Ultrahigh-Power Pseudocapacitors Based on Ordered Porous Heterostructures of Electron-Correlated Oxides

Sputter-Deposited Binder-Free Nanopyramidal Cr/γ-Mo2N TFEs for High-Performance Supercapacitors

Due to their outstanding power density, long cycle life and low cost, supercapacitors have gained much interest. As for supercapacitor electrodes, molybdenum nitrides show promising potential. Molybdenum nitrides, however, are mainly prepared as nanopowders via a chemical route and require binders for the manufacture of electrodes. Such electrodes can impair the performance of supercapacitors. Herein, binder-free chromium (Cr)-doped molybdenum nitride (Mo₂N) TFEs having different Cr concentrations are prepared via a reactive co-sputtering technique. The Cr-doped Mo₂N films prepared have a cubic phase structure of γ-Mo₂N with a minor shift in the (111) plane. While un-doped Mo₂N films exhibit a spherical morphology, Cr-doped Mo₂N films demonstrate a clear pyramid-like surface morphology. The developed Cr-doped Mo₂N films contain 0-7.9 at.% of Cr in Mo₂N lattice. A supercapacitor using a Cr-doped Mo₂N electrode having the highest concentration of Cr reveals maximum areal capacity of 2780 mC/cm², which is much higher than that of an un-doped Mo₂N electrode (110 mC/cm²). Furthermore, the Cr-doped Mo₂N electrode demonstrates excellent cycling stability, achieving ~ 94.6% capacity retention for about 2000 cycles. The reactive co-sputtering proves to be a suitable technique for fabrication of binder-free TFEs for high-performance energy storage device applications.

Vertically Aligned and Ordered Arrays of 2D MCo2S4@Metal with Ultrafast Ion/Electron Transport for Thickness-Independent Pseudocapacitive Energy Storage

Pseudocapacitance holds great promise for energy density improvement of supercapacitors, but electrode materials show practical capacity far below theoretical values due to limited ion diffusion accessibility and/or low electron transferability. Herein, inducing two kinds of straight ion-movement channels and fast charge storage/delivery for enhanced reaction kinetics is proposed. Very thick electrodes consisting of vertically aligned and ordered arrays of NiCo₂S₄-nanoflake-covered slender nickel columns (NCs) are achieved via a scalable route. The vertical standing ∼5 nm ultrathin NiCo₂S₄ flakes build a porous covering with straight ion channels without the "dead volume", leading to thickness-independent capacity. Benefiting from the architecture acting as a "superhighway" for ultrafast ion/electron transport and providing a large surface area, high electrical conductivity, and abundant availability of electrochemical active sites, the NiCo₂S₄@NC-array electrode achieves a specific capacity up to 486.9 mAh g^-1. The electrode even can work with a high specific capacity of 150 mAh g^-1 at a very high current density of 100 A g^-1. In particular, due to the advanced structure features, the electrode exhibits excellent flexibility with a unexpected improvement of capacity when being largely bent and excellent cycling stability with an obvious resistance decrease after the cycles. An asymmetric pseudocapacitor applying the NiCo₂S₄@NC-array as a positive electrode achieves an energy density of 66.5 Wh kg^-1 at a power density of 400 W kg^-1, superior to the most reported values for asymmetric devices with NiCo₂S₄ electrodes. This work provides a scalable approach with mold-replication-like simplicity toward achieving thickness-independent electrodes with ultrafast ion/electron transport for energy storage.

Dual High-Conductivity Networks via Importing a Polymeric Gel Electrolyte into the Electrode Bulk

To obtain high-ionic-conductivity and high-electron-conductivity electrode materials, we design novel dual high-conductivity networks through importing a polymeric gel electrolyte into the electrode bulk by doping gold nanoparticles, which endows the membrane electrode with not only a high electron conductivity of 1.66 s·cm^-1 but also a high ionic conductivity of 2.7 × 10^-2 s·cm^-1, as well as a good surface area capacitance of 1098 mF·cm^-2 at 0.5 mA·cm^-2. The membrane electrode shows great mechanical strength, high flexibility, and tremendous stability, and the design concept based on dual conductive networks could be also applied to other electrode systems and other energy storage fields.

Enhancing Capacitance Performance of Ti3C2Tx MXene as Electrode Materials of Supercapacitor: From Controlled Preparation to Composite Structure Construction

Ti3C2Tx, a novel two-dimensional layer material, is widely used as electrode materials of supercapacitor due to its good metal conductivity, redox reaction active surface, and so on. However, there are many challenges to be addressed which impede Ti3C2Tx obtaining the ideal specific capacitance, such as restacking, re-crushing, and oxidation of titanium. Recently, many advances have been proposed to enhance capacitance performance of Ti3C2Tx. In this review, recent strategies for improving specific capacitance are summarized and compared, for example, film formation, surface modification, and composite method. Furthermore, in order to comprehend the mechanism of those efforts, this review analyzes the energy storage performance in different electrolytes and influencing factors. This review is expected to predict redouble research direction of Ti3C2Tx materials in supercapacitors.

Heterostructural Graphene Quantum Dot/MnO2 Nanosheets toward High-Potential Window Electrodes for High-Performance Supercapacitors

The potential window of aqueous supercapacitors is limited by the theoretical value (≈1.23 V) and is usually lower than ≈1 V, which hinders further improvements for energy density. Here, a simple and scalable method is developed to fabricate unique graphene quantum dot (GQD)/MnO2 heterostructural electrodes to extend the potential window to 0-1.3 V for high-performance aqueous supercapacitor. The GQD/MnO2 heterostructural electrode is fabricated by GQDs in situ formed on the surface of MnO2 nanosheet arrays with good interface bonding by the formation of Mn-O-C bonds. Further, it is interesting to find that the potential window can be extended to 1.3 V by a potential drop in the built-in electric field of the GQD/MnO2 heterostructural region. Additionally, the specific capacitance up to 1170 F g-1 at a scan rate of 5 mV s-1 (1094 F g-1 at 0-1 V) and cycle performance (92.7%@10 000 cycles) between 0 and 1.3 V are observed. A 2.3 V aqueous GQD/MnO2-3//nitrogen-doped graphene ASC is assembled, which exhibits the high energy density of 118 Wh kg-1 at the power density of 923 W kg-1. This work opens new opportunities for developing high-voltage aqueous supercapacitors using in situ formed heterostructures to further increase energy density.

Three-Dimensional Binder-Free Nanoarchitectures for Advanced Pseudocapacitors

The ever-increasing energy demands for electrification of transportation and powering of portable electronics are driving the pursuit of energy-storage technologies beyond the current horizon. Pseudocapacitors have emerged as one of the favored contenders to fill in this technology gap, owing to their potential to deliver both high power and energy densities. The high specific capacitance of pseudocapacitive materials is rooted in the various available oxidation states for fast surface or near-surface redox charge transfer. However, the practical implementation of pseudocapacitors is plagued by the insulating nature of most pseudocapacitive materials. The wealth of the research dedicated to addressing these critical issues has grown exponentially in the past decade. Here, we briefly survey the current progress in the development of pseudocapacitive electrodes with a focus on the discussion of the recent most exciting advances in the design of three-dimensional binder-free nanoarchitectures, including porous metal/graphene-based electrodes, as well as metal-atom/ion-doping-enhanced systems, for advanced supercapacitors with comparable energy density to batteries, and high power density.

Recent Advances in Designing and Fabricating Self-Supported Nanoelectrodes for Supercapacitors

Owing to the outstanding advantages as electrical energy storage system, supercapacitors have attracted tremendous research interests over the past decade. Current research efforts are being devoted to improve the energy storage capabilities of supercapacitors through either discovering novel electroactive materials or nanostructuring existing electroactive materials. From the device point of view, the energy storage performance of supercapacitor not only depends on the electroactive materials themselves, but importantly, relies on the structure of electrode whether it allows the electroactive materials to reach their full potentials for energy storage. With respect to utilizing nanostructured electroactive materials, the key issue is to retain all advantages of the nanoscale features for supercapacitors when being assembled into electrodes and the following devices. Rational design and fabrication of self-supported nanoelectrodes is therefore considered as the most promising strategy to address this challenge. In this review, we summarize the recent advances in designing and fabricating self-supported nanoelectrodes for supercapacitors towards high energy storage capability. Self-supported homogeneous and heterogeneous nanoelectrodes in the forms of one-dimensional (1D) nanoarrays, two-dimensional (2D) nanoarrays, and three-dimensional (3D) nanoporous architectures are introduced with their representative results presented. The challenges and perspectives in this field are also discussed.