The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish "pseudocapacitive" and "battery" materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery-type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

As modern society develops, the need for clean energy becomes increasingly important on a global scale. Because of this, the exploration of novel materials for energy storage and utilization is urgently needed to achieve low-carbon economy and sustainable development. Among these novel materials, metal–organic frameworks (MOFs), a class of porous materials, have gained increasing attention for utilization in energy storage and conversion systems because of ultra-high surface areas, controllable structures, large pore volumes and tunable porosities. In addition to pristine MOFs, MOF derivatives such as porous carbons and nanostructured metal oxides can also exhibit promising performances in energy storage and conversion applications. In this review, the latest progress and breakthrough in the application of MOF and MOF-derived materials for energy storage and conversion devices are summarized, including Li-based batteries (Li-ion, Li–S and Li–O2 batteries), Na-ion batteries, supercapacitors, solar cells and fuel cells.

Metal–Organic Frameworks (MOFs) and MOF-Derived Materials for Energy Storage and Conversion

Creating oxygen-vacancies in MoO3-x nanobelts toward high volumetric energy-density asymmetric supercapacitors with long lifespan

Supercapacitors (SCs) have attracted much attention as energy storage devices due to their high power density, fast charge/discharge capability, and long cycling life. The core/shell structure design of the electrocapacitive material is one of the effective ways to achieve large surface area and high conductivity for providing more faradaic reaction sites and accelerating the charge transfer, respectively, and therefore to enhance the electrocapacitive performance of SCs. To better understand the core/shell structure, this review paper compares the material category, morphology, and synthesis methods for the core/shell structures as well as their electrochemical performances for the corresponding SCs. The electroactive materials applied in the core/shell structure include carbon materials, conducting polymers, metals, metal hydroxides, metal oxides and metal sulfides, while zero-dimensional, one-dimensional, two-dimensional, and three-dimensional structures are considered for the core/shell material. This review article outlines the most commonly used methods for making the core and shell materials over the past decade (2007–2018), and points out the most efficient combination of the material categories and morphologies for the core/shell structure. By understanding the details of the core/shell materials, more efficient design regarding the choices of material category and morphology can be achieved, and therefore better electrocapacitive performance for the resulting SCs can be realized.

A review of electrode materials based on core–shell nanostructures for electrochemical supercapacitors

Capacitors began their journey in 1745, and to date have advanced in the form of supercapacitors. Supercapacitors are one of the advanced forms of capacitors with higher energy density, bridging capacitors and batteries. The energy storage through the formation of an electrical double layer is pivotal for supercapacitor technology. Accordingly, to further improve the energy density, surface faradaic (pseudocapacitive) processes are employed, and henceforth, the journey of chemical supercapacitors commenced. Herein, the materials, mechanisms and fabrication of chemical supercapacitors based on metallic compounds and conducting polymers are discussed in detail. The inherent limitations of these materials are addressed, and the feasible mitigation measures are identified. Poor conductivity, slow diffusion kinetics and rapid structural disintegration over cycling are the common constraints of metallic compounds, which can be overcome by preparing conductive nanocomposites. Thus, versatile conductive nanocomposites of metal oxides, hydroxides, carbides, nitrides, phosphides, phosphates, phosphites, and chalcogenides are elaborated. A lack of structural integrity is the prime obstacle for the realization of conducting polymer-based supercapacitors, which may be solved by forming composites with robust support from carbonaceous materials or metallic compounds. Consequently, the composites of polyaniline, polypyrrole, polythiophene and polythiophene-derivatives are discussed. The historical accounts of early stages of works are emphasised in order to review the developmental pathways of chemical supercapacitors. The construction of full cells and their performance data are presented herein, which synchronize the behaviour of practical scaled-up devices. To the best of our knowledge, this review is the first holistic description of chemical supercapacitors based on metallic compounds and conducting polymers from the first reports to recent advancements.

Chemical supercapacitors: a review focusing on metallic compounds and conducting polymers

In recent years, the electrochemical properties of supercapacitors have been greatly improved due to continuous improvement in their composite materials. In this study, an urchin-like MgCo2O4@PPy/NF (MgCo2O4@polypyrrole/Ni foam) core–shell structure composite material was successfully developed as an electrode for supercapacitors. The MCP-2 composite material, obtained by a hydrothermal method and in situ chemical oxidative polymerization, shows a high specific capacitance of 1079.6 F g−1 at a current density of 1 A g−1, which is much higher than that of MC (783.6 F g−1) under the same conditions. Simultaneously, it has low resistance and an excellent cycling stability of 97.4% after 1000 cycles. Furthermore, an all-solid-state asymmetric supercapacitor (ASC) was assembled using MCP-2 as the positive electrode and activated carbon (AC) as the negative electrode. The MCP-2//AC ASC exhibits high specific capacitance (94 F g−1 at a current density of 0.4 A g−1), high energy density (33.4 W h kg−1 at a power density of 320 W kg−1), high volumetric energy density (17.18 mW h cm−3 at a volumetric power density of 0.16 W cm−3) and excellent cycling stability (retaining 91% of the initial value after 10 000 cycles). Simultaneously, the device has low leakage current and excellent self-discharge characteristics. All these results indicate that the MCP-2//AC ASC is a good energy storage device; it can support the function of two LEDs for 20 minutes. These results indicate that the MCP-2//AC ASC will play an important role in energy structures in the future.

An urchin-like MgCo2O4@PPy core-shell composite grown on Ni foam for a high-performance all-solid-state asymmetric supercapacitor.

PVP-assisted growth of Ni-Co oxide on N-doped reduced graphene oxide with enhanced pseudocapacitive behavior

Zeolitic imidazolate frameworks have stimulated great attention due to their potential applications in energy storage, catalysis, gas sensing, drug delivery etc. In this paper, the three-dimensional porous nanomaterial Co3O4/ZnCo2O4/CuO with hollow polyhedral nanocage structures and highly enhanced electrochemical performances was synthesized successfully by a zeolitic imidazolate framework-67 route. The composites hold the shape of the ZIF-67 templates well and the shell has multiple compositions. In the process, we first synthesized the nanostructure hydroxide precursors and then transformed them into the corresponding metal oxide composites by thermal annealing in air. In addition, the mass ratio of Zn to Cu in this material is discussed and optimized. We found that when the mass ratio is 3, the composite material has better electrochemical properties. When applied as an electrode material, Co3O4/ZnCo2O4/CuO-1 shows enhanced pseudocapacitive properties and good cycling stability compared with Co3O4/ZnCo2O4, Co3O4/CuO and Co3O4/ZnCo2O4/CuO-2, and Co3O4/ZnCo2O4/CuO-3. The assembled Co3O4/ZnCo2O4/CuO-1//AC hybrid device can be reversibly cycled in a large potential range of 0–1.6 V and can deliver a high energy density of 35.82 W h kg−1 as well as the maximum power density of 4799.25 W kg−1.

Designed formation of Co3O4/ZnCo2O4/CuO hollow polyhedral nanocages derived from zeolitic imidazolate framework-67 for high-performance supercapacitors

In this paper, the rational design and synthesis of ZIF-8-derived ternary ZnO/ZnCo2O4/NiO wrapped by nanosheets is introduced. Polyhedral ternary ZnO/ZnCo2O4/NiO composites surrounded by nanosheets with different compositions are successfully fabricated through in situ growth on ZIF-8 templates and subsequent thermal annealing in air. Electrochemical investigation reveals that when the molar ratio of nickel nitrate to cobalt nitrate is 1, the composite material is more outstanding, which shows a high specific capacitance of 1136.4 F g-1 at 1 A g-1 and excellent cycling stability of 86.54% after 5000 cycles. Moreover, the excellent performance of this material is also confirmed by assembling an asymmetric supercapacitor. The assembled hybrid device can reach a large potential range of 0-1.6 V and deliver a high energy density of 46.04 W h kg-1 as well as the maximum power density of 7987.5 W kg-1.

Chengxiang Huang

Papers

An urchin-like MgCo2O4@PPy core-shell composite grown on Ni foam for a high-performance all-solid-state asymmetric supercapacitor.

PVP-assisted growth of Ni-Co oxide on N-doped reduced graphene oxide with enhanced pseudocapacitive behavior

Designed formation of Co3O4/ZnCo2O4/CuO hollow polyhedral nanocages derived from zeolitic imidazolate framework-67 for high-performance supercapacitors

In situ growth of ZIF-8-derived ternary ZnO/ZnCo2O4/NiO for high performance asymmetric supercapacitors.

Synthesis of polyaniline/nickel oxide/sulfonated graphene ternary composite for all-solid-state asymmetric supercapacitor