Comparison of Thermal and Laser-Reduced Graphene Oxide Production for Energy Storage Applications

A way to obtain graphene-based materials on a large-scale level is by means of chemical methods for the oxidation of graphite to obtain graphene oxide (GO), in combination with thermal, laser, chemical and electrochemical reduction methods to produce reduced graphene oxide (rGO). Among these methods, thermal and laser-based reduction processes are attractive, due to their fast and low-cost characteristics. In this study, first a modified Hummer's method was applied to obtain graphite oxide (GrO)/graphene oxide. Subsequently, an electrical furnace, a fusion instrument, a tubular reactor, a heating plate, and a microwave oven were used for the thermal reduction, and UV and CO₂ lasers were used for the photothermal and/or photochemical reduction. The chemical and structural characterizations of the fabricated rGO samples were performed by Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy measurements. The analysis and comparison of the results revealed that the strongest feature of the thermal reduction methods is the production of high specific surface area, fundamental for volumetric energy applications such as hydrogen storage, whereas in the case of the laser reduction methods, a highly localized reduction is achieved, ideal for microsupercapacitors in flexible electronics.

Effect of Surface Treatment on Performance and Internal Stacking Mode of Electrohydrodynamic Printed Graphene and Its Microsupercapacitor

Microelectronic devices are developing rapidly in portability, wearability, and implantability. This puts forward an urgent requirement for the delicate deposition process of materials. Electrohydrodynamic printing has attracted academic and industrial attention in preparing ultrahigh-density microelectronic devices as a new noncontact, direct graphic, and low-loss thin film deposition process. In this work, a printed graphene with narrow line width is realized by combining the electrohydrodynamic printing and surface treatment. The line width of printed graphene on the hydrophobic treatment surface reduced from 80 to 28 μm. The resistivity decreased from 0.949 to 0.263 Ω·mm. Unexpectedly, hydrophobic treatment can effectively induce random stacking of electrohydrodynamic printed graphene, which avoids parallel stacking and agglomeration of graphene sheets. The performance of printed graphene is thus effectively improved. After optimization, a graphene planar supercapacitor with a printed line width of 28 μm is successfully obtained. Its capacitance can reach 5.39 mF/cm² at 50 mV/s, which is twice higher than that of the untreated devices. The device maintains 84.7% capacitance after 5000 cycles. This work provides a reference for preparing microelectronic devices by ultrahigh precision printing and a new direction for optimizing two-dimensional material properties through stacking adjustment.

Laser Processing of Flexible In-Plane Micro-supercapacitors: Progresses in Advanced Manufacturing of Nanostructured Electrodes

Flexible in-plane architecture micro-supercapacitors (MSCs) are competitive candidates for on-chip miniature energy storage applications owing to their light weight, small size, high flexibility, as well as the advantages of short charging time, high power density, and long cycle life. However, tedious and time-consuming processes are required for the manufacturing of high-resolution interdigital electrodes using conventional approaches. In contrast, the laser processing technique enables high-efficiency high-precision patterning and advanced manufacturing of nanostructured electrodes. In this review, the recent advances in laser manufacturing and patterning of nanostructured electrodes for applications in flexible in-plane MSCs are comprehensively summarized. Various laser processing techniques for the synthesis, modification, and processing of interdigital electrode materials, including laser pyrolysis, reduction, oxidation, growth, activation, sintering, doping, and ablation, are discussed. In particular, some special features and merits of laser processing techniques are highlighted, including the impacts of laser types and parameters on manufacturing electrodes with desired morphologies/structures and their applications on the formation of high-quality nanoshaped graphene, the selective deposition of nanostructured materials, the controllable nanopore etching and heteroatom doping, and the efficient sintering of nanometal products. Finally, the current challenges and prospects associated with the laser processing of in-plane MSCs are also discussed. This review will provide a useful guidance for the advanced manufacturing of nanostructured electrodes in flexible in-plane energy storage devices and beyond.

Femtosecond Laser-Fabricated Photonic Chips for Optical Communications: A Review

Integrated optics, having the unique properties of small size, low loss, high integration, and high scalability, is attracting considerable attention and has found many applications in optical communications, fulfilling the requirements for the ever-growing information rate and complexity in modern optical communication systems. Femtosecond laser fabrication is an acknowledged technique for producing integrated photonic devices with unique features, such as three-dimensional fabrication geometry, rapid prototyping, and single-step fabrication. Thus, plenty of femtosecond laser-fabricated on-chip devices have been manufactured to realize various optical communication functions, such as laser generation, laser amplification, laser modulation, frequency conversion, multi-dimensional multiplexing, and photonic wire bonding. In this paper, we review some of the most relevant research progress in femtosecond laser-fabricated photonic chips for optical communications, which may break new ground in this area. First, the basic principle of femtosecond laser fabrication and different types of laser-inscribed waveguides are briefly introduced. The devices are organized into two categories: active devices and passive devices. In the former category, waveguide lasers, amplifiers, electric-optic modulators, and frequency converters are reviewed, while in the latter, polarization multiplexers, mode multiplexers, and fan-in/fan-out devices are discussed. Later, photonic wire bonding is also introduced. Finally, conclusions and prospects in this field are also discussed.

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.

Progress and Perspectives in Designing Flexible Microsupercapacitors

Miniaturized flexible microsupercapacitors (MSCs) that can be integrated into self-powered sensing systems, detecting networks, and implantable devices have shown great potential to perfect the stand-alone functional units owing to the robust security, continuously improved energy density, inherence high power density, and long service life. This review summarizes the recent progress made in the development of flexible MSCs and their application in integrated wearable electronics. To meet requirements for the scalable fabrication, minimization design, and easy integration of the flexible MSC, the typical assembled technologies consist of ink printing, photolithography, screen printing, laser etching, etc., are provided. Then the guidelines regarding the electrochemical performance improvement of the flexible MSC by materials design, devices construction, and electrolyte optimization are considered. The integrated prototypes of flexible MSC-powered systems, such as self-driven photodetection systems, wearable sweat monitoring units are also discussed. Finally, the future challenges and perspectives of flexible MSC are envisioned.

A review of the microwave-assisted synthesis of carbon nanomaterials, metal oxides/hydroxides and their composites for energy storage applications

Currently, nanomaterials are considered to be the backbone of modern civilization. Especially in the energy sector, nanomaterials (mainly, carbon- and metal oxide/hydroxide-based nanomaterials) have contributed significantly. Among the various green approaches for the synthesis of these nanomaterials, the microwave-assisted approach has attracted significant research interest worldwide. In this context, it is noteworthy to mention that because of their enhanced surface area, high conducting nature, and excellent electrical and electrochemical properties, carbon nanomaterials are being extensively utilized as efficient electrode materials for both supercapacitors and secondary batteries. In this review article, we briefly demonstrate the characteristics of microwave-synthesized nanomaterials for next-generation energy storage devices. Starting with the basics of microwave heating, herein, we illustrate the past and present status of microwave chemistry for energy-related applications, and finally present a brief outlook and concluding remarks. We hope that this review article will positively convey new insights for the microwave synthesis of nanomaterials for energy storage applications.

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.

Catalytic methane technology for carbon nanotubes and graphene

The catalytic methane technology for the production of carbon nanotubes and graphene is summarized in this review.

Carbon-Based Electrode Materials for Microsupercapacitors in Self-Powering Sensor Networks: Present and Future Development

There is an urgent need to fulfill future energy demands for micro and nanoelectronics. This work outlines a number of important design features for carbon-based microsupercapacitors, which enhance both their performance and integration potential and are critical for complimentary metal oxide semiconductor (CMOS) compatibility. Based on these design features, we present CMOS-compatible, graphene-based microsupercapacitors that can be integrated at the back end of the line of the integrated circuit fabrication. Electrode materials and their interfaces play a crucial role for the device characteristics. As such, different carbon-based materials are discussed and the importance of careful design of current collector/electrode interfaces is emphasized. Electrode adhesion is an important factor to improve device performance and uniformity. Additionally, doping of the electrodes can greatly improve the energy density of the devices. As microsupercapacitors are engineered for targeted applications, device scaling is critically important, and we present the first steps toward general scaling trends. Last, we outline a potential future integration scheme for a complete microsystem on a chip, containing sensors, logic, power generation, power management, and power storage. Such a system would be self-powering.

Direct Writing Supercapacitors Using a Carbon Nanotube/Ag Nanoparticle-Based Ink on Cellulose Acetate Membrane Paper

In this work, we present a cellulose acetate membrane flexible supercapacitor prepared through a direct writing method. A carbon nanotube (CNT) and silver (Ag) nanoparticle were prepared into ink for direct writing. The composite electrode displayed excellent electrochemical and mechanical electrochemical performance. Furthermore, the CNT-Ag displayed the highest areal capacity of 72.8 F/cm³. The assembled device delivered a high areal capacity (17.68 F/cm³) at a current density of 0.5 mA/cm², a high areal energy (9.08–5.87 mWh/cm³) at a power density of 1.18–0.22 W/cm³, and showed no significant decrease in performance with a bending angle of 180°. The as-fabricated CNT/Ag electrodes exhibited good long-term cycling stability after 1000 time cycles with 75.92% capacitance retention. The direct writing was a simple, cost-effective, fast, and non-contact deposition method. This method has been used in current printed electronic devices and has potential applications in energy storage.

High-performance supercapacitors based on the carbon nanotubes, graphene and graphite nanoparticles electrodes

In this study, the three structures of the symmetric paper supercapacitors based on the carbon nanotubes (CNTs), graphite nanoparticles (GNPs) and graphene electrodes have been fabricated. In the supercapacitors was used of polyvinyl alcohol (PVA)/phosphoric acid (H₃PO₄) as a gel electrolyte and the BaTiO₃ film as a separator film. The carbon nanomaterials, gel electrolyte surface, and electrode films were characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The specific capacitance of the symmetric paper supercapacitors using charge-discharge technique and C-V curves at the voltage scan rates 20 mV/s and 150 mV/s have been investigated. The symmetric paper supercapacitor based on the CNTs electrode showed higher specific capacitance 411 F g⁻¹, compared to GNPs and graphene supercapacitors. Also by electrochemical impedance spectroscopy, the Nyquist curves of the symmetric paper supercapacitors have been plotted. For the symmetric paper supercapacitors based on the GNPs, graphene and CNTs electrodes the equivalent series resistance (ESR) resistance was 210 Ω, 96 Ω and 101 Ω respectively. The flexible symmetric paper supercapacitor based on BaTiO₃/PVA/CNTs structure denotes a new type of the flexible supercapacitor that can be applied to the soft electronic.

Ni(OH)2/CNTs hierarchical spheres for a foldable all-solid-state supercapacitor with high specific energy

Energy density and mechanical strength are crucial for practical usage of flexible supercapacitors. Herein, we demonstrate a flexible supercapacitor using Ni(OH)2/CNTs hierarchical spheres with high specific capacity (854 C g-1) and tough PBI-KOH solid polymer electrolyte. The integrated device shows high specific energy (50.6 W h kg-1) and good flexibility under folding tests.

Latest advances in supercapacitors: from new electrode materials to novel device designs

Notably, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years, related to theoretical understanding, material synthesis and device designs. Herein, we summarize the state-of-the-art progress toward mechanisms, new materials, and novel device designs for supercapacitors. Firstly, fundamental understanding of the mechanism is mainly focused on the relationship between the structural properties of electrode materials and their electrochemical performances based on some in situ characterization techniques and simulations. Secondly, some emerging electrode materials are discussed, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), MXenes, metal nitrides, black phosphorus, LaMnO₃, and RbAg₄I₅/graphite. Thirdly, the device innovations for the next generation of supercapacitors are provided successively, mainly emphasizing flow supercapacitors, alternating current (AC) line-filtering supercapacitors, redox electrolyte enhanced supercapacitors, metal ion hybrid supercapacitors, micro-supercapacitors (fiber, plane and three-dimensional) and multifunctional supercapacitors including electrochromic supercapacitors, self-healing supercapacitors, piezoelectric supercapacitors, shape-memory supercapacitors, thermal self-protective supercapacitors, thermal self-charging supercapacitors, and photo self-charging supercapacitors. Finally, the future developments and key technical challenges are highlighted regarding further research in this thriving field.

Flexible micro-supercapacitors prepared using direct-write nanofibers

A direct-write flexible micro-supercapacitor with high performance was demonstrated using a simple and versatile approach.

Synthesis of a hierarchical cobalt sulfide/cobalt basic salt nanocomposite via a vapor-phase hydrothermal method as an electrode material for supercapacitor

A hierarchical cobalt sulfide/cobalt basic salt nanocomposite shows excellent electrochemical performances as a supercapacitor.