Three Dimensional Interconnected Nanopores Current Collector for Drastically Enhanced Pseudocapacitive Performance

A facile and cost-efficient approach to functionalize raw carbon fiber paper (CFP) used for a self-standing capacitive electrode has been proposed here. Benefiting from the improved specific surface area and surface functional groups, the functionalized CFP (F-CFP) showed much enhanced capacitive performance, 3 orders of magnitude higher than that of the raw CFP. It delivered the areal capacitance of 1275 mF cm-2 at 5 mA cm-2 with a rather wide voltage window of 1.4 V (-0.4 to 1 V vs Ag/AgCl) in 0.5 M H2SO4. However, in a neutral 1 M Na2SO4 aqueous solution, although the areal capacitance of 1115 mF cm-2 at 3 mA cm-2 is slightly smaller, the potential window is much wider (2 V, -1 to 1 V vs Ag/AgCl), indicating a high overpotential of hydrogen evolution. The areal capacitance was still as high as 722 mF cm-2 at a very fast charge-discharge current density of 50 mA cm-2, and about 66% of the initial capacitance (at 3 mA cm-2) was remained in Na2SO4, indicating considerable rate capability.

Aqueous Fe-ion hybrid capacitors, with high safety, low cost, and environmental friendliness, have attracted considerable attention as an emerging energy storage device. However, Fe-ion-based energy storage systems still face challenges, such as narrow voltage windows, limited energy density, and harsh fabrication conditions. To address these issues, this work fabricates an aqueous Fe-ion hybrid capacitor using low-cost activated carbon as the anode, manganese dioxide (MnO2) as the cathode, with an FeSO4 + NH4Cl aqueous electrolyte, successfully expanding the voltage window to 0-1.2 V. The atomic molecular dynamics simulations confirm the potential of MnO2 as a cathode material, and the spiny nanostructured MnO2 shows a large specific surface area and a stable tunnel structure, which facilitates the intercalation/deintercalation of Fe2+ ions. Consequently, the assembled device achieved a specific capacitance of 835 mF cm-2 at 1 mA cm-2 and a surface energy density of 167 μWh cm-2 with a capacitance retention of 97.2% after 3000 cycles. Furthermore, to meet wearable electronics requirements, a flexible device was assembled by integrating a carboxymethyl cellulose-poly(vinyl alcohol) hydrogel soft-packaging material. The results show that the flexible device exhibits excellent bending resistance. The further assembled flexible Fe-ion supercapacitors demonstrate a high energy storage potential. Under a current density test of 1 mA cm-2, the calculated specific capacitance is 682.4 mF cm-2, and the areal energy density is 136.48 μWh cm-2. The further assembled flexible Fe-ion supercapacitors demonstrate high energy storage potential. Under a current density test of 1 mA cm-2, the calculated specific capacitance is 682.4 mF cm-2, and the areal energy density is 136.48 μWh cm-2. Similarly, in the cycling performance test, the device retains 92.6% of its capacity after 3000 cycles. This study provides technical references for the development and practical application of flexible Fe-ion-based energy storage devices.

Nanoporous Anodic Aluminum Oxide (AAO), its average diameters of porous were 50 nm, was obtained by two-step electrochemical anodization aluminum process. Highly ordered luminescence arrays formed by filling AAO nanopores with quercetin molecules has been studied by fluorescent spectroscopy in this work. AAO showed stronger adsorption capability of quercetin than Al3+-quercetin complex. The mechanism may be physical and chemical adsorption all together. Meanwhile, red shift of the maximum fluorescence peak of quercetin in AAO was observed. The molecular assemblies in the nanopore array are highly ordered and the fluorescence polarization dependence indicates a preferred molecular orientation along the pore axis. This maybe explains the mechanism of molecular luminescence depending on its environment.

Cobalt disulfide (CoS2) features highly active catalytic sites and is regarded as a promising candidate for electrocatalytic hydrogen evolution. In this study, molybdenum-doped cobalt disulfide (CoS2:Mo) was synthesized via a facile hydrothermal approach. XRD analysis confirms that the obtained samples crystallize in a cubic pyrite structure, with diffraction peaks consistently shifting towards lower angles. SEM characterization reveals that the samples exhibit microrod-like morphologies with an average size of approximately 1 μm. Integrated analyses from XRD, XPS, and EDS mapping demonstrate that Mo is uniformly distributed across the surface and successfully doped into the CoS2 lattice. Electrochemical measurements indicate that the CoS2:Mo sample delivers a low overpotential of 122 mV and a Tafel slope of 128 mV dec-1 at a current density of 10 mA cm-2 in alkaline media, significantly surpassing the performance of pure CoS2 and MoS2. Moreover, the CoS2:Mo exhibits an enhanced double-layer capacitance, with a Cdl value of 2.72 mF cm-2, superior to that of pure CoS2 (1.63 mF cm-2) and MoS2 (0.31 mF cm-2). Mo doping enhances conductivity and active sites, thereby boosting electrocatalysis. This work presents an effective strategy for the development of cost-efficient and high-performance non-precious metal electrocatalysts.

The development for environmentally friendly energy conversion and storage equipment has given rise to tremendous research efforts as a result of the growing requirements for environmental friendly resources and the rapid consumption of traditional fossil fuel. Herein, a novel hierarchical CoO/NiO-Cu@CuO heterostructure is successfully devised and synthesized. Cobalt/nickel ions are used to generate novel CoO/NiO-doped laminated CuO nanospheres through the facile in situ wet oxidation combined with cation exchange and calcination strategies. As a result, the electrochemical supercapacitance of the as-prepared CoO/NiO-Cu@CuO electrode can reach 875 C cm-2 (2035 mF cm-2), which exhibits much better electrochemical performance compared to other precursor electrodes at a same current density of 2 mA cm-2. Moreover, an excellent rate capacity of 1395 mF cm-2 (50 mA cm-2) can be achieved when measured at a relative high current density; 90.3% of the initial supercapacitance remains even after 5000 cycles. Furthermore, the as-prepared hierarchical hybrid of laminated CoO/NiO-CuO nanospheres in situ generated on three-dimensional (3D) porous Cu foam is applied to prepare a solid-state asymmetric supercapacitor equipment unit. The fabricated equipment unit shows an energy density of 69.3 W h kg-1 at a power density of 1080 W kg-1. Additionally, the commercially applied 2.5 V light-emitting-diode indicator with blue light can be energized for 4 min when two as-fabricated supercapacitor devices are in series connection. The unique hierarchical heterostructure of the novel laminated nanospheres combined with the 3D grid structure brings about the outstanding electrochemical capacitor performances. This strategy for the fabrication of hierarchical heterostructure electrodes could have an enormous potential for high-performance electrochemical equipment.

Electrochemical performance of the material depends on the morphology and structural characteristics of the material. Co3O4 samples shows the remarkable electrochemical performance owing to the high porosity, appropriate pore size distribution and novel architecture and reaction time effect of morphology. In this work, Co3O4 nanowires grown on Ni foam have been synthesized through a facile hydrothermal approach, revealing large capacitance of 2178.4 mF cm-2 at the current density of 2 mA cm-2 and cycling stability with 79.6% capacitance retention after 6000 cycles. The as-assembled device delivers excellent electrochemical performance for high specific capacitance of 356 mF cm-2 at the current density of 2 mA cm-2 and high cycle stability.

1. Introduction The market for portable and/or wearable electronics is continually expanding, owing to the rapid worldwide consumer adoption of smart phones. For this reason, electrode materials with lightweight and flexibility have attracted considerable attention as their power sources. Among them, we focus carbon cloth (CC), composed of numerous uniform activated carbon fibers, as the lightweight and inexpensive electrode material in EDLCs and the current collector to support pseudocapacitive materials. Basically, commercial or untreated CC (uCC) exhibits only poor electrochemical activity. However, the EDLC capacitance of CC can be enhanced by oxidizing it, followed by heating at high temperature under reductive gas atomspheres.1,2 In the present study, we doped nitrogen into CC, simultaneously with reduction, through a hydrothermal reaction of oxidized CC for the first time, and their electrochemical properties were explored. 2. Experimental A section of commercial CC, denoted hereafter as uCC, was oxidized in a solution of HNO3/H2SO4 mixed acids containing KMnO4. The thus-obtained CC (oxidized CC, oCC) was transferred to an autoclave with aqueous hydrazine/ammonia solution and hydrothermally treated at 160 °C, referring the procedure employed to reduce graphene oxide to graphene.3 The CC samples subjected to each process were characterized by XPS and other techniques. Electrochemical tests were made in aqueous electrolytes with and without K4[Fe(CN)6]. 3. Results and discussion XPS spectra in C 1s region of the CC samples presented four typical peaks assignable to C=C (284.6 eV), C-OH (286.2 eV), C=O (287.3 eV), O-C=O (288.7 eV). uCC consisted only of C=C (sp2 carbon). In oCC, the signal due to C=C remarkably decreased in intensity, while those related to the oxygen-containing groups (C-OH and C=O) were revealed. These components decreased again after the hydrothermal treatment with hydrazine/ammonia at 160 °C. At this time, the involvement of nitrogen was evidenced by the signals in N 1s region; i.e., graphitic-N (401.6 eV), pyrrolic-N (400.3 eV), and pyridinic-N (398.6 eV). To the best of our knowledge, this is the first report on the preparation of N-doped carbon cloth via a hydrothermal reaction. The product is denoted as N160-CC. The atomic ratio of N to C was estimated to be 0.11. The dominant species was pyridinic-N whose content increased with an increase in hydrothermal temperature from 25 °C to 160 °C. In CV measurements with K4[Fe(CN)6], the redox peaks of the [Fe(CN)6]4-/[Fe(CN)6]3- couple were detected with oCC and N160-CC, whereas they were hardly observed with uCC. The peak separation was narrower with N160-CC than with oCC, indicating faster electron transfer kinetics resulting from the simultaneous reduction and N-doping of oCC during the hydrothermal treatment. In galvanostatic charge/discharge tests in a 1.0 M H2SO4 solution. The areal capacitance (capacitance normarized by the unit area of the electrode) of N160-CC was estimated to be 136 mF cm-2 at 0.5 mA cm-2, being larger than that (76 mF cm-2) of the oCC piece before hydrothermal treatment. At the talk we will provide information regarding the effects of other strategies including electrochemical processing and chemical modification on the capacitance of CC. References (1) G. Wang, H. Wang, X. Lu, Y. Ling, M. Yu, T. Zhai, Y. Tong, Y. Li, Adv. Mater, 26, 2676 (2014). (2) S. Jiang, T. Shi, X. Zhan, H. Long, S. Xi, H. Hu, Z. Tang, J. Power Sources, 272, 16 (2014). (3) D. Long, W. Lei, L. Ling, J. Miyawaki, I. Mochida, S.-H. Yoon, Langmuir, 26, 16096 (2010).

In this work, a simple, single-step electrochemical method is used to decorate a ternary Polypyrrole-Cu2O-MoO3 (PPy-Cu2O-MoO3) nanocomposite on Graphene Oxide (GO) nanosheets formed on a Graphite Foil Electrode (GFE). The characterization confirmed successful synthesis of ternary nanocomposite, and electrochemical tests showed the electrode performed well as both a supercapacitor and a hydrogen evolution reaction (HER). The investigation of the hydrogen production reaction by the PPy-Cu2O-MoO3/GO/GFE shows that this electrode has a smaller Tafel slope and overpotential. Additionally, the PPy-Cu2O-MoO3/GO/GFE has a specific capacitance of 1010.30 mF cm-2 at 1mA cm-2 in a 0.5 M H2SO4 electrolyte solution. The evaluation of the fabrication of a symmetric solid-state supercapacitor device shows that the constructed device has an excellent capacitance of 596.5 mF cm-2 at 1mA cm-2 and a cyclic stability of 82.4% after 6000 GCD cycles. For hydrogen evolution reaction, the electrode demonstrated an overpotential of 361 mV at 10mA cm-2 and a Tafel slope of 142 mV dec-1, indicating favorable electrocatalytic activity. These results highlight the potential of the synthesized nanocomposite as a multifunctional electrode material for both energy storage and clean energy production.

Two-dimensional atomically thick materials, reduced graphene oxide (RGO), and layered molybdenum disulfide (MoS2 ) have been investigated as potential novel energy storage materials because of their distinct physicochemical properties. These materials suffer, however, from rapid capacity decay and low rate capability. This study describes a facile, binder-free approach to fabricate large-scale, 3D network structured MoS2 @carbon nanotube (CNT)/RGO composites for application in flexible supercapacitor devices. The as-obtained composites possess a hierarchical porosity, and an interconnected framework. The electrochemical supercapacitive measurements of the MoS2 @CNT/RGO electrode show a high specific capacitance of 129 mF cm-2 at 0.1 mA cm-2 . The symmetric supercapacitor devices based on the as-obtained composites exhibit a long lifetime (94.7 % capacitance retention after 10 000 cycles), and a high electrochemical performance (29.7 mF cm-2 ). The present experimental findings will lead to scalable, binder-free synthesis of MoS2 @CNT/RGO hybrid electrodes, with enhanced, flexible, supercapacitive performance, in portable and wearable energy storage devices.

Three-dimensional (3D) hybrid nanostructured arrays grown on a flexible substrate have recently attracted great attention owing to their potential application as supercapacitor electrodes in portable and wearable electronic devices. Here, we report an in situ conversion of Ni-Co active electrode materials for the fabrication of high-performance electrodes. Ni-Co carbonate hydroxide nanowire arrays on carbon cloth were initially synthesized via a hydrothermal method, and they were gradually converted to Ni-Co (oxy)hydroxide nanowire-supported nanoflake arrays after soaking in an alkaline solution. The evolution of the supercapacitor performance of the soaked electrode was investigated in detail. The areal capacitance increases from 281 mF cm-2 at 1 mA cm-2 to 3710 and 3900 mF cm-2 after soaking for 36 h and 48 h, respectively. More interestingly, the electrode also shows an increased capacitance with charge/discharge cycles due to the long-time soaking in KOH solution, suggesting novel cycling durability. The enhancement in capacitive performance should be related to the formation of a unique nanowire-supported nanoflake array architecture, which controls the agglomeration of nanoflakes, making them fully activated. As a result, the facile in situ fabrication of the hybrid architectural design in this study provides a new approach to fabricate high-performance Ni/Co based hydroxide nanostructure arrays for next-generation energy storage devices.

Laser-assisted reduction of graphene oxide (GO) has emerged as a promising method for fabricating conductive films and interdigitated electrodes onto flexible substrates, paving the way for high-performance flexible electronics and smart textiles. In this study, we demonstrate laser-reduced graphene oxide (LrGO) with an unprecedentedly low defect density (average D/G Raman band ratio of 0.1) onto bare heat-sensitive paper substrates. Under optimal lasing conditions, the paper-supported LrGO films exhibit a very low sheet resistance (<50 Ohm sq-1) and maintain good conductivity after multiple cycles of bending deformation. Increasing the laser power beyond the threshold required for GO reduction leads to a gradual decrease in defect density (as measured by Raman spectroscopy), while the surface chemistry (analyzed via X-ray Photoelectron Spectroscopy) remains largely unaffected. This suggests that excess photon energy primarily contributes to defect healing rather than further extending the reduction of LrGO. Three-electrode characterization in a 6 M KOH electrolyte revealed a maximum capacitance of 31.8 mF cm-2 at 10 mV s-1, while the capacitance of an interdigitated supercapacitor with PVA:H2SO4 electrolyte was 5.2 mF cm-2 at 0.02 mA cm-2. These values surpass those reported in recent studies on symmetric supercapacitors using LrGO electrodes on protected paper and other polymeric substrates, underlining the superior quality and potential of the electrodes developed in this work.

MXenes have extensive applications due to their different properties determined by intrinsic structures and various functional groups. Exploring different functional groups of MXenes leads to improved performance or potential applications. In this work, we prepared new Ti3C2PBrx (x=0.4-0.6) MXene with phosphorus functional groups (-P) through a two-step gas-phase reaction. The acquisition of -P is achieved by replacing bromine functional groups (-Br) of Ti3C2Br2 in the phosphorus vapor. After -Br is replaced with -P, Ti3C2PBrx MXene shows an improved areal capacitance (360 mF cm-2) at 20 mV s-1 compared with Ti3C2Br2 MXene (102 mF cm-2). At a current density of 5 mA cm-2 after 10000 cycles, the capacitance retention of Ti3C2PBrx MXene has not decreased. The pseudocapacitive enhancement mechanism has been discovered based on the dual redox sites of the functional groups -P and Ti.

The projection of sustainable, low-cost, and environmentally friendly energy technologies demands innovation of electrocatalysts utilizing earth-abundant materials. The current study aims to improve the catalytic activity of spinel zinc ferrite (ZF), which is an earth-abundant and economically viable material, via a doping strategy. The spinel ZF shows a weak catalytic activity for water splitting, whereas the substitution of Cu ions at octahedral sites results in improving the catalytic performance in both acidic and basic electrolytes. Structural characterization using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction demonstrates that, depending on the Cu concentration, Cu ions either incorporate into spinel Zn-ferrite oxide as doping agents or form CuO nanocomposites, where Cu-induced construction of a composite containing ZCF nanoparticles and CuO nanophase coexists. Substituting Zn with Cu in the octahedral sites of the ZF crystal structure leads to a decrease in the unit cell lattice parameter, and the crystal symmetry is impacted, including the creation of strain and dislocation density. HRTEM analyses provide evidence that the ZF particles nucleate and grow randomly due to the asymmetric reaction dynamics of spinel oxide and the lack of surfactant, while the ZCF nanoparticles are elongated in preferential orientation, forming oriented nanoparticles with a greater surface-to-volume ratio. To attain the current density of 10 mA cm-2, the nanocomposite of the ZCF-50 electrode shows the lowest overpotential of 280 mV for oxygen evolution reaction (OER) among other electrodes. The Tafel slope also decreases significantly in which the nanocomposite of ZCF-50 shows the lowest value of 80 mV dec-1. The measured double-layer capacitance (Cdl) for the nanocomposite structure of ZCF-50 offers the highest value of 27 mF cm-2, which indicates that the nanocomposite contains the largest electrochemically active surface area (ECSA). The catalytic activity of Cu-doped spinel ZCF for hydrogen evolution reaction is also evaluated. The nanocomposite of ZCF-50 shows the lowest onset overpotential of 60 mV compared to 200 mV for the ZF electrode. The obtained Cdl over cathodic potentials for the ZCF-50 electrode shows the highest value of 11.3 mF cm-2 compared with other electrodes. These results confirm that ZCF-50 contains the largest ECSA and highest electrochemical activity. Electrochemical impedance spectroscopy studies also demonstrate that the ZCF-50 electrode shows the lowest charge-transfer resistance, indicating that the catalytic OER is improved significantly at its interfaces. We realize that Cu doping into the ferrite structure and the formation of the CuO semishells synergistically can improve interparticle and transparticle charge transfer.

This research work aims to explore the potential usage of post-consumer waste expanded polystyrene (EPS) for the fabrication of self-standing electrodes by incorporating reduced graphene oxide (rGO) into it via a facile cost-effective mechanical mixing process. The π-π interaction between the expanded polystyrene and rGO is evidenced from FT-IR and Raman analysis. The elevated thermal stability of the EPS/rGO composite from thermogravimetric analysis (TGA) further confirms the interconnection between the rGO and EPS. This π-π stacking interaction between the rGO and the polystyrene molecules present in the polymer matrix enable the composite material to be interconnected throughout which is beneficial for the charge transport process. The symmetric coin cell supercapacitor fabricated using the EPS/rGO composite electrode can be operated with a high operating voltage of 1.6 V in aqueous KOH and Na2SO4 electrolytes. The devices fabricated with KOH and Na2SO4 electrolytes deliver an areal capacitance of 11.9 mF cm-2 and 10 mF cm-2 at the discharge current density of 0.1 mA cm-2. Further, the devices fabricated with the KOH and Na2SO4 electrolytes demonstrated remarkable rate capability of 87.1% and 99.5% after 10 000 continuous charge discharge cycles. This facile method of preparation without consuming energy or polluting the environment is a novel approach which can be scaled-up to large-scale fabrication of self-standing plastic electrodes for low-cost energy storage applications.

Carbon aqueous symmetric supercapacitors are attractive supercapacitor devices owing to the low-cost and high conductivity of porous carbons. However, the limited capacitive charge storage process in carbon symmetric capacitors in comparison to pseudocapacitors or asymmetric supercapacitors severely limit the energy density. The only effective strategy to enhance the energy density of carbon aqueous symmetric supercapacitor is to extend the cell voltage beyond the thermodynamic stability of aqueous electrolytes (1.23 V). Also, in recent studies, researchers have focused on preparing high performance carbon electrodes from commercial carbon fiber clothes by activating the surface usually through chemical and electrochemical routes. However, it is still a major challenge to realize a comprehensive electrochemical performance of high capacitances, excellent rate capability, stability and wide operational potential windows in the activated carbon fiber cloth electrodes. Herein, we successfully activated the surface of commercial carbon fiber clothes using a facile-one-step air calcination at 400 oC for 6 hours strategy to realize a remarkable improvement in surface area, porosity and electrochemical performance. The successful activation of the carbon fiber clothes is fully confirmed through BET tests, SEM, TEM, Raman and contact angle measurement tests. Specifically, the activated carbon exhibits a 450-fold increase in specific surface area (615 m2 g-1 for the activated carbon fiber clothes) as well as rich microporosity, excellent conductivity and superhydrophilicity, which would be beneficial in realizing easy electrolyte penetration and excellent capacitive performance. As expected, the activated carbon fiber cloth achieves ultrahigh areal capacitances of 1553 and 1600 mF cm-2 at 1 mA cm-2 in positive and negative potential windows, good rate capability and excellent cycling stability up to 20000 cycles in 1 M Na2SO4 electrolyte compared to a lowly capacitance of 1.53 mF cm-2 for the commercial carbon fiber clothes. Owing to the large overpotential for hydrogen and oxygen evolution, the activated carbon fiber cloth based symmetric supercapacitor is stable in a wide cell voltage of 2.0 V, displaying high volumetric and gravimetric energy densities of 7.62 mWh cm-3 and 18.2 Wh kg-1, respectively. The stability in this wide potential window is again confirmed using float voltage test method. These results not only report one of the best electrochemical performance of activated carbon fiber clothes, it also provides an effective and simple calcination activation method for realizing high capacitances and takes advantage of the wide operating voltage window to achieve high energy density in carbon textile supercapacitors. Importantly, our studies also showed that the electrochemical performances of metal oxides grown on carbon fiber clothes and annealed in air at 400 oC might be over-estimated as that temperature is sufficient to activate the surface of the carbon fibers.