Wood-Derived, Conductivity and Hierarchical Pore Integrated Thick Electrode Enabling High Areal/Volumetric Energy Density for Hybrid Capacitors.

For the proliferation of the supercapacitor technology, it is essential to attain superior areal and volumetric performance. Nevertheless, maintaining stable areal/volumetric capacitance and rate capability, especially for thick electrodes, remains a fundamental challenge. Here, for the first time, a rationally designed porous monolithic electrode is reported with high thickness of 800 µm (46.74 mg cm-2 , with high areal mass loading of NiCo2 S4 6.9 mg cm-2 ) in which redox-active Ag nanoparticles and NiCo2 S4 nanosheets are sequentially decorated on highly conductive wood-derived carbon (WC) substrates. The hierarchically assembled WC@Ag@NiCo2 S4 electrode exhibits outstanding areal capacitance of 6.09 F cm-2 and long-term stability of 84.5% up to 10 000 cycles, as well as exceptional rate capability at 50 mA cm-2 . The asymmetric cell with an anode of WC@Ag and a cathode of WC@Ag@NiCo2 S4 delivers areal/volumetric energy density of 0.59 mWh cm-2 /3.93 mWh cm-3 , which is much-improved performance compared to those of most reported thick electrodes at the same scale. Theoretical calculations verify that the enhanced performance could be attributed to the decreased adsorption energy of OH- and the down-shifted d-band of Ag atoms, which can accelerate the electron transport and ion transfer.

Understanding and Calibration of Charge Storage Mechanism in Cyclic Voltammetry Curves.

Noticeable pseudo-capacitance behavior out of charge storage mechanism (CSM) has attracted intensive studies because it can provide both high energy density and large output power. Although cyclic voltammetry is recognized as the feasible electrochemical technique to determine it quantitatively in the previous works, the results are inferior due to uncertainty in the definitions and application conditions. Herein, three successive treatments, including de-polarization, de-residual and de-background, as well as a non-linear fitting algorithm are employed for the first time to calibrate the different CSM contribution of three typical cathode materials, LiFePO4 , LiMn2 O4 and Na4 Fe3 (PO4 )2 P2 O7 , and achieve well-separated physical capacitance, pseudo-capacitance and diffusive contributions to the total capacity. This work can eliminate misunderstanding concepts and correct ambiguous results of the pseudo-capacitance contribution and recognize the essence of CSM in electrode materials.

Boosting Pseudocapacitive Behavior of Supercapattery Electrodes by Incorporating a Schottky Junction for Ultrahigh Energy Density.

Pseudo-capacitive negative electrodes remain a major bottleneck in the development of supercapacitor devices with high energy density because the electric double-layer capacitance of the negative electrodes does not match the pseudocapacitance of the corresponding positive electrodes. In the present study, a strategically improved Ni-Co-Mo sulfide is demonstrated to be a promising candidate for high energy density supercapattery devices due to its sustained pseudocapacitive charge storage mechanism. The pseudocapacitive behavior is enhanced when operating under a high current through the addition of a classical Schottky junction next to the electrode-electrolyte interface using atomic layer deposition. The Schottky junction accelerates and decelerates the diffusion of OH‒/K+ ions during the charging and discharging processes, respectively, to improve the pseudocapacitive behavior. The resulting pseudocapacitive negative electrodes exhibits a specific capacity of 2,114 C g-1 at 2 A g-1 matches almost that of the positive electrode's 2,795 C g-1 at 3 A g-1. As a result, with the equivalent contribution from the positive and negative electrodes, an energy density of 236.1 Wh kg-1 is achieved at a power density of 921.9 W kg-1 with a total active mass of 15 mg cm-2. This strategy demonstrates the possibility of producing supercapacitors that adapt well to the supercapattery zone of a Ragone plot and that are equal to batteries in terms of energy density, thus, offering a route for further advances in electrochemical energy storage and conversion processes.

Different metal cation-doped MnO2/carbon cloth for wide voltage energy storage.

Aqueous gel supercapacitors, as an important component of flexible energy storage devices, have received widespread attention for their fast charging/discharging rates, long cycle life and high electrochemical stability under mechanical deformation condition. However, the low energy density of aqueous gel supercapacitors has greatly hindered their further development due to the narrow electrochemical window and limited energy storage capacity. Therefore, different metal cation-doped MnO2/carbon cloth-based flexible electrodes herein are prepared by constant voltage deposition and electrochemical oxidation in various saturated sulphate solutions. The influence of different metal cations as K+, Na+ and Li+ doping and deposition conditions on the apparent morphology, lattice structure and electrochemical properties are explored. Furthermore, the pseudo-capacitance ratio of the doped MnO2 and the voltage expansion mechanism of the composite electrode are investigated. The specific capacitance and pseudo-capacitance ratio of the optimized δ-Na0.31MnO2/carbon cloth as MNC-2 electrode could be reached 327.55 F/g at 10 mV/s and 35.56% of the pseudo-capacitance, respectively. The flexible symmetric supercapacitors (NSCs) with desirable electrochemical performances in the operating range of 0-1.4 V are further assembled with MNC-2 as the electrodes. The energy density is 26.8 Wh/kg at the power density of 300 W/kg, while the energy density can still reach 19.1 Wh/kg when the power density is up to 1150 W/kg. The energy storage devices with high-performance developed in this work can provide new ideas and strategic support for the application in portable and wearable electronic devices.

A high-performance and flexible electrode film based on bacterial cellulose/polypyrrole/nitrogen-doped graphene for supercapacitors.

With the development and popularity of portable electronic devices, there is an urgent need for flexible energy storage devices suitable for mass production. We report freestanding paper electrodes for supercapacitors fabricated via a simple but efficient two-step method. Nitrogen-doped graphene (N-rGO) was first prepared via a hydrothermal method. This not only obtained nitrogen atom-doped nanoparticles but also formed reduced graphene oxide. Pyrrole (Py) was then deposited on the bacterial cellulose (BC) fibers as a polypyrrole (PPy) pseudo-capacitance conductive layer by in situ polymerization and filtered with nitrogen-doped graphene to prepare a self-standing flexible paper electrode with a controllable thickness. The synthesized BC/PPy/N15-rGO paper electrode has a remarkable mass specific capacitance of 441.9 F g-1, a long cycle life (96 % retention after 3000 cycles), and excellent rate performance. The BC/PPy/N15-rGO-based symmetric supercapacitor shows a high volumetric specific capacitance of 244 F cm-3 and a max energy density of 67.9 mWh cm-3 with a power density of 1.48 W cm-3, suggesting that they will be promising materials for flexible supercapacitors.

Enhanced Pseudo-Capacitance Process in Nanoarchitectural Layered Double Hydroxide Nanoarrays Hollow Nanocages for Improved Capacitive Deionization Performance.

Layered double hydroxides (LDHs) are perceived as a hopeful capacitive deionization (CDI) faradic electrode for Cl- insertion due to its tunable composition, excellent anion exchange capacity, and fast redox activity. Nevertheless, the self-stacking and inferior electrical conductivity of the two-dimensional structure of LDH lead to unsatisfactory CDI performance. Herein, the three-dimensional (3D) hollow nanocage structure of CoNi-layered double hydroxide/carbon composites is well designed as a CDI anode by cation etching of the pre-carbonized ZIF-67 template. C/CoNi-LDH has a unique 3D hollow nanocage structure and abundant pore features, which can effectively suppress the self-stacking of LDH sheets and facilitate the transport of ions. Moreover, the introduced amorphous carbon layer can act as a conductive network. When employed as the CDI anode, C/CoNi-LDH exhibited a high Cl- removal capacity of 60.88 mg g-1 and a fast Cl- removal rate of 18.09 mg g-1 min-1 at 1.4 V in 1000 mg L-1 NaCl solution. The mechanism of the Cl- intercalation pseudo-capacitance reaction of C/CoNi-LDH is revealed by electrochemical kinetic analysis and ex situ characterization. This study provides vital guidance for the design of high-performance electrodes for CDI.

Influence of Resorcinol to Sodium Carbonate Ratio on Carbon Xerogel Properties for Aluminium Ion Battery.

Carbon xerogels were synthesized using a soft-template route with resorcinol as the carbon source and sodium carbonate as the catalyst. The influence of the resorcinol to catalyst ratio in the range of 500-20,000 on pore structure, graphitic domains, and electronic conductivity of as-prepared carbon xerogels, as well as their performance in an aluminium ion battery (AIB), was investigated. After carbonization steps of the polymers up to 800 °C, all carbon samples exhibited similar specific volumes of micropores (0.7-0.8 cm³ g-1), while samples obtained from mixtures with R/C ratios lower than 2000 led to carbon xerogels with significantly higher mesopore diameters up to 6 nm. The best results, in terms of specific surface (1000 m² g-1), average pore size (6 nm) and reversible capacity in AIB cell (28 mAh g-1 @ 0.1 A g-1), were obtained with a carbon xerogel sample synthetized at a resorcinol to catalyst ratio of R/C = 500 (CXG500). Though cyclic voltammograms of carbon xerogel samples did not exhibit any sharp peaks in the applied potential window, the presence of both oxidation and a quite wide reduction peak in CXG500-2000 cyclic voltammograms indicated pseudocapacitance behaviour induced by diffusion-controlled intercalation/de-intercalation of AlCl4- ions into/from the carbon xerogel matrix. This was confirmed by shifting of the (002) peak towards lower 2θ angle values in the XRD pattern of the CXG500 electrode after the charging step in AIB, whereas the contribution of pseudocapacitance, calculated from half-cell measurements, was limited to only 6% of overall capacitance.

Design and synthesis of cellulose nanofiber-derived CoO/Co/C two-dimensional nanosheet toward enhanced and stable lithium storage.

Nano-sized two-dimensional carbonaceous materials have been widely used as the matrix for alloying-type and conversion-type anode materials for Li-ion batteries (LIBs) to improve structural stability and rate performance. However, relevant synthesis usually requires rigorous conditions and chronic reaction processes. Herein, we have designed a simple solvothermal reaction and heat treatment to prepare a novel CoO/Co/C two-dimensional nanosheet (CoO/Co/C 2DNS) by adopting cellulose nanofibers (CNFs) as the precursor. The unique characteristics of CNFs facilitate the uniform distribution of active materials on the surface and the construction of two-dimensional nanostructure via self-assembly. It is worth noting that CoO/Co/C 2DNS exhibits a striking synergistic effect since the porous 2D carbon framework offers additional pseudo-capacitance and enhances the electronic conductivity, while the ultrafine active materials encapsulated inside shorten the Li-ions diffusion pathways and relieve the volume change. Benefit from the unique structure, the composite anode delivered outstanding rate performance (∼500 mAh g-1 at 10 A g-1) and superior long-range cycling performance up to 800 cycles even at 2 A g-1. This work provides a new strategy for the synthesis of nano-sized 2D composite, offering a promising route to construct high performance conversion-type anodes for next-generation LIBs.

Green and Scalable Fabrication of Sandwich-like NG/SiOx/NG Homogenous Hybrids for Superior Lithium-Ion Batteries.

SiOx is considered as a promising anode for next-generation Li-ions batteries (LIBs) due to its high theoretical capacity; however, mechanical damage originated from volumetric variation during cycles, low intrinsic conductivity, and the complicated or toxic fabrication approaches critically hampered its practical application. Herein, a green, inexpensive, and scalable strategy was employed to fabricate NG/SiOx/NG (N-doped reduced graphene oxide) homogenous hybrids via a freeze-drying combined thermal decomposition method. The stable sandwich structure provided open channels for ion diffusion and relieved the mechanical stress originated from volumetric variation. The homogenous hybrids guaranteed the uniform and agglomeration-free distribution of SiOx into conductive substrate, which efficiently improved the electric conductivity of the electrodes, favoring the fast electrochemical kinetics and further relieving the volumetric variation during lithiation/delithiation. N doping modulated the disproportionation reaction of SiOx into Si and created more defects for ion storage, resulting in a high specific capacity. Deservedly, the prepared electrode exhibited a high specific capacity of 545 mAh g-1 at 2 A g-1, a high areal capacity of 2.06 mAh cm-2 after 450 cycles at 1.5 mA cm-2 in half-cell and tolerable lithium storage performance in full-cell. The green, scalable synthesis strategy and prominent electrochemical performance made the NG/SiOx/NG electrode one of the most promising practicable anodes for LIBs.

Intercalation pseudo-capacitance behavior of few-layered molybdenum sulfide in various electrolytes.

The insertion and de-insertion of ions in layered materials are important processes during the energy storage. The investigation on the preparation and electrochemical properties of layered materials attracts a lot of attention. Here, few-layered MoS2 nanosheets are prepared by solvothermally treating ammonium tetrathiomolybdate ((NH4)2MoS4) in the presence of hydrazine and cetyltrimethylammonium bromide (CTAB). The structure and electrochemical properties of few-layered MoS2 have been characterized and explored in detail. The results show that ions intercalation causes an interlayer spacing change of the obtained MoS2, and that the prepared few-layered MoS2 exhibits fairly good specific capacitance (330.8 F g-1 at 2.0 A g-1), a high rate capability (specific capacitance can still retain 256 F g-1 at 40 A g-1) and good cycle stability (capacitance can retain 88.8 % over 5000 cycles). Furthermore, different from most of the current literatures in which MoS2's capacitance is generally attributed to electric double-layer capacitance, the relationships between current responses and scan rates uncover that few-layered MoS2's capacitance mainly comes from intercalation pseudo capacitance. The result of this work predicts that few layered-MoS2 can be developed as a promising electrode material for energy storage.

Capacity Contribution Induced by Pseudo-Capacitance Adsorption Mechanism of Anode Carbonaceous Materials Applied in Potassium-ion Battery.

The intrinsic bottleneck of graphite intercalation compound mechanism in potassium-ion batteries necessitates the exploitation of novel potassium storage strategies. Hence, utmost efforts have been made to efficiently utilize the extrinsic pseudo-capacitance, which offers facile routes by employing low-cost carbonaceous anodes to improve the performance of electrochemical kinetics, notably facilitating the rate and power characteristics for batteries. This mini-review investigates the methods to maximize the pseudo-capacitance contribution based on the size control and surface activation in recent papers. These methods employ the use of cyclic voltammetry for kinetics analysis, which allows the quantitative determination on the proportion of diffusion-dominated vs. pseudo-capacitance by verifying a representative pseudo-capacitive material of single-walled carbon nanotubes. Synergistically, additional schemes such as establishing matched binder-electrolyte systems are in favor of the ultimate purpose of high-performance industrialized potassium-ion batteries.

Functionalization of Petroleum Coke-Derived Carbon for Synergistically Enhanced Capacitive Performance.

Petroleum coke is a valuable and potential source for clean energy storage if it could be modified legitimately and facilely. In the present study, porous carbon with high surface area and abundant oxygen-containing groups was prepared from petroleum coke by chemical activation and modification processes. The as-prepared carbon exhibits a high surface area (1129 m(2) · g(-1)) and stable micrographic structure. It presents a high specific capacitance and excellent rate performance in KOH electrolyte. Even at an ultrahigh current density of 50 A · g(-1), the specific capacitance of the prepared carbon can still reach up to an unprecedented value of 261 F · g(-1) with a superhigh retention rate of 81 %. In addition, the energy density of this material in aqueous electrolyte can be as high as 13.9 Wh · kg(-1). The high energy density and excellent rate performance ensure its prosperous application in high-power energy storage system.

Morphologically controlled preparation of CuO nanostructures under ultrasound irradiation and their evaluation as pseudocapacitor materials.

Various morphologies of copper oxide (CuO) nanostructures have been synthesized by controlling the reaction parameters in a sonochemical assisted method without using any templates or surfactants. The effect of reaction parameters including molar ratio of the reactants, reaction temperature, ultrasound exposure time, and annealing temperature on the composition and morphology of the product(s) has been investigated. The prepared samples have been characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDAX), and thermogravimetric analysis (TGA). It has been found that Cu2(OH)3NO3 nanoplatelets are achieved in mild conditions which can be then converted to various morphologies of CuO nanostructures by either using high concentrations of OH(-) (formation of nanorods), prolonging sonication irradiation (nanoparticles), or thermal treatment (nanospheres). Application of the prepared CuO nanostructures was evaluated as supercapacitive material in 1 M Na2SO4 solution using cyclic voltammetry (CV) in different potential scan rates ranging from 5 to 100 mV s(-1). The specific capacitance has been calculated using CV curves. It has been found that the pseudocapacitor performance of CuO can be tuned via employing morphologically controlled samples. Accordingly, the prolonged sonicated sample (nanoparticles) showed the high specific capacitance of 158 F.g(-1).