Application of metal-organic framework materials in supercapacitors

Supercapacitors, distinguished by their rapid charge-discharge capabilities, prolonged operational lifetimes, and eco-friendly nature, offer marked improvements over conventional energy storage technologies like batteries and fuel cells in the domain of electrochemical energy storage. The extensive interest in supercapacitors, particularly when augmented with metal-organic frameworks, is evidenced by a surge in academic literature, totaling over 2000 publications since the turn of the century, with nearly 350 studies published in 2023 alone, accumulating citations surpassing 73,802 instances (Fig. 1). This bibliometric trend underscores the escalating focus on MOFs in advancing energy storage and conversion technologies [1], reflecting academia’s pressing quest for next-generation, high-performance energy storage materials. The escalating global energy demand and the intermittent nature of renewable energy sources necessitate alternatives to conventional storage systems, positioning supercapacitors as a pivotal innovation. Their multifaceted applications span sectors from electric vehicles, where they augment battery systems to enhance vehicle acceleration and driving range [2], to renewable energy storage, playing a crucial role in stabilizing grid operations by matching supply and demand fluctuations. Furthermore, in smart grids, supercapacitors function as dynamic regulators, ensuring grid efficiency and reliability [2]. The ongoing exploration into supercapacitors confronts challenges, primarily centered around boosting energy and power densities. However, ongoing technological advancements are anticipated to overcome these hurdles, solidifying supercapacitors’ position as a cornerstone of future energy storage infrastructure, thereby contributing to a sustainable energy landscape for humanity. The integration of MOFs, with their tunable structures and vast surface areas, holds the key to unlocking these advancements, offering a pathway towards higher storage capacities and more efficient energy utilization.

MOFs are a class of porous crystalline materials that are composed of metal ions or clusters linked with organic ligands [3,4]. Characterized by their high specific surface area, tunable pore structure and abundant surface functional groups, MOFs demonstrate substantial potential for applications in energy storage, particularly in the field of supercapacitors [5]. MOF materials boast an exceptionally high specific surface area, furnishing a multitude of active sites for charge storage and release [6]. This attribute renders MOFs as prime contenders for electrode materials in supercapacitors, with the potential to markedly enhance their energy density. The active sites of MOFs are specific regions in their structures that can participate in chemical reactions or adsorption processes. For instance, the metal ions in MOF materials can have synergistic effects with organic molecules or electrons, making MOF materials better in catalysis, energy storage, and adsorption [170,171]. At present, the organic ligands in MOF materials produce some specific functional groups (e.g., carboxyl groups, etc.), which can react with other molecules through covalent bonds such as Van Der Waals forces, hydrogen bonds, and other covalent bonds. In some situations, defects or irregular structures of MOF materials can expose their metal ions or organic ligands, thus increasing the active sites [172]. Moreover, their pore structure is amenable to deliberate design and regulation, enabling the optimization of ion transport [5], This feature translates to improved charging and discharging rates, as well as superior cycling stability for supercapacitors [4]. Beyond their role in supercapacitors, MOF materials present a broad spectrum of application opportunities. Notably, their innate porosity positions them as key players in gas adsorption. The highly tunable pore architecture and surface functional group [7] of MOFs empower them to achieve efficient adsorption and separation of specific gases [8]. Such capabilities are particularly valuable in applications concerning gas storage, separation, and capture [8,9]. Zhou, et al. [6] proposed the development of a material capable of separating impurity propyne from propylene by comprehensively tuning the pore properties of MOF, and the material, they suggested, should possess high capacity and selectivity as well as thermal stability [10]. To achieve this, they utilized the tridentate ligand tris (pyridin-4-yl) amine (Tripa) and the metal ion TiF₆²⁻ in the synthesis of a MOF material known as ZNU-2 (where ZNU stands for Zhejiang Normal University). As observed in Fig. 2(a-d), ZNU-2 has a structure of cage-like pores combined with narrow channels, and this structure can provide a larger pore capacity and richer interaction sites for benchmark propargyl recovery and propylene purification, which is evident that the structure of the MOF material has an important influence on its performance. These characteristics are crucial for achieving the desired benchmarks in propyne recovery and propylene purification. The MOF material’s ability to meet these benchmarks is attributed to its architectural features that facilitate efficient separation processes. In addition, the MOF material can be used as a carrier for catalysts to achieve selective adsorption and catalytic conversion of reactants by embedding active species within its pores, which has important potential for catalytic applications [11]. Huang, et al. [12] used MOF-derived Cu-embedded N-doped mesoporous carbon as a carrier for PdAu nanocatalysts for ethanol fuel cell oxidation (Fig. 2 (e-h)), and this work plays a significant role in promoting the application of metal-organic skeletons in direct ethanol fuel cells, as well as giving inspiration for the design of more efficient catalyst carriers. Zeolite imidazolium framework-8 (ZIF-8) offers considerable advantages in the preparation of carbon carrier templates due to characteristics such as large specific surface area and good porosity. The material increases the ionic transport rate of the catalyst, which further improves the catalytic efficiency. Bimetallenes are typically synthesized by ligand-constrained methods to maintain their ultrathin two-dimensional structures [173]. The synergistic effect of different metal elements in metallocenes enhances their electrocatalytic performance. Similarly, the synergistic effect of different metals in the bimetallic MOF could optimize its electrocatalytic performance. Typically, to further improve its performance, transition metal elements (Ni, Co, and Fe) are added during the synthesis of ZIF-8, as pointed out in Fig. 2 (i-k). Also, due to the high specific surface area and porosity, MOF can carry a significant amount of drug molecules, realizing slow release and targeted release of drugs, improving drug efficacy, and reducing toxic side effects. At present, the high specific surface area enables MOF to be an excellent carrier for biosensors, thus improving the sensitivity of detection. For instance, the sensor prepared using modified MOF material (PPy@NU-1000) was able to detect levodopa with high sensitivity and selectivity and remained steady in the presence of interfering substances [174], which suggests that MOF material can be used as an excellent platform for constructing novel biosensors for disease diagnosis and drug monitoring. There are feasible applications of MOF materials in gas sensors, its rich structure and diverse physicochemical properties provide the possibility of adsorption and detection of gas molecules. Gas sensors prepared with MOF materials have excellent performance in terms of their sensitivity, selectivity, long-term stability, and power consumption efficiency [201]. For example, the growth of ZIF-8/ZIF-67 thin films on ZnO effectively reduces the selective effect of moisture on acetone in ZnO gas sensors due to the hydrophobicity of the film. This suggests that these optimized gas sensors are able to respond more rapidly to the presence of target gases and more accurately distinguish specific gas components from complex environments, while maintaining their stable and reliable performance over long periods of time and consuming less energy in performing monitoring tasks. Using MOF materials as the precursors [3] to obtain new porous structures can help the application of humidity sensors. e.g. Zhang et al. [202] synthesized 3D mesoporous Co₃O₄ hollow polyhedra with a special structure using typical ZIF-8 material as the precursor, which was used as the base material to fabricate high-performance humidity sensors. Co₃O₄ is hydrophilic, which makes it able to interact with water molecules quickly when it comes into contact with water molecules. Co₃O₄ is hydrophilic, which enables it to rapidly interact with water molecules when it comes into contact with them, thus effectively sensing changes in humidity. It also has a large specific surface area, which means that in the same volume, Co₃O₄ provide more surface to contact and react with water molecules in the external environment, greatly enhancing the sensor’s ability to sense humidity. In addition, its 3D porous hollow structure is a major advantage, this special structure is like a well-designed “sponge”, the porous structure not only increases the adsorption sites of water molecules but also provides a convenient channel for the diffusion of water molecules inside the material, so that the water molecules can quickly move in and out of the material, thus enabling the sensor to quickly respond to changes in humidity. The porous structure not only increases the number of adsorption sites for water molecules, but also provides a convenient channel for water molecules to diffuse inside the material, allowing them to move in and out of the material quickly, thus enabling the sensor to respond quickly to changes in humidity. These unique properties combine to give the sensor excellent humidity sensing performance. In response, the sensor is optimized to provide stable and accurate operation over a wide range of humidity, from 10 % – 95 % RH. In summary, MOF materials, as an attractive class of porous materials, show a wide range of applications in supercapacitors as well as gas adsorption and catalysts. With the in-depth study of the structure and properties of MOF materials, it is believed that they will play an increasingly important role in the fields of energy storage, environmental protection, and chemical synthesis, and contribute to the sustainable development of human society.

This work aims to investigate the performance and potential advantages of MOF materials in supercapacitors and to contribute to the development of the supercapacitor field. So, we have made a comprehensive summary of the synthesis and characterization methods of MOF materials, and compared with previous reports, we have updated and more comprehensive analysis, including the application of MOF materials in other fields, and the use of machine learning to assist the synthesis of MOF materials. At the same time, we analyzed and explained the mechanism, structure, and composition of supercapacitors, systematically compared the effects of different synthesis methods on the electrochemical performance of MOFs for the first time, made a more in-depth prospect for future research directions, and put forward specific suggestions. MOF materials are innovative and prospective as a new type of electrode material, and the energy density, cycle life, and reliability of supercapacitors can be significantly enhanced by carefully designing and synthesizing different types of MOF materials. In addition, MOF materials have good chemical stability and controllable electrochemical activity, which provide new ideas and methods for the design and optimization of supercapacitors. Therefore, in-depth analysis and summarization of MOF materials are of great significance to improve the performance of supercapacitors and make positive contributions to the development of energy storage and clean technology.