Charge Storage Performance Research Articles

3D Structured Graphene Anodes for Alkali Metal Ion Storage Applications

Electrochemical exfoliation of graphite into graphene has emerged as a promising manufacturing method for the fast production of graphene at a low cost. We will first discuss our recent efforts to understand the electrochemical exfoliation process of graphite under various electrolyte conditions to control the structures of graphene products.1 Then, we will introduce the stacking and defect engineering processes to assemble 3D structured graphene electrodes.2, 3 The 3D graphene anodes delivered enhanced capacities for Li-, Na-, K-ion storage due to the effective utilization of surface defects. In particular, the 3D graphene anodes can exhibit superior charge-storage performance under extremely low-temperature conditions. We reveal that the role of the surface-charge storage mechanism in improving the charge storage performance under low-temperature conditions.References Lee, H.; Choi, J. I.; Park, J.; Jang, S. S.; Lee, S. W. Role of anions on electrochemical exfoliation of graphite into graphene in aqueous acids. Carbon 2020, 167, 816-825.Lee, B.; Kim, M.; Kim, S.; Nanda, J.; Kwon, S. J.; Jang, H. D.; Mitlin, D.; Lee, S. W. High Capacity Adsorption—Dominated Potassium and Sodium Ion Storage in Activated Crumpled Graphene. Adv. Energy Mater. 2020, 10, 1903280.Lee, M. J.; Lee, K.; Lim, J.; Li, M.; Noda, S.; Kwon, S. J.; DeMattia, B.; Lee, B.; Lee, S. W. Outstanding Low-Temperature Performance of Structure-Controlled Graphene Anode Based on Surface-Controlled Charge Storage Mechanism. Adv. Funct. Mater. 2021, 31, 2009397.

Effect of Mass Transport on the Charge Storage Mechanism of Cobalt Hydroxide Nanosheets in an Aqueous Electrolyte

Although cobalt hydroxide (Co(OH)2) has received great attention as a layered electrode material for aqueous-based energy storage applications due to its high theoretical capacity, it is unstable and easily oxidized to be more stable phase in an aqueous medium. This leads to decrease in capacity and cycling performance, limiting its practical application. In addition, a wide range of electrolyte concentration (1-6M) has been utilized for Co(OH)2 electrode and the mechanisms related to charge storage performance were still unclear. To enhance the overall performance, a deep understanding of charge storage mechanisms of Co(OH)2 corresponding to mass transport and electrolyte concentration is needed. Herein, the hydrodynamic voltammetry experiment using rotating disk electrode (RDE) was performed to study the effect of mass transport of the ions in electrolytes on the charge storage mechanism of the as-electrodeposited cobalt hydroxide nanosheets in various concentrations of an aqueous potassium hydroxide (KOH) solution. The cycling performance related to rotation rates and electrolyte concentrations was also studied. Additionally, Operando X-ray absorption spectroscopy (XAS) was carried out to investigate the change in oxidation state of Co(OH)2 in different concentrations of KOH during charge/discharge processes. The set of information in this work will pay the way for further development of the cobalt-based energy storage technologies.

MXene‐Coupled Sandwich‐Like Polyaniline as Dual Conductive Electrode for Flexible All‐Solid‐State and Ionic‐liquid‐Based Supercapacitors with Superior Energy Density

Abstract2D conducting materials (2DCMs), such as graphene, coordination polymers including 2D‐metal‐organic frameworks, conducting organic polymer nanosheets, metal sheets, and their composites are promising candidates for energy storage and conversion applications. Rational design and preparation of 2DCMs usually involve specific templates (soft or hard) that often sacrifice conductivity and negatively impact the electrochemical charge storage performance. In this work, 2D MXene is used as the conductive substrate yielding sandwich‐like MXene‐polyaniline hybrids (MX@PANIs) for high‐performance supercapacitors (SCs) operating in aqueous, solid‐state, and ionic‐liquid‐based safe electrolytes. The MX@PANI‐based flexible all‐solid‐state SC using PVA‐H2SO4 delivers an excellent energy density of 28.6 W h kg–1 at 0.2 kW kg–1. Importantly, the synergistic effect of constituents in MX@PANI in ionic liquid (EMITFSI) electrolyte exhibits an impressive energy density performance of 139 W h kg–1 at a power density of 1.66 kW kg–1, exceeding most of the reported PANI and MXene‐based SCs. This piece of work offers a feasible solution to create the next generation energy‐storage devices using novel 2D hybrid nanocomposites working in safer electrolytes for high‐performance SCs.

Supercapacitor properties of V10O14(OH)2 and reduced graphene oxide hybrids: Experimental and theoretical insights

In this work, V10O14(OH)2 nanostructure and its hybrid with reduced graphene oxide (rGO/V10O14(OH)2) have been synthesized by a simple hydrothermal process, and the supercapacitance property has been explored. The material's crystal structure, surface morphology, elemental compositions, and oxidation state of metal atoms are properly characterized. The structure and electronic properties of V10O14(OH)2 and rGO/V10O14(OH)2 are presented using Density Functional Theory (DFT) investigations. Using the cyclic voltammetry (CV) technique, the capacitance of the rGO/V10O14(OH)2 hybrid was found to be 558 Fg−1 at a scan rate of 1 mV/s, which is 2.9 times greater than the capacitance of the V10O14(OH)2 material. The hybrid material provides 94% operational stability over 5000 cycles. From the Trassatti plot, the capacitance contribution of rGO/V10O14(OH)2 and V10O14(OH)2 are estimated, and found that the maximum capacitance contribution is due to diffusion-controlled pseudocapacitance phenomena, i.e., 87.5% and 95.9%, respectively. Enhanced electronic states near Fermi level and improved quantum capacitance for the hybrid structure justify the superior charge storage performance of the integrated structure (rGO/ V10O14(OH)2) as observed in the experiment. Also, the mobility of the electrolyte ions increases in the hybrid structure due to lower diffusion energy barrier leading to better charge transfer kinetics.

Synthesis of cross-linked Ni-Co oxide nanowires on graphene hydrogel with high capacitive performance for hybrid supercapacitors

Herein, Ni-Co oxide nanowire arrays on graphene supported by Ni foam are fabricated by a simple strategy. In this strategy, the Ni-Co oxide nanosheets are induced to evolve into ultra-fine nanowires with high length/diameter ratio through two-step hydrothermal process, which can effectively increase the surface area, enhance electrochemical activity and improve electron/charge transfer efficiency, thereby resulting in enhanced charge storage performance. It is worth noting that the graphene as a support help to optimize capacitive performance of Ni-Co oxides nanowires. As expected, Ni-Co oxide nanowires anchored on graphene show enhanced specific capacitance of 2619.4 F g−1, due to high-efficient synergistic effect of graphene. However, Ni-Co oxides nanowires directly anchored on Ni foam without graphene have a relatively low specific capacitance of 1950.7 F g−1 at 1 A g−1, but still higher that that of Ni-Co oxides nanosheets arrays (1632.4 F g−1). Furthermore, the hybrid supercapacitor assembled by the Ni-Co oxides nanowires anchored on graphene and nitrogen doped graphene materials shows a specific energy of 24.8 Wh kg−1 and great cycle stability of 90.3% retention after 8000 cycles. This work also provides a strategy to construct novel Ni-Co oxides materials with high charge storage performance for supercapacitor.

Drive towards Sonochemically Synthesized Ternary Metal Sulfide for High‐Energy Supercapattery

The present research focuses on, combining the chemistry of supercapacitors and batteries to get highly efficient hybrid energy storage devices. For projected drive, a novel battery‐grade electrode material is introduced. In this regard, binary and ternary metal sulfides with different compositions are synthesized via a facile sonochemical route. The electrode material Co(25%)Cu(75%)MnS (S4) reveals prime electrochemical performance, possessing a maximum specific capacity of 503.5 C g−1 while operating at 0.5 A g−1 in a three‐electrode assembly. The charge storage performance is further scrutinized in a two‐electrode assembly. A supercapattery device (S4//AC) is fabricated by coupling the Co(25%)Cu(75%)MnS (positive) and activated carbon (negative) electrodes. The device displays outstanding energy storage performance with an ultrahigh energy density of 88.71 Wh kg−1 along with a power density of 320 W kg−1. The maximum power density delivered by the assembled supercapattery device is 8000 W kg−1 in parallel with an energy density of 20 Wh kg−1. The supercapattery device also demonstrates an excellent cyclic stability potential of 84% after 3000 continuous charge/discharge cycles. The outstanding outcomes of the fabricated device reveals its potential applications in high‐performance energy storage technologies.

Stabilization of Orthorhombic CoSe2 by 2D-rGO/MWCNT Heterostructures for Efficient Hydrogen Evolution Reaction and Flexible Energy Storage Device Applications

In view of recent environmental concerns, development of sustainable as well as renewable energy sources, such as the production of hydrogen through electro-reduction of water and energy storage devices in particular high power density as well as eco-friendly supercapacitors have become exigency for energy security. Herein, cobalt selenides (CoSe2) and their carbon allotropic heterostructures have been synthesized and explored for bifunctional applications. The designed CoSe2/rGO/MWCNT heterostructure with an effective interface construction possessed enhanced HER (η20:131 mV, Tafel slope: 52 mV/dec, stable up to ∼8.5 h) in alkaline pH. Owing to its storage performance, the asymmetric CoSe2/rGO/MWCNT//F-MWCNT device also proved to have excellent high capacitance (23.73 F/g at mass normalized current density of 30 A/g), high energy density (44.64 Wh/kg at a power density of 22.32 W/kg @ 6 A/g), and good cycling stability (91.03% capacitance retention after 3000 cycles). The experimental data are also supported by the results from density functional theory simulations in terms of the computed overpotential for HER activity and the quantum capacitance for charge storage performance. The interactions between CoSe2 and carbon allotropes CNT/rGO are due to charge transfer from the Co 3d orbital of CoSe2 to the C 2p orbital. Because of this interaction, the density of states shows enhancement near the Fermi level for hybrid structures, which indicates an increase in the conductivity of the material. The theoretical computed overpotential for HER activity follows the trend CoSe2 > CoSe2/MWCNT > CoSe2/rGO > CoSe2/rGO/MWCNT, supporting the experimental data. Also, the quantum capacitance is highest for the ternary heterostructure CoSe2/rGO/MWCNT, justifying its superior charge storage performance as obtained from the experimental data. This work provides technological insights into the design of an efficient and cost-effective catalyst for sustainable hydrogen production and flexible energy storage applications.

Reinforced birnessite derived from spinel Mn3O4 for sustainable energy storage

The layered birnessite derivatives as intercalation cathode materials for rechargeable batteries and supercapacitors receive great attention due to the abundant, low-cost, highly safe, and environmentally friendly manganese element. However, the practical application of chemically synthesized birnessite in energy storage has been restricted by low specific capacity and poor cyclability because of the limited interlayer metal ions intercalation and inferior structural stability during cycling. In this focused review, we discuss the origin of unsatisfying charge storage performance of the chemically synthesized birnessite and disclose the reinforced birnessite structures derived from spinel Mn3O4 by in-situ electrochemical conversion. With enhanced structural stability and large interlayer distance, the electrochemically converted birnessite shows promising electrochemical performance in various batteries and supercapacitors. Finally, critical perspectives on the future development of layered birnessite from spinel Mn3O4 are provided, which may guide advanced electrode design for high-performance sustainable batteries.

Effect of Electrolyte Concentration on Electrochemical Performance of Bush Like α‐Fe2O3 Nanostructures

AbstractThe nano‐sized bush‐like α‐Fe2O3 particles were synthesized using sol‐gel auto combustion route. The structural characterization of obtained α‐Fe2O3 was performed by X‐ray diffraction (XRD) method to confirm its phase and crystallinity. The Scanning Electron Microscopy (SEM) confirmed the formation of bush‐like nanostructure which was further used for the electrochemical charge storage performance testing. The electrochemical performances of the synthesized electrode material were studied by using Cyclic Voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis in a three electrode configuration. The variation of specific capacitance values with the different molar concentration of electrolyte has been discussed. An enhancement in the specific capacitance has been observed up to 3 M KOH electrolyte solution. The maximum value of specific capacitance is 202.2 F g−1 which is obtained at a scan rate of 2 mV s−1 for 3 M aqueous KOH solution as electrolyte.

Binary composites of sonochemically synthesized cobalt phosphates/polyaniline for supercapattery devices

This work focuses on enhancing the electrochemical performance of cobalt phosphate (Co3(PO4)2) via polyaniline (PANI) for high performance hybrid energy storage devices also termed as supercapattery. For projected drive binary composites cobalt phosphate and PANI at different Wt% were made and composition of PANI was optimized. The composition reveals prime electrochemical performance possessing a maximum specific capacity of 638 C/g while operating at 0.8 A/g in three electrode assembly. The charge storage performance was further scrutinized in two electrode assembly, a supercapattery device is fabricated by coupling Co0.70 (PO4)2/PANI0.30 (S3) (positive) and activated carbon (negative) electrodes. The developed device exhibits exceptional energy storage performance with an ultrahigh specific energy (ES) of 53.2 Wh/kg along with a specific power (PS) of 432 W/kg at 0.5 A/g. The maximum specific power delivered by assembled supercapattery device is 6027 W/kg in parallel with an specific energy of 7.1 Wh/kg. The supercapattery device also demonstrates an outstanding cyclic stability potential of 97.6% and excellent columbic efficiency of 87% after 5000 continuous charge/discharge cycles. Furthermore, the Dunn's model is employed to calculate the capacitive and diffusive impact in total capacity of the device. The outstanding performance of the developed binary composites compared with previously reported literature, provides an efficient and preferable electrode material for potential applications in high-performance energy storage technologies.

Application of New Mxene Based Materials in Electrochemical Sensors

In the current research on the application of new Mxene based materials in electrochemical sensors, the research on the charge storage of new Mxene based materials is not comprehensive enough. The charge storage of new Mxene based materials under different scanning rates is studied in the form of experiments. According to the needs of the experiment, experimental reagents and instruments were selected to prepare Mxene solution and new Mxene based materials. Macroporous Mxene was prepared as the electrode of EES device to store and convert energy. The enzyme was immobilized on the inner surface of Ti3C2Tx to sense biological activity. The experimental results show that the charge storage performance of the new Mxene based materials is the best when the scanning rate is 15mVs-1, which is the most suitable for electrochemical sensors.

Molecular Engineering of Covalent Organic Framework Cathodes for Enhanced Zinc-Ion Batteries.

Covalent organic frameworks (COFs) are potentially promising electrode materials for electrochemical charge storage applications thanks to their pre-designable reticular chemistry with atomic precision, allowing precise control of pore size, redox-active functional moieties, and stable covalent frameworks. However, studies on the mechanistic and practical aspects of their zinc-ion storage behavior are still limited. In this study, a strategy to enhance the electrochemical performance of COF cathodes in zinc-ion batteries (ZIBs) by introducing the quinone group into 1,4,5,8,9,12-hexaazatriphenylene-based COFs is reported. Electrochemical characterization demonstrates that the introduction of the quinone groups in the COF significantly pushes up the Zn2+ storage capability against H+ and elevates the average (dis-)charge potential in aqueous ZIBs. Computational and experimental analysis further reveals the favorable redox-active sites that host Zn2+ /H+ in COF electrodes and the root cause for the enhanced electrochemical performance. This work demonstrates that molecular engineering of the COF structure is an effective approach to achieve practical charge storage performance.

Band-gap shrinked NiO@Co3O4 nanotubes as high-performance supercapacitor electrodes

Novel band-gap shrinked NiO@Co3O4 nanotube arrays on Ni foam have been fabricated via a two-step synthesis route. Hexagonal NiO nanotube arrays are obtained by electrodeposition of Ni nanoparticles onto ZnO nanoprisms followed by selective dissolution of ZnO. The inside and outside surfaces of the NiO nanotubes are homogeneously covered with Co3O4 nanosheets via hydrothermal treatment, which enlarges the surface area and accelerates electron transferring. Benefiting from the hierarchically hollow structure and the synergistic effects, the obtained NiO@Co3O4 nanotube arrays deliver a superior specific capacitance of 1769.2 F/g at 1 A/g and outstanding cycling stability (87.5% capacitance retention after 10,000 cycles at 20 A/g). Density functional theory calculations reveal that the disappearance of the band-gap of the NiO@Co3O4 composite is one intrinsic reason that the conductivity and the capacitive charge storage performance are enhanced significantly. The as-prepared NiO@Co3O4 nanotube arrays are highly promising electrodes for high-performance supercapacitors.

Reducing the Energy Band Gap of Cobalt Hydroxide Nanosheets with Silver Atoms and Enhancing Their Electrical Conductivity with Silver Nanoparticles.

Although cobalt hydroxide (Co(OH)2) has been attracting attention in several applications, its photoelectrochemical property has not yet been fully investigated. In this work, tuning the energy band gap of Co(OH)2 nanosheets with silver atoms and enhancing their electrical conductivity with silver nanoparticles were then focused. A Ag-doped α-Co(OH)2 thin film was successfully synthesized via an electrodeposition method. The optical properties of the as-prepared materials were characterized by UV–vis and fluorescence lifetime spectroscopies and further confirmed by density functional theoretical calculation. It was found that Ag atoms between adjacent layers of Co(OH)2 can reduce its electronic band gap to 2.45 eV (α-Co(OH)2) as compared to 2.85 eV of β-Co(OH)2. In terms of electrochemical properties, silver nanoparticles (AgNPs) can enhance the electrical conductivity of Co(OH)2 nanosheets, leading to faster charge transfer reducing the internal resistance and significantly increasing the overall charge storage performance. Interestingly, under light illumination, Ag-doped α-Co(OH)2 exhibits ca. 0.8 times lower charge storage capacity as compared to that under the dark condition. This is because the photoelectrons can be recombined with the generated holes in the conduction band. The charge storage mechanisms of Ag-doped α-Co(OH)2 operated under dark conditions and light irradiation were further studied and confirmed using in situ electrochemical X-ray absorption spectroscopy (XAS). Overall, the in situ XAS supports the electrochemical result. This finding may pave a way to further develop photoactive advanced functional materials of metal hydroxides and oxides.

A bipolar organic molecule toward a universal pseudocapacitive cathode for stable dual ion charge storage

Organic molecules are promising electrode materials for green and sustainable rechargeable electrochemical energy storage due to their tunable theoretical capacity and environmental friendliness. However, the commercialization of organic electrode materials is impeded by high solubility in electrolytes and inherently low electronic conductivity. Herein, a series of bipolar organic molecules, [5,10,15,20-tetra(ethynyl)porphinato] M(II) (MTEPP, M= Cu, Zn, Mg, 2H), is proposed as new cathode for highly stable dual ion charge storage. The stacking property of the porphyrin molecules is significantly influenced by complex ions and the mixed valence state of cation (Cu+ and Cu2+) renders better charge storage performance in functionalized porphyrin cathode. Reversible capacity of 219 mAh g−1 is obtained for CuTEPP cathode based on a four-electrons transfer and it can be stably cycled up to 1000 times in lithium based organic batteries. Ex-situ spectroscopies as well as simulation studies evidence the multiple electron transfer and the charge storage sites. Highly reversible capability (> 160 mAh g−1) and stable cyclability (> 300 cycles) is also achieved as cathode in both sodium and potassium based organic cells. This study offers a new approach for developing ultrastable and universal organic cathode for electrochemical energy storage.

Manganese oxide nanoflakes grown around cobalt oxide nanobundles with improved charge-storage performance for supercapacitors

Cobalt oxide nanobundle arrays (denoted as CoO) consisting of nanorods were homogeneously grown around the stainless steel wire mesh (SSWM) through a simple hydrothermal synthesis and subsequent heat treatment. The highly dispersed CoO can act as a supporting platform for the deposition of manganese oxide (MnO2) nanoflakes to engineer a CoO@MnO2 core-shell array structure. Without the CoO supports, the MnO2 was found to be prone to form aggregated nanoflakes on the SSWM substrate. CoO arrays with a one-dimensional nanorod skeleton can mitigate the aggregation of two-dimensional MnO2 nanoflakes. The CoO@MnO2 core-shell arrays integrate the advantages of abundant active edge sites, conductive networks for charge transfer, and pore channels for easy transport of electrolyte. The CoO@MnO2 electrode realizes a larger charge-storage capacity than the pristine MnO2 electrode in an aqueous sodium sulfate solution (1 M). The specific capacitances of CoO@MnO2 under 0.15 mA cm-2 and 7.50 mA cm-2 reach 79 mF cm-2 and 53 mF cm-2, respectively, much more than that of MnO2 (31 mF cm-2 and 20 mF cm-2). The CoO@MnO2 core-shell electrode shows a definite improvement in supercapacitive behavior compared to the pristine MnO2 electrode, resulting from reduced charge- and mass-transfer resistance during charge-storage process.

Ultra-stable Sb/hard carbon composite anodes with synergistic alkali-ion storage performances

Antimony based anode materials are attractive for alkali-ion battery applications due to their high theoretical capacity and lower insertion voltage. Herein, Sb/hard carbon composites are fabricated using a simple and scalable high-energy ball-milling approach and have been investigated as anodes for alkali-ion batteries. The structural and morphological studies reveal that Sb nanoparticles are embedded into hard carbon matrix, which provide mechanical buffer and conductive network to improve alkali-ion insertion kinetics. In sodium cells, the composite (HS5050) with optimized Sb loading delivers capacities of 250 mAh g−1 at 100 mA g−1 with excellent cycle life up to 800 cycles, compared to other composite and individual anodes. Remarkably, the same HS5050 provides higher capacities of 572 and 380 mAh g−1 in lithium and potassium cells, respectively with impressive cycle life. The enhanced alkali-ion charge storage performances of composite anode are attributed to the synergistic effect of Sb and hard carbon.

Highly controlled synthesis of nanoprickly nickel@nickel oxide formed on carbon black/reduced graphene oxide nanosheets: Charge-storage performance and electrocatalytic activity for methanol oxidation

Supercapacitive performance and electrocatalytic activity of the nickel@nickel oxide core@shells (Ni@NiO), formed with different composition, structure, and morphology via electroless deposition on the surface of carbon black/reduced graphene oxide nanosheets (CB/RGONs) in the presence of citrate ion are reported here. The conditions are controlled and the synthesis is carried out systematically via three different routes: (a) in the absence of CB and citrate ion, (b) just in the presence of CB, and (c) in the presence of both CB and citrate ion. Then, the synthesized composites were transferred onto the GC electrodes, leading to (a) GC-RGONs-PNi@NiO(Particles), (b) GC-RGONs-HENi@NiO(Hedgehog), and (c) GC-CB/RGONs-NPNi@NiO(Nanoprickly) systems, respectively. The synthesis process is followed by several surfaces and electrochemical techniques, from which the physicochemical behaviors of the prepared composites and electrode systems were determined, and the systems were characterized. Then, their activities for capacitive charge storage and electrooxidation of methanol were quantitatively studied. The composite fabricated under optimized conditions, Nanoprickly system, exhibited (i) the most efficient charge storage behavior with specific capacitance (Cs) of 2398 F g−1; and (ii) a large electrocatalytic activity for electrooxidation of methanol with peak current density of 154 A g−1 at 10 mV s−1, compared with values of 1451 & 1049 F g−1 and 72 & 58 A g−1 obtained for the composite ancestors, Hedgehog and Particle systems, respectively. The increase in the Cs and improvement in the electrocatalytic activity of the nanocomposite are attributed to the (i) crucial role of CB nanospacer, preventing graphene layers from restacking, (ii) role of RGONs, dispersing and stabilizing of Ni@NiO nanoprickly particles, and (iii) synergetic chemical coupling effect between NPNi@NiO and CB/RGONs.

Li-ion charge storage performance of wood-derived carbon fibers@MnO as a battery anode

Wood-derived carbons have been demonstrated to have large specific capacities as the anode materials of lithium-ion batteries (LIBs). However, these carbons generally show low tap density and minor volumetric capacity because of high specific surface area and pore volume. Combination with metal oxide is one of the expected methods to alleviate the obstacles of wood-derived carbons. In this work, the composites of MnO loaded wood-derived carbon fibers (CF@MnO) were prepared via a simple and environmentally friendly method, showing decreased specific surface area due to the generation of MnO nanoparticles on carbon fibers. Furthermore, the CF@MnO compostites exhibit superior electrochemical performance as anode materials of LIBs, which show high reversible capacity in the range of 529–734 mAh/g at a current density of 100 mA/g. The optimal CF@MnO product (MnO:carbon = 1:2) delivers reversible capacity of 734 and 265.3 mAh/g at current density of 100 and 2000 mA/g, respectively. Besides, the material presents outstanding stability with coulombic efficiency around 100% after 200 cycles at a high current density of 400 mA/g, revealing a potential as promising anode materials for high-performance LIBs.

Oxygen Nucleation of MoS2 Nanosheet Thin Film Supercapacitor Electrodes for Enhanced Electrochemical Energy Storage.

A direct thin film approach to fabricate large-surface MoS2 nanosheet thin film supercapacitors using the solution-based diffusion of thiourea into an anodized MoO3 thin film was investigated. A dense MoS2 nanosheet thin film electrode (D-MoS2 ) was obtained when the anodized MoO3 thin film was processed in a low thiourea solution concentration, whereas a highly porous MoS2 nanosheet thin film electrode (P-MoS2 ) was formed at a higher thiourea solution concentration. The charge storage performances of the D-MoS2 and P-MoS2 thin films displayed an unusual increase in capacitance on extended cycling, leading to a capacitance as high as around 5-8 mF cm-2 . X-ray diffraction and cross-sectional microscopy revealed the capacitance enhancements of the MoS2 supercapacitors are attributable to the nucleation of a new MoS2-x Ox phase upon cycling. For the D-MoS2 nanosheet thin film, the formation and growth of the MoS2-x Ox phase during cycling was accompanied by a volumetric expansion of the MoS2 layer. For the P-MoS2 thin film, the nucleation and growth of the MoS2-x Ox phase occurred in the pores of the MoS2 layer. The propagation of the MoS2-x Ox phase also shifted the charge storage process in both films from a diffusion-limited process to a capacitive-dominant process.