Elsevier

Energy Storage Materials

Volume 26, April 2020, Pages 1-22
Energy Storage Materials

Metal-organic frameworks derived porous carbon, metal oxides and metal sulfides-based compounds for supercapacitors application

https://doi.org/10.1016/j.ensm.2019.12.019Get rights and content

Abstract

The energy storage field has witnessed a dramatic expansion in terms of short charging time of supercapacitors, especially in the highly active electrode materials. Metal-organic frameworks (MOFs) materials have been extensively applied as precursors or templates for the synthesis of carbon, metal oxides/sulfides-based compounds as high-performance electrodes in supercapacitors. The MOFs-derived materials can be broadly divided into two groups: MOFs-derived porous carbon and MOFs-derived metal oxide/sulfide compounds. These MOFs-derived materials are characterized by unique porous structures, controllable structures, high surface area, good conductivity (MOFs-derived carbon) and outstanding electrochemical stability, meeting the requirements of the desirable specific capacitance and long-term cyclic performance of both electrochemical double layer capacitors and pseudocapacitors. However, MOFs-derived materials still suffer from the complex synthesis process and high production cost of MOFs precursors, difficulties in mass production, and unstable structure in highly corrosive electrolyte. In addition, the electrical conductivity of MOFs-derived metal oxides/sulfides needs to be further improved. In this review, we summarize the recent progresses in the development of MOFs-derived porous carbons and metal oxide/sulfide compounds and their applications in supercapacitors. The prospects and challenges of MOFs-derived carbon and metal oxide/sulfide compounds as highly effective and durable electrodes in supercapacitors are discussed.

Introduction

With the rapid expansion of the world’s economy, technology and industry, the demand for energy resources has continuously escalated and outpaced its growth rate, arousing worldwide attention on the development of renewable and sustainable energy and the protection of ecological environment. Serious issues associated with the excess use of conventional fossil fuels, such as increased greenhouse gas emission and ozone layer depletion, could only be resolved through the exploitation of renewable energy sources. Thus, it is critical to explore and develop clean, safe, and sustainable energy storage and conversion technologies in the future renewable energy based society. There have been substantial research activities in the development of a variety of energy generating technologies such as fuel cells and solar cells as well as energy storage technologies including water electrolysis, lithium ion batteries (LIBs), vanadium redox flow batteries and supercapacitors [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]].

Supercapacitors, also known as ultracapacitors or electrochemical capacitors, store electric energy on conducting materials in the form of electrical charges [[11], [12], [13]]. Based on the mechanism of charge storage, supercapacitors can be normally divided into two categories: electrochemical double layer capacitors (EDLCs) and pseudocapacitors. EDLCs store energy by electrostatic adsorption at the interface between electrode and electrolyte. EDLCs store energy with the formation of a double layer of electrolyte ions on the surface of conductive electrodes. Not restricted by the electrochemical charge transfer kinetics as found in traditional LIBs, EDLCs can thus operate at very high discharge and charge rates with lifetimes of over several million cycles. However, as compared to the energy that can be stored by LIBs, the energy stored by the state-of-the-art EDLCs is lower by an order of magnitude, limiting their widespread applications that require a high power density. In addition, the currently available electrode materials for supercapacitors, primarily based on porous activated carbon (AC), have energy density of about 4 to 5 ​W ​h ​kg−1, whereas that of lead acid batteries ranges from 26 to 34 ​W ​h ​kg−1 [14]. It remains difficult for EDLCs to achieve extensive applications due to their relatively high effective series resistance and low energy storage density. On the other hand, pseudocapacitors store charge through fast and reversible surface redox reactions. Compared with EDLCs, high-rate pseudocapacitors can offer higher power density and energy density. However, due to the presence of Faradaic processes, pseudocapacitors often suffer from relatively poor loops and partially irreversible reactions.

Porous carbons (PCs) are currently regarded as one of the most representative candidates in the selection of electrodes for EDLCs. Compared to the traditional carbonaceous electrode, PCs possess large surface areas, high conductivity, good chemical stability in acidic/alkaline environments, controllable pore structure through the synthesis steps and low cost for scale-up production [[15], [16], [17]]. The surface area along with the pore size distribution determine the electrochemical performance of PCs for use as supercapacitor electrodes [[18], [19], [20], [21]]. Therefore, various methods have been adopted and developed for the preparation of PCs to obtain the unique and desirable pore structures, including solid-state reaction, wet chemical approach [[22], [23], [24]], transformation from carbide precursors [[25], [26], [27]] and natural plants/biomass [28,29], and hard or soft template method [[30], [31], [32], [33]]. For instance, an ordered mesoporous carbon material was fabricated by the template method with carbon based CMK-3, showing long straight channels and a higher specific capacitance [34]. It has been reported that PCs with ultrahigh surface area of approximately 1200 ​m2 ​g−1 can be facilely prepared by poly (vinylidene chloride) carbonization at high temperatures with no activation or other additional processes, displaying high gravimetric capacitance (262 ​F ​g−1) as well as high electrode density [35].

In the case of pseudocapacitors, transition-metal oxides, for instance, amorphous RuO2 and IrO2 [[36], [37], [38], [39]], are known to exhibit outstanding properties. Since pseudocapacitance is primarily attributed to the surface redox reactions of active materials, nanosized and nanostructured electrode materials, which possess an increased number of active sites, have been in widespread use to improve the pseudocapacitance. For example, ultrathin mesoporous NiCo2O4 nanosheets were reported to have a high specific capacitance of 1450 ​F ​g−1 ​at a high current density of 20 ​A ​g−1 [40]. Lee and co-workers reported that NiCo2O4/3D graphene foam exhibited better electrochemical properties than the individual component, which is due to the synergistic contributions from the graphene substrate with high surface area and good electrical conductivity, as well as the considerable activity of NiCo2O4 with a flower-like morphology [41]. Our group also showed that NiO nanoparticles (NPs) supported on functionalized carbon nanotubes are highly active electrodes for supercapacitors [42].

Metal–organic frameworks (MOFs), also named as porous coordination polymers (PCPs), are a class of compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. Since their first discovery more than 20 years ago, MOFs have attracted significant research interests and attention in various applications owing to their high specific surface area, satisfactory porosity, tunable morphology and multifunctionalities [[43], [44], [45]]. Of importance, MOFs are considered as bifunctional materials with both sacrificial templates and metal precursors, and can thus play an important role in the construction of hollow micro/nanostructured materials with internal voids and functional shells [46]. MOFs with various characteristics of unique particle shapes, exceptional porosities, and surface functionalities can be synthesized by combining organic and inorganic constituents [47]. Up to now, hundreds or thousands of MOF materials have been synthesized and the number continues to grow [48], such as MIL-88B, ZIF-8, MOF-74, MOF-5, ZIF-78, ZIF-67 and Al-based porous coordination polymers (Al-PCPs) [[49], [50], [51], [52], [53], [54], [55]]. For these reasons, MOFs have been investigated as ideal materials or templates for the synthesis of PCs and metal compounds as electrode materials for batteries or supercapacitors. However, direct application of pristine MOFs as electrodes of supercapacitors is considered to be unsuitable due to their generally poor electrical conductivity. Thus, pyrolysis at high temperatures is one of the most common methods to carbonize MOF materials to increase the conductivity properties [56]. With selective pyrolysis in a controlled atmosphere, MOFs can be transformed into PCs or metal compounds featuring unique nanostructures. Parent MOFs can be prepared by the combinations of organic and inorganic compositions via facile coordination chemistry with the control and manipulation of the surface area, pore volume and porous structure. Compared with other carbon-based materials synthesized using traditional methods, templates or precursors, MOFs-derived carbons have outstanding merits with respect to a facile preparation with inherent diversity offering accurate control of the physicochemical properties. In the past decade, the application of MOFs-derived materials in the field of electrochemical conversion and energy storage has developed into a fast-expanding research area. Several comprehensive reviews are available, focusing on the application of MOFs-related or derived materials in areas such as electrocatalysis, photocatalysis, heterogeneous catalysis, gas adsorption and energy storage devices [11,12,[57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68]].

Despite the significant achievements, the precise control over the morphologies of MOFs-derived materials still appears to be difficult because the induction force is in shortage during structural transformations at increased temperatures [69]. Although single or binary metal compound or metal compound-carbon hybrid materials derived from MOFs have been successfully fabricated, their electrochemical properties are still not appropriate for many applications. For the construction of active materials with high surface area suitable for electrochemical applications, it is desirable to achieve the direct synthesis of highly functionalized MOFs-derived materials with nanosize and the growth of MOFs-based nanostructures embedded into unique substrates [[70], [71], [72]]. In this review, we present the recent advances in the field of supercapacitors with MOFs-derived materials as electrodes, including different inorganic materials which are prepared using MOFs as templates or precursors. The prospect of the MOFs-derived functional materials for both existing and emerging energy storage technologies is also discussed.

Section snippets

Synthesis of MOF precursors

The synthesis of MOF materials with desirable properties and morphologies strongly depends on the selection of starting materials (e.g., inorganic salts, organic ligands, and solvents) and reaction conditions (e.g., pH, temperature, concentration, and solvent polarity) [73]. To date, a wide range of synthetic approaches have been developed, including solvothermal, sonochemical, and mechanochemical processes, among many others.

In solvothermal synthesis process, solvents such as ethanol,

MOFs-derived porous carbon electrodes

According to the mechanism of EDLCs, the microporous structure plays a significant role in allowing the ion center to approach closer to the surface of a carbon electrode, resulting in enhanced capacitance. In addition, mesoporous structure can render an effective shuttle passage for ions to enter the narrow pores of the double-layer capacitor carbons while keeping their solvent shells unchanged. High surface area carbons originated from MOFs and MOFs-based composites have seen widespread

MOFs-derived porous metal oxides/sulfides-based electrodes

In the molecular structure of MOFs, metal centers are in coordination with organic ligands. The MOFs-derived metal compounds with unique structures can be prepared by controlled calcination under certain environments. Generally, metal compounds as pseudocapacitors have better energy density and electron capacity than conventional porous carbon materials. Ideal metal compound electrodes usually possess several requirements, such as good electrical conductivity, high porosity, large surface area,

MOFs-derived porous carbon/metallic compound hybrid materials

As mentioned above, the relatively low conductivity and severe agglomeration of MOFs-derived metal compound materials still have negative influence on the performance of supercapacitors. Thus, the introduction of highly active carbon-based additives with porous structures can promote the supercapacitor performance of MOF-derived electrodes (see Table 2). It is well known that graphene can act as a promising electrode material due to its outstanding electrochemical stability, high conductivity,

Conclusion and outlook

Recent advances of MOFs-derived carbon- and metal-based materials in supercapacitors applications have been reviewed. MOFs have become popular precursors or templates for the derivation of porous carbon, single/binary metal compounds or their composites due to the designable composition, tunable structure, controllable porosity, high surface area, and bifunctionality with both sacrificial templates and metal precursors. The unique porous structure as well as the highly active properties of the

Author contribution

Dr Yu Liu and Mr Xiaomin Xu write the draft, Prof San Ping Jiang and Prof Zongping Shao propose the content of review paper, read, and write the paper.

Declaration of competing interest

The authors of the manuscript titled “Metal-organic frameworks derived porous carbon and metal oxides-based compounds for supercapacitors application” declare no conflict of interest.

Acknowledgement

The work was supported by the Australian Research Council under the Discovery Project Scheme ((DP180100731 and DP180100568).

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