Rapid, all dry microfabrication of three-dimensional Co3O4/Pt nanonetworks for high-performance microsupercapacitors

Pushing the Electrochemical Performance Limits of Polypyrrole Toward Stable Microelectronic Devices

Conducting polymers have achieved remarkable attentions owing to their exclusive characteristics, for instance, electrical conductivity, high ionic conductivity, visual transparency, and mechanical tractability. Surface and nanostructure engineering of conjugated conducting polymers offers an exceptional pathway to facilitate their implementation in a variety of scientific claims, comprising energy storage and production devices, flexible and wearable optoelectronic devices. A two-step tactic to assemble high-performance polypyrrole (PPy)-based microsupercapacitor (MSC) is utilized by transforming the current collectors to suppress structural pulverization and increase the adhesion of PPy, and then electrochemical co-deposition of PPy-CNT nanostructures on rGO@Au current collectors is performed. The resulting fine patterned MSC conveyed a high areal capacitance of 65.9 mF cm-2 (at a current density of 0.1 mA cm-2), an exceptional cycling performance of retaining 79% capacitance after 10,000 charge/discharge cycles at 5 mA cm-2. Benefiting from the intermediate graphene, current collector free PPy-CNT@rGO flexible MSC is produced by a facile transfer method on a flexible substrate, which delivered an areal capacitance of 70.25 mF cm-2 at 0.1 mA cm-2 and retained 46% of the initial capacitance at a current density of 1.0 mA cm-2. The flexible MSC is utilized as a skin compatible capacitive micro-strain sensor with excellent electromechanochemical characteristics.

Advances in wearable textile-based micro energy storage devices: structuring, application and perspective

The continuous expansion of smart microelectronics has put forward higher requirements for energy conversion, mechanical performance, and biocompatibility of micro-energy storage devices (MESDs). Unique porosity, superior flexibility and comfortable breathability make the textile-based structure a great potential in wearable MESDs. Herein, a timely and comprehensive review of this field is provided according to recent research advances. The following aspects, device construction of textile-based MESDs (TMESDs), fabric processing of textile components and smart functionalization (e.g., mechanical reliability, energy harvesting, sensing, self-charging and self-healing, etc.) are discussed and summarized thoroughly. Also, the perspectives on the microfabrication processes and multiple applications of TMESDs are elaborated.

Updated Insights into 3D Architecture Electrodes for Micropower Sources

Microbatteries (MBs) and microsupercapacitors (MSCs) are primary on-chip micropower sources that drive autonomous and stand-alone microelectronic devices for implementation of the Internet of Things (IoT). However, the performance of conventional MBs and MSCs is restricted by their 2D thin-film electrode design, and these devices struggle to satisfy the increasing IoT energy demands for high energy density, high power density, and long lifespan. The energy densities of MBs and MSCs can be improved significantly through adoption of a 2D thick-film electrode design; however, their power densities and lifespans deteriorate with increased electrode thickness. In contrast, 3D architecture electrodes offer remarkable opportunities to simultaneously improve MB and MSC energy density, power density, and lifespan. To date, various 3D architecture electrodes have been designed, fabricated, and investigated for MBs and MSCs. This review provides an update on the principal superiorities of 3D architecture electrodes over 2D thick-film electrodes in the context of improved MB and MSC energy density, power density, and lifespan. In addition, the most recent and representative progress in 3D architecture electrode development for MBs and MSCs is highlighted. Finally, present challenges are discussed and key perspectives for future research in this field are outlined.

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.

Progress in Iron Oxides Based Nanostructures for Applications in Energy Storage

The demand for green and efficient energy storage devices in daily life is constantly rising, which is caused by the global environment and energy problems. Lithium-ion batteries (LIBs), an important kind of energy storage devices, are attracting much attention. Graphite is used as LIBs anode, however, its theoretical capacity is low, so it is necessary to develop LIBs anode with higher capacity. Application strategies and research progresses of novel iron oxides and their composites as LIBs anode in recent years are summarized in this review. Herein we enumerate several typical synthesis methods to obtain a variety of iron oxides based nanostructures, such as gas phase deposition, co-precipitation, electrochemical method, etc. For characterization of the iron oxides based nanostructures, especially the in-situ X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy are elaborated. Furthermore, the electrochemical applications of iron oxides based nanostructures and their composites are discussed and summarized.

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Recent Advances in High-Performance Microbatteries: Construction, Application, and Perspective

High-performance miniaturized energy storage devices have developed rapidly in recent years. Different from conventional energy storage devices, microbatteries assume the main responsibility for micropower supply, functionalization, and characterization platforms. Evolving from the essential goals for battery design of high power density, high energy density, and long lifetime, further practical demands for microbatteries (MBs) have been raised for the microfabrication technique and device design. Numerous studies have generally focused on specific aspects of the microelectrode structures or certain microfabrication techniques, while the connection from techniques to functional applications is rarely involved. This Review generally fills such blanks from an application-oriented perspective. First, some basic micromachining techniques with different compatible features are summarized. Afterward, device designs including diversified battery reaction types, configuration, and assembly are highlighted, as well as microbatteries serving powering resources or further complicated functional systems. Finally, through providing the overall design concept based on requirements in application, this Review offers innovative insights for further development of microbatteries.

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.

Surface Engineering of Carbon-Based Microelectrodes for High-Performance Microsupercapacitors

In this research, the enhancement in electrochemical performance of pyrolyzed carbon microelectrodes by surface modification is investigated. For the proposed microfabrication process, pyrolyzed carbon microelectrodes with multi-walled carbon nanotubes (MWCNTs) on their surface are obtained by developing GM-1060 photoresist in mixture of propylene glycol methyl ether acetate (PGMEA) and CNTs, and following pyrolysis of a micropatterned photoresist. Polyvinyl alcohol (PVA)/H2SO4 electrolyte (1 M) was applied to assemble this carbon/CNT microelectrode-based all-solid-state microsupercapacitor (carbon/CNT-MSC). The carbon/CNT-MSC shows a higher electrochemical performance compared with that of pyrolyzed carbon microelectrode-based MSC (carbon-MSC). The specific areal and volumetric capacitances of carbon/CNT-MSC (4.80 mF/cm2 and 32.0 F/cm3) are higher than those of carbon-MSC (3.52 mF/cm2 and 23.4 F/cm3) at the scan rate of 10 mV/s. In addition, higher energy density and power density of carbon/CNT-MSC (2.85 mWh/cm3 and 1.98 W/cm3) than those of carbon-MSC (2.08 mWh/cm3 and 1.41 W/cm3) were also achieved. This facile surface modification and optimization are potentially promising, being highly compatible with modern microfabrication technologies and allowing integration of highly electrically conductive CNTs into pyrolyzed carbon to assemble MSCs with improved electrochemical performance. Moreover, this method can be potentially applied to other high-performance micro/nanostructures and microdevices/systems.

High Energy Density Micro-Supercapacitor Based on a Three-Dimensional Bicontinuous Porous Carbon with Interconnected Hierarchical Pores

On-chip micro-supercapacitors (MSCs) have attracted great attention recently. However, the performance of MSCs is usually unsatisfactory because of the unreasonable pore structure. The construction of a three-dimensional (3D) interconnected porous carbon-based MSC by controllable activation is proposed. The porous monolithic carbon microelectrode activated by ZnO nanowires provides electron/ion bicontinuous conduction path. The fabricated MSC with this microelectrode rendered a high areal specific capacitance of 10.01 mF cm^-2, 6 times higher than that of pure pyrolyzed carbon-based MSC, 1.6-5 times higher than that of the MSC with porous carbon activated by ZnO nanoparticles because of its cross-linking macropore-mesopore-micropore structure and considerable areal atomic ratio. The optimization mechanism of the hierarchical channel pore for the electrochemical performance of MSCs is investigated in detail. Four kinds of electrolytes, including H₂SO₄, redox additive KI/H₂SO₄, LiCl, and LiTFSi, are employed for constructing MSCs. The voltage window of water in a salt electrolyte assembled LiTFSi-MSC is expanded to 2.5 V. The energy density of LiTFSi-MSC is 6 times higher than that of H₂SO₄-MSC, which can drive light-emitting diodes without serial or parallel connection. This high-performance 3D interconnected porous carbon-based MSC shows a great potential in applications for large-scale integration of micro-/nanodevices.

Recent Advances in Nanowire-Based, Flexible, Freestanding Electrodes for Energy Storage

The rational design of flexible electrodes is essential for achieving high performance in flexible and wearable energy-storage devices, which are highly desired with fast-growing demands for flexible electronics. Owing to the one-dimensional structure, nanowires with continuous electron conduction, ion diffusion channels, and good mechanical properties are particularly favorable for obtaining flexible freestanding electrodes that can realize high energy/power density, while retaining long-term cycling stability under various mechanical deformations. This Minireview focuses on recent advances in the design, fabrication, and application of nanowire-based flexible freestanding electrodes with diverse compositions, while highlighting the rational design of nanowire-based materials for high-performance flexible electrodes. Existing challenges and future opportunities towards a deeper fundamental understanding and practical applications are also presented.

Hydrogen evolution reactions boosted by bridge bonds between electrocatalysts and electrodes

The interfacial interactions between nanostructured electrode materials and electrodes play an important part in the performance enhancement of electrochemical energy devices. However, the mechanism of interfacial interactions, as well as its influence on device performance, still remains unclear and is rarely studied. In this work, a CoS₂ nanobelt catalyst assembled on Ti foil (CoS₂ nanobelts/Ti) is prepared through in situ chemical conversions and chosen as an example to probe the interfacial interactions between the CoS₂ catalyst and the Ti electrode, and the correlation between the interfacial interaction and the hydrogen evolution reaction (HER) performance. By a series of characterization studies and analyses, we propose that interfacial bridge bonds (Ti-S-Co and Ti-O-Co) in a covalent form may exist in the CoS₂ nanobelts/Ti as well as its precursor Co(OH)₃ nanobelts growing on Ti foil, which is further supported by density functional theory (DFT) calculations. Moreover, as a binder-free electrocatalytic electrode, the CoS₂ nanobelts/Ti shows boosted HER performance, including higher catalytic activity, and lower overpotential and Tafel slope, compared to its counterpart transformed from a solution-produced precursor. The HER performance enhancement is ascribed to the existence of interfacial bridge bonds that not only strengthen the electrode-catalyst mechanical integrity, but also serve as efficient charge transfer channels between the electrode and the catalyst, thus ensuring a stable and fluent electron transfer for the HER. Furthermore, the DFT calculations reveal that the CoS₂ nanobelts/Ti catalyst with interfacial covalent interactions can facilitate the adsorption of H⁺ ions/H₂ molecules and the desorption of H₂ molecules for an accelerated HER. This work provides a new insight into the interfacial interactions between electrodes and electrode materials in electrochemical devices, and paves the way for the rational design and construction of high-performance electrochemical devices for practical energy applications.

Two-dimensional materials for miniaturized energy storage devices: from individual devices to smart integrated systems

This review summarizes recent advances, key challenges and perspectives regarding two-dimensional materials for miniaturized energy storage devices.