Notably, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years, related to theoretical understanding, material synthesis and device designs. Herein, we summarize the state-of-the-art progress toward mechanisms, new materials, and novel device designs for supercapacitors. Firstly, fundamental understanding of the mechanism is mainly focused on the relationship between the structural properties of electrode materials and their electrochemical performances based on some in situ characterization techniques and simulations. Secondly, some emerging electrode materials are discussed, including metal–organic frameworks (MOFs), covalent organic frameworks (COFs), MXenes, metal nitrides, black phosphorus, LaMnO3, and RbAg4I5/graphite. Thirdly, the device innovations for the next generation of supercapacitors are provided successively, mainly emphasizing flow supercapacitors, alternating current (AC) line-filtering supercapacitors, redox electrolyte enhanced supercapacitors, metal ion hybrid supercapacitors, micro-supercapacitors (fiber, plane and three-dimensional) and multifunctional supercapacitors including electrochromic supercapacitors, self-healing supercapacitors, piezoelectric supercapacitors, shape-memory supercapacitors, thermal self-protective supercapacitors, thermal self-charging supercapacitors, and photo self-charging supercapacitors. Finally, the future developments and key technical challenges are highlighted regarding further research in this thriving field.

Latest advances in supercapacitors: from new electrode materials to novel device designs.

Reactive oxygen species (ROS) play an essential role in regulating various physiological functions of living organisms. The intrinsic biochemical properties of ROS, which underlie the mechanisms ne...

Reactive Oxygen Species (ROS)-Based Nanomedicine.

Electrocatalytic oxygen evolution reaction for energy conversion and storage: A comprehensive review

Recent advancements in supercapacitor technology

Metals and metal oxides are widely used as catalysts for materials production, clean energy generation and storage, and many other important industrial processes. However, metal-based catalysts suffer from high cost, low selectivity, poor durability, susceptibility to gas poisoning and have a detrimental environmental impact. In 2009, a new class of catalyst based on earth-abundant carbon materials was discovered as an efficient, low-cost, metal-free alternative to platinum for oxygen reduction in fuel cells. Since then, tremendous progress has been made, and carbon-based metal-free catalysts have been demonstrated to be effective for an increasing number of catalytic processes. This Review provides a critical overview of this rapidly developing field, including the molecular design of efficient carbon-based metal-free catalysts, with special emphasis on heteroatom-doped carbon nanotubes and graphene. We also discuss recent advances in the development of carbon-based metal-free catalysts for clean energy conversion and storage, environmental protection and important industrial production, and outline the key challenges and future opportunities in this exciting field. Reducing or even eliminating the need for precious-metal catalysts is crucial for the commercialization of clean energy technologies and various important industrial processes. Carbon materials have recently been shown to be cost-effective and efficient metal-free catalysts in clean energy generation and storage, environmental protection and chemical production.

Carbon-based metal-free catalysts

The high specific surface area and the excellent mechanical, electrical, optical and thermal properties of graphene make it an attractive component for high-performance stimuli-responsive or ‘smart’ materials. Complementary to these inherent properties, functionalization or hybridization can substantially improve the performance of these materials. Typical graphene-based smart materials include mechanically exfoliated perfect graphene, chemical vapour deposited high-quality graphene, chemically modified graphene (for example, graphene oxide and reduced graphene oxide) and their macroscopic assemblies or composites. These materials are sensitive to a range of stimuli, including gas molecules or biomolecules, pH value, mechanical strain, electrical field, and thermal or optical excitation. In this Review, we outline different graphene-based smart materials and their potential applications in actuators, chemical or strain sensors, self-healing materials, photothermal therapy and controlled drug delivery. We also introduce the working mechanisms of graphene-based smart materials and discuss the challenges facing the realization of their practical applications. Graphene and its macroscopic assemblies and composites are currently enabling a range of high-performance ‘smart’ materials that are responsive to various stimuli. In this Review, different graphene-based smart materials are described, along with their potential applications in actuators, chemical or strain sensors, self-healing materials, photothermal therapy and controlled drug delivery.

Graphene-based smart materials

Water oxidation to evolve oxygen is the key step in water splitting and is related to a variety of energy systems. Here, we report a facile electrodeposition process to immobilize nickel–iron layered double hydroxide (Ni–Fe LDH) nanoplates on three-dimensional electrochemically reduced graphene oxide (3D-ErGO) for water oxidation. This Ni–Fe LDH/3D-ErGO electrode has a three-dimensional interpenetrating network with Ni–Fe nanoplates uniformly decorated on graphene sheets. It has an electrochemically active surface area (EASA) 3.3 times that of conventional planar electrodes. The open porous structure of this electrode also makes its EASA fully accessible to the electrolyte for water oxidation and easy release of oxygen gas. This electrode can be directly used for catalysing the oxygen evolution reaction (OER) in alkaline media without using a binder and conductive additive, exhibiting a small overpotential of 0.259 V and a low Tafel slope of 39 mV dec−1. It outperforms the precious IrO2 catalyst in activity, kinetics, and electrochemical stability.

A high-performance three-dimensional Ni–Fe layered double hydroxide/graphene electrode for water oxidation

Reducing the energy consumption of a hydrogen evolution reaction (HER) at a platinum (Pt) electrode is important for the hydrogen economy. Herein, we report the loading of alpha- or beta-nickel hydroxide (α- or β-Ni(OH)2) nanostructures on the surface of a Pt electrode to improve its catalytic activity and stability for HER in alkaline electrolytes. Both experimental and theoretical studies reveal that β-Ni(OH)2 is a better co-catalyst of Pt than α-Ni(OH)2 for promoting the HER, attributed to the higher water dissociation ability of β-Ni(OH)2, as well as the stronger interactions between β-Ni(OH)2 and the Pt electrode. Particularly, the overpotential of the HER in 0.1 M KOH at 10 mA cm–2 is decreased from 278 mV at the Pt electrode to 92 mV at the β-Ni(OH)2/Pt electrode, and the Tafel slope decreased from 62 to 42 mV dec–1, correspondingly. The performance of the β-Ni(OH)2/Pt catalytic electrode surpasses that of most of the previously reported electrodes for the same purpose.

Hydrogen Evolution Reaction in Alkaline Media: Alpha- or Beta-Nickel Hydroxide on the Surface of Platinum?

The oxidation of water to produce oxygen gas is related to a variety of energy storage systems. Thus, the development of efficient, cheap, durable, and scalable electrocatalysts for oxygen evolution reaction (OER) is of great importance. Here, a high-performance OER catalyst, nitrogen and sulfur codoped graphite foam (NSGF) is reported. This NSGF is prepared from commercial graphite foil and directly applied as an electrocatalytic electrode without using a current collector and a polymeric binder. It exhibits an extremely low overpotential of 0.380 V to reach a current density of 10 mA cm−2 and shows fast kinetics with a small Tafel slope of 96 mV dec−1 in 0.1 m KOH. This electrocatalytic performance is superior or comparable to those of previously reported metal-free OER catalysts.

Nitrogen and Sulfur Codoped Graphite Foam as a Self‐Supported Metal‐Free Electrocatalytic Electrode for Water Oxidation

Alternating current (AC) line-filters are widely used to attenuate the leftover AC ripples in line-powered devices. However, the commercialized aluminum electrolytic capacitors (AECs) have low specific capacitances, making them usually the largest components in the electronic circuits of miniaturized, portable and/or flexible electronics. Herein, we report a scalable wet-process to fabricate an electrochemical capacitor (EC) using sulfuric acid treated commercially available poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (AT-PEDOT:PSS) as an electrode material and graphite foil as the current collector. It exhibited high areal (994 μF cm−2) and volumetric (16.6 F cm−3) specific capacitances at 120 Hz, ultrahigh-rate frequency response (phase angle = −83.6° at 120 Hz) with a short resistor–capacitor time constant of 0.15 ms, and an excellent electrochemical stability. Therefore, it is promising to replace AECs for AC line-filtering, and their performance is superior to those of the state-of-the-art ECs for this purpose.

Xiaowen Yu

Papers

Graphene-based smart materials

A high-performance three-dimensional Ni–Fe layered double hydroxide/graphene electrode for water oxidation

Hydrogen Evolution Reaction in Alkaline Media: Alpha- or Beta-Nickel Hydroxide on the Surface of Platinum?

Nitrogen and Sulfur Codoped Graphite Foam as a Self‐Supported Metal‐Free Electrocatalytic Electrode for Water Oxidation

An ultrahigh-rate electrochemical capacitor based on solution-processed highly conductive PEDOT:PSS films for AC line-filtering