1.Introduction

With the rapid development of miniaturized portable electronic devices,the requirements for microsized energy storage equipments that can be directly integrated with portable electronic devices are getting higher and higher.How ever,the currently commonly used energy storage devices,including lithium-ion batteries and supercapacitors,are too heavy and bulky to fit for miniaturized portable electronics[1-7].Therefore,developing small sized energy storage devices with special structures are highly desired.

Currently,the most commonly used power sources for portable electronics are micro-batteries,which usually store energy either by redox or by expansion-contraction reactions.Unfortunately,micro-batteries often suffer the obvious draw backs including limited lifetimes and low power density determined by the internal mechanisms of batteries.Microsupercapacitors(MSCs)represent one of the newly developed energy storage devices that can be used as power source for portable electronics.As we know,supercapacitors can provide higher power density than batteries and higher energy density than conventional capacitors, filling the gap between batteries and conventional capacitors[8-10].They are the promising devices comprising the advantages of fast charge/discharge rates,moderate energy density,long cycling life and so on[11-13].How ever,conventional supercapacitors are usually with sandwich structure or a spiral-wound con figuration,and are heavy and bulky[14].Contrary to conventional supercapacitors,MSCs can be directly fabricated into planar structures with on-chip microelectrode fingers with micro-or nano-scale sizes.Due to the short ion diffusion length,MSCs can deliver ultrahigh power densities that are usually many orders larger than that of batteries and conventional supercapacitors[15-19].Importantly,the compatibility of the MSCs with micro-electronic fabrication techniques ensures the MSCs to be easily integrated with functional microelectronic devices in the electronic circuit[20-28].With the structural advantages of microelectrodes,MSCs can be efficiently connected in series or in parallel on a single substrate to satisfy the voltage and current requirements of miniaturized portable electronic devices,therefore providing excellent microscale energy storage sources for future portable electronic devices.

This review summarizes the recent progress of metal oxides based on-chip MSCs.The synthetic methods,conditions of metal oxides nanostructures and their characteristics are introduced at first.Then,the recent developments of metal oxides based on-chip MSCs,with a focus on their fabrication techniques and electrochemical performances,are highlighted.We also discuss the further developments and prospects of metal oxides based on-chip MSCs.

对实验组40例原发性高血压患者采用综合护理干预：护理人员对患者实施心理护理，使患者保持积极、乐观心态配合治疗。护理人员要主动向患者介绍关于原发性高血压疾病治疗的知识，对于患者提出的疑问，护理人员要耐心、细致、专业的解答，打消患者的疑虑[2]。医护人员不仅要对患者的疾病状态、生理指标等进行密切观察，还要采用家访或者是电话访问的形式，对患者实施随访，及时了解患者的血压控制情况和坚持服药情况，督促患者谨遵医嘱完成后续的药物治疗。采用联合用药的方式，在控制患者血压平稳的状态下，减少患者服药种类，或者是选择新型的复方制剂，药物长效性突出，血压控制效果更好。

2.Device structures of on-chip MSCs

It is know n that traditional capacitor is made up of two paralleled metal polar plates at both ends and insulating dielectric in the middle(Fig.1a).When charging,voltage(potential difference)is formed between two metal plates(Fig.1d),while the middle material cannot perform the charge transporting due to its insulativity.Thus,conventional capacitor is not conductive,failing to meet the requirements of large circuit operation.The emergence of supercapacitor solves this problem,which offers a higher voltage windows and larger power densities.

遗尿症是儿科常见病，是指5岁及以上儿童在夜间不能控制排尿，且每月至少发生2次，并持续发生3个月以上者[1] 。本病不伴有任何尿路症状，仅以夜间遗尿为主，其发病机制尚未完全明了，而长期的遗尿会造成小儿出现一定的心理压力，也给家庭造成一定的负担，故而需要积极治疗。治疗小儿遗尿症的药物较多，如去氨加压素、甲氯芬酯、丙咪嗪、奥昔布宁、中药等，效果不尽相同。近年来笔者采用奥昔布宁联合醋酸去氨加压素治疗小儿遗尿症，取得了较好的效果，现报道如下。

Contrary to the first category,the second category of photolithography technique to get metal oxide based on-chip MSCs is the first patterning of metallic current collectors via photolithography and then the local deposition of active electrode materials onto the metallic current collectors.Kurra et al.presented on-chip planar interdigitated electrodes with Ni(OH)2 nanostructures using the photolithography technology[67].As demonstrated in Fig.7a,the fabrication starts from the photolithography pattern and deposition of interdigitated metal current collectors(Ni/Au/Ti).Before the lift-off process,thin films of Ni(OH)2 nano flakes were deposited on the metal layers via the chemical bath deposition process(CBD).The role of Ni/Au/Ti metal layers is to allow Ni(OH)2 nano flakes to homogeneously deposited on the interdigitated electrode during the CBD process.The SEM images of the finger electrodes after Ni(OH)2 deposition is shown in Fig.7b.Zoom-in image(Fig.7c)show s the uniform vertical grow th of Ni(OH)2 nanoflakes on the Ni/Pt/Ti current collectors.The device exhibits a maximum energy density of 21 m Wh/cm 3 at a power density of 262.5 m W/cm 3,as shown in Fig.7d.In recent years,electrodeposition,as a convenient method was widely used to synthesize active nanomaterials electrodes.An on-chip MSC with Au as the current collectors was recently reported by Li et al.using 3D bicontinuous nanoporous architecture and Mn O2 nanostructures[68].Among them,the unique Au nanostructures facilitated the transmission of electrons.

Usually,conventional supercapacitors and on-chip MSCs are quite different both in their structural designs and the ionic diffusion behaviors.As shown in Fig.1b,conventional supercapacitor has a sandwich structure,consisting of a current collector,active materials,and a separator that are sealed in organic or aqueous electrolyte liquid.The positive/negative electrodes are fabricated by mixing active materials with carbon black,poly(vinylidene fluoride)and organic solvent,and then coated onto current collector supports,such as Ni foam,Cu foam and carbon cloth.These supports act as both the substrate and the current collector.Besides the forming of slurry,directly growing the electrode materials on the current collectors provides a much more efficient way[48].Different with conventional supercapacitors,as shown in Fig.1b,typical on-chip MSCs consist of an interdigitated microelectrode microscale array(Fig.1c),which are made up of a positive and a negative electrode separated by an ionic conductor electrolyte,leading to a simplified device con figuration.The total size of the on-chip MSCs can be on the centimeter or even the micrometer scale,making them excellent candidates for power supply or energy storage components of miniaturized on-chip electronics.

Different structures between conventional supercapacitors and on-chip MSCs lead to different ionic diffusion behaviors.As shown in Fig.1e,owing to the unique structures of conventional supercapacitors,the mostly commonly used separator of conventional supercapacitors is porous cellulose filter membrane with the thickness of around about 300μm.The much thick separator makes the ion diffusion path between the positive electrode and the negative electrode much longer and the velocity greatly decreased,thus greatly affects the charge/discharge rates,leading to a higher equivalent series resistance(ESR).On the contrary,there is no separator in on-chip MSCs as can be seen in Fig.1f.The space between the microelectrodes can replace the commonly used separator and the electrolyte ions can easily transfer in the parallel direction of interface between the electrode and the electrolyte.With the development of microelectronic fabrication techniques,the distance between the microelectrode architectures can be efficiently adjusted and the average migration distance of ions can thus be well controlled,resulting in much larger power densities and faster frequency response than conventional supercapacitors[49-51].

次日一早，老板的奔驰果然准时到达东亭。已然准备好的阿里和罗爹爹看见小车都激动起来，他们都从来没有坐过这样的小车。灰色的金属漆，闪发着令人心跳的辉光。阿里一巴掌伸出去，意欲抚摸车身，司机说：“莫乱摸。”

Fig.1.(a,b,c)Difference structures and(c,d,f)ionic diffusion behaviors between a conventional capacitor,a conventional supercapacitor with sandwich structure and on-chip MSCs with an interdigitated finger structure.

Hollow spherical nanostructures attracted a great deal of attention due to their large surface area,and extra active sites,which are much useful for the improvement of the performance of supercapacitors.The addition of templates to the reaction solution is an effective way to get hollow nanospheres.For example,Du et al.synthesized Co3O4 hollow spheres with carbon spheres as the templates in a hydrothermal process follow ed with high temperature calcination[54].Rationally increasing the complexity of hollow sphere nanostructures will likely to further improve the electrochemical performance and endow them with new functionalities.Recently,Li et al.used a simple step-by-step hydrothermal method to prepare CoFe2O4 hollow microspheres using cyclodextrin as the template.The shell number and shell porosity of the hollow microspheres were well controlled by adjusting the hydrothermal synthesis parameters[55].As shown in Figs.2m-q,the enhancement of the pore in filtration and complexing ability of Fe3+,Co2+can be controlled by adjusting the ratio of water to ethanol.Multi-shelled hollow nanostructures can generally increase the electrochemical performance due to their higher weight fraction of active species.It was found that the capacitance of the triple-shelled CoFe2O4 microspheres is the highest one.

3.Synthetic procedures for metal oxide nanostructures

To fabricate on-chip MSCs,large scale synthesis of metal oxides electrodes is a vital step.Till now,several types of synthetic procedures have been developed to efficiently synthesize metal oxides with different structures,especially nanostructures as they usually have high surface area,excellent conductivity and high electrochemical stability,which greatly affect the performance of the on-chip MSCs in terms of specific capacitance,energy density and power density.

3.1.Hydrothermal methods

For the synthesis of nanomaterials,the most commonly used and efficient way is the hydrothermal method,which uses a special closed reactor filled with aqueous or organic solution as a reaction system.The equipments used usually includes a blast oven and a Te flon-lined autoclave by heating and pressuring the reaction system to create a relatively high temperature and high pressure reaction environment,so that the material is usually dissolved and recrystallized.Using hydrothermal method,a variety of nanostructures can be obtained,such as nanoparticles,nanosheets,nanospheres,nanorods and so on,by controlling the experimental conditions,including temperature,reaction time,pressure,reagents,etc.For example,various α-Mn O2 nanostructures have been successfully synthesized by a simple hydrothermal method with different reaction time[37].Adjusting the p H value is also an efficient way to change the morphologies of the nanomaterials.For instance,Wang et al.synthesize urchin-like NiCo2O4 nanostructures via a simple hydrothermal method free of any template and catalyst[52].

Fig.2.(a-c)SEM images and(d-f)schematic illustrations of the NiCo2O4 products obtained under a hydrothermal process with p H 5.5,6.1 and 6.8,respectively.(g)XRD pattern,(h)SEM image,and(i,j)TEM images of the urchin-like NiCo2O4 nanostructures.N2 adoption and desorption isotherms and pore size distribution curves of(k)the urchin-like NiCo2O4 nanostructures and(l)the straw-like NiCo2O4 bundle.Reproduced with permission[52].Copyright 2012,The Royal Society of Chemistry.(m)Illustration showing the enhancement of the pore in filtration and complexing ability of Fe3+,Co2+in infusion solutions of increasing ethanol content.TEM images of CoFe2O4 multishelled hollow microspheres after calcination at 550°C:(n)single-shelled,(o)double-shelled,(p)triple-shelled,and(q)quadruple-shelled hollow spheres.Reproduced with permission[55].Copyright 2016,Tsinghua University Press and Springer.

LightScribe DVD burner belong to a new kind of laser scribing technique and is also used to fabricate high-performance on-chip MSCs.El-Kady et al.proposed on-chip MSCs based on laser-scribed graphene(LSG)-Mn O2 by rationally designing the electrode microstructure[72].As shown in Fig.8e,the direct writing of LSG interdigital patterns was performed on GO films using a consumer-grade LightScribe DVD burner.The color of the surface of DVD changed from golden brown to black due to the formation of LSG framework.The LSG framework was subsequently in-situ coated with Mn O2 using an electrochemical deposition technique.The areal capacitance of the fabricated on-chip MSCs was about 400 m F/cm 2.Such a method can be easily scaled up to a large area,facilitating the on-chip MSCs to meet the voltage requirements of different types of electronic devices.Fig.8i show ed the fabrication of on-chip MSCs array consisting of 9 cells.Different amounts of on-chip MSCs were connected in series with the voltage changed,as shown in Fig.8j.Besides,Amiri et al.fabricated graphene/Zn O nanocomposite based on-chip MSCs[73].The DVD disk with GO/Zn O film was placed in a LightScribe DVD,and then carried out the laser scribing process.Following several steps,high-performance on-chip MSCs can be fabricated,which showed a high stack capacitance of 9 F/cm 3 at a current density of 150 m A/cm 2.

成名对一些人来说是惊喜，对一些人来说是预谋，对于朦胧来说却是突发情况。成名后他有点儿诚惶诚恐：“那种感觉是一下子多了很多人在看我，很害怕自己做得不够好。”《太子妃升职记》一夜爆红的时候，于朦胧也被推到了大众视野的正中央。他不是没有慌张过，只是学会了一点一点去适应，这个过程就是把他自己个性里那些容易被干扰到的部分想办法安抚好，给自己的心找一处舒服的位置，安顿下来。

As metal oxides based supercapacitors are generally governed by the fast reversible faradic process of redox-active materials and can provide much higher specific capacitances and energy density than the carbon based supercapacitors,we will only focus on the metal oxides based on-chip MSCs in this review.

3.2.Electrospinning synthesis

Besides,Niu et al.synthesized various types of mesoporous nanotubes and pea-like nanotubes by a gradient electrospinning and controlled pyrolysis methodology.The synthesized materials include multi-element oxides,binary-metal oxides and singlemetal oxides.The gradient distribution of low-/middle-/highmolecular-w eight poly(vinyl alcohol)in the precursor is the key point of this method to get the final products[57].1D nanostructures with much more complex structures can also be prepared using electrospin with different post-treatment processes.Liu et al.proposed a rationally designed hierarchical structure of Zn Co2O4@NiCo2O4 core-sheath nanowires synthesized through facile electrospinning combined with a simple co-precipitation method(Fig.3h)[58].The intensity of the nanosheets on the surface of the nanowires is mainly controlled by the hydrothermal treatment time( Figs.3i and j).Fig.3k show ed the TEM images of the Zn Co2O4@NiCo2O4 nanowires.It can be clearly seen that ultrathin NiCo2O4 nanosheets are grown uniformly on Zn Co2O4 backbones to form the composite nanostructure.The hierarchical Zn Co2O4@NiCo2O4 core-sheath nanow ires demonstrate high specific capacitance according to the following electrochemical investigations.

Fig.3.The synthetic procedures and corresponding results of(a-g)CoMn2O4,and(h-k)Zn Co2O4@NiCo2O4 with complex nanostructures via the electrospining techniques.Reproduced with permission[56].Copyright 2015,American Chemical Society;Reproduced with permission[58].Copyright 2015,Wiley Publishing Group.

In recent years,electrospinning synthesis of transition metal oxide nanostructures has become a new topic in the field of nanotechnology and supercapacitors applications.Electrospinning is an effective synthetic method to get one-dimensional(1D)nanostructures.The basic structure of electrospinning is shown in Fig.3a.In a typical electrospining process,a high voltage pow er supply,plastic syringe,a metal needle,controlled injection pump and a receiving substrate were used to controlled synthesis 1D nanostructures.Precursor solution containing a polymer and metallic salt was used and the solvent is volatile solvent such as ethanol or acetic acid.The basic working principle of the electrospining process is that the injection was pumped out of the precursor solution,which was under the action of the electric field force to accelerate the movement and split to form precursor nanofibers.The precursor nanofibers are deposited on the receiving substrate,which were then calcinated at high temperature to get the final nanostructures.1D nanostructures synthesized by the electrospinning method have uniform size and a very high specific surface area.By controlling the calcinating time of the precursor nanofibers from electrospinning can generate 1D nanostructures with different morphologies.For instance,Peng et al.demonstrated the synthesis of various ternary transition metal oxides(TMOs)using the electrospinning method,including CoMn2O4,NiCo2O4,Co Fe2O4,NiMn2O4 and Zn Mn2O4[56].Fig.3a shows the electronspining process of the TMOs with complex 1D nanostructures including tube-in-tube,nanotube,and solid 1D nanostructures.During this process,the key parameters are the precursor consisting two types of polymers with fine tuned ratios and the accurate control of the heating rate in the following calcination process.The precursor nanowires were first synthesized as shown in Fig.3b.Typical SEM and TEM images of the Co Mn2O4 tube-in-tube structures at a heating rate of 3°C/min were shown in Figs.3c and d.If the heating rate was set to 1°C/min,porous CoMn2O4 nanotubes were obtained as shown in Figs.3e and f.Further increase the heating rate to 5°C/min led to the collapse of the hollow structure and solid CoMn2O4 nanowires with the average diameter of around 252 nm were obtained,looking quite like colon(Fig.3g).

3.3.Other typical methods

Besides the hydrothermal and electrospining processes,there are also many other methods that are used to synthesize nanostructured electrodes for supercapacitors.For example,three kinds of hierarchical porous nanostructures assembled from ultrathin Mn O2 nano flakes were synthesized by Jiang et al.[59].They used a precipitation method with controlled temperature to achieve nanostructured samples with different sizes and crystallinities.Such a method is a quite simple and scalable strategy.Figs.4a and b show ed the SEM and TEM images of the Mn O2 products synthesized in an ice bath,which are loose porous nanospheres composed of interconnected small Mn O2 nanoflakes.When the reaction temperature was raised to room temperature,the obtained ultrathin Mn O2 nanoflakes possess a larger sizes than those in an ice bath ( Figs.4c and d).Further increase the temperature to 100°C using a hot plate resulted in the further grow th of the ultrathin Mn O2 nanoflakes,as depicted in Figs.4e and f.

Fig.4.(a-f)SEM and TEM images of Mn O2 nanostructures synthesized under different conditions(a,b)in an ice bath,(c,d)at room temperature,and(e,f)at 100°C.Reproduced with permission[59].Copyright 2012,The Royal Society of Chemistry.(g)Schematic illustration of the synthesis of MnO2-coated graphene electrodes and the corresponding SEM images of(h)graphene oxide and(j)Mn O2 nano flowers.Reproduced with permission[60].Copyright 2011,Elsevier B.V.

内部控制管理是实现国有企业健康稳定发展的基础，对国有企业实现有效的经营有着重要的意义。对此，就需要国有企业经营发展过程中，提高对内部控制措施的使用，以此来提高对企业内部控制管理的合理性，这就可以从以下几方面进行思考：

晨起8:00-10:00真空采血法采集肘部静脉血5 mL，立即送检。采用全自动生化分析仪，乳胶颗粒免疫比浊法检测CysC、酶联免疫吸附法测定检测RBP4。

In addition to the above synthetic methods,there are many other simple and convenient synthetic methods.For instance,Lei et al.synthesized 3D hierarchical flower-shaped nickel cobaltite(NiCo2O4)microspheres by a rapid and template-free microwaveassisted heating(MAH)re flux approach follow ed by calcination the precursors[61].Fig.5a show ed the schematic illustration of the morphological evolution of the flower-shaped microsphere precursors.They carried out time-dependent experiments to explore the formation mechanism of flower-shaped NiCo2O4 microspheres.As shown in Figs.5b-e,SEM images of the samples collected after the reaction temperature reached 100°C at different reaction times as follow s:2 min,5 min,10 min and 15 min.The figures proved that the morphology and size of the products are changed with increased reaction time.Xu et al.prepared the hybrid reduced graphene oxide-cobalt molybdate(RGO/CoMoO4)nanocomposites assisted by microwave-assisted synthesismethod[62].The formation process of the RGO/CoMoO4 nanocomposites was provided in Fig.5f. Figs.5g and h displayed that the RGO/CoMoO4 composites contain randomly distributed CoMoO4 nanoparticles on the graphene sheets.Compared with CoMoO4 nanoplatelets,the fine-separated Co Mo O4 nanoparticles on the graphene sheets in the RGO/CoMoO4 composites increase in surface area and number of active sites,which lead to better electrochemical performance.

Sol-gel method is also a common method for synthetic materials.Lan et al.presented a simple approach for the synthesis of graphene(Gr)/Fe2O3 aerogel by a sol-gel method and supercritical carbon dioxide drying technique[63].Synthesis of the Gr/Fe2O3 aerogel was illustrated in Fig.5i.Gr oxide was dispersed in N,N-dimethylformamide (DMF) firstly,then FeCl3⋅6H2O were added to the suspension form homogeneously mixed by stirring and ultrasonication.After wards,epichlorohydrin was added to mixture for the gelation process.The gel was then immersed into ethanol to replace DMF within the network of the gel.Finally,the gel was dried with supercritical CO2 and calcinated under N2 atmosphere to obtain the Gr/Fe2O3 aerogel.Fig.5j displayed the prepared Fe2O3 nanoparticles with sphere-like morphology dispersed on the Gr sheets.

4.Fabrication of metal oxide based on-chip MSCs

With the successful synthesis of metal oxide nanostructures on a large scale via different methods,on-chip MSCs are then fabricated.To date,several approaches have been explored for the manufacture of on-chip MSCs,including photolithography,laser scribing,and printing techniques,which will be discussed in detail in the following sections.

4.1.Photolithography technology

Photolithography is a well-established method to fabricate electronic devices in microelectronic industries.It also plays an important role in the fabrication of on-chip MSCs with precisely controlled patterns and resolutions.In order to produce metal oxide based on-chip MSCs with improved performance,the technical direction of the photolithography technology is to continuously improve its resolution and makes metal oxide electrodes locally deposited onto the microsized patterns.

Fig.5.(a)Schematic illustration of the morphological evolution of flower-shaped NiCo2O4 precursors.SEM images of the samples collected at different reaction times after the reaction temperature reached 100°C:(b)2 min,(c)5 min,(d)10 min,and(e)15 min.Reproduced with permission[61].Copyright 2014,American Chemical Society.(f)Illustration for the formation process of the RGO/CoMoO4 nanocomposites.(g)RGO/CoMoO4 at low magnification.(h)RGO/CoMoO4 at high magnification.Reproduced with permission[62].Copyright 2014,Elsevier B.V.(i)Illustration of the synthesis of the Gr/Fe2O3 aerogel.(j)TEM images of the Gr/Fe2O3 aerogel.Reproduced with permission[63].Copyright 2015,Springer Science Business Media Dordrecht.

Fabrication of metal oxide based on-chip MSCs via the photolithography technique can be classified into two categories.In the first category,the active metal oxide electrodes were first even spread on the device substrates,including both rigid and flexible substrates.Then interdigitated metallic collectors were deposited on the active materials using the photolithography technology.For example,Li et al.fabricated flexible on-chip MSCs with electrospun NiFe2O4 nanofibers as the active electrodes as shown in Fig.6a[64].During this process,electrospun NiFe2O4 nanofibers were first spin-coated on flexible polyethylene terephthalate(PET)substrate.In the spin-coating process,the thickness and homogeneity of the NiFe2O4 nano fibers film mainly depended on the concentration of the NiFe2O4 aqueous suspension,the speed and time of spin coater and the surface modification of the PET substrate.Following the spin-coating process,interdigital Ni current collectors were then patterned and deposited by a conventional photolithography technique and a magnetic control spluttering coating process.Finally,PVA/KOH gel electrolyte was spread on the integrated electrodes.The obtained on-chip MSCs exhibited a specific capacitance of 2.23 F/cm 3 at the scan rate of 100 m V/s,and still maintained 93.6%of its initial capacitance after 10,000 charge/discharge cycles.Besides,the onchip MSCs exhibited excellent mechanical flexibility. Figs.6b and c showed the CV curves and galvanostatic charge/discharge curves of the device after bending for different times.The two curves before and after bending are nearly coincident with each other,con firming the outstanding flexibility and mechanical stability of the on-chip MSCs.Besides metal oxide nanowires,two-dimensional(2D)metal oxides,metal hydroxides or composited nanostructures were also used as the active electrodes to fabricate onchip MSCs using similar device fabrication process.For example,Wu et al.used 2D Ni(OH)2 nanoplates to fabricate highperformance on-chip MSCs via a similar fabrication process[65].A specific capacitance of 8.80 F/cm 3 at the scan rates of 100 m V/s was obtained for the 2D Ni(OH)2 nanoplates based device.The device exhibited excellent long term electrochemical stability.Only about 0.20%of its original capacitance value was lost even after 10,000 charge/discharge cycles.Later,Gu et al.used reduced graphene oxide/Fe2O3 hollow nanospheres to fabricate flexible onchip MSCs via the similar process,which can be efficiently integrated with Cd S nanowire photodetectors to form a integrated photodetecting system[66].The composited based on-chip MSCs exhibited high specific capacitances of 11.57 F/cm 3 at a scan rate of 200 m V/s and excellent rate capability and robust cycling stability with capacitance retention of 92.08% after 32,000 charge/discharge cycles.After fully charged,the on-chip MSCs can be used as power source to power a CdS nanowire photodetector.

Fig.6.(a)Schematic illustration of the fabrication of flexible on-chip MSCs with NiFe2O4 nano fiber electrodes on PET substrate.(b)CV curves and(f)galvanostatic charge-discharge curves of the on-chip MSCs before and after bending for different times.Reproduced with permission[64].Copyright 2016,The Royal Society of Chemistry.

Generally,supercapacitors,including conventional supercapacitors and the new type on-chip MSCs,can be divided into two types:the electrical double-layer capacitors(EDLCs)and the pseudo-capacitors[29,30].EDLCs store energy via the reversible physical ion adsorption at the interface between the electrode material and the electrolyte.Carbon-based materials,such as active carbon[31],carbon nanotubes(CNTs)[32],and graphene[4,33-35],with large surface areas are mainly governed by this energy-storage mechanism and are commonly used as the electrode materials of EDLCs.On the contrary,pseudo-capacitors arising from the fast reversible faradic process of redox-active materials can provide much higher specific capacitances and energy density compared to EDLCs[29,36].Many transition metal oxides have been extensively studied as the electrode materials for pseudo-capacitors,such as Mn O2[37-39],Mn3O4[40],Co3O4[41,42],Fe3O4[43]and WO3[44],etc.Asymmetric supercapacitors are also fabricated using a double layer electrode as cathode and a pseudo-capacitive electrode as anode[45,46],which usually exhibit a significantly capacitive performance and enhanced energy density, filling the gap between batteries and supercapacitors[47].

For the second category,directly grow th of active nanomaterials after lithography not only fabricates the fabrication of symmetry on-chip MSCs,but also makes it possible to fabricate asymmetric on-chip MSCs.For instance,Liu et al.reported the fabrication of high-performance asymmetric on-chip MSCs using Mn O2 nanoflakes as the cathode and Fe2O3 nanoparticles as the anode[69].As shown in Fig.7e,the schematic diagram depicts the microfabrication process of the planar asymmetric on-chip MSCs.Using to obtain Interdigital micro-patterns were patterned by photolithography technology,follow ed with the deposition of a Cr/Ni layer as the current collector using a physical vapor deposition(PVD)process.A lift-off process was then conducted with acetone to remove the additional photoresist.Afterward,Mn O2 nanoflakes and Fe2O3 nanoparticles were electrodeposited on the fabricated microelectrodes by the electrophoretic deposition,respectively.As both the Mn O2 nano flakes and Fe2O3 nanoparticles were directly deposited on the Cr/Ni layer current collector,no additional binder was introduced.After electrodeposition,the device was annealed at 350°C under N2 for 3 h to convert the anode precursor to Fe2O3 nanoparticles.Finally,1 mol/L KOH was dropped onto the interdigital microelectrodes as the electrolyte.As fabricated on-chip MSCs delivered a maximum energy density of 12 m Wh/cm 3 at a power density of 1 W/cm 3 and 35μWh/cm 3 at 14.8 W/cm 3(Fig.7h).Similarly, Shen et al.fabricated asymmetric on-chip MSCs based on graphene quantum dots(GQDs)//MnO2 via the controllable electrodeposition[70].In the device,Mn O2 is an anode and GQDs is a negative electrode.In addition,Wang et al.proposed Mn O2 based on-chip MSCs with ultra-high areal capacitances.Au interdigital microelectrodes on SiO2 substrate w =ere obtained via photolithography process,and anodic deposition of Mn O2 was carried out on the microelectrode arrays.As-fabricated devices gave an ultra-high areal capacitance of 56.3 m F/cm 2 at a current density of 27.2 m A/cm 2,and an energy density of 5.01 m Wh/cm 3 at a power density of 12.02 m W/cm 3.

Fig.7.(a)Schematic illustration of the fabrication process for Ni(OH)2 MSCs devices.(b,c)SEM images and(d)Ragone plot of the specific volumetric energy density and power density of Ni(OH)2 based on-chip MSCs.Reproduced with permission[67].Copyright 2015,Wiley Publishing Group.(e)Schematic illustration of the fabrication process for planar asymmetry MSCs.(f,g)SEM images and(h)Ragone plot of the specific volumetric energy density and power density of the planar asymmetry on-chip MSCs.Reproduced with permission[69].Copyright 2016,Tsinghua University Press and Springer.

4.2.Laser scribing

In addition to the photolithography method,laser scribing technique is a promising protocol for the fabrication of on-chip MSCs electrodes with various shapes on different substrates.Compared with the photolithography technology,laser scribing method is a much simpler and efficient way to fabricate on-chip MSCs.By controlling the laser beam,it is possible to pattern the desired features.Using this technique,people can fabricate interdigital electrodes and then used these electrodes as promising candidates for energy storage devices.Fig.8a show ed the laser scribing fabrication of on-chip MSCs using a CO2 laser beam.The beam was used to convert the PI substrate into porous laserinduced graphene(LIG)interdigitated pattern with 12 in-plane interdigitated electrodes.Mn O2,FeOOH,and electrically conductive PANI were then electrodeposited on the surface of LIG[49].Fig.8b showed the cross-sectional SEM image of the LIG-Mn O2-2.5 h,in which Mn O2 was observed to deposit into the LIG layer.The performance of the device increased with increased electrodeposition time at a current density of 0.5 m A/cm 2,as can be seen in Fig.8c.The highest areal specific capacitance was 934 m F/cm 2.The fabricated on-chip MSCs exhibited excellent mechanical flexibility and stability.As shown in Fig.8d,the CV curves at different bending angles were nearly overlapping with each other,con firming the excellent stability of the device.

Similarly,Cai et al.reported high-performance flexible interdigitated carbon/TiO2 based MSCs,fabricated by combining a laser direct writing technique with electrophoretic deposition of TiO2 nanoparticles[71].The fabricated devices exhibited a high specific capacitance up to 27.3 m F/cm 2 at a typical current density of 0.05 m A/cm 2,excellent cycling stability,long-time stability,and mechanical stability.In the work,it was also found that the carbon/TiO2 devices exhibited strong response to UV light and can be charged to more than 100 m V under UV irradiation,thus forming photo-rechargeable on-chip MSCs.

The formation of urchin-like NiCo2O4 nanostructures is controlled by the p H values of the solution,which were carried out by controlling the mass of urea used during the hydrothermal process.As shown in Figs.2a and d,when the p H value was set to 5.5,nanorod bundles composed of numerous nanorods extruding from the two ends were obtained.When the p H value was increased to 6.1,pure straw-like nanorod bundles were obtained(Figs.2b and e).With the continuous increase of p H values,more and more nanorods extruding from the two ends of the straw-like nanorod bundles and finally urchin-like NiCo2O4 spheres were obtained( Figs.2c and f)at a p H value of 6.8.The crystal structure and composition of the synthesized urchin-like NiCo2O4 spheres are characterized by X-ray diffraction(XRD)and the corresponding pattern is depicted in Fig.2g,corresponding to pure NiCo2O4 phase.The scanning electron microscopy(SEM)and transmission electron microscopy(TEM)image of the urchin-like NiCo2O4 spheres were shown in Figs.2h-j,respectively.It was found that there are many spherical urchin-like NiCo2O4 nanostructures with diameters of about 5μm.From the high magnification TEM image(Fig.2j),the NiCo2O4 nanorods are of porous structures with typical diameters of 100-200 nm. Figs.2k and l showed the investigated results of nitrogen adsorption-desorption measurements,w hich show ed the speci fic surface areas and hierarchical porous nature of the as-prepared urchin-like NiCo2O4 nanostructures and the straw-like NiCo2O4 nanorod bundles,respectively.The Brunauer-Emmett-Teller(BET)surface area of the urchin-like NiCo2O4 nanostructures is calculated to be around 99.3 m 2/g,which is much larger than the BET surface area of the straw-like NiCo2O4 nanorod bundles(33.05 m 2/g).It was also reported that various porous NiO nanostructures including nanoslices,nanoplates and nanocolumns can be obtained by adjusting the p H value[53].

In some cases,in order to make the nanostructures better fit in the current collector of supercapacitors,so-called electrodeposition method was also developed to synthesize the required active electrode material.Fig.4g illustrates the electrodeposition method to get Mn O2-coated graphene electrodes[60].This technique can easily control the coating mass,thickness,uniformity,and morphology of the metal oxide film by simply adjusting the applied current,bath chemistry,and temperature. Figs.4h and i are the SEM images of the thin sheets-like graphene oxide(GO)before and after electrodeposition with Mn O2 nano flowers,respectively.

4.3.Printing technology

From the perspective of device fabrication and applications,in order to pursue a simple and convenient way to fabricate high performance on-chip MSCs,a low cost and scalable printing technology is much desirable.Compared with photolithography technology,laser scribing and other techniques,printing technique needs less cumbersome procedures and can efficiently produces devices on a very large scale[74-82].Among printing techniques,screen printing uses a wovenmesh to support an ink-blocking stencil to obtain a desired image.During the printing process,the ink can be pressed through the opening of the mesh to form the pattern on the substrate.Yang et al.recently proposed a screen printing method to large scale produce on-chip MSCs[74].As shown in Fig.9a,silver pastes were first screen printed on the substrate,then dried and annealed to form Ag electrodes.Subsequently,pre-prepared active material inks were printed on the Ag electrode,follow ed by drying to ensure that the active materials were well adhered to the Ag electrodes.Finally,the PVA/H3PO4 electrolyte sol was coated on top of the device to cover the channel area.Fig.9b demonstrated the on-chip MSCs printed on different substrates,including PET,glass and conventional A4 printing paper.The areal specific capacitance of the printed flexible on-chip MSCs on PET substrate is 3.80 m F/cm 2 at a current density of 20μA/cm 2(Fig.9c).The CV curves of the printed devices were basically in good consistence at a scan rate of 100 m V/s under different bending radii ranging from flat to 3.5 mm as shown in Fig.8d.Analogously,Zhu et al.synthesized CoO/CNT nanocomposites for high energy density flexible on-chip MSCs by the screen printing process[75].

在开始基西米河生态修复的基础上，组织人员成立了专门的研究机构，制定和提出了奥基乔比湖生态修复、大沼泽地恢复及建设、佛州湾生态修复、入海河口治理等一系列计划、方案，并在每个项目上投入了大量经费。

Fig.8.(a)Schematic diagram of the fabrication of LIG-MnO2 based on-chip MSCs and their(b)cross-sectional SEM images,(c)areal speci fic capacitance,and(d)CV curves under bending angles of 0°,45°,90°,135°,and 180°,respectively.Reproduced with permission[49].Copyright 2016,Wiley Publishing Group.(e-j)Fabrication of on-chip asymmetric MSCs using the Light Scribe DVD burner process with the LSG-MnO2 as the positive electrode and LSG as the negative electrode.Reproduced with permission[72].Copyright 2015,National Academy of Sciences.

Fig.9.(a)Schematic diagram of the fabrication of on-chip MSCs using the printing technique.(b)Photographsof the on-chip MSC sprinted on different substrates.(c)Specific energy vs.power density of the printed devices with various channel lengths.(d)Flexibility testing con figuration and(e)the CV of the printed devices with various bending radii with a scan rate of 100 m V/s.Reproduced with permission[74].Copyright 2014,IOP Publishing Ltd.(f)Schematic illustrations for the fabrication of on-chip MSCs with 3D-i-AMA stacked hybrid electrodes and single i-AMA sandwich hybrid electrode unit.(g-h)Photographs of large-area printed interdigital patterns on a flexible PET film.(i)Evolution of the areal capacitances of the devices based on 3D-i-AMA stacked hybrid electrodes versus current densities.(j)CV curves of the device under normal and bent conditions at a scan rate of 200 m V/s.Reproduced with permission[76].Copyright 2016,Wiley Publishing Group.

In addition to screen printing technique,laser printing is also a kind of printing technology that can be used to fabricate on-chip MSCs on a large scale.Hu et al.fabricated on-chip Mn O2-based MSCs with 3D interdigital Au/Mn O2/Au(3D-i-AMA)electrodes[76].Aschematic illustration for the fabrication process was shown in Fig.9f.In this process,the laser printing technique is used for printing interdigitated finger electrodes patterns on PET,follow ed by deposited a layer of Au film(～100 nm)on the surface of the pattern by magnetron sputtering technique.Then,Mn O2 active layers(～400 nm)were electrodeposited on the Au film before the final lift-off process.During this process,researchers can control the number of alternate magnetron sputtering and eletrodepostion to fabricate 3D interdigital single 3D-i-AMA electrodes,3D-i-AMA(tw o layers)and 3D-i-AMA(three layers).As shown in Fig.9g,the printed devices can be produced on a very large scale since no rigid template or toxic photoresist was applied.As-printed MSCs with 3D-i-AMA electrodes have the sizes located in centimeter scale,as can be seen in Fig.9h.It was found that the areal capacitance of the printed devices roughly increased in proportion to the layers,and a maximum areal capacitance was achieved for the on-chip MSCs based on 3D-i-AMA(three layer)stacked hybrid electrodes.The areal specific capacitance is 11.9 m F/cm 2 at a current density of 0.05 m A/cm 2.The laser printing technique developed in this work makes as-fabricated on-chip MSCs storage systems.

Fig.10.(a)Schematic diagram showing the fabrication process for PRG-MSCs with different geometries and(b-d)PRG-MSC with different device structures.(e)SEM image of a PRG film.(f)Change of the volumetric capacitance(CV)and areal capacitance(CA)of PRG-MSCs at difference scan rate.(g)Capacitance ratio as a function of bending angle.(h)CV curves of the PRG-MSCs measured at scan rates of 20-200 V/s.Reproduced with permission[83].Copyright 2017,American Chemical Society.(i)Schematic of the maskassisted fabrication of all-solid-state planar EG-MSCs.(j)Top-view and(k)cross-section SEM images of the MG film(2 m L dispersion of Mn O2 and EG).(l)Areal capacitance as a function of scan rate.(m)Capacitance retention as a function of bending angle of MG-MSCs-2.Insets are optical images of MG-MSCs-2 taken at 0°and 180°.Reproduced with permission[84].Copyright 2017,Elsevier B.V.

4.4.Other fabrication technology

Besides the above fabrication methods,some other methods were also developed to fabricate MSCs.For example,Wang et al.demonstrated the scalable fabrication of graphene-based monolithic MSCs by photochemical reduction and patterning of GO/TiO2 hybrid films.The method represents the distinguished advantages of high efficiency,low cost[83].Detailed fabrication process was shown in Fig.10a.With the help of a customized photomask,a 500 W high-pressure Hg lamp was used for the efficient photoreduction and patterning of the prepared GO/TiO2 films for 4 h at ambient conditions. Figs.10b-d displayed the fabricated MSC devices with different geometries and sizes.As shown in Fig.10e,after UV irradiation,the color of a pristine GO film changed from light brow n to black for a patterned photoreduced graphene oxide(PRG) film.The fabricated MSCs displayed high areal capacitance of 1.5 m F/cm 2,volumetric capacitance of 233.0 F/cm 3(Fig.10f),and outstanding flexibility(Fig.10g).High-rate MSCs were demonstrated by the authors using a hydrophobic electrolyte of ionic liquid,exhibiting excellent electrochemical performance with a wide electrochemical window of 3.0 V(Fig.10h).

Similarly,Qin et al.demonstrated the simplified fabrication of planar MSCs based on interdigital patterned films of 2D pseudocapacitive MnO2 nanosheets and electrochemically exfoliated graphene(EG)[84],as depicted in Fig.10i.EG dispersion was first filtrated on nylon membrane to form EG interdigital patterns as current collectors with the assistance of the customized mask.Hybrid interdigital patterns(MG film)electrodes were then directly prepared by deposition of 2D Mn O2 and EG on a nylon membrane.Third,PVA/LiCl electrolyte was drop-casted and solidified.SEM image of the MG films was displayed in Fig.10j,which revealed large-area continuity and uniformity surface of EG and Mn O2 nanosheets.It is demonstrated that such a technique was highly favorable for the construction of flexible MG-MSCs.Cross-section SEM image(Fig.10k)of the MG film clearly exhibited layer-stacked structure of EG and Mn O2.The areal capacitances of three different devices,namely,MG-MSCs-1,MG-MSCs-2 and MGMSCs-3 based on MG films with varied loading mass were shown in Fig.10l.MG-MSCs-3 was found exhibiting excellent performance with high areal capacitance of～355 m F/cm 2,and outstanding mechanical stability with～92%of initial capacitance even at a highly bending angle of 180°(Fig.10m).

5.Summary and outlook

Compared with conventional supercapacitors,researches on on-chip MSCs are relatively rapidly growing in recent years.Onchip MSCs possess remarkable features of miniaturization,high security,high performance,and easy integration to build an all-in-one integrated system.Although carbon-based on-chip MSCs have received extensive attention and have achieved remarkable results[23,77-79],metal oxide-based on-chip MSCs are thought to be an very important alternation as they can deliever much higher capacitance than carbon-based devices and have some other attractive features including low cost and environmentally friendly.This review summarized several important synthetic methods to get metal oxide nanostructures including hydrothermal,electrospun,etc.Particular emphasis was placed on recent progress in the fabrication of metal oxides based on-chip MSCs through several efficient techniques,mainly focusing on photolithography,laser scribing and printing techniques.

State-of-the-art results have shown that the flexibility and electrochemical performance of the metal oxides based on-chip MSCs depend not only on the intrinsic properties of the active electrode materials but also on the architecture of the devices.One major challenge remains for the metal oxides based on-chip MSCs is how to precisely control the size and thickness of the designed microelectrodes and the space between the microelectrodes in order to further increase the specific capacitance and energy density of an on-chip MSC to fit for real applications,while without sacrificing the cyclic stability and power densities.With the rapid development of line fabrication technology,in principle,the architecture and size of the interdigitated electrodes can be precisely controlled,thus needing further studies and investigations in the future.

Compared with the thickness of the separator in conventional supercapacitors,the space width of on-chip MSCs is much smaller and thus the diffusion path lengths of the electrolyte ions are greatly reduced,which resulted in more efficient utilization of the electrochemical surface area of the active materials.As a result,onchip MSCs have higher specific capacitance,columbic efficiency,rate performance,and knee frequency than conventional SCs.How ever,on-chip MSCs usually exhibited lower areal energy densities than conventional SCs because the mass of active materials is generally a very small fraction of the total w eight of the on-chip MSC device.

爱丁堡有一座标志性建筑——司各特纪念碑，纪念的是苏格兰著名诗人、“历史小说之父”司各特。他的代表作《艾凡赫》等开创了欧洲历史小说的先河。这座宏伟的纪念碑表达了人们对作家最崇高的敬意和最深切的怀念。

Although great progress has been achieved in the design and fabrication of metal oxides based on-chip MSCs,there are still many issues to be explored.For example,MSCs devices often suffer from low energy densities,which is proportional to the square of the voltage.Thus,higher energy densities may be obtained by enlarging the voltage of the MSCs.Development of stable solidstate electrolytes with good ion-transporting properties is a good way to realize higher voltage window.

Another issue that should be concerned for metal oxides based on-chip MSCs is that,for integrated systems,several MSCs were required to be connected in series to provide a stable voltage output for the functional sensor units.How ever,the current fabrication technology for MSCs is more suitable for the fabrication of individual device.How to optimize and adjust current used techniques to match the current microfabrication techniques that are widely used for electronic circuits and large-scale device integrations is still a challenge.Developing suitable processes to get integrated systems is also a challenge for the researchers to be resolved.

本文通过建立政府、金融机构、企业三方博弈模型，研究在绿色金融政策驱动下企业通过技术创新进行产业升级、开展绿色化的生产模式问题，通过对模型中关键影响因素分析发现：

Besides,most of the currently fabricated MSC devices were either fabricated on rigid or flexible substrates,which lack in stretchability,thus greatly limited their applications,especially in wearable electronics.Thus,developing MSC devices on stretchable substrates with special designed device structures to adapt to more working environment is highly desirable.

Biological compatibility is the development trend of electronic devices.It is very important especially for wearable and implantable devices.Inevitably,there is a pressing need for exploring more biodegradable substrates and non-toxic electrode materials to fabricate high performance MSCs for use in integrated implantable systems.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(Nos.51672308,61625404),the Beijing Natural Science Foundation(No.4162062),Beijing Municipal Science and Technology Project(No.Z1711000220000)and the Key Research Program of Frontiers Sciences,Chinese Academy of Sciences(No.QYZDY-SSW-JSC004).

根据测量数据，仪器设置用线性回归法求出标准工作曲线，其标准工作曲线为：y＝131.66 C－137.89，r＝0.999 7。

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