Single-Walled Carbon Nanotubes Wrapped CoFe2O4 Nanorods with

Subscriber access provided by Gothenburg University Library

Article

Single-walled Carbon Nanotubes Wrapped CoFe2O4 Nanorods with enriched oxygen vacancies for Efficient Overall Water Splitting Yu Ding, Jun Zhao, Wenqing Zhang, Jian Zhang, Xijie Chen, Fengchun Yang, and Xin Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01338 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 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 33 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 Applied Energy Materials

Single-Walled Carbon Nanotubes Wrapped CoFe2O4 Nanorods with Enriched Oxygen Vacancies for Efficient Overall Water Splitting Yu Ding‡, Jun Zhao‡, Wenqing Zhang‡, Jian Zhang, Xijie Chen, Fengchun Yang*, and Xin Zhang*

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, National Demonstration Center for Experimental Chemistry Education, College of Chemistry & Material Science, Northwest University, Xi’an 710127, China.

Keywords：Electrocatalysis, CoFe2O4, Single-walled carbon nanotubes, Synergistic effect, Oxygen vacancies, Overall water splitting

Abstract Developing highly active and stable non-noble-metal electrocatalysts for water splitting is the main title toward energy conversion systems. CoFe2O4 has been reported as promising oxygen evolution reaction (OER) electrocatalyst. However, the CoFe2O4 showed poor conductivity and sluggish electrocatalysis in hydrogen evolution reaction (HER). In this work a new hybrid

ACS Paragon Plus Environment

1

ACS Applied Energy Materials 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

Page 2 of 33

catalyst was synthesized based on single-walled carbon nanotubes (SWNTs) wrapped CoFe2O4 nanorods. The synergistic effect between CoFe2O4 nanorods and SWNTs can not only increase the oxygen vacancies and optimized the electronic structure of the composite, but also facilitate the adsorption and stabilization of hydroxyl, which could significantly improve both the OER and HER performance. Besides, the addition of SWNTs would reduce the aggregating and tubes bundling in the composites and further improve the stability of the hybrid. All the excellent performances made CoFe2O4/SWNTs as a promising bifunctional electrocatalyst for the OER and HER, as well as overall water splitting. 1. Introduction

Producing oxygen and hydrogen through electrolysis of water provides an advance and sustainable development for storing intermittent renewable energy.1-3 Water electrolysis requires efficient, high-stability catalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).4-8 At present, the noble metals-based electrocatalysts (Pt-, Ir-based materials) showed the best performance in water splitting. However, the high costs of the noble metal limited their large-scale application. Thus, the earth-abundant alternatives such as transition metals and their compounds have been widely investigated for better catalytic capacity.9-13

The transition metal oxides have been extensively studied as advanced electrocatalysts due to their diversified nanostructures. Compared with the single metal oxides, ferrite MFe2O4 (M = Co,


2

Page 3 of 33 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


Ni, Cu, etc.) showed potential OER electrocatalytic activity due to the electronic transition between the different valences of metals in the O-sites. They can also provide the necessary surface redox active metal centers for the adsorption and activation of electroactive species.14-18 Among the various ferrites, CoFe2O4 is one of the most attractive compounds due to its low cost, high abundance, low toxicity, and rich active sites. Unfortunately, the poor electrical conductivity and large volume expansion of pure CoFe2O4 in electrochemical processes limit its catalytic activity and stability severely.19-21 Thus, many researches focus to improve its electrocatalytic properties. For instance, Lu et al. reported the porous CoFe2O4/C nanorods arrays with high OER performance.22 Yan et al. synthesized the hollow CoFe2O4 nanospheres with higher surface area and rich oxygen vacancies, which showed excellent OER activity.23 However, rare could be referenced on the HER performance of CoFe2O4 catalysts, due to its intrinsic poor HER activity.

Single-walled carbon nanotubes (SWNTs), because of their high aspect ratio and electrical conductivity, combined with high mechanical strength and resilience, are attractive carbonaceous materials as solid supports for heterogeneous catalysts.24-28 Thus, we designed CoFe2O4/SWNTs hybrid catalyst for electrocatalytic water splitting. Beyond their superiority in physical and structural, the synergistic effect between CoFe2O4 and SWNTs enhance the electrocatalytic activity effectively as following: (i) to provide more oxygen vacancy in the CoFe2O4, which could further affect the electronic structure of Co and Fe in CoFe2O4/SWNTs and promote the


3


OER and HER activities.29,

30

Page 4 of 33

(ii) The SWNTs wrap on the CoFe2O4 nanorods could further

facilitate the adsorption and stabilization of hydroxyl and benefited HER.31,32 (iii) The strong interconnection between CoFe2O4 and SWNTs would reduce the aggregating and tubes bundling in the composites, greatly improve the stability and dispersibility.

Herein, we prepared the CoFe2O4 nanorods with diameters of ≈ 45 nm by hydrothermal process and compound with SWNTs via sonication. The CoFe2O4/SWNTs show larger active surface area and multiple active sites as well as the better conductivity and stability than CoFe2O4, which make this composite exhibited excellent electrocatalytic performance of both OER and HER with low overpotentials of 310 mV for OER and 263 mV for HER at 10 mA cm-2 in 1.0 M KOH. Moreover, this advanced electrocatalyst showed effective performance for overall water splitting with the cell voltage of 1.72V.

2. Experimental Section

2.1 Reagents and materials.

Pristine single-walled carbon nanotubes (carbonaceous purity 60%) were purchased from Carbon Solutions, Inc. Other chemical reagents, including cobalt chloride (CoCl2·6H2O), ferrous sulfate (FeSO4·7H2O), urea (CH4N2O), potassium hydroxide (KOH), Nafion (5 wt%), N, N-


4

Page 5 of 33 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


dimethylformamide (DMF) were purchased from Sigma-Aldrich. All solutions were dissolved by deionized water (18.3 M cm) produced from a Millpore water purification system.

2.2 Characterization.

The as-prepared catalysts’ crystallographic structure was obtained by X-ray diffraction (XRD, RigakuD/max-2400, USA) with high-intensity Cu Kα radiation (λ=1.5406A). Morphologies of the composites were determined using scanning electron microscopy (SEM, Hitachi S-4800 Japan) and Transmission electron microscopy (TEM, Tecnai G2 F30, FEI, USA). X-ray photoelectron spectroscopy (XPS, ESCALAB250xi, USA) was used to analyze the composition and chemical valence.

2.3 Preparation of CoFe2O4 nanorods

A solution of 0.5 mM CoCl2·6H2O and 1 mM FeSO4·7H2O was dissolved into 20 mL deionized water, then mixed with 5 mM urea to sonicate for 30 min. This mixed solution was transferred into Teflon-lined stainless-steel 25 mL autoclave. The autoclave closed tightly and maintained at 160 ℃ for 7 h in an electric oven. After cooling down to room temperature, the CoFe2O4 was washed by water and ethanol three times to remove unreacted residues and impurities. Finally, CoFe2O4 nanorods dried under vacuum at 60 ℃.


5


Page 6 of 33

2.4 Preparation of CoFe2O4/SWNTs

Firstly, the commercial SWNTs were purified in terms of an established procedure involving thermal annealed and nitric acid treatment.33 The pristine SWNTs (200 mg) were heated at 350℃ for 1 h in a furnace to get rid of the amorphous carbon. After cooled down to room temperature, the sample was refluxed in diluted nitric acid (2.6 M, 100 ml) overnight, then filtered and washed with deionized water until pH of the filtrate reached the value neutral (99 mg).

To prepare CoFe2O4/SWNTs complex, 1 mg CoFe2O4 nanorods and 2 mg SWNTs were added into 1.5 mL DMF for sufficient sonication ， so the mass ratio of CoFe2O4 and SWNTs in the composite is 1:2. After that 20 μL 5 wt% Nafion was added and mixed uniformly.

2.5 Electrochemical Measurements

All the electrochemical measurements of the materials were tested in the CHI 660D electrochemical workstation (CH Instruments). And the OER and HER activities of the asobtained materials were tested in a conventional three-electrode system, consisting of glassy carbon electrode (GCE) with a geometric area of 0.071 cm2 as a working electrode, a saturated Ag/AgCl electrode and a carbon rod or Pt wire were applied as the reference electrode and the counter electrode (carbon rod was employed for HER while Pt wire were used for OER), respectively. The potential values of the Ag/AgCl reference electrode were calibrated with


6

Page 7 of 33 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


respect to RHE in all measurements in different electrolyte solution, Evs. RHE=Evs. Ag/AgCl + 0.198 V + 0.059 pH. All data were reported without iR compensation. Before the modification, a working electrode was polished with 0.05 μm α-Al2O3 powder, and then ultrasonically rinsed with ethanol and deionized water, and dried in N2 at room temperature. The working electrode was modified using prepared catalysts by a general dropping 20 μl suspensions onto the GCE. The LSV polarization curves of OER and HER were measured in 1.0 M KOH electrolyte with the scan rate of 5 mV/s. Cyclic voltammograms were recorded at various scan rates (20 to 200 mV/s) to estimate the double layer capacitance. The electrochemical impedance spectroscopy (EIS) was investigated at 310 mV in the frequency range of 10-2 Hz to 105 Hz.

The overall water splitting was performed in a two-electrode system in 1.0 M KOH, the catalyst was loaded on two 1×0.5 cm carbon cloth by drop coating (loading amount: ~ 0.5 mg cm-2).

3. Results and Discussion


7


Page 8 of 33

Figure 1．SEM images of the (a) CoFe2O4 and (c) CoFe2O4/SWNTs. TEM images of the (b) CoFe2O4 and (d) CoFe2O4/SWNTs (The SWNTs were marked with dotted lines).


8

Page 9 of 33 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 2．(a) The XRD patterns of the CoFe2O4 and the CoFe2O4/SWNTs. The HR-XPS (b) Co 2p, (c) Fe 2p, and (d) O 1s spectrum of the CoFe2O4 and CoFe2O4/SWNTs.

Figure 1 showed The SEM and TEM images of the CoFe2O4 and CoFe2O4/SWNTs. From Figure 1a and 1b, the homogeneous nanorods with diameters of ≈ 45 nm were clearly observed. After being wound by SWNTs, as shown in Figure 1c and 1d, the scale and diameters of the nanorods were unchanged, which indicated the structural integrity of the CoFe2O4/SWNTs. In


9


Page 10 of 33

order to perform elemental analysis on as-prepared CoFe2O4, elemental analysis of the asprepared CoFe2O4 was conducted by the Inductive Coupled Plasma Emission Spectrometer (ICP) test (Table S1) which shows the result that the ratio of Co, Fe and O is 1:1.956:3.954, which is similar to the stoichiometry of the compounds CoFe2O4. In addition, there are no other interference elements in CoFe2O4, supporting the negation of impurities. XRD pattern in Figure 2a also presented the unchanged lattice planes of CoFe2O434 after modified by SWNTs, which demonstrated the addition of SWNTs would not affect the structure of CoFe2O4 nanorods in physical. We further analyze the composition and valence state of the CoFe2O4/SWNTs and CoFe2O4 by XPS (Figure S1). The presence of Co 2p, Fe 2p and their shake-up satellite peaks demonstrated the existence of CoFe2O4. Moreover, the XPS spectrum of O 1s was deconvoluted into three peaks: the peak O1 at 529.9 eV was the typical metal-oxygen bond, O2 at 530.9 eV corresponds to the oxygen vacancy, and O3 at 532.1 eV represented the peak of hydroxyl or surface oxygen adsorption.35, 36 The number of oxygen vacancies in pure CoFe2O4 nanorods and SWNTs/CoFe2O4 composite were compared by O2 at 530.9eV in XPS. The percentage of O2 in O 1s of the CoFe2O4/SWNTs composite is almost three times than pure CoFe2O4 (Figure S2a). There is a obvious peak at 531.9eV in the O 1s of SWNTs (Figure S2b). Notably, the O2 is inexistent in the SWNTs, indicating the intrinsic number of the oxygen vacancies would increase significantly in the CoFe2O4/SWNTs composite. The higher ratio of O2 peak in CoFe2O4/SWNTs may derive from the oxygen containing functional groups in SWNTs, except the intrinsic oxygen defect in CoFe2O4. The partial electrons of Fe and Co would transfer from


10

Page 11 of 33 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


the symmetrical electronic structure to the asymmetric 3d electronic structure in the oxygen vacancies.37,

38, 39

Thus, the enriched oxygen vacancies would optimizes the partial density of

states for 3d electrons (3d-PDOS) of Fe and Co in the CoFe2O4/SWNTs and further boost the improvement of electrocatalytic activity of reactive sites.


11


Page 12 of 33

Figure 3 ． (a) The OER polarization curves and (b) Tafel plots of CoFe2O4/SWNTs, SWNTs, CoFe2O4 and commercial RuO2 in 1.0 M KOH, and the loadings of different catalysts for electrochemical tests are all 20 μL with concentration of 2 mg/mL. (c) HER curves and (d)


12

Page 13 of 33 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


corresponding Tafel plots of CoFe2O4/SWNTs, SWNTs, CoFe2O4 and commercial Pt/C in 1.0 M KOH, and the loadings of different catalysts for electrochemical tests are all 20 μL with concentration of 2 mg/mL dispersed into DMF. (e) Nyquist plots of CoFe2O4/SWNTs, CoFe2O4 and SWNTs at potential of 1.6 V vs. RHE in 1.0 M KOH. (f) Calculated double-layer capacitance (Cdl) for CoFe2O4/SWNTs, CoFe2O4 and SWNTs.

The OER activity of CoFe2O4/SWNTs and CoFe2O4 was evaluated by linear scanning voltammetry (LSV) with a three-electrode system in 1.0 M KOH electrode solution. As shown in Figure 3a, the little anodic peak observed at 1.4V-1.5V versus RHE is likely to be related to the oxidation of Co2+ to Co3+.40 The CoFe2O4/SWNTs exhibited excellent OER performance with small potential of 310 mV at current density of 10 mA cm-2, lower than CoFe2O4 nanorods (355 mV) and SWNTs (510 mV), even lower than RuO2 (317 mV). Besides, the Tafel slope (Figure 3b) of CoFe2O4/SWNTs was calculated as 85 mV dec-1, lower than the CoFe2O4 nanorods (96 mV dec-1) and SWNTs (147 mV dec-1). Moreover, the exchange current densities (j0) are calculated from Tafel plots by an extrapolation method (Figure S3). The OER j0 of CoFe2O4/SWNTs is 2.4 × 10-3 mA cm-2, which is comparable with RuO2 (3.3 × 10-3 mA cm-2) and about 5.6 times than that ofCoFe2O4 (4.2 × 10-4 mA cm-2). This high performance should be due to the synergistic effect in the CoFe2O4/SWNTs system, which consisted of the interconnection in CoFe2O4 nanorods by SWNTs, more electron penetration and high defect of


13


Page 14 of 33

oxygen as demonstrated above in SEM, TEM and XPS results. These outstanding OER properties of CoFe2O4/SWNTs were also comparable with the recent CoFe2O4-based electrocatalysts.41-44 The HER properties of CoFe2O4/SWNTs were also investigated in Figure 3c and 3d. The overpotential at current density of 10 mA cm-2 and Tafel slope of CoFe2O4/SWNTs were 263 mV dec-1and 46 mV dec-1 respectively, which surpass CoFe2O4 nanorods (342 mV and 95 mV dec-1). The low Tafel slope of CoFe2O4/SWNTs indicated the HER followed the VolmerHeyrovsky mechanism.45 Additionally, the HER j0 (Figure S4b) of CoFe2O4/SWNTs (4.8 × 10-3 mA cm-2) is 10.7 times higher than CoFe2O4 (4.5×10-4 mA cm-2). We further measured the stability of CoFe2O4 and CoFe2O4/SWNTs during OER and HER process by the time-current method (i-t) (Figure S4). After 10 hours of operation, CoFe2O4/SWNTs can maintain the current density of 10 mA cm-2 with little change, and there is no significant change on the morphology (Figure S5). While for the CoFe2O4, there was a significant decrease of the current density, with severe collapse of the nanorod structure, which indicated the important role of the SWNTs in the CoFe2O4/SWNTs hybrid. Thus the excellent stability of CoFe2O4/SWNTs should derive from the robust stability of SWNTs and interaction in the composites to prevented the aggregation of CoFe2O4 nanorods. To shed light upon the superior electrocatalytic activity of CoFe2O4/SWNTs, the EIS of CoFe2O4/SWNTs and CoFe2O4 were recorded (Figure 3e). The charge transfer resistance (Rct) of CoFe2O4/SWNTs was only 8.835 Ω, smaller than CoFe2O4 and much smaller than the SWNTs, implying the faster kinetics of CoFe2O4/SWNTs,46, 47 which was attributed to the enhanced electron transfer, which sprang from the interconnection between CoFe2O4 and


14

Page 15 of 33 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


SWNTs. We further measured the double layer capacitance (Cdl) by using a simple cyclic voltammetry (CV) method (Figure S6) to estimate the electrochemically active surface area (ECSA) (Figure 3f), and the Cdl of CoFe2O4, CoFe2O4/SWNTs and SWNTs are 15.00 mF cm-2, 6.12 mF cm-2 and 18.57 mF cm-2, respectively (Figure 3f). Notably, the rare electrocatalytic active site in SWNTs leads the poor activity of it, but the larger ECSA of SWNTs significantly increase the active area and provide more active sites of the CoFe2O4/SWNTs, which was conducive to the electrocatalytic reaction process.48 We also change the mass ratio of CoFe2O4 and SWNTs to 1:1 and 1:3 respectively. And the OER, HER performances were tested then, which shows the ratio of 1:2 is the best (Figure S7). When the content of SWNTs is higher, the overpotentials of OER and HER were 324mV, 333mV at a current density of 10 mA/cm2 respectively (Figure S7a and S7b), the performances were worse. Meanwhile, the stabilities of the catalyst were also getting worse (Figure S7c and S7d), which might be the lack of the metallic active sites. When the SWNTs become less, the catalytic performance would get worse (322 mV for OER, 348 mV for HER at j=10 mA/cm2) (Figure S7a and S7b), the stabilities tested deteriorated (Figure S7c and S7d), indicating that SWNTs play an important role in the catalytic process. Moerover, both OER and HER have good repeatability with standard deviation of 0.829156 mV and 1.268611 mV, respectively. Based on all the results presented above, we assembled a full cell using CoFe2O4/SWNTs as both anode and cathode for overall water splitting in 1.0 M KOH. The polarization curve was shown in Figure 4a. In addition, CoFe2O4/SWNTs also showed good stability during the test, and maintained a current density of


15


Page 16 of 33

10 mA cm-2 after 10 hours, demonstrating that CoFe2O4/SWNTs had great potential as a water splitting catalyst.

The results described above showed improved electrocatalytic properties of CoFe2O4/SWNTs, which could be attributed to the synergistic effect between CoFe2O4 and SWNTs: (I) The oxygen-containing functional groups existing on acid-treated SWNTs could facilitate the combine of CoFe2O4 and SWNTs and prevent the agglomeration in CoFe2O4 nanorods simultaneously, thus improving the stability of the composite material. (II) The increased oxygen vacancies in CoFe2O4/SWNTs could optimize the 3d-PDOS of Co and Fe. (III) The link between CoFe2O4 nanorods and SWNTs could significant improved the active surface area and the lead rapid electron transfer, further reduced the charge transfer resistance. (IV) The adsorbing of OHby SWNTs was effective under alkaline conditions, thus greatly enhanced the kinetics of the reaction.


16

Page 17 of 33 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 4．(a) The polarization curve and (b) stability test of CoFe2O4/SWNTs||CoFe2O4/SWNTs catalysts for overall water splitting in 1.0 M KOH solution.

4. Conclusion

An excellent catalyst based on CoFe2O4/SWNTs composites was synthesized for high active electrocatalysis. After the composition of CoFe2O4 and SWNTs, the synergistic effect between CoFe2O4 and SWNTs can further increase the content of oxygen vacancies and optimize electronic structure of the composite, which could enhance the active sites, surface area, promote charge transfer in CoFe2O4/SWNTs and boosting the OER and HER effectively. In addition, SWNTs would greatly prevent the agglomeration as an efficient scaffold of the electrocatalysts and improve the stability of the electrocatalyst. Moreover, the high active and stable CoFe2O4/SWNTs composite can be further applied for efficient full water splitting. All the results demonstrate the strategy for enriched oxygen vacancies can boost a kind of promising catalysts based on MFe2O4/SWNTs.

ASSOCIATED CONTENT

Supporting Information. Characterization and electrochemical data.

AUTHOR INFORMATION


17


Page 18 of 33

Corresponding Author E-mail： Fengchun Yang, [email protected] Xin Zhang, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Funding Sources ACKNOWLEDGMENT We gratefully acknowledge the support from the National Natural Scientific Foundation of China (No. 21405120), the Shaanxi Provincial Science and Technology Development Funds (No. 2016KW-061), and China Geological Survey Project (No. DD20160308). This work was also supported by the “Top-rated Discipline” construction Scheme of Shaanxi higher education. REFERENCES (1) Turner, J.A. Sustainable Hydrogen Production. Science 2004, 305, 972-974.

(2) Du, N.; Wang, C.; Wang, X.; Lin, Y.; Jiang, J. and Xiong, Y. Trimetallic TriStar Nanostructures: Tuning Electronic and Surface Structures for Enhanced Electrocatalytic Hydrogen Evolution Adv. Mater. 2016, 28, 2077-2084.


18

Page 19 of 33 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


(3) Walter, M.G.; Warren, E.L.; McKone, J.R.; Boettcher, S.W.; Mi,Q.; Santori, E.A. and Lewis, N.S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473.

(4) Han, G.-Q.; Liu, Y.-R.; Hu, W.-H.; Dong, B.; Li, X.; Shang, X.; Chai, Y.-M.; Liu, Y.-Q. and Liu, C.-G. Three Dimensional Nickel Oxides/Nickel Structure by in Situ Eelectro-Oxidation of Nickel Foam as Robust Electrocatalyst for Oxygen Evolution Reaction. Appl. Surf. Sci. 2015, 359, 172-176.

(5) H, G.-Q.; Liu, Y.-R.; Hu, W.-B.; Dong, B.; Li, X.; Shang, X.; Chai, Y.-M.; Liu, Y.-Q. and Liu, C.-G. Crystallographic Structure and Morphology Transformation of MnO2 Nanorods as Efficient Electrocatalysts for Oxygen Evolution Reaction. J. Electrochem. Soc. 2016, 163, H67H73.

(6) Li, X.; Han, G.-Q.; Liu, Y.-R.; Dong, B.; Shang, X.; Hu, W.-H.; Chai, Y.-M.; Liu, Y.-Q. and Liu, C.-G. In situ Grown Pyramid Structures of Nickel Diselenides Dependent on Oxidized Nickel Foam as Efficient Electrocatalyst for Oxygen Evolution Reaction. Electrochim. Acta 2016, 205, 77-84.

(7) Zhang, B.; Xiao, C.; Xie, S.; Liang, J.; Chen, X. and Tang, Y. Iron−Nickel Nitride Nanostructures in Situ Grown on Surface-Redox-Etching Nickel Foam: Efficient and Ultrasustainable Electrocatalysts for Overall Water Splitting. Chem. Mater. 2016, 28, 6934-6941.


19


Page 20 of 33

(8) Chen, S.; Kang, Z.; Hu, X.; Zhang, X.; Wang, H.; Xie, J.; Zheng, X.; Yan, W.; Pan, B. and Xie, Y. Delocalized Spin States in 2D Atomic Layers Realizing Enhanced Electrocatalytic Oxygen Evolution. Adv. Mater. 2017, 29, 1701687.

(9) Feng, J.-X.; Xu, H.; Dong, Y.-T.; Lu, X.-F.; Tong, Y.-X. and Li, G.-R. Efficient Hydrogen Evolution Electrocatalysis Using Cobalt Nanotubes Decorated with Titanium Dioxide Nanodots. Angew. Chem. Int. Ed. 2017, 56, 2960 –2964.

(10) Feng, J.-X.; Ye, S.-H.; X. Han.; Tong, Y.-X. and Li, G.-R. Design and Synthesis of FeOOH/CeO2 Heterolayered Nanotube Electrocatalysts for the Oxygen Evolution Reaction. Adv. Mater. 2016, 28, 4698–4703.

(11) Feng, J.-X.; Wu, J.-Q.; Tong, Y.-X. and Li, G.-R. Efficient Hydrogen Evolution on Cu Nanodots-Decorated Ni3S2 Nanotubes by Optimizing Atomic Hydrogen Adsorption and Desorption. J. Am. Chem. Soc. 2018, 140, 610−617.

(12) Feng, J.-X.; Tong, S.-Y.; Tong, Y.-X. and Li, G.-R. Pt-like Hydrogen Evolution Electrocatalysis on PANI/CoP Hybrid Nanowires by Weakening the Shackles of Hydrogen Ions on the Surfaces of Catalysts. J. Am. Chem. Soc. 2018, 140, 5118−5126

(13) Pei, Z.; Li, H.; Huang, Y.; Xue, Q.; Huang, Y.; Zhu, M.; Wang, Z. and Zhi, C. Texturing in Situ: N,S-Enriched Hierarchically Porous Carbon as a Highly Active Reversible Oxygen Electrocatalyst. Energy. Environ. Sci. 2017, 10, 742-749.


20

Page 21 of 33 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


(14) Zhu, H.; Zhang, S.; Huang, Y.-X.; Wu, L. and Sun, S. Monodisperse MxFe3-xO4 (M = Fe, Cu, Co, Mn) Nanoparticles and Their Electrocatalysis for Oxygen Reduction Reaction. Nano. Lett. 2013, 13, 2947-2951.

(15) Wei, C.; Feng, Z.; Baisariyev, M.; Yu, L.; Zeng, L.; Wu, T.;Zhao, H.; Huang, Y.; Bedzyk, M.J.; Sritharan, T. and Xu, Z.J. Valence Change Ability and Geometrical Occupation of Substitution Cations Determine the Pseudocapacitance of Spinel Ferrite XFe2O4 (X = Mn, Co, Ni, Fe). Chem. Mater. 2016, 28, 4129-4133.

(16) Chen, J.; Zhao, D.; Diao, Z.; Wang, M.; Guo, L. and Shen, S. Bifunctional Modification of Graphitic Carbon Nitride with MgFe2O4 for Enhanced Photocatalytic Hydrogen Generation. ACS Appl. Mater. Interfaces 2015, 7, 18843-18848.

(17) Li, M.; Xiong, Y.; Liu, X.; Bo, X.; Zhang, Y.; Han, C. and Guo,L. Facile Synthesis of Electrospun MFe2O4 (M = Co, Ni, Cu, Mn) Spinel Nanofibers with Excellent Electrocatalytic Properties for Oxygen Evolution and Hydrogen Peroxide Reduction. Nanoscale 2015, 7, 89208930.

(18) Suen, N.-T.; Hung, S.-F.; Quan, Q.; Zhang, N.; Xu, Y.-J. and Ming, H. Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365.


21


Page 22 of 33

(19) Maruthapandian, V.; Mathankumar, M.; Saraswathy, V.; Subramanian, B. and Muralidharan, S. Study of the Oxygen Evolution Reaction Catalytic Behavior of CoxNi1–xFe2O4 in Alkaline Medium. ACS Appl. Mater. Interfaces 2017, 9, 13132-13141.

(20) Ji, X.; Hao, S.; Qu, F.; Liu, J.; Du, G.; Asiri, A.M.; Chen, L. and Sun, X. Core–shell CoFe2O4@Co–Fe–Bi Nanoarray: a Surface-Amorphization Water Oxidation Catalyst Operating at Near-Neutral pH. Nanoscale 2017, 9, 7714-7718.

(21) Li, P.; Ma, R.; Zhou,Y.; Chen, Y.; Zhou, Z.; Liu, G.; Liu, Q.; Peng, G.; Liang, Z. and Wang, J. In Situ Growth of Spinel CoFe2O4 Nanoparticles on Rod-Like Ordered Mesoporous Carbon for Bifunctional Electrocatalysis of both Oxygen Reduction and Oxygen Evolution. J. Mater. Chem. A 2015, 3, 15598-15606.

(22) Lu, F.; Gu, L.-F.; Wang, J.-W.; Wu, J.-X.; Liao, P.-Q. and Li, G.-R. Bimetal-Organic Framework Derived CoFe2O4/C Porous Hybrid Nanorod Arrays as High-Performance Electrocatalysts for Oxygen Evolution Reaction. Adv. Mater. 2017, 29, 1604437.

(23) Yan, K.-L.; Shang, X.; Liu, Z.-Z.; Dong, B.; Lu, S.-S.; Chi, J.-Q.; Gao, W.-K.; Chai, Y.-M. and Liu, C.-G. A Facile Method for Reduced CoFe2O4 Nanosheets with Rich Oxygen Vacancies for Efficient Oxygen Evolution Reaction. Int. J. Hydrogen. Energy 2017, 42, 24150-24158.

(24) Wang, D.Y.; Gong, M.; Chou, H.L.; Pan, C.J.; Chen, H.A.; Wu, Y.; Lin, M.C.; Guan, M.; Yang, J.; Chen, C.W.; Wang, Y.L.; Hwang, B.J.; Chen, C.C. and Dai, H. Highly Active and


22

Page 23 of 33 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


Stable Hybrid Catalyst of Cobalt-Doped FeS2 Nanosheets−Carbon Nanotubes for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 1587-1592.

(25) Deng, J.; Ren, P.; Deng, D.; Yu, L.; Yang, F. and Bao, X. Highly Active and Durable NonPrecious-Metal Catalysts Encapsulated in Carbon Nanotubes for Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 1919-1923.

(26) Li, D.J.; Maiti, U.N.; Lim, J.; Choi, D.S.; Lee, W.J.; Oh, Y.; Lee, G.Y. and Kim, S.O. Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction. Nano. Lett. 2014, 14, 1228-1233.

(27) Liang,Y.; Wang, H.; Diao, P.; Chang, W.; Hong, G.; Li, Y.; Gong, M.; Xie, L.; Zhou, J.; Wang, J.; Regier, T.Z.; Wei, F. and Dai, H. Oxygen Reduction Electrocatalyst Based on Strongly Coupled Cobalt Oxide Nanocrystals and Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134, 15849-15857.

(28) Gong, M.; Zhou, W.; Tsai, M.C.; Zhou, J.; Guan, M.; Lin, M.C.; Zhang, B.; Hu, Y.; Wang, D.Y.; Yang, J.; Pennycook, S.J.; Hwang, B.J. and Dai, H. Nanoscale Nickel Oxide/Nickel Heterostructures for Active Hydrogen Evolution Electrocatalysis. Nat. Commun. 2014, 5, 4695.

(29) Chen, J.Y.C.; Dang, L.; Liang, H.; Bi, W.; Gerken, J.B.; Jin, S.; Alp, E.E. and Stahl, S.S. Operando Analysis of NiFe and Fe Oxyhydroxide Electrocatalysts for Water Oxidation: Detection of Fe4+ by Mössbauer Spectroscopy. J. Am. Chem. Soc. 2015, 137, 15090-15093.


23


Page 24 of 33

(30) Zhao, S.; Wang, Y.; Dong, J.; He, C.-T.; Yin, H.; An, P.; Zhao,K.; Zhang, X.; Gao, C.; Zhang, L.; Lv, J.; Wang, J.; Zhang, J.; Khattak, A.M.; Khan, N.A.; Wei, Z.; Zhang, J.; Liu, S.; Zhao, H. and Tang, Z. Ultrathin Metal-Organic Framework Nanosheets for Electrocatalytic Oxygen Evolution. Nat. Energy 2016, 1, 16184.

(31) Suryanto, B.H.R.; Chen, S.; Duan, J. and Zhao, Chuan. Hydrothermally Driven Transformation of Oxygen Functional Groupsat Multiwall Carbon Nanotubes for Improved Electrocatalytic Applications. ACS Appl. Mater. Interfaces 2016, 8, 35513−35522

(32) Mamtani, K.; Jain, D.; Dogu, D.; Gustin, V.; Gunduz, S.; Co, A. and Ozkan, U. Insights Into Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) Active Sites for Nitrogen-Doped Carbon Nanostructures (CNx) in Acidic Media. Appl. Catal. B: Environ. 2018, 220, 88–97.

(33) Hu, H.; Zhao, B.; Itkis, M.E. and Haddon, R.C. Nitric Acid Purification of Single-Walled Carbon Nanotubes. J. Phy. Chem. B. 2003, 107, 13838-13842.

(34) Chandramohan, P.; Srinivasan, M.P.; Velmurugan, S. and Narasimhan, S.V. Cation Distribution and Particle Size Effect on Raman Spectrum of CoFe2O4. J. Solid State Chem. 2011, 184, 89-96.


24

Page 25 of 33 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


(35) Yin, J.; Li, Y.; Lv, F.; Lu, M.; Sun, K.; Wang, W.; Wang, L.; Cheng, F.; Li, Y.; Xi, P. and Guo, S. Oxygen Vacancies Dominated NiS2/CoS2 Interface Porous Nanowires for Portable Zn– Air Batteries Driven Water Splitting Devices. Adv. Mater. 2017, 29, 1704681.

(36) Bao, J.; Zhang, X.; Fan, B.; Zhang, J.; Zhou, M.; Yang, W.; Hu, X.; Wang, H.; Pan, B. and Xie, Y. Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation. Angew. Chem. Int. Ed. 2015, 54, 7399-7404.

(37) Odedairo, T.; Yan, X.; Yao, X.; Ostrikov, K.K. and Zhu, Z. Hexagonal Sphericon Hematite with High Performance for Water Oxidation. Adv. Mater. 2017, 29, 1703792.

(38) Liu, R.; Wang, Y.; Liu, D.; Zou, Y. and Wang, S. Water-Plasma-Enabled Exfoliation of Ultrathin Layered Double Hydroxide Nanosheets with Multivacancies for Water Oxidation. Adv. Mater. 2017, 29, 1701546.

(39) Liu, Y.; Yin, S. and Shen, P. Asymmetric 3d Electronic Structure for Enhanced Oxygen Evolution Catalysis. ACS Appl. Mater. Interfaces 2018, 10, 23131−23139.

(40) Burke, M.S.; Kast, M.G.; Trotochaud, L.; Smith, A.M. and Boettcher, S.W. Cobalt–Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648.


25


Page 26 of 33

(41) Wang, Y.; Xie, C.; Zhang, Z.; Liu, D.; Chen, R. and Wang, S. In Situ Exfoliated, N-Doped, and Edge-Rich Ultrathin Layered Double Hydroxides Nanosheets for Oxygen Evolution Reaction. Adv. Funct. Mater. 2018, 28, 1703363.

(42) Lin, X.; Li, X.; Li, F.; Fang, Y.; Tian, M.; An, X.; Fu, Y.; Jin, J. and Ma, J. Precious-MetalFree Co–Fe–Ox Coupled Nitrogen-Enriched Porous Carbon Nanosheets Derived From SchiffBase Porous Polymers as Superior Electrocatalysts for the Oxygen Evolution Reaction. J. Mater. Chem. A 2016, 4, 6505-6512.

(43) Liu, Y.; Li, J.; Li, F.; Li, W.; Yang, H.; Zhang, X.; Liu, Y. and Ma, J. A facile Preparation of CoFe2O4 Nanoparticles on Polyaniline-Functionalised Carbon Nanotubes as Enhanced Catalysts for the Oxygen Evolution Reaction. J. Mater. Chem. A 2016, 4, 4472-4478.

(44) Yan, W.; Cao, X.; Tian, J.; Jin, C.; Ke, K. and Yang, R. Nitrogen/Sulfur Dual-Doped 3D Reduced Graphene Oxide Networks-Supported CoFe2O4 with Enhanced Electrocatalytic Activities for Oxygen Reduction and Evolution Reactions. Carbon 2016, 99, 195-202.

(45) Yang, J.; Wang, X.; Li, B.; Ma, L.; Shi, L.; Xiong, Y. and Xu, H. Novel Iron/CobaltContaining Polypyrrole Hydrogel-Derived Trifunctional Electrocatalyst for Self-Powered Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1606497.


26

Page 27 of 33 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


(46) Sun, T.; Xu, L.; Yan, Y.; Zakhidov, A.A.; Baughman, R.H. and Chen, J. Ordered Mesoporous Nickel Sphere Arrays for Highly Efficient Electrocatalytic Water Oxidation. ACS Catal. 2016, 6, 1446-1450.

(47) Lu, C.; Tranca, D.; Zhang, J.; Hernández, F.R.; Su,Y.; Zhuang, X.; Zhang, F.; Seifert, G. and Feng, X. Molybdenum Carbide-Embedded Nitrogen-Doped Porous Carbon Nanosheets as Electrocatalysts for Water Splitting in Alkaline Media. ACS Nano 2017, 11, 3933-3942.

(48) Chen, Z.; Cummins, D.; Reinecke, B.N.; Clark, E.; Sunkara, M.K. and Jaramillo, T.F. Core– shell MoO3–MoS2 Nanowires for Hydrogen Evolution: A Functional Design for Electrocatalytic Materials. Nano. Lett. 2011, 11, 4168-4175.


27


Page 28 of 33

TOC


28

Page 29 of 33 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 1 79x66mm (300 x 300 DPI)



Figure 2


Page 30 of 33

Page 31 of 33 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



Figure 4 147x51mm (299 x 299 DPI)


Page 32 of 33

Page 33 of 33 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


TOC


Single-Walled Carbon Nanotubes Wrapped CoFe2O4 Nanorods with

Recommend Documents