Highly exposed ruthenium-based electrocatalysts from bimetallic metal-organic frameworks for overall water splitting

Author’s Accepted Manuscript Highly Exposed Ruthenium-based Electrocatalysts from Bimetallic Metal-organic Frameworks for Overall Water Splitting Tianjie Qiu, Zibin Liang, Wenhan Guo, Song Gao, Chong Qu, Hassina Tabassum, Hao Zhang, Bingjun Zhu, Ruqiang Zou, Yang Shao-Horn www.elsevier.com/locate/nanoenergy

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S2211-2855(18)30998-4 https://doi.org/10.1016/j.nanoen.2018.12.085 NANOEN3333

To appear in: Nano Energy Received date: 18 November 2018 Revised date: 17 December 2018 Accepted date: 28 December 2018 Cite this article as: Tianjie Qiu, Zibin Liang, Wenhan Guo, Song Gao, Chong Qu, Hassina Tabassum, Hao Zhang, Bingjun Zhu, Ruqiang Zou and Yang ShaoHorn, Highly Exposed Ruthenium-based Electrocatalysts from Bimetallic Metalorganic Frameworks for Overall Water Splitting, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.12.085 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly Exposed Ruthenium-based Electrocatalysts from Bimetallic Metal-organic Frameworks for Overall Water Splitting Tianjie Qiu,a,1 Zibin Liang,a,1 Wenhan Guo,a Song Gao,a Chong Qu,a Hassina Tabassum,a Hao Zhang,a Bingjun Zhu,a Ruqiang Zoua,* and Yang Shao-Hornb

a

Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, Department of Materials

Science and Engineering, College of Engineering, Peking University, Beijing 100871, China b

Department of Materials Science and Engineering, Department of Mechanical Engineering, Massachusetts

Institute of Technology, Cambridge, MA 02139, USA. *Corresponding author E-mail address: [email protected] (R. Zou) 1

These authors contributed equally to this work.

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ABSTRACT

The development of highly exposed active sites and highly active inexpensive electrocatalysts is important and challengeable for electrocatalytic hydrogen evolution reaction (HER). Herein, we report a bimetallic metalorganic frameworks based strategy to fabricate Ru-based electrocatalysts with high exposure of the Ru active sites (Ru-HPC, ruthenium-decorated hierarchical porous carbon) for high efficient hydrogen evolution. Remarkably, Ru-HPC achieves a current density of 25 mA cmgeo-2 at an overpotential of 22.7 mV, and shows an ultrahigh turnover frequency of 1.79 H2 s-1, which is almost twice higher than that of commercial Pt/C. The electrochemical HER performance of Ru-HPC surpasses most of the electrocatalysts reported so far in alkaline solutions. Besides, a two-electrode water splitting device is constructed with Ru-HPC and porous RuO2 (obtained by oxidizing Ru-HPC) as electrode materials, which achieves a current density of 10 mA cm-2 at only 1.53 V. This work provides a facile strategy to fabricate high-performance metal-carbon hybrid electrocatalysts with abundant exposed active sites from bimetallic MOFs, which can hopefully be applied to many other metals-carbon hybrids for various electrochemical applications in the foreseeable future.

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Highlights Bimetallic MOF enables low amount of Ru usage and hierarchically porous structure. Highly exposed active sites contribute to ultrahigh HER activity in alkaline solution. Ru-based catalyst exhibits an extremely low HER overpotential of 22.7 mV at 25 mA/cm2.

Graphical Abstract Ru-based electrocatalysts from bimetallic metal-organic frameworks with extremely high electrochemical active surface areas showed excellent performance toward hydrogen evolution reaction and overall water splitting exceeding commercial benchmark catalysts, which shows great potential for large-scale industrial applications.

Keywords: Ruthenium; Metal-organic frameworks; Electrocatalysis; Hydrogen evolution reaction; Water splitting

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1. Introduction The search for high-activity electrocatalysts to split water in a large scale, which can produce high-purity hydrogen as clean and sustainable substitutes to traditional non-renewable fossil fuels, has been a tough but imperative research topic [1-4]. Generally, to efficiently catalyze water splitting, there are two strategies to overcome the sluggish kinetics and high overpotentials of the two electrocatalytic water splitting processes, namely, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [5, 6]: improving the intrinsic activity of the active sites and increasing the number of the exposed active sites of electrocatalysts [710]. It has been demonstrated that the highly exposed catalytic active sites of materials could be realized by nanostructuring, dispersing nanoparticles on supports, shape-engineering, etc. [7, 11-17]. Very recently, as cheaper alternatives to Pt, Ru-based materials have been investigated as electrocatalysts for HER in virtue of the excellent anti-corrosion properties and theoretically high intrinsic activity of Ru with a similar bond strength with hydrogen of ~ 65 kcal mol-1 [18-23]. To maximize the activity of Ru-based material for HER, the methods to disperse Ru nanoparticles on C2N [19], N-doped carbon [24], CoP [25], and hierarchically ordered carbon [26] with small size and to alloy ruthenium with other transition metals [22, 27-29] to regulate the charge distribution of the alloy surface were developed. However, the active sites are always buried below the surface, which results in low exposure of active sites and low mass activity. The design and controllable synthesis of low content Ru-based catalysts supported on hierarchically porous carbon with large exposure of active sites for HER electrocatalysis have been rarely reported. Metal-organic frameworks (MOFs), constructed with metal nodes and organic ligands to form a highly ordered crystal structure, have been widely used as templates to synthesize nanostructured metal/carbon materials with high activity for electrocatalysis [30-33]. Utilizing MOFs as self-sacrificial templates to fabricate metal/carbon materials via facile pyrolysis can offer many advantages for electrocatalysis: (1) The MOFsderived nanostructured metal/carbon materials can to some extent inherit the high surface area and porosity of the MOF precursors, which are beneficial for mass transfer and ion diffusion during electrochemical processes. (2) By delicate synthesis, the atomically distributed metal nodes in MOFs can be converted into ultrafine and

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uniformly distributed metal particles in the corresponding MOFs-derived carbon architectures. (3) The carbon component generated from the decomposition of organic ligands of MOFs can not only serve as skeletons for metal nanoparticles anchoring but also promote electron transfer within the whole structure [34-36]. Considering that the high surface area/porosity and the uniform distribution of the ultrafine metal nanoparticles can lead to high exposure of the active sites, using Ru-based MOFs as templates to design and synthesize Rubased materials can be an effective strategy to improve their electrocatalytic activity by increasing the number of the exposed Ru active sites. Ru-MOFs constructed with 1,3,5-benzenetricarboxylic acid (H3BTC) ligands and paddle wheel Ru metal nodes with mixed valence (Ru-MOF) had same crystalline structure as Cu3(BTC)2 (Cu-MOF), which enabled the fabrication of bimetallic MOFs (CuRu-MOF) with Ru and Cu nodes atomically dispersing throughout the whole MOF crystals [37-43]. Bearing in mind that the metal content is too high in mono-metallic Ru-MOF that will result in large metal particle size and low carbon porosity after pyrolysis, we developed a bimetallic CuRuMOF-based strategy to synthesize highly exposed ultrafine Ru nanoparticles anchored on hierarchically porous carbon (Ru-HPC) for efficient and durable HER electrocatalysis. The utilization of CuRu-MOF as precursors played unique and important roles in this MOF-template strategy: (1) Ru and Cu sites were uniformly distributed in the metal nodes and the Ru sites were insulated by the Cu sites and organic ligands of CuRu-MOF, which could prevent the aggregation of Ru during pyrolysis and lead to the formation of uniformly distributed ultrafine Ru nanoparticles in Ru-HPC. (2) The high surface area and rich porosity of CuRu-MOF may be partially inherited by Ru-HPC, and the removal of the Cu particles can generate a large amount of meso/macropores in Ru-HPC, which led to a high exposure of the ultrafine Ru nanoparticles and facile mass transport. (3) The hierarchically porous carbon derived from the organic ligands of CuRu-MOF can serve as a conductive substrate to anchor the Ru nanoparticles, promoting the electron/mass transfer and suppressing the aggregation of Ru. As a consequence of the unique structure, Ru-HPC showed an extremely high electrochemical surface area and high active sites density. When employed as an electrocatalyst for HER in alkaline solution, Ru-HPC with Ru content of only 5.55 wt% showed extremely high activity and durability exceeding those of the commercial 20% Pt/C. The mass current density of the Ru-HPC electrocatalyst was as 5

high as 7.8 A mgRu-1 at overpotential of 50 mV, nearly 16 times higher than that of the commercial Pt/C (0.46 A mgPt-1), exceeding most of the reported HER catalysts in alkaline solution. Moreover, Ru-HPC can be converted into the porous RuO2 network (P-RuO2) composed of ultrafine RuO2 nanoparticles with abundant exposed active sites after simple pyrolysis in air, which showed superior electrocatalytic performance for OER over the commercial RuO2 counterpart. Remarkably, the two-electrode water splitting device constructed with Ru-HPC (for HER) and P-RuO2 (for OER) as electrode materials showed ultrahigh electrocatalytic activity with a cell voltage of 1.53 V at 10 mA cm-2 along with excellent long-term durability, which outperformed the device constructed with commercial 20% Pt/C and RuO2 and ranked at the top of the reported water splitting electrolysers.

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Fig. 1. Schematic illustration of the synthetic strategy of Ru-HPC.

2. Experimental section 2.1. Materials Ruthenium chloride hydrate (RuCl3∙xH2O) was purchased from Ourchem. Copper nitrate trihydrate (Cu(NO3)2∙3H2O) and LiCl·H2O were purchased from Xilong Scientific. Trimesic acid (H3BTC) was purchased from Beijing Ouhe Technology. Iron chloride hexahydrate (FeCl3∙6H2O) was purchased from 7

Sinopharm Chemical Reagent. Glacial acetic acid and acetic anhydride were purchased from Beijing Tong Guang Fine Chemical Company. All chemicals were used as received without further purification. 2.2. Synthesis of CuRu-MOF CuRu-MOF was synthesized by previously reported methods [44, 45]. In a typical preparation of the CuRuMOF, 0.94 g of CuNO3·3H2O and 0.258 g of hydrous RuCl3 were dissolved in 25 ml of ethanol, which was then mixed with 25 ml of DMF containing 0.5 g of 1,3,5-benzenetricarboxylic acid. Then the mixed solution was heated at 60 ⁰ C for 24 h under continuous stirring. The product, CuRu-MOF, was centrifuged and washed with DMF and anhydrous ethanol for several times and dried at 160 ⁰ C for 8 h under dynamic vacuum. The changing amount Ru-MOFs were synthesized by the same method and Ru amounts were changing with 0.129 g and 0.516 g. 2.3. Synthesis of Ru-MOF [Ru2(OOCCH3)4Cl] was first synthesized. Hydrous RuCl3 (0.5 g) and LiCl·H2O (0.5 g) were dissolved in glacial acetic acid (17.5 mL) and acetic anhydride (3.5 mL). Subsequently, the mixture was refluxed for 24 hours at 140 ⁰ C. The resulting powder was filtered and washed with acetone several times and dried under dynamic vacuum. To synthesize the Ru-MOF, 0.17 g of the resulting [Ru2(OOCCH3)4Cl] and 0.101 g of 1,3,5benzenetricarboxylic acid were mixed with 4 ml of deionized water and 0.7 ml of acetic acid in a 20 ml Teflon vessel. The vessel was then sealed in an autoclave and heated at 160 ⁰ C for 3 days. The final product was washed with water and ethanol for several times and dried at 160 ⁰ C for 8 h under dynamic vacuum. 2.4. Synthesis of Cu-MOF In a typical preparation of the Cu-MOF, 0.94 g of CuNO3·3H2O was dissolved in 25 ml of ethanol, which was then mixed with 25 ml of DMF containing 0.5 g of 1,3,5-benzenetricarboxylic acid. Then the mixed solution was heated at 60 ⁰ C for 24 h under continuous stirring. The product, Cu-MOF, was centrifuged and washed with DMF and anhydrous ethanol for several times and dried at 160 ⁰ C for 8 h under dynamic vacuum. 2.5. Synthesis of Ru-HPC The CuRu-MOF was annealed in a tube furnace at 500 ⁰ C for 1 hour with a heating rate of 5 ⁰ C min-1 and subsequently at 700 ⁰ C for 2 hours under Ar atmosphere with a heating rate of 2 ⁰ C min-1. The resulting 8

powder (CuRu-C) was then immersed in 0.1 M FeCl3 solution for 2 days to remove the Cu. After the immersion, the product was washed with a large amount of deionized water. The final product was denoted as Ru-HPC. 2.6. Synthesis of Ru-C The Ru-MOF was annealed in a tube furnace at 500 ⁰ C for 1 hour with a heating rate of 5 ⁰ C min-1 and subsequently at 700 ⁰ C for 2 hours under Ar atmosphere with a heating rate of 2 ⁰ C min-1. The final product was denoted as Ru-C. 2.7. Synthesis of P-RuO2 The as-synthesized Ru-HPC was pyrolyzed at 400 ⁰ C for 4 hours in air with a heating rate of 10 ⁰ C min-1 and P-RuO2 was obtained. 2.8. Synthesis of C-RuO2 The as-synthesized Ru-C was pyrolyzed at 400 ⁰ C for 4 hours in air with a heating rate of 10 ⁰ C min-1 and C-RuO2 was obtained. 2.9. Characterization The crystallographic structures of the materials were obtained using a Rigaku SmartLab X-ray diffractometer operating at 45 kV and 200 mA, using Cu Kα radiation (λ=1.5406 Å) at room temperature. The microstructure and morphology were examined by using a field emission scanning electron microscope (SEM, Hitachi S-4800). Transmission electron microscope (TEM) images were taken on FEI Tecnai F30 microscopes. Element content analysis was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo IRIS Intrepid). The surface characterization of elemental electronic states was measured by X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd Axis Ultra X-ray Photoelectron spectrometer) with a monochromatic aluminum Kα X-ray source. The nitrogen sorption isotherms of the materials were measured within the pressure range 0-1 atm at 77K using a Quantachrom Autosorb-IQ gas adsorption analyzer. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) model and pore size distribution was calculated using the non-local density functional theory (NLDFT) model. Thermogravimetric analysis (TGA) was carried out in the synthetic air. Gas chromatography (GC, Agilent 7890B) was carried out for the hydrogen and oxygen detection. 2.10. Electrochemical measurements 9

All the electrochemical measurements were carried out in a standard three-electrode system at an electrochemical station (CHI 760E). Ag/AgCl (saturated KCl solution) electrode and glassy carbon rod electrode were used as the reference electrode and counter electrode, respectively. To prepare the catalyst ink, 2 mg sample and 20 µl 5 wt% Nafion solution were dispersed in 980 µl ethanol by sonication for at least 1 h to form a homogeneous slurry. Then, the mixture was dripped onto the surface of a glassy carbon electrode with a diameter of 5 mm. The final loading for all electrocatalysts was 0.2 mg cm-2. For HER and OER, linear sweep voltammetry with a scan rate of 5 mV s-1 was conducted in 1 M KOH or 0.5 M H2SO4 for all samples. Water electrolysis measurement was carried out in a standard two-electrode system by using Ru-HPC and P-RuO2 supported on carbon fiber paper (CFP) with a geometric area of 0.8 cm-2 as cathode and anode respectively with a mass loading of 0.4 mg cm-2. For comparison, Ru-C and C-RuO2, commercial 20% Pt/C and commercial RuO2 were also tested for water splitting under the same conditions. All polarization curves were 95% iR corrected. 3. Results and discussion The synthetic process of Ru-HPC was schematically illustrated in Fig. 1. First, the bimetallic CuRu-MOF was synthesized using Cu2+ and Ru3+ as metal sources and 1, 3, 5-benzenetricarboxylic acid (H3BTC) as an organic ligand. The monometallic Cu-MOF and Ru-MOF were also synthesized for comparison. X-ray diffraction (XRD) patterns showed that all of the MOFs had a similar crystal structure, indicating that both Ru and Cu nodes were simultaneously introduced in CuRu-MOF without changing the framework structure (Fig. 2a) [45]. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that CuRu-MOF were octahedral nanocrystals (Fig. S1). Thermal treatment of CuRu-MOF under Ar atmosphere was conducted and led to a pyrolysis production of Cu and Ru-doped carbon (CuRu-C) (Fig. 1), in which large Cu particles with size up to hundreds of nanometers and small Ru nanoparticles with size of several nanometers were separately embedded in carbon substrates derived from the decomposition of the organic ligands (Fig. S2). For comparison, Ru-doped carbon (Ru-C) was also prepared by pyrolysis of Ru-MOF. Afterwards, the inactive Cu particles of CuRu-C were removed by FeCl3 solution to obtain Ru-embedded hierarchically porous carbon network (Ru-HPC). The XRD pattern of Ru-HPC showed characteristic peaks for 10

hexagonal Ru, while the characteristic peaks for Cu disappeared, indicating the successful removal of Cu by FeCl3 treatment (Fig. 2c). The Ru-HPC possessed a large Brunauer-Emmett-Teller (BET) surface area of 461.77 m2 g-1 revealed by the N2 sorption measurements, which was 45 times higher than that of CuRu-C (10.45 m2 g-1, Fig. 2b) and the pore size distribution of Ru-HPC indicated the generation of abundant micro/mesopores. The results of the BET surface area and pore distribution confirmed that the removal of Cu can lead to the formation of hierarchical structures of Ru-HPC. High-resolution X-ray photoelectron spectroscopy (XPS) spectra for Ru 3p showed that Ru-HPC has peaks at 461.7 and 483.7 eV corresponding to Ru 3p3/2 and 3p1/2 of Ru (0), respectively.[46-48] The Ru content in Ru-HPC was measured to be 5.55 wt% by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

Fig. 2. Characterizations of Ru-based MOFs and their derived materials. a) XRD patterns of CuRu-MOF, Ru-MOF, Cu-MOF and the simulated one with Fm

m space group from the crystallographic information file of Cu-MOF. b) The N2

adsorption/desorption isotherms of Ru-HPC and CuRu-C. Inset: corresponding pore size distributions for Ru-HPC and CuRu-C calculated using NLDFT method, respectively. c) XRD patterns of Ru-HPC and Ru-C. d) High resolution XPS spectra for Ru 3p of Ru-HPC.

SEM images showed that Ru-HPC possessed hierarchically porous structure derived from the inherent porosity of MOF precursor (Fig. 3a). Moreover, a large amount of meso/macropores were observed in the porous Ru-HPC which were generated after the removal of the Cu particles (Fig. 3b). As illustrated in TEM

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images, ultrafine Ru nanoparticles with an average diameter of 2.87 nm were uniformly distributed on the hierarchically porous carbon network (Fig. 3c and Fig. S3). Notably, the ultrafine Ru nanoparticles were highly exposed on the surface of the carbon substrate (Fig. 3d). The 0.234 and 0.214 nm spacings observed in the high-resolution TEM (HRTEM) images of Ru-HPC corresponded to the Ru (100) and (002), respectively, which was consistent with the XRD analysis (Fig. 3e). High-angle annular dark-field scanning TEM (HAADFSTEM) image and the corresponding elemental mapping analysis of Ru-HPC further confirmed the existence of Ru nanoparticles on carbon substrate (Fig. 3f).

Fig. 3. Morphology and structure characterizations of Ru-HPC. a) and b) SEM images of the hierarchically porous structures of Ru-HPC. Inset: the enlarged region of the dashed rectangular area, which showed abundant mesopores. c) TEM image of the Ru-HPC. Inset showed the ultrafine Ru nanoparticles. d) and e) HRTEM images of Ru-HPC. f) HAADF-STEM image and the corresponding elemental mapping images of Ru-HPC.

The electrochemical HER performance of Ru-HPC was measured in 1 M KOH using a standard threeelectrode configuration. As shown in linear sweep voltammetry (LSV) curves (Fig. 4a), Ru-HPC showed ultralow overpotentials of 22.7 and 44.6 mV at current densities of 25 and 50 mA cm geo-2 (normalized by

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geometrical surface area), respectively, which were nearly half of those of commercial 20% Pt/C (45.4 and 84.4 mV at 25 and 50 mA cmgeo-2 respectively) outperforming most of the recently reported Ru-based and other electrocatalysts for HER (Table S1). To get more reliable results, HER performances of Ru-HPC obtained from four different syntheses were measured, which exhibited good repeatability (Fig. S4) and Ru-HPC also showed good applicability with no big differences between different alkaline solutions for HER (Fig. S5). The corresponding Tafel slope of Ru-HPC was 33.9 mV decade-1, even lower than commercial 20% Pt/C (41 mV decade-1), indicating a Volmer-Tafel pathway with recombination of chemisorbed hydrogen atoms as the ratelimiting step (Fig. 4b). The small Tafel slope of Ru-HPC suggested that the HER rate increased dramatically as the overpotential increased, which is desirable for practical application. In comparison, the overpotential at 25 mA cmgeo-2 of CuRu-C was 161.8 mV higher than Ru-HPC with a larger Tafel slope of 154.5 mV decade-1, suggesting that the removal of Cu particles by FeCl3 was essential to enhance the HER performance, which led to generation of a large amount of meso/macropores and the exposure of Ru nanoparticles. The high exposure of Ru nanoparticles increased the utilization of the Ru active sites, which endowed Ru-HPC with high mass activity based on low metal content with a mass current density of 7.8 A mgRu-1 at an overpotential of 50 mV, 16 times higher than that of commercial 20% Pt/C (0.46 A mgPt-1, Fig. 4c). Furthermore, different original Ru amounts were studied for alkaline HER performance during the synthesis of CuRu-MOF. As shown in Fig. S6, variation of original ruthenium amount didn’t change the frameworks of CuRu-MOFs and all XRD patterns of CuRu-MOFs were well matched with simulated Cu-MOF. After pyrolysis and removing Cu, the HER performance was improved and reached maximum with the original Ru amount of 0.258 g as the original Ru amount increasing. However, the performance decreased when original Ru amount was higher than 0.258 g, which probably resulted from partial aggregation of Ru particles when Ru amount increased. Exchange current density is another important kinetic parameter that reflects the intrinsic electrocatalytic activity. The exchange current density of Ru-HPC obtained from extrapolating Tafel plots was 6.59 mA cmgeo-2, 1.8 times higher than that of commercial Pt/C (2.37 mA cmgeo-2) and the mass exchange current density was 2.34 A mgRu-1, 4.6 times higher than that of Pt/C (0.42 A mgPt-1), indicating superior electrocatalytic HER activity of Ru-HPC over Pt/C in alkaline solution (Fig. S7). 13

Fig. 4. Electrocatalytic HER tests in 1 M KOH solution and 0.5 M H 2SO4 a) Polarization curves and b) Tafel plots of Pt foil, commercial 20% Pt/C catalyst, CuRu-C and Ru-HPC in 1 M KOH solution. c) Mass activity of Ru-HPC and Pt/C in 1 M KOH. d) Polarization curves of Ru-HPC before and after 10 h current-time dependent test in 1 M KOH solution and the inset showed the amperometric current-time curve. e) Polarization curves and f) Tafel plots of commercial 20% Pt/C catalyst, CuRu-C and Ru-HPC in 0.5 M H2SO4 solution. All the tests were 95% iR corrected.

The electrocatalytic HER durability of Ru-HPC was evaluated by cyclic voltammetry (CV) cycling test. The polarization curve of Ru-HPC showed negligible change after 3000 CV cycles, indicating high HER durability of Ru-HPC (Fig. S8). The high durability of Ru-HPC was further confirmed by amperometric current-time curve measurement (Fig. 4d). The current density remained stable after long-term operation for 10 h, and the polarization curve tested after amperometric current-time curve measurement showed negligible degradation, demonstrating long-term HER stability of Ru-HPC. In contrast, the commercial Pt/C suffered from severe current density loss during the stability test (Fig. S9). The excellent electrocatalytic HER performance of Ru-HPC proved the presented bimetallic MOF-templated strategy to be an efficient method to synthesize highly active and durable electrocatalysts. The uniform

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distribution and adjustable concentration of Ru sites in CuRu-MOF led to the formation of ultrafine Ru nanoparticles in Ru-HPC, and hierarchically porous structure with abundant micro/meso/macropores originated from the porous nature of CuRu-MOF precursor and the removal of Cu particles can greatly expose the ultrafine Ru nanoparticles, which resulted in a large amount of exposed active sites for efficient electrocatalysis. To identify the active centers of the Ru-HPC, thiocyanate ions (SCN-) [19, 49, 50], which are known to poison active metal centers due to their strong binding ability with metal atoms, were added into the electrolyte. The current decreased dramatically with the addition of SCN- ions due to the blocking of the Ru, demonstrating the Ru components are active sites in Ru-HPC (Fig. S10). Electrochemical active surface area (ECSA) and active site density were measured by copper underpotential deposition (UPD) method to further confirm the highly exposed nature of the Ru components in Ru-HPC (Fig. S11). It was measured that Ru-HPC had an active site density of 0.390×10-3 mol gmetal-1 and an ECSA of 385.57 m2 gmetal-1, surpassing those of commercial Pt/C (active site density of 0.332×10-3 mol gmetal-1 and ECSA of 76.30 m2 gmetal-1), confirming the existence of abundant active sites on the highly exposed Ru nanoparticles in Ru-HPC. Turnover frequency (TOF) values that can reveal the intrinsic electrocatalytic activity were calculated from the current density of the polarization curve. The TOF values of Ru-HPC at 25 mV and 100 mV (vs. RHE) were 1.79 H2 s-1 and 9.20 H2 s-1, much higher than that of Pt/C (0.67 H2 s-1 at 25 mV and 5.01 H2 s-1 at 100 mV). For comparison, without Cu components in the Ru-MOF precursor to synthesize Ru-C, Ru-C showed larger Ru particles and higher Ru amount than Ru-HPC with an average particle size of 4.26 nm along with particle aggregation and lower BET surface area of 144.33 m2 g-1 than Ru-HPC (Fig. S12). As a consequence, Ru-C had lower exposure of the Ru active sites evidenced by the much lower active site density (0.137×10-3 mol gmetal-1) and ECSA (9.03 m2 gmetal-1) than those of Ru-HPC, resulting in lower HER activity than Ru-HPC (Fig. S13 and Table S2). To further understand the superiority of Ru-based catalysts for alkaline HER, cyclic voltammetry (CV) cycling test and amperometric current-time test for Ru-C were also conducted. Ru-C showed negligible performance decrease, which indicated Ru-C had high durability similar to Ru-HPC (Fig. S14). Both the stability tests of Ru-HPC and Ru-C indicated ruthenium can preserve better durability than commercial Pt/C. Synergistic effect between conducted carbon substrate and crystalline Ru plays important roles for the high 15

durability, which prevents the agglomeration and dissolution of Ru, and hierarchically porous structure of RuHPC further contributes to distribute nanoparticles more dispersed avoiding aggregation. In this method to fabricate Ru-HPC catalyst, Cu was removed by Fe3+ etching, in which trace Cu and Fe originated from the adsorbed metal ions cannot be avoided. To understand the influence of trace Cu and Fe on the electrochemical HER performance of Ru-HPC, Ru-C that didn’t contain Cu and Fe was immersed in 0.1 M FeCl3 and CuCl2 solution to introduce Fe and Cu (Fig. S15). No obvious difference was observed between the HER performances of Ru-C before and after the introduction of Fe and Cu, which indicated the trace Cu and Fe exerted negligible influence on HER performance. The ultrahigh HER performance, durable stability and economic benefit of low Ru amount endow Ru-based catalysts with the potentials to serve as good substitutes to commercial Pt/C for alkaline HER electrocatalysis. In addition, Ru-HPC also performed remarkable HER activity in an acidic media of 0.5 M H2SO4 solution. Ru-HPC showed an overpotential of 61.6 mV at 10 mA cmgeo-2, only 22.2 mV higher than that of Pt/C (39.4 mV), and was 261.1 mV lower than that of CuRu-C (Fig. 4e) and 113.4 mV lower than that of Ru-C (Fig. S16 and Table S2). Moreover, Ru-HPC displayed a Tafel slope of 66.8 mV dec-1, 88.3 mV dec-1 lower than that of CuRu-C (Fig. 4f). Also, the exchange current density of Ru-HPC (1.12 mA cmgeo-2) was much higher than those of CuRu-C and commercial Pt/C (Fig. S17). The TOF value of Ru-HPC was 0.18 H2 s-1 at 25 mV, close to commercial Pt/C (0.19 H2 s-1). These results suggested that Ru-HPC also had high HER activity in acidic solution.

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Fig. 5. Characterizations and OER tests of P-RuO2 derived from Ru-HPC. a) and b) SEM images of P-RuO2. c) TEM image of PRuO2. Inset: HRTEM image of P-RuO2. d) XRD pattern of the P-RuO2. e) Polarization curves and f) Tafel plots of P-RuO2, C-RuO2 and commercial RuO2 for OER. All the tests were 95% iR corrected.

Taking advantages of uniform distribution of the ultrafine Ru nanoparticles on hierarchically porous carbon substrate, Ru-HPC can be converted into porous RuO2 networks with abundant exposed active sites through pyrolysis at 400 °C in air (P-RuO2), which can be used as an efficient electrocatalyst for OER. For comparison, Ru-C was also converted into RuO2 (denoted as C-RuO2) using the same method. As revealed by SEM and TEM images, P-RuO2 had hierarchically porous network structure constructed with ultrafine RuO2 nanoparticles (Fig. 5a, 5b, and 5c). The spacing of lattice distances shown in Fig. 5c were 0.255 nm and 0.318 nm, which could be attribute to the (110) and (101) planes of RuO2 in coincidence with the well match of XRD pattern of P-RuO2 with RuO2 crystalline structure (Fig. 5d). The mean size of RuO2 nanoparticles was about 5.64 nm (Fig. S18). And the N2 sorption measurement also confirmed the existence of abundant meso/macropores in P-RuO2 derived from the inherent porosity of Ru-HPC (Fig. S19). The OER performance of P-RuO2 was investigated in 1 M KOH solution. LSV curve of P-RuO2 showed an overpotential of only 310

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mV at 10 mA cmgeo-2, which was 30 mV lower than that of C-RuO2 and 60 mV lower than that of commercial RuO2 (Fig. 5e and Table S3). Also, the Tafel slope of P-RuO2 (60.7 mV dec-1) was 15.8 mV dec-1 lower than that of C-RuO2 (76.5 mV dec-1) and 13.0 mV dec-1 lower than that of commercial RuO2 (73.7 mV dec-1), indicating excellent electrocatalytic OER performance of P-RuO2 (Fig. 5f). Besides, the polarization curve of PRuO2 displayed negligible change after 2000 cycles CV test with an excellent stability (Fig. S20). The high OER activity of P-RuO2 can be attributed to the hierarchically porous structure that can lead to high exposure of the active sites on the surface of ultrafine RuO2 nanoparticles. P-RuO2 had higher BET surface area of 77.2 m2 g-1 than that of C-RuO2 (41.9 m2 g-1), as C-RuO2 was composed of aggregated RuO2 particles with larger particle size than that of P-RuO2 (Fig. S21). Moreover, the electrochemical double-layer capacitance (Cdl) was measured to estimate the ECSA (Fig. S22). P-RuO2 possessed a Cdl value of 13.26 mF cm-2, which was 2.2 times larger than that of C-RuO2 (4.09 mF cm-2), indicating a much large ECSA and higher exposure of the RuO2 active sites of P-RuO2 than C-RuO2. These results proved the important roles of the Cu component for exposure of more active sites, as the Cu sites can lead to smaller Ru/RuO2 particle size and the removal of Cu particles can generate abundant meso/macropores. In this work, Ru-HPC and P-RuO2 showed ultrahigh activity and excellent durability superior to Ru-C and CRuO2, and even commercial Pt and RuO2 catalysts. Benefitting from bimetallic CuRu-MOF template strategy, the higher activity and more economic benefit of Ru-HPC than Ru-C originated from many advantages: (1) RuHPC possessed well-crystalline Ru nanoparticles with small particle size, which contributed to high performance and good durability. (2) The hierarchically porous structure of Ru-HPC displayed high surface area and rich porosity, which can prevent particle aggregation, facilitate mass transport, and expose more active sites. (3) Conductive carbon substrate benefited charge transfer, which accelerate the reaction kinetics. (4) Low Ru amount usage endowed Ru-HPC with more economic benefit as compared to Ru-C. Inheriting the advantageous properties from Ru-HPC, P-RuO2 also showed a much higher OER performance than that of CRuO2, even commercial RuO2 with suitable porous structure contributing to fast mass transport and smaller particle size compared leading to expose more active sites. Based on good performance of Ru-HPC and P-RuO2 for HER and OER, respectively, an overall water splitting device with two-electrode configuration was 18

fabricated (denoted as Ru-HPC║P-RuO2, Fig. 6a and Fig. S23). For comparison, a water splitting cell constructed with Ru-C║C-RuO2 and commercial Pt/C║RuO2 was also made. As shown in Fig. 6b, the RuHPC║P-RuO2 cell achieved a current density of 10 mA cm-2 at a cell voltage of 1.53 V, much lower than those of Ru-C║C-RuO2 (1.60 V) and commercial Pt/C║RuO2 (1.62 V), and was in the forefront of the recently reported water splitting devices (Fig. 6c and Table S4). At the cell voltage of 1.7 V, Ru-HPC║P-RuO2 exhibited much higher current density and generated H2 and O2 much more rapidly than the commercial Pt/C║RuO2, revealing the outstanding electrocatalytic activity of Ru-HPC and P-RuO2 for overall water splitting (Fig. 6d and Movie S1). The generated H2 and O2 by Ru-HPC║P-RuO2 cell were further quantified by gas chromatography (GC), which showed that Ru-HPC║P-RuO2 produced much more H2 and O2 with molar ratio of nearly 2:1 than that of the commercial Pt/C║RuO2 (Fig. 6e and Fig. S24). The Ru-HPC║P-RuO2 cell also showed desirable long-term durability. A steady current density was observed during amperometric current-time measurement at cell voltage of 1.65 V for over 24 h, and after the amperometric current-time measurement the LSV curve showed negligible change, exhibiting the scalability of the Ru-HPC║P-RuO2 cell for industrial applications (Fig. 6f). The excellent water splitting performance endowed Ru-based catalysts the potential to serve as a good substitute to commercial Pt/C for large-scale water splitting in the foreseeable future

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Fig. 6. Overall water splitting performance of Ru-HPC║P-RuO2 device. a) Schematic diagram of an electrolyzer for water splitting. b) Polarization curves of Ru-HPC║P-RuO2, Ru-C║C-RuO2 and Commercial Pt/C║RuO2 for overall water splitting. c) Cell voltage of Ru-HPC║P-RuO2, Ru-C║C-RuO2 and Commercial Pt/C║RuO2 at 10 mA cm-2 for overall water splitting, with other reported water splitting electrocatalysts in 1 M KOH. d) Amperometric current-time curves of water splitting at a cell voltage of 1.7 V for Ru-HPC║P-RuO2, Ru-C║C-RuO2 and commercial Pt/C║RuO2. e) The produced hydrogen and oxygen volume of Ru-HPC║PRuO2 and commercial Pt/C║RuO2 at a cell voltage of 1.7 V. f) Polarization curves of Ru-HPC║P-RuO2 before and after 1.7 V and 1.65 V current-time test respectively. Inset: amperometric current-time curves for Ru-HPC║P-RuO2 at a cell voltage of 1.65 V for 24 h. All the tests were 95% iR corrected.

4. Conclusions In summary, we developed a bimetallic MOF-templated strategy to synthesize ultrafine Ru nanoparticles with high exposure anchored on hierarchically porous carbon network for efficient HER electrocatalysis. Utilization of the bimetallic CuRu-MOF as template resulted in small particle size of the Ru nanoparticles and the removal of Cu particles generated abundant meso/macropores, which led to a large amount of exposed Ru active sites for HER electrocatalysis. Ru-HPC showed extremely high HER activity outperforming the commercial Pt/C catalyst and even most of the reported electrocatalysts in alkaline solution. Ru-HPC had high active site density 20

(0.390×10-3 mol gmetal-1) and ECSA (385.57 m2 gmetal-1), which confirmed the high exposure of the Ru active sites. The ultrafine Ru nanoparticles can be further converted into ultrafine RuO2 nanoparticles with porous network structure for efficient OER electrocatalysis. Using these two Ru-based electrocatalysts with abundant active sites, a two-electrode device was constructed for efficient and durable water splitting electrocatalysis. This work provided a facile method to utilize bimetallic MOF as a precursor for the synthesis of metal-carbon hybrid electrocatalysts with abundant exposed active sites. This strategy can be hopefully applied to many other metal-based materials synthesis for various electrocatalysis applications. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51772008), the National Key Research and Development Program of China (2017YFA0206701), the National Program for Support of Top-notch Young Professionals, and the Changjiang Scholar Program.

Appendices Appendix A. Supporting Information Supplementary data associated with this article. Appendix Movie S1. Video of the two-electrode water splitting device.

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Tianjie Qiu is currently a master candidate under the supervision of Professor Ruqiang Zou at the College of Engineering, Peking University. He received his bachelor degree from College of Chemistry, Nankai University. His research interests focus on the synthesis metal–organic frameworks derived porous nanomaterials for energy conversion applications.

Zibin liang is currently a Ph.D. candidate under the supervision of Professor Ruqiang Zou at the College of Engineering, Peking University. His research interest is the synthesis of functional nanomaterials based on metal–organic frameworks for energy storage and conversion applications.

Wenhan Guo obtained his BS degree in Department of Material Science and Engineering, College of Engineering, Peking University in 2014. Currently he’s a PhD candidate under the supervision of Prof. Ruqiang Zou in the same department. His research interests include the synthesis and design of metal organic frameworks and their applications in energy storage and conversion.

Song Gao is an assistant research fellow of the College of Engineering, Peking University, China. She received her Ph.D. from Harbin Institute of Technology. She research interest is the fabrication of metal-organic frameworks derived nanomaterials and energy storage and conversion.

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Chong Qu received his Ph.D. degree from the College of Engineering, Peking University, China. He received his bachelor degree from School of Chemistry and Chemical Engineering at Nanjing University in 2013. He joined Prof. Meilin Liu’s group as a GTPKU joint program student at School of Materials Science and Engineering, Georgia Institute of Technology in 2014. His research interests include electrochemical materials based on crystalline porous frameworks for supercapacitors and Li-ion battery.

Hassina Tabassum is currently a postdoctoral research fellow at the College of Engineering, Peking University, China. She received his Ph.D. degree from the College of Engineering, Peking University. Her research interests include nanomaterials synthesis and their energy storage and conversion applications.

Hao Zhang is now a post-doctoral researcher in College of Engineering, Peking University, China. He received his Ph.D. from Peking University. His research interests focus on perovskites and antiperovskites in energy storage.

Bingjun Zhu is currently a postdoctoral research fellow at the College of Engineering, Peking University, China. He received his Ph.D. degree from the Department of Chemistry, University College London, UK, and his Master of Engineering degree from the School of Engineering and Materials Science, Queen Mary, University of London, UK, and his Bachelor of Engineering degree from the School of Engineering and Materials Science, Beihang University, China. His research interests include metal– organic frameworks, biomass, polymers, and their derivatives for energy and environmental applications.

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Ruqiang Zou is a Professor of Materials Science and Engineering at the College of Engineering, Peking University, China. He received his Ph.D. from Kobe University and the National Institute of Advanced Industrial Science and Technology (AIST), Japan. His research interests focus on the controllable preparation of nanoporous materials for green energy utilization.

Yang Shao-Horn is W. M. Keck Professor of Energy, Professor of Mechanical Engineering and Professor of Materials Science and Engineering at MIT. Professor ShaoHorn teaches and conducts research in the area of surface science, catalysis/electrocatalysis, and design of materials and processes for electrochemical energy storage.

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