Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES
Bio-inspired engineering of cobalt-phosphonate nanosheets for robust hydrogen evolution reaction Zhongsheng Cai, Yi Shi, Song-Song Bao, Yang Shen, Xing-Hua Xia, and Li-Min Zheng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04276 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Bio-Inspired Engineering of Cobalt-Phosphonate Nanosheets for Robust Hydrogen Evolution Reaction Zhong-Sheng Cai,‡ a Yi Shi,‡ b Song-Song Bao,a Yang Shen,a Xing-Hua Xia,* b and Li-Min Zheng* a a
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, P. R. China b
State Key Laboratory of Analytical Chemistry for Life Science and Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China
ABSTRACT: The conversion of water to hydrogen (hydrogen evolution reaction, HER) in regenerative fuel cells promises an environmentally-friendly framework for smoothing out the energy crisis. However, addressing this issue depends on the development of efficient electrocatalyst design to replace precious platinum catalysts. Herein, we construct a cobaltphosphonate coordination polymer nanosheets, bio-inspired by the active center engineering and coordination environments of catalytic sites in natural enzyme, which catalyzes HER at an overpotential of 84 mV in neutral condition. We propose that the introduction of phosphate group tunes the electron density of cobalt ion center and synergistically accelerates HER in bi-functional activation mechanism of water molecules. This research sheds light on the relationship between the electrocatalyst structures and their catalytic activities, a concept that has broad implications for the design of efficient electrocatalysts in energy conversion. KEYWORDS: water splitting, hydrogen evolution reaction, electrocatalysis, cobalt coordination polymer
achieve efficient hydrogen generation from water.8-17 Up to data, many promising non-noble electrocatalysts have been widely researched, including the integration of metals (metal alloys, core-shell metals, and single-atom metal-substrate nanomaterials), transition metal inorganic compounds (metal oxides, chalcogenides, carbides, borides, nitrides and phosphides), together with molecular complexes.8-17 Most of these nanomaterials are restricted to perform high HER activities in strong acidic or alkaline electrolytes. With the aim of avoiding the use of strong acids or bases to reduce their environmental impacts and to increase the biocompatibility, the seeking for HER catalysts with high efficiency and catalytic stability in neutrally aqueous systems is very essential but still remains a great challenge.
INTRODUCTION Hydrogen has emerged as a green and sustainable fuel that promises an environmentally friendly alternative to meet the demand for future global energy.1-3 It has been commercially produced in electrocatalytic system either through electrolyzing chlor-alkali as side product, or directly through electrolyzing water as main product (hydrogen evolution reaction, HER).4-7 Unfortunately, the electrocatalytic HER is yet hampered by three fundamental limitations: considerably low thermal efficiency, short lifetime of electrode material, and extreme lack of costeffective replacement catalysts for noble metals (i.e., Pt, Pd, and Rh). Natural hydrogenase enzymes, with iron and/or nickel metal ion as part of a metallomacrocyclic complex surrounded by protein matrix, catalyze HER at its thermodynamic potential in neutral aqueous solution with high turnover frequency (over 9000 s-1). However, the oxygen-sensitive hydrogenase normally exhibits much lower faradaic efficiency in electrocatalysis and tends to deactivate once faced with extreme conditions of temperature and pressure, which largely suppresses its practical application. In the pursuit of enabling a hydrogen economy, much attention has been paid in artificial systems, which focuses on designing alternative electrocatalysts based on abundant and inexpensive elements on earth to
Coordination polymers (CPs), composed of metal ions as nodes and organic ligands as linkers, are an intriguing class of materials with a broad range of applications.18-20 These materials can be efficiently heterogeneous catalysts for organic reactions21,22 and photochemical water splitting.23,24 However, owing to their low electrical conductivity and less accessible active centers25, few researches has reported the electrocatalysis of CPs for HER. Due to structural characteristics, two-dimensional CPs with large surface-to-volume-ratio surface possess very well-defined crystal structure and specific active sites. Considering that
1 ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
transition metal ions (TMδ+) in center are directly linked with organic ligands, the electron density of TMδ+ can be easily tuned by the neighboring redox-inactive organic group, which dramatically influences its electrocatalytic activity toward HER. Transition metals in Period Four (Mn, Fe, Co, Ni, Cu) are the earth-abundant metal elements and well-recognized as the active sites in enzymes. Thus, it is expected to well design an artificial transition metal ion-based CPs for HER with high performance.
Page 2 of 10
participates in many important living processes such as phosphorylation. When protein is integrated with phosphate group, the charge, hydrophilicity/hydrophobicity, or active site of the protein will be tuned, leading to the changed structure. As a result of phosphorylation, the activation and function of protein will be consequently modulated. Herein, we propose a combined bottom-up and top-down methodology to synthesize HER electrocatalysts by directly self-assembling phosphate-group organic compound ((3-methoxyphenyl)phosphonic acid, with cobalt salt abbreviated as 3-moppH2) (Co(CH3COO)2) in aqueous solution under sonication at room temperature. Thus, synthesis of ultrathin Co(3mopp)(H2O) nanosheets (abbreviated as 1Co-ns) on a large scale can be achieved. 1Co-ns exhibits superior electrocatalytic activity toward HER in neutral media, as confirmed by the overpotential of 84 mV and the Tafel slope of 48 mV/dec. This result by far exceeds the state-of-theart inorganic or organic compounds and is comparable with commercial Pt/C. We propose that the introduction of phosphate group matches the electron density of cobalt ion to HER and synergistically accelerates HER in bifunctional mechanism. This research would shed light on the relationship between the electrocatalyst structure and activity, which promises a strategy to design efficient electrocatalysts for energy conversion.
In order to rationally design electrocatalysts, both thermodynamics and kinetics should be under consideration to ensure a fast proceeding of reactions. Protoncoupled electron transfer (PCET) is very fundamental to energy conversion processes in biological system (i.e. photosynthesis, respiration, and N2 fixation in organisms).26,27 In the PCET process, many approaches existed to modulate the thermodynamics and kinetics of electron and proton transfer independently at the electrode-solution interface.28 Inspired by this concept in living system, we have previously designed active zinc-doped MoS2 for robust H2 evolution. The large enhancement can be attributed to the synergistic effect of electronic effect (energy level matching) and morphological effect (rich active sites) via thermodynamic and kinetic acceleration, respectively.29 Energy level matching reflects the feasibility of electron transfer, which contributes to HER thermodynamically. In the catalysis of enzyme, the electronic density of metal ion center will be modulated by the residue chemical groups or neighboring peptides to the appropriate value matching the energy level of reacting species, thus realizing the feasibility of electron transfer between substrates and products.30 Bearing the mind of “energy level matching” from nature, biomimetics of enzyme encourage researchers to design functional artificial analogues of enzymes.31-34 In heterogeneous electrocatalysis, engineering the energy level of electrocatalyst comparable to the target reaction often helps to enhance the electrocatalytic activity owing to energy level matching. Tuning the electronic density of TMδ+ active sites has emerged as an effective strategy to manipulate their energy level, thus optimizing their electrocatalytic activities. However, energy level matching is only an essential prerequisite for high electrocatalytic activity, which will also be codetermined by many other complicated factors (e.g., coordination environments, number of active sites, and conductivity).29 As well as the consideration of thermodynamics (energy level matching), kinetics also plays an essential role in chemical reaction. Hydrogenase uses pendant bases that are proximate to the metal centers as active sites. The metal centers and the basic ligands act as the hydride-acceptor and proton-acceptor centers, respectively.17,35 This heteroatom structure affords synergistic effect for activation of H2 molecule, which accelerates HER kinetically.36,37
EXPERIMENTAL SECTION Materials and physical measurements. The reagents and solvents were obtained from commercial sources. The morphologies of samples were characterized on a transmission electron microscope (TEM, JEM-2100, Japan) by droping sample solutions on Cu-grids. Scanning electron microscopic (SEM, S-4800, Japan) photos were obtained on Si wafers. Infrared scptra were collected on a Bruker TENSOR 27 IR spectrometer using ATR mode in a range of 600–4000 cm-1. Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8 ADVANCE XRD instrument with Cu-Kα radiation. The XPS spectra were measured on a PHI 5000 Versa Probe instrument. Elemental analysis was carried out on an elemental analyzer (Elementar Vario MICRO). TG analysis was carried out using a METTLER TOLEDO TGA/DSC 1 STARe under a nitrogen flow within 30-500 °C at a heating rate of 10 °C min-1. The sonication bath was performed using a Hechuang Ultrasonic KH-600 KDB instrument at a frequency of 600 W. Synthesis of the phosphonate ligand 3-moppH2. The ligand was synthesized through the traditional Arbuzov reaction with a NiCl2 catalyst. Triethyl phosphite (11 mmol) was added dropwise to a mixture of 10 mmol bromide and 1 mmol NiCl2 in a 100 mL three-neck bottle under the flow of N2 at 150 °C during a period of ca. 0.5 h. The mixture was further heated for 5 h, and then cooled to the room temperature. Next, HCl (50 mL, 1 M) was added to the brown mixture which was extracted with
In our biomimic design, we attempt to utilize a normal chemical group to manipulate the energy level of TMδ+ as active site for HER. Phosphate group is biocompatible and
2
ACS Paragon Plus Environment
Page 3 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
ethyl acetate (50 mL) for three times. The organic layers were summed and dried by Na2SO4. Afterwards, the solvent was removed by distillation. After addition of 20 mL concentrated HCl to the resultant oil, reflux for 24 h was performed. Removal of the solvent resulted in a pale yellow solid followed by washing with CH3CN and drying in air. A white solid was obtained. Yield: 66.8 %. Elemental anal. calcd. for C7H9O4P: C, 44.69; H, 4.82 %. Found: 43.99; H, 4.89 %.
revealed that the weight loss in the temperature range of 110 - 230 °C was 7.2 %, which was close to the theoretical value for releasing one coordinated water molecule (6.9 %). Synthesis of 1Ni. NiCl2·6H2O (47 mg, 0.2 mmol), 3moppH2 (37 mg, 0.2 mmol) and urea (24 mg, 0.4 mmol) were added to 5 mL of distilled water and kept at 100 °C for 4 h. A yellow crystalline sample was obtained and was washed with distilled water and dried in air. Yield: 42.1 mg (80 % based on Ni). Elemental anal. calcd. for C7H9NiO5P: C, 31.99; H, 3.45. Found: C, 32.25; H, 3.74 %. Thermal analysis revealed that the weight loss in the temperature range of 190 - 280 °C was 6.9 %, which was close to the theoretical value for releasing one coordinated water molecule (6.8 %).
Synthesis of 1Co. Route a: CoCl2·6H2O (47 mg, 0.2 mmol), 3-moppH2 (37 mg, 0.2 mmol) and urea (24 mg, 0.4 mmol) were added to 5 mL of distilled water and kept at 100 °C for 4 h. A red crystalline sample was obtained, washed with distilled water and dried in air. Yield: 38.4 mg (73 % based on Co). Elemental anal. calcd. for C7H9CoO5P: C, 31.96; H, 3.45. Found: C, 32.17; H, 3.54 %. Thermal analysis revealed that the weight loss within 120 - 210 °C was 6.9 %, which is close to the theoretical value for releasing one coordinated water molecule (6.8 %). Route b: Co(CH3COO)2·4H2O (49 mg, 0.2 mmol) and 3-moppH2 (37 mg, 0.2 mmol) were added to 5 mL of distilled water and kept at room temperature (20 - 30 °C) for one day without disturbance. A red polycrystalline sample was obtained and was washed with distilled water and dried in air. Yield: 20.6 mg (39 % based on Co).
Synthesis of 1Cu. Cu(CH3COO)2·H2O (40 mg, 0.2 mmol) and 3-moppH2 (37 mg, 0.2 mmol) were added to 5 mL of distilled water and kept at room temperature (20 - 30 °C) for one day without disturbance. A green polycrystalline sample was obtained and was washed with distilled water and dried in air. Yield: 43.3 mg (81 % based on Cu). Elemental anal. calcd. for C7H9CuO5P: C, 31.41; H, 3.39. Found: C, 30.89; H, 3.33 %. Thermal analysis revealed that the weight loss in the temperature range of 130 - 200 °C was 6.6 %, which was close to the theoretical value for releasing one coordinated water molecule (6.7 %).
Synthesis of 1Co-ns. Co(CH3COO)2·4H2O (49 mg, 0.2 mmol) and 3-moppH2 (37 mg, 0.2 mmol) were added to 5 mL of distilled water and kept under sonication (600 W) at room temperature (20 - 30 °C) for 1.5 h. Red floccule were obtained and were washed with distilled water. Yield: 16.3 mg (31 % based on Co). Elemental anal. calcd. for C7H9CoO5P: C, 31.96; H, 3.45. Found: C, 31.59; H, 3.28 %. Thermal analysis revealed that the weight loss in the temperature range of 120-210 oC was 6.9 %, which was close to the theoretical value for releasing one coordinated water molecule (6.8 %).
Determination of crystal structures. A single crystal of dimensions 0.05 × 0.04 × 0.03 mm3 for 1Co was chosen for indexing and collecting intensity data using a diffractometer (Bruker SMART APEX CCD) with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). The Siemens SAINT program was used to integrate the data. Correction of absorption was applied. The structure was figured out using direct methods and then optimized on F2 by the full-matrix least-squares using SHELXTL. The non-hydrogen atoms were calculated using the Fourier maps and were anisotropically optimized. The isotropic vibration parameters relating to the non-hydrogen atoms to which they were bonded were used to isotropically refine all the H atoms. The crystallographic data and the selected bond lengths and angles of compound 1Co are listed in Table S1 and Table S2, respectively. CCDC 1529373 containing the supplementary crystallographic data of the materials in this paper can be obtained from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of 1Mn. Mn(CH3COO)2·4H2O (49 mg, 0.2 mmol) and 3-moppH2 (37 mg, 0.2 mmol) were added to 5 mL of distilled water and kept at room temperature (20 30 °C) for one day without disturbance. A colorless polycrystalline sample was obtained and was washed with distilled water and dried in air. Yield: 38.2 mg (74 % based on Mn). Elemental anal. calcd. for C7H9MnO5P: C, 32.45; H, 3.50. Found: C, 31.91; H, 3.40 %. Thermal analysis revealed that the weight loss in the temperature range of 120 - 200 °C was 7.0 %, which was close to the theoretical value for releasing one coordinated water molecule (6.9 %).
HER electrochemical analyses. All electrochemical experiments were performed on a CHI 660E (Chenhua, China) at room temperature. Prior to the electrochemical measurements, glassy carbon (GC) electrode (diameter: 3 mm) was polished using a 0.05 mm alumina powder. A 4 μL suspension of the as-synthesized catalysts (2 mg mL−l samples dispersed using ethanol) was droped on the cleaned GC electrode following by covering a Nafion film. An Ag/AgCl (saturation KCl) electrode was used as the
Synthesis of 1Fe. (NH4)2Fe(SO4)2·6H2O (80 mg, 0.2 mmol), 3-moppH2 (37 mg, 0.2 mmol), urea (30 mg, 0.5 mmol) and Vitamin C (10 mg) were added to 5 mL of distilled water and kept at 100 °C for 4 h. A colorless crystalline sample was obtained and was washed with distilled water and dried in air. Yield: 39.6 mg (76 % based on Fe). Elemental anal. calcd. for C7H9FeO5P: C, 32.34; H, 3.49. Found: C, 32.69; H, 3.36 %. Thermal analysis
3
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
reference and a graphite rod was used as the counter electrode. The potentials of HER were converted to values in reference to RHE (reversible hydrogen electrode) in all the figures. In 0.1 M N2-saturated tris(hydroxymethyl)aminomethane-HNO3 (pH = 7.4) neutral aqueous solution, the open circuit potential of the Ag/AgCl electrode remained at 0.703 V [E(RHE) = E(Ag/AgCl)+0.703 V], showing a slight variation of 0.0003 V (Figure S1). The composition of the artificial seawater contained 26.5 g L−1 NaCl + 0.73 g L−1 KCl + 24 g L−1 MgCl2 + 3.3 g L−1 MgSO4 + 1.1 g L−1 CaCl2 + 0.2 g L−1 NaHCO3 + 0.28 g L−1 NaBr (pH = 7.02).
Page 4 of 10
ca. 1.5 nm (Figures 2D~2E and Figures S8~S10). The thickness corresponds to a single-layer of the 1Co structure.
Gas Chromatography Characterization. The reaction product of H2 in the artificial seawater was measured using a gas chromatograph (GC-2014, SHIMADZU) equipped with a separation column (MS-13X, 80/100 mesh, 3.2*2.1 mm*2.0 m) and a thermal conductivity detector (TCD). Nitrogen was used as the carrier gas in the chromatograph. The parameters were set as follows: column temperature (80 °C); TCD temperature (100 °C); bridge current (60 mA).
Figure 1. The schematic synthesis of 1Co and 1Co-ns. (A) Synthetic routes of the 1Co and 1Co-ns samples. (B) Packing structure of the 1Co along the b-axis. A single inorganic layer in the ac plane is also shown. (C) Proposed stacking of the layers in 1Co-ns. (D) TEM image of 1Co-ns.
RESULTS AND DISCUSSION With the framework of our design, Co(CH3COO)2 and 3moppH2 were selected as the precursor of TMδ+ center and organic group, which results in compound 1Co by reacting in water at room temperature (route a, Figures 1A and 1B). Scanning electron microscopic (SEM) image shows the obvious bulk morphology of 1Co (Figure S2). The same reaction under sonication (Figures 1A and 1C) leads to a new formation of light floccules (Figure S3). SEM image (Figure S4) of the floccules exhibits a fluffy and soft cotton-like sructure composed of Co, O, P, and C elements (Figures S4 and S5). The floccules show an identical infrared (IR) spectrum and powder X-ray diffraction (PXRD) pattern to those of the bulk sample 1Co (Figures 2A and 2B). The appearance of a few reflections at 2θ = 5.3, 10.8 and 16.2°, assigned to the (020), (040) and (060) facets, indicates the highly preferential orientation of the material (Figure 2A). The X-ray photoelectron spectroscopy (XPS) spectra of both 1Co and the floccules were also studied (Figure 2C and Figures S6~S7). Two broad sets of signals corresponding to the 2p3/2 (781.9 eV) and 2p1/2 (797.8 eV) core levels of the CoII ions are observed in both cases. Combined with the elemental results, we can conclude that the floccules are the same material as 1Co. Interestingly, the floccules transform into a gel after the addition of a small amount of water. The gel becomes a clear solution when a large amount of water is added (Figure 2A, inset). The clear solution demonstrates the Tyndall effect, suggesting that the 1Co nanosheets (1Co-ns) are formed. Indeed, the ultrathin nanosheet morphology of 1Co-ns is supported by TEM image (Figure 1D). Atomic force microscopy (AFM) images reveal that the nanosheets are quite uniform with a lateral size of ca. 1 μm and a thickness of
In order to characterize the specific structure of 1Co-ns, single crystals of 1Co suitable for the structural characterization can also be obtained through the hydrothermal reaction of CoCl2 and 3-moppH2 at 100 °C (route b), as confirmed by both PXRD and IR spectroscopy measurements (Figures S11 and S12). This material crystallizes in the monoclinic space group P21/n.38 The asymmetric unit of 1Co consists of one CoII, one 3-mopp2- and one coordinated H2O (Figure 2F). The Co atom with a distorted octahedral geometry was surrounded by five oxygen atoms from the phosphonate ligands (O1, O2, O1C, O2B and O3A) and one water molecule (O1W). The O-Co-O angles and the Co-O bond lengths are within the range of 66.3(1)-174.1(1)° and 2.055(2)-2.278(2) Å, respectively (Table S2). The CoII ions are each chelated and bridged by the phosphonate groups, forming a layered structure containing Co4O4 squares, which is reminiscent of perovskite-type layered structures (Figure 1). Within the layer, the nearest Co···Co distance is 3.701(1) Å, and the Co-O-Co angles are 119.6(1)-122.0(1)°. The Me-O-phenyl groups are appended at both sides of the layer, and disordered over three sites. The interlayer distance is ca. 16.2 Å, and weak van der Waals interactions dominate between the layers.
4
ACS Paragon Plus Environment
Page 5 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Figure 2. Characterizations of the catalysts. (A) PXRD patterns of the floccules (1Co-ns) and the simulated pattern for 1Co. Inset: Photographs of the gel and solution after the addition of excess water. The Tyndall effect is observed in the latter. (B) IR spectra of 1Co-ns and 1Co. (C) XPS spectra of the Co bands for samples 1Co-ns and 1Co. AFM image (D) of 1Co-ns with the corresponding height value (E). (F) Building unit of the 1Co structure.
Obviously, sonication interferes with the stacking of the layers during the self-assembly process of 1Co, resulting in aggregation of the nanosheets in the form of floccules. The aggregates can be simply dispersed in water to form single-layer nanosheets. This approach is facile, taking the advantages of both bottom-up and top-down methods, and can be conveniently scaled for large-scale production. The successful preparation of 1Co-ns monolayer nanosheets enables the exposure of more active sites and implies the potential application for efficient electrocatalytic HER.
loading amount of 2 mg/mL exhibits the superior HER activity with the onset potential of ca. -0.084 V against RHE, which is better than those of other loading amounts (Figure S13). Thus, the optimized loading amount of 2 mg/mL is determined. It is worth noting that the onset potential of ca. -0.084 V on 1Co-ns is much higher than those of other MOF-based HER electrocatalysts,39-41 indicating its superior electrocatalytic properties. For comparison, the electrocatalytic activity of 1Co with the same loading amount of 2 mg/mL toward HER was also investigated (black line, Figure 3A). As shown, HER occurs with an onset potential of ca. -0.186 V. This value is considerably more negative than that for 1Co-ns, which could be ascribed to the greater exposure of the catalytic active sites and improved charge transport in the twodimensional nanosheet catalysts.3,29,42 Notably, compared to the chemically exfoliated MoS2 nanosheets (ce-MoS2), 1Co-ns shows superior HER electrocatalytic activity under neutral conditions, which is comparable to the activity of commercial Pt/C (Figure S14).
The electrocatalytic activity toward HER of the assynthesized 1Co-ns modified on GC electrode was investigated with a three-electrode system in N2-saturated tris(hydroxymethyl)aminomethane-HNO3 (tris-HNO3) neutral aqueous solution (pH=7.4). Quantitative analysis on surface-modified electrodes is very important for the newly developed two-dimensional nanomaterial. We have first checked the effect of the loading amount of 1Co-ns on the HER activity (Figure S13). As control, the polarization curve of a blank GC electrode is also displayed for comparison and shows negligible HER catalytic activity as expected (blue line, Figure 3A). However, the 1Co-ns-modified electrode displays a considerable cathodic current when the potential reaches more negative values, suggesting the occurrence of the hydrogen reduction reaction (red line, Figure 3A). It is shown that the 1Co-ns-modified GC electrode with a
The Tafel slope is an important parameter revealing the rate-limiting step during the HER process. Three principal steps have been reported to be involved in HER: the Volmer (discharge), Heyrovsky (electrochemical desorption), and Tafel (recombination desorption) steps with the Tafel slopes of 116, 38, and 29 mV/dec, respectively.43 The HER mechanism can be described in two pathways: (1) the Volmer−Heyrovsky mechanism
5
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(H2O + e− = Hads + OH− and H2O + Hads + e− = H2 + OH−), and/or (2) the Volmer−Tafel mechanism (H2O + e− = Hads + OH− and Hads + Hads = H2). The mechanisms imply that the process of adsorption/desorption of the H atoms at the surface of catalyst is competitive, they should be balanced for facilitating HER. The Tafel slopes of 1Co and 1Co-ns derived from the early stages of the corresponding HER polarization curves are 77 and 48 mV/dec, respectively (Figure 3B). Thus, similar rection mechaism can be suggested from the Tafel slopes. The mismatch among the data and any value of the three steps (29, 38, or 116 mV/dec) for HER43 are attributed to the complexity of the HER mechanism of the present system, indicating that the surface coverage of the adsorbed H atoms (ΘH) on the Co-based catalysts would be higher and thus other reaction besides the Volmer reaction codetermin the HER rate. Deuterated effect in HER rate is usually utilized to further clarify the contribution of the Volmer−Heyrovsky and the Volmer−Tafel mechanism. Electrochemical measurement of HER on 1Co-ns was carried out in the D2O solution of tris-HNO3 (pH=7.4). We find that the HER activity of 1Co-ns in D2O is inferior to that in H2O, as confirmed by the increased overpotential (162 mV vs. 84 mV) and Tafel slope (76 mV/dec vs. 48 mV/dec) (Figure S15). The recombination desorption/Tafel reaction (Dads + Dads = D2) is a surface-dominated process and less dependent on the water layer, while the electrochemical desorption process/Heyrovsky reaction (D2O + Dads + e− = D2 + OD−) is more dependent on the diffusion process of water.44 In the Heyrovsky reaction, D2O molecule diffuses to the catalyst surface, and then D2O picks up electrons together with the fixed Dads on catalyst, finally generating D2 adsorbed on the electrode surface. Larger molar mass results in slower diffusion of D2O, which will largely influence the Heyrovsky process. The observed Tafel slope of ca. 48 mV/dec is close to the theoretical value of Heyrovsky step, and more importantly it is largely influenced by the deuterated effect, which suggests that electrochemical desorption is the ratelimiting step and thus the Volmer-Heyrovsky mechanism is more operative in the HER catalyzed by 1Co-ns.
Page 6 of 10
(Table S4).45-49 However, three protonation states make phosphonic acid ligands more sensitive, and hence unstable in acidic and base solutions. Protonation weakens the coordinate bond, which may further result in the collapse of skeleton within the phosphonate CPs. The effect of the solution pH on the HER performance of 1Co-ns was also studied. As shown in Figures S16 and S17, the catalyst shows poor HER activity under acidic or basic conditions. This pH dependent catalytic activity suggests that 1Co-ns is a good HER catalyst around neutral conditions, which implies that 1Co-ns is an ideal HER catalyst for the reduction of earth-abundant seawater. To investigate this possibility, we measured the HER activity of 1Co-ns in artificial seawater. Clearly, the catalyst shows a good performance for hydrogen production from seawater with an overpotential of 205 mV (Figure 3D). Quantitative analysis has also been performed for 1Co-ns-modified electrodes in seawater (Figure S18). The product of H2 catalyzed by 1Co-ns in the artificial seawater was measured using a gas chromatography (GC). The configuration was bubbled with N2 for 1 h before gas-collecting operation (Figure S19A). As shown, the product shows the same GC peak position located at ca. 1.25 min as that of the standard H2 sample (Figure S19B) and no other peaks can be observed, indicating only H2 is formed by 1Co-ns in the artificial seawater. Note that the electrochemical activity of 1Co-ns toward HER in the seawater declined compare to that in the tris-HNO3 neutral solution (overpotential: 205 mV vs. 84 mV). Seawater contains high concentrations of cations and anions that may poison the catalyst and result in the corrosion of the surface. This is a common phenomenon and also exists in many other researches (Table S5).
Moreover, the turnover frequency (TOF, as the number of HER accomplished at each active site and per second) was measured to reveal the intrinsic catalytic activity. Based on the catalyst loadings determined by the inductively coupled plasma optical emission spectroscopy (ICP-OES) (Co = 8.15×10-9 mol/μL), a minimum TOF of 0.023 s−1 was calculated according to the method described in supporting information. However, we note that this value represents an under-estimated activity of 1Co-ns, as only surface exposed sites are responsible for HER. In addition, the 1Co-ns catalyst shows exceptional stability, as indicated by the negligible decline in the overpotential and current density within 1000 potential scans (Figure 3C). The low overpotential, high activity, and good stability of 1Co-ns are superior to the state-ofthe-art HER electrocatalysts for use in neutral pH water
Figure 3. Electrochemical characterization of various catalysts toward HER. (A) HER polarization curves obtained on several catalysts measured in 0.1 M tris-HNO3 (pH = 7.4); (B) Tafel plots of the corresponding electrocatalysts derived from the early stages of the HER polarization curves; (C) stability measurements by recording the polarization curves for the Co-ns catalyst before and after
6
ACS Paragon Plus Environment
Page 7 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
1,000 potential scans in 0.1 M tris-HNO3 (pH 7.4) with 20 -1 mVs ; (D) HER polarization curves of various catalysts (as indicated in the figure) in artificial seawater (pH=7.02) at a -1 scan rate of 20 mVs .
above, the HER process on 1Co-ns mainly goes through the Volmer−Heyrovsky mechanism, which relies on the one primary discharge step (H2O + e− = Hads + OH−) and the other desorption step (H2O + Hads + e− = H2 + OH−). During the initial step, a H2O molecule tends to absorb on the catalyst surface, forming a hydrogen bond to the catalyst (Hads). Then, the second step receives another attack of H2O molecule and releases molecular hydrogen through the electrochemical desorption processes. Due to the synergistic catalysis of H2O molecules on the cobalt site in this complex, the HER kinetics depends largely on both steps, which in turn validates the low Tafel slope of 1Co-ns (48 mV/dec).
The essence of overpotential decrease for HER is the manipulation of electronic density of the active sites in electrocatalyst more comparable to this reaction. In this case, electrons tend to exchange between electrocatalyst and H2O, which activates HER thermodynamically.25 To further corroborate this mechanism, we explored the effect of metal center on the electrocatalytic activities toward HER. Different transition metal ions MII (M = Mn, Fe, Ni, and Cu) electrocatalysts were synthesized through the same strategy as 1Co. The PXRD patterns, IR and XPS spectra, thermogravimetric (TG) curves (Figures S20S26) and elemental analysis (EA) reveal that the 1M (M = Mn, Fe, Ni, Cu) samples have the same layered structure as the 1Co compound with a formula of M(3-mopp)(H2O). As shown in Figure 4A, the resultant 1M compounds exhibit varied electrocatalytic activities toward HER. The electrocatalytic currents at -0.55 V (vs. RHE) against the number of d electrons in the transition metal ions show a volcano trend with the peak located at 7 (inset in Figure 4A). According to the volcano theory, the free energy of H adsorption on a good catalyst is neither too high nor too low (ΔGoH ≅ 0 eV), which contributes to HER.31 During the HER process in the present system, more (e.g., CuII ions) or fewer (e.g., MnII ions) d electrons leads to difficulty in activating the substrate by strong or weak chemisorption. For example, the MnII ion has the fewest d electrons, when the lone pair of electrons on H2O molecule are injected into the d orbitals of the MnII ions, the hydrogen release step occurs very slowly. However, an excess of d electrons in the CuII ions causes difficulties in binding H2O molecules. Excellent activity can thus be achieved with an optimized number of d electrons in the transition metal ions, which is also supported by recently reported results (i.e., perovskites with a filling electron number of eg approaching 1 in the octahedral geometry showing the highest electrocatalytic activity).50 Even though the phosphonate CPs display poor HER activity under acidic condition, similar trend is also shown in the relationship between the HER activity and the number of d electrons (Figure S27). These results rule out the possibility that the various activities of the different transition metal ions MII electrocatalysts might be derived from the chemical structures, and hence further confirm the effect of metals at neutral pH. The excellent HER catalytic activity of 1Co-ns might also be ascribed to the activation effect on H2O molecules through the synergistic interaction between Co···O and O···H, which is roughly represented in Figure 4B. In this bi-functional activation mechanism of water molecules, the kinetics depend on a fine balance among the water dissociation rate on CoII, the recombination of Hads on Oδ- and the rate of desorption of OHads which helps accommodate the adsorption of the next water molecules. As mentioned
Figure 4. Plausible mechanism of HER on 1Co-ns. (A) HER polarization curves obtained on several catalysts with different metal centers in 0.1 M tris-HNO3 (pH = 7.4) at 20 -1 mVs . Inset: relationship between the HER electrocatalytic activity and the number of d electrons of the center II II II II II transition metal (Mn , Fe , Co , Ni , and Cu ). The dashed volcano line is shown for guidance only. (B) Schematics for II the under-coordinated Co active centers in HER and the plausible mechanism.
CONCLUSION In summary, we propose that manipulation of the electronic density of active sites in electrocatalysts is an efficient means to enhance the electrocatalytic activity. Cobalt-phosphonate coordination polymer nanosheets (1Co-ns) have been designed as a model system to understand the fundamental relationship between the nanostructure and the electrocatalytic activity toward HER. Our results suggest that 1Co-ns exhibits the superior electrochemical activity toward HER with an overpotential of 84 mV and a Tafel slope of 48 mV dec−1 in neutral condition. The large enhancement can be ascribed to the synergistic effect of matched energy level and bifunctional activation of water molecules. Ongoing efforts are concentrated on modifying specific metal center ion with different ligand to adjust its electronic density and the catalytic activities.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.
7
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Details of TOFs calculation, and supporting data results (Figures S1-S27 and Tables S1-S5)
Page 8 of 10
performance for overall water splitting. Electrochim. Acta 2017, 242, 355–363. (12) Chen, W. F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Hydrogen-evolution catalysts based on non-noble metal nickelmolybdenum nitride nanosheets. Angew. Chem. Int. Ed. 2012, 51, 6131–6135. (13) Andreiadis, E. S.; Jacques, P. A.; Tran, P. D.; Leyris, A.; Chavarot-Kerlidou, M.; Jousselme, B.; Matheron, M.; Pécaut, J.; Palacin, S.; Fontecave, M.; Artero, V. Molecular engineering of a cobalt-based electrocatalytic nanomaterial for H₂ evolution under fully aqueous conditions. Nat. Chem. 2013, 5, 48–53. (14) Vrubel, H.; Hu, X. Molybdenum boride and carbide catalyze hydrogen evolution in both acidic and basic solutions. Angew. Chem. Int. Ed. 2012, 124, 12875–12878. (15) Fan, Z.; Luo, Z.; Huang, X.; Li, B.; Chen, Y.; Wang, J.; Hu, Y.; Zhang, H. Synthesis of 4H/fcc noble multimetallic nanoribbons for electrocatalytic hydrogen evolution reaction. J. Am. Chem. Soc. 2016, 138, 1414–1419. (16) Fei, H.; Dong, J.; Arellano-Jiménez, M. J.; Ye, G.; Dong Kim, N.; Samuel, E. L. G.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J.; Yacaman, M. J.; Ajayan, P. M.; Chen, D.; Tour, J. M. Atomic cobalt on nitrogen-doped graphene for hydrogen generation. Nat. Commun. 2015, 6, 8668. (17) Jiang, P.; Liu, Q.; Liang, Y. H.; Tian, J. Q.; Asiri, A. M.; Sun, X. P. A cost-effective 3D hydrogen evolution cathode with high catalytic activity: FeP nanowire array as the active phase. Angew. Chem. Int. Ed. 2014, 53, 12855–12859. (18) Furukawa, H.; Cordova, K. E.; O'Keeffe, M.; Yaghi, O. M. The chemistry and applications of metal-organic frameworks. Science 2013, 341, 1230444. (19) Li, B.; Wen, H. M.; Cui, Y.; Zhou, W.; Qian, G.; Chen, B. Emerging multifunctional metal-organic framework materials. Adv. Mater. 2016, 28, 8819–8860. (20) Sun, L.; Campbell, M. G.; Dincǎ , M. Electrically conductive porous metal-organic frameworks. Angew. Chem. Int. Ed. 2016, 55, 3566–3579. (21) Dhakshinamoorthy, A.; Asiri, A. M.; Garcia, H. Metal– organic frameworks as catalysts for oxidation reactions. Chem. Eur. J. 2016, 22, 8012–8024. (22) García-García, P.; Moreno, J. M.; Díaz, U.; Bruix, M.; Corma, A. Organic-inorganic supramolecular solid catalyst boosts organic reactions in water. Nat. Commun. 2016, 7, 10835. (23) Fateeva, A.; Chater, P. A.; Ireland, C. P.; Tahir, A. A.; Khimyak, Y. Z.; Wiper, P. V.; Darwent, J. R.; Rosseinsky, M. J. A water-stable porphyrin-based metal-organic framework active for visible-light photocatalysis. Angew. Chem. Int. Ed. 2012, 51, 7440–7444. (24) Meyer, K.; Ranocchiari, M.; van Bokhoven, J. A. Metal organic frameworks for photo-catalyticwater splitting. Energy Environ. Sci. 2015, 8, 1923–1937. (25) Mahmood, A.; Guo, W.; Tabassum, H.; Zou, R. Metal organic framework-based nanomaterials for electrocatalysis. Adv. Energy Mater. 2016, 6, 1600423. (26) Weinberg, D. R. Proton-coupled electron transfer. Chem. Rev. 2012, 112, 4016–4093. (27) Costentin, C.; Robert, M.; Savéant, J. M. Update 1 of: Electrochemical approach to the mechanistic study of pro-
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected];
[email protected] Author Contributions ‡
Z.-S.C. and Y.S. contributed equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (2017YFA0206500) and the National Natural Science Foundation of China (21731003, U1532110, 21635004).
REFERENCES (1) Dresselhaus, M. S.; Thomas, I. L. Alternative energy technologies. Nature 2001, 414, 332–337. (2) Turner, J. A. Sustainable hydrogen production. Science 2004, 305, 972–974. (3) Shi, Y.; Wang, J.; Wang, C.; Zhai, T. T.; Bao, W. J.; Xu, J. J.; Xia, X. H.; Chen, H. Y. Hot electron of Au nanorods activates the electrocatalysis of hydrogen evolution on MoS2 nanosheets. J. Am. Chem. Soc. 2015, 137, 7365–7370. (4) Sun, Y. F.; Gao, S.; Lei, F. C.; Liu, J. W.; Liang, L.; Xie, Y. Atomically-thin non-layered cobalt oxide porous sheets for highly efficient oxygen-evolving electrocatalysts. Chem. Sci. 2014, 5, 3976–3982. (5) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-doped carbon nanomaterials as non-metal electrocatalysts for water oxidation. Nat. Commun. 2013, 4, 2390. (6) Jiang, J.; Liu, Q. X.; Zeng, C. M.; Ai, L. H. Cobalt/molybdenum carbide@N-doped carbon as a bifunctional electrocatalyst for hydrogen and oxygen evolution reactions. J. Mater. Chem. A 2017, 5, 16929–16935. (7) Ai, L. H.; Tian, T.; Jiang, J. Ultrathin graphene layers encapsulating nickel nanoparticles derived metal−organic frameworks for highly efficient electrocatalytic hydrogen and oxygen evolution reactions. ACS Sustainable Chem. Eng. 2017, 5, 4771−4777. (8) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 2013, 135, 9267–9270. (9) Tian, T.; Huang, L.; Ai, L. H.; Jiang, J. Surface anionrich NiS2 hollow microspheres derived from metal–organic frameworks as a robust electrocatalyst for the hydrogen evolution reaction. J. Mater. Chem. A 2017, 5, 20985–20992. (10) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. Nat. Mater. 2012, 11, 963–969. (11) Ai, L. H.; Niu, Z. G.; Jiang, J. Mechanistic insight into oxygen evolution electrocatalysis of surface phosphate modified cobalt phosphide nanorod bundles and their superior
8
ACS Paragon Plus Environment
Page 9 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
ton-coupled electron transfer. Chem. Rev. 2010, 110, PR1– PR40. (28) Edmund C. M. T.; Christopher J. B.; Nicholas A. K.; Li, Y.; Gewargis1, J. P.; Zimmerman, S. C.; Hosseini, A.; Gewirth, A. A. Proton transfer dynamics control the mechanism of O2 reduction by a non-precious metal electrocatalyst. Nat. Mater. 2016, 15, 754–759. (29) Shi, Y.; Zhou, Y.; Yang, D. R.; Xu, W. X.; Wang, C.; Wang, F. B.; Xu, J. J.; Xia, X. H.; Chen, H. Y. Energy level engineering of MoS2 by transition-metal doping for accelerating hydrogen evolution reaction. J. Am. Chem. Soc. 2017, 139, 15479–15485. (30) Masa, J.; Schuhmann, W. Electrocatalysis and bioelectrocatalysis–Distinction without a difference. Nano Energy 2016, 29, 466–475. (31) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jorgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Norskov, J. K. Biomimetic hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. J. Am. Chem. Soc. 2005, 127, 5308–5309. (32) Alan, L. G.; Vincent, A.; Bruno, J.; Phong, D. T.; Nicolas, G.; Romain, M.; Aziz, F.; Serge, P.; Marc, F. From hydrogenases to noble metal-free catalytic nanomaterials for H2 production and uptake. Science 2016, 326, 1384–1387. (33) Miguel, C. A.; Michael, L. S.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, H. C.; Tsai, M. L.; He, J. H.; Jin, S. Efficient hydrogen evolution catalysis using ternary pyrite-type cobalt phosphosulphide. Nat. Mater. 2015, 14, 1245–1251. (34) Wang, J.; Wang, K.; Wang, F. B.; Xia, X. H. Bioinspired copper catalyst effective for both reduction and evolution of oxygen. Nat. Commun. 2014, 5, 5285. (35) Liu, Q.; Tian, J. Q.; Cui, W.; Jiang, P.; Cheng, N. Y.; Abdullah, M. A.; Sun, X. P. Carbon nanotubes decorated with CoP nanocrystals: A highly active non-noble-metal nanohybrid electrocatalyst for hydrogen evolution. Angew. Chem. Int. Ed. 2014, 53, 6710–6714. (36) Jirkovský, J. S.; Malliakas, C. D.; Lopes, P. P.; Danilovic, N.; Kota, S. S.; Chang, K. C.; Genorio, B.; Strmcnik, D.; Stamenkovic, V. R.; Kanatzidis, M. G.; Markovic, N. M. Design of active and stable Co-Mo-Sx chalcogels as pH-universal catalysts for the hydrogen evolution reaction. Nat. Mater. 2016, 15, 197–203. (37) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in activity for the water electrolyser reactions on 3d M (Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat. Mater. 2012, 11, 550–557. (38) Crystal data of 1Co: C7H9CoO5P, monoclinic, space group P21/n, a = 4.8553(4), b = 32.281(2), c = 5.6773(4) Å, β = 3 91.788(1)°, V = 889.39(11) Å , Z = 4, R1 = 0.0339, wR2 = 0.0658 for I > 2σ(I). (39) Wu, Y. P.; Zhou, W.; Zhao, J.; Dong, W. W.; Lan, Y. Q.; Li, D. S.; Sun, C. H.; Bu, X. H. Surfactant-assisted phaseselective synthesis of new cobalt MOFs and their efficient electrocatalytic hydrogen evolution reaction. Angew. Chem. Int. Ed. 2017, 56, 13001–13005. (40) Dong, R.; Pfeffermann, M.; Liang, H.; Zheng, Z.; Zhu, X.; Zhang, J.; Feng, X. Large-area, free-standing, twodimensional supramolecular polymer single-layer sheets for highly efficient electrocatalytic hydrogen evolution. Angew. Chem. Int. Ed. 2015, 54, 12058–12063.
(41) Dong, R.; Zheng, Z.; Tranca, D. C.; Zhang, J.; Chandrasekhar, N.; Liu, S.; Zhuang, X.; Seifert, G.; Feng, X. Immobilizing molecular metal dithiolene–diamine complexes on 2D metal–organic frameworks for electrocatalytic H2 production. Chem. Eur. J. 2017, 23, 2255– 2260. (42) Deng, J.; Li, H.; Wang, S.; Ding, D.; Chen, M.; Liu, C.; Tian, Z.; Novoselov, K. S.; Ma, C.; Deng, D.; Bao, X. Multiscale structural and electronic control of molybdenum disulfide foam for highly efficient hydrogen production. Nat. Commun. 2017, 8, 14430. (43) Bockris, J. O. M.; Potter, E. C. The mechanism of the cathodic hydrogen evolution reaction. J. Electrochem. Soc. 1952, 99, 169–186. (44) Tang, Q.; Jiang, D. E. Mechanism of hydrogen evolution reaction on 1T-MoS2 from first-principles. ACS Catal. 2016, 6, 4953−4961. (45) Cobo, S.; Heidkamp, J.; Jacques, P. A.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; Fontecave, M.; Artero, V. A Janus cobalt-based catalytic material for electro-splitting of water. Nat. Mater. 2012, 11, 802–807. (46) Jiang, N.; Bogoev, L.; Popova, M.; Gul, S.; Yano, J.; Sun, Y. Electrodeposited nickel-sulfide films as competent hydrogen evolution catalysts in neutral water. J. Mater. Chem. A 2014, 2, 19407–19414. (47) Sun, Y.; Liu, C.; Grauer, D. C.; Yano, J. J.; Long, R.; Yang, P. C.; Chang, J. Electrodeposited cobalt-sulfide catalyst for electrochemical and photoelectrochemical hydrogen generation from water. J. Am. Chem. Soc. 2013, 135, 17699– 17702. (48) Tran, P. D.; Nguyen, M.; Pramana, S. S.; Bhattacharjee, A.; Chiam, S. Y.; Fize, J.; Field, M. J.; Artero, V.; Wong, L. H.; Loo, J.; Barber, J. Copper molybdenum sulfide: a new efficient electrocatalyst for hydrogen production from water. Energy Environ. Sci. 2012, 5, 8912–8916. (49) Zhuang, M. H.; Ou, X. W.; Dou, Y. B.; Zhang, L. L.; Zhang, Q. C.; Wu, R. Z.; Ding, Y.; Shao, M. H.; Luo, Z. T. Polymer-Embedded Fabrication of Co2P Nanoparticles encapsulated in N,P-doped graphene for hydrogen generation. Nano lett. 2016, 16, 4691–4698. (50) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Yang, S. H. Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal-air batteries. Nat. Chem. 2011, 3, 546–550.
9
ACS Paragon Plus Environment
ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
10
ACS Paragon Plus Environment
Page 10 of 10
