Transparent and Flexible Supercapacitors with Networked Electrodes

Conductive MXene/Polymer Composites for Transparent Flexible Supercapacitors

Transparent flexible energy storage devices are limited by the trade-off among flexibility, transparency, and charge storage capability of their electrode materials. Conductive polymers are intrinsically flexible, but limited by small capacitance. Pseudocapacitive MXene provides high capacitance, yet their opaque and brittle nature hinders their flexibility and transparency. Herein, the development of synergistically interacting conductive polymer Ti3C2Tx MXene/PEDOT:PSS composites is reported for transparent flexible all-solid-state supercapacitors, with an outstanding areal capacitance of 3.1 mF cm-2, a high optical transparency of 61.6%, and excellent flexibility and durability. The high capacitance and high transparency of the devices stem from the uniform and thorough blending of PEDOT:PSS and Ti3C2Tx, which is associated with the formation of O─H…O H-bonds in the composites. The conductive MXene/polymer composite electrodes demonstrate a rational means to achieve high-capacity, transparent and flexible supercapacitors in an easy and scalable manner.

MnO2 Nanoparticles Decorated PEDOT:PSS for High Performance Stretchable and Transparent Supercapacitors

With the swift advancement of wearable electronics and artificial intelligence, the integration of electronic devices with the human body has advanced significantly, leading to enhanced real-time health monitoring and remote disease diagnosis. Despite progress in developing stretchable materials with skin-like mechanical properties, there remains a need for materials that also exhibit high optical transparency. Supercapacitors, as promising energy storage devices, offer advantages such as portability, long cycle life, and rapid charge/discharge rates, but achieving high capacity, stretchability, and transparency simultaneously remains challenging. This study combines the stretchable, transparent polymer PEDOT:PSS with MnO2 nanoparticles to develop high-performance, stretchable, and transparent supercapacitors. PEDOT:PSS films were deposited on a PDMS substrate using a spin-coating method, followed by electrochemical deposition of MnO2 nanoparticles. This method ensured that the nanosized MnO2 particles were uniformly distributed, maintaining the transparency and stretchability of PEDOT:PSS. The resulting PEDOT:PSS/MnO2 nanoparticle electrodes were gathered into a symmetric device using a LiCl/PVA gel electrolyte, achieving an areal capacitance of 1.14 mF cm-2 at 71.2% transparency and maintaining 89.92% capacitance after 5000 cycles of 20% strain. This work presents a scalable and economical technique to manufacturing supercapacitors that combine high capacity, transparency, and mechanical stretchability, suggesting potential applications in wearable electronics.

Metallic Micro-Nano Network-Based Soft Transparent Electrodes: Materials, Processes, and Applications

Soft transparent electrodes (TEs) have received tremendous interest from academia and industry due to the rapid development of lightweight, transparent soft electronics. Metallic micro-nano networks (MMNNs) are a class of promising soft TEs that exhibit excellent optical and electrical properties, including low sheet resistance and high optical transmittance, as well as superior mechanical properties such as softness, robustness, and desirable stability. They are genuinely interesting alternatives to conventional conductive metal oxides, which are expensive to fabricate and have limited flexibility on soft surfaces. This review summarizes state-of-the-art research developments in MMNN-based soft TEs in terms of performance specifications, fabrication methods, and application areas. The review describes the implementation of MMNN-based soft TEs in optoelectronics, bioelectronics, tactile sensors, energy storage devices, and other applications. Finally, it presents a perspective on the technical difficulties and potential future possibilities for MMNN-based TE development.

Overcoming the Trade-Off between Optical Transmittance and Areal Capacitance of Transparent Supercapacitors for Practical Application

It is substantially challenging for transition metal oxide nanoparticle (NP)-based electrodes for supercapacitors to achieve high transparency and large capacity simultaneously due to the inherent trade-off between optical transmittance (T) and areal capacitance (C_A ). This study demonstrates how this trade-off limitation can be overcome by replacing some electrode NPs with transparent tin oxide (SnO₂ ) NPs. Although SnO₂ NPs are non-capacitive, they provide effective paths for charge transport, which simultaneously increase the C_A and T_550nm of the manganese oxide (Mn₃ O₄ ) NP electrode from 11.7 to 13.4 mF cm^-2 and 82.1% to 87.4%, respectively, when 25 wt% of Mn₃ O₄ are replaced by SnO₂ . The obtained C_A values at a given T are higher than those of the transparent electrodes previously reported. An energy storage window fabricated using the mixed-NP electrodes exhibits the highest energy density among transparent supercapacitors previously reported. The improved energy density enables the window to operate various electronic devices for a considerable amount of time, demonstrating its applicability in constructing a reliable and space-efficient building-integrated power supply system.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Cohesively Enhancing the Conductance, Mechanical Robustness, and Environmental Stability of Random Metallic Mesh Electrodes via Hot-Pressing-Induced In-Plane Configuration

Flexible transparent conductive electrode (FTCE) is highly desirable due to the fast-growing flexible optoelectronic devices. Several promising FTCEs based on metal material have been developed to replace conventional indium tin oxide (ITO). The random metal mesh is considered to be one of the competitive candidates. However, obtaining feasible random metal mesh with low sheet resistance, high transparency, good mechanical durability, and strong environmental stability is still a great challenge. Here, a random metal mesh-based FTCE with an in-plane structure, achieved by a facile hot-pressing process, is demonstrated. The hot-pressing process enables the fabrication of highly conductive FTCE with improved mechanical robustness and environmental stability. The in-plane FTCE shows a low sheet resistance of 1.63 Ω·sq^-1 with an 80.6% transmittance, low relative resistance increase (RRI) of 7.9% after 240 h 85 °C/85% RH test, and low RRI of 8.0% after 10⁵ cycles of bending test. Besides, various applications of the in-plane FTCE were demonstrated, including the flexible heater, flexible touch screen, and flexible electroluminescence. We anticipate that these results will spark interest in in-plane random metal mesh electrodes and enable the application of random metal mesh in flexible optoelectronic devices.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.

PEDOT:PSS-glued MoO3 nanowire network for all-solid-state flexible transparent supercapacitors

Flexible transparent supercapacitors (FTSCs) are essential for the development of next-generation transparent electronics, however, a significant challenge is to achieve high-areal-capacitance FTSCs without sacrificing optical transparency. Herein, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-glued MoO₃ nanowires anchored on the Ag nanofiber (AgNF) network are employed as FTSC film electrodes, in which the AgNF network provides primary conducting pathways and guarantees rapid electron transport, while wide-bandgap semiconductor MoO₃ nanowires glued by the ultrathin PEDOT:PSS layer provide abundant redox-active sites to store energy. Benefiting from the PEDOT:PSS as the conducting glue to promote the connection at the junctions between AgNFs and MoO₃ nanowires, the as-prepared AgNFs/MoO₃/PEDOT:PSS (AMP) film electrode demonstrates a high transmittance (82.8%) and large areal capacitance (15.7 mF cm^-2), and has outperformed all the transparent conductive films known to date. Even after 11 000 charge/discharge cycles, the capacitance still remains at 92.4% of the initial value. The assembled all-solid-state FTSC device delivers an energy density of 0.623 μW h cm^-2, a power density of 40 μW cm^-2, and excellent mechanical robustness, implying a great potential in high performance FTSCs.

Interstitial boron-doped mesoporous semiconductor oxides for ultratransparent energy storage

Realizing transparent and energy-dense supercapacitor is highly challenging, as there is a trade-off between energy storing capability and transparency in the active material film. We report here that interstitial boron-doped mesoporous semiconductor oxide shows exceptional electrochemical capacitance which rivals other pseudocapacitive materials, while maintaining its transparent characteristic. This improvement is credited to the robust redox reactions at interstitial boron-associated defects that transform inert semiconductor oxides into an electrochemically active material without affecting its transparency. By precisely tuning the level of doping, the pseudocapacitive reactivity of these materials is optimized, resulting in a volumetric capacitance up to 1172 F cm-3. Attributing to such efficient charge storage utilization on the active film, the fabricated transparent supercapacitor delivers a maximum areal energy density of 1.36 × 10-3 mWh cm-2 that is close to those of conventional pseudocapacitive materials, with nearly 100% capacitance retention after 15000 cycles and ultrahigh transparency (up to 85% transmittance at 550 nm). In addition, this device shows excellent durability and flexibility with multiple optional outputs, demonstrating the potential as a transparent energy supply in planar electronics.

Constructed Ag NW@Bi/Al core-shell nano-architectures for high-performance flexible and transparent energy storage device

Flexible and transparent energy storage devices (FTESDs) have recently attracted much attention for use in wearable and portable electronics. Herein, we developed an Ag nanowire (NW) @Bi/Al nanostructure as a transparent negative electrode for FTESDs. In the core-shell nanoarchitecture, the Ag NW percolation network with excellent conductivity contributes superior electron transport pathways, while the unique nanostructure provides an effective interface contact between the current collector and electroactive material. As a result, the electrode delivers a high capacity of 12.36 mF cm-2 (3.43 μA h cm-2) at 0.2 mA cm-2. With a minor addition of Al, the coulombic efficiency of the electrode remarkably increases from 65.1% to 83.9% and the capacity retention rate improves from 53.8% to 91.9% after 2000 cycles. Moreover, a maximum energy density of 319.5 μW h cm-2 and a power density of 27.5 mW cm-2 were realized by an interdigital structured device with a transmittance of 58% and a potential window of 1.6 V. This work provides a new negative electrode material for high-performance FTESDs in the next-generation integrated electronics market.

Transparent Flexible Heteroepitaxy of NiO Coated AZO Nanorods Arrays on Muscovites for Enhanced Energy Storage Application

Transparent flexible energy storage devices are considered as important chains in the next-generation, which are able to store and supply energy for electronic devices. Here, aluminum-doped zinc oxide (AZO) nanorods (NRs) and nickel oxide (NiO)-coated AZO NRs on muscovites are fabricated by a radio frequency (RF) magnetron sputtering deposition method. Interestingly, AZO NRs and AZO/NiO NRs are excellent electrodes for energy storage application with high optical transparency, high conductivity, large surface area, stability under compressive and tensile strain down to a bending radius of 5 mm with 1000 bending cycles. The obtained symmetric solid-state supercapacitors based on these electrodes exhibit good performance with a large areal specific capacitance of 3.4 mF cm^-2 , long cycle life 1000 times, robust mechanical properties, and high chemical stability. Furthermore, an AZO/NiO//Zn battery based on these electrodes is demonstrated, yielding a discharge capacity of 195 mAh g^-1 at a current rate of 8 A g^-1 and a discharge capacity of over 1000 cycles with coulombic efficiency to 92%. These results deliver a concept of opening a new opportunity for future applications in transparent flexible energy storage.

Active and Deformable Organic Electronic Devices based on Conductive Shape Memory Polyimide

Smart, deformable, and transparent electrodes are a significant part of flexible optoelectronic devices. In this work, a novel approach to making highly transparent, smooth, and conductive shape memory polyimide hybrids has been proposed. Colorless shape memory polyimide (CSMPI) with high optical transparency and high heat resistance is served as the substrate for flexible electronic devices for the first time. A hybrid (Au/Ag) metal grid electrode embedded in CSMPI (BMG/CSMPI) is first fabricated via self-cracking template and solution-coating, the advantages of which include ultrasmooth surface, superior mechanical flexibility and durability, strong surface adhesion, and excellent chemical stability due to the unique embedded hybrid structure. The resulting white polymer light emitting diodes (WPLEDs) based on BMG/CSMPI with shape memory effect are active and deformable, and are converted from 2D device into 3D devices depending on its variable stiffness characteristics. The deformed 3D devices could actively recover to the original shape upon heating. Furthermore, ultrathin and flexible 3D optoelectronic devices fabricated using shape memory polymers can promote the development of advanced optoelectronic applications in the future.

Controlling Electrode Spacing by Polystyrene Microsphere Spacers for Highly Stable and Flexible Transparent Supercapacitors

Transparent polymer electrolytes such as poly(vinyl alcohol)-based H⁺, Li⁺, K⁺, and Na⁺ gels have been widely used as both an electrolyte and a separator for flexible transparent supercapacitors (FTSCs). However, these gels sandwiched between the electrodes in FTSCs are easily compressed under bending and compression due to their viscous flow behavior, resulting in the deformation of electrode spacing and the unstable capacitance performance. To resolve this issue, herein, we introduce monodispersed polystyrene (PS) microspheres into PVA-LiCl polymer gel electrolytes as spacers to precisely control the electrode spacing during the assembly of FTSCs using single-walled carbon nanotubes/indium tin oxide-polyethylene terephthalate (ITO-PET) or MnO₂/multiwalled carbon nanotubes/ITO-PET as transparent electrodes. The electrode spacing could be tuned by varying the diameter of PS microspheres, for example, 20, 40, and 80 μm. More importantly, the PS microsphere spacers protect the gel electrolyte from the squeeze when bending takes place, allowing the stable performance output by FTSCs under a bending state. After repeating bending tests, the capacitance remains 95.6%, indicating the high stability and flexibility of the devices with the assistance of PS microsphere spacers.

Charge-Transfer-Modulated Transparent Supercapacitor Using Multidentate Molecular Linker and Conductive Transparent Nanoparticle Assembly

One of the most critical issues in preparing high-performance transparent supercapacitors (TSCs) is to overcome the trade-off between areal capacitance and optical transmittance as well as that between areal capacitance and rate capability. Herein, we introduce a TSC with high areal capacitance, fast rate capability, and good optical transparency by minimizing the charge transfer resistance between pseudocapacitive nanoparticles (NPs) using molecular linker- and conductive NP-mediated layer-by-layer (LbL) assembly. For this study, bulky ligand-stabilized manganese oxide (MnO) and indium tin oxide (ITO) NP multilayers are LbL-assembled through a ligand exchange reaction between native ligands and small multidentate linkers (tricarballylic acid). The introduced molecular linker substantially decreases the separation distance between neighboring NPs, thereby reducing the contact resistance of electrodes. Moreover, the periodic insertion of ITO NPs into the MnO NP-based electrodes can lower the charge transfer resistance without a meaningful loss of transmittance, which can significantly improve the areal capacitance. The areal capacitances of the ITO NP-free electrode and the ITO NP-incorporated electrode are 24.6 mF cm^-2 (at 61.6% transmittance) and 40.5 mF cm^-2 (at 60.8%), respectively, which outperforms state of the art TSCs. Furthermore, we demonstrate a flexible symmetric solid-state TSC that exhibits scalable areal capacitance and optical transmittance.

Self-Standing Metallic Mesh with MnO2 Multiscale Microstructures for High-Capacity Flexible Transparent Energy Storage

Flexible transparent electrochemical supercapacitors are critical components for the rapid development of fully flexible transparent electronics; however, typical flexible transparent supercapacitor electrodes store limited energy due to the requirements of transparency. Self-standing core-shell structure metal oxide mesh electrodes with metal oxide as active "shell" and metallic mesh as current collector "core" are efficient for simultaneously achieving high capacity, flexibility, and transparency. In this work, we perform a morphology-controlled electrodeposition of MnO₂ on a self-standing flexible transparent metallic Ni mesh electrode to achieve a high-capacity flexible transparent supercapacitor electrode. Under optimized conditions, the MnO₂ nanosheet-composed flowerlike multiscale microstructure was constructed. The open, loose, and porous MnO₂ multiscale microstructure "shell" and high electrical conductivity of self-standing metallic mesh "core" synergistically enable efficient ionic and electronic transport and meanwhile retain high structural stability. The metal oxide mesh electrode yields an outstanding areal capacitance of 1.15 F/cm² at an optical transmittance of 69.4% and excellent cycling stability. The symmetric solid-state supercapacitor device exhibits a high areal capacitance value (78.46 mF/cm²), superior cycling life, as well as high optical transmittance and mechanical flexibility, superior to the most reported flexible transparent supercapacitors. This work provides a comprehensive understanding on how to achieve high-capacity flexible transparent supercapacitor electrodes and solid-state devices.

Electrochemical energy storage devices for wearable technology: a rationale for materials selection and cell design

Compatible energy storage devices that are able to withstand various mechanical deformations, while delivering their intended functions, are required in wearable technologies. This imposes constraints on the structural designs, materials selection, and miniaturization of the cells. To date, extensive efforts have been dedicated towards developing electrochemical energy storage devices for wearables, with a focus on incorporation of shape-conformable materials into mechanically robust designs that can be worn on the human body. In this review, we highlight the quantified performances of reported wearable electrochemical energy storage devices, as well as their micro-sized counterparts under specific mechanical deformations, which can be used as the benchmark for future studies in this field. A general introduction to the wearable technology, the development of the selection and synthesis of active materials, cell design approaches and device fabrications are discussed. It is followed by challenges and outlook toward the practical use of electrochemical energy storage devices for wearable applications.

Metal Oxide and Hydroxide-Based Aqueous Supercapacitors: From Charge Storage Mechanisms and Functional Electrode Engineering to Need-Tailored Devices

Energy storage devices that efficiently use energy, in particular renewable energy, are being actively pursued. Aqueous redox supercapacitors, which operate in high ionic conductivity and environmentally friendly aqueous electrolytes, storing and releasing high amounts of charge with rapid response rate and long cycling life, are emerging as a solution for energy storage applications. At the core of these devices, electrode materials and their assembling into rational configurations are the main factors governing the charge storage properties of supercapacitors. Redox-active metal compounds, particularly oxides and hydroxides that store charge via reversible valence change redox reactions with electrolyte ions, are prospective candidates to optimize the electrochemical performance of supercapacitors. To address this target, collaborative investigations, addressing different streams, from fundamental charge storage mechanisms and electrode materials engineering to need-tailored device assemblies, are the key. Over the last few years, significant achievements in metal oxide and hydroxide-based aqueous supercapacitors have been reported. This work discusses the most recent achievements and trends in this field and brings into the spotlight the authors' viewpoints.

Stretchable, transparent and imperceptible supercapacitors based on Au@MnO2 nanomesh electrodes

A wearable supercapacitor achieved high transparency of 82.1% and an excellent areal capacitance of 0.53 mF cm⁻², together with high stretchability (160% strain).

Automated Scalpel Patterning of Solution Processed Thin Films for Fabrication of Transparent MXene Microsupercapacitors

A simple and generic strategy is proposed to pattern thin films deposited by a solution processable route. A soft approach based on an automated scalpel technique is developed to engrave thin films in a single step for sculpting functional planar devices. MXenes-the emerging family of 2D transition metal carbides and nitrides-combine metallic conductivity and hydrophilicity, enabling solution processing of transparent conducting electrodes (TCEs) under ambient conditions. Scalable dip coating is employed to process titanium carbide, Ti₃ C₂ , MXene thin films with excellent optoelectronic properties, achieving electrical Figure of merit up to 14. Automated scalpel engraving is adopted to fabricate transparent and semi-transparent MXene microsupercapacitors in a single step, hitherto not reported. Combining TCE and pseudocapacitive characteristics, MXene devices show excellent capacitive storage capabilities at high rates, without the aid of external metal current collectors. This technique allows for maskless patterning of solution processed thin films without losing intrinsic physicochemical properties and can be extended to fabricate heterostructured hybrid devices out of solution processable materials.

Patterning Islandlike MnO2 Arrays by Breath-Figure Templates for Flexible Transparent Supercapacitors

Although plenty of active materials could be used as supercapacitor electrodes, only limited ones have been engineered to construct transparent supercapacitors. Specially, it is a great challenge to make opaque metal oxides, which often own high energy density, into transparent films. Here, we demonstrate a novel approach to fabricate transparent MnO₂ films for flexible transparent supercapacitors. By utilizing breath-figure polymer films with ordered pores as template, arrays of MnO₂ islands were electrochemically deposited, with high light transmission. The thickness and interspace distance of MnO₂ island arrays could be adjusted by tuning deposition time so that the capacitance and transparency of the electrodes are changed accordingly. Such island array structure can effectively eliminate the internal stress existing in the composite film to avoid cracks during bending operation. The assembled transparent supercapacitor shows a transmittance of 44% at 550 nm and can yield a high capacitance of 4.73 mF/cm² at a current density of 50 μA/cm², demonstrating high flexibility and stability.