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Core-Shell ZIF-8@ZIF-67 Derived CoP Nanoparticles-Embedded N-doped Carbon Nanotube Hollow Polyhedron for Efficient Over-all Water Splitting Yuan Pan, Kaian Sun, Shoujie Liu, Xing Cao, Konglin Wu, Weng-Chon Cheong, Zheng Chen, Yu Wang, Yang Li, Yunqi Liu, Dingsheng Wang, Qing Peng, Chen Chen, and Yadong Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12420 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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Journal of the American Chemical Society

Core-Shell ZIF-8@ZIF-67 Derived CoP Nanoparticles-Embedded Ndoped Carbon Nanotube Hollow Polyhedron for Efficient Overall Water Splitting Yuan Pan,†,§,∆ Kaian Sun,§,∆ Shoujie Liu,†,⊥,∆ Xing Cao,† Konglin Wu,†,⊥ Weng-Chon Cheong,† Zheng Chen,† Yu Wang,† Yang Li,† Yunqi Liu,§ Dingsheng Wang,† Qing Peng,† Chen Chen,*,† Yadong Li† †

Department of Chemistry, Tsinghua University, Beijing, 100084, China State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China), Qingdao, Shandong, 266580, China ⊥ College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, China KEYWORDS: ZIF-8@ZIF-67; CoP; N-doped carbon nanotube hollow polyhedron; electrocatalyst; water splitting §

ABSTRACT: The construction of highly active and stable non-noble metal electrocatalysts for hydrogen and oxygen evolution reactions electrocatalysts is a major challenge for overall water splitting. Herein, we report a novel hybrid nanostructure with CoP nanoparticles (NPs) embedded in N-doped carbon nanotube hollow polyhedron (NCNHP) through a pyrolysis-oxidationphosphidation strategy derived from core-shell ZIF-8@ZIF-67. Benefiting from the synergistic effects between highly active CoP NPs and NCNHP, the CoP/NCNHP hybrid exhibited outstanding bifunctional electrocatalytic performances. When the CoP/NCNHP was employed as both anode and cathode for overall water splitting, a potential as low as 1.64 V was needed to achieve the current density of 10 mA·cm-2, and it still exhibited superior activity after continuously working for 36 h with nearly negligible decay in potential. Density functional theory calculations indicated that the electron transfer from NCNHP to CoP could increase the electronic states of Co d-orbital around the Fermi level, which could increase binding strength with H, and therefore improve the electrocatalytic performance. The strong stability is attributed to high oxidation resistance of CoP surface protected by the NCNHP.

the hybridization with porous carbon materials and the doping of heteroatoms. Zeolitic imidazolate frameworks (ZIFs), such as ZIF-67 and ZIF-8, are a novel class of porous materials with zeolite-like 3D topological structures, abundant carbon and nitrogen ligands and high metal ion contents, and are expected to be good candidates as the precursor template to design various porous nanostructured metal-carbon hybrid materials19 and the corresponding phosphide derivatives. And among various nanostructure, nanotube structures showed well-defined inner channels and large surface area, which is benificial for charge and mass transport in electrocatalysis.20 However, up to now, the design of novel hybrid nanostructure between TMPs and heteroatom-doped carbon materials as efficient bifunctional catalyst is still a challenging task and has rarely been reported. Inspired by the above considerations, we designed a novel hybrid nanostructure composed of CoP nanoparticles (NPs) embedded in N-doped carbon nanotube hollow polyhedron (NCNHP) through a pyrolysis-oxidation-phosphidation strategy derived from core-shell structured ZIF-8@ZIF-67, and the electrochemical measurements suggested that it could work as an efficient bifunctional electrocatalyst toward water splitting. The seed ZIF-8 derived N-doped carbon (Zn will evaporate when the temperature goes above 900 °C21) acts as the

INTRODUCTION Electrochemical hydrogen and oxygen productions from water splitting is of paramount significance for solving the energy crisis and environmental problems.1 Noble metal Ptbased materials have been recognized as excellent catalysts for hydrogen evolution reaction (HER), while RuO2 and IrO2 exhibit remarkable performance for oxygen evolution reaction (OER).2 However, the large-scale utilization of these catalysts is limited owing to the low abundance and high cost of nobel metals. Therefore, developing highly active and stable nonnoble metal bifunctional electrocatalysts including carbides,3 oxides,4 sulfides,5 selenides,6 nitrides7 and phosphides8 is the key for reducing cost and enhancing the catalytic efficiency for water splitting. Transition metal phosphides (TMPs, M = Fe9, Co10, Ni11, 12 Cu , Mo13, W14) have been reported as efficient catalysts for HER due to their similar feature with hydrogenase15. Meanwhile, recent studies also indicated that the OER also can be catalyzed by TMPs16. In order to maximize catalytic efficiency of TMPs-based catalysts, the nanostructure and composition of catalysts should be optimized to expose more catalytically active sites for water splitting17. Previous studies18 indicated that the catalytic performance of TMPs can be improved via

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nanostructured hollow framework to promote the diffusion kinetics22. CoP-embedded N-doped carbon, which is transformed from the outer shell ZIF-67 derived Co-embedded Ndoped carbon materials, is responsible for catalyzing the HER and OER process. We found that numerous carbon nanotubes (CNTs) could be obtained on the surface of hollow polyhedron after the pyrolysis treatment of ZIF-8@ZIF-67. The CNTs played a positive role in the electrocatalytic reactions. To the best of our knowledge, the current hybrid nanostructure between CoP NPs and NCNHP is reported for the first time.The CoP/NCNHP catalyst exhibits remarkable bifunctional electrocatalytic performances for both HER and OER. When the CoP/NCNHP catalyst was employed as both anode and cathode in a two-electrode cell for overall water splitting, a potential as low as 1.64 V was needed to achieve the current density of 10 mA·cm-2, and it also exhibited superior stability after continuous working for 36 h. Density functional theory (DFT) calculations indicated that the electron transfer from NCNHP to CoP could increase the electronic states of Co d-orbital around the Fermi level, which in turn could increase binding strength of H, and therefore improve the electrocatalytic performance. Additionally, the strong stability of CoP/NCNHP catalyst is attributed to high oxidation resistance of CoP surface protected by the NCNHP.

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structure of ZIF-8@ZIF-67. Then, the Co/NCNHP sample was obtained after the pyrolysis treatment of ZIF-8@ZIF-67 at 900 °C in flowing Ar, followed by acid etching to remove any accessible Co species on its surface. As shown in the XRD pattern of Co/NCNHP (Figure 1a), three peaks located at 44.3°, 51.5°, and 76.1° can be observed, corresponding to the (111), (200), and (220) facets of face-centered-cubic (fcc) Co (PDF #15-0806). The relatively sharp peak located at 25.8° can be assigned to (002) facet of graphitic carbon. SEM (Figure S5), TEM (Figure S6) and HAADF-STEM EDS mapping images (Figure S7) indicate that the Co/NCNHP shows a hollow polyhedron morphology with rough surface anchored with numerous CNTs, and the distribution of C, N, Co elements are even. During this pyrolysis process, the produced gas facilitated metal ions to move outside, promoting the formation of hollow polyhedra25. Previous report suggested that the formation of CNTs were mainly due to the catalytic effect of Co NPs26. However, we found that the pyrolysis treatment of sole ZIF-67 or ZIF-8 could not produce CNTs (Figure S8-S11), but numerous CNTs could be obtained after the pyrolysis treatment of ZIF-8@ZIF-67, indicating that the evaporation of Zn at high pyrolysis temperature accelerates the growth of CNT, and this process might be catalyzed by Co NPs. After subsequent low-temperature oxidation in air and phosphidation in

Ar with NaH2PO2, the Co/NCP and Co/NCNHP samples were converted into corresponding Co3O4/NCP, Co3O4/NCNHP, CoP/NCP, and CoP/NCNHP, respectively. The detailed characterizations were shown in Figure S12-S17. The XRD pattern of Co3O4/NCNHP (Figure 1a) shows the peaks at 31.2°, 36.7°, 44.7°, 59.2°, and 65.1°, corresponding to the (220), (311), (400), (511), and (440) facets of Co3O4 (PDF #74-2120). After phosphidation, the XRD peaks of CoP/NCNHP (Figure 1a) become weaker, the peaks at 31.6°, 36.2°, 48.2°, and 56.6° can be assigned to the (011), (111), (211), and (301) facets of CoP (PDF #29-0497). The hollow polyhedron made of carbon nanotube also remained and showed high porosity (Figure 1bc, Figure S17). The HRTEM image of the CoP/NCNHP (Figure 1d) revealed fringe spacings of about 1.89 Å and 2.49 Å, corresponding to the (211) and (111) lattice planes of CoP, respectively. The selected area electron diffraction (SAED) (Figure 1e) indicated that the major diffraction rings matched well with the CoP/NCNHP. The smallest diffraction ring could be attributed to the (002) plane of carbon. HAADFSTEM and EDS elemental mapping (Figure 1f) also revealed that C, N, Co and P elements were evenly distributed over the entire NCNHP. X-ray photoelectron spectroscopy (XPS) of the synthesized CoP/NCNHP catalyst revealed that the binding energy

Scheme 1. Schematic illustration of the synthesis process of CoP/NCNHP.

RESULTS AND DISCUSSION Synthesis and Characterization of CoP/NCNHP Catalyst. Scheme 1 illustrates the synthesis process of the CoP/NCNHP catalyst. In brief, the core-shell ZIF-8@ZIF-67 was synthesized by seed epitaxial growth due to the similar topological structure and unit cell parameters of ZIF-8 (a = b = c = 16.9910 Å)23 and ZIF-67 (a = b = c = 16.9589 Å)24, which was confirmed by X-ray diffraction (XRD) (Figure S1). Scanning electron microscopy (SEM) (Figure S2) and transmission electron microscopy (TEM) (Figure S3) images revealed that ZIF-8@ZIF-67, ZIF-8, and ZIF-67 showed well-defined rhombic dodecahedral structure with smooth surfaces and uniform size distributions. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) elemental mapping images and line scan analyses (Figure S4) revealed that C and N distributed over the entire sample, but Zn and Co were mainly located at the center and outside of the sample, which suggested the successful synthesis of the core-shell


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Journal of the American Chemical Society between that of Co2+ and Co3+. The Fourier-transformed (FT) k3-weighted EXAFS spectra (Figure 2e) of CoP/NCNHP catalyst showed two main peaks at 1.7 Å and 2.5 Å, corresponding to the Co-P and Co-Co coordination shells, respectively. The quantitative coordination configuration of Co atom in CoP/NCNHP catalyst can be obtained by EXAFS fitting, as shown in Figure S19 and Table S1. The experimental total CoP coordination number and the Co-P average bond length of CoP/NCNHP catalyst are very close to bulk CoP and literature reported values27. Additionally, a Co-N shell also can be found by fitting, which indicates that Co atom was coordinated by one N atom, further demonstrating the formation of CoP embedded in NCNHP. The pore structure information of CoP/NCNHP catalyst was characterized by N2 sorption. As shown in Figure 2f, the typical type-IV isotherm with a distinct hysteresis loop was observed for CoP/NCNHP catalyst, indicating abundant mesoporous structure. The CoP/NCNHP catalyst shows a large Brunauer-Emmett-Teller (BET) surface area of 86.5 m2·g-1 and pore volume of 0.56 cm3·g-1. Moreover, the pore size distribution curves (inset of Figure 2f) indicate that the sizes of micropore and mesopore are mainly concentrated in 1.12 nm and 9.65 nm, respectively. The porosity of CoP/NCNHP catalyst is benficial for charge and mass transport for electrocatalysis28. Hydrogen Evolution Reaction of CoP/NCNHP Catalyst. The electrocatalytic HER performance of the CoP/NCNHP catalyst was first investigated in 0.5 M H2SO4 and 1 M KOH using a standard three-electrode setup. For comparison, a series of reference samples including NCP, Co/NCP, Co/NCNHP, CoP/NCP catalysts and commercial 20% Pt/C were also examined. Figure 3a and Figure 3d show the linear sweep voltammetry (LSV) curves of all samples with a scan rate of 5 mV·s-1 at room temperature. As expected, 20% Pt/C catalyst shows the highest HER activity. The CoP/NCNHP catalyst shows higher catalytic activity with small overpotential of 140 mV and 115 mV at the current density of 10 mA·cm-2 for HER than that of CoP/NCP (234 mV and 148 mV), Co/NCNHP (245 mV and 168 mV), Co/NCP (296 mV and 243 mV), NCP (586 mV and 628 mV) catalysts in 0.5 M H2SO4 and 1 M KOH, respectively. The corresponding Tafel slopes of CoP/NCNHP catalyst (53 mV·dec-1 and 66 mV·dec-1) are also smaller than that of CoP/NCP (71 mV·dec-1 and 86 mV·dec-1), Co/NCNHP (94 mV and 101 mV), Co/NCP (135 mV·dec-1 and 105 mV·dec-1), NCP (369 mV·dec-1 and 274 mV·dec-1) in 0.5 M H2SO4 and 1 M KOH, respectively (Figure 3b and Figure 3e), revealing more favorable electrocatalytic kinetics on CoP/NCNHP catalyst for HER. The values of Tafel slope of CoP/NCNHP catalyst also indicate that the HER process undergo a Volmer-Heyrovesky pathway29. The current CoP/NCNHP catalyst exhibits higher HER catalytic performance compared with many other reported catalysts (Table S2-S3). In addition, the Nyquist plots (Figure S20) also indicate that CoP/NCNHP catalyst shows smaller semicircle diameter than that of Co/NCNHP catalyst, suggesting a favorable charge transfer resistance (Rct) for CoP/NCNHP catalyst in both acid (-0.16 V, Rct = 17.35 Ω) and alkaline media (-0.16 V, Rct = 135.7 Ω). However, the Co/NCNHP catalysts exhibit high Rct under the same conditions (Table S4). The electrochemical active

(BE) of the Co 2p3/2 peaks (Figure 2a) located at 778.8, 782.2 and 786.1 eV, which could be assigned to the Co-P in CoP, oxidized Co species and satellite peak, respectively. The Co 2p1/2 peaks of CoP/NCNHP catalyst have similar peak assignment with Co 2p3/2. The BE of the P 2p peaks (Figure 2b) located at 129.6, 130.4 and 134.5 eV, corresponding to the P 2p3/2, P 2p1/2 in CoP and oxidized P species, respectively. Compared with the BE of metallic Co (778.1 eV) and elemental phosphorus (130 eV), the BE of Co 2p3/2 has positive shift and P 2p3/2 has negative shift, respectively, indicating Co and P in CoP/NCNHP catalyst have a partial positive charge (δ+) and negative charge (δ-), respectively. Three N-types (Figure 2c) including graphitic (401.5 eV, 39.65 %), pyrrolic (400.1 eV, 44.52 %), and pyridinic (398.8 eV, 15.83 %) could be distinguished from N 1s spectrum. The existence of pyrrolic-N and pyridinic-N species in CoP/NCNHP catalyst also can improve the electrocatalytic activity by interacting with H+ for HER. Furthermore, no signal peak can be found from the Zn 2p spectrum (Figure S18), which indicates that Zn species have been evaporated completely.

Figure 1. (a) XRD patterns of the Co/NCNHP, Co3O4/NCNHP and CoP/NCNHP catalysts. (b) SEM, (c) TEM, (d) HRTEM, (e) SAED, (f) HAADF-STEM elemental mapping of CoP/NCNHP catalyst. The inset in (b): the magnified SEM image. The inset in (d): the fast Fourier transformation of the selected area in red box. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were carried out to provide further insights on the electronic structure and coordination environment of the Co atoms in CoP/NCNHP catalyst (Figure 2d-e). It can be seen that the Co K-edge XANES of CoP/NCNHP is very different compared with Co foil, CoO and Co3O4. The absorption edge position of CoP/NCNHP catalyst is located between that of CoO and Co3O4, which indicates that Co in CoP/NCNHP catalyst carries positive charge and the valence of Co atom is situated



Figure 2. (a) Co 2p, (b) P 2p, and (c) N 1s spectra of CoP/NCNHP catalyst. (d) XANES spectra and (e) Fourier transform (FT) at the Co K-edge of CoP/NCNHP, CoP bulk, CoO, Co3O4 samples and Co foil. (f) N2 adsorption-desorption isotherm of CoP/NCNHP catalyst. The inset in (f) is the corresponding micropore (left) and mesoporous (right) size distribution curves. surface area (ECSA) of the catalysts was reflected from the double-layer capacitance (Cdl),30 which can be obtained by cyclic voltammetry (CV) with different scan rates from 40 to 300 mV·s-1 in a non-faradic potential range (Figure S21a-b) in 1 M KOH. The Cdl of CoP/NCNHP catalyst (16.8 mF·cm-2) is higher than that of Co/NCNHP catalyst (10.2 mF·cm-2) (Figure S21c), which suggests that CoP/NCNHP catalyst has large ECSA, revealing the excellent electrocatalytic activity for HER. Furthermore, the CoP/NCNHP catalyst exhibits higher Cdl value in 1 M KOH than 0.5 M H2SO4 (11.9 mF·cm-2) (Figure S21d-e). The stability of CoP/NCNHP catalyst is further evaluated by CV scanning 1000 cycles and long-term chronoamperometry. As shown in Figure 3c and Figure 3f, a weak decay can be observed by comparing the LSV curves before and after 1000 cycles. Additionally, the current-time curves (inset of Figure 3c and Figure 3f) display that the catalytic activities of CoP/NCNHP catalyst could be retained for at least 24 h and 20 h in 0.5 M H2SO4 and 1 M KOH, respectively. Additionally, SEM, TEM, HAADF-EDS mapping image indicate that the hollow polyhedron made of carbon nanotube morphology and the element composition of the CoP/NCNHP catalyst showed barely any change after long-term electrolysis (Figure S22 a~e). XPS analysis (Figure S22 f~h) also suggested the chemical states after electrochemical testing is nearly not changed, demonstrating the good stability of the

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CoP/NCNHP catalyst which may attribute to oxidation resistance of NCNHP to surface CoP. Generally, the electrocatalyst exhibits higher catalytic activity for HER in acid solution than alkaline solution, owing to the favorable electrocatalytic kinetics in acid solution. However, we are surprise for the better HER electrocatalytic activity for CoP/NCNHP hybrid catalyst in 1 M KOH than in 0.5 M H2SO4, which may underscore the key role that Co palyed in the Heyrovsky and Tafel steps31 in HER process. Furthermore an increased Cdl of the CoP/NCNHP in 1 M KOH than 0.5 M H2SO4 was observed. Oxygen Evolution Reaction of CoP/NCNHP Catalyst. The OER catalytic performance of the CoP/NCNHP catalyst was then assessed in 1 M KOH solution. NCP, Co/NCP, Co/NCNHP, CoP/NCP catalysts, commercial RuO2 and IrO2 were also tested for comparison. Figure 3g shows the LSV curves of all the samples with a scan rate of 5 mV·s-1. The CoP/NCNHP catalyst exhibits the highest electrocatalytic activity with the lowest overpotential of 310 mV to achieve the current density of 10 mA·cm-2 compared with CoP/NCP (360 mV), Co/NCNHP (580 mV), Co/NCP (610 mV) catalysts, commercial RuO2 (500 mV) and IrO2 (560 mV), respectively, further indicating the superior catalytic behavior for OER of CoP/NCNHP catalyst. The corresponding Tafel curves (Figure 3h) of these catalysts also show the smallest Tafel value (70 mV·dec-1) of CoP/NCNHP compared with CoP/NCP (77 mV·dec-1), Co/NCNHP (161 mV·dec-1), Co/NCP (163 mV·dec-1), NCP (370 mV·dec-1), suggesting higher OER rates of CoP/NCNHP catalyst. The Nyquist plots (Figure S23) also shows that the CoP/NCNHP catalyst (1.57 V, Rct = 79.3 Ω) exhibits smaller charge-transfer resistance than that of Co/NCNHP catalyst (1.57 V, Rct = 4360 Ω), suggesting the positive effect of phosphidation. The above results prove that


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CoP/NCNHP catalyst is also a remarkable catalyst that outperforms most of reported Co-based non-noble metal electrocatalysts for OER in alkaline media (Table S5). The stability test of CoP/NCNHP catalyst was further performed in O2-saturated 1M KOH by CV scanning 1000 cycles and long-term chronoamperometry (Figure 3i). The LSV curve shows negligible change after 1000 cycles and the electrocatalytic activity can maintain at least of 19 h. Moreover, the morphology also can be preserved after the long-term electrolysis (Figure S24).

sponding to an overpotential of 420 mV) to achieve the current density of 10 mA·cm-2 (Figure 4a). Large numbers of bubbles were produced from both cathode (H2) and anode (O2) during electrolysis (Video S1). The overall water splitting performance of CoP/NCNHP catalyst is superior to most of reported non-noble metal electrocatalysts (Table S6). The water electrolyzer durability was examined at different current densities from 10 to 100 mA·cm-2 (Figure 4b). With the increase of current density, the cell voltage increases accordingly. In each

Figure 3. (a, d, g) LSV and (b, e, h) Tafel curves of CoP/NCNHP and the compared samples in 0.5 M H2SO4, 1 M KOH for HER and 1 M KOH for OER with a scan rate of 5 mV·s-1 at room temperature, respectively. (c, f, i) LSV curves of CoP/NCNHP catalyst before and after 1000 CV cycles in the stability test over in 0.5 M H2SO4, 1 M KOH for HER and 1 M KOH for OER, respectively. The inset in (c), (f), and (i) shows the time-dependent of current density curves over CoP/NCNHP catalyst during electrolysis at -0.15, -0.12, and 1.55 V, respectively. Overall Water Splitting of CoP/NCNHP Catalyst. The above results indicate that CoP/NCNHP catalyst can serve as a bifunctional electrocatalyst for overall water splitting with superior activity and stability. Therefore, we evaluated its overall water splitting activity in 1 M KOH using twoelectrode configuration using CoP/NCNHP catalyst as both a cathode for HER and an anode for OER (inset of Figure 4a). The CoP/NCNHP catalyst exhibits superior overall water splitting performance with a cell voltage of 1.64 V (corre-

increment of current density, the voltage could stabilize rapidly. When the current density decreased to 20 mA·cm-2, the cell voltage was also restored well accordingly and stable rapidly. These electrolysis process could be sustained for at least 36 h, revealing the outstanding stability of the CoP/NCNHP catalyst for overall water splitting. DFT Calculation. For further insights on the excellent electrocatalytic performance of CoP/NCNHP hybrid catalyst, DFT calculations about the density of states (DOS), charge density distributions and H* adsorption free energies (∆GH*, * denotes an adsorption site) were performed by constructing theoretical models. The model of CoP/NCNHP hybrid catalyst was built by encapsulating CoP in N-doped graphene in comparison with pristine CoP. The DOS (Figure 4c) and charge transfer images (Figure 4d-e, Figure S25) of CoP/NCNHP hybrid catalyst shows that the electron transfer from NCNHP to CoP can increase the electronic states of Co d-orbital around the Fermi level. Pervious study indicated that Co d-orbital electrons make the major contribution to the H bonding formation on CoP (101) as the favorable binding site is the Co-Co bridge site.32 However, the weak H chemisorption of Co-Co bridge



could inhibit the higher HER activity. Therefore, the increased electronic states induced by NCNHP leads to the increase of the binding strength of H, improving the HER performance.33 Moreover, the NCNHP can also inhibited the surface oxidation of CoP. To confirm the inhibition effect, the ∆GH* on pristine CoP and surface-oxidized CoP were calculated (Figure S26). The ∆GH* is a reasonable descriptor of hydrogen evolution activity for HER catalysts.34, 35 For an ideal electrocatalyst, a moderate H adsorption free energy is close to zero (∆GH* ≈ 0) , which is in favor of the adsorption and desorption steps, thus facilitating the proton-electron-transfer process.36 The value of ∆GH* for CoP, surface-oxidized-50% CoP and surface-oxidized-100% CoP are -0.22, 0.29 and 0.42 eV, respectively. With the increase of oxidation degree, the HER activity decrease gradually. Therefore, combining the above evidences, the NCNHP can not only increase the HER activity, but also improve the stability.

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worked as the actual active species. As conductive support, CoP can provide an effective electron pathway to the active CoOOH shell which accelerated the whole OER process16a,37. (ii) The unique hollow polyhedron made of N-doped carbon nanotube of the CoP/NCNHP catalyst displayed a high surface area which can provide more active sites for the electrocatalytic process. The abundant pore structure promotes the contact between the CoP/NCNHP catalyst and electrolyte, leading to fast charge and mass transport during the electrolysis process20. The doping of N atoms into carbon frames brings additional defects active sites which is favor of the electrocatalysis38. (iii) The CoP nanoparticles were embedded in carbon matrix strongly, inhibiting the aggregation during electrocatalytic process and improving the stability. (iv) DFT calculation results further indicate that the electron transfer from NCNHP to CoP can increase the electronic states of Co d-orbital around the Fermi level, which can increase binding strength of H, thus improving the electrocatalytic performance. Overall, taking advantage of the synergetic effects of between highly active CoP NPs and the hollow polyhedron made of N-doped carbon nanotube, the CoP/NCNHP hybrid has been considered as a highly efficient and stable bifunctional electrocatalyst for water splitting.

CONCLUSION In summary, we elaborately designed a novel hybrid nanocomposite CoP/NCNHP through a pyrolysis-oxidationphosphidation strategy using core-shell structured ZIF8@ZIF-67 as precursors. The CoP/NCNHP hybrid catalyst exhibits superior bifunctional electrocatalytic performances. The needed overpotentials are 140 and 115 mV for HER in 0.5 M H2SO4 and 1 M KOH to reach the current density of 10 mA·cm-2, for OER in 1 M KOH the overpotential is 310 mV, and the catalyst could maintain excellent stability for both HER and OER. The needed cell potential is 1.64 V to achieve the current density of 10 mA·cm-2 when the CoP/NCNHP catalyst is employed as both anode and cathode for overall water splitting in a two-electrode cell. It also exhibits robust stability after continuously working for 36 h with nearly negligible decay in potential. Both experiments and DFT calculations demonstrated that the strong synergistic effect between highly active CoP NPs and the N-doped carbon nanotube hollow polyhedron effectively promotes the electrocatalytic performance. The current work shed a new insight into the design and synthesis of highly active, stable and novel structured TMPs/carbon catalyst for electrocatalysis applications.

Figure 4. (a) LSV curve of the CoP/NCNHP || CoP/NCNHP electrode in 1 M KOH at 5 mV·s-1 with iR compensation in a two-electrode system. (b) Chronopotentiometric curve of water electrolysis at different current densities in 1 M KOH. The inset in (a) is the digital photograph of the evolution of H2 and O2 gas from the electrodes during electrolysis. (c) Calculated DOS curves for pure CoP and CoP/NCNHP. The charge density distribution images of CoP/NCNHP: (d) Top and (e) side view. (f) The calculated free-energy diagram of the HER on CoP, surface-oxidized-50% CoP, surface-oxidized-100% CoP, respectively. The above results suggest that the CoP/NCNHP catalyst is a promising bifunctional electrocatalyst for water splitting. The superior electrocatalytic activity and stability can be attributed to the following considerations: (i) XPS and XANES results indicate that the Co and P species in CoP/NCNHP catalyst have a partial positive charge (δ+) and a negative charge (δ-), indicating the covalent nature of the Co-P bond, which is similar to those of proton acceptors and hydride acceptors in hydrogenase, improving the HER activity of CoP/NCNHP catalyst28. The superior OER activity of the CoP/NCNHP for OER may be consistent with the reported mechanism. Co atoms on the surface of the catalyst were ﬁrst partially oxidized to CoOOH, and the formed CoP/CoOOH core-shell structure

EXPERIMENTAL SECTION Synthesis of ZIF-8. For the synthesis of ZIF-8, Zn(NO3)2·6H2O (5.95 g, 0.02 mol) and 2-methylimidazole (2MeIm, 6.16g, 0.075 mol) were dissolved in 150 mL methanol (MeOH) to form two clear solution, respectively. Then, the solution of 2-MeIm was subsequently poured into the solution of Zn(NO3)2·6H2O. After mixing and stirring at room temperature for 24 h, the white precipitates were centrifuged and washed with MeOH several times and dried at 60 °C. Synthesis of Core-Shell ZIF-8@ZIF-67. For the synthesis of ZIF-8@ZIF-67, the ZIF-8 (0.5 g) was dispersed in 100 mL MeOH to form a mixture solution. Co(NO3)2·6H2O (5.82


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g, 0.02 mol) and 2-MeIm (6.16g, 0.075 mol) were dissolved in 100 mL MeOH to form two clear solution, respectively. Then, the solution of Co(NO3)2·6H2O was subsequently poured into the mixture solution of ZIF-8, and the solution of 2-MeIm was further poured into the above mixture solution. After mixing and stirring at room temperature for 24 h, the light purple precipitates were centrifuged and washed with MeOH several times and dried at 60 °C.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions ∆ Y. P., K. S. and S. L. contributed equally.

Notes The authors declare no competing financial interests.

Synthesis of Co/NCNHP. The ZIF-8@ZIF-67 was placed in a tube furnace and heated to 920 °C with a ramp rate of 2 °C·min-1 and kept for 3 h in flowing Ar. The obtained black powders were washed in 0.8 M H2SO4 solution at room temperature for 12 h to remove any accessible Co species on its surface, yielding Co/NCNHP.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21573119, 21590792, 21521091, 21390393, U1463202), China Ministry of Science and Technology under Contract of 2016YFA (0202801) and China Postdoctoral Science Foundation (2017M610076). We thank the 1W1B station for XAFS measurement in Beijing Synchrotron Radiation Facility (BSRF).

Synthesis of Co3O4/NCNHP. The Co/NCNHP was placed in a tube furnace and heated to 350 °C with a ramp rate of 2 °C·min-1 and kept for 4 h in Air to yield Co3O4/NCNHP.

REFERENCES

Synthesis of CoP/NCNHP. The Co3O4/NCNHP (20 mg) was placed at the middle of a porcelain boat and 1.0 g of NaH2PO2 was placed at the upstream side. The porcelain boat was put in a tube furnace and heated to 300 °C with a ramp rate of 2 °C·min-1 and kept for 2 h in Ar to yield CoP/NCNHP.

(1) a) Wang, J; Xu, F.; Jin, H.; Chen, Y.; Wang, Y. Adv. Mater. 2017, 29, 1605838; b) Kuang M.; Wang Q.; Han P.; Zheng G. Adv. Energy Mater. 2017, 7, 1700193; c) Yang H. B.; Miao J.; Hung S. F.; Chen J.; Tao H. B.; Wang X.; Zhang L.; Chen R.; Gao J.; Chen H. M.; Dai L.; Liu B. Sci. Adv. 2016, 2, e1501122. (2) Gray, H. B. Nat. Chem. 2009, 1, 7. (3) Wu, H. B.; Xia, B. Y.; Yu, L.; Yu, X. Y.; Lou, X. W. Nat. Commun. 2015, 6, 6512. (4) a) Zhang, C.; Antonietti, M.; Fellinger, T. P. Adv. Funct. Mater. 2014, 24, 7655-7665; b) Tao H. B.; Fang L.; Chen J.; Yang H. B.; Gao J.; Miao J.; Chen S.; Liu B. J. Am. Chem. Soc. 2016, 138, 99789985; c) Wang H. Y.; Hung S. F.; Chen H. Y.; Chan T. S.; Chen H. M.; Liu B. J. Am. Chem. Soc. 2016, 138, 36-39. (5) Liu, D.; Lu, Q.; Luo, Y.; Sun, X.; Asiri, A. M. Nanoscale 2015, 7, 15122-15126. (6) Sun, C.; Dong, Q.; Yang, J.; Dai, Z.; Lin, J.; Chen, P.; Huang, W.; Dong, X. Nano Res. 2016, 9, 2234-2243. (7) Chen, W. F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Angew. Chem. Int. Ed. 2012, 51, 6131-6135. (8) a) Li, Y.; Zhang, H.; Jiang, M.; Zhang, Q.; He, P.; Sun, X. Adv. Funct. Mater. 2017, 27, 1702513; b) Zhao, Z.; Schipper, D. E.; Leitner, A. P.; Thirumalai, H.; Chen, J.; Xie, L.; Qin, F.; Alam, M. K.; Grabow, L. C.; Chen, S.; Wang, D.; Ren, Z.; Wang, Z.; Whitmire, K. H.; Bao, J. Nano Energy 2017, 39, 444; c) Yan, L.; Cao, L.; Dai, P.; Gu, X.; Liu, D.; Li, L.; Wang, Y.; Zhao, X. Adv. Funct. Mater. 2017, 27, 1703455; d) Pu Z.; Amiinu I. S.; Kou Z.; Li W.; Mu S. Angew. Chem. Int. Ed. 2017, 56, 11559-11564. (9) a) Jiang, P.; Liu, Q.; Liang, Y.; Tian, J.; Asiri, A. M.; Sun, X. Angew. Chem. Int. Ed. 2014, 53, 12855-12859. b) Callejas, J. F.; McEnaney, J. M.; Read, C. G.; Crompton, J. C.; Biacchi, A. J.; Popczun, E. J.; Gordon, T. R.; Lewis, N. S.; Schaak, R. E. ACS Nano 2014, 8, 11101-11107. (10) a) Huang, Z.; Chen, Z.; Chen, Z.; Lv, C.; Humphrey, M. G.; Zhang, C. Nano Energy 2014, 9, 373-382; b) Pan, Y.; Lin, Y.; Chen, Y.; Liu, Y.; Liu, C. J. Mater. Chem. A 2016, 4, 4745-4754; c) Huang, J.; Li, Y.; Xia, Y.; Zhu, J.; Yi, Q.; Wang, H.; Xiong, J.; Sun, Y.; Zou, G. Nano Res. 2017, 10, 1010-1020. (11) a) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. J. Am. Chem. Soc. 2013, 135, 9267-9270; b) Wang, S.; Zhang, L.; Li, X.; Li, C.; Zhang, R.; Zhang, Y.; Zhu, H. Nano Res. 2017, 10, 415-425. (12) Tian, J.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. Angew. Chem. Int. Ed. 2014, 53, 9577-9581. (13) a) Xiao, P.; Sk, M. A.; Thia, L.; Ge, X.; Lim, R. J.; Wang, J. Y.; Lim, K. H.; Wang, X. Energy Environ. Sci. 2014, 7, 2624-2629; b)

Materials Characterization. XRD was carried out with a Rigaku D/max 2500Pc X-ray powder diffractometer with monochromatized Cu Kα radiation (λ =1.5418 Å). SEM was performed on a Hitachi S-4800 electron microscope. TEM was operated by a Hitachi-7700 working at 100 kV. HRTEM and HAADF-STEM were carried out by a JEOL JEM-2100F field emission electron microscope working at 200 kV. XPS was performed on a ULVAC PHI Quantera microscope. N2 adsorption-desorption experiments were carried out at 77 K on a Quantachrome SI-MP Instrument. XAFS spectra were obtained at 1W1B station in BSRF (Beijing Synchrotron Radiation Facility, P. R. China) operated at 2.5 GeV with a maximum current of 250 mA. XAS measurements at the Co Kedge were performed in fluorescence mode using a Lytle detector. All samples were pelletized as disks of 13 mm diameter with 1 mm thickness using graphite powder as a binder. Electrochemical tests. All the measurements were carried out on a CHI 760E electrochemical workstation (CH Instruments, Inc., Shanghai) with a standard three-electrode setup. A glassy carbon electrode (GCE, 4 mm in diameter) used as the support for the working electrode. The Ag/AgCl electrode in saturated KCl and graphite rod were served as the reference electrode and counter electrode, respectively. The HER performance was evaluated in N2-saturated 0.5 M H2SO4 and 1 M KOH solutions, respectively. The OER performance was evaluated in O2-saturated 1 M KOH solution. The detailed test method was shown in Supporting Information.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental procedures; DFT calculations methods, and additional figures and tables (PDF).



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(26) a) Chen, Y. M.; Li, X. Y.; Park, K.; Song, J.; Hong, J. H.; Zhou, L. M.; Mai, Y. W.; Huang, H. T.; Goodenough, J. B. J. Am. Chem. Soc. 2013, 135, 16280-16283; b) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. Nat. Energy 2016, 1, 15006; c) Huang, X. K.; Cui, S. M.; Chang, J. B.; Hallac, P. B.; Fell, C. R.; Luo, Y. T.; Metz, B.; Jiang, J. W.; Hurley, P. T.; Chen, J. H. Angew. Chem. Int. Ed. 2015, 54, 1490-1493. (27) Ha, D. H.; Moreau, L. M.; Bealing, C. R.; Zhang, H.; Hennig, R. G.; Robinson, R. D. J. Mater. Chem. 2011, 21, 11498-11510. (28) a) Liu, Z. Q.; Cheng, H.; Li, N.; Ma, T. Y.; Su, Y. Z. Adv. Mater. 2016, 28, 3777-3784; b) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Angew. Chem. Int. Ed. 2014, 53, 6710-6714. (29) Jin, H.;Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. J. Am. Chem. Soc. 2015, 137, 2688-2694. (30) Wang, J.; Ji, L.; Chen, Z. ACS Catal. 2016, 6, 6987-6992. (31) a) Kong S.; Wang H.; Lu Z.; Cui Y. J. Am. Chem. Soc. 2014, 136, 4897-4900; b) Staszak-Jirkovsky J.; 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. Nat. Mater. 2016, 15, 197-203. (32) Tang, C.; Gan, L.; Zhang, R.; Lu, W.; Jiang, X.; Asiri, A. M.; Sun, X.; Wang, J.; Chen, L. Nano Lett. 2016, 16, 6617-6621. (33) Zhou, L.; Shao, M.; Li, J.; Jiang, S.; Wei, M.; Duan, X. Nano Energy, 2017, 41, 583-590. (34) Skúlason, E.; Karlberg, G. S.; Rossmeisl, J.; Bligaard, T.; Greeley, J.; Jónsson, H.; Nørskov, J. K. Phys. Chem. Chem. Phys. 2007, 9, 3241-3250. (35) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. J. Am. Chem. Soc. 2005, 127, 5308-5309. (36) Greeley, J.; Nørskov, J. K. Surf. Sci. 2007, 601, 1590-1598. (37) He P.; Yu X. Y.; (David) Lou X. W. Angew. Chem. 2017, 129, 3955-3958. (38) a) Fei H.; Dong J.; Arellano-Jiménez M. J.; Ye G.; Kim N. D.; Samuel E. L. G.; Peng Z.; Zhu Z.; Qin F.; Bao J.; Yacaman M. J.; Ajayan P. M.; Chen D.; Tour J. M. Nat. Commun. 2015, 6, 8668; b) Zhuang M.; Ou X.; Dou Y.; Zhang L.; Zhang Q.; Wu R.; Ding Y.; Shao M.; Luo Z. Nano Lett. 2016, 16, 4691.

Yang, J.; Zhang, F.; Wang, X.; He, D.; Wu, G.; Yang, Q.; Hong, X.; Wu, Y.; Li, Y. Angew. Chem. Int. Ed. 2016, 55, 12854-12858. (14) a) Du, H.; Gu, S.; Liu, R.; Li, C. M. J. Power Sources 2015, 278, 540-545; b) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X. ACS Catal. 2015, 5, 145-149. (15) Liu, P.; Rodriguez, J. A. J. Am. Chem. Soc. 2005, 127, 1487114878. (16) a) Zhang, G.; Wang, G.; Liu, Y.; Liu, H.; Qu, J.; Li, J. J. Am. Chem. Soc. 2016, 138, 14686-14693; b) Zhu, Y. P.; Liu, Y. P.; Ren, T. Z.; Yuan, Z. Y. Adv. Funct. Mater. 2015, 25, 7337-7347. (17) Song, J.; Zhu, C.; Xu, B. Z.; Fu, S.; Engelhard, M. H. Ye, R.; Du, D.; Beckman, S. P.; Lin, Y. Adv. Energy Mater. 2016, 7, 1601555. (18) a) Liang, H. W.; Bruller, S.; Dong, R.; Zhang, J.; Feng, X. L. Nat. Commun. 2015, 6, 7992; b) Strickland, K.; Miner, E.; Jia, Q. Y.; Tylus, U.; Ramaswamy, N.; Liang, W. T.; Sougrati, M. T.; Jaouen, F.; Mukerjee, S. Nat. Commun. 2015, 6, 7343. (19) Kuo C. H.; Tang Y.; Chou L. Y.; Sneed B. T.; Brodsky C. N.; Zhao Z.; Tsung C. K. J. Am. Chem. Soc. 2012, 134, 14345-14348. (20) a) Liu, Z. Q.; Cheng, H.; Li, N.; Ma, T. Y.; Su, Y. Z. Adv. Mater. 2016, 28, 3777-3784; b) Peng, S.; Li, L.; Han, X.; Sun, W.; Srinivasan, M.; Mhaisalkar, S. G.; Cheng, F.; Yan, Q.; Chen, J.; Ramakrishna, S. Angew. Chem. Int. Ed. 2014, 53, 12594-12599. (21) Yin, P.; Yao, T.; Wu, Y.; Zheng, L.; Lin, Y.; Liu, W.; Ju, H.; Zhu, J.; Hong, X.; Deng, Z.; Zhou, G.; Wei, S.; Li, Y. Angew. Chem. Int. Ed. 2016, 55, 10800-10805. (22) a) Tang, J.; Salunkhe, R. R.; Liu, J.; Torad, N. L.; Imura, M.; Furukawa, S.; Yamauchi, Y. J. Am. Chem. Soc. 2015, 137, 15721580; b) Chen, Y. Z.; Wang, C.; Wu, Z. Y.; Xiong, Y.; Xu, Q.; Yu, S. H.; Jiang, H. L. Adv. Mater. 2015, 27, 5010-5016; c) Liu, S.; Wang, Z.; Zhou, S.; Yu, F.; Yu, M.; Chiang, C. Y.; Zhou, W.; Zhao, J.; Qiu, J. Adv. Mater. 2017, 29, 1700874. (23) Park, K. S.; Ni, Z.; Côte, A. P.; Choi, J. Y.; Huang, R. D.; UribeRomo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 10186-10191. (24) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Science 2008, 319, 939-943. (25) a) Hu, L.; Huang, Y. M.; Zhang, F. P.; Chen, Q. W. Nanoscale 2013, 5, 4186-4190; b) Wu, H. B. Wei, S.; Zhang, L.; Xu, R.; Hng, H. H.; Lou, X. W. Chem. Eur. J. 2013, 19, 10804-10808.


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Scheme 1. Schematic illustration of the synthesis process of CoP/NCNHP. 47x12mm (300 x 300 DPI)

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Figure 1. (a) XRD patterns of the Co/NCNHP, Co3O4/NCNHP and CoP/NCNHP catalysts. (b) SEM, (c) TEM, (d) HRTEM, (e) SAED, (f) HAADF-STEM elemental mapping of CoP/NCNHP catalyst. The inset in (b): the magnified SEM image. The inset in (d): the fast Fourier transformation of the selected area in red box. 85x86mm (600 x 600 DPI)



Figure 2. (a) Co 2p, (b) P 2p, and (c) N 1s spectra of CoP/NCNHP catalyst. (d) XANES spectra and (e) Fourier transform (FT) at the Co K-edge of CoP/NCNHP, CoP bulk, CoO, Co3O4 samples and Co foil. (f) N2 adsorption-desorption isotherm of CoP/NCNHP catalyst. The inset in (f) is the correspond-ing micropore (left) and mesoporous (right) size distribution curves. 111x70mm (600 x 600 DPI)


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Figure 3. (a, d, g) LSV and (b, e, h) Tafel curves of CoP/NCNHP and the compared samples in 0.5 M H2SO4, 1 M KOH for HER and 1 M KOH for OER with a scan rate of 5 mV·s-1 at room temperature, respectively. (c, f, i) LSV curves of CoP/NCNHP catalyst before and after 1000 CV cycles in the stability test over in 0.5 M H2SO4, 1 M KOH for HER and 1 M KOH for OER, respectively. The inset in (c), (f), and (i) shows the timedependent of current density curves over CoP/NCNHP catalyst during electrolysis at -0.15, -0.12, and 1.55 V, respectively. 120x81mm (600 x 600 DPI)



Figure 4. (a) LSV curve of the CoP/NCNHP || CoP/NCNHP elec-trode in 1 M KOH at 5 mV·s-1 with iR compensation in a two-electrode system. (b) Chronopotentiometric curve of water electrolysis at different current densities in 1 M KOH. The inset in (a) is the digital photograph of the evolution of H2 and O2 gas from the electrodes during electrolysis. (c) Calculated DOS curves for pure CoP and CoP/NCNHP. The charge density distribution images of CoP/NCNHP: (d) Top and (e) side view. (f) The calculated free-energy diagram of the HER on CoP, surface-oxidized-50% CoP, sur-face-oxidized-100% CoP, respectively. 72x63mm (600 x 600 DPI)


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