Hierarchically Structured Nb_(2)O_5 Microflowers with Enhanced Capacity and Fast-Charging Capability for Flexible Planar Sodium Ion Micro-Supercapacitors

Planar Na ion micro-supercapacitors(NIMSCs) that offer both high energy density and power density are deemed to a promising class of miniaturized power sources for wearable and portable microelectron-ics. Nevertheless, the development of NIMSCs are hugely impeded by the low capacity and sluggish Na ion kinetics in the negative electrode.Herein, we demonstrate a novel carbon-coated Nb_(2)O_5 microflower with a hierarchical structure composed of vertically intercrossed and porous nanosheets, boosting Na ion storage performance. The unique structural merits, including uniform carbon coating, ultrathin nanosheets and abun-dant pores, endow the Nb_(2)O_5 microflower with highly reversible Na ion storage capacity of 245 mAh g^(-1) at 0.25 C and excellent rate capability.Benefiting from high capacity and fast charging of Nb_(2)O_5 microflower, the planar NIMSCs consisted of Nb_(2)O_5 negative electrode and activated car-bon positive electrode deliver high areal energy density of 60.7 μWh cm^(-2),considerable voltage window of 3.5 V and extraordinary cyclability. Therefore, this work exploits a structural design strategy towards electrode materials for application in NIMSCs, holding great promise for flexible microelectronics.

Recent advances and key opportunities on in-plane micro-supercapacitors:From functional microdevices to smart integrated microsystems

The popularization of portable,implantable and wearable microelectronics has greatly stimulated the rapid development of high-power planar micro-supercapacitors(PMSCs).Particularly,the introduction of new functionalities(e.g.,high voltage,flexibility,stretchability,self-healing,electrochromism and photo/thermal response)to PMSCs is essential for building multifunctional PMSCs and their smart selfpowered integrated microsystems.In this review,we summarized the latest advances in PMSCs from various functional microdevices to their smart integrated microsystems.Primarily,the functionalities of PMSCs are characterized by three major factors to emphasize their electrochemical behavior and unique scope of application.These include but are not limited to high-voltage outputs(realized through asymmetric configuration,novel electrolyte and modular integration),mechanical resilience that includes various feats of flexibility or stretchability,and response to stimuli(self-healing,electrochromic,photo-responsive,or thermal-responsive properties).Furthermore,three representative integrated microsystems including energy harvester-PMSC,PMSC-energy consumption,and all-in-one selfpowered microsystems are elaborately overviewed to understand the emerging intelligent interaction models.Finally,the key perspectives,challenges and opportunities of PMSCs for powering smart microelectronics are proposed in brief.

Carbon-Based Flexible and All-Solid-State Micro-supercapacitors Fabricated by Inkjet Printing with Enhanced Performance

By means of inkjet printing technique, flexible and all-solid-state micro-supercapacitors(MSCs) were fabricated with carbon-based hybrid ink composed of graphene oxide(GO,98.0vol.%) ink and commercial pen ink(2.0vol.%). A small amount of commercial pen ink was added to effectively reduce the agglomeration of theGO sheets during solvent evaporation and the following reduction processes in which the presence of graphite carbon nanoparticles served as nano-spacer to separate GO sheets. The printed device fabricated using the hybrid ink,combined with the binder-free microelectrodes and interdigital microelectrode configuration, exhibits nearly 780%enhancement in areal capacitance compared with that of pure GO ink. It also shows excellent flexibility and cycling stability with nearly 100% retention of the areal capacitance after 10,000 cycles. The all-solid-state device can be optionally connected in series or in parallel to meet the voltage and capacity requirements for a given application.This work demonstrates a promising future of the carbonbased hybrid ink for directly large-scale inkjet printing MSCs for disposable energy storage devices.

Aqueous high-voltage all 3D-printed micro-supercapacitors with ultrahigh areal capacitance and energy density

With the rapid development of integrated and miniaturized electronics,the planar energy storage devices with high capacitance and energy density are in enormous demand.Hence,the advanced manufacture and fast fabrication of microscale planar energy units are of great significance.Herein,we develop aqueous planar micro-supercapacitors(MSCs) with ultrahigh areal capacitance and energy density via an efficient all-3 D-printing strategy,which can directly extrude the active material ink and gel electrolyte onto the substrate to prepare electrochemical energy storage devices.Both the printed active carbon/exfoliated graphene(AC/EG) electrode ink and electrolyte gel are highly processable with outstanding conductivity(~97 S cm^(-1) of electrode;-34.8 mS cm^(-1) of electrolyte),thus benefiting the corresponding shaping and electrochemical performances.Furthermore,the 3 D-printed symmetric MSCs can be operated stably at a high voltage up to 2.0 V in water-in-salt gel electrolyte,displaying ultrahigh areal capacitance of2381 mF cm^(-2) and exceptional energy density of 331 μWh cm^(-2),superior to previous printed micro energy units.In addition,we can further tailor the integrated 3 D-printed MSCs in parallel and series with various voltage and current outputs,enabling metal-free interconnection.Therefore,our all-3 D-printed MSCs place a great potential in developing high-power micro-electronics fabrication and integration.

General synthesis of hollow mesoporous conducting polymers by dual-colloid interface co-assembly for high-energy-density micro-supercapacitors

Rational design and precise regulation over the morphology, structure, and pore size of functional conducting mesoporous polymers with enriched active sites and shorten electron–ion transport pathway are extremely important for developing high-performance micro-supercapacitors (MSCs), but still remain a great challenge. Herein, a general dual-colloid interface co-assembly strategy is proposed to fabricate hollow mesoporous polypyrrole nano-bowls (mPPy-nbs) for high-energy-density solid-state planar MSCs. By simply adjusting the size of block copolymer micelles, the diameter of polystyrene nanospheres and the amount of pyrrole monomer, mesopore size of the shell, void and shell thickness of mPPy-nbs can be simultaneously controlled. Importantly, this strategy can be further utilized to synthesize other hollow mesoporous polymers, including poly(tris(4-aminophenyl)amine), poly(1,3,5-triaminobenzene) and their copolymers, demonstrative of excellent universality. The structurally optimized mPPy-nb exhibits high specific surface area of 122 m^(2) g^(−1)and large capacitance of 225 F g^(−1) at 1 mV s^(−1). Furthermore, the MSCs assembled by mPPy-nbs deliver impressive volumetric capacitance of 90 F cm^(−3) and energy density of 2.0 mWh cm^(−3), superior to the most reported polymers-based MSCs. Also, the fabricated MSCs present excellent flexibility with almost no capacitance decay under varying bending states, and robust serial/parallel self-integration for boosting voltage and capacitance output. Therefore, this work will inspire the new design of mesoporous conducting polymer materials toward high-performance microscale supercapacitive devices.

Scalable and fast fabrication of graphene integrated micro-supercapacitors with remarkable volumetric capacitance and flexibility through continuous centrifugal coating

Microscale electrochemical energy storage devices,e.g., micro-supercapacitors(MSCs),possessing tailored performance and diversified form factors of lightweight,miniaturization,flexibility and exceptional integration are highly necessary for the smart power sources-unitized electronics.Despite the great progress,the fabrication of MSCs combining high integration with high volumetric performance remains largely unsolved.Herein,we develop a simple,fast and scalable strategy to fabricate graphene based highly integrated MSCs by a new effective continuous centrifugal coating technique.Notably,the resulting highly conductive graphene films can act as not only patterned microelectrodes but also metal-free current collectors and interconnects,endowing modular MSCs with high integrity,remarkable flexibility,tailored voltage and capacitance output,and outstanding performance uniformity.More importantly,the strong centrifugal force and shear force generated in continuous centrifugal coating process lead to graphene films with high alignment,compactness and packing density,contributing to excellent volumetric capacitance of ~31.8 F cm^(-3) and volumetric energy density of ~2.8 mWh cm^(-3),exceeding most reported integrated MSCs.Therefore,our work paves a novel way for simple and scalable fabrication of integrated MSCs and offers promising opportunities as standalone microscale power sources for new-generation electronics.

Ultrahigh-rate and high-frequency MXene micro-supercapacitors for kHz AC line-filtering

Harnessing energy from the environment promotes the rapid development of micro-power generators and relevant power management modules of alternating current (AC) line-filtering to obtain a stabilized direct current (DC) output for storage and use. Micro-supercapacitors (MSCs) with miniaturized volume and high-frequency response are regarded as a critical component in filtering circuits for microscale power conversion. Here, we reported the fabrication of the wafer-sized planar MSCs (M-MSCs) based on 2D Ti_(2)C_(2)T_(6) MXene using a photolithography technique. The M-MSCs exhibited an areal capacitance of 153 μF cm^(-2) and a frequency characteristic (f_(0)) of 5.6 k Hz in aqueous electrolyte. Moreover, by employing suitable ionic liquid as electrolyte, the voltage window was expanded to 2 V and the f_(0) could be pushed to 6.6 k Hz relying on the electrical double-layer mechanism and lower adsorption energy while maintaining quasi-rectangular cyclic voltammogram curves at 5000 V s^(-1). Furthermore, the integrated MSCs pack was constructed and exhibited excellent rectifying ability by filtering various highfrequency 5000 Hz AC signals with different waveforms into stable DC outputs. Such ultrahigh-rate and high-voltage M-MSCs module for k Hz AC line-filtering would be potentially integrated with customizable electronics to realize on-chip rectifiers in high-density integrated circuit.

Low-temperature and high-voltage planar micro-supercapacitors based on anti-freezing hybrid gel electrolyte

Micro-supercapacitors(MSCs)are considered as highly competitive power sources for miniaturized electronics.However,narrow voltage window and poor anti-freezing properties of MSCs in conventional aqueous electrolytes lead to low energy density and limited environmental adaption.Herein,we report the construction of low-temperature and high-energy-density MSCs based on anti-freezing hybrid gel electrolytes(HGE)through introducing ethylene glycol(EG)additives into aqueous LiCl electrolyte.Since EG partially destroys hydrogen bond network among water molecules,the HGE exhibits maximum electrochemical stability window of 2.7 V and superior anti-freezing features with a glass transition temperature of-62.8℃.Further,the optimized MSCs using activated carbon microelectrodes possess impressive volumetric capacitance of 28.9 F cm^(-3)and energy density of 10.3 mWh cm^(-3)in the voltage of 1.6 V,2.6 times higher than MSCs tested in 1.2 V.Importantly,the MSCs display 68.3%capacitance retention even at-30℃ compared to the value at 25℃,and ultra-long cyclability with 85.7%of initial capacitance after 15,000 times,indicating extraordinary low-temperature performance.Besides,our devices offer favorable flexibility and modular integration.Therefore,this work provides a general strategy of realizing flexible,safe and anti-freezing microscale power sources,holding great potential towards subzero-temperature microelectronic applications.

Laser fabrication of functional micro-supercapacitors

Due to the rapid development of portable,wearable and implantable electronics in the fields of mobile communications,biomonitoring,and aerospace or defense,there is an increasing demand for miniaturized and lightweight energy storage devices.Micro-supercapacitors(MSCs)possessing long lifetime,high power density,environment friendliness and safety,have attracted great attention recently.Since the performance of the MSCs is mainly related to the structure of the active electrode,there is a great need to explore the efficient fabricating strategies to deterministically coordinate the structure and functionality of microdevices.Considering that laser technology possesses many superior features of facility,high-precision,low-cost,high-efficiency,shape-adaptability and maneuverability,herein we summarize the development of laser technologies in MSCs manufacturing,along with their strengths and weaknesses.The current achievements and challenges are also highlighted and discussed,aiming to provide a valuable reference for the rational design and manufacture of MSCs in the future.

Drying-Mediated Self-Assembly of Graphene for Inkjet Printing of High-Rate Micro-supercapacitors

Scalable fabrication of high-rate micro-supercapacitors(MSCs)is highly desired for on-chip integration of energy storage components.By virtue of the special self-assembly behavior of 2D materials during drying thin films of their liquid dispersion,a new inkjet printing technique of passivated graphene micro-flakes is developed to directly print MSCs with 3D networked porous microstructure.The presence of macroscale through-thickness pores provides fast ion transport pathways and improves the rate capability of the devices even with solid-state electrolytes.During multiple-pass printing,the porous microstructure effectively absorbs the successively printed inks,allowing full printing of 3D structured MSCs comprising multiple vertically stacked cycles of current collectors,electrodes,and sold-state electrolytes.The all-solid-state heterogeneous 3D MSCs exhibit excellent vertical scalability and high areal energy density and power density,evidently outperforming the MSCs fabricated through general printing techniques.

Understanding the Coffee ring Effect on Self-discharge Behavior of Printed micro-Supercapacitors

Printed micro-supercapacitor exhibits its flexibility in geometry design and integration,showing unprecedented potential in powering the internet of things and portable devices.However,the printing process brings undesired processing defects(e.g.,coffee ring effect),resulting in severe self-discharge of the printed micro-supercapacitors.The impact of such problems on device performance is poorly understood,limiting further development of microsupercapacitors.Herein,by analyzing the self-discharge behavior of fully printed micro-supercapacitors,the severe self-discharge problem is accelerated by the ohmic leakage caused by the coffee ring effect on an ultrathin polymer electrolyte.Based on this understanding,the coffee ring effect was successfully eradicated by introducing graphene oxide in the polymer electrolyte,achieving a decline of 99%in the self-discharge rate.Moreover,the micro-supercapacitors with uniformly printed polymer electrolyte present 7.64 F cm^(-3)volumetric capacitance(14.37 mF cm^(-2)areal capacitance),exhibiting about 50%increase compared to the one without graphene oxide addition.This work provides a new insight to understand the relationship between processing defects and device performance,which will help improve the performance and promote the application of printed micro-supercapacitors.

2.4 V ultrahigh-voltage aqueous MXene-based asymmetric micro-supercapacitors with high volumetric energy density toward a self-sufficient integrated microsystem

Two-dimensional MXenes are key high-capacitance electrode materials for micro-supercapacitors(MSCs)catering to integrated microsystems.However,the narrow electrochemical voltage windows of conventional aqueous electrolytes(≤1.23 V)and symmetric MXene MSCs(typically≤0.6 V)substantially limit their output voltage and energy density.Highly concentrated aqueous electrolytes exhibit lower water molecule activity,which inhibits water splitting and consequently widens the operating voltage window.Herein,we report ultrahigh-voltage aqueous planar asymmetric MSCs(AMSCs)based on a highly concentrated LiCl-gel quasi-solid-state electrolyte with MXene(Ti3C2Tx)as the negative electrode and MnO_(2) nanosheets as the positive electrode(MXene//MnO_(2)-AMSCs).The MXene//MnO_(2)-AMSCs exhibit a high voltage of up to 2.4 V,attaining an ultrahigh volumetric energy density of 53 mWh cm−3.Furthermore,the in-plane geometry and the quasi-solid-state electrolyte enabled excellent mechanical flexibility and performance uniformity in the serially/parallel connected packs of our AMSCs.Notably,the MXene//MnO_(2)-AMSC-based integrated microsystem,in conjunction with solar cells and consumer electronics,could efficiently realize simultaneous energy harvesting,storage,and conversion.The findings of this study provide insights for constructing high-voltage aqueous MXene-based AMSCs as safe and self-sufficient micropower sources in smart integrated microsystems.

Interfacial assembly of 2D polydopamine/graphene heterostructures with well-defined mesopore and tunable thickness for high-energy planar micro-supercapacitors

Two-dimensional(2D)mesoporous pseudocapacitive polymer/graphene heterostructures combine the advanced merits of 2D materials and mesoporous materials,possessing unique nanosheet structure,large specific surface area(SSA),abundant oxygen/nitrogen-containing groups,desirable electrical conductivity and admirable electrochemical redox activity,and hold great potential for constructing high-performance planar micro-supercapacitors(MSCs).Herein,we demonstrate the interfacial assembly of 2D mesoporous polydopamine/graphene(mPDG)heterostructures with well-defined mesopore structure(12 nm)and adjustable thickness(7.5–14.1 nm)for planar high-energy pseudocapacitive MSCs.Attributed to medium thickness,exposed mesopore of 12 nm and large SSA of 108 m^(2)/g,the m PDG with 10.8 nm thickness reveals prominent mass capacitance of 419 F/g and impressive cycling stability with~96%capacitance retention after 5000 cycles.Furthermore,the symmetric mPDG-based MSCs with“water-in-salt”gel electrolyte present wide voltage window of 1.6 V,superior volumetric energy density of 11.5 mWh/cm^(3),outstanding flexibility and self-integration ability.Therefore,this work offers a new platform of controllably synthesizing 2D mesoporous heterostructures for high-performance MSCs.

Biologically inspired anthraquinone redox centers and biomass graphene for renewable colloidal gels toward ultrahigh-performance flexible micro-supercapacitors

Biomass carbon and small redox biomolecules are attractive materials for green,sustainable energy storage devices owing to their environmentally friendly,low-cost,scalable,and novel sources.However,most devices manufactured using these materials have low specific capacitance,poor cycle stability,short lifetime,complexity,and low precision of device fabrication.Herein,we report the directed self-assembly of mononuclear anthraquinone(MAQ)derivatives and porous lignin-based graphene oxide(PLGO)into a renewable colloidal gel through noncovalent interactions.These self-assembled gel electrode materials exhibited high capacitance(484.8 F g^(−1) at a current density of 1 A g^(−1))and could be further printed as flexible micro-supercapacitors(FMSCs)with arbitrary patterns and a relatively high resolution on specific substrates.The FMSCs exhibited excellent areal capacitance(43.6 mF cm^(−2)),energy and power densities(6.1μWh cm^(−2) and 50μW cm^(−2),respectively),and cycle stability(>10,000 cycles).Furthermore,the printed FMSCs and integrated FMSC arrays exhibited remarkable flexibility while maintaining a stable capacitance.The proposed approach can be applied to other quinone biomolecules and biomass-based carbon materials.This study provides a basis for fabricating green and sustainable energy storage device architectures with high capacitance,long-term cycling,high scalability,and high precision.

Direct mask-free fabrication of patterned hierarchical graphene electrode for on-chip micro-supercapacitors

Graphene-based electrodes with rational structural design have shown extraordinary prospect for en-hanced electrical double-layer capacitance of micro-supercapacitors(MSCs).Herein,a facile fabrication method for flexible planar MSCs based on hierarchical graphene was demonstrated by using a laser-treated membrane for electrode patterning,complemented with hierarchical electrode configuration tak-ing full advantages of size-determined functional graphene.The in-plane interdigital shape of MSCs was defined through vacuum filtration with the assistance of the functionalized polypropylene(PP)mem-brane.The hierarchical graphene films were built by macroscopic assembly based on size effect of differ-ent lateral sized graphene sheets(rGO-LSL).The sample of MSCs based on rGO-L SL(MSCs-LSL)exhibited excellent volumetric capacitance of 6.7 F cm^(−3) and high energy density of 0.37 mWh cm−3.The MSCs-LSL presented superb flexibility and cycling stability with no capacitance deteroriated after 2000 cycles.This newly developed fabrication strategy is of good scalability and designability to manufacture flexible elec-trode for MSCs with customized shapes,while the construction of hierarchical graphene can enlighten the structural design of analogous two-dimensional materials for potential advanced electronics.

Ultraconformable Integrated Wireless Charging Micro-Supercapacitor Skin

Conformable and wire-less charging energy storage devices play important roles in enabling the fast development of wearable,non-contact soft electronics.However,current wire-less charging power sources are still restricted by limited flexural angles and fragile connection of components,resulting in the failure expression of performance and constraining their fur-ther applications in health monitoring wearables and moveable artificial limbs.Herein,we present an ultracompatible skin-like integrated wireless charging micro-supercapacitor,which building blocks(including electrolyte,electrode and substrate)are all evaporated by liquid precursor.Owing to the infiltration and permeation of the liquid,each part of the integrated device attached firmly with each other,forming a compact and all-in-one configuration.In addition,benefitting from the controllable volume of electrode solution precursor,the electrode thickness is easily regulated varying from 11.7 to 112.5μm.This prepared thin IWC-MSC skin can fit well with curving human body,and could be wireless charged to store electricity into high capacitive micro-supercapacitors(11.39 F cm-3)of the integrated device.We believe this work will shed light on the construction of skin-attachable electronics and irregular sensing microrobots.

Vertically stacked holey graphene/polyaniline heterostructures with enhanced energy storage for on-chip micro-supercapacitors

Planar micro-supercapacitors （MSCs） have drawn extensive research attention owing to their unique structural design and size compatibility for microelectronic devices. Graphene has been widely used to improve the performance of microscale electrochemical capacitors. However, investigations of an intrinsic electrochemical mechanism for graphene-based microscale devices are still not sufficient. Here, micro-supercapacitors with various typical architectures are fabricated as models to study the graphene effect, and their electrochemical performance is also evaluated. The results show that ionic accessibility and adsorption are greatly improved after the introduction of the holey graphene intermediate layer. This study provides a new route to understand intrinsic electrochemical behaviors and possesses exciting potential for highly efficient on-chip micro-energy storage.

Capacitance and voltage matching between MnO2 nanoflake cathode and Fe2O3 nanoparticle anode for high-performance asymmetric micro-supercapacitors

Planar micro-supercapacitors show great potential as the energy storage unit in miniaturized electronic devices. Asymmetric structures have been widely inves- tigated in micro-supercapacitors, and carbon-based materials are commonly applied in the electrodes. To integrate different metal oxides in both electrodes in micro-supercapacitors, the critical challenge is the pairing of different faradic metal oxides. Herein, we propose a strategy of matching the voltage and capadtance of two faradic materials that are fully integrated into one high-performance asymmetric micro-supercapacitor by a facile and controllable fabrication process. The fabricated micro-supercapacitors employ MnO2 as the positive active material and Fe203 as the negative active material, respectively. The planar asymmetric micro-supercapacitors possess a high capacitance of 60 F-cm-3, a high energy density of 12 mW.h.cm-3, and a broad operation voltage range up to 1.2 V.

Advances on three-dimensional electrodes for micro-supercapacitors:A mini-review

Owing to the high power density,long cycle life and maintain-free,micro-supercapacitors(MSCs)stand out as preferred miniaturized energy source for themiscellaneous autonomous electronic components.However,the shortage of energy density is the main stumbling block for their practical applications.To solve this energy issue,constructing a three-dimensional(3D)electrode within the limited footprint area is proposed as a new solution for improving the energy storage capacity ofMSCs.In the last few years,extensive efforts have been devoted to developing 3D electrodes for MSCs,and significant progress and breakthrough have been achieved.While,there is still lack of systematic summary on the 3D electrode design strategies.To this end,it is imperative to outline the basic design conception,summarize the current states,and discuss the future research about 3D electrodes in MSCs based on the latest development.

Mesoporous polypyrrole-based graphene nanosheets anchoring redox polyoxometalate for all-solid-state micro-supercapacitors with enhanced volumetric capacitance

Micro-supercapacitors（MSCs） have emerged as one competitive candidate of high-performance, flexible, safe,portable and wearable energy storage devices. However, improving their electrochemical performance from electrode materials to assembled devices still remains huge challenges.Here, we for the first time synthesized two-dimensional（2D）,ultrathin, mesoporous polypyrrole-based graphene nanosheets uniformly anchored with redox polyoxometalate（mPPy@rGO-POM） by soft template approach. Further, using a layer-by-layer deposition and mask-assisted technique, the compactly stacked and sandwich-like hybrid film（mPGM）based on pseudocapacitive mPPy@rGO-POM nano sheets and electrochemically exfoliated graphene was directly fabricated as binder-and additive-free interdigital electrodes for all-solid-state planar micro-supercapacitors（mPGM-MSCs）. Notably, the resulted mPGM-MSCs exhibited outstanding areal capacitance（115 mF cm^-2）, remarkably enhanced volumetric capacitance（137 F cm^-3 at 1 mV s^-1） in comparison with MSCs based on the films of mPPy@rGO without POM anchoring（95 F cm^-3）, and non-porous polypyrrole-graphene（68 F cm^-3）.Further, mPGM-MSCs disclosed robust mechanical flexibility with ～96% of capacitance retention at a highly bending angle of 180°, and impressive parallel or serial interconnection for boosting capacitance or voltage output. As a consequence, our proposed strategy of filling the redox species into mesoporous graphene and other 2D nanosheets will open up new ways to manufacture high-compact and flexible energy storage devices ranging from supercapacitors to batteries.