The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

As energy storage devices, supercapacitors that are also called electrochemical capacitors possess high power density, excellent reversibility and long cycle life. The recent boom in electronic devices with different functions in transparent LED displays, stretchable electronic systems and artificial skin has increased the demand for supercapacitors to move towards light, thin, integrated macro- and micro-devices with transparent, flexible, stretchable, compressible and/or wearable abilities. The successful fabrication of such supercapacitors depends mainly on the preparation of innovative electrode materials and the design of unconventional supercapacitor configurations. Tremendous research efforts have been recently made to design and construct innovative nanocarbon-based electrode materials and supercapacitors with unconventional configurations. We review here recent developments in supercapacitors from nanocarbon-based electrode materials to device configurations. The advances in nanocarbon-based electrode materials mainly include the assembly technologies of macroscopic nanostructured electrodes with different dimensions of carbon nanotubes/nanofibers, graphene, mesoporous carbon, activated carbon, and their composites. The electrodes with macroscopic nanostructured carbon-based materials overcome the issues of low conductivity, poor mechanical properties, and limited dimensions that are faced by conventional methods. The configurational design of advanced supercapacitor devices is presented with six types of unconventional supercapacitor devices: flexible, micro-, stretchable, compressible, transparent and fiber supercapacitors. Such supercapacitors display unique configurations and excellent electrochemical performance at different states such as bending, stretching, compressing and/or folding. For example, all-solid-state simplified supercapacitors that are based on nanostructured graphene composite paper are able to maintain 95% of the original capacity at a 180° folding state. The progress made so far will guide further developments in the structural design of nanocarbon-based electrode materials and the configurational diversity of supercapacitor devices. Future developments and prospects in the controllable assembly of macroscopic nanostructured electrodes and the innovation of unconventional supercapacitor configurations are also discussed. This should shed light on the R&D of supercapacitors.

Unconventional supercapacitors from nanocarbon-based electrode materials to device configurations

Electrochemical capacitors (best known as supercapacitors) are high‐performance energy storage devices featuring higher capacity than conventional capacitors and higher power densities than batteries, and are among the key enabling technologies of the clean energy future. This review focuses on performance enhancement of carbon‐based supercapacitors by doping other elements (heteroatoms) into the nanostructured carbon electrodes. The nanocarbon materials currently exist in all dimensionalities (from 0D quantum dots to 3D bulk materials) and show good stability and other properties in diverse electrode architectures. However, relatively low energy density and high manufacturing cost impede widespread commercial applications of nanocarbon‐based supercapacitors. Heteroatom doping into the carbon matrix is one of the most promising and versatile ways to enhance the device performance, yet the mechanisms of the doping effects still remain poorly understood. Here the effects of heteroatom doping by boron, nitrogen, sulfur, phosphorus, fluorine, chlorine, silicon, and functionalizing with oxygen on the elemental composition, structure, property, and performance relationships of nanocarbon electrodes are critically examined. The limitations of doping approaches are further discussed and guidelines for reporting the performance of heteroatom doped nanocarbon electrode‐based electrochemical capacitors are proposed. The current challenges and promising future directions for clean energy applications are discussed as well.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.202001239

Heteroatom-Doped and Oxygen-Functionalized Nanocarbons 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.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700887

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

Interconnected highly graphitic carbon nanosheets derived from wheat stalk as high performance anode materials for lithium ion batteries

Developing low‐cost and high‐efficiency catalysts for sustainable hydrogen production through electrocatalytic hydrogen evolution reaction (HER) is crucial yet remains challenging. Here, a strategy is proposed to fill Ni‐vacancy (Niv) sites of dual‐deficient NiO (D‐NiO‐Pt) deliberately created by Ar plasma with homogeneously distributed Pt atoms driven by oxygen vacancies (Ov). The incorporated Pt atoms filling the Niv reduce the formation energy to increase crystal stability, and subsequently combine with additional Ov to tune the electronic structure of the surrounding Ni sites. Thus, a more ideal hydrogen adsorption free energy (ΔGH*) closer to 0 of Ni sites and Pt sites can be achieved. As a result, the D‐NiO‐Pt electrode achieves superior mass activity of ≈1600 mA mg−1 (normalized by platinum) and nearly negligible loss of activity during long‐term operation, which is much better than as‐prepared Pt‐containing NiO catalysts without plasma treatment. A low overpotential of 20 mV is required for the D‐NiO‐Pt at 10 mA cm−2 in alkaline HER, outperforming that of the commercial Pt/C. In addition, the universal access to the other Ni‐based compounds including nickel phosphide (Ni2P), nickel sulfide (Ni0.96S), and nickel selenide (NiSe2) is also demonstrated by employing a vacancy‐driven Pt filling mechanism.

Atomic‐Level Platinum Filling into Ni‐Vacancies of Dual‐Deficient NiO for Boosting Electrocatalytic Hydrogen Evolution

A high-performance supercapacitor was successfully developed using graphene-based 3D hybrid nanostructured electrodes. The 3D hybrid nanostructure, consisting of vertically oriented few-layer graphene (VFG) grown on an alveolate Pt film with high-density nanocups, was synthesized by a simple and efficient “one-step” method. The 3D VFG-nanocup hybrid structured electrodes showed a high specific surface area for ion transmission and storage, which contributes to the enhancement of areal capacitance by accommodating more charges in a given footprint area than that of conventional plane-structured electrodes. Electrochemistry results indicate that the 3D VFG-nanocup hybrid structured electrodes exhibit a high specific capacitance up to 1052 μF cm−2 (three times that of the VFG-plane, at approximately 337 μF cm−2), and a good cycling stability with about 93% capacitance retention after 3000 cycles. These easily fabricated, high-performance 3D hybrid nanostructured electrodes offer great promise in energy storage device applications.

Vertically oriented few-layer graphene-nanocup hybrid structured electrodes for high-performance supercapacitors

Corrosion behavior of stainless steel-tungsten carbide joints brazed with AgCuX (X = In, Ti) alloys

Pristine graphene with a 3D structure is desired for use in graphene-based supercapacitors, yet the very poor wettability of such graphene in water has limited its practical application. Here we report a way to simultaneously realize the 3D structure and good wettability in vertically-oriented few-layered graphene (VFG) grown by plasma-enhanced chemical vapor deposition. Based on scanning and transmission electron microscopic, Raman spectroscopic, contact angle (CA) and electrochemical analyses, a mechanism to explain the improved performance of VFG-based supercapacitors by defect-stimulated increases in wettability is proposed. The CA of our VFG samples notably reduces from 131° to 73° as the content of surface defects increases, which confirms that the wettability of VFG is markedly improved with an increased density of surface defects. Electrochemical results indicate that the VFG samples with a high density of defects exhibit high specific capacities of up to 704 μF cm−2 and a good cycling stability with about 91.2% capacitance retention after 3000 cycles. The excellent supercapacitor performance of the VFG samples with a high density of defects makes them attractive candidates for ultrathin high-performance supercapacitor electrodes.

Jing Huang Lin

Papers

Atomic‐Level Platinum Filling into Ni‐Vacancies of Dual‐Deficient NiO for Boosting Electrocatalytic Hydrogen Evolution

Vertically oriented few-layer graphene-nanocup hybrid structured electrodes for high-performance supercapacitors

Corrosion behavior of stainless steel-tungsten carbide joints brazed with AgCuX (X = In, Ti) alloys

A high-performance supercapacitor of vertically-oriented few-layered graphene with high-density defects

Low resistance VFG-Microporous hybrid Al-based electrodes for supercapacitors