NiCo2O4-rGO as Highly Efficient

Subscriber access provided by University of Sussex Library

Article

MOF-Derived NiO/NiCo2O4 and NiO/NiCo2O4-rGO as Highly Efficient and Stable Electrocatalysts for Oxygen Evolution Reaction Yanying Wang, Zhaoyi Zhang, Xin Liu, Fang Ding, Ping Zou, Xianxiang Wang, Qingbiao Zhao, and Hanbing Rao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03221 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 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 52 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 Sustainable Chemistry & Engineering

MOF-Derived NiO/NiCo2O4 and NiO/NiCo2O4-rGO as Highly Efficient and Stable Electrocatalysts for Oxygen Evolution Reaction

Yanying Wang1 †, Zhaoyi Zhang1 †, Xin Liu1 †, Fang Ding2, Ping Zou1, Xianxiang Wang1, Qingbiao Zhao3*, Hanbing Rao1* 1

College of Science, Sichuan Agricultural University, Xin Kang Road, Yucheng

District, Ya’an 625014, China, P.R.China 2

Nanshan District Key Lab for Biopolymers and Safety Evaluation, Shenzhen Key

Laboratory of Polymer Science and Technology, Guangdong Research Center for Interfacial Engineering of Functional Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P.R. China 3

Key Laboratory of Polar Materials and Devices, Ministry of Education, Department

of Electronic Engineering, East China Normal University, Shanghai, 200241, P.R. China

†These authors contributed equally to the work. *Corresponding

authors

Qingbiao Zhao E-mail addresses: [email protected] Hanbing Rao E-mail addresses: [email protected]

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

ABSTRACT We developed a hydrothermal synthesis and high temperature calcination method to synthesize the metal-organic framework (MOF) derived NiCo2O4/NiO and NiCo2O4/NiO-rGO. These composites were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Brunner-Emmet-Teller measurements (BET). This work combines excellent catalytic efficiency of the transition metal with large specific surface area and the large number of pores of the MOF. Consequently, catalysts for oxygen evolution with high efficiency and low cost were obtained. Both materials show lower overpotential in 430 mV and 350 mV under 10 mA·cm-2. The Tafel slopes are 49 mV·dec-1 and 66 mV·dec-1 with very small performance attenuation for long-term catalytic reaction. Therefore, the catalysts reported herein possess great potential for application in oxygen evolution reaction.

Key words: MOF-derived NiO/NiCo2O4, oxygen evolution reaction, electrocatalysis, metal-organic frameworks, energy conversion


Page 2 of 52

Page 3 of 52 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


INTRODUCTION In recent years, the increasing energy crisis and environmental pollution have drawn more and more attention and demand novel solutions.1, 2 Traditional energy (mostly fossil fuels) is mainly made of hydrocarbons, which generates large amounts of carbon dioxide, carbon monoxide, nitrogen oxides, sulfur oxides and respirable suspended particulates. Therefore, it is necessary to develop low-emission clean energy to ease the upcoming energy crisis and promote environmental protection.3 Among them, hydrogen energy is considered to be an alternative with great potential. Hydrogen energy possesses multiple advantages, including good combustion performance, high productivity, recyclability and generating water as the combustion product.4 5, 6 A key semi-reaction in electrochemical hydrolysis is the oxygen evolution reaction (OER). For OER, there is a high activation barrier in the transfer of four electrons and four protons, which is the bottleneck in the conversion of water into O2 and H2.7,

8

Ruthenium dioxide (RuO2) and iridium dioxide (IrO2) have been

extensively studied as OER catalysts. However, the high cost and scarcity of the noble metals have limited their application. Additionally, the instability of their catalytic performance makes them unsuitable for large-scale production.9 To solve this problem, large amount of efforts have been made for the development of efficient, durable and low cost alternatives by using electrocatalysts based on non-noble metal, such as transition metal oxides,10 sulphides,11,

12

phosphides,13,

14

borates,15,

16

and metal

alloys.17, 18 Recently, metal-organic frameworks (MOFs) have emerged as a new type of porous materials with metal ions as the center combined with organic ligands. Due to the high specific surface areas, adjustable pores and good chemical tunability, they are widely applied in many fields including gas storage, catalysis, sensing and separation. Compared with traditional synthesis methods, more synthetic strategies are used to synthesize MOFs, such as DNA-templated synthesis19 and electrochemical synthesis20. MOFs derived metal oxides generally exhibit features such as large surface area,



graded pore size, and hydrophobic surfaces.21, 22 MOFs derived metal oxides take advantage of the synergistic effects from the two types of functional components, thus they not only retain the excellent structural characteristics of the framework materials, but also can possess excellent conductive and catalytic properties23, 24. Among the transition metal oxides, NiCo2O4, has a promising application prospect in the fields of lithium ion batteries25, supercapacitors26and electrocatalysts27 and optoelectronic devices. NiCo2O4 has an AB2O4 spinel structure where Ni cations occupy octahedral sites and Co cations are uniformly distributed between octahedral and tetrahedral sites.28 For the metal electrodes and alloy electrodes, the surface will produce a layer of metal oxides after being used for a long time. The oxide layer will increase the internal resistance of the electrode, affecting the charge transfer efficiency and leading to decrease in the catalytic performance. The spinel transition metal oxide, which is a ternary oxide, as the electrode material can avoid this situation. Furthermore, the catalytic efficiency of the complex oxides of spinel typically exceeds those of the binary oxides. Additionally, NiCo2O4 is inexpensive and convenient to synthesize. Thus, it is expected to be an excellent material to replace the noble metal based catalyst for water electrolysis. In this study, we developed a feasible method to prepare MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO by hydrothermal synthesis and high-temperature calcination (Scheme 1). Both the good catalytic properties of metal oxides and the large specific surface area of the MOF contribute to its excellent performance in OER catalytic reactions. Therefore, the composites of MOF-derived NiCo2O4/NiO with MOFs and MOF-derived NiCo2O4/NiO-rGO possess high potential for application in the field of hydrogen production.


Page 4 of 52

Page 5 of 52 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


Scheme. 1 Schematic illustration for the preparation process of MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO.

EXPERIMENTAL SECTION Materials Graphite powder (99.85%) was purchased from KeLong. Ni(NO3)2·6H2O and Co(NO3)2·6H2O were purchased from Aladdin Co. Ltd. KMnO4 (99.0%), H2SO4 (98.0%), KOH (85.0%), NaNO3 (84.99%), Methanol (99.5%) and DMF (99.5%) were obtained from KeLong. 2,5-Dihydroxyterephthalic acid (2.5-DHTA 97%) was



supplied by Bide Pharmatech Ltd. All chemicals were used as received. Deionized water (18.25 MΩ·cm) was used throughout this study. Preparation of graphene oxide Graphene oxide was prepared using the Hummers method.29 Specifically, the graphite powder and NaNO3 were dispersed in concentrated sulfuric acid with continuous stirring with the reaction temperature maintained below 20°C. The mixture was maintained at 5°C for 1 hour after KMnO4 was added. Afterwards, the mixture was transferred to a water bath and stirred for 30 minutes at 35°C. Then, deionized water was slowly added. Finally, the reaction was maintained at 98°C for 15 minutes. Distilled water and 30% hydrogen peroxide were added, and a golden suspension formed. The product was washed several times with 5% HCl under centrifugation, then washed with deionized water until the solution became neutral. The final product was collected and dried under vacuum for 24 hours, and graphene oxide was obtained. Synthesis of Ni-MOF74 and Ni-MOF74/GO composite Ni-MOF-74 was prepared according to previously reports.30 0.2216 g 2-5-dihydroxyterephthalic acid and 1.077 g Ni(NO3)2·6H2O were added to a 100 ml tetrafluoroethylene lined stainless steel autoclave charged with 25 mL deionized water, 25 mL ethanol and 25 mL DMF. The mixture was stirred at room temperature and the reaction was carried out at 120°C for 24 hours. When the reaction completed, the solid was centrifuged and washed several times with methanol and DMF. Finally, the collected product was placed in a vacuum drying oven and dried at 80°C for 24 hours. The resulting yellow powder was Ni-MOF74. The synthesis procedure for the Ni-MOF74/GO composite was similar to that of Ni-MOF74. The only difference was replacing the 25 mL deionized water with 10 mg graphene oxide dispersed in 25 ml deionized water. Synthesis of MOF-derived NiCo2O4/NiO and MOF- derived NiCo2O4/NiO-rGO The prepared Ni-MOF74 or Ni-MOF74 GO (1.0 g) was dispersed in 25 ml DMF, then Co(NO3)2·6H2O (0.556 g) was added and stirred at room temperature for 10 minutes. The mixture was heated at 80 °C with an oil bath for 2 hours under stirring. ACS Paragon Plus Environment

Page 6 of 52

Page 7 of 52 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


Then the solid product was separated with centrifugation and was washed several times with methanol and DMF. The separated products were dried in a vacuum drying oven for 24 hours at 80°C. Finally, MOF-derived NiCo2O4/NiO or MOF-derived NiCo2O4/NiO-rGO were obtained. Characterization The XRD pattern was collected with a D/Max-RA diffractometer (DX-2700, Dan Dong, China) with Cu Kα-radiation (λ = 0.1548 nm), which was operated at 40 kV and 100 Ma. JSM4800F (JEOL, Japan) was used for SEM characterization. JEOL2100F (JEOL, Japan) was used for TEM characterization, XPS spectrums were collected with ESCALAB 250Xi (Boyue, Shanghai, China). Electrochemical measurement All

electrochemical

measurements

were

performed

on

a

CHI760E

electrochemical workstation with a three-electrode method, for which a platinum (Pt) plate served as the counter electrode and a Ag/AgCl electrode was the reference electrode. The electrolyte was 1.0 M KOH aqueous solution. The current density was normalized to the geometrical area. The measured potential versus Ag/AgCl was performed with iR compensation enabled, and was converted to a reversible hydrogen electrode (RHE) scale according to the Nernst equation: ERHE = EAg/AgCl + 0.059 pH+ 0.199 Typically, 4 mg of catalyst powder, 1 mL of solvent (water and ethanol, 4:1, v/v) and 80 µL of Nafion solution (5 wt% in water) were mixed. The suspension was immersed in an ultrasonic bath for 30 minutes, and a homogeneous ink was generated. The working electrode was prepared by depositing 5 µL of catalyst ink onto a glass carbon electrode. The onset potentials were determined from the intersection of the tangents of OER current (J = 1.0 mA·cm−2) and the polarization curve baseline. Cyclic voltammetry was run for 50 cycles at 50 mV·s−1 to stabilize the system before data collection. Linear sweep voltammetry (LSV) of the catalyst was performed at a scan rate of 5 mV·s−1, while Tafel plot was conducted at a slow rate of 1 mV·s−1. Electrochemical impedance spectroscopies were carried out at 1.5 versus RHE for OER, where a frequency ranged from 105 to 0.01 Hz with a 5 mV AC dither was used. ACS Paragon Plus Environment


Cyclic voltammetry method was used to determine the electrochemical double-layer capacitances (Cdl). Electrochemically active surface area was evaluated from the slope of the plot of the charging current versus the scan rate, which was directly proportional to Cdl.

RESULTS AND DISCUSSION Figure 1(A) shows the XRD pattern of Ni-MOF74 and Ni-MOF74/GO composite. The crystallinity of Ni-MOF74 is confirmed by XRD analysis. The XRD pattern of Ni-MOF74 and the Ni-MOF74/GO composite are consistent with the the calculated XRD pattern of MOF74 structure, indicating that the topological structure is retained and both are isomorphic.31 As shown in Figure 1(B), the same diffraction peaks can be obtained in the XRD peak of MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO. One group of peaks were indexed to (200), (220), (311) of cubic NiO (JCPDS standard ratio 73-1532); another group of peaks were indexed to (111), (220), (311), (222), (400), (422), (511), and (440) planes of NiCo2O4 (JCPDS standard comparison card 73-1702). The XRD patterns indicate the formation of the mixed MOF-derived NiCo2O4/NiO phases.

Figure. 1 (A) The XRD patterns of Ni-MOF74(a) and Ni-MOF74-rGO(b) (B) The XRD patterns of MOF-derived NiCo2O4/NiO(a) and MOF-derived NiCo2O4/NiO-rGO(b).

The morphologies and structures were characterized by SEM and TEM. Figure 2 shows the SEM images of Ni-MOF74, Ni-MOF74/GO composite, MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO. As shown in Figure 2(A) and


Page 8 of 52

Page 9 of 52 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


(B), the morphology of a Ni-MOF74 crystallite is similar to a shuttle, and many crystals form a flower-like cluster by accumulation. The topological structure of the crystal is close to the ETO topology structure of MOF-2.32 In contrast to Figure 2(A) and (B), the flaky structure on the Ni-MOF74 crystal can be clearly observed in Figure 2(C) and (D). The formation of this structure is due to the derivative structure produced by the addition of graphene oxide in the synthesis of MOF. The good mechanical properties and large specific surface area of graphene oxide can enhance the adsorption and catalytic properties of Ni-MOF74. A large number of cobalt ion are incorporated into the surface and interior of the framework, due to the large pore size of the Ni-MOF74 and many unsaturated metal sites (Figure 2(E) and (F)). These cobalt ions react with the nickel ions in the MOF under calcination, with 2,5-dihydroxyterephthalic acid producing free hydroxyl groups during the calcination process, thus MOF-derived NiCo2O4/NiO composites is generated. Due to the growth of the NiCo2O4 crystal and the cracking of the MOF structure at high temperature, the urchin-like MOF-derived NiCo2O4/NiO ball structure shown in Figure 2(E) formed on the basis of the original framework. The rod-like structure of MOF-derived NiCo2O4/NiO can be clearly observed in Figure 2(F). As shown in Figure 2(G) and (H), due to the addition of graphene oxide, the cobalt ions originally bound to the interior of the framework are largely anchored by the oxygen-containing functional groups on the graphene oxide surface，thus many MOF-derived NiCo2O4/NiO particles were grown on the graphene oxide sheets. Therefore, MOF-derived NiCo2O4/NiO mostly formed on the graphene oxide sheet and the surface of the framework during the calcination, and the original Ni-MOF74 structure was retained.



Figure 2 SEM images of (A)-(B)Ni-MOF74, (C)-(D)Ni-MOF74-rGO, (E)-(F)MOF-derived NiCo2O4/NiO and (G)-(H)MOF-derived NiCo2O4/NiO-rGO.

Figure 3(A), shows the SEM image of a large cluster (~5 µm in diameter) composed of many shuttle-like Ni-MOF74 crystals, with a few partial bulk crystals without full growth. Figure 3(B) shows the morphology of the graphene oxide layer. Since graphene oxide is grown on the surface of the crystal, the aggregation between the multiple crystals is weak, and the crystal cluster is larger. A MOF-derived NiCo2O4/NiO microsphere is shown in Figure 3(C). In accordance with previous SEM image, it also has a rod-like structure. Since the MOF-derived NiCo2O4/NiO microsphere does not have a preferred growth orientation, an urchin-like spherical structure eventually formed. Compared with Figure 3(B), Figure 3(D) shows that the area covered by graphene oxide is reduced due to the partial thermal decomposition during the calcination process. In addition, a large amount of nickel-cobalt precursor is anchored on the surface of graphene, and they formed oxides during the calcination, which were bonded to the surface of the framework structure. Because of the adhesion of graphene, the framework structure is basically preserved, and many free MOF-derived NiCo2O4/NiO particles or free MOF-derived NiCo2O4/NiO-rGO sheet fragments can be observed.


Page 10 of 52

Page 11 of 52 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


Figure. 3. TEM images of (A)Ni-MOF74, (B)Ni-MOF74-rGO, (C)MOF-derived NiCo2O4/NiO and (D)MOF-derived NiCo2O4/NiO-rGO.

Figure 4(A) shows a crystal of Ni-MOF74 with a size of about 500 nm and surrounded by some partially grown MOF crystals. As shown in Figure 4(B), there are some MOF crystals and graphene oxide grown on them, also with some small MOF crystals on the graphene oxide. This is possibly due to that during the reaction, a small amount of ligands were bound to the graphene. Some of the crystals collapsed during the growth process. The hydroxyl groups contained on the detached small crystals were non-covalently bound to the functional groups on the graphene oxide surface and were therefore adsorbed on graphene. Figure 4(C) shows the rod-like structure composed of MOF-derived NiCo2O4/NiO particles ranging from 10 to 20 nm in size. There are also carbon fibers formed from organic framework ligands that were pyrolyzed due to high-temperature calcination. A large amount of MOF-derived



NiCo2O4/NiO particles generated in calcination appeared, as shown in Figure 4(D). Since the MOF-derived NiCo2O4/NiO particles mostly grew on the surface of the MOF and the graphene, the framework structure was preserved. Additionally, the outer framework and graphene oxide sheets contain more particles, while the middle parts of the framework contain less, thus it can be inferred that the surface is nickel cobalt oxide/nickel oxide particles, and the interior is mostly a carbon skeleton formed from the organic framework.

Figure 4. 200 nm TEM images of (A)Ni-MOF74, (B)Ni-MOF74-rGO, (C)MOF-derived NiCo2O4/NiO and (D)MOF-derived NiCo2O4/NiO-rGO.

The lattice fringes of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO crystals were characterized with HRTEM (Figure 5A and 5B). Four distinct lattice fringes can be identified in Figure 5A. The fringes with inter-layer


Page 12 of 52

Page 13 of 52 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


distances of 0.209 and 0.241 nm correspond to the d-spacing of the (200) and (111) planes of cubic NiO, while the fringes with inter-layer distances of 0.244 and 0.468 nm correspond to the (311) and (111) planes of NiCo2O4. In Figure 5B, five distinct lattice fringes can be identified. The fringes with inter-layer distances of 0.209 and 0.241 nm correspond to the d-spacing of the (200) and (111) planes of cubic NiO, and the fringes with inter-layer distances of 0.234, 0.244 and 0.468 nm correspond to the (311), (222) and (111) planes of cubic NiCo2O4.33 These data are consistent with those from XRD patterns.

Figure 5. Structure characterization of (A) and (B) HRTEM image, (C) and (D) EDS-elemental mapping of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO, respectively.

Elemental mapping was used to study the MOF-derived NiCo2O4/NiO and ACS Paragon Plus Environment


MOF-derived NiCo2O4/NiO-rGO materials (Figure. 6). The elemental mapping of MOF-derived NiCo2O4/NiO demonstrates the uniform distribution of Ni and Co. In the MOF-derived NiCo2O4/NiO-rGO, Ni is mostly located in the framework structure, whereas Co is largely distributed in the graphene oxide sheets. These results indicate that the two types of metal oxides (NiCo2O4 and NiO) are seamlessly integrated at atomic scale.

Figure 6. EDS-elemental mapping of (A) and (E) Ni, (B) and (F) O, (C) and (G) Co, (D) and (H) C of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO, respectively

X-ray photoelectron spectroscopy (XPS) was used to study MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGOO. As shown in Figure 7(A), the XPS spectra show that both of them contain the elements of nickel, cobalt, carbon, nitrogen, and oxygen. The XPS spectra were analysed with a Gaussian fitting method. The Ni2p spectra in MOF-derived NiCo2O4/NiO show fitting peaks at 854. 0 eV and 871.80 eV that correspond to the Ni2+ ions located in octahedral sites, and the fitting peaks at 855.70 eV and 873.40 eV correspond to Ni3+ ions located in tetrahedral sites. The two satellite broad peaks were attributed to the presence of both Ni2+ and Ni3+ ions. The Ni2p spectra in MOF-derived NiCo2O4/NiO-rGO sheets correspond to Ni2+ and Ni3+ at 853.9 and 871.4 eV and 855.6 and 873.0 eV, respectively34(Figure 7(B)and(C)). In the Co2p spectra, two types of Co species were observed in NiCo2O4/NiO and assigned to the species containing Co2+ and Co3+. The fitting peaks ACS Paragon Plus Environment

Page 14 of 52

Page 15 of 52 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


at 779.70 and 794.60 eV were attributed to Co3+, while the other two fitting peaks at 780.80 and 796.00 eV were assigned to Co2+. For MOF-derived NiCo2O4/NiO-rGO, the peaks at 853.9 and 871.4 eV correspond to Co3+, and those at 855.6 and 873.0 eV correspond to Co2+ (Figure 7(D)and(E)).35 The coexistence of divalent and trivalent states for both Co and Ni means that Ni2+ and Co3+ were partially oxidized and reduced, respectively. In NiCo2O4/NiO, when the Ni2+ in NiCo2O4 and NiO were partially oxidized, it is likely that part of the generated electrons reduced the Co3+ into Co2+, while part of the electrons occupy the the oxygen vacancies.10 The O1s spectra for MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO are compared in Figure 7(F) and (G). Three peaks were identified. O1 at 529.5 and 529.45 eV are of typical metal oxygen bonds, corresponding to Ni-O and Co-O.36 The O2 located at 531.2 eV is typically due to crystal defects, chemisorbed oxygen or lattice oxygen under coordination.37 O3 has a weak binding peak at 533.4 and 532.7 eV, which can be due to physical or chemical adsorption of water in the free state.38 The C1s spectra for MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO are compared in Figure 7(H) and (I). Four peaks were identified, C2 at 284.8 eV and 284.83 eV is composed of the sp2-hybridized graphitic carbon. The C2 at 286.4 eV and 286.33 eV belong to hydroxyl/epoxy groups. C3 at 288.5 eV and 288.15 eV correspond to carboxyl group in 2.5-DHTA and GO, respectively. C4 at 289.2eV in Figure 7(I) correspond to carbonyl group in GO.39



Page 16 of 52

Figure 7 (A) XPS spectra of the MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO nanocomposite, Ni2p region in (B)MOF-derived NiCo2O4/NiO and (C)MOF-derived NiCo2O4/NiO-rGO, Co2p region in(D) MOF-derived NiCo2O4/NiO and(E) MOF-derived NiCo2O4/NiO-rGO, O1s region in(F) MOF-derived NiCo2O4/NiO and(G) MOF-derived NiCo2O4/NiO-rGO, C1s region in(H) MOF-derived NiCo2O4/NiO and(I) MOF-derived NiCo2O4/NiO-rGO.

Brunner-Emmet-Teller

measurements

(BET)

and

Barrett-Joyner-Halenda

methods (BJH) were used to evaluate the specific surface area and pore size of the MOF-derived NiCo2O4/NiO and the MOF-derived NiCo2O4/NiO-rGO. As shown in figure 8(A), the specific surface area of MOF-derived NiCo2O4/NiO is 89 m²·g-1, while the specific surface area of MOF-derived NiCo2O4/NiO-rGO is as large as 151 m²·g-1. Thus, the MOF-derived NiCo2O4/NiO-rGO has a larger specific surface area. As shown in Figure 8(B), the pores of MOF-derived NiCo2O4/NiO are mostly distributed in the mesoporous level (2 nm~50 nm), and contain a small portion of micropores (50 nm). The average pore volume of MOF-templated NiCo2O4/NiO composed of rod-like fibers is 0.22 cm3·g-1, slightly larger than that of MOF-derived NiCo2O4/NiO-rGO composites (~0.20 cm3·g-1). The ACS Paragon Plus Environment

Page 17 of 52 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


average pore size of MOF-derived NiCo2O4/NiO spheres is 10.01 nm, while that of MOF-derived NiCo2O4/NiO-rGO is only 5.33 nm. Clearly, the MOF-derived NiCo2O4/NiO microspheres have larger pores than MOF-derived NiCo2O4/NiO-rGO. Since the overall pore volumes are almost the same, MOF-derived NiCo2O4/NiO-rGO has a greater number of mesopores. The larger quantities of mesopores lead to more contact area with the electrolyte, facilitating the adsorption and catalytic reaction.

Figure 8 (A) N2 adsorption–desorption isotherms of MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiOnanocomposite. and (B) Pore size distributions

The OER activities of the prepared catalyst were investigated in a standard three-electrode cell with 1.0 M KOH aqueous as the electrolyte. Figure 9(A) shows the linear sweep voltammetry (LSV) curves at a scan rate of 5 mV·s−1. Both NiCo2O4/NiO-rGO and NiCo2O4/NiO electrode exhibits high electrocatalytic activity. At 10 mA·cm−2, the MOF-derived NiCo2O4/NiO-rGO achieves the lowest overpotential of 340 mV, slightly lower than that of RuO2 (370 mV). The overpotential of MOF-derived NiCo2O4/NiO (390 mV) is slightly higher than RuO2. MOF-derived NiCo2O4/NiO-rGO also has a lower onset potential of 1.51 V compared to MOF-derived NiCo2O4/NiO (1.57 V) and RuO2 (1.52 V). NiCo2O4/NiO generates active species of NiOOH and CoOOH under alkaline conditions. The have a large surface area due to the template function of the MOF during the preparation of MOF derived NiCo2O4/NiO. The mechanism of NiCo2O4/NiO catalytic reaction is as the following:40 NiO + OH− ↔ NiOOH + eACS Paragon Plus Environment

(1)


NiCo2O4 + OH− + H2O ↔ NiOOH + 2CoOOH + e-

Page 18 of 52

(2)

The broad oxidation peak was due to the formation of the active species NiOOH and CoOOH from NiO and NiCo2O4 through oxidation of Ni2+ and accompanying formation of CoOOH in the alkaline medium, respectively. It can be seen that due to the inclusion of graphene, the high specific surface area and high pore diameter structure of the MOF are retained, the internal resistance of the MOF-derived NiCo2O4/NiO-rGO polarization curve is less, and less energy is consumed to cross the catalytic activation energy barrier. Therefore, MOF-derived NiCo2O4/NiO-rGO has impressive catalytic performance. The MOF-derived NiCo2O4/NiO microspheres have a slower rate of electron migration relative to the MOF-derived NiCo2O4/NiO-rGO, and the overpotential generated by crossing the catalytic activation energy barrier is higher, therefore better catalytic performance of MOF-derived NiCo2O4/NiO-rGO was obtained.In addition, the catalytic kinetics of the catalyst can be evaluated with the Tafel slope. The data from the polarization curve can be evaluated by the Tafel equation (η = b log|j| + a). The initial b represents the Tafel slope. A smaller Tafel slope is expected to produce a certain response current density at a smaller overpotential, thereby reducing power consumption. Tafel slope is derived from Tafel derivation of the steady-state polarization curve. From Figure 9(B), Tafel slope of MOF-derived NiCo2O4/NiO is only 49 mV·dec-1, while that of MOF-derived NiCo2O4/NiO-rGO composite is 66 mV·Dec-1. In contrast, the Tafel slope of RuO2 is 75 mV·dec-1. Although the MOF-derived NiCo2O4/NiO microsphere has a large overpotential, it has the smallest Tafel slope and generates a higher current at a given overpotential. The reason for the small Tafel slope is likely related to its specific surface and pore size. The specific surface area of MOF-derived NiCo2O4/NiO is less than MOF-derived NiCo2O4/NiO-rGO, but its pore volume and pore size are both larger than the latter, resulting in a smaller Tafel slope. This is consistent with the BET analysis.


Page 19 of 52 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


Figure. 9 Steady state polarization plots for the OER (A) of Ni-MOF74, Ni-MOF74 GO, RuO2, MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO nanocomposite catalysts recorded saturated 1.0 M KOH at 900 r/min, respectively. And (B) Tafel plots at low overpotential during the OER

Two important parameters for evaluation of the efficiency of the OER catalyst are η10 (also η at higher current densities if available) and Tafel slope. η10 is the potential for the current at 10 mA·cm-2 in LSV. The η10 and Tafel slope values of MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO were compared with those of other OER catalysts reported in recent years (Table 1). Clearly, MOF-derived NiCO2O4/NiO-rGO has better performances compared to other Ni/Co based OER catalysts. Table 1. Comparison of OER performances: present work vs. literature.

Catalyst

Eonset(V)

Eη10(V)

Tafel slope

Electrolyte

Reference

(mV·dec-1) Co3O4

1.51

1.586

58

0.1M KOH

41

NiCo2O4

N/A

1.64

79

1M KOH

42

CoMn LDH

1.51

1.56

71

1M KOH

43

porous NiCo2O4

1.53

1.60

63.4

0.1M KOH

44

1.54

1.61

60

1M KOH

45

nanoneedles

nanosheets Zn/Co hydroxy sulfate



Page 20 of 52

1.57

1.649

113

1M KOH

46

1.565

1.65

65.46

1M KOH

47

1.43

N/A

86

0.1M KOH

48

Fe1.3Mn0.7P

1.67

N/A

53

0.1M KOH

49

Ni0.69Co0.31P

1.49

1.59

85

1M KOH

50

Ag/NiO

N/A

1.53

93

1M KOH

51

MOF-derived

1.58

1.64

49

1M KOH

This work

1.51

1.57

66

1M KOH

This work

Ni-Co LDH nanosheets Sphere like NiCo2O4 Co3O4 modified MnO2

NiCo2O4/NiO microspheres MOF-derived NiCo2O4/NiO-rGO

The electrochemically active surface area of the catalyst is measured by the electrochemical double-layer capacitance (Cdl), which is proportional to the active site of the catalyst52. The Cdl values of the samples were determined based on the cyclic voltammograms recorded in a non-Faradaic potential window (0.92-1.11 V vs.RHE) at increasing scan rates (1-50 mV·s-1). Figure 10 and Fig. 11(A) show that the double-layer capacitances of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO composites are high, based on the slope from the linear relationship between the current density and the scan rate. The CoOOH and NiOOH formed by NiCo2O4/NiO under alkaline conditions are the main active materials of the catalyst, coupled with its excellent electrical conductivity, leading to the remarkable catalytic effect. NiCo2O4/NiO-rGO, with the binding of graphene oxide and the retained framework morphology, has a larger specific surface area. Thus, as shown in the Cdl data，NiCo2O4/NiO-rGO has a greater linear slope and better catalytic performance.


Page 21 of 52 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


Figure. 10 Cyclic voltammograms of (A)MOF-derived NiCo2O4/NiO-rGO and (B)MOF-derived NiCo2O4/NiO measured at different scan rates from 1 to 50 mV·s−1 in 1 M KOH.

Electrochemical impedance spectroscopy studies were performed to investigate the electrocatalytic kinetics. The applied potential of 1.51 V vs. RHE ensures oxygen evolution reactions of all the samples can proceed. As the arc is generated at the electrode/electrolyte interface, a high frequency arc is formed. The arc diameter is a measure of the charge transfer resistance. Because the oxygen evolution reaction requires a rapid transfer of charge at the pole/electrolyte interface, a smaller arc means less resistance and a faster rate of electron transfer. As shown in Figure 11(B), the resistance of the Ni-MOF74, Ni-MOF74/GO composite, MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO increase sequentially. Since OER occurs on the surface between the electrode and the solution, the diameter of the resulting high-frequency arc corresponds to the charge transfer resistance. Since the MOF material is formed by coordination of an organic ligand and a metal ion, in which the organic ligand has poor conductivity, it is not sufficient for the electrons to pass through. The MOF-derived NiCo2O4/NiO material formed on MOF allow better charge transfer. After the binding of graphene, the electrical activity at the junction between the metal oxide and the electrolyte is improved and the conductivity is enhanced. The trend of the arc diameter is in good agreement with those of LSV, and the arc of MOF-derived NiCo2O4/NiO-rGO is significanlty smaller than those of other samples.



Figure. 11 (A) Plots of the corresponding current density at 1.10 V vs the scan rate and (B) Nyquist plots of all samples at applied potential of 1.51 V vs. RHE.

The long term stability is very important, so we used the long-term accelerated stress test (AST) to evaluate the stability of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO catalysts in a 0.1 M NaOH solution at 25°C in N2 atmosphere at a scan rate of 900 r/min with a potential of 0-1.9 V vs. RHE. Figure 12(A) is the polarization curve of the MOF-derived NiCo2O4/NiO-rGO after initial polarization curve and scanning 1000 cycles of cyclic voltammetry, and the polarization current attenuation is approximately 20%. Figure 12(B) shows the steady-state polarization curve of MOF-derived NiCo2O4/NiO before and after 1000 cycles of cyclic voltammetry. The current decay is ~16%. In contrast, the conventional commercial catalysts for precious metals have a current decay of approximately 40% in the long-term accelerated stress test53. Therefore, the present nickel cobalt oxide/nickel oxide and their composite materials have better stability than common commercial catalysts.


Page 22 of 52

Page 23 of 52 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


Fig. 12 Stability tests of the OER for the (A) MOF-derived NiCo2O4/NiO-rGO and (B) MOF-derived NiCo2O4/NiO nanocomposite catalysts. Testing conditions are in N2 saturated 0.1 M NaOH at 25°C and 900r/min with continuous potential cycling: 0-1.9 V vs. RHE.

Finally, the long-term chronoamperometric curve was used to evaluate the durability of MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO at a current density of 10 mA·cm-2. The results are shown in Figure 13. After testing for 18,000 seconds, the current density of MOF-derived NiCo2O4/NiO decreased for 15.6% while that of MOF-derived NiCo2O4/NiO-rGO slightly increased for 2.9%. These results show that the two materials have good stability under long-term durability tests, thus they can function as oxygen evolution catalysts for a long time.

Figure 13 Chronoamperometric stability tests of (A)MOF-derived NiCo2O4/NiO-rGO and (B)MOF-derived NiCo2O4/NiO nanocomposite conducted in 1 M KOH at 1.51 V and 1.58V (vs. RHE), respectively.



CONCLUSION The composite material of MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO were synthesized by a hydrothermal and calcination method. NiCo2O4/NiO and NiCo2O4/NiO-rGO have low overpotentials at 390 mV and 340 mV at 10 mA·cm-2, and good Tafel slopes of 49 mV·dec-1 and 66 mV·dec-1, respectively. NiCo2O4/NiO and NiCo2O4/NiO-rGO produces a large amount of NiOOH and CoOOH during the catalytic process, which not only serves as a good catalytic center, but also provides a high-speed passage for the electron transfer, significantly improving the efficiency of oxygen evolution reaction. The low-cost, durable, and highly efficient electrodes of MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO possesses high potential for application in oxygen evolution. This work can lead to new advances in the field of oxygen evolution for replacing precious metal-based catalysts.

ACKNOWLEDGMENTS This work was supported by a grant from the Two-way Support Programs of Sichuan Agricultural University (Project No. 03570113) and the Education Department of Sichuan Provincial, PR China (Grant No. 16ZA0039). The authors sincerely acknowledge the anonymous reviewers for their insights and comments to further improve the quality of the manuscript.

REFERENCES 1.

Li, D.; Baydoun, H.; Kulikowski, B.; Brock, S. L. Boosting Catalytic Performance of Iron Phosphide

Nanorods for the Oxygen Evolution Reaction by Incorporation of Manganese. Chem. Mat. 2017, 29 (7), 3048-3054. 2.

Liang, H.; Gandi, A. N.; Anjum, D. H.; Wang, X.; Schwingenschlögl, U.; Alshareef, H. N.

Plasma-Assisted Synthesis of NiCoP for Efficient Overall Water Splitting. Nano Lett. 2016, 16 (12), 7718-7725. 3.

Tang, K.; Wang, X.; Wang, M.; Xie, Y.; Zhou, J.; Yan, C. Ni/Fe Ratio Dependence of Catalytic

Activity in Monodisperse Ternary Nickel Iron Phosphide for Efficient Water Oxidation. ChemElectroChem. 2017, 4 (9), 2150-2157.


Page 24 of 52

Page 25 of 52 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


4.

Xiao, X.; Huang, D.; Fu, Y.; Wen, M.; Jiang, X.; Lv, X.; Li, M.; Gao, L.; Liu, S.; Wang, M.; Zhao, C.;

Shen, Y. Engineering NiS/Ni2P Heterostructures for Efficient Electrocatalytic Water Splitting. ACS Appl. Mater. Interfaces. 2018, 10 (5), 4689-4696. 5.

Peng, X.; Wang, L.; Hu, L.; Li, Y.; Gao, B.; Song, H.; Huang, C.; Zhang, X.; Fu, J.; Huo, K. In situ

segregation of cobalt nanoparticles on VN nanosheets via nitriding of Co2V2O7 nanosheets as efficient oxygen evolution reaction electrocatalysts. Nano Energy. 2017, 34, 1-7. 6.

Xu, H.; Feng, J.-X.; Tong, Y.-X.; Li, G.-R. Cu2O-Cu Hybrid Foams as High-Performance

Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media. ACS Catal. 2017, 7 (2), 986-991. 7.

Li, F.; Bu, Y.; Lv, Z.; Mahmood, J.; Han, G. F.; Ahmad, I.; Kim, G.; Zhong, Q.; Baek, J. B. Porous

Cobalt Phosphide Polyhedrons with Iron Doping as an Efficient Bifunctional Electrocatalyst. Small. 2017, 13 (40), 1701167. 8.

Qian, M.; Cui, S.; Jiang, D.; Zhang, L.; Du, P. Highly Efficient and Stable Water-Oxidation

Electrocatalysis with a Very Low Overpotential using FeNiP Substitutional-Solid-Solution Nanoplate Arrays. Adv. Mater. 2017, 29 (46), 1704075. 10.1002/adma.201704075. 9.

Kim, J. H.; Youn, D. H.; Kawashima, K.; Lin, J.; Lim, H.; Mullins, C. B. An active nanoporous Ni(Fe)

OER electrocatalyst via selective dissolution of Cd in alkaline media. Appl. Catal. B-Environ. 2018, 225, 1-7. 10. Liu, D.; Zhang, C.; Yu, Y.; Shi, Y.; Yu, Y.; Niu, Z.; Zhang, B. Hydrogen evolution activity enhancement by tuning the oxygen vacancies in self-supported mesoporous spinel oxide nanowire arrays. Nano Res. 2017, 11 (2), 1-11. 11. Yan, K.; Lu, Y. Direct Growth of MoS2 Microspheres on Ni Foam as a Hybrid Nanocomposite Efficient for Oxygen Evolution Reaction. Small. 2016, 12 (22), 2975. 12. Zhang, J.; Hu, Y.; Liu, D.; Yu, Y.; Zhang, B. Enhancing Oxygen Evolution Reaction at High Current Densities on Amorphous-Like Ni-Fe-S Ultrathin Nanosheets via Oxygen Incorporation and Electrochemical Tuning. Adv. Sci. 2017, 4 (3), 1600343. 13. Wan, H.; Li, L.; Chen, Y.; Gong, J.; Duan, M.; Liu, C.; Zhang, J.; Wang, H. One pot synthesis of Ni12P5 hollow nanocapsules as efficient electrode materials for oxygen evolution reactions and supercapacitor applications. Electrochim. Acta. 2017, 229, 380-386. 14. Zhang, L.; Chang, C.; Hsu, C. W.; Chang, C. W.; Lu, S. Y. Hollow nanocubes composed of well-dispersed mixed metal-rich phosphides in N-doped carbon as highly efficient and durable electrocatalysts for the oxygen evolution reaction at high current densities. J. Mater. Chem. A. 2017, 5 (37), 19656-19663 15. Bediako, D. K.; Surendranath, Y.; Nocera, D. G. Mechanistic studies of the oxygen evolution reaction mediated by a nickel-borate thin film electrocatalyst. J. Am. Chem. Soc. 2013, 135 (9), 3662-3674. 16. Ren, X.; Ge, R.; Liu, D.; Zhang, Y.; Wu, D.; Sun, X.; Du, B.; Wei, Q. Cobalt-borate nanowire array as a high-performance catalyst for oxygen evolution reaction in near-neutral media. J. Mater. Chem. A. 2017, 5 (16), 7291-7294. 17. Geng, J.; Kuai, L.; Kan, E.; Sang, Y.; Geng, B. Hydrothermal Synthesis of a rGO Nanosheet Enwrapped NiFe Nanoalloy for Superior Electrocatalytic Oxygen Evolution Reactions. Chem.-Eur. J. 2016, 22 (41), 14480-14483. 18. Saha, S.; Ganguli, A. K. FeCoNi Alloy as Noble Metal-Free Electrocatalyst for Oxygen Evolution Reaction (OER). Chemistryselect. 2017, 2 (4), 1630-1636. 19. Yang, D.; Guo, Z.; Tang, Y.; Miao, P. Poly(thymine)-Templated Selective Formation of Copper



Nanoparticles for Alkaline Phosphatase Analysis Aided by Alkyne–Azide Cycloaddition “Click” Reaction. ACS Appl. Nano Mater. 2018, 1 (1), 168-174. 20. Miao, P.; Tang, Y.; Han, K.; Wang, B. Facile synthesis of carbon nanodots from ethanol and their application in ferric(iii) ion assay. J. Mater. Chem. A.2015, 3 (29), 15068-15073. 21. Li, S. L.; Xu, Q. Metal–organic frameworks as platforms for clean energy. Energy Environ. Sci. 2013, 6 (6), 1656-1683. 22. Sun, J. K.; Xu, Q. Functional materials derived from open framework templates/precursors: synthesis and applications. Energy Environ. Sci. 2014, 7 (7), 2071-2100. 23. Yan, L.; Cao, L.; Dai, P.; Gu, X.; Liu, D.; Li, L.; Wang, Y.; Zhao, X. Metal-Organic Frameworks Derived Nanotube of Nickel–Cobalt Bimetal Phosphides as Highly Efficient Electrocatalysts for Overall Water Splitting. Adv. Funct. Mater. 2017, 27 (40), 1703455. 10.1002/adfm.201703455. 24. Jayaramulu, K.; Masa, J.; Tomanec, O.; Peeters, D.; Ranc, V.; Schneemann, A.; Zboril, R.; Schuhmann, W.; Fischer Roland, A. Nanoporous Nitrogen-Doped Graphene Oxide/Nickel Sulfide Composite Sheets Derived from a Metal-Organic Framework as an Efficient Electrocatalyst for Hydrogen and Oxygen Evolution. Adv. Funct. Mater. 2017, 27 (33), 1700451. 25. Hou, X.; Xue, S.; Liu, M.; Shang, X.; Fu, Y.; He, D. Hollow irregular octahedra-like NiCo2O4 cages composed of mesoporous nanosheets as a superior anode material for lithium-ion batteries. Chem. Eng. J. 2018, 350, 29-36. 26. Wang, N.; Sun, B.; Zhao, P.; Yao, M.; Hu, W.; Komarneni, S. Electrodeposition preparation of NiCo2O4 mesoporous film on ultrafine nickel wire for flexible asymmetric supercapacitors. Chem. Eng. J. 2018, 345, 31-38. 27. Rao, H.; Zhang, Z.; Ge, H.; Liu, X.; Zou, P.; Wang, X.; Wang, Y. Enhanced amperometric sensing using a NiCo2O4/nitrogen-doped reduced graphene oxide/ionic liquid ternary composite for enzyme-free detection of glucose. New J. Chem. 2017, 41 (9), 3667-3676. 28. Yu, Z.; Li, H.; Zhang, X.; Liu, N.; Tan, W.; Zhang, X.; Zhang, L. Facile synthesis of NiCo2O4 @Polyaniline core-shell nanocomposite for sensitive determination of glucose. Biosens. Bioelectron. 2016, 75, 161-165. 29. Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80 (6), 1339-1339. 30. Fu, Y.; Sun, D.; Qin, M.; Huang, R.; Li, Z. Cu(II)-and Co(II)-containing metal-organic frameworks (MOFs) as catalysts for cyclohexene oxidation with oxygen under solvent-free conditions. RSC Adv.2012, 2 (8), 3309-3314. 31. Wang, L. J.; Deng, H.; Furukawa, H.; Gándara, F.; Cordova, K. E.; Peri, D.; Yaghi, O. M. Synthesis and characterization of metal-organic framework-74 containing 2, 4, 6, 8, and 10 different metals. Inorg. Chem. 2014, 53 (12), 5881-5883. 32. Guillerm, V.; Kim, D.; Eubank, J. F.; Luebke, R.; Liu, X.; Adil, K.; Lah, M. S.; Eddaoudi, M. ChemInform Abstract: A Supermolecular Building Approach for the Design and Construction of Metal-Organic Frameworks. Chem. Soc. Rev. 2014, 43 (16), 6141-6172. 33. Zhang, C.; Yu, J. S. Morphology-Tuned Synthesis of NiCo2O4-Coated 3D Graphene Architectures Used as Binder-Free Electrodes for Lithium-Ion Batteries. Chem.-Eur. J. 2016, 22 (13), 4422-4430. 34. Nardi, K. L.; Yang, N.; Dickens, C. F.; Strickler, A. L.; Bent, S. F. Creating Highly Active Atomic Layer Deposited NiO Electrocatalysts for the Oxygen Evolution Reaction. Adv. Energy Mater. 2015, 5 (17), 1500412. 35. Zhu, C.; Fu, S.; Du, D.; Lin, Y. Facilely Tuning Porous NiCo2O4 Nanosheets with Metal


Page 26 of 52

Page 27 of 52 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


Valence-State Alteration and Abundant Oxygen Vacancies as Robust Electrocatalysts Towards Water Splitting. Chem.-Eur. J. 2016, 22 (12), 4000-4007. 36. Salunkhe, R. R.; Jang, K.; Yu, H.; Yu, S.; Ganesh, T.; Han, S. H.; Ahn, H. Chemical synthesis and electrochemical analysis of nickel cobaltite nanostructures for supercapacitor applications. J. Alloy. Compd. 2011, 509 (23), 6677-6682. 37. Choudhury, T.; Saied, S. O.; Sullivan, J. L.; Abbot, A. M. Reduction of oxides of iron, cobalt, titanium and niobium by low-energy ion bombardment. J. Phys. D-Appl. Phys. 1989, 22 (8), 1185. 38. Roginskaya, Y. E.; Lubnin, E. N.; Ulitina, Y. E.; Lopukhova, G. T. S.; Morozova, O. V. Characterization of Bulk and Surface Composition of CoxNi1 -xOy Mixed Oxides for Electrocatalysis. Langmuir. 1997, 13 (17), 4621-4627. 39. Meng, L.; Xia, Y.; Liu, W.; Zhang, L.; Zou, P.; Zhang, Y. Hydrogen microexplosion synthesis of platinum nanoparticles/nitrogen doped graphene nanoscrolls as new amperometric glucose biosensor. Electrochim. Acta. 2015, 152, 330-337. 40. Liang, J.; Wang, Y.; Wang, C. C.; Lu, S. Y. In-situ Formation of NiO on Ni Foam Prepared with a Novel Leaven Dough Method as an Outstanding Electrocatalyst for Oxygen Evolution Reaction. J. Mater. Chem. A. 2016, 4 (25), 9797-9806. 41. Ranaweera, C. K.; Zhang, C.; Bhoyate, S.; Kahol, P. K.; Ghimire, M.; Mishra, S. R.; Perez, F.; Gupta, B. K.; Gupta, R. K. Flower-shaped cobalt oxide nano-structures as an efficient, flexible and stable electrocatalyst for the oxygen evolution reaction. Materials Chemistry Frontiers. 2017, 1 (4), 1580-1584. 42. Shi, H.; Zhao, G. Water Oxidation on Spinel NiCo2O4 Nanoneedles Anode: Microstructures, Specific Surface Character, and the Enhanced Electrocatalytic Performance. J. Phys. Chem. C 2014, 118 (45), 25939-25946. 43. Fang, S.; Hu, X. Ultrathin Cobalt-Manganese Layered Double Hydroxide Is an Efficient Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2014, 136 (47), 16481-16484. 44. Zhu, Y.; Zhou, W.; Yu, J.; Chen, Y.; Liu, M.; Shao, Z. Enhancing Electrocatalytic Activity of Perovskite Oxides by Tuning Cation Deficiency for Oxygen Reduction and Evolution Reactions. Chem. Mat. 2016, 28 (6), 1691-1697. 45. Dutta, S.; Ray, C.; Negishi, Y.; Pal, T. Facile Synthesis of Unique Hexagonal Nanoplates of Zn/Co Hydroxy Sulfate for Efficient Electrocatalytic Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces. 2017, 9 (9), 8134. 46. Jiang, J.; Zhang, A.; Li, L.; Ai, L. Nickel-cobalt layered double hydroxide nanosheets as high-performance electrocatalyst for oxygen evolution reaction. J. Power Sources 2015, 278, 445-451. 47. Devaguptapu, S. V.; Hwang, S.; Karakalos, S.; Zhao, S.; Gupta, S.; Su, D.; Xu, H.; Wu, G. Morphology Control of Carbon-Free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media. ACS Appl. Mater. Interfaces. 2017, 9 (51), 44567-44578. 48. Du, G.; Liu, X.; Zong, Y.; Hor, T. S.; Yu, A.; Liu, Z. Co3O4 nanoparticle-modified MnO2 nanotube bifunctional oxygen cathode catalysts for rechargeable zinc-air batteries. Nanoscale. 2013, 5 (11), 4657-4661. 49. Li, D.; Baydoun, H.; Kulikowski, B.; Brock, S. L. Boosting the Catalytic Performance of Iron Phosphide Nanorods for the Oxygen Evolution Reaction by Incorporation of Manganese. Chem. Mat. 2017, 29 (7), 3048-3054. 50. Yin, Z.; Zhu, C.; Li, C.; Zhang, S.; Zhang, X.; Chen, Y. Hierarchical nickel-cobalt phosphide yolk-shell spheres as highly active and stable bifunctional electrocatalysts for overall water splitting. Nanoscale.



2016, 8 (45), 19129-19138. 51. Iqbal, M. Z.; Kriek, R. J. Silver/Nickel Oxide (Ag/NiO) Nanocomposites Produced Via a Citrate Sol-Gel Route as Electrocatalyst for the Oxygen Evolution Reaction (OER) in Alkaline Medium. Electrocatalysis. 2018, 9 (3), 279-286. 52. Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135 (28), 10274-10277. 53. Devaguptapu, S. V.; Hwang, S.; Karakalos, S.; Zhao, S.; Gupta, S.; Su, D.; Xu, H.; Wu, G. Morphology Control of Carbon-Free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media. ACS Appl. Mater. Interfaces. 2017, 9 (51),

Figure captions Scheme. 1 Schematic illustration for the preparation process of MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO. Fig. 1 The XRD patterns of (A) Ni-MOF74(a), Ni-MOF74-rGO(b)and (B )MOF-derived NiCo2O4/NiO(a), MOF-derived NiCo2O4/NiO-rGO(b). Fig. 2 SEM images of (A)-(B) Ni-MOF74, (C)-(D) Ni-MOF74-rGO, (E)-(F)MOF-derived NiCo2O4/NiO and (G)-(H) MOF-derived NiCo2O4/NiO-rGO. Fig. 3. 1-2 µm TEM images of (A)Ni-MOF74, (B)Ni-MOF74-rGO, (C)MOF-derived NiCo2O4/NiO and (D)MOF-derived NiCo2O4/NiO-rGO. Fig. 4. 200 nm TEM images of (A) Ni-MOF74, (B)Ni-MOF74-rGO, (C)MOF-derived NiCo2O4/NiO and (D)MOF-derived NiCo2O4/NiO-rGO. Fig. 5. Structure characterization of (A)and(B) HRTEM image, (C)and(D) EDS-elemental mapping of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO, respectively.


Page 28 of 52

Page 29 of 52 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


Fig. 6. EDS-elemental mapping of (A)and(E) Ni, (B)and(F) O, (C)and(G) Co, (D)and(H) C of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO, respectively Fig. 7 (A)XPS spectra of the MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO nanocomposite, Ni2p region in(B)MOF-derived NiCo2O4/NiO and (C)MOF-derived NiCo2O4/NiO-rGO, Co2p region in (D)MOF-derived NiCo2O4/NiO and (E)MOF-derived NiCo2O4/NiO-rGO, O1s region in (F)MOF-derived NiCo2O4/NiO and (G)MOF-derived NiCo2O4/NiO-rGO , C1s region in (H)MOF-derived NiCo2O4/NiO and (I)MOF-derived NiCo2O4/NiO-rGO. Fig. 8 (A)N2 adsorption–desorption isotherms of MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO nanocomposite. and (B) Pore size distributions Fig. 9 Steady state polarization plots for the OER (A) of Ni-MOF74、Ni-MOF74 GO、RnO2、MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO nanocomposite catalysts recorded saturated 1.0 M KOH at 900r/min, respectively. And (B) Tafel plotsat low overpotential during the OER Fig. 10 Cyclic voltammograms of (A)MOF-derived NiCo2O4/NiO-rGO and (B)MOF-derived NiCo2O4/NiO measured at different scan rates from 1 to 50 mV·s−1 in 1 M KOH. Fig. 11 (A)Plots of the corresponding current density at 1.10 V vs the scan rate and (B)Nyquist plots of all samples at applied potential of 1.51 V vs. RHE. Fig. 12 Stability tests of the OER for the (A)MOF-derived NiCo2O4/NiO-rGO and (B)MOF-derived NiCo2O4/NiO nanocomposite catalysts. Testing conditions are in N2 saturated 0.1 M NaOH at 25°C and 900r/min with continuous potential cycling: 0-1.9 V vs. RHE. Fig. 13 Chronoamperometric stability tests of (A)MOF-derived NiCo2O4/NiO-rGO and (B)MOF-derived NiCo2O4/NiO nanocomposite conducted in 1 M KOH at 1.51 V and 1.58V (vs. RHE), respectively.



Scheme. 1

Fig. 1


Page 30 of 52

Page 31 of 52 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


Fig. 2

Fig. 3



Fig. 4


Page 32 of 52

Page 33 of 52 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


Fig. 5



Fig. 6

Fig. 7


Page 34 of 52

Page 35 of 52 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


Fig. 8

Fig. 9

Fig. 10



Fig. 11

Fig. 12

Fig. 13


Page 36 of 52

Page 37 of 52 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


Table 1. Comparison of OER performances: present work vs. literature.

Catalyst

Eonset(V)

Eη10(V)

Tafel slope

Electrolyte

Reference

(mV·dec-1) Co3O4

1.51

1.586

58

0.1M KOH

41

NiCo2O4

N/A

1.64

79

1M KOH

42

CoMn LDH

1.51

1.56

71

1M KOH

43

porous NiCo2O4

1.53

1.60

63.4

0.1M KOH

44

1.54

1.61

60

1M KOH

45

1.57

1.649

113

1M KOH

46

1.565

1.65

65.46

1M KOH

47

1.43

N/A

86

0.1M KOH

48

Fe1.3Mn0.7P

1.67

N/A

53

0.1M KOH

49

Ni0.69Co0.31P

1.49

1.59

85

1M KOH

50

Ag/NiO

N/A

1.53

93

1M KOH

51

NiCo2O4/NiO

1.58

1.64

49

1M KOH

This work

NiCo2O4/NiO-rGO 1.51

1.57

66

1M KOH

This work

nanoneedles

nanosheets Zn/Co hydroxy sulfate Ni-Co LDH nanosheets Sphere like NiCo2O4 Co3O4 modified MnO2

microspheres



Table of Contents Graphic

Schematic illustration for the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO in OER.


Page 38 of 52

Page 39 of 52 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


Scheme. 1 Schematic illustration for the preparation process of MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO. 84x47mm (300 x 300 DPI)



Fig. 1 The XRD patterns of (A) Ni-MOF74(a), Ni-MOF74-rGO(b)and (B )MOF-derived NiCo2O4/NiO(a), MOFderived NiCo2O4/NiO-rGO(b). 84x47mm (300 x 300 DPI)


Page 40 of 52

Page 41 of 52 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


Fig. 2 SEM images of (A)-(B) Ni-MOF74, (C)-(D) Ni-MOF74-rGO, (E)-(F)MOF-derived NiCo2O4/NiO and (G)(H) MOF-derived NiCo2O4/NiO-rGO. 84x47mm (300 x 300 DPI)



Fig. 3. 1-2 µm TEM images of (A)Ni-MOF74, (B)Ni-MOF74-rGO, (C)MOF-derived NiCo2O4/NiO and (D)MOFderived NiCo2O4/NiO-rGO. 84x47mm (300 x 300 DPI)


Page 42 of 52

Page 43 of 52 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


Fig. 4. 200 nm TEM images of (A) Ni-MOF74, (B)Ni-MOF74-rGO, (C)MOF-derived NiCo2O4/NiO and (D)MOFderived NiCo2O4/NiO-rGO. 84x47mm (300 x 300 DPI)



Fig. 5. Structure characterization of (A)and(B) HRTEM image, (C)and(D) EDS-elemental mapping of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO, respectively. 84x47mm (300 x 300 DPI)


Page 44 of 52

Page 45 of 52 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


Fig. 6. EDS-elemental mapping of (A)and(E) Ni, (B)and(F) O, (C)and(G) Co, (D)and(H) C of the MOF-derived NiCo2O4/NiO and MOF-derived NiCo2O4/NiO-rGO, respectively 84x47mm (300 x 300 DPI)



Fig. 7 (A)XPS spectra of the MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO nanocomposite, Ni2p region in(B)MOF-derived NiCo2O4/NiO and (C)MOF-derived NiCo2O4/NiO-rGO, Co2p region in (D)MOFderived NiCo2O4/NiO and (E)MOF-derived NiCo2O4/NiO-rGO, O1s region in (F)MOF-derived NiCo2O4/NiO and (G)MOF-derived NiCo2O4/NiO-rGO , C1s region in (H)MOF-derived NiCo2O4/NiO and (I)MOF-derived NiCo2O4/NiO-rGO. 84x47mm (300 x 300 DPI)


Page 46 of 52

Page 47 of 52 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


Fig. 8 (A)N2 adsorption–desorption isotherms of MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO nanocomposite. and (B) Pore size distributions 84x47mm (300 x 300 DPI)



Fig. 9 Steady state polarization plots for the OER (A) of Ni-MOF74、Ni-MOF74 GO、RnO2、MOF-derived NiCo2O4/NiO-rGO and MOF-derived NiCo2O4/NiO nanocomposite catalysts recorded saturated 1.0 M KOH at 900r/min, respectively. And (B) Tafel plotsat low overpotential during the OER 84x47mm (300 x 300 DPI)


Page 48 of 52

Page 49 of 52 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


Fig. 10 Cyclic voltammograms of (A)MOF-derived NiCo2O4/NiO-rGO and (B)MOF-derived NiCo2O4/NiO measured at different scan rates from 1 to 50 mV•s−1 in 1 M KOH. 84x47mm (300 x 300 DPI)



Fig. 11 (A)Plots of the corresponding current density at 1.10 V vs the scan rate and (B)Nyquist plots of all samples at applied potential of 1.51 V vs. RHE. 84x47mm (300 x 300 DPI)


Page 50 of 52

Page 51 of 52 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


Fig. 12 Stability tests of the OER for the (A)MOF-derived NiCo2O4/NiO-rGO and (B)MOF-derived NiCo2O4/NiO nanocomposite catalysts. Testing conditions are in N2 saturated 0.1 M NaOH at 25°C and 900r/min with continuous potential cycling: 0-1.9 V vs. RHE. 84x47mm (300 x 300 DPI)



Fig. 13 Chronoamperometric stability tests of (A)MOF-derived NiCo2O4/NiO-rGO and (B)MOF-derived NiCo2O4/NiO nanocomposite conducted in 1 M KOH at 1.51 V and 1.58V (vs. RHE), respectively.

84x47mm (300 x 300 DPI)


Page 52 of 52

NiCo2O4-rGO as Highly Efficient

Recommend Documents