Engineering 2D Architectures toward High‐Performance Micro‐Supercapacitors

The rise of micro‐supercapacitors is satisfying the demand for power storage in portable devices and wireless gadgets. But the miniaturization of the energy‐storage components is significantly limited by their energy density. Electrode materials with adequate electrochemical active surfaces are therefore required for improving performance. 2D materials with ultralarge specific surface areas offer a broad portfolio of the development of high‐performance micro‐supercapacitors in spite of their several critical drawbacks. An architecture engineering strategy is therefore developed to break these natural limits and maximize the significant advantages of these materials. Based on the approaches of phase transformation, intercalation, surface modification, material hybridization, and hierarchical structuration, 2D architectures with improved conductivity, enlarged specific surface, enhanced redox activity, as well as the unique synergetic effect exhibit great promise in the application of miniaturized supercapacitors with highly enhanced performance. Herein, the architecture engineering of emerging 2D materials beyond graphene toward optimizing the performance of micro‐supercapacitors is discussed, in order to promote the application of 2D architectures in miniaturized energy‐storage devices.     

Pencil‐Drawing Skin‐Mountable Micro‐Supercapacitors

In this study, integrated plaster‐like micro‐supercapacitors based on medical adhesive tapes are fabricated by a simple pencil drawing process combined with a mild solution deposition of MnO₂. These solid micro‐supercapacitors not only exhibit excellent stretchability, flexibility, and biocompatibility, but also possess outstanding electrochemical performances, such as exceptional rate capability and cycling stability. Hence they may act as skin‐mountable and thin‐film energy storage devices of high efficiency to power miniaturized and wearable electronic devices.     

Zn‐Ion Hybrid Micro‐Supercapacitors with Ultrahigh Areal Energy Density and Long‐Term Durability

On‐chip micro‐supercapacitors (MSCs), as promising power candidates for microdevices, typically exhibit high power density, large charge/discharge rates, and long cycling lifetimes. However, as for most reported MSCs, the unsatisfactory areal energy density (<10 µWh cm^?2) still hinders their practical applications. Herein, a new‐type Zn‐ion hybrid MSC with ultrahigh areal energy density and long‐term durability is demonstrated. Benefiting from fast ion adsorption/desorption on the capacitor‐type activated‐carbon cathode and reversible Zn stripping/plating on the battery‐type electrodeposited Zn‐nanosheet anode, the fabricated Zn‐ion hybrid MSCs exhibit remarkable areal capacitance of 1297 mF cm^?2 at 0.16 mA cm^?2 (259.4 F g^?1 at a current density of 0.05 A g^?1), landmark areal energy density (115.4 µWh cm^?2 at 0.16 mW cm^?2), and a superb cycling stability without noticeable decay after 10 000 cycles. This work will inspire the fabrication and development of new high‐performance microenergy devices based on novel device design.     

The Road Towards Planar Microbatteries and Micro‐Supercapacitors: From 2D to 3D Device Geometries

The rapid development and further modularization of miniaturized and self‐powered electronic systems have substantially stimulated the urgent demand for microscale electrochemical energy storage devices, e.g., microbatteries (MBs) and micro‐supercapacitors (MSCs). Recently, planar MBs and MSCs, composed of isolated thin‐film microelectrodes with extremely short ionic diffusion path and free of separator on a single substrate, have become particularly attractive because they can be directly integrated with microelectronic devices on the same side of one single substrate to act as a standalone microsized power source or complement miniaturized energy‐harvesting units. The development of and recent advances in planar MBs and MSCs from the fundamentals and design principle to the fabrication methods of 2D and 3D planar microdevices in both in‐plane and stacked geometries are highlighted. Additonally, a comprehensive analysis of the primary aspects that eventually affect the performance metrics of microscale energy storage devices, such as electrode materials, electrolyte, device architecture, and microfabrication techniques are presented. The technical challenges and prospective solutions for high‐energy‐density planar MBs and MSCs with multifunctionalities are proposed.     

One‐Step Synthesis of Monodispersed Mesoporous Carbon Nanospheres for High‐Performance Flexible Quasi‐Solid‐State Micro‐Supercapacitors

Cost‐effective synthesis of carbon nanospheres with a desirable mesoporous network for diversified energy storage applications remains a challenge. Herein, a direct templating strategy is developed to fabricate monodispersed N‐doped mesoporous carbon nanospheres (NMCSs) with an average particle size of 100 nm, a pore diameter of 4 nm, and a specific area of 1093 m² g^?1. Hexadecyl trimethyl ammonium bromide and tetraethyl orthosilicate not only play key roles in the evolution of mesopores but also guide the assembly of phenolic resins to generate carbon nanospheres. Benefiting from the high surface area and optimum mesopore structure, NMCSs deliver a large specific capacitance up to 433 F g^?1 in 1 m H₂SO₄. The NMCS electrodes–based symmetric sandwich supercapacitor has an output voltage of 1.4 V in polyvinyl alcohol/H₂SO₄ gel electrolyte and delivers an energy density of 10.9 Wh kg^?1 at a power density of 14014.5 W kg^?1. Notably, NMCSs can be directly applied through the mask‐assisted casting technique by a doctor blade to fabricate micro‐supercapacitors. The micro‐supercapacitors exhibit excellent mechanical flexibility, long‐term stability, and reliable power output.     

Direct Laser‐Patterned Micro‐Supercapacitors from Paintable MoS2 Films

Micrometer‐sized electrochemical capacitors have recently attracted attention due to their possible applications in micro‐electronic devices. Here, a new approach to large‐scale fabrication of high‐capacitance, two‐dimensional MoS₂ film‐based micro‐supercapacitors is demonstrated via simple and low‐cost spray painting of MoS₂ nanosheets on Si/SiO₂ chip and subsequent laser patterning. The obtained micro‐supercapacitors are well defined by ten interdigitated electrodes (five electrodes per polarity) with 4.5 mm length, 820 μm wide for each electrode, 200 μm spacing between two electrodes and the thickness of electrode is ～0.45 μm. The optimum MoS₂‐based micro‐supercapacitor exhibits excellent electrochemical performance for energy storage with aqueous electrolytes, with a high area capacitance of 8 mF cm^?2 (volumetric capacitance of 178 F cm^?3) and excellent cyclic performance, superior to reported graphene‐based micro‐supercapacitors. This strategy could provide a good opportunity to develop various micro‐/nanosized energy storage devices to satisfy the requirements of portable, flexible, and transparent micro‐electronic devices.     

Inkjet‐Printed High‐Performance Flexible Micro‐Supercapacitors with Porous Nanofiber‐Like Electrode Structures

Flexible planar micro‐supercapacitors (MSCs) with unique loose and porous nanofiber‐like electrode structures are fabricated by combining electrochemical deposition with inkjet printing. Benefiting from the resulting porous nanofiber‐like structures, the areal capacitance of the inkjet‐printed flexible planar MSCs is obviously enhanced to 46.6 mF cm^?2, which is among the highest values ever reported for MSCs. The complicated fabrication process is successfully averted as compared with previously reported best‐performing planar MSCs. Besides excellent electrochemical performance, the resultant MSCs also show superior mechanical flexibility. The as‐fabricated MSCs can be highly bent to 180° 1000 times with the capacitance retention still up to 86.8%. Intriguingly, because of the remarkable patterning capability of inkjet printing, various modular MSCs in serial and in parallel can be directly and facilely inkjet‐printed without using external metal interconnects and tedious procedures. As a consequence, the electrochemical performance can be largely enhanced to better meet the demands of practical applications. Additionally, flexible serial MSCs with exquisite and aesthetic patterns are also inkjet‐printed, showing great potential in fashionable wearable electronics. The results suggest a feasible strategy for the facile and cost‐effective fabrication of high‐performance flexible MSCs via inkjet printing.     

Design of Architectures and Materials in In‐Plane Micro‐supercapacitors: Current Status and Future Challenges

The rapid development of integrated electronics and the boom in miniaturized and portable devices have increased the demand for miniaturized and on‐chip energy storage units. Currently thin‐film batteries or microsized batteries are commercially available for miniaturized devices. However, they still suffer from several limitations, such as short lifetime, low power density, and complex architecture, which limit their integration. Supercapacitors can surmount all these limitations. Particularly for micro‐supercapacitors with planar architectures, due to their unique design of the in‐plane electrode finger arrays, they possess the merits of easy fabrication and integration into on‐chip miniaturized electronics. Here, the focus is on the different strategies to design electrode finger arrays and the material engineering of in‐plane micro‐supercapacitors. It is expected that the advances in micro‐supercapacitors with in‐plane architectures will offer new opportunities for the miniaturization and integration of energy‐storage units for portable devices and on‐chip electronics.     

Tungsten Oxide@Polypyrrole Core–Shell Nanowire Arrays as Novel Negative Electrodes for Asymmetric Supercapacitors

Among active pseudocapacitive materials, polypyrrole (PPy) is a promising electrode material in electrochemical capacitors. PPy‐based materials research has thus far focused on its electrochemical performance as a positive electrode rather than as a negative electrode for asymmetric supercapacitors (ASCs). Here high‐performance electrochemical supercapacitors are designed with tungsten oxide@PPy (WO₃@PPy) core–shell nanowire arrays and Co(OH)₂ nanowires grown on carbon fibers. The WO₃@PPy core–shell nanowire electrode exhibits a high capacitance (253 mF/cm²) in negative potentials (–1.0–0.0 V). The ASCs packaged with CF‐Co(OH)₂ as a positive electrode and CF‐WO₃@PPy as a negative electrode display a high volumetric capacitance up to 2.865 F/cm³ based on volume of the device, an energy density of 1.02 mWh/cm³, and very good stability performance. These findings promote the application of PPy‐based nanostructures as advanced negative electrodes for ASCs.