Flexible solid-state supercapacitors (FSSCs) are frontrunners in energy storage device technology and have attracted extensive attention owing to recent significant breakthroughs in modern wearable electronics In this study, we review the state-of-the-art advancements in FSSCs to provide new insights on mechanisms, emerging electrode materials, flexible gel electrolytes and novel cell designs The review begins with a brief introduction on the fundamental understanding of charge storage mechanisms based on the structural properties of electrode materials The next sections briefly summarise the latest progress in flexible electrodes (ie, freestanding and substrate-supported, including textile, paper, metal foil/wire and polymer-based substrates) and flexible gel electrolytes (ie, aqueous, organic, ionic liquids and redox-active gels) Subsequently, a comprehensive summary of FSSC cell designs introduces some emerging electrode materials, including MXenes, metal nitrides, metal–organic frameworks (MOFs), polyoxometalates (POMs) and black phosphorus Some potential practical applications, such as the development of piezoelectric, photo-, shape-memory, self-healing, electrochromic and integrated sensor-supercapacitors are also discussed The final section highlights current challenges and future perspectives on research in this thriving field

Towards flexible solid-state supercapacitors for smart and wearable electronics

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of “bio-integrated” technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in f...

Bio-Integrated Wearable Systems: A Comprehensive Review

Over the past decade, electrochemical energy storage (EES) devices have greatly improved, as a wide variety of advanced electrode active materials and new device architectures have been developed. These new materials and devices should be evaluated against clear and rigorous metrics, primarily based on the evidence of real performances. A series of criteria are commonly used to characterize and report performance of EES systems in the literature. However, as advanced EES systems are becoming more and more sophisticated, the methodologies to reliably evaluate the performance of the electrode active materials and EES devices need to be refined to realize the true promise as well as the limitations of these fast-moving technologies, and target areas for further development. In the absence of a commonly accepted core group of metrics, inconsistencies may arise between the values attributed to the materials or devices and their real performances. Herein, we provide an overview of the energy storage devices from conventional capacitors to supercapacitors to hybrid systems and ultimately to batteries. The metrics for evaluation of energy storage systems are described, although the focus is kept on capacitive and hybrid energy storage systems. In addition, we discuss the challenges that still need to be addressed for establishing more sophisticated criteria for evaluating EES systems. We hope this effort will foster ongoing dialog and promote greater understanding of these metrics to develop an international protocol for accurate assessment of EES systems.

Towards establishing standard performance metrics for batteries, supercapacitors and beyond.

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

Mixed metal sulfides (MMSs) have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), hybrid supercapacitors (HSCs), metal–air batteries (MABs), and water splitting. Compared with monometal sulfides, MMSs exhibit greatly enhanced electrochemical performance, which is largely originated from their higher electronic conductivity and richer redox reactions. In this review, recent progresses in the rational design and synthesis of diverse MMS-based micro/nanostructures with controlled morphologies, sizes, and compositions for LIBs, SIBs, HSCs, MABs, and water splitting are summarized. In particular, nanostructuring, synthesis of nanocomposites with carbonaceous materials and fabrication of 3D MMS-based electrodes are demonstrated to be three effective approaches for improving the electrochemical performance of MMS-based electrode materials. Furthermore, some potential challenges as well as prospects are discussed to further advance the development of MMS-based electrode materials for next-generation electrochemical energy storage and conversion systems.

Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

To achieve high-performance wearable supercapacitors (SCs), a new class of flexible electrodes with favorable architectures allowing large porosity, high conductivity, and good mechanical stability is strongly needed. Here, this study reports the rational design and fabrication of a novel flexible electrode with nanotube-built multitripod architectures of ternary metal sulfides' composites (FeCo2S4–NiCo2S4) on a silver-sputtered textile cloth. Silver sputtering is applicable to almost all kinds of textiles, and S2− concentration is optimized during sulfidation process to achieve such architectures and also a complete sulfidation assuring high conductivity. New insights into concentration-dependent sulfidation mechanism are proposed. The additive-free FeCo2S4–NiCo2S4 electrode shows a high specific capacitance of 1519 F g−1 at 5 mA cm−2 and superior rate capability (85.1% capacitance retention at 40 mA cm−2). All-solid-state SCs employing these advanced electrodes deliver high energy density of 46 W h kg−1 at 1070 W kg−1 as well as achieve remarkable cycling stability retaining 92% of initial capacitance after 3000 cycles at 10 mA cm−2, and outstanding reliability with no capacitance degradation under large twisting. These are attributed to the components' synergy assuring rich redox reactions, high conductivity as well as highly porous but robust architectures. An almost linear increase in capacitance with devices' area indicates possibility to meet various energy output requirements. This work provides a general, low-cost route to wearable power sources.

Wearable High-Performance Supercapacitors Based on Silver-Sputtered Textiles with FeCo2S4–NiCo2S4 Composite Nanotube-Built Multitripod Architectures as Advanced Flexible Electrodes

A significant advance toward the design and fabrication of a novel hierarchical supercapacitor electrode consisting of FeCo2S4-tubes with well-defined square cross-section and intersecting nanosheets built porous shells on a 3D porous Ni backbone via controlled sulfidation is reported. This general method allows template-free synthesis of metal sulfides tubular structures with polygonal cross-sections and also fine control over the nanostructure leading to both maximized porosity and saturation sulfidation. New insights into concentration and time dependent sulfidation reaction kinetics are proposed. The FeCo2S4 electrode achieves a specific capacitance reaching 2411 F g-1 at 5 mA cm-2 and good rate capability, which are superior over those for nanotube arrays of other ternary transition metal sulfides. This is attributed to rich redox reactions, the highly porous but robust architecture as well as high electrical conductivity. Especially such porous shells effectively avoid “dead volume”, thus improve the utilization ratio of the electrode material. Asymmetric solid-state device applying the FeCo2S4 as positive electrode and N-doped graphene hydrogel film as negative electrode has a high cell voltage of 1.6 V and thus delivers considerably higher energy density of 76.1 W h kg-1 (at 755 W kg-1) than those reported for similar devices.

General Controlled Sulfidation toward Achieving Novel Nanosheet-Built Porous Square-FeCo2S4-Tube Arrays for High-Performance Asymmetric All-Solid-State Pseudocapacitors

Rich‐Mixed‐Valence NixCo3−xPy Porous Nanowires Interwelded Junction‐Free 3D Network Architectures for Ultrahigh Areal Energy Density Supercapacitors

Hierarchically porous hexagonal microsheets constructed by well-interwoven MCo2S4 (M = Ni, Fe, Zn) nanotube networks via two-step anion-exchange for high-performance asymmetric supercapacitors

The invention provides a high-capacitance flexible electrode material of a foamy copper supported CuO nanometer sheet and a manufacturing method, and introduces an application on an aspect of a supercapacitor electrode. Ammonium persulfate and sodium hydroxide are taken as raw materials, and a one-step hydrothermal method is used to carry out oxidation etching on a copper surface so that surfacelayer copper is converted into a copper oxide nanostructure. CuO is a sheet shape structure, a thickness is about 20nm and a width is about 200nn. The nanometer sheets are mutually intersected and uniformly and tightly cover a surface of a foamy copper three-dimensional skeleton so as to form a porous CuO coating layer. A thickness of the CuO coating layer can be accurately controlled through reaction time and a concentration of a mixed solution. The manufactured foamy copper supported CuO nanometer sheet material can be directly applied to the super capacitor electrode, a binder is not needed, and characteristics of a high specific capacitance, high cycle stability and excellent flexibility are possessed. In addition, operation of the manufacturing method is simple, cost is low, the method is easy to control and large-scale production can be realized.

Juan Wu

Papers

Wearable High-Performance Supercapacitors Based on Silver-Sputtered Textiles with FeCo2S4–NiCo2S4 Composite Nanotube-Built Multitripod Architectures as Advanced Flexible Electrodes

General Controlled Sulfidation toward Achieving Novel Nanosheet-Built Porous Square-FeCo2S4-Tube Arrays for High-Performance Asymmetric All-Solid-State Pseudocapacitors

Rich‐Mixed‐Valence NixCo3−xPy Porous Nanowires Interwelded Junction‐Free 3D Network Architectures for Ultrahigh Areal Energy Density Supercapacitors

Hierarchically porous hexagonal microsheets constructed by well-interwoven MCo2S4 (M = Ni, Fe, Zn) nanotube networks via two-step anion-exchange for high-performance asymmetric supercapacitors

High-capacitance flexible electrode material of foamy copper supported CuO nanometer sheet and manufacturing method