High-performance Micro-supercapacitors Research Articles

Screen-printing fabrication of high volumetric energy density micro-supercapacitors based on high-resolution thixotropic-ternary hybrid interdigital micro-electrodes

We demonstrate a general and scalable screen-printing technique for the one-step construction of high-performance micro-supercapacitors.

High performance microsupercapacitors based on a nano-micro hierarchical carbon electrode by direct laser writing

We have demonstrated a direct laser writing (DLW) process that uses a femtosecond laser to fabricate a nano-micro hierarchical structure for a large capacitance microsupercapacitor (MSC) electrode. By applying a two-photon polymerization-based DLW technique, a photoresist (PR) nano-pillar pattern was fabricated on a pre-defined PR interdigitated electrode (PR-IDE) pattern to form a nano-micro hierarchical structure. Carbon pyrolysis converted a PR-IDE with a nano-micro hierarchical structure to a PR derived carbon (PRC)-IDE while maintaining the aspect ratio of the pillar structure. The electrochemical performance of the PRC-MSC is improved by introducing the nano-pillar pattern to the PRC-IDE, resulting in larger areal capacitance of the as-fabricated PRC-IDE compared to the PRC-IDE with a micropattern only. The PRC-IDE with a nano-micro hierarchical structure in this study could be further applied as a backbone electrode structure for a high power pseudo-capacitor.

Synthesis of P-Doped and NiCo-Hybridized Graphene-Based Fibers for Flexible Asymmetrical Solid-State Micro-Energy Storage Device.

Fiber supercapacitors (FSCs) are promising energy storage devices in portable and wearable smart electronics. Currently, a major challenge for FSCs is simultaneously achieving high volumetric energy and power densities. Herein, the microscale fiber electrode is designed by using carbon fibers as substrates and capillary channels as microreactors to space-confined hydrothermal assembling. As P-doped graphene oxide/carbon fiber (PGO/CF) and NiCo2 O4 -based graphene oxide/carbon fiber (NCGO/CF) electrodes are successfully prepared, their unique hybrid structures exhibit a satisfactory electrochemical performance. An all-solid-state PGO/CF//NCGO/CF flexible asymmetric fiber supercapacitor (AFSC) based on the PGO/CF as the negative electrode, NCGO/CF hybrid electrode as the positive electrode, and poly(vinyl alcohol)/potassium hydroxide as the electrolyte is successfully assembled. The AFSC device delivers a higher volumetric energy density of 36.77 mW h cm-3 at a power density of 142.5 mW cm-3 . In addition, a double reference electrode system is adopted to analyze and reduce the IR drop, as well as effectively matching negative and positive electrodes, which is conducive for the optimization and improvement of energy density. For the AFSC device, its better flexibility and electrochemical properties create a promising potential for high-performance micro-supercapacitors. Furthermore, the introduction of the double reference electrode system provides an interesting method for the study on the electrochemical performances of two-electrode systems.

All-printed solid-state substrate-versatile and high-performance micro-supercapacitors for in situ fabricated transferable and wearable energy storage via multi-material 3D printing

Micro-supercapacitors are a kind of state-of-the-art energy storage devices and have great potential to be developed in portable and wearable electronics. Here, we report a novel strategy for scalable fabrication of all-printed solid-state micro-supercapacitors with multilayer structure via multi-material 3D printing technique. To obtain electrode materials, we synthesize polypyrrole/polyaniline coaxis nanotubes by utilizing the self-degraded template polymerization method and in situ electrochemical polymerization. The as-prepared aqueous composite electrode ink shows significant shear thinning behavior and excellent processability: a complex electrode patterning with 484 retractions was proved. In addition, the inks are well-adapted to various substrates (paper, texitle, plastic film etc), allowing the device to be printed onto different substrates, even directly on the desired surface. As a conceptual exhibition, we design two schemes of in situ fabricated energy storage devices to meet the ever-changing energy needs and expand the application range of micro-supercapacitors. The transferable tape-type micro-supercapacitors can be stuck to any surfaces where energy is needed immediately. The in situ fabricated wearable micro-supercapacitor on lab-gown shows promising performance and excellent flexibility.

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.

Laser direct writing of carbon-based micro-supercapacitors and electronic devices

Micro-supercapacitors (MSCs) are recognized as potential power supply units for on-chip micro-devices because they possess not only the advantages of supercapacitors but also the simplified packaging processes and compatibility with integrated circuits. Laser direct writing technique has been demonstrated as an effective tool for the fabrication of micro-supercapacitors and electronic devices using different materials. However, the high-performance micro-supercapacitors and the integration with electronic devices even the portable or wearable self-powered systems fabricated by the laser direct writing technique or in combination with other techniques still need to be demonstrated. In this paper, we will introduce our recent studies on the following several parts: the preparation of carbon-based micro-supercapacitors by laser direct writing on polyimide films in air, the improvement of the carbon MSC by laser direct writing in Ar, the carbon/TiO2 MSC with photo-rechargeable capability, the carbon/Au MSC with high-rate charge–discharge performance, and the in-plane integrated circuits with MSCs and sensors using the laser direct writing technique combining with other methods.

Stretchable array of high-performance micro-supercapacitors charged with solar cells for wireless powering of an integrated strain sensor

The aim of this paper is to report on the fabrication of a stretchable array of high-performance solid-state micro-supercapacitors (MSCs), which can be charged with integrated, commercial Si-based solar cells (SCs). This would facilitate the powering of an integrated strain sensor. The planar MSCs comprised electrodes of potentiostatically deposited polypyrrole, on spray-coated multi-walled carbon nanotube film, and a gel-type electrolyte of LiCl/polyvinyl alcohol with a redox additive of 1-methyl-3-propylimidazolium iodide. The fabricated MSC achieved an areal capacitance of 5.17 mF cm−2. After 5000 charge/discharge cycles, the MSC retained 80% of their initial capacitance. A strain sensor was fabricated utilizing a composite film of fragmentized graphene foam and polydimethylsiloxane. Such fabricated twelve parallel connected MSCs, a strain sensor, and SCs were integrated on a single deformable polymer substrate with embedded stiff platforms of negative epoxy series resist (SU-8) via long serpentine interconnections of polyimide encapsulated Ti/Pt metal film for mechanical stability under stretching. After 1000 repetitive biaxial stretching/releasing cycles by 30%, no noticeable change was observed in the charge/discharge behavior of the MSC array. Furthermore, both the photo-charge/discharge characteristics and electrochemical performance remained stable. When the whole integrated system was attached to the wrist, the integrated strain sensor could detect both externally applied strain and the arterial pulse using the energy stored in the MSCs from the SCs.

Stretchable Array of Wirelessly Charged High Performance Micro-Supercapacitors with Solar Cells for Wireless Powering of the Integrated Strain Sensor

Along with the increasing demands for wearable health care and biomedical applications, there has been extensive research on the development of various stretchable electronic devices with mechanical stability even under deformations caused by body or skin movements. Also, energy storage devices which can be integrated with the stretchable active sensor devices have been actively studied for the practical utilization of wearable electronics without any wire-connected external power sources. Among various candidates for integrated energy storage devices, high performance solid-state micro-supercapcitors (MSCs) have been extensively investigated. Furthermore, integration of those MSCs with photovoltaic devices is expected to provide the power for the sensors, leading to the wirelessly driven skin-attachable health monitoring sensor system. In this work, we report on the fabrication of stretchable array of high performance solid-state MSCs which can be charged via integration with Si-based commercial solar cell so that those can power the integrated strain sensor. The planar MSC consists of electrodes of potentiostatically deposited polypyrrole on spray coated multi-walled carbon nanotubes film and gel-type electrolyte of LiCl/polyvinyl alcohol with redox additive of 1-Methyl-3-propylimidazolium iodide. The fabricated MSC shows areal capacitance 5.2 mF cm-2 at a current density of 100 µA cm-2. The MSC retains 80% of initial capacitance after 5000 charge/discharge cycles at a current density of 100 µA cm-2. A strain sensor was fabricated with a composite film of fragmentized graphene foam and PDMS. Such fabricated MSCs, strain sensor, and the solar cell were integrated on a single deformable polymer substrate with embedded stiff platforms of SU-8 via long serpentine interconnections of Ti/Pt metal film for mechanical stability under stretching. After 1000 repetitive biaxial stretching/releasing cycles by 30%, there appeared no noticeable change in the charge/discharge behaviors. Furthermore, the photo-charge/discharge characteristics and electrochemical performance remained stable under biaxial stretching of 30 %. With the energy stored in MSCs using the solar cell, the integrated strain sensor could detect the externally applied strain and the arterial pulse after attachment of the whole integrated system onto the wrist of the author. This work successfully demonstrates the potential application of our stretchable self-charging power/sensor system to skin-attachable health monitoring devices.

Highly Self-Healable 3D Microsupercapacitor with MXene-Graphene Composite Aerogel.

High-performance microsupercapacitors (MSCs) with three-dimensional (3D) structure provide an effective approach to improve the ability of energy storage. Because the electrodes with 3D structure are generally easily destroyed under mechanical deformation in practical applications, we fabricated a self-healable 3D MSC consisting of MXene (Ti3C2T x)-graphene (reduced graphene oxide, rGO) composite aerogel electrode by wrapping it with a self-healing polyurethane as an outer shell. The MXene-rGO composite aerogel combining large specific surface area of rGO and high conductivity of the MXene can not only prevent the self-restacking of the lamella structure but also resist the poor oxidization of MXene to a degree. The MSC based on a 3D MXene-rGO aerogel delivers a large area specific capacitance of 34.6 mF cm-2 at a scan rate of 1 mV s-1 and an outstanding cycling performance with a capacitance retention up to 91% over 15 000 cycles. The 3D MSC presents an excellent self-healing ability with specific capacitance retention of 81.7% after the fifth healing. The preparation of this self-healable 3D MSC can provide a method for designing and manufacturing next-generation long-life multifunctional electronic devices further to meet the requirements of sustainable development.

High-Performance Microsupercapacitors Based on Bioinspired Graphene Microfibers.

The miniaturization of portable electronic devices has fueled the development of microsupercapacitors that hold great potential to complement or even replace microbatteries and electrolytic capacitors. In spite of recent developments taking advantage of printing and lithography, it remains a great challenge to attain a high energy density without sacrificing the power density. Herein, a new protocol mimicking the spider's spinning process is developed to create highly oriented microfibers from graphene-based composites via a purpose-designed microfluidic chip. The orientation provides the microfibers with an electrical conductivity of ∼3 × 104 S m-1, which leads to a high power density; the energy density is sustained by nanocarbons and high-purity metallic molybdenum disulfide. The microfibers are patterned in-plane to fabricate asymmetric microsupercapacitors for flexible and on-chip energy storage. The on-chip microsupercapacitor with a high pattern resolution of 100 μm delivers energy density up to the order of 10-2 W h cm-3 and retains an ultrahigh power density exceeding 100 W cm-3 in an aqueous electrolyte. This work provides new design of flexible and on-chip asymmetric microsupercapacitors based on microfibers. The unique biomimetic microfluidic fabrication of graphene microfibers for energy storage may also stimulate thinking of the bionic design in many other fields.

Enriched carbon dots/graphene microfibers towards high-performance micro-supercapacitors

A dot-sheet structured strategy enabling construction of carbon dots/graphene fiber toward high-performance micro-supercapacitors through a microfluidic spinning technique is developed.

High Performance Micro-Supercapacitors Integrated on Both Silicon Chips and Flexible Polyimide Substrate

Modern microelectronic systems, such as electronic papers, smart clothes, sensors or biomedical devices, demand new energy storage devices to achieve their tasks (1, 2). While batteries suffer from limited power capabilities and cyclability, owing to the faradic mechanisms involved during energy delivery and harvesting, electrochemical double layer supercapacitors (or so called EDLCs) can handle fast charge and discharge over 1,000,000 cycles (3). Indeed, EDLCs store energy via reversible adsorption of ions from an electrolyte at the surface of high-surface-area carbons, or by fast redox reactions occurring at the surface of metal oxide or polymer electrodes (4). However, standard electrode preparation, consisting in mixing the active material with a polymer binder, is not compatible with micro-fabrication processes. Therefore, high performance micro-supercapacitors have to be prepared using non wet processing routes. In this work, nanoporous carbide-derived carbon (CDC) films were integrated onto silicon chips by removing under chlorine atmosphere the metallic atoms of a thin film titanium carbide (TiC) precursor sputtered on silicon wafers (5). The TiC precursor could be patterned through reactive ion etching, thus leading to interdigitated on-chip CDC-based micro-supercapacitors after partial chlorination. The as-prepared devices provided capacitance values up to 410 F.cm-3 (205 mF.cm-2) in 1M H2SO4 with more than 50% of the initial capacitance preserved for 0.9 s discharge time (6). Full chlorination could lead, in certain conditions, to self-supported CDC films of several mm², which were transferred on PET. The flexible CDC electrode delivered 240 mF.cm-2 in sulfuric acid (6). A second strategy consisted in the preparation of flexible micro-supercapacitors via laser-writing of commercial RuO2 on polyimide substrate. Indeed, laser-writing provides facile and scalable engineering with low cost (7), as less than 1 mg.cm-2 of active material are needed in micro-electrodes. Thus, RuO2 interdigitated electrodes were obtained from laser-writing of a precursor mixture of commercial RuO2.xH2O powder and HAuCl4.3H2O salt, spin-coated onto KaptonTM. Laser-writing led to current collector-free and mechanically stable flexible interdigitated RuO2 electrodes. Capacitance values of ~ 30 mF.cm-2 were recorded at 20 mV.s-1 in 1M H2SO4 for the flexible device. This work open the way for the design of high performance micro-devices at large scale. 1. S. Zhai, H. E. Karahan, L. Wei, Q. Qian, A. T. Harris, A. I. Minett, S. Ramakrishna, A. K. Ng, and Y. Chen, Textile energy storage: Structural design concepts, material selection and future perspectives. Energy Storage Mater. 3, 123–139 (2016). 2. J. R. Miller, Valuing reversible energy storage. Science. 335, 1312–1313 (2012). 3. P. Simon and Y. Gogotsi, Materials for electrochemical capacitors. Nat. Mater. 7, 845–854 (2008). 4. T. Brousse, D. Belanger, and J. W. Long, To Be or Not To Be Pseudocapacitive? J. Electrochem. Soc. 162, A5185–A5189 (2015). 5. J. Chmiola, C. Largeot, P. L. Taberna, P. Simon, and Y. Gogotsi, Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors. Science. 328, 480–483 (2010). 6. P. Huang, C. Lethien, S. Pinaud, K. Brousse, R. Laloo, V. Turq, M. Respaud, A. Demortière, B. Daffos, P. L. Taberna, B. Chaudret, Y. Gogotsi, and P. Simon, On-chip and freestanding elastic carbon films for micro-supercapacitors. Science. 351, 691–695 (2016). 7. M. F. El-Kady and R. B. Kaner, Direct laser writing of graphene electronics. ACS Nano. 8, 8725–8729 (2014).

Simplified fabrication of high areal capacitance all-solid-state micro-supercapacitors based on graphene and MnO2 nanosheets

All-solid-state micro-supercapacitors are acknowledged as a very promising class of microscale energy storage devices for directly integrating portable and wearable electronics. However, the improvement of electrochemical performance from materials to devices still remains tremendous challenges. Here, we demonstrate a novel and universal mask-assisted filtration technology for the simplified fabrication of all-solid-state planar micro-supercapacitors (MSCs) based on interdigital patterns of 2D pseudocapacitive MnO2 nanosheets and electrochemically exfoliated graphene film as both electrode and current collector, and polyvinyl alcohol/LiCl gel as electrolyte. Remarkably, the resulting MSCs exhibit outstanding areal capacitance of ∼355mF/cm2, which is among the highest values reported in the state-of-the-art MSCs. Meanwhile, MSCs possess exceptionally mechanical flexibility as high as ∼92% of initial capacitance even at a highly bending angle of 180°, excellent cyclability with a capacitance retention of 95% after 3000 cycles, and impressive serial or parallel integration for modulating the voltage or capacitance. Therefore, our proposed strategy of simplified construction of MSCs will pave the ways for utilizing graphene and analogous pseudocapactive nanosheets in high-performance MSCs.

Performance enhancement of micro-supercapacitor by coating of graphene on silicon nanowires at room temperature

Fabrication of high performance micro-supercapacitor (μ-SC) based on Silicon nanowires (SiNWs) at low temperatures by simple, effective and a scalable technique is always desired. In this regard, the performance of the μ-SC was greatly improved by employing a novel architecture developed by coating SiNWs with Graphene (rGO) in the present work. This coating of Graphene was deposited by an easy and efficient technique, Electrophoretic Deposition (EPD), at the room temperature with no intricate steps. These rGO coated SiNWs electrodes show as much as four times larger capacitance (240 μF/cm2) than the bare SiNWs (60 μF/cm2). Moreover, the key advantage of the architecture is that both rGO and SiNWs structurally complement each other enhancing the electrode performance - EPD deposited rGO fills the empty space between SiNWs by making a bridge type structure, increasing the surface area of the electrodes whereas SiNWs prevent restacking and aggregation of the rGO nano-sheets and also provide support to rGO nano-sheets to stand vertically over the substrate. One more notable bonus of using EPD technique is that the wires are selectively coated. SiNWs were synthesized at a temperature of 320 °C by the Hot-wire assisted SiNW growth technique and deposition of Graphene oxide (GO) and its reduction into reduced Graphene oxide (rGO) was carried out at the room temperature. The results suggest that rGO coated SiNWs could be a promising candidate as an electrode for on-chip integrable μ-SC application.

(Invited) Wafer Level Fabrication Process of High Performance Micro-Supercapacitors

Nowadays, miniaturized power sources are key devices for providing autonomy to smart and connected sensors. To reach this goal, the current trend is to fabricate micro-storage unit such as microbatteries (MB) or microsupercapacitors (MSC) at the wafer level. According to the limited surface of such device, the metrics should be reported as normalized to the footprint area (mF, µWh and mW per cm2). With such metrics, the average energy density of most of MSC1,2 (Onions like carbon, graphene) is found to be close to 1 µWh/cm2which is not enough to get autonomous miniaturized and connected sensors. Within this study, two different approaches will be reported to significantly improve the performances of MSC. One attractive way to improve the areal energy density of MSC is to enhance the material mass loading while keeping low the footprint area. To meet this requirement3, on chip fabrication of 3D MSC is proposed. Wafer level fabrication process on high area enlargement factor 3D scaffold (AEF # 50) is achieved. Step conformal deposition of MnO2 thin film (electrophoretic deposition) on this 3D template is performed and the MSC are tested either in aqueous electrolyte (0.5M Na2SO4) or in EMI-TFSI aprotic ionic liquids (AILs). The areal energy density of the proposed micro-device is close to 10 µWh/cm2 while maintaining high areal power density of 10 mW/cm2. Carbon derived carbide (CDC) based MSC is the second followed way to propose on chip high performance device. The fine-tuned chlorination of TiC thin films deposited and patterned as interdigitated planar electrodes is performed in the framework of this study4. CDC based MSC were directly fabricated on silicon wafer. The corresponding areal energy density is around 1 µWh/cm2 for aqueous based device (1M H2SO4) and higher than 20 µWh/cm2 for MSC tested in organic electrolyte (2M EMI-BF4 in ACN). The power density is measured to be close to 100 mW/cm2. Finally, MSC based on vanadium nitride5 thin films grown either by sputtering technique (for planar MSC) or by atomic layer deposition technology (for 3D MSC) will be reported. The objective of these studies is to clearly find the suitable technology allowing to propose the fabrication of high areal energy density, small footprint area MSC and to move toward solid state configuration.

C-MEMS Based Micro-Power: From Symmetric to Battery-Type Hybrid Microsupercapacitors

Given the rapid boom in development of miniaturized electronic devices, there has been an ever-increasing demand for high performance rechargeable micro-power devices. At the heart of micro-power are microsupercapacitors, which have great potential as standalone power devices or even replace microbatteries depending upon the application. Carbon microelectromechanical systems (C-MEMS) a technique, in which a patterned photoresist is pyrolyzed in order to produce carbon microelectrode platforms, has been proved to be a powerful approach for fabricating high-performance microsupercapacitors. The as-fabricated carbon electrodes can further be electrochemically activated or even decorated with different nanostructures as such carbon nanotubes, graphene or conducting polymers such as polypyrrole (PPy) in order to further enhance the electrochemical performance of the microsupercapacitors. In general, supercapacitors with identical anode and cathode are referred to as “symmetric” while the ones with different anode and cathode are referred to as “asymmetric” or colloquially as “hybrid”. The latter type of capacitor combining a faradaic type battery electrode and an electrochemical double layer type electrode has gained significant scientific interest in recent years. Intuitively such a capacitor would combine the benefits of the two very disparate electrodes, such as a wide operating potential, high power, high energy along with high cycle longevity. In previous works, our group has mostly focused on integrating nanostructures onto C-MEMS platforms for mostly symmetric microsupercapacitors and at present we are focusing on C-MEMS platforms for hybrid battery-type microsupercapacitors. Detailed works will be presented at the conference.

Focused Ion Beam Reduced Graphene Oxide Micro-Supercapacitors with Unprecedented Energy and Power Densitites

Miniaturization of energy storage devices with high energy and power densities can reduce the footprint of micro-devices being used in our daily life. Micro-supercapacitors with planar geometries offer several advantages, such as, the ability to control and reduce the distances ions travel between two electrodes, easy integration to micro devices as the separator and the electrodes are in the same plane, and offer the potential of being extended into 3D without compromising the inter-electrode separation distances. In this study, we have used focused ion beam technology (which is capable of producing reduced graphene oxide patterns and complex shapes in insulating films of graphene oxide with a possible spatial resolution down to ~20 nm in a mask-less and direct write approach) to directly write miniaturized planar electrodes of reduced graphene oxide on films of graphene oxide (GO). We have optimised the ion-beam irradiation and have investigated the influence of ion beam irradiation on the resultant microstructure and the electrochemical properties of the resultant reduced graphene oxide films. Subsequently, using the optimized ion-beam irradiation, interdigitated electrode designs (40 μm long and 3.5 μm wide fingers with ultra-small inter-electrode spacing of 1 μm) have been generated, which have demonstrated a large capacitance (102 mF/cm2), ultra-small time response (0.03 ms), low equivalent series resistance (0.35 mΩ cm2), and have retained 95 % of the specific capacitance after 1000 cycles at an ultrahigh current density of 45 mA/cm2. These performance metrics show unprecedented improvements on several aspects of supercapacitor performance over existing reports due to the miniaturized electrode dimensions and minimal damage to the graphene sheets. We believe our results can provide opportunities for large-scale fabrication of arrayed, planar, high performance micro-supercapacitors with a small environmental footprint.

A step forward into hierarchically nanostructured materials for high performance micro-supercapacitors: Diamond-coated SiNW electrodes in protic ionic liquid electrolyte

Silicon nanowires (SiNWs) were successfully coated by uniform, adherent and homogenous ultra-thin crystalline diamond films through microwave enhanced chemical vapor deposition (MWCVD). The as-grown functionalized nanowires were employed as electrodes in a symmetric micro-supercapacitor (MSC) using a protic ionic liquid electrolyte [triethylammonium bis(trifluoromethylsulfonyl)imide; Et3NH TFSI]. The electrochemical performance of the device delivered a specific capacitance of 1.5mFcm−2 and a power density of 25mWcm−2 using an enlarged cell voltage of 4V. Furthermore, a remarkable cycling stability was evaluated after 1·106galvanostatic cycles at a high current density of 10mAcm−2 with an excellent capacitive behavior. These results confirm that diamond-coated SiNW micro-supercapacitors exhibit very promising performances dealing with MSCs based on CVD-grown SiNWs.

Titanium nitride films for micro-supercapacitors: Effect of surface chemistry and film morphology on the capacitance

Electrochemical capacitors (EC) in the form of packed films can be integrated in various electronic devices as power source. A fabrication process of EC electrodes, which is compatible with micro-fabrication, should be addressed for practical applications. Here, we show that titanium nitride films with controlled porosity can be deposited on flat silicon substrates by reactive DC-sputtering for use as high performance micro-supercapacitor electrodes. A superior volumetric capacitance as high as 146.4 F cm−3, with an outstanding cycling stability over 20,000 cycles, was measured in mild neutral electrolyte of potassium sulfate. The specific capacitance of the films as well as their capacitance retentions were found to depend on thickness, porosity and surface chemistry of electrodes. The one step process used to fabricate these TiN electrodes and the wide use of this material in the field of semiconductor technology make it promising for miniaturized energy storage systems.

A general strategy for the fabrication of high performance microsupercapacitors

We propose a generic strategy for microsupercapacitor fabrication that integrates layers of reduced graphene oxide (rGO) and pseudocapacitive materials to create electrode heterostructures with significantly improved cycling stability and performance. Our approach involves a combination of photolithography and a simple transfer method of free-standing reduced graphene oxide film onto an Au/patterned photoresist bilayer. The resulting stack (rGO/Au/patterned resist/substrate) is then used for the electrochemical deposition of various pseudocapacitive materials before the final step of lift-off. To prove the viability of this method, we have successfully fabricated microsupercapacitors (MSCs) with the following interdigitated electrode heterostructures: MnO2/rGO, Co(OH)2/rGO and PANI/rGO. These MSCs show better performance and cycling stability compared to the single layer, (i.e., rGO-free) counterparts. The interdigitated electrode heterostructures result in MSCs with energy densities in the range of 3–12mWh/cm3 and power densities in the range of 400–1200mW/cm3, which is superior to the Li thin film batteries (E=10mWh/cm3), carbon, and metal oxide based MSCs (E=1–6mWh/cm3) while device energy densities are in the range of 1.3–5.3mWh/cm3, corresponding power densities are in the range of 178–533mW/cm3. These results can be explained by a facilitated nucleation model, where surface topology of the rGO film creates a favorable environment for the nucleation and growth of pseudocapacitive materials with strong interfacial contacts and enhanced surface area. This approach opens up a new avenue in fabricating MSCs involving a variety of heterostructures combining electrical double layer carbon type with Faradaic pseudocapacitive materials for enhanced electrochemical performance.