Boosting the Energy Density of Flexible Solid-State Supercapacitors

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Boosting the Energy Density of Flexible Solid-State Supercapacitors via Both Ternary NiV2Se4 and NiFe2Se4 Nanosheet Arrays Thanh Tuan Nguyen, Jayaraman Balamurugan, Vanchiappan Aravindan, Nam Hoon Kim, and Joong Hee Lee Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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Chemistry of Materials

Boosting the Energy Density of Flexible Solid-State Supercapacitors via Both Ternary NiV2Se4 and NiFe2Se4 Nanosheet Arrays Thanh Tuan Nguyen†, Jayaraman Balamurugan†, Vanchiappan Aravindan‡, Nam Hoon Kim*†, and Joong Hee Lee*†§ Advanced Materials Institute of BIN Convergence Technology (BK21 Plus Global) & Dept. of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea ‡ Department of Chemistry, Indian Institute of Science Education and Research (IISER), Tirupati-517507, India § Carbon Composite Research Centre, Department of Polymer - Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea †

ABSTRACT: Due to the demand for sustainable energy storage devices, investigating high energy density and cost-effective electrodes for flexible supercapacitors (SCs) is essential; however, the emergence of such high-performance electrodes to fulfill the requirements of industrial sectors remains a highly challenging task. Herein, we successfully demonstrated the preparation of ternary metal selenides of nickel-vanadium selenide (NixV3-xSe4) and nickel-iron selenide (NixFe3-xSe4) series by a simple and low-cost hydrothermal, followed by selenization for flexible asymmetric SC (ASC). The impacts of Ni2+ are studied and shown to lead to significant enhancement in electrochemical properties which varied with the stoichiometric ratio of Ni: V/Fe in NixV3-xSe4 and NixFe3-xSe4 nanosheet arrays. The optimized NiV2Se4 and NiFe2Se4 electrodes displayed high specific capacities (329 and 261 mAh g1, respectively, at 1 mA cm2), excellent rate performances (capacity retentions of about 79.33% and 77.78%, respectively, even at 50 mA cm2), and outstanding cycling stability (98.6 % and 97.9 % capacity retention after 10 000 cycles, respectively). Most notably, NiV2Se4//NiFe2Se4 ASC provides an excellent energy density of 73.5 Wh kg−1 at the power density of 0.733 kW kg−1 and superior cycling performance (96.6% capacity retention after 10 000 cycles). The high-performance nanostructured flexible ASCs show promise in portable electronics and zero-emission transportation.

1. Introduction

profile,10 which certainly hinders their potential usefulness as an electrode material from a commercialization point of view.

Renewable energy storage and conversion technologies are of immense significance in addressing energy management in response to worldwide environmental changes.1-3 Among the existing energy storage systems, one of the most promising candidates are SCs, which have attracted extensive interest in the research community because of their numerous advantages of ultra-fast rechargeability, high power density, round-trip efficiency, and tremendous cycling stability.4,5 However, the fact that they lag in energy density is worth mentioning as a major setback for such fascinating SCs. In order to achieve a high-energy SC, the exploitation of battery-type electrode in the SC assembly is the crucial component for realizing the same, i.e., construction of SCs in an asymmetric fashion.6 Consequently, numerous efforts have been devoted to identifying suitable battery-type electrodes with desired electrochemical properties to achieve high energy density. For example, conducting polymers and metal oxides are widely studied as battery-type electrode materials for ASC because of their unique properties such as superior Faradic redox kinetics7,8 and ultra-high specific capacities as compared to commercially available carbonaceous materials (e.g., activated carbon, AC).9 While metal oxides show excellent electrochemical activities, they exhibit poor electrical conductivity, low rate performances, and inferior cycling

Recently, metal selenides have been regarded as a new class of battery-type electrodes because of their superior electronic conductivity, enriched redox active sites, lower bandgap, large specific surface area (SSA), and ultra-high specific capacity as compared to their corresponding metal oxide analogs.11,12 Among the reported metal selenide-based electrodes, vanadium selenide13, and iron selenide14 are promising candidates for energy storage applications due to their catalytic activities and electrical conductivities. The earlier studies were found to be three times higher for selenides than for their corresponding oxides.15 However, the electrochemical activities of vanadium selenide (V3Se4) and iron selenide (Fe3Se4) are still unsatisfying, specifically in terms of their poor rate capability and cycling stability because of their deprived structural integrity and synergistic effects, which eventually hinders their potential use. Therefore, it is essential and necessary to fine-tune the structural aspect, e.g., by preparing hierarchical nanoarchitectures with excellent conductivity, large SSA, unique porous networks, and electrochemical stability. These features certainly allow the metal selenides (V3Se4 and Fe3Se4) to achieve higher energy density without conceding cycling stability and power density.15 Further, the hierarchical nanostructures should be sensibly altered with enhanced active sites which eventually

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circumvent the “dead-mass” of the active electrodes, i.e., binder and current collector. The use of bi-metal alloys as an active material is considered to be one of the versatile routes for investigating the novel functions, such as synergistic effect, electronic configuration, and electrochemical activity.16,17 For example, Alshareef et al.18 presented the alloying effect of (Ni, Co)0.85Se for improving the SC performance by tuning the morphology without altering the crystal structure. Hu et al.4 reported the utilization of bi-metal alloy and its importance in the electrochemical activity of nanoporous (NixCo1−x)9Se8 with tunable porosity and controllable compositions. It is important to note that the specific capacity and rate performances fully depend on the stoichiometric ratios of Co: Ni in the (NixCo1−x)9Se8 nanodendrite arrays. Inspired by these factors, herein, we have proposed the electrochemical properties of the V3Se4 and Fe3Se4 nanostructures, and they may be boosted by the introduction of some promising element such as nickel (Ni). The choice of Ni is predominantly based on its natural abundance, cost-effective nature, and the fact that it has multiple oxidation states19-21 to succeed in the process of establishing NixV3-xSe4 and NixFe3-xSe4 with tailored nanostructures. Therefore, NixV3-xSe4 and NixFe3-xSe4 nanostructures are anticipated to be better electrodes for flexible ASC due to their unique electrochemical properties. There has been no study to date focused on free-standing NixV3xSe4 and NixFe3-xSe4 electrodes for the construction of flexible solid-state SCs. On the other hand, the oriented growth between the substrates and foreign catalysts could impede the charge transport properties during the electrochemical performances. Conceivable active material aggregation over the surface of the electrode may reduce the electroactive sites and increase the “dead mass,” consequently resulting in underperforming ASC. Beyond the Se-based active material preparation, the morphology and structure of the electrodes will significantly influence the electrochemical performance. To date, numerous nanostructures such as nanotubes, nanoparticles, nanowires, nanosheets, and the arrays of various substrates have been investigated for SC applications.22 However, most of these reports are of nanostructures in powdered form, which requires a binder and conductive additive to formulate the electrodes for SC applications. The passive components, like a binder and conductive additive, certainly impede the ion accessibility to the porous architecture, which often leads to higher electrical equivalent series resistance (ESR) values and inferior performance. This problem can be easily addressed by the direct growth of active material on a 3D porous metallic substrate, which not only increases the electron transport kinetics but also provides more porosity, which in turn transforms more active sites for ion accessibility. Enhancing the cycling life of the ternary metal selenide without experiencing any drop in the electrical conductivity remains a challenging task. Moreover, the rigid nature of 3D porous metallic substrate hinders their application in flexible and wearable energy storage devices in modern electronics. In order to overcome this problem, carbon fiber cloth (CFC) substrates are considered to be the perfect choice due to their excellent flexibility, high electrical conductivity, ultra-high chemical integrity regardless of the electrolyte medium, and low manufacturing cost.23-25 This is further addressed by the electrode materials grown over the CFC, which not only enhances the flexibility but also offers more voids to grow the

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nanostructures with hierarchical architectures for extended active sites by increasing the ion accessibility. Finally, the poor ionic conductivity and leakage of the electrolyte certainly reduce the cycling life. The detachment and dissolution of the active materials in a liquid electrolyte often occur, and eventually ruin the SC performance, hindering its potential use. Therefore, the usage of solid-state or gel-electrolyte is strongly encouraged. Interestingly, gel electrolyte often translates shape versatility, which logically led us to fabricate a flexible ASC assembly. This flexible configuration not only overcomes the conductivity issue of the active materials but also hampers the inevitable side reaction with electrolyte. Therefore, it is possible to enhance the energy density with outstanding cycling stability without compromising power capability. Herein, we would like to present high-performance flexible electrodes based on the hierarchical design of porous NixV3xSe4 and NixFe3-xSe4 nanosheet arrays (NAs) fabricated by a simple and scalable hydrothermal approach followed by an effective selenization process. This hierarchical NixV3-xSe4 and NixFe3-xSe4 NAs with tunable stoichiometric ratios of Ni: V/Fe is investigated in detail. Our present work aims to enhance the SSA, unique porous architectures, and fine-tune the electronic structure by Ni-substitution, which subsequently facilitates the ion transport kinetics and boosts the electroactive sites for SCs. As stated previously, both NixV3-xSe4 and NixFe3-xSe4 electrodes possess numerous benefits such as excellent conductivity, exceptional electrochemical properties, low-cost, environmental friendliness, and mechanical robustness. Most importantly, the highly flexible solid-state ASC assembled using the optimal NiV2Se4 and NiFe2Se4 concerning the positive and negative electrodes improve the energy and power densities, since these are essential for SCs in practical applications. In order to validate its immense ability as a power source for the practical use, the flexible ASC device was investigated for its proficiency in powering green light emitting diodes (LED) and effectively run a toy fan. Thus, we trust that the current synthesis method can be simply interpreted for the general route to design various ternary metal selenide-based NAs for next-generation flexible energy storage technologies. 2. Results and Discussion 2.1. Structural and Morphological Characterizations of NixV3-xSe4 and NixFe3-xSe4 NAs. The overall synthesis process for the NixV3-xSe4 and NixFe3-xSe4 NAs (0  x  2) is schematically presented in Figure 1. In the first step, the NixV3-x and NixFe3-x LDH precursors are directly grown on CFC by a simple hydrothermal approach. Then, both the NixV3-x and NixFe3-x LDH precursors are successfully converted to selenides while retaining their original structures.26 In order to investigate the effect of Ni substitution into the V and Fe places of NixV3-xSe4 and NixFe3-xSe4 nanostructures, a sequence of the electrodes was fabricated with different stoichiometric ratios and studied from an energy storage perspective. Further, to validate the synergistic effect of these hierarchical 3D nanostructures, we fine-tuned the reaction time in the selenization process. The successful formation of the NixV3-xSe4 nanostructures was investigated by X-ray diffraction (XRD) and the results are shown in Figure 2a. The XRD patterns of NixV3-xSe4 (x = 0) are consistent with those of V3Se4 phase (PDF#01-087-2431), and the renowned reflections can be denoted to (202), (114), and (310) planes at the angles of 2 = 33.01, 42.95, and 51.44, respectively.


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Figure 1. Schematic representation for the design and fabrication of hierarchical NixV3-xSe4 and NixFe3-xSe4 nanostructures for solidstate ASC. The NixV3-xSe4 (x = 1) exhibits an XRD pattern similar to that of NixV3-xSe4 (x = 0). Besides, the presence of new peaks was observed at 36.9, 50.1, and 57.1, and these correspond to the (203), (115), and (206) planes of NiV2Se4 (PDF#01-073-1478). Note that the appearance of Ni peak demonstrates the perfect formation of the NixV3-xSe4 (x = 1) phase, where increasing the concentration of the Ni in the ternary metal selenide encourages the structural transformation from one phase to another phase, which is consistent with the results of previous report.27 As presented in Figure 2a, the (202), (114), and (310) planes of the NixV3-xSe4 (x = 1, 1.5, and 2) NAs significantly shifted towards lower angles with the Ni content. In addition, the (310) diffraction peak exhibits a well-organized shape with symmetric aspects. The lattice parameter value (a) of the NixV3xSe4 (x = 1, 1.5, and 2) samples increases from 6.19 to 6.25 Å, which is consistent with the consecutive shift of the (112) peak, as illustrated in Figure 2a. This linear tendency of this variation obeys Vegard's law well,28 serving as important evidence that Ni2+ are effectively substituted into form the bimetal alloy of the NixV3-xSe4.29 In consideration of the very close ionic radii of Ni2+ (0.69 Å) and V4+ (0.67 Å), nickel may efficiently replace vanadium in the crystal lattice of V3Se4, which is clear from the deviation in the lattice parameter values.30 The elemental compositions and purity of the NixV3-xSe4 (x = 1) NAs were examined by X-ray photoelectron spectroscopy (XPS) and presented in Figure S1ac. The high-resolution Ni 2p peaks (Figure S1a) are de-convoluted to 2p1/2, 2p3/2, and corresponding satellite peaks. The Ni-Se is shown to be originated at 871.1 and 853.4 eV, which corresponds to the 2p1/2 and 2p3/2 peaks, respectively.31 These results suggest that the Ni has been effectively doped into the NixV3-xSe4 (x = 1) NAs crystal lattice. The high-resolution V 2p peaks (Figure S1b) are de-convoluted into two pairs of 2p1/2, 2p3/2 of V4+ and V5+. In the case of the high-resolution Se 3d (Figure S1c), two pairs of 3d3/2, 3d5/2 peaks are designated to NiSe and VSe phases.32 Similar structural studies were carried out for the NixFe3-xSe4 (0  x  2), which is used as a negative terminal in ASC assembly. For NixFe3-xSe4 (x = 0), the XRD diffraction patterns are consistent with the literature (PDF#010-071-2250, Figure

2b). The perceived diffraction peaks at 32.9, 43.8, and 51.1 can be assigned to the (112), (114), and (310) planes, respectively. Not much deviation can be observed in the XRD pattern for the NixFe3-xSe4 (x = 1) as compared to NixFe3-xSe4 (x = 0); however, there is the appearance of a new reflection at 28.9, which is indexed to the (200) plane of NiFe2Se4 (PDF#03-065-2338), indicating the formation of NixFe3-xSe4 phase. Further increases in the Ni concentration (x = 1.5) are observed a mixture of the two different phases.27 At higher Ni contents (NixFe3-xSe4 (x = 2)), the diffraction peaks are in good agreement with PDF#04-006-5240. The structure is wellmatched with a monoclinic structure. The diffraction peaks of the (112), (114), and (310) planes of the NixFe3-xSe4 (x = 1, 1.5, and 2) NAs expressively move toward lower angles with the content of the Ni increment from 32.9 to 32.4. As presented in Figure 2b, The NixFe3-xSe4 diffraction patterns reveal the increase in the order of lattice parameter a from 4.31 to 5.95 Å, which is in good agreement with the successive shift of the (112) peak, and the linearity well conforms with Vegard's law,28 serving as authoritative proof that Ni ions occupy the transition metal site, i.e., Fe and ensure the bimetal alloy formation. Based on the properties of the close crystal radii of Ni2+ (0.69 Å) and Fe3+ (0.63 Å), Ni may be an appropriate substitute into the Fe atoms in the essential location of the Fe3Se4 crystal lattice. The enlargement of the lattice parameter (Figure 2b) should be attributed to the slightly larger ion radii of Ni2+ (0.69 Å) than Fe3+ (0.63 Å).26 The purity and elemental composition of the NixFe3-xSe4 (x = 1) NAs is further examined by XPS, and the results are shown in Figure S2ac. The highresolution Ni 2p peaks (Figure S2a) are de-convoluted to 2p1/2, 2p3/2, and corresponding satellite peaks. The 2p1/2 and 2p3/2 peaks of the Ni-Se can be originated at 871.3 and 853.9 eV, respectively.31 The results indicate that the Ni has been successfully doped into the NixFe3-xSe4 (x = 1) NAs crystal lattice. The Fe 2p peaks deconvoluted into two pairs of 2p1/2, 2p3/2 (Figure S2b). The peak is observed at 707.5 eV, corresponding to the FeSe in the NixFe3-xSe4 (x = 1).26 The Se 3d deconvoluted into the pairs of 3d3/2, 3d5/2 peaks nominated to NiSe and FeSe can be seen in Figure S2c.32



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Figure 2. (a) XRD patterns and their corresponding structural formation of NixV3-xSe4 NAs, and (b) XRD patterns and their corresponding structural formation of NixFe3-xSe4 NAs. The morphological features of the NixV3-xSe4 nanostructures were studied by field emission scanning electron microscope (FE-SEM) and shown in Figures 3a, b, and S3. As shown in Figure 3a and S3, with tunable Ni/V ratios, all NixV3-xSe4 (x = 0, 1, 1.5, and 2) display the growth of numerous nanoarchitectures over the CFC. All electrodes except pure V3Se4@CFC exhibit hierarchical nanostructures with unique networks comprised of plentiful NAs. Their microscopic morphology displays excellent similarity with their corresponding LDHs (Figure S4), indicating that ultra-thin NixV3-xSe4 (x = 1) NAs were effectively retained after the selenization process. The chemical composition of NixV3-xSe4 (x = 1) NAs is validated by energy dispersive X-ray spectroscopy (EDAX, Figure 3c) and well supported by the results of inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. Furthermore, the homogeneous distribution of the Ni, V, and Se elements was revealed by SEMEDS color mapping analysis, and the results are appended in Figure S5. Figures 3d-f show transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images of the hierarchical NixV3-xSe4 (x = 1) NAs. The TEM image of NixV3-xSe4 (x = 1) displays wrinkled and large size, demonstrating that ultra-thin and defect free NAs (Figure 3d). The selected area electron diffraction (SAED) pattern of NixV3-xSe4 (x = 1) NAs shows a sequence of bright diffraction rings (inset of Figure 3d) which ensures the crystalline nature of the prepared compound. The HR-TEM (Figure 3e) indicates the existence of numerous nanopores in the hierarchical nanostructures which are developed during the selenization process.33 The existence

of such hierarchical nanopores has a profound effect on the supercapacitor perspective by shortening the diffusion pathways, enhancing the electroactive sites, and boosting the electron/ion transport kinetics. The exclusive porous features of the NixV3-xSe4 (x = 1) NAs was further examined by N2 sorption isotherm and are shown in Figure S6. The NixV3-xSe4 (x = 1) NAs exhibit type IV isotherm with a hysteresis loop indicating the mesoporous nature of the NAs.34 Impressively, the NixV3-xSe4 (x = 1) NAs possess a high SSA of 96.7 m2 g1 as well as a pore size distribution with an average size of 3.74 nm. The SSA of the NixV3-xSe4 (x = 1) NAs is superior to those reported on the metal selenide-based materials, such as HCo0.85Se (74 m2 g1)35 and Fe2CoSe4 (81.21 m2 g1).31 The rich lattice fringes with interplanar spacings of NAs of 0.265 nm is associated with the (112) planes of the NixV3-xSe4 (x = 1) NAs. The accomplishments of NixV3-xSe4 (x = 1) NAs are further supported by conducting scanning TEMenergy dispersive Xray spectroscopy (STEMEDS) studies (Figure 3g). It is clear that the elements such as Ni, V, and Se are homogeneously distributed throughout the mapped region of NixV3-xSe4 (x = 1) NAs. It is important to note that all of the elemental signals detected from the assigned areas were found to be very thin, in which the presence of Se is observed. This result suggests that oxygen atoms from the LDH precursors are successfully replaced by Se atoms during the efficient selenization process. By contrast, the signals for the Ni atom are somewhat weaker than the V signals, which is due to their lower level of substitution. This result is consistent with those of EDAX and ICP-OES analyses.


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Figure 3. Morphological characterizations of the NixV3-xSe4 (x = 1) NAs: (a) SEM image, (b) high-resolution SEM image, (c) EDAX of NixV3-xSe4 (x = 1) NAs, (d, e) TEM images (inset shows the corresponding SAED pattern), (f) HR-TEM image of NixV3-xSe4 (x = 1) NAs (inset shows the corresponding FFT), and (f) STEM image of NixV3-xSe4 (x = 1) NAs and its corresponding EDS color mappings of NiK, V-K, and Se-K. The thickness of the NixV3-xSe4 (x = 1) NAs was further verified by atomic force microscopy (AFM), and corresponding pictures are presented in Figure S7. The AFM image undoubtedly proves that the NixV3-xSe4 (x = 1) NAs has a thickness of 3.8 nm with rough surface features (roughness factor is 1.3 nm), which is consistent with morphological observations. The thermal stability of NixV3-xSe4 (x = 1) NAs was investigated by thermogravimetric analysis (TGA) and the results are shown in Figure S8. Thermal decomposition occurred from 212–575 C indicating that the selenides are slowly transformed to oxide. Further, the TGA reveals that the elemental composition in the NixV3-xSe4 (x = 1) NAs is estimated to be 51%. Similarly, the detailed morphological investigation was undertaken for NixFe3-xSe4 nanostructures, and the results are presented in Figures 4 and S9. The FE-SEM images of the NixFe3-x LDH (x = 0, 1, 1.5, and 2) precursors show that the nanostructures are vertically grown over CFC with various Ni/Fe ratios (Figure S10). All the compositions, except for pure Fe3Se4, showed NAs interconnected with each other and

developing a hierarchical superstructure. Following the selenization process, a reduction in sheet thickness is observed as compared to their corresponding LDH precursors. Nevertheless, the NAs retained their original sheet-like morphology well. Interestingly, exclusive porosity and alteration of the NAs are observed after the selenization process. Among the compositions, the NixFe3-xSe4 (x = 1) NAs shows ultra-thin hierarchical nanostructures, which eventually help improve electron transportation and boost the rate capability. The EDAX (Figure 4c) analysis confirms the existence of Ni, Fe, and Se elements with an atomic ratio of Ni: Fe in the NixFe3-xSe4 (x = 1) NAs, which is very close to the Ni: Fe feed ratios in the initial synthesis process (1: 2). This atomic ratio of the Ni: Fe in NixFe3-xSe4 (x = 1) NAs is in good agreement with ICP-OES (Table S1). TEM was further used to examine the intrinsic morphology and internal structure of the NixFe3-xSe4 (x = 1) NAs, and the results are presented in Figure 4df. The TEM image of NixFe3-x (x = 1) LDH precursor is also recorded for comparison and are shown in Figure S11.



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Figure 4. Morphological characterizations of the NixFe3-xSe4 (x = 1) NAs: (a) SEM image, (b) high-resolution SEM image, (c) EDAX of NixFe3-xSe4 (x = 1) NAs, (d, e) TEM images (inset shows the corresponding SAED pattern), (f) HR-TEM image of NixFe3-xSe4 (x = 1) NAs (inset shows corresponding FFT), and (g) STEM image of NixFe3-xSe4 (x = 1) NAs and its corresponding EDS color mappings of Ni-K, Fe-K, and Se-K. The TEM image reveals that NixFe3-x LDH (x = 1) precursor exhibit a non-porous and smooth surface morphology, which drastically changed into the rough and mesopores (i.e., 10 to 30 nm) state after the selenization. The formation of the mesopores in the NixFe3-xSe4 (x = 1) NAs is supposed to be related with the superior dissolution of the residual/unreacted LDH precursors. The unique porous feature was further investigated by N2 sorption isotherm analysis, and the results are presented in Figure S12. The NixFe3-xSe4 (x = 1) NAs shows type IV isotherm with an exclusive hysteresis loop, demonstrating the mesoporous structure of the NAs.34 The NixFe3-xSe4 (x = 1) NAs presents a BET SSA of 84.7 m2 g1, with a pore size distribution in an average size of 4.53 nm. The obtained BET surface area is superior to that of reported ternary selenide nanostructures, such as Ni0.75Fe0.25Se2 (11.2 m2 g1)36 and Ni0.89Co0.11Se2 (37.32 m2 g1).34 The SAED pattern (inset of Figure 4d) exposes the polycrystalline nature of the NixFe3-xSe4 (x = 1) NAs. As shown in Figure 4f, the designated lattice fringes with d spacings of 0.268 nm can be indexed to the (111) plane of the NiFe2Se4 phase. The STEM-EDS mapping further proves the homogeneous distribution of the Ni, Fe, and

Se in the matrix of NixFe3-xSe4 (x = 1) NAs (Figure 4g). As expected, the elemental signals of the Ni are weaker than the signals of Fe due to the lower concentration of the former element as compared to the latter and parallel the EDAX and ICP-OES observations. The AFM study results showed a sheet thickness of 4.0 nm (Figure S13) and held an exclusive roughness around 1.5 nm. The TGA and DTA curves (Figure S14) further validated that the thermal stability of NixFe3-xSe4 (x = 1) NAs and also confirms that the estimated elemental composition is 49.7%. These studies further reveal that both NixV3-xSe4 (0  x  2) and NixFe3-xSe4 (0  x  2) NAs are comprised of almost similar morphological features with larger SSA and unique porous structures. Such nanostructures were successfully grown on CFC substrate by the topological transformation of their corresponding LDH precursors and directly employed as an integrated 3D electrode for energy storage systems such as flexible solid-state SCs. 2.2. Electrochemical Performance of the NixV3-xSe4 and NixFe3-xSe4. Bearing in mind the 3D hierarchical nanostructures with exclusive features, the as-obtained NixV3-xSe4 and NixFe3-


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xSe4

electrodes are anticipated to exhibit better electrochemical performances. The electrochemical performances of the NixV3-xSe4 and NixFe3-xSe4 electrodes with various x values were first evaluated at room temperature in the standard three-electrode configurations comprised of aqueous 3 M KOH as the electrolyte, Hg/HgO, Pt sheet as the reference and counter electrodes, respectively. It is well known that the mass of the electroactive materials for the design and fabrication process is imperative to investigate the electrochemical properties, specifically power capability. The mass loading of the electrodes was calculated before and after in-situ growth of the active materials (as given in the experimental section). In order to examine the electrochemical performance of the NixV3-xSe4 (positive terminal), NixV3-xSe4 electrodes with various x values with an operating voltage window from 0 to 0.7 V are studied at a constant scan rate of 50 mV s1, and the results are shown in Figure S15a. Further, bare CFC is also subjected for the investigation to estimate the contribution in the electrochemical activity of NixV3-xSe4 electrodes but displayed a negligible current response as compared to those of the other electrodes. Among them, NixV3xSe4 (x = 1) electrode was shown to deliver a larger CV curve area and higher redox peak intensity than the other NixV3-xSe4 (x = 0, 1.5, 2) compositions, indicating that NixV3-xSe4 (x = 1) electrode has excellent redox reaction kinetics, which eventually translates to high specific capacity. The electrodes exhibit excellent redox behavior with a pair of peaks, which is attributed to the strong redox characteristics of the Ni2+/Ni3+ and V4+/V5+ couple.18, 37 The electrochemical reaction mechanism of NixV3-xSe4 electrode is proposed as follows:26 NixV3-xSe4 + (33-x)OH = NixV3-xOH9-x + 4SeO32- + 12H2O + (25-x)e (1) NixV3-xSe4 + (41-x)OH = NixV3-xOH9-x + 4SeO42- + 16H2O + (33-x)e (2)

The CV curves of NixV3-xSe4 electrodes at various scan rates of 10 to 100 mV s−1 are presented in Figures 5a and S15b-d. As depicted in Figure 5a, when the scan rate increases, all of the CV curves maintain their shapes with a dramatic increase in the current density, demonstrating that the NixV3-xSe4 (x =1) electrode holds perfect redox characteristics and it is capable of delivering high rate performance. While the scan rates increase from 10 to 100 mV s−1, the anodic and cathodic peaks were somewhat shifted towards positive and negative potential window sides, owing to the polarization behavior of the electrode materials during the electrochemical process. In order to further study the energy storage properties, the galvanostatic charge-discharge (GCD) measurement was carried out for the NixV3-xSe4 electrodes at the current density of 1 mA cm2, and the results are illustrated in Figure S16a. It is obvious that all the electrodes delivered a typical GCD with battery type characteristics. As expected, the NixV3-xSe4 (x = 1) electrode showed a higher discharge time of 1623 s, which is higher than those of NixV3-xSe4 (x = 0; 601 s), NixV3-xSe4 (x = 1.5; 1229 s), and NixV3-xSe4 (x = 2; 1497 s) electrodes. The GCD curves of NixV3-xSe4 (0  x  2) electrodes at different current density from 1 to 50 mA cm−2 in the working voltage window of 0 to 0.6 V are presented in Figures 5b and S16b-d. Observation showed that the all GCDs exhibit typical non-linear behavior, which clearly indicates that the charge storage is accomplished by the redox process.38 Moreover, the GCDs showed a pair of redox charge-discharge plateaus, which is in good agreement with CVs. All GCDs are symmetric in nature, regardless of the applied current-rate, indicating excellent reversibility of the system, i.e., Coulombic efficiency, owing to the ultra-fast electron transportation provided by the CFC

backbone.39 In addition, the NixV3-xSe4 (x = 1) showed a lower voltage (IR) drop, indicating a relatively ultra-small ESR which certainly boosted the power capability of SCs based of the NixV3-xSe4 (x = 1) NAs.40 These results further confirm that the partial substitution of Ni into NixV3-xSe4 NAs plays a vital role in improvement of the electrochemical properties. The relationships between the specific capacity and current density of NixV3-xSe4 electrodes are shown in Figure 5c. The specific capacities of the NixV3-xSe4 (x = 1) electrode as high as 329, 320, 311, 302, 297, 286, 279, 271, 266, and 261mAh g−1 at the current densities of 1, 3, 5, 8, 10, 15, 20, 30, 40, and 50 mA cm−2, respectively. The NixV3-xSe4 (x = 1) electrode shows high specific capacities of 329 mAh g1 at the current density of 1 mA cm2 as compared to ~141, 220, and 261 mAh g–1 for x =0, 1.5, and 2, respectively. Impressively, 79.33% of capacity is maintained at the high current density of 50 mA cm2, indicating that the NixV3-xSe4 (x = 1) electrode holds excellent rate capability as compared to the other phases NixV3-xSe4 (x = 0, 1.5, and 2). The rate capability of the NixV3-xSe4 (x = 1) electrode not only offers better high current and capacity retention characteristics than the other compositions prepared, but they are also superior to those of recently reported Ni-based electrodes such as NiCo2O4@NiCo2O4/carbon textile (about 42.1% capacity retained at 20 mA cm2)41 and FeCo2S4–NiCo2S4/silver coated cloth (85.1% retained at 40 mA cm2).42 This superior specific capacity and rate capability of the NixV3-xSe4 (x = 1) electrode is mainly ascribed to the high synergistic interaction ensuing from the numerous electroactive sites existing in NixV3-xSe4 (x = 1) NAs and ultra-fast electron transport availed from CFC support. In addition, this hierarchical nanostructure with unique porous architectures shortened the ion pathways to simplify the electrolyte flux across the interface, which simplifies the charge transport and ion diffusion without any rise in ESR due to the presence of polymeric binder, and some extend to conductive additives are worth mentioning. In order to further understand the electrical conductivity and electron transport kinetics, the electrochemical impedance spectroscopy (EIS) study was conducted on the NixV3-xSe4 (0  x  2) electrodes. As shown in Figure S17, the Nyquist plot of all NixV3-xSe4 electrodes with x = 0, 1, 1.5, and 2 displayed a small ESR, indicating that the ultra-fast ion/charge transport kinetics between NixV3-xSe4 nanostructures and aqueous KOH electrolyte. Most significantly, the perceived charge-transfer resistance (Rct) values are in the following order: NixV3-xSe4 (x = 1)  NixV3-xSe4 (x = 1.5)  NixV3-xSe4 (x = 2)  NixV3-xSe4 (x = 0). This clearly indicates that the optimal substitution of Ni in the NixV3-xSe4 (x = 1) NAs phase leads to the lowest Rct values.41 The cycling stability of the electrodes is another crucial factor in establishing the SCs in shape versatile configuration, such as the flexible and wearable perspective. The cycling stability of the NixV3-xSe4 electrodes were examined at the higher current density of 30 mA cm2 with 10 000 successive charge-discharge cycles and are presented in Figures 5d and S18. The NixV3-xSe4 (x = 1) electrode retained 98.6% of capacity after 10 000 consecutive GCD tests, which is greater than those of the other NixV3-xSe4 (x = 0, 1.5, 2) electrodes. In order to further prove the ultra-long cycling performance along with structural honesty, the first 10 and last 10 GCDs of the NixV3-xSe4 (x = 1) electrode are illustrated in the inset of Figure 5d. A minor deviation is observed at the 10 000th cycle related to the first cycle, demonstrating the highly synergistic interaction between the CFC and electroactive materials. The corresponding morphological analysis (FE-SEM) and XRD pattern of the NixV3-



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xSe4

(x = 1) electrode after long-term cycling stability testing were analyzed and the results are presented in Figure S19.

Figure 5. Electrochemical performance of NixV3-xSe4 and NixFe3-xSe4 electrodes: (a) CV curves of NixV3-xSe4 (x=1) electrode with different scan rates ranging from 10 to 100 mV s1, (b) GCD curves of NixV3-xSe4 (x=1) electrode with various current densities ranging from 1 to 50 mA cm2, (c) specific capacity vs. current density of NixV3-xSe4 (x = 0, 1, 1.5, and 2) electrodes, (d) cycling performance of NixV3-xSe4 (x = 1) electrode (inset shows GCD curves: 1st to 10th cycles and 9991st to 10 000th cycles), (e) CV curves of NixFe3-xSe4 (x = 1) electrode with different scan rates ranging from 10 to 100 mV s1, (f) GCD curves of NixFe3-xSe4 (x = 1) electrode with various current densities ranging from 1 to 50 mA cm2, (g) specific capacity vs. current density of NixFe3-xSe4 (x = 0, 1, 1.5, and 2) electrodes, and (h) cycling performance of NixFe3-xSe4 (x = 1) electrode (inset shows GCD curves: 1st to 10th cycles and 9991st to 10 000th cycles). The morphology and structural features of the NixV3-xSe4 (x = 1) NAs were well-maintained after the 10 000 continuous GCD cycles, representing its outstanding stability and structural

integrity. The EIS test was conducted for the NixV3-xSe4 (x = 1) electrode after long-term cycling stability, and the result is shown in Figure S20. It showed a marginal increase in Rct values after the 10 000 consecutive charge-discharge cycles


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test.44 The amazing electrochemical properties of the NixV3-xSe4 (x = 1) electrode have logically led us to employ as the positive electrode to design the flexible solid-state ASC for various industrial purposes. Aside from the exploration of the positive electrode, we have also intended to develop a new type of NixFe3-xSe4 electrodes and employed it as a negative side to boost the energy density of the flexible solid-state ASC as an alternative for industrially-used, traditional AC. The structural and morphological investigations demonstrate that the NixFe3-xSe4 electrodes possess hierarchical superstructure, large SSA, and exclusive mesoporous frameworks. In order to investigate the electrochemical properties of the NixFe3-xSe4 (negative terminal), the CV curves of NixFe3-xSe4 electrodes and bare CFC were studied in the operating window of 1.0 to 0.0 V at a scan rate of 50 mV s1 as shown in Figure S21a. Apparently, the contribution of bare CFC presents a negligible capacitance behavior as compared to those of other electrodes. However, the shapes of the CV traces of NixFe3-xSe4 electrodes differ from those of electrode materials which obey the non-Faradaic process, indicating that their capacity behavior is mainly dominated by the redox kinetics, i.e., Faradaic behavior. Interestingly, the NixFe3-xSe4 (x =1) electrode exhibits a larger CV area and excellent redox peak current intensity than the other compositions investigated (NixFe3-xSe4, x = 0, 1.5, 2), indicating that the NixFe3-xSe4 (x =1) electrode possesses ultrahigh specific capacity. This superior specific capacity mainly originates from its large SSA, unique porous networks, and high synergistic effect of the hierarchical NixFe3-xSe4 (x =1) NAs with CFC support. The CV profiles of the NixFe3-xSe4 electrodes at different scan rates from 10 to 100 mV s1 are shown in Figures 5e and S21b-d. As shown in Figure 5e, all CV curves showed a couple of symmetrical and a pair of typical redox peaks, indicating its superb electrochemical reversibility. When the sweep rate rises from 10 to 100 mV s1, a significant enhancement of current intensity is noted with a slight shift in the peak positions. Nevertheless, the NixFe3-xSe4 (x = 1) electrode preserves and maintain the shape of CV curves, even at a high scan rate of 100 mV s1, indicating an excellent synergistic interaction between NixFe3-xSe4 (x = 1) NAs and the CFC substrate. In contrast to positive electrodes, the reaction kinetics of the NixFe3-xSe4 electrodes depend on the utilization of Fe3+/Fe2+ redox couple.45 Therefore, the electrochemical reaction mechanism of NixFe3-xSe4 electrode is proposed as follows: NixFe3-xSe4 + (33-x)OH = NixFe3-xOH9-x + 4SeO32- + 12H2O + (25-x)e (3) NixFe3-xSe4 + (41-x)OH = NixFe3-xOH9-x + 4SeO42- + 16H2O + (33-x)e (4)

The corresponding GCD test was carried out for the NixFe3-xSe4 electrodes with different x values at a current density of 1 mA cm2, and the results are presented in Figure S22a. As expected, NixFe3-xSe4 NAs electrodes presented the non-linear batterytype behavior due to the redox activity of Fe3+/Fe2+ couple and were in good agreement with CV curves. Among the compositions investigated, the NixFe3-xSe4 (x = 1) electrode exhibits a largest discharge time of 3378 s, which is a much longer duration than those of the other NixFe3-xSe4 electrodes such as NixFe3-xSe4 (x = 0; 1073 s), NixFe3-xSe4 (x = 1.5; 2738 s), and NixFe3-xSe4 (x = 2; 1768 s). The typical voltage plateaus observed from the GCDs of NixFe3-xSe4 electrodes are in good agreement with the redox peaks perceived from CVs. The typical GCD profiles of the NixFe3-xSe4 electrode at various current densities are shown in Figures 5f and S22b-d. All GCD

curves showed a pair of redox charge-discharge plateaus, which is in good agreement with CVs. In addition, the NixFe3xSe4 (x = 1) showed a lower IR drop, indicating a relatively ultra-small ESR which certainly boosted the power capability of SCs.40 These results demonstrate that the effective Ni substitution into NixFe3-xSe4 NAs plays a major role in enhancement for the electrochemical behaviors. The specific capacities of the NixFe3-xSe4 electrodes were evaluated using the GCDs at various current densities, and the results are shown in Figure 5g. The specific capacities of the NixFe3-xSe4 (x = 1) electrode as high as 261, 252, 245, 239, 236, 229, 222, 212, 207, and 203 mAh g−1 at the current densities of 1, 3, 5, 8, 10, 15, 20, 30, 40, and 50 mA cm−2, respectively. The NixFe3-xSe4 (x = 1) electrode clearly exhibits superior redox activity with a high specific capacity of 261 mAh g−1 at a current density of 1 mA cm2, which is much higher than which is higher than NixFe3-xSe4 (x = 0) (116 mAh g−1), NixFe3-xSe4 (x = 1.5) (209 mAh g−1), and NixFe3-xSe4 (x = 2) (160 mAh g−1). At a high current density of 50 mA cm−2, the NixFe3-xSe4 (x = 1) electrode still maintain an excellent specific capacity value of 203 mAh g−1 with a retention of 77.78%, which is much higher than those of other NixFe3-xSe4 (x = 0, 1.5, 2) electrodes (Figure 5g). This result clearly suggests the superior rate capability of the NixFe3-xSe4 (x = 1) electrode. Additionally, EIS analysis was carried out for the NixFe3-xSe4 electrodes in order to obtain additional evidence regarding the electrochemical properties of the electrode materials, and the resultant Nyquist plots are presented in Figure S23. In the high-frequency region, the semi-circle radius and impedance spectrum intercept on the real axis stands for the Rct as well as ESR related to the redox reaction electron transfer kinetics internal of the electrode as well as at the interface between the active electrode and the electrolyte, respectively.46 The Rct of the NixFe3-xSe4 (x = 1) electrode is smaller than those of the rest of the compositions studied. The Rct value of the NixFe3-xSe4 (x = 1) electrode is only 0.85 , which is significantly lower than that of other electrodes such as NixFe3-xSe4 (x =0) (1.47 ), NixFe3-xSe4 (x = 1.5) (1.01 ), and NixFe3-xSe4 (x = 2) (1.23 ). In the lowfrequency region, the slop and straight line indicate the Warburg resistance (Rw). The Rw appears basically from the electrolyte ions diffusive resistance into the heart of the active electrode materials.45 As a result, it is clear that the NixFe3-xSe4 (x =1) electrode displays a perfectly straight line along with imaginary axis, demonstrating low Rw for ultra-fast diffusion of the ions during the electrochemical performance. This is predominantly ascribed to the fact that the hierarchical NixFe3xSe4 (x = 1) NAs perpendicular grown over the CFC, and their numerous mesopores developed throughout the NixFe3-xSe4 nanoarchitectures (during selenization process) or branch channels developed by interconnecting each NAs, which are appropriate for the fast charge-discharge characteristics of the SCs. Notably, long-term cycling life has been significantly enhanced at the same time. The NixFe3-xSe4 (x =1) electrode exhibits outstanding cycling stability with a capacity retention of over 97.9% after 10 000 consecutive charge-discharge cycles at a current density of 30 mA cm−2 (Figure 5h). The obtained results are much better than those of the other NixFe3-xSe4 (x = 0, 1.5, 2) electrodes compared (Figure S24). The enhanced cycling stability could be mainly ascribed to the hierarchical architectures of NixFe3-xSe4 (x = 1) NAs with extraordinary SSA and unique porous networks offer to shorten the ion pathways so as to improve the electron/ion transportation and stabilize the structure of NixFe3-xSe4 (x = 1) NAs during long-term cycling. In order to further understand the electrochemical



stability of the NixFe3-xSe4 (x = 1) electrode, we plotted the first 10 and last 10 GCD cycles, and the results are presented in the inset of Figure 5h. Impressively, the shape of the last 10 cycles is well maintained as compared to that of the initial cycles during the GCD measurement with a negligible change, indicating the exceptional structural stability in the aqueous KOH electrolyte. Further, the EIS plot of the NixFe3-xSe4 (x = 1) electrode was examined before and after continuous 10 000 GCDs as shown in Figure S25. As expected, no obvious changes in the low-frequency vertical line as well as the high-frequency semicircle are noted, which strongly parallels the stability of the negative electrode in aqueous media. The XRD and SEM analyses were carried out for the NixFe3-xSe4 (x = 1) NAs after 10 000 charge-discharge cycles and the results are illustrated in Figure S26. The structure and morphology of the NixFe3-xSe4 (x = 1) NAs were well-preserved after long-term cycling, as is evident from the above studies. Based on the electrochemical performance in the individual electrochemical properties, we suggest that is a suitable negative electrode for the flexible solid-state ASCs instead of well-established AC. We strongly believe that the superior electrochemical performances of both NixV3-xSe4 (x = 1) and NixFe3-xSe4 (x = 1) NAs can be mainly attributed to the following aspects: (i) the presence of hierarchical nanostructures with ultra-thin NAs can offer large SSA and plentiful exposed active sites, which has been clearly validated through FE-SEM and BET observations. The unique porous and hierarchical superstructures effectively increase the electrolyte flux, which eventually fills the void spaces between the NAs and CFC. As a result, the wellorganized interfacial contact between electrolyte and active materials which facilitates the facile redox activity will be developed. (ii) The exclusive porous networks with standardized chemical distribution aid the formation of an efficient pathway for charge transport. The plentiful active sites provide the charge-transport kinetics through well interconnected NAs (highway) to the CFC current collector.47 (iii) This current design and fabrication strategy ensure the free-standing, binder-free nanostructures, as well as the high synergistic interaction of the active NixV3-xSe4 (x = 1) /NixFe3xSe4 (x = 1) NAs and CFC conductive substrates, which could avoid the incorporation of “dead-mass” elements, like a binder and conductive additive, without compromising the electrochemical performance. (iv) The direct growth of the NAs also lowers the Rct because of the reducing the distance between the CFC current collector and the active materials which act as a superhighway to boost the ion/electron transport kinetics.48 (v) The electrochemical features of Ni, V, and Fe compounds in aqueous KOH electrolyte exhibit a welldefined Faradaic behavior, which is well-consistent with previous reports.1,49 Overall, the remarkable features and satisfactory electrochemical performance of hierarchical nanostructures in the three-electrode configuration dictated us to assemble the solid-state ASC in a versatile shape configuration. Therefore, the ASC is assembled in a flexible solid-state configuration with NixV3-xSe4 (x = 1) NAs (i.e., NiV2Se4) and NixFe3-xSe4 (x = 1) NAs (i.e., NiFe2Se4) as positive and negative electrodes, respectively, and deliberated the details in the upcoming sections. 2.3. Electrochemical Performance of the Flexible NiV2Se4//NiFe2Se4 ASC. In order to further investigate the practical aspects of the prepared electrodes for energy storage applications, a solid-state flexible ASC is assembled, in which

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the NiV2Se4 act as a positive electrode, NiFe2Se4 act as a negative electrode, and PVA/KOH serve as a gel electrolyte (Figure 6a). Based on the electrochemical performance of the individual electrode in the three-electrode set-up, the mass ratio of the positive to negative electrodes was fixed at ~1.32 (the details are in the experimental section). Generally, supercapacitors fabricated asymmetrically are anticipated to work up to 1.6 V. Taking advantage of employing two Faradaic type electrodes (NiV2Se4 as positive and NiFe2Se4 as negative, Figure S27), figuring out the best and most suitable operating potential window which be optimized. Therefore, the ASC was subjected into a sequence of CVs with various operating potentials at a constant scan rate of 50 mV s−1 (Figure S28). As expected, the flexible ASC holds the stable operating potential window up to 1.8 V. Moreover, the operating potential window exceeds 1.8 V, a noticeable polarization occurs, which may be due to the oxygen evolution reactions.50 In order to further confirm the best operating potential of flexible ASC, GCDs with multiple potential windows were performed at a current density of 10 mA cm−2, and the results are illustrated in Figure S29. Among the various investigated potential windows, the highest specific capacity of 92 mAh g−1 is observed for 1.6 V. When the potential window exceeds 1.6 V, the ASC fails to retain its symmetric behavior, which in turn provides ultra-low Coulombic efficiency.44 On the other hand, CV profiles of the flexible ASC at different sweep rates in the operating window of 0 to 1.8 V are presented in Figure 6b. Noticeably, all of the CV curves comprise a pair of symmetrical redox peaks that are ascribed to highly reversible redox kinetics related to the storage mechanism described above. Further, there is no noticeable distortion in CVs shape, demonstrating satisfactory rate performance as the result of the effective ion/electron transport kinetics on active electrodes. It is worth mentioning that the redox reactions are taking place within a potential window of 1.6 V regardless of the scan rates, although we tested up to 1.8 V. Hence, we limited the testing operating potential window of 1.6 V for GCD studies. Accordingly, the GCD studies were performed for NiV2Se4//NiFe2Se4-based ASC at various current densities, and the results are illustrated in Figure 6c. All of the GCDs showed almost symmetric chargedischarge features, demonstrating that the electrodes hold exceptional Coulombic efficiency and reversibility. The presence of non-linear GCD curves further confirms that the capacity of the ASC device mainly originates from the Faradaic characteristics, which is in good agreement with CV observations. The specific and volumetric capacities of the flexible ASC are calculated based on the function of the current density and presented in Figures 6d and S30, respectively. It can be realized that the specific and volumetric capacities gradually decreased with the increase of current density, which is due to the limited participation of the active material during redox reactions with poor ion diffusion.46 Based on the active material loading, the flexible ASC exhibits a specific capacity as high as 92 mAh g−1 and a corresponding volumetric capacity of 1.5 mAh cm–3 at the current density of 3 mA cm−2. In order to rationalize the superior electrochemical performance of the NiV2Se4 //NiFe2Se4-based ASC, we fabricated another kind of ASC, in which NiV2Se4 serves as the positive electrode and conventional AC serves as the negative electrode by keeping the experimental condition constant, and the electrochemical performance was studied, and the results are illustrated in Figure S31.


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Figure 6. Electrochemical performance of NiV2Se4//NiFe2Se4 ASC device: (a) Schematic illustration of the assembled ASC with NiV2Se4 and NiFe2Se4 electrodes, (b) CV curves of the flexible ASC at different scan rates ranging from 10 to 100 mV s1, (c) GCD curves of the flexible ASC with various current densities ranging from 1 to 50 A g1, (d) specific capacity values as a function of applied current density for the flexible ASC, (e) cycling performance of the flexible ASC exemplified at the current density of 20 mA cm2 with 10 000 successive charge-discharge cycles (inset shows the first and last 10 GCD cycles of the flexible ASC), and (f) Ragone plot of flexible ASC as compared with reported literature (inset shows the photographic images of the two series connected flexible ASCs glowing a green LED and run a toy fan). Remarkably, NiV2Se4//NiFe2Se4-based ASC holds about 79.7% of capacity retention at the higher current density (30 mA cm−2) and 74.6% for the current density of 50 mA cm−2. The capacity retention of NiV2Se4//NiFe2Se4 ASC is significantly higher than that of NiV2Se4//AC (about 68.1% at the current density of 30 mA cm−2). Nonetheless, the NiV2Se4//NiFe2Se4based ASC not only outperformed the AC-based assembly but was also superior to the previously reported solid-state ASC

(Table S2). As mentioned, the superior electrochemical properties of the NiFe2Se4 negative electrode are due to the hierarchical nanostructures, unique porous networks, and more electro-active sites. Therefore, we conclude that the newly developed NiFe2Se4 negative electrode would be one of the best choices for a potential electrode for the construction of high energy density SCs instead of AC.



As an important requirement for regulating the durability of the flexible ASC for commercial applications, the long-term cycling stability is crucial and necessary. Hence, the assembled NiV2Se4//NiFe2Se4 ASC device is subjected to long-term cycling life behavior at the high current density of 20 mA cm−2, and the results are presented in Figure 6e. The capacity slightly increased during the initial 500 cycles, which is due to the sequential penetration of electrolyte into the interior frameworks of the active NAs.49 Most importantly, only 1.7% of a capacity drop is noticed after 5000 charge-discharge cycles, and the flexible ASC still holds 96.6% of its initial capacity after 10 000 successive charge-discharge cycles. The observed results are significantly higher than those of the NiV2Se4//AC ASC, and it retains only 76.6% of its initial capacity after 10 000 successive charge-discharge cycles, which is inferior to the commercial requirement of 80% retention. The long-term cycling stability study clearly demonstrates the outstanding stability of the NiV2Se4//NiFe2Se4 ASC device, and the performance is also superior to those of other reported ASC devices (Table S2). Figure S32 shows the capacity retention and Coulombic efficiency of NiV2Se4//NiFe2Se4 ASC with 10 000 successive charge-discharge cycles. The Coulombic efficiency of the NiV2Se4//AC ASC device was calculated using the following Equation (5) =

𝑡𝑑 𝑡𝑐

 100%

(5)

where td and tc are the discharging and charging time of the flexible ASC, respectively. The Coulombic efficiency remain about 99% after 10 000 successive charge-discharge cycles. This study further confirms that the flexible ASC has highly reversible surface redox kinetics. The EIS was carried out for the NiV2Se4//NiFe2Se4 ASC device before and after cycling stability test and the results are illustrated in Figure S33. In the real axis, the Nyquist shows an internal resistance around 0.67 Ω, further authorizing that our NiV2Se4//NiFe2Se4 ASC device retains very small internal resistance. Besides, the Nyquist plot clearly reveals that the ASC device also offers ultra-low diffusion resistance, which indicates that the flexible ASC holds excellent electrical conductivity. The Bode plot of the NiV2Se4//NiFe2Se4 ASC before and after cycling stability test are shown in Figure S34. The “knee frequency” showed the capacitive behavior at phase angle of 45 was carried for before and after stability test. A very minimal changes was observed in the relaxation time () is about 18 ms when compared the before cycling stability test (11 ms), which is fully consistent with previous report.51 This result further demonstrates that the ASC holds excellent charge-discharge behavior as well as exceptional cycling performance. To further prove the outstanding cycling performance of the flexible ASC, the morphological and structural investigations for both NiV2Se4 and NiFe2Se4 electrodes were examined after 10 000 charge-discharge cycles and the results are presented in Figures S35-37. Interestingly, both electrodes preserve their original structure, which is one of the prime reasons for the outstanding cyclability of the NiV2Se4//NiFe2Se4 ASC device. It is also worth mentioning that, regarding the stability of the CFC support, it often translates to a rise in resistance due to the electrochemical oxidation and corrosion. The excellent electrical conductivity of the CFC frameworks not only aids the growth of the hierarchical NiV2Se4 and NiFe2Se4 NAs and also prevents the structural collapse from the original structure

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under such harsh current testing during the prolonged cycling. In addition, it accelerates the effective charge transport medium for the active materials, regardless of the aqueous or (quasi) solid-state interfaces. Figure 6f displays a Ragone plot of the assembled NiV2Se4//NiFe2Se4 ASC device. Interestingly, the highest specific energy density of flexible NiV2Se4//NiFe2Se4 ASC attains up to 73.5 Wh kg−1 at a power density of 0.733 kW kg−1, which is much higher than the NiV2Se4//AC ASC device (46.2 Wh kg−1 at a power density of 0.711 kW kg−1), and also superior to previously reported ASC devices[38,52-56] (Figure 6f and Table S2). In addition, the NiV2Se4//NiFe2Se4 ASC device holds about 72.24% of the energy density (53.1 Wh kg−1) retained at a high power density of 19.11 kW kg−1, which is much higher than NiV2Se4//AC ASC device (63.84% of the energy density 29.52 Wh kg−1 retention at 9.97 kW kg−1). In order to further authorize potential applicability of flexible ASC device, two flexible solid-state ASCs are connected in series to attain a voltage of 3.2 V. After charging the flexible ASC devices, and they can power a green LED to glow for more than 5 minutes and also effectively run a fan for about 1 minute (inset in Figure 6f). As one of the basic requirements for proving wearability, flexibility testing was carried out for the NiV2Se4//NiFe2Se4 ASC device. Digital photographs of our flexible ASC with different bending and twisted angles are shown in Figure S38, demonstrating that flexible ASC holds outstanding structural honesty and integrity. As depicted in Figure S39, the CV curve of the flexible ASC exhibits a slight capacity degradation with different bending and twisting angles. In order to further evaluate the flexibility of the ASC, the electrochemical profile was verified under bending and twisting circumstances for every 1500 cycles, and the results are illustrated in Figure S40. This cycling stability test under bending and twisting test certifies the shape versatility and outstanding mechanical resilience of the flexible ASC performing under highly flexible environments. This performance undoubtedly cultivates the flexible NiV2Se4//NiFe2Se4 ASC as a gifted aspirant for textilebased energy systems, such as rechargeable batteries and SCs, from the commercial viewpoint.57 In addition, such remarkable achievements will authorize our flexible ASC to possess essential ability and be considered a contender to fulfill the practical aspects for the high energy/power density requirements in the energy storage sectors. The mechanism of high-capacity NiV2Se4//NiFe2Se4 ASC is schematically illustrated in Figure S41. The OH− ions from the aqueous KOH electrolyte are expected to be constantly involved in the active materials for richer redox reaction kinetics. In particular, when the flexible ASC is charged, the positive terminal of NiV2Se4 NAs involves the reaction with OH and is described as follows: NixV3-xSe4 + (33-x)OH = NixV3-xOH92  x + 4SeO3 + 12H2O + (25-x)e ; NixV3-xSe4 + (41-x)OH = NixV32 xOH9-x + 4SeO4 + 16H2O + (33-x)e , and the negative terminal of NiFe2Se4 NAs involves the effective reaction process of NixFe3 2-x + 12H O + (25-x)e; xSe4 + (33-x)OH = NixFe3-xOH9-x + 4SeO3 2 NixFe3-xSe4 + (41-x)OH = NixFe3-xOH9-x + 4SeO42- + 16H2O + (33x)e.25 Nevertheless, during the discharge process of the flexible ASC, the electrochemical reactions occurred in a reverse direction. Based on electrochemical performance, one can clearly understand the hierarchical NAs and the synergistic effect of the strategy explanation for the exceptional storage capability of the ASC fashion in the following characteristics: (i)

12 ACS Paragon Plus Environment

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Both NiV2Se4 and NiFe2Se4 NAs are directly grown over highly conductive CFC by a hydrothermal and subsequent selenization process, which significantly endorses a strong interaction and excellent binding with CFC current collector. This eventually avoids the use of conductive agents or binders, resulting in a noticeable reduction in the net weight of the ASC along with suppression of the ESR; in other words, eliminating the “dead-mass” loading in the active electrode materials. (ii) As specific healthy supports, CFC can serve as a mechanical barrier that sustains the volume variation observed during the conversion of selenides into hydroxides upon the chargedischarge process, and efficiently protects the corrosion of the electroactive materials. Moreover, the CFC not only provides a network for fast electron transportation but also donates the high SSA for growing the active materials homogeneously and creating them with fully active electrochemical accessibility, resulting in a high degree of utilization. (iii) The hierarchical NiV2Se4 and NiFe2Se4 NAs are homogeneously anchored on the CFC and hold the unique porous between the adjacent NAs, which can act as an “ion reservoir” for enhancing the penetration of the flux by accelerating the charge carriers and solvated ions. (iv) The NiV2Se4 and NiFe2Se4 NAs themselves also have unique porous features, which can deliver abundant electroactive sites for ultra-fast and reversible richer redox kinetics. (v) The accurately balancing cell voltage windows and well-matched specific capacities of the positive and negative electrodes, i.e., mass balance, boost the energy and power densities of the flexible solid-state ASC. Therefore, the contributions from both the NiV2Se4 and NiFe2Se4 electrodes with hierarchical nanostructures the single electrodes and ASC fashion exhibiting superior electrochemical performances. 3. Conclusions In summary, for the first time, we established a process for growing hierarchical NixV3-xSe4 and NixFe3-xSe4 nanostructures with unique porous networks directly on CFC, which can be effectively used as prospective electrodes for the fabrication of flexible solid-state SC in an asymmetric fashion. Under the optimized conditions, we observed that both NiV2Se4 and NiFe2Se4 NAs are homogeneously grown over the CFC, which provides not only efficient conductive support but also delivers rapid pathways for electron transportation and shorten the ion diffusion pathways, which eventually results in superior rate capability. As a result, hierarchical NiV2Se4 and NiFe2Se4 electrodes delivered an ultra-high specific capacity and excellent rate performances with ultra-long durability. Most importantly, a flexible ASC was assembled with NiV2Se4 as the positive electrode and NiFe2Se4 as the negative electrode with an operating potential window of 1.6 V. This flexible ASC exhibits amazing electrochemical performance, specifically in terms of an excellent specific energy density of 73.5 Wh kg−1 at 0.733 kW kg−1 and an outstanding cycling stability (96.6% of capacity retention after 10 000 GCD). This study not only suggests a novel strategy for designing high-performance ternary metal selenide-based electrodes for flexible ASC with promising application perspective, i.e., high energy/power densities but also opens up a new pathway for the progression of next-generation wearable energy storage systems for industrial applications. 4. Experimental Section

4.1. Synthesis of the NixV3-x LDH Precursors: The CFC (Hesenbio Ltd. Shanghai, China) was cut into the size of 2 cm × 5 cm and washed with acetone, ethanol, and water for 30 min each, respectively. Next, NiCl2.6H2O (1 mmol, 99.9%, SigmaAldrich) and NH4VO3 (2 mmol, 99%, Sigma-Aldrich) were dissolved in 48 mL of DI water, then NH4OH solution (3 mL, 28% NH3 in H2O, Sigma-Aldrich) was added and stirred for 30 min to obtain a clear solution. The mixture was then transferred into a Teflon-lined autoclave with a glass slidesupported CFC in a perpendicular direction. The autoclave was sealed and heated at 180 C for 6 h. The samples were then repeatedly washed with DI water and ethanol to remove the unwanted impurities and dried at 60 C for 12 h in a vacuum oven. The as-obtained material is denoted as NixV3-x LDH (x=1) precursors. For comparison, NixV3-x LDH precursors with different Ni: V stochiometric ratios (x= 0, 1.5, and 2) were also synthesized through a similar synthesis protocol. 4.2. Synthesis of NixFe3-x LDH Precursors: First, NiCl2.6H2O (1 mmol), FeCl3.6H2O (2 mmol, 99%, Sigma-Aldrich), CO(NH2)2 (12 mmol, 98%, Sigma-Aldrich), and NH4F (6 mmol, 98%, Alfa Aesar) were dissolved in 33 mL of DI water. Once a clear solution had formed, ethylene glycol (17 mL, 99%, Daejung, Korea) was added to the above mixture, which was then continuously stirred for 30 mins. Next, the solution was transferred to a Teflon-lined autoclave along with a glass slidesupported CFC in a vertical direction. The autoclave was sealed, and then the reaction was carried out at 120 C for 12 h. After that, the sample was washed with DI water and ethanol and dried at 60 C for 12 h in a vacuum oven. The as-obtained material is designated as NixFe3-x LDH (x=1) precursors. Similarly, the NixFe3-x LDH precursors with various Ni: Fe stoichiometric ratios (x= 0, 1.5, and 2) were also prepared using the similar synthesis procedure for comparison. 4.3. Synthesis of the NixV3-xSe4 and NixFe3-xSe4: The NixV3-x LDH@CFC and NixFe3-x LDH@CFC precursors were immersed into 50 mL ethanol containing Na2SeO3 (0.2 g, 98%, SigmaAldrich) and N2H4.H2O (2 mL, 98%, Sigma-Aldrich). The selenization process was carried out at 180 C for 2 h. After completion of the reaction, the samples were washed with DI water and ethanol, and vacuum dried at 60 C for 12 h. The asobtained materials were denoted as NixV3-xSe4 and NixFe3-xSe4 NAs. In order to calculate the mass loading, the samples were weighed before and after the reaction. To avoid experimental errors, we prepared five samples for each electrode and calculate the average value of mass loading for the samples. The mass loadings of the NixV3-xSe4 (x = 0, 1, 1.5, and 2) samples were calculated to be 1.2, 1.5, 1.4, and 1.7 mg cm–2, respectively. In the case of NixFe3-xSe4 (x = 0, 1, 1.5, and 2), the samples were estimated to be 1.5, 1.8, 1.9, and 1.6 mg cm–2, respectively. 4.4. Structural Characterizations: The morphological analyses of the as-obtained electrodes were conducted out using an FE-SEM (JEOL JSM-6701F, Japan) and the structure of the electrode materials were examined using a TEM (JEMARM200F, Japan) installed in the Center for University-Wide Research Facilities (CURF) at Chonbuk National University. The elemental compositions were estimated by EDAX (SUPRA 40 VP; Carl Zeiss, Germany) and ICP-OES (J-A1100; Jarrell-Ash Company, Japan). The XRD patterns were obtained using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ



= 0.154 nm) at a scan rate of 2 C min1. A Micromeritics ASAP 2020 was used to investigate the BET surface area and pore size distribution of the as-obtained electrode materials. XPS (Theta Probe; Thermo Fisher Scientific, UK) was used to investigate the surface features of the prepared electrode materials. AFM was obtained with a Park NX10 (Park System Corporation, Korea). TGA was carried out under air atmosphere using a Q50 (TA Instrument, USA) with a heating rate of 5 C min1. 4.5. Electrochemical Characterizations: The electrochemical performances of the formulated electrodes were performed on a CH660E (CH Instruments, Inc., USA). In the three-electrode configuration, the CFC-supported electrodes were directly used as a working electrode, while Hg/HgO and Pt foil were used as a reference and counter electrodes, respectively. The EIS was conducted in the frequency range from 100 kHz to 0.01 Hz with an applied a.c. amplitude of 10 mV. 4.6. Fabrication of Flexible Solid-State ASCs: The flexible ASCs were assembled with NiV2Se4 NAs as the positive electrode, NiFe2Se4 NAs as the negative electrode, and NKK TF40 as the separator (40 µm thickness). The gel (KOH/PVA) was prepared as described in our previous report.44 First, the NKK membrane was immersed into the KOH/PVA gel electrolyte for 15 min and sandwiched between the NiV2Se4 NAs@CFC and NiFe2Se4 NAs@CFC electrodes.

The specific capacity (CS, mAh g−1) and areal capacity (CA, mAh cm−2) of the materials were calculated from the GCD curves using Equations (6) and (7), respectively, as follows:

CA =

2𝐼  𝑉𝑑𝑡

(6)

𝑚𝑉 2𝐼  𝑉𝑑𝑡

(7)

𝑆𝑉

where I is the discharge current (A); m = the mass of active materials grown on CFC (i.e., NixV3-xSe4 or NixFe3-xSe4) for three-electrode configurations (g), whereas the total masses of the NiV2Se4 and NiFe2Se4 electrodes were considered for the flexible solid-state ASC device (g); t is the discharge time (h); V is the potential window (V); and S is the area of the electrode (cm2). 4.8. Volumetric Capacity:

P =

𝐼  𝑉𝑑𝑡

(9)

𝑚  3.6 E 𝑡

 3600

(10)

4.10. Flexible Solid-State ASC Mass Balance: In order to achieve the optimum mass ratio of NiV2Se4 positive to NiFe2Se4 negative electrodes, we examined the charge balance using the given equation, q+ = q−. The mass of the positive electrode and negative electrode were tuned according to the following equation (11): 𝑚+ 𝑚―

=

C―  V― C+  V+

(11)

where C is the specific capacity (investigated through three electrode configuration), m is the mass of the (+) positive and (–) negative electrodes, and V is the voltage range for positive and negative electrodes, respectively. In the flexible solid-state ASC configuration, the optimum mass loading between NiV2Se4 positive and NiFe2Se4 negative electrodes was fixed at 1.32, and the potential window was restricted to ~1.6 V to escalate the inevitable side reactions.

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.” Additional materials characterization including XPS, FE-SEM, SEM-EDS, BET, AFM, TGA-DTA, TEM, XRD (after stability test), CV, GCD, and EIS for the comparative electrodes. Bode plot, digital photographs for ASC devices with different bending and twisting states, stability test for flexible ASC with bending and twisting states, mechanism for ion diffusion for flexible ASC. Table for elemental composition determination for electrode materials. Table for flexible solid-state ASC device properties comparison with the reported literature.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Prof J.H. Lee) and [email protected] (Prof N.H. Kim)

ORCiD cm3)

The volumetric capacity (CV, mAh of the flexible ASC was examined using the following Equation (8) CV =

E =

ASSOCIATED CONTENT

4.7. Specific/Areal Capacity:

CS =

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2𝐼  𝑉𝑑𝑡 𝑉𝑣𝑜𝑙𝑉

(8)

Thanh Tuan Nguyen: 0000-0002-4014-6923 Jayaraman Balamurugan: 0000-0003-3505-5522 Vanchiappan Aravindan: 0000-0003-1357-7717 Nam Hoon Kim: 0000-0001-5122-9567 Joong Hee Lee: 0000-0001-9281-0489

Author Contributions

where Vvol is the total volume of the flexible ASC.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

4.9. Energy Density and Power Density: The energy density (E, Wh kg1) and power density (P, W kg1) of the flexible ASCs were investigated by equations (9) and (10), respectively, as follows:

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This research was supported by the Basic Science Research Program (Grant No. 2017R1A2B3004917) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT of the Republic of Korea.

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