Scalable Production of Graphene Inks via Wet‐Jet Milling Exfoliation for Screen‐Printed Micro‐Supercapacitors

The miniaturization of energy storage units is pivotal for the development of next‐generation portable electronic devices. Micro‐supercapacitors (MSCs) hold great potential to work as on‐chip micro‐power sources and energy storage units complementing batteries and energy harvester systems. Scalable production of supercapacitor materials with cost‐effective and high‐throughput processing methods is crucial for the widespread application of MSCs. Here, wet‐jet milling exfoliation of graphite is reported to scale up the production of graphene as a supercapacitor material. The formulation of aqueous/alcohol‐based graphene inks allows metal‐free, flexible MSCs to be screen‐printed. These MSCs exhibit areal capacitance (C_areal) values up to 1.324 mF cm^?2 (5.296 mF cm^?2 for a single electrode), corresponding to an outstanding volumetric capacitance (C_vol) of 0.490 F cm^?3 (1.961 F cm^?3 for a single electrode). The screen‐printed MSCs can operate up to a power density above 20 mW cm^?2 at an energy density of 0.064 µWh cm^?2. The devices exhibit excellent cycling stability over charge–discharge cycling (10 000 cycles), bending cycling (100 cycles at a bending radius of 1 cm) and folding (up to angles of 180°). Moreover, ethylene vinyl acetate‐encapsulated MSCs retain their electrochemical properties after a home‐laundry cycle, providing waterproof and washable properties for prospective application in wearable electronics.     

Multilayer‐Folded Graphene Ribbon Film with Ultrahigh Areal Capacitance and High Rate Performance for Compressible Supercapacitors

Limited by 2D geometric morphology and low bulk packing density, developing graphene‐based flexible/compressible supercapacitors with high specific capacitances (gravimetric/volumetric/areal), especially at high rates, is an outstanding challenge. Here, a strategy for the synthesis of free‐standing graphene ribbon films (GRFs) for high‐performance flexible and compressible supercapacitors through blade‐coating of interconnected graphene oxide ribbons and a subsequent thermal treatment process is reported. With an ultrahigh mass loading of 21 mg cm^?2, large ion‐accessible surface area, efficient electron and ion transport pathways as well as high packing density, the compressed multilayer‐folded GRF films (F‐GRF) exhibit ultrahigh areal capacitance of 6.7 F cm^?2 at 5 mA cm^?2, high gravimetric/volumetric capacitances (318 F g^?1, 293 F cm^?3), and high rate performance (3.9 F cm^?2 at 105 mA cm^?2), as well as excellent cycling stability (109% of capacitance retention after 40 000 cycles). Furthermore, the assembled F‐GRF symmetric supercapacitor with compressible and flexible characteristics, can deliver an ultrahigh areal energy density of 0.52 mWh cm^?2 in aqueous electrolyte, almost two times higher than the values obtained from symmetric supercapacitors with comparable dimensions.     

Hierarchical Ni–Co Hydroxide Petals on Mechanically Robust Graphene Petal Foam for High‐Energy Asymmetric Supercapacitors

A hierarchical structure consisting of Ni–Co hydroxide nanopetals (NCHPs) grown on a thin free‐standing graphene petal foam (GPF) has been designed and fabricated by a two‐step process for pseudocapacitive electrode applications. The mechanical behavior of GPFs has been, for the first time to our knowledge, quantitatively measured from in situ scanning electron microscope characterization of the petal foams during in‐plane compression and bending processes. The Young's modulus of a typical GPF is 3.42 GPa, indicating its outstanding mechanical robustness as a nanotemplate. The GPF/NCHP electrodes exhibit volumetric capacitances as high as 765 F cm^?3, equivalent to an areal capacitance of 15.3 F cm^?2 and high rate capability. To assess practical functionality, two‐terminal asymmetric solid‐state supercapacitors with 3D GPF/NCHPs as positive electrodes are fabricated and shown to exhibit outstanding energy and power densities, with maximum average energy density of ≈10 mWh cm^?3 and maximum power density of ≈3 W cm^?3, high rate capability (a capacitance retention of ≈60% at 100 mA cm^?2), and excellent long‐term cyclic stability (full capacitance retention over 15 000 cycles).     

Flexible Cellulose Paper‐based Asymmetrical Thin Film Supercapacitors with High‐Performance for Electrochemical Energy Storage

Cellulose paper (CP)‐based asymmetrical thin film supercapacitors (ATFSCs) have been considered to be a novel platform for inexpensive and portable devices as the CP is low‐cost, lightweight, and can be rolled or folded into 3D configurations. However, the low energy density and poor cycle stability are serious bottlenecks for the development of CP‐based ATFSCs. Here, sandwich‐structured graphite/Ni/Co₂NiO₄‐CP is developed as positive electrode and the graphite/Ni/AC‐CP as negative electrode for flexible and high‐performance ATFSCs. The fabricated graphite/Ni/Co₂NiO₄‐CP positive electrode shows a superior areal capacitance (734 mF/cm² at 5 mV/s) and excellent cycling performance with ≈97.6% C_sp retention after 15 000 cycles. The fabricated graphite/Ni/AC‐CP negative electrode also exhibits large areal capacitance (180 mF/cm² at 5 mV/s) and excellent cycling performance with ≈98% C_sp retention after 15 000 cycles. The assembled ATFSCs based on the sandwich‐structured graphite/Ni/Co₂NiO₄‐CP as positive electrode and graphite/Ni/AC‐CP as negative electrode exhibit large volumetric C_sp (7.6 F/cm³ at 5 mV/s), high volumetric energy density (2.48 mWh/cm³, 80 Wh/kg), high volumetric power density (0.79 W/cm³, 25.6 kW/kg) and excellent cycle stability (less 4% C_sp loss after 20 000 cycles). This study shows an important breakthrough in the design and fabrication of high‐performance and flexible CP‐based electrodes and ATFSCs.     

Substrate‐Free and Shapeless Planar Micro‐Supercapacitors

Micro‐supercapacitors (MSCs), albeit powerful, are unable to broaden their potential applications primarily because they are not as flexible and morphable as electronics. To address this problem, a universal strategy to fabricate substrate‐free, ultrathin, shapeless planar‐MSCs with high‐performance tenability under serious deformation is put forward. These represent a new class of “all‐inside‐one” film supercapacitors, achieved by encapsulating two‐dimensional interdigital microelectrodes within chemically cross‐linked polyvinyl‐alcohol‐based hydrogel electrolyte containing graphene oxide (GO). GO nanosheets significantly improve ionic conductivity, enhance the capacitance, and boost robustness of hydrogel electrolyte. Consequently, the entire MSC, while being only 37 µm thick, can be crumpled and its shape can self‐adjust through fluid channel ten times smaller than its original size without any damage, demonstrating shapelessness. Using MXene as active material, high single‐cell areal capacitance of 40.8 mF cm^?2 is achieved from microelectrodes as thin as 5 µm. Furthermore, to demonstrate wide applicability of this protocol, screen‐printed graphene‐based highly integrated MSCs connecting nine cells in series are fabricated to stably output a high voltage of 7.2 V while crumpling them from 0.11 to 0.01 cm^?3, manifesting superior performance uniformity. This protocol allows the coexistence of high performance with incredible flexibility that may greatly diversify MSCs' applications.     

Wood‐Inspired Morphologically Tunable Aligned Hydrogel for High‐Performance Flexible All‐Solid‐State Supercapacitors

Oriented microstructures are widely found in various biological systems for multiple functions. Such anisotropic structures provide low tortuosity and sufficient surface area, desirable for the design of high‐performance energy storage devices. Despite significant efforts to develop supercapacitors with aligned morphology, challenges remain due to the predefined pore sizes, limited mechanical flexibility, and low mass loading. Herein, a wood‐inspired flexible all‐solid‐state hydrogel supercapacitor is demonstrated by morphologically tuning the aligned hydrogel matrix toward high electrode‐materials loading and high areal capacitance. The highly aligned matrix exhibits broad morphological tunability (47–12 µm), mechanical flexibility (0°–180° bending), and uniform polypyrrole loading up to 7 mm thick matrix. After being assembled into a solid‐state supercapacitor, the areal capacitance reaches 831 mF cm^?2 for the 12 µm matrix, which is 259% times of the 47 µm matrix and 403% times of nonaligned matrix. The supercapacitor also exhibits a high energy density of 73.8 µWh cm^?2, power density of 4960 µW cm^?2, capacitance retention of 86.5% after 1000 cycles, and bending stability of 95% after 5000 cycles. The principle to structurally design the oriented matrices for high electrode material loading opens up the possibility for advanced energy storage applications.     

Ultralight and Binder‐Free All‐Solid‐State Flexible Supercapacitors for Powering Wearable Strain Sensors

Flexible energy storage devices play a pivotal role in realizing the full potential of flexible electronics. This work presents high‐performance, all‐solid‐state, flexible supercapacitors by employing an innovative multilevel porous graphite foam (MPG). MPGs exhibit superior properties, such as large specific surface area, high electric conductivity, low mass density, high loading efficiency of pseudocapacitive materials, and controlled corrugations for accommodating mechanical strains. When loaded with pseudocapacitive manganese oxide (Mn₃O₄), the MPG/Mn₃O₄ (MPGM) composites achieve a specific capacitance of 538 F g^?1 (1 mV s^?1) and 260 F g^?1 (1 mV s^?1) based on the mass of pure Mn₃O₄ and entire electrode composite, respectively. Both are among the best of Mn₃O₄‐based supercapacitors. The MPGM is mechanically robust and can go through 1000 mechanical bending cycles with only 1.5% change in electric resistance. When integrated as all‐solid‐state symmetric supercapacitors, they offer a full cell specific capacitance as high as 53 F g^?1 based on the entire electrode and retain 80% of capacitance after 1000 continuous mechanical bending cycles. Furthermore, the all‐solid‐state flexible supercapacitors are incorporated with strain sensors into self‐powered flexible devices for detection of both coarse and fine motions on human skins, i.e., those from finger bending and heart beating.     

Sputtered Titanium Carbide Thick Film for High Areal Energy on Chip Carbon‐Based Micro‐Supercapacitors

The areal energy density of on‐chip micro‐supercapacitors should be improved in order to obtain autonomous smart miniaturized sensors. To reach this goal, high surface capacitance electrode (>100 mF cm^?2) has to be produced while keeping low the footprint area. For carbide‐derived carbon (CDC) micro‐supercapacitors, the properties of the metal carbide precursor have to be fine‐tuned to fabricate thick electrodes. The ad‐atoms diffusion process and atomic peening effect occurring during the titanium carbide sputtering process are shown to be the key parameters to produce low stress, highly conductive, and thick TiC films. The sputtered TiC at 10^?3 mbar exhibits a high stress level, limiting the thickness of the TiC‐CDC electrode to 1.5 µm with an areal capacitance that is less than 55 mF cm^?2 in aqueous electrolyte. The pressure increase up to 10^?2 mbar induces a clear reduction of the stress level while the layer thickness increases without any degradation of the TiC electronic conductivity. The volumetric capacitance of the TiC‐CDC electrodes is equal to 350 F cm^?3 regardless of the level of pressure. High values of areal capacitance (>100 mF cm^?2) are achieved, whereas the TiC layer is relatively thick, which paves the way toward high‐performance micro‐supercapacitors.     

Layered Silicon‐Based Nanosheets as Electrode for 4 V High‐Performance Supercapacitor

Silicon‐based materials have shown great potential and been widely studied in various fields. Unlike its unparalleled theoretical capacity as anodes for batteries, few investigations have been reported on silicon‐based materials for applications in supercapacitors. Here, an electrode composed of layered silicon‐based nanosheets, obtained through oxidation and exfoliation, for a supercapacitor operated up to 4 V is reported. These silicon‐based nanosheets show an areal specific capacitance of 4.43 mF cm^?2 at 10 mV s^?1 while still retaining a specific capacitance of 834 µF cm^?2 even at an ultrahigh scan rate of 50 000 mV s^?1. The volumetric energy and power density of the supercapacitor are 7.65 mWh cm^?3 and 9312 mW cm^?3, respectively, and the electrode can operate for 12000 cycles in a potential window of 4 V at 2 A g^?1, while retaining 90.6% capacitance. These results indicate that the silicon‐based nanosheets can be a competitive candidate as the supercapacitor electrode material.     

Micro‐Supercapacitors Based on Interdigital Electrodes of Reduced Graphene Oxide and Carbon Nanotube Composites with Ultrahigh Power Handling Performance

A novel method for fabricating micro‐patterned interdigitated electrodes based on reduced graphene oxide (rGO) and carbon nanotube (CNT) composites for ultra‐high power handling micro‐supercapacitor application is reported. The binder‐free microelectrodes were developed by combining electrostatic spray deposition (ESD) and photolithography lift‐off methods. Without typically used thermal or chemical reduction, GO sheets are readily reduced to rGO during the ESD deposition. Electrochemical measurements show that the in‐plane interdigital design of the microelectrodes is effective in increasing accessibility of electrolyte ions in‐between stacked rGO sheets through an electro‐activation process. Addition of CNTs results in reduced restacking of rGO sheets and improved energy and power density. Cyclic voltammetry (CV) measurements show that the specific capacitance of the micro‐supercapacitor based on rGO–CNT composites is 6.1 mF cm^?2 at 0.01 V s^?1. At a very high scan rate of 50 V s^?1, a specific capacitance of 2.8 mF cm^?2 (stack capacitance of 3.1 F cm^?3) is recorded, which is an unprecedented performance for supercapacitors. The addition of CNT, electrolyte‐accessible and binder‐free microelectrodes, as well as an interdigitated in‐plane design result in a high‐frequency response of the micro‐supercapacitors with resistive‐capacitive time constants as low as 4.8 ms. These characteristics suggest that interdigitated rGO–CNT composite electrodes are promising for on‐chip energy storage application with high power demands.