Well-Ordered Oxygen-Deficient CoMoO4 and Fe2O3 Nanoplate

Subscriber access provided by HACETTEPE UNIVERSITESI KUTUPHANESI

Article

Well-ordered Oxygen-deficient CoMoO4 and Fe2O3 Nanoplate Arrays on 3D Graphene Foam: towards Flexible Asymmetric Supercapacitor with Enhanced Capacitive Properties Kai Chi, Zheye Zhang, Qi-Ying Lv, Chuyi Xie, Jian Xiao, Fei Xiao, and Shuai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14810 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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 38

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 Materials & Interfaces

Well-ordered Oxygen-deficient CoMoO4 and Fe2O3 Nanoplate Arrays on 3D Graphene Foam: towards Flexible Asymmetric Supercapacitor with Enhanced Capacitive Properties Kai Chi†,§, Zheye Zhang†,§, Qiying Lv†, Chuyi Xie†, Jian Xiao†, Fei Xiao*,†, Shuai Wang*,†,‡ †

Key laboratory of Material Chemistry for Energy Conversion and Storage, Ministry

of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China ‡

State Key Laboratory of Digital Manufacturing Equipment and Technology,

Ministry of Education, Flexible Electronics Research Center (FERC), School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China

ABSTRACT In this work, we report the development of well-ordered hydrogenated CoMoO4 (H-CoMoO4) and hydrogenated Fe2O3 (H-Fe2O3) nanoplate arrays on 3D graphene foam (GF), and explore their practice application as binder-free electrodes in

1

ACS Paragon Plus Environment


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

assembling flexible all-solid-state asymmetric supercapacitor (ASC) device. Our results show that the monolithic 3D porous GF prepared by solution casting method using Ni foam template possesses large surface area, superior electrical conductivity, and sufficient surface functional groups, which not only facilitate in situ growth of CoMoO4 and Fe2O3 nanoplates on it, but also contribute the double-layer capacitance of resultant supercapacitor. And the well-ordered pseudocapacitive metal oxide nanoplate arrays standing up on 3D GF scaffold can provide efficient space and shorten the length for electrolyte diffusion from outer into the inner region of electrode material for Faradaic energy storage. Furthermore, one of our major findings is that the introduction of oxygen vacancies in CoMoO4 and Fe2O3 nanoplates by hydrogenation treatment can increase their electronic conductivity as well as improve their donor density and surface properties, which give rise to a substantially improved electrochemical performance. Benefited from the synergistic contributions of different component in the nanohybrid electrode, the resultant flexible ASC device with GF/H-CoMoO4 as the positive electrode and GF/H-Fe2O3 as the negative electrode achieves a wide operation voltage of 1.5 V and the maximum volumetric specific capacitances of 3.6 F cm-3, which is two times larger than that of the Ni/GF/CoMoO4//Ni/GF/Fe2O3 device (1.8 F cm-3), and the rate capability is up to 70% as the current density increase from 2 to 200 mA cm-3. Moreover, the Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device also exhibits a high energy density of 1.13 mWh cm-3 and a high power density of 150 kW cm-3, good mechanical flexibility with the decrease of capacitance less than 4% after being bended inward to different angles and bended inward to 90° for 200 times, and cycling stability of 93.1% capacitance retention after 5000 cycling. 2


Page 2 of 38

Page 3 of 38

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


KEYWORDS: Hydrogenated transition metal oxide; Ordered nanoplate arrays; Three-dimension graphene foam; Binder-free flexible electrode; All-solid-state asymmetric supercapacitor

1. INTRODUCTION Supercapacitors (SCs) have gained tremendous research interest owing to their high power density, fast charge/discharge rate, high reliability, and long cycling life.1-3 Specially, the flexible SCs demonstrate significant predominance such as light weight, excellent reliability, mechanical flexibility, environment friendliness and safety, which can be served as high-efficient power sources to fulfill the requirements of next-generation wearable and portable electronics.4-6 Nonetheless, the energy density of current available SCs is still very low. In order to increase the energy density of the SC system, the assembly of flexible asymmetric SCs (ASCs) has been exploited to extend the operating voltage window of the SC system. The development of flexible ASC devices requires the rational design of electrode construction and preparation of highly active electrode materials. The strategy for loading highly active electrode materials on monolithic 3D porous graphene-based substrates, e.g., graphene foam (GF), graphene aerogel (GA), graphene hydrogel (GH) will be effective in developing advanced flexible ASC devices.7-9 These graphene-based macroscopic materials assembled from continuously interconnected individual graphene nanosheets possess macroporous structure, low mass density, exceptional mechanical properties, high stability and conductivity, large surface area and abundant surface functional groups. 3



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 active electrode materials grown on these 3D graphene substrates can be directly utilized as integrated electrodes, which avoid the utilization of any polymer binder and conductive additives, and therefore meet the requirements of flexible and light-weight supercapacitor architecture.10-14 Recently, great progresses have been achieved in the development of active electrode materials based on a range of transition metal oxides owing to their large specific capacitance and high energy density originated from their multiple oxidation states for pseudocapacitance. For instance, cobalt oxide (Co3O4) has exhibited an extremely high theoretical specific capacitance of 3600 F g-1 and excellent capability retention.15 And manganese dioxide (MnO2) also offers several advantages such as high specific capacitance of 1370 F g-1, earth-abundance, cheapness, and eco-friendliness.16-18 These properties make them promising candidate as positive electrode materials for ASC devices. Furthermore, as an alternative negative electrode material, iron oxide (e.g., Fe2O3) has been extensively investigated due to their excellent capacitive performances in negative potential.19-22 In recent time, binary metal oxides, e.g., NiCo2O4, ZnCo2O4, MnMoO4, NiMoO4 and CoMoO4, have demonstrated superior electrochemical properties to single-component oxides due to their feasible oxidation states and good electrical conductivity.23-27 Among them, binary CoMoO4, which combines the salient features of extremely high theoretical specific capacitance of cobalt oxide and rich polymorphism and reversible small ions storage of molybdenum oxide, has exhibited improved overall performances including high capacitance, high energy density, and excellent durability.28,29 Despite the achievements in developing different single or binary transition metal oxides as highly active electrode materials for ASC system, these 4


Page 4 of 38

Page 5 of 38

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


semiconductive metal oxides exhibit low power density attributed to their low electrical conductivity and insufficient electron transfer at high rates. As a result, tremendous research efforts have been devoted to solve these problems. For instant, developing nanostructured metal oxide, e.g., nanoparticles, nanowires, porous nanoflakes and hollowed nanospheres, can increase the contact area of electrode/electrolyte and favor the active species diffusion and electron transport process, thus enable fast redox reaction at high rates.30-33 Meanwhile, modifying metal oxides by doping metal ions into their crystal structure and/or integrating metal oxide with high-electrically-conductive nanomaterials have been demonstrated to be operative strategies to increase the specific capacitance and improve the conductivity and cycling ability.34-36 Significantly, it has been demonstrated that hydrogenating metal oxides by controlled introduction of oxygen vacancy states can improve their electrical conductivity and facilitate the surface redox reactions kinetics,37-40 which is anticipated to be a facile, effective and reliable strategy to improve the capacitive properties of resultant electrode materials. In this work, we develop well-ordered hydrogenated metal oxide i.e., hydrogenated CoMoO4 (H-CoMoO4) and hydrogenated Fe2O3 (H-Fe2O3) nanoplate arrays standing up on 3D GF, and explore their practice application as positive electrode and negative electrodes, respectively, for assembling flexible all-solid-state ASC device. Fig. 1 schematically illustrates the preparation procedure of the proposed flexible ACS based on 3D GF supported H-CoMoO4 (GF/H-CoMoO4) and GF supported H-Fe2O3 (GF/H-Fe2O3). The monolithic 3D porous GF was prepared by solution casting method using Ni foam as the template. The as-obtained GF material possesses superior electrical conductivity, large surface area and sufficient surface 5



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

functional groups, which not only facilitate the in situ growth of CoMoO4 and Fe2O3 nanoplates on GF scaffold, but also contribute to the double-layer capacitance of resultant ASC system. H-CoMoO4 and H-Fe2O3 nanoplate arrays were directly grown on 3D GF substrate by a facile hydrothermal synthesis, followed by hydrogenation treatment. The well-ordered metal oxide nanoplate arrays with high surface to volume ratio can facilitate the surface reactions and species transport from electrode surface to subsurface. Furthermore, the direct growth of H-CoMoO4 and H-Fe2O3 nanoplate arrays on GF enables the structure integration and good electrical contact of active materials and electrode substrate toward binder-free flexible electrodes, which reduces Ohmic polarization as well as improves the rate capability and cycling stability. Therefore, the as-prepared GF/H-CoMoO4 and GF/H-Fe2O3 electrodes exhibit fast charge transport pathway, low contact resistance and unique mechanical properties. More importantly, our findings reveal that the introduction of oxygen vacancies in H-CoMoO4 and H-Fe2O3 nanoplates via thermal decomposition of precursor under H2/Ar atmosphere can increase their electronic conductivity as well as improve their donor density as well as the surface properties, and give rise to a substantially improved electrochemical performance, which is much better than those obtained in pure Ar atmosphere. Under the optimized condition, the resultant GF/H-CoMoO4 electrode and GF/H-Fe2O3 electrode exhibit high areal capacitance up to 5360 mF cm-2 and 694 mF cm-2, respectively, with excellent rate capability. The ASC device based on GF/H-CoMoO4 (positive electrode) and GF/H-Fe2O3 (negative electrode) achieves a high energy density of 1.13 mWh cm-3 at a current density of 5 mA cm-3, a high power density of 150 kW cm-3 at a current density of 200 mA cm-3, and good cycling stability. After 5000 times charge-discharge cycling, the flexible 6


Page 6 of 38

Page 7 of 38

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


ASC remains 93.1% of its initial specific capacitance.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ni/GF material GO was prepared based on a modified Hummers method. Ni foam was used as a 3D skeleton template and a reducing agent at the same time to prepare GF. Briefly, a piece of Ni foam (2 cm × 4 cm) was sequentially washed with acetone, HCl, ethanol and deionized water. After that, the Ni foam was immersed into 3 mg mL-1 GO suspension at 60 oC for 12 h.41 During this process, GO has been thermally reduced and deposited on Ni foam to form Ni/GF. The as-obtained Ni/GF was taken out and repeatedly rinsed with deionized water.

2.2. Synthesis of Ni/GF/H-Fe2O3 and Ni/GF/H-CoMoO4 materials Ni/GF/H-CoMoO4 was synthesized by a facile hydrothermal process and further hydrogenation treatment. Typically, the Ni/GF (2 cm × 4 cm) was immersed into 50 mM Co(NO3)2·6H2O and 50 mM Na2MoO4 solution in Teflon-lined stainless steel autoclave liner, which was sealed and maintained at 180 oC for 6 h, and then cooled to room temperature. The as-prepared samples were cleaned with distilled water and ethanol, and annealed at different temperatures (i.e., 300, 400 and 500 oC) in H2/Ar (H2:5 sccm, Ar:100 sccm) atmosphere for 1 h, and the Ni/GF/H-CoMoO4 was obtained. For comparison, the Ni/GF/CoMoO4 was also prepared by annealed in Ar atmosphere under the same procedure. 7



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

For the preparation of Ni/GF/H-Fe2O3, the Ni/GF was immersed into a mixture containing 50 mM Fe(NO3)3·9H2O, 50 mM Na2SO4, 100 mM urea in Teflon-lined stainless steel autoclave liner, which was sealed and maintained at 120 oC for 6 h, and then cooled to room temperature. The as-obtained samples were also annealed at different temperatures (i.e., 200, 300 and 400 oC) in H2/Ar (H2: 5 sccm; Ar: 100 sccm) atmosphere for 1 h. For comparison, the Ni/GF/Fe2O3 was also prepared by annealed in Ar atmosphere under the same procedure. The mass of active material on electrode have been precisely weighted. In detail, the average quality of Ni foam (2 cm × 4 cm) before and after hydrothermal growth and hydrogenation was measured by 5 decimal analytical balance for 10 times. The weight gain has been taken as the mass of the active material on Ni foam electrode substrate.

2.3. Assembly of ASC device For the preparation of flexible ASC device, a PVA/KOH gel electrolyte has been obtained by mixing 6 g of PVA with 60 mL of KOH solution (1 M), which was heated to 90

o

C under stirring until it turn to transparent. Second, the as-obtained

Ni/GF/H-CoMoO4 and Ni/GF/H-Fe2O3 electrodes were immersed into the PVA/KOH gel for 2 min, and then taken out and assembled together with a cellulose separator sandwiched in between. After vaporize the excess water, the all-solid-state ASC was obtained.

2.4. Materials characterizations

8


Page 8 of 38

Page 9 of 38

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 morphology and structure of products were characterized with a field-emission scanning electron microscope (SEM, FEI, Nova NanoSEM 450). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a TECNAI G2 20 U-Twin instrument (Netherlands) operated at an acceleration voltage of 200 kV. The samples subjected to TEM measurements were ultrasonically dispersed in ethanol and drop-cast onto a carbon-coated 200-mesh copper grid and subsequently dried at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos-Axis spectrometer with monochromatic Al Kα (1486.71 eV) X-ray radiation (15 kV and 10 mA) and a hemispherical electron energy analyzer. XRD patterns were recorded using a diffractometer (X' Pert PRO, Panalytical B.V., Netherlands) equipped with a Cu Kα radiation source (λ= 1.5406 Å).

2.5. Electrochemical measurements The electrochemical tests were performed with a CHI 760E electrochemical workstation. The electrochemical measurement of single-electrode was performed using a three-electrode system, with Ni/GF/H-CoMoO4 and Ni/GF/H-Fe2O3 electrodes as the working electrode, Pt foil as the counter electrode and Hg/HgO electrode (SCE) as the reference electrodes. The electrolyte is. KOH solution (3 M). Electrochemical impedance spectroscopy (EIS) was carried out in a frequency range of 10-2 ~ 106 Hz. For the leakage current test, the device was first charged to 1.5 V at 5 mA cm-3, and then the potential was kept at 1.5 V for 2 h. For the self-discharge test, the device was first charged to 1.5 V at 5 mA cm-3 and kept at 1.5 V for 15 min. Each 9



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

electrode was tested for six times, and the average value was figured out.

3. RESULTS AND DISCUSSION 3.1. Morphological and structural characterization The 3D GF material, which serves as a large surface-area and highly conductive support for high loading of electroactive nanomaterials, was synthesized by solution casting using 3D Ni foam scaffold as the template. The SEM images show that the graphene nanosheets are well assembled on 3D porous Ni foam substrate, leading to the formation of well-defined “skeleton/skin” architecture, where Ni network acts as the skeleton and graphene coating acts as the skin (Fig. 2a). From the high-magnification SEM images shown in Fig. 2a inset, we can find that the graphene coating on Ni foam substrate exhibits characteristic wrinkle morphology originated from the nature of graphene nanosheets. The structure of GF by etching Ni substrate of Ni/GF was characterized by XRD and Raman spectra. The XRD pattern reveals a decreased interlayer distance from 8.42 Å to 3.69 Å since the oxygen-containing groups on GO are eliminated to form reduced GO material in GF (Supporting information, Fig. S1a). In Raman spectrum, the intensity ratio of D to G peaks (ID/IG) for GO and GF are 0.92 and 1.26, respectively (Fig. S1b), which is attributed to the increased sp2 domains in GF sample. Furthermore, the XPS C1s spectrum of GF samples shows that the intensity of the C–O peak decreases significantly while the C– C peak becomes predominant in comparison with GO sample (Fig. S2), indicating the successful reduction of GO during the interfacial redox process. For the preparation of Ni/GF/H-CoMoO4 materials, H-CoMoO4 nanoplates were 10


Page 10 of 38

Page 11 of 38

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


in situ grown on Ni/GF support by hydrothermal reaction and subsequent hydrogenation treatment. As shown in the SEM image (Fig. 2b and Fig. S3), both CoMoO4 and H-CoMoO4 nanoplates are standing on Ni/GF support and propping up each other to form well-ordered opened-up array on Ni/GF scaffold. TEM image shows that H-CoMoO4 deposited on Ni/GF exhibits typical crystal structure of ultrathin nanoplate (Fig. 2c), as confirmed by the selected area electron diffraction (SAED) pattern (Fig. 2c inset). The HRTEM image shown in Fig. 2d reveals the lattice fringes of 0.26 and 0.27 nm, corresponding to the (-222) and (-131) plane of CoMoO4, respectively. The Ni/GF/H-Fe2O3 has also been prepared by hydrothermal reaction and hydrogenation treatment. During the hydrothermal procedure, the precursor of H-Fe2O3 nanoplates, i.e., FeOOH nanoplates were in situ grown on Ni/GF support to form Ni/GF/FeOOH (Fig. S2). For comparison, Ni/GF/Fe2O3 was prepared by annealing treatment in Ar atmosphere. Although both of Ni/GF/Fe2O3 and Ni/GF/H-Fe2O3 exhibit 3D opened network constructed by interconnected Fe2O3 nanoplates, the thickness of H-Fe2O3 nanoplates becomes much thinner than that of Fe2O3 nanoplates (Fig. 2e and 2f), and numerous holes can be observed on the H-Fe2O3 nanoplates from the higher-magnification SEM image (Fig. 2f inset) and TEM image (Fig. 2g), which is originated from the introduced oxygen vacancies by thermally annealing in H2/Ar atmosphere. The crystal structure of H-Fe2O3 has also been confirmed by the SAED pattern. The HRTEM image in Fig. 2h shows that H-Fe2O3 species is composed of a significant amount of amorphous phase and some crystalline domains, where the lattice fringes of 0.25 nm and 0.20 nm are corresponding to the (311) and (400) plane of FeO and Fe2O3, respectively. The structural and compositional properties of different samples are explored by 11



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 12 of 38

XRD and XPS characterizations. Fig. 3a displays the XRD pattern of Ni/GF/H-CoMoO4, with Ni/GF/CoMoO4 as the control. Both of these two samples present several distinct diffraction peaks at 2θ value of 13.2o, 19.1o, 23.2o, 26.5o, 28.4o, 32.2o, 33.6o, 36.8o, 38.7o, 40.2o, 43.5o, 45.2o, 47.2o, 52.3o, 53.6o, 54.7o, 58.6o and 60.2o, which were attributed to the reflections of (001), (-201), (021), (002), (-311), (-131), (-222), (400), (040), (003), (-422), (113), (421), (-204), (-531), (440), (024) and (-424) planes indexed to the monoclinic rutile-type CoMoO4 (JCPDS Card No.00-021-0868), respectively,

indicating

that

the

crystallographic

phase

is

retained

for

Ni/GF/H-CoMoO4 after hydrogenation. Noticeable, in comparison to Ni/GF/CoMoO4, there is a slight shift of the peaks towards lower diffraction angles for Ni/GF/H-CoMoO4, suggestive of the expanded interplanar spacing for H-CoMoO4. This may be caused by the increase of lattice constant can be attributed to the substitution of the smaller Mo4+ ions (0.65 Å) onto the Mo6+(0.59 Å) lattice sites. The XRD patterns of GF/H-Fe2O3 and GF/Fe2O3 have been shown in Fig. 3b, which exhibit the typical diffraction peaks closely match those of FeO·Fe2O3 (JCPDS 00-007-0322) and α-Fe2O3 (JCPDS 00-087-1164) for H-Fe2O3 and Fe2O3, respectively, indicating the effective hydrogenation and introduction of oxygen vacancies for H-Fe2O3. In addition, there is a small diffraction peaks at 2θ value of 44.1o, corresponding to (101) diffraction peaks of graphitic carbons in GF substrate.42 XPS measurement has also been performed to acquire the compositional information of different samples. As shown in Fig. 3c, the Co 2p3/2 core-level spectra of CoMoO4 and H-CoMoO4 samples show the binding energy of 781.8 eV assigned to Co2+ oxidation state, and no any significant difference in Co 2p XPS spectra of CoMoO4 and H-CoMoO4, indicating that the hydrogenation treatment does not 12


Page 13 of 38

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


change the oxidation state of Co species in H-CoMoO4 samples. In the Mo 3d XPS spectra of CoMoO4 and H-CoMoO4 samples, two peaks assigned to Mo 3d3/2 and Mo 3d5/2 can be observed at the binding energies of 235.3 and 232.2 eV, respectively (Fig. 3d), with a splitting width of 3.1 eV. This is in line with those reported for Mo6+.43,44 What’s more, for H-CoMoO4 sample, there are two additional peaks centered at 230.0 and 233.2 eV, which match well to the 3d5/2 and 3d3/2 of Mo4+ ions, respectively. This indicates that during the hydrogenation process, Mo6+ species has been partial reduced to Mo4+ species.38 Fig. 3e shows the core-level XPS spectra of Fe 2p for Fe2O3 and H-Fe2O3 nanoplate array on Ni/GF. The XPS spectrum of Fe2O3 sample exhibits typical Fe 2p3/2 and 2p1/2 peaks assigned to Fe3+ at the binding energies of 711.4 and 724.7 eV, respectively.45 Additionally, the core level XPS spectrum of Fe 2p shows an additional peak centered at 708.4 eV for H-Fe2O3 sample, which is attributed to Fe2+.46 This demonstrates that under the hydrogenation treatment, Fe3+ species has been partially reduced to Fe2+ species for H-Fe2O3 sample. The core level XPS spectra of O 1s for the Fe2O3 and H-Fe2O3 samples also display two distinct peaks centered at about 530.3 eV and 531.8 eV (Fig. 3f), which correspond to the lattice oxygen and oxygen defects, respectively.19 Evidently, in comparison to that of the Fe2O3 sample, the peak assigned to the oxygen defects exhibits higher intensity for H-Fe2O3 sample, confirming that the H-Fe2O3 sample possesses more oxygen defects than Fe2O3 sample.

3.2. Electrochemical characterization 13



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 14 of 38

3.2.1. Positive electrode material To explore the practical application of Ni/GF/H-CoMoO4 and Ni/GF/H-Fe2O3 electrodes for electrochemical energy storage, the electrochemical properties of these hybrid electrodes have been characterized in three-electrode system, using the proposed hybrid electrodes as the working electrode. Fig. 4a shows the CV curve of Ni/GF/H-CoMoO4 electrode in a 3 M KOH aqueous solution at a scan rate of 10 mV s-1, using Ni/GF/CoMoO4 electrode as the control. The CV curves of these two electrodes exhibit a pair of well-defined redox peaks, signifying that a Faradaic redox characteristics of their capacitive behaviors. This reveals that Co2+/Co3+ redox couple has been involved in electron transfer process and accounts for quasi reversible redox reaction process on electrode, which may by mediated by the OH- ions in the alkaline electrolyte. However, Mo atoms in CoMoO4 do not involve in electron transfer process and contribute to any capacitance. However, after introducing oxygen defects in CoMoO4, Mo6+ species has been partial reduced to Mo4+ species, as demonstrated by XPS analysis, which increase the electrical conductivity of resultant electrode material. This is confirmed by CV characterization, as shown in Fig. 4a that Ni/GF/H-CoMoO4 electrode exhibits substantially larger peak current associated to Co2+/Co3+

redox

than

that

of

Ni/GF/CoMoO4,

indicative

of

a

superior

pseudocapacitive behavior of H-CoMoO4 material. This is because the induced oxygen vacancies into CoMoO4 can significantly increase its donor density, and give rise to an enhanced conductivity and surface redox reaction reactivity of CoMoO4 species. Furthermore, the oxygen vacancies can act as active sites for the redox reaction of CoMoO4 species, thus improves its capacitive performance. As the oxygen vacancies in the nanohybrid materials have an important effect on 14


Page 15 of 38

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


their electrochemical properties, the influence of hydrogenation temperature on the capacitive performance of the H-CoMoO4 electrode has also been investigated by CV and GCD measurements. Our results show that as the annealing temperature increases from 300 oC to 500 oC, the calculated areal capacitance firstly increases and reaches to its maximum value of 5.36 F cm-2 at the annealing temperature of 400 oC, and further decreases. Therefore, the annealing temperature of 400oC was chosen in the following experiments (Fig. S5). The electrochemical performances of Ni/GF/H-CoMoO4, Ni/GF/CoMoO4, Ni/H-CoMoO4 and Ni/GF electrodes have further been investigated by GCD measurements at a current density of 1 mA cm-2 (Fig. 4b), which show that the charge/discharge curve of Ni/GF/H-CoMoO4 electrode is more symmetric and substantially prolonged than that of other thee control electrodes. Fig. 4c illustrates the calculated areal capacitances as a function of current densities of all hybrid electrodes. The Ni/GF/H-CoMoO4 electrode not only yields the largest areal capacitance up to 5.36 F cm-2 (1472 F g-1) at 1 mA cm-2 among all of these four electrodes, but also exhibits the highest good rate capability with 79.6% retention of the initial capacitance when the current density increases from 1 to 40 mA cm-2, which is much higher than that of Ni/GF/CoMoO4 electrodes (~38.9%). Moreover, the charge storage capacity of Ni/GF/CoMoO4 (3.89 F cm-2) electrode is significantly higher than that of Ni/GF (0.63 F cm-2) the Ni/CoMoO4 (1.186 F cm-2) electrodes at 1 mA cm-2, indicating the synergistic effect between GF substrate and electroactive CoMoO4 nanoplates on its surface in improving the capacitive properties. After charged and discharged for 3000 times, the capacitance of Ni/GF/H-CoMoO4 electrode remains 92.3%, which indicates that the nanohybrid electrode material 15



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

possesses good stability (Fig.S8a). Fig. 4d shows the EIS behaviors of Ni/GF/H-CoMoO4 and Ni/GF/CoMoO4 electrodes. In the impedance spectrum, the semicircle in the high frequency region and the curve slope in the low frequency region represent the charge transfer resistance and diffusional-limited processes, respectively. The Nyquist plots of both Ni/GF/H-CoMoO4 and Ni/GF/CoMoO4 electrodes display a nearly ideal straight line in the low-frequency region, indicating to a very low resistance for electrolyte and ion/proton diffusion. And there are no any apparent semicircles observed in the low-frequency region, which reveals the low charge-transfer resistance of the nanohybrid electrodes. The equivalent series resistance for Ni/GF/H-CoMoO4 electrode is about 0.79 Ω, which is smaller than that of Ni/GF/CoMoO4 electrode (0.81 Ω). From the EIS study, it appears that the introduction of oxygen vacancies in H-CoMoO4 has decreased the resistance for charge transfer and ion diffusion, and accelerated the Faradaic reaction with higher power, which agree well with the results of CV and GCD measurements.

3.2.2. Negative electrode material The electrochemical properties of Ni/GF/H-Fe2O3 as the negative electrode material for ASC have also been characterized in three-electrode electrochemical system. The experimental optimization shows that Ni/GF/H-Fe2O3 electrode obtained at the hydrothermal temperature of 300oC exhibits the largest areal capacitance and highest rate capability from the CV and GCD measurements, indicating a substantial improvement of electrochemical capacitance obtained under the optimal conditions 16


Page 16 of 38

Page 17 of 38

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. S6). Moreover, the CV profiles of Ni/GF/H-Fe2O3 electrode remain similar shape as the scan rates increase from 10 to 200 mV s-1 (Fig. 4e), and the GCD curves also remain similar shape as the current densities increase from 1 to 40 mA cm-2 (Fig. S7). These demonstrate the excellent high-rate capability of Ni/GF/H-Fe2O3 electrode. Fig. 4f shows the charge/discharge curves of Ni/GF/H-Fe2O3, Ni/GF/Fe2O3, Ni/Fe2O3 and Ni/GF electrodes collected at a current density of 1 mA cm-2. And the calculated areal capacitance of these electrodes as a function of current density has been shown in Fig. 4g. The areal capacitance of Ni/GF/Fe2O3 is calculated to be 572 mF cm-2 at 1 mA cm-2, which is much larger than that of Ni/Fe2O3 (141 mF cm-2) and Ni/GF (40 mF cm-2) electrodes, demonstrating the synergistic contribution of double-layer capacitance of GF and pseudocapacitance of Fe2O3 nanoplates in improving the total capacitance of the resultant nanohybrid electrode. Moreover, the Ni/GF/H-Fe2O3 electrode exhibits the largest areal capacitance up to 694 mF cm-2 (177 F g-1) and good capacitance retention of 54.5% when the current densities increase from 1 to 20 mA cm-2, which is much better than that of Ni/GF/Fe2O3 electrode (12.1%). This indicates that the hydrogenation treatment can increase their electronic conductivity as well as improve their donor density and surface properties of H-Fe2O3 nanomaterial, which enhance the capacitance and the rate of surface redox reactions of Fe2O3. Furthermore, the Nyquist plots show that the semicircle diameter in the high frequency region of Ni/GF/H-Fe2O3 electrode is much smaller than that of Ni/GF/Fe2O3 electrode (Fig. 4h), demonstrative of the lower charge transfer resistance for Ni/GF/H-Fe2O3 electrode. The equivalent series resistance for Ni/GF/H-Fe2O3 electrode is calculated to be about 0.63 Ω, which is smaller than that of Ni/GF/Fe2O3 electrode (0.95 Ω). Furthermore, the capacitance of Ni/GF/H-Fe2O3 electrode is 17



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

attenuated by only 5.9% after charged and discharged for 3000 times, demonstrating its good cycle stability (Fig. S8b). 3.3. Assembly of flexible all-solid-state asymmetric supercapacitor Owing to the excellent performance of Ni/GF/H-CoMoO4 and Ni/GF/H-Fe2O3, a flexible all-solid-state ASC device has been assembled, using the Ni/GF/H-CoMoO4 as positive electrode and Ni/GF/H-Fe2O3 as negative electrode (denoted as Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3). In comparison, the control CoMoO4//Fe2O3 ASC device has also been prepared. As shown in Fig. 5a, the operating cell voltage of the as-fabricated Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device has fully utilized the different potential windows of Ni/GF/H-CoMoO4 and Ni/GF/H-Fe2O3 electrodes to maximize the operation voltage extended to 1.5 V. And the CV curves of the as-prepared ASC device do not show any distortion as the scan rates increase from 10 to 200 mV s-1, demonstrating its good capacitive performance (Fig. 5a inset). Fig. 5b shows the GCD curves of Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device at different current densities. The ASC device possesses the maximum volumetric specific capacitances of 3.6 F cm-3, which is two times larger than that of the Ni/GF/CoMoO4//Ni/GF/Fe2O3 device (1.8 F cm-3). Moreover, a remarkably high rate capability has been achieved by the Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device, which retains about 70% of its initial capacitance as the current density increase from 2 to 200 mA cm-3 (Fig. 5c). The EIS behaviors of the ASC device have further been investigated. The Nyquist plots in the high frequency region shows low internal resistance with ESR of 1.03 Ω (Fig. 5d inset). And in the low frequency region, the Nyquist plots exhibits a straight line (Fig. 5d), representing the diffusion limited electron transfer process of the ASC device. 18


Page 18 of 38

Page 19 of 38

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


For the application of Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device in flexible electronics, its mechanical properties are of significance and should be evaluated in detail. The flexibility testing has been performed by bending the flexible all-solid-state ASC device inward to angles of 30°, 90° and 120°, and bending it inward to 90° for several times. The results show that after being bended inward to different angles and bended inward to 90° for 200 times, the decrease of capacitance for the flexible ASC device are 2.4% and 4%, respectively (Fig. 5e), which indicates the

good

mechanical

flexibility

and

strength

of

the

Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device. After 5000 times charge-discharge cycling, the Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device still remains 93.1% of its initial specific capacitance (Fig. 5f). The superior mechanical and electrochemical properties of the Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device are benefited from the strong coupling between the active metal oxide nanoplate array and 3D GF scaffold. In our work, the polycrystalline Ni/GF support possesses a number of points and line defects, which can strongly affect the molecule adsorptions for the nucleation and facilitate in situ growth of CoMoO4 and Fe2O3 nanoplates on GF substrate during the hydrothermal process, which results in strong coupling between the active metal oxide nanoplate array and the 3D GF substrate. Furthermore, three symmetric devices connected in series can light up a yellow light emitting diode (Fig. 5f inset), illustrative of its potential application in energy storage. The supercapacitive properties of energy density and power density for Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device have been estimated. The maximum volumetric energy density of Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device is calculated to

be

1.13

mWh

cm-3,

which

is

substantially

19


higher

than

that

of


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 20 of 38

Ni/GF/CoMoO4//Ni/GF/Fe2O3 (0.57 mWh cm-3). And its energy density is also higher than that of a commercially available supercapacitor (5.5 V, 100 mF, 0.55 mWh cm-3) and the recently reported ASCs, e.g., carbon/MnO2 fiber (0.22 mWh cm-3),47 HZM based SC (0.04 mWh cm-3),48 MnO2//Fe2O3 (0.55 mWh cm-3),19 N-Fe2O3//MnO2 (0.41 mWh cm-3),20 H-TiO2@MnO2//H-TiO2@C (0.3 mWh cm-3),40 TiN-SC (0.05 mWh cm-3).49

And

the

mass-energy

density

and

power

density

of

Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device are also higher than those of the previously reported works (Fig. S9). Furthermore, the leakage current during self-discharge, which will cause voltage decay of a charged supercapacitor over time, has also been measured. Fig. 6b shows the self-discharge curves obtained immediately after charging the device to Umax, where the time required for the voltage across the supercapacitor changing from Umax to 1/2 Umax has been measured. The leakage current significantly drops from 67 mA to 215 µA and retains an output voltage of 0.75 V after 44 h for our flexible ASC device, which is superior to that of commercial supercapacitors (i.e., 21 h).50,51

4. CONCLUSION In summary, we present the development of flexible ACS device based on well-ordered H-CoMoO4 and H-Fe2O3 nanoplate arrays standing up on 3D porous GF scaffold. This strategy demonstrates several advantages including: i) 3D GF electrode substrate possesses large surface area and abundant surface functional groups, good conductivity and unique mechanical properties, which not only serves as an ideal support to facilitate in situ growth of active metal oxide nanomaterials on it and keep 20


Page 21 of 38

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


them stable during the mechanical and electrochemical testing, but also contributes to the double-layer capacitance of resultant ASC system; ii) The well-ordered metal oxide nanoplate arrays on 3D GF provide large contact area of electrode/electrolyte and shorten the diffusion length for active species; iii) The introduction of oxygen vacancies into metal oxide nanoplates has significantly increased the conductivity and accelerated the kinetics of the surface redox reactions, which gives rise to an improve electrochemical performance; iv) The flexible ASC device with Ni/GF/H-CoMoO4 as the positive electrode and Ni/GF/H-Fe2O3 as the negative electrode exhibits a high operating voltage of 1.5 V, which increases the energy density. Therefore, the as-obtained flexible all-solid-state ASC device achieves a maximum energy density of 1.13 mWh cm-3 at a current density of 5 mA cm-3, and a maximum power density of 150 kW cm-3 at a current density of 200 mA cm-3. It can be envisioned that our strategies for the rational design of electrode construction and controllable synthesis of active electrode materials will open a new horizon in the development of advanced flexible electrochemical energy storage devices, which will contribute to the advance of the next-generation portable/wearable personal electronics.

ASSOCIATED CONTENT Supporting Information

XRD patterns of GO and GF. Raman spectra of GO and GF. C 1s spectra of GO and GF. SEM image of Ni/GF/CoMoO4 and Ni/GF/FeOOH. CV curves, Galvanostatic charge/discharge curves and Areal capacitance as a function of current density for 21



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

CoMoO4

and

Fe2O3

hydrogenated

at

various

temperatures.

Page 22 of 38

Galvanostatic

charge/discharge curves of Ni/GF/H-Fe2O3 electrode at various current densities. Cycling behavior of Ni/GF/H-CoMoO4 and Ni/GF/H-Fe2O3 electrodes. Ragone plot of the device and some other devices from previous literature for comparison.

AUTHOR INFORMATION Corresponding Author *S. Wang. E-mail: [email protected]. *F. Xiao. E-mail: [email protected].

Author Contributions §

Both the authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Project 22


Page 23 of 38

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


No. 51572094 and 21305048). We thank the Analytical and Testing Center of Huazhong University of Science and Technology, the Wuhan National Laboratory for Optoelectronics.

REFERENCES (1) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845-854.

(2) Wang, G. P.; Zhang, L.; Zhang, J. J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828.

(3) Lin, T. Q.; Chen, I. W.; Liu, F. X.; Yang, C. Y.; Bi, H.; Xu, F. F.; Huang, F. Q. Nitrogen-Doped

Mesoporous

Carbon

of

Extraordinary

Capacitance

for

Electrochemical Energy Storage. Science, 2015, 350, 1508-1513.

(4) Wang, X. F.; Lu, X. H.; Liu, B.; Chen, D.; Tong, Y. X.; Shen, G. Z. Flexible Energy-Storage Devices: Design Consideration and Recent Progress. Adv. Mater. 2014, 26, 4763-4782.

(5) Yang, P. H.; Mai, W. J. Flexible Solid-state Electrochemical Supercapacitors. Nano Energy. 2014, 8, 274-290.

(6) Yan, J.; Wang, Q.; Wei, T.; Fan, Z. J. Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities. Adv. Energy Mater. 2014, 4, 1300816. 23



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 24 of 38

(7) Zhang, Z. Y.; Xiao, F.; Qian, L. H.; Xiao, J. W.; Wang, S.; Liu, Y. Q. Facile Synthesis of 3D MnO2-Graphene and Carbon Nanotube-Graphene Composite Networks

for

High-Performance,

Flexible,

All-Solid-State

Asymmetric

Supercapacitors. Adv. Energy Mater. 2014, 4, 1400064.

(8) Chi, K.; Zhang, Z. Y.; Xi, J. B.; Huang, Y. A.; Xiao, F.; Wang, S.; Liu, Y. Q. Freestanding

Graphene

Paper

Supported

Three-Dimensional

Porous

Graphene-Polyaniline Nanocomposite Synthesized by Inkjet Printing and in Flexible All-Solid-State Supercapacitor. ACS Appl. Mater. Interfaces. 2014, 6, 16312-16319.

(9) Zhu, C.; Liu,T. Y.; Qian, F.; Han, T. Y. J.; Duoss, E. B.; Kuntz, J. D.; Spadaccini, C. M.; Worsley, M. A.; Li, Y. Supercapacitors Based on Three-Dimensional Hierarchical Graphene Aerogels with Periodic Macropores. Nano Lett. 2016, 16, 3448-3456.

(10) Ji, J. Y.; Li, Y.; Peng, W. C.; Zhang, G. L.; Zhang, F. B.; Fan, X. B. Advanced Graphene-Based Binder-Free Electrodes for High-Performance Energy Storage. Adv. Mater. 2015, 27, 5264-5279.

(11) Li, Q.; Mahmood, N.; Zhu, J. H.; Hou, Y. L.; Sun, S. H. Graphene and Its Composites with Nanoparticles for Electrochemical Energy Applications. Nano Today. 2014, 9, 668-683.

(12) Wang, X. L.; Shi, G. Q. Flexible Graphene Devices Related to Energy Conversion and Storage. Energy Environ. Sci. 2015, 8, 790-823.

24


Page 25 of 38

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


(13) Sun, Y. M.; Fang, Z.; Wang, C. X.; Zhou, A. J.; Duan, H. W. Incorporating Nanoporous Polyaniline into Layer-by-layer Ionic Liquid-Carbon Nanotube-Graphene Paper: towards Freestanding Flexible Electrodes with Improved Supercapacitive Performance. Nanotechnology. 2015, 26, 374002.

(14) Dou, S.; Tao, L.; Huo, J.; Wang, S. Y.; Dai, L. M. Etched and Doped Co9S8/Graphene Hybrid for Oxygen Electrocatalysis. Energy Environ. Sci. 2016, 9, 1320-1326.

(15) Salunkhe, R. R.; Bastakoti, B. P.; Hsu, C. T.; Suzuki, N.; Kim, J. H.; Dou, S. X.; Hu, C. C.; Yamauchi, Y. Direct Growth of Cobalt Hydroxide Rods on Nickel Foam and Its Application for Energy Storage. Chem. Eur. J. 2014, 20, 3084-3088.

(16) Toupin, M.; Brousse, T.; Be´langer, D. Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor. Chem. Mater. 2004, 16, 3184-3190.

(17) Sun, Y. M.; Fang, Z.; Wang, C. X.; Rakhitha Malinga Ariyawansha, K. R.; Zhou, A. J.; Duan, H. W. Sandwich-Structured Nanohybrid Paper Based on Controllable Growth of Nanostructured MnO2 on Ionic Liquid Functionalized Graphene Paper as a Flexible Supercapacitor Electrode. Nanoscale. 2015, 7, 7790-7801.

(18) Sun, Y. M.; Cheng, Y. B.; He, K.; Zhou, A. J.; Duan, H. W. One-step Synthesis of Three-Dimensional Porous Ionic Liquid-Carbon Nanotube-Graphene Gel and

25



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

MnO2-Graphene Gel as Freestanding Electrodes for Asymmetric Supercapacitors. RSC Adv. 2015, 5, 10178-10186.

(19) Yang, P. H.; Ding, Y.; Lin, Z. Y.; Chen, Z. W.; Li, Y. Z.; Qiang, P. F.; Ebrahimi, M.; Mai, W. J.; Wong, C. P.; Wang, Z. L. Low-Cost High-Performance Solid-State Asymmetric Supercapacitors Based on MnO2 Nanowires and Fe2O3 Nanotubes. Nano Lett. 2014, 14, 731-736.

(20) Lu, X. H.; Zeng, Y. X.; Yu, M. H.; Zhai, T.; Liang, C. L.; Xie, S. L.; Balogun, M. S.; Tong, Y. X. Oxygen-Deficient Hematite Nanorods as High-Performance and Novel Negative Electrodes for Flexible Asymmetric Supercapacitors. Adv. Mater. 2014, 26, 3148-3155.

(21) Guan, C.; Liu, J. L.; Wang, Y. D.; Mao, L.; Fan, Z. X.; Shen, Z. X.; Zhang, H.; Wang, J. Iron Oxide-Decorated Carbon for Supercapacitor Anodes with Ultrahigh Energy Density and Outstanding Cycling Stability. ACS Nano. 2015, 9, 5198-5207.

(22) Ma, Z. L.; Huang, X. B.; Dou, S.; Wu, J. H.; Wang, S. Y. One-Pot Synthesis of Fe2O3 Nanoparticles on Nitrogen-Doped Graphene as Advanced Supercapacitor Electrode Materials. J. Phys. Chem. C. 2014, 118, 17231-17239.

(23) Gao, Z.; Yang, W. L.; Wang, J.; Song, N. N.; Li, X. D. Flexible All-Solid-State Hierarchical NiCo2O4/Porous Grapheme Paper Asymmetric Supercapacitors with An Exceptional Combination of Electrochemical Properties. Nano Energy. 2015, 13, 306-317. 26


Page 26 of 38

Page 27 of 38

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


(24) Qiu, K. W.; Lu, Y.; Zhang, D. Y.; Cheng, J. B.; Yan, H. L.; Xu, J. Y.; Liu, X. M.; Kim, J. K.; Luo, Y. S. Mesoporous, Hierarchical Core/Shell Structured ZnCo2O4/MnO2 Nanocone Forests for High-Performance Supercapacitors. Nano Energy, 2015, 11, 687-696.

(25) Mai, L. Q.; Yang, F.; Zhao, Y. L.; Xu, X.; Xu, L.; Luo, Y. Z. Hierarchical MnMoO4/CoMoO4 Heterostructured Nanowires with Enhanced Supercapacitor Performance. Nat. Commun. 2011, 2, 381.

(26) Peng, S. J.; Li, L. L.; Wu, H. B.; Madhavi, S.; Lou, X. W. Controlled Growth of NiMoO4 Nanosheet and Nanorod Arrays on Various Conductive Substrates as Advanced Electrodes for Asymmetric Supercapacitors. Adv. Energy Mater. 2015, 5, 1401172.

(27) Yu, X. Z.; Lu, B. A.; Xu, Z. Super Long-Life Supercapacitors Based on the Construction of Nanohoneycomb-Like Strongly Coupled CoMoO4-3D Graphene Hybrid Electrodes. Adv. Mater. 2014, 26, 1044-1051.

(28) Guo, D.; Zhang, H. M.; Yu, X. Z.; Zhang, M.; Zhang, P.; Li, Q. H.; Wang, T. H. Facile Synthesis and Excellent Electrochemical Properties of CoMoO4 Nanoplate Arrays as Supercapacitors. J. Mater. Chem. A. 2013, 1, 7247-7254.

(29) Li, M.; Xu, S. H.; Cherry, C.; Zhu, Y. P.; Wu, D. J.; Zhang, C.; Zhang, X. L.; Huang, R.; Qian, R. J.; Wang, L. W.; Chu, P. K. Hierarchical 3-Dimensional CoMoO4

27



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

Nanoflakes on A Macroporous Electrically Conductive Network with Superior Electrochemical Performance. J. Mater. Chem. A. 2015, 3, 13776-13785.

(30) Han, L. J.; Tang, P. Y.; Zhang, L. Hierarchical Co3O4@PPy@MnO2 Core-Shell-Shell Nanowire Arrays for Enhanced Electrochemical Energy Storage. Nano Energy. 2014, 7, 42-51.

(31) Zhao, Y. F.; Ran, W.; He, J.; Huang, Y. Z.; Liu, Z. F.; Liu, W.; Tang, Y. F.; Zhang, L.; Gao, D. W.; Gao, F. M. High-Performance Asymmetric Supercapacitors Based on Multilayer MnO2/Graphene Oxide Nanoflakes and Hierarchical Porous Carbon with Enhanced Cycling Stability. Small. 2015, 11, 1310-1319.

(32) Xia, X. H.; Zhang, Y. Q.; Fan, Z. X.; Chao, D. L.; Xiong, Q. Q.; Tu, J. P.; Zhang, H.; Fan, H. J. Novel Metal@Carbon Spheres Core-Shell Arrays by Controlled Self-Assembly of Carbon Nanospheres: A Stable and Flexible Supercapacitor Electrode. Adv. Energy Mater. 2015, 5, 1401709.

(33) Zhu, C. R.; Yang, P. H.; Chao, D. L.; Wang, X. L.; Zhang, X.; Chen, S.; Tay, B. K.; Huang, H.; Zhang, H.; Mai, W. J.; Fan, H. J. All Metal Nitrides Solid-State Asymmetric Supercapacitors. Adv. Mater. 2015, 27, 4566-4571.

(34) Zeng, Y. X.; Han, Y.; Zhao, Y. T.; Zeng, Y.; Yu, M. H.; Liu, Y. J.; Tang, H. L.; Tong, Y. X.; Lu, X. H. Advanced Ti-Doped Fe2O3@PEDOT Core/Shell Anode for High-Energy Asymmetric Supercapacitors. Adv. Energy Mater. 2015, 5, 1402176.

28


Page 28 of 38

Page 29 of 38

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) Lv, Q. Y.; Wang, S.; Sun, H. Y.; Luo, J.; Xiao, J.; Xiao, J. W.; Xiao, F.; Wang, S. Solid-State Thin-Film Supercapacitors with Ultrafast Charge/Discharge Based on N-Doped-Carbon-Tubes/Au-Nanoparticles-Doped-MnO2 Nanocomposites. Nano Lett. 2016, 16, 40-47.

(36) Li, R. Z.; Wang, Y. M. Zhou, C.; Wang, C.; Ba, X.; Li, Y. Y.; Huang, X. T.; Liu, J. P. Carbon-Stabilized High-Capacity Ferroferric Oxide Nanorod Array for Flexible Solid-State Alkaline Battery-Supercapacitor Hybrid Device with High Environmental Suitability. Adv. Funct. Mater. 2015, 25, 5384-5394.

(37) Zhai, T.; Xie, S. L.; Yu, M. H.; Fang, P. P.; Liang, C. L.; Lu, X. H.; Tong, Y. X. Oxygen Vacancies Enhancing Capacitive Properties of MnO2 Nanorods for Wearable Asymmetric Supercapacitors. Nano Energy. 2014, 8, 255-263.

(38) Yan, X. D.; Tian, L. H.; Murowchick, J.; Chen, X. B. Partially Amorphized MnMoO4 for Highly Efficient Energy Storage and the Hydrogen Evolution Reaction. J. Mater. Chem. A. 2016, 4, 3683-3688.

(39) Xiao, X.; Peng, Z. H.; Chen, C.; Zhang, C. F.; Beidaghi, M.; Yang, Z. H.; Wu, N.; Huang, Y. H.; Miao, L.; Gogotsi, Y.; Zhou, J. Freestanding MoO3-x Nanobelt/Carbon Nanotube Films for Li-Ion Intercalation Pseudocapacitors. Nano Energy. 2014, 9, 355-363.

29



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

(40) Lu, X. H.; Yu, M. H.; Wang, G. M.; Zhai, T. Xie, S. L.; Ling, Y. C.; Tong, Y. X.; Li, Y. H-TiO2@MnO2//H-TiO2@C Core-Shell Nanowires for High Performance and Flexible Asymmetric Supercapacitors. Adv. Mater. 2013, 25, 267-272.

(41) Zhang, Z. Y.; Chi, K.; Xiao, F.; Wang, S. Advanced solid-state asymmetric supercapacitors based on 3D graphene/MnO2 and graphene/polypyrrole hybrid architectures. J. Mater. Chem. A. 2015, 3, 12828-12835.

(42) Fayos, J. Possible 3D Carbon Structures as Progressive Intermediates in Graphite to Diamond Phase Transition. J. Solid. State. Chem. 1999, 148, 278-285.

(43) Xu, X. W.; Shen, J. F.; Li, N.; Ye, M. X. Microwave-Assisted Synthesis of Graphene/CoMoO4 Nanocomposites with Enhanced Supercapacitor Performance. J. Alloys. Compd. 2014, 616, 58-65.

(44) Xia, X. F.; Lei, W.; Hao, Q. L.; Wang, W. J.; Wang, X. One-Step Synthesis of CoMoO4/Graphene Composites with Enhanced Electrochemical Properties for Supercapacitors. Electrochim. Acta. 2013, 99, 253-261. (45) Yamashita, T.; Hayes, P. Analysis of XPS Spectra of Fe2+ and Fe3+ Ions in Oxide Materials. Appl. Surf. Sci. 2008, 254, 2441-2449.

(46) Fujii, T.; Groot, F. M. F. de.; Sawatzky, G. A. In Situ XPS Analysis of Various Iron Oxide Films Grown by NO2-Assisted Molecular-Beam Epitaxy. Phys. Rev. B. 1999, 59, 3195-3202.

30


Page 30 of 38

Page 31 of 38

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


(47) Xiao, X.; Li, T. Q.; Yang, P. H.; Gao, Y.; Jin, H. Y.; Ni, W. J.; Zhan, W. H.; Zhang, X. H.; Cao, Y. Z.; Zhong, J. W.; Gong, L.; Yen, W. C.; Mai, W. J.; Chen, J.; Huo, K. F.; Chueh, Y. L.; Wang, Z. L.; Zhou, J. Fiber-Based All-Solid-State Flexible Supercapacitors for Self-Powered Systems. ACS Nano. 2012, 6, 9200-9206.

(48) Yang, P. H.; Xiao, X. Li, Y. Z.; Ding, Y.; Qiang, P. F.; Tan, X. H.; Mai, W. J.; Lin, Z. Y.; Wu, W. Z.; Li, T. Q.; Jin, H. Y.; Liu, P. Y.; Zhou, J.; Wong, C. P.; Wang, Z. L. Hydrogenated ZnO Core-Shell Nanocables for Flexible Supercapacitors and Self-Powered Systems. ACS Nano. 2013, 7, 2617-2626.

(49) Lu, X. H.; Wang, G. M.; Zhai, T.; Yu, M. H.; Xie, S. L.; Ling, Y. C.; Liang, C. L.; Tong, Y. X.; Li, Y. Stabilized TiN Nanowire Arrays for High-Performance and Flexible Supercapacitors. Nano Lett. 2012, 12, 5376-5381.

(50) El-Kady, M. F.; Kaner, R. B. Scalable Fabrication of High-Power Graphene Micro-Supercapacitors for Flexible and on-Chip Energy Storage. Nat. Commun. 2013, 4, 1475.

(51) Yang, P. H.; Chao, D. L.; Zhu, C. R.; Xia, X. H.; Zhang, Y. Q.; Wang, X. L.; Sun, P.; Tay, B. K.; Shen, Z. X.; Mai, W. J.; Fan, H. J. Ultrafast-Charging Supercapacitors Based on Corn-Like Titanium Nitride Nanostructures. Adv. Sci. 2016, 3, 1500299.

31



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 captions

Figure 1 Schematic illustration of the synthesis of Ni/GF/H-CoMoO4 and Ni/GF/H-Fe2O3 electrodes, and the assembly of flexible solid-state ASC device with Ni/GF/H-CoMoO4 as the positive electrode and Ni/GF/H-Fe2O3 as the negative electrode.

32


Page 32 of 38

Page 33 of 38

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) SEM image of Ni/GF. (b) SEM image of well-ordered H-CoMoO4 nanoplates array on Ni/GF. (c) TEM image and (d) HRTEM image of H-CoMoO4 nanoplates, inset of (c) is the corresponding SAED pattern. SEM image of well-ordered (e) Fe2O3 and (f) H-Fe2O3 nanoplates arrays on Ni/GF. (g) TEM image and (h) HRTEM image of H-Fe2O3 nanoplates, inset of (g) is the corresponding SAED pattern.

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 (a) XRD patterns of the CoMoO4 and H-CoMoO4 nanoplate arrays on Ni/GF, inset of (a) is the magnified region showing the detectable shift for these different samples. (b) XRD patterns of the Fe2O3 and H-Fe2O3 nanoplate arrays on Ni/GF. XPS spectra of (c) Co 2p and (d) Mo 3d for CoMoO4 and H-CoMoO4 nanoplate arrays on Ni/GF. And XPS spectra of (e) Fe 2p and (f) O 1s for Fe2O3 and H-Fe2O3 nanoplate arrays on Ni/GF. 34


Page 34 of 38

Page 35 of 38

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) CV curves of Ni/GF/H-CoMoO4 and Ni/GF/CoMoO4 electrodes at a scan rate of 10 mV s-1. (b) GCD curves of Ni/GF/H-CoMoO4, Ni/GF/CoMoO4, Ni/ CoMoO4 and Ni/GF electrodes at a current density of 1 mA cm-2. (c) Plot of the current density against the areal capacitance of different nanohybrid electrodes obtained from the GCD curves. (d) Nyquist plots of Ni/GF/H-CoMoO4 and Ni/GF/CoMoO4 electrodes. (e) CV curves of Ni/GF/H-Fe2O3 electrodes at diffident scan rate from 10 to 200 mV s-1. (f) GCD curves of Ni/GF/H-Fe2O3, Ni/GF/Fe2O3, Ni/Fe2O3 and Ni/GF electrodes at a current density of 1 mA cm-2. (g) Plot of the current density against the areal capacitance of different nanohybrid electrodes obtained from the GCD curves. (h) Nyquist plots of Ni/GF/H-Fe2O3 and Ni/GF/Fe2O3 electrodes, inset of (h) is the enlarged Nyquist plot in the high-frequency region.

35



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 5 (a) CV curves of Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device in different scan voltage windows, inset of (a) is the ASC device at different scan rates. (b) GCD curves of the ASC device collected at different current densities. (c) Volumetric capacitance of Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 and Ni/GF/CoMoO4//Ni/GF/Fe2O3 devices as a function of current density. (d) Nyquist plots of Ni/GF/H-CoMoO4//Ni/ GF/H-Fe2O3 device, inset of (d) is the enlarged Nyquist plot in the high-frequency region. (e) Specific capacitance retention ratio of the flexible all-solid-state ASC device after bending inward to different angles or being bent repeatedly. (f) Cycling behavior of the ASC device at a current density of 15 mA cm-2.

36


Page 36 of 38

Page 37 of 38

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

6

(a)

Ragone

plots

of

the

flexible

all-solid-state

Ni/GF/H-CoMoO4//Ni/GF/H-Fe2O3 device compared with the selected previous solid-state supercapacitors. (b) Leakage current and self-discharge characteristics of the all-solid-state ASC device.

37



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 of Content

38


Page 38 of 38

Well-Ordered Oxygen-Deficient CoMoO4 and Fe2O3 Nanoplate

Recommend Documents