Caiwei Shen

A Review of On-Chip Micro Supercapacitors for Integrated Self-Powering Systems

Miniaturized self-powering systems that integrate both energy harvesters and energy storage units as the power sources are essential to realize maintenance-free wireless sensor networks, implantable medical devices, and active radio frequency identification systems. On-chip micro supercapacitors (MSCs) are promising candidates for energy storage in such systems by providing high power densities, fast charge/discharge rates, and long cycle life. Researchers have been improving the performances, especially energy and power densities, of MSCs in recent years. This paper reviews the fundamental working mechanisms and design considerations of on-chip MSCs with special emphasis on the advantages of 3-D configurations. Typical fabrication methods are summarized, and their effects on the device performance and system integration are analyzed. In particular, the power generation of micro energy harvesters and the power consumption of typical wireless micro systems are surveyed, providing the basic and targeting performance requirements of future MSCs that can be integrated with them. [2017-0069]

Titanium Disulfide Coated Carbon Nanotube Hybrid Electrodes Enable High Energy Density Symmetric Pseudocapacitors

While electrochemical supercapacitors often show high power density and long operation lifetimes, they are plagued by limited energy density. Pseudocapacitive materials, in contrast, operate by fast surface redox reactions and are shown to enhance energy storage of supercapacitors. Furthermore, several reported systems exhibit high capacitance but restricted electrochemical voltage windows, usually no more than 1 V in aqueous electrolytes. Here, it is demonstrated that vertically aligned carbon nanotubes (VACNTs) with uniformly coated, pseudocapacitive titanium disulfide (TiS2 ) composite electrodes can extend the stable working range to over 3 V to achieve a high capacitance of 195 F g-1 in an Li-rich electrolyte. A symmetric cell demonstrates an energy density of 60.9 Wh kg-1 -the highest among symmetric pseudocapacitors using metal oxides, conducting polymers, 2D transition metal carbides (MXene), and other transition metal dichalcogenides. Nanostructures prepared by an atomic layer deposition/sulfurization process facilitate ion transportation and surface reactions to result in a high power density of 1250 W kg-1 with stable operation over 10 000 cycles. A flexible solid-state supercapacitor prepared by transferring the TiS2 -VACNT composite film onto Kapton tape is demonstrated to power a 2.2 V light emitting diode (LED) for 1 min.

Flexible micro-supercapacitors prepared using direct-write nanofibers

Herein, we demonstrated direct-write flexible micro-supercapacitors prepared by the near-field electrospinning of polypyrrole (PPy) nanofibers on a patterned metal electrode deposited on a flexible polymer substrate. The continuous porous nanofiber network with a pseudocapacitance effect greatly increases the capacitance and facilitates ion transport in the electrodes. The prototype based on the lab-synthesized PPy nanofibers shows a high capacitance of 0.48 mF cm−2 when the total area of both electrodes and electrolyte is considered, and the performance is not affected when it is bent to different extents. The micro-supercapacitors were experimentally proved to be highly flexible with excellent electrochemical performance and cycling stability. Our approach is simple, versatile, and compatible with different substrates for the direct integration of energy storage devices in flexible microsystems.

Wearable woven supercapacitor fabrics with high energy density and load-bearing capability

Flexible power sources with load bearing capability are attractive for modern wearable electronics. Here, free-standing supercapacitor fabrics that can store high electrical energy and sustain large mechanical loads are directly woven to be compatible with flexible systems. The prototype with reduced package weight/volume provides an impressive energy density of 2.58 mWh g−1 or 3.6 mWh cm−3, high tensile strength of over 1000 MPa, and bearable pressure of over 100 MPa. The nanoporous thread electrodes are prepared by the activation of commercial carbon fibers to have three-orders of magnitude increase in the specific surface area and 86% retention of the original strength. The novel device configuration woven by solid electrolyte-coated threads shows excellent flexibility and stability during repeated mechanical bending tests. A supercapacitor watchstrap is used to power a liquid crystal display as an example of load-bearing power sources with various form-factor designs for wearable electronics.

Laser‐Induced Molybdenum Carbide–Graphene Composites for 3D Foldable Paper Electronics

Versatile and low-cost manufacturing processes/materials are essential for the development of paper electronics. Here, a direct-write laser patterning process is developed to make conductive molybdenum carbide-graphene (MCG) composites directly on paper substrates. The hierarchically porous MCG structures are converted from fibrous paper soaked with the gelatin-mediated inks containing molybdenum ions. The resulting Mo3 C2 and graphene composites are mechanically stable and electrochemically active for various potential applications, such as electrochemical ion detectors and gas sensors, energy harvesters, and supercapacitors. Experimentally, the electrical conductivity of the composite is resilient to mechanical deformation with less than 5% degradation after 750 cycles of 180° repeated folding tests. As such, the direct laser conversion of MCGs on papers can be applicable for paper-based electronics, including the 3D origami folding structures.

Ultrathin Coaxial Fiber Supercapacitors Achieving High Energy and Power Densities

Fiber-based supercapacitors have attracted significant interests because of their potential applications in wearable electronics. Although much progress has been made in recent years, the energy and power densities, mechanical strength, and flexibility of such devices are still in need of improvement for practical applications. Here, we demonstrate an ultrathin microcoaxial fiber supercapacitor (μCFSC) with high energy and power densities (2.7 mW h/cm3 and 13 W/cm3), as well as excellent mechanical properties. The prototype with the smallest reported overall diameter (∼13 μm) is fabricated by successive coating of functional layers onto a single micro-carbon-fiber via a scalable process. Combining the simulation results via the electrochemical model, we attribute the high performance to the well-controlled thin coatings that make full use of the electrode materials and minimize the ion transport path between electrodes. Moreover, the μCFSC features high bending flexibility and large tensile strength (more than 1 GPa), which make it promising as a building block for various flexible energy storage applications.

Direct-write polymeric strain sensors with arbitary contours on flexible substrates

Here we demonstrate the direct-write all-polymer nanofibers as uniaxial strain sensors for flexible and wearable devices. Compared to the state-of-art strain sensors, the unique characteristics include: direct-write all polymeric stain sensors for the first time on flexible substrates; local strain sensing with arbitrary contours and designated sensing directions; and 10X higher gauge factor than that of commercial metal strain sensors.

Solid-state flexible micro supercapacitors by direct-write porous nanofibers

Solid-state flexible micro supercapacitors based on porous and conducting polymer nanofibers via the direct-write, near-field electrospinning process have been constructed. Testing results have shown a capacitance of 0.3mF/cm2, 30X larger as compared with those of flat electrodes. Key innovations of this work include: (1) densely-packed, porous 3D nanostructures with conductive nanofibers via the near-field electrospinning process; (2) flexible solid-state micro electrodes with high energy density using the pseudocapacitive effect; and (3) simple yet versatile manufacturing process compatible with various substrates and surfaces. As such, this technology is readily available to make practical MEMS energy storage devices.

A coaxial carbon fiber micro supercapacitor

This paper presents a working solid-state coaxial carbon fiber micro supercapacitor with more than 100X smaller diameter as compared with the state-of-art, including: (1) an overall size of 10μm in diameter; (2) well-controlled electrodeposition and mechanical pulling processes for electrolyte and electrode coatings toward high volumetric energy density and power density; and (3) scalable and low-cost processes. As such, we believe this technology can lead to direct integration of energy storage devices in fabrics for future wearable electronic systems.