Design and Fabrication of Printed Paper-Based Hybrid Micro-Supercapacitor by using Graphene and Redox-Active Electrolyte

Ultraconformable Integrated Wireless Charging Micro-Supercapacitor Skin

Conformable and wireless charging energy storage devices play important roles in enabling the fast development of wearable, non-contact soft electronics. However, current wireless 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 further 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 112.5 μm. 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.

Aramid Nanofibers/Reduced Graphene Oxide Composite Electrodes with High Mechanical Properties

In this work, aramid nanofibers (ANFs)/reduced graphene oxide (ANFs/RGO) film electrodes were prepared by vacuum-assisted filtration, followed by hydroiodic acid reduction. Compared with thermal reduced ANFs/RGO, these as-prepared film electrodes exhibit a combination of mechanical and electrochemical properties with a tensile strength of 184.5 MPa and a volumetric specific capacitance of 134.4 F/cm³ at a current density of 0.125 mA/cm², respectively. In addition, the film electrodes also show a superior cycle life with 94.6% capacitance retention after 5000 cycles. This kind of free-standing film electrode may have huge potential for flexible energy-storage devices.

A Review of Fabrication Technologies for Carbon Electrode-Based Micro-Supercapacitors

The very fast evolution in wearable electronics drives the need for energy storage micro-devices, which have to be flexible. Micro-supercapacitors are of high interest because of their high power density, long cycle lifetime and fast charge and discharge. Recent developments on micro-supercapacitors focus on improving the energy density, overall electrochemical performance, and mechanical properties. In this review, the different types of micro-supercapacitors and configurations are briefly introduced. Then, the advances in carbon electrode materials are presented, including activated carbon, carbon nanotubes, graphene, onion-like carbon, and carbide-derived carbon. The different types of electrolytes used in studies on micro-supercapacitors are also treated, including aqueous, organic, ionic liquid, solid-state, and quasi-solid-state electrolytes. Furthermore, the latest developments in fabrication techniques for micro-supercapacitors, such as different deposition, coating, etching, and printing technologies, are discussed in this review on carbon electrode-based micro-supercapacitors.

Printed Zinc Paper Batteries

Paper electronics offer an environmentally sustainable option for flexible and wearable systems and perfectly fit the available printing technologies for high manufacturing efficiency. As the heart of energy-consuming devices, paper-based batteries are required to be compatible with printing processes with high fidelity. Herein, hydrogel reinforced cellulose paper (HCP) is designed to serve as the separator and solid electrolyte for paper batteries. The HCP can sustain higher strain than pristine papers and are biodegradable in natural environment within four weeks. Zinc-metal (Ni and Mn) batteries printed on the HCP present remarkable volumetric energy density of ≈26 mWh cm-3 , and also demonstrate the feature of cuttability and compatibility with flexible circuits and devices. As a result, self-powered electronic system could be constructed by integrating printed paper batteries with solar cells and light-emitting diodes. The result highlights the feasibility of hydrogel reinforced paper for ubiquitous flexible and eco-friendly electronics.

A Redox-Mediator-Integrated Flexible Micro-Supercapacitor with Improved Energy Storage Capability and Suppressed Self-Discharge Rate

To effectively improve the energy density and reduce the self-discharging rate of micro-supercapacitors, an advanced strategy is required. In this study, we developed a hydroquinone (HQ)-based polymer-gel electrolyte (HQ-gel) for micro-supercapacitors. The introduced HQ redox mediators (HQ-RMs) in the gel electrolyte composites underwent additional Faradaic redox reactions and synergistically increased the overall energy density of the micro-supercapacitors. Moreover, the HQ-RMs in the gel electrolyte weakened the self-discharging behavior by providing a strong binding attachment of charged ions on the porous graphitized carbon electrodes after the redox reactions. The micro-supercapacitors with HQ gel (HQ-MSCs) showed excellent energy storage performance, including a high energy volumetric capacitance of 255 mF cm-3 at a current of 1 µA, which is 2.7 times higher than the micro-supercapacitors based on bare-gel electrolyte composites without HQ-RMs (b-MSCs). The HQ-MSCs showed comparatively low self-discharging behavior with an open circuit potential drop of 37% compared to the b-MSCs with an open circuit potential drop of 60% after 2000 s. The assembled HQ-MSCs exhibited high mechanical flexibility over the applied external tensile and compressive strains. Additionally, the HQ-MSCs show the adequate circuit compatibility within series and parallel connections and the good cycling performance of capacitance retention of 95% after 3000 cycles.

A seamlessly integrated device of micro-supercapacitor and wireless charging with ultrahigh energy density and capacitance

Microdevice integrating energy storage with wireless charging could create opportunities for electronics design, such as moveable charging. Herein, we report seamlessly integrated wireless charging micro-supercapacitors by taking advantage of a designed highly consistent material system that both wireless coils and electrodes are of the graphite paper. The transferring power efficiency of the wireless charging is 52.8%. Benefitting from unique circuit structure, the intact device displays low resistance and excellent voltage tolerability with a capacitance of 454.1 mF cm⁻², superior to state-of-the-art conventional planar micro-supercapacitors. Besides, a record high energy density of 463.1 μWh cm⁻² exceeds the existing metal ion hybrid micro-supercapacitors and even commercial thin film battery (350 μWh cm⁻²). After charging for 6 min, the integrated device reaches up to a power output of 45.9 mW, which can drive an electrical toy car immediately. This work brings an insight for contactless micro-electronics and flexible micro-robotics.

Miniaturized energy storage devices integrated with wireless charging bring opportunities for next generation electronics. Here, authors report seamlessly integrated wireless charging micro-supercapacitors with high energy density capable of driving a model electrical car.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish "pseudocapacitive" and "battery" materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery-type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.

Superacid-doped polyaniline as a soluble polymeric active electrolyte for supercapacitors

Using polyaniline as a soluble electrochemically active additive in an electrolyte has the advantages of high pseudocapacitance and good cycle stability of polyaniline, however, the challenge is how to make polyaniline soluble in the electrolyte. In this study, we prepare a solution of polyaniline in N-methylpyrrolidone by protonating polyaniline with trifluoromethyl sulfonic acid. Spectroscopic and electrochemical results indicate that the weak binding interaction, between trifloromethyl sulfonate ions and protonated polyaniline chains, increases the solubility of trifloromethyl sulfonic acid doped polyaniline. An active electrolyte system composed of 15 mg mL-1 polyaniline and 0.4 M trifluoromethyl sulfonic acid in N-methylpyrrolidone is developed. With the active electrolyte and reduced graphene oxide as the electrodes, the fabricated supercapacitor shows a higher specific capacitance than the corresponding electric double-layer supercapacitors. Because the volume change and hydrolyzation of polyaniline, which are the main causes of the performance degradation in polyaniline-based supercapacitors, are avoided, the present supercapacitor exhibits an excellent cycle stability of 100% capacitance retention after 10 000 cycles. This work demonstrates the possibility of directly using a conductive polymer as an active electrolyte in supercapacitors with high cycle stability.

2D Graphene-Based Macroscopic Assemblies for Micro-Supercapacitors

Rapid development of portable and wearable electronic devices has triggered increased research interest in small-scale power sources, especially in micro-supercapacitors (MSCs) because of their high power densities, long service life, and ability to be charged and discharged quickly. Graphene, an ideal two-dimensional energy-storage electrode material with good conductivity, high quantum capacitance, and large specific surface area, can be used as a building block for MSCs with multi-dimensional architectures. Considerable efforts have been devoted to constructing structures with different dimensions for advanced graphene-based MSCs (GMSCS). In this Review, we summarize the recent progress of graphene-based macroscopic assemblies in MSCs, including 1D fiber GMSCs, 2D planar GMSCs; and 3D in-plane or stacked GMSCs, and discuss the relationship between the structures and applications of the devices. In addition, future prospects and challenges in the MSCs are also discussed.

Miniaturized Energy Storage Devices Based on Two-Dimensional Materials

A growing demand for miniaturized biomedical sensors, microscale self-powered electronic systems, and many other portable, wearable, and integratable electronic devices is continually stimulating the rapid development of miniaturized energy storage devices (MESDs). Miniaturized batteries (MBs) and supercapacitors (MSCs) were considered to be suitable energy storage devices to power microelectronics uninterruptedly with reasonable energy and power densities. However, in addition to similar challenges encountered with electrode materials in conventional energy storage devices, their performances are also greatly affected by microfabrication technologies, as well as the challenges of how to realize stable and high-performance MESDs in such a limited footprint area. Benefiting from the unique architectural engineering of two-dimensional materials and the emergence of precise and controllable microfabrication techniques, the output electrochemical performances of MSCs and MBs are improving rapidly. This minireview summarizes recent advances in MSCs and MBs built from two-dimensional materials, including electrode/device configuration designs, material synthesis, microfabrication processes, smart function incorporations, and system integrations. An introduction to configurations of the MESDs, from linear fibrous shapes, planar sandwich thin-film or interdigital structures, to three-dimensional configurations, is presented. The fundamental influences of the electrode material and configuration designs on the exhibited MB/MSC electrochemical performances are also highlighted.

Tungsten Nitride Nanodots Embedded Phosphorous Modified Carbon Fabric as Flexible and Robust Electrode for Asymmetric Pseudocapacitor

Owing to the excellent physical properties of metal nitrides such as metallic conductivity and pseudocapacitance, they have recently attracted much attention as competitive materials for high-performance supercapacitors (SCs). However, the voltage window for metal nitride-based symmetric SCs is limited (0.6-0.8 V) in aqueous electrolyte due to the oxidation at high negative potentials. In this respect, ultra-small tungsten nitride particles onto the phosphorous modified carbon fabric (W₂ N@P-CF) are engineered as a promising hybrid electrode for pseudocapacitors. Additionally, the fact that the W₂ N@P-CF electrode can operate in the negative potential region is exploited to design asymmetric pseudocapacitors by coupling with a polypyrrole on carbon fabric (PPy@CF) as the positive electrode. Remarkably, the W₂ N@P-CF//PPy@CF asymmetric cell can be cycled in a wide voltage window of 1.6 V that is almost two times higher than that of metal nitrides symmetric SCs. The pseudocapacitive behavior with matching different potential regions of W₂ N@P-CF and PPy@CF, considerably enhance performance of asymmetric device. The device delivers high volumetric capacity (7.1 F cm^-3 ), high energy (2.54 mWh cm^-3 ), power densities, and good cycling stability (88%) over 20 000 cycles. Thus, pseudocapacitive metal nitride-based devices hold a great promise to provide high voltage and improved energy density in aqueous electrolyte.

High-Performance Symmetrical Supercapacitor with a Combination of a ZIF-67/rGO Composite Electrode and a Redox Additive Electrolyte

The synthesis of a highly porous composite of ZIF-67 and reduced graphene oxide (rGO) using a simple stirring approach is reported. The composite has been investigated as an electrode to be assembled in a supercapacitor. In the presence of an optimized redox additive electrolyte (RAE), that is, 0.2 M K3[Fe(CN)6] in 1 M Na2SO4, the ZIF-67/rGO composite electrode has combined the properties of improved conductivity, high specific surface area, and low resistance. The proposed composite electrode in the three-electrode system shows an ultrahigh specific capacitance of 1453 F g-1 at a current density of 4.5 A g-1 within a potential window of -0.1 to 0.5 V. Further, the ZIF-67/rGO composite electrode was used to fabricate a symmetrical supercapacitor whose operation in the presence of the RAE has delivered high values of specific capacitance (326 F g-1 at a current density of 3 A g-1) and energy density (25.5 W h kg-1 at a power density of 2.7 kW kg-1). The device could retain about 88% of its initial specific capacitance after 1000 repeated charge-discharge cycles. The practical usefulness of the device was also verified by combining two symmetrical supercapacitors in series and then lighting a white light-emitting diode (illumination for 3 min). This study, for the first time, reports the application of a ZIF-based composite (ZIF-67/rGO) in the presence of an RAE to design an efficient supercapacitor electrode. This proposed design is also scalable to a flexible symmetric device delivering high values of specific capacitance and energy density.