, and electrochemical water splitting is an attractive and viable approach to produce H21-2. The most significant prerequisite for hydrogen evolution reaction (HER) is to explore electrochemical catalysts with high efficiency and robust stability3. Although Pt-group metals so far have been proven to be the most effective electrocatalysts with low overpotential for HER, the high-cost and scarcity heavily inhibit their potential for large-scale application4-5. Therefore, the highly efficient and low-cost electrochemical catalysts are still being vigorously pursued to 2


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realize hydrogen economy. Typically, the electrochemical HER includes two steps: the Volmer reaction for reducing H+ to Hads attached on catalyst surface, and the Heyrovsky or Tafel reaction for the desorption of Hads to generate H2 gas6-7. In recent decades, considerable efforts have been concentrated on transition metal carbides to replace Pt-based electrocatalysts, such as WC, W2C, Mo2C and MXenes materials8, due to their highly similar d-band electronic structures to Pt group metals9-11. Among them, Mo2C has been extensively investigated as a promising alternative12-14. However, previous reports have revealed that Mo2C has a relatively negative hydrogen binding energy (△GH*), meaning that the Mo-H bonds on Mo2C surface are so strong that the H+ reduction (Volmer step) is favorable but the desorption of Hads (Heyrovsky/Tafel step) is seriously constrained6, 15-17. It has been proven that combining Mo2C with carbon substrate is an efficient route to boost the catalytic activity18-21, mainly because of the following two aspects: 1) nanocarbon could provide enough attachments for the Mo2C particles and favor the fast electron transfer; 2) the conjunction with carbon should downshift the d-band center of Mo atoms by inducing the electron transfer from Mo active sites to carbon, aiming to weaken the strong negative Mo-H bonds as close to zero as possible22-25. Generally, the carbon precursors used to prepare Mo2C can be subdivided into three major categories: hydrocarbon gas, carbohydrate and amine. For the traditional synthesis, hydrocarbon (CH4) is usually chosen as the carbon source, coupled with hydrogen as reducing agent to convert MoOx to Mo2C26-28. Such mixed gases are flammable and environmentally unfriendly, undoubtedly causing potential safety hazard during the annealing process29. Compared to carbohydrate such as glucose25, 30-32

and chitosan33, nitrogen-containing amine (dopamine34-36, dicyandiamide22, 37-38,

aniline39-41 and melamine42) used as the carbon precursor usually involves the doping of N into final carbon support, which would help to regulate the electronic structure of carbon to boost catalytic activity. Nevertheless, high temperature carbonization usually causes serious agglomeration and excessive growth of Mo2C particles, 3



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inevitably leading to the decrease of specific surface area (generally less than 250 m2 g-1, as shown in Table S1) and the sacrifice of electrochemical active sites. Compared to the above-mentioned carbon precursors, recently biomasses, such as soybean43, bacterial cellulose24 and shells of sunflower seeds44, were reported to be employed to prepare carbon materials, because of the following properties: (1) being low-cost, environmentally friendly and easy to realize large-scale production; (2) allowing the heteroatoms-doping (N, S or P) into the final carbon support. However, similar to abovementioned other solid carbon precursors, the bulk biomass with low surface area usually lacks porosity, and has to be treated by physical grinding or ball-milling to mix with Mo source. Therefore, exploring novel porous biomass as the carbon precursor and developing proper mixing approach are extensively required for preparation transition metal carbides catalysts. Although great progress has been made for improving HER performance based on complete non-precious metals (NPM), very few NPM catalysts have been reported to be competitive to Pt-based catalysts29, 45-46. Very recently, ruthenium (Ru) has been explored as potential substitutes towards Pt catalysts, because of its relatively lower cost of only 1/25 of Pt metal47, high acid-resistance ability and Pt-like bonding strength with hydrogen45-46,

48-51

. Normally, Ru is considered as superior

electrocatalyst for oxygen evolution reaction, while the research about its underlying performance toward HER is rare. Furthermore, combining low-content Ru with NPM catalysts to achieve cost-efficient electrocatalysts with Pt-like HER performance is a highly promising strategy but is still a considerable challenge. Herein, highly uniform Ru-doped Mo2C nanoparticles embedded in popcorn-derived hierarchically porous carbon matrix were readily synthesized via a cost-efficient and scalable method. The preparation included two steps: (1) KOH-assisted mixing of porous popcorn with Mo/Ru sources and subsequent freeze-drying; (2) annealing treatment under inert atmosphere. During the annealing process, the carbonization of popcorn, the in-situ growth of Mo2C particles and the reduction of Ru3+ were realized simultaneously. Such synthetic technology involved environmentally friendly and low cost biomass 4


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precursor, as well as simple preparation process, making it promising to scale up. In particular, the as-prepared Mo2C@Ru catalyst exhibited low overpotentials of 24.6 mV and 66.5 mV to drive the current densities of 10 and 50 mA cm-2, respectively. The outstanding Pt-like HER performance is congruent with DFT calculations, which prove that the synergistic effect between the hetero Ru atoms and Mo2C can help optimize the hydrogen adsorption energy and thereby enhance electrocatalytic activity towards HER.

2. Experimental section 2.1 Chemicals and reagents. Ammonium molybdate ((NH4)6Mo7O24·4H2O), sulfuric acid (H2SO4), potassium hydroxide (KOH), ruthenium(III) chloride anhydrous (RuCl3) and 10 wt% Ru/C catalyst were purchased from Aladdin Industrial Corp. Platinum on graphitized carbon (20 wt% Pt/C) was purchased from Johnson Matthey. The Nafion® 212 membrane and Nafion ionomer solution (5 wt%) and IrO2 were provided by DuPont. The carbon papers were provided by Ce Tech Co., Ltd. All chemicals were of reagent grade and used without further purification. All solutions were prepared using deionized water (18.2 MΩ cm). 2.2 Preparation of Mo2C@Ru embedded in carbon matrix. Typically, the popcorn was prepared by heating corns in an oriental sealed vessel with manual rotation, which was elaborated in previous reports52-53. The moisture in the corns was vaporized during heating, so that the corns were popped when the pressure was released suddenly by opening the tap of vessel with a loud noise. Hereafter, 300 mg popcorn was immersed in 10 ml of an aqueous solution containing 100 mg KOH, and the mixture was ultrasonicated for 30 min to form a “popcorn-hominy” solution. Then, each of the 100, 200, 300, 400 and 500 mg (NH4)6Mo7O24·4H2O was dispersed in the as-obtained solution under continuous stirring. After agitated stirring for 6 h, the uniform products were freeze-dried to evaporate the water. After drying, the solid mixture was further transferred to a tube furnace for annealing treatment at 750°C for 2 h in N2 atmosphere with a heating rate of 5°C min-1. Subsequently, the as-prepared 5



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black materials were washed with a H2SO4 solution (1 M) to remove inorganic impurities, then washed with deionized water, and dried at 60°C in vacuum for 10 h. The obtained products are labeled as Mo2C-x, where the x is the mass of the added (NH4)6Mo7O24·4H2O. As for the Mo2C@Ru composite, before the freeze-drying process, 35 mg of RuCl3 was dispersed in the hominy solution containing 300 mg (NH4)6Mo7O24·4H2O. For comparison, the sole Ru catalyst supported on popcorn-derived carbon was also prepared following the same procedures, but without the addition of Mo precursor. 2.3 Materials characterization. The X-ray diffraction (XRD) patterns of the prepared powder were performed on Bruker D8 diffractometer using Cu Kα radiation (λ=1.5418 Å) and the data were collected in the range of 10-80° with a scanning rate of 0.02 deg min-1. The scanning electron microscope (SEM) images were obtained with a TESCAN system. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) spectroscopy were conducted using a FEI Tecnai G2 F30 instrument with an acceleration voltage of 300 kV. Nitrogen adsorption-desorption isotherms were measured on a Quadrasorb SI-MP at 77 K. The X-ray photoelectric spectroscopy (XPS) measurements were performed on a Perkin-Elmer Model PHI 5600 XPS system. Raman spectra were obtained using a LabRAM HR Evolution spectrometer with a laser excitation wavelength of 532 nm. The inductively coupled plasma (ICP) measurement was performed on an Agilent Technologies 7700 series instrument. 2.4 Electrochemical measurements. Electrochemical measurements were performed with a Solartron electrochemical workstation, equipped with Compact Pine Rotator system. A typical three-electrode configuration at room temperature was used, where a glassy carbon rotating disk electrode (RDE) with a diameter of 5 mm (disk geometric area of 0.196 cm2), a graphite rod and a reversible hydrogen electrode (RHE) served as the working electrode, counter electrode and reference electrode, respectively. The catalyst ink was prepared by dispersing 4 mg of each catalyst in 1 mL of 1:3 (v/v) water/ethanol mixture solvents along with 40 µL Nafion solution (5 6


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wt%), and the mixture was sonicated for 30 min. Then, 14 µL of this solution was drop-casted onto the RDE, producing a catalyst loading of 0.275 mg cm-2. An electrode using commercial Ru/C (10 wt%) catalyst, as a reference catalyst, was prepared by the same way. In addition, an electrode with commercial Pt/C (20 wt%) catalyst was also prepared, but the added mass was 2 mg. Current density was normalized to the geometrical area of the working electrode. Linear sweep voltammetry (LSV) was collected in 0.5 M H2SO4 with the scan rate of 5 mV s-1 on the RDE under 1600 rpm. The Tafel slopes were fitted according to the Tafel equation (η = a + b log(j)), where η (mV) indicates the applied overpotential, j (mA cm-2) denotes the current density and b (mV dec-1) is the Tafel slope. Long-term stability tests were performed by continuous cyclic voltammetry from -0.20 to +0.15 V at a sweep rate of 100 mV s-1. In order to obtain the solution resistance, the electrochemical impedance spectroscopy measurement was performed with frequency from 0.01 Hz to 100 kHz and an amplitude of 10 mV. The final solution resistance was 7.1 Ω and all the potentials reported in this paper were manually iR-corrected. 2.5 Computational details. Density functional theory (DFT) calculations were performed using the Vienna Ab-initio Simulation Package (VASP)54 and the perdew-Burke-Ernzerh (PBE) of exchange-correlation functional correction55. The calculated lattice parameters of Mo2C were: a = 4.72 Å，b = 6.00 Å，c = 5.20 Å. A 2 x 2 x1 molybdenum carbide supercell with a vacuum layer of 20 Å was constructed. The cutoff energy was 500 eV during calculation. The adsorption free energy of H* (∆GH*) is calculated as follows: ∆GH* = ∆EH* + ∆EZPE - T∆S where ∆EH*, ∆EZPE and ∆S are the binding energy of atomic hydrogen, zero point energy correction and entropy change of H* adsorption, respectively. In addition, the value of ∆S was obtained by ∆S = S(H*) -1/2 S(H2) and ∆EZPE was estimated according to the equation ∆EZPE = ∆EZPE (H*) - 1/2 ∆EZPE(H2). T is the system temperature of 300 K, so the value of T∆S is -0.205 eV, in agreement with previous report38. Furthermore, the calculated ∆EZPE here is 0.289 eV, which is very close to previously 7



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reported value by Norskov et al56.

3. Results and discussions 3.1 Preparation and characterization

Figure 1. Schematic illustration of the synthetic procedure for the Mo2C@Ru embedded in 3D porous popcorn-derived carbon matrix. The typical preparation procedure of the Mo2C@Ru products is demonstrated in Figure 1. As described in previous reports52-53, the popcorn was prepared by the “fast-explosion” process with an oriental sealed container, which has been a traditional technology in China with a long history. In brief, the corn was placed in a sealed iron container and then heated, where the moisture inside the corn was vaporized under high temperature. Then, accompanied by the rapid temperature drop and pressure release with the opening of the lid of the sealed vessel, the corn was 8


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boomed to become delicious popcorn. The boomed popcorn reveals a 3D honeycomb-like porous structure, and after directly carbonization treatment the structure remains well, as shown in the SEM image (Figure 1). Compared with other biomass used as carbon precursors, popcorn presents abundant porosity and large specific surface area. Instead of using traditional high-energy-consuming grinding or ball-milling process, we carried out the liquid mixing procedure with the assistance of KOH to realize the complete mixing of Mo, Ru and carbon precursors. The porous structure of popcorn was more easily dismembered and homogeneously dissolved by the KOH activation to form a “popcorn-hominy” solution, favoring the following complete contact with Ru-Mo precursors. It should be pointed out that the Ru3+ cation tends to absorb Mo7O246- in alkaline solution due to the electrostatic attraction42, allowing the subsequent Ru doping on Mo2C, rather than the growth of Ru separate particles on carbon support (further discussion is provided below). In particular, the carbon etching by KOH under high-temperature could further help produce more porosity and suppress the supply of carbon atoms, thus leading to the desired inhibition on the excessive growth of Mo2C@Ru particles. Powder X-ray diffraction (XRD) measurements were performed to characterize the crystalline structure of the prepared electrocatalysts. As shown in Figure 2f, for the five samples with different Mo precursor contents, similar diffraction peaks located at 34.4°, 38.0°, 39.5°, 52.2°, 61.7°, 69.7°, 74.9° and 75.8° correspond well to the characteristic (021), (200), (121), (221), (040), (321), (240) and (142) lattice planes of Mo2C (JCPDS card no. 72-1683, a = 4.72 Å，b = 6.00 Å，c = 5.20 Å), respectively. In addition, the characteristic peaks of Mo2C gradually become stronger with the increase of Mo content. No other diffraction peaks belonging to other crystal structure were detected, suggesting the high phase purity of the as-synthesized electrocatalysts. Moreover, shaded by the strong peaks of Mo2C, the diffractions belonging to carbon were undiscovered.

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(a)

Mo2C-200

500 nm

(d)

Mo2C-300

500 nm

500 nm

(h)

(e) Mo 2C-500

Mo2C-400

500 nm

500 nm

(f)

(c)

(b) Mo2C-100

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(g)

Figure 2. (a-e) SEM images; (f) XRD patterns; (g) N2 adsorption-desorption isotherms and (h) the corresponding pore size distribution of Mo2C-x samples (x = 100, 200, 300, 400 and 500). In order to detect the carbon matrix structures, we conducted Raman measurement that is a powerful non-destructive tool to distinguish the crystal structure of carbon. As shown in Figure S1, the characteristic peaks located at around 662.0 cm-1, 816.5 cm-1 and 991.6 cm-1 are ascribed to the Mo2C25, 57. In addition, two apparent peaks at 1350 ± 10 cm-1 and 1579 ± 10 cm-1 can be observed, which can be assigned to the D and G bands of carbon, respectively30, 44. Furthermore, with the increase of the Mo content, the peaks corresponding to Mo2C become more distinct, while the intensities of D and G bands show a significant decrease, suggesting that more carbon atoms were sacrificed to form Mo2C particles. The morphologies of the as-prepared Mo2C products with the addition of various 10


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amounts of Mo precursor were investigated by SEM. As demonstrated in Figure 2a, the sample Mo2C-100 presents a 3D porous structure with abundant pores, which could be favorable for HER by facilitating the transport of electrolyte on the surface and fast release of hydrogen gas. Moreover, the SEM results (Figure 2a-e) confirm that with the increase the content of Mo precursor, the porous structures gradually become blocked. In order to achieve further information about the specific surface area and pore-structure of the as-synthesized products with various Mo2C content, N2 adsorption-desorption isotherm measurements were conducted. As shown in Figure 2g, the isotherm curves show the features of both type-I isotherms and H4 type hysteresis curves, suggesting that the samples contain typical micropores and mesopores52. Such a hierarchically porous structure was investigated in detail by the Barrett-Joy-Halenda (BJH) and Horvath-Kawazoe (H-K) methods52. According to the pore size distribution curves (Figure 2h), the size of the major pores is less than 1 nm, typically belonging to micropores, which is mainly related to the activation and chemical etching by KOH during annealing process. As summarized in Table S2, with the increase of added mass of (NH4)6Mo7O24·4H2O, the final specific surface area of products presents a descending trend from 1079.9 to 20.1 m2 g-1. Similarly, a decrease of total pore volume from 0.9854 to 0.0547 cm3 g-1 for Mo2C-x samples is also observed. Notably, Mo2C-300 sample exhibits a specific surface area of 274.9 m2 g-1, a total pore volume of 0.2505 cm3 g-1 and an average pore size of 3.64 nm.

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(b)

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(c)

Mo

200 nm

200 nm

N

Ru

1 µm

(d)

200 nm

200 nm

200 nm

(f)

(e)

Mo2C (121) Mo2C (121)

d=0.23 nm

500 nm

(g)

10 nm

(h)

d=0.23 nm

3 nm

(i)

Figure 3. (a, b) SEM images; (c) STEM and corresponding element mappings (Mo, Ru, N); (d, e) TEM images and (f) HRTEM image of Mo2C@Ru; (g) XPS summaries of Mo2C@Ru and Mo2C; the corresponding high-resolution XPS signals of (h) N1s and (i) C1s for Mo2C@Ru. Then, considering the best catalytic performance of Mo2C-300 catalyst, which will be further discussed below, it was consequently chosen to combine with RuCl3 to prepare Mo2C@Ru hybrid catalyst. Because of the low-content of Ru, Mo2C@Ru hybrid catalyst shown in Figure 3a, b exhibits similar porous morphology to that of pure Mo2C-300 product. The 3D hierarchically porous structure was further confirmed using TEM, as clearly shown in Figure 3d. From Figure 3e, it can be seen that the Mo2C@Ru nanoparticles with a uniform diameter size of 3-8 nm are homogeneously dispersed on the popcorn-derived carbon support. Moreover, in the high-resolution TEM image (Figure 3f), the interplanar distance of 0.23 nm, corresponding to the (121) plane of Mo2C are observed. Attributed to the KOH 12


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etching, such hybrid particles exhibit no obvious aggregation or excessive growth even during high-temperature annealing process. Furthermore, the STEM in Figure 3c and corresponding elemental mapping clearly show the uniform distribution of Mo, Ru and N elements, proving the homogenous Ru-doping on Mo2C and uniform N-doping in carbon matrix. In addition, the clear diffraction peaks for the hybrid Mo2C@Ru catalyst, corresponding to Mo2C were detected in XRD results (Figure S2), while no obvious Ru peaks were identified. The pure Ru particles located on popcorn-derived carbon matrix were also prepared following similar procedures except for the addition of Mo precursor. The XRD pattern of the pure Ru sample is provided in Figure S3, showing strong metallic Ru phase (JCPDS 06-0663). Therefore, we can draw a conclusion that during the co-existence of Ru3+ and Mo7O246-, they are inclined to attach to each other due to the electrostatic force, thus resulting in the Ru doping on Mo2C particles, instead of the uncontrolled individual nucleation and growth of Ru particles. In order to further investigate the elemental composition and chemical bonding state, especially the bonding contact between Mo2C and Ru of the as-prepared catalyst, XPS measurements were also carried out. As shown in Figure 3g, the elements of Mo, N, C, O are all identified in the XPS surveys of Mo2C and Mo2C@Ru. Meanwhile, two peaks at binding energies of 461.7 eV and 483.9 eV corresponding to the Ru3p3/2and Ru3p1/2 can be observed for Mo2C@Ru, indicating the effective reduction of Ru3+ to metallic Ru by annealing treatment58. The high-resolution XPS spectrum of N1s for Mo2C@Ru (Figure 3h) is fitted into three peaks located at 401.1, 399.4 and 397.9 eV, that are ascribed to graphitic-N, pyrrolic-N and pyridinic-N, respectively, confirming the doping of hetero-atom N into carbon support59. As can be seen in Figure 3i, the high-resolution C1s spectrum implies the presence of C-C bonds, oxygen-containing C-O, C=O and O-C=O bonds, as well as molybdenum bonded carbon at a lower binding energy of 283.9 eV30. In addition to the carbon-bonded oxygen species, most of oxygen atoms are introduced by the inevitable oxidation of molybdenum on the surface when exposed to air42. As for the Mo3d spectrum (Figure 13



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S4), an obvious red-shift can be observed in Mo2C@Ru compared to that of pure Mo2C, which can be explained by the strong electron interactions of Ru and Mo2C, and thus proves that the Ru atoms tend to anchor on the Mo2C particles instead of individual growth. The Ru-doping resulted in the lower binding energy, suggesting the enriched electrons around Mo60. Therefore, the incorporation of Ru atoms on Mo2C nanocrystals could alter the electron structure and help neutralize the strong negative binding of Mo2C with hydrogen, which will be elaborated by the calculation results below. 3.2 Electrochemical characterization The electrocatalytic HER activities of as-prepared nanocatalysts were measured using a standard three-electrode setup in 0.5 M H2SO4 with continuous purging of N2. All the HER polarization curves were presented with iR-correction for further analysis. As shown in Figure 4a, in spite of the absence of Mo precursor, the Mo2C-0 sample exhibits a current density of 5 mA cm-2 with a potential of 414.1 mV, which is ascribed to the N-doping in the popcorn-derived carbon matrix57. For driving a current density of 10 mA cm-2, the Mo2C-300 catalyst only requires an overpotential of 172.9 mV, smaller than that for Mo2C-100 (383.8 mV), Mo2C-200 (196.8 mV), Mo2C-400 (226.9 mV) and Mo2C-500 (239.9 mV). It can be explained that Mo2C-100 and Mo2C-200 do not contain sufficient Mo2C particles to sustain the catalytic reaction; on the contrary too high Mo loadings in Mo2C-400 and Mo2C-500 samples cause the loss of porosity and specific surface area; that is, the Mo2C-300 catalyst possesses the optimal content of Mo2C among the five catalysts. Owing to the 3D hierarchically porous structure that favors fast mass transfer, the Mo2C-300 catalyst requires a potential of 219.2 mV to reach a high current density of 50 mA cm-2. Tafel plots were obtained from the polarization curves to help understand the underlying mechanism for the inherently high HER activity. As shown in Figure 4b, the Mo2C-300 catalyst presents the smallest Tafel slope of 58.4 mV dec-1, which is lower than 120 mV dec-1, suggesting a Volmer-Heyrovsky kinetic reaction process30, 44.

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(a)

(b)

(c)

(d)

Figure 4. (a) Polarization curves recorded in 0.5 M H2SO4 at a scan rate of 5 mV s-1 and (b) corresponding Tafel curves of Mo2C-x samples. (c) Polarization curves of Mo2C@Ru, Mo2C-300, Ru, commercial Ru/C and commercial Pt/C. (d) Polarization curves of Mo2C@Ru after 1, 2000 and 5000 cycles. Inset represents the current-time chronoamperometric response of Mo2C@Ru at an overpotential of 20 mV for 10 h. For comparison, the HER catalytic measurements of sole Ru, hybrid Mo2C@Ru, commercial Ru/C and commercial Pt/C were also conducted. Commercial 20 wt% Pt/C sample only requires a low potential of 46.6 mV to afford a current density of 50 mA cm-2, showing the best performance as expected. Notably, the hybrid Mo2C@Ru exhibits potentials of 24.6 mV and 66.5 mV to drive the current densities of 10 and 50 mA cm-2, respectively. Based on the ICP result, the mass concentration of Ru is 7.6 wt%. Obviously, the hybrid Mo2C@Ru catalyst possesses better electrochemical HER performance than that of pure Mo2C-300, pure Ru particles and the commercial Ru/C catalyst, and as well as the majority of reported HER catalysts as listed in Table S3. 15



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The detailed comparison of corresponding overpotentials for the whole samples to drive the current densities of 10 and 50 mA cm-2 are clearly displayed by bar diagrams in Figure S5. Moreover, to measure the durability of Mo2C@Ru catalyst, the cycling test was conducted and only a slight decay was observed in Figure 4d. Additionally, current-time chronoamperometric results shows that the current of the Mo2C@Ru catalyst remains stable with a fixed overpotential of 20 mV for 10 hours (inset of Figure 4d), confirming its robust stability. 3.3 Discussion and theoretical calculation In summary, such excellent electrochemically HER performance of the hybrid Mo2C@Ru particles embedded in popcorn-derived 3D porous carbon matrix (Figure 5a) could be ascribed to the following aspects: Firstly, as shown in the TEM results, the Mo2C@Ru particles were uniformly and homogeneously dispersed on the biomass-derived porous carbon support, which could favor the exposure of electrochemical active sites, thus promoting the availability of electrocatalyst. Secondly, the popcorn-derived 3D carbon matrix is rich of pores and possesses large specific surface area. Meanwhile, the popcorn is used as carbon source for the in-situ growth of Mo2C particles, ensuring the tight affinity between Mo2C and carbon matrix. Thirdly, both Mo2C and Ru possess electrochemical activity for catalyzing H2 production. Last but most importantly, the synergistic effect between Mo2C and hetero-atom Ru plays a significant role in promoting the hydrogen evolution, which will be further discussed below with DFT calculations.

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(a)

(b)

Figure 5. (a) Schematic illustration of the 3D porous Mo2C@Ru towards hydrogen evolution. (b) Calculated free-energies of hydrogen adsorption of Ru, Mo2C, Mo2C@Ru and Pt. In order to gain a further insight into the high electrocatalytic activity of the Mo2C@Ru, we carried out a series of DFT calculations by building the relevant theoretical models. The H* adsorptions on pure Ru, Mo2C and Mo2C@Ru (Mo site and Ru site) are shown in Figure S6, respectively. The adsorption energy of H species (∆EH*), the relevant contributions to the free energy (∆ZPE and T∆S), and the free energy of adsorbed H* (∆GH*) on surface of different models are provided in Table S4. It is well known that the metal-hydrogen bond (M-H) is an essential factor for hydrogen evolution. Typically, the closer to zero for the value of Gibbs free energy (∆GH*), the better HER catalytic performance should be obtained. For example, the most efficient Pt catalyst possesses a low |∆GH*| of 0.09 eV61. As shown in Figure 5b, the ∆GH* value of pure Ru is 0.369 eV, indicating that pure Ru has weak proton binding and easy desorption of hydrogen gas. On the contrary, Mo2C exhibits a largely negative ∆GH* value of -0.360 eV, clearly proving that the H* adsorption on the surface is too strong, which is in accordance with previous result33. Interestingly, coupling Ru atoms on the Mo2C crystals leads to ∆GH* of -0.234 eV for Mo site and 0.245 eV for Ru site, respectively. Therefore, we can draw a conclusion that both Mo2C and Ru are active centers for catalyzing H2 evolution. Furthermore, compared to the individual Mo2C and Ru, the Mo and Ru active centers in the hybrid Mo2C@Ru 17



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catalyst present smaller absolute value of ∆GH*, thus producing a relatively mediated adsorption-desorption behavior.

4. Conclusion In summary, we have proposed a facile and scalable synthesis method for Ru-doped Mo2C nanoparticles over popcorn-derived N-doped carbon matrix. Such uniquely open and porous structure of carbon support allows the highly uniform anchoring of Mo2C@Ru particles, thus favoring the exposure of electrochemical active sites. In particular, the Ru-based catalyst exhibited an extremely low overpotential of 24.6 mV to drive the current density of 10 mA cm-2, which is very close to that of commercial Pt/C and superior to the-state-of-art NPM-based catalysts. Furthermore, DFT results have proven that the synergistic effect between Mo2C and hetero-atom Ru plays essential roles in modulating the adsorption strength with H species. The scalable preparation technology, the relative low-cost, and outstanding Pt-like electrochemical HER performance prove that combining Ru with biomass-derived transition metal carbides is a novel and effective strategy to fabricate highly efficient alternatives to Pt for hydrogen production.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Further Raman spectra, XRD patterns, high-resolution spectrum of Mo3d, comparison of HER performance and the theoretical models for DFT calculation. AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected] 18


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ORCID Zhen Zhang: 0000-0003-4095-0206 Author Contributions All authors have given approval to the final version of the manuscript. ||

These authors contributed equally to the work.

Notes The authors declare no competing financial interest. Acknowledgment This work was financially supported by the Shenzhen Key Laboratory project (ZDSYS201603311013489), Development and Reform Commission of Shenzhen Municipality 2017 (No. 1106), Guangdong Innovative and Entrepreneurial Research Team

Program

(2016ZT06N500),

Shenzhen

Peacock

Plan

(KQTD2016022620054656), the National Key Research and Development Program of China (2017YFB0102701), and Development and Reform Commission of Shenzhen Municipality 2017 (No. 1181).

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Scalable synthesis of ruthenium-based electrocatalyst as a promising

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